Behavior and mechanism investigation of separating Pt and Ir by liquid–liquid extraction using a mixed [C6bet]Br/[C6mim][NTF2] system

Abstract

In the process of recovering platinum group metals by extraction, separation of Pt and Ir is very difficult because Pt(IV) and Ir(IV) show similar chemical properties and extraction behaviours in hydrochloric acid medium. In this paper, a betaine-based ionic liquid [C₆bet]Br was firstly applied to the separation of Pt(IV) and Ir(IV). [C₆bet]Br showed remarkable precipitation performance of Pt(IV) and Ir(IV). The impactful extraction was realized by importing [C₆mim][NTF₂] as the hydrophobic phase with extraction efficiencies of ca. 99% and 88% under optimal conditions. A selective reduction process for Ir(IV) was firstly proposed for the separation of these two similar PGMs by using hydroxylamine hydrochloride (NH₂OH·HCl). The mechanism of extracting Pt(IV)/Ir(IV) using [C₆bet]Br was confirmed to be an anion-exchange process by spectroscopic analysis (UV-Vis, FT-IR, ¹H NMR) and the method of continuous variation. The electrostatic potential map of [C₆bet]Br was calculated for the first time to confirm the anion-exchange mechanism. 8 M HCl was effective for the back extraction of Pt(IV) and Ir(IV).