JP2002224643A - Method for treating alkaline fly ash containing heavy metal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for treating heavy metal-containing alkaline fly ash, by which heavy metals can be prevented surely from being eluted from the treated ash by adding a comparatively small amount of a chemical to the heavy metal-containing alkaline fly ash and stabilizing the pH of the treated ash at the adequate pH easily. SOLUTION: This method for treating the heavy metal-containing alkaline fly ash comprises a step of treating the heavy metal-containing alkaline fly ash by using water, a dithiocarbamic acid-based heavy metal immobilizing agent and carbon dioxide.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating heavy metal-containing alkali fly ash, and more particularly to a method for treating heavy metal-containing alkali fly ash to prevent elution of heavy metal and scale components from the treated product.

[0002]

2. Description of the Related Art When household waste and industrial waste are incinerated in an incinerator, waste gas is generated. Since this waste gas contains dust containing heavy metals such as lead volatilized in the incinerator and strongly acidic gases such as hydrogen chloride (HCl) and sulfur oxides (SO _X ), these waste gases are used. After processing, it is released into the atmosphere. Fly ash is a dust component discharged after the treatment of the waste gas, and its composition varies depending on the treatment applied to the waste gas, and can be roughly classified into the following two types for each treatment method.

[0003] One treatment method is to use HCl or SO in waste gas.
_This is a method in which a strongly acidic gas such as _X is neutralized by adding an alkali powder such as slaked lime, and then dust is captured by a dust collector. Dust (fly ash) discharged by this method contains unreacted (excess) alkali powder such as slaked lime in addition to dust volatilized in the incinerator and a neutralization reaction product.
Shows strong alkalinity and is called alkaline fly ash.

Another method is a method of removing dust in waste gas by a dust collector and then neutralizing (absorbing) and removing acidic gas with an aqueous alkali solution such as caustic soda. Ash is almost neutral and is called neutral fly ash.

[0005] These fly ash are effectively used for landfill, ground construction, or the like, or are disposed of. In this case, since both alkali fly ash and neutral fly ash contain heavy metals such as lead and cadmium, it is necessary to perform a treatment for preventing the elution of the contained heavy metals in the effective use or disposal of the fly ash. is there.

In particular, alkali fly ash contains a large amount of alkali such as unreacted slaked lime, so that the pH of the discharged liquid is high and lead and the like are easily eluted. In addition, elution of scale-forming components such as calcium may cause scale disturbance in fly ash treatment sewage facilities and water collection facilities in landfills. For this reason, for alkali fly ash, it is particularly important to perform a treatment for preventing elution of heavy metals and scale-forming components.

As one of the chemical treatment methods for the alkali fly ash, there is a treatment method using a combination of carbon dioxide and a phosphate-based heavy metal fixing agent (Japanese Patent Application Laid-Open Nos. 8-155417 and 11-188331). . This method uses carbon dioxide gas to reduce the alkali components in fly ash, thereby reducing the required amount of heavy metal fixing agent to reduce fly ash treatment costs, and further reducing exposure to acid rain. This is an extremely effective treatment method that can also suppress the elution of heavy metals and the elution of scale-forming components.

In order to treat alkali fly ash by this method, it is necessary to mix and react alkali fly ash, water, carbon dioxide, and a heavy metal fixing agent. First, a casing and an alkali fly ash supply port, a processing fly ash discharge port, a liquid supply port, a carbon dioxide gas-containing gas supply port and a gas discharge port that open to the casing,
An alkali fly ash treatment device provided with a mixing / stirring device driven at a low speed and a shearing / dispersing device driven at a high speed provided in the casing has been proposed (Japanese Patent Application No. Hei 11-243467).
issue. Hereinafter, it is referred to as “first application”. ).

Conventionally, as a heavy metal fixing agent, a dithiocarbamic acid-based heavy metal fixing agent is also known in addition to the above-mentioned phosphoric acid-based heavy metal fixing agent.

[0010]

In the method of treating alkali fly ash with water, carbon dioxide and a phosphate-based heavy metal fixing agent,
The appropriate range of the addition amount of the phosphate heavy metal fixing agent is relatively narrow, and if this addition amount is small, elution of the heavy metal cannot be prevented,
Conversely, if the amount added is too large, the pH of the treated ash will be too low, causing a problem of elution of heavy metals, and in particular, a problem of increasing the elution amount of Cd.

Further, the dithiocarbamic acid-based heavy metal fixing agent has a drawback that when used alone, when the heavy metal content in fly ash is high, the required addition amount is extremely large and the drug cost increases. .

The present invention solves the above-mentioned conventional problems, can prevent the elution of heavy metals with a relatively small amount of added chemicals, and can easily stabilize the pH of treated ash to an appropriate pH. Accordingly, an object of the present invention is to provide a method for treating heavy metal-containing alkali fly ash, which can reliably prevent the elution of heavy metals from treated ash.

[0013]

According to the present invention, there is provided a method for treating heavy metal-containing alkali fly ash, which comprises kneading water and a dithiocarbamic acid-based heavy metal fixing agent to the heavy metal-containing alkali fly ash, and supplying carbon dioxide gas. And mixing.

A dithiocarbamic acid-based heavy metal fixing agent can provide an excellent heavy metal elution prevention effect. However, in the case of a dithiocarbamic acid-based heavy metal fixing agent alone, a heavy metal content in fly ash increases. There is a drawback that the required addition amount of the dithiocarbamic acid-based heavy metal fixing agent becomes very large.

On the other hand, in the present invention, since the amount of heavy metals such as Pb to be eluted can be reduced by lowering the alkalinity in fly ash by passing carbon dioxide gas, the dithiocarbamic acid-based heavy metal is fixed. The required addition amount of the agent can be reduced, and the elution of heavy metals such as Pb and Cd can be reliably suppressed.

Further, compared with a method in which an acidic solution-based heavy metal fixing agent such as phosphoric acid or sulfuric acid is used in combination with carbon dioxide, for example, an aqueous solution of dithiocarbamate is alkaline, so that the pH of the eluate is not lowered. Elution of Pb and Cd can be suppressed. Moreover, since the dithiocarbamic acid-based heavy metal fixing agent has a particularly high effect of fixing Cd,
Even when the pH of the eluate is excessively lowered due to excessive aeration of carbon dioxide gas, elution of Cd can be reliably prevented.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the method for treating heavy metal-containing alkali fly ash according to the present invention will be described in detail below.

In the present invention, heavy metal-containing alkali fly ash is treated using water, a dithiocarbamic acid-based heavy metal fixing agent, and carbon dioxide gas.

The processing procedure will be described below.

First, water is added to the heavy metal-containing alkali fly ash and kneaded so as to be sufficiently mixed. Here, the amount of water added to the alkali fly ash is 5 to 50% by weight, especially 10 to 50% by weight, based on the efficiency of the subsequent neutralization reaction.
It is preferably 30% by weight.

[0021] Carbon dioxide gas is sufficiently ventilated to the treated ash kneaded as described above.

The alkalinity of the fly ash after the carbonation treatment by aeration of the carbon dioxide gas slightly varies depending on the alkalinity of the fly ash to be treated, the heavy metal component and the composition in the fly ash, but preferably 10 to 80 mg-CaCO 3. ₃ / g-ash.

After the above carbon dioxide gas aeration, a predetermined amount of a dithiocarbamic acid-based heavy metal fixing agent is added to fly ash, and the mixture is sufficiently stirred. The addition amount of the dithiocarbamic acid-based heavy metal fixing agent is 1 to 20% by weight, especially 2 to 1% by weight based on fly ash.
It is preferably 0% by weight.

The dithiocarbamic acid-based heavy metal fixing agent may be added in a water kneading step before the carbon dioxide gas is passed, or may be kneaded before and after the carbon dioxide gas is passed.

Examples of the dithiocarbamic acid-based heavy metal fixing agent used in the present invention include diethyldithiocarbamic acid and salts thereof (eg, sodium salt), piperazinedithiocarbamic acid and salts thereof, and polyaminedithiocarbamic acid and salts thereof. Among them, an aqueous solution of dithiocarbamate is preferably used as a liquid from the viewpoint of handleability.

The dithiocarbamic acid-based heavy metal fixing agent is preferably added and kneaded as an aqueous solution having a concentration of 20 to 50% by weight. When such an aqueous solution of a dithiocarbamic acid-based heavy metal fixing agent is used, it is desirable to consider the amount of water brought in as the aqueous solution as kneading water.

In the present invention, a conventional heavy metal fixing agent can be used in combination. In this case, the heavy metal fixing agent that can be used in combination is a phosphate-based heavy metal fixing agent or a clay-based fixing agent such as activated clay. Examples include heavy metal fixing agents, polymer heavy metal fixing agents such as starch, cement heavy metal fixing agents, and acidic solution heavy metal fixing agents such as sulfuric acid. The addition amount of the heavy metal fixing agent is 1 to 30 with respect to fly ash.
% By weight. When these heavy metal fixing agents are used in combination, the heavy metal fixing agent may be added at the same time as the dithiocarbamic acid-based heavy metal fixing agent, and may be separately discharged from the dithiocarbamic acid-based heavy metal fixing agent after passing the gas containing carbon dioxide gas. It may be added before the process.

Such a method for treating heavy metal-containing alkali fly ash according to the present invention can be easily carried out using the alkali fly ash treatment apparatus of the prior application.

The construction of the alkali fly ash treatment apparatus of the prior application will be described below with reference to FIG.

FIG. 1 is a perspective view showing the alkali fly ash treatment apparatus of the prior application, and shows a state where a cover of a working port is removed.

A cylindrical casing 3 is arranged between the pair of side plates 1 and 2. Flanges 4 are provided around both end surfaces of the casing 3, respectively.
Are fixed to the side plates 1 and 2 by bolts or welding. Therefore, both end surfaces of the casing 3 are sealed by these side plates 1 and 2.

A rotary shaft 5 is arranged at the axis of the casing 3, and a plurality of stirring blades 6 are attached to the rotary shaft 5. The stirring blade 6 includes an arm 6a extending radially from the rotary shaft 5, blades 6b and 6c fixed to the arm and intersecting in a V-shape, and a tie plate 6d connecting the blades 6b and 6c. It is of the plow type. The blades 6b and 6c rotate in the direction of rotation of the rotating shaft 5 (θ direction).
Are connected in a V-shape such that the space between them (the space in the direction parallel to the axis of the rotating shaft 5) increases toward the downstream side in the rotation direction.

A plurality of the stirring blades 6 are provided at intervals in the axial direction with respect to the rotating shaft 5. The mounting phases of the stirring blades 6 in the rotation direction are different. Preferably, the phases are equally divided in this rotation direction (for example, when two stirring blades 6 are provided, the phases are different by about 180 ° and the stirring blades 6 When they are provided, they are arranged with a phase difference of about 120 °).

Both ends of the rotary shaft 5 extend outside the side plates 1 and 2 through shaft seals 7 provided on the side plates 1 and 2, and are supported by a shaft support 8. This shaft support 8 is supported by side plates 1 and 2 via a bracket 9.

Outside the side plate 1, a motor 10,
A power transmission unit 11 including a chain, a belt or a gear, and a rotary shaft driving device including a reduction gear 12 are provided. The rotating shaft driving device is supported by the side plate 1.

In the casing 3, a shear dispersion blade 13 is provided. In the present embodiment, the shear dispersion blade 13 has a plurality of blades, for example, four blades fixed to the tip side of the high-speed rotation shaft 14. Although not shown, the high-speed rotation shaft 14 extends out of the casing 3 via a shaft sealing portion, and is connected to a high-speed rotation motor. This shear dispersion blade 13
Is arranged between the swirl passing areas of the mixing and stirring blades 6 and 6 of the rotating shaft 5 and does not contact the mixing and stirring blade 6.

The upper surface of the casing 3 is provided with a supply port 16 for alkaline fly ash, a supply port 17 for liquid, a supply port 18 for gas containing carbon dioxide, and a discharge port 19 for gas.
These supply ports and discharge ports may be provided on the side surface or the like. An outlet 20 for the processing fly ash is provided on the lower surface of the casing 3. Each of the fly ash supply port 16 and the fly ash outlet 20 may be provided with a fly ash supply / discharge device such as a rotary feeder or a gate valve, or may be provided with a simple openable / closable lid. The fly ash supply port 16 may be provided with a hopper.
The liquid supply port 17 is connected to a liquid quantitative supply mechanism, and the carbon dioxide gas supply port 18 is connected to a mechanism capable of supplying a carbon dioxide gas quantitatively.

A work port 21 is provided on a side surface of the casing 3, and maintenance inside the casing 3 can be performed through the work port 21. The working port 21 is normally closed by a detachable or openable / closable lid (not shown) which is curved at the same curvature as the cylindrical casing 3.

In order to treat the alkali fly ash using the alkali fly ash treatment apparatus shown in FIG. 1, for example, a predetermined amount of the alkali fly ash is supplied from the supply port 16 into the casing 3 and the rotating shaft 5 and the high-speed rotating shaft 14 are supplied. While rotating, a predetermined amount of water is supplied little by little from the supply port 17 to mix fly ash and water. Next, a carbon dioxide-containing gas (carbon dioxide or a gas containing carbon dioxide and another gas) from the carbon dioxide-containing gas supply port 18.
Is supplied into the casing 3 and the rotating shaft 5 and the high-speed rotating shaft 14 are rotated while discharging the gas from the outlet 19 to cause the carbon dioxide gas to react with the alkali (particularly Ca (OH) ₂ ). The high-speed rotating shaft 14 may be stopped at the beginning of the ventilation of the carbon dioxide-containing gas, and may be driven after a predetermined time has elapsed.

Next, while rotating the rotating shaft 5 and the high-speed rotating shaft 14, a predetermined amount of an aqueous solution of a dithiocarbamic acid-based heavy metal fixing agent is supplied from the supply port 17 and mixed.

Alternatively, a predetermined amount of alkaline fly ash is supplied into the casing 3 from the supply port 16, and a predetermined amount of an aqueous solution of a dithiocarbamic acid-based heavy metal fixing agent is supplied to the supply port while rotating the rotating shaft 5 and the high-speed rotating shaft 14. 17 to casing 3
Feed into and mix. Thereafter, while rotating the rotating shaft 5 and the high-speed rotating shaft 14, the gas containing carbon dioxide is supplied into the casing 3 from the supply port 18, and the gas is discharged from the inside of the casing 3 through the discharge port 19. In this case as well, the high-speed rotation may be stopped at the beginning of the ventilation of the carbon dioxide-containing gas, and the rotation may be performed after a predetermined time has elapsed after the start of the ventilation.

In either method, after the processing for a predetermined time, the processed fly ash is discharged from the fly ash discharge port 20 while rotating the rotating shaft 5 and the high-speed rotating shaft 14. When granulating the treated fly ash, the high-speed rotating shaft 14 is stopped and the rotating shaft 5
Alone to form granules, and the fly ash can be discharged from the fly ash outlet 20.

In this alkali fly ash treatment apparatus, the rotating shaft 5 with stirring blades is rotated at low speed, and the high speed rotating shaft 14 with shear dispersion blades 13 is rotated at high speed while stirring the entire fly ash in the casing 3. As a result, the fly ash is sheared and dispersed, so that the fly ash in the casing 3 is efficiently mixed and stirred with the aqueous solution and comes into contact with the carbon dioxide gas-containing gas efficiently.
Therefore, fly ash processing can be performed very efficiently. In addition, rotating at high speed is a small shear dispersion blade 1
3, the rotating shaft 5 with the large stirring blade 6 is rotated at a low speed, so that the rotational driving power of the shear dispersion blade 13 and the stirring blade 6 can be small.

[0044]

The present invention will be described more specifically below with reference to examples and comparative examples.

The fly ash treated in the following Examples and Comparative Examples is municipal solid waste fly ash, and its appearance and components are as shown in Table 1.

[0046]

[Table 1]

Example 1 Fly ash was processed by the following processing procedure using the fly ash processing apparatus shown in FIG.

12.0 kg of fly ash was put into the casing 3 through the fly ash supply port 16. Then, 3.0k of demineralized water
While injecting g (25% by weight based on fly ash) from the liquid supply port 17, the rotating shaft 5 and the high-speed rotating shaft 14 were driven for 3 minutes to uniformly mix fly ash and water. The rotation speed of the rotation shaft 5 is 100 rpm, and the rotation speed of the high-speed rotation shaft 14 is 20 rpm.
00 pm.

While continuing to rotate the rotating shaft 5 and the high-speed rotating shaft 14, carbon dioxide gas having a CO ₂ concentration of 10% was aerated at 300 L / min for 60 minutes from the carbon dioxide gas-containing gas supply port 18.

The fly ash was discharged from the fly ash outlet 20 60 minutes after gas aeration, and the P-alkalinity in the fly ash was measured. The results are shown in Table 2. The method for measuring P-alkalinity in fly ash is as follows. First, 1 g of fly ash was collected, 1000 mL of demineralized water was added, and the mixture was stirred for 5 minutes. Then, a predetermined amount of the mixed solution was collected, and 2-3 drops of a phenolphthalein solution were added as an indicator. Subsequently, titration was performed using 0.02 N sulfuric acid until the color became colorless, and the P-alkalinity in fly ash was determined from the titer.

50 mg of the discharged fly ash was collected in a 500 mL polybeaker, a 35% by weight aqueous solution of potassium piperazine dithiocarbamate was added at a ratio shown in Table 2, and kneaded with a spatula for about 3 minutes to mix well with the fly ash.

[0052] The obtained treated ash was subjected to the Environment Agency Notification No. 13 test to measure the pH and Pb and Cd concentrations of the eluate. For comparison, a dissolution test was similarly performed for a sample to which no aqueous solution of potassium piperazine dithiocarbamate was added. The results are shown in Table 2 and FIGS.

Comparative Example 1 In Example 1, the fly ash after kneading water was discharged without passing the carbon dioxide gas, and the P-alkalinity was measured. The results are shown in Table 2.

An aqueous solution of potassium piperazine dithiocarbamate was added to the fly ash in the same manner as in Example 1 at the ratio shown in Table 2 and kneaded.
The results are shown in Table 2 and FIGS.

Comparative Example 2 The same treatment as in Example 1 was carried out except that an aqueous solution of 80% by weight of orthophosphoric acid was added at a ratio shown in Table 2 instead of the aqueous solution of potassium piperazine dithiocarbamate. The ash dissolution test was performed, and the results are shown in Table 2 and FIG.
-4 are shown.

[0056]

[Table 2]

The following can be seen from Table 2 and FIGS.

That is, in the combined treatment of carbon dioxide and the dithiocarbamic acid-based heavy metal fixing agent in Example 1, the pH of the eluate is between 11 and 12 even if the amount of the dithiocarbamic acid-based heavy metal fixing agent is increased. Pb and Cd can be immobilized with a stable amount of 2.0% by weight or more.

On the other hand, in the case of the dithiocarbamic acid-based heavy metal fixing agent alone in Comparative Example 1, 6.5% by weight or more of the dithiocarbamic acid-based heavy metal fixing agent is required to reduce the amount of Pb eluted. .

In the combined treatment of carbon dioxide and orthophosphoric acid in Comparative Example 2, the elution amount p
H decreased and the Cd elution amount increased.

[0061]

As described in detail above, according to the method for treating alkali fly ash containing heavy metals of the present invention, elution of heavy metals can be prevented with a relatively small amount of added chemicals, and the pH of the treated ash can be easily adjusted. Can be stabilized to an appropriate pH,
Thereby, elution of heavy metals from the treated ash can be reliably prevented.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an alkali fly ash treatment apparatus of the prior application.

FIG. 2 is a graph showing the relationship between the amount of drug added and the pH of the eluate in Example 1 and Comparative Examples 1 and 2.

FIG. 3 is a graph showing a relationship between a drug addition amount and a Pb elution amount in Example 1 and Comparative Examples 1 and 2.

FIG. 4 is a graph showing a relationship between a drug addition amount and Cd elution amount in Example 1 and Comparative Examples 1 and 2.

[Explanation of symbols]

 Reference numerals 1 and 2 Side plate 3 Casing 5 Rotating shaft 6 Stirrer blade 10 Motor 12 Reducer 13 Shear dispersion blade 14 High-speed rotating shaft 16 Alkali fly ash supply port 17 Liquid supply port 18 Carbon dioxide gas-containing gas supply port 19 Gas discharge port 20 Fly ash Outlet 21 Working port

Claims (1)

[Claims] 1. A method for treating heavy metal-containing alkali fly ash, comprising: adding and kneading water and a dithiocarbamic acid-based heavy metal fixing agent to heavy metal-containing alkali fly ash; and supplying and mixing carbon dioxide gas.