Toward green and efficient recycling of Au(III), Pd(II) and Pt(IV) from acidic medium using UCST-type ionic liquid

The development of efficient ionic liquids (ILs) for disposable separation and recovery of various precious metals was of great importance, but still faced difficult challenges. A phase change temperature-controlled system composed of IL and HCl solution was developed for the extraction and separation of Au(III), Pd(II) and Pt(IV) from secondary resources. UCST (upper critical solution temperature)-type IL ([1,4,7-TMTA][Tf2N]) possessed outstanding selectivity of precious metals and the property of temperature-controlled phase transition. The ability of ILs to form solvated hydrogen bonds was confirmed by quantum-chemical calculation to be correlated with their corresponding UCST. Different experimental parameters were optimized to explore the extraction behavior of IL. It is noteworthy that homogeneous liquid-liquid extraction system (HLLE) constructed by UCST-type IL could significantly enhance the extraction kinetic of Au(III), Pd(II) and Pt(IV) by decreasing phase interface resistance. For Au(III), Pd(II) and Pt(IV), when the system temperatures were higher than 40 °C, 49 °C and 65 °C, the maximum values were 98.5%, 98.3% and 93.2% at the highest efficiency. In addition, the extraction mechanism of anion exchange was revealed by UV–vis, IR, NMR and LC-MS spectrometry. At last, Au(III), Pd(II) and Pt(IV) could be selectively separated from multi-metal solutions where impurity ions coexist by K2C2O4, NH3·H2O and CS(NH2)2/HCl, respectively. After 5 cycles, the extraction performance of [1,4,7-TMTA][Tf2N] maintained. In summary, this paper overcame the obstacle of slow kinetics of solvent extraction technology, and pointed out a way for polymetallic co-extraction and separation in HLLE system.