Carbon dioxide removal method

Abstract

To improve the generation efficiency of magnesium hydroxide.SOLUTION: In a recovery method for recovering magnesium by electrolyzing a water solution at least containing magnesium ions to produce and recover magnesium hydroxide that is insoluble in water, an electrolytic cell having an anode and a cathode is partitioned by a cation exchange membrane into an anode-side cell and a cathode-side cell, and the water solution containing magnesium ions is introduced into the cathode-side cell to electrolyze the solution while restricting the movement of anions from the cathode-side cell to the anode-side cell.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention provides Carbon dioxide removal method for removing carbon dioxide contained in seawater Regarding. [Background technology]

[0002] Conventionally, there has been a magnesium recovery method and magnesium recovery device in which an electrolysis treatment vessel having an inlet, an outlet, an anode and a cathode on the upper and lower wall surfaces, and a partition wall and a diaphragm separating the upper and lower sides, is immersed in the sea to perform electrolysis (electrolysis), and magnesium is recovered by precipitating it as magnesium hydroxide in the cathode electrolyzed water (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-57230 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in Patent Document 1, hydroxide ions, which are anions generated in the cathodic electrolyzed water by electrolysis, can pass through the diaphragm and migrate to the anodic electrolyzed water, which causes a problem of low efficiency in generating magnesium hydroxide.

[0005] The present invention has been made in view of these problems, and aims to provide a magnesium recovery method and magnesium recovery apparatus that can improve the efficiency of producing magnesium hydroxide. [Means for solving the problem]

[0006] In order to solve the above problems, the carbon dioxide removal method according to claim 1 of the present invention comprises: A carbon dioxide removal method for removing carbon dioxide contained in seawater by electrolyzing the seawater, comprising: an electrolytic cell having an anode and a cathode is partitioned by a cation exchange membrane into an anode-side cell having the anode and a cathode-side cell having the cathode; The seawater is introduced into the cathode-side tank, and electrolysis is carried out in a state where the movement of anions from the cathode-side tank to the anode-side tank is restricted. It is characterized by the following. According to this feature, a carbon dioxide removal method for removing carbon dioxide contained in seawater can be provided.

[0008] Claims of the invention 2 Described in Carbon dioxide removal The method is as claimed 1 Described in Carbon dioxide removal 1. A method comprising: The anode side tank contains aqueous solution Lead Enter R It is characterized by the following. According to this feature, it is possible to prevent harmful chlorine gas from being generated on the anode side due to electrolysis. do.

[0009] A carbon dioxide removal method according to claim 3 of the present invention is the carbon dioxide removal method according to claim 2, The aqueous solution containing no chloride ions is an aqueous sodium sulfate solution. It is characterized by the following. According to this feature, by using sodium sulfate, which is easily available and inexpensive, it is possible to suppress an increase in the cost of recovering magnesium due to the use of an aqueous solution that does not contain chloride ions. A carbon dioxide removal method according to a fourth aspect of the present invention is the carbon dioxide removal method according to the first aspect, wherein the electrolysis is carried out at a volumetric electricity quantity of 3600 C / L or less. It is characterized by the following. According to this feature, almost all of the carbon dioxide contained in seawater as bicarbonate ions can be precipitated as calcium carbonate.

[0017] Furthermore, the present invention may have only the invention-specific matters set forth in the claims of the present invention, or may have the invention-specific matters set forth in the claims of the present invention as well as configurations other than the invention-specific matters. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a configuration diagram of a magnesium recovery device according to a first embodiment of the present invention. [Figure 2] 1 is a diagram showing the configuration of an electrolytic cell in a magnesium recovery device according to a first embodiment of the present invention. FIG. [Figure 3] FIG. 2 is a diagram showing flow paths used in the electrolysis stack 2 according to the first embodiment of the present invention. [Figure 4] 3 is a table showing test conditions in the first embodiment of the present invention. [Figure 5] 1 is a graph showing the relationship between current and magnesium ion concentration. [Figure 6] 1 is a graph showing the relationship between current and calcium ion concentration. [Figure 7] 1 is a graph showing the relationship between the concentration of magnesium ions and the quantity of electricity per unit volume. [Figure 8] 1 is a graph showing the relationship between calcium ion concentration and unit volume electricity quantity. [Figure 9] 10 is a table showing measured concentrations of magnesium ions and calcium ions at each flow rate and each unit volume of electricity. [Figure 10] 1 is a graph showing changes in pH value in the anode side chamber 20a and the cathode side chamber 20b. [Figure 11] (a) is a mapping SEM image of a precipitate with a unit volume electrical charge of 3600 C / L, and (b) is a mapping SEM image of a precipitate with a unit volume electrical charge of 14400 C / L. [Figure 12] (a) shows the EDS analysis results of a sediment with a unit volume electrical charge of 3600 C / L, and (b) shows the EDS analysis results of a sediment with a unit volume electrical charge of 14400 C / L. [Figure 13]10 is a graph showing the relationship between magnesium ions and the unit volume electricity quantity for each deaerated seawater and undegassed seawater. [Figure 14] 10 is a graph showing the relationship between calcium ions and the unit volume electricity quantity for each deaerated seawater and non-deaerated seawater. [Figure 15] 1 is a table showing the measured values ​​of magnesium ions and calcium ions at each unit volume of electricity during electrolysis of each degassed seawater. [Figure 16] (a) is a mapping SEM image of sediment from seawater that has been degassed by boiling, (b) is a mapping SEM image of sediment from seawater that has been degassed by adding acid, and (c) is a mapping SEM image of sediment from non-degassed seawater. [Figure 17] (a) shows the EDS analysis results of sediments from seawater that had been degassed by boiling, (b) shows the EDS analysis results of sediments from seawater that had been degassed by adding acid, and (c) shows the EDS analysis results of sediments from non-degassed seawater. [Figure 18] FIG. 10 is a configuration diagram of a magnesium recovery device according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] Examples of the present invention are described below.

[0020] FIG. 1 is a diagram showing the configuration of an example of a magnesium recovery apparatus to which the magnesium recovery method of the present invention is applied.

[0021] As shown in FIG. 1 , the magnesium recovery apparatus of the first embodiment is mainly composed of an electrolysis stack 2, which is an electrolysis cell (electrolytic cell) for performing electrolysis; a DC power supply unit (power supply means) 1 for supplying power for electrolysis (electrolysis power) to an anode 2 a and a cathode 2 c provided in the electrolysis stack 2; a seawater tank 7 and a sodium sulfate aqueous solution tank 8 for storing seawater (deep ocean water) and a 5% by weight aqueous sodium sulfate solution to be supplied to the electrolysis stack 2; a cathode solution tank 10 and an anode solution tank 9 for storing treated seawater and aqueous sodium sulfate solution electrolyzed in the electrolysis stack 2; and a filtration device 16 for filtering the treated seawater stored in the cathode solution tank 10 and recovering magnesium hydroxide, which is magnesium hydroxide contained in the treated seawater without settling. The magnesium recovery apparatus of the first embodiment is capable of performing electrolysis continuously.

[0022] The lower part of the electrolysis stack 2 is connected to the sodium sulfate aqueous solution tank 8 and the seawater tank 7 by an inlet pipe 13a and an inlet pipe 13c, and the sodium sulfate aqueous solution and seawater stored in the sodium sulfate aqueous solution tank 8 and the seawater tank 7 are introduced into the electrolysis stack 2 from the lower part of the electrolysis stack 2 and discharged from the upper part of the electrolysis stack 2.

[0023] As shown in FIG. 1 , variable flow rate pumps 14a and 14c are provided on the inlet pipes 13a and 13c, respectively. The flow rates of the aqueous sodium sulfate solution and seawater supplied to the electrolysis stack 2 by the pumps 14a and 14c can be individually adjusted by inverter power supplies 6a and 6c for operating the pumps 14a and 14c. In the first embodiment, as described below, the variable flow rate pumps 14a and 14c are, for example, MG204XPD17-10S, MAGPON GEAR (trade name of Nikkiso Eiko Co., Ltd.). However, the present invention is not limited to this, and pumps with fixed flow rates may also be used. The inverter power supplies 6a and 6c may be selected appropriately depending on the pumps used.

[0024] 1, flow meters 3a and 3c are provided on the paths of the inlet pipes 13a and 13c to measure the flow rates of the aqueous sodium sulfate solution and seawater supplied to the electrolysis stack 2 via the inlet pipes 13c and 13a, and valves 4a and 4c are also provided. Note that, for example, a handy logger GL200A (trade name, manufactured by Graphtec Corporation) can be used as the flow meters 3a and 3c.

[0025] In this embodiment 1, as shown in FIG. 1, a spare seawater tank 11 capable of storing deep-sea water is connected to the seawater tank 7, and a degassing treatment device 12 for degassing carbon dioxide contained in the deep-sea water is provided between the seawater tank 7 and the spare seawater tank 11.

[0026] The degassing treatment device 12 of this embodiment 1 has two degassing treatment systems inside: a boiling degassing system that heats and boils deep-ocean water (seawater) to degas carbon dioxide, and an acid addition degassing system that adds acid to deep-ocean water (seawater) to degas carbon dioxide, and these can be switched between for use.

[0027] Meanwhile, the upper part of the electrolytic stack 2 is connected to the anode solution tank 9 and the cathode solution tank 10 by discharge pipes 15a and 15c, and the treated sodium sulfate aqueous solution and seawater that have been electrolyzed in the electrolytic stack 2 and discharged are stored in the anode solution tank 9 and the cathode solution tank 10.

[0028] As mentioned above, the catholyte solution tank 10 is connected to the filtration device 16, and the magnesium hydroxide contained in the treated seawater without settling in the catholyte solution tank 10 is filtered out.

[0029] Fig. 2 is a diagram showing the configuration of the electrolysis stack 2 of the present embodiment 1. As shown in Fig. 2, the electrolysis stack 2 of the present embodiment 1 is an electrolysis cell (electrolysis cell) that includes an anode 2a and a cathode 2c and is partitioned by an ion exchange membrane 2b provided between the anode 2a and the cathode 2c.

[0030] Specifically, a cation exchange membrane 2b is placed in the center of the electrolytic cell by pressing it with silicone rubber, thereby dividing the electrolytic cell into an anode-side cell 20a and a cathode-side cell 20b. In this embodiment, a CMB (product name manufactured by Astom Corporation) capable of exchanging sodium ions, which are monovalent cations, is used as the cation exchange membrane, but the present invention is not limited to this, and an appropriate membrane may be selected depending on the electrolyte, particularly the valence of the cations, dissolved in the aqueous solution introduced into the anode-side cell 20a.

[0031] Furthermore, the concentration of the aqueous sodium sulfate solution supplied from the aqueous sodium sulfate solution tank 8 to the anode-side tank 20a of the electrolysis stack 2 is preferably a concentration sufficient to prevent a neutralization reaction caused by hydrogen ions transported from the anode side to the cathode side. Specifically, as a result of studies conducted by measuring the pH value of the solution obtained at the outlet of the electrolysis stack 2, it is preferable to use an aqueous sodium sulfate solution with a high concentration of 5% by weight.

[0032] The anode 2a and cathode 2c provided in the electrolytic stack 2 are platinum-plated titanium plates, which are titanium plates plated with platinum, because they are not corroded by aqueous solutions during electrolysis, have high electrical conductivity, excellent mechanical strength, and are inexpensive. These platinum electrodes have high chemical stability, and platinum does not dissolve in aqueous solutions even during electrolysis. The thickness of the platinum-plated layer may be any appropriate thickness that does not leave missing portions (defects) in the plating layer. While the electrodes in this embodiment have a platinum layer formed on their surface by plating, the present invention is not limited to this. If cost and mechanical strength are not an issue, the electrodes themselves may be entirely platinum plates, or the electrodes may be made of a carbon material. In this case, the surface of the carbon material is preferably coated with a conductive material such as a nanoporous metal. In this embodiment, a platinum-plated titanium plate is used as the anode 2a provided in the electrolytic stack 2, but the present invention is not limited to this. If the ionization tendency of these anodes 2a is greater than that of silver (Ag), the electrodes will be oxidized and dissolved. Therefore, as long as the ionization tendency of the material is smaller than that of silver (Ag), the electrodes may be electrodes having a material other than platinum, such as gold (Au) or carbon (C), on at least the surface. Furthermore, since a reduction reaction occurs on the cathode side, any metal can be used for the cathode 2c provided in the electrolysis stack 2.

[0033] Here, the flow path used in the electrolysis stack 2 in the present embodiment 1 is shown in Fig. 3. The flow path was formed by hollowing out the center of a 5 mm thick plate-shaped silicone rubber, as shown in Fig. 3, with electrodes disposed on one side and a cation exchange membrane disposed on the other side.

[0034] Specifically, two silicone rubber plates with platinum-plated titanium arranged in the center of one side were prepared, and a cation exchange membrane 2b was sandwiched between the other sides of the two silicone rubber plates and pressure-bonded to form the electrolytic stack 2 shown in Figure 2.

[0035] Therefore, by placing an electrode on one side of the central part of the flow channel, the area where the electrode is placed becomes the area where a chemical reaction occurs due to electrolysis.The dimensions of the area where the chemical reaction occurs are 600 mm in length, 5 mm in depth, and 20 mm in width.

[0036] Although FIG. 3 illustrates the flow paths as extending horizontally, in the first embodiment, the flow paths are arranged so as to be substantially perpendicular to the ground, and each aqueous solution is supplied from below in the direction opposite to the force of gravity. This makes it possible to prevent clogging of the flow paths due to the production of magnesium hydroxide in the cathode-side tank 20b when the flow paths are arranged horizontally.

[0037] An electrolysis reaction occurs when an aqueous sodium sulfate solution is introduced into the anode-side chamber 20a of the silicone rubber electrolysis stack 2 formed as described above, and seawater is introduced into the cathode-side chamber 20b, while a current is passed between the anode 2a and the cathode 2c. Hydroxide ions generated at the cathode 2c by the electrolysis of water react with magnesium ions in the seawater, resulting in the deposition of magnesium hydroxide.

[0038] Furthermore, because the cation exchange membrane 2b is used in this embodiment, hydroxide ions, which are anions generated at the cathode 2c, cannot pass through the cation exchange membrane 2b and cannot migrate to the anode-side chamber 20a. This makes it possible to efficiently increase the pH value in the cathode-side chamber 20b with a small amount of power, thereby efficiently producing magnesium hydroxide. Similarly to hydroxide ions, chloride ions, which are anions contained in seawater introduced into the cathode-side chamber 20b, cannot pass through the cation exchange membrane 2b and cannot migrate to the anode-side chamber 20a. Therefore, electrolysis is performed without chloride ions in the anode-side chamber 20a. Therefore, chlorine gas is not generated at the anode 2a, and oxygen gas is generated at the anode 2a. Hydrogen gas is generated at the cathode 2c.

[0039] Next, magnesium recovery conditions were tested using the apparatus of the first embodiment described above. In this test, the flow rates of seawater and the sodium sulfate aqueous solution were set to the same value, and three flow rates of 20 ml / min, 40 ml / min, and 60 ml / min were selected, as shown in Figure 4. Furthermore, the test was conducted by varying the quantity of electricity per unit volume of seawater according to each flow rate. In this test, the quantity of electricity was set so as not to exceed the limiting current density, so that water dissociation did not occur in the boundary layer of the cation exchange membrane 2b. Details of the test conditions are shown in Figure 4. The quantities of electricity per unit volume of seawater (hereinafter referred to as "unit volume quantity of electricity") were selected as 3600 C (coulomb) / L (liter), 7200 C / L, 10800 C / L, and 14400 C / L.

[0040] In addition, the test was carried out using non-degassed seawater without operating the degassing device 16 so that the effect of the degassing device 16 would be clear in the comparison described below.

[0041] In the test, electrolyzed seawater containing precipitate was collected in a glass beaker from the outlet of electrolysis stack 2 after electrolysis reached a steady state. The beaker was sealed and left for 30 minutes until the chemical reaction in the electrolyzed seawater had fully progressed. The fully reacted solution was then filtered through filter paper to separate the precipitate from the aqueous solution. Water-soluble compounds in the seawater, such as sodium chloride, were rinsed three times with pure water to prevent them from adhering to the precipitate. The precipitate was then dried, and its surface condition and constituent elements were analyzed using a scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). The filtered aqueous solution was diluted with ultrapure water to an appropriate ratio for each ion in a dilution device. The diluted solution was then analyzed for the concentration of each element using inductively coupled plasma (ICP).

[0042] Figure 5 is a graph showing the relationship between current and magnesium ion concentration. As shown in Figure 5, the magnesium ion concentration decreases as the flow rate decreases. This is because, when the amount of electricity is the same, the amount of hydroxide ions generated at the cathode 2c is also the same, so when the flow rate is reduced, more hydroxide ions are contained per unit volume. On the other hand, the magnesium ion concentration decreases as the applied current increases. This is because, when the applied current is increased, the amount of hydroxide ions generated by the electrode reaction also increases, and the amount of magnesium ions that react with hydroxide ions to form magnesium hydroxide also increases.

[0043] Figure 6 is a graph showing the relationship between current and calcium ion concentration. The relationship between calcium concentration and flow rate, like that of magnesium ions, decreased with a decrease in flow rate. Furthermore, as shown in Figure 6, the calcium production reaction is divided into two stages. In the first stage, there is a slight decrease in calcium concentration simultaneously with the production of magnesium hydroxide. This is because, as electrolysis proceeds, the pH value of the seawater in the cathode tank 20b increases, and carbonate ions present in the seawater are converted to carbonate ions, which then react with calcium ions and precipitate as calcium carbonate. In the second stage, the magnesium ions are consumed by the reaction of magnesium ions with hydroxide ions, and the calcium concentration decreases. This is because, as the magnesium hydroxide reaction progresses, the magnesium ions in the seawater decrease proportionally, and the proportion of calcium ions increases. As a result, a chemical reaction between calcium ions and hydroxide ions begins, and calcium ions precipitate as calcium hydroxide, thereby decreasing the calcium ion concentration.

[0044] FIG. 9 is a table showing measured concentrations of magnesium ions and calcium ions at each flow rate and each unit volume of electricity, and graphs based on these measurements are shown in FIGS.

[0045] Figure 7 is a graph showing the relationship between the measured magnesium ion concentration and the unit volume charge. With the unit volume charge represented on the horizontal axis, the ion concentration remains consistent at the same unit volume charge regardless of the flow rate. This shows that the generation of hydroxide ions at the cathode 2c is proportional to the current, so the change in magnesium ion concentration depends on the volume charge. On the other hand, in the case of undegassed seawater, it can be seen that the magnesium ions in the seawater can be almost completely recovered at a unit volume charge between 10,000 C / L and 14,400 C / L, more specifically, at approximately 12,000 C / L, calculated from the slope of the graph.

[0046] Figure 8 is a graph showing the relationship between the measured calcium ion concentration and the unit volume charge. Unlike magnesium ions, calcium ions have a small unit volume charge, and so their concentration decreases slightly in the early stages of electrolysis. However, it remains almost unchanged between 3600 C / L and 7200 C / L, and then decreases significantly from 10000 C / L (more specifically, 10800 C / L). This is because, in the early stages of electrolysis, in parallel with the production of magnesium hydroxide, the pH value increases due to the production of hydroxide ions, causing the carbon dioxide contained in seawater to react with calcium, resulting in the precipitation of calcium carbonate, and a slight decrease in the calcium ion concentration, as will be described later. After all the carbon dioxide contained in seawater has been consumed, calcium carbonate does not precipitate, so the calcium ions do not decrease, and magnesium hydroxide precipitates.The concentration of calcium ions does not decrease much until the concentration reaches approximately 10,000 C / L (more specifically, 10,800 C / L), at which point the magnesium ions are almost all consumed.It can be seen that the concentration decreases after almost all the magnesium ions in seawater have been consumed, as the calcium ions are consumed as calcium hydroxide.

[0047] Next, changes in the pH values ​​in the anode-side tank 20a and the cathode-side tank 20b are shown in Fig. 10. As the quantity of electricity per unit volume increases, hydrogen ions are generated in the anode 2a, causing the pH value of the sodium sulfate aqueous solution in the anode-side tank 20a to decrease. On the other hand, as the quantity of electricity per unit volume increases at the cathode 2c, hydroxide ions are generated, causing the pH value of the seawater in the cathode-side tank 20b to increase.

[0048] As shown in Figure 10, it can be seen that the pH value depends on the unit volume charge (C / L) input to the electrolysis stack 2. Furthermore, because the solubility products of magnesium hydroxide and calcium hydroxide are different and the pH environment required for the reaction is also different, the pH value of the seawater in the cathode-side tank 20b changes in two stages. The pH value of the seawater in the cathode-side tank 20b remains at 9.8 under conditions where the unit volume charge is between 3600 C / L and 10000 C / L, and increases again when it exceeds 10000 C / L (more specifically, 10800 C / L). This is because hydroxide ions are consumed by the production of magnesium hydroxide in the range of 3600 C / L to 10000 C / L (more specifically, 10800 C / L), so the pH value of the seawater in the cathode tank 20b remains at 9.8. When the concentration exceeds 10000 C / L, calcium hydroxide begins to be produced as magnesium ions decrease, and the pH value rises.

[0049] Next, the analysis results of the recovered precipitate are shown. In order to verify the components of the precipitate, the surface condition of the precipitate generated in the cathode-side chamber 20b during electrolysis was observed using a scanning electron microscope (SEM), and at the same time, the constituent elements were analyzed using energy dispersive X-ray spectroscopy (EDS). Based on the results of energy dispersive X-ray spectroscopy (EDS), SEM image mapping was performed.

[0050] Figure 11(a) is a mapping SEM image of a precipitate with a unit volume electrical charge of 3600 C / L at a flow rate of 40 ml / min, and Figure 11(b) is a mapping SEM image of a precipitate with a unit volume electrical charge of 14400 C / L at a flow rate of 40 ml / min. In the mapping SEM image, the light gray areas with smoothness are magnesium compounds, and the dark gray areas with relatively large crystals and no smoothness are calcium compounds.

[0051] Based on Figures 7 and 8, as well as Figures 12, 13, and 14, which show the test results after degassing (described below), it is believed that the calcium compound in Figure 11(a) is calcium carbonate, and the calcium compound in Figure 11(b) is calcium hydroxide. Furthermore, as shown in Figure 8, there is almost no decrease in calcium ion concentration when comparing the unit volume electrical quantity of 3600 C / L with the unit volume electrical quantity of 7200 C / L, so it is believed that a mapping SEM image of the precipitate similar to that of Figure 11(a) for the unit volume electrical quantity of 3600 C / L will also be obtained when the unit volume electrical quantity is 7200 C / L. With a unit volume electrical charge of 12,000 C / L, magnesium can be completely recovered. However, not only magnesium but also calcium in the seawater precipitates, resulting in a low purity of the recovered magnesium. Therefore, magnesium purification is required in a later process. Furthermore, at a unit volume electrical quantity of 10,800 C / L, almost all of the magnesium can be recovered. Although calcium precipitates in addition to magnesium in seawater, the amount of precipitated calcium is small, and the purity of the magnesium is high. Therefore, near this unit volume electrical quantity, the magnesium recovery rate is high and calcium precipitation is low, so high-purity magnesium can be recovered with high efficiency, and this unit volume electrical quantity is sufficient to obtain magnesium that is practical for industrial magnesium applications. Furthermore, in the range of unit volume electrical charge of 3600 C / L to 7200 C / L, the recovery rate of magnesium is lower than in the case of the unit volume electrical charge of 10800 C / L mentioned above, but there is almost no calcium precipitation and the purity of magnesium is the highest. This unit volume electrical charge is effective when high purity of magnesium is required for applications such as pharmaceuticals and reagents.

[0052] Next, the results of the component analysis by energy dispersive X-ray analysis (EDS) are shown in Figure 12. Note that the carbon content in the precipitate was not measured because EDS measurement requires a seal containing carbon. Therefore, the percentages in the EDS analysis results are values ​​excluding carbon.

[0053] Figure 12(a) shows the EDS analysis results of a precipitate with a unit volume electrical charge of 3600 C / L at a flow rate of 40 ml / min, and Figure 12(b) shows the EDS analysis results of a precipitate with a unit volume electrical charge of 14400 C / L at a flow rate of 40 ml / min.

[0054] As can be seen from Figure 12, the EDS analysis results detected only cations, calcium ions and magnesium ions. This is because highly soluble compounds, such as sodium chloride and potassium chloride, present in the seawater in the cathode-side tank 20b were washed away into the pure water during filtration. Furthermore, as shown in Figure 10, there was no increase in the pH value of the seawater between the unit volume electrical quantities of 3600 C / L and 10000 C / L (more specifically, 10800 C / L), which can be attributed to the fact that the hydroxide ions produced by electrolysis were consumed in the chemical reaction that produced magnesium hydroxide.

[0055] Furthermore, since calcium ions begin to decrease after most of the magnesium ions have been recovered, it can be determined that the calcium compound produced in the two steps is calcium hydroxide.

[0056] Next, it will be explained below how the purity of the recovered magnesium hydroxide can be improved by degassing the carbon dioxide in the seawater.

[0057] As described above, when hydroxide ions are generated by electrolysis in the cathode-side tank 20b and the pH value rises, magnesium carbonate precipitates in parallel with the precipitation of magnesium ions in the seawater, and the magnesium carbonate becomes mixed with the recovered magnesium ions, reducing the purity of the magnesium hydroxide.

[0058] Therefore, in order to prevent magnesium carbonate from being mixed in, it is effective to carry out a degassing process to remove carbonate ions, which are carbon dioxide, from seawater.

[0059] These degassing processes include a boiling degassing process in which seawater is heated to a boil before being supplied to the electrolysis stack 2 to degas the carbon dioxide, since the solubility of carbon dioxide, which is a gas, decreases as the liquid temperature increases according to Henry's law, and an acid addition degassing process in which the pH value of the seawater is changed to an acidic value by adding an acid, thereby degassing the carbonate ions as carbon dioxide, since the abundance ratio of carbonate ions varies depending on the pH value of the solution.

[0060] Therefore, in this embodiment 1, as described above, the degassing treatment device 12 is provided with two degassing treatment systems: a boiling degassing system that performs boiling degassing treatment, and an acid addition degassing system that performs acid addition degassing treatment, and it is possible to switch between each degassing treatment.

[0061] Although the degassing treatment device 12 of this embodiment 1 is exemplified as having two systems, the present invention is not limited to this, and may have only one of these systems, or may be capable of performing both treatments of the two systems in an overlapping manner.

[0062] In the boiling degassing process, seawater is heated to 100°C for 5 minutes. In the acid degassing process, the pH value of seawater is lowered to 3 by adding hydrochloric acid, and bicarbonate ions are converted to carbon dioxide and removed from the seawater. In the case of acid addition, the bicarbonate ions are neutralized by hydroxide ions generated by electrolysis.

[0063] In the first embodiment, the seawater degassed by the degassing device 12 was stored in the seawater tank 7 for about 24 hours (one day) before being supplied to the electrolysis stack 2 for electrolysis.

[0064] For electrolysis, the aqueous solution collected from the outlet of the cathode-side chamber 20b of the electrolysis stack 2 at a flow rate of 40 ml / min was filtered to separate the precipitate from the aqueous solution, after which the aqueous solution was subjected to component analysis using inductively coupled plasma (ICP).The surface condition and constituent elements of the precipitate were also analyzed using a scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS).

[0065] Fig. 15 is a table showing the measured values ​​of magnesium ions and calcium ions at each unit volume of electricity in the electrolysis of each degassed seawater, Fig. 13 is a graph showing the relationship between magnesium ions and unit volume of electricity in each degassed seawater and non-degassed seawater, and Fig. 14 is a graph showing the relationship between calcium ions and unit volume of electricity in each degassed seawater and non-degassed seawater. Note that for non-degassed seawater, the measurement data at a flow rate of 40 ml / min in Fig. 9 was used.

[0066] The graph in Figure 13 shows that the rate of magnesium hydroxide production in boiling degassed seawater is slightly faster than in non-degassed seawater. Furthermore, the graph in Figure 14 shows that the calcium ions in boiling degassed seawater do not decrease between unit volume electrical quantities of 0 C / L and 7200 C / L. Therefore, it can be seen that the calcium compound produced in the first stage in non-degassed seawater is calcium carbonate, produced by the increase in the pH value of the seawater.

[0067] Therefore, the graph in Figure 13 can be understood as the amount of electricity being used to recover only the magnesium ions in the seawater, with almost no effect of calcium carbonate due to the calcium ions contained in the seawater.From this graph in Figure 13, it can be seen that by keeping the amount of electricity per unit volume in the electrolysis of magnesium in seawater at around 10,000 C / L (more specifically 10,800 C / L), it is possible to recover almost all of the magnesium in the seawater while recovering high-purity magnesium hydroxide with significantly little loss of purity due to the precipitation of calcium hydroxide.

[0068] On the other hand, as shown in the graph in Figure 14, in the case of acid-added degassing treatment, the amount of calcium carbonate produced in the first stage was slightly reduced. Adding acid to seawater increases the concentration of carbon dioxide. However, although some carbon dioxide is released to the outside due to its solubility, it is not possible to remove all of the carbon dioxide. Therefore, when electrolysis is performed, the pH value of the solution rises again due to the electrolysis of water, and the carbon dioxide returns to carbonate ions, reacts with calcium ions, and precipitates as calcium carbonate.

[0069] As described above, boiling degassing is an excellent degassing treatment for preventing calcium carbonate from being mixed in, but boiling seawater requires a lot of energy and requires a large amount of equipment, which may increase the treatment cost, whereas acid addition degassing treatment only involves adding acid, which makes it possible to suppress increases in treatment cost. Therefore, which degassing treatment to select can be appropriately selected based on the balance between the purity of the magnesium hydroxide to be recovered and the acceptable treatment cost. Furthermore, other degassing methods may be used in addition to boiling degassing and acid addition degassing.

[0070] Next, mapping SEM images of sediments from degassed seawater are shown in Figure 16. Figure 16(a) is a mapping SEM image of sediments from seawater that had been subjected to boiling degassing, Figure 16(b) is a mapping SEM image of sediments from seawater that had been subjected to acid-addition degassing, and Figure 16(c) is a mapping SEM image of sediments from non-degassed seawater, which are the same images as Figure 11(a). Note that the unit volume electrical charge of each mapping SEM image shown in Figure 16 is 3600 C / L; similar mapping SEM images would be obtained even if the unit volume electrical charge was 7200 C / L.

[0071] As shown in Figure 16(c), calcium compounds are scattered on the surface of the sediment from seawater that has not been degassed, but the dark gray areas are almost nonexistent in Figures 16(a) and (b), which shows that the amount of calcium compounds has clearly decreased in the sediment from seawater that has been subjected to boiling degassing or additive degassing treatments.

[0072] Figure 17 shows the results of energy dispersive X-ray analysis (EDS) of the components of the sediment from degassed seawater. Figure 17(a) shows the EDS analysis results of the sediment from boiling degassed seawater, Figure 17(b) shows the EDS analysis results of the sediment from acid-degassed seawater, and Figure 17(c) shows the EDS analysis results of the sediment from non-degassed seawater.

[0073] Comparing the EDS analysis results of the undegassed seawater sediment shown in Figure 17(c) with the EDS analysis results of the boiling degassed seawater sediment shown in Figure 17(a) and the acid-added degassed seawater sediment shown in Figure 17(b), the calcium element abundance ratio in the degassed seawater sediment was significantly reduced. This proves that the boiling and acid-added methods are effective for producing calcium carbonate. Furthermore, the EDS analysis results show that the molar ratio of magnesium ions to calcium ions in the degassed sediment was 30.54:0.41 for the boiling degassed sediment and 31.23:0.28 for the acid-added sediment. This demonstrates that magnesium compounds with a purity of 99% can be recovered by degassing. Furthermore, since the magnesium to oxygen molar ratio in magnesium compounds that do not contain calcium carbonate is 1:2, it is clear that the magnesium compound is magnesium hydroxide. Embodiment 2

[0074] 18 is a diagram showing the configuration of the magnesium recovery device of the embodiment 2. The magnesium recovery device of the embodiment 2 is mainly characterized by the following two points.

[0075] The first point is that in the above-mentioned first embodiment, in order to reduce calcium carbonate mixed in the magnesium hydroxide precipitate, the carbon dioxide contained as bicarbonate ions in seawater is degassed to reduce calcium carbonate, whereas in the second embodiment, the carbon dioxide contained as bicarbonate ions in seawater is removed as calcium carbonate by the first-stage electrolysis.

[0076] The second advantage is that the aqueous sodium sulfate solution used in the anode-side tank 20a during electrolysis is regenerated and reused.

[0077] As shown in Fig. 18, the magnesium recovery apparatus of the second embodiment is mainly composed of two electrolysis stacks (electrolysis tanks) 21, 22 and one regeneration tank 30. In Fig. 18, "P" indicates a pump, and "FM" indicates a flow meter. In Fig. 18, due to space limitations, the DC power supply 1 shown in Fig. 1 and the inverter power supplies 6a, 6c that drive the pumps are omitted.

[0078] The two electrolysis stacks 21 and 22 have the same configuration as the electrolysis stack 2 used in the first embodiment, which is partitioned into an anode-side chamber 20a and a cathode-side chamber 20b by a cation exchange membrane 2b, as shown in FIGS. 2 and 3.

[0079] As shown in FIG. 18 , the first electrolysis stack 21 performing the first electrolysis and the second electrolysis stack 22 performing the second electrolysis are connected in series by piping such that the sodium sulfate aqueous solution in the anode-side chamber 20a of the first electrolysis stack 21 is introduced into the anode-side chamber 20a of the second electrolysis stack 22, and the seawater in the cathode-side chamber 20b of the first electrolysis stack 21 is introduced into the cathode-side chamber 20b of the second electrolysis stack 22.

[0080] A first filtration device 40 is provided on the piping path connecting the cathode-side chamber 20b of the first electrolysis stack 21 and the cathode-side chamber 20b of the second electrolysis stack 22. Calcium hydroxide and magnesium hydroxide deposited by electrolysis in the first electrolysis stack 21 are filtered by the first filtration device 40, and the filtered seawater is supplied to the cathode-side chamber 20b of the second electrolysis stack 22.

[0081] Furthermore, as shown in FIG. 18 , the magnesium recovery apparatus of the second embodiment is provided with a configuration in which the electrodes are removed from the electrolytic stack 2 shown in FIG. 2 , that is, a regeneration tank 30 having two compartments separated by a cation exchange membrane, and the regeneration tank 30 is connected so that one compartment is supplied with the aqueous sodium sulfate solution discharged from the anode-side tank 20 a of the second electrolytic stack 22, and the other compartment is supplied with seawater discharged from the cathode-side tank 20 b of the second electrolytic stack 22.

[0082] The aqueous sodium sulfate solution regenerated in one compartment of the regeneration tank 30 is supplied to the anode-side tank 20a of the first electrolysis stack 21 and reused, as shown in FIG. 18, while the seawater discharged from the other compartment of the regeneration tank 30 is discharged into the sea.

[0083] Next, the process flow in the magnesium recovery apparatus of the second embodiment will be described. Undeaerated seawater (deep ocean water) stored in a seawater tank is supplied by a pump to the cathode-side tank 20b of the first electrolysis stack 21, and a regenerated aqueous sodium sulfate solution is supplied to the anode-side tank 20a of the first electrolysis stack 21. Note that the sodium sulfate concentration in the regenerated aqueous sodium sulfate solution may be measured, and if the concentration is insufficient, i.e., does not reach a weight concentration of 5%, sodium sulfate may be added to adjust the concentration to a sufficient level.

[0084] Then, in the first electrolysis stack 21, primary electrolysis is performed at a unit volume electrical quantity of 3600 C / L, as shown in Fig. 18. The reason for performing electrolysis at 3600 C / L is that, as described in the first embodiment, the calcium ion concentration remains almost unchanged over the period from 3600 to 7200 C / L in the graph of Fig. 8 showing changes in calcium ion concentration, and therefore it is believed that precipitation of calcium carbonate by bicarbonate ions (carbon dioxide) contained in seawater is already completed by the time the unit volume electrical quantity reaches 3600 C / L.

[0085] That is, by performing primary electrolysis with a unit volume electrical quantity of 3600 C / L, almost all of the carbon dioxide contained as bicarbonate ions in seawater is consumed in the precipitation of calcium carbonate, so that the seawater after primary electrolysis contains almost no bicarbonate ions (carbon dioxide), and even if the seawater is further electrolyzed, almost no calcium carbonate is precipitated. In this state where almost no calcium carbonate is precipitated, secondary electrolysis can be performed to precipitate magnesium hydroxide. Therefore, the primary electrolysis (degassing (calcium removal) step) in Embodiment 2 corresponds to the carbon dioxide removal step of the present invention.

[0086] The seawater that has undergone primary electrolysis is filtered in the first filtration device 40, and calcium carbonate and magnesium hydroxide precipitated in the primary electrolysis are separated from the seawater. The seawater that is the filtrate after separation is supplied to the cathode-side tank 20b of the second electrolysis stack 22.

[0087] Then, in the second electrolysis stack 22, secondary electrolysis is performed at a unit volume electricity quantity of 8400 C / L, as shown in Fig. 18. The reason for performing electrolysis at 8400 C / L is that, as described above in the first embodiment, if the total unit volume electricity quantity of the primary electrolysis and secondary electrolysis exceeds 12000 C / L, the amount of calcium hydroxide precipitated increases, causing calcium hydroxide to be mixed into the recovered magnesium hydroxide, thereby preventing a decrease in the purity of the recovered magnesium hydroxide. If calcium hydroxide is removed and magnesium hydroxide is purified in a subsequent process, electrolysis may be performed at a total unit volume electrical charge of 12,000 C / L or more (e.g., 14,400 C / L), which allows complete recovery of magnesium from seawater. Even when electrolysis is performed at a total unit volume electrical charge of 10,800 C / L, a small amount of calcium (calcium hydroxide) precipitates, as described in embodiment 1. Therefore, if it is desired to further increase the purity of magnesium hydroxide, electrolysis may be performed at a total unit volume electrical charge of 7,200 C / L, which is the level before calcium (calcium hydroxide) begins to precipitate. In this case, the recovery rate of magnesium from seawater will be slightly lower than when the total unit volume electrical charge is 10,800 C / L, but highly pure magnesium can be recovered.

[0088] By filtering the seawater that has undergone secondary electrolysis in the second electrolysis stack 22 in this manner using the second filtration device 41, it is possible to recover almost all of the magnesium contained in the seawater as insoluble magnesium hydroxide, and since the recovered magnesium hydroxide contains almost no calcium carbonate or calcium hydroxide, it is possible to recover high-purity magnesium hydroxide.

[0089] After the secondary electrolysis, the seawater and the aqueous sodium sulfate solution are supplied to each compartment of the regeneration tank 30, as shown in FIG.

[0090] In this regeneration tank 30, as shown in FIG. 18 , sodium ions that migrated into the seawater by the first and second electrolysis pass through the cation exchange membrane and return to the aqueous sodium sulfate solution, thereby regenerating the aqueous sodium sulfate solution. At the same time, hydrogen ions in the aqueous sodium sulfate solution, which has been made highly concentrated by the first and second electrolysis, pass through the cation exchange membrane and migrate into the seawater, where they react with and neutralize hydroxide ions in the seawater, which has been made highly concentrated by the first and second electrolysis, thereby significantly lowering the pH value of the seawater. The seawater with the lowered pH value can then be discharged back into the sea, with only magnesium having been recovered from the original seawater.

[0091] As described above, according to the embodiment, the cation exchange membrane 2b separates the anode-side tank 20a having the anode 2a from the cathode-side tank 20b having the cathode 2c. Seawater, which is an aqueous solution containing magnesium ions, is introduced into the cathode-side tank 20b, and electrolysis is performed in a state in which the movement of hydroxide ions, which are anions, from the cathode-side tank 20b to the anode-side tank 20a is restricted. This makes it possible to efficiently increase the hydroxide ion concentration in the cathode-side tank 20b through electrolysis using little power, thereby improving the efficiency of magnesium hydroxide production.

[0092] Furthermore, it is possible to prevent hydroxide ions generated in the cathode-side tank 20b from migrating to the anode-side tank 20a and coming into contact with hydrogen ions, which are cations generated in the anode-side tank 20a, thereby preventing a decrease in the power efficiency of electrolysis. This improves the power efficiency of electrolysis, and since the generation of magnesium hydroxide depends on the quantity of electricity per unit volume, it is also possible to accurately grasp the state of recovery (precipitation) by electrolysis based on the quantity of electricity per unit volume.

[0093] Furthermore, according to the above-described embodiment, seawater, which is readily available and inexpensive, is used as the aqueous solution containing at least magnesium ions, so that magnesium can be recovered inexpensively.

[0094] Furthermore, according to the above-described embodiment, the use of a cation exchange membrane restricts the movement of chloride ions to the anode-side tank 20a, and the use of a sodium sulfate aqueous solution, which is non-chlorine-based electrolyzed water that does not contain chloride ions, in the anode-side tank 20a prevents harmful chlorine gas from being generated in the anode-side tank 20a. This makes it possible to suppress the increase in the cost of treating chlorine gas and the cost of recovering magnesium that would otherwise be caused by the use of non-chlorine-based electrolyzed water.

[0095] Furthermore, according to the second embodiment described above, in the regeneration tank 30, sodium ions, which are cations reduced from the sodium sulfate aqueous solution by electrolysis, are returned from seawater to the sodium sulfate aqueous solution to be regenerated, and the regenerated sodium sulfate aqueous solution can be reused, thereby further reducing the cost of recovering magnesium.

[0096] Furthermore, according to the above-described embodiment 1, the carbon dioxide contained in the seawater as bicarbonate ions is removed in the degassing treatment device 12 by boiling degassing or acid-addition degassing, thereby reducing the amount of calcium carbonate that gets mixed into the recovered magnesium hydroxide and increasing the purity of the recovered magnesium hydroxide.

[0097] Furthermore, according to the first embodiment described above, dissolved carbon dioxide gas can be easily removed by performing boiling degassing, in which seawater is heated to a boil.

[0098] Furthermore, according to the second embodiment described above, carbon dioxide contained in seawater as bicarbonate ions is removed by performing the first electrolysis and recovering it as calcium carbonate, so that not only can high-purity magnesium hydroxide be recovered, but calcium carbonate can also be recovered.

[0099] Furthermore, according to the above-mentioned second embodiment, the total unit volume electric energy applied to the seawater electrolysis is 14,400 (3,600 + 10,800) C / L, so that it is possible to recover almost all of the magnesium ions contained in the seawater, and it is also possible to prevent calcium hydroxide contained in the seawater from being recovered together with magnesium hydroxide, which would result in a decrease in the purity of magnesium.

[0100] Furthermore, according to the above-described embodiment, the electrodes constituting the anode 2a and the cathode 2c are titanium electrodes having a platinum layer on their surfaces, and therefore, it is possible to prevent metal ions constituting the electrodes from being contained in seawater after magnesium recovery, which would have a significant impact on the environment.

[0101] Although the embodiments of the present invention have been described above with reference to the drawings, the specific configuration is not limited to these embodiments, and the present invention also includes modifications and additions that do not deviate from the gist of the present invention.

[0102] For example, in the above embodiment, the aqueous solution containing at least magnesium ions is seawater, but the present invention is not limited to this. The aqueous solution containing magnesium ions may be obtained by, for example, treating magnesium minerals or the like, or may be an aqueous solution obtained by treating seawater to make it fresh water using a reverse osmosis membrane.

[0103] Furthermore, in the above embodiment, an example is given of a form in which seawater, which is an aqueous solution containing at least magnesium ions, is continuously treated, but the present invention is not limited to this. For example, these treatments may be performed in a batch form in which the electrolysis stack 2 is filled with seawater and electrolysis is performed, and then the electrolyzed seawater is replaced with new seawater, and treatment is performed in units of seawater that can be stored in the electrolysis stack 2.

[0104] Furthermore, in the above embodiment, an example is shown in which a sodium sulfate aqueous solution, which is non-chlorine-based electrolyzed water, is introduced into the anode-side tank 20a in order to prevent the generation of harmful chlorine gas. However, the present invention is not limited to this. In cases where chlorine gas can be treated or where it is desired to utilize the generated chlorine gas, seawater may be introduced into the anode-side tank 20a, just like the cathode-side tank 20b.

[0105] Furthermore, in the above embodiment, sodium sulfate, which is readily available and inexpensive, is used as the non-chlorine electrolyzed water that does not contain chloride ions to prevent the generation of chlorine gas. However, the present invention is not limited to this. The non-chlorine electrolyzed water may be a sulfate other than sodium, or may be sodium nitrate or a nitrate other than sodium. Any aqueous solution of a water-soluble salt may be used as long as it does not generate chlorine gas when introduced into the anode-side tank 20a and electrolyzed. If the aqueous solution introduced into the cathode-side tank 20b contains a cation such as sodium ion in seawater, a sulfate or nitrate containing the same cation as the cation may be used. While aqueous solutions of sulfates and nitrates are common, aqueous solutions of phosphates, oxalates, and chromates other than sulfates and nitrates may also be used. Furthermore, any cation, such as sodium or potassium, may be used to form a salt with these acids. However, if a monovalent cation exchange membrane is used, the cation must be monovalent.

[0106] In addition, in the above embodiment, the weight concentration of the sodium sulfate aqueous solution is exemplified as 5%, but these concentrations may be determined appropriately depending on the type of non-chlorine-based electrolyzed water used, and the concentration may be set so that almost no hydrogen ions generated in the anode-side tank 20a move to the cathode-side tank 20b, and only cations contained in the non-chlorine-based electrolyzed water move to the cathode-side tank 20b.

[0107] Furthermore, in the above-mentioned second embodiment, an example is given in which seawater that has undergone secondary electrolysis is supplied to a tank different from the tank to which the aqueous sodium sulfate solution in the regeneration tank 30 is supplied. However, the present invention is not limited to this. Instead of the seawater that has undergone secondary electrolysis, untreated seawater may be supplied, or an aqueous solution exclusively for regeneration that contains a high concentration of cations, such as sodium ions, that have been reduced in non-chlorine electrolyzed water by electrolysis may be supplied.

[0108] Furthermore, in the second embodiment, the pH value of the discharged seawater is not adjusted, but if the pH value is high, the pH value may be adjusted to a value close to that of actual seawater by adding an acid, for example.

[0109] In the above embodiment, both the anode 2a and the cathode 2c are platinum-plated titanium electrodes, assuming that if deposits such as calcium carbonate or magnesium hydroxide adhere to the cathode 2c, the deposited deposits can be peeled off by switching the polarity. However, the present invention is not limited to this, and a metal electrode that does not have a platinum layer on its surface may be used for the cathode 2c.

[0110] Furthermore, although not particularly implemented in the above embodiment, an additive capable of inhibiting deposition of deposits on the electrodes may be added to the aqueous solution such as seawater supplied to the cathode-side tank 20b.

[0111] Furthermore, although not specifically implemented in the above embodiment, a third electrolysis stack may be provided between the second electrolysis stack 22 and the regeneration tank 30, and the seawater after the second electrolysis and an aqueous sodium sulfate solution may be supplied to the third electrolysis stack and electrolyzed, thereby recovering calcium in the seawater as calcium hydroxide.

[0112] Although not specifically implemented in the above embodiment, it goes without saying that hydrogen and oxygen produced by electrolysis may be recovered and utilized.

[0113] Furthermore, in the above embodiment, an example is given of a configuration in which the degassing treatment device 16 is provided exclusively, but the present invention is not limited to this. For example, these degassing treatment devices may be provided separately, and for example, seawater used for cooling the power plant may be used as degassed seawater. [Industrial Applicability]

[0114] As an example of the application of the present invention, since seawater from which magnesium has been recovered by electrolysis does not contain carbon dioxide (bicarbonate ions), it can also be used to absorb industrially produced carbon dioxide into the seawater. [Explanation of symbols]

[0115] 1 DC power supply 2. Electrolysis stack 2a anode 2b Cation exchange membrane 2c cathode 3a flow meter 3c flow meter 4a Valve 4c Valve 6a inverter power supply 6c Inverter Power Supply 7 Seawater Tank 8. Sodium sulfate aqueous solution tank 9 Anode solution tank 10. Cathode solution tank 11 Spare seawater tank 12 Degassing equipment 13a Inlet piping 13c Inlet piping 14a Pump 14c pump 16 Filtration device 20a Anode side tank 20b Cathode side tank 21 First electrolysis stack 22 Second electrolysis stack 30 Regeneration tank 40 First filtration device 41 Second filtration device

Claims

1. A carbon dioxide removal method for removing carbon dioxide contained in seawater by electrolyzing the seawater, comprising: an electrolytic cell having an anode and a cathode is partitioned by a cation exchange membrane into an anode-side cell having the anode and a cathode-side cell having the cathode; The seawater is introduced into the cathode-side tank, and electrolysis is carried out in a state where the movement of anions from the cathode-side tank to the anode-side tank is restricted. A method for removing carbon dioxide.

2. An aqueous solution containing no chloride ions is introduced into the anode-side tank.

2. The carbon dioxide removal method according to claim 1 .

3. The aqueous solution not containing chloride ions is an aqueous sodium sulfate solution.

3. The carbon dioxide removal method according to claim 2.

4. The electrolysis is carried out with a volumetric charge of 3600 C / L or less.

2. The carbon dioxide removal method according to claim 1 .

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