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

A 'green' single-step separation process, involving a phosphonium phosphinate functionalized ionic liquid (FIL) in Cmim·NTf, 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 Cmim·NTf was found to be somewhat lower compared to that seen in Cmim·NTf. In Cmim·NTf, the extraction proceeded via the neutral NbOF(R)(R) and TaOF(R)(R) species predominated by a 'solvation' mechanism at 2 M HNO, where both the cationic and anionic parts of the FIL took part in the metal ion extraction. However, in the case of Cmim·NTf, the extraction proceeded via a cation exchange mechanism involving the mono-positive species viz. [NbO(R)], [TaO(R)]. 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 Cmim ion from the ionic liquid phase to the aqueous phase. The processes were spontaneous, exothermic involving outer sphere complexation. The radiolytic stabilities of the Cmim·NTf-based solvent systems were poorer than those of the solvents based on Cmim·NTf. 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.