Direct leaching of sphalerite has been considered as a cost effective and an environmentally benign alternative to the traditional two-step roasting-leaching approach. Yet the slow leaching rate due to the formation of surface passivating phases remains the main challenge. Here, we studied the mechanism and kinetics of sphalerite leaching in the temperature range of 35-130 degrees C using three oxidants, Fe-2(SO4)(3), FeCl3, and Fe(NO3)(3), and observed distinctly different surface passivation and significantly different leaching rate. Leaching using Fe-2(SO4)(3) was the slowest, due to the formation of passivating surface layers of sulfur at the early stage and hydrated (Fe,Zn)-sulfates at the later stage. The formation of hydrated (Fe,Zn)-sulfates reduced Zn extraction by up to 20%, leading to incomplete Zn extraction. Using FeCl3, leaching was faster than in Fe-2(SO4)(3). However, the formation of surface sulfur also caused passivation. Leaching in Fe(NO3)(3) was the fastest, as the initially formed surface sulfur was quickly oxidized to sulfuric acid and hence passivation was negligible. Using Fe(NO3)(3), complete Zn extraction from 106 to 150 mu m sphalerite particles took 7 days at 35 degrees C, 2 days at 70 degrees C, 5 h at 90 degrees C, and 1 h at 130 degrees C. This is about one order of magnitude and two orders of magnitude faster than leaching in FeCl3 and Fe-2(SO4)(3), respectively. The observed leaching behaviors were in agreement with changing activation energy as a function of leaching extent, analyzed by the modified 'time-to-a-given-fraction' method. In the case of Fe-2(SO4)(3) and FeCl3, leaching was controlled by phase-boundary reactions at the early-to-middle stages, but changed to diffusion control at the later stage after the formation of passivating surface layers; in the case of Fe (NO3)(3), leaching was controlled by phase boundary reactions over the entire leaching process. This work demonstrates that Fe(NO3)(3) is the more efficient oxidant than FeCl3 and Fe-2(SO4)(3) for fast leaching of sphalerite at low temperatures with minimum surface passivation.
