Separation of cobalt and nickel via solvent extraction with Cyanex-272: Batch experiments and comparison of mixer-settlers and an agitated column as contactors for continuous counter-current extraction

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The separation of cobalt and nickel was carried out via solvent extraction using the commercial extractant bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex-272). Solvent extraction experiments were performed on a batch scale to optimize the extractant concentration, saponification degree, equilibration time, organic-to-aqueous phase ratio (O/A), and stripping conditions. High selectivity of cobalt over nickel was obtained at an extractant concentration as low as 0.2 M (or 20% v/v), with a saponification degree of 40% and a phase ratio O/A of 1/1. Furthermore, eleven diluents, covering different aliphatic– aromatic compositions were tested to evaluate their effect on the separation of cobalt and nickel. Cobalt extraction efficiencies were not significantly impacted by the nature of the diluent. However, the co-extraction of nickel was approximatively 10% higher when toluene was used as a diluent in comparison with the aliphatic diluent Shell GS270. The process was validated in continuous mode using a battery of mixer-settlers and an agitated Kühni column. High recovery of cobalt and negligible co-extraction of nickel were achieved in both cases. The specificities, advantages, and challenges of the operation of both types of liquid-liquid extraction equipment were further discussed.