An Explanation for Zinc Electrowinning Process

An Explanation for Zinc Electrowinning Process

Electrowinning is an electrolytic process that deposits high purity zinc metal on the cathode by applying external electrical energy. In the electrowinning process, zinc reduction and reaction occur on the cathode, and water is decomposed at the anode surface to release oxygen. The latter reaction is known as the oxygen evolution reaction (OER). Although these two reactions are the main redox reactions in the zinc electrowinning process, there are some other reactions happening on the surface of the electrodes. Proton reduction, also known as hydrogen evolution, takes place at the cathode surface. This reaction is undesirable since it consumes electrons and reduces the zinc deposition current efficiency. In spite of the higher standard reduction potential of proton than zinc, the hydrogen evolution proceeds at significantly lower rates at the electrowinning operating conditions. This is the result of the high hydrogen evolution overpotential on the surface of zinc, which makes this reaction difficult to occur. Co-deposition of some other elements, such as cobalt and copper, which have low overpotential for the hydrogen evolution reaction, decreases the electrowinning current efficiency.
 
Some of the impurities, such as manganese and chloride ions, react at the anode surface during the electrowinning process, which will be explained later. The zinc electrowinning process usually operates in highly acidic electrolytes and at high current densities.

The electrical energy consumption in the electrowinning process is approximately 80% of the power requirement for the entire zinc refinery process. The electrowinning process is operated under constant current density, therefore, the EW cell potential determines the energy consumption of this process.