Thermodynamics and physical properties of an ionic liquid-based metal extraction process

In this study a LLX process for the extraction of cobalt by the IL [P₈₈₈₈][Oleate] is analysed in terms of relevant thermodynamic parameters. The process can be considered a typical example of transition metal extraction by an ionic liquid. Conductivity and chemical (FTIR) analyses indicate that Co²⁺ complexes with the IL. Three different models are evaluated, all different with respect to the actual Co²⁺ species that complexes with the IL, as well as the Co²⁺:IL stoichiometry. Based on simulations we identified CoCl₂ as the Co species that enters and complexes with the IL, in a Co²⁺:IL ratio of 1:2. The complexation reaction between the Co-species and the IL is an endothermic, entropy-driven reaction. The influence of the feed composition on Co²⁺ extraction is investigated, including the effect of the nature of the accompanying anion as well as the presence of a salting out cation agent. The higher Co²⁺ extraction from a NO₃⁻ medium is due to the stronger interaction between Co(NO₃)₂ and the IL, reflected by a higher equilibrium constant of Co(NO₃)₂ compared to CoCl₂. Differences in dehydration enthalpy between the ion species involved may contribute as well. Similar effects play a role when comparing uptake rates in solutions containing both Co²⁺ and Na⁺, with Co²⁺ extraction clearly preferred over that of Na⁺. Observed differences in Co²⁺ uptake in the presence of a salting-out agent (NaCl, KCl and NH₄Cl) can be explained in terms of the hydration energy of the salting out cation, the higher this hydration energy, the higher the Co²⁺ uptake by the IL.