Modeling of the Competition between Uranyl Nitrate and Nitric Acid upon Extraction with Tri-n-butyl Phosphate

Abstract

Uranium is a strategic element and plays an important role in energy resources. A H2O-HNO3-UO2(NO3)2-TBP (tri-n-butyl phosphate)-diluent system is commonly used for uranium separation and purification in liquid-liquid extraction. Uranyl nitrate is promoted by the existence of nitrate at low HNO3 concentrations but is inhibited at high HNO3 concentrations. Considering the competitive extraction between HNO3 and UO2(NO3)2, a generic extraction model is developed. The activities of components in the aqueous phase were estimated using Pitzer models. The thermodynamic equilibrium constants and Pitzer parameters were regressed by experimental data. The resulting model was able to successfully predict uranyl nitrate, nitric acid, and water extraction over a large range of conditions (U, 0-1.8 mol/L; HNO3, 0-10 mol/L; TBP, 5-100 vol %) within average absolute relative deviations of 11.2, 15.7, and 23.8%, respectively. The predicted results show that water and nitric acid were extracted as di-solvates HNO3·(TBP)2·H2O and (TBP)2·2H2O at low nitric acid concentrations, with the formation of mono-solvates HNO3·TBP and HNO3·TBP·H2O as the acid concentration increased. Uranyl nitrate was shown to be rejected from the organic phase as the formation of HNO3·TBP and HNO3·TBP·H2O in acid was extracted at high acid concentrations.