Lead Anodes Performance in Nickel Electrowinning

"Lead and its alloys are proper candidates for anodes in electrowinning processes mainly because they are insoluble in the leach solutions and relatively cheap. However, the annual cost of corrosion problems associated with these anodes is millions of dollars. In this study lead-silver and lead-calcium anodes were tested under typical nickel electrowinning conditions and the effect of chloride on the corrosion performance of the anode materials was investigated. Corrosion rates of the anodes were calculated using the discharge plateau (reduction of lead dioxide to lead sulfate) after galvanostatic experiments. It was found that lead calcium anodes have the highest corrosion rate both in the presence and absence of chloride. Surface morphology of the electrodes was studied using SEM and the corrosion products were characterized using XRD.IntroductionLead (Pb) is usually the preferred material for commercial anodes since it is insoluble in the leach solution and the costs are manageable. One advantage of using an insoluble material as an anode is the higher purity of the cathodic deposits and better quality of the end product. During nickel electrowinning, metallic nickel is deposited on the cathode while oxygen evolution reaction (OER) takes place on the anode. In practice, relatively high potentials are necessary for the reaction to proceed at the desired rate because of the high overpotential of OER on lead. Lead anodes get oxidized in typical electrowinning conditions and usually have two to three years of lifetime. Although the price of each anode is insignificant, the overall cost of corrosion of these anodes is considerable, which is due to the large number of anodes used in a plant, typically two to three thousand. Therefore, corrosion is a significant problem associated with lead anodes in commercial nickel electrowinning plants and industries show interest in addressing this problem. Corrosion of lead electrodes in acidic environments has been previously the subject of many research studies [I-7]. It has been reported that, during the anodic polarization of lead, several phases may be present on the surface, depending on the potential range that is being studied [8]. Lander showed that lead is oxidized at potentials higher than 0.252 YsHE with tetragonal lead monoxide being the main reaction product [9]. At 0.666 YsHE lead can transform to lead dioxide; however, it is not stable and transforms to lead sulfate and lead oxide. At potentials higher than 1.684 YsHE, lead dioxide is eventually a stable phase and forms a protective layer on the surface [I 0, 11]. Under conditions of a typical nickel electrowinning plant, usually a 200 A/m2 current density is passed through the electrodes."