JP2001009420A - Treatment of fly ash - Google Patents

Abstract

PROBLEM TO BE SOLVED: To simply and surely neutralize fly ash which is collected from a waste incinerator exhaust gas and contains acidic substances such as hydrogen chloride by kneading the fly ash and a slurry-like aqueous solution containing a neutralizing agent together. SOLUTION: This treatment process comprises: collecting dust containing acidic substances such as hydrogen chloride, from a waste incinerator exhaust gas with a dust collector 1; recovering the dust as fly ash in a fly ash hopper 2; introducing the recovered fly ash from the fly ash hopper 2 into a kneader 7; in the kneader 7, kneading the fly ash, a slurry-like aqueous solution which contains magnesium hydroxide and is supplied from an injection line 4, and water supplied from another injection line 5 together, to subject the fly ash to neutralization treatment; and thereafter, discharging the neutralized fly ash from a discharge port 8 as the treated fly ash; wherein if necessary, a chelating agent, cement or the like can be injected from an injection line 6 to mix the chelating agent or cement with the fly ash being treated. Thus, the acidic substances in fly ash can surely be subjected to the neutralization treatment and also, this neutralization treatment can simply and surely be performed by using only a kneader.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a fly ash treatment method for neutralizing fly ash in an exhaust gas treatment device of a waste incineration plant. More specifically, the present invention relates to a fly ash treatment method for simply and surely neutralizing fly ash containing an acidic substance such as hydrogen chloride collected from exhaust gas by kneading it with a slurry-like aqueous solution containing a neutralizing agent.

[0002]

2. Description of the Related Art Generally, waste incineration facilities include an incinerator, a device for cooling exhaust gas, a dust collector for collecting dust from exhaust gas, and a device for removing acid gas such as hydrogen chloride in exhaust gas (slaked lime blowing device). , A scrubber such as caustic soda), and a device for removing harmful components such as sulfur oxides, nitrogen oxides, and dioxins may be added. When the concentration of acidic gas in the exhaust gas is high, the exhaust gas from the incinerator is cooled and then the dust is removed by the dust collector in order to suppress the dust collector from becoming large,
Next, the acidic gas in the exhaust gas is removed by the acidic gas removing device. Dust removed by dust collector (ie fly ash)
Since they contain an acidic gas and the like, they are neutralized with an alkali component and then landfilled as waste. The buried fly ash does not pollute the environment because the pH of the leachate becomes almost neutral when it comes into contact with rainwater or the like. On the other hand, when the concentration of acidic gas in the exhaust gas is relatively low, after neutralizing the exhaust gas from the incinerator by injecting an alkali such as slaked lime into the exhaust gas,
A method for collecting dust is performed. In any case, the fly ash is completely neutralized and discarded without any acid gas or unreacted alkali remaining in the fly ash.

[0003] With regard to the neutralization of fly ash collected after neutralizing the exhaust gas from the incinerator with alkali, for example, Japanese Patent Laid-Open No. 1-284381 discloses that fly ash is mixed with water in a dissolution tank. A fly ash treatment method is disclosed in which a slurry is formed, and then an exhaust gas containing carbon dioxide gas is blown in a neutralization tank to neutralize unreacted alkali components, and further dehydrated by a dehydrator to obtain treated fly ash. . JP-A-8-108038 discloses that fly ash is dissolved in water in a fly ash dissolving tank, and an exhaust gas containing carbon dioxide gas is blown together with the calcium carbonate seed crystal recycled in the fly ash detoxifying tank to unreact. A fly ash treatment method is disclosed in which a neutralized alkali component is further treated and dehydrated by a dehydrator to obtain treated fly ash.

All of these techniques require a plurality of processing equipment such as a dissolving tank, a neutralizing tank, a dehydrator, etc., or a fly ash dissolving tank, a fly ash detoxifying tank, a dehydrator, etc. Therefore, there has been a problem that the neutralization process is extremely complicated since appropriate management is also required. on the other hand,
The same applies to the neutralization of fly ash collected from the exhaust gas from the incinerator, since it is necessary to neutralize the fly ash with alkali and then neutralize the unreacted alkali. There was a problem. Also, if the concentration of acid gas in the exhaust gas fluctuates due to the composition of the waste to be incinerated, determine the amount of alkali used while monitoring the neutralization status by sampling fly ash in batches. There is also a problem that the operation is often complicated. Furthermore, when fly ash is neutralized with alkali such as slaked lime, slaked lime is strongly alkaline, so it is difficult to accurately add a neutralization equivalent of alkali. There is also a problem that the degree of pH greatly varies from pH = 7.

On the other hand, when the fly ash contains water-soluble heavy metals, the fly ash is usually kneaded with cement and water and solidified in addition to the neutralization treatment of the acid gas, and then buried as waste. Can be In this case, the water-soluble heavy metals in the fly ash are converted into hydroxides and insolubilized due to the alkalinity (about pH = 12) of the cement. Will not pollute. However, when the fly ash contains a large amount of alkali components such as unreacted slaked lime, the pH increases and heavy metals such as lead are easily eluted, so that an expensive chelating agent or the like may be further added. .

Regarding solidification of heavy metals, see, for example,
-248423 contains slaked lime and magnesium hydroxide as main components, and further contains cement, neutralizing agent, inorganic adsorbent,
Chelating agent, water glass, phosphate, and a treating agent consisting of a compound that generates a hardly soluble or insoluble compound by reacting with harmful metal compounds such as lead, and after treating the exhaust gas by blowing in the exhaust gas treatment process, A fly ash treatment method is disclosed in which fly ash obtained by collecting soot and dust generated from the exhaust gas with a dust collector or the like is kneaded with water as required. In this case, the harmful heavy metal can be enclosed in the fly ash while simultaneously capturing the acidic gas in the exhaust gas.

[0007] In the conventional exhaust gas treatment technology, technology has mainly been established mainly from the viewpoint of solidification of heavy metals due to the recent increase in efforts to deal with environmental problems. In addition, the self-disposal responsibility of the discharge company is clearly specified, and many establishments of production processes that do not use heavy metals have been found, and the waste treatment technology that does not contain heavy metals has been increasing. Under these circumstances, it has become important today to establish technology from the viewpoint of pH control of leachate. However, in the prior art, as described above, the step of neutralizing the fly ash is inadequate in that the step of neutralizing the fly ash cannot be accurately neutralized with an acidic substance. A way was desired.

[0008]

SUMMARY OF THE INVENTION The present invention provides a fly ash treatment method that simply and reliably neutralizes fly ash containing acidic substances such as hydrogen chloride collected from exhaust gas from a waste incineration plant. Things.

[0009]

Means for Solving the Problems With respect to fly ash collected from the exhaust gas of a waste incineration plant, the present inventors have determined the concentration of the acidic substance in the exhaust gas and the content of the acidic substance in the fly ash. As a result of intensive studies on the relationship, physical properties of the neutralizing agent, and neutralization conditions, a slurry-like aqueous solution containing a slightly excessive amount of the neutralizing agent in the dust containing the acidic substance collected from the exhaust gas in excess of the expected equivalent of the acidic substance In particular, the present inventors have found that fly ash can be simply and reliably neutralized using only a kneader by kneading and neutralizing a slurry aqueous solution of magnesium hydroxide.

That is, the present invention relates to an exhaust gas treatment apparatus for a waste incineration plant, in which fly ash containing acidic substances such as hydrogen chloride collected from exhaust gas is kneaded with a slurry aqueous solution containing a neutralizing agent. And a method for treating fly ash. Further, the present invention provides the above fly ash treatment method, wherein the above-mentioned neutralizing agent comprises magnesium hydroxide.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The fly ash treatment method of the present invention is a method for treating fly ash containing an acidic substance, particularly an acidic substance consisting of hydrogen chloride, collected from exhaust gas from a waste incineration plant with a neutralizing agent, preferably water. It is kneaded with a slurry-like aqueous solution containing a neutralizing agent composed of magnesium oxide.

(Fly ash) Fly ash targeted by the present invention is obtained by collecting dust from exhaust gas from a waste incineration facility with a dust collector. The collected dust, that is, fly ash, contains not only dust but also acidic substances generated from incinerated waste, particularly acidic substances composed of hydrogen chloride. The content of acidic substances in fly ash is determined when the concentration in exhaust gas is
Several tens to several hundreds per unit weight of fly ash in terms of hydrogen chloride
ppm.

(Neutralizing agent) As the neutralizing agent used in the present invention, a known alkali component can be used without particular limitation, but it is important that the neutralizing agent has a low solubility in water. Preferably, it is magnesium hydroxide, and its solubility in 100 g of water is 0.0009 g (1
8 ° C.). The neutralizing agent used in the present invention desirably has a large specific surface area from the viewpoint of the reaction efficiency with an acidic substance.

The neutralizing agent used in the present invention is used as a slurry aqueous solution because of its low solubility in water. By using the neutralizing agent as a slurry-like aqueous solution, transfer of the neutralizing agent, workability such as mixing with fly ash can be facilitated, and when kneaded with fly ash,
The acidic substances on the fly ash particle surface can be reliably neutralized. Further, it is possible to prevent fly ash from scattering during the kneading operation.

That is, by using a neutralizing agent having low solubility in water as a slurry-like aqueous solution, the surface of the fly ash particles is slightly covered with water when kneaded with fly ash, and the neutralizing agent is mixed with this aqueous layer. And a neutralization reaction with an acidic substance mainly composed of hydrogen chloride can be carried out. The neutralizer quickly dissolves in a thin aqueous layer on the surface of the fly ash particles up to its solubility, and can instantaneously neutralize with an acidic substance mainly composed of hydrogen chloride. Consumed neutralizing agent is quickly supplied through the water layer that thinly covers the fly ash particle surface, so the neutralization reaction proceeds while acidic substances are present on the fly ash particle surface,
The fly ash can be reliably neutralized. On the other hand, a neutralizing agent such as slaked lime has a solubility in water (100 g of water).
0.155 g (20 ° C.), and strong alkaline, making it difficult to accurately add a neutralizing equivalent of a neutralizing agent. It varies greatly from 7.

The slurry-like aqueous solution of the neutralizing agent can be obtained by mixing usually 1 to 30 parts by weight, preferably 2 to 10 parts by weight of the neutralizing agent with respect to 100 parts by weight of water. If the mixing ratio of the neutralizing agent is less than 1 part by weight, a large amount of a slurry-like aqueous solution of the neutralizing agent is required to neutralize the fly ash, which is not economical. On the other hand, if it exceeds 30 parts by weight, the fly ash becomes clay-like and slurry-like, and a large amount of power is required for kneading, which deteriorates operability.

The neutralizing agent used in the present invention is preferably magnesium hydroxide, but the magnesium hydroxide is not particularly limited, and a known one can be used. In addition, a slurry product commercially available for flue gas desulfurization can be diluted with water and used according to the use conditions. By adjusting the degree of dilution, the sedimentation of magnesium hydroxide can be suppressed, so that a more uniform concentration can be prepared. Furthermore, the effect of suppressing corrosion of equipment can be expected by dilution.

Magnesium hydroxide has slightly weaker alkalinity than slaked lime and caustic soda which are generally used for neutralizing exhaust gas, and has a much higher solubility in water than slaked lime. Small and hardly soluble in water. Furthermore, magnesium hydroxide itself has no significant harm. Therefore, when neutralizing acidic substances,
Even if magnesium hydroxide is used in a slight excess, the problem of secondary pollution of the environment due to discharge of unreacted magnesium hydroxide is extremely small.

The slurry aqueous solution containing a neutralizing agent used in the present invention may be, if desired, a cement, an inorganic adsorbent, a chelating agent, water glass, or the like, as long as the performance is not impaired.
A metal stabilizer or the like can be included.

Examples of the cements include calcined gypsum, Portland cement, early-strength cement, jet cement, blast furnace cement, alumina cement and the like. By including cements, reliable solidification can be achieved.

Examples of the inorganic adsorbent include various types of activated clay, synthetic silicic acid, natural silicic acid products, activated carbon and the like, and powdery aluminum silicate and powdery silicon dioxide having a specific surface area of 200 m ² / g or more are desirable. Further, from the viewpoint of industrial availability, those having a specific surface area of 200 to 1000 m ² / g are desirable. However, inorganic adsorbents having a specific surface area of 200 m ² / g or more are bulky and difficult to handle, so that it is preferable to perform processing such as granulation in advance. By including the inorganic adsorbent, the diffusion of various harmful substances can be suppressed.

As a chelating agent, -OH, -CSS
H, -SH, = NH, -COOH, etc. -NH _2, a compound having a chelating ligand structure against harmful metals,
And salts thereof, but dimethyldithiocarbamate, diethyldithiocarbamate, dibutyldithiocarbamate, and tannate are particularly preferred from the viewpoint of lead elution prevention performance. Industrially available, Ash Clean C-3 manufactured by EBARA CORPORATION
00, New Epolva 800 manufactured by Miyoshi Oil & Fat Co., Ltd.
Hydion manufactured by Nippon Soda Co., Ltd., Alsite L-101 manufactured by Fujisashi Co., Ltd., Organite 2050 manufactured by Organo Co., Ltd., Sumichelate AC-2 manufactured by Sumitomo Chemical Co., Ltd.
And Oriental Giken Co., Ltd .; By including a chelating agent, heavy metals can be immobilized.

As the water glass, general-purpose water-soluble silicate,
That is, sodium silicate, potassium silicate and the like can be exemplified. Sodium silicate is desirable from the viewpoint of lead elution prevention performance and price. Examples of the metal stabilizer include sulfides such as sodium sulfide, carbonates, oxalates, sulfates, and chromates. The metal stabilizer reacts with a harmful metal compound such as lead to form a hardly soluble or insoluble compound.

(Fly Ash Treatment Method) The fly ash treatment method of the present invention comprises kneading the above-described aqueous solution of the neutralizing agent slurry and dust collected from exhaust gas, that is, fly ash. . Hereinafter, the fly ash treatment method of the present invention will be described with reference to the drawings.

FIG. 1 is a view for explaining a main part of a fly ash processing apparatus for carrying out the fly ash processing method of the present invention. In FIG. 1, dust collected by the dust collector 1 is collected as fly ash by the fly ash hopper 2. The collected fly ash is mixed with a kneader 7
Here, after being kneaded with a slurry aqueous solution containing magnesium hydroxide from the injection line 4 and water from the injection line 5 to be neutralized, it is discharged from the outlet 8 as treated fly ash. In addition, a chelating agent, cement, or the like can be added from the injection line 7 as needed.

In the fly ash treatment method of the present invention, the use of a kneader (for example, a twin-screw kneader) causes the fly ash particles to move around each other, so that the mass transfer is performed very quickly and the neutralizing agent is used. Contact is made sufficiently,
Implementation of the present invention can be facilitated.

The mixing ratio of the neutralizing agent to the fly ash may be at least the equivalent of the acidic substance in the fly ash, and is usually 1.2 to 2 times the equivalent in terms of reaction efficiency and economical efficiency at the time of kneading. Preferably it is 1.4 to 1.8 times. By kneading a neutralizing agent equal to or more than the equivalent of the acidic substance with the fly ash, the acidic substance in the fly ash can be reliably neutralized. Unreacted neutralizing agent is discharged together with fly ash after the neutralization treatment. When the compounding ratio is less than 1.2 times, the neutralization of the acidic substance becomes insufficient. Also, 2
If it exceeds twice, the acidic substance can be surely neutralized, but the economic efficiency is inferior due to the large number of unreacted neutralizing agents.

Further, as described above, the content of the acidic substance in the fly ash may be several tens to several tens per unit weight in terms of hydrogen chloride when the concentration in the exhaust gas is up to about several percent.
Although it is 00 ppm, when the composition of waste fluctuates greatly and the content of acidic substances in fly ash fluctuates greatly, fly ash is sampled appropriately and the content of acidic substances in fly ash is measured. It is necessary to set the amount of the neutralizing agent to be used.

[0029]

EXAMPLES The present invention will be described more specifically based on examples. In addition, this invention is not limited at all by the following Examples. Examples 1 to 3 and Comparative Example 1
The following were used as fly ash and the neutralizing agent used in ~ 4.

(1) Fly ash: Dust collected from exhaust gas from an exhaust gas treatment device was used as fly ash in an incineration facility that mainly incinerates waste liquid containing an organic chlorine compound. (2) Slurry aqueous solution of magnesium hydroxide: 300 g of magnesium hydroxide (manufactured by Ube Materials Co., Ltd.)
Was mixed with 9700 g of water and stirred at room temperature for 30 minutes to obtain a slurry-like aqueous solution containing magnesium hydroxide. (3) Magnesium hydroxide: Magnesium hydroxide (Ube Materials Co., Ltd.) was used. (4) Caustic soda aqueous solution: caustic soda (Wako Pure Chemical Industries, Ltd.)
1/20 to 1 / 50N aqueous solution of the reagent (special grade manufactured by Toshiba Corporation) was used. (5) Slaked lime aqueous solution: A 4% by weight aqueous solution of slaked lime (calcium hydroxide) (manufactured by Maruo Calcium Co., Ltd.) was used.

(Examples 1 to 3) In the fly ash treatment apparatus shown in FIG. 1, a fly ash and a neutralizing agent were neutralized at a kneading ratio shown in Table 1. The kneading was performed for about 2 hours with a residence time of 3 to 10 minutes using a biaxial kneader installed in the building. Next, the pH of the leachate was measured for the obtained fly ash after the neutralization treatment. The results are shown in Table 1.

[0032]

[Table 1]

The method for measuring acidic substances in fly ash and the method for measuring the pH of leachate from treated fly ash are as follows. (1) Method for measuring acidic substances in fly ash Water and a sample (fly ash collected from exhaust gas) were measured with water 100
Take the Erlenmeyer flask with a stopper in a ratio of 10 parts by weight of the sample to the parts by weight, quickly close the stopper, make the mixed solution 100 ml or more, and minimize the head space of the stoppered Erlenmeyer flask. After stirring at room temperature (approximately 20 ° C.) for about 3 minutes using a magnet stirrer, the supernatant liquid left standing for about 5 minutes was collected. Then, the supernatant was subjected to neutralization titration, and the content of the acidic substance was calculated as hydrogen chloride. The results were expressed in terms of the weight (mg / kg) of hydrogen chloride per unit weight of the sample (fly ash). In the neutralization titration, a 1/50 aqueous solution of sodium hydroxide for volumetric analysis was used, and phenolphthalein was used as an indicator.

(2) Method for measuring pH of leachate Water and a sample (fly ash after neutralization treatment) were mixed at a ratio of 10 parts by weight of the sample (fly ash) to 100 parts by weight of water, and a triangle with a stopper was used. Take the flask, stopper quickly, and adjust the mixed solution to 100 ml or more, and minimize the head space of the stoppered Erlenmeyer flask at room temperature (approximately 20 ° C.) using a magnetic stirrer. After stirring for about 3 minutes, the supernatant liquid left standing for about 5 minutes was collected. The pH of the supernatant was measured according to JIS-K-0802.

(Comparative Examples 1 to 4) Fly ash was neutralized in the same manner as in Examples 1 to 3 except that the mixing ratio of the fly ash and the neutralizing agent was set to the ratio shown in Table 2. The leachate pH was measured for the fly ash after the neutralization treatment. The results are shown in Table 2.

[0036]

[Table 2]

As shown in the results of Table 1, Examples 1 to
Regarding the fly ash after the neutralization treatment of No. 3, despite the fact that the amount of the neutralizing agent used is equal to or more than the equivalent of the acidic substance, the pH of the leachate is
Was 7.1 to 7.6, which was almost neutral. This clearly shows that the acidic substances in the fly ash were reliably neutralized, and that the amount of the used neutralizing agent was appropriate.

On the other hand, as shown in the results in Table 2, the fly ash after the neutralization treatment obtained in Comparative Example 1 uses the neutralizing agent in an amount equal to or less than the equivalent of the acidic substance. The pH of the leachate was 5.9, indicating acidic. On the other hand, in the fly ash after the neutralization treatment obtained in Comparative Examples 2 to 4, the acidic substances were neutralized despite the use of the same equivalent of the neutralizing agent as in Examples 1 to 3. The pH of the leachate was 9.1 to 1
0.1 and alkaline. From this, it is clear that this method is not suitable as a method for neutralizing fly ash.

[0039]

According to the fly ash treatment method of the present invention, dust containing acidic substances is collected from the exhaust gas, and the neutralizing agent of the present invention is added to the dust in a slightly excessive amount from the expected amount of the acidic substances. By adopting a configuration in which the slurry-like aqueous solution containing the mixture is directly kneaded and neutralized, the acidic substances in the fly ash can be reliably neutralized, and the medium can be easily and reliably used only with a kneader. The effect that sum processing can be performed is produced. Furthermore, since the remaining neutralizing agent has very low solubility in water, it has little effect on leachate, and is a substance with little danger. Therefore, the problem of secondary pollution of the environment is extremely small.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a fly ash processing device.

[Explanation of symbols]

Reference Signs List 1 dust collector 2 fly ash hopper 3 rotary valve 4 injection line of aqueous slurry containing magnesium hydroxide 5 injection line of water 6 injection line of chelating agent, cement, etc. 7 kneader 8 discharge of fly ash after neutralization treatment exit

Claims (2)

[Claims] 1. An exhaust gas treatment device for a waste incineration plant, wherein fly ash containing an acidic substance such as hydrogen chloride collected from an exhaust gas is kneaded with a slurry-like aqueous solution containing a neutralizing agent. Fly ash treatment method. 2. The fly ash treatment method according to claim 1, wherein said neutralizing agent comprises magnesium hydroxide.