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66 PCB007 MAGAZINE I DECEMBER 2018 dustry standards, which are intended as an in- dicator of regulatory compliance and commit- ment to environmental procedures that follow a reuse-recover hierarchy and seek to reduce wastes to landfills and monitor transbound- ary movement. Resale of components typical- ly offers the highest value and does not gen- erate any new waste. After the initial sort, po- tentially hazardous components such as bat- teries must be removed. Components likely to contain recoverable levels of precious metals should also be targeted such as high-density PCBs, integrated circuits (ICs), memory chips, and PM connectors and pins (Figure 1). Most recyclers serve primarily as collection points and include some type of sorting or shredding operations. They may shred or pul- verize materials before sending them to fur- ther processing operations. A few are vertical- ly integrated and incorporate chemical recov- ery and/or thermal reduction methods to pre- pare marketable metals for recovery. Tight labor markets and rising wages put pressure on e-scrap recyclers to limit disassem- bly and/or rely more on automated methods. Along these lines, Phase II of a trial on a new method for disassembling PCBs is underway with the U.S. Environmental Protection Agen- cy (EPA) and due for completion at the end of February. The equipment uses infrared (IR) to melt solder connections as PCBs travel on a conveyor and are then vibrated to loosen com- ponents [7] . This may improve recovery of re- usable board-level components such as ICs, as well as improve returns by segregating devic- es containing precious metals for refining or further processing or refining. However, sig- nificant copper remains in the substrate with a requirement to process. Downstream charg- es are the same, or in some cases, more since both the per-pound value and attractiveness as a smelter feedstock are reduced. Chemical Methods of Metals Recovery from PCBs Chart 2 shows a generalized method of re- covering metals from recycled PCBs via hydro- metallurgy [6] . This wet chemical recovery pro- cess involves leaching followed by a concen- tration and purification step and may include further metal recovery. Leaching solutions may vary in their oxidation-reduction potential de- pending on the reaction. They may also re- quire additional energy for heating to speed or extend the reaction, and may use chelating agents to extract certain metals selectively. Recovery may proceed in sequence from the top, or leaching processes may be used for tar- geted materials. Leaching agents include nitric acid and hydrochloric acid (aqua regia). They also include salts such as sodium cyanide that form a strong base when dissolved in water. These are all hazardous chemicals and pose health and environmental risks if not proper- ly stored and handled. Sodium cyanide solu- tion reacts violently with acid and can produce highly toxic and flammable hydrogen cyanide gas. Electrowinning selectively separates metals from an ionic solution using electrolysis. For example, with direct current, copper or gold can be electroplated from the solution. Elec- trowinning can be used in the solder leaching step to remove trace hazardous chemicals such as selenium and lead. In the PM recovery process, chloride leach- ing may be used to recover palladium or po- tassium iodide used to recover platinum. Gold and silver are recovered by acid leaching, and the solution is then filtered. Activated carbon adsorption can also be used in a separate step Figure 1: High-value electronic scrap containing precious metals.