Composition of Defluorinated Phosphate

phate rocks. This paper deals with the composition of the defluorinated materials and its relation to reversion. Defluorinated phosphate rock, apart f...
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Composition of Defluorinated Phosphate W. L. HILL, S. B. HENDRICKS, M. E. JEFFERSON, AND D. S. REYNOLDS

Synthetic calcined phosphates approximating the composition of defluorinated phosphate rock were prepared by heating mixtures of a calcium phosphate, calcium carbonate, and quartz at 1400" C. The composition and reverting properties of the synthetic phosphates were studied, and the results were used as a basis for interpreting similar results for defluorinated phosphate rock. The composition of the products has a marked effect on the extent of reversion that occurs when the material is annealed between 400" and 1200" C. Thus, considering only materials that can be prepared from Caa(P04)2and CaSiOe alone, maximum reversion occurs in the neigh-

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HE preceding article (page 1294) described the reverting characteristics of several defluorinated phosphate rocks. This paper deals with the composition of the defluorinated materials and its relation t o reversion. Defluorinated phosphate rock, apart from 2 t o 4 per cent of iron and aluminum oxides, consists almost entirely of PzO6, CaO, and Sios; therefore, as a first approximation, it may be regarded as belonging to the ternary system of these components. Accordingly, synthetic materials, simulating the composition of defluorinated phosphate as regards the relative proportions of PzOc, CaO, and SiOn, can be prepared from pure ingredients. The products thus obtained were more readily amenable t o detailed study because of the absence of the minor constituents of natural phosphates and their possible influence on the behavior of the fluorine-free product. They were, therefore, used extensively in this investigation in order t o determine the effect of composition on the nature of the product and t o obtain a basis of comparison for the behavior of defluorinated phosphate rock. The results fall under two divisions, depending upon whether they relate directly to calcined synthetic mixtures of pure materials or to defluorinated phosphate rock. The composition and behavior of the synthetic mixtures are discussed first. The investigation was not intended t o be a phase-rule study and cannot be so regarded. However, such a comprehensive investigation is projected, and therefore certain experiments reported here were not as extensive as they would have been otherwise. Materials and Method The synthetic mixtures were prepared from precipitated hydroxyapatite (9) or calcium pyrophosphate prepared by igniting Schering-Kahlbaum dicalcium phosphate at 900' C., c. P. calcium carbonate (CaO, 55.70 per cent), and 200-mesh purified quartz flour (9). The alumina was prepared by heating recrystallized c. P. aluminum nitrate slowly to 500" C. and holding it at this temperature until the oxides of nitrogen were expelled. The calcium carbonate and alumina each contained less than

borhood of the composition represented by the formula 3Ca3(PO&CazSi04. As a result of the formation of solid solutions on an extensive scale, the distinct phases observed in the synthetic phosphates and defluorinated rocks is greatly limited, and no phase was observed in the one that was not also identified in the other. Besides glass and an occasional trace of apatite, the phases identified in the unannealed materials are a-tricalcium phosphate, silicocarnotite (5Ca0.P,05.Si02),and a phase B of unknown composition. Apatite appears as an important constituent in reverted samples, and the composition of the apatite phase Iormed in this way is variable.

0.05 per cent of alkalies; the other reagents were alkali-free. All the materials passed a 100-mesh sieve. Accurately weighed amounts of the ingredients were mixed by hand and ignited at 800' C. The mixture wa5 then thoroughly agitated in a ball mill for 5 hours. This mixture in 2- to 3-gram portions was calcined a t 1400" C. in wet air in a furnace of the same type and under the same conditions as are used in this laboratory for defluorinating phosphate rock. The air-quenched calcined product was ground to pass a 100-mesh sieve. In the few preliminary experiments reported, the mixthres were calcined only 30 minutes. In order to approach equilibrium conditions more closely, the period of heating was extended in later work to 4.5, 6.5, and 8.5 hours. Accordingly, the mixture was heated initially for 30 minutes, ground, sieved, and reheated for periods of 2 hours with intervening grinding and sieving. When the mixtures were treated in this manner little was gained by extending the total time of heating beyond 4.5 hours. The final product was analyzed to ensure that it had the desired ultimate composition In the annealing experiments, 0.6 gram of the calcined product was placed in the furnace a t the chosen temperature and removed at the end of 30 minutes. The annealed samples were brushed through a 100-mesh sieve; as a rule, grinding was unnecessary. Citrate-insoluble phosphorus in the calcined synthetic mixtures and their annealed products was determined in the manner described in previous papers of this series. However, on account of the rather wide ranges in the percentages of phosphorus and calcium, the size of the sample was varied so as to keep the ratio of total calcium to citrate equivafent to the ratio of 1 gram of hydroxyapatite [ C ~ I ~ ( O H ) ~ ( PtoO 100 ~ ) ~ml. ] of citrate solution. The nature of the phases present in the calcined and annealed materials was ascertained by examination with the petrographic microscope and by x-ray powder diffraction photographs. Indices of refraction were determined by the immersion method, using white light in most instances. However, monochromatic illumination, NaD, was used when the homogeneity of the samples justified it. The phases encountered were generally very finely divided, and in no case were the crystals sufficiently well developed to permit a determination of other optical constants. The usefulness of the microscopic method was considerably limited by the presence of inhomogeneous phases, the fine-grained character of the samples, and the fact that the indices of certain phases were almost identical. X-ray powder diffraction photographs were made with CuK radiation filtered through nickel foil to remove the Kg line. 1299

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The phases were identified by comparing the photograph of the unknown material with photographs made from samples known to be optically homogeneous. Here again the presence of several phases limited the application of the method, as did also the similarity of the patterns of certain phases. A combination of the x-ray and optical methods, though far better than either alone, was not as satisfactory as usual.

1200" C. (6),and it has also been shown to form solid solutions in the ternary system CaO-PzOsSiOz (8). These solid solutions, as well as those of silicocamotite, were observed in the calcined products examined by the authors. Tricalcium phosphate readily forms in mixtures of Ca2P207 and CaO a t 1400" C. I In order to prepare materials that were sufficiently homoComposition of Calcined Synthetic Mixtures geneous for x-ray and microscopic standards, it was necessary to heat the mixtures for 4.5 hours or longer. With calcined The compositions of the calcined mixtures, in terms of mixtures on the join C ~ ~ ( P O ~ ) Z - ~ C ~ O . Phomogeneous ~O~.S~O~, Caa(P04)2, CaO, and SiOz, are shown with the aid of trisamples of a-C3 were obtained up to the composition 11.2 angular coordinates in Figure 1. As a convenient means (Figure 1). One mixture with the composition 11.4 yielded of reference each of the points representing the several synhomogeneous ol-Cs, whereas all other mixtures of this comthetic mixtures was assigned an arbitrary number ranging position showed two phases. This anomalous behavior from 1to 33, designated the composition number. Likewise, probably indicates that the boundary of the region of homothe points representing the several defluorinated phosphate geneous a-Cs a t 1400" C. lies very close to 11.4, in which rocks are indicated by the numbers ranging from 1476 to event some difficulty in duplicating results would be ex1497. Mixtures in which hydroxyapatite was used are inpected. The B phase, usually accompanied by a-Cs or Ss, dicated in the tables by the letter A preceding the compoappeared in samples between 11.4 and 11.7, inclusive, a resition number; calcium pyrophosphate was used in mixtures gion that includes the composition (11.65) represented by the not so designated. formula 3Caa(POJzCazSiO4. Strictly homogeneous samples At first a cursory examination of that portion of the terof Si were not obtained. a-Cs was always present, usually nary system CasPzOd3aO-SiO2, which includes the approxiin minute amounts, although it was quite prominent in inmate compositions of the phosphate rocks studied in the crustations on materials between the compositions 11.7 and preceding paper, was made by heating synthetic mixtures a t 12.0. This condition, which was somewhat less marked in 1400" C. for only 30 minutes, It was evident from this precalcium pyrophosphate mixtures than in those prepared liminary study that, whereas the compositions above the join from hydroxyapatite, casts some doubt on the limiting comCas(PO4)~5Ca0.PzO6.SiOs (Figure 1) contained only a-Ca and position of the Sa phase. However, this phase was not deglass, a t least three phases might appear along this joinnamely, tricalcium phosphate (CJ, silicocarnotite (SS), tected in any unannealed sample below 11.6, and its region of homogeneity at 1400" C. probably extends down to about and a compound (B) of unknown composition. The latter 11.7. proved to be the same as the X phase prepared and photoNone of the compositions between 11.0 and 13.0, inclusive, graphed by Bredig, Franck, and Fuldner (4). Silicocarnoshowed the presence of glass; this was also true of compotite was noted by these authors and others working on basic sitions between 11.0 and 12.0 that had been heated 8.5 hours slags (1,2, 3, 8). Since the work of Blome (3), its composition has been represented by the formula 5 C a 0 . P 2 0 ~ . S i 0 ~ , a t 1400" C., followed by heating a t 1500" C. for 4.5 hours. Accordingly, it would seem that even a t equilibrium the eutecalthough as a result of solid solution formation (8) the comtics affecting this part of the system must lie above 1500' C. position of the homogeneous phase varies widely. TricalOn the other hand, compositions above the C3-S6 join all cium phosphate (C3) is a well-known phase of the binary showed glass with a-C3 as the crystalline phase at 1400' C., system CaO-PzOs (11, 1%'). The anhydrous salt exists in and compositions 23 and 33 fused almost completely. two forms with an inversion point in the neighborhood of

OF CALCINED SYNTHETIC MIXTURESAND FIQURE1. COMPOSITION

PHATE R O C K S

OF

DEFLUORINATED PHOB-

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1301

and again becomes low in composition 13.0, which corresponds with silicocarnotite. As already indicated, anhydrous tricalciiim phosphate is known in two forms, the high-temperature or a-form and the low-temperature or &form, with an inversion point in the neighborhood of 1200" C. (6). Since the a-form is very soluble in neutral ammonium citrate, whereas the p-form is only moderately soluble, the solubility CUNC for a-Cas(PO4), annealed at different temperatures below 1400" C. would be expected to have a steep slope around 1100" to 1200' C. At lower temperatures theinversion becomes sluggish and finally ceases to take place within any reasonable time. ConseI t I t I I I I t quently, the curve exhibits a dip (Figure 3a), over a tempera.30.20 .IO .05 0 .05 .IO .20 30 ture range helow 1200° C. Small amounts of impurities may FIGURE2. X-RAY POWDERDIFFRACTWN PKOTOQRAPKE have a profound effect on the rate of inversion, which would (CliKa RADIATION)OF PHASESPRESENTIN CALCINED change the position of the minimum or even alter the &ape SYNTHETIC MIXTUBES AND DEFLUOBINATED PlZoSPEATE of the entire curve. For example, Bredig, Franck, and ROCKS Fiildner (6) found that a-tricalcium phosphate containing

Indices of refraction for the various phases are Listed in Table I, and x-ray powder diffraction photographs are re produced in Figure 2. Although the lines in the diffraction patterns of solid solutions of a-C, (for example, composition 11.2) are not displaced in comparison with pure a-tricalcium phosphate, the refractive indices are considerably enhanced. Likewise, solid solutions in the Sa series showed a change in refractive index without trustworthy alteration of the diffraction pattern. Furthermore, the Sa phase with the lower refractive index also has the lower birefringence. The B phase showed no variation of refractive index, but this observation does not exclude the possibility of a limited series of solid solutions. TABLE

1. OFIlCAC PROPERTW8 O F PHASE8 IDENTIFIED I N SYNTHETIC MATERIALS

Phaae

Compositioo

os, g Ct

Solid aolution ll.ZR

SI

Solid eolution 11.7Composition 13 5Ce0.PzOr8iOl

m.C*dPo+ Unknown

6s A(O1Ih CadOH)dPO& * Figure 1.

Refiaotive Indices (Nan light) n

1.588 1.598 1.007 1.021 1.832 1.6413 ( 5 )

^i

1.591 1,603 1.611 1.025 1.843 1.6452 ( 8 )

It is apparent from the foregoing discussion that the phases, particularly in compositions 11.7 to 12.0, wore not in true equilibrium. However, the behavior of the materials was sufficiently reproducible, and the conditions simulated those of the calcined phosphate rocks closely enough to serve as a basis for interpreting the behavior of the letter. Thephases identified in one series of experiments with mixtures on the join Ca~(PO~)r5Ca0.P20~.SiO~ are listed in the third column of Table 11.

Reversion of Calcined Synthetic Materials The effect of wet annealing on the citrate solubility of the phosphorus in a-tricalcium phosphate and five selected calcined mixtures is shown graphically in Figure 3, and the phases identified in the products are listed in Table 11. The compositions all lie on the join Ca3(PO~)~5Ca0.PIO~SiOP, and the C U N ~ S furnish a vivid picture of the profound effect 1200 1400 that composition has on the annealing properties of calcined 1empeiature. 'C, products. Thus, leaving pure tricalcium phosphate out of FIGURE3. EFFECTOF ~ A L ~ N(30O MINUTEBM consideration for the moment, the change in citrate soluWETAIR) ON THE CITEATESomsimm OF CALCINZD bility (reversion) on annealing, which is smll in composition S~~THET MIXTORES IC 11.0, increases with the amounts of SiOl and CaO in orthosiliThe aolubility of the unsnnaslad materiels is s h o m on the ordinete cate proportions, passes through a maximum a t about 11.65, at 140W C.

TABLE 11. EFFECT OF ANNEALING ON Composition No. A1l.O A11.4 11.68 12.0 13.0 11.66 11.66 4

b C

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1302

+ AlrOtc

Mixture Calcined 4.5 Hr. at 1400° C. p Phaaesb 99.7 a-Cs 91.1 B a-Ca 95.8 B:Sr 99.3 SS 100.0 869x 95.8 98.7

€39

B'

'

THE

PHASES PRESENT AND THE CITRATE SOLUBILITY OF SYNTHETIC MATERIALS"

I

'2-7

---1300°

Phases 3 . 1 a-CI,Al

-A9

2:0 B;Si' 0 . 0 sS,x

.......

$ 22.'3o

-1200° -Ap

8.2 7.6 66.2 17.0 0.5

Calcined Mixture Annealed 30 C.-.llOOo C . 7 ~ 1 0 Phases -Ap Phases -Ap a-Cs,A 7 . 5 a-Ca,Al 15.6 a-C3,A 51.4 63 0 A1,X 78.6 A i X ' 8019 58 1 A1:Sr 35.2 Si,X' 2.9

......

7 8 . 6 AaX 7 7 . 9 A1:Aa

6 6 . 2 A1,X 6 3 . 7 A1,Ar

B B's'

THE

PHOSPHORUS IN CALCINED

Minutes at: 0 C.0 ~ --700° C.,---600° C.Phases -Ap Phases -4p Phases a-Ca,X 17.5b a-Cs, Xb 2 1 . 7 a-Cs,X A a-Cs 19.56 S6,Ab 9 8 SsA A;,X 6.9 Ss,X 8:C Sr,'X &,A 5.6 8s Ai 6 . 4 Ss,Ai 5.6 Sa:X s6,X 8.4

....

8 0 . 9 Aa,X 7 8 . 8 As,Ai

6.9 41.5

86,X %,As

8.4 35.8

ssx Sr:X

All experiments were made in a wet atmosphere. Annealed rat 800' C. Composition 11.65 to which 1.3% of Ah01 was added prior to calcination.

TABLE111. EFFECTOB ANNEALINGON --Defluorinated

THE

PHASES PRESENT AND THE CITRATE SOLUBILITY OF DEFLUORINATED PHOSPEATP ROCKS"

Phosphate R o c k - 7 ---Defluorinated 1200° '2.No. Saurce p Phases - A q Phases 1491 Fla.landpebble 9 3 . 7 a-Ca,G 4 1 . 0 A,a-Ca,G 1492 Fla. land pebble 9 6 . 1 a-Cs G 2 4 a-Ca G 1494 Term. brown rock 9 9 . 5 a-C::G 52:7 As,A;,a-C$ 1478 Tenn. brownrock 9 0 . 0 a-Ca G A 6 5 . 3 A,X 1476d Tenn. brown rock 8 4 . 1 a-Ca:G:X ..... 1495 Wyo. rock 90 2 SsG 1496 Mont. rock 9 S : l a-'Cs,G 26:5 a-Cs,A' ..... 1497 Idaho rock 9 9 . 5 B,G ..

--

..

Solubility data were taken from the preceding paper. ,Annealed at 1000° C. 0

b

Defluorinated Phosphate Rock DryPhosphate Rock Wet-Annealed 30 Min. at:--Annealed 30 Min. at: -llOOo C.c-900' C . 7 --700° C.--1200° C . 7 ---llOOo C.A v Phases -Ap Phases -Ap Phases -A9 Phases -4p Phasea 2 7 . 9 a-Ca,Ai 6 . 2 b a-Ca,Gb 3 3 . 1 a-CtAi.G +0.7C a-Cs,G" ..... 2 . 2 a-Cs:G l i : 5 a%,'Ai 26'8b a-Ca,Alb 3 6 . 3 a-Cs,As 3:4b a:C;,'Gb ..... ..... 1 2 . 6 a-Cs,A 74:5 A'ULC; ..... , 57:6 A.,>:Ci 6 7 . 2 A;,Al,Sr 56156 Al,Aa,Sse 66:2C A'a'Ai'SaC 4.01 Sa,Gf 4 . 2 a-bs,b 16:7 a:C;,'A 40:SA;B" 19:2 B;A" 6 7 . 3 Ai,As i : o B',G'*

.. .. .. .. .. .. ..

..

The x-ray diffraction pattern of the apatite phase observed in the annealed products was closely similar to that of hydroxyapatite, CadOH)z(PO.&-,and fluorapatite, CaloFz(POJs. The characteristics of this pattern are such as to permit easy recognition when it is superimposed on other patterns; unfortunately, it is quite insensitive to variations in composition. On the other hand, microscopic examination, though less trustworthy on account of the small particle size and the

.....

..

..

. ,. .

.......

Annealed at 600° C.

excess CaO was not converted into the &form by annealing a t 1000" C. in a dry atmosphere, although in the presence of water vapor the change did occur with the formation of some hydroxyapatite. Furthermore, when silica is introduced into the lattice [a condition that may be attained by calcining the appropriate mixture of Ca2P207, CaC03, and SiOz-for example, composition 1 (Figure 1) consisting of a-Co and glass], the a-C3 thus obtained does not invert to P-C, a t 1000" C. even in 9 wet atmosphere. As might be expected, silica does not readily enter the lattice unless the tricalcium phosphate is actually formed in contact with it. For example, a-Cs, obtained by heating a mixture of P-Ca3(PO4)2 and quartz flour to 1400" C., inverted to 0-c~ as though silica were absent, when the calcined material was annealed below 1200" C. I n view of the dimorphous nature of anhydrous tricalcium phosphate and the difference between the citrate solubilities of the two forms, it would seem logical to suggest, as did Reynolds et al. (IO), that the change from the soluble to the less soluble form is responsible for reversion of defluorinsted phosphate rock when it is cooled slowly, particularly in the case of products in which a-Cais the predominant phosphate phase. Actually &C3 was not observed in any of the annealed silica-bearing synthetic products. Therefore, the inversion of tricalcium phosphate seems to have no bearing upon the reversion of defluorinated phosphate rock: this conclusion is confirmed by the data for defluorinated rocks discussed in a later section. On the other hand, decrease in citrate solubility of silica-bearing products (Figure 3) was invariably accompanied by the appearance of an apatite phase (Table 11); the amount of this relatively insoluble phase was sufficient to account for the observed reversion

(-.b).

..

c

Annealed at 800' C.

d

Fused phosphate rock.

0

-4nnealed at 950' C.

similarity in optical properties of the apatite phase to some samples of SS, is particularly useful in indicating variable composition of the apatite phase. Refractive indices of the apatite varied from near 1.620 to 1.650 with a birefringence of 0.005 to 0.010, samples with higher indices having the higher birefringence. The wet-annealed materials all gained in weight and showed a maximum increase in the region of maximum reversion as regards both composition and temperature. This gain in weight (water) never exceeded 1 per cent of the weight of the unannealed sample, whereas the formation of the Calo(OH)2(POd)eequivalent of the total phosphorus in oalcined mixture 11.65, for example, requires the absorption of about 1.6 per cent of water. The water content of the unannealed material, as indicated by the ignition loss a t 1400" C. in a dry atmosphere, amounted to only about 0.1 per cent. Accordingly, it is obvious that the apatite phase cannot be pure hydroxyapatite; nevertheless, water is an important constituent of the wet-annealed samples. The reversion of calcined mixture 13, in which SSis the predominant phase, was small with a maximum around 400" C. (Figure 3c). As the phase became richer in Ca, reversion increased; at composition 12.0, apatite became the principal phase for a narrow temperature interval (Table 11). Similarly, composition 11, a homogeneous a-Cs,showed limited reversion with the maximum a t 600" C. (Figure 3a). Materials between 11.0 and 12.0 had a very complex behavior. Thus, a t 11.4 the predominant phase was B, a-Cs, A, and Sg, respectively, as the annealing temperature was lowered. SS, accompanied by A, seemed to be the chief phase appearing in the materials annealed a t 400" t o 700" C. Results obtained with calcined mixtures a t other points on the join (Figure 1) agree in essential details with those listed in Table 11. Although the A phase is quite variable in composition, there seems to be no entirely safe criterion for predicting the particular member that will appear a t a given temperature. Double minima, such as are shown by the solubility curves of the defluorinated phosphate rocks discussed in the preceding paper, are absent in the curves for the synthetic materials, but the observed broad minima (Figures 3b and

NOVEMBER, 1937

INDUSTRIAL AND ENGINEERING CHEMISTRY

4) may have been prsduced b y the appearance in varying proportions of two different apatite phases that have different temperature intervals of reversion. This point is discussed more fully in a later section. A few annealing experiments were made in a dry atmosphere. I n the case of defluorinated phosphate rocks that showed reversion with dry annealing,t the minima on the solubility curves occur a t substantially the same temperature as the high-temperature minima of the curves for the wetannealed materials. Wherever the latter are broad, the former usually approach the high-temperature side of the dip. Using this observation as a guide, samples of the calcined synthetic materials, respectively, were annealed in dry air a t the temperature of the minimum on the solubility curve for wet annealing. The results of dry annealing show slight losses in weight (0.05 per cent) and practically negligible changes in citrate solubility, the greatest reversion (7 per cent) again occurring in composition 11.65. According to the foregoing results it seems safe to conclude that in the region near compositions 11.4 to 11.7 four different homogeneous phases may exist-namely, Sg, B, a-C3, and A, water probably being required for the last one. Extensive solid solutions are the rule, and as a result of this behavior synthetic materials, as well as defluorinated phosphate rock, even though they are not a t equilibrium, are characterized by the presence of only a few phases.

Effect of Aluminum Oxide on Composition of Calcined and Annealed Materials Aluminoapatite was reported by Bredig, Franck, and Fuldner (6) ; since phosphate rock contains notable amounts of aluminum, this apatite may be expected as a constituent of defluorinated phosphate rock. Accordingly, an aluminum-

10

I

I

bearing synthetic material was prepared by adding 1.3 per cent of aluminum oxide to composition 11.65 and calcining the resultant mixture as was done with the other materials. For convenient comparison the solubility curves of composition 11.65, with and without aluminum oxide, are shown in Figure 4; the identified phases, as well as the reversion, are given in the last two lines of Table 11. The presence of aluminum oxide not only broadened the minimum of the solubility curve, but it also gave a more 1 Defluorineted phosphates belonging to type 111, Figures l A , l D , and 2 of the preceding paper.

1303

gradual slope a t the lower temperatures. The observed phases were, in general, the same in the two materials, although B and A3 were somewhat more prevalent in the aluminiferous products. It is of special interest to note that no phase was found that had not already been observed in the alumina-free materials. Apparently the aluminum oxide readily enters crystalline phases Sg, B, and A, forming complex solid solutions, and thus the number of phases is greatly limited.

Phases in Defluorinated Phosphate Rocks and Their Annealed Products The phases observed in eight of the defluorinated phosphate rocks from the experiments reported in the preceding paper are summarized in Table 111,where the reversion a t the several temperatures is also shown. The phases present in the defluorinated phosphates are the same as those observed in the synthetic materials. All of the unannealed materials contained some glass, and a-Cs was usually the predominant phase. The presence of glase is not surprising, because phosphate rock has other components than CaO, P z O ~and , Si02. The composition of the defluorinated rock, as regards CaO, PzOS, and Si02, also lies on the Si02 side of the join Caa(P04)r5Ca0.PzO~Bi02, in a region (Figure 1) where glass was always observed in the synthetic mixtures. Furthermore, since the compositions lie chiefly in regions in which a-Ca was observed in the synthetic materials, a-C3 was to be expected as an initial phase in most cases. Two of the materials, 1478 and 1487, do not follow this generality. I n the former, a-Ce predominated although this composition is near 11.65 where B should be expected (Figure 1 and Table 11); in the latter, B appeared rather than a-Ca. Without attempting to explain this discrepancy it will be noted that, as a consequence of the much shorter time of heating (less than 1 hour), the defluorinated phosphates probably did not approach equilibrium conditions as closely as did the synthetic materials. Although the phases found in defluorinated phosphate are the same as those observed in the synthetic mixtures, two prominent differences in behavior are to be noted. These characteristics of the rock products (Figures 1, 2, and 3 of the preceding paper) that were not observed in synthetic materials are: (a) Some of the rock products undergo reversion when they are annealed in a dry atmosphere, and (b) the citrate-solubility curves for annealed materials often show two minima. One defluorinated phosphate (1492) failed to show appreciable reversion under any conditions of annealing. It is one of the three most siliceous compositions and lies very near the apatite-silica join, on which synthetic materials showed very limited reversion. Phosphates 1492, 1495, and 1497, containing a-C3, SS,and B as the principal phases, respectively, were the only ones that did not show reversion with dry annealing. Reversion with dry annealing was accompanied by the appearance of an apatite phase. Therefore, it would appear that water is unnecessary for the formation of an apatite phase in these systems. The solubility curves of the phosphates that show reversion in dry air exhibit only one minimum, and this corresponds closely with a minimum on the solubility curve for wet annealing (Figures 1 and 2 of the preceding paper). The minima on the solubility curves for wet-annealed products correspond with maxima of the curves for water absorption (Figure 3 of the preceding paper). I n the case of materials that show two intervals of marked reversion (minima), the greater absorption of water occurs a t the lowtemperature minimum, although the greater reversion usually occurs a t the high-temperature minimum. Thus, recalling

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INDUSTRIAL AND ENGINEERING CHEMISTRY

for comparison the behavior of the materials when they were dry-annealed, it appears that water is essential to the reversion that produces the low-temperature minima; its presence, though a promoting factor and in certain cases (phosphate 1497) also the determining factor, is not always necessary for the reversion that results in the high-temperature minima. Accordingly, a t the latter minima the observed reversion, always greater in wet- than in dry-annealed products, may be regarded as the resultant of the changes occurring with dry annealing (modified and enhanced, as they probably are, by the presence of water vapor) and the alterations accompanying the absorption of water. These observations alone furnish good evidence that reversion involves the formation of a t least two citrate-insoluble phases. One of these phases requires the presence of water, and the formation of the other is enhanced by water. Analysis of x-ray diffraction patterns of the annealed products shows that both phases are apatites, but it does not permit their differentiation. Some differences are apparent in their optical properties, but these do not lead to an unambiguous distinction of the phases. Thus, phosphate 1497, annealed a t the low-temperature minimum (700" C.), contained both A and &; the material annealed a t the hightemperature minimum ( 1 1 0 0 O C . ) contained only A, as the apatite phase. The apatite containing the greater amount of water would be expected to show the higher refractive indices, and this expectation is apparently met in the instances cited. Unfortunately, the conditions are not quite so clear in the case of sample 1495, for here A3 is the predominant phase a t both minima, with A occurring as a minor constituent. However, in phosphate 1495 annealed a t 950" C., the annealing temperature for maximum citrate solubility between the two minima, was the principal and As the secondary phase. On the basis of the results a t hand it is permissible to point only to the possibility of explaining the double minima by a variation in the composition of the apatite phase. One instance of pronounced incrustation was noted in the defluorinated phosphates. Although phosphate 1476, wetannealed a t l l O O o C., contained a-CSand A in about equal amounts, the citrate solubility was very low, indicating complete reversion. The a-Cs was present as large crystals, which were covered with a layer of finely divided apatite, and the low solubility was undoubtedly a result of this physical characteristic. Phosphate 1476 was prepared by defluorinating phosphate rock while it was in the molten condition (7), whereas the other materials were prepared by deBuorinating rock a t or below the sintering temperature.

VOL. 29, NO. 11

Examination was made of a number of materials obtained by reheating a t 1400" C. samples of defluorinated phosphate that had been reverted a t lower temperatures (Table I11 of the preceding paper). The results showed that the predominant phase initially present was re-formed in all cases.

Acknowledgment Grateful acknowledgment is made to Lewis F. Rader, Jr., for assistance in the preparation and analysis of the synthetic materials during the early stages of the investigation.

Nomenclature = =

= = = = = =

citrate solubility of phosphorus, per cent of the tota phosphorus change in citrate solubility, in per cent of total phosphorus, when a calcined product is annealed or otherwise treated further; if the change is negative, the phenomenon is called reversion apatite phase, in general apatite phase, with refractive indices below 1.635 apatite phase with refractive indices between 1.635 and 1.640 apatite phase with refractive index above 1.640 compound of unknown composition tricalcium phosphate phase, with the prefixes and p indicating high- and low-t emperature forms, respectively (Y

= glass = silicocarnotite =

phase ( ure silicocarnotite has the composition ~ c a o . p , o J i o , ) unidentified material, probably A or Ss

Literature Cited Bainbridge, F., Iron Steel Inst. (London), Carnegie Schol. Mem., 10, 1-40 (1920). Behrendt, G., and Wentrup, H., Arch. Eisenh.iLttenwesen, 7 , 95-102 (1933).

Blome, H., Metallurgie, 7, 659-67, 698-705 (1910). Bredig, M. A., Franok, H. H., and Filldner, H., 2. Elektrochem., 38, 158-64 (1932). Ibid., 39, 959-69 (1933). Burri, C., Jacob, J., Parker, R. L., and Struntz, H., Sweiz. Min. Pet. Mittl., 15, 327-39 (1935). Curtis, H. A., Copson, R. L., Brown, E. H., and Pole, G. R., IND.ENQ.CHEM.,29,766-70 (1937). Korber, F., and Tromel, G., 2. Elektrochem., 38, 578-82 (1932). Marshall, H. L., Reynolds, D. S., Jacob, K. D., and Rader, L. F., Jr., IND.ENQ.CHEM.,27, 205-9 (1935). Reynolds, D. S., Jacob, K. D., Marshall, H. L., and Rader, L. F.,Jr., Ibid., 27,87-91 (1935). Schneiderhohn, H., Mitt. Kaiser-Wilhelm-Inst. Eisenforsch. Dilsseldorf, 14, 34-6 (1932). Tromel, G., Ibid., 14, 25-34 (1932). RECEIVED July 28, 1937.

ACETATE YARNPLANT, TENNESSEE EASTMAN CORPORATION, KINQSPORT, TENN.