Separation of uranium(VI) from tri- and tetravalent elements in phosphoric acid solutions

Abstract

SummaryA new method of separation of uranyl ions from di-, tri- and tetravalent cations such as thorium, zirconium, cerium(IV) and/or lanthanides(III) involving phosphoric acid medium is described in this paper. The process is based on the difference of solubility between various phosphates: M2(PO4)2(HPO4)·H2O, M(HPO4)2·H2O, M(OH)PO4(where M is a tetravalent cation) or APO4·nH2O (where A is a trivalent cation) on the one hand, and the uranyl phosphate pentahydrate (UO2)3(PO4)2·5H2O, on the other hand. Taking into account the associated solubility constants, the conditions of separation were calculated then experimented for a mixture of Th4+, Ce4+(as a plutonium surrogate), Zr4+, La3+, Ba2+, Sr2+and UO22+. Tetravalent and trivalent cations phosphates were precipitated in 2 mol L−1nitric acid medium while uranyl ion and divalent cations remained in the solution. In a second step, (UO2)3(PO4)2·5H2O was precipitated at pH ≈ 1.5 and thereby separated from divalent cations present in the solution.