Process for treating refractory gold ores by roasting under oxidizing conditions

Microscopic gold inclusions in sulfide minerals cannot typically be estimated by conventional cyanidation. The gold ore must be pretreated to oxidize sulfide minerals associated with gold and to remove other minerals and chemical compounds that interfere in the cyanidation leaching process. The recovery of gold from refractory sulfidic-carbonaceous ores after roasting pretreatment was studied. With the addition of an oxygen-containing gas (e. g., air or oxygen-enriched air), sulfides of iror, zinc, copper and other metals are transfomzed into solid oxides or sulfates and gaseous sulfur dioxide at elevated temperatures (e. g., 400 degrees C to 800 degrees C). When the sulfides are oxidized individual par-tides lose internal volume and become porous, allowing leaching solutions access to the gold. The structural characteristics of the calcine have a strong influence on the eventual recovery of gold by cyanidation. These characteristics are strongly dependent on the roasting conditions and roasting method. The optimum combination of particle size, roasting temperature, reaction rime and oxygen concentration in the off-gas was determined for a typical refractory sulfidic-carbonaceous are from the Carlin Trend in Nevada. The tests were run using a 150-mm laboratory rotating tube furnace and 160- and 200-mm fluid-bed pilot roasters. Laboratory results indicated that the maximum gold extraction was 92 %, with the head and the leach residue containing 5, 7 and 0.4 g/t (0.166 and 0.013 oz/st) Au, respectively. The maximum extraction was achieved under the following roasting coinditions: a top particle size of 150 mu m, a residence time of 2 hrs, a roasting temperature of 475 degrees C and with 40% oxygen (by volume) in the off-gas. Similar results were obtained from circulating fluid bed (CFB) pilot-plant testing. The maximum gold extraction was 90%, with the head and leach residue containing of 5.1 and 0. 5 g/t (0.149 and 0. 015 oz/st) Au, respectively. These results were achieved with the same particle size and oxygen concentration as in the laboratory testing but with the roasting temperature of 500 degrees to 525 degrees C and a residence time of 30 min.