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The work presented in this thesis focuses on developing and understanding practices and reagents that can be more efficient at rare earth element (REE) recovery and separation than current industrial reagents and practices. With demand for REEs rapidly increasing due to their use in modern technologies and clean energy production, the sustainable supply of these elements has become vitally important for society. The concepts and difficulties that underpin current REE recovery are introduced in Chapter 2 followed by an appraisal of how an ammonium ionic liquid (IL) transports these elements from an aqueous phase into an immiscible organic phase during solvent extraction. Prior to this work the extraction of REEs using ILs was known but an understanding of the chemical transport mechanisms was limited. A large variety of analytical, spectroscopic and computational techniques have been used: ICP-OES measurements confirmed that REE extraction from nitrate solutions is maximised under low acid, high salt conditions with preferential extraction of the lighter REEs occurring; Karl-Fischer water content measurements confirmed only modest water transport, allowing for (reverse)-micelle formation in the organic phase to be rejected. The combined experimental, analytical and computational data alludes to the transport of REEs through a microhydration mechanism pathway. Organic phase REEs comprising both inner-sphere bound water molecules and nitrate anions are encapsulated and stabilised in the organic phase by multiple lipophilic IL cations through a hydrogen-bonding network. Chapter 3 builds upon the understanding of REE extraction by ILs gained in Chapter 2. The extraction of REEs using neutral reagents (malonoamides and diglycolamides) is introduced, highlighting their effectiveness but also their tendency to form undesirable precipitates (3rd phase formation). Incorporating chemical characteristics of these reagents such as the amide functional group into an IL results in a stronger extractant with improved selectively for lighter REEs and no solubility issues. The effective separation of lighter REEs (e.g. Ce or Nd) from heavier (e.g. Tb or Dy) REEs using an amide functionalised IL is presented with potential industrial applications discussed. Experimental, analytical and computational work suggests microhydrated REEs are extracted and a more extensive hydrogen-bonding network comprising amides, nitrate anions and water molecules enhances the stability of the formed organic phase assemblies. Chapter 4 compares the leaching of REEs from eudialyte, a zirconate REE mineral by highly acidic solutions against mildly acidic solutions containing the bacterialstrain Acidithiobacillus thiooxidans. The percentage of REEs leached using mild conditions is noticeably reduced over highly acidic conditions. Even so, and with no pH adjustment, the leached lighter REEs could be effectively separated from other elements within the leach solutions using an amide functionalised IL, potentially providing a more environmentally friendly REE purification process. Chapter 5 explores the development of multiple variations of the amide functionalised ILs introduced in Chapter 3, highlighting their similarities and differences. The variants include the incorporation of more amide functionalities and increasing the lipophilicity of the ILs. While initial screening of these reagents did not indicate any improvements over the ILs introduced in Chapter 3, the experimental data collected for these reagents helps to validate the importance of the amido-ammonium function.

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.

Chapter 1 consists of a review of solvent extraction of metals, primarily focused on the fundamental chemical processes involved in the technique, and techniques previously applied to the elucidation of extraction mechanisms. Chapter 2 contains an explanation and discussion of the theoretical and experimental techniques employed in obtaining the results presented in later chapters. In Chapter 3, computational studies are presented which aim to explore the modes of action of two novel classes of reagent which have been found to function efficaciously as extractants for precious metals, and rationalise trends in the extractive strength and selectivity of these reagents observed through experiment. DFT formation energy calculations were found to reproduce experimentally-determined trends in the extractive strength of a series of amidoamine and amidoether extractants for platinum, as well as the apparent selectivity of these reagents for hexachloroplatinate (PtCl62−) over chloride, which is found to arise due to a wide array of hydrogen-bond donor groups within the extractant which leads to preferential association to larger anions. Favourability of protonation is found to be the primary determiner of extractive strength in reagents of this class. A contrasting mode of action was revealed for a class of selective amide extractants for gold, with DFT and classical molecular dynamics methods indicating the formation of large supramolecular aggregates of extractant units and tetrachloroaurate (AuCl4−) anions during the extraction process. Molecular dynamics results are consistent with, and allow the rationalisation of, results obtained from slope analysis and EXAFS experiments. Aggregate stability is found to be greater for primary amide extractants than for secondary or tertiary amide analogues, which is proposed as the origin of the superior selectivity for gold exhibited by primary amide extractants. In Chapter 4, the mode of action of commercial phosphinic acid extractant CYANEX® 272 in cobalt(II) extraction is investigated using a variety of techniques including inductively-coupled plasma optical emission spectroscopy (ICP-OES), 31P{1H} NMR spectroscopy, mass spectrometry and DFT geometry optimisation calculations. Extraction is found to proceed via differing mechanisms depending on the concentration of unassociated phosphinic acid in the system, with extended polymeric structures being formed at high levels of cobalt extraction, providing an explanation for observed increases in organic phase viscosity under such conditions. Similar techniques are applied in Chapter 5 to the case of iron(III) extraction by CYANEX® 272. A single mechanism was found to persist across all levels of Fe(III) extraction, though extraction experiments and 31P{1H} NMR spectroscopy indicate that anions other than phosphinate must be incorporated into the extracted species. Results from ICP-OES and mass spectrometry suggest that species incorporating SO42− or HSO4− anions as well as O2− or OH− anions are formed during the extraction process. Finally, in Chapter 6, a novel computational method for determination of the stability of metalate anions in aqueous solutions is discussed. Preliminary results from solution-phase DFT formation energy calculations suggest that inner-sphere co-ordination of chloride and nitrate anions to La3+ in solution is greatly disfavoured compared to formation of outer-sphere complexes, consistent with literature reports that inner-sphere co-ordination of these anions to lanthanum is not observed in solution, and experimental results suggesting that solvent extraction of lanthanum metalates is unviable. Disfavouring of inner-sphere co-ordination to La3+ is calculated to be less substantial for sulfate anions, reflective of experimental observations that inner-sphere co-ordination of sulfate to La3+ can occur to a smaller degree. Similar, disfavouring of inner-sphere chloride association to Cd2+ and Hg2+ (which is known to occur in solution) is calculated to be significantly smaller than to La3+, suggesting that such calculations could serve as qualitative predictors of the viability of formation of solution-phase inner-sphere complexes of any given metals and anions.

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.

This thesis explores the use of mass spectrometry to define the strengths, and understand solution phase speciation of phenolic oxime-based solvent extractants of the types used in the hydrometallurgical recovery of copper. Chapter 1 reviews briefly the current extraction technology for copper and focuses on hydrometallurgy and the use of phenolic oximes such as 5-nonylsalicylaldoxime. The modification of the latter to improve extraction efficiency is discussed, focussing on the introduction of X-substituents in the 3 position of the benzene ring. Modern mass spectrometry techniques are also discussed with a focus on their application to inorganic systems and their use in achieving the aims of this thesis, as defined above. The work described in chapter 2 involves the development of collision induced dissociation (CID) techniques to determine the relative gas phase stabilities of copper complex anions of the type [Cu(L)(L-H)]-, where LH is a 5-alkyl-3-X-2- hydroxybenzaldehyde oximes and X a range of substituents. The importance of interligand interactions in the outer-coordination sphere and their influence on gas phase anion stability, as defined by CID, is reported. The work described in Chapter 2 on CID is extended in chapter 3 and looks at the effect of charge, of ligand type, LH, and of the nature of the metal on the stability of ionic forms of [M(L)2] complexes, where LH is extended to include 5-alkyl-2- hydroxyphenylethanone oximes. The effects of substitution at the azomethine carbon atom and at the 3-position of the benzene ring and of variation of the nature of the metal on the ion dissociation mechanisms are shown to have a major influence on ion stability under CID conditions. In chapter 4 density functional theory calculations have been used to investigate the influences of substitution at the azomethine carbon atom and at the 3-position of the benzene ring and of variation of the nature of the metal on the gas phase structures of the neutral complexes, [M(L)2]. Gas phase deprotonation and dimerisation enthalpies of the ligands, LH, and enthalpies of formation of [M(L)2] complexes have been calculated and correlates with experimentally determined ligand extraction strength. The ligand type has been extended to include 3-X-2-hydroxybenzaldehyde hydrazones, which have previously been shown to have lower distribution coefficients for copper than the analogous 3-X-substituted oximes. The calculated gas phase formation enthalpies for [M(L)2] show a strong correlation with the strengths as extractants LH, measured as their pH0.5 values for metal uptake. Chapter 5 considers whether mass spectrometry can be used to define the solution equilibria when two different oxime-based ligands, LXH and LYH, compete for Cu(II) in a single phase solution. It has been established that shifts in the relative peak intensities of deprotonated ions derived from the Cu(II) complexes, [Cu(LX)2], [Cu(LY)2] and [Cu(LX)(LY)] reflect changes in the solution composition. The work described in chapter 6 extends the study of solution phase speciation using mass spectrometry. When the Cu(II) and proton concentrations of solutions were varied distinct changes in the resulting electrospray mass spectra were observed and the resulting species were identified using CID and high resolution mass spectrometry. A novel, [Cu3(L-H)3-μ3-O/OH]- species is determined to be a major component of solutions where Cu(II) concentrations are equal to/greater than the LH concentration. Various 3-X-2-hydroxybenzaldehyde oximes (X = CH2NR2) were synthesised. The incorporation of a protonatable arm in the 3-position enabled trinuclear complexes, [Cu3(L-H)3-μ3-OH], to be isolated and fully characterized, including two X-ray determined crystal structures.

This thesis concerns the development of new types of solvent extractants for use in the hydrometallurgical recovery of base metals, and addresses the ligand design features which are needed to control the strength, transport efficiency and selectivity of these extractants. Chapter 1 provides background on the development of extractive metallurgy, e.g. pyrometallurgy and hydrometallurgy, and introduces the history, basic terminology and various processes and reagents involved. Solvent extraction as a hydrometallurgical technique to achieve the recovery of base metals is discussed in most detail. The design criteria of extractants are highlighted as the focus of the whole thesis. Chapter 2 investigates the potential of salicylaldehyde hydrazones as cation exchange extractants for hydrometallurgical recovery of copper, and studies substituent effects, e.g. electronic, steric and particularly “buttressing” of ligand-ligand hydrogen bonding, on the strength and efficacy of the extractants. A series of 3-substituted (X) and N-substituted salicylaldehyde hydrazones has been developed. Solvent extraction experiments show that the 3-substitution can increase the distribution coefficient of the methylhydrazone for copper extraction by more than three orders of magnitudes along the series (X) Me < OMe ≤ H < Br < NO2. Both the phenol acidity and intermolecular hydrogen bonding are significantly influenced by the 3-substituent as judged by a systematic NMR study on the solution speciation of the free ligands. Electron-withdrawing groups which also act as hydrogen bond acceptors (X = NO2 or Br) are particularly effective in enhancing the strength of the methylhydrazones. Compared to the commercially applied salicylaldoximes, the hydrazones are weaker extractants, and the strength varies in the order: oximes > methylhydrazones > phenylhydrazones. This order probably arises from a combination of variations in the phenol acidities and the strength of the hydrogen bonding motifs, which are also probed in the free ligands using NMR techniques. Chapter 3 considers reagents capable of extracting metal salts and deals with the development of polytopic salicylaldimine ligands bearing pendant tertiary amine groups for the hydrometallurgical recovery of zinc chloride. These extractants show extremely high transport efficiency of metal salts with more than two moles of ZnCl2 loaded per mole of ligand, and high chloride over sulfate selectivity in solvent extraction experiments. The unusual multiple loading suggests the extraction of chlorozincate as anion species, which is supported by the elemental analysis and ESI MS spectra of the formed complexes. The zinc chloride dependent extraction and stripping experiments further indicate that the extraction process is controlled by the Cl- activities in the aqueous solutions. 1H NMR studies show that two different complexes are successively formed in solution as the ligand to zinc stoichiometry is increased to 1L : 2ZnCl2, with the first formed by extracting the zinc cation at low ZnCl2 concentrations and the second likely resulting from the further extraction of chlorozincate at high ZnCl2 concentrations. An extraction mechanism is proposed in which a tritopic assembly [ZnLCl2] forms by binding the Zn2+ cation with the N2O2 2- site of the “salen” type ligands and two Cl- anions with the protonated pendant amine groups and then a ditopic assembly [ZnL(ZnCl4)] forms by extracting the [ZnCl4]2- anion when the reagent is contacted with high tenor ZnCl2 feed. The possible formation of tritopic assemblies [ZnL(ZnCl3)2] accounts for Zn-loadings higher than 200%. The ligand design features such as the benefits of combining cation and anion binding sites in the same molecule are indicated by comparing the polytopic ligands with a series of dual host pairs of the salen ligand and hydrophobic amines. The polytopic ligands show potential for industrial application as ZnCl2 extractants. Finally chapter 4 focuses on the anion binding and selectivity involving simple anions and chlorometallates in solvent extraction by the polytopic ligands discussed in chapter 3 and their metal complexes. A novel and reliable method for chloride analysis in the presence of complexing metal cations has been developed using excess silver(I) to precipitate chloride and then analyze the silver content by ICP-OES technique. Multiple loadings of chloride are achieved by the polytopic ligands, which confirms the uptake of metal salt as ZnCl2, and is desirable for excellent material balances in hydrometallurgical processes. The anion binding of the copper-only complex [Cu(L-2H)] supports the proposed extraction mechanism by indicating that one mole of zinc is extracted in the likely form of chlorozincate anions when the cation binding site is occupied by Cu2+. The polytopic ligands and their copper-only complexes [Cu(L-2H)] show the same anion selectivity following the order [ZnCl4]2- > Cl- > SO4 2-, which can be explained by the hydration energies of these anions. Extractions from ZnCl2, ZnSO4, Zn(NO3)2 or mixed feed solutions indicate that the zinc transport efficiency is also dependent on the nature of counter anions.

The work outlined in this thesis was sponsored, in part, by Anglo American and concerns the development of new technologies to achieve the concentration and separation of base metal values in chloride hydrometallurgical circuits. New processes for the production of zinc, cobalt and nickel aim to use solvent extraction to achieve the separation of metal values in highly concentrated acid chloride feeds containing iron and this thesis involves new extractants for potential use in these circuits. Anion exchange solvent extraction was chosen as the most practical approach and so a range of new reagents are described which remove zinc(II), cobalt(II) and iron(III) chlorometallates from acid chloride solutions via the reaction: nL(org) + nH+ + MClx n- [(LH)nMClx](org) Chapter 1 reviews the literature which concerns base metal chloride hydrometallurgy, presents a range of commercial processes and discusses the chemistry which underpins them. This chapter also outlines the new Anglo American circuits and the general approach to the design of base metal chlorometallate extractants. In Chapter 2, the analytical methods are discussed. These methods include the solvent extraction experiments that were used to define the behaviour of the new ligands and the techniques that were employed to examine the interactions between an extractant and a chlorometallate anion. Chapter 3 presents a series of five new amido-functionalised pyridine reagents that were designed to investigate the affect of hydrogen bond donor functionality on the extraction of zinc, cobalt and iron chlorometallates. The pyridine nitrogen atom is sterically hindered in the new reagents to suppress formation of inner-sphere complexes. Solvent extraction performance was found to vary considerably with hydrogen bond donor functionality and ligand structure. The ligand 2-(4,6-di-tertbutylpyridin- 2-yl)-N,N’-dihexylmalonamide (L2) was the strongest and most efficient extractant in this series and this was attributed to a ‘proton chelate’ six-membered ring interaction between the malonamide oxygens and the protonated pyridine nitrogen that resulted in a pre-organised array of N-H and C-H donors that could interact favourably with the chlorometallate anion. Chapter 4 explores a series of six new tertiary amine-based ligands which contain varying amido-functionality, e.g. 3-(di-2-ethylhexylamino)-N-hexylpropanamide (MAA). Zinc, cobalt and iron chlorometallate extraction studies show the amide and malonamide-functionalised ligands are notably stronger than the tertiary alkylamine control, tris-2-ethylhexylamine (TEHA). Platinum(IV) extraction is also discussed, showing that some of the new reagents are more efficient than the tren-based ligands previously described,{Bell Katherine, 2008 #93} which were the most efficient known. The enhanced extraction performance of the new ‘MAA-type’ ligands was again attributed to the formation of a ‘proton chelate’ six-membered ring forming [(LH)2MCl4] assemblies in the organic phase. Conditions have been identified which would allow separation of Fe(III), Co(II) and Zn(II) in circuits which use the ‘MAAtype’ reagents. Chapter 5 explores a series of three new malonamide reagents which contain varying alkyl-chain functionality, e.g. N,N’-dimethylhexylpentadecylmalonamide (M1), which are thought to extract chlorometallate anions via protonation of the carbonyl oxygens. Zinc, cobalt and iron chlorometallate extraction studies demonstrate that the malonamide ligands show high efficiency and selectivity for iron over zinc and cobalt. Performance as chlorometallate extractants was found to vary considerably with ligand structure and hydrogen bond donor functionality in all three ligand series, with a number of ligands showing potential for commercial application. Analysis of anion-host interactions suggests that chlorometallate binding in the organic phase probably proceeds via an array of both N-H and C-H weak hydrogen bonding interactions between the extractant and the outer-sphere of the metallate complex.

The thesis deals with the modification of salicylaldoxime-based reagents used in hydrometallurgical extraction, addressing rational ligand design to tune copper(II) extractant strengths and also the development of reagents which are capable of transporting transition metal salts. Chapter 1 reviews current solvent extractant technology for metal recovery, including the limited knowledge of the effect of substituents on extractive efficacy. Advances in leaching technology have led to systems wherein increases in process efficiency could be obtained using reagents which can transport both a transition metal cation and its attendant anion(s), and the potential advantages of metal salt extractants are discussed. The problems encountered when trying to extract hydrophilic anions selectively into organic media are also considered. Chapter 2 discusses techniques used in industry to tune reagent properties, many of which depend on the importance of H-bonding in non-polar solvents. Synthesis of a series of 5-alkyl-3-X-2-hydroxybenzaldehyde oximes (X = a range of substituents) is described and copper extraction experiments are reported. 3-Substitution is found to alter reagent strength by two orders of magnitude, with 3-bromo-5-tert-butyl-2- hydroxybenzaldehyde oxime the strongest extractant. An analysis of X-ray structures of several ligands and copper(II) complexes is given in an attempt to establish whether trends in the solid state structures can account for variations in extractant strength. A more detailed analysis of the hydrogen bonding in salicylaldoximato copper(II) complexes and ligand dimers is carried out in Chapter 3, with the aim of defining how substituent effects could be used to design reagents with appropriate extractive behaviour. 3-X-2-Hydroxybenzaldehyde oximes with no 5-alkyl substituent are synthesised and subjected to a detailed study by X-ray crystallography and computational techniques, which, alongside evidence provided by CID-MS experiments, suggest that the dominant substituent effect in determining extractant strength is the ability to “buttress” the pseudomacrocyclic hydrogen bonding motif involving the oximic hydrogen and phenolic oxygen. Ligands with 3-substituents capable of accepting H-bonds were found to be stronger extractants than those which could not, and the steric hindrance afforded by bulky substituents made 3,5-di-tert-butyl-2-hydroxybenzaldehyde oxime the weakest extractant. Ligand acidity is also noted to have a significant effect on reagent strength, with electronwithdrawing substituents lowering the pKa of the phenolic proton and increasing extractive efficacy. Chapter 4 focuses on metal salt extraction, and the development of selective, robust and hydrolytically stable reagents. Six novel extractants, based on a salicylaldoxime scaffold with a pendant dialkylaminomethyl arm, are described. Only 5-tert-butyl-3- dihexylaminomethyl-2-hydroxybenzaldehyde oxime and 3-tert-butyl-5- dihexylaminomethyl-2-hydroxybenzaldehyde oxime have sufficient solubility to be effective reagents. The former extracts CuCl2 and ZnCl2 in a highly efficient manner, with one mole of metal salt extracted per mole of ligand, twice the expected capacity. X-ray structure determination of complexes of the related ligand 5-tertbutyl- 2-hydroxy-3-piperidin-1-ylmethylbenzaldehyde oxime defines the binding mode, with the chloride anions bound to the inner sphere of the metal cations. Loading and stripping experiments show it to be an extractant with potential commercial application. Cation and anion selectivity of the two extractants defined above is the focus of Chapter 5, which begins with an overview of techniques and attempts to attenuate the Hofmeister bias, the main factor in the selective extraction of hydrophilic anions into organic media. pH loading profiles show the 3- dihexylaminomethyl isomer to be an effective CuCl2 and CuSO4 extractant, but the cation extractive efficacy of the 5-isomer is hampered by the 3-tert-butyl group. Both ligands are found to be selective for Cl- > SO4 2-, following the Hofmeister bias. Further information on anion binding is provided by solid state structures of copper salt complexes, showing that in all cases the copper(II) cation interacts in some way with the anion. Cation extraction is affected significantly by the anion present, with FeIII selectively extracted against CuII in the presence of SO4 2- which is consistent with cation-anion interactions having great influence on the overall stability of the ligand-metal salt assembly.

Oxidation developed from the surfaces into the cores of the microwaved particles. Metallic particles were also formed during microwave exposure. Lihir gold ore, in which gold was finely disseminated in pyrite and marcasite, was an extremely refractory gold ore. Without pretreatment, only 37-39% of the gold could be extracted with sodium cyanide. However, this was improved after the head ores or floatation concentrates were pretreated by microwave radiation. 74.581.2% of the gold was extracted from the microwave treated head ore. The hydrometallurgical pretreatment of pyrite and marcasite in a microwave field and a conventional heating environment was also investigated.I,n a nitric acid solution, pyrite and marcasite can be rapidly leached. Reaction temperature and the concentration of HNO3 had a significant influence on decomposition rate. Marcasite had a substantially higher i decomposition rate than pyrite. Microwave heating could promote the dissociation of marcasite and pyrite, compared with conventional heating. This was caused by special volumetric heating during microwave exposure that may induce local overheating or improve the interactions between the high dielectric loss minerals and the leaching solution. Kinetic investigations show that the decomposition of both the minerals in a nitric acid medium is controlled by chemical reactions on the surfaces of particles. The decomposition is a second order reaction with respect to nitric acid concentration. Less than 5- 7% of the decomposed sulphur was transformed into elemental sulphur during the leaching of both the minerals

Selective coordination to zinc over other members of the first row transition metal series is important for the commercial application of lipophilic ligands for the hydrometallurgic recovery of the metal. In order to achieve selectivity for zinc(n) over other ions, particularly copper(II) and iron(III), ligands have to be designed which take advantage of zinc's preferred tetrahedral coordination geometry and the borderline donor atom preference of the metal. With such ligands in mind, three new classes of ligand system have been designed and synthesised based on the benzimidazole, quinoline and pyridine ring systems with appended phosphinic and thiophosphinic acid anionic donor groups. Benzimidazole ligands which bind zinc in a 2:1 manner in solution have been made and their complexation ability assessed by ESMS, (^31)p NMR, liquid-liquid extraction, fluorescence and UV absorption spectroscopic methods. A bisbenzimidazole ligand with a C(_3) spacer in the 2,2' position was synthesised and shown to bind zinc initially in a 2:1 manner, at low metal concentrations, and predominantiy as a 1:1 species at higher metal concentrations. The formation constant for the ML complex was shown to be logK(_ML)=5 by analysis of the NMR titration curve, which was in close agreement with the value obtained from liquid-liquid extraction studies. A directly linked 2,2'- bisbenziraidazole system was also synthesised as an extension to previous work, however isolation of the target ligand proved to be difficult due to the insolubility of the desired bis-acid.8-(Quinolinyl)phenylphosphinic acid and 8-(2-metiiylquinolinyl)phenylphosphinic acid were also synthesised and their solution complexation behaviour studied in detail. The unsubstituted ligand appeared to form 1:1 complexes with zinc at all the metal concentrations studied, and the absence of the methyl substituent does not inhibit coordination to the ferric ion. In contrast the metiiyl substituted ligand initially forms a 2:1 L:M complex, and 1:1 complexes at higher metal concentrations. The initial ML(_2) complex probably involves coordination of two of the phosphinic acid moieties. Three pyridyl derived ligand systems were also synthesised varying the bulk of the C-6 substituent and the effect of the phosphinic versus the thiophosphinic acid moiety towards zinc coordination was examined. Both of the methyl appended ligands were shown to bind zinc in a 2:1 manner, with the thiophosphinic acid exhibiting a greater avidity for zinc. In contrast, the unsubstituted ligand predominantly forms 1:1 complexes at all metal concentrations.

Abstract: Increased demand for lithium products for use in lithium ion batteries has led to a search for new lithium resources in recent years in order to meet projected future consumption. One such potential lithium resource is low lithium bearing brines that are discharged from hydraulically fractured oil and gas wells as flowback and produced water (FPW). In this way, hydraulic fracturing presents an opportunity to turn what is normally considered a wastewater into a lithium resource. In this research, two manganese-based adsorbents that are selective toward lithium were prepared using a co-precipitation method and were employed for lithium recovery from FPW. At optimized conditions, lithium uptake reached 18 mg g-1, with a 80% lithium recovery within 30 minutes. The recovered lithium was isolated and concentrated to 15 mM in an acidic final product. The degree of sorbent loss during acid desorption of lithium were significantly higher for sorbents used to recover lithium from FPW as compared to recovery from a synthetic lithium-bearing brine (4.5% versus 0.8%); here, I propose that organic molecules present in the FPW reduce manganese in the sorbent structure during lithium sorption, leading to increased sorbent loss through reductive dissolution. Systematic characterization including wet chemical manganese valence measurements along with EXAFS and XPS show that tetravalent manganese in the sorbent is reduced during lithium sorption, and subsequently dissolves during acid desorption. Partial removal of these organic molecules by nanofiltration leads to decreased sorbent dissolution in acid. I show that dissolved organic molecules represent a critical control on the reductive dissolution of manganese-based lithium ion exchange sorbents.

Rare earth elements (REEs) are a group of 15 elements in the lanthanide series along with scandium and yttrium. They are often grouped together because of their similar chemical properties. As a result of their increased application in advanced technologies and electronics including electric vehicles, the demand of REEs and other critical elements has increased in recent decades and is expected to significantly grow over the next decade. As the majority of REEs are produced and utilized within the manufacturing industry in China, concerns over future supplies to support national defense technologies and associated manufacturing industries has generated interest in the recovery of REEs from alternate sources such as coal and recycling. A solvent extraction (SX) process and circuit was developed to concentrate REEs from dilute pregnant leach solutions containing low concentrations of REEs and high concentrations of contaminant ions. The separation processes used for concentrating REEs from leachates generated by conventional sources are not directly applicable to the PLS generated from coal-based sources due to their substantially different composition. Parametric effects associated with the SX process were evaluated and optimized using a model test solution produced based on the composition of typical pregnant leach solution (PLS) generated from the leaching of pre-combustion, bituminous coal-based sources. Di-2(ethylhexyl) phosphoric acid (DEHPA) was used as the extractant to selectively transfer the REEs in the PLS from the aqueous phase to the organic phase. The tests performed on the model PLS found that reduction of Fe3+ to Fe2+ prior to introduction to the SX process provided a four-fold improvement in the rejection of iron during the first loading stage in the SX circuit. The performances on the model system confirmed that the SX process was capable of recovering and concentrating the REEs from a dilute PLS source. Subsequently, the process and optimized parametric values were tested on a continuous basis in a pilot-scale facility using PLS generated from coal coarse refuse. The continuous SX system was comprised of a train of 10 conventional mixer settlers having a volume of 10 liters each. A rare earth oxide (REO) concentrate containing 94.5% by weight REO was generated using a two- stage (rougher and cleaner) solvent extraction process followed by oxalic acid precipitation. The laboratory evaluations using the model PLS revealed issues associated with a third phase formation. Tributyl Phosphate (TBP) is commonly used as a phase modifier in the organic phase to improve the phase separation characteristics and prevent the formation of a third phase. The current study found that the addition of TBP affected the equilibrium extraction behavior of REE as well as the contaminant elements., The effect on each metal was found to be different which resulted in a significant impact on the separation efficiency achieved between individual REEs as well as for REEs and the contaminant elements. The effect of TBP was studied using concentrations of 1% and 2% by volume in the organic phase. A Fourier Transform Infrared (FTIR) analysis on the mixture of TBP and DEHPA and experimental data quantifying the change in the extraction equilibrium for each element provided insight into their interaction and an explanation for the change in the extraction behavior of each metal. The characteristic peak of P-O-C from 1033 cm-1 in pure DEHPA to 1049 cm-1 in the 5%DEHPA-1%TBP mixture which indicated that the bond P-O got shorter suggesting that the addition of TBP resulted in the breaking of the dimeric structure of the DEHPA and formation of a TBP-DEHPA associated molecule with hydrogen bonding. The experimental work leading to a novel SX circuit to treat dilute PLS sources was primarily focused on the separation of REEs from contaminant elements to produce a high purity rare earth oxide mix product. The next step in the process was the production of individual REE concentrates. To identify the conditions needed to achieve this objective, a thermodynamic model was developed for the prediction of distribution coefficients associated with each lanthanide using a cation exchange extractant. The model utilized the initial conditions of the system to estimate the lanthanide complexation and the non-idealities in both aqueous and organic phases to calculate the distribution coefficients. The non-ideality associated with the ions in the aqueous phase was estimated using the Bromley activity coefficient model, whereas the non-ideality in the organic phase was computed as the ratio of the activity coefficient of the extractant molecule and the metal extractant molecule in the organic phase which was calculated as a function of the dimeric concentration of the free extractant in the organic phase. To validate the model, distribution coefficients were predicted and experimentally determined for a lanthanum chloride solution using DEHPA as the extractant. The correlation coefficient defining the agreement of the model predictions with the experimental data was 0.996, which is validated the accuracy of the model. As such, the developed model can be used to design solvent extraction processes for the separation of individual metals without having to generate a large amount of experimental data for distribution coefficients under different conditions.

Nine empirical equations that estimate saturated hydraulic conductivity as a func- tion of grain size in well-graded sands with gravels having large uniformity coecients (U > 50) are evaluated by comparing their accuracy when predicting observed conduc- tivities in constant head permeability tests. According to the ndings of this thesis, in decreasing order of accuracy these equations are: USBR (Vukovic and Soro, 1992; USBR, 1978), Hazen (Hazen, 1892), Slichter (Slichter, 1898), Kozeny-Carman (Carrier, 2003), Fair and Hatch (Fair and Hatch, 1933), Terzaghi (Vukovic and Soro, 1992), Beyer (Beyer, 1966), Kruger (Vukovic and Soro, 1992), and Zunker (Zunker, 1932). These re- sults are based on multiple constant head permeability tests on two samples of granular material corresponding to well-graded sands with gravels. Using the USBR equation sat- urated hydraulic conductivities for a statistical population of 874 samples of well-graded sands with gravels forming 29 loads from a heap leaching mine in northern Chile are calculated. Results indicate that, using the USBR equation, on average the hydraulic conductivity of the leaching heaps has a two standard deviation range between 0.18 and 0.15 cm/s. Permeability tests on the actual material used in the heaps provided by the mine shows that the results presented in this thesis are consistent with actual observa- tions and represent saturated conductivities in heaps up to 3 m high under a pressures of up to 62 Kpa. In future work hydraulic conductivities can be combined with water retention curves, discharge rates, irrigation rates, porosities, and consolidation so as to evaluate the relationship between copper yields and the hydraulic conductivities of the heap.

Polonium-210 is a radioactive daughter of uranium-238. It has a relatively short half-life and, as a consequence, it is very dangerous to handle even in microgram quantities because of intense radiation effects. These properties have made it difficult to obtain reliable data on its chemical behaviour and in particular its aqueous chemistry is poorly understood. To overcome the shortfalls in our knowledge of polonium behaviour, this thesis has addressed pertinent issues by the undertaking of extensive research to develop a thermochemical database for the element. Such data is vital in trying to predict and understand the behaviour of the element in solution for a given set of conditions. Because of the paucity of reliable data in the literature, much of the data presented here has been derived theoretically using linear free energy relationships. To validate the derived data, pH-potential diagrams were constructed for several practical systems. The pH-potential diagram provides a compact, pictorial summary of electron-transfer, proton-transfer and electron-and-proton-transfer reactions, favoured on thermodynamic grounds. Use of these diagrams, in conjunction with pratical observations and measurements, has resulted in a better understanding of the aqueous chemistry of polonium in both the simple metal-water system and the more complicated systems of many industrial processes. In particular, the work highlighted that the chemistry of polonium is often similar to that of lead. This is in contrast with the literature which has indicated that the behaviour of polonium is analogous to that of its Group(VI) homologues, tellurium and selenium. The present work has shown that polonium has characteristics of both lead and tellurium, depending upon conditions.