Effect of Extracting Phase Composition on Cobalt Recovery from Chloride-Rich Media: Transition from Molecular to Ionic and Eutectic Solvents

Cobalt (Co) is an essential metal for sustainable technologies, including electric vehicles and energy storage systems. Its selective recovery from complex leachates remains a challenge due to the co-presence of nickel (Ni) and manganese (Mn). This work examines the influence of the extracting phase composition on the solvent extraction (SX) of Co from highly acidic chloride media (pH ≈ 0.86, 6–12 M LiCl) containing Ni and Mn. Trioctylphosphine oxide (TOPO) was used as the extractant, dissolved in three different media: dodecane, the ionic liquid (IL) 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide ([C₈mim][Tf₂N]), and the hydrophobic eutectic solvent (HES) TOPO:decanoic acid (1:1). The extraction of Co increased with LiCl concentration, consistent with the formation of anionic Co–chloride complexes, while Ni extraction remained negligible due to its high hydration energy. Slope analysis confirmed a 2:1 TOPO:Co stoichiometry for all systems, indicating a solvation-controlled extraction mechanism. Both the IL and the HES improved Co recovery compared with the molecular diluent, achieving high selectivity with minimal Mn co-extraction. Volatility tests showed severe solvent loss for dodecane, whereas [C₈mim][Tf₂N] and TOPO:decanoic acid exhibited excellent thermal stability (<2% mass loss) and low water uptake (<2%). These results highlight the potential of IL and HES as sustainable alternatives to conventional organic diluents. Among the systems studied, TOPO:decanoic acid (1:1) displayed the best balance of extraction efficiency, selectivity, and stability, representing a promising medium for Co recovery from chloride solutions.