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Yanhua Zhang et al. / Procedia Environmental Sciences 16 (2012) 560 – 568 563 The recovery of gold and copper from waste PCBs with H2O2-H2SO4 solution was studied by Zhu [30]. The influence of concentration and amound of sulfuric, solid-liquid ratio, the amound of hydrogen peroxide and reaction time on the reaction were investigated. The result indicated that it is unfavorable for recovering rate of gold and copper when acidity is too high or too low. For 10 g waste material, under the condition of 1:2 solid-liquid ratio, 20 ml hydrogen peroxide 20 ml sulfuric acid (1:3), all of the gold and copper were almost recovered. Desquamation rate of gold and recovering rate of copper reaches 98.75% and 99.43%. 2.2. Cyanide leaching Cyanide as lixiviant for gold has dominated in the mining industry for more than one century for its advantages such as cheaper reagent, less dosage, operating in an alkaline solution. Its principle is to dissolve gold and silver of the PCB surface into the solution by alkali metal cyanide, and then to recover gold and silver by reduction from the cyanide solution. Cyanide leaching is only effective to leach gold and/or silver at the surface of PCBs. Those gold and/or silver wrapped inside PCBs are hard to dissolve by cyanide process [31]. Although cyanide leaching is still dominant in the gold mining industry, plenty of wastewater containing cyanide is produced in the leaching process, which is harmful for both operators and surrounding environment. And cyanidation has a long production cycle for its slow leaching rate. Therefore, metallurgical researchers have provided multiple non-cyanide leaching processes in recent years, and some of these processes have made significant progress that they are almost to be used in commercial production. Currently, more attention has been paid to the study of several non-cyanide leaching process, including thiourea leaching, thiosulfate leaching and halide leaching. 2.3. Thiourea leaching Thiourea was first synthesized in 1868, also known as sulfurized urea. It is an organic complexing agent with reducibility, being able to form white crystal of complexes with many metal ions. Under acidic conditions, with the presence of oxidant (usually Fe3+), thiourea and gold will form soluble cationic complexes. Comparing with other leaching reagent, thiourea shows much poorer stability. It is easy to decompose to sulfide and cyanamide in alkaline solution, and cayanamide can be further converted to urea; and it is easy to be oxidized into disulfide formamidine and elemental sulfur and other various products in acidic solution. Therefore, it’s very important to select suitable oxidant and the oxidant concentration so that gold is oxidized as much as possible into the solution but thiourea is oxidized as little as possible. Although disulfide formamidine, as the product of the initial stage of oxidation, is help to accelerate leaching rate. However, with the increase of disulfide formamidine concentration, it is prone to irreversible decomposition to generate elemental sulfur, which will form a stable passivation layer on the surface of gold particles, hindering gold dissolution. At the same time, a great deal of chemical reagent is consumed. That is the very reason why acidic thiourea leaching has not been used widely and industrially up to now [32]. Thiourea leaching gold and silver from waste circuit boards was carried out by Xu [33], and the influence of leaching time, react temperature, thiourea concentration, Fe3+ concentration and material particle size on gold leaching rates were investigated. The results showed that, gold and silver leaching rates can reach 90.87% and 59.82% respectively under the best conditions.PDF Image | leaching precious metals from waste printed circuit boards
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