584 Fe (CN)6 -4, Co (CN) 6 -4, Au (CN) 2 - → HCN, CN- 3. CARO’S ACID PROCESS Peroxymonosulfuric acid was first described by Heinrich Caro (1834–1910), who prepared it in 1898. Laboratory-scale preparation involves the combination of chlorosulfuric acid and hydrogen peroxide. H2O2 + ClSO2OH ↔ H2SO5 + HCl (2) Large-scale production is usually carried out in situ due to its instability. According to Martin’s patent, peroxymonosulfuric acid is produced by reacting sulfuric acid >85% with hydrogen peroxide >50% H2SO4 + H2O2 ↔ H2SO5 + H2O (3) Maximum efficiency is obtained with a molar ratio: H2SO4 / H2O2 from 1.5 to 3.0 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)
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