It is important to mention that chlorine was one of the original reagents employed to dissolve gold. The optimum gold leach takes place under acid conditions in a narrow ph range. The main idea of flash chlorination is to oxidize organic carbon and pyrites from carbonaceous and pyritic refractory gold ores in order to get satisfactory gold recoveries by cyanidation. The main reactions are below,
Cl2 + H2O = HCl + HOCl
CaCO3 + 2HCl = CaCl2 + H2O + CO2
CaCO3 + 2HOCl = Ca(OCl)2 + H2O + CO2
2FeS2 + 15Cl2 + 20H2O = Fe2O3.H2O + 4H2SO4 + 30HCl
CaCO3 + H2SO4 = CaSO4 + H2O + CO2
According to these reactions, organic carbon is oxidized gaseous chlorine or HOCl and carbonates are neutralized. Since preg-robbing carbon is neutralized, the material is appropriate for cyanidation. According to some studies, hypochlorite ion is the main responsible of the neutralization of carbonaceous compounds is most the time 60-70% of this carbon is oxidized and the difference is deactivated. The oxidation of pyritic gold ores produces gypsum that controls scale build up during the chlorination process. The oxidation of pyrite depends of the pH and the final consumption of chlorine is influenced by the kinetics of the reactions indicated above.
Carbonates react during flash chlorination and only a portion of the organic carbon and sulphide sulphur is oxidized. The preg-robbing carbon is deactivated and the sulphide is converted to sulphate. By treating refractory gold ores with flash chlorination the recovery of gold is increased from 5 to 80%. According to Newmont plant results, the extraction of gold was increased near 5% over the standard Carlin Chlorination process and the throughput was increased 15% due the high proportion of carbonaceous ores with gold content detected in the deposit. Next figure shows the Potential – pH Equilibrium Diagram for the system Au-H2O-Cl.
Potential – pH Equilibrium Diagram for the system Au-H2O-Cl at 25 oC