A typical silver chloride contents 58-68% AgCl, 12-16% CuCl, 1.5-4% PbCl2 and 12-22% NaCl. The sodium chloride is basically produced during De-Golding and considering its high solubility, it is removed without major problem. The leaching of CuCl and PbCl2 is an important part of the process and obviously the related problems of the solubility of silver chloride. The solubility of copper chloride is improved in the presence of sodium chloride and hydrochloric acid, but at the same time the solubility of lead chloride decreased, both undesirable effects that must be faced. The leaching process tries to oxidize CuCl to CuCl2, which is highly soluble. Sodium Chlorate (NaClO3) is a good oxidant, relatively cheap and does not introduce any new salts.
6CuCl + NaClO3 + 6HCl = 6CuCl2 + NaCl + 3H2O
This reaction is relatively fast and the considerable percentage of sodium chloride present in the AgCl is active in removing some of the copper chloride at the quenching stage under acidic conditions. Approximately, 35-45% of the CuCl is leached out in this way and the remaining is dissolved as shown in the prior equation. The solubility of lead chloride is relatively high at low chloride ion concentration and at high temperatures. It is easier to leach out lead chloride by oxidation. It is important to mention that the solubility of silver chloride increases at high NaCl or HCl concentrations and also with temperature. Nevertheless, with the low chloride ion concentration, even at elevated temperatures, the solubility is not optimum and less silver chloride is passed to the effluent.
A problem encountered is the formation of colloidal silver chloride as the concentration of the electrolytes present in the leach solution decreases. Colloidal silver chloride can be eliminated or at least make to coagulate by the addition of small dosages of HCl. Coagulation is better when temperature is increased. It has been noted that traces of gold remain in the crude silver chloride. Chlorine is formed during the oxidation of CuCl to CuCl2 according to the following reaction,
6HCl + NaClO3 = 3Cl2 + NaCl + 3H2O
The chlorine produced is available to dissolve trace amounts of gold. The dissolved gold is later recovered in a cementation stage through all effluents pass.