PRESSURE OXIDATION OF ARSENOPYRITE AND PYRITE IN ALKALINE-SOLUTIONS

Alkaline pressure oxidation of a sulfide concentrate assaying 60% arsenopyrite and 40% pyrite was investigated. A comparative analysis of the effect of pH, temperature, oxygen partial pressure and particle size on the oxidation rate of both sulfides is presented. The results show that the reaction rates are extremely favored by the increase of pH up to a value of 13.5. The oxidation rate of pyrite increases as oxygen partial pressure increases, while the oxidation rate of arsenopyrite becomes nearly independent of this variable above 202.6 kPa. Apparent activation energies equal to 15.0 and 27.2 kJ/mol were determined for arsenopyrite and pyrite, respectively. High levels of oxidation were achieved working with relatively-coarse particle sizes (-212 + 150-mu-m), under moderate temperature (393 K) and oxygen partial pressure (405.2 kPa). Microprobe analysis revealed that porous layers of iron oxides precipitate on the sulfide particles at high pH, while thin and apparently dense oxide coatings are formed under neutral conditions. Hematite is the main iron oxide formed in all experimental conditions. Sulfates and arsenates were detected only during precipitation from neutral or slightly acid solutions.