INDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1944 (3)
Caw, H. H., and Beckman, A. O., J . Optical SOC.Am., 31, 692
(14)
(1941).
Polg&r,A., and Zechmeiater, L., J. Am. @hem. Boc., 64, 1856 (1942).
Denny, F. E., Contrib. Boyce Thompson Znst., 12, 309 (1942). Dutton, H. J., Bailey, G . F., and Kohake, Eleanor, IND. ENG. CHEM., 35, 1173 (1943). (6) Frapa, G. S., and Kernrnerer, A. R., Assoc. Oficial Agr. Chem.,
Reeve, R. M., Food Industries, 14, No. 12, 51 (1942). Reeve, R. M., Food Research, 8, 137 (1943). Silker, R. E., Schrenk, W. G . , and King, H. H., IND.ENO. CHmM., ANAL. ED., 16, 513 (1944). (18) Wall, M. E., and Kelley, E. G., Ibid., 15, 18 (1943). (19) Williams, K. T., Bickoff, Emanuel, and Van Sandt, Walter, J . Biol. C h m . , 91, 105 (1931). (20) Wiseman, H. C., Kane, E. A,, Shinn, L. A., and Cary. C. A.. J. Agr. Research, 57, 635 (1938).
(4) (5)
(15) (16) (17)
22, 190 (1939). (7)
835
Kemmerer, A. R., and Frapa, G. S., IND.ENG.CHEY.,ANAL.
ED., 15, 714 (1943). (8) Kernohan, George, Science, 90,623 (1939). (9) Moore, L. A., IND.ENO.CHEY.,ANAL.ED., 12, 726 (1940). (10) Moore, L. A., and Ely, Ray, Zbid., 13, 600 (1941). (11) Mornal, P. W., Byers, L. W., and Miller, E. J., IND.ENO. CHEM.,35, 794 (1943). (12) Pepkowits, L.P., J . Biol. Chem., 149, 465 (1943). (13) Peterson, W. J., Hughes, J. S.,and Freeman, H. F., IND.ENG. CHEM., ANAL.ED.,9, 71 (1937).
P R ~ ~ E X Tbefore E D the Divisions of Biological and of Agricultural and Food CHEMICAL SOCIQTY, Chemistry at the 107th Meeting of the AMBRICAN Cleveland, Ohio. Contribution 286, Department of Chemistry, Kansas State College. This work is supported by the Kansas Industrial Development Commission
CALCIUM METAPHOSPHATE Effect of Impurities on Fusibility, Citrate Solubility, and Hygroscopicity 4
f
G. L. FREAR, E. F. DEESE', AND J. W. LEFFORGE2 Tennessee Valley Authority, Wilson Dam, Ala.
ERTILIZER-grade calcium metaphosphate is produced by the Tennessee Valley Authority in full-scale units by burning elemental phosphorus with a moderate excess of air and bringing the hot products of combustion into contact with rock phosphate. The high-temperature reaction of the P p 0 6 and rock phosphate yields molten calcium metaphosphate which collects in the bottom of the furnace. After tappiag and chilling, the product is obtained in the form of vitreous lumps. The composition of the product depends upon the composition of the rock phosphate used and upon the conditions within the furnace. A typical analysis is: 66.3% P z O ~25.2% , CaO, 4.5% Sios, 2.1% FeOa, 1.8% AlgOs, and 0.4% F; practically the entire PSOScontent is soluble in neutral ammonium citrate. Numerous pot tests and field tests by state agricultural experiment stations have demonstrated that vitreous calcium metaphosphate is a valuable phosphatic fertilizer on most soils. An account of the early developments in the manufacture of calcium metaphosphate by TVA, a description of a full-scale calcium metaphosphate furnace, and a report of some of the solubility characteristics of the product have been published (6, 6, IO). Later publications presented further developments in processes for producing metaphosphates and a study of the rate of reaction of phosphorus pentoxide with rock phosphate (4,7). The present paper gives results of more detailed laboratory studies of some: of the properties of calcium metaphosphate that relate to its manufacture and use as a fertilizer.
F
FUSIBILITY
When gases containing phosphorus pentoxide are brought into contact with rock phosphate in a high-temperature furnace, the absorption of PzO6 proceeds on the exterior portion of the rock until a composition is reached which fuses at the prevailing temperature. The molten material flows toward the bottom of the furnace; it may absorb more Pzo5 on its way. The minimum Present address, School of Medicine, Emory University, Atlanta, Ga. Present address, Tennessee Vrtlley Authority, Godwin Plant, Columbia, Tenn 1
2
Fertilizer-grade calcium metaphosphate, which is n o w produced in full-scale units at the TVA Fertilizer Works by reacting PsO6 with rock phosphate at high temperatures, has approximately the following composition: 66.3% P206,25.2% CaO, 4.5% SiOz, 1.8% AlnOs, 2.1% Fez08 0.4% F. The product is vitreous unless it contains an excess of CaO in which case it is partly crystalline, presumably as a result of the formation of some CazPZO,. When the product carries an excess of P z O ~it , is somewhat hygroscopic, and in practice ground limestone is added to prevent caking. Laboratory studies are reported on changes in fusibility, citrate solubility, hygroscopicity, and tendency to crystallize when additions of PzO6, CaO, SiOz, AlzOt, and Fa08 are made to pure calcium metaphosphate.
Pa06content that confers fusibility and, consequently, the composition of the product depend upon the temperature of the furnace. To establish fusibility-cornpositon relations useful in the manufacture of calcium metaphosphate, the fusion temperature of compositions in the binary system CaO-P206 was determined over the range from 17.5% Ca0-82.5% P z O ~to 49% Ca0-51% Pz06, or in terms of mole fractions, from 0.35 CaO0.65 P20sto 0.71 Ca0-0.29 P206. A few determinations were made to ascertain the fusibility of compositions obtained by adding Si02 to calcium metaphosphate.
MATERIALS. The calcium metaphosphate used in melting point determinations was generally prepared by fusing in platinum crucibles the dehydration product of monocalcium phosphate monohydrate that had been made from Iceland spar (56.0y0 CaO) and reagent-grade phosphoric acid, and that had been recrystallized from the phosphoric acid solution. Other CaO-P?OS Compositions were obtained by fusion of resublimed P z O ~with calcium metaphosphate or Iceland spar. The compositions containing Si02 were obtained by fusing rock crystal (100.0% SiOg)
INDUSTRIAL AND ENGINEERING CHEMISTRY
836
I
COO,
Figure 1.
miwr
410.4' C., sodium chloride Sol", gold 1063", and diopside 1391'. The samples were usually held in the furnace 15 to 60 minutes before ~quenching. Owing to the possibility of volatilizing Pz06, long exposure of Pz06-rich compositions to high temperatures wae avoided.
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40
Vol. 36. No. 9
45
50
PER CENT
Temperature of Complete Fusion of Pure CaO-Pz06 Compositions
with a CaO-PzOs composition that had a ratio of PzOa to CaO slightly greater than that sought in the product, since some PzO, was lost by volatilization while making the product homogeneous. By observing the weight loss as heating progressed, it was possible to stop the heating when the desired composition had been attained. Prolonged heating was necessary since each composition was alternately fused and ground until microscopic examination indicated that a homogeneous glass had been obtained. Each glass was analyzed in duplicate: the P z O was ~ determined gravimetrically as magnesium pyrophosphate, the CaO was determined by permanganate titration of the sulfuric acid solutions of the calcium oxalate precipitates, and the Si02 was determined by dehydrating with fuming perchloric acid and weighing the residue as SiOz. Prior to the fusibility measurements, most of the compositions were crystallized by prolonged heating at temperatures slightly below the fusion range. Observations of the weight loss indicated that substantially no PzOa was volatilized during crystallization. MEASUREMEKTS. The liquidus temperatures, or points of complete fusion, were determined in all cases by the method of quenching. Charges of a few milligrams of each composition were suspended in platinum foil envelopes in a constant-temperature furnace and then quenched in mercury. The lowest temperature at which the quenched samples, according to microscopic examination, were vitreous was regarded as the point of complete fusion. The temperature of the specimen was measured by a platinum to platinum-rhodium (10% Rh) thermocouple mounted with its junction close to the platinum envelope and connected to a Leeds & Xorthrup Type K potentiometer. The thermocouples were compared frequently with a standard thermocouple calibrated a t the melting points of the following substances: zinc
The results of fusion temperature determinations in the system CaO-PZOb are shown in Figure 1. The curve represents the points of complete fusion in the composition range studied. The liquidus data, except for the compositions that contained more than 459/, CaO, have been revised t o conform with the recent measurements of Hill, Faust, and Reynolds (8), whose results were kindly made available to the authors prior to publication. The two sets of determinations of liquidus temperature generally differed by less than 10" C. except for the compositions that contained more than 77% PzOa ( A , Figure l),as in the determination of the Ca0.2PzOa liquidus where the exclusion of moisture is important. Information concerning the solid-phase equilibria is given by Hill, Faust, and Reynolds (8). Extending from 1720" C., the melting point of tricalcium phosphate ( I I ) , through the portion of the C~O-PZOF,system shown in Figure I, there is a general decrease in the fusion temperature with increasing PzO6 content, except in comparatively small intervals between each eutectic and the pure compound of next higher P~06content. I n the temperature range 1292" to 984" C., which includes the temperatures most suitable for the formation of metaphosphate, the liquid compositions formed by reaction of phosphorus pentoxide with the solid calcium phosphates do nol become solid upon further increase in P z O contents. ~ Thus, in a calcium metaphosphate furnace the liquid formed on the surface of the rock phosphate can, in flowing to the hearth, absorb additional Pz06without danger of solidification, provided its temperature is maintained. The fusion diagram indicates that, if phosphorus pentoxide reacts with tricalcium phosphate a t temperatures above 1200" C. ( B , Figure I), products that contain less than 64YGP ~ O (SP z O S ~ CaO mole ratio 0.70) may be obtained. These composition., are FIGURE 2 TEMPERATURE OF COMPLETE FUSION
A
68
0-LIQUIOUS POINT IN THE SYSTEM C a O - P z O r S r O 2 8 - FERTILIZER- GRADE CALCIUM1 METAPHOSPHATE
TABLEI. FUSIBILITY OF FERTILIZER-GRADE CALCIVMMETAPHOSPHATES~ Sample No.
A B C a
,-
Pi06
CaO
Si02
65.61 65.62 63.41
22.35 24.87 25.93
5.35 4.04 4.27
Composition, Weight yo Fez08 MnO Kz0
AlzOa 2.59 2.44 2.72
2.05 1.57 1.88
0.30 0.12
0.10
0.07 0.04 0.09
NaiO
0.28 0.20 0.35
F 0.37 0.39 0.52
Total 98.97 99.29 99.27
Refractive Liquidus Index of TEmp., Glass
c.
1.542 1.546 1.546
900 958 1021
Data supplied by oourtesy of U. S. Department of Agriculture, Bureau of Plant Industry. Soils, and Agricultural Engineering.
Primary Crystal Probably 8-CaO. Pa01 Probably 5-CaO. Pa05
.................
September, 1944
INDUSTRIAL AND ENGINEERING CHEMISTRY
undesirable since, upon cooling, they readily form crystalline calcium pyrophosphate which is not citrate soluble. Calcium pyrophosphate appeared in the product of a semiworks-scale furnace if the temperature was much above 1200" C. (6). At high temperatures equilibrium between the vapor pressure of phosphorus pentoxide from the product and the partial pressure of phosphorus pentoxide in the gas phase also may limit the PZO5content of the product. The vapor pressure equilibrium was not qtudied in the present work. Below 1200" C. there is a considerable range of temperature within which fusions of approximately the composition of calcium metaphosphate may be obtained. If the temperature is too !ow, however, the proportion of Pzo5 may be excessive and the resultant product hygroscopic. Thus, a t 950' C. in the system CaO-PZ06 (C, Figure 1)only compositions that contain more than 74.5% Pzo5 (Pz05/CaO mole ratio 1.15) are completely fused. Previous publications (6, 7 ) indicated that temperatures of at least l l O O o C. are desirable in a calcium metaphosphate furnace, h c e the increased viscosities of the products at lower temperatures are conducive to excessive P z O ~contents. On the other hand, the high viscosity of calcium metaphosphate fusions a t the temperature of melting is advantageous for, upon cooling, these compositions crystallize so slowly that devitrification and consequent loss in citrate solubility rarely occur in large-scale operation. The influence of the impurities SiOz, Fe203, and AlzOa upon the fusibility of calcium metaphosphate was not studied extensively. The liquidus temperatures of three samples of fertilizer-grade calcium metaphosphate, whose compositions are given in Table I, and the liquidus temperatures of a few CaO-PzO5-Si02 composiLions are shown in Figure 2. The data for the CaO-Pz05-Si02 compositions indicate that the temperature of complete fusion of calcium metaphosphate is lowered between 30" and 40" C. by the addition of 5% SiOz. Little further change in fusibility was observed as the silica content was increased up to 19.5%; at that composition, evidence of a second isotropic phase with a refractive index of 1.46 was noted. Occurrence a t 1400" C. of a second liquid phase with this refractive index a t approximately this composition was reported recently by Barrett and McCaud=y ( 2 ) Data not included in this paper indicate that the presence of Alz03 raises the fusion temperature of compositions approximating calcium metaphosphate, and that the presence of Si02 counteracts the effect of Al208. The effect of Fez03 upon the fusion temperature appeared to be less than that of AlzO3. CITRATE SOLUBI LlTY
The development of a new fertilizer material would be extremely slow if the availability of the material as plant food were gaged solely by plant growth tests. Of the chemical tests of availability of phosphatic fertilizers that are not readily soluble in water, the solubility of PzOs in neutral ammonium citrate seemed more appropriate for calcium metaphosphate than the solubility in citric acid; the ammonium citrate solubility was known to be applicable to the water-insoluble fraction of concentrated superphosphate, whereas the citric acid method is applicable t o basic slag which contains a much higher proportion of CaO. The choice of the citrate solubility test as a criterion of availability was supported by the following consideration: Whereas the form of calcium metaphosphate obtained by slowly heating monocalcium phosphate t o 600-650" C. was reported t o be both citrate insoluble and ineffective as a phosphate fertilizer ( 3 ) ,vitreous calcium metaphosphate, although largely insoluble in 2% citric acid, was found to be citrate soluble and effective in supplying PZOS to plants on neutral or acid soils (9). The composition of fertilizer-grade calcium metaphosphate varies with the composition of the rock phosphate used in its manufacture and with operating conditions in the furnace, especially temperature. Information was desirable therefore concerning
837
MOLE RATIO Rz%/CmO
Figure 3. Citrate Solubility of CaOPzOl Glasses as Affected by Incorporation of AI203 and Fez03
the influence of variations in proportions of constituents upon the citrate solubility of the product. T o systematize the results, the effect of variations in the proportions of the principal ingredients in crude calcium metaphosphate upon its solubility were studied by fusing pure calcium metaphosphate with different proportions of added CaO, Pz06, SiOl, AlzOa, and FezOa. The effect of crystallization upon the citrate solubility of several compositions also was determined. I n preparing the compositions for solubility studies, reagentgrade chemicals were used. CaO, SiOz, Fe203,and A1203generally were added to pure calcium metaphosphate in the proportions desired in the final product; P20jwas used in excess of the desired proportion, and the final proportion was approached by following the weight loss during prolonged heating in platinum crucibles a t 1200" C. or higher, as explained in the section on fusibility. T;Then the necessary quantity of PzOa had been volatilized, the crucible was withdrawn from the furnace and air-cooled to room temperature without removing the contents. I n this manner there were obtained in homogeneous glassy form about 8 grams of each composition in three series having Pz05/Ca0mole ratios of 0.8, 1.0, and 1.2. Some attempts were made to prepare the compositions with the PzOb/CaO mole ratio of 0.8 from calcium metaphosphate without adding CaO. The volatilization of so much PZOsrequired exceptionally long heating, and some of the charge crept over the edge of the crucible. Since the material outside the crucible lost Pz05 rapidly, calculation of the composition of the crucible contents from the weight loss was inexact. The PzOj/CaO mole ratio in the latter preparations varied in the range 0.8 to 0.95. The compositions of most of the other preparations, with indicated exceptions, were calculated from the initial weight of the ingredients and the weight loss. Analytical determinations of P,Os and CaO in representative
Vol. 36, No. 9
INDUSTRIAL AND ENGINEERING CHEMISTRY
838
CaO mole ratios of 0.8 t o 0.95, 1.0, and 1.2 are shown in Figure 3. TABLE 11. CITRATESOLUBILITY OF P205IN VITREOUS COMPOSI- Alumina was fused with the P206-Ca0 glasses without depressing TIONS OF P205, CaO, Si02, Fe8O3, AND AlaOI the solubility, provided the proportion did not exceed 0.03 to 0.04 ------Composition ' yo Total mole AlzOaper mole CaO. Further increase in the proportion Mole Ratio P20s of A 1 2 0 a caused the citrate solubility of Pa05 to decrease in the Weight % ' PgOa/ AlnOa/ FezOa/ 8i02/ CitratePa05 CaO Alp08 FeOa Si02 CaO CaO CsO CaO Sol. proportion 2 to 3 moles of citrate-insoluble P 2 0 6 per mole of ad0 . 8 0 0 0 67 33 0 0 0 100 ditional AlnOs. Substitution of one third or one half the A&Oa 32 4.3 0 0 89 0.8 0.075 0 0 64
-
._
63
31
3.4
2.6
0
62
ai
4.6
0
2.6
72 70 69
0.8
0.06
0.8 0.08 0.8-0.95 0 0.8-0.95 0.04 0.8-0.95 0.075 0.8-0.95 0.10 0 . S O .95 0.13 0.8--0.950.15 0.8-0.95 0.06 0.8-0.95 0.09 0.8-0.95 0 . 0 8 0.8-0.95 0.08 0.8-0,95 0.08 0.8-0.95 0.08
0
0
0 0
0.08 0 0
0
0
0 0.03 0.045 0 0 0 0
0 0 0 0.04 0.08 0.16 0.24
96 100 97 87 81 71 67 83 70 94 97 98 98
0 0 0 0 0 0 0.04 0.06
0 0 0 0 0 0 0 0
100 99 93 85 70 62 75 98
0 0
0 0
65
28 28 27 27 26 26 27 26
0 2.0 3.7 4.9 7.2 9.1 3.9 5.6
0 0 0 0 0 0 3.0 4.3
1.0
0 0.04 0.075 0.10 0.15 0.20 0 08 0 12
67 66 65 65 63 61
27 26 26 25 25 24
4.8 4.7 4.7 4.6 4.5 4.4
0 0 0 0 0
1.4 2.8 4.1 5.4 7.9 10.0
1.0 1 .o 1.0 1.0 1 .o 1.0
0.10 0.10 0.10 0.10 0.10 0.10
0 0 0 0 0 0
0.05 0.10 0.15 0.20 0.30 0.40
90 94 96 99 100 99
67 66 64 62 61
27 26 25 24 24
2.4 2.3 2.3 2.2 2.2
3.8 0 3.7 2.8 3.6 5.4 3.5 7.8 3.4 10.0
1 *o 1.0 1.0 1.0 1.0
0.05 0.05 0.05 0.05 0.05
0.05 0.05 0.05 0.05 0.05
0 0.10 0.20 0.30 0.4
89 97 99 100 100
75 74 72 71 69 70
25 24 24 23 23 23
0 1.8 4.3 6.3 8.2 4.2
0 0
0 0.04 0.10 0.15 0.20 0.10
0 0 0
0 0.05
0 0 0 0 0 0
100 98
0 3.3
1.2 1.2 1.2 1.2 1.2 1.2
69 68 66 63
23 22 22 21
6.2 6.1 5.9 6.7
0 0 0 0
1.2 1.2 1.2 1.2
0.15 0.15 0.15 0.15
0 0 0 0
0.075 0.15 0.30 0.45
92 94 100 99
69.2" 68.6' 66.1' 65.0' 62.2' 88.6"
30.0 28.2 26.8 26.0 25.4 23.3
0
0 0 0 0 6.6 0
0.78 0.96 0.98 0.99 0.97 1.16
0 0 0 0 0
8.13 0.04 0.09 0.13 0.09 0.13
0 0
0 0 0.24 0
23 97 82 64 95 70
5.1 10.7 13.6 20.1
0.91 0.98 0.96 0.95
0 0
0 0 0
0
0
0.16 0.39 0.50 0.82
100 100 100 99
68
67 65 66
0 0 0
0 0
66.6" 29.0 0
63.7 25.6 0 f31.1° 25.2 0 55,6' 23.0 0 a
0
0 0
10.8 3.6 6.7 9.4 6.4 8.4 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0
0 1.8 3.6 6.9 10.0
1.0 1.0 1.0 1.0 1.0
83
0.03
1 .o 1 .o
0 0
0
84 69 56 71
by an equimolar quantity of Fe20s did not appreciably affect the citrate solubility of PzOb, but complete replacement of AI2O8by FeaOa appeared to cause further decrease in solubility. The desolubilizing influence of A1208 and Fez08 upon CaO-PsOa glasses was offset largely by incorporating silica into the glass, as shown by Figure 4. Usually 2 to 3 moles of Si02 were required per mole of AlaOs or Fe2Os. Thus, in terms of weight percentage, if more than 2% %os is present, the combined percentages of Al208and Fe20sshould not exceed the percentage of Si02 in the sample. I n the crude calcium nietaphosphate the percentage of Si02 generally exceeds that of RzOS,and the citrate insolubility observed is probably due to variables in manufacture other than the ratio of RaOsto SiO2. The citrate solubility of shbstantially pure vitreous calcium metaphosphate (Table 11) was unaffected by the incorporation in the glass of 14% SiOt, or 0.5 mole of Si02 per mole of CaO. The effect of crystallinity upon citrate solubility is indicated in Table 111. All of the CaO-P205 glasses showed about the same high solubility, whereas the sohbility of the crystdine samples generally was lower and decreased progressively as the P2O6/Ca0 ratio decreased. The depression in solubility due to crystallization is apparent from comparison of fertilizer-grade samples B and C. It is fortunate that calcium metaphosphate, even in a high state of purity, can readily be cooled from the molten state without crystal!ization. This property is in marked contrast to the behavior of the pyrophosphate which tends to dystallise partially, even upon drastic quenching.
Composition determined by analysis.
glasses agreed'with the calculated compositions within 3%. The degree of oxidation of the iron in the products was not determined. Calculations showed, however, that even if all the iron were reduced to the ferrous state, failure to correct the apparent P a 0 6 loss by the resultant loss of oxygen in unanalyzed samples in no case would cause an error of more than 1% in the calculated PnOs content. The agreement between the results of calculations and the analyses was commensurate with the degree of reproducibility of the citrate solubility tests. I n grinding the glasses to -80 mesh, an effort was made to treat all samples uniformly so as to obtain substantially the same size distributions in the composition%. The citrate-insoluble PZOSwas determined by using 1-gram samples of -80 mesh material to 100 ml. of neutral ammonium citrate solution. The citrate digestion was made with inclusion of filter paper pulp. Large groups of citrate solubility determinations were made a t a time t o minimize the possibility of irregularities in the analytical procedure. The results of citrate solubility determinations on the vitreous products are given in Table 11. The influence of.additions of Alaos and FezOs upon the citrate solubility of glasses with Pz06/
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MOLE RATIO S i q / R a O S '
Figure 4. Citrate Solubility of CaO-PzOrR?Oa Glasses as Affected by Incorporation of Si02
INDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1944
TABLE 111.
compn.
CITRATE SOLUBILITY AS AFFECTEDBY CRYSTALLIZATION Mole YoTotal Ratio '!'nos
weight % CaO PaOn 2 0 . 6 79.4 24.7 7 5 . 3 24.7 7 5 . 3
CitratePnoa' CaO Phases Present Sol. 1.62 Crystals: 2Ca0.3PeOs 100 0 1 . 2 0 Glass 100 0 1 . 2 0 Crystals: PCa0.3PzOsB D-CaO. PnOs 82 19 2 6 . 0 74.0 0 1.12 Cryetals: 2Ca0.3PzOr & B-CaO. PZOS 60 0-8 8 8 . 2 71.8 0 1.01 Glass 100 BD 30 70 0 0.9 Crvstals: B-ClrO.Pa06 solid soln. 15 79 R-1 31.8 68.2 0 0 . 8 5 Glass and trdrnelitea 100 0 . 8 0 Glass D-10 a3.0 67.0 0 0 . 6 2 Glass trGmelite, and 8R-2 38.8 61.2 0 2C;O. PeOs solid s o h . 42 7 0 . 4 9 Crystals: 8-2CaO.PeOs R-3 44.6 55.4 0 100 Ab 2 2 . 4 65.6 5 . 4 1 . 1 6 Glass 100 Bb 24.9 65.6 4 . 0 1.04 Glass 83 25.9 6 3 . 4 4 . 3 0 . 9 7 Glass and crystals Cb Trdmelite is a series of solid solutions identified by Hill, Fauat, and Reynolds ( 8 ); i t lies between metaphosphate solid solution and cornpodition 3ct10.2P206. b Fertilizer-grade products. analyaes by courtesy of U. 5. Depwttuent of Agriculture, Bureau of Plant industry, Soils, and .4gricultural Engineering.
No. P-1O1A D-12 D-12
Si02 0
HYGROSCOPICITY
Although lumps of vitreous calcium metaphosphate having a PzOs/CaO mole ratio of 1.0 give little evidence of absorbing moisture or caking upon exposure to the atmosphere, compositions of high P20s/Ca0 ratios are markedly hygroscopic, more so if the product is finely ground. To obtain further information in regard to the influence of composition upon hygroscopicity, comparisons were made of the hygroscopicity of representative composifions that had been prepared for the determinations of citrate solubility. For this purpose 3-gram samples of -80 mesh materials were spread in thin layers on 9-cm. watch glasses. The watch glasses were mounted on racks in 1-gallon cans which were tightly closed with friction tops. The cans were placed in a 30" C. constant-temperature air bath. An atmosphere of 59% relative humidity was maintained by including in each can shallow dishes containing saturated aqueous solutions of ammonium nitrate. The rate of absorption of moisture by the different compositions was observed by noting periodically the increases in weight of the samples during a 45-day period. Comparisons of the weights gained in 9 days were typical of those in other intervals. In a 9 d a y exposure to the humid atmosphere, pure CaOP2Oa compositions having Pt06/Ca0 mole ratios of 0.8-1.0 and of 1.2 gained, respectively, 1-2 mg. and 10 mg.; that is, the weight increased 0.03 to 0.3%. When no silica was present, the introduction of 0.10-0.15 mole of bo, per mole of CaO reduced the moisture absorption about half, the reduction being even greater in the compositions having the P~06/Ca0mole ratio of 1.2. The addition to compositions containing R ~ 0 8of 2-3 moles of Si02 per mole of &Os increased the moisture absorption to about that of pure CaO-PgOs compositions of the same PZO6/CaO mole ratio. The extent of caking after 45-day exposure to the humid atmosphere showed parallelism to the weight gain. None of the compositions with P206/CaO mole ratios of 0.8-0.95 caked appreciably, whereas the caking a t the mole ratio 1.2 was more proper nounced than at 1.0. Introduction of 0.15-0.20 mole of RZOS mole of CaO eliminated caking at the PzOs/CaO mole ratios of 1.0-1.2 under the conditions of test. The introduction of 2-3 moles of Si02 per mole of RsOs in the latter compositions increased the tendency to cake. Measurements of the crushing strengths of cakes formed from different fertilizer materials containing up to 1% moisture upon storage at pressures of 12 pounds per square inch for 7 days were reported by Adams and Ross ( I ) . Their tests showed that the hardness of cake formed from calcium metaphosphate was greater than that from superphosphate, but much less than from potassium chloride, urea, ammonium sulfate, or sodium nitrate.
839
If exposed to the weather, calcium metaphosphate slowly dissolves in rain water. Tests of crude lump calcium metaphosphate exposed in the open in 2-3 inch layers showed that a sample with a Pzo5/c&o mole ratio of 1.26 was dissolved several times as r a p idly as one with a ratio of 0.94. During the period of a year, in which the rainfall was 39 inches, the sample with the P20r/Ca0 mole ratio of 0.94 lost about 11% of its weight and 12% of its P205 content; the lumps became coated with an easily friable layer but did not disintegrate. The sample with the P20s/CacP mole ratio of 1.26 was reduced in the same time to a porous crusty mass. The caking of impure calcium metaphosphate glasses in which the ratio of PtOs to CaO is not too high Can be offset by the addition of substances like calcium carbonate, which react with the products of hydrolysis of calcium metaphosphate to form nonhygroscopic compounds. Previous work (IO) had shown t h a t mixtures of one part of - 100 mesh calcium metaphosphate with three parts of 100 mesh limesbone could be stored for 75 days in s moist atmosphere without gain in weight or loss of free-flowing property, Figure 5 gives the results of hygroscopicity tests upon crude -80 mesh calcium metaphosphates, with and without the addition of 5% of -100 mesh limestone as conditioner. The limestone-conditioned sample having the P2Os/CaO mole ratio of 1.13 in 28 days gained only 0.02% in weight and remained free flowing; another sample of the same calcium metaphosphate without limestone gained 11% in weight and became gummy. Laboratory tests of longer duration at higher humidity indicate the desirability of using 10% limestone. The effectiveness of ground limestone as a conditioner has been checked with 35-mesh calcium metaphosphate in storage tests under warehouse conditions. If the PpOs/CaO mole ratio did not exceed 1.0 and the particles of limestone were uniformly distributed, the materids generally remained granular for at least a year. The nature of the hydrolysis of calcium metaphosphate has not been investigated completely, but some preliminary observations have been made. Well-defined crystals of monocalcium phosphate monohydrate, accompanied by small amounts of appa-
CXPOSWIE
m.45
DAIS
Figure 5. Effect of Ground Limestone (-100 mesh) on Hygroscopicity of Fertilizer-Grade Calcium Metaphosphate (-80 mesh) at 30" C. and 59% Relative Humidity
IN D U S T R I A L A N D E N G IN E E R I N G C H E M I S T R Y
840
rently amorphous substances, were found on the surface of beads of vitreous compositions of P205/CaO mole ratio 1.4-1.9; these compositions had been made by reaction of rock phosphate with Pz05in excess and had been stored in capped bottles in the laboratory for five years. Autoclaving of pure, finely ground calcium metaphosphate with small amounts of water for a few minutes a t 180" C. and then cooling gave practically complete conversion of metaphosphate to monocalcium phosphate. In boiling dilute acids, complete hydrolysis of calcium metaphosphate to orthophosphates requires several hours, and in aqueous extracts a t room temperature the hydrolysis may continue for months (IO). I n the presence of limestone the hydrolysis of calcium metaphosphate is followed by formation of dicalcium phosphate, which was identified in the solids obtained by boiling an aqueous slurry of the stoichiometric proportions of pure calcium carbonate and calcium metaphosphate. ACKNOWLEDGMENT
The authors acknowledge the encouragement and aid of R. L, Copson, J. W. H. Aldred, E. H. Brown, and other members of the TVA Chemical Engineering Staff in obtaining the data and preparing this paper. They are grateful also for the helpful advice of J. F. Schairer and other members of the staff of the Geo-
Vol. 36, No. 9
physical Laboratory, Carriegie Institution of Washington. The generous cooperation of 8. B. Hendricks, K. I>. Jacob, and W. L Hill of the-U. S. Department of Agrirulture is acknowledged. LlTER4TURE CITED
(1) Adams, J. R., and Ross, 1%'. H., IND.ENG.CHEM.,33, 121-7 (1941). (2) Bariett, R. L., and McCaughey, W. J., Am. Mineral., 27, 680 95 (1942). (3) Bartholomew, R. P., and Jacob, K. D., J . Assoc. Oficiai A ~ T Chem., 16, 698-611 (1933). (4) Copson, R. L., Pole, G. R., and Baskervill, W. H., IND. E ~ G CHEM.,34, 26-32 (1942). (5) Curtis, H. A,, Copson, R. L., and Ahrams, A. J., Chem. & Met Ena.. 44. 140-2 (1937). (6) CurtG,'H. A., Copson, R. L., Abrams, A. J., and Junkins, J X.. Ibid., 45, 318-22 (1938). (7) Frear, G . L., and Hull, L. H., IND. ENG.CHZM.,33, 1560 6 (1941). (8) Hill, W. L., Faust, G . T., and Reynolds, D. S., A m . J . Sci., 212 457 (1944). (9) Jacob, K. D., and Ross, W. H., J. Agr. Research, 61, 539 bU (1940). (10) MacIntire, W. H., Hardin, L. J., and Oldham, F. D., IND ENG. CHEM.,29, 224-34 (1937).
(11) Tromel, G., Mitt. Kaiser-Wilhelm Inst. Eisenforsch. Dusseldorf 14,25-34 (1932); Stahk u. B k n , 63, 21-30 (1943).
RATE OF SEDIMENTATION Suspensions of Uniform-Size Angular Particles
HAROLD €3. STEINOUR Portland Cement Association, Chicago, 111.
Rates of sedimentation are reported for suspensions of closely sized emery particles, both flocculated and nonflocculated. Except for the value of an experimental constant, one rate equation applies to both states, provided the flocculated suspensions are highly concentrated. Comparison with previous tests on uniform spheres indicates that a portion of the liquid suspension medium is carried down with the angular emery particles during their fall, whether the suspensions are flocculated or not. The question as to 3s hether this liquid is bound to the particles or simply stagnant is studied, and evidence is shown to support the latter view.
I
N T H E first article of this series (15) equations were developed for the sedimentation rates of dispersions of uniform spheres. Under conditionswhere the Stokes law gives the velocity of a single isolated sphere, the velocity at anx concentration of spheres is given by Equation 24 of the first paper: Q = vieeZ10--%.82(1--e)
where the function 10-1.82(1-e) is empirical. At values of c between 0.3 and 0.7, Equation 24 is practically equivalent to Equation 23 (16): €3 Q = 0.123Va -_ 1 . 5
which was derived, except for the value of the proportionality constant, by using the hydraulic radius to denote the size of the flow space around the spheres and by assuming that no additional variable shape factor was needed. I n the present article the sedimentation of angular particles is considered, starting with the equations for spheres. The experiments on which the study is based embraced both the disperse and flocculated states. Low Reynolds numbers (Table 1V) ensured viscous flow in the displaced liquid. Particles of very small size were used td permit flocculation; they were closely sized in order to obtain uniform settlement in the disperse or nonflocculated state. Flocculated suspensions in which many sizes are present will be treated in a third article.
SEDIMENTATION TESTS ON EMERY POWDERS
A commercial emery powder was fractionated by air separation; two fractions called A and B were used in sedimentation testa. Their densities were 3.79 and 3.77 grams per cc.,. respectively. The appearance of emery A is shown by Figure 1. Size analyses (Table I) were obtained for both roducts by an adaptation of the Wagner turbidimeter method 6 8 ) . The sedimentation tests Rere made in water. At first, in tests in which flocculation was to be avoided, no dispersing agent was added, for the fresh powders did not flocculate. However, as a safeguard 0.1274 sodium hexametaphosphate was used in later tests. Absence of flocculation was shown by direct observations with a binocular microscope, and by the firmnevs and constant density of the sediments produced by different initial concentrations of solid. I n all tests in which flocculation was desired, zinc sulfate was used, chiefly a t 0.12%. All suspensions were mixed for 2 minutes with an electrical mixer and, except as noted in the tables, were tested in a cylindrical glass jar about 100 mm. in diameter, filled to a height of 40 to 60 mm. These dimensions were chosen in order that effects of Ta!l friction would be negligible at the center of the jar (14). A micrometer microscope reading to 0.001 mm. was used to follow the change in level of the suspended solids. Readings were timed 1' C. to 1 second. Temperatures were 24' In all tests on flocculated suspensions the readings were taken on a float placed centrally in the jar (Figure 2). The float was like one used by Powers (f4),a thin disk of Bakelite with a thread-like glass stem attached to its upper face. The densities were such that the float rode a t the plane of separation of the suqpension and a layer of vvater which was placed on top in
