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

Abstract

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.

Keywords

• Ionic liquids
• Solvent extraction
• Metal extraction
• Thermodynamics
• Statistical analysis
• Hydration energy