Title: Study of Fluoroapatite reactions;
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Title: Study of Fluoroapatite reactions; the effect of metaphosphate melts on fluorine-containing substances ..
Alternate Title: Fluoroapatite reactions, Study of
Physical Description: 97 leaves. : ; 28 cm.
Language: English
Creator: Valdsaar, Herbert, 1925-
Publication Date: 1956
Copyright Date: 1956
 Subjects
Subject: Apatite   ( lcsh )
Fluorine   ( lcsh )
Chemistry thesis Ph. D
Dissertations, Academic -- Chemistry -- UF
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
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Thesis: Dissertation -- University of Florida.
Bibliography: Bibliography: leaves 93-96.
Additional Physical Form: Also available on World Wide Web
General Note: Manuscript copy. Typed on one side of leaf only.
General Note: Vita.
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Bibliographic ID: UF00098029
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 - 000568466
oclc - 13663313
notis - ACZ5200

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STUDY OF FLUOROAPATITE REACTIONS

THE EFFECT OF METAPHOSPHATE MELTS ON
FLUORINE-CONTAINING SUBSTANCES






By

HERBERT VALDSAAR


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











UNIVERSITY OF FLORIDA
JANUARY, 1956

















































UNIVERSITY OF FLORIDA


3 1262 08552 2877




















TABLE OF CONTENTS


Page


LIBT OF TABLTS *.

INTRODUCTION


The Occurrence and Use of Apatites
The Fluorine Problem . .
The Objectives of Further Studies


SYNTHESIS OF FLUOROAPATITE . . *


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* 0 0 0 0
* 0 0 0


0 0 0


Synthesis Methods * * * .
Recommendations of the TVA Group .
Synthesis of Chemically Pure Fluoroapatit& .
A Simplified Procedure for Fluoroapatite Synthesis


ANALYSIS OF FLUOROAPATITE


0 0 0 0 0 0 0 0 0


Preparation of Analysis Standards
Calcium Determination . .
Phosphorus Determination . .
Fluorine Determination . .


0 0


* 0 0
* 0 0 0 0
* 0 0 0 0
* 0 0 0 0


THE MOLECULAR STRUCTURE OF FLUOROAPATITE .* .

SSOLUBILITY OF FLUOROAPATITE IN AQUEOUS SOLUTIONS

General Behavior . . * *
The Effect of Sodium Carbonate Solutions on
Fluoroapatite .* * . . .

REACTIONS OF FLUOROAPATITE 'ITH A GASEOUS PHASE ,

Formation of Fluozoapatite and Hydroxyapatite
Reactions with Hydrogen-Containing Gases .*
Experimental Tests with Gases 0 * *
Reduction of Fluoroapatite by a Gas Phase *
Conclusions * * *


* 0

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. 26


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Page


REACTIONS THE SOLID STATE . 0 0 0 .. 38

Types of Solid State Reactions . . * 38
Thermodynamic Considerations . . . . 39
Exchange Reactions Between a Pair of Salts . . 40

REACTIONS OF FLUOROAPATITE If MELTS .* . .* * 44

Melts of Neutral Salts * *. 44
Molts of Basic Salts * * * * 5
Melts of Acidic Salts . . . . . 5

rTTERACTION OF SODIUM METAPHCSPHATE WITH FLUORID1]S . 46

Preliminary Observations . . . 46
Literature Survey . *. . * * 46
Objectives of Study of the Metaphosphate Melts . 47
Selected Metaphosphate Systems . * . 49
Characteristics of the Evolved Gases * * 50
Construction of the Vacuum Furnaces .* * * 54
Method of Measurement with the Vacuum Furnaces * 57
Experimental Results of the Vacuum Measurements . 58
Experimental Procedure at Atmospheric Pressure * 60
Standard Mixtures and Conditions * . * 62
Experimental Results of the Measurements at 700C with
the Standard Compositions * . 0 64
Variation in Composition at 7000C. . . . 65
Temperature Dependence * * * 72
Reactions Involving Other Cations . * 72

INTERACTION OF METAPHOSPHATE WITH FLUOROAPATITE . . 75

Experimental Conditions * * * * * 75
General Results * * . * * 75
Temperature Dependence . . . * 77
Effect of Supplementary Materials , . 79

EXPER TS IN THI CRUCIBLE FURNACE . . . . 81

Solubility of Salts in Molten Sodium Motaphosphate 81
Weight Loss of Standard Mixtures and the Component
Salts * * * e 8 82
Microscopic Observations . * 83










Page

SUMMARY OF THE EXPERIMENTAL RESULT 'ITH MTAPHiSPHATE MELTS 84

Reaction Formnllas * * *. 84
The Major Factors and Their Influence Upon the
Reactions * * . . . 84

PHYSICAL ITERPRETATIONS . . . . . 87

Metaphosphate Melts . * . * *. 87
Effect of the Type of Cation * * 88
Extension of the Reaction Concept * * * 89

CONCLUSION. * . 90

BIBLIOGRAPHY . . . . . . 93

ACKOcTI DGET:IFJS * . * 0 * 97

BIOGC;A'IC/L NOTE . . . . 98















LIST OF TABLES


Table Page

1 Composition of Pure Fluoroapatite Samples . 14

2 Composition of Fluoroapatite Samples Prepared by
the Simplified Procedure . . . 16

3 Free Energies of Formation of Fluoroapatite and
Hydroxyapatite at Four Temperatures .* * 31

4 Standard Free Energy Changes of Reactions of
Fluoroapatite with a Gaseous Phaso .* * 34

5 Rate of Gas Evolution from Metaphosphate Melts in
the Vacuum Furnace * * * * 59

6 Effect of Experimental Conditions at 7000C. on the
Evolution of Fluorine-Containing Gases from
Metaphosphate Melts . . . . 66

7 Compositions of the Experimental Metaphosphate-
Fluoride Mixtures . . 69

8 Evolution of Fluorine-Containing Gases from
Metaphosphate Melts at Various Compositions 73

9 Evolution of Fluorine-Containing Gases from
Metaphosphate Melts at Various Temperatures 73

10 Evolution of Fluorine-Containinp Gases from
Metaphosphate Melts in the Presence of Other
Cations * . . 74

11 Compositions of Metaphosphate Melts Containing
Fluoroapatite *. . * . 76

12 Evolution of Fluorine-Containing Gases from Meta-
phosphate Melts Containing Fluoroapatite 78
















INTRODUCTION


The Occurrence and Use of Apatites


One of man's most useful minerals is phosphate rock. Its major

use as the source for phosphate fertilizers is well known. Phosphate

rock is primarily composed of a group of minerals called apatites, the

most typical representative of which is fluoroapatitel with the formula

Ca10F2 (PO)6. Natural apatites vary greatly not only in kind and variety

of impurities present, but also in the composition of the apatite itself.

This is true even for two mineral samples from the same deposit.

The phosphate rock deposits of varying composition can be found

in many places throughout the world. The North American continent, how-

ever, is particularly rich in this mineral, which in this case consists

essentially of submicrocrystalline fluoroapatite containing some excess

of fluorine (27), and is usually found in an admixture of iron, aluminum,

and silica compounds.

The structure of crystalline apatites has been studied, and it

has proved to be remarkably stable, permitting a variety of unusual types

of substitutions involving a considerable number of ions. The most im-

portant of these changes occur where the fluoride ion in fluoroapatite is


In this work the prefix "fluoro-" is applied in the term
"fluoroapatite" in conformity with the general usage of this prefix to
designate compounds containing fluorine (e.g., fluoroaluminates). The
Tennessee Valley Authority investigators use "fluorapatite", and some

1







2

substituted partly or fully by hydroxyl, chlorine or carbonate Croups,

forming the respective apatites. The occurrence of extensive series of

solid solutions among these apatite type structures are also very common.

The apatites are readily formed in solid state reactions as voll as

in aqueous solutions in which they are quite stable. For example, a

suspension of hydrated tricalcium phosphate may be used to induce the

formation of fluoroapatite by the precipitation of solute fluoride ions,

thus effecting complete removal of fluorides from ground waters (49).

Ever since the phosphate fertilizer industry existed, numerous in-

vestigators have been seeking better industrial procedures for processing

phosphate rock, but in recent decades more attention is being paid to

these materials as chemical systems. Basic studies have been started,

and are being carried out by several independent groups, each interested

in their omw particular applications. Considering the complexity of

phosphate systems, the research done until now has been only a modest

beginning in disentangling the variety of problems related to these

materials.

In this country the Tennessee Valley Authority has done extensive

and excellent work in the field of phosphates in developing new industrial

procedures as well as conducting basic studies of the phosphatic compounds.

The major purpose of their work has been the increase of the phosphorus

availability in the processed materials. The results of their findings

have been made public in a series of reports and articles (18b, 68).

The study of the structure of apatites has deserved considerable


other authors prefer "fluor-apatite" or "fluoro-apatite".







3

attention on the part of medical people and crystallographers particu-

larly since the discovery that the mineral constituent of bone and of

the enamel and dentine of teeth is essentially hydroxyapatite, as

established by X-ray crystal analysis (h).

In recent years a number of new practical applications have been

found for synthetical apatite type materials. For example, the alkaline

earth halophosphates possess very favorable characteristics as fluore-

scent materials, and are already in wide-apread use in fluorescent lamps

in England (55).


The Fluorine Problem

In the near future ride attention will be directed to the fact that

the North-American phosphate rock constitutes a practically inexhaustible

source for fluorine-containing compounds. Presently this element is won

from fluorspar (CaF2), but since the domestic resources of this mineral

in highgrade purity are being rapidly depleted, it has been placed on the

list of critical materials by the Strategic Materials Committee of the

U. S. Government (21), and rccomended for stockpiling.

At this time no efficient and economical industrial process is

known for the extraction of fluorine from phosphate rock.

The fertilizer industry has long been concerned about this problem

for its own reasons, since the presence of fluoride exerts a deleterious

effect on the availability of phosphorus in the soil and acts as a

poison if fluorine-containing phosphates are used as cattle-feed

supplement.

The known and practiced methods for the extraction of fluorine-
containing components from phosphate rock are:










1. dissolution of the mineral in strong aqueous acids, and the

precipitation of fluorine as fluoride, or silicofluoride (18b);

2. treatment of phosphate rock with sulfuric acid and heating (23);

3. defluorination of the mineral at high temperatures, above

1300C., in the presence of silica and water vapor (17, 67).

Part of the extracted fluorine, in the form of hydrogen fluoride

and silicon tetrafluoride, can be recovered from the flue gases by the

adsorption of hydrogen fluoride in (18a) lime-packed towers, and by

scrubbing silicon tetrafluoride with alkaline solution (52). In the

latter case fluorosilicates will be formed.

The above-mentioned procedures have been used only to a minor

extent, and it is improbable that any of them could be adopted to a

wide-scale economic industrial process for the purpose of fluorine

extraction.


The Objectives of Further Studies

More knowledge about the apatite systems is necessary for the

development of now approaches uhich would enable more efficient bene-

ficiation of all the materials present in phosphate rock. This can be

accomplished only by thorough fundamental studies, which should be

extended to a large number of phosphate systems under a variety of

conditions.

Almost all previous work has been done on natural minerals, and

although much of a general nature has been published very little precise

scientific knowledge is available on phosphate rock. A reproducible

fluoroapatite has only been prepared within the past several years and








5

the rates of reactions of this pure material with other chemicals have

not yet been studied.

The contribution of a small group of investigators must of necessity

remain rather limited considering the demands of the field. It was,

however, hoped that this work would help to emphasize the need to look

at the problem as a whole, and add to the basic knowledge concerning

these important materials.

The general objective was set to initiate a broad program of study

of apatites and other related materials for the purpose of finding new

basic facts about the physical properties of well defined phosphatic

systems, their reactivities and rates of reactions. The coordination of

the available information from various sources would form a part of this

task.

This dissertation summarizes parts of the work which was carried

out with the purpose of studying the possibilities of novel fluoroapatite

reactions below 1000C,. with special attention to the behavior of fluorine.

One of the favorable decomposition reactions was then selected for a

closer study.

Furthermore, there was the preliminary task of choosing a feasible

method for the production of chemically pure fluoroapatite, and the

comparison of suitable analysis methods for these materials.

The subject matter in this dissertation is composed of three in-

dependent parts.

In the following three chapters the synthesis, the analysis and the

crystallographic properties of fluoroapatite are described.

The chapters in the middle section discuss the general possibilities








6

of fluoroapatite reactions with chemicals in various phases. The refer-

ences to the known reactions have been included, and the results of a

variety of experiments carried out in this laboratory have been

presented, he standard free energy changes vere calculated for a

number of reactions and the results have been given.

The third and major part of the dissertation describes and dis-

cusses the interaction of metaphosphate melts with fluorine-containing

substances. The experimental procedures for the study of the special

reactions were developed in this laboratory.















SYNTIHESI OF FLUOROAPATITE


Synthesis Methods


Tricalcium phosphate combines readily with pulverulent calcium

fluoride forming mixtures which register characteristic properties of

fluoroapatite. Numerous investigators have observed this reaction under

various conditions.

The process may take place in aqueous solutions, although in this

case the precipitate is always contaminated with other phosphates (6).

Stoichiometric amounts of tricalcium phosphate and calcium fluor-

ide, intimately mixed, produce fluoroapatite after gradual heating of the

mixture (5) to 9500C.

Fluoroapatite has been prepared by the reaction of hydroxyapatite

with calcium fluoride in the solid state (8) above 6000C.

Wallaeys has synthesized mixed fluoro-, chloro-, and hydroxyapa-

tites. The structure and isomorphism of these compounds are described

(70, 71).

The phase diagram of the system calcium fluoride tricalcium

phosphate has been studied by R. Nacken. According to him fluoroapatite

will be formed in a molten mixture of calcium fluoride with an excess of

tricalcium phosphate (50, 5S4).

In the foregoing preparations the identity of fluoroapatite was
usually established by X-ray analysis, and little was mentioned about the
chemical purity of the products or the presence of other phases. In
7










this respect the latest and the most reliable procedure has been given

by the TVA scientists under the direction of Kelly L. Elmore, Chief of

Research Branch, Division of Chemical Development. It was decided to

adopt their method for the synthesis of fluoroapatite, hydroxyapatite,

and tricalcium phosphate as described by Dr. K. L. Eliore (14).


Recommendations of the TVA Group

The folloirting directions are an excerpt from a personal letter by

Dr. K. L. Elmore, April 1, 1953.

Prepare a stock of raw materials as folloust

Calcium Carbonate: Recrystallize reagent-grade calcium nitrate
and precipitate calcium carbonate with ammonium carbonate.
Calcium Fluoride: Distill reagent-grade hydrofluoric acid in
a platinum still and precipitate calcium fluoride with precipitated
calcium carbonate.
Calcium Metaphosphate: Recrystallize reagent-grade monocalcium
phosphate monohydrate. Heat the monocalcium phosphate carefully
to 6000C. in a platinum boat to form crystalline calcium
metaphosphate.
Tricalcium Phosphate: Mix calcium metaphosphate and calcium
carbonate in stoichiometric proportions to form tricalcium
phosphate. Heat the mixture in a platinum boat at 1100C, for
five hours. It may be necessary to make adjustments in composition.
Hydroxyapatite: Precipitate a composition approximating
hydroxyapatite by the method of Rathje (57). Dry the precipitate
at 105C. and then heat in a platinum boat at 9500C. in an
atmosphere of 20 per cent steam and 80 per cent nitrogen for at
least 72 hours to promote crystal growth.
Fluoroapatitoe Prepare a stoichiometric mixture of tricalcium
phosphate and calcium fluoride. Heat the mixture in a platinum
boat at 13500C. for at least 30 minutes in a stream of dry
nitrogen. Again, adjustments of composition may be necessary.

Complete reaction requires grinding and intimate mixing. The
resulting products have small crystal size. In a separate
publication (13) the conditions are described by which large,
well defined crystals of fluoroapatik can be produced from
sodium fluoride flux in a vacuum furnace heated by induction.
Difficulties were encountered in keeping the crystals completely
free of flux. In none of the preparations was it advisable to
prepare more than fifty grams in one batch.







9

The first task of the present study was to prepare various samples

of fine crystalline fluoroapatite according to the TVA recommendations.

The directions given above are quite generally worded, and it was found

that an extensive review of the literature on the subject along with

actual experiments had to be carried out in order to determine the in-

fluencing factors in the preparation of the materials needed.

The procedure for fluoroapatite synthesis can be divided into seven

separate steps:

1. Recrystallization of monocalcium phosphate monohydrate.

2. Conversion of monocalcium phosphate monohydrate into metaphosph-

ate,

3. Recrystallization of calcium nitrate tetrahydrate.

4. Distillation of an-monium carbonate and precipitation of

calcium carbonate.

5. Ignition for formation of tricalcium phosphate.

6. Precipitation of calcium fluoride,

7. Ignition for formation of fluoroapatite.

In the following section some of the essential details of each step

will be pointed out along with major references for background infor-

mation.


Synthesis of Chemically Pure Fluoroapatite

Recrystallization of Monocalcium Phosphate Monohydrate.-The mono-

calcium phosphate obtainable on the market usually contains free phos-

phoric acid, sometimes up to $ per cent, although the salt may appear

quite dry. If the commercial material contains only monocalcium







10

phosphate as a solid phase, a satisfactory acid-free product can be

obtained by washing out the free acid with a suitable organic solvent.

Frequently, however, the C.P, salt carries a small. quantity of dical-

cium phosphate, which can be removed only by recrystallization from a

solution of the proper composition (9).

Monocalcium phosphate monohydrate, Fisher Certified Reagent,

was used in this laboratory, and recrystallized according to the direc-

tions of V. L. Hill and S. B. Hendricks (32).

It is difficult to remove every trace of excess phosphoric acid

from the crystals. Washing with alcohol and ether has been used for

removing (64) the excess acid, but acetone (31, 60) is to be preferred.

Acetone was dried over anhydrous potassium carbonate and filtered before

use.

Conversion of Monocalcium Phosphate Monohydrate into

Metaphosphate.-Heating of monocalcium phosphate produces varying results,

depending upon whether charges are heated directly or in a step-wise

fashion to the selected temperature. Step-wise heating gives somewhat

more reproducible results, particularly when hydrated monocalcium phos-

phate is used as a starting material. This salt, when it is heated

rapidly, melts incongruently near 150C., and the accompanying rapid

loss of water vapor causes frothing; frequently with loss of material

from the container, and formation of a tough mass. Fusion can be

avoided (33) by first heating the charge at 1250C. to expel water of

crystallization, but further heating in the range 2000 to 6000C. yields

an unpredictable mixture of phases consisting of glasslike amorphous

material, and one or more of at least three crystalline phases. This








11

mixture, however, can be converted to stable (-calcium metaphosphate by

heating between 600 and 70000C,

In this laboratory, about a fifty gram lot of pure, acid-free

monocalcium phosphate monohydrate was heated, first at 1250C. for two

hours to expel water of crystallization, and then at 4000C. for two

hours; after reheating between 600 and 70000., the substance was cooled

in air. The product obtained by this method (34) had a sandy crystalline

appearance and could be easily pulverized.

Recrystallization of Calcium Nitrate Tetrahydrate.-Calcium

nitrate, Mallinckrodt Analytical Reagent, contains as major impurity a

total of 0.20 per cent of magnesium and alkali salts. Richards and

Honigschmid proved (50) that such impurities can be removed by the re-

crystallization of the nitrate.

Calcium nitrate is a deliquescent and very soluble salt with a

great tendency to form supersaturated solutions. Owing to the ready

solubility of calcium nitrate in organic solvents, the crystals could not

be freed from solution by washing with alcohol, acetone, pyridine, etc.

(3).

In this laboratory, about 203 grams of calcium nitrate tetra-

hydrate were dissolved in 25 ml. of water in a platinum dish and heated

to about 1000C. The solution was cooled overnight in a refrigerator and

seeded with a small crystal of calcium nitrate tetrahydrate. The crystal

formation progressed very rapidly in cool solutions. Suction on a

fritted-glass funnel was applied to separate the mother liquor from the

crystals.







12

Distillation of Anmonium Carbonate and Precipitation of

Calcium Carbonate.-Pure annonium carbonate, Mallinckrodt Analytical

Reagent, was distilled in a glass apparatus and condensed in a flask

with ice-cold water. The fresh a.monium carbonate solution was added

to the calcium nitrate solution, and the precipitated calcium carbonate

was filtered and washed with plenty of distilled water. The product

was preserved in a platinum dich in which it was first heated to 105$C.

for a couple of hours and then at 00C. for a short time.

Ignition for Formation of Tricalcium Phosphate.-Calciu- meta-

phosphate and calcium carbonate were mixed in stoichiometric proportions

and heated in a platinum shell at 1150C, for five hours. There was

always some loss of phosphorus durinC the ignition, thus the Ca/P ratio

in the product was usually somewhat higher than the theoretical value,

The ratio can be corrected by an addition of a right amount of

calcium metaphosphate to the product and by the repetition of furnace

treatment.

The necessary quantity of calcium metaphosphate can be calculated

from the following equation:

W(Ca) + r(Ca) X 1.906
W(P) + r(P) X

W(Ca) weight of calcium in the sample by analysis

W(P) weight of phosphorus in the sample by analysis

r(Ca) ratio of Ca/Ca(P03)2 0,20238

r(P) ratio of P2/Ca(PO3)2 0.31290

X amount of calcium metaphosphate necessary







13

After rearrangement and insertion of the numerical values the

formula has this expression:

w(Ca) 1.9406 W(P) ,.
O.40o78
For example, in one case 0.060 gr. of calcium metaphosphate had

to be added to five grams of tricalcium mretaphosphate to obtain the

correct Ca/P ratio which is 1.9406.

Precipitation of Calcium Fluoride.-Crystalline calcium fluoride

was produced by adding a slight stoichiometric excess of hydrogen fluoride

in aqueous solution to a suspension of precipitated calcium carbonate.

'Ieighings and other operations were carried out in platinum dishes. The

product was washed with abundant water, dried at 1100C. for several hours,

and then heated to about 7000C. until the weight remained constant.

At higher temperatures, calcium fluoride volatilizes in notice-

able quantities. Thus the loss in grams per sq. cm. of surface for 24

hours of calcium fluoride heated in air has been reported (2) to be 6.3

ag. at 12500C. and 23.5 mg. at 1400C.

Ignition for Formation of Fluoroapatite.-The heating unit in use

was a Burrell High Temperature Electric Furnace, Model L-G-M, maximum

temperature 14000C.

Stoichiometric mixtures of tricalcium phosphate and calcium

fluoride were heated in a platinum shell for about an hour at 13500C.

The charge was covered by another platinum shell which in its inverted

position rested with the edge on the even floor of the furnace, whereas

its bottom touched the upper edge of the first shell. Thus, there was

formed a doubly isolated space with hindered diffusion between the space







14

above the charge and the furnace atmosphere. The first shell with the

charge was adjusted a little above the floor of the furnace, supported

by pieces of firebrick, and underneath it was a platinum cover with

calcium fluoride in order to provide a vapor phase of the salt at high

temperatures. The heating proceeded gradually in the course of about

ten hours. Nitrogen, dried by the passage through a long tube of silica

gel was led into the furnace during the ignition, primarily for the

purpose of keeping the furnace atmosphere as dry as possible.

The results of various samples indicated that some fluorine-

containing compound had been volatilized during the treatment, and that

usually the phosphorus content was also lowered as compared with the

initial composition.

Three mixtures were then prepared containing an excess of calcium

fluoride 105, 110, and 115 per cent respectively, as compared with

the stoichiometric requirement. The following table presents the

compositions of the three preparations.

TABLE 1

COMPOSITION OF PURE FLUOROAPATITE SAMPLES

Percentage by Weight of the Component Elements
Sample
Calcium Phosphorus Fluorine
O0.10 1 0.10 10.08

105% CaF2 39.94 18.32 3.56
110% CaF2 io.o0 18.20 3.74
115% CaF2 40.25 18.18 3.96
Theoretical 39.735 18.428 3.767







15

By adding ten per cent excess calcium fluoride, the Ca/P ratio

has to be newly adjusted by a proper amount of calcium metaphosphate.

This quantity can be calculated from a formula expressing the Ca/P ratio

in fluoroapatite.

The initial formula is quite long; after the substitution of

numerical values and contraction it is:

0.0090866 W(Ca-P) X

W(Ca-P) = weight of tricalcium phosphate in grams

X additional amount of calcium metaphosphate

needed in grams


A Simplified Procedure for Fluoroapatite Synthesis

The procedure described previously for the synthesis of

chemically pure fluoroapatite yields good results, but the preparation

of starting materials is laborious. Need was felt for a simplified

procedure which would facilitate the preparation of fluoroapatite in

sizeable quantities with good quality and acceptable purity.

A new procedure was developed which utilized the commercially

available quality chemicals and dispensed with the recrystallization of

salts.

Materials,-The following materials were used monocalcium phos-

phate monohydrate, Mallinckrodt's Analytical Reagent, was held in dry

acetone over a period of three months. First, the solvent was changed

every other day; afterwards once a week. Acetone was removed carefully

from the salt by drying at 500C.

Mallinckrodt's calcium carbonate (300 g.) and Baker's calcium







16

fluoride (200 g.), both Analytical Reagents, were treated for three

months rith approximately twenty liters of water, in each case, at room

temperature. After the treatment, the salts were dried at 140OC. and

calcium fluoride was further ignited at 6000C. for two hours.

Results.-Using these three salts as starting materials, five lots

of fluoroapatite were prepared following the standard heating procedures.

The appearance and chemical properties of the new products were identical

with those of the previous preparations after the first ignition. The

lots were designated by letters a-e. The analysis results are presented

in the following table,


COMPOS I"I0B
BY


TABLE 2

OF FLUOROAPATITE SAMPLES PR PARED
THE SIMPLIFIED PROCEDURE


Percentage by Weight of the Component Elements
Sample Ratio
Ca/P
P Calcium Phosphorus Fluorine
o0.10 1 0.10 10.03


a 2.137 39.42 18.36 3.42

b 2.145 39.47 18.41 3.42

c 2.184 39.11 17.50 3.60

d 2.179 39.66 18.20 3.67

e 2.203 39.78 18.06 3.65

Theo-
retical 2.1563 39.735 18.428 3.77







17

Identification of Fluoroapatite.-*-icroscopic observations

rendered positive proof concerning the identity of the products, and gave

an indication regarding the uniformity of the materials.

The available instrument was a Bausch & Loob Petrographic Micro-

scope, Model LC, with accessories.

Fine grains of the material were irnersed in various oils of

known refractive indices, and the movement of Becke lines was observed.

The small crystal size prohibited the determination of the interference

figures, but the characteristic birefringence could easily be followed.

Weak birefringence persisted uniformly throughout the material.

All preparations exnibited identical crystallographic ana optical

properties. The mwas of the material possessed a refractive index in the

range of 1.630 to 1.632, which agrees with the values,,/u= 1.6325 and

,-r 1.630, reported by R. Nacken ($4) for artificial fluoroapatite.

Minor amounts (a fraction of one per cent) of different crystals

were observed, the optical properties of which suggested the presence of

some unroacted calcium fluoride and tricalcium phosphate.

The material can be cleaned by treating it first with sodium car-

bonate solution (2 g. in 100 ml.) at aboat 700C. for several hours, with

subsequent standing in a two per cent citric acid solution at room tem-

perature. The total loss of the material is small, and the product has a

uniform appearance under the raicroscope as clcn fluoroapatite.















ANALYSEB OF FLUOROIPATITE


The analysis of fluoroapaLite constitutes a problem of deter-

mining quantitatively phosphorus, calcium, and fluorine in the presence

of one another, and practically in the absence of other elements except

in the smallest quantities.

A very extensive literature has been accumulated on the subject

of the analysis of phosphate rock and phosphatic fertilizers. The

described procedures are quite involved because of the necessity of

taking account of the numerous interfering elements.

In the case of pure fluoroapatite, one is not concerned with

such a multitude of co*mronents, and it was hoped that simpler and less

time-consinuln procedures could be found. For tnat purpose, the favor-

able features of various methods were combined and tested on the phos-

phatic materials. Only volumetric and graviretric determrtnations were

considered. Due attention was paid to the widely used official or

recornended procedures of the Bureau of Standards and of the Association

of Official Agricultural Cacmists.


Preparation of Analysis Standards

Calcium,-Calcium carbonate, C.P. Reagent, can be used with

advantage as a standard substance after washing with ample distilled

water and drying at 1100C. to constant weight.

Phosphorus.-Potassium dihydrogen phosphate is being widely used

18







19

as a standard substance for preparing solutions of known phosphate con-

centration. Purification beyond the reagent grade can be effected by

recrystallization (37). As a test for the purity of the material, the

loss on ignition should not be less than 13.13 per cent nor more than

13.33 per cent.

Fluorino.-Sodium fluoride, Reagent Grade, dried for one hour at

1050C., offers a satisfactory standard for fluorine determinations.


Calcium Determination

Practically all methods for calcium determination, whether by

volumetric or gravimetric procedures, start with the precipitation of

calcium as calcium oxalate monohydrate. It has been shcwn experimentally

that calcium ion can be quantitatively precipitated in the presence of

phosphate and fluoride ions (up to about 5% fluorine), and some other

elements as they usually occur in the phosphate rock.

Seven methods of calcium precipitation were recorded, and five

of them (25, 35a, 36b, 46, 72) tested. The procedures differed from

each other in the volume of the sample in the concentrations of the rea-

gents, the indicators used, and the temperature of ammonium oxalate

during the addition.

Volumetric Determination.-Precipitation (l4a) followed by titra-

tion according to the description by Kolthoff and Sandell (l4b), is a

simple and recommendable procedure. The accuracy is about one per cent

or somewhat better.

The procedure by Lingano (46) is the latest and claims the

highest accuracy, about 0.1 per cent. The method is relatively fast, but








20

needs more experimental verification.

In determining the proper endpoint of the titration with perman-

ganate, the recommendations of Fowler and Bright (22) should be observed.

Gravimetric Determination.-Under suitable precipitation condi-

tions, calcium can be weighed in the form of calcium oxalate monohydrate

dried at room temperature, according to Sandell and Kolthoff (41b).

Accuracy is about 0.2 per cent.

washburn and Shear (72) dried the precipitate at 105C. and

obtained good results as confirmed by the experiments in this laboratory.

Accuracy is approximately the same as in the previous case.

The most accurate weighing form for calcium ion is considered to

be calcium carbonate. The accuracy amounts to 0.1 per cent or less in

the absence of interfering ions (7h). The precipitate has to be heated

at about 5000C. for one hour.

Calcium sulfate is a good weighing form for calcium (l4a) if it

can be directly precipitated from the solution free from other earth

alkali elements. The ignition temperature does not have to be closely

observed (the range is from 1050 to 8900.).

This method was used most frequently in this laboratory with

good results which ;ere checked by the carbonate method. Accuracy was

about 0,1 per cent.


Phosphorus Determination

In the presence of calcium, two courses are open for the preci-

pitation of phosphorus in tho form of orthophosphate.

1) The precipitation of maWnesium ammonium phosphate may be







21

made in the presence of much ammonium citrate, which prevents the pre-

cipitation of calcium and other phosphates.

2) Phosphorus may first be separated from the interfering

elements by precipitation as ammonium phosphomolybdate in acid medium.

This precipitate may then be dissolved in ammonia and magnesium amumonium

phosphate precipitated.

The first method as described by Hoffman and Lundell (36a)

promises to be a more rapid procedure and gives (h7) equally accurate

results.

The second method has been in most wide use in many practical

procedures for separating phosphorus from a multitude of interfering

elements. Besides, it is the basis for the ordinary volumetric deter-

mination of phosphorus (1).

Gravimetric Determination.-The most accurate results, within 0.1

per cent of the total phosphate, are obtained by igniting the ammonium

magnesium phosphate at high temperatures to pyrophosphate (35b).

Alternatively, the ammonium magnesium phosphate hexahydrate may

be weighed directly after drying the precipitate at room temperature (19),

or at about 370C., according to NJashburn and Shear (72) who obtained good

accuracy (about 0.2 per cent). This method, however, as employed in

this work was not as reliable as the pyrophosphate procedure, primarily

because of the difficulties in obtaining a reproducible tare weight for

the Pyrex fritted glass crucibles. In the course of ordinary phosphorus

analysis, the fritted glass crucibles lost first up to 1.0 mg. in weight

per determination and later on about 0.1 to 0.5 mg. per determination.

These crucibles were also somewhat hygroscopic and gained in weight up







22

to one milligram standing in room temperature.

Volumetric Determination.-In the volumetric method the ammonium

pphophomolybdate is dissolved in a known amount of standard sodium hy-

droxide solution, and the excess of the latter is back-titrated with

standard acid, phenolphthalein being used as indicator. The accuracy of

the volumetric procedure is hardly any better than one per cent. Several

attempts have been made to improve the procedure, for example, Kassner

(39) claims very good accuracy, about 0.1 per cent, for his method

employing mixed indicators.


Fluorine Determination

For the determination of fluorine in phosphatic materials, the

most tested and the most widely used method is that outlined by Willard

and V'inter (75), which has undergone numerous modifications by a host of

other investigators. A good summary about the modifications of the

Millard and Uinter fluorine determination method has been given by

keynolds and Hill (58). The method used in this laboratory corresponded

closely to the procedure as given by Willard and Winter and modified by

Reynolds and Hill.














THE MOLECULAR STRUCTURE OF 'L'Jr.JOA2ATITE


The unit cell and space group of the crystal lattice of fluoro-

apatite have been determined by the following investigators:

Leonhardt (h5), St. Naray-Szabo (63), Mehmel (50), Beevers and

McIntyre (4).

Apatite is a typical example of the crystal class C6h; etching

experiments and Laue photographs confirm this symmetry. The unit cell

contains 42 atoms, ordinarily written in the form CaF2.3 Ca3(PO)2 .

The unit cell of the apatite structure has two equal edges inclined at

1200 to one another. The length of these edces is 9.37 Ao in the case

of fluoroapatite. The third edge is at right angles to these and has a

length of 6.88 A,

The cell possesses vertical symmetry axes of three kinds:

1) passing through the points (two-thirds, one-third) and (one-third,

two-thirds) of the cell are three-fold rotation axes; 2) passing through

the corners of the unit cell are hexagonal screw axes; and 3) passing

through the halfway points of the cell edges and through the center are

two-fold screw axes.

There are also eight centers of symmetry and two mirror planes

parallel to the xy plane.










The atomicI surroundings are as follows:

Fluorine three Ca at one level

Phosphorus tetrahedron of four 0

Calcium I (on three-fold axis) nine 0

Calcium II (on reflection planes) an irregular
polyhedron of one F and five 0

Oxygen tetrahedron of one P and three Ca

While investigating the compositions of some western phosphate

rocks Hendricks, et al. (28), carried out a number of X-ray diffraction

measurements on the group of compounds structurally similar to fluoro-

apatite. The compositions and the crystallographic properties of these

minerals and of some artificial products support the suggestion that the

fluoride ion in fluoroapatite can isomorphously be replaced by carbonate,

sulfate, silicate, or hydroxyl groups; and by oxygen, chlorine, bromine,

or iodine ions.

Furthermore, the crystallographers have provided evidence that

the calcium ion in the lattice can be replaced completely or partly by

the ions of the folloirinr elements: Be, Mg, Sr, Ba, Pb, Zn, Ni, Cd, Cu,

Fe, 1n, Al, Mo, or Cr; and the phosphorus ion by V, As, Sb, S, C, Si, or

Al ions.

Fluoroapatite is the stablest of all these substituted structures

both thermally and chemically. The specific feature of the fluoroapatite

structure is the arrangement of calcium and phosphate ions in such a way

that channels exist within the crystal into which the fluorine ions fit


1The elements are present in the form of ions: F- 0-2 ,
P+5, Ca+2.







25

with close packing. This fact evidently accounts for the remarkable

stability of fluoroapatite, and makes it the substance to which the

numerous other structurally similar compounds can be compared in their

chemical reactivity and physical properties.















SOLUBILITY OF FL'UJ.rAP2ITE EI AQUTJUS SOLUTIONS


General Behavior

Fluoroapatite can be regarded as a basic substance due to its

high Ca to P ratio, and therefore it may be expected to react more

readily with acidic materials.

In aqueous solutions, the mineral acids decompose fluoroapatite

slowly at room temperature, faster at higher temperatures.

TVA is currently carrying out experiments on the solubility of

fluoroapatite in various concentrations of phosphoric acid at 2$5C. and

50C., including its rate of solution at a pH rarge between one and

four (15).

The solubility of fluoroapatite in solutions of organic acids is

rather limited. No reliable data are available yet about such systems.

Alkaline solutions under ordinary conditions do not affect

fluoroapatite appreci bly.

In respect to the reactions of alkaline solutions with naturally

occurring materials, the latest developments are the studies of Japanese

investigators (38) concerning the effect of alkali treatment of phosphate

rock by extracting fluorine with sodium hydroxide solution under pressure.

Fluoroapatite is very slightly soluble in water. The solubility

product, calculated from heat capacity and heat of solution is 3.73x10-117

at 250C. This value was calculated by the TVA investigators (15).











The Effect of Sodium Carbonate Solutions on Fluoroapatite

In the course of the preparatory work in this laboratory, there

was some interest in finding a solution of suitable composition which

would dissolve crystalline calcium fluoride in preference to fluoro-

apatite. The data on the relative solubilities of these two materials

in organic acids are incomplete, ani1 since there was no intention on our

part to study the solubility of fluoroapatite in acidic solutions, con-

sideration was given to sodium carbonate solutions. A series of experi-

ments were carried out in order to determine the effect of sodium car-

bonate solutions on finely powdered fluoroapatite unde' varied conditions.

The results are presented in the following section.

Experimental Results. -The extent of fluoroapatite solubility was

followed by -, osphate analysis. The materials wre treated with sodium

carbonate solution and filtered, The phosphate was precipitated in the

filtrate with a~noni~m molybdate. After washing with dilute potassium

nitrate solution, the precipitate was titrated with standard sodium

hydroxide.

Tro grades of fineness of apatite powder a) 250-300 mesh, and

b) finer than 300 mesh, were used for the following experiments.

Five grams of each material were let stand for three weeks at

room temperature in 100 ml. of water with two grars of sodium carbonate,

with occasional stirring The solid matter was separated from the

solution by filtration. The phosphate was precipitated in the filtrate,

and the titration gave the following values: a) 1.1 mg. and b) 1.5 mg.

phosphorus.








28

The residue on the filter paper was similarly treated for five

more weeks. The respective titration results were a) 2.7 mg. and

b) 2.8 mg. phosphorus.

In the next experiment, the two solutions with 1) 1.0 g. and

2) 0.5 g. of fluoroapatite (250-300 mesh) were boiled with two grams of

sodium carbonate in 100 ml. of solution for two hours and filtered at

room temperature. The results were: 1) 0.9 mg. and 2) 0.8 mg. phos-

phorus determined in the filtrates.

Repeating the procedure with the residues after filtration and

with new solutions, the following values were found: 1) 0.8 mg. and

2) 0.6 mr. phosphorus.

A higher carbonate concentration increased the phosphate concen-

tration in the solution, and filtering of the solution while hot gave

somewhat higher values.

The amount of phosphorus determined in the solution after treat-

ment was in the order of one to two per cent of the total phosphorus

present in the fluoroapatite sample (184 mg. in one gram of fluoroapatite)

Conclusions.-The results lead to the conclusion that sodium car-

bonate solutions affect the solubility of fluoroapatite. The total

effect does not exceed two per cent with one treatment at the given con-

centration of sodium carbonate.

Several factors influenced the rate and extent of the increased

solubility of fluoroapatite, but generally there seemed to be an equili-

brium value of about two to three milligrams of phosphorus in 100 ml of a

solution containing two grams of sodium carbonate. This value was

approached faster on boiling of the solution, and slower upon standing at









room temperature.

Solubility of Calcium Fluoride and Tricalcium Phosphate. -The

solubility of calcium fluoride and tricalcium phosphate was also deter-

mined in similar sodium carbonate solutions.

After boiling 0.100 g. of calcium fluoride with one gram of

sodium carbonate for two hours in 200 ml. of water, 75 per cent of the

total fluorine could be titrated with thorium nitrate.

Similarly, tricalcium phosphate is partly decomposed by sodium

carbonate. A sample of 0.930 g. was boiled for two hours with two

grams of sodium carbonate in 100 ml. of solution. Phosphorus precipi-

tated. in the filtrate amounted to 13 mg., that is, 1l.3 per cent of the

total phosphorus.















REACTIONS OF FLJ,3OAPATIi'Z KITH A CASEOUS PHASE


The possibilities of fluoroapatite reactions with a gaseous

phase and the formation of gaseous products in such reactions are dis-

cussed in this chapter. A number of standard free energy changes were

calculated and are tabulated for reactions involving materials for which

the thermodynamic data were available. The major sources for the thermo-

dynamic values used throughout this chapter are listed in the biblio-

graphy (40, 43, 53, 56, 61).

First it was necessary to calculate the free energies of forma-

tion of fluoroapatite and hydroxyapatite.


Formation of Fluoroapatite and Hydroxyapatite

The values for the standard free energy of formation, and the

standard heat of formation of fluoroapatite and hydroxyapatite have not

yet been published by K. L. Elmore, et al., who have determined the heat

capacities and the solubilities (15, 16a,b) of these two apatites.

The data, however, can be calculated from the available solubility

constants according to the equations:

A Fo(aq.) -aF(c) -RT In Ksop (at 298.160K.)


6 F(c) -= Hapatite T(Sopatite- 1SO elements)

By utilizing the heat capacity data, the free energy functions

of these two apatites can be formed and used up to 15000K. For the sake







31

of simplicity, the numerical values for only four temperatures have

been tabulated.


TABLE 3
FREE ENERGIES OF FO iION OF FLUOROAPATITE AND
!IYD ZYAPATITE AT FOUR T"Ti.ATIRES

Z Ho Fo Per Gram Formula Height

298.160K. 298.160K. 50OK. 10000K. 15000K.

Fluoroapatite 3266.6 3083.4 2960.8 2646.9 2330.!5

Hydroxyapatite 3212.h 3020.2 2891.4 2561.6 2232.1

The conventional standard states were used for the elements

except that the standard state for phosphorus was the metastable solid

white modification at 298.160K., and gaseous P4 at higher temperatures.

The standard free energy changes for the reaction

3 Ca3(i'O)2 + CaF2 .F CalO2(PO4)6
at the four temperatures are:-

a Fo(kcal.): 9.6 (298.160K.), 9.2 (5000K.)

-10.5 (1000K.), 4.o (15000K.)

The standard free energy values for the above reaction exhibit a

maximum which is probably due to the phase changes of calcium fluoride

and tricalciun phosphate near 1000K.

The corresponding reaction for the formation of hydroxyapatite

3 Ca3(PO4)2 + Ca(OH)2 -.. Ca10(OH)2(PO)6
gives the following standard free energy changes:-

A oF(kcal.)t 9.6 (298.16K.), 9.6 (500K.)

-11.7 (1000K.), -19.3 (5oo00K.)







32

For the higher temperatures it is assumed that H of Ca(OH)2

does not change and that the change in heat capacity of reactants and

products is zero.


Reactions with Hydrogen-Containing Gases

The first group of reactants to be considered consisted of

hydrogen-containing gases, and the reaction was assumed to produce gas-

eous hydrogen fluoride. The numerical values of the standard free

energy changes of the following reactions designated by small letters,

are presented for the four temperatures in Table 4.

The conversion of fluoroapatite into hydroxyapatite upon reaction

with water vapor

(a) CalOF2(PO4)6 + 2 H20 -- Cal(OH)2(PO4)6 + 2 HF

evidently does not proceed in the given temperature range, a conclusion

which is supported by experimental evidence. The addition of acidic

oxides iould facilitate the decomposition of the original structure, for

example, by using silica:

(b) CaloF2(P04)6 + H20 + SiOC2 CaSi03 + 3 Ca3(PO4)2 + 2 HF

The actual decomposition of fluoroapatite with silica in the

presence of water vapor takes place above 1500K. The other oxides, the

addition of which may exert similar beneficial effect are: Fe203, A1203,

B203, TiO2, etc.

The possible side reaction in the presence of silica (h4)s

4 HF + Si02 ---- 2 H20 + SiF4

has a positive standard free energy change at higher temperatures.







33

A Fo(kcal.): 17.2 (298.160K.), 13.4 ( 500K.)

2.7 ( 1000oK. ), + 8.8 (1500oo.).

In the following examples not all the reaction products could be

predicted with certainty. They were assigned arbitrarily for the pur-

pose of providing an indication about the relative tendencies for vari-

ous possible chemical reactions.

According to the arbitrary scheme

Cal0F2(PO)6 + H2Y CaY + 3 Ca3(P04)2 + 2 HF
all the standard free energy values are positive foz the hydrogen-

containing gases, including HCl(c), lBr(d), H2S(e), H3N(f), etc. I T

two Cl, two Br, one S, two-thirds N.

The above scheme refers to the improbable condition that trical-

cium phosphate remain intact. If it is assumed that it is rather calcium

fluoride which remains among the products, the strongly acidic gases

should decompose fluoroapatite at low temperatures.

The reaction with hydrogen chloride, for example, according to the

following equation

(g) CaloF2(PO0)6 + 6 HC1 CaF2 + 3 CaC12 + 6 CaRIPO

has a negative standard free energy change at room temperature.

With hydrogen fluoride, the reaction should be far more energetic

at the same temperature.

(h) CalF2(PO)6 + 6 HF r. 4 CaF2 + 6 CaHPO4

An excess of gas would probably further the reaction with the

formation of monocalcium phosphate and more calcium halide. A large

excess of the gas, however, should not readily yield the following

products:










TABIE 4

STANDARD FREE ENERGY CHANGES OF REACTIONS OF
FLUOROAPATITE WITH A (ASTQTJS PHASE


A Fo in Kilocalories Per Gram-Formnla Weight
of Fluoroapatite
Reaction
298.160K. 5000. 10000K. 15000K.


+ 42.9

+ 47.1

+ 23.5

+ 25.0

+ 53.3

+130.9

- 4.6

- 48.2

+240.6

+150

-677.6

-10o.4

-490.8

-762.0

-695.7


+ 43.7

+ 40.2

+ 22.2

+ 2,.1

+ 46.7

+121.5




+312.5



-631.7

-425.3

-380.5


+ 45.8

+ 26.1

+ 21.2

+ 23.4

+ 33.6

+101.4


+ 44.7

+ 5.3

+ 9.8

+ 12.5

+ 13.3

+ 73.9


+438.0


-459.4


-540.3

-4o6.2

-151.5


-60o.3


-"44.6







35

(i) Ca10 2(PO )6 + 36 HG1 -U 6 PO1.3 + 9 CaCl *

18 H20 + CaF2


(j) CalF2(P04)6 + 36 HP .. 6 POF3 + 10 CaF2 + 18 H20


Experimental Tests with Gases

A number of tests were carried out in order to find experimental

verification of the above-mentioned conclusions regarding the reactions

with hydrogen-containing gases.

Several samples of finely -round (300 mesh) synthetic fluoro-

apatite were subjected to the atmosphere of hydrogen and ammonia in the

temperature range of 500 to 10000C. for the duration of ten to twelve

hours in an electrically heated tube furnace. Similar experiments were

performed in the nitrogen atmosphere, which was saturated with water

vapor at room temperature, with and without the presence of silica dust

added to the sample.

Traces of fluorine-containing components could not be detected

in any case in the exit gases which were passed through an alkaline

solution.

The sensitivity of the test was about 8xlO'5g. fluorine, i.e.,

1 to 4000 of the total fluorine available in the sample.


Reduction of Fluoroapatite by a Gas Phase

The next group of reactions refers to the reduction of fluoro-

apatite in contact with an active gas phase. The numerical values of

the free energy changes are presented in Table 4.

Molecular hydrogen gives strongly positive standard free energy







36

changes in the given temperature range for the reaction

(k) CalOF2(PO4)6 + 15 H2 CaF2 + 9 CaO + 5/4 P4 + 15 H20

Molecular chlorine is also ineffective in the decomposition

of pure fluoroapatite if the reaction is written as follows:

(1) Cal0F2(P04)6 + 18 C12 .-9,6 POC13 + 9 CaC12 + 9 02 + CaF2

In contact with atomic chlorine (m), or hydrogen (n), the re-

duction yielding the same products as given above should occur readily.

An efficient low-temperature decomposition of fluoroapatite

with a mixture of chlorine and carbon monoxide will probably take place

according to the following schemes

(o) CaloF2(PO4)6 + 18 CO + 18 C12 -. POC13 + 9 CaC12 +

18 002 + CaF2

Conclusions

For the reaction of fluoroapatite with hydrogen-containing

gases, it is improbable that hydrogen fluoride will be formed as a

reaction product below 100000C.

The decomposition of fluoroapatite structure may take place with

strongly acidic gases at low temperatures, whereas fluorine remains in

the solid phase, most probably as calcium fluoride.

The reduction of fluoroapatite without additives is not easy

to effect under 10000C., and takes rather drastic conditions as the use


lit has been reported (11) that natural phosphatic nodules can
be defluorinated in the presence of elementary chlorine at about l000 to
6000C. The volatile products are silicon tetrafluoride and phosphoryl
chloride. In the presence of carbon and silica defluorination can be
carried out at 5OO0C.







37

of atomic hydrogen, or chlorine.

In special cases a mixture of reactive gases, e.g., chlorine

and carbon monoxide, might be efficient for a low temperature reduction.
















RACTIOUDS 1 T'Mi SOLID STATE


Types of Solid State Reactions


Many types of solid state reactions of matter are mnown (10).

The simplest and most frequently occurring type is the additive

reaction A + B ---qtA, where A rand B may be elements or compounds. A

large group of reactions in this category are the combinations of an

acidic reactant with a reactant of basic nature to yield a salt. For

example, CaO + SiO2 --~ CaSiO3.

The second mode of reaction between two solids is by exchange

of constituents according to one of the following schemes:

(a) A + BC AC + B (b) AB + CD -~- AD + BC

(c) ABX + CB -- CBX + AB

In these reactions which involve diffusion through two product

layers, it is possible to have a considerable variety of phase arrange-

ments determined by the miscibilities of product and reactant phases,

and by the reaction mechanism.

Examples of reactions according to scheme (a) are found in the

exchange of the metal of a salt for a more electropositive one.

Reactions conforming to scheme (b) are exemplified by the

following: CaO + NiC2 CaC12 + NiO; CaO + PbC12 ->. CaCl + PbOj

or in combination with other reactions.

Reactions according to scheme (c) comprise a large group of

38









39

reactions in which an oxygen-containing acid group is exchanged between

one basic oxide and another: MeO + Me'XOn -- Met0 + MeXOn.

The reaction temperatures in the last group, as in the others,

show a correlation with the basic nature of the oxide involved; moreover,

in contrast to the other types described, these reactions have in common

the fact that the reaction temperatures for a given basic oxide are

approximately constant and independent of the nature of the other reac-

tant (carbonate, sulfate, phosphate, silicate).

The above principles were considered with a Tew to possible

application on fluoroapatite by way of inducing permanent changes in the

apatite lattice. In effect the application of these principles means

that calcium should combine with acidic components other than fluoride

and phosphate, for example, silicates and sulfates. In the presence of

additional cations, e.g., sodium and magnesium, mixed salts and new

phases could be formed.


Thermodynamic Considerations

Except for cases of miscibility of reactants and product phases,

equilibrium is usually not possible in reactions in the solid state, and

they proceed exothermally until at least one of the reactants is

completely consumed. In practice, of course, complete reaction is

difficult to achieve.

In a number of cases, the comparison of free energies of the

reactants and of the products may give an indication about the tendency

for a solid state reaction. Thermodynamic data were compiled for various

groups of compounds over a range of temperatures for the purpose of free








ho

energy calculations. Awareness, however, was maintained of the limita-

tions of such an approach because of incomplete data, and for other

reasons. For example, even if the free energy relationships should be

favorable, the thermodynamic requirements are by no means a safe cri-

terion for determining which phases actually will be formed in a

reacting system. Since the particle motion in solids is strongly

resisted and requires large energies of activation, unstable phases

might form and coexist with other phases for practically unlimited

times if the temperature is sufficiently low. Even when the reaction

products are the thermodynamically required phases, they may appear in

unstable states characterized by an excess energy content. Nevertheless,

the intention was only to establish a broad scale for various groups of

compounds in their relative readiness to react with fluoroapatite, inso-

far as there wore proper data available. The results are summarized

in one of the following sections.

Apart from some preliminary tests, laboratory experimentation

was not carried out with regard to the solid state reactions for lack of

suitable analysis equipment and facilities for X-ray diffraction

photographs.

Studies of the solid state reactions could profitably be com-

bined with the determinations of diffusion and migration of the ions in

the fluoroapatite lattice as measured by the electric conductivity of

the crystal.


Exchange Reactions between a Pair of Salts

A large number of solid state reactions are known in which an







4l

exchange of cations (or anions) takes place between a pair of salts at

moderate temperatures with a measurable rate (10).

It is conceivable that the addition of some suitable salts to

the apatite system might induce changes in the initial structure of the

material whether by solid or liquid state reactions. It is possible that

new phases would result which show increased solubility or enhanced sus-

ceptibility to the gas phase reactions. Fron this point of view, various

possibilities were considered and many standard free energy changes were

compared. The problem is essentially to find a salt, the anion of which

would readily combine with calcium, and the cation of which would have a

strong affinity toward fluoride or phosphate.

Amonf the most suitable compounds for the above-mentioned

purposes are chlorides, sulfates, and sulfides; furthermore, silicates,

borates, etc. First the tendency for fluoride exchange was considered

in the simplified manner for calcium fluoride.

Chlorides.-The general reaction is:

CaF2 + XC12 -> CaC12 + X

Of the nonvolatile chlorides, only beryllium and magnesium chlo-

ride exhibited negative standard free energy changes for the given

reaction.

JeCl2 --AF(kcal.): 6.7 (298.160K.), 16.6 (500K.),

-16.2 (1000K.).

IMgC12 -- AF(kcal.) -12.3 (298.160K.), -13.5 (500K.),
-16.7 (10000K.) -16.9 (15000K.).

Other cations considered were: Li, Na, K, Sr, Ba, Fe, Po, Cu,

Co, Ni, Sn, and Al.







h2

Herein must be included the possibility that the two gaseous

chlorides, silicon tetrachloride and boron trichloride, might effect an

exchange of fluoride for chloride in the solid phase, at least according

to the standard free energy equations. The respective values are per

one mole of calcium fluoride:

SiCG --AF(kcal.): -10.1 (298.160K.), -16.0 (5000K.).

BC13 -aFo(kcal.): -15.1 (298.160K.).

Sulfates.-The exchange of fluoride for sulfate between a pair of

salts is generally unfavorable, except for magnesium sulfate. The

cations considered were: Na, K, Mg, Zn, Fb, Fe, Mn, Cu, and Al. The

free energy chance for the reaction

CaFG + XiSO4-1 CaS04 + XF2
is positive in the order of 8 to 15 kcal. (for magnesium sulfate, it is

negative by 2.2 kcal.) at room temperature per one mole of calcium

fluoride, and 0 to 9 kcal. at 1000K. The values decrease regularly

with increasing temperature. All, 'o and Cu offered the lowest positive

free energy changes.

Sulfides.-For most sulfides the thermodynamic data for higher

temperatures are deficient. At room temperature only Na, K, and Mg-

sulfides exhibit negative standard free energy changes in the order of

h to 6 kcal. for the reaction

CaF2 + XS -- XF2 + CaS

The cations considered were: Ma, K, Be, Mg, Ca, Ba, B, Al, t1, Fo, Co,

Ni, Cu, Zn, Sn, and Pb.

1An aqueous solution of aluminum sulfate undergoes double
decomposition with calcium fluoride (24).







h3

Orthophosphates.-The meager data available on ort'.ohos siteste s

prohibited the extension of sirnilar comparicons to the exchange of phos-

phate ion for other anions.2

Conclusion.-The possibilities for solid state reactions remain

inconclusive until proper experimental testing, or until more thermo-

dynanic data will become available.

Under actual experimental conditions, the formation of mixed

salts, like CaC1F and NaCaPO4 and the existence of solid solutions

must be considered, which further complicates the thermodynanic evalua-

tion of possible solid state reactions.

2t has been proved experimentally (26) that Sr and Ba replace

Ca in phosphates (also in sulfates and silicates) in the solid state

reactions between 5000 and 600C.















REACTIONS OF FLUOROAPATITE IN MELTS


A number of materials could be used as additives for high tem-

perature reactions for the purpose of inducing changes in the fluoro-

apatite structure. The pure synthetic fluoroapatite has not yet been

used for such experiments, but the fusion of phosphate rock with addition

agents has been the object of many investigators.

Among the materials used there have been: furnace slag, alkali

silicates, alkaline earth silicates, sodium carbonate, silica, alumina,

magnesia, alkalies, magnesium silicates, magnesium sulfate, langbeinito,

polyhalite, etc., and the mixtures of these. The individual references

are listed in the following sources (7, 18b). The reaction temperatures

are usually above 10000C., and no defluorination takes place.

Various experiments were carried out in this laboratory with

synthetic fluoroapatite in salt melts.


Melts of Neutral Salts

The neutral salt melts can possibly induce partial replacements

in the fluoroapatite lattice.

For example: one gram of fluoroapatite with five grams of sodium

chloride kept in a platinum crucible for an hour at 90000C yielded eight

per cent of its total fluorine after leaching the product with water. A

comparable amount of chlorine was found in the water-insoluble residue.











Melts of Basic Salts

The basic salt melts decompose fluoroapatite at high temperatures.

Fluoroapatite-sodium carbonate mixtures, with and without the

presence of powdered silica, were heated to the maximum temperature of

925C. in the course of six hours. The fusion products were leached

with water, and the solutions were analyzed for phosphate. The extrac-

tions contained about one third to one fourth of the total phosphate in

the sample. The value for released phosphate in the melts with silica

was higher than in the melts without silica.

Fluoroapatite-sodium tetraborate mixtures at 10000C. effected

the decomposition of the starting material, and about ten per cent of

the available fluorine was volatilized (possibly as boron trifluoride).


Melts of Acid Salts

The acidic melts should offer the best promise for the decom-

position of fluoroapatite at medium temperatures.

First of all, metaphosphates may be considered because of their

relatively strong acidity and convenient melting temperatures. The

natural relationship of metaphosphates to the other phosphate systems

would allow easier interpretation of the results, insofar as such systems

have been observed in other studies. The high thermal stability and

ease of preparation of metaphosphates speak in favor of their use.

In consideration of the above-mentioned advantages the latter

part of this work will be devoted to the study of metaphosphate inter-

actions with fluoroapatite, and other fluorine-containing materials.
















INTERACTION OF SODIUM METAPHOSPHATE -'ITH FLUORIDES


Preliminary Observations

In the course of the studies about the reactions of fluorides in

melts, it ras observed that an interaction takes place between polyphos-

phate and fluoride ions with the formation of a phosphorus-fluorine

containing gas (or Gases), in addition to the evolution of hydrogen

fluoride.

The initial system observed was calcium fluoride in sodium meta-

phosphate melt. Soon, however, it became evident that other fluorides,

e.g., sodium and potassium behaved similarly.

A series of preliminary experiments was then carried out with

calcium fluoride and sodium fluoride mixed with sodium metaphosphate and

heated in dry nitrogen atmosphere for the purpose of determining the

major factors affecting the rate and extent of the formation of volatile

fluorine-containing gases.

The reaction started considerably below the melting point of any

single component present, and was easily noticeable because of the

immediate formation of white dense fumes as soon as the gas from the

furnace came in contact with water or atmospheric moisture.


Literature Survey

The survey of literature revealed that the above-mentioned

fluoride-metaphosphate systems have not been systematically studied,

46







h7

except that the addition of fluorides to metaphosphates, usually in

stoichiometric amounts, is the standard procedure for the production of

the respective monofluorophoophates (29) which are known to decompose at

higher temperatures with the reduction in the fluorine content of the

salts (30). According to the information received from the Ozark-

Mahoning Company concerning the fluorophosphate salts (73), neither the

decomposition rates of these salts nor the reaction products have been

investigated.

Tarbutton, et al. (65), let calcium fluoride react with phos-

phorus pentoxide at temperatures ranging from 5000C. to 100000., and

identified phosphoryl trifluoride (PO3) as the predominant volatile

product of the reaction, along with some small amounts of phosphorus

trifluoride (PF3).

Gerard Montel (51) studied the same system, CaF2-P205, over a

wider range of compositions, and confirmed the results of the previous

investigators.

These authors were primarily interested in the end products of

the reaction and did not carry out studies of melts related to the rate

of the formation of volatile fluorine compounds.


Objectives of Study of the Metaphosphate Melts

It was here attempted to determine systematically the major

factors which affect the rate and extent of the evolution of volatile

fluorine compounds from the metaphosphate melts containing fluorides.

The further intention was to determine whether the same relation-

ships are valid for the decomposition of fluoroapatite in contact with








48

molten metaphosphate, and for the subsequent volatilization of fluorine-

containing compounds.

The major variables were composition and temperature. The other

factors considered were: duration of heating, presence of water vapor

in the furnace atmosphere, size of charge, surface of the melt, rate of

gas flow above the melt, pressure, typo and mode of preparation of the

used salts, particle size, and way of packing.

For the fluoroapatite tests the effect of added silica, sulfates,

and sodium tetraborate to the metaphosphate melts was observed.

Use was made of three furnaces and experimental arrangements for

obtaining the necessary data.

1. The vacuum furnace was constructed for the purpose of

finding out the following information:

a) whether it was possible to remove all of the bound

water from the materials below the reaction temperature;

b) the absolute pressure above the melt and the rate of

gas evolution from the nelt;

c) the identity of the gases involved.

2. The tube furnace was used to measure the rate of gas evolu-

tion at constant temperature with varying composition and in controlled

atmosphere. It was also used for heating the closed nickel tube which

was attached to the vacuum system.

3. The crucible furnace served for the visual observation of

the melts, and for the determination of the molting range of the mixtures.

Also larger amounts of salts could be used for varying the depth of the

charge.








49

The use of each method, and the actual experimental conditions

are to be described separately in the following sections.


Selected Metaphosphate Systems

From among the numerous possibilities the following systems were

chosen for a closer study. The reasons for the particular choices are

indicated after each system.

1. NaF-NaPO Convenient temperature range. The single

components have the following melting points:

NaPO3 6200C., NaF appr. 9900C, Both salts

are nonhygroscopic, provided that the meta-

phosphate is in the form of crystalline tri-

metaphosphate. Much is known about sodium

metaphosphate melts as such in respect to

the molecular weight distribution, density,

surface tension, and viscosity (69).

2. CaF2-NaPO3 -The presence of two types of cations.
and
NaF-Ca(PO3)2 Effect of a bivalent cation replacing the

monovalent cation. Relatively low reaction

temperature, about 5000 to 6000C.

3. Fluoroapatite-A special case of the previous system.
NaPO3
Na and The effect of crystal structure on the
NaPO -Ca
(PO4)2-CaF2 reactivity.
Some additional systems were also tested, e.g., calcium fluoride-

calcium metaphosphate, sodium metaphosphate various metal fluorides.

In preparing the materials for reaction, the mixing had to be







50

unifrom and the particle size defined. The standard sieves were used,

250 and 300 mesh, from the Newark Wire Cloth Company, Newark, New Jersey.


Characteristics of the Evolved Gases

General Observations.-The gas evolved from the melt strongly

etched a silica tube and after passing through water, made it acidic.

The presence of fluoride (or fluorosilicate) ions as well as phosphate

ions could be proved in such a solution.

The solid adsorbents (soda lime, Ascarite, etc.) did not prove

to be reliable for the total capture of the gases at a moderate flow

rate, but a dilute alkaline solution cooled to ice temperature, was

satisfactory for the absorption of the gas mixture for analysis.

The ratio of phosphorus to fluorine was relatively small in the

analysis solution. Most of the fluorine was evidently evolved as

hydrogen fluoride. Since the atmosphere above the melt was dry nitrogen,

the hydrogen-containing component must have been provided by the ever-

present chemically bound water in the melt.

The phosphorus-fluorine ratio in the trap was directly related

to the amount of moisture present in the furnace. If the atmosphere

above the melt was saturated with water vapor, the phosphorus content was

very small, although sufficient for a qualitative proof, about 0.1 mg.

If the charge was heated gradually over a period of several

hours, the P to F ratio became higher but not more than about one to

seven, expressed in mole ratio. The gradual heating in the stream of

dry nitrogen evidently removed some of the surface moisture of the con-

stituent salts before the reaction started.







51

Separation of the Gaseous Phosphorus-Fluorine Containing

Component.-Attempts were made to find a suitable adsorbent which would

separate hydrogen fluoride from the rest of the gas mixture.

In one experiment, the gas from the furnace was passed through

a 3-cm. depth of phosphoric acid (85%, Reagent Grade) at the gas flow

rate of 80 ml. per minute. About seventy per cent of the total fluorine

had been absorbed but the P to F ratio still remained very low. Con-

centrated sulfuric acid used under the same conditions let more gas

through unabsorbed and the P to F ratio was approximately the same as

before.

Reagent Grade sodium fluoride powder was moistened and baked in

an oven at 110C. for 24 hours. The dry aggregate was powdered to 250

mesh and filled into a narrow copper tube 100 cm. long. At a slow rate

of gas flow, 25 ml. per minute, all phosphorus and fluorine-containing

components of the gas mixture were adsorbed.

Next, the same narrow copper tube was filled with sodium fluoride

pellets and an increased gas flow rate was applied, 100 ml. per minute.

The source of the gas was 1.5 g. of calcium fluoride-sodium metaphosphate

mixture heated between 4000 and 7600C. for three hours.

The analysis of the trap gave 1.0 milli-equivalents of fluorine

and 0.33 milli-equivalents of phosphorus. A duplicate confirmed the

result of a phosphorus-fluorine ratio one to three.

This proves that sodium fluoride pellets adsorb hydrogen

fluoride preferentially and at a moderate gas flow rate, a part of the

phosphorus-fluorine compound passes through the adsorbent.









There are three known phosphorus-fluorine containing gases1:

POF3, PF 3, PF5; PO2F is only assumed to exist (62).

Since it is highly improbable that the found P to F ratio

results from a correct mixture of PF5 and PO2F, only the first two

gases nay be considered.

Phosphorus Trifluoride.-Phosphorus trifluoride (PF ) is

absorbed by aqueous bases producing fluorophosphate which reacts readily

with potassium permanganate or bromine. Repeated attempts with these

two reagents failed to yield evidence for a reduction-oxidation reaction,

and therefore the only possible conclusion is that the major component

of the phosphorus-fluorine containing gas is phosphoryl fluoride, even

though the possibility of tracks of other gases need not be denied.

Phosphoryl Trifluoride.-Phosphoryl trifluoride hydrolyzes (42)

by steps according to the following scheme:
HR20 20 H20
POF. -- P02F2 HPO3F -- H3P
HF HF

In an alkaline medium the respective salts which are readily

soluble will be forced. Aqueous solutions of the alkali difluorophos-

phatoos react neutral but become acidic upon longer standing even at low

temperatures. Heating accelerates the hydrolysis. The first products

of deco.'iposition are monofluorophosphates and they may be obtained in

quantitative yields by boiling aqueous solutions of alkali difluorophoe-

phates with very dilute solutions of the corresponding hydroxides.


excluding other elements, except oxygen, from the composition







53

The alkali monofluorophosphate solutions are neutral toward

phenolphthalein but alkaline to methyl orange. Neutral or weakly

alkaline solutions are not hydrolyzod on boiling for one hour. Heating

in strongly alkaline solution results in rapid hydrolysis, and fast

decomposition also occurs within a few minutes while heating in a

strongly acidic solution.

As an example of phosphoryl trifluoride neutralization in dilute

sodium hydroxide solution the following equation is presented:

POF3 + 4 NaOH -- Na2PO3F + 2 NaF + 2 H20

One molecule of phosphoryl trifluoride reacts with four equi-

valents of alkali. Phcnolphthalein is used as an indicator. In acidic

solution the hydrolysis would go one step further.

Difluorophosphoric Acid.-Luring the experiments with the tube

furnace at lower temperatures, it Tas noticed that small droplets of a

colorless liquid had condensed on the valls of the polyethylene tubing

near the end of the nickel tube. These droplets were rinsed with small

volumes of cool nitronI solution and thus removed from the tube. After

standing for a few hours at ice-temperature, a slight colored precipi-

tate was formed. The precipitates of several runs were combined, dried,

weighed, and analyzed for fluorine. The weight ratio of fluorine to the

whole sample (14.8 ng.) was about 1 to 10. The theoretical ratio for

the compound nitron-difluorophosphoric acid (one molecule) is 10 to 109.

It may be assumed that the condensed material consisted primarily

of difluorophosphoric acid2 which is the first decomposition product of

phosphoryl trifluoride with water,

INitron is a shorter name for diphenyl-endo-anilo-hydro-triazole,
which is an analytical reagent.
2The boiling point of difluorophosphoric acid (5) is appr. 1100G.










Construction of the Vacuum Furnaces

The Furnace for Crucibles.-A vacuum furnace was constructed

which could accommodate a platinum crucible. Since the design was

unique for a furnace of this size, the details of construction will be

described.

Nichrone V resistance wire (B&S 24) was wound around a specially

made nonporous porcelain core in a helix one-eighth inch in diameter and

nine inches long (when closed). The total resistance was 20 ohms and

the current was, for example, four amperes at 7000C. The dimensions of

the core were: the diameter inside one inch, outside one and three-

eighths inches, height two and three-quarters inches. The core was

surrounded by two concentric radiation shields made of nickel sheet,

one-one hundred twentieth inch thick. The diameter of the inner shield

was two and one-quarter inches, of the outer shield two and eleven-

sixteenths inches, the height three and three-quarters inches. The two

shields were separated by porcelain spheres through which short pieces of

nichrome wire held them together. There were also horizontal shields,

two above and two underneath the core. The core and the shields were

suspended by nichromo wire from three steel support rods eight inches

high and one-eighth inch in diameter. Two of these wires served also for

the conduction of current to the furnace. The support rods were screwed

into the base which was a brass plate five-sixteenths inch thick. The

connections through the base for the heating current as well as for the

thermocouple wires, were made with the help of Stupakoff Kovar-Glass

Terminals, Type FC, soldered through the plate.









The whole furnace was enclosed by a glass dome three and seven-

eighths inches in diameter and ten inches high which rested in a groove

in the brass plate. The dimensions of the grove were outside diameter

four and one-eighth inches, one-quarter inch wide, one-eighth inch deep.

The vacuum seal between the plate and the glass was achieved by using

the vacuum wax Apiezon W, which softens at about 800C.

The platinum crucible rested in the middle of the furnace on a

hollow porcelain rod through which the thermocouple junction was led up

to the crucible without touching it. Through the center of the plate

went a copper tube, three-eighths inch in diameter and fifty inches long

which connected the furnace with the rest of the vacuum system. The

furnace itself rested on a thermally insulated tripod which was firmly

attached to the desk.

The furnace rendered good service at low pressures and reached

high temperatures readily. The bottom plate did not become warmer than

5oC., even when the furnace was at 8000C. for several hours. The tem-

perature control was affected noticeably by the pressure changes, and

was not better than SoC. Baking of the furnace over a longer period of

time at moderate temperatures did not remove the adsorbed gases completely

and with an increase in temperature, slight degassing continued. Intro-

duction of hydrogen into the system before opening it to the atmosphere

improved subsequent evacuations.

According to the available data regarding phosphoryl trifluoride

(65), it was not supposed to react with the furnace materials under the

given conditions. Some etching of the porcelain core, however, became

noticeable after the experiments, and silicon tetrafluoride was found to








56

be present in the liquid air trap. Since it was possible that the de-

corposition of the phosphorus-fluorine containing gas took place because

of high temperature, and the presence of siliceous materials, another

type of furnace was made use of for a number of experiments.

The Nickel-Tube lurnace.-An all-nickel tube (International Nickel

Co., Mark:53965) was placed into a Hoskins electric tube furnace (Type

FH 303 A, 15 V, 37 A) and connected to the copper tube leading to the

vacuum system with Apiezcn W wax. From the closed end of the nickel

tube, a smaller tube of the same material, silver soldered to the larger

one, led up to the middle of the furnace and housed the thermocouple.

The dimensions of the tutes were as follows: larger tube had the length

thirty-three and one-half inches, and the inside diameter fifteen-

sixteenths inch; tho smaller tube had the length nine and one-half inches,

and the inside diameter three-quarters inch.

A Sorgel electric. transformer (Type IFS, sec. volts 17; 32.3 A)

belonged to the tube furr.ace as an accessory.

The thermocouples were chromel-alumel junctions standardized

against the melting points of pure inorganic salts, and compared to the

other couples. The potertiometer used with the thermocouples was from

Wheelco Instruments Co., Model 310.

The now apparatuE had a smaller volume, 365 cc., as compared to

1600 cc. of the former. It also offered better dogassinp characteris-

tics and closer temporatire control. The disadvantage was that the

size of the charge was limited by the capacity of the platinum boat.

The platinum boat was placed into a larger zirconia boat so as to avoid

the contact of platinum iith the nickel tube.







57

This furnace operated in a satisfactory manner and an example

of the measurements is presented in the next section.


Method of Measurement with the Vacuum Furnaces

When the furnace was below 300C., a vacuum of the order of

103 anm. was reached in a couple of hours. At higher temperatures it

took considerably longer, about a day.

The vacuum system on the rack consisted of a McLeod gauge, a

mercury manometer, three traps, and provisions for removing material

from the system. The one-stage mercury diffusion pump was backed by a

Welch Duo-Peal vacuum pump. As a safety precaution, the water flowing

through the cooling jacket of the diffusion pump operated a relay

regulating the heating current of the diffusion pump, thus securing safe

over-night operation of the pump without supervision.

The rate of the gas evolution was determined in the following

manner:- 'Thile the system was open to the pumps, the pressure was

measured with the McLeod gauge. Then the pumps were cut off by closing

a stopcock and letting the system stand for five minutes (or some other

known period of time), whereupon the pressure was again measured. The

liquid air trap was opened simultaneously with the closing of the furnace.

After 5 to 15 minutes, the pressure was determined again. The difference

between the two last measurements made it possible to calculate the

amount of condensible gases evolved from the melt in the given time

period at a known temperature. After such a series of measurements, the

system, including the furnace, was pumped empty through the other liquid

air trap.







58

The volumes were: the McLeod gauge 282 cc., the rest of the

glass system minus the traps 312 cc., a single trap 18 cc., and the

furnace 365 cc.


Experimental Results of the Vacuum Measurements

The results showed that water was held very tenaciously by the

melts, and that even prolonged heating (12 hours and longer) under

reduced pressure (10-3 mm.) in the temperature range from 3000C. to

5000C., did not remove the hydrogen-containing constituent completely.

Then the temperature was raised above 500., the reaction

gradually started and gases condensible in the liquid air trap were

present in the vacuum system.

The rate of gas evolution did not present any sharp maxima in

its temperature dependence. It usually showed somewhat higher values

immediately after the rise in temperature but then levelled off to a

rate which decreased slowly at a given temperature.

For example, a charge of 5.0 grams of the standard sodium

fluoride-sodium metaphosphate mixture with the platinum boat in the

nickel tube was preheated under vacuum for twelve hours. V;ith a gradual

rise in temperature above 500C., the following rates were observed:










TABLE 5

RATE OF GAS EVOLUTION FROM .ETAPH.JGPHATE ITLLTS
IN THE VACUUM FURNACE


Temperature Time Period Above Rate Expressed in Remarks
OC. 5000C. in Minutes Moles of Gas per
i5 Minute x105
10.1


510 60 1.7 Steady Temp.

530 90 3.7 Steady Temp.

S40 120 3.8 Steady Temp.

580 150 3.9

594 160 5.6 Increased Temp.
628 270 5.0 Steady Temp.

630 300 5.0 Steady Temp.

713 380 11.2 Increased Temp.

716 480 3.9 Steady Temp.

790 560 5.6 Increased Temp.

800 630 2.8 Steady Temp.


These rates refer to the mixture of gases the pressure of which

was measured in the system. The analysis showed that silicon tetra-

fluoride was the major component in the gas phase, with about ten per

cent of a phosphorus-containing component.

Traces of hydrogen-containing compound (probably in the form of

water) still present in the melt evidently interfere with the assumed

formation of phosphoryl trifluoride, and produce hydrogen fluoride








60

instead which in turn attacks the siliceous parts of the apparatus with

the formation of silicon totrafluoride and water. This water may attach

itself to phosphoryl trifluoride, causing the decomposition of the

latter to difluorophosphoric acid and hydrogen fluoride.

Thus, even the smallest traces of water vapor may decompose most

of the phosphoryl trifluoride, the total quantity of which is rather

small to begin with in th3 order of about 7x10-3 moles of gas for the

given charge of five gram3.

In the above example, the trap contained 1.18x10"3 moles of gas.

The analytical determinatLon gave the following values: fluorine 4.6x10-3

equivalents, phosphorus 0.13x103 equivalents, and silica.

No etching was visible in the other parts of the glass apparatus

except in or near the liquid air trap where most of the decomposition of

the gaseous components seemed to have taken place.

The other experiments in the vacuum furnaces gave similar

results.


Experimental Procedure at Atmospheric Pressure

Furnace.-The same tube furnace as described previously served for

the series of experiments presented in the following text. First a

Sillimanite tube, Coors Porcelain, was used, but since there was some

contamination of the outcoming gas with silicon tetrafluoride, it was

replaced with a nickel tube. The dimensions were length twenty-five

and one-half inches, inside diameter fifteen-sixteenths inch; to this

another smaller nickel tube was attached: length fourteen inches, inside

diameter one-half inch. A copper nipple was silver-soldered to the free










end of this tube. It was provided with threads for the adjustment of

polyethylene tubing, twenty inches long, which led to the bottom of a

one-liter polyethylene flask. Another tube through the cap led to the

second trap. The other end of the larger nickel tube was closed with a

hard rubber stopper through which passed a short piece of nickel tubing

of a small diameter, On the furnace side of the stopper, asbestos tape

was wrapped around the incoming end of the small nickel tube in order to

protect the rubber from the heat radiation.

The temperature was measured with a protected chromel-alumel

thermocouple in a porcelain sheath. After a steady temperature state

was reached, the thermocouple was withdrawn for the duration of the

experiment.

The furnace atmosphere was formed by Linde's dry nitrogen, which

was passed through a four foot column of silica gal before entering the

furnace.

Treatment of the Gaseous Reaction Products.-The outcoaing reaction

products diluted with nitrogen entered the polyethylene bottle which was

immersed in ice water, and contained 50 to 75 ml. 0.1 normal sodium hy-

droxide solution. The absorption of the acidic components of the gas

took place very readily, and only small amounts of it reached the next

flask. The collection of the gas continued at least half an hour beyond

the time of the removal of the boat from the furnace, since it was

observed that some of the acidic components of the reaction products

condensed or were adsorbed on the walls near the cooler outgoing end of

the nickel tube. Heating this end with gas flame removed practically all

the adsorbed materials, and an additional 5 to 15 per cent of the total









was usually recovered.

The sodium hydroxide solution in the traps was back-titrated

with a dilute nitric acid solution, and the number of equivalents neu-

tralized by the furnace gas was thus determined. All the solutions

with condensed products were collected and analyzed for fluorine and

phosphorus. Frequent blank runs were carried out between the experiments

for checking purposes.


Standard Mixtures and Conditions

On the basis of the preliminary tests, a set of standard con-

ditions ias chosen and defined with which the effect of varied experi-

mental factors could be compared.

Standard Conditions.-The standard conditions were as follows:

A charge of 1.50 g. in a platinum boat was held in the hot zone of the

furnace, at 700oC. (- 5$C.), for one hour while a stream of dry nitrogen

swept the furnace at a rate of 110 to 130 ml. per minute. The platinum

boat (length 7.5 cm., depth 1.0 cm.) was lying in a zirconia boat

surrounded by a cylindrical sheath of thin nickel sheet, to which was

attached a piece of nichromo wire for the centering and fast removal of

the charge. The surface of the molten charge was about 6 sq. cm., and

the depth about 3 to 5 mm.

Standard Compositions.-There were two standard compositions one

with sodium fluoride and the other with calcium fluoride. In terms of

equivalents, the compositions were identical in respect to the fluoride

and the metaphosphate content, viz., 18.9 mole per cent of fluoride and

31.1 mole per cent of notaphosphate, counting the anions and cations







63

separately in forrdna one hundred per cent. The original mixture, which

was also used for the vacuum furnace experiments, had resulted from

mixing one grati of sodium fluoride with four grams of sodium metaphos-

phate.

The natorials were finely ground and intimately mixed. W
mixture was transferred into the platinum boat, the powder was not

pressed in too tightly, only the top was smoothed with a spatula. The

charge was dried for about thirty minutes with the boat in an oven at

1500C,, and was then directly placed into the hot ,one of the furnace.

Samples of Sodiun Metaphosphate.-Three differently prepared

samples of the same salt, Mallinckrodt's monosodium orthophosphate mono-

hydrate, Analytical Reagent, were used for comparison. According to the

mode of dehydration, the three preparations were designated as (a), (b),

and (c) samples.

NaPO3 (a) Heated at 600-620oC. for two hours; stored in


NaPO3 (b)








NaPO3 (c)


a screw-capped bottle for four months.

Heated gradually up to 8000C., let stand at this

temperature for twelve hours, cooled slowly to

about 600OC., held there for two hours. The

salt was removed from the furnace at 4000G. and

used immediately.

Heated slowly in the course of three hours to

500C. The product was used immediately.










Experimental Results of the Measurements at 7000C.
rwith the Standard Co-:rposit.ions

The purpose of the first series of experiments was to follow the

ef ects of various factors, other than temperature and composition, on

the reduction in the fluorine content of the samples, and to establish

the limits of r eroducibility of the data. Table 6 presents the results,

expressed in terrn of equivalents neutralized in the traps.

There is a marked difference in the values of sodium and calcium

fluoride compositions, -hich runs consistently through all tests, the

mixture with calcium fluoride always offering a higher yield. Since this

difference is too large to be ascribed to the influence of varying

moisture content, or the presence of impurities in the respective salts,

it must result from a stronger effect of the calcium ion to further the

reaction.

The mode of sodium metaphosphate preparation may also change the

yield considerably. However, it is not so much the length of heating of

the compound as the aging of the material which seers to play a role.

Much, undoubtedly, depends upon the way of cooling of the product, since

this determines the crystallinity and the hygroscopic properties of the

resultant sodium metaphosphate.

The presence of water vapor in the atmosphere exerted benefidal

influence upon the reduction in the fluorine-content of the samples.

Doubling of the reaction time did not improve the yield significantly;

and the rate of gas flow seemed to be immaterial in the rivcn range.

The reproducibility of the single runs ranged from two to four

per cent deviation from the moan. Usually the accuracy was somewhat








65

better for the mixtures with the calcium component than with sodium.

All sodium netaphosphate relts showed spr cding of the melt on the walls

of the boat, IUtt ith sodiu~ fluoride-sodiui mi '-hspht t.o mixtures the

te ndncy was particularly pronounced. Several erratic values, usually

coinciding -A th spilli, j or strong spi ~ i,. of the 'alt,had to be

discarded.

The yield was not proportional to the size of the charge and

generally suffered a relative decrease when the charge increased.

Fluorine analysis in the trap solution cave usually a quite

close value to the nuliber of equivalents neutralized by nitric acid,

although at tines it was slightly lower. The tro values could be con-

sidered practically equal after the correction for a blank was made.

The presence of the phosphorus-con' ai:in i component vwa evident

in all runs, although the phosphorus-content in the trap was rather low,

seldom exceeding 0.2 milli-equivalcnts, and sometimes only sufficient

for a qualitative proof.


Variation in Composition at 7000C.

The next series of experiments dealt with the variation of com-

position of the melts. The results rere expressed in percentages of

fluorine expelled from these melts. The total fluorine content of the

sample counted as one hundred per cent.

The previously used salts, sodium fluoride and calcium fluoride,

were nixed in varying proportions. In two cases calcium metaphosphate

replaced sodium metaphosphate. For comparison, use was made of a

commercial disodium monofluorophosphate salt.













TABLE 6

EFFECT OF YXPrTfIlEUIAL CONDITIONS AT 7000C. ON THE EVOLUTION OF
FLUORINE-CONTAIfING GASIS FROM METAPHCSPHATE MELTS


Composition of Milli-equivalents Special
the Charge Titrated in Traps Conditions


NaF-NaPO3 (a)


Mean


NaF-NaPO3 (b)


Mean


5.49

5.70

5.58

5.74

5.63 1 .14

6.71

4.31

4.24

7.01

5.72

4.02

3.71

3.96

3.90 1 .15

4.05


Standard

Standard

Standard

Standard



Presence of water vapor

One-gram charge

One-gram charge

Two-gram charge

Double Gas Velocity

Standard

Standard

Standard


NaF-NaPO3 (c)


Double time














TABLE 6--Continued


Composition of Milli-equivalents Special
the Charge Titrated in Traps Conditions


CaF2-NaPO3 (a)


Mean


6.77

6.62

6.57

6.65 .12


6.81

4.39


CaF2-NaP03 (b)


Mean


7.58

5.55

$.33

5.39

5.h2 .12


Double time

One-gram charge

Two-gram charge

Standard

Standard

Standard


CaF2-NaPO3 (c)


Standard

Standard

Standard


5.39


Standard







68

The sources for the used chemicals were:

NaF Herck, Reagent Grade, dried at 1100C.

CaF2 "Baker Analyzed", rinsed with water, ignited at

8000C. for an hour.

NaPO3 Preparation (a), previously described.

Ca(PO3)2 Prepared from monocalciun phosphate monohydrate,

Fisher Certified Reoaent. Described previously.

Na2PO3F Ozark & Mahoning Company, dried at 1100C.

The tests were carried out under standard conditions as defined

in the preceding section. At least duplicate samples wore tested of each

composition and Table 8 presents the mean values of the reduction in the

fluorine-content of the samples.

The results revealed that an increasing P03 to F mole ratio

furthered strongly the fluorine volatilization. The absolute quantity

of the removed fluorine was smaller for the composition of the mole ratio

1 to 1, than either for a moderate excess of sodium fluoride or of

sodium metaphosphate. The reason must be related to the salt formation

at this mole ratio. The tendency for any reaction of the fluoride ions

is lowered in such a case, since fluoride forms a part of the salt

(Na2PO3F).

The ready-made compound, obtained from Ozark & Mahoning Co.,

gave a yield of volatile fluorine-containing compounds comparable to that

of a salt mixture of equal composition. A somewhat higher value of the

yield from the former may have been due to a different moisture content,

The difference in the behaviour of molts containing calcium and

sodium, as observed before, persisted throughout the varying compositions.













COMPOS ITIOI OF THE EXMPE ITAL :L TTAPHOSPHATE-FLUORIDE MIXTURES

Per Cent in Elements Milli-equivalents
Equivalents and PO3 Per Cent in "ei-Lht of Fluorine in a
of the Salt Component Equivalents Per Cent 1.5 g. sap1le


NaF NaPO3

37.8 62.2
(standard)

CaF2 NaPO3

37.8 62.2
(standard)

NaF NaPO3

62.2 37.8

CaF2 NaPO3

62.2 37.8


NaF NaPO3

20.0 80.0

CaF2 NaPO3

20.0 80.0


NaF Ca(P03)2

o0.o 60.0


Na
F
P03


Na
Ca
F
P03

Na
F
P03

Na
Ca
F
PO3

Na
F
P0
p?3
Na
Ca
F
P03
Na
Ca
F
PO3


50.0
18.9
31.1


31.1
18.9
18.9
31.1

50.0
31.1
18.9

18.9
31.1
31.1
18.9

50.0
10.0


140.0
10.0
10.0


20.0
30.0
20.0
30.0


29.0
9.1
61.9


18.3
9.6
9.2
62.9

35.6
18.3
46.1

13.8
19.9
18.8
47.5
25.5
4.2
70.3

20.6
4.5
4.3
70.6

12.0
15.8
10.0
62.2


7.15




7.26







14.85



3.32




3.37



7.90

















TABLE 7-Continued


Per Cent in Elements Milli-equivalents
Equivalents and PO3 Per Cent in Weight of Fluorine in a
of the Salt Component Equivalents Per Cent 1.5 g. sample


CaF2 Ca(P3)2

4o.o 60.0


NaF NaPO3

50.0 50.0


CaF2 NaPO3

50.0 50.0


Ca
F
PO0

Na
F
P03

Na
Ca
F


50.0
20.0
30.0


50.0
25.0
25.0


25.0
25.0
25.0


26.7
10.1
63.2


32.0
13.2
51.8


16.3
14.2
13.5


8.00




10.ho




10.65












TABLE 8

EVOLUTION OF FLUORINE-CONTAINING OASES FROM METAPHC6PHATE MELTS
AT VARIOUS COMPOSITION' I


Composition Milli-equivalents Per Cent by Yeight
Per Cent in Milli-equivalents of Fluorine Present of Fluorine Removed
Equivalents Titrated 0.15 in a 1.5 g. Sample 1 3 (average)


NaF NaPO3

20.0 80.0

37.8 62.2

50.0 50.0

62.2 37.8

CaF2 NaPO

20.0 80.0

37.8 62.2

5o.o 50.0

62.2 37.8

NaF Ca(P03)2

IO.0 60.0

CaF2 Ca(P03)2

Io.o 60.0


3.10

5.63

4.61

5.89


3.25

6.65

5.78

7.00


6.74


3.32

7.15

o10.40

14.43


93.5

78.8

4U.3

04.8



96.5

92.2

54.2

47.2



85.5


3.37

7.26

10.65

14.85


7.90


2.34


8.00


29.3

50.0


Na2PO3F 5.20


I0.o40







72

The stoichiometrically analogous mixtures made up from the salt

pairs CaF2-NaPO3 and Ca(P03)2-NaF gave rather close values in respect to

the reduction in the fluorine-content of the samples. This is an

indication that the effective composition of the melt is significant

rather than the combination of initially added salts.

Even the calcium metaphosphate-calcium fluoride system lost part

of its fluorine-content at this temperature, in spite of the high melting

points of the individual components. In this case no melting had taken

place; the powder was slightly sintered, however.


Temperature Dependence

The increase in temperature generally enhanced the reaction, and

the fluorine-content of the samples was successively decreased under the

conditions of the standard procedure.

Table 9 shows the respective mean values for the experiments

with the standard mixtures of calcium and sodium fluoride with sodium

metaphosphate. The melts with calcium fluoride exhibit higher yields in

the fluorine-containing gases.


Reactions Involving Other Cations

Since the interaction between fluorides and metaphosphates was

assumed to be a general reaction, it was considered interesting to let

sodium metaphosphate react with fluorides other than sodium and calcium

salts.

Some of the available materials wore tried and samples were

prepared which corresponded to the composition of the standard samples in

respect to fluorine equivalents. The experimental conditions remained










TABLE 9

EVOLUTION OF FLUORINE-CONTAINING GASES FR~i I4ETAPHOSPHATE
MELTS AT VARIOUS TEIPERATUPJS


Percentage by eight of Fluorine Removed
Composition 0...0
600C. 700C. 80000.


NaF NaPO .
a a62.8 5.0 78.8-3.0 85.5+3.0
37.8- 62.2%


GaR, -NaPO
2 3 72.514.0 92.212.0 97.012.0

37.8 -62.2%


the same.

The sources for the other fluorides were

KF General Chemical Division, anhydrous.

BaF2 "Baker's Analyzed", anhydrous.

AlF3 A.D. Mackay, Inc.

ZrFj A.D. Mackay, Inc.

The accompanying chemical analysis of the fluorides gave the

stoichiometric fluoride values for anhydrous potassium fluoride and

barium fluoride. Aluminum fluoride and zirconium fluoride contained an

undetermined amount of moisture and less fluoride than expected from the

formula. Therefore, the titration data for the last two salts present

actually the minimum calculated percentage for fluoride removed from the

test samples. Potassium fluoride is a very hygroscopic salt, therefore,

the weighing and mixing of this salt was performed in a dry box and







74
closed containers. However, while transferring the mixed charge into the

furnace, some atmospheric moisture may have been adsorbed by the salt.


TABLE 10

EVOLUTION OF FLUORINE-CONTAINING GASTS FROM METAPH06PHATE MELTS
IN THE PRESENCE OF OTiHE CATIONS


Fluorine Present Percentage by Weight
Milli-equivalents in the Sample of Fluorine Removed
Fluoride Titrated (duplicates) (calc.) mg. :2.5


KF 5.94 5.62 135 93.0

BaF2 4.19 4.36 105 77.2

AlF3 3.82 3.78 126 53.5

ZrF4 l.30 4.18 118 64.0

CaF2 5$.2 (mean) 138 92.2

NaF 3.90 (mean) 136 78.8


The extent of the reduction in the fluorine-content of the

mentioned fluorides is comparable to that of sodium and calcium fluoride.

The results are tabulated in the above table.















INTIPLACTION OF ::;.AP?-6'SP -TE -ITH FLUCROAPATITE


Experimental Conditicns

As the preliminary tests had indicated, a destructive decomposi-

tion of fluoroapatite takes place when the material is in contact with

sodium metaphosphato above 55000.

An independent series of measurements was performed in order to

establish the extent of fluorine removal from fluoroapatite under

standard conditions. Subsequently, the variations in temperature and

composition, the presence of moist atmosphere, and supplementary materials

were imposed upon the system.

The standard composition in this case consisted of fifty per cent

(by weight) of sodium metaphosphate (preparation c) with finely ground

(300 mesh) fluoroapatite (preparation b).

All samples were tested at least twice, and the tabulated percen-

tages present the respective mean values. Fluorine analysis was carried

out in each case in the trap solution, as well as in the residue of the

sample. No spreading of the melt occurred with these compositions, and

the reaction product could easily be removed from the boat,


General Results

The results showed that a considerable proportion of fluorine

was removed from the 1.5 g. charge within one hour at 70000. Two other

ratios of fluorospatite to metaphosphate (see Table 11) were tested, but

75










TABLE 11

COaIPOSITIONS OF METAPHOSPHATE MELTS
CONTAINING FLUOROAPATITE


Percentage by Weight
Composition of Fluorine Removed


50% fluoroap. 50% NaPO3 . . . .. . 56.9+-2.0

251 fluoroap. 75% NaP03 . . . .. 43.4- 14.0

75, fluoroap. 25% NaPO3 . . . . . 33.4 14.0

25% fluoroap. 25% Ca3(PO4)2 -50% NaP03 . 53.51 1.o

25% fluoroap. 25% hydroxyap.-50% NaPO3 . . 51.22 4.O



in either case the yields were lower. The probable explanation is that

with the smaller amount of sodium metaphosphate, the acidity of the melt

is lower, and with the larger proportion of metaphosphate, the viscosity

of the melt may exert a retarding influence on the rate of fluorine

removal. The latter assumption was supported by the experiment in which

tricalcium phosphate replaced 25 per cent of sodium metaphosphato and

effected an improvement in the yield of fluorine-containing gases rather

than a decrease.

The use of hydroxyapatite in place of tricalcium phosphate did

not change the result. The amount of hydrogen which hydroxyapatite may

contribute is small (about 0.3 mg.), compared with the rest of bound

water. Hydroxyapatite was prepared according to the method of Rathje

(57).

A mixture of calcium fluoride and tricalcium phosphate was made







77

up with sodium metaphosphate, which was analogous to the standard

fluoroapatite-sodium metaphosphate in respect to its stoichiometric

composition. The extent of defluorination in the first sample was

larger than for the fluoroapatite charge; the difference, however, was

rather small.

The relationships between the conditional factors and the extent

of defluorination in fluoroapatite melts were expected to be somewhat

different from the results of the fluoride group, primarily because of

the relatively low fluorine content (about 1.8 per cent by weight) of the

fluoroapatite charges. Besides, solid phases were always present in the

fluoroapatite mixtures, whereas previously a majority of the fluoride

melts had formed a homogeneous molten phase.


Temperature Dependence

The most interesting result appeared in the temperature variation

of the experiments, where a maximum reduction in the fluorine-content of

the samples was displayed at about 6IOC. The reaction became noticeable

at about 550C., and proceeded best just above the normal melting point

of sodium metaphosphate (approximately 6200C.). With rising temperature,

the extent of defluorination of the melt decreased until 800C. was

reached. From there on the decrease in the fluorine-content of the

samples gained again gradually with increasing temperature. See Table 12.

Evidently there are two different processes involved regarding

the reduction in the fluorine-content of the melts. The first one is the

decomposition of fluoroapatite, and the gradual removal of the fluorine-

containing volatile products, e.g., phosphoryl trifluoride, from the










TABLE 12

EVOLUTION OF FLUORINE-CONTAINING GAS0S FROM
CONTAINING FLUOROAPATITE


METAPHOSPHATE MELTS


Percentage by 'eight of Fluorine Removed
Temperature
C. ...... ..--------------
oC.
5 Water Vapor Silica CaF2 and
Standard Present Present Ca3(P0)2
Conditions 3.0 5.0 o2.0


575 47.7 1 5.0

600 65.2 1 h.o

640 79.6 1 2.0 80.7 72.9 82.0

700 56.9 2.0 65.0 75.1 69.0

800 50.4 1 2.0 57.2 61.9 65.6

900 59.2 1t .o

1000 63.01 5.0



melt. This process is presumably favored by an increase in temperature.

The other reaction may be the interaction of hydrogen-containing com-

ponents (e.g. water) in the melt or at the surface of the melt with

fluorine-containing components, facilitating the formation and expulsion

of hydrogen fluoride. This effect rmst be proportional to the concen-

tration of hydrogen-containing components.

Upon fast insertion of the material into the hot zone of the furnace

at higher temperatures, part of the surface-adsorbed moisture may bo

vaporized and removed before it can participate in the reaction in the

molten phase. Just above the melting point of sodium metaphosphate a







79

relatively larger amount of water may be trapped by the incipient melt.

This assumption is supported by the observation that the presence

of water vapor in the atmosphere above the melt favors fluorine evolution

as hydrogen fluoride. The effect of moisture probably consists in the

acceleration of the process. For example, a charge treated for two hours

in a dry atmosphere gave about the same yield of fluorine-containing

gases as the charge which was held for one hour in a wet atmosphere.


The Effect of Supplementary Materials

Small supplements, 0.1 e. of silicic acid (containing 20 per cent

water) added to 1.5 standard fluoroapatite charge, induced an improve-

ment in the expulsion of fluorine-containing Cases above 70000. Below

that temperature such an effect was not evident. Some of the silica was

found in the trap after the reaction, consequently it had participated

in the reaction and had been volatilized as silicon ttrafluoride.

The other added salts (0.2 g. to each standard sample), sodium

sulfate and sodium tetraborate, did not further the defluorination of

the melt but rather suppressed it somewhat. Calcium sulfate o'ave a better

yicld, possibly because of the presence of some residual water of consti-

tution. The respective mean values uere: sodium sulfate 53 per cent,

sodium tetraborate 53 per cent, and calcium sulfate 72 per cent of the

total fluorine content of the sample removed at 7000C.

The sources for chemicals were:

Na2SO4 Mallinckrodt's Analytical rea;-rnt, anhydrous.

CaSOh.2 H20 Coleman & Bell Co., C.P., dried slowly and

ignited at 5000C.









NaI2B4O7



Sio2.xR20


80

Borax, Baker's Analyzed, C.P., dried first at

150C. and then at 300C.

MTallinckrodt's Analytical Reagent.














EXPERIENTS IN TIM C RUPCIBZ. FURNACE


Solubility of Salts in Molten Sodium Metaphosphate

The furnace in operation was an electric multiple unit crucible

furnace from Hevy Duty Electric Co. (Type 82, 115V, 725'). Platinum

crucibles were used in all experiments.

First the solubility of various salts in sodium metaphosphate

melt was tested.

Small amounts of calcium fluoride disappeared in the melt quite

rapidly at the beginning; later on, the process became slower, and after

seven hours at 7000-8500C., about 2.5 g. of the salt had dissolved in

7.5 g. of sodium metaphosphate.

Tricalcium phosphate displayed a similar behavior in the same

temperature range. In the course of six hours about twenty per cent by

weight had dissolved in the melt.

Sodium fluoride was very readily soluble so long as there was not

any excess of sodium fluoride. At the mole ratio 1 to 1, the molt grad-

ually disappeared and crystalline salts were formed.

Fluoroapatito was very slow in becoming a homogeneous part of

the melt. Possibly other solid phases of phosphates replaced the

original structure since the decomposition of the material took un-

doubtedly place.

For example, in a mixture of 1.23 g. of fluoroapatite and 6.13 g.







82

sodium metaphosphate held for two hours at 8500C., the molten part was

analyzed separately from the crystalline phase. The glassy part of the

melt contained more than half of the total fluorine and about thirteen

per cent of it had been volatilized. In this experiment, fluoroapatite

powder was laid at the bottom of the crucible, and the decomposition

products had to diffuse through a layer of 12 nn. of viscous melt before

reaching the surface.

In another experiment in which the layer was even deeper, less

than ten per cent of the total fluorine had been volatilized in the

same period of time.

If a similar melt was held overnight for sixteen hours at about

8000C., half of the total fluorine still remained in the melt.

Analogous experiments with small amounts of sodium or calcium

fluoride gave better yields of volatilized fluorine, the percentage of

which was still considerably lower than in the case of tube furnace

experiments.


Weight Loss of Standard Mixtures
and the Component Salts

The rate of weight loss for various samples was determined in

another series of experiments.

The standard compositions of sodium and calcium fluoride with

sodium metaphosphate at 7000C. gave closely similar results. The total

weight loss for a 1.5 g. charge was about 120 mg.

The highest weight loss, about one fourth of the total, occurred

during the first quarter of an hour. In the last quarter, the rate was

down to about four per cent of the total. During the first hour only








83

half of the total weight loss had taken place. From then on there

continued a rather steady rate of weight loss for further twelve hours,

decreasing gradually.

Additional tests confirmed the observation that after the first

hour, the weight loss for a given sample had reached a slow steady state

for a number of hours.

The weight loss of the standard charges upon drying in the oven

at 1500C. for two hours was small, 1.1 to 1.4 mg. per 1.5 g. charge.

The ignited calcium fluoride, which was used in making up the

mixtures, lost 0.45 per cent of its weight in 20 hours at 6000-7000C.

The respective loss for sodium fluoride was 0.39 per cent in 15 hours at

6000-800C.

A charge of 1.2 .g of sodium metaphosphate held in the tube

furnace at 750C0. for two hours lost 0.1 nig. of its weight.


Microscopic Observations

Some of the crystalline phases which separated during the course

of reaction, or upon cooling of the above-mentioned melts, were subjected

to study under a petrographic microscope.

An excess of metaphosphate forms a glass when chilled rapidly,

and on occasions the crystalline parts of a melt could be observed

embedded in the optically isotropic glass, thus obviating the need to

separate the crystals from the rest of the melt.

A variety of phases were observed, the type of which depended

primarily on the composition of the melt and the rate of cooling. For

example, in the system calcium fluoride-sodium metaphosphate the presence

of calcium pyrophosphate was positively proved,














SUMMARY OF TiH EXPERIMENTAL RESULTS WITH IIfTAPHCSPHATE MELTS


Reaction Formulas

In all cases when sodium metaphosphate comes into contact with

fluorides or fluoroapatite, above the temperature of about 550C.,

reactions take place which liberate a considerable part of fluorine in

the form of volatile compounds. Most of this fluorine is in the form

of hydrogen fluoride. The hydrogen component is provided by the melt

which always maintains small amounts of water, even though the consti-

tuent salts wore dried and heat-treated before mixing. Evidence also

points to the presence of phosphoryl trifluoride in the exit gas.

The basic reactions for fluoride are assumed to be:

2 P03- + 2 F- + H20 2 HF + P22 HF

5 P03 + 3 F" POF3 + 2 P2-

The same reactions for fluoroapatite give the following equations:

8 PO3" + (PO-3)6(F")2 + H20 --> 7 P20"4 + 2 HF

28 PO3- + (Po-3)18(F-)6 -> 2 POF3 + 22 P204

In the case of the standard fluoroapatite-metaphosphate mixture

(50 to 50%) at least five per cent of the material must have remained in

the form of orthophosphate.


The Major Factors and Their Influence Upon the Reactions

The major factors which influence the extent of the reduction in

the fluorine content are composition and temperature. The rate of

86







85

defluorination of the melt is markedly affected by the depth of the layer

through which the decomposition products have to diffuse and by the pre-

sence of hydrogen-containing compounds as constituents of the melt.

For a given temperature the percentage of removed fluorine in-

creases as the acidity of the melt increases. Herein the relative

acidity of a metaphosphate melt (in respect to fluoride ion) is defined

by the concentration of P-O-P bonds as they occur in polyphosphates

(straight chains), and polymetaphosphates (cyclic structures). There is

a rather abrupt increase in the percentage of removed fluorine as soon as

the mole ratio of metaphosphate to fluoride exceeds one to one.

The rate of formation of fluorine-containing gases is highest

immediately after a mixture has reached the reaction temperature. After

the first hour the rate has slowed considerably, and under favorable

conditions more than ninety per cent of the original fluoride in the

sample has been volatilized.

Complete removal of the fluoride component of the melts was

reached in no case with the standard compositions, not even at high tem-

peratures (10000C.) or with prolonged heating (12 hours and longer) under

reduced pressure.

A large excess of metaphosphate, particularly if it formed a deep

layer, impaired strongly the rate of expulsion of fluorine-containing

gases. This effect must be due to the fact that the viscous melt impedes

the transfer of the reaction products. This assumption was also supported

by the observation that the total amount of fluorine removed was not

directly proportional to the size of the charge, but decreased as the

charge increased.








86
A rise in temperature generally favored the rate and extent of

defluorination. Special conditions prevailed at low concentrations of

fluorine in the presence of solid phosphate phases, as in the case of

fluoroapatite melts. A maximum was exhibited in the formation of

fluorine-containing gases just above the normal melting point of sodium

metaphosphate (about 620C).

The presence of water vapor in the furnace atmosphere was

efficacious in promoting the evolution of hydrogen fluoride, but what

seemed to be more significant was the concentration of the hydrogen con-

stituent in the melt.

There was a consistent and marked difference in the yield between

the melts containing sodium and calcium ions. Such a variation empha-

sizes the effect which different cations may have on the reaction, or

for example, in respect to the viscosity of the melt.

The decomposition of fluoroapatite by metaphosphate was almost

complete, judging from the comparison between the standard samples of

fluoroapatite and the respective mixtures of calcium fluoride and tri-

calcium phosphate.

Small supplements of silica enhanced the defluorination by the

fornmtion of silicon ttrafluoridc; the addition of calcium sulfate gave

only slightly better results than the standard mixture, whereas sodium

sulfate and tetraborate did not have any beneficial effect under the

given conditions.















T 'I3ICAI NTr~T-TIT! Ii


Metaphosphate telts

The reactions of fluoride ions in salt melts at high termeratures

can be interpreted in the light of acid-base theory extended to non-

aqueous systems. From the point of view of the Lewis theory, the fluoride

ion acts as a strong base capable of reacting with high temperature acids,

which can be coordinatively unsaturated molecules like silica, or ions

such as metaphosphate.

Ions with medium ionic potential (B, Si, P) preferably tend to

form structures with oxygen bridges, thus building up macromolecules which

react as acids at higher temperatures. These so-called polyacids are

characterized by the linking of the single groups to large complexes by

means of oxygen bridges of the type X-O-X. The transition of a polyacid

to its corresponding base consists in breaking of an oxygen bridge, and

a simultaneous binding of an oxygen atom. The acid-base reaction is thus

connected with a disintegration of the macromolecular structure, as for

example, in the case of metaphosphates and silicates.

These macromolecules are built up of a network of PO4 and SiO4

tetrahedra, in the interstices of which are interspersed cations, such as

sodium or calcium. In the melt there is a certain degree of freedom of

motion for the individual cation, or tetrahedron, and undoubtedly there

exists a dynamic equilibrium while a continuous rearrangement and inter-

change of tetrahedra takes place (69). The occurrence of single

87







88

phosphate ions in metaphosphate melts is rather improbable.


Effect of the Type of Cations

A distinct relationship exists between the acidity of the poly-

acids and the cations present. Flood and Forland (20) working with

pyrosulfates, concluded that the stability of the oxygen bridge S-O-S is

greatly influenced by the polarizing power of the cation. The size of

the cation is the decisive factor for the number of oxygen bridges broken

per cation, provided that the polarizing power of the cation exceeds a

certain amount.

A similar set of relationships may be expected to exist also in

the metaphosphate melts which would explain the difference between the

effect of sodium and calcium ions. The ionic radii of these ions are

relatively close (Na0,.98; Ca1l.06), but since the polarizing power of

calcium ion is larger owing to its double charge, the ease of breaking

the P-O-P bonds should be enhanced by calcium ions. Barium ion should

not be as effective as calcium ion because of its larger size and

decreased power of polarization. Such a difference was observed experi-

mentally. Because of the presence of mdsture and uncertain chemical

composition, the tests with other cations were not reliable enough to

enable an extension of such conclusions to a variety of systems.

The problems regarding the effect of an individual cation on the

volatilization of fluorino-containing compounds, as related to its size

and charge, from salt melts, would form a special study which would

necessitate the use of refined experimental methods and a complete

exclusion of moisture from the starting materials.










Extension of the Reaction Concept

Besides cations, also anions other than metaphosphates could be

considered in systems which would exhibit similar phenomena of anion-

fluoride interaction in an acidic melt with the formation of volatile

fluorine-containing compounds. Borates and silicates evidently form such

systems. Furthermore, pyrosulfates form fluorosulfonates with fluorides,

and it is known that fluorosulfonates decompose with the formation of

sulfuryl difluoride (66).

Possibly even bichromates and retavanadates could behave

similarly; they might, for example, form the products chromyl fluoride

(Cr02F2), and vanadyl trifluoride (VOF3).

The interesting subject the possibilities and conditions of

formation of volatile covalent fluorine compounds in salt melts has

not been previously treated from a general point of view, most probably

because of lack of experimental data. It certainly would offer a new

rewarding field of experimentation, and possibly contribute to the theory

of the properties of salt melts.
















CONCLUSION


The study of the various factors affecting the stability and in-

ertness of fluoroapatite toward chemical reactions as compared with

related materials, as TOll as with the component salts, should reveal

interesting relationships between structure and reactivity which may

prove to be meaningful in a wider context.

A significant feature of the fluoroapatite structure lies in the

fact that it is chemically more resistant than either one of its compo-

nent salts, calcium fluoride and tricalcium phosphate, or other minerals

of similar structure. Since the physical and chemical properties of

fluoroapatite are determined by the structure as well as by the size and

properties of the component ions, further detailed studies of those

physically determined facts may be profitable.

Recent developments in the investigation of phosphatic materials

have provided reliable methods for the synthesis of reproducible fluoro-

apatite. Thus it has become possible to study the chemical reactivity

of this material in its pure form. With the simplified method of fluoro-

apatite synthesis as described in this work, sufficient quantities of it

can be prepared in reasonable time. The quality control of the product

may be carried out by chemical analysis and microscopic observations.

The experiments in this laboratory confirmed the fact that pure

fluoroapatite is a very stable substance and chemically quite inert

90







91

toward most chemicals even at elevated temperatures. The general

possibilities of fluoroapatite reactions with chemicals in various phases

were studied, and the following conclusions were reached for each phase.

The consideration of gas-solid reactions does not give much

promise for the formation of volatile fluorine-containing compounds among

the products below 10000C, The destruction of the fluoroapatite struc-

ture, however, should be possible at relatively low temperatures in the

atmosphere of highly acidic gases or mixtures of strong reducing agents.

The addition of suitable solid reactants, for example, silica and

carbon would facilitate the rate of such reactions with a gaseous phase.

According to the available thermodynamic data, there are few

indications for solid-solid reactions below 10000C, for the modification

of the fluoroapatite structure with ordinary salts, but the possibility

of special effects by tho added materials is by no means excluded.

The melts should offer the widest variety of reactions with

fluoroapatite. The neutral salt melts can possibly induce partial

replacements in the fluoroapatite lattice. The basic salts react with

fluoroapatite at higher temperatures and the acidic melts do the same in

a lower temperature range.

Suitable combinations of two or three types of reactions as

indicated above may give rise to fluoroapatite reactions proceeding at a

rapid rate in the range of moderate temperatures. Such systems, however,

will become too complicated for the deduction of new facts regarding the

characteristics of the pure fluoroapatite structure.

The interaction of sodium metaphosphate with fluorides and

fluoroapatite was studied in greater detail and a number of factors







92

influencing the reactions were considered. The major variables were

composition and temperature. In all cases when sodium metaphosphate

comes into contact with fluorides or fluoroapatite above the temperature

of about 550C., reactions take place which liberate a considerable part

of fluorine in the form of volatile compounds, mostly as hydrogen

fluoride.

A special contribution of this work lies in the experimental

proof that at least one gas containing phosphorus and fluorine, is

released from the melt. Evidence points to phosphoryl trifluoride (POF3)

as a component among the gaseous products of the reaction. The relative

amount of hydrogen fluoride as compared to phosphoryl trifluoride depends

on the concentration of combined hydrogen in the melt and water vapor in

the atmosphere above the melt.

The interaction of metaphosphate melts with fluorides is a

general reaction independent of the type of fluoride and produces the

same major products in the gaseous phase. There are indications, however,

that the type of cation influences the rate of the reaction. For example,

calcium ions seem to be generally more effective in furthering the gas

evolution from the melt than the sodium ions.

The fluoroapatite structure is destroyed by sodium metaphosphate

above 550C., and under favorable conditions the fluorine content of the

sample is reduced by eighty per cent within an hour. The yield in the

fluorine-containing gases shows a maximum at about 6400C. The effect of

various added materials (e.g. silica) upon the reaction was determined.

The results have been interpreted in the light of the acid-base

theory according to the Lewis concept, and the possibilities for further

extension of such studies have been indicated.














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