Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 83 of 707

Fig. 2. Double-stage rinse. tion. Then someone tells us that they hate to break this bad news ... but years ago when the experiment was started they had forgotten to add the 1 part plating solu- tion; effectively, we had started with 1,000 parts water instead. Our intuition tells us that the concentration in the overflow today can't possibly be dramatically influenced by the starting conditions all those years ago, and our intuition is right: the system long ago reached a condition so close to equilibrium that the differ- ence is unmeasurable. This is what we mean when we say a process approaches an equilibrium con- dition; it really doesn't matter whether the vat originally contained the desired 1:999 mixture, or was pure water, or was straight plating solution. If we have added liquid in the ratio of 1 part plating solution to 999 parts water for a substantial enough period of time, then for all practical purposes the concentration in the vat is 1 part to 999, regardless of the initial conditions. This is the real-world sit- uation in the majority of rinsing applications. SALT IN EQUALS SALT OUT Mohler succinctly described the equilibrium condition for a rinse tank with one simple equation whose logic is irrefutable: SALT IN = SALT OUT The chief mechanism which brings SALT IN to a rinse tank is the clinging film of solution, which adheres to the workpiece when it is removed from the process 82

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