Novel octanethioic acid alkyl amide extractants: Synthesis, their highly selective separation for palladium, and the extraction mechanism from chloride media

Abstract

Highly selective extraction of Pd from leaching liquor of spent auto-catalysts is a research hotspot in recent years. In this study, three novel octanethioic acid alkyl amide extractants (alkyl = butyl TA-804, hexyl TA-806 and 2-ethylhexyl TA-826) were designed, synthesized and characterized by 1H NMR, FT-IR and ESI-HRMS. The effect of contact time, extractant concentration, HCl concentration and diluents on Pd selective extraction and separation by these three extractants were investigated. The stripping behaviors of the loaded Pd in the organic phase were examined. The McCabe-Thiele diagrams of Pd extraction and stripping were drawn. The extraction mechanism was studied through methods of stoichiometry calculation, slope analysis, FT-IR and ESI-HRMS. In the end, the Pd separation from simulated leaching liquor of spent auto-catalysts by TA-804, TA-806 and TA-826 was carried out. The results show TA-826 has much slow Pd extraction kinetics than TA-804 and TA-806. They all have highly efficient extraction and separation performance for Pd from chloride media at acidities of [HCl] ≤ 3 mol/L. When the HCl concentration is ≥4 mol/L, Fe and Pt begin to be extracted. At any acidities, Rh, Zn, Al, La, Zr, Co, Cu, Ni, Ce and Mg are not extracted. The studied octanethioic acid alkyl amides extract Pd from chloride media via complexation mechanism. Two extractant molecules quantitatively extract one Pd. The loaded Pd are easily and efficiently stripped by thiourea. They can efficiently extract and highly separate Pd from leaching liquor of spent auto-catalysts at acidity of 3 mol/L HCl.