UBC Theses and Dissertations
Development of Pb-MnO₂ composite anodes for electrowinning application : electrochemical and corrosion evaluations Mohammadi, Maysam
Electrowinning is the last step of the extraction of zinc in which aqueous zinc ions are electrodeposited to produce highly pure zinc metal. This process operates in highly acidic solutions and at high current densities. Conventional lead-based anodes used in the zinc electrowinning are associated with high corrosion rates and oxygen evolution overpotential in the electrowinning process. These result in some problems such as Pb contaminating the zinc cathode, shortened anode lifetime, and high energy consumption in the process. Lead-based composite anodes have been introduced to address these issues by using electroactive oxide particles dispersed in the Pb anode. Manganese ions, which typically exist in the zinc electrowinning electrolyte, can influence the anode performance, depending on their concentration and the anode material. The main aim of this work was to develop a new composite anode using MnO₂ particles in a Pb matrix to improve the anode performance in the zinc electrowinning operating conditions in Mn-free and Mn-containing electrolytes. A combination of electrochemical and analytical methods was used to understand the oxidation, electrocatalytic activity, and corrosion performance of the Pb-MnO₂ composite anodes compared to the conventional PbAg anode. Potentiometric titration was utilized to measure the oxidation rate of manganese ions. The anodic corrosion layers and the MnO₂ deposited layers were characterized by Scanning Electron Microscopy and X-Ray Diffraction. A new electrochemical method was also developed for an on-site investigation of the deposited MnO₂ layer on the anodes. The Pb-MnO₂ composite anodes showed higher catalytic activity and better corrosion resistance than the PbAg anode under the zinc electrowinning operating conditions. The Pb–MnO₂ anode can decrease the energy consumption in the electrowinning process in the Mn-free electrolyte by up to 5%. The composite particles were not effective in decreasing the anode potential in the Mn-containing electrolyte. The MnO₂ particles catalyzed Mn(III) disproportionation, contributing to the deposition of a uniform, adherent, and protective MnO₂ layer. Formation of this layer decreased the lead dissolution and manganese consumption rates. The Pb-MnO₂ composite anode can be put into service without any pre-treatment since it quickly develops a stable MnO₂ layer and generates very low MnO₂ mud.
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