JP2012005935A - Method of neutralizing alkaline wastewater - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of neutralizing alkaline wastewater capable of efficiently subjecting alkaline wastewater containing wood waste and the like generated in a paper-making/pulp factory to neutralization treatment, and excelling in economic efficiency.SOLUTION: Alkaline wastewater containing wood waste and the like generated in a paper-making/pulp factory is subjected to neutralization treatment in a state containing the wood waste and the like by a gas-liquid contact treatment with exhaust gas generated in a boiler installed in the factory in a water storage tank 20. The gas-liquid contact treatment between the alkaline wastewater and the boiler exhaust gas in the treatment tank 20 is performed by a combination of a self-suction type exhaust gas diffusion device 30 and a wastewater agitation device 40 arranged in the vicinity of the diffusion device.

Description

  The present invention relates to an alkaline wastewater neutralization method for efficiently and economically neutralizing alkaline wastewater generated in large quantities in a paper / pulp factory.

  In a paper / pulp factory, a large amount of sodium hydroxide is used in a pulp digesting process or a disaggregation process when recycling used paper. For this reason, a large amount of alkaline waste water is generated from the paper / pulp mill and neutralized. Only the neutralization treatment with sulfuric acid solution is actually performed as the neutralization treatment (Patent Document 1). The reason is as follows.

  Alkaline wastewater is neutralized by gas-liquid contact with acidic gases such as carbon dioxide (Patent Document 2), but the alkaline wastewater generated in a paper / pulp factory contains a large amount of slurry-like wood chips. It is mixed in. For this reason, in gas-liquid contact using a mixing device such as a mixer or an ejector, the mixing devices are clogged, making the mixing device difficult to use in a short period of time. For this reason, in the gas-liquid contact using the mixing device, it is necessary to remove the wood chips and the like in the alkaline wastewater in advance, but this method is not actually performed because the processing costs enormous costs. .

  Another gas-liquid contact method is performed by a diffuser in a water tank. In this method, acidic gas such as carbon dioxide gas is injected from an air diffuser into alkaline drainage in a water tank. Although there is no risk of clogging, the efficiency is extremely low because the bubble diameter of the acidic gas supplied from the air diffuser into the alkaline waste water is large. For this reason, even though this method is theoretically considered, it is not actually performed.

  As yet another gas-liquid contact method, the present applicant has developed a method for neutralizing alkaline drainage such as boiler blow drainage with carbon dioxide-containing gas such as boiler exhaust gas, and has been proposed in Patent Document 3. This method uses a neutralization treatment apparatus in which a gas-liquid contact tower is continuously provided on a water tank. And after injecting alkaline wastewater into the tower from the upper part of the gas-liquid contact tower, injecting carbon dioxide-containing gas into the alkaline wastewater in the water storage tank, and contacting with the alkaline wastewater in the water storage tank and the gas-liquid contact tower , Discharged from the top of the tower. This method is effective for neutralization treatment of alkaline wastewater that does not contain solids such as boiler blow wastewater, but for treatment of alkaline wastewater that is generated in a paper / pulp factory and contains a large amount of slurry-like wood chips, etc. It is not suitable because wood chips and the like are clogged in the gas-liquid contact tower.

  For these reasons, only the neutralization treatment with sulfuric acid solution is actually performed as the neutralization treatment of the alkaline drainage generated in the paper / pulp factory. However, in this method, since the discharge amount of alkaline waste water is large, the amount of sulfuric acid solution used is also large, and the sulfuric acid solution cost is a big problem. For this reason, alkaline wastewater containing a large amount of slurry-like solid foreign matter such as alkaline wastewater generated in a paper / pulp factory does not use chemicals such as sulfuric acid and does not require prior removal of solid foreign matter. Development of a neutralization method is awaited.

JP 2001-081520 A JP 2002-066654 A JP 2009-279465 A

  An object of the present invention is to provide an alkaline drainage neutralization method excellent in economy that can efficiently neutralize alkaline drainage containing wood chips and the like produced in a paper / pulp factory.

  In order to achieve the above-mentioned object, the present inventors have made improvements in order to apply an alkali drainage neutralization apparatus (Patent Document 3) using boiler exhaust gas developed by themselves to neutralize alkaline drainage generated in a paper / pulp factory. We started and examined many things from various directions. As a result, the following facts were found.

  Many boilers are used in paper / pulp mills. Conventionally, these boilers have been oil-fired boilers that use heavy oil as fuel. However, due to the recent increase in fuel oil costs and environmental constraints such as carbon dioxide emission control, conversion to biomass boilers has been attempted. Recently, most of the boilers are occupied by this biomass boiler. The exhaust gas from the biomass boiler contains more carbon dioxide than the exhaust gas from the oil-fired boiler, but the carbon dioxide is not added to the carbon dioxide emission. In addition, the exhaust gas from biomass boilers does not contain sulfur, so it is not subject to legal regulations. For these reasons, the exhaust gas from biomass boilers, which are rapidly increasing in many paper / pulp mills, is suitable for the neutralization treatment of alkaline wastewater generated in paper / pulp mills.

  However, as described above, slurry-like wood chips and the like contained in alkaline drainage generated in a paper / pulp factory are a major obstacle to neutralization treatment with boiler exhaust gas. Therefore, the present inventors have focused on a gas-liquid contact method in a water storage tank in which wood waste or the like in alkaline drainage does not become an obstacle, and tried to improve the above-described alkaline drainage neutralization device (Patent Document 3). As a result, it became clear that the following devices are effective.

  The first is to remove the gas-liquid contact tower on the water tank that may cause clogging in the alkaline wastewater neutralizer. The second is to compensate for the reduction in efficiency caused by removing the gas-liquid contact tower with the device of the diffuser in the water tank. Specifically, a boiler exhaust gas diffuser is combined with a waste water agitator, and preferably a self-priming diffuser is used as the diffuser.

  A self-priming diffuser is a device that rotates an impeller provided with a gas discharge port at the rotation shaft between adjacent blades at high speed in the liquid to be treated. When the downstream side becomes negative pressure, the gas sucked through the rotating shaft is ejected into the liquid from each discharge port, and the ejected gas is sheared by each blade, so that fine gas around the impeller A large amount of bubbles are ejected and brought into gas-liquid contact with the liquid to be treated. This self-priming diffuser is very efficient because it ejects a large amount of fine bubbles at high speed, but it still does not make up for the removed gas-liquid contact tower. Therefore, the liquid to be treated filled with fine bubbles is forcibly stirred by a stirring device separate from the self-priming diffuser. As a result, it has been clarified that the alkaline drainage produced in the paper / pulp factory and containing the slurry-like wood waste is neutralized by the boiler exhaust gas while containing the wood waste.

  The alkaline wastewater neutralization method of the present invention has been made based on such knowledge. Alkaline wastewater generated in a paper / pulp factory and containing wood chips and the like is generated in a water tank with a boiler installed in the factory. By the gas-liquid contact treatment with the exhaust gas, neutralization is performed in a state containing wood chips and the like.

  The boiler used in the alkaline wastewater neutralization method of the present invention is preferably an exhaust gas from a biomass boiler that does not contain sulfur, but contains a large amount of carbon dioxide, and the carbon dioxide is not added to the carbon dioxide emission. It is also possible to use exhaust gas from a gas-fired boiler with a low sulfur content.

  It is effective to perform the gas-liquid contact treatment between the alkaline waste water and the boiler exhaust gas in the treatment tank by a combination of the exhaust gas diffuser and the waste water agitator disposed in the vicinity of the diffuser. Since the waste water agitation device needs to stir the waste water sufficiently containing the fine bubbles of the exhaust gas, it is necessary to be disposed in the vicinity of the exhaust gas aeration device. A configuration in which the exhaust gas diffuser and the waste water agitator are both rotary vane type and arranged close to the same axis is preferable from the viewpoint of rational and high efficiency, and the rotary vane type exhaust gas diffuser arranged coaxially is preferable. A configuration in which the air device and the waste water stirring device are driven by a common motor is more preferable. As the exhaust gas diffuser, the above-described self-priming exhaust gas diffuser is preferable.

  The method for neutralizing alkaline wastewater according to the present invention neutralizes alkaline wastewater generated in a paper / pulp factory and containing wood chips with exhaust gas generated in a boiler installed in the factory, so that no chemical solution for neutralization is required. It is economically superior. Since neutralization is performed by gas-liquid contact in the water storage tank and clogging factors such as wood chips are eliminated, it is possible to eliminate the trouble of removing the wood chips and the like from the alkaline drainage in advance, and this is also economical. The structure of the water storage tank becomes simple, and it is excellent in economic efficiency from this point. Therefore, the alkaline wastewater neutralization method of the present invention is very economical.

It is a conceptual diagram of the alkaline waste water neutralization method which shows 1st Embodiment of this invention. It is a conceptual diagram of the alkaline waste water neutralization method which shows 2nd Embodiment of this invention. It is a conceptual diagram of the alkaline waste water neutralization method which shows 3rd Embodiment of this invention. It is a conceptual diagram of the alkaline waste water neutralization method which shows 4th Embodiment of this invention. It is a conceptual diagram of the alkaline waste water neutralization method which shows 5th Embodiment of this invention.

  Hereinafter, first to fifth embodiments of the present invention will be described with reference to the drawings.

  As shown in FIG. 1, the alkaline wastewater neutralization method of the first embodiment is used for neutralization treatment of alkaline wastewater generated in a papermaking / pulp factory, and more specifically, in the papermaking / pulp factory. The neutralization treatment is performed with the exhaust gas from the biomass boiler installed in the plant. For this neutralization treatment, the alkaline waste water is introduced into the water storage tank 20 by the liquid feed pump 10. The alkaline drainage is in a state containing slurry-like wood chips and the like, and its pH is about 10 for example.

  The water storage tank 20 is provided with a neutralization promoting device that combines a self-priming exhaust gas diffuser 30 and a wastewater agitator 40. The self-priming exhaust gas diffusing device 30 includes a motor 31 installed on the water storage tank 20, a vertical hollow support shaft 32 extending downward from the motor 31 and inserted into the water storage tank 20, and a hollow support shaft 32. And a rotary vane 33 that is rotatably attached to the distal end portion. The rotary blade 33 includes a plurality of blades extending radially from the rotation center portion and gas discharge holes provided in the rotation center portion between adjacent blades. It is rotationally driven by a motor 31 through a vertical rotating shaft that penetrates the inside. Each blade in the rotary blade 33 is a flat plate parallel to the center of rotation.

  By rotating the rotary blade 33 at high speed in the alkaline drainage in the water storage tank 20, the boiler exhaust gas is sucked into the hollow support shaft 32 from the boiler exhaust gas flue 50 and provided between adjacent blades of the rotary blade 33. Each gas discharge hole is discharged as fine bubbles into the alkaline drainage. The principle is as described above.

  The waste water agitation device 40 is composed of a rotary blade 41 provided on the front end side of the rotary blade 33 in the self-priming exhaust gas diffuser 30. The rotating blade 41 is concentrically and integrally connected to the rotating blade 33, and is driven by the motor 31 together with the rotating blade 33. That is, the motor 31 of the self-priming exhaust gas diffusing device 30 also serves as a driving source for the rotary blade 41 that constitutes the drainage stirring device 40. The structure of the rotary blade 41 is the same as that of the rotary blade 33, in which a plurality of plate-like blades parallel to the rotation center extend radially from the rotation center portion. The number of blades in the rotary blade 41 is smaller than that of the rotary blade 33, for example, four of the half of the eight rotary blades 33.

  Here, the self-priming exhaust gas diffusing device 30 and the waste water agitating device 40 are vertical types having a vertical rotation center, and the rotary blades 33 and 41 are located in the vicinity of the bottom in the water tank 20.

  In operation, the motor 31 in the liquid feed pump 10 and the self-priming exhaust gas diffuser 30 is driven. By driving the liquid feed pump 10, alkaline drainage containing wood chips and the like is introduced into the water storage tank 20 from the top of the tank. By driving the motor 31 in the self-priming exhaust gas diffuser 30, the rotary blades 33 constituting the rotary blade 33 and the waste water agitator 40 in the self-priming exhaust gas diffuser 30 are rotated at high speed. Due to the high speed rotation of the rotary blade 33, a vacuum state is established between adjacent blade plates, and the boiler exhaust gas from the exhaust gas flue 50 of the biomass boiler passes through the hollow support shaft 32 to form a plurality of gases formed at the rotation center of the rotary blade 41. It is discharged from the discharge hole into the alkaline drainage, and fine bubbles of the exhaust gas are diffused to the surroundings.

  At the same time, the alkaline drainage near the rotary blade 41 is agitated by the high-speed rotation of the rotary blade 41. Since the rotary blade 41 is connected to the rotary blade 33 and rotates at a high speed in the immediate vicinity of the rotary blade 33, the alkaline drainage containing a large amount of fine bubbles of exhaust gas at high density is forcibly agitated. For this reason, the alkaline drainage is neutralized with high efficiency by the carbon dioxide in the exhaust gas. As the fine bubbles of the exhaust gas move away from the rotary blade 33, the bubble diameter increases and the density decreases. Therefore, even if stirring is performed with the rotary blade 41 away from the rotary blade 33, the efficiency of the neutralization treatment does not increase. It is meaningful to stir the alkaline drainage in the immediate vicinity of the rotary blade 33 by the rotary blade 41 provided continuously with the rotary blade 33.

  Thus, the alkaline wastewater containing slurry-like wood chips and the like produced in the paper / pulp factory is efficiently and economically neutralized without using sulfuric acid with the boiler exhaust gas while containing the wood chips and the like.

  The neutral waste water after neutralization treatment in the water storage tank 20 is sequentially discharged out of the tank from the top of the water tank 20 on the side opposite to the alkaline drain introduction side. The exhaust gas after being used for the neutralization process and consuming the carbon dioxide gas is discharged out of the tank from an exhaust pipe 21 erected on the upper part of the water storage tank 20.

  Next, the alkali waste water neutralization method after 2nd Embodiment is demonstrated.

  As shown in FIG. 2, the alkaline wastewater neutralization method of the second embodiment is that the self-priming exhaust gas diffuser 30 and the wastewater agitator 40 are in a horizontal type with a horizontal center of rotation. It differs from the alkaline drainage neutralization method. The self-priming exhaust gas diffusing device 30 and the waste water stirring device 40 are installed in the vicinity of the bottom of the water storage tank 20, and the functions are substantially the same as those of the alkaline waste water neutralization method of the first embodiment.

  In the alkaline drainage neutralization method of the third embodiment, as shown in FIG. 3, the motor 31 in the self-priming exhaust gas diffusing device 30 is installed on the tank of the water storage tank 20 together with the rotary blades 33 and 41 by a pedestal 70. The point directly connected to the rotary blades 33 and 41 is different from the alkaline drainage neutralization method of the first embodiment and the second embodiment. The gantry 70 has a configuration that does not hinder the flow of alkaline drainage in the water tank 20. The function is substantially the same as the alkaline wastewater neutralization method of the first embodiment and the second embodiment.

  As shown in FIG. 4, the alkaline drainage neutralization method of the fourth embodiment is different in that the blower blower 60 is interposed in a conduit that leads the boiler exhaust gas from the exhaust flue 50 to the self-priming exhaust gas diffuser 30. It differs from the alkaline wastewater neutralization method of the embodiment. By interposing the blower blower 60, more boiler exhaust gas can be supplied to the self-priming exhaust gas diffuser 30, and the neutralization capacity in the water storage tank 20 can be improved.

  In the alkaline drainage neutralization method of FIG. 4, the underlying system is that of FIG. 1, but it goes without saying that the system of FIG. 2 and the system of FIG. 3 are also effective.

  The alkaline wastewater neutralization method of the fifth embodiment is different from the alkaline wastewater neutralization method of other embodiments in that the water storage tank 20 is multistaged (tandemized). The water storage tank 20 here is partitioned into an upstream side and a downstream side by a weir 22, and each is equipped with a combination of a self-priming exhaust gas diffuser 30 and a waste water agitator 40.

  In this alkaline wastewater neutralization method, neutralization treatment capacity is improved because neutralization treatment with boiler exhaust gas is repeatedly performed on alkaline wastewater. Again, the underlying system is that of FIG. 1, but it goes without saying that the system of FIG. 2, the system of FIG. 3, and the system of FIG. 4 are also effective.

  Furthermore, although the number of processing stages is two in FIG. 5, it is possible to make it three or more, and it goes without saying that the processing efficiency is further improved.

[Example 1]
The alkaline wastewater neutralization method of the first embodiment shown in FIG. 1 was actually implemented. The water to be treated is an alkaline drainage generated in a paper / pulp factory, and contains a large amount of slurry-like wood chips and the like. The pH of the water to be treated is 11.8. The volume of the water storage tank was 5 m ³ , and the amount of water to be treated was 150 m ³ / h. The residence time in the tank to be treated is about 2 minutes. On the other hand, as the boiler exhaust gas, the combustion exhaust gas generated in the biomass boiler was used. The carbon dioxide gas concentration (CO2 concentration) in the boiler exhaust gas is 10 vol%, and the supply amount to the self-priming exhaust gas diffuser is 300 m ³ / h. As a result, the amount of carbon dioxide (CO2) blown into the water to be treated is 30 m ³ / h. The rotation speed of the rotating blades in the self-priming exhaust gas diffuser and the waste water agitator is 1800 rpm, and the output of the motor as the power source is about 14 kW.

The pH of the treated water discharged from the water storage tank was 7.6. The theoretical amount of carbon dioxide gas (CO2 amount) required to adjust the treated water to pH 7.6 is 21 m ² / h, and it can be said that the treatment efficiency and economy are very high.

[Reference example]
In Example 2, the waste water stirring device was removed, and only a self-priming exhaust gas diffuser was used. The pH of the treated water discharged from the water storage tank rose to 9.0.

[Example 2]
The alkaline wastewater neutralization method of the fourth embodiment shown in FIG. 4 was actually carried out. Specifically, in Example 1, an air blower having an output of 2.2 kW is interposed in a conduit for leading boiler exhaust gas from the exhaust gas flue to the self-priming exhaust gas diffuser to supply carbon dioxide concentration (CO2 concentration). The amount was increased to 400 m ³ / h. The pH of the treated water discharged from the water storage tank dropped to 6.9.

Example 3
The alkaline wastewater neutralization method of the fifth embodiment shown in FIG. 5 was actually carried out. Specifically, the interior of the water tank having a capacity of 5 m ³ used in Example 1 was divided into two chambers to perform a two-stage treatment. The capacity of each chamber was 2.5 m ³ , and the same combination of the self-priming exhaust gas diffuser and the waste water agitator as in Example 1 was provided, and boiler exhaust gas was supplied by 150 m ³ / h. The total supply amount of boiler exhaust gas is 300 m ³ / h (150 m ³ / h × 2), which is the same as that in the first embodiment. The pH of the treated water discharged from the water storage tank was 7.0, which was higher than that in Example 1.

DESCRIPTION OF SYMBOLS 10 Liquid feed pump 20 Water storage tank 30 To-be-processed water circulation system 21 Exhaust pipe 22 Weir 30 Self-priming type exhaust gas diffuser 31 Motor 32 Hollow support shaft 33 Rotary blade 40 Drainage stirring apparatus 41 Rotary blade 50 Exhaust gas flue 60 Conduit 61 Blower 70 frame

Claims (6)

  Neutralizing alkaline wastewater, including wood chips, produced in a paper / pulp factory, in a water storage tank, containing wood chips, etc. by gas-liquid contact treatment with exhaust gas generated in a boiler installed in the factory. A method for neutralizing alkaline wastewater.   The alkaline wastewater neutralization method according to claim 1, wherein the boiler is a biomass boiler or a gas fired boiler.   3. A method for neutralizing alkaline drainage according to claim 1 or 2, wherein the gas-liquid contact treatment between the alkaline drainage and the boiler exhaust gas in the treatment tank is performed in the waste gas agitator and in the vicinity of the diffuser. Alkaline wastewater neutralization method performed in combination with equipment.   The alkaline wastewater neutralization method according to claim 3, wherein the exhaust gas aeration device and the wastewater agitation device are both rotary blade type and are arranged close to each other on the same axis.   5. The alkaline drainage neutralization method according to claim 4, wherein the rotary vane type exhaust gas diffuser and the drainage agitator arranged coaxially are driven by a common motor. The alkaline wastewater neutralization method according to any one of claims 3 to 5, wherein the exhaust gas diffusion device is a self-priming exhaust gas diffusion device.

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