Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 265 of 331

be tested to determine its metal content and treated accordingly.) Air Salt bath systems normally are enclosed to contain heat and minimize bath evaporation. They also incorporate air exhaust systems for removing any incidental smoke and fumes during stripping. because of the speed of the stripping process the instantaneous smoke that maybe generated requires high-volume exhaust rates. Air cleaning equipment must be capable of removing the fine particulate solids present in the exhaust stream while handling the volume requirements of the system. This usually is accomplished with wet scrubbers or bag houses. In- line systems normally do not require this equipment. Solid Wastes Reaction products formed during salt bath paint stripping are 100% inorganic. Paint is thermochemically oxidized to water vapor, carbon dioxide, and nitrogen. Inorganics in the paint (opacifiers, filters, and inorganic pigments) usually react with the bath to form their alkaline analog. Some of the reaction products are in their gaseous form at the bath's operating temperature and simply leave the bath. Some of the products continue to react with the bath to form alkali car- bonates. Once the bath becomes saturated with these by products, they start to precipitate as a sludge or solid waste. It is important to note that the bath con- tinues to strip paint even when saturated. The bath does not have to be dumped; it will remain effective through periodic additions of process chemicals. The sludge resulting from the stripping operation is comprised of alkali car- bonates and any inorganic components, which may have been present in the paint. It normally is classified as a corrosive because of its high alkalinity. Restricted metals may be present if these elements were present in the original paint. In the past the sludges from a salt bath were disposed of as hazardous waste in approved landfills. Today, these sludges frequently are point-source treated in- house. The sludges, which are freely water-soluble, may be dissolved in water and then treated. Some systems utilize the alkaline quench/rinse waters for all or part of sludge dissolution. Once in solution, the sludge may be pH adjusted and metal re- duction/precipitation used, if required. After filtration and clarification, the water is usually acceptable for discharge. The solid residue obtained during filtration will not be corrosive, but should be subjected to toxic characteristic leaching procedure (TCLP) testing if heavy metals were initially present to determine its ultimate dis- posal classification. An alternative to point-source treatment is to direct the sludge solution to the plant's central waste treatment facility for beneficial use or for pH adjustment and metal removal. SUMMATION Historically, molten salt baths have been used to strip heavy deposits of coatings from part carriers. New families of difficult-to-remove hybrid paints and the in- creased use of powder coating have broadened applications. Finishing processes, such as electrostatic coating, also have created new demands for 100% coating re- moval because of the need of a good ground between a part and fixture; however, the most significant change in molten salt paint stripping is improved equipment technology. Salt baths now can be integrated into finishing systems to provide continuous, in-line stripping. Dual-function, in-line systems also provide the ca- pability to strip part carriers and salvage finished parts in a common bath. 264

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