586 Table 2 - Maximum Permissible Levels for Liquid Effluent Emissions in Mining and Metallurgical Units (Source: MEM) Parameter Value at any time Annual average value pH Greater than 6 and less than 9 Greater than 6 and less than 9 Suspended solids (mg/l) 50 25 Lead (mg/l) 0.4 0.2 Copper (mg/l) 1.0 0.3 Zinc (mg/l) 3.0 1.0 Iron (mg/l) 2.0 1.0 Arsenic (mg/l) 1.0 0.5 Total cyanide (mg/l)* 1.0 1.0 *Total cyanide equivalent to 0.1 mg/L of free cyanide and 0.2 mg/L of weak acid dissociable cyanide. 5. TESTS PERFORMED According to the theory of free cyanide destruction in cyanidation pulps with Caro’s Acid, the fundamental parameter is the molar ratio H2SO4 / H2O2, which must remain between 1.5 and 3.0 to ensure rapid reaction kinetics and efficient cyanide destruction. Starting in 2007, tests were conducted with molar ratios between 1.0 and 2.3, achieving average total cyanide values of 6.0 ppm in the tailings dam by year-end. With an average molar ratio H2SO4 / H2O2 = 1.43, total cyanide was reduced by 253% compared to the average values from 2002 to 2006. Between 2008 and 2010, under the principle of continuous improvement, the objective was to reduce total cyanide concentrations in the tailings dam to comply with national legislation. Final concentrations reached 0.14 ppm, with a molar ratio H2SO4 / H2O2 = 1.23. Figure 1 - Molar ratio H2SO4 / H2O2 Vs Total Cyanide (ppm) in the tailings dam. 18 17 16 12 13 6 1.68 0.31 0.14 1.33 1.58 1.38 1.22 1.22 1.43 1.40 1.27 1.23 0 0.4 0.8 1.2 1.6 2 0 4 8 12 16 20 2002 2003 2004 2005 2006 2007 2008 2009 2010 Molar ratio H2SO4 / H2O2 Total NaCN (ppm) Total NaCN (ppm) H2SO4/H2O2
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