Redistribution of impurities in commercial wet-process acid

Abstract

Fifty-four commercial wet-process phosphoric acid (WPA) products were characterized to determine the redistribution of each chemical component between the solid and liquid fractions. The acid samples ranged in concentration from 24–70% P2O5 and represent production from the major U.S. phosphate rock fields (western United States, North Carolina, and Florida). The liquid and solid phases were separated and analyzed within two to three days of production and thus do not reflect long-term storage. The type and distribution of the compounds in the solid phase were dependent upon both the P2O5 concentration and that of 10 impurity components (Fe, Al, Mg, Na, K, Ca, Si, F, NH3-N, and S). The precipitates between 24 and 41% P2O5 are primarily gypsum, (Fe, Al)3(K, Na, NH4)H8(PO4)6·6H2O, and the AlF 6 3- and SiF 6 2- compounds of sodium, potassium, calcium, and magnesium. From 42–58% P2O5, the precipitates include greatly reduced amounts of most of the fluorides from the 28–41% P2O5 region and significant quantities of (Fe, Al)3(K, Na, NH4)H14(PO4)8·4H2O. Between 58 and 70% P2O5, the precipitates are primarily acid ferric phosphates and alkaline earth and ferrous pyrophosphates admixed with minor amounts of hemihydrate. Model equations were developed to estimate the concentration of a specific impurity in terms of overall solution composition. These equations exhibited correlation coefficients in the 60–80% range and allow prediction of trends for sludge formation.