Process for treating refractory gold ores by roasting under oxidizing conditions

Microscopic gold inclusions in sulfide minerals cannot typically be estimat ed by conventional cyanidation. The gold ore must be pretreated to oxidize sulfide minerals associated with gold and to remove other minerals and chem ical compounds that interfere in the cyanidation leaching process. The reco very of gold from refractory sulfidic-carbonaceous ores after roasting pret reatment was studied. With the addition of an oxygen-containing gas (e. g., air or oxygen-enriched air), sulfides of iror, zinc, copper and other meta ls 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 cha racteristics of the calcine have a strong influence on the eventual recover y of gold by cyanidation. These characteristics are strongly dependent on t he roasting conditions and roasting method. The optimum combination of part icle size, roasting temperature, reaction rime and oxygen concentration in the off-gas was determined for a typical refractory sulfidic-carbonaceous a re from the Carlin Trend in Nevada. The tests were run using a 150-mm labor atory rotating tube furnace and 160- and 200-mm fluid-bed pilot roasters. L aboratory 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 0.01 3 oz/st) Au, respectively. The maximum extraction was achieved under the fo llowing 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%, w ith 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 par ticle size and oxygen concentration as in the laboratory testing but with t he roasting temperature of 500 degrees to 525 degrees C and a residence tim e of 30 min.