Optimizing the Oxi-chlorination Leaching Process of Stibnite by Ozone

Acidic chloride leaching of antimony sulfide (stibnite) requires an oxidizing agent to have high efficiency. Among various oxidants, ozone gas (O3) is believed to be so effective, due to its high oxidation potential (2.07 V). This study aims to investigate thermodynamic aspects of injecting ozone gas into stibnite oxi-chlorination leaching system. Furthermore, a thermodynamic modelling is utilized to optimize the operational parameters of leaching process, including hydrochloric acid concentration,pH, and temperature. For 4.5 M hydrochloric acid concentration, elemental sulfur has a chance to be produced during the process, at potentials lower than about 0.4 V. Higher potentials lead to the production of HSO4 - ion, which is the most favorable condition for oxi-chlorination leaching process. However, increasing the potential to values more than 0.9 V will interfere the dissolution, because of antimony oxide production. Based on thermodynamic modelling, it has been determined that a minimum hydrochloric acid concentration of 2.9 M is required for dissolution process to start. According to this critical concentration, the antimony sulfide will not be dissolved at pH values higher than -0.46, even by introducing ozone gas. Raising temperature is favorable for leaching progress from both thermodynamic and kinetic viewpoints. However, it is not appropriate for this oxi-chlorination system, due to adverse effect on solubility extent and decomposition rate of ozone gas. Therefore, the optimum temperature is determined to be 25 ˚C.

Keywords:

Antimony , Stibnite , Oxi-chlorination , Ozone , Eh-pH diagram.