Recycling Zinc from Metal Oxide Varistors Through Leaching and Cementation of Cobalt and Nickel

Metal oxide varistors (MOVs) are housed in a surge arrestor and composed of zinc oxide (>90 wt%) and other metal oxides such as antimony, bismuth, cobalt, manganese, and nickel. Due to the high concentration of zinc in MOVs, it is a better choice to recycle them as opposed to landfilling. This research set out to determine if cementation could be used as a purification step to remove co-leached metals, leading to a purified leachate suitable for zinc electrowinning. Zinc was leached from crushed MOVs using dilute sulfuric acid, which avoided co-leaching of antimony and bismuth but required further purification to remove co-leached cobalt and nickel. In further purification of the leachate, cementation was investigated. Initial findings suggest that the cobalt concentration can be reduced by over 50 % (200 mg/L) and nickel concentration reduced by over 90 % (90 mg/L), by optimizing the activator (Sb/Cu) concentration, temperature, pH, and surface area of zinc dust. Further investigations into optimized batch addition of zinc and copper–antimony activators verified that nearly 92 % (>390 mg/L) of the cobalt and all nickel (100 mg/L) can be removed from the acidic varistor leachate. These results suggest that cementation by addition of zinc dust can be used for purification of zinc solutions containing over 400 mg/L cobalt and 100 mg/L nickel and thus preparation of the solutions for zinc electrowinning.