For over 15 years, membrane separation processes have been very effectively utilized as essential components of electrodeposition (ED) paint processes. ED is the accepted method of applying the undercoat paint layer to new automobiles and appliances. As cross-flow membrane and ED paint technologies mature, new applications combining the two have been developed. A major American automobile manufacturer now uses reverse osmosis (RO) to treat ultrafiltration (UF) permeate generated by an ED paint process, resolving a waste disposal problem while gaining significant economic benefit through reuse of recovered materials. Laboratory data used to develop and optimize this application are discussed. A case history is presented emphasizing the technological and economic considerations of a UF/RO application.
Electrodeposition Paint Basics
A major automobile manufacturer utilizes a large ED paint operation to apply undercoat paint to car bodies and other automobile parts. ED paint processes apply an electrical current to a large paint bath containing approximately 60,000 gallons of 15-20% dispersed paint solids and 2-4% paint solvents. A current of opposite charge (anodic vs cathodic) is applied to pretreated metal parts which are carried on mobile racks. The parts are slowly dragged through the bath, completely immersing them for 1-2 minutes. The anodic and cathodic charges of the paint bath and metal parts allow a portion of the paint to chemically and electrically adhere to the metal surfaces. This is a particularly effective method of coating an entire auto body with a uniform thickness of paint. A series of spray rinses and immersion rinse tanks are then used to remove the non-adhered paint from the metal parts.
ED Paint Using Ultrafiltration
The design and chemical composition of these rinse tanks are critical to continuous ED paint processing. A significant portion of non-adhered paint is 'dragged out' from the paint bath and is caught in the rinse tanks. The rinse tanks are designed so that a portion of liquid continuously cascades back to the previous tank, and eventually back to the paint bath itself. Hence, the rinse stations, which are usually about the same size as the paint bath, recover excess non-adhered paints from the metal parts which are then cascaded to the paint bath for reuse. The final rinse station, however, uses deionized (DI) water which is drained out rather than cascaded back to prior rinse tanks. The rinse water from the final station, containing mg/L levels of paint and solvents, is drained because addition of this high volume of DI water to the system would significantly dilute the paint bath.
A portion of the initial paint bath fluid is continuously fed to a UF unit, which concentrates the larger paint solids while passing water and paint solvents. The concentrated paint solids are returned to the paint bath and the permeate, which consists of paint solvents and water, is used to feed the first two rinse stages. In this way, the cascading rinse water does not dilute the paint bath. UF permeate is a particularly good initial rinsing agent because it contains these paint solvents. The final rinse station uses DI water to wash the solvents from the auto bodies.
Several cleaning tanks precede the paint bath and strip oils and grease from the parts and pretreat them with phosphate or chrome solutions. The parts carry some dissolved salts from these cleaning tanks into the paint, raising the conductivity of the paint bath over a period of time. ED paint baths require operation within a specific conductivity range. When the conductivity exceeds the pre-established limit, a portion of the UF permeate is drained or 'purged.' The volume of water and solvent lost during purging is replaced by addition of fresh solvent and water to the paint bath. The UF permeate purge, which is typically discharged to sewer or waste treatment, contains 2-4% paint solvents and thus has high levels of biological oxygen demand (BOD), chemical oxygen demand (COD) and volatile organic compounds (VOC). These BOD, COD and VOC levels often lead to environmental waste disposal problems for industrial manufacturers. Figure 1 illustrates a basic flow schematic incorporating UF in the ED paint process.