Decontamination of uranium oxides from solid wastes by supercritical CO 2 fluid leaching method using HNO 3–TBP complex as a reactant

Abstract

A new method referred to as supercritical CO 2 fluid leaching (SFL), which is based on selective dissolution of uranium oxides with the supercritical CO 2 fluid containing the HNO 3–tri- n-butyl phosphate (TBP) complex, has been developed for the removal and recovery of uranium from solid wastes contaminated by uranium oxides. A recommended SFL procedure has been established consulting experimental results of the complexation efficiency of the uranium oxides and the dissolution efficiency of U(VI)–TBP complex at different pressures. The recommended method consists of complexation process at 60 °C and 15 MPa for 150 min using the supercritical CO 2 containing the HNO 3–TBP complex and dissolution process at 60 °C and 20 MPa using a flow of the neat supercritical CO 2. The complexation–dissolution cycle is repeated twice. The feasibility of the recommended procedure to the removal of uranium from the solid wastes was demonstrated using synthetic wastes that were a mixture of standard sea sand (50 g) and UO 2 or U 3O 8 powders (20–200 mg of U). The decontamination factor of UO 2 or U 3O 8 was determined to be higher than 5×10 2. The solid waste after the SFL treatment was dry and contained enough low concentrations of TBP and HNO 3.