Ionic liquids (ILs) have been presented as suitable candidates for metal extraction in the hydrometallurgy. It has already been proven that they have an adjustable selectivity towards metal ions. However, industrial applications of ILs are limited due to their high price.Therefore, regeneration of ILs is necessary. In this study the regeneration of the fatty acid based IL tetraoctylammonium oleate was investigated, because in a previous study we obtained excellent metal salt extraction efficiencies with this benign IL. Several methods for regeneration were investigated, i.e. electro-deposition and chemical regeneration. Electrodeposition turned out to be unfeasible for this IL. Chemical regeneration showed that the metal ions (Zn, Co and Mn) can be selectively back-extracted, so that metals can also be separated from each other in this step. The best stripping solution was aqueous sodium oxalate, which allowed the IL to be immediately reused.
Aqueous waste streams of the metallurgical industry often contain considerable concentrations of metal salts. Previous research showed that the metal chloride salts of zinc(ii), manganese(ii) and iron(iii) can be recovered by solvent extraction using a sustainable and renewable fatty acid based ionic liquid as the extractant. In this paper, the extraction mechanism of Zn(ii), Co(ii) and Ni(ii) from chloride media has been studied systematically. The metal extraction performances of the precursors, sodium oleate and tetraoctylammonium chloride, were compared to the extraction performance of the ionic liquid tetraoctylammonium oleate. Slope analysis experiments were performed to determine the number of ionic liquid molecules involved in the extraction. The experimental data showed that Co(ii) and Ni(ii) were extracted in the pH range from 6 to 8 by the formation of negatively charged metal carboxylate complexes with tetraalkylammonium counter ions. In contrast, Zn(ii) gets extracted as a mixed metal chloride carboxylate anionic complex with tetraalkylammonium counter ions. This extraction mechanism was supported by EXAFS measurements.
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