Application of Ultrafiltration (UF) System in Electrophoretic Coating

Electrophoretic coating involves immersing the workpiece and corresponding electrode in a water-soluble paint and applying an electric current. Through the physicochemical action generated by the electric field, the resin, pigments, and fillers in the paint uniformly precipitate and deposit on the workpiece surface (acting as an electrode) to form a water-insoluble film. The ultrafiltration (UF) system and UF membranes play a critical role in this process. Proper adoption and promotion of ED-RO technology can significantly enhance the utilization of electrophoretic paint and water resources while drastically reducing electrophoretic wastewater discharge. Correct use and maintenance of UF equipment maximize system efficiency. Post-electrophoretic UF rinsing removes floating paint adhered to the film surface, improving appearance quality and enabling paint recovery.

1. Overview of Ultrafiltration System

Ultrafiltration (UF) is a membrane separation technology driven by a pressure differential across the UF membrane, operating on the principle of mechanical sieving. Operating pressure typically ranges from 0.1 to 0.6 MPa, with pore sizes of 1 nm to 0.1 μm and molecular weight cutoff of 500 to 1,000,000 Da.

In the UF process for electrophoretic paint, when the paint contacts the UF membrane, water and inorganic salts pass through, while resin and pigment-sized molecules are retained in the paint liquid and returned to the electrophoretic bath.

2. Application of UF Membranes in Electrophoretic Coating

UF membranes are key equipment in electrophoretic coating lines. Their primary functions are:

• Extract deionized water and paint solvents from the bath via UF to provide rinse water for electrophoretic workpieces.

• Wash off excess paint adhered to the workpiece surface and return it to the bath.

Benefits:

• Recycles paint carried out on workpiece surfaces, achieving closed-loop circulation and saving ~30% on paint procurement costs.

• Discharges a portion of UF permeate to remove impurity ions introduced during coating, maintaining bath conductivity and pH within specified limits.

• Reuses UF permeate as rinse water instead of deionized water, nearly eliminating paint discharge and significantly reducing wastewater treatment burden and environmental pollution.

3. Operating Principle and Conditions of UF System

3.1 UF Principle

UF removes impurity ions from the electrophoretic bath and effectively manages cationic and anionic contaminants introduced in a closed loop, ensuring paint stability and film integrity.

UF is a membrane permeation separation technology that purifies, separates, or concentrates solutions. It functions as a sieving process based on membrane pore size. Driven by pressure across the membrane, only water, inorganic salts, and small molecules pass through, while suspended solids, colloids, proteins, and microorganisms are retained. UF operates continuously under low pressure.

Under pressure differential, the bath liquid passes through the UF unit. The permeate flows to the UF tank, with ~30% of circulation coming from the auxiliary tank before returning to the main bath. Minimum paint feed to the UF system should be 10× design permeate rate, with 20× being optimal.

3.2 Normal Startup and Operation Steps

1. Confirm no pressure in outlet lines; close paint supply, rinse inlet/outlet, and permeate-to-storage valves.

2. Open paint circulation, permeate discharge, and all pressure gauge isolation valves.

3. Open UF pump shaft seal coolant inlet/outlet valves (0.2 MPa).

4. Start paint supply pump.

5. Slightly open paint inlet to slowly fill the system.

6. Gradually open inlet valve until baseline pressure reaches 0.15 MPa, then start the pump.

7. Adjust inlet and outlet valves until differential pressure reaches 0.2 MPa (inlet: 0.35 MPa, outlet: 0.15 MPa).

8. Check for membrane leaks; replace O-rings or membrane modules if needed.

9. Permeate discharge time should be at least 10 minutes.

10. Open permeate-to-storage valve and close discharge valve.

3.3 Precautions During UF Operation

1. Handle membrane modules, housings, and accessories gently to avoid impact damage.

2. Unexpected shutdowns (power failure, etc.) must not exceed 2 hours. Clean membranes immediately if prolonged to prevent paint sedimentation and clogging.

3. Record all cleaning and electrophoresis system parameters for troubleshooting.

4. Inlet-outlet pressure differential should be <0.08 MPa. Replace 25 μm filter bags if exceeded.

5. Never run UF system with permeate valve closed.

6. Do not start pump with discharge valve open to avoid membrane damage.

7. Avoid excessive permeate flow; adjust valves within specified rates.

8. Minimize chemical carryover from pretreatment. Final rinse before electrophoresis should have drip conductivity <10 μS/cm.

Post-electrophoretic UF rinsing removes floating paint, improves appearance, and recovers paint. Equipment is similar to post-phosphating water rinse systems. Final pure water rinse removes impurity ions to prevent contamination defects. To avoid secondary flow marks in crevices due to insufficient rinsing, full immersion rinsing is required.

UF rinse typically uses 2–3 stages to reduce paint carryout. For example:

• Vehicle surface area: 80–100 m²

• Paint carryout per vehicle: 7–10 L (20% solids)

• 1st rinse: 4–5% solids → 2nd rinse: <1%

• Fresh UF permeate: <0.5% solids

With ED-RO (UF + reverse osmosis), purified permeate replaces deionized water for final rinse, achieving true closed-loop post-rinse, drastically reducing wastewater and significantly improving paint utilization.

UF permeate contains water and paint cosolvents. Used in rinsing, this closed-loop system compensates for paint loss and lowers bath conductivity.

4. UF Equipment Cleaning Process and Steps

Cleaning solution: Concentrated cleaner : pure water = 1:99, temperature 38–43°C, pH 2.0–2.2. Due to high viscosity, circulate pure water in the cleaning tank using a pump until >32°C, add cleaner, circulate to 35°C, then adjust pH to 2.0 with HCl. Open cleaning valves for membrane core cleaning, maintaining pH ≥2.2.

Strict adherence to process is essential. After prolonged use, UF membrane failures may occur: filter bag clogging, insufficient flow/pressure differential. Track cleaning per SOP to identify and resolve issues, optimizing process and improving quality.

4.1 Precautions

1. Clean when permeate flow drops to 70% of normal; delays cause irreversible clogging.

2. Precision pre-filter (25 μm) prevents membrane clogging. Replace bags per SOP and clean filter/piping regularly.

3. Clean immediately after unexpected shutdowns to prevent membrane blockage.

4. Operate strictly per SOP; log all UF parameters.

4.2 Cleaning Steps

1. Close paint inlet/outlet valves of target UF module.

2. Fully drain paint from module.

3. Open cleaning pump inlet/outlet valves.

4. Open main cleaning inlet/outlet valves to membrane tubes.

5. Open target module cleaning inlet/outlet valves.

6. Start cleaning pump.

7. Flush with flowing deionized water until effluent is clear.

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