How Lead Base Anode is Used in the Electrowinning Process

How Lead Base Anode is Used in the Electrowinning Process?

Electrowinning is the most energy-intensive process. The cost of electrical energy for electrowinning can constitute up to 80% of the total energy cost of recovering metals from concentrates. Furthermore, electrowinning is also the most crucial stage in the process in relation to the quality of the final product.

In electrowinning, the most significant contribution to the overall energy consumption is directly connected to the processes at the anodes, where the current oxidizes water and produces oxygen gas. This process requires significant overvoltage and the contribution of the anodic potential to the overall cell voltage is between 50 and 70% in zinc electrowinning.  

In addition, the main consequence of the oxygen evolution reaction on the anode is corrosion of the lead-based anodes which are universally used in such operations. How Lead Base Anode is Used in the Electrowinning Process? Initially, a non-conducting layer of lead sulfate is formed on the anode surface, followed by the formation of lead hydrated oxides and finally a conducting lead dioxide layer that aids in oxygen evolution. As corrosion of the substrate continues, internal stress develops, and cracking of the protective inner layer may occur. The evolved oxygen causes flaking of the corrosion product from the anode. Dissolved lead ions and suspended lead dioxide and sulfate particles contaminate the electrodeposited copper and zinc. The increasingly tight specifications for lead in both copper and zinc cathodes have highlighted the importance of minimizing and controlling the corrosion of the anodes. The use of a diaphragm in the electrowinning of nickel and cobalt assists in minimizing this problem with these metals.

Despite the high operating potentials and susceptibility to corrosion, lead-based alloys still dominate the base metal electrowinning operations. As pure lead is mechanically weak, the lead must be alloyed in order to improve its mechanical and corrosion properties. Common lead alloys include lead-calcium-tin (Pb-Ca-Sn anodes) which are used in the electrowinning of copper and nickel and lead-silver (Pb-Ag anodes) used in the production of zinc. Significant research and development are required in order to improve the performance of these alloys in terms of reduced overpotentials and corrosion rates, both of which are susceptible to the presence of certain other metal ions in the electrolytes. Thus, manganese is added to zinc electrolytes and cobalt to copper electrolytes. In particular, manganese may be present in the copper and zinc electrolytes as an impurity and manganese ions are often added to zinc electrolytes in order to reduce the operating potential and minimize corrosion of the anodes. However, there is also evidence that the presence of low concentrations of manganese ions may increase the corrosion of anodes in copper tank houses. The mechanisms involved in these apparent conflicting effects are not well understood. Thus an understanding of the role of manganese in the anode reactions on lead alloy anodes could assist in the optimization of the electrowinning processes for both copper and zinc with possible application to the electrowinning of nickel.

In addition to the major reaction of oxygen evolution, manganese ions may also be oxidized at the lead alloy anodes, thus generating either soluble species such as Mn3+ and MnO4- ions or insoluble oxides such as MnO2 and possibly MnOOH. The formation of layers of these oxides on the lead anode surface may assist in minimizing the disintegration of the anode and also modify the kinetics of the oxygen evolution reaction. However, excessive amounts of MnO2 scale have a tendency to break away from the anode causing increased corrosion of the underlying lead anode. Moreover, the anodes must be cleaned periodically to remove the deposit which may otherwise cause short-circuiting between the anode and the cathode and increase the energy consumption.