JPH09294916A - Method for treating exhaust gas and dust - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a treatment method in which heavy metals contained in dust generated during the incineration of waste are sealed to be prevented from eluting. SOLUTION: In the exhaust gas treatment apparatus of waste incineration facilities, slaked lime of at least 30m<2> /g specific surface area is sprayed into exhaust gas, and dust containing heavy metals such as Pb which is generated from the exhaust gas and captured by an electric precipitator and a bag filter is added with A liquid containing a soluble silicate aqueous solution as a main component, B liquid chiefly containing an aqueous solution containing a multivalent metal cation which reacts with a hydroxide ion and produces insoluble or poorly soluble hydroxide, and water as required. In this method, heavy metals contained in the dust generated during the incineration of waste are sealed surely to be prevented from eluting.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heavy metal (Pb, Cd, Cr ⁶⁺ , As, contained in soot and dust produced from exhaust gas generated during waste incineration in a waste incinerator.
Hg, Cu, Zn, etc.) is effective in stabilizing exhaust gas and dust,
In particular, the present invention relates to a treatment method that is effective in stabilizing soot and dust discharged from municipal solid waste incineration facilities, in which it is difficult to prevent the elution of lead (Pb).

[0002]

2. Description of the Related Art Conventionally, for the disposal of wastes containing harmful substances such as heavy metals, humidification treatment is performed to prevent the wastes from scattering during transportation, and cement is mainly used for solidification molding of the wastes. Processing has taken place. However,
These treatments have various problems from the viewpoint of suppressing the elution of harmful heavy metals from waste, and unless careful attention is paid to their handling, heavy metals will elute from the treated waste and secondary pollution will occur. May occur.

As described above, it has become clear domestically and internationally that it is difficult for the conventional method to suppress the elution of harmful metals from wastes containing heavy metals such as Pb. Therefore, there has been a demand for a treatment method in which heavy metals and the like are reliably enclosed even when landfill waste is disposed of on land and are not re-eluted.

[0004] Typical waste incineration facilities are typically an incinerator, a heat exchanger for cooling exhaust gas, a water spray device,
It is composed of a slaked lime blowing device (blowing into the exhaust gas guide pipe) for the purpose of neutralizing acidic gas such as hydrogen chloride gas in the exhaust gas, and a dust collector for capturing the soot and dust generated from the exhaust gas. A device or equipment for removing harmful components such as sulfur oxides (SOx), nitrogen oxides (NOx) and dioxins may be attached.

Further, the soot and dust generated during the incineration of waste contains dust, a reaction product of slaked lime and an acid gas such as hydrogen chloride gas, unreacted slaked lime, and heavy metals such as Pb. There is. This soot and dust is subjected to intermediate treatment so that heavy metals do not elute, and then landfilled at the final disposal site. The problem of elution of heavy metals such as Pb relating to the waste treatment as described above is particularly remarkable in the soot and dust that is generated from the exhaust gas at the time of incineration of the waste such as general waste and is captured by the dust collector.

[0006]

DISCLOSURE OF THE INVENTION In view of the present situation of such waste treatment, in particular, dust disposal generated during waste incineration, the present invention reliably stabilizes various heavy metals in soot and suppresses their elution. It is an object of the present invention to provide a method for treating exhaust gas and soot and dust that can be used.

[0007]

DISCLOSURE OF THE INVENTION As a result of intensive studies made by the present inventors for the purpose of solving the above problems in the prior art, as a result, soot and dust or a large amount of heavy metals that cannot substantially prevent the elution of heavy metals such as Pb. We have found that soot and dust that requires stabilizers have common characteristics. That is, it was found that these soot and dust are alkaline because they contain a large amount of unreacted slaked lime, promote elution of amphoteric metals such as Pb, and inhibit the effect of the heavy metal stabilizer. . In other words, in the exhaust gas treatment device of the incinerator, because it meets the emission standard value of acidic gas such as hydrogen chloride gas,
Slaked lime (calcium hydroxide) is blown in as a neutralizing agent. However, since the reaction efficiency of conventionally used slaked lime with acid gas is not always sufficient, the present situation is that an excess amount of slaked lime exceeding the equivalent amount has been blown into the generated acid gas. Therefore, the soot and dust produced from the exhaust gas in which such a large amount of slaked lime is blown contains a large amount of unreacted slaked lime. Therefore, compared to slaked lime that has been used conventionally, the specific surface area is high,
By using slaked lime with excellent reaction efficiency with acidic gas in an exhaust gas treatment device to reduce the amount of unreacted slaked lime contained in soot and dust, it is possible to easily suppress the elution of heavy metals such as Pb. The present invention has been completed based on the idea.

That is, the present invention is, in an exhaust gas treatment apparatus attached to a waste incineration facility, for the purpose of neutralizing and reducing acidic gas such as hydrogen chloride gas, slaked lime (calcium hydroxide) having a specific surface area of 30 m ² / g or more. ) Is blown into the exhaust gas, and this slaked lime is blown to produce it from the exhaust gas after treatment,
Soot and dust containing heavy metals such as Pb captured by electric dust collectors and bag filters, and solution A, which is mainly composed of an aqueous solution of soluble silicate, and insoluble or sparingly soluble hydroxide that reacts with hydroxide ions. The present invention is directed to a method for treating exhaust gas and dust, which is characterized in that an aqueous solution containing a polyvalent metal cation that produces the above is mixed with the solution B as a main component, and water is further added and mixed if necessary. .

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The method for treating exhaust gas and dust according to the present invention will be described in more detail. First, the slaked lime having a specific surface area of 30 m ² / g or more used in the present invention is preferably the larger the specific surface area in consideration of the reaction efficiency with an acidic gas such as hydrogen chloride gas, but from the viewpoint of industrial availability,
Those having a specific surface area of about 30 to 60 m ² / g are preferable. Slaked lime having such a specific surface area is, for example,
It can be produced by the method disclosed by 6-8194 or the like. This method of producing slaked lime is a method for slaked lime by strongly and uniformly mixing finely lumped or crushed lightly calcined lime with a slaking liquid consisting of water and an organic solvent that delays the reaction. To do.
Thus, in the method for producing dry calcium hydroxide, 30 to 50 parts by volume of water and 50 to 7 of organic solvent are used.
Mixing of 0 volume part of the slaking liquid and lime is performed in a mixing container at a temperature of 45 ° C. or lower. The reaction mixture is then transferred to the main reaction vessel in which it is heated to 50-7 by a heating device.
The temperature is 0 ° C. and the final reaction is carried out in the second reaction vessel. At that time, the ratio of the lime to the slaking liquid in the mixing container was 85 to 11 in the second reaction container.
The choice is made so that a reaction temperature of 0 ° C. is achieved. Further, in a subsequent degassing step, the adhering solvent is removed from the finished calcium hydroxide by applying a vacuum and / or purging with an inert gas. As such slaked lime having a high specific surface area, specifically, the use of slaked lime “Tamacalc” manufactured by Okutama Kogyo Co., Ltd. is industrially suitable. The effect of the high specific surface area slaked lime can be confirmed if the specific surface area is 20 m ² / g or more, but the effect is more remarkable as the specific surface area is higher.
It is preferably 0 m ² / g or more. It should be noted that any material other than "tamacalc" can be used as long as it has a specific surface area within the above range, and is within the scope of the present invention.

The specific surface area used in the present invention is 30 m ²
The amount of slaked lime used in the amount of 1 g / g or more must be appropriately set according to the concentration of hydrogen chloride gas in the exhaust gas, the temperature of the exhaust gas, the flow rate, and the concentration of hydrogen chloride gas finally discharged to the outside. Compared to JIS special slaked lime used (specific surface area 14.5 m ² / g), the specific surface area is about 2
Since it is double, the reaction efficiency with hydrogen chloride gas is also high,
The concentration of hydrogen chloride gas finally discharged can be made approximately the same as in the case of JIS special slaked lime with an addition amount of about half (weight) of JIS special slaked lime.

Although the effects of the solutions A and B in the soot and dust treatment of the present invention are not always clear, they can be estimated as follows.

First, the action of the liquid A will be described. When the dust containing heavy metals such as Pb is treated by the method of the present invention, the soluble silicate in the solution A and the heavy metal ions such as Pb eluted from the dust react with each other to form insoluble heavy metal silicates. Elution can be suppressed (the following formula (1)).

Pb ²⁺ + SiO ₃ ^2- → PbSiO ₃ (1)

In addition, silicate and leached from soot and dust,
Whether a polyvalent metal ion such as Ca derived from unreacted slaked lime reacts and a heavy metal is included in the reaction product,
Heavy metal is adsorbed on the surface of the reaction product and stabilized.

Next, the action of the liquid B will be described. When the dust containing heavy metals such as Pb is treated by the method of the present invention, the hydroxide ion eluted from the dust and the polyvalent metal cation in the liquid B are reacted by the following reaction.
An insoluble or sparingly soluble hydroxide is produced (the following formulas (2) and (3)) to lower the pH of soot and dust.

M ^{2+ 12} OH ⁻ → M (OH) ₂ (2)

M ³⁺ + 3OH ⁻ → M (OH) ₃ (3)

At this time, if the pH is lowered to a region where the solubility of the heavy metal to be treated is sufficiently low, the elution of Pb can be suppressed only by the action of the solution B. Also, in the exhaust gas treatment, using high specific surface area slaked lime,
This effect is very likely to occur when the amount of unreacted slaked lime in the dust is reduced. However, heavy metals contained in soot and dust are not only amphoteric metals such as Pb, but also many heavy metals that are newly eluted with a decrease in pH. Therefore, depending on the pH range changed by the action of the solution B, the elution of the heavy metal which has not been eluted originally may be promoted.

In this respect, according to the method of the present invention, even if the pH of the solution B does not decrease until the elution of heavy metals is sufficiently suppressed,
By using the liquid together, a synergistic effect can be expected and a high stabilizing performance of heavy metals is exhibited. That is, by slightly adjusting the pH range of the waste from the alkaline side to the acidic side, the action of the above-mentioned liquid A is dramatically accelerated. This is because the morphology of the reaction product between the liquid A and polyvalent metal ions such as Ca eluted from the soot and dust is different depending on the pH, and the pH is slightly lowered from the highly alkaline region, resulting in soluble silicic acid. The gel time of the aqueous salt solution is significantly reduced. Under such conditions, it is considered that the reaction between the liquid A and the heavy metal ions is promoted, and the adsorption ability and the inclusion ability of the produced reaction product with respect to the heavy metal are promoted. Further, the reaction product produced by the reaction between the polyvalent metal cation in the liquid B and the silicate in the liquid A contains the heavy metal more than the reaction product with the polyvalent metal ion eluted from the dust. It seems that it produces a reaction product more suitable for adsorption. Therefore, in the present invention, A
Even if the performances of the liquid and the liquid B are insufficient, by using the both together, a synergistic effect is exhibited and it becomes possible to exhibit high performance.

The aqueous solution of the soluble silicate used in the present invention may be a general-purpose one, and examples thereof include lithium silicate, sodium silicate (sodium silicate), potassium silicate and ammonium silicate. Generally, the solid content of soluble silicate is M ₂
O · nSiO ₂ (M: cation of silicate), n
(SiO ₂ / M ₂ O composition ratio) represents the molar ratio of the silicate. Since the performance of the aqueous solution of soluble silicate depends on the solid content thereof, the performance is higher as the concentration of silicate is higher when the addition amount is the same. Its concentration depends on the type of soluble silicate,
The upper limit of the concentration that can be adjusted differs depending on the molar ratio and the temperature of the solution. On the other hand, the viscosity increases as the concentration of the aqueous solution of the soluble silicate increases, and the handling property may be significantly reduced. Therefore, it is necessary to set the concentration of the aqueous solution of the soluble silicate after sufficiently considering the performance and the handling property.

Although various types of soluble silicates are industrially produced as exemplified above, sodium silicate is used in view of stabilizing performance of heavy metals, industrial availability, and price. Preferably there is. It is known that the soluble silicate generally has a faster gelation reaction with heavy metals and polyvalent metals as the molar ratio is higher. In the case of sodium silicate, the molar ratio of commercially available products is in the range of about 0.5 to 4.2, but the higher the molar ratio, the better the performance. Therefore, the molar ratio should be about 2.0 or more. Is preferred. Further, considering the stabilizing performance of heavy metals, industrial availability, price, handling property, etc., it is generally preferable in the present invention to select JIS standard product sodium silicate No. 3.

Most of the hydroxide ions eluted from the soot and dust are derived from slaked lime that is blown in to neutralize the acid gas generated during waste incineration. Therefore, with respect to the polyvalent metal cation contained in the liquid B, the hydroxide produced by the reaction with the hydroxide ion needs to have a lower solubility than calcium hydroxide. Any polyvalent metal cation may be selected as long as this condition is satisfied.
Considering industrial availability and ease of handling, aluminum ion, magnesium ion, iron (II) ion,
It will be mostly selected from iron (III) ions. Among them, in particular, industrial availability, heavy metal stabilization performance,
Considering the price, aluminum sulfate (sulfate band), aluminum chloride, polyaluminum chloride (PA
C), sodium aluminate, iron (II) sulfate, iron (II) chloride
I), magnesium sulfate and magnesium chloride are preferred. In addition, these substances can be used in combination of two or more, if necessary, and are within the scope of the present invention. Also,
It is also preferable to use a waste liquid discharged from a chemical plant or the like as the polyvalent metal cation from the viewpoint of recycling useful substances.

When the pH of the liquid B is low, corrosion may be promoted depending on the material of the equipment such as the pipe and the kneader that use the liquid B. If this causes a problem, it is preferable to change the material of the equipment or to adjust the pH of the solution B from the neutral to the alkaline range before use, which is within the scope of the present invention. Further, it is preferable to use sodium aluminate without paying attention to these points.

The amounts of the liquids A and B added in the present invention are the characteristics of the soot and dust to be treated, in particular, the amount of heavy metal elution without treatment, the amount of unreacted slaked lime remaining, the target allowable elution amount of heavy metal, and Varies depending on the ratio of addition of liquid A and liquid B. Practically, selecting the addition amounts of the A liquid and the B liquid so that the cost for achieving the target elution amount is the lowest is a factor that determines the addition amount. In the normal case, with respect to 100 parts by weight of soot and dust, about 3 to 20 parts by weight of the solution A and 5 to 30 parts by weight of the solution B are used, and the elution amount of the heavy metal is equal to or less than the legal regulation value. Can be suppressed.

In the present invention, the method of adding the liquids A and B can be arbitrarily selected. The preferred addition method differs depending on the properties of the soot and dust, and therefore it is preferable to study it beforehand in a preliminary test or the like.

In carrying out the method for treating exhaust gas and dust according to the present invention, there is a possibility that a handling problem may occur especially in a cold region. That is, when the temperature at the time of using the liquid A is remarkably lowered, the viscosity is remarkably increased and the fluidity is lowered. Also in the liquid B, a salt of a polyvalent metal may be precipitated when the temperature is lowered. In such a case, if the storage tank for the treatment agent is kept warm, the problem is almost solved. Moreover, such a use method is also within the scope of the present invention.

The method for treating exhaust gas and soot and dust of the present invention shows a sufficient effect by adding liquid A and liquid B and, if necessary, water to soot and dust and mixing them. Further, kneading is also a more preferable method, if necessary.
In addition, curing the treated material for a certain period of time at a certain temperature or higher is an effective means for further exhibiting the effects of the treatment agents of the liquids A and B, and is within the scope of the present invention. This is the above-mentioned A used as a treating agent in the present invention.
By the contact of liquid and liquid B with the soot and dust component,
It suggests that the reaction starts instantly and the effect is exerted, but thereafter the reaction gradually progresses. Generally, the longer the curing time, the better. Further, it is possible to further develop the effect by heating the treated material during curing. Therefore, by curing the mixture obtained by kneading the dust and the treatment agent at a predetermined temperature, the curing time can be shortened and the treated product can be promptly discarded.

[0028]

The method for treating exhaust gas and dust according to the present invention,
Although the mechanism for stabilizing heavy metals such as Pb is not always clear, it can be estimated as follows. That is, since Pb compounds generally tend to be easily eluted in an alkaline atmosphere, by carrying out the method for treating exhaust gas and soot of the present invention, 1) by using slaked lime having a high specific surface area, It becomes possible to reduce the amount of unreacted slaked lime, and elution of hydroxide ions can be suppressed. 2) Since the elution of hydroxide ions is suppressed, the action of the solution B is easily exhibited, and the addition amount can be reduced. 3) As a result, the action of the liquid A is promoted and excellent heavy metal stabilizing performance is exhibited. As described above, according to the present invention, the synergistic effect of the combined use of the exhaust gas treatment with slaked lime having a high specific surface area and the soot and dust treatment with the liquids A and B enables more effective elution of heavy metals such as Pb in the soot and dust. It is considered to be preventable.

[0029]

EFFECTS OF THE INVENTION By treating exhaust gas and dust produced during incineration of waste such as municipal waste using the treatment method of the present invention, heavy metals in dust, particularly Pb, are stabilized and the amount of elution is reduced. To do. In addition, the mixture of the waste material and the treating agent obtained in the present invention can be reused as a material having a very small amount of heavy metal eluted, such as a roadbed material or an aggregate of cement, and can be a valuable resource.

[0030]

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited thereto.

(Comparative Example 1 and Example) In a municipal solid waste incineration facility, when the incinerator and the exhaust gas treatment device were operated while adjusting the blowing amount of slaked lime so that the concentration of hydrogen chloride gas in the outdoor exhaust gas was constant. Soot and dust were collected and treated. Two types of soot and dust were collected, when JIS special slaked lime was used as slaked lime (Comparative Example 1) and when slaked lime “Tamacalc” manufactured by Okutama Kogyo Co., Ltd. was used (Example). To the above-mentioned two kinds of soot and dust, the amounts of liquids A and B shown in Table 1 and water, respectively, were added and kneaded. After curing at 20 ° C. for 1 day,
A Pb dissolution test was carried out according to the Environmental Agency Notification Method 13 (Japan). The results are shown in Table 1.

[0032]

[Table 1]

From the results shown in Table 1 above, it is possible to effectively elute Pb by carrying out the method for treating soot and dust of the present invention on the soot and dust collected from the exhaust gas treated with slaked lime having a large specific surface area. It has become clear that That is, in the case of dust collected by performing the exhaust gas treatment by the conventional method such as JIS special slaked lime, even if the Pb elution amount cannot be less than 0.3 mg / L of the landfill standard value, the exhaust gas treatment of the present invention is performed to collect. In the end,
The performance of the heavy metal stabilizer is remarkably exhibited, and it can be set to the standard value or less.

(Comparative Examples 2 and 3) In the same manner as in Comparative Example 1 and Example above, JIS special slaked lime was used as slaked lime (Comparative Example 2) and slaked lime "Tamacalc" manufactured by Okutama Kogyo Co., Ltd. was used. For the two types of soot and dust in (Comparative Example 3), the amounts of liquids A and B shown in Table 2 below, respectively
After the liquid and water were added and kneaded, the mixture was cured at 20 ° C. for 1 day, and then a Pb dissolution test according to the Environmental Agency Notification No. 13 method was performed. The results are shown in Table 2.

[0035]

[Table 2]

From the results shown in Table 2 above, it is effective that the dust collected from the exhaust gas treated with slaked lime having a large specific surface area is treated with the aqueous solution of the soluble silicate and the organic chelating agent. It was revealed that the elution of Pb could not be suppressed. That is, it has been clarified that by carrying out the exhaust gas treatment method and the soot and dust treatment method of the present invention, a synergistic effect is exhibited and the heavy metal stabilization first performance is improved.

From these results, it is clear that the method for treating exhaust gas and dust according to the present invention is extremely effective for preventing the elution of heavy metals in dust generated during waste incineration.

Claims (7)

[Claims] 1. An exhaust gas treatment apparatus attached to a waste incineration facility, in which slaked lime having a specific surface area of 30 m ² / g or more is blown into the exhaust gas for the purpose of neutralizing and reducing acidic gas such as hydrogen chloride gas in the exhaust gas, Liquid A, which is an aqueous solution of soluble silicate as a main component, is added to the dust containing heavy metals such as Pb, which is generated from the exhaust gas after being treated by blowing in this slaked lime and is captured by an electrostatic precipitator or a bag filter, and water. Characteristically, an aqueous solution containing a polyvalent metal cation that reacts with an oxide ion to form an insoluble or sparingly soluble hydroxide is mixed with the solution B as a main component, and water is further added if necessary. Exhaust gas and dust treatment method. 2. The method for treating exhaust gas and dust according to claim 1, wherein a mixture of soot and dust, solution A and solution B is kneaded. 3. The method for treating exhaust gas and dust according to claim 1 or 2, wherein a mixture or kneaded product of dust, liquid A and liquid B is cured. 4. The method for treating exhaust gas and dust according to claim 1, wherein the soluble silicate is sodium silicate. 5. The method for treating exhaust gas and dust according to claim 4, wherein the sodium silicate is JIS standard sodium silicate No. 3. 6. The polyvalent metal cation is aluminum ion, magnesium ion, iron (II) ion, and iron.
The method for treating exhaust gas and dust according to claim 1, wherein the method is at least one selected from (III) ions. 7. The liquid B comprises aluminum sulfate, aluminum chloride, polyaluminum chloride, sodium aluminate, iron (II) sulfate, iron (III) chloride, magnesium sulfate,
2. The method for treating exhaust gas and dust according to claim 1, further comprising at least one selected from magnesium chloride and magnesium chloride.