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 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.

The Extraction Mechanism of Zirconium and Hafnium in the MIBK-HSCN System

“…This is consistent with the trend that the distribution ratio of metals decreased with the increase in initial HCl concentration when the initial concentration of HCl was higher than 0.5 M, which will be described in Section 3.2.4. Referring to the study of Tan et al [36], the decrease of K 3 (Zr or Hf) was considered to be caused by the changes in metal complexes formed at different HCl concentrations, which might be due to the competition between the formation of HSCN and metal complexes described above [15]. Since the equilibrium constant for Zr was more sensitive to the change in HCl concentration, the influence of HCl concentration on the complexation reaction of SCN − /Zr was more significant than that of SCN − /Hf.…”

Unraveling the coordination approach of Eu(iii) in cyphos nitrate ionic liquid – a comprehensive luminescence spectroscopy study

“…It was mentioned that at a low initial concentration of nitric acid in the aqueous phase, both nitric acid and water are predominantly extracted by self-association of TBP. [50][51][52][53] With the further increase in the concentration of nitric acid, a marked increase in the formation of TBP monomer species dominates over dimer complexes leading to the formation of strong TBP•HNO 3 species. A similar explanation might be implemented in the present case, since the organic phase extracts a higher amount of nitric acid with a steady decrease in water activity in the ionic liquid phase after 1 M. Besides, ionic liquid too forms monomer species ([P 66614 ][NO 3 ]•HNO 3 ) during the acid extraction process as confirmed by the slope analysis method of the extraction data presented in Fig.…”

Speciation and organic phase structure in nitric acid extraction with trioctylamine

“…27 In the case of TBP, IR spectroscopy and chemical equilibrium modelling studies have been performed to support the competitive nature of acid extraction and have been extended to explain the effect of background salts on extraction. [28][29][30] Speciation and intermolecular interactions in water-HNO 3 -TBP-alkane system have been studied using IR spectroscopy, quantum mechanical calculations, and molecular dynamics simulations showing that water is loosely held in the solution forming hydrogen bonds with other polar solutes that includes a chain-like conformation of TBP. (HNO 3 ) 2 .…”