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Yanhua Zhang et al. / Procedia Environmental Sciences 16 (2012) 560 – 568 565 2.5. Halide leaching Halide leaching includes chloride leaching, bromide leaching and iodide leaching. Their common merits are high leaching rate. Gold forms both Au+ and Au3+ complexes with chloride, bromide and iodide depending on the solution chemistry conditions. Only chlorine/chloride has been applied industrially on a significant scale of the halides. But chloride leaching requires special stainless steel and rubber-lined equipments to resist the highly corrosive conditions. The vapor pressure of bromine is 10kPa at 0 and 28kPa at 35 . Special equipment is required for safety and health risks in bromide leaching process, and that restricts it from industrial application [41]. Iodide leaching has the following advantages: (1) quick leaching; (2) good selectivity, less leaching of base metal; (3) easy to regenerate iodide, iodine being reduced while recovering gold in anode region; (4) no corrosion, for iodide leaching in a weakly alkaline medium; (5) no toxicity. Moreover, the complexes formed by gold and iodine are the most stable complexes formed by gold and halogen [42]. Research on iodide leaching gold from discard PCBs was carried out by Xu [43]. According to the result, the rational parameters for iodide leaching gold are 1.0%~1.2% iodide concentration, n(I2) : n(I-) =1:8 ~ 1:10, H2O2 concentration 1%~2%, leaching time 4 hrs, solid to liquid 1:10, leaching gold at normal temperature (25 ), solution pH=7. Gold leaching rate can reach at about 95%. Iodide leaching is a promising technology in non-cyanide leaching process. However, iodide leaching consumes a great deal of reagent, and iodine is relatively expensive, and efficiency of electrolytic deposition of gold needs to be improved. All these problems must be solved before iodide leaching gold is applied in industry. 3. Comparison of various leaching processes As one of the most popular methods for multi-criteria decision-making, the analytic hierarchy process (AHP) has been successfully used in a variety of fields [44-47]. Here, three-scale AHP is applied to determine the weight of evaluation indexes of different leaching methods. Based on the basic score of each leaching method offered in Table 2 and diagram of hierarchical structure in Fig. 2, comparison matrixes are given in Table 3 and judgment matrixes in Table 4. And then the weight of different indexes are calculated in Table 5. Final score in Table 6 is the basic score multiplied by its relevant weight. Table 2 Basic scores of different evaluation indexes of variou s leaching methods Leaching method Cyanide Aquaregia Thiourea Thiosulfate Chloride Bromide Iodide Economic feasibility Environmental impact Toxicity 0 3 4 4 3 3 5 Research level Reliability 5 5 4 2 4 2 3 Leaching rate 3 4 4 2 5 5 5 Reagent cost Corrosive 5 5 4 0 4 4 2 5 4 0 2 2 3 5PDF Image | leaching precious metals from waste printed circuit boards
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