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Pollution Prevention and Control Technologies for Plating Operations
Section 3 - Chemical Recovery
3.3 VACUUM EVAPORATORS
3.3.3 Applications and Restrictions
Two common configurations for the application of vacuum evaporators to chemical recovery are shown in Exhibit 3-10. In configuration VE-1, a closed-loop process is achieved using a four stage rinse system with the feed from the first rinse being concentrated by the evaporator and directed to the plating bath. A solution purification step is shown, which removes bath contaminants that would buildup in the bath due to the closed-loop process. Typical solution purification technologies used for this purpose include ion exchange and/or carbon filtration. PS 125 employs this configuration using a cation exchange unit to remove contaminants from its decorative chromium plating rinse water/drag-out. PS 124 has a similar arrangement. PS 082 installed a cation exchange unit and electrolytic purification unit that are connected to a storage tank. The use of multiple-stage rinsing is nearly always required with evaporator applications in order to minimize the quantity of water to be evaporated. The survey respondents used a minimum of two and a maximum of four rinsing stations. The second configuration (VE-2) shows an open process, where a small portion of the drag-out is not recovered. Also shown in VE-2, is a direct bleed from the bath to the evaporator. This may be required for ambient or low temperature baths, where there is a limited surface evaporation rate and insufficient "head-room" in the plating tank to return the concentrated drag-out/rinse water.
Vacuum evaporators are applied to the recovery of a wide range of plating solutions. They are especially applicable in situations where atmospheric evaporators are either technically or economically impractical. This includes: (1) the recovery of heat sensitive chemicals (e.g., cyanide plating baths); (2) the recovery of chemicals that are sensitive to air oxidation (e.g., cyanide plating baths or the stannous tin bath); (3) low or ambient temperature plating solutions where there is no appreciable surface evaporation; (4) the recovery of solutions that contain volatile components; and/or (5) where high evaporation rates (e.g., >20 to 40 gph) are necessary to achieve recovery and atmospheric evaporators become too expensive (i.e., energy cost) to operate (ref. 299).
The results of the Users Survey and Vendors Survey showed that vacuum evaporators are applied to a range of plating and finishing solutions. These identified applications are shown in Exhibit 3-11.
Although vacuum evaporators may provide an energy savings over the atmospheric types of evaporators, neither one is economically practical to purchase or operate where large volumes of low concentration solutions are involved. In those cases, ion exchange or reverse osmosis are the more cost effective methods of recovery (ref. 299).