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

A 'green' single-step separation process, involving a phosphonium phosphinate functionalized ionic liquid (FIL) in C₈mim·NTf₂, 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 C₄mim·NTf₂ was found to be somewhat lower compared to that seen in C₈mim·NTf₂. In C₈mim·NTf₂, the extraction proceeded via the neutral NbOF₃(R₄P⁺)(R₂POO^-) and TaOF₃(R₄P⁺)(R₂POO) species predominated by a 'solvation' mechanism at 2 M HNO₃, where both the cationic and anionic parts of the FIL took part in the metal ion extraction. However, in the case of C₄mim·NTf₂, the extraction proceeded via a cation exchange mechanism involving the mono-positive species viz. [NbO(R₂POO^-)₂]⁺_IL, [TaO(R₂POO^-)₂]⁺_IL. 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 C₄mim⁺ ion from the ionic liquid phase to the aqueous phase. The processes were spontaneous, exothermic involving outer sphere complexation. The radiolytic stabilities of the C₈mim·NTf₂-based solvent systems were poorer than those of the solvents based on C₄mim·NTf₂. 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.