EFFECT OF U(Ⅵ) CONCENTRATION ON EQUILIBRIUM AND KINETICS IN FLOW-EXTRACTION OF U(Ⅵ) IN HNO_3 / SUPERCRITICAL CO_2 + TBP SYSTEM

摘要

The distribution ratio D of U(Ⅵ) was determined in the supercritical CO_2 extraction system with the nitric acid solution containing 10~(-3) ― 10~(-1) M U(Ⅵ) and the supercritical CO_2 containing 0.1 ― 0.5 M tributylphosphate (TBP) at 60°C and 15 MPa. The distribution behavior was found to be practically independent on the U(Ⅵ) concentration, though the D decreased slightly with an increase of U(Ⅵ) concentration, which was attributed to a decrease of the equilibrium concentration of free TBP in the supercritical CO_2 phase. The solubility of UO_2(NO_3)_2(TBP)_2 in the supercritical CO_2 phase was determined to be higher than 7 x 10~(-2) M and a suppression of the D due to the solubility limitation of the extracted complex in the supercritical CO_2 was unlikely to occur under the extraction condition examined. The extraction-rate curve was measured in the flow-extraction system. The distribution ratio D_(flow) of U(Ⅵ) in the flow-extraction system was calculated and compared with the D. The D_(flow,max) agreed well with the D indicating that the extraction equilibrium of U(Ⅵ) was attained rapidly even in the flow-extraction performance.