The reactions between gold and hypochlorite leach solutions containing hydrochloric acid and sodium chloride are demonstrated below:
4Au + 16NaOCl + 10H2O → 4HAuCl4 + 16NaOH + 5O2……………………………………(1)
4HAuCl4 + 4NaCl → 4NaAuCl4 + 4HCl…………………………………………(2)
16NaOH + 16HCl → 16NaCl + 16H2O…………………………………………(3)
Overall reaction of equations (1), (2) and (3) can be summarized as follows:
4Au + 16NaOCl + 12HCl → 4NaAuCl4 + 12NaCl + 6H2O + 5O2……………………(4)
It is obvious that both hydrochloric acid and sodium chloride have affected the gold dissolution in hypochlorite solution.
Leaching pure gold with an acidic sodium hypochlorite solution has been demonstrated to be effective. The initial rate of gold dissolution was extremely rapid. In a leaching solution containing 1 g/L NaCl, 1 % HCl and 2 % NaOCl, nearly 90 % of the gold was extracted in only 30 minutes. Additions of hydrochloric acid will reduce the sodium hypochlorite requirement A leach solution containing 1 g/L NaCl, 5 % HCl and 1 % NaOCl achieved nearly the same per cent gold extraction under the same conditions. Raising the leach solution temperature to about 55°C will also enhance the gold dissolution.
Extracting gold from an ore required more reagents and longer retention time because sulphide and gangue minerals consumed sodium hypochlorite and hydrochloric acid. In a leach solution containing 20 % HCl and 20 g/L NaCl, 97 % of the gold was extracted in 6 hours with 90 kg/t available chlorine and 95 % of the gold was extracted in 18 hours with 45 kg/t available chlorine.
For a refractory ore, pretreatment is required in order to expose the gold surface to the leach solution. An acidic hypochlorite solution can be added to the autoclave for pressure oxidation and extraction of the gold at the same time. It required 50 % sodium hypochlorite or 120 kg/t available chlorine to extract 97 % of the gold from the ore. When 3 % HCl and 10 g/L NaCl was added to the leach solution, the same amount of gold could be extracted with 24 kg/t available chlorine under the same conditions. However, the reagent requirement may be further reduced by replacing sodium hypochlorite with calcium hypochlorite. In a leach solution containing 10 kg/t Ca(OCl)2 or 5 kg/t available chlorine, 3 % HCl and 2 kg/t NaCl, 95 % of the gold was extracted in 90 minutes oxidation at 210°C. Silver extraction was 98 % under the same conditions. This silver extraction can not be achieved by conventional pressure oxidation and cyanidation.