The U.S. Environmental Protection Agency’s (EPA) Water Infrastructure Resiliency and Finance Center, in collaboration with the...
Deionization provides quality water for painting method
There are many niche markets for the water treatment dealer to pursue. A multitude of industries rely on a consistent source of deionized water for manufacturing and production, and as a source of makeup water for power generation and process water.
Electrocoating is a painting method used by many manufacturers today. It is an electrodeposition process that applies coatings to conductive surfaces using an aqueous paint bath. The process uses water-based paints and primers, and relies on pure water for the rinsing steps.
The electrocoat (or ecoat) process works by putting an electrical charge on a part and immersing it in a bath with paint particles of an opposite charge. Since opposite electrical charges attract, the part is completely covered with a layer of the paint particles. The beauty of elecrocoating is that the part is completely and evenly coated. The voltage applied during the process controls the thickness of the coating.
The advantages that ecoat has over traditional spray applications of paint is that it evenly coats any irregular shapes, nooks and crannies. The uniform coating is free of drip marks or sag lines. There is no paint booth so there are no problems with overspray and fumes. Also, without paint vapor present, VOCs are eliminated or present only at very low levels. Most times operators do not have to wear special breathing apparatus. Fire hazards are greatly reduced since it is a waterborne system.
The parts to be coated are first cleaned of any oils, greases, polishing compounds, etc., by alkaline cleaners and vigorous rinsing. Different coating chemicals require different degrees and amounts of pretreatment. Some systems require an activator or phosphate dip before the part undergoes it's final rinse before ecoat.
Since the ecoat process is an aqueous-based process, there is no drying off of the parts required after the final pretreatment rinse with deionized (DI) water. The wet parts can go directly into the ecoat bath.
After the ecoat deposition, the parts undergo additional DI water rinses. This may include a static DI rinse, counterflow DI rinse and virgin DI rinse. The static rinse is merely a DI water rinse tank that the part is first passed through to rinse off the bulk of any dragout. The counterflow DI rinses are recirculating rinse tanks that have the freshest (or virgin) DI water entering the final tank. The dragout and initial rinse waters can be cycled back to the paint bath to maintain a high rate of transfer of paint solids to the parts being coated.
In addition to the ecoat rinses, the ecoat paint itself requires DI water as it's primary constituent. A “premix,” high solids paint concentrate is blended with DI water to form the final paint bath. The bath final makeup is in the range of
The final step of the ecoat process is a baking at elevated temperatures. It allows the process to fully cure and provides the ultimate in corrosion protection by melting the resin polymer present in the paint mixture.
The deionized water specifications will vary according to the paint manufacturer's requirements but, generally speaking, DI water with a specific conductance of less than 10 micromhos (equivalent to a resistance of 100,000 ohms) is needed. On most city water supplies, this can be achieved using a two-bed deionizer or a reverse osmosis system.
The paint manufacturer also may have a silica requirement that would necessitate the use of strong base anion resins or mixed-bed resins.
Separate bed deionizers (cation followed by anion) offer the most efficient method of ion removal on low to moderate TDS waters. Consider RO for higher TDS waters. Separate bed resins have higher capacity than mixed beds and also offer advantages of easier regeneration compared to mixed beds.
In a normal operating two-bed demineralizer, the effluent quality--measured as conductivity or resistivity--primarily is indicated by the sodium leakage of the cation unit. The sodium exits a weak base anion unit two-bed system as a salt, thereby having a neutral effluent pH. A two-bed deionizer that uses a strong base anion resin for the second bed will have a slightly higher pH, depending on the sodium leakage, because the sodium ion from the cation unit leakage exits the deionizer as sodium hydroxide.
Weak-base anion resins have high operating capacity and good regeneration efficiency. And since they only neutralize acids, they do not excessively increase the effluent pH returning to the rinse tanks.
Strong-base anion resins offer complete removal of all anions including carbon dioxide and silica. The use of strong-base anion resin results in higher quality water than can be produced by weak-base anion resins. The complete removal capability comes at the expense of operating capacity and regeneration efficiency.
Mixed beds (cation and anion resin mixed in a single tank) offer the simplicity and economy of a single tank system and also offer neutral pH and exceptional water quality. The advantages are counterbalanced by somewhat poorer operating capacity and greatly complicated regeneration. Mixed beds are popular with service exchange companies because of their simplicity of operation and the elimination of the need for a second tank.
Your resin supplier can predict which effluent quality can be achieved by a two-bed deionizer. The information needed to calculate the anticipated quality of treated water includes
If an uninterrupted flow of water is required, any permanent inhouse system would require a two train ion exchange system or a means of storing DI water to accommodate times when a train is in regeneration. For low flow requirements, the water treatment dealer may want to propose service DI tanks or a combination of RO polished by service DI tanks.