Effect of Direct and Indirect Pre-Oxidation of Gold Ore from Doup North Sulawesi using Ozone Gas on the Gold Extraction in the Cyanidation Process

Refractory gold ore is a type of gold ore that is difficult to process when treated through a conventional cyanidation process. Pre-oxidation using ozone gas is one of the pre-treatment methods that can be used to improve gold recovery from sulfide-type refractory ore. This study used gold ore samples from Doup, North Sulawesi, Indonesia, to study the effect of direct and indirect pre-oxidation using ozone gas on gold recovery and subsequently treated by a cyanidation process. Direct and indirect pre-oxidation experiments were carried out at different pHs and using two raw materials for ozone generation (air and O 2 ). The slurry obtained from the pre-oxidation experiments was filtered, and the filtrate was analyzed for the concentration of dissolved Cu, Fe, and Zn using atomic absorption spectroscopy (AAS); meanwhile, the residue from each pre-oxidation experiment was leached by cyanidation to obtain a pregnant leaching solution (PLS), which was analyzed for its gold concentration using AAS. The percentages of dissolved Cu, Fe, and Zn during pre-oxidation increased with the pre-oxidation time when the acidity was maintained at pH 2, using pure O 2 as the ozone raw material. The direct pre-oxidation process at pH 2 using air as the ozone raw material increased gold extraction from 79.08% to 82.78% in the cyanidation process. The pre-oxidation experiments performed without controlling the pH resulted in lower gold dissolutions than the pre-oxidation conditions at pH 2. Possible causes of the lower gold dissolution were discussed.