Dual hydroxide regeneration method with carbonate-like layer structure, acid exhaust gas treatment method, and acid exhaust gas treatment equipment.

Abstract

The invention relates to a method for regenerating a double Mg-Al carbonate structure in which at least some of the carbonate ions between the layers of the double Mg-Al carbonate structure are exchanged with other anions including a step for the double Mg-Al carbonate structure in which the double Mg-Al carbonate structure is in contact with a carbon dioxide-containing gas in the presence of an alkali.

Description

Method for regenerating carbonate-type layered double hydroxide, method for treating acidic exhaust gas, and acidic exhaust gas treatment equipment

[0001] The present invention relates to a method for regenerating a carbonate layered double hydroxide used in the treatment of acidic exhaust gas, a method for treating acidic exhaust gas using the regeneration method, and an acidic exhaust gas treatment facility.

[0002] Combustion exhaust gases generated in combustion facilities such as thermal power plants and waste incineration facilities contain harmful acidic substances such as hydrogen chloride, sulfur oxides, and nitrogen oxides. There are various treatment technologies for removing the acidic substances from acidic exhaust gases containing the acidic substances.

[0003] The present inventors have proposed a method and agent for treating acidic exhaust gases that utilizes a carbonate-type Mg—Al-based layered double hydroxide as an efficient treatment technology capable of simultaneously treating and removing multiple types of acidic substances. The carbonate-type Mg—Al-based layered double hydroxide is a treatment agent that can be regenerated and reused. Therefore, after using the carbonate-type Mg—Al-based layered double hydroxide to treat acidic exhaust gases, the used layered double hydroxide can be regenerated and reused.

[0004] Patent Document 1 describes a method for regenerating a layered double hydroxide that has been used to treat acidic exhaust gas by mixing the used layered double hydroxide with an aqueous carbonate solution. Patent Document 2 describes a method for regenerating a layered double hydroxide that has been used to treat acidic exhaust gas by contacting the layered double hydroxide with a gas that has been treated with the layered double hydroxide and from which acidic gases other than carbon dioxide have been removed by the layered double hydroxide.

[0005] JP 2016-190199 A JP 2021-090913 A

[0006] According to the regeneration method described in Patent Document 1, the spent layered double hydroxide must be mixed with an aqueous carbonate solution, necessitating the installation of a regeneration facility for the spent layered double hydroxide outside the combustion facility. In this case, the spent layered double hydroxide and the regenerated carbonate-type Mg—Al-based layered double hydroxide must be transported between the combustion facility and the layered double hydroxide regeneration facility, and the layered double hydroxide regeneration facility must be installed separately from the combustion facility, resulting in significant labor and cost burdens. Furthermore, if the regeneration equipment for the spent layered double hydroxide is installed within the combustion facility, it will also be necessary to install and connect a storage tank and a mixing tank for the aqueous carbonate solution, which will necessitate an increase in the scale of the combustion facility.

[0007] The regeneration method described in Patent Document 2 does not require the installation of a large-scale layered double hydroxide regeneration facility within the combustion facility, but has the problem of making it difficult to obtain a sufficient regeneration effect.

[0008] The present invention aims to provide a method for regenerating carbonate layered double hydroxide, a method for treating acidic exhaust gas, and an acidic exhaust gas treatment facility, which are capable of efficiently regenerating used carbonate layered double hydroxide into Mg—Al layered double hydroxide after the layered double hydroxide has been used in the treatment of acidic exhaust gas, without requiring large-scale facilities.

[0009] As a result of extensive research, the present inventors have found that it is possible to efficiently regenerate used layered double hydroxide into a Mg—Al carbonate layered double hydroxide by contacting, in the presence of an alkali, used layered double hydroxide remaining after the use of a carbonate Mg—Al layered double hydroxide in the treatment of acidic exhaust gas with post-treatment gas containing carbon dioxide treated with the carbonate Mg—Al layered double hydroxide. The present invention is based on this finding.

[0010] That is, the present invention provides the following [1] to [7]. [1] A method for regenerating a used Mg—Al-based carbonate layered double hydroxide in which at least a portion of the interlayer carbonate ions of the carbonate layered double hydroxide have been exchanged for other anions, comprising contacting the used Mg—Al-based carbonate layered double hydroxide with a carbon dioxide-containing gas containing carbon dioxide in the presence of an alkali. [2] The used Mg—Al-based carbonate layered double hydroxide is the used Mg—Al-based carbonate layered double hydroxide obtained after contacting an acidic exhaust gas containing carbon dioxide and acidic substances other than carbon dioxide with the carbonate layered double hydroxide to remove the acidic substances from the acidic exhaust gas to obtain a treated gas, and the carbon dioxide-containing gas containing carbon dioxide is the treated gas. [3] The method for regenerating a used Mg—Al-based carbonate layered double hydroxide according to [1] above, wherein the carbon dioxide-containing gas is the treated gas.

[0011] [3] A method for regenerating spent Mg—Al-based carbonate layered double hydroxide according to [2] above, wherein the acidic exhaust gas is combustion exhaust gas generated from an incinerator. [4] A method for treating acidic exhaust gas, comprising: an acidic exhaust gas treatment step of contacting an acidic exhaust gas containing carbon dioxide and acidic substances other than carbon dioxide with an Mg—Al-based carbonate layered double hydroxide to remove the acidic substances from the acidic exhaust gas to obtain a treated gas; and a regeneration step of regenerating the spent Mg—Al-based carbonate layered double hydroxide produced in the acidic exhaust gas treatment step, wherein the regeneration step is carried out by the method according to [2] or [3] above.

[0012] [5] An acidic exhaust gas treatment facility comprising: a storage container for storing a carbonate Mg-Al-based layered double hydroxide; a supply pipe for supplying an acidic exhaust gas containing carbon dioxide and acidic substances other than carbon dioxide to the storage container; a discharge pipe for discharging treated gas discharged after the acidic substances have been treated in the storage container; and a regeneration device for regenerating the carbonate Mg-Al-based layered double hydroxide in the storage container, wherein the regeneration device comprises an alkali addition device for adding alkali to the storage container and a return pipe for returning the treated gas to the storage container. [6] The acidic exhaust gas treatment facility according to [5] above, wherein the alkali addition device comprises an alkaline aqueous solution storage tank for storing an alkaline aqueous solution and an alkaline aqueous solution inlet pipe connecting the alkaline aqueous solution storage tank to the storage container. [7] The acidic exhaust gas treatment facility according to [5] or [6] above, wherein the regeneration device is connected to the storage container and comprises a waste liquid discharge device for discharging the alkaline aqueous solution that has flowed into the storage container to the outside of the storage container.

[0013] The present invention provides a method for regenerating carbonate layered double hydroxide, a method for treating acidic exhaust gas, and an acidic exhaust gas treatment facility, which can efficiently regenerate used carbonate layered double hydroxide into Mg—Al layered double hydroxide after using the carbonate layered double hydroxide in the treatment of acidic exhaust gas, without requiring large-scale facilities.

[0014] 1 is a schematic diagram illustrating a regeneration process of spent layered double hydroxide in a first storage vessel 120a using an incineration facility equipped with acidic exhaust gas treatment equipment according to an embodiment of the present invention; FIG. 2 is a schematic diagram illustrating a regeneration process of spent layered double hydroxide in a second storage vessel 120b using an incineration facility equipped with acidic exhaust gas treatment equipment according to an embodiment of the present invention; FIG. 3 is a graph showing the results of powder X-ray diffraction measurement;

[0015] The method for regenerating carbonate-type Mg—Al-based layered double hydroxide, the method for treating acidic exhaust gas, and the equipment for treating acidic exhaust gas according to the present invention will be described below, partly with reference to the drawings. In this specification, "acidic substances other than carbon dioxide" may be simply referred to as "acidic substances." In other words, in the present invention, "acidic substances" means "acidic substances other than carbon dioxide."

[0016] [Method for regenerating used Mg—Al-based carbonate layered double hydroxide] The method for regenerating used Mg—Al-based carbonate layered double hydroxide according to the present invention is a method for regenerating used Mg—Al-based carbonate layered double hydroxide in which at least a portion of the interlayer carbonate ions of the carbonate Mg—Al-based carbonate layered double hydroxide have been exchanged with other anions, and comprises contacting the used Mg—Al-based carbonate layered double hydroxide with a carbon dioxide-containing gas that contains carbon dioxide in the presence of an alkali.

[0017] In this way, by contacting a used Mg—Al-based carbonate layered double hydroxide with a carbon dioxide-containing gas containing carbon dioxide in the presence of an alkali, the used layered double hydroxide can be efficiently regenerated into a Mg—Al-based carbonate layered double hydroxide. The reason for this is unclear, but it is presumed to be as follows. Specifically, the Mg—Al-based carbonate layered double hydroxide has a structure in which hydroxide base layers and intermediate layers composed of interlayer carbonate ions and interlayer water are alternately laminated. Furthermore, in the used Mg—Al-based carbonate layered double hydroxide, some or all of the interlayer carbonate ions have been exchanged for anions derived from acidic substances. By contacting the used Mg—Al-based carbonate layered double hydroxide with a carbon dioxide-containing gas containing carbon dioxide in the presence of an alkali to regenerate the used Mg—Al-based carbonate layered double hydroxide, the presence of the alkali promotes the dissolution of carbonate ions into the interlayer water, facilitating ion exchange between anions derived from acidic substances and carbonate ions derived from carbon dioxide in the treated gas. This is thought to improve the regeneration efficiency.

[0018] In the method for regenerating a used Mg—Al-based carbonate layered double hydroxide according to a first aspect, the used Mg—Al-based carbonate layered double hydroxide is used Mg—Al-based carbonate layered double hydroxide after an acidic exhaust gas containing carbon dioxide and acidic substances other than carbon dioxide has been brought into contact with the carbonate Mg—Al-based carbonate layered double hydroxide to remove the acidic substances from the acidic exhaust gas to obtain a treated gas, and the carbon dioxide-containing gas containing carbon dioxide is the treated gas.

[0019] That is, the method for regenerating spent Mg—Al-based carbonate layered double hydroxide according to the first aspect is a method for regenerating spent Mg—Al-based carbonate layered double hydroxide after contacting an acidic exhaust gas containing carbon dioxide and acidic substances other than carbon dioxide with an Mg—Al-based carbonate layered double hydroxide to remove the acidic substances from the acidic exhaust gas and obtain a treated gas, in which the used Mg—Al-based carbonate layered double hydroxide is contacted with the treated gas in the presence of an alkali. In this case, by using the treated gas obtained by the Mg—Al-based carbonate layered double hydroxide as the carbon dioxide-containing gas for regeneration, there is an advantage in that a separate carbon dioxide-containing gas for regeneration is not required.

[0020] <Carbon dioxide-containing gas containing carbon dioxide> The carbon dioxide-containing gas containing carbon dioxide is not particularly limited, but as described above, a treated gas is preferred from the viewpoint that it is not necessary to separately prepare a carbon dioxide-containing gas for regeneration.

[0021] <Acidic Exhaust Gas> Acidic exhaust gas contains carbon dioxide and acidic substances other than carbon dioxide. Examples of the acidic substances include hydrogen chloride, sulfur dioxide, and other SO ｘ , NO such as nitrogen dioxide ｘCarbon dioxide is also an acidic substance. However, in this specification, carbon dioxide is not included in the "acidic substance" in order to distinguish carbon dioxide from acidic substances other than carbon dioxide. Acidic exhaust gases are not particularly limited as long as they contain carbon dioxide and acidic substances, but examples include combustion exhaust gases generated from incinerators at combustion facilities such as thermal power plants and incineration facilities. Because such combustion exhaust gases contain a large amount of carbon dioxide, the treated gas obtained by contacting the combustion exhaust gas with a carbonate-type Mg—Al-based layered double hydroxide to remove acidic substances also contains a large amount of carbon dioxide. Therefore, when used carbonate-type Mg—Al-based layered double hydroxide is regenerated using this treated gas, the regeneration can be carried out efficiently. The carbon dioxide content in the acidic exhaust gas is preferably 1 to 20 vol%, more preferably 5 to 18 vol%, and even more preferably 10 to 15 vol%. When the acidic exhaust gas concentration is 1 vol% or higher, the reactivity of the ion exchange reaction is increased, and when a used carbonate-type Mg—Al-based layered double hydroxide is regenerated using a treated gas derived from the acidic exhaust gas, regeneration can be performed efficiently. The content of acidic substances in the acidic exhaust gas is preferably 10 to 5,000 vol ppm, more preferably 200 to 3,000 vol ppm, even more preferably 300 to 1,500 vol ppm, and even more preferably 500 to 1,000 vol ppm. The content of water in the acidic exhaust gas is preferably 0 to 30 vol%, more preferably 5 to 25 vol%, and even more preferably 10 to 20 vol%. The total amount of nitrogen, oxygen, carbon dioxide, acidic substances, and water in the acidic exhaust gas is preferably 90 vol% or higher, more preferably 95 vol% or higher, and even more preferably 99 vol% or higher.

[0022] <Carbonate-type Mg—Al-based layered double hydroxide> The carbonate-type Mg—Al-based layered double hydroxide is a hydroxide base layer ([Mg 2+ 1-x Al 3+ x (OH)2]), and an intermediate layer consisting of interlayer carbonate ions and interlayer water ([(CO3 2- ) x / 2The carbonate-type Mg-Al-based layered double hydroxide is a non-stoichiometric compound in which the hydroxide basic layer has a positive charge equivalent to x, and carbonate ions exist in the intermediate layer as negative ions (anions) with a negative charge that compensates for this. The carbonate-type Mg-Al-based layered double hydroxide can be easily oxidized by, for example, hydrogen chloride, SO while maintaining the hydroxide basic layer. ｘ , NO ｘ Therefore, the catalyst can be suitably used for acidic exhaust gas treatment, which removes acidic substances from the acidic exhaust gas.

[0023] When a Mg—Al-based carbonate layered double hydroxide is used for treating acidic exhaust gases, it may be used in combination with a layered double hydroxide other than the Mg—Al-based carbonate layered double hydroxide, or with a chemical other than the layered double hydroxide, such as calcium hydroxide (slaked lime), calcium oxide, sodium bicarbonate (baking soda), sodium carbonate, dolomite hydroxide, lightly burned dolomite, aluminum hydroxide, aluminum oxide, magnesium hydroxide, magnesium oxide, etc. However, from the viewpoint of efficiently regenerating and reusing the Mg—Al-based carbonate layered double hydroxide, it is preferable not to mix it with other layered double hydroxides or chemicals.

[0024] Although carbonate-type Mg—Al-based layered double hydroxides exist as naturally occurring clay minerals in the form of hydrotalcite, synthetic powders are typically used. The synthesis method is not particularly limited, and known methods (e.g., the method described in Patent Document 1) can be used. For example, carbonate-type Mg—Al-based layered double hydroxides can be obtained by adding dropwise an aqueous solution of magnesium nitrate (Mg(NO)) and aluminum nitrate (Al(NO)) in a molar ratio of Mg / Al = 2 / 1 to an aqueous sodium carbonate (NaCO) solution while maintaining the pH at 10.5.

[0025] <Used Mg—Al-based carbonate layered double hydroxide (anionic Mg—Al-based layered double hydroxide)> When a carbonate Mg—Al-based layered double hydroxide is used to treat acidic exhaust gas and the acidic substances are trapped between the layers, the interlayer carbonate ions are exchanged for other anions derived from the acidic substances, such as chloride ions, sulfate ions, and nitrate ions, to form an anionic Mg—Al-based layered double hydroxide (used carbonate Mg—Al-based layered double hydroxide). The anionic Mg—Al-based layered double hydroxide produced in this manner has a reduced ability to remove acidic exhaust gas. For this reason, the anionic Mg—Al-based layered double hydroxide is regenerated into a carbonate Mg—Al-based layered double hydroxide by anion exchange and reused.

[0026] <Treated Gas> The treated gas is an acidic exhaust gas containing carbon dioxide and acidic substances other than carbon dioxide, in which some or all of the acidic substances have been exchanged for carbon dioxide derived from interlayer carbonate ions in the carbonate-type Mg—Al-based layered double hydroxide. The carbon dioxide content in the acidic exhaust gas is preferably 1 to 20 vol%, more preferably 5 to 18 vol%, and even more preferably 10 to 15 vol%. A carbon dioxide content of 1 vol% or more allows for efficient regeneration of spent carbonate-type Mg—Al-based layered double hydroxide using the treated gas derived from the acidic exhaust gas. The acidic substance content in the acidic exhaust gas is preferably 100 vol ppm or less, more preferably 10 vol ppm, and even more preferably 1 vol ppm. The moisture content in the acidic exhaust gas is preferably 0 to 30 vol%, more preferably 5 to 25 vol%, and even more preferably 10 to 20 vol%. The total amount of nitrogen, oxygen, carbon dioxide, acidic substances, and water in the acidic exhaust gas is preferably 90 vol % or more, more preferably 95 vol % or more, and even more preferably 99 vol % or more.

[0027] <Regeneration Method> The method for regenerating spent Mg—Al-based carbonate layered double hydroxide according to the present invention involves contacting the spent Mg—Al-based carbonate layered double hydroxide with the post-treatment gas in the presence of an alkali. Since the post-treatment gas contains carbon dioxide that originated in the acidic exhaust gas in addition to the carbon dioxide derived from the interlayer carbonate ions, anions derived from the acidic substances present in the spent Mg—Al-based carbonate layered double hydroxide can be efficiently anion-exchanged with carbonate ions. Furthermore, the anion exchange is carried out in the presence of an alkali. Therefore, compared to anion exchange carried out in the absence of an alkali, anions derived from the acidic substances present in the spent Mg—Al-based carbonate layered double hydroxide can be efficiently anion-exchanged with carbonate ions.

[0028] The alkali is preferably sodium hydroxide or potassium hydroxide, more preferably sodium hydroxide. The proportion of alkali relative to 100 parts by mass of used carbonate Mg—Al-based layered double hydroxide is preferably 1 to 20 parts by mass, more preferably 1 to 10 parts by mass, even more preferably 2 to 10 parts by mass, and even more preferably 2 to 5 parts by mass. When the proportion is 1 part by mass or more, anions derived from the acidic substances present in the used carbonate Mg—Al-based layered double hydroxide can be efficiently anion-exchanged with carbonate ions. When the proportion is 20 parts by mass or less, elution of constituent elements (e.g., Al) of the carbonate Mg—Al-based layered double hydroxide can be prevented, and the crystal structure of the carbonate Mg—Al-based layered double hydroxide can be maintained.

[0029] When regenerating spent Mg—Al-based carbonate layered double hydroxide, it is preferable to add an alkaline aqueous solution to the spent Mg—Al-based carbonate layered double hydroxide and then contact the spent Mg—Al-based carbonate layered double hydroxide with the treated gas. This allows the treated gas to be contacted with the spent Mg—Al-based carbonate layered double hydroxide while the alkali is uniformly present in the spent Mg—Al-based carbonate layered double hydroxide, thereby more efficiently regenerating the spent Mg—Al-based carbonate layered double hydroxide. However, the addition of the alkaline aqueous solution to the spent Mg—Al-based carbonate layered double hydroxide and the contact of the spent Mg—Al-based carbonate layered double hydroxide with the treated gas may be carried out simultaneously, or the alkaline aqueous solution may be added during the contact with the treated gas. The alkali concentration of the alkaline aqueous solution is preferably 0.1 to 20% by mass, more preferably 1 to 10% by mass, even more preferably 2 to 10% by mass, and even more preferably 2 to 5% by mass. When the alkali concentration is 0.1% by mass or more, the amount of alkaline aqueous solution used can be reduced due to the low alkali concentration. When the alkali concentration is 20% by mass or less, excellent handleability is achieved. The ratio of the alkaline aqueous solution to 100 parts by mass of used carbonate-type Mg—Al-based layered double hydroxide is preferably 1 to 1,000 parts by mass, more preferably 10 to 500 parts by mass, even more preferably 50 to 200 parts by mass, and even more preferably 80 to 120 parts by mass. When the alkali concentration is 1 part by mass or more, the concentration of the alkaline aqueous solution can be reduced, resulting in excellent handleability. When the alkali concentration is 1,000 parts by mass or less, the amount of alkaline aqueous solution used can be reduced.

[0030] The temperature during the regeneration, i.e., the treatment in which the used carbonate Mg—Al-based layered double hydroxide is brought into contact with the treated gas in the presence of an alkali, is preferably 20 to 100°C, more preferably 60 to 95°C, even more preferably 70 to 90°C, and even more preferably 75 to 85°C. Within this range, the anionic Mg—Al-based layered double hydroxide can be efficiently regenerated into a carbonate Mg—Al-based layered double hydroxide. The regeneration time is preferably 10 minutes to 10 hours, more preferably 30 minutes to 8 hours, and even more preferably 1 to 5 hours. A regeneration time of 10 minutes or longer ensures sufficient regeneration of the anionic Mg—Al-based layered double hydroxide into a carbonate Mg—Al-based layered double hydroxide. A regeneration time of 10 hours or less allows for rapid regeneration.

[0031] [Acidic Exhaust Gas Treatment Method] The method for treating acidic exhaust gas according to the present invention comprises: an acidic exhaust gas treatment step of bringing acidic exhaust gas containing carbon dioxide and acidic substances into contact with a carbonate Mg—Al-based layered double hydroxide to remove the acidic substances from the acidic exhaust gas and obtain a treated gas; and a regeneration step of regenerating spent carbonate Mg—Al-based layered double hydroxide produced in the acidic exhaust gas treatment step, wherein the regeneration method described above is carried out in the regeneration step.

[0032] This method for treating acidic exhaust gas allows the carbonate-type Mg—Al-based layered double hydroxide to capture acidic substances between its layers while retaining the hydroxide base layer. This allows the acidic substances to be effectively removed from the acidic exhaust gas. Furthermore, as explained above in the description of the method for regenerating spent carbonate-type Mg—Al-based layered double hydroxide, this regeneration step allows the spent layered double hydroxide to be efficiently regenerated into a carbonate-type Mg—Al-based layered double hydroxide.

[0033] [Acidic Exhaust Gas Treatment Equipment] The acidic exhaust gas treatment equipment of the present invention comprises: a storage container for storing a carbonate Mg—Al-based layered double hydroxide; a supply pipe for supplying an acidic exhaust gas containing carbon dioxide and an acidic substance to the storage container; a discharge pipe for discharging the treated gas discharged after the acidic substance has been treated in the storage container; and a regeneration device for regenerating the carbonate Mg—Al-based layered double hydroxide in the storage container, wherein the regeneration device comprises an alkali addition device for adding alkali to the storage container and a return pipe for returning the treated gas to the storage container. By using this acidic exhaust gas treatment equipment, the above-mentioned acidic exhaust gas treatment method can be efficiently carried out.

[0034] The alkali addition device preferably includes an alkaline aqueous solution storage tank for storing an alkaline aqueous solution and an alkaline aqueous solution inlet pipe connecting the alkaline aqueous solution storage tank to the storage container. This allows the above-mentioned acidic exhaust gas treatment method to be carried out more efficiently. Furthermore, the regeneration device preferably includes a waste liquid discharge device connected to the storage container and discharging the alkaline aqueous solution that has flowed into the storage container out of the storage container. This allows the above-mentioned acidic exhaust gas treatment method to be carried out more efficiently.

[0035] [Specific Examples of Acidic Exhaust Gas Treatment Equipment and Acidic Exhaust Gas Treatment Method] <Acidic Exhaust Gas Treatment Equipment> Figure 1 is a first schematic diagram illustrating an incineration facility equipped with an acidic exhaust gas treatment equipment according to one embodiment of the present invention. The incineration facility 1 shown in Figure 1 includes an incinerator 11, a boiler 12, a gas cooler 13, a dust collector 14, a layered double hydroxide storage device 30, an induced draft fan 15, a chimney 16, and pipes 21 to 26 that sequentially connect these to form a flue. The incineration facility 1 also includes an alkali addition device 130, a waste liquid discharge device 140, and a regeneration gas supply device 150, which will be described later.

[0036] The layered double hydroxide storage device 30 includes a first three-way valve V1, a second three-way valve V2, a first pipe 111a and a second pipe 111b connecting the first three-way valve V1 and the second three-way valve V2 in parallel, a first storage container 120a provided midway along the first pipe 111a, and a second storage container 120b provided midway along the second pipe 111b. Of the three connection ports of the first three-way valve V1, the connection port to which the first pipe 111a and the second pipe 111b are not connected is connected to pipe 24. Of the three connection ports of the second three-way valve V2, the connection port to which the first pipe 111a and the second pipe 111b are not connected is connected to pipe 25.

[0037] The first pipe 111a comprises a pipe 111a1 upstream of the first storage container 120a and a pipe 111a2 downstream of the first storage container 120a. That is, the first three-way valve V1 and the first storage container 120a are connected by the pipe 111a1, and the first storage container 120a and the second three-way valve V2 are connected by the pipe 111a2. The second pipe 111b comprises a pipe 111b1 upstream of the second storage container 120b and a pipe 111b2 downstream of the second storage container 120b. That is, the first three-way valve V1 and the second storage container 120b are connected by the pipe 111b1, and the second storage container 120b and the second three-way valve V2 are connected by the pipe 111b2. The first storage container 120a and the second storage container 120b contain carbonate type Mg—Al-based layered double hydroxide.

[0038] The alkali addition device 130 has an alkaline aqueous solution storage tank 131, an alkali supply three-way valve V5, a pipe 132 connecting the three-way valve V5 and the alkaline aqueous solution storage tank 131, a pipe 133a connecting the three-way valve V5 and the first storage container 120a, and a pipe 133b connecting the three-way valve V5 and the first storage container 120b.

[0039] The waste liquid discharge device 140 has a waste liquid storage container 141, a three-way valve V6 for discharging alkali, a pipe 142 connecting the three-way valve V6 to the waste liquid storage container 141, a pipe 143a connecting the three-way valve V6 to the first storage container 120a, and a pipe 143b connecting the three-way valve V6 to the second storage container 120b.

[0040] A third three-way valve V3 is installed midway along the piping 26 connecting the induced draft fan 15 and the chimney 16, and a return piping 151 is connected to the remaining connection port of the third three-way valve V3. The return piping 151 branches into a return piping 152a and a return piping 152b via a fourth three-way valve V4. The return piping 152a is connected to the first storage container 120a, and the return piping 152b is connected to the second storage container 120b. A heat exchanger 151a is installed midway along the return piping 151. One end of an exhaust gas piping 153a is connected to the first storage container 120a, and one end of an exhaust gas piping 153b is connected to the second storage container 120b. The other ends of the exhaust gas piping 153a and the exhaust gas piping 153b join together to form an exhaust gas piping 154, the other end of which is connected to the chimney 16.

[0041] The third three-way valve V3, the heat exchanger 151a, the fourth three-way valve V4, the pipe 151, the pipe 152a, the pipe 152b, the pipe 153a, the pipe 153b, and the pipe 154 constitute a regeneration gas supply device 150.

[0042] The alkali addition device 130, the waste liquid discharge device 140, and the regeneration gas supply device 150 constitute a regeneration device 160. The layered double hydroxide storage device 30 and the regeneration device 160 constitute an acidic exhaust gas treatment facility 170.

[0043] The incineration facility 1 shown in Figures 2 and 3 is the same as the incineration facility 1 shown in Figure 1. However, as will be described later, Figures 1 to 3 differ in the operating state of the incineration facility 1. In Figures 1 to 3, fluid flows through the parts of the piping indicated by thick lines, and no fluid flows through the parts indicated by thin lines.

[0044] <Acidic Exhaust Gas Treatment Method> <<First Step: Regeneration Step in First Storage Container (FIG. 1)>> First, as shown in FIG. 1 , three-way valve V5 is operated to switch pipe 132 to a state where it is connected to pipe 133a but not to pipe 133b. Furthermore, three-way valve V6 is operated to switch pipe 142 to a state where it is connected to pipe 143a but not to pipe 143b. This causes an alkaline aqueous solution to be supplied from alkali addition device 130 to first storage container 120a, which then permeates into the used carbonate Mg—Al-based layered double hydroxide in first storage container 120a, supplying alkali into the carbonate Mg—Al-based layered double hydroxide. Furthermore, the alkaline aqueous solution discharged from first storage container 120a is discharged into waste liquid storage container 141.

[0045] 1, the first three-way valve V1 is operated to switch the state in which pipe 24 is connected to pipe 111b but not to pipe 111a. The second three-way valve V2 is operated to switch the state in which pipe 25 is connected to pipe 111b but not to pipe 111a. The three-way valve V3 is operated to switch the state in which pipe 26 is connected to return pipe 151 but not to chimney 16. The three-way valve V4 is operated to switch the state in which return pipe 151 is connected to pipe 152a but not to pipe 152b.

[0046] As a result, acidic exhaust gas discharged from the incinerator 11 passes through the boiler 12, gas cooler 13, dust collector 14, and first three-way valve V1 before being introduced into the second storage vessel 120b, where acidic substances contained in the acidic exhaust gas are removed by the carbonate Mg-Al-based layered double hydroxide. The treated gas from which the acidic substances have been removed is then discharged from the second storage vessel 120b. The discharged treated gas passes through the second three-way valve V2, induced draft fan 15, third three-way valve V3, and return pipe 151, and is adjusted to an appropriate temperature by the heat exchanger 151a before being introduced into the first storage vessel 120a through the fourth three-way valve V4 and return pipe 152a.

[0047] The treated gas introduced into the first storage container 120a contains carbon dioxide derived from the acidic exhaust gas discharged from the incinerator and carbon dioxide supplied from the carbonate Mg—Al-based layered double hydroxide through ion exchange with the acidic substances contained in the acidic exhaust gas. The carbonate Mg—Al-based layered double hydroxide is regenerated by ion exchange between carbonate ions derived from the carbon dioxide contained in the treated gas and anions derived from the acidic substances present in the spent carbonate Mg—Al-based layered double hydroxide. Because an alkaline aqueous solution is supplied to the first storage container 120a during this regeneration, the spent carbonate Mg—Al-based layered double hydroxide is regenerated in the presence of alkali, resulting in efficient regeneration.

[0048] The treated gas introduced into the first storage container 120 a passes through the pipes 153 a and 154 and is discharged from the chimney 16 .

[0049] <<Second Step: Acidic Exhaust Gas Treatment Step in Second Storage Container (FIG. 2)>> After the used carbonate Mg—Al-based layered double hydroxide in the first storage container 120a is regenerated, the third three-way valve V3 is operated to switch the pipe 26 to a state in which it is connected to the chimney 16 but not to the return pipe 151. The three-way valves V5 and V6 are also operated to switch to their "closed" positions. As a result, the acidic exhaust gas discharged from the incinerator 11 passes through the boiler 12, gas cooler 13, dust collector 14, and first three-way valve V1 and is introduced into the second storage container 120b, where the acidic substances contained in the acidic exhaust gas are removed by the carbonate Mg—Al-based layered double hydroxide. The treated gas from which the acidic substances have been removed is discharged from the second storage container 120b, passes through the second three-way valve V2, the induced draft fan 15, and the third three-way valve V3, and is then discharged into the atmosphere from the chimney 16. In this way, the acidic exhaust gas treatment step is carried out in the second storage vessel 120b. When the ion exchange capacity of the carbonate Mg—Al-based layered double hydroxide in the second storage vessel 120b decreases as the second step continues, the carbonate Mg—Al-based layered double hydroxide in the second storage vessel 120b is regenerated in the next third step.

[0050] <<Third Step: Regeneration Step in Second Storage Container (FIG. 3)>>

[0051] As shown in Figure 3, three-way valve V5 is operated to switch pipe 132 to a state where it is connected to pipe 133b but not to pipe 133a. Furthermore, three-way valve V6 is operated to switch pipe 142 to a state where it is connected to pipe 143b but not to pipe 143a. This causes an alkaline aqueous solution to be supplied from alkali addition device 130 to second storage container 120b, where it permeates into the used carbonate Mg-Al-based layered double hydroxide in second storage container 120b, supplying alkali to the carbonate Mg-Al-based layered double hydroxide. Furthermore, the alkaline aqueous solution discharged from second storage container 120b is discharged into waste liquid storage container 141. Furthermore, in Figure 3, first three-way valve V1 is operated to switch pipe 24 to a state where it is connected to pipe 111a but not to pipe 111b. Furthermore, the second three-way valve V2 is operated to switch the state so that the pipe 25 is in communication with the pipe 111a but not in communication with the pipe 111b. Furthermore, the three-way valve V3 is operated to switch the state so that the pipe 26 is in communication with the return pipe 151 but not in communication with the chimney 16. Furthermore, the three-way valve V4 is operated to switch the state so that the return pipe 151 is in communication with the pipe 152b but not in communication with the pipe 152a.

[0052] As a result, acidic exhaust gas discharged from the incinerator 11 passes through the boiler 12, gas cooler 13, dust collector 14, and first three-way valve V1 before being introduced into the first storage vessel 120a, where acidic substances contained in the acidic exhaust gas are removed by the carbonate Mg-Al-based layered double hydroxide. The treated gas from which the acidic substances have been removed is then discharged from the first storage vessel 120a. The discharged treated gas passes through the second three-way valve V2, induced draft fan 15, third three-way valve V3, and return pipe 151, and is adjusted to an appropriate temperature by the heat exchanger 151a. After that, the discharged treated gas passes through the fourth three-way valve V4 and return pipe 152b and is introduced into the second storage vessel 120b.

[0053] The treated gas introduced into the second storage container 120b contains carbon dioxide derived from the acidic exhaust gas discharged from the incinerator and carbon dioxide supplied from the carbonate Mg—Al-based layered double hydroxide through ion exchange with the acidic substances contained in the acidic exhaust gas. The carbonate Mg—Al-based layered double hydroxide is regenerated by ion exchange between carbonate ions derived from the carbon dioxide contained in the treated gas and anions derived from the acidic substances present in the spent carbonate Mg—Al-based layered double hydroxide. Because an alkaline aqueous solution is supplied to the second storage container 120b during this regeneration, the spent carbonate Mg—Al-based layered double hydroxide is regenerated in the presence of alkali, resulting in efficient regeneration.

[0054] The treated gas introduced into the second storage container 120b passes through the pipes 153b and 154 and is discharged from the chimney 16.

[0055] <<Fourth Step: Acidic Exhaust Gas Treatment Step in First Storage Container (Not Shown)>> After the used carbonate Mg—Al-based layered double hydroxide in the second storage container 120b is regenerated, the third three-way valve V3 is operated to switch the pipe 26 to a state in which it is connected to the chimney 16 but not to the return pipe 151. The three-way valves V5 and V6 are also operated to switch to their "closed" positions. As a result, the acidic exhaust gas discharged from the incinerator 11 passes through the boiler 12, gas cooler 13, dust collector 14, and first three-way valve V1 and is introduced into the first storage container 120a, where the acidic substances contained in the acidic exhaust gas are removed by the carbonate Mg—Al-based layered double hydroxide. The treated gas from which the acidic substances have been removed is discharged from the first storage container 120a, passes through the second three-way valve V2, the induced draft fan 15, and the third three-way valve V3, and is then discharged into the atmosphere from the chimney 16. In this way, the acidic exhaust gas treatment process is carried out in the first storage container 120a. When the ion exchange capacity of the carbonate Mg—Al-based layered double hydroxide in the first storage container 120a decreases as the fourth process continues, the first process described above is carried out to regenerate the carbonate Mg—Al-based layered double hydroxide in the first storage container 120a.

[0056] By repeating the above steps 1 to 4, the used carbonate Mg—Al-based layered double hydroxide can be regenerated while the incineration facility is continuously operated.

[0057] In the first step, the supply of the alkaline aqueous solution to the first storage container 120a and the supply of the treated gas to the first storage container 120a may be started simultaneously, or the supply of the alkaline aqueous solution may be started first, or the supply of the treated gas may be started first, but it is preferable that the supply of the alkaline aqueous solution be started first. In the first step, the three-way valve V6 may be closed, and the treated gas may be supplied to the first storage container 120a and discharged from the pipe 153a while the alkaline aqueous solution is stored in the first storage container 120a. Alternatively, the three-way valve V6 may be operated so that the pipe 142 is connected to the pipe 143a but not to the pipe 143b, and the alkaline aqueous solution may be supplied to the first storage container 120a, brought into contact with the used carbonate Mg—Al-based layered double hydroxide, and then discharged from the pipe 143a, while the treated gas is supplied to the first storage container 120a and discharged from the pipe 153a. In this case, the alkaline aqueous solution may be passed through the first storage container 120a, and then the treated gas may be passed through the first storage container 120a, or the passing of the solution and the aeration may be performed simultaneously.

[0058] Similarly, in the third step, the timing at which the supply of the alkaline aqueous solution to the second storage container 120b and the timing at which the supply of the treated gas to the second storage container 120b are started may be simultaneous, or the supply of the alkaline aqueous solution may be started first, or the supply of the treated gas may be started first, but it is preferable that the supply of the alkaline aqueous solution be started first. Also, in the third step, the three-way valve V6 may be kept in the "closed" state, and the treated gas may be supplied to the second storage container 120b with the alkaline aqueous solution stored in the second storage container 120b and discharged from the pipe 153b. Alternatively, by operating the three-way valve V6 so that the pipe 142 is in communication with the pipe 143b but not with the pipe 143a, the alkaline aqueous solution may be supplied to the second storage container 120b, brought into contact with the used carbonate Mg—Al-based layered double hydroxide, and then discharged from the pipe 143b, and the treated gas may be supplied to the second storage container 120b and discharged from the pipe 153b. In this case, the alkaline aqueous solution may be passed through the second storage container 120b, and then the treated gas may be vented into the second storage container 120b, or the passing of the solution and the venting may be performed simultaneously.

[0059] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0060] Preparation Example 1 of Chlorine-Type Mg—Al-Based Layered Double Hydroxide (Spent Carbonate-Type Mg—Al-Based Layered Double Hydroxide) An acrylic resin column having an inner diameter of 40 mm was packed with a carbonate-type Mg—Al-based layered double hydroxide (Kyoward (registered trademark) 500 PL, manufactured by Kyowa Chemical Industry Co., Ltd., synthetic hydrotalcite), and hydrogen chloride gas was introduced into air to adjust the hydrogen chloride concentration to about 1000 vol ppm at 100° C. The hydrogen chloride-containing gas was passed through the column until hydrogen chloride was detected at the outlet of the column, thereby obtaining a chloride-type Mg—Al-based layered double hydroxide (spent carbonate-type Mg—Al-based layered double hydroxide). The formation of the chloride-type Mg—Al-based layered double hydroxide was confirmed by powder X-ray diffraction measurement (powder X-ray diffractometer: "MiniFlex 600" manufactured by Rigaku Corporation) and ion chromatography (ion chromatograph: "Dionex Integrion" manufactured by Thermo Fisher Scientific K.K.).

[0061] Example 1 20 g of the chloride-type Mg—Al-based layered double hydroxide prepared in Preparation Example 1 above was packed into an acrylic resin column with an inner diameter of 40 mm. 20 g of a 4 mass % aqueous sodium hydroxide solution was added to the top of the column. The flow rates of each gas were adjusted with a mass flow controller, and water and carbon dioxide were introduced into air to adjust the mixed gas to a water content of 20 vol %, a carbon dioxide concentration of 13 vol %, and a temperature of 80°C. This mixed gas was passed through the column for 3 hours at a superficial velocity of 1.0 m / min, to perform a regeneration test.

[0062] The chlorine content in the Mg—Al-based layered double hydroxide before and after the regeneration test was measured by ion chromatography, and the chloride ion desorption rate was calculated based on the measurement results using the following formula to evaluate the regeneration efficiency of the carbonate-type Mg—Al-based layered double hydroxide. The results are shown in Table 1. The higher the chloride ion desorption rate, the higher the regeneration efficiency from the chloride-type Mg—Al-based layered double hydroxide to carbonate-type Mg—Al-based layered double hydroxide. Chloride ion desorption rate (%) = [(B - A) / B] x 100, where A represents the chloride ion content in the mixed gas after the regeneration test, and B represents the chloride ion content in the mixed gas before the regeneration test.

[0063] In addition, powder X-ray diffraction measurements were performed on the Mg—Al-based layered double hydroxide before and after the regeneration treatment test. The results are shown in Figure 4. Table 2 shows the results of identifying each component based on the results of powder X-ray diffraction measurements. When chloride ions are desorbed by regeneration, the chloride ions decrease and the carbonate ion content increases. In Figure 4, A shows the powder X-ray diffraction results of the Mg—Al-based layered double hydroxide after the regeneration treatment test of Example 1, B shows the powder X-ray diffraction results of the Mg—Al-based layered double hydroxide after the regeneration treatment test of Comparative Example 1, which will be described later, and C shows the powder X-ray diffraction results of the Mg—Al-based layered double hydroxide before the regeneration treatment test of Example 1. The peak pattern and interlayer distance (d ００３ ) and the structure of the Mg—Al-based layered double hydroxide was confirmed.

[0064] Comparative Example 1 The same operations and measurements were carried out as in Example 1, except that 20 g of the 4% by mass aqueous sodium hydroxide solution was not added from the top of the column. The results are shown in Tables 1 and 2 and FIG. 4.

[0065]

[0066] From the test results of the above examples and comparative examples, it was confirmed that the present invention can efficiently regenerate used Mg—Al-based carbonate layered double hydroxide (chlorine-type Mg—Al-based layered double hydroxide) into a Mg—Al-based carbonate layered double hydroxide.

[0067] 1 incineration facility 11 incinerator 12 boiler 13 gas cooler 14 dust collector 15 induced draft fan 16 chimney 30 layered double hydroxide storage device 120a first storage container 120b second storage container 130 alkali addition device 131 alkaline aqueous solution storage tank 140 waste liquid discharge device 141 waste liquid storage container 150 regeneration gas supply device 151a heat exchanger 160 regeneration device 170 acidic exhaust gas treatment equipment V1 first three-way valve V2 second three-way valve V3 third three-way valve V4 fourth three-way valve V5 alkali supply three-way valve V6 alkali discharge three-way valve

Claims

1. A method for regenerating spent Mg-Al-based carbonate layered double hydroxide in which at least a portion of the interlayer carbonate ions of the carbonate layered double hydroxide have been exchanged for other anions, comprising contacting the spent Mg-Al-based carbonate layered double hydroxide with a carbon dioxide-containing gas containing carbon dioxide in the presence of an alkali.

2. The method for regenerating used Mg-Al-based carbonate layered double hydroxide according to claim 1, wherein the used Mg-Al-based carbonate layered double hydroxide is obtained after an acidic exhaust gas containing carbon dioxide and acidic substances other than carbon dioxide has been brought into contact with the carbonate Mg-Al-based carbonate layered double hydroxide to remove the acidic substances from the acidic exhaust gas and obtain a treated gas, and the carbon dioxide-containing gas containing carbon dioxide is the treated gas.

3. The method for regenerating used carbonate-type Mg-Al-based layered double hydroxide according to claim 2, wherein the acidic exhaust gas is combustion exhaust gas generated from an incinerator.

4. A method for treating acidic exhaust gas, comprising: an acidic exhaust gas treatment step of contacting an acidic exhaust gas containing carbon dioxide and acidic substances other than carbon dioxide with a carbonate-type Mg-Al-based layered double hydroxide to remove the acidic substances from the acidic exhaust gas and obtain a treated gas; and a regeneration step of regenerating the spent carbonate-type Mg-Al-based layered double hydroxide produced in the acidic exhaust gas treatment step, wherein the regeneration step involves implementing the regeneration method described in claim 2 or 3.

5. Acidic exhaust gas treatment equipment comprising: a storage container for storing a carbonated Mg-Al-based layered double hydroxide; a supply pipe for supplying an acidic exhaust gas containing carbon dioxide and acidic substances other than carbon dioxide to the storage container; a discharge pipe for discharging treated gas discharged after the acidic substances have been treated in the storage container; and a regeneration device for regenerating the carbonated Mg-Al-based layered double hydroxide in the storage container, wherein the regeneration device has an alkali addition device for adding alkali to the storage container and a return pipe for returning the treated gas to the storage container.

6. The acidic exhaust gas treatment facility according to claim 5, wherein the alkali addition device has an alkaline aqueous solution storage tank for storing an alkaline aqueous solution, and an alkaline aqueous solution inlet pipe connecting the alkaline aqueous solution storage tank and the storage container.

7. An acidic exhaust gas treatment facility as described in claim 5 or 6, wherein the regeneration device is connected to the storage container and has a waste liquid discharge device that discharges the alkaline aqueous solution that has flowed into the storage container out of the storage container.

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