A FUNDAMENTAL STUDY OF THE ACIDIC PRESSURE OXIDATION OF ORPIMENT AND PYRITE AT HIGH TEMPERATURE

The acidic pressure oxidation of pyrite (FeS2) and orpiment (As2S3) as a pretreatment for refractory gold ores has been investigated to better understand the reaction chemistry and kinetics. This study evaluated the impacts of variables such as retention time, temperature, particle size, oxygen partial pressure, pulp density, sulfuric acid concentration, and cupric ions on both minerals. Additionally, the influence of ferric ions on the oxidation rate of As(III) to As(V) was assessed. Findings showed that during the oxidation of orpiment, most arsenic remained in the trivalent state after 2 hours at temperatures between 170 and 230°C, with slow subsequent oxidation to As(V) under oxygen presence. Notably, while cupric ions had no effect on As(III) oxidation, ferric ions significantly accelerated the conversion rate. Sulfate became the primary product only at temperatures exceeding 190°C, with oxidation kinetics primarily governed by product layer diffusion across the 170-210°C range, displaying an Arrhenius activation energy of 22.2 kJ/mol. In contrast, for pyrite oxidation, ferric was identified as the initial product at higher temperatures, with the reaction order indicating dependence on pulp density and oxygen partial pressure. An activation energy of 33.2 kJ/mol was recorded for pyrite oxidation as well.