WO2022030529A1

WO2022030529A1 - Method for fixing carbon dioxide

Info

Publication number: WO2022030529A1
Application number: PCT/JP2021/028899
Authority: WO
Inventors: 隆雄中垣; コーリアダムマイヤズ; 翔藤本; 良彦加藤; 悟平野; 基頼早水; 慶明三保
Original assignee: 学校法人早稲田大学; 日揮グローバル株式会社; 株式会社ササクラ
Priority date: 2020-08-05
Filing date: 2021-08-04
Publication date: 2022-02-10
Also published as: JPWO2022030529A1

Abstract

A method for fixing carbon dioxide, the method comprising: a first step S1 for passing seawater or brine through a nano-filtration membrane to produce NF membrane concentrated liquid that has been concentrated as a result of not passing through the nano-filtration membrane; a second step S2 for adding an alkali to the NF membrane concentrated liquid produced in the first step S1 so as to fix carbon dioxide through a reaction with an alkaline-earth metal contained in the NF membrane concentrated liquid and to cause deposition of an alkaline-earth metal carbonate crystal; and a third step S3 for collecting the alkaline-earth metal carbonate crystal deposited in the second step S2 from the NF membrane concentrated liquid by solid-liquid separation.

Description

Carbon dioxide fixation method

The present invention relates to a method for immobilizing carbon dioxide on an alkaline earth metal.

As global warming becomes more serious, it is required to suppress the temperature rise, and the goal is to reduce anthropogenic carbon dioxide (CO ₂ ) emissions to zero as an evaluation model. As a means for achieving the above-mentioned goal, a CO ₂ immobilization method can be mentioned.
As an effective means of the CO ₂ immobilization method, there is a method of using the alkaline earth metals Mg and Ca to bond and immobilize these alkaline earth metals and CO ₂ . However, the conventional method using ore containing alkaline earth metal requires a treatment linked to CO ₂ emission such as high temperature and high pressure and addition of chemicals, so that CO ₂ emission is often generated in the entire process.
In addition, Mg and Ca are also contained in seawater and waste liquid irrigation from seawater desalination plants, and for example, a CO ₂ immobilization method using seawater has been proposed (see, for example, Patent Documents 1 and 2). ..

Japanese Unexamined Patent Publication No. 2005-21870 Japanese Unexamined Patent Publication No. 2010-12354

As a CO ₂ immobilization method using seawater or brine, many methods have been studied by injecting CO ₂ into seawater or brine, but it is formed around Mg ²⁺ ions, which are divalent cations with a small ion diameter. Due to the strong hydrated shell and the presence of cations (Na ⁺ , K ⁺ ) competing for carbon dioxide chloride, there is a problem that the efficiency of CO ₂ immobilization in the liquid phase is reduced. As a solution to this problem, it has been the mainstream to use a means for raising the pH by adding an alkali, which is difficult to recycle, such as Ca (OH) ₂ . However, these measures to accelerate the reaction with CO ₂ result in positive CO ₂ emissions throughout the process, given the CO ₂ emissions resulting from energy consumption and additive production as a life cycle assessment. ..
Even with the techniques of

Patent Documents

1 and 2, it is difficult to make the CO ₂ emission amount negative as a whole process because pH adjustment, wastewater treatment, etc. are required.
As described above, a problem of the CO ₂ immobilization method using seawater or brine is the inhibition of the reaction with CO ₂ due to the presence of molecules and ions other than Mg and Ca. Therefore, in the process of separating Mg and Ca from seawater or brackish water, the CO ₂ reduction capacity must be evaluated in consideration of the CO ₂ emissions derived from each unit operation.

Therefore, the present invention provides a method for immobilizing carbon dioxide on an alkaline earth metal, which enhances the carbon dioxide reduction capacity while considering the amount of carbon dioxide emissions.

The object of the present invention is a first step of passing seawater or alkaline water through a nanofiltration membrane to generate a concentrated NF membrane concentrate without permeating the nanofiltration membrane, and the first step. The second step is to add an alkali to the generated NF film concentrate, react the alkaline earth metal contained in the NF film concentrate with carbon dioxide to immobilize it, and precipitate alkaline earth metal charcoal oxide crystals. This is achieved by a method for immobilizing carbon dioxide, which comprises a third step of solid-liquid separation and recovery of the alkaline earth metal charcoal oxide crystals precipitated in the second step from the NF film concentrate. In this method for immobilizing carbon dioxide, the first step is a salt-making step of concentrating an NF membrane permeate that has permeated the nanofiltration membrane and recovering the precipitated sodium chloride crystals, and a chloride-making step of recovering the chloride. It is preferable to include an electrodialysis step of separating an acid solution and an alkaline solution by electrodialyzing a solution of sodium crystals, and in the second step, the alkaline solution obtained in the electrodialysis step is used as an NF membrane concentrate. It is preferable to add it.

In the first step, by performing the salt-making step only on a part of the NF membrane permeate, the alkaline solution produced in the electrodialysis step can be reduced to only the amount required in the second step. Alternatively, in the first step, the alkaline solution produced in the electrodialysis step is obtained by bypassing a part of seawater or brackish water without passing through the nanofiltration membrane and performing the salt making step. Only the amount required for the process can be used.

It is preferable that the third step includes a neutralization step of neutralizing the filtrate after recovering the alkaline earth metal charcoal oxide crystals from the NF membrane concentrate with the acid solution obtained in the electrodialysis step.

The salt-making step is a film treatment step of producing a concentrated membrane treatment concentrate without permeating the reverse osmosis membrane by passing water through the reverse osmosis membrane, and a membrane treatment step. It is preferable to include a crystallization step of heating and evaporating the obtained membrane treatment concentrate to precipitate sodium chloride crystals. In the third step, the recovered alkaline earth metal charcoal oxide crystals are obtained in the crystallization step. It is preferable to include a washing step of washing with washing water containing the distilled water.

The second step can include an evaporation concentration step of precipitating calcium sulfate crystals by evaporating and concentrating the NF film concentrate in which alkaline earth metal charcoal oxide crystals are precipitated, and the third step is said to be described above. The alkaline earth metal charcoal oxide crystals and calcium sulfate crystals precipitated in the second step can be recovered by solid-liquid separation from the NF film concentrate. In this case, in the third step, at least one crystal of sodium chloride, potassium chloride and sodium sulfate is precipitated from the filtrate after recovering alkaline earth metal charcoal oxide crystals and calcium sulfate crystals from the NF membrane concentrate. It is preferable to include a crystal recovery step for recovering the crystals.

It is preferable that the crystal recovery step includes a concentrated crystallization step of precipitating and recovering sodium chloride crystals by evaporating and concentrating the filtrate. It is preferable that the crystal recovery step includes a cooling crystallization step of recovering the precipitated crystals by cooling crystallization of the filtrate. In the cooling crystallization step, the first cooling crystallization step of recovering the potassium chloride crystals precipitated by the cooling crystallization of the filtrate and the first cooling crystallization step of the filtrate having undergone the first cooling crystallization step. It is preferable to include a second cooling crystallization step of recovering the precipitated sodium sulfate crystals by cooling crystallization at a temperature lower than the cooling crystallization temperature of the cooling crystallization step. It is preferable that the crystal recovery step includes a step of merging a part of the filtrate after crystal recovery with the NF membrane concentrate in which the second step is performed to neutralize the balance of the filtrate after crystal recovery.

In the third step, it is preferable to perform the crystal recovery step by merging the washed water obtained by washing the recovered alkaline earth metal charcoal oxide crystals and calcium sulfate crystals with the filtrate. The washing water preferably contains distilled water produced in the evaporation concentration step.

The first step is an evaporation concentration step of precipitating calcium sulfate crystals by evaporating and concentrating an NF membrane concentrate that has been concentrated without permeating through the nanofiltration membrane, and a calcium sulfate crystals precipitated by the evaporation concentration step. It can be provided with a solid-liquid separation step of solid-liquid separation and recovery from the NF membrane concentrate.

According to the present invention, it is possible to provide a method for immobilizing carbon dioxide on an alkaline earth metal, which enhances the carbon dioxide reduction capacity while considering the amount of carbon dioxide emissions.

It is a process flow diagram for demonstrating the carbon dioxide fixation method which concerns on 1st Embodiment of this invention. It is a figure which shows an example of the change of various ion amounts in the processing flow shown in FIG. It is a figure which shows the modification of some steps in the processing flow shown in FIG. It is a process flow diagram for demonstrating the carbon dioxide fixation method which concerns on 2nd Embodiment of this invention. It is a figure which shows the modification of the processing flow shown in FIG.

The method for immobilizing carbon dioxide of the present invention provides a method for immobilizing carbon dioxide to an alkaline earth metal contained in seawater or brackish water. In the present invention, "alkaline earth metal" means a broad range including Mg and Be, which are elements of Group 2 of the periodic table, in addition to Ca, Sr, Ba, and Ra. In particular, it is preferable to contain at least Mg as an alkaline earth metal from the viewpoint of ease of reaction with CO ₂ and the expectation that the carbonate obtained by the reaction can be used for various purposes.

Further, "seawater or brine" is an aqueous solution containing ions of an alkaline earth metal such as magnesium ion (Mg ²⁺ ) and calcium ion (Ca ²⁺ ). Seawater or brine usually contains ions that make up at least one crystal selected from calcium sulfate, sodium chloride, potassium chloride and sodium sulfate, in addition to the ions of alkaline earth metals. Specifically, seawater or brine usually contains at least one ion selected from chloride ion (Cl ⁻ ), sulfate ion ( ^{SO 4-2} ₎ , sodium ion (Na ⁺ ), and potassium (K ⁺ ). I'm out.

As the above-mentioned seawater or brackish water, those obtained from at least one selected from seawater, salt lakes, and industrial wastewater can be used. In addition to seawater, salt lakes and industrial wastewater, river water, rainwater, treated sewage water, and accompanying water from oil and gas fields can also be used as long as alkaline earth metals are contained. More specifically, as irrigation, irrigation using salt lakes, waste liquid irrigation discharged by desalination and salt formation processes, recovery of valuable resources using seawater and salt lakes, collection of valuable resources from chemical factories, etc. Examples include industrial wastewater.

From the viewpoint of containing a large amount of Mg as described above, reducing the environmental load and easing to reduce CO ₂ emissions, brackish water is obtained from brackish water obtained from a desalination device using seawater and a process of producing salt from seawater. It is preferably at least one selected from brackish water and brackish water obtained from the process of recovering lithium from a salt lake.

<First Embodiment>
Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a processing flow diagram for explaining a method for immobilizing carbon dioxide according to the first embodiment of the present invention. In the first embodiment, the treatment target is seawater, but the same treatment can be performed in the case of brackish water. As shown in FIG. 1, in the method for immobilizing carbon dioxide according to the first embodiment, seawater is passed through a nanofiltration membrane (NF membrane) to concentrate the NF membrane without permeating the NF membrane. Alkaline is added to the first step S1 to generate the liquid and the NF membrane concentrate produced in the first step S1, and carbon dioxide is reacted with the alkaline earth metal contained in the NF membrane concentrate to immobilize the liquid. The second step S2 for precipitating alkaline earth metal charcoal oxide crystals and the third step S3 for solid-liquid separation and recovery of the alkaline earth metal charcoal oxide crystals precipitated in the second step S2 from the NF membrane concentrate. I have.

<S1: First step>
In the first step S1, seawater was supplied to the NF membrane unit by a medium pressure pump or the like and passed through the NF membrane to concentrate the NF membrane permeable liquid that had permeated the NF membrane and the NF membrane without permeating the NF membrane. Produces an NF membrane concentrate.

Since the NF membrane suppresses the permeation of divalent or higher ions, the monovalent ion has the property of being easily permeated. Therefore, the NF membrane concentrate contains an alkaline earth metal that is the target of carbon dioxide immobilization. As much remains, the concentrations of Na ⁺ , K ⁺ , etc. that may interfere with this immobilization are reduced. Therefore, it is possible to easily and efficiently fix carbon dioxide to alkaline earth metals contained in seawater (or brine), and it is possible to suppress the generation of carbon dioxide in the entire process including the post-process. .. FIG. 2 shows an example of various ion amounts (mg / h) contained in seawater, NF membrane permeate and NF membrane concentrate when seawater is supplied at a flow rate of 100 m ³ / h.

<S11: Salt making process>
On the other hand, since the NF membrane permeate contains a large amount of monovalent ions Na ⁺ and Cl ⁻ , in the first step S1, the sodium chloride (NaCl) crystals precipitated by concentrating the NF membrane permeate. The salt-making step S11 is provided. In the salt-making step S11 of the present embodiment, the NF membrane permeate is supplied to the reverse osmosis membrane (RO membrane) unit by a high-pressure pump or the like and water is passed through the RO membrane, so that the membrane is concentrated without permeating the RO membrane. The membrane treatment step S111 for producing a treatment concentrate and the crystallization step S112 for supplying the produced membrane treatment concentrate to a crystal can and heating and evaporating to precipitate NaCl crystals are provided. The vapor discharged from the crystal can is condensed by a condenser or the like to become distilled water, which is merged with the membrane-treated permeate liquid that has permeated the RO membrane and used as production water or the like. The concentrated liquid in the crystal can is partially discharged from the crystal can as a slurry liquid containing NaCl crystals, and dehydrated by a centrifuge or the like to recover the NaCl crystals. Since the NF membrane ^permeate contains almost no SO _4-2 , it can be concentrated at a high concentration by the low energy membrane treatment step S111.

The membrane treatment step S111 is not limited to the treatment using the RO membrane, and may be another treatment using a semipermeable membrane, or may be a combination of a plurality of membrane treatments. For example, as shown in FIG. 3, the membrane treatment step S111 concentrates the NF membrane permeate with the RO membrane to generate the RO membrane concentrate, and the RO membrane concentrate step S113 and the RO membrane concentrate are separated by a semipermeable membrane. It is possible to provide a composite membrane treatment step S114 that supplies the semipermeable membrane unit to the high pressure chamber and further concentrates the RO membrane concentrate by utilizing the pressure difference from the recovered liquid passing through the low pressure chamber. As the recovery liquid supplied to the low pressure chamber, a part of the RO membrane concentrate that has passed through the high pressure chamber can be used, and the recovery liquid that has passed through the low pressure chamber is used as the NF membrane permeate before the RO membrane concentration step S113. Can be merged. Further, as shown in FIG. 3, the evaporation treatment step S115 in which the membrane treatment concentrate produced in the membrane treatment step S111 is evaporated and concentrated by a horizontal tube type evaporator or the like between the membrane treatment step S111 and the crystallization step S112. May be provided.

<S12: Confluence process>
As shown in FIG. 1, the first step S1 includes a merging step S12 in which the filtrate after collecting the NaCl crystals in the salt making step S11 is merged with the above-mentioned NF membrane concentrate, whereby the waste liquid system is provided. It is possible to reduce the environmental load by suppressing the discharge to the outside. Alkaline earth metals such as magnesium to be recovered are contained not only in the NF membrane concentrate but also in the NF membrane permeate. Therefore, by providing the above-mentioned merging step S12, carbon dioxide in the second step S2 is provided. It is possible to increase the recovery rate of alkaline earth metals required for carbon immobilization. From the viewpoint of increasing the purity of the NaCl crystal obtained in the crystallization step S112, it is preferable to increase the amount of the filtrate to be merged with the NF film concentrate by the merging step S12 as much as possible.

<S13: Electrodialysis process>
The first step S1 includes an electrodialysis step S13 for electrodialyzing the NaCl crystals produced in the salt making step S11. In the electrodialysis step S13, for example, a bipolar membrane electrodialysis apparatus can be used, and the solution obtained by dissolving the NaCl crystals obtained in the crystallization step S112 in water is separated into an HCl solution and a NaOH solution. Electrodialysis preferably utilizes renewable energy such as solar energy and can suppress CO ₂ emissions in the entire process. In order to increase the purity of the NaCl solution to which electrodialysis is performed, it is preferable to adsorb impurities such as magnesium and calcium contained in the NaCl solution to the chelate resin to sufficiently reduce their concentrations (for example, 1 ppm or less).

The NaOH solution obtained in the electrodialysis step S13 can be suitably used as an alkali to be added in the second step S2. The flow path of the NF membrane permeate produced in the first step S1 is branched so that the amount of the alkaline solution produced in the electrodialysis step S13 is only the amount required in the second step S2. The salt making step S11 may be performed only partially. As a result, the energy consumption required for the production of alkali can be suppressed, and CO ₂ emissions in the entire process can be reduced. It is preferable to use renewable energy for the generation of alkali as described above. The remainder of the NF membrane permeate not used in the salt making step S11 can be used in another step such as a desalination process, and the NF membrane permeate not used in the separate step may be discharged into the ocean or the like.

In the first step S1, a part of seawater or brackish water is bypassed without passing through the NF membrane so that the amount of the alkaline solution produced in the electrodialysis step S13 is only the amount required in the second step S2. The salt-making step S11 may be performed on a part or all of the NF film permeation liquid in which the amount of production is suppressed. The seawater or brine that bypasses the NF membrane can be used for immobilization of carbon dioxide in the second step S2 after merging with the NF membrane concentrate.

<S2: Second step>
In the second step S2, the pH of the NF membrane concentrate is adjusted to the alkaline side (for example, pH 9 to 10) by adding an alkali to the NF membrane concentrate produced in the first step S1 and stored in the storage tank. By blowing a gas containing carbon dioxide into this NF membrane concentrate and bringing it into gas-liquid contact by bubbling, carbon dioxide is reacted with the alkaline earth metal contained in the NF membrane concentrate and immobilized. The gas containing carbon dioxide may be the atmosphere, or may be the exhaust gas of various combustion devices. The concentration of carbon dioxide contained in the gas is not limited, but for example, the concentration of carbon dioxide contained in the gas is about 100% by volume from the atmosphere. In order to maintain the pH during the reaction of alkaline earth metals and carbon dioxide, the addition of alkali to the NF membrane concentrate may be carried out not only before bubbling the gas containing carbon dioxide but also during bubbling. When bubbling, the reaction efficiency between alkaline earth metals and carbon dioxide can be improved by blowing fine bubbles of carbon dioxide (fine bubbles such as microbubbles and ultrafine bubbles).

The method of gas-liquid contact between the NF membrane concentrate and carbon dioxide is not only the method of blowing CO ₂ gas into the NF membrane concentrate, but also the CO ₂ gas in a single-stage or multi-stage desulfurization tower, a degassing tower, or the like. A method of spraying the NF film concentrate inside with a spray nozzle or a tray may be used, and various known gas-liquid contacts are taken in consideration of the reaction rate, the reaction amount, the CO ₂ concentration in the gas such as exhaust gas, and the like. The device can be used.

As the alkali added to the NF membrane concentrate in the second step S2, it is preferable to use the NaOH solution obtained in the electrodialysis step S13 as described above, and the increase in CO ₂ emissions due to the separate generation of the alkali is increased. It can be suppressed. However, the NF membrane concentrate has a lower concentration of Na ⁺ , K ⁺ , etc. than the original solution, and it is easy to immobilize carbon dioxide. Therefore, what is the alkali obtained in the electrodialysis step S13? Different alkalis may be used, or the alkali obtained in the electrodialysis step S13 may be used in combination with another alkali.

<S3: Third step>
The NF film concentrate that has undergone the second step S2 becomes a slurry liquid in which alkaline earth metal carbon dioxide crystals such as MgCO ₃ and CaCO ₃ are precipitated by the reaction between the alkaline earth metal and carbon dioxide. In the third step S3, the alkaline earth metal charcoal oxide crystals contained in this slurry liquid are separated into solid and liquid by a solid and liquid separating device such as a centrifuge and recovered.

<S31: Neutralization process>
The third step S3 includes a neutralization step S31 for neutralizing the filtrate after recovering the alkaline earth metal charcoal oxide crystals from the NF film concentrate. Since the filtrate of the NF membrane concentrate is usually pH 9 or higher due to the addition of alkali, it should be discharged as it is to the outside of the system such as the ocean by adding an acid to neutralize it (for example, pH 7 to 8). Will be possible. As the acid added to the filtrate, it is preferable to use the HCl solution obtained in the electrodialysis step S13, and it is possible to suppress an increase in CO ₂ emissions associated with the separate generation of the acid.

<S32: Washing process>
In the third step S3, the recovered alkaline earth metal charcoal oxide crystals are washed with washing water to dissolve Na ⁺ , K ⁺ , etc. adhering to the alkaline earth metal charcoal oxide crystals in the washing water and remove them. The water washing step S32 is provided. The washing water used in the washing step S32 preferably contains distilled water obtained in the above crystallization step S12, thereby suppressing an increase in CO ₂ emissions associated with the separate generation of washing water. Can be done. In the present embodiment, a part of the produced water obtained by merging distilled water with the NF membrane permeate is used as washing water.

The washed alkaline earth metal charcoal oxide crystals such as MgCO ₃ and CaCO ₃ can be suitably used as building materials such as concrete and cement. Therefore, the present invention can also provide a method for producing an alkaline earth metal charcoal oxide salt using a method for immobilizing carbon dioxide.

The method for immobilizing carbon dioxide of the present embodiment can easily and efficiently immobilize carbon dioxide with respect to alkaline earth metals contained in seawater or irrigation water, and at the same time, alkalis and acids generated in each step can be used. Since the immobilization of carbon dioxide can be completed in the system by using distilled water or the like, the carbon dioxide reduction capacity can be enhanced while considering the carbon dioxide emissions in the entire process.

<Second embodiment>
FIG. 4 is a processing flow diagram for explaining a method for immobilizing carbon dioxide according to a second embodiment of the present invention. The method for immobilizing carbon dioxide according to the second embodiment shown in FIG. 4 is similar to the first embodiment, in which seawater is passed through the NF membrane to concentrate the NF membrane without permeating the NF membrane. Alkaline is added to the first step S1 to generate the liquid and the NF membrane concentrate produced in the first step S1, and the alkaline earth metal contained in the NF membrane concentrate is reacted with carbon dioxide to be immobilized. The second step S2 for precipitating alkaline earth metal charcoal oxide crystals and the third step S3 for solid-liquid separation and recovery of the alkaline earth metal charcoal oxide crystals precipitated in the second step S2 are performed. I have. In the second embodiment, the treatment target is seawater, but the same treatment can be performed in the case of brackish water, and in particular, it can be preferably used when the content of Ca or Mg is high. In FIG. 4, the same steps as in FIG. 1 are designated by the same reference numerals, and detailed description thereof will be omitted.

<S21: Evaporation concentration step>
As shown in FIG. 4, in the second step S2, the slurry liquid in which carbon dioxide is immobilized on the alkaline earth metal contained in the NF film concentrate and the alkaline earth metal carbon oxide crystals are precipitated is evaporated in the evaporation can. The evaporative concentration step S21 for precipitating calcium sulfate crystals (CaSO _4.2H ₂ O) by concentrating is provided. The slurry solution before evaporation and concentration contains Ca ²⁺ , Na ⁺ , K ⁺ , etc. However, since calcium sulfate crystals have backsolubility whose solubility decreases as the temperature rises, Ca ²⁺ is precipitated. On the other hand, Na ⁺ and K ⁺ are concentrated to a concentration ratio at which NaCl and KCl do not precipitate while maintaining the operating temperature of evaporation concentration so that Na + and K + do not precipitate. The operating temperature for evaporation concentration is preferably 70 to 90 ° C, for example set to 80 ° C.

In the evaporation concentration step S21, in order to prevent the generation of scale of calcium sulfate, it is preferable to add a seed crystal of CaSO _4.2H ₂ O to the slurry liquid to promote crystal growth centered on the seed crystal. CaSO _4.2H ₂ O produced in the evaporation concentration step S21 can be preferably used for this seed crystal.

<S3: Third step>
In the third step S3, alkaline earth metal carbon oxide crystals such as MgCO ₃ and CaCO ₃ and calcium sulfate crystals precipitated in the second step S2 are separated from the NF film concentrate by a solid-liquid separator and recovered. .. The recovered alkaline earth metal charcoal oxide crystals and calcium sulfate crystals are washed with washing water in the washing step S32 in the same manner as in the first embodiment.

In the water washing step S32 of the second embodiment, a part of the distilled water generated in the evaporation concentration step S21 is used as washing water. As the washing water, in addition to the distilled water produced in the evaporation concentration step S21, the production water produced in the salt making step S11 or the distilled water produced in the concentration crystallization step S331 described later may be used.

<S33: Crystal recovery process>
The third step S3 is a crystal recovery step of precipitating and recovering sodium chloride, potassium chloride and sodium sulfate crystals from the filtrate after recovering alkaline earth metal charcoal oxide crystals and calcium sulfate crystals from the NF film concentrate. It is equipped with S33. The filtrate in which the crystal recovery step S33 is performed is preferably only a part of the filtrate obtained by solid-liquid separating alkaline earth metal charcoal oxide crystals and calcium sulfate crystals, and the balance of the solid-liquid separated filtrate is impurities. In order to suppress the increase in concentration, it is preferable to discharge the liquid to the outside of the system through the neutralization step S31.

Washing water obtained by washing alkaline earth metal charcoal oxide crystals and calcium sulfate crystals in the washing step S32 may be combined with the filtrate in which the crystal recovery step S33 is performed. The crystal recovery step S33 includes a concentrated crystallization step S331 that precipitates and recovers sodium chloride crystals by evaporating and concentrating the filtrate, and a cooling crystallization step S332 that recovers the precipitated crystals by cooling crystallization of the filtrate. And.

<S3311: Concentrated crystallization step>
In the concentration crystallization step S331, the filtrate is supplied to an evaporative can and heated to evaporate and concentrate to precipitate crystals containing sodium chloride (NaCl) as a main component, and then sodium chloride crystals are produced by a solid-liquid separator. It is done separately. The operating temperature for evaporation concentration is preferably 60 to 80 ° C, for example set to 70 ° C.

<S332: Cooling crystallization step>
In the cooling crystallization step S332, the filtrate that has undergone the concentrated crystallization step S331 is supplied to the cooling crystallization can and cooled to a predetermined cooling crystallization temperature while stirring to precipitate crystals of the target impurities. After that, the crystals are separated by a solid-liquid separator. The cooling crystallization step S332 includes a first cooling crystallization step S3321 for recovering crystals containing potassium chloride (KCl) as a main component by cooling crystallization of the NF film concentrate, and a first cooling crystallization step S3321. A second method for recovering sodium sulfate crystals (Na ₂ SO _4.10H ₂ O) precipitated by cooling crystallization of the filtrate that has passed through the above steps at a temperature lower than the cooling crystallization temperature of the first cooling crystallization step S3321. It is provided with a cooling crystallization step S3322. The cooling crystallization temperature of the first cooling crystallization step S3321 is a temperature at which crystals mainly composed of KCl precipitate but crystals of Na ₂ SO _4.10H ₂ O do not precipitate, and is 33 to 40 ° C. Preferably, it is set to, for example, 36 ° C. The cooling crystallization temperature of the second cooling crystallization step S3322 is a temperature at which crystals of Na ₂ SO ₄・ 10H ₂ O precipitate, and is set to, for example, 0 to 10 ° C.

The filtrate that has passed through the crystal recovery step S33 is alkaline, and a part of the filtrate can be combined with the NF membrane concentrate in which the second step S2 is performed. Further, the remainder of the filtrate that has passed through the crystal recovery step S33 can be neutralized with an acid such as the HCl solution obtained in the electrodialysis step S13 by the neutralization step S34 and discharged to the outside of the system.

The crystal recovery step S33 does not need to perform all of the concentrated crystallization step S331, the first cooling crystallization step S3321 and the second cooling crystallization step S3322, and is selected from sodium chloride, potassium chloride and sodium sulfate from the filtrate. Only necessary steps may be appropriately selected depending on the components of seawater or brackish water to be treated so that at least one kind of crystal can be precipitated and recovered.

<First modification>
FIG. 5 is a diagram showing a modified example of the processing flow shown in FIG. In the carbon dioxide fixation method shown in FIG. 5, the first step S1 of the carbon dioxide fixation method shown in FIG. 1 further includes an evaporation concentration step S14 and a solid-liquid separation step S15, and the other steps include. , The same as the carbon dioxide fixation method shown in FIG.

<S14: Evaporation concentration step>
In the evaporation concentration step S14, calcium sulfate crystals are precipitated by evaporating and concentrating the NF film concentrate in the evaporation can before the carbon dioxide is reacted with the alkaline earth metal and immobilized in the second step S2. The evaporative concentration step S14 may be performed on the NF membrane concentrate before the merging step S12, but the filtrate that merges with the NF membrane concentrate in the merging step S12 contains a small amount of calcium. As shown in 5, it is preferable to perform the evaporation concentration step S14 on the NF membrane concentrate that has undergone the merging step S12. In order to suppress the scale of the evaporation can, it is preferable to add calcium sulfate crystals as seed crystals to the NF membrane concentrate. As the seed crystal, for example, calcium sulfate crystals recovered in the solid-liquid separation step S15 described later can be used. As a method for suppressing the scale of the evaporation can, it is also preferable to add an acid to the NF membrane concentrate to adjust the pH. As the acid, for example, the HCl solution obtained in the above electrodialysis step S13 can be used. The distilled water produced in the evaporation concentration step S14 can be used, for example, as the washing water in the washing step S32 of the third step S3.

<S15: Solid-liquid separation step>
In the solid-liquid separation step S15, the calcium sulfate crystals precipitated in the evaporation concentration step S14 are solid-liquid separated from the NF film concentrate by a solid-liquid separation device and recovered. The recovered calcium sulfate crystals can be used, for example, as gypsum.

By providing the above-mentioned evaporation concentration step S14 and solid-liquid separation step S15, the following merits occur in the second step S2. First, since the amount of the NF membrane concentrate in which the second step S2 is performed can be reduced, the amount of alkali (NaOH) to be added can be reduced, and the power consumption of the electrodialysis step S13 can be suppressed. At the same time, it is possible to make a reaction tank such as a storage tank for gas-liquid contact compact. Further, since the reaction efficiency of carbon dioxide increases due to the increase in the concentration of the alkaline earth metal in the NF film concentrate in which the second step S2 is performed, the amount of carbon dioxide bubbling is reduced and the consumption required for bubbling is increased. Power can be suppressed. Further, since calcium is recovered as calcium sulfate crystals from the NF membrane concentrate before the second step S2, the precipitation of CaCO ₃ is suppressed in the second step S2, and the purity of the recovered MgCO ₃ can be increased. ..

<Second modification>
The evaporation concentration step S14 and the solid-liquid separation step S15 shown in FIG. 5 can also be applied to the carbon dioxide immobilization method shown in FIG. 4, and the evaporation concentration step S14 is before the second step S2 shown in FIG. By performing the solid-liquid separation step S15, the same effect as described above can be obtained. In this case, the evaporation concentration step S21 shown in FIG. 4 is unnecessary, and the NF membrane concentrate that has passed through the second step S2 is solid-liquid separated by the third step S3. Since the carbon dioxide immobilization method shown in FIG. 4 includes an evaporation concentration step S14 and a solid-liquid separation step S15, the NF membrane concentrate that has undergone the second step S2 contains almost no CaSO ₄ and CaCO ₃ . In the third step S3, MgCO ₃ can be recovered with high purity.

S1 1st step S11 Salt making step S111 Film treatment step S112 Crystallization step S12 Confluence step S13 Electrodialysis step S14 Evaporation concentration step S15 Solid-liquid separation step S2 2nd step S21 Evaporation concentration step S3 3rd step S31 Neutralization step S32 Washing step S33 Crystal recovery step S331 Concentrated crystallization step S332 Cooling crystallization step S3321 First cooling crystallization step S3322 Second cooling crystallization step S34 Neutralization step

Claims

The first step of producing a concentrated NF membrane concentrate without permeating the nanofiltration membrane by passing seawater or brine through the nanofiltration membrane.
Alkaline is added to the NF film concentrate produced in the first step, and carbon dioxide is reacted with the alkaline earth metal contained in the NF film concentrate to immobilize the NF film concentrate to precipitate alkaline earth metal charcoal oxide crystals. The second step to make it
A method for immobilizing carbon dioxide, which comprises a third step of solid-liquid separation and recovery of alkaline earth metal charcoal oxide crystals precipitated in the second step from the NF film concentrate.
The first step is a salt-making step of concentrating the NF membrane permeate that has permeated the nanofiltration membrane to recover the precipitated sodium chloride crystals, and electrodialysis of the solution of the sodium chloride crystals recovered in the salt-making step. Equipped with an electrodialysis step to separate acid and alkaline solutions by
The method for immobilizing carbon dioxide according to claim 1, wherein the second step is to add the alkaline solution obtained in the electrodialysis step to the NF membrane concentrate.
The second aspect of claim 2, wherein in the first step, only a part of the NF membrane permeate is subjected to the salt-making step, so that the amount of the alkaline solution produced in the electrodialysis step is limited to the amount required in the second step. Carbon dioxide immobilization method.
In the first step, the alkaline solution produced in the electrodialysis step is obtained in the second step by bypassing a part of seawater or brackish water without passing it through the nanofiltration membrane and performing the salt making step. The method for immobilizing carbon dioxide according to claim 2 or 3, wherein only the required amount is used.
The third step includes a neutralization step of neutralizing the filtrate after recovering the alkaline earth metal charcoal oxide crystal from the NF membrane concentrate with the acid solution obtained in the electrodialysis step, according to claims 2 to 4. The method for immobilizing carbonic acid according to any one of the above.
The salt-making step is a membrane treatment step of producing a concentrated membrane treatment concentrate without permeating the reverse osmosis membrane by passing water through the reverse osmosis membrane, and a membrane treatment step. It is provided with a crystallization step of heating and evaporating the prepared membrane treatment concentrate to precipitate sodium chloride crystals.
The dioxide according to any one of claims 2 to 5, wherein the third step comprises a washing step of washing the recovered alkaline earth metal charcoal oxide crystals with washing water containing distilled water obtained in the crystallization step. Carbon immobilization method.
The second step includes an evaporative concentration step of precipitating calcium sulfate crystals by evaporating and concentrating the NF film concentrate in which alkaline earth metal carbon oxide crystals are precipitated.
The carbon dioxide immobilization according to claim 1 or 2, wherein in the third step, the alkaline earth metal charcoal oxide crystals and calcium sulfate crystals precipitated in the second step are separated from the NF film concentrate by solid and liquid and recovered. Method.
In the third step, at least one crystal of sodium chloride, potassium chloride and sodium sulfate is precipitated and recovered from the filtrate after collecting alkaline earth metal charcoal oxide crystals and calcium sulfate crystals from the NF membrane concentrate. The method for immobilizing carbon dioxide according to claim 7, further comprising a crystal recovery step.
The method for immobilizing carbon dioxide according to claim 8, wherein the crystal recovery step comprises a concentrated crystallization step of precipitating and recovering sodium chloride crystals by evaporating and concentrating the filtrate.
The method for immobilizing carbon dioxide according to claim 8 or 9, wherein the crystal recovery step comprises a cooling crystallization step of recovering the crystals precipitated by cooling crystallization of the filtrate.
In the cooling crystallization step, the first cooling crystallization step of recovering the potassium chloride crystals precipitated by the cooling crystallization of the filtrate and the first cooling crystallization step of the filtrate having undergone the first cooling crystallization step. The method for immobilizing carbon dioxide according to claim 10, further comprising a second cooling crystallization step of recovering the sodium sulfate crystals precipitated by cooling crystallization at a temperature lower than the cooling crystallization temperature of the cooling crystallization step.
The crystal recovery step includes a step of merging a part of the filtrate after crystal recovery with the NF membrane concentrate in which the second step is performed to neutralize the balance of the filtrate after crystal recovery according to claim 8. 11. The method for immobilizing carbon dioxide according to any one of 11.
The dioxide according to any one of claims 8 to 12, wherein in the third step, washing water obtained by washing the recovered alkaline earth metal charcoal oxide crystals and calcium sulfate crystals is combined with a filtrate to carry out the crystal recovery step. Carbon immobilization method.
The method for immobilizing carbon dioxide according to claim 13, wherein the washing water contains distilled water produced in the evaporation concentration step.
The first step is an evaporation concentration step of precipitating calcium sulfate crystals by evaporating and concentrating an NF membrane concentrate that has been concentrated without permeating the nanofiltration membrane, and a calcium sulfate crystals precipitated by the evaporation concentration step. The method for immobilizing carbon dioxide according to claim 1 or 2, further comprising a solid-liquid separation step of solid-liquid separation and recovery from the NF membrane concentrate.