Composition and Properties of Superphosphate Volatilization of Fluorine in Superphosphate Manufacture' K. D. JACOB, H. L. MARSHALL, D. S. REYNOLDS, AND T. H. TREME-ARNE
Fluosilitcaes are obtained principally as byproducts of t h e manufacture of superphosphates. I n the production of ordinary superphosphate about 11 to 42 per cent of the fluorine i n the phosphate rock is volatilized during the mixing and denning operations, and the greater portion of the volatilized fluorine can be easily recovered as aqueous solutions of hydrofluosilicic acid. Also, small quantities of fluorine are volatilized during the subsequent handling of the fresh superphosphate. I n general, the percentage volatilization of the fluorine tends to decrease w i t h increase i n the phosphorus content of the rock. With rocks of the same phosphorus content, the fluorine volatilization is somewhat higher from Tennessee brown rock than from Florida land pebble. Owing principally to the much smaller quantities of fluorine in furnace-made phosphoric acid, double superphosphates produced with phosphoric acid made by wet processes usually contain considerably more fluorine than do tholse produced with acid made b y furnace processeii. Studies of t h e distribution of fluorine among the callcium sulfate waste and the intermediate andi final products of the manufacture of double superphosphates by the wet method showed cumulative volatilizations of fluorine ranging from 36 to 54 per cent of t h e total fluorine in all the phosphate rock used throughout the process.
Bureau of Plant Industry, U. S. Department of Agriculture, Beltsville, Md.
HE commercial grades of domestic phosphate rock that contain more than 66 per cent €3. P. Laa(30.2 per cent P&) carry more than 3 per cent but usually less than
T
4 per cent of fluorine, and a portion of the fluorine is evolved in the manufacture of superphosphate. At a number of domestic plants that produce ordinary superphosphate, as well as a t some that manufacture double superphosphate (also called triple or treble superphosphate), the volatilized fluorine is recovered more or less completely as hydrofluosilicic acid and is marketed chiefly as barium, zinc, ammonium, magnesium and sodium fluosilicates, principally the latter. At most of the plants, however, the volatilized fluorine either is allowed to escape t o the atmosphere or is absorbed in water and the solution run to waste. I n recent years, especially since the outbreak of the war and the initiation of the defense program, there has been a large increase in the domestic demand for fluosilicates. This, together with the marked decrease in importations, has caused a sharp rise in prices, with the result that more superphosphate plants have undertaken the recovery of fluorine. As published information relating t o fluorine volatilization in phosphate fertilizer manufacture is almost nonexistent, representative data showing the quantities of fluorine evolved in the production of superphosphate a t a considerable number of plants would seem to have a particular interest at the present time. The evolution of fluorine begins in the mixing pan and continues in a varying degree during subsequent operations; thus, in order to obtain comparable volatilization data, the samples of the products must be taken a t the same stage in the manufacturing process. Even this precaution does not afford data that are comparable in the strictest sense, because manufacturing practices-for example, concentration of the sulfuric acid, acid-rock ratio, and time the charge remains in the mixer-vary from plant to plant. I n the case of ordinary superphosphate, samples of the den superphosphate and the rock used in its preparation were obtained and analyzed for P205 and fluorine. I n the more complex case of double superphosphate, samples of the rock and of the products a t each stage of the process were obtained and analyzed for P206,fluorine, and CaO. I n most instances the interval between sampling and analysis was a t least one week, but in the meantime the samples were kept in closed containers and
lost little, if any, fluorine. Except :as indicated otherwise, the samples were obtained from commercial operations; the latter were each assigned a plant number for identification and use in the tables. I n case a company operates more than one plant, the numerical part of the plant number is the same, whereas the different plants are indicated. by letters. For example, 9a, 9b, etc., indicate plants operated by company No. 9. The methods of calculating the fluorine volatilizations are described in connection with the presentation of results.
Fluorine Volatilization i n the Manufacture of Ordinary SuperphosphRte The data for ordinary superphosphate manufacture, as determined on samples of den superphosphates, are assembled in the ascending order of the P2O6 conlknt of the rock in Table I. The fluorine in the superphosphate expressed as a fraction of the fluorine in the rock is obtained by dividing the fluorineP206ratio of the superphosphate by the 1Ruorine-P~O5ratio of the rock. Subtraction of this quotient from 1 and multiplication of the result by 100 gives the volatilization in per
Previous papers in this series appeared as follows: J . Assoc. Oficial (1934), 18, 244-60 (1935), 19, 328-38 (1936); IND. ENQ.CHEX, 28, 440-7 (19361, 32, 1128-35, 1224-32, 1631-6 (1940). 1 Bone phosphate of lime, a trade term for the tricalcium phosphate equivalent of the PlOa. 1
Agr. Chent., 17, 487-505
122
INDUSTRIAL AND ENGINEERING CHEMISTRY
June, 1942 ~~
OF FLUORINE IN THE MANUFACTURE OF ORDINARY TABLE I. VOLATILIZATION SUPERPHOSPHATES
Plant No. a
Phosphate Rockb Sample No. PiOa, % F, %
Superphosphate Sample Total NO. PSOS, % F, %
F Volatilized* As % Be lb.
1959 1900 1958 1957 2010 2012 1986 1980 1884 1978 1976 1867 2016
Den Superphosphates from Fla. Land Pebble 18.72 1.54 3.74 68 31.27 20.10 1.54 3.45 50 32.79 20 18.81 1.59 3.78 32.85 1965 19.45 1.49 3.62 33.17 19.05 1.49 2009 3.74 33.27 1.74 2011 20.25 3.88 33.43 1.70 1985 20.00 3.81 33.45 20.83 1.75 1979 3.84 33.54 1.93 1883 20.93 3.89 34.12 2.01 1977* 20.95 3.68 34.28 22.45 1.93 1975 3.84 34.94 1.96 1866 3.81 21.85 35.47 1.70 2015 20.50 3.90 36.33
70 7b Ig 17b
1965 1969 1889 1984 1967 2008 1974 1988
Den Superphosphates from 1964 32.44 3.47 3.47 1968 32.68 1888 33.53 3.92 3.58 1983 33.54 1866 33.65 3.59 2007 33.72 3.73 3.70 1973 33.90 1987 3.60 34.19
20a 9b 9s 4 17a 7a 16
1879 1893 1895 1882 1875 1891 1873
Cured Superphosphates from Fla. Land Pebble 1.71 1877 21.00 3.81 32.32 20.48 1.78 3.84 1892 32.82 1.86 1894 20.43 33.02 3.88 21.00 1.59 1881 33.47 3.85 1.74 1874 20.48 3.87 33.59 20.83 1.41 3.68 1890 33.80 1.87 20.78 1872 34.03 3.88
31.0 25.7 22.6 34.2d 20.2 37.W 21.1
23.6 19.7 17.6 26.3d 20.3 27.81 16.4
18 9d 6 90
1871 1899 1869 1897
Cured Superphosphates from Tenn. Brown Rook 20.30 1.46 3.66 1870 34.53 1.62 21.50 3.78 1898 34.84 1.46 1868 21.20 34.92 3.72 21.43 1.67 3.79 1896 34.96
32.6d 30.7 35.8 28.1
23.9d 23.2 26.6 21.3
27 23
Cured Superphosphates from Idaho and Mont. Phosphates .~ 1933 20.33 1.34 35.4dsh 1887 21.45 1.50 33.5dvh 1930i 3i:ii 3'.'3z 1927 20.30 1.32 38.3
la
7a
la 15 12 19 13a 4 13a
5 11
3a
2
21a 8 3b 10
:
...
31.2 27.2d 26.6 29.8 30.4 26.0 25.4 26.6d 19.1 10.7 21.7 16.5d 22.7
23.3 18.W 20.1 21.6 22.7 20.2 19.3 20.4d 14.9 7.9 16.7 12.W 17.7
Tenn. Brown Rock 42.1 19.85 1.23 2.65 -34.11 18.58 36.24 19.83 1.48 30.9 19.80 1.46 42.1 19.75 1.22 1.42 33.1 19.20 35.7 19.95 1.40 1.68 28.1 22.20
29.2 f 28.4d 22.1 30.2 24.7 26.4 20.2
...
... zK.4
a Plant numbers are different from those used by Marshall et al. (IO) in a previous paper of this series. b Moisture-free basis 105" C.). c Results in column are in per cent of the total fluorine; those in column B are in vounds Der 2000 vounds of moisture-free rock. d Plaht recovGed fluorine. e Oberphos f Super hoiphate contained a larger quantity of fluorine than did the rook. I Rock %om Cpnda, Idaho, was used. h Estimated with use of an average F-Pa06 ratio of 0.102 (range 0.101 to 0.103) for 6 8am lea of rook from the Conda Idaho mine. i gock from Garriaon, Mont.: was d e d .
d
~
cent of the total fluorine in the rock. Given the fluorine content of the rock and the fraction of this fluorine that is volatilized, the method for converting the volatilization to pounds per ton of rock is obvious. With den superphosphates (Table I) the fluorine volatilizations from Florida land pebble range from 10.7 (Oberphos) to 31.2 per cent and average 21.4 per cent, whereas those from Tennessee brown rock range from 28.1 to 42.1 per cent and average 35.5 per cent. I n the case of one brown-rock material (plant 8), the analytical data show a larger quantity of fluorine in the superphosphate than in the parent rock, which gives a negative result for fluorine volatilization. This variance was not due'to errors in the analyses, and the authors were unable to obtain any information that would explain it. The data for the den superphosphates from Florida land pebble (Table I) plainly show a tendency of the fluorine volatilization to decrease as the phosphorus content of the rock increases; this trend appears t o be much less pronounced with Tennessee brown-rock materials. The tendency of fluorine volatilization to decrease with increase in the grade of the rock is shown better by a comparison of the results obtained with rocks below and above 73.5 per cent B. P. L. (33.66 per cent P2O6). Thus the respective averages for Florida land pebble are 27.9 and 20.0 per cent, not including the Oberphos
723
which is made by a method radically different from all other processes of superphosphate manufacture; the averages for Tennessee brown rock are 37.8 and 32.3 per cent. A part of the fluorine evolved in the mixing pan and den is recovered a t some plants, as already pointed out. The paucity of data for plants that recover fluorine do not, however, permit any conclusions as to the relative volatilizations under recovery and nonrecovery conditions. Volatilization results for superphosphates cured under factory conditions are also given in Table I. The average volatilization of fluorine (28.4 per cent) for the cured land-pebble superphosphates, which were made from rocks containing 32.32 to 34.03 per cent Pz06,is slightly higher than that (26.9 per cent) for the first nine samples of the den materials made from similar grades of rock. It appears, therefore, that small quantities of fluorine are volatilized from land-pebble superphosphates during the period of curing in the pile. On the other hand, the average volatilization (31.8 per cent) for the cured brown-rock superphosphates is lower than that (35.5 per cent) for the den superphosphates. It should be noted, however, that all the rocks used in making the cured Tennessee superphosphates contained more phosphorus than did those from which the den materials were made. Since volatilization of the fluorine tends to decrease with increase in the grade of the rock, as previously pointed out, this is probably a factor in the lower results with the cured superphosphates. The chief value of the foregoing comparison is to emphasize the indication that only a relatively small part of the total volatilization occurs after the superphosphate leaves the den. The data in Table I1 show that less than 10 per cent of the fluorine in den superphosphate made from Florida land pebble is volatilized during the granulation of this material. Samples of about 50 pounds of each material were granulated experimentally by the method of rotary drying (6).
Fluorine Volatilization in Relation to Conditions of Ordinary Superphosphate Manufacture Considerable data relating to the conditions of manufacture are available for a number of the superphosphates in Table I. These data (Table 111) include the mechanical analysis of the phosphate rock, the concentration and temperature of the sulfuric acid, the acid-rock ratio, the type of process or den, and the time the superphosphate remained in the den after the latter was filled. The pertinent features TABLE11. VOLATILIZATIONOF FLUORINE IN EXPERIMENTAL OF DEN SUPERPHOSPHATES PREPARED FROM GRANULATION FLORIDA LAND-PEBBLE PHOSPHATE % ' of Total F Vola-
tilized during Granulation, in: Den Superphos- Max. Temp. Granulated SuperDen during phosphate phate. superPhosGranylaTotal Total phosphate rocks PzOS, % F, % phates tion, C. PzOs, % F. % 18.81 1.69 94 20.88 1.63 7.6 5.6 100-105 21.00 1.67 18.72 1.64 3.3 2.3 8.3 b 107 21.35 1.66 19.22 1.63 The first two materials were made at different times by plant la; the third was made by plant 7a. b No sample available.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
724
TABLE111.
Plan1
Sample
SO,
Xo.
9b
1893 1895 1957 2010 2012 1986 1980 1875 1891 1873 1978 1976 2016
9a 15 12 19 13a 4 17a 7a 16 5 11
2
21a 8 3b 10 7c 7b 1: 18 9d 90
1965 1969 1889 1984 1967 2008 1974 1988 1871 1899 1857
VOLATILIZATION OF
Xoisture,
ycD
1.0
1.1
0.5 1.5 0.8
1.0
1.5 0.9 0.8Oi 1.7 1.5 0.84j 0.9 1.0
1.3 0.9 0.8 1.5 0.95 0.95 2.1 1.0 0.8 1.1
FLUORINE U N D E R DIFFERENT COKDITIOXSOF
F,
ycb
Ycb
0.226 mm., 71
32.62 33.02 33.17 33,27 33,43 33.45 33.54 33.59 33.80 34.03 34.28 34.94 36.33
3.84 3.86 3.62 3.74 3.88 3.81 3.84 3.87 3.68 3.88 3.68 3.84 3.90
04.0 94.0 97.0 57.0 97.5 98.5 99.5 59.8
32.44 32.68 33.53 33.54 33.65 33.72 33.90 84.19 34.53 34.84 34.96
3.47 3.47 3.92 3.58 3.59 3.73 3.70 3.60 3.66 3.78 3.79
PiOa,
..
Vol. 34, No. 6
ORDINARYSUPERPHOSPHATE MAKPFACTURE
mesh, 0,147, mm., I (
.. 6910 61.0
81.0 68.5 93.5 92.0 80-85
99:0 95.0 98.0
8i:O
97.0 98.0
80.5
66.5 87.5
81.0
si: 0
99: 5 99.0 96.5 92.5 99.0
89.0 80.0 67.0 91.0
9; 0 94 0
..
Florida Land Pebble 54.5 (68.9) 100 (37.8) 54.5 (68.9) 115 (46.1) 56.0 (69.6) 120 (48 I91 56.0 (71.2) 140 (60,O) 54.8 (69.3) 111 (43.9) 56.0 (71.2) 120 (48.9) 56.5 (72.0) 140 (60.0) 55.0 (65.6) 120 (48.9) 5510 55.0 (69.6) 140 (60.0) 56.0 (71.2) 125 (51.7) 52.7 (66.2) 120 (48.9) 42:O 36.5 56.0 (71.2) 130 (54,4) 36.5 56.0 (71.2) 151 (66.1)
85.7 85.7 86.1 86.3 83.6 82.9 82.0 89.3 81.8 84.8 89.9 84.2 83.0
95.0 96.5 98.8 93.2 94.9 94.9 100,o 91.7 97.1 95.7 96.3 95.0
Box Box Broadfield Box Box Box Sturtevant Broadfield Box Silo Oberphos Broadfield Box
Tennessee Brown Rock 36.0 55.2 (69.9) 120 (48.9) 31.5 52.0 (65.1) 125 (51.7) 55.2 (69.9) 135 (57,2) 54.5 (68.9) 37:5 ..... 55.0 (69.6) 41 .O 96 (35,5) 32.5 53.0 (66.6) 140 (60.0) 24.5 55.0 (69.6) 109 (42.8) 5 6 . 0 (71.2) 114 ( 4 5 . 5 ) 21.5 54.5 (68.9) 120 (48.9) 5610 54 .,7 (69.2) 120 (48.9) 5 4 . ~(69.2) 120 (48.9) 52.0
88.1 89.8 91.0 87.0 82.9 90.9 87.6 83.0 88.9 89.9 89.9
59.1 94.1 102.4 96.3 92.8 97.4 98.1 95.0 98.5 100.0
Sturtevant Box Sven8k.z Box Box Box Box Broadfield Box Box Baker
50.0 50.0 53.0 37.5 39.0 42.0 41.0
95.0
100.0
1b2 h r . 1--2 hr. ;SO min. 5 hr. 16 hr.
5--6 hr. 1 hr. 2 1 min. 6 hr. 24 hr.b
32 min. 40 min.
20 min.
20 min. 24 hr.k 1 hr. .
.
I
.
.
2 hr. hr.
