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562 Yanhua Zhang et al. / Procedia Environmental Sciences 16 (2012) 560 – 568 2. Leaching of precious metals from waste PCBs 2.1. Pretreatment Pretreatment is of necessity prior to leaching of precious metals from waste PCBs including physical pretreatments and chemical pretreatments. Physical pretreatment is to mechanically break and crush PCB scrap after dismantling the electronic components, and then according to the differences of physical characteristics of metals and nonmetals, to use one of the methods, including pneumatic separation, magnetic separation, screening, eddy current separation, and electrostatic separation, to obtain the enrichment of metals and nonmetals. Chemical pretreatment is to dissolve base metals such as copper, lead and tin in solution, leaving precious metals in residue, and then to isolate and purify precious metals from the residue. The commonly used chemical pretreatment is nitric acid pretreatment or sulfuric acid & hydrogen peroxide pretreatment. Fig. 1 shows typical flow chart of recovery of precious metals from waste PCBs. Waste PCB Physical Pretreatments Crushed Metal Enrichment Chemical Pretreatments Sulfuric Acid & Hydrogen Peroxide Copper Solution Recovery of Copper Nitric Acid Leaching Reagent Decoppered Meterials Gold-bearing Residue Gold Solution Chemical Reduce Coarse Gold Refining Pure Gold Silver Nitrate Recovery of Silver Fig.1. Typical flow chart of recovery of precious metals from waste PCBs Mecucci [28] used nitric acid of 1 - 6 mol/L to extract copper and lead from PCBs. Precipitation of tin as H2SnO3 (metastannic acid) occurred at acid concentrations above 4 mol/L. And preliminary galvanostatic electrolysis from simulated leaching solutions had investigated the feasibility of electrodeposition of copper and lead at different concentrations of HNO3. The nitric acid solutions with different concentration were employed [29] for leaching metals from the waste printed circuit boards. The effects of nitric acid concentration on disassembling times of the electric components and dissolving rate of metals were investigated Results indicated that the metals were apt to dissolve in solution with higher nitric acid concentration they all dissolved quickly except Cu which did not dissolve in the leachate when nitric acid concentration was lower than 2.73 mol/L. The dissolution rate of Cu, Fe and Zn could reach to 50%, 80% and 93% when the nitric acid concentration was 4.55 mol/LPDF Image | leaching precious metals from waste printed circuit boards
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