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

As antimony is typically present in industrial and commercial products only in small amounts, the concentration of antimony in waste types is low and a limited amount of antimony is currently recycled. One product relatively rich in antimony is the metal oxide varistor (MOV) used for overvoltage protection in electric circuits. To increase the antimony concentration, the MOV was pulverized (< 65 μm) and leached, resulting in an insoluble MOV residue containing 186 ± 2 mg/g of antimony. This work investigates the thermal decomposition and carbothermal reduction of pure metal oxides (Sb₂O₃, Bi₂O₃, and ZnO) and MOV residue. Thermogravimetric (TG) analysis was used in order to propose a temperature range in which it is possible to separate antimony oxide from the MOV residue. TG results indicate that during thermal decomposition of pure metal oxides, sublimated antimony oxide can be recovered at 650 °C, leaving Bi₂O₃ and ZnO unreacted. The addition of carbon caused mainly volatilization, with some reduction, of Sb₂O₃ and reduction of Bi₂O₃ to occur at nearly the same temperature, approximately 600 °C. However, volatilization of Bi was not troublesome below 800 °C due to slow kinetics. Thermal decomposition of antimony from the MOV residue was not possible in the temperature range studied (< 1000 °C), while carbothermal reduction to the MOV residue revealed antimony volatilization occurred near 800 °C.