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 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.