Chapter 11
Removing Heavy Metals from Phosphoric Acid and Phosphate Fluid Fertilizers Organic and Inorganic Reagents 1
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V. M . Norwood III and L. R. Tate Chemical Research Department, Tennessee Valley Authority, National Fertilizer and Environmental Research Center, Muscle Shoals, AL 35660-1010 Heavy metals in wet-process phosphoric acid (WPA) and phosphate fluid fertilizers are an environmental concern in the United States and Europe. To address this concern, several organic and inorganic reagents were evaluated as precipitants for heavy metals in a 10-34-0 (N-P2O5-K2O) fluid fertilizer and WPA. Trisodium trithiocyanuric acid (TMT-15), sodium polythiocarbonate (Thio-Red II), and sodium trithiocarbonate (5% Na2CS3) precipitated arsenic, cadmium, copper, mercury, lead, and zinc from 10-34-0. Ammonium cyanurate was ineffective in removing cadmium from 10-34-0. Thio-Red II and 5% Na2CS3 precipitated mercury, lead, cadmium, copper, and chromium from WPA. A water-insoluble starch xanthate adsorbed mercury, copper, and lead from 10-34-0 and WPA. Sodium sulfide, sodium polysulfide, and potassium ferrocyanide were tested as inorganic precipitants. The polysulfide was twice as effective as the sulfide alone, and concentrations of less than 10 ppm of arsenic, cadmium, mercury, and lead were achieved in 10-34-0. Ferrocyanide reduced the concentrations of cadmium and nickel to less than 10 ppm in WPA.
Phosphate fluid fertilizers of various grades [e.g., 10-34-0 (N-P2O5-K2O)] are prepared by ammoniating merchant-grade superphosphoric acid. The superphosphoric acid is produced by concentrating merchant-grade orthophosphoric acid (54% P2O5) in a vacuum or atmospheric concentrator. Similarly, merchant-grade orthophosphoric acid is produced by concentrating wet-process phosphoric acid (WPA, 30% P 2 O 5 ) . WPA is manufactured by acidulating the apatite content of phosphate rock with sulfuric acid to produce phosphoric acid and insoluble calcium sulfate dihydrate (gypsum). The majority of the metallic impurities originally present in the phosphate rock, such as iron, aluminum, magnesium, zinc, lead, cadmium, manganese, nickel, chromium, vanadium, mercury, and arsenic, are acid-soluble, and these become partitioned between the WPA (80%) and the solid waste gypsum (20%) during the acidulation step. Consequently, some portion of these metallic impurities also is found in phosphate fluid 1Current address: 1101 Market Street, MR-3A, Chattanooga, TN 37402 This chapter not subject to U.S. copyright Published 1992 American Chemical Society
In Environmental Remediation; Vandegrift, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
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ENVIRONMENTAL REMEDIATION
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fertilizers. Table I summarizes the levels of heavy metal impurities found in phosphoric acids and 10-34-0 phosphate fluid fertilizers manufactured in Florida, North Carolina, and the Western states (Wakefield, Z.T., TVA Bulletin Y-159, Tennessee Valley Authority, unpublished data).
Table I. Concentrations of Heavy Metals in Phosphoric Acids and Phosphate Fertilizers Heavy Metals (ppm) P2O5 Source wt % Cd Cu Zn Mn Ni Pb Cr V Central Florida Acid 51.6 6 12 70 346 36 5 86 212 10-34-0 34.0 4 8 46 228 24 3 57 140 North Florida Acid Superacid 10-34-0
47.8 69.3 34.0
8 11 5
15 98 371 19 149 610 9 73 299
13 1 82 112 25
