Strength, transport efficiency and selectivity of novel extractants for the recovery of base metals
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Date
2009Author
Lin, Tai
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Abstract
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.