Selective extraction equilibria of Sc(III), Y(III), Fe(III) and Al(III) from acidic media with toluene mixture of deep eutectic solvent (DES) composed of TOPO and isostearic acid.

A deep eutectic solvent (DES) composed of isostearic acid and TOPO was newly found out and investigated for selective recovery of scandium. The four elements used in this study are scandium, iron, yttrium, and aluminum. It was difficult to separate the four elements owing to the overlap of extraction behavior when isostearic acid or TOPO alone in toluene was used, respectively. However, scandium was able to be selectively extracted from other metals using DES prepared by 1:1 mol ratio of isostearic acid and TOPO without toluene. In DES consisting of isostearic acid and TOPO, synergistic and blocking effects with three extractants acted on the extraction selectivity for scandium. Both effects are proved also from the fact that scandium was easily stripped with diluted acidic solutions such as 2 M HCl and H2SO4. Thus, DES extracted selectively scandium and back-extraction was easily achieved. In order to elucidate these phenomena above, the extraction equilibria of Sc(III) using DES dissolved in toluene was investigated in detail. The extraction mechanism of scandium by DES in toluene indicates that the extracted chemical species vary with pH, and that trivalent scandium is extracted by forming stable metal complexes with DESs consisting of five molecules of isostearic acid and five molecules of TOPO.