BACTERIAL OXIDATION CONDITIONS FOR GOLD EXTRACTION FROM OLYMPIAS REFRACTORY ARSENICAL PYRITE CONCENTRATE

The arsenical pyrite concentrate produced at the Olympias mine, Macedonia, Greece, assays approximately Fe: 40%, S: 40%, As: 12% and Au: 26 g/t. Mineralogically, it consists mainly of pyrite (68-70%) and arsenopyrite (23-26%), the former includes both arsenic-free and arsenian pyrite. Gold is mainly associated with the arsenopyrite and arsenian pyrite. The concentrate is highly refractory in nature, as direct cyanidation yields gold recovery lower than 10%. An oxidative pretreatment step is, therefore, necessary before cyanidation, in order to break up the sulphide lattice and liberate gold. The present paper aims at elucidating the effect of the leaching conditions on the bacterial oxidation of the Olympias concentrate. This research subject is of particular interest because selective oxidation of the arsenopyrite and arsenical pyrite fractions may result in high gold recoveries without the need for total sulphur oxidation. Based on the above, this study is focused on the factors that enhance preferential arsenopyrite oxidation. It has shown that preferential oxidation of arsenopyrite is observed especially at short retention times and pulp densities in excess of 10% solids. Arsenopyrite oxidation is complete at EMF values of 480-550 mV, while the oxidation of pyrite is observed to commence at higher EMF values, where the oxidation of arsenopyrite is almost complete. High ferric iron concentrations in solution enhance arsenopyrite but have an adverse affect on pyrite oxidation rates. When operating at constant pH values in the range 1.0-1.2, selective oxidation of arsenopyrite is observed, while pyrite oxidation proceeds at higher pH values, around 1.5. The indirect mechanism is deduced to play a significant role in the bio-oxidation of arsenopyrite, while pyrite oxidation is mainly attributed to direct bacterial attack.