Title Page
 Table of Contents
 Literature review
 Experimental plan
 Materials and methods
 Suggestions for future work
 Biographical sketch

Title: Selective flotation of dolomite from apatite using sodium oleate as the collector
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Permanent Link: http://ufdc.ufl.edu/UF00089976/00001
 Material Information
Title: Selective flotation of dolomite from apatite using sodium oleate as the collector
Physical Description: Book
Language: English
Creator: Chanchani, Rajen
Publisher: Rajen Chanchani
Publication Date: 1984
 Record Information
Bibliographic ID: UF00089976
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: alephbibnum - 000466485
oclc - 11588786

Table of Contents
    Title Page
        Page i
        Page ii
        Page iii
    Table of Contents
        Page iv
        Page v
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        Page vii
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        Page 10
    Literature review
        Page 11
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    Experimental plan
        Page 45
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    Materials and methods
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    Suggestions for future work
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    Biographical sketch
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Full Text







To my family


The author wishes to express his sincerest gratitude to

Dr. B. M. Moudgil for his invaluable guidance in this study.

The author is also very grateful to Dr. E. Dow Whitney

and Dr. D. O. Shah for their encouragement and constructive


Appreciation is due also to Dr. G. Y. Onoda, Dr. F. N.

Blanchard,and Dr. C. D. Batich for the helpful discussions;

to Mr. Rob Pelick for his assistance in measuring the pore

size distribution of the minerals; to Mr. Richard Crockett

for his assistance at various stages of the experiments.

The author is very grateful to the University of

Florida and the Florida Institute of Phosphate Research

(through grant no. 82-02-023) for providing the financial

support of this study. Any opinions, findings, conclusions

or recommendations expressed in this publication are those

of the author and do not necessarily reflect the views of

the Florida Institute of Phosphate Research.

The author also wishes to acknowledge International

Minerals & Chemicals Corp., Bartow, Florida for providing

the mineral samples and Carpco, Inc., Jacksonville, Florida

for their help in preparing the apatite sample.



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

ABSTRACT .......... .... .... ............ ... ... ... vi


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

1.1. Statement of the Problem................ 1
1.2. Objective of this Study.................. 9

II. LITERATURE REVIEW .......... ................... 11

2.1. Characteristics of the Minerals.'......... 11
2.2. Past Investigations of Apatite/Dolomite
Separation............................... 12
2.3. Basic Principles......................... 22

III. EXPERIMENTAL PLAN............................. 45

3.1. Selection of the Separation Technique.... 45
3.2. Selection of Minerals and
Preparation Methods...................... 46
3.3. Outline of the Investigation............. 46
3.4. Selection of the Experimental Techniques. 48
3.5. Selection of Experimental Variables...... 50

IV. MATERIALS AND METHODS.......................... 52

4.1. Materials................................ 52
4.2. Methods.................................. 73

V. RESULTS........................................ 98

5.1. Flotation Studies..................... 98
5.2. Oleate Adsorption Studies................ 135
5.3. Zeta Potential Measurements............. 149
5.4. Infrared Spectroscopy Studies........... 159
5.5. Bubble Size Measurements................ 162
5.6. Solubility of the Minerals............... 169
5.7. Depletion of Oleate Ions in the presence
Calcium and Magnesium Cations............ 174

VI. DISCUSSION...................................... 177
6.1. Flotation after Conventional Conditioning 178
6.2. Flotation after Two-Stage Conditioning... 217

VII. SUGGESTIONS FOR FUTURE WORK.................... 239

VIII. CONCLUSIONS...... .............................. 243

8.1. Flotation after Conventional Conditioning 243
8.2. Flotation after Two-Stage Conditioning... 244

LIST OF REFERENCES............ ....................... 246


SINGLE MINERALS.......................... 259

MIXED MINERALS......................... 260

BIOGRAPHICAL SKETCH................... ............ 262

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




April, 1984

Chairman: Dr. E. Dow Whitney
Major Department: Materials Science and Engineering

The major problem with the future utilization of

phosphate deposits in South Florida is the dolomite

impurity, which interferes with the subsequent processing of

the phosphate mineral, apatite, into fertilizers. The

purpose of this study is to achieve and understand the

separation of dolomite from apatite by selective flotation.

Flotation behavior of single and mt tests in the pH

range of 7 to 11, was not observed in mixed mineral systems.

To understand the loss of selectivity in the mixed minerals,

electrokinetic, oleate adsorption and

solubility studies were conducted. The loss of selectivity

in the mixed mineral system at pH 7 to 10 is attributed to

the depletion of oleate by precipitation with cations,

dissolved mainly from dolomite. At pH 11, the selectivity

in mineral mixtures is lost due to possible modification of

the apatite surface when in contact with dolomite.

A new technique of two-stage conditioning prior to

flotation has been developed. This method involves

conditioning the feed at pH 10 with sodium oleate followed

by reconditioning at a lower pH. Selective flotation of

dolomite from apatite was observed both for single and mixed

minerals by reconditioning at pH lower than 4.5. To

understand the mechanisms of observed selective flotation,

further studies involving oleate adsorption, infra-red

spectroscopy, and solubility of the minerals were conducted.

Selective flotation of dolomite after two-stage conditioning

is attributed to the combined effect of higher oleate

adsorption on dolomite and the hydrolysis of the adsorbed

oleate species to oleic acid at lower pH values. This

technique has a potential for commercial application in the

flotation scheme currently practiced by the industry without

any additional reagent cost.



1.1. Statement of the Problem

Improved agricultural techniques in industrial nations

as well as in third world countries have led to a steady

growth in the production of fertilizers. To keep up with

this demand, world production of phosphate rock, 95 percent

of which is converted to fertilizers, also has steadily

increased. In 1952, the world production of phosphate rock

was 22.4 mm tons, as compared with 119.6 mm tons in 1982. In

the year 2000 the demand for the rock is expected to be 296.9

mm tons(1).

In the past, about 30 to 50 percent of the world supply

of phosphate rock came from the United States and 80 percent

of the U.S. production was contributed by the Florida

phosphate industry. The demand and supply for U.S. phosphate

rock from 1979 and the trend for next two decades are shown

in Table 1. The foreseeable deficit of phosphate minerals

after about a decade has led to the development of known

resources by new technology.

Until the present, the production of phosphate rock in

Florida has been from the Bone Valley formation. The

technology for processing the rock from this deposit has been

Supply and demand for

Year Supply Demand Deficit Surplus

1979 51.3 53.7 2.4
1980 54.7 55.0 0.3
1981 58.1 56.4 1.7
1982 60.9 57.8 3.1
1983 70.0 59.3 10.7
1984 72.1 60.8 11.3
1985 75.9 63.7 12.2
1986 82.8 63.9 18.9
1987 83.1 65.5 17.6
1988 85.0 67.3 17.7
1989 83.5 69.0 14.5
1990 84.1 70.7 13.4
1991 80.5 72.4 8.1
1992 77.7 74.3 3.4
1993 70.6 76.2 5.6
1994 71.5 78.2 6.7
1995 72.0 80.3 8.9
1996 68.8 82.4 13.6
1997 64.5 84.5 20.0
1998 66.3 86.8 20.5
1999 65.6 89.0 23.4
2000 63.3 91.3 28.0

Source: U. S. Bureau of Mine
A Supply-Demand Anal
May 19, 1981
[after reference(1)]

ysis of the U. S. Phosphate

Table 1.

phosphate rock in United

adequately developed to obtain a suitable product required

for the manufacture of the fertilizers. The pebble product,

-20,+1 mm, is produced by simply desliming and sizing and the

concentrate is obtained by a double stage froth flotation

circuit(2-4) When these deposits are depleted, the

replacement production will come from the southern extension

of the central district shown in Figure 1(5-12).

The future deposits in the southern extension consist of

two distinct geological ore types often referred to as upper

and lower zone. As compared to the currently processed ores

from the Bone Valley, the yield and grade of the phosphate

rock from south Florida are significantly lower. If these

ores were processed by conventional techniques, products

having the specifications shown in Table 2 would be obtained.

The products from the lower zone would be inferior in grade

to that of the upper zone.

The major problem in the production of fertilizers from

phosphate rock from the southern extension is the presence of

dolomitic (Ca Mg carbonate) impurities, usually reported as

weight percent MgO. The MgO content of the currently

produced phosphate rock is less than 0.5 percent. By

comparison, the deposits from the South Florida district

would yield phosphate products ranging from 0.2 to 25 percent

in MgO if processed by the conventional techniques(13-19).

Greater dolomite impurity in the rock is undesirable because

it interferes with the chemical processing as discussed




Figure 1.

Phosphate deposits in the current mining
district in Central Florida and its Southern
extension [after Bernardi and Hall(5)].


Table 2. Comparison of the chemical analyses of the
phosphate rock from Central Florida and its
Southern extension.

Southern Extension Central Florida
Upper Zone Lower Zone Bone Valley
reference (18) (18) (5)

Pebble (+16 mesh)
BPL 20 77 5 70 67

Feed (-16,+150
mesh) BPL 10 48 10 44 27

Concentrate (-16,
+150 mesh) BPL 55 80 54 73 73

% MgO-Pebble 0.2 10 0.5 25 0.4

% MgO-Concentrate 0.2 4 0.2 13 0.3

% Iron & Aluminum
Oxide Pebble 1.4 8 1 8 2.5

% Iron & Aluminum
Oxide-Concentrate 1.3 3.5 1.3 3.5 2.3

% Insol Pebble 2 50 3 70 7.6

A. Phosphoric acid is an intermediate compound during

the chemical processing of the phosphate rock into

fertilizers. The viscosity of this acid

exponentially increases(22) with percent MgO as

illustrated for three grades of phosphoric acid in

Figure 2. The higher viscosity results in higher

pumping costs and in certain cases it could make

pumping impossible.

B. Magnesium ions also precipitate complex salts that

would plug the process filters used to separate the

calcium sulfate by-product.

C. The maximum possible grade of fertilizer is also

downgraded in relation to the amount of dolomite.

D. During the production of phosphoric acid, the

carbonates in the rock would react with sulfuric

acid without yielding any useful product, thus

increasing the consumption of sulfuric acid.

It is widely accepted that the tolerance limit of MgO in

phosphate rock for fertilizer production is 1 percent. If

economical methods are not developed to eliminate dolomitic

impurities from the phosphate resources in Florida, it is

estimated that approximately 2 billion tons(23) of phosphate

rock cannot be mined. As a result, the phosphate rock would

not only be costly, but to maintain the same level of

production larger acreage have to be disturbed.


P. 104.4% H-304

75% H PO

.L-'50% HPO4-

Conc. of Mg,wt. %.

Figure 2.

Viscosity of simulated wet process phosphoric
acids [after Cate and Deming(22)].










Phosphate rock, which is essentially carbonate-

fluorapatite(24,25), could contain magnesium in one of the

following three possible ways.

i) Ionic substitution in the apatite lattice

ii) Second phase dolomite in the apatite matrix

iii) Discrete dolomite particles.

Magnesium present as the ionic substitute in the

lattice cannot be removed by any of the physical methods of

separation. The dolomite phase in the apatite matrix can be

physically removed provided it is liberated by further

grinding. Discrete dolomite particles can be separated by

the physical methods.

No detailed study of the distribution of magnesium ions

in the South Florida phosphate mineral has been reported.

One such investigation(25) is under way and the results are

yet to be reported. However it has been suggested(24) that

the amount of magnesium present as the substitute for Ca+2 in

the apatite lattice is well below the tolerance limit.

Furthermore, a few studies investigating the separation of

dolomite from apatite in the deposits of South Florida have

illustrated that significant reduction in MgO content can be

achieved by physical separation(13-19). This could be

possible only if a major fraction of the magnesium is present

as second phase dolomite either in apatite matrix or as

discrete particles.

In the past, several investigators have concentrated on

the separation of calcite from apatite which is the major

problem in other phosphate deposits of the world(26-51). The

efforts to eliminate dolomite impurities from south Florida

apatite have been limited to a few engineering studies which

were undertaken by the phosphate industry(6,15,16,19,52,53),

the U. S. Bureau of Mines(17,54) and the Tennessee Valley

Authority(55). These investigations were conducted only

under specific conditions and their results indicate that the

methods developed are either inefficient or uneconomical for

commercial application. However, these studies provide a

direction for the future research to eliminate dolomitic

impurities from phosphate rock.

It should be noted that no complete and systematic basic

studies involving mineral-water-surfactant systems using

phosphate minerals from South Florida have been conducted.

Such a study would increase the fundamental understanding of

the current processes so that they could be further improved.

1.2. Objective of This Study

The overall objective of this study is to develop a

suitable technique for achieving effective separation of

dolomite from south Florida apatite and to elucidate the

mechanisms of separation. Specifically, the objective was to

investigate in two phases the selective flotation of dolomite

from apatite using sodium oleate as the collector. The aim

of the first phase was to test the selective floatability of

apatite and dolomite under a wide range of pH and collector

concentration by conventional conditioning. In the second


phase, the objective was to investigate selective flotation

of these minerals after two-stage conditioning. The two-

stage process involved first conditioning the solids at

alkaline pH and then reconditioning at a lower pH.


2.1. Characteristics of the Minerals

2.1.1. Characteristics of Apatites in Phosphate Rock

The principal phosphate mineral in all commercial

phosphate rocks is apatite, which varies in physical,

chemical and crystallographic properties in different

deposits. The compositions of apatites in crystalline

sedimentary deposits, which are encountered in south Florida

are essentially fluorapatite [Ca5(PO4)3F] with extensive

substitutions of carbonate for phosphate and fluorine and of

other metals for calcium(56-58). These carbonate-

fluorapatites (also called francolite) have been assigned the

following formula(24)


where x is one fourth of the total number of carbon atoms.

The lattice structure of apatite is hexagonal and the lengths

of crystallographic axes are dependent on the extent of

substitution of other ions.

The carbonate apatite in most sedimentary phosphate

rock, including the south Florida deposits, is composed of

complex aggregates of microcrystals of size ranging from 0.02

to 0.2 microns(56,59). This morphology of the minerals

results in a very high specific surface area.


2.1.2. Characteristics of Dolomite

The chemical formula of dolomite is Ca Mg (CO3)2(60).

The crystal structure of dolomite is a rhombohedron with

cations at the corners and centers of the faces of the unit
cell and the CO3 ions at the center of the edges as well as

in the center of the cell. This mineral is known to have a

large surface area due to its porous nature.

2.2. Past Investigations of Apatite/Dolomite

Some of the physical properties of the apatite and the

dolomite minerals are shown in Table 3. Since both the

minerals are nonconducting and nonmagnetic, the separation

techniques based on these characteristics would not be

feasible. The specific gravities of both minerals are not

sufficiently different for the separation by conventional

gravity separation techniques. However,the 'apparent'

densities of the two minerals have been found sufficiently

different for the possible application of heavy media

separation(15). Another difference in the two minerals is

their hardness. Since dolomite is the softer mineral,

separation of dolomite after selective attritioning has been

attempted(16). Other methods of dolomite separation are

based on surface chemistry, namely selective froth flotation

and flocculation. The flotation studies to remove dolomite

from south Florida apatite have been limited to a few

laboratory scale tests under specific

Table 3. Comparison of physical properties of apatite and

Apatite Dolomite

Specific Gravity 3.1 3.35 2.86

Hardness (Mohs scale) 5 3.5 4

Magnetic nonmagnetic nonmagnetic

Electrical nonconducting nonconducting


conditions(6,15,16,19,52-55). However, several past

investigators(26-51) have studied the selective flotation of

apatite from calcite, which is the major impurity in other

phosphate deposits of the world. Selective flocculation has

not yet been attempted to remove dolomite from apatite. Some

of the relevant past studies are discussed below.

2.2.1. Heavy Media Separation

It was determined by Lawyer et al.(15) of the

International Minerals and Chemical Corporation that +1 mm

size fraction dolomite pebbles had more internal pores than

apatite pebbles of the same size. As a result, the dolomite

yielded a lower 'apparent' specific gravity than phosphate

pebbles because of the trapped air or moisture in these

pores. Measurements of numerous dolomite pebbles have

confirmed the true specific gravity in the range of 2.67-

2.84, and 'apparent' specific gravity was determined to vary

from 1.38 to 2.32. Porosities of dolomite has been estimated

to vary from 14.1 to 51.4 percent. The porosity differences

in the two minerals have permitted the heavy media separation

using an aqueous suspension of ferrosilicon or magnetite.

Laboratory scale heavy media separation using the Erickson

Cone showed a considerable promise as a processing tool for

separating dolomite from pebble phosphate in +1 mm size

fraction. However, the effective separation in the size

fraction smaller than 20 mesh was not achieved by this

technique. In the pilot plant tests, using a Dyna Whirlpool

(DWP) heavy media circuit, dolomite separation from apatite

was successful, but at phosphate recoveries of 52 to 74

percent only.

2.2.2. Selective Attritioning

As indicated in Table 3, hardness of the apatite in Mohs

scale is higher than that of dolomite. It was also confirmed

by Iwasaki and Soto(61) that a constant agitation of mineral

slurries containing 33 percent solids resulted in a

relatively higher attritioning of dolomite. Predali et

al.(16) have also investigated the removal of dolomite by

selective attritioning. In this study three different size

fractions, -32,+250 mesh, -16,+32 mesh and +1 mm pebbles were

tested separately. Attritioning was done in a cell which was

fitted with a turbine comprising two propellers rotating at

940 rpm. It was also found that a cleaner phosphate product

could be obtained by introducing a weak grinding step in the

circuit. Forty to 60 percent of the dolomite was eliminated

by this method with phosphate losses of below 10 percent for

-1 mm fraction and 27 percent for +1mm fraction. To obtain

further reduction in MgO content, these investigators

recommended subsequent selective flotation of dolomite.

2.2.3. Selective Flotation of Minerals

As mentioned earlier, only a few studies have been

reported on selective flotation of dolomite from south

Florida apatite. However, several investigators have studied

the separation of calcite from foreign phosphate deposits.

Since the chemical composition of calcite (CaCO3) is

analogous to dolomite [Ca Mg (CO3)2], the results of some of

these investigations are discussed below.

Both cationic flotation using amines and anionic

flotation using mostly fatty acids or their salts have been

studied in the past. Cationic flotation

An extensive investigation to separate francolite from

dolomite by cationic flotation was undertaken by the

International Minerals and Chemical Corporation in the late

seventies(14,46,52,53). The process utilizes the cationic

collectors, usually primary amines in combination with

kerosene, as the extender. For adequate removal of dolomite,

a rougher float followed by several cleaner flotation stages

was suggested. The requirement of several flotation stages

to reduce the MgO content to below 1 percent also decreases

the process efficiency. Another disadvantage of this process

is that the major component, francolite, which often

constitutes over 90 percent weight of the feed, must be


To improve the efficiency and to explore the possibility

of reversing the flotation procedure Iwasaki and Soto(61)

continued the above study on dolomite and apatite from south

Florida. They concluded that there is a chemical

interaction between the amine collector and the phosphate

ions present on the francolite surface. The selectivity was

attributed to the lower solubility of the amine-phosphate

complex formed on the apatite as compared to the amine-

carbonate complex that could form on dolomite. Thus, they

concluded that selective flotation of dolomite, instead of

apatite, can not be achieved by using the cationic collector.

Another study on Egyptian ores by Hanna(27), should also

be noted. These attempts to selectively float francolite

from calcite using cationic collectors were not successful.

Nonselectivity of flotation in this case was attributed to

the lack of significant differences in the adsorption power

of amine salts on the two minerals.

In order to improve the selective flotation of calcite

and apatite using cationic collectors, several investigators

studied the mechanisms of adsorption of amines on these

minerals. Some of the noteworthy studies include those of

Taggart and Arbiter(28), Klassen and Mokrousov(29), Cases et

al.(30), Solnyshkin and Cheng(31), Kuz'kin and Cheng(32) and

Levinskii(33). These studies found that amines chemically

interact with both apatites and carbonates to form complexes

of mineral anion and collector cation. Anionic flotation

Four major studies pertaining to the separation of

dolomite from south Florida phosphate rock were conducted at

International Minerals and Chemicals Corp.(15), Minemet

Recherche, France(16), U.S. Bureau of Mines(17,54) and

Tennessee Valley Authority(55). The results of these studies

as well as some of the investigations of similar minerals

from other domestic and foreign deposits are discussed below.

At the International Minerals and Chemical Corporation,

Lawyer et al.(15) examined selective flotation of dolomite

using several anionic collectors, including fatty acids and

their soaps, sodium alkyl sulfates, and petroleum sulfonates.

These collectors were used at pH 5.6-6.2 with inorganic

depressants such as phosphates and fluorides. It was

observed that sodium tripolyphosphate and hexametaphosphate

with an anionic collector (unidentified because of pending

patent application) yielded the best results. This technique

was tested in the laboratory and pilot plant. The

application of this process to size fraction finer than 48

mesh and containing less than 2 percent MgO resulted in less

than 1 percent MgO product at BPL recoveries in the range of

50 to 90 percent.

Predali et al., at Minemet Recherche, France(16),

developed an anionic flotation scheme to eliminate residual

carbonate in the phosphate rock in which dolomite was

partially separated by selective attritioning. After the

flotation feed had been washed, the phosphate concentrate was

subjected to conditioning with anionic collector and a

depressant for phosphate particles at pH 5.5. The reagents

used have not been identified in the literature. The final

product, containing less than 1 percent MgO, was obtained

with BPL recoveries of 48 to 85 percent. Higher recoveries

were obtained for particles finer than 32 mesh.

Llewellyn, Davis, and Sullivan at the U.S. Bureau of

Mines(17) conducted studies to recover phosphate from four

dolomitic southern Florida deposits. These samples contained

5.8 to 10.2 weight percent P205 and 1.7 to 4.8 percent MgO.

In the conventional one-step fatty acid-fuel oil batch

flotation tests at pH 9.2 to 9.6, phosphate was floated while

silica and carbonates were depressed. It was observed that

the addition of sodium silicate aided in further depression

of the dolomite particles. The rougher concentrate was

further cleaned three times. The process yielded phosphate

rock containing 29 to 31.3 percent P205, 0.47 to 1.36 percent

MgO and 4.0 to 6.4 percent insol on a laboratory batch scale.

The flotation recovery of P205 ranged from 72.4 to 96.1

percent. Since the final grade acceptable for the

manufacture of fertilizers could not be achieved by

flotation, the investigators recommended additional removal

of dolomite by SO2 leaching.

In another study by the U.S. Bureau of Mines, Rule et

al.(62-64) developed a technique to selectively float

carbonates from western phosphates. The feed material was

conditioned with fluosilicic acid to depress phosphate

minerals and with an aqueous fatty acid emulsion at slightly

acidic pH. The fluosilicic acid renders the surface of the

phosphate minerals hydrophillic and prevents adsorption of

the fatty acid. The fatty acid then becomes a selective

collector for carbonate minerals. The laboratory flotation

tests yielded concentrates containing about 0.75 percent MgO

with flotation recoveries of 70 to 80 percent P205.

A recent patent(55) issued to the Tennessee Valley

Authority describes the use of alkyl phosphonic acids to

depress Florida phosphate while floating dolomite with oleic

acid as the collector at pH of 6 to 7.

The anionic flotation of calcium and magnesium carbonate

impurities from finely ground (60-80 percent -200 mesh)

western U.S. phosphorites, using various phosphate and

fluoride compounds as the phosphate mineral depressant at

slightly acidic conditions have been developed by Cominco

Ltd. (65-68). Phosphate concentrates analyzing less than 1.0

percent MgO were obtained from dolomite flotation feeds

containing 1.6 percent MgO with P205 recoveries of 85 percent

or higher.

Johnston and Leja(43) determined that calcite and

dolomite floated at pH 6,using oleic acid as the collector,

whereas fluorapatite is fully depressed in the presence of

phosphate ions at that pH. The difference in flotation

behavior may be explained by the characteristic hydrogen

bonded phosphate ions that adsorb strongly on apatite. On

the other hand, it is suggested that the weaker hydrogen

bonding is occurring on the dolomite due to CO2 gas evolution

from carbonates in the acidic solutions.

Smani et al.(47) have reported separation of calcite

from a phosphate ore obtained from a Moroccan deposit using

0.9 Kg/ton (2 lb./ton) of fatty acid and a few hundred grams

of a 2:1 mixture of aluminum sulfate and tartaric acid as the

phosphate depressant. The best result indicated an upgrading

of a 29 percent P205 feed to 34 percent P205 concentrate with

95 percent P205 recovery at pH 5 to 6.

Mitzmager and coworkers(48) have reported that the

addition of monosodium phosphate significantly improved the

flotation of calcite from phosphate using fatty acid as the

collector. The calcite used in this investigation was from

Israel's Negev region.

Calcite in the -100 mesh phosphate rock from eastern

Turkey was separated by Onal(49) using phosphoric acid and

sodium oleate. However, the minimum MgO content of the

concentrate achieved was 1.4 percent, which is higher than

what is considered to be acceptable by the fertilizer

producers in the United States.

The other noteworthy studies include that of Belash(35),

Hanna et al.(36), and Ratobylskaya(37). These studies like

the ones discussed above have shown that selective flotation

of dolomite can be achieved at slightly acidic conditions

provided a suitable phosphate depressant is used with the

fatty acid collector.

Investigation by Predali(69) on the flotation of

dolomite and magnesite from Kosice deposits, Czekoslovakia

should also be noted. He studied flotation as a function of

pH and fatty acid chain length. The flotation of carbonates

in the acidic pH range increased as chain length of the

collector decreased from 18 to 12. On the other hand in the

alkaline pH range, the recovery of carbonates was higher with

the sodium oleate (18 carbon chain) than with sodium laurate

(12 carbon chain).

The low separation efficiency and additional cost of the

apatite surface modifiers are some of the major limitations

in applying the anionic flotation scheme on a commercial

scale. It should be noted that none of the studies involving

anionic flotation of minerals from South Florida deposits was

reported for the complete pH range (pH 3 to pH 11) or under a

wide range of collector concentrations. Furthermore, none of

these studies attempted to explain the mechanisms of

selective flotation.

The basic principles of aqueous suspensions and

flotation of minerals are discussed next.

2.3. Basic Principles

The froth flotation(70-74) is a process for separating

different solids based on their physico-chemical nature.

This process has permitted the mining of low grade and

complex ores which would have been worthless if more

conventional methods, like gravity concentration, were relied

upon. Flotation of mineral particles results from the

attachment of gas bubbles to the solids while they are

suspended in aqueous solution. The float fraction, which is

the fraction of the minerals attached to the gas bubbles, is

collected in the froth that is later skimmed off. Although

this process has been in use for seventy years, the basic

principles are still not completely understood.

The fundamental requirement for particles to float is

that they should be preferentially wetted by gas and not

water, i.e. the mineral surface should be hydrophobic(75-77).

Only a small fraction of minerals like sulfur are naturally

hydrophobic. In the case of most of the other minerals

hydrophobicity has to be imparted onto the surface with a

surfactant(78) that will selectively adsorb on the mineral to

be floated. This surfactant, also called the collector, is

added to the mineral slurry to achieve its adsorption at the

solid/liquid interface. The success of the flotation process

lies in obtaining the selective hydrophobic character of the

minerals to be separated. The particle bubble attachment and

froth stability are also important factors that would

influence the efficiency of the flotation process.

Apatite and dolomite minerals are both salt type

minerals, whose solubility is lower than that of simple salt

minerals, but higher than that of oxide and silicate

minerals. Separation of these minerals is extremely

complex(26) because of their dissolution.

To understand the flotation properties, it is necessary

to examine the solubility, the aging behavior and

electrokinetic behavior of these minerals and the adsorption

of various organic and inorganic species present in the

system that could influence the various interfaces, namely

solid/liquid, and liquid/gas.

2.3.1. Solubility of the Minerals

Solubility of the minerals is of importance because of

the the role of dissolved ions in determining both the

chemical composition of the aqueous phase and the charge

characteristic of the mineral interface. When a salt is in

contact with water, lattice ions will be transferred to the

solution until the chemical potentials in both phases are

identical. Several studies(79-84) have been attempted in an

effort to understand the solubility behavior of apatite, but

none of them is adequate. The major discrepancies in the

data are attributed to the impurity ions in the lattice or

the added electrolytes. However Saleeb and deBruyn(85) have

shown that a constant solubility product can be obtained if

precautions are taken to obtain a stoichiometric compound.

In the case of dolomite the published pK values are in the

range of 16.5 to 19.5(79).

In most salt-type minerals such as calcite, dolomite, and

apatite dissolved ions in solution undergo various hydrolyses

and complex reactions.

In aqueous slurry, fluorapatite undergoes dissolution to

produce Ca+, HPO4 -2, and F-1 ions, which would undergo

further hydrolysis and complex formations according to the

reactions given below(86).

Ca2 + OH-1 CaOH+1 (1)

CaOH+1 + OH-1 # Ca(OH)2 (2)

Ca(OH)(aq) Ca(OH)(s) (3)
2(aq) 2(s)

PO4 3 + H20 # HPO4-2 + OH-1 (4)

HPO4-2 + H204 H2PO41 -+ OH-1 (5)

H2PO4-1 + H20 H3PO4 + OH-1 (6)
Ca+2 + HPO4-2 t CaHPO4 (7)
+2 -1 +1
Ca+ + H2PO4- CaH2PO4 (8)

F-1 + H20 + HF + OH1 (9)

Ca+2 + 2F-1 i CaF2 (10)

Calcite, when dissolved in water, will undergo the

following reactions(87).

CaCC Ir CaCo
CaCO3(s) Ca3(aq) (11)
+2 -2
CaCo q Ca2 + CO32 (12)
3(aq) 3
-2 -
CO3-2 + H20 HCO3-1 + OH-1 (13)
-1 -
HCO3-1 + H20 0 H2CO3 + OH-1 (14)

H2CO3 = CO2(g) + H0 (15)

Ca+2 + HCO3-1 CaHCO3+1 (16)

CaHCO3 +1 # H+1 + CaCO (17)
3 3(aq)
Ca+2 + OH-1 V CaOH+1 (18)

CaOH+1 + OH-1 Ca(OH)2(aq) (19)

Ca(OH)2(aq) & Ca(OH)2(s) (20)
2(aq) 2(s)
For the dolomite-water-CO2 system the following

reactions should be added to the reactions 13-20(88).

CaMg(CO3)2 C Ca+2 + Mg+2 + 2Co3-2 (21)
+2 -1 +1
Mg2 + HCO3-1 MgHCO3 (22)

MgHCO3+1 0 MgCO3(aq) + H+1 (23)
Mg+2 + OH-1 0 MgOH+1 (24)

MgOH+1 + OH-1 v Mg(OH)2(s) (25)

MgOH+1 + OH-1 # Mg(OH)2(aq) (26)

2.3.2. Aging Behavior of Mineral Slurries

When the dry minerals are suspended in water, the

dissolution of the minerals followed by the hydrolysis of the

dissolved ions and complex formation reactions, as presented

in the previous section, causes the pH of the slurry to

shift with time to an equilibrium value. This is often

referred to as the "aging phenomenon". It is of importance

in the present investigation since pH is the most critical

flotation variable. The aging behavior of apatites and

carbonates has been studied by the previous investigators.

Somasundaran(86) observed that the pH of the apatite

slurries decreased with time if the initial value was greater

than 7, whereas slurry pH increased if the initial value was

lower than 7. No significant changes were observed if the

starting pH was 7. He also noted- that it was necessary to

condition the minerals for two weeks to obtain constancy in

pH and zeta potential as a function of aging time. It should

be further noted that not only the slurry pH but also the

zeta potential (obtained by streaming potential) changed in a

similar manner. The isoelectric point, defined as the pH of

zero zeta potential value, shifted from pH 4 to 6 as

equilibrium was reached. Iwasaki and Soto(61) observed an

equilibrium pH value of about 6.7 for apatite from Florida


Agar and Somasundaran(87) studied the calcite-water

system. They observed a trend similar to that of the apatite-

water system. The equilibrium pH was found to be 8.2, which

was in agreement with the value calculated from thermodynamic

data. However, the isoelectic point of the mineral had an

initial value of 10.8, which shifted to 9.5 upon


Predali and Cases(88) studied the aging phenomena of

magnesite and dolomite from Kosice, Czekoslovakia. They

obtained equilibrium pH values very close to the calculated

isoelectric point of the solution, namely 8.46 and 7.96 for

magnesite and dolomite, respectively. Iwasaki and Soto(61)

determined an equilibrium pH value of about 8.4 for dolomite

from Florida.

2.3.3. Electrochemical Aspects of Mineral-Water Interface Electrical double layer

When a mineral surface is brought into contact with

water, it acquires a surface charge(79,89-92). The possible

charging mechanisms are ionisation, ion adsorption and ion

dissolution. Besides this charging mechanism, the charge

characteristics of the mineral particles can be altered by

the adsorption of the dipolar molecules from the solution.

The surface charge on the solid surfaces influences the

distribution of the dissolved ions in the aqueous media.

Ions of opposite charge (counter ions) are attracted towards

the surface and the ions of like charge (co-ions) are

repelled from the surface. This, together with the thermal

effects, leads to the formation of an electric double layer.

This layer is made up of a charged surface and a neutralising

excess of counter ions over co-ions distributed in a diffuse

manner. The theory of double layer deals with the magnitude

of the electrical potentials which occur on the charged

surfaces. This is an important step toward understanding

several experimental observations such as electrokinetic

behavior of minerals and electrostatic forces governing the

adsorption of organic or inorganic ions. Details of the

electrical double layer phenomenon can be found in several

research papers(79,89-92). Quantitative determination of the potential at the
mineral-water interface

It is widely accepted that the measurement of the

electrokinetic effect is representative of the potential at

solid/liquid interface. The electrokinetic phenomenon is the

mobility of the charged particles caused by applying an

electric field and conversely an electric field induced by

the relative motion of such a surface. In each case the slip

plane between the double layer and the medium is involved.

Thus the electrokinetically measured value, called zeta

potential, is the potential at the plane of shear in the

double layer and strictly speaking not the surface potential.

In the absence of any direct means of measuring the actual

surface potential, zeta potential has provided valuable

information about the nature of solid/liquid interfaces.

There are four electrokinetic methods of measuring zeta

potential, namely electrophoretic mobility, streaming

potential, electro-osmosis, and sedimentation potential.

Electrophoretic mobility is the most developed method of

measuring zeta potential because reproducible results could

be obtained with relative ease.

Electrophoresis is the movement of charged colloidal

particles in suspension under a potential gradient. The

velocity of the particles can be the measured by a

microscope. This motion of a particle under applied field

directly proportional to its zeta potential:

S = ( )(4n )

1 = v/H


where 5 = zeta potential

y = electrophoretic mobility

n = viscosity of the fluid

e = dielectric constant

v = velocity of the particles in the

electrical field

H = Applied potential drop per unit length Isoelectric point of the minerals

In the past two decades, several attempts(84-87,93-101)

have been reported to characterize the electrokinetic

behavior and to understand the surface charge generation

mechanism for apatite and calcite. A few attempts have also

been reported(88,102-104) for dolomite. All investigators

have found H and OH to be the potential determining ions

for these minerals. In other words the value and sign of the

zeta potential is dependent on the pH of the slurry. A

particular mineral-aqueous system has a characteristic

isoelectric point pH at which the zeta potential has a zero

value. Below or above this pH, the mineral has either

positive or negative potential. Thus, the isoelectric point

is the parameter that is often used to compare the

electrokinetic behavior of different mineral slurries. The

reported values of the isoelectric point for apatites

(hydroxyapatite and fluorapatite), calcite, and dolomite are

summarized in Table 4, Table 5, and Table 6, respectively.

As seen from these data, there is a wide range of variations

observed in the results obtained by different investigators.

This could be attributed to the differences in the

experimental techniques, sample preparation, and the

impurities present on the mineral surfaces. Mechanisms for surface charge generation

The mechanisms of surface charge generation for salt-

type minerals are basically different from those of simple

oxide minerals(26,75,77,86-88,105-108), owing to the

relatively high solubility of the former. In addition to any

preferential dissolution of the constituent species, the

surface charge development is also .influenced by the

formation of various complex species due to the hydrolysis

reactions discussed in section 2.3.1. These ionic species

may be produced at the solid/liquid interface or in the bulk

Isoelectric point of apatite.

Sample pH Method IEP electrolyte Ref.
description modifier

1. Fluorapatite
Aged Hours

2. Fluorapatite
Indian Ocean

3. Fluorapatite

4. Fluorapatite

5. Francolite

6. Fluorapatite

7. Fluorapatite

8. Hydroxy-

9. Hydroxy-

10. Hydroxy-

11. Apatite


HC10 /







Streaming pH 5.6

Electro- pH 3.5

Electro- pH 5.5











pH 3.9-

pH 6.7





pH 6.5 KNO

pH 8.5


pH 7.15 KNO3

pH 6.5 KNO3

pH 3.8












Table 4.

Table 5. Isoelectric point of calcite.

Sample pH Method IEP electrolyte Ref.
description modifier

1. Natural Electro- pH 10.8 _(100)
Aged Hours phoresis

2. Natural HNOI/ Streaming pH 10.8- KNO3 (87)
Iceland Spar NaOH potential 9.5
Aged Days

3. Natural HC1/ Streaming pH 5-6 (45)
Morrocco NaOH potential

4. Natural NaOH Electro- pH 9.5 (101)
Micro- phoresis

5. Natural HC104/ Electro- pH 8.2 NaClO4 (95)
Broken Hill NaOH

6. Natural Electro- pH 10.1 (93)

Table 6. Isoelectric point of dolomite

Sample pH Method IEP electrolyte Ref.
description modifier

1. Natural HC1/ Streaming pH<7 KC1 (88)
Kosice, NaOH Potential

2. Natural Streaming pH<6 (103)
Bela Stena Potential


solution and subsequently adsorb on the mineral in amounts

proportional to their concentration in the solution.

Besides the H+, OH and constituent ions of the

minerals, other 'foreign' ions, both inorganic and organic,

could also affect the zeta potential of the minerals by their

adsorption(75,109-111). The adsorption of polyvalent metal

cations which results in significant alteration of the

minerals surface charge characteristics is known(112,113) to

occur when the cations involved are hydrolyzable. In the

case of quartz, a charge reversal, from negative to positive,

was observed in the presence of Pb+2(114), Fe 2(112) and

Mg+2(114-118) at the pH values corresponding to the

hydrolysis of these cations. In flotation or flocculation

processes, the surfactants or polymers are added so that the

particles will acquire the desired surface properties. The

charge characteristics of the adsorbed organic ions are also

known(111,119) to have an influence on the electrokinetic

behavior of the minerals.
2.3.4. Adsorption of Surfactants

In flotation, the adsorption of surface active agents on

solid/liquid and liquid/ gas interfaces is of significance.

In the literature, the adsorption of the flotation

collectors, as well as their interactions with coadsorbents,

like extenders, brothers. and polymers(74,91,120-126), has

been reported. Since this study does not include the use of

any other surface active agents, only the collector

adsorption on mineral/water and water/air interfaces will be

discussed. Adsorption on mineral-water interface

Except for some naturally occurring hydrophobic

minerals, hydrophobicity on most minerals to be floated is

achieved by adsorption of the collectors at the solid/liquid

interface. A suitable collector and adsorption conditions

are established so that the selective hydrophobicity is

obtained for the particular minerals to be separated.

Adsorption mechanisms. Adsorption of the surfactants or

their complexes in solution occurs on the minerals due to the

interaction between the solute and surface species on the

solid. Density of adsorption of the surfactant is commonly

interpreted using the Stern-Graham equation(127-130):

6 = 2r C exp (- AGoads /RT)

where rF = adsorption density in the plane 6 ,which is

at the distance of closest approach of counter ions to the


r = the effective radius of the adsorbed ion.

C = the bulk concentration of the adsorbate.

R = gas constant.

T = absolute temperature.

AG ads= the driving force for adsorption. This is the

sum of a number of contributing interactions between the

solute and the mineral surface as shown by the equation

AG0 = AG0 + AG0 + AG0 + AG0 + AG0
ads elec chem c-c c-s H
+ AGo

AG0 = electrostatic interaction between the electrical
field on the mineral/water interface and the charge on the

surfactant or its complex.

AGo = chemical interaction due to covalent bonding of
surfactant to the mineral surface specie.

AG = lateral interaction term between the long carbon
chain of the adsorbed surfactant.

AG = interaction terms between the hydrocarbon chains
of the adsorbed surfactant and the hydrophobic sites on the


AGH = interaction due to the adsorption of surfactant

by hydrogen bonding.

AGo 0= dissolvation term due to displacement of any
species from the interface due to adsorption.

The adsorption of surfactants at the solid/liquid

interface can be classified into the two general categories,

physical adsorption or chemisorption. These categories have

been arbitrarily defined by the amount of adsorption energies

involved. If the free energy of adsorption is less than 10

kcal/mole(106), then adsorption is physical, whereas it is

chemisorption if the energy of adsorption is higher. Except

for the adsorption by covalent bonding chemisorptionn), all

other adsorption mechanisms, mentioned in the previous

paragraph, are physical. Physical adsorption is reversible,

whereas chemisorption is irreversible. For a particular

mineral-water-collector system, the adsorption could be

physical, chemical or a combination of both.

The major factor that determines the type and the amount

of adsorption beside the syrface charge of the minerals is

the nature of the surfactant. Even for the same surfactant,

adsorptive power may vary as a function of pH because of

changes in the oleate solution chemistry(129). Furthermore,

for a particular salt-type mineral-water-oleate system, the

adsorption by surface precipitation of oleate also has been

recently found to be the major contributing factor(131).

Influence of oleate solution chemistry. In addition to

micellization and precipitation, certain surfactants, like

fatty acids and amines, undergo hydrolysis and associative

interaction to form dimers, trimers and acid-soap

complexes(132,133). The concentration of the hydrolyzed

species and various complexes is influenced most by the

solution pH. Each of these species would not only have

different adsorption behavior but would also induce varying

degree of hydrophobicity on the mineral surface. The

solution chemistry of oleate has been studied in detail by

Somasundaran et al.(134-137). They constructed an

equilibrium diagram, based on estimated and experimentally

determined reaction constants, showing the activities of

various species of oleate at a concentration of 3 x 10-

kmol/m3. It is seen further in Figure 3 that the

precipitation of oleic acid occurs below pH 7.78. The

maximum acid-soap complex formation also occurs at this pH.

3 4 5 6 7 8 9 10 II


Figure 3.

Oleate species distribution as a function of pH
[after Ananthpadmanabhan and Somasundaran(136)].

12 13

In spite of the low numerical value of the acid-soap complex

concentration, it is sufficient to affect the adsorption and

flotation. The surface tension studies on oleate solution as

a function of pH suggest the presence of highly surface

active species in the pH range at which these complexes are

expected to be present in maximum amount. Also, in the case

of hematite, the narrow alkaline pH range in which flotation

is observed was found to coincide with the pH at which these

complexes are present. Moreover, as shown in Figure 4(136),

a number of minerals have exhibited maximum flotation with

fatty acid collector around this pH range. These studies

suggest an active role of acid-soap complex in adsorption and

flotation of minerals. The adsorption of monomers and dimers

could also occur at alkaine pH. However, at higher values,

pH 11 or above, there would be an increased competition for

adsorption from OH ions(139). It should be noted that the

contribution of oleate dimers in making the mineral surface

hydrophobic would depend on the orientation of the adsorbed


Precipitation of oleate soaps. Oleate anions are known

to precipitate readily with multivalent cations present in

the solution(140). This is attributed to high pK value of

the precipitation reactions as shown for the calcium and

magnesium oleate precipitates in Table 7(141-143). The

precipitation could occur in the bulk solution or at the

mineral/water interface. The bulk precipitation would


Figure 4.

Flotation recovery of minerals as a function
of pH using fatty acid collector, 1 Kg/ton.
1) columbite 2) zircon 3) tantalite 4)
ilmenite 5) rutile 6) garnet, 7) tourmaline,
8) albite, 9) perovskite [after
Ananthpadmanabhan and Somasundaran (136)].

Table 7. Solubility product of calcium oleate and
magnesium oleate.

ref. 141 ref. 142 ref. 143

Calcium Oleate 3.98 x 10-13 2.51 x 10-16 2.04 x 10-20

Magnesium Oleate 1.58 x 10-11 6.31 x 10-16 4.68 x 10-19

deplete the free oleate anions available for adsorption. On

the other hand, the precipitation on the mineral surface

would increase the oleate adsorption at the interface, thus,

it would be an additional mechanism for adsorption. Since

the precipitation requires the presence of dissolved cations,

it would be significant for the salt-type minerals. On the

basis of adsorption isotherm for calcite at pH 10 as

illustrated in Figure 5(145), this mechanism was

recently(131,144) proposed. The slope of the isotherm

increased several fold beyond a certain minimum collector

concentration. It has been suggested, that due to the

soluble nature of the salt-type minerals, there is a higher

concentration of dissolved cations at the interface. As a

consequence in such systems, the oleate precipitation could

occur on the surface at lower collector concentration than in

the bulk. It should be noted that a similar adsorption

isotherm was also observed for magnesite by a Predali and

Cases(146). The mechanisms of surface precipitation

phenomenon are not yet completely understood. Adsorption on solution-gas interface

The attachment of particles to gas bubbles is the most

important step in the flotation process. If more gas bubbles

are available for attachment, the flotation kinetics could be

further improved. In a pure liquid, the smaller bubbles

coalesce to form larger bubbles, whereas in surfactant

solutions, a higher number of finer and more stable bubbles





Figure 5.

Eq. Conc. of Oleate, Kmol/m3

Adsorption isotherm of oleate on calcite at pH
9.6 [after Somasundaran(145)].

are present. The adsorption of the collector on the

liquid/gas interface, therefore, aids the flotation kinetics

by increasing the number of stable gas bubbles available for

particle attachment. It has also been suggested that the

adsorbed collector at the L/G interface could affect the

induction time for particle-bubble attachment. The

phenomenon of particle-bubble attachment is not sufficiently

well understood to isolate this role of the collector

adsorption at L/G interface in the overall flotation process.

Orientation of the collector molecules adsorbed at the

water/air interface is such that the polar group stays in the

solution and the nonpolar hydrocarbon chain points toward the

gas phase resulting in lower surface tension. Measurement of

surface tension, therefore, represents one of the common

methods of determining adsorption surfactants at the L/G

interface. The surface tension measurements are mostly

conducted under equilibrium conditions. In the flotation

process, a bubble is only a fraction of a second old and

adsorption on its surface can be different from that

determined under equilibrium conditions. The effect of the

dynamic nature of surface tension of oleate solution has been

illustrated in a previous investigation(147).


3.1. Selection of the Mineral Separation Technique

Past investigations of some of the separation methods

(refer to Section 2.2.) suggest that selective flotation is

the technique of greatest potential for separating these

minerals. For a better process efficiency in a large-scale

operation, the selective flotation of the minor mineral,

dolomite, is preferred(15). Cationic flotation using amines

as the collectors, as attempted in the past, yielded lower

process efficiency, as well as resulted in the flotation of

the major mineral, apatite. On the other hand, anionic

flotation resulted in the selective flotation of dolomite.

Although the past studies on the anionic flotation did not

result in an economically feasible technique for a commercial

application, they have demonstrated a potential for

eliminating dolomite impurities in the phosphate rock. Thus

anionic flotation, using sodium oleate as the collector, has

been selected for investigation in the current study. It

should be noted that sodium oleate, which solubilizes readily

in water, was selected because oleic acid is the major

constituent of the fatty acid collectors currently used by

the mineral industry.

3.2. Selection of Minerals and Preparation Methods

Selection of the natural minerals from South Florida

deposits for a systematic study was governed by the following


A. Since the objective of this study is to develop a

technique to eliminate dolomite impurity in south Florida

phosphate rock, the choice of natural minerals from the same

location was considered to be appropriate.

B. Synthetic dolomite and carbonate-fluorapatite are

not commercially available. In the laboratory, they could be

synthesized only in a powder (-200 mesh size) form, which is

not a suitable size for flotation tests.

The sample preparation method was selected so that these

minerals are not exposed to any surfactants prior to testing.

The dolomite pebbles, which did not contain any significant

apatite impurity, required only a washing step to remove the

clays. On the other hand, since apatite pebbles of

sufficient purity could not be obtained, the flotation feed

containing the mixture of apatite and silica was used.

Electrostatic separation, instead of the conventional

flotation using amines or fatty acids, was selected as a

method to remove silica from the feed.

3.3. Outline of The Investigation

The following steps were followed in the current study:

A. Characterization of the mineral samples by

determining their chemical compositions, surface

area, solubility and aging behavior.

B. Flotation tests by conventional conditioning under

a wide range of pH and collector concentration. It

should be noted that all earlier studies(15-

17,54,55) were conducted at slightly acidic pH

conditions using an additional surface active agent

to depress apatite flotation. This part of the

current study was planned to determine if selective

flotation of the minerals could be obtained at

other pH values under a wider range of collector

concentrations without using any other types of

surface additives.

C. Flotation tests by two-stage conditioning which

involve first conditioning at high alkaline pH and

then reconditioning at a lower pH. Reconditioning

was done both with and without reducing collector

concentration. In the preliminary tests on calcite

and alumina, it was determined(148) that after two-

stage conditioning, there was selective flotation

of carbonate. Therefore, it was also planned to

test the selective flotation of the dolomite from

apatite by similar technique. It should be noted

that there is a need to test new conditioning

methods of achieving surface hydrophobicity,

because conventional conditioning techniques, using

some of the less costly and easily available

collectors like fatty acids, are not effective.

D. Elucidation of mechanisms involved in the observed

selective flotation by oleate adsorption studies,

electrokinetic measurements, determination of the

nature of the adsorbed species, and froth


3.4. Selection of the Experimental Techniques

The following experimental techniques were selected.

3.4.1. Flotation Apparatus for flotation tests

A Hallimond cell was selected for flotation studies.

This device was preferred because it is easier to control the

parameters like flotation time, agitation, air flow rate and

collection of the float fraction in the Hallimond cell than

with other laboratory scale flotation apparatus. It is also

recognized, however, that a direct correlation of Hallimond

cell results to a laboratory cell in terms of the percent

recovery may not be achieved, but it is expected that the

trends in the flotation behavior will be reproduced in the

bench scale tests(149,150). Particle size of the minerals

Generally, the size fraction -16,+150 mesh is used for

the flotation of phosphate rock. However, at the extremes of

this size range, the flotation recovery usually becomes

sensitive to the particle size. To eliminate the possible

effect of particle size on the recovery, a narrow size range

-65,+100 mesh was selected. As discussed in Chapter IV, this

size range was also found to be suitable for the oleate

adsorption experiments.

3.4.2. Oleate Adsorption Studies

These tests were conducted to determine the amount of

oleate adsorbed on the surface by altering the conditioning

or reconditioning variables. Oleate adsorption measurement

using solution depletion method was selected.

3.4.3. Electrokinetic Measurements

The determination of zeta potential of minerals can lead

to a better understanding of oleate adsorption

characteristics and thus their flotation behavior. In this

study, the electrophoretic technique of measuring the zeta

potential was selected.

3.4.4. Determination of the Nature of the Adsorbed
Oleate Species

To explain the differences in flotation behavior under

different pH conditions, it is necessary to identify the

nature of the oleate species adsorbed. Since the oleate

adsorption studies discussed earlier are not expected to

yield any information about the nature of the adsorbed

species, transmission IR spectroscopy was selected to

identify the adsorbed species.

3.4.5. Bubble and Froth Characteristics

Knowledge of changes in the size of the bubbles and the

characteristics of froth would aid in understanding their

role on the flotation behavior. Unlike past investigators,

who extrapolated the surface tension measurements to the

flotation conditions, the in-situ determination of bubble

size and froth characteristics in the Hallimond cell was

selected to study the role of liquid/gas interface on the

flotation recovery.

3.5. Selection of Experimental Variables

Based on the preliminary tests and the data reported in

the literature, the following variables were selected.

3.5.1. pH

pH of the slurry is one of the important variables for

flotation because the surface charge characteristics of the

minerals and the solution chemistry of the collectors are

governed by this parameter. By correlating the flotation,

oleate adsorption, electrokinetic behavior of the minerals

and solution chemistry of the collectors as a function of pH,

the observed selectivity in mineral flotation can be

understood. The pH range, 3 to 11, which is feasible in

practice, was selected for testing.

3.5.2. Collector Concentration

Collector concentration was selected as the system

variable since this factor is known to influence the

adsorption and flotation processes. On the basis of the

preliminary flotation tests, the range of collector

concentration studied was determined to be in the range of 0
-4 3
to 1.87 x 104 kmol/m However oleate adsorption tests were

conducted up to 4.7 x 10-4 kmol/m3

3.5.3. Presence of Foreign Ions

Since both apatite and dolomite are soluble, the

dissolved ions from one mineral could affect the surface

charge behavior of the other mineral. Wherever necessary the

apatite tests were conducted with the ions dissolved from

dolomite and vice versa.

3.5.4. Ionic Strength

Ionic strength effects due to modifications in pH and to

the presence of the dissolved ions were tested up to 3 x 10-2


3.5.5. Apatite:Dolomite Ratio in the Feed

Since the final objective of the study is to separate

the minerals from their mixture ranging from low (5 wt.

percent) to high (50 percent) dolomite content, the

individual minerals, as well as mineral mixtures, in these

ratios were selected as the flotation feed.


4.1. Materials

4.1.1. Mineral Preparation Preparation of apatite

The flotation feed sample, containing 10.7 percent

P205, from South Florida phosphate deposits was supplied by
the International Minerals and Chemical Corp., Bartow,

Florida. This sample, consisting of -16,+150 mesh size

fraction of phosphate rock and silica grains, was obtained

after desliming and sizing to remove the fines (-150 mesh)

and the coarse pebbles (+16 mesh) associated with the


Silica, the gangue mineral in the feed sample, was

separated electrostatically at Carpco, Inc., Jacksonville,

Florida. The sample was first dried at 100 OC and then fed

into the electrostatic separator. The scheme of the

separation is shown in Figure 6. The final sample, observed

under an optical microscope, did not contain free silica

grains. The removal of silica upgraded the P205 content of

the feed from 10.7 percent to 36.78 percent yielding only 6

Ibs. of the final product out of the total feed of 37 lbs.

The particle size analysis of the cleaned sample is reported

in Table 8.

Dry Feed

_ Francolite


Final Product

Figure 6 Electroseparation scheme to remove quartz
from apatite.

Table 8. Size analysis of apatite concentrate after
removal of quartz by electroseparator.

Size Range weight Percent

-16, +20 1.3

-20, +35 24.9

-35, +65 49.9

-65, +100 19.6

-100, +150 3.5

-150 1.6

The cleaned sample was confirmed by x-ray diffraction

to be carbonate-fluorapatite (also called francolite). In

this study the carbonate-fluorapatite sample will be

referred to as apatite. Preparation of dolomite

Dolomite, +1/2 inch hand-picked pebbles from South

Florida deposits, was supplied by the International Minerals

and Chemical Corp., Bartow, Florida. From this large batch

of sample, tan colored pebbles were further sorted out.

X-ray diffraction tests confirmed that the mineral was


Both apatite and dolomite samples were ground and

sieved to yield -65,+100 mesh and -325 mesh size fractions.

The coarser size fraction was used for flotation and oleate

adsorption tests and the finer sample was utilized in zeta

potential and IR spectroscopy studies. The procedures for

preparing these samples are outlined below.

The coarser size fractions, -65,+100 mesh, were

prepared in an alumina mortar and pestle. To improve the

yield of the particles in this narrow size range, care was

taken to avoid overgrinding by frequently screening the

samples. In spite of this measure, 58 percent of the

apatite and 71 percent of the dolomite samples were lost as

a -100 mesh fraction. The ground samples were washed 10

times with triple distilled water, dried at 50 C and stored

in a glass bottle.

The -325 mesh samples were prepared by grinding in a

steel rod mill for six hours. With a hand magnet the excess

iron impurities in the ground powders, that may have been

introduced from the steel mill, were removed. These samples

were also stored in a glass bottle.

4.1.2. Mineral Characterization Chemical analysis

The chemical analysis of the apatite and the dolomite

conducted by Thornton Laboratories Inc., Tampa, Florida is

reported in Table 9. It is clear from the data presented

that the amount of MgO (0.12 percent) in the apatite and the

amount of P205 (1.62 percent) in the dolomite are not

significant. The major impurity in the cleaned samples is

silica which is reported as acid insolubles (3.34 percent in

apatite and 5.11 percent in dolomite). The other minor

constituents are Fe 03 and A1203, which are both less than 1

percent. The CO2 (1.62 percent) and fluorine (3.82 percent)

content in apatite confirm that it is a carbonate-

fluorapatite. Surface area measurements

The surface area was measured by B.E.T. technique using

nitrogen as the adsorbate in the Quantasorb Sorption System,

supplied by Quantachrome Corp., New York. The measured

surface areas of the -65,+100 mesh size fraction of the

minerals are reported as follows.

Table 9. Chemical analysis of apatite and dolomite.

Mineral P 05 Fe2 0 A103 Acid MgO CaO CO F Organic
% % % Insoluble % % % % Carbon
% %

Apatite 36.78 0.42 0.93 3.34 0.12 50.35 1.68 3.82 0.19

Dolomite 1.67 0.94 0.96 5.11 17.58 32.33 42.16 0.21 0.06
(loss of ign.)


Apatite: 18.83 m2/g

18.42 m2/g

Dolomite: 4.19 m 2/g

4.29 m2/g

The surface areas of the minerals are higher than

expected for nonporous particles in this size range. The

presence of fine open pores in both the minerals was

confirmed by pore size distribution which was determined by

a mercury porosimeter, supplied by Quantachrome Corp., New

York. The pore size distributions of apatite and dolomite

are reported in Figures 7 and 8, respectively. The pore

size distribution of the two minerals is similar, except for

10 percent higher number of pores of size lower than 200

angstrom in apatite as compared to dolomite. Due to high

surface area of the minerals, flotation and oleate

adsorption tests could be conducted on the same size

fraction. Scanning electron microscopy

Apatite and dolomite particles (-65,+100 mesh) were

aged for two hours at pH 10 in a similar manner as in

flotation and adsorption tests. After drying at 50 0C, the

samples were observed, as illustrated in Figure 9, under the

SEM at 100x and 10,000x magnification. High surface

porosity of both the minerals is further confirmed in the

micrographs at 10,000x magnification. It is to be noted

that apatite particles are composed of aggregates of




I 40


iO-3 16-2 ICo I o o I 1 2

Pore Size, microns

Pore size distribution on apatite.

Figure 7.


0 -
3 40

I Po3 re2 Size microns 02

Pore Size, microns

Pore size distribution on dolomite.

Figure 8.






Figure 9.

SEM micrographs of apatite and dolomite (-65, +100
mesh size fraction).


submicron size crystals. A similar characteristic of

carbonate apatite has been observed in most sedimentary
phosphate rocks() Aging behavior

The soluble minerals, such as apatite and dolomite, are

known to exhibit the aging phenomenon, i.e. pH of an aqueous

slurry of these minerals will vary as a function of time.

Since pH is an important variable in the present study, it

was decided to examine the aging behavior of the minerals by

monitoring the pH of a 1 wt. percent suspension as a

function of time. These tests were conducted using -65,+100

and -325 mesh material.

Aging behavior of apatite. The aging data at different

levels of initial pH are presented in Figures 10 and 11 for

-65,+100 and -325 mesh, respectively. With -65,+100 mesh

size fraction, the equilibrium pH was reached after

approximately 600 minutes. The equilibrium value of pH 6

and 7 was observed with the slurry of initial pH 4 and 10,

respectively. No significant shift in pH was observed in

the case of slurry with initial value of pH 7. For -325

mesh size fraction, the system attains equilibrium

instantaneously in the acidic pH range. Aging behavior of

-325 mesh fraction at pH 10 was found to be similar to that

of the coarser size fraction. Similar pH shifts toward

equilibrium, also observed by previous investigators(86,61),

have been attributed to the dissolution of the mineral.


Particle Size:
Pulp Density:

-65, 100 mesh
I wt.%

initial pH

0--- 10.1

--- 7.0
.... ..... 57

--C-- 4.0

-- ..A l-. .-.-.-s- --7C.- ..- 7.-. 7. 7 .
--- "- ---.-------. . .- 0

) 500

1000 1500

Aging Time, min.

2000 2500
2000 2500

Figure 10.

Effect of aging on pH of 1 wt. percent apatite
slurry (size fraction -65,+100 mesh).

11.0 r




I 7.0




!I I




Particle Size: 325 mesh
Pulp Density: I wt. %

Initial pH

-0-- 10.0
-0- 7.0
..... ..... 5.7

-" -- "- 3.9

0 500 1000 1500 2000

Aging Time, min.

Figure 11.

Effect of aging on pH of 1 wt. percent apatite
slurry (size fraction -325 mesh).






I. 7.0






Since the mineral solubility is higher in acidic pH range

and for finer size fraction, the equilibration pH under

these conditions is therefore also attained faster. Effect

of ionic strength on the aging behavior of apatite was also

investigated. It is clear from Figure 12 that the aging

behavior in the presence of 3 x 10-2 kmol/m3 KNO3 remained

the same as that in the absence of the electrolyte.

Aging behavior of dolomite. The aging behavior of

dolomite is plotted in Figures 13 and 14 for -65,+100 mesh

and -325 mesh size fractions. Size fraction -65,+100 mesh

reached an equilibrium value of pH 8.2 after 800 minutes.

In the case of the finer size fraction (-325 mesh), the

equilibrium value of pH 8.5 was attained after 1800 minutes.

The equilibrium pH range 8.2 to 8.5 corresponds closely to

the equilibrium value reported in the literature(88,61). As

illustrated for dolomite in Figure 15, the presence of 3 x

10 kmol/m KNO3 did not affect its aging behavior.

From the aging data it is observed that equilibration

time of 8 hours or more would result in a narrow pH range of

6 to 8 thus eliminating pH as a variable. For -65,+100 mesh

size fraction of apatite and dolomite, if the mineral slurry

was aged for 2 hours, there would be a possibility of

studying the effect of wider pH range. After 2 hours of

aging, the pH drift during the next 25 minutes, which is the

maximum duration of flotation tests, would not be more than

+0.1 pH units. It was decided, therefore, that all the

Particle Size =-325 mesh
Pulp Density = Wt %
0 0 Kmol/m3 KNO3
a 3X10-2 Kmol/O KNO





Aging Time, min.

Figure 12.

Effect of aging on pH of 1 wt. percent apatite
slurry with and without KNO3 (size fraction
-325 mesh).







5.0 -


Particle Size: 65, + 100 mesh
Pulp Density: I wt.%



--- -
<>--- -

Initial pH



0 500

Figure 13.

1000 1500

Aging Time; min.



Effect of aging on pH of 1 wt. percent
dolomite slurry (size fraction -65,+100 mesh).


Particle Size:

Pulp Density:

- 325 mesh

I wt.%

Initial pH

SI I I i I I I I





Aging Time, min.

Figure 14.

Effect of aging
dolomite slurry

on pH of 1 wt. percent
(size fraction -325 mesh).












Particle Size=-325 mesh
Pulp Density=l Wt.%
0 0 Kmol / m KNO
3X2 3
y A 3X10 Kmol/m KNO




Figure 15.

Effect of aging on pH of 1 wt. percent
dolomite slurry with and without KNO3
fraction -325 mesh).









Aging Time, min.




samples be pre-aged for two hours at the desired pH prior to

flotation and adsorption tests. Solubility of the minerals

The solubility of the two minerals was measured by

analysing the dissolved cations in the supernatant of the 1

wt. percent slurry of -65,+100 mesh size fraction after

aging it for two hours. The cations in the solution were

measured by atomic absorbtion (Perkin Elmer 6000

spectrophotometer). The dissolved cations from apatite and

dolomite as a function of pH are plotted in Figure 16. It

is to be noted from these results that dissolution of the

dolomite is significantly higher than that of apatite.

Apatite is slightly soluble in the pH range of 7 to 10 and

negligibly soluble at PH 11. On the other hand the

solubility of dolomite is significant in the pH range 7 to

11. At pH below 7, the solubility of both the minerals

increases exponentially as the pH decreases. The solubility

of individual and mixed minerals at various pH levels is

shown in Table 10.

4.1.3. Chemicals

Purified sodium oleate from Fisher Scientific Company

was used in this study. ACS certified grade potassium

hydroxide and nitric acid were used to modify the pH. Also,

ACS grade calcium and magnesium nitrate standards, potassium

carbonate, potassium phosphate, and 99.999 percent potassium

nitrate were used in this investigation.

Suspension aging time
= 2 hrs

\ ^Dolomite


7 8

II 12

Figure 16.

Effect of pH on the amount of cations
dissolved from apatite and dolomite (-65,+100
mesh) 1 wt. percent slurry.


3 4





Table 10.

Amount of dissolved calcium and magnesium
ions in the supernatant of apatite and
dolomite slurries after aging for two hours.

pH mineral Ca+ Mg+
(ppm) (ppm)

11 Apatite 0.1 0.0
Dolomite 1.4 0.0

Apatite 0.3 0.0
10+3 Dolomite 1.2 0.4
-- 1:1 mixture 0.9 0.3
95:5 mixture 0.3 0.0

Apatite 0.6 0.2
7+0.7 Dolomite 5.0 0.0
- 1:1 mixture 3.3 1.8
95:5 mixture 0.7 0.3

Apatite 8.0 0.7
Dolomite 70 32
4- 1:1 mixture 50 15
95:5 mixture 10 1.4

1 ppm Ca+

= 2.5 x 10-5 kmol/m3

1 ppm Mg+ = 4.1 x 10-5 kmol/m3

4.1.4. Water

In this study, deionized, triple distilled water of

less than 1.2 micromhos specific conductivity was used.

4.2. Methods

4.2.1. Flotation Tests The Hallimond cell apparatus

Flotation tests were conducted using a modified

Hallimond cell(149,150) as sketched in Figure 17. The

complete Hallimond cell flotation arrangement is shown in

Figure 18. Since 'prepurified' nitrogen containing less

than 1 ppm CO2 was used, no further attempt was made to

purify the gas. The gas was first passed through a column

of Drierite D, to remove moisture from the gas stream. The

sparger T2 is provided to remove the solids from the gas.

The liquid traps, T1 and T3, prevent water from entering the

rest of the circuit. Next the nitrogen gas enters a 50

litre glass reservoir, R. This large volume helps to

maintain a constant gas pressure, which is measured by the

manometer, M. A solenoid valve, V, is provided to turn the

gas supply on and off by a remote timer. The gas flow to

the flotation cell is regulated by a flowmeter, F. The

outgoing gas from the flowmeter is connected to the

Hallimond cell. The magnetic stirrer, preadjusted to a

fixed speed, can also be turned on and off by the remote

timer. The arrangement is designed so that the gas supply

to the cell and the magnetic stirrer can be turned on/off

simultaneously or individually by the timer.

Teflon i
Stirring Bar

'ork S topper

Figure 17.

Hallimond Flotation Cell

Hallimond flotation cell.

Hallimond Cell Flotation Arrangement

Arrangement for nitrogen gas supply and Hallimond cell.


Cell Flotation

Arran gement

Figure 18.

Prior to flotation, the mineral slurries were

conditioned in 100 ml volumetric flasks by tumbling around

an horizontal axis at 28 rpm. Flotation test procedure

The flotation test procedure consists of the following


A. Solution preparation

B. Aging of the mineral slurry

C. Conditioning

D. Flotation

Solution preparation. A stock solution of 1 x 10-2

kmol/m3 sodium oleate was prepared. After 15 days, a white

curd-like precipitate was observed at the air/liquid

interface. The effect of this precipitate was tested by

obtaining flotation recovery as a function of stock solution

age. It was observed as shown in Figure 19 that the

flotation of apatite and dolomite was not affected by using

up to three-week-old stock solution. However, as a

precaution, solutions more than 5 days old were not used in

this study. Approximately 30 minutes prior to the

conditioning of the minerals this stock solution was diluted

to 2 x 10-3 kmol/m3 sodium oleate concentration for further


Some of the experiments involved using the solutions

containing calcium nitrate, magnesium nitrate, potassium

carbonate and potassium nitrate. The solutions of desired

Stock Solution

3 Dolomite

h Apatite

IXIO-2Kmol/n3 Sodium Oleate.

(1.45 X 10-4 Kmol/m3 Sodium Oleote)

(5.61 X 105 kmol/m3 Sodium Oleate)

Figure 19.

S5 10 15 20 25

Stock Solution Aging Time,days

Effect of sodium oleate stock solution age on the
flotation of apatite and dolomite.








concentrations of these inorganic chemicals were prepared

just prior to testing.

Aging of the mineral slurry. It was noted in the

earlier section on aging characteristics of the mineral

slurries that pH of the apatite and dolomite slurries shift

with time. In order to perform the flotation experiments

over a wide pH range without significant shifts in pH, the

optimum aging time of 2 hours was determined to be

necessary. The detailed procedure for aging the mineral

slurry is described below.

One gram sample of apatite, dolomite or their mixtures

were suspended in 107 + 1 ml (the total volume of 100 ml

volumetric flask) of water. The pH of this water was

preadjusted so that the desired pH was obtained after

conditioning. The samples were aged without any agitation

for 120 + 5 minutes at room temperature 25 + 1 0C.

Conditioning. Flotation experiments were conducted

after reagentizing the minerals with sodium oleate by

conventional (one- stage) and two-stage conditioning. The

detailed conditioning procedures were as the following:

A. In conventional (one-stage) conditioning, the

minerals were treated with reagents directly at the pH at

which they were floated. First, the collector solution (2 x

10-3 kmol/m3) was added to the volumetric flask containing

the aged slurry. This was done by first pouring out 40 ml

of the supernatant in a clean glass beaker and adding a

desired volume of the sodium oleate solution. The

supernatant in the glass beaker was immediately transferred

to the flask containing the remaining mineral suspension.

Next the mineral slurry with the collector was tumbled in

the conditioner for a desired duration at room temperature.

It was determined that flotation recovery, as shown in

Figure 20, was not affected significantly by conditioning

time of up to 10 minutes. To allow adequate time of mixing

with minimum attritioning effects, the conditioning time of

five minutes was selected. After conditioning, the pH of

the supernatant was measured.

In one series of tests, the aging and conditioning at

pH 10 of apatite and dolomite samples were conducted in a

specially constructed glass container, as shown in Figure

21, instead of a standard 100 ml volumetric flask. This

container was built so that apatite and dolomite could be

conditioned together without the minerals coming into

physical contact with eachother.

B. In two-stage conditioning, the mineral slurry was

first conditioned with the desired amount of the collector

at pH 10 for 2.5 minutes followed by reconditioning for 2.5

minutes at a lower pH. The second stage conditioning was

done with and without reducing the collector concentration.

It should be noted that the total tumbling time of 5 minutes

was maintained both in the case of one-stage and two-stage

conditioning techniques to obtain similar attritioning

100 r


0 5 1(


Figure 20.

Time, min

Effect of conditioning time on the flotation
of apatite and dolomite at pH 10.


0 Dolomite

A Apatite

Sodium Oleate Conc. = 1.87XIO04 Kmol/m3

Conditioning PH =I0.1 0.2





Figure 21.


400 Mesh
Copper Screen.

Pyrex Glass
100 ml Flasks (Glued).
Diagram of the cell designed to treat apatite and
dolomite together without contacting each other.

effects. The first stage of conditioning at pH 10 was done

as described earlier in the case of conventional

conditioning. Next, 75 ml of the supernatant (S) was

transferred to a clean glass beaker. In order to

recondition the mineral slurry at a lower pH only, the pH of

the supernatant (S) was adjusted to a lower value before it

was poured back into the original flask containing the

solids. Another series of samples was tested by

reconditioning at lower pH with reduced collector

concentration. This was achieved by replacing the

supernatant (S) with triple distilled water of preadjusted

pH (lower than pH 10).

Flotation. After the mineral slurry was conditioned

with the desired amount of sodium oleate, the pH of the

supernatant was measured prior to flotation. The mineral

slurry was next transferred to the Hallimond cell, which was

then connected to the rest of the cell arrangement (see

Figure 18). With the solenoid valve open for 25 seconds and

the flotation cell valve V1 closed, the system was

pressurized with nitrogen gas. This step was necessary to

avoid the significant time lag (10 to 15 seconds) between

turning on the solenoid valve and observing the gas bubbles

emerging from the bottom of the cell. Next the nitrogen

gas, preadjusted to flow at 48+ 3 ml per minute, and the

magnetic stirrer were turned on for exactly 60 seconds,

using the automatic timer. Simultaneously the valve, V1,

was opened manually. This would start the flotation

process, in which the floated particles were lifted up by

the gas bubbles and collected in the stem of the cell.

After the flotation test was completed, the cork at the

bottom of the stem was opened to drain out the floated

material into a clean glass beaker and the pH of this slurry

was measured. The float and sink fraction were filtered on

pre-weighed filter paper and dried overnight at 50 0C in

air. The percent flotation recovery was calculated based on

the dried weight (and P205 content in the case of mixed

minerals) as shown in Appendix 1.

All the experimental parameters were maintained

constant unless otherwise stated.

4.2.2. Oleate Adsorption Tests

Since the oleate adsorption tests were conducted on -

65,+100 mesh size fraction, the samples were conditioned by

the same techniques as used for the flotation tests. The

oleate adsorption was measured by the solution depletion

method. The oleate concentration in the supernatant was

determined by Gregory's method(151) as outlined in Appendix

2. In conducting these tests three main problems were

encountered and resolved as described below:

A. The maximum detectable limit of oleate

concentration in the aqueous solution was determined to be 1

x 10-4 kmol/m3. Gregory's method involves the determination

of oleate concentration by comparing the optical densities

of unknown samples to those of the standards. As shown in

Figure 22, the optical density of oleate solution varies

nonlinearly with the concentration above 1 x 10-4 kmol/m3

All the samples were appropriately diluted so that the final

oleate concentration was within the linear range of optical


B. As observed in Figure 23, at values below pH 10,

the oleate concentration in centrifuged samples was found to

be significantly lower than the actual amount, whereas for

uncentrifuged solutions the oleate content by analysis was

the same as the actual amount added. The discrepancy in

oleate analysis under aidicic pH conditions could be due to

phase separation of the oleic acid. This was confirmed by

observing an oily layer above water after centrifuging the

sodium oleate solution at pH 5 or lower. It should be noted

that since -65,100 mesh size fraction was used for

adsorption experiments, centrifugation of the suspension was

not necessary to obtain a supernatant sample for oleate

analysis. It was also shown by analyzing a known amount of

the added sodium oleate to the supernatant that any trace

amounts of mineral fines generated during the conditioning

step did not interfere with the oleate analysis.

Furthermore, oleate analyses of the uncentrifuged samples

was also observed after they were stored for a prolonged

time. The oleate content of the samples which were

transferred for analysis after 12 hours, as shown in Table

5 10 15

Conc. of Sodium

Figure 22.

Oleate, X l-5Kmol/m3

Effect of sodium oleate concentration on the optical
density of its extract prepared by Gregory's








. 0.

O Not Centrfuged
O Centrfuged at 15000 rpm
for 3 hrs

Total Sodium Oleate Cone. =9.4 X 10




Figure 23.

Oleate analysis of sodium oleate standards as a function
of pH with and without centrifugation.


11, was reported to be lower than that of samples taken for

analysis immediately after conditioning. The lower oleate

analysis could also be attributed to the phase separation

upon prolonged storage. Based on the above findings, the

supernatant from the conditioned slurries was immediately

transferred to 75 ml glass tubes for the analysis.

C. Calcium and magnesium ions, dissolved from these

minerals (refer to Figure 16) can bring about the

precipitation of calcium or magnesium oleate soaps in the

alkaline pH range upon addition of sodium oleate. The

precipitation of Ca/Mg oleates would result in a decrease of

the free oleate ions. It should be noted that in Gregory's

method of analysis, the pH of the solution is raised to

11.5. Thus, if calcium or magnesium ions are present in the

oleate solution at any pH, the precipitation of the soaps

would result in lower surfactant analysis. To illustrate

this, 1.87 x 10-4 kmol/m3 sodium oleate solution containing

60 ppm Ca+2 and 35 ppm Mg+2 ions at pH 3.7 (Sol L) was

analyzed. It was determined to contain 1.36x10-4 kmol/m3

oleate ions, instead of 1.87 x 10-4 kmol/m3. It should be

noted that calcium and magnesium oleate precipitates are

very fine. Calcium oleate would settle to the bottom of the

tube only after centrifuging at 15000 rpm, for more than 2

hours. On the other hand the magnesium oleate precipitate

flocculated to a gel type mass, which did not settle upon

centrifuging at 15000 rpm for up to three hours. Therefore,

Table 11. Effect of prolonged storage of collector
solution on its oleate analysis.

Oleate Analysis, Kmol/m3
samples samples
transferred transferred
immediately 12 hours after

1. 1.57 x 10-4 1.40 x 10-4

2. 1.66 x 10-4 1.51 x 10-4

3. 1.26 x 10-4 1.04 x 10-4


to prevent the depletion of the oleate ions due to

precipitation in the bulk solution, the modified Gregory'

method using EDTA(151) was followed. EDTA is a sequestering

agent for the cations and thus its presence would prevent

the precipitation of oleate soaps. The effectiveness of

this modified method was confirmed by observing the correct

oleate analysis of the above solution (Sol L) containing

calcium and magnesium ions. It should be noted that

analyzing by this method would include the oleate ions as

well as any oleate soaps that may have formed in the bulk

solution. Knowing the analyzed oleate content in the

supernatant and the initial amount added, the true oleate

adsorption on the minerals would be obtained.

The above procedures were incorporated in analyzing the

known sodium oleate standards (1.87 x 10-4 kmol/m3) as set

forth in Table 12. (Since the oleate content in the above

standards exceeded the maximum limit of 1 x 10-4 kmol/m ,

the solutions were diluted by half.) The analysis within 95

percent confidence interval was 9.40 x 10-5 + o.41 x 10-5

kmol/m3. This is an accuracy of within 4 percent.

After the supernatant was analyzed, the amount of

oleate adsorbed on the minerals was calculated as

micromole/g as shown in Appendix 3.

4.2.3. Electrokinetic Measurements

Electrophoretic mobility was measured by a Rank II

electrophoresis apparatus. Two tenths gram of -325 mesh

Table 12. Reproducibility of the oleate analysis

pH Added Cations, ppm Oleate Analysis
+2 kmol/m
Ca Mg




Standard Deviation
95% Confidence Interval

9.4 x
9.4 x
0.4 x




10 5


10 +

Initial amount of sodium oleate in the solution=9.4 x 105



sample was suspended in 100 ml of 3 x 10-2 kmol/m3 KNO3

solution of a desired pH in a glass cylinder covered with

Teflon tape. The container was inverted 10 times and

contents allowed to settle for 10 minutes; 60 ml of the

supernatant containing fine solids was transferred to

another container. After aging the samples for two hours,

they were sonicated in Bransan sonicator (model B-52, 240

watts) for five minutes. After measuring pH of the sample,

the electrophoretic mobility was determined at a number of

electrode voltage settings before the particles settled in

the cell. The pH of the slurry was measured again. Zeta

potential was calculated from the average electrophoretic

mobility values using the following equation.

S= 4(1 ) n/c

where 5 = zeta potential

1i = electrophoretic mobility

n = viscosity

S= dielectric constant

Assuming 5 = 78.5 and n = 0.89 cps under the

experimental conditions, the above relation would reduce to

E= 12.8 (u ).

4.2.4. Transmission Infra-red Spectroscopy

Transmission IR spectroscopy was done on -325 mesh

mineral samples. The sample preparation technique was

similar to that used by Peck et al.(152) A brief

description of it is as follows. One gram of -325 mesh

sample was suspended in 100 ml of water at the desired pH in

a glass cylinder covered with Teflon tape. The suspension

was agitated by inverting the cylinder 10 times and then

allowing contents to settle for 5 minutes. Eighty ml of the

supernatant containing fine particles was reagentized with

50 ml of 10-2 kmol/m3 sodium oleate solution at pH 10 and

reconditioned at pH 4 to 5. In another test, the minerals

were conditioned at pH 4 to 5. The samples obtained at

different stages of the conditioning were centrifuged at

15000 rpm for 5 minutes. The settled solids in the

centrifuge tube were dried at 50 0C for 2 days in the air,

and 0.01 g of dried apatite and 0.005 g of dolomite were

mixed with 2 g of KBr with a diamonite mortar and pestle.

After the KBr mixture had been dried overnight in a vacuum

oven at 80 o C, 0.5-inch pellets were pressed. A MX-1 FT IR

NICOLET Spectrometer was used to obtain the transmission IR

spectra of these pellets.

4.2.5. Bubble Size Measurements

The size of the bubbles in the Hallimond cell was

measured photographically as follows. To facilitate the

viewing of the bubbles, the top part of the flotation cell

was modified as shown in Figure 24. The camera was arranged

as shown in Figure 25. The cell was filled with the

collector solution without any solids in it. After the gas

was turned on for 30 seconds, the photograph of the bubbles

was taken so that solid/liquid interface would appear at the


Stirring Bar

Figure 24.

Diagram of modified Hallimond cell used in
measuring the size of the bubbles.


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