Influence of the extracting phase composition on cobalt extraction from concentrated chloride media: From molecular diluents to ionic and eutectic solvents

The energy transition demands sustainable and selective processes for recovering critical cobalt (Co) from complex secondary resources. This work evaluates the influence of extracting phase composition on Co(II) extraction from concentrated acidic chloride media (initial pH ≈ 0.85 before LiCl addition, 6–12 M LiCl), comparing conventional molecular diluents with ionic and eutectic matrices containing trioctylphosphine oxide (TOPO) as the extractant. Three systems were compared: 200 mM TOPO in n-dodecane/1-octanol, 200 mM TOPO in the ionic liquid (IL) [C 8 mim][Tf2N], and a hydrophobic eutectic solvent (HES) of equimolar TOPO:decanoic acid. All systems achieved high Co extraction (>93%) at 10 M LiCl, with the IL providing the highest selectivity ( log α Co / Ni = 4.03), followed by the HES (3.77). Notably, the HES enabled a substantially higher Co loading capacity due to its greater TOPO concentration, although it showed lower extraction efficiency when normalized per mole of extractant. The dodecane phase presented a robust extraction and stripping behavior, but suffered hydrodynamic failure via third-phase formation and significant volatile losses (~90% at 50 °C for 128 h). Conversely, while ionic and eutectic systems effectively suppressed solvent volatilization, they exhibited elevated dynamic viscosities (98.1 and 70.1 mPa ∙ s, respectively) and significant aqueous phase losses for the IL (1334 ppm). Furthermore, a qualitative toxicity assessment and economic analysis revealed sharp trade-offs, balancing the more favorable estimated acute-toxicity profile and low cost of n-dodecane against the high cost and less favorable estimated acute-toxicity profile of the IL, positioning the HES as a balanced compromise among the evaluated systems.