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<br /> <br /> <br /> <br /> <br />r <br /> <br /> <br /> <br /> <br /> <br />1 <br /> <br /> <br /> <br /> <br /> <br />BACKGROUND <br />In the production of gold, cyanide (CN) is an essential compound which, after <br />use, becomes a toxic waste and must be destroyed. <br />The two places where destruction must occur are in tailings ponds end heap leach <br />pads. Each have different requirements. <br />TAILINGS PONDS <br />Untreated tailings slurries typically contain 90-150 ppm CN If left untreated <br />CN would be very slowly oxidized to CN0 (cyanate) and then hydrolyzed to ammonia <br />and carbon dioxide, provided the pH is above 9.5. <br />Below pH 9.5 HCN is predominant due to hydrolysis: <br />CN + HBO=HCN + OH <br />HCN is then slowly oxidized to hydrogen cyanate by: <br />2 HCN + O2 f 2HCN0 <br />Hydrogen cyanate would then be further hydrolysed to ammonia and carbon dioxide. <br />The problem, of course, is that unenhanced oxidation is so slow that an <br />insignificant change occurs in CN- levels during the short dwell-time (6-72 <br />hours) of water in the pond prior to recycle. <br />These untreated high levels of CN are highly toxic and the resulting high death <br />rate in water fowl and other animals is well known from early-operation days. <br />A reduction of CN~ to below 50 ppm, although not mandated, is strongly suggested <br />by the EPA. Most operators however, feel that 50 ppm may also be dangerous and <br />prefer to be below 20 ppm. The federal Department of Fish and Wildlife believe <br />(from verbal communication with Dr. Isler) that the toxicity of CN- also depends <br />upon the presence of other metals such as Hg and Cu whose presence enhances <br />toxicity. <br />All tailings ponds, today, are treated to reduce CN to "acceptagle" levels. <br />These levels range from 5 ppm to 25 ppm (from operation to operation). Some of <br />the methods used include: <br />1. Dilution <br />2. Oxidation with hydrogen peroxide to the cyanate at a cost of about $5 per <br />1000 gal of tailings pond water. <br />3. Oxidation with the alkaline chlorination process at a cost of about $8 <br />per 1000 gal. <br />4. Oxidation by S0~ which is more expensive than the alkaline chlorination. <br />5. Complexing CN with Fe~~ (typically ferrous sulphate heptahydrate) at a <br />cost of less than $1 per 1000 gal. <br /> <br />