This is an extremely exothermic reaction, with rapid mixing, applied directly to the effluent discharge line, and it appears clear and colorless. Caro’s Acid is a well-known cyanide-degrading agent widely used for treating effluents from precious metal cyanidation processes. Caro’s Acid readily degrades WAD cyanides and free cyanide from cyanidation tailings, since these dissociate at pH 9; This pH results from the addition of Caro’s Acid, which lowers the tailings pulp pH, typically ranging between 10.5 and 11.5. Caro’s Acid is unstable and may decompose into sulfuric acid, a process favored at high temperatures, H2SO5 → H2SO4 + O2 (4) Caro’s Acid oxidizes cyanide to cyanate through the following reaction CN- + H2SO5 ↔ CON- + H2SO4 (5) The reaction kinetics between Caro’s Acid and free cyanide occur within 1 to 5 minutes Residual hydrogen peroxide present in Caro’s Acid solution (not converted) also reacts with free cyanide to form cyanates H2O2 + CN- → CON- + H2O (6) 4. OPTIMIZATION OF FREE CYANIDE DESTRUCTION IN TAILINGS WITH CARO’S ACID AT ANTAPITE PROCESSING PLANT CIP cyanidation tailings at the Antapite processing plant, containing 40 - 42% solids and an average of 250 ppm of free cyanide, are treated with Caro’s Acid directly in the discharge line to destroy free cyanide before disposal in the tailings dam. Since the beginning of operations at Antapite, effluent discharge has been maintained at “zero,” due to the scarcity of water resources in the area. Therefore, it is necessary to recirculate clarified solution from the tailings dam, free of cyanide, to sustain continuous operations at the processing plant in a sustainable manner. The characteristics of the recirculated solution (“industrial water”), after treatment with Caro’s Acid and reused in the processing plant between 2002 and 2006, are shown in Table 1.
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