Manganese-Chloride Interactions on Pb–Ag Anode Behaviour in Synthetic Sulfuric Acid Electrolytes

Manganese in electrolyte has both beneficial and detrimental effects in Zn electrowinning. Mn oxidizes to form MnO₂ on Pb–Ag anodes, cell walls and pipes. MnO₂ reduces anode corrosion but also leads to short circuiting and maintenance issues. MnO₂ is thought to interact with chloride ions and produce oxidized chlorine species. The interactions between Mn and Cl are not well understood. Therefore, two sets of 45+ day bench scale experiments were conducted to investigate the effects of the manganese to chloride ratio on anode corrosion rate and electrolyte chemistry using rolled Pb–Ag anodes. Daily manganese and chloride losses from the electrolyte appeared to be correlated. Increasing the average Mn/Cl⁻ ratio from ~7:1 to ~11:1 reduced the anode corrosion rate. Increasing the ratio above 11 did not reduce the corrosion rate further. Anode scales produced with Mn/Cl⁻ ratios at and above 11:1 contained some γ–MnO₂ while scales formed with ratios less than 11:1 did not.