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Electrochemistry of sulfur dioxide relevant to the electrowinning of metals from hydrometallurgical leach liquors

Dawson, Jodee N. (2000) Electrochemistry of sulfur dioxide relevant to the electrowinning of metals from hydrometallurgical leach liquors. PhD thesis, Murdoch University.

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Electrowinning (EW) is becoming an increasingly important method of recovering copper from aqueous acidic sulfate solutions. In conventional practice copper is plated onto type 316 stainless steel cathodes and the anode reaction is oxygen evolution on a lead alloy. Conventional Cu EW is energy intensive and this is largely due to the anode reaction. The energy requirements can be significantly decreased however, by changing the anode reaction to the oxidation of SO2. Only a few previous investigators have used this process as the anode reaction in Cu EW. Whilst some information can be gleaned from those studies there are still many questions concerning the effects of SO2 on the energy requirements and also the nature/quality of the copper deposits. The oxidation of SO2 is itself poorly understood. This thesis examines these issues with a view to implementing S02 oxidation as the anode reaction in Cu EW.

In the first part of this thesis a literature review on speciation in the S(IV)-H20, S(V)- H2O and S(VI)-H20 systems is presented The review shows that speciation in each of these systems is pH dependent and that some species are sensitive to atmospheric oxidation and/or photochemical decomposition. Some experiments that confirm the latter issue are also presented.

In the second part of this thesis the results from a detailed investigation of the oxidation of S(IV) at Pt and carbon electrodes over a wide pH range are presented. The nature of the electrode surface, most notably its degree of oxidation, is found to have a significant effect on the oxidation of S(IV). When a H2SO4-pretreated Pt electrode is scanned from 0.20 V (pH 10.5), 0.30 V (pH 5.7) or 0.40 V (pH 2.3) to 1.20 V, a poorly-defined S(IV) oxidation peak is observed near 0.7 V followed by a well-defined S(IV) oxidation peak near 0.9 V. When the scan is reversed a broad S(IV) oxidation peak is observed near 0.6 V but only in acidic solutions. If the surface becomes too heavily oxidized then the S(IV) oxidation peaks diminish/disappear. Reduction of the Pt oxide however, restores the activity of the electrode towards S(IV) oxidation. When a polished carbon electrode is scanned from 0.00 V to more positive potentials, S(IV) oxidation peaks are observed near 0.7 ±0.1 V and 1.25 ± 0.1 V at pH > ca. 7 but only the latter process is observed in more acidic solutions. Oxidation of the carbon surface likewise inhibits S(IV) oxidation but reduction of the graphitic oxide only partially restores its activity. X-ray photoelectron spectroscopy was used to confirm the effect of graphitic oxide formation/reduction on S(IV) oxidation. The results of some Raman spectroscopic analyses are also presented and they show that the products of S(IV) oxidation at a carbon electrode are HSO4- and SO42' at acidic pH and SO42' and S2O62- at alkaline pH. The effects of some cations commonly found in copper EW solutions on the oxidation of S(IV) at a carbon electrode at pH 2.3 were also investigated. Of the cations tested, only Fe2+ was found to affect S(IV) oxidation by shifting the S(IV) oxidation peak to less positive potentials.

The final part of this thesis presents a detailed study of the copper EW process using SO2 oxidation as the anode reaction. Particular attention is given to the effects of SO2 on the energy requirements and the nature/quality of the copper deposit. The results show that, whilst the energy requirements decrease by about 70 % when compared to the conventional process, the deposits are of a poor quality unless a diaphragm cell is used to keep SO2 away from the depositing copper.

Item Type: Thesis (PhD)
Murdoch Affiliation: Division of Science and Engineering
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Singh, Pritam and Hefter, Glenn
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