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CHAPTER 3 HALIDE LEACHING Using a halide system provides the possibility of direct leaching and recovery of precious metals. Chlorination was extensively introduced to the gold extraction industry in the 1800s, prior to cyanide leaching, for the treatment of gold sulfide minerals and refractory gold ores. Two other important halide leaching reagents are bromine and iodine. Generally, the major advantage of halide reagents in the gold leaching is their powerful oxidizing ability, leading to a high dissolution rate compared to alkaline cyanide leaching. 3.1 Overview of Halide Leaching of Gold The group of chemicals referred to as halides is composed of the elements fluorine, chlorine, bromine, and iodine. Among them, given that fluorine cannot dissolve gold, chlorine, bromine, and iodine are the three common halides that are studied extensively to dissolve precious metals. Basically, a halide dissolves gold in aqueous solutions to form soluble Au(I) and the more stable Au(III) halide complexes. The reaction of gold with halogens (chlorine, bromine and iodine) is expressed as follows: (Kelsall et al., 1993). Au + 2X- = AuX2- + e (3.1) Au + 4X- = AuX4- + 3e (3.2) X = Cl, Br, and I Given that both Au(I) and Au(III) are B-type metal ions, the stability of their complexes tends to decrease as the electronegativity of the ligand donor atom increases. This leads to the stability orders of gold halogen complexes such as I- > Br-> Cl- (Nicol et al. 1992). However, the stability of gold halide is dependent on the solution pH, halide concentration, solution oxidation potential and the presence of reductants (such as metals) in the material (Aylmore, 2005). Furthermore, to avoid the re-precipitation of metallic gold from the solution, it is necessary to maintain a high solution potential. 3.2 Chlorination Prior to the introduction of cyanidation, chlorine was widely used as a leaching reagent to treat gold ores and concentrates (Marsden et al., 1992). The oxidation of gold in chloride solutions 21PDF Image | HYDROMETALLURGICAL TREATMENT OF E-SCRAP
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