The concept of hypochlorite treatment prior to cyanidation in the heap leaching of low grade carbonaceous gold ores is discussed. Laboratory column leach tests using a low grade Carlin-type carbonaceous ore were performed to investigate the effects of pH, hypochlorite consumption and concentration, solution flow rate, finer grinding, agglomeration, curing time, and bed height on total gold extraction. At high pH, hypochlorite stability increases and thus, hypochlorite consumption decreases. At ambient temperature, total gold extraction is proportional to hypochlorite consumption. However, at some consumption level addition of hypochlorite does not increase gold extraction. Gold recoveries of 10% and 80% are achieved at 12 and 167 lb/ton hypochlorite consumption, respectively. Increasing hypochlorite concentration increases the rate of extraction. Crushing the ore to a finer size and agglomerating with cement decreases hypochlorite consumption by 60%. On the other hand, agglomerating and curing the cement-ore mixture with hypochlorite solution has no significant effect on total gold extraction. Hypochlorite consumption decreases with increasing solution flow rate. In addition, using calcium hypochlorite results in higher gold extraction than using sodium hypochlorite at a fixed hypochlorite consumption.; For a given hypochlorite consumption, gold extraction increases with decreasing ore bed height. The high hypochlorite consumption is due to oxygen depletion as the solution percolates through the column, i.e., oxidation of gangue materials. These side reactions are reduced at sub-ambient temperatures resulting in a substantial further decrease in hypochlorite consumption. The optimum temperature range and additional reduction of hypochlorite consumption by introducing the reagent into the column at different elevations will be discussed. Finally, a brief operating cost evaluation is given.
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