Solvent extraction technology for the separation and purification of niobium and tantalum: A review

In this paper, solvent extraction technology for the separation and purification of niobium and tantalum has been extensively reviewed. Although a variety of solvents have been investigated for niobium and tantalum solvent extraction, only four have been commercially used in the production of niobium and tantalum from their raw materials. These are methyl iso-butyl ketone (MIBK), tri-butyl phosphate (TBP), cyclohexanone (CHN) and 2-Octanol (2-OCL). In industrial operations, the separation and purification of niobium and tantalum by solvent extraction are all performed in the presence of fluoride to enable the formation of fluoro-complexes which can combine with the organic molecules. MIBK is the most commonly used extractant due to its low density, low viscosity and other properties, which makes it possible to obtain high purity products. Tertiary amines such as Alamine® 336 (mainly tri-octylamine) are promising extractants for purifying niobium and tantalum from very low HF concentrations or in oxalic acid solutions where HF is not present. Further investigation is required to indentify aqueous complex systems and solvent extraction systems to enable the purification of niobium and tantalum without the use of fluorides.