Calculation of equilibrium species for the aqueous solution systems of UO2SO4−U(SO4)2−H2SO4−HF and UO2Cl2−UCl4−HCl−HF at 298 K

Abstract

Speciation models for aqueous solutions of UO2SO4−U(SO4)2−H2SO4−HF and UO2Cl2−UCl4−HCl−HF were proposed based on chemical reaction equilibria, mass balances, charge balance, and stoichiometry of UF4(s). The equilibrium concentrations of uranium and fluoride species in these solutions were calculated at 298 K, and are of relevance to the electrolytic reduction of U(VI), followed by the precipitation of UF4(s). In these calculations, the reduction ratios of U(VI) were set at 25, 50, 75, and 100 pct. In the sulfate system the stable domains of U4+, U(SO4) n 4−2n , UF n 4−n , and UF4(s) as U(IV) species and UO 2 2+ , UO2(SO4) n 2−2n , and UO2F n 2−n , as U(VI) species are strongly dependent on theC T(F)/C T(U(IV)) value. On the other hand, the stable domains of U4+ UCl3+, UF n /4−n , and UF4(s) as U(IV) species and UO 2 2+ , UO2Cl+, and UO2F n 2−n as U(VI) species are also strongly affected by theC T(F)/C T(U(IV)) ratio in the chloride system. The initiation and precipitation of UF4(s) in both the sulfate and chloride systems are a function of the reduction ratio of U(VI). The higher the reduction ratio, the lower theC T(F)/C T(U(IV)) values required. Compared to the chloride system, UF4(s) precipitation in the sulfate system starts at a lower value ofC T(F)/C T(U(IV)). The addition of an excess amount of HF does not cause the dissolution of UF4(s) precipitates because HF is a weak acid.