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AUG- 9-95 WED 16:G3 CROWN TECHNOLOG`! FAX N0, 3179459096 P. 03 INTRODUCTION The environmental impacts of heap leach ore dumps and tailings ponds generated by the cyanide processing of precious metals are the subject of Increased public and regulatory attention. Avian mortalities and groundwater ccntaminatlon are problems of con- cern to the precious metals mining industry. Cyanide destruction or complexing is one method of mitigating these problems. Chlorine, hydrogen peroxide or sulfur dioxide have been used to destroy cyanide. The use of ferrous sulfate to complex cyanide present in precious metals milling solutions provides the mining industry with a more cost effective method of treatment. In one western state, mines may be required to maintain tailings ponds at cyanide concentrations lower than 60 ppm sodium cyanide. This level would be required to pre- vent bird mortalities. Ferrous sulfate can be used effectively and cheaply to control cyanide in tailings ponds at concentrations less than 50 ppm. This publication provides the mine operator with Information concerning the applica- tion and use of ferrous sulfate for the complexing of cyanide in precious metals mill soutlons. CHEMISTRY The complexing of cyanide by ferrous sulfate into ferrocyanide has been known since the early twentieth century. The conversion of sodium cyanide to ferrocyanide takes place according to the reaction: FeSO4 + 6NaCN -- Na4Fe(CN)6 + Na2SO4 The ferrocyanides are also known as Prussian or Turnbulls blue and are among the most stable cyanide complexes in aqueous solutions. Ferrocyanide is virtually nontoxic, it also being the product of the oral antidote for cyanide ingestion. Work conducted by Schiller (1) indicates the reaction to be pH dependent (Chart 1), the lowest concentration of Free cyanide in solution being obtained at a pH of about 8S. However, in general the pH of tailings solutions do not need to be adjusted. Schiller's work, treating pure potassium cyanide solutions, also showed that use of ferrous sulfate alone would not produce cyanide concentrations in solution lower than 3.8 to 5.6 ppm sodium cyanide. Reduction of cyanide in solution to lower levels requires the addition of other chemicals - sodium bisulfite or quaternary ammonium compounds. — 1 --