Chapter 8
Recovery of Phosphates from Elemental Phosphorus-Bearing Wastes Ronald E . Edwards, Jack M. Sullivan, and Oscar E . Moore
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Environmental Research Center, Tennessee Valley Authority, P.O. Box 1010, Muscle Shoals, AL 35662-1010
A process for oxidizing aqueous elemental phosphorus containing residues (sludges) to produce orthophosphate containing slurries suitable for subsequent reaction with ammonia to produce nitrogen and phosphate containing plant nutrient products. It comprises reacting aqueous elemental phosphorus containing residues with certain special mixtures of concentrated nitric acid and sulfuric acid to effect the conversion of the elemental phosphorus into mostly orthophosphoric acid and very little orthophosphorus acid with relative ratios of the two acids being dependent upon the mole ratio of H SO :HNO employed in the processing. The resulting aqueous reaction intermediate is neutralized with ammonia during processing to a fluid or solid plant nutrient product. Prior to the conversion to products, the aqueous reaction intermediate may be subjected to a solids separation step to remove insoluble salts of certain environmentally undesirable metals, such as Pb, Cd, Ba, and Cr. 2
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3
Historically high purity elemental phosphorus has been used for the production of a number of industrial products including detergents, dentifrice products, food additives, phosphatizing agents, fertilizers, and a whole host of phosphorus containing chemicals (1). During war time, huge quantities of elemental phosphorus is required for the production of ammunitions and incendiary bombs (2). Practically all elemental phosphorus was produced by the electric furnace method which also produced quantities of elemental phosphorus bearing wastes. In recent years most of the furnaces were decommissioned and some of the above mentioned uses of elemental phosphorus have been replaced by less pure phosphates produced from purified wet process phosphoric acid. However, large quantities of the elemental phosphorus bearing wastes still exist at most of the operational and non-operational phosphorus furnace sites, military installations, and industrial sites that used elemental phosphorus
This chapter not subject to U.S. copyright Published 1995 American Chemical Society Tedder;Pohland; Emerging Technologies in Hazardous Waste Management V ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
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8. EDWARDS ET AL.
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Recovery of Phosphates from Wastes
as a raw material. As of 1992, there was an estimated total of 400 acre-feet of these phosphorus bearing wastes in the form of sludges awaiting treatment in the United States (3). The objective of this research was to develop an environmentally sound process to safely process elemental phosphorus bearing wastes by minimizing or eliminating all processing by-products and producing a usable, and hopefully marketable, product other than elemental phosphorus. This process development study was initiated by the Tennessee Valley Authority (TVA) to develop technology for the processing of elemental phosphorus containing by-products produced by the electric furnaces operated until 1976 at Muscle Shoals, Alabama. This technology could be useful at numerous other facilities throughout the United States that once produced elemental phosphorus by the electric furnace process or utilized phosphorus as a raw material. Background of Phosphorus Wastes In the electric furnace process for producing elemental phosphorus, agglomerated phosphate-containing ore (apatite) is reacted with coke at temperatures of 1200 to 1500 °C. Silica is added to the reaction mixture to serve as a flux to remove calcium silicate slag. A simplified equation for the overall process is shown below (4). 2Ca (P0 ) + 6 S i 0 3
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+ 10 C = P + 10 CO + 6 (CaO-Si0 ) 4
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(1)
The gaseous phosphorus and carbon monoxide, along with impurities (generally SiF and dust), are passed to an electrostatic precipitator where a major portion of the dust is removed at temperatures above the dew point of phosphorus. The gaseous phosphorus and carbon monoxide then proceed to condensing columns where the phosphorus is condensed to a liquid by water sprays maintained at 45-55 °C. The uncondensed carbon monoxide exits the sprayers and may be burned as a fuel. Ferrophosphorus is tapped from the furnace as a by-product for use in steel production. The calcium silicate slag is removed as a waste product containing no elemental phosphorus. The water-covered liquid phosphorus runs to a sump where a small amount of impurity sludge collects at the interface between the water and the liquid phosphorus. This sludge layer consists primarily of hydrated silica, fluorosilcates, dust, and emulsified phosphorus. A portion of the entrained phosphorus is recovered from the sludge by centrifugation and a limited amount of the sludge is recycled to the furnace. However, the physical and chemical properties of the residue prevents its complete recycle and the remaining sludge must be treated as a waste or processed by some other means. In past practice, phosphorus producers often stored such by-product sludges in ponds, sumps, or tanks for future processing. Due to certain environmental considerations associated with the chemical properties or characteristics of elemental phosphorus, such practices are now discouraged and methods for the complete recovery, or conversion of the elemental phosphorus values in such sludges to benign and useful products, are needed (2,4,5). 4
Tedder;Pohland; Emerging Technologies in Hazardous Waste Management V ACS Symposium Series; American Chemical Society: Washington, DC, 1995.
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EMERGING TECHNOLOGIES IN HAZARDOUS WASTE MANAGEMENT V
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The elemental phosphorus containing sludge is a very non-homogeneous material varying in consistency from thin slurries to a thick mud. Upon aging hard crystalline layers may also form. Sludges that are newly formed can contain elemental phosphorus concentrations as high as 50-60% by weight. However, sludges that have settled for several years (which includes most sludges stored at the old furnace locations) will have elemental phosphorus concentrations of generally 5-15% by weight. These sludges usually contain small amounts of various heavy metals including iron, aluminum, lead, arsenic, barium, cadmium, and others. A typical analysis of phosphorus sludge located at Muscle Shoals, Alabama, is provided (Table I).
Table I. Typical Analysis of Phosphorus Sludge Concentration Range Mean 6.1-37.5 Total P, wt % 20.2 0.7 - 32.9 Elemental P, Wt % 11.0 1.0-5.5 Fe, wt % 2.6 0.9 - 3.4 C, wt % 2.0 0.1-0.4 A l , wt % 0.2 70 - 600 Pb, ppm 310 20-70 As. ppm 42 0-40 Ba, ppm 19 0-60 Cd, ppm 30
