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

An aqueous biphasic system (ABS) based on the ionic liquid (IL)tributyltetradecylphosphonium chloride ([P444,14]Cl) has been developed for theseparation of the Sm/Co, Nd/Fe, Eu/Zn, and La/Ni rare-earth element andtransition metal pairs. Phase diagrams of the new ABSs based on [P444,14]Cl-CoCl2/SmCl3-H2O were constructed, and these showed that ABS formation wasfacilitated by higher temperatures (e.g., 50 degrees C). The effects of salting-out agents(NaCl, LiCl, and HCl) and the amount of IL on metal extractions wereinvestigated for the separation of samarium and cobalt. The graph showing thevariation of the distribution ratio of cobalt as a function of the chlorideconcentrations exhibited a maximum, while the distribution ratios for samarium stayed close to 0 at low chloride concentrations andthen gradually increased with increasing chloride concentration. Ferric and zinc ions could be fully transferred to the IL phasewithout contamination by rare-earth elements at low concentrations of salting-out agents, whereas the maximum in Ni/La selectivitywas observed at 8 mol/kg of LiCl in the system. Cobalt was removed from the loaded IL phase by precipitation stripping withNa2CO3, and the quantitative removal of cobalt could be achieved at a Na2CO3to Co molar ratio of 1.5:1. Infrared spectraconfirmed that the precipitate was pure CoCO3without contamination by the IL. By selecting economic and environmental-friendlyILs, the IL-based ABS can be used for development of sustainable green processes for metal separations