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Title: Exploring the solvent extraction of rhodium and gold
Author: Nicolson, Rebecca Michelle
ISNI:       0000 0004 8508 8768
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2019
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This work explores the solvent extraction of precious metals, specifically rhodium and gold, from chloride solution, with the aims of understanding how existing extractants work and designing new extractants. A wide variety of analytical techniques are employed, demonstrating how they can be used together to assess extraction ability and provide insight into the identity of the extracted species. Computational techniques are also used; their implementation, often in conjunction with experiment, can identify the interactions which allow extraction to occur and explain differences in extraction behaviour. Chapter 3 focuses on understanding how amidoamine extractants interact with the Rh(III) complexes present in chloride solution and thus enable their extraction. Experimental analysis shows that at low concentrations of Rh(III), the extracted species is RhCl5(H2O).(LH)2, but at high concentrations a di-nuclear species, Rh2Cl9.(LH)3, is present in the organic phase and on aging an inner-sphere complex, RhCl3L, can form. The mode of extraction is found to differ from that of a simple amine-based extractant. Computational modelling explores the extraction behaviour of a series of amidoamine extractants, finding they have different anion binding modes depending on the number of intra-extractant hydrogen bonds they can form. This results in bis- and tris-amidoamines having a more suitable binding mode for the Rh(III) aquo-chloridometalate compared to a mono-amidoamine, which has a more suitable binding mode for chloride. Calculated formation energies are broadly in agreement with experimental results and suggest selectivity for [RhCl5(H2O)]2− over chloride is the source of the success of this class of extractants. This work presents a full mode of action analysis of the bis-amidoamine and rationalises why the amidoamine reagents were the first effective Rh(III) extractants from chloride solution: they are proton-chelating reagents, which can adopt a binding mode that is selective for larger, more charge-diffuse anions. Building on the work of Chapter 3, a theoretical screening study of other mono- and bisamidoamines (or amido-quaternary ammonium compounds) is presented in Chapter 4. Different potential binding modes of the molecules are explored computationally, and it is found that, where possible, N-H to anion binding is generally more favourable than intramolecule proton chelation and C-H to anion binding, with only one exception to this rule. Interestingly, this exception proves to have the most favourable energy of exchanging chloride for [RhCl5(H2O)]2−, suggesting that it would be the most effective at extracting the Rh(III) metalate. All other molecules are theoretically poorer extractants of Rh(III) and, based on a comparison of the energies of formation, it appears that the reason for this is less favourable association with [RhCl5(H2O)]2−, more favourable association with chloride, or a combination of the two. This work highlights the importance of a favourable C-H to anion, or "soft", binding mode in the selective extraction of Rh(III) metalate over chloride, and how small structural changes to the extractant can drastically alter the favourability of this type of binding mode. Chapter 5 explores the possibility of using polyamine-based Rh precipitants or reagents based on them for Rh extraction. It is found that "precipitant" molecules with long chain Rgroups added are capable of extracting Rh(III) metalate very well, even from solutions of high HCl concentration. At high HCl concentrations, the most likely Rh(III) species extracted from the aqueous phase is [RhCl6]3−, with which the extractant is expected to associate in the outer-sphere. In contrast, it is found that an inner-sphere complex forms upon extraction from solutions of very low chloride concentrations. Extraction from mixed Rh(III)/Pt(IV) aqueous solutions is also investigated, but no selectivity for Rh(III) over Pt(IV) is found. Rh(III) stripping from the loaded organic is investigated using a number of reagents, with ammonium hydroxide solution found to be the most effective. The reagent designed and tested has the potential to be used for the solvent extraction of Rh industrially, offering extraction at the higher HCl concentrations typically used in existing metal recovery flowsheets. Industrially, Au(III) solvent extraction from chloride solution is well established, however, some of the reagents used are still not well understood. Chapter 6 aims to explain the extraction mechanism, and particularly the role of water, in Au(III) recovery with an industrial reagent via classical molecular dynamics simulations, which allow the assembly of the extracted species to be viewed and analysed. Experimental conditions can be modelled, but, in addition, so can other, non-experimental conditions to permit a better understanding of the extraction behaviour. Analysis of the output structures suggests that water's primary role in extraction is as the positive charge carrier. In some systems, where chloride is considered as the anion, the water partially hydrates the extracted anion, acting as a mediating agent between the electronegative functional groups on the extractant and the anion. This new insight into the nature of the extraction assemblies provides a greater understanding of the role of water in the extraction mechanism, and the means by which the extractant transports [AuCl4]‒ into the organic phase, information which can be used for informed modification of existing extraction processes and development of new reagents.
Supervisor: Morrison, Carole ; Love, Jason Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: efficiency ; hydrometallurgy ; solvent extraction ; industrial reagents ; rhodium recovery ; amidoamine reagents ; gold extractants ; rhodium