System for producing magnesium chloride

Abstract

To provide a system that enhances the recovery rate of magnesium ions from water to be treated.SOLUTION: A system for producing magnesium chloride comprises: a first removal part for reducing the concentration of sulfate ions in water to be treated using seawater as a raw material by electrodialysis; at least one second removal part for reducing the concentration of sodium ions in the water to be treated in which the concentration of sulfate ions has been reduced by discharging from the first removal part; a concentration part for concentrating the water to be treated in which the concentration of sodium ions has been reduced by discharging from the second removal part and producing a slurry with crystallized magnesium chloride; and a recycling line for supplying waste liquid discharged from the concentration part to a position upstream from the at least one second removal part.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] In the method of the above-mentioned Patent Document 1, magnesium ions (Mg 2+When separating ions other than magnesium ions, wastewater containing other ions is generated. This wastewater is usually discharged to the outside. However, because it is difficult to completely separate magnesium ions from other ions, magnesium ions that should be left in the treated water until crystallization is performed are also contained in the wastewater. As a result, there is a problem that the recovery rate of magnesium ions from the treated water is insufficient. Therefore, there is a demand to increase the recovery rate of magnesium ions from the treated water, which is made from seawater, as much as possible.

[0005] The present disclosure has been made to solve the above-mentioned problems, and aims to provide a magnesium chloride production system that is capable of recovering magnesium ions as magnesium chloride at a high recovery rate from treated water in which sulfate ions have been reduced by costly electrodialysis. [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 that reduces the sulfate ion concentration in treated water made from seawater by electrodialysis, at least one second removal section that reduces the sodium ion concentration in the treated water discharged from the first removal section and having the reduced sulfate ion concentration, a concentrating section that concentrates the treated water discharged from the second removal section and having the reduced sodium ion concentration to produce a slurry in which magnesium chloride crystallizes, and a recycle line that supplies waste liquid discharged from the concentrating section to a position upstream of at least one of the second removal sections. Effect of the Invention

[0007] According to the magnesium chloride production system of the present disclosure, magnesium ions from treated water in which sulfate ions have been reduced by costly electrodialysis can be recovered as magnesium chloride at a high recovery rate. [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. [Figure 4] FIG. 1 is a schematic diagram showing a magnesium chloride manufacturing system according to a fourth 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 amount of these salts that precipitate is the largest for 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 manufacturing system 1 of the present embodiment includes a first removal section 10, a second removal section 20, a concentration section 50, a dilution section 70, a recycle line LR, and an additional recycle line LA.

[0012] The first removal unit 10 reduces the sulfate ion concentration in the water to be treated W, which is made from seawater, by electrodialysis. 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 a concentrating unit 50, which will be described later. 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 concentrating unit 50. In addition, second wastewater EW2 is discharged from each nanofiltration membrane.

[0020] The concentrating section 50 concentrates the sodium-ion reduced water NW, which is the water W to be treated that has been discharged from the second removal section 20 and has a reduced sodium ion concentration. In this way, the concentrating section 50 produces a slurry S in which magnesium chloride is crystallized. The concentrating section 50 is connected to the second removal section 20. The concentrating section 50 is supplied with the fourth sodium-ion reduced water NW4, which is the sodium-ion reduced water NW in which the sodium ion concentration has been reduced the most, via the second treated water line L2. 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 sodium-ion reduced water NW4 by heating, reducing pressure, blowing air, or a combination of these to evaporate the water.

[0021] The concentrating section 50 concentrates the quaternary sodium ion reduced water NW4, so that the ions dissolved in the quaternary sodium ion reduced water NW4 become salt 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 supplying the water to the concentrating section 50. Therefore, in the concentrating section 50, in addition to a small amount of iron oxide and calcium carbonate, the target magnesium chloride precipitates, and a slurry S in which magnesium chloride is crystallized is obtained. In addition, in the concentrating section 50, 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. In other words, in the concentrating section 50, when the slurry S is generated, the waste liquid FW and the distilled water DW are generated.

[0022] 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.

[0023] The recycle line LR supplies the waste liquid FW discharged from the concentration section 50 to a position upstream of at least one second removal section 20. In other words, the recycle line LR supplies the waste liquid FW discharged from the concentration section 50 so as to return it to a position upstream of the concentration section 50 itself. The recycle line LR is preferably connected so as to return the waste liquid FW to a position where the ion ratio (magnesium ion / sodium ion), which is the ratio of magnesium ion and sodium ion in the circulating water to be treated W, is lower. Therefore, the recycle line LR is preferably connected so as to return the waste liquid FW to a position where the concentration of magnesium ion is low and the concentration of sodium ion is high in the circulating water to be treated W. In addition, the recycle line LR of this embodiment also supplies distilled water DW together with the waste liquid FW. The recycle line LR supplies the waste liquid FW to a position upstream of at least the most downstream nanofiltration membrane arranged at the most downstream among the multiple nanofiltration membranes. In other words, the recycle line LR supplies it to a position upstream of the fourth nanofiltration membrane 24, which is the most downstream nanofiltration membrane. Specifically, the recycle line LR is connected to the concentration section 50 and the third connection line LC3. The recycle line LR supplies the waste liquid FW and distilled water DW to the third sodium ion reduced water NW3 before being supplied to the fourth nanofiltration membrane 24 so as to be mixed with each other.

[0024] The additional recycle line LA supplies the second wastewater EW2, which is wastewater discharged from at least one second removal unit 20, to a position upstream of at least one second removal unit 20. The additional recycle line LA supplies the second wastewater EW2 discharged from one nanofiltration membrane among the plurality of nanofiltration membranes to a position upstream of an upstream nanofiltration membrane located upstream of one nanofiltration membrane among the plurality of nanofiltration membranes. In other words, the additional recycle line LA supplies the second wastewater EW2 discharged from one nanofiltration membrane so as to return it to a position upstream of the one nanofiltration membrane that discharged it. As with the recycle line LR, the additional recycle line LA is preferably connected to return the second wastewater EW2 to a position where the ion ratio (magnesium ion / sodium ion), which is the ratio of magnesium ions and sodium ions in the circulating second wastewater EW2, is lower. Therefore, the additional recycle line LA is preferably connected to return the second wastewater EW2 to a position where the concentration of magnesium ions is lower and the concentration of sodium ions is higher in the circulating second wastewater EW2. The additional recycle line LA in this embodiment supplies the second wastewater EW2 discharged from the fourth nanofiltration membrane 24 to a position further upstream of the first nanofiltration membrane 21, which is an upstream nanofiltration membrane located further upstream of the fourth nanofiltration membrane 24. The additional recycle line LA connects the fourth nanofiltration membrane 24 and the first treated water line L1.

[0025] 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.

[0026] The discharged sulfate ion reduced water SW is supplied to the first nanofiltration membrane 21 through the first treated water line L1. At this time, the dilution water AW supplied from the dilution unit 70 is also supplied to the first treated water line L1. Furthermore, the second wastewater EW2 discharged from the fourth nanofiltration membrane 24 is also supplied to the first treated water line L1 through the additional recycle line LA. As a result, the first nanofiltration membrane 21 is supplied with sulfate ion reduced water SW diluted with the dilution water AW and mixed with the second wastewater EW2 discharged from the fourth nanofiltration membrane 24. The first nanofiltration membrane 21 generates the first sodium ion reduced water NW1 in which the sodium ion concentration is reduced, and the second wastewater EW2 which is the remainder. The second wastewater EW2 generated by the first nanofiltration membrane 21 is discharged to the outside.

[0027] The generated first sodium ion reduced water NW1 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 is also supplied to the first connection line LC1. As a result, the first sodium ion reduced water NW1 diluted with the dilution water AW is supplied to the second nanofiltration membrane 22. The second nanofiltration membrane 22 generates second sodium ion reduced water NW2 in which the sodium ion concentration is further reduced, and second wastewater EW2 which is the remainder. The second wastewater EW2 generated by the second nanofiltration membrane 22 is discharged to the outside.

[0028] 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 is also supplied to the second connection line LC2. As a result, the second sodium ion reduced water NW2 diluted with the dilution water AW 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.

[0029] The third sodium ion reduced water NW3 thus produced is supplied to the fourth nanofiltration membrane 24 via the third connection line LC3. At this time, the waste liquid FW discharged from the concentrating section 50 is also supplied to the third connection line LC3 via the recycle line LR. As a result, the third sodium ion reduced water NW3 mixed with the waste liquid FW is supplied to the fourth nanofiltration membrane 24. The fourth nanofiltration membrane 24 produces fourth sodium ion reduced water NW4 in which the sodium ion concentration is further reduced, and the remainder, second wastewater EW2. The second wastewater EW2 produced by the fourth nanofiltration membrane 24 is supplied to the first treated water line L1 via the additional recycle line LA.

[0030] The produced quaternary sodium ion-reduced water NW4 is supplied to the concentrating section 50 via the second treated water line L2. As a result, the quaternary sodium ion-reduced water NW4 is supplied to the concentrating section 50. In the concentrating section 50, a slurry S in which magnesium chloride is crystallized from the quaternary sodium ion-reduced water NW4 is obtained. The waste liquid FW and distilled water DW are discharged as the remainder from the concentrating section 50 via the recycle line LR to the third connecting line LC3.

[0031] (Action and effect) In the magnesium chloride manufacturing system 1 having the above configuration, a mixed liquid of the waste liquid FW and distilled water DW generated in the process of generating the slurry S in which magnesium chloride is crystallized in the concentrating section 50 is supplied to the third sodium ion reduced water NW3 before being supplied to the fourth nanofiltration membrane 24 by the recycle line LR. Then, the mixed liquid is mixed with the third sodium ion reduced water NW3 and supplied to the fourth nanofiltration membrane 24. After that, magnesium chloride is taken out again through the treatment in the fourth nanofiltration membrane 24 and the concentrating section 50, and is returned to the fourth nanofiltration membrane 24. In this way, by circulating the waste liquid FW and distilled water DW discharged from the concentrating section 50, magnesium that was not removed in one treatment in the concentrating section 50 and remained in the waste liquid FW can be recovered. As a result, magnesium ions from the sulfate ion reduced water SW in which sulfate ions are reduced in the first removal section 10, which is a costly electrodialysis, can be recovered as magnesium chloride at a high recovery rate.

[0032] 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.

[0033] Furthermore, the recycle line LR is connected to a third connection line LC3, which is located upstream of the fourth nanofiltration membrane 24, which is the most downstream nanofiltration membrane located at the most downstream of the multiple nanofiltration membranes. When multiple nanofiltration membranes are provided, the ion ratio (Mg 2+ / Na + ) is the highest. The ion ratio of the waste liquid FW is also higher than the ion ratio of the third sodium ion-reduced water NW3. In other words, by merging the waste liquid FW with a high ion ratio with the third sodium ion-reduced water NW3 with a relatively high ion ratio, it is possible to suppress magnesium loss and improve the recovery rate of magnesium ions.

[0034] Further, the second wastewater EW2 discharged from the fourth nanofiltration membrane 24 is supplied to the first treated water line L1 by the additional recycle line LA. The second wastewater EW2 returned to the first treated water line L1 is mixed with the sulfate ion reduced water SW and the dilution water AW, and is sent to the first nanofiltration membrane 21. Thereafter, the second wastewater EW2 is again processed by the first nanofiltration membrane 21, the second nanofiltration membrane 22, and the third nanofiltration membrane 23 to reduce sodium ions, and to recover magnesium ions, and is returned to the fourth nanofiltration membrane 24. In this way, the second wastewater EW2 discharged from the fourth nanofiltration membrane 24 is circulated, so that the magnesium ions remaining in the second wastewater EW2 can be recovered. This makes it possible to suppress discharge of magnesium ions from the second wastewater EW2 and improve the recovery rate of magnesium ions.

[0035] Particularly in this embodiment, the additional recycle line LA supplies the second wastewater EW2 discharged from the fourth nanofiltration membrane 24 to the first treated water line L1, which is further upstream of the first nanofiltration membrane 21, which is located at the most upstream of the multiple nanofiltration membranes. The treated water W having the lowest ion ratio among the multiple nanofiltration membranes is the sulfate ion reduced water SW. Therefore, the loss at the fourth nanofiltration membrane 24 can be suppressed by re-recovering it with the first nanofiltration membrane 21, and the recovery rate of magnesium ions can be improved.

[0036] 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 reducer 90.

[0037] As shown in FIG. 2, the magnesium chloride manufacturing system 1A of the second embodiment further includes a sulfate ion reducing section 90 that reduces the amount of sulfate ions in the waste liquid FW flowing through the recycle line LR.

[0038] The sulfate ion reduction unit 90 of the second embodiment reduces the amount of sulfate ions in the waste liquid FW that returns to the second removal unit 20 through the recycle line LR by partially discharging the waste liquid FW flowing through the recycle line LR. The sulfate ion reduction unit 90 has a discharge valve 91 and an ion concentration detection unit 92.

[0039] The discharge valve 91 is disposed in the recycle line LR. For example, the discharge valve 91 is disposed in a position upstream of the position where the waste liquid FW and the distilled water DW join in the recycle line LR and downstream of the concentrating section 50. The discharge valve 91 may be disposed in the recycle line LR. Therefore, the discharge valve 91 may be disposed downstream of the position where the waste liquid FW and the distilled water DW join. The discharge valve 91 is capable of discharging the waste liquid FW flowing through the recycle line LR to the outside. The discharge valve 91 in this embodiment is set to discharge the waste liquid FW based on the detection result of the ion concentration detection section 92. Specifically, the discharge valve 91 is opened to discharge the waste liquid FW to the outside of the recycle line LR when the sulfate ion concentration in the treated water W acquired by the ion concentration detection section 92 meets a predetermined standard. In this embodiment, the sulfate ion concentration of the quaternary sodium ion reduced water NW4 supplied to the concentrating section 50 is set as a predetermined standard value that can be regarded as causing calcium sulfate scale in the concentrating section 50. Therefore, when the detection result of the ion concentration detection unit 92 exceeds the standard, the discharge valve 91 is opened to discharge the waste liquid FW from the recycle line LR. On the other hand, when the detection result of the ion concentration detection unit 92 does not exceed the standard, the discharge valve 91 is closed to not discharge the waste liquid FW but to allow it to flow through the recycle line LR.

[0040] The ion concentration detection unit 92 acquires information on the sulfate ion concentration in the treated water W discharged from the second removal unit 20 and before being supplied to the concentration unit 50. The ion concentration detection unit 92 detects the sulfate ion concentration of the quaternary sodium ion reduced water NW4. The ion concentration detection unit 92 is disposed in the second treated water line L2. When the detected sulfate ion concentration exceeds the above-mentioned standard, the ion concentration detection unit 92 sends a signal to the discharge valve 91 to open the valve as the condition is satisfied. When the detected sulfate ion concentration does not exceed the standard, the ion concentration detection unit 92 sends a signal to the discharge valve 91 to close the valve as the condition is not satisfied.

[0041] (Action and effect) Since the removal rate of sulfate ions in the nanofiltration membrane of the second removal unit 20 is extremely high, sulfate ions are not sufficiently discharged from the second waste liquid EW2 even when the circulating waste liquid FW and distilled water DW are circulated for a long period of time. In addition, by providing the recycle line LR and the additional recycle line LA, sulfate ions contained in the waste liquid FW and the waste liquid EW2 are recirculated in the system, and sulfate ions are concentrated in the system. Therefore, the sulfate ion concentration in the circulating water to be treated W becomes relatively high. As a result, there is a possibility that scale due to calcium sulfate will be precipitated in the concentration unit 50. However, in the magnesium chloride manufacturing system 1A of the second embodiment, the sulfate ion reduction unit 90 can discharge sulfate ions out of the system, so that the sulfate ion concentration concentrated in the circulating water to be treated W can be reduced. That is, the amount of sulfate ions in the waste liquid FW and the distilled water DW can be reduced. Therefore, the precipitation of scale in the concentration unit 50 can be suppressed.

[0042] The sulfate ion reduction unit 90 also includes an ion concentration detection unit 92 that acquires the sulfate ion concentration of the quaternary sodium ion reduced water NW4, and a discharge valve 91 that discharges the waste liquid FW flowing through the recycle line LR to the outside based on the detection result of the ion concentration detection unit 92. When the detection result of the ion concentration detection unit 92 exceeds the standard, the discharge valve 91 discharges the waste liquid FW from the recycle line LR. Therefore, while appropriately grasping the sulfate ion concentration, it is possible to effectively reduce the sulfate ion concentration only when the sulfate ion concentration exceeds the standard. This makes it possible to suppress the concentration of sulfate ions and effectively suppress the precipitation of scale in the concentration unit 50.

[0043] <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, 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 a sulfate ion reducer 90B is different from that of the second embodiment.

[0044] In the magnesium chloride manufacturing system 1B of the third embodiment, a sulfate ion reducer 90B has a barium adder 93 and a barium remover 94, as shown in FIG.

[0045] The barium addition section 93 adds barium chloride (BaCl2) to the water to be treated W discharged from the second removal section 20 and before being supplied to the concentration section 50. The barium addition section 93 of this embodiment adds barium chloride to the quaternary sodium ion reduced water NW4. The barium addition section 93 is disposed in the second treated water line L2. In the barium addition section 93, it is preferable that the amount of barium added is determined according to the concentration of sulfate ions in the quaternary sodium ion reduced water NW4. By adding barium chloride to the quaternary sodium ion reduced water NW4, barium sulfate (BaSO4) is generated in the second treated water line L2.

[0046] The barium removal unit 94 removes barium sulfate from the water to be treated W to which barium chloride has been added. The barium removal unit 94 of this embodiment removes barium sulfate from the quaternary sodium ion-reduced water NW4 to which barium chloride has been added. The barium removal unit 94 is disposed in the second treated water line L2, between the barium addition unit 93 and the concentrating unit 50. The barium removal unit 94 separates barium sulfate from the quaternary sodium ion-reduced water NW4 by crystallization. The quaternary sodium ion-reduced water NW4 from which barium sulfate has been separated is supplied to the concentrating unit 50.

[0047] (Action and effect) In the magnesium chloride manufacturing system 1B of the third embodiment, the sulfate ion reduction unit 90B has a barium addition unit 93 that adds barium chloride to the quaternary sodium ion reduced water NW4, and a barium removal unit 94 that removes barium sulfate from the quaternary sodium ion reduced water NW4 to which barium chloride has been added. By adding barium chloride to the quaternary sodium ion reduced water NW4 by the barium addition unit 93, the sulfate ions in the quaternary sodium ion reduced water NW4 react with barium to become barium sulfate. This makes it possible to reduce the sulfate ions in the quaternary sodium ion reduced water NW4. Furthermore, the barium removal unit 94 can remove the generated barium sulfate while it is being supplied to the concentrating unit 50. Therefore, it is possible to reduce the sulfate ion concentration while suppressing the influence on the concentrating unit 50. This makes it possible to effectively suppress the concentration of sulfate ions and the precipitation of scale in the concentrating unit 50.

[0048] <Fourth embodiment> Next, a magnesium chloride production system 1C according to a fourth embodiment of the present disclosure will be described. In the fourth embodiment described below, components common to the first to third embodiments are denoted by the same reference numerals in the drawings and will not be described. In the fourth embodiment, the configuration of a sulfate ion reducer 90C is different from those of the second and third embodiments.

[0049] In the magnesium chloride manufacturing system 1C of the fourth embodiment, a sulfate ion reduction section 90C has a sulfate ion removal device 95 as shown in FIG.

[0050] The sulfate ion removal device 95 is capable of selectively removing only sulfate ions in the waste liquid FW. The sulfate ion removal device 95 is disposed in the recycle line LR. The sulfate ion removal device 95 is disposed, for example, upstream of the position where the waste liquid FW and the distilled water DW join in the recycle line LR and downstream of the concentrating section 50. The sulfate ion removal device 95 may be disposed in the recycle line LR. Therefore, the sulfate ion removal device 95 may be disposed downstream of the position where the waste liquid FW and the distilled water DW join. In this embodiment, the sulfate ion removal device 95 removes only sulfate ions from the waste liquid FW without adding a chemical to the waste liquid FW. For example, a known saltwater desalting device can be used as the sulfate ion removal device 95. The saltwater desalting device is capable of removing sulfate ions brought in from raw salt in the saltwater. The sulfate ion removal device 95 is not limited to the saltwater desalting device, and may be a device using adsorption or a chromatograph.

[0051] (Action and effect) In the magnesium chloride manufacturing system 1C of the third embodiment, the sulfate ion reduction section 90C has a sulfate ion removal device 95 that can selectively remove only sulfate ions in the waste liquid FW. Therefore, similarly to the third embodiment, it is possible to reduce the sulfate ion concentration without using a chemical agent and while suppressing the effect on the concentrating section 50. This makes it possible to suppress the concentration of sulfate ions in the waste liquid FW and effectively suppress the precipitation of scale in the second removal section 20 and the concentrating section 50.

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

[0053] For example, the recycle line LR is not limited to being connected to the third connection line LC3 as in this embodiment. The recycle line LR may have any structure as long as it can supply the waste liquid FW discharged from the concentration section 50 to a position upstream of at least one second removal section 20. Therefore, the recycle line LR may be connected to any one of the first treated water line L1, the first connection line LC1, and the second connection line LC2.

[0054] Moreover, the magnesium chloride production systems 1, 1A, 1B, and 1C are not limited to having the additional recycle line LA. The magnesium chloride production systems 1, 1A, 1B, and 1C may be configured without the additional recycle line LA. Furthermore, the additional recycle line LA is not limited to a configuration in which the second wastewater EW2 discharged from the third nanofiltration membrane 23 is supplied to the first treated water line L1. The additional recycle line LA may be configured to supply the second wastewater EW2 discharged from one nanofiltration membrane so as to return it to an upstream of the one nanofiltration membrane itself that discharged it. Therefore, the additional recycle line LA may be connected to the second nanofiltration membrane 22 or the fourth nanofiltration membrane 24. Moreover, the additional recycle line LA may be connected to the first connection line LC1 or the second connection line LC2.

[0055] 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.

[0056] 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.

[0057] Furthermore, the concentrating section 50 may have another configuration as long as it can obtain the slurry S containing magnesium chloride. For example, the concentrating section 50 may obtain the slurry S containing magnesium chloride by utilizing sedimentation or the like.

[0058] Moreover, the sulfate ion reducer 90, 90B, 90C described in the second to fourth embodiments is not limited to a configuration having only one of the components. For example, the magnesium chloride manufacturing system 1, 1A, 1B, 1C may simultaneously have, as the sulfate ion reducer 90, 90B, 90C, a plurality of the discharge valve 91 and ion concentration detector 92, the barium adder 93 and barium remover 94, and the sulfate ion remover 95.

[0059] <Additional Notes> The magnesium chloride production systems 1, 1A, 1B, and 1C described in the respective embodiments can be understood, for example, as follows.

[0060] (1) The magnesium chloride manufacturing system 1, 1A, 1B, 1C according to the first embodiment comprises a first removal section 10 which reduces the sulfate ion concentration in water to be treated W made from seawater by electrodialysis, at least one second removal section 20 which 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, a concentrating section 50 which concentrates the water to be treated W discharged from the second removal section 20 and having the reduced sodium ion concentration to produce a slurry S in which magnesium chloride is crystallized, and a recycle line LR located upstream of at least one second removal section 20, which supplies waste liquid FW discharged from the concentrating section 50.

[0061] According to this configuration, the waste liquid FW generated in the process of generating the slurry S in which magnesium chloride is crystallized in the concentrating section 50 is supplied to the water to be treated W before being supplied to the second removal section 20 by the recycle line LR. Then, the waste liquid FW is mixed with the water to be treated W and supplied to the second removal section 20. Thereafter, magnesium chloride is extracted again through the treatment in the second removal section 20 and the concentrating section 50, and is returned to the second removal section 20. In this way, by circulating the waste liquid FW discharged from the concentrating section 50, magnesium that was not removed in one treatment in the concentrating section 50 and remains in the waste liquid FW can be recovered. As a result, magnesium ions can be recovered as magnesium chloride at a high recovery rate from the sulfate ion reduced water SW in which sulfate ions have been reduced in the first removal section 10, which is a costly electrodialysis.

[0062] (2) The magnesium chloride production systems 1, 1A, 1B, and 1C of the second aspect are the magnesium chloride production systems 1, 1A, 1B, and 1C of (1), in which the second removal section 20 is arranged in plurality, each having a nanofiltration membrane that separates monovalent ions and polyvalent ions from the treated water W in which the sulfate ion concentration has been reduced in the first removal section 10, and the recycle line LR supplies the waste liquid FW to a position upstream of at least the most downstream nanofiltration membrane arranged in the most downstream position among the plurality of nanofiltration membranes.

[0063] According to this configuration, in the recycle line LR, the waste liquid FW is supplied to a position upstream of the most downstream nanofiltration membrane that is disposed at the most downstream of the multiple nanofiltration membranes. When multiple nanofiltration membranes are provided, the ion ratio (Mg 2+ / Na + ) is the highest. The ion ratio of the waste liquid FW is also higher than the ion ratio of the third sodium ion-reduced water NW3. In other words, by merging the waste liquid FW with a high ion ratio with the third sodium ion-reduced water NW3 with a relatively high ion ratio, it is possible to suppress magnesium loss and improve the recovery rate of magnesium ions.

[0064] (3) The magnesium chloride production system 1, 1A, 1B, 1C of the third aspect is the magnesium chloride production system 1, 1A, 1B, 1C of (1) or (2), in which the recycle line is connected so as to return the waste liquid to a position where the ion ratio, which is the ratio of magnesium ions to sodium ions in the circulating waste liquid, is lower.

[0065] (4) A magnesium chloride production system 1, 1A, 1B, 1C according to a fourth aspect is any one of the magnesium chloride production systems 1, 1A, 1B, 1C of (1) to (3), in which the second removal section 20 is arranged in a plurality of nanofiltration membranes that separate monovalent ions and polyvalent ions from the treated water W in which the sulfate ion concentration has been reduced in the first removal section 10, and further includes an additional recycle line LA that supplies wastewater discharged from one of the plurality of nanofiltration membranes to an upstream position of an upstream nanofiltration membrane that is located upstream of one of the plurality of nanofiltration membranes.

[0066] According to this configuration, the additional recycle line LA supplies the wastewater discharged from the first nanofiltration membrane to the upstream nanofiltration membrane located upstream of the first nanofiltration membrane. The wastewater FW supplied to the upstream nanofiltration membrane is mixed with the water to be treated W and sent to the upstream nanofiltration membrane. Thereafter, the wastewater is again processed by the upstream nanofiltration membrane to reduce sodium ions, and magnesium ions are recovered and returned to the first nanofiltration membrane. In this way, the wastewater discharged from the first nanofiltration membrane is circulated, so that the magnesium ions remaining in the wastewater can be recovered. This makes it possible to suppress the discharge of magnesium ions from the second wastewater EW2 and improve the recovery rate of magnesium ions.

[0067] (5) The magnesium chloride production systems 1, 1A, 1B, and 1C of the fifth aspect are the magnesium chloride production systems 1, 1A, 1B, and 1C of (4), in which the additional recycle line LA supplies the wastewater to an upstream position relative to the most upstream nanofiltration membrane that is located at the most upstream position among the multiple nanofiltration membranes.

[0068] According to this configuration, the additional recycle line LA supplies wastewater discharged from one nanofiltration membrane to a position upstream of the most upstream nanofiltration membrane that is located most upstream among the multiple nanofiltration membranes. The water to be treated W with the lowest ion ratio among the water to be treated W supplied to the multiple nanofiltration membranes is the water to be treated W supplied to the most upstream nanofiltration membrane. The water to be treated W with the lowest ion ratio among the water to be treated W supplied to the multiple nanofiltration membranes is the sulfate ion reduced water SW. Therefore, the loss at the fourth nanofiltration membrane 24 can be reduced by re-recovering it with the first nanofiltration membrane 21, thereby improving the recovery rate of magnesium ions.

[0069] (6) A magnesium chloride production system 1, 1A, 1B, 1C according to a sixth aspect is any one of the magnesium chloride production systems 1, 1A, 1B, 1C of (1) to (5), and further includes a sulfate ion reduction section 90, 90B, 90C that reduces the amount of sulfate ions in the waste liquid FW that returns to at least one of the second removal sections 20 through the recycle line LR.

[0070] According to this configuration, the amount of sulfate ions in the waste liquid FW is reduced by the sulfate ion reducing sections 90, 90B, and 90C. Therefore, the deposition of scale in the concentrating section 50 can be suppressed.

[0071] (7) The magnesium chloride production system 1, 1A, 1B, 1C of the seventh aspect is the magnesium chloride production system 1, 1A, 1B, 1C of (6), wherein the sulfate ion reduction unit 90 is arranged in the recycle line LR and has a discharge valve 91 that discharges the waste liquid FW flowing through the recycle line LR to the outside, and an ion concentration detection unit 92 that acquires information on the sulfate ion concentration in the treated water W discharged from the second removal unit 20 and before being supplied to the concentration unit 50, and the discharge valve 91 is opened to discharge the waste liquid FW to the outside when the sulfate ion concentration in the treated water W acquired by the ion concentration detection unit 92 meets a predetermined standard.

[0072] According to this configuration, the sulfate ion concentration can be appropriately grasped and effectively reduced only when the sulfate ion concentration meets a predetermined standard, thereby suppressing wasteful discharge of the waste liquid FW and effectively suppressing precipitation of scale in the concentration unit 50.

[0073] (8) The magnesium chloride production system 1, 1A, 1B, 1C of the eighth aspect is the magnesium chloride production system 1, 1A, 1B, 1C of (6) or (7), wherein the sulfate ion reduction section 90B has a barium addition section 93 that adds barium chloride to the treated water W discharged from the second removal section 20 and before being supplied to the concentration section 50, and a barium removal section 94 that removes barium sulfate from the treated water W to which the barium chloride has been added.

[0074] According to this configuration, the barium addition unit 93 adds barium chloride to the water to be treated W, so that sulfate ions in the water to be treated W react with the barium to become barium sulfate. This makes it possible to reduce the sulfate ions in the water to be treated W. Furthermore, the barium removal unit 94 can remove the generated barium sulfate while it is being supplied to the concentrating unit 50. Therefore, it is possible to reduce the sulfate ion concentration while suppressing the impact on the concentrating unit 50. This makes it possible to suppress the concentration of sulfate ions and effectively suppress the precipitation of scale in the concentrating unit 50.

[0075] (9) A magnesium chloride manufacturing system 1, 1A, 1B, 1C according to a ninth aspect is any one of the magnesium chloride manufacturing systems 1, 1A, 1B, 1C of (6) to (8), in which the sulfate ion reduction section 90C is arranged in the recycle line LR and has a sulfate ion removal device 95 capable of selectively removing only the sulfate ions in the waste liquid FW.

[0076] According to this configuration, it is possible to reduce the sulfate ion concentration without using any chemicals and while suppressing the effect on the concentrating section 50. This makes it possible to suppress the concentration of sulfate ions and effectively suppress the deposition of scale in the concentrating section 50. [Explanation of symbols]

[0077] 1, 1A, 1B, 1C…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 50...Concentration section 70…Dilution section LR…Recycling line LA…Additional recycling line 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 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 FW: Waste liquid DW: Distilled water S: Slurry AW: Diluted water 90, 90B, 90C... Sulfate ion reduction section 91...Discharge valve 92...Ion concentration detector 93...Barium-added section 94…Barium removal section 95...Sulfate ion removal device

Claims

1. a first removal unit that reduces a sulfate ion concentration in the water to be treated, the water being made from seawater, by electrodialysis; At least one 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; a concentrating section for concentrating the water to be treated, which has been discharged from the second removal section and has a reduced sodium ion concentration, to produce a slurry in which magnesium chloride is crystallized; A magnesium chloride manufacturing system comprising: a recycle line located upstream of at least one of the second removal sections, for supplying waste liquid discharged from the concentration section.

2. The second removal unit is arranged in plurality and has a nanofiltration membrane that separates monovalent ions and polyvalent ions from the water to be treated in which the sulfate ion concentration has been reduced in the first removal unit, The magnesium chloride manufacturing system according to claim 1 , wherein the recycle line supplies the waste liquid to a position upstream of at least a most downstream nanofiltration membrane that is disposed most downstream among the plurality of nanofiltration membranes.

3. 3. The magnesium chloride manufacturing system according to claim 1 or 2, wherein the recycle line is connected so as to return the waste liquid to a position where the ion ratio, which is the ratio of magnesium ions to sodium ions in the circulating waste liquid, is lower.

4. The second removal unit is arranged in plurality and has a nanofiltration membrane that separates monovalent ions and polyvalent ions from the water to be treated in which the sulfate ion concentration has been reduced in the first removal unit, 3. The magnesium chloride production system according to claim 1 or 2, further comprising an additional recycle line that supplies wastewater discharged from one of the nanofiltration membranes of the plurality of nanofiltration membranes to an upstream position of an upstream nanofiltration membrane located upstream of the one of the plurality of nanofiltration membranes.

5. The magnesium chloride manufacturing system according to claim 4 , wherein the additional recycle line supplies the wastewater to a position upstream of a most upstream nanofiltration membrane that is located at the most upstream position among the plurality of nanofiltration membranes.

6. The magnesium chloride manufacturing system according to claim 1 or 2, further comprising a sulfate ion reduction section that reduces the amount of sulfate ions in the waste liquid that returns to at least one of the second removal sections through the recycle line.

7. The sulfate ion reduction unit is a discharge valve disposed in the recycle line and configured to discharge the waste liquid flowing through the recycle line to the outside; an ion concentration detection unit that acquires information on the sulfate ion concentration in the water to be treated that has been discharged from the second removal unit and is not yet supplied to the concentration unit; The magnesium chloride manufacturing system of claim 6, wherein the discharge valve is opened to discharge the waste liquid to the outside when the sulfate ion concentration in the treated water obtained by the ion concentration detection unit meets a predetermined standard.

8. The sulfate ion reduction unit includes a barium addition unit that adds barium chloride to the water to be treated that is discharged from the second removal unit and before being supplied to the concentration unit; The magnesium chloride manufacturing system according to claim 6 , further comprising: a barium removal section that removes barium sulfate from the water to be treated to which the barium chloride has been added.

9. 7. The magnesium chloride manufacturing system according to claim 6, wherein the sulfate ion reduction section is disposed in the recycle line and has a sulfate ion removal device capable of selectively removing only sulfate ions in the waste liquid.

Images