Which impurities are removed from zinc sulfate solution before electrolysis?

To ensure the successful electrolytic deposition of zinc, the zinc sulfate solution must undergo a rigorous two-stage purification process to remove impurities that would otherwise interfere with the process by lowering the hydrogen overvoltage.1

The purification stages are as follows:

• Removal of Iron: In the first stage, iron is removed from the solution as a solid residue. This is typically achieved by precipitating it as jarosite (a basic ferric sulfate) or as the oxides goethite or hematite.1
• Cementation: In the second stage, other metallic impurities—specifically copper, nickel, cadmium, cobalt, and germanium—are removed from the solution through a process of cementation using zinc dust.1

This extreme level of purification is a critical necessity, as the presence of these impurities, even at very low concentrations, can cause a drastic reduction in the hydrogen overvoltage, leading to the production of hydrogen at the cathode instead of the desired zinc deposition.1