Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Figure 5. Contaminant concentration in two-tank and three-tank counterflow rinses. for the critical final rinse, but the same rinse water is reused for the initial and least critical rinse. In Figure 2, if counterflow rinsing was the only additional water- saving method used, there would be a 50% reduction in water use (6 gpm) com- pared to Figure 1 (12 gpm). The second, less commonly used method of reducing water use is called reac- tive rinsing. It is a method of taking rinse water around a process tank to a pre- vious rinse tank. The example in Figure 2 shows acid rinse water (acid rinse 1) flowing to the last alkaline rinse tank (alkaline rinse 2). The acid contained in this rinse water would normally be sent to waste treatment. With reactive rinsing, the acid from acid rinse 1 now goes to alkaline rinse 2 and neutralizes the residual alkalinity in that water. Any rinse water from alkaline rinse 2 being dragged out by parts and racks to the acid tank will now contain acid which previously would have been wasted. Therefore, no acid is being neutralized by alkaline dragout to the acid tank, and acid previously lost in acid rinse 1 now has some recovery by the reactive rinse flow. Figure 2 has a total water use of 3 gpm compared to the original flow rate of 12 gpm. The cost savings is $5,400 per year at $5/1,000 gallons for an 8-hour-per-day, 50- week operation, if 9 gpm is saved. Oftentimes, when the rinse appears to be inadequate, companies assume that the best method of improving an immersion rinse is to increase the flow rate. However, rinse flow rates can be deceptive in that high flow rates might not be as helpful as expected. Figure 3 displays rinse tank concentration over time at var- ious flow rates. The initial conditions are: 100 gallon rinse tank, incoming (dragout) solution concentration of 100 grams/gallon, and a dragout volume per rack of 0.05 gallons. It is apparent from Figure 3 that the rinse tank does not dilute the dragged-in chemical very rapidly. Even the 25 gpm flow rate takes 566

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