Impact of Hydrophobic Ionic Liquid Mixtures on Cobalt (II) Extraction from Sulfate Media at Low pH: Stoichiometry and Operating Variables Study

Global demand for cobalt (Co) is increasing due to its essential role in lithium-ion batteries for electric vehicles and renewable energy storage systems. However, efficient extraction remains challenging because of the complex composition of ores and the associated environmental concerns. This study examines the stoichiometry of Co extraction from a sulfated acidic aqueous phase containing Co, nickel (Ni), manganese (Mn), and iron (Fe) at low pH levels. Two ionic liquid (IL) combinations were identified as optimal for extraction and selectivity at pH values of 0.85 and 3.28: trioctylmethylammonium benzoate ([Toma][Ba]) dissolved in 1-octyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide ([Omim][Tf₂N]) and trioctylmethylammonium di(2-ethylhexyl)phosphate ([Toma][D2EHP]) in 1-methyl-1-octylpyrrolidinium bis(trifluoromethyl)sulfonyl)imide ([Ompy][Tf₂N]), respectively. The results show improved Co extraction and selectivity at acidic pH levels compared with conventional organic phases, such as D2EHPA/CYANEX 272 dissolved in kerosene. This approach could potentially eliminate the need for Fe precipitation steps prior to solvent extraction. Co extraction was found to follow an ion exchange mechanism, as confirmed through experimental assays and theoretical calculations. Analysis of operating variables revealed that increasing temperature decreases Co extraction, while higher extractant concentrations significantly enhance Co recovery (by approximately 50%), although with reduced selectivity.