JP4756415B2 - Gas processing method - Google Patents

Description

【００３０】
分析結果は、いずれも、水銀の分析値は基準値として定められている０．００５ｍｇ／Ｌ、鉛の水質に関する排水基準値０．１ｍｇ／Ｌを超えておらず問題なかった。
【００３１】
また、１段目の集塵器のバグフィルタより回収して得られた飛灰及び副生塩処理による凝集沈殿物は、溶融炉に戻しスラグ化した。これにより、飛灰及び副生塩処理による凝集沈殿物は路盤材にリサイクルできた。２段目の集塵器のバグフィルタより回収して得られた副生塩は、本例１により無害化されたので、下水に放流処分した。以上により、固形廃棄物をなくすことができた。
【００３２】
［例２（比較例）］
例１と同じ副生塩１５０ｋｇを水８５０ｋｇに溶解させて１０００ｋｇの水溶液を得た。このときの水溶液のｐＨは９．２であった。液のｐＨを調整せず、キレート剤を１００ｐｐｍ添加して撹拌した。その後、濃度３８質量％塩化鉄（ＩＩＩ）溶液を有効成分換算で２００ｐｐｍ添加、撹拌し、その後、高分子凝集剤濃度０．１質量％の水溶液を５０００ｇ、すなわち、固形換算で副生塩溶液に対して、５ｐｐｍ添加して処理した液の上澄みを濾過し、その濾液と、濾別物の溶出試験による溶出液を例１と同様にして分析した。結果を表２に示す。
【００３３】
【表２】

【００３４】
分析結果より溶出試験については、定められている基準値０．００５ｍｇ／Ｌを達成できなかった。同時に鉛の測定も例１と同様にして実施し、同様に基準値を達成できなかった。これは、重金属類の水酸化物や酸化物が再溶解したことによるものである。したがって、ｐＨ７超で液を処理する場合、分離固形分の埋め立て処分には、さらに分離固形分に重金属固定剤を充分に混合させる処置をとらない限り不適当であることがわかった。また、沈殿の沈降速度も例１よりも遅く、圧密性も弱かった。
【００３５】
【発明の効果】
本発明のガスの処理方法により、廃棄物焼却炉等の排出ガスに、二段の集塵器を使用し、さらに、酸性成分の中和除去剤として炭酸水素ナトリウム粉末を使用することにより、中和処理の際、発生する副生塩中の重金属類、特に水銀を効果的に除去することができる。
【図面の簡単な説明】
【図１】廃棄物焼却炉の排ガスの処理工程を示す図。
【符号の説明】
１：廃棄物焼却炉
２：廃熱ボイラ
３：１段目の集塵器
４：２段目の集塵器
５：煙突
６：溶解槽
７：反応槽
８：分離機[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for treating gas discharged from municipal waste and / or industrial waste incinerators.
[0002]
[Prior art]
Conventionally, in order to neutralize and remove acidic components such as hydrogen chloride, hydrogen fluoride or sulfur oxide contained in exhaust gas from industrial waste incinerators, it is known to use slaked lime as an acidic component remover. ing. In this case, since it is necessary to use slaked lime in excess of 3 to 4 times the reaction equivalent, solid waste such as reaction products generated during the neutralization treatment and unreacted acidic component remover ( Hereinafter, in the present specification, there are disadvantages such as an increase in by-product salts) and the production of calcium salt scales when these by-product salts are treated as an aqueous solution.
[0003]
In addition to the acidic components, the combustion exhaust gas discharged from a waste incinerator or the like may contain a plurality of heavy metal components including mercury depending on the object to be treated. For this reason, even when the exhaust gas is treated, heavy metals may be contained in the by-product salt. Therefore, when the by-product salt is dissolved in water and discarded, the concentration of the heavy metal component satisfies at least the drainage standard. It is necessary to treat heavy metals to reduce the value.
[0004]
Hazardous heavy metals whose water quality standards are regulated include mercury, chromium, cadmium, selenium, arsenic, and lead. The standard values in the wastewater are 0.005 mg / L for mercury and chromium. 0.5 mg / L and other heavy metals are 0.1 mg / L, and it is necessary to be less than the reference value.
[0005]
As a method for removing heavy metals in an aqueous solution, for example, a method is known in which a chelating agent is added to an aqueous solution to form a hardly soluble precipitate and then separated. However, conventionally used low molecular weight chelating agents have the disadvantages of low removal ability and small flocs produced. If the removal capability is low, a large amount of addition is required, and the tolerance for fluctuations in the concentration of heavy metals in the inflowing aqueous solution also decreases. In addition, if the generated floc is small, it is difficult to separate by sedimentation, and there is a risk of outflow to the subsequent process, and it is necessary to install important equipment. Therefore, when a low molecular weight chelating agent is used in the by-product salt aqueous solution, it is difficult to effectively reduce the concentration of residual heavy metals in the solution below the standard value, especially in mercury with strict drainage standards, and the effect is insufficient. It was necessary to install a chelate resin tower in the latter stage. That is, a conventional chelating agent is a low molecular weight chelating agent having a molecular weight of about 100 to several hundreds as typified by sodium diethyldithiocarbamate, and a chelating resin for mercury (trade name: Eporus Z-7, It was essential to install a packed tower such as Miyoshi Oil & Fats. In addition, chelating agents are generally expensive, and reducing the amount used has been a problem.
[0006]
[Problems to be solved by the invention]
The present invention is a gas treatment that efficiently removes heavy metals, particularly mercury, in an aqueous solution in which a by-product salt generated during the neutralization treatment is dissolved, with a small amount of a chelating agent and reduces the residual concentration in the aqueous solution. The purpose is to provide a method.
[0007]
[Means for Solving the Problems]
In the present invention, exhaust gas containing one or more acidic components and heavy metals selected from the group consisting of hydrogen chloride, hydrogen fluoride, and sulfur oxides is removed after fly ash removal using a first-stage dust collector. Sodium bicarbonate powder with a particle size of 20 μm or less is sprayed to produce a by-product salt containing heavy metals, the by-product salt is separated by a second-stage dust collector, and the resulting by-product salt is dissolved in water. After the aqueous solution is adjusted to pH 3-7, the aqueous solution is composed of iron (III) chloride, iron (II) sulfate, iron (III) sulfate, aluminum sulfate, polyaluminum chloride and sodium aluminate. One or more selected coprecipitation agents and a water-soluble polymer chelating agent are added to form an insoluble solid containing heavy metals so that the mercury concentration in the aqueous solution is 0.005 mg / L or less. Provide processing methods
[0008]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, fly ash contained in exhaust gas from a municipal waste incineration plant or the like is separated by a first-stage dust collector, and then sodium bicarbonate powder is sprayed into the exhaust gas to produce two-stage by-product salt. Separate with eye dust collector.
[0009]
In the present invention, the adoption of a two-stage dust collector is advantageous in that the first-stage dust collector collects only fly ash, so that it is ash-fused to recycle roadbed materials and building materials. . Furthermore, since the by-product salt from which fly ash has been removed can be collected in the second-stage dust collector, the by-product salt can be dissolved and detoxified. As the first-stage dust collector and the second-stage dust collector, a bag filter, an electric dust collector, a cyclone dust collector, a scrubber, or the like can be used.
[0010]
In the present invention, sodium hydrogen carbonate powder is used as an agent for removing acidic components in the gas. The average particle size of the sodium hydrogen carbonate powder needs to be 20 μm or less. If the average particle size exceeds 20 μm, the reaction efficiency with the acidic component contained in the gas is lowered, and the injection amount is increased, which is not preferable. The average particle size is preferably 10 μm or less. Sodium hydrogen carbonate powder is dispersed in a gas to be treated at about 150 to 300 ° C., reacted with an acidic component in the gas, and collected by a second-stage dust collector. Sodium hydrogen carbonate powder is thermally decomposed to sodium carbonate, which reacts with acidic components in the gas. Since sodium hydrogencarbonate powder is injected into the exhaust gas in excess of the reaction equivalent, it remains as sodium carbonate in the by-product salt.
[0011]
Sodium bicarbonate consists of sodium chloride, which is a reaction product with hydrogen chloride in exhaust gas, sodium fluoride, which is a reaction product with hydrogen fluoride, and sodium sulfate and sodium sulfite, which are reaction products with sulfur oxides. These are both water-soluble. In addition, since sodium carbonate is water-soluble, these by-product salts can be dissolved in water and discarded, which not only simplifies processing operations but also reduces the amount of solid waste that needs to be landfilled. be able to.
[0012]
The by-product salt obtained in the present invention preferably has a sodium carbonate content of 30% by mass or less. If the content of sodium carbonate is more than 30% by mass, it is not preferable because it is wasteful in cost, and it is not preferable because the amount of acid required during pH adjustment described later increases.
[0013]
In the present invention, when an aqueous solution of by-product salt is prepared, the dissolution concentration is preferably 10 to 25% by mass. If the dissolution concentration is less than 10% by mass, the amount of liquid increases and the processing equipment becomes larger, which is not preferable. On the other hand, if the dissolution concentration exceeds 25% by mass, the viscosity of the aqueous solution becomes high, and it becomes difficult to perform gravity separation of the insoluble solid content to be described later, which is not preferable. The dissolution concentration of by-product salt is particularly preferably 13 to 20% by mass.
[0014]
In the present invention, the aqueous solution obtained from the by-product salt is adjusted to pH 3-7. It is preferable to adjust the pH by adding an acid. The pH of the by-product salt aqueous solution is the ability to form a chelate between the chelating agent and heavy metals, the formation of sparingly soluble hydroxides of heavy metals, the re-elution of heavy metals from insoluble solids containing heavy metals, and the by-product salt aqueous solution. Affects the solubility of carbon dioxide. It is not preferable that the aqueous solution of the by-product salt is less than pH 3 because the amount of alkali necessary for finally making the waste water in the neutral region increases. When the pH is higher than 7, it is not preferable because dissolved carbon dioxide tends to generate bubbles during the separation operation of the insoluble solid formed by addition of the chelating agent and may interfere with the sedimentation operation. In particular, when sodium hydrogen carbonate is used, the unreacted component is present as sodium carbonate having high solubility, and therefore, the concentration of carbon dioxide in the aqueous solution tends to increase, which is not preferable. The pH of the aqueous solution is particularly preferably 4-6. In addition, in this specification, pH means the value measured at 25 degreeC with the pH meter.
[0015]
Moreover, it does not specifically limit about the kind of acid to add, Since inorganic acids, such as hydrochloric acid and a sulfuric acid, are cheap, they are preferable. In the case of organic acids, wastewater when treated as wastewater may raise the BOD and COD of wastewater and may be an impurity in the final product even when used as an industrial raw material. This is not preferable because it increases the nitrogen component.
[0016]
In the present invention, since most of the heavy metals are removed by the first stage fly ash removal operation, the amount of chelating agent used can be greatly reduced. After the fly ash removal operation in the first stage, after dissolving the by-product salt in water and adjusting the pH to 3-7, heavy metals and especially mercury are added by adding specific chelating agents and coprecipitation agents. Can be effectively removed.
[0017]
The chelating agent used in the present invention is a water-soluble polymer chelating agent. Specifically, those made of a water-soluble polymer containing a thiol group and a dithiocarbamic acid group are preferable. A polymer compound having an average molecular weight of about 80,000 to 120,000 is preferable. For example, trade name Epoflock L-1 manufactured by Miyoshi Oil & Fats Co., Ltd., trade name Aclean M manufactured by Asahi Glass Engineering Co., Ltd., and the like can be given.
[0018]
For injection of the chelating agent, a general metering pump or the like can be used. The addition amount of the chelating agent is preferably 10 ppm by mass or more, and particularly preferably 30 to 400 ppm by mass per mass of the aqueous solution. If the added amount is less than 10 ppm by mass, it is not preferable because the removal of heavy metals contained may be insufficient, and if the added amount is large, it is not preferable because it is wasteful and economically disadvantageous.
[0019]
In the present invention, the coprecipitation agent is at least one selected from the group consisting of iron (III) chloride, iron (II) sulfate, iron (III) sulfate, aluminum sulfate, polyaluminum chloride and sodium aluminate. Iron (III) chloride or aluminum sulfate is particularly preferred. The addition amount of the coprecipitation agent is preferably 50 ppm by mass or more per mass of the byproduct salt aqueous solution. If the amount of the coprecipitation agent to be added is less than 50 ppm by mass, it is not preferable because removal of the heavy metals contained may be insufficient. The amount added is particularly preferably 100 to 500 ppm by mass, and more preferably 100 to 200 ppm by mass.
[0020]
In the present invention, it is preferable to use a polymer flocculant to promote sedimentation of insoluble solids. Examples of the polymer flocculant include anionic polymer flocculants such as sodium polyacrylate, nonionic polymer flocculants such as polyacrylamide, and cationic polymer flocculants such as polyacrylic acid esters. Specifically, the Kurita Kogyo brand name Kurifix CP-933 etc. are mentioned. Specific effects of the polymer flocculant include an increase in the separation rate and an improvement in the clarification of the separated liquid, and these effects can reduce the size and simplify the equipment. The concentration of the flocculant is preferably 1 to 20 ppm by mass, and particularly preferably 2 to 15 ppm by mass with respect to the by-product salt aqueous solution.
[0021]
In the present invention, the insoluble solid content can be separated from the aqueous solution of the by-product salt by a generally known method such as sedimentation separation and centrifugation utilizing gravity, or filtration. Among gravity separations, those using centrifugal force include basket-type centrifuges and screw decanter centrifuges, and those using filtration include filter presses, belt-type dehydrators, leaf filters, and Hundback filters. Is mentioned. In general, a thickener type separation method is often employed from the viewpoint of reducing equipment costs.
[0022]
In the present invention, the aqueous solution after the heavy metal is removed is measured by a method defined in the Water Pollution Control Law.
[0023]
In the present invention, the heavy metal comprises at least one selected from the group consisting of mercury, chromium, cadmium, selenium, arsenic, and lead. According to the present invention, mercury is 0.005 mg / L or less. As for other metals, chromium is preferably 0.5 mg / L or less, and cadmium, selenium, arsenic and lead are preferably 0.1 mg / L or less. According to the present invention, heavy metals satisfy the drainage standard value for water quality.
[0024]
Hereinafter, although the specific aspect of this invention is demonstrated along FIG. 1, this invention is not limited to the following example.
[0025]
FIG. 1 illustrates a waste incinerator for treating general waste discharged from homes and the like. The exhaust gas containing acidic components generated from the waste incinerator 1 is heat recovered by the waste heat boiler 2 and then fly ash is removed by the first stage dust collector 3. Next, sodium hydrogen carbonate powder is added in a dry manner and then introduced into the second stage dust collector 4. The exhaust gas filtered by the second stage dust collector 4 is released from the chimney 5 into the atmosphere. The fly ash captured by the first-stage dust collector 3 can be returned to the gasification melting furnace, put into the ash melting furnace, or recycled to the raw material for roadbed materials and building materials. The by-product salt captured by the second-stage dust collector 4 is dissolved in water in the dissolution tank 6 and adjusted to pH 3 to 7 with hydrochloric acid. Furthermore, in the reaction vessel 7, a water-soluble polymer chelating agent was added to the by-product salt aqueous solution, then a coprecipitation agent such as iron chloride or aluminum sulfate was added, and after the reaction, a polymer flocculant was further added. Later, the insoluble solid content is removed by the separator 8 to finally obtain a treatment liquid having a mercury concentration of 0.005 mg / L or less that satisfies the effluent standard value for water quality.
[0026]
The structure of the dissolution tank for dissolving the by-product salt collected from the second stage dust collector is not particularly limited, but external circulation consisting of a stirrer for accelerating the dissolution or a pump or piping in place of it. It is desirable to have a device. Water used for dissolution can be variously used such as tap water, industrial water, ion exchange water, demineralized water and the like. Further, it may be a salt already dissolved in salt such as seawater, process liquid or waste liquid.
[0027]
【Example】
In the combustion treatment process of municipal waste by a gasification melting furnace that employs a bag filter in each of the two-stage dust collectors exemplified in FIG. 1, the exhaust gas discharged from the incinerator is discharged by the first-stage dust collector. After removing fly ash, sodium bicarbonate powder having an average particle size of 9 μm was injected, and the by-product salt obtained by collecting from the bag filter of the second stage dust collector was treated. Mercury is analyzed using a reduction vaporizer MVU-1A type manufactured by Shimadzu Corporation and a mercury analysis unit for UV-1200, in accordance with the method according to the Notification of the Environment Agency established in accordance with the Enforcement Regulations of the Water Pollution Control Law. Measured by vaporized atomic absorption. About the analysis of lead, it measured by the ICP emission spectrometry using ICP emission-spectral-analysis apparatus SPS1500R by Seiko Denshi Kogyo. In addition, the dissolution test was conducted according to the method shown in Notification No. 13 of the Environment Agency.
[0028]
[Example 1 (Example)]
150 kg of by-product salt was dissolved in 850 kg of water to obtain 1000 kg of an aqueous solution.
To this, hydrochloric acid having a concentration of 35% by mass was added to adjust the pH to 7.0, and then 100 ppm of a chelating agent (trade name: Epofloc L-1, manufactured by Miyoshi Oil & Fats Co., Ltd.) was added and stirred. Thereafter, 200 ppm iron chloride (III) solution having a concentration of 38% by mass in terms of active ingredient was added and stirred. 1 ppm by weight of a 1% by weight aqueous solution was added in an amount of 5 ppm in terms of solid to the by-product salt solution. Table 1 shows the results obtained by filtering the supernatant of the added and treated liquid and analyzing the filtrate and the eluate by the elution test of the filtered product.
[0029]
[Table 1]

[0030]
As for the analysis result, in all cases, the analysis value of mercury did not exceed 0.005 mg / L defined as the standard value and the drainage standard value 0.1 mg / L for the water quality of lead, and there was no problem.
[0031]
Further, the fly ash collected from the bag filter of the first-stage dust collector and the aggregated precipitate by the by-product salt treatment were returned to the melting furnace and turned into slag. Thereby, the aggregate deposit by fly ash and byproduct salt treatment could be recycled to the roadbed material. By-product salt obtained by collecting from the bag filter of the second-stage dust collector was rendered harmless in this Example 1, and was discharged into sewage. As a result, solid waste could be eliminated.
[0032]
[Example 2 (comparative example)]
150 kg of the same by-product salt as in Example 1 was dissolved in 850 kg of water to obtain 1000 kg of an aqueous solution. The pH of the aqueous solution at this time was 9.2. Without adjusting the pH of the solution, 100 ppm of a chelating agent was added and stirred. Then, 200 ppm of 38 mass% iron chloride (III) solution in terms of active ingredient was added and stirred, and then 5000 g of an aqueous solution with a polymer flocculant concentration of 0.1 mass% was converted into a by-product salt solution in solid conversion. On the other hand, the supernatant obtained by adding 5 ppm was filtered, and the filtrate and the eluate obtained by the elution test of the separated product were analyzed in the same manner as in Example 1. The results are shown in Table 2.
[0033]
[Table 2]

[0034]
From the analysis results, it was not possible to achieve the defined reference value of 0.005 mg / L for the dissolution test. At the same time, lead was measured in the same manner as in Example 1, and the reference value could not be achieved. This is due to the redissolution of hydroxides and oxides of heavy metals. Accordingly, it has been found that when the liquid is processed at a pH of more than 7, it is not suitable for landfill disposal of the separated solid content unless a treatment for sufficiently mixing the heavy metal fixing agent into the separated solid content is taken. Further, the sedimentation rate of the precipitate was slower than that of Example 1, and the compactness was weak.
[0035]
【The invention's effect】
By using a two-stage dust collector for exhaust gas from a waste incinerator or the like, and using sodium hydrogen carbonate powder as a neutralizing and removing agent for acidic components by the gas treatment method of the present invention, During the sum treatment, heavy metals, particularly mercury, in the by-product salt generated can be effectively removed.
[Brief description of the drawings]
FIG. 1 is a diagram showing an exhaust gas treatment process in a waste incinerator.
[Explanation of symbols]
1: Waste incinerator 2: Waste heat boiler 3: First stage dust collector 4: Second stage dust collector 5: Chimney 6: Dissolution tank 7: Reaction tank 8: Separator

Claims (5)

After removing fly ash from an exhaust gas containing one or more acidic components selected from the group consisting of hydrogen chloride, hydrogen fluoride and sulfur oxides and heavy metals using a first stage dust collector, the average particle size is 20 μm or less. The sodium bicarbonate powder is sprayed to produce a by-product salt containing heavy metals, the by-product salt is separated by a second stage dust collector, and the resulting by-product salt is dissolved in water to form an aqueous solution. After the aqueous solution is adjusted to pH 3-7, the aqueous solution is selected from the group consisting of iron (III) chloride, iron (II) sulfate, iron (III) sulfate, aluminum sulfate, polyaluminum chloride and sodium aluminate. A method for treating a gas, which comprises adding a coprecipitate of at least seeds and a water-soluble polymer chelating agent to generate an insoluble solid containing heavy metals so that the mercury concentration in the aqueous solution is 0.005 mg / L or less. The gas treatment method according to claim 1, wherein the water-soluble polymer chelating agent comprises a water-soluble polymer containing a thiol group and a dithiocarbamic acid group. The addition amount of the said water-soluble polymer chelating agent is 10 mass ppm or more with respect to the said aqueous solution, and the addition amount of the said coprecipitation agent is 50 mass ppm or more with respect to the said aqueous solution. Gas processing method. The gas treatment method according to claim 1, wherein a polymer flocculant is added to the aqueous solution together with the water-soluble polymer chelating agent and the coprecipitation agent. The gas treatment method according to any one of claims 1 to 4, wherein the heavy metal is at least one selected from the group consisting of mercury, chromium, cadmium, selenium, arsenic, and lead.

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