THE SOLTTBILITT O F THE PHOSPHATES O F C h L C I U l I I S AQL-EOL-S SOLUTIOSS O F SULFUR DIOXIDE BY 11.. \I. \IEB.LXE, J. T. D O B B I S S A X D F. K . C.IXERON
Introduction
It has been kno-,vn for many years that phosphate rock suspended in water is niade more soluble by absorption of sulfur dioxide, and that the resulting solution contains the base and acid in a ratio differing from the ratio in the suspended solid. h number of attempts have been niadt to utilizc these facts in the preparation of acid or "super" phosphates; particularly, for the fertilizer industry. The larger part of the plant investment in the fertilizer industry is in the lead chambers of the acid plants, and the cost of oxidizing sulfur dioxide i 5 a dominating factor in determining the price of phosphatic fertilizers. K i t h a lower fisetl charge against a smaller plant investment and a lowcr manufacturing charge by eliminating the oxidation of sulfur dioxide. a chcaper fertilizer would be possible. Lloreover, it should be possible to treat the ran' rock "as rninrd" and eliminate the expensive washing process deemed necessary in the present practice, whereby fifty per cent or more of the phosphorus goes to the tailings pond. Again, in the vicinity of some of our Kestern smelters are readily accessible deposits of phosphorites. The sulfur dioxide emitted in the smelting operations is a liability. The geographical position of these western phosphates IesFens much their present economic importance. Both sulfur dioxide and phosphorites xould become important assets if a satisfactory method of direct treatment of one by the other can be devised. There is no prospect that any significant fraction of the sulfur dioxide can ever be converted economically into sulfuric acid unless a change in metallurgical practices will afford a much richer stack gas. Tirelli' claims t o have made a superphosphate in the laboratory, but regards the method as inapplicable t o commercial practice. Sestiniz tried mixing chlorine with sulfur dioxide, but again the process failed practically. The three-component system CaO-P20j-H20 was studied by Cameron and coworkers, the results with a summary of those of previous workers being brought together in a departmental bulletin3 Cameron and Bell' found that a t 2 5 C'. there is a range of high concentrations of phosphoric acid at, which the stable solid phase in contact with the solutions is the mono-calcium phosphate, the solubility of the salt decreasing with increasing concentration of the acid. There is a second range of lower :Chem. Abs., 1; 1601 ;19071; Rass. X n . 26, I O I - O ~116-20. ; I n d . Chim., 12, 49-j3; J. Soc. Chem. I n d . , 31. 293 : 1 9 1 21 , U. 9. Department of Agricultiire. Bureau of Soils. Bulletin S o . 41 (1907). J. Am. Chern. Soc., 27, I ~ ~ 1~190j). I Z
K915
962
W. &I. MEBASE, J. T. D O B B I S S A S D F. K . C A V E R O S
concentrations of the acid in contact with di-calcium phosphate as the stable solid phase. Finally there is a range of yet lower concentrations of phosphoric acid, in contact with a series of solid solutions, one limiting solid solution being calcium hydroxide, the other being di-calcium phosphate. S o range of concentration exists a t which the hypothetical tricalcium phosphate is the stable solid phase, nor is there any evidence for the existence of the tetra-calcium phosphate described by \Tiley and Krug' and by Otto.? Bassett3 confirmed the work of Cameron and Bell, but, objecting to the concept of a series of solid solutions, postulates the probable existence of "oxy" and "hydroxy" apatites in contact with liquid solutions of low content of phosphoric acid. Lorah, Tartar, and Wood4 claim to have precipitated tri-calcium phosby adding ammonium hydroxide t o a solution of monophate (Ca3(P04)*, calcium phosphate, a t such a rate that the supernatant liquid remains continuously very slightly alkaline (presumably t o phenol phthalein). From their own work as well as that of Rassett,j Kolthoff,6 Jolibois,' Jolibois and Maze-Sencier,8 and Hayashi and i\Iatsui,Q they concluded this precipitate selectively adsorbs calcium hydroxide from the supernatant solution. They think these adsorption complexes become, or are accompanied by the formation of, solid solutions; but, that the limiting member on the alkaline end is the hydroxy-apatite of Bassett. Cameron and McCaugheylO obtained chlor-apatite and chlor-spodiosite by dissolving calcium phosphates in calcium chloride, fluor-apatite by dissolving calcium fluoride in molten di-sodium phosphate, but were unable to obtain calcium tetra-phosphate. Clifford and Cameron" recently obtained a series of solid solutions of calcium oxide and arsenic acid in contact with dilute aqueous solutions of arsenic acid. Gerland" found dicalcium phosphate t o be very soluble in sulfurous acid, but was unable to isolate a definite compound from t'he solution. Dr. S. C. Collins13 in a study of the rapor tensions of this system a t the laboratory of the State Teachers' College, Johnson City, Tenn., has encountered this difficulty, and our own results to be presently described show that equilibrium conditions are not obtained readily when dicalcium phosphate is a solid phase in contact with solutions of sulfur dioxide. T i t h tri-calcium phosphate J. Anal. Chem., 5, 685 (1891). Chem. Ztg., 18, 2 2 5 (1887). 32.anal. Chem.. 59, I - s ~ , (1908:; J. Chem. Suc., 111; 620-42 (1917). 4 J. Am. Chem. Sue., 51, 1097 (1929). J. Chem. SOC., 111. 620 (1917). 7 Chem. TVeekblad, 12, 662 (191j). 7 Compt. rend., 169, 1161 (1919). 8 Compt. rend., 181, 369 (192j:. J. SOC. Chem. Ind., (Japan), 29, I i j (192j). 10 J. Phys. Chem., 15, 464 (1911). 11 Ind. Eng. Chem., 21, 69, (1929). I* J. prakt. Chem. ( 2 1 , 4, 97 (1871 11 13 Private Communication. Q
CALCIUM PHOSPHATES 9 S D AQUEOUS SCLFUR DIOXIDE
963
Gerland obtained solutions in contact with a solid to TThich he ascribes the formula Ca3.P20E.SOz.zHz0.His analyses were not convincing. He must have had two solid phases, probably calcium sulfite and dicalciuni phosphate' or a phosphate approximating it in composition. All attempts to develop a commercial practice in this direction have proven futile. A search of the literature shows that there has not been developed the necessary scientific data on which to base such a practice.
Experiments Yreliminary trials showed that equilibrium conditions of solubility were finally obtained only after long standing with this system. I t mas also found that the temperature of the laboratory in which this work was done varied but little from a mean of about 2 6 T . Consequently the containers were not immersed in a water bath. A. series of wide-mouth bottles of j o o cc. capacity each was fitted with rubber stoppers carrj4ng appropriate inlet and outlet tubes of glass. Into each was placed about zoo ccs. of a solution of phosphoric acid, the several concentrations varying over a n4de range. hppropriate solid mixtures of the phosphates of calcium and calcium carbonate were added until a solid persisted in each bottle after saturation with sulfur dioxide. This last, obtained from a commercial cylinder, was bubbled through the solutions from time t o time until a stable state was obtained. The bottles containing the higher proportions of phosphoric acid were not brought to this st,ate in less than six weeks, while those containing the lowest proportions appeared to come to equilibrium within a fortnight. With the higher concentrations of phosphoric acid, diffusion was slo~v,necessitating frequent shakings. There appeared to be three series of solutions. One in which there was a large excess of phosphoric acid to calcium appeared to contain but one solid phase, a clear crystalline body. At the other end where the lower ratios of phosphoric acid to calcium were obtained, there also appeared to be but one solid phase, a homogeneous, white, amorphous body. Between these two series was a third, each bott'le of vhich appeared to contain two solids in distinct layers, the upper opaque and very finely divided, the lower transparent and fair-sized trystals. The solutions in the middle series had the straw yellow color previously noted by Gerland. Samples of the solutions for analysis were obtained with much difficulty, due to the volatility of the sulfur dioxide. K i t h a small pipette a sample was quickly transferred to a weighing bottle which was a t once stoppered tightly and then weighed. The bottle was opened under cold water in a 600 cc. beaker, the solution further diluted to definite volume and aliquots withdrawn for analysis. As soon as possible after the dilution and mixing, the sulfur dioxide was determined by titrating with a 0 . 0 5 1; solution of iodine. Calcium was determined by adding ammonium oxalate to known excess, then adding ammonia until the solution was alkaline in reaction, and allowRotondi: Annali di Chimica, 74, 128 (1882).
964
T. 11. MERASE, J. T. DOBBIXS A S D
F. B. C.UIEROS
ing it t o stand over night. During this time all calcimn \yap precipitated as oxalate, although some of it a t first canie tiown as phosphate. The precipitatp was filtered, washed, and transferred n i t h the filter paper to a beaker, and dissolved with dilute sulfuric acid. The solution was then titrated with a 0.ISpernianganate solution, tlie filter paper offering no difficulty if care were taken t o avoid a large escess of sulfuric acid. To detcriiiinc the phosphoric acid, the aliquot \vas first treated with nitric acid and brought to a. boil in o:der t o csprl or completely oxidize the sulfur dioxide. V-liile yet amount of a soiution o started a t once, hut half tate was filtered and wa monium nitrate. It \\as solution of pota&m hydrositie. The solution of ami~ioniuinmolybdate vias prepared hy dissolving the salt in a concentrated solution of arninonia, and t h m adding nitric acid tu faint acidity. To insure complete precipitation of the phosphorus a large eseess of the molybdate was alivayc used. Identification of the solid phases p ented additiorial difficulties to that involving loss of sulfur dioxide in sampling. Xttemptu t o use a chlor-ion as a "tell-talc" in finding the aniount? of liquid phase components adhering t o t h r solids proved to be quite impracticable. I n most cases, the liquid phase was so dense that tlie xeiglit of liquid t o weight of solid was too high in any sample that could br pipetted to give a chlorine percentage in the residue that would differ markedly from the percentage in the liquid phase alone. Dobbins and Gilreathl finding that the composition of the solid precipitated from a solution containing calcium and pho3phoric acid, when an excess of ammonia is added, has appriisiniately the proportions in the formula Ca3(P04)?, proposed t o use the ratio of phosphoric acid precipitated t o t'hat remaining in soliition as a critrrion of the composition of the calcium phosphate from which the solution is made. This lyas used to identify the solids of the third series, but no sat ctoq- riiotlification of the procedure could be devised applicable to the case Tvherc the precipitate. are of necessity a mixture of t,wo solid phasrs along a bounclar>- curve! or three in contact with a constant solution. Consrquentl>- thc solids were filtered an rapidly and completely as possible, dried by pressure b e t w e n filter paper?, and transferred to stoppiwd wighing bottlw. C'onsidcration of the r e d t i ; obtained fruni the serics just dwcrihed niatlc evident the desir:ihilit> finding the efftlct of added quantities of sulfur Thi. end could be obtninecl Pither by increasing dioxide i n t.he !iquid ph the presure in the x i p o r phase, or. more cunwnient!y, tJy absorbing the sulfui, tliosidc at a IOIWY tc!ripc~ruture. . h o t h e r ccriw of wlutiuns and solidx-as prepared and saturatcJ befoi.c, a t tlie te~iiper:itureof an ice chest. approximating o s ( ' .
CALCIUM PHOSPHATES h S D AQvEOuS SULFTR DIOXIDE
965
TABLE I Composition of Liquid Phases in Contact with Pairs of Solids in the System: Ca0-P205-SOi-H20 2 5°C
SO
Per cent
CaO
Per cent Per cent P?O* SO?
Isotherm Per cent H?O
I
I .92
0.00
4.57
2
I .2j
1.30
I .60
3 4 5
I
I .63
I
.80 .63 .5s
I
2.11
8 9
2 .jo
3. 2 8 3 .-I5
.62
4.10
.36 2.83 2.65 I .66 I .92 2.18
i3
4 38
2.jj
90.32
3.80 4.33 5.94 6.11 6.11 j .86
4.45
88.;; 85 .9i
.54
I
1.57
I
z
IO
2
I1
3 .oo 3.52 4.34 4.80 5 40
I2
13 1-1 15
16
-
-3 ' > 1
2
93.51 95.83 94.30 93 97 94.19 92.95 92.33 9 1 .IO
6.18 7 .I8 4.32
82 . w
2.j5
8j.i4
84.i;
4.00
84.ji
2.10
84.14 84.2j 83 .82 83 3-1 82.69 82.80
Solid Phases
Calcium Sulfite Calcium Sulfite and Solid solution. ,f
3
>' I $
1'
19
,) >* l?
Di-Calcium Phosphate and IIono-Calcium Phosphate
20
6.08 6.50 5 5-1 5 .6-1
21
5.22
7.68 9.OS 9.34 9.61 10.64
22
4.81
I O .j.+
I .6j
23
5.52
11
0.35 I .35
81.42
I1 . q j
0.40
lI.9j
0 . 2 0
82 . o j 81.83
,,
16 . Y 3
2 . j j
j4.25
28 29 30
5.61 6.12 6 02 6.25 6 js 6.45 6.25
11.62 .62
82.j1
24
!, ,,
18.11 2 1 .93
2.20
0.20
;I
1)
2 2 .OI
0.40
71.34
f,
31
j.j6
2j.77
0
32 33 34
5.95
28.90
0 .00
6.20
2 Y .2-1
0 00
.64 5.57
33 . i 8 36.80
0.00
Ii
I8 I9
25
26 2 i
35
j
0.20
I 30 I .41
1.45
.oo
0.00
72.94 .42
66.47
65.15 64.56 60.58 5j.63
11 11
,, >
f
,,
IIono-Calcium Phosphate ,I
,, , ,I
W. 11. MEBAXE, J. T. DOBBIXS A S D F. K. CA1lEROS
966
TABLE I (Continued) Composition of Liquid Phases in Contact with Pairs of Solids in the System: CaO-P205-S02-H10 0°C Isotherm Per cent
Per cent
SO.
CaO
P?O$
Per cent SO?
36
0.03 I .42
o .oo 0.67
4.03
9j .94
0,43
97.84
I .j2
1.23
1.22
1.58 1.63 1.91 3.29 6 .OS 13.73 13.52
2.09
9j.83 94.02 93.61 92.84 84.86 81. j 4 77.71 78.j4
37
38 39 40
2 .31 2.40
41
2.jO
42 43 14 45
4.8j
4.55 6.43 6.46
2.36 2.75
7.00
7 .63 2.13
1.48
Per cent
Solid Phases
HxO
Calcium Sulfite and Solid Solution ,I ,I
,, If
,, Di-Calcium Phosphate and Mono-Calcium Phosphate
Discussion The analytical results of the investigation of the liquid phases are assembled in Table I. Included are the results of t,he examination of the solid phases, given in the last column. These results are charted in Fig. I as the orthographic projection on the Ca0-P205-H20plane of the figure obtained by plotting the percentages of CaO, P205,SO2, and H 2 0 on an equilateral tetrahedron. The crosses (s)indicate the isotherms for 2 j°C. The circles ( 0 ) indicate the isotherms for 0°C. .It concentrations of phosphoric acid (PCOj) above 27.j per cent, the sulfur dioxide content of the liquid phases is vanishingly small, and absent in the solid phases. Hence, suIfur dioxide ceases to be a component, the system becomes a three-component one, and there is but one solid phase along the boundary. The curve is not shown in the figure. The figures for sulfur dioxide content are the results of careful analysis. I t is possible that equilibrium between solid and liquid phases had not been attained. It is more probable that our technique in preparing samples for analysis was inadequate. Any different method was impracticable. The results can not have any absolute value. They do, however, show the order of magnitude, and that each isotherm shows a maximurn solubility for sulfur dioxide in contact \.vith the pairs of solid phases. Analysis of the solid phase in bottle S o . I gives the composition represented by the formula CaSOa. S o other sulfite is known. Hence, it appears safe t o assume that calcium sulfite is one of the solid phases in all the solids containing sulfur dioxide. This assumption made, the analyses show that the second solid phase in the series S o s . 2-16and Xos. 36-43is a member of a series of solid solutions, a limiting solid solution in each case being di-calcium phosphate. These conclusions are supported by the figures in Table 11, in which are given the mole ratios t o phosphoric acid of the lime in excess of that equivalent t o sulfurous acid found to be present.
CALCIUM PHOSPHATES AND AQUEOCS SrLFCR DIOXIDE
967
The liquid solutions in contact with solid mixtures of mono- and di-calcium phosphate appeared to come to equilibrium slowly and uncertainly. As noted above we, as other investigators, had difficulty in obtaining consistent data. Several hypotheses to account for these difficulties were investigated.
FIG. I
Solubility isotherms for calcium phosphate in aqueous solutions of sulphur dioxide Projection on the H?O-CaO-P205plane from the equilateral tetrahedron representing H20-C'aO-PnOj-S02.
TABLE I1 Ratios of Calcium Oxide to Phosphoric .inhydride in the Solid Phosphates in Contact with Aqueous Solutions saturated with Sulfur Dioxide. Moles CaO-Noles PO, to Moles P:Oj
2j2C
O'C.
Solution S o .
Ratio
Solution S o .
Ratio
2
6.13 4.59
37 38
4.90 4.72
4.51 4,oi
39 40
4.41
3 4 3
I1 I2
3.31 2 .84
21
I .9S
22
1.72
34
I .OJ
4.33
968
Vi 11 XEBAXE, J. T. DOBBISS APiD F. K. CAMEROS
They seem to be due to the fact that in withdrawing samples for analysis sulfur dioxide escapes very readily from the sample and a t the same time lime and phosphoric acid precipitate. Hence the difficulty in drawing representative liquid samples. The isotherm for 2 j°C at a pressure of one atmosphere having been determined, and offering no espect,ation of the effective separation of lime and phosphoric acid, the desirability of determining the effect of higher concentrations of sulfur dioxide waa apparent. To this end two procedures appeared possible. To increase the pressure in the vapor phase by increasing the partial pressure of sulfur dioxide, and a t the same time have a n uniform pressure in a series of bottles involved serious experimental difficulties. The I
TIC;. 2
Soluhilitr isothprnis Tor ralcium phosphate in aqueow solutions of siilphur dioxide compared with the lo(-ifor hypothetical solutions of dicalcirim and monocalrinm phosphate, respectively.
alternative iiiethocl of dcteriiiining the possible isotherms a t 0°C was adopted. h s a matter of fact, the conccxntrationi of qulfur dioxide in the liquid phases were but slightly higher than in the solutions maintained at 2 j"C. But the relatiye solubility of lime 1va5 much incrpased, at leapt in the solutions in contact with calcium wlfite as a solitl ph2i.e. It appears, therefore, that increasing the partial pressure of sulfur dioxide in the vapor phase will decrease the ratio of pho3phoric acid to lime in thr liquid pliasp. For tiit, particular purpose of this inve4gation it is convenient' t o plot the result. as in Fig. 2 . Ordinates represent per cent calcium oxide and c per cent of phosphoric acid. T h c ~lines passing through the origin, lahellctl 11 and are t h loci of points rcprwentine ~olutionrcontaining phoqihoric anhyclritle and calciuni ositlc in the ratios of iiiuno- and di-calcium phopphatc. rrsptlctiv(~1y. It i- apparcnt that starting n i t h such a product as pho;phate rock, contact n.ith ~ a t w n t c :rqucous ~l rolution.; of siilfur tliosiilc ~vould 11eyt1r prntlucc. ;i (:a() a:. corre.ponds t o mono-calciuni pho.sphate. It is fea~iiilr.however by siic-
CALCIUM PHOSPHATES A T D AQUEOUs SULFUR DIOXIDE
969
cessive (or counter current) decantation t o obtain a sdution with a sorne\Yhat higher ratio than corresponds t o di-calcium phosphate, the most favorable ratio being with a wlution containing about percr.nt phoaphoric anhydride. -4 roughly approsinixtc hut conservat ire calculation shows that more than 96 percent of the water must be evaporated before the solution w o d d be sufficiently concentrated for mono-calcium phosphate to precipitate. But meanxhile nearly So percent of the phosphoric nnhydride would have precipitated a3 the di-calcium salt. Considering these fact;, that a close chemical coiitrol would be necessary, that large volumes n i w t be handled and evaporated t o produce a moderately priced product. the procedure holds no promise as a practicable plant process. The narrow margins of concentration attainable is a quite sufficient explanation of the failures of previous investigators and inventors.
Summary The addition of sulfur dioxide to aqueous suspensions of the phosphates of calciuni increases the s o l u b k y of the calcium oxide and of the phosphoric acid. The solubility (:f the calciuni oxide is inereawl more than the solu2. bility of the phosphoric acid by adding sulfur dioxide. 3 . Xt 0°C or at Z j O C there are three solubility cur responding to the lowest concentrations of phosphoric acid, the solubility of sulfur dioxide in the liquid phase, rises to a niaxinium and then falls again. The solid p h a s e in contact with these ~ o l u t i o nare ~ calcium sulfite and a series of solid solution3 of calcium oxide and phosphoric acid, B limiting rnemher of the serics being di-calciuni phosphate. On the second solubility curve. the concentration of sulfur dioxide again p es through a niaxiniuni. The solid phases in contact with the Tolutions represented by this curve are mono- and di-calcium phosphates. A third curvc for solutions with relatively high concentrations of phosphoric acid, shoT? negligible and approximately constant content of sulfur dioxide in the liquid phase. Llono-calcium phosphatc alone is the solid phase in contact with the solutions represented by this curve. 4. Trratment with sulfur dioxide of an aqueous suspension of a basic phosphate of calciiirii can yield a solution in n.hich the ratio of phosphoric acid to calciuni oxide is somewhat higher than t h ratio in di-calcium phosphate. Evaporation of this solution t o a concentration a t ~vhichmono-calcium phosphate would he a 5rable solid phase involves the 10,:s of 9j percent of the water and the lose of 80 percent of the pholphoric acid as di-calcium phosphate. -4s a plant niethod the procedure is iiiipracticable. j . The facts d ~ i c l o p e dexplain the failuw of preiious investigators and inventors t o develop the procedure :ind obtain mono-calcium phosphate or superphosphates. I.
CniLersify of Soriii Carohcr. Chapel Hill, .\-orth L'nrollr!a.
