Carbon dioxide fixation method

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

[Technical Field]

[0001] The present invention relates to a method for fixing carbon dioxide on an alkaline earth metal. [Background technology]

[0002] As global warming becomes more serious, there is a need to curb temperature rises, and as an assessment model, the goal is to reduce anthropogenic carbon dioxide (CO2) emissions to zero. One method for achieving this goal is to fix CO2. One effective method for CO2 fixation is to use alkaline earth metals such as Mg and Ca to bind and fix CO2. However, conventional methods that use ores containing alkaline earth metals require treatments that result in CO2 emissions, such as high temperature and pressure and the addition of chemicals, and in many cases the entire process results in CO2 emissions. Furthermore, Mg and Ca are also contained in seawater and brine wastewater from seawater desalination plants, and a method for CO2 fixation using seawater has been proposed (see, for example, Patent Documents 1 and 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-21870 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-125354 Summary of the Invention [Problem to be solved by the invention]

[0004] In CO2 fixation methods using seawater or brine, methods of injecting CO2 into seawater or brine have been widely studied, but the use of Mg, a divalent cation with a small ionic diameter, has not been well studied. 2+ The strong hydration shell formed around the ion and the cation (Na + , K.+ ), there is a problem that the efficiency of CO2 fixation in the liquid phase decreases due to the presence of cations. The mainstream solution to this problem has been to increase the pH by adding alkalis such as Ca(OH)2, which are difficult to recycle. However, when considering energy consumption and CO2 emissions resulting from additive production as part of a life cycle assessment, these methods for promoting the reaction with CO2 result in a net increase in CO2 emissions throughout the process. Even with the technologies of Patent Documents 1 and 2, pH adjustment, wastewater treatment, etc. are required, making it difficult to reduce CO2 emissions throughout the process. As mentioned above, one problem with CO2 fixation methods using seawater or brine is that the presence of molecules and ions other than Mg and Ca inhibits the reaction with CO2. Therefore, in the process of separating Mg and Ca from seawater or brine, the CO2 reduction potential must be evaluated taking into account the CO2 emissions resulting from each unit operation.

[0005] Therefore, the present invention provides a method for immobilizing carbon dioxide on alkaline earth metals, which enhances the carbon dioxide reduction capacity while taking into account the amount of carbon dioxide emitted. [Means for solving the problem]

[0006] The above-mentioned object of the present invention is achieved by a method for fixation of carbon dioxide, comprising: a first step of passing seawater or brackish water through a nanofiltration membrane to produce an NF membrane concentrate that is concentrated without passing through the nanofiltration membrane; a second step of adding an alkali to the NF membrane concentrate produced in the first step to react carbon dioxide with the alkaline earth metal contained in the NF membrane concentrate to immobilize carbon dioxide and precipitate alkaline earth metal carbonate crystals; and a third step of recovering the alkaline earth metal carbonate crystals precipitated in the second step from the NF membrane concentrate by solid-liquid separation. teeth, The first step inThe method preferably comprises a salt production step in which the NF membrane permeate liquid that has permeated the nanofiltration membrane is concentrated to recover precipitated sodium chloride crystals, and an electrodialysis step in which the solution of sodium chloride crystals recovered in the salt production step is subjected to electrodialysis to separate an acid solution from an alkaline solution, and the second step preferably comprises adding the alkaline solution obtained in the electrodialysis step to the NF membrane concentrated liquid.

[0007] The first step Generated with By subjecting only a portion of the NF membrane permeate to the salt production process, the amount of alkaline solution produced in the electrodialysis process can be reduced to only the amount required for the second process. In By performing the salt production process by bypassing a portion of the seawater or brine without passing it through the nanofiltration membrane, the amount of alkaline solution produced in the electrodialysis process can be reduced to only the amount required for the second process.

[0008] The third step in It is preferable to provide a neutralization step in which the filtrate obtained after recovering the alkaline earth metal carbonate crystals from the NF membrane concentrate is neutralized with the acid solution obtained in the electrodialysis step.

[0009] The salt production process preferably comprises a membrane treatment process in which the NF membrane permeate liquid is passed through a reverse osmosis membrane to produce a membrane-treated concentrate that is concentrated without passing through the reverse osmosis membrane, and a crystallization process in which the membrane-treated concentrate produced in the membrane treatment process is heated and evaporated to precipitate sodium chloride crystals, and the third process in It is preferable to provide a water washing step in which the recovered alkaline earth metal carbonate crystals are washed with wash water containing the distilled water obtained in the crystallization step.

[0010] The second step in The present invention can further include an evaporation and concentration step of evaporating and concentrating the NF membrane concentrate in which alkaline earth metal carbonate crystals have been precipitated, to thereby precipitate calcium sulfate crystals. The third step is to concentrate the alkaline earth metal carbonate crystals precipitated in the second step and The precipitated material was collected during the evaporation and concentration process. The calcium sulfate crystals can be recovered from the NF membrane concentrated liquid by solid-liquid separation. In this case, the third step in It is preferable to provide a crystal recovery step in which crystals of at least one of sodium chloride, potassium chloride and sodium sulfate are precipitated and recovered from the filtrate obtained after recovering alkaline earth metal carbonate crystals and calcium sulfate crystals from the NF membrane concentrate.

[0011] The crystal recovery step preferably comprises a concentration and crystallization step in which the filtrate is evaporated and concentrated to precipitate and recover sodium chloride crystals. The crystal recovery step preferably comprises a cooling and crystallization step in which the filtrate is cooled and crystallized to recover the precipitated crystals. The cooling and crystallization step preferably comprises a first cooling and crystallization step in which potassium chloride crystals are recovered by cooling and crystallizing the filtrate, and a second cooling and crystallization step in which sodium sulfate crystals are recovered by cooling and crystallizing the filtrate that has been subjected to the first cooling and crystallization step at a temperature lower than the cooling and crystallization temperature of the first cooling and crystallization step. The crystal recovery step in It is preferable to include a step of combining a part of the filtrate after the crystal recovery with the NF membrane concentrate to be subjected to the second step, and neutralizing the remainder of the filtrate after the crystal recovery.

[0012] The third step in The crystal recovery step is preferably carried out by combining the washing water used to wash the recovered alkaline earth metal carbonate crystals and calcium sulfate crystals with the filtrate, and the washing water preferably contains distilled water produced in the evaporation and concentration step.

[0013] The first step in The method may include an evaporation and concentration step in which calcium sulfate crystals are precipitated by evaporating and concentrating the NF membrane concentrate that has been concentrated without passing through the nanofiltration membrane, and a solid-liquid separation step in which the calcium sulfate crystals precipitated in the evaporation and concentration step are recovered from the NF membrane concentrate by solid-liquid separation. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a method for immobilizing carbon dioxide on alkaline earth metals, which enhances the carbon dioxide reduction capacity while taking into account the amount of carbon dioxide emitted. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a process flow diagram illustrating a method for fixation of carbon dioxide according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a diagram showing an example of changes in the amounts of various ions in the processing flow shown in FIG. [Figure 3] FIG. 2 is a diagram showing a modification of some steps in the processing flow shown in FIG. [Figure 4] FIG. 1 is a process flow diagram illustrating a method for fixation of carbon dioxide according to a second embodiment of the present invention. [Figure 5] FIG. 2 is a diagram showing a modification of the processing flow shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0016] The carbon dioxide fixation method of the present invention provides a method for fixating carbon dioxide to alkaline earth metals contained in seawater or brine. In the present invention, "alkaline earth metal" refers to a broad range of elements 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 that the alkaline earth metal contains at least Mg, from the viewpoints of ease of reaction with CO and the fact that the carbonate obtained by the reaction can be expected to be used in various applications.

[0017] "Seawater or brine" also contains magnesium ions (Mg 2+ ), calcium ions (Ca 2+ Seawater or brine is an aqueous solution containing ions of alkaline earth metals such as calcium sulfate, sodium chloride, potassium chloride, and sodium sulfate. In addition to alkaline earth metal ions, seawater or brine usually contains at least one ion that forms a crystal selected from calcium sulfate, sodium chloride, potassium chloride, and sodium sulfate. Specifically, seawater or brine contains chloride ions (Cl- ), sulfate ions (SO4 2- ), sodium ions (Na + ), potassium (K + ) usually contains at least one ion selected from the group consisting of:

[0018] The seawater or brine can be obtained from at least one selected from seawater, salt lakes, and industrial wastewater. In addition to seawater, salt lakes, and industrial wastewater, river water, rainwater, treated sewage water, and produced water from oil and gas fields can also be used, as long as they contain alkaline earth metals. More specific examples of brine include waste brine discharged from water production processes using salt lakes, desalination, and salt production processes, recovery of valuable materials using seawater and salt lakes, and industrial wastewater from chemical plants.

[0019] From the viewpoints of being rich in Mg as described above, reducing the environmental load, and facilitating the reduction of CO2 emissions, the brine is preferably at least one selected from the group consisting of brine obtained from a freshwater production system using seawater, brine obtained from a process for producing salt from seawater, and brine obtained from a process for recovering lithium from a salt lake.

[0020] First Embodiment An embodiment of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a process flow diagram illustrating a carbon dioxide fixation method according to a first embodiment of the present invention. In the first embodiment, seawater is treated, but similar treatment can be performed with brine. As shown in FIG. 1, the carbon dioxide fixation method according to the first embodiment includes a first step S1 in which seawater is passed through a nanofiltration membrane (NF membrane) to produce an NF membrane concentrate that is concentrated without passing through the NF membrane; a second step S2 in which an alkali is added to the NF membrane concentrate produced in the first step S1 to react carbon dioxide with the alkaline earth metal contained in the NF membrane concentrate to immobilize it, thereby precipitating alkaline earth metal carbonate crystals; and a third step S3 in which the alkaline earth metal carbonate crystals precipitated in the second step S2 are recovered from the NF membrane concentrate by solid-liquid separation.

[0021] <S1: Step 1> In Step 1 (S1), seawater is supplied to the NF membrane unit by a medium-pressure pump or the like and passed through the NF membrane to generate NF membrane permeate that has permeated through the NF membrane and NF membrane concentrate that has been concentrated without permeating through the NF membrane.

[0022] Since the NF membrane has the property of suppressing the permeation of ions with a valence of 2 or more while allowing the permeation of monovalent ions, a large amount of alkaline earth metals that are the target of carbon dioxide immobilization remains in the NF membrane concentrate, and there is a risk of interfering with this immobilization, such as Na + , K + and so on are reduced in concentration. Therefore, carbon dioxide can be easily and efficiently immobilized on the alkaline earth metals contained in seawater (or brackish water), and the generation of carbon dioxide throughout the process including subsequent steps can be suppressed. Fig. 2 shows an example of the amounts (mg / h) of various ions contained in seawater, NF membrane permeate, and NF membrane concentrate when seawater is supplied at a flow rate of 100 m 3 / h.

[0023] <S11: Salt production step> On the other hand, since the NF membrane permeate contains a large amount of monovalent ions such as Na + and Cl - , Step 1 (S1) includes a salt production step (S11) of concentrating the NF membrane permeate to recover precipitated sodium chloride (NaCl) crystals. The salt production step (S11) of the present embodiment includes a membrane treatment step (S111) of supplying the NF membrane permeate to a reverse osmosis membrane (RO membrane) unit by a high-pressure pump or the like and passing it through the RO membrane to generate a membrane treatment concentrate that has been concentrated without permeating through the RO membrane, and a crystallization step (S112) of supplying the generated membrane treatment concentrate to a crystallization tank, heating and evaporating it to precipitate NaCl crystals. The vapor discharged from the crystallization tank is condensed by a condenser or the like to become distilled water, which is merged with the membrane treatment permeate that has permeated through the RO membrane and used as produced water or the like. A part of the crystallization tank concentrate concentrated in the crystallization tank is discharged from the crystallization tank as a slurry liquid containing NaCl crystals, and dehydrated by a centrifuge or the like to recover NaCl crystals. The NF membrane permeate contains SO4 2-Since it contains almost none, it can be concentrated at a high concentration by the low-energy membrane treatment step S111.

[0024] The membrane treatment step S111 is not limited to the treatment using an RO membrane, and may be other treatments using a semipermeable membrane, or may be a combination of multiple membrane treatments. For example, as shown in FIG. 3, the membrane treatment step S111 includes an RO membrane concentration step S113 in which the NF membrane permeate is concentrated by an RO membrane to generate an RO membrane concentrate, and the RO membrane concentrate is supplied to the high-pressure chamber of a semipermeable membrane unit separated by a semipermeable membrane, and the RO membrane concentrate is further concentrated by utilizing the pressure difference with the recovered liquid passing through the low-pressure chamber. The recovered liquid supplied to the low-pressure chamber can utilize a part of the RO membrane concentrate that has passed through the high-pressure chamber, and the recovered liquid that has passed through the low-pressure chamber can be merged with the NF membrane permeate before the RO membrane concentration step S113. Further, as shown in FIG. 3, an evaporation treatment step S115 for evaporating and concentrating the membrane treatment concentrate generated in the membrane treatment step S111 by a horizontal tube evaporator or the like may be provided between the membrane treatment step S111 and the crystallization step S112.

[0025] <S12: Confluence step> As shown in FIG. 1, the first step S1 includes a confluence step S12 in which the filtrate after recovering NaCl crystals in the salt production step S11 is merged with the above NF membrane concentrate, thereby suppressing the discharge of waste liquid outside the system and reducing the environmental burden. Since alkaline earth metals to be recovered such as magnesium are contained not only in the NF membrane concentrate but also in the NF membrane permeate, by providing the above confluence step S12, the recovery rate of alkaline earth metals required for carbon dioxide immobilization in the second step S2 can be increased. From the viewpoint of increasing the purity of the NaCl crystals obtained in the crystallization step S112, it is preferable to make the amount of filtrate merged with the NF membrane concentrate by the confluence step S12 as large as possible.

[0026] <S13: Electrodialysis step> The first step S1 includes an electrodialysis step S13 of electrodialyzing the NaCl crystals produced in the salt production step S11. The electrodialysis step S13 can use, for example, a bipolar membrane electrodialysis device to separate the solution obtained by dissolving the NaCl crystals obtained in the crystallization step S112 in water into HCl solution and NaOH solution. Electrodialysis preferably utilizes renewable energy such as solar energy and can suppress CO2 emissions throughout the process. In order to increase the purity of the NaCl solution for which electrodialysis is performed, it is preferable to adsorb impurities such as magnesium and calcium contained in the NaCl solution onto a chelating resin to sufficiently reduce their concentrations (for example, 1 ppm or less).

[0027] The NaOH solution obtained in the electrodialysis step S13 can be suitably used as the alkali added in the second step S2. In order for the amount of the alkali solution generated in the electrodialysis step S13 to be only the required amount in the second step S2, the flow path of the NF membrane permeate generated in the first step S1 may be branched, and the salt production step S11 may be performed only on a part of the NF membrane permeate. Thereby, it is possible to suppress the energy consumption required for the generation of the alkali and reduce the CO2 emissions throughout the process. Note that it is preferable to utilize renewable energy for the generation of the alkali as described above. The remainder of the NF membrane permeate not used in the salt production step S11 can be used in another process such as a freshwater production process, and the NF membrane permeate not used in another process may be discharged into the ocean or the like.

[0028] In order for the amount of the alkali solution generated in the electrodialysis step S13 to be only the required amount in the second step S2, in the first step S1, a part or all of the NF membrane permeate with a suppressed production amount may be bypassed without passing a part of seawater or brackish water through the NF membrane, and the salt production step S11 may be performed thereon. The seawater or brackish water bypassing the NF membrane can be used for carbon dioxide immobilization in the second step S2 after merging into the NF membrane concentrate.

[0029] <S2: Second Step> In the second step S2, by adding an alkali to the NF membrane concentrate produced in the first step S1, the pH of the NF membrane concentrate is adjusted to the alkaline side (for example, pH 9 - 10) and stored in a storage tank. Then, a gas containing carbon dioxide is blown into this NF membrane concentrate to cause gas-liquid contact by bubbling, so that the alkaline earth metals contained in the NF membrane concentrate react with carbon dioxide and are immobilized. The gas containing carbon dioxide may be air, or may be exhaust gas from various combustion devices, etc. Also, there is no limit to the carbon dioxide concentration contained in the gas. For example, the carbon dioxide concentration contained in the gas is about atmospheric to 100% by volume. In order to maintain the pH during the reaction of the alkaline earth metal and carbon dioxide, the addition of the 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, by blowing fine bubbles of carbon dioxide (fine bubbles such as microbubbles and ultrafine bubbles), the reaction efficiency between the alkaline earth metal and carbon dioxide can be improved.

[0030] The method of causing gas-liquid contact between the NF membrane concentrate and carbon dioxide, in addition to the method of blowing CO2 gas into the NF membrane concentrate, may also be a method of spraying the NF membrane concentrate in the CO2 gas with a spray nozzle or tray in a single-stage or multi-stage desulfurization tower, degassing tower, etc. Considering the reaction rate, reaction amount, CO2 concentration in the gas such as exhaust gas, etc., various known gas-liquid contact devices can be used.

[0031] It is preferable to use the NaOH solution obtained in the electrodialysis step S13 as the alkali added to the NF membrane concentrate in the second step S2, which can suppress the increase in CO2 emissions associated with separately generating the alkali. However, since the concentration of Na + , K + etc. in the NF membrane concentrate is reduced compared to the original solution, and it is easy to immobilize carbon dioxide, an alkali different from the alkali obtained in the electrodialysis step S13 may be used, or the alkali obtained in the electrodialysis step S13 and another alkali may be used in combination.

[0032] <S3: Third Step> The NF membrane concentrate after the second step S2 becomes a slurry liquid in which alkaline earth metal carbonate crystals such as MgCO3 and CaCO3 are precipitated by the reaction of an alkaline earth metal and carbon dioxide. In the third step S3, the alkaline earth metal carbonate crystals contained in this slurry liquid are separated and recovered by a solid-liquid separation device such as a centrifuge.

[0033] <S31: Neutralization step> The third step S3 includes a neutralization step S31 of neutralizing the filtrate after recovering the alkaline earth metal carbonate crystals from the NF membrane concentrate. Since the filtrate of the NF membrane concentrate usually has a pH of 9 or more by adding an alkali, it becomes possible to discharge it directly outside the system such as the ocean by adding an acid for neutralization (for example, pH 7 to 8). It is preferable to use the HCl solution obtained in the electrodialysis step S13 as the acid to be added to the filtrate, and it is possible to suppress an increase in the amount of CO2 emissions associated with separately generating the acid.

[0034] <S32: Washing step> In the third step S3, the recovered alkaline earth metal carbonate crystals are washed with washing water to dissolve and remove Na + , K + etc. in the washing water. The washing water used in the washing step S32 preferably contains the distilled water obtained in the above crystallization step S12, and thereby it is possible to suppress an increase in the amount of CO2 emissions associated with separately generating the washing water. In this embodiment, a part of the produced water obtained by merging distilled water into the NF membrane permeate is used as the washing water.

[0035] The washed alkaline earth metal carbonate crystals such as MgCO3 and CaCO3 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 carbonate salt using a method for fixing carbon dioxide.

[0036] The carbon dioxide fixation method of the present embodiment can easily and efficiently fix carbon dioxide to the alkaline earth metals contained in seawater or brackish water, and can complete the fixation of carbon dioxide within the system using alkalis, acids, distilled water, etc. generated in each process. Therefore, the carbon dioxide reduction ability can be enhanced while considering the carbon dioxide emissions throughout the process.

[0037] <Second Embodiment> FIG. 4 is a process flow diagram for explaining the carbon dioxide fixation method according to the second embodiment of the present invention. The carbon dioxide fixation method of the second embodiment shown in FIG. 4, similar to the first embodiment, includes a first step S1 of generating a NF membrane concentrate that is concentrated without passing through the NF membrane by passing seawater through the NF membrane, and a second step S2 of adding an alkali to the NF membrane concentrate generated in the first step S1 to react and fix carbon dioxide to the alkaline earth metals contained in the NF membrane concentrate, and precipitating alkaline earth metal carbonate crystals. It also includes a third step S3 of solid-liquid separation and recovery of the alkaline earth metal carbonate crystals precipitated in the second step S2 from the NF membrane concentrate. Also in the second embodiment, although the treatment target is seawater, the same treatment can be performed in the case of brackish water, and it can be preferably used particularly when the contents of Ca and Mg are high. In FIG. 4, the same steps as those in FIG. 1 are denoted by the same reference numerals, and detailed descriptions are omitted.

[0038] <S21: Evaporation Concentration Step> As shown in FIG. 4, the second step S2 includes an evaporation concentration step S21 of precipitating calcium sulfate crystals (CaSO4·2H2O) by evaporating and concentrating the slurry liquid in which carbon dioxide is fixed to the alkaline earth metals contained in the NF membrane concentrate and alkaline earth metal carbonate crystals are precipitated in an evaporation pan. The slurry liquid before evaporation concentration contains Ca 2+ , Na + , K + etc. However, since calcium sulfate crystals have an inverse solubility in which the solubility decreases as the temperature rises, while Ca 2+ precipitates, Na + , K +While maintaining the operating temperature of evaporation concentration so that precipitation does not occur, concentrate to a concentration multiple at which NaCl and KCl do not precipitate. The operating temperature of evaporation concentration is preferably 70 to 90 °C, and is set to, for example, 80 °C.

[0039] In the evaporation concentration step S21, in order to prevent the generation of calcium sulfate scale, it is preferable to add seed crystals of CaSO4·2H2O to the slurry liquid to promote crystal growth with the seed crystals as nuclei. It is preferable to preferably use CaSO4·2H2O generated in the evaporation concentration step S21 for these seed crystals.

[0040] <S3: Third step> In the third step S3, alkaline earth metal carbonate crystals such as MgCO3 and CaCO3 and calcium sulfate crystals precipitated in the second step S2 are separated from the NF membrane concentrate by a solid-liquid separator and recovered. The recovered alkaline earth metal carbonate crystals and calcium sulfate crystals are washed with washing water in a washing step S32 as in the first embodiment.

[0041] In the 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. In addition to the distilled water generated in the evaporation concentration step S21, the washing water may use the production water generated in the salt production step S11 or the distilled water generated in the concentration crystallization step S331 described later.

[0042] <S33: Crystal recovery step> The third step S3 includes a crystal recovery step S33 in which crystals of sodium chloride, potassium chloride, and sodium sulfate are precipitated and recovered from the filtrate after recovering alkaline earth metal carbonate crystals and calcium sulfate crystals from the NF membrane concentrate. The filtrate in which the crystal recovery step S33 is performed is preferably only a part of the filtrate obtained by solid-liquid separation of alkaline earth metal carbonate crystals and calcium sulfate crystals, and the remainder of the filtrate obtained by solid-liquid separation is preferably discharged out of the system through the neutralization step S31 in order to suppress an increase in the impurity concentration.

[0043] The filtrate in which the crystal recovery step S33 is performed may be combined with the washing water obtained by washing the alkaline earth metal carbonate crystals and calcium sulfate crystals in the washing step S32. The crystal recovery step S33 includes a concentration crystallization step S331 in which sodium chloride crystals are precipitated and recovered by evaporating and concentrating the filtrate, and a cooling crystallization step S332 in which the crystals precipitated by cooling crystallization of the filtrate are recovered.

[0044] <S331: Concentration Crystallization Step> The concentration crystallization step S331 is performed by supplying the filtrate to an evaporator and heating it to evaporate and concentrate it, precipitating crystals mainly composed of sodium chloride (NaCl), and then separating the sodium chloride crystals by a solid-liquid separation device. The operating temperature for evaporation and concentration is preferably 60 to 80°C, and is set to, for example, 70°C.

[0045] <S332: Cooling Crystallization Step> The cooling crystallization step S332 is performed by supplying the filtrate that has passed through the concentration crystallization step S331 to a cooling crystallization tank, cooling it to a predetermined cooling crystallization temperature while stirring to precipitate crystals of the target impurities, and then separating these crystals by a solid-liquid separation device. The cooling crystallization step S332 includes a first cooling crystallization step S3321 in which potassium chloride (KCl)-based crystals are recovered by cooling and crystallizing the NF membrane concentrate, and a second cooling crystallization step S3322 in which sodium sulfate crystals (Na2SO4·10H2O) precipitated by cooling and crystallizing the filtrate that has passed through the first cooling crystallization step S3321 at a temperature lower than the cooling crystallization temperature of the first cooling crystallization step S3321 are recovered. The cooling crystallization temperature of the first cooling crystallization step S3321 is a temperature at which KCl-based crystals precipitate while Na2SO4·10H2O crystals do not precipitate, and is preferably 33 to 40°C, and is set to, for example, 36°C. Also, the cooling crystallization temperature of the second cooling crystallization step S3322 is a temperature at which Na2SO4·IOH2O crystals precipitate, and is set to, for example, 0 to 10°C.

[0046] The filtrate obtained through the crystal recovery step S33 is alkaline and can be partially merged with the NF membrane concentrate in which the second step S2 is performed. Also, the remainder of the filtrate obtained 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 outside the system.

[0047] The crystal recovery step S33 does not necessarily need to perform all of the concentration crystallization step S331, the first cooling crystallization step S3321, and the second cooling crystallization step S3322. Depending on the components of the seawater or brine to be treated, only the necessary steps may be appropriately selected as long as at least one crystal selected from sodium chloride, potassium chloride, and sodium sulfate can be precipitated and recovered from the filtrate.

[0048] <First Modified Example> FIG. 5 is a diagram showing a modified example of the treatment flow shown in FIG. 1. The carbon dioxide immobilization method shown in FIG. 5 is such that the first step S1 of the carbon dioxide immobilization method shown in FIG. 1 further includes an evaporation concentration step S14 and a solid-liquid separation step S15, and the other steps are the same as the carbon dioxide immobilization method shown in FIG. 1.

[0049] <S14: Evaporation Concentration Step> In the evaporation concentration step S14, before reacting carbon dioxide with an alkaline earth metal for immobilization in the second step S2, calcium sulfate crystals are precipitated by evaporating and concentrating the NF membrane concentrate in an evaporation tank. The evaporation concentration step S14 may be performed on the NF membrane concentrate before the merging step S12. However, since the filtrate that merges into the NF membrane concentrate in the merging step S12 contains some calcium, as shown in FIG. 5, it is preferable to perform the evaporation concentration step S14 on the NF membrane concentrate that has passed through the merging step S12. For scale inhibition in the evaporation tank, it is preferable to add calcium sulfate crystals as seed crystals to the NF membrane concentrate. As such seed crystals, for example, the calcium sulfate crystals recovered in the solid-liquid separation step S15 described later can be used. As a method for scale inhibition in the evaporation tank, it is also preferable to add an acid to the NF membrane concentrate to adjust the pH. As such an acid, for example, the HCl solution obtained in the electrodialysis step S13 described above can be used. The distilled water generated 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.

[0050] <S15: Solid-Liquid Separation Step> In the solid-liquid separation step S15, the calcium sulfate crystals precipitated in the evaporation concentration step S14 are separated from the NF membrane concentrate by a solid-liquid separation device and recovered. The recovered calcium sulfate crystals can be used, for example, as gypsum.

[0051] The inclusion of the evaporation concentration step S14 and the solid-liquid separation step S15 provides the following advantages in the second step S2. First, the amount of NF membrane concentrated solution subjected to the second step S2 can be reduced, thereby reducing the amount of alkali (NaOH) added. This reduces power consumption in the electrodialysis step S13 and allows for the compactness of reaction tanks, such as storage tanks, where gas-liquid contact takes place. Furthermore, the increase in the concentration of alkaline earth metals in the NF membrane concentrated solution subjected to the second step S2 increases the reaction efficiency of carbon dioxide, thereby reducing the amount of carbon dioxide bubbling and reducing the power consumption required for bubbling. Furthermore, calcium is recovered from the NF membrane concentrated solution as calcium sulfate crystals before the second step S2, thereby suppressing the precipitation of CaCO3 in the second step S2 and increasing the purity of the recovered MgCO3.

[0052] <Second Modification> The evaporation concentration step S14 and solid-liquid separation step S15 shown in Fig. 5 can also be applied to the carbon dioxide fixation method shown in Fig. 4, and by performing the evaporation concentration step S14 and solid-liquid separation step S15 before the second step S2 shown in Fig. 4, the same effects as those described above can be achieved. In this case, the evaporation concentration step S21 shown in Fig. 4 is unnecessary, and the NF membrane concentrated liquid that has undergone the second step S2 is subjected to solid-liquid separation in the third step S3. By including the evaporation concentration step S14 and solid-liquid separation step S15 in the carbon dioxide fixation method shown in Fig. 4, the NF membrane concentrated liquid that has undergone the second step S2 contains almost no CaSO4 or CaCO3, and therefore high-purity MgCO3 can be recovered in the third step S3. [Explanation of symbols]

[0053] S1 1st process S11 Salt production process S111 Membrane treatment process S112 Crystallization process S12 Merging process S13 Electrodialysis process S14 Evaporation and concentration process S15 Solid-liquid separation process S2 2nd process S21 Evaporation and concentration process S3 3rd process S31 Neutralization process S32 Water washing process S33 Crystal recovery process S331 Concentration crystallization process S332 Cooling crystallization process S3321 First cooling crystallization process S3322 Second cooling crystallization process S34 Neutralization process

Claims

1. A first step of passing seawater or brackish water through a nanofiltration membrane to produce an NF membrane concentrate that is concentrated without permeating the nanofiltration membrane; a second step of adding an alkali to the NF membrane concentrate produced in the first step, reacting carbon dioxide with the alkaline earth metal contained in the NF membrane concentrate to immobilize it, and precipitating alkaline earth metal carbonate crystals; and a third step of recovering the alkaline earth metal carbonate crystals precipitated in the second step from the NF membrane concentrated liquid by solid-liquid separation.

2. a salt production step of concentrating the NF membrane permeate that has permeated the nanofiltration membrane in the first step to recover precipitated sodium chloride crystals; and an electrodialysis step of electrodialyzing the solution of sodium chloride crystals recovered in the salt production step to separate an acid solution from an alkaline solution, 2. The method for fixation of carbon dioxide according to claim 1, wherein the second step comprises adding the alkaline solution obtained in the electrodialysis step to the NF membrane concentrate.

3. 3. The method for fixation of carbon dioxide according to claim 2, wherein the salt production step is performed on only a portion of the NF membrane permeate produced in the first step, so that only the amount of alkaline solution produced in the electrodialysis step is required in the second step.

4. 4. The method for fixation of carbon dioxide according to claim 2 or 3, wherein the salt production step is carried out by bypassing a portion of the seawater or brine without passing it through the nanofiltration membrane in the first step, so that only the amount of alkaline solution produced in the electrodialysis step is required in the second step.

5. 5. The method for fixation of carbon dioxide according to claim 2, further comprising a neutralization step of neutralizing a filtrate obtained after recovering alkaline earth metal carbonate crystals from the NF membrane concentrate in the third step with the acid solution obtained in the electrodialysis step.

6. The salt production process includes a membrane treatment step in which the NF membrane permeate liquid is passed through a reverse osmosis membrane to produce a membrane-treated concentrate that is concentrated without passing through the reverse osmosis membrane, and a crystallization step in which the membrane-treated concentrate produced in the membrane treatment step is heated and evaporated to precipitate sodium chloride crystals, 6. The method for fixation of carbon dioxide according to claim 2, further comprising a water washing step of washing the alkaline earth metal carbonate crystals recovered in the third step with wash water containing distilled water obtained in the crystallization step.

7. an evaporation and concentration step of evaporating and concentrating the NF membrane concentrated liquid from which alkaline earth metal carbonate crystals have been precipitated in the second step to precipitate calcium sulfate crystals, 3. The method for fixation of carbon dioxide according to claim 1 or 2, wherein the third step comprises recovering the alkaline earth metal carbonate crystals precipitated in the second step and the calcium sulfate crystals precipitated in the evaporation and concentration step from the NF membrane concentrated solution by solid-liquid separation.

8. 8. The method for fixation of carbon dioxide according to claim 7, further comprising a crystal recovery step of precipitating and recovering crystals of at least one of sodium chloride, potassium chloride and sodium sulfate from the filtrate remaining after recovering alkaline earth metal carbonate crystals and calcium sulfate crystals from the NF membrane concentrate in the third step.

9. 9. The method for fixation of carbon dioxide according to claim 8, wherein the crystal recovery step includes a concentration and crystallization step of evaporating and concentrating the filtrate to precipitate and recover sodium chloride crystals.

10. 10. The method for fixation of carbon dioxide according to claim 8, wherein the crystal recovery step comprises a cooling and crystallization step of cooling and crystallizing the filtrate to recover the precipitated crystals.

11. 11. The method for fixation of carbon dioxide according to claim 10, wherein the cooling and crystallization step comprises: a first cooling and crystallization step of cooling and crystallizing the filtrate to recover precipitated potassium chloride crystals; and a second cooling and crystallization step of cooling and crystallizing the filtrate that has been subjected to the first cooling and crystallization step at a temperature lower than the cooling and crystallization temperature of the first cooling and crystallization step to recover precipitated sodium sulfate crystals.

12. 12. The method for fixation of carbon dioxide according to claim 8, further comprising a step of combining a part of the filtrate after the crystal recovery step with the NF membrane concentrate to be subjected to the second step, and neutralizing the remainder of the filtrate after the crystal recovery step.

13. 13. The method for fixation of carbon dioxide according to any one of claims 8 to 12, wherein the crystal recovery step is carried out by combining the wash water used to wash the alkaline earth metal carbonate crystals and calcium sulfate crystals recovered in the third step with the filtrate.

14. The method for fixation of carbon dioxide according to claim 13, wherein the washing water contains distilled water produced in the evaporation and concentration step.

15. the method comprises an evaporation and concentration step of evaporating and concentrating the NF membrane concentrate that has been concentrated in the first step without passing through the nanofiltration membrane to precipitate calcium sulfate crystals, and a solid-liquid separation step of recovering the calcium sulfate crystals precipitated in the evaporation and concentration step from the NF membrane concentrate by solid-liquid separation, 2. The method for fixation of carbon dioxide according to claim 1, wherein the second step comprises adding an alkali to the NF membrane concentrated liquid that has been subjected to the solid-liquid separation step.

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