Anion-exchange separation in hydrochloric acid solutions for the ultrahigh purification of cobalt

Recent applications in advanced electronics have increased the practical importance of ultrahigh-purity cobalt. Anion exchange in HCl media is considered an efficient alternative to the combination of the conventional purification methods. Anion-exchange distribution functions have been determined for cobalt and the main impurity elements by the technique of batch equilibration. Spectrophotometric and elution studies confirmed the established anion-exchange behavior patterns. Based on the new and the available distribution functions, combined with the assessment of thermodynamic stability, a procedure of anion-exchange separation has been devised to eliminate virtually all the impurities from the cobalt-chloride solution. The major shortcoming of previously proposed anion-exchange procedures-failing to separate copper sufficiently-has been corrected by the introduction of a preliminary step under reduced conditions, removing copper from the solution by sorption in the monovalent state. The rest of the impurities are eliminated in a second anion-exchange step by applying rinsing and elution stages under oxidizing conditions. The optimum parameters of the procedure have been determined according to performance characteristics (purification ratios, yields, and volume efficiencies) derived from the analysis of elution curves obtained with laboratory-scale ion-exchange columns. Special computer programs have been developed to facilitate thermodynamic simulation, analytical correction, and data processing.