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

[Image: see text] Uranium is a strategic element and plays an important role in energy resources. A H(2)O–HNO(3)–UO(2)(NO(3))(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 HNO(3) concentrations but is inhibited at high HNO(3) concentrations. Considering the competitive extraction between HNO(3) and UO(2)(NO(3))(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; HNO(3), 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 HNO(3)·(TBP)(2)·H(2)O and (TBP)(2)·2H(2)O at low nitric acid concentrations, with the formation of mono-solvates HNO(3)·TBP and HNO(3)·TBP·H(2)O as the acid concentration increased. Uranyl nitrate was shown to be rejected from the organic phase as the formation of HNO(3)·TBP and HNO(3)·TBP·H(2)O in acid was extracted at high acid concentrations.