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 rare earths from the transition metals copper, cobalt, iron, manganese and zinc by solvent extraction with the ionic liquid trihexyl(tetradecyl)phosphonium chloride has been developed. The solvent extraction process is carried without the use of organic diluents or extra extraction agents and it can be applied as a sustainable hydrometallurgic method for removing transition metals from neodymium-ironboron 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 concentration 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10 6 and 8.010 5 and, 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 [FeCl 4 ]is lower, and less depending on the feed concentration, than in the case with a tetracobaltate(II) anion [CoCl 4 ] 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. Table of contents An environmentally friendly hydrometallurgic method for the separation of the transition metals iron, cobalt, copper, manganese and zinc from the rare earths neodymium and samarium with the undiluted ionic liquid trihexyl(tetradecyl)phosphonium chloride has been developed.

Supported Ionic Liquid Membranes for Metal Separation

“…Therefore, undiluted Cyphos IL 101 can be used as both extractant and diluents when contacting with aqueous phase [53]. Undiluted Cyphos IL 101 was used for extractions of Co(II), Fe(III) and rare earths [53][54][55][56]. Extraction mechanism of metal ions with undiluted Cyphos IL101 is same as that of Cyphos IL 101 diluted in organic solvent.…”

Regeneration of the ionic liquid tetraoctylammonium oleate after metal extraction

“…The percentage stripped from the organic phase was calculated by: (2) where is the metal concentration in the IL before stripping ( ) and is the metal concentration in the aqueous phase after the stripping process.…”

“…It has already been proven that some ILs present excellent metal extraction efficiencies. [2][3][4][5][6] Recently, non-fluorinated fatty acid based ILs were synthesized in the search for biocompatible, non-toxic metal selective ILs. 7 Oleic acid and linoleic acid were selected as anion due to their affinity towards metals combined with the voluminous and hydrophobic tetraalkylammonium cations to prevent the loss of the IL to the water phase.…”

Titanium alkylphosphate functionalised mesoporous silica for enhanced uptake of rare-earth ions

“…3,4 The SX process involves the use of organic extractants which transfer the REE ions from the aqueous to the organic phase. 5 Usually, the organic phase consists of a diluent (kerosene) to dissolve the extractant, reduce the viscosity of the loaded organic phase and facilitate the hydrodynamics of the extracted species. Tri-n-butyl phosphate (TBP) is a classic solvating extractant used in the separation of REE.…”