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Effect of sodium chloride on the selective flotation of dolomite from apatite

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Title:
Effect of sodium chloride on the selective flotation of dolomite from apatite
Creator:
Ince, Dursun E., 1951-
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[s.n.]
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English
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xv, 168 leaves : ill. ; 28 cm.

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Subjects / Keywords:
Adsorption ( jstor )
Apatites ( jstor )
Calcium ( jstor )
Dolomite ( jstor )
Ions ( jstor )
Magnesium ( jstor )
Minerals ( jstor )
pH ( jstor )
Phosphates ( jstor )
Sodium ( jstor )
Apatite -- Florida ( lcsh )
Dissertations, Academic -- Materials Science and Engineering -- UF
Dolomite -- Florida ( lcsh )
Flotation ( lcsh )
Materials Science and Engineering thesis Ph. D
Salt ( lcsh )
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bibliography ( marcgt )
non-fiction ( marcgt )

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Thesis:
Thesis (Ph. D.)--University of Florida, 1987.
Bibliography:
Bibliography: leaves 159-167.
General Note:
Typescript.
General Note:
Vita.
Statement of Responsibility:
by Dursun E. Ince.

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University of Florida
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Copyright [name of dissertation author]. Permission granted to the University of Florida to digitize, archive and distribute this item for non-profit research and educational purposes. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder.
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EFFECT OF SODIUM CHLORIDE ON THE SELECTIVE
FLOTATION OF DOLOMITE FROM APATITE












BY

DURSUN E. lNCE

















A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY


UNIVERSITY OF FLORIDA 1987





















This work is dedicated to the

memory of my father, the late HIDIR INCE,

without whose inspiration, encouragement and special admiration for education this

work would have never been attempted.
















ACKNOWLEDGMENTS

I wish to express my sincere gratitude and respect to Professor B. M. Moudgil, my major advisor, for his invaluable help, guidance and encouragement during the course of this research.

I am very grateful to Professor F. N. Blanchard for his guidance, valuable comments, and to Professors 0. 0. Shah, E. D. Whitney, D. E. Clark, H. A. Laitinen and J. H. Simmons for very helpful discussions and comments.

I am most grateful to Professor C. T. Johnston and Mrs. L. D. Applewhite for discussions and help in FT-IR study.

With due gratitude I wish to acknowledge the encouragement,

helpful comments and constructive criticism of Drs. T. V. Vasudevan, H. Soto and Wen-Keng Shih.

I am thankful to M. May, A. Zutshi, J. Ransdell, Y. C. Cheng and J. Rogers for their help at various stages, and to Mrs. G. Keim for her help in preparing the manuscript.

Special love and appreciation is due to my wife Sevgi for her

support, help and encouragement and to my two lovely daughters, Elif and Ebru, for their patience throughout the course of this study.

Finally, I wish to thank Agrico Chemical Company and International Minerals and Chemicals Corporation for supplying the mineral samples used in this study and to acknowledge Florida Institute of Phosphate Research (Grants #82-02-023 and #85-02-067) for providing financial iii









support. Any opinions, findings, and conclusions or recommendations expressed in this work are those of the author and do not necessarily reflect the views of the Florida Institute of Phosphate Research.















































iv















TABLE OF CONTENTS


Page

ACKNOWLEDGEMENTS . . . . . . . . . . . . iii

LIST OF TABLES . . . . . . . . . . . . . ix

LIST OF FIGURES . . . . . . . . . . . . xi

ABSTRACT . . . . . . . . . . . . . . xiv


CHAPTERS


I INTRODUCTION . . . . . . . . . . . . 1


II BACKGROUND . . . . . . . . . . . . 6

Characteristics of Salt-Type Minerals . . . . . 6
Characteristics of Apatites in Florida Phosphorites . 7
Characteristics of Dolomite . . . . . . . 7
Solubility of Apatite and Dolomite . . . . . . 8
Surface Charge of Apatite and Dolomite . . . . . 9
Aging Behavior of Apatite and Dolomite . . . . . 10
Separation Studies . . . . ... . . . . . 11
Methods Based on Physical Properties . . . . . 11
Separation Based on Physico-Chemical Properties . . 12
Selective flocculation . . . . . . . 12
Selective flotation . . . . . . . 12
Flotation in the Presence of Salts . . . . . . 16


III EXPERIMENTAL . . . . . . . . . . . . 18

Materials . . . . . . . . . . . . 18
Minerals . . . . . . . . . . . 18
Apatite . . . . . . . . . . 18
Dolomite . . . . . . . . . . 18
Reagents . . . . . . . . . . . . 19
Other Chemicals . . . . . . . . . . 19
Methods . . . . . . . . . . . . . 20
Flotation . . . . . . . . . . . 20

V









Electrokinetic Measurements. .. ...... .......23
Solution Preparation .. ....... ...........24
Oleate Adsorption Tests. .. ....... ........24
Mineral Dissolution Tests. .. ...... ........27
Solubility Product Determination. .... ........29
FT-IR Tests. .. ...... ......... ......29
Contact Angle Measurements .. ....... .......31
Experimental Plan. .... ........ ...........31
Selection, Preparation and Characterization of
the Minerals. .. ... ........ .........32
Selection of the Surfactant. .. ...... .......32
Selection of the Experimental Techniques .. .......32
Flotation. .... ........ .........32
Electrokinetic measurements. ... .........33
Adsorption .. ... ......... ........33
Determination of the nature of the adsorbed
surfactant species. .. ...... ........34
Mineral dissolution studies. .... ........34
Contact angle. .... ........ .......34
Experimental Approach. .. ....... .........35


IV RESULTS. .... ........ ........ .......36

Characterization of Minerals. .. ...... .........36
Chemical Analysis .. ... ......... .......36
Surface Area and Porosity. .. ...... ........36
X-ray Analysis. .... ........ .........40
Surface Chemical Characterization .. ... ........40
Flotation Studies.....................40
Flotation Studies with Dodecylamine Hydrochloride ... 40
Single mineral flotation behavior. ..............40
Flotation of apatite-dolomite mixture with
dodecylamine .. ... ......... ......50
Flotation Studies Using Sodium Oleate as the
Collector. .. ...... ......... .......50
Single mineral flotation tests .. ... .......52
Mixed minerals .. ... ........ .......52
Flotation tests in the presence of NaCl using
dodecylamine as the collector. .. ... ......56
Flotation studies in the presence of NaCl using
sodium oleate as the collector .. .. .......59
Flotation studies in the presence of KCl and NaF 67
Electrokinetic Studies .. ... ........ ........67
Effect of Salt Addition on the Zeta Potential
of Apatite.....................67
Effect of Salts on the Zeta Potential of Dolomite . 70
Role of NaCl in the Reversal of Surface Charge
of Apatite......... ..............76
Substitution of sodium for calcium in the
apatite lattice .. ....... .........76
Adsorption Studies .. ... ........ ...........82

vi









Single Minerals Adsorption Tests ..... ........... 82
Apatite-oleate system .... .............. ... 82
Dolomite-oleate system .... .............. ...85
Mixed Mineral Adsorption Studies ..... ........... 90
Characterization of the Adsorbed Oleate Species
by FT-IR Spectroscopy ........................... ..90
IR Spectra of Pure Oleate Species ............. ....93
Nature of the Adsorbed Species at pH 10 .... ........ 96
Apatite-oleate system .... .............. ... 96
Dolomite-oleate system .... .............. ...96
Nature of the Adsorbed Species at pH 4 .. ........ .. 99
Apatite-oleate system .... .............. ... 99
Dolomite-oleate system .... .............. ...99
Oleate Species Adsorbed in the Presence of NaCl .... 102
Apatite-NaCl-oleate system ..... ............ 102
Dolomite-NaCl-oleate system ..... ........... 102


V DISCUSSION ..... .... ........................ ...105

Solution Properties of Dodecylamine and Oleate ......... ...105
Apatite-Dolomite Flotation Using Dodecylamine as
the Collector ......... ...................... 109
Apatite-Dodecylamine System .... .............. ..109
Dolomite-Dodecylamine System .... ............. ..110
Flotation of Apatite and Dolomite Mixture with
Dodecylamine ................................ 111
Changes in the surface charge and surface
chemical composition .... .............. ..112
Surface coating ...... ................. ...114
Surfactant depletion by precipitation ... ...... 114
Flotation of Apatite and Dolomite Using Sodium Oleate
as the Collector ..... ..................... 116
Evaluation of the Results and Alternatives ... ...... 116
Effect of NaCl on the Selective Flotation of
Apatite Using Dodecylamine as the Collector ....... 119
Effect of NaCl on the Separation of Dolomite From
Apatite Using Sodium Oleate as the Collector ..... ...120
Mechanism of Selective Flotation of Dolomite from Apatite 121
Effect of NaCl on the Zeta Potential of Apatite
and Dolomite ...... ..................... ...121
Role of NaCl in the reversal of surface charge
of apatite ...... ................... ...122
Dolomite structure ....... ................ 126
Adsorption of Oleate on Apatite and Dolomite .... ........ 128
Effect of Conditioning pH .... ............... ...128
Effect of NaCl Addition on Adsorption ........... ...131
Adsorption in the Mixed Mineral System .. ........ .. 133
Effect of Salt on Adsorption in the Mixed Minerals
System ........ ....................... ...133
Nature of the Adsorbing Surfactant Species ........... ...134
Apatite-Oleate System ..... ................. ...134

vii








Dolomite-Oleate System. .... ........ ....136
Preferential formation of magnesium oleate ....138
Contact Angle Studies .. ....... ........ ...145
Mechanism of Selective Flotation of Dolomite from
Apatite in the Presence of NaCl .. ........ ....147


VI CONCLUSIONS. .... ........ ........ ....149


VII SUGGESTIONS FOR FUTURE RESEARCH. .... ........ ..155


REFERENCES. .. ...... ........ ......... ...159

BIOGRAPHICAL SKETCH. .... ........ ........ ...168





































viii















LIST OF TABLES


Table No. Page

1 Sodium Oleate Distribution in Various Streams
During Adsorption Tests at pH 4.5 ............. ..25

2 Characteristics of Apatite and Dolomite Samples . 37

3 Flotation Results of 50:50 Apatite/Dolomite
Mixture Using Dodecylamine as the Collector ....... 51

4 Flotation Results of 50:50 Apatite-Dolomite
Mixture Using Sodium Oleate as the Collector ....... 54

5 Effect of NaCl on the Single Mineral Flotation
of Apatite and Dolomite with Dodecylamine
Hydrochloride as the Collector ............... ...57

6 Results of Mixed Mineral Flotation in the
Presence of NaCl Using Dodecylamine as the
Collector at pH 6.3 ..... ................. ...58

7 Results of Mixed Mineral Flotation Tests in the
Presence of NaCl Using Sodium Oleate as the
Collector ......... ...................... 60

8 Apatite Dissociation at pH 4 with and without
Added Sodium Chloride ....... ................ 79

9 Effect of NaCl on Unit Cell Dimensions of Apatite
Conditioned at pH 4 ..... ................. ...80

10 Determination of Substitution of Sodium for
Calcium in the Apatite Structure .............. ..81

11 Effect of Sodium Chloride Addition on Oleate
Adsorption on Apatite and Dolomite at pH 4.0 ....... 92

12 Dissolution of Calcium and Magnesium from
Dolomite at pH 4.0 with and without NaCl
Addition ....... ....................... ..113




ix









13 Adsorption and Flotation Results with Zeta
Potential Values for Apatite and Dolomite
with and without NaCl at PH 4.0 .. ... .......132

14 Solubility Product of Calcium and Magnesium
Oleate .. .. ...... ........ .......140

15 Electrostatic Interaction Energy of Calcium and
Magnesium with Oleate. .. ...... ........144

16 Contact Angle and Flotation Recovery of Apatite
and Dolomite at PH 4.0 .. .. ...... .......146









































x














LIST OF FIGURES

Figure No. Page

1 Location of Florida phosphate deposits .. .. ......2

2 A schematic diagram of a modified Hallimond Cell . 21

3 Hallimond cell flotation arrangement .. .. ... ..22

4 Apparatus for mixed mineral adsorption studies . . 28

5 SEM micrograph of apatite (65x100 mesh size
fraction), a) 200X; b) 1000X .. .. .. .. . ....38

6 SEM micrograph of dolomite (65x100 mesh size
fraction), a) lOOX; b) 100OX .. .. ...... ....39

7 X-ray diffractogram of apatite .. .. ...... ...41

8 X-ray diffractogram of dolomite. .. ...... ...42

9 Zeta potential of apatite as a function of pH . ..43

10 Zeta potential of dolomite as a function of pH .. 44

11 Flotation of apatite as a function of pH. .. ... ..46

12 Dolomite flotation as a function of pH. .. ... ...47

13 Flotation of apatite and dolomite (single minerals)
as a function of pH .. ... ......... .....48

14 Flotation of apatite and dolomite (single minerals)
as a function of pH .. ... ......... .....49

15 Flotation of apatite and dolomite (single minerals)
as a function of pH .. ... ......... .....53

16 Effect of sodium chloride addition on apatite and
dolomite flotation .. .. ...... ..........61

17 Effect of sodium oleate concentration on apatite
and dolomite (single minerals) flotation with and
without NaCl addition .. ... ........ ....62



X i









18 Flotation of apatite and dolomite as a function of
sodium oleate concentration in the absence and
presence of sodium chloride (single minerals) . . 63

19 Effect of pH on flotation of apatite and dolomite
(single minerals) in the presence of sodium
chloride. .. ... ........ ........ ..65

20 Flotation recovery of apatite and dolomite (mixed
minerals) as a function of pH in the presence of
sodium chloride. .. ...... ......... ..66

21 Effect of KCl addition on flotation of apatite and
dolomite as a function of pH .. .. ...... ....68

22 Apatite and dolomite (single minerals) flotation
with and without NaF addition at pH 4 .. ... ....69

23 Zeta potential of apatite with and without NaCl
addition. .. .. ......... ........ ..71

24 Zeta potential of apatite as a function of pH
with and without KCl addition. .. ....... ...72

25 Zeta potential of apatite as a function of pH with
and without NaF addition. .. ... ........ ..73

26 Zeta potential of dolomite as a function of pH with
and without sodium chloride. .. ....... .....74

27 Effect of KCl on the zeta potential of dolomite . 75

28 Zeta potential of dolomite in the presence and
absence of NaF as a function of pH .. .. .... ...77

29 Oleate adsorption on apatite as a function of
conditioning pH. .. ....... ........ ..83

30 Oleate adsorption on apatite as a function of
initial oleate concentration at pH 4.0 and 10.0 . 84

31 Oleate adsorption on apatite at pH 4.0 in the
absence and presence of sodium chloride .. ... ...86

32 Adsorption of oleate on dolomite as a function of
conditioning pH. .. ...... ......... ..87

33 Oleate adsorption on dolomite as a function of
initial oleate concentration at pH 4.0 and 10.0 . 88 34 Adsorption of oleate on dolomite at pH 4.0, with
and without added sodium chloride. .. ...... ..89

xii









35 Oleate adsorption on apatite and dolomite (single
and mixed minerals) as a function of pH .. ... ...91

36 Diffuse reflectance IR spectra of Mg-, Ca- and
Na-oleate (400-4000 cm-1 range). .. ....... ..94

37 Diffuse reflectance IR spectra of Mg-, Ca- and
Na-oleate (1200-1800 wavenumber region) .. .... ..95

38 Diffuse reflectance IR spectra of treated and
untreated apatite at pH 10.0 .. .. ...... .....97

39 IR spectra of untreated and treated dolomite at
pH 10.0 .. ... ........ ......... ..98

40 Diffuse reflectance IR spectra of treated and
untreated apatite at pH 4.0 .. ... ..........100

41 IR spectra of treated and untreated dolomite
at pH 4.0 .. ... ........ ........ ..101

42 Diffuse reflectance IR spectra of apatite
(treated and untreated) and the difference
spectrum at pH 4.0 in the presence of NaCl. .. .....103

43 Diffuse reflectance IR spectra of dolomite at
pH 4.0 in the presence of sodium chloride. .. ....104

44 Dodecylamine species distribution as a function
of pH. Total amine concentration, 1.6 x 10-2 M . 107

45 Oleate species distribution as a function of pH
Total oleate concentration, 4.0 x 10-5 M .. .......108

46 Crystal structure of dolomite, c-axis vertical
a) Layered structure b) Stereoscopic projection
of the hexagonal unit cell for dolomite (a = 4.81 A,
c = 16.00 A). .. ... ......... .......127

47 Correlation between oleate adsorption and flotation
for apatite as a function of pH .. ... .......129

48 Correlation between oleate adsorption and flotation
for dolomite as a function of pH. .. .. ........130

49 Difference IR spectra of apatite-oleate system at
pH 10, and at pH 4 in the presence and absence
of sodium chloride .. .. ...... ..........135

50 Difference IR spectra of dolomite-oleate system at
pH 10, and at pH 4 with and without sodium chloride
addition. .. .. ......... ........ ..137

xiii
















Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy


EFFECT OF SODIUM CHLORIDE ON THE SELECTIVE FLOTATION OF DOLOMITE FROM APATITE



BY

DURSUN E. INCE

December, 1987

Chairman: B. M. Moudgil
Major Department: Materials Science and Engineering

Selective flotation of dolomite from apatite was investigated

using dodecylamine hydrochloride and sodium oleate as the collector in the absence and presence of inorganic modifiers such as sodium chloride.

Separation of dolomite from apatite was anticipated from the single mineral experiments under various PH and collector concentrations. Flotation selectivity in the mixtures, however, was found to be limited. Attempts to control the flotation response of these minerals by addition of salts such as NaCl and KCl indicated that selectivity is possible. Upon optimization of the process parameters, the best selectivity was obtained at pH 4, in the presence of sodium chloride, using sodium oleate as the collector.

Electrokinetic and adsorption studies were conducted to elucidate the mechanism of observed selectivity. Zeta potential measurements xiv









demonstrated that the surface charge of apatite is reversed below its isoelectric point (pH 5.4) in the presence of sodium chloride. This was attributed to the increased rate of calcium dissolution and sodium substitution in the apatite structure.

Adsorption studies confirmed that the amount of oleate adsorbed on apatite decreases in the presence of sodium chloride relative to that in distilled water, while on dolomite the amount adsorbed remained unchanged.

FT-IR spectra of apatite in distilled water indicated the presence of oleic acid and calcium oleate on the surface. In the presence of NaCi, however, calcium oleate was not detected. This was ascribed to the surface charge reversal, which adversely influenced adsorption of anionic oleate species, in addition to the depletion of calcium sites by selective dissolution. FT-IR spectra of dolomite indicated the presence of magnesium oleate besides oleic acid, with and without the salt addition. Formation of magnesium oleate in preference to calcium oleate on the dolomite surface was explained in terms of higher charge density and electronegativity of magnesium ions,. and relatively higher rate of calcium dissolution.

Contact angle measurements on apatite and dolomite indicated a good correlation with adsorption and flotation results in the absence and the presence of sodium chloride.










xv
















CHAPTER I
INTRODUCTION

The Florida phosphate rock deposits are located in the central and northern land pebble districts. The central land pebble district, as shown in Figure 1, constitutes the major phosphate rock producing region and underlies 2600 square miles in Polk, Hillsborough, Hardee, Manatee and DeSoto Counties. Until recently, the phosphate production has been confined to the Bone Valley of the central district. These deposits, however, are being depleted and the mining will shift to the southern extension of the central district. Phosphate rock from the southern extension (lower zone matrix) is lower in grade and contains significant quantities of dolomitic limestone (Ca, Mg carbonate) impurities in addition to quartz and clays (Lawver et al., 1982a). Beneficiation of Florida phosphate rock by a "double float" or Crago flotation process (Crago, 1940) has been in use commercially since 1937. However, with the conventional processing techniques, dolomite, usually reported as weight percent MgO, cannot be selectively removed from apatite, the phosphate mineral. On the other hand, it is generally agreed that more than 1% MgO in the final phosphate concentrate would present problems during chemical processing to manufacture phosphoric acid, an intermediate product used in the production of fertilizers. The presence of MgO in quantities greater than the specified amount would:


1

















Hammon JAC SONVLLEI
"c 0
jBake
1 Count Columbia
County

Hillb rough Polk Co nty County r-- Ak LAN TAM BA TOW
Manate T County .Hardee C
\Te Soto ounty
Northern District

Central District MIAMI

Southern Extension






Figure 1. Location of Florida phosphate deposits.









3

1. Increase the viscosity of the phosphoric acid resulting in

higher pumping costs (Cate and Deming, 1970);

2. Cause an increase in sulfuric acid consumption and

induce foaming;

3. Form excessive sludge during phosphoric acid production;

4. Precipitate as complex salts (MgNH4P205) and lead to

clogging of filters (Becker, 1983);

5. Interfere in the production of certain super phosphates

(Becker, 1983).

Phosphate rock, which is essentially carbonate-fluorapatite,

(Fantel and Rosenkranz 1983; Whippo and Murowchick, 1967), contains magnesium impurities in one of the following forms:

1. Ionic substitution in the apatite structure;

2. Second phase dolomite in apatite;

3. Discrete dolomite particles.

The MgO content in francolite (carbonate-fluorapatite) occurs both as fine inclusions of dolomite and as Mg in the francolite structure, according to Lawver et al. (1982a). The range of "lattice Mg" calculated as MgO was found to be 0.40% from 31 samples studied by Lawyer and co-workers. Recently, Blanchard et al. (1986) have conducted a systematic investigation using 18 samples from Florida phosphate fields. Measurements of dolomite content by X-ray diffraction and chemical analyses of these samples indicated that the average excess MgO content above that accounted for by dolomite is about 0.57% by weight. This represents a reasonable estimate of the amount of MgO substituting in the apatite structure. Additionally, McClellan (1980)









4

showed that the "a" unit-cell dimension for carbonate-fluorapatite can also be used to estimate the Mg substitution for Ca. The average unit-cell dimensions measured for 10 samples corresponded to 0.55% (by weight) MgO in the apatite, in a good agreement with the average of

0.57% MgO estimated by Blanchard and co-workers (1986).

Physical methods of separation obviously cannot remove the

substituted magnesium. However, the second phase dolomite and discrete particles upon liberation can be separated from apatite by physical and physi co-chemi cal processes.

Separation of dolomite from apatite by selective flotation has been the focal point of research in the last decade, because physical methods such as gravity and magnetic separation have not shown much promise in beneficiating dolomitic phosphate ores. Flotation studies by the phosphate industry (Lawyer et al., 1978; Snow, 1979; Dufour et al., 1980; Lawyer et al., 1980; Lawyer et al., 1982b), the U.S Bureau of Mines (Llewellyn et al., 1982 and 1984) and the Tennessee Valley Authority (Lehr and Hsieh, 1981) have resulted in the development of a number of processes. It should be pointed out that these studies evolved from engineering applications, and an understanding of the fundamentals which govern the selectivity of the proposed processes were not fully established. This is believed to be a serious limitation in the optimization of these processes. Nevertheless, past efforts have provided a direction for developing suitable separation techniques.

A systematic study involving an apatite/dolomite-anionic surfactant system was conducted by Moudgil and Chanchani (1985a, 1985b and 1985c), and Chanchani (1984) which has resulted in the development of a "two-








5

stage conditioning" process. In this process, the feed is conditioned at pH 10 followed by reconditioning at a pH lower than 4.5 before flotation. Dolomite is selectively floated out following the two-stage conditioning process. Bench-scale optimization of this process using natural samples appears promising.

Flotation separation of apatite from dolomite using cationic collectors has also been studied by researchers at International Minerals and Chemicals Company (Lawver et al., 1980; and Snow, 1979), followed by Soto and Iwasaki (1985 and 1986). The role of the dissolved mineral species, flotation pH and solution chemistry of the surfactant, however, was not taken into account in the above studies.

The surface modifiers used in the past for apatite separation from carbonates are mostly phosphate salts, which are uneconomical. Also, changes in the characteristics of the matrix (ore), even from the same location, have been observed. It is conceivable that a given flotation scheme would be applicable to a specific ore. It is imperative, therefore, to develop flotation processes which possibly would have applicability to a wide variety of ores. Additionally, understanding the mechanism of selectivity would be helpful in overcoming the difficulties in processing different ores and ensuring the usefulness of the method for treating ores of different characteristics.
















CHAPTER II
BACKGROUND

Characteristics of Salt-Type Minerals

Salt-type minerals such as apatite and dolomite are characterized by solubilities higher than those of most oxides and silicates, but lower than simple salt minerals such as halite and sylvite. Flotation of such minerals from the associated gangue and from each other is of major practical importance. For example, apatites constitute the largest tonnage of any raw material beneficiated by froth flotation techniques in the United States as well as several other countries (Hanna and Somasundaran, 1976).

Separation of the salt-type minerals from oxide and silicate

minerals has been achieved and used commercially. However, separation of these minerals from each other is complex and the problems involved remain unresolved in many cases. It has been reported that the differences between flotation characteristics of various salt-type minerals may not be greater than those between samples of a single mineral from different deposits (Sorensen, 1973). The similarities in the flotation response of these minerals is generally attributed to their comparable surface chemical behavior. In addition, interaction of dissolved mineral species with collector molecules is considered to contribute to the poor selectivity. Consequently, it has been suggested that the use of inorganic or organic modifying agents might result in


6









7

the desired selectivity in these systems (Klassen and Mokrousov, 1973; Joy and Robinson, 1964).



Characteristics of Apatites in Florida Phosphorites

Phosphorites are sedimentary rocks with 15 to 20% P205 content. In Florida deposits, apatite is the most abundant phosphorite mineral generally found in the form of carbonate-fluorapatite (Calo(P04)6X(C03)X FO4xF2) (McClellan, 1980). The carbonate-fluorapatite (also known as francolite) can have extensive substitutions such as carbonate for phosphate; and other cations such as Mg, Na, Mn and K for calcium (Lehr et al., 1967; McConnell, 1952, and McConnell and Gruner, 1940). Apatite has a hexagonal lattice structure and lattice parameters are dependent on the extent of substitution. It is generally composed of microcrystals which vary in size from 0.02 to 0.20 microns (Lehr et al., 1967; Smith and Lehr, 1966). High surface area of apatites has therefore been attributed to this crystal size. In addition, much of the south Florida phosphate rock is composed of grains which are mixtures of apatite, dolomite and other constituents. Phosphorite sediments are complex because they are the product of several different sedimentary systems, and are formed by intermixing of phosphates, carbonates, organic matter, glauconite, terrigenous and siliceous sediments (Riggs, 1979).



Characteristics of Dolomite

Dolomite along with calcite is the most abundant carbonate mineral. The crystal structure of dolomite is similar to that of calcite. The









8

description of the dolomite [CaMg(C03)21 structure is provided by retaining the calcite (CaC03) structure, but simply substituting Mg atoms for the Ca atoms in every other cation layer. The alternating CaMg arrangement of dolomite has some similarities with calcite, but the c-glide present in calcite is destroyed. Dolomite has a rhombohedral crystal structure with a Ca-0 bond length of 2.38 A, and Mg-O bond length of 2.08 R. This results in oxygen lying closer to Mg than Ca in the dolomite structure (Reeder and Sheppard, 1984)



Solubility of Apatite and Dolomite

Charge characteristics of the solid/liquid interface and chemical composition of the aqueous phase depends on the solubility of the minerals. When the minerals come into contact with water, the constituent species such as Ca++, Poi- and F from apatite, and Ca++, Mg and COJ- from dolomite will be transferred from the structure into,"/ the solution. This dissolution will continue until the chemical potentials of the species in solution and the solid phases reach an equilibrium. The solubility of the salt-type minerals often varies over a wide range, because the chemical potentials of the species in solution and in the solid are affected by a number of factors such as degree of hydration, solid solution formation and presence of other components in solid or solution. It is known that both apatite and dolomite, in contrast to simple salts, dissolve with their ions undergoing various hydrolysis and complex formation reactions.

There have been a number of publications on the solubility of apatite with marked differences in the data reported. Hanna and









9

Somasundaran (1976) have attributed these discrepancies to crystal structure modifications, the presence of impurities, and added electrolytes. Saleeb and de Bruyn (1972), however, have shown that a constant solubility product can be obtained if a stoichiometric compound is prepared. These investigators have reported a pK50 value of 119.1 for fluorapatite.

In the case of dolomite, the lack of understanding of its precipitation process has contributed to the discrepancy of the solubility products reported by different investigators. The published pK50 values are in the range of 16.5 to 19.5 for dolomite (Stumm and Morgan, 1981).



Surface Charge of Apatite and Dolomite

It is known that in the case of insoluble oxides the surface charge is mostly determined by OH- and H+, i.e., by the pH of the suspension in the absence of any specifically adsorbing ions. The charge of the ionic solids such as silver iodide is considered to be the result of preferential dissolution or adsorption of Ag+ or I-. The magnitude and sign of the surface charge is determined by the solution concentration of the constituent ions.

Surface charge development on minerals such as apatite and dolomite is much more complex. It is controlled by preferential dissolution of constituent species, and their hydrolysis products in addition to OH- and H+ (Saleeb and de Bruyn, 1972) ions. It has also been shown that the impurity ions present as substituents for lattice ions produce significant changes in the electrokinetic characteristics.








10

Somasundaran (1968) obtained an isoelectric (IEP) point of pH 4 for natural apatite that was partially saturated with fluoride. It has been reported that the nature of pretreatment plays an important role on the IEP of apatite (Somasundaran, 1972). The IEP values given in the literature for fluorapatite and hydroxyapatite vary between pH 3.8 and

8.5. Chanchani (1984) found the IEP of Florida apatite to be at pH 5.5. In the case of dolomite, Predali and Cases (1973) have determined the IEP at pH less than 7 using a natural sample from Kosice, Czechoslovakia. Chanchani (1984) has reported two IEPs (pH 5.5 and pH 10.5) for dolomite from Florida. The second IEP at pH 10.5 was attributed to the precipitation of hydrolysis products such as magnesium hydroxide on the dolomite surface.



Aging Behavior of Apatite and Dolomite

The soluble minerals such as apatite and dolomite exhibit aging phenomenon, i.e., a change in the pH of the aqueous slurry as a function of time. Aging behavior of apatite and dolomite samples from Florida has been examined by Chanchani (1984) using 1 wt % suspension of these minerals. It was shown that in the case of apatite equilibrium is reached after about 600 minutes. Initial pH of 4 and 10 was observed to shift to a value of 6 and 7. respectively, after equilibration. Similar pH shifts for apatite were also observed by Somasundaran (1968) which were attributed to the dissolution of the mineral. In the case of dolomite, the equilibrium pH was found to be between pH 8.2 and 8.5.








11

Separation Studies

The past research efforts to develop a suitable technique for separation of carbonates from apatite can be divided into two broad categories, those in which differences in physical properties such as specific gravity, conductivity, hardness, etc., were utilized and those where surface chemical properties of the minerals were exploited to achieve the desired separation. A brief discussion of these efforts is presented below.



Methods Based on Physical Properties

Apatite and dolomite have relatively close physical properties such as specific gravity and hardness, thereby making it difficult to achieve their separation based on these properties. Both of the minerals are also nonmagnetic. Hence, the separation techniques based on such properties are not feasible. However, the "apparent" densities of the two minerals have been found to be sufficiently different for the possible application of heavy media separation (Lawver et al., 1982a). The difference in the specific gravity is attributed to the highly porous nature of the large dolomite particles as compared to the same size apatite particles. The effectiveness of the heavy media separation process is reported to be limited to the coarse size (pebble size) only, which is found in lesser quantity in the southern district. Selective attritioning has also been attempted by Soto and Iwasaki (1986). Results from these studies indicated that only 40-60% of the dolomite can be eliminated and that flotation is necessary for further reduction of MgO in the apatite concentrate.








12



Separation Based on Physico-Chemical Properties Selective flocculation

Selective flocculation of dolomite from apatite was attempted for the first time by Moudgil and Shah (1986) using polyethylene oxide (PEO) as the flocculent. It was shown that PEO flocculates dolomite, but not apatite in single mineral tests. However, mixed mineral tests did not exhibit the expected selectivity. The loss in selectivity in the case of mixed minerals was attributed to polymer-induced entrapment, which occurs due to a limited affinity of the polymer for the inert mineral (apatite). Incorporation of the polymer-coated apatite particles into dolomite flocs was explained on the basis of the higher probability of polymer bridging due to the larger number of "active" sites on dolomite as compared to apatite particles. Further efforts are underway to reduce the adsorption of the polymer on apatite so as to achieve the desired separation.



Selective flotation

Separation of dolomitic limestone from sedimentary phosphates has been the subject of studies by various researchers. Due to the complex structure and the presence of different amorphous and porous phosphates in these sedimentary deposits, separation of carbonates such as calcite and dolomite from phosphates has not always been feasible. The amount and type of impurities present have shown considerable variation even within the same ore deposit. Despite these problems, development of a








13

number of flotation processes has been reported to separate dolomitic impurities using both cationic and anionic collectors.

Cationic flotation of apatite from dolomite. Flotation of apatite (francolite) from dolomite using cationic collectors was investigated by the International Minerals and Chemicals Corporation in the late seventies and early eighties (Snow, 1979; Baumann and Snow, 1980; and Lawver, 1980). These investigators developed a process which reduces the MgO content of the conventionally floated (double flotation) material to 1% or less yielding more than 90% BPL recoveries. This process involved a rougher float followed by several cleaner stages using a primary aliphatic amine in combination with kerosene as the collector.

The above process was later studied by Soto and Iwasaki (1985 and 1986) to elucidate the mechanisms involved. These researchers concluded that there is a stronger chemical interaction between the cationic collector and the phosphate ions present at the apatite surface. The selectivity was attributed to the lower solubility of the reagentphosphate compound as compared to that of the reagent-carbonate complex formed. Other noteworthy studies with cationic collectors have been conducted to separate calcite from apatite (Hanna, 1975, and Samani et al., 1975).

Anionic flotation of dolomite from apatite. Separation of

calcite and in some cases dolomite from phosphate ores using anionic collectors has been studied extensively in the past decade. A detailed review of these studies has recently been presented by Moudgil and









14

Somasundaran (1986) and Chanchani (1984). A brief summary of recent developments is presented below.

Flotation of dolomite from apatite in the presence of inorganic depressants such as phosphates and fluorides at pH 5.6-6.2 was studied by Lawyer et al. (1984) using fatty acids and their soaps, including petroleum sulfonates. Reportedly, the best results were obtained with sodium tripolyphosphate and hexametaphosphate using a proprietary anionic collector. This process resulted in phosphate concentrates containing less than 1% MgO at 50-90% BPL recoveries from a feed of less than 48 mesh size fraction containing about 2% MgO.

Hsieh and Lehr (1985) at TVA used diphosphonic acid to depress apatite while floating dolomite with oleic acid. This process reduced the MgO content of the concentrate to less than 1% from a feed containing 1.9% MgO at 83% apatite recovery. In another process developed by TVA (1983). H2S04 was added to the concentrate to separate calcareous phosphate ores. Selective flotation of carbonates was attributed to differential desorption of the fatty acid collector on the phosphate mineral. No experimental data was, however, presented to support this hypothesis.

Llewellyn et al. (1982) at U.S. Bureau of Mines depressed

dolomite by the addition of sodium silicate and floated apatite at pH 9.2-9.6. In cases where the MgO content of the final concentrate was not reduced to less than 1%, further removal of dolomite by S0 leaching was recommended. Rule et al. (1970 and 1985), also at USBM, depressed apatite with fluosilicic acid while floating carbonaceous impurities using fatty acid emulsion under slightly acidic pH conditions.









15

Dufour et al. (1980) at Minemet Recherche, France, depressed phosphate at pH 5.5 and floated dolomite after attritioning; however, the reagents used were not disclosed. The apatite recoveries ranged from 48 to 85 percent with the final product containing less than 1% MgO.

Johnston and Leja (1978) also selectively floated dolomite at pH 6, using oleic acid as the collector by depressing fluorapatite by phosphate ions. The difference in flotation was explained on the basis of adsorption of phosphate ions onto apatite by hydrogen bonding. Dolomite flotation was attributed to hydrogen bonding of oleic acid on dolomite in addition to CO2 gas evolution.

In an effort to develop new reagents that interact selectively, Eu and Somasundaran (1986) used alizarin red S, a dye that stains calcite, and achieved effective separation of calcite from apatite with sodium oleate as the collector. They found that alizarin red S adsorbs more on apatite than calcite and consequently acts as an apatite depressant.

Other noteworthy studies include that of Bushel et al. (1970); Onal (1973); Ratobylskaya et al. (1975); Lawyer et al. (1978, 1980, 1981); Kiukkola (1980); Dufour et al. (1980); Baumann and Snow (1980); Houot and Polgaire (1980); Lehr and Hsieh (1981); Clerici et al. (1984); Rao and coworkers (1985), and Atalay et al. (1985).

It should, however, be noted that most of the above studies have evolved from engineering studies. Hence, the fundamental principles governing these processes were not studied which are required for developing more efficient techniques.









16

A systematic and thorough study of apatite-dolomite separation using anionic collectors has been conducted by Moudgil and Chanchani (1985a and 1985b). The surface chemical and dissolution characteristics of the minerals were taken into account in addition to solution properties of the surfactant (sodium oleate), and a "two-stage flotation" process was developed. This process involves conditioning the feed at pH 10 followed by reconditioning at a pH less than 4.5 to selectively float out dolomite. Bench scale testing of this process has yielded favorable results (Moudgil, 1987).



Flotation in the Presence of Salts

As mentioned above apatite-dolomite separation has been studied

(Atalay et al., 1985; Rule et al., 1985; Hsieh and Lehr, 1985; Lawyer et al., 1984; Llewellyn et al., 1982; Johnston and Leja, 1978; and Onal, 1973) using phosphate salts as depressants for apatite. However, no systematic studies are reported on the role of the added salts on the flotation behavior of these minerals. It should be noted that in these studies phosphate minerals were depressed in the presence of phosphate salts.

Maslow (1971) and Strel'tsyn et al. (1967) have reported the adverse effect of salt addition on apatite flotation in apatitenepheline ore. These workers found that when NaCl addition exceeded 0.5 kg/ton, the ore was suppressed at pH 9, but it was overcome by addition of NaOH and sodium silicate. Additionally, Gruber et al. (1986) have reported successful separation of carbonates such as calcite from apatite in sea water from the Santo Domingo phosphate deposit in Baja









17

California Sur. Apatite was found to be depressed at pH less than 5.5 in sea water when fatty acid was used as the collector. In the presence of fresh water, apatite was found to be naturally depressed at pH 3.0 and below. No efforts, however, were made to study the mechanism involved in such a separation.















CHAPTER III

EXPERIMENTAL

Materials

Minerals

Apatite

A sample of high-grade phosphate rock (apatite) was procured from Agrico Chemical Company (Mulberry, Florida). This sample (16x150 mesh) was screened to obtain a 65x100 mesh fraction, which was deslimed, dried and passed through an electrostatic separator after heating to 140 C to remove the silica grains. The 65x100 mesh sample was used for flotation and adsorption studies. FT-IR and electrokinetic studies, however, were conducted on a portion of this sample which was ground to

-325 mesh.



Dolomite

This sample, supplied by International Minerals and Chemicals Corporation (Bartow, Florida), was hand picked and crushed using a Chipmunk crusher and hand ground to maximize the yield of 65x100 mesh fraction. The dolomite was deslimed and dried at 140 C before removing silica by electrostatic separator. Electrokinetic and FT-IR studies were conducted on a portion of this sample ground to -325 mesh. The samples were stored in a glass jar and used as required.




18










19

Samples of completely crystalline New Jersey dolomite and

fluorapatite obtained from Geology Department, University of Florida (Blanchard, 1987) were used for contact angle measurements. Chemical analysis of the apatite sample indicated 37.51% P205. 0.14% MgO and

1.15% acid insoluble. Crystalline New Jersey dolomite analyzed 17.73% MgO, 0.36% P205 and 0.95% acid insoluble.



Reagents

Dodecylamine hydrochloride obtained from Eastman Kodak Company, and purified sodium oleate purchased from Fisher Scientific Company, were used as the cationic and anionic collectors, respectively, for flotation experiments.

A mixture of unlabeled and 14C labeled oleic acid was used for the adsorption experiments. Unlabeled oleic acid (gold label grade) was obtained from Aldrich Chemical Company. 14C labeled oleic acid was purchased from ICN Pharmaceuticals in nitrogen-sealed ampules of 0.1 mCi.



Other Chemicals

All other chemicals such as calcium and magnesium standards, HN03, KOH, NaOH, NaCl, KCl, NaF, etc., were of reagent grade purchased from Fisher Scientific Company.

Triple distilled water of less than 1.2 micromhos conductivity was used in the flotation experiments. All other solutions were prepared using triple distilled water deaerated by bubbling nitrogen for two hours.









20



Methods

Flotation

Flotation experiments were conducted using a modified Hallimond cell (Fuerstenau et al., 1957; and Modi and Fuerstenau, 1960) shown in Figure 2. This cell consists of two parts connected by a ground glass joint. The lower part of the cell is fitted with a fritted glass, having uniform pore size. A Teflon-coated magnetic stirring bar is used to maintain the particles in suspension. In the Hallimond cell technique, the hydrodynamic variables such as agitation, airflow rate, and flotation time can be controlled.

The complete Hallimond cell flotation assembly is illustrated in

Figure 3. The details of the set-up and operation are similar to those used by Moudgil (1972), Ananthapadmanabhan (1980) and Chanchani (1984) and are only briefly described below.

Nitrogen gas was used for the flotation experiments, which was

first purified by passing through an ascarite column (D) for removing the CO2 (see Figure 3). Subsequently, the gas passes through a water trap (T1) to remove any ascarite fines carried over. Traps (T2 and T3) are provided to prevent back suction of water to the ascarite column. The purified gas is passed to a 50 liter glass reservoir (R), which acts as a buffer tank to supply the gas at a constant pressure. The pressure in the tank is measured by the manometer (M). The outlet of the gas reservoir is connected to the Hallimond cell (HC) through a solenoid valve (V) and flow meter (F). The solenoid valve is energized using a







21









4?











29/42 1 cm
9mm Tubing Jointd
Concentrate -""
Stem Stirr
Bar
Fritted Glass
Purified
Cork Stopper Nitrogen
Magnetic Stirrer







Figure 2. A schematic diagram of a modified Hallimond Cell.










22




























4-1 cl; Z- 00 LU Z-1









23

timer, which can be set to allow the gas flow and stirring for any length of time.

The apatite, dolomite, or the mixture of these minerals were aged in 100 ml volumetric flask for 20 minutes and conditioned after reagent addition for a period of 5 minutes by slow tumbling at 8 rpm. Whenever required, minerals were aged in the salt solution of specified concentration. The PH of the suspension was measured before aging and after conditioning in all cases and the latter is reported as the PH of flotation. The suspension was transferred to the Hallimond cell with minimum turbulence and floated for one minute using nitrogen gas at a flow rate of 50 ml/min. The floated and unfloated fractions were dried at 50 OC, and flotation recovery was calculated based on the weight of the dried samples. In the case of the mixtures, float and sink fractions were pulverized and leached in aqua-regia before analyzing for P205 and MgO contents.



El ectroki neti c Measurements

Zeta potential measurements were conducted using a Pen Kem Model 501 Laser Zee Meter. Suspensions of 0.1 wt % apatite or dolomite were prepared using -325 mesh sample. These were stirred with a Tefloncoated magnetic bar for approximately 21 hours at natural PH. Fifty milliliter samples of the aged suspension were equilibrated at the desired pH for 3 hours before they were transferred to the cell for zeta potential measurements.









24

Solution Preparation

Stock solutions of 5.0x10-3 kmol/m3 sodium oleate or dodecylamine hydrochloride were prepared in deaerated distilled water as required for flotation tests. The solution pH was adjusted to approximately 11.5 and

8.0 for oleate and dodecylamine, respectively, using NaOH or HCl. It was diluted daily and used as needed, and the stock solution was used for 5 days only.

Carbon-14 labeled radioactive oleic acid solution was also prepared as described above for the unlabeled sodium oleate solution after evaporating benzene. The stock solutions were refrigerated and used up to one week. Solutions of desired concentrations were obtained by mixing 14C labeled oleic acid with unlabeled oleate solution.



Oleate Adsorption Tests

To ensure correct adsorption measurements, the solid samples were rinsed with distilled water of the same pH value after the adsorption experiments to remove any precipitated or entrapped oleic acid from the solids. Control tests were conducted to check the oleic acid coating on the surface of the vials. The results presented in Table 1 indicated that approximately 20% of the initial collector present in the solution reported as possible coating on the vial surface. Out of the total, only 5% was determined to adsorb on the mineral (apatite) and less than 2% was found to be in the rinse solution at pH 4.5. This revealed that there was not any significant desorption of oleic acid as a consequence of rinsing of the solids. It should be noted that if the solution depletion method were to be followed in this case, the amount adsorbed










25












0 C 1
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26

on the mineral would have been calculated to be five-fold higher, resulting in misleading conclusions. In addition, adsorption of oleate on quartz was measured at pH 4 to ensure that the results obtained with apatite and dolomite do not include any coating of oleic acid on the surface. Since quartz is not expected to absorb oleate at pH 4, any adsorption of oleic acid can be presumed to be physical coating. Results of these tests indicated that there was no physical coating of oleate on quartz, for up to a sodium oleate concentration of 6.0x10-4 kmol/m3.

The amount of oleate or oleic acid adsorbed was therefore

determined in this study by direct counts on the solids in the oleic acid precipitation region, i.e., at pH 8.0 and below. Above this pH value, the amount adsorbed was determined by the solution depletion method because test results indicated that oleate adsorption on the vials is less than 0.5% of the total amount present.

Single mineral adsorption tests were conducted by adding 11 ml of sodium oleate solution of the desired pH to 0.1 g of 65xlOO mesh mineral sample in a 20 ml polypropylene vial. The samples were then conditioned by tumbling at 8 rpm for 5 minutes. The amount adsorbed on the minerals at pH 8 and below (oleic acid precipitation region) was determined by direct counts on the solids after cocktail addition. To determine the amount adsorbed above pH 8, as mentioned earlier the solution depletion method was followed. The solids were first allowed to settle for 5 minutes and the supernatant was analyzed for the amount of surfactant depleted. This method was followed because no major surfactant precipitation was anticipated at pH 8 and above.









27

The mixed mineral adsorption tests were conducted in a glass cell

arrangement as shown in Figure 4. This cell design allows mixed mineral conditioning of apatite and dolomite in the same medium without coming in contact with each other. A 0.5 g, 65xlOO mesh sample was used for these tests. The sample volume in each side of the cell was maintained at 50 ml and the conditioning was identical to that of single mineral experiments with respect to pulp density, conditioning time, and tumbling speed. Adsorption tests in the presence of sodium chloride were conducted in the same manner after aging the mineral samples in

2.0x10-2 kmol/m3 NaCl.

Analysis of the 14C labeled oleic acid was performed using a Beckman Model LS 1800 liquid scintillation counter after mixing the solids or solution with a scintillation cocktail, "Scintiverse-II" (obtained from Fisher Scientific Company). In the case of mixed minerals, the amount adsorbed was also determined directly on the solids in the entire pH range since the solution depletion method could not be followed.



Mineral Dissolution Tests

The dissolution of ions from apatite or dolomite at a given pH

value was determined by agitating one gram of 65xlOO mesh solids in 100 ml of distilled water for a known time interval using a slow speed (8 rpm) tumbler. At the end of the tumbling period, a 10 ml sample of the supernatant was withdrawn for analysis. The supernatant was centrifuged at 734 G (2500 rpm) for 30 minutes to remove fines created during agitation, before calcium and magnesium ion analysis was carried out











28
























41 (n

0
-W
CL
0





CL


E




IC

o
4






CL
CL









LL-









29

using a Perkin Elmer Plasma II inductively coupled plasma (ICP) emission spectrometer.



Solubility Product Determination

The solubility products of calcium and magnesium oleate at pH 10

were determined using the nephelometric method. Known concentrations of calcium and magnesium solutions were prepared from 1000 ppm standards and were mixed with the sodium oleate solution of desired concentration. The mixtures were initially stirred for a period of one minute and then allowed to stand for 10 minutes before making turbidity measurements using a Hach Model 2100 turbidimeter. The instrument was calibrated by measuring the turbidity of solutions containing known concentrations of sodium oleate and calcium/magnesium ions. The solubility product (Ks) was obtained using the concentrations of calcium/magnesium and oleate ions, at which a sharp increase in turbidity was observed.



FT-IR Tests

Samples used for FT-IR analysis were prepared as follows: A one gram, -325 mesh size fraction, sample was suspended in 100 ml of distilled water in a 100 ml volumetric flask and the pH was adjusted to the desired value using HN03 or KOH. After 20 minutes of aging, the supernatant was partly replaced by the surfactant solution such that a concentration of 5.0x10-3 kmol/m3 sodium oleate was obtained. The suspension, with the added surfactant, was then tumbled for one hour at 8 rpm. At the end of the conditioning period, the solids were separated from the supernatant by centrifuging the sample at 734 G (2500 rpm) for










30

30 minutes. Next, the solids were resuspended twice in distilled water of the same pH value as the conditioning solution and centrifuged again to remove the entrapped or free oleate solution from the solids. The pure apatite and dolomite samples were also treated identically and centrifuged the same way to remove water. The characterization of adsorbed oleate species in the presence of sodium chloride was carried out by aging the solids in 2x10-2 kmol/m3 NaCl solution, followed by conditioning in a 5x10-3 kmol/m3 sodium oleate solution of the same NaCl concentration used during aging. The solid samples were then dried at 50 OC for 24 hours and stored in a vacuum desiccator until used.

Freshly ground, dried KCl was used as a reference throughout these experiments. The calcium and magnesium oleate precipitates were obtained by mixing sodium oleate solution with those of calcium and magnesium chloride, respectively. The reagentized pure mineral and the precipitate samples were first mixed with 90% KC1 before introduction into the diffuse reflectance cell.

The diffuse reflectance IR spectra were obtained on a BOMEM DA3.10 Fourier Transform Infrared Spectrometer equipped with a 25 cm path length Michelson interferometer fitted to a KBr beam splitter. The optical interferometer is connected to a high speed vector processor, which performs the Fourier Transform and numerical fitting of the collected interferograms. The spectra were recorded with a spectral resolution of 0.5 cm/sec. Typically, 64 to 256 scans were obtained under vacuum conditions to minimize interference from atmospheric moisture and carbon dioxide.









31

Contact Angle Measurements

The samples for contact angle measurements were prepared by cutting the rock using a diamond saw and mounting them on an epoxy base. The surface of these samples were abraded using sandpaper with 400 grids and polished using diamond cloth. The reagentization of samples was similar to the method used for FT-IR analysis, except that aging and conditioning in this case were performed by simply immersing the solid in solution for 10 minutes. The solution was agitated using a magnetic stirrer bar. After conditioning, the solid samples were dried in an oven at 30 OC before measuring the contact angle. The water droplets were dispensed onto the sample surface using a micro-syringe. The contact (tangent) angle formed between a "sessile" drop and the mineral surface was determined directly, using a NRL Contact Angle Goniometer Model 100-00. Following each test, the samples were abraded and repolished to remove the surface coating before making subsequent contact angle measurements.



Experimental Plan

The experimental plan employed in this investigation involved a) Selection, preparation, and characterization of the minerals; b) Selection of surfactant;

c) Selection of the experimental techniques; and d) The experimental approach.









32

Selection, Preparation, and Characterization of the Minerals

Natural apatite and dolomite samples from Florida phosphate

deposits were selected. Both apatite and dolomite samples were crushed and ground to obtain 65xl00 mesh size fractions for flotation experiments. A portion of these samples was ground to -325 mesh for electrokinetic and FT-IR spectroscopic studies. The samples were characterized for chemical composition, surface area and porosity, and surface charge behavior. X-ray analysis and Scanning Electron Microscopy studies were also carried out on the samples.



Selection of the Surfactant

Sodium oleate (anionic) and dodecylamine hydrochloride (cationic) surfactants, which constitute the active components of fatty acid and fatty amine collectors, respectively, were used in this study. It should be noted that both of these surfactants hydrolyze significantly. Nature of the dominant species present therefore would be governed by the pH of the solution.



Selection of the Exoerimental Techniques

The experimental techniques selected are described in the following section.



Flotation

Microflotation tests using a Hallimond cell was selected for

flotation studies. This technique enables close control of hydrodynamic variables such as the agitation, air flow rate, and flotation time. It









33

should be noted that a direct correlation of Hallimond cell results to a laboratory cell in terms of percent recovery, etc., may not be appropriate, but the trends obtained are expected to be similar.



Electrokinetic measurements

Zeta potential measurements with and without modifiers in combination with dissolution and flotation can lead to a better understanding of the various species adsorbed at the solid/liquid interface. Zeta potential of the minerals was measured by the electrophoretic mobility technique using a Pen Kem Model 501 Laser Zee Meter.



Adsorpti on

These tests were conducted to obtain information on the amount of oleate/oleic acid adsorbed on the mineral surface under different experimental conditions. Oleate adsorption measurements using 14C labeled oleic acid were conducted using a Beckman Model LS 1800 liquid scintillation counter.

To avoid experimental artifacts in the adsorption studies, the following precautions were taken:

(1) Oleate adsorption tests were conducted using the same 65x100 mesh

size fraction that was employed for flotation experiments.

(2) Reagentizing time of 5 minutes was maintained for both adsorption

and flotation to avoid variations due to kinetics involved.

(3) The exact PH value of oleic acid precipitation depends on the

total oleate concentration and the reaction constants used. Hence,









34

the low solubility of oleic acid in the neutral and acidic PH range

was taken into account to ensure correct analyses.



Determination of the nature of the adsorbed surfactant species

Adsorption studies are not expected to yield any information about the nature of the various oleate species such as ionic oleate monomers or acid-soap complex. Such information can be helpful in explaining the differences in flotation behavior of these minerals under different PH conditions. FT-IR spectroscopy was employed to study the nature of the adsorbed species, using a BOMEM 0A3.1O model instrument.



Mineral dissolution studies

These tests can provide information about the dissolution

characteristics of various ionic species from the minerals which can be helpful in explaining the surface charge and adsorption behavior. Analysis of the dissolved ions such as calcium and magnesium was carried out using a Perkin Elmer Plasma II, inductively coupled plasma (ICP) emission spectrometer.



Contact angle

These measurements under different experimental conditions can

yield information about relative hydrophobicity of the mineral surfaces. Contact angle measurements were conducted with and without salt addition, using a NRL Contact Angle Goniometer (Model 100-00).









35

The Experimental Approach

The experimental approach involved the following steps:

(1) Flotation tests to identify the conditions for selectivity.

a) Study of the flotation response of apatite and dolomite

individually to determine the reagent (dodecylamine and oleate) concentration and pH values where differences in the flotation

behavior are maximum.

b) Flotation of the mixed minerals to evaluate the selectivity

predicted by the single mineral tests, and investigation of the

reasons if observed selectivity is not achieved.

c) To select appropriate experimental conditions based on (b) to

achieve the desired separation.

(2) Elucidate the mechanisms by conducting the following studies:

a) Electrokinetic measurements as a function of pH with and without

chemical additives to establish the surface charge.

b) Adsorption tests under conditions selected on the basis of

flotation experiments.

c) A study of the nature of the surfactant species by FT-IR under

the conditions used for adsorption tests.

d) Evaluation of the relative hydrophobicity of the mineral

surfaces by contact angle measurements.















CHAPTER IV
RESULTS

Characteristics of Minerals

Chemical Analysis

Chemical analyses of the apatite and dolomite samples were

conducted using a Perkin Elmer Plasma Il inductively coupled plasma (ICP) emission spectrometer. It is clear from the data presented in Table 2, that the apatite sample is essentially free of dolomite and vice versa. The major impurity occurring in these samples is silica, which is reported as acid insoluble.



Surface Area and Porosity

The surface area and pore size distribution of the minerals were determined using nitrogen gas as the adsorbate with a Quantachrome Autosorb-6 unit. Surface area measurements of apatite and dolomite presented in Table 2 indicated that these samples are highly porous. Pore size distribution of apatite and dolomite samples revealed that as much as 95% of their surface area is contributed by pores less than 400 A in diameter. Average pore radius for both apatite and dolomite was determined to be 82 A. SEM micrographs of 65xlOO mesh apatite and dolomite samples presented in Figures 5 and 6, respectively, further confirm the high surface porosity. It is observed from these micrographs that the pores in the apatite sample extend to the surface.


36









37











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b) Figure 5. SEM micrograph of apatite (65x100 mesh size fraction),
a) 200X; b) 1000X.








39 a)



















b) Figure 6. SEM micrograph of dolomite (65x100 mesh size fraction), a)
IOOX; b) 100OX.









40

Dolomite, on the other hand, appears to have a relatively rough surface morphology, but does not reveal any features of its porosity.



X-Ray Analysis

X-ray diffraction analysis of apatite, presented in Figure 7,

indicated the presence of a very small amount of quartz, however, even after repeated scans no dolomite was found to be associated with apatite. The dolomite sample exhibited the characteristic peak of dolomite along with minor peaks for quartz and feldspar as seen in Figure 8.



Surface Chemical Characterization

The zeta potential measurements were made using a Pen Kem Model 501 Laser Zee Meter. The isoelectric point (IEP) of apatite, as shown in Figure 9, is at pH 5.4. This value is similar to that reported by Chanchani (1984) and Somasundaran (1968). In the case of dolomite, two isoelectric points, at pH 5.3 and pH 11.1, are observed (Figure 10). The second IEP at pH 11.1 has been attributed to the presence of hydroxylated magnesium species on the dolomite surface (Chanchani, 1984; Iwasaki and Krishnan, 1983; and Balajee and Iwasaki, 1969).



Flotation Studies

Flotation Studies with Dodecylamine Hydrochloride Single mineral flotation behavior

Flotation response of single mineral apatite and dolomite was examined at two concentrations of dodecylamine hydrochloride as a









41




















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45

function of pH. These tests were followed by mixed mineral flotation tests under selected experimental conditions.

Apatite-dodecylamine system. Results of apatite flotation as a

function of pH at two levels of dodecylamine concentration are presented in Figure 11. It is observed that at a dodecylamine hydrochloride concentration of 1.0x10-3 kmol/m3, apatite recovery is 100% between pH 4 and 10, and decreases precipitously beyond pH 10. However, at a dodecylamine concentration of 1.6xi0-4 kmol/m3, apatite recovery exhibits two maxima at pH 6 and 9.8, the flotation of apatite at pH 6 being about 50% of that at pH 9.8.

Dolomite-dodecylamine system. Flotation response of dolomite as a function of pH at a dodecylamine hydrochloride concentration of 1.0x10-3 and 1.6x10-4 kmol/m3 is illustrated in Figure 12. The amount floated is observed to increase sharply at a dodecylamine concentration of 1.0x10-3 kmol/m3 above the first IEP at pH 5.3 and to reach a 100% level at pH

5.8. However, flotation recovery starts decreasing above pH 9, descending to 25% at pH 11, the value at which dolomite exhibits the second IEP (refer to Figure 10).

The amount of apatite and dolomite floated as a function of pH at a dodecylamine hydrochloride concentration of 1.0x10-3 kmol/m3 is compared in Figure 13. It is indicated from these results that apatite can possibly be recovered selectively from its mixture with dolomite at pH less than 4.5. At a dodecylamine concentration of 1.6x10-4 kmol/m3, another region of selectivity occurs at pH 9.8, as illustrated in Figure 14.








46





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50

Attempts were therefore made to achieve the separation of apatite from dolomite by conducting flotation tests using synthetic mixtures of apatite and dolomite under the above specified pH conditions.



Flotation of apatite-dolomite mixture with dodecylamine

Results of 50:50 apatite and dolomite mixed mineral flotation

experiments under selected pH conditions and collector concentrations are summarized in Table 3.

Although a preferential flotation of apatite was observed at pH

9.8 and pH 4.1 at both levels of dodecylamine concentrations, the magnitude of the selectivity predicted by the single mineral experiments was not realized.

The difference in the flotation response of these minerals at pH

9.8 and at a dodecylamine concentration of 1.6x,0-4 kmol/m3 also did not correspond to the single mineral test results. In general, results of mixed mineral experiments with dodecylamine as the collector indicated depression of apatite and activation of dolomite in the selectivity ranges predicted by the single mineral tests.



Flotation Studies Using Sodium Oleate as the Collector

Flotation behavior of apatite and dolomite using sodium oleate as the collector was examined as a function of pH. Mixed mineral tests, were also conducted under specific pH conditions.









51


TABLE 3



Flotation Results of 50:50 Apatite/Dolomite Mixture Using Dodecylamine as the Collector





Flotation Collector Chemical Analysis of Apatite Dolomite
pH Conc. Float Fraction, % Recovery Reject
(kmol/m3) P205 MgO (Weight %) (Weight %)


4.10.2 1.0x10-3 26.5, 26.6 4.2, 4.2 72.3, 74.7 53.4, 55.7

9.80.2 1.6x10-4 25.4, 25.9 3.8, 4.0 53.3, 51.8 37.6, 38.2




Feed: 1 gram 65x100 mesh fraction 18.0% P205, 9.5% MgO









52

Single mineral flotation tests

Results of flotation tests as a function of pH using sodium oleate as the collector are presented in Figure 15. At a sodium oleate concentration of 4.OX10-5 kmol/m3, flotation recovery of dolomite was observed to be 100% in the acidic pH range (pH 4.0-5.5). Dolomite recovery remained at 10-15% range between pH 7 and 10 and was seen to increase above pH 10.

Apatite flotation in the acidic pH range, on the other hand,

exhibited a maximum at pH 4. The recovery of apatite increased sharply above pH 6, and was observed to be 100% between pH 7 and 10.5. No apatite flotation was observed between pH 5 and 6 at this level of sodium oleate addition.

As observed from Figure 15, selective flotation of dolomite from apatite or vice versa was predicted in the following pH ranges.

1) Flotation of dolomite from apatite at pH 5 to 6.

2) Flotation of apatite from dolomite between pH 7 and 10.



Mixed minerals

Results of the mixed mineral flotation tests using a 50:50 apatite and dolomite mixture under given pH conditions are summarized in Table

4. It is observed that at pH 5.3, even though dolomite recovery was greater than 95%, a significant amount of apatite (33%) also reported in the float fraction. Consequently, apatite recovery in the sink fraction was reduced to approximately 67%. In the alkaline pH range, apatite recovery decreased to 68% from 100% observed in the single mineral tests. On the other hand, dolomite flotation remained at the level










53










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54




TABLE 4



Flotation Results of 50:50 Apatite-Dolomite
Mixture Using Sodium Oleate as the Collector





Flotation Chemical Analysis Apatite Dolomite
pH of Concentrate, % Recovery Reject
P205 MgO (Weight %) (Weight %)

5.20.2 32.82, 33.36 1.22, 1.28 67.8, 66.1 96.0, 95.3

8.10.2 31.40, 30.64 1.32, 1.36 69.2, 68.6 92.4, 91.6




Collector Conc., 4.0x10-5 kmol/m3
Flotation Feed: I gram, 65x100 mesh size fraction, 18.0% P205, 9.5% MgO









55

predicted by the single mineral results. It appears that the changes in the flotation response of apatite are the major cause for the loss of selectivity in the mixed mineral system when sodium oleate is used as the collector.

It is clear from the results presented that the loss of selectivity in the mixed mineral system is primarily due to the activation or depression of either apatite or dolomite depending on whether dodecylamine hydrochloride or sodium oleate is used as the collector. Moudgil and Chanchani (1985a and 1985b), and Soto and Iwasaki (1985 and 1986) have provided detailed explanations for the loss of selectivity in this system.

In the case of the apatite/dolomite-amine system, Soto and Iwasaki (1985) proposed that the adsorption of amine on dolomite is mainly controlled by the electrostatic attraction in addition to weak chemical interaction. The fact that flotation recovery of both apatite and dolomite decreased sharply below their respective IEPs (at pH -5.4), when dodecylamine was used (refer to Figure 14) and increased in the case of sodium oleate (Figure 15), indicated that the coloumbic forces play a significant role in the adsorption of the collector on the respective substrates. It should, therefore, be possible to modify flotation behavior of dolomite by the addition of indifferent electrolytes such as sodium chloride.









56

Flotation tests in the presence of NaCi using dodecylamine as the
collector

Single minerals flotation tests. Experiments conducted in the natural pH range (pH 6.7) as shown in Table 5, indicated 85-90% flotation recovery of apatite at a dodecylamine concentration of

1.6x,0Q4 kmol/m3 and a NaCl concentration of 5.0x10'1 kmol/m3. Apatite recovery was observed to be only 50% under identical conditions without NaCl addition (refer to Figure 11). In contrast, dolomite recovery under similar experimental conditions remained at the 15-20% level. It should be noted that the change expected in the flotation behavior of dolomite in the presence of sodium chloride was not observed, but more selective flotation of apatite was realized.

Mixed mineral flotation tests. Flotation of 88:12 apatitedolomite mixtures was conducted to determine the selectivity predicted by the single mineral experiments in the presence of NaCl at natural pH value. Results presented in Table 6 demonstrated that at a dodecylamine concentration of 1.6x10-4 kmol/m3 apatite can be selectively recovered from the mixture leaving dolomite in the sink fraction.

It should be noted that with dodecylamine hydrochloride, apatite, the major mineral, is floated leaving dolomite in the sink. In practice, however, flotation of the minor constituent is desired. Further test work therefore, was conducted using sodium oleate as the collector, which under slightly acidic pH conditions is known to yield flotation of dolomite leaving apatite in the sink fraction.









57




TABLE 5



Effect of NaCl on the Single Mineral
Flotation of Apatite and Dolomite with
Dodecylamine Hydrochloride as the Collector






Mineral Amount Floated, Weight %

Without Salt With Salt


Apatite 50.0, 48.0 84.5, 90.8

Dolomite 17.0, 20.0 15.7, 20.2



Collector Conc., 1.6x10-4 kmol/m3
NaCl Conc., 5.0x10-I kmol/m3
Feed: 1 gram, 65x100 mesh size fraction
Flotation pH: 6.7









58




TABLE 6



Results of Mixed Mineral Flotation in the Presence of
NaCl Using Dodecylamine as the Collector at pH 6.3





Test Chemical Analysis Apatite Recovery Dolomite Reject
of Float Fraction, % in Float in Sink
No. P205 MgO (Weight %) (Weight %)


1 33.65 0.97 79.0 71.8

2 33.49 1.02 79.8 69.8

3 33.59 1.03 81.2 69.1



Collector Conc., 1.6x10-4 kmol/m3
NaCl Conc., 5.0x101- kmol/m3
Feed: 1 gram 88/12 Apatite-Dolomite Mixture
pH: 6.3









59

Flotation studies in the presence of NaCi using sodium oleate as the
collector

Results of mixed mineral flotation tests summarized in Table 7

indicated selective recovery of dolomite from the apatite and dolomite mixture at pH 4 in the presence of 5.0x101l kmol/m3 NaCl and at a sodium oleate concentration of 4.0x10-5 kmol/m3. Single mineral tests without NaCi addition indicated 60% apatite and 95-100% dolomite recovery at the same pH and collector concentration (see Figure 15). In the presence of NaCl apatite was found to be depressed without any significant effect on the flotation of dolomite. In order to achieve maximum separation of dolomite single mineral tests were conducted to determine the optimum pH, salt and collector concentration.

Optimum salt concentration for apatite depression. Mixed mineral results indicated the best selectivity at pH 4, in the presence of NaCl (see Table 7). Therefore, further tests to determine the optimum concentration of NaCl were also conducted at this pH value. It was determined that optimum results are obtained in the presence of 2.0x10-2 kmol/m3 sodium chloride addition (see Figure 16). It is to be noted that dolomite flotation is not affected by NaCl addition under the given experimental conditions.

Optimum collector concentration. Results presented in Figure 17 and 18, indicated that at pH 10, apatite requires less collector to float with and without salt addition, as compared to dolomite, and vice versa at pH 4. The difference in the amount of sodium oleate required to float 100% apatite or dolomite without NaCl addition was determined to be about five-fold, and it more than doubled in the presence of sodium chloride. From the data in Figure 17, the optimum sodium oleate









60



TABLE 7



Results of Mixed Mineral Flotation Tests in the
Presence of NaCl Using Sodium Oleate as the Collector





Flotation Chemical Analysis, % Apatite Recovery Dolomite Reject
pH P205 MgO Weight % Weight %



10.9 18.0 9.5 100.0 0.0

8.2 18.5 9.5 100.0 0.0

6.6 18.5 9.5 100.0 0.0

4.0 33.3, 33.8 0.64, 0.48 96.0, 94.8 96.6, 97.6



Collector Conc., 4.0x10-5 kmol/m3 NaCl Conc., 5.0x10-1 kmol/m3 Flotation Feed: 1 gram 50:50 Apatite-Dolomite Mixture, 18.0% P205, 9.5% MgO









61









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64

concentration was determined to be 4.0x10-5 kmol/m3, which incidentally coincides with that used for single mineral experiments conducted without NaCl addition. It is to be noted that at this level of sodium oleate concentration at pH 4, apatite appears to be completely depressed whereas dolomite flotation approaches 100%.

Optimum pH for separation. Results of single mineral apatite and dolomite flotation tests as a function of pH are plotted in Figure 19. It is observed that apatite remains depressed up to pH 4.2, but becomes activated above this pH value.

Apatite flotation behavior in the presence of NaCl at pH 9.5 and above is significantly different from that observed in distilled water (refer to Figure 15). On the contrary, dolomite recovery, for the most part, appears to be the same as that obtained in distilled water.

Following the above tests, flotation response of a 50:50 apatitedolomite mixture was studied as a function of pH in the presence of

2.0x10-2 kmol/m3 NaCl and at a sodium oleate concentration of 4.0x10-5 kmol/m3.

In general, the selectivity predicted by the single mineral tests (see Figure 19) was maintained in the mixed mineral systems (see Figure 20). In addition, during the flotation of the mixed minerals, a more effective apatite depression was realized in the pH range of 4.0-4.5 as compared to single mineral test results. Optimum separation was obtained at pH 4.00.2 where more than 95% apatite was recovered with 95% or more dolomite rejection. The MgO content of the sink fraction (concentrate) was analyzed to be less than 0.7% from a feed containing

9.5% MgO.








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67



Flotation studies in the presence of KCl and NaF

To examine the effect of additional salts such as KCl and NaF on

the flotation behavior of apatite and dolomite, single and mixed mineral tests were conducted. Results of the experiments with KCl in the pH range of 3.0-5.5, using sodium oleate as the collector, are illustrated in Figure 21. It is seen that in the case of single minerals about 80% of dolomite is floated at pH 4 with only 5-10% apatite reporting in the float fraction. The best results with the 50:50 mixture, however, were obtained at pH 4.5, where 75% of dolomite and 5-10% apatite was recovered in the float fraction. These tests were conducted in the presence of 2.0x10-2 kmol/m3 KCI (the optimum concentration for NaCl).

Comparison of apatite and dolomite flotation with and without

2.0x10-2 kmol/m3 NaF, as shown in Figure 22, indicated that dolomite depression is relatively higher than that of apatite. Due to the greater depression of dolomite with NaF, the difference in the floatability gap of these minerals narrowed. In fact, some exploratory mixed mineral experiments conducted at pH 4 using sodium oleate as the collector indicated that in the presence of NaF, either both of these minerals were depressed or floated together.



Electrokinetic Studies
Effect of Salt Addition on the Zeta Potential of Apatite

Zeta potential of apatite was measured as a function of pH in the presence of 2.0x10-2 kmol/m3 NaCl (optimum concentration for apatite depression), KCI and NaF. As seen from the results plotted in Figures







68












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70

23-25, the zeta potential of apatite in the presence of added salts exhibited negative values in the entire pH range (4-11) examined, indicating reversal of the surface charge below the IEP (pH 5.4). It should be noted that ionic strength was not maintained constant because, as discussed in a previous study by Chanchani (1984), KNO3 additions of up to 1.0x10-2 kmol/m3 did not significantly affect flotation recovery of either apatite or dolomite.

The zeta potential of apatite in the presence of NaCl in the pH range of 6 to 9 was found to be less negative than that in distilled water. In the presence of KC1, at pH 6 and above, the zeta potential of apatite was measured to be nearly the same as that in distilled water. The fact that both NaCl and KCl reversed the surface charge indicated that they are not indifferent electrolytes for apatite. On the other hand, because F- is a lattice ion and therefore a potential determining ion (Somasundaran and Wang, 1984)' NaF was not expected to act as an indifferent electrolyte for apatite. NaF decreased the zeta potential of apatite in the entire pH range (4-11) examined. It also rendered the surface more negative as compared to NaCl or KCl.



Effect of Salts on the Zeta Potential of DolomiteThe zeta potential of dolomite was measured in the presence of salts as a function of pH. Zeta potential versus pH curves for dolomite in the presence of NaCl and KCl, shown in Figures 26 and 27, respectively, indicated that the IEP of dolomite (at pH 5.3) is not influenced by NaCl or KCl addition. The zeta potential values measured below IEP did not appear to show any measurable change. Above the IEP,









71













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76

however, both NaCi and KCl reduced the zeta potential of dolomite from

-25 mV to -15 mV between pH 8 and 10. This decrease in the value of zeta potential can be attributed to the compression of the electrical double layer. It is suggested that the compression of the electrical double layer occurred in the presence of the added electrolytes also below the IEP. However, due to the low zeta potential values, these changes could not be measured. In the case of sodium fluoride, however, zeta potential of dolomite, as shown in Figure 28, was found to be reversed below its IEP, as was the case for apatite. This is possibly due to the adsorption of fluoride ions on the surface Ca++ and Mg++ sites for dolomite since the solubility of CaF2 and MgF2 (1.6x10'10 and

8.4x10-8, is rather low.

It should be mentioned that the maximum flotation selectivity was achieved in the presence of NaCl, and the minimum with NaF. Further studies were therefore, conducted to understand the reason for charge reversal in the presence of only NaCl.



Role of NaCl in the Reversal of Surface Charge of Apatite

Charge reversal of apatite in the presence of sodium chloride could be due to the selective dissolution of calcium and/or the substitution of sodium for calcium sites in the apatite structure.



Substitution of sodium for calcium in the apatite lattice

Calcium, in addition to POJk, OH- and H+, is the potential

determining ion (PDI) for apatite (Somasundaran, 1968; Samani et al., 1975, and Somasundaran and Wang, 1984). Selective dissolution of












77





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78

calcium from apatite can therefore render the surface more negatively charged. Results of Ca++ and P0J- dissolution from apatite as a function of time with and without NaCi addition are summarized in Table

8. An increase in Ca++ and a decrease in P0J-dissolution in NaCl solution relative to that of distilled water is observed from these results. It is therefore possible that selective dissolution of calcium can lead to surface charge reversal of apatite. This preferential dissolution, however, was believed to be occurring as a result of sodium substitution in the apatite structure, since dissolution was found to be incongruent in the presence of added salt. This possibility was studied by 1) measuring the change in the unit cell dimensions of apatite; 2) analyzing the mineral sample for sodium content before and after treatment with NaCl solution.

The change in the lattice parameters of apatite was studied by

computerized X-ray diffraction method. Results of the tests, summarized in Table 9, indicated 0.0060.001 A decrease in the unit cell "a" dimension of apatite when conditioned with NaCl solution. This change was anticipated to be small, because the ionic radius of Na+ (0.95 A) is only slightly smaller than that of Ca++ (0.99 A) (Brescia et al., 1966). These tests were followed by determination of sodium uptake by apatite. Results shown in Table 10, indicated that the amount of sodium present in apatite after conditioning and rinsing is stoichiometrically equivalent to the amount of calcium dissolved from apatite suggesting mole per mole substitution of Na+ for Ca++ in the apatite structure.









79




TABLE 8



Apatite Dissociation at pH 4 with and without Added Sodium Chloride






Time Amount of Ions Dissolved, kmol/m3
(iue)In Distilled Water In 2.0x10-2 kmol/m3
NaC 1

Ca++ POj- Ca++ POj05 8.75x,0-4 3.05x10-4 1.05xj103 9.45x10-5

30 1.60x10-3 6.00xj104 2.23x10-3 2.21x,0-4

60 2.25x,0-3 8.40x10-4 3.53xj103 3.57x10-4








80




TABLE 9



Effect of NaCl on Unit Cell Dimensions
of Apatite Conditioned at pH 4





Salt Conc. Measured Unit Cell Dimension ( )

(kmol/m3) "a" "c"



None 9.348, 9.346 6.893, 6.894

7.5x10-4 9.339, 9.340 6.895, 6.893

2.0x10-2 9.342, 9.340 6.892, 6.894













81


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82

Adsorption Studies

In order to investigate the mechanism of observed flotation

selectivity, adsorption of sodium oleate on apatite and dolomite was studied. Adsorption experiments were conducted under conditions identical to the flotation tests so that a meaningful correlation between the two could be established.



Single Minerals Adsorption Tests

Apatite-oleate system

The amount of oleate adsorbed on apatite as a function of

conditioning pH is plotted in Figure 29 at three levels of sodium oleate concentration. Two peaks, a larger peak at pH 8 and a smaller one around pH 5, are observed. It is seen that the amount of oleate adsorbed on apatite increased with an increase in the amount of sodium oleate added.

Adsorption data on apatite as a function of sodium oleate concentration at pH 4 and pH 10 was also obtained. Results presented in Figure 30 indicated that the amount of oleate adsorbed at pH 10 is about five-times higher than that at pH 4, i.e., at a sodium oleate concentration of 2.0x10-4 kmol/m3). At a lower sodium oleate concentration, however, this ratio appears to be different.

Effect of NaCl on sodium oleate adsorption on apatite. Selective flotation of dolomite from apatite was found to be enhanced in the presence of sodium chloride at pH 4 (see Figure 19 and 20). Adsorption experiments were therefore conducted as a function of sodium oleate concentration in order to determine the effect of NaCl (2.0x10-2









83













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85

kmol/m3) on the amount of oleate adsorbed on apatite. Results of the adsorption experiments illustrated in Figure 31, indicated a decrease in the amount of oleate adsorbed on apatite in the presence of sodium chloride as compared to that obtained in distilled water.



Dol omi te-ol eate system

The amount of oleate adsorbed on dolomite as a function of pH is plotted in Figure 32 at three levels of sodium oleate concentration. Adsorption behavior similar to that of apatite (refer to Figure 29) is observed, however, the maxima in the alkaline pH range is shifted to pH 10 as compared to pH 8 for apatite. The amount of oleate adsorbed on dolomite at pH 10 is comparable to that on apatite at pH 8. In the acidic pH range, a smaller peak is observed at pH 4. Oleate adsorption on dolomite was found to be minimal between pH 5.5-8.5. The amount adsorbed was higher at higher dosages of sodium oleate.

Tests were also conducted at pH 4 and pH 10 to determine the amount adsorbed on dolomite as a function of sodium oleate concentration. It is observed from Figure 33, that the amount adsorbed at pH 10 is approximately three-fold higher than that at pH 4 under most of the concentration range examined.

Effect of NaCl on sodium oleate adsorption on dolomite. Results of adsorption experiments, at pH 4 in the presence of 2.0x10-2 kmol/m3 NaCl, as shown in Figure 34, indicated no measurable change in the amount adsorbed on dolomite as compared to that in distilled water. It should be noted that oleate adsorption on dolomite is nearly twice as




Full Text
100
80
60
40
20
0
50/50 Mixed Minerals
Dolomite
A Apatite
-5 3
4.0x10 kmol/m
Sodium Oleate
-2 3
2.0x10 kmol/m
6 7 8
FLOTATION pH
9
10
1 1
1 2
cn
cn
Flotation recovery of apatite and dolomite (mixed minerals) as a function of pH
in the presence of sodium chloride.


This work is dedicated to the
memory of my father,
the late HIDIR INCE,
without whose inspiration, encouragement
and special admiration for education this
work would have never been attempted.


115
At pH 10 and dodecylamine concentration of 1.6xl04 kmol/m3,
apatite recovery, however, was observed to decrease while dolomite
flotation increased (see Table 3). Since, in the case of mixtures,
there are more cations (Ca++ and Mg++) present as compared to single
mineral alone; apatite is expected to acquire a less negative surface
charge in the mixture. Consequently, adsorption of ionic dodecylamine
species on apatite possibly decreased leading to reduced flotation
recovery. On the other hand, dolomite flotation under similar
conditions increased probably as a result of adsorption of phosphate
ions dissolving from apatite that renders its surface more negatively
charged (Chanchani, 1984).
Results of mixed mineral tests at pH 4 and at a dodecylamine
concentration of l.OxlO'3 kmol/m3 indicated a significant increase in
the recovery of dolomite, and only a slight decrease in the apatite
flotation (see Table 3). Thus it appears that dodecylamine adsorption
on dolomite increased substantially in the mixture. The increased
dissolution of apatite in the acidic pH range can lead to adsorption of
more phosphate ions on dolomite causing its surface to acquire a more
negative surface charge. It should be noted that the solubility of Mg-
phosphate is less than that of Ca-phosphate and, therefore, more
phosphate ion adsorption on the dolomite surface can be expected.
Dodecylamine adsorption on dolomite therefore can be presumed to have
increased under these conditions resulting in higher flotation.


ZETA POTENTIAL, mV
30
20
O
Dolomite in Water
~2 3
Dolomite in 2.0x10 kmol/m NaF
10
-10
-20
-30 L
\
L
x6
8
10 / 12
pH
1 4
Figure 28. Zeta potential of dolomite in the presence and absence of NaF as a function of pH.


12
Separation Based on Physico-Chemical Properties
Selective flocculation
Selective flocculation of dolomite from apatite was attempted for
the first time by Moudgil and Shah (1986) using polyethylene oxide (PEO)
as the flocculant. It was shown that PEO flocculates dolomite, but not
apatite in single mineral tests. However, mixed mineral tests did not
exhibit the expected selectivity. The loss in selectivity in the case
of mixed minerals was attributed to polymer-induced entrapment, which
occurs due to a limited affinity of the polymer for the inert mineral
(apatite). Incorporation of the polymer-coated apatite particles into
dolomite floes was explained on the basis of the higher probability of
polymer bridging due to the larger number of "active" sites on dolomite
as compared to apatite particles. Further efforts are underway to
reduce the adsorption of the polymer on apatite so as to achieve the
desired separation.
Selective flotation
Separation of dolomitic limestone from sedimentary phosphates has
been the subject of studies by various researchers. Due to the complex
structure and the presence of different amorphous and porous phosphates
in these sedimentary deposits, separation of carbonates such as calcite
and dolomite from phosphates has not always been feasible. The amount
and type of impurities present have shown considerable variation even
within the same ore deposit. Despite these problems, development of a


Figure 12. Dolomite flotation as a function of pH.


support. Any opinions, findings, and conclusions or recommendations
expressed in this work are those of the author and do not necessarily
reflect the views of the Florida Institute of Phosphate Research.
i v


136
the absence of added salt at pH 4 were identified to be oleic acid and
calcium oleate. However, a higher calcium oleate peak is observed,
suggesting greater adsorption of ionic oleate species as compared to
neutral oleic acid molecules. These species appear to be consistent
with the solution properties of sodium oleate. Although calcium oleate
and oleic acid were detected on the apatite surface in distilled water
at pH 4, in the presence of sodium chloride, oleic acid appeared to be
the only adsorbing specie. The change in the nature of the surfactant
species adsorbing on apatite is anticipated because of the selective
calcium dissolution and its substitution by sodium at pH 4. In
addition, the reversal of the surface charge of apatite in the presence
of NaCl was expected to adversely affect the adsorption of the anionic
oleate species.
Dolomite-Oleate System
The difference spectrum obtained from the diffuse reflectance FT-IR
spectra of treated and untreated dolomite at pH 10 and pH 4 is presented
in Figure 50. The spectra at pH 10 indicated the presence of magnesium
oleate. At pH 4, oleic acid and magnesium oleate were both observed in
the absence and presence of NaCl, indicating no change upon salt
addition. This result was expected, since it was shown that flotation,
electrokinetic and adsorption behavior of dolomite was not affected to
any significant extent by sodium chloride addition.
In summary, it has been established, from the above discussion that
formation of calcium oleate occurs on apatite at both pH 4 and pH 10.
Adsorption of oleic acid, however, was observed at pH 4 only. It was


76
however, both NaCl and KC1 reduced the zeta potential of dolomite from
-25 mV to -15 mV between pH 8 and 10. This decrease in the value of
zeta potential can be attributed to the compression of the electrical
double layer. It is suggested that the compression of the electrical
double layer occurred in the presence of the added electrolytes also
below the IEP. However, due to the low zeta potential values, these
changes could not be measured. In the case of sodium fluoride, however,
zeta potential of dolomite, as shown in Figure 28, was found to be
reversed below its IEP, as was the case for apatite. This is possibly
due to the adsorption of fluoride ions on the surface Ca++ and Mg++
sites for dolomite since the solubility of CaF2 and MgF2 (1.6x10"^ and
8.4x10-8) is rather low.
It should be mentioned that the maximum flotation selectivity was
achieved in the presence of NaCl, and the minimum with NaF. Further
studies were therefore, conducted to understand the reason for charge
reversal in the presence of only NaCl.
Role of NaCl in the Reversal of Surface Charge of Apatite
Charge reversal of apatite in the presence of sodium chloride could
be due to the selective dissolution of calcium and/or the substitution
of sodium for calcium sites in the apatite structure.
Substitution of sodium for calcium in the apatite lattice
Calcium, in addition to P0$~, OH" and H+, is the potential
determining ion (PDI) for apatite (Somasundaran, 1968; Samani et al.,
1975, and Somasundaran and Wang, 1984). Selective dissolution of


CHAPTER I
INTRODUCTION
The Florida phosphate rock deposits are located in the central and
northern land pebble districts. The central land pebble district, as
shown in Figure 1, constitutes the major phosphate rock producing
region and underlies 2600 square miles in Polk, Hillsborough, Hardee,
Manatee and DeSoto Counties. Until recently, the phosphate production
has been confined to the Bone Valley of the central district. These
deposits, however, are being depleted and the mining will shift to the
southern extension of the central district. Phosphate rock from the
southern extension (lower zone matrix) is lower in grade and contains
significant quantities of dolomitic limestone (Ca, Mg carbonate)
impurities in addition to quartz and clays (Lawver et al., 1982a).
Beneficiation of Florida phosphate rock by a "double float" or Crago
flotation process (Crago, 1940) has been in use commercially since
1937. However, with the conventional processing techniques, dolomite,
usually reported as weight percent MgO, cannot be selectively removed
from apatite, the phosphate mineral. On the other hand, it is generally
agreed that more than 1% MgO in the final phosphate concentrate would
present problems during chemical processing to manufacture phosphoric
acid, an intermediate product used in the production of fertilizers.
The presence of MgO in quantities greater than the specified amount
would:
1


9
Somasundaran (1976) have attributed these discrepancies to crystal
structure modifications, the presence of impurities, and added
electrolytes. Saleeb and de Bruyn (1972), however, have shown that a
constant solubility product can be obtained if a stoichiometric compound
is prepared. These investigators have reported a pKS0 value of 119.1
for fluorapatite.
In the case of dolomite, the lack of understanding of its
precipitation process has contributed to the discrepancy of the
solubility products reported by different investigators. The published
pKS0 values are in the range of 16.5 to 19.5 for dolomite (Stumm and
Morgan, 1981).
Surface Charge of Apatite and Dolomite
It is known that in the case of insoluble oxides the surface charge
is mostly determined by 0H and H+, i.e., by the pH of the suspension in
the absence of any specifically adsorbing ions. The charge of the ionic
solids such as silver iodide is considered to be the result of
preferential dissolution or adsorption of Ag+ or I. The magnitude and
sign of the surface charge is determined by the solution concentration
of the constituent ions.
Surface charge development on minerals such as apatite and
dolomite is much more complex. It is controlled by preferential
dissolution of constituent species, and their hydrolysis products in
addition to OH- and H+ (Saleeb and de Bruyn, 1972) ions. It has also
been shown that the impurity ions present as substituents for lattice
ions produce significant changes in the electrokinetic characteristics.


148
other hand, calcium oleate formation was not detected, which has been
attributed to depletion of surface calcium sites in addition to
electrostatic repulsion due to surface charge reversal. It was
established by contact angle measurements that, in the presence of NaCl
at pH 4, the apatite surface remains hydrophilic when 4.0x10"^ kmol/nP
sodium oleate is added as the collector. Apatite flotation, therefore,
is suppressed under these conditions.
Surface charge, adsorption behavior and flotation response of
dolomite, unlike that of apatite, was not affected to any significant
extent by the addition of sodium chloride. The contact angle on
dolomite surface also did not change significantly to alter its
hydrophobicity. Consequently, dolomite flotation remained high while
apatite was depressed resulting in the desired selective separation of
dolomite from apatite.


117
however, shown that only limited separation can be achieved in the
mixtures. To achieve high recovery and selectivity, previous
investigators (Lawver et al., 1982b; Llewellyn et al., 1982; Dufour et
al., 1980; Johnston and Leja, 1978; Dahl in and Fergus, 1978; Samani et
al., 1975; Onal, 1973; and Rule et al., 1970) have suggested the use of
surface modifying agents. Phosphate salts have generally been used to
depress apatite in most of these studies. A detailed study of the
mineral/surfactant interaction was conducted by Moudgil and Chanchani
(1985a and 1985b), and Soto and Iwasaki (1985 and 1986) who studied
flotation of apatite and dolomite with anionic and cationic surfactant
systems, respectively.
Moudgil and Chanchani (1985a) concluded that there is a strong
chemical interaction between oleate and apatite which has been
attributed to the chemisorption of oleate anions on the calcium sites on
the surface of apatite in the pH range of 7 to 10. In addition,
adsorption of acid-soap complexes was also considered in the alkaline pH
range. It was indicated that oleate adsorption on dolomite is mostly
governed by electrostatic attraction. However, the possibility of
chemisorption of oleate ions on surface calcium and magnesium sites was
not ruled out in the alkaline pH range (Chanchani, 1984).
Soto and Iwasaki (1985) indicated that the adsorption of
dodecylamine on dolomite is essentially due to the electrostatic
attraction along with some chemisorption. It was noted in the above
study that the specific interaction of the dodecylamine ion is stronger
for phosphate as compared to the carbonate anion.


OLEATE ADSORBED, >4mol/g
INITIAL OLEATE CONO., kmol/m3
Figure 30. Oleate adsorption on apatite as a function of initial oleate concentration at
pH 4.0 and 10.0.


KUBELKA-MUNK UNITS
Figure 36.
Diffuse reflectance IR spectra of Mg-, Ca- and Na-oleate
(400-4000 cm~i range).


154
The contact angle measurements, determined in this study,
indicated a good correlation with the flotation and oleate adsorption
results in the absence and the presence of sodium chloride. It was
determined that NaCl addition reduces the hydrophobicity of apatite at
pH 4 without affecting that of dolomite to any significant extent.


33
should be noted that a direct correlation of Hallimond cell results to a
laboratory cell in terms of percent recovery, etc., may not be
appropriate, but the trends obtained are expected to be similar.
Electrokinetic measurements
Zeta potential measurements with and without modifiers in
combination with dissolution and flotation can lead to a better
understanding of the various species adsorbed at the solid/liquid
interface. Zeta potential of the minerals was measured by the
electrophoretic mobility technique using a Pen Kern Model 501 Laser Zee
Meter.
Adsorption
These tests were conducted to obtain information on the amount of
oleate/oleic acid adsorbed on the mineral surface under different
experimental conditions. Oleate adsorption measurements using
labeled oleic acid were conducted using a Beckman Model LS 1800 liquid
scintillation counter.
To avoid experimental artifacts in the adsorption studies, the
following precautions were taken:
(1) Oleate adsorption tests were conducted using the same 65x100 mesh
size fraction that was employed for flotation experiments.
(2) Reagentizing time of 5 minutes was maintained for both adsorption
and flotation to avoid variations due to kinetics involved.
(3) The exact pH value of oleic acid precipitation depends on the
total oleate concentration and the reaction constants used. Hence,


93
without NaCl addition. Johnston and Leja (1978), and Chanchani (1984)
studied the same system by IR spectroscopy, however, the nature of the
species adsorbed on dolomite (oleic acid, or calcium- and magnesium-
oleate) could not be established in these studies due to strong
absorption of this mineral between 2000 and 1000 cm'-'-. The capability
of the currently available FT-IR spectroscopy equipment at University of
Florida to obtain difference spectrum by subtracting the pure mineral
spectrum from that of the mineral-surfactant permitted identification of
these species on dolomite in the present study.
IR Spectra of Pure Oleate Species
The diffuse reflectance IR spectra of sodium oleate and of calcium
and magnesium oleate precipitates in the 400 to 4000 wavenumber region
are illustrated in Figure 36. The IR spectra in the 1200 to 1800 cm"1
region have been enlarged in Figure 37 to highlight the differences
between the asymmetrical stretching frequencies of the carboxylate group
for the respective precipitates.
The main features of the pure oleate species spectra illustrated in
Figure 36, are the asymmetrical stretching vibration of CH2 groups at
2930 cm'1 and the asymmetric vibration of CH3 group at 2859 wavenumber.
Most of the bands in the region from 1400 to 1800 cm"'- are related to
the carboxylate groups. The characteristic band for sodium oleate was
located at 1560 cm"' as illustrated in Figure 37. Calcium oleate and
magnesium oleate bands were found at 1563 and 1583 cm"1, respectively.
The large difference between the bands for calcium and magnesium oleate
enables identification of these species on the mineral surface.


Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
EFFECT OF SODIUM CHLORIDE ON THE SELECTIVE
FLOTATION OF DOLOMITE FROM APATITE
BY
DURSUN E. INCE
December, 1987
Chairman: B. M. Moudgil
Major Department: Materials Science and Engineering
Selective flotation of dolomite from apatite was investigated
using dodecylamine hydrochloride and sodium oleate as the collector in
the absence and presence of inorganic modifiers such as sodium chloride.
Separation of dolomite from apatite was anticipated from the single
mineral experiments under various pH and collector concentrations.
Flotation selectivity in the mixtures, however, was found to be limited.
Attempts to control the flotation response of these minerals by addition
of salts such as NaCl and KC1 indicated that selectivity is possible.
Upon optimization of the process parameters, the best selectivity was
obtained at pH 4, in the presence of sodium chloride, using sodium
oleate as the collector.
Electrokinetic and adsorption studies were conducted to elucidate
the mechanism of observed selectivity. Zeta potential measurements
xiv


TABLE 15
Electrostatic Interaction Energy of Calcium and Magnesium with Oleate
Separation
Distance (A)
Interaction Energy, kJ/mole
Energy Ratio
Mg-01eate
Ca-01eate
Mg-01eate/Ca-01eate
8.0
-0.145
-0.138
1.051
4.0
-0.486
-0.448
1.085
2.0
-1.429
-1.244
1.149
1.0
-3.458
-2.801
1.235
contact
-17.504
-11.202
1.563


EFFECT OF SODIUM CHLORIDE ON THE SELECTIVE
FLOTATION OF DOLOMITE FROM APATITE
BY
DURSUN E. INCE
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL
FOR THE
FULFILLMENT OF THE REQUIREMENTS
DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
1937


127
Ca(COj)6
Mg(C03)6
Dolomite
C*Mg(C03)j
a)
b)
Figure 46. Crystal structure of dolomite, c-axis vertical, a) Layered
structure b) Stereoscopic projection of the hexagonal unit
cell for dolomite (a = 4.81 A, c = 16.00 A)


TABLE 11
Effect of Sodium Chloride Addition on Oleate Adsorption
on Apatite and Dolomite at pH 4.
.0
Amount of Oleate Adsorbed
(pmol/g x
10)
Mineral
Single Mineral
Mi xed
Mineral
Apatite
5.1, 5.6
4.0,
4.3
Dolomite
9.1, 9.5
9.6,
10.0
rv>
Total Oleate Concentration = 3.6x10^ kmol/m^


126
Dolomite structure
Although the use of NaCl was intended for compression of the EDL
and reduction of the electrostatic attraction between the mineral
surface and the surfactant, in the preceding discussion, it was shown
that it did not act as an indifferent electrolyte for apatite. However,
no changes were observed in the zeta potential of dolomite below its
IEP. The possible reasons for this behavior are as follows.
The dolomite structure is similar to the calcite structure in many
respects. The hexagonal unit cell of dolomite basically retains the
calcite structure, but simply substitutes Mg atoms for Ca atoms in
alternating cation layers (see Figure 46). In this structure the c-
glide present in calcite is destroyed by the alternating Ca-Mg
arrangement, making it unfit for substitution of certain cations such as
sodium and potassium.
Single-crystal X-ray structure refinements of dolomite from
different locations conducted by Reeder and Wenk (1983) and Effenburger
et al. (1981) indicated substitutions of Fe and Mn for Mg, and Mg for
Ca or vice versa. However, substitution of alkali cations such as Na
and K was not observed, which confirmed the above hypothesis. Reeder
(1983) has also shown that the smaller rhombohedral cell of calcite
(CaCC^) which is similar to that of the dolomite structure, only favors
incorporation of the smaller cations (Mg, Fe, Mn, Zn and Cu) whereas the
large cell of the orthorhombic aragonite (CaC03) allows preferential
substitution of larger cations such as Sr, Ba, Na and U. It has also
been indicated that Na and K ions are not compatible with dolomite


LIST OF FIGURES
Figure No. Page
1 Location of Florida phosphate deposits 2
2 A schematic diagram of a modified Hallimond Cell . 21
3 Hallimond cell flotation arrangement 22
4 Apparatus for mixed mineral adsorption studies. ... 28
5 SEM micrograph of apatite (65x100 mesh size
fraction), a) 200X; b) 1000X 38
6 SEM micrograph of dolomite (65x100 mesh size
fraction), a) 100X; b) 1000X 39
7 X-ray diffractogram of apatite 41
8 X-ray diffractogram of dolomite 42
9 Zeta potential of apatite as a function of pH ... 43
10 Zeta potential of dolomite as a function of pH. . 44
11 Flotation of apatite as a function of pH 46
12 Dolomite flotation as a function of pH 47
13 Flotation of apatite and dolomite (single minerals)
as a function of pH 48
14 Flotation of apatite and dolomite (single minerals)
as a function of pH 49
15 Flotation of apatite and dolomite (single minerals)
as a function of pH 53
16 Effect of sodium chloride addition on apatite and
dolomite flotation 61
17 Effect of sodium oleate concentration on apatite
and dolomite (single minerals) flotation with and
without NaCl addition 62
XI


Dolomite-Oleate System 136
Preferential formation of magnesium oleate .... 138
Contact Angle Studies 145
Mechanism of Selective Flotation of Dolomite from
Apatite in the Presence of NaCl 147
VI CONCLUSIONS 149
VII SUGGESTIONS FOR FUTURE RESEARCH 155
REFERENCES 159
BIOGRAPHICAL SKETCH 168
vi i i


<0
I-
*
z
3
I
<
*
-I
LU
OS
3
*
o
co
Figure 42. Diffuse reflectance IR spectra of apatite (treated and untreated) and the difference
spectrum at pH 4.0 in the presence of NaCl.


38
Figure 5. SEM micrograph of apatite (65x100 mesh size fraction),
a) 200X; b) 1000X.


TABLE 14
Solubility Product of Calcium and Magnesium Oleate
Product
Present Study Fuerstenau and Palmer, (1976) Du Reitz, (1957)
Calcium Oleate
2.40xl0'16 2.51xl0-16 3.98X10"13
Magnesium Oleate
3.02X10'16 6.31xl016 1.58xl0-11


139
Preferential exposure of magnesium ions. The breakage of mineral
particles, like most other substances, occurs along surfaces of least
resistance. In most minerals, the strength of chemical bonds is not
uniform in all directions. The chemical bonding along certain planes
may be weaker, resulting in breakage along that interface rather than at
random. The cleavage of the rhombohedral dolomite structure, like many
other carbonate minerals, occurs along the {104} plane. The relative
tendency of dolomite to develop cleavage along this plane has been
considered to be ideal (Zoltai and Stout, 1974). If there were no
preferred planes of weakness that are controlled by the crystal
structure, the mineral would break along a random fracture such as in
the case of apatite. It can be seen, upon examination of the cleavage
plane of orthorhombic dolomite structure, that equal numbers of both
calcium and magnesium ions would be exposed. The selective magnesium
oleate formation, therefore, cannot be explained on the basis of
exposure of more magnesium ions during fracture.
Solubility of magnesium and calcium oleate. The solubility product
of calcium and magnesium oleate, as determined by the nephelometric
method, are given in Table 14, including the literature values. Results
obtained in the present study are in good agreement with those of
Fuerstenau and Palmer (1976). It is, however, observed that the
solubility product of magnesium oleate is higher as compared to that of
calcium oleate. Assuming that this observation can be extrapolated to
the mineral surface, preferential formation of calcium oleate rather
than magnesium oleate should have been observed which is contrary to the
results obtained in the present investigation.


23
timer, which can be set to allow the gas flow and stirring for any
length of time.
The apatite, dolomite, or the mixture of these minerals were aged
in 100 ml volumetric flask for 20 minutes and conditioned after reagent
addition for a period of 5 minutes by slow tumbling at 8 rpm. Whenever
required, minerals were aged in the salt solution of specified
concentration. The pH of the suspension was measured before aging and
after conditioning in all cases and the latter is reported as the pH of
flotation. The suspension was transferred to the Hallimond cell with
minimum turbulence and floated for one minute using nitrogen gas at a
flow rate of 50 ml/min. The floated and unfloated fractions were dried
at 50 C, and flotation recovery was calculated based on the weight of
the dried samples. In the case of the mixtures, float and sink
fractions were pulverized and leached in aqua-regia before analyzing for
P2O5 and MgO contents.
Electrokinetic Measurements
Zeta potential measurements were conducted using a Pen Kern Model
501 Laser Zee Meter. Suspensions of 0.1 wt % apatite or dolomite were
prepared using -325 mesh sample. These were stirred with a Teflon-
coated magnetic bar for approximately 21 hours at natural pH. Fifty
milliliter samples of the aged suspension were equilibrated at the
desired pH for 3 hours before they were transferred to the cell for zeta
potential measurements.


LOG (ACTIVITY OF THE SPECIES)
Figure 44. Dodecylamine species distribution as a function of pH. Total amine
concentration, 1.6 x 10~^ M.


AMOUNT FLOATED, WT%
100
80
60
40
20
0
Figure 19.
FLOTATION pH
Effect of pH on flotation of apatite and dolomite (single minerals) in the presence
of sodium chloride.


15
Dufour et al. (1980) at Minemet Recherche, France, depressed
phosphate at pH 5.5 and floated dolomite after attritioning; however,
the reagents used were not disclosed. The apatite recoveries ranged
from 48 to 85 percent with the final product containing less than 1%
MgO.
Johnston and Leja (1978) also selectively floated dolomite at pH
6, using oleic acid as the collector by depressing fluorapatite by
phosphate ions. The difference in flotation was explained on the basis
of adsorption of phosphate ions onto apatite by hydrogen bonding.
Dolomite flotation was attributed to hydrogen bonding of oleic acid on
dolomite in addition to COg gas evolution.
In an effort to develop new reagents that interact selectively,
Fu and Somasundaran (1986) used alizarin red S, a dye that stains
calcite, and achieved effective separation of calcite from apatite with
sodium oleate as the collector. They found that alizarin red S adsorbs
more on apatite than calcite and consequently acts as an apatite
depressant.
Other noteworthy studies include that of Bushel et al. (1970);
Onal (1973); Ratobylskaya et al. (1975); Lawver et al. (1978, 1980,
1981); Kiukkola (1980); Dufour et al. (1980); Baumann and Snow (1980);
Houot and Polgaire (1980); Lehr and Hsieh (1981); Clerici et al. (1984);
Rao and coworkers (1985), and Atalay et al. (1985).
It should, however, be noted that most of the above studies have
evolved from engineering studies. Hence, the fundamental principles
governing these processes were not studied which are required for
developing more efficient techniques.


AMOUNT FLOATED, WT%
100
£)-0-
O
-
80
Singla Mlnarala
O Dolomita
A Apatita
Sodium Olaata Cone.,
4.0x10 kmol/m3
0 163 152 151 1.0
SODIUM CHLORIDE CONC.( kmol/m3
cn
Figure 16. Effect of sodium chloride addition on apatite and dolomite flotation.


99
at 1480-1450, 1415, 896-875 and 727 cm"^. Dolomite bands identified by
Gadsden are apparently in a good agreement. The positions of the major
apatite and dolomite bands did not indicate any change after treatment
with either the collector or sodium chloride.
Nature of the Adsorbed Species at pH 4
Apatite-oleate system
The diffuse reflectance spectra of treated and untreated apatite
and the difference spectrum at pH 4 are exhibited in Figure 40. The
difference spectrum indicated the presence of oleic acid and calcium
oleate with bands at 1717 and 1563 wavenumbers, respectively, the
intensity for calcium oleate being relatively higher than that for oleic
acid.
Dolomite-oleate system
The IR spectra of treated and untreated dolomite at pH 4 are
illustrated in Figure 41. The difference spectrum in this case
indicated the existence of magnesium oleate and oleic acid on the
dolomite surface with bands at 1582 and 1717 cm-1, respectively. In
this case the intensity of the oleic acid band appeared to be higher as
compared to that of magnesium oleate. It is to be noted that at both pH
4 and pH 10, no calcium oleate formation on dolomite surface was
observed from the spectra obtained.


156
apatite was observed to be more negative in the alkaline pH range.
Between pH 6 and 8, however, it was less negative indicating possible
compression of the electrical double layer as a result of higher ionic
strength. It is known from the aging studies (Chanchani, 1984) that
apatite reaches equilibrium at pH 7.0-7.5. The slope of the pH shift
versus time from the aging studies gives information on dissolution
rates, which appear to be higher in the acidic and basic pH ranges.
Hence, a study of sodium substitution for calcium in the apatite
structure at pH 7 and pH 10, as was done at pH 4 in the present study
can possibly give information about the changes in the zeta potential
behavior. In addition, the data might be helpful in explaining why
apatite is depressed above pH 9 in the presence of sodium chloride as
has been observed in this study and reported also by Maslow (1971) and
Strel'tsyn et al. (1967).
Equilibrium adsorption studies of collectors such as oleate on
apatite and dolomite when combined with the heat of adsorption data can
provide significant information about the mechanism of adsorption.
Adsorption of oleate on apatite has been studied by Moudgil et al.
(1987). Currently, there is no heat of adsorption data available on
apatite-oleate or dolomite-oleate systems. Therefore, it is recommended
that the heat of adsorption of oleate be determined on these minerals
under various experimental conditions, using a microcalorimeter. In
addition, oleate adsorption on apatite and dolomite as a function of
temperature (even though it can only be done in a narrow temperature
range) should be conducted to find out the energy of activation
required for adsorption of oleate on each of the minerals. It is


128
structure (Blanchard, 1987). This might explain the reasons for NaCl
acting as an indifferent electrolyte in the case of dolomite.
Adsorption of Oleate on Apatite and Dolomite
Effect of Conditioning pH
Adsorption of sodium oleate on apatite and dolomite was determined
to explain the reasons for the selective flotation of dolomite from
apatite in the presence of NaCl. Sodium oleate adsorption on single
minerals as a function of pH is shown in Figures 47 and 48 for apatite
and dolomite, respectively. A good correlation between adsorption and
flotation is observed from the results. Similar results were reported
by Chanchani (1984). It is observed that the amount of adsorbed oleate
required for flotation is dependent on the pH of the
mineral/collector/water system and varies significantly for apatite and
dolomite. It has been suggested that acid-soap complexes are the
predominant species adsorbing on apatite between pH 7 and 9, with
possible chemisorption of oleate monomers and dimers onto the surface
calcium sites (Chanchani, 1984). In the pH range of 7 to 11, the zeta
potential of apatite has a nearly constant negative value. Thus, the
adsorption of the anionic oleate species is expected to be adversely
affected. In the acidic pH range (pH 6 and lower), a combination of
physical and specific interaction forces has been suggested by Johnston
and Leja (1978) to be responsible for the observed flotation behavior.
Unlike apatite, adsorption of oleate on dolomite has been suggested to
be largely influenced by the electrostatic forces (Chanchani, 1984).


AMOUNT FLOATED, WT%
FLOTATION pH
Figure 21. Effect of KC1 addition on flotation of apatite and dolomite as a
function of pH.


ZETA POTENTIAL. mV
25
Apatite in Distilled Water
15 -
pH
GJ
25
Figure 9. Zeta potential of apatite as a function of pH.


16
A systematic and thorough study of apatite-dolomite separation
using anionic collectors has been conducted by Moudgil and Chanchani
(1985a and 1985b). The surface chemical and dissolution characteristics
of the minerals were taken into account in addition to solution
properties of the surfactant (sodium oleate), and a "two-stage
flotation" process was developed. This process involves conditioning
the feed at pH 10 followed by reconditioning at a pH less than 4.5 to
selectively float out dolomite. Bench scale testing of this process has
yielded favorable results (Moudgil, 1987).
Flotation in the Presence of Salts
As mentioned above apatite-dolomite separation has been studied
(Atalay et al., 1985; Rule et al., 1985; Hsieh and Lehr, 1985; Lawver et
al., 1984; Llewellyn et al., 1982; Johnston and Leja, 1978; and Onal,
1973) using phosphate salts as depressants for apatite. However, no
systematic studies are reported on the role of the added salts on the
flotation behavior of these minerals. It should be noted that in these
studies phosphate minerals were depressed in the presence of phosphate
salts.
Maslow (1971) and Strel'tsyn et al. (1967) have reported the
adverse effect of salt addition on apatite flotation in apatite-
nepheline ore. These workers found that when NaCl addition exceeded 0.5
kg/ton, the ore was suppressed at pH 9, but it was overcome by addition
of NaOH and sodium silicate. Additionally, Gruber et al. (1986) have
reported successful separation of carbonates such as cal cite from
apatite in sea water from the Santo Domingo phosphate deposit in Baja


2
Figure 1. Location of Florida phosphate deposits.


This dissertation was submitted to the Graduate Faculty of the College
of Engineering and to the Graduate School and was accepted as partial
fulfillment of the requirements for the degree of Doctor of Philosophy.
December 1987
iLJuJl d-
Dean, College of Engineering
Dean, Graduate School


KUBELKA-MUNK UNITS
Figure 41. IR spectra of treated and untreated dolomite at pH 4.0.


112
b) Coating of the mineral particles by precipitates or slimes
generated during conditioning;
c) Depletion of the surfactant species by complexation and
precipitation with dissolved ionic species.
Changes in the surface charge and surface chemical composition
The surface charge and surface chemical composition of the minerals
can undergo modifications due to the dissolution of lattice ions.
Moudgil and Chanchani (1985a) and Chanchani (1984) have reported that
the Ca++ and Mg++ ions dissolving from dolomite cause a significant
reduction in the flotation of apatite when sodium oleate is used as the
collector. It was also found that when these ions are present at
1.0x10^ kmol/m3 level, they can reverse the sign of the zeta potential
of apatite. In addition, it was established that the zeta potential of
dolomite is affected by the phosphate ions, even at concentrations as
low as l.OxlO-3 kmol/m3 level. Amankonah et al. (1986), in their study
of the apatite-calcite system, have found that the IEP of apatite
interchanges with that of calcite when conditioned in calcite
supernatant, despite the fact that the IEP of these minerals were
determined to be four pH units apart in distilled water (at pH 6.5 and
10.5, for apatite and calcite, respectively). In the present study,
the amounts of Ca++ and Mg++ dissolving from dolomite and calcium from
apatite were determined to be 2.7xl0-3 and l.lxlO-3 kmol/m3,
respectively, at pH 4 after 5 minutes of conditioning (Table 12 and
Table 8). It is therefore possible that both the surface charge and
surface chemical composition of apatite and dolomite could have been


20
Methods
Flotation
Flotation experiments were conducted using a modified Hallimond
cell (Fuerstenau et al., 1957; and Modi and Fuerstenau, 1960) shown in
Figure 2. This cell consists of two parts connected by a ground glass
joint. The lower part of the cell is fitted with a fritted glass,
having uniform pore size. A Teflon-coated magnetic stirring bar is used
to maintain the particles in suspension. In the Hallimond cell
technique, the hydrodynamic variables such as agitation, airflow rate,
and flotation time can be controlled.
The complete Hallimond cell flotation assembly is illustrated in
Figure 3. The details of the set-up and operation are similar to those
used by Moudgil (1972), Ananthapadmanabhan (1980) and Chanchani (1984)
and are only briefly described below.
Nitrogen gas was used for the flotation experiments, which was
first purified by passing through an ascarite column (D) for removing
the CO2 (see Figure 3). Subsequently, the gas passes through a water
trap (T^) to remove any ascarite fines carried over. Traps (T2 and T3)
are provided to prevent back suction of water to the ascarite column.
The purified gas is passed to a 50 liter glass reservoir (R), which acts
as a buffer tank to supply the gas at a constant pressure. The pressure
in the tank is measured by the manometer (M). The outlet of the gas
reservoir is connected to the Hallimond cell (HC) through a solenoid
valve (V) and flow meter (F). The solenoid valve is energized using a


OLEATE ADSORBED, ^mol/g
5.0 p
4.0 -
3.0 -
2.0 -
1.0 -
O
101
Figure 33.
O Dolomite at pH 10.0
Dolomite at pH 4.0

o
10 4
INITIAL OLEATE CONC., kmol/m'
Oleate adsorption on dolomite as a function of initial oleate concentration
at pH 4.0 and 10.0.
oo
00


55
predicted by the single mineral results. It appears that the changes in
the flotation response of apatite are the major cause for the loss of
selectivity in the mixed mineral system when sodium oleate is used as
the collector.
It is clear from the results presented that the loss of selectivity
in the mixed mineral system is primarily due to the activation or
depression of either apatite or dolomite depending on whether
dodecylamine hydrochloride or sodium oleate is used as the collector.
Moudgil and Chanchani (1985a and 1985b), and Soto and Iwasaki (1985 and
1986) have provided detailed explanations for the loss of selectivity in
this system.
In the case of the apatite/dolomite-amine system, Soto and Iwasaki
(1985) proposed that the adsorption of amine on dolomite is mainly
controlled by the electrostatic attraction in addition to weak chemical
interaction. The fact that flotation recovery of both apatite and
dolomite decreased sharply below their respective IEPs (at pH -5.4),
when dodecylamine was used (refer to Figure 14) and increased in the
case of sodium oleate (Figure 15), indicated that the coloumbic forces
play a significant role in the adsorption of the collector on the
respective substrates. It should, therefore, be possible to modify
flotation behavior of dolomite by the addition of indifferent
electrolytes such as sodium chloride.


8
description of the dolomite [CaMgiCC^^] structure is provided by
retaining the calcite (CaCC^) structure, but simply substituting Mg
atoms for the Ca atoms in every other cation layer. The alternating Ca-
Mg arrangement of dolomite has some similarities with calcite, but the
c-glide present in calcite is destroyed. Dolomite has a rhombohedral
crystal structure with a Ca-0 bond length of 2.38 8, and Mg-0 bond
length of 2.08 8. This results in oxygen lying closer to Mg than Ca in
the dolomite structure (Reeder and Sheppard, 1984)
Solubility of Apatite and Dolomite
Charge characteristics of the solid/liquid interface and chemical
composition of the aqueous phase depends on the solubility of the
minerals. When the minerals come into contact with water, the
constituent species such as Ca++, P0$ and F" from apatite, and Ca++,
Mg++ and C0^ from dolomite will be transferred from the structure into
the solution. This dissolution will continue until the chemical
potentials of the species in solution and the solid phases reach an
equilibrium. The solubility of the salt-type minerals often varies over
a wide range, because the chemical potentials of the species in solution
and in the solid are affected by a number of factors such as degree of
hydration, solid solution formation and presence of other components in
solid or solution. It is known that both apatite and dolomite, in
contrast to simple salts, dissolve with their ions undergoing various
hydrolysis and complex formation reactions.
There have been a number of publications on the solubility of
apatite with marked differences in the data reported. Hanna and


TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS iii
LIST OF TABLES ix
LIST OF FIGURES xi
ABSTRACT xiv
CHAPTERS
I INTRODUCTION 1
II BACKGROUND 6
Characteristics of Salt-Type Minerals 6
Characteristics of Apatites in Florida Phosphorites . 7
Characteristics of Dolomite 7
Solubility of Apatite and Dolomite 8
Surface Charge of Apatite and Dolomite 9
Aging Behavior of Apatite and Dolomite 10
Separation Studies 11
Methods Based on Physical Properties 11
Separation Based on Physico-Chemical Properties .... 12
Selective flocculation 12
Selective flotation 12
Flotation in the Presence of Salts 16
III EXPERIMENTAL 18
Materials 18
Minerals 18
Apatite 18
Dolomite 18
Reagents 19
Other Chemicals 19
Methods 20
Flotation 20
v


19
Samples of completely crystalline New Jersey dolomite and
fluorapatite obtained from Geology Department, University of Florida
(Blanchard, 1987) were used for contact angle measurements. Chemical
analysis of the apatite sample indicated 37.51% P2O5, 0.14% MgO and
1.15% acid insoluble. Crystalline New Jersey dolomite analyzed 17.73%
MgO, 0.36% P2O5 and 0.95% acid insoluble.
Reagents
Dodecylamine hydrochloride obtained from Eastman Kodak Company, and
purified sodium oleate purchased from Fisher Scientific Company, were
used as the cationic and anionic collectors, respectively, for flotation
experiments.
A mixture of unlabeled and ^C labeled oleic acid was used for the
adsorption experiments. Unlabeled oleic acid (gold label grade) was
obtained from Aldrich Chemical Company. ^C labeled oleic acid was
purchased from ICN Pharmaceuticals in nitrogen-sealed ampules of 0.1
mCi.
Other Chemicals
All other chemicals such as calcium and magnesium standards, HNO3,
KOH, NaOH, NaCl, KC1, NaF, etc., were of reagent grade purchased from
Fisher Scientific Company.
Triple distilled water of less than 1.2 micromhos conductivity was
used in the flotation experiments. All other solutions were prepared
using triple distilled water deaerated by bubbling nitrogen for two
hours.


21
Figure 2. A schematic diagram of a modified Hallimond Cell.


TABLE 1
Sodium Oleate Distribution in Various Streams
During Adsorption Tests at pH 4.5
Initial Total
Oleate Concentration*
(kmol/m^)
Final Distribution of Oleate, %
On Mineral
On Vial
In Solution
In Rinse
Solution
1.88xl0"4
19.3, 20.0
5.7, 5.5
73.5, 72.7
1.6,
1.8
7.53xl0"4
22.5, 22.0
4.8, 4.9
72.2, 72.5
0.5,
0.6
*Total oleate concentration was assumed to be 100%.


3
1. Increase the viscosity of the phosphoric acid resulting in
higher pumping costs (Cate and Deming, 1970);
2. Cause an increase in sulfuric acid consumption and
induce foaming;
3. Form excessive sludge during phosphoric acid production;
4. Precipitate as complex salts (MgNH4P25) and lead to
clogging of filters (Becker, 1983);
5. Interfere in the production of certain super phosphates
(Becker, 1983).
Phosphate rock, which is essentially carbonate-fluorapatite,
(Fantel and Rosenkranz 1983; Whippo and Murowchick, 1967), contains
magnesium impurities in one of the following forms:
1. Ionic substitution in the apatite structure;
2. Second phase dolomite in apatite;
3. Discrete dolomite particles.
The MgO content in francolite (carbonate-fluorapatite) occurs both
as fine inclusions of dolomite and as Mg in the francolite structure,
according to Lawver et al. (1982a). The range of "lattice Mg"
calculated as MgO was found to be 0.40% from 31 samples studied by
Lawver and co-workers. Recently, Blanchard et al. (1986) have
conducted a systematic investigation using 18 samples from Florida
phosphate fields. Measurements of dolomite content by X-ray diffraction
and chemical analyses of these samples indicated that the average excess
MgO content above that accounted for by dolomite is about 0.57% by
weight. This represents a reasonable estimate of the amount of MgO
substituting in the apatite structure. Additionally, McClellan (1980)


67
Flotation studies in the presence of KC1 and NaF
To examine the effect of additional salts such as KC1 and NaF on
the flotation behavior of apatite and dolomite, single and mixed mineral
tests were conducted. Results of the experiments with KC1 in the pH
range of 3.0-5.5, using sodium oleate as the collector, are illustrated
in Figure 21. It is seen that in the case of single minerals about 80%
of dolomite is floated at pH 4 with only 5-10% apatite reporting in the
float fraction. The best results with the 50:50 mixture, however, were
obtained at pH 4.5, where 75% of dolomite and 5-10% apatite was
recovered in the float fraction. These tests were conducted in the
presence of 2.0x10"^ kmol/m^ KC1 (the optimum concentration for NaCl).
Comparison of apatite and dolomite flotation with and without
2.0x10-2 kmol/m^ NaF, as shown in Figure 22, indicated that dolomite
depression is relatively higher than that of apatite. Due to the
greater depression of dolomite with NaF, the difference in the
floatability gap of these minerals narrowed. In fact, some exploratory
mixed mineral experiments conducted at pH 4 using sodium oleate as the
collector indicated that in the presence of NaF, either both of these
minerals were depressed or floated together.
Electrokinetic Studies
Effect of Salt Addition on the Zeta Potential of Apatite
Zeta potential of apatite was measured as a function of pH in the
presence of 2.0x10^ kmol/m^ NaCl (optimum concentration for apatite
depression), KC1 and NaF. As seen from the results plotted in Figures


85
kmol/m3) on the amount of oleate adsorbed on apatite. Results of the
adsorption experiments illustrated in Figure 31, indicated a decrease in
the amount of oleate adsorbed on apatite in the presence of sodium
chloride as compared to that obtained in distilled water.
Dolomite-oleate system
The amount of oleate adsorbed on dolomite as a function of pH is
plotted in Figure 32 at three levels of sodium oleate concentration.
Adsorption behavior similar to that of apatite (refer to Figure 29) is
observed, however, the maxima in the alkaline pH range is shifted to pH
10 as compared to pH 8 for apatite. The amount of oleate adsorbed on
dolomite at pH 10 is comparable to that on apatite at pH 8. In the
acidic pH range, a smaller peak is observed at pH 4. Oleate adsorption
on dolomite was found to be minimal between pH 5.5-8.5. The amount
adsorbed was higher at higher dosages of sodium oleate.
Tests were also conducted at pH 4 and pH 10 to determine the amount
adsorbed on dolomite as a function of sodium oleate concentration. It
is observed from Figure 33, that the amount adsorbed at pH 10 is
approximately three-fold higher than that at pH 4 under most of the
concentration range examined.
Effect of NaCl on sodium oleate adsorption on dolomite. Results of
adsorption experiments, at pH 4 in the presence of 2.0xl02 kmol/m3
NaCl, as shown in Figure 34, indicated no measurable change in the
amount adsorbed on dolomite as compared to that in distilled water. It
should be noted that oleate adsorption on dolomite is nearly twice as


142
ionicity, can be evaluated from the electronegativity values of calcium
(1.00), magnesium (1.31), and oxygen (3.44), using the following
expression (Atkins, 1982):
Ionicity % = 16[Xa Xb] + 3.5[Xa Xb]2
The % ionicity of the calcium and magnesium oleate bonds were
computed to be 60% and 50%, respectively. These calculations
demonstrate that magnesium-oleate bond would be relatively more
covalent than the calcium-oleate bond. The extremely strong ionic
"bond" is readily disrupted in a medium of high dielectric constant,
such as water, in marked contrast to covalent bonds, which even though
weaker, are not generally disintegrated by a solvent such as water.
Charge density difference. Smaller ions or molecules have a higher
charge density, i.e., charge/volume ratio, in contrast to the larger
ones. The energy of interaction (ip) between an ion and a dipole is a
function of 1/r2, where r is the ionic distance between the center of
the ion and the dipole molecule. The interaction energy (cp) between an
oleate anion (oleate forms a permanent dipole) and a calcium (r^a =
0.71 ft) or magnesium cation (r^g = 0.51 ft) on the dolomite surface can
be calculated using the following expression (Israelachvi1i, 1985):
q u Cos0
(p = NAv(J/mole)
4 tt 2 £ r
where, q = electronic charge (= ze, where z is valence and e is
the electron charge) of the ion


30
20
10
0
10
20
30
Dolomita in Distilled Water
-P*
Figure 10. Zeta potential of dolomite as a function of pH.


32
Selection, Preparation, and Characterization of the Minerals
Natural apatite and dolomite samples from Florida phosphate
deposits were selected. Both apatite and dolomite samples were crushed
and ground to obtain 65x100 mesh size fractions for flotation
experiments. A portion of these samples was ground to -325 mesh for
electrokinetic and FT-IR spectroscopic studies. The samples were
characterized for chemical composition, surface area and porosity, and
surface charge behavior. X-ray analysis and Scanning Electron
Microscopy studies were also carried out on the samples.
Selection of the Surfactant
Sodium oleate (anionic) and dodecylamine hydrochloride (cationic)
surfactants, which constitute the active components of fatty acid and
fatty amine collectors, respectively, were used in this study. It
should be noted that both of these surfactants hydrolyze significantly.
Nature of the dominant species present therefore would be governed by
the pH of the solution.
Selection of the Experimental Techniques
The experimental techniques selected are described in the following
section.
Flotation
Microflotation tests using a Hallimond cell was selected for
flotation studies. This technique enables close control of hydrodynamic
variables such as the agitation, air flow rate, and flotation time. It


147
is not expected to greatly affect the trends observed, the magnitude of
the changes determined for the minerals can be different than those
indicated by flotation and adsorption measurements.
Mechanism of Selective Flotation of
Dolomite From Apatite in the Presence of NaCI
It was mentioned earlier that the selectivity predicted by the
single mineral flotation tests was found to be limited in the case of
mixtures using either oleate or dodecylamine as the collector. The loss
in the selectivity was attributed to the surface charge modification as
well as to the complexation and precipitation of the collector species
with dissolved ions from the minerals, which reduced the effective
collector concentration in the system. Sodium chloride was therefore
added to minimize the effect of the electrical double layer in the
adsorption process. Consequently, selective flotation of dolomite was
achieved from apatite.
It was shown that in the presence of sodium chloride the surface
charge of apatite reverses below its IEP. This was ascribed to the
substitution of Na+ for Ca++ in the apatite structure. Substitution of
Na+ for Ca++ was established through chemical analysis of the samples in
addition to determining the changes in the unit cell parameters.
Subsequently, it was shown that the amount of collector adsorbed on the
apatite in the presence of NaCI decreases by about 40% as compared to
that in distilled water. Furthermore, the adsorbed surfactant species
were identified to be different in the presence of sodium chloride.
Calcium oleate and oleic acid were determined to form on apatite surface
at pH 4 in the absence of added salt. In the presence of NaCI, on the


13
number of flotation processes has been reported to separate dolomitic
impurities using both cationic and anionic collectors.
Cationic flotation of apatite from dolomite. Flotation of
apatite (francolite) from dolomite using cationic collectors was
investigated by the International Minerals and Chemicals Corporation in
the late seventies and early eighties (Snow, 1979; Baumann and Snow,
1980; and Lawver, 1980). These investigators developed a process which
reduces the MgO content of the conventionally floated (double flotation)
material to 1% or less yielding more than 90% BPL recoveries. This
process involved a rougher float followed by several cleaner stages
using a primary aliphatic amine in combination with kerosene as the
collector.
The above process was later studied by Soto and Iwasaki (1985 and
1986) to elucidate the mechanisms involved. These researchers concluded
that there is a stronger chemical interaction between the cationic
collector and the phosphate ions present at the apatite surface. The
selectivity was attributed to the lower solubility of the reagent-
phosphate compound as compared to that of the reagent-carbonate complex
formed. Other noteworthy studies with cationic collectors have been
conducted to separate calcite from apatite (Hanna, 1975, and Samani et
al., 1975).
Anionic flotation of dolomite from apatite. Separation of
calcite and in some cases dolomite from phosphate ores using anionic
collectors has been studied extensively in the past decade. A detailed
review of these studies has recently been presented by Moudgil and


27
The mixed mineral adsorption tests were conducted in a glass cell
arrangement as shown in Figure 4. This cell design allows mixed mineral
conditioning of apatite and dolomite in the same medium without coming
in contact with each other. A 0.5 g, 65x100 mesh sample was used for
these tests. The sample volume in each side of the cell was maintained
at 50 ml and the conditioning was identical to that of single mineral
experiments with respect to pulp density, conditioning time, and
tumbling speed. Adsorption tests in the presence of sodium chloride
were conducted in the same manner after aging the mineral samples in
2.0x10^ kmol/m^ NaCl.
Analysis of the labeled oleic acid was performed using a
Beckman Model LS 1800 liquid scintillation counter after mixing the
solids or solution with a scintillation cocktail, "Scintiverse-II"
(obtained from Fisher Scientific Company). In the case of mixed
minerals, the amount adsorbed was also determined directly on the solids
in the entire pH range since the solution depletion method could not be
followed.
Mineral Dissolution Tests
The dissolution of ions from apatite or dolomite at a given pH
value was determined by agitating one gram of 65x100 mesh solids in 100
ml of distilled water for a known time interval using a slow speed (8
rpm) tumbler. At the end of the tumbling period, a 10 ml sample of the
supernatant was withdrawn for analysis. The supernatant was centrifuged
at 734 G (2500 rpm) for 30 minutes to remove fines created during
agitation, before calcium and magnesium ion analysis was carried out


34
the low solubility of oleic acid in the neutral and acidic pH range
was taken into account to ensure correct analyses.
Determination of the nature of the adsorbed surfactant species
Adsorption studies are not expected to yield any information about
the nature of the various oleate species such as ionic oleate monomers
or acid-soap complex. Such information can be helpful in explaining the
differences in flotation behavior of these minerals under different pH
conditions. FT-IR spectroscopy was employed to study the nature of-the
adsorbed species, using a BOMEM DA3.10 model instrument.
Mineral dissolution studies
These tests can provide information about the dissolution
characteristics of various ionic species from the minerals which can be
helpful in explaining the surface charge and adsorption behavior.
Analysis of the dissolved ions such as calcium and magnesium was carried
out using a Perkin Elmer Plasma II, inductively coupled plasma (ICP)
emission spectrometer.
Contact angle
These measurements under different experimental conditions can
yield information about relative hydrophobicity of the mineral surfaces.
Contact angle measurements were conducted with and without salt
addition, using a NRL Contact Angle Goniometer (Model 100-00).


KUBELKA-MUNK UNITS
1563
Figure 40. Diffuse reflectance IR spectra of treated and untreated apatite at pH 4.0.


153
Adsorption of oleate and oleic acid at pH 4, and of oleate alone at
pH 10, were detected on dolomite from FT-IR results. It was determined
that magnesium oleate forms on the dolomite surface in preference to
calcium oleate at both pH 4 and pH 10. The nature of the adsorbed
species, however, remained the same in the presence of sodium chloride
at pH 4. This finding is again consistent with flotation and
electrokinetic behavior of dolomite in the presence of NaCl which
remained unchanged from that in distilled water. In the past, it has
been postulated that oleate would adsorb on both Ca++ and Mg++ sites on
the dolomite surface. Consequently, the inability to separate dolomite
from apatite was attributed to the presence of the common cation (Ca++).
In this study, for the first time, it has been established that this is
not necessarily the case. Furthermore, it has been established that
presence of two different cations in the same structure ( e.g., Ca++ and
Mg++ in dolomite) cannot be assumed to be the adsorption sites for the
anionic collector specie such as oleate.
Preferential formation of magnesium oleate on dolomite was
explained in terms of higher charge density of magnesium ions as
compared to calcium. Higher calcium dissolution from dolomite as
compared to magnesium ions and higher electronegativity of magnesium
also contributed to the formation of only magnesium oleate on the
dolomite surface. Additionally, higher electronegativity of magnesium
ions can lead to more covalent bond formation with oleate, which is
stronger in an aqueous environment, thus resulting in preferential
magnesium oleate formation on the surface of dolomite.


11
Separation Studies
The past research efforts to develop a suitable technique for
separation of carbonates from apatite can be divided into two broad
categories, those in which differences in physical properties such as
specific gravity, conductivity, hardness, etc., were utilized and those
where surface chemical properties of the minerals were exploited to
achieve the desired separation. A brief discussion of these efforts is
presented below.
Methods Based on Physical Properties
Apatite and dolomite have relatively close physical properties
such as specific gravity and hardness, thereby making it difficult to
achieve their separation based on these properties. Both of the
minerals are also nonmagnetic. Hence, the separation techniques based
on such properties are not feasible. However, the "apparent" densities
of the two minerals have been found to be sufficiently different for the
possible application of heavy media separation (Lawver et al., 1982a).
The difference in the specific gravity is attributed to the highly
porous nature of the large dolomite particles as compared to the same
size apatite particles. The effectiveness of the heavy media separation
process is reported to be limited to the coarse size (pebble size)
only, which is found in lesser quantity in the southern district.
Selective attritioning has also been attempted by Soto and Iwasaki
(1986). Results from these studies indicated that only 40-60% of the
dolomite can be eliminated and that flotation is necessary for further
reduction of MgO in the apatite concentrate.


160
Blanchard, F. N., Goddard, R. E., and Saffer, B., 1986, "Application of
Quantitative X-Ray Diffraction Analysis Combined with Other
Analytical Methods to the Study of High-Magnesium Phosphorites,"
Proceedings of the Thirty-fourth Annual Conference on Applications
of X-Ray Analysis, Snowmass, Colorado, pp. 235-242.
Brescia, F., Arents, J., Meislich, H. and Turk, A., 1966,
"Fundamentals of Chemistry: A Modern Introduction," Academic
Press, New York.
Bushel 1, C. H. G., Hirsch, H. E. and Laner, R. M., 1970, "Phosphate
Flotation Process," Canadian Patent 833,611.
Cate, W. E. and Deming, M. E., 1970, "Effect of Impurities on Density
and Viscosity of Simulated Wet-Process Phosphoric Acid," J. Chem.
and Eng. Data, Vol. 15, pp. 290-295.
Chanchani, R., 1984, "Selective Flotation of Dolomite from Apatite
Using Sodium Oleate as the Collector," Ph.D. Dissertation.
University of Florida, Gainesville, Florida.
Clerici, C., Frisa Morandini, A., Mancini, A. and Mancini, R., 1984,
"Flotation of a Phosphate Rock with Carbonate-Quartz Gangue,"
Reagents in the Mineral Industry, Jones, M. J. and Oblatt, R.,
Eds., IMM, London, pp. 221-225.
Crago, A., 1940, "Process of Concentrating Phosphate Minerals," U.S.
Patent No. 2,293,640.
Dahlin, D. C. and Fergus, A. J., 1978, "Flotation of Carbonate and
Silica Minerals from Partially Altered Phosphate Rock of the
Phosphoria Formation," Technical/Economic Conference, Orlando,
Florida, pp. 37-47.
Dufour, P., Pelletier, B., Predali, J. J. and Ranchin, G., 1980,
"Beneficiation of South Florida Phosphate Rock with Higher
Carbonate Content," Proceedings, 2nd International Congress on
Phosphorous Compounds, Boston, pp. 247-267.
Du Reitz, C., 1957, "Fatty Acids in Flotation," Progress in Mineral
Dressing, Trans., IV Int'l. Min. Proc. Congress, Stockholm,
Almquist & Wiksell, Pub. pp. 417-433.
Effenberger, H., Mereiter, K. and Zemann, J., 1981, "Crystal Structure
Refinements of Magnesite, Calcite, Rhodochrosite, Siderite,
Smitsonite and Dolomite, with Discussion of Some Aspects of the
Stereochemistry of Calcite-type Carbonates," Z. Kristallogr.,
Vol. 156, pp. 233-243.
Fantel, R. J. and Rosenkranz, R. D., 1983, "The Availability of
Phosphate Rock from the Southeastern United States," AIME Annual
Meeting, Atlanta, Georgia, Preprint No. 83-87.


KUBELKA-MUNK UNITS
1717
Figure 43. Diffuse reflectance IR spectra of dolomite at pH 4.0 in the presence of
sodium chloride.


35 Oleate adsorption on apatite and dolomite (single
and mixed minerals) as a function of pH 91
36 Diffuse reflectance IR spectra of Mg-, Ca- and
Na-oleate (400-4000 cm-1 range) 94
37 Diffuse reflectance IR spectra of Mg-, Ca- and
Na-oleate (1200-1800 wavenumber region) 95
38 Diffuse reflectance IR spectra of treated and
untreated apatite at pH 10.0 97
39 IR spectra of untreated and treated dolomite at
pH 10.0 98
40 Diffuse reflectance IR spectra of treated and
untreated apatite at pH 4.0 100
41 IR spectra of treated and untreated dolomite
at pH 4.0 101
42 Diffuse reflectance IR spectra of apatite
(treated and untreated) and the difference
spectrum at pH 4.0 in the presence of NaCl 103
43 Diffuse reflectance IR spectra of dolomite at
pH 4.0 in the presence of sodium chloride 104
44 Dodecylamine species distribution as a function
of pH. Total amine concentration, 1.6 x 10"^ M . 107
45 Oleate species distribution as a function of pH
Total oleate concentration, 4.0 x 10^ M 108
46 Crystal structure of dolomite, c-axis vertical
a) Layered structure b) Stereoscopic projection
of the hexagonal unit cell for dolomite (a = 4.81 A,
c = 16.00 A) 127
47 Correlation between oleate adsorption and flotation
for apatite as a function of pH 129
48 Correlation between oleate adsorption and flotation
for dolomite as a function of pH 130
49 Difference IR spectra of apatite-oleate system at
pH 10, and at pH 4 in the presence and absence
of sodium chloride 135
50 Difference IR spectra of dolomite-oleate system at
pH 10, and at pH 4 with and without sodium chloride
addition 137
xi i i


no
occurs at pH 10. As mentioned earlier, this complex is highly surface
active than the monomer or the dimer because of the increase in its
molecular size and single charge (Ananthapadmanabhan et al., 1979) and
its low intrinsic solubility (Pugh, 1986).
The peaks observed in the flotation response of apatite are
explained as follows. The flotation peak at pH 10 can be ascribed to
the maxima in the concentration of iono-molecular complex. As the pH
decreases, the concentration of the complex (RNH2 RNH3)*" decreases
approaching a value of 1.0x10^ kmol/m^ at pH 8 at a total dodecylamine
concentration of 1.6x10^ kmol/m^ (see to Figure 44). Below pH 8, the
amine monomers and dimers are observed to become the dominant species
which can adsorb on the negatively charged apatite surface through
electrostatic attraction in addition to the adsorption due to specific
interaction (Soto and Iwasaki, 1985). The reasons for the smaller peak
at pH 6 in the flotation recovery of apatite, however, are not clear. A
gradual decrease in apatite flotation below pH 6 can be due to the onset
of the electrostatic repulsion between the collector cation and the
positively charged apatite surface.
Polomite-Dodecylamine System
Flotation recovery of dolomite using dodecylamine as the collector
as a function of pH (refer to Figure 12) appears to follow its zeta
potential behavior (Figure 10), suggesting that the adsorption of
dodecylamine on dolomite is mainly governed by the electrostatic
attraction. Considering that both the substrates and the collector are
positively charged below the IEP of dolomite, no flotation would be


82
Adsorption Studies
In order to investigate the mechanism of observed flotation
selectivity, adsorption of sodium oleate on apatite and dolomite was
studied. Adsorption experiments were conducted under conditions
identical to the flotation tests so that a meaningful correlation
between the two could be established.
Single Minerals Adsorption Tests
Apatite-oleate system
The amount of oleate adsorbed on apatite as a function of
conditioning pH is plotted in Figure 29 at three levels of sodium oleate
concentration. Two peaks, a larger peak at pH 8 and a smaller one
around pH 5, are observed. It is seen that the amount of oleate
adsorbed on apatite increased with an increase in the amount of sodium
oleate added.
Adsorption data on apatite as a function of sodium oleate concent
ration at pH 4 and pH 10 was also obtained. Results presented in
Figure 30 indicated that the amount of oleate adsorbed at pH 10 is about
five-times higher than that at pH 4, i.e., at a sodium oleate
concentration of 2.0xl0-4 kmol/m3). At a lower sodium oleate
concentration, however, this ratio appears to be different.
Effect of NaCl on sodium oleate adsorption on apatite. Selective
flotation of dolomite from apatite was found to be enhanced in the
presence of sodium chloride at pH 4 (see Figure 19 and 20). Adsorption
experiments were therefore conducted as a function of sodium oleate
concentration in order to determine the effect of NaCl (2.0xl0-2


AMOUNT FLOATED, WT%
Figure 11. Flotation of apatite as a function of pH.


CHAPTER IV
RESULTS
Characteristics of Minerals
Chemical Analysis
Chemical analyses of the apatite and dolomite samples were
conducted using a Perkin Elmer Plasma II inductively coupled plasma
(ICP) emission spectrometer. It is clear from the data presented in
Table 2, that the apatite sample is essentially free of dolomite and
vice versa. The major impurity occurring in these samples is silica,
which is reported as acid insoluble.
Surface Area and Porosity
The surface area and pore size distribution of the minerals were
determined using nitrogen gas as the adsorbate with a Quantachrome
Autosorb-6 unit. Surface area measurements of apatite and dolomite
presented in Table 2 indicated that these samples are highly porous.
Pore size distribution of apatite and dolomite samples revealed that as
much as 95% of their surface area is contributed by pores less than 400
X in diameter. Average pore radius for both apatite and dolomite was
determined to be 82 X. SEM micrographs of 65x100 mesh apatite and
dolomite samples presented in Figures 5 and 6, respectively, further
confirm the high surface porosity. It is observed from these
micrographs that the pores in the apatite sample extend to the surface.
36


122
presence of NaCI as compared to distilled water alone. A similar trend
was also observed in the presence of KC1 (Figure 24), even though KC1
appeared to render the surface less negative at pH 5 and below as
compared to NaCI.
In the presence of NaF, unlike the trends observed with NaCI and
KC1, dolomite indicated a surface charge reversal below its IEP (Figure
28). Apatite also exhibited a negative zeta potential value in the
presence of NaF (Figure 25), even more than that observed with NaCI and
KC1. Both apatite and dolomite were either depressed or floated
together with sodium oleate in the presence of NaF as illustrated in
Figure 22.
It is concluded that not all of the salts mentioned above act as
indifferent electrolytes for apatite. However, they appear to be
indifferent for dolomite with the exception of NaF which also depressed
the flotation of dolomite. Since the best selectivity was observed only
when sodium chloride was added to the system, the mechanism of the
surface charge reversal for apatite, at pH 4, will be discussed only in
the presence of this salt.
Role of NaCI in the reversal of surface charge of apatite
The surface charge of a substrate, in the absence of any other
added salt, is determined by the concentration of potential determining
ions (PDI) in solution. For apatite the PDI's are its lattice ions such
as Ca++ and P0| or their reaction products in water including H+ and
OH". The mechanism by which PDI determine the surface charge of


TABLE 10
Determination of Substitution of Sodium for Calcium in the Apatite Structure
Sodium Chloride
Concentration
(kmol/m3)
Treatment
After
Conditioning
Sodium
Depletion
(kmol/m3)
Calciurn
Dissolution
(kmol/m3)
Apatite Unit Cell
Cell "a" Dimensions
(A)
None
None
None
8.9xl0-4
9.347
2.0xl0"2
None
4.7xl0-3
2.8xl0"3
9.340
2.0xl0'2
Rinsed
2.4xl0"3
2. 7xl0~3
9.340


57
TABLE 5
Effect of NaCl on the Single Mineral
Flotation of Apatite and Dolomite with
Dodecylamine Hydrochloride as the Collector
Mineral
Amount Floated,
Weight %
Without Salt
Wi th
Sal t
Apatite
50.0, 48.0
84.5,
90.8
Dolomite
17.0, 20.0
15.7,
20.2
Collector Cone., 1.6xl04 kmol/m^
NaCl Cone., 5.0x10"! kmol/m^
Feed: 1 gram, 65x100 mesh size fraction
Flotation pH: 6.7


150
dolomite which are known to depress apatite flotation, as well as to the
depletion of oleate ions by precipitation as calcium and magnesium
oleate.
Additional mixed mineral experiments were conducted in the
presence of sodium chloride using either anionic or cationic collector
to minimize the extent of the electrostatic attraction between the
surfactant ions and the mineral surface by compressing the electrical
double layer, in order to achieve the collector adsorption on these
minerals predominantly by specific interaction forces. Consequently,
the desired selective separation of dolomite from apatite or vice versa
was achieved depending on the type of the collector used.
Upon optimization of the relevant process parameters such as pH,
collector and salt concentration, it was determined that more than 95%
of the dolomite can be removed from a 50:50 apatite-dolomite mixture at
a BPL recovery of 95% or higher using sodium oleate as the collector.
Other salts such as KC1 and NaF were also evaluated. The best
selectivity, however, was obtained in the presence of sodium chloride,
with no selectivity when NaF was added to the system.
Electrokinetic, adsorption, FT-IR and contact angle studies were
conducted to elucidate the mechanism for the observed selective
flotation of dolomite from apatite with sodium oleate.
It was demonstrated by the zeta potential measurements that, in the
presence of sodium chloride, the surface charge of apatite reverses
below its IEP (pH 5.4). It was also established that the surface charge
reversal for apatite results from increased Ca++ dissolution which is
substituted by Na+ in the apatite structure. This was based on the


SODIUM OLEATE ADSORBED, imol/g
oo
CONDITIONING pH
Figure 32. Adsorption of oleate on dolomite as a function of conditioning pH.


116
Flotation of Apatite and Dolomite Using
Sodium Oleate as the Collector
Results of flotation tests conducted as a function of pH using
4.0xlCT5 kmol/m3 sodium oleate as the collector (see Figure 15)
predicted selectivity under the following conditions.
Flotation of apatite from dolomite between pH 7 and pH 10;
Flotation of dolomite from apatite in the pH range of 5 to 6.
Apatite recovery in the mixed mineral system, under alkaline pH
conditions, decreased significantly as compared to that of the single
mineral tests, while under acidic pH conditions (pH 5 to 6) it increased
(refer to Table 4). Similar results were obtained by Moudgil and
Chanchani (1985a) in the apatite-dolomite system. It was determined by
these investigators that selectivity is mostly affected by changes in
the flotation response of apatite. They attributed the loss in the
selectivity to the presence of excess Ca++ and Mg++ ions dissolved from
dolomite as well as to the depletion of oleate by precipitation as
calcium and magnesium oleate. As mentioned above, there are more
dissolved species present in the mixture as compared to single mineral
apatite because of higher solubility of dolomite. It should be noted
that precipitate formation would reduce the effective collector
concentration in the system. The dissolved Ca++ and Mg++ ions are
expected to adversely affect apatite flotation as has been determined by
Moudgil and Chanchani (1985a).
Evaluation of the Results and Alternatives
Flotation separation of apatite and dolomite was predicted under
certain experimental conditions by the single mineral results. It was,


119
increase or decrease adsorption of the collector on the mineral surface
due to electrostatic interaction depending on the effect of the
electrolyte added. The electrostatic attraction between the mineral
surface and the ionic surfactant species can be altered by compressing
the electrical double layer, e.g., by adding an (indifferent)
electrolyte. The electrical double layer compression on apatite and
dolomite can lead to selective flotation of apatite with the collector
such as dodecylamine hydrochloride due to its more preferential
adsorption for apatite.
Sodium chloride was used to manipulate the surface charge of the
two minerals. Its effect on flotation of apatite and dolomite with
dodecylamine hydrochloride or sodium oleate as the collector are
discussed next.
Effect of NaCl on the Selective Flotation of
Apatite Using Dodecylamine as the Collector
Results of apatite and dolomite flotation tests (see Table 6)
indicated selective recovery of apatite with dodecylamine as the
collector in the presence of sodium chloride. Dolomite was observed to
be depressed in the presence of added salt.
Subsequently, the possibility of floating dolomite and depressing
apatite by utilizing anionic collector such as sodium oleate was
investigated. In practice, flotation of the minor mineral (i.e.,
dolomite) is preferred over that of the major mineral (i.e., apatite),
because it would generally require less collector.


30
30 minutes. Next, the solids were resuspended twice in distilled water
of the same pH value as the conditioning solution and centrifuged again
to remove the entrapped or free oleate solution from the solids. The
pure apatite and dolomite samples were also treated identically and
centrifuged the same way to remove water. The characterization of
adsorbed oleate species in the presence of sodium chloride was carried
out by aging the solids in 2xl0-^ kmol/m^ NaCl solution, followed by
conditioning in a 5x10^ kmol/m^ sodium oleate solution of the same NaCl
concentration used during aging. The solid samples were then dried at
50 C for 24 hours and stored in a vacuum desiccator until used.
Freshly ground, dried KC1 was used as a reference throughout these
experiments. The calcium and magnesium oleate precipitates were
obtained by mixing sodium oleate solution with those of calcium and
magnesium chloride, respectively. The reagentized pure mineral and the
precipitate samples were first mixed with 90% KC1 before introduction
into the diffuse reflectance cell.
The diffuse reflectance IR spectra were obtained on a B0MEM DA3.10
Fourier Transform Infrared Spectrometer equipped with a 25 cm path
length Michelson interferometer fitted to a KBr beam splitter. The
optical interferometer is connected to a high speed vector processor,
which performs the Fourier Transform and numerical fitting of the
collected interferograms. The spectra were recorded with a spectral
resolution of 0.5 cm/sec. Typically, 64 to 256 scans were obtained
under vacuum conditions to minimize interference from atmospheric
moisture and carbon dioxide.


114
affected when conditioned simultaneously in the mixture. As a
consequence, the surface charge of apatite can be less negative or even
positive in the alkaline pH range, whereas that of dolomite would become
more negative. Hence, adsorption of dodecylamine on apatite can be
expected to decrease due to electrostatic repulsion between the surface
and the cationic collector, thus leading to its reduced flotation as
compared to that in the single mineral flotation tests.
Surface coating
Soto and Iwasaki (1986) and Dufour et al. (1980) have reported
that dolomite is softer than apatite. Consequently, dolomite slimes
could be generated during the conditioning stage which could affect
flotation of apatite. Moudgil and Chanchani (1985a) tested this
hypothesis by floating apatite in the supernatant of dolomite with and
without fines present and observed no change, indicating that dolomite
fines do not affect apatite flotation.
Surfactant depletion by precipitation
Soto and Iwasaki (1985) determined the solubilities of the salts
formed upon reaction of dodecylamine with phosphate and carbonate ions
to be 1.5xl0-3 and 5.0xl0-3, respectively. In the alkaline pH range,
where the solubility of these minerals is low, complexation and
precipitation of dodecylamine ions with anions such as P0$ and C0^" is
not expected to occur to any significant extent and, therefore, could
not explain the loss in the selectivity as observed.


54
TABLE 4
Flotation Results of 50:50 Apatite-Dolomite
Mixture Using Sodium Oleate as the Collector
Flotation
Chemical
Analysis
Apatite
Dolomite
pH
of Concentrate, %
Recovery
Reject
P25
MgO
(Weight %)
(Weight %)
5.20.2
32.82, 33.36
1.22, 1.28
67.8, 66.1
96.0, 95.3
8.10.2
31.40, 30.64
1.32, 1.36
69.2, 68.6
92.4, 91.6
Collector Cone., 4.0xl05 kmol/m3
Flotation Feed: 1 gram, 65x100 mesh
size fraction, 18.0% P2O5, 9.5% MgO


45
function of pH. These tests were followed by mixed mineral flotation
tests under selected experimental conditions.
Apatite-dodecylamine system. Results of apatite flotation as a
function of pH at two levels of dodecylamine concentration are presented
in Figure 11. It is observed that at a dodecylamine hydrochloride
concentration of 1.0x10^ kmol/m^, apatite recovery is 100% between pH 4
and 10, and decreases precipitously beyond pH 10. However, at a
dodecylamine concentration of 1.6x10^ kmol/m^, apatite recovery
exhibits two maxima at pH 6 and 9.8, the flotation of apatite at pH 6
being about 50% of that at pH 9.8.
Polomite-dodecylamine system. Flotation response of dolomite as a
function of pH at a dodecylamine hydrochloride concentration of 1.0x10^
and 1.6xl0-4 kmol/m3 is illustrated in Figure 12. The amount floated is
observed to increase sharply at a dodecylamine concentration of 1.0x10^
kmol/m^ above the first IEP at pH 5.3 and to reach a 100% level at pH
5.8. However, flotation recovery starts decreasing above pH 9,
descending to 25% at pH 11, the value at which dolomite exhibits the
second IEP (refer to Figure 10).
The amount of apatite and dolomite floated as a function of pH at
a dodecylamine hydrochloride concentration of 1.0x10^ kmol/m^ is
compared in Figure 13. It is indicated from these results that apatite
can possibly be recovered selectively from its mixture with dolomite at
pH less than 4.5. At a dodecylamine concentration of 1.6x10^ kmol/m^,
another region of selectivity occurs at pH 9.8, as illustrated in
Figure 14.


102
Oleate Species Adsorbed in the Presence of NaCl
As stated earlier, selective flotation of dolomite from apatite was
achieved at pH 4. The adsorbed oleate species at this pH value,
therefore, were also characterized in the presence of sodium chloride.
Apatite-NaCl-oleate system
The diffuse reflectance IR spectra obtained on apatite at pH 4 are
presented in Figure 42. To study the effect of NaCl, both pure and
treated apatite samples were first conditioned in 2.0x10^ kmol/m^ NaCl
solution. The treated apatite sample was then conditioned in 5.0x10^
kmol/m^ sodium oleate solution also prepared in 2.0x10^ kmol/m^ NaCl.
The difference spectrum in Figure 42, indicated the presence of only
oleic acid on apatite surface. It is to be noted that both calcium
oleate and oleic acid were observed on apatite in the absence of sodium
chloride at pH 4.
Polomite-NaCl-oleate system
The IR spectra of untreated and treated dolomite in the presence of
2.0x10"^ kmol/m^ NaCl solution at pH 4 are presented in Figure 43. The
bands in the difference IR spectrum of Figure 43, at 1717 and 1582
wavenumbers are the same as those obtained without salt addition,
indicating that the nature of the adsorbed species on dolomite (oleic
acid and magnesium oleate) remains unchanged.


131
In the acidic pH range (below pH 5) oleate adsorption on dolomite
is higher than that on apatite under identical experimental conditions.
It is to be noted that dolomite flotation is also higher than apatite
under similar pH conditions. The higher adsorption and flotation
response of dolomite cannot be explained on the basis of electrostatic
interactions alone, because both minerals have the same magnitude of
zeta potential at pH 5 and below. Possible reasons for the observed
behavior could be the faster adsorption kinetics of oleate on dolomite
as mentioned earlier. This would decrease the collector available for
adsorption on apatite and consequently results in its reduced flotation.
Effect of NaCl Addition on Adsorption
The amount of oleate adsorbed on apatite as a function of sodium
oleate concentration at pH 4 (refer to Figure 31) in the presence of
NaCl was observed to decrease at all concentrations as compared to that
in distilled water. On the other hand, adsorption on dolomite remained
unaffected in the presence of NaCl (see Figure 34). A summary of the
amount adsorbed, flotation recovery and zeta potential values determined
for apatite and dolomite is presented in Table 13. The amount adsorbed
on apatite is seen to decrease by about 40% in the presence of NaCl.
The decrease in adsorption on apatite as described earlier is attributed
to the surface charge reversal in addition to depletion of calcium sites
by substitution of sodium. It is to be noted that ionic oleate species
were expected to adsorb on the calcium sites on the apatite surface.
The negative zeta potential at pH 4 in the presence of NaCl would result


143
M = dipole moment of the molecule
0 = orientation angle between the ion and dipole molecule
(0=0 and Cos0 = 1 when the ion and the dipole molecule are
oriented)
^Av = Avogadro's number (6.022x10^ mol-*-)
I = permittivity of the free space (8.854x10-*-^ C^J"-*-m-*-)
£ = dielectric constant of the medium (e = 78 for water)
At contact r = R/2 + r^on where R = dipole length (point
charge is assumed for the dipole molecule)
The dipole moment of oleate, calculated using the bond angle (124.3
degrees), and the dipole moments of C=0 (0.74D) and C-0 (2.5D), was
found to be 1.51D (=1.51 x 3.336x10-^ Cm). The bond lengths for C=0
(1.23 X) and C-0 (1.22 X) were used to calculate the dipole length (R)
which was determined to be 0.57 K. The interaction energy values
between the dipole molecule (oleate) and the ions (calcium and
magnesium) presented in Table 15, indicate that the interaction energy
is long range and that it becomes greater than kT before the contact is
made between the ion and the dipole. It is also seen from this data
that the interaction energy between the dipole and the magnesium ion is
greater than that of calcium and oleate for the same distance of
separation between the outer shells. Upon contact, the interaction
energy between the magnesium-oleate pair is 56% greater than that of
calcium-oleate. Thus, the preferential formation of the magnesium-
oleate specie on dolomite can be attributed also to the difference in
the charge densities of the respective species.


CHAPTER V
DISCUSSION
The adsorption characteristics of a surfactant and the resultant
flotation behavior of the minerals depends on the nature and the
solution properties of the collector in addition to parameters such as
surface composition and surface charge of the substrate. The chemical
and physical properties of the collector are determined by the
hydrocarbon chain and the ionic head. The ionic head determines
whether the collectors are strong electrolytes, that ionize completely
in solution, or weak electrolytes, which ionize only slightly and
hydrolyze in solution to form various species including the neutral
molecules. A discussion of the solution chemistry behavior of sodium
oleate and dodecylamine hydrochloride is presented below.
Solution Properties of Dodecylamine Hydrochloride and Sodium Oleate
The important role of the solution chemistry of collectors such as
dodecylamine and oleic acid in governing the flotation has been well
documented by Ananthapadmanabhan (1980), Somasundaran and
Ananthapadmanabhan (1979a), and Ananthapadmanabhan et al. (1979).
Using the thermodynamic data of Ananthapadmanabhan (1980), species
distribution diagrams for dodecylamine and oleic acid were generated in
the relevant concentration ranges.
105


SODIUM OLEATE ADSORBED, jumol/g
Figure 29. Oleate adsorption on apatite as a function of conditioning pH.


118
Based on the results obtained in the present study and those
reported in the literature, the following conclusions can be reached
about the adsorption mechanism:
a) Flotation of dolomite as a function of pH with either dodecyl-
amine or sodium oleate as the collector mostly appeared to fol
low its electrokinetic behavior. This indicated that the sur
factant adsorption on dolomite is governed by the electrical
double layer forces, i.e., electrostatic attraction for the most
part.
b) The fact that dolomite flotation with sodium oleate was higher
than that of apatite below pH 5, despite the fact that both
minerals have the same magnitude of zeta potential, indicates
that the collector adsorption is more specific for dolomite.
Higher flotation of dolomite in the acidic pH range was also
reported by Moudgil and Chanchani (1985a), Johnston and Leja
(1978) and Ratobylskaya et al. (1975). It is possible that more
oleic acid molecules adsorb on the dolomite surface along with
charged oleate species due to the higher density of surface
hydroxyl groups on the surface of this mineral as has been
pointed out by Shah (1986).
c) Apatite flotation as a function of pH with either of the
collectors did not seem to correlate completely with its
electrokinetic behavior.
It appears from the above discussion that the selectivity in the
apatite-dolomite system could be improved by manipulation of the
electrical double layer forces. Such a change can be expected to


CHAPTER II
BACKGROUND
Characteristics of Salt-Type Minerals
Salt-type minerals such as apatite and dolomite are characterized
by solubilities higher than those of most oxides and silicates, but
lower than simple salt minerals such as halite and sylvite. Flotation
of such minerals from the associated gangue and from each other is of
major practical importance. For example, apatites constitute the
largest tonnage of any raw material beneficiated by froth flotation
techniques in the United States as well as several other countries
(Hanna and Somasundaran, 1976).
Separation of the salt-type minerals from oxide and silicate
minerals has been achieved and used commercially. However, separation
of these minerals from each other is complex and the problems involved
remain unresolved in many cases. It has been reported that the
differences between flotation characteristics of various salt-type
minerals may not be greater than those between samples of a single
mineral from different deposits (Sorensen, 1973). The similarities in
the flotation response of these minerals is generally attributed to
their comparable surface chemical behavior. In addition, interaction of
dissolved mineral species with collector molecules is considered to
contribute to the poor selectivity. Consequently, it has been suggested
that the use of inorganic or organic modifying agents might result in
6


TABLE 12
Dissolution of Calcium and Magnesium from Dolomite at pH 4.0
With and Without NaCl Addition
Time
(Minutes)
Sodium Chloride
Cone.
(kmol/ni3 x 102)
Amount Dissolved, kmol/m3
Concentration Ratio
[Ca++]
[Mg++]
[Ca++]/[Mg++]
05
None
1. 41xl0-3
1.21xl0"3
1.165, 1.133
1.45xl0"3
1.28xl0-3
30
None
1.87xl0'3
1.68xl0~3
1.113, 1.109
1.83xl0-3
1.65xl0'3
05
2.0
4.60xl0'4
4.00xl0"4
1.150, 1.200
5.05xl0"4
4.21xl0~4
30
2.0
8.95xl0-4
7.88xl0-4
1.136, 1.156
7.95xl0-4
6.88xl0-4


KUBELKA-MUNK UNITS
400 1400 2400 3400
WAVENUMBER,cm1
Figure 50. Difference IR spectra of dolomite-oleate system at pH 10, and at pH 4
with and without sodium chloride addition.


LOG (ACTIVITY OF THE SPECIES)
-2
SOLUTION pH
o
oo
Figure 45. Oleate species distribution as a function of pH. Total oleate
concentration, 4.0 x 10^ M.


165
Rao, D. V., Narayanan, M. K., Nayak, U. B., Ananthapadmanabhan, K. P.
and Somasundaran, P., 1985, "Flotation of Calcareous Mussorie
Phosphate Ore," Inter. J. of Mineral Processing, Vol. 14,
pp. 57-66.
Ratobylskaya, L. D., Eigeles, M. A., Kuznetsov, V. P., Volova, M. L.,
Sokolov, Yu. F., Lyubimova, E. I. and Grebnev, A. N., 1975,
"Development and Industrial Introduction of New Concentration
Process for Phosphorites of Complex Mineral Composition,"
Proceedings, 11th IMPC, Seminar on Beneficiation of Lean Phosphate
with Carbonate Gangue, Cagliari, Italy, pp. 167-186.
Reeder, R. J., 1983, "Crystal Chemistry of the Rhombohedral Carbonates"
In: Reviews in Mineralogy, R. J. Reeder, Ed., Vol. 11, pp. 1-47.
Reeder, R. J., and Sheppard, C. E., 1984, "Variation of Lattice
Parameters in Some Sedimentary Dolomites," Am. Mineralogy,
Vol 69, pp. 520-527.
Reeder, R. J. and Wenk, H. R., 1983, "Structure Refinements of Some
Thermally Disordered Dolomites," Amer. Mineralogy, Vol. 68,
pp. 769-776.
Riggs, S. R., 1979, "Petrology of the Tertiary Phosphorite System of
Florida," Economic Geology, Vol. 74, pp. 195-220.
Rule, A. R. and Dallenbach, C. B., 1985, "Beneficiation of Complex
Phosphate Ores Containing Carbonate and Silica Gangue,"
Proceedings, XVth IMPC, Cannes, France, Vol. 3, pp. 380-389.
Rule, A. R., Gruzensky, W. G. and Stickney, W. A., 1970, "Removal of
Magnesium Impurities from Phosphate Rock Concentrate," U.S. Dept,
of the Interior, U.S. Bureau of Mines, RI 7362.
Saleeb, F. Z., and de Bruyn, P. L., 1972, "Surface Properties of
Alkaline Earth Apatites," Electrochem. Chem. and Interfacial
Chemistry, Vol. 37, pp. 99-118.
Shah, B. D., 1986, "Selectivity in Mixed Mineral Flocculation: Apatite-
Dolomite System." M.S. Thesis, University of Florida, Gainesville.
Smani, M. S., Blazy, P. and Cases, J. M., 1975, "Beneficiation of
Sedimentary Moroccan Phosphate Ores, Part 1-4," Trans., SME/AIME,
Vol 258, pp. 168-182.
Smith, J. P. and Lehr, J. R., 1966, "An X-Ray Investigation of
Carbonate Apatites," Journal of Agriculture and Food Chem.,
Vol. 14, pp. 342-349.
Snow, R. E., 1979, "Beneficiation of Phosohate Ores," U.S. Patent
No. 4,144,969.


I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
Brij M. Moudgil, Chairman
Professor of Materials Science
and Engineering
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation f/jr the degree of
Doctor of Philosophy.
E. Dow Whitney
Professor of Materials Science
and Engineering
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
i.o. s
Dinesh 0. Shah
Professor of Chemical
Engineering
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
/L
I f (AL
David E. Clark
Professor of Materials Science
and Engineering
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly .presentation and is fully
adequate, in scope and quality, as a dissert^tioh for the degree of
Doctor of Philosophy.
Frank N. Blanchard
Professor of Geology


35
The Experimental Approach
The experimental approach involved the following steps:
(1) Flotation tests to identify the conditions for selectivity.
a) Study of the flotation response of apatite and dolomite
individually to determine the reagent (dodecylamine and oleate)
concentration and pH values where differences in the flotation
behavior are maximum.
b) Flotation of the mixed minerals to evaluate the selectivity
predicted by the single mineral tests, and investigation of the
reasons if observed selectivity is not achieved.
c) To select appropriate experimental conditions based on (b) to
achieve the desired separation.
(2) Elucidate the mechanisms by conducting the following studies:
a) Electrokinetic measurements as a function of pH with and without
chemical additives to establish the surface charge.
b) Adsorption tests under conditions selected on the basis of
flotation experiments.
c) A study of the nature of the surfactant species by FT-IR under
the conditions used for adsorption tests.
d) Evaluation of the relative hydrophobicity of the mineral
surfaces by contact angle measurements.


REFERENCES
Amankonah, J. 0., Somasundaran, P. and Ananthapadmanabhan, K. P.,
1986, "Effect of Dissolved Mineral Species on the Electrokinetic
Behavior of Calcite and Apatite," Colloids and Surfaces, Vol. 15,
pp. 335-353.
Ananthapadmanabhan, K. P., 1980, "Associative Interactions in
Surfactant Solution and Their Role in Flotation," Doctor of Eng.
Science, Columbia University, New York.
Ananthapadmanabhan, K. P. and Somasundaran, P., 1985, "Surface
Precipitation of Inorganics and Surfactants and its Role in
Adsorption and Flotation," Colloids and Surfaces, Vol. 13,
pp. 151-167.
Ananthapadmanabhan, K. P., Somasundaran, P. and Healy, T. W., 1979,
"Chemistry of Oleate and Amine Solutions in Relation to
Flotation," Trans., AIME, Vol. 266, pp. 2003-2009.
Atalay, U., Dogan, M. Z., Ozbayoglu, G. and Duman, H., 1985,
"Beneficiation of Low Grade Tasit Phosphate Ore from Turkey,"
Proceedings, World Congress on Non-Metallic Minerals, Belgrade,
Yugoslavia, pp. 389-396.
Atkins, P. W., 1982, "Physical Chemistry," 2nd Ed., W. H. Freeman and
Company, San Francisco, California.
Balajee, S. R. and Iwasaki, I., 1969, "Adsorption Mechanism of Starches
in Flotation and Flocculation of Iron Ores," Trans., SME/AIME,
Vol. 244.
Baumann, A. N. and Snow, R. E., 1980, "Processing Techniques for
Separating MgO Impurities from Phosphate Products," Proceedings,
2nd Int. Congr. Phosphorus Compounds, Boston, pp. 269-280.
Becker, P., 1983, "Phosphates and Phosphoric Acid," Marcel Dekker,
Inc., New York.
"Beneficiation of High Carbonate Phosphate Ores," 1983, In: New
Developments in Fertilizer Technology, TVA Publication, pp.48-51,
159


OLEATE ADSORBED, Amol/gx100
100
80
60
40
20
O
Figure 47. Correlation between oleate adsorption and flotation for apatite as a
function of pH.
AMOUNT FLOATED, WT%


141
Selective leaching of calcium ions from dolomite. During
dissolution of the minerals, different constituents can dissolve at
varying rates, resulting in a change in both the surface chemical
composition of the solids and the ionic concentration in the solution.
The possibility of differential calcium or magnesium dissolution from
dolomite was examined by conducting dissolution kinetic tests at pH 4
using the 65x100 mesh fraction of the mineral. The amount of calcium
and magnesium ions leached into the solution were determined as a
function of time. The results obtained (shown in Table 12) indicated a
higher rate (greater than 11%) of calcium dissolution relative to that
of magnesium from dolomite, with and without NaCl addition. As a
consequence of selective leaching, it is possible that a relatively
higher concentration of magnesium sites would result on the dolomite
surface. The probability of magnesium oleate formation, therefore, can
be expected to be higher, although calcium oleate formation cannot be
ruled out.
Electronegativity effect. Electronegativity of an element is the
attraction of the nucleus of its atom for the electrons in the outer
shell. Using the electronegativity values of the elements involved in
bonding, the type and strength of the bond can be determined. If the
difference in the electronegativity values between two elements is large
(e.g., 2 or more) the bonds in the compound will be largely ionic in
nature. When the difference is small, a covalent bond would exist,
which is stronger in an aqueous medium. Upon adsorption of oleate
anions on dolomite, a calcium and/or a magnesium oleate bond is
expected to form. The polarity of the bond formation, i.e., the %


Single Minerals Adsorption Tests 82
Apatite-oleate system 82
Dolomite-oleate system 85
Mixed Mineral Adsorption Studies 90
Characterization of the Adsorbed Oleate Species
by FT-IR Spectroscopy 90
IR Spectra of Pure Oleate Species 93
Nature of the Adsorbed Species at pH 10 96
Apatite-oleate system 96
Dolomite-oleate system 96
Nature of the Adsorbed Species at pH 4 99
Apatite-oleate system 99
Dolomite-oleate system 99
Oleate Species Adsorbed in the Presence of NaCl .... 102
Apatite-NaCl-oleate system 102
Dolomite-NaCl-oleate system 102
V DISCUSSION 105
Solution Properties of Dodecylamine and Oleate 105
Apatite-Dolomite Flotation Using Dodecylamine as
the Collector 109
Apatite-Dodecylamine System 109
Dolomite-Dodecylamine System 110
Flotation of Apatite and Dolomite Mixture with
Dodecylamine Ill
Changes in the surface charge and surface
chemical composition 112
Surface coating 114
Surfactant depletion by precipitation 114
Flotation of Apatite and Dolomite Using Sodium Oleate
as the Collector 116
Evaluation of the Results and Alternatives 116
Effect of NaCl on the Selective Flotation of
Apatite Using Dodecylamine as the Collector 119
Effect of NaCl on the Separation of Dolomite From
Apatite Using Sodium Oleate as the Collector 120
Mechanism of Selective Flotation of Dolomite from Apatite 121
Effect of NaCl on the Zeta Potential of Apatite
and Dolomite 121
Role of NaCl in the reversal of surface charge
of apatite 122
Dolomite structure 126
Adsorption of Oleate on Apatite and Dolomite 128
Effect of Conditioning pH 128
Effect of NaCl Addition on Adsorption 131
Adsorption in the Mixed Mineral System 133
Effect of Salt on Adsorption in the Mixed Minerals
System 133
Nature of the Adsorbing Surfactant Species 134
Apatite-01eate System 134
vi 1


56
Flotation tests in the presence of NaCI using dodecylamine as the
col 1ector
Single minerals flotation tests. Experiments conducted in the
natural pH range (pH 6.7) as shown in Table 5, indicated 85-90%
flotation recovery of apatite at a dodecylamine concentration of
1.6xl04 kmol/m^ and a NaCI concentration of 5.0xl0-*- kmol/m^. Apatite
recovery was observed to be only 50% under identical conditions without
NaCI addition (refer to Figure 11). In contrast, dolomite recovery
under similar experimental conditions remained at the 15-20% level. It
should be noted that the change expected in the flotation behavior of
dolomite in the presence of sodium chloride was not observed, but more
selective flotation of apatite was realized.
Mixed mineral flotation tests. Flotation of 88:12 apatite-
dolomite mixtures was conducted to determine the selectivity predicted
by the single mineral experiments in the presence of NaCI at natural pH
value. Results presented in Table 6 demonstrated that at a
dodecylamine concentration of 1.6xl0-4 kmol/m3 apatite can be
selectively recovered from the mixture leaving dolomite in the sink
fraction.
It should be noted that with dodecylamine hydrochloride, apatite,
the major mineral, is floated leaving dolomite in the sink. In
practice, however, flotation of the minor constituent is desired.
Further test work therefore, was conducted using sodium oleate as the
collector, which under slightly acidic pH conditions is known to yield
flotation of dolomite leaving apatite in the sink fraction.


ACKNOWLEDGMENTS
I wish to express my sincere gratitude and respect to Professor B.
M. Moudgil, my major advisor, for his invaluable help, guidance and
encouragement during the course of this research.
I am very grateful to Professor F. N. Blanchard for his guidance,
valuable comments, and to Professors D. 0. Shah, E. D. Whitney, D. E.
Clark, H. A. Laitinen and J. H. Simmons for very helpful discussions and
comments.
I am most grateful to Professor C. T. Johnston and Mrs. L. D.
Applewhite for discussions and help in FT-IR study.
With due gratitude I wish to acknowledge the encouragement,
helpful comments and constructive criticism of Drs. T. V. Vasudevan, H.
Soto and Wen-Keng Shih.
I am thankful to M. May, A. Zutshi, J. Ransdell, Y. C. Cheng and
J. Rogers for their help at various stages, and to Mrs. G. Keim for her
help in preparing the manuscript.
Special love and appreciation is due to my wife Sevgi for her
support, help and encouragement and to my two lovely daughters, El if
and Ebru, for their patience throughout the course of this study.
Finally, I wish to thank Agrico Chemical Company and International
Minerals and Chemicals Corporation for supplying the mineral samples
used in this study and to acknowledge Florida Institute of Phosphate
Research (Grants #82-02-023 and #85-02-067) for providing financial
i i i


134
to be significantly different from that of the single mineral system in
the presence of sodium chloride. Adsorption on apatite, unlike that on
dolomite, however, decreased by about 20% in the case of mixtures as
compared to the single mineral tests. Flotation of apatite was also
observed to be depressed more in the mixed mineral system. The same
observation was also made by Chanchani (1984), who reported that the
rate of oleate adsorption on dolomite is higher during the first 5
minutes of conditioning. Thus, the amount available for adsorption on
apatite decreased because of its slower adsorption kinetics.
In summary, it was seen that oleate adsorption on apatite and
dolomite varies as a function of pH. Oleate adsorption on dolomite in
the acidic pH range was determined to be higher than that of apatite.
It was also established that adsorption of oleate on dolomite remains
unchanged in the presence of sodium chloride whereas that on dolomite
decreases by about 40%. Selective flotation of dolomite from apatite
can be partially ascribed to the decreased oleate adsorption on apatite.
However, as mentioned earlier, the nature of the surfactant species
adsorbing on the surface is also important, since it could impart
different degrees of hydrophobicity. Identification of the nature of
adsorbed species using FT-IR spectroscopy is discussed next.
Nature of the Adsorbing Surfactant Species
Apatite-01eate System
The difference IR spectra of treated and untreated apatite at pH 10
and at pH 4 are depicted in Figure 49. It can be seen that calcium
oleate forms on the apatite surface at pH 10. The adsorbed species in


kubelka-munk units
1563
Figure 38.
Diffuse reflectance IR spectra of treated and untreated apatite
at pH 10.0.


pH
Figure 26.
Zeta potential of dolomite as a function of pH with and without sodium chloride.


BIOGRAPHICAL SKETCH
Dursun E. Ince was born in Tunceli, Turkey, on December 23, 1951.
Upon graduating from high school in Izmir, in June 1970, he entered the
Technical University of Istanbul the same year, and received a B.S. in
mining and mineral engineering in June 1974. Following one year of work
in Turkey, he moved to the United States for graduate study and went to
the University of Wisconsin-Madison, where he completed a M.S. degree in
mineral and metallurgical engineering in May 1978. He was associated
with the Mineral Engineering Department of the Pennsylvania State
University before joining Union Carbide Corporation in Niagara Falls,
N.Y., in December 1979, where he worked as a research engineer for three
years. In January 1984, he entered the Ph.D. program in the materials
science and engineering at the University of Florida.
168


106
The chemical equilibria used to calculate the distribution of the
species for dodecylamine hydrochloride were as follows:
RNH$ t RNH2 + H+ pKa = 10.63
2RNH3 t (RNH3)^+ pKd = -2.08
RNH2 + RNH3 t (RNH2.RNH3)+ pKad = -3.12
RNH2(i) t RNH2(ap) PKsoi = 4.69
The species versus pH diagram for a total dodecylamine
hydrochloride concentration of 1.6x10"^ kmol/m^ is illustrated in Figure
44. The maximum in amine-aminium complex formation at this
concentration is observed at pH 10. The pH of maximum flotation for
minerals such as quartz has been found to correspond to the pH of
formation of maximum amine-aminium complex (Ananthapadmanabhan, 1980).
It has been shown by Pugh (1986) and Ananthapadmanabhan (1980) that the
surface activities of the association complexes such as amine-aminium
are higher and, even if present in small amounts, they can make
significant contributions to the flotation process.
The species distribution diagram for a total oleate concentration
of 4.0x10^ kmol/m^ using the following chemical equilibria
(Ananthapadmanabhan, 1980) is presented in Figure 45.
RH(1) t RH(aq)
RH(aq) R~+H+
2R" t r£"
RH + R t RoH"
P^sol 7-60
pKa = 4.95
pKd = -3.70
PKad = -7.10


Figure 4. Apparatus for mixed mineral adsorption studies.


4
showed that the "a" unit-cell dimension for carbonate-fluorapatite can
also be used to estimate the Mg substitution for Ca. The average "a"
unit-cell dimensions measured for 10 samples corresponded to 0.55% (by
weight) MgO in the apatite, in a good agreement with the average of
0.57% MgO estimated by Blanchard and co-workers (1986).
Physical methods of separation obviously cannot remove the
substituted magnesium. However, the second phase dolomite and discrete
particles upon liberation can be separated from apatite by physical and
physico-chemical processes.
Separation of dolomite from apatite by selective flotation has been
the focal point of research in the last decade, because physical
methods such as gravity and magnetic separation have not shown much
promise in beneficiating dolomitic phosphate ores. Flotation studies by
the phosphate industry (Lawver et al., 1978: Snow, 1979; Dufour et al.,
1980; Lawver et al., 1980; Lawver et al., 1982b), the U.S Bureau of
Mines (Llewellyn et al., 1982 and 1984) and the Tennessee Valley
Authority (Lehr and Hsieh, 1981) have resulted in the development of a
number of processes. It should be pointed out that these studies
evolved from engineering applications, and an understanding of the
fundamentals which govern the selectivity of the proposed processes were
not fully established. This is believed to be a serious limitation in
the optimization of these processes. Nevertheless, past efforts have
provided a direction for developing suitable separation techniques.
A systematic study involving an apatite/dolomite-anionic surfactant
system was conducted by Moudgil and Chanchani (1985a, 1985b and 1985c),
and Chanchani (1984) which has resulted in the development of a two-


80
60
40
20
0
-I I L.
-6
0
SODIUM OLEATE CONC., kmol/mv
O'!
rv>
17. Effect of sodium oleate concentration on apatite and dolomite (single
minerals) flotation with and without NaCl addition.


ZETA POTENTIAL, mV
20
10
Apatite in Water
Apatite in 2.0x102 kmol/m3
KCl Solution
Figure 24. Zeta potential of apatite as a function of pH with and without KC1 addition.


167
Whippo, R. and Murowchick, B. L., 1967, "The Crystal Chemistry of Some
Sedimentary Apatites," Trans., SME/AIME, Vol. 238, pp. 257-263.
Zoltai, T. and Stout, J. H., 1984, "Mineralogy: Concepts and
Principles," Burgess Publishing Company, Minneapolis, Minnesota.


0000
4860
3840
2940
2160
1 500
960
540
240
0
Dolomite
h
-P*
rv>
5 26 27 28 29 30 31 32 33 34 35
TWO-THETA
Figure 8. X-ray diffractogram of dolomite.


152
Oleate adsorption as a function of pH on single and mixed minerals
indicated a good correlation with their flotation behavior.
The amount of oleate adsorbed on apatite at pH 4 in the presence of
sodium chloride decreased by 40% as compared to that in distilled water.
This decrease was anticipated in terms of the reversal of surface charge
which was expected to adversely affect adsorption of anionic oleate
species due to electrostatic repulsion. Adsorption on dolomite under
similar conditions remained unaltered in distilled water and in the
presence of sodium chloride.
Oleate adsorption on apatite at pH 4 in the case of mixed minerals
indicated a 20% decrease relative to that of single mineral results.
This was attributed to the faster adsorption rate of oleate on dolomite
as compared to apatite, which subsequently decreased the effective
oleate concentration and led to the enhanced selective flotation of
dolomite from apatite.
In order to determine the nature of the adsorbed surfactant
species on the apatite and dolomite surfaces, FT-IR spectroscopic
studies were conducted. It was established that formation of calcium
oleate occurs on apatite at both pH 4 and pH 10. Adsorption of oleic
acid was observed at pH 4 only, which is consistent with the solution
chemistry behavior of oleic acid.
It was also established that calcium oleate does not form on the
apatite surface at pH 4 in the presence of sodium chloride. This has
been ascribed to the depletion of Ca++ sites on apatite surface as a
result of Na+ substitution. In addition, the surface charge reversal
also adversely affected adsorption of anionic oleate species.


29
using a Perkin Elmer Plasma II inductively coupled plasma (ICP) emission
spectrometer.
Solubility Product Determination
The solubility products of calcium and magnesium oleate at pH 10
were determined using the nephelometric method. Known concentrations of
calcium and magnesium solutions were prepared from 1000 ppm standards
and were mixed with the sodium oleate solution of desired concentration.
The mixtures were initially stirred for a period of one minute and then
allowed to stand for 10 minutes before making turbidity measurements
using a Hach Model 2100 turbidimeter. The instrument was calibrated by
measuring the turbidity of solutions containing known concentrations of
sodium oleate and calcium/magnesium ions. The solubility product (Ks)
was obtained using the concentrations of calcium/magnesium and oleate
ions, at which a sharp increase in turbidity was observed.
FT-IR Tests
Samples used for FT-IR analysis were prepared as follows: A one
gram, -325 mesh size fraction, sample was suspended in 100 ml of
distilled water in a 100 ml volumetric flask and the pH was adjusted to
the desired value using HNO3 or K0H. After 20 minutes of aging, the
supernatant was partly replaced by the surfactant solution such that a
concentration of 5.0x10"^ kmol/m^ sodium oleate was obtained. The
suspension, with the added surfactant, was then tumbled for one hour at
8 rpm. At the end of the conditioning period, the solids were separated
from the supernatant by centrifuging the sample at 734 G (2500 rpm) for


Ill
expected. Flotation of dolomite (10-15%) below the isoelectric point,
at a dodecylamine concentration of 1.6xl04 kmol/m3 therefore indicates
the presence of weak specific interaction in addition to the coulombic
attraction. The heat of reaction of dodecylamine with phosphate and
carbonate anions (-15.1 and -3.3 kJ/mol for apatite and dolomite,
respectively) as determined by Soto and Iwasaki (1985) supports this
hypothesis. This indicates that the collector adsorbs more
preferentially on apatite than dolomite when both of the minerals are
present. The flotation results obtained in this study and those of Soto
and Iwasaki indicate compatible trends.
Flotation of Apatite and Dolomite Mixture with Dodecylamine
The single mineral flotation results indicated that apatite can
possibly be selectively floated out from dolomite at pH a less than 4.5
at dodecylamine concentration of l.OxlO-3 kmol/m3, or at pH 9.8 and
dodecylamine concentration of 1.6x10"^ kmol/m3. However, the
selectivity predicted by the single mineral flotation data of apatite
and dolomite was found to be limited in the case of their mixture (refer
to Table 3). The loss in selectivity was observed to be due to the
lower flotation of apatite and increased recovery of dolomite in the
float fraction as compared to the results of the single minerals tests.
This could be due to one or more of the following reasons:
a) Change in the interfacial potential or the chemical composition
of the minerals due to dissolution or adsorption of dissolved
species;


80
TABLE 9
Effect of NaCl on Unit Cell Dimensions
of Apatite Conditioned at pH 4
Salt Cone. Measured Unit Cell Dimension (K)
(kmol/m2)
II II
a
"c"
None
9.348, 9.346
6.893, 6.894
7.5xl04
9.339, 9.340
6.895, 6.893
2.0xl0-2
9.342, 9.340
6.892, 6.894


64
concentration was determined to be 4.0xl0-5 kmol/m3, which incidentally
coincides with that used for single mineral experiments conducted
without NaCl addition. It is to be noted that at this level of sodium
oleate concentration at pH 4, apatite appears to be completely depressed
whereas dolomite flotation approaches 100%.
Optimum pH for separation. Results of single mineral apatite and
dolomite flotation tests as a function of pH are plotted in Figure 19.
It is observed that apatite remains depressed up to pH 4.2, but becomes
activated above this pH value.
Apatite flotation behavior in the presence of NaCl at pH 9.5 and
above is significantly different from that observed in distilled water
(refer to Figure 15). On the contrary, dolomite recovery, for the most
part, appears to be the same as that obtained in distilled water.
Following the above tests, flotation response of a 50:50 apatite-
dolomite mixture was studied as a function of pH in the presence of
2.0xl0-3 kmol/m3 NaCl and at a sodium oleate concentration of 4.0xl0~3
kmol/m3.
In general, the selectivity predicted by the single mineral tests
(see Figure 19) was maintained in the mixed mineral systems (see Figure
20). In addition, during the flotation of the mixed minerals, a more
effective apatite depression was realized in the pH range of 4.0-4.5 as
compared to single mineral test results. Optimum separation was
obtained at pH 4.00.2 where more than 95% apatite was recovered with
95% or more dolomite rejection. The MgO content of the sink fraction
(concentrate) was analyzed to be less than 0.7% from a feed containing
9.5% MgO.


40
Dolomite, on the other hand, appears to have a relatively rough surface
morphology, but does not reveal any features of its porosity.
X-Ray Analysis
X-ray diffraction analysis of apatite, presented in Figure 7,
indicated the presence of a very small amount of quartz, however, even
after repeated scans no dolomite was found to be associated with
apatite. The dolomite sample exhibited the characteristic peak of
dolomite along with minor peaks for quartz and feldspar as seen in
Figure 8.
Surface Chemical Characterization
The zeta potential measurements were made using a Pen Kern Model 501
Laser Zee Meter. The isoelectric point (IEP) of apatite, as shown in
Figure 9, is at pH 5.4. This value is similar to that reported by
Chanchani (1984) and Somasundaran (1968). In the case of dolomite, two
isoelectric points, at pH 5.3 and pH 11.1, are observed (Figure 10).
The second IEP at pH 11.1 has been attributed to the presence of
hydroxylated magnesium species on the dolomite surface (Chanchani, 1984;
Iwasaki and Krishnan, 1983; and Balajee and Iwasaki, 1969).
Flotation Studies
Flotation Studies with Dodecylamine Hydrochloride
Single mineral flotation behavior
Flotation response of single mineral apatite and dolomite was
examined at two concentrations of dodecylamine hydrochloride as a


OLEATE ADSORBED, ^mol/gx 100
Figure 48. Correlation between oleate adsorption and flotation for dolomite as a
function of pH.
AMOUNT FLOATED, WT%


18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
63
65
66
68
69
71
72
73
74
75
77
83
84
86
87
88
89
Flotation of apatite and dolomite as a function of
sodium oleate concentration in the absence and
presence of sodium chloride (single minerals) . .
Effect of pH on flotation of apatite and dolomite
(single minerals) in the presence of sodium
chloride
Flotation recovery of apatite and dolomite (mixed
minerals) as a function of pH in the presence of
sodium chloride
Effect of KC1 addition on flotation of apatite and
dolomite as a function of pH
Apatite and dolomite (single minerals) flotation
with and without NaF addition at pH 4
Zeta potential of apatite with and without NaCl
addition
Zeta potential of apatite as a function of pH
with and without KC1 addition
Zeta potential of apatite as a function of pH with
and without NaF addition
Zeta potential of dolomite as a function of pH with
and without sodium chloride
Effect of KC1 on the zeta potential of dolomite . .
Zeta potential of dolomite in the presence and
absence of NaF as a function of pH
Oleate adsorption on apatite as a function of
conditioning pH
Oleate adsorption on apatite as a function of
initial oleate concentration at pH 4.0 and 10.0 . .
Oleate adsorption on apatite at pH 4.0 in the
absence and presence of sodium chloride
Adsorption of oleate on dolomite as a function of
conditioning pH
Oleate adsorption on dolomite as a function of
initial oleate concentration at pH 4.0 and 10.0 . .
Adsorption of oleate on dolomite at pH 4.0, with
and without added sodium chloride
XI 1


158
Inhibition of calcium oleate formation, in the presence of sodium
chloride, was attributed to the depletion of surface Ca++ sites as a
result of Na+ substitution in the apatite structure. It is not clear if
any alteration occurred in the amount of oleic acid adsorbed on the
surface due to the presence of salt. This can be investigated by
quantitative FT-IR spectroscopy if a calibration curve can be
established for the amount of oleic acid adsorbed from its band in the
spectrum.


KUBELKA-MUNK UNITS
Figure 37.
Diffuse reflectance IR spectra of Mg-, Ca- and Na-oleate
(1200-1800 wavenumber region).


25
20
15
10
5
0
-5
10
15
20
25
Apatite in Distilled Water
2
A Apatite in 2.0x10 kmol/m NaCI
Figure 23. Zeta potential of apatite with and without NaCI addition.


KUBELKA-MUNK UNITS
Figure 39. IR spectra of untreated and treated dolomite at pH 10.0.


124
calcium in the apatite structure can similarly render the surface more
negatively charged.
Calcium dissolution and sodium substitution in apatite structure.
As a result of sodium substitution for calcium, the amount of calcium
dissolved from apatite was expected to increase with an anticipated
decrease in the concentration of sodium in solution. A possible change
in the lattice parameters was also expected. It was, however, realized
that the change in the lattice parameters would be rather diminutive
since the ionic radii of sodium (0.95 8) is only slightly smaller than
that of calcium (0.99 K).
Examination of the amount of calcium dissolved from apatite with
and without added NaCl as shown in Table 8, indicated that during the
first 5 minutes (the conditioning time for flotation) calcium
dissolution increased by 1.2 times in the presence of NaCl relative to
that in distilled water. This increase in calcium dissolution in the
presence of NaCl along with a decrease in the rate of P0$ dissolution
suggested that sodium is possibly substituting for calcium in the
apatite structure. The reasons for decreased P0$~ dissolution are not
yet clear. However, it is possible that the rate of dissolution of
phosphate ions decreased because of the increased ionic strength of the
solution. Nevertheless, this process is partly responsible for
development of a negative surface charge besides increased calcium
dissolution. The incongruent solubility of apatite in the presence of
NaCl, i.e., higher Ca/P ratio, was also observed by Levinskas and Neuman
(1955). These investigators attributed the phenomena to Na+


24
Solution Preparation
Stock solutions of 5.0xl0-3 kmol/m3 sodium oleate or dodecylamine
hydrochloride were prepared in deaerated distilled water as required for
flotation tests. The solution pH was adjusted to approximately 11.5 and
8.0 for oleate and dodecylamine, respectively, using NaOH or HC1. It
was diluted daily and used as needed, and the stock solution was used
for 5 days only.
Carbon-14 labeled radioactive oleic acid solution was also prepared
as described above for the unlabeled sodium oleate solution after
evaporating benzene. The stock solutions were refrigerated and used up
to one week. Solutions of desired concentrations were obtained by
mixing labeled oleic acid with unlabeled oleate solution.
Oleate Adsorption Tests
To ensure correct adsorption measurements, the solid samples were
rinsed with distilled water of the same pH value after the adsorption
experiments to remove any precipitated or entrapped oleic acid from the
solids. Control tests were conducted to check the oleic acid coating on
the surface of the vials. The results presented in Table 1 indicated
that approximately 20% of the initial collector present in the solution
reported as possible coating on the vial surface. Out of the total,
only 5% was determined to adsorb on the mineral (apatite) and less than
2% was found to be in the rinse solution at pH 4.5. This revealed that
there was not any significant desorption of oleic acid as a consequence
of rinsing of the solids. It should be noted that if the solution
depletion method were to be followed in this case, the amount adsorbed


17
California Sur. Apatite was found to be depressed at pH less than 5.5
in sea water when fatty acid was used as the collector. In the presence
of fresh water, apatite was found to be naturally depressed at pH 3.0
and below. No efforts, however, were made to study the mechanism
involved in such a separation.


TABLE 13
Adsorption and Flotation Results,
and Zeta Potential Values for Apatite and Dolomite
with and without NaCI at pH 4.0
Mineral
Salt Cone.
(kmol/m3)
Amount Adsorbed
(pmol/g x 10+2)
Amount Floated
(Weight %)
Zeta Potential
(mV)
Apatite
None
3.3
55-60
+4
Apatite
2.0xl0"2
1.8
0-3
-13
Dolomite
None
5.8
95-100
+5
Dolomite
2.OxlO-2
5.8
95-100
+4
Oleate Cone., 4.0x10^ kmol/m^


CHAPTER VI
CONCLUSIONS
Flotation response of apatite and dolomite has been studied using
dodecylamine hydrochloride (cationic) and sodium oleate (anionic) as the
collector, with particular emphasis on separating dolomite from apatite
to obtain a phosphate concentrate of less than 1% MgO.
Selective separation of apatite from dolomite, using dodecylamine
as the collector, was predicted by the single mineral flotation results.
However, the selectivity was observed to be limited in the case of mixed
minerals. The loss in selectivity was attributed to decreased flotation
of apatite and increased recovery of dolomite due to surface charge
modification of these minerals as a result of their dissolution. It
should be noted that there are more dissolved ions present in the mixed
mineral system as compared to the single mineral system. In addition,
faster adsorption kinetics of dodecylamine on dolomite are believed also
to affect the resultant flotation behavior.
Separation of apatite from dolomite in the alkaline pH range (pH 7-
10) and that of dolomite from apatite in the acidic pH range (pH 5-6)
was anticipated from the single mineral tests when sodium oleate was
used as the collector. Mixed mineral tests, however, indicated
activation of apatite in the acidic pH range and depression of it under
alkaline pH conditions, limiting the selectivity. This has been
attributed to the presence of excess Ca++ and Mg++ ions dissolved from
149


i- s oo wz
Microflotation Arrangement
V F
ro
rv>
Figure 3
Hallimond cell flotation arrangement.


10
Somasundaran (1968) obtained an isoelectric (IEP) point of pH 4
for natural apatite that was partially saturated with fluoride. It has
been reported that the nature of pretreatment plays an important role on
the IEP of apatite (Somasundaran, 1972). The IEP values given in the
literature for fluorapatite and hydroxyapatite vary between pH 3.8 and
8.5. Chanchani (1984) found the IEP of Florida apatite to be at pH 5.5.
In the case of dolomite, Predali and Cases (1973) have determined the
IEP at pH less than 7 using a natural sample from Kosice,
Czechoslovakia. Chanchani (1984) has reported two IEPs (pH 5.5 and pH
10.5) for dolomite from Florida. The second IEP at pH 10.5 was
attributed to the precipitation of hydrolysis products such as magnesium
hydroxide on the dolomite surface.
Aging Behavior of Apatite and Dolomite
The soluble minerals such as apatite and dolomite exhibit aging
phenomenon, i.e., a change in the pH of the aqueous slurry as a function
of time. Aging behavior of apatite and dolomite samples from Florida
has been examined by Chanchani (1984) using 1 wt % suspension of these
minerals. It was shown that in the case of apatite equilibrium is
reached after about 600 minutes. Initial pH of 4 and 10 was observed to
shift to a value of 6 and 7, respectively, after equilibration.
Similar pH shifts for apatite were also observed by Somasundaran (1968)
which were attributed to the dissolution of the mineral. In the case of
dolomite, the equilibrium pH was found to be between pH 8.2 and 8.5.


166
Snow, R. E., 1982, "Flotation of Phosphate Ores Containing Dolomite,"
U.S. Patent No. 4,364,824.
Somasundaran, P., 1968, "Zeta Potential of Apatite in Aqueous Solutions
and its Change During Equilibration," J. Colloid Interface
Science, Vol. 27, No. 4, pp. 659-666.
Somasundaran, P., 1972, "Pretreatment of Mineral Surfaces and its
Effect on Their Properties," In Clean Surfaces, Their Preparation
and Characterization for Interfacial Studies, Marcel Dekker,
New York, pp. 285-306.
Somasundaran, P. and Ananthapadmanabhan, K. P., 1979a, "Solution
Chemistry of Surfactants and the Role of it in Adsorption and
Froth Flotation in Mineral-Water systems." In: Solution Chemistry
of Surfactants, K. L. Mittal, Ed., Vol. 2. Plenum Press. New York,
pp. 17-38.
Somasundaran, P., and Ananthapadmanabhan, K. P., 1979b, "Physico-
Chemical Aspects of Flotation," Trans. Indian Inst. Metals,
Vol. 32, p. 2.
Somasundaran, P. and Wang, Y. H. C., 1984, "Surface Chemical
Characteristics and Adsorption Properties of Apatite," in
Adsorption and Surface Chemistry of Hydroxyapatite, D. N. Misra,
Ed., Plenum Press, New York, pp. 129-149.
Sorensen, E., 1973, "On the Adsorption of Some Anionic Collectors on
Fluoride Minerals," J. Colloid and Interface Science, Vol. 45,
No. 3, pp. 601-607.
Soto, H. and Iwasaki, I., 1985, "Flotation of Apatite from Calcareous
Ores with Primary Amines," Mineral and Metallurgical Processing,
Vol. 2, pp. 160-166.
Soto, H. and Iwasaki, I., 1986, "Selective Flotation of Phosphates from
Dolomite Using Cationic Collectors. Part II. Effect of Particle
Size, Abrasion and pH," Inter. J. of Mineral Processing, Vol. 16,
pp. 17-27.
Stoll, W. R. and Neuman, W. F., 1956, "The Uptake of Sodium and
Potassium Ions by Hydrated Hydroxyapatite," J. Am. Chem. Soc.,
Vol. 78, p. 1585.
Strel'tsyn, G. S., Pudov, V. F., Kostritsyn, V. N. and Klimenko, V. Y.,
1967, "Lowering the Harmful Effect of Sodium Chloride on Flotation
of Apatite-Nephel ine Ore," Obogashch. Rud., 12(3), 13-14 (Russ).
Stumm, W. and Morgan, J. J., 1981, "Aquatic Chemistry," 2nd Edition.
A Wiley-Interscience Pub., New York.


59
Flotation studies in the presence of NaCI using sodium oleate as the
collector
Results of mixed mineral flotation tests summarized in Table 7
indicated selective recovery of dolomite from the apatite and dolomite
mixture at pH 4 in the presence of 5.0xl0--*- kmol/m3 NaCI and at a sodium
oleate concentration of 4.0x10^ kmol/m3. Single mineral tests without
NaCI addition indicated 60% apatite and 95-100% dolomite recovery at
the same pH and collector concentration (see Figure 15). In the
presence of NaCI apatite was found to be depressed without any
significant effect on the flotation of dolomite. In order to achieve
maximum separation of dolomite single mineral tests were conducted to
determine the optimum pH, salt and collector concentration.
Optimum salt concentration for apatite depression. Mixed mineral
results indicated the best selectivity at pH 4, in the presence of NaCI
(see Table 7). Therefore, further tests to determine the optimum
concentration of NaCI were also conducted at this pH value. It was
determined that optimum results are obtained in the presence of 2.0x10^
kmol/m3 sodium chloride addition (see Figure 16). It is to be noted
that dolomite flotation is not affected by NaCI addition under the given
experimental conditions.
Optimum collector concentration. Results presented in Figure 17
and 18, indicated that at pH 10, apatite requires less collector to
float with and without salt addition, as compared to dolomite, and vice
versa at pH 4. The difference in the amount of sodium oleate required
to float 100% apatite or dolomite without NaCI addition was determined
to be about five-fold, and it more than doubled in the presence of
sodium chloride. From the data in Figure 17, the optimum sodium oleate


96
Nature of the Adsorbed Species at pH 10
Apatite-oleate system
The diffuse reflectance IR spectra of untreated apatite and the one
reagentized with 5.0x10^ kmol/m^ sodium oleate at pH 10 are shown in
Figure 38 along with the difference spectrum. The asymmetrical
stretching frequencies of CH3 group and CH2 groups, the bands at 2859
and 2930 cm'-'-, are observed in the difference spectrum as well as in the
spectrum of treated apatite and are not masked by the bands from the
mineral. The band observed in the difference spectrum suggests the
formation of calcium oleate on the apatite surface.
The diffuse reflectance IR spectra of pure apatite (i.e.,
untreated) as shown in Figure 38, indicated bands at 1090, 1054, 967,
604, and 574 cm-'-. These bands are in good agreement with those
identified by Gadsden (1975) at 1100-1080, 1050, 970-960 and 580 cm'1,
respectively. Apatite spectra was also characterized by Gnosh (1978)
with bands at 1040, 610 and 570 cm'1.
Dolomite-oleate system
The IR spectra of dolomite at pH 10 with and without adsorbed
oleate, along with the difference spectrum are illustrated in Figure 39.
This spectrum, in agreement with the magnesium oleate spectra, displayed
a band at 1582 cm"1, which indicated formation of magnesium oleate on
dolomite surface at pH 10.
The pure dolomite spectrum obtained in this study (Figure 39) shows
bands at 1460, 1413, 879 and 728 cm'1. Gadsden identified these bands


26
on the mineral would have been calculated to be five-fold higher,
resulting in misleading conclusions. In addition, adsorption of oleate
on quartz was measured at pH 4 to ensure that the results obtained with
apatite and dolomite do not include any coating of oleic acid on the
surface. Since quartz is not expected to absorb oleate at pH 4, any
adsorption of oleic acid can be presumed to be physical coating.
Results of these tests indicated that there was no physical coating of
oleate on quartz, for up to a sodium oleate concentration of 6.0xl0-4
kmol/m^.
The amount of oleate or oleic acid adsorbed was therefore
determined in this study by direct counts on the solids in the oleic
acid precipitation region, i.e., at pH 8.0 and below. Above this pH
value, the amount adsorbed was determined by the solution depletion
method because test results indicated that oleate adsorption on the
vials is less than 0.5% of the total amount present.
Single mineral adsorption tests were conducted by adding 11 ml of
sodium oleate solution of the desired pH to 0.1 g of 65x100 mesh mineral
sample in a 20 ml polypropylene vial. The samples were then conditioned
by tumbling at 8 rpm for 5 minutes. The amount adsorbed on the minerals
at pH 8 and below (oleic acid precipitation region) was determined by
direct counts on the solids after cocktail addition. To determine the
amount adsorbed above pH 8, as mentioned earlier the solution depletion
method was followed. The solids were first allowed to settle for 5
minutes and the supernatant was analyzed for the amount of surfactant
depleted. This method was followed because no major surfactant
precipitation was anticipated at pH 8 and above.


EFFECT OF SODIUM CHLORIDE ON THE SELECTIVE
FLOTATION OF DOLOMITE FROM APATITE
BY
DURSUN E. INCE
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL
FOR THE
FULFILLMENT OF THE REQUIREMENTS
DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
1937

This work is dedicated to the
memory of my father,
the late HIDIR INCE,
without whose inspiration, encouragement
and special admiration for education this
work would have never been attempted.

ACKNOWLEDGMENTS
I wish to express my sincere gratitude and respect to Professor B.
M. Moudgil, my major advisor, for his invaluable help, guidance and
encouragement during the course of this research.
I am very grateful to Professor F. N. Blanchard for his guidance,
valuable comments, and to Professors D. 0. Shah, E. D. Whitney, D. E.
Clark, H. A. Laitinen and J. H. Simmons for very helpful discussions and
comments.
I am most grateful to Professor C. T. Johnston and Mrs. L. D.
Applewhite for discussions and help in FT-IR study.
With due gratitude I wish to acknowledge the encouragement,
helpful comments and constructive criticism of Drs. T. V. Vasudevan, H.
Soto and Wen-Keng Shih.
I am thankful to M. May, A. Zutshi, J. Ransdell, Y. C. Cheng and
J. Rogers for their help at various stages, and to Mrs. G. Keim for her
help in preparing the manuscript.
Special love and appreciation is due to my wife Sevgi for her
support, help and encouragement and to my two lovely daughters, El if
and Ebru, for their patience throughout the course of this study.
Finally, I wish to thank Agrico Chemical Company and International
Minerals and Chemicals Corporation for supplying the mineral samples
used in this study and to acknowledge Florida Institute of Phosphate
Research (Grants #82-02-023 and #85-02-067) for providing financial
i i i

support. Any opinions, findings, and conclusions or recommendations
expressed in this work are those of the author and do not necessarily
reflect the views of the Florida Institute of Phosphate Research.
i v

TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS iii
LIST OF TABLES ix
LIST OF FIGURES xi
ABSTRACT xiv
CHAPTERS
I INTRODUCTION 1
II BACKGROUND 6
Characteristics of Salt-Type Minerals 6
Characteristics of Apatites in Florida Phosphorites . 7
Characteristics of Dolomite 7
Solubility of Apatite and Dolomite 8
Surface Charge of Apatite and Dolomite 9
Aging Behavior of Apatite and Dolomite 10
Separation Studies 11
Methods Based on Physical Properties 11
Separation Based on Physico-Chemical Properties .... 12
Selective flocculation 12
Selective flotation 12
Flotation in the Presence of Salts 16
III EXPERIMENTAL 18
Materials 18
Minerals 18
Apatite 18
Dolomite 18
Reagents 19
Other Chemicals 19
Methods 20
Flotation 20
v

Electrokinetic Measurements 23
Solution Preparation 24
Oleate Adsorption Tests 24
Mineral Dissolution Tests 27
Solubility Product Determination 29
FT-IR Tests 29
Contact Angle Measurements 31
Experimental Plan 31
Selection, Preparation and Characterization of
the Minerals 32
Selection of the Surfactant 32
Selection of the Experimental Techniques 32
Flotation 32
Electrokinetic measurements 33
Adsorption 33
Determination of the nature of the adsorbed
surfactant species 34
Mineral dissolution studies 34
Contact angle 34
Experimental Approach 35
IV RESULTS 36
Characterization of Minerals 36
Chemical Analysis 36
Surface Area and Porosity 36
X-ray Analysis 40
Surface Chemical Characterization 40
Flotation Studies 40
Flotation Studies with Dodecylamine Hydrochloride ... 40
Single mineral flotation behavior 40
Flotation of apatite-dolomite mixture with
dodecylamine 50
Flotation Studies Using Sodium Oleate as the
Collector 50
Single mineral flotation tests 52
Mixed minerals 52
Flotation tests in the presence of NaCl using
dodecylamine as the collector 56
Flotation studies in the presence of NaCl using
sodium oleate as the collector 59
Flotation studies in the presence of KC1 and NaF 67
Electrokinetic Studies 67
Effect of Salt Addition on the Zeta Potential
of Apatite 67
Effect of Salts on the Zeta Potential of Dolomite ... 70
Role of NaCl in the Reversal of Surface Charge
of Apatite 76
Substitution of sodium for calcium in the
apatite lattice 76
Adsorption Studies 82
vi

Single Minerals Adsorption Tests 82
Apatite-oleate system 82
Dolomite-oleate system 85
Mixed Mineral Adsorption Studies 90
Characterization of the Adsorbed Oleate Species
by FT-IR Spectroscopy 90
IR Spectra of Pure Oleate Species 93
Nature of the Adsorbed Species at pH 10 96
Apatite-oleate system 96
Dolomite-oleate system 96
Nature of the Adsorbed Species at pH 4 99
Apatite-oleate system 99
Dolomite-oleate system 99
Oleate Species Adsorbed in the Presence of NaCl .... 102
Apatite-NaCl-oleate system 102
Dolomite-NaCl-oleate system 102
V DISCUSSION 105
Solution Properties of Dodecylamine and Oleate 105
Apatite-Dolomite Flotation Using Dodecylamine as
the Collector 109
Apatite-Dodecylamine System 109
Dolomite-Dodecylamine System 110
Flotation of Apatite and Dolomite Mixture with
Dodecylamine Ill
Changes in the surface charge and surface
chemical composition 112
Surface coating 114
Surfactant depletion by precipitation 114
Flotation of Apatite and Dolomite Using Sodium Oleate
as the Collector 116
Evaluation of the Results and Alternatives 116
Effect of NaCl on the Selective Flotation of
Apatite Using Dodecylamine as the Collector 119
Effect of NaCl on the Separation of Dolomite From
Apatite Using Sodium Oleate as the Collector 120
Mechanism of Selective Flotation of Dolomite from Apatite 121
Effect of NaCl on the Zeta Potential of Apatite
and Dolomite 121
Role of NaCl in the reversal of surface charge
of apatite 122
Dolomite structure 126
Adsorption of Oleate on Apatite and Dolomite 128
Effect of Conditioning pH 128
Effect of NaCl Addition on Adsorption 131
Adsorption in the Mixed Mineral System 133
Effect of Salt on Adsorption in the Mixed Minerals
System 133
Nature of the Adsorbing Surfactant Species 134
Apatite-01eate System 134
vi 1

Dolomite-Oleate System 136
Preferential formation of magnesium oleate .... 138
Contact Angle Studies 145
Mechanism of Selective Flotation of Dolomite from
Apatite in the Presence of NaCl 147
VI CONCLUSIONS 149
VII SUGGESTIONS FOR FUTURE RESEARCH 155
REFERENCES 159
BIOGRAPHICAL SKETCH 168
vi i i

LIST OF TABLES
Table No. Page
1 Sodium Oleate Distribution in Various Streams
During Adsorption Tests at pH 4.5 25
2 Characteristics of Apatite and Dolomite Samples ... 37
3 Flotation Results of 50:50 Apatite/Dolomite
Mixture Using Dodecylamine as the Collector 51
4 Flotation Results of 50:50 Apatite-Dolomite
Mixture Using Sodium Oleate as the Collector 54
5 Effect of NaCl on the Single Mineral Flotation
of Apatite and Dolomite with Dodecylamine
Hydrochloride as the Collector 57
6 Results of Mixed Mineral Flotation in the
Presence of NaCl Using Dodecylamine as the
Collector at pH 6.3 58
7 Results of Mixed Mineral Flotation Tests in the
Presence of NaCl Using Sodium Oleate as the
Collector 50
8 Apatite Dissociation at pH 4 with and without
Added Sodium Chloride 79
9 Effect of NaCl on Unit Cell Dimensions of Apatite
Conditioned at pH 4 80
10 Determination of Substitution of Sodium for
Calcium in the Apatite Structure 81
11 Effect of Sodium Chloride Addition on Oleate
Adsorption on Apatite and Dolomite at pH 4.0 92
12 Dissolution of Calcium and Magnesium from
Dolomite at pH 4.0 with and without NaCl
Addition 113
IX

13 Adsorption and Flotation Results with Zeta
Potential Values for Apatite and Dolomite
with and without NaCl at pH 4.0 132
14 Solubility Product of Calcium and Magnesium
Oleate 140
15 Electrostatic Interaction Energy of Calcium and
Magnesium with Oleate 144
16 Contact Angle and Flotation Recovery of Apatite
and Dolomite at pH 4.0 146
x

LIST OF FIGURES
Figure No. Page
1 Location of Florida phosphate deposits 2
2 A schematic diagram of a modified Hallimond Cell . 21
3 Hallimond cell flotation arrangement 22
4 Apparatus for mixed mineral adsorption studies. ... 28
5 SEM micrograph of apatite (65x100 mesh size
fraction), a) 200X; b) 1000X 38
6 SEM micrograph of dolomite (65x100 mesh size
fraction), a) 100X; b) 1000X 39
7 X-ray diffractogram of apatite 41
8 X-ray diffractogram of dolomite 42
9 Zeta potential of apatite as a function of pH ... 43
10 Zeta potential of dolomite as a function of pH. . 44
11 Flotation of apatite as a function of pH 46
12 Dolomite flotation as a function of pH 47
13 Flotation of apatite and dolomite (single minerals)
as a function of pH 48
14 Flotation of apatite and dolomite (single minerals)
as a function of pH 49
15 Flotation of apatite and dolomite (single minerals)
as a function of pH 53
16 Effect of sodium chloride addition on apatite and
dolomite flotation 61
17 Effect of sodium oleate concentration on apatite
and dolomite (single minerals) flotation with and
without NaCl addition 62
XI

18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
63
65
66
68
69
71
72
73
74
75
77
83
84
86
87
88
89
Flotation of apatite and dolomite as a function of
sodium oleate concentration in the absence and
presence of sodium chloride (single minerals) . .
Effect of pH on flotation of apatite and dolomite
(single minerals) in the presence of sodium
chloride
Flotation recovery of apatite and dolomite (mixed
minerals) as a function of pH in the presence of
sodium chloride
Effect of KC1 addition on flotation of apatite and
dolomite as a function of pH
Apatite and dolomite (single minerals) flotation
with and without NaF addition at pH 4
Zeta potential of apatite with and without NaCl
addition
Zeta potential of apatite as a function of pH
with and without KC1 addition
Zeta potential of apatite as a function of pH with
and without NaF addition
Zeta potential of dolomite as a function of pH with
and without sodium chloride
Effect of KC1 on the zeta potential of dolomite . .
Zeta potential of dolomite in the presence and
absence of NaF as a function of pH
Oleate adsorption on apatite as a function of
conditioning pH
Oleate adsorption on apatite as a function of
initial oleate concentration at pH 4.0 and 10.0 . .
Oleate adsorption on apatite at pH 4.0 in the
absence and presence of sodium chloride
Adsorption of oleate on dolomite as a function of
conditioning pH
Oleate adsorption on dolomite as a function of
initial oleate concentration at pH 4.0 and 10.0 . .
Adsorption of oleate on dolomite at pH 4.0, with
and without added sodium chloride
XI 1

35 Oleate adsorption on apatite and dolomite (single
and mixed minerals) as a function of pH 91
36 Diffuse reflectance IR spectra of Mg-, Ca- and
Na-oleate (400-4000 cm-1 range) 94
37 Diffuse reflectance IR spectra of Mg-, Ca- and
Na-oleate (1200-1800 wavenumber region) 95
38 Diffuse reflectance IR spectra of treated and
untreated apatite at pH 10.0 97
39 IR spectra of untreated and treated dolomite at
pH 10.0 98
40 Diffuse reflectance IR spectra of treated and
untreated apatite at pH 4.0 100
41 IR spectra of treated and untreated dolomite
at pH 4.0 101
42 Diffuse reflectance IR spectra of apatite
(treated and untreated) and the difference
spectrum at pH 4.0 in the presence of NaCl 103
43 Diffuse reflectance IR spectra of dolomite at
pH 4.0 in the presence of sodium chloride 104
44 Dodecylamine species distribution as a function
of pH. Total amine concentration, 1.6 x 10"^ M . 107
45 Oleate species distribution as a function of pH
Total oleate concentration, 4.0 x 10^ M 108
46 Crystal structure of dolomite, c-axis vertical
a) Layered structure b) Stereoscopic projection
of the hexagonal unit cell for dolomite (a = 4.81 A,
c = 16.00 A) 127
47 Correlation between oleate adsorption and flotation
for apatite as a function of pH 129
48 Correlation between oleate adsorption and flotation
for dolomite as a function of pH 130
49 Difference IR spectra of apatite-oleate system at
pH 10, and at pH 4 in the presence and absence
of sodium chloride 135
50 Difference IR spectra of dolomite-oleate system at
pH 10, and at pH 4 with and without sodium chloride
addition 137
xi i i

Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
EFFECT OF SODIUM CHLORIDE ON THE SELECTIVE
FLOTATION OF DOLOMITE FROM APATITE
BY
DURSUN E. INCE
December, 1987
Chairman: B. M. Moudgil
Major Department: Materials Science and Engineering
Selective flotation of dolomite from apatite was investigated
using dodecylamine hydrochloride and sodium oleate as the collector in
the absence and presence of inorganic modifiers such as sodium chloride.
Separation of dolomite from apatite was anticipated from the single
mineral experiments under various pH and collector concentrations.
Flotation selectivity in the mixtures, however, was found to be limited.
Attempts to control the flotation response of these minerals by addition
of salts such as NaCl and KC1 indicated that selectivity is possible.
Upon optimization of the process parameters, the best selectivity was
obtained at pH 4, in the presence of sodium chloride, using sodium
oleate as the collector.
Electrokinetic and adsorption studies were conducted to elucidate
the mechanism of observed selectivity. Zeta potential measurements
xiv

demonstrated that the surface charge of apatite is reversed below its
isoelectric point (pH 5.4) in the presence of sodium chloride. This was
attributed to the increased rate of calcium dissolution and sodium
substitution in the apatite structure.
Adsorption studies confirmed that the amount of oleate adsorbed on
apatite decreases in the presence of sodium chloride relative to that in
distilled water, while on dolomite the amount adsorbed remained
unchanged.
FT-IR spectra of apatite in distilled water indicated the presence
of oleic acid and calcium oleate on the surface. In the presence of
NaCl, however, calcium oleate was not detected. This was ascribed to
the surface charge reversal, which adversely influenced adsorption of
anionic oleate species, in addition to the depletion of calcium sites by
selective dissolution. FT-IR spectra of dolomite indicated the presence
of magnesium oleate besides oleic acid, with and without the salt
addition. Formation of magnesium oleate in preference to calcium oleate
on the dolomite surface was explained in terms of higher charge density
and electronegativity of magnesium ions, and relatively higher rate of
calcium dissolution.
Contact angle measurements on apatite and dolomite indicated a good
correlation with adsorption and flotation results in the absence and the
presence of sodium chloride.
xv

CHAPTER I
INTRODUCTION
The Florida phosphate rock deposits are located in the central and
northern land pebble districts. The central land pebble district, as
shown in Figure 1, constitutes the major phosphate rock producing
region and underlies 2600 square miles in Polk, Hillsborough, Hardee,
Manatee and DeSoto Counties. Until recently, the phosphate production
has been confined to the Bone Valley of the central district. These
deposits, however, are being depleted and the mining will shift to the
southern extension of the central district. Phosphate rock from the
southern extension (lower zone matrix) is lower in grade and contains
significant quantities of dolomitic limestone (Ca, Mg carbonate)
impurities in addition to quartz and clays (Lawver et al., 1982a).
Beneficiation of Florida phosphate rock by a "double float" or Crago
flotation process (Crago, 1940) has been in use commercially since
1937. However, with the conventional processing techniques, dolomite,
usually reported as weight percent MgO, cannot be selectively removed
from apatite, the phosphate mineral. On the other hand, it is generally
agreed that more than 1% MgO in the final phosphate concentrate would
present problems during chemical processing to manufacture phosphoric
acid, an intermediate product used in the production of fertilizers.
The presence of MgO in quantities greater than the specified amount
would:
1

2
Figure 1. Location of Florida phosphate deposits.

3
1. Increase the viscosity of the phosphoric acid resulting in
higher pumping costs (Cate and Deming, 1970);
2. Cause an increase in sulfuric acid consumption and
induce foaming;
3. Form excessive sludge during phosphoric acid production;
4. Precipitate as complex salts (MgNH4P25) and lead to
clogging of filters (Becker, 1983);
5. Interfere in the production of certain super phosphates
(Becker, 1983).
Phosphate rock, which is essentially carbonate-fluorapatite,
(Fantel and Rosenkranz 1983; Whippo and Murowchick, 1967), contains
magnesium impurities in one of the following forms:
1. Ionic substitution in the apatite structure;
2. Second phase dolomite in apatite;
3. Discrete dolomite particles.
The MgO content in francolite (carbonate-fluorapatite) occurs both
as fine inclusions of dolomite and as Mg in the francolite structure,
according to Lawver et al. (1982a). The range of "lattice Mg"
calculated as MgO was found to be 0.40% from 31 samples studied by
Lawver and co-workers. Recently, Blanchard et al. (1986) have
conducted a systematic investigation using 18 samples from Florida
phosphate fields. Measurements of dolomite content by X-ray diffraction
and chemical analyses of these samples indicated that the average excess
MgO content above that accounted for by dolomite is about 0.57% by
weight. This represents a reasonable estimate of the amount of MgO
substituting in the apatite structure. Additionally, McClellan (1980)

4
showed that the "a" unit-cell dimension for carbonate-fluorapatite can
also be used to estimate the Mg substitution for Ca. The average "a"
unit-cell dimensions measured for 10 samples corresponded to 0.55% (by
weight) MgO in the apatite, in a good agreement with the average of
0.57% MgO estimated by Blanchard and co-workers (1986).
Physical methods of separation obviously cannot remove the
substituted magnesium. However, the second phase dolomite and discrete
particles upon liberation can be separated from apatite by physical and
physico-chemical processes.
Separation of dolomite from apatite by selective flotation has been
the focal point of research in the last decade, because physical
methods such as gravity and magnetic separation have not shown much
promise in beneficiating dolomitic phosphate ores. Flotation studies by
the phosphate industry (Lawver et al., 1978: Snow, 1979; Dufour et al.,
1980; Lawver et al., 1980; Lawver et al., 1982b), the U.S Bureau of
Mines (Llewellyn et al., 1982 and 1984) and the Tennessee Valley
Authority (Lehr and Hsieh, 1981) have resulted in the development of a
number of processes. It should be pointed out that these studies
evolved from engineering applications, and an understanding of the
fundamentals which govern the selectivity of the proposed processes were
not fully established. This is believed to be a serious limitation in
the optimization of these processes. Nevertheless, past efforts have
provided a direction for developing suitable separation techniques.
A systematic study involving an apatite/dolomite-anionic surfactant
system was conducted by Moudgil and Chanchani (1985a, 1985b and 1985c),
and Chanchani (1984) which has resulted in the development of a two-

5
stage conditioning" process. In this process, the feed is conditioned
at pH 10 followed by reconditioning at a pH lower than 4.5 before
flotation. Dolomite is selectively floated out following the two-stage
conditioning process. Bench-scale optimization of this process using
natural samples appears promising.
Flotation separation of apatite from dolomite using cationic
collectors has also been studied by researchers at International
Minerals and Chemicals Company (Lawver et al., 1980; and Snow, 1979),
followed by Soto and Iwasaki (1985 and 1986). The role of the
dissolved mineral species, flotation pH and solution chemistry of the
surfactant, however, was not taken into account in the above studies.
The surface modifiers used in the past for apatite separation from
carbonates are mostly phosphate salts, which are uneconomical. Also,
changes in the characteristics of the matrix (ore), even from the same
location, have been observed. It is conceivable that a given flotation
scheme would be applicable to a specific ore. It is imperative,
therefore, to develop flotation processes which possibly would have
applicability to a wide variety of ores. Additionally, understanding
the mechanism of selectivity would be helpful in overcoming the
difficulties in processing different ores and ensuring the usefulness of
the method for treating ores of different characteristics.

CHAPTER II
BACKGROUND
Characteristics of Salt-Type Minerals
Salt-type minerals such as apatite and dolomite are characterized
by solubilities higher than those of most oxides and silicates, but
lower than simple salt minerals such as halite and sylvite. Flotation
of such minerals from the associated gangue and from each other is of
major practical importance. For example, apatites constitute the
largest tonnage of any raw material beneficiated by froth flotation
techniques in the United States as well as several other countries
(Hanna and Somasundaran, 1976).
Separation of the salt-type minerals from oxide and silicate
minerals has been achieved and used commercially. However, separation
of these minerals from each other is complex and the problems involved
remain unresolved in many cases. It has been reported that the
differences between flotation characteristics of various salt-type
minerals may not be greater than those between samples of a single
mineral from different deposits (Sorensen, 1973). The similarities in
the flotation response of these minerals is generally attributed to
their comparable surface chemical behavior. In addition, interaction of
dissolved mineral species with collector molecules is considered to
contribute to the poor selectivity. Consequently, it has been suggested
that the use of inorganic or organic modifying agents might result in
6

7
the desired selectivity in these systems (Klassen and Mokrousov, 1973;
Joy and Robinson, 1964).
Characteristics of Apatites in Florida Phosphorites
Phosphorites are sedimentary rocks with 15 to 20% P2O5 content.
In Florida deposits, apatite is the most abundant phosphorite mineral
generally found in the form of carbonate-fluorapatite (Ca^o(PO4)6-
x(C03)x Fo 4X^2^ (McClellan, 1980). The carbonate-fluorapatite (also
known as francolite) can have extensive substitutions such as carbonate
for phosphate; and other cations such as Mg, Na, Mn and K for calcium
(Lehr et al., 1967; McConnell, 1952, and McConnell and Gruner, 1940).
Apatite has a hexagonal lattice structure and lattice parameters are
dependent on the extent of substitution. It is generally composed of
microcrystals which vary in size from 0.02 to 0.20 microns (Lehr et
al., 1967; Smith and Lehr, 1966). High surface area of apatites has
therefore been attributed to this crystal size. In addition, much of
the south Florida phosphate rock is composed of grains which are
mixtures of apatite, dolomite and other constituents. Phosphorite
sediments are complex because they are the product of several different
sedimentary systems, and are formed by intermixing of phosphates,
carbonates, organic matter, glauconite, terrigenous and siliceous
sediments (Riggs, 1979).
Characteristics of Dolomite
Dolomite along with calcite is the most abundant carbonate mineral.
The crystal structure of dolomite is similar to that of calcite. The

8
description of the dolomite [CaMgiCC^^] structure is provided by
retaining the calcite (CaCC^) structure, but simply substituting Mg
atoms for the Ca atoms in every other cation layer. The alternating Ca-
Mg arrangement of dolomite has some similarities with calcite, but the
c-glide present in calcite is destroyed. Dolomite has a rhombohedral
crystal structure with a Ca-0 bond length of 2.38 8, and Mg-0 bond
length of 2.08 8. This results in oxygen lying closer to Mg than Ca in
the dolomite structure (Reeder and Sheppard, 1984)
Solubility of Apatite and Dolomite
Charge characteristics of the solid/liquid interface and chemical
composition of the aqueous phase depends on the solubility of the
minerals. When the minerals come into contact with water, the
constituent species such as Ca++, P0$ and F" from apatite, and Ca++,
Mg++ and C0^ from dolomite will be transferred from the structure into
the solution. This dissolution will continue until the chemical
potentials of the species in solution and the solid phases reach an
equilibrium. The solubility of the salt-type minerals often varies over
a wide range, because the chemical potentials of the species in solution
and in the solid are affected by a number of factors such as degree of
hydration, solid solution formation and presence of other components in
solid or solution. It is known that both apatite and dolomite, in
contrast to simple salts, dissolve with their ions undergoing various
hydrolysis and complex formation reactions.
There have been a number of publications on the solubility of
apatite with marked differences in the data reported. Hanna and

9
Somasundaran (1976) have attributed these discrepancies to crystal
structure modifications, the presence of impurities, and added
electrolytes. Saleeb and de Bruyn (1972), however, have shown that a
constant solubility product can be obtained if a stoichiometric compound
is prepared. These investigators have reported a pKS0 value of 119.1
for fluorapatite.
In the case of dolomite, the lack of understanding of its
precipitation process has contributed to the discrepancy of the
solubility products reported by different investigators. The published
pKS0 values are in the range of 16.5 to 19.5 for dolomite (Stumm and
Morgan, 1981).
Surface Charge of Apatite and Dolomite
It is known that in the case of insoluble oxides the surface charge
is mostly determined by 0H and H+, i.e., by the pH of the suspension in
the absence of any specifically adsorbing ions. The charge of the ionic
solids such as silver iodide is considered to be the result of
preferential dissolution or adsorption of Ag+ or I. The magnitude and
sign of the surface charge is determined by the solution concentration
of the constituent ions.
Surface charge development on minerals such as apatite and
dolomite is much more complex. It is controlled by preferential
dissolution of constituent species, and their hydrolysis products in
addition to OH- and H+ (Saleeb and de Bruyn, 1972) ions. It has also
been shown that the impurity ions present as substituents for lattice
ions produce significant changes in the electrokinetic characteristics.

10
Somasundaran (1968) obtained an isoelectric (IEP) point of pH 4
for natural apatite that was partially saturated with fluoride. It has
been reported that the nature of pretreatment plays an important role on
the IEP of apatite (Somasundaran, 1972). The IEP values given in the
literature for fluorapatite and hydroxyapatite vary between pH 3.8 and
8.5. Chanchani (1984) found the IEP of Florida apatite to be at pH 5.5.
In the case of dolomite, Predali and Cases (1973) have determined the
IEP at pH less than 7 using a natural sample from Kosice,
Czechoslovakia. Chanchani (1984) has reported two IEPs (pH 5.5 and pH
10.5) for dolomite from Florida. The second IEP at pH 10.5 was
attributed to the precipitation of hydrolysis products such as magnesium
hydroxide on the dolomite surface.
Aging Behavior of Apatite and Dolomite
The soluble minerals such as apatite and dolomite exhibit aging
phenomenon, i.e., a change in the pH of the aqueous slurry as a function
of time. Aging behavior of apatite and dolomite samples from Florida
has been examined by Chanchani (1984) using 1 wt % suspension of these
minerals. It was shown that in the case of apatite equilibrium is
reached after about 600 minutes. Initial pH of 4 and 10 was observed to
shift to a value of 6 and 7, respectively, after equilibration.
Similar pH shifts for apatite were also observed by Somasundaran (1968)
which were attributed to the dissolution of the mineral. In the case of
dolomite, the equilibrium pH was found to be between pH 8.2 and 8.5.

11
Separation Studies
The past research efforts to develop a suitable technique for
separation of carbonates from apatite can be divided into two broad
categories, those in which differences in physical properties such as
specific gravity, conductivity, hardness, etc., were utilized and those
where surface chemical properties of the minerals were exploited to
achieve the desired separation. A brief discussion of these efforts is
presented below.
Methods Based on Physical Properties
Apatite and dolomite have relatively close physical properties
such as specific gravity and hardness, thereby making it difficult to
achieve their separation based on these properties. Both of the
minerals are also nonmagnetic. Hence, the separation techniques based
on such properties are not feasible. However, the "apparent" densities
of the two minerals have been found to be sufficiently different for the
possible application of heavy media separation (Lawver et al., 1982a).
The difference in the specific gravity is attributed to the highly
porous nature of the large dolomite particles as compared to the same
size apatite particles. The effectiveness of the heavy media separation
process is reported to be limited to the coarse size (pebble size)
only, which is found in lesser quantity in the southern district.
Selective attritioning has also been attempted by Soto and Iwasaki
(1986). Results from these studies indicated that only 40-60% of the
dolomite can be eliminated and that flotation is necessary for further
reduction of MgO in the apatite concentrate.

12
Separation Based on Physico-Chemical Properties
Selective flocculation
Selective flocculation of dolomite from apatite was attempted for
the first time by Moudgil and Shah (1986) using polyethylene oxide (PEO)
as the flocculant. It was shown that PEO flocculates dolomite, but not
apatite in single mineral tests. However, mixed mineral tests did not
exhibit the expected selectivity. The loss in selectivity in the case
of mixed minerals was attributed to polymer-induced entrapment, which
occurs due to a limited affinity of the polymer for the inert mineral
(apatite). Incorporation of the polymer-coated apatite particles into
dolomite floes was explained on the basis of the higher probability of
polymer bridging due to the larger number of "active" sites on dolomite
as compared to apatite particles. Further efforts are underway to
reduce the adsorption of the polymer on apatite so as to achieve the
desired separation.
Selective flotation
Separation of dolomitic limestone from sedimentary phosphates has
been the subject of studies by various researchers. Due to the complex
structure and the presence of different amorphous and porous phosphates
in these sedimentary deposits, separation of carbonates such as calcite
and dolomite from phosphates has not always been feasible. The amount
and type of impurities present have shown considerable variation even
within the same ore deposit. Despite these problems, development of a

13
number of flotation processes has been reported to separate dolomitic
impurities using both cationic and anionic collectors.
Cationic flotation of apatite from dolomite. Flotation of
apatite (francolite) from dolomite using cationic collectors was
investigated by the International Minerals and Chemicals Corporation in
the late seventies and early eighties (Snow, 1979; Baumann and Snow,
1980; and Lawver, 1980). These investigators developed a process which
reduces the MgO content of the conventionally floated (double flotation)
material to 1% or less yielding more than 90% BPL recoveries. This
process involved a rougher float followed by several cleaner stages
using a primary aliphatic amine in combination with kerosene as the
collector.
The above process was later studied by Soto and Iwasaki (1985 and
1986) to elucidate the mechanisms involved. These researchers concluded
that there is a stronger chemical interaction between the cationic
collector and the phosphate ions present at the apatite surface. The
selectivity was attributed to the lower solubility of the reagent-
phosphate compound as compared to that of the reagent-carbonate complex
formed. Other noteworthy studies with cationic collectors have been
conducted to separate calcite from apatite (Hanna, 1975, and Samani et
al., 1975).
Anionic flotation of dolomite from apatite. Separation of
calcite and in some cases dolomite from phosphate ores using anionic
collectors has been studied extensively in the past decade. A detailed
review of these studies has recently been presented by Moudgil and

14
Somasundaran (1986) and Chanchani (1984). A brief summary of recent
developments is presented below.
Flotation of dolomite from apatite in the presence of inorganic
depressants such as phosphates and fluorides at pH 5.6-6.2 was studied
by Lawver et al. (1984) using fatty acids and their soaps, including
petroleum sulfonates. Reportedly, the best results were obtained with
sodium tripolyphosphate and hexametaphosphate using a proprietary
anionic collector. This process resulted in phosphate concentrates
containing less than 1% MgO at 50-90% BPL recoveries from a feed of less
than 48 mesh size fraction containing about 2% MgO.
Hsieh and Lehr (1985) at TVA used diphosphonic acid to depress
apatite while floating dolomite with oleic acid. This process reduced
the MgO content of the concentrate to less than 1% from a feed
containing 1.9% MgO at 83% apatite recovery. In another process
developed by TVA (1983), H2SO4 was added to the concentrate to separate
calcareous phosphate ores. Selective flotation of carbonates was
attributed to differential desorption of the fatty acid collector on
the phosphate mineral. No experimental data was, however, presented to
support this hypothesis.
Llewellyn et al. (1982) at U.S. Bureau of Mines depressed
dolomite by the addition of sodium silicate and floated apatite at pH
9.2-9.6. In cases where the MgO content of the final concentrate was
not reduced to less than 1%, further removal of dolomite by SO2 leaching
was recommended. Rule et al. (1970 and 1985), also at USBM, depressed
apatite with fluosilicic acid while floating carbonaceous impurities
using fatty acid emulsion under slightly acidic pH conditions.

15
Dufour et al. (1980) at Minemet Recherche, France, depressed
phosphate at pH 5.5 and floated dolomite after attritioning; however,
the reagents used were not disclosed. The apatite recoveries ranged
from 48 to 85 percent with the final product containing less than 1%
MgO.
Johnston and Leja (1978) also selectively floated dolomite at pH
6, using oleic acid as the collector by depressing fluorapatite by
phosphate ions. The difference in flotation was explained on the basis
of adsorption of phosphate ions onto apatite by hydrogen bonding.
Dolomite flotation was attributed to hydrogen bonding of oleic acid on
dolomite in addition to COg gas evolution.
In an effort to develop new reagents that interact selectively,
Fu and Somasundaran (1986) used alizarin red S, a dye that stains
calcite, and achieved effective separation of calcite from apatite with
sodium oleate as the collector. They found that alizarin red S adsorbs
more on apatite than calcite and consequently acts as an apatite
depressant.
Other noteworthy studies include that of Bushel et al. (1970);
Onal (1973); Ratobylskaya et al. (1975); Lawver et al. (1978, 1980,
1981); Kiukkola (1980); Dufour et al. (1980); Baumann and Snow (1980);
Houot and Polgaire (1980); Lehr and Hsieh (1981); Clerici et al. (1984);
Rao and coworkers (1985), and Atalay et al. (1985).
It should, however, be noted that most of the above studies have
evolved from engineering studies. Hence, the fundamental principles
governing these processes were not studied which are required for
developing more efficient techniques.

16
A systematic and thorough study of apatite-dolomite separation
using anionic collectors has been conducted by Moudgil and Chanchani
(1985a and 1985b). The surface chemical and dissolution characteristics
of the minerals were taken into account in addition to solution
properties of the surfactant (sodium oleate), and a "two-stage
flotation" process was developed. This process involves conditioning
the feed at pH 10 followed by reconditioning at a pH less than 4.5 to
selectively float out dolomite. Bench scale testing of this process has
yielded favorable results (Moudgil, 1987).
Flotation in the Presence of Salts
As mentioned above apatite-dolomite separation has been studied
(Atalay et al., 1985; Rule et al., 1985; Hsieh and Lehr, 1985; Lawver et
al., 1984; Llewellyn et al., 1982; Johnston and Leja, 1978; and Onal,
1973) using phosphate salts as depressants for apatite. However, no
systematic studies are reported on the role of the added salts on the
flotation behavior of these minerals. It should be noted that in these
studies phosphate minerals were depressed in the presence of phosphate
salts.
Maslow (1971) and Strel'tsyn et al. (1967) have reported the
adverse effect of salt addition on apatite flotation in apatite-
nepheline ore. These workers found that when NaCl addition exceeded 0.5
kg/ton, the ore was suppressed at pH 9, but it was overcome by addition
of NaOH and sodium silicate. Additionally, Gruber et al. (1986) have
reported successful separation of carbonates such as cal cite from
apatite in sea water from the Santo Domingo phosphate deposit in Baja

17
California Sur. Apatite was found to be depressed at pH less than 5.5
in sea water when fatty acid was used as the collector. In the presence
of fresh water, apatite was found to be naturally depressed at pH 3.0
and below. No efforts, however, were made to study the mechanism
involved in such a separation.

CHAPTER III
EXPERIMENTAL
Materials
Minerals
Apatite
A sample of high-grade phosphate rock (apatite) was procured from
Agrico Chemical Company (Mulberry, Florida). This sample (16x150 mesh)
was screened to obtain a 65x100 mesh fraction, which was deslimed, dried
and passed through an electrostatic separator after heating to 140 C to
remove the silica grains. The 65x100 mesh sample was used for
flotation and adsorption studies. FT-IR and electrokinetic studies,
however, were conducted on a portion of this sample which was ground to
-325 mesh.
Dolomite
This sample, supplied by International Minerals and Chemicals
Corporation (Bartow, Florida), was hand picked and crushed using a
Chipmunk crusher and hand ground to maximize the yield of 65x100 mesh
fraction. The dolomite was deslimed and dried at 140 C before removing
silica by electrostatic separator. Electrokinetic and FT-IR studies
were conducted on a portion of this sample ground to -325 mesh. The
samples were stored in a glass jar and used as required.
18

19
Samples of completely crystalline New Jersey dolomite and
fluorapatite obtained from Geology Department, University of Florida
(Blanchard, 1987) were used for contact angle measurements. Chemical
analysis of the apatite sample indicated 37.51% P2O5, 0.14% MgO and
1.15% acid insoluble. Crystalline New Jersey dolomite analyzed 17.73%
MgO, 0.36% P2O5 and 0.95% acid insoluble.
Reagents
Dodecylamine hydrochloride obtained from Eastman Kodak Company, and
purified sodium oleate purchased from Fisher Scientific Company, were
used as the cationic and anionic collectors, respectively, for flotation
experiments.
A mixture of unlabeled and ^C labeled oleic acid was used for the
adsorption experiments. Unlabeled oleic acid (gold label grade) was
obtained from Aldrich Chemical Company. ^C labeled oleic acid was
purchased from ICN Pharmaceuticals in nitrogen-sealed ampules of 0.1
mCi.
Other Chemicals
All other chemicals such as calcium and magnesium standards, HNO3,
KOH, NaOH, NaCl, KC1, NaF, etc., were of reagent grade purchased from
Fisher Scientific Company.
Triple distilled water of less than 1.2 micromhos conductivity was
used in the flotation experiments. All other solutions were prepared
using triple distilled water deaerated by bubbling nitrogen for two
hours.

20
Methods
Flotation
Flotation experiments were conducted using a modified Hallimond
cell (Fuerstenau et al., 1957; and Modi and Fuerstenau, 1960) shown in
Figure 2. This cell consists of two parts connected by a ground glass
joint. The lower part of the cell is fitted with a fritted glass,
having uniform pore size. A Teflon-coated magnetic stirring bar is used
to maintain the particles in suspension. In the Hallimond cell
technique, the hydrodynamic variables such as agitation, airflow rate,
and flotation time can be controlled.
The complete Hallimond cell flotation assembly is illustrated in
Figure 3. The details of the set-up and operation are similar to those
used by Moudgil (1972), Ananthapadmanabhan (1980) and Chanchani (1984)
and are only briefly described below.
Nitrogen gas was used for the flotation experiments, which was
first purified by passing through an ascarite column (D) for removing
the CO2 (see Figure 3). Subsequently, the gas passes through a water
trap (T^) to remove any ascarite fines carried over. Traps (T2 and T3)
are provided to prevent back suction of water to the ascarite column.
The purified gas is passed to a 50 liter glass reservoir (R), which acts
as a buffer tank to supply the gas at a constant pressure. The pressure
in the tank is measured by the manometer (M). The outlet of the gas
reservoir is connected to the Hallimond cell (HC) through a solenoid
valve (V) and flow meter (F). The solenoid valve is energized using a

21
Figure 2. A schematic diagram of a modified Hallimond Cell.

i- s oo wz
Microflotation Arrangement
V F
ro
rv>
Figure 3
Hallimond cell flotation arrangement.

23
timer, which can be set to allow the gas flow and stirring for any
length of time.
The apatite, dolomite, or the mixture of these minerals were aged
in 100 ml volumetric flask for 20 minutes and conditioned after reagent
addition for a period of 5 minutes by slow tumbling at 8 rpm. Whenever
required, minerals were aged in the salt solution of specified
concentration. The pH of the suspension was measured before aging and
after conditioning in all cases and the latter is reported as the pH of
flotation. The suspension was transferred to the Hallimond cell with
minimum turbulence and floated for one minute using nitrogen gas at a
flow rate of 50 ml/min. The floated and unfloated fractions were dried
at 50 C, and flotation recovery was calculated based on the weight of
the dried samples. In the case of the mixtures, float and sink
fractions were pulverized and leached in aqua-regia before analyzing for
P2O5 and MgO contents.
Electrokinetic Measurements
Zeta potential measurements were conducted using a Pen Kern Model
501 Laser Zee Meter. Suspensions of 0.1 wt % apatite or dolomite were
prepared using -325 mesh sample. These were stirred with a Teflon-
coated magnetic bar for approximately 21 hours at natural pH. Fifty
milliliter samples of the aged suspension were equilibrated at the
desired pH for 3 hours before they were transferred to the cell for zeta
potential measurements.

24
Solution Preparation
Stock solutions of 5.0xl0-3 kmol/m3 sodium oleate or dodecylamine
hydrochloride were prepared in deaerated distilled water as required for
flotation tests. The solution pH was adjusted to approximately 11.5 and
8.0 for oleate and dodecylamine, respectively, using NaOH or HC1. It
was diluted daily and used as needed, and the stock solution was used
for 5 days only.
Carbon-14 labeled radioactive oleic acid solution was also prepared
as described above for the unlabeled sodium oleate solution after
evaporating benzene. The stock solutions were refrigerated and used up
to one week. Solutions of desired concentrations were obtained by
mixing labeled oleic acid with unlabeled oleate solution.
Oleate Adsorption Tests
To ensure correct adsorption measurements, the solid samples were
rinsed with distilled water of the same pH value after the adsorption
experiments to remove any precipitated or entrapped oleic acid from the
solids. Control tests were conducted to check the oleic acid coating on
the surface of the vials. The results presented in Table 1 indicated
that approximately 20% of the initial collector present in the solution
reported as possible coating on the vial surface. Out of the total,
only 5% was determined to adsorb on the mineral (apatite) and less than
2% was found to be in the rinse solution at pH 4.5. This revealed that
there was not any significant desorption of oleic acid as a consequence
of rinsing of the solids. It should be noted that if the solution
depletion method were to be followed in this case, the amount adsorbed

TABLE 1
Sodium Oleate Distribution in Various Streams
During Adsorption Tests at pH 4.5
Initial Total
Oleate Concentration*
(kmol/m^)
Final Distribution of Oleate, %
On Mineral
On Vial
In Solution
In Rinse
Solution
1.88xl0"4
19.3, 20.0
5.7, 5.5
73.5, 72.7
1.6,
1.8
7.53xl0"4
22.5, 22.0
4.8, 4.9
72.2, 72.5
0.5,
0.6
*Total oleate concentration was assumed to be 100%.

26
on the mineral would have been calculated to be five-fold higher,
resulting in misleading conclusions. In addition, adsorption of oleate
on quartz was measured at pH 4 to ensure that the results obtained with
apatite and dolomite do not include any coating of oleic acid on the
surface. Since quartz is not expected to absorb oleate at pH 4, any
adsorption of oleic acid can be presumed to be physical coating.
Results of these tests indicated that there was no physical coating of
oleate on quartz, for up to a sodium oleate concentration of 6.0xl0-4
kmol/m^.
The amount of oleate or oleic acid adsorbed was therefore
determined in this study by direct counts on the solids in the oleic
acid precipitation region, i.e., at pH 8.0 and below. Above this pH
value, the amount adsorbed was determined by the solution depletion
method because test results indicated that oleate adsorption on the
vials is less than 0.5% of the total amount present.
Single mineral adsorption tests were conducted by adding 11 ml of
sodium oleate solution of the desired pH to 0.1 g of 65x100 mesh mineral
sample in a 20 ml polypropylene vial. The samples were then conditioned
by tumbling at 8 rpm for 5 minutes. The amount adsorbed on the minerals
at pH 8 and below (oleic acid precipitation region) was determined by
direct counts on the solids after cocktail addition. To determine the
amount adsorbed above pH 8, as mentioned earlier the solution depletion
method was followed. The solids were first allowed to settle for 5
minutes and the supernatant was analyzed for the amount of surfactant
depleted. This method was followed because no major surfactant
precipitation was anticipated at pH 8 and above.

27
The mixed mineral adsorption tests were conducted in a glass cell
arrangement as shown in Figure 4. This cell design allows mixed mineral
conditioning of apatite and dolomite in the same medium without coming
in contact with each other. A 0.5 g, 65x100 mesh sample was used for
these tests. The sample volume in each side of the cell was maintained
at 50 ml and the conditioning was identical to that of single mineral
experiments with respect to pulp density, conditioning time, and
tumbling speed. Adsorption tests in the presence of sodium chloride
were conducted in the same manner after aging the mineral samples in
2.0x10^ kmol/m^ NaCl.
Analysis of the labeled oleic acid was performed using a
Beckman Model LS 1800 liquid scintillation counter after mixing the
solids or solution with a scintillation cocktail, "Scintiverse-II"
(obtained from Fisher Scientific Company). In the case of mixed
minerals, the amount adsorbed was also determined directly on the solids
in the entire pH range since the solution depletion method could not be
followed.
Mineral Dissolution Tests
The dissolution of ions from apatite or dolomite at a given pH
value was determined by agitating one gram of 65x100 mesh solids in 100
ml of distilled water for a known time interval using a slow speed (8
rpm) tumbler. At the end of the tumbling period, a 10 ml sample of the
supernatant was withdrawn for analysis. The supernatant was centrifuged
at 734 G (2500 rpm) for 30 minutes to remove fines created during
agitation, before calcium and magnesium ion analysis was carried out

Figure 4. Apparatus for mixed mineral adsorption studies.

29
using a Perkin Elmer Plasma II inductively coupled plasma (ICP) emission
spectrometer.
Solubility Product Determination
The solubility products of calcium and magnesium oleate at pH 10
were determined using the nephelometric method. Known concentrations of
calcium and magnesium solutions were prepared from 1000 ppm standards
and were mixed with the sodium oleate solution of desired concentration.
The mixtures were initially stirred for a period of one minute and then
allowed to stand for 10 minutes before making turbidity measurements
using a Hach Model 2100 turbidimeter. The instrument was calibrated by
measuring the turbidity of solutions containing known concentrations of
sodium oleate and calcium/magnesium ions. The solubility product (Ks)
was obtained using the concentrations of calcium/magnesium and oleate
ions, at which a sharp increase in turbidity was observed.
FT-IR Tests
Samples used for FT-IR analysis were prepared as follows: A one
gram, -325 mesh size fraction, sample was suspended in 100 ml of
distilled water in a 100 ml volumetric flask and the pH was adjusted to
the desired value using HNO3 or K0H. After 20 minutes of aging, the
supernatant was partly replaced by the surfactant solution such that a
concentration of 5.0x10"^ kmol/m^ sodium oleate was obtained. The
suspension, with the added surfactant, was then tumbled for one hour at
8 rpm. At the end of the conditioning period, the solids were separated
from the supernatant by centrifuging the sample at 734 G (2500 rpm) for

30
30 minutes. Next, the solids were resuspended twice in distilled water
of the same pH value as the conditioning solution and centrifuged again
to remove the entrapped or free oleate solution from the solids. The
pure apatite and dolomite samples were also treated identically and
centrifuged the same way to remove water. The characterization of
adsorbed oleate species in the presence of sodium chloride was carried
out by aging the solids in 2xl0-^ kmol/m^ NaCl solution, followed by
conditioning in a 5x10^ kmol/m^ sodium oleate solution of the same NaCl
concentration used during aging. The solid samples were then dried at
50 C for 24 hours and stored in a vacuum desiccator until used.
Freshly ground, dried KC1 was used as a reference throughout these
experiments. The calcium and magnesium oleate precipitates were
obtained by mixing sodium oleate solution with those of calcium and
magnesium chloride, respectively. The reagentized pure mineral and the
precipitate samples were first mixed with 90% KC1 before introduction
into the diffuse reflectance cell.
The diffuse reflectance IR spectra were obtained on a B0MEM DA3.10
Fourier Transform Infrared Spectrometer equipped with a 25 cm path
length Michelson interferometer fitted to a KBr beam splitter. The
optical interferometer is connected to a high speed vector processor,
which performs the Fourier Transform and numerical fitting of the
collected interferograms. The spectra were recorded with a spectral
resolution of 0.5 cm/sec. Typically, 64 to 256 scans were obtained
under vacuum conditions to minimize interference from atmospheric
moisture and carbon dioxide.

31
Contact Angle Measurements
The samples for contact angle measurements were prepared by cutting
the rock using a diamond saw and mounting them on an epoxy base. The
surface of these samples were abraded using sandpaper with 400 grids and
polished using diamond cloth. The reagentization of samples was similar
to the method used for FT-IR analysis, except that aging and
conditioning in this case were performed by simply immersing the solid
in solution for 10 minutes. The solution was agitated using a magnetic
stirrer bar. After conditioning, the solid samples were dried in an
oven at 30 C before measuring the contact angle. The water droplets
were dispensed onto the sample surface using a micro-syringe. The
contact (tangent) angle formed between a "sessile" drop and the mineral
surface was determined directly, using a NRL Contact Angle Goniometer
Model 100-00. Following each test, the samples were abraded and re
polished to remove the surface coating before making subsequent contact
angle measurements.
Experimental Plan
The experimental plan employed in this investigation involved
a) Selection, preparation, and characterization of the minerals;
b) Selection of surfactant;
c) Selection of the experimental techniques; and
d) The experimental approach.

32
Selection, Preparation, and Characterization of the Minerals
Natural apatite and dolomite samples from Florida phosphate
deposits were selected. Both apatite and dolomite samples were crushed
and ground to obtain 65x100 mesh size fractions for flotation
experiments. A portion of these samples was ground to -325 mesh for
electrokinetic and FT-IR spectroscopic studies. The samples were
characterized for chemical composition, surface area and porosity, and
surface charge behavior. X-ray analysis and Scanning Electron
Microscopy studies were also carried out on the samples.
Selection of the Surfactant
Sodium oleate (anionic) and dodecylamine hydrochloride (cationic)
surfactants, which constitute the active components of fatty acid and
fatty amine collectors, respectively, were used in this study. It
should be noted that both of these surfactants hydrolyze significantly.
Nature of the dominant species present therefore would be governed by
the pH of the solution.
Selection of the Experimental Techniques
The experimental techniques selected are described in the following
section.
Flotation
Microflotation tests using a Hallimond cell was selected for
flotation studies. This technique enables close control of hydrodynamic
variables such as the agitation, air flow rate, and flotation time. It

33
should be noted that a direct correlation of Hallimond cell results to a
laboratory cell in terms of percent recovery, etc., may not be
appropriate, but the trends obtained are expected to be similar.
Electrokinetic measurements
Zeta potential measurements with and without modifiers in
combination with dissolution and flotation can lead to a better
understanding of the various species adsorbed at the solid/liquid
interface. Zeta potential of the minerals was measured by the
electrophoretic mobility technique using a Pen Kern Model 501 Laser Zee
Meter.
Adsorption
These tests were conducted to obtain information on the amount of
oleate/oleic acid adsorbed on the mineral surface under different
experimental conditions. Oleate adsorption measurements using
labeled oleic acid were conducted using a Beckman Model LS 1800 liquid
scintillation counter.
To avoid experimental artifacts in the adsorption studies, the
following precautions were taken:
(1) Oleate adsorption tests were conducted using the same 65x100 mesh
size fraction that was employed for flotation experiments.
(2) Reagentizing time of 5 minutes was maintained for both adsorption
and flotation to avoid variations due to kinetics involved.
(3) The exact pH value of oleic acid precipitation depends on the
total oleate concentration and the reaction constants used. Hence,

34
the low solubility of oleic acid in the neutral and acidic pH range
was taken into account to ensure correct analyses.
Determination of the nature of the adsorbed surfactant species
Adsorption studies are not expected to yield any information about
the nature of the various oleate species such as ionic oleate monomers
or acid-soap complex. Such information can be helpful in explaining the
differences in flotation behavior of these minerals under different pH
conditions. FT-IR spectroscopy was employed to study the nature of-the
adsorbed species, using a BOMEM DA3.10 model instrument.
Mineral dissolution studies
These tests can provide information about the dissolution
characteristics of various ionic species from the minerals which can be
helpful in explaining the surface charge and adsorption behavior.
Analysis of the dissolved ions such as calcium and magnesium was carried
out using a Perkin Elmer Plasma II, inductively coupled plasma (ICP)
emission spectrometer.
Contact angle
These measurements under different experimental conditions can
yield information about relative hydrophobicity of the mineral surfaces.
Contact angle measurements were conducted with and without salt
addition, using a NRL Contact Angle Goniometer (Model 100-00).

35
The Experimental Approach
The experimental approach involved the following steps:
(1) Flotation tests to identify the conditions for selectivity.
a) Study of the flotation response of apatite and dolomite
individually to determine the reagent (dodecylamine and oleate)
concentration and pH values where differences in the flotation
behavior are maximum.
b) Flotation of the mixed minerals to evaluate the selectivity
predicted by the single mineral tests, and investigation of the
reasons if observed selectivity is not achieved.
c) To select appropriate experimental conditions based on (b) to
achieve the desired separation.
(2) Elucidate the mechanisms by conducting the following studies:
a) Electrokinetic measurements as a function of pH with and without
chemical additives to establish the surface charge.
b) Adsorption tests under conditions selected on the basis of
flotation experiments.
c) A study of the nature of the surfactant species by FT-IR under
the conditions used for adsorption tests.
d) Evaluation of the relative hydrophobicity of the mineral
surfaces by contact angle measurements.

CHAPTER IV
RESULTS
Characteristics of Minerals
Chemical Analysis
Chemical analyses of the apatite and dolomite samples were
conducted using a Perkin Elmer Plasma II inductively coupled plasma
(ICP) emission spectrometer. It is clear from the data presented in
Table 2, that the apatite sample is essentially free of dolomite and
vice versa. The major impurity occurring in these samples is silica,
which is reported as acid insoluble.
Surface Area and Porosity
The surface area and pore size distribution of the minerals were
determined using nitrogen gas as the adsorbate with a Quantachrome
Autosorb-6 unit. Surface area measurements of apatite and dolomite
presented in Table 2 indicated that these samples are highly porous.
Pore size distribution of apatite and dolomite samples revealed that as
much as 95% of their surface area is contributed by pores less than 400
X in diameter. Average pore radius for both apatite and dolomite was
determined to be 82 X. SEM micrographs of 65x100 mesh apatite and
dolomite samples presented in Figures 5 and 6, respectively, further
confirm the high surface porosity. It is observed from these
micrographs that the pores in the apatite sample extend to the surface.
36

TABLE 2
Characteristics
of Apatite
and Dolomite Samples
Chemical
Analysis, %
Size Fraction
Surface Area
Mineral
(Mesh)
p25
MgO
Insol.
CaO
m2/g
Apatite
65x100
35.28
0.28
2.14
42.08
11.5
Dolomite
65x100
0.90
18.86
3.12
27.01
6.0

38
Figure 5. SEM micrograph of apatite (65x100 mesh size fraction),
a) 200X; b) 1000X.

39
Figure 6. SEM micrograph of dolomite (65x100 mesh size fraction), a)
100X; b) 1000X.

40
Dolomite, on the other hand, appears to have a relatively rough surface
morphology, but does not reveal any features of its porosity.
X-Ray Analysis
X-ray diffraction analysis of apatite, presented in Figure 7,
indicated the presence of a very small amount of quartz, however, even
after repeated scans no dolomite was found to be associated with
apatite. The dolomite sample exhibited the characteristic peak of
dolomite along with minor peaks for quartz and feldspar as seen in
Figure 8.
Surface Chemical Characterization
The zeta potential measurements were made using a Pen Kern Model 501
Laser Zee Meter. The isoelectric point (IEP) of apatite, as shown in
Figure 9, is at pH 5.4. This value is similar to that reported by
Chanchani (1984) and Somasundaran (1968). In the case of dolomite, two
isoelectric points, at pH 5.3 and pH 11.1, are observed (Figure 10).
The second IEP at pH 11.1 has been attributed to the presence of
hydroxylated magnesium species on the dolomite surface (Chanchani, 1984;
Iwasaki and Krishnan, 1983; and Balajee and Iwasaki, 1969).
Flotation Studies
Flotation Studies with Dodecylamine Hydrochloride
Single mineral flotation behavior
Flotation response of single mineral apatite and dolomite was
examined at two concentrations of dodecylamine hydrochloride as a

INTENSITY (COUNTS/SECOND)
TWO-THETA
Figure 7. X-ray diffractogram of apatite.

0000
4860
3840
2940
2160
1 500
960
540
240
0
Dolomite
h
-P*
rv>
5 26 27 28 29 30 31 32 33 34 35
TWO-THETA
Figure 8. X-ray diffractogram of dolomite.

ZETA POTENTIAL. mV
25
Apatite in Distilled Water
15 -
pH
GJ
25
Figure 9. Zeta potential of apatite as a function of pH.

30
20
10
0
10
20
30
Dolomita in Distilled Water
-P*
Figure 10. Zeta potential of dolomite as a function of pH.

45
function of pH. These tests were followed by mixed mineral flotation
tests under selected experimental conditions.
Apatite-dodecylamine system. Results of apatite flotation as a
function of pH at two levels of dodecylamine concentration are presented
in Figure 11. It is observed that at a dodecylamine hydrochloride
concentration of 1.0x10^ kmol/m^, apatite recovery is 100% between pH 4
and 10, and decreases precipitously beyond pH 10. However, at a
dodecylamine concentration of 1.6x10^ kmol/m^, apatite recovery
exhibits two maxima at pH 6 and 9.8, the flotation of apatite at pH 6
being about 50% of that at pH 9.8.
Polomite-dodecylamine system. Flotation response of dolomite as a
function of pH at a dodecylamine hydrochloride concentration of 1.0x10^
and 1.6xl0-4 kmol/m3 is illustrated in Figure 12. The amount floated is
observed to increase sharply at a dodecylamine concentration of 1.0x10^
kmol/m^ above the first IEP at pH 5.3 and to reach a 100% level at pH
5.8. However, flotation recovery starts decreasing above pH 9,
descending to 25% at pH 11, the value at which dolomite exhibits the
second IEP (refer to Figure 10).
The amount of apatite and dolomite floated as a function of pH at
a dodecylamine hydrochloride concentration of 1.0x10^ kmol/m^ is
compared in Figure 13. It is indicated from these results that apatite
can possibly be recovered selectively from its mixture with dolomite at
pH less than 4.5. At a dodecylamine concentration of 1.6x10^ kmol/m^,
another region of selectivity occurs at pH 9.8, as illustrated in
Figure 14.

AMOUNT FLOATED, WT%
Figure 11. Flotation of apatite as a function of pH.

Figure 12. Dolomite flotation as a function of pH.

AMOUNT FLOATED, WT%
FLOTATION pH
Figure 13. Flotation of apatite and dolomite (single minerals) as a function of pH.

AMOUNT FLOATED, WT%
Figure 14. Flotation of apatite and dolomite (single minerals) as a function of pH.

50
Attempts were therefore made to achieve the separation of apatite
from dolomite by conducting flotation tests using synthetic mixtures of
apatite and dolomite under the above specified pH conditions.
Flotation of apatite-dolomite mixture with dodecylamine
Results of 50:50 apatite and dolomite mixed mineral flotation
experiments under selected pH conditions and collector concentrations
are summarized in Table 3.
Although a preferential flotation of apatite was observed at pH
9.8 and pH 4.1 at both levels of dodecylamine concentrations, the
magnitude of the selectivity predicted by the single mineral experiments
was not realized.
The difference in the flotation response of these minerals at pH
9.8 and at a dodecylamine concentration of 1.6xl0-4 kmol/m^ also did not
correspond to the single mineral test results. In general, results of
mixed mineral experiments with dodecylamine as the collector indicated
depression of apatite and activation of dolomite in the selectivity
ranges predicted by the single mineral tests.
Flotation Studies Using Sodium Oleate as the Collector
Flotation behavior of apatite and dolomite using sodium oleate as
the collector was examined as a function of pH. Mixed mineral tests,
were also conducted under specific pH conditions.

51
TABLE 3
Flotation Results of 50:50 Apatite/Dolomite
Mixture Using Dodecylamine as the Collector
Flotation
Col 1ector
Chemical Analysis of
Apatite
Dolomite
pH
Cone.
(kmol/m3)
Float Fraction, %
Recovery
Reject
P205
MgO
(Weight %)
(Weight %)
4.10.2
l.OxlO'3
26.5, 26.6
4.2, 4.2
72.3, 74.7
53.4, 55.7
9.80.2
1.6xl0'4
25.4, 25.9
3.8, 4.0
53.3, 51.8
37.6, 38.2
Feed: 1 gram 65x100 mesh fraction
18.0% P205, 9.5% MgO

52
Single mineral flotation tests
Results of flotation tests as a function of pH using sodium oleate
as the collector are presented in Figure 15. At a sodium oleate
concentration of 4.0x10^ kmol/m^, flotation recovery of dolomite was
observed to be 100% in the acidic pH range (pH 4.0-5.5). Dolomite
recovery remained at 10-15% range between pH 7 and 10 and was seen to
increase above pH 10.
Apatite flotation in the acidic pH range, on the other hand,
exhibited a maximum at pH 4. The recovery of apatite increased sharply
above pH 6, and was observed to be 100% between pH 7 and 10.5. No
apatite flotation was observed between pH 5 and 6 at this level of
sodium oleate addition.
As observed from Figure 15, selective flotation of dolomite from
apatite or vice versa was predicted in the following pH ranges.
1) Flotation of dolomite from apatite at pH 5 to 6.
2) Flotation of apatite from dolomite between pH 7 and 10.
Mixed minerals
Results of the mixed mineral flotation tests using a 50:50 apatite
and dolomite mixture under given pH conditions are summarized in Table
4. It is observed that at pH 5.3, even though dolomite recovery was
greater than 95%, a significant amount of apatite (33%) also reported in
the float fraction. Consequently, apatite recovery in the sink fraction
was reduced to approximately 67%. In the alkaline pH range, apatite
recovery decreased to 68% from 100% observed in the single mineral
tests. On the other hand, dolomite flotation remained at the level

AMOUNT FLOATED, WT%
cn
OJ
Figure 15. Flotation of apatite and dolomite (single minerals) as a function of pH.

54
TABLE 4
Flotation Results of 50:50 Apatite-Dolomite
Mixture Using Sodium Oleate as the Collector
Flotation
Chemical
Analysis
Apatite
Dolomite
pH
of Concentrate, %
Recovery
Reject
P25
MgO
(Weight %)
(Weight %)
5.20.2
32.82, 33.36
1.22, 1.28
67.8, 66.1
96.0, 95.3
8.10.2
31.40, 30.64
1.32, 1.36
69.2, 68.6
92.4, 91.6
Collector Cone., 4.0xl05 kmol/m3
Flotation Feed: 1 gram, 65x100 mesh
size fraction, 18.0% P2O5, 9.5% MgO

55
predicted by the single mineral results. It appears that the changes in
the flotation response of apatite are the major cause for the loss of
selectivity in the mixed mineral system when sodium oleate is used as
the collector.
It is clear from the results presented that the loss of selectivity
in the mixed mineral system is primarily due to the activation or
depression of either apatite or dolomite depending on whether
dodecylamine hydrochloride or sodium oleate is used as the collector.
Moudgil and Chanchani (1985a and 1985b), and Soto and Iwasaki (1985 and
1986) have provided detailed explanations for the loss of selectivity in
this system.
In the case of the apatite/dolomite-amine system, Soto and Iwasaki
(1985) proposed that the adsorption of amine on dolomite is mainly
controlled by the electrostatic attraction in addition to weak chemical
interaction. The fact that flotation recovery of both apatite and
dolomite decreased sharply below their respective IEPs (at pH -5.4),
when dodecylamine was used (refer to Figure 14) and increased in the
case of sodium oleate (Figure 15), indicated that the coloumbic forces
play a significant role in the adsorption of the collector on the
respective substrates. It should, therefore, be possible to modify
flotation behavior of dolomite by the addition of indifferent
electrolytes such as sodium chloride.

56
Flotation tests in the presence of NaCI using dodecylamine as the
col 1ector
Single minerals flotation tests. Experiments conducted in the
natural pH range (pH 6.7) as shown in Table 5, indicated 85-90%
flotation recovery of apatite at a dodecylamine concentration of
1.6xl04 kmol/m^ and a NaCI concentration of 5.0xl0-*- kmol/m^. Apatite
recovery was observed to be only 50% under identical conditions without
NaCI addition (refer to Figure 11). In contrast, dolomite recovery
under similar experimental conditions remained at the 15-20% level. It
should be noted that the change expected in the flotation behavior of
dolomite in the presence of sodium chloride was not observed, but more
selective flotation of apatite was realized.
Mixed mineral flotation tests. Flotation of 88:12 apatite-
dolomite mixtures was conducted to determine the selectivity predicted
by the single mineral experiments in the presence of NaCI at natural pH
value. Results presented in Table 6 demonstrated that at a
dodecylamine concentration of 1.6xl0-4 kmol/m3 apatite can be
selectively recovered from the mixture leaving dolomite in the sink
fraction.
It should be noted that with dodecylamine hydrochloride, apatite,
the major mineral, is floated leaving dolomite in the sink. In
practice, however, flotation of the minor constituent is desired.
Further test work therefore, was conducted using sodium oleate as the
collector, which under slightly acidic pH conditions is known to yield
flotation of dolomite leaving apatite in the sink fraction.

57
TABLE 5
Effect of NaCl on the Single Mineral
Flotation of Apatite and Dolomite with
Dodecylamine Hydrochloride as the Collector
Mineral
Amount Floated,
Weight %
Without Salt
Wi th
Sal t
Apatite
50.0, 48.0
84.5,
90.8
Dolomite
17.0, 20.0
15.7,
20.2
Collector Cone., 1.6xl04 kmol/m^
NaCl Cone., 5.0x10"! kmol/m^
Feed: 1 gram, 65x100 mesh size fraction
Flotation pH: 6.7

58
TABLE 6
Results of Mixed Mineral Flotation in the Presence of
NaCl Using Dodecylamine as the Collector at pH 6.3
Test
No.
Chemical
of Float
p2o5
Analysis
Fraction, %
MgO
Apatite Recovery
in Float
(Weight %)
Dolomite Reject
in Sink
(Weight %)
1
33.65
0.97
79.0
71.8
2
33.49
1.02
79.8
69.8
3
33.59
1.03
81.2
69.1
Collector Cone., 1.6x10"^ kmol/m^
NaCl Cone., 5.0x1o--1- kmol/m^
Feed: 1 gram 88/12 Apatite-Dolomite Mixture
pH: 6.3

59
Flotation studies in the presence of NaCI using sodium oleate as the
collector
Results of mixed mineral flotation tests summarized in Table 7
indicated selective recovery of dolomite from the apatite and dolomite
mixture at pH 4 in the presence of 5.0xl0--*- kmol/m3 NaCI and at a sodium
oleate concentration of 4.0x10^ kmol/m3. Single mineral tests without
NaCI addition indicated 60% apatite and 95-100% dolomite recovery at
the same pH and collector concentration (see Figure 15). In the
presence of NaCI apatite was found to be depressed without any
significant effect on the flotation of dolomite. In order to achieve
maximum separation of dolomite single mineral tests were conducted to
determine the optimum pH, salt and collector concentration.
Optimum salt concentration for apatite depression. Mixed mineral
results indicated the best selectivity at pH 4, in the presence of NaCI
(see Table 7). Therefore, further tests to determine the optimum
concentration of NaCI were also conducted at this pH value. It was
determined that optimum results are obtained in the presence of 2.0x10^
kmol/m3 sodium chloride addition (see Figure 16). It is to be noted
that dolomite flotation is not affected by NaCI addition under the given
experimental conditions.
Optimum collector concentration. Results presented in Figure 17
and 18, indicated that at pH 10, apatite requires less collector to
float with and without salt addition, as compared to dolomite, and vice
versa at pH 4. The difference in the amount of sodium oleate required
to float 100% apatite or dolomite without NaCI addition was determined
to be about five-fold, and it more than doubled in the presence of
sodium chloride. From the data in Figure 17, the optimum sodium oleate

60
TABLE 7
Results of Mixed Mineral Flotation Tests in the
Presence of NaCI Using Sodium Oleate as the Collector
Flotation
pH
Chemical
P25
Analysis, %
MgO
Apatite Recovery
Weight %
Dolomite Reject
Weight %
10.9
18.0
9.5
100.0
0.0
8.2
18.5
9.5
100.0
0.0
6.6
18.5
9.5
100.0
0.0
4.0
33.3, 33.
8 0.64, 0.48
96.0, 94.8
96.6, 97.6
Collector Cone., 4.0x10^ kmol/m^
NaCI Cone., 5.0xl0_1 kmol/m^
Flotation Feed: 1 gram 50:50 Apatite-Dolomite
Mixture, 18.0% P205, 9.5% MgO

AMOUNT FLOATED, WT%
100
£)-0-
O
-
80
Singla Mlnarala
O Dolomita
A Apatita
Sodium Olaata Cone.,
4.0x10 kmol/m3
0 163 152 151 1.0
SODIUM CHLORIDE CONC.( kmol/m3
cn
Figure 16. Effect of sodium chloride addition on apatite and dolomite flotation.

80
60
40
20
0
-I I L.
-6
0
SODIUM OLEATE CONC., kmol/mv
O'!
rv>
17. Effect of sodium oleate concentration on apatite and dolomite (single
minerals) flotation with and without NaCl addition.

100
80
60
40
20
0
/ 8/

AA
/ /
/ 8
/
A
A
pH 4.0
NaCI Cone., Single Minerals
2.Ox 1 o a
I I I Hi
I . A J-A 1
-6
0
-5
10
-4
10
13
-2
10
SODIUM OLEATE CONC., kmol/m'
Flotation of apatite and dolomite as a function of sodium oleate concentration
in the absence and presence of sodium chloride (single minerals).
CT
OJ

64
concentration was determined to be 4.0xl0-5 kmol/m3, which incidentally
coincides with that used for single mineral experiments conducted
without NaCl addition. It is to be noted that at this level of sodium
oleate concentration at pH 4, apatite appears to be completely depressed
whereas dolomite flotation approaches 100%.
Optimum pH for separation. Results of single mineral apatite and
dolomite flotation tests as a function of pH are plotted in Figure 19.
It is observed that apatite remains depressed up to pH 4.2, but becomes
activated above this pH value.
Apatite flotation behavior in the presence of NaCl at pH 9.5 and
above is significantly different from that observed in distilled water
(refer to Figure 15). On the contrary, dolomite recovery, for the most
part, appears to be the same as that obtained in distilled water.
Following the above tests, flotation response of a 50:50 apatite-
dolomite mixture was studied as a function of pH in the presence of
2.0xl0-3 kmol/m3 NaCl and at a sodium oleate concentration of 4.0xl0~3
kmol/m3.
In general, the selectivity predicted by the single mineral tests
(see Figure 19) was maintained in the mixed mineral systems (see Figure
20). In addition, during the flotation of the mixed minerals, a more
effective apatite depression was realized in the pH range of 4.0-4.5 as
compared to single mineral test results. Optimum separation was
obtained at pH 4.00.2 where more than 95% apatite was recovered with
95% or more dolomite rejection. The MgO content of the sink fraction
(concentrate) was analyzed to be less than 0.7% from a feed containing
9.5% MgO.

AMOUNT FLOATED, WT%
100
80
60
40
20
0
Figure 19.
FLOTATION pH
Effect of pH on flotation of apatite and dolomite (single minerals) in the presence
of sodium chloride.

100
80
60
40
20
0
50/50 Mixed Minerals
Dolomite
A Apatite
-5 3
4.0x10 kmol/m
Sodium Oleate
-2 3
2.0x10 kmol/m
6 7 8
FLOTATION pH
9
10
1 1
1 2
cn
cn
Flotation recovery of apatite and dolomite (mixed minerals) as a function of pH
in the presence of sodium chloride.

67
Flotation studies in the presence of KC1 and NaF
To examine the effect of additional salts such as KC1 and NaF on
the flotation behavior of apatite and dolomite, single and mixed mineral
tests were conducted. Results of the experiments with KC1 in the pH
range of 3.0-5.5, using sodium oleate as the collector, are illustrated
in Figure 21. It is seen that in the case of single minerals about 80%
of dolomite is floated at pH 4 with only 5-10% apatite reporting in the
float fraction. The best results with the 50:50 mixture, however, were
obtained at pH 4.5, where 75% of dolomite and 5-10% apatite was
recovered in the float fraction. These tests were conducted in the
presence of 2.0x10"^ kmol/m^ KC1 (the optimum concentration for NaCl).
Comparison of apatite and dolomite flotation with and without
2.0x10-2 kmol/m^ NaF, as shown in Figure 22, indicated that dolomite
depression is relatively higher than that of apatite. Due to the
greater depression of dolomite with NaF, the difference in the
floatability gap of these minerals narrowed. In fact, some exploratory
mixed mineral experiments conducted at pH 4 using sodium oleate as the
collector indicated that in the presence of NaF, either both of these
minerals were depressed or floated together.
Electrokinetic Studies
Effect of Salt Addition on the Zeta Potential of Apatite
Zeta potential of apatite was measured as a function of pH in the
presence of 2.0x10^ kmol/m^ NaCl (optimum concentration for apatite
depression), KC1 and NaF. As seen from the results plotted in Figures

AMOUNT FLOATED, WT%
FLOTATION pH
Figure 21. Effect of KC1 addition on flotation of apatite and dolomite as a
function of pH.

80
60
40
20
0
/>/*
/ A
pH 4.0
NaF Cone., Singla Mineral
(kmol/m3) Dolomite Apatite
A None
2.Ox12
o A
/ *
j i hi ..u*i Qj iin
J i i t i.inii
0
r 6
i65
i64
,-o3
SODIUM OLEATE CONC., kmol/m
CT
LO
22.
Apatite and dolomite (single minerals) flotation with and without NaF
addition at pH 4.

70
23-25, the zeta potential of apatite in the presence of added salts
exhibited negative values in the entire pH range (4-11) examined,
indicating reversal of the surface charge below the IEP (pH 5.4). It
should be noted that ionic strength was not maintained constant because,
as discussed in a previous study by Chanchani (1984), KNO3 additions of
up to 1.0xl0"2 kmol/m^ did not significantly affect flotation recovery
of either apatite or dolomite.
The zeta potential of apatite in the presence of NaCl in the pH
range of 6 to 9 was found to be less negative than that in distilled
water. In the presence of KC1, at pH 6 and above, the zeta potential of
apatite was measured to be nearly the same as that in distilled water.
The fact that both NaCl and KC1 reversed the surface charge indicated
that they are not indifferent electrolytes for apatite. On the other
hand, because F is a lattice ion and therefore a potential determining
ion (Somasundaran and Wang, 1984) NaF was not expected to act as an
indifferent electrolyte for apatite. NaF decreased the zeta potential
of apatite in the entire pH range (4-11) examined. It also rendered the
surface more negative as compared to NaCl or KC1.
Effect of Salts on the Zeta Potential of Dolomite
The zeta potential of dolomite was measured in the presence of
salts as a function of pH. Zeta potential versus pH curves for
dolomite in the presence of NaCl and KC1, shown in Figures 26 and 27,
respectively, indicated that the IEP of dolomite (at pH 5.3) is not
influenced by NaCl or KC1 addition. The zeta potential values measured
below IEP did not appear to show any measurable change. Above the IEP,

25
20
15
10
5
0
-5
10
15
20
25
Apatite in Distilled Water
2
A Apatite in 2.0x10 kmol/m NaCI
Figure 23. Zeta potential of apatite with and without NaCI addition.

ZETA POTENTIAL, mV
20
10
Apatite in Water
Apatite in 2.0x102 kmol/m3
KCl Solution
Figure 24. Zeta potential of apatite as a function of pH with and without KC1 addition.

ZETA POTENTIAL, mV

pH
Figure 26.
Zeta potential of dolomite as a function of pH with and without sodium chloride.

ZETA POTENTIAL, mV
30
20
10
-20
-30 L
Dolomite in water
_ -2 3
O Dolomite in 2.0 x 10 kmol/m
KCI solution
cn
Figure 27. Effect of KCI on the zeta potential of dolomite.

76
however, both NaCl and KC1 reduced the zeta potential of dolomite from
-25 mV to -15 mV between pH 8 and 10. This decrease in the value of
zeta potential can be attributed to the compression of the electrical
double layer. It is suggested that the compression of the electrical
double layer occurred in the presence of the added electrolytes also
below the IEP. However, due to the low zeta potential values, these
changes could not be measured. In the case of sodium fluoride, however,
zeta potential of dolomite, as shown in Figure 28, was found to be
reversed below its IEP, as was the case for apatite. This is possibly
due to the adsorption of fluoride ions on the surface Ca++ and Mg++
sites for dolomite since the solubility of CaF2 and MgF2 (1.6x10"^ and
8.4x10-8) is rather low.
It should be mentioned that the maximum flotation selectivity was
achieved in the presence of NaCl, and the minimum with NaF. Further
studies were therefore, conducted to understand the reason for charge
reversal in the presence of only NaCl.
Role of NaCl in the Reversal of Surface Charge of Apatite
Charge reversal of apatite in the presence of sodium chloride could
be due to the selective dissolution of calcium and/or the substitution
of sodium for calcium sites in the apatite structure.
Substitution of sodium for calcium in the apatite lattice
Calcium, in addition to P0$~, OH" and H+, is the potential
determining ion (PDI) for apatite (Somasundaran, 1968; Samani et al.,
1975, and Somasundaran and Wang, 1984). Selective dissolution of

ZETA POTENTIAL, mV
30
20
O
Dolomite in Water
~2 3
Dolomite in 2.0x10 kmol/m NaF
10
-10
-20
-30 L
\
L
x6
8
10 / 12
pH
1 4
Figure 28. Zeta potential of dolomite in the presence and absence of NaF as a function of pH.

78
calcium from apatite can therefore render the surface more negatively
charged. Results of Ca++ and P0$" dissolution from apatite as a
function of time with and without NaCl addition are summarized in Table
8. An increase in Ca++ and a decrease in P0$dissolution in NaCl
solution relative to that of distilled water is observed from these
results. It is therefore possible that selective dissolution of calcium
can lead to surface charge reversal of apatite. This preferential
dissolution, however, was believed to be occurring as a result of sodium
substitution in the apatite structure, since dissolution was found to be
incongruent in the presence of added salt. This possibility was studied
by 1) measuring the change in the unit cell dimensions of apatite; 2)
analyzing the mineral sample for sodium content before and after
treatment with NaCl solution.
The change in the lattice parameters of apatite was studied by
computerized X-ray diffraction method. Results of the tests, summarized
in Table 9, indicated 0.006*0.001 8 decrease in the unit cell "a"
dimension of apatite when conditioned with NaCl solution. This change
was anticipated to be small, because the ionic radius of Na+ (0.95 K) is
only slightly smaller than that of Ca++ (0.99 K) (Brescia et al., 1966).
These tests were followed by determination of sodium uptake by apatite.
Results shown in Table 10, indicated that the amount of sodium present
in apatite after conditioning and rinsing is stoichiometrically
equivalent to the amount of calcium dissolved from apatite suggesting
mole per mole substitution of Na+ for Ca++ in the apatite structure.

79
TABLE 8
Apatite Dissociation at pH 4 with
and without Added Sodium Chloride
Time
(Minutes)
NaCl
Amount of Ions Dissolved, kmol/m3
In Distilled Water
In 2.0xl02 \
cmol/m3
Ca++
POj"
Ca++
poj-
05
8.75xl0'4
3.05xl0-4
1.05xl0'3
9.45xl0"5
30
1.60xl0"3
6.OOxlO-4
2.23xl0"3
2.21xl04
60
2.25xl0'3
8.40xl0-4
3.53X10'3
3.57xl0"4

80
TABLE 9
Effect of NaCl on Unit Cell Dimensions
of Apatite Conditioned at pH 4
Salt Cone. Measured Unit Cell Dimension (K)
(kmol/m2)
II II
a
"c"
None
9.348, 9.346
6.893, 6.894
7.5xl04
9.339, 9.340
6.895, 6.893
2.0xl0-2
9.342, 9.340
6.892, 6.894

TABLE 10
Determination of Substitution of Sodium for Calcium in the Apatite Structure
Sodium Chloride
Concentration
(kmol/m3)
Treatment
After
Conditioning
Sodium
Depletion
(kmol/m3)
Calciurn
Dissolution
(kmol/m3)
Apatite Unit Cell
Cell "a" Dimensions
(A)
None
None
None
8.9xl0-4
9.347
2.0xl0"2
None
4.7xl0-3
2.8xl0"3
9.340
2.0xl0'2
Rinsed
2.4xl0"3
2. 7xl0~3
9.340

82
Adsorption Studies
In order to investigate the mechanism of observed flotation
selectivity, adsorption of sodium oleate on apatite and dolomite was
studied. Adsorption experiments were conducted under conditions
identical to the flotation tests so that a meaningful correlation
between the two could be established.
Single Minerals Adsorption Tests
Apatite-oleate system
The amount of oleate adsorbed on apatite as a function of
conditioning pH is plotted in Figure 29 at three levels of sodium oleate
concentration. Two peaks, a larger peak at pH 8 and a smaller one
around pH 5, are observed. It is seen that the amount of oleate
adsorbed on apatite increased with an increase in the amount of sodium
oleate added.
Adsorption data on apatite as a function of sodium oleate concent
ration at pH 4 and pH 10 was also obtained. Results presented in
Figure 30 indicated that the amount of oleate adsorbed at pH 10 is about
five-times higher than that at pH 4, i.e., at a sodium oleate
concentration of 2.0xl0-4 kmol/m3). At a lower sodium oleate
concentration, however, this ratio appears to be different.
Effect of NaCl on sodium oleate adsorption on apatite. Selective
flotation of dolomite from apatite was found to be enhanced in the
presence of sodium chloride at pH 4 (see Figure 19 and 20). Adsorption
experiments were therefore conducted as a function of sodium oleate
concentration in order to determine the effect of NaCl (2.0xl0-2

SODIUM OLEATE ADSORBED, jumol/g
Figure 29. Oleate adsorption on apatite as a function of conditioning pH.

OLEATE ADSORBED, >4mol/g
INITIAL OLEATE CONO., kmol/m3
Figure 30. Oleate adsorption on apatite as a function of initial oleate concentration at
pH 4.0 and 10.0.

85
kmol/m3) on the amount of oleate adsorbed on apatite. Results of the
adsorption experiments illustrated in Figure 31, indicated a decrease in
the amount of oleate adsorbed on apatite in the presence of sodium
chloride as compared to that obtained in distilled water.
Dolomite-oleate system
The amount of oleate adsorbed on dolomite as a function of pH is
plotted in Figure 32 at three levels of sodium oleate concentration.
Adsorption behavior similar to that of apatite (refer to Figure 29) is
observed, however, the maxima in the alkaline pH range is shifted to pH
10 as compared to pH 8 for apatite. The amount of oleate adsorbed on
dolomite at pH 10 is comparable to that on apatite at pH 8. In the
acidic pH range, a smaller peak is observed at pH 4. Oleate adsorption
on dolomite was found to be minimal between pH 5.5-8.5. The amount
adsorbed was higher at higher dosages of sodium oleate.
Tests were also conducted at pH 4 and pH 10 to determine the amount
adsorbed on dolomite as a function of sodium oleate concentration. It
is observed from Figure 33, that the amount adsorbed at pH 10 is
approximately three-fold higher than that at pH 4 under most of the
concentration range examined.
Effect of NaCl on sodium oleate adsorption on dolomite. Results of
adsorption experiments, at pH 4 in the presence of 2.0xl02 kmol/m3
NaCl, as shown in Figure 34, indicated no measurable change in the
amount adsorbed on dolomite as compared to that in distilled water. It
should be noted that oleate adsorption on dolomite is nearly twice as

SODIUM OLEATE ADSORBED, yumol/g
Figure 31. Oleate adsorption on apatite at pH 4.0 in the absence and presence of
sodium chloride.

SODIUM OLEATE ADSORBED, imol/g
oo
CONDITIONING pH
Figure 32. Adsorption of oleate on dolomite as a function of conditioning pH.

OLEATE ADSORBED, ^mol/g
5.0 p
4.0 -
3.0 -
2.0 -
1.0 -
O
101
Figure 33.
O Dolomite at pH 10.0
Dolomite at pH 4.0

o
10 4
INITIAL OLEATE CONC., kmol/m'
Oleate adsorption on dolomite as a function of initial oleate concentration
at pH 4.0 and 10.0.
oo
00

SODIUM OLEATE ADSORBED, >umol/g
INITIAL OLEATE CONC., kmol/m3
Figure 34. Adsorption of oleate on dolomite at pH 4.0, with and without added
sodium chloride.

90
much higher than that on apatite in the absence of NaCl, and three times
as much in the presence of it.
Mixed Minerals Adsorption Studies
Adsorption in mixed mineral systems has not been studied in the
past because of experimental difficulties. The problem was overcome
during the present study by utilizing a special cell arrangement (see
Figure 4). Results of the mixed mineral adsorption tests as a function
of pH at a sodium oleate concentration of 3.6xl04 kmol/m^ along with
those of single minerals, are presented in Figure 35. An increase in
oleate adsorption on dolomite and a decrease on apatite in the pH range
of 7-10 is observed. In the acidic pH range, however, the trend is
observed to be reversed. It is clear from the results presented that
the adsorption behavior in mixed minerals systems follows the trends
exhibited by the single mineral tests.
Adsorption of oleate on apatite and dolomite at pH 4 (single and
mixed minerals) in the presence of NaCl was also examined. Results of
these tests, summarized in Table 11, indicated that adsorption on
dolomite in the presence of sodium chloride remains unchanged when
conditioned with apatite. The amount adsorbed on apatite, however, was
observed to decrease by more than 20%, indicating that, in the mixture,
dolomite adsorbs more oleate than apatite in the presence of sodium
chloride.
Characterization of the Adsorbed Oleate
Species by FT-IR Spectroscopy
FT-IR spectroscopic studies were conducted to characterize the
nature of the oleate species absorbed by apatite and dolomite with and

SODIUM OLEATE ADSORBED, >imol/g
Figure 35. Oleate adsorption on apatite and dolomite (single and mixed minerals) as
a function of pH.

TABLE 11
Effect of Sodium Chloride Addition on Oleate Adsorption
on Apatite and Dolomite at pH 4.
.0
Amount of Oleate Adsorbed
(pmol/g x
10)
Mineral
Single Mineral
Mi xed
Mineral
Apatite
5.1, 5.6
4.0,
4.3
Dolomite
9.1, 9.5
9.6,
10.0
rv>
Total Oleate Concentration = 3.6x10^ kmol/m^

93
without NaCl addition. Johnston and Leja (1978), and Chanchani (1984)
studied the same system by IR spectroscopy, however, the nature of the
species adsorbed on dolomite (oleic acid, or calcium- and magnesium-
oleate) could not be established in these studies due to strong
absorption of this mineral between 2000 and 1000 cm'-'-. The capability
of the currently available FT-IR spectroscopy equipment at University of
Florida to obtain difference spectrum by subtracting the pure mineral
spectrum from that of the mineral-surfactant permitted identification of
these species on dolomite in the present study.
IR Spectra of Pure Oleate Species
The diffuse reflectance IR spectra of sodium oleate and of calcium
and magnesium oleate precipitates in the 400 to 4000 wavenumber region
are illustrated in Figure 36. The IR spectra in the 1200 to 1800 cm"1
region have been enlarged in Figure 37 to highlight the differences
between the asymmetrical stretching frequencies of the carboxylate group
for the respective precipitates.
The main features of the pure oleate species spectra illustrated in
Figure 36, are the asymmetrical stretching vibration of CH2 groups at
2930 cm'1 and the asymmetric vibration of CH3 group at 2859 wavenumber.
Most of the bands in the region from 1400 to 1800 cm"'- are related to
the carboxylate groups. The characteristic band for sodium oleate was
located at 1560 cm"' as illustrated in Figure 37. Calcium oleate and
magnesium oleate bands were found at 1563 and 1583 cm"1, respectively.
The large difference between the bands for calcium and magnesium oleate
enables identification of these species on the mineral surface.

KUBELKA-MUNK UNITS
Figure 36.
Diffuse reflectance IR spectra of Mg-, Ca- and Na-oleate
(400-4000 cm~i range).

KUBELKA-MUNK UNITS
Figure 37.
Diffuse reflectance IR spectra of Mg-, Ca- and Na-oleate
(1200-1800 wavenumber region).

96
Nature of the Adsorbed Species at pH 10
Apatite-oleate system
The diffuse reflectance IR spectra of untreated apatite and the one
reagentized with 5.0x10^ kmol/m^ sodium oleate at pH 10 are shown in
Figure 38 along with the difference spectrum. The asymmetrical
stretching frequencies of CH3 group and CH2 groups, the bands at 2859
and 2930 cm'-'-, are observed in the difference spectrum as well as in the
spectrum of treated apatite and are not masked by the bands from the
mineral. The band observed in the difference spectrum suggests the
formation of calcium oleate on the apatite surface.
The diffuse reflectance IR spectra of pure apatite (i.e.,
untreated) as shown in Figure 38, indicated bands at 1090, 1054, 967,
604, and 574 cm-'-. These bands are in good agreement with those
identified by Gadsden (1975) at 1100-1080, 1050, 970-960 and 580 cm'1,
respectively. Apatite spectra was also characterized by Gnosh (1978)
with bands at 1040, 610 and 570 cm'1.
Dolomite-oleate system
The IR spectra of dolomite at pH 10 with and without adsorbed
oleate, along with the difference spectrum are illustrated in Figure 39.
This spectrum, in agreement with the magnesium oleate spectra, displayed
a band at 1582 cm"1, which indicated formation of magnesium oleate on
dolomite surface at pH 10.
The pure dolomite spectrum obtained in this study (Figure 39) shows
bands at 1460, 1413, 879 and 728 cm'1. Gadsden identified these bands

kubelka-munk units
1563
Figure 38.
Diffuse reflectance IR spectra of treated and untreated apatite
at pH 10.0.

KUBELKA-MUNK UNITS
Figure 39. IR spectra of untreated and treated dolomite at pH 10.0.

99
at 1480-1450, 1415, 896-875 and 727 cm"^. Dolomite bands identified by
Gadsden are apparently in a good agreement. The positions of the major
apatite and dolomite bands did not indicate any change after treatment
with either the collector or sodium chloride.
Nature of the Adsorbed Species at pH 4
Apatite-oleate system
The diffuse reflectance spectra of treated and untreated apatite
and the difference spectrum at pH 4 are exhibited in Figure 40. The
difference spectrum indicated the presence of oleic acid and calcium
oleate with bands at 1717 and 1563 wavenumbers, respectively, the
intensity for calcium oleate being relatively higher than that for oleic
acid.
Dolomite-oleate system
The IR spectra of treated and untreated dolomite at pH 4 are
illustrated in Figure 41. The difference spectrum in this case
indicated the existence of magnesium oleate and oleic acid on the
dolomite surface with bands at 1582 and 1717 cm-1, respectively. In
this case the intensity of the oleic acid band appeared to be higher as
compared to that of magnesium oleate. It is to be noted that at both pH
4 and pH 10, no calcium oleate formation on dolomite surface was
observed from the spectra obtained.

KUBELKA-MUNK UNITS
1563
Figure 40. Diffuse reflectance IR spectra of treated and untreated apatite at pH 4.0.

KUBELKA-MUNK UNITS
Figure 41. IR spectra of treated and untreated dolomite at pH 4.0.

102
Oleate Species Adsorbed in the Presence of NaCl
As stated earlier, selective flotation of dolomite from apatite was
achieved at pH 4. The adsorbed oleate species at this pH value,
therefore, were also characterized in the presence of sodium chloride.
Apatite-NaCl-oleate system
The diffuse reflectance IR spectra obtained on apatite at pH 4 are
presented in Figure 42. To study the effect of NaCl, both pure and
treated apatite samples were first conditioned in 2.0x10^ kmol/m^ NaCl
solution. The treated apatite sample was then conditioned in 5.0x10^
kmol/m^ sodium oleate solution also prepared in 2.0x10^ kmol/m^ NaCl.
The difference spectrum in Figure 42, indicated the presence of only
oleic acid on apatite surface. It is to be noted that both calcium
oleate and oleic acid were observed on apatite in the absence of sodium
chloride at pH 4.
Polomite-NaCl-oleate system
The IR spectra of untreated and treated dolomite in the presence of
2.0x10"^ kmol/m^ NaCl solution at pH 4 are presented in Figure 43. The
bands in the difference IR spectrum of Figure 43, at 1717 and 1582
wavenumbers are the same as those obtained without salt addition,
indicating that the nature of the adsorbed species on dolomite (oleic
acid and magnesium oleate) remains unchanged.

<0
I-
*
z
3
I
<
*
-I
LU
OS
3
*
o
co
Figure 42. Diffuse reflectance IR spectra of apatite (treated and untreated) and the difference
spectrum at pH 4.0 in the presence of NaCl.

KUBELKA-MUNK UNITS
1717
Figure 43. Diffuse reflectance IR spectra of dolomite at pH 4.0 in the presence of
sodium chloride.

CHAPTER V
DISCUSSION
The adsorption characteristics of a surfactant and the resultant
flotation behavior of the minerals depends on the nature and the
solution properties of the collector in addition to parameters such as
surface composition and surface charge of the substrate. The chemical
and physical properties of the collector are determined by the
hydrocarbon chain and the ionic head. The ionic head determines
whether the collectors are strong electrolytes, that ionize completely
in solution, or weak electrolytes, which ionize only slightly and
hydrolyze in solution to form various species including the neutral
molecules. A discussion of the solution chemistry behavior of sodium
oleate and dodecylamine hydrochloride is presented below.
Solution Properties of Dodecylamine Hydrochloride and Sodium Oleate
The important role of the solution chemistry of collectors such as
dodecylamine and oleic acid in governing the flotation has been well
documented by Ananthapadmanabhan (1980), Somasundaran and
Ananthapadmanabhan (1979a), and Ananthapadmanabhan et al. (1979).
Using the thermodynamic data of Ananthapadmanabhan (1980), species
distribution diagrams for dodecylamine and oleic acid were generated in
the relevant concentration ranges.
105

106
The chemical equilibria used to calculate the distribution of the
species for dodecylamine hydrochloride were as follows:
RNH$ t RNH2 + H+ pKa = 10.63
2RNH3 t (RNH3)^+ pKd = -2.08
RNH2 + RNH3 t (RNH2.RNH3)+ pKad = -3.12
RNH2(i) t RNH2(ap) PKsoi = 4.69
The species versus pH diagram for a total dodecylamine
hydrochloride concentration of 1.6x10"^ kmol/m^ is illustrated in Figure
44. The maximum in amine-aminium complex formation at this
concentration is observed at pH 10. The pH of maximum flotation for
minerals such as quartz has been found to correspond to the pH of
formation of maximum amine-aminium complex (Ananthapadmanabhan, 1980).
It has been shown by Pugh (1986) and Ananthapadmanabhan (1980) that the
surface activities of the association complexes such as amine-aminium
are higher and, even if present in small amounts, they can make
significant contributions to the flotation process.
The species distribution diagram for a total oleate concentration
of 4.0x10^ kmol/m^ using the following chemical equilibria
(Ananthapadmanabhan, 1980) is presented in Figure 45.
RH(1) t RH(aq)
RH(aq) R~+H+
2R" t r£"
RH + R t RoH"
P^sol 7-60
pKa = 4.95
pKd = -3.70
PKad = -7.10

LOG (ACTIVITY OF THE SPECIES)
Figure 44. Dodecylamine species distribution as a function of pH. Total amine
concentration, 1.6 x 10~^ M.

LOG (ACTIVITY OF THE SPECIES)
-2
SOLUTION pH
o
oo
Figure 45. Oleate species distribution as a function of pH. Total oleate
concentration, 4.0 x 10^ M.

109
It is noted from the diagram (Figure 45) that oleic acid formation
at 4.0x10^ kmol/m3 oleate concentration begins to occur at pH 8. The
maximum in the acid-soap complex, (RC00H.RC00)-, concentration is also
observed at this pH value.
Apatite-Dolomite Flotation Using Dodecylamine as the Collector
Apatite-Dodecylamine System
Flotation of apatite with dodecylamine as a function of pH at two
levels of dodecylamine is illustrated in Figure 11. It is observed that
at a dodecylamine concentration of l.OxlCT^ kmol/m^, apatite flotation
is 100% both above and below its IEP (pH 5.4). At a lower collector
concentration (1.6xl0-^ kmol/m^), two flotation peaks, at pH 4 and pH
10, were obtained.
Continued apatite flotation below its IEP was attributed to the
chemical interaction between the collector ion and the mineral surface,
as has been pointed out by Soto and Iwasaki (1985). It is presumed that
the hydrophobic interactions of the hydrocarbon chains also play a role
in the adsorption process in addition to possible hydrogen bonding. It
is equally important, however, to realize the role of the solution
chemistry of the surfactant in the interpretation of these results, as
has been emphasized by Ananthapadmanabhan (1980) and Somasundaran and
Ananthapadmanabhan (1979b).
Analysis of the species distribution diagram for the 1.6xl04
kmol/m^ total dodecylamine concentration, as shown in Figure 44,
indicated that the maxima in the iono-molecular complex, (RNH2-RNH3)+,

no
occurs at pH 10. As mentioned earlier, this complex is highly surface
active than the monomer or the dimer because of the increase in its
molecular size and single charge (Ananthapadmanabhan et al., 1979) and
its low intrinsic solubility (Pugh, 1986).
The peaks observed in the flotation response of apatite are
explained as follows. The flotation peak at pH 10 can be ascribed to
the maxima in the concentration of iono-molecular complex. As the pH
decreases, the concentration of the complex (RNH2 RNH3)*" decreases
approaching a value of 1.0x10^ kmol/m^ at pH 8 at a total dodecylamine
concentration of 1.6x10^ kmol/m^ (see to Figure 44). Below pH 8, the
amine monomers and dimers are observed to become the dominant species
which can adsorb on the negatively charged apatite surface through
electrostatic attraction in addition to the adsorption due to specific
interaction (Soto and Iwasaki, 1985). The reasons for the smaller peak
at pH 6 in the flotation recovery of apatite, however, are not clear. A
gradual decrease in apatite flotation below pH 6 can be due to the onset
of the electrostatic repulsion between the collector cation and the
positively charged apatite surface.
Polomite-Dodecylamine System
Flotation recovery of dolomite using dodecylamine as the collector
as a function of pH (refer to Figure 12) appears to follow its zeta
potential behavior (Figure 10), suggesting that the adsorption of
dodecylamine on dolomite is mainly governed by the electrostatic
attraction. Considering that both the substrates and the collector are
positively charged below the IEP of dolomite, no flotation would be

Ill
expected. Flotation of dolomite (10-15%) below the isoelectric point,
at a dodecylamine concentration of 1.6xl04 kmol/m3 therefore indicates
the presence of weak specific interaction in addition to the coulombic
attraction. The heat of reaction of dodecylamine with phosphate and
carbonate anions (-15.1 and -3.3 kJ/mol for apatite and dolomite,
respectively) as determined by Soto and Iwasaki (1985) supports this
hypothesis. This indicates that the collector adsorbs more
preferentially on apatite than dolomite when both of the minerals are
present. The flotation results obtained in this study and those of Soto
and Iwasaki indicate compatible trends.
Flotation of Apatite and Dolomite Mixture with Dodecylamine
The single mineral flotation results indicated that apatite can
possibly be selectively floated out from dolomite at pH a less than 4.5
at dodecylamine concentration of l.OxlO-3 kmol/m3, or at pH 9.8 and
dodecylamine concentration of 1.6x10"^ kmol/m3. However, the
selectivity predicted by the single mineral flotation data of apatite
and dolomite was found to be limited in the case of their mixture (refer
to Table 3). The loss in selectivity was observed to be due to the
lower flotation of apatite and increased recovery of dolomite in the
float fraction as compared to the results of the single minerals tests.
This could be due to one or more of the following reasons:
a) Change in the interfacial potential or the chemical composition
of the minerals due to dissolution or adsorption of dissolved
species;

112
b) Coating of the mineral particles by precipitates or slimes
generated during conditioning;
c) Depletion of the surfactant species by complexation and
precipitation with dissolved ionic species.
Changes in the surface charge and surface chemical composition
The surface charge and surface chemical composition of the minerals
can undergo modifications due to the dissolution of lattice ions.
Moudgil and Chanchani (1985a) and Chanchani (1984) have reported that
the Ca++ and Mg++ ions dissolving from dolomite cause a significant
reduction in the flotation of apatite when sodium oleate is used as the
collector. It was also found that when these ions are present at
1.0x10^ kmol/m3 level, they can reverse the sign of the zeta potential
of apatite. In addition, it was established that the zeta potential of
dolomite is affected by the phosphate ions, even at concentrations as
low as l.OxlO-3 kmol/m3 level. Amankonah et al. (1986), in their study
of the apatite-calcite system, have found that the IEP of apatite
interchanges with that of calcite when conditioned in calcite
supernatant, despite the fact that the IEP of these minerals were
determined to be four pH units apart in distilled water (at pH 6.5 and
10.5, for apatite and calcite, respectively). In the present study,
the amounts of Ca++ and Mg++ dissolving from dolomite and calcium from
apatite were determined to be 2.7xl0-3 and l.lxlO-3 kmol/m3,
respectively, at pH 4 after 5 minutes of conditioning (Table 12 and
Table 8). It is therefore possible that both the surface charge and
surface chemical composition of apatite and dolomite could have been

TABLE 12
Dissolution of Calcium and Magnesium from Dolomite at pH 4.0
With and Without NaCl Addition
Time
(Minutes)
Sodium Chloride
Cone.
(kmol/ni3 x 102)
Amount Dissolved, kmol/m3
Concentration Ratio
[Ca++]
[Mg++]
[Ca++]/[Mg++]
05
None
1. 41xl0-3
1.21xl0"3
1.165, 1.133
1.45xl0"3
1.28xl0-3
30
None
1.87xl0'3
1.68xl0~3
1.113, 1.109
1.83xl0-3
1.65xl0'3
05
2.0
4.60xl0'4
4.00xl0"4
1.150, 1.200
5.05xl0"4
4.21xl0~4
30
2.0
8.95xl0-4
7.88xl0-4
1.136, 1.156
7.95xl0-4
6.88xl0-4

114
affected when conditioned simultaneously in the mixture. As a
consequence, the surface charge of apatite can be less negative or even
positive in the alkaline pH range, whereas that of dolomite would become
more negative. Hence, adsorption of dodecylamine on apatite can be
expected to decrease due to electrostatic repulsion between the surface
and the cationic collector, thus leading to its reduced flotation as
compared to that in the single mineral flotation tests.
Surface coating
Soto and Iwasaki (1986) and Dufour et al. (1980) have reported
that dolomite is softer than apatite. Consequently, dolomite slimes
could be generated during the conditioning stage which could affect
flotation of apatite. Moudgil and Chanchani (1985a) tested this
hypothesis by floating apatite in the supernatant of dolomite with and
without fines present and observed no change, indicating that dolomite
fines do not affect apatite flotation.
Surfactant depletion by precipitation
Soto and Iwasaki (1985) determined the solubilities of the salts
formed upon reaction of dodecylamine with phosphate and carbonate ions
to be 1.5xl0-3 and 5.0xl0-3, respectively. In the alkaline pH range,
where the solubility of these minerals is low, complexation and
precipitation of dodecylamine ions with anions such as P0$ and C0^" is
not expected to occur to any significant extent and, therefore, could
not explain the loss in the selectivity as observed.

115
At pH 10 and dodecylamine concentration of 1.6xl04 kmol/m3,
apatite recovery, however, was observed to decrease while dolomite
flotation increased (see Table 3). Since, in the case of mixtures,
there are more cations (Ca++ and Mg++) present as compared to single
mineral alone; apatite is expected to acquire a less negative surface
charge in the mixture. Consequently, adsorption of ionic dodecylamine
species on apatite possibly decreased leading to reduced flotation
recovery. On the other hand, dolomite flotation under similar
conditions increased probably as a result of adsorption of phosphate
ions dissolving from apatite that renders its surface more negatively
charged (Chanchani, 1984).
Results of mixed mineral tests at pH 4 and at a dodecylamine
concentration of l.OxlO'3 kmol/m3 indicated a significant increase in
the recovery of dolomite, and only a slight decrease in the apatite
flotation (see Table 3). Thus it appears that dodecylamine adsorption
on dolomite increased substantially in the mixture. The increased
dissolution of apatite in the acidic pH range can lead to adsorption of
more phosphate ions on dolomite causing its surface to acquire a more
negative surface charge. It should be noted that the solubility of Mg-
phosphate is less than that of Ca-phosphate and, therefore, more
phosphate ion adsorption on the dolomite surface can be expected.
Dodecylamine adsorption on dolomite therefore can be presumed to have
increased under these conditions resulting in higher flotation.

116
Flotation of Apatite and Dolomite Using
Sodium Oleate as the Collector
Results of flotation tests conducted as a function of pH using
4.0xlCT5 kmol/m3 sodium oleate as the collector (see Figure 15)
predicted selectivity under the following conditions.
Flotation of apatite from dolomite between pH 7 and pH 10;
Flotation of dolomite from apatite in the pH range of 5 to 6.
Apatite recovery in the mixed mineral system, under alkaline pH
conditions, decreased significantly as compared to that of the single
mineral tests, while under acidic pH conditions (pH 5 to 6) it increased
(refer to Table 4). Similar results were obtained by Moudgil and
Chanchani (1985a) in the apatite-dolomite system. It was determined by
these investigators that selectivity is mostly affected by changes in
the flotation response of apatite. They attributed the loss in the
selectivity to the presence of excess Ca++ and Mg++ ions dissolved from
dolomite as well as to the depletion of oleate by precipitation as
calcium and magnesium oleate. As mentioned above, there are more
dissolved species present in the mixture as compared to single mineral
apatite because of higher solubility of dolomite. It should be noted
that precipitate formation would reduce the effective collector
concentration in the system. The dissolved Ca++ and Mg++ ions are
expected to adversely affect apatite flotation as has been determined by
Moudgil and Chanchani (1985a).
Evaluation of the Results and Alternatives
Flotation separation of apatite and dolomite was predicted under
certain experimental conditions by the single mineral results. It was,

117
however, shown that only limited separation can be achieved in the
mixtures. To achieve high recovery and selectivity, previous
investigators (Lawver et al., 1982b; Llewellyn et al., 1982; Dufour et
al., 1980; Johnston and Leja, 1978; Dahl in and Fergus, 1978; Samani et
al., 1975; Onal, 1973; and Rule et al., 1970) have suggested the use of
surface modifying agents. Phosphate salts have generally been used to
depress apatite in most of these studies. A detailed study of the
mineral/surfactant interaction was conducted by Moudgil and Chanchani
(1985a and 1985b), and Soto and Iwasaki (1985 and 1986) who studied
flotation of apatite and dolomite with anionic and cationic surfactant
systems, respectively.
Moudgil and Chanchani (1985a) concluded that there is a strong
chemical interaction between oleate and apatite which has been
attributed to the chemisorption of oleate anions on the calcium sites on
the surface of apatite in the pH range of 7 to 10. In addition,
adsorption of acid-soap complexes was also considered in the alkaline pH
range. It was indicated that oleate adsorption on dolomite is mostly
governed by electrostatic attraction. However, the possibility of
chemisorption of oleate ions on surface calcium and magnesium sites was
not ruled out in the alkaline pH range (Chanchani, 1984).
Soto and Iwasaki (1985) indicated that the adsorption of
dodecylamine on dolomite is essentially due to the electrostatic
attraction along with some chemisorption. It was noted in the above
study that the specific interaction of the dodecylamine ion is stronger
for phosphate as compared to the carbonate anion.

118
Based on the results obtained in the present study and those
reported in the literature, the following conclusions can be reached
about the adsorption mechanism:
a) Flotation of dolomite as a function of pH with either dodecyl-
amine or sodium oleate as the collector mostly appeared to fol
low its electrokinetic behavior. This indicated that the sur
factant adsorption on dolomite is governed by the electrical
double layer forces, i.e., electrostatic attraction for the most
part.
b) The fact that dolomite flotation with sodium oleate was higher
than that of apatite below pH 5, despite the fact that both
minerals have the same magnitude of zeta potential, indicates
that the collector adsorption is more specific for dolomite.
Higher flotation of dolomite in the acidic pH range was also
reported by Moudgil and Chanchani (1985a), Johnston and Leja
(1978) and Ratobylskaya et al. (1975). It is possible that more
oleic acid molecules adsorb on the dolomite surface along with
charged oleate species due to the higher density of surface
hydroxyl groups on the surface of this mineral as has been
pointed out by Shah (1986).
c) Apatite flotation as a function of pH with either of the
collectors did not seem to correlate completely with its
electrokinetic behavior.
It appears from the above discussion that the selectivity in the
apatite-dolomite system could be improved by manipulation of the
electrical double layer forces. Such a change can be expected to

119
increase or decrease adsorption of the collector on the mineral surface
due to electrostatic interaction depending on the effect of the
electrolyte added. The electrostatic attraction between the mineral
surface and the ionic surfactant species can be altered by compressing
the electrical double layer, e.g., by adding an (indifferent)
electrolyte. The electrical double layer compression on apatite and
dolomite can lead to selective flotation of apatite with the collector
such as dodecylamine hydrochloride due to its more preferential
adsorption for apatite.
Sodium chloride was used to manipulate the surface charge of the
two minerals. Its effect on flotation of apatite and dolomite with
dodecylamine hydrochloride or sodium oleate as the collector are
discussed next.
Effect of NaCl on the Selective Flotation of
Apatite Using Dodecylamine as the Collector
Results of apatite and dolomite flotation tests (see Table 6)
indicated selective recovery of apatite with dodecylamine as the
collector in the presence of sodium chloride. Dolomite was observed to
be depressed in the presence of added salt.
Subsequently, the possibility of floating dolomite and depressing
apatite by utilizing anionic collector such as sodium oleate was
investigated. In practice, flotation of the minor mineral (i.e.,
dolomite) is preferred over that of the major mineral (i.e., apatite),
because it would generally require less collector.

120
Effect of NaCI on the Separation of Dolomite
From Apatite Using Sodium Oleate as the Collector
Dolomite was selectively floated out from the mixture of apatite
and dolomite in the presence of 5.0xl0l | concentration of 4.0x10^ kmol/m^ at pH 4 (Table 7). In these tests,
more than 95% of the dolomite was floated out at 95% or higher apatite
recovery level. The MgO content of the concentrate (sink fraction) was
reduced to less than 0.7% from a feed containing 9.5% MgO. Mixed
mineral flotation experiments without the use of NaCI under similar
conditions yielded apatite recovery of 45%, with the concentrate
analyzing 1.7% MgO from the same 50:50 apatite/dolomite feed. Thus, it
was shown that apatite can be effectively depressed in the presence of
sodium chloride without influencing the flotation behavior of dolomite.
In further test work, the optimum salt concentration (Figure 16),
oleate concentration (Figures 17 and 18), and pH of separation (Figure
19) was determined through single mineral flotation studies. Additional
mixed mineral flotation experiments as a function of pH revealed that
dolomite can be selectively separated from apatite at pH 4.0 in the
presence of 2.0x10^ kmol/m^ NaCI at a sodium oleate concentration of
4.0x10^ kmol/m^ (Figure 20).
The single and mixed mineral flotation tests under the above
experimental conditions were also conducted by replacing NaCI with KC1
or NaF (Figure 21 and 22). Unlike the results obtained with NaCI,
dolomite flotation was found to be lower along with lower apatite
recovery when KC1 was used. In the case of NaF, both apatite and
dolomite were found to be depressed.

121
Mechanism of Selective Flotation of Dolomite From Apatite
Sodium chloride addition was expected to modify the magnitude of
the surface charge on apatite and dolomite particles possibly by
compressing the electrical double layer. It was also expected to
subsequently effect the adsorption process. Surfactant adsorption and
the resultant flotation of apatite and dolomite was observed to have
been affected as was presented in the previous chapter. Reasons for
these changes are discussed in the following sections.
Effect of NaCI on the Zeta Potential of Apatite and Dolomite
Zeta potential of dolomite in the presence of NaCI decreased above
its IEP (pH 5.3) which is indicative of compression of the electrical
double layer (EDL) by the added salt (Figure 26). It should be noted
that possible EDL compression also occurred below the IEP, but the
change was not detected due to the low values of zeta potential measured
in this pH range. It can, therefore, be concluded that NaCI acts as an
indifferent electrolyte for dolomite. A similar behavior was also
recorded in the presence of KC1 which too can be considered as an
indifferent electrolyte (Figure 27) for dolomite.
Zeta potential of apatite in the presence of NaCI, on the other
hand, was observed to be negative in the entire pH range (4-11) examined
(Figure 23), indicating that this salt does not act as an indifferent
electrolyte for apatite. The effect of salt addition on the surface
charge behavior of apatite was more striking in the acidic pH range
possibly because of its higher solubility under such conditions.
Apatite exhibited a less negative zeta potential value at pH 7 in the

122
presence of NaCI as compared to distilled water alone. A similar trend
was also observed in the presence of KC1 (Figure 24), even though KC1
appeared to render the surface less negative at pH 5 and below as
compared to NaCI.
In the presence of NaF, unlike the trends observed with NaCI and
KC1, dolomite indicated a surface charge reversal below its IEP (Figure
28). Apatite also exhibited a negative zeta potential value in the
presence of NaF (Figure 25), even more than that observed with NaCI and
KC1. Both apatite and dolomite were either depressed or floated
together with sodium oleate in the presence of NaF as illustrated in
Figure 22.
It is concluded that not all of the salts mentioned above act as
indifferent electrolytes for apatite. However, they appear to be
indifferent for dolomite with the exception of NaF which also depressed
the flotation of dolomite. Since the best selectivity was observed only
when sodium chloride was added to the system, the mechanism of the
surface charge reversal for apatite, at pH 4, will be discussed only in
the presence of this salt.
Role of NaCI in the reversal of surface charge of apatite
The surface charge of a substrate, in the absence of any other
added salt, is determined by the concentration of potential determining
ions (PDI) in solution. For apatite the PDI's are its lattice ions such
as Ca++ and P0| or their reaction products in water including H+ and
OH". The mechanism by which PDI determine the surface charge of

123
apatite in water can be represented by the following reactions as
discussed by Somasundaran and Wang (1984):
C¡+ + OH" t CaOH+ (1)
CaOH+ + OH" t Ca(0H)2(aq) (2)
Ca(0H)2(aq) ^ Ca(0H)2(Surf) (3)
H3P04 H+ + H2P04' (4)
H2P04" t H+ + HPOj" (5)
HPOj" H+ POj" (6)
It is clear that when the pH is increased the equilibrium will be
driven toward the right hand side in equations 1 to 6. This would
increase the concentration of negative PDI's in solution which will
render the surface negatively charged. When the pH is decreased, these
reactions will move in the opposite direction and the surface will
become positively charged.
It was shown that in the presence of NaCl the surface charge of
apatite reverses below its IEP and attains a negative zeta potential
value (Figure 23). This change was believed to have resulted from the
selective dissolution of calcium and substitution of sodium in the
apatite structure. Such process, i.e., substitution of Na+ for Ca++ in
the structure, could lead to the development of a more negative surface
charge because of the difference in their valency. For example, if in
an array of solid Si02 tetrahedra, an atom of Si (4+ valency) is
replaced by an A1 atom (3+ valency), a negatively charged frame work
will result (Stumm and Morgan, 1981). Substitution of sodium for

124
calcium in the apatite structure can similarly render the surface more
negatively charged.
Calcium dissolution and sodium substitution in apatite structure.
As a result of sodium substitution for calcium, the amount of calcium
dissolved from apatite was expected to increase with an anticipated
decrease in the concentration of sodium in solution. A possible change
in the lattice parameters was also expected. It was, however, realized
that the change in the lattice parameters would be rather diminutive
since the ionic radii of sodium (0.95 8) is only slightly smaller than
that of calcium (0.99 K).
Examination of the amount of calcium dissolved from apatite with
and without added NaCl as shown in Table 8, indicated that during the
first 5 minutes (the conditioning time for flotation) calcium
dissolution increased by 1.2 times in the presence of NaCl relative to
that in distilled water. This increase in calcium dissolution in the
presence of NaCl along with a decrease in the rate of P0$ dissolution
suggested that sodium is possibly substituting for calcium in the
apatite structure. The reasons for decreased P0$~ dissolution are not
yet clear. However, it is possible that the rate of dissolution of
phosphate ions decreased because of the increased ionic strength of the
solution. Nevertheless, this process is partly responsible for
development of a negative surface charge besides increased calcium
dissolution. The incongruent solubility of apatite in the presence of
NaCl, i.e., higher Ca/P ratio, was also observed by Levinskas and Neuman
(1955). These investigators attributed the phenomena to Na+

125
substitution for Ca++, and postulated that the surface charge would
possibly be influenced.
Substitution of sodium for calcium in the apatite structure was
investigated in the present study by determining the unit cell
parameters (Table 9) and analyzing the treated and untreated apatite
samples for their sodium content (Table 10). The unit cell dimension
"a" as measured by computerized X-ray diffraction technique indicated a
decrease of 0.0060.001 ft with a standard deviation of 0.001 ft. The
difference, as indicated earlier, was anticipated to be small since the
ionic radii of sodium and calcium are comparable in size.
In addition to lattice parameters, sodium substitution was
confirmed through chemical analysis of the samples treated in the
absence and presence of NaCl. It was shown that the amount of excess
Na+ present in apatite is approximately stoichiometrically equivalent to
the excess calcium dissolved from apatite in the sodium chloride
solution. Stoichiometric substitution of sodium for calcium in the
apatite structure was also observed by Stoll and Neuman (1956), who
studied uptake of Na+ from solution. They postulated that Na+ ions
substitute in the apatite structure and possibly result in the
compression of the electrical double layer. In the present study,
attempts were made by using ESCA to determine if there is any excess
sodium present on the surface of apatite after treatment, but no
conclusive evidence was obtained. This is because the apatite sample
used in this investigation contained sodium (possibly as a substitute
for calcium) in the structure.

126
Dolomite structure
Although the use of NaCl was intended for compression of the EDL
and reduction of the electrostatic attraction between the mineral
surface and the surfactant, in the preceding discussion, it was shown
that it did not act as an indifferent electrolyte for apatite. However,
no changes were observed in the zeta potential of dolomite below its
IEP. The possible reasons for this behavior are as follows.
The dolomite structure is similar to the calcite structure in many
respects. The hexagonal unit cell of dolomite basically retains the
calcite structure, but simply substitutes Mg atoms for Ca atoms in
alternating cation layers (see Figure 46). In this structure the c-
glide present in calcite is destroyed by the alternating Ca-Mg
arrangement, making it unfit for substitution of certain cations such as
sodium and potassium.
Single-crystal X-ray structure refinements of dolomite from
different locations conducted by Reeder and Wenk (1983) and Effenburger
et al. (1981) indicated substitutions of Fe and Mn for Mg, and Mg for
Ca or vice versa. However, substitution of alkali cations such as Na
and K was not observed, which confirmed the above hypothesis. Reeder
(1983) has also shown that the smaller rhombohedral cell of calcite
(CaCC^) which is similar to that of the dolomite structure, only favors
incorporation of the smaller cations (Mg, Fe, Mn, Zn and Cu) whereas the
large cell of the orthorhombic aragonite (CaC03) allows preferential
substitution of larger cations such as Sr, Ba, Na and U. It has also
been indicated that Na and K ions are not compatible with dolomite

127
Ca(COj)6
Mg(C03)6
Dolomite
C*Mg(C03)j
a)
b)
Figure 46. Crystal structure of dolomite, c-axis vertical, a) Layered
structure b) Stereoscopic projection of the hexagonal unit
cell for dolomite (a = 4.81 A, c = 16.00 A)

128
structure (Blanchard, 1987). This might explain the reasons for NaCl
acting as an indifferent electrolyte in the case of dolomite.
Adsorption of Oleate on Apatite and Dolomite
Effect of Conditioning pH
Adsorption of sodium oleate on apatite and dolomite was determined
to explain the reasons for the selective flotation of dolomite from
apatite in the presence of NaCl. Sodium oleate adsorption on single
minerals as a function of pH is shown in Figures 47 and 48 for apatite
and dolomite, respectively. A good correlation between adsorption and
flotation is observed from the results. Similar results were reported
by Chanchani (1984). It is observed that the amount of adsorbed oleate
required for flotation is dependent on the pH of the
mineral/collector/water system and varies significantly for apatite and
dolomite. It has been suggested that acid-soap complexes are the
predominant species adsorbing on apatite between pH 7 and 9, with
possible chemisorption of oleate monomers and dimers onto the surface
calcium sites (Chanchani, 1984). In the pH range of 7 to 11, the zeta
potential of apatite has a nearly constant negative value. Thus, the
adsorption of the anionic oleate species is expected to be adversely
affected. In the acidic pH range (pH 6 and lower), a combination of
physical and specific interaction forces has been suggested by Johnston
and Leja (1978) to be responsible for the observed flotation behavior.
Unlike apatite, adsorption of oleate on dolomite has been suggested to
be largely influenced by the electrostatic forces (Chanchani, 1984).

OLEATE ADSORBED, Amol/gx100
100
80
60
40
20
O
Figure 47. Correlation between oleate adsorption and flotation for apatite as a
function of pH.
AMOUNT FLOATED, WT%

OLEATE ADSORBED, ^mol/gx 100
Figure 48. Correlation between oleate adsorption and flotation for dolomite as a
function of pH.
AMOUNT FLOATED, WT%

131
In the acidic pH range (below pH 5) oleate adsorption on dolomite
is higher than that on apatite under identical experimental conditions.
It is to be noted that dolomite flotation is also higher than apatite
under similar pH conditions. The higher adsorption and flotation
response of dolomite cannot be explained on the basis of electrostatic
interactions alone, because both minerals have the same magnitude of
zeta potential at pH 5 and below. Possible reasons for the observed
behavior could be the faster adsorption kinetics of oleate on dolomite
as mentioned earlier. This would decrease the collector available for
adsorption on apatite and consequently results in its reduced flotation.
Effect of NaCl Addition on Adsorption
The amount of oleate adsorbed on apatite as a function of sodium
oleate concentration at pH 4 (refer to Figure 31) in the presence of
NaCl was observed to decrease at all concentrations as compared to that
in distilled water. On the other hand, adsorption on dolomite remained
unaffected in the presence of NaCl (see Figure 34). A summary of the
amount adsorbed, flotation recovery and zeta potential values determined
for apatite and dolomite is presented in Table 13. The amount adsorbed
on apatite is seen to decrease by about 40% in the presence of NaCl.
The decrease in adsorption on apatite as described earlier is attributed
to the surface charge reversal in addition to depletion of calcium sites
by substitution of sodium. It is to be noted that ionic oleate species
were expected to adsorb on the calcium sites on the apatite surface.
The negative zeta potential at pH 4 in the presence of NaCl would result

TABLE 13
Adsorption and Flotation Results,
and Zeta Potential Values for Apatite and Dolomite
with and without NaCI at pH 4.0
Mineral
Salt Cone.
(kmol/m3)
Amount Adsorbed
(pmol/g x 10+2)
Amount Floated
(Weight %)
Zeta Potential
(mV)
Apatite
None
3.3
55-60
+4
Apatite
2.0xl0"2
1.8
0-3
-13
Dolomite
None
5.8
95-100
+5
Dolomite
2.OxlO-2
5.8
95-100
+4
Oleate Cone., 4.0x10^ kmol/m^

133
in an electrostatic repulsion between the oleate anion and the apatite
surface, thereby reducing the amount of oleate adsorbed on apatite.
Unlike apatite, the zeta potential and the adsorption behavior of
dolomite was not affected by sodium chloride addition. Some variation
in the adsorption and flotation of dolomite as a result of the
electrical double layer compression cannot be ruled out. However, they
were not significant enough to yield measurable changes.
Adsorption in the Mixed Mineral System
Adsorption of oleate on apatite and dolomite in the single and
mixed mineral systems was measured at a sodium oleate concentration of
3.6xl0~4 kmol/m^ using the special cell arrangement described earlier
(Figure 35). The influence of pH on adsorption of oleate on apatite and
dolomite can be ascertained from the data given in Figure 35. Mixed
mineral adsorption behavior is found to follow the trends exhibited by
the single minerals with only minor changes. Single mineral adsorption
and flotation behavior are similar to those observed by Chanchani
(1984). For example, the decrease in the amount adsorbed on apatite and
the increase on dolomite, between pH 7 and 9, corresponds with the
decreased apatite and increased dolomite flotation in the mixtures. It
can therefore, be concluded that the predictions made on the basis of
single mineral test results are valid for mixed mineral systems.
Effect of Salt on Adsorption in the Mixed Minerals Systems
On the basis of the adsorption data given in Table 11, the amount
adsorbed on dolomite during mixed mineral conditioning does not appear

134
to be significantly different from that of the single mineral system in
the presence of sodium chloride. Adsorption on apatite, unlike that on
dolomite, however, decreased by about 20% in the case of mixtures as
compared to the single mineral tests. Flotation of apatite was also
observed to be depressed more in the mixed mineral system. The same
observation was also made by Chanchani (1984), who reported that the
rate of oleate adsorption on dolomite is higher during the first 5
minutes of conditioning. Thus, the amount available for adsorption on
apatite decreased because of its slower adsorption kinetics.
In summary, it was seen that oleate adsorption on apatite and
dolomite varies as a function of pH. Oleate adsorption on dolomite in
the acidic pH range was determined to be higher than that of apatite.
It was also established that adsorption of oleate on dolomite remains
unchanged in the presence of sodium chloride whereas that on dolomite
decreases by about 40%. Selective flotation of dolomite from apatite
can be partially ascribed to the decreased oleate adsorption on apatite.
However, as mentioned earlier, the nature of the surfactant species
adsorbing on the surface is also important, since it could impart
different degrees of hydrophobicity. Identification of the nature of
adsorbed species using FT-IR spectroscopy is discussed next.
Nature of the Adsorbing Surfactant Species
Apatite-01eate System
The difference IR spectra of treated and untreated apatite at pH 10
and at pH 4 are depicted in Figure 49. It can be seen that calcium
oleate forms on the apatite surface at pH 10. The adsorbed species in

WAVENUMBER, cm1
Figure 49. Difference IR spectra of apatite-oleate system at pH 10, and at pH 4 in the
presence and absence of sodium chloride.
CJ

136
the absence of added salt at pH 4 were identified to be oleic acid and
calcium oleate. However, a higher calcium oleate peak is observed,
suggesting greater adsorption of ionic oleate species as compared to
neutral oleic acid molecules. These species appear to be consistent
with the solution properties of sodium oleate. Although calcium oleate
and oleic acid were detected on the apatite surface in distilled water
at pH 4, in the presence of sodium chloride, oleic acid appeared to be
the only adsorbing specie. The change in the nature of the surfactant
species adsorbing on apatite is anticipated because of the selective
calcium dissolution and its substitution by sodium at pH 4. In
addition, the reversal of the surface charge of apatite in the presence
of NaCl was expected to adversely affect the adsorption of the anionic
oleate species.
Dolomite-Oleate System
The difference spectrum obtained from the diffuse reflectance FT-IR
spectra of treated and untreated dolomite at pH 10 and pH 4 is presented
in Figure 50. The spectra at pH 10 indicated the presence of magnesium
oleate. At pH 4, oleic acid and magnesium oleate were both observed in
the absence and presence of NaCl, indicating no change upon salt
addition. This result was expected, since it was shown that flotation,
electrokinetic and adsorption behavior of dolomite was not affected to
any significant extent by sodium chloride addition.
In summary, it has been established, from the above discussion that
formation of calcium oleate occurs on apatite at both pH 4 and pH 10.
Adsorption of oleic acid, however, was observed at pH 4 only. It was

KUBELKA-MUNK UNITS
400 1400 2400 3400
WAVENUMBER,cm1
Figure 50. Difference IR spectra of dolomite-oleate system at pH 10, and at pH 4
with and without sodium chloride addition.

138
also shown that calcium oleate does not form on apatite in the presence
of sodium chloride. In the case of dolomite, oleate at pH 10, and both
oleate and oleic acid adsorption at pH 4 were observed. However, the
adsorbed oleate specie on dolomite, unlike that on apatite, was
identified to be magnesium oleate. The nature of the adsorbing species
on dolomite was not affected by the presence of NaCl. Preferential
formation of magnesium oleate could possibly explain the differences in
the flotation performance of apatite and dolomite as they would probably
impart different degrees of hydrophobicity on the respective substrate.
In addition, such information could be utilized to develop more specific
surfactants for apatite/dolomite separation. Possible reasons for
magnesium oleate formation on dolomite in preference to calcium oleate
are discussed below.
Preferential formation of magnesium oleate
Dolomite, CaMg(C03)2, is composed of alternating layers of calcium
and magnesium carbonate and, therefore, both calcium and magnesium
oleate formation would normally be expected. Selective formation of
magnesium oleate on the dolomite surface can be due to one or more of
the following reasons:
(1) preferential exposure of magnesium ions during fracture,
(2) lower solubility of the magnesium oleate complex,
(3) selective leaching of calcium from the dolomite surface,
(4) electronegativity effect, and
(5) charge density difference.

139
Preferential exposure of magnesium ions. The breakage of mineral
particles, like most other substances, occurs along surfaces of least
resistance. In most minerals, the strength of chemical bonds is not
uniform in all directions. The chemical bonding along certain planes
may be weaker, resulting in breakage along that interface rather than at
random. The cleavage of the rhombohedral dolomite structure, like many
other carbonate minerals, occurs along the {104} plane. The relative
tendency of dolomite to develop cleavage along this plane has been
considered to be ideal (Zoltai and Stout, 1974). If there were no
preferred planes of weakness that are controlled by the crystal
structure, the mineral would break along a random fracture such as in
the case of apatite. It can be seen, upon examination of the cleavage
plane of orthorhombic dolomite structure, that equal numbers of both
calcium and magnesium ions would be exposed. The selective magnesium
oleate formation, therefore, cannot be explained on the basis of
exposure of more magnesium ions during fracture.
Solubility of magnesium and calcium oleate. The solubility product
of calcium and magnesium oleate, as determined by the nephelometric
method, are given in Table 14, including the literature values. Results
obtained in the present study are in good agreement with those of
Fuerstenau and Palmer (1976). It is, however, observed that the
solubility product of magnesium oleate is higher as compared to that of
calcium oleate. Assuming that this observation can be extrapolated to
the mineral surface, preferential formation of calcium oleate rather
than magnesium oleate should have been observed which is contrary to the
results obtained in the present investigation.

TABLE 14
Solubility Product of Calcium and Magnesium Oleate
Product
Present Study Fuerstenau and Palmer, (1976) Du Reitz, (1957)
Calcium Oleate
2.40xl0'16 2.51xl0-16 3.98X10"13
Magnesium Oleate
3.02X10'16 6.31xl016 1.58xl0-11

141
Selective leaching of calcium ions from dolomite. During
dissolution of the minerals, different constituents can dissolve at
varying rates, resulting in a change in both the surface chemical
composition of the solids and the ionic concentration in the solution.
The possibility of differential calcium or magnesium dissolution from
dolomite was examined by conducting dissolution kinetic tests at pH 4
using the 65x100 mesh fraction of the mineral. The amount of calcium
and magnesium ions leached into the solution were determined as a
function of time. The results obtained (shown in Table 12) indicated a
higher rate (greater than 11%) of calcium dissolution relative to that
of magnesium from dolomite, with and without NaCl addition. As a
consequence of selective leaching, it is possible that a relatively
higher concentration of magnesium sites would result on the dolomite
surface. The probability of magnesium oleate formation, therefore, can
be expected to be higher, although calcium oleate formation cannot be
ruled out.
Electronegativity effect. Electronegativity of an element is the
attraction of the nucleus of its atom for the electrons in the outer
shell. Using the electronegativity values of the elements involved in
bonding, the type and strength of the bond can be determined. If the
difference in the electronegativity values between two elements is large
(e.g., 2 or more) the bonds in the compound will be largely ionic in
nature. When the difference is small, a covalent bond would exist,
which is stronger in an aqueous medium. Upon adsorption of oleate
anions on dolomite, a calcium and/or a magnesium oleate bond is
expected to form. The polarity of the bond formation, i.e., the %

142
ionicity, can be evaluated from the electronegativity values of calcium
(1.00), magnesium (1.31), and oxygen (3.44), using the following
expression (Atkins, 1982):
Ionicity % = 16[Xa Xb] + 3.5[Xa Xb]2
The % ionicity of the calcium and magnesium oleate bonds were
computed to be 60% and 50%, respectively. These calculations
demonstrate that magnesium-oleate bond would be relatively more
covalent than the calcium-oleate bond. The extremely strong ionic
"bond" is readily disrupted in a medium of high dielectric constant,
such as water, in marked contrast to covalent bonds, which even though
weaker, are not generally disintegrated by a solvent such as water.
Charge density difference. Smaller ions or molecules have a higher
charge density, i.e., charge/volume ratio, in contrast to the larger
ones. The energy of interaction (ip) between an ion and a dipole is a
function of 1/r2, where r is the ionic distance between the center of
the ion and the dipole molecule. The interaction energy (cp) between an
oleate anion (oleate forms a permanent dipole) and a calcium (r^a =
0.71 ft) or magnesium cation (r^g = 0.51 ft) on the dolomite surface can
be calculated using the following expression (Israelachvi1i, 1985):
q u Cos0
(p = NAv(J/mole)
4 tt 2 £ r
where, q = electronic charge (= ze, where z is valence and e is
the electron charge) of the ion

143
M = dipole moment of the molecule
0 = orientation angle between the ion and dipole molecule
(0=0 and Cos0 = 1 when the ion and the dipole molecule are
oriented)
^Av = Avogadro's number (6.022x10^ mol-*-)
I = permittivity of the free space (8.854x10-*-^ C^J"-*-m-*-)
£ = dielectric constant of the medium (e = 78 for water)
At contact r = R/2 + r^on where R = dipole length (point
charge is assumed for the dipole molecule)
The dipole moment of oleate, calculated using the bond angle (124.3
degrees), and the dipole moments of C=0 (0.74D) and C-0 (2.5D), was
found to be 1.51D (=1.51 x 3.336x10-^ Cm). The bond lengths for C=0
(1.23 X) and C-0 (1.22 X) were used to calculate the dipole length (R)
which was determined to be 0.57 K. The interaction energy values
between the dipole molecule (oleate) and the ions (calcium and
magnesium) presented in Table 15, indicate that the interaction energy
is long range and that it becomes greater than kT before the contact is
made between the ion and the dipole. It is also seen from this data
that the interaction energy between the dipole and the magnesium ion is
greater than that of calcium and oleate for the same distance of
separation between the outer shells. Upon contact, the interaction
energy between the magnesium-oleate pair is 56% greater than that of
calcium-oleate. Thus, the preferential formation of the magnesium-
oleate specie on dolomite can be attributed also to the difference in
the charge densities of the respective species.

TABLE 15
Electrostatic Interaction Energy of Calcium and Magnesium with Oleate
Separation
Distance (A)
Interaction Energy, kJ/mole
Energy Ratio
Mg-01eate
Ca-01eate
Mg-01eate/Ca-01eate
8.0
-0.145
-0.138
1.051
4.0
-0.486
-0.448
1.085
2.0
-1.429
-1.244
1.149
1.0
-3.458
-2.801
1.235
contact
-17.504
-11.202
1.563

145
Contact Angle Studies
Contact angle measurements were carried out in order to determine
the relative hydrophobicity of apatite and dolomite at pH 4, with and
without sodium chloride addition. These tests were conducted at a
sodium oleate concentration of 4.0x10^ kmol/m^ as summarized in Table
16. Without any surfactant or salt addition, a contact angle of
approximately 42 degrees was measured on both of the minerals. The
contact angles of apatite and dolomite following conditioning in sodium
oleate solution were measured to be 58 and 91 degrees, respectively,
indicating greater hydrophobicity of the surface of dolomite as compared
to that of apatite under similar experimental conditions.
In the presence of 2.0x10"^ kmol/m^ sodium chloride and at a sodium
oleate concentration of 4.0x10"^ kmol/m^, the contact angles were 37 and
85 degrees for apatite and dolomite, respectively, indicating that the
apatite surface has been rendered more hydrophilic in the presence of
NaCl at pH 4. The hydrophobicity of dolomite was not affected to any
significant extent.
It has been shown in this study that the flotation, adsorption and
the contact angle values are in good agreement for apatite and
dolomite, both with and without salt addition at pH 4. It should,
however, be noted that the apatite and dolomite samples used in contact
angle studies were from different locations than those employed for
flotation and adsorption measurements and were of highly crystalline and
nonporous nature. These samples were selected to minimize any
hysteresis that could possibly result from the porous nature of the
samples utilized in flotation, adsorption and other studies. While this

TABLE 16
Contact Angle and Flotation Recovery
of Apatite and Dolomite at pH 4.0
Mineral
Oleate Concentration
(kmol/m5)
Sodium Chloride
Cone, (kmol/m5)
Contact
Angle
(degrees)
Flotation
Recovery
(%)
Apatite
None
None
42*
None
Apatite
4.OxlO"5
None
58
55-60 ;
Apatite
4.OxlO"5
2.OxlO"2
37
0-3
Dolomite
None
None
41
None
Dolomite
4.OxlO"5
None
91
95-100
Dolomite
4.OxlO"5
2.OxlO"2
85
95-100
*It is to be
samples from
noted that contact angle and flotation
different sources
measurements were
conducted using apatite
and dolomite

147
is not expected to greatly affect the trends observed, the magnitude of
the changes determined for the minerals can be different than those
indicated by flotation and adsorption measurements.
Mechanism of Selective Flotation of
Dolomite From Apatite in the Presence of NaCI
It was mentioned earlier that the selectivity predicted by the
single mineral flotation tests was found to be limited in the case of
mixtures using either oleate or dodecylamine as the collector. The loss
in the selectivity was attributed to the surface charge modification as
well as to the complexation and precipitation of the collector species
with dissolved ions from the minerals, which reduced the effective
collector concentration in the system. Sodium chloride was therefore
added to minimize the effect of the electrical double layer in the
adsorption process. Consequently, selective flotation of dolomite was
achieved from apatite.
It was shown that in the presence of sodium chloride the surface
charge of apatite reverses below its IEP. This was ascribed to the
substitution of Na+ for Ca++ in the apatite structure. Substitution of
Na+ for Ca++ was established through chemical analysis of the samples in
addition to determining the changes in the unit cell parameters.
Subsequently, it was shown that the amount of collector adsorbed on the
apatite in the presence of NaCI decreases by about 40% as compared to
that in distilled water. Furthermore, the adsorbed surfactant species
were identified to be different in the presence of sodium chloride.
Calcium oleate and oleic acid were determined to form on apatite surface
at pH 4 in the absence of added salt. In the presence of NaCI, on the

148
other hand, calcium oleate formation was not detected, which has been
attributed to depletion of surface calcium sites in addition to
electrostatic repulsion due to surface charge reversal. It was
established by contact angle measurements that, in the presence of NaCl
at pH 4, the apatite surface remains hydrophilic when 4.0x10"^ kmol/nP
sodium oleate is added as the collector. Apatite flotation, therefore,
is suppressed under these conditions.
Surface charge, adsorption behavior and flotation response of
dolomite, unlike that of apatite, was not affected to any significant
extent by the addition of sodium chloride. The contact angle on
dolomite surface also did not change significantly to alter its
hydrophobicity. Consequently, dolomite flotation remained high while
apatite was depressed resulting in the desired selective separation of
dolomite from apatite.

CHAPTER VI
CONCLUSIONS
Flotation response of apatite and dolomite has been studied using
dodecylamine hydrochloride (cationic) and sodium oleate (anionic) as the
collector, with particular emphasis on separating dolomite from apatite
to obtain a phosphate concentrate of less than 1% MgO.
Selective separation of apatite from dolomite, using dodecylamine
as the collector, was predicted by the single mineral flotation results.
However, the selectivity was observed to be limited in the case of mixed
minerals. The loss in selectivity was attributed to decreased flotation
of apatite and increased recovery of dolomite due to surface charge
modification of these minerals as a result of their dissolution. It
should be noted that there are more dissolved ions present in the mixed
mineral system as compared to the single mineral system. In addition,
faster adsorption kinetics of dodecylamine on dolomite are believed also
to affect the resultant flotation behavior.
Separation of apatite from dolomite in the alkaline pH range (pH 7-
10) and that of dolomite from apatite in the acidic pH range (pH 5-6)
was anticipated from the single mineral tests when sodium oleate was
used as the collector. Mixed mineral tests, however, indicated
activation of apatite in the acidic pH range and depression of it under
alkaline pH conditions, limiting the selectivity. This has been
attributed to the presence of excess Ca++ and Mg++ ions dissolved from
149

150
dolomite which are known to depress apatite flotation, as well as to the
depletion of oleate ions by precipitation as calcium and magnesium
oleate.
Additional mixed mineral experiments were conducted in the
presence of sodium chloride using either anionic or cationic collector
to minimize the extent of the electrostatic attraction between the
surfactant ions and the mineral surface by compressing the electrical
double layer, in order to achieve the collector adsorption on these
minerals predominantly by specific interaction forces. Consequently,
the desired selective separation of dolomite from apatite or vice versa
was achieved depending on the type of the collector used.
Upon optimization of the relevant process parameters such as pH,
collector and salt concentration, it was determined that more than 95%
of the dolomite can be removed from a 50:50 apatite-dolomite mixture at
a BPL recovery of 95% or higher using sodium oleate as the collector.
Other salts such as KC1 and NaF were also evaluated. The best
selectivity, however, was obtained in the presence of sodium chloride,
with no selectivity when NaF was added to the system.
Electrokinetic, adsorption, FT-IR and contact angle studies were
conducted to elucidate the mechanism for the observed selective
flotation of dolomite from apatite with sodium oleate.
It was demonstrated by the zeta potential measurements that, in the
presence of sodium chloride, the surface charge of apatite reverses
below its IEP (pH 5.4). It was also established that the surface charge
reversal for apatite results from increased Ca++ dissolution which is
substituted by Na+ in the apatite structure. This was based on the

151
Chemical analyses of Ca++ and Na+ for apatite with and without NaCl
addition and measurement of the lattice parameter ("a" dimension) of the
unit cell. Chemical analysis indicated that a stoichiometric
substitution of Na+ occurs for Ca++ in the structure. Thus, replacement
of the bivalent cations (Ca++) from the surface sites by monovalent
cations (Na+) led to the development of a negative surface charge on
apatite. The surface charge of dolomite, unlike that of apatite,
remained unchanged below its IEP (pH 5.3).
Adsorption experiments were conducted on apatite and dolomite using
sodium oleate in the absence and presence of sodium chloride.
Associative interactions in oleate solutions were taken into account in
this study and accordingly, a new technique was developed to measure the
adsorption directly on the mineral surfaces in the oleic acid
precipitation region by using labeled surfactant (oleic acid). This
technique has yielded reliable measurements of the amount of oleate
adsorbed on the mineral surface which in the past has led in some cases
to misleading conclusions (Marinakis and Shergold, 1985; Gutierrez and
Iskra, 1977; and Pope and Sutton, 1973).
In the present study, a new cell arrangement was also utilized to
measure the amount of oleate adsorbed on the mineral surfaces in the
case of the mixtures. This was made possible as a consequence of the
direct measurement technique explained above. Determination of the
amount of oleate adsorbed in the case of mixed minerals allows one to
determine the effect of dissolved ionic species from one mineral on the
flotation behavior of the other minerals constituting the sample.

152
Oleate adsorption as a function of pH on single and mixed minerals
indicated a good correlation with their flotation behavior.
The amount of oleate adsorbed on apatite at pH 4 in the presence of
sodium chloride decreased by 40% as compared to that in distilled water.
This decrease was anticipated in terms of the reversal of surface charge
which was expected to adversely affect adsorption of anionic oleate
species due to electrostatic repulsion. Adsorption on dolomite under
similar conditions remained unaltered in distilled water and in the
presence of sodium chloride.
Oleate adsorption on apatite at pH 4 in the case of mixed minerals
indicated a 20% decrease relative to that of single mineral results.
This was attributed to the faster adsorption rate of oleate on dolomite
as compared to apatite, which subsequently decreased the effective
oleate concentration and led to the enhanced selective flotation of
dolomite from apatite.
In order to determine the nature of the adsorbed surfactant
species on the apatite and dolomite surfaces, FT-IR spectroscopic
studies were conducted. It was established that formation of calcium
oleate occurs on apatite at both pH 4 and pH 10. Adsorption of oleic
acid was observed at pH 4 only, which is consistent with the solution
chemistry behavior of oleic acid.
It was also established that calcium oleate does not form on the
apatite surface at pH 4 in the presence of sodium chloride. This has
been ascribed to the depletion of Ca++ sites on apatite surface as a
result of Na+ substitution. In addition, the surface charge reversal
also adversely affected adsorption of anionic oleate species.

153
Adsorption of oleate and oleic acid at pH 4, and of oleate alone at
pH 10, were detected on dolomite from FT-IR results. It was determined
that magnesium oleate forms on the dolomite surface in preference to
calcium oleate at both pH 4 and pH 10. The nature of the adsorbed
species, however, remained the same in the presence of sodium chloride
at pH 4. This finding is again consistent with flotation and
electrokinetic behavior of dolomite in the presence of NaCl which
remained unchanged from that in distilled water. In the past, it has
been postulated that oleate would adsorb on both Ca++ and Mg++ sites on
the dolomite surface. Consequently, the inability to separate dolomite
from apatite was attributed to the presence of the common cation (Ca++).
In this study, for the first time, it has been established that this is
not necessarily the case. Furthermore, it has been established that
presence of two different cations in the same structure ( e.g., Ca++ and
Mg++ in dolomite) cannot be assumed to be the adsorption sites for the
anionic collector specie such as oleate.
Preferential formation of magnesium oleate on dolomite was
explained in terms of higher charge density of magnesium ions as
compared to calcium. Higher calcium dissolution from dolomite as
compared to magnesium ions and higher electronegativity of magnesium
also contributed to the formation of only magnesium oleate on the
dolomite surface. Additionally, higher electronegativity of magnesium
ions can lead to more covalent bond formation with oleate, which is
stronger in an aqueous environment, thus resulting in preferential
magnesium oleate formation on the surface of dolomite.

154
The contact angle measurements, determined in this study,
indicated a good correlation with the flotation and oleate adsorption
results in the absence and the presence of sodium chloride. It was
determined that NaCl addition reduces the hydrophobicity of apatite at
pH 4 without affecting that of dolomite to any significant extent.

CHAPTER VII
SUGGESTIONS FOR FUTURE RESEARCH
It has been suggested that the difficulty in devising a selective
separation process for the salt-type minerals such as apatite and
dolomite is due partially to their solubility behavior, which
consequently modifies the surface charge and the surface chemical
composition (Ananthapadmanabhan and Somasundaran, 1985). Although no
direct evidence has been provided, it has been proposed that the surface
composition of apatite is altered by the presence of dolomite, or vice
versa, in the mixture. It should be noted that the indirect
measurements such as conditioning of one of the minerals in the
supernatant of the other, does not depict the actual conditions, since
the dynamic process of dissolution, readsorption, etc., is not truly
represented by such an approach. It is, therefore, recommended that the
mixed mineral conditioning, for flotation and other experiments, be
conducted in the special cell arrangement used for the adsorption
experiments in the present study. Following the same method, it should
be possible to prepare samples for surface analysis techniques such as
ESCA and FT-IR, which would provide data about the extent of surface
transformations.
The experimental results of this study indicated that the zeta
potential of apatite is reversed below its IEP (pH 5.4) in the presence
of salts such as sodium chloride. In addition, the zeta potential of
155

156
apatite was observed to be more negative in the alkaline pH range.
Between pH 6 and 8, however, it was less negative indicating possible
compression of the electrical double layer as a result of higher ionic
strength. It is known from the aging studies (Chanchani, 1984) that
apatite reaches equilibrium at pH 7.0-7.5. The slope of the pH shift
versus time from the aging studies gives information on dissolution
rates, which appear to be higher in the acidic and basic pH ranges.
Hence, a study of sodium substitution for calcium in the apatite
structure at pH 7 and pH 10, as was done at pH 4 in the present study
can possibly give information about the changes in the zeta potential
behavior. In addition, the data might be helpful in explaining why
apatite is depressed above pH 9 in the presence of sodium chloride as
has been observed in this study and reported also by Maslow (1971) and
Strel'tsyn et al. (1967).
Equilibrium adsorption studies of collectors such as oleate on
apatite and dolomite when combined with the heat of adsorption data can
provide significant information about the mechanism of adsorption.
Adsorption of oleate on apatite has been studied by Moudgil et al.
(1987). Currently, there is no heat of adsorption data available on
apatite-oleate or dolomite-oleate systems. Therefore, it is recommended
that the heat of adsorption of oleate be determined on these minerals
under various experimental conditions, using a microcalorimeter. In
addition, oleate adsorption on apatite and dolomite as a function of
temperature (even though it can only be done in a narrow temperature
range) should be conducted to find out the energy of activation
required for adsorption of oleate on each of the minerals. It is

157
presumed that the data obtained from such studies, when used in
conjunction with electrokinetic results, would probably yield
information for the selective adsorption only on the desired mineral
species.
The diffuse reflectance FT-IR spectra of dolomite indicated the
presence of magnesium oleate on dolomite at pH 4 and 10. On the other
hand, calcium oleate formation was observed on apatite at both pH
values. In addition, both mineral spectra indicated the presence of
oleic acid at pH 4. It is desirable to determine if the same species
will form on these minerals when conditioned together using the special
cell arrangement. This approach can also be extended to zeta potential
measurements to establish the effect of dissolved mineral species on the
surface charge modification in the mixture. Use of different size
fractions is an alternative for the insoluble oxides; however, the
possible effects of abrasion raise doubts about such a procedure.
A simulation of the physical process of collector adsorption on
certain surface sites (e.g., that of oleate on surface magnesium sites
of dolomite as has been determined from the FT-IR studies) can provide
details about the closeness of the approach of the surfactant molecule
to the surface sites. Therefore, it is desirable to simulate such a
process using the computer.
Results of the adsorption of oleate on apatite at pH 4 in the
presence of sodium oleate indicated a 40% decrease in the amount
adsorbed on the surface relative to those of distilled water.
Subsequent FT-IR studies indicated that oleate adsorption on surface
calcium sites is inhibited on apatite, but oleic acid still adsorbs.

158
Inhibition of calcium oleate formation, in the presence of sodium
chloride, was attributed to the depletion of surface Ca++ sites as a
result of Na+ substitution in the apatite structure. It is not clear if
any alteration occurred in the amount of oleic acid adsorbed on the
surface due to the presence of salt. This can be investigated by
quantitative FT-IR spectroscopy if a calibration curve can be
established for the amount of oleic acid adsorbed from its band in the
spectrum.

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Rule, A. R. and Dallenbach, C. B., 1985, "Beneficiation of Complex
Phosphate Ores Containing Carbonate and Silica Gangue,"
Proceedings, XVth IMPC, Cannes, France, Vol. 3, pp. 380-389.
Rule, A. R., Gruzensky, W. G. and Stickney, W. A., 1970, "Removal of
Magnesium Impurities from Phosphate Rock Concentrate," U.S. Dept,
of the Interior, U.S. Bureau of Mines, RI 7362.
Saleeb, F. Z., and de Bruyn, P. L., 1972, "Surface Properties of
Alkaline Earth Apatites," Electrochem. Chem. and Interfacial
Chemistry, Vol. 37, pp. 99-118.
Shah, B. D., 1986, "Selectivity in Mixed Mineral Flocculation: Apatite-
Dolomite System." M.S. Thesis, University of Florida, Gainesville.
Smani, M. S., Blazy, P. and Cases, J. M., 1975, "Beneficiation of
Sedimentary Moroccan Phosphate Ores, Part 1-4," Trans., SME/AIME,
Vol 258, pp. 168-182.
Smith, J. P. and Lehr, J. R., 1966, "An X-Ray Investigation of
Carbonate Apatites," Journal of Agriculture and Food Chem.,
Vol. 14, pp. 342-349.
Snow, R. E., 1979, "Beneficiation of Phosohate Ores," U.S. Patent
No. 4,144,969.

166
Snow, R. E., 1982, "Flotation of Phosphate Ores Containing Dolomite,"
U.S. Patent No. 4,364,824.
Somasundaran, P., 1968, "Zeta Potential of Apatite in Aqueous Solutions
and its Change During Equilibration," J. Colloid Interface
Science, Vol. 27, No. 4, pp. 659-666.
Somasundaran, P., 1972, "Pretreatment of Mineral Surfaces and its
Effect on Their Properties," In Clean Surfaces, Their Preparation
and Characterization for Interfacial Studies, Marcel Dekker,
New York, pp. 285-306.
Somasundaran, P. and Ananthapadmanabhan, K. P., 1979a, "Solution
Chemistry of Surfactants and the Role of it in Adsorption and
Froth Flotation in Mineral-Water systems." In: Solution Chemistry
of Surfactants, K. L. Mittal, Ed., Vol. 2. Plenum Press. New York,
pp. 17-38.
Somasundaran, P., and Ananthapadmanabhan, K. P., 1979b, "Physico-
Chemical Aspects of Flotation," Trans. Indian Inst. Metals,
Vol. 32, p. 2.
Somasundaran, P. and Wang, Y. H. C., 1984, "Surface Chemical
Characteristics and Adsorption Properties of Apatite," in
Adsorption and Surface Chemistry of Hydroxyapatite, D. N. Misra,
Ed., Plenum Press, New York, pp. 129-149.
Sorensen, E., 1973, "On the Adsorption of Some Anionic Collectors on
Fluoride Minerals," J. Colloid and Interface Science, Vol. 45,
No. 3, pp. 601-607.
Soto, H. and Iwasaki, I., 1985, "Flotation of Apatite from Calcareous
Ores with Primary Amines," Mineral and Metallurgical Processing,
Vol. 2, pp. 160-166.
Soto, H. and Iwasaki, I., 1986, "Selective Flotation of Phosphates from
Dolomite Using Cationic Collectors. Part II. Effect of Particle
Size, Abrasion and pH," Inter. J. of Mineral Processing, Vol. 16,
pp. 17-27.
Stoll, W. R. and Neuman, W. F., 1956, "The Uptake of Sodium and
Potassium Ions by Hydrated Hydroxyapatite," J. Am. Chem. Soc.,
Vol. 78, p. 1585.
Strel'tsyn, G. S., Pudov, V. F., Kostritsyn, V. N. and Klimenko, V. Y.,
1967, "Lowering the Harmful Effect of Sodium Chloride on Flotation
of Apatite-Nephel ine Ore," Obogashch. Rud., 12(3), 13-14 (Russ).
Stumm, W. and Morgan, J. J., 1981, "Aquatic Chemistry," 2nd Edition.
A Wiley-Interscience Pub., New York.

167
Whippo, R. and Murowchick, B. L., 1967, "The Crystal Chemistry of Some
Sedimentary Apatites," Trans., SME/AIME, Vol. 238, pp. 257-263.
Zoltai, T. and Stout, J. H., 1984, "Mineralogy: Concepts and
Principles," Burgess Publishing Company, Minneapolis, Minnesota.

BIOGRAPHICAL SKETCH
Dursun E. Ince was born in Tunceli, Turkey, on December 23, 1951.
Upon graduating from high school in Izmir, in June 1970, he entered the
Technical University of Istanbul the same year, and received a B.S. in
mining and mineral engineering in June 1974. Following one year of work
in Turkey, he moved to the United States for graduate study and went to
the University of Wisconsin-Madison, where he completed a M.S. degree in
mineral and metallurgical engineering in May 1978. He was associated
with the Mineral Engineering Department of the Pennsylvania State
University before joining Union Carbide Corporation in Niagara Falls,
N.Y., in December 1979, where he worked as a research engineer for three
years. In January 1984, he entered the Ph.D. program in the materials
science and engineering at the University of Florida.
168

I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
Brij M. Moudgil, Chairman
Professor of Materials Science
and Engineering
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation f/jr the degree of
Doctor of Philosophy.
E. Dow Whitney
Professor of Materials Science
and Engineering
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
i.o. s
Dinesh 0. Shah
Professor of Chemical
Engineering
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
/L
I f (AL
David E. Clark
Professor of Materials Science
and Engineering
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly .presentation and is fully
adequate, in scope and quality, as a dissert^tioh for the degree of
Doctor of Philosophy.
Frank N. Blanchard
Professor of Geology

This dissertation was submitted to the Graduate Faculty of the College
of Engineering and to the Graduate School and was accepted as partial
fulfillment of the requirements for the degree of Doctor of Philosophy.
December 1987
iLJuJl d-
Dean, College of Engineering
Dean, Graduate School



151
Chemical analyses of Ca++ and Na+ for apatite with and without NaCl
addition and measurement of the lattice parameter ("a" dimension) of the
unit cell. Chemical analysis indicated that a stoichiometric
substitution of Na+ occurs for Ca++ in the structure. Thus, replacement
of the bivalent cations (Ca++) from the surface sites by monovalent
cations (Na+) led to the development of a negative surface charge on
apatite. The surface charge of dolomite, unlike that of apatite,
remained unchanged below its IEP (pH 5.3).
Adsorption experiments were conducted on apatite and dolomite using
sodium oleate in the absence and presence of sodium chloride.
Associative interactions in oleate solutions were taken into account in
this study and accordingly, a new technique was developed to measure the
adsorption directly on the mineral surfaces in the oleic acid
precipitation region by using labeled surfactant (oleic acid). This
technique has yielded reliable measurements of the amount of oleate
adsorbed on the mineral surface which in the past has led in some cases
to misleading conclusions (Marinakis and Shergold, 1985; Gutierrez and
Iskra, 1977; and Pope and Sutton, 1973).
In the present study, a new cell arrangement was also utilized to
measure the amount of oleate adsorbed on the mineral surfaces in the
case of the mixtures. This was made possible as a consequence of the
direct measurement technique explained above. Determination of the
amount of oleate adsorbed in the case of mixed minerals allows one to
determine the effect of dissolved ionic species from one mineral on the
flotation behavior of the other minerals constituting the sample.


52
Single mineral flotation tests
Results of flotation tests as a function of pH using sodium oleate
as the collector are presented in Figure 15. At a sodium oleate
concentration of 4.0x10^ kmol/m^, flotation recovery of dolomite was
observed to be 100% in the acidic pH range (pH 4.0-5.5). Dolomite
recovery remained at 10-15% range between pH 7 and 10 and was seen to
increase above pH 10.
Apatite flotation in the acidic pH range, on the other hand,
exhibited a maximum at pH 4. The recovery of apatite increased sharply
above pH 6, and was observed to be 100% between pH 7 and 10.5. No
apatite flotation was observed between pH 5 and 6 at this level of
sodium oleate addition.
As observed from Figure 15, selective flotation of dolomite from
apatite or vice versa was predicted in the following pH ranges.
1) Flotation of dolomite from apatite at pH 5 to 6.
2) Flotation of apatite from dolomite between pH 7 and 10.
Mixed minerals
Results of the mixed mineral flotation tests using a 50:50 apatite
and dolomite mixture under given pH conditions are summarized in Table
4. It is observed that at pH 5.3, even though dolomite recovery was
greater than 95%, a significant amount of apatite (33%) also reported in
the float fraction. Consequently, apatite recovery in the sink fraction
was reduced to approximately 67%. In the alkaline pH range, apatite
recovery decreased to 68% from 100% observed in the single mineral
tests. On the other hand, dolomite flotation remained at the level


14
Somasundaran (1986) and Chanchani (1984). A brief summary of recent
developments is presented below.
Flotation of dolomite from apatite in the presence of inorganic
depressants such as phosphates and fluorides at pH 5.6-6.2 was studied
by Lawver et al. (1984) using fatty acids and their soaps, including
petroleum sulfonates. Reportedly, the best results were obtained with
sodium tripolyphosphate and hexametaphosphate using a proprietary
anionic collector. This process resulted in phosphate concentrates
containing less than 1% MgO at 50-90% BPL recoveries from a feed of less
than 48 mesh size fraction containing about 2% MgO.
Hsieh and Lehr (1985) at TVA used diphosphonic acid to depress
apatite while floating dolomite with oleic acid. This process reduced
the MgO content of the concentrate to less than 1% from a feed
containing 1.9% MgO at 83% apatite recovery. In another process
developed by TVA (1983), H2SO4 was added to the concentrate to separate
calcareous phosphate ores. Selective flotation of carbonates was
attributed to differential desorption of the fatty acid collector on
the phosphate mineral. No experimental data was, however, presented to
support this hypothesis.
Llewellyn et al. (1982) at U.S. Bureau of Mines depressed
dolomite by the addition of sodium silicate and floated apatite at pH
9.2-9.6. In cases where the MgO content of the final concentrate was
not reduced to less than 1%, further removal of dolomite by SO2 leaching
was recommended. Rule et al. (1970 and 1985), also at USBM, depressed
apatite with fluosilicic acid while floating carbonaceous impurities
using fatty acid emulsion under slightly acidic pH conditions.


AMOUNT FLOATED, WT%
cn
OJ
Figure 15. Flotation of apatite and dolomite (single minerals) as a function of pH.


TABLE 2
Characteristics
of Apatite
and Dolomite Samples
Chemical
Analysis, %
Size Fraction
Surface Area
Mineral
(Mesh)
p25
MgO
Insol.
CaO
m2/g
Apatite
65x100
35.28
0.28
2.14
42.08
11.5
Dolomite
65x100
0.90
18.86
3.12
27.01
6.0


163
Lehr, J. R. and Hsieh, S. S., 1981, "Beneficiation of High Carbonate
Phosphate Ores," U.S. Patent. 4,287,053.
Lehr, J. R., McClellan, G. H. and Smith, J. P., 1967, "Characterization
of Apatite in Commercial Phosphate Rock," International Colloquium
on Solid Inorganic Phosphates, Toulouse, France.
Levinskas, G. J. and Neuman, W. F., 1955, "The Solubility of Bone
Mineral, I. Solubility Studies of Synthetic Hydroxyapatite,"
J. Phys. Chem., Vol. 59, p. 164.
Llewellyn, T. 0., Davis, B. E. and Sullivan, G. E., 1984,
"Beneficiation of Florida Dolomite Phosphate Ores," Mineral and
Metallurgical Processing, Vol. 1, No. 1, pp. 43-48.
Llewellyn, T. 0., Davis, B. E., Sullivan, G. V. and Hansen, J. P.,
1982, "Beneficiation of High-Magnesium Phosphate From South
Florida," U.S. Dept, of the Interior, U.S. Bureau of Mines,
RI 8609.
Marinakis, K. I. and Shergold, H. L., 1985, "The Mechanism of Fatty
Acid Adsorption in the Presence of Fluorite, Calcite and Barite,"
Inter. J. of Mineral Processing, Vol. 14, pp. 161-176.
Maslow, A. D., 1971, "Flotation of Apatite in the Presence of Sodium
Chloride," Vop. Teor. Prakt. Obogashch. Rud., 159-164 (Russ).
McClellan, G. H., 1980, "Mineralogy of Carbonate-Fluorapatites," J.
Geol. Soc., London, Vol. 137, pp. 675-681.
McConnell, D., 1952, "The Crystal Chemistry of Carbonate Apatites and
Their Relationship to the Composition of Calcified Tissues,"
J. Dental Research, Vol. 31, pp. 53-63.
McConnell, D. and Gruner, J. W., 1940, "The Problem of the Carbonate
Apatites, III. Carbonate-Apatite from Magnet Cove, Arkansas," Am.
Mineralogist, Vol. 25, pp. 157-164.
Mishra, R. K., Chander, S. and Fuerstenau, D. W., 1980, "Effect of
Ionic Surfactants on the Electrophoretic Mobility of
Hydroxyapatite," Colloids and Surfaces, Vol. 1, pp. 105-119.
Modi, H. J., and Fuerstenau, D. W., 1960, "Flotation of Corundum: An
Electrochemical Interpretation," Trans., SME/AIME, Vol. 217,
pp. 381-387.
Moudgil, B. M., 1972, "Absorption of Hydrocarbon Gases in Aqueous
Surfactant Solutions and Their Effect on Flotation," MS Thesis,
Columbia University, New York.


TABLE 16
Contact Angle and Flotation Recovery
of Apatite and Dolomite at pH 4.0
Mineral
Oleate Concentration
(kmol/m5)
Sodium Chloride
Cone, (kmol/m5)
Contact
Angle
(degrees)
Flotation
Recovery
(%)
Apatite
None
None
42*
None
Apatite
4.OxlO"5
None
58
55-60 ;
Apatite
4.OxlO"5
2.OxlO"2
37
0-3
Dolomite
None
None
41
None
Dolomite
4.OxlO"5
None
91
95-100
Dolomite
4.OxlO"5
2.OxlO"2
85
95-100
*It is to be
samples from
noted that contact angle and flotation
different sources
measurements were
conducted using apatite
and dolomite


78
calcium from apatite can therefore render the surface more negatively
charged. Results of Ca++ and P0$" dissolution from apatite as a
function of time with and without NaCl addition are summarized in Table
8. An increase in Ca++ and a decrease in P0$dissolution in NaCl
solution relative to that of distilled water is observed from these
results. It is therefore possible that selective dissolution of calcium
can lead to surface charge reversal of apatite. This preferential
dissolution, however, was believed to be occurring as a result of sodium
substitution in the apatite structure, since dissolution was found to be
incongruent in the presence of added salt. This possibility was studied
by 1) measuring the change in the unit cell dimensions of apatite; 2)
analyzing the mineral sample for sodium content before and after
treatment with NaCl solution.
The change in the lattice parameters of apatite was studied by
computerized X-ray diffraction method. Results of the tests, summarized
in Table 9, indicated 0.006*0.001 8 decrease in the unit cell "a"
dimension of apatite when conditioned with NaCl solution. This change
was anticipated to be small, because the ionic radius of Na+ (0.95 K) is
only slightly smaller than that of Ca++ (0.99 K) (Brescia et al., 1966).
These tests were followed by determination of sodium uptake by apatite.
Results shown in Table 10, indicated that the amount of sodium present
in apatite after conditioning and rinsing is stoichiometrically
equivalent to the amount of calcium dissolved from apatite suggesting
mole per mole substitution of Na+ for Ca++ in the apatite structure.


SODIUM OLEATE ADSORBED, >umol/g
INITIAL OLEATE CONC., kmol/m3
Figure 34. Adsorption of oleate on dolomite at pH 4.0, with and without added
sodium chloride.


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INGEST IEID EELV8BTBN_NHEM00 INGEST_TIME 2014-10-06T23:52:53Z PACKAGE AA00025731_00001
AGREEMENT_INFO ACCOUNT UF PROJECT UFDC
FILES


SODIUM OLEATE ADSORBED, >imol/g
Figure 35. Oleate adsorption on apatite and dolomite (single and mixed minerals) as
a function of pH.


80
60
40
20
0
/>/*
/ A
pH 4.0
NaF Cone., Singla Mineral
(kmol/m3) Dolomite Apatite
A None
2.Ox12
o A
/ *
j i hi ..u*i Qj iin
J i i t i.inii
0
r 6
i65
i64
,-o3
SODIUM OLEATE CONC., kmol/m
CT
LO
22.
Apatite and dolomite (single minerals) flotation with and without NaF
addition at pH 4.


13 Adsorption and Flotation Results with Zeta
Potential Values for Apatite and Dolomite
with and without NaCl at pH 4.0 132
14 Solubility Product of Calcium and Magnesium
Oleate 140
15 Electrostatic Interaction Energy of Calcium and
Magnesium with Oleate 144
16 Contact Angle and Flotation Recovery of Apatite
and Dolomite at pH 4.0 146
x


CHAPTER III
EXPERIMENTAL
Materials
Minerals
Apatite
A sample of high-grade phosphate rock (apatite) was procured from
Agrico Chemical Company (Mulberry, Florida). This sample (16x150 mesh)
was screened to obtain a 65x100 mesh fraction, which was deslimed, dried
and passed through an electrostatic separator after heating to 140 C to
remove the silica grains. The 65x100 mesh sample was used for
flotation and adsorption studies. FT-IR and electrokinetic studies,
however, were conducted on a portion of this sample which was ground to
-325 mesh.
Dolomite
This sample, supplied by International Minerals and Chemicals
Corporation (Bartow, Florida), was hand picked and crushed using a
Chipmunk crusher and hand ground to maximize the yield of 65x100 mesh
fraction. The dolomite was deslimed and dried at 140 C before removing
silica by electrostatic separator. Electrokinetic and FT-IR studies
were conducted on a portion of this sample ground to -325 mesh. The
samples were stored in a glass jar and used as required.
18


157
presumed that the data obtained from such studies, when used in
conjunction with electrokinetic results, would probably yield
information for the selective adsorption only on the desired mineral
species.
The diffuse reflectance FT-IR spectra of dolomite indicated the
presence of magnesium oleate on dolomite at pH 4 and 10. On the other
hand, calcium oleate formation was observed on apatite at both pH
values. In addition, both mineral spectra indicated the presence of
oleic acid at pH 4. It is desirable to determine if the same species
will form on these minerals when conditioned together using the special
cell arrangement. This approach can also be extended to zeta potential
measurements to establish the effect of dissolved mineral species on the
surface charge modification in the mixture. Use of different size
fractions is an alternative for the insoluble oxides; however, the
possible effects of abrasion raise doubts about such a procedure.
A simulation of the physical process of collector adsorption on
certain surface sites (e.g., that of oleate on surface magnesium sites
of dolomite as has been determined from the FT-IR studies) can provide
details about the closeness of the approach of the surfactant molecule
to the surface sites. Therefore, it is desirable to simulate such a
process using the computer.
Results of the adsorption of oleate on apatite at pH 4 in the
presence of sodium oleate indicated a 40% decrease in the amount
adsorbed on the surface relative to those of distilled water.
Subsequent FT-IR studies indicated that oleate adsorption on surface
calcium sites is inhibited on apatite, but oleic acid still adsorbs.


60
TABLE 7
Results of Mixed Mineral Flotation Tests in the
Presence of NaCI Using Sodium Oleate as the Collector
Flotation
pH
Chemical
P25
Analysis, %
MgO
Apatite Recovery
Weight %
Dolomite Reject
Weight %
10.9
18.0
9.5
100.0
0.0
8.2
18.5
9.5
100.0
0.0
6.6
18.5
9.5
100.0
0.0
4.0
33.3, 33.
8 0.64, 0.48
96.0, 94.8
96.6, 97.6
Collector Cone., 4.0x10^ kmol/m^
NaCI Cone., 5.0xl0_1 kmol/m^
Flotation Feed: 1 gram 50:50 Apatite-Dolomite
Mixture, 18.0% P205, 9.5% MgO


demonstrated that the surface charge of apatite is reversed below its
isoelectric point (pH 5.4) in the presence of sodium chloride. This was
attributed to the increased rate of calcium dissolution and sodium
substitution in the apatite structure.
Adsorption studies confirmed that the amount of oleate adsorbed on
apatite decreases in the presence of sodium chloride relative to that in
distilled water, while on dolomite the amount adsorbed remained
unchanged.
FT-IR spectra of apatite in distilled water indicated the presence
of oleic acid and calcium oleate on the surface. In the presence of
NaCl, however, calcium oleate was not detected. This was ascribed to
the surface charge reversal, which adversely influenced adsorption of
anionic oleate species, in addition to the depletion of calcium sites by
selective dissolution. FT-IR spectra of dolomite indicated the presence
of magnesium oleate besides oleic acid, with and without the salt
addition. Formation of magnesium oleate in preference to calcium oleate
on the dolomite surface was explained in terms of higher charge density
and electronegativity of magnesium ions, and relatively higher rate of
calcium dissolution.
Contact angle measurements on apatite and dolomite indicated a good
correlation with adsorption and flotation results in the absence and the
presence of sodium chloride.
xv


50
Attempts were therefore made to achieve the separation of apatite
from dolomite by conducting flotation tests using synthetic mixtures of
apatite and dolomite under the above specified pH conditions.
Flotation of apatite-dolomite mixture with dodecylamine
Results of 50:50 apatite and dolomite mixed mineral flotation
experiments under selected pH conditions and collector concentrations
are summarized in Table 3.
Although a preferential flotation of apatite was observed at pH
9.8 and pH 4.1 at both levels of dodecylamine concentrations, the
magnitude of the selectivity predicted by the single mineral experiments
was not realized.
The difference in the flotation response of these minerals at pH
9.8 and at a dodecylamine concentration of 1.6xl0-4 kmol/m^ also did not
correspond to the single mineral test results. In general, results of
mixed mineral experiments with dodecylamine as the collector indicated
depression of apatite and activation of dolomite in the selectivity
ranges predicted by the single mineral tests.
Flotation Studies Using Sodium Oleate as the Collector
Flotation behavior of apatite and dolomite using sodium oleate as
the collector was examined as a function of pH. Mixed mineral tests,
were also conducted under specific pH conditions.


133
in an electrostatic repulsion between the oleate anion and the apatite
surface, thereby reducing the amount of oleate adsorbed on apatite.
Unlike apatite, the zeta potential and the adsorption behavior of
dolomite was not affected by sodium chloride addition. Some variation
in the adsorption and flotation of dolomite as a result of the
electrical double layer compression cannot be ruled out. However, they
were not significant enough to yield measurable changes.
Adsorption in the Mixed Mineral System
Adsorption of oleate on apatite and dolomite in the single and
mixed mineral systems was measured at a sodium oleate concentration of
3.6xl0~4 kmol/m^ using the special cell arrangement described earlier
(Figure 35). The influence of pH on adsorption of oleate on apatite and
dolomite can be ascertained from the data given in Figure 35. Mixed
mineral adsorption behavior is found to follow the trends exhibited by
the single minerals with only minor changes. Single mineral adsorption
and flotation behavior are similar to those observed by Chanchani
(1984). For example, the decrease in the amount adsorbed on apatite and
the increase on dolomite, between pH 7 and 9, corresponds with the
decreased apatite and increased dolomite flotation in the mixtures. It
can therefore, be concluded that the predictions made on the basis of
single mineral test results are valid for mixed mineral systems.
Effect of Salt on Adsorption in the Mixed Minerals Systems
On the basis of the adsorption data given in Table 11, the amount
adsorbed on dolomite during mixed mineral conditioning does not appear


123
apatite in water can be represented by the following reactions as
discussed by Somasundaran and Wang (1984):
C¡+ + OH" t CaOH+ (1)
CaOH+ + OH" t Ca(0H)2(aq) (2)
Ca(0H)2(aq) ^ Ca(0H)2(Surf) (3)
H3P04 H+ + H2P04' (4)
H2P04" t H+ + HPOj" (5)
HPOj" H+ POj" (6)
It is clear that when the pH is increased the equilibrium will be
driven toward the right hand side in equations 1 to 6. This would
increase the concentration of negative PDI's in solution which will
render the surface negatively charged. When the pH is decreased, these
reactions will move in the opposite direction and the surface will
become positively charged.
It was shown that in the presence of NaCl the surface charge of
apatite reverses below its IEP and attains a negative zeta potential
value (Figure 23). This change was believed to have resulted from the
selective dissolution of calcium and substitution of sodium in the
apatite structure. Such process, i.e., substitution of Na+ for Ca++ in
the structure, could lead to the development of a more negative surface
charge because of the difference in their valency. For example, if in
an array of solid Si02 tetrahedra, an atom of Si (4+ valency) is
replaced by an A1 atom (3+ valency), a negatively charged frame work
will result (Stumm and Morgan, 1981). Substitution of sodium for


WAVENUMBER, cm1
Figure 49. Difference IR spectra of apatite-oleate system at pH 10, and at pH 4 in the
presence and absence of sodium chloride.
CJ


AMOUNT FLOATED, WT%
Figure 14. Flotation of apatite and dolomite (single minerals) as a function of pH.


138
also shown that calcium oleate does not form on apatite in the presence
of sodium chloride. In the case of dolomite, oleate at pH 10, and both
oleate and oleic acid adsorption at pH 4 were observed. However, the
adsorbed oleate specie on dolomite, unlike that on apatite, was
identified to be magnesium oleate. The nature of the adsorbing species
on dolomite was not affected by the presence of NaCl. Preferential
formation of magnesium oleate could possibly explain the differences in
the flotation performance of apatite and dolomite as they would probably
impart different degrees of hydrophobicity on the respective substrate.
In addition, such information could be utilized to develop more specific
surfactants for apatite/dolomite separation. Possible reasons for
magnesium oleate formation on dolomite in preference to calcium oleate
are discussed below.
Preferential formation of magnesium oleate
Dolomite, CaMg(C03)2, is composed of alternating layers of calcium
and magnesium carbonate and, therefore, both calcium and magnesium
oleate formation would normally be expected. Selective formation of
magnesium oleate on the dolomite surface can be due to one or more of
the following reasons:
(1) preferential exposure of magnesium ions during fracture,
(2) lower solubility of the magnesium oleate complex,
(3) selective leaching of calcium from the dolomite surface,
(4) electronegativity effect, and
(5) charge density difference.


58
TABLE 6
Results of Mixed Mineral Flotation in the Presence of
NaCl Using Dodecylamine as the Collector at pH 6.3
Test
No.
Chemical
of Float
p2o5
Analysis
Fraction, %
MgO
Apatite Recovery
in Float
(Weight %)
Dolomite Reject
in Sink
(Weight %)
1
33.65
0.97
79.0
71.8
2
33.49
1.02
79.8
69.8
3
33.59
1.03
81.2
69.1
Collector Cone., 1.6x10"^ kmol/m^
NaCl Cone., 5.0x1o--1- kmol/m^
Feed: 1 gram 88/12 Apatite-Dolomite Mixture
pH: 6.3


120
Effect of NaCI on the Separation of Dolomite
From Apatite Using Sodium Oleate as the Collector
Dolomite was selectively floated out from the mixture of apatite
and dolomite in the presence of 5.0xl0l | concentration of 4.0x10^ kmol/m^ at pH 4 (Table 7). In these tests,
more than 95% of the dolomite was floated out at 95% or higher apatite
recovery level. The MgO content of the concentrate (sink fraction) was
reduced to less than 0.7% from a feed containing 9.5% MgO. Mixed
mineral flotation experiments without the use of NaCI under similar
conditions yielded apatite recovery of 45%, with the concentrate
analyzing 1.7% MgO from the same 50:50 apatite/dolomite feed. Thus, it
was shown that apatite can be effectively depressed in the presence of
sodium chloride without influencing the flotation behavior of dolomite.
In further test work, the optimum salt concentration (Figure 16),
oleate concentration (Figures 17 and 18), and pH of separation (Figure
19) was determined through single mineral flotation studies. Additional
mixed mineral flotation experiments as a function of pH revealed that
dolomite can be selectively separated from apatite at pH 4.0 in the
presence of 2.0x10^ kmol/m^ NaCI at a sodium oleate concentration of
4.0x10^ kmol/m^ (Figure 20).
The single and mixed mineral flotation tests under the above
experimental conditions were also conducted by replacing NaCI with KC1
or NaF (Figure 21 and 22). Unlike the results obtained with NaCI,
dolomite flotation was found to be lower along with lower apatite
recovery when KC1 was used. In the case of NaF, both apatite and
dolomite were found to be depressed.


LIST OF TABLES
Table No. Page
1 Sodium Oleate Distribution in Various Streams
During Adsorption Tests at pH 4.5 25
2 Characteristics of Apatite and Dolomite Samples ... 37
3 Flotation Results of 50:50 Apatite/Dolomite
Mixture Using Dodecylamine as the Collector 51
4 Flotation Results of 50:50 Apatite-Dolomite
Mixture Using Sodium Oleate as the Collector 54
5 Effect of NaCl on the Single Mineral Flotation
of Apatite and Dolomite with Dodecylamine
Hydrochloride as the Collector 57
6 Results of Mixed Mineral Flotation in the
Presence of NaCl Using Dodecylamine as the
Collector at pH 6.3 58
7 Results of Mixed Mineral Flotation Tests in the
Presence of NaCl Using Sodium Oleate as the
Collector 50
8 Apatite Dissociation at pH 4 with and without
Added Sodium Chloride 79
9 Effect of NaCl on Unit Cell Dimensions of Apatite
Conditioned at pH 4 80
10 Determination of Substitution of Sodium for
Calcium in the Apatite Structure 81
11 Effect of Sodium Chloride Addition on Oleate
Adsorption on Apatite and Dolomite at pH 4.0 92
12 Dissolution of Calcium and Magnesium from
Dolomite at pH 4.0 with and without NaCl
Addition 113
IX


121
Mechanism of Selective Flotation of Dolomite From Apatite
Sodium chloride addition was expected to modify the magnitude of
the surface charge on apatite and dolomite particles possibly by
compressing the electrical double layer. It was also expected to
subsequently effect the adsorption process. Surfactant adsorption and
the resultant flotation of apatite and dolomite was observed to have
been affected as was presented in the previous chapter. Reasons for
these changes are discussed in the following sections.
Effect of NaCI on the Zeta Potential of Apatite and Dolomite
Zeta potential of dolomite in the presence of NaCI decreased above
its IEP (pH 5.3) which is indicative of compression of the electrical
double layer (EDL) by the added salt (Figure 26). It should be noted
that possible EDL compression also occurred below the IEP, but the
change was not detected due to the low values of zeta potential measured
in this pH range. It can, therefore, be concluded that NaCI acts as an
indifferent electrolyte for dolomite. A similar behavior was also
recorded in the presence of KC1 which too can be considered as an
indifferent electrolyte (Figure 27) for dolomite.
Zeta potential of apatite in the presence of NaCI, on the other
hand, was observed to be negative in the entire pH range (4-11) examined
(Figure 23), indicating that this salt does not act as an indifferent
electrolyte for apatite. The effect of salt addition on the surface
charge behavior of apatite was more striking in the acidic pH range
possibly because of its higher solubility under such conditions.
Apatite exhibited a less negative zeta potential value at pH 7 in the


164
Moudgil, B. M., 1987, "Separation of Dolomite From the South Florida
Phosphate Rock-Phase II," Report Submitted to Florida Institute of
Phosphate Research, Bartow, Florida. July, 1987.
Moudgil, B. M. and Chanchani, R., 1985a, "Flotation of Apatite and
Dolomite Using Sodium Oleate as the Collector," Mineral and
Metallurgical Processing, Vol. 2, No.l, pp. 13-19.
Moudgil, B. M. and Chanchani, R., 1985b, "Selective Flotation of
Dolomite From Francolite Using Two-Stage Conditioning," Mineral
and Metallurgical Processing, Vol. 2, No. 1, pp. 19-25.
Moudgil, B. M. and Chanchani, R., 1985c, "Beneficiation of Complex
Phosphate Ores from South Florida by Two Stage Conditioning
Process," Proceedings, XVth IMPC, Cannes, France, Vol. 3,
pp. 357-366.
Moudgil, B. M. and Shah, B. D., 1987, "Single and Mixed Mineral
Flocculation Behavior of Apatite and Dolomite," In: Flocculation
in Biotechnology and Separation Systems, Y. A. Attia, Ed.
Elsevier, New York, pp. 729-739.
Moudgil, B. M., Shah, B. D., and Soto, H. S., 1987, "Loss of Selecti
vity in Apatite-Dolomite Flocculation," Minerals and Metallurgical
Processing", Vol. 4, No. 1, pp. 27-31.
Moudgil, B. M. and Somasundaran, P., 1986, "Advances in Phosphate
Flotation", In: Advances in Mineral Processing, P. Somasundaran,
Ed., SME/AIME Pub., Littleton, Colorado, pp. 426-441.
Moudgil, B. M., Vasudevan, T.V. and Blaakmeer, J., 1987, "Adsorption of
Oleate on Apatite," Mineral and Metallurgical Processing, Vol. 4,
No. 1, pp. 50-54.
Onal, G., 1973, "Mazidag Low-Grade Calcareous Phosphate Ore Flotation,"
Proceedings of Cento Symposium on the Mining and Beneficiation of
Fertilizer Minerals, pp. 171-180.
Pope, M. I. and Sutton, D. I., 1973, "The Correlation Between Froth
Flotation Response and Collector Adsorption from Aqueous
Solution," Powder Technology, Vol. 7, pp. 271-279.
Predali, J. J. and Cases, J. M., 1973, "Zeta Potential of Magnesium
Carbonate in Inorganic Electrolytes," J. of Colloid Interface
Science, Vol. 45, No. 3, pp. 449-458.
Pugh, R. J., 1986, "The Role of the Solution Chemistry of Dodecylamine
and Oleic Acid Collectors in the Flotation of Fluorite," Colloids
and Surfaces, Vol. 18, pp. 19-41.


109
It is noted from the diagram (Figure 45) that oleic acid formation
at 4.0x10^ kmol/m3 oleate concentration begins to occur at pH 8. The
maximum in the acid-soap complex, (RC00H.RC00)-, concentration is also
observed at this pH value.
Apatite-Dolomite Flotation Using Dodecylamine as the Collector
Apatite-Dodecylamine System
Flotation of apatite with dodecylamine as a function of pH at two
levels of dodecylamine is illustrated in Figure 11. It is observed that
at a dodecylamine concentration of l.OxlCT^ kmol/m^, apatite flotation
is 100% both above and below its IEP (pH 5.4). At a lower collector
concentration (1.6xl0-^ kmol/m^), two flotation peaks, at pH 4 and pH
10, were obtained.
Continued apatite flotation below its IEP was attributed to the
chemical interaction between the collector ion and the mineral surface,
as has been pointed out by Soto and Iwasaki (1985). It is presumed that
the hydrophobic interactions of the hydrocarbon chains also play a role
in the adsorption process in addition to possible hydrogen bonding. It
is equally important, however, to realize the role of the solution
chemistry of the surfactant in the interpretation of these results, as
has been emphasized by Ananthapadmanabhan (1980) and Somasundaran and
Ananthapadmanabhan (1979b).
Analysis of the species distribution diagram for the 1.6xl04
kmol/m^ total dodecylamine concentration, as shown in Figure 44,
indicated that the maxima in the iono-molecular complex, (RNH2-RNH3)+,


Electrokinetic Measurements 23
Solution Preparation 24
Oleate Adsorption Tests 24
Mineral Dissolution Tests 27
Solubility Product Determination 29
FT-IR Tests 29
Contact Angle Measurements 31
Experimental Plan 31
Selection, Preparation and Characterization of
the Minerals 32
Selection of the Surfactant 32
Selection of the Experimental Techniques 32
Flotation 32
Electrokinetic measurements 33
Adsorption 33
Determination of the nature of the adsorbed
surfactant species 34
Mineral dissolution studies 34
Contact angle 34
Experimental Approach 35
IV RESULTS 36
Characterization of Minerals 36
Chemical Analysis 36
Surface Area and Porosity 36
X-ray Analysis 40
Surface Chemical Characterization 40
Flotation Studies 40
Flotation Studies with Dodecylamine Hydrochloride ... 40
Single mineral flotation behavior 40
Flotation of apatite-dolomite mixture with
dodecylamine 50
Flotation Studies Using Sodium Oleate as the
Collector 50
Single mineral flotation tests 52
Mixed minerals 52
Flotation tests in the presence of NaCl using
dodecylamine as the collector 56
Flotation studies in the presence of NaCl using
sodium oleate as the collector 59
Flotation studies in the presence of KC1 and NaF 67
Electrokinetic Studies 67
Effect of Salt Addition on the Zeta Potential
of Apatite 67
Effect of Salts on the Zeta Potential of Dolomite ... 70
Role of NaCl in the Reversal of Surface Charge
of Apatite 76
Substitution of sodium for calcium in the
apatite lattice 76
Adsorption Studies 82
vi


145
Contact Angle Studies
Contact angle measurements were carried out in order to determine
the relative hydrophobicity of apatite and dolomite at pH 4, with and
without sodium chloride addition. These tests were conducted at a
sodium oleate concentration of 4.0x10^ kmol/m^ as summarized in Table
16. Without any surfactant or salt addition, a contact angle of
approximately 42 degrees was measured on both of the minerals. The
contact angles of apatite and dolomite following conditioning in sodium
oleate solution were measured to be 58 and 91 degrees, respectively,
indicating greater hydrophobicity of the surface of dolomite as compared
to that of apatite under similar experimental conditions.
In the presence of 2.0x10"^ kmol/m^ sodium chloride and at a sodium
oleate concentration of 4.0x10"^ kmol/m^, the contact angles were 37 and
85 degrees for apatite and dolomite, respectively, indicating that the
apatite surface has been rendered more hydrophilic in the presence of
NaCl at pH 4. The hydrophobicity of dolomite was not affected to any
significant extent.
It has been shown in this study that the flotation, adsorption and
the contact angle values are in good agreement for apatite and
dolomite, both with and without salt addition at pH 4. It should,
however, be noted that the apatite and dolomite samples used in contact
angle studies were from different locations than those employed for
flotation and adsorption measurements and were of highly crystalline and
nonporous nature. These samples were selected to minimize any
hysteresis that could possibly result from the porous nature of the
samples utilized in flotation, adsorption and other studies. While this


ZETA POTENTIAL, mV
30
20
10
-20
-30 L
Dolomite in water
_ -2 3
O Dolomite in 2.0 x 10 kmol/m
KCI solution
cn
Figure 27. Effect of KCI on the zeta potential of dolomite.


CHAPTER VII
SUGGESTIONS FOR FUTURE RESEARCH
It has been suggested that the difficulty in devising a selective
separation process for the salt-type minerals such as apatite and
dolomite is due partially to their solubility behavior, which
consequently modifies the surface charge and the surface chemical
composition (Ananthapadmanabhan and Somasundaran, 1985). Although no
direct evidence has been provided, it has been proposed that the surface
composition of apatite is altered by the presence of dolomite, or vice
versa, in the mixture. It should be noted that the indirect
measurements such as conditioning of one of the minerals in the
supernatant of the other, does not depict the actual conditions, since
the dynamic process of dissolution, readsorption, etc., is not truly
represented by such an approach. It is, therefore, recommended that the
mixed mineral conditioning, for flotation and other experiments, be
conducted in the special cell arrangement used for the adsorption
experiments in the present study. Following the same method, it should
be possible to prepare samples for surface analysis techniques such as
ESCA and FT-IR, which would provide data about the extent of surface
transformations.
The experimental results of this study indicated that the zeta
potential of apatite is reversed below its IEP (pH 5.4) in the presence
of salts such as sodium chloride. In addition, the zeta potential of
155


90
much higher than that on apatite in the absence of NaCl, and three times
as much in the presence of it.
Mixed Minerals Adsorption Studies
Adsorption in mixed mineral systems has not been studied in the
past because of experimental difficulties. The problem was overcome
during the present study by utilizing a special cell arrangement (see
Figure 4). Results of the mixed mineral adsorption tests as a function
of pH at a sodium oleate concentration of 3.6xl04 kmol/m^ along with
those of single minerals, are presented in Figure 35. An increase in
oleate adsorption on dolomite and a decrease on apatite in the pH range
of 7-10 is observed. In the acidic pH range, however, the trend is
observed to be reversed. It is clear from the results presented that
the adsorption behavior in mixed minerals systems follows the trends
exhibited by the single mineral tests.
Adsorption of oleate on apatite and dolomite at pH 4 (single and
mixed minerals) in the presence of NaCl was also examined. Results of
these tests, summarized in Table 11, indicated that adsorption on
dolomite in the presence of sodium chloride remains unchanged when
conditioned with apatite. The amount adsorbed on apatite, however, was
observed to decrease by more than 20%, indicating that, in the mixture,
dolomite adsorbs more oleate than apatite in the presence of sodium
chloride.
Characterization of the Adsorbed Oleate
Species by FT-IR Spectroscopy
FT-IR spectroscopic studies were conducted to characterize the
nature of the oleate species absorbed by apatite and dolomite with and


31
Contact Angle Measurements
The samples for contact angle measurements were prepared by cutting
the rock using a diamond saw and mounting them on an epoxy base. The
surface of these samples were abraded using sandpaper with 400 grids and
polished using diamond cloth. The reagentization of samples was similar
to the method used for FT-IR analysis, except that aging and
conditioning in this case were performed by simply immersing the solid
in solution for 10 minutes. The solution was agitated using a magnetic
stirrer bar. After conditioning, the solid samples were dried in an
oven at 30 C before measuring the contact angle. The water droplets
were dispensed onto the sample surface using a micro-syringe. The
contact (tangent) angle formed between a "sessile" drop and the mineral
surface was determined directly, using a NRL Contact Angle Goniometer
Model 100-00. Following each test, the samples were abraded and re
polished to remove the surface coating before making subsequent contact
angle measurements.
Experimental Plan
The experimental plan employed in this investigation involved
a) Selection, preparation, and characterization of the minerals;
b) Selection of surfactant;
c) Selection of the experimental techniques; and
d) The experimental approach.


ZETA POTENTIAL, mV


AMOUNT FLOATED, WT%
FLOTATION pH
Figure 13. Flotation of apatite and dolomite (single minerals) as a function of pH.


162
Iwasaki, I., and Krishnan, S. V., 1983, "Heterocoagulation Versus
Surface Precipitation in Quartz-Mg(0H)2 System," SME/AIME Annual
Meeting, Atlanta, Georgia, Preprint No. 83-113.
Johnston, D. J. and Leja, J., 1978, "Flotation Behavior of Calcium
Phosphates and Carbonates in Orthophosphate Solution," IMM
Trans., Vol. 87, pp. C237-242.
Joy, A. S. and Robinson, A. J., 1964, "Flotation," In Recent Progress
in Surface Science, J. F. Danielli, K. G. A. Pankhurst, and A. C.
Riddiford, Eds., Academic Press, New York, Vol. 2, pp. 169-260.
Jung, R. F., 1976, "Oleic Acid Adsorption at the Geothite-Water
Interface," M.S. Thesis, University of Melbourne, Australia.
Kiukkola, K., 1980, "Selective Flotation of Apatite from Low-grade
Phosphorus Ore Containing Calcite, Dolomite and Phlogopite,"
Proceedings, 2nd Int. Congr. Phosphorus Compounds, Boston, April,
pp. 219-229.
Klassen, V. J. and Mokrousov, V. A., 1973, "An Introduction to Theory
of Flotation," London, Butterworths.
Lawver, J. E., Bernardi, J. P., McKereghan, G. F., Raulerson, J. D.,
Lynch, D. and Hearon, R. S., 1984, "New Techniques in
Beneficiation of the Florida Phosphates of the Future," Mineral
and Metallurgical Processing, Vol. 1, No. 2, pp. 89-106.
Lawver, J. E., McClintock, W. 0. and Snow, R. E., 1978, "Beneficiation
of Phosphate Rock: A State of the Art Review," Miner. Sci. Eng.,
Vol. 10, pp. 278-294.
Lawver, J. E., McClintock, W. 0. and Snow, R. E., 1980, "Method of
Beneficiating Phosphate Ores," U.S. Patent 4, 189,103.
Lawver, J. E., Murowchick, B. L. and Snow, R. E., 1978, "Beneficiation
of South Florida High Carbonate Phosphorites," ISMA, Technical
Economic Conferences, Orlando, Florida, Preprints TA/78/1.
Lawver, J. E., Raulerson, J. D. and Cook, C. C., 1980, "New Techniques
in Beneficiation of Phosphate Rock," Trans., SME/AIME, Vol. 268,
pp. 1787-1801.
Lawver, J. E., Wiegel, R. L., Snow, R. E. and Hwang, C. L., 1982a,
"Phosphate Reserves Enhancement by Beneficiation," Mining
Congress Journal, Vol. 68, pp. 27-31.
Lawver, J. E., Wiegel, R. L., Snow, R. E. and Hwang, C. L., 1982b,
"Beneficiation of Dolomitic Florida Phosphate Reserves," XIV
International Mineral Processing Congress, Toronto, Canada.


INTENSITY (COUNTS/SECOND)
TWO-THETA
Figure 7. X-ray diffractogram of apatite.


7
the desired selectivity in these systems (Klassen and Mokrousov, 1973;
Joy and Robinson, 1964).
Characteristics of Apatites in Florida Phosphorites
Phosphorites are sedimentary rocks with 15 to 20% P2O5 content.
In Florida deposits, apatite is the most abundant phosphorite mineral
generally found in the form of carbonate-fluorapatite (Ca^o(PO4)6-
x(C03)x Fo 4X^2^ (McClellan, 1980). The carbonate-fluorapatite (also
known as francolite) can have extensive substitutions such as carbonate
for phosphate; and other cations such as Mg, Na, Mn and K for calcium
(Lehr et al., 1967; McConnell, 1952, and McConnell and Gruner, 1940).
Apatite has a hexagonal lattice structure and lattice parameters are
dependent on the extent of substitution. It is generally composed of
microcrystals which vary in size from 0.02 to 0.20 microns (Lehr et
al., 1967; Smith and Lehr, 1966). High surface area of apatites has
therefore been attributed to this crystal size. In addition, much of
the south Florida phosphate rock is composed of grains which are
mixtures of apatite, dolomite and other constituents. Phosphorite
sediments are complex because they are the product of several different
sedimentary systems, and are formed by intermixing of phosphates,
carbonates, organic matter, glauconite, terrigenous and siliceous
sediments (Riggs, 1979).
Characteristics of Dolomite
Dolomite along with calcite is the most abundant carbonate mineral.
The crystal structure of dolomite is similar to that of calcite. The


125
substitution for Ca++, and postulated that the surface charge would
possibly be influenced.
Substitution of sodium for calcium in the apatite structure was
investigated in the present study by determining the unit cell
parameters (Table 9) and analyzing the treated and untreated apatite
samples for their sodium content (Table 10). The unit cell dimension
"a" as measured by computerized X-ray diffraction technique indicated a
decrease of 0.0060.001 ft with a standard deviation of 0.001 ft. The
difference, as indicated earlier, was anticipated to be small since the
ionic radii of sodium and calcium are comparable in size.
In addition to lattice parameters, sodium substitution was
confirmed through chemical analysis of the samples treated in the
absence and presence of NaCl. It was shown that the amount of excess
Na+ present in apatite is approximately stoichiometrically equivalent to
the excess calcium dissolved from apatite in the sodium chloride
solution. Stoichiometric substitution of sodium for calcium in the
apatite structure was also observed by Stoll and Neuman (1956), who
studied uptake of Na+ from solution. They postulated that Na+ ions
substitute in the apatite structure and possibly result in the
compression of the electrical double layer. In the present study,
attempts were made by using ESCA to determine if there is any excess
sodium present on the surface of apatite after treatment, but no
conclusive evidence was obtained. This is because the apatite sample
used in this investigation contained sodium (possibly as a substitute
for calcium) in the structure.


161
Fuerstenau, D. W., 1962, Ed., "Froth Flotation," 50th Anniversary
Vol., AIME, New York.
Fuerstenau, D. W., Metzger, P. H. and Seele, G. D., 1957, "How to Use
this Modified Hallimond Tube," Eng. and Mining Journal, Vol. 158,
pp. 93-95.
Fuerstenau, M. C. and Palmer, B. R., 1976, "Anionic Flotation of Oxides
and Silicates," in Flotation, A. M. Gaudin Memorial Vol., SME/AIME,
New York, pp. 149-196.
Fu, E. and Somasundaran, P., 1986, "Alizarin Red S as a Flotation
Modifying Agent in Calcite-Apatite Systems," Inter. J. of Mineral
Processing, Vol. 18, pp. 287-296.
Gadsden, J. A., 1975, "Infrared Spectra of Minerals and Related
Inorganic Compounds," London, Butterworths.
Gnosh, S. N., 1978, "Infrared Spectra of Some Selected Minerals, Rocks
and Products," Journal of Material Science, Vol. 18,
pp. 1877-1876.
Gruber, G. A., Raulerson, J. O.and Farias, R. P., 1986, "Adapting
Technology to Beneficate a Low Grade Phosphate Ore," AIME Annual
Meeting, New Orleans, Louisiana. Preprint No. 86-53.
Gutierrez, C. and Iskra, J., 1977, "The Action of Neutral Oleic Acid in
the Flotation of Hematite" Inter. J. of Mineral Processing,
Vol. 4, pp. 163-171.
Hanna, H. S., 1975, "Role of Cationic Collectors on Selective Flotation
of Phosphate Ore Constituents," Powder Technology, Vol. 12, No.l,
pp. 57-64.
Hanna, H. S. and Somasundaran, P., 1976, "Flotation of Salt-Type
Minerals," in Flotation, A. M. Gaudin Memorial Vol. SME/AIME, New
York, pp. 197-272.
Houot, R. and Polgaire, J. L., 1980, "Inverse Flotation Beneficiation
of Phosphate Ores," Proceedings, 2nd Int. Congr. Phosphorus
Compounds, Boston, April, pp. 231-246.
Hsieh, S. S. and Lehr, J. R., 1985, "Beneficiation of Dolomitic Idaho
Phosphate Rock by the TVA Diphosphoric Acid Depressant Process,"
Minerals and Metallurgical Processing, Vol. 2, pp. 10-13.
Israelachvi 1 i, J. N., 1985, "Intermolecular and Surface Forces,"
Academic Press, Inc., Orlando, Florida.


39
Figure 6. SEM micrograph of dolomite (65x100 mesh size fraction), a)
100X; b) 1000X.


70
23-25, the zeta potential of apatite in the presence of added salts
exhibited negative values in the entire pH range (4-11) examined,
indicating reversal of the surface charge below the IEP (pH 5.4). It
should be noted that ionic strength was not maintained constant because,
as discussed in a previous study by Chanchani (1984), KNO3 additions of
up to 1.0xl0"2 kmol/m^ did not significantly affect flotation recovery
of either apatite or dolomite.
The zeta potential of apatite in the presence of NaCl in the pH
range of 6 to 9 was found to be less negative than that in distilled
water. In the presence of KC1, at pH 6 and above, the zeta potential of
apatite was measured to be nearly the same as that in distilled water.
The fact that both NaCl and KC1 reversed the surface charge indicated
that they are not indifferent electrolytes for apatite. On the other
hand, because F is a lattice ion and therefore a potential determining
ion (Somasundaran and Wang, 1984) NaF was not expected to act as an
indifferent electrolyte for apatite. NaF decreased the zeta potential
of apatite in the entire pH range (4-11) examined. It also rendered the
surface more negative as compared to NaCl or KC1.
Effect of Salts on the Zeta Potential of Dolomite
The zeta potential of dolomite was measured in the presence of
salts as a function of pH. Zeta potential versus pH curves for
dolomite in the presence of NaCl and KC1, shown in Figures 26 and 27,
respectively, indicated that the IEP of dolomite (at pH 5.3) is not
influenced by NaCl or KC1 addition. The zeta potential values measured
below IEP did not appear to show any measurable change. Above the IEP,


5
stage conditioning" process. In this process, the feed is conditioned
at pH 10 followed by reconditioning at a pH lower than 4.5 before
flotation. Dolomite is selectively floated out following the two-stage
conditioning process. Bench-scale optimization of this process using
natural samples appears promising.
Flotation separation of apatite from dolomite using cationic
collectors has also been studied by researchers at International
Minerals and Chemicals Company (Lawver et al., 1980; and Snow, 1979),
followed by Soto and Iwasaki (1985 and 1986). The role of the
dissolved mineral species, flotation pH and solution chemistry of the
surfactant, however, was not taken into account in the above studies.
The surface modifiers used in the past for apatite separation from
carbonates are mostly phosphate salts, which are uneconomical. Also,
changes in the characteristics of the matrix (ore), even from the same
location, have been observed. It is conceivable that a given flotation
scheme would be applicable to a specific ore. It is imperative,
therefore, to develop flotation processes which possibly would have
applicability to a wide variety of ores. Additionally, understanding
the mechanism of selectivity would be helpful in overcoming the
difficulties in processing different ores and ensuring the usefulness of
the method for treating ores of different characteristics.


79
TABLE 8
Apatite Dissociation at pH 4 with
and without Added Sodium Chloride
Time
(Minutes)
NaCl
Amount of Ions Dissolved, kmol/m3
In Distilled Water
In 2.0xl02 \
cmol/m3
Ca++
POj"
Ca++
poj-
05
8.75xl0'4
3.05xl0-4
1.05xl0'3
9.45xl0"5
30
1.60xl0"3
6.OOxlO-4
2.23xl0"3
2.21xl04
60
2.25xl0'3
8.40xl0-4
3.53X10'3
3.57xl0"4


SODIUM OLEATE ADSORBED, yumol/g
Figure 31. Oleate adsorption on apatite at pH 4.0 in the absence and presence of
sodium chloride.


51
TABLE 3
Flotation Results of 50:50 Apatite/Dolomite
Mixture Using Dodecylamine as the Collector
Flotation
Col 1ector
Chemical Analysis of
Apatite
Dolomite
pH
Cone.
(kmol/m3)
Float Fraction, %
Recovery
Reject
P205
MgO
(Weight %)
(Weight %)
4.10.2
l.OxlO'3
26.5, 26.6
4.2, 4.2
72.3, 74.7
53.4, 55.7
9.80.2
1.6xl0'4
25.4, 25.9
3.8, 4.0
53.3, 51.8
37.6, 38.2
Feed: 1 gram 65x100 mesh fraction
18.0% P205, 9.5% MgO


100
80
60
40
20
0
/ 8/

AA
/ /
/ 8
/
A
A
pH 4.0
NaCI Cone., Single Minerals
2.Ox 1 o a
I I I Hi
I . A J-A 1
-6
0
-5
10
-4
10
13
-2
10
SODIUM OLEATE CONC., kmol/m'
Flotation of apatite and dolomite as a function of sodium oleate concentration
in the absence and presence of sodium chloride (single minerals).
CT
OJ