Composition and Properties of Superphosphate Effect of Degree of Acidulation on the Curing Processes H. L. MARSHALL AND W. L. HILL Bureau of Agricultural Chemistry and Engineering, U. S. Department of Agriculture, Washington, D. C. Results are given for the distribution of phosphorus, fluorine, and RzO, elements among water-soluble, citrate-soluble, and citrate-insoluble compounds, as affected by the degree of acidulation and by the age and amount of liquid phase, in thirty-one superphosphates prepared from various types of phosphatic material with widely different acidulations. Superphosphates are classified on the basis of resultant net acidulation and the free acid-water ratio
T
HIS paper presents the results of a study of thirty-one superphosphates. The materials are compared and classified in accordance with the general methods developed in a previous article (9). The discussion is concerned primarily with the relation of composition t o the changes that occur in superphosphate, and the wide variety of superphosphates that have been studied in detail gives the work a general application. Detailed data relating to the amount and distribution of water in superphosphate (the factor that to a great extent determines the physical characteristics of the product) will appear in a later article. The data and discussion apply only to superphosphates that have not been mixed with other materials subsequent to their manufacture. Materials and Methods The composition and acidulation values of the superphosphates are given in Table I. The materials include both ordinary and double superphosphates comprised in three experimental series, two of which were conducted in this bureau. The other series, concerned only with double superphosphate prepared in the laboratory, is described by Newton and Copson (11). The first series, consisting of ordinary superphosphate prepared in the laboratory by a method described by Hill and Hendricks (6) and commercially produced ordinary and double superphosphates, was studied several years ago. Only a part of the data was published at that time (6). The second series, studied recently, includes sets of laboratory preparations in which the acidulation was varied and one commercially prepared ordinary superphosphate. The methods of producing the laboratory pre arations of the two series differ only in minor details. In the fatter series the batch was stirred until the tem erature of the mixture reached its peak value (about 3 minutes7 and was then cured 5 hours in
into four types, only one of which contains dicalcium phosphate. Alterations in the distribution of phosphorus and R203as the superphosphate cures arise from (a) dissolution of residual raw phosphate, ( b ) deposition of phosphorus from the liquid phase either as monocalcium phosphate, as dicalcium phosphate, or, when Rz03 elements are present, as water-insoluble complex calcium Rz03 phosphates, and ( c ) diminution in the quantity of solution. an electrically heated oven a t this peak temperature and spread out t o cool and dry overnight; in the older experiments the mixture was stirred as long as possible, the batch was cured 5 hours on the steam bath, and the roduct was cooled by allowing it to stand overnight in a coverefbeaker. In both cases the product was put throu h a 20-mesh sieve and stored in closed containers at 20' t o 30' The acidulation characteristics of the raw phosphates are given in Table 11. The concentration of the ingredient sulfuric acid was GG.63 per cent (53" Be.), The concentration of the sulfuric acid used in the manufacture of the commercially prepared material probably ranged from 68.1 t o 71.2 per cent (54" to 56OB6.). Extensive analyses were made on the products at the ages of 1, 10, 30, and 90 days. The water and ammonium citrate extracts of the superphosphates were repared as specified in the official method (current at the time? for determining available phosphorus in acidulated materials. Accordingly, 2 grams of sam le and a 30-minute digestion in the citrate solution ( 1 ) was useain the first (2-30) series (indicated in the figures by shaded symbols), whereas 1 gram of sample and a 60-minute digestion in the citrate solution (2) was used in the second (1-60) and the third (TVA) series. In order to obtain sufficiently large aliquots of the sample for analysis in the 1-60 series, extracts of two samples were combined and made up to volume. Citrate-soluble aluminum, iron, and fluorine were determined in the citrate extract. The first two were determined by evaporating an aliquot, burning o f f the citrate by gradually raising the temperature to 500" C., redissolving in hydrochloric acid, and then proceeding in the usual manner; fluorine was determined by distillation with phosphoric acid and titration with thorium nitrate. Free acids and free water were determined by ether extraction (5, 7 ) . Total phosphorus, sulfur, calcium, aluminum, and iron were determined by methods previously outlined ( 6 ) . Total fluorine and citrate-insoluble phosphorus and calcium were determined by standard procedures. In the case of the laboratory preparations of double superphosphate, the free acid was determined (11 ) by extraction with acetone, and the result represents the total free acids in terms of the HsP04 equivalent.
8.
Extensive analytical data are not given in this paper; only the analyses for total constituents a t one age and the acidulation coefficients are listed (Table I). The results are pre1224
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INDUSTRIAL AND ENGINEERING CHEMISTRY
1225
TABLEI. DESCRIPTION AND ACIDULATION COEFFICIESTSOF SUPERPHOSPHATES Series4
No.
Description of Superphosphate Source P2OS
7c 1-60
2-30 1-60 2-30 1-60 1-60 2-30
A
F %
AlzOs
Fez0d
V
U
Fluorine Correction, UF'
Resultant Acidulation, U UF'
%
%
n
R
%
% 92.0 98.4 105.8 113.0 119.7 11s 105 117 83 101
Imposed A'cidulat@ /O
Ordinary Superphosphate (Age 9 0 Days) Prepared in Laboratory 22. :34 2.14 0 66 0.44 85.6 21.39 1.96 0.64 0.42 90.9 20.06 1.94 0.56 0.40 96.2 19.41 1.91 0.57 0.39 101.5 19.56 1.87 0.54 0.40 106.9 18.97 1.72 0.56 0.38 106.3 20.41 2.01 0.69 1.50 95.6 18.48 1.58 0.63 1.35 105.5 21.37 2.10 0.77 0.58 78.6 19.62 1.84 0.71 0.53 93.0
1 2 3 4 5 ES6 6 ES7 7 8
Fla. land pebble
81.3 88 2 95 0 IO? 0 109 0 108 0 94 0 107.6 71.9 90.8
10 7
ES8 ES9 ES5 ESlO
N a u r u Island rock 20.74 1 34 c 0.15 100.0 100.0 N a u r u Island rock Si02 20 10 1 30 C 0.15 100.0 100.0 Bone ash 22 07 , or more conveniently for the present purpose by the w;)]in the concentration of the aqueous H8POd [ p > / ( p ; liquid phase. AMOUNT OF SOLUTION.The variations in amount of solution phase with age and acidulation of Florida land-pebble superphosphate prepared in the laboratory under uniform conditions is shown by the curves in Figure 7a. In this set of materials the effect of age is more pronounced a t the intermediate acidulations. At the higher acidulations the effect is less because of the near completeness of conversion of raw phosphate, whereas a t the lowest acidulation it is diminished as a result of the slow rate of conversion of raw phosphate in the presence of only a small amount of solution. The extension of the curves (Figure 7a) towards still lower acidulations would seem to indicate that conversion would practically cease, if the amount of solution should fall to about 2 per cent of the superphosphate. This conclusion is supported (a) by the high conversion observed in the material a t 84 per cent acidulation (Figure 5 ) , which contained a relatively large amount of liquid phase (Figure 7 a ) , in comparison with that observed in the authors' laboratory preparation a t 92 per cent acidulation (Figure l),which carried a much smaller amount of solution (Figure 7 a ) , and (b) by the results for the laboratory preparations of double superphosphate (Figures 2 and 7 a ) . On the other hand, as indicated by the results for double superphosphate that was stored in a sealed can, the mere presence of 8 to 10 per cent of solution does not necessarily mean a high conversion of raw phosphate.
VOL. 32, NO. 9
+
+
is
+$
55
I 55
I
60 , per cent
70
I
55
I
1 b
45
I
I
5 35
c
wC
z
0
-2 25
L.
5
25
I
1
40 50 Concenfrofron of Aqueous H,PO~,
20
8& 20 9 h
30
+f
I
FIGURE8.
CONCENTRATION OF FLUORINE IN SOLUTION PHASEOF ORDINARYSUPERPHOSPHATE Materials are identified by the superphosphate numbers given in Table I.
a k
%
s
L I i 45
O 45
lo
&i
Q
5" K s 3 %
*
o
I 1 b. Concenfrafjon I
I
I
75
85
4
\
l 3
I
I
I
~~~~
95
I05
//5
/z5
Resultant Acidulation, ,u +,uF:per cent
FIGURE 7. AQUEOUS PHOSPHORIC ACID IN SOLUTION PHASE OF SUPERPHOSPHATE
CONCENTRATION OF SOLUTION.The effect of the age of ordinary superphosphate on its relative amounts of free acid and water (Figure 7b) shows wide variation a t the lower acidulations. Both the free acid and free water decrease, and the resultant alteration in the concentration depends upon the relative rates a t which these two quantities change. Although the laboratory preparations of ordinary superphosphate, as well as the commerical preparations that were cured in the laboratory, did not lose water to the atmosphere during storage, the quantity of water volatilized during the preparation of the laboratory superphosphates (curves) increased markedly as the acidulation decreased. Had this excessive loss by volatilization not occurred, the lower part of the 90day curve would presumably have taken the form indicated by the dotted line (Figure 7 b ) . The laboratory preparations of double superphosphate, which were stored in light paper bags ( I I ) , lost water to the atmosphere, and consequently the concentration of acid in the solution phase increased with the period of storage (Figure 7b). FLUORINEAND R2O3CONTENTS.The concentration of fluorine in the solution phase of three classes of superphosphate a t different ages is shown in Figure 8. These results
SEPTEMBER, 1940 1001,
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INDUSTRIAL AND ENGINEERING CHEMISTRY I
class
I
I
I
I
1231
Accordingly, the direction of the observed change in any given instance would be determined by the process that takes place most rapidly.
Citrate-Insoluble Phosphorus as a Measure of Residual Raw Phosphate
As a measure of the residual raw phosphate in superphosphate, the observed citrate-insoluble phosphorus is subject t o generally recognized errors arising from (a) more or less dissolution of the raw-phosphate constituent during the preparation of the water and citrate extracts and perhaps (b) in rare instances the presence of other phosphorus compounds, formed either in the course of manufacture and storage of the superphosphate or during the preparation of the extracts, that are incompletely dissolved by the citrate reagent. Apart from the first-mentioned analytical uncertainty, the latter error as manifested by a n increase in citrate-insoluble phosphorus with the age of the superphosphate, is observable only if i t exceeds the simultaneous decrease in the amount of residual raw phosphate. Accordingly, superphosphates of class I would afford the better chance to observe such a phenomenon. Kone of the materials considered here showed a n increase in citrate-insoluble phosphorus with the age of the superphosphate (Figures 1, 2, and 5 ) , although increases did, for no readily apparent reason, occur in certain double superphosphates studied by Newton and Copson (11). Shisji (16) found that mixtures containing cured ordinary superphosphate and amounts of aluminum and iron oxides up to 10 per cent showed no change in citrate-insoluble phosphorus during storage for 60 days at 25" C., and that similar mixtures stored a t 60' C. showed only small increases in citrate-insoluble phosphorus. Concentration of Aqueous H3P04, &+
per cent
FIGURE9. CONCENTRATION OF RIOs ELEMENTS IN SUPERPHOSPHATE SOLUTION
Curing Processes THE
Materials are identified by the superphosphate numbers given in Table I; dotted curve connects the points corresponding to the age of 30 days. indicate little connection between the fluorine content and the concentration of free phosphoric acid in the solution phase. I n Figure 9 the water-soluble & 0 3 is assumed to be a part of the liquid phase of superphosphate. Accordingly, the Reo3 content of the liquid phase in class I materials (Figure 9) does not show a consistent trend with time or with concentration of acid. A connection between the water-soluble Rz03and the composition of the liquid phase, which, as mentioned above, is suggested by the similarity of the curves in Figures 6 and 7b, is not exhibited with any consistency by the materials of class I. On the other hand, the curves for superphosphates of class I1 (Figure 9) show a greater regularity. content either increased with age or decreased only The Rz03 slightly. Furthermore, the cured materials (30 days old) fall along a smooth curve (dotted curve) that corresponds with a n increase in R& content with the acid concentration. The irregularity in the direction of the changes in the duorine and Rz03 concentrations (Figures 8 and 9) is such as might be expected to arise from two simultaneous opposing processes. The one, most readily observed in class I materials, tends toward a reduction in the amount of the constituent in the solution phase by the separation of solid salts. The other, observable only in materials that contain raw phosphate, tends toward a n increase in concentration of the constituent as a result of continued dissolution of raw phosphate.
The reactions initiated during the mixing of the ingredients tend to proceed to completion as the mixture ages. The superphosphate is thus said to undergo curing. The reactions whereby the raw phosphate and other constituents of the rock dissolve in the liquid phase constitute the primary process, whereas the separation of salts from the liquid phase are to be regarded as secondary processes. The primary process feeds the constituents of the rock into the liquid phase, from which they subsequently separate in a form more or less consistent with the equilibrium requirements a t the moment precipitation occurs. Apart from such factors as fineness of the ingredient rock and the reactivity of its phosphate constituents, as well as surface coatings formed in the superphosphate, the rate and extent of the primary process is influenced by the temperature and within limits by the composition and quantity of liquid phase. I n the presence of a relatively large quantity of solution, such as is obtained a t high acidulations (Figure 7a),this process may go to virtual completion within a few days, in which case the physical characteristics of the product are poor as a consequence of the large amount of liquid that can be diminished appreciably only by a reduction in the free water. I n the case of class IB superphosphates, drying concentrates the liquid phase with respect to free phosphoric acid and thereby increases the hygroscopicity of the material, with the result that any improvement in mechanical condition gained by drying such a material is more or less temporary. At low acidulations, on the other hand, the liquid phase tends to disappear as a result of the continuous diminution in the amounts of both free water and free phosphoric acid-the one largely by volatilization, the other by reaction with raw phosphate. In the event of unchecked loss of water from the curing superphosphate, the amount of liquid phase
INDUSTRIAL AND ENGINEERING CHEMISTRY
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may soon become too small (Figure 7a) to support the primary process a t a significant rate; in this case the final product, although its mechanical condition may be excellent, contains a n excessive amount of raw phosphate (Figures 1, 2, and 5). Industry has for the most part compromised by adopting intermediate acidulations (Figure 5 ) . The conditions for the precipitation of phosphate salts from solution are such that the amount of monobasic phosphates increases in curing superphosphates of classes I and IIAthat is, in commercial superphosphate as it is commmonly prepared at present. The monocalcium phosphate content of materials in these classes may increase (Figures 1 and 5) or decrease (Figure 2), depending presumably upon the extent (limited by the Rz03content) and the rate of formation of water-insoluble complex monobasic phosphates. Unless the quantity of Rz03elements in the superphosphate exceeds the saturation limit of the solution phase, the deposition of compounds of these elements, and hence citrate-soluble Rz03 phosphate, will not occur. I n superphosphate belonging to class IIB, dicalcium phosphate instead of monocalcium phosphate separates from the liquid phase. This class of material, rarely met in practice, is obtained by the retention of the free water, so that as free acid is consumed, with the accompanying separation of monocalcium phosphate, the composition of the liquid phase eventually becomes favorable to the separation of dicalcium phosphate. From this point a continuance of the primary process to complete conversion necessitates the decomposition of some monocalcium phosphate formed in the first stage of curing in order to supply the requisite free acid for the conversion of the residual raw phosphate. Since the concentration of free acid in the liquid phase a t the critical point, where dicalcium phosphate formation becomes possible, increases markedly with the temperature (9),an increase in the temperature of the curing superphosphate should favor conversion. Meager data (not given in this paper) obtained by the authors on preparations a t about 70 per cent acidulation promise limited success in the attainment of satisfactory conversion at very low acidulations. The state of curing observed in samples of one superphosphate that were stored a t different temperatures for 90 days is shown in Table V. The lower R203content of the liquid phase in the material a t the higher temperature is in accord with the observations noted by Sanfourche and Focet ( I S ) in connection with their preparation of complex calcium iron phosphates, and the smaller proportion of monocalcium phosphate a t the higher temperature is in agreement with common experience in the superphosphate industry.
Best Acidulation From a purely theoretical viewpoint the ideal acidulation value for superphosphate that is to be marketed in countries where the presence of citrate-soluble phosphates is not a detriment to the sale of the material would be obtained by proportioning the acid and rock so as to make the resultant acidulation (u up#)a few units higher than 50 per cent. From a practical point of view, however, and apart from any subsequent specialized processing to which the material may be subjected, the most favorable acidulation corresponds with the proportion of acid and rock that yields a product in good physical condition with the attainment of the highest conversion, consistent with the economy of the plant, of the phosphorus to available forms in the cured product. Much of the history of superphosphate manufacture is concerned with the evolution of methods for acid economy by a n ever closer approach to the ideal acidulation. I n the commercial superphosphate of today the resultant acidulation is usually greater than 100 per cent, although lower acidulation is successfully practiced a t some plants
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TABLEv. EFFECTOF TEMPERATURE ON THE CURING ORDINARYSUPERPHOSPHATE^ (Age 90 Days) Proportion of PzOs Present Jlonocalcium phosFree phate acid
Total as:
OF
Liquid Phase
Fez08 Free Concn. equivalent of Freeacid of Storage free aqueous acid water-sol. Temp. water acid fhorinea RzOsb 70 of superc. % % % phosphate % M Q . p e r gram 20-30 56.6 12.0 6.8 10.5 29.5 5 71 14.2 6.2 12.0 30.7 3 42 60-70 51.5 a Commercially mixed material (No. 1423, Table I) was aged i n t h e laboratory (6). b Expressed in milligrams per gram of aqueous phosphoric acid i n liquid phase (preceding column). Raw phosphate
+
(Figure 5 ) . To a limited extent a further lowering of the acidulation with the attainment of satisfactory conversion would seem possible and perhaps also practical. I n view of previous discussion, progress in this direction, if i t is attainable at all, will be realized by the retention of sufficient free water to provide an adequate amount of liquid phase for the ensuing reactions, and by the maintenance of the temperature high enough to permit the utilization of the potential acidity of the monocalcium phosphate, made available by the decomposition of this monobasic phosphate into dicalcium phosphate and free phosphoric acid, for the conversion of the residual raw phosphate. On account of the relatively high free-water content, artificial drying of the product would doubtless be necessary and could be accomplished conveniently as a step in the manufacture of granulated superphosphate.
Nomenclature = fluorine content of the superphosphate p = phosphorus content (P,Os) of phosphate rock p ‘ / = free HsPO, in superphosphate R = dilution factor for phosphate rock u = net imposed acidulation (disregarding presence of fluorine) UP’ = correction for net acidulation effect of fluorine
F’
+
v
= net resultant acidulation (u u p ) at which monocalcium phosphate fraction of p reaches (with increasing u) its highest value in equilibrium system = gross imposed acidulation (disregarding presence of
vo
=
usOs
=
u
U,
UP’
=
+
fluorine) fraction of ingredient acid required for 1 0 0 ~ acidulation o that is consumed by metallic constituents in excess of tricalcium phosphate proportions gross acidulation effect of sulfate content of phosphate rock
= free water in superphosphate
Literature Cited Assoc. Official Agr. Chem., Methods of Analysis, 3rd ed., pp. 161 8 (1 w m . -\----I.
Ibid., 4th ed., pp. 21-3 (1935). Austin, W. R., IND.ENG.CHEM.,15, 1037-8 (1923). Hill, W. L., and Beeson, K. C., J . Atsoc. Oflciat A g r . Chem., 18, 244-60 (1935). Ibid., 19, 328-38 (1936). Hill, W. L., and Hendricks, S. B., IWD. EXQ.CHEM.,28, 440-7 (1936). Hill, W. L., and Jacob, K. D., J . Astroc. Oi9ickzZ Agr. Chem., 17, 487-505 (1934). Jacob, K. D., and others, U. S. Dept. Agr., Tech. Bull. 364 (1933). Marshall, H. L., and Hill, W. L., IND. ENQ.CHEM.,32, 1128 (1840). Marshall, H. L., et al, IND.ENG.CHEM.,25, 1253-9 (1933). Newton, R. H., and Copson, R. L., Ibid., 28, 1182-6 (1936). Sanfourche, A.. Bull. SOC. chim., [4] 53, 1580-94 (1933). Sanfourche, A.. and Focet, B., Ibid., [4] 53, 1517-22 (1933). Shaji, T., and others, J . SOC.Chem. I n d . , Japan, 35,Suppl. binding 130-4 (1932). Shaji, T., and Susuki, E., Ibid., 35, Suppl. binding 417-21 (1932).
