Electrodialysis system and method for cleaning electrodialysis device

Abstract

An electrodialysis system 1A comprises: an electrodialysis device 50 that, when being energized, generates an alkali liquid and an acidic liquid from a treatment target liquid; and a cleaning mechanism 20 that cleans an alkali chamber 7 of the electrodialysis device 50 for generating the alkali liquid. After the electrodialysis device 50 has been energized until the pH of a treated liquid in the alkali chamber 7 becomes 7 or higher, the energizing operation is stopped, and the cleaning mechanism 20 performs cleaning by passing water through the alkali chamber 7.

Description

Electrodialysis system and method for cleaning electrodialysis equipment

[0001] The present invention relates to an electrodialysis system and a method for cleaning an electrodialysis device.

[0002] Electrodialysis devices are used to recover acid from acid-containing waste liquids (e.g., acid regeneration waste liquids from ion exchange resin towers). Patent Document 1 describes an example of this type of electrodialysis device, which produces an acid solution and an alkaline solution from a salt solution by electrodialysis. In this electrodialysis device, the alkaline chamber is washed with an acid solution having a pH of 3.5 or less to prevent clogging due to scales of polyvalent cations such as calcium.

[0003] Patent Document 2 describes a recovery device that recovers acid from acid regeneration wastewater of an acidic ion exchanger using an electrodialysis device. In this recovery device, in order to suppress scale formation, the voltage and / or current of the electrodialysis device is controlled so that the pH of the acid regeneration wastewater treated by the electrodialysis device is 7 or less.

[0004] Japanese Patent No. 7356200 Japanese Patent Application Laid-Open No. 2017-217596

[0005] However, in the electrodialysis apparatus described in Patent Document 1, acid is used to wash the alkaline chamber. Therefore, from the viewpoint of acid recovery, there is a problem that the acid recovery efficiency decreases by the amount of acid used to wash the alkaline chamber. In the recovery apparatus described in Patent Document 2, electrodialysis is stopped at a pH of about 7, so there is a problem that the acid component in the waste liquid cannot be sufficiently recovered.

[0006] An object of the present invention is to provide an electrodialysis system and a method for cleaning an electrodialysis apparatus that can prevent clogging of the alkaline chamber with scale while suppressing a decrease in acid recovery efficiency and without impairing the recovery of acid components.

[0007] To achieve the above object, one aspect of the present invention provides an electrodialysis system comprising an electrodialysis device that generates an acid solution and an alkaline solution from a liquid to be treated by passing current through it, and a cleaning mechanism that cleans an alkaline chamber of the electrodialysis device that produces the alkaline solution, wherein current is passed through the electrodialysis device until the pH of the treated liquid in the alkaline chamber reaches 7 or higher, and then the current application operation is stopped, and the cleaning mechanism passes water through the alkaline chamber to clean it.

[0008] Another aspect of the present invention provides a method for cleaning an electrodialysis device that produces an acid solution and an alkaline solution from a liquid to be treated by applying current to the electrodialysis device, characterized in that the electrodialysis device is energized until the pH of the alkaline solution in the alkaline chamber producing the alkaline solution reaches 7 or higher, and then the energization operation is stopped and the alkaline chamber is washed with water.

[0009] According to the present invention, it is possible to suppress a decrease in the efficiency of acid recovery, and to prevent clogging of the alkaline chamber with scale without impairing the recovery of acid components in the waste liquid.

[0010] 1 is a schematic diagram showing the configuration of an electrodialysis system according to a first embodiment of the present invention; FIG. 2 is a schematic diagram showing the configuration of an electrodialysis system according to a second embodiment of the present invention; FIG. 3 is a schematic diagram showing the configuration of a primary pure water production apparatus to which the electrodialysis system shown in FIG. 1 is applied; and FIG. 4 is a schematic diagram showing the configuration of a primary pure water production apparatus to which the electrodialysis system shown in FIG.

[0011] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the components described in the embodiments are merely examples and are not intended to limit the scope of the present invention.

[0012] (First embodiment) Figure 1 is a schematic diagram showing the configuration of an electrodialysis system according to a first embodiment of the present invention. In Figure 1, solid and dashed arrows indicate pipes (or flow paths), and solid open arrows indicate signal lines for transmitting control signals and the like. The intersection of two pipes indicates that they are not connected and intersect while being separated from each other. Dotted arrow A indicates the direction (or path) of flow of wash water.

[0013] Referring to FIG. 1, the electrodialysis system 1A of this embodiment includes an electrodialysis device 50 that produces an acid solution and an alkaline solution from a liquid to be treated by electrodialysis, a power supply device 11 that supplies electricity to the electrodialysis device 50, and a cleaning mechanism 20 that cleans the alkaline chamber 7 in which the alkaline solution is produced in the electrodialysis device 50. The power supply device 11 supplies electricity to the electrodialysis device 50 until the pH of the liquid to be treated in the alkaline chamber 7 reaches 7 or higher, and then stops the power supply operation, and the cleaning mechanism 20 passes water through the alkaline chamber 7 to clean it. The "water" passed through the alkaline chamber 7 is preferably clear water, particularly water with a low bicarbonate ion concentration, in order to clean hardness scale. The bicarbonate ion concentration is 50 mg-CaCO . ３ / L or less is preferable, and reverse osmosis (RO) permeate water or pure water is more preferable. The energization operation and cleaning operation are controlled by the control device 10, but are not limited to this. For example, the energization and cleaning operations may be started and stopped manually.

[0014] The configuration of the electrodialysis system 1A will be described in detail below. The electrodialysis device 50 has an anode 3a and a cathode 3b, which are energized by a power supply 11. Ion exchange partition walls 4, one surface of which has an anion exchange function and the other surface of which has a cation exchange function, and anion exchange membranes 5 are alternately arranged between the anode 3a and the cathode 3b. A plurality of chambers are provided, each partitioned by the ion exchange partition walls 4 and the anion exchange membranes 5.

[0015] When the alkaline chamber 7 and the acid chamber 8 of the electrodialysis device 50 are not filled with an ion exchanger, the ion exchange partition wall 4 may have any configuration as long as it is capable of dissociating water. For example, the ion exchange partition wall 4 may be a bipolar membrane, a membrane in which a cation exchange membrane is superimposed with a separate anion exchange membrane, or a membrane in which a cation exchange membrane and a separate anion exchange membrane are superimposed with a polyvalent metal adsorbed on at least one of the membranes.

[0016] When the alkaline chamber 7 and the acid chamber 8 of the electrodialysis device 50 are filled with ion exchangers (in this case, the electrodialysis device is also called an electrodeionization device, or EDI (electrodeionization) device) the ion exchange partition 4 may have any configuration as long as water dissociation is possible by contact between the ion exchange partition alone or the ion exchange partition and the filled ion exchanger. Examples of the ion exchange partition 4 include a bipolar membrane, a membrane in which a cation exchange membrane is stacked with a separate anion exchange membrane, a membrane in which a cation exchange membrane and a separate anion exchange membrane are stacked with each other, with a polyvalent metal adsorbed on at least one of the membranes, a cation exchange membrane, an anion exchange membrane, a cation exchange membrane adsorbed with a polyvalent metal, and an anion exchange membrane adsorbed with a polyvalent metal. When a cation exchange membrane, an anion exchange membrane, a cation exchange membrane adsorbed with a polyvalent metal, or an anion exchange membrane adsorbed with a polyvalent metal is used, water dissociation can be achieved by filling the membrane with an ion exchanger having a counterpart ion exchange function. For example, water dissociation can be achieved by combining a cation exchange membrane with an anion exchanger.

[0017] The bipolar membrane may have a structure in which, for example, an anion exchange membrane and a cation exchange membrane are stacked together with an intermediate layer (catalyst) formed between the two exchange membranes. As the ion exchange membrane having a polyvalent metal adsorbed thereon, one having particles adsorbed thereon containing at least one of aluminum silicate, magnesium silicate, calcium silicate, calcium magnesium silicate, aluminosilicate, and silicate minerals can be used.

[0018] The multiple chambers provided between the anode 3a and the cathode 3b include an anode chamber 6a, a cathode chamber 6b, an alkaline chamber 7, and an acid chamber 8 for producing an acid solution. The anode chamber 6a is partitioned by the anode 3a and the ion exchange partition wall 4. The cathode chamber 6b is partitioned by the cathode 3b and the ion exchange partition wall 4. The acid chamber 8 is adjacent to the alkaline chamber 7 on the anode side, with an anion exchange membrane 5 sandwiched between them. Two pairs of an acid chamber 8 and an alkaline chamber 7 are disposed between the anode chamber 6a and the cathode chamber 6b, but this is not limiting. The number of pairs of an acid chamber 8 and an alkaline chamber 7 may be one or three or more.

[0019] The electrodialysis system 1A has a stock solution tank 12, a wastewater tank 13, and a recovered water tank 14. The stock solution tank 12 stores the stock solution to be treated. The stock solution tank 12 is connected to a pipe 12c for supplying the stock solution, and this pipe 12c is provided with a valve 2b. The supply of the stock solution to the stock solution tank 12 can be controlled by opening and closing the valve 2b. For example, acid regeneration wastewater from an ion exchange resin tower can be supplied from the pipe 12c to the stock solution tank 12 at a predetermined timing. Note that a pump may be used instead of the valve 2b, and the supply of the stock solution to the stock solution tank 12 may be controlled by turning the pump on and off.

[0020] The raw liquid tank 12 is connected to the alkaline chamber 7 via a pipe 12a, and the liquid to be treated stored in the raw liquid tank 12 can be supplied to the alkaline chamber 7. Although not shown, a pump is provided in the pipe 12a, and the treatment liquid treated in the alkaline chamber 7 is configured to circulate between the raw liquid tank 12 and the alkaline chamber 7. In this embodiment, an alkaline liquid of a predetermined concentration can be obtained by circulating the treatment liquid. Note that the treatment liquid refers to the water at the outlet of the alkaline chamber 7. Furthermore, the treatment liquid in the alkaline chamber 7 having a pH of 7 or higher is called an alkaline liquid.

[0021] The drainage tank 13 is used to discharge the alkaline solution produced in the alkaline chamber 7 and the waste cleaning solution used to clean the alkaline chamber 7. A drainage pipe 12b is connected to the outlet side of the alkaline chamber 7 in the pipe 12a. The pipe 12b communicates with the drainage tank 13, and the alkaline solution flowing through the pipe 12a can be supplied from the pipe 12b to the drainage tank 13. A valve 2c is provided in the pipe 12b, and by opening and closing this valve 2c, the alkaline solution can be supplied to the drainage tank 13 at a predetermined timing.

[0022] The recovery water tank 14 is used to recover the acid solution produced in the acid chamber 8. The recovery water tank 14 is connected to the acid chamber 8 via a pipe 14a. Although not shown, a pump is provided in the pipe 14a, so that the acid solution produced in the acid chamber 8 circulates between the recovery water tank 14 and the acid chamber 8. The recovery water tank 14 is also connected to a pipe 14b, and pure water (PW) can be supplied to the recovery water tank 14 via this pipe 14b. In this embodiment, an acid solution of a predetermined concentration can be obtained by performing electrodialysis while supplying and circulating pure water.

[0023] The cleaning mechanism 20 has a pipe 21 for supplying cleaning water. The pipe 21 is connected to a portion of the pipe 12a on the inlet side of the alkaline chamber 7. The cleaning water is supplied to the alkaline chamber 7 via the pipe 21 and a portion of the pipe 12a. The cleaning waste liquid used to clean the alkaline chamber 7 is supplied to the drainage tank 13 via the pipe 12b. The pipe 21 is provided with a valve 2a. The supply of cleaning water to the alkaline chamber 7 can be controlled using this valve 2a and a valve 2c provided on the pipe 12b. Although not shown, a pump is provided on the pipe 21, which makes it possible to supply cleaning water to the alkaline chamber 7 at a predetermined space velocity. The portion of the pipe 12a, the pipe 12b, and the valve 2c constitute part of the cleaning mechanism 20.

[0024] The electrodialysis device 50 may have a so-called EDI structure in which the alkaline chamber 7 and the acid chamber 8 are filled with an ion exchanger. In the EDI, the alkaline chamber 7 is preferably filled with an ion exchanger having at least an anion exchange function to transfer chloride (Cl), a type of electrolyte component. For example, the ion exchanger filled in the alkaline chamber 7 preferably includes an anion exchanger or an anion exchange resin. Meanwhile, the acid chamber 8 may be filled with any ion exchanger, such as a cation exchange resin or an anion exchange resin. The ion exchanger may be treated to adsorb a polyvalent metal.

[0025] An electrode solution flows through the anode chamber 6a and the cathode chamber 6b. The anode chamber 6a and the cathode chamber 6b are configured so that the electrode solution circulates. Depending on the electrode material, sodium hydroxide, saline solution, or the like is used as the electrode solution. In the case of EDI, pure water is preferably used as the electrode solution.

[0026] Next, the operation of the electrodialysis system 1A of this embodiment will be described in detail. Below, the operating steps for batch operation will be described as an example. In batch operation, a fixed amount of stock solution is received, circulation and electrodialysis are performed, and when the acid concentration reaches a predetermined value, the acid solution is discharged and new stock solution is received again.

[0027] The operating steps of the electrodialysis system 1A include a stock solution introduction step, a first circulation dialysis step, an intermediate cleaning step, a second circulation dialysis step, a discharge step, and a cleaning step. Each step is described in detail below. In the following description, it is assumed that the electrodialysis device 50 has an EDI structure in which the alkaline chamber 7 and the acid chamber 8 are each filled with an ion exchange resin.

[0028] (Stock solution introduction step) The control device 10 controls the valves 2b and 2d to be open and the valves 2a and 2c to be closed. A predetermined amount of stock solution is supplied from the pipe 12c to the stock solution tank 12, and a predetermined amount of pure water is supplied from the pipe 14b to the recovery water tank 14.

[0029] (First circulation dialysis step) The control device 10 closes all of the valves 2a to 2d and passes the liquid to be treated stored in the raw liquid tank 12 through the alkaline chamber 7. The control device 10 also causes the power supply device 11 to apply electricity to perform electrodialysis. Here, the liquid to be treated contains cations (H ＋ , Na ＋ , Ca ２＋ , Mg ２＋ etc.) and anions (Cl － , S.O. ４ ２－ etc.)

[0030] When the anode 3a and the cathode 3b are energized, electrodialysis begins. In the alkaline chamber 7, anions adsorbed on the anion exchange resin move to the anode 3a side, permeate the anion exchange membrane 5, and move to the acid chamber 8. Water dissociation occurs on the cathode side of the alkaline chamber 7 (ion exchange partition wall 4), and H ＋ moves through the ion exchange partition 4 to the acid chamber 8 adjacent to the cathode 3b side.

[0031] Pure water is supplied to the acid chamber 8 and circulated, and an acid solution is produced in the acid chamber 8 by electrodialysis. The acid solution produced in the acid chamber 8 is, for example, hydrochloric acid, sulfuric acid, etc. (sometimes both hydrochloric acid and sulfuric acid are mixed). Here, as the electrodialysis proceeds in the acid chamber 8, H ＋ and Cl － is supplied to generate hydrochloric acid, and when the concentration reaches a predetermined value (for example, 4%), the hydrochloric acid is recovered.

[0032] On the other hand, in the alkaline chamber 7, the acid in the circulating treatment liquid is gradually removed as the electrodialysis progresses, and the OH generated by water dissociation is gradually removed. － As a result, the pH of the treatment liquid in the alkaline chamber 7 increases and becomes alkaline. For example, the liquid to be treated (e.g., acid regeneration waste liquid) supplied to the alkaline chamber 7 contains cations (H ＋ , Na ＋ , Ca ２＋ , Mg ２＋ etc.) and anions (Cl － , S.O. ４ ２－ As the electrodialysis proceeds, the OH ions supplied from the ion exchange partition 4 are mixed in the alkaline chamber 7. － By H ＋ is neutralized, and then OH － As the pH of the treatment solution in the alkaline chamber 7 increases, the pH of the treatment solution in the alkaline chamber 7 increases. If the pH of the treatment solution in the alkaline chamber 7 exceeds 7 and reaches the alkaline region, there is a high risk of scale such as calcium hydroxide or magnesium hydroxide being generated.

[0033] Here, it is assumed that the pH of the treatment liquid in the alkaline chamber 7 reaches 7 or higher before the hydrochloric acid circulating between the recovered water tank 14 and the acid chamber 8 reaches a predetermined concentration.

[0034] (Intermediate Cleaning Step) The control device 10 stops the power supply from the power supply device 11 and controls the valves 2a and 2c to be open and the valves 2b and 2d to be closed. After the power supply is stopped, the cleaning mechanism 20 passes clear water through the alkaline chamber 7 to clean it. The cleaning waste liquid used to clean the alkaline chamber 7 is supplied to the drainage tank 13 via the pipe 12b. The cleaning waste liquid stored in the drainage tank 13 is drained to any location outside the system.

[0035] The space velocity of the water passing through the alkaline chamber 7 is 400 h -1 That's it, 1000h -1 The following is preferable: Within this range of space velocity, the alkaline chamber 7 can be washed at a flow rate that washes away scale and does not excessively increase the differential pressure between the inlet and outlet of the alkaline chamber 7.

[0036] If the cleaning time is too short, sufficient cleaning effect may not be obtained. On the other hand, if the cleaning time is extended, the electrodialysis time per batch operation period will be shortened accordingly, resulting in a decrease in the amount of acid recovered. In addition, if the cleaning time is extended, a large amount of water will be used for cleaning. In consideration of these points, the cleaning time is preferably in the range of 1 minute to 60 minutes, and more preferably in the range of 2 minutes to 10 minutes.

[0037] (Second circulating dialysis step) After the cleaning waste liquid is discharged, the control device 10 closes all of the valves 2a to 2d, passes the liquid to be treated stored in the raw liquid tank 12 through the alkaline chamber 7, and applies electricity using the power supply device 11. The operation of the alkaline chamber 7 and the acid chamber 8 during electrodialysis is as described in the first circulating dialysis step.

[0038] Here, it is assumed that the pH of the treatment liquid in the alkaline chamber 7 reaches 7 or more, and the hydrochloric acid circulating between the recovered water tank 14 and the acid chamber 8 reaches a predetermined concentration (for example, 4%).

[0039] (Discharge Process) When the hydrochloric acid reaches a predetermined concentration, the control device 10 stops the power supply 11 and controls the valve 2c to be open and the valves 2a, 2b, and 2d to be closed. The alkaline solution circulating between the stock solution tank 12 and the alkaline chamber 7 is supplied to the drainage tank 13 via the pipe 12b. The alkaline solution stored in the drainage tank 13 is then discharged to any location outside the system. The discharged alkaline solution can be reused as a neutralizing agent.

[0040] The hydrochloric acid circulating between the recovery water tank 14 and the acid chamber 8 is stored in the recovery water tank 14. The hydrochloric acid stored in the recovery water tank 14 is discharged to a recovery destination outside the system. The recovered hydrochloric acid can be reused, for example, as a regenerant for the ion exchange resin tower.

[0041] (Cleaning step) After recovering the hydrochloric acid, the control device 10 controls the valves 2a and 2c to be open and the valves 2b and 2d to be closed. Thereafter, the cleaning mechanism 20 passes clear water through the alkaline chamber 7 to clean it. The cleaning waste liquid used to clean the alkaline chamber 7 is supplied to the drainage tank 13 via the pipe 12b. The cleaning waste liquid stored in the drainage tank 13 is drained to any location outside the system. The space velocity and cleaning time are as described in the intermediate cleaning step.

[0042] The above-described operating process of the electrodialysis system 1A provides the following advantageous effects. By using the cleaning mechanism 20 to clean the alkaline chamber 7 of the electrodialysis device 50 with clear water, the risk of clogging due to scale can be reduced, enabling stable operation of the electrodialysis system 1A. As a result of examining the risk of clogging due to scale, it was found that cleaning the alkaline chamber 7 with water can significantly reduce the impact of clogging due to scale that would interfere with the electrodialysis treatment. This is presumably because, even if the pH of the treatment liquid reaches 7 or higher, alternating between electrodialysis treatment and water cleaning makes it easy to flush or dissolve the scale with water before it develops. Furthermore, particularly when the target is a liquid with a low bicarbonate ion concentration (50 mg-CaCO ), such as the acid regeneration wastewater of an ion exchange resin, the risk of clogging due to scale can be reduced. ３ / L or less), the main scale that forms is not carbonates such as calcium carbonate but hydroxide scale derived from hardness components, which are soft, so a more significant effect can be expected.

[0043] Furthermore, because the alkaline chamber 7 is washed with clear water, a decrease in the efficiency of acid recovery can be suppressed compared to washing with acid (Patent Document 1). Furthermore, because electrodialysis is performed until the pH of the treatment liquid in the alkaline chamber 7 reaches 7 or higher, the electrodialysis time can be extended compared to stopping electrodialysis at around pH 7 (Patent Document 2), and a larger amount of acid components in the waste liquid can be recovered.

[0044] Furthermore, batch operation allows the liquid to be treated (waste liquid) to be circulated and electrodialysis to be performed repeatedly, so the maximum current value applied at one time between the anode 3a and the cathode 3b can be kept low, allowing for the miniaturization of the device.

[0045] In the operation of the electrodialysis system 1A, the alkaline chamber 7 is cleaned twice, once in an intermediate cleaning step and once in a cleaning step, but this is not limited to this. The intermediate cleaning step may be omitted or may be performed two or more times as long as it is possible to prevent clogging of the alkaline chamber 7 with scale and suppress a decrease in acid recovery efficiency without impairing the recovery of acid components from the wastewater.

[0046] The power supply unit 11 may be energized at either a constant current or a constant voltage. A constant current allows for easy treatment of the wastewater to a predetermined concentration in one batch operation before discharging it. On the other hand, a constant voltage may prevent the wastewater from being treated to the predetermined concentration if the resistance (membrane resistance) between the anode 3a and the cathode 3b increases. For this reason, it is preferable to energize the power supply unit 11 at a constant current.

[0047] Pressure gauges may be installed at the inlet and outlet of the alkaline chamber 7, and the cleaning mechanism 20 may clean the alkaline chamber 7 when the differential pressure, which is the difference between the measured values ​​of these pressure gauges, exceeds a threshold. It has been found that recovery by cleaning is possible as long as the amount of scale generated does not cover the ion exchange membrane surface, significantly reducing the amount of current flow, or severely block the flow path of the alkaline chamber, causing an increase in the differential pressure, making treatment impossible. Specifically, cleaning using this method enabled stable operation when the amount of scale generated was such that the water flow differential pressure in the alkaline chamber was within a range of +100 kPa or less compared to the initial value. Furthermore, cleaning using this method may be performed when the current amount at a constant voltage is 50 to 80% of the initial value. Furthermore, the pH of the treatment solution in the alkaline chamber 7 can be measured, and the cleaning time can be adjusted based on the measured value to a range that does not cause blockage by scale.

[0048] While performing electrodialysis until the pH of the treatment solution in the alkaline chamber 7 becomes higher can increase the amount of acid recovered, it also increases the risk of clogging due to scale. As a result of investigating this, it was found that cleaning the alkaline chamber 7 when the pH of the treatment solution in the alkaline chamber 7 is in the range of 7 to 12 can suppress clogging due to scale while providing a larger amount of acid recovered. This is thought to be because the solubility of calcium hydroxide, a major scale component, drops sharply when the pH exceeds 12.

[0049] The cleaning mechanism 20 may be configured to clean the acid chamber 8 and the electrode chambers (6a, 6b) with clear water. Alternatively, the cleaning mechanism 20 may be configured to clean the alkaline chamber 7 with clear water and then clean the alkaline chamber 7 with acid. For example, if the acid regeneration wastewater from the ion exchange resin tower is stored in the raw liquid tank 12 as the raw liquid (liquid to be treated), a portion of the raw liquid may be used for acid cleaning. In this case, the amount of acid recovered will be reduced, but since most of the scale is drained with clear water before being cleaned with acid, the amount of acid recovered can be increased compared to the methods described in Patent Documents 1 and 2. The acid may be hydrochloric acid or sulfuric acid contained in the raw liquid, or hydrochloric acid or sulfuric acid may be added separately. The higher the concentration, the greater the cleaning effect, but the greater the amount of acid required. Therefore, sulfuric acid or hydrochloric acid adjusted to a pH of about 1 to 2 is more preferable.

[0050] The timing of collecting the acid solution may be determined using the pH, conductivity or current value of the acid solution, a timer, or the like.

[0051] In the electrodialysis system 1A of this embodiment, the electrodialysis device 50 has an EDI structure in which the alkaline chamber 7 and the acid chamber 8 are filled with ion exchange resin, but this is not limiting. The electrodialysis device 50 may also have an ED (electrodialysis) structure in which the alkaline chamber 7 and the acid chamber 8 are not filled with ion exchange resin. The ED structure can also perform the same operations as the EDI structure, and its effects are basically the same.

[0052] In the ED structure, scale is generated over the entire surface of the ion exchange membrane. In this case, the scale deposits thickly on the surface of the ion exchange membrane, which can increase the voltage applied between the anode 3a and the cathode 3b. In contrast, in the EDI structure, scale also deposits on the surface of the filled ion exchanger (resin), resulting in thin, widespread deposition of scale. This allows for a larger surface area for scale generation than the ED structure, suppressing the increase in voltage applied between the anode 3a and the cathode 3b, and also making it easier to peel or dissolve the scale by cleaning. From the perspective of suppressing the increase in voltage, the EDI structure is preferable.

[0053] Second Embodiment Figure 2 is a schematic diagram showing the configuration of an electrodialysis system according to a second embodiment of the present invention. In Figure 2, solid and dashed arrows indicate pipes (or flow paths), and solid open arrows indicate signal lines for transmitting control signals and the like. The intersection of two pipes indicates that they are not connected and intersect while being separated from each other. Dotted arrow A indicates the direction (or path) of flow of wash water.

[0054] Like the electrodialysis system 1A, the electrodialysis system 1B of this embodiment also includes an electrodialysis device 51 that produces an acid solution and an alkaline solution from the liquid to be treated by electrodialysis. In the electrodialysis system 1B, the power supply 11 energizes the alkaline chamber 7 of the electrodialysis device 51 until the pH of the liquid to be treated reaches 7 or higher. After the power supply is stopped, the cleaning mechanism 20 cleans the alkaline chamber 7 by passing water through it. However, the structure of the electrodialysis system 1B and the arrangement of the piping and valves are different from those of the electrodialysis system 1A.

[0055] The configuration of the electrodialysis system 1B is described in detail below. In the electrodialysis device 51, an ion exchange partition wall 4, an anion exchange membrane 5, and a cation exchange membrane 30 are disposed, in this order from the anode 3a side, between the anode 3a and the cathode 3b. The electrodialysis device 51 has a plurality of chambers partitioned by the ion exchange partition wall 4, the anion exchange membrane 5, and the cation exchange membrane 30. The plurality of chambers includes an alkaline chamber 7, an acid chamber 8, and a deionization chamber 9 to which the liquid to be treated is supplied.

[0056] The acid compartment 8 is disposed adjacent to the anode side of the deionization compartment 9, with the anion exchange membrane 5 sandwiched therebetween. The alkaline compartment 7 is disposed adjacent to the cathode side of the deionization compartment 9, with the cation exchange membrane 30 sandwiched therebetween. An ion exchange partition wall 4 is disposed on the anode side of the acid compartment 8 and the cathode side of the alkaline compartment 7, respectively. In this embodiment, two sets of an alkaline compartment 7, an acid compartment 8, and a deionization compartment 9 are disposed between the anode compartment 6a and the cathode compartment 6b, but this is not limited thereto. The number of sets of an alkaline compartment 7, an acid compartment 8, and a deionization compartment 9 may be one or three or more. The ion exchange partition wall 4, the anion exchange membrane 5, the alkaline compartment 7, and the acid compartment 8 are the same as those described in the first embodiment.

[0057] The raw liquid tank 12 communicates with a pipe 12c, which is provided with a valve 2b. The raw liquid tank 12 communicates with the desalting compartment 9 via a pipe 12a, so that the liquid to be treated stored in the raw liquid tank 12 can be supplied to the desalting compartment 9. Although not shown, a pump is provided in the pipe 12a, and the desalted water produced in the desalting compartment 9 is configured to circulate between the raw liquid tank 12 and the desalting compartment 9.

[0058] The alkaline chamber 7 is in communication with the wastewater tank 13 via a pipe 22. Although not shown, a pump is provided in the pipe 22, and the treated solution from the alkaline chamber 7 is configured to circulate between the wastewater tank 13 and the alkaline chamber 7. A valve 2c is provided at the inlet of the pipe 22 to the wastewater tank 13. By circulating the solution while undergoing electrodialysis, an alkaline solution of a predetermined concentration can be obtained.

[0059] The recovered water tank 14 is in communication with the acid chamber 8 via a pipe 14a. Although not shown, a pump is provided in the pipe 14a, and the acid solution produced in the acid chamber 8 is circulated between the recovered water tank 14 and the acid chamber 8. The recovered water tank 14 is also in communication with a pipe 14b, and pure water (PW) can be supplied to the recovered water tank 14 via this pipe 14b. The raw solution tank 12, wastewater tank 13, and recovered water tank 14 are the same as those described in the first embodiment. Pure water is supplied to the recovered water tank 14.

[0060] In this embodiment, the pipe 21, a portion of the pipe 22, and the valves 2a and 2c constitute the cleaning mechanism 20. The pipe 21 is connected to a portion of the pipe 22 on the inlet side of the alkaline chamber 7. Cleaning water is supplied to the alkaline chamber 7 via the pipe 21 and a portion of the pipe 22. The cleaning waste liquid used to clean the alkaline chamber 7 is supplied to the drainage tank 13 via a portion of the pipe 22. The supply of cleaning water to the alkaline chamber 7 can be controlled using the valves 2a and 2c. Cleaning water can be supplied to the alkaline chamber 7 at a predetermined space velocity using a pump (not shown).

[0061] The electrodialysis device 51 preferably has an EDI structure in which the alkaline chamber 7, acid chamber 8, and deionization chamber 9 are each filled with an ion exchanger. The deionization chamber 9 is required to transfer anions and cations. Therefore, the ion exchanger filled in the deionization chamber 9 preferably contains an anion exchanger or an anion exchange resin. For example, the deionization chamber 9 may be filled with an ion exchanger containing a mixed-bed resin of an anion resin and a cation resin. On the other hand, the alkaline chamber 7 and the acid chamber 8 can be filled with an ion exchanger containing any resin. In this embodiment, the alkaline chamber 7 is filled with an anion exchange resin, and the acid chamber 8 is filled with a cation resin.

[0062] Next, the operation of the electrodialysis system 1B of this embodiment will be specifically described. Hereinafter, the operation process for batch operation will be described as an example. The batch operation is the same as that described in the first embodiment.

[0063] The operating steps of the electrodialysis system 1B include a raw solution introduction step, a first circulation dialysis step, an intermediate cleaning step, a second circulation dialysis step, a discharge step, and a cleaning step. Each step will be described in detail below. In the following description, it is assumed that the electrodialysis device 51 has an EDI structure.

[0064] (Stock solution introduction step) The control device 10 controls the valves 2b and 2d to be open and the valves 2a, 2c, and 2e to be closed. A predetermined amount of stock solution is supplied from the pipe 12c to the stock solution tank 12, and a predetermined amount of pure water is supplied from the pipe 14b to the recovery water tank 14.

[0065] (First circulation dialysis step) The control device 10 closes all of the valves 2a to 2e and passes the liquid to be treated stored in the raw liquid tank 12 through the deionization chamber 9. The control device 10 also causes the power supply device 11 to apply electricity to perform electrodialysis. Here, the liquid to be treated contains cations (H ＋ , Na ＋ , Ca ２＋ , Mg ２＋ etc.) and anions (Cl － , S.O. ４ ２－ etc.)

[0066] Electrodialysis begins when a current is applied between the anode 3a and the cathode 3b. In the deionization compartment 9, anions move through the anion exchange membrane 5 to the acid compartment 8 adjacent to the anode side, and cations move through the cation exchange membrane 30 to the alkaline compartment 7 adjacent to the cathode side. The deionization compartment 9 produces deionized water (pure water).

[0067] An acid solution is produced in the acid chamber 8 by electrodialysis. The acid solution produced in the acid chamber 8 is, for example, hydrochloric acid, sulfuric acid, etc. (both hydrochloric acid and sulfuric acid may be mixed). Here, as the electrodialysis proceeds in the acid chamber 8, H ＋ and Cl － is supplied to generate hydrochloric acid, and when the concentration reaches a predetermined value (for example, 4%), the hydrochloric acid is recovered.

[0068] In the alkaline chamber 7, as the electrodialysis progresses, OH generated by water dissociation － As the electrodialysis progresses, the OH supplied from the ion exchange partition 4 increases. － By H ＋ is neutralized, and then OH － As the pH of the treatment solution in the alkaline chamber 7 increases, the pH of the treatment solution in the alkaline chamber 7 increases. If the pH of the treatment solution in the alkaline chamber 7 exceeds 7 and reaches the alkaline region, there is a high risk of scale such as calcium hydroxide or magnesium hydroxide being generated.

[0069] Here, it is assumed that the pH of the treatment liquid in the alkaline chamber 7 reaches 7 or higher before the hydrochloric acid circulating between the recovered water tank 14 and the acid chamber 8 reaches a predetermined concentration.

[0070] (Intermediate Cleaning Step) The control device 10 stops the power supply from the power supply device 11 and controls the valves 2a and 2c to be open and the valves 2b, 2d, and 2e to be closed. After the power supply is stopped, the cleaning mechanism 20 passes clear water through the alkaline chamber 7 to clean it. The cleaning waste liquid used to clean the alkaline chamber 7 is supplied to the drainage tank 13 via the pipe 22. The cleaning waste liquid stored in the drainage tank 13 is drained to any location outside the system.

[0071] In the electrodialysis system 1B of this embodiment, similarly to the electrodialysis system 1A, the space velocity of the water passing through the alkaline chamber 7 is 400 h-1 That's it, 1000h -1 The following is preferred: The cleaning time is preferably in the range of 1 to 60 minutes, more preferably in the range of 2 to 10 minutes.

[0072] (Second circulating dialysis step) After the cleaning waste liquid is discharged, the control device 10 closes all of the valves 2a to 2d, passes the liquid to be treated stored in the raw liquid tank 12 through the deionization chamber 9, and applies electricity using the power supply device 11. The operations of the alkaline chamber 7, the acid chamber 8, and the deionization chamber 9 during electrodialysis are as described in the first circulating dialysis step.

[0073] Here, it is assumed that the pH of the treatment liquid in the alkaline chamber 7 reaches 7 or more, and the hydrochloric acid circulating between the recovered water tank 14 and the acid chamber 8 reaches a predetermined concentration.

[0074] (Discharge Process) When the hydrochloric acid reaches a predetermined concentration, the control device 10 stops power supply from the power supply device 11 and controls the valve 2e to be open and the valves 2a to 2d to be closed. The hydrochloric acid circulating between the recovered water tank 14 and the acid chamber 8 is stored in the recovered water tank 14. The hydrochloric acid of the predetermined concentration stored in the recovered water tank 14 is discharged to a recovery destination outside the system. The recovered hydrochloric acid can be reused, for example, as a regenerant for the ion exchange resin tower.

[0075] The alkaline solution produced in the alkaline chamber 7 is stored in the drainage tank 13. The alkaline solution stored in the drainage tank 13 is discharged to any location outside the system. The discharged alkaline solution can be reused as a neutralizing agent.

[0076] (Cleaning step) After recovering the hydrochloric acid, the control device 10 controls the valves 2a and 2c to be open and the valves 2b, 2d, and 2e to be closed. Thereafter, the cleaning mechanism 20 passes clear water through the alkaline chamber 7 to clean it. The cleaning waste liquid used to clean the alkaline chamber 7 is supplied to the drainage tank 13 via the pipe 22. The cleaning waste liquid stored in the drainage tank 13 is drained to any location outside the system. The space velocity and cleaning time are as explained in the intermediate cleaning step.

[0077] The above-described operation steps of electrodialysis system 1B also achieve the same effects as those of electrodialysis system 1A. In electrodialysis system 1B, the intermediate cleaning step may be omitted or may be performed two or more times as long as it is possible to prevent clogging of alkaline chamber 7 with scale and suppress a decrease in the acid recovery efficiency and recovery amount without impairing the recovery of acid components in the wastewater.

[0078] Furthermore, the configurations and modifications described in the section on the effects of electrodialysis system 1A can be applied to electrodialysis system 1B as long as they do not impair operation. For example, in electrodialysis system 1B, either an EDI structure or an ED structure can be applied to the electrodialysis device 51. However, in the ED structure, as desalination progresses in the desalination compartment 9, the water approaches pure water, which may make it difficult for current to flow. In contrast, in the EDI structure, the desalination compartment 9 is filled with an ion exchanger (resin), so current flows through the ion exchanger (resin) even as desalination progresses and the water approaches pure water. From the perspective of electrodialysis stability, the EDI structure is preferable. Note that electrodialysis system 1B can be called a pure water production system because the desalination compartment 9 produces desalination water (pure water).

[0079] In the above-described electrodialysis systems 1A and 1B, the flow directions through the anode 3a and cathode 3b are not limited to those shown in the drawings. For example, in the electrodialysis system 1A shown in FIG. 1 , the flow direction of the treated liquid in the alkaline chamber 7 and the flow direction of the acid liquid in the acid chamber 8 during electrodialysis are the same, but this is not a limitation. The flow directions of the treated liquid (alkaline liquid) in the alkaline chamber 7 and the acid liquid in the acid chamber 8 during electrodialysis may be countercurrent. Furthermore, in the electrodialysis system 1B shown in FIG. 2 , the flow directions of the demineralized water in the demineralization chamber 9, the treated liquid in the alkaline chamber 7, and the acid liquid in the acid chamber 8 during electrodialysis are the same, but this is not a limitation. For example, the flow directions of the demineralized water in the demineralization chamber 9 and the treated liquid (alkaline liquid) in the alkaline chamber 7 during electrodialysis may be countercurrent. Similarly, the flow directions of the demineralized water in the demineralization chamber 9 and the acid liquid in the acid chamber 8 during electrodialysis may be countercurrent.

[0080] By using a countercurrent flow in electrodialysis systems 1A and 1B, for example, the residence time of highly charged components (hardness components) in the alkaline chamber 7 is shortened, thereby suppressing scale formation. For example, in electrodialysis system 1B, hardness components (divalent calcium ions and magnesium ions) move to the alkaline chamber 7 before monovalent ions. When the flow directions of the deionization chamber 9 and the alkaline chamber 7 are countercurrent, the hardness components move to the alkaline chamber 7 near the inlet side of the deionization chamber 9. Because the inlet side of the deionization chamber 9 is adjacent to the outlet side of the alkaline chamber 7, the hardness components are discharged from the alkaline chamber 7 immediately after moving to the alkaline chamber 7. This shortens the residence time of the hardness components in the alkaline chamber 7.

[0081] The electrodialysis systems 1A and 1B described above can be suitably applied to a system for recovering acid from the acid regeneration wastewater of an ion-exchange resin tower. A primary pure water production apparatus incorporating either the electrodialysis system 1A or 1B will be described below.

[0082] Figure 3 is a schematic diagram showing the configuration of a primary pure water production system to which an electrodialysis system 1A is applied. Referring to Figure 3, the primary pure water production system 100A includes an electrodialysis system 1A, a K tower 40, an A tower 41, a regenerated wastewater tank 42, and wastewater treatment equipment 43. The K tower 40 is an ion exchange resin tower packed with a cation exchange resin (e.g., a strongly acidic cation exchange resin). The A tower 41 is an ion exchange resin tower packed with an anion exchange resin (e.g., a strongly basic anion exchange resin). The electrodialysis system 1A has the structure shown in Figure 1.

[0083] Raw water such as industrial water or groundwater is subjected to pretreatment such as clarification as necessary, and then passed through K tower 40 and A tower 41. K tower 40 removes cation components, and A tower 41 removes anion components, thereby producing primary pure water.

[0084] The K tower 40 is regenerated with an acid such as hydrochloric acid. Here, hydrochloric acid with a concentration of about 4% is produced from hydrochloric acid with a concentration of, for example, 35%. The K tower 40 is regenerated using hydrochloric acid with a concentration of about 4% as a regenerant. The K tower 40 discharges acid regeneration waste liquid. Here, the acid regeneration waste liquid contains cations (H＋ , Na ＋ , Ca ２＋ , Mg ２＋ etc.) and anions (Cl － etc.)

[0085] The electrodialysis system 1A recovers acid from the acid regeneration waste liquid discharged from the K tower 40. In the electrodialysis system 1A, the acid regeneration waste liquid is supplied to the raw liquid tank 12. The acid liquid (here, hydrochloric acid with a concentration of about 4%) generated in the acid chamber 8 by electrodialysis is recovered and reused as a regenerant. As scale derived from hardness components is generated in the alkaline chamber 7 during electrodialysis, the cleaning mechanism 20 cleans the alkaline chamber 7 with clear water. The cleaning waste liquid used to clean the alkaline chamber 7 is discharged to the regeneration wastewater tank 42 via the same route as the alkaline liquid generated in the alkaline chamber 7.

[0086] Tower A 41 is regenerated with an alkali such as sodium hydroxide. Here, sodium hydroxide with a concentration of about 3% is produced from sodium hydroxide with a concentration of, for example, 25%. Tower A 41 is regenerated using sodium hydroxide with a concentration of about 3% as a regenerant. Tower A 41 discharges alkali regeneration waste liquid. The alkali regeneration waste liquid discharged from Tower A 41 is supplied to regeneration wastewater tank 42.

[0087] In the regeneration wastewater tank 42 , the washing wastewater and alkaline solution from the electrodialysis system 1 A are mixed with the alkaline regeneration wastewater from the A tower 41 as needed, and the resulting mixture is supplied to the wastewater treatment facility 43 .

[0088] It is preferable to operate the electrodialysis system 1A in batch mode in accordance with the regeneration cycle of the K column 40. The frequency of cleaning the alkaline chamber 7 is preferably approximately once every 12 to 48 hours, in accordance with the regeneration cycle of the K column 40. Shorter cleaning intervals increase the amount of cleaning water used and reduce the operating time of the electrodialysis system 1A. Conversely, longer cleaning intervals increase the risk of scaling in the alkaline chamber 7. It is preferable to set the cleaning interval taking these points into consideration. This reduces the amount of scale of hardness components (divalent calcium ions and magnesium ions) to a certain level, and by cleaning the alkaline chamber 7 with water, the impact of scale blockage that could interfere with the electrodialysis process can be significantly reduced.

[0089] If the concentration of hardness components in the acid regeneration wastewater from the K tower 40 is higher than 1000 mg / L, the alkaline chamber 7 of the electrodialysis system 1A will need to be cleaned more frequently and for longer periods of time, resulting in a decrease in the acid recovery efficiency. In consideration of this, the concentration of hardness components in the acid regeneration wastewater from the K tower 40 is preferably about 100 to 1000 mg / L.

[0090] In the electrodialysis system 1A, the stock solution before electrodialysis is acidic, so after water washing, a portion of the stock solution may be used to wash the alkaline chamber 7. Alternatively, waste acid from the factory may be used for washing instead of the stock solution. Examples of waste acid include waste hydrochloric acid and waste sulfuric acid. When the concentration of the waste acid is several tens of wt %, it may be diluted to a few wt % before use for washing.

[0091] Figure 4 is a schematic diagram showing the configuration of a primary pure water production system to which an electrodialysis system 1B is applied. Referring to Figure 4, the primary pure water production system 100B includes an electrodialysis system 1B, a K tower 40, an A tower 41, a reclaimed wastewater tank 42, and wastewater treatment equipment 43. The electrodialysis system 1B has the structure shown in Figure 2. Other than the electrodialysis system 1B, the configuration is the same as that of the primary pure water production system 100A shown in Figure 3. The same components as those in the primary pure water production system 100A are designated by the same reference numerals, and detailed descriptions thereof will be omitted.

[0092] The electrodialysis system 1B recovers acid from the acid regeneration waste liquid discharged from the K tower 40. In the electrodialysis system 1B, the acid regeneration waste liquid is supplied to the raw liquid tank 12. The acid liquid (here, 4% hydrochloric acid) produced in the acid chamber 8 by electrodialysis is recovered and reused as a regenerant. As scale derived from hardness components is produced in the alkaline chamber 7 during electrodialysis, the cleaning mechanism 20 cleans the alkaline chamber 7 with clear water. The cleaning waste liquid used to clean the alkaline chamber 7 is discharged to the regeneration wastewater tank 42 via the same route as the alkaline liquid produced in the alkaline chamber 7.

[0093] In the primary pure water production system 100B, it is preferable to perform batch operation of the electrodialysis system 1B in accordance with the regeneration cycle of the K tower 40. The frequency and intervals of cleaning, the hardness components in the acid regeneration waste liquid, and cleaning using a portion of the stock solution are as described for the primary pure water production system 100A.

[0094] In the primary pure water production systems 100A and 100B described above, most of the anion components in the acid regeneration waste liquid are chloride ions, and do not contain carbonate ions, etc. Therefore, in the electrodialysis systems 1A and 1B, there is a low risk of blockage due to carbonate scale such as calcium carbonate, and most of the scale becomes soft hydroxide scale, which is relatively easy to wash away with water.

[0095] Although the present invention has been described above with reference to the embodiments, the present invention is not limited to the above embodiments. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the present invention.

[0096] This application claims priority based on Japanese Patent Application No. 2024-106311, filed July 1, 2024, the disclosure of which is incorporated herein in its entirety by reference.

[0097] 1A, 1B Electrodialysis system 2a to 2e Valves 3a Anode 3b Cathode 4 Ion exchange partition 5 Anion exchange membrane 6a Anode chamber 6b Cathode chamber 7 Alkaline chamber 8 Acid chamber 9 Deionization chamber 10 Control device 11 Power supply device 20 Cleaning mechanism 30 Cation exchange membrane 50, 51 Electrodialysis device

Claims

1. An electrodialysis system comprising: an electrodialysis device that generates an acid solution and an alkaline solution from a liquid to be treated by passing an electric current through it; and a cleaning mechanism that cleans an alkaline chamber of the electrodialysis device that produces the alkaline solution, wherein the electrodialysis device is energized until the pH of the treated liquid in the alkaline chamber reaches 7 or higher, and then the energization operation is stopped, and the cleaning mechanism passes water through the alkaline chamber to clean it.

2. The electrodialysis system according to claim 1, wherein the electrodialysis device comprises: an alkaline chamber partitioned by the ion exchange partitions and the anion exchange membrane, to which the liquid to be treated is supplied, and an acid chamber partitioned by the ion exchange partitions and the anion exchange membrane, adjacent to the anode side of the alkaline chamber via the anion exchange membrane, and for producing the acid solution.

3. The electrodialysis system according to claim 2, wherein the alkaline compartment and the acid compartment are each filled with an ion exchanger, and the ion exchanger filled in the alkaline compartment includes an anion exchanger.

4. The electrodialysis system according to claim 2, wherein the flow direction of the acid solution in the acid compartment and the flow direction of the treatment solution or alkaline solution in the alkaline compartment are countercurrent to each other.

5. The electrodialysis system according to claim 1, wherein the electrodialysis device comprises an ion exchange partition wall, an anion exchange membrane, and a cation exchange membrane arranged in this order from the anode side between an anode and a cathode, and comprises: a deionization compartment partitioned by the anion exchange membrane and the cation exchange membrane and to which the liquid to be treated is supplied; an alkaline compartment partitioned by the ion exchange partition wall and the cation exchange membrane and adjacent to the cathode side of the deionization compartment across the cation exchange membrane; and an acid compartment partitioned by the ion exchange partition wall and the anion exchange membrane and adjacent to the anode side of the deionization compartment across the anion exchange membrane, and for producing the acid solution.

6. The electrodialysis system according to claim 5, wherein the alkaline compartment, the acid compartment, and the deionization compartment are each filled with an ion exchanger, and the ion exchanger filled in the deionization compartment includes an anion exchanger.

7. The electrodialysis system according to claim 5, wherein the flow direction of the desalted water in the demineralizing compartment and the flow direction of the treated liquid or alkaline liquid in the alkaline compartment are countercurrent to each other.

8. An electrodialysis system according to any one of claims 1 to 7, wherein the liquid to be treated is an acid regenerated waste liquid from an ion exchange resin tower.

9. An electrodialysis system according to any one of claims 1 to 7, wherein the cleaning mechanism cleans the alkaline chamber with water and then cleans the alkaline chamber with water containing an acid.

10. The space velocity of the water passing through the alkaline chamber is 400 h -1 The electrodialysis system according to any one of claims 1 to 7.

11. A method for cleaning an electrodialysis device that produces an acid solution and an alkaline solution from a liquid to be treated by passing an electric current through the electrodialysis device, comprising passing an electric current through the electrodialysis device until the pH in the alkaline chamber producing the alkaline solution reaches 7 or higher, then stopping the electric current passage and washing the alkaline chamber with water.

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