Baghouse Dusts

One way to eliminate the generation of hazardous baghouse dusts is to use pigments in paste form instead of dry powders. Pigments in paste form are dry pigments that have been wetted or mixed with resins. Since these pigments are wet, less or no dust is generated when the package is opened. In addition, most pigments in paste form are supplied in

'Confidential source 1985: Personal communication.

ZE.I. Du Pont de Nemours & Co. 1985: Personal communication.

drums that can be recycled, and therefore eliminate the waste due to empty bags. While this method would increase the amount of pigment handling occurring at the supplier's facility, it can be argued that the overall number of handling/transfer points for dry powder would be greatly reduced along with the probability of spills and dust generation. In addition to use of pigments in paste form, scheduled baghouse hopper emptying could be performed to reduce the amount of hazardous waste generated. The major portion of solid waste collected by air pollution equipment in a paint manufacturing shop is typically titanium dioxide and extenders. Hazardous pigments make up only a small fraction of the waste and would only be present during colored paint production. Since many facilities produce colored paints on a non-continuous basis (i.e., once a week or once a month), scheduled cleaning of the equipment to coincide with colored paint production would allow for segregating the waste that contained hazardous pigments from that which did not.

9.133 Spills

Spills are due to accidental or inadvertent discharges usually occurring during transfer operations or due to equipment failures. Spilled paint and the resulting clean up wastes are usually discharged to the wastewater treatment system or directly drummed for disposal. If the plant has floor drains, large quantities of water are used to clean up water-based paint spills. Dry cleaning methods (using sawdust) are employed for cleaning of solvent-containing spills or for water-based spills where floor drains are not available. Source reduction methods are similar to those for off-spec paint, and include increased automation* better worker supervision and equipment maintenance. Also useful are dry cleanup methods such as sawdust and mopping in which the waste is not discharged to drains. By closing floor drains, employees are discouraged from grabbing a hose and washing down the area (Wapora 1975, USEPA 1979). In addition, a large decrease in wastewater is achieved since the floors cannot be routinely washed.

9.13.4 Equipment Cleaning Wastes

Equipment cleaning is responsible for most of the waste generated during the paint manufacturing process. Following the production of either solvent or water-based paints, considerable waste or "clingage" remains affixed to the sides of the preparation tanks. The three specific methods of tank cleaning used in the paint industry are solvent washing for solvent-based paint, caustic washing for either solvent or water-based paint, and water washing for water-based paint. Wastewater and liquid wastes resulting from cleaning are usually collected and used in the next compatible batch of paint as part of the formulation, collected and used with or without treatment for cleaning until spent, or disposed with or without treatment as wastewater or as a solid or liquid waste in drums. Sludges from settling tanks are drummed and disposed of as solid waste. Spent recycled rinsewater is drummed and disposed of a solid waste after the solubles content prohibits further use.

The percentage of solvent-based and water-base paints produced is the most important factor that affects the volume of process wastewater generated and discharged at paint plants. Due to their greater use of water-wash, plants producing 90 percent or more waterbase paint discharge more wastewater than plants producing 90 percent or more solventbase paint. Additional factors influencing the amount of wastewater produced include the pressure of the rinse water, spray head design, and the existence or absence of floor drains. Where no troughs or floor drains exist, equipment is often cleaned externally by hand with rags; when wastewater drains are present, there is a greater tendency to use hoses. Several plants have closed floor drains to force use of dry clean-up methods and discourage excessive water use.

Waste associated with equipment cleaning represents the largest source of waste in a paint facility. Methods that reduce the need or frequency of tank cleaning or allow for reuse of the cleaning solutions are the most effective. Source reduction methods include:

9.13.4.1 Use of Mechanical Devices Such As Rubber Wipers. In order to reduce the amount of paint left clinging to the walls of a mix tank, rubber wipers are used to scrape the side of the tank. This operation requires manual labor and hence the percentage of waste reduction is a function of the operator. Since the benefits will be offset by increased labor, mechanization/automation should be considered. Many new mixers are available that are designed with automatic wall scrapers (Weismantel 1985). These mixers can be used with either flat or conical bottom cylindrical mix tanks.

9.13.4J Use of High Pressure Spray Heads and Limiting Wash/Rinse Time. After scraping the tank walls, high pressure spray hoses can be used in place of regular hoses to clean water-based paint tanks. Based on studies (USEPA 1979), high pressure wash systems can reduce water use by 80 to 90 percent In addition, high pressure sprays can remove partially dried-on paint so that the need for caustic is reduced. Tanks used for making solvent-based paints normally employ a built-in high pressure cleaning system. At Lilly, in High Point, N.C., a high pressure cleaning system was installed in several mix tanks. By continuously pumping a fixed amount of solvent into a tank until it was clean, the overall volume of solvent required for cleaning was reduced (Kohl 1984).

9.13.43 Use of Teflon3 Lined Tanks to Reduce Adhesion and Improve Drainage. The reduced amount of "clingage" will make dry cleaning more attractive. This method is probably applicable only to small batch tanks amenable to manual cleaning.

9.13.4.4 Use of Plastic or Foam Tigs" to Clean Pipes. It was reported that much of the industry is currently using plastic or foam "pigs" (slugs) to clean paint from pipes4. The "pig" is forced through the pipe from the mixing tank to the filling machine hopper. The "pig" pushes ahead paint left clinging to the walls of the pipe. This, in turn, increases yield and reduces the subsequent degree of pipe cleaning required. Inert gas is used to propel the "pig" so as to minimize drying of paint inside the pipe. Piping runs and the launcher and catcher equipment must be carefully designed so as to prevent plugging, spills, sprays, and potential injuries.

9.13.4.5 Scheduling. By scheduling paint production for long runs or by cycling from light to dark colors, the need for equipment cleaning could be reduced.

"^Registered trademark of E.I. Du Pont de Nemours & Co.

4E.I. Du Pont de Nemours & Co. 1986: Personal communication. National Paint and Coating Association 1986: Personal communication.

9.13.4.6 Avoiding Drying of Residue. For facilities using small portable mix tanks for water-based paints, immediate cleaning after use would reduce the amount of paint drying in the tank and hence reduce the need for caustic. Frequently, dirty equipment is sent to a central cleaning operation where it waits until a given shift (usually night) to be cleaned . Meanwhile, residual paint dries up, often necessitating the use of caustic solution for cleaning. Avoidance of accumulation of dirty tanks can be achieved by designing and performing the cleaning operation to handle any peak load continuously. This would reduce the need for caustic cleaning and promote water cleaning instead.

9.13.5 Off-Specification Paint

Most off-spec paint is generated by small shops manufacturing specialty paints. Better operating practices are useful in reducing off-spec products. These practices include better operator training, closer operator supervision, more rigorous equipment inspection and maintenance, and, where feasible, increased use of automated manufacturing systems.

9.13.6 Filter Cartridges

Spent filter cartridges are produced during the paint loading operation. These cartridges are designed to remove undispersed pigment from the paint during loading and are saturated with paint when removed. Hence, waste minimization and economy both call for as small a cartridge as possible so as to reduce the amount of paint lost and the capital spent for the filters. If frequent filter plugging is a problem, then it should be first addressed from the standpoint of improving pigment dispersion, and not from the standpoint of increasing filter area.

Viable alternatives to cartridge filters include bag filters and metal mesh filters. Metal mesh filters are available in very fine micron sizes and they can be cleaned and reused. Since it is very important to minimize all wastes, the issue of mesh filter cleaning waste reuse or recycling would need to be addressed before switching to these filters.

9.13.7 Product Substitution

By altering the composition of paint, its overall toxicity can be reduced. In weighing the environmental pros and cons of toxic pigment substitution with less toxic alternatives, it must be noted that the use of heavy metal pigments in product coatings extends the durability of the product (NPCA 1986). It is also important to note that substitution of pigments in existing formulations may alter both the color and performance of the coating; hence, it must be done with the approval of changed specifications by a customer.

While the paint industry has eliminated the use of lead to a large extent, a market still exists for red lead primers. Most plants produce a wide variety of paints, and typically, red lead primers represent a small percentage of plants' total output A commitment by management not to produce red lead primer would eliminate the need to manage lead pigments. The loss of this market would have to be weighed against handling and disposal costs for each facility. Because of the increasing regulations on the use of all lead pigments, some plants have ceased production of lead primer6.

Chrome yellow is used mainly in traffic paint and provides a very bright color. In order to avoid the use of chrome yellow in other products, some companies have switched to organic pigments, which tend to be expensive, or to yellow iron oxide, which generates less environmental risk, but produces a "dirtier," duller color. One such company is Environmental Inks and Coatings Corporation in Morganton, N.C. (Kohl 1984). A waste reduction commitment by management would be required for this substitution, in light of the added expense versus disposal cost savings. Customer's specifications stand in the way of total elimination of chrome yellow use, since it is required to be used in yellow traffic paint7.

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