Evaluation of a Phosphinate Functionalized Ionic Liquid for the Separation of Nb and Ta from Nitric Acid Feed Conditions

Abstract

A 'green' single-step separation process, involving a phosphonium phosphinate functionalized ionic liquid (FIL) in C 8 mim•NTf 2 , has been developed for highly encouraging improvements in the mutual separation of Nb and Ta with a maximum separation factor of ∼48 at 2 M nitric acid. The separation factor in C 4 mim•NTf 2 was found to be somewhat lower compared to that seen in C 8 mim•NTf 2 . In C 8 mim•NTf 2 , the extraction proceeded via the neutral NbOF 3 (R 4 P + )(R 2 POO − ) and TaOF 3 (R 4 P + )(R 2 POO − ) species predominated by a 'solvation' mechanism at 2 M HNO 3 , where both the cationic and anionic parts of the FIL took part in the metal ion extraction. However, in the case of C 4 mim•NTf 2 , the extraction proceeded via a cation exchange mechanism involving the mono-positive species viz.Only the phosphinate group of the FIL was directly involved in the binding to the metal ion. The charge neutrality was maintained by the exchange of the C 4 mim + ion from the ionic liquid phase to the aqueous phase. The processes were spontaneous, exothermic involving outer sphere complexation. The radiolytic stabilities of the C 8 mim•NTf 2 -based solvent systems were poorer than those of the solvents based on C 4 mim•NTf 2 . Aqueous solutions of EDTA-guanidine carbonate or DTPA-guanidine carbonate showed promising back extraction ability though three contacts of these organic phases were required for more than 99.99% stripping of the metal ion. The reusability of these solvent systems was evaluated. After four consecutive cycles, a maximum of only 8% reduction in the extraction efficiency of Ta was noticed, while for Nb it was less than 4% for Nb.