Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Fig. 5. Recovery from a high-flow rinse. value of 50%, such that each load processed returns half of its drag-out to the process tank. There are many technologies for recovering process solutions or their metal val- ues from the rinsewater, for example evaporation and electrowinning (see chapter elsewhere in this Guidebook on recovery technologies). In most cases, the greater the concentration in the recovery rinse, the greater the efficiency of the recovery process, so that maximizing the concentration in the recovery rinses may be a goal of the rinsing strategy. This usually will suggest decoupling the recovery rinse from the other rinses, rather than employing the counterflow principle because, with the recovery rinse being of a high concentration, a very low flow rate is desirable to minimize the amount of chemical carried off in the overflow. The more dilute final rinses can have a fairly sub- stantial flow without carrying much chemical out of the system. To illustrate this fact, consider Fig. 5, which is the same as Fig. 2 except that evap- orative losses in the process tank make it possible to return one-half of the volume of rinsewater from the recovery tank. Operated in this fashion, and comparing the results with Fig. 2, the benefit of the evaporation is shown to be negligible. In Fig. 6, however, we reduce the flow rate in the recovery rinse so that the con- centration can build up. Then the same small volume of rinsewater returned to the plating tank carries a significant amount of plating salt back with it; using the numbers in our example, the plating solution lost to drag-out is cut by 25%. Note that this advantage was not gained without penalty: although we reduced 86

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