Cobalt-nickel separation: the extraction of cobalt(II) and nickel(II) by Cyanex 301, Cyanex 302 and Cyanex 272

Cyanex 301, Cyanex 302 and Cyanex 272 were used to extract cobalt(II) and nickel(II) from a sulphate medium in order to elucidate any separation properties these phosphinic acid extractants display. All the reagents extracted cobalt selectively, Cyanex 302 exhibiting better separation characteristics than Cyanex 272, which in turn showed a higher selectivity than did Cyanex 301. The separation (pH0.5Ni-pH0.5Co) found for Cyanex 302 was 2.6 pH units, compared to 1.7 pH units for Cyanex 272 and 1.1 pH units for Cyanex 301. Based on these results and the fact that Cyanex 301 would need strong acid to strip it, an experiment was conducted using Cyanex 302 and Cyanex 272 to separate cobalt(II) and nickel(II) from a solution containing 100 g/l NiSO4·6H2O and 2 g/l CoSO4·7H2O with favourable results. Vapour pressure osmometry was used to determine the degree of aggregation of the extractants in toluene. Both Cyanex 272 and Cyanex 302 are dimeric and Cyanex 301 exists as a monomer. Slope analysis methods were used to determine the nature of the cobalt(II) and nickel(II) complexes extracted.