Leaching Kinetics of Zinc from Metal Oxide Varistors (MOVs) with Sulfuric Acid

The leaching kinetics of zinc from zinc oxide-based metal oxide varistors (MOVs) was investigated in H2SO4 at atmospheric pressure. Kinetics experiments were carried out at various agitation speeds, particle sizes, initial H2SO4 concentrations, and reaction temperatures. It was determined that the leaching rate of zinc was independent of agitation speed above 300 rpm and also independent of particle size below 105 μm, whereas it dramatically increased with an increasing H2SO4 concentration. Except for when the H2SO4 concentration was varied, the m-values were almost constant at varying agitation speeds (m-values: 0.554–0.579), particle sizes (m-values: 0.507–0.560) and reaction temperature (m-values: 0.530–0.560) conditions. All of the m-values in these experiments were found to be below 0.580. Therefore, it is proposed that the extraction of zinc is a diffusion-controlled reaction. The leaching kinetics followed the D3 kinetic equation with a rate-controlling diffusion step through the ash layers, and the corresponding apparent activation energy was calculated as 20.7 kJ/mol in the temperature range of 313 K to 353 K.