Selective Co/Ni Separation in Sulfate Media by Continuous Counter‑Current Membrane‑Assisted Solvent Extraction

A continuous counter-current membrane-assisted (CCCMA) extractor used wettable membranes (hydrophobic or hydrophilic properties) to separate organic and aqueous phases. In this study, the CCCMA extractor was evaluated as an alternative to conventional gravity-based phase-disengagement mixer-settlers for solvent extraction (SX) of critical metals, targeting cobalt (Co) and nickel (Ni). The performance of the CCCMA–SX system was investigated for the hydrometallurgical separation of Co from Ni in sulfate media using bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex 272) as the extractant. The effects of operating conditions, including the organic-to-aqueous (O/A) flowrate ratio, extractant concentration, degree of saponification, initial pH, initial Co/Ni ratio, and number of extraction stages, were assessed. Cobalt stripping utilizing the CCCMA–SX system was evaluated, with H2SO4 being the stripping solution. The applicability of the system for different SX chemistry, including Ni extraction from ammoniacal solution utilizing 2-hydroxy-5-nonylacetophenone oxime (LIX 84-I) extractant, was also demonstrated. The results from this study indicate that CCCMA–SX can improve process continuity and lower phase entrainment, while maintaining extraction efficiency and selectivity as in conventional process. This study also highlights key technical challenges for implementation of the CCCMA–SX system.