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to detoxification using AVR at this time. <br />On June 15, 1992, hydrogen peroxide treatment of the free water pool in the lower pond was <br />started, and the use of the AVR circuit was discontinued. Under alkaline conditions, hydrogen <br />peroxide oxidizes both free and WAD cyanide species. Free cyanide is oxidized to form <br />cyanate. Cyanate subsequently hydrolyzes to yield ammonia and carbonate. Welaldy complexed <br />metal cyanides oxidized by hydrogen peroxide include copper, zinc, and nickel. The weakly <br />complexed metal cyanides are oxidized to form cyanate and metallic hydroxide precipitates. <br />Cyanate contlnues to break down into ammonia and carbonate. Hydrogen peroxide treatment <br />of the free water pool in the lower pond was used in conjunction with Cytox treatment of the <br />tailings slurry. The maximum beach elevation of the lower pond at the time hydrogen peroxide <br />treatment was started was 8,555. <br />On July 18, 1992, the free water pool in the upper pond was treated with hydrogen peroxide. <br />No fresh tailings were deposited in the upper pond so the hydrogen peroxide treatment primarily <br />affected the cyanide concentration of the free water pool. <br />During interim detoxification the free water pools were sampled and analyzed Qaily for WAD <br />cyanide. Total cyanide sampling and analysis also occurred daily once approptfiate laboratory <br />procedures were implemented. The analyses were submitted on a weekly basis tb the Division. <br />2.1.4 Long-Term Detoxification - INCO Circuit <br />Under alkaline conditions, sulfur dioxide in the presence of a copper catalyst oxidizes both free <br />and WAD cyanide. Free cyanide is oxidized to cyanate which hydrolyzes to ammonia and <br />carbonate. Metal cyanide complexes are oxidized to form cyanate and metal hydroxides. <br />Copper, zinc, and nickel hydroxides precipitate as insoluble solid compounds. Iron cyanide <br />removal is initiated by reduction of ferricyanide to ferrocyanide. Ferrocyanidd then forms a <br />metallo-ferrocyanide complex which is precipitated. Calcium sulfate or gypsurd is formed as <br />a by-product of the reactions. These reactions are shown on Figures 2.3, 2.4, and 2.5. <br />6 <br />