Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 555 of 707

Fig. 10. Electrolytic metal recovery schematic. Ion exchange is a chemically driven separation process. It is an ideal and useful separation method for collecting low concentrations of ionic materials, such as metal salts, from dilute rinsewater. This characteristic differentiates it from all of the previously discussed methods where relatively low flow rates and high con- centrations of recoverable materials must be maintained. From a recovery standpoint, ion exchange is not capable of producing a "highly" concentrated stream for recycle (20-25 g/L is a practical limit). It is also difficult to optimize the split between recovered metal salts and excess regenerant acid, which is intolerable in the plating bath. Also noteworthy is the fact that a waste stream containing excess regenerant must be dealt with, as shown in Figure 8. Ion Exchange NONRECOVERY METHODS Nonrecovery or indirect recovery methods do not return concentrate to the originating process; thus, they obviate any concern over accumulation of impu- rities or the primary metal in the bath. The result is a "decoupling" of the recov- ery process from the basic manufacturing operation, which may be a consider- able benefit if downtime or process upsets cannot be tolerated. A general schematic is given in Figure 9. In certain instances, these nonreturn processes may also allow recovery from process bath losses other than drag-out (i.e., purification losses or plating bath desludging waste). This is in sharp contrast to the previous category of recovery methods, which can actually increase losses to purification or sludge removal oper- 554

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