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gold, silver, and palladium. For instance, the gold content in PCBs is 35–50 times higher than gold ore (Syed, 2012), even though it has been noticed that the gold content in printed circuit boards is decreasing (Cui et al., 2016). Table 2.1 Comparison of potential leaching reagents for base metals (Tuncuk et al., 2012; Yazici et al., 2014; Yazici et al., 2015; Zhu et al., 2012, Cui et al., 2016)) Sulfuric acid Chloride Aqua regia Ionic liquids Pros Highly selective, low reagent cost, well established process for copper ore Fast kinetics at room temperature, high solubility and activity of base metals, low toxicity Fast kinetics, effective Thermally stable, environmentally friendly Cons At elevated temperature, corrosive Excessive corrosion, difficult electrowinning of copper, poor quality of copper High reagent cost, highly corrosive, low selectivity High cost, excessive dosage Extraction of precious metals from PCBs includes leaching, purification and recovery. Precious metals are noble in the normal environment and require high oxidation potential during leaching. The most common leaching reagents for precious metal leaching include cyanide, thiourea and thiosulfate because they form stable metal complexes (Tuncuk et al., 2012). Table 2.2 and Table 2.3 summarize the alternatives to cyanide and make a comparison between several common reagents for gold extraction. Senanayake (2004) summarized a series of equations (Equation 2.3-2.7) to illustrate the mechanism of gold complex formation regarding different lixiviants, and also showed the linear correlations of stability constants of Au(I)-complexes, following the order: CN− > HS− > S2O32− > SC(NH2)2 > OH− > I− > SCN− > SO32− > Br− >Cl− (Senanayake, 2004) 4Au + O2 + 2H2O + 8NaCN = 4NaAu(CN)2 + 4NaOH 2Au + H2O2 + 4L− = 2Au(I)L2 + 2OH− (L = Cl−, S2O32−, SC(NH2)2) 2Au + L2 + 2L− = 2Au(I)L2 (L = Cl, Br, I, SCN, SC(NH2)2) Au + 1.5L2 + L− = Au(III)L4 (L = Cl, Br, I) Au + Cu(II) or Fe(III) + 2L = Au(I)L2 + Cu(I) or Fe(II) 17 (2.3) (2.4) (2.5) (2.6) (2.7)PDF Image | HYDROMETALLURGICAL TREATMENT OF E-SCRAP
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