Electrodialysis device and electrodialysis process

Abstract

The present invention relates to an electrodialysis device and an electrodialysis process, wherein the electrodialysis device comprises: at least one pair of bipolar membranes disposed adjacent to each of an anode and a cathode; an acid chamber in which cation dialysis membranes and anion dialysis membranes are alternately arranged, and which is disposed between the bipolar membranes and the anion dialysis membranes and receives an acid solution from an acid tank; a salt solution chamber which is disposed between the anion dialysis membranes and the cation dialysis membranes and receives a salt solution from a salt tank; a base chamber which is disposed on the cation dialysis membranes and the bipolar membranes and receives a base chamber solution from a base tank; and a pump member disposed to be connected to the acid tank and the salt tank.

Description

Electrodialysis device and electrodialysis process

[0001] The present invention relates to the production of lithium hydroxide, and to an electrodialysis device and an electrodialysis process.

[0002] The recent rapid growth of the IT and electric vehicle markets has led to a significant increase in demand for lithium, a key raw material for secondary batteries. The lithium market for secondary batteries is heavily concentrated in Korea, China, and Japan. However, Korea relies entirely on imports, necessitating a stable supply and demand. Accordingly, research into lithium production is underway, focusing on the development of lithium extraction technologies from ore and brine. A demonstration plant for producing lithium carbonate and lithium hydroxide from ore is currently in operation in Korea. Several companies in South America and China are producing large quantities of lithium, and research into lithium production is also underway in Korea.

[0003] The existing lithium production process in Korea extracts lithium from ore in the form of a lithium sulfate solution and then uses electrodialysis to produce lithium hydroxide. This lithium hydroxide solution is then crystallized to produce lithium hydroxide monohydrate (LiOH-H2O). The electrodialysis process, a key step in converting lithium sulfate into lithium hydroxide, requires stable and efficient operation.

[0004] The technical problem to be solved by the present invention is to provide an electrodialysis device that maintains the pH of salt introduced for operation in a stable and efficient manner.

[0005] Another technical problem to be solved by the present invention is to provide an electrodialysis process that maintains the pH of salt introduced for operation in a stable and efficient manner.

[0006] According to one embodiment of the present invention, an electrodialysis device may include at least one pair of bipolar membranes arranged adjacent to an anode and a cathode, respectively, and an acid chamber in which cation dialysis membranes and anion dialysis membranes are alternately arranged, an acid tank arranged between the bipolar membranes and the anion dialysis membrane and receiving an acid reaction solution from an acid tank, a salt solution chamber arranged between the anion dialysis membrane and the cation dialysis membrane and receiving a salt solution from a salt tank, a base chamber arranged between the cation dialysis membrane and the bipolar membrane and receiving a base reaction solution from a base tank, and a pump member connected to the acid tank and the salt tank.

[0007] In one embodiment, the pump member can supply an acid solution to the brine chamber within the acid tank. In one embodiment, the pump member can be arranged to connect a solution supply portion arranged at the upper portion of the acid tank and a solution inlet portion arranged at the upper portion of the brine tank.

[0008] In one embodiment, the pump member may include a driving unit that operates the pump member. In one embodiment, the driving unit may include a control unit that controls the operation of the pump member according to the pH of the salt solution chamber.

[0009] In one embodiment, the control unit may operate the pump member to supply an acid solution from the acid tank to the salt tank when the pH of the salt solution chamber is 3.0 or higher. In one embodiment, the control unit may control to stop the operation of the pump member to prevent the acid solution from being supplied from the acid tank to the salt solution chamber when the pH of the salt solution chamber is 1.5 or lower. In one embodiment, a plurality of unit pairs may be arranged in series.

[0010] In another embodiment of the present invention, an electrodialysis process comprises an electrodialysis apparatus including at least one pair of bipolar membranes disposed adjacent to each of an anode and a cathode, and cation dialysis membranes and anion dialysis membranes alternately disposed, an acid room disposed between the bipolar membranes and the anion dialysis membrane, a salt room disposed between the anion dialysis membrane and the cation dialysis membrane, and a base room disposed between the cation dialysis membrane and the bipolar membrane, wherein the electrodialysis process may include a step of supplying an acid solution of the acid room to the salt room.

[0011] In one embodiment, the step of supplying the acid solution of the acid chamber to the brine chamber may be supplied from an acid tank that supplies the acid chamber reaction solution to the brine chamber. In one embodiment, the step of supplying the acid solution of the acid chamber to the brine chamber may be such that when the pH of the brine chamber is 3 or higher, the acid solution of the acid chamber may be supplied to the brine chamber.

[0012] In one embodiment, the step of supplying the acidic solution of the delivery room to the brine chamber may not supply the acidic solution to the brine chamber if the pH of the brine chamber is 1.5 or lower. In one embodiment, in the step of supplying the acidic solution of the delivery room to the brine chamber, the acidic solution may be an acidic solution circulated in the delivery room.

[0013] According to one embodiment of the present invention, an electrodialysis device includes a pump member arranged to connect a delivery room and a salt solution room, thereby efficiently maintaining the pH of salt introduced for operation in a stable and efficient manner.

[0014] According to another embodiment of the present invention, the electrodialysis process includes a step of supplying an acidic solution from a salt chamber to a salt solution chamber, thereby efficiently maintaining the pH of the salt introduced for operation in a stable and efficient manner.

[0015] Figure 1 illustrates an electrodialysis device according to one embodiment of the present invention.

[0016] FIG. 2 is a simplified drawing of the interior of a stack according to one embodiment of the present invention.

[0017] Figure 3 shows the change in pH and current of salt as the acid dosing pump operates.

[0018] Figure 4 shows the change in pH over time in the salt tank of the demonstration plant.

[0019] Figures 5 and 6 show the change in pH over time after the acid dosing pump is installed.

[0020] The terms first, second, and third, etc., are used to describe, but are not limited to, various parts, components, regions, layers, and / or sections. These terms are used only to distinguish one part, component, region, layer, or section from another part, component, region, layer, or section. Accordingly, a first part, component, region, layer, or section described below may be referred to as a second part, component, region, layer, or section without departing from the scope of the present invention.

[0021] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the present invention. As used herein, the singular forms "singular" and "comprising" include plural forms as well, unless the context clearly dictates otherwise. The word "comprising" as used herein specifies a particular feature, region, integer, step, operation, element, and / or component, but does not exclude the presence or addition of other features, regions, integers, steps, operations, elements, and / or components.

[0022] When a part is referred to as being "on" or "over" another part, it can be directly on or over the other part, or there may be other parts intervening. Conversely, when a part is referred to as being "directly on" another part, there are no other parts intervening.

[0023] Additionally, % in this specification means weight % unless otherwise specified.

[0024] Although not defined otherwise, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention pertains. Terms defined in commonly used dictionaries are further interpreted to have meanings consistent with the relevant technical literature and the present disclosure, and are not to be construed as ideal or overly formal unless otherwise defined.

[0025] Hereinafter, embodiments of the present invention will be described in detail. However, these are presented as examples and the present invention is not limited thereto, and the present invention is defined solely by the scope of the claims set forth below.

[0026] According to one embodiment of the present invention, an electrodialysis device may include a unit pair that is repeatedly stacked and positioned between an anode and a cathode connected to a rectifier. The unit pair may have a configuration in which a bipolar membrane, an anion dialysis membrane, and a cation dialysis membrane are sequentially positioned. More specifically, the unit pair may have a configuration in which a bipolar membrane, a spacer gasket, an anion dialysis membrane, a spacer gasket, a cation dialysis membrane, and a spacer gasket are sequentially positioned.

[0027] The unit pairs of the above configuration may be sequentially adjacently positioned. Among two adjacent unit pairs, the spacer gasket positioned at the rear end of the cation dialysis membrane of the unit pair positioned at the front may be positioned adjacent to the biopolar membrane of the unit pair positioned adjacent to the rear end.

[0028] In one embodiment, the electrodialysis device may include at least one pair of bipolar membranes arranged adjacent to each of an anode and a cathode, and an acid chamber in which cation dialysis membranes and anion dialysis membranes are alternately arranged, an acid chamber arranged between the bipolar membranes and the anion dialysis membrane and receiving an acid reaction solution from an acid tank, a salt solution chamber arranged between the anion dialysis membrane and the cation dialysis membrane and receiving a salt solution from a salt tank, and a base chamber arranged between the cation dialysis membrane and the bipolar membrane and receiving a base reaction solution from a base tank.

[0029] Specifically, a lithium salt solution is injected into the salt chamber, an acid solution is injected into the acid chamber, and a base solution is injected into the base chamber to perform bipolar electrodialysis.

[0030] At this time, when voltage is applied to the bipolar electrodialysis device to cause current to flow, an acid solution is formed in the acid chamber, a lithium hydroxide aqueous solution is formed in the base chamber, and a deionized water containing residual lithium salt that did not move through the membrane from the introduced lithium salt aqueous solution can be formed in the salt solution chamber.

[0031] Specifically, the formation process of the acid solution and lithium hydroxide aqueous solution is as follows.

[0032] First, the acid reaction solution introduced into the acid chamber is hydrolyzed on the surface of the bipolar membrane of the acid chamber and decomposed into hydrogen ions and hydroxide ions. At this time, the decomposed hydroxide ions move to the anode cell, and the decomposed hydrogen ions move between the bipolar membrane and the anion dialysis membrane. Meanwhile, the anions of the lithium salt aqueous solution introduced between the anion dialysis membrane and the cation dialysis membrane pass through the anion dialysis membrane and move between the bipolar membrane and the anion dialysis membrane. Then, the hydrogen ions and the anions are concentrated between the first bipolar membrane and the anion dialysis membrane to form an acid solution.

[0033] Thereafter, the process by which lithium hydroxide solution is formed is as follows. The base reaction solution introduced into the base chamber is hydrolyzed in the water-splitting catalyst layer of the bipolar membrane of the base chamber, and is decomposed into hydrogen ions and hydroxide ions. At this time, the decomposed hydrogen ions move to the cathode cell, and the decomposed hydroxide ions move between the cation dialysis membrane and the bipolar membrane. Meanwhile, the lithium ions of the lithium salt solution introduced between the anion dialysis membrane and the cation dialysis membrane pass through the cation dialysis membrane and move to the base chamber into which it is introduced. Then, the hydroxide ions and the lithium ions are concentrated in the base chamber to form a lithium hydroxide solution.

[0034] More specifically, the acid group meets the hydrogen hydrolyzed in the bipolar membrane on the positive cell side and is converted into an acid, and the lithium ions moving to the negative electrode through the cation dialysis membrane are converted into hydroxyl groups (OH) generated in the bipolar membrane. -) and is converted into a lithium hydroxide (LiOH) solution.

[0035] According to one embodiment, the overall reaction formula may be as follows.

[0036] Li2SO4(aq) ⇔ 2Li + (base chamber) + SO4 2- (acid chamber)

[0037] H2O ⇔ H + (acid chamber) + OH - (base chamber)

[0038] Li2SO4(aq)+ 2H2O ⇔ 2LiOH(aq, base chamber) + H2SO4(aq, acid chamber)

[0039] At this time, the high-concentration lithium salt aqueous solution injected into the base chamber is decomposed into lithium ions and acid groups and escapes, and a low-concentration lithium salt aqueous solution with some remaining is generated and can be discharged outside the electrodialysis device.

[0040] In one embodiment, the lithium salt may be, for example, sulfate (Li2SO4), and the acid may be sulfuric acid (H2SO4). Meanwhile, the acid discharge solution may be a low-concentration acid or water, such as deionized water, and the base discharge solution may be a low-concentration lithium hydroxide aqueous solution or water, such as deionized water.

[0041] In one embodiment, the lithium salt solution formed in the base chamber may be injected into the unit pair positioned adjacent to the rear end, thereby performing the same electrodialysis as the previous unit pair. In this way, the electrodialysis device according to one embodiment may be operated by sequentially arranging a plurality of unit pairs.

[0042] In one embodiment, the lithium hydroxide-containing base solution formed and discharged in the base chamber of the electrodialysis device flows into the base tank, the acid solution formed and discharged in the acid chamber flows into the acid tank, and the deionized water formed and discharged in the salt solution chamber flows into the salt tank. In this way, the acid tank, the salt tank, and the base tank may be components through which the solutions introduced and discharged into and from the acid chamber, the salt solution chamber, and the base chamber, respectively, and the discharged solutions are circulated.

[0043] In one embodiment, partitions are installed within the acid tank, the salt solution tank, and the base tank so that a portion of the solutions flowing into each tank can be discharged to the outside, and the remainder can be mixed with water or a salt solution, such as deionized water, that is additionally introduced separately and circulated to the electrodialysis device.

[0044] In one embodiment, the electrodialysis device may include a pump member connected to an acid tank and a base tank. The pump member may be a member that supplies an acid solution from a delivery chamber to the salt solution chamber. Specifically, the pump member supplies an acid solution provided from the delivery chamber to the salt solution tank, and the acid solution may be supplied from the salt solution tank to the salt solution chamber.

[0045] Specifically, the salt solution passes through the cation membrane and the anion membrane, respectively. + ions and SO4 2- Ions pass through. In the bipolar membrane, water is decomposed into H + Wow OH - They become ions and move to the anode and cathode respectively. At this time, the pH of the salt solution injected is changed by the H that diffuses into the acid and base chambers respectively. + Wow OH - The amount of ions can be maintained in balance.

[0046] At this time, the direction of cation or anion movement may vary depending on the state of the membranes. In one embodiment, if a pinhole is formed in the membrane, a physical flow of solution occurs through the pinhole, resulting in a problem in which the pH of the saline solution chamber cannot be maintained at a constant level.

[0047] According to one embodiment of the present invention, the pH of the salt solution can be maintained at 2 to 3.0, specifically, 2 to 2.5. Specifically, by maintaining the pH of the salt solution at 2 to 2.5, the current applied to the membrane can be prevented from changing when there is a change in pH, thereby stabilizing the operation of the process, preventing the performance of the ion exchange membrane from deteriorating, and providing the advantage of stable operation in the upstream process when the desalination solution is recycled.

[0048] In one embodiment, the pH of the salt solution can be maintained by the pump member. Specifically, the pump member can control the pH of the salt solution within the salt solution chamber to be maintained uniformly by supplying an acid solution within the acid tank to the salt solution chamber.

[0049] In one embodiment, the pump member may be arranged to connect a solution supply portion disposed at the upper portion of the acid tank and a solution inlet portion disposed at the upper portion of the salt solution tank. By having the pump member disposed at the upper portions of the acid tank and the salt solution tank, there is an advantage in that the precise amount of acid solution can be supplied to the salt solution tank.

[0050] In one embodiment, the electrodialysis device may include a drive unit that operates the pump member. The drive unit is electrically connected to the pump member and can control the operation of the pump member. For example, if an opening / closing structure is arranged within the pump member, the drive unit may assist in opening and closing the opening / closing structure.

[0051] In one embodiment, the driving unit may include a control unit that controls the operation of the pump unit according to the pH of the salt solution chamber. Specifically, the pump unit is a unit for maintaining the pH of the salt solution chamber at 2 to 3, and when the pH of the salt solution chamber is 3 or higher, the pump unit may be controlled to supply an acidic solution from the acid tank to lower the pH of the salt solution in the salt solution chamber. Specifically, when the pH of the salt solution chamber is 3 or higher, the pump unit may transfer the acidic solution of the acid tank to the salt solution tank so that the pH of the salt solution chamber is controlled to be lower than 3.

[0052] In one embodiment, the control unit can control the operation of the pump member to stop when the pH of the salt solution chamber is 1.5 or lower, thereby preventing the supply of acidic solution from the acid tank to the salt solution chamber. In this way, the control unit can continuously control the pH of the salt solution chamber to be maintained within a predetermined range, for example, within a range of 2 to 3.

[0053] In one embodiment, the pump member may further include a sensor unit for measuring the pH of the salt solution near the salt solution tank. The sensor unit may be disposed near the salt solution tank of the pump member, more specifically near the solution inlet, to measure the pH of the salt solution circulating in the salt solution tank. Specifically, the sensor unit may measure the pH of the salt solution, and if the pH of the salt solution is 3 or higher, the pump member may be started, and if the pH of the salt solution is 2 or lower, the pump member may be stopped.

[0054] In another embodiment of the present invention, a bipolar electrodialysis process is a bipolar electrodialysis process for separating lithium ions by electrodialyzing a lithium ion-containing solution in an electrodialysis apparatus including at least one pair of bipolar membranes arranged adjacent to each of an anode and a cathode, and anion dialysis membranes and cation dialysis membranes alternately arranged, an acid room arranged between the bipolar membranes and the anion dialysis membrane, a salt room arranged between the anion dialysis membrane and the cation dialysis membrane, and a base room arranged between the cation dialysis membrane and the bipolar membrane, the bipolar electrodialysis process may include a step of supplying an acid solution of the acid room to the salt room.

[0055] In one embodiment, the step of supplying the acid solution of the acid chamber to the brine chamber may be performed by supplying the acid solution from an acid tank that supplies the acid chamber reaction solution to the brine chamber. Specifically, the acid solution of the acid chamber may be supplied from the acid tank to the brine tank, and the acid solution may be supplied from the brine tank to the brine chamber.

[0056] In one embodiment, the step of supplying the acidic solution of the delivery room to the brine chamber may include supplying the acidic solution of the delivery room to the brine chamber when the pH of the brine chamber is 3 or higher. In one embodiment, the step of supplying the acidic solution of the delivery room to the brine chamber may include not supplying the acidic solution to the brine chamber when the pH of the brine chamber is 1.5 or lower. In this way, when the pH of the brine chamber increases, the acidic solution may be uniformly maintained within a predetermined range in order to control the concentration of the brine chamber.

[0057] In one embodiment, the acid solution may be an acid solution circulated in the acid chamber. Specifically, the acid solution may be supplied to the brine chamber by a pumping member connected to the acid tank and the brine tank, and the description thereof is the same as that described above, to the extent that it does not contradict the description.

[0058]

[0059] To further illustrate the present invention, examples of the present invention are described. The following examples are merely exemplary of the present invention, and the present invention is not limited to the examples described below.

[0060]

[0061] <Experimental Example>

[0062] Figure 1 illustrates an electrodialysis device according to one embodiment of the present invention.

[0063] Referring to Fig. 1, the electrodialysis device of the present invention supplies a salt solution to a pump member (salt tank) connected to press 1. DI water is supplied countercurrently to the base tank and acid tank from press 3. The supplied solution circulates to the upper stack, and ions move through an ion exchange membrane within the stack.

[0064] FIG. 2 is a simplified drawing of the interior of a stack according to one embodiment of the present invention.

[0065] Referring to Figure 2, a salt solution enters the stack and passes through the cation membrane and the anion membrane to form Li + ions and SO4 2- Ions pass through. In the bipolar membrane, water is decomposed, and H+ and OH- ions move to the anode and cathode, respectively. LiOH is produced in the base chamber, and a sulfuric acid solution is produced in the acid chamber.

[0066] At this time, the pH of the salt introduced into the electrodialysis process in the conventional lithium process is adjusted by controlling the concentration of base and acid to diffusion H + Wow OH - It is controlled by balancing the amount of DIW (Base DI Water) and DIW (Acid DI Water) input to control the concentration of base and acid within the process. At this time, when the process is normal, pH control is easy, but when pinholes occur in the cation membrane or holes occur due to burning, control is impossible.

[0067] In contrast, the present invention supplies an acid solution to the salt solution room by connecting a delivery room and a salt solution room and disposing a pump member.

[0068] Figure 3 shows the change in pH and current of salt as the acid dosing pump operates.

[0069] Referring to FIG. 3, it can be seen that as the acid dosing pump, specifically, the pump member of the present invention, operates, the current decreases over time and the pH of the salt increases over time.

[0070] Figure 4 shows the change in pH over time in the salt tank of the demonstration plant.

[0071] Figure 4 illustrates the pH change over time of an electrodialysis device that does not include the pump member of the present invention. Specifically, Figure 4 shows PDS data for the pH of a salt tank of a conventional demo plant. More specifically, the data in Figure 4 represents data for 40 days, with values ​​automatically measured every minute.

[0072] When operating a conventional demonstration plant, specifically when Astom's anion membrane was applied, the pH was maintained at a constant level of 1.5. However, from approximately 25,000 minutes onward, the pH increase became uncontrollable. This indicates that, due to the long-term operation of the electrodialysis device, certain problems, such as pinholes or holes caused by burning, occurred in the cation membrane, making pH control difficult.

[0073] Figures 5 and 6 show the pH change over time of an electrodialysis device with a pump member installed.

[0074] Figure 5 shows the change in salt pH over time in an electrodialysis device of the present invention equipped with a pump member. Referring to Figure 5, it can be confirmed that, unlike the conventional demonstration plant of Figure 4, the pH is maintained at approximately 1.5.

[0075] Figure 6 shows the y-axis scale of Figure 5 adjusted, specifically, when the pump element starts operating at pH 1.6 and stops operating at pH 1.5, a sawtooth-shaped graph was confirmed between pH 1.5 and 1.6 when the process was operated after setting. Through this, it was confirmed that there is an advantage in that automation of the process for maintaining salt pH is also possible.

[0076] In this way, the electrodialysis device of the present invention includes a pump member that supplies an acidic solution from a delivery chamber to a salt solution chamber, and since the pH of the salt solution chamber is controlled by the pump member, the pH of the salt can be maintained at a constant level without the addition of separate raw materials or additional processes. In addition, it has the advantage of being able to maintain the pH of the salt uniformly even when the electrodialysis process is performed for a long time and cracks occur in the cation membrane.

[0077]

[0078] The present invention is not limited to the above-described embodiments, but can be manufactured in a variety of different forms. Those skilled in the art will appreciate that the present invention can be implemented in other specific forms without altering the technical spirit or essential characteristics of the present invention. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive.

Claims

1. At least one pair of bipolar membranes arranged adjacent to each of the anode and cathode, and cation dialysis membranes and anion dialysis membranes are arranged alternately, An acid chamber positioned between the bipolar membrane and the anion dialysis membrane and supplied with acid solution from an acid tank; A salt solution chamber positioned between the anion dialysis membrane and the cation dialysis membrane and supplied with a salt solution from a salt tank; A base chamber arranged on the above cation dialysis membrane and the above bipolar membrane and supplied with a base chamber solution from a base tank; and An electrodialysis device comprising a pump member connected to the acid tank and the salt tank.

2. In paragraph 1, The above pump member is an electrodialysis device that supplies an acid solution in the acid tank to the salt solution room.

3. In paragraph 1, An electrodialysis device in which the pump member is arranged to connect a solution supply portion arranged at the upper part of the acid tank and a solution injection portion arranged at the upper part of the salt solution tank.

4. In paragraph 1, An electric dialysis device including a driving unit that operates the above pump member.

5. In paragraph 4, An electrodialysis device, wherein the driving unit includes a control unit that controls the operation of the pump unit according to the pH of the saline solution chamber.

6. In paragraph 5, The above control unit, when the pH of the dye chamber is 3.0 or higher, An electrodialysis device that supplies an acid solution from the acid tank to the salt tank by driving the pump member.

7. In paragraph 6, The above control unit, when the pH of the dye chamber is 1.5 or lower, An electrodialysis device that controls the operation of the above pump member to prevent the supply of acid solution from the acid tank to the salt solution room.

8. In paragraph 1, An electric catapult device in which multiple unit pairs are arranged in series.

9. A bipolar electrodialysis process for separating lithium ions by electrodialyzing a lithium ion-containing solution in an electrodialysis device including at least one pair of bipolar membranes arranged adjacent to each of an anode and a cathode, and cation dialysis membranes and anion dialysis membranes alternately arranged, an acid room arranged between the bipolar membranes and the anion dialysis membrane, a salt room arranged between the anion dialysis membrane and the cation dialysis membrane, and a base room arranged between the cation dialysis membrane and the bipolar membrane, wherein: An electrodialysis process comprising a step of supplying an acidic solution from the above acid chamber to the above salt chamber.

10. In paragraph 8, The step of supplying the acid solution of the above acid chamber to the above brine chamber is an electrodialysis process in which the acid solution is supplied from an acid tank that supplies the acid chamber reaction solution to the above acid chamber.

11. In paragraph 8, The step of supplying the acidic solution of the above-mentioned acidic chamber to the above-mentioned salt solution chamber is an electrodialysis process in which the acidic solution of the above-mentioned acidic chamber is supplied to the above-mentioned salt solution chamber when the pH of the above-mentioned salt solution chamber is 3 or higher.

12. In paragraph 8, The step of supplying the acidic solution of the above-mentioned acid room to the above-mentioned salt solution room is an electrodialysis process in which the acidic solution is not supplied to the above-mentioned salt solution room when the pH of the above-mentioned salt solution room is 1.5 or lower.

13. In paragraph 8, In the step of supplying the acid solution of the above acid room to the above salt room, The above acidic solution is an electrodialysis process in which the acidic solution is circulated in the acid chamber.

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