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pushing the development of a substitute that is more effective and environmentally friendly (Cui et al., 2016). Thiourea, NH2CSNH2, is considered a most promising alternative to cyanide regarding the leaching of precious metals due to its fast leaching rate and non-toxicity. Thiourea leaching is conducted at pH = 1.5 and the reaction is shown as Eq. 2.4. The problems with thiourea leaching are its high cost and consumption due to the poor stability of thiourea. Li et al. (2012) examined the thiourea leaching of gold and silver from PCBs as functions of particle size, temperature, and thiourea concentration, as well as Fe3+ concentration. The study indicated that the optimum conditions for gold leaching were 24 g/L thiourea and 0.6% of Fe3+ for 2 h. It is also proved that thiourea is less toxic and highly efficient (Li et al., 2012). Birloaga et al. reported that 69% of gold was extracted under the conditions of 20g/L thiourea, 6g/L Fe3+, and 10 g/L H2SO4, as well as 600rpm (Birloage et al., 2014). Yin et al. (2014) compared thiourea leaching to iodine leaching of gold from PCBs. It appeared that 93.5% gold was able to be leached directly by iodine without pretreatment, while thiourea leaching of gold was carried out after copper leaching, which resulted in the high consumption of thiourea (Yin et al., 2014) Table 2.3 Comparison of potential leaching reagents for gold (Tuncuk et al., 2012; Zhang et al., 2012; Petter et al., 2014, Cui et al., 2016) Cyanide Thiourea Thiosulfate Halide Aqua regia Pros Highly effective, low reagent dosage and cost Less toxic, high reaction rate, less interference ions High selectivity, non-toxic and non- corrosive, fast leaching rate High leaching rate, high selectivity, relatively healthy and safe except for bromine Fast kinetics, low reagent dosage Cons Difficult to process wastewater, environmental risk, low kinetics Poorer stability, high consumption, more expensive than cyanide, downstream metal recovery High consumption of reagent, downstream metal recovery Highly corrosive for chlorine, high consumption for iodine Strongly oxidative and corrosive, difficult to deal with downstream 19PDF Image | HYDROMETALLURGICAL TREATMENT OF E-SCRAP
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