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

Abstract

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 copperantimony 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.

Recovery of Antimony: A Laboratory Study on the Thermal Decomposition and Carbothermal Reduction of Sb(III), Bi(III), Zn(II) Oxides, and Antimony Compounds from Metal Oxide Varistors

“…The MOV under investigation in this study was composed of sintered metal oxides of zinc, antimony (3.2 mol%), bismuth (2.3 mol%), cobalt (\ 1 mol%), nickel (\ 1 mol%), and manganese (\ 1 mol%) [15,16]. The microstructure of the MOV contains three phases: the most dominant phase is that of the zinc oxide grains; the bismuth-rich phase encompasses the zinc oxide grains; and the antimony-rich phase is located within the bismuth oxide phase.…”

“…The MOV under investigation in this study is composed of sintered zinc oxide (88.2 wt%), antimony trioxide (4.4 wt%), bismuth trioxide (5.1 wt%), cobalt trioxide (\ 1 wt%), nickel oxide (\ 1 wt%), and manganese carbonate (\ 1 wt%) [15,16]. Antimony oxide is used in the varistor to reduce the average size of the ZnO grain [17][18][19].…”

“…Sulfuric acid was used because the leachate, once separated from the insoluble residue, could be sold to a zinc electrowinning facility for the production of secondary metallic zinc. It was demonstrated that the leachate contained low concentrations of cobalt and nickel, which can be removed by cementation [16], as well as some manganese. In previous work, the concentration of zinc in the leachate was shown to be 1 mol/l but can be increased to suit the needs of the electrowinning facility [16].…”

“…It was demonstrated that the leachate contained low concentrations of cobalt and nickel, which can be removed by cementation [16], as well as some manganese. In previous work, the concentration of zinc in the leachate was shown to be 1 mol/l but can be increased to suit the needs of the electrowinning facility [16]. Other mineral acids as well as organic acids could also have been used for zinc leaching [15], but from those leachates, zinc or zinc oxide would have been recovered by precipitation or solvent extraction.…”

“…Other mineral acids as well as organic acids could also have been used for zinc leaching [15], but from those leachates, zinc or zinc oxide would have been recovered by precipitation or solvent extraction. The process for pretreatment of the MOV is described in detail in previous work by Gutknecht et al [16].…”