TEXT FOR OVERHEADS -4

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1 Fig. 6 Associated Costs of Drag-out TEXT FOR OVERHEADS -4 Figures 6-10 ZF2y.y PDF The chart depicts estimated costs associated with each gallon of dragout. To these figures you must add analyzed costs, labor costs, taxes, insurance, and utilities. For a medium sized chrome or zinc plating shop the total cost of operating an end-of-pipe treatment system can be well over $200 per day. This also does not take into consideration the cost of long an short term liability connected with the generated sludges. These costs were excerpted from the Proceedings of the Fourth Conference on Advanced Pollution Control for the Metal Finishing Industry. EPA- 600/ Cincinnati, Ohio Fig. 7 The Dilution Effect of Dragout While dragout of process solution is both harmful and wasteful it has the beneficial effect of removing "poisons" from the plating bath as well as plating salts. A typical example of this is in the contamination of the plating bath solution by impure anodes. The metal dissolved from the anodes to replace metal in solution that has been plated onto the work piece is not usually 100% pure. Continued dissolution of the anodes yields higher concentrations of impurities. The bath will reach a critical or threshold concentration of impurities unless these can be removed. The most common removal method is through dragout. Figure 7 illustrates the results of contaminants building in a tank with and without dragout. The dragout effectively bends the straight line over and holds it down to a constant value. This effect reaches a steady state as dissolved impurities enrich the solution at the same rate dragout removes them. This is known as a "steady state" or the "limiting state". The limiting effect holds for impurities dissolving from the workpiece, the tank walls, heating coils, and the plating salts themselves or in the water; and for impurities produced by the electrochemical process.

2 -y. Fig. 8 Concentration Build up in Stagnant and Running Rinses In figure 8 the interaction between dragin and part rinsing can be seen graphically. The saw tooth pattem of each graph results from the interaction of these to processes. A moment after the part is dipped into the rinse tank, the plating solution starts leaving the tank with the water flow. The concentration of salts rises during the fist few seconds the work is immersed but falls off at an exponential rate until the next piece of work is introduced. When the equilibrium state is reached a horizontal zig zag line is produced. It looks like this because the amount of plating salts being washed off each rack will be removed by the water flow before the next rack is introduced. The average concentration in the tank after a steady state has been reached can be calculated through the use of calculus to find the area under one of the saw teeth and dividing it by the time interval. Fig. 9 Optimized Rinse Tank Design Figure 9 shows the details needed for an optimized rinse tank. Items to be noted include the aeration pipes and their location, inflow and outflow pipe locations, their relation to the flow of work pieces, and the overflow weir. What is not shown but of equal value is the drip boards and drain racks. The drip boards would angle back into the previous tanks and the drain racks overhang the tanks enough to be convenient but not get in the way of the workflow. Fig. 10 Dragout Tank Design Figure 10 illustrates a possible dragout tank design and location. The idea is to locate the tank close to the plating tank to facilitate ease of use. In some cases very large plating tanks can simply be subdivided if the extra space is no longer used. If there is no room for the dragout tank in the layout of the line it can be hung off the front of the plating tanks. Of course this would not work for automated lines.

3 I Treatment Sludge 1 I cu 11 CuCN $1.75 $0.75 $0.50 $3.00 Ni Cr Zn ZnCN 1.50 I I 3.35 I oo oo /r--cd I 1.IO I 2, I Figure 6 Costs Associated with Dragout

4 Figure 6---Concentration Effect in Plating Baths Limiting value: CP (1 + E/x) > Danger threshhold (deposit affected) Fluctuations produce a concentration band Time b Cp = concentration of the substance in the process solution E evaporation rate in gpm x = dragout rate in gallons per unit time Evaporating water -- Dragin Plating Rep1 acemen t water I tank Work pat h b Plating bath dragout

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6 Concentration Build Up in Stagnant Rinse Figure 8 Time,-+ Concentration Build Up in Running Rinse Time

7 Figure 9 Proper Rinse Tank Design

8 Figure IO InstallacLon of a Small Dtagout Tank

9 ONE RINSE TANK 4 P Configuration 1 : Single overflow THREE RINSE TANKS Configuration 5: Three-stage parallel FOUR RINSE' TANKS 4 Configuration 10 Four-stage parallel TWO RINSE TANKS Configuration 6: Three-stage series Configuration 11: Four-stage series Configuration 2: Two-stage parallel Configuration 7: Two drag-out. one overflow Configuration 12: Three drag-out. one overflow Configuration 3: Two-stage series n Configuration 4: 0 out one overflow 4 e drag Configuration 8: One drag-out. two-stage series c a n l c 0 Configuration 9: Drag-in/dragout one overflow Configuration 13: Two dragout two-stage series Configuration 14: One drag-out three-stage series Configuration 15: Drag-in/drag-out two-stage series Note.-Decreasing heights of shading show that metals concenlrations decrease.

10 . " Table 8. Drag-out for Various Rack Configurations Vertical parts, well drained Vertical parts, poorly drained Vertical parts, very poorly drained Horizontal parts, well drained Horizontal parts, very poorly drained Cupshaped parts, very poorly drained to > 24.0 Source: Durney,19&1. Table 9. Flow Rates for Five Rinsing Systems Svstem Single rinse Two rinses in series, equal flow of k h rinse water to each tank Three rinses in series, equal flow of fresh rinse water to each tank Two counterflow rinses, fresh water feed to wcond tank only Three counterflow rinses, fresh water feed to third tank only ' flow(gpmj Source: Durney,1984.

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