Recovery of Au(III) from Acidic Chloride Media by Homogenous Liquid–Liquid Extraction with UCST-Type Ionic Liquids

Three novel ionic liquids were synthesized. Their binary mixtures with water exhibit tunable thermomorphic behaviors involving monophasic–biphasic transition. Through electrostatic potential maps, we predicted the ability of the three ionic liquids to form solvated hydrogen bonding to estimate the upper critical solution temperature (UCST) ordering of their binary mixture with water. Then, experimental verifications were carried out of the prediction. Three homogeneous liquid–liquid extraction systems were used to extract Au(III) from acidic chloride media based on the thermodynamic property. By comparing the UCST and extraction efficiency of the three systems, the N-carboxymethyl-ethyl-ester-N-methylmorpholinium bis(trifluoromethylsulfonyl)imide ([EtbetmMor][Tf2N]) was selected for subsequent systematic extraction study. High-efficiency extraction and extraordinary selectivity of gold were achieved by optimizing a series of factors affecting the extraction process. The extraction mechanism was confirmed in further. Finally, the extracted gold could be recovered from the ionic liquid phase by precipitation stripping with oxalic acid. The [EtbetmMor][Tf2N] could be regenerated simply by mixing the raffinate containing [Tf2N]− with the [HbetmMor]Cl phase separated with gold after stripping. It is reasonable to think that the environmental friendliness and high efficiency of this new extraction system make it a strong candidate in the field of gold recycling.