Ultrafiltration System (UF) Application In Electrophoretic Coating Industrial
Electrophoretic coating is a coating method in which the workpiece and the corresponding electrode are put into the water-soluble coating, connected to the power supply, and the resin, pigment and filler in the coating are uniformly precipitated and deposited on the surface of the coating as the electrode to form a water-insoluble coating film by relying on the physical and chemical action generated by the electric field.
Ultrafiltration and ultrafilter is an important role in the process of electrophoretic coating. Better use and promotion of ED-RO technology can further improve the utilization rate of electrophoretic coating and water resources, and greatly reduce the discharge of electrophoretic sewage. Correct and effective use and maintenance of ultrafiltration equipment, improve the utilization rate of ultrafiltration system. The purpose of ultrafiltration cleaning (UF) after electrophoresis is to remove the floating paint attached to the surface of the electrophoretic coating, improve the appearance quality of the coating and recover the electrophoretic coating.
Brief description of ultrafiltration system
Ultrafiltration (UF) is a membrane separation technology. It is a solution separation process based on the principle of mechanical screening, which is driven by the pressure difference between the two sides of UF membrane. The use pressure is usually (0.1 ~ 0.6)MPa, the separation pore diameter is 1nm ~ 0.1μm, and the retention molecular weight is about 500 ~ 1000000. In the process of electrophoretic paint UF, when the electrophoretic paint comes into contact with the UF membrane, the solution and inorganic salts can pass through the UF membrane, while the molecules similar to the size of resin and pigment cannot pass through the UF membrane, and are retained in the paint solution and transported back to the electrophoresis tank.
Application of ultrafilter in electrophoretic coating
Ultrafilter is the key equipment in the electrophoresis coating line. The main purpose of ultrafilter is to obtain deionized water and paint solvent from the electrophoresis tank through the UF device, provide washing water for the electrophoresis parts, and clean the excess electrophoretic paint attached to the electrophoresis workpiece and return to the electrophoresis tank. In this way, the electrophoretic paint from the workpiece surface can be recycled, and the closed-circuit circulation can be realized, saving 30% of the cost of purchasing electrophoretic paint for enterprises. Secondly, part of the UF liquid can be discharged to eliminate the impurity ions brought into the electrophoresis tank during the painting process, so that the impurity content of the working liquid of the electrophoresis tank can be kept within the range of conductance and pH value specified in the process, and the UF liquid can be recycled instead of deionized water as the washing water of the workpiece after electrophoresis, and the electrophoresis paint is basically not discharged. It avoids the wastewater treatment load discharged by direct cleaning with deionized water and reduces environmental pollution.
Working principle of ultrafiltration system
Ultrafiltration is used to remove the impurity ions in the electrophoretic paint working liquid and effectively close the cation and anion pollution source, to ensure the stability of the paint solution and the integrity of the paint film. Ultrafiltration (UF) is a membrane permeation separation technology that can purify, separate or concentrate solutions. The ultrafiltration process can be understood as a screening process related to the membrane pore size. With the pressure on both sides of the membrane as the driving force and the ultrafiltration membrane as the filter medium, under a certain pressure, when the solution flows through the membrane surface, only water, inorganic salts and small molecules are allowed to pass through the membrane, while the suspended solids, colloids, proteins and microorganisms and other macromolecules in the tissue water are allowed to pass through, so as to achieve the purpose of purification, separation and concentration of the solution. Ultrafiltration is a continuous, low-pressure operation.
The electrophoresis paint tank liquid is filtered through the ultrafiltration filter and the side ultrafiltration membrane under the pressure difference, and flows through the liquid to the ultrafiltration tank. About 30% of the circulation of the ultrafiltration liquid is from the electrophoresis auxiliary tank, and then returns to the electrophoresis tank. The minimum amount of paint into the ultrafiltration system is 10 times of the designed water yield, and the optimal amount of paint is 20 times of the designed water yield.
Ultrafiltration system operation procedures
1) Confirm that there is no pressure on the outlet pipeline, close the paint supply valve, cleaning inlet valve, cleaning outlet valve, through the liquid to the storage tank and cleaning tank valve;
2) Open the paint circulation valve, through the liquid discharge valve, all pressure gauge isolation valves;
3) Open the coolant inlet and outlet valve of ultrafiltration pump shaft seal (pressure is 0.2MPa);
4) Start the paint supply pump;
5) Slightly open the paint inlet to slowly inject the paint into the Ultra filtration system;
6) Slowly open the paint inlet valve until the pressure on the baseline reaches 0.15MPa, and then open the pump.
7) Slowly adjust the inlet and outlet valves until the pressure difference reaches 0.2MPa, and the inlet and outlet pressures are 0.35MPa and 0.15MPa respectively.
8) Check whether the Ultra filtration membrane leaks. If there is still leakage, the O-ring and membrane group should be replaced in time.
9) The discharge time of permeable liquid should be at least 10min.
10) Open the valve through the liquid to the storage device, and then close the separation liquid discharge valve.
Precautions in use of ultrafiltration plant1) Ultrafiltration components, enclosures and accessories should be handled lightly to prevent damage;
2) The time of sudden power failure or other unexpected shutdown shall not exceed 2h; If the shutdown time is too long, the ultrafiltration membrane element should be cleaned immediately to avoid the deposition of paint blocking the ultrafiltration membrane element;
3) Record the parameters of cleaning system and electrophoresis system in detail to prevent and solve problems;
4) The pressure difference between the inlet and outlet of the filter should be < 0.08MPa, if > 0.08MPa, the filter bag of ultrafiltration should be replaced in time, and the filtration accuracy is less than or equal to 25μm;
5) Do not run the ultrafiltration system when the liquid valve is closed;
6) Do not start the pump when the discharge valve is open to avoid pressure damage to the ultrafiltration membrane;
7) Large amount of filtrate is prohibited, and the opening valve should be adjusted under the specified amount of filtrate.
8) Minimize the introduction of chemical agents in the metal pretreatment process, and the dripping conductivity of the last rinse before entering the electrophoresis tank should be lower than 10us/cm.
The ultrafiltration cleaning system (UF) used after electrophoresis is to remove the floating paint attached to the surface of the electrophoretic coating, improve the appearance of the coating and recover the electrophoretic paint. The equipment used is the same as the water washing equipment after pre-treatment phosphating. The purpose of final washing with pure water is to remove impurity ions and prevent the production of pollution stains and film coating. In order to prevent the secondary flow mark in the gap after inadequate cleaning, it is necessary to have a full immersion washing process. The ultrafiltration wash is generally carried out 2-3 times to reduce the amount of paint taken out, for example: The car body area is 80-100 square meters, and each car body takes out 7-10L of 20% solid liquid. When the solid content of the first water wash is 4%-5%, the second water wash may reach less than 1%. The solid content of fresh ultrafiltration liquid is generally less than 0.5%. In the case of removing impurity ions and solids), UE-RO liquid is used instead of pure water for final pure water washing, so as to realize the real fully closed electrophoresis post-cleaning, which can greatly reduce the discharge of electrophoresis sewage and significantly improve the utilization rate of electrophoresis coating. Remove the impurity ions in the electrophoretic paint working solution and the cation and anion pollution source brought in by the effective closed circuit to ensure the stability of the paint solution and the integrity of the paint film.
In electrophoretic paint system, the permeation solution produced by ultrafiltration contains water and the solubilizing substance of paint. This looped circulation system can compensate for the loss of paint when the liquid is used in the washing process of paint. Through the liquid can reduce the conductivity of paint.
Cleaning process and steps of electrophoretic ultrafiltration equipment
Concentrated cleaning solution and pure water ratio 1:99, control the temperature between 38~43℃, PH=2~2.2, because the cleaning solution is sticky, so in the cleaning tank to use the cleaning pump to circulate the pure water to the temperature above 32℃, and then add the cleaning solution to circulate to 35℃, add hydrochloric acid to adjust the PH value to PH=2, and then open the cleaning valve for membrane core cleaning. Keep the PH at least 2.2 during cleaning.
The ultrafiltration system should be operated in strict accordance with the process requirements. After replacing the ultrafiltration membrane in the ultrafiltration system for a period of time, there is a failure: the filter bag is blocked, and the flow pressure difference of the ultrafiltration membrane cannot meet the process requirements. Therefore, in view of the above problems, the operator should be tracked in the process of cleaning in strict accordance with the operation requirements and process parameters, so as to find the cause and eliminate the fault. To optimize the process, improve the process, to improve the quality of products.
Notes:
1) When the permeability of the ultrafiltration membrane component drops to 70% of the normal value, it must be cleaned. If the cleaning is delayed, it will lead to blockage, and it will be difficult to restore the original flow in serious cases;
2) The ultrafiltration system is equipped with a precision filter to prevent the blockage of membrane elements, with an accuracy of 25μm. The filter bag should be replaced strictly according to the process requirements, and the filter and pipeline should be cleaned regularly;
3) Unexpected shutdown due to failure, cleaning should be carried out in time according to regulations to prevent blockage of membrane parts;
4) Operate in strict accordance with the operation rules and record the operating parameters of UF ultrafiltration system in detail for reference.
Steps of cleaning1) Close the paint inlet and outlet valves of the ultrafiltration membrane assembly to be cleaned;
2) Thoroughly discharge the paint in the ultrafiltration membrane assembly needed to be cleaned;
3) Open the inlet and outlet valve of the cleaning pump;
4) Open the cleaning inlet and outlet valve of the cleaning pipe into the membrane pipe;
5) Open the cleaning inlet and outlet valves of the required ultrafiltration membrane assembly;
6) Start the cleaning pump;
7) Clean the paint in the ultrafiltration membrane with flowing deionized water (visual inspection is clear water);
8) Prepare the cleaning solution according to the cleaning formula (pH value is adjusted to 2~3, test paper), cycle 2h~3h, clean until the penetration reaches 600L/h, and drain the cleaning solution;
9) The temperature of the liquid during cleaning should not exceed 45℃. If the temperature is too high, the machine should be stopped to cool down and then continue cleaning, and the cleaning liquid should be discharged;
10) Clean with flowing deionized water for 30min to drain the cleaning liquid. If the blockage is serious, the cleaning time can be extended.
11) Seal the membrane with deionized water or UF solution for use.
The ultrafiltration system controls the concentration of impurity ions brought into the workpiece to ensure the coating quality. In the operation of this system, attention should be paid to the continuous operation of the system once it is run, and intermittent operation is strictly prohibited to prevent the drying of the ultrafiltration membrane. The dried resin and pigment are attached to the ultrafiltration membrane and cannot be cleaned thoroughly, which will seriously affect the water permeability and service life of the ultrafiltration membrane. The water output rate of the ultrafiltration membrane decreases with the running time, so it should be cleaned once for 30 ~ 40 consecutive days to ensure the ultrafiltration water required for ultrafiltration leaching and washing.
Concluding Remarks
Electrophoretic coating is a special coating formation method, which is the most practical construction technology for waterborne coatings. It has the characteristics of water-soluble, non-toxic and easy automatic control, so it is necessary to recycle the electrophoretic paint and reduce waste. Electrophoretic coating recovery generally adopts ultrafiltration or reverse osmosis, but its working principle is fundamentally different, the use of process generally has two kinds: ultrafiltration UF: polymer and electrolyte separation, also can be said to be paint water separation, working pressure of about 1Kg, about 3 times the osmotic pressure, the membrane pore is large. Reverse osmosis RO: purify or concentrate the solution, that is, realize the separation of organic matter and inorganic matter. The working pressure is about 25Kg, which is about 50 times the osmotic pressure, and the membrane hole is small.
