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

Abstract

The present invention comprises: a first removal unit that discharges a concentrated water in which magnesium ions are concentrated and the concentration of sulfate ions is reduced by electrodialysis; a pH adjustment unit that adjusts the pH of the concentrated water discharged from the first removal unit to be within the range of 2<pH<5; a second removal unit that is supplied with a portion of the concentrated water having a pH adjusted by the pH adjustment unit and that discharges sodium reduced water; 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 has a nanofiltration membrane that separates monovalent ions and polyvalent ions from the portion of the concentrated water having an adjusted pH.

Description

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

[0001] This application claims priority to Japanese Patent Application No. 2023-215821, filed on December 21, 2023, the contents of which are incorporated herein by reference.

[0002] Conventionally, as described in Patent Document 1, a method for recovering magnesium chloride from seawater involves removing sulfate ions (SO ) from seawater by electrodialysis (ED). ４ ２－ ), and then sodium ions (Na ＋ In this method, sulfate ions and sodium ions are removed from treated water such as seawater to separate magnesium chloride (MgCl ２ The aqueous solution containing magnesium chloride as the main component is concentrated by crystallization to precipitate magnesium chloride.

[0003] Japanese Patent Application Laid-Open No. 2021-109791

[0004] In the method of Patent Document 1, when filtering with a nanofiltration membrane, the separation performance of the membrane changes depending on the pH of the solution passing through the nanofiltration membrane. Therefore, due to pH fluctuations, there is a possibility that magnesium ions will not be sufficiently recovered from the treated water, which is made from seawater. On the other hand, while it is possible to adjust the pH by adding an acidic chemical to keep the pH within a predetermined range, it is also desirable to reduce the amount of chemical added.

[0005] The present disclosure provides a magnesium chloride production system and an operating method for the magnesium chloride production system that can reduce the amount of chemicals added, suppress the effect of pH, and recover magnesium ions with high efficiency from treated water made from seawater.

[0006] The magnesium chloride production system according to the present disclosure includes a first removal unit that uses seawater as a raw material to reduce sulfate ions in water to be treated by electrodialysis and discharges concentrated water in which the concentration of the sulfate ions has been reduced and magnesium ions has been concentrated; 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 be within a range of 2 < pH < 5; a second removal unit that receives a portion of the concentrated water whose pH has been adjusted by the pH adjustment unit, reduces the concentration of sodium ions contained in the portion of the concentrated water whose pH has been adjusted, and discharges reduced-sodium water in which the concentration of sodium ions has been reduced; and a concentration unit that concentrates the reduced-sodium water discharged from the second removal unit to produce a slurry in which magnesium chloride is crystallized, wherein the second removal unit has a nanofiltration membrane that separates monovalent ions and polyvalent ions from the portion of the concentrated water whose pH has been adjusted.

[0007] The method of operating the magnesium chloride production system according to the present disclosure includes a first removal unit that reduces sulfate ions in water to be treated using seawater as a raw material by electrodialysis, and discharges concentrated water in which the sulfate ions have been reduced and magnesium ions have been concentrated, and diluted water in which the sulfate ions have been concentrated and the magnesium ions have been diluted; 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; and a method of operating a magnesium chloride production system according to the present disclosure, wherein the method includes: a first removal unit that electrodialyzes sulfate ions in water to be treated using seawater as a raw material, and discharges concentrated water in which the sulfate ions have been reduced and magnesium ions have been concentrated, and diluted water in which the sulfate ions have been concentrated and the magnesium ions have been diluted; 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 section that reduces the concentration of sodium ions in the concentrated water by discharging the reduced-sodium water, and a concentration section that concentrates the reduced-sodium water discharged from the second removal section to produce a slurry in which magnesium chloride crystallizes; wherein the second removal section has a nanofiltration membrane that separates monovalent ions and polyvalent ions from a portion of the concentrated water, the nanofiltration membrane having an isoelectric point within the range of 2<pH<5, and the pH of the concentrated water is adjusted to between 2 and 5 by the pH adjustment section.

[0008] According to the magnesium chloride production system and the operating method of the magnesium chloride production system disclosed herein, it is possible to reduce the amount of chemicals added, suppress the effect of pH, and highly efficiently recover magnesium ions from treated water made from seawater.

[0009] It is a schematic diagram showing a magnesium chloride production system according to a first embodiment. It is a schematic diagram showing a magnesium chloride production system according to a second embodiment. It is a schematic diagram showing a first removal unit according to a third embodiment.

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

[0011] <First embodiment> (Magnesium chloride production system) The magnesium chloride production system reduces sulfate ions and sodium ions in seawater and extracts magnesium chloride (MgCl ２ When seawater is concentrated, various types of salts are precipitated. Specifically, the salts that precipitate when seawater is concentrated include a small amount of iron oxide (Fe ２ O ３ ) is precipitated. Then, calcium carbonate (CaCO ３ ) precipitates, and calcium sulfate (CaSO ４ ), sodium chloride (NaCl), magnesium sulfate (MgSO ４ ) precipitate in this order. The amount of precipitated salts is the largest among these salts, followed by magnesium sulfate, calcium sulfate, calcium carbonate, and iron oxide. Magnesium chloride, which is the target of production, precipitates after these salts have precipitated. Furthermore, when we look at the salts that precipitate before magnesium chloride, sulfate ions (SO ) contained in seawater precipitate. ４ ２－ ) can be selectively reduced, the precipitation of magnesium sulfate and calcium sulfate can be suppressed. ＋) can be selectively reduced, the precipitation of sodium chloride can be suppressed. Therefore, in the magnesium chloride production system 1 of this embodiment, the amount of magnesium chloride precipitation is increased by selectively reducing sulfate ions and sodium ions in seawater.

[0012] The magnesium chloride production system 1 of this 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 seawater-based water W by electrodialysis. The first removal unit 10 is an electrodialysis cell having multiple different types of membranes. For example, the first removal unit 10 of this embodiment is an electrodialysis cell having four types of membranes between an anode 11 (see FIG. 3 ) and a cathode 12 (see FIG. 3 ): a cation exchange membrane, a monovalent-selective anion exchange membrane, a monovalent-selective cation exchange membrane, and an anion exchange membrane. Here, the term "cation exchange membrane" refers to an ion exchange membrane that allows cations to pass through regardless of valence and blocks anions. The term "monovalent-selective anion exchange membrane" refers to an ion exchange membrane that selectively allows monovalent anions to pass through and blocks polyvalent anions and cations regardless of valence. The term "monovalent-selective cation exchange membrane" refers to an ion exchange membrane that selectively allows monovalent cations to pass through and blocks polyvalent cations and anions regardless of valence. Furthermore, the term "anion exchange membrane" refers to an ion exchange membrane that allows anions to pass through regardless of valence and prevents cations from passing through. The first removal unit 10 can also employ a known electrodialysis tank. The electrodialysis tank removes divalent or higher anions, thereby reducing the concentration of sulfate ions in the water to be treated W. The water to be treated W includes seawater as well as concentrated seawater obtained by removing water from seawater and concentrating it. Concentrated seawater is, for example, a concentrate obtained by subjecting seawater to reverse osmosis membrane treatment and separating the water. The water to be treated W is supplied to the first removal unit 10 from the sea via a first supply line L1. The water to be treated W is supplied to the first removal unit 10 without subjecting seawater or concentrated seawater to decarbonation treatment.

[0014] In the first removal unit 10, the water to be treated W is supplied and subjected to electrodialysis, whereby the concentration of divalent anions such as sulfate ions is reduced, and magnesium ions (Mg２＋ ) is discharged. Specifically, concentrated water CW concentrated with calcium ions, magnesium ions, and chloride ions is discharged from the first removal unit 10. The amount of concentrated water CW discharged from the first removal unit 10 is approximately 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 an overflow circulation line LO. In the overflow circulation line LO, the concentrated water CW is sent vertically upward, then circulated downward, and returned to the first removal unit 10. The concentrated water CW returned to the first removal unit 10 is electrodialyzed again. The overflow circulation line LO is connected to a second supply line L2. A portion 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] In addition, in the first removal unit 10, sodium ions and chloride ions (Cl － The dilution water AW is passed through an RO membrane (not shown) and then merged with the pH-adjusted concentrated water CPW (described later) immediately before flowing into the second removal unit 20, or is sent to the first supply line L1 and returned to the first removal unit 10 without passing through the RO membrane, or is merged with the first wastewater EW1 and then discarded.

[0016] The pH adjustment unit 30 adds a pH adjuster to the concentrated water CW discharged from the first removal unit 10. By adding the pH adjuster, the pH adjustment unit 30 adjusts the pH of the concentrated water CW to within the range of 2 < pH < 5. In the pH adjustment unit 30 of this embodiment, the pH adjuster is added to the overflow circulation line LO. The concentrated water CW to which the pH adjuster has been added has its pH adjusted to become pH-adjusted water PW, which flows directly through the overflow circulation line LO 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 portion of the concentrated water CW whose pH has been adjusted in the pH adjustment unit 30. The pH-adjusted concentrated water CPW is concentrated water CW treated by the first removal unit 10 in a state in which the pH-adjusted water PW and 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. In other words, 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 in which the concentration of sulfate ions has been reduced. The second removal unit 20 hardly reduces the concentration of magnesium ions in the pH-adjusted concentrated water CPW.

[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. At least one second removal unit 20 is arranged downstream of the first removal unit 10 (downstream in the flow direction of the water W to be treated). In the magnesium chloride production system 1 of this embodiment, only one second removal unit 20 is arranged. The second removal unit 20 of this embodiment has a nanofiltration membrane (NF membrane). The nanofiltration membrane is a membrane that allows only monovalent ions to pass through, thereby separating monovalent ions and polyvalent ions. The nanofiltration membrane separates monovalent ions and polyvalent ions from the pH-adjusted concentrated water CPW in which the sulfate ion concentration has been reduced in the first removal unit 10. Note that the nanofiltration membrane of this embodiment has an isoelectric point within the range of 2 < pH < 5, regardless of the positive or negative zeta potential (charge characteristic of the membrane surface). The pH-adjusted concentrated water CPW is treated by such a nanofiltration membrane to separate it into sodium-ion-reduced water NW, which has a sodium ion concentration reduced compared to the pH-adjusted concentrated water CPW, and second wastewater EW2, which has a sodium ion concentration increased compared to the pH-adjusted concentrated water CPW. The sodium-ion-reduced water NW discharged from the second removal unit 20 is sent to a third supply line L3. The third supply line L3 is connected to the concentration unit 50.

[0019] The concentrating unit 50 concentrates the sodium-ion-reduced water NW discharged from the second removal unit 20 and having a reduced sodium ion concentration. As a result, the concentrating unit 50 produces a slurry S in which magnesium chloride is crystallized. The concentrating unit 50 is connected to the second removal unit 20 via a third supply line L3. The sodium-ion-reduced water NW is supplied to the concentrating unit 50 via the third supply line L3. A known crystallization device can be used as the concentrating unit 50. For example, the concentrating unit 50 can be configured to treat the sodium-ion-reduced water NW by heating, reducing pressure, blowing air, or a combination of these to evaporate the water.

[0020] The concentrating unit 50 concentrates the sodium ion-reduced water NW, causing the ions dissolved in the sodium ion-reduced water NW to precipitate as salt. In this embodiment, sulfate ions and sodium ions are reduced by the first removal unit 10 and the second removal unit 20 before the water is supplied to the concentrating unit 50. Therefore, in the concentrating unit 50, the target magnesium chloride precipitates in addition to small amounts of iron oxide and calcium carbonate, and a slurry S in which magnesium chloride is crystallized is obtained. Furthermore, when the concentrating unit 50 produces the slurry S, a waste liquid FW that does not contain the slurry S is generated.

[0021] Furthermore, the slurry S in which magnesium chloride is crystallized and obtained in the magnesium chloride production system 1 is used to produce metallic magnesium in the magnesium production system 100. The magnesium production system 100 has a production unit 60 in addition to the magnesium chloride production system 1 described above.

[0022] The production section 60 is an apparatus for separating water from the slurry S in which magnesium chloride has been crystallized, to obtain magnesium chloride. The slurry S is supplied from the concentration section 50 to the production section 60 via a fourth supply line L4.

[0023] In the generation unit 60, a configuration can be adopted in which the slurry S is treated by heating, reducing pressure, blowing air, or a combination of these to evaporate water. The magnesium chloride generated in the generation unit 60 is sent to the electrolysis unit 70 via a fifth supply line L5.

[0024] The electrolysis unit 70 is a device for obtaining metallic magnesium by molten salt electrolysis of magnesium chloride. The electrolysis unit 70 can be configured using a known molten salt electrolysis facility.

[0025] Furthermore, in the magnesium chloride production system 1 described above, the pH adjustment of the concentrated water CW by the pH adjustment unit 30 is switched depending on the type of nanofiltration membrane in the second removal unit 20. Specifically, in the operation method of the magnesium chloride production system 1, when the nanofiltration membrane 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 near the isoelectric point between 2 and 5. In addition, in the operation method of the magnesium chloride production system 1, 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 to between 2 and 5, but does not adjust the pH of 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, the pH is adjusted only to minimize the amount of chemicals, such as pH 4.5, regardless of the isoelectric point. In other words, only when the nanofiltration membrane of 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 described above, water to be treated W, such as seawater supplied from the sea, is sent via a first supply line L1 to a first removal unit 10. The first removal unit 10 performs electrodialysis on the supplied water to be treated W, and discharges concentrated water CW in which magnesium ions are concentrated, dilution water AW in which the concentration of divalent anions such as sulfate ions is reduced, and first wastewater EW1 as the remainder. The dilution water AW and first wastewater EW1 produced in the first removal unit 10 are discharged to the outside.

[0027] The discharged concentrated water CW is sent to the overflow circulation line LO. Here, if the nanofiltration membrane of the second removal unit 20 has 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 of the concentrated water CW is adjusted to within the range of 2 < pH < 5. The concentrated water CW, which has been pH-adjusted to become pH-adjusted water PW, is circulated through 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 via 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, which have been treated again in the first removal unit 10, are discharged as concentrated water CW with their pH 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 portion 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 portion of the concentrated water CW whose pH has been adjusted and sent to the second supply line L2, is supplied to the second removal unit 20. As a result, the second removal unit 20 produces sodium-ion-reduced water NW, in which the sodium ion concentration has been reduced, and the remainder, second wastewater EW2. The second wastewater EW2 produced in the second removal unit 20 is discharged to the outside.

[0029] The sodium ion-reduced water NW produced in the second removal unit 20 is supplied to the concentrating unit 50 via a third supply line L3. As a result, the sodium ion-reduced water NW is supplied to the concentrating unit 50. In the concentrating unit 50, the sodium ion-reduced water NW is concentrated to obtain a slurry S in which magnesium chloride crystallizes. Furthermore, waste liquid FW is produced as the remainder and is discharged to the outside.

[0030] Furthermore, the slurry S in which magnesium chloride produced in the concentration unit 50 is crystallized is supplied to the production unit 60 via a fourth supply line L4. In the production unit 60, water is separated from the slurry S to produce magnesium chloride. Furthermore, the magnesium chloride produced in the production unit 60 is supplied to the electrolysis unit 70 via a fifth supply line L5, and metallic magnesium is produced.

[0031] (Effects) In the magnesium chloride production system 1 configured as described above, electrodialysis in the first removal unit 10 reduces the sulfate ion concentration, and concentrates magnesium ions to produce concentrated water CW. The pH of this concentrated water CW is adjusted to a range of 2 < pH < 5. The pH-adjusted concentrated water CW is then circulated through the first removal unit 10. Furthermore, pH-adjusted concentrated water CPW, a portion of the pH-adjusted concentrated water CW, passes through the second removal unit 20 and the concentrating unit 50, thereby producing a slurry S in which magnesium chloride crystallizes. In this way, adjusting the pH of the concentrated water CW to a range of 2 < pH < 5 prevents magnesium ions from being reduced when sodium ions are reduced using a nanofiltration membrane in the second removal unit 20. In other words, the second removal unit 20 prevents a decrease in the magnesium concentration contained in the sodium ion-reduced water NW. Therefore, a decrease in the amount of magnesium chloride contained in the slurry S that can be recovered in the concentrating unit 50 can be prevented. This allows magnesium ions to be recovered from the water to be treated W with high efficiency.

[0032] In particular, by keeping the pH below 5 (pH<5), it is possible to suppress the generation of scale in the first removal section 10, the second removal section 20, and the concentration section 50 due to calcium components and magnesium components contained in the concentrated water CW. Furthermore, by keeping the pH above 2 (2<pH), it is possible to suppress corrosion of metal parts used in the first removal section 10, the second removal section 20, the concentration section 50, and various lines (pipes).

[0033] The pH adjuster is added not to the water to be treated W supplied to the first removal unit 10, but to the concentrated water CW discharged from the first removal unit 10. The amount of concentrated water CW is extremely small compared to the amount of water to be treated W supplied to the first removal unit 10. Therefore, the amount of pH adjuster to be added can be reduced. This reduces the amount of chemicals added, suppresses the effect of pH, and enables highly efficient recovery of magnesium ions from the water to be treated W made from seawater.

[0034] Furthermore, in this embodiment, the concentrated water CW is circulated while its pH is adjusted. In other words, by mixing the pH-adjusted water PW, whose pH has already been adjusted, with the concentrated water CW newly generated from the water to be treated W, the mixed solution has a lower pH than when only the concentrated water CW newly generated from the water to be treated W is mixed. Therefore, the concentrated water CW circulating through the overflow circulation line LO has a low pH. This allows for further reduction in the amount of pH adjuster added by the pH adjustment unit 30 when adjusting the pH to within the range of 2<pH<5.

[0035] In addition, when a pH adjuster is directly added to the water to be treated W, the hydrogen ions (H ＋ ) increases. Therefore, during electrodialysis in the first removal unit 10, the ratio of hydrogen ions that permeate the cation exchange membrane and migrate to the concentrated water CW increases. As a result, the amount of magnesium ions that migrate to the concentrated water CW decreases, and the concentration of magnesium ions in the concentrated water CW may decrease. In other words, there is a possibility that the recovery efficiency of magnesium ions from the water to be treated W may decrease. However, in this embodiment, a pH adjuster is added to the concentrated water CW, so this phenomenon can be suppressed.

[0036] Furthermore, the nanofiltration membrane of the second removal section 20 has an isoelectric point within the range of 2 < pH < 5. Nanofiltration membranes have a charge characteristic on the membrane surface. The lower the pH of the solution passing through the nanofiltration membrane, the higher the zeta potential. The inventors have discovered that there is a correlation between the pH and the magnesium ion removal rate with the nanofiltration membrane. Specifically, the inventors have found that the lower the pH of the solution is in a range close to the isoelectric point of nanofiltration, the more the magnesium ion removal rate improves. 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 section 20 can be improved.

[0037] Furthermore, in the magnesium chloride production system 1, as 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 this time, whether or not to adjust the pH of the concentrated water CW in the pH adjustment unit 30 is determined depending on the type of nanofiltration membrane after replacement (whether or not the membrane has an isoelectric point within the range of 2 < pH < 5). Specifically, if the nanofiltration membrane does not have an isoelectric point within the range of 2 < pH < 5, pH adjustment to a specific value such as the isoelectric point is not performed, allowing the efficient use of pH additives. Therefore, it is possible to reduce the amount of chemicals added, suppress the effects of pH, and highly efficiently recover magnesium ions from the treated water W made from seawater.

[0038] Second Embodiment Next, a magnesium chloride production system 1A according to a second embodiment of the present disclosure will be described. In the second embodiment described below, components common to the first embodiment will be denoted by the same reference numerals in the drawings, and description thereof will be omitted. In the second embodiment, the magnesium chloride production system 1A differs from the first embodiment in that carbon dioxide dissolved in the pH-adjusted concentrated water CPW is reduced.

[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 amount of carbon dioxide dissolved in the pH-adjusted concentrated water CPW. In other words, the decarbonation unit 80 is not located upstream of the first removal unit 10 in the direction of flow of the water W to be treated, but is located only downstream in the direction of flow of the water W to be treated. The decarbonation unit 80 is located 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 located between the first removal unit 10 and the second removal unit 20 will be described as an example. The decarbonation unit 80 is located midway along the second supply line L2. The decarbonation unit 80 can have a known configuration, such as a decarbonation tower. For example, the decarbonation unit 80 first adds an acid to the pH-adjusted concentrated water CPW. The acid added to the pH-adjusted concentrated water CPW is preferably the same chemical as the pH adjuster. An example of the acid added in the decarbonation unit 80 is hydrochloric acid. Thereafter, in the decarbonation unit 80, the pH-adjusted concentrated water CPW is aerated to remove the carbon dioxide contained in the pH-adjusted concentrated water CPW. In the decarbonation unit 80, the pH-adjusted concentrated water CPW is aerated and the decarbonation treatment is continued until the pH of the pH-adjusted concentrated water CPW falls within the range of 2 < pH ≦ 4. This is because, by reducing the pH to 4 or less, it can be assumed that the carbon dioxide has been removed from the pH-adjusted concentrated water CPW, which is a solution. The pH-adjusted concentrated water CPW decarbonated in the decarbonation unit 80 is supplied to the second removal unit 20 via the second supply line L2.

[0041] (Effects) In the magnesium chloride production system 1A of the second embodiment, the decarbonation unit 80 reduces the amount of carbon dioxide dissolved in the pH-adjusted concentrated water CPW. Therefore, sodium ion-reduced water NW from which the carbon dioxide has been removed is supplied to the concentrating unit 50, and a slurry S in which magnesium chloride crystallizes is produced. As a result, when the slurry S is produced by concentration in the concentrating unit 50, the amount of calcium carbonate produced as an impurity in the slurry S can be reduced. In particular, by performing the decarbonation treatment on the concentrated water CW discharged from the first removal unit 10 rather than the water to be treated W supplied to the first removal unit 10, the amount of acid added for the decarbonation treatment can be reduced. This reduces the amount of chemicals added during the decarbonation treatment while suppressing the production of calcium carbonate as an impurity.

[0042] In this embodiment, the decarbonation unit 80 is disposed between the first removal unit 10 and the second removal unit 20. This allows carbon dioxide to be removed from the pH-adjusted concentrated water CPW before it passes through the nanofiltration membrane. This prevents carbonate scale from forming on the nanofiltration membrane.

[0043] In this embodiment, the decarbonation unit 80 is disposed between the first removal unit 10 and the second removal unit 20, but it may also be disposed between the second removal unit 20 and the concentrating unit 50. The flow rate of the sodium ion-reduced water NW discharged from the second removal unit 20 is lower than the flow rate of the pH-adjusted concentrated water CPW supplied to the second removal unit 20. Therefore, by disposing the decarbonation unit 80 between the second removal unit 20 and the concentrating unit 50, the amount of acid added can be further reduced. This makes it possible to significantly reduce the amount of chemicals added while suppressing the production of calcium carbonate, an impurity.

[0044] Furthermore, the decarbonation unit 80 adds acid to adjust the pH of the pH-adjusted concentrated water CPW to within the range of 2<pH≦4 and then aerates it. Therefore, the pH is already adjusted to within the range of 2<pH<5 by the pH adjustment unit 30, and the decarbonation unit 80 adjusts the pH to within the range of 2<pH≦4. This means that there is almost no need to add acid. In other words, the amount of acid added can be greatly reduced. This makes it possible to significantly reduce the amount of chemicals added while suppressing the production of calcium carbonate, an impurity.

[0045] Third Embodiment Next, a magnesium chloride manufacturing system 1B according to a third embodiment of the present disclosure will be described. In the third embodiment described below, components common to the first and second embodiments are denoted by the same reference numerals in the drawings, and description thereof will be omitted. In the third embodiment, the internal configuration of a 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 has 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 membranes 15 and the second membranes 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 arranged alternately with the first membranes 15 at intervals between the anode 11 and the cathode 12. The first membranes 15 are cation exchange membranes. The second membranes 16 are monovalent selective anion exchange membranes or nanofiltration membranes.

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

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

[0049] Furthermore, sulfate ions that could not permeate the second membrane 16 remain in the dilution chamber 101. Furthermore, small amounts of calcium ions and the like remain in the dilution chamber 101 without permeating the first membrane 15. Dilution water AW is discharged from the dilution chamber 101.

[0050] (Effects) In the magnesium chloride production system 1B of the third embodiment, a cation exchange membrane is used as the first membrane 15 in the first removal unit 10B, and a monovalent-selective anion exchange membrane or a monovalent-selective nanofiltration membrane is used as the second membrane 16. 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 monovalent-selective anion exchange membrane and the monovalent-selective nanofiltration membrane selectively allow monovalent ions to pass through and block polyvalent ions. This allows for greater removal of sulfate ions, which are divalent anions. In other words, the first removal unit 10B has a structure specialized for removing sulfate ions. Furthermore, in the first removal unit 10B, only a large number of pairs of membranes, the first membrane 15 and the second membrane 16, are disposed between the anode 11 and the cathode 12. This reduces the overall membrane area in the first removal unit 10B, thereby reducing the cost of the membranes in the first removal unit 10B. Furthermore, since many pairs of membranes are arranged between the anode 11 and the cathode 12, the total number of membranes (thickness of the part through which electricity flows) between the anode 11 and the cathode 12 is smaller than in electrodialysis cells using three or four types of membranes. As a result, the power cost required for electrodialysis can also be reduced. As a result, a large number of sulfate ions can be removed in the first removal unit 10B at low cost.

[0051] Furthermore, a monovalent-selective anion exchange membrane or a monovalent-selective nanofiltration membrane is used as second membrane 16, but a monovalent-selective nanofiltration membrane is significantly less expensive than a monovalent-selective anion exchange membrane. Therefore, by using a monovalent-selective nanofiltration membrane instead of a monovalent-selective anion exchange membrane, a large number of sulfate ions can be removed in first removal unit 10B at lower costs.

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

[0053] Note that the magnesium chloride production systems 1, 1A, and 1B are not limited to the above-described configuration as long as a portion of the concentrated water CW whose pH has been adjusted in the pH adjustment unit 30 is supplied to the second removal unit 20. For example, the first removal units 10 and 10B do not have to be configured to supply the concentrated water CW to the overflow circulation line LO and circulate the pH-adjusted concentrated water CW. That is, the pH adjustment unit 30 does not have to be configured to add a pH adjuster to the overflow circulation line LO. For example, the pH adjustment unit 30 may be configured to add a pH adjuster to the second supply line L2. Furthermore, in addition to the pH adjustment unit 30 that adds a pH adjuster to the concentrated water CW in the overflow circulation line LO, the system may further include a pH adjustment unit 30 that adds a pH adjuster to the second supply line L2.

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

[0055] The concentrating unit 50 may have any other configuration as long as it can obtain the slurry S containing magnesium chloride. For example, the concentrating unit 50 may obtain the slurry S containing magnesium chloride by utilizing sedimentation or the like.

[0056] <Additional Notes> The magnesium chloride production systems 1, 1A, and 1B described in the respective embodiments can be understood, for example, as follows.

[0057] (1) The magnesium chloride production system 1, 1A, 1B according to the first aspect includes a first removal unit 10, 10B that reduces sulfate ions in water to be treated W, which is made from seawater as a raw material, by electrodialysis and discharges concentrated water CW in which the concentration of the sulfate ions has been reduced and magnesium ions have been concentrated; a pH adjustment unit 30 that adds a pH adjuster to the concentrated water CW discharged from the first removal unit 10, 10B to adjust the pH of the concentrated water CW to within a range of 2<pH<5; and a pH adjustment unit 30 that adjusts the pH of the concentrated water CW, the pH of which has been adjusted by the pH adjustment unit 30, to within a range of 2<pH<5. The system is equipped with a second removal section (20) that receives a portion of water (CW), reduces the concentration of sodium ions contained in the portion of concentrated water (CW) whose pH has been adjusted, and discharges sodium-reduced water whose concentration of sodium ions has been reduced, and a concentration section (50) that concentrates the sodium-reduced water discharged from the second removal section (20) to produce a slurry (S) in which magnesium chloride is crystallized, and the second removal section (20) has a nanofiltration membrane that separates monovalent ions and polyvalent ions from the portion of concentrated water (CW) whose pH has been adjusted.

[0058] With this configuration, the pH of the concentrated water CW is adjusted to within the range of 2 < pH < 5, thereby preventing magnesium ions from being reduced when sodium ions are reduced using the nanofiltration membrane in the second removal unit 20. In other words, the second removal unit 20 prevents a decrease in the magnesium concentration contained in the sodium ion-reduced water NW. Therefore, a decrease in the amount of magnesium chloride contained in the slurry S recovered in the concentrating unit 50 can be prevented. This allows for highly efficient recovery of magnesium ions from the water to be treated W. Furthermore, the pH adjuster is added to the concentrated water CW discharged from the first removal unit 10, 10B, rather than to the water to be treated W supplied to the first removal unit 10, 10B. The amount of concentrated water CW is significantly smaller than the amount of water to be treated W supplied to the first removal unit 10, 10B. Therefore, the amount of pH adjuster added can be reduced. This reduces the amount of chemicals added, suppresses the effects of pH, and allows for highly efficient recovery of magnesium ions from the water to be treated W, which is made from seawater.

[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), in which the nanofiltration membrane has an isoelectric point within the range of 2<pH<5.

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

[0061] (3) The magnesium chloride production system 1, 1A, 1B relating to the third aspect is the magnesium chloride production system 1, 1A, 1B of (1) or (2), and further includes a decarbonation section 80 that reduces the carbon dioxide dissolved in a portion of the concentrated water CW whose pH has been adjusted.

[0062] According to this configuration, sodium ion-reduced water NW from which carbon dioxide has been removed is supplied to the concentrating unit 50, and a slurry S in which magnesium chloride crystallizes is produced. As a result, when the slurry S is produced by concentration in the concentrating unit 50, the amount of calcium carbonate produced as an impurity in the slurry S can be reduced. In particular, by performing the decarbonation treatment on the concentrated water CW discharged from the first removal units 10 and 10B rather than the water to be treated W supplied to the first removal units 10 and 10B, the amount of acid to be added can be reduced. This makes it possible to reduce the amount of chemicals added while suppressing the production of calcium carbonate as an impurity.

[0063] (4) The magnesium chloride production system 1, 1A, 1B relating to the fourth aspect is the magnesium chloride production system 1, 1A, 1B of (3), in which the decarbonation section 80 is arranged between the first removal section 10, 10B and the second removal section 20 or between the second removal section 20 and the concentration section 50.

[0064] According to this configuration, the decarbonation unit 80 is disposed between the first removal unit 10, 10B and the second removal unit 20. This allows carbon dioxide to be removed from the pH-adjusted concentrated water CPW before it passes through the nanofiltration membrane. This prevents the formation of carbonate scale in the nanofiltration membrane. Furthermore, 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 reduced. This allows the amount of chemicals added to be significantly reduced while suppressing the generation of calcium carbonate, an impurity.

[0065] (5) The magnesium chloride production system 1, 1A, 1B according to the fifth aspect is the magnesium chloride production system 1, 1A, 1B of (3) or (4), in which the decarbonation section 80 adds an acid to a portion of the concentrated water CW whose pH has been adjusted to adjust the pH to within the range of 2<pH≦4, and then aerates the water.

[0066] With this configuration, the pH is already adjusted to within the range of 2<pH<5 by the pH adjusting unit 30, and the pH is then adjusted to within the range of 2<pH≦4. Therefore, there is almost no need to add acid. In other words, the amount of acid to be added can be greatly reduced. This makes it possible to significantly reduce the amount of chemicals added while suppressing the generation of calcium carbonate, an impurity.

[0067] (6) The magnesium chloride production system 1, 1A, 1B according to a sixth aspect is any one of the magnesium chloride production systems (1) to (5), wherein the first removal section 10B has an anode 11, a cathode 12, a first membrane 15 arranged between the anode 11 and the cathode 12, and a second membrane 16 arranged alternately with the first membrane 15 at a distance between them between the anode 11 and the cathode 12, 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 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 this configuration, only two types of membranes, the first membrane 15 and the second membrane 16, are disposed between the anode 11 and the cathode 12. Monovalent-selective anion exchange membranes and monovalent-selective nanofiltration membranes selectively allow monovalent ions to pass through while blocking polyvalent ions. This allows for greater removal of sulfate ions, a divalent anion. In other words, the first removal unit 10B is specialized for sulfate ion removal. Furthermore, in the first removal unit 10B, only a large number of pairs of membranes, the first membrane 15 and the second membrane 16, are disposed between the anode 11 and the cathode 12. This reduces the overall membrane area in the first removal unit 10B, thereby reducing the cost of the membranes in the first removal unit 10B. Furthermore, because a large number of pairs of membranes are disposed between the anode 11 and the cathode 12, the total number of membranes (thickness of the electrically conducting portion) between the anode 11 and the cathode 12 is smaller than in electrodialysis cells using three or four types of membranes. As a result, the power costs for electrodialysis can also be reduced. As a result, first removal unit 10B can remove a large amount of sulfate ions while keeping costs down.

[0069] (7) A method of operating the magnesium chloride production systems 1, 1A, 1B according to the seventh aspect includes a first removal unit 10, 10B that reduces sulfate ions in water to be treated W made from seawater by electrodialysis, and discharges concentrated water in which the sulfate ions have been reduced and magnesium ions have been concentrated, and diluted water in which the sulfate ions have been concentrated and the magnesium ions have been diluted; a pH adjustment unit 30 that adds a pH adjuster to the concentrated water CW discharged from the first removal unit 10, 10B to adjust the pH of the concentrated water CW to between 2 and 5; and a method of operating a magnesium chloride production system 1, 1A, 1B according to the seventh aspect, which includes a first removal unit 10, 10B that reduces sulfate ions in water to be treated W made from seawater by electrodialysis, and discharges concentrated water in which the sulfate ions have been reduced and magnesium ions have been concentrated, and diluted water in which the sulfate ions have been concentrated and the magnesium ions have been diluted; a pH adjustment unit 30 that adjusts the pH of the concentrated water CW to between 2 and 5; and a method of operating a magnesium chloride production system 1, 1A, 1B that receives a portion of the concentrated water CW whose pH has been adjusted by the pH adjustment unit 30, and adjusts the sodium content of the portion of the concentrated water CW. and a concentrating section (50) that concentrates the sodium-reduced water discharged from the second removal section (20) to produce a slurry (S) in which magnesium chloride is crystallized. The method for operating a magnesium chloride production system (1, 1A, 1B) includes: a second removal section (20) that reduces the concentration of sodium ions in the concentrated water (CW) and discharges the sodium-reduced water in which the concentration of sodium ions has been reduced; and a concentrating section (50) that concentrates the sodium-reduced water discharged from the second removal section (20) to produce a slurry (S) in which magnesium chloride is crystallized. The second removal section (20) has a nanofiltration membrane that separates monovalent ions and polyvalent ions from a portion of the concentrated water (CW) whose pH has been adjusted. When the nanofiltration membrane has an isoelectric point within the range of 2<pH<5, the pH of the concentrated water (CW) is adjusted to between 2 and 5 by the pH adjusting section (30).

[0070] With this configuration, whether or not to adjust the pH of the concentrated water CW in the pH adjusting unit 30 can be determined depending on the type of nanofiltration membrane after replacement (whether the nanofiltration membrane has an isoelectric point within the range of 2 < pH < 5). Therefore, if the nanofiltration membrane does not have an isoelectric point within the range of 2 < pH < 5, the pH is not adjusted, allowing the pH additive to be used efficiently.

[0071] According to the magnesium chloride production system and the operating method of the magnesium chloride production system disclosed herein, it is possible to reduce the amount of chemicals added, suppress the effect of pH, and highly efficiently recover magnesium ions from treated water made from seawater.

[0072] DESCRIPTION OF SYMBOLS 1, 1A, 1B...Magnesium chloride production system 10, 10B...First removal section 20...Second removal section 50...Concentration section 30...pH adjustment section L1...First supply line LO...Overflow circulation line L2...Second supply line L3...Third supply line 100...Magnesium production system 60...Generation section 70...Electrolysis section 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 wastewater CPW...pH-adjusted concentrated water NW...Sodium ion-reduced water EW2...Second wastewater FW...Waste liquid S...Slurry 80...Decarbonation section 11...Anode 12...Cathode 15...First membrane 16...Second membrane 101...Dilution chamber 102...Concentration chamber

Claims

1. A magnesium chloride production system comprising: a first removal unit that reduces sulfate ions in water to be treated, which is made from seawater as a raw material, by electrodialysis, and discharges concentrated water in which the concentration of the sulfate ions has been reduced and magnesium ions has been concentrated; 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 be within the range of 2 < pH < 5; a second removal unit that receives a portion of the concentrated water whose pH has been adjusted by the pH adjustment unit, reduces the concentration of sodium ions contained in the portion of the concentrated water whose pH has been adjusted, and discharges sodium-reduced water in which the concentration of sodium ions has been reduced; and a concentration unit that concentrates the sodium-reduced water discharged from the second removal unit to produce a slurry in which magnesium chloride is crystallized, wherein the second removal unit has a nanofiltration membrane that separates monovalent ions and polyvalent ions from the portion of the concentrated water whose pH has been adjusted.

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

3. A magnesium chloride manufacturing system as described in claim 1 or claim 2, further comprising a decarbonation section for reducing carbon dioxide dissolved in a portion of the concentrated water whose pH has been adjusted.

4. A magnesium chloride production system as described in claim 3, wherein the decarbonation section is disposed between the first removal section and the second removal section or between the second removal section and the concentration section.

5. A magnesium chloride manufacturing system as described in claim 3, in which the decarbonation section adds an acid to a portion of the concentrated water whose pH has been adjusted to adjust the pH to within the range of 2<pH≦4 before aerating it.

6. A magnesium chloride manufacturing system as described in claim 1 or claim 2, wherein the first removal section has an anode, a cathode, a first membrane arranged between the anode and the cathode, and a second membrane arranged alternately with a gap between the first membrane and the anode and the cathode, and only two types of membranes, the first membrane and the second membrane, are arranged between the anode and the cathode, the first membrane is a cation exchange membrane, and the second membrane is a monovalent selective anion exchange membrane or a monovalent selective nanofiltration membrane.

7. A method for operating a magnesium chloride production system comprising: a first removal unit which reduces sulfate ions in seawater-based water to be treated by electrodialysis, and discharges concentrated water in which the sulfate ions have been reduced and magnesium ions have been concentrated, and diluted water in which the sulfate ions have been concentrated and the magnesium ions have been diluted; a pH adjustment unit which 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 which is supplied with a portion of the concentrated water whose pH has been adjusted by the pH adjustment unit, reduces the concentration of sodium ions contained in the portion of the concentrated water, and discharges sodium-reduced water in which the concentration of sodium ions has been reduced; and a concentration unit which concentrates the sodium-reduced water discharged from the second removal unit to produce a slurry in which magnesium chloride is crystallized, wherein the second removal unit has a nanofiltration membrane which separates monovalent ions and polyvalent ions from the portion of the concentrated water whose pH has been adjusted, A method for operating a magnesium chloride production system, in which the pH of the concentrated water is adjusted to a value between 2 and 5 by the pH adjustment unit when the nanofiltration membrane has an isoelectric point within a range of 2<pH<5.

Images