December, 1933
INDUSTRIAL AND ESGISEERING CHEMISTRY
present may be, owing to an efficient washing step, restricted to any colloidal occluded sulfur and to sulfur which may be in combination with the cellulose present. Consideration of the analytical methods and their results indicate that such sulfur is of two types. One of these, the free sulfur, must then be in the form of organic sulfides or of occluded colloidal sulfur in order to give hydrogen sulfide upon analysis. It is difficult to understand why, if this were in the form of colloidal sulfur, it was not completely extracted by benzene in series C and D but was completely extracted by water. If in the form of organic sulfides, it might well be hydrolyzed to soluble compounds by water and yet remain insoluble in benzene. It is indicated then that the free sulfur probably consists of sulfide compounds with cellulose or cellulose residues. I n the same way the combined sulfur may be compounds of the higher oxides of sulfur with cellulose--for example,
1321
the sulfate, formed by the direct action of residual spin bath acid. Sulfates of cellulose are, in general, quite soluble in water or are a t least readily hydrolyzed thereby. It would be difficult to account for the fact that benzene would extract half of the total combined sulfur if it were in the form of adsorbed inorganic salts.
CONCLUSION The general picture derivable from the results obtained is that there is a portion of the sulfur present in some form readily removed by almost any aqueous solution. The best guess as to the form of this is that of easily hydrolyzable or water-soluble sulfates of organic residues. The residue, which is more difficult to remove, is assumed to be in the form of some sulfide combination with cellulose. RECEIVED July 27, 1933.
Volatilization of Fluorine in Manufacture of Phosphorus and Phosphoric Acid by Furnace Processes D. S. REYNOLDSAND K. D. JACOB, Bureau of Chemistry and Soils, Washington, D. C. HOSPHATE rock is the principal source of the fluosilicates-chiefly the barium, magnesium, and sodium salts-used for various agricultural, industrial, and technical purposes, such as the control of plant pests ( I ) , the moth-proofing of fabrics ( I d ) , the preservation and hardening of cement and concrete ( 7 ) ,the treatment of rubber latex (4), the manufacture of glass (19) and enamels (20), the electrolytic refining of lead (6) and antimony (RZ),and laundry purposes (24). The commercial grades of domestic phosphate rock usually contain approximately 3.3 to 3.9 per cent fluorine (8, 10, 13, 1'9, and the average fluorine content of all phosphate rock produced in this country at present is probably close to 3.7 per cent. In the manufacture of ordinary superphosphate approximately 25 per cent of the fluorine in the phosphate rock, equivalent to about 20 pounds of fluorine per long ton of rock, is volatilized ( I O ) , principally as silicon 1etrafluoride, and the greater portion can be recovered in the form of hydrofluosilicic acid by washing the gases with water. I n normal years of fertilizer production approximately 20,000 short tons of fluorine are volatilized in the manufacture of superphosphate in the United States. However, only a portion of the total fluorine volatili,zed is recovered, the majority of the plants allowing it to go to waste. I n recent years furnace methods for the production of high-grade phosphoric acid (5, 12, 23) have comc into prominence in this country, and it is anticipated that these methods will play an important part in the future phosphate fertilizer industry. Klugh (12) states that the fluorine volatilized from phosphate rock during the electric furnace manufacture of phosphoric acid has a very strong corrosive effect on all refractories commonly used in building conduits for handling the hot furnace gases, and it is known that a certain amount of fluorine, principally in the form of silicon tetrafluoride, escapes from the Cottrell precipitators used for the recovery of the phosphoric acid. Data on the quantity of fluorine volatilized in the manufacture of phosphorus and phosphoric
acid by furnace processes are of interest from the standpoint of furnace operation and also in connection with the possible recovery of fluorine compounds from the furnace gases. Inasmuch as such data were not available in the literature, several experiments and analyses were made to obtain information on the subject. TABLEI. VOLATILIZATIOS OF FLUORISE FROM SMALL MIXTURES OF FLORIDA LAKD-PEBBLE PHOSPHATE, CARBON, AND SILICA (Mixtures heated f o r 0.5 hour in a graphite tube furnace in a n atmosphere of dry nitrogen) COMPOSITION O F X f I X T T R E TEMP. F \'OLATILIZED P VOLATILIZED 8 . 5 grams phosphate rock,b 1.5 gram8 carbon
5.95 grams phosphate rock, 2.55 grams silica,C 1.5 grams carbon
c.
%"
%"
1100
6.6
6.8
1300
14.3
49.1
1300
16.3
96.2
1400
9.7
98.8
a Percentage of total in original phosphate rock. b Containing 3.87 per cent fluorine, 8.98 per cent total silica, and 31.62 per cent PnOn. c I n addition to the silica present in the phosphate rock.
EXPERIMENTS IS LABORATORY FURNACE I n these experiments 10-gram mixtures of 200-mesh Florida land-pebble phosphate and carbon, and of pebble phosphate, carbon, and silica (quartz) were heated in an atmosphere of dry nitrogen for 0.5 hour in the graphite-tube resistance furnace previously described (3, 9 ) . The volatilization of fluorine was determined from the total fluorine content of the residue as compared with that of the original charge. The fluorine content of the phosphate rock, and also that of the other phosphate rocks and the sintered phosphate matrix discussed elsem-here in this paper, was determined by the TTillard and Winter method (25, 26). This method may give low results, however, when used on slags containing acid-decomposable silicates. Consequently, all
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IN DUSTR IAL AND EN GI NEER IN G CHE M ISTR Y
fluorine determinations on slags and furnace residues were made by the fusion-acid extraction method (16). The latter usually gives somewhat lower results than the Willard and Winter method on highly phosphatic materials, such as phosphate rock. As shown in Table I, only 6.6 to 16.3 per cent of the fluorine in the phosphate rock was volatilized from charges heated under the above-mentioned conditions a t 1100" to 1400" C., although 6.8 to 98.8 per cent of the phosphorus was volatilized under the same conditions. VOLATILIZATION OF FLUORINE IN ELECTRIC FURNACE MANUFACTURE OF PHOSPHORIC ACID Samples of Tennessee brown-rock and Florida land-pebble phosphates and of the slags produced therefrom in the manufacture of phosphoric acid by the electric furnace process, were kindly furnished by the Swann Chemical Company, Anniston, Ala., in December, 1930. These were analyzed for fluorine, phosphoric oxide, and calcium oxide, and the percentages of fluorine and phosphorus volatilized were calculated from the differences in the fluorine-calcium oxide and the phosphoric oxide-calcium oxide ratios in the phosphate rocks and in the corresponding furnace slags, respectively. These calculations do not take into consideration the calcium oxide, phosphoric oxide, and fluorine in the silica flux and in the reducing agent (coke). These materials do not contain important quantities of phosphorus and fluorine, and, according to Klugh (11),the calcium content of the materials used a t Anniston is insignificant in comparison with that of the phosphate rock. Under normal operating conditions volatilization of important quantities of calcium oxide does not occur. Consequently, the total calcium oxide content of the slag is substantially the same as that of the phosphatic material charged to the furnace.
Val. 25, No. 12
to phosphoric oxide in the original matrix from which the sintered material was prepared, it is known that a certain amount of fluorine is volatilized during the sintering process. TABLE111.
VOLATILIZATION O F FLUORINE IN BLAsT MANUFACTURE OF PHOSPHORUS
MATERIAL Sintered Florida landpebble phosphate matrix Furnace slag 1 Furnace slag 2
F
COMPOSITION P10a CaO
FURNACE
F VOLA-
P VOLA-
TILIZED
TILIZED
%
%
%
%"
%b
1.90 1.78
25.83 3.55 2.14
35.06 44.96 45.74
26:9 19.3
93.6
2.00
89.3
a Calculated from the difference between the ratios of fluorine t o calcium oxide in the sintered phosphate matrix a n d t h e furnace slag, respectively b Calculated from the difference b e t a e e n t h e ratios of phosphoric oxide to calcium oxide in the sintered phosphate matrlx and the furnace slag, respectively
The volatilization of fluorine during the blast furnace smelting of the sintered matrix was determined in the same manner as outlined for the electric furnace process. The results (Table 111) on two slags show 26.9 and 19.3 per cent volatilization of the fluorine charged to the furnace. The yield of slag from this particular raw material is approximately 80 per cent. Consequently, if the volatilization is calculated from the fluorine content of the slag as compared with that of the original phosphate material, the results show 25.0 per cent volatilization in the case of slag 1 and 15.8 per cent with slag 2. These figures are much lower than those (40.6 to 97.5 per cent of the fluorine in the original phosphate) reported by Britzke and Pestov ( 2 ) as having been obtained in experiments in the production of phosphoric acid in a fuel-fired shaft-type furnace by smelting mixtures of Russian phosphorite, silica, and carbonaceous material a t 1 1 0 0 O to 1540' C. The reason for the high volatilization of fluorine reported by Britzke and Pestov is not evident, but it may have been due, a t least in part, to T a m 11. VOLATILIZATION OF FLUORINE IN ELECTRIC FURNACE failure of the analytical methods, used by these investigators, MANUFACTUR~ OF PHOSPHORIC ACID to account for all the fluorine present in the furnace residues COMPOSITION F VOLA- P VOLAand slags (16). MATERIAL F P ~ O S CaO TILIZED TILIZED The results given in Tables I1 and I11 indicate that in the % % % %" %a Tennessee brown-rock smelting of phosphate rock approximately twice as much fluorine is volatilized in the blast furnace process as in the electric furnace process. As shown by Reynolds and Jacob phosphate Furnace slag 4.05 ( I @ , the moisture content of the furnace atmosphere has a Calculated from t h e difference between t h e ratios of ,fluorine t o calcium very pronounced effect on the volatilization of fluorine from oxide i n t h e phosphate rock a n d t h e furnace slag, respectively. mixtures of phosphate rock and silica a t high temperatures; b Calculated from t h e difference between t h e ratios of phosphoric oxide to calcium oxide in t h e phosphate rock a n d t h e furnace slag, respectively. this is probably a contributing factor in the greater volatilization of fluorine in the blast furnace process. However, As shown in Table 11, 10.8 per cent of the fluorine was the relative amount of fluorine volatilized, even in the blastvolatilized from the Tennessee brown-rock phosphate and furnace process, is no greater than in the manufacture of 7.3 per cent from the FIorida land-pebble. If the voIatiliza- ordinary superphosphate (10). In the furnace processes, as tion of fluorine is calculated from the fluorine content of the in the manufacture of ordinary superphosphate, the fluorine slag as compared with that of the phosphate rock, assuming seems to be volatilized principally in the form of silicon tetrathat 2.548 pounds of slag are produced per pound of phos- fluoride. phoric oxide charged to the furnace as stated by Klugh (11), the results show a volatilization of 13.2 per cent of the fluorine RECOVERY OF FLUORINE from the Tennessee rock and 10.5 per cent from the Florida pebble. As far as the authors are aware, no attempt has been made to recover commercially the fluorine volatilized in any of VOLATILIZATION OF FLUORINE IN BLASTFURNACE MANUthe furnace processes for phosphorus and phosphoric acid, FACTURE OB PHOSPHORUS and no information is available on the percentage of the Samples of sintered Florida land-pebble phosphate matrix volatilized fluorine that can be recovered. In the electric and the slags produced therefrom in the manufacture of furnace process, as operated at Anniston, Ala., a small porphosphorus by the blast furnace process, were kindly fur- tion of the fluorine is dissolved in the phosphoric acid renished by the Coronet Phosphate Company, Pembroke, Fla., covered in the Cottrell precipitators; according to several in February, 1931. The ratio of fluorine to phosphoric oxide old analyses (18, 21) the acid carries approximately 0.01 to in the sintered matrix was 0.074 which is considerably lower 0.03 per cent fluorine, but a recent analysis (Sa)showed 0.06 than the ratios, 0.110 to 0.130, usually found in the ordinary per cent fluorine in precipitator acid containing 88.3 per cent commercial grades of washed Florida pebble phosphate (8). H3P04. This figure represents the fluorine content of crude Although information is not available on the ratio of fluorine acid which is then further treated for removal of fluorine.
1~'liiorincIussw prdmiily occur in the gas Inailis ahead of tlie Cottrcll 11recijiitators, and it is probable that only that part of the fluorine which oscapea in the gaseous condition f r m the precipitators would be available for recovery. Similar losses also occur in tho other furnace proeesscfi. The available data indicate that under the present cmditions of plant operation there is nat inucli promise
