Strength, transport efficiency and selectivity of novel extractants for the recovery of base metals
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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.