JP2873095B2 - Treatment method of waste water in final washing tank in cationic electrodeposition coating - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for treating waste liquid from a final washing tank in cationic electrodeposition coating.

BACKGROUND ART Electrodeposition coating is widely applied by utilizing its features such as easy management and economical efficiency, and there is a remarkable progress in technology. In particular, more than 15 years have passed since the conversion from anionic electrodeposition paints to cationic electrodeposition paints, and almost 100% of them have become cationic electrodeposition paints in the automotive field today. Cationic electrodeposition paints are not limited to the automotive field,
It has been introduced in a wide range of fields such as general metal products, electric products, and industrial machinery.

The background of the development of cationic electrodeposition coating suitable for coating of automobiles and the like as described above is the development of cationic electrodeposition coating with excellent UF (ultrafiltration) characteristics, and UF suitable for cationic electrodeposition coating. There has been the development of membranes and the development of UF systems that make use of these.

In the cationic electrodeposition coating system thus developed, a washing step is provided after the object to be coated is electrodeposited. In other words, the object to be coated is an electrodeposited undried coating film that has been electrodeposited in an electrodeposition tank. The coating is completed by drying and baking in a drying furnace. Negatively charged counter ions are attached.
If drying and baking are carried out while keeping this as it is, poor finishes such as sagging, nagare, sima, and buttocks will occur. In order to avoid this, in a general electrodeposition coating system, after electrodeposition, a washing step is performed, followed by drying and baking.

In the washing step after the electrodeposition, the electrodeposition bath liquid or the washing waste liquid containing a relatively large amount of paint is filtered, the recovered liquid containing the paint components is returned to the electrodeposition tank, and the filtrate is returned as washing water and recirculated. The process is divided into two steps, a closed-circuit rinsing step to be used and a final rinsing step provided as a separate step as a subsequent step. In the former method, the coating material adhered to the coating material can be recovered and washed by washing the coating material with a filtrate obtained by treating the coating material in the electrodeposition tank with UF. The latter is an extremely important step in performing advanced washing, and the final wash is performed using pure water, industrial water, tap water, etc., and dilute paint components and counters that cannot be washed by the former are used. At present, ions and the like are washed, and the washing waste liquid is discharged outside the system.

However, the amount of water required for the final washing is extremely large, and since the washing waste liquid contains a small amount of miscellaneous ions in the paint, it cannot be discharged to the outside. From the standpoint of cost and economy, special treatment technology for the final washing waste liquid is required.

However, since the waste liquid in the final washing step contains almost no organic acid or solubilizing agent and is mostly water, the slightly contained paint component is easily precipitated. Therefore, this washing wastewater is purified by membrane treatment.
Recycling is difficult and has not been put to practical use because it causes aggregation and clogging of the film in the film processing apparatus.

Japanese Patent Application Laid-Open No. 7-207495 discloses a technique for reusing the waste liquid of the final washing step without discarding it. This publication discloses that the UF filtrate obtained by performing UF filtration on the washing water in the pure water washing step, which is the final washing step, is used as the final washing water. However, this publication states that UF
There is no mention of treatment conditions, especially measures for maintaining UF filtration stably for a long time, and of course, there is no description about adjusting pH to obtain UF filtration stability.

DISCLOSURE OF THE INVENTION The present invention can treat the waste liquid in the final cleaning step, which was difficult to treat the waste liquid as described above, without causing agglomeration and clogging in the membrane treatment apparatus, and concentrates the paint residue remaining in the waste liquid. A technology for purifying a large amount of washing wastewater and making it reusable as washing water to make it a closed system that does not drain any water to the outside, or purifying it to a state where it can be drained. The purpose is to provide.

The present invention relates to a method for treating a waste liquid discharged from a final washing tank in a cationic electrodeposition coating step, wherein the waste liquid in the concentration tank 6 is constantly maintained at a pH of 6.4 or less, and the waste liquid is filtered through a filtration device 9 having a semipermeable membrane. The present invention relates to a method for treating a waste water in a final washing tank, wherein the liquid is separated into a concentrated liquid containing a paint component and a filtrate by circulating the liquid.

In the present specification, the term final rinsing tank or final cleaning step is used for final finish cleaning provided independently of a closed circuit composed of an electrodeposition tank and a cleaning tank generally employed in an electrodeposition coating step. Washing tank or washing step.

The present invention is characterized in that all or a part of the filtrate obtained by separation by the semipermeable membrane is returned to the final washing tank.

Further, the present invention is characterized in that the concentrated liquid is extracted from the concentrated tank when the concentration of the concentrated liquid in the circulating waste liquid concentration tank reaches a maximum of 200 times.

In another embodiment, the waste water discharged from the final washing tank in the cationic electrodeposition coating step is circulated to a filtration device provided with a semipermeable membrane, thereby separating into a concentrated solution containing a paint component and a filtrate. In the treatment of the tank waste liquid, the semi-permeable membrane is backwashed using a filtrate to which 300 to 3,000 ppm of an organic acid is added, and the pH of the concentrated liquid in the concentration tank is constantly maintained at 6.4 or less. The present invention relates to a method for treating waste liquid.

In the cationic electrodeposition coating step, the pH of water in the final washing step is usually about 6.0 to 7.0. The amount of paint taken out is about 0.05-0.2% by weight in the washed water, and the pH is
It has risen to about 6.5 to 7.0. When this is directly applied to a membrane treatment apparatus, the paint particles are usually aggregated and deposited on the filtration membrane. In the present invention, in order to prevent this, the pH of the water in the concentration tank 6 is adjusted.
Is always 6.4 or less, preferably 6.0 or less, more preferably 4.
The mixture is adjusted to 5 to 6.0 and passed through a filtration device 9 provided with a semipermeable membrane, particularly a UF membrane. By simply adjusting the pH of the washing waste liquid to 6.4 or less, the paint in the waste liquid exists stably even under the pressure and shear conditions in the membrane treatment apparatus, and does not cause aggregation or clogging in the apparatus. . The concentrated solution obtained by the membrane treatment in this manner can be continuously circulated through the membrane and concentrated, and can be concentrated up to about 400 times in volume ratio. There is no trouble such as clogging.

In addition, all of the filtrate can be recovered and returned to the final washing tank as final washing water, so that it can be circulated and reused, whereby a substantially complete closed circuit that does not discharge washing waste liquid to the outside can be obtained. In addition, the filtrate can be entirely or partially discharged to the outside without concern about environmental pollution. When the filtrate is used as a part of the final washing water, surprisingly, such use does not cause fine clumping of the paint on the object to be coated, so-called bumps, and does not stain the coating film.

On the other hand, the circulating concentrate that has been treated and concentrated by the filtration device 9 having a semipermeable membrane gradually increases the concentration of paint components and other solids, but the concentration of the concentrate in the concentration tank is up to 200%.
At the stage when the number of times has been reached, withdraw from the concentration tank. The extracted concentrated liquid is a small amount, and if necessary, can be further concentrated and safely incinerated. If the solution remains stable, it can be returned to the electrodeposition tank, which is more economically advantageous.

In the above description, the enrichment means a rate of change based on the initial operation state, and can be expressed as a volume ratio, a nonvolatile matter concentration ratio, a conductive material content ratio, and the like. It is expressed as The volume ratio is defined as the ratio of the total volume of waste liquid sent to the concentration tank to be treated to the volume of the concentrated liquid left in the concentration tank.

Since it is difficult to adjust the pH in the concentration tank at the start of operation, a certain amount of washing waste liquid accumulates in the concentration tank. The pH is adjusted to 6.0 or less, preferably 4.5 to 5.5.

In order to stabilize the amount of filtration by a filtration device provided with a semipermeable membrane, the filtration device may be backwashed with an acid. Backwash
The concentration is constantly performed using a filtrate to which about 300 to 3,000 ppm of an organic acid is added, and the pH in the concentration tank is constantly maintained at 6.4 or less.

The final washing tank may be a single tank or a multi-stage type in which two or three tanks are arranged in series. In the case of a multi-stage type, countercurrent washing is preferred in which the washing water flows from the last tank to the first tank, contrary to the flow of the object to be coated. In this case, the washing waste liquid is discharged from the first tank, and the filtrate from the filtration device may be used to wash the object directly on the final tank of the final washing tank, or may be used in the final tank of the final washing tank. May be returned.

RO (reverse osmosis) membrane, UF membrane, MF (microfiltration)
There are membranes, etc., but when filtering waste liquid, UF membrane is most suitable. RO membranes have excellent ability to remove paint components, but have low processing capacity per unit time and are economically undesirable. Conversely, for example, an MF membrane having a pore size equal to or larger than the UF has an excellent membrane treatment capacity, but a large amount of paint components enter the filtrate, and do not satisfy the drainage water quality standard for drainage, and when the filtrate is reused as final washing water. However, there is a problem in that the cleaning effect is poor and the paint is minutely solidified on the object to be coated, that is, so-called bumps are generated, and the coating is stained. The UF membrane has no problem in practical use of both the processing capacity and the components in the filtrate. Molecular weight cutoff for UF membrane
About 3,000 to 1,000,000, and any material such as polyacrylonitrile, polysulfone, polyolefin, and chemically modified products thereof may be used. The suitability of the material, the molecular weight cut-off, etc. differs depending on the liquid quality, so it is preferable to select the material by conducting a preliminary investigation.

If it is necessary to obtain a filtrate closer to pure water, the UF-treated filtrate may be further subjected to RO treatment as shown in FIG. As a result, water with a lower BOD can be obtained.

PH of wastewater from final washing tank in cationic electrodeposition coating
Is about 6.5 to 7 as described above. Therefore, in the present invention, the pH in the concentration tank is adjusted to 6.4 or less using an acid. Suitable acids include organic acids such as carboxylic acids, sulfonic acids, sulfates, phosphates, phenols and the like, with water-soluble carboxylic acids being particularly preferred. Specifically, formic acid, acetic acid, propionic acid, butyric acid, lactic acid, citric acid, maleic acid, fumaric acid, oxalic acid, malic acid, tartaric acid, oxaloacetic acid, malonic acid, nitrile acetic acid, phthalic acid, trimellitic acid, and inorganic acid Examples thereof include sulfamic acid and carbonic acid. Particularly preferred are formic acid, acetic acid, lactic acid and the like. Alternatively, the polar liquid discharged from the diaphragm device may be used.

In the treatment of the waste liquid from the final washing tank of the present invention, a suitable pre-filter may be provided before the semipermeable membrane.

To make the description easier to understand, the invention will now be described with reference to FIG.

In the cationic electrodeposition coating, an object 18 attached to a conveyor 17 is continuously flowed, and first enters the electrodeposition tank 1.
The object 18 to which the electrodeposition coating has been performed is then sequentially transferred to a UF filtration three-stage recovery washing step line including a first washing tank 2, a second washing tank 3, and a third washing tank 4. In the UF filtrate recovery water washing step, the filtrate obtained by filtering the coating liquid in the electrodeposition tank 1 through the UF device 8 enters the third water washing tank 4, and the second water washing tank 3,
It flows to the washing tank 2 in the countercurrent direction with the object to be coated, and is returned to the electrodeposition tank, so that the non-electrodeposited paint is collected simultaneously with the washing of the object. In each washing tank, the washing liquid in the tank is circulated by showering on the object to be coated, but excess washing liquid overflows and is sent to the immediately preceding washing tank.

The object to be coated that has left the third washing tank 4 is further washed with pure water, working water, or the like in the final washing tank 5, and then enters a drying and baking process.

The present invention is carried out in the final rinsing tank 5 in the final rinsing step of the above general coating-rinsing step. In order to facilitate understanding, a step of membrane-treating the wastewater from the washing tank 5 will be described. Since the paint components, miscellaneous ions and the like from the previous process are brought into the final washing tank 5 together with the article 18 to be coated, the washing water in the tank 5 is contaminated. The contaminated cleaning liquid cannot be subsequently used as final rinsing water and cannot be discarded as it is. Therefore, the contaminated cleaning liquid is extracted into the concentration tank 6 to separate and remove contaminants as follows. Thickening tank 6
The washing waste liquid extracted to the above is treated with a membrane treatment device 9 after the pH is constantly adjusted to 6.4 or less by adding an acid. The concentrated solution is further circulated through the membrane processing device 9 and can be concentrated to about 400 times in volume ratio. The filtrate is received in the filtrate tank 7, and all or a part of the filtrate is returned to, for example, the washing tank 5, or is directly showered on the object and reused. Further, the filtrate entering the filtrate tank 7 can be reused after being filtered once again through the RO treatment device 10. On the other hand, the film processing apparatus 9
Is extracted from the concentration tank when the concentration of the concentrated liquid in the concentration tank reaches a maximum of 200 times, as described in detail above.

FIG. 2 illustrates a method of treating waste water in the final washing tank when the final washing tank includes a plurality of tanks (three tanks are illustrated as an example). The object to be coated which has been electrodeposited and washed in the previous step enters the first tank of the final washing tank. The object to be cleaned washed here is then transported to the second tank and the third tank in order, while the washing water is sent in countercurrent from the last tank to the first tank. The filtrate from the membrane treatment device 9 or 10 is supplied to the third tank (FIG. 2).
24) (or after directly washing the object to enter the third tank), the object sent from the second tank is repeatedly washed, and the overflowing washing waste liquid is discharged to the second tank (23 in FIG. 2). )
Here, similarly, the object to be coated sent from the first tank (22 in FIG. 2) is repeatedly washed. As described above, the waste liquid from each tank overflows to the immediately preceding tank, the waste liquid from the first tank (22 in FIG. 2) is led to the concentration tank, and is applied to a filtration device. At least a part of the filtrate is transferred to the final tank of the final washing tank. Back to.

In FIG. 2, the filtrate after the membrane treatment is applied to the final tank of the final washing tank (FIG. 2).
However, if it is desired to finally wash with pure water, the filtrate is not returned to the final tank but is returned to an earlier tank (for example, 22 and 23 in FIG. 2), and pure water is added to the third tank. Water may be introduced for washing.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow sheet showing a flow of waste water treatment in a final washing tank in an electrodeposition coating process.

FIG. 2 shows a flow sheet of the final washing waste liquid treatment using a multi-stage (for example, three tanks) final washing tank.

The numbers in the figure mean the following: 1. Electrodeposition tank, 2. First rinsing tank, 3. Second rinsing tank, 4. Third rinsing tank, 5. Final rinsing tank, 6. Concentration tank, 7. Filtrate tank, 8.UF equipment, 9.
Filtration device with semi-permeable membrane, 10.RO device, 11, 12, 13, 1
4, 15, 16. Pump, 17. Conveyor, 18. Coated object, 19. Washing shower device, 20. Concentrate, 21. Filtrate, 22. Final washing tank 1st tank, 23. Final washing tank 2nd Vessel, 24. Third Rinse Vessel Third Vessel Aspects of the Invention The following examples illustrate the invention.

In the following examples, the non-volatile component in the concentrated liquid means a residual component when the concentrated liquid is dried at 110 ° C. and reaches a constant weight.

Example 1 NV (concentration of nonvolatile components) = 20% by weight, pH = 6.5, MEQ = 25
In the cationic electrodeposition paint being managed (Nippon Paint Co., Ltd.: Power Top (registered trademark) U-2500) of 10 m ³ reactor with coating conditions: liquid temperature = 28 ° C., CV (painted voltage) 250V, energization time = Washing of the electrodeposited line in 2-3 minutes is performed in three stages of UF recovery washing and one stage of final washing.
The object to be coated after UF recovery water washing was washed in a final washing tank with pure water having an electric conductivity of 2 μs / cm. At this time, depending on the paint components brought in, the waste liquid in the final washing tank was NV = 0.1%, pH = It was 6.5.

The waste liquid was introduced into a concentration tank, adjusted to pH = 5.0 using acetic acid, and then subjected to filtration provided with a UF membrane (Microza (registered trademark) KCV3010, manufactured by Asahi Kasei Corporation).

In this case, the filtration conditions are as follows: input pressure = 2.5 kg / cm ² , output pressure = 0.5 kg / c
It was m ^2.

The filtrate amount was 12 / min at the initial stage, and was 6 / min after the concentration was performed up to 100 times, and the filtration stability was excellent.

Although the waste liquid subjected to filtration was 5000, the filtrate was subjected to a UF membrane to obtain a filtrate 4950 and a concentrated liquid 50, and the concentrated liquid was discharged to another container.

The filtrate is NV = 0.04%, pH = 5.2, and the concentrated solution is NV = 6.2%, pH
= 6.3.

The filtrate was returned to the final washing tank again and used as a substitute for pure water, but the finish was good, and the re-dissolution test of the object did not cause any problem.

The concentrate extracted in another container was evaporated to dryness, reduced in volume and incinerated.

The operation was extremely stable even though the filtrate was reused in the continuous repetition test, and could be continued without trouble even after 6 months.

Example 2 The waste liquid from the pure water washing tank used in Example 1 was adjusted to pH = 5.0 using an electrodeposition tank polar solution (main component of about 0.05% acetic acid), and then UF membrane (manufactured by Asahi Kasei Kogyo Co., Ltd.) Microza (registered trademark) KCV3010).

In this case, the filtration conditions are as follows: input pressure = 2.5 kg / cm ² , output pressure = 0.5 kg / c
It was m ^2.

The filtrate amount was 12 / min at the initial stage, and 3 / min after concentration was performed up to 200 times, and the filtration stability was excellent.

Although the waste liquid subjected to the filtration was 5,000, it was subjected to a UF membrane to obtain a filtrate 4975 and a concentrated liquid 25.

The filtrate is NV = 0.05%, pH = 5.2, the concentrate is NV = 10%, pH
= 6.4.

The filtrate was returned to the final washing tank again and used as a substitute for pure water, but the finish was good, and the re-dissolution test of the object did not cause any problem.

Example 3 In Example 2, the concentrated liquid was returned to the electrodeposition tank. The operation condition was stable even after returning to the electrodeposition tank, and the surface finish of the product after baking finish was good.

Example 4 The total volume of the waste liquid from the pure water washing tank used in Example 1 was 50
The solution was supplied to a concentration tank of No. 00, adjusted to pH 6.0 with acetic acid, and then supplied to a UF membrane (Microza (registered trademark) KCV-3010 manufactured by Asahi Kasei Kogyo Co., Ltd.).

In this case, the filtration conditions are as follows: input pressure = 2.5 kg / cm ² , output pressure = 0.5 kg / c
It was m ^2. During the filtration operation, acid backwashing was performed once every 30 minutes for 30 seconds. In the acid backwashing, a filtrate obtained by adding 300 ppm of acetic acid to the filtrate was used as a backwashing liquid at a time of 15 times. As a result, the pH of the concentrate was constantly adjusted to the range of 6.0 to 6.3. The filtrate amount was 12 / min at the initial stage, 10 / min after the concentration was performed up to 10 times, and 6 / min after the concentration was further performed up to 100 times, and the filtration stability was excellent. Filtration was continued for 4 days, 5000 days a day.
Each of the filtrates was obtained, and while being sent to the washing tank, an amount of waste liquid from the pure water washing tank corresponding to the amount of the filtrate was constantly supplied to the concentration tank. On the fifth day, the supply of the waste liquid from the pure water washing tank was stopped, and the 5000 concentrated solution was further concentrated by batch concentration treatment for 250 minutes.
Concentrated. (Concentration magnification: 100 times) The total amount of waste liquid subjected to filtration was 25,000, but the filtrate
4750 and a concentrate 250 were obtained.

The initial filtrate of the batch concentration on day 5 (at the time of 5-fold concentration) is NV
= 0.04%, pH = 5.9, final concentrated solution (at 100-fold concentration)
Was NV = 6.1%, pH = 6.2. This filtrate was used as a substitute for pure water while always being returned to the final washing tank, but the finish was good and there was no problem in the re-dissolution test of the object to be coated.

Example 5 The same test as in Example 4 was performed, and a part of the obtained UF filtrate was supplied to an RO membrane (SU-700, manufactured by Toray Industries, Inc.). The RO filtrate was showered on the object to be coated at the time of leaving the final washing tank, and the dripping water was collected in the final washing tank. The finish was good, and the re-dissolution test of the object did not cause any problem.

The RO filtration was performed at an operating pressure of 15 kg / cm ² , the RO filtrate was collected at 3 / min, and the RO concentrate was sent to a washing tank. RO
The water quality of the filtrate was NV = 0.001% or less, pH = 6.4, and conductivity = 2 μs / cm.

Comparative Example 1 The waste water from the final washing tank of Example 1 was applied to a UF membrane (Microsa (registered trademark) KCV3010, manufactured by Asahi Kasei Kogyo Co., Ltd.) in the same manner as in Example 1 without any pH adjustment, and the concentrated solution and the filtrate were used. And separation. Concentrate pH from 6.5
Rose to 7.0. The amount of filtrate from the UF membrane was 12 / min at first, but gradually decreased as circulating concentration continued.
At the time of double concentration, the concentration dropped to 1 / min or less, and the filtration stability was insufficient.

Comparative Example 2 Operating under the same conditions as in Example 1, NV =
0.1%, pH = 6.5 waste liquid is added with acetic acid to pH = 6.0,
This was subjected to UF treatment in the same manner as in Example 1. The filtrate volume by the UF membrane was 12 / min at first, but the pH of the concentrate gradually increased as the circulating concentration was continued, while the filtrate volume gradually decreased. Is 1 / min
As a result, the UF process could not be continued.

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Claims (9)

(57) [Claims] 1. A method for treating a waste liquid discharged from a final washing tank in a cationic electrodeposition coating step, wherein the waste liquid in a concentration tank 6 is provided with a semipermeable membrane while the pH of the waste liquid is constantly maintained at 6.4 or less. A method for treating waste water in a final washing tank, wherein the liquid is separated into a concentrated liquid containing a paint component and a filtrate by circulating through a filtration device 9. 2. The final washing tank according to claim 1, wherein all or a part of the filtrate obtained by separation by the semipermeable membrane is returned to the final washing tank and reused as final washing water. Waste liquid treatment method. 3. A certain amount of washing waste liquid is accumulated in the concentration tank 6,
3. The final washing according to claim 1, wherein the pH of the liquid in the concentration tank is adjusted to 6.0 or less by adding an acid at an early stage of the operation before the circulation to the filtration device 9 is started. Treatment method for tank waste liquid. 4. The method according to claim 1, wherein when the concentration of the circulating waste liquid in the concentration tank reaches a maximum of 200 times, the concentrated liquid is withdrawn from the concentration tank. The method for treating a final washing tank waste liquid described in Crab. 5. When a plurality of tanks are used as a final washing tank,
The waste liquid from each of the final washing tanks is returned to the immediately preceding tank, the waste liquid from the first tank of the final washing tank is put into a concentration tank, and the filtrate is passed through a filtration device.At least a part of the filtrate is returned to the final tank of the final washing tank. The method for treating waste water in a final washing tank according to any one of claims 1 to 4, wherein the waste water is reused as final washing water. 6. A final washing tank which separates a waste liquid discharged from the final washing tank in the cationic electrodeposition coating step into a concentrated liquid containing a paint component and a filtrate by circulating the waste liquid through a filtration device provided with a semipermeable membrane. In the treatment of waste liquid, the semi-permeable membrane is 300 ~ 3,0
Backwashing with a filtrate to which 00 ppm of organic acid is added, wherein the pH of the concentrated solution in the concentration tank is constantly maintained at 6.4 or lower. Processing method. 7. The method for treating waste water in a final washing tank according to any one of claims 1 to 6, wherein the pH of the waste water in the final washing tank is adjusted using an extreme solution, an organic acid, sulfamic acid, or carbonic acid. 8. The method for treating waste water in a final washing tank according to claim 1, wherein the semipermeable membrane is a UF membrane. 9. The method according to claim 1, wherein the separation by the semipermeable membrane is a combination of separation by UF and separation by RO.