Magnesium chloride production system and method for operating magnesium chloride production system

Abstract

To recover magnesium ions with high efficiency from treated water made from seawater, by suppressing an effect of pH while reducing an addition amount of chemicals.SOLUTION: A magnesium chloride production system includes: a first removal unit for discharging concentrated water in which a concentration of sulfate ions is reduced by electrodialysis and magnesium ions are concentrated; a pH adjustment unit that adds a pH adjuster to the concentrated water discharged from the first removal unit to adjust pH of the concentrated water to fall within a range of 2<pH<5; a second removal unit to which a portion of the concentrated water having pH adjusted by the pH adjustment unit is supplied and which discharges sodium-reduced water having a reduced concentration of sodium ions; and a concentration unit that concentrates the sodium-reduced water discharged from the second removal unit and generates a slurry in which magnesium chloride is crystallized. The second removal unit includes a nanofiltration membrane that separates monovalent ions and multivalent ions from a portion of the concentrated water with pH adjusted.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a magnesium chloride production system and a method for operating the magnesium chloride production system.

Background Art

[0002] Conventionally, as a method for recovering magnesium chloride from seawater, as described in Patent Document 1, after separating sulfate ions (SO4 2- ) in seawater by electrodialysis (ED), a method of separating sodium ions (Na + ) by pressure filtration using a nanofiltration membrane (NF) is known. In this method, by removing sulfate ions and sodium ions from treated water such as seawater, an aqueous solution mainly composed of magnesium chloride (MgCl2) is obtained. Magnesium chloride is precipitated by concentrating the aqueous solution mainly composed of magnesium chloride by crystallization.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, in the method of Patent Document 1 described above, when performing filtration with a nanofiltration membrane, the separation performance of the membrane changes depending on the pH of the solution permeating through the nanofiltration membrane. Therefore, due to fluctuations in pH, there is a possibility that magnesium ions cannot be sufficiently recovered from the treated water using seawater as a raw material. On the other hand, in order to keep the pH within a predetermined range, it is also conceivable to adjust the pH by adding an acidic chemical, but it is also required to suppress the amount of chemical added.

[0005] The present disclosure has been made to solve the above problems, and an object thereof is to provide a magnesium chloride production system capable of efficiently recovering magnesium ions from treated water using seawater as a raw material while reducing the amount of chemicals added and suppressing the influence of pH, and an operation method of the magnesium chloride production system.

Means for Solving the Problems

[0006] To solve the above problems, the magnesium chloride production system according to the present disclosure reduces sulfate ions in treated water using seawater as a raw material by electrodialysis, and discharges concentrated water in which the concentration of the sulfate ions is reduced and magnesium ions are concentrated. A first removal unit; a pH adjustment unit that adds a pH adjuster to the concentrated water discharged from the first removal unit to adjust the pH of the concentrated water within a range of 2 < pH < 5; a part of the concentrated water whose pH is adjusted by the pH adjustment unit is supplied, and the concentration of sodium ions contained in a part of the concentrated water whose pH is adjusted is reduced, and a second removal unit that discharges sodium-reduced water in which the concentration of the sodium ions is reduced; and the sodium-reduced water discharged from the second removal unit is concentrated to generate a slurry in which magnesium chloride crystallizes. A concentrating unit, wherein the second removing unit has a nanofiltration membrane that separates monovalent ions and polyvalent ions from a part of the concentrated water whose pH is adjusted.

[0007] Moreover, the operation method of the magnesium chloride production system according to the present disclosure reduces sulfate ions in the water to be treated using seawater as a raw material by electrodialysis, and discharges concentrated water in which the sulfate ions are reduced and magnesium ions are concentrated, and diluted water in which the sulfate ions are concentrated and the magnesium ions are diluted. A first removal unit, a pH adjustment unit that adds a pH adjuster to the concentrated water discharged from the first removal unit and adjusts the pH in the concentrated water to between 2 and 5, and a part of the concentrated water whose pH has been adjusted by the pH adjustment unit is supplied. A second removal unit that reduces the concentration of sodium ions contained in a part of the concentrated water and discharges sodium-reduced water with a reduced sodium ion concentration, and a concentration unit that concentrates the sodium-reduced water discharged from the second removal unit to generate a slurry in which magnesium chloride has crystallized. In the operation method of the magnesium chloride production system, the second removal unit has a nanofiltration membrane that separates monovalent ions and polyvalent ions from a part of the concentrated water whose pH has been adjusted, and when the nanofiltration membrane has an isoelectric point in the range of 2 < pH < 5, the pH adjustment unit adjusts the pH in the concentrated water to between 2 and 5.

Effects of the Invention

[0008] According to the magnesium chloride production system and the operation method of the magnesium chloride production system of the present disclosure, it is possible to recover magnesium ions from the water to be treated using seawater as a raw material with high efficiency while reducing the amount of chemical added and suppressing the influence of pH.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0010] Hereinafter, with reference to the accompanying drawings, embodiments for implementing a magnesium chloride production system according to the present disclosure will be described. However, the present disclosure is not limited to only this embodiment.

[0011] <First Embodiment> (Magnesium Chloride Production System) The magnesium chloride production system reduces sulfate ions and sodium ions in seawater and produces magnesium chloride (MgCl2) from seawater. When concentrating seawater, various types of salts precipitate. Specifically, as salts that precipitate when concentrating seawater, first, a small amount of iron oxide (Fe2O3) precipitates due to the difference in solubility in water. Next, calcium carbonate (CaCO3) precipitates, and then calcium sulfate (CaSO4), sodium chloride (NaCl), and magnesium sulfate (MgSO4) precipitate in order. At that time, the precipitation amounts of these salts are in the order of the most sodium chloride, followed by magnesium sulfate, calcium sulfate, calcium carbonate, and iron oxide. And the magnesium chloride to be produced precipitates after these salts have precipitated. Furthermore, focusing on the salts that precipitate before magnesium chloride, if sulfate ions (SO4 2- ) contained in seawater can be selectively reduced, the precipitation of magnesium sulfate and calcium sulfate can be suppressed. Similarly, if sodium ions (Na + ) contained in seawater can be selectively reduced, the precipitation of sodium chloride can be suppressed. Therefore, in the magnesium chloride production system 1 of the present embodiment, by selectively reducing sulfate ions and sodium ions in seawater, the precipitation amount of magnesium chloride is increased.

[0012] The magnesium chloride production system 1 of the present embodiment includes a first removal unit 10, a pH adjustment unit 30, a second removal unit 20, and a concentration unit 50.

[0013] The first removal unit 10 reduces sulfate ions in the water to be treated W using seawater as a raw material by electrodialysis. The first removal unit 10 is an electrodialysis cell having a plurality of different types of membranes. For example, the first removal unit 10 of the present embodiment is an electrodialysis cell having four types of membranes, namely a cation exchange membrane, a monovalent selective anion exchange membrane, a monovalent selective cation exchange membrane, and an anion exchange membrane, between an anode 11 (see FIG. 3) and a cathode 12 (see FIG. 3). Here, the "cation exchange membrane" refers to an ion exchange membrane that allows cations to permeate regardless of valence and does not allow anions to permeate. The "monovalent selective anion exchange membrane" refers to an ion exchange membrane that selectively allows monovalent anions to permeate and does not allow polyvalent anions and cations regardless of valence to permeate. The "monovalent selective cation exchange membrane" refers to an ion exchange membrane that selectively allows monovalent cations to permeate and does not allow polyvalent cations and anions regardless of valence to permeate. Further, the "anion exchange membrane" refers to an ion exchange membrane that allows anions to permeate regardless of valence and does not allow cations to permeate. Note that the first removal unit 10 can also employ a known electrodialysis cell. By removing anions with a valence of two or more using the electrodialysis cell, the concentration of sulfate ions in the water to be treated W is reduced. The water to be treated W includes, in addition to seawater, concentrated seawater obtained by removing and concentrating water from seawater. The concentrated seawater corresponds to, for example, a concentrated solution generated by subjecting seawater to reverse osmosis membrane treatment and separating water. The water to be treated W is supplied to the first removal unit 10 via the first supply line L1 from the sea. The water to be treated W is supplied to the first removal unit 10 without performing a decarbonation treatment on the seawater or concentrated seawater.

[0014] In the first removal unit 10, by supplying the water to be treated W and performing electrodialysis, the concentration of divalent anions such as sulfate ions is reduced, and magnesium ions (Mg 2+)-concentrated concentrated water CW is discharged. Specifically, concentrated water CW in which calcium ions, magnesium ions, and chloride ions are concentrated is discharged from the first removal unit 10. The amount of concentrated water CW discharged from the first removal unit 10 is about several percent of the amount of water to be treated W supplied to the first removal unit 10. The concentrated water CW discharged from the first removal unit 10 is sent to the overflow circulation line LO. In the overflow circulation line LO, the concentrated water CW is sent upward in the vertical direction and then circulated downward, and returned to the first removal unit 10 again. The concentrated water CW returned to the first removal unit 10 is electrodialyzed again. The overflow circulation line LO is connected to the second supply line L2. A part of the concentrated water CW flowing through the overflow circulation line LO is sent to the second supply line L2. The second supply line L2 is connected to the second removal unit 20.

[0015] Also, in the first removal unit 10, separately from the concentrated water CW, dilution water AW with a reduced concentration of monovalent ions such as sodium ions and chloride ions (Cl - ) and first wastewater EW1 in which sulfate ions, sodium ions, and chloride ions are concentrated are discharged. The dilution water AW is either merged into the pH-adjusted concentrated water CPW described later immediately before flowing into the second removal unit 20 after passing through an RO membrane (not shown), or sent to the first supply line L1 and returned to the first removal unit 10 without passing through the RO membrane, or merged with the first wastewater EW1 and discarded.

[0016] The pH adjustment unit 30 adds a pH adjuster to the concentrated water CW discharged from the first removal unit 10. The pH adjustment unit 30 adjusts the pH of the concentrated water CW to be within the range of 2 < pH < 5 by adding the pH adjuster. In the pH adjustment unit 30 of the present embodiment, the pH adjuster is added to the overflow circulation line LO. The concentrated water CW to which the pH adjuster is added has its pH adjusted to become pH-adjusted water PW, and flows through the overflow circulation line LO as it is and is sent to the first removal unit 10.

[0017] The second removal unit 20 is supplied with pH-adjusted concentrated water CPW, which is a part of the concentrated water CW whose pH has been adjusted by the pH adjustment unit 30. The pH-adjusted concentrated water CPW is the concentrated water CW that has been treated by the first removal unit 10 in a state where the pH-adjusted water PW and the newly supplied water to be treated W are mixed. The second removal unit 20 reduces the concentration of sodium ions contained in the pH-adjusted concentrated water CPW. That is, the second removal unit 20 reduces the concentration of sodium ions in the pH-adjusted concentrated water CPW discharged from the first removal unit 10 and having the concentration of sulfate ions reduced. In the second removal unit 20, the concentration of magnesium ions in the pH-adjusted concentrated water CPW is hardly reduced.

[0018] The second removal unit 20 is connected to the first removal unit 10. The pH-adjusted concentrated water CPW is supplied to the second removal unit 20 via the overflow circulation line LO and the second supply line L2. The second removal unit 20 is arranged at least one in the subsequent stage (the downstream position in the flow direction of the water to be treated W) of the first removal unit 10. In the magnesium chloride production system 1 of the present embodiment, only one second removal unit 20 is arranged. The second removal unit 20 of the present embodiment has a nanofiltration membrane (NF membrane). The nanofiltration membrane is a membrane capable of separating monovalent ions and polyvalent ions by allowing only monovalent ions to pass through. The nanofiltration membrane separates monovalent ions and polyvalent ions from the pH-adjusted concentrated water CPW in which the concentration of sulfate ions has been reduced in the first removal unit 10. Note that the nanofiltration membrane of the present embodiment is a membrane having an isoelectric point within the range of 2 < pH < 5 regardless of the positive or negative of the zeta potential (the charge characteristics of the membrane surface). By treating the pH-adjusted concentrated water CPW with such a nanofiltration membrane, it is separated into sodium ion-reduced water NW having a lower concentration of sodium ions than the pH-adjusted concentrated water CPW and second wastewater EW2 having an increased concentration of sodium ions than the pH-adjusted concentrated water CPW. The sodium ion-reduced water NW discharged from the second removal unit 20 is sent to the third supply line L3. The third supply line L3 is connected to the concentration unit 50.

[0019] The concentration unit 50 concentrates the sodium ion-reduced water NW that is discharged from the second removal unit 20 and has a reduced sodium ion concentration. As a result, the concentration unit 50 generates a slurry S in which magnesium chloride has crystallized. The concentration unit 50 is connected to the second removal unit 20 via the third supply line L3. The sodium ion-reduced water NW is supplied to the concentration unit 50 via the third supply line L3. As the concentration unit 50, a known crystallization device can be adopted. For example, the concentration unit 50 can adopt a configuration in which the sodium ion-reduced water NW is processed by heating, depressurizing, blowing air, and combinations thereof to evaporate moisture.

[0020] By concentrating the sodium ion-reduced water NW, the ions dissolved in the sodium ion-reduced water NW become salts and precipitate in the concentration unit 50. Here, in the present embodiment, sulfate ions and sodium ions are reduced by the first removal unit 10 and the second removal unit 20 before being supplied to the concentration unit 50. Therefore, in the concentration unit 50, in addition to a small amount of iron oxide and calcium carbonate, the target magnesium chloride precipitates, and a slurry S in which magnesium chloride has crystallized is obtained. Also, in the concentration unit 50, when the slurry S is generated, a waste liquid FW that does not contain the slurry S is generated.

[0021] Further, the slurry S in which magnesium chloride has crystallized, obtained in the magnesium chloride production system 1, is used for the production of metallic magnesium in the magnesium production system 100. The magnesium production system 100 includes a production unit 60 in addition to the above-described magnesium chloride production system 1.

[0022] The production unit 60 is a device that separates water from the slurry S in which magnesium chloride has crystallized to obtain magnesium chloride. The slurry S is supplied from the concentration unit 50 to the production unit 60 via the fourth supply line L4.

[0023] In the generation unit 60, the slurry S can be processed by heating, reducing the pressure, blowing air, and combinations thereof to evaporate moisture. The magnesium chloride generated in the generation unit 60 is sent to the electrolysis unit 70 via the fifth supply line L5.

[0024] The electrolysis unit 70 is a device that performs molten salt electrolysis of magnesium chloride to obtain metallic magnesium. As the configuration of the electrolysis unit 70, known molten salt electrolysis equipment can be adopted.

[0025] In the magnesium chloride production system 1 as described above, depending on the type of the nanofiltration membrane in the second removal unit 20, the operation switches the implementation of pH adjustment of the concentrated water CW by the pH adjustment unit 30. Specifically, the operation method of the magnesium chloride production system 1 is such that when the nanofiltration membrane has an isoelectric point within the range of 2 < pH < 5, the pH adjustment unit 30 adjusts the pH in the concentrated water CW to near the isoelectric point between 2 and 5. Also, the operation method of the magnesium chloride production system 1 is such that when the nanofiltration membrane does not have an isoelectric point within the range of 2 < pH < 5, the pH adjustment unit 30 adjusts the pH between 2 and 5 but does not adjust the pH in the concentrated water CW to a specific value such as the isoelectric point. For example, when the nanofiltration membrane does not have an isoelectric point within the range of 2 < pH < 5, regardless of the isoelectric point, it is only adjusted so that the chemical amount such as pH 4.5 is minimized. That is, only when the nanofiltration membrane in the second removal unit 20 has an isoelectric point within the range of 2 < pH < 5, the pH adjustment unit 30 adjusts the pH of the concentrated water CW to the isoelectric point.

[0026] In the magnesium chloride production system 1 as described above, the water to be treated W such as seawater supplied from the sea is sent to the first removal unit 10 via the first supply line L1. The first removal unit 10 performs electrodialysis on the supplied water to be treated W, discharges the concentrated water CW in which magnesium ions are concentrated, the diluted water AW in which the concentration of divalent anions such as sulfate ions is reduced, and the remaining first wastewater EW1. The diluted water AW and the first wastewater EW1 generated in the first removal unit 10 are discharged to the outside.

[0027] Also, the discharged concentrated water CW is sent to the overflow circulation line LO. Here, when the nanofiltration membrane of the second removal unit 20 is a membrane having an isoelectric point within the range of 2 < pH < 5, hydrochloric acid is added as a pH adjuster to the concentrated water CW flowing through the overflow circulation line LO by the pH adjustment unit 30. As a result, the pH in the concentrated water CW is adjusted within the range of 2 < pH < 5. The concentrated water CW whose pH has been adjusted to become pH-adjusted water PW is circulated by the overflow circulation line LO and supplied again to the first removal unit 10. Therefore, the circulated pH-adjusted water PW and the water to be treated W sent through the first supply line L1 are simultaneously supplied to the first removal unit 10. Thereafter, the pH-adjusted water PW and the water to be treated W that have been treated again in the first removal unit 10 are discharged as concentrated water CW whose pH has been adjusted. In this way, the concentrated water CW continues to circulate while its pH is adjusted by the pH adjustment unit 30.

[0028] Furthermore, a part of the concentrated water CW whose pH has been adjusted is sent from the overflow circulation line LO to the second supply line L2. The pH-adjusted concentrated water CPW, which is a part of the concentrated water CW whose pH has been adjusted and is sent to the second supply line L2, is supplied to the second removal unit 20. As a result, in the second removal unit 20, sodium ion-reduced water NW with a reduced sodium ion concentration and the remaining second wastewater EW2 are generated. The second wastewater EW2 generated in the second removal unit 20 is discharged to the outside.

[0029] Also, the sodium ion-reduced water NW generated in the second removal unit 20 is supplied to the concentration unit 50 via the third supply line L3. As a result, the sodium ion-reduced water NW is supplied to the concentration unit 50. In the concentration unit 50, the sodium ion-reduced water NW is concentrated, and a slurry S in which magnesium chloride has crystallized is obtained. Also, waste liquid FW is generated as the remainder and discharged to the outside.

[0030] In addition, the slurry S in which magnesium chloride crystallized in the concentration unit 50 is supplied to the generation unit 60 via the fourth supply line L4. In the generation unit 60, water is separated from the slurry S, and magnesium chloride is generated. Further, the magnesium chloride generated in the generation unit 60 is supplied to the electrolysis unit 70 via the fifth supply line L5, and metallic magnesium is generated.

[0031] (Function and effect) In the magnesium chloride production system 1 having the above configuration, concentrated water CW in which the concentration of sulfate ions is reduced and magnesium ions are concentrated is discharged by electrodialysis in the first removal unit 10. The pH of this concentrated water CW is adjusted within the range of 2 < pH < 5. Then, the concentrated water CW with the adjusted pH circulates with respect to the first removal unit 10. Further, a pH-adjusted concentrated water CPW, which is a part of the concentrated water CW with the adjusted pH, passes through the second removal unit 20 and the concentration unit 50, thereby obtaining a slurry S in which magnesium chloride has crystallized. In this way, by adjusting the pH of the concentrated water CW within the range of 2 < pH < 5, when reducing sodium ions by a nanofiltration membrane in the second removal unit 20, it is possible to suppress the reduction of magnesium ions. That is, it is possible to suppress a decrease in the concentration of magnesium contained in the sodium-ion-reduced water NW in the second removal unit 20. Therefore, it is possible to suppress a decrease in the amount of magnesium chloride contained in the slurry S that can be recovered in the concentration unit 50. As a result, magnesium ions can be recovered from the water to be treated W with high efficiency.

[0032] In particular, by setting the pH to less than 5 (pH < 5), it is possible to suppress the generation of scale in the first removal unit 10, the second removal unit 20, and the concentration unit 50 due to the calcium component and magnesium component contained in the concentrated water CW. Further, by having the pH exceed 2 (2 < pH), it is possible to suppress the corrosion of the first removal unit 10, the second removal unit 20, and the concentration unit 50, as well as the metal parts used in various lines (pipes).

[0033] Instead of adding the pH adjuster to the water to be treated W supplied to the first removal unit 10, the pH adjuster is added to the concentrated water CW discharged from the first removal unit 10. The amount of the concentrated water CW is extremely small compared to the amount of the water to be treated W supplied to the first removal unit 10. Therefore, the amount of the pH adjuster to be added can be suppressed. As a result, while reducing the chemical addition amount, the influence of pH can be suppressed, and magnesium ions can be efficiently recovered from the water to be treated W using seawater as a raw material.

[0034] Furthermore, in the present embodiment, the concentrated water CW is circulated while its pH is being adjusted. That is, by mixing the pH-adjusted water PW whose pH has already been adjusted and the concentrated water CW newly generated from the water to be treated W, the pH of the mixed solution is suppressed compared to the case of only the concentrated water CW newly generated from the water to be treated W. Therefore, the concentrated water CW circulating through the overflow circulation line LO has a low pH. As a result, when adjusting the pH within the range of 2 < pH < 5, the amount of the pH adjuster added by the pH adjustment unit 30 can be further suppressed.

[0035] Also, when the pH adjuster is directly added to the water to be treated W, the concentration of hydrogen ions (H + ) in the water to be treated W increases. Therefore, during electrodialysis in the first removal unit 10, the ratio of hydrogen ions that permeate through the cation exchange membrane and move to the concentrated water CW increases. As a result, the amount of magnesium ions moving to the concentrated water CW may decrease, and the magnesium ion concentration of the concentrated water CW may decrease. That is, there is a possibility that a phenomenon occurs in which the recovery efficiency of magnesium ions from the water to be treated W decreases. However, in the present embodiment, since the pH adjuster is added to the concentrated water CW, such a phenomenon can be suppressed from occurring.

[0036] Further, the nanofiltration membrane of the second removal unit 20 is a membrane having an isoelectric point within the range of 2 < pH < 5. The nanofiltration membrane has a charge characteristic on the membrane surface. And the lower the pH of the solution passing through the nanofiltration membrane, the higher the zeta potential becomes. In the nanofiltration membrane, the inventors have found that there is a correlation between this pH and the removal rate of magnesium ions. Specifically, it is known to the inventors that the lower the pH of the solution is within the range close to the isoelectric point of nanofiltration, the particularly improved the removal rate of magnesium ions becomes. On the other hand, in the present invention, the nanofiltration membrane has an isoelectric point within the range of 2 < pH < 5. Therefore, the removal rate (recovery rate) of magnesium ions in the second removal unit 20 can be improved.

[0037] Also, in the magnesium chloride production system 1, when the operation continues over a long period of time, it becomes necessary to replace the membranes used in the first removal unit 10 and the second removal unit 20. That is, the nanofiltration membrane used in the second removal unit 20 is also replaced. At that time, depending on the type of the nanofiltration membrane after replacement (whether it is a membrane having an isoelectric point within the range of 2 < pH < 5), the implementation of pH adjustment of the concentrated water CW in the pH adjustment unit 30 is switched. Specifically, when the nanofiltration membrane is not a membrane having an isoelectric point within the range of 2 < pH < 5, by not performing pH adjustment up to a specific value such as the isoelectric point, the pH additive can be used efficiently. Therefore, while reducing the addition amount of the chemical, suppressing the influence of pH, magnesium ions can be recovered from the water to be treated W using seawater as a raw material with high efficiency.

[0038] <Second Embodiment> Next, the magnesium chloride production system 1A according to the second embodiment of the present disclosure will be described. In the second embodiment described below, for the components common to the first embodiment, the same reference numerals are given in the drawings and the description thereof is omitted. The second embodiment is different from the first embodiment in that the magnesium chloride production system 1A reduces the carbonic acid dissolved in the pH-adjusted concentrated water CPW.

[0039] As shown in Fig. 2, the magnesium chloride production system 1A of the second embodiment further includes a decarbonation unit 80.

[0040] The decarbonation unit 80 reduces the carbonic acid dissolved in the pH-adjusted concentrated water CPW. That is, the decarbonation unit 80 is not arranged at a position upstream of the first removal unit 10 in the flow direction of the water to be treated W, but only at a position downstream of the water to be treated W in the flow direction. The decarbonation unit 80 is arranged between the first removal unit 10 and the second removal unit 20 or between the second removal unit 20 and the concentration unit 50. In the second embodiment, the decarbonation unit 80 arranged between the first removal unit 10 and the second removal unit 20 will be taken as an example for explanation. The decarbonation unit 80 is arranged in the middle of the second supply line L2. The decarbonation unit 80 can adopt a known configuration such as a decarbonation tower, for example. For example, the decarbonation unit 80 first adds an acid to the pH-adjusted concentrated water CPW. As the acid added to the pH-adjusted concentrated water CPW, the same chemical as the pH adjuster is preferable. Examples of the acid added in the decarbonation unit 80 include hydrochloric acid. Then, in the decarbonation unit 80, the carbonic acid contained in the pH-adjusted concentrated water CPW is removed by aerating the pH-adjusted concentrated water CPW. In the decarbonation unit 80, the pH-adjusted concentrated water CPW is aerated until the pH thereof is within the range of 2 < pH ≤ 4, and the decarbonation treatment is continued. This is because it can be considered that carbonic acid has been removed from the pH-adjusted concentrated water CPW which is a solution by setting the pH to 4 or less. The pH-adjusted concentrated water CPW decarbonated by the decarbonation unit 80 is supplied to the second removal unit 20 via the second supply line L2.

[0041] (Function and effect) In the magnesium chloride production system 1A of the second embodiment, the decarbonation unit 80 reduces the carbonic acid dissolved in the pH-adjusted concentrated water CPW. Therefore, the sodium ion-reduced water NW in a state where carbonic acid has been removed is supplied to the concentration unit 50, and a slurry S in which magnesium chloride has crystallized is generated. As a result, when concentrating in the concentration unit 50 to generate the slurry S, the amount of calcium carbonate generated as an impurity in the slurry S can be reduced. In particular, by performing a decarbonation treatment on the concentrated water CW discharged from the first removal unit 10 instead of the water to be treated W supplied to the first removal unit 10, the amount of acid added for the decarbonation treatment can be suppressed. Thereby, while reducing the amount of chemicals added in the decarbonation treatment, the generation of calcium carbonate as an impurity can be suppressed.

[0042] Also, in the present embodiment, the decarbonation unit 80 is disposed between the first removal unit 10 and the second removal unit 20. Therefore, carbonic acid can be removed from the pH-adjusted concentrated water CPW before passing through the nanofiltration membrane. Accordingly, the generation of carbonate scale on the nanofiltration membrane can be suppressed.

[0043] Note that in the present embodiment, the decarbonation unit 80 is disposed between the first removal unit 10 and the second removal unit 20, but it may be disposed between the second removal unit 20 and the concentration unit 50. Between the pH-adjusted concentrated water CPW supplied to the second removal unit 20 and the sodium ion-reduced water NW discharged from the second removal unit 20, the flow rate of the sodium ion-reduced water NW is smaller. Therefore, by disposing the decarbonation unit 80 between the second removal unit 20 and the concentration unit 50, the amount of acid added can be further suppressed. Thereby, while significantly reducing the amount of chemicals added, the generation of calcium carbonate as an impurity can be suppressed.

[0044] Further, the decarbonation unit 80 adjusts the pH of the pH-adjusted concentrated water CPW to 2 < pH ≤ 4 by adding an acid and then aerating it. Therefore, the pH is already adjusted within the range of 2 < PH < 5 in the pH adjustment unit 30, and then it is adjusted to the range of 2 < pH ≤ 4. As a result, almost no acid needs to be added. That is, the amount of acid to be added can be very much suppressed. Thereby, while greatly reducing the chemical addition amount, the generation of calcium carbonate as an impurity can be suppressed.

[0045] <Third Embodiment> Next, the magnesium chloride production system 1B of the third embodiment according to the present disclosure will be described. In the third embodiment described below, the components common to the first and second embodiments are denoted by the same reference numerals in the drawings, and the description thereof will be omitted. In the third embodiment, the internal configuration of the first removal unit 10B is different from that of the first and second embodiments.

[0046] In the magnesium chloride production system 1B of the third embodiment, as shown in FIG. 3, the first removal unit 10B is an electrodialysis cell having two different types of membranes. The first removal unit 10B of the third embodiment includes an anode 11, a cathode 12, a plurality of first membranes 15, and a plurality of second membranes 16. That is, in the first removal unit 10B, only two types of membranes, the first membrane 15 and the second membrane 16, are arranged between the anode 11 and the cathode 12. A plurality of first membranes 15 are arranged at intervals between the anode 11 and the cathode 12. A plurality of second membranes 16 are alternately arranged at intervals from the first membranes 15 between the anode 11 and the cathode 12. The first membrane 15 is a cation exchange membrane. The second membrane 16 is a monovalent selective anion exchange membrane or a nanofiltration membrane.

[0047] Further, in the first removal unit 10B, a plurality of concentration chambers 102 and a plurality of dilution chambers 101 are formed by the alternately arranged first membranes 15 and second membranes 16 between the anode 11 and the cathode 12. The concentration chamber 102 and the dilution chamber 101 are adjacent to each other via the first membrane 15 or the second membrane 16.

[0048] In the concentration chamber 102, calcium ions (Ca 2+ ) and magnesium ions that have permeated through the first membrane 15, which is a cation exchange membrane, from the adjacent dilution chamber 101 flow in. Further, chloride ions that have permeated through the second membrane 16, which is a monovalent selective anion exchange membrane or a monovalent selective nanofiltration membrane, from the dilution chamber 101 located adjacent on the opposite side flow into the concentration chamber 102. Both the inlet and the outlet of the concentration chamber 102 are connected to the overflow circulation line LO. As a result, in the concentration chamber 102, concentrated water CW in which calcium ions, magnesium ions, and chloride ions are concentrated is discharged. The concentrated water CW discharged from the concentration chamber 102 circulates through the pH adjustment unit 30 by the overflow circulation line LO and is supplied to the concentration chamber 102 again.

[0049] Also, sulfate ions that could not permeate through the second membrane 16 remain in the dilution chamber 101. Further, a small amount of remaining calcium ions and the like that did not permeate through the first membrane 15 remain in the dilution chamber. Diluted water AW is discharged from the dilution chamber 101.

[0050] (Function and effect) In the magnesium chloride production system 1B of the third embodiment, in the first removal unit 10B, a cation exchange membrane is used as the first membrane 15, and a monovalent-selective anion exchange membrane or a monovalent-selective nanofiltration membrane is used as the second membrane 16. And only two types of membranes, the first membrane 15 and the second membrane 16, are arranged between the anode 11 and the cathode 12. The monovalent-selective anion exchange membrane and the monovalent-selective nanofiltration membrane selectively permeate monovalent ions and do not permeate polyvalent ions. Therefore, it is possible to remove more sulfate ions, which are divalent anions. That is, the first removal unit 10B has a structure specialized for removing sulfate ions. Further, in the first removal unit 10B, only a large number of a pair of membranes, the first membrane 15 and the second membrane 16, are arranged between the anode 11 and the cathode 12. Therefore, the membrane area of the entire membrane in the first removal unit 10B is reduced, and the cost of the membrane in the first removal unit 10B can be suppressed. Further, since a large number of a pair of membranes are arranged between the anode 11 and the cathode 12, the total number of membranes (the thickness of the portion through which electricity flows) between the anode 11 and the cathode 12 is smaller than that of an electrodialysis cell using three or four types of membranes. As a result, the power cost for electrodialysis can also be suppressed. Thus, in the first removal unit 10B, it is possible to remove a large amount of sulfate ions while suppressing the cost.

[0051] Also, a monovalent-selective anion exchange membrane or a monovalent-selective nanofiltration membrane is used as the second membrane 16, and the monovalent-selective nanofiltration membrane is much lower in cost than the monovalent-selective anion exchange membrane. Therefore, by using the monovalent-selective nanofiltration membrane instead of the monovalent-selective anion exchange membrane, it is possible to remove a large amount of sulfate ions while suppressing the cost more in the first removal unit 10B.

[0052] (Other Embodiments) As described above, the embodiments of the present disclosure have been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and design changes and the like within the scope not departing from the gist of the present disclosure are also included.

[0053] In the magnesium chloride production systems 1, 1A, and 1B, if a part of the concentrated water CW whose pH is adjusted by the pH adjustment unit 30 is supplied to the second removal unit 20, it is not limited to the above-described configuration. For example, the first removal units 10, 10B do not necessarily have a configuration in which the concentrated water CW is supplied to the overflow circulation line LO to circulate the concentrated water CW whose pH is adjusted. That is, the pH adjustment unit 30 does not necessarily have a structure in which a pH adjuster is added to the overflow circulation line LO. For example, the pH adjustment unit 30 may have a structure in which a pH adjuster is added to the second supply line L2. Further, in addition to the pH adjustment unit 30 that adds a pH adjuster to the overflow circulation line LO for the concentrated water CW, a structure may be provided that further includes a pH adjustment unit 30 that adds a pH adjuster to the second supply line L2.

[0054] Also, the second removal unit 20 is not limited to a structure having a single nanofiltration membrane as in this embodiment. The second removal unit 20 may have a multi-stage structure having a plurality of nanofiltration membranes. In that case, in the multi-stage nanofiltration membranes, a pH adjustment unit 30 may be arranged at the supply part of each nanofiltration membrane.

[0055] Also, the concentration unit 50 may have another configuration as long as a slurry S containing magnesium chloride is obtained. For example, the concentration unit 50 may obtain a slurry S containing magnesium chloride by using sedimentation or the like.

[0056] <Supplementary Note> The magnesium chloride production systems 1, 1A, and 1B described in each embodiment are understood as follows, for example.

[0057] (1) The magnesium chloride production systems 1, 1A, and 1B according to the first aspect reduce sulfate ions in the water to be treated W using seawater as a raw material by electrodialysis, and discharge concentrated water CW in which the concentration of the sulfate ions is reduced and magnesium ions are concentrated. A first removal unit 10, 10B; a pH adjustment unit 30 that adds a pH adjuster to the concentrated water CW discharged from the first removal units 10, 10B to adjust the pH of the concentrated water CW within the range of 2 < pH < 5; a part of the concentrated water CW whose pH is adjusted by the pH adjustment unit 30 is supplied, and the concentration of sodium ions contained in a part of the concentrated water CW whose pH is adjusted is reduced, and a second removal unit 20 that discharges sodium-reduced water in which the concentration of the sodium ions is reduced; and a concentration unit 50 that concentrates the sodium-reduced water discharged from the second removal unit 20 to generate a slurry S in which magnesium chloride crystallizes. The second removal unit 20 has a nanofiltration membrane that separates monovalent ions and polyvalent ions from a part of the concentrated water CW whose pH is adjusted.

[0058] According to such a configuration, by adjusting the pH of the concentrated water CW within the range of 2 < pH < 5, when the sodium ions are reduced by the nanofiltration membrane in the second removal unit 20, it is possible to suppress the reduction of magnesium ions. That is, it is possible to suppress the decrease in the concentration of magnesium contained in the sodium-reduced water NW in the second removal unit 20. Therefore, it is possible to prevent the amount of magnesium chloride contained in the slurry S that can be recovered in the concentration unit 50 from decreasing. As a result, magnesium ions can be recovered from the water to be treated W with high efficiency. And instead of the water to be treated W supplied to the first removal units 10, 10B, a pH adjuster is added to the concentrated water CW discharged from the first removal units 10, 10B. The amount of the concentrated water CW is extremely small compared to the amount of the water to be treated W supplied to the first removal units 10, 10B. Therefore, the amount of the pH adjuster to be added can be suppressed. As a result, while reducing the amount of chemical added, the influence of pH can be suppressed, and magnesium ions can be recovered from the water to be treated W using seawater as a raw material with high efficiency.

[0059] (2) The magnesium chloride production systems 1, 1A, and 1B according to the second aspect are the magnesium chloride production systems 1, 1A, and 1B of (1), wherein the nanofiltration membrane is a membrane having an isoelectric point within the range of 2 < pH < 5.

[0060] In the nanofiltration membrane, the inventors have found that there is a correlation between pH and the removal rate of magnesium ions. Specifically, it is known to the inventors that the lower the pH of the solution in the range close to the isoelectric point of nanofiltration, the particularly higher the removal rate of magnesium ions. In contrast, in the present invention, the nanofiltration membrane has an isoelectric point within the range of 2 < pH < 5. Therefore, the removal rate (recovery rate) of magnesium ions in the second removal unit 20 can be improved.

[0061] (3) The magnesium chloride production systems 1, 1A, and 1B according to the third aspect are the magnesium chloride production systems 1, 1A, and 1B of (1) or (2), further comprising a decarbonation unit 80 for reducing the carbonic acid dissolved in a part of the concentrated water CW whose pH has been adjusted.

[0062] According to such a configuration, sodium ion-reduced water NW in a state where carbonic acid has been removed is supplied to the concentration unit 50, and a slurry S in which magnesium chloride has crystallized is generated. As a result, when concentrating in the concentration unit 50 to generate the slurry S, the amount of calcium carbonate generated as an impurity contained in the slurry S can be reduced. In particular, by performing decarbonation treatment on the concentrated water CW discharged from the first removal units 10 and 10B instead of the water to be treated W supplied to the first removal units 10 and 10B, the amount of acid added can be suppressed. Thereby, while reducing the amount of chemical added, the generation of calcium carbonate as an impurity can be suppressed.

[0063] (4) The magnesium chloride production systems 1, 1A, and 1B according to the fourth aspect are the magnesium chloride production systems 1, 1A, and 1B of (3), wherein the decarbonation unit 80 is disposed between the first removal units 10 and 10B and the second removal unit 20 or between the second removal unit 20 and the concentration unit 50.

[0064] According to such a configuration, the decarbonation unit 80 is disposed between the first removal units 10 and 10B and the second removal unit 20. Therefore, carbonic acid can be removed from the pH-adjusted concentrated water CPW before passing through the nanofiltration membrane. Thus, the generation of carbonate scale on the nanofiltration membrane can be suppressed. Further, by disposing the decarbonation unit 80 between the second removal unit 20 and the concentration unit 50, the amount of acid to be added can be further suppressed. Thereby, while greatly reducing the chemical addition amount, the generation of calcium carbonate as an impurity can be suppressed.

[0065] (5) The magnesium chloride production systems 1, 1A, and 1B according to the fifth aspect are the magnesium chloride production systems 1, 1A, and 1B according to (3) or (4), wherein the decarbonation unit 80 adds an acid to a part of the concentrated water CW whose pH has been adjusted, adjusts the pH to within the range of 2 < pH ≤ 4, and then performs aeration.

[0066] According to such a configuration, the pH is already adjusted to within the range of 2 < PH < 5 by the pH adjustment unit 30, and then the pH is adjusted to within the range of 2 < pH ≤ 4. Therefore, almost no acid needs to be added. That is, the amount of acid to be added can be very much suppressed. Thereby, while greatly reducing the chemical addition amount, the generation of calcium carbonate as an impurity can be suppressed.

[0067] (6) The magnesium chloride production systems 1, 1A, and 1B according to the sixth aspect are the magnesium chloride production systems according to any one of (1) to (5), wherein the first removal unit 10B includes an anode 11, a cathode 12, a first membrane 15 disposed between the anode 11 and the cathode 12, and a second membrane 16 disposed alternately with a space from the first membrane 15 between the anode 11 and the cathode 12. Only two types of membranes, the first membrane 15 and the second membrane 16, are disposed between the anode 11 and the cathode 12. The first membrane 15 is a cation exchange membrane, and the second membrane 16 is a monovalent selective anion exchange membrane or a monovalent selective nanofiltration membrane.

[0068] According to such a configuration, only two types of membranes, i.e., the first membrane 15 and the second membrane 16, are arranged between the anode 11 and the cathode 12. Monovalent selective anion exchange membranes and monovalent selective nanofiltration membranes selectively allow monovalent ions to permeate and do not allow polyvalent ions to permeate. Therefore, it is possible to remove more sulfate ions, which are divalent anions. That is, the first removal unit 10B has a structure specialized for the removal of sulfate ions. Further, in the first removal unit 10B, only a large number of a pair of membranes, i.e., the first membrane 15 and the second membrane 16, are arranged between the anode 11 and the cathode 12. Therefore, the membrane area as a whole in the first removal unit 10B is reduced, and the cost of the membranes in the first removal unit 10B can be suppressed. Further, since a large number of a pair of membranes are arranged between the anode 11 and the cathode 12, the total number of membranes (the thickness of the portion through which electricity flows) between the anode 11 and the cathode 12 is smaller than that in an electrodialysis cell using three or four types of membranes. As a result, the power cost for electrodialysis can also be suppressed. Thus, in the first removal unit 10B, it is possible to remove a large amount of sulfate ions at a reduced cost.

[0069] (7) The operation method of the magnesium chloride production systems 1, 1A, and 1B according to the seventh aspect is to reduce sulfate ions in the water to be treated W using seawater as a raw material by electrodialysis, and discharge concentrated water in which the sulfate ions are reduced and magnesium ions are concentrated, and diluted water in which the sulfate ions are concentrated and the magnesium ions are diluted, from the first removal units 10 and 10B. A pH adjuster is added to the concentrated water CW discharged from the first removal units 10 and 10B to adjust the pH in the concentrated water CW to between 2 and 5. A part of the concentrated water CW whose pH has been adjusted by the pH adjustment unit 30 is supplied, the concentration of sodium ions contained in a part of the concentrated water CW is reduced, and a second removal unit 20 that discharges sodium-reduced water with the reduced sodium ion concentration is provided. The sodium-reduced water discharged from the second removal unit 20 is concentrated to generate a slurry S in which magnesium chloride has crystallized. In the operation method of the magnesium chloride production systems 1, 1A, and 1B including a concentration unit 50, the second removal unit 20 has a nanofiltration membrane that separates monovalent ions and polyvalent ions from a part of the concentrated water CW whose pH has been adjusted, and when the nanofiltration membrane is a membrane having an isoelectric point within the range of 2 < pH < 5, the pH adjustment unit 30 adjusts the pH in the concentrated water CW to between 2 and 5.

[0070] According to such a configuration, depending on the type of the nanofiltration membrane after replacement (whether it is a membrane having an isoelectric point within the range of 2 < pH < 5), the implementation of pH adjustment of the concentrated water CW in the pH adjustment unit 30 is switched. Therefore, when the nanofiltration membrane is not a membrane having an isoelectric point within the range of 2 < pH < 5, by not performing pH adjustment, the pH additive can be used efficiently.

Explanation of symbols

[0071] 1, 1A, 1B... Magnesium chloride production systems 10, 10B... First removal units 20... Second removal unit 50... Concentration unit 30... pH adjustment unit L1... First supply line LO... Overflow circulation line L2…Second supply line L3…Third supply line 100…Magnesium production system 60…Generation unit 70…Electrolysis unit L4…Fourth supply line L5…Fifth supply line W…Water to be treated CW…Concentrated water AW…Diluted water PW…pH-adjusted water EW1…First waste water CPW…pH-adjusted concentrated water NW…Sodium ion-reduced water EW2…Second waste water FW…Waste liquid S…Slurry 80…Decarbonation unit 11…Anode 12…Cathode 15…First membrane 16…Second membrane 101…Dilution chamber 102…Concentration chamber

Claims

1. A first removal unit that reduces sulfate ions in the water to be treated using seawater as a raw material by electrodialysis and discharges concentrated water in which the concentration of the sulfate ions is reduced and magnesium ions are concentrated; A pH adjustment unit that adds a pH adjuster to the concentrated water discharged from the first removal unit and adjusts the pH of the concentrated water within a range of 2 < pH < 5; A second removal unit to which a part of the concentrated water whose pH has been adjusted by the pH adjustment unit is supplied, reduces the concentration of sodium ions contained in a part of the concentrated water whose pH has been adjusted, and discharges sodium-reduced water in which the concentration of the sodium ions is reduced; A concentration unit that concentrates the sodium-reduced water discharged from the second removal unit to generate a slurry in which magnesium chloride has crystallized; and The second removal unit is a magnesium chloride production system having a nanofiltration membrane that separates monovalent ions and polyvalent ions from a part of the concentrated water whose pH has been adjusted.

2. The magnesium chloride production system according to claim 1, wherein the nanofiltration membrane is a membrane having an isoelectric point within a range of 2 < pH < 5.

3. The magnesium chloride production system according to claim 1 or claim 2, further comprising a decarbonation unit that reduces carbonic acid dissolved in a part of the concentrated water whose pH has been adjusted.

4. The magnesium chloride production system according to claim 3, wherein the decarbonation unit is disposed between the first removal unit and the second removal unit or between the second removal unit and the concentration unit.

5. The magnesium chloride production system according to claim 3, wherein the decarbonation unit adds an acid to a part of the concentrated water whose pH has been adjusted to adjust the pH within a range of 2 < pH ≤ 4 and then aerates it.

6. The first removal unit has an anode, a cathode, a first membrane disposed between the anode and the cathode, and a second membrane disposed alternately with a space from the first membrane between the anode and the cathode; Only two types of membranes, the first membrane and the second membrane, are disposed between the anode and the cathode; The first membrane is a cation exchange membrane; The magnesium chloride production system according to claim 1 or claim 2, wherein the second membrane is a monovalent-selective anion exchange membrane or a monovalent-selective nanofiltration membrane.

7. Reducing sulfate ions in the water to be treated using seawater as a raw material by electrodialysis, discharging concentrated water in which the sulfate ions are reduced and magnesium ions are concentrated, and diluted water in which the sulfate ions are concentrated and the magnesium ions are diluted, a first removal unit; A pH adjustment unit that adds a pH adjuster to the concentrated water discharged from the first removal unit to adjust the pH of the concentrated water to between 2 and 5; A second removal unit that is supplied with a part of the concentrated water whose pH has been adjusted by the pH adjustment unit, reduces the concentration of sodium ions contained in a part of the concentrated water, and discharges sodium-reduced water with a reduced sodium ion concentration; A method for operating a magnesium chloride production system comprising a concentration unit that concentrates the sodium-reduced water discharged from the second removal unit to produce a slurry in which magnesium chloride crystallizes, The second removal unit has a nanofiltration membrane that separates monovalent ions and polyvalent ions from a part of the concentrated water whose pH has been adjusted; A method for operating a magnesium chloride production system in which the pH adjustment unit adjusts the pH of the concentrated water to between 2 and 5 when the nanofiltration membrane has an isoelectric point within the range of 2 < pH < 5.

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