Magnesium chloride production system

Abstract

To provide a system that reduces the amount of water to be treated during crystallization.SOLUTION: A magnesium chloride production system comprises: a first removal part for reducing the concentration of sulfate ions in water to be treated using seawater as a raw material; a second removal part for reducing the concentration of sodium ions in the water to be treated having the concentration of sulfate ions reduced by discharging from the first removal part; an OARO separation part for supplying the water to be treated having the same concentration to both sides of a semipermeable membrane and separating the water to be treated having the concentration of sodium ions reduced by discharging from the second removal part into concentrated water and low concentration water; and a concentration part for further concentrating the concentrated water discharged from the OARO separation part to produce a slurry in which magnesium chloride is crystallized.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a system for producing magnesium chloride. [Background technology]

[0002] Conventionally, as described in Patent Document 1, a method for recovering magnesium chloride from seawater involves the use of electrodialysis (ED) to remove sulfate ions (SO4 2- ), and then sodium ions (Na + ) is known. In this method, sulfate ions and sodium ions are removed from treated water such as seawater to obtain an aqueous solution mainly composed of magnesium chloride (MgCl2). The aqueous solution mainly composed of magnesium chloride is concentrated by crystallization to precipitate magnesium chloride. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-109791 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the method of Patent Document 1, the energy consumption during crystallization is large. Therefore, if the amount of water to be treated during crystallization increases, there is a problem that the energy required during crystallization becomes enormous. Therefore, there is a demand for reducing the amount of water to be treated during crystallization.

[0005] The present disclosure has been made to solve the above-mentioned problems, and aims to provide a magnesium chloride production system that can reduce the amount of water to be treated during crystallization. [Means for solving the problem]

[0006] In order to solve the above problems, the magnesium chloride manufacturing system of the present disclosure comprises a first removal section which reduces the sulfate ion concentration in water to be treated which is made from seawater as a raw material; a second removal section which reduces the sodium ion concentration in the water to be treated which has been discharged from the first removal section and has had the sulfate ion concentration reduced; an OARO separation section which supplies the water to be treated which has the same concentration to both sides of a semipermeable membrane and separates the water to be treated which has been discharged from the second removal section and has had the sodium ion concentration reduced into concentrated water and low-concentration water; and a concentration section which further concentrates the concentrated water discharged from the OARO separation section to produce a slurry in which magnesium chloride is crystallized. Effect of the Invention

[0007] According to the magnesium chloride production system of the present disclosure, the amount of water to be treated during crystallization can be reduced. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic diagram showing a magnesium chloride production system according to a first embodiment. [Diagram 2] FIG. 1 is a schematic diagram showing a magnesium chloride production system according to a second embodiment. [Diagram 3] FIG. 1 is a schematic diagram showing a magnesium chloride production system according to a third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] 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.

[0010] First Embodiment (Magnesium chloride manufacturing system) The magnesium chloride manufacturing system 1 reduces sulfate ions and sodium ions in seawater and manufactures magnesium chloride (MgCl2) from seawater. When concentrating seawater, various types of salts precipitate. Specifically, as for the salts that precipitate when concentrating seawater, a small amount of iron oxide (Fe2O3) is precipitated first due to differences in solubility in water. Next, calcium carbonate (CaCO3) precipitates, and then calcium sulfate (CaSO4), sodium chloride (NaCl), and magnesium sulfate (MgSO4) precipitate in that order. At this time, the largest amount of these salts precipitates is sodium chloride, followed by magnesium sulfate, calcium sulfate, calcium carbonate, and iron oxide, in that order. The magnesium chloride to be manufactured precipitates after these salts have precipitated. Furthermore, when focusing on the salts that precipitate before magnesium chloride, sulfate ions (SO4 2- ) can be selectively reduced, the precipitation of magnesium sulfate and calcium sulfate can be suppressed. Similarly, the sodium ions (Na + ) can be selectively reduced, the precipitation of sodium chloride can be suppressed. Therefore, in the magnesium chloride production system 1 of the present embodiment, the amount of magnesium chloride precipitation is increased by selectively reducing sulfate ions and sodium ions in seawater.

[0011] The magnesium chloride production system 1 of the present embodiment includes a first removal section 10, a second removal section 20, an OARO separation section 40, a concentrating section 50, and a diluting section .

[0012] The first removal unit 10 reduces the sulfate ion concentration in the water to be treated W, which is made from seawater. The first removal unit 10 is an electrodialysis layer having two different types of ion exchange membranes. Specifically, the first removal unit 10 is a device having a cation exchange membrane (without selectivity) that can pass all cations regardless of valence, and an anion exchange membrane with monovalent selectivity that does not pass multivalent anions and passes only monovalent anions. The first removal unit 10 can also adopt a known electrodialysis cell. The electrodialysis cell removes divalent or higher anions, thereby reducing the sulfate ion concentration 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. The concentrated seawater corresponds to, for example, a concentrated liquid obtained by subjecting seawater to a reverse osmosis membrane treatment and separating the water. Furthermore, the water to be treated W also includes a liquid obtained by subjecting seawater or concentrated seawater to a decarbonation treatment to reduce carbon dioxide. The water to be treated W is supplied from the sea to the first removal section 10 via a supply line LS.

[0013] When the to-be-treated water W is supplied to the first removal unit 10 and electrodialysis is performed, sulfate ion reduced water SW, which is the to-be-treated water W in which the concentration of divalent anions such as sulfate ions has been reduced, is discharged. Meanwhile, a liquid in which sulfate ions have been concentrated, separated from the sulfate ion reduced water SW, is discharged from the first removal unit 10 as first wastewater EW1.

[0014] The second removal unit 20 reduces the sodium ion concentration in the water to be treated W discharged from the first removal unit 10 and in which the sulfate ion concentration has been reduced. The second removal unit 20 is connected to the first removal unit 10. The sulfate ion reduced water SW is supplied to the second removal unit 20 via the first treated water line L1. At least one second removal unit 20 is disposed in the rear stage of the first removal unit 10 (downstream in the flow direction of the water to be treated W). In the magnesium chloride manufacturing system 1 of this embodiment, a plurality of (four) second removal units 20 are disposed. The second removal unit 20 of this embodiment is a nanofiltration membrane (NF membrane). The nanofiltration membrane is a membrane that allows only monovalent ions to pass therethrough, thereby enabling separation of monovalent ions and polyvalent ions. The nanofiltration membrane separates monovalent ions and polyvalent ions from the sulfate ion reduced water SW in which the sulfate ion concentration has been reduced in the first removal unit 10. As a result, the sulfate ion reduced water SW is separated into sodium ion reduced water NW, which is the treated water W in which the sodium ion concentration is reduced more than that of the sulfate ion reduced water SW by being treated with the nanofiltration membrane, and second wastewater EW2 in which the sodium ion concentration is increased more than that of the sulfate ion reduced water SW.

[0015] In this embodiment, the multiple second removal units 20 have a first nanofiltration membrane 21, a second nanofiltration membrane 22, a third nanofiltration membrane 23, and a fourth nanofiltration membrane 24. The first nanofiltration membrane 21, the second nanofiltration membrane 22, the third nanofiltration membrane 23, and the fourth nanofiltration membrane 24 are arranged in sequence from the upstream in the flow direction of the water to be treated W. In other words, the first nanofiltration membrane 21 is arranged at a position closest to the first removal unit 10. Moreover, the first nanofiltration membrane 21, the second nanofiltration membrane 22, the third nanofiltration membrane 23, and the fourth nanofiltration membrane 24 in this embodiment have the same structure.

[0016] The first nanofiltration membrane 21 is a part of the second removal unit 20 connected to the first removal unit 10 by a first treated water line L1. The first sodium ion-reduced water NW1, whose sodium ion concentration has been reduced by the first nanofiltration membrane 21, is sent to the second nanofiltration membrane 22.

[0017] The second nanofiltration membrane 22 is part of the second removal section 20 connected to the first nanofiltration membrane 21 by a first connection line LC1. The second sodium ion-reduced water NW2, which has a sodium ion concentration reduced by the second nanofiltration membrane 22 compared to the first sodium ion-reduced water NW1, is sent to the third nanofiltration membrane 23.

[0018] The third nanofiltration membrane 23 is a part of the second removal section 20 connected to the second nanofiltration membrane 22 by a second connection line LC2. The third sodium ion-reduced water NW3, which has a sodium ion concentration reduced by the third nanofiltration membrane 23 compared to the second sodium ion-reduced water NW2, is sent to the fourth nanofiltration membrane 24.

[0019] The fourth nanofiltration membrane 24 is a part of the second removal unit 20 connected to the third nanofiltration membrane 23 by a third connection line LC3. The fourth nanofiltration membrane 24 is disposed at a position closest to the OARO separation unit 40 described below. The fourth sodium ion reduced water NW4, which has a sodium ion concentration reduced by the fourth nanofiltration membrane 24 compared to the third sodium ion reduced water NW3, is sent to the OARO separation unit 40. In addition, the second wastewater EW2 is discharged from each nanofiltration membrane.

[0020] The OARO separation unit 40 separates the water W to be treated, which is discharged from the second removal unit 20 and has a reduced sodium ion concentration, into concentrated water and low-concentration water by the OARO method. The OARO separation unit 40 has a semipermeable membrane (not shown). The OARO separation unit 40 is supplied with water W to be treated at the same concentration on both sides of the semipermeable membrane. Here, the "same concentration" is theoretically a concentration at which the difference in osmotic pressure between the liquids supplied to both sides of the semipermeable membrane is equal to or less than the withstand pressure of the membrane (generally about 7 MPa) and the semipermeable membrane can function. In other words, the "same concentration" does not mean a completely identical concentration, but a concentration that allows a certain range of deviation that can ensure an osmotic pressure difference that allows the semipermeable membrane to function. As a result, in the OARO separation unit 40, by applying pressure to only one side of the semipermeable membrane in this state, osmotic pressure does not act between the water W to be treated separated by the semipermeable membrane by the OARO method, and water permeation occurs even with a slight applied pressure, and ions other than water molecules in the water W to be treated are separated, making it possible to highly concentrate the water W. The OARO separation section 40 of this embodiment has an RO membrane 41, a first OARO membrane 42, a second OARO membrane 43, a third OARO membrane 44, and a fourth OARO membrane 45. The RO membrane 41, the first OARO membrane 42, the second OARO membrane 43, the third OARO membrane 44, and the fourth OARO membrane 45 are arranged in sequence from the upstream in the flow direction of the water to be treated W. In this embodiment, the first OARO membrane 42, the second OARO membrane 43, the third OARO membrane 44, and the fourth OARO membrane 45 have the same structure.

[0021] The RO membrane 41 is disposed at a position closest to the first removal unit 10. The RO membrane 41 is connected to the fourth nanofiltration membrane 24 by the second treated water line L2. The RO membrane 41 is disposed between the fourth nanofiltration membrane 24 and the first OARO membrane 42. Before the OARO process is performed in the OARO separation unit 40, the RO membrane 41 desalinates the quaternary sodium ion reduced water NW4 by reverse osmosis to concentrate the ions in the quaternary sodium ion reduced water NW4. As a result, the RO membrane 41 generates RO concentrated water RW that is concentrated so that the ion concentration is higher than that of the quaternary sodium ion reduced water NW4, and permeate water PW in which ions are removed from the quaternary sodium ion reduced water NW4. The RO concentrated water RW is sent from the RO membrane 41 to the first OARO membrane 42. The permeate water PW is sent from the RO membrane 41 to the dilution unit 70 described later.

[0022] The first OARO membrane 42 is connected to the RO membrane 41 by a reverse osmosis line LRO. The first OARO membrane 42 increases the ion concentration of the RO concentrate RW by the OARO method using the RO concentrate RW and the second return liquid BW2 described later. In the first OARO membrane 42, the RO concentrate RW is supplied from the RO membrane 41 to one side of the semipermeable membrane, and the second return liquid BW2 is supplied from the second OARO membrane 43 to the other side of the semipermeable membrane. As a result, the first OARO membrane 42 generates the first OARO water OW1, which is concentrated water that has been concentrated to have a higher ion concentration than the RO concentrate RW and reduced in volume, and the first return liquid BW1, which is the remainder and is low-concentration water. The concentration of the first return liquid BW1 is equal to or lower than that of the quaternary sodium ion-reduced water NW4. The first OARO water OW1 is sent from the first OARO membrane 42 to the second OARO membrane 43. The first return liquid BW1 is returned to the second treated water line L2 via the first return line LB1 and sent again to the RO membrane 41 together with the quaternary sodium ion-reduced water NW4.

[0023] The second OARO membrane 43 is connected to the first OARO membrane 42 by the first OARO line LO1 and the second return line LB2. The second OARO membrane 43 increases the ion concentration of the first OARO water OW1 by the OARO method using the first OARO water OW1 and the third return liquid BW3 described later. In the second OARO membrane 43, the first OARO water OW1 is supplied from the first OARO membrane 42 to one side of the semipermeable membrane, and the third return liquid BW3 is supplied from the third OARO membrane 44 to the other side of the semipermeable membrane. As a result, the second OARO membrane 43 generates the second OARO water OW2, which is concentrated and reduced in amount so that the ion concentration of ions other than water molecules is increased compared to the first OARO water OW1, and the second return liquid BW2, which is the remainder and is low-concentration water. The concentration of the second return liquid BW2 is equivalent to that of the first OARO water OW1. The second OARO water OW2 is sent from the second OARO membrane 43 to the third OARO membrane 44. The second return liquid BW2 is sent to the first OARO membrane 42 from the second return line LB2.

[0024] The third OARO membrane 44 is connected to the second OARO membrane 43 by the second OARO line LO2 and the third return line LB3. The third OARO membrane 44 increases the ion concentration of the second OARO water OW2 by the OARO method using the second OARO water OW2 and the fourth return liquid BW4 described later. In the third OARO membrane 44, the second OARO water OW2 is supplied from the second OARO membrane 43 to one side of the semipermeable membrane, and the fourth return liquid BW4 is supplied from the fourth OARO membrane 45 to the other side of the semipermeable membrane. As a result, the third OARO membrane 44 produces the third OARO water OW3, which is concentrated and reduced in amount so that the ion concentration of non-water molecules is increased compared to the second OARO water OW2, and the third return liquid BW3, which is the remaining low-concentration water. The third OARO water OW3 is sent from the third OARO membrane 44 to the third OARO membrane 44. The third return liquid BW3 is sent from the third return line LB3 to the second OARO membrane 43.

[0025] The fourth OARO membrane 45 is connected to the third OARO membrane 44 by the third OARO line LO3 and the fourth return line LB4. The fourth OARO membrane 45 increases the ion concentration of the third OARO water OW3 by the OARO method using the third OARO water OW3 and the fourth OARO water OW4 described later. In the fourth OARO membrane 45, the third OARO water OW3 is supplied from the third OARO membrane 44 to one side of the semipermeable membrane, and the fourth OARO water OW4 generated by the fourth OARO membrane 45 itself is supplied to the other side of the semipermeable membrane. As a result, the fourth OARO membrane 45 generates the fourth OARO water OW4, which is concentrated and reduced in amount so that the ion concentration of non-water molecules is increased compared to the third OARO water OW3, and the fourth return liquid BW4, which is the remaining low-concentration water. The fourth OARO water OW4 is sent from the fourth OARO membrane 45 to the concentration section 50. A part of the fourth OARO water OW4 is returned to the fourth OARO membrane 45 through the circulation line LR, not to the concentration section 50. A valve for adjusting the return amount of the fourth OARO water OW4 is disposed in the circulation line LR. The fourth return liquid BW4 is sent to the third OARO membrane 44 through the fourth return line LB4.

[0026] The concentrating section 50 concentrates the fourth OARO water OW4, which is the water W to be treated discharged from the OARO separation section 40. As a result, the concentrating section 50 generates a slurry S in which magnesium chloride is crystallized. The concentrating section 50 is connected to the OARO separation section 40. The concentrating section 50 is supplied with the fourth OARO water OW4, which is the sodium ion reduced water NW in which the sodium ion concentration is most reduced and which is the most concentrated and reduced in volume, via the third treated water line L3. A known crystallization device can be used as the concentrating section 50. For example, the concentrating section 50 can be configured to treat the fourth OARO water OW4 by heating, reducing pressure, blowing air, or a combination of these to evaporate the water.

[0027] In the concentrating section 50, the ions dissolved in the fourth OARO water OW4 become salts and precipitate. Here, in this embodiment, sulfate ions and sodium ions are reduced by the first removal section 10 and the second removal section 20 before being supplied to the concentrating section 50. Therefore, in the concentrating section 50, magnesium chloride precipitates, and a slurry S in which magnesium chloride is crystallized is obtained. In addition, in the concentrating section 50 of this embodiment, when the slurry S is generated, the waste liquid FW that does not contain the slurry S and the distilled water DW generated in the process of generating the slurry S are discharged. That is, in the concentrating section 50, the waste liquid FW and the distilled water DW are generated when the slurry S is generated. The waste liquid FW and the distilled water DW are discharged to the outside of the concentrating section 50.

[0028] The dilution section 70 supplies the dilution water AW to the second removal section 20. In this embodiment, the dilution water AW is a liquid (e.g., pure water) having a lower concentration than the destination treated water W. The dilution section 70 supplies the dilution water AW to the first treated water line L1, the first connection line LC1, and the second connection line LC2.

[0029] In the magnesium chloride production system 1 as described above, water to be treated W, such as seawater supplied from the sea, is sent to the first removal unit 10 via a supply line LS. The first removal unit 10 performs electrodialysis on the supplied water to be treated W, and discharges sulfate ion reduced water SW, which is the water to be treated W in which the concentration of divalent anions, such as sulfate ions, has been reduced, and first wastewater EW1, which is the remainder. The first wastewater EW1 generated in the first removal unit 10 is discharged to the outside.

[0030] The discharged sulfate ion reduced water SW is supplied to the first nanofiltration membrane 21 via the first treated water line L1. At this time, the dilution water AW supplied from the dilution section 70 is also supplied to the first treated water line L1. Furthermore, the permeated water PW discharged from the RO membrane 41 is also supplied to the first treated water line L1. As a result, the sulfate ion reduced water SW diluted with the dilution water AW and the permeated water PW is supplied to the first nanofiltration membrane 21. The first nanofiltration membrane 21 generates first sodium ion reduced water NW1 in which the sodium ion concentration has been reduced, and second wastewater EW2, which is the remainder. The second wastewater EW2 generated by the first nanofiltration membrane 21 is discharged to the outside.

[0031] The first sodium ion reduced water NW1 thus produced is supplied to the second nanofiltration membrane 22 via the first connection line LC1. At this time, the dilution water AW supplied from the dilution section 70 and the permeated water PW discharged from the RO membrane 41 are supplied to the first connection line LC1. As a result, the first sodium ion reduced water NW1 diluted with the dilution water AW and the permeated water PW is supplied to the second nanofiltration membrane 22. The second nanofiltration membrane 22 produces second sodium ion reduced water NW2 in which the sodium ion concentration has been further reduced, and second wastewater EW2, which is the remainder. The second wastewater EW2 produced by the second nanofiltration membrane 22 is discharged to the outside.

[0032] The generated second sodium ion reduced water NW2 is supplied to the third nanofiltration membrane 23 via the second connection line LC2. At this time, the dilution water AW supplied from the dilution unit 70 and the permeated water PW discharged from the RO membrane 41 are supplied to the second connection line LC2. As a result, the second sodium ion reduced water NW2 diluted with the dilution water AW and the permeated water PW is supplied to the third nanofiltration membrane 23. The third nanofiltration membrane 23 generates third sodium ion reduced water NW3 in which the sodium ion concentration is further reduced, and second wastewater EW2 which is the remainder. The second wastewater EW2 generated by the third nanofiltration membrane 23 is discharged to the outside.

[0033] The generated third sodium ion-reduced water NW3 is supplied to the fourth nanofiltration membrane 24 via the third connection line LC3. As a result, the fourth nanofiltration membrane 24 generates fourth sodium ion-reduced water NW4 having a further reduced sodium ion concentration and the remainder, second wastewater EW2. The second wastewater EW2 generated by the fourth nanofiltration membrane 24 is discharged to the outside.

[0034] The generated quaternary sodium ion reduced water NW4 is supplied to the OARO separation unit 40 via the second treated water line L2. In the OARO separation unit 40, the quaternary sodium ion reduced water NW4 is supplied to the RO membrane 41. The RO membrane 41 generates RO concentrate RW and permeate PW. The RO concentrate RW is sent from the RO membrane 41 to the first OARO membrane 42 via the reverse osmosis line LRO. The permeate PW is sent from the RO membrane 41 to the first treated water line L1, the first connection line LC1, and the second connection line LC2 together with the dilution water AW.

[0035] In addition, the RO concentrate RW supplied to the first OARO membrane 42 via the reverse osmosis line LRO is separated into the first OARO water OW1 and the first return liquid BW1. The first OARO water OW1 is sent from the first OARO membrane 42 to the second OARO membrane 43 via the first OARO line LO1. The first return liquid BW1 is returned to the second treated water line L2 via the first return line LB1 and sent to the RO membrane 41 again.

[0036] The first OARO water OW1 supplied to the second OARO membrane 43 through the first OARO line LO1 is separated into the second OARO water OW2 and the second return liquid BW2. The second OARO water OW2 is sent from the second OARO membrane 43 to the third OARO membrane 44 through the second OARO line LO2. The second return liquid BW2 is returned to the first OARO membrane 42 through the second return line LB2.

[0037] The second OARO water OW2 supplied to the third OARO membrane 44 via the second OARO line LO2 is separated into the third OARO water OW3 and the third return liquid BW3. The third OARO water OW3 is sent from the third OARO membrane 44 to the fourth OARO membrane 45 via the third OARO line LO3. The third return liquid BW3 is returned to the second OARO membrane 43 via the third return line LB3.

[0038] The third OARO water OW3 supplied to the fourth OARO membrane 45 through the third OARO line LO3 is separated into the fourth OARO water OW4 and the fourth return liquid BW4. The fourth OARO water OW4 is sent from the fourth OARO membrane 45 through the third treated water line L3 to the concentration section 50. The fourth return liquid BW4 is returned to the third OARO membrane 44 through the fourth return line LB4.

[0039] The fourth OARO water OW4 sent to the concentration section 50 through the third treated water line L3 is concentrated, and a slurry S in which magnesium chloride crystallizes is obtained in the concentration section 50. In addition, the waste liquid FW and distilled water DW are discharged from the concentration section 50 to the outside as the remainder.

[0040] (Action and effect) In the magnesium chloride production system 1 having the above configuration, the 4th sodium ion reduced water NW4 discharged from the second removal unit 20 and having a reduced sodium ion concentration is separated into concentrated water and low concentration water by the OARO method by the OARO separation unit 40. As a result, the concentrated water is concentrated so that the concentration of ions other than water molecules increases. That is, the concentrated water has a reduced water content compared to the 4th sodium ion reduced water NW4, which is the original liquid to be treated. Then, the 4th OARO water OW4, which is the concentrated water with a reduced water content, is supplied to the concentrating unit 50. Therefore, the amount of the water to be treated W to be treated in the concentrating unit 50 is reduced compared to when the 4th sodium ion reduced water NW4 is directly supplied to the concentrating unit 50. This makes it possible to reduce the amount of the water to be treated W during crystallization of magnesium chloride. As a result, the cost during crystallization of magnesium chloride can also be reduced.

[0041] In addition, in this embodiment, the OARO separation section 40 has multiple OARO membranes, such as a first OARO membrane 42, a second OARO membrane 43, a third OARO membrane 44, and a fourth OARO membrane 45. Therefore, the fourth sodium ion reduced water NW4 is concentrated multiple times (four times in this embodiment), and the water content of the fourth OARO water OW4 can be significantly reduced. Therefore, the amount of the water to be treated W to be treated in the concentration section 50 is further reduced. This makes it possible to further reduce the amount of the water to be treated W during the crystallization of magnesium chloride. As a result, the cost during the crystallization of magnesium chloride can also be further reduced.

[0042] In addition, the second removal section 20 has a plurality of nanofiltration membranes, such as a first nanofiltration membrane 21, a second nanofiltration membrane 22, a third nanofiltration membrane 23, and a fourth nanofiltration membrane 24. This makes it possible to increase the removal rate of sodium ions from the water to be treated W. As a result, when magnesium chloride is crystallized in the concentrating section 50, precipitation of sodium chloride is suppressed, and the recovery rate of magnesium ions can be further increased.

[0043] 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 will not be described. In the second embodiment, the magnesium chloride production system 1A is different from the first embodiment in that it further includes a sulfate ion removal device 80.

[0044] The magnesium chloride manufacturing system 1A of the second embodiment further includes a sulfate ion removal device 80 capable of selectively removing only sulfate ions in the water to be treated W supplied to the OARO concentrating section 55, as shown in FIG.

[0045] The sulfate ion removal device 80 of the second embodiment selectively removes only sulfate ions from the treated water that has been treated in the second removal section 20 and is not yet supplied to the OARO concentrating section 55. In this embodiment, the sulfate ion removal device 80 removes only sulfate ions from the quaternary sodium ion reduced water NW4 without adding any chemical to the quaternary sodium ion reduced water NW4. As the sulfate ion removal device 80, for example, a known salt water desalination device can be used. The salt water desalination device is capable of removing sulfate ions brought in from raw salt in the salt water. Note that the sulfate ion removal device 80 is not limited to a salt water desalination device, and may be a device that uses adsorption or a chromatograph.

[0046] The sulfate ion removal device 80 is disposed between the second removal section 20 and the OARO separation section 40. The sulfate ion removal section is disposed in the second treated water line L2. In this way, the sulfate ion removal section removes sulfate ions from the quaternary sodium ion-reduced water NW4 before it is supplied to the RO membrane 41.

[0047] (Action and effect) In the concentrated water generated by the OARO separation unit 40, the concentration of sulfate ions also increases due to concentration. In particular, in a structure having multiple OARO membranes as in this embodiment, the concentration of sulfate ions in the fourth OARO water OW4, which is the final concentrated water, becomes very high. As a result, there is a possibility that calcium sulfate scale will be precipitated in the concentration unit 50 or the OARO membrane along the way. However, in the magnesium chloride production system 1A of the second embodiment, only sulfate ions in the fourth sodium ion reduced water NW4 flowing through the second treated water line L2 are selectively removed by the sulfate ion removal device 80. Therefore, the sulfate ion concentration in the fourth OARO water OW4, which is the concentrated water, is reduced by the sulfate ion removal device 80 without using any chemicals. Therefore, it is possible to suppress the precipitation of scale in the concentration unit 50 or the OARO membrane along the way.

[0048] <Third embodiment> Next, a magnesium chloride production system 1B according to a third embodiment of 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 will not be described. In the third embodiment, the configuration of the concentrating section 50B is different from that of the first embodiment.

[0049] In the magnesium chloride production system 1B of the third embodiment, as shown in Fig. 3, the concentrating section 50B also utilizes the OARO method to concentrate the water to be treated W. The concentrating section 50B has a crystallization section 51, a turbidity section 54, and an OARO concentrating section 55.

[0050] The crystallization unit 51 precipitates and crystallizes magnesium chloride. The crystallization unit 51 evaporates water by heating, reducing pressure, blowing air, or a combination of these. The crystallization unit 51 of this embodiment has a reaction tank 52 and a precipitate unit 53.

[0051] The reaction tank 52 stores the treated water W sent from the OARO separation section 40. The reaction tank 52 stores the fourth OARO water OW4 sent from the fourth OARO membrane 45 through the third treated water line L3. The stored fourth OARO water OW4 is sent to the settling section 53.

[0052] The settling section 53 separates the fourth OARO water OW4 stored in the reaction tank 52 into a slurry S containing high concentration magnesium chloride particles and a separated liquid XW containing low concentration magnesium chloride particles by settling. This results in a slurry S in which magnesium chloride is crystallized. The separated liquid XW is sent to the turbidity section 54.

[0053] In the turbidity section 54, the separated liquid XW is subjected to solid-liquid separation treatment (turbidity treatment). In the turbidity section 54, the separated liquid XW is subjected to solid-liquid separation by filtration treatment using, for example, a multi-layer filtration membrane, an ultrafiltration membrane, filtration sand, etc. The discharged liquid YW, which is the separated liquid XW that has been subjected to the turbidity treatment, is sent to the OARO concentration section 55.

[0054] The concentrating section 50B of the third embodiment is not limited to a structure having the turbidity section 54. The concentrating section 50B may have any structure as long as it can cause magnesium chloride to precipitate and crystallize. For example, the turbidity section 54 may not be provided when a sufficient amount of crystal fine particles can be dissolved in the settling section 53.

[0055] The OARO concentrating section 55 is supplied with the discharged liquid YW discharged from the crystallization section 51. In the OARO concentrating section 55, the concentration of magnesium chloride in the discharged liquid YW discharged from the crystallization section 51 is increased by the OARO method. The OARO concentrating section 55 has a semipermeable membrane (not shown) like the OARO separation section 40. The OARO concentrating section 55 is supplied with discharged liquid YW of approximately the same concentration on both sides of the semipermeable membrane. As a result, in the OARO concentrating section 55, by applying pressure to only one side of the semipermeable membrane in this state, osmotic pressure does not act between the discharged liquid YW separated by the semipermeable membrane by the OARO method, and water permeation occurs even with a slight applied pressure, and ions other than water molecules in the discharged liquid YW are separated, making it possible to highly concentrate. The OARO concentrating section 55 supplies the saturated liquid ZW, which is the discharged liquid YW with an increased concentration of magnesium chloride, to the crystallization section 51. The OARO concentrating section 55 of this embodiment has one OARO membrane. The OARO membrane has the same structure as the first OARO membrane 42, the second OARO membrane 43, the third OARO membrane 44, and the fourth OARO membrane 45. Therefore, in the OARO concentration section 55, the OARO method is performed on the discharge liquid YW from which crystalline fine particles and the like have been removed by the turbidity treatment in the turbidity section 54. As a result, magnesium chloride remaining in the discharge liquid YW is concentrated, and a saturated liquid ZW having a higher magnesium chloride concentration than the discharge liquid YW is generated. The saturated liquid ZW contains magnesium chloride in a supersaturated state. The saturated liquid ZW is returned to the reaction tank 52. In addition, the low-concentration waste liquid LW having a low concentration of magnesium chloride generated when the saturated liquid ZW is generated is sent to the circulation line LR and supplied to the fourth OARO membrane 45.

[0056] (Action and effect) In the magnesium chloride production system 1B of the third embodiment, the concentrating section 50B has an OARO concentrating section 55 that concentrates the discharge liquid YW discharged from the crystallization section 51. In particular, in this embodiment, a saturated liquid ZW containing magnesium chloride in a supersaturated state is generated by the OARO concentrating section 55. The saturated liquid ZW having a high concentration of magnesium chloride as a result of being concentrated in the OARO concentrating section 55 is supplied to the reaction tank 52 of the crystallization section 51. By supplying the liquid having a high concentration of magnesium chloride to the reaction tank 52, the precipitation of magnesium chloride can be further promoted. As a result, the energy consumption of the crystallization section 51 can be reduced.

[0057] Furthermore, the saturated liquid ZW discharged from the OARO concentrating section 55 is supplied to the reaction tank 52, and the low-concentration waste liquid LW is supplied to the fourth OARO membrane 45. This allows the fluid discharged from the OARO concentrating section 55 to be returned to the system without being discharged to the outside. This allows the recovery rate of magnesium ions in the concentrating section 50 to be increased.

[0058] In addition, the low-concentration waste liquid LW separated from the saturated liquid ZW by the OARO concentration section 55 is sent to the circulation line LR and supplied to the fourth OARO membrane 45. In other words, the low-concentration waste liquid LW having a low concentration of magnesium chloride can be used for concentration by the fourth OARO membrane 45.

[0059] (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 that do not depart from the gist of the present disclosure are also included.

[0060] For example, the magnesium chloride manufacturing systems 1, 1A, and 1B may simultaneously include the sulfate ion removing device 80 of the second embodiment and the concentrating section 50B of the third embodiment.

[0061] Furthermore, the dilution unit 70 is not limited to a configuration in which it supplies the dilution water AW only to the first treated water line L1, the first connection line LC1, and the second connection line LC2. The dilution unit 70 may also supply the dilution water AW to the third connection line LC3. Furthermore, the dilution unit 70 may be configured not to supply the dilution water AW to any of the first treated water line L1, the first connection line LC1, and the second connection line LC2.

[0062] In addition, the distilled water DW discharged from the concentration unit 50 is not limited to being merged with the waste liquid FW and treated. The distilled water DW discharged from the concentration unit 50 may be supplied to a different location separately from the waste liquid FW.

[0063] Moreover, the OARO separation unit 40 is not limited to the configuration of this embodiment. For example, the OARO separation unit 40 is not limited to a structure having OARO membranes in multiple stages. The OARO separation unit 40 may have a structure having only one OARO membrane. Even if the OARO separation unit 40 has multiple OARO membranes, it is not limited to four as in this embodiment. The OARO separation unit 40 may have three or less OARO membranes, or may have five or more OARO membranes. Furthermore, the OARO separation unit 40 is not limited to a structure having an RO membrane 41 in front of an OARO membrane. In other words, the OARO separation unit 40 may have an OARO membrane without having an RO membrane 41.

[0064] <Additional Notes> The magnesium chloride production systems 1, 1A, and 1B described in the respective embodiments can be understood, for example, as follows.

[0065] (1) The magnesium chloride manufacturing system 1, 1A, 1B according to the first embodiment includes a first removal section 10 that reduces the sulfate ion concentration in the water to be treated W made from seawater, a second removal section 20 that reduces the sodium ion concentration in the water to be treated W discharged from the first removal section 10 and having the reduced sulfate ion concentration, an OARO separation section 40 that supplies the water to be treated W of approximately the same concentration to both sides of a semipermeable membrane and separates the water to be treated W discharged from the second removal section 20 and having the reduced sodium ion concentration into concentrated water and low-concentration water by the OARO method, and a concentration section 50, 50B that further concentrates the concentrated water discharged from the OARO separation section 40 to produce a slurry S in which magnesium chloride is crystallized.

[0066] According to this configuration, the water W to be treated, which is discharged from the second removal unit 20 and has a reduced sodium ion concentration, is separated into concentrated water and low-concentration water by the OARO method using the OARO separation unit 40. As a result, the concentrated water is concentrated so that the concentration of ions other than water molecules increases. In other words, the concentrated water has a reduced water content compared to the original liquid to be treated. Then, the concentrated water with a reduced water content is supplied to the concentrating units 50 and 50B. Therefore, the amount of the water W to be treated in the concentrating units 50 and 50B is reduced compared to when the original liquid to be treated is directly supplied to the concentrating units 50 and 50B. This allows the amount of water W to be treated during crystallization of magnesium chloride to be reduced. As a result, the cost of crystallizing magnesium chloride can also be reduced.

[0067] (2) The magnesium chloride production system 1, 1A, 1B of the second embodiment is the magnesium chloride production system 1, 1A, 1B of (1), wherein the OARO separation section 40 has multiple OARO membranes arranged between the second removal section 20 and the concentration section 50, 50B.

[0068] According to such a configuration, the water to be treated W is concentrated multiple times, and the water content of the concentrated water can be significantly reduced. Therefore, the amount of the water to be treated W to be treated in the concentration section 50 is further reduced. This makes it possible to further reduce the amount of the water to be treated W during crystallization of magnesium chloride. As a result, the cost during crystallization of magnesium chloride can also be further reduced.

[0069] (3) The magnesium chloride production system 1, 1A, 1B of the third aspect is the magnesium chloride production system 1, 1A, 1B of (1) or (2), further comprising a dilution section 70 that supplies a liquid having a lower concentration than the water to be treated W to the second removal section 20, and the OARO separation section 40 has an RO membrane 41 arranged between the second removal section 20 and the OARO membrane, and the RO membrane 41 desalinates the water to be treated W by reverse osmosis before performing the OARO method to produce concentrated RO concentrate RW and permeate PW from which ions have been removed, and sends the permeate PW to the dilution section 70.

[0070] (4) A magnesium chloride production system 1, 1A, 1B relating to a fourth aspect is any one of the magnesium chloride production systems 1, 1A, 1B of (1) to (3), and further includes a sulfate ion removal device 80 that is arranged between the second removal section 20 and the OARO separation section 40 and is capable of selectively removing only sulfate ions in the treated water W that is supplied to the OARO separation section 40.

[0071] With this configuration, only sulfate ions are selectively removed from the water W to be treated that is supplied to the OARO separation unit 40. Therefore, the sulfate ion concentration in the concentrated water is reduced by the sulfate ion removal device 80 without using any chemicals. This makes it possible to suppress the deposition of scale in the concentration units 50 and 50B.

[0072] (5) A magnesium chloride production system 1, 1A, 1B according to a fifth aspect is any one of the magnesium chloride production systems 1, 1A, 1B of (1) to (4), in which the concentrating section 50B includes a crystallization section 51 which causes the magnesium chloride to settle and crystallize, and an OARO concentrating section 55 which receives an effluent YW discharged from the crystallization section 51, supplies the effluent YW of the same concentration to both sides of a semipermeable membrane, and increases the concentration of the magnesium chloride in the effluent YW, and the OARO concentrating section 55 supplies the effluent YW with the increased concentration of magnesium chloride to the crystallization section 51.

[0073] According to this configuration, the discharge liquid YW having a high concentration of magnesium chloride as a result of being concentrated in the OARO concentrating section 55 is supplied to the reaction tank 52 of the crystallization section 51. By supplying the liquid having a high concentration of magnesium chloride to the reaction tank 52, the precipitation of magnesium chloride can be further promoted. This allows the energy consumption of the crystallization section 51 to be reduced. [Explanation of symbols]

[0074] 1, 1A, 1B…Magnesium chloride production system 10...First removal section 20…Second removal section 21…First nanofiltration membrane 22...Second nanofiltration membrane 23…Third nanofiltration membrane 24...Quaternary nanofiltration membrane 40…OARO separation section 41...RO membrane 42…First OARO membrane 43…Second OARO membrane 44…Third OARO membrane 45...Fourth OARO membrane 50...Concentration section 70…Dilution section LS…Supply line L1: First treated water line LC1: First connection line LC2: Second connection line LC3…Third connecting line L2: Second treated water line LRO: Reverse osmosis line LO1…First OARO Line LO2: Second OARO line LO3…Third OARO Line L3: Third treated water line LB1: First return line LB2: Second return line LB3: Third return line LB4: Fourth return line LR: Circulation line W: Water to be treated SW: Sulfate ion reduced water EW1…First wastewater NW: Sodium ion reduced water NW1: Reduced sodium ion water NW2: Dibasic sodium ion reduced water NW3: Water with reduced sodium ions NW4: Quaternary sodium ion reduced water EW2…Second wastewater RW…RO concentrated water PW…Permeated water OW1…First OARO water OW2…Second OARO Water OW3…3rd OARO Water OW4…Fourth OARO Water BW1: First return solution BW2: Second return solution BW3: Third return solution BW4: Fourth return solution LW: Low-concentration waste liquid FW: Waste liquid DW: Distilled water S: Slurry AW: Diluted water 80...Sulfate ion removal device 50B…Concentration section 51…Crystallization section 52...Reaction tank 53…Submerged section 54...Slow turbidity part 55…OARO enrichment section XW…separation liquid YW…Drained liquid ZW…Saturated liquid LW: Low-concentration waste liquid

Claims

1. A first removal unit that reduces a sulfate ion concentration in the water to be treated that is made from seawater; A second removal unit that reduces a sodium ion concentration in the water to be treated that has been discharged from the first removal unit and has a reduced sulfate ion concentration; an OARO separation section that separates the treated water discharged from the second removal section and having a reduced sodium ion concentration into concentrated water and low-concentration water by an OARO method in which the treated water having the same concentration is supplied to both sides of a semipermeable membrane; a concentrating section that further concentrates the concentrated water discharged from the OARO separation section to produce a slurry in which magnesium chloride is crystallized.

2. 2. The magnesium chloride production system according to claim 1, wherein the OARO separation section has a plurality of OARO membranes disposed between the second removal section and the concentration section.

3. A dilution unit that supplies a liquid having a lower concentration than the water to be treated to the second removal unit, the OARO separation unit has an RO membrane disposed between the second removal unit and the OARO membrane, The magnesium chloride manufacturing system of claim 2, wherein the RO membrane desalinates the treated water by reverse osmosis to produce concentrated RO concentrated water and permeate water from which ions have been removed before carrying out the OARO method, and sends the permeate water PW to the dilution section.

4. The magnesium chloride production system according to claim 1 or 2, further comprising a sulfate ion removal device disposed between the second removal section and the OARO separation section and capable of selectively removing only sulfate ions in the treated water supplied to the OARO separation section.

5. The enrichment section comprises: a crystallization section for precipitating and crystallizing the magnesium chloride; an OARO concentration section to which the discharged liquid discharged from the crystallization section is supplied, the discharged liquid having the same concentration is supplied to both sides of a semipermeable membrane, and the concentration of the magnesium chloride in the discharged liquid is increased by an OARO method; 3. The magnesium chloride manufacturing system according to claim 1, wherein the OARO concentrating section supplies the discharged liquid having an increased concentration of the magnesium chloride to the crystallization section.

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