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occurs at potentials above 1.2 V, and this requires the addition of strong oxidants, such as Cl2 and HClO. As shown in Eq. 3.1 and 3.2, two gold chloride complexes can be produced, depending on the concentration of chloride. The dissolution rate of gold with chlorine is much faster than that with cyanide (Gökelma et al. 2016). Nicol et al. reported that the rate of gold dissolution by chlorination is approximately two orders of magnitude greater than in cyanidation, because the solubility of chlorine is greater than that of oxygen in aqueous solutions (Nicol et al. 1992). Chlorine can be regenerated in slurries and solutions by NaCl electrolysis or by the oxidation of hydrochloric acid via MnO2. The high dissolution of gold in the chloride solution is achieved by the low pH, high chloride and chlorine concentrations, elevated temperatures, and large ore surface area. Due to the high toxicity and corrosivity of chlorine, as well as the high consumption of chlorine, chloride gold extraction was largely replaced by cyanidation in 1888 (Marsden et al., 1992). However, chlorination processes are being utilized for refining gold-rich materials with an absence of reductants, such as anode slimes (Aylmore, 2005). However, in the last few decades, chlorine has received attention again, especially for refractory gold ores. The refractory gold ore discussed herein is defined as ore that produces less than 80% gold extraction by standard cyanidation techniques. The refractoriness can be attributed by the organic carbon content, sulfide content, sulfide encapsulation, and silicon encapsulation (Seymour et al. 1989). In the late 1960s, researchers started to recognize that aqueous chlorination could be used to destroy carbonaceous material (Afenya, 1991). Viñals et al. (1995) investigated the kinetic mechanism of gold in the chloride solution. The kinetics were found to be chemically controlled, and the reaction order was 1 with respect to the total chlorine concentration, and 1.5 with respect to the total chloride concentration (Viñals et al., 1995). In US Patent 7,682,420 B2, a leaching process via chloride ions and ferric ions was proposed to leach gold from a copper sulfide ore containing gold or a silicate ore containing gold (Abe et al., 2010). In their study, the ore was first leached to reduce the copper content down to 7.9% or less, and then the leaching residue was treated by the leaching solution containing chloride ions and ferric ions. The major advantages of this process are the high gold extraction, the low oxidation and reduction potential required, and the low reagent consumption. Moreover, Hasab et al. (2014) extracted 96% gold from a highly refractory pyritic concentrate by a three-stage chloride-hypochlorite leaching, followed by a hydrochloric acid washing. 22PDF Image | HYDROMETALLURGICAL TREATMENT OF E-SCRAP
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