Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 86 of 707

mathematical techniques designed for the solution of simultaneous equations; but today, with the ready availability of personal computers, recursion (a fancy name for trial and error) will quickly yield the answer directly from the above equa- tions without complicated mathematical manipulations. Note that our analysis is for a theoretical counterflow arrangement. Real- world experience indicates that many counterflow rinse tanks do not operate prop- erly in that the placing of a load of work into the initial rinse impels a backwash of relatively dirty water into the final rinse. Care must be exercised to ensure that the gravity head is sufficient, and that the piping is free of air locks and large enough to ensure that the actual rinse tanks function according to the theoreti- cal model. Where counterflow rinsing is desired, but the requisite gravity head absent, it is possible to convey the rinsewater from one tank to another with a simple device called an air lift, which creates an artificial gravity head through the principle that highly aerated water is significantly lighter than an equivalent volume of normal rinsewater. Where space constraints or production realities preclude adding additional rinse tanks, it may be possible to install a bank of spray nozzles above the rinse tank to serve as a second rinse. To adhere to the counterflow principle of fresh water being used only in the final stage, the spray should be activated only while the work is exit- ing from the rinse tank. IT'S ALL FOR NAUGHT Because in an equilibrium situation, "SALT IN = SALT OUT," none of the illus- trated rinse tank arrangements accomplishes anything toward changing the fact that all of the salt dragged out of the process tank eventually goes on to treat- ment. In all cases, the only thing varying is how much water is consumed, but min- imizing waste and the amount of metal requiring treatment is usually an impor- tant goal, and is best achieved through actions directed at preventing the solution from being dragged out of the process tank in the first place. Slow withdrawal of the workpieces from the process tank, and ample drainage time, are keys. Racking the parts to minimize pockets, and tilting the pieces to encour- age drainage, will prove beneficial. Orienting the parts so that the low point is a cor- ner, rather than an edge, slashes drag-out. Studies have shown that small flat plates dragged out only one sixth as much solution when racked diagonally. Maintenance of plating racks is important so that bubbles in the coating do not exaggerate drag-out. Poorly maintained racks may drag out more solution than the work itself. In barrel plating, specifying a large perforation size will aid drainage. It is the nature of the plastics used in the construction of plating bar- rels, when subjected to the tumbling action of the work, to peen over and block the perforations. An investment in replacement barrels, or labor to redrill the holes, will probably prove beneficial. RECOVERY SYSTEMS All of the solution that has left the process tank is not necessarily lost. It may be pos- sible to return a portion of the solution, or at least its metal value, to the process tank. One way to do this is with a drag-in/drag-out tank, which consists of a stagnant rinse tank that the work enters immediately before and immediately after the process tank. The concentration in this tank approaches an equilibrium 85

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