JP4536482B2 - Operation method of membrane filtration device - Google Patents

Description

  The present invention relates to a method for operating a membrane filtration apparatus for treating waste water discharged from a final water washing tank in a cationic electrodeposition coating process.

  Cationic electrodeposition coating is widely used for painting automobile bodies, automobile parts, electrical products, building materials, etc. The electrodeposition coating system consists of an electrodeposition process for forming a coating film electrochemically on the object to be coated, a cleaning process for washing off non-electrodeposited paint, and a drying and baking process for curing the coating film. In general, the washing process is roughly divided into a UF filtrate recovery washing process and a final washing process. In the former, the coating object is washed with a UF filtrate obtained by filtering the coating material in the electrodeposition tank with UF, and the paint components physically adhered to the coating object are washed off and collected in the electrodeposition tank. It is a process. The latter is a process of performing final cleaning using pure water or industrial water, and trace amounts of paint components and contaminant ions that could not be washed out in the UF filtrate recovery water washing process are washed away. And the washing water used for washing | cleaning at the last washing process is discharged | emitted out of the process as waste water. A large amount of water is required for the final washing, which is the final washing. When the object to be coated is an automobile body, 100 L / min or more of water is newly supplied as washing water, and after the washing, it is discharged out of the process. Yes. Since this wastewater contains a small amount of paint components and impurity ions, it must be finally treated by some method. Moreover, it is also the loss of a coating material, and generally a loss amount is equivalent to 5% of the coating material physically taken out by the coating object from an electrodeposition tank.

  In recent years, a membrane separation method has been proposed as a treatment method for such waste water, and the filtrate is separated into a filtrate and a concentrate by a membrane filtration device. The filtrate is circulated and used as washing water, and the concentrate is returned to the electrodeposition tank and collected as a paint. A method of reuse is known (WO96 / 07775 pamphlet, Patent Document 1). Usually, when the concentrate from the membrane filtration device is recovered as a paint and the filtrate is recycled as washing water, the final washing water contains pure water (to prevent the accumulation of contaminating ions in the electrodeposition coating process). Ion exchange water) is used. And generally, the water quality of ion exchange water is managed by electric conductivity or TOC. However, ion-exchanged water contains a large amount of fine particles, organic substances derived from the raw water of ion-exchanged water, and eluate from ion-exchange resin. This is probably because the membrane is contaminated, but there is a problem that the filtration capacity is greatly reduced. Even when the cleanness of ion-exchanged water is increased by membrane filtration or the like, there are the following problems at the initial stage of water flow, and a reduction in filtration capacity cannot be avoided.

Patent Document 1 discloses a method for adjusting the pH of wastewater by adding acid to wastewater as a method for maintaining the ability of the membrane filtration device stably, but the dry solid content concentration (hereinafter referred to as NV) is low. In this case, since the pH is greatly lowered by the addition of a small amount of acid, there is a problem that the acid is not substantially added and the film is contaminated. Thus, when the water in the final washing tank is renewed and the operation of the membrane filtration apparatus is started, or when the wastewater NV at the start of operation is low, such as in the initial stage of painting, the filtration capacity decreases over time. As a result, they have found the present invention.
WO96 / 07775 pamphlet

  An object of the present invention is to provide an operation method for stabilizing the filtration capacity by suppressing the deterioration of the filtration capacity with the passage of time when the waste water discharged from the final water washing step of electrodeposition coating is treated with a membrane filtration device. To do.

The present inventors have found that the above problem can be solved by adjusting the NV of wastewater in the wastewater receiving tank 6 to 0.1 wt% or more and then passing it through a membrane filtration device, and have reached the present invention.
The present invention is as follows.
1. Waste water discharged from the final washing tank in the cationic electrodeposition coating process is applied to a membrane filtration device and separated into a filtrate and a concentrated liquid. The filtrate is recycled as washing water and returned to the electrodeposition tank for recovery as a paint. In the method to be used, either a replenishing paint , a coating for an electrodeposition tank, a washing water for a UF recovery washing tank, or a mixture thereof is added to adjust the dry solids concentration of the waste water to 0.1% wt or more. Then, the method for operating the membrane filtration device is characterized in that water passage is started to the membrane filtration device.

  According to the present invention, when waste water discharged from the final water washing step of electrodeposition coating is treated with a membrane filtration device, it is possible to suppress a decrease in filtration ability with time and stabilize the filtration ability.

Hereinafter, the present invention will be described with a focus on preferred embodiments.
FIG. 1 is a flow diagram showing an example of a cationic electrodeposition coating process equipped with a membrane filtration device for treating waste water from the final washing tank. In the figure, an object to be coated 16 is attached to a conveyor 15 and first enters the electrodeposition tank 1 and is electrodeposited. Next, the first rinsing tank 2, the second rinsing tank 3, and the third rinsing tank 4 are led to the UF filtrate recovery rinsing step (inside the dotted line in FIG. 1), finally led to the final rinsing tank, and washed with ion-exchanged water. Is done. Waste water from the final rinsing tank 5 is passed through a membrane filtration device and separated into a concentrate and a filtrate by the filtration membrane module 9. The filtrate is usually stored in the filtrate storage tank 7 and then recycled mainly to the final washing tank. The concentrated liquid is returned to a tank having a higher NV than the concentrated liquid, such as the electrodeposition tank 1 or the first washing tank 2.

In the above, the present invention solves the above problem by adjusting the NV of wastewater in the wastewater receiving tank 6 to 0.1 wt% or more and then passing it through a membrane filtration device. By the way, all NV shown in this invention is the weight% of the residue at the time of heat-drying at 105 degreeC for 3 hours.
In the present invention, the adjustment of NV can be preferably performed by adding any one of the replenishing paint, the paint in the electrodeposition tank 1, the rinsing water in the UF recovery water washing tanks 2 to 4, or a mixture thereof. The paint added here is recovered as a concentrated solution at any time, so there is no loss. In addition, a replenishment paint means the paint at the time of replenishing the electrodeposition tank with the paint consumed by painting.
In the present invention, it is necessary to adjust the wastewater NV to 0.1 wt% or more before passing water. When NV is less than 0.1 wt%, the membrane is directly contaminated with impurities in ion-exchanged water, and / or until the pH changes sharply due to addition of a small amount of acid, until NV in the wastewater, that is, the paint component can be dispersed. Although it is considered that no acid is added, the filtration capacity is greatly reduced at the beginning of water flow. Moreover, even if NV of wastewater rises to 0.1 wt% or more, the fall of filtration capability continues.

  On the other hand, according to the method of the present invention, impurities in ion-exchanged water are included in the NV component, and the NV component has a buffering action against pH change. However, the permeation ability is stabilized over time after initially decreasing. In the present invention, NV is preferably 5 wt% or less, more preferably 3% or less. Since the filtration capacity of the liquid containing the electrodeposition paint is dependent on the concentration, if the NV exceeds 5 wt%, the reduction of the filtration capacity may increase. Furthermore, if water is passed at a concentration of NV of 5 wt% or higher, the filtration capacity may not be easily restored even if it is returned to a lower NV.

Next, the NV adjustment of the wastewater of the present invention is particularly effective when the water is first passed through the membrane module, that is, at the time of starting the operation. Good. For example, if the final washing tank is regularly renewed or not painted, the inside of the membrane filtration device is replaced with filtrate, and the device is shut down, but after such water is restarted It is not necessary to use the method of the present invention. The required water passage time is at least 30 minutes or more, preferably 1 hour or more, and the accumulated filtrate amount per membrane area is at least 50 L / hr / m ² or more, preferably 100 L / hr / m ² or more. The present invention can be widely applied to the UF membrane regardless of the structure, material, and hole diameter of the membrane module.

  The wastewater NV rises as painting is performed, so it is possible to wait for water to reach 0.1 wt% or more, but until then, recycle the filtered water as flush water. Therefore, a large amount of ion exchange water is consumed, which is inefficient. At the same time, since the amount of drainage also increases, it is preferable to perform NV adjustment by adding either the replenishing paint of the present invention, the paint of the electrodeposition tank, the flush water of the UF recovery flush tank, or a mixture thereof.

The membrane filtration test apparatus shown in FIG. 2 was installed in the final water washing step for electrodeposition coating of automobile parts, and the following tests were conducted. First, 400 L of final washing waste water of NV 0.02 wt% was received in a 500 L waste water receiving tank, and 3.6 L of an electrodeposition tank paint (NV 20.5%) was added thereto to adjust NV to 0.2 wt%. Next, water was passed through the membrane filtration device while maintaining the water level of the wastewater receiving tank with the liquid level controller. As the membrane module, one module having a membrane area of 4.7 m ² filled with a hollow fiber membrane made of a polyacrylonitrile polymer and having an outer diameter of 1.4 mm, an inner diameter of 0.8 mm, and a molecular weight cut off of 13,000 was used. In addition, 10 vol% acetic acid is prepared in the acid storage tank, the lower limit value of the pH of the filtrate is set to 5.3, the upper limit value is set to 5.5, and the pH is maintained in this range by acid injection. The concentrated liquid was dispensed 0.5 L at a frequency of once every 5 minutes by the concentrated liquid recovery pump. The concentrated liquid was returned to the electrodeposition tank, and the filtrate was discharged out of the apparatus. The inlet pressure of the membrane module was set to 0.25 MPa, the outlet pressure was set to 0.05 MPa, and the filtration capacity was measured. Table 1 shows the transition of the filtration capacity and the NV of the final flush wastewater (water received in the wastewater receiving tank).

[Comparative Example 1]
Table 1 also shows the change in filtration capacity and NV of the final washing wastewater when the same test as in Example 1 was performed except that the wastewater NV was not adjusted before passing through the membrane filtration device.

  The present invention can be suitably used in the field of cationic electrodeposition coating wastewater treatment.

It is a flowchart which shows an example of the electrodeposition coating process and membrane filtration apparatus which this invention makes object. It is a flowchart which shows the membrane filtration test apparatus used in the Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electrodeposition tank 2 1st UF filtrate collection | recovery washing tank 3 2nd UF filtrate collection | recovery washing tank 4 3rd UF filtrate collection | recovery washing tank 5 Final washing tank 6 Waste water receiving tank 7 Filtrate reservoir 8 Collected washing UF module 9 Final filtration washing membrane filter module 10 circulator pump for recovered water washing UF 11 circulation pump for final water washing waste water membrane filtration device 12 filtrate recovery pump for final water washing waste water membrane filtration device 13 concentrate recovery pump for final water washing waste water membrane filtration device 14 ion exchange water 15 conveyor 16 coating object

Claims (1)

Waste water discharged from the final washing tank in the cationic electrodeposition coating process is applied to a membrane filtration device and separated into a filtrate and a concentrated liquid. The filtrate is recycled as washing water and returned to the electrodeposition tank for recovery as a paint. In the method to be used, either a replenishing paint, an electrodeposition tank paint, a UF recovery water washing tank washing water, or a mixed solution thereof is added to the waste water, and the dry solid concentration of the waste water is increased to 0.1 wt% or more. A method for operating the membrane filtration device, wherein after the adjustment, water passage through the membrane filtration device is started.

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