Composition and Properties of
Superphosphate EFFECT OF ALUMINUM AND IRON CONTENT ON CURING BEHAVIOR H . L. MARSHALL AND
w.
L . HILL
Mathieson Chemical Corp., Baltimore, M d . , U . S. D e p a r t m e n t of Agriculture, Beltsville, M d .
T h e data accumulated during research work indicated that aluminum and iron were effective in the curing of superphosphate. In several cases the effect on phosphate solubility seemed to warrant a detailed study of the reaction of aluminum and iron. When the iron and aluminum content of a phosphate rock or a superphosphate system is greater than 59’0 RzOa, the soluble phosphorus fraction decreases with age. If iron is the predominant RzOa value, the citrate-soluble fraction of phosphorus increases upon curing. If aluminum is the predominant RzOs value, the citrate-insoluble phosphorus increases upon curing at the expense of the available phosphorus originally formed upon acidulation. There is evidence that a calcium iron or calcium aluminum phosphate complex of variant solubility is formed. This information is vital in guiding the producer of superphosphate in his selection of rock and acid to be used, for maximum “water solubility of phosphorus” or ccavaiIable phosphorus” after curing.
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T
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HE influence of the iron and aluminum content of phosphate rock on the physical character and phosphorus solubility of superphosphate made therefrom has been studied by several workers during the past 30 years (2, IS, 16, 17). I n a preceding paper of this series (11) sundry superphosphates were found to contain citrate-soluble phosphorus pentoxide and R203 in mole ratios that approach 2 to 1, particularly a t the higher Rz03contents. Interest was thus turned to a systematic study of the effects of controlled amounts of aluminum and of iron in the phosphate rock on the physical character and the phosphate solubility pattern.of the supetphosphate. Although this work was interrupted before the contemplated study had been concluded, the results obtained possess sufficient interest to merit a place in the literature.
overnight (20 hours). The cooled product was then excavated from the beaker, u t through a 20-mesh sieve, mixed well, and stored in tightly cyosed screw-cap bottles at laboratory temperature. The analytical samples of the day-old superphosphate were weighed out immediately after screening and mixing. Thereafter, for periodical analysis the product was removed from container and thoroughly mixed before samples were taken for analysis. Total, water-soluble, and citrate-insoluble constituents were determined (1, 11) on the day-old superphosphate and after it had been stored for 5, 10, 15, 20, and 30 days. The phosphorus content of the superphosphates at ages of 1 day and 30 days is shown in the last two columns of Table 11. The phosphorus pentoxide content, 19.2 t o 20.9% in day-old products, increased somewhat with age to 19.7 to 21.4%, presumably as a consequence of drying occasioned by the intervening samplings. INFLUENCE OF ALUMINUM AND IRON COMPOUNDS ON PHYSICAL CONDITION
The readiness with which the day-old superphosphate passed a 20-mesh screen is the basis on which the physical characters of the products are rated in Table 11. I n the table “good”
TABLE I.
Ocean Island
The phosphate rock was an Ocean Island rock of moderately high fluorine content that was very low in Rz03. Raw rock for acidulation was prepared by thoroughly mixing in a ball mill the chosen amounts of aluminum phosphate and/or iron phosphate with the quantity of rock needed for a batch of superphosphate. The analyses of these phosphate8 are given in Table
I.
July 1952
P.UU Materials prepared by Jamb ( 8 ) .
MATERIALS
Aluminum Phosphate
Iron Phosphate
...
...
1236Q
1237a
TABLE 11. PHYSICAL CONDITION OF SUPERPHOSPHATE IK RELATION TO RzOa CONTENT O F RAWPHOSPHATE Raw Phosphate AlzOs, % 0.04 0.04 0.04 0.04
The raw phosphate was acidulated with 66.6% (53’ Be.) sulfuric acid of technical grade, the amount of acidulant being governed by the adjusted composition of the raw rock, so as to keep the degree of acidulation 100% (10). The mixing procedure was the same as that used previously (11). The rock (50 grams) and acid, both at room temperature, were mixed by hand in a 200-ml. beaker until the temperature of the mixture reached its peak value (68’ to 80’ C. within 3 minutes), whereupon the covered beaker was put in an electrically heated oven set a t the same temperature for a curing period of 5 hours. The material was removed from the oven and allowed to stand otherwise undisturbed
Rock 1566
Constituent
a
MATERIALS AND METHOD
PERCENTAGE COMPOSITION O F PHOSPH.4TE
0.04 1.0 2.5 5.0 7.5 10.0 1.0 2.0 1.5 3.0 2.0 4.0
FezOs,
% 0.4 2.5 5.0 7.5 10.0 0.4 0.4
0.4 0.4 0.4 2.0 1 .o 1.5 3.0
4.0 2.0
FeOs
equivalent,
% 0.46 2.66 5.06 7.56 10.06 2.0 4.3 8.2 12.2 16.1 3.6 4.1 3.9 7.7
7.1 8.3
INDUSTRIAL AND ENGINEERING CHEMISTRY
Superphosphate Physical Total T z a F condition PZOS PzOa of daj-old at 1 day, at 30 days, pro uct % % 19.4 19.7 Good Good 19.2 19.9 Good 19.8 20.0 20.4 20.5 Fair 20.6 21.4 Poor Good 19.2 19.7 Good 19.6 20.7 20.7 21.0 Fair Poor 20.9 22.5 Very poor .. .. Good 19.5 19.7 Good 20.5 20.0 Good 19.4 19.7 Fair 20.7 21.4 20.6 20.6 Fair Fair 20.4 20.9
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The products became fair when the added iron content of the rock was 7.5y0,whereas the corresponding value for added aluminum was 5%. Viewed more generally, the physical condition of the superphosphate dropped t o a fair rating when the ferric oxide equivalent of the aluminum and iron oxides in the raw rock reached about 7.5%. Free phosphoric acid and free water were determined by ether extraction (6, 8) on all the products at different ages. However, the results, not shown in the tables or figures, were somewhat erratic with poor reproducibility and seemingly meaningless because of uncertainties as to suitability of the procedure for high R203 contents. The results for free phosphoric acid, being lower for high Rz03 contents, ranged from 1.4 to 15.0% in day-old products and from 2.4 to about 15% in products 30 days old. Oddly enough, the first figure in each of the foregoing ranges is for the same material (7.5% alumina in raw rock). Such an increase in determined free acid with age, though not inconceivable, needs further verification after the analytical methods have been carefully re-examined. The results for free water, though more consistent, ran extremely high, being 16 t o 22% in day-old products and 13 to 18% in materials 30 days old.
(A) WATER-SOLUBLE PHOSPHORUS
90
60 -I
c4 ?
50
8
50
M 0”
n*
40
30
INFLUENCE OF ALUMINUM Ahl) IRON COdPOU>DS 01 PHOSPHATE SOLUBILITY
20
0
0
0
I 0
5
10 15 20 AGE Of SUPERPHOSPHATE. DAYC
I
I
25
30
Figure 1. Influence of Ferric Oxide Additions to Rock ,on Water- and Citrate-Soluble Phosphorus in Superphosphate AlaOj content small
applic. to friable products that passed the sieve without difficulty, “fair” to tbow showing some stickiness and requiring more than usual carc 1’1 handling, “poor” to those requiring great patience anti and “very poor” t o material that could not be sieved. All ih , ~day-uld materials were thixotropic and required reasonable care duiing the screening operation. I ’ ~ ’ ,
Observed changes in phosphate solubility with the R,Oi content of the rock and age of the superphosphate are depicted in Figures 1 to 4. In the near absence of &03,water solubility of the phosphorus increased with curing of the superphosphate :it the expense of both citrate-soluble and citrate-insoluble (mainly lock) phosphorus, as illustrated by the curves a t 0.4% ferric oxide (Figures l A , l B , and 3A). With 501, of iron in the absence of aluminum, the water solubility remained nearly independent of age (Figure l A ) , whereas citrate solubility increased slightly with age (Figure IB), apparently a t the expense of citrateinsoluble phosphorus (Figure 3A). At higher iron contents citrate solubility increased at the expense of both water-soluble and citrate-insoluble phosphorus only during the fist 15 days of curing. Thereafter, the insoluble form showed a slow increase that was apparently derived from both of the other forms.
IO 0
so
20 C
1
I
AIz41N RAW ROCK o
0
A
I6
2.5 d 5.0 % 7.5 %
I2
6 10
4
0
5
15 20 AGE Of SUPERPHOSPHATE, DAYS
25
30
[Figure 2. Influence of Alumina Additions to Rock on LWater- and Citrate-Soluble Phosphorus in Superphosphate Fer01 content small
1538
0
5
IO 15 20 AGE OF SUPERPHOSPHATE, DAYS
25
30
Figure 3. Influence of Ferric Oxide and Alumina Additions (Singly) to Rock on Citrate-Insoluble Phosphorus in Superphosphate
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
Vol. 44, No. 7
Phosphorus and Phosphate
I -I
F
2
I
7 A I
0
5
I
4
I
,'
20 AGE Of SUPERPHOSPHATE, DAYS IO
PS
26
30
Figure 4. Influence of Ferric Oxide and Alumina Additions (Jointly) to Rock on Citrate-Insoluble Phosphorus in Superphosphate
iron compound is t o be expeeted, and the results for citrateinsoluble phosphorus in aluminum-bearing superphosphate (Figure 3 B ) point to a lower solubility than was found for the iron compound. The view that &Oa compound formation is responsible for the observed changes in water-soluble phosphorus is supported by the results for water-soluble ferric oxide (Figure 5 A ) . The proportions of water-soluble iron decrease with increase in the amount of iron in the material, and in materials with the higher iron contents also with age, in a manner that is fairly consident with the phosphate solubility pattern (Figure 1A). Support is equally good for water-soluble aluminum (Figure 5B), though the curves fall much closer together as a consequence of the small equivalent weight of aluminum in comparison with iron.
The effect of alumina (Figures 2A, 2B, and 3B) differs from that of ferric oxide mainly in the direction of watrer solubility changes, which alxays increased during the first half of the curing period, and in relatively small changes in citrate solubility, accompanied by marked increases in citrate-insoluble phosphorus during the latter half of the curing period. The results seem t o indicate a marked tendency for water-soluble phosphorus t o go over t o a citrate-soluble condition during the latter half of the curing period. The influence of aluminum on the development of insoluble phosphorus was enhanced by the presence of iron (Figure 4).
20
i- \
DEPENDENCE O F SOLUBILITY CHANGES ON COMPOUND FORMATION
t
In the absence of Rz03the increase in water solubility during curing arises from the reaction, slowly continuing to complet,ion, whereby monocalcium phosphate is formed. This process also occurs when Rp03is present, but with sufficient amounts of R20a the normal expected increase in water solubility is more than compensated by the simultaneous formation of water-insoluble phosphates of iron and aluminum. Very good evidence points to a partially citrate-soluble calcium iron phosphate, rather than t o ferric phosphate as supposed formerly (16),as the waterinsoluble iron phosphate that forms in superphosphate. Sanfourche (13)concluded that this iron phosphate was a complex phosphate of calcium and iron, and he with Focet (14)prepared, under solution conditions very similar t o those obtaining in superphosphate, gel-like materials that approached the composition of Ca[HiFe(P04)2]e. Similarly, the authors (11) succeeded in preparing a coarsely crystalline compound having this composition but also includinn 6 moles of water (Table 111). The citrate solubility of its phosphorus is above SO%, which is more than sufficient t o account for the observed citrate solubilities of superphosphate (Figure 1B). TABLE111. PERCENTAGE COMPOSITION OF CRYSTALLINE CALCIUM IRON PHOSPHATE@ Constituent CaO
FeeOa. total
Citrate-inaol. PeOs, total Water-sol. Citrate-insol. Hz0 crystal. Hz0 tots1
-
Ca[HzFe(POa)ale.
6Ha0 8.71
Laboratory Preparation
44:io
1366-d 9.58 24.35 6.90 44.70
1li:ig 22.39
7.15; 15.59 21.87s
24.80
...
0.70
100
80 -I
2
c 0
60
1
0
Figure 5.
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5
IO
I5
20
25
I
30
AGE OF SUPERPHOSPHATE, DAYS Dependence of Water-Soluble Aluminum and Iron on Age of Superphosphate
The magnitude of the changes in phosphate solubility arising from RzOa compound formation as the superphosphates cure also depends on the proportion of liquid in the superphosphate. In preparing the materials under dhcussion the acid-rock ratio, and therewith the amount of solution phase, was higher by 5 t o 10% than is umal in Taetory practice, deliberately made so, in order t o work within the acidulation range for maximal effects ( 1 1 , p. 1229). Larger amounts of liquid can hold more Rz08 in solution permanently, whereas the smaller amounts of liquid obtained with moderate underacidulation, having less holding capacity, neceesitate more rapid separation of R203compounds from solution, with the result t h a t variations in the phosphate solubility with age are greatly reduced. APPLICATIOR TO SUPERPHOSPHATE TECHNOLOGY
The situation with respect t o the compound responsible for the influence of aluminum on phosphate solubility is less satisfactory. An aluminum compound analogous to the foregoing July 1952
I n marketing phosphate rock for use in Superphosphate manufacture in countries that recognize water solubility only, the seller guarantees a maxlmum R2Oa content ranging from 0.3 t o 3%, depending on the quality of rook afforded by the mine ( 4 ) . This
I N D U S T R I A L A N D E N G INEERING CHEMISTRY
1539
restriction is imposed because of the lower water solubility ’of superphosphate containing aluminum and iron compounds. Furthermore, factory practice in these countries is to use relatively high acid-rock ratios, which yield a product with a proportionately large amount of liquid phase. On the basis of the preceding discussion it would appear that the RnO, content of the rock is being restricted t o the amount that can be held in solution in the superphosphate. On the other hand, wherever superphosphate is sold on the basis of the sum of the water- and citrate-soluble portions of the phosphorus, the maximum workable amounts of R20s in the rock are determined by the amount of phosphorus the producer can afford to lose as a citrate-insoluble form and/or by the physical condition of the product. I n this study the character of the dayold superphosphates changed from “good” to “fair” between Rz03 contents equivalent to about 5 to 7.5% ferric oxide (Table XI), which may be considered tentatively as the upper limit with respect t o physical condition The indication is that the same limit also holds fairly well with respect to citrate-insoluble phosphorus, provided the Rz08is substantially all ferric oxide (Figure 3 A ) . If, as in the usual case, a considerable part of the R z O is ~ alumina, the limit amount becomes smaller (Figures 3R and 4). Acidulation grades of phosphate rock used in the United States rarely exceed 5 to 6% total aluminum and iron oxides. Aluminum and iron may also get into superphosphate as impurities in the sulfuric acid when certain waste and spent acids are utilized. A producer, who happens to be using a rock of high RzOacontent, would probably experience difficulty with the use of such spent acids, whereas his neighbor, whose rock s ~ ~ ~is~ low p l in y R2O3,could get along nicely with them. This
caution seems timely just n o r in the face of present restrictions on the supply of virgin sulfuric acid to the fertilizer induAtry and the accompanying need for the ut.ilization of all waste and spent acids in the interest of sulfur economy. REFERENCES
(1) Assoc. Offic. Agr. Chemists, “Official and Tentative Methods of Analysis,” 6th ed., 1940. (2) Austin, W.Z., IND.ENG.CHEW,15, 1037-8 (1923). (3) Bartholornew, R. P., and Jacob, K. D., J . Assoc. O s c . Agr. Chemists, 16, 598-611 (1933). (4) Gray, A. N., “Phosphates and Superphosphates,” 2nd ed., London, E. T. Heron 85 Co., 1944. (5) Hill, W. L., and Beeson K. C . , J . Assoc. O s c . Agr. Chemists, 18, 244-60 (1935). (6) Ibid., 19, 328-38 (1936). (7) Hill, W.L.. and Hendricks, S. B., IND.ENG.(>HIM., 28, 440-7 (1936). ( 8 ) Hill, W. L., and Jacob, K. D., J . Assoc. Ofic.A g r . Chemisls, 17, 487-505 (1934). (9) Jacob, IT. D., et al., IND.ENG.CHEM.,34, 722-8 (1942). (lo) Marshall, H. L., and Hill, W. L., Ihid., 32,1128-36 (1940). (11) Ibid., 32, 1224-32 (1940). (12) Marshall, H. L., et al., Ibid., 32, 1 6 3 2 4 (1940). (13) Sanfourche, A., Bull. soc. c h i m . , [4] 53, 1580-94 (1933). (14) Sanfourche, A , , and Focet, B., Ibid., 53, 1517-22 (1933). (15) Schucht, L., “Die Fabrikation des Superphosphates,” 4th ed., Braunschweig, Fried. Vieweg & Sohn, 1926. (16) Shoji, T., et al., J . SOC.Chem. Ind., Japan, 35, Suppl. binding^ 130-4 (1932). (17) Ibid., pp. 417-21. RECEIVED for review October 20, 1951. ACCEPTED May 6, 1952. Presented before t h e Division of Fertilizer Chemistry a t t h e 120th Meeting of the AMERICAN CHEMICAL SOCIETY, New York, N. Y. Work done about 10 years ago, while senior author was in the employ of the Department of Agriculture. Previous publications in this series are (5-f8).
Quick-Curing of Superphos G. L. BRIDGER AND EDWIN C. K;IPUSTA1 D e p a r t m e n t of Chemical and Mining Engineering, Iowa S t a t e College, A m e s , Iowa Studies were made of the quick-curing and granulation of ordinary swperphosphate i n a laboratory Roto-Louvre dryer. When the superphosphate was made by treating phosphate rock with 71.8qo sulfuric acid, a concentration typical of that used commercially, drying conditions were not found which would produce acceptable conversion of the phosphorus pentoxide to an available form. When steam and steam-air mixtures were used as the drying agents, better results were obtained with air; this indicated the desirability of adequate moisture during curing, but the conversion was still not high enough. When 40 to 6070 sulfuric acid was used for preparation of the
mperphosphate, the fresh material had a substantially higher conversion than that made with the more concentrated acid, and this conversion was increased by drying with air. Maximum conversions were obtained when the product temperature was kept below 215“ F. The conversion of superphosphates made and quick-cured under these conditions was coniparable to that achieved commercially. The quick-cured products were granulated during drying, and were superior in physical characteristics to storage-cured superphosphate. Products made with the more dilute acids granulated better than those made with iicid of commercial concentration.
N THE most widely used process for manufacturing ordinary superphosphate (6, l a ) , pulverized rock phosphate containing 31 to 34y0phosphorus pentoxide reacts with 0.82 to 0.95 pound of 56 to 50’ BB. (71 to 62%) sulfuric acid per pound of rock (14, 16) in a shallow pan mixer. While fluid, the mixture is discharged into a den, where solidification takes place. The resulting porous solid is then disintegrated and transferred to a storage pile for curing. During the curing period, which may be from 4 to 12 weeks ( 6 ) , the reactions initiated in the mixer and continued in the den are allowed t o go to completion, with a resultant increase in available phosphorus peptoxide in the superphosphate. [Available phosphorus pentoxide is the sum of that which is soluble in water and in a neutral ammonium citrate solution
under certain prescribed conditions (a).] The cured pyoduci is then disintegrated and shipped. The quality of the finished product is judged by and its price is determined by its available phosphorus pentoxide content, and a measure of the efficiency of the process is the conversion of the phosphorus pentoxide in the rock phosphate to the available form. Although the storage curing process has proved t o be satisfactory and economical, it possesses inherent disadvantages which might be eliminated by use of a suitable quick-curing process. The advantages of such a process are: (1) The product could be shipped directly, thus reducing the required storage facilities and working capital tied up in the inventory stock, (2) opportunities for producing a granular product should be greater, (3) operating conditions in the mixing step could be chosen with
1
Present address, National Fertilizer Association, Washington. D. C.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 7
