Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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on part carriers result in incon- sistent part grounding and high- er rejects. An in-line salt bath eliminates the costs associated with these problems. Part Salvage Costs: A molten salt bath system can be designed to permit parts to be salvaged in the same bath used to strip racks.This eliminates the need for additional cleaning equipment and lowers reject rates. Outsourcing Costs: With process- Fig. 3. Dual-purpose salt bath stripping system for stripping racks and salvaging parts in the same salt bath. (Continuous, in-line stripping of paint racks and hangers at upper left; batch stripping of parts for salvage and repainting at upper right. es like burn-off ovens, it general- ly is necessary to outsource paint stripping operations. Molten salt baths allow this task to be per- formed in-house, usually result- ing in significant cost savings. Process Chemical Costs: The two factors that contribute to process chemical consumption are the amount of paint stripped and the small amount of drag-out that oc- curs when work is removed from the bath. Both factors can accurately be predicted to support cost analysis. Energy Costs: Most molten salt bath systems are gas heated (electrically heat- ed systems are available). Steady-stage BTU consumption is about the same as for a large residential furnace, i.e., 100,000 to 150,000 BTUs. At $0.55/therm, the average energy cost for a 4 x 6 x 4-ft deep salt bath operating at 650°F is about $0.68/hr. This is below the energy consumption of a similar size water solution system operated at or near its boiling point. Because there are no sol- vents or diluents used in a molten salt bath, there are no evaporative losses or the associated energy penalties. By product Handling Costs: By products remaining in a salt bath are 100% inor- ganic and water soluble. They are easily and economically treated for disposal or can be used to treat plant waste streams. ENVIRONMENTAL IMPACT Water A water quench/rinse is an integral part of a molten salt bath system. It is used to cool parts and to remove a thin salt film that remains on parts after stripping. The salts in this film are water soluble and quickly go into solution. Since most stripping formulations contain alkaline materials the quench/rinse water quick- ly rises to a pH of 11 to 12. Usually, pH adjustment with a mineral acid is all that is required prior to discharge. If a plant has a central waste treatment facility the alkaline rinse waters may be used to neutralize acidic waste streams from oth- er finishing operations. (If the coatings being stripped contain heavy metal pig- ments, the quench/rinse water may also contain these metals; the water should 263

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