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NPR ~{~ ~6~~~4 species if discharged. Chlorine can be neutralized by treating with thiosulfate or hydrogen peroxide. Hypochlorite reacts preferentially with thiocyanate (CNS-) present in solutions over free cyanide. In solutions having significant thiocyanate concentrations, unless sufficient hypochlorite is used, free cyanide concentra- tions can remain little changed (Mathre and DeVries 1981). In the leaching of sulfide ores the formation of [hiocyanate is common. Thiocyanate interference, under conditions where thiocyanates aze readily formed, may require from two to four dines the anticipated amount of chlorine to destroy free cyanide. Oxidation by hydrogen (HBO,) peroxide is also employed as a treatment for the selective destruction of free cyanide and the weak-acid- dissociable cyanide complexes. This method has the advantage of producing no undesirable by-products. The reaction of hydrogen peroxide with sodium cyanide was first recognized eazly in this century. The rate of this reaction is relatively slow even in the presence of excess hydrogen peroxide. Masson (1907) showed that the reduction of cyanide concentration in solution from 50 ppm to less than 0.5 ppm requires more than eight hours even when the initial molar ratio of hydrogen peroxide to cyanide is as great as 5:1. In the late 1960's it was discovered that this reaction can be significantly speeded up by the addition of soluble copper salts. Since copper is often an accessory mineral to ores containing gold and silver, soluble copper salts may be naturally present in sufficient amounts to help accelerate the oxidation of cyanide. Most of the copper eventually precipitates out of solution during the reactions. Ordinary hydrogen peroxide does not have a long shelf life. Stabilized hydrogen peroxide mixtures, such as Du Pont's KASTONE R, are specially formulated for treating cyanide wastes. Under certain conditions the addition of formaldehyde is also effective in increasing the rate of cyanide oxidation. Refer to the Du Pont publication included in this handbook for additional discussion of both alkaline chlori- nation and peroxide treatment. The availability of sufficient quantities of hydrogen peroxide to treat large volumes of solution in an emergency situation, such as a case where ponds are about to overtop, must be taken into consideration, particularly in remote areas. Hydrogen peroxide treamtent tray be the preferred method of treatmenr. in circumstances where sufficient treatment time is available and where water quality concerns would necessitate minimizing chlorine levels in discharged effluents. Another treatment method, if time permits, in- volves ferrous sulfate, also known as copperas. Cyanide is convened to fetrocyanide, and to some degree to ferricyanide, also called Prussian blue. The latter compound is quite in- soluble. These compounds aze relatively stable and less toxic than free cyanide. Ferrous sulfate is effective oncenna - am e m so anon, bit its effecavenecc _ w en trea rn v¢ low c amide ncentranons. ere anve reaction rates are an tole involving alkaline chlori- nation. The optimum solution pH for reaction is reported to be near 8.5 (Schiller 1983, using potassium cyanide solutions). But in order for a substantial amount of the product to be converted to Prussian blue, a stable precipitate, Schiller (1983) reports results from experi- mental data that the solution mmst be kept below about pH 5. Reed et al. (1971) concluded that this method for cyanide removal requires close control of solution pH, and is effective on pure cyanide solutions. In neaz-emergency situations this treatment method may trot be practical except when there would be xtleast a day or more of available treatment time. Moreover, without further treatment by alkaline chlori- nation or with peroxide, rosidual cyanide concenaadons may still exceed discharge limits. 28 t