An environmentally friendlier approach to hydrometallurgy: highly selective separation of cobalt from nickel by solvent extraction with undiluted phosphonium ionic liquids

A green solvent extraction process for the separation of cobalt from nickel, magnesium and calcium in chloride medium was developed, using undiluted phosphonium-based ionic liquids as extractants. Cobalt was extracted to the ionic liquid phase as the tetrachlorocobaltate(II) complex, leaving behind nickel, magnesium and calcium in the aqueous phase. Manganese is interfering in the separation process. The main advantage of this ionic liquid extraction process is that no organic diluents have to be added to the organic phase, so that the use of volatile organic compounds can be avoided. Separation factors higher than 50 000 were observed for the cobalt/nickel separation from 8 M HCl solution. After extraction, cobalt can easily be stripped using water and the ionic liquid can be reused as extractant, so that a continuous extraction process is possible. Up to 35 g L-1 of cobalt can be extracted to the ionic liquid phase, while still having a distribution coefficient higher than 100. Instead of hydrochloric acid, sodium chloride can be used as a chloride source. The extraction process has been upscaled to batch processes using 250 mL of ionic liquid. Tri(hexyl) tetradecylphosphonium chloride, tri(butyl)tetradecylphosphonium chloride, tetra( octyl) phosphonium bromide, tri( hexyl) tetradecylphosphonium bromide and Aliquat 336 have been tested for their performance to extract cobalt from an aqueous chloride phase to an ionic liquid phase. Tri( hexyl) tetradecylphosphonium chloride (Cyphos IL 101) turned out to be the best option as the ionic liquid phase, compromising between commercial availability, separation characteristics and easiness to handle the ionic liquid.