583 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. 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. 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. Weak Acid Dissociable Cyanide (WAD) Weak cyanide complexes, or WAD, can dissociate in solution and release environmentally significant concentrations of free cyanide. These complexes include cadmium, copper, nickel, zinc, and silver cyanide complexes. CYANIDE WAD → CYANIDE FREE Zn (CN)4 -2, Cd (CN) 4 -2, Cu CN) 4 -2 Ni (CN) 4 -2 Ag (CN) 2 - → HCN, CN- The degree of dissociation of WAD cyanides depends on solution pH, temperature, light intensity, dissolved solids, and the concentration of weak complexes. Strongly Complexed Cyanides Strong cyanide complexes degrade much more slowly compared to WAD cyanides. Iron, cobalt, and gold cyanide complexes are strong and stable in solution. STRONGLY COMPLEXED → CYANIDE FREE
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