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1 <br />3 ' <br />of the monitoring sampling and analysis protocol. Permit levels still appear to be violated, ' <br />although there is significant uncertainty about the validity of the analytical procedures to <br />accurately determine free cyanide. The Board will allow the violation to continue until the <br />new cyanide: detoxification process (INCO' Process) is fully operational. , <br />2.2 Cyanide Detoxification Procedures ' <br />A number of cyanide treatment/reuse processes are available for detoxification of ryanide <br />use in the beneficiation and processing of gold and silver ores. In general terms, cyanide can , <br />be destroyed via natural degradation, biological processes, and oxidation. Only one rerycling <br />methods is currently in use. <br /> , <br />Acidification/Volatilization/Reneutralization (AVR) is a recycling technique in which the <br />barren solution is acidified in an enclosed vessel under negative pressure to the point where <br /> <br />ryanide volatilizes as hydrogen cyanide. Cyanide recoveries of 95-97% have been achieved , <br />at a pH of 2.5. Hydrogen cyanide gas is then circulated through a sodium hydroxide solution <br />to recapture the hydrogen cyanide as sodium cyanide for reuse in leaching. The residual <br /> <br />barren solution is reneutralized with a caustic and sent to a tailings impoundment In , <br />addition to recovery of cyanide, metals are precipitated from solution as hydroxides. The <br />process is applicable to solutions; problems associated with the chemical and physical <br /> <br />properties of low-pH slurries have prohibited the used of AVR on tailings slurries, although , <br />recent research has focused on making the process viable at higher pH values. Although <br />the pK for the reaction HCN (aq.) = H* + CN~ is 9.36 (25°C), much lower pH values are <br /> <br />required for efficient volatilization of hydrogen cyanide in the AVR process. AVR was ' <br />attempted at the San Luis facility, but high copper ]evels apparently rendered the process <br />unsuitable. ' <br />Natural degradation can transform cyanide to non-toxic compounds through processes <br />including photodecomposition, acidification by COZ and subsequent volatilization, oxidation , <br />by oxygen, dilution, adsorption onto solids, and biological action. Decomposition products <br />such as thioryanate and ryanate are relatively non-toxic. The toxicity of different metal- <br />cyanide complexes is variable and not very well understood. Homestake currently employs , <br />a biological process in Lead, South Dakota, where cyanide concentrations have been <br />reduced, after natural degradation, to 0.2 ppm. Natural degradation is enhanced under well <br />oxygenated conditions with ample sunlight. These conditions can be met in the surface of , <br />solution ponds and tailings impoundments. If solutions containing cyanide leak to <br />groundwater, natural degradation is considerably less effective. In addition, metals <br />associated with cyanide are likely to be transported further than uncomplexed metals, which ' <br />would have higher affinities for adsorption onto mineral surfaces. <br /> 1 <br />RCG/Hagler, Badly, inc ' <br /> <br />