Separation of Rare Earths and Transition Metals Using Ionic-Liquid-Based Aqueous Biphasic Systems

Abstract

An aqueous biphasic system (ABS) based on the ionic liquid (IL) tributyltetradecylphosphonium chloride ([P444,14]Cl) has been developed for the separation of the Sm/Co, Nd/Fe, Eu/Zn, and La/Ni rare-earth element and transition metal pairs. Phase diagrams of the new ABSs based on [P444,14]Cl–CoCl2/SmCl3–H2O were constructed, and these showed that ABS formation was facilitated by higher temperatures (e.g., 50 °C). The effects of salting-out agents (NaCl, LiCl, and HCl) and the amount of IL on metal extractions were investigated for the separation of samarium and cobalt. The graph showing the variation of the distribution ratio of cobalt as a function of the chloride concentrations exhibited a maximum, while the distribution ratios for samarium stayed close to 0 at low chloride concentrations and then gradually increased with increasing chloride concentration. Ferric and zinc ions could be fully transferred to the IL phase without contamination by rare-earth elements at low concentrations of salting-out agents, whereas the maximum in Ni/La selectivity was observed at 8 mol/kg of LiCl in the system. Cobalt was removed from the loaded IL phase by precipitation stripping with Na2CO3, and the quantitative removal of cobalt could be achieved at a Na2CO3 to Co molar ratio of 1.5:1. Infrared spectra confirmed that the precipitate was pure CoCO3 without contamination by the IL. By selecting economic and environmental-friendly ILs, the IL-based ABS can be used for development of sustainable green processes for metal separations.