Dissolution behavior of willemite in the (NH4)2SO4–NH3–H2O system

The leaching behavior of high purity willemite in ammoniacal solution was investigated under a range of experimental conditions with the view of optimizing zinc extraction. The results showed that decreasing the initial silica concentration while increasing the temperature, total ammonia/ammonium sulfate concentration and liquid/solid ratio resulted in a greater leaching efficiency. In the leaching process, zinc and silica in willemite were found to be simultaneously dissolved in the solution, followed by the precipitation of amorphous silica. The solubility of silica in the (NH4)2SO4–NH3–H2O system was very low with a saturated concentration of 0.3 g/L. In addition, the precipitation rate of dissolved silica in the leaching solution was found to be significantly slow, making the willemite difficult to leach in the (NH4)2SO4–NH3–H2O system, especially at a low liquid/solid ratio.