Non-aqueous solvent extraction of indium from an ethylene glycol feed solution by the ionic liquid Cyphos IL 101: speciation study and continuous counter-current process in mixer–settlers†
A solvometallurgical process for the separation of indium(III) and zinc(II) from ethylene glycol solutions using the ionic liquid extractants Cyphos IL 101 and Aliquat 336 in an aromatic diluent has been investigated. The speciation of indium(III) in the two immiscible organic phases was investigated by Raman spectroscopy, infrared spectroscopy, EXAFS and 115In NMR spectroscopy. At low LiCl concentrations in ethylene glycol, the bridging (InCl3)2(EG)3 or mononuclear (InCl3)(EG)2 complex is proposed. At higher lithium chloride concentrations, the first coordination sphere changes to two oxygen atoms from one bidentate ethylene glycol ligand and four chloride anions ([In(EG)Cl4]−). In the less polar phase, indium(III) is present as a tetrahedral [InCl4]− complex independent of the LiCl concentration. After the number of theoretical stages had been determined using a McCabe–Thiele diagram for extraction by Cyphos IL 101, the extraction and scrubbing processes were performed in lab-scale mixer–settlers to test the feasibility of working in continuous mode. Indium(III) was extracted quantitatively in four stages, with 19% co-extraction of zinc(II). The co-extracted zinc(II) was scrubbed selectively in six stages using an indium(III) scrub solution. Indium(III) was recovered from the loaded less polar organic phase as indium(III) hydroxide (98.5%) by precipitation stripping with an aqueous NaOH solution.