1992-04-02_REVISION - M1988112 - Laserfiche WebLink

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' 2. Reasons for High CN Levels ' a. Free CN - Free CN levels are high because the system installed to address this problem, namely the AVR, is working inefficiently. Attached in Section 2 is a full description of how the AVR works, by Mudder, who is probably the authority on the subject. In essence, one needs an air-flow rate of approximately 500 ' times the slurry flow-rate. At San Luis this implies an air flow of 80,000 cfm. The air must be evenly distributed with high surface area (small bubbles) and exposure time should 6e ' 1-3 hours. Just a 1 hour exposure time would require a tower/reactor volume of 10,000 cu. ft. The levels of free CN in the tailings pond indicate the ' inefficiency of the AVR which, given air flow rate and the fact that mechanical agitation has no effect upon HCN volatization, is probably causing whatever volatization does ' occur, to do so only when the H2SOy is introduced. b. WAD CN - The WAD cyanide is almost entirely a copper complex since Zn is less than 0.1 ppm in the slurry. The copper is ' present in the ore and hence in the tailings and since San Luis has no system for removing Cu, there it remains. ' B. RESOLUTION OF SAN LUIS PROBLEMS BEI believes that both problems (free CN and WAD CN1 can be relatively ' easily solved and that a two-step solution be implemented and later integrated into one continuous step (as long as you are working in the high copper ore). ' 1. Steo 1: Free CN Elimination There are a number of approaches that can be taken that will resolve ' the free CN problem; these include: AVR Inco Process ' Alkaline Chlorination Peroxide Ferrous Sulfate ' Cytox (BEI) A broad summary of these methods is attached in Section 3, except ' for AVR (covered earlier) and Cytox (covered below). Free CN removal should occur first because it is the easiest problem to resolve and its resolution will dramatically reduce the risk of 1 SEA