A new approach to mathematical description of the extraction of HNO3, U(VI), and other hexavalent actinides with tributyl phosphate in a mixture with paraffins

Abstract

The isotherm of uranyl nitrate extraction with TBP in diluent without salting-out agents with the formation of the disolvate is described by a chemical equation with the concentration constant of 10 without correction coefficients in a wide range of component concentrations. The concentration of “free” TBP in the dependence of the extraction of U(VI) microamounts on the acid concentration was calculated using this value, and it was shown that the formal stoichiometric ratio TBP: HNO3 in the extract is 0.75. To describe this effect, a set of reactions was suggested, taking into account the role of extracted water, and the corresponding concentration equilibrium constants were calculated without corrections. The values obtained differ significantly from those suggested previously. The constants obtained adequately describe the joint extraction of U(VI) and HNO3 at the acid concentration of 0.3–8 M and extract loading with U(VI) of up to 85% of its capacity. At low extract loadings and acid concentration lower than 1 M, weak hydrolysis of uranyl ion and extraction of U(VI) hydrate solvate were taken into account. For the extraction of Np(VI) and Pu(VI) with TBP in diluent, the extraction constants that do not vary with the U(VI) and HNO3 concentrations were calculated taking into account the formation of the Np(VI) and Pu(VI) hydrate solvates.