Designing reagents for the solvent extraction of critical metal resources
Item statusRestricted Access
Embargo end date31/12/2100
Doidge, Euan Douglas
The work in this thesis aims to develop new systems for the more efficient recovery of metals from aqueous solution using solvent extraction. Understanding the underlying coordination chemistry to improve hydrometallurgical methods is crucial in order to meet the demand for critical metals for use in modern technologies, reduce the environment impact of recovery from primary mining deposits, and recycle valuable metals from secondary sources (e.g. mobile phones, WEEE). Chapter 2 examines the use of a simple primary amide that can load gold and other chloridometalates into a toluene phase through an outer-sphere mechanism. The loading of a variety of metals/metalloids from varying [HCl] is reported, highlighting the selectivity for gold over other metalates and chloride due to a combination of speciation of those metals and the relative ease of extraction of lower charged species (the Hofmeister bias). The advantages in loading/stripping, toxicity and mass balances compared to commercial alternatives are also outlined, in particular the efficacy of separating gold from a mixed-metal solution representative of those found in WEEE. The mode of action of the primary amide (and secondary/tertiary analogues) is determined using slope analysis, Karl-Fischer water determinations, NMR and MS measurements, EXAFS and computational models. The extraction occurs by the dynamic assembly of multiple amide ligands and gold metalates to generate supramolecular clusters held together through hydrogen-bonding and electrostatic interactions. The secondary and tertiary amides are found to be able to extract monoanionic metalates in a similar manner as the primary amide, although clustering occurs to a lesser extent. Whilst the secondary and tertiary amides are stronger gold extractants than the primary amide, they are not observed to be as successful when extracting from a mixed-metal solution. Instead, a 3rd phase is seen to form from these amides and some metals at higher metal concentrations, which removes the ligands from solution and prevents successful extraction of gold. Chapter 3 builds on an observation in Chapter 2 that a synergistic combination of a simple primary amide and an amine can extract chloridometalates that are typically difficult to solvent extract, such as iridium(III) and rhodium(III). These metalates, complexes with increased anionic charge and varying speciation in aqueous solution, are typically recovered last in a metal production flowsheet. The combination of a primary amide and primary amine was found to be the most effective at extracting the chloridometalates; the strength and strippability of the system is of particular interest in the context of rhodium(III) recovery as this metal currently is not extracted in commercial circuits. The mode of action of the system is investigated using similar techniques to Chapter 2, and reveals that the amine is the more important component of the synergistic mixture compared to the amide, with an improvement in extraction observed when both components are present. Rh(III) is extracted as a mixture of RhCl6 3– and RhCl5(OH2)2– complexes, dependent on the initial [HCl] concentration and the age of the initial aqueous solution. Chapter 4 investigates the feasibility of the recovery of lanthanides as anionic metalates from chloride-, nitrate- or sulfate-rich feeds. Reagents that have been found to be strong chloridometalate extractants, fragmented versions of these, and ‘classic’ commercial outer-sphere reagents are studied. The variations of ligand, anion type and concentration, proton concentration and solvent for the extraction of lanthanides is investigated. However, despite these permutations, no extraction of lanthanides is observed due to the difficulty in extracting more highly hydrated species and the lack of stability of the metalates in aqueous solution.