Method for treating exhaust gas containing fly ash generated from the incineration or melting of waste.

Abstract

This invention provides an exhaust gas treatment method that can reduce the amount of heavy metals, such as lead, that are ultimately leached when disposing of incinerated fly ash or molten fly ash that has varying properties. [Solution] The exhaust gas treatment method of the present invention includes a stabilization step of obtaining stabilized fly ash by stabilizing the exhaust gas, and an immersion step of immersing the stabilized fly ash in water.

Description

Technical Field

[0001] The present invention relates to a method for treating exhaust gas containing fly ash generated by incineration or melting treatment of waste.

Background Art

[0002] Conventionally, methods for insolubilizing heavy metals eluted from incineration fly ash generated by incineration treatment of waste or melting fly ash generated by melting treatment of waste have been studied. Here, fly ash obtained through various insolubilization treatments has come to be subjected to landfill treatment at a final disposal site.

[0003] Patent Document 1 discloses a composite treatment agent and a treatment method for treating exhaust gas and fly ash, which can stabilize various heavy metals contained in the exhaust gas and fly ash generated by incineration of general waste. This composite treatment agent is a chemical agent that performs two types of treatments, namely neutralization and stabilization of waste, and is said to be effective against alkaline fly ash in a high-temperature environment exceeding 160°C and a humid environment of 30 to 50%.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] When performing incineration or melting treatment of waste, the properties of the exhaust gas can vary greatly, such as when switching the waste lot. Therefore, simply using the treatment agent described in Patent Document 1 is insufficient as an insolubilization treatment for heavy metals due to variations in the contents of the waste. If the insolubilization treatment is insufficient, there is a risk that heavy metals such as lead will elute during landfill of the fly ash. There are various heavy metals that need to be considered, and the present inventors started from establishing a method for reducing the lead elution amount, paying particular attention to the elution of lead from fly ash.

[0006] Therefore, the present invention aims to provide an exhaust gas treatment method that can reduce the amount of heavy metals, such as lead, that are ultimately leached when disposing of incinerated fly ash or molten fly ash that has varying properties. [Means for solving the problem]

[0007] As a result of diligent research to solve the above problems, the inventors conceived of a method to stabilize fly ash by stabilizing it, and then washing the stabilized fly ash by immersing it in water. The gist of the exhaust gas treatment method containing fly ash generated by the incineration or melting of waste according to the present invention is as follows.

[0008] In other words, the present invention encompasses the following embodiments. [1] A method for treating exhaust gas containing fly ash generated by incineration or melting of waste, A stabilization step to obtain stabilized fly ash by stabilizing the exhaust gas, The immersion step involves immersing the stabilized fly ash in water to obtain a mixture, A processing method that includes this. [2] The stabilization step includes supplying an inorganic agent to the exhaust gas to obtain neutralized fly ash, The treatment method according to [1], comprising the step of kneading the neutralized fly ash with water to obtain hydrolyzed and kneaded fly ash. [3] Stabilized fly ash is obtained by allowing the hydrated and kneaded fly ash to stand until the elution pH of the fly ash is 9.50 or higher and 11.5 or lower, in the stabilization step, as described in [2]. [4] Stabilized fly ash is obtained by allowing the hydrated and kneaded fly ash to stand until the elution pH of the hydrated and kneaded fly ash is 0.20 to 1.50 lower than the elution pH of the neutralized fly ash, as described in [2]. [5] The stabilization step is to obtain the stabilized fly ash by letting the water-mixed fly ash stand for 20 hours or more and for 1 week or less, as described in [2]. [6] The treatment method according to [1] or [2], wherein in the immersion step, the stabilized fly ash is washed with water at a weight of 0.5 to 10.0 times its weight. [7] The treatment method according to [1] or [2], wherein in the immersion step, the stabilized fly ash is washed with water at a weight of 2.0 to 5.0 times its weight. [8] The treatment method according to [1] or [2], further comprising a solid-liquid separation step after the immersion step, wherein the mixture is separated into solid and liquid components to obtain fly ash after washing. [Effects of the Invention]

[0009] The present invention provides an exhaust gas treatment method that can reduce the amount of heavy metals, such as lead, that are ultimately leached when disposing of incinerated fly ash or molten fly ash with varying properties. [Brief explanation of the drawing]

[0010] [Figure 1] This is an example of a waste melting treatment exhaust gas treatment process according to the present invention. [Modes for carrying out the invention]

[0011] [Definition] Prior to describing embodiments of the present invention, fly ash and exhaust gas as used herein will be explained.

[0012] -Fly ash- In this specification, fly ash at each stage of the process is referred to as incineration fly ash, molten fly ash, neutralized fly ash, hydrolyzed fly ash, stabilized fly ash, and washed fly ash. Incineration fly ash refers to fly ash contained in exhaust gas produced when general waste or industrial waste is incinerated. Molten fly ash refers to fly ash contained in exhaust gas produced by melting treatment. Neutralized fly ash is obtained by adding inorganic chemicals to the incineration fly ash or molten fly ash contained in these exhaust gases to neutralize them. Hydrolyzed fly ash is obtained by adding water to neutralized fly ash and kneading it. Stabilized fly ash is obtained by letting hydrolyzed fly ash stand for a certain period of time. Washed fly ash is obtained by immersing stabilized fly ash in water and then separating the solid and liquid components.

[0013] —Exhaust gas— In this specification, the exhaust gas means both the incineration exhaust gas generated when incinerating general waste or industrial waste and the melting exhaust gas generated when melting the main ash and fly ash of the incineration of general waste or industrial waste. This exhaust gas contains, in addition to the acidic gas generated during incineration or melting, incineration fly ash resulting from the above-mentioned incineration process or melting fly ash resulting from the melting process as solids.

[0014] ―Elution pH― In this specification, the elution pH is measured using the filtrate of the dispersion obtained by adding purified water 10 times the mass ratio to the dried fly ash or the water-mixed kneaded fly ash and stirring. Specifically, after shaking this dispersion at normal temperature and pressure for 6 hours, the filtrate obtained by filtering using a glass fiber filter paper (GF / B) with a mesh size of φ1 μm is measured.

[0015] Hereinafter, embodiments of the present invention (hereinafter referred to as "the present embodiment") will be described in detail. However, the present invention is not limited to the following description and can be implemented with various modifications within the scope of the gist.

[0016] (Method for treating exhaust gas) The waste used in the present embodiment described below is incineration ash obtained after incinerating general waste or industrial waste. Through a series of steps of subjecting this incineration ash to a melting process, the present embodiment will be described in detail. An overview of the process for treating the exhaust gas generated in the melting process of the incineration ash, which is the waste, is shown in FIG. 1. The method for treating exhaust gas according to the present invention includes at least a stabilization step of obtaining stabilized fly ash by stabilizing the exhaust gas and an immersion step of immersing the stabilized fly ash in water.

[0017] <Stabilization step> In the stabilization step, the melting fly ash contained in the exhaust gas is stabilized to obtain stabilized fly ash. By this step, the water-soluble harmful substances contained in the melting fly ash are reduced. The stabilization step preferably includes a step of supplying an inorganic agent to the exhaust gas to obtain neutralized fly ash and a step of kneading the neutralized fly ash and water to obtain water-kneaded fly ash.

[0018] <<Step of obtaining neutralized fly ash>> Exhaust gas generally flows into the atmosphere through a flue. At this time, fly ash is collected in a dust collection and filtration device called a bag filter installed in the flue. In the process of obtaining neutralized fly ash, it is preferable to blow an inorganic agent consisting of slaked lime, aluminum hydroxide, and activated carbon into the flue. At this time, the amount of inorganic agent blown in and the mixing ratio of slaked lime, aluminum hydroxide, and activated carbon in the inorganic agent should be adjusted according to the properties of the exhaust gas. The mixing ratio may be set to suppress the discharge of HCl or SOx to the final outlet to the atmosphere through the flue, or it may be set to suppress the amount of heavy metals leached from the fly ash.

[0019] In this process, fly ash in exhaust gas mixed with inorganic chemicals is collected and recovered as neutralized fly ash using a bag filter.

[0020] <<Process for obtaining water-mixed fly ash>> In the process of obtaining hydrolyzed fly ash, the neutralized fly ash is stabilized. That is, the neutralized fly ash obtained in the previous process is further homogenized, harmful substances are immobilized, and the ash is stabilized. In this process, for example, the neutralized fly ash is mixed with 10 to 50% by mass of water relative to the neutralized fly ash, and the mixture is kneaded using a fly ash quantitative supply device and kneading equipment. Hydrolyzed fly ash can be obtained from the mixture thus kneaded.

[0021] The obtained hydrolyzed fly ash is allowed to stand to ensure sufficient insolubilization and stabilization. This standing can be done at room temperature under atmospheric conditions. Preferably, the standing is allowed to continue until the elution pH of the hydrolyzed fly ash is between 9.50 and 11.5. This is because the ionization of lead-containing hydroxides is suppressed when the elution pH of the hydrolyzed fly ash is between 9.50 and 11.5. From a similar viewpoint, it is more preferable to let the standing process until the elution pH of the hydrolyzed fly ash is between 10.3 and 11.5, even more preferable to let it stand until it is between 10.3 and 11.2, and even more preferable to let it stand until it is between 10.3 and 11.0. This standing process yields stabilized fly ash from the hydrolyzed fly ash.

[0022] Furthermore, it is preferable to allow the hydrated fly ash to stand until its elution pH falls to a range of 0.20 to 1.50 compared to the elution pH of the neutralized fly ash. By allowing the hydrated fly ash to stand, a reaction proceeds in which it reacts with unreacted slaked lime and calcium chloride hydroxide, which are alkali sources in the fly ash, to produce Friedel's salt [3CaO·Al2O3·CaCl2·10H2O], a type of cement hydrate, and the elution pH of the hydrated fly ash decreases. At this time, if the elution pH of the hydrated fly ash has fallen to a range of 0.20 to 1.50 compared to the elution pH of the neutralized fly ash, it can be determined that the reaction is sufficiently complete. Lead hydroxide is produced by this reaction. Therefore, by dissolving the remaining water-soluble salt in water in a later immersion step and removing it, the lead component can be efficiently reduced throughout the entire process. From a similar viewpoint, it is more preferable to allow the mixture to stand until the elution pH of the hydrolyzed fly ash falls to 0.20 or higher compared to the elution pH of the neutralized fly ash, even more preferable to allow it to stand until it falls to 0.20 or higher, even more preferable to allow it to stand until it falls to 1.50 or lower, and even more preferable to allow it to stand until it falls to 1.50 or lower.

[0023] Furthermore, in order to satisfy either or both of the above-mentioned range of elution pH and decrease in pH of the hydrolyzed fly ash, it is preferable to allow the hydrolyzed fly ash to stand for 20 hours to 1 week. As described above, by allowing the hydrolyzed fly ash to stand, the reaction in which Ca(OH)2 is converted to CaClOH proceeds, and the elution pH of the hydrolyzed fly ash decreases. This reaction follows reaction equation 1 or reaction equation 2 below. In this case, when allowed to stand at room temperature in an atmospheric environment, these reactions generally converge after allowing the hydrolyzed fly ash to stand for 20 hours to 1 week. (Reaction equation 1): Ca(OH)2 + HCl → CaClOH + H2O (Reaction equation 2): 2Ca(OH)2 + 2CaClOH + 2Al(OH)3 + 4H2O →3CaO·Al2O3·CaCl2·10H2O

[0024] <Soaking process> In the immersion process, a mixture is obtained by immersing the stabilized fly ash obtained in the stabilization process in water. The stabilized fly ash may solidify and become lumpy when left standing in an atmospheric environment. In this case, the lumpy stabilized fly ash can be crushed as needed. The crushed stabilized fly ash can then be subjected to the immersion treatment. The immersion treatment is carried out by adding a predetermined amount of water to the stabilized fly ash and stirring. When performing the immersion treatment in actual operation, a predetermined amount of water may be added to a stirring tank, and the stabilized fly ash may be added little by little while stirring to maintain the dispersion of the stabilized fly ash. When performing the treatment in a laboratory, the immersion treatment can be carried out in the same way using a beaker and a stirrer. The immersion time is preferably 1 minute or more, more preferably 2 minutes or more, and even more preferably 30 minutes or more.

[0025] In this case, it is preferable to immerse the stabilized fly ash in water that is 0.5 to 20.0 times its weight, and more preferably 2.0 to 5.0 times its weight. By immersing it in water that is 0.5 times or more its weight, water-soluble salts that were not completely insoluble in the water-adding and kneading process can be separated from the stabilized fly ash.

[0026] Furthermore, after the immersion step, it is preferable to perform a solid-liquid separation step to separate the mixture into solid and liquid components and obtain washed fly ash. After the solid-liquid separation step, it is preferable to perform a moisture content adjustment step to adjust the moisture content of the washed fly ash. The solid-liquid separation step and the moisture content adjustment step will be described below.

[0027] <Solid-liquid separation process> In the solid-liquid separation process, the mixture obtained in the immersion process can be separated into solid and liquid components to obtain fly ash after washing. In actual operation, this can be done using a filter press, etc., while in a laboratory setting, it can be done using a Nutsch filter, suction bottle, and filter paper. The amount of filtrate obtained after solid-liquid separation is small. By mixing the filtrate with a chemical agent using simple equipment such as a stirring tank, the lead ions contained in the filtrate can be insolubilized.

[0028] <Moisture content adjustment process> In the moisture content adjustment process, the moisture content of the washed fly ash recovered in the solid-liquid separation process is adjusted. The moisture content is preferably adjusted to a level that maintains a dewatered cake-like consistency suitable for transport and does not generate dust. To adjust the moisture content of the washed fly ash, in actual operation, for example, the washed fly ash can be dried in a dryer set to 110°C, and in a laboratory setting, for example, the washed fly ash can be dried in a dryer set to 110°C. The drying time can be adjusted as appropriate to achieve the above conditions. In this way, by immersing the stabilized fly ash in water, performing solid-liquid separation, and adjusting the moisture content, the final volume of landfill fly ash can be reduced. The volume reduction of fly ash is expressed as the residue rate (weight of washed fly ash / weight of stabilized fly ash). The residue rate of the landfill fly ash is preferably 60% or less, more preferably 55% or less, and even more preferably 50% or less.

[0029] Furthermore, it is preferable to carry out the stabilization and immersion processes so that the weight of water-soluble chlorides contained in the fly ash after washing is between 0% and 15% by weight.

[0030] Furthermore, it is preferable to carry out the stabilization and immersion steps so that the weight of water-soluble sulfites contained in the fly ash after washing is between 0% and 10% by weight.

[0031] Furthermore, it is preferable to perform the stabilization and immersion steps so that the weight of water-soluble calcium salts contained in the washed fly ash is between 0% and 5% by weight. The weight of water-soluble calcium salts contained in the washed fly ash can be determined by measuring the amount of CaO in a sample taken from the washed fly ash and in a sample obtained by further washing the same weight of washed fly ash with 10 times the amount of water, using X-ray fluorescence analysis, and then calculating the difference in the amount of CaO measured from the two samples divided by the weight of the washed fly ash.

[0032] In this way, the exhaust gas containing molten fly ash undergoes at least the stabilization and immersion processes described above, resulting in washed fly ash in which at least lead among the heavy metals has been reduced. [Examples]

[0033] In the examples, exhaust gas generated during the melting process of incinerated ash from general waste and industrial waste was used.

[0034] The amount of exhaust gas generated during the melting process of incinerated ash is 28,400 Nm³ of wet gas. 3 The inorganic chemical was injected into the flue through which the exhaust gas passed at a rate of 160 kg / hour. Three types of inorganic chemicals with the chemical compositions shown in Table 1 were used for injection into the exhaust gas.

[0035] [Table 1]

[0036] The amount of solid matter generated in the exhaust gas was approximately 300 kg / hour, and the amount of neutralized fly ash collected and recovered by a bag filter after the addition of inorganic chemicals was approximately 460 kg / hour. 5 kg of the recovered neutralized fly ash was mixed with 1.5 kg of water to obtain hydrolyzed fly ash.

[0037] After drying the hydrolyzed fly ash in an air atmosphere at 105°C for 24 hours, the components of the decomposed hydrolyzed fly ash were analyzed using inductively coupled plasma atomic emission spectroscopy (ICP-OES). The analysis results are shown in Table 2.

[0038] [Table 2]

[0039] (Preliminary experiment) For water-mixed fly ash samples 1-6, which differed only in the lot obtained during actual operation, the progression of eluted pH was examined at each time interval from day 0 (immediately after sample collection) to day 5. The progression of pH at each elapsed time is shown in Table 3. From these results, it was confirmed that the pH decreased over time, and in all samples, the pH was 11.0 or lower from day 4 (48 hours) onward. [Table 3]

[0040] (Example 1) The inorganic agent used was "Agent 1" listed in Table 1. Stabilized fly ash was obtained using the method described above, and after being allowed to stand for 4 days, it was washed with water three times its weight. Specifically, 600 ml of purified water was added to 200.0 g of stabilized fly ash, and the dispersion treatment was carried out by stirring in a beaker at a rotation speed of 400 rpm for 30 minutes. In actual operation, samples were taken from the neutralized fly ash once every hour, and the above treatment was performed on each neutralized fly ash sample to obtain a total of six stabilized fly ash samples.

[0041] For each of the six stabilized fly ash samples, the immersion process described above was carried out in the laboratory to obtain washed fly ash. The analysis of heavy metal leaching from each fly ash sample was performed by adding purified water to the fly ash solids to make a solid-liquid ratio of 1:10, shaking at room temperature and pressure for 6 hours, and then filtering the filtrate using a φ1 μm glass fiber filter paper (GF / B). For the analysis of heavy metals (lead, cadmium, etc.), quantitative analysis was performed by ICP mass spectrometry after thermal decomposition with nitric acid, in accordance with the testing method of the Ministry of the Environment Notification No. 13. Similarly, the pH analysis of the obtained filtrate was performed to obtain the leaching pH value. Table 4 shows the analysis results of the leaching pH value and lead leaching value for each of the six samples of stabilized fly ash and washed fly ash.

[0042] (Example 2) The aforementioned treatment was carried out under the same conditions as in Example 1, except that the mixing ratio of the inorganic agent was changed to the value shown in "Agent 2" in Table 1. During actual operation, samples of the neutralized fly ash were taken once every three hours, yielding a total of seven stabilized fly ash samples. Similar to Example 1, Table 4 shows the analysis results for the elution pH value and lead elution value of each of the seven stabilized fly ash and post-wash fly ash samples.

[0043] (Example 3) The aforementioned treatment was carried out under the same conditions as in Example 1, except that the mixing ratio of the inorganic agent was changed to the value shown in "Agent 3" in Table 1. Neutralized fly ash samples were taken every 3 hours during actual operation, yielding a total of three stabilized fly ash samples. Similar to Example 1, Table 4 shows the analysis results for the elution pH value and lead elution value of each of the three stabilized fly ash and post-wash fly ash samples.

[0044] (Example 4) The above treatment was carried out under the same conditions as in Example 1, except that the solid-liquid ratio during washing was set to the values ​​shown in Table 4. The analysis results of the elution pH value and lead elution value for each sample when the solid-liquid ratio was changed are shown in Table 4.

[0045] (Example 5) In Examples 1-4, the process from the immersion step of stabilized fly ash onward was carried out on a laboratory scale, but a demonstration test was conducted on an actual operational scale. 2000 kg of stabilized fly ash obtained under scaled-up conditions of Example 1 was added to the melting treatment exhaust gas generated during actual operation. 3 The mixture was washed in a stirring tank at 71 rpm for 30 minutes. After washing, the slurry was injected into a horizontal fully automatic pressure dewatering machine at a pressure setting of approximately 0.4 MPa to dewater it and separate the solids, thereby obtaining solid material. The stabilized fly ash before washing and the fly ash after washing were analyzed for elution pH and lead elution values ​​in the same manner as in Examples 1 to 4 described above. The analytical values ​​are shown in Table 4.

[0046] [Table 4]

[0047] The experimental results above confirm that washing the stabilized fly ash reduces the lead leaching level of the washed fly ash by more than 20% compared to the stabilized fly ash in all samples. Furthermore, in all examples, the Cd leaching level did not exceed the landfill leaching standard value of 0.09 mg / L.

[0048] Furthermore, lead ions contained in the washing solution can be immobilized separately in a stirring tank by iron coprecipitation and chelation. Specifically, they can be immobilized by an iron coprecipitation reaction by adding ferric chloride manufactured by Toagosei Co., Ltd. to the washing solution, and by immobilization through chelation by adding "Wellclean K-800" manufactured by Kurita Water Industries Ltd. [Industrial applicability]

[0049] The present invention provides an exhaust gas treatment method that can reduce the amount of heavy metals, such as lead, that are ultimately leached when disposing of incinerated fly ash or molten fly ash with varying properties.

[0050] [Contribution to the United Nations-led Sustainable Development Goals (SDGs)] The SDGs have been proposed to realize a sustainable society. One embodiment of this invention is considered to be a technology that can contribute to "No. 7: Affordable and Clean Energy" and "No. 12: Responsible Consumption and Production," among others.

Claims

1. A method for treating exhaust gas containing fly ash generated by incineration or melting of waste, A stabilization step to obtain stabilized fly ash by stabilizing the exhaust gas, The immersion step involves immersing the stabilized fly ash in water to obtain a mixture, A processing method that includes this.

2. The stabilization step includes supplying an inorganic agent to the exhaust gas to obtain neutralized fly ash, The processing method according to claim 1, comprising the step of kneading the neutralized fly ash with water to obtain hydrolyzed and kneaded fly ash.

3. The treatment method according to claim 2, wherein the stabilization step involves allowing the water-mixed fly ash to stand until the elution pH becomes 9.50 or higher and 11.5 or lower to obtain stabilized fly ash.

4. The treatment method according to claim 2, wherein the stabilization step involves allowing the hydrated and kneaded fly ash to stand until the elution pH of the hydrated and kneaded fly ash is 0.20 to 1.50 lower than the elution pH of the neutralized fly ash.

5. The processing method according to claim 2, wherein the stabilization step involves allowing the water-mixed fly ash to stand for 20 hours or more and 1 week or less to obtain the stabilized fly ash.

6. The processing method according to claim 1, wherein in the immersion step, the stabilized fly ash is washed with water at a weight of 0.5 to 10.0 times its weight.

7. The processing method according to claim 1, wherein in the immersion step, the stabilized fly ash is washed with water at a weight of 2.0 to 5.0 times its weight.

8. The processing method according to claim 1, further comprising a solid-liquid separation step after the immersion step, in which the mixture is separated into solid and liquid components to obtain fly ash after washing.

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