Developing new processes for the solvent extraction of precious metals

Abstract

The work presented in this thesis focuses on developing and understanding new solvent extraction processes for the recovery and separation of precious metals (PMs), specifically rhodium, from aqueous hydrochloric acid solutions. Rhodium is a rare and extremely valuable metal, widely used in catalytic converters as well as jewellery and other industrial and chemical applications. It is found alongside other PMs, both in virgin ores and secondary recycled sources, from which it must be separated. However, it is typically recovered at the end of an industrial refining process, using single-use precipitants, which contributes to the high economic and environmental cost of rhodium production.

Chapter 1 provides an overview of metal separation using solvent extraction, describing the various modes of action into which such processes are typically classified, and current methods for the separation of precious metals within a typical refining flowsheet are also outlined. The background literature investigating rhodium speciation in aqueous hydrochloric acid solutions and developing existing rhodium extractants is also discussed. The aims of this thesis are also set out, namely to understand the mode of action of rhodium solvent extraction using amine and amide synergists, and to develop upon this understanding to design efficient, selective and industrially viable processes for the solvent extraction of rhodium and other precious metals.

A range of amide and amine extractants were investigated as extractants for rhodium in this work. In Chapter 2, the mode of action for rhodium extraction by a synergistic mixture of a simple primary amine (2-ethylhexylamine, LA) and a simple primary amide (3,5,5- trimethylhexanamide, L1) was evaluated. Spectroscopic and computational techniques showed that two rhodium complexes 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 rhodium through the nitrogen atom. It is shown that additional protonated amines and exogenous chloride anions cluster around the chloridometalates in the organic phase, shielding the charge-dense anions. By extracting two different rhodium complexes, the synergistic mixture is able to adapt to the aqueous phase speciation of rhodium and so maximise extraction.