Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 262 of 331

and bronze. Metals that cannot best ripped include magnesium, which may ig- nite in contact with the oxidizing salt, and low-melting die cast alloys, which may distort at the bath-operating temperature or suffer surface blistering due to high levels of dissolved gases in the coating. Maintenance stripping nor- mally is performed at 800 to 900°F. Salvage stripping normally is performed at 600 to 700°F. The temperatures can vary, depending up on the substrate ma- terial and coatings to be removed. PROCESS CHEMICALS The process chemicals used in a salt bath are a mixture of inorganic salts. These salts are heated to a temperature above their melting point to form the working bath. Because process chemicals contain no solvents or diluents, there are no evaporative losses, fumes, or odors from the molten salt bath when it is idle and at operating temperature. Only during stripping operations are there any gaseous emissions from a bath. As noted, the emissions are CO2 and water. PROCESS EQUIPMENT The basic molten salt bath system consists of a salt bath furnace (a double-wall tank fabricated from mild steel, see Fig. 1), ambient-temperature water quench/rinse tank, and integral hooding. Depending upon application requirements, a user also may specify ventilation equipment, automated sludge-handling systems, and work-handling automation. The size of equipment is dictated by the size of the workpieces to be stripped, the amount of material to be removed, and production requirements. Until recently, most salt baths were stand-alone, batch systems capable of serving multiple coating lines. This no longer is the case. Metal finishers can now purchase salt bath systems designed for integration into a metal-finishing line to provide continuous, inline stripping of hooks and racks following every pass through the finishing sta- tion. (See Fig. 2.) In some in- stances a single continuous sys- tem also can serve multiple coat- ing lines, depending upon the Fig. 1. Typical molten salt bath paint and powder stripping system. 261

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