Metal Finishing Guide Book


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The primary purpose of the RO system as a pretreatment for Company XYZ is to remove the hard water ions such as magnesium and calcium before this water is used in the various plating line tanks. If the city water was used directly in the plating rinse tanks, the magnesium and calcium would be removed by the ion-exchange system but would needlessly reduce the life of the ion exchange resins. Figure 2 shows the RO system at Company XYZ. It should be noted that a typical RO system is about 50% efficient since it relies on pressure to push the pure water through the RO membrane (against the osmotic pressure), leaving the hard water ions behind. Therefore, 100 gallons of incoming water produces about 50 gallons of Figure 4. DI system for one of the plating rinse tanks. low ion water and 50 gallons of high ion wastewater. Each rinse tank and the chrome exhaust scrubber have dedicated sets of ionexchange columns (DI) to remove dissolved metals and other ionic impurities as the water in each system recirculates through the tank and the columns. The pump on each DI tank is very small, resulting in low electrical use. The DI units start with particulate filters, followed by carbon filters for particulate and organic material removal. Next, water is passed through the DI columns to remove the dissolved metal ions and other cations and anions. These columns eventually become saturated with ions and must be sent out for regeneration where the ions are stripped off the active sites on the DI resin and are ready for another cycle of use. Only the DI columns used for the chromium and copper plating rinses go out as hazardous waste compared to the previous process, where the sludge from all the tanks went out as hazardous waste since there was no rinse water segregation. (Figure 3 shows schematically how the active sites on the DI resin beads act to pick up anions or cations from the rinse water.) The DI systems remove the metal ions from the plating tank rinses and the metal ions and salts from the cleaning rinses and acid rinses. Regenerating the ion exchange columns is the means of removing the metals from the resin columns and allows the columns and resins to be reused. There are transportation and treatment costs associated with each column regeneration, and costs approximately $300 per DI column regeneration. Figure 4 shows one of the skid-mounted DI systems at Company XYZ used for one of the rinse tanks. Since the column regeneration costs are a major portion of the new system's operating cost, conservative estimates were used to determine the DI tank life. All the DI tanks have lasted longer than the estimates. There was one start-up issue that caused the scrubber DI tanks to have a much shorter life. Fine "silt" from the scrubber was being flushed out during start-up and caused physical plugging of the DI tanks. After the initial purge 676

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