Reaction of Calcined Phosphate Rock with Alkali Carbonate
Solutions' KEXNETH A. KOBE, WILLIA;\T S. HAJI3\I, AND ALEX4iYDER LEIPPER
University of Washington, Seattle, Wash.
I
EARLIER paper Kobe and Doumani (0') reported a study of the reaction of sodium carbonate solution on precipitated tricalcium phosphate and bone ash. They found no reaction with a Montana phosphate rock, but a few unreported experiments showed the ready metathesis of a calcined phosphate rock. This paper reports the study of a calcined or defluorinated phosphate rock with solutions of alkali metal carbonates. Investigators in the Phosphate Division of the United States Bureau of Agricultural Chemistry and Engineering have published a series of papers on the defluorination of phosphate rock by a calcination process (8, 13, 14, 16). In their investigations silica is mixed with the ground rock and heated to about 1400" C., and steam is passed through the mixture. It was found that better than 95 per cent of the fluorine could be volatilized from the mixture, and that from 85 to 95 per cent of the phosphorus present could be converted to a citrate-soluble form. The variables affecting this conversion were studied in detail (8, 13, 14, 15). The Tennessee Valley Authority ( 2 ) has conducted semiplant experiments on a similar process in which the phosphate rock is fused and treated with water vapor. Over 90 per cent of the fluorine was volatilized and over 80 per cent of the phosphate was converted to a citrate-soluble form. It is believed that this method for treating phosphate rock will yield available P,os a t a cost lower than any process operating at present. As pointed out in the first paper (6),the previous literature on the reaction of calcium phosphates with sodiuni carbonate solution is unreliable. Kobe and Doumani stress the variable factors of source, heat treatment, particle size, and effective reaction surface as those controlling the transposition of Ppos from the solid phase to solution. Considerably more work is reported in the literature on the reaction of ammonium carbonate with calcium phosphate. Hackspill and Claude ( 4 ) treated precipitated tricalcium phosphate with ammonium carbonate solution and found that 4 per cent of the P20s transposed after agitation for 50 hours. X o reaction was found with natural phosphate rock. Treatment of secondary calcium phosphate with ammonium carbonate or hydroxide causes reversion to tricalcium phosphate. Two patents were granted illeyers (10, 11) for the treatment of a phosphate rock calcined with sodium bisulfate or sulfate by his method. The aqueous suspension of the calcined phosphate rock in an autoclave is treated with ammonia and carbon dioxide under pressure for 6 to 8 hours. Extractions are given as 80 to 90 per cent. ,I' AN
1 This is the second paper in a series on IMetathesis between Solids and Solutions; the first appeared in 1937 (8).
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A defluorinated phosphate rock was treated with solutions of alkali carbonates, and the extent of transposition of the PzO6 was determined when the steady state was attained. The amount transposed increased with both concentration and amount of excess carbonate, amounting to 84 per cent with four equivalents of both sodium and ammonium carbonate. This corresponds closely to the citrate-soluble PSOS in the rock. Conversion decreases with increased temperature. The rate of conversion is influenced by the method of agitation, as grinding or shaking. The mechanism of reaction appears to be different for sodium and ammonium carbonates. Methods of separating the trisodium and triammonium phosphates from the solution are discussed.
Experimental Procedure Calcined phosphate rock was supplied by Darling and Company. Its analysis is as follows: Chemical Analysis, % Total PlOs 37.2 Citrate-insol. Pros
% of PnOs citrate sol. Silica
5.55
85.0 5.86
-Screen Mesh
48 70 100
140
200
-' 200
Analysis% retained
2.8 10.3 18.0 14.7
11.4 42.8
1oo.o
Methods of analysis for total P20, and citrate-insoluble PnOa were those of the A. 0. A. C. (1) except that the alkali solution was standardized against a phosphate rock sample of the National Bureau of Standards. The alkali carbonates used were commercial chemicals. The alkali carbonate solutions mrere standardized by analysis. Weighed quantities of phosphate rock were added t o a known volume, and the suspension was allowed t o react under one of the methods t o be described. To determine the amount of P206 transposed, a sample of the slurry was centrifuged and a volumetric sample was pipetted from the clear liquid for a PZOS determination. When sodium carbonate and sodium phosphate were t o be determined in the same solution, the method of Kobe and Leipper ( 7 ) was used. The agitation and grinding t o give increased surface freshly exposed t o the solution were accomplished in three different ways. An Abbe ball mill was used for some runs with sodium carbonate, but the rinding action was too slow. A Charlotte laboratory-type c8loid mill was employed for most of the runs with sodium carbonate. It has the advantage of extremely ra id grinding under temperature controlled by the water jacget around the stator. A shaking machine as illustrated by Doumani and Kobe (3) was used for many of the runs with ammonium carbonate. The influence of the equipment will be discussed with the reagent used.
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Reaction with Sodium Carbonate The reaction of calcined phosphate rock with sodium carbonate solutions was carried out in the Charlotte colloid mill. The effect of concentration and excess sodium carbonate was determined by adding the calculated quantity of rock to 500 ml. of sodium carbonate solution and grinding a t 25' C. until successive analyses showed no increase in Pz05transposed. The results are given in Figure 1. In
CONCENTRATION Na,CO,
FIGURE 1. EFFECT OF CONCENTRATION AND EXCESS(ONE, Two, AND FOUREQUIVALENTS) OF SODIUM CARBONATEON PZOS TRANSPOSED AT 25" C.
the lower curve one equivalent of sodium carbonate was used per equivalent of P205in the rock, in the middle curve two equivalents, in the upper curve four equivalents. The Pz05transposed increases with the concentration of the solution and the amount of excess sodium carbonate. From the law of mass action the ratios of conversions would be predicted as 1:2:4, whereas that found a t maximum concentration is 1:1.86:3.60. I n the less concentrated solutions these ratios are much lower. The maximum transposition of 83.6 per cent is close to the 85 per cent citrate-soluble Pz05 in the rock. Temperature markedly affects the transposition, which decreases with increasing temperature as shown in Figure 2. One equivalent of rock was ground with 500 ml. of a 2.7 M sodium carbonate solution, constant temperature being maintained in the water jacket of the colloid mill. The runs a t 50", 75", and 90" C. first reached a maximum and then decreased until a steady state was attained. The effect of time of grinding is shown in Figure 3. This decrease in conversion a t higher temperatures may be accounted for by hydrolysis of the calcium phosphate or sodium carbonate, and by formation of the more insoluble and less reactive hydroxyapatites. The reaction of the sodium phosphate with the calcium carbonate to remove PzOsfrom the solution is possible, as has been noted with the ammonium phosphates (4, 1.9). Sodium silicate was found to be in solution, as evidenced by the presence of silica in the analytical samples. The calcium silicate formed during the defluorination process evidently undergoes metathesis with the sodium carbonate solution. The amount of silica is increased by the time of contact,
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as grinding in the ball mill over long periods of time gave more silica than the short grinding period in the colloid mill. The loss of sodium carbonate from the solution was noted when a sodium carbonate balance was made in the solution. This loss amounted to as much as 12 per cent in the concentrated solutions and decreased with the lower concentration. I n the causticization of sodium carbonate with lime, a loss of sodium carbonate is found due to the formation of NazCa(C0&.5HzO (gaylussite), which is stable below 40°, or SazCa(COa)z, which is stable above 40' ( 5 , 9 ) . It was found that when precipitated calcium carbonate was ground in the colloid mill with sodium carbonate solution, sodium carbonate disappeared from the solution. Although the loss was not so great as when phosphate rock was used, the actual formation of calcium carbonate in the latter case might cause the simultaneous formation of the double salt.
Reaction w i t h A m m o n i u m Carbonate The fact that transposition of PzO5 with sodium carbonate decreased at higher temperatures was accounted for by the hydrolysis of the sodium carbonate and the formation of hydroxyapatites which are not reactive. A more neutral salt, such as ammonium carbonate, s h o u Id a t t a c k t h e phosphate rock with greater facility. When the calcined phosphate rock was ground in the colloid mill with ammonium carbonate solutions, lower c o n v e r s i o n s were o b t a i n e d , but the results were irregular and irreproducible. When the m i x t u r e w a s subjected to grinding TEMPERATURE - * C . action in the ball m i l l , g r e a t e r conFIGURE 2. EFFECT OF TEMPERATURE versions were obtained over the longer time of reaction, but again the results were irregular and irreproducible. The method finally used consisted of placing 500 ml. of ammonium carbonate solution in a 1-liter bottle, adding the calculated amount of calcined phosphate rock, and shaking the mixture in a mechanical shaker (3) until the steady state was attained. The time required was from 48 to 150 hours, conversion being more rapid in concentrated solutions. The reaction went rapidly a t first, and about 80 per cent of the total transposition took place in one quarter of the total time necessary to attain the
HOURS OF GRINDING
FIGURE 3.
EFFECTOF TIMEOF GRINDING AT VARIOUS TEMPERATURES
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INDUSTRIAL AND ENGINEERING CHEMISTRY
steady state. The results are shown in Figure 4. I n the lower curve one equivalent of ammonium carbonate was used, in the middle curve two equivalents, and in the upper curve four equivalents. The P20jtransposed increases with concentration, but reaches a maximum and then decreases, which does not occur with sodium carbonate. Also P20j transposed increases with excess carbonate, and all conversions are greater than the corresponding ones with sodium carbonate. Again, the maximum conversion of 84 1 per cent is close to the 85 per cent citrate-soluble P205. In the determinations using one equivalent a t higher concentrations, the slurry is very ~ i s c o u sbecause of the formation of a voluminous white precipitate. This was thought a t first to be calcium carbonate formed in the reaction. It was separated from the phosphate rock by the more rapid settling rate of the latter, and analysis of the precipitate showed the presence of phosphate. This probably is tricalcium phosphate. since previous workers (4,12) showed that ammonium phosphate reacts with calcium carbonate to form the tricalcium phosphate. This reversion apparently does not take place in solutions more dilute in phosphate ion. Because of the high partial pressure of ammonia over the solution, no runs mere made a t temperatures above 2 5 ” .
Reaction w i t h Potassium Carbonate It was desirable to determine the transposition of P205in potassium carbonate solutions. This reaction was carried out a t 25” C. in the same manner as with ammonium carbonate. The conversions were very low compared to both sodium and ammonium carbonates. The maximum conversion with two equivalents was 13 per cent, and with four equivalents 17 per cent. S o further work was done. Separation of Reaction Products Under the optimum conditions of conversion with sodium carbonate, the solution produced has a molar ratio of sodium c a r b o n a t e to trisodium phosphate of approximately 4 . 5 t o 1. K o b e a n d Leipper gave the data for the system trisodium phosphate- sodium carbonate-water (7‘) a n d showed how t h e s e components can be separated. No double salt was found in the system. Diammonium p h o s p h a t e is t h e m o s t s o l u b l e and triammonium phosphate is the least soluble salt, its solubility d e c r e a s i n g rauidlv as the ammonia concentration is increased. Thus it is possible to precipitate the triammonium phosphate FIGCRE 4. EFFECT OF CONCENTRATION as a white crystalAXD EXCESS(ONE, Two, AND FOUR EQUIVALENTS) OF AMMONIUM CARBON- line salt by adding A T E O N P 2 0 5 TRANSPOSED AT 25’ c. either gaseous ammonia or ammonia solu tion to the liquor separated from the phosphate rock. The trisalt changes to the disalt with loss of ammonia a t
OF
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LIQUOR
FIGURE 5. PRECIPITATION OF TRIAMMONIUM PHOSPHATE FROM REACTION SOLUTIOX BY THE ADDITION OF AMMONIA
ordinary temperatures. T o a solution produced from a run with two equivalents of ammonium carbonate (56 per cent conversion) containing 0.39 mole of phosphate ion per liter was added 28 per cent ammonia solution; the resulting solution was analyzed for phosphate ion after each addition. The results are shown in Figure 5 . Up to 96 per cent of the phosphate can be removed from the solution in this way.
Conclusions 1. The reaction of sodium and ammonium carbonates on a defluorinated phosphate rock to give soluble phosphates is favored by high excesses of carbonate, high concentration of solution, and low temperature. 2. The amount of P20jtransposed does not exceed the citrate-soluble P2OSin the rock. 3. The mechanism of the reaction appears to differ between sodium and ammonium carbonates. With the former, grinding is beneficial during reaction; with the latter it retards the reaction. 4. Sodium carbonate is lost from the reacting solution by the formation of a double salt with calcium carbonate. Literature Cited (1) Assoc. Official Agr. Chem., Official and Tentative Methods of Analysis, p. 17 (1930). (2) Curtis, Copson, Brown, and Pole, IND.ENG.CHEM.,29, 766-70 (1937). (3) Doumani and Kobe, Ibid., 31, 264-5 (1939). (4) Hackspill and Claude, C h i m i e et industrie, Special No., 453-7 (Feb., 1929). (5) Hou, “Manufacture of Soda”, A. C. S. Monograph 65, p. 201, New York, Chemical Catalog Co., 1933. (6) Kobe and Doumani, 2. anorg. allgem. Chem., 232, 319-24 (1937). ( i ) Kobe and Leipper, ISD.ESG. CHEM.,32, 198-203 (1940). (8) Marshall, Reynolds, Jacob, and Rader, I b i d . , 27, 205-9 (1935). (9) Mellor, “Comprehensive Treatise on Inorganic and Theoretical Chemistry”, Vol. 11, p . 499, London, Longmans, Green and Cn. 1R22. - -., - -- . (10) Meyers, U. S. Patent 1,760,990 (June 3, 1930). (11) Ibid., 1,846,347 (Feb. 23, 1932). (12) Muckenberger, Z. anorg. a2lgern. Chem., 169, 81-95 (1928). (13) Reynolds, Jacob, Marshall, and Rader, IND.EXG.CHEM.,27, 87-91 (1935). --, (14) Reynolds, Jacob, and Rader, Ibid.. 26, 406-12 (1934). (15) Reynolds, Marshall, Jacob, and Rader, Ibid., 28, 678-82 (1936). PRESENTED before the Western Chemical Congress, San Francisco, Calif. \ -
