a clarified solution surface. In addition, the tailings dam serves as a containment reservoir for rainwater, storing sufficient volume to ensure operational continuity, while maintaining zero discharge. At the start of operations at Antapite in 2001, free cyanide in tailings pulp from the processing plant was treated using hydrogen peroxide with copper sulfate catalyst. Cyanide destruction performance reduced concentrations from 250 ppm to 120 ppm, with operating costs of $1.51/t. of ore treated for hydrogen peroxide and 0.23$/t. for copper sulfate. The total cost was therefore 1.74$/t., demonstrating the process to be inefficient and costly. Under these conditions of high cyanide content, reuse of the solution in the processing plant was impossible due to its high toxicity. Beginning in October 2002, the Caro’s Acid process (a mixture of sulfuric acid and hydrogen peroxide) was implemented, achieving cyanide destruction from 250 ppm down to 18 ppm at lower cost. This improvement was due to the process’s high reaction kinetics and reduced hydrogen peroxide consumption, making it feasible for further optimization. 2. CHEMISTRY OF THE CYANIDATION PROCESS The cyanidation process for gold follows Elsner’s equation: 4Au + 8NaCN + O2 + 2H2O → 4NaAu (CN)2 + 4NaOH (1) At Antapite, for the recovery of gold as Au (CN-)2, the CIP process is used, and the cyanide contained in the tailings pulp is discharged into the tailings dam. 2.1. Cyanide in Solutions Three main forms of cyanide compounds may be present in residual effluents or process solutions: ▪ Free Cyanide ▪ Weak Acid Dissociable Cyanide (WAD) ▪ Strongly Complexed Cyanide Together, these constitute TOTAL CYANIDE. 2.1.1. Free Cyanide The term free cyanide applies both to the cyanide ion (CN-) dissolved in process water and to hydrogen cyanide (HCN) formed in solution. These are the most toxic forms, but also the easiest to remove from solutions through treatment processes and natural attenuation mechanisms.
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