Extraction of Uranium and Lanthanides from Their Oxides with a High-Pressure Mixture of TBP-HNO_3- H_2O-CO_2

摘要

The solubility of tri-n-butylphosphate (TBP) complex with nitric acid in supercritical CO_2 (SF-CO_2) is very high and (HNO_3)_(1.8)(H_2O)_(0.6)TBP is completely miscible in SF-CO_2 if the mixture is pressurized more than 14 MPa at 323 K. A mixture of 0.5 mol·dm~(-3) UO_2(NO_3)_2(TBP)_2 and SF-CO_2 is also miscible if the mixture is pressurized more than 13 MPa at 323 K. Uranium was extractable from the solid matrices of UO_2 with the high pressure solution of TBP-HNO_3-H_2O-CO_2 at 323 K and 13 MPa. Molecular ratios of HNO_3 to TBP in the high pressure mixture affected extraction rates and the higher ratio gave the higher extraction rates. Currently an application of supercritical fluid extraction to the nuclear fuel reprocessing is under investigation in Japan. Another promising field of application is treatment of uranium wastes. Decontamination factor of UO_2 or U_3O_8 was determined to be higher than 3xl0~2 for the decontamination using the simulated uranium waste. The solid waste after the supercritical fluid leaching treatment was dry and contained low concentrations of TBP and HNO_3.