Application of Unified Model with TBP Dilution to Description of Extraction of Nitrates of Tetravalent Zirconium and Hafnium Actinides from Nitrate Solutions

Abstract

Published and authors’ own data have been used to create a unified mathematical model describing the distribution of uranyl nitrate, nitric acid, and tetravalent elements in TBP with consideration for its dilution in their distribution in neutral and acid systems in a wide range of component concentrations. As the base values of the concentration equilibrium constants were taken their values calculated for pure TBP. In this case, raising the diluent concentration leads to a linear dependence of the logarithms of the extraction constants on the TBP concentration with a parameter a the value of which reflects the influence exerted by the nature of a diluent. It is assumed that, simultaneously, there occur several complexation reactions in the aqueous and organic phases and a nonstoichiometric physicochemical interaction of the components. The values of the equilibrium constants of the extraction reactions and the corresponding parameters a were determined for tetravalent thorium, uranium, plutonium, neptunium, zirconium, and hafnium, which made it possible to describe their extraction with TBP solutions in a hydrocarbon diluent at concentration of the extractive agent of 5 to 100 vol %. The error in calculations of the distribution coefficients in terms of the model does not exceed 12%.