Recovery of uranium from phosphoric acid by means of supported liquid membranes

Abstract

U(VI) was transported at 23 ± 1°C from 5–6 M phosphoric acid solutions through liquid membranes of kerosene solutions of di(2-ethylhexyl) phosphoric acid and trioctyl phosphine oxide (D2EHPA/TOPO) supported on porous polytetrafluoroethylene to a solution of phosphoric acid of equal or greater molarity containing ferrous ion as a reducing agent. The ferrous ion could be omitted when the higher molarity acid was used. The uranium flux was proportional to the U(VI) concentration. The overall resistivity of the membranes to uranium flux had a diffusional component that was proportional to the membrane thickness and an interfacial component that resulted from rate-limiting uranium complexation/decomplexation kinetics. The interfacial component accounted for over 80% of the resistivity of a membrane 75 μm thick. Increasing the temperature to 60°C only slightly diminished the interfacial resistivity. A theoretical model was constructed that accommodated data obtained from uranium transport through the membranes and through quiescent layers of phosphoric acid and D2EHPA/TOPO in kerosene. The average uranium flux from simulated solutions of wet-process phosphoric acid at 90% uranium transfer was estimated to be 1.3 × 10 −11 mol cm −2 sec −1, or 0.09 lb ft −2 yr −1. The flux was judged to be too low for supported liquid membranes to be competitive with liquid/liquid extraction for recovery of uranium from wet-process phosphoric acid.