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r5 <br />BACKGROUND <br />In the production of gold, cyanide (CN) is en essential compound which, after <br />use, becomes a toxic caste and must be destroyed. <br />The two places where destruction must occur are in tailings ponds and heap leach <br />pads and both 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 CNO" (cyanate) and then hydrolyzed to <br />_ ammonia and carbon dioxide. These reactions are slow, they ari catalyzed by <br />sunlight and enhanced by bubbling and if the pond pH is in excess of 9.5 it will <br />be the predominant reaction. <br />~6 . ~ <br />Below pH 9.5 HCN is predominant due to hydrolysis: <br />CN" + HZO = HCN + OH" <br />HCN can be slowly oxi zed (as described above) to hydrogen cyanate by: <br />2 HCN + 0~ ~ 2 HCNO <br />Hydrogen cyanate would then be hydrolyzed to ammonia and Carbon dioxide. <br />The problem,~ ~o~f cou1~se , is that unenhanced oxidation is so slow that an <br />insignificant ct ah nge occurs in CN levels during the short dwl11-time (B-72 <br />Hours) of the 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 waterfowl and other animals is wail known from early-opeiration days. <br />A reduction of CN to below 50 ppm, although not mandated, is strollgty 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 Department of Fish and Wildlife believe that the <br />toxicity of CN also depends upon the presence of other metals such as Hg and Cu <br />whose presence enhances toxicity. <br />A77 tailings ponds, today, are treated to reduce CN" to "acceptable" Levels. <br />These levels range from 5 ppm to 25 ppm (from company to company). Some of the <br />_ methods used include: <br />i. Dilution. <br />2. Oxidation with hydrogen peroxide to the cyanate at a cost of about <br />E5 per 1000 gal of tailings pond water. <br />3. Oxidation with the alkaline chlorination process ate cost of about <br />E8 per 1000 gal, <br />4, Oxidation by S02 which is more expensive than the alkaline <br />_ chlorination. <br />5. Complexing CN~ with Fe+' (typically ferrous sulphate heptahydrate) <br />at a cost of less than S1 per 1000 gal. <br />z <br />