Samarium/cobalt separation by solvent extraction with undiluted quaternary ammonium ionic liquids

Abstract

SmCo permanent magnets are used in the automotive and aircraft industries. They are preferable over NdFeB magnets for applications above 150 °C. End-of-life SmCo magnets are a valuable secondary source of cobalt and samarium. This study presents solvent extraction systems without addition of molecular diluents. Undiluted quaternary ammonium ionic liquids with chloride, thiocyanate and nitrate anions were used to extract metals from aqueous feed solutions with high chloride concentrations in order to separate the main metals (Sm/Co/Cu) present in SmCo magnets. A chloride/chloride extraction was compared with two split-anion extraction systems (i.e. chloride in the aqueous phase and either nitrate or thiocyanate in the organic phase). The advantage of split–anion extraction is that it can be done from a chloride solution, but with the extraction behavior of nitrate or thiocyanate solutions. Split-anion extraction was in this work applied to the separation of rare earth elements/transition metals instead of previous applications to the mixtures of rare earths. For all the systems, the selectivity of extraction, scrubbing and stripping from a multi-element solution were investigated. High separation factors (>7500) were found for Co/Sm separations. Co(II) and Cu(II) extracted in the chloride/chloride and chloride/nitrate systems could be stripped simply by water. However, stripping of the loaded ionic liquid in the chloride/thiocyanate system with water was not feasible. The extraction mechanism for the different ionic liquids was studied by extended X–ray absorption fine structure (EXAFS). A possible explanation on the extraction behavior of samarium ions in the presence of transition metal ions (cobalt and copper) is given.