Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 556 of 707

Fig. 11. Combined ion exchange and electrolytic metal recovery system schematic. ations by increasing the frequency with which they must be performed. Electrolytic Metal Recovery In the metal-finishing industry electrolytic metal recovery (EMR) is both a use- ful and a familiar electrochemical process technique that applies special elec- troplating equipment to reduce the concentration of dissolved metals in many types of process solutions such as plating rinse water and dumped baths. Removing metal in solid form avoids the need to treat and convert the metal con- tent of such process solutions to sludge. In the mining industry, EMR is referred to as electrowinning. Recent advances in EMR cell design now make it possible to reduce the met- al concentration of spent electroless baths and rinsewater prior to waste treatment and to recover metal from chloride or ammoniacal etch solutions while con- currently regenerating the etch baths. There are three common embodiments of EMR in commercial use in the plating industry: 1. "Extractive" methods, which aim primarily to remove the metal from the recovery rinse but with little regard to byproduct value, are depicted in Figure 10. One of these deposits the metal on a sacrificial plastic starter cathode. The cost of the starter cathode and the undesirability of intro- ducing plastic to a smelter or secondary recovery operation are a sig- nificant offset to any resale value of the metal. Another type of extractive cell produces a spongy or powdery deposit, which is removed as a sludgelike material (usually from the bottom of the recovery cell) and is usually of little or no value. The high surface area of the powder exposes a significant portion of the metal to oxidation. The 555

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