Environmental Impact

Most cleaning salts are formulated from alkali metal salts, as previously described. Some of these ingredients may be highly alkaline and corrosive. Process chemicals formulated for salt bath cleaning applications do not contain restricted or heavy metals in their "fresh" condition.

The byproducts or sludge that must be removed from an operating bath will contain the materials that were processed in it. For example, byproducts formed from paint stripping applications will contain heavy metals if the paints stripped in them contained heavy metals. Likewise, byproducts formed during the stripping of chromium carbide will contain significant amounts of hexavalent chromium (Cr6+) due to the reaction between an oxidizing molten salt and the chromium content in the original coating being removed.

Once byproducts are removed from a bath, they solidify upon cooling into a dense solid. Most sludges are freely soluble in water, allowing subsequent treatment operations to be readily performed. In the absence of heavy or restricted metals, the pH level is often the only adjustment that is necessary. This is usually accomplished by the controlled addition of a mineral acid such as sulfuric acid. In facilities where other metal finishing operations are performed, the alkaline values of the sludge solution are often used to adjust the pH level of acidic streams from other processes. When heavy metals are present in the sludge, more involved waste treatment procedures are necessary. These commonly include reduction of oxidized metal species (for example, the reduction of hexavalent chromium to trivalent chromium, with subsequent pH adjustment, metals precipitation, and filtration/separation of solids). Numerous proprietary and nonproprietary approaches may be used for in-plant treatment of sludge. Sludges may also be disposed of off-site in an approved disposal facility. Representative samples would require testing for corrosivity, restricted metals, and so on, as dictated by applicable regulations, to determine their ultimate disposal classification.

Ultrasonic Cleaning

Jeff Hancock, Blue Wave Ultrasonics

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