METHOD AND SYSTEM FOR TREATING CIRCULATING WATER CIRCULATING THROUGH WET PAINT BOOTH

Abstract

An adequate control of a pH of circulating water circulating through a wet paint booth enables an efficient coagulation treatment of a paint included in the circulating water. A method for treating circulating water circulating through a wet paint booth and including a paint includes treating the paint included in the circulating water by adding an alkaline solution of a phenolic resin to the circulating water, and adjusting a pH of the circulating water to 6.5 to 8.0. An amount of the alkaline solution of the phenolic resin added to the circulating water is increased when the pH of the circulating water is a predetermined value of 8.0 or less. An acidic aluminum salt is further added to the circulating water. An amount of the acidic aluminum salt added to the circulating water is increased when the pH of the circulating water is a predetermined value of 6.5 or more.

Claims

1. A method for treating circulating water circulating through a wet paint booth and including a paint, the method comprising treating a paint included in the circulating water by adding an alkaline solution of a phenolic resin to the circulating water, and adjusting a pH of the circulating water to 6.5 to 8.0, wherein an amount of the alkaline solution of the phenolic resin added to the circulating water is increased when the pH of the circulating water is a predetermined value of 8.0 or less.

2. The method for treating circulating water circulating through a wet paint booth according to claim 1, wherein the alkaline solution of the phenolic resin is used as an alkaline agent, and the pH of the circulating water is adnjusted to 6.5 to 8.0 without using another alkaline agent.

3. The method for treating circulating water circulating through a wet paint booth according to claim 1, wherein an acidic aluminum salt is further added to the circulating water, and wherein an amount of the acidic aluminum salt added to the circulating water is increased when the pH of the circulating water is a predetermined value of 6.5 or more.

4. The method for treating circulating water circulating through a wet paint booth according to claim 3, wherein the acidic aluminum salt is used as an acidic agent, and the pH of the circulating water is adjusted to 6.5 to 8.0 without using another acidic agent.

5. A method for treating circulating water circulating through a wet paint booth and including a paint, the method comprising adding an alkaline solution of a phenolic resin and an acidic aluminum salt to the circulating water, and adjusting a pH of the circulating water to 6.5 to 8.0, wherein an amount of the alkaline solution of the phenolic resin added to the circulating water is increased when the pH of the circulating water is 6.8 or more and 8.0 or less, and wherein an amount of the acidic aluminum salt added to the circulating water is increased when the pH of the circulating water is 6.5 or more and 7.2 or less.

6. A method for treating circulating water circulating through a wet paint booth and including a paint, the method comprising coagulating the paint included in the circulating water by adding an alkaline solution of a phenolic resin and an acidic aluminum salt to the circulating water, wherein the alkaline solution of the phenolic resin and the acidic aluminum salt are separately added to the circulating water such that the pH of the circulating water is adjusted to 6.5 to 8.0.

7. The method for treating circulating water circulating through a wet paint booth according to claim 6, wherein the alkaline solution of a phenolic resin is used as an alkaline agent, the acidic aluminum salt is used as an acidic agent, and the pH of the circulating water is adjusted to 6.5 to 8.0 without using another pH-adjusting agent.

8. The method for treating circulating water circulating through a wet paint booth according to claim 6, wherein the amount of the alkaline solution of the phenolic resin added to the circulating water is increased when the pH of the circulating water is less than 6.5, and wherein the amount of the acidic aluminum salt added to the circulating water is increased when the pH of the circulating water is more than 8.0.

9. The method for treating circulating water circulating through a wet paint booth according to claim 3, wherein the acidic aluminum salt is at least one selected from the group consisting of aluminum sulfate, aluminum chloride, polyaluminum chloride, basic aluminum chloride, and aluminum nitrate.

10. The method for treating circulating water circulating through a wet paint booth according to claim 1, wherein a cationic polymer is further added to the circulating water.

11. The method for treating circulating water circulating through a wet paint booth according to claim 1, wherein a polymer coagulant is further added to the circulating water in order to perform a coagulation treatment, after the alkaline solution of the phenolic resin; the alkaline solution of a phenolic resin and the acidic aluminum salt; or the alkaline solution of a phenolic resin, the acidic aluminum salt, and the cationic polymer is added to the circulating water.

12. A system for treating circulating water circulating through a wet paint booth and including a paint, the system comprising: a circulating water pit where the circulating water flows such that the circulating water circulates between the pit and the wet paint booth; a pH-measuring device that measures a pH value of the circulating water; and a chemical-addition device that adds a treatment chemical to the circulating water in accordance with the measured pH value, wherein the system further comprises: a measurement tank provided with a pH-meter; a circulating water-feed device that feeds part of the circulating water to the measurement tank; a clarified water-feed device that feeds clarified water to the measurement tank; and a controlling device that controls the circulating water-feed device, the clarified water-feed device, and the chemical-addition device, wherein the controlling device controls the circulating water-feed device to feed the circulating water to the measurement tank and the clarified water-feed device to feed the clarified water to the measurement tank in an alternating manner, and wherein the pH-meter of the measurement tank measures the pH of the circulating water in a period during which the circulating water-feed device feeds the circulating water to the measurement tank.

13. The system for treating circulating water circulating through a wet paint booth according to claim 12, wherein the measurement tank includes a retention section in which water fed to the measurement tank is retained and an overflow section into which water retained in the retention section overflows, wherein the pH-meter of the measurement tank is disposed in the retention section, and wherein the circulating water-feed device is arranged to feed the circulating water to the retention section, and the clarified water-feed device is arranged to feed the clarified water to the retention section.

14. The system for treating circulating water circulating through a wet paint booth according to claim 12, wherein the controlling device controls the addition of a chemical in accordance with a pH value, the pH value being measured with the pH-meter after a lapse of a predetermined amount of time from the time when the circulating water-feed device starts feeding the circulating water to the measurement tank.

15. The system for treating circulating water circulating through a wet paint booth according to claim 12, the system further comprising a device that cleans the pH-meter with the clarified water fed by the clarified water-feed device to the measurement tank.

16. The system for treating circulating water circulating through a wet paint booth according to claim 12, wherein the controlling device controls the clarified water-feed device such that the feeding of the clarified water is stopped after the clarified water-feed device has fed the clarified water to the measurement tank, and subsequently controls the circulating water-feed device such that the circulating water-feed device starts feeding the circulating water to the measurement tank after a lapse of a predetermined amount of time from the time when the feeding of the clarified water is stopped.

17. The method for treating circulating water circulating through a wet paint booth according to claim 1, wherein the alkaline solution of the phenolic resin is added as a treatment chemical, or the alkaline solution of the phenolic resin and the acidic aluminum salt are added as a treatment chemical, with the system for treating circulating water circulating through a wet paint booth and including a paint, the system comprising: a circulating water pit where the circulating water flows such that the circulating water circulates between the pit and the wet paint booth; a pH-measuring device that measures a pH value of the circulating water; and a chemical-addition device that adds a treatment chemical to the circulating water in accordance with the measured pH value, wherein the system further comprises: a measurement tank provided with a pH-meter; a circulating water-feed device that feeds part of the circulating water to the measurement tank; a clarified water-feed device that feeds clarified water to the measurement tank; and a controlling device that controls the circulating water-feed device, the clarified water-feed device, and the chemical-addition device, wherein the controlling device controls the circulating water-feed device to feed the circulating water to the measurement tank and the clarified water-feed device to feed the clarified water to the measurement tank in an alternating manner, and wherein the pH-meter of the measurement tank measures the pH of the circulating water in a period during which the circulating water-feed device feeds the circulating water to the measurement tank.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0062] FIG. 1 is a schematic system diagram illustrating an example of a system for treating water circulating through a wet paint booth according to an embodiment of the present invention.

[0063] FIG. 2 is a graph illustrating changes in the pH values of circulating water samples with time, which are measured in Example 1 and Comparative Example 1.

[0064] FIG. 3 is a schematic diagram illustrating a test system used in Examples.

DESCRIPTION OF EMBODIMENTS

[0065] Embodiments of the present invention are described below in detail.

[0066] [Method for Treating Water Circulating through Wet Paint Booth]

[0067] A method for treating circulating water circulating through a wet paint booth according to a first embodiment of the present invention includes treating a paint included in the circulating water by adding an alkaline solution of a phenolic resin to the circulated water, and adjusting the pH of the circulating water to 6.5 to 8.0. The amount of the alkaline solution of a phenolic resin added to the circulating water is increased when the pH of the circulating water is a predetermined value of 8.0 or less. An acidic aluminum salt is preferably further added to the water circulating through a wet paint booth. In such a case, the amount of the acidic aluminum salt added to the circulating water is increased when the pH of the circulating water is a predetermined value of 6.5 or more.

[0068] A method for treating circulating water circulating through a wet paint booth according to a second embodiment of the present invention includes coagulating a water-based paint and/or a solvent-based paint included in the circulating water by adding an alkaline solution of a phenolic resin and an acidic aluminum salt to the circulating water. The alkaline solution of a phenolic resin and the acidic aluminum salt are separately added to the circulating water such that the pH of the circulating water is adjusted to 6.5 to 8.0.

[0069] <Mechanism of Action>

[0070] Water-based paints are anionic. Reducing the pH of the circulating water results in a reduction in the anion degree of a water-based paint. This increases the likelihood of the coagulation of the water-based paint. Furthermore, the activity of an anionic surfactant is reduced, which limits the foaming of the water-based paint.

[0071] An alkaline solution of a phenolic resin is anionic. Charge neutralization increases the affinity of the alkaline solution of a phenolic resin for mainly hydrophobic (nonionic) substances, that is, nonionic surfactants and resins included in paints. The anion degree of an alkaline solution of a phenolic resin is also reduced under an acidic condition. This reduces the amount of cationic polymer required for charge neutralization.

[0072] The acidic aluminum salt is cationic in the neutral to acidic region. Thus, the acidic aluminum salt causes an anionic paint and hydrophilic emulsoids and SS particles (mainly anions) produced as a result of hydrolysis, oxidative degradation, or biodegradation of the paint to be coagulated by charge neutralization. The acidic aluminum salt is also responsible for the charge neutralization of the alkaline solution of a phenolic resin.

[0073] In the case where an alkaline solution of a phenolic resin is used for coagulation and, in particular, in the case where an alkaline solution of a phenolic resin and an acidic aluminum salt are used in combination for coagulation, the coagulation treatment is preferably performed in the neutral or acidic region. Since the alkaline solution of a phenolic resin is alkaline and the acidic aluminum salt is acidic, they can be used for adjusting pH.

[0074] In the first embodiment of the present invention, an alkaline solution of a phenolic resin, which is alkaline, is used as a coagulant serving also as an alkaline agent. Moreover, an acidic aluminum salt, which is acidic, is used as a coagulant serving also as an acidic agent. In the first embodiment, the amounts of the above coagulants are each adjusted in response to the fluctuations in the pH of the circulating water so as to be within a specific range in which the coagulant produces its coagulation effect such that the pH of the circulating water is adjusted to 6.5 to 8.0 and preferably 6.8 to 7.2. This enables the pH of the circulating water to be adjusted to an optimum value without using a pH-adjusting agent.

[0075] When an alkaline solution of a phenolic resin is mixed with an acidic aluminum salt and the resulting mixture is used as a pH-adjusting agent, it is not possible to address fluctuations in the pH of the circulating water, and the use of an additional pH-adjusting agent is needed. Therefore, for adjusting pH, the alkaline solution of a phenolic resin and the acidic aluminum salt are not mixed together to be used in the form of a single agent but each separately added to water circulating through a wet paint booth.

[0076] In the second embodiment of the present invention, the alkaline solution of a phenolic resin and the acidic aluminum salt are not mixed together to be used in the form of a single agent but each separately added to water circulating through a wet paint booth. In the second embodiment of the present invention, an alkaline solution of a phenolic resin, which is alkaline, is used as a coagulant serving also as an alkaline agent, and an acidic aluminum salt, which is acidic, is used as a coagulant serving also as an acidic agent. The amounts of the above coagulants are each adjusted in response to the fluctuations in the pH of the circulating water so as to be within a specific range in which the coagulant produces its coagulation effect such that the pH of the circulating water is adjusted to 6.5 to 8.0 and preferably 6.8 to 7.2. This enables the pH of the circulating water to be adjusted to the optimum pH value without using a pH-adjusting agent.

[0077] A method for treating circulating water circulating through a wet paint booth according to the present invention is described taking, as an example, a case where an alkaline solution of a phenolic resin and an acidic aluminum salt are used in combination, the alkaline solution of a phenolic resin serving as an alkaline agent, the acidic aluminum salt serving as an acidic agent, any alkaline agent other than the alkaline solution of a phenolic resin is not used, any acidic agent other than the acidic aluminum salt is not used, and the amounts of the above agents are controlled such that the pH of the circulating water is adjusted to 6.5 to 8.0. In the present invention, an alkaline agent other than the alkaline solution of a phenolic resin and an acidic agent other than the acidic ammonium salt may be used in combination with the alkaline solution of a phenolic resin and the acidic ammonium salt.

[0078] Performing the treatment by using an alkaline solution of a phenolic resin as an alkaline agent but not any alkaline agent other than the alkaline solution of a phenolic resin and using an acidic aluminum salt as an acidic agent but not any acidic agent other than the acidic aluminum salt eliminates the need to use a pH-adjusting agent. This reduces the number of the types of the chemicals used, the amounts of chemicals used, and the amounts of time and effort required for chemical management and chemical feed control. Furthermore, the treatment cost can be reduced. In addition, it is not necessary to use hazardous chemicals such as acids and alkalis. This improves the workability. Thus, the above treatment method is markedly advantageous from an industrial viewpoint.

[0079] <Alkaline Solution of Phenolic Resin>

[0080] The phenolic resin included in the alkaline solution of a phenolic resin is a condensate produced by the condensation of a phenol such as a monohydric phenol (e.g., phenol, cresol, or xylenol) with an aldehyde such as formaldehyde or a modified product of the condensate which has not yet been cured by crosslinking. Specific examples of such a phenolic resin include the following. The phenolic resin may be of a novolac type or a resole type. The following phenolic resins may be used alone or in combination of two or more.

[0081] 1) Condensate of phenol and formaldehyde

[0082] 2) Condensate of cresol and formaldehyde

[0083] 3) Condensate of xylenol and formaldehyde

[0084] 4) Alkyl-modified phenolic resin produced by the alkylation of the phenolic resin described in any one of 1) to 3)

[0085] 5) Polyvinyl phenol

[0086] The alkali used for preparing the alkaline solution of a phenolic resin is commonly sodium hydroxide (NaOH) and/or potassium hydroxide (KOH). The concentration of the alkali in the alkaline solution of a phenolic resin is preferably 1% to 25% by weight. The concentration of the phenolic resin in the alkaline solution of a phenolic resin is preferably 1% to 50% by weight. For increasing the concentration of the phenolic resin in the alkaline solution of a phenolic resin, the alkaline solution may be heated to about 70 C. to 80 C. such that the phenolic resin is dissolved in the alkaline solution.

[0087] The alkaline solution of a phenolic resin commonly has a pH of about 10 to 13, that is, alkalinity.

[0088] <Acidic Aluminum Salt>

[0089] Examples of the acidic aluminum salt include basic aluminum chloride, aluminum sulfate, aluminum chloride, polyaluminum chloride, and aluminum nitrate. The above acidic aluminum salts may be used alone or in combination of two or more.

[0090] <Amounts of Chemicals Used>

[0091] The amounts of alkaline solution of a phenolic resin and acidic aluminum salt used are each adjusted so as to be within the range described below in response to the variation in the pH of water circulating through a wet paint booth.

[0092] The pH of water circulating through a wet paint booth is adjusted to 6.5 to 8.0 and preferably 6.8 to 7.2, which are the pH region in which the coagulation effects of the alkaline solution of a phenolic resin and the acidic aluminum salt are maximized. For adjusting the pH of the circulating water, it is preferable to control the amounts of the alkaline solution of a phenolic resin and the acidic aluminum salt without using, as pH-adjusting agents, an acidic agent such as an inorganic acid (e.g., sulfuric acid or hydrochloric acid) or an organic acid and an alkaline agent such as sodium hydroxide, potassium hydroxide, or ammonia.

[0093] Specifically, the amounts of the alkaline solution of a phenolic resin and the acidic aluminum salt which need to be used in the coagulation treatment are determined in advance. When the pH of the circulating water is changed and reaches a predetermined pH value of 8.0 or less, the amount of the alkaline solution of a phenolic resin, which is alkaline, is increased within the range described below so as to be larger than the predetermined amount in order to achieve a pH of 6.5 or more and preferably 6.8 or more. When the pH of the circulating water is changed and reaches a predetermined pH value of 6.5 or more, the amount of the acidic aluminum salt, which is acidic, is increased within the range described below so as to be larger than the predetermined amount in order to achieve a pH of 8.0 or less and preferably 7.2 or less. Adjusting the amounts of the alkaline solution of a phenolic resin and the acidic aluminum salt used in response to the variation in the pH of the circulating water in the above manner enables the pH of the circulating water to be adjusted to the optimum pH without using, as pH-adjusting agents, an acidic agent such as sulfuric acid and an alkaline agent such as sodium hydroxide in addition to the above coagulants.

[0094] When the pH of the circulating water is brought to fall within the above preferable range as a result of an increase in the amount of alkaline solution of a phenolic resin or the acidic aluminum salt used, the amount of the alkaline solution of a phenolic resin or the acidic aluminum salt used is reduced to the predetermined amount.

[0095] In order to adjust the pH of water circulating through a wet paint booth to 6.5 to 8.0 and preferably 6.8 to 7.2, the amount of alkaline solution of a phenolic resin used may be increased when the pH of the circulating water is less than 6.5 and preferably less than 6.8. In order to further reduce the variation in the pH of the circulating water, the amount of alkaline solution of a phenolic resin used may be increased when the pH of the circulating water is 6.8 or more and 8.0 or less.

[0096] For the same purpose, the amount of acidic aluminum salt used may be increased when the pH of the circulating water is more than 8.0 and preferably more than 7.2. In order to further reduce the variation in the pH of the circulating water, the amount of acidic aluminum salt used may be increased when the pH of the circulating water is 6.5 or more and 7.2 or less.

[0097] <Amounts of Alkaline Solution of Phenolic Resin Used>

[0098] The amount of alkaline solution of a phenolic resin added to water circulating through a wet paint booth varies depending on the properties of the water circulating through a wet paint booth, the type of the paint included in the water circulating through a wet paint booth, and the content of the paint in the circulating water and is preferably 1 mg/L or more and particularly preferably 5 mg/L or more in terms of the content of active component (resin solid content) relative to the amount of the circulating water. The amount of the alkaline solution of a phenolic resin added to the circulating water is preferably 0.1% by weight or more and is particularly preferably 0.5% by weight or more of the amount (solid content) of the paint included in the circulating water in terms of the content of active component.

[0099] Setting the proportion of amount of alkaline solution of a phenolic resin added to the circulating water to be equal to or higher than the above proportion enables a sufficient coagulation effect, a sufficient adhesion-reduction effect, and a sufficient anti-foaming effect to be achieved.

[0100] Adding an excessive amount of alkaline solution of a phenolic resin to the circulating water does not increase the advantageous effects as appropriate to the amount of alkaline solution of a phenolic resin added to the circulating water but disadvantageously increases the costs of chemicals, the amount of coagulated sludge, and the like.

[0101] The amount of alkaline solution of a phenolic resin added to water circulating through a wet paint booth is preferably 1000 mg/L or less, more preferably 1 to 200 mg/L, and particularly preferably 5 to 200 mg/L in terms of the content of active components. The amount of alkaline solution of a phenolic resin added to the circulating water is preferably 100% by weight or less and particularly preferably 0.5% to 10% by weight of the amount of the paint included in the circulating water in terms of the amount of active components.

[0102] The amount of alkaline solution of a phenolic resin added to the circulating water may temporarily exceed the above upper limit when the amount of alkaline solution of a phenolic resin added to the circulating water is increased in order to control the pH of the circulating water.

[0103] <Amount of Acidic Aluminum Salt Used>

[0104] The amount of acidic aluminum salt added to water circulating through a wet paint booth varies depending on the properties of the water circulating through a wet paint booth, the type of the alkaline solution of a phenolic resin added to the water circulating through a wet paint booth, the amount of alkaline solution of a phenolic resin added to the circulating water, and the presence of a cationic polymer used in combination and is preferably about 1 to 1000 mg/L, more preferably about 1 to 200 mg/L, and particularly preferably about 5 to 200 mg/L in terms of the content of active component relative to the amount of the circulating water. The amount of the acidic aluminum salt added to the circulating water is preferably about 0.1% to 100% by weight, is more preferably about 0.5% to 50% by weight, and is particularly preferably about 2.0% to 20% by weight of the amount of the paint included in the circulating water in terms of the content of active component.

[0105] Setting the amount of acidic aluminum salt added to the circulating water to be equal to or larger than the above lower limit enables a sufficient coagulation effect, a sufficient adhesion-reduction effect, and a sufficient anti-foaming effect to be achieved by the addition of the acidic aluminum salt. Adding the acidic aluminum salt to the circulating water in an amount exceeding the above upper limit does not increase the effect of the acidic aluminum salt as appropriate to the amount of acidic aluminum salt added to the circulating water but disadvantageously increases the costs of chemicals, the amount of coagulated sludge, and the like.

[0106] The amount of acidic aluminum salt added to the circulating water may temporarily exceed the above upper limit when the amount of acidic aluminum salt added to the circulating water is increased in order to adjust the pH of the circulating water.

[0107] <Cationic Polymer>

[0108] In the present invention, a cationic polymer may be used in addition to an alkaline solution of a phenolic resin and an acidic aluminum salt. Using a cationic polymer in combination with an alkaline solution of a phenolic resin and an acidic aluminum salt enables a further suitable coagulation effect, a further suitable adhesion-reduction effect, and a further suitable anti-foaming effect to be achieved.

[0109] Examples of the cationic polymer include, but are not limited to, organic coagulants having a weight-average molecular weight of 1000 to 1 million and preferably 5000 to 300 thousand, such as dimethyldiallylammonium chloride, alkylamine-epichlorohydrin condensate, polyethyleneimine, alkylene dichloride-polyalkylene polyamine condensate, dicyandichloride-polyalkylene polyamine condensate, DMA (dimethylaminoethyl methacrylate), and DADMAC (diallyldimethylammonium chloride).

[0110] The above cationic polymers may be used alone or in a mixture of two or more.

[0111] In the case where a cationic polymer is used in combination with an alkaline solution of a phenolic resin and an acidic aluminum salt, the amount of cationic polymer added to the water circulating through a wet paint booth varies depending on the properties of the circulating water, the type and amount of the alkaline solution of a phenolic resin used, the amount of the acidic aluminum salt used, and the like and is preferably set to about 5 to 100 mg/L, more preferably about 5 to 50 mg/L, and particularly preferably about 10 to 30 mg/L relative to the amount of water circulating through a wet paint booth in terms of the amount of active components. The amount of cationic polymer added to the circulating water is preferably about 0.01% to 10% by weight, more preferably about 0.05% to 5% by weight, and particularly preferably about 0.5% to 2% by weight of the amount of paint included in the circulating water in terms of the amount of active components.

[0112] Setting the amount of cationic polymer added to the circulating water to be equal to or larger than the above lower limit increases the coagulation effect, the adhesion-reduction effect, and the anti-foaming effect of the addition of the cationic polymer to a sufficient level. Adding the cationic polymer to the circulating water in an amount exceeding the above upper limit does not increase the effect of the cationic polymer as appropriate to the amount of cationic polymer added to the circulating water. Adding an excessively large amount of cationic polymer to the circulating water causes particles to electrically repel one another due to the excess cations. This leads to an insufficient coagulation. Adding a large amount of cationic polymer to the circulating water also disadvantageously increases the costs of chemicals, the amount of coagulated sludge, and the like.

[0113] <Coagulation Treatment by Addition of a Chemical>

[0114] A method for adding an alkaline solution of a phenolic resin, an acidic aluminum salt, and a cationic polymer, which may be optionally used as needed, to the circulating water circulating through a wet paint booth is not limited. The above agents may be added to the circulating water system about once or twice every day in an intermittent manner or in a continuous manner. The alkaline solution of a phenolic resin, the acidic aluminum salt, and the cationic polymer for coagulation treatment are each desirably fed to the circulating water continuously in a consistent amount with a pump. The addition of the alkaline solution of a phenolic resin and the acidic aluminum salt, which serve also as pH-adjusting agents, is preferably controlled in accordance with the pH of water circulating through a wet paint booth measured.

[0115] The position at which the alkaline solution of a phenolic resin, the acidic aluminum salt, and the cationic polymer are added to the circulating water is not limited; the above agents may be added to the circulating water at any position. In normal cases, an alkaline solution of a phenolic resin and an acidic aluminum salt that serve as coagulants are preferably added to piping that connects the circulating water pit to the wet paint booth, through which the circulating water passes. An alkaline solution of a phenolic resin and an acidic aluminum salt that serve as pH-adjusting agents are preferably added to the circulating water pit. Alternatively, the above pH-adjusting agents may also be added to the circulating water on the entry side of a separation tank to which the circulating water is returned.

[0116] The order of the addition of the phenolic resin, the acidic aluminum salt, and the cationic polymer optionally used as needed is not limited. The alkaline solution of a phenolic resin, the acidic aluminum salt, and the cationic polymer optionally used as needed may be added to the circulating water simultaneously at the same position or at different timings at different positions. In the case where the separation of coagulated sludge is performed with a floatation separation apparatus or a centrifugal separation apparatus, the cationic polymer may be added to the circulated water at a position upstream of the separation apparatus.

[0117] In the present invention, the alkaline solution of a phenolic resin and the acidic aluminum salt are not mixed together to form a single agent but separately used at least when being added to the circulating water for adjusting pH. The expression the alkaline solution of a phenolic resin and the acidic aluminum salt are separately added used herein means that these agents are not mixed together to form a single agent but separately added to the circulating water. The agents may be separately added to the circulating water at the same position at the same timing.

[0118] The cationic polymer may be added to the circulating water separately from the alkaline solution of a phenolic resin and the acidic aluminum salt. Alternatively, the cationic polymer may be mixed with the acidic aluminum salt, and the resulting mixture may be added to the circulating water.

[0119] Since the alkaline solution of a phenolic resin and the acidic aluminum salt serve also as pH-adjusting agents, the device that feeds the alkaline solution of a phenolic resin and the acidic aluminum salt is preferably capable of operating in cooperation with a pH-meter that measures the pH of water circulating through a wet paint booth. In such a case, the pH-meter may measure the pH of the circulating water at any position, such as in the circulating water pit or at the outlet of the circulation pump. The pH of the circulating water is preferably measured in a measurement tank other than the circulating water pit by using the system for treating water circulating through a wet paint booth according to the present invention, which is described below.

[0120] Upon the addition of the alkaline solution of a phenolic resin and the acidic aluminum salt or the addition of the alkaline solution of a phenolic resin, the acidic aluminum salt, and the cationic polymer, a paint included in the circulating water is quickly insolubilized and coagulate to form flocs. The flocs formed by coagulation are separated and recovered by, for example, flotation separation, wedge wire screening, rotary screening, bar screening, cyclone separation, or a method in which a centrifugal separating apparatus, a filtering apparatus, or the like is used.

[0121] The coagulated sludge separated and recovered by the above method is dewatered by gravity drainage or an ordinary method and subsequently disposed of by incineration and landfill. According to the present invention, an alkaline solution of a phenolic resin and an acidic aluminum salt are used also as pH-adjusting agents for adjusting the pH of the circulating water to an optimum pH value. This reduces the amount of chemicals required, the amount of sludge generated, and the cost of the disposal of the sludge.

[0122] In the coagulation treatment according to the present invention, subsequent to a coagulation treatment in which the alkaline solution of a phenolic resin and the acidic aluminum salt are added to the circulating water, or the alkaline solution of a phenolic resin, the acidic aluminum salt, and the cationic polymer are added to the circulating water, a polymer coagulant including a water-soluble polymer having a weight-average molecular weight of commonly more than one million and preferably five million or more may be further added to the circulating water in order to increase the size of the flocs.

[0123] Examples of the polymer coagulant include publicly known anionic polymer coagulants, cationic polymer coagulants, and zwitterionic polymer coagulants. The above polymer coagulants may be used alone or in combination of two or more.

[0124] In the case where a polymer coagulant is used, the amount of polymer coagulant added to the circulating water is determined appropriately so as to be 0.1% to 10% by weight and preferably 0.5% to 2% by weight of the amount of excess paint such that a suitable coagulation effect is achieved.

[0125] The method for treating water circulating through a wet paint booth according to the present invention may be used, with effect, for treating water circulating through a wet paint booth which includes a water-based paint, water circulating through a wet paint booth which includes a solvent-based paint, and water circulating through a wet paint booth which includes a water-based paint and a solvent-based paint.

[0126] The method for treating water circulating through a wet paint booth according to the present invention is preferably implemented with a system for treating water circulating through a wet paint booth according to the present invention, which is described below.

[0127] [System for Treating Water Circulating through Wet Paint Booth]

[0128] A system for treating water circulating through a wet paint booth according to the present invention is described below with reference to FIG. 1.

[0129] FIG. 1 is a schematic diagram illustrating an example of a system for treating water circulating through a wet paint booth according to an embodiment of the present invention, where Reference Numeral 1 denotes a wet paint booth, Reference Numeral 2 denotes a circulating water pit, Reference Numeral 3 denotes a measurement tank, Reference Numeral 4 denotes a sludge-recovering apparatus, Reference Numeral 5 denotes a recovered sludge-storage tank, and Reference Numeral 6 denotes a controlling apparatus. The controlling apparatus 6 is controlled by, for example, an ON-OFF control system or a PID control system, which are feedback control systems.

[0130] The circulating water contained in the circulating water pit 2 is drawn with a circulation pump P and fed to the wet paint booth 1 through piping 10. Circulation piping 12 is branched from the piping 10 in order to uniformly stir the circulating water contained in the circulating water pit 2. Part of the circulating water drawn with the circulation pump P is transported through the piping 12 and sprinkled over the surface of the circulating water contained in the circulating water pit 2. The piping 10 is provided with piping 13 and piping 14 that are connected thereto, through which the alkaline solution of a phenolic resin and the acidic aluminum salt, respectively, which serve as coagulants, are fed to the circulating water in predetermined amounts.

[0131] The circulating water circulating through a wet paint booth is fed to the wet paint booth 1 through the piping 10, collects excess paint in the wet paint booth 1, and is returned to the circulating water pit 2 through the piping 11 and the piping 12.

[0132] The circulating water pit 2 is provided with a device 21 for the addition of the alkaline solution of a phenolic resin and a device 22 for the addition of the acidic aluminum salt, which add the alkaline solution of a phenolic resin and the acidic aluminum salt, respectively, to the circulating water contained in the pit for adjusting pH. The above chemical-addition devices are each constituted by a chemical-storage tank, a chemical-feed pump, chemical-feed piping, and the like, which are not illustrated in the drawing. The controlling apparatus 6 controls the amounts of chemicals added to the circulating water. The control of the addition of the chemicals may be done by controlling the action of the chemical-feed pumps, switching the valves of the chemical-feed valves, or adjusting the degree of opening of the valves.

[0133] An overflow wall 31 partitions the measurement tank 3 into a retention section 32 and an overflow section 33. In the retention section 32, a pH-meter 34 and a water gauge (level switch) 35, which prevents a malfunction (measuring pH when the measurement tank 3 does not contain the circulating water) of the pH-meter 34, are disposed. The controlling apparatus 6 receives the pH value measured with the pH-meter 34 and the signal detected with the water gauge 35.

[0134] The overflow wall 31 has a V-shape notch portion formed in the vicinity of the center of the upper edge of the overflow wall 31. Water retained in the retention section 32 overflows through the notch portion, enters the overflow section 33, and discharges through an outlet 36 disposed on the overflow-section-33-side surface of the measurement tank 3. The discharged water is fed to the circulating water pit 2 through piping 16.

[0135] The retention section 32 of the measurement tank 3 receives, in addition to part of the circulating water fed through circulating water-feed piping 14 branched from the piping 12, clarified water, such as industrial water, fed through clarified water-feed piping 15. The feed piping 14 and the feed piping 15 are provided with on-off valves 14A and 15A, respectively. The controlling apparatus 6 controls the opening and closing of the valves 14A and 15A.

[0136] A device 37 that cleans the pH sensor included in the pH-meter 34 is disposed at the edge of the clarified water-feed piping 15. An example of the cleaning device 37 is a device that includes a plurality of cleaning nozzles through which the clarified water fed through the clarified water-feed piping 15 is ejected at a high pressure. The cleaning device 37 may optionally include an air nozzle through which air is ejected in order to clean the pH sensor with the clarified water and air bubbles.

[0137] The pH-meter 34 and the water gauge 35 are arranged in the retention section 32 of the measurement tank 3 at an adequate interval. The retention section 32 may have any volume sufficient to clean the pH-meter 34 with the cleaning device 37. The overflow section 33 may have a size sufficient to allow the overflowing water to discharge through the outlet 36 smoothly. For example, the storage capacity of the retention section 32 may be about 5 to 10 L. When the retention section 32 has such a small capacity, it is possible to force water stored in the retention section 32 to discharge from the retention section 32 in a short time and fill the retention section 32 with new circulating water by, for example, feeding circulating water through the circulating water-feed piping 14 at a high flow rate of about 5 to 50 L/min and preferably about 10 to 20 L/min. The same applies to the case where clarified water, such as industrial water, is fed through the clarified water-feed piping 15.

[0138] The circulating water contained in the circulating water pit 2 is drawn through piping 17 and, after a polymer coagulant has been added to the circulating water through piping 18, subjected to a coagulative separation treatment in the sludge-recovering apparatus 4. The coagulative separation water produced in the sludge-recovering apparatus 4 is returned to the circulating water pit 2 through piping 19. The resulting coagulated sludge is fed to the recovered sludge-storage tank 5 through piping 20.

[0139] In the system for treating water circulating through a wet paint booth according to the present invention, the feeding of the circulating water to the measurement tank 3 through the circulating water-feed piping 14 and the feeding of the clarified water, such as industrial water, to the measurement tank 3 through the clarified water-feed piping 15 are alternately done by controlling the on-off valves 14A and 15A in response to the signal sent from the controlling apparatus 6. In the present invention, a step in which the pH of the circulating water is measured and adjusted and a step in which the pH-meter is cleaned and the system is put on standby are alternately repeated in the following manner.

[0140] <pH-Measurement and pH-Adjustment Step>

[0141] Upon the on-off valve 14A being opened (the on-off valve 15A is closed) in response to the signal sent from the controlling apparatus 6, the circulating water is fed into the measurement tank 3 through the circulating water-feed piping 14. Since the retention section 32 of the measurement tank 3 has a small capacity, feeding the circulating water to the measurement tank 3 through the circulating water-feed piping 14 at a high flow rate enables water contained in the retention section 32 to be replaced with the circulating water in a short time and the retention section 32 to be filled with the circulating water. The water forced to discharge from the retention section 32 is returned to the circulating water pit 2 through the overflow wall 31, the overflow section 33, the outlet 36, and the piping 16.

[0142] Upon the retention section 32 being filled with the circulating water as a result of the feeding of the circulating water and the water gauge 35 detecting an increase in the level of the circulating water, a detection signal is sent to the controlling apparatus 6. Upon receiving the signal, the controlling apparatus 6 sends a measurement signal to the pH-meter 34, and the pH-meter 34 measures the pH of the circulating water. The amount of time from the start of the feeding of the circulating water to the start of the measurement by the pH-meter 34 is normally about 1 to 5 minutes.

[0143] The controlling apparatus 6 receives the pH value measured by the pH-meter 34. The controlling apparatus 6 sends a chemical-feed signal to the device 21 for the addition of the alkaline solution of a phenolic resin or the device 22 for the addition of the acidic aluminum salt in accordance with the received pH value in order to adjust the pH of the circulating water. When the measured pH value is lower than the predetermined value, the chemical-feed signal is sent to the device 21 for the addition of the alkaline solution of a phenolic resin. When the measured pH value is higher than the predetermined value, the chemical-feed signal is sent to the device 22 for the addition of the acidic aluminum salt.

[0144] The pH-meter 34 may measure the pH of the circulating water continuously or intermittently.

[0145] The amount of time required by the pH-measurement and pH-adjustment step is not limited. However, if the amount of time required by the pH-measurement and pH-adjustment step is excessively small, it may not be possible to adequately adjust the pH of the circulating water. If the amount of time required by the pH-measurement and pH-adjustment step is excessively large, the pH-meter 34 may be contaminated, which reduces the accuracy of the measurement of pH. The amount of time required by each pH-measurement and pH-adjustment step, which includes the amount of time elapsed from the start of the feeding of the circulating water to the start of the measurement by the pH-meter 34, is preferably about 5 to 60 minutes and preferably about 10 to 30 minutes.

[0146] In order to prevent excessive amounts of alkaline solution of a phenolic resin and acidic aluminum salt from being added to the circulating water as a result of a malfunction of the pH-meter 34, it is preferable to set the maximum operation time of the chemical-feed pumps of the device 21 for the addition of the alkaline solution of a phenolic resin and the device 22 for the addition of the acidic aluminum salt in the controlling apparatus 6 and control the devices 21 and 22 with a timer such that the above chemicals are not added beyond the maximum operation time.

[0147] <pH-Meter-Cleaning and Standby Step>

[0148] Subsequent to the pH-measurement and pH-adjustment step, the on-off valve 14A is closed and the on-off valve 15A is opened in response to the signal sent from the controlling apparatus 6 in order to stop the feeding of the circulating water and feed clarified water, such as industrial water, to the measurement tank 3 through the clarified water-feed piping 15. The clarified water fed through the clarified water-feed piping 15 is ejected through the jet nozzles of the cleaning device 37 toward the pH sensor of the pH-meter 34 in order to clean the pH sensor.

[0149] When the clarified water is fed into the measurement tank 3 for cleaning the pH-meter, since the retention section 32 has a small capacity, feeding the clarified water to the measurement tank 3 at a high flow rate of about 5 to 50 L/min and preferably about 10 to 20 L/min enables the circulating water contained in the retention section 32 to be replaced with clarified water in a relatively short time and the retention section 32 to be filled with the clarified water. The circulating water forced to discharge from the retention section 32 is fed to the circulating water pit 2 through the overflow wall 31, the overflow section 33, the outlet 36, and the piping 16.

[0150] After the retention section 32 has been filled with the clarified water as a result of the feeding of the clarified water, the on-off valve 15A is closed in order to stop the feeding of the clarified water and the following measurement-standby step is conducted.

[0151] The amount of time required for cleaning the pH sensor of the pH-meter 34 by the feeding of the clarified water is normally about 1 to 5 minutes.

[0152] Subsequent to the feeding of the clarified water and the cleaning of the pH-meter 34, the system is put on standby for a predetermined amount of time while the retention section 32 is filled with the clarified water. Conducting this standby step reduces a time lag that occurs when the pH of the circulating water is measured in the next pH-measurement and pH-adjustment step.

[0153] Since the circulating water pit has a large capacity, the alkaline solution of a phenolic resin or the acidic aluminum salt that is added to the circulating water pit 2 from the device 21 for the addition of the alkaline solution of a phenolic resin or the device 22 for the addition of the acidic aluminum salt, respectively, for adjusting the pH of the circulating water may fail to be diffused immediately over the entirety of the pit 2. That is, the pH of the circulating water is not always uniform in the pit. It is also considered that an alkaline or acidic component of the chemical added to the circulating water may gradually elute from the sludge deposited in the circulating water pit 2 and cause the pH of the circulating water to fluctuate locally in the circulating water pit 2.

[0154] If the standby step is omitted and the next pH-measurement and pH-adjustment step is conducted immediately after the pH-meter 34 has been cleaned with clarified water, the above-described nonuniformity and fluctuations in the pH of the circulating water in the circulating water pit 2 may affect the measured pH value. That is, it is not possible to measure the correct pH value.

[0155] Conducting the above-described standby step allows the circulating water contained in the circulating water pit 2 to be circulated by the circulation pump P through the circulation piping 12 during the standby step. This makes the properties of the circulating water contained in the circulating water pit 2 to be uniform. As a result, the nonuniformity and local fluctuations in the pH of the circulating water are reduced, and the pH value measured in the next pH-measurement and pH-adjustment step sufficiently reflects the pH of the circulating water contained in the system. Thus, the pH of the circulating water can be adjusted adequately in accordance with the measured pH value.

[0156] If the amount of time required by the standby step is excessively small, the above-described advantageous effects of conducting the standby step may fail to be achieved at a sufficient degree. If the amount of time required by the standby step is excessively large, the amount of time required by the pH-measurement and pH-adjustment step disadvantageously becomes relatively small. The amount of time required by the standby step, which is conducted subsequent to the steps of cleaning the pH-meter, is preferably about 10 to 50 minutes and particularly preferably about 20 to 30 minutes.

[0157] Subsequent to the standby step, the above-described pH-measurement and pH-adjustment step is conducted. In this manner, the pH-measurement and pH-adjustment step and the pH-meter-cleaning and standby step are alternately repeated.

[0158] FIG. 1 illustrates an example of the system for treating water circulating through a wet paint booth according to an embodiment of the present invention. However, the system for treating water circulating through a wet paint booth according to the present invention is not limited to that illustrated in FIG. 1.

[0159] Although an alkaline solution of a phenolic resin and an acidic aluminum salt are used as pH-adjusting agents for adjusting pH in FIG. 1, the alkaline solution of a phenolic resin may be replaced with a general-purpose alkaline agent, and the acidic aluminum salt may be replaced with a general-purpose acidic agent. The positions at which the above-described chemicals are added to the circulating water are not limited. A cationic polymer may optionally added to the circulating water.

EXAMPLES

[0160] The present invention is described more specifically with reference to Examples.

[0161] [Chemicals Used]

[0162] The following treatment chemicals were used. Alkaline solution of a phenolic resin: Kuristuck B-310 produced by Kurita Water Industries Ltd. (aqueous NaOH solution of a phenolic resin (novolac phenol-formaldehyde polycondensate), phenolic resin concentration: 32 weight %, NaOH concentration: 5 weight %, pH: 12) (hereinafter, referred to as B-310)

[0163] Acidic aluminum salt: aqueous aluminum sulfate solution (Al.sub.2(SO.sub.4).sub.3 concentration: 27 weight %) (hereinafter, referred to as Al.sub.2(SO.sub.4).sub.3)

[0164] Acid: 10-weight % aqueous sulfuric acid solution (hereinafter, referred to as sulfuric acid)

[0165] Cationic polymer: Kuristuck B-450 produced by Kurita Water Industries Ltd. (alkylamine-epichlorohydrin condensate, weight-average molecular weight: 100 thousand) (hereinafter, referred to as B-450)

[0166] Hereinafter, the term stock solution of a treatment chemical refers to the above solution (aqueous solution) of the chemical.

Example 1

[0167] Water circulating through a wet paint booth was treated under the following conditions with the system for treating water circulating through a wet paint booth as illustrated in FIG. 1.

[0168] A paint included in the water circulating through a wet paint booth was collected under the following conditions.

[0169] Amount of paint (for automotive body, water-based) fed: 4 g/min

[0170] Circulating water: mixture of water circulating in an automobile assembly plant and tap water

[0171] Amount of water stored in circulating water pit 2: 800 L

[0172] Amount of circulating water (flow rate of water circulated through piping 11): 100 L/min

[0173] Predetermined amount of Al.sub.2(SO.sub.4).sub.3 added through piping 14: 0.12 g/min (in terms of the amount of stock solution)

[0174] Predetermined amount of B-310 added through piping 13: 0.16 g/min (in terms of the amount of stock solution)

[0175] The conditions of the measurement tank 3 were as follows.

[0176] Hereinafter, the chemical-feed pump included in the device 21 for the addition of the alkaline solution of a phenolic resin is referred to as B-310 pump, and the chemical-feed pump included in the device 22 for the addition of the acidic aluminum salt is referred to as Al.sub.2(SO.sub.4).sub.3 pump.

[0177] Volume of retention section 32 of measurement tank 3: 5 L

[0178] Flow rate at which industrial water was fed to measurement tank 3: 10 L/min

[0179] Amount of time during which industrial water was fed to measurement tank 3 (amount of time during which pH-meter was cleaned): 2 minutes

[0180] Amount of standby time: 28 minutes

[0181] Flow rate at which circulating water was fed to measurement tank 3: 10 L/min

[0182] Amount of time during which circulating water was fed to the measurement tank 3: 30 minutes

[0183] Amount of time elapsed from the start of feeding of circulating water to the measurement of pH: 2 minutes

[0184] Predetermined pH value: 6.8 to 7.2

[0185] Chemical-feed control conditions: The B-310 pump was turned on upon the pH of the circulating water falling below 6.8 and turned off upon the pH of the circulating water reaching 7.0. The Al.sub.2(SO.sub.4).sub.3 pump was turned on upon the pH of the circulating water exceeding 7.2 and turned off upon the pH of the circulating water reaching 7.0.

[0186] Flow rate of B-310 pump: 0.1 g/min (in terms of the amount of stock solution)

[0187] Flow rate of Al.sub.2(SO.sub.4).sub.3 pump: 0.1 g/min (in terms of the amount of stock solution)

[0188] Maximum operating time of B-310 pump: 100 minutes

[0189] Maximum operating time of Al.sub.2(SO.sub.4).sub.3 pump: 100 minutes

[0190] The circulating water was fed to the measurement tank 3. After a lapse of two minutes from the time when the feeding of the circulating water was started, the pH of the circulating water was measured with the pH-meter 34 and adjusted. The amount of time required by the pH measurement and pH adjustment was 28 minutes. Thus, the amount of time during which the circulating water was fed to the measurement tank 3 was 30 minutes in total.

[0191] In the pH-measurement and pH-adjustment step, B-310 or Al.sub.2(SO.sub.4).sub.3 was added to the circulating water as needed in accordance with the chemical-feed control conditions described above.

[0192] Subsequent to the pH-measurement and pH-adjustment step, industrial water was fed to the measurement tank 3 for two minutes in order to clean the pH-meter 34. The system was subsequently put on standby for 28 minutes while the measurement tank 3 was filled with the industrial water. Subsequent to the pH-meter-cleaning and standby step, the pH-measurement and pH-adjustment step was again conducted. In this manner, the pH-measurement and pH-adjustment step and the pH-meter-cleaning and standby step were alternately repeated.

[0193] Table 1 and FIG. 2 show a change in the pH of the circulating water measured with the pH-meter 34 with time.

[0194] Table 2 summarizes the amounts of B-310 and Al.sub.2(SO.sub.4).sub.3 (amounts of chemicals fed with the B-310 pump and the Al.sub.2(SO.sub.4).sub.3 pump, respectively) required for adjusting pH in a 12-hour operation.

Comparative Example 1

[0195] The system was operated as in Example 1, except that industrial water was not fed to the measurement tank 3, and the pH of the circulating water was measured and adjusted while the circulating water was continuously fed to the measurement tank 3 at a flow rate of 10 L/min. Table 1 and FIG. 2 show a change in the pH of the circulating water with time. Table 2 summarizes the amounts of B-310 and Al.sub.2(SO.sub.4).sub.3 required for adjusting pH.

TABLE-US-00001 TABLE 1 Measured pH value Time elapsed Comparative (hr) Example 1 example 1 0.0 6.99 7.00 0.5 6.95 6.95 1.0 6.90 6.91 1.5 6.86 6.84 2.0 6.80 6.79 2.5 6.78 6.90 3.0 6.90 7.00 3.5 6.98 7.10 4.0 6.94 7.22 4.5 6.90 7.15 5.0 6.84 7.00 5.5 6.79 6.80 6.0 6.89 6.88 6.5 6.97 6.98 7.0 6.95 7.10 7.5 6.90 7.15 8.0 6.84 7.14 8.5 6.76 7.09 9.0 6.88 7.04 9.5 6.96 6.98 10.0 6.96 6.92 10.5 6.91 6.87 11.0 6.86 6.81 11.5 6.80 6.90 12.0 6.90 6.98

TABLE-US-00002 TABLE 2 Amount of chemical used (g/12 hr) B-310 Al.sub.2(SO.sub.4).sub.3 Example 1 12 0 Comparative 18 5.5 example 1

[0196] The results shown in Tables 1 and 2 confirm that, according to the present invention, the pH of the circulating water can be adjusted with consistency and certainty while reducing fluctuations in the pH of the circulating water and the amounts of chemicals used.

[0197] In Example 1 and Comparative Example 1, the pH of a pH standard solution (pH: 6.81) was measured with the pH-meter 34 prior to and subsequent to the 12-hour operation. Table 3 summarizes the results. It was confirmed that, while the accuracy of the pH-meter was reduced due to the contamination of the pH-meter in Comparative Example 1, the accuracy of the pH-meter was maintained at a sufficient level in Example 1 since the pH-meter had been cleaned.

TABLE-US-00003 TABLE 3 Before After operation operation Example 1 6.81 6.81 Comparative 6.81 6.85 example 1

Example 2

[0198] The treatment was performed as in Example 1, except that the chemical-feed control conditions were changed as follows. As a result, the pH of the circulating water was able to be adjusted with consistency and certainty as in Example 1.

[0199] Chemical-feed control conditions: The B-310 pump was turned on upon the pH of the circulating water falling below 6.9 and turned off upon the pH of the circulating water reaching 7.0. The Al.sub.2(SO.sub.4).sub.3 pump was turned on upon the pH of the circulating water exceeding 7.1 and turned off upon the pH of the circulating water reaching 7.0. Examples 3 and 4 and Comparative Examples 2 to 4

[0200] Into a bottle having a volume of 500 ml, 300 mL of tap water and 0.6 mL of a water-based paint (water-based paint for automotive bodies: silver metallic (acryl-based)) were charged. The chemicals shown in Table 4 were further added to the bottle in the specific amounts shown in Table 4 (note that, any chemical was not used in Comparative Example 2). Subsequently, a cap was put on the bottle. The bottle was then shaken 60 times every 30 seconds. The entirety of the contents of the bottle was transferred into a beaker, and the pH and the colloid equivalent (determined with a PCD (particle charge detector) produced by Mitek) of the treatment liquid were measured. Subsequently, the following foaming test, turbidity measurement, and secondary coagulation test were conducted.

[0201] Amounts of chemicals added in Table 4 are each determined in terms of the amount of the product (stock solution). The same applies to Tables 5 to 7.

[0202] <Foaming Test>

[0203] Into a 1-liter graduated cylinder, 300 mL of the treatment liquid was charged. A bubbling test of the treatment liquid was conducted.

[0204] In the bubbling test, air was blown into the treatment liquid contained in the graduated cylinder at a rate of 1.5 L/min with a spherical air stone in order to bubble the treatment liquid. Thus, the foaming ability and defoaming ability of the treatment liquid were determined as described below.

[0205] <Foaming Ability>

[0206] The amount (mL) of bubbles was measured after a lapse of two minutes from the time when the bubbling was started.

[0207] When the amount of bubbles exceeded 700 mL within 2 minutes, the number of seconds required for the amount of bubbles to exceed 700 mL was measured. The larger the number of seconds required for the amount of bubbles to exceed 700 mL, the greater the anti-foaming effect.

[0208] <Defoaming Ability>

[0209] Subsequent to performing bubbling, the treatment liquid was left to stand for two minutes, and the amount (mL) of the remaining bubbles was measured.

[0210] In the case where the bubbles disappeared within two minutes, the number of seconds required for the bubbles to disappear was measured. The smaller the number of seconds required for the bubbles to disappear, the higher the defoaming ability of the treatment liquid.

[0211] <Turbidity Measurement>

[0212] The treatment liquid was filtered through a Whatman filter paper No. 41 (particle retention: 20 to 25 microns). The turbidity of the filtrate was measured with a turbidimeter.

[0213] <Secondary Coagulation Test>

[0214] To the treatment liquid, 1 mL (13 mg/L in terms of the concentration of active constituents) of 1-weight % solution of a cationic polymer coagulant (copolymer of acrylamide and 2-(acryloyloxy)ethyltrimethylammonium chloride (weight-average molecular weight: 8 million)) was added. The state of flocs was determined and evaluated in accordance with the following criteria.

[0215] <Coagulation Effect>

[0216] Good: Suitable flocs were formed, that is, the treatment liquid had a good coagulation property.

[0217] Poor: Flocs were not formed, that is, the coagulation effect was not obtained.

[0218] Table 4 summarizes the results.

[0219] In a coagulation treatment of a water-based paint, the achievement of the primary coagulation of the paint (i.e., low turbidity of the filtrate) and the anti-foaming property are the most important evaluation items.

TABLE-US-00004 TABLE 4 Anti-foaming effect Defoaming ability Amount of Amount of Amount of Amount of Amount of chemical added bubbles bubbles bubbles bubbles Turbidity (% relative to weight of paint) Colloid after reached after reached of Secondary Sulfuric equivalent 2 minutes 700 mL 2 minutes 0 mL filtrate coagulation B-310 Al.sub.2(SO.sub.4).sub.3 acid B-450 pH (meq/L) (mL) (sec) (mL) (sec) (degree) property Comparative 7.4 0.1 700 or more 55 200 120 or 100 or Poor example 2 more more Comparative 4 5.8 2.2 6.97 0.03 300 0 34 1.0 Good example 3 Example 3 2 2 0.5 6.97 0.015 350 0 29 0.5 Good Example 4 4 2 2.0 7.99 0.005 350 0 30 0.5 Good Comparative 6 2.5 8.04 0.005 350 0 32 1.0 Good example 4

[0220] In the treatment of a water-based paint, the colloid equivalent of the treatment water significantly affects the treatment effect. Thus, the colloid equivalent of water treated in each test was controlled to be substantially equal.

[0221] The results shown in Table 4 confirm that the turbidity of the filtrate was low, that is, the treatment liquid was clear, in Examples 3 and 4, where aluminum sulfate was used. In the foaming test, a large amount of bubbles were generated in Comparative Example 2, where no treatment was performed, while the amounts of bubbles generated in Examples 3 and 4 and Comparative Examples 3 and 4 were substantially equal to one another.

[0222] The above results confirm that adjusting the pH of the treatment liquid to about 7 reduced the amounts of chemicals required and that using aluminum sulfate in combination reduced the turbidity of the filtrate without using a hazardous substance such as sulfuric acid.

Example 5 and Comparative Examples 5 to 8

[0223] A coagulation treatment was performed as in Example 3, except that an intercoating paint (for automotive-body coating: gray (polyester-based)) was used instead of a water-based paint, and the amounts of chemicals were set as shown in Table 5 (any chemical was not used in Comparative Example 5). Subsequently, the evaluation tests were conducted as in Example 3. Table 5 summarizes the results.

TABLE-US-00005 TABLE 5 Anti-foaming effect Defoaming ability Amount of Amount of Amount of Amount of Amount of chemical added bubbles bubbles bubbles bubbles Turbidity (% relative to weight of paint) Colloid after reached after reached of Secondary Sulfuric equivalent 2 minutes 700 mL 2 minutes 0 mL filtrate coagulation B-310 Al.sub.2(SO.sub.4).sub.3 acid B-450 pH (meq/L) (mL) (sec) (mL) (sec) (degree) property Comparative 7.2 0.11 700 or more 50 100 300 100 or Poor example 5 more Comparative 6 6.5 2.2 6.55 0.03 300 0 50 1.5 Good example 6 Example 5 4 3.5 6.54 0.005 300 0 55 0.5 Good Comparative 6 3.0 2.0 8.1 0.01 350 0 60 0.5 Good example 7 Comparative 10 3.0 8.3 0.01 300 0 65 2.0 Good example 8

[0224] The results shown in Table 5 confirm that, in the treatment of the intercoating paint, the turbidity of the filtrate was low, that is, the treatment liquid was clear, in Example 5 and Comparative Example 7, where aluminum sulfate was used. In the foaming test, a large amount of bubbles were generated in Comparative Example 5, where no treatment was performed, while the amounts of bubbles generated in Example 5 and Comparative Examples 6 to 8 were substantially equal to one another. Adjusting the pH of the treatment liquid to about 7 reduced the amounts of chemicals required. In the treatment of the intercoating paint, the treatment was performed in a suitable manner even without using B-450 (cationic polymer) as in Example 5. Using aluminum sulfate in combination reduced the turbidity of the filtrate without using a hazardous substance such as sulfuric acid.

Example 6 and Comparative Examples 9 to 12

[0225] A test was conducted using the test system illustrated in FIG. 3. In the test system, circulating water contained in a circulating water tank 41 having a capacity of 50 L was caused, by a pump P, to circulate and flow downward along a water-screen plate 42 disposed above the circulating water tank, on which a paint was sprayed. In FIG. 3, Reference Numeral 43 denotes a paint-spraying apparatus, Reference Numeral 51 denotes circulation piping, Reference Numeral 52 denotes drainage piping through which the circulating water was discharged to the outside, Reference Numeral 53 denotes exhaust piping, V.sub.1 and V.sub.2 denote a valve, and F denotes an exhaust fan.

[0226] With 50 L of tap water, 100 mL of a water-based paint (for automotive-body coating: silver metallic (acryl-base)) was mixed. To the mixture, the chemicals shown in Table 6 were added in the amounts shown in Table 6 (note that, any chemical was not used in Comparative Example 9). Subsequently, the system was actuated. A solvent-based paint (automotive body clear solvent-based paint) was sprayed at a rate of 2 g/minute for 30 minutes. Then, the system was paused.

[0227] The adhesion of the treatment sludge that came to the water surface of the circulating water tank was inspected with fingers and evaluated in accordance with the following criteria.

<Evaluation Criteria>

[0228] Good: Adhesion of the sludge was not confirmed [0229] Poor: Adhesion of the sludge was confirmed

[0230] The measurement of the pH of the treatment liquid, the measurement of the amount of charge, the foaming test, the measurement of the turbidity of the filtrate, and the secondary coagulation test were conducted as in Example 3. In the secondary coagulation test, the amount of cationic polymer coagulant added was set to 13 mg/L in terms of the concentration of active components.

[0231] Table 6 summarizes the results.

[0232] The most important evaluation item in the treatment of a solvent-based paint is whether the adhesion of the treatment sludge was confirmed or not. The most important evaluation items in the treatment of a water-based paint are whether the primary coagulation of the paint was achieved or not (whether the turbidity of the filtrate is low or not) and whether the treatment liquid had an anti-foaming property or not.

TABLE-US-00006 TABLE 6 Anti-foaming Defoaming effect ability Amount Amount Amount of of Amount of of Amount of chemical added bubbles bubbles bubbles bubbles (% relative to weight of paint) Colloid after reached after reached Turbidity Secondary B- Sulfuric B- equivalent 2 minutes 700 mL 2 minutes 0 mL of filtrate coagulation 310 Al.sub.2(SO.sub.4).sub.3 acid 450 pH (meq/L) Adhesion (mL) (sec) (mL) (sec) (degree) property Comparative 7.3 0.11 X 700 or 70 150 120 or 100 or Poor example 9 more more more Comparative 3 1.5 0.3 6.90 0.00 300 0 28 1.0 Good example 10 4 5.5 2.5 Example 6 3 0.7 0.3 7.05 0.005 300 0 28 0.5 Good 2 2 0.5 Comparative 3 0.3 0.3 8.25 0.005 330 0 29 0.5 Good example 11 4 2 3.0 Comparative 3 0.3 8.40 0.005 320 0 29 1.0 Good example 12 6 3.2 The proportion of the amount of chemical relative to the amount of solvent-based paint is in the upper row. The proportion of the amount of chemical relative to the amount of water-based paint is in the lower row.

[0233] The results shown in Table 6 confirm that the turbidity of the filtrate was low in Example 6 and Comparative Example 11, where aluminum sulfate was used. That is, the treatment liquid was clear in Example 6 and Comparative Example 11. In the foaming test, a large amount of bubbles were generated in Comparative Example 9, where no treatment was performed, while the amounts of bubbles generated in Example 6 and Comparative Examples 10 to 12 were substantially equal to one another. A reduction in the adhesion of a solvent-based paint was achieved at substantially the same level in each of Example 6 and Comparative Examples 10 to 12.

[0234] The above results confirm that adjusting the pH of the treatment liquid to the optimum pH by using aluminum sulfate in combination further reduced the amounts of chemicals required.

Examples 7 and 8 and Comparative Examples 13 to 15

[0235] A coagulation treatment was performed as in Example 6, except that a mixture (1:1) of a water-based paint (for automotive-body coating: white (acryl-based)) and a water-based, intercoating paint (for automotive-body coating: white (polyester-based)) was used instead of the water-based paint, and the amounts of chemicals added were set as shown in Table 7 (note that, any chemical was not used in Comparative Example 13). Subsequently, evaluations were made as in Example 6.

[0236] Table 7 summarizes the results.

TABLE-US-00007 TABLE 7 Anti-foaming effect Defoaming ability Amount Amount Amount of of Amount of of Amount of chemical added bubbles bubbles bubbles bubbles (% relative to weight of paint) Colloid after reached after reached Turbidity Secondary B- Sulfuric B- equivalent 2 minutes 700 mL 2 minutes 0 mL of filtrate coagulation 310 Al.sub.2(SO.sub.4).sub.3 acid 450 pH (meq/L) Adhesion (mL) (sec) (mL) (sec) (degree) property Comparative 7.2 0.15 X 700 or 80 130 120 or 100 or Poor example 13 more more more Comparative 3 1.5 0.3 7.00 0.005 350 0 45 1.5 Good example 14 5 5.0 2.5 Example 7 3 0.7 6.70 0.005 300 0 40 0.5 Good 4 3.5 Example 8 3 0.3 0.3 8.00 0.01 330 0 55 0.5 Good 5 2.5 2.5 Comparative 3 0.3 8.40 0.005 320 0 55 3.0 Good example 15 8 3.0 The proportion of the amount of chemical relative to the amount of solvent-based paint is in the upper row. The proportion of the amount of chemical relative to the amount of water-based paint is in the lower row.

[0237] The results shown in Table 7 confirm that the turbidity of the filtrate was low in Examples 7 and 8, where aluminum sulfate was used. That is, the treatment liquid was clear in Examples 7 and 8. In the foaming test, a large amount of bubbles were generated in Comparative Example 13, where no treatment was performed, while the amounts of bubbles generated in Examples 7 and 8 and Comparative Examples 14 and 15 were substantially equal to one another. In addition, a reduction in the adhesion of a solvent-based paint was achieved at substantially the same level in each of Examples 7 and 8 and Comparative Examples 14 and 15.

[0238] The above results confirm that adjusting the pH of the treatment liquid to the optimum pH by using aluminum sulfate in combination further reduced the amounts of chemicals required.

[0239] Although the present invention has been described in detail with reference to particular embodiments, it is apparent to a person skilled in the art that various modifications can be made therein without departing from the spirit and scope of the present invention.

[0240] The present application is based on Japanese Patent Application No. 2014-149506 filed on Jul. 23, 2014, and Japanese Patent Application No. 2015-17142 filed on Jan. 30, 2015, which are incorporated herein by reference in their entirety.

REFERENCE SIGNS LIST

[0241] 1 WET PAINT BOOTH

[0242] 2 CIRCULATING WATER PIT

[0243] 3 MEASUREMENT TANK

[0244] 4 SLUDGE-RECOVERING APPARATUS

[0245] 5 RECOVERED SLUDGE-STORAGE TANK

[0246] 6 CONTROLLING APPARATUS

[0247] 21 DEVICE FOR THE ADDITION OF THE ALKALINE SOLUTION OF A PHENOLIC RESIN

[0248] 22 DEVICE FOR THE ADDITION OF THE ACIDIC ALUMINUM SALT

[0249] 31 OVERFLOW WALL

[0250] 32 RETENTION SECTION

[0251] 33 OVERFLOW SECTION

[0252] 34 pH-METER

[0253] 35 WATER GAUGE

[0254] 41 CIRCULATING WATER TANK

[0255] 42 WATER-SCREEN PLATE

[0256] 43 PAINT-SPRAYING APPARATUS