Metal Finishing Guide Book


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the forming process or handling; cleaning of the parts or assembled product with oils, greases, and other soils present due to the forming processes (such as drawing compounds and lubricants); and conversion coatings (such as iron and zinc phosphates), which are applied to promote enhanced paint performance and provide corrosion inhibition. A similar sequence of events is used in plastics manufacture, with cleaners, alkaline and acidic, used to remove shop soils and mold release agents, and conditioning agents applied to promote better paint adhesion. Because most of these processes are aqueous-based, a number of opportunities exist for contaminants to enter the waste stream. From the derusting or pickling operations, extremely low pH solutions, often high in iron and other dissolved metals, require eventual disposal. Alkaline cleaner solutions contain surfactants, which are present to help remove/disperse oils and greases but can themselves add to the organic pollutants requiring removal in waste treatment. In addition ,the more alkaline caustic-based cleaners require pH neutralization when treated for disposal. These cleaners may also contain chelants, which are chemical compounds present to tie up metal fines and particulates in the water solution. When sent to treatment, these chelants may prevent the easy precipitation of metals. The tank solutions of alkaline cleaners will also contain high levels of oils and greases coming from the drawing compounds, etc., being removed as soils. Conversion coating baths are typically at a low pH during use. Depending on whether the process is iron or zinc phosphating, there will be a high concentration of that particular metal when the tank is dumped; in either case, there will be a large amount of phosphates, both soluble and insoluble (in the form of sludge). In cleaning and prepaint treatment systems one of the most important process steps is the clear water rinse. These rinses may be continuously overflowed or recirculated, or a combination of the two.The rinse stages will gradually become contaminated with the same materials as the chemical process stages due to carry over and drag-out from stage to stage. The final stage of a multiple stage washer often is used to apply a rust inhibitory material or other final sealing rinse material. These treatments can include chromium, zinc, and other exotic metals, which may require special treatment for removal. Paint Operations Once the formed parts are cleaned and pretreated, they are ready for painting. Sprayed liquid paint is applied in an apparatus called a spray booth,which is typically a water system. Although these booths are closed systems, with the sludge removal taking place at the booth site, there maining water from cleanout of the booth is often pumped directly to waste treatment for disposal. This water may have a high pH (if caustic-based detackifying chemicals are in use) or high dissolved solids (if a polymer system is in use). Surfactants, miscible solvents,and other debris may also be present. When water-based paints are sprayed, it may be more practical to continuously cycle the dispersed paint-and-water mixture directly to waste treatment. The waste treatment scheme has to be adjusted to account for this other material. ADDITIONAL SOURCES OF OPERATIONAL CONTAMINANTS There are a number of other processes, which may be in use inorganic finishing operations that will significantly impact waste treatment. 709

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