Extraction of Nb(v) by quaternary ammonium-based solvents: toward organic hexaniobate systems

Solvent extraction of Nb(v) from alkaline aqueous media using quaternary ammonium solutions, especially Aliquat® 336 diluted in an aliphatic diluent, was investigated. The hexaniobate ions (H_xNb₆O₁₉^x-8) were extracted into the organic phase with very high yields at room temperature and within a few minutes, affording easy access to organic solutions of hexaniobates. Several parameters were found to influence the extraction of H_xNb₆O₁₉^x-8 including the nature and concentration of alkali cations, confirming subtle effects previously described for polyoxoniobates such as ion-pairing with alkali ions. The extraction of H_xNb₆O₁₉^x-8 with Aliquat® 336 is also influenced if competing anions are present in the aqueous phase (NO₃^-, Cl^-, C₂O₄^2-, SO₄^2- and CO₃^2-) and varies with the pH mainly due to the competitive extraction of hydroxide ions at high pH. The co-extraction of sodium ions with H_xNb₆O₁₉^x-8 was observed as well as the co-extraction of water molecules, suggesting a self-association of the extractant. The proposed liquid-liquid extraction generic system paves the way for innovative niobium (and potentially tantalum) hydrometallurgical processes and it may also afford more direct routes for exploring the chemistry of hexaniobates in organic solvents.