March, 1946
INDUSTRIAL AND ENGINEERING CHEMISTRY
effect of oxygen appears to be due largely to a lowerin of the sulfur dioxide level by this reaction. However, at 49 only 3045% of the observed oxygen effect can be explained in this way. Oxygen uptake is nonenzymatic. 3. The rate of oxygen uptake is independent of sulfur dioxide level, whereas the rate of sulfur dioxide oxidation to sulfate increases with the partial pressure of oxygen. It therefore appears that the latter reaction takes place in the fruit rather than in the gas phase. 4. The rate of oxygen consumption is decreased by low moisture. Fruit of 25% moisture content consumes ox gen about ten times more rapidly than 10% moisture fruit. Txe detrimental effect of oxygen is correspondingly much greater at the higher moisture levels. 5. The results emphasize the importance of limiting the quantity of oxy en t o which fruit is exposed t o less than 15 mg. per 100 grams fry fruit. This can be readily accomplished by packing the fruit solidly in airtight containers or by vacuum packing. 6. There is a nonenzymatic production of carboa dioxide during storage which, under most conditions, bears a constant relation t o darkening. For a given lot of fruit a constant amount of carbon dioxide (usually 35-40 mg. er 100 grams of dry fruit) is produced during the storage life. kowever, increasing the partial pressure of oxygen to which the fruit is exposed increases the carbon dioxide production in relation t o darkening. 7. Carbon dioxide production results in the develo ment of "swells" in cans of dried apricots which are hermetical$ sealed. Pressure development is favored by low sulfur dioxide concentrations, tight packing, and canning in an inert gas.
8.
329
,8. The rate of oxygen uptake by fruit is generally greater than the rate of carbon dioxide production; hence, when fruit is sealed in air, pressure development due t o carbon dioxide formation is at first compensated by a decrease in pressure due to the uptake of oxygen. Ap roximately 20 ml. carbon dioxide per 100 grams dry fruit are prosuced during the edible storage life; therefore, if the ratio of oxygen t o fruit is greater than 20 ml. per 100 grams, no swells are observed until after the fruit becomes inedible. 9. Temperature is the most important factor influencing the rates of oxygen consumption, carbon dioxide production, sulfur dioxide disappearance, and darkening. The rates are increased about four times for every 10' C. rise in temperature over the range 22-49' C. LITERATURE CITED
(1) Nichols, P. F., Mrak, E. M., and Bethel, R., Food Research, 4, 67-74 (1939).
( 2 ) Nichols, P: F., and Reed, H. M., Western Canner & Packer, 23, 6 , l l - 1 3 (1931); Frudt Products J.,22, 206-8,247-9 (1943). (3) Stadtman, E. R., Barker, H. A.,Haas, Victoria, and Mrak, E. M., IND. ENO.CHBM.,38, to be published (1946).
(4) Stadtman, E. R., Barker, H. A., Mrak, E. M., and Mackinney, G., Ibid., 38,99-103 (1946). (6) Western Regional Research Lab., Informtion Sheet AIC-47 (1944).
REPORT on a joint research project of the Quartermaster General's Offioe, U. 8. Army, and the University of California.
Thermal Defluorination of Superphosphate E. J. FOX,W. L. HILL, K. D. JACOB, AND D. S. REYNOLDS Bureau of Plant Industry,
U.S. Department of Agriculture, Beltsaille, M d .
A
N IMPENDING shortage of feed-grade phosphates-chiefly bone meal with small annual tonnages of dicalcium phosphate, bone char, and thermally defluoriaated superphosphatebecame evident a t the close of 1942 (8) in the face of the scheduled increase in food production for war. A hurried search ensued for phosphates of low fluorine content for use as substitutes for bone meal. A satisfactory feed supplement must be very low in fluorine on account of the toxicity of this element to animals
ferences between commercially prepared products. A laboratory study was needed t o determine the effect of temperature, furnace atmosphere, and composition of superphosphate on the rate and extent of fluorine volatilization and on the nature of the defluorinated product. The results of this investigation, designed t o meet only the immediate needs of the problem, are reported here. MATERIALS AND METHOD
1
Deceased.
Superphosphate can be defluorinated to the extent of 80 to 99%by heating in a ventilated space for a comparatively short period at 600' C. or above. The character of-the product obtained from ordinary superphosphate depends to a large extent upon the temperature to which i t is heated and, to a less extent, upon the duration of the heating period at temperatures above 600'. Below 600' C. longer periods of heating are required to eliminate the fluorine. A moist atmosphere is beneficial at temperatures below 300' (to limit dehydration) and above 700'
-
tory for two years or longer. The samples were put through a 35-
(to prevent fluorapatite formation), but has no apparent effect at intermediate temperatures. The principal crystalline phosphorus compounds formed are calcium metaphosphate ( W O O ' C.), calcium pyrophosphate (600-800°), and tricalcium orthophosphate (800-1000'). The thermal conversion of meta- to pyro- and orthophosphates is accompanied by the volatilization df sulfur oxides. The tricalcium phosphate so formed compares favorably with bone meal as a mineral supplement for animal feeds.
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Vol. 38, No. 3
alumel thermocouple placed between the Pyrex tube and the furnace tube; the temperature of the charge was measured by two other similar couples, one placed in the center of the charge, the other 10 mm. from the bottom of the charge, and about the same distance from the tube wall. Steam without dilution with air was passed into the lower end of the tube a t the rate of 0.25 gram per minute. The exit gases were led through a watercooled condenser 350 mm. long, and the condensate was caught in a flask. Figure 1 shows the temperatures observed in a typical 4-hour run on 100 grams of superphosphate, VI here the constancy of the furnace temperature is much better than the =t5 O C. usually observed in such runs. The superphosphate and heated products were analyzed for PzOs, CaO, SO3, and fluorine by standard methods, with
Temperature of chor
due precaution in the case of the heated materials to ensure complete solution of the phosphorus and its conFigure 1. Temperatures Observed in Typical Run with Low Temperature Furnace (100-Gram Charge) version to orthophosphate. Phosphorus soluble in hydrochloric acid was determined by digesting l gram of sample with 100 ml. of 0.4% HCl a t room temperature for 1 hour (15). mesh sieve (0.42-mm. openings) and thoroughly mixed. Table I I n the low temperature work the quantities of fluorine volagives analyses of the samples. tilized in the different periods during the run were deTwo furnace methods were adopted. The high temperature termined by direct titration of the condensate for each procedure was used with or without steam over the entire temperiod and summation of the separate results. The total perature range studied; the low temperature procedure was used volatilization measured in this way agreed rather closely with only a t 300" or below. that calculated from the residual fluorine in the heated charges. HIGHTEMPERATURE PROCEDURE. A weighed charge (usually 2 grams) of superphosphate contained in a platinum boat (3 inches long) was heated a t a constant furnace temperature in a stream of air (120 cc. per minute, normal temperature and pressure) in a horizontal tube (1 inch bore) furnace of the type used Z - g r o r n charger healed 30 minutes by Reynolds and others (16). At the end of the heating period, 0 F l o r i d 0 l a n d - p e b b l e supeipho6phole Na.2261 In atmoaphsrlc air usually 30 minutes reckoned from the moment the boat was in" '' "humldllied (0.zsgm.of n Z O per minute) serted, the boat was withdrawn, cooled in a desiccator, and reX Tennessee brown-rack ruperpl,a,phote No.2280 lnotmorpherie 01 weighed. The furnaced charge was ground to pass an 80-mesh EO F/P s l l u o r i n e to p h o s p h o r u s w e i q h t r o t l o sieve (0.175-mm. openings) and preserved in a screw-cap glass vial. The temperature, referred to the International Scale, was measured with a shielded chromel-alumel thermocouple, disposed with the junction directly above the center of the charge, and a portable potentiometer that permitted the electromotive force to be read to the nearest 0.1 mv. The thermocouple was calibrated by comparison with a standard thermocouple of platinum and platinum-lO% rhodium, that had been recently calibrated by the Kational Bureau of Standards. The temperature of the furnace was maintained constant within *3" C. by a potentiometer controller that in the beginning of the work operated on the temperature-measuring thermocouple. Later it was found desirable t o operate the controller on another thermocouple placed outside of the inner tube of the furnace and, thereby, remove the controlling element from disturbances arising from heat changes during chemical reaction in the charge. Low TEMPERATURE PROCEDURE. The superphosphate was charged into a Pyrex tube, 480 mm. long and 40 mm. in diameter, with a 30-mm. portion of the lower end drawn down to 8-mm. diameter, and provided with a coarse fritted-glass disk, 250 mm. from the top, to support the loosely packed charge of 10 to 100 grams (10 to 100 mm. high). The constricted lower end of the tube was attached by a rubber stopper to a 500-cc. flask which 3 served as a steam generator. The loaded tube was heated in a vertical position by an electric resistance furnace 450 mm. in Figure 2. Volatilization of Fluorine from Ordinary length. The temperature controller operated on a chromelSuperphosphate at High Temperatures 0
30
60
90
I20
150
180
210
240
Time, minute$
I'
"
March, 1946
33B
INDUSTRIAL AND ENGINEERING CHEMISTRY
----
a. Phosphorus and S u l f u r SO3 P s o l u b l e In
I
L
0
1
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0.4% H C I
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-
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Figure 3. Effect of Heating Period on Extent of Reactions in Ordinary Superphosphate at 500', 794', and 1000° C.
This method of following the volatilization of fluorine could not be used in the high-temperature work on account of the disturbing effect of large quantities of sulfur acids in the condensate. REMOVAL OF FLUORINE
3FQ
Figure 2 shows the extent of fluorine volatilization from a Florida land-pebble and a Tennessee brown-rock superphosphate, heated for 30 minutes at different temperatures in the range 100" t o 1000' C. in accordance with the high-temperature procedure. The land-pebble superphosphate showed somewhat higher volatilization of fluorine than did the brown-rock superphosphate. This difference in behavior is probably due t o different amounts of unattacked phosphate rock in the two types of material, since apatite fluorine was not expelled from a mixture of superphosphate and phosphate rock under the same conditions. The landpebble and brown-rock superphosphates, respectively, contained 0.08 and 1.07%of citrate-insoluble PzO6 (Table I). An optimal temperature for fluorine removal is 800" C., at which the residual fluorine was 0.02 and 0.18% in the land-pebble and brown-rock materials, respectively. The somewhat lower volatilizations above 800" are attributable t o apatite formation which is made possible by the increased basicity of the charge arising from sulfur volatilization. I n this temperature range the addition of water vapor to the furnace atmosphere had a beneficial effect on fluorine removal, whereas in the range 500' to 700' C. a humid atmosphere was without noticeable effect. The results obtained with wet air by this procedure a t 400' and below were vitiated by the condensation of water in the furnace tube and are not shown in Figure 2.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
1 0 0 - g r a m charge. furnoce temp.
0
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Figure 4. Volatilization of Fluorine from Ordinary and Double Superphosphates at Low Temperatures with Steam
I I
!
\
95c
The course and extent of fiuorine volatilization in 30-minute heating periods were not altered n-hen the weight of charge was varied from 1 to 5 grams. Figure 3 shows the effect of length of heating period on fluorine volatilization a t 500°, 794 O, and 1000' C. At 500" 80% of the fluorine was volatilized in a 15-minute heating period; a t 794", about 93% in 5 minutes. Fluorine was removed equally well by heating 100-gram charges in a muffle furnace for 3 hours. Figure 4 gives typical results for the removal of fluorine a t temperatures below 300 O C. in accordance with the low temperature procedure from ordinary and double superphosphates prepared from Florida land pebble. One of these volatilizationtime curves is also shown with the temperature-time curves in Figure 1. A heating period of 1.5 to 2 hours a t 280" C. was required to remove 80 to 90% of the fluorine. The optimal furnace temperature would appear to lie in the range 260" to 280" C. The data (Figure 4) for ordinary superphosphate show a marked drop in volatilization below 260 ', and other. results (not given here) obtained on 10-gram charges show the same reduced volatilization below 260 O and also a somewhat lowered volatilization above 280 The latter effect is attributed to the rapid stripping of acid hydrogen from the charge and the consequent retarding of the relatively slow volatilization reaction which, in this temperature range, may properly be viewed as an acid-distillation process. Reduction in the size of the charge resulted in increased volatilization during the first part of the heating period, but did not alter appreciably the total volatilization attained a t the end of 2 hours (Figure 4). I n the low tcmperature heating, as was observed in the high temperature cxperiinents, Tennessee brown-rock superphosphate showed lower volatilization than did Florida land-pebble superphosphate; this result was also reported by MacIntire and othcrs (fd) and attrikuted by them to the prcsence of larger quantities of iioii compounds in brown-rock superphosphate. I n the case of the materials used in this study, however, the larger quantity of residual fluorine in the heated brown-rock superphosphate arises, a t least in part, from the larger indicated amount of unattaclred rock in this material. Results for low temperature volatilization of fluorine from double superphosphate (Figure 4) agreed rather closely with those for ordinary superphosphate. The displacement of the curves for the two types of material arises from the choice of initial time. In the case of ordinary superphosphate, time was counted from the beginning of heating, whereas in the case of double superphosphate it v a s (more properly) counted from the moment the first drop of condensate formed, which was 25 to 30 minutes after the start of heating. The sample of double superphosphate was a lumpy material, which permitted some experiments with different mesh sizes. The results (Figure 4) indicate 10 to 30 mesh as the most favorable grain size.
'.
Ordinory s u p e r p h o s p h a t e No.2261 ( 3 5 - m e a h )
aloe
Vol. 38, No. 3
I
CHEMICAL REACTIONS
78.57,
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Figure 5. Dehydration and "Solubility" Curves of a Sample of Monocalcium Phosphate Monohydrate (Free HIPO,, 0.2%)
The w i g h t losses sustained by ordinary superphosphate whcn heated to temperatures below about 575' C. are limited mainly to those associated with the dehydration of monocalcium phosphate, the evolution of fluorine, and the combustion of organic matter. When pure monocalcium phosphate monohydrate is heated in accordaim with the high temperature procedure2, it loses its water of crystallization and of constitution in the range 100-500" C. I t is thereby converted into calcium metapliosphates which is only slightly soluble in dilute hydrochloric acid (Figure 5 ) . Different samples of monocalcium phosphate heated under similar conditions did not follow the same course during decomposition, Consequently, it cannot be assumed that the monocalcium phosphate in ordinary superphosphate, even In order t o avoid frothing as a result of partial fusion the charges heated a t the higher temperatures were preheated a t 225". f
INDUSTRIAL AND ENGINEERING CHEMISTRY
March, 1946 I
I
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“Solubility” of Phosphorus in 0.4% Hydrochloric Acid in Relation to Composition of Heated Material
if unaffected by other salts present, would follow the same course of decomposition as that shown in Figure 5. If one assumes that the fluorine volatilized below 575 O C. was initially bound with calcium, it follows that some calcium pyrophosphate may also be formed in this temperature range by the reactions resulting in fluorine evolution. Thus, as Figure 6 shows, the estimated CaO-PZ06 mole ratio of the phosphatic component in superphosphate 2261 defluorinated at 500” C. is 1.2, which would correspond with an amount of calcium pyrophosphate equivalent to 20% of the total PzOS. Reactions a t temperatures above 575 ’ C. involve the calcium metaphosbhate formed a t lower temperatures and the calcium sulfate component of superphosphate, yielding successively normal calcium pyro- and orthophosphates with the evolution of SOa (Figure 7). The extent of these reactions depends primarily upon the temperature to which the charge is heated (Figures 6 and 7) and, t o a less degree, upon the duration of the heating period (Figure 3). The formation of hydroxylapatite at temperatures above about 900” C. accounts for the liberation of sulfur oxides in excess of that which would be liberated by the formation of only tricalcium phosphate (Figure 6). .
333
of the calcium metaphosphate found in ordinary superphosphate heated to 500” C. is presumed to be the anhydrite-like modification*, since the diffraction patterns of the heated superphosphates were indistinguishable from that of anhydrite. An attempt t o isolate the metaphosphate by leaching such mixtures with water until the washings were substantially constant in composition yielded slightly soluble residues containing approximately two moles of calcium sulfate per mole of calcium metaphosphate, suggestive of either a double salt or a solid solution. On the other hand, in mixtures of &calcium pyrophosphate and calcium sulfate obtained by heating superphosphate t o 750-800” C. and identified by their x-ray diffraction patterns, the calcium sulfate component exhibited normal solubility in water and could be removed by leaching. Both beta and alpha modifications of tricalcium orthophosphate, as well as apatite, were identified in superphosphate heated a t the higher tempersr tures. The optical properties and the thermal stability ranges of the high temperature forms of calcium meta- and pyrophosphates were described in recent publications (7, 9). ANIMAL FEED SUPPLEMENTS
The solubility of defluorinated superphosphates in dilute hydrochloric acid has been suggested (6, 9, 16) as a rough measure
.
IDENTIFICATION OF PRODUCTS
The compounds mentioned were identified by S. B. Hendricks by means of x-ray powder diffraction methods, in one or more modifications, in both laboratory and commercial preparations of heated superphosphates and in heated synthetic mixtures of monocalcium phosphate and calcium sulfate. I n the case of heated charges of monocalcium phosphate the diffraction pat’terns of a t least two compounds intermediate between anhydrous monocalcium phosphate and calcium metaphosphate were found, but their compositions have not yet been established. The form
8 Two lowtemperature modifications of calcium metaphosphate oan be obtained by heating monocalcium phosphate a t 400’ to 500° C.: both invert to the beta form a t higher temperatures. One of the low-temperature forms has a diffraction pattern closely similar to that of anhydrite, which makes a clear-cut identification impossible when considerable anhydrite is present. Sulfate-containing double superphosphate (presumably also ordinary buperphosphate) gives the anhydrite-like modification, whereas pure monocalcium phosphate and sulfate-free double superphosphate usually yield the other low-temperature modific8tion.
3
Figure 7. Volatilization of Water, Fluorine, and Sulfur from Ordinary Superphosphate at Different Temperatures Two-gram ohargem of Florida land-pebble muperphoiphats 2261 heated 30 minut-
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2 - g r a m charger h e o t e d 3 0 m 5 n u l e s 0 F l o r i d 0 l a n d - p e b b l e S U P e r ~ h o S P h C t eNo. 2 2 6 1 X T e n n e s s e e b r a w n - r o c k superpb,osphote No. 2 2 8 0
Vol. 38, No. 3
rcgards yield and content of calcium oxide, sulfate, and phosphorus; but the heated products show considerable variation in the percentage of total phosphorus soluble in dilute acid, particularly at 785" C. where volatilization of fluorine was maximum. -4n increase in temperature from 785' to 1085' C. increased '.soluble" phosphorus approximately twofold. None of the materials heated above 500" C. showed any marlced tendency to absorb moisture from the air. L i m i t d moisture absorption, however, was noted in materials heated at lower temperatures (la), notably monocalcium phosphate heated a t 400" C. INDUSTRIAL APPLICATIONS
20-
10-
0
I
.
P u r e n - c P ( ~ o ~ / ~
I
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Figure 8. Effect of Heating Temperature 011 "Solubility" of Phosphorus in 0.4% Hydrochloric Acid
of the possible utilization of the phosphorus by animals. The way in which this property of heated ordinary superphosphates was affected (a) by the temperatures a t which the materials were processed is shown graphically in Figures 6 and 8, and ( b ) by the duration of the heating period in Figure 3. In the case of double superphosphate (data not given here), the soluble phosphorus decreases with increasing temperature in a manner analogous to the behavior of pure monocalcium phosphate (Figure 5 ) . However, by raising the temperature until the metaphosphate is fused, a more soluble product than the unfused material may be obtained. The low-temperature range (below 500" C.) of the curves in Figure 8 corresponds to the conversion of monocalcium phosphate (Figure 5) into a less soluble compound, presumably the anhydrite-like calcium metaphosphate. For the peculiar up and down fluctuations observed in the intermediate temperature range (500-800" C.), there is no ready explanation other than the reaction of calcium metaphosphate with calcium sulfate, which results in the formation of calcium pyrophosphate at about 780". The high temperature range (800-llOO0 C.) of the curves corresponds to the conversion of calcium pyrophosphate into normal calcium orthophosphatc. Feeding experiments with rats (5) showcd"that superphosphate defluorinated by the low temperature procedure at 300" C. was a fair source of phosphorus, and that material defluorinated a t 1000a by the high temperature procedure was good, comparing favorably with bone meal. On the other hand, materials prepared a t 600" and 760" C. were poor sources of phospliorns; this finding supports the estimates based on clieinical tests. Table I shows the yields and analyses of products obtained by heating small charges of a numbcr of different supcrphosphates a t 585", 788", and 1085" C., together with pertinent data on the original materials. The results are fairly consistent as
T o obtain a product of low fluorine content that compares favorably with bone meal as a phosphorus supplement in animal feeds, it appears necessary for the producer of defiuorinated ordinary superphosphate to operate at a temperature high enouy!~ (around 1000° C.) to form tricalcium phosphate. This condition is imposed by the poor nutritive value of crystalline calcium mcta- and pyrophosphates obtained by heating superphosphate at 600" to 800" C. and by the long heat,ing period required at low temperatures to produce a low-fluorine material of only mcdium value. Operation a t higher temperatures probably would necessitate some means of sulfur disposal other than discharge of the gases to the atmosphere. By defluorinating at about 600' and t,hen raising the temperature to 1000" C., 80% or more of the evolved fluorine can be separated from the evolved sulfur. If double superphosphate is used instead of ordinary superphosphate, the operating temperature must) be either low (under 300" C.) with the provision of steam, in order to avoid complete dehydration of thc acid phosphate, or else high enough (about 1000") t o fuse the material. I n the first case the product will have a composition intermediate between monocalcium phosphate and calcium metaphosphate, whereas in thc latter it will be a metaphosphate glass. LITERATURE CITED (1) Bird, H. R., and others, J . Assoc. O#iciul A g r . Chem., 28, 118-29 (1945). (2) Board of Am. Feed Mfrs., Rept. of Investigations Made by
Planning Comm. of Feed Industry on Feed Supplies and Demand in Relation to Govt. Goals, Jan. 12, 1943. (3) Burberry, A. T., Brit. Patent 3327 (1912). (4) Butt, C. A , , U. S. Patent 2,360,197 (Oct. 10, 1944). (5) Ellis, N. R., and others, J . Assoc. Oficial Agr. Chem., 28, 12942 (1945). ( 6 ) Foss, A,, U. S.Patent 1,994,070 (March 12, 1935). (7) . , Hill. W. L.. Faust, G. T., and Reynolds, D. S., Am. J . Sci., 242, 457-77, 542-62 (1944). (8) Hill, W. L., and Hendricks, S. B., IND.ESG. CHEM.,28, 440-7 (1936). (9) Hill, W.L., Reynolds, D. S., Hendricks, S. B., and Jacob, K. D., J . ASSGC. Oflcial A g r . Chem., 28, 105-18 (1945). (10) Huber, H., German Patent 681,645 (Sept. 27, 1939). (11) Jacob, K. D., Feedstufls, 16, No. 7, 18-20, 22-4, 26, 28, 30-2 (1944). (12) Maclntire, TI', H., Hardin, L. J., and Hammond, J. W., J . Asaoc. Oficial Agr. Chem., 24, 477-89 (1941). (13) Mitchell, €1. H., Natl. Research Council, Reprint Circ. Series 113 (1942). (14) Xewberry, S . B., and Barrett, H. N., U.S.Patent 995,028 (June 13, 1911). (15) Reynolds, D. S., Hill-W. L., and Jacob, K. D., J . ASSGC. Oficial A g r . Chem., 27,559-71 (1944). (16) Reynolds, D. S., Jacob, K. D., and Rader, L. F., Jr., IND. ENG. CHEM.,26, 406-12 (1934). (17) Reynolds, D. S., Pinckney, R. M., and Hill, TI', L., J . Assoc. Oficial Agr. Chem., 26, 564-75 (1943). (18) Rupp, V. R., U. S. Patent 1,712,404 (May 7,1929). (19) Shoeld, M., Ibid., 2,288,112 (June 30, 1942). (20) I b i d . , 2,328,884 (Sept. 7, 1943); Reissue 22,500 (June 20, 1944). (21) Wight, E. H., and Anderson, D. L., Ibid., 2,234,511 (March 11, 1941). PRESEXTED before the Division of Fertilizer Chemistry at the 108th Meeting of the AMERICAN C H m i I c a S O C I E T Yin h'ew York, N. Y .
