Simple Amides and Amines for the Synergistic Recovery of Rhodium from Hydrochloric Acid by Solvent Extraction

Abstract

The separation and isolation of many of the platinum group metals (PGMs) is currently achieved commercially using solvent extraction processes. The extraction of rhodium is problematic however, as a variety of complexes of the form [RhCln(H2O)6‐n](n−3)− are found in hydrochloric acid, making it difficult to design a reagent that can extract all the rhodium. In this work, the synergistic combination of a primary amine (2‐ethylhexylamine, LA) with a primary amide (3,5,5‐trimethylhexanamide, L1) is shown to extract over 85 % of rhodium from 4 M hydrochloric acid. Two rhodium complexes are shown to reside in the organic phase, the ion‐pair [HLA]3[RhCl6] and the amide complex [HLA]2[RhCl5(L1)]; in the latter complex, the amide is tautomerized to its enol form and coordinated to the rhodium centre through the nitrogen atom. This insight highlights the need for ligands that target specific metal complexes in the aqueous phase and provides an efficient synergistic solution for the solvent extraction of rhodium.

Sustainable and Selective Modern Methods of Noble Metal Recycling

“…Rh III extraction is challenging and generally presents the last separation step due to the complexity of the Rh III species of consequence from the preceding steps. Combination of inner and outer ligands can be applied for efficient transportation of Rh III to organic phase [71–73] . Single ligand separation of PGMs, Cu II and base metals are also known with thioureas like 2‐mercaptobenzimidazole [74] (Figure 4e) and 1‐hexyl‐3‐methyimidazole‐2‐thione [75] (Figure 4f).…”

“…Combination of inner and outer ligands can be applied for efficient transportation of Rh III to organic phase. [71][72][73] Single ligand separation of PGMs, Cu II and base metals are also known with thioureas like 2-mercaptobenzimidazole [74] (Figure 4e) and 1-hexyl-3-methyimidazole-2-thione [75] (Figure 4f). These act as inner sphere ligands for Pt II and outer sphere ligands for Pt IV (or even Rh III ).…”