Removal of transition metals from rare earths by solvent extraction with an undiluted phosphonium ionic liquid: separations relevant to rare-earth magnet recycling

Abstract

An environmentally friendly process for the separation of the transition metals copper, cobalt, iron, manganese and zinc from rare earths by solvent extraction with the ionic liquid trihexyl(tetradecyl)phosphonium chloride has been developed. The solvent extraction process is carried out without the use of organic diluents or extra extraction agents and it can be applied as a sustainable hydrometallurgical method for removing transition metals from neodymium–iron–boron or samarium–cobalt permanent magnets. The recycling of rare earths is of high importance because of the possible supply risk of these elements in the near future. The method was tested for the removal of cobalt and iron from samarium and neodymium, respectively. The highest distribution ratios for cobalt and iron were found with 8.5 and 9 M HCl. At the tested conditions, the concentrations of neodymium and samarium in the ionic liquid were below 0.5 mg L⁻¹ (0.5 ppm), even for feed concentrations of 45 g L⁻¹. The separation factors of Nd/Fe and Sm/Co are 5.0 × 10⁶ and 8.0 × 10⁵, respectively. The percentage extraction of iron is still higher than 99.98% at loadings of the ionic liquids with 70 g L⁻¹ of iron. The viscosity of the ionic liquid containing the tetrachloroferrate(III) complex [FeCl₄]⁻ is lower, and less depending on the feed concentration, than in the case with a tetrachlorocobaltate(II) anion [CoCl₄]²⁻. After extraction, cobalt can be stripped very easily from the ionic liquid phase with water. However, due to the very high distribution ratio, iron could only be stripped by forming a water-soluble iron complex with ethylenediaminetetraacetic acid (EDTA). Also the possibility to extract chromium, nickel, aluminium, calcium and magnesium with trihexyl(tetradecyl)phosphonium chloride has been investigated, but the distribution ratios of these elements are very low in the tested conditions.