Leaching Process Of Willemite In (NH₄)₂SO₄-NH₃-H₂O System

Zinc oxide ores were stepwise developed and utilized along with the rapidly consumption of zinc sulfide ore resources. It was comparatively suitable to leaching high-alkaline gangue zinc oxide ore with ammonia system, in which a lot of research had been carried out. It was accepted that zinc oxide and zinc carbonate was easy to leach in this system, while a high willemite content ore would result in a poor leaching ratio. But it was unclear of the ammonia leaching mechanism of willemite and the reasons of willemite difficult to leach as yet.In this paper, the leaching of willemite in (NH4)2SO4-NH3-H2O system was investigated in details. The results show that the reaction formula of the leaching is as follows, Zn2Si04(s)+(2i-4)NH3(aq)+4NH4+ =2[Zn(NH3)i]2++SiO2(s)+2H2O(1), i= 1～4. In the leaching process, zinc and silica in willemite dissolve simultaneously into the solution, and then silica precipitates from the solution in the form of amorphous SiO2. The solubility of silica in (NH4)2SO4-NH3-H2O system was measured to be very low, i.e., only on the order of 0.3 g/L; additionally, the precipitation rate of dissolved silica from the leaching solution is slow extraordinarily, may resulting in willemite difficult to leach in (NH4)2SO4-NH3-H2O system, especially under the condition of a lower liquid to solid mass ratio. Increasing the initial mass ratio of liquid to solid in the leaching slurry from 5 to 500 results a sharp raise of zinc leaching efficiency from 2.72% to 84.15%.The leach process obeys the grain model with porous diffusion control, which supposes that the diffusion and chemical reaction take place, not only on the outer surface of the particles, but also on the out surface together with the cavities of the porous particles. Kinetic studies demonstrate that the leach process is controlled by the porous diffusion, determining the apparent active energy and reaction order to be 71.35kJ/mol and 4.27, respectively.Further investigation indicated that in comparision to willemite, the leaching of amorphous zinc silicate was easier in (NH4)2SO4-NH3-H2O system. Mechanical activation converted willemite into amorphous zinc silicate, decreased its crystallinity, and so that enhanced its leaching process in (NH4)2SO4-NH3-H2O system. There was a large reduction in peak intensity of XRD patern of willemite with increasing activating time which manifests decrease in content of crystalline phase of the mineral. When willemite was mechanically activated for 20 min, the crystallinity decreased from 100% to 32%, the zinc leaching ratio increased from 3.9% to 64%, while the powder particle changed only a little.