The Problems of Lead Anodes in Copper Electrowinning

The Problems of Lead Anodes in Copper Electrowinning

Historically, lead anodes are used in copper electrowinning due to lead's abundance, good conductivity, corrosion resistance, and relatively low cost. The corrosion resistance of these anodes is a product of the formation of PbO2 via PbSO4 under oxidative conditions during electrowinning. This passivation layer is attributed to the added benefit of self-regeneration in the event of a short circuit between electrodes.

Pb-Ca-Sn and Pb-Sr-Sn anodes were developed to replace Pb-Sb anodes as corrosion issues developed in the cells, and pure lead presented a creeping problem at electrowinning temperatures, leading to cracking of the oxide layer and accelerated corrosion. While the dissolution of the anode is reduced by the above alloys, anode corrosion remains a problem and results in short anode lifetimes and Pb contamination of the copper cathode. To combat these problems, cobalt is generally added to the electrolyte. Electrolyte bleed from the tank house, necessary to control other impurities, results in the loss of cobalt, constituting a continuous process expense.

The expenses of electricity and cobalt addition, the corrosion of lead anodes, and the contamination of copper cathode material have led to the development of alternative, non-lead anode materials. Principle among these materials has been precious metal oxide-coated titanium, to which a number of tank houses have been successfully transitioned. These coated titanium anodes (CTAs) offer several benefits over common lead anodes. The most important benefit is the reduced voltage necessary for electrowinning, which results from the higher activity of the coating. This quality is reported to reduce energy consumption by as much as 15%. Further, because coated titanium anodes contain no lead the addition of cobalt to the electrolyte, the cleaning of lead sludge from the cells, and cathode contamination by Pb are eliminated. These benefits are offset by the cost of electrodes and work required to repair anodes damaged by short-circuiting.

Because the active coating does not develop in-situ but is manufactured, it does not possess the self-regenerating quality of the lead oxide layer of lead anodes.