JP2024152742A - Carbon Dioxide Removal Method - Google Patents

Abstract

[Problem] To improve the efficiency of magnesium hydroxide production. [Solution] In a magnesium recovery method in which poorly water-soluble magnesium hydroxide is generated and recovered by electrolysis of an aqueous solution containing at least magnesium ions, an electrolytic cell having an anode and a cathode is partitioned by a cation exchange membrane into an anode-side cell having an anode and a cathode-side cell having a cathode, and an aqueous solution containing magnesium ions is introduced into the cathode-side cell to perform electrolysis in a state in which the movement of anions from the cathode-side cell to the anode-side cell is restricted. [Selected Figure] Figure 1

Description

Application for application of Article 30, Paragraph 2 of the Patent Act has been filed. Published on November 17, 2018 in Heliyon, Vol. 4, No. 11, No. e00923 (published by Elsevier Ltd.)

The present invention relates to a magnesium recovery method and magnesium recovery device.

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 and a diaphragm separating the upper and lower sides, is dropped into the sea to perform electrolysis (electrolysis), and magnesium is recovered by precipitating magnesium hydroxide in the cathode electrolyzed water (see, for example, Patent Document 1).

JP 2012-57230 A

However, in Patent Document 1, the negative ions, or hydroxide ions, generated in the cathodic electrolytic water by electrolysis can pass through the diaphragm and migrate to the anodic electrolytic water, resulting in a problem of low efficiency in generating magnesium hydroxide.

The present invention was made with a focus on these problems, and aims to provide a magnesium recovery method and magnesium recovery device that can improve the efficiency of magnesium hydroxide production.

In order to solve the above problems, the magnesium recovery method according to claim 1 of the present invention comprises the steps of:
A method for recovering magnesium, comprising electrolyzing an aqueous solution containing at least magnesium ions to generate and recover magnesium hydroxide, which is poorly soluble in water, the method comprising the steps of:
The 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, and the aqueous solution is introduced into the cathode-side cell to perform electrolysis in a state in which movement of anions from the cathode-side cell to the anode-side cell is restricted.
According to this feature, the hydroxide ions generated in the cathode side tank are prevented from migrating to the anode side tank, and the hydroxide ion concentration in the cathode side tank can be efficiently increased, thereby improving the efficiency of magnesium hydroxide production.

The magnesium recovery method according to claim 2 of the present invention is the magnesium recovery method according to claim 1,
The aqueous solution is characterized in that it is seawater.
This feature allows magnesium to be recovered from seawater at low cost.

The magnesium recovery method according to claim 3 of the present invention is the magnesium recovery method according to claim 1 or 2,
The electrolytic solution is characterized in that non-chlorine-based electrolytic water not containing chlorine ions is introduced into the anode side tank, and the aqueous solution is introduced into the cathode side tank.
According to this feature, it is possible to prevent harmful chlorine gas from being generated on the anode side due to electrolysis, and therefore it is possible to suppress an increase in the cost of recovering magnesium due to the cost of treating chlorine gas.

The magnesium recovery method according to claim 4 of the present invention is the magnesium recovery method according to claim 3,
The non-chlorine electrolytic water is characterized in that it is an aqueous solution of sodium sulfate.
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 that would otherwise be caused by using non-chlorine-based electrolytic water.

The magnesium recovery method according to claim 5 of the present invention is the magnesium recovery method according to claim 3 or 4,
The method is characterized by including a regeneration step in which an aqueous regeneration solution containing cations transferred from the non-chlorine electrolyzed water in the anode side tank to the cathode side tank by electrolysis for recovering the magnesium hydroxide is introduced into one compartment of a regeneration tank partitioned by a cation exchange membrane, and the non-chlorine electrolyzed water after electrolysis for recovering the magnesium hydroxide is introduced into the other compartment of the regeneration tank, thereby transferring the cations from the aqueous regeneration solution to the non-chlorine electrolyzed water, thereby regenerating the water.
According to this feature, the non-chlorine electrolytic water can be regenerated and reused, and the cost of recovering magnesium can be reduced.

The magnesium recovery method according to claim 6 of the present invention is the magnesium recovery method according to any one of claims 1 to 5,
The method is characterized by including a carbon dioxide removal step of reducing the concentration of carbon dioxide dissolved in the aqueous solution, and electrolyzing the aqueous solution in which the carbon dioxide concentration has been reduced in the carbon dioxide removal step to recover magnesium.
According to this feature, the amount of calcium, which is an impurity, contained in the recovered magnesium can be reduced, and the purity of the magnesium can be increased.

The magnesium recovery method according to claim 7 of the present invention is the magnesium recovery method according to claim 6,
The carbon dioxide gas removing step is characterized by including a heating step of heating the aqueous solution.
According to this feature, the dissolved carbon dioxide gas can be easily removed.

The magnesium recovery method according to claim 8 of the present invention is the magnesium recovery method according to claim 6 or claim 7,
The carbon dioxide gas removing step is characterized by including a step of producing calcium carbonate by electrolyzing the aqueous solution.
This feature allows the dissolved carbon dioxide gas to be removed.

The magnesium recovery method according to claim 9 of the present invention is the magnesium recovery method according to any one of claims 1 to 8,
The aqueous solution is seawater, and the amount of electricity applied per liter of seawater in electrolysis for recovering magnesium is 12,000 coulombs or less.
According to this feature, it is possible to prevent the calcium hydroxide contained in the aqueous solution from being recovered together with the magnesium hydroxide, which would result in a decrease in the purity of magnesium.

The magnesium recovery method according to claim 10 of the present invention is the magnesium recovery method according to any one of claims 1 to 9,
The electrode constituting the anode is characterized in that it is an insoluble electrode having a substance with a smaller ionization tendency than silver.
According to this feature, it is possible to prevent ions of the material constituting the electrode from being contained in the aqueous solution after magnesium recovery, which would have a significant impact on the environment.

The magnesium recovery apparatus according to claim 11 of the present invention comprises:
an electrolytic cell having an anode and a cathode;
A power supply means capable of supplying power for electrolysis to the anode and the cathode;
A recovery means capable of recovering magnesium hydroxide which is sparingly soluble in water produced by electrolysis;
A magnesium recovery apparatus comprising:
the electrolytic cell is partitioned by a cation exchange membrane into an anode-side cell having the anode and a cathode-side cell having the cathode,
The present invention is characterized by comprising a supply means for supplying an aqueous solution containing at least magnesium ions to the cathode side tank.
According to this feature, the hydroxide ions generated in the cathode side tank are prevented from migrating to the anode side tank, and the hydroxide ion concentration in the cathode side tank can be efficiently increased, thereby improving the efficiency of magnesium hydroxide production.

The present invention may have only the invention-specific features described in the claims of the present invention, or may have the invention-specific features described in the claims of the present invention as well as features other than the invention-specific features.

1 is a configuration diagram of a magnesium recovery device according to a first embodiment of the present invention. FIG. FIG. 2 is a diagram showing the configuration of an electrolytic cell in the magnesium recovery device according to the first embodiment of the present invention. FIG. 2 shows flow paths used in the electrolysis stack 2 according to the first embodiment of the present invention. 4 is a table showing test conditions in the first embodiment of the present invention. 1 is a graph showing the relationship between current and magnesium ion concentration. 1 is a graph showing the relationship between current and calcium ion concentration. 1 is a graph showing the relationship between magnesium ion concentration and unit volume charge. 1 is a graph showing the relationship between calcium ion concentration and electrical charge per unit volume. 11 is a table showing measured concentrations of magnesium ions and calcium ions at each flow rate and each unit volume charge. 1 is a graph showing changes in pH value in the anode side chamber 20a and the cathode side chamber 20b. 13A is a mapping SEM image of a precipitate having a unit volume electrical charge of 3600 C/L, and FIG. 13B is a mapping SEM image of a precipitate having a unit volume electrical charge of 14400 C/L. 13A shows the results of EDS analysis of a precipitate having a unit volume electrical charge of 3600 C/L, and FIG. 13B shows the results of EDS analysis of a precipitate having a unit volume electrical charge of 14400 C/L. 1 is a graph showing the relationship between magnesium ions and unit volume electricity in each deaerated seawater and non-deaerated seawater. 1 is a graph showing the relationship between calcium ions and unit volume electricity in each deaerated seawater and non-deaerated seawater. 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. (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. (a) is the result of EDS analysis of sediment from seawater that had been subjected to boiling degassing treatment, (b) is the result of EDS analysis of sediment from seawater that had been subjected to acid-addition degassing treatment, and (c) is the result of EDS analysis of sediment from non-degassed seawater. FIG. 4 is a configuration diagram of a magnesium recovery device according to a second embodiment of the present invention.

An embodiment of the present invention is described below.

EMBODIMENT 1

Figure 1 is a diagram showing an example of a magnesium recovery device that applies the magnesium recovery method of the present invention.

As shown in FIG. 1, the magnesium recovery device of this embodiment 1 is mainly composed of an electrolysis stack 2, which is an electrolysis tank (electrolysis tank) for performing electrolysis, a DC power supply unit (power supply means) 1 for supplying power for electrolysis (electrolysis power) to the anode 2a and cathode 2c 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% weight concentration aqueous sodium sulfate solution 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, and is a device capable of performing electrolysis continuously.

The lower part of the electrolysis stack 2 is connected to the sodium sulfate aqueous solution tank 8 and the seawater tank 7 by inlet pipes 13a and 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 and discharged from the upper part of the electrolysis stack 2.

As shown in FIG. 1, pumps 14a and 14c capable of varying the flow rate are provided on the paths of the inlet pipes 13a and 13c corresponding to the respective inlet pipes 13a and 13c, and the flow rates of the sodium sulfate aqueous 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 this embodiment 1, as described later, for example, MG204XPD17-10S, MAGPON GEAR (product name of Nikkiso Eiko Co., Ltd.) are used as the pumps 14a and 14c capable of varying the flow rate because it is necessary to change the flow rate to consider the recovery conditions, but the present invention is not limited to this, and a pump that cannot vary the flow rate may be used. In addition, the inverter power supplies 6a and 6c may be appropriately selected depending on the pump to be used.

As shown in FIG. 1, flow meters 3a and 3c are provided on the inlet pipes 13a and 13c to measure the flow rates of the sodium sulfate aqueous solution and seawater supplied to the electrolysis stack 2 via the inlet pipes 13c and 13a, and valves 4a and 4c are also provided. As the flow meters 3a and 3c, for example, a Handy Logger GL200A (product name manufactured by Graphtec Corporation) can be used.

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 tank 11.

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

On the other hand, the upper part of the electrolysis stack 2 is connected to the anode solution tank 9 and the cathode solution tank 10 by the discharge pipes 15a and 15c, and the treated sodium sulfate aqueous solution and seawater discharged after electrolysis in the electrolysis stack 2 are stored in the anode solution tank 9 and the cathode solution tank 10.

As mentioned above, a filtration device 16 is connected to the cathode solution tank 10, so that the magnesium hydroxide contained in the treated seawater is filtered out without settling in the cathode solution tank 10.

Figure 2 is a diagram showing the configuration of the electrolysis stack 2 of this embodiment 1. As shown in Figure 2, the electrolysis stack 2 of this 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.

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

The concentration of the sodium sulfate aqueous solution supplied from the sodium sulfate aqueous 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 an investigation into measuring the pH value of the solution obtained at the outlet of the electrolysis stack 2, it is found that a high concentration sodium sulfate aqueous solution with a weight concentration of 5% is sufficient.

The anode 2a and the cathode 2c provided in the electrolytic stack 2 are preferably platinum-plated titanium plates, which are titanium plates plated with platinum, because they are not corroded by the aqueous solution by electrolysis, have high electrical conductivity, are excellent in mechanical strength, and are inexpensive. The platinum electrodes have high chemical stability, and platinum does not dissolve in the aqueous solution even by electrolysis. The thickness of the platinum plating layer may be any appropriate thickness that does not cause missing parts (defects) in the plating layer. In the first embodiment, the electrodes have a platinum layer formed on the surface by plating, but the present invention is not limited to this. If there are no problems with the price or mechanical strength, the electrodes themselves may be platinum plates or may be made of a carbon material. In this case, it is preferable that the surface of the carbon material is 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 anode 2a has an ionization tendency equal to or greater than that of silver (Ag), the electrode will be oxidized and dissolved. Therefore, as long as the ionization tendency of the material is smaller than that of silver (Ag), the electrode may have a material other than platinum, such as gold (Au) or carbon (C), on at least the surface.
In addition, since a reduction reaction occurs on the cathode side, any metal can be used for the cathode 2c provided in the electrolytic stack 2.

The flow path used in the electrolysis stack 2 in this embodiment 1 is shown in Figure 3. The flow path is made by hollowing out the center of a 5 mm-thick plate-shaped silicone rubber as shown in Figure 3, with an electrode disposed on one side and a cation exchange membrane disposed on the other side.

Specifically, two silicone rubber plates with platinum-plated titanium arranged in the center of one side are prepared, and the cation exchange membrane 2b is sandwiched between the other sides of the two silicone rubber plates and pressed to form the electrolytic stack 2 shown in Figure 2.

Therefore, by placing an electrode on one side of the central part of the flow path, the area in which the electrode is placed becomes the area where a chemical reaction occurs through 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.

In addition, in FIG. 3, the flow paths are illustrated as being horizontal, but in this embodiment 1, the flow paths are arranged so as to be approximately vertical to the ground, and each aqueous solution is supplied from below in the direction opposite to the force of gravity. In this way, clogging of the flow paths caused by the production of magnesium hydroxide in the cathode side tank 20b can be prevented when the flow paths are arranged horizontally.

When an aqueous sodium sulfate solution is introduced into the anode tank 20a of the silicone rubber electrolysis stack 2 formed as described above and seawater is introduced into the cathode tank 20b, an electrolysis reaction occurs when 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, and magnesium hydroxide is precipitated.

Furthermore, since the cation exchange membrane 2b is used in this embodiment 1, hydroxide ions, which are anions generated at the cathode 2c, cannot pass through the cation exchange membrane 2b and cannot move to the anode side tank 20a, so it is possible to efficiently increase the pH value of the cathode side tank 20b with a small amount of electricity, and magnesium hydroxide can be efficiently generated. In addition, chloride ions, which are anions contained in the seawater introduced into the cathode side tank 20b, cannot pass through the cation exchange membrane 2b like hydroxide ions and cannot move to the anode side tank 20a, so electrolysis is performed without chloride ions being contained in the anode side tank 20a, so 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.

Next, the magnesium recovery conditions were tested using the device 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 sets of flow rates of 20 ml/min, 40 ml/min, and 60 ml/min were selected as shown in FIG. 4. Furthermore, the test was performed by changing the amount of electricity per unit volume of seawater according to each flow rate. In this test, the amount of electricity was set so as not to exceed the limit current density, so that dissociation of water does not occur in the boundary layer of the cation exchange membrane 2b. Details of the test conditions are shown in FIG. 4. The amount of electricity per unit volume of seawater (hereinafter referred to as the amount of electricity per unit volume) was selected to be 3600 C (coulomb)/L (liter), 7200 C/L, 10800 C/L, and 14400 C/L.

In addition, the tests were conducted using non-degassed seawater without operating the degassing device 16 so that the effect of the degassing device 16 would become clear in the comparison described below.

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

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 decreased, more hydroxide ions are contained in a unit volume. On the other hand, the concentration of magnesium ions 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 become magnesium hydroxide also increases.

Figure 6 is a graph showing the relationship between current and calcium ion concentration. The relationship between calcium concentration and flow rate was the same as that of magnesium ions, and the calcium concentration decreased with the decrease in flow rate. Furthermore, the calcium production reaction is divided into two stages as shown in Figure 6. In the first stage, the concentration slightly decreases at the same time as the production of magnesium hydroxide. This is because the pH value of the seawater in the cathode tank 20b increases as electrolysis is performed, and the 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 concentration of calcium decreases after magnesium ions are consumed due to the reaction between magnesium ions and hydroxide ions. This is because as the reaction of magnesium hydroxide progresses, the magnesium ions in the seawater decrease proportionally, and the proportion of calcium ions increases, so a chemical reaction between calcium ions and hydroxide ions begins, and calcium ions precipitate as calcium hydroxide, thereby decreasing the concentration of calcium ions.

Figure 9 is a table showing the 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 Figures 7 and 8.

Figure 7 is a graph showing the relationship between the measured magnesium ion concentration and the unit volume charge. With the unit volume charge on the horizontal axis, the ion concentration is 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 un-deaerated 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 about 12,000 C/L, calculated from the slope of the graph.

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 the concentration decreases slightly in the early stages of electrolysis, but there is almost no change between 3600 C/L and 7200 C/L, and then there is a large decrease from 10,000 C/L (more specifically, 10,800 C/L). This is because in the early stages of electrolysis, in parallel with the production of magnesium hydroxide, as will be described later, the pH value increases due to the production of hydroxide ions, causing the carbon dioxide contained in the seawater to react with calcium, resulting in the precipitation of calcium carbonate, and a slight decrease in the calcium ion concentration. After all the carbon dioxide in the 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 approaches 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 calcium ion concentration decreases after almost all the magnesium ions in the seawater have been consumed, as the calcium ions are consumed as calcium hydroxide.

Next, the changes in pH value in the anode side tank 20a and the cathode side tank 20b are shown in Figure 10. As the unit volume of electricity increases, hydrogen ions are generated in the anode 2a, so the pH value of the sodium sulfate aqueous solution in the anode side tank 20a decreases. On the other hand, at the cathode 2c, as the unit volume of electricity increases, hydroxide ions are generated, so the pH value of the seawater in the cathode side tank 20b increases.

As shown in Fig. 10, it can be seen that the pH value depends on the unit volume charge (C/L) input to the electrolysis stack 2. Furthermore, since 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 of unit volume charge between 3600 C/L and 10000 C/L, and increases again when the unit volume charge 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 side tank 20b remains at 9.8. When the concentration exceeds 10000 C/L, calcium hydroxide starts to be produced as magnesium ions decrease, and the pH value rises.

Next, the analysis results of the recovered precipitate are shown. In order to verify the composition of the precipitate, the surface condition of the precipitate formed in the cathode side tank 20b during electrolysis was observed using a scanning electron microscope (SEM), and at the same time, the composition of the constituent elements was analyzed using energy dispersive X-ray analysis (EDS). Based on the results of energy dispersive X-ray analysis (EDS), SEM images were mapped.

Figure 11(a) is a mapping SEM image of a precipitate with a unit volume 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 charge of 14400 C/L at a flow rate of 40 ml/min. In the mapping SEM image, the light grey areas that are smooth are magnesium compounds, and the dark grey areas that are relatively large crystals and not smooth are calcium compounds.

7 and 8, and Fig. 12, Fig. 13, Fig. 14, which are the test results by degassing described later, the calcium compound part in Fig. 11(a) is considered to be calcium carbonate, and the calcium compound in Fig. 11(b) is considered to be calcium hydroxide. Also, as shown in Fig. 8, the calcium ion concentration is almost the same when comparing the unit volume charge of 3600 C/L and the unit volume charge of 7200 C/L, so it is considered that the mapping SEM image of the precipitate similar to that of Fig. 11(a) for the unit volume charge of 3600 C/L can be obtained in the case of the unit volume charge of 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 seawater precipitates, resulting in low purity of the recovered magnesium. Therefore, magnesium needs to be refined in a post-process.
In addition, at a unit volume electrical quantity of 10,800 C/L, almost all of the magnesium can be recovered. Although not only magnesium but also calcium precipitates in seawater, the amount of precipitated calcium is small, and the purity of magnesium is high. Therefore, near this unit volume electrical quantity, the magnesium recovery rate is high and calcium precipitation is small, so that magnesium of high purity can be recovered with high efficiency, and this unit volume electrical quantity is sufficient to obtain practical magnesium for industrial magnesium applications.
In addition, in the range of unit volume electrical charge of 3600 C/L to 7200 C/L, the recovery rate of magnesium is lower than the above-mentioned unit volume electrical charge of 10800 C/L, but there is almost no calcium precipitation and the purity of magnesium is the highest. This is an effective unit volume electrical charge when magnesium purity is required for applications such as medicines and reagents.

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 a seal containing carbon is required for energy dispersive X-ray analysis (EDS) measurement. Therefore, the percentages in the EDS analysis results show values excluding carbon.

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

As can be seen from Figure 12, only the positive ions calcium ions and magnesium ions were detected in the EDS analysis results. 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. Also, 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 concluded to be because the hydroxide ions generated by electrolysis were consumed in the chemical reaction that produced magnesium hydroxide.

Furthermore, since calcium ions begin to decrease after magnesium ions have been almost completely recovered, it can be determined that the calcium compound produced in the two stages is calcium hydroxide.

Next, we will explain how the purity of the recovered magnesium hydroxide can be improved by degassing the carbon dioxide in seawater.

As described above, when hydroxide ions are generated by electrolysis in the cathode 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.

For this reason, 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.

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

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.

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

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

In this embodiment, the seawater degassed by the degassing device 12 was stored in the seawater tank 7 for about 24 hours (one day) and then supplied to the electrolysis stack 2 for electrolysis.

For electrolysis, the aqueous solution collected from the outlet of the cathode side tank 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 precipitate was also subjected to surface analysis and elemental analysis using a scanning electron microscope (SEM) and energy dispersive X-ray spectrometry (EDS).

Figure 15 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 13 is a graph showing the relationship between magnesium ions and unit volume of electricity for each degassed seawater and non-degassed seawater, and Figure 14 is a graph showing the relationship between calcium ions and unit volume of electricity for each degassed seawater and non-degassed seawater. For non-degassed seawater, the measurement data for a flow rate of 40 ml/min in Figure 9 was used.

The graph in Figure 13 shows that the rate of magnesium hydroxide production in boiled and degassed seawater is slightly faster than in non-degassed seawater. Furthermore, the graph in Figure 14 shows that in boiled and degassed seawater, calcium ions 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.

The graph in Figure 13 can therefore be understood as indicating that the amount of electricity is 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 unit volume electricity quantity 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 calcium hydroxide precipitation.

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 proportion of carbon dioxide present. However, although some carbon dioxide was 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, which react with calcium ions and precipitate as calcium carbonate.

Thus, boiling degassing is an excellent degassing process for preventing calcium carbonate from being mixed in, but boiling seawater requires a lot of energy and the equipment is large, which may increase processing costs. On the other hand, acid addition degassing only involves adding acid, which can suppress increases in processing costs. Therefore, which degassing process to select can be chosen appropriately based on the purity of the magnesium hydroxide to be recovered and the acceptable processing costs. Furthermore, other degassing methods may be used in addition to boiling degassing and acid addition degassing.

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

As shown in Figure 16 (c), calcium compounds are scattered on the surface of the sediment from seawater that has not been degassed, but there are almost no dark gray areas in Figures 16 (a) and (b), and it can be seen 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.

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

Comparing the EDS analysis results of the sediment of seawater without degassing shown in Figure 17(c) with the EDS analysis results of the sediment of seawater that was subjected to boiling degassing shown in Figure 17(a) and the EDS analysis results of the sediment of seawater that was subjected to acid-addition degassing shown in Figure 17(b), the abundance ratio of calcium element in the sediment of the degassed seawater was significantly reduced. This proved that the boiling method and the acid-addition method are effective for the production of calcium carbonate. In addition, the EDS analysis results show that the molar ratio of magnesium ion to calcium ion in the sediment that was subjected to degassing treatment was 30.54:0.41 for the boiling degassing treatment and 31.23:0.28 for the acid-addition method. Therefore, it can be seen that a magnesium compound with a purity of 99% can be recovered by the degassing treatment. Furthermore, since the molar ratio of magnesium to oxygen is 1:2 in magnesium compounds that do not contain calcium carbonate, it can be seen that the magnesium compound is magnesium hydroxide.

EMBODIMENT 2

Figure 18 is a diagram showing the configuration of the magnesium recovery device of embodiment 2. The magnesium recovery device of embodiment 2 has the following two main features:

The first point is that in the above-mentioned embodiment 1, in order to reduce calcium carbonate mixed into the magnesium hydroxide precipitate, the carbon dioxide contained as bicarbonate ions in the seawater is degassed to reduce calcium carbonate, whereas in the present embodiment 2, the carbon dioxide contained as bicarbonate ions in the seawater is removed as calcium carbonate by the first stage of electrolysis.

The second point is that the sodium sulfate aqueous solution used in the anode tank 20a during electrolysis is regenerated and reused.

As shown in FIG. 18, the magnesium recovery device of this embodiment 2 is mainly composed of two electrolytic stacks (electrolytic cells) 21, 22 and one regeneration cell 30. In FIG. 18, "P" indicates a pump and "FM" indicates a flow meter. In FIG. 18, the DC power supply unit 1 shown in FIG. 1 and the inverter power supplies 6a, 6c that drive the pumps are omitted due to space limitations.

The two electrolysis stacks 21 and 22 have the same configuration as the electrolysis stack 2 used in embodiment 1, which is partitioned into an anode side tank 20a and a cathode side tank 20b by a cation exchange membrane 2b, as shown in Figures 2 and 3.

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, as shown in FIG. 18, so that the sodium sulfate aqueous solution in the anode side tank 20a of the first electrolysis stack 21 is introduced into the anode side tank 20a of the second electrolysis stack 22, and the seawater in the cathode side tank 20b of the first electrolysis stack 21 is introduced into the cathode side tank 20b of the second electrolysis stack 22.

A first filtration device 40 is provided on the piping path connecting the cathode side tank 20b of the first electrolysis stack 21 and the cathode side tank 20b of the second electrolysis stack 22. The calcium hydroxide and magnesium hydroxide precipitated 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 tank 20b of the second electrolysis stack 22.

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

The sodium sulfate aqueous solution regenerated in one compartment of the regeneration tank 30 is supplied to the anode 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.

Next, the process flow in the magnesium recovery device of this embodiment 2 will be described. Un-deaerated seawater (deep ocean water) stored in a seawater tank is supplied to the cathode side tank 20b of the first electrolysis stack 21 by a pump, and 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 of the regenerated aqueous sodium sulfate solution may be measured, and if the concentration is insufficient, not reaching a weight concentration of 5%, sodium sulfate may be added to adjust the concentration to a sufficient level.

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

In other words, by performing primary electrolysis with a unit volume electrical charge of 3600 C/L, almost all of the carbon dioxide contained as bicarbonate ions in the seawater is consumed in the precipitation of calcium carbonate, so 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) process) of embodiment 2 corresponds to the carbon dioxide gas removal process of the present invention.

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

Then, in the second electrolysis stack 22, secondary electrolysis is performed at a unit volume electrical 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, when the total unit volume electrical quantity of the primary electrolysis and secondary electrolysis exceeds 12000 C/L, the precipitation of calcium hydroxide increases, causing calcium hydroxide to be mixed into the recovered magnesium hydroxide, thereby preventing a decrease in the purity of the recovered magnesium hydroxide.
In addition, when calcium hydroxide is removed in a later process and magnesium hydroxide is refined, electrolysis may be performed at a total unit volume electricity quantity of 12000 C/L or more (for example, 14400 C/L) that allows complete recovery of magnesium from seawater. Even when electrolysis is performed at a total unit volume electricity quantity of 10800 C/L, as described in embodiment 1, a small amount of calcium (calcium hydroxide) precipitates, so if it is desired to further increase the purity of magnesium hydroxide, electrolysis may be performed at a total unit volume electricity quantity of 7200 C/L 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 electricity quantity is 10800 C/L, but magnesium can be recovered at a high purity.

In this way, by filtering the seawater that has undergone secondary electrolysis in the second electrolysis stack 22 using the second filtration device 41, almost all of the magnesium contained in the seawater can be recovered as insoluble magnesium hydroxide, and since the recovered magnesium hydroxide contains almost no calcium carbonate or calcium hydroxide, high-purity magnesium hydroxide can be recovered.

The seawater and sodium sulfate aqueous solution that have completed secondary electrolysis are then supplied to each compartment of the regeneration tank 30, as shown in Figure 18.

In this regeneration tank 30, as shown in FIG. 18, the sodium ions that have migrated to 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, the hydrogen ions in the aqueous sodium sulfate solution, which has become highly concentrated by the first and second electrolysis, pass through the cation exchange membrane and migrate to the seawater side, where they react with and neutralize the hydroxide ions in the seawater, which has become 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 be discharged into the sea with only magnesium recovered from the original seawater.

According to the above-mentioned embodiment, the cation exchange membrane 2b is used to separate the anode side tank 20a having the anode 2a and 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 allows the hydroxide ion concentration in the cathode side tank 20b to be efficiently increased by electrolysis using less power, thereby improving the efficiency of magnesium hydroxide production.

In addition, 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 positive ions, generated in the anode side tank 20a, which would result in 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 unit volume of electricity, the status of recovery (precipitation) by electrolysis can be accurately grasped based on the unit volume of electricity.

In addition, according to the above-mentioned embodiment, seawater, which is easily available and inexpensive, is used as the aqueous solution containing at least magnesium ions, so magnesium can be recovered inexpensively.

In addition, according to the above-mentioned embodiment, the movement of chlorine ions to the anode side tank 20a is restricted by using a cation exchange membrane, and since the anode side tank 20a uses a sodium sulfate aqueous solution, which is non-chlorine electrolytic water that does not contain chlorine ions, it is possible to prevent harmful chlorine gas from being generated in the anode side tank 20a, and therefore it is possible to suppress the increase in the cost of treating chlorine gas and the cost of recovering magnesium due to the use of non-chlorine electrolytic water.

In addition, according to the above-mentioned embodiment 2, in the regeneration tank 30, sodium ions, which are cations that have been reduced from the sodium sulfate aqueous solution by electrolysis, are returned from seawater to the sodium sulfate aqueous solution and regenerated, and the regenerated sodium sulfate aqueous solution can be used repeatedly, which further reduces the cost of recovering magnesium.

In addition, according to the above-mentioned 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, so the amount of calcium carbonate that gets mixed into the recovered magnesium hydroxide can be reduced, and the purity of the recovered magnesium hydroxide can be increased.

Furthermore, according to the above-mentioned embodiment 1, dissolved carbon dioxide gas can be easily removed by performing boiling degassing, in which seawater is heated to a boil.

In addition, according to the above-mentioned embodiment 2, carbon dioxide contained in seawater as bicarbonate ions is removed by performing the first electrolysis and recovering it as calcium carbonate, so that high-purity magnesium hydroxide can be recovered and calcium carbonate can also be recovered.

In addition, according to the above-mentioned embodiment 2, the total unit volume power applied to the seawater electrolysis is 14,400 (3,600 + 10,800) C/L, so 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.

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

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.

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, for example, an aqueous solution containing magnesium ions obtained by processing magnesium minerals, or may be, for example, an aqueous solution obtained by converting seawater into fresh water using a reverse osmosis membrane.

In addition, the above embodiment illustrates 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 the seawater that can be stored in the electrolysis stack 2 is treated in units of seawater.

In addition, in the above embodiment, a sodium sulfate aqueous solution, which is non-chlorine electrolytic water, is introduced into the anode side tank 20a to prevent harmful chlorine gas from being generated. 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 chlorine gas generated, seawater may be introduced into the anode side tank 20a in the same way as the cathode side tank 20b.

In the above embodiment, sodium sulfate, which is easy to obtain and inexpensive, is used as the non-chlorine electrolytic water that does not contain chlorine ions to prevent the generation of chlorine gas, but the present invention is not limited to this. The non-chlorine electrolytic water may be a sulfate other than sodium, or may be sodium nitrate or a nitrate other than sodium, and may be an aqueous solution of a water-soluble salt that does not generate chlorine gas when the aqueous solution is 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 the sodium ion in the case of seawater, a sulfate or nitrate containing a cation common to the cation may be used. Although an aqueous solution of these sulfates or nitrates is common, an aqueous solution of phosphates, oxalates, and chromates other than sulfates and nitrates may also be used. The cations that form salts with these acids can be of any type, such as sodium, potassium, etc. However, if a monovalent cation exchange membrane is used, it is necessary to use monovalent cations.

In addition, in the above embodiment, the weight concentration of the sodium sulfate aqueous solution is 5%. However, these concentrations can be appropriately determined depending on the type of non-chlorine electrolytic water used. The concentration should be such 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 electrolytic water move to the cathode side tank 20b.

In addition, in the above embodiment 2, an example is given in which seawater that has undergone secondary electrolysis is supplied to a tank different from the tank to which the sodium sulfate aqueous solution in the regeneration tank 30 is supplied, but 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 the non-chlorine electrolyzed water by electrolysis may be supplied.

In addition, in the above embodiment 2, 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 closer to that of actual seawater by adding acid, etc.

In addition, in the above embodiment, a configuration is illustrated in which 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 polarity is reversed to remove the deposited deposits. However, the present invention is not limited to this, and for the cathode 2c, a metal electrode that does not have a platinum layer on its surface may be used.

In addition, although not specifically implemented in the above embodiment, an additive capable of inhibiting adhesion of precipitates to the electrodes may be added to the aqueous solution, such as seawater, supplied to the cathode side tank 20b.

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 to recover calcium in the seawater as calcium hydroxide.

In addition, although not specifically implemented in the above embodiment, it goes without saying that the hydrogen and oxygen produced by electrolysis may be collected and used.

In addition, in the above embodiment, a dedicated degassing treatment device 16 is illustrated, 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.

As an example of how the present invention can be used, 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.

1 DC power supply unit 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 device 13a Inlet pipe 13c Inlet pipe 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

The present invention relates to a carbon dioxide removal method for removing carbon dioxide contained in seawater .

In order to solve the above problems, the carbon dioxide removal method according to claim 1 of the present invention comprises the steps of:
A carbon dioxide removal method for removing carbon dioxide contained in seawater by electrolyzing the seawater , comprising the steps of:
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 in which the movement of anions from the cathode-side tank to the anode-side tank is restricted.
According to this feature, a carbon dioxide removal method for removing carbon dioxide contained in seawater can be provided .

A method for removing carbon dioxide according to claim 2 of the present invention is the method for removing carbon dioxide according to claim 1 ,
The method is characterized in that an aqueous solution not containing chloride ions is introduced into the anode side tank.
According to this feature, it is possible to prevent harmful chlorine gas from being generated on the anode side due to electrolysis.

A method for removing carbon dioxide according to claim 3 of the present invention is the method for removing carbon dioxide according to claim 2 , comprising the steps of:
The aqueous solution not containing chloride ions is characterized by being an aqueous solution of sodium sulfate.
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, which would otherwise be caused by using an aqueous solution that does not contain chloride ions .

Claims (1)

A method for recovering magnesium, comprising electrolyzing an aqueous solution containing at least magnesium ions to generate and recover magnesium hydroxide, which is poorly soluble in water, the method comprising the steps of:
A method for recovering magnesium, comprising: partitioning an electrolytic cell having an anode and a cathode by a cation exchange membrane into an anode-side cell having the anode and a cathode-side cell having the cathode; introducing the aqueous solution into the cathode-side cell; and performing electrolysis while restricting the movement of anions from the cathode-side cell to the anode-side cell.

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