Improving base metal electrowinning

In zinc electrowinning, Mn oxidizes to form MnO2 on Pb-Ag anodes, cell walls and pipes. MnO2 reduces anode corrosion but also leads to short circuits 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. 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. Increasing the average Mn/Cl- ratio from ~7:1 to ~11:1 reduced the anode corrosion rate. Anode scales produced with Mn/Cl- ratios at or above 11:1 contained some γ-MnO2 while scales formed with ratios less than 11:1 did not. In copper electrowinning, the effects of temperature, power outages and concentrations of cobalt and manganese on anode corrosion and current efficiency have not been well investigated. Benchscale tests were used to investigate these effects. Anode corrosion in all experiments was minimal probably due to the presence of at least 150 mg/L Co in each electrolyte. Cathode current efficiency typically ranged between 90 and 95% for the experiments. The range of power outage, temperature and concentrations of Mn and Co tested did not produce a significant effect on current efficiency. Anode potential estimation equations for industrial tankhouse operations where cobalt is added after solvent extraction and where cobalt concentration is naturally high due to the ores being processed were developed using lab scale tests. These models have been combined with other reported equations to calculate the energy consumption of electrowinning systems for Pb-Ca-Sn anodes in synthetic sulfuric acid electrolytes --Abstract, page iv