Alkaline Pressure Oxidation of Pyrite in the Presence of Silica -- Characterization of the Passivating Film

Alkaline pressure oxidation, particularly in the presence of trona as additive, can be used to oxidize high carbonate refractory gold ores as it prevents the formation of CO2 in the autoclave. However, the presence of silica in the ore can lead to the encapsulation of pyrite due to the formation of a passive layer. This phenomenon occurs due to the high solubility of silica in alkaline solutions and its subsequent re-precipitation on the reacting pyrite surface. The present study investigated the chemical composition and thickness of the passive layer on a rotating pyrite surface in an aqueous slurry containing silica sand, sodium carbonate and calcium carbonate at 230°C and under 7 bar of oxygen overpressure. Results obtained from XPS and SEM show that a concentration of 2.5 g/L sodium carbonate gave the maximum thickness of passivation on pyrite and that the passive layer consisted primarily of silicates and aluminosilicates.