Efficient separation of vanadium and chromium by the complexation with sulfate ions in solvent extraction using EHEHPA

The significant chemical similarity between vanadium and chromium complicates the process of efficient extracting V(Ⅳ) from the solution containing Cr(Ⅲ) to prepare high-purity vanadyl sulfate electrolyte. In this study, the separation of vanadium and chromium with 2-ethylhexyl phosphoric acid-2-ethylhexyl ester (EHEHPA) was enhanced by increasing the concentration of SO 4 2− . Under the extraction equilibrium pH of 2.5, the extraction of Cr(III) was suppressed by increasing the concentration of SO 4 2− anions, which hardly affected the extraction efficiency of V(IV), achieving a separation factor as high as 5780.6 at SO 4 2− concentration of 1.75 mol·L −1 . Furthermore, after pretreatment of the water-leaching solution of sodium-roasted vanadium slag, extraction was carried out under the conditions of equilibrium pH of 1.8 and SO 4 2− concentration of about 2.00 mol·L −1 , and after stripping, the vanadyl sulfate electrolyte containing 2.12 mol·L −1 of vanadium and only 0.11 mg·L −1 of chromium was obtained. It was proved that Cr(Ⅲ) mainly extracted in the form of CrSO 4 + into organic phase by one EHEHPA dimer as [CrSO 4 (H 2 O)] + under higher SO 4 2− concentrations (e.g., ≥1.50 mol·L −1 ), while Cr(Ⅲ) was extracted by two EHEHPA dimers as Cr 3+ without SO 4 2− in the solution. Based on the density functional theory, the complex structures of Cr(III) were optimized and the energy changes of Cr(III) extraction reactions were calculated, indicating the complexation between SO 4 2− and Cr(III) under higher SO 4 2− concentrations resulting in the inhibition of Cr(III) extraction, and thus enhancing the separation of vanadium and chromium.