Phosphate Fertilizers by Reversion of Defluorinated Phosphate at Temperatures below 1400' C.' Depending upon their reverting characteristics when they are annealed at temperatures below 1400 O C., defluorinated phosphates can be divided into three general classes-namely, those which show (a) no marked reversion in either a wet or a dry atmosphere, (b) marked reversion in a wet atmosphere only, and (c) marked reversion in both wet and dry atmospheres. Reversion is usually much greater in a wet atmosphere than in a dry one. I n general, the reversion is greatest in products annealed between 700" and
I
N A PRECEDING paper (4) it was shown that the citrate
solubility of a sample of calcined (defluorinated) Florida pebble phosphate decreased markedly when the product was cooled slowly from 1400O t o 1200 O C. and lower temperatures. Also, the decrease in solubility (reversion) was much greater when the phosphate was cooled in wet air than when it was cooled in dry air. Inasmuch as the reversion of the phosphate during cooling may have an idportant bearing on the commercial preparation of defluorinated phosphate, a further investigation was made of the factors involved. The present paper gives the results of a study of the behavior of nine defluorinated phosphates (eight calcined phosphates and one fused phosphate) on annealing in wet and in dry atmospheres at temperatures ranging from 1400' down to 200" C.
Materials and Method Calcined phosphates 1491 to 1497 were prepared in the laboratory by defluorinating 2.5-gram charges of 40-80 mesh (0.381-0.175 mm.) particles of phosphate rock a t 1400" C. in wet air (0.8 gram of water in 120 cc. of air per minute), as de1
For previous papers in this series, see literature citations 3, 4, 6,and 6.
1200"C., but serious reversion may occur at temperatures as low as 200" C. The citrate solubility curves of certain wetannealed products show two pronounced minima at about 600" to 800" C. and 1100" to 1200" C., respectively. The rate of reversion is rapid during the first 15 minutes of annealing and usually decreases markedly thereafter. Defluorinated phosphates in which reversion of the phosphorus has occurred as a result of slow cooling can be resolubilized by reheating at 1400" C. for a few minutes and rapidly cooling the product.
scribed in previous papers ( 4 5 ) )and allowing the products to cool quickly in the laboratory atmosphere. The products from the individual runs were ground to pass a 40-mesh sieve and thoroughly mixed. Calcined phosphate 1478 was prepared experimentally in a direct, oil-fired rotary kiln by heating a commercial grade of Tennessee brown-rock phosphate a t approximately 1400" C. The fused phosphate 1476 was prepared experimentally by the Tennessee Valley Authority by blowing wet air through molten Tennessee brown-rock phosphate (1). The last two materials were ground to pass an 80-mesh sieve. Partial chemical analyses of t h e defluorinated ph6sphates are given in Table I. The annealing experiments with the laboratory-prepared calcined phosphates (1491 to 1497) were carried out in the following manner : A 1-gram charge of the calcined phosphate (contained in a platinum boat 7.6 cm. long, 1.3 cm. wide, and 0.9 cm. deep) was heated at 1400' C . for 10 minutes in a horizontal tube furnace (6) which was connected with an annealing furnace of the same type through a continuous refractory porcelain tube. After the preliminary heating at 1400' C. the boat was pulled directly into
OF DEFLUORINATED PHOSPHATE ROCK TABLEI. COMPO~ITION
Time of Total Heating in --P*OsDefluorination Total Citrate-sol. F cao 0iOz Min % % % , % % 11.66 0.011 50.55 32.24 34.13 40 Fla land pebble C No. 790-C 1491 11.75 0.012 48.45 35.78 37.24 40 1492 Fla'land pebble'd Bartow No. 1447 5.57 0.015 51.90 36.61 37.54 Tedn. brown rodk, Wales,'No. 907 30 1493 5.29 0.015 52.13 36.60 38.00 40 Tenn. brown rook, Mountpleasant, NO. 909 1494 5.33 0.09 52.80 33.76 37.25 f Tenn. brown rockC 1478' 22.53 0.12 43.12 24.24 28.96 1476P Tenn. brown rockc f 6.00 0.045 54.60 32.13 37.48 45 Wyo rock Cokeville No. 948 i495 6.12 0.012 53.05 37.91 38.86 40 1496 Mon't. rock, Garrison: No. 1252 6.49 0.012 52.85 36.37 38.09 45 Idaho rock, Conda, No. 1253 1497 0 Total Fa. b Not oorrected for Ti, etc. 0 From ri deposit of unknown location. d Concentrated b y oiling. and tabling. f Unknown. I Fused phosphate prepared experlmentally by TVA. e Prepared experimentally in a direct oil-fired rotary kiln. Sample No.
Source of Defluorinated Phosphate
.
1294
F ~ ~ O P AlrOab
%
%
2.06 0.81 3.22 3.14 3.05 2.65 1.13 1.03 0.97
0.84 1.76 1.37 1.38 1.02 0.82 0.72 1.06 1.05
Calcination Process H. L. MARSHALL, D. S. REYNOLDS, K. D. JACOB, AND T. H. TREMEXRNE Bureau of Chemistry and Soils, U. 8. Department of Agriculture, Washington, D. C.
the annealing furnace, and the charge was annealed under the desired conditions. The boat was then removed from the furnace, allowed to cool in the laboratory atmosphere until redness disappeared, and finally cooled to room temperature in a desiccator. The nature of the furnace atmosphere was the same during the preliminary heating a t 1400" C. as in the subsequent annealing period. These conditions simulated those that may be encountered in commercial practice, in that the charge was cooled directly from 1400" C. t o the annealing temperature. Several comparative experiments indicated, however, that practically the same results are usually obtained by cooling the charge to room temperature after the preliminary heating at 1400" C . and then reheating it at the annealing temperature. In the experiments with calcined phosphate 1478 and fused phosphate 1476, the preliminary heating at 1400" C. was omitted and the charge was heated directly to the annealing temperature. Temperatures were measured by means of platinum and platinum-rhodium thermocouples which were calibrated at frequent intervals against a standard couple of the National Bureau of Standards. The heating period was measured from the time the boat was put in place in the furnace. In the wetair experiments water was supplied at the rate of 0.8 gram in 120 cc. of carbon-dioxide-free air per minute. The air was supplied from the laboratory compressed-air system. In the dry-air experiments the charges were heated in carbon-dioxidefree air (120 cc. per minute) which had been dried by passing in succession through towers containing anhydrous calcium chloride, concentrated sulfuric acid (three towers), and anhydrous magnesium perchlorate. The gain in weight of a tube of phosphorus pentoxide inserted in the system a t the inlet to the furnaces did not exceed 2.5 mg. and usually averaged about 1.0 mg. in 6 hours. Suitable precautions were taken to prevent the entrance of moisture into the system from the exit end of the system. Analytical samples were ground to pass a 100-mesh sieve. Fluorine and citrate-insoluble phosphorus were determined by the methods used in previous studies ( 6 ) in this series. The citrate digestions were made in the presence of filter-paper pulp (2).
solubilities of the original materials are given on the zero ordinates. I n accordance with the results of annealing experiments (Figures 1and 2) defluorinated phosphates can be divided into three general types or classes, depending upon their reverting characteristics, as follows: Type 1.. No marked reversion in either a wet or a dry atmosphere (Figure 1B). Type 11. Marked reversion in a wet atmosphere only (Figures 1C and E ) . Type 111. Marked reversion in both wet and dry atmospheres (Figures l A , lD, and 2 ) .
The writers have found no example of a defluorinated phosphate that shows marked reversion in a dry atmosphere but not in a wet atmosphere. I n the case of the defluorinated phosphates that showed marked reversion in a dry atmosphere (type 111), the solubility curves (Figures l A , l D , and 2) exhibit only one welldefined minimum which occurs in the range 1100" to 1300" C., usually at about 1200" C. I n some cases, however, there is a slight indication of another minimum a t lower temperatures (Figures 1A and 2B). Marked reversion of the phosphorus to the citrate-insoluble condition did not occur a t temperatures below 809" C. and usually not below 1000" C. The majority of the solubility curves of the wet-annealed phosphates show two well-defined minima (Figures lC, lE, 2A, 2B, 2C) a t approximately 600" to 800" C. and 1050" to 1250" C., respectively, and with one exception (Figure 1E) the greatest decrease in solubility occurs in the region of the high-temperature minimum. I n the case of the wet-annealed phosphates that show only one pronounced minimum in the solubility curves (Figures l A , l D , and 2 0 ) the minima occur I n duplicate experiments the results for the citrate solubility a t approximately 1100" to 1200" C. I n addition to the of the phosphorus agreed remarkably well; the differences did very marked minima shown by the solubility curves for the not exceed 5 and were usually less than 1.5 per cent. Therewet-annealed phosphates (types I1 and 111), many of the ported results represent the averages of two or more expericurves (Figures l A , l D , 2A, 2B, and 2 0 ) show pronounced ments, which in many cases were run in different furnaces and kinks a t about 1000" to 1150" C. The kink usually occurs on widely separated days. on the steep portion of the curve as it nears the minimum at about 1100" to 1250" C. Several of the experiments a t temEffect of Annealing peratures in the region of the kinks in the curves were repeated The citrate solubilities 'of laboratory-prepared calcined in different furnaces and in the same furnace on widely sepaphosphates that were annealed for 30 minutes a t temperatures rated days with closely agreeing results. Similar kinks do not ranging from 200" to 1400" C. are shown in Figures 1, 2A, occur in the curves for the dry-annealed phosphates. I n and 2B. The initial solubilites shown on the zero ordinates of other experiments (results not shown here) increasing the time these figures were obtained on charges that had been given of preliminary heating of calcined phosphate 1493 (Figure 2A) the preliminary 10-minute heating a t 1400" C.; these solua t 1400" C. from 10 minutes to 4 hours caused an increase in bilities are usually higher than those shown in Table I. The the reversion of the phosphorus in samples subsequently wetother results were obtained on charges that had received the annealed for 30 minutes a t 600" to 1300" C. The solubility 10-minute preliminary heating a t 1400" C. plus the 30-minute curves of wet-annealed samples of this material always annealing at the indicated temperature. showed two minima, regardless of the time of heating a t Figures 2C and D show the effect of annealing for 30 min1400" C. or of subsequent annealing (30 minutes t o 5 hours). utes on the citrate solubilities of calcined phosphate that was Marked reversion of the phosphorus to the citrate-insoluprepared in an experimental rotary kiln and of fused phosble condition usually did not occur in a wet atmosphere at phate that was prepared experimentally by the TVA. The temperatures below 500" C. I n one case (Figure lC), how1295
1296
INDUSTRIAL AND ENGINEERING CHEMISTRY
VOL. 29, NO. 11
FIQURE 1. EFFECTOF ANNEALING (30 MINUTES)ON THE CITRATESOLUBILITY OF DEFLUORINATED PHOSPHATE ROCK FROM FLORIDA, WYOMING, MONTANA, AND IDAHO
ever, considerable reversion occurred a t 200" C . With the exception of Figure 1B the solubility curves for the wetannealed phosphates lie considerably below those for the corresponding dry-annealed phosphates in the temperature range 600" t o 1200" C. and frequently in the range 500" to 1300" C. The defluorinated Tennessee brown-rock phosphates (Figure 2), which were prepared from different samples of rock, show approximately the same general behaviors on annealing.
FIGURE2. EFFECTOF ANNEALING(30 MINUTES)ON THE CITRATESOLUBILITY OF DEFLUORINATED PHOSPHATE ROCK FROM TENNESSEE
This is not the case, however, with the defluorinated Florida pebble phosphates (Figures 1A and B). With the exception of the fused phosphate rock, all the ma-
INDUSTRIAL AND ENGINEERING CHEMISTRY
NOVEMBER, 1937
1297
Temperature
FIGURE3. CHANGEIN WEIGHTOF CALCINED PHOSPHATE ON ANNEALING (30 MINUT~S) IN WET AIR
FIGURE4. terials showed small losses in weight on annealing for 30 minutes in dry air. The losses usually amounted to less than 0.2 per cent (generally less than 0.1 per cent) and were approximately the same as those shown by corresponding charges that had received only the preliminary 10-minute heating a t 1400" C . On the other hand, the fused phosphate consistently showed small increases in weight (0.01 to 0.18 per cent) on annealing in dry air a t 500' to 1350" C. Inasmuch as the increase could not be traced to absorption of water from the
REVERSIONRATE OF CALCINEDPHOSPHATE AT 800'AND 1200" c .
Eurnace atmosphere, it is believed to be due to changes in the state of oxidation of certain constituents (principally iron) of the fused phosphate. I n no case was there any relation between the change in weight of the charge and the citrate solubility of the phosphorus. The wet-annealed phosphates usually showed small but progressive increases in weight as the citrate solubility of the
INDUSTRIAL AND ENGINEERING CHEMISTRY
1298
phosphorus approached a minimum. Furthermore those samples which show two well-defined minima in the solubility curves also show two maxima in the increases in weight (Figure 3), and for a given sample the temperatures at which minima occur in the so1ubilit)y usually coincide with those a t which maxima occur in the increase in weight and vice versa. The largest increases in weight usually occur a t approximately 600" to 800" C., corresponding to the low-temperature minima in solubility, whereas the lowest solubilities usually occur a t about 1050" to 1250" C. corresponding to the high-temperature minima in the solubility curves and roughly to the second maxima in the increases in weight. With the materials that show kinks in the solubility at about looo" to 11500 " (Figures lD, 2A, 2B, and 20), the high-temperature maxima in the increases in weight usually occur a t temperatures nearer those of the kinks than those of the high-temperature minima in the solubility curves.
TABLE111. SOLUBILIZATION OF REVERTED PHOSPHATE" Defluorination treatment
a
Source
Citrate soly. of PlOab
%
0
%
%
%
%
.. ..
.. ..
._ ... .
.. ..
n ~ ~ ~ ~ ~phosphate ~ d heated & ~at 14000 ~ $C, for ~ 10~ minutes, t ~ ~ Original defluorinated phosphate heated direct from room temperature to 1200" C. of PnOs in original defluorinated phosphate.
Practical Applications of Data
Citrate Soly. of PsOa in Material Annealed for: 1 hr. 2 hr. 4 hr.
%
%
Citrate Soly. of P106 before Reversionb
d Solubility
OF CALCINED PHOSPHATE ON ANNEALING TABLE XI. REVERSION FOR 1 TO 4 HOURS AT 1200' (2.0
Sample No.
Citrate Citrate Soly. pf Pt01 Anneal- soly. after Reheating at ing of 1400' C. for: Sample No. and source of defluorinated material temp. PzOs 5 min. 15 min. 30 min. O C .
Minima usually occur a t or near 1200" C. in the solubility curves of those phosphates (types I1 and 111) which show marked reversion on annealing for 30 minutes (Figures 1 and 2). At 800 " C. marked reversion does not occur in any of the materials on annealing for 30 minutes in dry air, whereas the reversion in wet air varies considerably with the different materials and is usually much smaller than the maximum reversion of the material. Accordingly these temperatures were selected for a study of the reversion rate of several calcined phosphates. If the solubility of the phosphorus decreases a t all, it decreases most rapidly during the first 15-30 minutes of annealing (Figure 4). Although a further decrease occurs with longer periods of annealing, the change is usually small after the first hour (Table 11). An exception occurs in the case of calcined phosphate 1497, prepared from Idaho rock, which showed further marked reversion on annealing a t 1200' C. for 2 to 4 hours in a wet atmosphere. In this case the high-temperature minimum in the 30-minute curve (Figure 1E) occurs a t 1100"C. whereas with the other materials the minima occur at or near 1200' C.
Calcined Phosphate
Reverted Phosphate
Heated in Dry Air Calcined 1491 Fla. pebble 1200 6 0 . 6 94.4 93.5 94.1 93.7 Calc/ned 1494: Tenn. brown rock 1200 63.3 100.0 99.2 Calcined 1496, Mont. rock 1200 81.4 97.8 98.2 Heated in Wet Air Calclojned 1491 Fla. pebble 1200 52 0 .. 94.1 93.6 Caloined 1494: Tenn. brown rock 1200 40.8 , 99.4 99.6 Calcined 1478 Tenn brown rock 1200; 24.7 98.8 90.6d Fused 1476: Tenn: brown rock 1200 19.8 . .. 87.8 83.7d Calcined 1496 Mont. rock 1200 71.6 98.4 98.1 Calcined 1495' Wyo. rock 1100 23.0 9i:g 9i:g 94 8 90.2 Caloined 1497: Idaho rock 700 32.2 98.9 98 4 99.9 99.5 a 1-Gram charges heated. at 1400' C. for 10 minutes, pulled directly into another furnace, reverted by heating at indicated temperature for 30 minutes, cooled to room temperature,
Reversion Rate of Calcined Phosnhate
c
VOL. 29, NO. 11
%
%
Heated in Dry Air 1491 Fla. pebble 790-C 93.7 66.7 66.0 56.4 1496 Wyo. rock 948 96.6 86.9 84.4 81.9 1496 Mont. rook 1252 98.2 75.1 73.4 70.9 Heated in Wet Air 1491 Fla. pebble 790-C 93.6 48.2 44.8 44.5 1494 Tenn. brownrock909 99.5 39.8 38.7 35.0 1496 Mont. rock 1262 98.1 64.6 63.2 60.2 1497 Idaho rock 1263 99.6 74.8 62.8 46.2 0 1-Gram charges heated at 1400O C. for 10 minutes, then pulled into another furnace and annealed. b Original aaloined phosphate heated at 1400' C. for 10 minutes.
Solubilization of Reverted Phosphate As shown in Table 111, the citrate-insoluble phosphorus formed by annealing defluorinated phosphates a t temperatures below 1400" C. can be converted into the citratesoluble condition by reheating the material a t 1400" C. The resolubilization of the phosphorus is rapid and appears to be almost complete in 5 minutes or less.
In the commerical production of defluorinated phosphate, provision should be made for rapidly cooling the product to temperatures below 400" C. after it is removed from the furnace. Otherwise, serious reversion of the phosphorus to the citrate-insoluble condition may and probably will occur. In many cases serious reversion will occur if the temperature of the product is allowed t o remain a t 800" to 1200"C. for 5 minutes. Also, the defluorinated material should not, in general, be held in the furnace after the temperature has fallen below about 1350" C. Under otherwise comparable conditions, reversion is usually much greater in products that are cooled slowly in wet air than in those cooled slowly in dry air. It is probable, however, that under factory conditions serious reversion will not occur in produots that are cooled rapidly-for example, by quenching in water. Certain defluorinated phosphates show little or no reversion when they are cooled slowly in either dry or wet air. Defluorinated phosphates in which reversion of the phosphorus has occurred as a result of slow cooling, can be resolubilized by reheating a t 1400" C. for a few minutes and rapidly cooling the product.
Literature Cited (1) Curtis, H. A., Copson, R. L., Brown, E. H., and Pole, G. R., IND.ENG.CHEM.,29, 766-70 (1937). (2) Jacob, K. D., Rader, L. F., Jr., and Tremearne, T. H., J . Assoc. Oficial Agr. Chem., 19, 449-72 (1936). (3) Marshall, H. L.,Reynolds, D. S., Jacob, K. D., and Rader, L. F., Jr., IND. ENQ.CHEM., 27, 205-9 (1935). (4) Reynolda, D. S., Jacob, K. D., Marshall, H. L., and Rader, L. F., Jr., Zbid., 27, 87-91 (1935). (5) Reynolds, D. S., Jacob, K. D., and Rader, L. F., Jr., Ibid., 26, 406-12 (1934). (6) Reynolds, D. S., Marshall, H. L., Jacob, K. D., and Rader, L. F., Jr., Ibid., 28, 678-82 (1936). RECEIVEDJuly 28, 1937. Presented before the Division of Fertilizer Chemistry at the 92nd Meeting of the American Chemical Society, Pitbburgh, Pa., September 7 to 11, 1936.
