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564 Yanhua Zhang et al. / Procedia Environmental Sciences 16 (2012) 560 – 568 Research by Wu [34] also showed that selectively leaching gold and silver from PCB scrap in acid thiourea solution is possible and effective. The optimal leaching condition was the ratio of solid to liquid is 1:10, stirring condition 300 rpm, leaching temperature 20 - 25 , leaching time 1 h, thiourea concentration 12 g/L, Fe3+ concentration 0.8%, pH =1.5. The leaching yields of gold and silver were 91.4% and 80.2%, respectively. Results of the experiment of leaching gold from waste PCBs with thiourea by Zhong [35] showed that suitable leaching conditions are as follows: solid-liquid 1:5, leaching temperature 35 , thiourea concentration 10 g/L, Fe3+ concentration 0.3%, H2SO4 concentration 5%, leaching period 1h. Comparing with cyanide leaching, thiourea leaching gold has the advantages of quick leaching speed, low-toxic, high efficiency, environmental friendly and less interference ions. Commercial application of thiourea leaching has been hindered by the following three factors [36]: (1) it is more expensive than cyanide; (2) its consumption in gold processing is high because thiourea is readily oxidized in solution; and (3) the gold recovery step requires more development. However, it was implied that the high costs attached to leaching are likely due to the thiourea process still being in an infancy stage. 2.4. Thiosulfate leaching There are two kinds of thiosulfate commonly used in leaching gold, one is sodium thiosulfate, the other is ammonium thiosulfate. Gold and thiosulfato will form a stable complex with the presence of oxygen. Thiosulfate is stable in alkaline medium, because tetrathionate, the oxidation product of thiosulfate, will turn into thiosulfate again under alkaline conditions in about 60%. But the pH of the solution cannot to be too high, otherwise it is prone to disproportionation reaction turning thiosulfato into sulfidion. The behavior of cathodic process of leaching gold with thiosulfate has been investigated by Jiang [37]. Results show that ammonia reacts with gold ions on anodic surface of gold; the formed Au(NH3)2+ is substituted by S2O32- after entering solution to form more stable Au(S2O3)2-. The kinetics of sodium thiosulfate leaching gold was studied by Moore [38], results show that it is linear relationship between gold solubility and temperature when temperature between 45 to 85 . Taking into account that thiosulfate will be prone to decomposition at high temperature, the suitable temperature is between 65 to 75 . The kinetics of electrochemical reaction of thiosulfate leaching under ammonia medium was researched by Heath [39], results show that leaching rate depends on the concentration of thiosulfate, Cu2+ and ammonia. The higher the concentration of reagents, the faster the leaching reaction goes. Yen [40] found that the dissolution of copper minerals is the main reason for the increase of thiosulfate consumption. Thiosulfate leaching process, whose solution is an ammoniacal solution, is suitable for handling gold mine rich of alkaline components, especially for gold ore or gold concentrate containing copper, manganese and/or arsenic which is sensitive to cyanide leaching. Thiosulfate leaching has the advantages of high selectivity, nontoxic and non-corrosive. The principal problem with thiosulfate leaching is the high consumption of reagent during extraction. It is reported a loss of up to 50% of thiosulfate in ammoniacal thiosulfate solutions containing copper. High reagent consumption renders most thiosulfate systems uneconomical overall, in spite of their potential environmental benefits.PDF Image | leaching precious metals from waste printed circuit boards
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