Extraction of Uranium(VI) in Nitric Acid Solution with Supercritical Carbon Dioxide Fluid Containing Tributylphosphate

Abstract

Extraction behaviors of U(VI) and selected fission product elements from HN0 3 solution into a supercritical C0 2 fluid containing tributylphosphate (TBP) were studied under equilibrium and dynamic extraction conditions. Uranium(VI) was found to be extracted in the supercritical C0 2 phase as a form of U0 2 (N0 3 ) 2 (TBP) 2 with a distribution ratio of, e.g., 2.2±0.3 for the extraction of 2 X10" 4 to 2 X 10" 2 M U(VI) from 3 M HN0 3 into supercritical C0 2 -0.3 M TBP mixture at 60°C and 15 MPa. The distribution ratio of U(VI) increased with an increase of HN0 3 concentration (0.1-6 M) or TBP (0.1-0.3 M), along with a decrease of pressure (10-40 MPa) at 60 °C, and with an increase of the temperature (40-80°C) at 15 MPa. A salting-out effect enhancing the distribution of U(VI) was observed by addition of a fairly high concentration of LiN0 3 to the aqueous phase. Results of dynamic extraction using a continuous flow of supercritical C0 2 -TBP through the aqueous nitric acid solution showed that >98% of U(VI) could be exctracted from 3 M HN0 3 +3 M LÍN0 3 solution into supercritical C0 2 -0.08 M TBP (60°C, 15 MPa). Main fission product elements such as lanthanides, Cs, Sr, Ba, Zr, Mo, and Pd were hardly extracted into the supercritical C0 2 phase under this condition, which suggests that the supercritical C0 2 extraction is a feasible method for the separation and recovery of uranium from these elements. Since rapid and complete removal of the C0 2 medium from the extracts is attained by gasification of C0 2 , the amount of the organic solvent waste generated in the extraction process can be largely minimized. Advantages of the supercritical C0 2 extraction in its application to the separation process in the field of nuclear technology are discussed.

Modelling of the Extraction of Uranium with Supercritical Carbon Dioxide.

“…The parameters obtained are given in Table 1. The product of ha and D/a 2 can be shown to be equal to K c F/3V and by using the experimental flow rates and sample volumes (6 ml and 1.4 ml per min for the present data and 5 ml and 1.1 ml per min, 12) respectively) the values of the partition coefficient, K c , can be obtained and these are shown in the final column of the table. Bearing in mind that the model is limited, the relative values of the results can be discussed.…”

“…12) Since the modelling indicates the process is mainly controlled by diffusion in the aqueous phase, it is then further implied that the better performance of the carbon dioxide system is due to better contact between the fluids, perhaps due to smaller bubble sizes. A possible explanation is that, because of the larger density difference with the aqueous phase (carbon dioxide at 150 bar and 60…”

Dissolution Behavior of Uranium Oxides with Supercritical CO2Using HNO3-TBP Complex as a Reactant

“…The species UO 2 (NO 3 ) 2 (TBP) 2 was confirmed as the extracted species in the supercritical CO 2 extraction system 13) identically to the species in the traditional solvent extraction system of U(VI) from nitric acid solution into organic solvent containing TBP.…”

Studies on solubility of uranyl complexes in supercritical carbon dioxide and its controlling factors using UV-visible and17O- and19F-NMR spectroscopy

“…Several studies have demonstrated that MA(III), Ln(III), and uranyl ion (U(VI)) can be extracted from solid or liquid materials to scCO 2 as soluble complexes with chelating ligands such as organophosphorus compounds and b-diketones [7][8][9][10][11][12]. The solubility of various metal complexes including U(VI) in scCO 2 has been investigated, and the results have indicated that fluorination of coordinated ligands drastically enhances the solubility of metal complexes by several orders of magnitude compared with non-fluorinated analogues [13][14][15][16][17][18][19].…”