T H E REACTION OF SULFUR OXIDES WITH PHOSPHATE ROCK L. W. ROSS'
AND H . C . L E W I S
Georgia Institute of Technology, Atlanta, Ga. Reaction of SO2 and so3 with phosphate rock in fixed and fluidized beds has shown that rapid conversion is possible with SOa, but that the reaction with SO2 is very slow. The SO8 reaction, however, exhibits upper limits on the attainable conversion, depending on conditions. Best conversion with SO3 is obtained a t 325' =t 5' C. with admixture of air and steam.
HE conventional trcple superphosphate process is greatly Tcomplicated by the role played by water. Water is added to the process stream at two points, only to be removed a t great expense in the evaporators for concentrating phosphoric acid and in the triple superphosphate dryers. In addition, triple superphosphate plants are highly prone to corrosion and air pollution problems arising from acid mist. For these reasons many investigators have sought to produce phosphatic fertilizers by contact of phosphate rock with anhydrous sulfur oxides. The first such process reported is that of Giana ( 4 ) ,in which the product is calcium pyrophosphate, produced a t 800' C . :
Ca3(POd?
+ SO2 +
-
+
l/202 Cas04 Ca2P207 (1) (9) (€9 (4 (4 Hughes and Cameron (5) performed a n extensive study with phosphate rock using both gaseous SOa (produced by vaporizing HzS04) and SO2 ac reactants. Conversions of P205 were 90y0to citrate-soluble form after 5 hours of contact with SO3 a t 500' to GOO0 C . , and about two thirds to water-soluble form by SO2contact for 10 hours a t 350' to 450' C. Baumgarten and Brandenburg (2) are the only investigators to claim pure phosphorus oxide as product. They brought potassium phosphates in contact with liquid sos. Pompowski (7) used dilute SO2 gas mixtures, and obtained conversions approaching those of H ~ g h e sand Cameron, but contact times were extremely long. Scheel (9) had previously claimed a similar process. T h e objective of the investigation described here was to alter the nature of phosphate rock by contact with sulfur oxides to produce (if possible) a phosphatic product competitive with existing plant nutrients. Previous investigators had overlooked the possibiliiies of fluidized beds for enhanced gassolid contact; pure SO3 was formerly unavailable in bulk quantities for process application ; and no previous investigators had explored the role (possibly catalytic) played by water. T h e investigation described here attempted to take advantage of these neglected factors. A minor objective was to produce free phosphorus oxide, which had been observed in gaseous form in preliminary experiments.
(4
Experimental
Fluidized-bed contact of phosphate rock with gaseous
SOP and SO2 was conducted in a stainless steel reactor 0.43 inch in inside diameter and 22 inches in length, holding a charge of 5 to 6 grams of phosphate rock. Fixed-bed contact was performed in a variety of glass arrangements.
Sulfur
Present address, Catholic University of America, Washington, D.C.
trioxide was supplied by boiling 20% oleum, and sulfur dioxide was supplied from a cylinder. Details are reported by Ross ( 8 ) . Analyses were conducted according to AOAC official methods ( 7 ) in the cases of phosphorus and sulfur, and according to standard methods for calcium (70) and fluorine ( 3 ) . Throughputs of so3 were measured by absorption and subsequent titration; SO2, air, and steam were metered. T h e composition of the phosphate rock used in this investigation is given in Table I. Results and Discussion
Liquid so3 contact with phosphate rock was shown to yield C a s 0 4 and free phosphorus oxide, but the P2OS is present in highly polymeric, nonvolatile form. T h e Ca-F linkages of the fluorapatite in the phosphate rock are undisturbed; C a s 0 4 forms entirely from calcium originally associated with phosphorus. The evidence for these conclusions is the material balance, plus x-ray diffraction demonstration of CaS04. Liquid SO3 contact produces a small amount of gaseous PlOla under certain conditions. The amount is consistent with the hypothesis that sulfuric acid is formed from SO3 in the phosphate rock by combination with moisture, and then phosphoric acid is formed by reaction and ultimately decomposed by desiccant action of excess SO3. The small quantities prevented a reliable material balance to prove this hypothesis. Gaseous SO3 contact in the fluidized bed yielded the results reported in Table 11. Contact with SOP alone yielded the results in Table IIA, which exhibit a maximum conversion of PZOSto a n availability (A0.4C definition) of 8 to 9%. I n Table IIB, it is seen that SOa-air mixtures offer no improvement in conversion, but the experimental difficulties of fluidization are greatly reduced by air admixtures. Conversion was observed to be insensitive to SOa dilution down to about 5% SO3 (molar). Table I I C reveals the ultimate capabilities of the SO3-steam-air system-about 55% conversion of P2O5 to available form. Most of the conversion occurred within 5 Table 1.
Typical Analysis of Phosphate Rock Employed in Present Investigation Weight Weight Constituent 7% Constituent 5% 33.88 0.361 pzo5 40.9 1.46 Ca 3.25 Free moisture 0.67 F S 0,37 Water-soluble 0.02 4.67 P&;Q Si02 AI 0.373 Citrate-soluble 2.42 Fe 0.855 P?OP a By A O A C oficial method.
VOL. 6
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OCTOBER 1967
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Table II. Typical Results of Fluidized-Bed Contact of Sulfur Trioxide with Phosphate Rock Total Temp., Time, P z O ~ , A.P.A.a, F, s, O c. Min. W t . 70 W t . 70 W t . % W t . y! A. Contact with SO3 Alone 250-500 .. 30.42 3.10 4.01 8.66 310 5.58 3.84 1.65 30 32.68 286-320 30 27.89 8.99 3.10 1.76 297-324 30 30.76 8.66 3.80 3.49 3.60 4.14 27 5-3 31 30 30.43 8.53 294-302 30 31.44 9.24 3.60 3.13 327-332 15 31.24 9.04 3.44 3.73 ... ... 320 30 30.3 7.72 B. Contact with SO3 and Air 330 15 28.6 6.9
31.9 8.4 ... ... 28.8 9.2 ... ... 320 30 30.6 6.7 5 29.1 8.5 ... ... 325 320 10 29.6 8.6 ... ... C. Contact with SO3,Steam, and Air 320 15 22.1 10.46 ... ... 325 15 24.4 12.74 ... ... 335 3 23.1 12.30 ... ... 330 3 26.0 11.47 ... ... 330 1 31.4 9.40 ... ... 330 2 30.5 8.94 ... ... 320-340 5 28.8 10.09 ... 325 10 21.2 9.56 ... ... 330 10 29.3 10.60 2.98 10.0 325 15 23.7 13.12 1.70 10.3 15 24.7 13.73 1.70 5.6 325 ... ... 325 15 23.4 12.33 a Available phosphoric acid, sum of water-soluble and citrate-soluble PsOs by A O A C dejnitions. .
I
.
minutes, and all occurred within 15 minutes. The upper limit of conversion is probably due to a film of reacted material o n the particles of phosphate rock, but this phenomenon was not studied. The reaction temperature was usually about 325’ C. O b servation indicated that conversions dropped sharply above 340” C. This corresponds closely to the maximum tempera-
ture of the vapor-liquid region of the Hz0-S03system a t 1 atm. (6). Therefore the conversion limit a t 340’ C. is probably due to absence of liquid, since the fluidizing phase cannot wet the rock above this temperature. Stable fluidization was possible between about 315’ and 340’ C., and this dictated the choice of 325” C. as normal operating temperature. Sulfur dioxide produced only minor conversion of PZOS to available form a t contact times up to 1 hour. Detailed results are reported by Ross ( 8 ) . The product of the runs reported in Table I I C was brought in contact with a series of extractant solutions, in a n effort to discover whether or not the product \Yould yield more PzOs than the AOAC “availability.” The results may be summarized as follows: Basic and neutral solutions reduce the PpOb availability, and acid solutions improve the PpOb availability slightly (to 16.1% with KZS04 saturated solution a t 65’ C.). Both results are to be expected of highly polymerized PZOb. Ac knowledgrnent
The authors acknowledge the support of the Tennessee Corp. for this research. W. 0. Land, Jr., gave valuable assistance with experimental equipment. Literature Cited (1) Association of Official Agricultural Chemists, Washington, D. C., “Official Methods of Analysis,” 9th ed., 1966. (2) Baumgarten, P., Brandenburg, C., Chem. Ber. 72, 555-63 (1939). ( 3 ) Furman, N. H., ed., “Standard Methods of Chemical Analysis,” Vol. I, 6th ed., pp. 442-4, Van Nostrand, Princeton, N. J., 1962. (4) Giana, E., German Patent 219,680 (1907). ( 5 ) Hughes, A. E., Cameron, F. K., Ind. Eng. Chcm. 23, 1262-71 (1931). (6) Luchinskii, G. P., Zh. Fiz. Khim. 30,1207-22 (1956). (7) Pompowski, T., Zesrty Nauk. Politechn. Gdansk., Chem. 4, NO. 26, 3-28 (1962). (8) Ross, L. W., Ph.D. thesis, Georgia Institute of Technology, 1966. ( 9 ) Scheel, K., German Patent 966,264 (1957). (10) Snell, F. D., Biffen, F. M., “Commercial Methods of Analysis,” rev. ed., pp. 216-18, Commercial Publishing Co., New York, 1964. RECEIVED for review September 23, 1966 ACCEPTED May 1, 1967
FLUIDIZED BED DISPOSAL OF FLUORINE JOHN T. HOLMES, LOWELL B. KOPPEL,’ AND ALBERT A. JONKE Argonne National Laboratory, Argonne, Ill. 60439
s
volatility processes for the recovery of unspent fissionable and fertile materials from nuclear reactor fuels grow toward future commercial application, environmental contamination and waste disposal considerations will require evaluation of existing methods or development of new methods for the disposal of the toxic gaseous reagents used in the process. This paper describes the development of a new method for the disposal of fluorine. The work is part of a continuing effort a t the Argonne National Laboratory to develop methods for the disposal of gaseous fluoride volatility reagents and of volatile fission product compounds. The first requirement is that a fluorine disposal system have a high efficiency for the removal of fluorine from a gas stream.
A
FLUORIDE
Present address, Purdue University, West Lafayette, Ind. 408
l & E C PROCESS D E S I G N AND DEVELOPMENT
I t should also be economic, involve simple equipment and procedures, and c,onsume a minimum amount of chemical reactant. If the process is to be used in a nuclear fuel reprocessing plant, it should have a product which is suitable for packaging and storage as radioactive waste (preferably a free-flowing solid), and be able to remove fission product compounds associated with the fluorine-containing gas stream. Existing methods for fluorine disposal include the reaction of fluorine with liquids, gases, or solids. Gas scrubbers employing caustic solution are in common use for the disposal of fluorine (Liimatainen and Levenson, 1953; Slesser and Schram, 1951 ; Stainker, 1956). These scrubbers are efficient, but produce large volumes of liquid wastes, which is considered undesirable for radioactive applications. The reaction of fluorine with gases such as hydrogen, hydrocarbons (Long,
