Electrodialysis method using bipolar membranes

Abstract

A bipolar electrodialysis method using an electrodialysis apparatus with a bipolar membrane (BP) and a cation exchange membrane (C) disposed between an anode (10) and a cathode (11) to generate acid and alkali from salt, characterized in that when the electrodialysis apparatus is de-energized, the alkali chamber facing the anion exchanger (a) of the bipolar membrane (BP) is acid-washed with an acidic aqueous solution with a pH of less than 3.5.

Description

Technical Field

[0001] This invention relates to an electrodialysis method using bipolar membranes (bipolar electrodialysis method), and to a bipolar electrodialysis method for generating alkali and acid from salt. Background Technology

[0002] Bipolar membranes are composite membranes made by bonding cation exchange membranes and anion exchange membranes together, and they have the function of dissociating water into protons and hydroxide ions.

[0003] Utilizing the unique function of bipolar membranes, cation exchange membranes, and even anion exchange membranes, are assembled together with bipolar membranes in electrodialysis devices to perform electrodialysis, thereby generating acids and bases from various salts. Therefore, this bipolar electrodialysis method is used in the production of various acids and bases.

[0004] In the following description, cation exchange membranes with bipolar membranes are referred to as cation exchangers, and anion exchange membranes with bipolar membranes are referred to as anion exchangers, to distinguish them from cation exchange membranes and anion exchange membranes used independently.

[0005] However, in electrodialysis apparatuses used in the manufacture of acids and bases using bipolar membranes as described above, a structure is formed in which a bipolar membrane and a cation exchange membrane are arranged adjacent to each other between the anode and cathode. For example, in an electrodialysis apparatus with a basic two-chamber structure (sometimes called a two-chamber electrodialysis apparatus), a cation exchange membrane and a bipolar membrane are alternately arranged between the anode and cathode. The bipolar membrane and the cation exchange membrane adjacent to the anode side form the alkali chamber, and the bipolar membrane and the cation exchange membrane adjacent to the cathode side form the acid chamber. Moreover, the acid chamber also serves as the salt chamber (acid-salt mixture chamber) for supplying the aqueous solution of the salt to be used in electrodialysis. Furthermore, in electrodialysis devices with a basic three-compartment structure (sometimes called three-compartment electrodialysis devices), a cation exchange membrane, a bipolar membrane, and an anion exchange membrane are sequentially arranged between the anode and cathode. The bipolar membrane and the cation exchange membrane adjacent to the anode side form an alkaline compartment, the bipolar membrane and the anion exchange membrane adjacent to the cathode side form an acidic compartment, and further, the cation exchange membrane on the anode side and the anion exchange membrane located on its further anode side form a salt compartment. That is, in these electrodialysis devices, an alkaline compartment is formed on the anion exchanger side of the bipolar membrane, and an acidic compartment is formed on the cation exchanger side of the bipolar membrane.

[0006] In an electrodialysis apparatus with this structure, a voltage is applied between the anode and cathode to conduct electricity. For example, in a two-chamber electrodialysis apparatus, if an aqueous solution of salt is circulated to the acid chamber, the salt dissociates into cations and anions. Therefore, the cations pass through the cation exchange membrane to the alkaline chamber, where they react with hydroxide ions (OH-) supplied from the bipolar membrane. - The anion combines with the protons (H+) supplied from the bipolar membrane to form a base, leaving the anion in the acid chamber.+ The salts combine to form an acid. Thus, the acid concentration gradually increases in the acid chamber and the alkali concentration increases in the alkali chamber. The acid is removed from the acid chamber and the alkali is removed from the alkali chamber, thereby obtaining acid and alkali from the salt supplied to the acid chamber.

[0007] In the production of acids and bases as described above, it is known that polyvalent cations such as calcium and magnesium sometimes exist as impurities in the aqueous solution of salt circulated to the acid chamber. If acid or base production is carried out by passing an electric current in the presence of such polyvalent cations, hydroxides of the polyvalent cations will precipitate on the surface or inside the cation exchange membrane. Due to the increased resistance between the electrodes, the current-carrying efficiency gradually decreases, and the cation exchange membrane may be damaged in some cases. To prevent such adverse conditions, various techniques for removing the precipitation of hydroxides of polyvalent cations have been proposed.

[0008] For example, patent documents 1 to 5 propose techniques for acid cleaning of cation exchange membranes using various methods, although not using bipolar membrane electrodialysis. In these methods, the hydroxides precipitated in the cation exchange membrane can be removed by acid cleaning.

[0009] In addition, Patent Document 6 discloses a method for performing electrodialysis after removing polyvalent cations from a brine solution, using a bipolar electrodialysis apparatus with a bipolar membrane. Patent Document 7 discloses that the electrodialysis apparatus can be safely operated if the content of magnesium and calcium in the chelating resin is below 0.1 ppm.

[0010] Patent document 8 proposes a method in which acid is added to the brine solution supplied to the acid chamber (or acid salt mixing chamber) during bipolar electrodialysis, thereby keeping the pH of the brine chamber at least less than 1 and preventing the precipitation of hydroxides of polyvalent cations.

[0011] Furthermore, Patent Document 9 discloses a method for supplying an inorganic acid with a pH less than 1 to a portion of the acid chamber (salt chamber) under energized conditions when producing organic acids via bipolar electrodialysis.

[0012] The above method can suppress the rise in membrane voltage of cation exchange membranes.

[0013] However, if bipolar electrodialysis is performed continuously over a long period, polyvalent metal ions will be detected in the bipolar membrane, which may degrade the quality of the alkaline solution. The aforementioned patent literature makes no mention of the contamination caused by polyvalent metal ions in the alkaline solution; in fact, these methods cannot prevent the degradation of the alkaline solution's quality.

[0014] Furthermore, since bipolar membranes are composite membranes formed by bonding anion exchange membranes and cation exchange membranes, as disclosed in Patent Document 10, there is a problem of water swelling (bubbling) easily occurring at the interface between the anion exchanger and the cation exchanger. In bipolar electrodialysis, this bubbling also causes an increase in the membrane voltage of the bipolar membrane. For example, in electrodialysis, the power supply is sometimes occasionally stopped while the membrane is installed. When this power supply stops, the circulation of acid, alkali, and salt solutions to each chamber of the electrodialysis apparatus usually stops as well. At this time, most of the acid, alkali, and salt solutions present in each chamber are discharged and returned to their respective tanks, but some remain in each chamber. If this situation continues, especially in the lower part where the acid, alkali, and salt solutions remain, water swelling and bubbling sometimes occur in the bipolar membrane. Once such bubbling occurs in the bipolar membrane, the membrane voltage increases in the next restart, and the flow of acid, alkali, and salt solutions deteriorates, ultimately making normal operation of the apparatus difficult. In the methods described in the aforementioned patent documents 1 to 9, it is also difficult to prevent bubbling from occurring on such bipolar films.

[0015] In addition, as a means to prevent foaming on the bipolar membrane, Patent Document 11 proposes an electrodialysis method, characterized in that the bipolar membrane is washed with water when the operation of the electrodialysis device equipped with the bipolar membrane is stopped (when the power is stopped) to form a state in which there is no alkali or acid.

[0016] However, even with the aforementioned countermeasures, foaming on the bipolar membrane was confirmed. For example, in bipolar electrodialysis, due to the presence of polyvalent ions as impurities in the brine, periodic acid cleaning is performed to suppress precipitation in the cation exchange membrane. After about six months of operation, the voltage increased, so the membrane was unpacked, and foaming was confirmed to have occurred in the bipolar membrane.

[0017] Existing technical documents

[0018] Patent documents

[0019] Patent Document 1: Japanese Patent Application Publication No. 58-202003

[0020] Patent Document 2: Japanese Patent Application Publication No. 52-149295

[0021] Patent Document 3: Japanese Patent Application Publication No. 57-169096

[0022] Patent Document 4: Japanese Patent Application Publication No. 51-67292

[0023] Patent Document 5: Japanese Patent Application Publication No. 61-263647

[0024] Patent Document 6: Japanese Patent Application Publication No. 63-65912

[0025] Patent Document 7: Japanese Patent Application Publication No. 9-135698

[0026] Patent Document 8: Japanese Patent No. 3151042

[0027] Patent Document 9: Japanese Patent Application Publication No. 2010-269288

[0028] Patent Document 10: Japanese Patent Application Publication No. 2002-306975

[0029] Patent Document 11: Japanese Patent Application Publication No. 5-163594 Summary of the Invention

[0030] The problem the invention aims to solve

[0031] Therefore, the object of the present invention is to provide a method for bipolar electrodialysis that can be performed for a long period of time without the quality degradation of the generated alkaline solution, without replacing the bipolar membrane and using the same bipolar membrane.

[0032] Another object of the present invention is to provide a bipolar electrodialysis method that also effectively prevents foaming on the bipolar membrane.

[0033] Solution for solving the problem

[0034] According to the present invention, a bipolar electrodialysis method is provided, which uses an electrodialysis apparatus. While applying a voltage between the anode and cathode, a brine solution is circulated into the electrodialysis apparatus to perform electrodialysis, continuously generating acid and alkali from the salt. In the electrodialysis apparatus, a bipolar membrane with one side being an anion exchanger and the other side being a cation exchanger is disposed between the anode and cathode. The bipolar membrane is configured such that the anion exchanger side is located on the anode side, with the anion exchanger side of the bipolar membrane serving as the alkali chamber and the cation exchanger side serving as the acid chamber.

[0035] Its features are,

[0036] After a certain period of time, the power supply is stopped. Without removing the bipolar membrane and cation exchange membrane from the electrodialysis device, the alkaline chamber is acid-cleaned with an acidic aqueous solution with a pH below 3.5.

[0037] After acid cleaning, the power is turned on again to perform electrodialysis.

[0038] In the bipolar electrodialysis method of the present invention, the following manner is appropriately adopted.

[0039] (1) The electrodialysis device has a basic dual-chamber structure in which a cation exchange membrane and a bipolar membrane are alternately arranged between the anode and the cathode, and the brine solution is circulated to the acid chamber.

[0040] (2) The electrodialysis device also includes an anion exchange membrane with a three-chamber basic structure. The three-chamber basic structure has a cation exchange membrane, a bipolar membrane and an anion exchange membrane arranged sequentially between the anode and the cathode. A salt chamber is formed by the anion exchange membrane and the cation exchange membrane, and the salt water solution is circulated to the salt chamber.

[0041] (3) While maintaining the acid concentration in the alkali chamber at a level not higher than the acid concentration in the acid chamber adjacent to the alkali chamber across the bipolar membrane, the acid cleaning is performed.

[0042] (4) During the acid cleaning, the acid for cleaning is first fed into the acid chamber, and then the acid for cleaning is fed into the alkali chamber with a delay.

[0043] (5) After the acid cleaning is completed, the acid used for cleaning the alkaline chamber is first discharged and replaced with a water washing solution, and then the acid used for cleaning the acid chamber is discharged and replaced with a water washing solution after a delay.

[0044] The effects of the invention

[0045] In the bipolar electrodialysis method using the electrodialysis apparatus of the present invention, which is equipped with a bipolar membrane and a cation exchange membrane, an important feature is that, when the power supply is stopped, the alkaline chamber (i.e., the chamber facing the anion exchanger of the bipolar membrane) is acid-cleaned with an acidic aqueous solution with a pH of 3.5 or lower. That is, for the method of the present invention, cleaning the alkaline chamber means that the acid cleaning is performed while the bipolar membrane and cation exchange membrane are installed in the apparatus to form the alkaline chamber. Methods for removing the membrane and cleaning it with acid are known, as are methods for acid cleaning the alkaline chamber while the membrane is installed, but a means of cleaning the alkaline chamber when the power supply is stopped while the membrane is installed is completely unknown.

[0046] According to the present invention, by acid cleaning the alkali chamber in this way, the quality reduction of the alkali solution due to bipolar membrane fouling caused by polyvalent metal cations can be effectively prevented.

[0047] In addition, during acid cleaning of the alkali chamber, by maintaining the acid concentration in the alkali chamber below that in the acid chamber, it is possible not only to suppress the rise in membrane voltage of the cation exchange membrane caused by the precipitation of hydroxides of polyvalent metal ions, but also to effectively suppress the occurrence of foaming on the bipolar membrane. Attached Figure Description

[0048] Figure 1 This is a schematic diagram illustrating an example of a dual-chamber electrodialysis apparatus used to implement the bipolar electrodialysis method of the present invention.

[0049] Figure 2 This is a schematic diagram illustrating an example of a three-chamber electrodialysis apparatus used to implement the bipolar electrodialysis method of the present invention.

[0050] Figure 3 This is a schematic diagram illustrating another example of a two-chamber electrodialysis apparatus used to implement the bipolar electrodialysis method of the present invention.

[0051] Figure 4 This is a schematic diagram illustrating another example of a three-chamber electrodialysis apparatus used to implement the bipolar electrodialysis method of the present invention. Detailed Implementation

[0052] <Electrodialysis Equipment and Electrodialysis>

[0053] The bipolar electrodialysis method of this invention uses at least a bipolar membrane and a cation exchange membrane, but depending on the structure of the electrodialysis device used, it is broadly classified into a two-chamber type (having a basic two-chamber structure) and a three-chamber type (having a basic three-chamber structure). Figure 1 and Figure 3 The schematic structure of a dual-chamber electrodialysis device is shown in the figure. Figure 2 and Figure 4 The diagram shows a schematic structure of a three-chamber electrodialysis device.

[0054] Dual-chamber electrodialysis unit;

[0055] Refer to an example of a two-chamber electrodialysis device. Figure 1 In this apparatus, a bipolar membrane (BP) and a cation exchange membrane (C) are used as membranes. The apparatus includes an electrodialysis tank, which is indicated as a whole by reference numeral 1. In addition, along with a brine tank 3 for storing an aqueous solution of salt as raw material, an alkali tank 5 and an acid tank 7 for cleaning are provided. These liquids are circulated in predetermined chambers inside the electrodialysis tank 1 via predetermined circulation piping.

[0056] The electrodialysis cell 1 is equipped with multiple bipolar membranes (BP) and cation exchange membranes (C), with the BP and C membranes alternately arranged between the anode 10 and the cathode 11. Figure 1 In this example, four bipolar membranes (BP) and three cation exchange membranes (C) are provided. The area housing the anode 10 is called the anode chamber 13, which is separated from the cathode 11 side by the bipolar membranes (BP). The area housing the cathode 11 is called the cathode chamber 15, which is also separated from the anode 10 side by the bipolar membranes (BP). Electrolytes (anolytes or catholytes) are circulated from a predetermined electrode tank (not shown) to the anode chamber 13 and cathode chamber 15, respectively.

[0057] It should be noted that the bipolar membrane BP is configured such that the anion exchanger a side faces the anode 10 side, and the cation exchanger c side faces the cathode side.

[0058] Acid chamber 17 and alkaline chamber 19 are alternately formed in a manner that is adjacent to each other, using bipolar membrane BP and cation exchange membrane C arranged alternately as described above. That is, the structure is represented as anode-(BP-C)n-BP-cathode, and the smallest repeating unit composed of bipolar membrane BP and cation exchange membrane C is called a battery cell, where n is the number of repeating layers of the battery cell. Figure 1 In an example where the number of battery cells n is 3, three acid cells 17a, 17b, and 17c are sequentially formed from the anode 10 toward the cathode 11 via a bipolar membrane BP and a cation exchange membrane C adjacent to the anode 10. Similarly, three alkaline cells 19a, 19b, and 19c are sequentially formed from the anode 10 toward the cathode 11 via the bipolar membrane BP and the cation exchange membrane C adjacent to the anode 10. The acid cells 17a and alkali cells 19a and 19b are fixed to a frame, such as a filter press, with the cation exchange membrane C sandwiched between them. Of course, the number of battery cells n is not limited to 3; in industrial implementations, it varies depending on the scale of implementation, but is typically set to a considerable number.

[0059] It should be noted that, in Figure 1 In the example, bipolar membranes BP are arranged facing each other on the anode 10 and cathode 11. However, in this invention, as long as multiple acid chambers 17 are formed, the arrangement of the bipolar membrane BP and the cation exchange membrane C is not limited. For example, the cation exchange membrane C can also be arranged facing the anode 10 or the cathode 11. The operating temperature of the electrodialysis apparatus is typically 20°C to 60°C.

[0060] The dual-chamber type is a bipolar electrodialysis method that uses an electrodialysis apparatus with this structure to perform bipolar electrodialysis. While circulating the electrode solution in the anode chamber 13 and the cathode chamber 15, a predetermined brine solution (i.e., feed solution) is circulated from the brine tank 3 to each acid chamber 17a-17c via the brine circulation piping 20, and an alkaline solution is circulated from the alkaline tank 5 to the alkaline chambers 19a-19c via the alkaline circulation piping 21, thereby producing the acid for electrodialysis. The current density between the electrodes is 3 A / dm³. 2 ~50A / dm 2 Around. Typically, such a two-chamber electrodialysis system is suitable for supplying an aqueous solution of organic acid salts as a salt solution to the acid chamber (acid salt mixing chamber) to obtain organic acid salts as weak acids.

[0061] That is, in the salt solution supplied to acid chambers 17a-17c, the salt dissociates into cations and anions, and the anions react with protons (H+) supplied from the bipolar membrane BP. + They combine to form acids. On the other hand, Na... +The cations pass through the cation exchange membrane C and are transferred to the adjacent alkaline chamber 19, where they react with hydroxide ions (OH-) supplied from the bipolar membrane BP. - The salt and alkali solutions combine to form an alkali solution. Therefore, if the aqueous salt solution and the aqueous alkali solution are circulated in the acid chamber 17 and the alkali chamber 19, the acid concentration gradually increases in the acid chamber 17, and the alkali concentration gradually increases in the alkali chamber 19. Thus, when the acid concentration in each of the acid chambers 17a-17c reaches a predetermined concentration, an aqueous solution of the target acid can be obtained by recovery from the drain side of the salt solution circulation pipe 20. Similarly, when the alkali concentration in each of the alkali chambers 19a-19c reaches a predetermined concentration, a high-concentration aqueous alkali solution can also be obtained by recovery from the drain side of the alkali solution circulation pipe 21.

[0062] It should be noted that in this method, since a salt solution is supplied to the acid chamber 17, the acid chamber 17 is sometimes also referred to as the salt chamber or acid salt mixing chamber.

[0063] In addition, Figure 1 In the diagram, the cleaning acid circulation piping 23 connected to the acid tank 7 is only circulated to the alkali chamber 19c and acid chamber 17c on the right, but in reality, it is also circulated to the alkali chambers 19a and 19b and the acid chambers 17a and 17b. A portion of the cleaning acid circulation piping 23 is omitted because if all the piping were shown, the diagram would become complex and difficult to interpret.

[0064] In addition, as an alkaline solution that is circulated from the alkaline tank 5 to each alkaline chamber 19, an aqueous solution of cations transferred from the acid chamber 17 to the alkaline chamber 19 via the cation exchange membrane C is used, such as an aqueous solution of sodium hydroxide.

[0065] It should be noted that, in Figure 1 In order to perform acid cleaning as described later, a dedicated acid tank 7 is provided for acid cleaning.

[0066] Three-chamber electrodialysis unit;

[0067] In a three-compartment electrodialysis device, in addition to the bipolar membrane BP and the cation exchange membrane C, an anion exchange membrane A is also used as the membrane.

[0068] In the electrodialysis cell 1 of such an electrodialysis device, such as Figure 2 As shown, the chamber 17 formed by the bipolar membrane BP and the cation exchange membrane C is divided by the anion exchange membrane A disposed therebetween. The anode side (anion exchanger side) of the bipolar membrane BP is the base chamber 19, and the cathode side (cation exchanger side) of the bipolar membrane BP is the acid chamber 17-2. The salt chamber 17-1 is adjacent to the acid chamber 17-2 through the anion exchange membrane A.

[0069] In such an electrodialysis tank 1, a brine solution is supplied from the brine tank 3 to the brine chamber (raw material chamber) 17-1 via a circulation pipe 20, and a dilute aqueous solution of the same type of acid as the intended acid is supplied from the acid tank 25 to the acid chamber 17-2 via a circulation pipe 30. An alkaline aqueous solution is supplied from the alkali tank 5 to the alkali chamber 19 via a circulation pipe 21. Acid is generated through electrodialysis under energized conditions. Typically, this three-chamber electrodialysis system is suitable for supplying an aqueous solution of inorganic salts to the brine chamber as the brine solution, obtaining a strong inorganic acid in the acid chamber, and obtaining a strong alkali in the alkali chamber.

[0070] That is, due to the dissociation of the salt supplied to salt chamber 17-1, cations are transferred to base chamber 19 via cation exchange membrane C and anions are transferred to acid chamber 17-2 via anion exchange membrane A by electrolysis. Therefore, in acid chamber 17-2, anions react with protons (H+) generated from bipolar membrane BP. + The acid is generated by combining with the acid, and the acid concentration of the acid solution supplied to acid chamber 17-2 gradually increases. When the concentration reaches a certain level, the acid is recovered, thereby obtaining the target acid.

[0071] Furthermore, in alkali chamber 19, exactly the same as in the two-chamber method, the cation reacts with the hydroxide ions (OH-) generated by the bipolar membrane BP. - The alkaline solution is generated by combining with the alkaline solution in the alkaline chamber 19. The alkaline concentration gradually increases. When the concentration reaches a certain level, the solution is recovered, thereby obtaining the target alkaline solution.

[0072] It should be noted that, in Figure 2 In the example, reference numeral 7 in the attached figure refers to the acid tank used for acid cleaning.

[0073] In addition, Figure 2 In the middle, the cleaning acid circulation piping 23 connected to the acid tank 7 is only circulated through the right-side alkali chamber 19, acid chambers 17-1, and 17-2, but in reality, it is also circulated through the left-side alkali chamber 19, acid chambers 17-1, and 17-2. Figure 1 Similarly, a portion of the acid circulation piping 23 for cleaning is omitted because if all the piping were represented, the diagram would become complex and difficult to interpret.

[0074] In the aforementioned two-compartment and three-compartment bipolar electrodialysis methods, the bipolar membrane BP is a composite ion exchange membrane with a structure consisting of a cation exchange membrane C and an anion exchange membrane A bonded together. As mentioned earlier, the anion exchanger a side is typically positioned facing the anode 10 side, while the cation exchanger c side is positioned facing the cathode 11 side. There are no particular limitations on such a bipolar membrane BP, and known membranes can be used.

[0075] It should be noted that, although in Figure 1 and Figure 2Although not shown in the diagram, in order to adjust the acid concentration of the alkali chamber 19 and the acid chamber 17 (17-2) and thus enable water cleaning, ion-exchange water, tap water and other water can be circulated in the alkali chamber 19, the acid chamber 17 (17-2) and the salt chamber 17-1.

[0076] The manufacturing method of bipolar film BP is not limited to this, but the following method is representative.

[0077] (1) A method for bonding and curing cation exchange membranes and anion exchange membranes together with a mixture of polyethyleneimine and epichlorohydrin (Japanese Patent Publication No. 32-3962);

[0078] (2) A method for bonding cation exchange membranes and anion exchange membranes using an ion exchange adhesive (Japanese Patent Publication No. 34-3961);

[0079] (3) A method for pressing together a paste-like mixture of ion exchange resin, anion or cation exchange resin and thermoplastic material coated with micro powder on cation exchange membrane and anion exchange membrane (Japanese Patent Publication No. 35-14531).

[0080] (4) A method of manufacturing a cation exchange membrane by coating the surface of the membrane with a paste-like substance composed of vinylpyridine and an epoxy compound and irradiating it with radiation (Japanese Patent Publication No. 38-16633).

[0081] (5) A method for cross-linking an anion exchange membrane by irradiating it with ionizing radiation after attaching sulfonic acid-type polymeric electrolytes and allylamines to the surface of the membrane (Japanese Patent Publication No. 51-4113);

[0082] (6) A method for depositing a dispersion system of ion exchange resins with opposite charges on the surface of an ion exchange membrane and a mixture of a parent polymer (Japanese Patent Application Publication No. 53-37190);

[0083] (7) A method of clamping a sheet of styrene and divinylbenzene impregnated with polyethylene film onto a stainless steel frame, sulfonating one side, removing the sheet and chloromethylating the remaining part, followed by amination treatment (US Patent No. 3562139).

[0084] (8) A method for bonding two membranes by treating the interface between anion exchange membrane and cation exchange membrane with an inorganic compound (Japanese Patent Application Publication No. 59-47235).

[0085] The cation exchange membrane C can be a known cation exchange membrane, for example, a cation exchange membrane that has sulfonic acid group, carboxylic acid group, phosphonic acid group, sulfate ester group, phosphate ester group, etc. as cation exchange groups, or a cation exchange membrane in which various ion exchange groups exist.

[0086] Furthermore, the cation exchange membrane C can be a cation exchange membrane obtained by either polymerization or condensation, and can be either a homogeneous membrane or a heterogeneous membrane, and may also appropriately have a reinforcing core material. Additionally, it can be formed from known resins such as hydrocarbon resins and fluorine resins. Moreover, as long as the content is 5A / dm... 2 For substances that function essentially as cation exchange membranes and have a current density of 70% or higher for electrodialysis of 2N- saline aqueous solutions, substances commonly referred to as zwitterion exchange membranes can also be used. Furthermore, when the cation exchange membrane C faces the anode 10, a fluorine-based cation exchange membrane C with high acid resistance is preferred.

[0087] As the anion exchange membrane A used in the three-compartment system, conventionally known anion exchange membranes can also be used. For example, anion exchange membranes containing quaternary ammonium groups, primary amino groups, secondary amino groups, tertiary amino groups, pyridinium groups, pyridinium groups, or mixtures of these ion exchange groups can be used. Furthermore, like cation exchange membranes, this anion exchange membrane can be obtained by either polymerization or condensation, and can be either a homogeneous membrane or a heterogeneous membrane, and can appropriately have a reinforcing core material. Additionally, it can be formed from known resins such as hydrocarbon resins and fluorine resins. It is only necessary to further react the 2N-sodium chloride solution at 5A / dm³... 2 Electrodialysis with a current density of over 70% essentially functions as an anion exchange membrane, and can also use substances known as zwitterion exchange membranes.

[0088] It should be noted that anion exchange membranes tend to allow acid to pass through easily, so it is preferable to use anion exchange membranes that are difficult for acid to pass through.

[0089] Furthermore, the electrodes used as anode 10 and cathode 11 can be known electrodes used in the electrochemical industry, such as water electrolysis and salt electrolysis. For example, anode 10 is usually made of nickel, iron, lead, platinum, graphite, etc., and cathode 11 is usually made of nickel, iron, stainless steel, etc.

[0090] Furthermore, the electrolyte supplied to the anode chamber 13 and the cathode chamber 15 is a suitable electrolyte depending on the electrode materials forming the anode 10 and the cathode 11. An example of such a combination is described below.

[0091] Anode solution;

[0092] Nickel or iron-sodium hydroxide aqueous solution

[0093] Lead-sulfuric acid aqueous solution

[0094] Platinum-sulfuric acid or sodium sulfate aqueous solution

[0095] Graphite-salt water solution

[0096] catholyte;

[0097] Nickel, iron, or stainless steel - sodium hydroxide, sodium sulfate

[0098] Or a salt solution

[0099] Furthermore, in the aforementioned bipolar electrodialysis, water-soluble salts of the target acids and bases are used as the sources of these acid and base generation. Examples include sodium chloride, potassium chloride, lithium chloride, ammonium chloride, sodium sulfate, potassium sulfate, ammonium sulfate, sodium nitrate, and potassium nitrate.

[0100] Of course, this bipolar electrodialysis can also be applied to the production of organic acids from aqueous solutions of organic acid salts. As such organic acid salts, formate, acetate, oxalate, trichloroacetate, dichloroacetate, monochloroacetate, mercaptoacetate, monochloroacetate, malonate, propionate, L-lactate, D-lactate, fumarate, maleate, succinate, tartrate, butyrate, phenolate, citrate, ascorbate, picrate, picolinate, benzoate, salicylate, etc., can be used as the cation forming the salt, with Na being preferred. + K + NH4 + wait.

[0101] Acid cleaning

[0102] Incidentally, in the aforementioned bipolar electrodialysis, unlike membrane replacement, sometimes the power supply is stopped while the membrane is installed (i.e., the operation of the device is halted). For example, after the device has been running for a certain period of time and the target amount of acid or alkali has been obtained, the operation of the device is temporarily stopped.

[0103] Furthermore, power is cut off when it becomes necessary to stop the operation of the equipment. For example, the equipment may be shut down after a certain period of operation, depending on the workshop's production schedule. In addition, from the viewpoint of preventing membrane performance degradation and restoring performance, the operating voltage is constantly monitored, and if an abnormal rise in voltage is detected, it may be suspected that impurities have contaminated the membrane, leading to a shutdown.

[0104] In this invention, an important feature is that the alkali chamber is acid-cleaned with an aqueous solution of acid with a pH of 3.5 or lower when the power supply is stopped. From the viewpoint of fully preventing the degradation of the alkali solution, it is effective to perform the acid cleaning by stopping the power supply after the device has been running for at least one hour, particularly three hours or more. Alternatively, it is effective to perform the acid cleaning by stopping the power supply after the voltage during operation has increased by at least 1.2 times, particularly 1.5 times or more, relative to the initial voltage.

[0105] On the other hand, even if the operation time is too long, the effect of preventing the degradation of alkaline solution quality caused by acid cleaning cannot be fully realized. Therefore, it is effective to set the operation time to no more than 100 hours, and in particular, no more than 120 hours. If expressed by the other references mentioned above, it is effective to set the operation time to no more than 4 times, and in particular no more than 5 times, the voltage during operation relative to the initial voltage.

[0106] That is, sometimes polyvalent metal ions are contained as impurities in the salt. If the device is operated continuously for a long time, these polyvalent metal ions may sometimes be detected in the alkali chamber 19 within the device. The presence of such polyvalent metal ions in the alkali chamber 19 can lead to a decrease in the quality of the product. In the present invention, in either the two-chamber or three-chamber type, by acid cleaning the alkali chamber with the strong acid aqueous solution when the power is stopped, the decrease in alkali quality caused by the accumulation of polyvalent metal ions in the alkali chamber 19 can be prevented. Furthermore, the precipitation of hydroxides of polyvalent metal ions in the alkali chamber 19 and other membrane performance degradation caused by these factors can be effectively prevented.

[0107] The acid solution used in acid cleaning is not particularly limited as long as it has high solubility for multivalent ions and a pH (measured at the operating temperature of electrodialysis) of 3.5 or lower. For example, aqueous solutions of inorganic acids such as hydrochloric acid, nitric acid, and sulfuric acid can be used, with hydrochloric acid being particularly preferred. The pH of the aforementioned acid solution is more preferably 2 or lower. However, when the pH of the acid solution is greater than 3.5, the effect of multivalent metal cations on reducing fouling of the bipolar membrane is insufficient.

[0108] In addition, water used for adjusting acidic aqueous solutions (water used for dilution) can be exemplified by, for example, ion-exchanged water and tap water.

[0109] It should be noted that, for the cleaning acid mentioned above, when using the same type of acid circulating in acid chamber 17 during bipolar electrodialysis, simply switch the valve on the acid circulation pipe 23 to supply the acid stored in acid tank 7 to alkali chamber 19. Alternatively, if the acid circulating in acid chamber 17 is a weak acid such as an organic acid (especially in dual-chamber systems, where weak acids are mostly used), a dedicated tank for cleaning acid should be provided, and the cleaning acid should be supplied to alkali chamber 19 from this tank. Of course, the use of weak acids (organic acids) is not limited to dual-chamber systems; the same applies to three-chamber systems.

[0110] However, when the power is stopped, during the acid cleaning of the alkali chamber 19 as described above, if the acid concentration in alkali chamber 19 is higher than that in the adjacent acid chamber 17, it has been experimentally confirmed that foaming easily occurs in the bipolar membrane BP. That is, the anion exchanger a of the bipolar membrane BP faces the alkali chamber 19. Therefore, the acid supplied to the alkali chamber 19 diffuses and permeates into the anion exchanger a. At this time, if the acid concentration in the adjacent acid chamber 17 is low, water enters the bipolar membrane BP from the cation exchanger c side due to osmotic pressure. The inventors believe that, as a result, water accumulates at the interface between the anion exchanger a and the cation exchanger c of the bipolar membrane BP, causing foaming.

[0111] Therefore, the acid cleaning performed in this invention is preferably carried out while maintaining the acid concentration in the alkali chamber 19 at a level not higher than that in the acid chamber 17. For example, if the acid concentration in the alkali chamber 19 is 1 mol / L, the acid concentration in the acid chamber 17 can also be 2 mol / L. However, if the concentration is lower than 1 mol / L, such as 0.5 mol / L, foaming is more likely to occur. When the acid concentration is 0 mol / L (i.e., water), the osmotic pressure difference increases, and foaming is accelerated.

[0112] Typically, acid cleaning of the alkali chamber 19 can effectively suppress foaming in the bipolar membrane BP by maintaining a concentration difference between the acid concentration in the acid chamber 17 and the acid concentration in the alkali chamber 19, particularly at least 0.2 mol / L. To effectively clean the alkali chamber 19, a high concentration of acid is sufficient; in this case, the acid concentration in the acid chamber 17 should also be maintained at a high concentration, above that in the alkali chamber 19.

[0113] Here, the time during which the acid concentration in the alkali chamber is maintained at a state not higher than that in the acid chamber is most preferably the entire cleaning period of the alkali chamber. However, as long as it is a very short time that does not significantly affect the effect of preventing foaming, specifically as long as it is within about 1 minute, even if there is a period that does not satisfy the above-mentioned relationship between the acid concentrations of the alkali chamber and the acid chamber, it is allowed as a substantially maintained period in this invention.

[0114] <Acid Cleaning Steps>

[0115] In this invention, the acid cleaning described above is performed with the power off and the bipolar membrane BP, cation exchange membrane C, and then anion exchange membrane A installed. However, as described above, the acid cleaning is performed while ensuring that the acid concentration in the alkali chamber 19 does not exceed the acid concentration in the acid chamber 17 (17-2).

[0116] That is, during acid cleaning in the alkali chamber 19, water washing and acid cleaning are also performed in other chambers. However, the acid concentration in the alkali chamber 19 is adjusted periodically by valves on each pipe to ensure that the acid concentration in the alkali chamber 19 does not exceed that in the acid chamber 17 (17-2). This is the same for both the two-chamber and three-chamber methods.

[0117] For example, the entire chamber can be drained, and water (ion-exchanged water) can be added to the entire tank to circulate the water throughout the chamber.

[0118] Next, an inorganic acid of a certain concentration for cleaning is added to the acid tank (or cleaning acid tank), and the inorganic acid (e.g., hydrochloric acid) of a certain concentration is circulated in the acid chamber 17 (17-2). Thus, the acid chamber 17 (17-2) is acid-cleaned to remove calcium ions and other polyvalent metal ions or their hydroxides that are attached to the cation exchange membrane C.

[0119] Following the acid cleaning of acid chamber 17 (17-2), acid cleaning of alkali chamber 19 and then salt chamber 17-1 is performed. That is, since the acid concentration of acid chamber 17 (17-2) is set to a certain value, the acid concentration of alkali chamber 19 can be set to not exceed the acid concentration of acid chamber 17 (17-2). Specifically, water circulation in alkali chamber 19 and then salt chamber 17-1 is stopped, and after draining, an inorganic acid (e.g., hydrochloric acid) with a concentration lower than that supplied to acid chamber 17 (17-2) is supplied from the acid tank (or the acid tank for cleaning) to alkali chamber 19 and then salt chamber 17-1 for acid cleaning.

[0120] The acid cleaning of the alkali chamber 19 is carried out for a certain period of time. Importantly, before the acid cleaning of the acid chamber 17 (17-2), the acid cleaning of the alkali chamber 19 is stopped, the liquid is drained from the alkali chamber 19, and then water is added to circulate it. In this case, the salt chamber 17-1 is simultaneously drained and circulated with water along with the alkali chamber 19. Thus, the acid concentration in the alkali chamber 19 will not exceed the acid concentration in the acid chamber 17 (17-2).

[0121] Finally, after draining the acid chamber 17 (17-2), the water is circulated and drained from the entire chamber after a certain period of time. Thus, the entire process when the power is turned off is completed, and bipolar electrodialysis can be started again.

[0122] It should be noted that in the above process, the three-chamber salt chamber 17-1 is drained, washed with water, and acid-washed simultaneously with the alkali chamber 19, but the salt chamber 17-1 can be drained, washed with water, and acid-washed simultaneously with the acid chamber 17-2.

[0123] In addition, not only the alkali chamber 19 and salt chamber 17 faced by the ion exchange membrane (cation exchange membrane or anion exchange membrane), but also the alkali tank 5 and alkali circulation piping 21, and the salt tank 3 and salt circulation piping 20 are cleaned with acid. It is expected that the quality of the generated alkali will be improved. Therefore, it is more preferable to pre-send the cleaning acid to the alkali tank 5, which has been cleaned with water after the alkali solution is discharged, and the salt tank 3, which has been cleaned with water after the salt solution is discharged, and to circulate and clean them using the alkali circulation piping 21 and the salt circulation piping 20, respectively.

[0124] This invention is not limited to using the above-described method. Figure 1 and Figure 2 The method shown can be modified to implement the same method as the device shown.

[0125] For example, it is not efficient to perform acid cleaning by emptying the alkali tank 5 and acid tank 25, which contain alkali and acid products manufactured by the method of the present invention. Therefore, a branch of the cleaning acid circulation pipe 23 connected to the dedicated acid cleaning tank is connected to the acid cleaning tank via a predetermined valve. Figure 3 In the (dual-chamber method), it is connected to the alkali circulation piping 21 and the brine circulation piping 20. Additionally, in Figure 4 In the (three-chamber method), the acid circulation pipe 23 for cleaning is connected via predetermined valves to the alkali circulation pipe 21, the brine circulation pipe 20, and the acid circulation pipe 30 connected to the acid tank 25 containing the product acid. Here, pure water is circulated through the circulation pipes 20, 21, and 30 connected to the brine tank 3, alkali tank 5, and acid tank 25 via valves, allowing each tank (3, 5, 25) to be washed independently of the acid cleaning process. This also allows acid cleaning to be performed without emptying the alkali tank 5 and the acid tank 25.

[0126] By performing the cleaning steps described above, not only can the voltage rise of the cation exchange membrane C be suppressed by cleaning the cation exchange membrane C, but also the occurrence of bubbles and the rise of membrane voltage on the bipolar membrane BP can be reliably suppressed.

[0127] Example

[0128] <Experimental Example 1>

[0129] Prepare the following membrane.

[0130] Bipolar film BP;

[0131] The bipolar membrane prone to bubbling disclosed in Comparative Example 1 of Japanese Patent Application Publication No. 2002-306975

[0132] Cation exchange membrane C;

[0133] CM-2 manufactured by ASTOM Co., Ltd.

[0134] Anion exchange membrane A;

[0135] ACM made by ASTOM Co., Ltd.

[0136] like Figure 2 Thus, a benchtop pressure filter type bipolar membrane electrodialysis tank 1 is fabricated by sequentially arranging a cation exchange membrane C, a bipolar membrane BP, and an anion exchange membrane A between a pair of anodes and cathodes, forming an alkaline chamber 19, an acid chamber 17-2, and a salt chamber 17-1. It should be noted that the number of cation exchange membranes C is 11, the number of bipolar membranes BP and anion exchange membranes A is 10 each, and the effective membrane area of ​​each membrane is 0.55 dm². 2 .

[0137] In addition, to measure the membrane potential, platinum foils were typically inserted into the anode and cathode sides of the fourth bipolar membrane BP and the sixth cation exchange membrane C from the anode side, and connected to a voltmeter, respectively.

[0138] Prepare 5 liters of 0.1 mol / L hydrochloric acid aqueous solution in acid tank 25.

[0139] Prepare 5 liters of sodium chloride solution with a concentration of 2 mol / L in salt tank 3. Add calcium chloride to achieve a calcium content of 250 ppm and add hydrochloric acid to achieve a pH of approximately 3.

[0140] Prepare 5 liters of 0.1 mol / L sodium hydroxide aqueous solution in alkaline solution tank 5.

[0141] It should be noted that no dedicated acid tank for cleaning was provided.

[0142] Thus, with liquid contained in each tank, the liquid is circulated from each tank to the acid chamber 17-2, salt chamber 17-1, and alkali chamber 19 at a linear velocity of 6 cm / sec. 5 liters of sodium sulfate aqueous solution (containing 490 g of sodium sulfate) are circulated in the anode chamber 13 and the cathode chamber 15, respectively.

[0143] At 40℃ and a current density of 8A / dm 2 Electrodialysis will then be performed.

[0144] After 3 hours, the membrane potential of the bipolar membrane remained unchanged, but the membrane potential of the cation exchange membrane increased to three times its initial value. It was assumed that calcium hydroxide scale had formed, therefore the power supply was stopped and acid cleaning was performed.

[0145] The acid cleaning is performed according to the following steps.

[0146] (1) Drain the entire room.

[0147] (2) Clean each tank and add pure water, and simultaneously supply pure water to the whole room for 5 minutes of water circulation.

[0148] (3) Stop water circulation only in acid chamber 17-2 and drain the water.

[0149] (4) Add 1 mol / L of hydrochloric acid with a pH of about 0 to acid tank 25 and start the circulation again.

[0150] (5) After running for 5 minutes, stop the water circulation in alkali chamber 19 and salt chamber 17-1 and drain the water.

[0151] (6) Add 1 mol / L of hydrochloric acid with a pH of approximately 0 to alkaline solution tank 5 and brine tank 3, and start the cycle again.

[0152] (7) After 60 minutes of operation, stop the acid circulation in the alkali chamber and salt chamber and discharge the acid.

[0153] (8) Add pure water to alkaline solution tank 5 and brine tank 3 and start the circulation again.

[0154] (9) After running for 5 minutes, stop the acid circulation in acid chamber 17-2 and discharge the acid.

[0155] (10) Add pure water to acid tank 25 and start the circulation again.

[0156] (11) After running for 5 minutes, stop the circulation of the whole room and drain the water.

[0157] Next, bipolar electrodialysis was performed under the aforementioned operating conditions.

[0158] Five minutes after operation began, the membrane potential of cation exchange membrane C was confirmed to have returned to its initial value. At this state, 10 bipolar membranes (BP) were examined, but no bubbling was observed. Additionally, 11 cation exchange membranes were also examined, but no abnormalities were found in their appearance.

[0159] In addition, elemental analysis was performed on the sixth cation exchange membrane C1 from the anode side using a fluorescence X-ray analyzer, but calcium was below the detection limit.

[0160] Comparative Experiment

[0161] Using the same type of membrane and electrodialysis cell as in Example 1, and operating under the same conditions, the membrane potential of cation exchange membrane C still increased to three times its initial value after approximately 3 hours. It should be noted that at this point, no change was observed in the membrane potential of the bipolar membrane BP.

[0162] After the power was turned off, the alkaline chamber was cleaned using saline solution according to the following steps.

[0163] (1) Drain the entire room.

[0164] (2) Clean each tank and add pure water, and simultaneously supply pure water to the whole room for 5 minutes of water circulation.

[0165] (3) Stop water circulation only in acid chamber 17-2 and drain the water.

[0166] (4) Add salt water with pH 4 to acid tank 25 and start the circulation again.

[0167] (5) After running for 5 minutes, stop the water circulation in alkali chamber 19 and salt chamber 17-1 and drain the water.

[0168] (6) Add salt water with pH 4 to alkaline solution tank 5 and salt solution tank 3, and start the circulation again.

[0169] (7) After 60 minutes of operation, stop the circulation of brine in the alkali chamber and salt chamber and drain the brine.

[0170] (8) Add pure water to alkaline solution tank 5 and brine tank 3, and start the circulation again.

[0171] (9) After running for 5 minutes, stop the salt water circulation in acid chamber 17-2 and drain the salt water.

[0172] (10) Add pure water to acid tank 25 and start the circulation again.

[0173] (11) After running for 5 minutes, stop the circulation of the whole room and drain the water.

[0174] Next, bipolar electrodialysis was performed under the aforementioned operating conditions.

[0175] Five minutes after operation began, the membrane potential of cation exchange membrane C was measured, and the result showed that it only recovered to about 2.7 times the initial value.

[0176] <Experimental Example 2>

[0177] Using the same type of membrane and electrodialysis cell as in Example 1, and operating under the same conditions, the membrane potential of cation exchange membrane C still increased to three times its initial value after approximately 3 hours. It should be noted that at this point, no change was observed in the membrane potential of the bipolar membrane BP.

[0178] After power is turned off, perform acid cleaning according to the following steps.

[0179] (1) Drain liquid from each tank, then add pure water and simultaneously supply pure water to the whole room for 5 minutes of water circulation.

[0180] (2) Stop water circulation only in alkaline chamber 19 and drain water.

[0181] (3) Add 1 mol / L of hydrochloric acid to alkaline solution tank 5 and start the cycle again.

[0182] (4) After running for 5 minutes, stop the water circulation in acid chamber 17-2 and salt chamber 17-1 and drain the water.

[0183] (5) Add 1 mol / L of hydrochloric acid with a pH of approximately 0 to acid tank 25 and salt tank 3, and start the cycle again.

[0184] (6) After 60 minutes of operation, stop the acid circulation in acid chamber 17-2 and salt chamber 17-1 and discharge the acid.

[0185] (7) Add pure water to acid tank 25 and salt tank 3, and start the circulation again.

[0186] (8) After running for 5 minutes, stop the acid circulation in the alkali chamber 19 and discharge the acid.

[0187] (9) Add pure water to alkaline solution tank 5 and start the circulation again.

[0188] (10) After running for 5 minutes, stop the circulation of the whole room and drain the water.

[0189] Next, bipolar electrodialysis was performed under the same operating conditions as in Experimental Example 1.

[0190] Five minutes after operation began, the membrane potential of cation exchange membrane C was confirmed to have returned to its initial value. However, the membrane potential of bipolar membrane BP increased to twice its initial value. Upon unpacking and observing bipolar membrane BP under these conditions, bubbling was observed in all 10 membranes. It should be noted that 11 cation exchange membranes C were also observed, but no abnormalities were found in their appearance. Elemental analysis of the sixth cation exchange membrane C from the anode side using a fluorescence X-ray analyzer revealed that calcium levels were below the detection limit.

[0191] <Experimental Example 3>

[0192] Using the same type of membrane and electrodialysis cell as in Experimental Example 1, bipolar electrodialysis was performed under the same operating conditions as in Experimental Example 1.

[0193] Therefore, the membrane potential of cation exchange membrane C still increased to three times its initial value after about 3 hours. It should be noted that at this time, no change was observed in the membrane potential of bipolar membrane BP.

[0194] Here, without acid cleaning, the bipolar membrane was unpacked and observed; no abnormalities were found in its appearance. Furthermore, if the sixth cation exchange membrane was removed from the anode side, its surface was washed with water, and elemental analysis was performed using a fluorescence X-ray analyzer; calcium was detected.

[0195] <Experimental Example 4>

[0196] In this example, using Figure 3 The illustrated dual-chamber electrodialysis apparatus performs bipolar electrodialysis.

[0197] Prepare the following membrane.

[0198] Bipolar film BP;

[0199] The bipolar membrane prone to bubbling disclosed in Comparative Example 1 of Japanese Patent Application Publication No. 2002-306975

[0200] Cation exchange membrane C;

[0201] CMB made by ASTOM Co., Ltd.

[0202] like Figure 3 In this way, a benchtop pressure filter type bipolar membrane electrodialysis cell 1 was constructed, in which a cation exchange membrane C and a bipolar membrane BP were sequentially arranged between a pair of anodes and cathodes to form an alkaline chamber 19 and an acid chamber 17.

[0203] It should be noted that the number of cation exchange membranes C is set to 11, and the number of bipolar membranes BP is set to 10, with an effective membrane area of ​​0.55 dm² for each membrane. 2 .

[0204] In addition, to measure the membrane potential, platinum foils were typically inserted into the anode and cathode sides of the fourth bipolar membrane BP and the sixth cation exchange membrane C from the anode side, and connected to a voltmeter, respectively.

[0205] Prepare 500 mL of sodium lactate solution with a concentration of 1 equivalent / L in salt tank 3, and add calcium chloride in such a way that the calcium content is 250 ppm.

[0206] Prepare 500 ml of a sodium hydroxide aqueous solution with a concentration of 0.5 equivalents / L in alkaline solution tank 5.

[0207] Prepare 5 liters of hydrochloric acid solution with a pH of approximately 2 and a concentration of 0.01 equivalents / liter in the acid tank 7 used for cleaning.

[0208] Thus, with liquid contained in each tank, liquid is supplied from the brine tank 3 to the acid chamber 17 and from the alkali tank to the alkali chamber 19 at a linear velocity of 6 cm / sec. 5 liters of 0.1 equivalence / L sodium hydroxide aqueous solution are circulated in the anode chamber 13 and the cathode chamber 15. The operation is carried out at 25°C and a current density of 8 A / dm³. 2 Electrodialysis will now begin.

[0209] After 30 minutes, although no change was observed in the brine conductivity of the bipolar membrane, the membrane potential of the cation exchange membrane rose to 1.5 times its initial value. Assuming scale formation, the current was stopped, and acid cleaning was initiated.

[0210] The acid cleaning was performed according to the following steps. Figure 3 middle

[0211] (1) Drain the entire room.

[0212] (2) Close the outlet valves under each tank, open the pure water valve, and supply pure water to the alkali chamber 19 via the brine circulation pipe 20, acid chamber 17, and alkali circulation pipe 21.

[0213] (3) Close the pure water valve leading to acid chamber 17 and drain the water from acid chamber 17 and brine circulation piping 20.

[0214] (4) 0.01 mol / L of hydrochloric acid with a pH of about 2 is introduced from the cleaning acid tank 7 into the brine circulation pipe 20 via the cleaning acid circulation pipe 23 to start the acid cleaning cycle of the acid chamber 17.

[0215] (5) After running for 5 minutes, close the pure water valve to the alkali chamber 19 and drain the water from the alkali chamber 19 and the alkali circulation pipe 21.

[0216] (6) 0.01 mol / L of hydrochloric acid with a pH of about 2 is introduced from the cleaning acid tank 7 into the alkali circulation pipe 21 via the cleaning acid circulation pipe 23 to start the acid cleaning cycle of the alkali chamber 19.

[0217] (7) After 60 minutes of operation, stop the acid cleaning cycle of the alkali chamber 19 and drain the acid from the alkali chamber 19 and the alkali circulation piping 21.

[0218] (8) Open the pure water valve connected to the alkaline solution circulation pipe 21 to supply pure water.

[0219] (9) After 5 minutes of operation, stop the acid cleaning cycle of acid chamber 17 and drain the acid from acid chamber 17 and salt solution circulation piping 20.

[0220] (10) Open the pure water valve connected to the brine circulation pipe 20 to supply pure water.

[0221] (11) After 5 minutes of operation, stop supplying pure water to the whole room and drain the water from the whole room and the piping.

[0222] Next, bipolar electrodialysis was performed under the aforementioned operating conditions.

[0223] Five minutes after operation began, the membrane potential of cation exchange membrane C was confirmed to have returned to its initial value. At this state, 10 bipolar membranes (BP) were examined, but no bubbling was observed. Additionally, 11 cation exchange membranes were also examined, but no abnormalities were found in their appearance.

[0224] In addition, elemental analysis was performed on the sixth cation exchange membrane C1 from the anode side using a fluorescence X-ray analyzer, but calcium was below the detection limit.

[0225] Explanation of reference numerals in the attached figures

[0226] 1: Electrodialysis tank, 3: Brine tank, 5: Alkali tank, 7: Acid tank (or acid tank for acid cleaning), 10: Anode, 11: Cathode, 17: Acid chamber, 17-1: Brine chamber, 17-2: Acid chamber, 19: Alkali chamber, BP: Bipolar membrane, a: Anion exchanger, c: Cation exchanger, C: Cation exchange membrane.

Claims

1. A bipolar electrodialysis method using an electrodialysis apparatus, wherein a voltage is applied between the anode and cathode while a brine solution is circulated into the electrodialysis apparatus to perform electrodialysis, continuously generating acid and alkali from the salt. In the electrodialysis apparatus, a bipolar membrane with one side being an anion exchanger and the other side being a cation exchanger is disposed between the anode and cathode. The bipolar membrane is configured such that the anion exchanger side is located on the anode side, with the anion exchanger side of the bipolar membrane serving as the alkali chamber and the cation exchanger side serving as the acid chamber. Its features are, After a certain period of time, the power supply is stopped. Without removing the bipolar membrane and cation exchange membrane from the electrodialysis device, the alkaline chamber is acid-washed with an acidic aqueous solution with a pH below 2. After acid cleaning, the current was turned on again for electrodialysis, and... During the acid cleaning process, cleaning acid is first introduced into the acid chamber, and then cleaning acid is introduced into the alkali chamber with a delay. This maintains the acid concentration in the alkali chamber at a level no higher than that in the acid chamber while performing the acid cleaning. After the acid cleaning is completed, the acid used for cleaning the alkali chamber is first drained, and the liquid in the alkali chamber is replaced with a water washing solution. Then, the acid used for cleaning the acid chamber is drained after a delay, and the liquid in the acid chamber is replaced with a water washing solution.

2. The bipolar electrodialysis method according to claim 1, wherein, The electrodialysis device has a basic two-chamber structure with alternating cation exchange membranes and bipolar membranes arranged between the anode and cathode, and the brine solution is circulated to the acid chamber.

3. The bipolar electrodialysis method according to claim 1, wherein, The electrodialysis device also includes an anion exchange membrane with a three-chamber basic structure. The three-chamber basic structure has a cation exchange membrane, a bipolar membrane and an anion exchange membrane arranged sequentially between the anode and the cathode. A salt chamber is formed by the anion exchange membrane and the cation exchange membrane, and the brine solution is circulated to the salt chamber.

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