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

The arsenical pyrite concentrate produced at the Olympias mine, Macedo nia, Greece, assays approximately Fe: 40%, S: 40%, As: 12% and Au: 26 g/t. Mineralogically, it consists mainly of pyrite (68-70%) and arseno pyrite (23-26%), the former includes both arsenic-free and arsenian py rite. Gold is mainly associated with the arsenopyrite and arsenian pyr ite. The concentrate is highly refractory in nature, as direct cyanida tion yields gold recovery lower than 10%. An oxidative pretreatment st ep is, therefore, necessary before cyanidation, in order to break up t he sulphide lattice and liberate gold. The present paper aims at eluci dating the effect of the leaching conditions on the bacterial oxidatio n 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 h as shown that preferential oxidation of arsenopyrite is observed espec ially at short retention times and pulp densities in excess of 10% sol ids. Arsenopyrite oxidation is complete at EMF values of 480-550 mV, w hile the oxidation of pyrite is observed to commence at higher EMF val ues, where the oxidation of arsenopyrite is almost complete. High ferr ic iron concentrations in solution enhance arsenopyrite but have an ad verse affect on pyrite oxidation rates. When operating at constant pH values in the range 1.0-1.2, selective oxidation of arsenopyrite is ob served, 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 attri buted to direct bacterial attack.