JP4092384B2 - Electrodeposition wastewater treatment method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
TECHNICAL FIELD The present invention relates to an electrodeposition coating wastewater treatment method, and more particularly to a method for treating wastewater generated in a cationic electrodeposition coating process.
[0002]
[Prior art]
The electrodeposition coating method is one of the methods for forming a coating film on the surface of a metal product. A uniform coating film can be formed even on an object having a complicated shape. Often used for painting.
In this electrodeposition coating, an object to be coated is immersed in a water-based paint, a direct current is applied to the object to be coated with the anode or cathode as the object to be coated, and the electrode opposite to the object to be painted as a coating tank. This is a method for forming a coating film. When the object to be coated becomes an anode, anion electrodeposition coating, and conversely, when the object to be coated becomes a cathode, it is called cationic electrodeposition coating, but there is no metal elution from the object to be coated, and it has excellent rust resistance. Cationic electrodeposition coating is often used for undercoating.
[0003]
Compared to anionic electrodeposition paints that contain carboxyl groups in the resin molecules and are rendered water-soluble by neutralization with quaternized ammonium salts and amines, cationic electrodeposition paints are secondary amines for epoxy group-containing resins, etc. It is made water-soluble by adding a suitable curing agent and neutralizing with an acid. For this reason, the cationic electrodeposition paint contains quaternized ammonium salts and amines.
[0004]
On the other hand, in the electrodeposition coating method, the electrodeposition coating liquid itself is water-based, and a large amount of wastewater is generated in the pretreatment and posttreatment, so that the wastewater treatment is extremely important.
Conventional electrodeposition coating will be described with reference to FIG. FIG. 3 is a flowchart of a conventional electrodeposition coating method. The product flow is indicated by a solid line, and the drainage route is indicated by a broken line.
Metal objects such as automobile parts are products in which a coating film is coated on the metal surface through a pretreatment process, an electrodeposition coating process, and a posttreatment process. The pretreatment step sequentially passes through the alkaline degreasing tank 2 and the water washing tank 2a for degreasing the surface of the object to be coated, and the chemical conversion treatment tank 3 and the water washing tank 3a for performing chemical conversion coating using zinc phosphate or the like. It is the process of setting it as the surface suitable for electrodeposition coating film formation by this. In the electrodeposition coating step, the pre-treatment object is immersed in a water-soluble or hydrophilic emulsion cationic electrodeposition coating liquid in the electrodeposition tank 1, and a water-insoluble resin film is formed by applying a DC voltage. It is a process. The post-processing step is a step of washing and removing the electrodeposition coating liquid adhering to the surface of the object to be coated pulled up from the electrodeposition coating liquid in the washing tank 1a. A plurality of washing tanks 1a, 2a and 3a are usually provided.
[0005]
The waste water from the washing tanks 1a and 2a is a low phosphorus concentration-containing wastewater containing quaternized ammonium salts of several tens of ppm, and the waste water from the washing tank 3a is a high phosphorus concentration containing wastewater containing zinc phosphate. Conventionally, drainage from each washing tank is merged and stored in the drainage adjustment tank 4 because pH adjustment is facilitated. After that, for example, an inorganic flocculant is added in a coagulating sedimentation tank 5 such as a pressurized flotation type and separated into flotation scum and treated water, and the treated water is further sent to the activated sludge tank 6 for treatment. The floating scum is treated as dewatered sludge. Conventionally, as the inorganic flocculant, metal salts such as aluminum salts such as aluminum chloride, aluminum sulfate, and aluminum hydroxide, and iron salts such as iron chloride, iron sulfate, and iron hydroxide are used.
[0006]
[Problems to be solved by the invention]
However, since it is necessary to add a large amount of the inorganic flocculant for dephosphorization, there is a problem that the amount of settled sludge increases and secondary contamination such as generation of a large amount of sludge is likely to occur.
Moreover, since the low phosphorus concentration drainage and the high phosphorus concentration drainage are merged, the phosphorus concentration in the wastewater stored in the drainage adjustment tank 4 is relative to that of the high phosphorus concentration drainage alone. As a result, the phosphorus concentration decreases. As a result, the reaction rate between the dephosphorizing agent and the contained phosphorus becomes slow, and if the wastewater treatment is performed in a short time, the removal efficiency of phosphorus may be inferior. Even if the wastewater containing low phosphorus concentration and the wastewater containing high phosphorus concentration are separated and dephosphorized, there is still a problem that a large amount of an inorganic flocculant for dephosphorization is required. If the amount of the inorganic flocculant added increases, the phosphorus concentration tends to decrease, but conversely, the concentration of other anions increases, resulting in a secondary contamination source that leads to environmental destruction.
Furthermore, the addition of a large amount of an inorganic flocculant has a problem of lowering the activity in the activated sludge treatment step, and there is a problem that the efficiency of removing nitrogen and residual phosphorus is deteriorated.
[0007]
The present invention has been made in order to cope with such problems. When wastewater generated by electrodeposition coating is treated, phosphorus removal efficiency is increased and sludge generation is reduced, which leads to environmental destruction. An object of the present invention is to provide an electrodeposition coating wastewater treatment method that prevents secondary contamination and does not lower the activity in the activated sludge treatment process.
[0008]
[Means for Solving the Problems]
The electrodeposition coating wastewater treatment method of the present invention is a dephosphorization step for removing phosphorus from wastewater containing low phosphorus concentration and wastewater containing high phosphorus concentration generated in the electrodeposition coating step, and an activity of treating with activated sludge after the dephosphorization step. In the electrodeposition coating wastewater treatment method comprising a sludge treatment step, the dephosphorization step separates the low phosphorus concentration-containing wastewater and the high phosphorus concentration-containing wastewater, and at least the high phosphorus concentration-containing wastewater is at least in the coagulation sedimentation tank. It is characterized by dephosphorization by reacting with a phosphorus remover comprising an aqueous solution containing magnesium ions in which an anion of a weakly acidic acid is coexisting with orthophosphoric acid .
[0010]
Further, the anion is a carbonate ion.
[0011]
By reacting the separated high phosphorus concentration wastewater with a phosphorus removal agent in the coagulation sedimentation tank, the phosphorus component can be effectively removed from the high phosphorus concentration wastewater containing zinc phosphate and the like. In addition, by using an aqueous solution containing magnesium ions in which an anion of an acid that is weaker than orthophosphoric acid coexists as a phosphorus removing agent, it reacts with phosphate ions and the like, so that hardly soluble or insoluble phosphorus compounds are easily precipitated. The amount of dewatered sludge that is finally generated can be reduced.
Moreover, when carbonate ion is used as an anion of acid, release of carbon dioxide, which causes global warming, is suppressed and effective use can be achieved along with removal of phosphorus.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
The electrodeposition coating according to the present invention will be described with reference to FIG. 1 particularly from the viewpoint of wastewater treatment. FIG. 1 is a flowchart of an electrodeposition coating method. The product flow is indicated by a solid line, and the drainage route is indicated by a broken line.
The electrodeposition coating according to the present invention may be either anionic electrodeposition coating or cationic electrodeposition coating, but is particularly suitable for cationic electrodeposition coating.
Wastewater containing high phosphorus concentration and containing zinc phosphate generated from the washing tank 3a in the cationic electrodeposition coating is directly sent to the coagulation sedimentation tank 5a through the drainage adjustment tank 4a. Moreover, the low phosphorus concentration containing waste water which generate | occur | produces from the water-washing tanks 2a and 1a merges with the waste water adjustment tank 4b, and is sent to the coagulation sedimentation tank 5b. Waste water from the coagulation sedimentation tanks 5a and 5b is finally treated in the activated sludge layer 6.
[0013]
In the present invention, the wastewater containing high phosphorus concentration refers to wastewater after the object to be coated treated with a treatment liquid containing a phosphorus component is washed with water, for example, zinc phosphate generated when the chemical conversion coating film is washed with water. Contains wastewater. More specifically, it means wastewater containing 10 ppm or more of phosphorus. In addition, wastewater containing low phosphorus concentration refers to wastewater after washing an object to be treated that has been treated with a treatment liquid that is essentially free of phosphorus components, excluding phosphorus components contained as impurities, such as electrodeposition coating. It refers to the drainage after washing with water after alkaline degreasing. More specifically, wastewater containing less than 10 ppm phosphorus.
[0014]
Wastewater containing high phosphorus concentration collected in the coagulation sedimentation tank 1 is reacted with a phosphorus removing agent. As the coagulation sedimentation tank 1, any of a so-called slurry circulation type, a slurry blanket type, and a mixed type of both can be used. In the present invention, since the phosphorus removing agent is added in the form of a slurry, a rapid agglomeration precipitation tank which is a kind of slurry blanket type is preferred. Such a rapid flocculation settling tank is shown in FIG. FIG. 2 is a cross-sectional view of a rapid flocculation settling tank. The rapid agglomeration sedimentation tank has an inner tank 8 having a stirring means 9 and an air blowing means 10 disposed in the lower part of the stirring means 9, and the circulation amount of the waste water 3b containing high phosphorus concentration is air. It is adjusted by the amount of blowing. A high phosphorus concentration drainage 3b and a phosphorus removal agent 7 are injected and mixed in the lower part of the rapid flocculation sedimentation tank and circulated through the rapid flocculation sedimentation tank, so that flocculation and clarification by floc sedimentation are performed at the same time. Sewage sludge is taken out from the lower part of the treated water.
The treated water from the rapid flocculating sedimentation tank is combined with the treated water from the flocculating sedimentation tank 2 and sent to the activated sludge tank 6 for treatment.
[0015]
The phosphorus removing agent according to the present invention is an aqueous solution containing magnesium ions in which an anion of an acid that is weaker than orthophosphoric acid coexists. The magnesium ion may be a divalent magnesium cation that can react with the phosphorus component.
In addition, by coexisting with the anion of at least a weakly acidic acid than orthophosphoric acid, the solubility of magnesium ions can be improved, and a poorly soluble or insoluble phosphorus compound can be efficiently generated, and floc can be precipitated and separated. Can do.
Such an aqueous solution containing a magnesium cation can be obtained, for example, by dissolving magnesium carbonate or magnesium acetate in water, or by blowing carbon dioxide into a magnesium hydroxide slurry or magnesium oxide slurry.
[0016]
In particular, the method of blowing carbon dioxide into the magnesium hydroxide slurry is suitable for the present invention because carbon dioxide generated in the aeration tank of the activated sludge treatment apparatus can be used as a carbon dioxide source.
The magnesium hydroxide slurry liquid can be easily obtained by reacting magnesium chloride, magnesium sulfate, magnesium nitrate or the like with an alkali such as aqueous ammonia or sodium hydroxide.
[0017]
Examples of the acid that is weaker than orthophosphoric acid include carbonic acid, acetic acid, ethylenediaminetetraacetic acid, and bicarbonate.
Among these, carbonic acid capable of increasing the solubility of magnesium ions is preferable, and carbonic acid is preferably generated by blowing carbon dioxide gas.
[0018]
By introducing a phosphorus removing agent into the rapid flocculation settling tank, a hardly soluble or insoluble phosphorus compound is precipitated, and dephosphorization is performed by separating the phosphorus compound.
When the phosphorus compound is precipitated, the pH of the aqueous solution containing phosphorus-containing ions is preferably adjusted, and in that case, the pH is 7.0 to 9.5, preferably 7.5 to 8.5. This is because when the pH is acidic, carbonate ions tend to be carbon dioxide gas, and when the pH is alkaline, magnesium hydroxide tends to precipitate, and in either case, the magnesium ion concentration becomes small. The pH can be adjusted with an alkaline aqueous solution such as aqueous ammonia or aqueous sodium hydroxide.
[0019]
As the phosphorus compound slightly soluble or insoluble, varies depending ions containing phosphorus, for example, in the case of phosphate ion, magnesium phosphate _{(Mg 3 (PO 4) 2} ) so that the precipitates. The solubility of this magnesium phosphate is 1 × 10 ^-10 g / l.
[0020]
The phosphorus removing agent according to the present invention can further enhance the phosphorus removal efficiency by adding a substance capable of increasing the magnesium ion solution concentration. Examples of such substances include alkylamines such as ammonia, dimethylamine, and diethylamine, alkanolamines such as monoethanolamine, diethanolamine, and triethanolamine, and ammonium salts such as ammonium chloride, ammonium sulfate, and ammonium bromide. Can do.
Among such ammonia and amines, ammonia that can be easily removed by secondary treatment is preferred for the present invention.
[0021]
The phosphorus removing agent according to the present invention is preferably a method in which carbon dioxide gas is blown into a magnesium hydroxide slurry or magnesium oxide slurry as a starting material, and carbon dioxide is preferably dissolved in a saturated or supersaturated state. Specific examples of the method of dissolving include a method of saturating a magnesium hydroxide slurry liquid under pressure in an autoclave and a method of blowing carbon dioxide gas into the magnesium hydroxide slurry liquid. Carbon dioxide gas can be used by appropriately pretreating gas discharged from combustion gas or activated sludge aeration tank, which is a global environmental problem.
[0022]
In addition, the phosphorus removal agent which concerns on this invention can be used also as a phosphorus removal agent of the low phosphorus concentration containing waste_water | drain generate | occur | produced from the water-washing tanks 2a and 1a. In the case of wastewater containing low phosphorus concentration, treatment with a polymer flocculant enables easy denitrification and dephosphorization in the subsequent activated sludge treatment. When a polymer flocculant is used, an adverse effect of an inorganic flocculant such as aluminum sulfate acting on activated sludge can be suppressed. As the polymer flocculant, an anion, a cation, or an amphoteric flocculant can be used, but an anionic polymer flocculant is preferable in view of the surface potential of the produced magnesium phosphate.
[0023]
The treated water that has been solid-liquid separated in the coagulation sedimentation tank is guided to the activated sludge tank, and the settled sludge becomes dewatered sludge. A phosphorus remover can be produced by bringing carbon dioxide gas generated by the reaction between activated sludge and treated water into contact with the magnesium hydroxide slurry solution. By this method, it is possible to effectively use the carbon dioxide gas generated in the activated sludge tank.
[0024]
【Example】
Reference Example Production of phosphorus remover;
Magnesium hydroxide (500 g) was charged into 10 kg of pure water and sufficiently stirred to obtain a slurry. The slurry was maintained at 15 ° C., and carbon dioxide gas was blown into the slurry at atmospheric pressure for 10 hours. Then, it filtered with the filter press and obtained the filtrate as a phosphorus removal agent. The dissolved magnesium ion concentration was measured by chelate titration and found to be 6500 ppm.
[0025]
Example 1
Undercoat coating of automobile parts was performed by cationic electrodeposition coating, and the wastewater generated at that time was separated into wastewater containing low phosphorus concentration and wastewater containing high phosphorus concentration.
Wastewater containing low phosphorus concentration is wastewater that has been washed with automobile parts that have passed through an alkaline degreasing tank, and wastewater that has been washed with parts after electrodeposition coating, and the treatment amount is 1500 t / day. Wastewater containing high phosphorus concentration is wastewater that has been washed through the chemical treatment tank. The phosphorus concentration is 780ppm, and the treatment amount is 200t / day.
[0026]
Wastewater containing high phosphorus concentration is temporarily stored in the drainage adjustment layer, and the treatment flow rate, pH, etc. are adjusted and moved to the coagulation reactor. The pH was 7.5-8.5. 340 ppm of phosphorus removal agent was added as the amount of magnesium per treatment flow rate unit. In the agglomeration reaction tank, a precipitate of magnesium phosphate salt is generated, and the supernatant liquid after solid-liquid separation is subjected to activated sludge treatment.
[0027]
On the other hand, wastewater containing low phosphorus concentration is temporarily stored in the drainage adjustment layer, and the treatment flow rate, pH, etc. are adjusted and moved to the coagulation reactor. The pH was 6.5-8.5. In the agglomeration reaction tank, anionic flocculant (trade name of Alcy Flock, Nippon Alcy Co., Ltd.) was added at 5 ppm per unit of treatment flow rate, and solid-liquid separation was performed by a pressurized flotation method. The supernatant is combined with the supernatant from the wastewater containing high phosphorus concentration and subjected to activated sludge treatment.
[0028]
The treated water treated with activated sludge had a phosphorus concentration of 0.5 to 1 ppm, a COD of 50 ppm or less, and an SS value of 50 ppm or less.
The amount of dewatered sludge separated into solid and liquid was 1/2 or less compared to the amount of dehydrated sludge of Comparative Example 2 described below.
[0029]
Comparative Example 1
The same cationic electrodeposition coating as in Example 1 was performed, and the waste water treatment was performed as follows.
Wastewater containing high phosphorus concentration and wastewater containing low phosphorus concentration are merged and introduced into a drainage adjustment tank, and the treatment flow rate, pH, etc. are adjusted and moved to a coagulation reaction tank. The pH was 6.5-7.5. In the agglomeration reactor, 500 ppm of aluminum sulfate was added per unit of treatment flow rate, and solid-liquid separation was performed by the pressure flotation method. The supernatant is subjected to activated sludge treatment.
The treated water treated with activated sludge had a phosphorus concentration of 20 to 35 ppm, a COD of 100 ppm or more, and an SS value of 50 to 100 ppm.
[0030]
Comparative Example 2
Waste water treatment was carried out by the same method as in Comparative Example 1 except that the amount of aluminum sulfate added was 12000 ppm.
The treated water treated with activated sludge had a phosphorus concentration of 1.5 to 1.8 ppm, a COD of 100 ppm or more, and an SS value of 50 to 100 ppm. However, the amount of dewatered sludge separated into solid and liquid was more than twice that of Comparative Example 1.
[0031]
【The invention's effect】
The electrodeposition coating wastewater treatment method of the present invention separates low phosphorus concentration-containing wastewater and high phosphorus concentration-containing wastewater, and at least the high phosphorus concentration-containing wastewater is an acid that is at least weaker than orthophosphoric acid in the coagulation sedimentation tank. Since the dephosphorization is carried out by reacting with a phosphorus removing agent comprising an aqueous solution containing magnesium ions in which the anions coexist, the amount of the inorganic flocculant used can be greatly reduced. As a result, the amount of dewatered sludge is reduced, and further, the activity of the activated sludge is not inhibited, so that denitrification and dephosphorization by the activated sludge can be improved. Moreover, phosphorus removal efficiency can be further increased, and the amount of chemicals used for wastewater treatment can be greatly reduced.
[0033]
Furthermore, since carbonate ions are used as anions of acids that are weaker than orthophosphoric acid, the removal of phosphorus and the release of carbon dioxide, which is a cause of global warming, are suppressed, and its effective use is reduced. I can plan.
[Brief description of the drawings]
FIG. 1 is a flowchart of an electrodeposition coating method.
FIG. 2 is a cross-sectional view of a rapid flocculation settling tank.
FIG. 3 is a flowchart of a conventional electrodeposition coating method.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Electrodeposition tank 2 Alkali degreasing tank 3 Chemical conversion treatment tank 4 Drainage adjustment tank 5 Coagulation sedimentation tank 6 Activated sludge tank 7 Phosphorus removal agent 8 Inner tank 9 Stirring means 10 Air blowing means

Claims (2)

Electrodeposition coating wastewater comprising a dephosphorization step for removing phosphorus from wastewater containing low phosphorus concentration and wastewater containing high phosphorus concentration generated in the electrodeposition coating step, and an activated sludge treatment step for treating with activated sludge after the dephosphorization step In the processing method,
In the dephosphorization step, the wastewater containing low phosphorus concentration and the wastewater containing high phosphorus concentration are separated, and at least the wastewater containing high phosphorus concentration is concentrated in a coagulating sedimentation tank , and an acid that is at least weakly acidic than orthophosphoric acid is used. An electrodeposition coating wastewater treatment method comprising dephosphorization by reacting with a phosphorus removal agent comprising an aqueous solution containing magnesium ions in which ions coexist .   2. The electrodeposition coating wastewater treatment method according to claim 1, wherein the anion is a carbonate ion.

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