EXTRACTION OF URANIUM(VI) IN NITRIC-ACID SOLUTION WITH SUPERCRITICAL CARBON-DIOXIDE FLUID CONTAINING TRIBUTYLPHOSPHATE

Extraction behaviors of U(VI) and selected fission product elements fr om HNO3 solution into a supercritical CO2 fluid containing tributylpho sphate (TBP) were studied under equilibrium and dynamic extraction con ditions. Uranium(VI) was found to be extracted in the supercritical CO 2 phase as a form of UO2 (NO3)(2)(TBP)(2) with a distribution ratio of , e.g., 2.2+/-0.3 for the extraction of 2 X 10(-4) to 2 X 10(-2) M U(V I) from 3 M HNO3 into supercritical CO2-0.3 M TBP mixture at 60 degree s C and 15 MPa. The distribution ratio of U(VI) increased with an incr ease of HNO3 concentration (0.1-6 M) or TBP (0.1-0.3 M), along with a decrease of pressure (10-40 h4Pa) at 60 degrees C, and with an increas e of the temperature (40-80 degrees C) at 15 MPa. A salting-out effect enhancing the distribution of U(VI) was observed by addition of a fai rly high concentration of LiNO3 to the aqueous phase. Results of dynam ic extraction using a continuous flow of supercritical CO2-TBP through the aqueous nitric acid solution showed that >98% of U(VI) could be e xtracted from 3M HNO3+3 M LiNO3 solution into supercritical CO2-0.08 M TBP (60 degrees C, 15 MPa). Main fission product elements such as lan thanides, Cs, Sr, Ba, Zr, Mo, and Pd were hardly extracted into the su percritical CO2 phase under this condition, which suggests that the su percritical CO2 extraction is a feasible method for the separation and recovery of uranium from these elements. Since rapid and complete rem oval of the CO2 medium from the extracts is attained by gasification o f CO2, the amount of the organic solvent waste generated in the extrac tion process can be largely minimized. Advantages of the supercritical CO2 extraction in its application to the separation process in the fi eld of nuclear technology are discussed.