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Resources 2014, 3 162 Limitations of Hydrometallurgy Route Hydrometallurgical routes have been successfully used to recover PMs from e-waste. However, these processes are associated with certain disadvantages that limit their application on the industrial scale. Some common limitations of hydrometallurgical methods for recovering PMs from e-waste are listed here [47,55,56]: Overall, hydrometallurgical routes are slow and time consuming and impact recycling economy. There are concerns regarding the economy of hydrometallurgical routes compared to pyrometallurgical processes for the extraction of PMs from e-waste. Mechanical processing of e-waste takes longer to reduce size for efficient dissolution. It is reported that 20% PM is lost by mechanical force during the liberation process that contributes to a significant loss in the overall revenue. Cyanide is a dangerous leachant and should therefore be used with high safety standards. It can cause contamination of rivers and seawater, especially near gold mines, which poses serious health risks to the inhabitants. Halide leaching is difficult to implement due to strong corrosive acids and oxidizing conditions. Specialized equipment made of stainless steel and rubbers is required for leaching of gold using halide agents from e-waste. The use of thiourea leachants is limited in gold extraction due to its high cost and consumption. Moreover, further developments are required to improve the current technology of thiourea-based gold leaching. The consumption of thiosulfate is comparatively higher and the overall process is slower, which limits its application for gold extraction from ores as well as from e-waste. There are risks of PM loss during dissolution and subsequent steps, therefore the overall recovery of metals will be affected. 2.1.2. Pyrometallurgical Processes Pyrometallurgical processes for recovering metals from various waste materials have been used during the last two decades. Smelting in furnaces, incineration, combustion and pyrolysis are typical e-waste recycling processes. State-of-the-art smelters and refineries can extract valuable metals and isolate hazardous substances efficiently. Such recycling facilities can close the loop of valuable metals and reduce environmental impact arising from large quantities of e-waste. Currently, e-waste recycling is dominated by pyrometallurgical routes [57], whereas the steel industry embraces the ferrous fractions for the recovery of iron, and the secondary aluminum industry takes the aluminum fractions. Pyrometallurgical processes work with the steps of liberation, separation/upgrading and purification that are fundamentally similar to those of mechanical or hydrometallurgical routes. However, the liberation of valuable metals is not achieved by leaching, crushing or grinding but by smelting in furnaces at high temperatures. In these processes, metals are sorted by exploiting their chemical and metallurgical properties, e.g., PMs are segregated into a solvent metal phase (copper or lead). The metal fractions separated during the preprocessing of e-waste are composed of Fe, Al, Cu, Pb and PMs. After Fe and Al, Cu and Pb are the main constituents of a typical e-waste. Therefore, it isPDF Image | Metal Extraction Processes for Electronic Waste
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