Phosphorus Pentoxide Reversion of Ammoniated Superphosphate SOME EFFECTS OF FLUORINE R. C. DATIN, E. A. WORTHINGTONf AND G.' L. POUDRIERZ The Solvay Process Division, Nitrogen Section, Allied Chemical & Dye Corp., Hopewell, Va.
T
HE addition of ammonia and ammonia-containing solutions to auperphoephate, as neutralizing agents and as economical
sources of nitrogen, has been widely practiced by the fertilizer industry for twenty years. Experience has indicated that there ie a practical limit to the amount of ammonia that can be added without causing appreciable quantities of the phosphorus pentoxide present to be classed as unavailable-Le., insoluble in ammonium citrate solution under the conditions of the A.O.A.C. procedure (I). How closely this practical limit approaches the guantity of ammonia that superphosphate is capable of absorbing depends upon the conditions in effect during ammoniation and subsequent storage of the products, including temperature ( 4 , 7, 8,18),water content (6,7-9), and thepreeence of liming materials
The equipment consisted of a 1-cubic-foot stainless steel ribbon mixer provided with a dump door for discharge of the superphosphate, a sulfuric acid tank, a "denning" tray to receive the superphosphate from the mixer, and an exhaust fan. Acid a t 85" C. was run rapidly into the solid tumbling in the mixer. After about 1 minute, when the foaming had started to subside, the charge was dumped into the tray, which wm immediately placed in an oven maintaiaed a t 85" C. The superphosphates were bagged a t the end of 18 hours' denning and allowed to cure for 1 month beforeammoniation.
(8, 6).
An intriguing suggestion by MacIntire (18, 13),that reversion to citrate-insoluble phosphorus pentoxide is caused by reaction of tricalcium phosphate with calcium fluoride in the superphosphate to form fluorapatite, 3C&(PO4)2.CaFz,has been the subject of many investigations, but the data reported ape conflicting. MacIntire found in 1940 that a fluorine-free triple superphosphate made from phosphoric acid and Appalachian marble ( I O ) and an ordinary superphosphate made from a fused phosphate rock of 0.003% fluorine (11) could be ammoniated to 6 to 7% ammonia without phosphorus pentoxide reversion. In a later paper (14), however, he reported that the ammoniation of a superphosphate made from a fused phosphate rock of 0.02% fluorine yielded a product with 19% of its phosphorus pentoxide insoluble in citrate. Other workers (6, 16, 18)have shown that in the case of simulated superphosphates made from pure monocalcium phosphate and calcium sulfate, reversion occurs after ammoniation only when fluorides are present. The authors considered that it would be of interest to compare the reversion to citrate-insoluble phosphorus pentoxide of a group of superphwphates made from commercial raw materials containing varlous amounts of fluorine, other factors being kept as nearly equal as possible. Since no adequate method other than fusion waa found for defluorinating superphosphates without a t the same time causing excessive reversion, the superphosphates were prepared in the laboratory, all from the same batch of Florida phosphate rock. PREPARATION OF SUPERPHOSPHATES
The were prepared in 50-p0und batches from Florida phosphate rock and sulfuric acid with a concentration of 60 to 90% in the apparatus shown in Figure 1. Since it had been found to increase the volatilization of fluorine, kieselguhr waa added to half of the samples before acidulation. The quantity of acid used waa 5 % in excess of the theoretical requirement as computed by the method of Marshall and Hill (16)-i.e., the sulfurio acid-phosphate rock weight ratio was 0.61. 1
Present address, Kaiser Aluminum & Chemical Corp., Permanente,
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Present address, Naval Proving Grounds, Dahlgren, Va.
Calif.
82" 0 I A . X
17"
UNLOAOI No 0008 TRPll
Figure 1. Experimental Equipment for Superphosphate Manufacture
Since the fluorine contents of the superphosphates were not so low as desired, portions of each sample were put back in the oven for 45 days additional denning a t 85" C. The analyses of the phosphate rock and of the superphosphates after denning for 18 hours and 45 days are given in Table I. Phosphoric acid (P&) analyses were made according to the methods of the A.O.A.C. ( I ) , by the of Reynolds and Hill ( l r ) ,and free acid ftR HaPo, by the method of Hill and Beeson AMMONIATION
The SuPerphoaphates were ammoniated using the apparatus shown in Figure 2, consisting of a gas-tight, stainless sfeel drum, which waa 4 inches deep and 14 i n c h in diameter and waa rotated at 24 revolutions per minub. Nitrogen solution composed of 65.0% ammonium nitrate, 21.7% ammonia, and 13.3% wa-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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TABLEI. LABORATORY-PREPARED SUPERPHOSPHATES (Sulfuric acid-phosphate rocka weight Analysis-Weight Superphosphates Denned 18 Hours a t 85' C. Citrateinsol. Total Free acid PzOs F as H I P O ~ PnOs
ratio = 0.61) %, Dry Basis Superphosphates Denned 45 D a y s a t 85' C. CitrateTotal HzO-sol. insol. Free acid PnOs Pion Pa06 F as HsPOl
Sample No.
Kieselguhr Added, yo of Rock
76 77 78 79
0 0 2 2
2.02 1.97 1.50 1.50
60.3% Acid Used in Preparation of Superphosphate 1.25 3.63 21.78 0.19 6.26 1.27 4.50 21.23 0.08 7.74 3.96 21.23 0.08 0.70 7.16 0.86 3.12 21.30 0.11 5.79
20.60 21.00 21.10 21.11
15.48 15.00 15.55 15.20
0.12 0.12 0.13 0.66
80 81 82 83
0 0 2 2
65.5% Acid Used in Preparation of Superphosphate 1.21 3.81 22.07 0.17 6.84 1.81 0.92 7.98 21.22 0.17 1.64 11.53 0.67 6.44 20.50 0.30 11.05 1.18 6.23 21.22 0.66 2.30 0.25 1.31
21.05 20.38 21.20 21.70
15.61 14.95 15.85 16.48
0.55 0.80 0.30 0.31
84 85 86 87
0
1.54 1.53 1.07 1.05
20.60 20.71 22.00 22.20
16.11 16.20 17.20 17.35
0.37 0.15 0.10 0.15
88 89 90 91
0
1.57 1.28 0.93 0.99
21.80 21.50 20.90 21.70
17.70 17.58 17.96 18.05
0.20 0.20 0.24 0.17
92 93 94 95
0
21.40 21.80 22.10 22.10
18.05 18.50 18.40 18.05
0.51 0.73 1.05 1.13
0 2 2
0 2 2
0 2 2
70% Acid Used in Preparation of Superphosphate 20.46 0.82 8.44 14.90 1.26 21.59 0.97 4.04 11.14 0.87 0.87 2.98 20.81 0.97 6.66 0.84 2.97 21.80 1.05 7.52 80.8% Acid Used i n Preparation of Superphosphate 13.50 21.43 2.03 1.08 5.86 18.77 20.00 2.30 0.82 9.28 21.03 2.82 0.51 4.19 20.85 4.50 0.57 3.15
90.0% Acid Used in Preparation of Superphosphate 0.81 3.52 1.41 21.80 3.69 21.51 0.47 5.23 3.74 1.34 21.76 0.55 3.00 4.42 0.91 2 1 . 4 3 4 . 6 4 0 . 9 2 2.68 0.92
Analysis of phosphate rock (dry basis) 0.80% HzO, a n d 0.93% Fez08. 0
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Vol. 44, No. 4 the phosphorus pentoxide contents of the c o r r e s p o n d i n g ammoniated products. On the other hand, for those materials denned 45 days, and of a lower fluorine level (0.47 to 1.27% as compared with 0.91 to 2.02%), a plot of the perc e n t a g e of t o t a l c i t r a t e insoluble phosphorus pentoxide after ammoniation against the percentage of fluorine (Figure 3) revealed a trend of decreasing availability with increasing fluorine, although it was obvious that other factors affecting availability were operable. STATISTICAL METHOD. A convenient means of analyzing data of this type involves the calculation of regression coefficients and from them the equation for the straight line best fitting the data ( 3 ) y =a
+
bl(Z bJ(W
- 2 ) + bL(2 - z ) + - E ) + , , , (1)
where y is the dependent variable; x, z, 20, etc. are independent variables; E , Z , E , etc., are means; and a, b,, bz, bs, etc., are constants, the partial regression coefficients. RESULTS. Since the samples mere all prepared from the same phosphate rock and were treated in the same manner with respect to ammoniation, water content, and conditions of storage, it was assumed that the only important variables were the percentages of fluorine, free acid, water-soluble phosphorus pentoxide, and citrate-soluble phosphorus pentoxide in the original superphosphates. The statistical analysis showed that the effects of watersoluble and citrate-soluble phosphorus pentoxide in the ranges encountered were negligible, but that the reversion to citrate insolubility can be expressed as a linear function of fluorine content,
34.30% PzOs, 48.70% CaO, 1.16% AlzOa, 3.70% F, 1.28% SO,,
ter was delivered into a £ batch of superphosphate at a constant rate of 260 ml. per minute (0.53 cubic feet per hour) through an aluminum tube having 9 equally spaced '/ea-inch holes. The water contents of the superphosphates were adjusted prior to ammoniation SO that the end products would contain 10% water, thus making the water present during ammoniation nearly the same for all samples. For all samples of superphosphate prepared in the laboratory, the extent of ammoniation was j pounds of free ammonia per 20 pounds of available phosphorus pentoxide, yielding products of about 8-17-0 grade. After ammoniation the products were stored in sealed bottles for 30 or 66 days a t TABLE11. PHOSPHORUS PEXTOXIDE REVERSION O F AMMONIATED LABORATORY-PREPARED 50" C. S a m p l e s c a r e f u l l y SUPERPHOSPHATES~ drawn from these bottles were Superphosphates Denned 45 Days a t 85' C.C dried at 50" C. for about 2 ~Superphosphates Denned (18 Hr. a t 85' C.b -1st Ammoniation--2nd Ammoniationhours, ground in a MikroCitrateCitratsCitrateCitrateinsol. Citrateinsol. Citrateinsol. Samplmill to pass a 30-mesh Total insol. ye of Total insol. % of Total insol. Ye of screen, and analyzed for total Superphosphate Pzos, PzOs, total Paos, Pzos, total PaOa, PsOa, total No. % % PaOa % % Pa06 % % PZOb Sample and c i t r a t e - i n s o lu b 1e phos17.65 2.51 14.2 76 17.10 3.53 20.6 3.67 21.5 17.05 phorus pentoxide b y t h e 17.24 1.70 9.9 77 16.98 3.11 18.3 3.26 19.1 17.05 17.18 1.57 9.2 78 16.93 1.88 11.1 10.6 17.33 1.83 A.O.A.C. procedures ( I ) , 17.83 2.26 12.7 79 17.13 2.77 16.2 2.59 15.0 17.25 modified to include continuous 17.71 2.37 13.4 80 17.18 2.58 15.0 3.00 17.2 17.40 1 6 . 8 4 1.31 7 . 8 81 1 7 . 1 0 2 . 0 9 1 2 . 2 1.88 11.1 1 6 . 9 3 agitation during the citrate di17.00 1.24 7.3 82 17.05 0.37 2.2 17.53 2.04 11.6 83 8.0 17.65 1.10 gestion and the use of Shimer 6.3 17.85 1.42 16.67 1.69 10.1 84 11.7 1.90 16.90 2.26 13.4 17.00 tubes for filtering the digestate. 16.46 3.09 18.8 85 15.3 17.03 2.60 17.08 2.70 15.8 15.9 86 18.08 2.59 14.4 17.40 2.49 14.3 2.7.5 17.30 The results are in Table 11. 15.6 87 17.26 1.52 8.8 17.70 17.60 2.82 16.0 2.77 7
DISCUSSION
I n regard to the superphosphates prepared in the laboratory, no correlation was found between the residual fluorine of those denned 18 hours and the availability of
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17 05 13.4 2.28 88 16.95 2.30 13.6 7.9 1.34 89 16.90 17.05 1.38 8.2 9.0 1.55 90 17.20 17.40 1.64 9.4 2.00 11.8 91 17.00 17.15 2.00 11.7 15.2 2.67 92 17.55 2.51 14.5 17.50 1.11 6.5 93 17.23 17.23 0.96 5.6 1.54 94 17.25 8.9 17.40 1.24 7.1 16.5 2.93 95 17.78 17.5 17.80 3.12 a Ammoniated at r a t e of 5 pounds of ammonia per 20 pounds of available phosphorus pentoxide; products contained 10% water. 6 Stored 66 days a t 50' C.; values a r e means of uadruplicates 8 Stored 30 davs a t 50° C.: values are means of juplicates.
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1952 DRUM
COVER
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free acid as H3POain the original superphosphate (mean = 4.48). All three regression coefficients are significant to the 0.1% level, or in other words the probability of obtaining the same results in the absence of correlation is less “r“*Oco”.Lc than one in a thousand. Considering samples of mean free-acid content, the reversion after ammoniation varied from 7.0 i 2.3 to 19.5 =k 2.3,with a mean difference of 12.5 f 4.6, as the fluorine changed from 0.47 to 1.27%. This difference is significant to the 1% level. The lines drawn in Figure 3 were calculated for samples of mean, lowest, and highest free-acid content. CONCLUSIONS.The excellent correlation of reversion with fluorine content in the range 0.47 to 1.27% is in agreement with MacIntire’s theory of fluorapatite formation ( l a ) and is strong evidence that the removal of fluorine from superphosphate would lower the phosphorus pentoxide reversion after ammoniation and storage. However, the lack of such correlation for samples of laboratory-prepared superphosphates having fluorine contents in range of 0.91 to 2.02%, respectively, suggests that fluorine above a certain percentage has no further effect on reversion. The data are insufficient to permit a precise statement regarding the critical level, but it is obvious that to be effective a high order of removal would be necessary. Such processing does not now seem economically justified. It should be noted, however, that drying and cooling of ammoniated products might be an alternative method for preventing reversion from this source, through restriction of temperature and moisture during storage. The relatively high reversions encountered in this study are in large degree attributable to the severe conditions of storage, particularly the high water content. &,,*
Figure 2.
Ammoniating Apparatus
provided the variation in free acid is taken into account The partial regression coefficients and their standard deviations, and the applicable equation, are
7i
5:
12.80 i 2.29, bl
y = 12.80
=
15.66 f 1.96,
- 0.75 =I=0.19
bi
+ 15.66(~- 0.84) - 0.75(~- 4.48)
or 3.01
+ 15.66s- 0.752
where y is the percentage of the total citrate-insoluble phosphorus pentoxide after ammoniation, 5 is the percentage of fluprine in the original superphosphate (mean = 0.84),and z is the percentage of
LITERATURE CITED
(1) Assoc. Offic. Agr. Chemists, “Methods of Analysis,” 7th ed., Washington, D. C., 1950. (2) Beeson, K. C., Am. FertiEizer,81, No. 10, 5 (1934). (3) Fischer, R. A., “Statistical Methods for Research Workers,” 10th ed., pp. 128-66, New York, Hafner Publishing Co., 1948. (4) Hardesty, J. O., and Ross, W. H., IND. ENQ.CHEM.,29, 1283 (1937). (5) Hardesty, J. O., Ross, W. H., and Adams, J. R., J. Assoc. Ofic. Agr. Chemists, 26, 203 (1943). (6) Hill, W. L., and Beeson, K. C., Ibid., 1 8 , 2 4 4 (1935). (7) Jones, R. M., and Rohner, L. V., Ibid., 25,195 (1942). ( 8 ) Keenen, F. G., IND. ENG.CHEM.,22,1378 (1930). (9) Keenen, F. G., and Morgan, W. A,, Ibid., 29,197 (1937). (10) MacIntire, W. H., and Hardin, L. J., Ibid., 3 2 , 8 8 (1940). (11) MacIntire, W. H., and Hardin, L. J., J. Assoc. Ofic. Agr. Chemists, 23,388 (1940). (12) MacIntire, W. H., Hardin, L. J., and Oldham, F. D., IND. ENG.CHEM.,2 8 , 4 8 (1936). ’(13) MacIntire, W. H., Hardin, L. J., Oldham, F. D., and Hammond, J. W., Ibid., 29,758 (1937). (14) MacIntire, W. H., Marshall, H. L., and Shank, R. C., J. Assoc. Ofic. Agr. Chemists, 27, 413 (1944). (15) Marshall, H. L., and Hill, W. L., IND.ENG.CHEM.,32, 1128 (1940). (16) Rader, L. F., Jr., and Ross, W. H., J . Assoc. Ofic.Agr. Chemists, 22, 400 (1939). (17) Reynolds, D. S., and Hill, W. L., IND. ENG.CHEM.,ANAL.ED., 11, 21 (1939). (18) Ross, W. H., Rader, L. F., Jr., and Beeson, K. C., J. Assoc. Ofic.Agr. Chemists, 21,258 (1938).
1 1
0 0.2
0.4
0.6
0.8
Figure 3.
1.2
1.0
PERCENT F L U O R I N E I* SUP6RPHOSPHATE,
X
Effect of Fluorine on Reversion of Ammoniated Superphosphates
RECEIVED for review August 23, 1951. A C C ~ P T ENovember D 20,1951. Presented &s part of the Bymposium on Fertilizer Technology before the Division of Fertilizer Chemistry at the 120th Meeting of the AMERICAN CHEMICAL SOCIETY, New York.
