Water treatment system and method

Abstract

The electrodialysis apparatus of the present invention comprises a bipolar membrane and an anion exchange membrane alternately arranged between an anode and a cathode, and includes: an anode chamber defined by an anode and a bipolar membrane; a cathode chamber defined by a cathode and a bipolar membrane; and at least one set of acid chambers and alkali chambers disposed adjacently between the anode chamber and the cathode chamber, separated by an anion exchange membrane. The acid chamber is defined by an anion exchange membrane and a bipolar membrane disposed on the anode chamber side and is supplied with water to generate an acid solution through electrodialysis. The alkali chamber is defined by an anion exchange membrane and a bipolar membrane disposed on the cathode chamber side and is supplied with a treated solution containing acid and alkali to generate an alkali solution through electrodialysis.

Description

Technical Field

[0001] This invention relates to an electrodialysis apparatus, a water treatment system, and a method. Prior Technology

[0002] Developing technologies for reusing neutral salts, waste acids, and waste alkalis discharged as byproducts or waste from various processes is an important issue for various factories. For example, the semiconductor manufacturing process generates large quantities of waste liquid from the discharge of a mixture of hydrofluoric acid (HF) and buffered hydrofluoric acid (HF + NH4F: BHF). Technologies for recovering acid (hydrofluoric acid) or alkali (ammonia: NH4OH) from this waste liquid have been under review. This recovery also includes the recovery of fluorine (F) or ammonia (NH3).

[0003] Regarding the method for recovering hydrofluoric acid (or fluorine) from wastewater containing fluorine and ammonium (NH4+) (hereinafter, sometimes referred to as the treated liquid), the conventional method to date is the coagulation and precipitation method using calcium hydroxide (Ca(OH)2). However, the coagulation and precipitation method has the problem of generating a large amount of sludge (sludge) containing unreacted calcium hydroxide or fluorides that have not been separated by precipitation and require further treatment.

[0004] On the other hand, regarding the method of recovering ammonia from the treated liquid, the conventional method is the steam stripping method. However, the steam stripping method requires the addition of alkali to adjust the pH, which increases the cost of chemicals and the TDS (Total Dissolved Solids) value of the wastewater.

[0005] To address these issues, for example, Patent Document 1 proposes a method for recovering hydrofluoric acid and ammonia (or ammonia gas) from the treated liquid using conventional electrodialysis.

[0006] Patent Document 1 describes a water treatment system that uses an electrodialysis device to separately generate acid, alkali, and desalination solutions from the treated liquid. However, in water treatment systems used in semiconductor device manufacturing processes where the primary purpose is to recover acid and alkali from waste liquid, desalination solutions may not be necessary. Furthermore, equipment for further processing the generated desalination solutions may also be required. Therefore, water treatment systems that include such desalination solutions as byproducts may potentially increase the overall cost of the system. [Previous Technical Documents] [Patent Literature]

[0007] Patent Document 1: Japanese Patent No. 3519112 Summary of the Invention

[0008] The present invention was made to solve the problems of the prior art as described above and aims to provide an electrodialysis apparatus, water treatment system and method for recovering acid and alkali from the treated liquid at low cost.

[0009] To achieve the above objectives, the electrodialysis apparatus of the present invention is an electrodialysis apparatus for treating a liquid containing acid and alkali. The electrodialysis apparatus is configured to alternately arrange a bipolar membrane and an anion exchange membrane between an anode and a cathode and has: an anode chamber defined by the aforementioned anode and the aforementioned bipolar membrane; a cathode chamber defined by the aforementioned cathode and the aforementioned bipolar membrane; and at least one set of acid chambers and alkali chambers, which are disposed adjacent to each other between the aforementioned anode chamber and the aforementioned cathode chamber through the aforementioned anion exchange membrane. The acid chamber is defined by the aforementioned anion exchange membrane and the aforementioned bipolar membrane disposed on the side of the aforementioned anode chamber and is supplied with water to generate an acid solution by electrodialysis. The alkali chamber is defined by the aforementioned anion exchange membrane and the aforementioned bipolar membrane disposed on the side of the aforementioned cathode chamber and is supplied with the aforementioned liquid to generate an alkali solution by the aforementioned electrodialysis.

[0010] The water treatment system of the present invention comprises: the aforementioned electrodialysis apparatus; a pure water tank for storing the water supplied to the aforementioned acid chamber; a treated liquid tank for storing the treated liquid supplied to the aforementioned alkali chamber; an acid circulation path for returning and circulating an acid mixture discharged from the aforementioned acid chamber, comprising the aforementioned acid solution generated by the aforementioned electrodialysis and the aforementioned water remaining unrelated to the generation of the aforementioned acid solution, to the aforementioned pure water tank; and an alkali circulation path for returning and circulating an alkali mixture discharged from the aforementioned alkali chamber, comprising the aforementioned alkali solution generated by the aforementioned electrodialysis and the aforementioned treated liquid remaining unrelated to the generation of the aforementioned alkali solution, to the aforementioned treated liquid tank. The device includes a current measuring device for measuring the current flowing in the electrodialysis apparatus during the electrodialysis process; and a control device for controlling the operation of the pure water tank, the treated liquid tank, the acid circulation path, and the alkali circulation path, and receiving the current value measured by the current measuring device. The control device circulates the acid mixture using the acid circulation path and the alkali mixture using the alkali circulation path during the electrodialysis process. The electrodialysis ends when the current value remains within a predetermined range for a predetermined time. Then, the acid mixture in the pure water tank is discharged as the acid solution, and the alkali mixture in the treated liquid tank is discharged as the alkali solution.

[0011] The water treatment method of the present invention is used to treat a liquid containing acid and alkali. The method includes the following steps: preparing an electrodialysis device, which has a bipolar membrane and an anion exchange membrane alternately arranged between an anode and a cathode and has: an anode chamber defined by the aforementioned anode and the aforementioned bipolar membrane; a cathode chamber defined by the aforementioned cathode and the aforementioned bipolar membrane; and at least one set of acid chambers and alkali chambers, which are disposed adjacent to each other between the aforementioned anode chamber and the aforementioned cathode chamber through the aforementioned anion exchange membrane; supplying water to the aforementioned acid chamber defined by the aforementioned anion exchange membrane and the aforementioned bipolar membrane disposed on the side of the aforementioned anode chamber and generating an acid solution by electrodialysis; supplying the aforementioned liquid to be treated to the aforementioned alkali chamber defined by the aforementioned anion exchange membrane and the aforementioned bipolar membrane disposed on the side of the aforementioned cathode chamber and generating an alkali solution by the aforementioned electrodialysis. Simple Explanation of the Diagram

[0012] Figure 1 is a block diagram showing a structural example of a water treatment system in the first embodiment.

[0013] Figure 2 is a schematic diagram showing the general structure of the electrodialysis device shown in Figure 1.

[0014] Figure 3 is a block diagram showing a structural example of one of the second embodiments of the water treatment system.

[0015] Figure 4 is a block diagram showing an example of the connection of the current measuring device in the water treatment system of the second embodiment.

[0016] Figure 5 is a block diagram showing a structural example of one of the third embodiments of the water treatment system.

[0017] Figure 6 is a block diagram showing a structural example of one of the fourth embodiments of the water treatment system.

[0018] Figure 7 is a graph showing the variation of the ratio (molar ratio) of fluoride ions and ammonium in the acid mixture and the alkali mixture in the examples.

[0019] Figure 8 is a graph showing the changes in conductivity of the acid mixture and the alkali mixture in the embodiment.

[0020] Figure 9 is a graph showing the changes in the current value and cumulative current in the electrodialysis apparatus of the embodiment. Implementation

[0021] The invention will now be illustrated with figures. (First Implementation Form)

[0022] The first embodiment illustrates an example of a water treatment system incorporating the electrodialysis apparatus of the present invention.

[0023] Figure 1 is a block diagram showing a structural example of a water treatment system according to a first embodiment, and Figure 2 is a schematic diagram showing the general structure of the electrodialysis device of the present invention shown in Figure 1.

[0024] As shown in Figure 1, the water treatment system of the first embodiment includes: a treated liquid tank 11 for storing the treated liquid; a pure water tank 12 for storing water (pure water: H2O); an electrodialysis device 13 for being supplied with the treated liquid and water and for generating acid and alkali solutions from the treated liquid and water through electrodialysis; a power supply device 14 for supplying a predetermined DC voltage required for electrodialysis to the electrodialysis device 13; an acid solution tank 15 for storing the acid solution generated by the electrodialysis device 13; an alkali solution tank 16 for storing the alkali solution generated by the electrodialysis device 13; and a control device 17 for controlling the operation of the entire water treatment system shown in Figure 1.

[0025] The treated liquid tank 11, pure water tank 12, acid tank 15, and alkali tank 16 are respectively connected to the electrodialysis apparatus 13 through flow paths 18 having pumps and valves (not shown). The control device 17 is connected to the power supply device 14 and the pumps and valves in each flow path 18 via conventional wired and wireless communication equipment, and can control the operation of the power supply device 14 and the pumps and valves in each flow path 18. The control device 17 controls the on / off state of the power supply device 14 and uses the pumps and valves in each flow path 18 to control the supply and stop of the treated liquid from the treated liquid tank 11 to the electrodialysis apparatus 13, the supply and stop of water from the pure water tank 12 to the electrodialysis apparatus 13, the supply and stop of acid from the electrodialysis apparatus 13 to the acid tank 15, and the supply and stop of alkali from the electrodialysis apparatus 13 to the alkali tank 16.

[0026] In the water treatment system shown in Figure 1, the control device 17 supplies the required amount of treated liquid and pure water from the treated liquid tank 11 and the pure water tank 12 to the electrodialysis device 13. A DC voltage is applied to the electrodialysis device 13 by the power supply device 14, and electrodialysis is performed for a predetermined time. Then, at the end of the electrodialysis, the acid solution produced by the electrodialysis device 13 is recovered to the acid solution tank 15, and the alkali solution produced by the electrodialysis device 13 is recovered to the alkali solution tank 16. The control device 17 can be implemented by an information processing device (computer), which includes: a CPU (Central Processing Unit) that processes according to a predetermined program; a main memory that temporarily stores the information or data required by the CPU processing; a secondary memory (auxiliary memory) that stores the program and the aforementioned information or data; a communication device for sending and receiving information externally; various input devices such as a touchpad and keyboard; and various output devices such as a display device and a printer. The control device 17 does not need to be frequently connected to the water treatment system of the present invention. For example, it can be connected to the device to be changed only when the settings of the power supply device 14 and the pumps and valves of each flow path 18 are changed.

[0027] The liquid to be treated stored in the treated liquid tank 11 is a waste liquid obtained by mixing hydrofluoric acid (HF) and buffered hydrofluoric acid (BHF) discharged from, for example, a semiconductor device manufacturing process. At this time, the acid solution produced by the electrodialysis device 13 is hydrofluoric acid and the alkali solution is ammonia.

[0028] As shown in Figure 2, the electrodialysis apparatus 13 of the present invention has a multi-chamber structure formed by alternately arranging a bipolar membrane (BP membrane) 133 and an anion exchange membrane (A membrane) 134, which serve as ion exchange membranes, between an anode (+) 131 and a cathode (-) 132. The electrodialysis apparatus 13 includes: an anode chamber 135 defined by an anode 131 and a BP membrane 133; a cathode chamber 136 defined by a cathode 132 and a BP membrane 133; and at least one set of acid chambers 137 and alkali chambers 138 disposed between the anode chamber 135 and the cathode chamber 136. Figure 2 shows a structural example of three sets of acid chambers 137 and alkali chambers 138 disposed between the anode chamber 135 and the cathode chamber 136.

[0029] The anode 131 and cathode 132 use, for example, nickel (Ni) electrodes and titanium (Ti) platinum (Pt) electroplating electrodes. The anode chamber 135 and cathode chamber 136 are filled with electrode solutions formed from, for example, sodium hydroxide (NaOH) solution and sodium sulfide (Na2SO4) solution. A set of acid chambers 137 and alkali chambers 138 are adjacent to each other across membrane A 134, with acid chamber 137 positioned on the anode 131 side and alkali chamber 138 positioned on the cathode 132 side. Acid chamber 137 is defined by membrane A 134 and a BP membrane 133 positioned on the anode 131 side and is supplied with water (pure water: H2O) from a pure water tank 12. Alkali chamber 138 is defined by membrane A 134 and a BP membrane 133 positioned on the cathode 132 side and is supplied with a treated solution from a treated solution tank 11.

[0030] Membrane A 134 is an ion exchange membrane that allows anions to pass through while blocking cations. Membrane BP 133 is a composite membrane obtained by bonding a cation exchange membrane and membrane A. A cation exchange membrane is an ion exchange membrane that allows cations to pass through while blocking anions.

[0031] The BP membrane 133 exhibits a rectifying effect, allowing current flow when a positive potential difference is applied to the cation exchange membrane side and a negative potential difference (forward voltage) is applied to the A membrane side, while only a tiny current flows when a potential difference in the opposite direction (reverse voltage) is applied. However, when the reverse voltage exceeds a predetermined critical value, the water (H2O) inside the BP membrane 133 ionizes, producing hydrogen ions and hydroxyl ions, allowing a large current to flow. Most BP membranes 133 are respectively disposed between the anode 131 and the cathode 132 to apply the reverse voltage.

[0032] The electrodialysis apparatus 13 of this embodiment shown in Figure 2 starts electrodialysis by applying a predetermined DC voltage between the anode 131 and the cathode 132 by a power supply device 14 with the anode 131 side being positive and the cathode 132 side being negative.

[0033] At the start of electrodialysis, water within each BP membrane 133 is ionized into hydrogen ions (H+) and hydroxyl ions (OH-). The hydrogen ions then move to the acid chamber 137 (or cathode chamber 136), while the hydroxyl ions move to the alkali chamber 138 (or anode chamber 135). In the alkali chamber 138, the water is ionized by the treatment solution (HF, NH4F) into hydrogen ions (H+), fluoride ions (F-), and ammonium ions (NH4+). The fluoride ions, acting as anions, then pass through membrane A 134 and move to the acid chamber 137 adjacent to the anode 131.

[0034] As a result, hydrogen ions obtained by ionization of the BP membrane 133 in the acid chamber 137 combine with fluoride ions moving from the alkali chamber 138 to produce hydrofluoric acid (HF), which is discharged outside and recovered by the acid tank 15. On the other hand, ammonium and hydroxyl ions obtained by ionization of the BP membrane 133 on the cathode side in the alkali chamber 138 combine to produce ammonia water (NH4OH), which is discharged outside and recovered by the alkali tank 16.

[0035] Furthermore, as shown in Figure 2, during electrodialysis, hydroxyl ions obtained by ionization through the BP membrane 133 move to the anode chamber 135, and hydrogen atoms obtained by ionization through the BP membrane 133 move to the cathode chamber 136. Therefore, when the same electrode solution is used in the anode chamber 135 and the cathode chamber 136, the hydrogen ions and hydroxyl ions in each chamber can be balanced, for example, by circulating the electrode solution between the anode chamber 135 and the cathode chamber 136.

[0036] According to the water treatment system of the first embodiment, by alternately arranging a bipolar membrane (BP membrane) 133 and an anion exchange membrane (A membrane) 134 between the anode 131 and the cathode 132 and using an electrodialysis device 13 with an acid chamber 137 and an alkali chamber 138 to perform electrodialysis, an acid solution (hydrofluoric acid) and an alkali solution (ammonia) can be generated from the treated liquid and pure water.

[0037] Therefore, as described in Patent Document 1, acid and alkali can be recovered from the treated liquid without including desalination liquid as a byproduct, which may increase costs. Thus, acid and alkali can be recovered from the treated liquid at a low cost. (Second Implementation Form)

[0038] Figure 3 is a block diagram showing a structural example of one of the second embodiments of the water treatment system.

[0039] As shown in Figure 3, the water treatment system of the second embodiment differs from the water treatment system of the first embodiment in that it has an acid circulation path 21 that circulates the solution discharged from the acid chamber 137 of the electrodialysis device 13 back to the pure water tank 12 and an alkali circulation path 22 that circulates the solution discharged from the alkali chamber 138 of the electrodialysis device 13 back to the treated liquid tank 11.

[0040] In the water treatment system of the first embodiment shown in Figure 1, an acid mixture consisting of hydrofluoric acid (HF) produced by electrodialysis and pure water that has not been ionized by the BP membrane 133 can be discharged to the outside through the acid chamber 137 of the electrodialysis device 13. Similarly, an alkaline mixture consisting of ammonia water produced by electrodialysis and the treated liquid containing ammonium that has not combined with hydroxyl ions and fluoride ions that have not moved to or returned from the acid chamber 137 can be discharged to the outside through the alkaline chamber 138 of the electrodialysis device 13. That is, an acid mixture consisting of acid produced by electrodialysis and water (pure water) remaining unrelated to the production of the acid can be discharged from the acid chamber 137, and an alkaline mixture consisting of alkaline solution produced by electrodialysis and the treated liquid remaining unrelated to the production of the alkaline solution can be discharged from the alkaline chamber 138.

[0041] Therefore, in the second embodiment of the water treatment system, when electrodialysis is performed, the acid mixture discharged from the acid chamber 137 of the electrodialysis device 13 is returned to the pure water tank 12 via the acid circulation path 21 and then supplied back to the acid chamber 137 from the pure water tank 12. During electrodialysis, the acid mixture is concentrated by circulating it through the acid circulation path 21. When the acid mixture concentration reaches a predetermined value (or a predetermined range), it can be discharged (or extracted) from the pure water tank 12 as acid (hydrofluoric acid) for recovery. The acid recovered from the pure water tank 12 can be stored in the acid tank 15 as shown in FIG3. Pure water is resupplyed from an external tank (not shown) to the pure water tank 12 where the acid mixture has been discharged.

[0042] Similarly, in the second embodiment of the water treatment system shown in Figure 2, when electrodialysis is performed, the alkaline mixture discharged from the alkaline chamber 138 is returned to the treated liquid tank 11 via the alkaline circulation path 22 and then supplied back to the alkaline chamber 138 from the treated liquid tank 11. During electrodialysis, the alkaline mixture is concentrated to a certain alkaline (ammonia) concentration by circulating it using the alkaline circulation path 22. When the alkaline (ammonia) concentration reaches a predetermined value (or a predetermined range), the alkaline mixture can be discharged (or extracted) from the treated liquid tank 11 and recovered as alkaline (ammonia). The alkaline solution recovered from the treated liquid tank 11 can be stored in the alkaline tank 16 as shown in Figure 3. The treated liquid is then resupplied from an external tank (not shown) to the treated liquid tank 11 where the alkaline mixture has been discharged.

[0043] The acid mixture in the pure water tank 12 and the alkali mixture in the treated liquid tank 11 can be discharged at the same time or, for example, at different times depending on the acid concentration of the acid mixture and the alkali concentration of the alkali mixture.

[0044] Pumps and valves (not shown), controllable by control device 17, are respectively disposed in acid circulation path 21 and alkali circulation path 22. Control device 17 can use these pumps and valves to control the circulation and stopping of the acid mixture in acid circulation path 21 and the circulation and stopping of the alkali mixture in alkali circulation path 22. Furthermore, valves (not shown), controllable by control device 17, are disposed in treated liquid tank 11 and pure water tank 12. Control device 17 can use these valves to control the discharge and stopping of alkali mixture from treated liquid tank 11 and the discharge and stopping of acid mixture from pure water tank 12.

[0045] In the second embodiment of the water treatment system, when the required amount of treated liquid and water is supplied to the electrodialysis apparatus 13 from the treated liquid tank 11 and the pure water tank 12 and electrodialysis begins, the control device 17 uses the acid circulation path 21 to circulate the acid mixture discharged from the acid chamber 137 and the alkali circulation path 22 to circulate the alkali mixture discharged from the alkali chamber 138. On the other hand, when electrodialysis ends, the control device 17 stops the circulation of the acid mixture using the acid circulation path 21 and the circulation of the alkali mixture using the alkali circulation path 22, respectively. Then, the acid mixture in the pure water tank 12 is discharged and stored in the acid solution tank 15, and the alkali mixture in the treated liquid tank 11 is discharged and stored in the alkali solution tank 16. Other structures are the same as those in the first embodiment of the water treatment system shown in FIG1, so their description is omitted.

[0046] In such a structure, electrodialysis using the electrodialysis device 13 can be performed for a predetermined amount of time, as exemplified in the first embodiment. However, in the water treatment system of the second embodiment, the time to end electrodialysis is determined by observing the change in the current value flowing between the anode 131 and the cathode 132 of the electrodialysis device 13.

[0047] Furthermore, although shown in the embodiments described later, the point at which electrodialysis ends can also be determined by separately observing the changes in conductivity of the acid mixture and the alkali mixture. However, in order to measure conductivity using a conventional conductivity meter, the metal electrode portion of the conductivity meter must be inserted into both the acid mixture and the alkali mixture. In this case, there is a risk of corrosion of the electrode portion by hydrofluoric acid or other substances contained in the acid mixture; therefore, measures such as fluorine coating must be taken to protect the electrode portion from corrosion.

[0048] On the other hand, the current flowing between the anode 131 and cathode 132 of the electrodialysis apparatus 13 can be measured without immersing a portion of the current sensor or galvanometer (electrode portion) into the acid or alkali mixture, thus eliminating the need for corrosion protection measures. Furthermore, since electrodialysis typically uses a current sensor or galvanometer to monitor the current flowing between the anode 131 and cathode 132 of the electrodialysis apparatus 13, the timing for ending electrodialysis is determined by the change in this current value, eliminating the need for new measuring instruments such as conductivity meters. Therefore, the timing for ending electrodialysis should be determined by observing the change in the current flowing between the anode 131 and cathode 132 of the electrodialysis apparatus 13.

[0049] As shown in Figure 4, the water treatment system of the second embodiment includes a current measuring device 30 connected in series with the power supply unit 14 and the electrodialysis unit 13. The current measuring device 30 has a current sensor or ammeter that measures the current flowing between the power supply unit 14 and the electrodialysis unit 13, and transmits the current value measured by the current sensor or ammeter to the control unit 17 frequently or at predetermined intervals (e.g., approximately several seconds to several minutes) using conventional wired or wireless communication equipment. Figure 4 shows a structural example of a water treatment system independently having the current measuring device 30, but the current measuring device 30 can be a structure installed within the power supply unit 14 or a structure installed within the control unit 17.

[0050] Although also shown in the examples described later, at the start of electrodialysis, the alkaline mixture becomes alkaline due to the decrease in fluoride ion concentration and the increase in hydroxyl ion concentration obtained through ionization via the BP membrane. Furthermore, because the ammonium in the alkaline mixture becomes free ammonium, which does not contribute to conductivity, the conductivity gradually decreases and stabilizes at a low value after a period of time. On the other hand, the acidic mixture gradually increases in conductivity due to the decrease in low-conductivity pure water and the increase in high-conductivity hydrogen and fluoride ions, and stabilizes at a relatively high value after a period of time.

[0051] Therefore, the current flowing in the electrodialysis unit 13 gradually increases at the start of electrodialysis, then decreases at a certain point, and finally stabilizes at a relatively low value. Electrodialysis ends when the current value remains stable at this relatively low value for a predetermined period of time within a predetermined range. Whether the current value remains within the predetermined range for the predetermined period of time can be determined, for example, by whether the slope of the current value change is within the predetermined range. Therefore, the required concentration of hydrofluoric acid and ammonia water using electrodialysis can be completed in a minimal time. Thus, acid and alkali solutions can be efficiently recovered from the treated liquid.

[0052] The control device 17 of this embodiment stores the current value received by the current measuring device 30 and observes its changes, and can terminate electrodialysis when the slope of the change in the current value becomes within a predetermined range, as described above.

[0053] The water treatment system according to the second embodiment, in addition to having the same effect as the water treatment system of the first embodiment, can also efficiently recover acid and alkali from the treated liquid. (Third Implementation Form)

[0054] Figure 5 is a block diagram showing a structural example of one of the third embodiments of the water treatment system.

[0055] As shown in Figure 5, the third embodiment of the water treatment system differs from the first and second embodiments in that the concentrated liquid obtained by the reverse osmosis membrane device 40 is supplied through the treated liquid tank 11 to the alkali chamber 138 of the electrodialysis device 13 as the treated liquid. Figure 5 shows a structural example of the reverse osmosis membrane device 40 in the first embodiment of the water treatment system shown in Figure 1, but the reverse osmosis membrane device 40 can be the structure of the second embodiment of the water treatment system shown in Figure 3.

[0056] The flow path 41 connecting the reverse osmosis membrane unit 40 and the treated liquid tank 11 includes a pump and valve (not shown) that can control the supply and stop of concentrate from the reverse osmosis membrane unit 40 to the treated liquid tank 11 via a control device 17. The control device 17 and the pump and valve in the flow path 41 are connected via conventional wired or wireless communication equipment. In this embodiment, the control device 17 controls the supply and stop of concentrate to the treated liquid tank 11 by controlling the pump and valve in the flow path 41.

[0057] The reverse osmosis membrane unit 40 is a device that uses a conventional reverse osmosis (RO) membrane to produce two solutions: permeate (usually pure water) obtained by removing the solute from the supplied solution, and a concentrate obtained by concentrating the solute. Waste liquid obtained by mixing, for example, hydrofluoric acid (HF) and buffered hydrofluoric acid (BHF), is supplied to the reverse osmosis membrane unit 40. At this time, the concentrated solution obtained by concentrating hydrofluoric acid (HF) and ammonium fluoride (NH4F) is output from the reverse osmosis membrane unit 40. Other structures are the same as the water treatment system shown in the first embodiment of FIG1 or the second embodiment shown in FIG3, therefore their description is omitted.

[0058] According to the water treatment system of the third embodiment, the concentrated solution obtained by the reverse osmosis membrane device 40 is supplied to the electrodialysis device 13 as the treated solution, which reduces the volume of the treated solution supplied to the electrodialysis device 13. Therefore, in addition to the same effects as the first or second embodiment, the electrodialysis device 13 can also be miniaturized. Therefore, it is expected that the overall cost of the water treatment system will be further reduced. Furthermore, the higher the concentration of fluoride ions and ammonium ions in the concentrated treated solution, the more conducive it is to the miniaturization of the electrodialysis device 13, which is ideal. For example, it is preferable that the concentrations of fluoride ions and ammonium ions in the treated solution be 1000 mg / L or more and 5000 mg / L or more, respectively. (Fourth Implementation Form)

[0059] Figure 6 is a block diagram showing a structural example of one of the fourth embodiments of the water treatment system.

[0060] As shown in Figure 6, the water treatment system of the fourth embodiment has a structure that differs from the water treatment systems of the first to third embodiments in that it has a fluorine recovery device 51 for recovering fluorine from acid (hydrofluoric acid) stored in acid tank 15 and an ammonia recovery device 52 for recovering ammonia from alkaline solution (ammonia water) stored in alkaline tank 16.

[0061] Fluorine recovery device 51 can be, for example, a structure that recovers fluorine by reacting fluoric acid obtained from acid tank 15 with a calcium compound (e.g., calcium hydroxide) to form solid calcium fluoride (CaF2). Furthermore, ammonia recovery device 52 can be a structure that recovers ammonia gas by distilling ammonia water obtained from alkali tank 16.

[0062] Figure 6 shows a structural example of a fluorine recovery device 51 recovering fluorine from acid (hydrofluoric acid) stored in an acid tank 15, and an ammonia recovery device 52 recovering ammonia from alkaline solution (ammonia water) stored in an alkaline tank 16. The fluorine recovery device 51 can recover fluorine from the acid discharged from the acid chamber 137 of the electrodialysis device 13, and the ammonia recovery device 52 can recover ammonia from the alkaline solution discharged from the alkaline chamber 138 of the electrodialysis device 13.

[0063] Furthermore, Figure 6 shows a structural example of the water treatment system of the first embodiment shown in Figure 1, which includes a fluoride recovery device 51 and an ammonia recovery device 52. The fluoride recovery device 51 and ammonia recovery device 52 shown in Figure 6 can be the structure of the water treatment system of the second embodiment shown in Figure 3. In this case, the fluoride recovery device 51 can recover fluoride from the acid mixture (acid solution) discharged from the pure water tank 12, and the ammonia recovery device 52 can recover ammonia from the alkaline mixture (ammonia water) discharged from the treated liquid tank 11. Furthermore, the fluoride recovery device 51 and ammonia recovery device 52 shown in Figure 6 can be the structure of the water treatment system of the third embodiment shown in Figure 5.

[0064] The flow path 53 connecting the acid tank 15 and the fluorine recovery device 51 has a pump and valve (not shown) that can control the supply and stop of acid from the acid tank 15 to the fluorine recovery device 51 via the control device 17. Similarly, the flow path 54 connecting the alkali tank 16 and the ammonia recovery device 52 has a pump and valve (not shown) that can control the supply and stop of alkali from the alkali tank 16 to the ammonia recovery device 52 via the control device 17. The control device 17 and the pumps and valves of the flow paths 53 and 54 are connected via conventional wired or wireless communication equipment. In this embodiment, the control device 17 controls, for example, the supply and stop of acid to the fluorine recovery device 51 and the supply and stop of alkali to the ammonia recovery device 52 by controlling the pumps and valves of the flow paths 53 and 54.

[0065] According to the water treatment system of the fourth embodiment, by installing a fluoride recovery device 51 and an ammonia recovery device 52, not only acid (hydrofluoric acid) and alkaline solution (ammonia water) can be recovered, but also fluoride and ammonia gas can be recovered. Therefore, in addition to the same effects as the first to third embodiments, fluoride and ammonia gas can also be recovered from the treated liquid. [Example]

[0066] The embodiments of the present invention will then be illustrated with figures.

[0067] This embodiment uses the water treatment system of the second embodiment shown in Figure 3, and performs electrodialysis under the conditions shown in Table 1 below.

[0068]

[0069] As shown in Table 1, in this embodiment, a 1 mol sodium hydroxide (1N-NaOH) solution is used as the electrode solution. Pure water (H2O) is supplied to the acid chamber 137 of the electrodialysis apparatus 13 and circulated thereon. BHF waste liquid (buffered hydrofluoric acid waste liquid), obtained from mixed hydrofluoric acid and buffered hydrofluoric acid, is supplied to the alkali chamber 138 and circulated thereon. Furthermore, the pure water and BHF waste liquid are kept at room temperature (20 to 25°C). Next, the fluoride ion concentration of the acid mixture and the ammonium concentration of the alkali mixture, the conductivity of the acid mixture and the alkali mixture, and the current flowing in the electrodialysis apparatus 13 and their cumulative current are measured separately during electrodialysis.

[0070] Figure 7 shows the variation in the molar ratio of fluoride ions and ammonium in the acid and alkali mixtures of the embodiment. Figure 8 shows the variation in the conductivity of the acid and alkali mixtures of the embodiment. Figure 9 shows the variation in the flowing current value and cumulative current in the electrodialysis apparatus of the embodiment. Figures 7 to 9 respectively show an example of the experimental results of this embodiment.

[0071] Ion movement through the ion exchange membrane is essentially controlled by the electrodialysis described above. However, when there is a concentration difference between the solutions in two adjacent chambers separated by the ion exchange membrane, ions gradually move through the membrane via the known diffusion phenomenon caused by this concentration difference. That is, in the structure shown in Figure 2, where water (pure water: H2O) is supplied to the acid chamber 137 and the treated solution (HF, NH4F) is supplied to the alkali chamber 138, fluoride ions (F-) and ammonium ions (NH4+) diffuse from the high-concentration alkali chamber 138 to the low-concentration acid chamber 138, respectively. The ion movement obtained by diffusion occurs independently of whether electrodialysis is being performed or not. Figure 7 shows an example of fluoride ions and ammonium ions that have moved from the alkali chamber 138 to the acid mixture before the start of electrodialysis.

[0072] As shown in Figure 7, at the start of electrodialysis, fluoride ions (F-) migrate from the alkaline mixture to the acidic mixture as described above, thus increasing the proportion of fluoride ions in the acidic mixture over time. Then, after a period of time, when the amount of fluoride ions (F-) migrating from the alkaline mixture decreases, the increase in fluoride ions in the acidic mixture stops. Furthermore, since ammonium (NH4+) does not migrate during electrodialysis, the proportion of ammonium in the alkaline mixture increases due to the migration of fluoride ions (F-) from the alkaline mixture to the acidic mixture. In Figure 7, the proportion of ammonium in the acidic mixture temporarily decreases and then gradually increases over time, but this indicates that the increase in the proportion of ammonium in the acidic mixture is due to the aforementioned diffusion phenomenon.

[0073] As described above, at the start of electrodialysis, in the alkaline mixture, due to the decrease in fluoride ions and the increase in ammonium ions, the conductivity gradually decreases as shown in Figure 8 and stabilizes at a low value after a period of time. On the other hand, in the acidic mixture, due to the increase in hydrogen ions and fluoride ions, the conductivity of the acidic mixture gradually increases as shown in Figure 8 and stabilizes at a relatively high value after a period of time.

[0074] Therefore, as shown in Figure 9, the current value flowing in the electrodialysis device 13 gradually increases at the start of electrodialysis, then decreases at a certain point, and then stabilizes at a relatively low value. As shown in Figures 8 and 9, in this embodiment, approximately 40 minutes after the start of electrodialysis, the conductivity of the alkali mixture and the acid mixture stabilizes, and the current value flowing in the electrodialysis device 13 also stabilizes at a relatively low value.

[0075] As shown in Figure 9, when the current value stabilizes at a relatively low value and the current continues to flow in the electrodialysis apparatus 13, the cumulative current increases. However, once the current value stabilizes at a relatively low value, electrodialysis does not contribute to the concentration of hydrofluoric acid and ammonia, so it is not necessary to continue the electrodialysis. Furthermore, as shown in Figure 7, in the acid mixture, the ammonium ratio gradually increases over time due to diffusion. Therefore, if the fluoride ion ratio in the acid mixture stops increasing, it is best to stop the electrodialysis at that point and discharge the acid mixture and the alkali mixture separately from the electrodialysis apparatus 13. That is, electrodialysis is best stopped when the current value flowing in the electrodialysis apparatus 13 stabilizes at a relatively low value, for example, when the slope of the current value change becomes within a predetermined range.

[0076] Through this embodiment, the inventors confirmed that even if the electrodialysis device 13 has only one set of acid chamber 137 and alkali chamber 138, it is possible to obtain an acid solution with sufficient fluoride ion concentration and an alkali solution with sufficient ammonium concentration from the treated liquid and pure water.

[0077] While the invention of this application has been described above with reference to the embodiments and examples, the invention of this application is not limited to the above embodiments. Various modifications to the structure or details of the invention can be made within the scope of the invention in a manner understandable to those skilled in the art.

[0078] 11: Tank for the liquid being treated

[0079] 12: Pure water tank

[0080] 13: Electrodialysis device

[0081] 14: Power supply device

[0082] 15: Acid tank

[0083] 16: Alkali solution tank

[0084] 17: Control device

[0085] 18,41,53,54:Flow path

[0086] 21: Acid Cycle

[0087] 22: Alkali circulation circuit

[0088] 30: Current measuring device

[0089] 40: Reverse osmosis membrane device

[0090] 51: Fluorine Recovery Unit

[0091] 52: Ammonia Recovery Unit

[0092] 131: Anode (+)

[0093] 132: Cathode (-)

[0094] 133: Bipolar membrane (BP membrane)

[0095] 134: Anion exchange membrane (A membrane)

[0096] 135: Anode Chamber

[0097] 136: Cathode Chamber

[0098] 137: Acid Chamber

[0099] 138: Alkali Chamber

Claims

1. A water treatment system comprising: an electrodialysis apparatus for treating a liquid containing acid and alkali, wherein a bipolar membrane and an anion exchange membrane are alternately disposed between an anode and a cathode, and comprising: an anode chamber defined by the anode and the bipolar membrane; a cathode chamber defined by the cathode and the bipolar membrane; and at least one set of acid chambers and alkali chambers disposed adjacent to each other between the anode chamber and the cathode chamber, spaced apart by the anion exchange membrane; the acid chamber being defined by the anion exchange membrane and the bipolar membrane disposed on the anode chamber side, and supplied with water to generate an acid solution by electrodialysis; the alkali chamber being defined by the anion exchange membrane and the bipolar membrane disposed on the cathode chamber side, and supplied with the liquid to generate an alkali solution by electrodialysis; a pure water tank for storing the water supplied to the acid chamber; and a liquid to be treated tank for storing the liquid to be treated supplied to the alkali chamber. An acid circulation circuit returns the acid mixture discharged from the acid chamber, containing the acid solution produced by the electrodialysis and residual water unrelated to the production of the acid solution, back to the pure water tank for circulation; an alkali circulation circuit returns the alkali mixture discharged from the alkali chamber, containing the alkali solution produced by the electrodialysis and residual alkali solution unrelated to the production of the alkali solution, back to the treated liquid tank for circulation; a current measuring device measures the current flowing in the electrodialysis apparatus during electrodialysis; and a control device controls the operation of the pure water tank, the treated liquid tank, the acid circulation circuit, and the alkali circulation circuit, and receives the current value measured by the current measuring device. The control device circulates the acid mixture using an acid circulation circuit and the alkali mixture using an alkali circulation circuit during electrodialysis. The electrodialysis ends when the current value remains within a predetermined range for a predetermined time. Then, the acid mixture in the pure water tank is discharged as the acid solution, and the alkali mixture in the treated liquid tank is discharged as the alkali solution. The acid solution is hydrofluoric acid, and the alkali solution is ammonia.

2. As in request item 1, the water treatment system, wherein, The control device terminates the electrodialysis when the slope of the change in the current value becomes within a predetermined range.

3. The water treatment system of claim 1 or 2 further includes a reverse osmosis membrane device, which uses a reverse osmosis membrane to supply the concentrate obtained by concentrating the solute to the alkaline chamber as the liquid to be treated.

4. The water treatment system of claim 1 or 2, wherein the treated liquid contains fluoride and ammonium.

5. The water treatment system of claim 4, wherein the liquid being treated is waste liquid discharged during the manufacturing process of a semiconductor device.

6. The water treatment system of claim 4 further comprises: a fluoride recovery device that recovers fluoride by reacting hydrofluoric acid, which is the acid solution produced by the electrodialysis device, with a calcium compound; and an ammonia recovery device that recovers ammonia by distilling ammonia water, which is the alkali solution produced by the electrodialysis device.

7. As in request item 4, the water treatment system, wherein, The concentrations of fluoride ions and ammonium in the treated liquid are both above 1000 mg / L.

8. The water treatment system of claim 7, wherein the concentration of fluoride ions and the concentration of ammonium contained in the treated liquid are both above 5000 mg / L.

9. A water treatment method for treating a liquid containing acid and alkali, the water treatment method comprising the following steps: preparing an electrodialysis apparatus, a pure water tank, a liquid to be treated tank, an acid circulation path, an alkali circulation path, a current measuring device, and a control device; the electrodialysis apparatus having a bipolar membrane and an anion exchange membrane alternately arranged between an anode and a cathode, and having: an anode chamber defined by the anode and the bipolar membrane; a cathode chamber defined by the cathode and the bipolar membrane; and at least one set of acid chambers and alkali chambers arranged adjacent to each other between the anode chamber and the cathode chamber, separated by the anion exchange membrane; the pure water tank storing the water supplied to the acid chamber; the liquid to be treated tank storing the liquid to be treated supplied to the alkali chamber; the acid circulation path returning an acid mixture discharged from the acid chamber, comprising acid produced by electrodialysis and residual water unrelated to the production of the acid mixture, to the pure water tank for circulation; The alkali circulation system returns the alkali mixture discharged from the alkali chamber, containing the alkali solution produced by the electrodialysis and residual alkali solution unrelated to the production of the alkali solution, back to the treated liquid tank for circulation; the current measuring device measures the current flowing in the electrodialysis apparatus during the electrodialysis; the control device controls the operation of the pure water tank, the treated liquid tank, the acid circulation system, and the alkali circulation system, and receives the current value measured by the current measuring device; water is supplied to the acid chamber defined by the anion exchange membrane and the bipolar membrane disposed on the anode chamber side, and the acid solution is produced by the electrodialysis; the treated liquid is supplied to the alkali chamber defined by the anion exchange membrane and the bipolar membrane disposed on the cathode chamber side, and the alkali solution is produced by the electrodialysis; The control device circulates the acid mixture using an acid circulation circuit and the alkali mixture using an alkali circulation circuit during electrodialysis. The electrodialysis ends when the current value remains within a predetermined range for a predetermined time. Then, the acid mixture in the pure water tank is discharged as the acid solution, and the alkali mixture in the treated liquid tank is discharged as the alkali solution. The acid solution is hydrofluoric acid, and the alkali solution is ammonia.

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