Metal Finishing Guide Book


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environmental controls WATER POLLUTION CONTROL FOR PAINT BOOTHS BY ALAN MONKEN As a nonrenewable resource, water and its conservation are of prime importance in the metal finishing industry. The reduction and control of water pollution in the manufacturing process is an important area for improvement; one of the most critical areas of industrial water usage is the paint shop. Current technology can be utilized to reduce water consumption and improve the efficiency of water use. To put this technology in perspective, it is necessary to explore the use of water in the paint shop, the available chemical means to deal with water pollutants, and the mechanical means of removing these pollutants. THE PAINT SHOP The function of the paint shop is to apply an organic coating (i.e., paint) to a substrate (i.e., metal or plastic) for protective and decorative reasons. The paint can be applied in various forms, including dry powder, solvent-diluted formulations, and waterborne formulations. The application method can vary widely, two of the most common being through spraying or through immersion. In the case of immersion or dip-paint systems, very little paint waste is generated. The drawback to immersion painting, however, is that variations in paint colors and types are severely limited within the same operation. Spray systems allow a great deal of flexibility in the types and colors of coatings that can be applied. The downside of spray systems is that not all of the paint sprayed comes into contact with the work surface. The paint that misses the production part is commonly known as overspray. If the process being used is powder painting, the oversprayed paint can most typically be collected and reused (the ability for capturing this excess powder typically is designed into the powder paint spraying system or booth). If the process involves liquid paint, however, the paint overspray quickly changes from an asset to a liability as it becomes paint waste. Although it is possible to collect this oversprayed paint on dry-filter media, the most common collection/removal method is the use of the waterwash paint booth. WATERWASH PAINT BOOTHS The primary function of a paint booth, whether wet or dry, is to remove the paint overspray from the air of the work environment; secondarily, it functions to remove the paint solids from the air stream, allowing any volatile solvent vapors to be expelled from the work area. Dry-filter booths make use of media sometimes resembling conventional furnace dust filters to screen out the tacky paint solids,which are actually the organic portion of the paint responsible for providing the coating. This media can quickly plug, reducing the effectiveness of the removal process. The media, once saturated with paint, is disposed of, typically as flammable waste. Waterwash booths perform the same function but use water as the medium of capturing the paint overspray and the resultant waste material. Although there is a wide variety of variations in waterwash paint booth styles and types, the two basic categories of design are side-draft and downdraft booths. Draft here refers to the way the air movement is directed through the system to draw the paint overspray in for capture. Side-draft booths, most common in small noncontinuous metal finishing and manufacturing operations, typically function by pulling a mixture of paint overspray and air through a mobile water 658

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