Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 543 of 707

Fig. 1. General recovery schematic for return methods: evaporation, reverse osmosis, electrodialysis, ion exchange. tical, two factors should be examined: 1. In most cases there is a tendency for harmful impurities to accumulate over time from drag-out return. These impurities can be metals or other cations or anions dragged into the bath. Or, they can be electrolytic breakdown products normally generated during bath operation. Examples of the lat- ter would be the formation of carbonate through anodic oxidation of cyanide or the generation of undesirable organic breakdown products formed through the electrolytic breakdown of brighteners, wetting agents, grain refiners, etc. 2. In baths that use soluble anodes, the primary metal generally has a tenden- cy to "grow" or to accumulate in the bath. This generally occurs because the electrochemical efficiency for anodic dissolution is higher than is the efficiency of cathodic deposition and/or because the bath itself has a solubilizing effect on the anodes during periods of inactivity. In many cases both of these effects are fortunately minimized or controlled by the routine loss of bath through drag-out, filtration, purification, and by the removal of suspended solids and sludge. In some baths, however, such as bright nickel, the accumulation of impurities can be a problem in spite of the normal losses from maintenance and purification procedures. When a high percentage of drag-out is returned by any of the technologies that will be reviewed, it may mean that the accumulation of cationic contaminants will become evident more quickly or more frequently, requiring a purposeful bleed- off of plating bath that is obviously somewhat counterproductive. In regard to impurity accumulation, complete return of drag-out necessitates purifica- 542

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