Treatment method for water to be treated

Abstract

To recover strong acid by effectively separating both acids (strong acid and weak acid) from water to be treated, which contains acids with different degrees of dissociation and has a low heavy metal content, in a simple process.SOLUTION: The present invention provides a method for treating water to be treated, and a related technique thereof, in which a strong acid solution containing acid A that has permeated through anion exchange membranes 2 and 4 is obtained by electrodialysis using an electrodialysis device 10 provided with bipolar membranes 1, 3, 5 each having anion exchange layers 1a, 3a, 5a and cation exchange layers 1b, 3b, 5b, and anion exchange membranes 2, 4, for treating water to be treated containing the acid A and acid B having a larger acid dissociation constant than that of the acid A and having a heavy metal content of 1,000 ppm or less at a pH (X) that is larger than an acid dissociation constant pKaA of the acid A and smaller than an acid dissociation constant pKaB of the acid.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for treating water to be treated. [Background technology]

[0002] 2. Description of the Related Art It is known that electrodialysis using an electrodialysis apparatus is used as a desalination process, such as for producing fresh water from seawater.

[0003] Patent Document 1 describes a process in which an acid mixture such as a nitric acid and hydrofluoric acid waste liquid containing heavy metals (specific values ​​disclosed in the document as the minimum values ​​are iron (Fe) 12g / L, nickel (Ni) 0.2g / L, and chromium (Cr) 2g / L in Example 4) is supplied to a first electrodialysis device, and one of the acids contained in the acid mixture, specifically hydrofluoric acid, is passed through the deoxidation chamber of the first electrodialysis device, while the other acid is moved to the concentration chamber side of the first electrodialysis device (Claim 1 and Example 4). This process is referred to as Process 1 in Patent Document 1.

[0004] Patent Document 1 describes a process in which one of the acids is passed through a deoxidizing chamber of a first electrodialysis device, and the other acid, specifically nitric acid, is moved to the concentration chamber 3 side of the first electrodialysis device, and then the one of the acids (hydrofluoric acid) passing through the deoxidizing chamber is neutralized with an alkali to precipitate and separate heavy metals, etc. (Claim 1 and Examples). This process is referred to as Process 2 in Patent Document 1.

[0005] Thereafter, the filtrate obtained by the precipitation and separation of the heavy metals and the like is supplied to a second electrodialysis device equipped with a bipolar membrane, a cation exchange membrane, and an anion exchange membrane to separate and recover one of the acids, and the desalted water passed through the second electrodialysis device is supplied to the concentration compartment side (containing the other acid, specifically nitric acid) of the first electrodialysis device, or water is supplied to the concentration compartment side to separate and recover the other acid from the concentration compartment of the first electrodialysis device (Claim 1). This step is referred to as Step 3 in Patent Document 1.

[0006] Patent Document 1 states that the invention described in Patent Document 1 can be applied to mixed acids that are mixtures of two or more acids with different degrees of dissociation (

[0075] ). Patent Document 1 also states that it has become possible to separately recover each of the two types of acids in a mixed acid (

[0078] ).

[0007] The bipolar membrane used in the second electrodialysis device is a composite membrane in which a cation exchange membrane and an anion exchange membrane are bonded together, and has the function of dissociating water into protons and hydroxide ions. An electrodialysis device equipped with this is used to produce acids and alkalis from neutral salts (for example, Patent Document 2). [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 9-10557 [Patent Document 2] JP 2009-39694 A Summary of the Invention [Problem to be solved by the invention]

[0009] Nitric acid waste is widely generated in industry (e.g., silver powder production). Given the current demand for SDGs, it is desirable to recover and recycle nitric acid from this waste.

[0010] Patent Document 1 describes that it has become possible to separately recover the two types of acids in a mixed acid. Note that the invention of Patent Document 1 targets an acid mixture containing a considerable amount of heavy metals (a specific concentration value of several tens of g / dL) to be treated.

[0011] For this reason, one of the acids (hydrofluoric acid) that passed through the deoxidizing chamber in process 1 contains a large amount of heavy metals, which are precipitated with alkali in process 2. Then, in process 3, the one of the acids is recovered and the desalted water generated in the process is mixed with nitric acid. There are wastewater standards for heavy metals from the viewpoint of environmental load, and they need to be recovered by precipitating them as in process 2 above.

[0012] As described above, the invention of Patent Document 1 is intended to treat acid waste liquid having a very high heavy metal content, and therefore requires a long process consisting of steps 1 to 3. However, this method is not suitable as a method for treating acid waste liquid generated in industry that does not have a very high heavy metal content.

[0013] One objective of the present invention (corresponding to embodiment 1 described below) is to provide a method for effectively separating and recovering strong acids from treated water containing acids with different degrees of dissociation and having a low heavy metal content (the acid with the smaller acid dissociation constant pKa will be referred to as a strong acid in a relative comparison sense, and the acid with the larger pKa will be referred to as a weak acid; the same applies below) in a simple process.

[0014] Another object of the present invention (the invention corresponding to embodiment 2 described below) is to provide a method for effectively separating strong acids and weak acids in a simple process and recovering the strong acids from treated water that contains the above-mentioned multiple acids, has a pH equal to or greater than the acid dissociation constant pKa of the weak acids, and has a low heavy metal content.

[0015] Yet another object of the present invention (corresponding to embodiment 3 described below) is to provide a method for effectively separating strong acids and weak acids and recovering the strong acids in a simpler process for treated water that contains the above-mentioned multiple acids, has a pH equal to or greater than the acid dissociation constant pKa of the weak acids, and has a low heavy metal content. [Means for solving the problem]

[0016] The present invention is as follows. [1] The acid dissociation constant pKa of the acid A is determined for a water to be treated, the water containing an acid A and an acid B having a larger acid dissociation constant than the acid A, and the heavy metal content is 1000 ppm or less. A and the acid dissociation constant pKa of the acid B is B The method for treating water to be treated comprises electrodialyzing the water at a smaller pH (X) using a bipolar membrane having an anion exchange layer and a cation exchange layer, and an electrodialysis device having an anion exchange membrane, thereby obtaining a strong acid solution containing acid A that has permeated the anion exchange membrane.

[0017] [2] The present invention relates to a method for preparing a solution containing an acid A and an acid B having a larger acid dissociation constant than that of the acid A, the solution having a heavy metal content of 1000 ppm or less, and a pH value equal to or lower than the acid dissociation constant pKa of the acid B. B A first electrodialysis step in which the above-mentioned water to be treated is electrodialyzed using an electrodialysis device equipped with a bipolar membrane having an anion exchange layer and a cation exchange layer to obtain an acid solution having a lower pH than the water to be treated and containing the acid A and the acid B; With respect to the acid solution, the acid dissociation constant pKa of the acid A A and the acid dissociation constant pKa of the acid B is B a second electrodialysis step in which the solution is electrodialyzed at a pH (X) smaller than the above pH (X) using an electrodialysis device equipped with a bipolar membrane having an anion exchange layer and a cation exchange layer, and an anion exchange membrane, to obtain a strong acid solution containing acid A that has permeated the anion exchange membrane; The method for treating water to be treated comprises the steps of:

[0018] [3] The present invention relates to a method for preparing a solution containing an acid A and an acid B having a larger acid dissociation constant than that of the acid A, the solution having a heavy metal content of 1000 ppm or less, and a pH value equal to or lower than the acid dissociation constant pKa of the acid B. B The method includes an electrodialysis step of electrodialyzing the above-mentioned treated water using an electrodialysis device including a bipolar membrane having an anion exchange layer and a cation exchange layer, a cation exchange membrane, and an anion exchange membrane, In the early stage of the electrodialysis step, a high pH solution having a higher pH than the water to be treated before the start of the electrodialysis is obtained, while the pH of the water to be treated is adjusted to the acid dissociation constant pKa Aand the acid dissociation constant pKa of the acid B is B Lowering the pH to a smaller value (X), In the latter stage of the electrodialysis process, the acid A in the water to be treated having the pH (X) is permeated through the anion exchange membrane to obtain a strong acid solution. A method for treating the water to be treated.

[0019] [4] The electrodialysis device includes a plurality of the bipolar membranes, The method for treating water to be treated according to any one of [1] to [3], wherein the anion exchange membrane is disposed between two of the bipolar membranes.

[0020] [5] the electrodialysis apparatus has a membrane structure in which the bipolar membrane, the anion exchange membrane, and the cation exchange membrane are repeatedly arranged in this order; the anion exchange membrane faces the cation exchange layer of the bipolar membrane and faces the cation exchange membrane; The method for treating water to be treated according to [3], wherein the cation exchange membrane faces the anion exchange layer of the bipolar membrane.

[0021] [6], The method for treating water to be treated according to any one of [1] to [5], wherein the acid A is at least one selected from the group consisting of nitric acid, hydrochloric acid, and sulfuric acid, and the acid B is at least one selected from the group consisting of formic acid and acetic acid.

[0022] [7] The pKa B from the pKa A The method for treating water to be treated according to any one of [1] to [6], wherein the value obtained by subtracting

[0023] [8] The pH(X) to the pKa A The method for treating water to be treated according to any one of [1] to [7], wherein the value obtained by subtracting

[0024] [9] The pKa B The method for treating water to be treated according to any one of [1] to [8], wherein the value obtained by subtracting the pH(X) from the pH(X) is 0.4 or more.

[0025]

[10] The method for treating water to be treated according to any one of [1] to [9], wherein Q1 is 30 times or more of Q2, where Q1 is an amount of acid A permeating through the anion exchange membrane and Q2 is an amount of acid B permeating through the anion exchange membrane.

[0026]

[11] The method for treating water to be treated according to any one of [1] to

[10] , wherein the concentration α of the acid A in the strong acid solution is 52 g / L or more.

[0027]

[12] The method for treating water to be treated according to any one of [1] to

[11] , wherein the content of heavy metals in the water to be treated that is subjected to the electrodialysis is 100 ppm or less.

[0028]

[13] The electrical resistance of the anion exchange membrane is 10 Ωcm 2 The method for treating water to be treated according to [3] above. Effect of the Invention

[0029] According to the present invention (corresponding to embodiment 1 described below), it is possible to effectively separate both acids (strong acid and weak acid) from treated water containing acids with different degrees of dissociation in a simple process, and recover the strong acid.

[0030] According to the present invention (corresponding to embodiment 2 described below), for treated water containing the above-mentioned multiple acids and having a pH equal to or higher than a predetermined value, strong acids and weak acids can be effectively separated in a simple process, and the strong acid can be recovered.

[0031] According to the present invention (corresponding to embodiment 3 described below), for treated water containing multiple acids and having a pH equal to or higher than a predetermined value, strong acids and weak acids can be effectively separated and the strong acid recovered in a simpler process. [Brief description of the drawings]

[0032] [Figure 1] FIG. 1 is a diagram showing an outline of an electrodialysis apparatus and electrodialysis according to the first embodiment. [Diagram 2] FIG. 2 is a partially enlarged view of FIG. 1 in the first embodiment. [Diagram 3] FIG. 3 is a diagram showing an overview of an electrodialysis device and electrodialysis used in the first electrodialysis step in the second embodiment. [Figure 4] FIG. 4 is a partially enlarged view of FIG. 3 in the second embodiment. [Diagram 5] FIG. 5 is a diagram showing an outline of an electrodialysis device and electrodialysis according to the third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] Hereinafter, an embodiment of the present invention will be described, but the present invention is not limited to the following embodiment.

[0034] In the description of the first embodiment, a case will be described in which the treatment target is water to be treated that contains an acid A and an acid B having an acid dissociation constant larger than that of the acid A. The first embodiment utilizes the basic mechanism of the present invention, and the present invention is not limited to the first embodiment.

[0035] In the explanation of the second and third embodiments, the case where the treatment target is water to be treated that contains acid A and acid B and has a pH value equal to or higher than a predetermined value will be explained.

[0036] In the second embodiment, the case where electrodialysis is performed twice on the water to be treated will be described. Specifically, the first electrodialysis produces an acid solution containing acids A and B and having a lower pH than the water to be treated, and the second electrodialysis separates the acid solution into a strong acid solution and a weak acid solution. On the other hand, in the description of the third embodiment, a case will be described in which the two electrodialysis steps in the second embodiment are combined into one step.

[0037] The content of the configuration of the invention of Embodiment 1 can be incorporated into the inventions of Embodiments 2 and 3 described below. Therefore, in Embodiments 2 and 3 described below, the description of the configuration similar to that of Embodiment 1 may be omitted.

[0038] In this specification, unless otherwise specified, the unit of "concentration" is g / L. "X to Y" represents X or more and Y or less (X and Y are numerical values satisfying X < Y).

[0039] [Embodiment 1] The method for treating water to be treated according to Embodiment 1 includes acid A and acid B having an acid dissociation constant larger than that of acid A, and for the water to be treated having a heavy metal content of 1000 ppm or less, the acid dissociation constant pKa of acid A A is larger and the acid dissociation constant pKa of acid B B is smaller, and has an electrodialysis step of performing electrodialysis using an electrodialysis device including a bipolar membrane having an anion exchange layer and a cation exchange layer, and an anion exchange membrane at pH (X). By this electrodialysis step, acid A in the water to be treated selectively permeates through the anion exchange membrane to obtain a strong acid solution. Further, among the water to be treated, a weak acid solution is obtained as a residual solution that did not permeate through the anion exchange membrane such as acid B.

[0040] The water to be treated according to Embodiment 1 is not limited as long as it contains acid A and acid B having an acid dissociation constant larger than that of acid A. The acid dissociation constant (pKa) in this specification is a value in water at 25°C.

[0041] On the other hand, in Embodiment 1, an anion exchange membrane and a bipolar membrane are used for separating a strong acid (acid A) and a weak acid (acid B). The heavy metal content in the water to be treated before the electrodialysis step targeted by Embodiment 1 is 1000 ppm or less, and these membranes are not easily clogged. The heavy metal content is preferably 500 ppm or less, more preferably 100 ppm or less, still more preferably 50 ppm, and particularly preferably 10 ppm or less. In the present invention, the heavy metal refers to a metal having a specific gravity of 4 or more.

[0042] The pKa Bfrom the pKa A The value obtained by subtracting pKa is usually 0.8 or more, and preferably 4 or more. B -pKa A If the pKa is large, it is easy to carry out electrodialysis at a pH (pH(X) described later) that allows a difference in the degree of dissociation between the two acids, i.e., acid A and acid B, and to separate them. The pKa is usually 16 or less. When the water to be treated contains multiple types of acid A, the pKa B -pKa A When calculating pKa A The highest pKa of the acid B is used as the standard. If a specific acid among multiple types is to be recovered, the pKa of that acid is used as the standard. If multiple types of acid B are included, the acid with the smallest pKa is used as the standard. The above is based on the pH(X) and pKa A Difference and pKa B The same applies to the difference between pH(X) and pH(X).

[0043] The acids A and B are not particularly limited as long as their acid dissociation constants are far apart from each other to a certain extent as described above and can be separated by electrodialysis at pH(X) as described below. In addition, the acid A in the present invention is typically an acid with a small pKa, such as nitric acid, specifically an acid with a pKa of 2.5 or less (usually the acid dissociation constant pKa of acid A is 2.5 or less). A (pKa is -5 or more). Acid A is, for example, nitric acid (pKa A =-1.8), hydrochloric acid (pKa A =-3.7) and sulfuric acid (pKa A =1.96). In the present invention, the acid B is typically an acid having a large pKa, such as formic acid, specifically an acid having a pKa of 3.3 or more (usually the acid dissociation constant pKa B is 12 or less). Acid B is, for example, formic acid (pKa B =3.75) and acetic acid (pKa B =4.8). Hereinafter (including the description of the invention in embodiments 2 and 3), an embodiment may be described using an example in which acid A is nitric acid and acid B is formic acid.

[0044] In addition, acids A and B are dissociated into ions at a predetermined ratio in water such as the water to be treated according to the pH of the water. As described below, in the present invention, electrodialysis is performed in a state where most of acid A is dissociated (in the form of a conjugate base), and the conjugate base passes through an anion exchange membrane, and is then given a proton, which can be recovered as acid A. In the present invention, if a substance measured as a component of various liquids such as the water to be treated or a strong acid liquid is itself an acid A as exemplified above, it is considered to be acid A. In the case of an ion (conjugate base), if a proton is added so that the charge is zero, the ion is considered to be acid A. The same applies to acid B.

[0045] In the water to be treated according to the present embodiment 1, the concentration α of acid A is not particularly limited, but is, for example, 20 to 95 g / L. The concentration β of acid B is also not particularly limited, but is, for example, 2 to 30 g / L. The water to be treated may contain other components, and examples of such components include formaldehyde, ammonia, sodium hydroxide, hexamethylenetetramine, and alcohol compounds. These can be easily separated from acid A in the electrodialysis step according to the present embodiment 1 by utilizing the difference in charge.

[0046] In the electrodialysis step, the acid dissociation constant pKa of the acid A is A and the acid dissociation constant pKa of the acid B is greater than B The electrodialysis is performed using a bipolar membrane equipped with an anion exchange layer and a cation exchange layer, and an electrodialysis device equipped with an anion exchange membrane, at a pH (X) smaller than the value of (1). If the pH of the water to be treated is not pH (X), hydrochloric acid, sodium hydroxide, or the like is added to adjust the pH to (X), and then electrodialysis is performed.

[0047] pH(X) is calculated based on the acid dissociation constant pKa of the acid A. A and the acid dissociation constant pKa of the acid B is B The reason for making it smaller is that acid A dissociates (e.g., nitric acid is converted to H + and NO3 -This is because acid B does not dissociate into 1HCOOH (formic acid remains as HCOOH, for example), whereas acid B does not dissociate into 1HCOOH (formic acid remains as HCOOH, for example).

[0048] The pH(X) is then subtracted from the pKa to ensure that the two have different acid dissociation behaviors. A is preferably 0.4 or more (usually less than 4), B The value obtained by subtracting the pH(X) from the above is preferably 0.4 or more (usually less than 4). It is more preferable that both regulations are satisfied.

[0049] The reason for setting the pH (X) as above and the mechanism of separation of acids A and B by electrodialysis in the present embodiment 1 will be described in detail below.

[0050] FIG. 1 is a plan view showing an overview of an electrodialysis device 10 and electrodialysis that can be employed in the first embodiment. FIG. 2 is a partially enlarged view of FIG. 1 in the first embodiment.

[0051] For the contents not described below, for example, the electrodialysis device 10 may be a known electrodialysis device, and the bipolar membrane may be a known bipolar membrane. For example, the bipolar membrane described in Patent Document 2 may be used. A known anion exchange membrane may also be used.

[0052] 1, an electrodialysis device 10 that can be used in the first embodiment includes anion exchange membranes 2 (left side) and 4 (right side). The anion exchange membranes 2 and 4 face the cation exchange layers 1b and 3b of the bipolar membranes 1 (left end) and 3 (center).

[0053] As shown in FIG. 1, the electrodialysis device 10 in the first embodiment includes a plurality of the bipolar membranes, and the anion exchange membranes 2 and 4 are disposed between two of the bipolar membranes (the bipolar membranes and the anion exchange membranes are disposed alternately and repeatedly).

[0054] As shown in Fig. 1, in the electrodialysis device 10 of the first embodiment, the bipolar membranes 1 and 5 are arranged at both ends of a repeating structure in which the anion exchange membranes and the bipolar membranes are alternately arranged. The anode 11 and the cathode 12 of the electrodialysis device 10 are arranged so as to further sandwich the bipolar membranes 1 and 5 at both ends.

[0055] In FIG. 1, the anode 11 is placed on the far left, the left-end bipolar membrane 1 is placed to its right, the left-side anion exchange membrane 2 is placed to its right, the central bipolar membrane 3 is placed to its right, the right-side anion exchange membrane 4 is placed to its right, the right-end bipolar membrane 5 is placed to its right, and the cathode 12 is placed on the far right.

[0056] In Fig. 1, a flow path (1) is formed between the left-end bipolar membrane 1 and the left anion exchange membrane 2. A flow path (2) is formed between the left anion exchange membrane 2 and the central bipolar membrane 3. A flow path (3) is formed between the central bipolar membrane 3 and the right anion exchange membrane 4. A flow path (4) is formed between the right anion exchange membrane 4 and the right-end bipolar membrane 5.

[0057] The water to be treated is supplied from the water tank 6 to the flow paths (2) and (4). Focusing on the flow path (2), by setting the pH (X) as described above, in the water to be treated flowing through the flow path (2), nitric acid (acid A) is converted into protons, H + and NO3 - On the other hand, formic acid (acid B) does not dissociate and remains as HCOOH (Figure 2(a)).

[0058] When the power supply of the electrodialysis device 10 is turned on, a current flows approximately perpendicular to the anion exchange membrane and the bipolar membrane, and the NO3 - passes through the left anion exchange membrane 2 and moves toward the anode 11 on the left side of FIG. 1 (FIG. 2(b)).

[0059] Here, the anion exchange membranes 2 and 4 may be conventionally known, but their electrical resistance should be less than 2 Ωcm. 2 It is preferable that the resistance is 10Ωcm or more. 2More preferably, the electrical resistance is 60 Ωcm or more. 2 This makes it more selective for only anions to pass, and NO3 - and HCOOH can be clearly separated.

[0060] However, the left end bipolar membrane 1 is disposed in front of the anode 11. Therefore, NO3 - tries to move toward the anode 11, but is repelled by the left end bipolar membrane 1 and remains in the flow channel (1) (FIG. 2(c)).

[0061] On the other hand, the cation exchange layer 1b of the left end bipolar membrane 1 faces the left anion exchange membrane 2, and H + is supplied to flow path (1). As a result, NO3 - and H + and combine to become nitric acid. In the first embodiment, nitric acid itself is obtained, not a nitrate. This means that in the first embodiment, it is not necessary to perform a step of obtaining nitric acid from a metal salt of nitric acid. If the water in the flow path (1) is collected in the strong acid liquid tank 7, a liquid (strong acid liquid) in which the acid A has selectively migrated from the water to be treated is obtained.

[0062] The formic acid remains in the flow path (2). Moreover, the state is not dissociated with the acid and remains as HCOOH. This means that the water in the flow path (2) contains formic acid itself, not formate salts. If the water in the flow path (2) is collected in the water tank 6, a liquid (weak acid liquid) with a lower concentration of acid A than before electrodialysis is obtained. If a certain amount of water to be treated is put into the water tank 6 and the water (or water being treated) is continuously circulated in the flow shown in FIG. 1, the liquid passing through the water tank 6 will change to a liquid with a lower concentration of acid A (weak acid liquid). Therefore, in this specification, the water tank 6 is also called the weak acid liquid tank 6.

[0063] Explaining with reference to Fig. 1, the water to be treated is supplied from the water to be treated tank 6 to flow path (2) and flow path (4), passes through both flow paths, and is then returned to the water to be treated tank 6. Nitric acid is then produced in flow path (1) and flow path (3), and the nitric acid moves together with moisture to the strong acid tank 7. The moisture is supplied from the strong acid tank 7 to flow paths (1) and flow path (3) together with nitric acid, and newly produced nitric acid (which has moved from flow paths (2) and (4) and has been given protons) is taken in and moved again to the strong acid tank 7.

[0064] At the same time, in streams (2) and (4), NO3 - The water is then transferred to the tank 6 for treated water. The water is then re-supplied from the tank 6 to the flow paths (2) and (4) for circulation. - As a result of this migration of nitric acid into flow paths (1) and (3), the pH of the water to be treated flowing through flow paths (2) and (4) increases, the proportion of nitric acid that dissociates increases, and this migration into flow paths (1) and (3) progresses further.

[0065] In addition, H generated by dissociation of nitric acid in flow path (2) + 1. The OH ions supplied from the central anion exchange layer 3a arranged on the left side of the central bipolar membrane 3 in FIG. - It combines with to become water.

[0066] The space between the anode 11 and the left-end bipolar membrane 1 (the left-end anion exchange layer 1a) is usually filled with an electrode solution (e.g., an aqueous sodium hydroxide solution). The same is true for the space between the cathode 12 and the right-end bipolar membrane 5 (the right-end cation exchange layer 5b).

[0067] A known pump may be used to circulate the water to be treated or the water being treated.

[0068] According to the first embodiment, various operations such as steps 2 and 3 in Patent Document 1 are not required, and an increase in costs can be avoided, and the strong acid (acid A) can be satisfactorily separated by a single operation (the electrodialysis step). In other words, according to the first embodiment, it is possible to effectively separate both acids (strong acid and weak acid) from the water to be treated that contains acids with different degrees of dissociation, and recover the strong acid (acid A) with a small amount of the weak acid, acid B, mixed therein.

[0069] Specifically, the electrodialysis process produces a strong acid solution having a large ratio δ (α / β) between the concentration α of acid A and the concentration β of acid B, and a weak acid solution having a smaller ratio δ compared to the treated water before the electrodialysis process.

[0070] As explained above, what is supplied to the water to be treated tank 6 is basically the water to be treated, and acid A alone is not supplied. In other words, acid A (e.g., nitric acid) that was originally contained in the water to be treated is separated from acid B and recovered in the strong acid solution.

[0071] As explained above, what is supplied to the tank 6 for treated water is basically the water to be treated, and acid B alone is not supplied. In other words, acid B (e.g., formic acid) originally contained in the water to be treated is separated from acid A and recovered in the weak acid solution.

[0072] The value of the ratio δ (α / β) in the strong acid solution obtained in the present embodiment 1 is preferably 10 or more, more preferably 30 or more. The ratio δ is usually 300 or less, and for example, 200 or less. The concentration α of acid A in the strong acid solution is preferably 52 g / L or more (usually 180 g / L or less). The content of heavy metals in the water to be treated is low as described above, and no metal is added in the electrodialysis of the present embodiment 1, so the content of heavy metals in the strong acid solution is low (specifically, usually 1000 ppm or less, preferably 500 ppm or less, more preferably 100 ppm or less, even more preferably 50 ppm, and particularly preferably 10 ppm or less). It is more preferable to satisfy all of the above specifications for the ratio δ, concentration α, and heavy metal content. The concentration β of acid B in the weak acid solution may be 2 g / L or more (usually 30 g / L or less).

[0073] In addition, since the present embodiment 1 can effectively separate acids A and B in the water to be treated, when the amount of acid A permeating the anion exchange membrane in the electrodialysis step is Q1 and the amount of acid B permeating the anion exchange membrane is Q2, Q1 is preferably 30 times or more, more preferably 40 times or more (usually 180 times or less) than Q2. In this specification, the amount of acid A, etc. permeated through various membranes can be determined from the concentration and liquid volume of each liquid, such as a strong acid liquid, before and after electrodialysis.

[0074] The conditions for the electrodialysis step are not limited. An example is as follows.

[0075] Voltage of current applied during electrodialysis: 25~35V Current applied during electrodialysis: Maximum 4.4 A (change as appropriate depending on the acid A concentration α and acid A / B ratio δ in the target strong acid solution) Time for applying electric current: Change appropriately depending on the target acid A concentration α and acid A / B ratio δ in the strong acid solution. It also varies depending on the amount of water to be treated, but as an example, the time per unit amount of water to be treated is in the range of 0.5 to 4 hours / L. Temperature at which electrodialysis is carried out (temperature of the water to be treated): Taking into consideration the effects on various membranes such as the anion exchange membrane used in this embodiment, it can be set to room temperature, for example, 2 to 40°C.

[0076] The end point of electrodialysis can be adjusted by the target concentration α of acid A in the strong acid solution. As electrodialysis progresses, the electrical conductivity of the strong acid solution increases, which can be used as an indicator of the progress of electrodialysis. In addition, if acid A is one whose density can be clearly distinguished from that of water (and the treated water contains few impurities), such as nitric acid (specific gravity: 1.5), the specific gravity of the strong acid solution increases as electrodialysis progresses, approaching the specific gravity of acid A, which can be used as an indicator of the progress of electrodialysis.

[0077] FIG. 1 shows an example in which two combinations of a left end bipolar membrane 1 and a left anion exchange membrane 2 (in other words, a combination of a flow path (1) and a flow path (2)) are arranged and a right end bipolar membrane 5 is provided. On the other hand, there is no limit to the number of combinations, and for example, the number of combinations may be 10 to 200 and a right end bipolar membrane 5 may be provided. In this case, the water to be treated is supplied to the flow paths (even numbers), and acid A is generated in the flow paths (odd numbers). In other words, the water to be treated tank 6 (later weak acid liquid tank) is connected to the flow paths (even numbers), and the strong acid liquid tank 7 is connected to the flow paths (odd numbers).

[0078] At the start of electrodialysis, the flow paths (1) and (3) may be filled with, for example, pure water, and acid A flows in as the electrodialysis proceeds. At the start of electrodialysis, the flow paths (1) and (3) may be filled with a mixture of a strong acid solution and pure water obtained by carrying out the first embodiment separately (the mixing ratio of the strong acid solution:pure water may be, for example, 1:0.5 to 1:4). In this way, a current easily flows between the anode 11 and the cathode 12 in the electrodialysis device 10, and the efficiency of the electrodialysis can be improved.

[0079] [Embodiment 2] The method for treating water according to this embodiment includes a first electrodialysis step and a second electrodialysis step. In carrying out either electrodialysis step, an electrodialysis device 10 having the configuration shown in FIG. 1 may be used. When using the same device, the device may be washed after the first electrodialysis step and then the second electrodialysis step is carried out. The contents of the first electrodialysis step and the second electrodialysis step are as follows.

[0080] In the first electrodialysis step, a solution containing an acid A and an acid B having a larger acid dissociation constant than that of the acid A, a heavy metal content of 1000 ppm or less, and a pH of the acid dissociation constant pKa of the acid B is used. B The water to be treated, which has a pKa of 10 or more (usually 12 or less), is subjected to electrodialysis using an electrodialysis device 10 equipped with a bipolar membrane having an anion exchange layer and a cation exchange layer to obtain an acid solution having a lower pH than the water to be treated and containing the acid A and the acid B. The water to be treated generally contains alkaline substances, and when the pH of the water to be treated is 10 or more, the pKa of the acid solution is 10 or more. B By carrying out the electrodialysis, in addition to the acid solution, an alkaline solution (mainly containing the alkaline substance) having a higher pH than the water to be treated is obtained.

[0081] The acid A and the acid B are the same as those in the first embodiment of the invention, and examples of the alkaline substance include ammonia, sodium hydroxide, and hexamethylenetetramine. Furthermore, the water to be treated may contain other components, and examples of such components include formaldehyde and alcohol compounds. The alkaline substance is a substance that dissolves in water and exhibits alkalinity, and dissociates in the water to be treated at a predetermined ratio depending on the pH of the water. When a cation exchange membrane is used in the first electrodialysis step or in the third embodiment described below, the positive ions dissociated from the alkaline substance permeate the cation exchange membrane, and then hydroxide ions are supplied from the bipolar membrane to regenerate the alkaline substance (the alkaline substance is recovered). In the present invention, regarding the substance X measured as a component of various liquids such as the water to be treated and a strong acid liquid, if it is an alkaline substance itself, it is an alkaline substance, and if it is an ion, if a hydroxide ion is added to the substance so that the charge is zero, the ion is considered to be an alkaline substance.

[0082] The heavy metal content in the treated water before the first electrodialysis process, which is the subject of this embodiment 2, is 1000 ppm or less, as in the case of the invention of embodiment 1, preferably 500 ppm or less, more preferably 100 ppm or less, even more preferably 50 ppm or less, and particularly preferably 10 ppm or less.

[0083] In the second electrodialysis step, the acid dissociation constant pKa of the acid A is measured for the acid solution obtained in the first electrodialysis step. A and the acid dissociation constant pKa of the acid B is B At a pH (X) smaller than the above, electrodialysis is performed using a bipolar membrane having an anion exchange layer and a cation exchange layer, and an electrodialysis device having an anion exchange membrane, and the acid A in the acid solution is made to permeate through the anion exchange membrane to obtain a strong acid solution. As a result, a weak acid solution is also obtained as a residual liquid after the acid A is discharged from the acid solution.

[0084] In the second embodiment, an alkaline substance and an acid (a mixture of acid A and acid B) are separated by a first electrodialysis step. The obtained mixture of acid A and acid B is subjected to a second electrodialysis step to separate acid A and acid B. This separation of acid A and acid B is almost the same as that in the first embodiment, and various configurations of the first embodiment can be applied. Therefore, the first electrodialysis step will be mainly described here.

[0085] FIG. 3 is a diagram showing an overview of an electrodialysis device 10 and electrodialysis used in the first electrodialysis step in the second embodiment. FIG. 3 differs from FIG. 1 in that the strong acid solution tank 7 is replaced with an acid solution tank 8. FIG. 3 illustrates a configuration including anion exchange membranes 2 and 4, but in the second embodiment, a configuration including cation exchange membranes 2' and 4' may be used. In the case of a configuration including anion exchange membranes, acids A and B permeate the anion exchange membrane in the first electrodialysis step (alkaline substances do not permeate the membrane and remain), and in the case of a configuration including a cation exchange membrane, alkaline substances permeate the cation exchange membrane in the first electrodialysis step (acids A and B do not permeate the membrane and remain). In either case, the alkaline substances and the acid are separated, and an acid solution is obtained. Since the water to be treated is a mixture of acid and alkaline substances, its pH is basically not very high, and most of the alkaline substances are dissociated. FIG. 4 is a partially enlarged view of FIG. 3 in the second embodiment.

[0086] In the first electrodialysis step, the water to be treated is supplied from the water to be treated tank 6 to the flow paths (2) and (4). Focusing on the flow path (2), as described above, the pH of the water to be treated is adjusted to the acid dissociation constant pKa of the acid B. B Since the concentration of nitric acid in the treated water flowing through the flow path (2) is higher than 12 (usually 12 or less), the nitric acid is converted into protons, H + and NO3 - Formic acid dissociates into H + and H.C.O.O. - It dissociates into acid (Figure 4(a)).

[0087] When the power of the electrodialysis device 10 is turned on, the NO3 - and H.C.O.O. - passes through the left anion exchange membrane 2 and moves toward the anode 11 on the left side of FIG. 3 (FIG. 4(b)).

[0088] However, a bipolar membrane is placed in front of the anode 11. Therefore, NO3 - and H.C.O.O. - tries to move toward the anode 11, but is repelled by the left-end bipolar membrane 1 and remains in the flow channel (1) (FIG. 4(c)).

[0089] On the other hand, the left end cation exchange layer 1b of the left end bipolar membrane 1 faces the left anion exchange membrane 2. + is supplied to flow path (1). As a result, NO3 - and H.C.O.O. - and H + and combine to form nitric acid and formic acid. In the second embodiment, nitric acid is obtained instead of nitrates, and formic acid is obtained instead of formates. This means that in the second embodiment, it is not necessary to perform the step of obtaining nitric acid from nitrates and the step of obtaining formic acid from formates. If the water in the flow path (1) is collected in the acid solution tank 8, an acid solution mainly containing nitric acid and formic acid is obtained (FIG. 4(d)). In this solution, the ratio ε (=γ / (α+β)) of the concentration γ of the alkaline substance to the sum of the concentration α of acid A and the concentration β of acid B is low.

[0090] The alkaline substance remains in the flow path (2). If there is any charged alkaline substance, it will move toward the cathode 12 on the right side of Figure 3 when a current is passed through it, but it will be repelled by the central bipolar membrane 3 in Figure 3. Substances that are not charged will not move.

[0091] If the moisture in the flow path (2) is collected in the water to be treated tank 6, an alkaline liquid containing mainly alkaline substances is obtained. In this liquid, the ratio ε (=γ / (α+β)) of the concentration γ of the alkaline substance to the sum of the concentration α of acid A and the concentration β of acid B is high. If a certain amount of water to be treated is poured into the water to be treated tank 6 and the water to be treated is continuously circulated in the flow shown in FIG. 3, a liquid with a high ratio ε (alkaline liquid) will accumulate in the water to be treated tank 6. In other words, the role of the water to be treated tank 6 changes to that of an alkaline liquid tank.

[0092] Explaining with reference to Fig. 3, the water to be treated is supplied from the water tank 6 to flow path (2) and flow path (4), passes through both flow paths, and is then returned to the water tank 6. Nitric acid and formic acid are then produced in flow path (1) and flow path (3), and the nitric acid and formic acid move together with water to the acid tank 8. These are then supplied from the tank 8 to flow paths (1) and (3), and newly produced nitric acid and formic acid are taken up in the flow paths and moved again to the acid tank 8. In other words, by electrodialyzing the water to be treated using the electrodialysis device 10 having the configuration of embodiment 2, a liquid with a small specific ε is stored in the acid tank 8.

[0093] In the invention of embodiment 2, the acid solution obtained by the above-mentioned first electrodialysis step is subjected to a second electrodialysis step. The content of the second electrodialysis step is the same as that of the electrodialysis step of embodiment 1, and therefore a description thereof will be omitted.

[0094] According to embodiment 2, the composition contains the above-mentioned acids, and the pH is the acid dissociation constant pKa B As described above, acid A can be effectively separated from the water to be treated.

[0095] Specifically, the first electrodialysis step produces an acid solution having a lower pH (and a lower specific ε) than the water to be treated, and an alkaline solution having a higher pH (and a higher specific ε) than the water to be treated.Then, the second electrodialysis step produces a strong acid solution containing acid A that has permeated the anion exchange membrane, and a weak acid solution containing acid B as a residual liquid after acid A has been discharged from the acid solution.

[0096] The value of the ratio δ (α / β) in the strong acid solution obtained in the present embodiment 2 is preferably 10 or more, more preferably 30 or more, and particularly preferably 100 or more. The ratio δ is usually 300 or less, and for example, 200 or less. The concentration α of acid A in the strong acid solution is preferably 52 g / L or more (usually 180 g / L or less). The content of heavy metals in the water to be treated is low as described above, and no metal is added in the electrodialysis of the present embodiment 2, so the content of heavy metals in the strong acid solution is low (specifically, usually 1000 ppm or less, preferably 500 ppm or less, more preferably 100 ppm or less, even more preferably 50 ppm or less, and particularly preferably 10 ppm or less). It is more preferable to satisfy all of the above specifications for the ratio δ, concentration α, and heavy metal content. The concentration β of acid B in the weak acid solution may be 2 g / L or more (usually 30 g / L or less).

[0097] As explained above, what is added to the water-to-be-treated tank 6 is basically the water-to-be-treated, and no alkaline substance is added in the second embodiment. In other words, the alkaline substance originally contained in the water to be treated is concentrated in the alkaline liquid.

[0098] [Embodiment 3] The method for treating water to be treated according to the present embodiment is a method for treating water to be treated, which comprises: acid A; and acid B having a larger acid dissociation constant than that of acid A; the content of heavy metals is 1000 ppm or less; and the pH is equal to or lower than the acid dissociation constant pKa of acid B. B The method includes an electrodialysis step in which the above-described water to be treated is electrodialyzed by an electrodialysis device equipped with a bipolar membrane having an anion exchange layer and a cation exchange layer, a cation exchange membrane, and an anion exchange membrane.

[0099] The acid A and the acid B are the same as those in the first embodiment of the present invention. BThis is because the water to be treated usually contains alkaline substances, such as ammonia, sodium hydroxide, and hexamethylenetetramine. Furthermore, the water to be treated may contain other components, such as formaldehyde and alcohol compounds.

[0100] The heavy metal content in the treated water before the electrodialysis process, which is the subject of this embodiment 3, is 1000 ppm or less, as in the case of the invention of embodiment 1, preferably 500 ppm or less, more preferably 100 ppm or less, even more preferably 50 ppm or less, and particularly preferably 10 ppm or less.

[0101] In the early stage of the electrodialysis process, a high pH solution (alkaline solution) having a higher pH than the water to be treated before the start of the electrodialysis is obtained, while the pH of the water to be treated is adjusted to the acid dissociation constant pKa A and the acid dissociation constant pKa of the acid B is B Lower the pH to a smaller value (X).

[0102] In the later stage of the electrodialysis step, the acid A in the water to be treated, which has reached the pH (X), is permeated through an anion exchange membrane to obtain a strong acid solution.

[0103] In the third embodiment, as in the second embodiment, the water to be treated contains acid A and acid B having a larger acid dissociation constant than acid A, and has a pH value equal to or higher than a predetermined value (and a small heavy metal content). The water to be treated is electrodialyzed by electrodialysis, and the first and second electrodialysis steps in the second embodiment are carried out in one step by one electrodialysis device. The first electrodialysis step is carried out in the early stage of the electrodialysis step, and the second electrodialysis step is carried out in the later stage of the electrodialysis step. Because it is a one-step process using one electrodialysis device, strong acids, weak acids, and alkaline substances can be effectively separated in a simple step. The contents of the second embodiment can be used for the invention of the third embodiment, as long as they are not described in the description of this embodiment.

[0104] FIG. 5 is a diagram showing an overview of the electrodialysis device 10 and electrodialysis according to the third embodiment.

[0105] 1, the electrodialysis device 10 of the first embodiment is provided with a left cation exchange membrane 2' between the left anion exchange membrane 2 and the central bipolar membrane 3. The cation exchange membrane 2' faces the anion exchange layer 3a of the central bipolar membrane 3. The cation exchange layer 3b of the central bipolar membrane 3 faces the right anion exchange membrane 4.

[0106] In Figure 5, the anode 11 is placed on the far left, the left-end bipolar membrane 1 is placed to its right, the left-side anion exchange membrane 2 is placed to its right, the left-side cation exchange membrane 2' is placed to its right, the central bipolar membrane 3 is placed to its right, the right-side anion exchange membrane 4 is placed to its right, the right-side cation exchange membrane 4' is placed to its right, the right-end bipolar membrane 5 is placed to its right, and the cathode 12 is placed on the far right.

[0107] In other words, the electrodialysis apparatus 10 in embodiment 3 comprises a plurality of the bipolar membranes 1, 3, 5, anion exchange membranes 2, 4, and cation exchange membranes 2', 4', the anion exchange membranes 2, 4 face the cation exchange layers 1b, 3b of the bipolar membranes 1, 3 and face the cation exchange membranes 2', 4', the cation exchange membranes 2', 4' face the anion exchange layers 3a, 5a of the bipolar membranes 3, 5, and the bipolar membranes 1, 5 are arranged at both ends of the arrangement of these membranes.

[0108] In FIG. 5, a flow path (1) is formed between the left-most bipolar membrane 1 and the left anion exchange membrane 2. A flow path (2) is formed between the left anion exchange membrane 2 and the left cation exchange membrane 2'. A flow path (2') is formed between the left cation exchange membrane 2' and the central bipolar membrane 3. A flow path (3) is formed between the central bipolar membrane 3 and the right-side anion exchange membrane 4. A flow path (4) is formed between the right-side anion exchange membrane 4 and the right-side cation exchange membrane 4'. A flow path (4') is formed between the right-side cation exchange membrane 4' and the right-side bipolar membrane 5.

[0109] FIG. 5 shows an example in which two sets of a combination of a left end bipolar membrane 1, a left anion exchange membrane 2, and a left cation exchange membrane 2' (in other words, a combination of a flow path (1), a flow path (2), and a flow path (2')) are arranged and a right end bipolar membrane 5 is provided. On the other hand, there is no limit to the number of sets of the combination (membrane structure in which the three types of membranes are repeatedly arranged), and for example, the number of sets may be 10 to 200 sets and a right end bipolar membrane 5 may be provided. In this case, the water to be treated is supplied to the flow path (even number (no ')), the alkaline liquid is generated in the flow path (even number (')) in the above-mentioned early stage, and the strong acid liquid is generated in the flow path (odd number) in the above-mentioned later stage. In other words, the water to be treated tank 6 (later weak acid liquid tank) is connected to the flow path (even number), the alkaline liquid tank 9 is connected to the flow path (even number (')), and the strong acid liquid tank 7 is connected to the flow path (odd number).

[0110] In the early stage of the electrodialysis process, the water to be treated is supplied from the water tank 6 to the flow paths (2) and (4). The following description focuses mainly on the flow path (2). When the water to be treated is supplied, as described above, the pH of the water to be treated is equal to or lower than the acid dissociation constant pKa of acid B. B That is all. Furthermore, since the water to be treated is a mixture of acid and alkaline substances, its pH is generally not very high (for example, below 10), and most of the alkaline substances are dissociated. Therefore, in the water to be treated flowing through flow path (2), the alkaline substances permeate the cation exchange membrane 2' and move to flow path (2'). On the other hand, it is thought that the acid components permeate the anion exchange membrane 2 and move to flow path (1), and although the mechanism is unclear, in this early stage, the movement of alkaline substances occurs preferentially, and the pH of the water to be treated decreases. As a result, the dissociation rate of alkaline substances increases, and further movement of alkaline substances occurs.

[0111] In this way, in the early stage of the electrodialysis process, the pH of the water to be treated is adjusted to the acid dissociation constant pKa A and the acid dissociation constant pKa of the acid B is B The pH drops to a smaller pH(X) and the later stages of the electrodialysis process begin.

[0112] By setting the pH (X) as described above in the later stage of the electrodialysis process, nitric acid is converted into protons, H + and NO3 - While formic acid is not dissociated into HCOOH, the formic acid remains as HCOOH. The subsequent steps are almost the same as those in the electrodialysis step of the first embodiment or the second electrodialysis step of the second embodiment, and therefore will not be described here. It is considered that even at this stage, some permeation of the alkaline substance through the cation exchange membrane 2' occurs.

[0113] Explaining with reference to Fig. 5, the water to be treated is supplied from the water to be treated tank 6 to the flow path (2) and the flow path (4), passes through both flow paths, and is then returned to the water to be treated tank 6. Then, in the early stage of the electrodialysis process, alkaline substances migrate to the flow path (2') and the flow path (4'), and the alkaline substances move to the alkaline liquid tank 9.

[0114] As the uptake (concentration) of alkaline substances into the flow paths (2') and (4') and the alkaline solution tank 9 progresses, the pH of the water passing through the flow paths (2) and (4) decreases to the above pH (X). Then, the later stage of the electrodialysis process begins.

[0115] In the later stage of the electrodialysis process, NO3 - In the flow path, protons supplied from the cation exchange layers 1b and 3b of the bipolar membranes 1 and 3 and NO3 - Nitric acid is generated from the nitric acid, and the nitric acid moves to the strong acid tank 7. That is, by electrodialyzing the water to be treated by the electrodialysis device 10, a strong acid solution containing nitric acid at a high concentration is stored in the strong acid tank 7.

[0116] At the same time, in streams (2) and (4), NO3 - The weak acid liquid is obtained as water with a relatively increased concentration of formic acid due to the decrease in the concentration of the formic acid. The weak acid liquid moves to the tank 6 for treated water.

[0117] According to the third embodiment, various operations such as those in steps 2 and 3 of Patent Document 1 are not required, and an increase in cost can be avoided. In addition, the strong acid A can be sufficiently separated by a single operation (the electrodialysis step). In other words, according to the third embodiment, a mixture containing multiple acids and having a pH of 0.1 to 0.25 is prepared by the method according to the third embodiment. B From the water to be treated as described above, strong acids and weak acids (as well as alkaline substances) can be effectively separated and the strong acids can be recovered through a simple process.

[0118] The value of the ratio δ (α / β) in the strong acid solution obtained in the third embodiment is preferably 10 or more, more preferably 30 or more, and particularly preferably 80 or more. The ratio δ is usually 300 or less, and for example, 200 or less. The concentration α of acid A in the strong acid solution is preferably 52 g / L or more (usually 180 g / L or less). The content of heavy metals in the water to be treated is low as described above, and no metal is added in the electrodialysis of the third embodiment, so the content of heavy metals in the strong acid solution is low (specifically, usually 1000 ppm or less, preferably 500 ppm or less, more preferably 100 ppm or less, even more preferably 50 ppm or less, and particularly preferably 10 ppm or less). It is more preferable to satisfy all of the above specifications for the ratio δ, concentration α, and heavy metal content. The concentration β of acid B in the weak acid solution may be 2 g / L or more (usually 30 g / L or less). The ratio ε (γ / (α+β)) in the alkaline solution is, for example, 3 to 30.

[0119] Incidentally, a preferred combination is that the strong acid A is nitric acid, the weak acid B is formic acid, and the alkaline substance is ammonia. Ammonia has a small molecular size and easily passes through the cation exchange membrane. In other words, it is thought that the movement of the alkaline substance between the flow paths is prioritized compared to other substances, and a rapid transition from the early stage to the later stage of the electrodialysis process is achieved.

[0120] A conventionally known cation exchange membrane can be used. If the resistance of the membrane is low, ammonia will preferentially move between the channels. Therefore, the electrical resistance of the cation exchange membrane is set to 8 Ω cm. 2The lower limit is not limited, but it is preferably 1 Ωcm. 2 It is.

[0121] Conventionally known anion exchange membranes can be used, but if the resistance of the membrane is high, anions will not pass through easily, and as a result, ammonia will preferentially move between the channels. For this reason, the electrical resistance of the anion exchange membrane is set to 10 Ω cm 2 There is no upper limit, but it is preferably 60 Ωcm. 2 It is.

[0122] The technical scope of the present invention is not limited to the above-described embodiments, but also includes forms in which various modifications and improvements are made within the scope that can derive specific effects obtained by the constituent elements of the invention and their combinations. EXAMPLES

[0123] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to these examples. Furthermore, the following comparative examples do not mean conventional examples.

[0124] In the following Examples and Comparative Examples, various measurements were carried out as follows. Metal content: Measured using ICP-OES. pH: Measured at 25°C using a pH meter and electrode HORIBA D-73. ·EC (Electrical Conductivity): Measured using an EC meter and electrode TOA DKK CM-31P. Nitric acid concentration: Measured using a Tosoh IC-8100EX ion chromatograph. Formic acid concentration: Measured using a Tosoh IC-8100EX ion chromatograph. Ammonia concentration: Measured using a Tosoh IC-8100EX ion chromatograph. HMT (hexamethylenetetramine) concentration: Measured using a Tosoh IC-8100EX ion chromatograph. Cl concentration: Measured using a Tosoh IC-8100EX ion chromatograph. Electrical resistance: Equilibrated in 0.5 mol / L saline and measured at 25°C using an alternating current.

[0125] [Comparative Example 1] Nitric acid wastewater generated during silver powder production was filtered using a DOW nanofilter (NF) (to separate HMT), and the resulting filtrate was treated with a reverse osmosis (RO) membrane to separate water from the filtrate, and the resulting concentrated water was used as the water to be treated. The composition of the water to be treated is as shown in Table 1 below. The heavy metal content in the water to be treated was 100 ppm or less. EC represents electrical conductivity. The pKa of nitric acid (acid A) in water at 25°C is -1.8, and the pKa of formic acid (acid B) is 3.75. The pH of the water to be treated is in the range of 8 to 9.

[0126] [Table 1]

[0127] This water to be treated was subjected to electrodialysis using the electrodialysis device 10 shown in Fig. 1. The electrodialysis conditions were as follows. Electrodialysis device 10: ASTOM Bipolar membrane electrodialysis device ASILYSER BPED EX-3B Anion exchange membrane: Astom AHA (electrical resistance is 4.1Ωcm 2 ) Bipolar membrane: Astom BP1-EX *10 anion exchange membranes and 11 bipolar membranes were used. Voltage: Max 25V ·Current: Max 4.4A *The maximum values ​​of voltage and current were determined as described above, and the voltage was increased until the first maximum value of voltage or current was reached. In the initial state, 1 L of the water to be treated is added to the water to be treated tank 6. In the strong acid tank 7, 0.5 L of pure water (0.5 times the amount of water to be treated) is added at the initial stage. Temperature of the liquid flowing through each tank and the electrodialysis device 10 during electrodialysis: 20 to 37°C

[0128] The changes in pH of the liquid in the tank 6 for treated water and in the composition of the liquid in the tank 7 for strong acid solution due to the electrodialysis described above are shown in Table 2 below. The amount of Na was not measured. [Table 2]

[0129] Thirty minutes after the start of electrodialysis, the ratio δ of nitric acid to formic acid in the strong acid tank 7 was high, about 20. On the other hand, the nitric acid concentration α was not very high, at 50 g / L, and when the treatment was continued and the nitric acid concentration reached 90 g / L, the ratio had dropped to about 6. That is, nitric acid that had not been sufficiently separated from formic acid was obtained in the strong acid tank 7.

[0130] [Example 1] In Example 1, the contents corresponding to the first embodiment were carried out. The acid solution with a sufficiently increased nitric acid concentration (nitric acid concentration: 91 g / L, formic acid concentration: 15 g / L, pH: 0, heavy metal content: 100 ppm or less) obtained in Comparative Example 1 after 60 minutes of electrodialysis was further subjected to electrodialysis using the electrodialysis device 10 of FIG. 1. The electrodialysis conditions were the same as those of Comparative Example 1, except that the acid solution obtained after 60 minutes of electrodialysis in Comparative Example 1 was initially charged into the water tank 6 and that the strong acid tank was added with an equal amount of pure water to the water to be treated. The changes in pH of the liquid in the water tank 6 and the changes in the composition of the liquid in the strong acid tank 7 due to the above electrodialysis are shown in Table 3 below. The heavy metal content of the liquid in the strong acid tank 7 at the end of the electrodialysis was 100 ppm or less. The amount of Na in the liquid was not measured.

[0131] [Table 3]

[0132] In Example 1, the ratio (Q1 / Q2) of the permeation amounts of nitric acid and formic acid in the strong acid solution in the strong acid solution tank 7 was high, at 40 or more, both 30 minutes and 60 minutes after the start of electrodialysis.

[0133] [Example 2] In Example 2, the contents corresponding to the embodiment 2 were carried out. The first electrodialysis step and the second electrodialysis step as referred to in the embodiment 2 were carried out on water to be treated (containing nitric acid and formic acid and having a pH in the range of 8 to 9) different from that in Comparative Example 1. Both electrodialysis steps were carried out using the same electrodialysis device 10 as that used in Comparative Example 1 and Example 1.

[0134] In the first electrodialysis step, 3.5 L of the water to be treated (heavy metal content: 100 ppm or less) was initially added to the water to be treated tank 6, and 3.5 L of pure water was initially added to the strong acid tank . The first electrodialysis step yielded an acid solution with a nitric acid concentration of 87 g / L and a formic acid concentration of 5.1 g / L. After the electrodialysis device 10 was washed, the second electrodialysis step was carried out under the same electrodialysis conditions as in Example 1 (the electrodialysis time was 270 minutes). The changes in the composition of the solution in the treated water tank 6 and the composition of the solution in the strong acid solution tank 7 due to the second electrodialysis step are shown in Table 4 below. The heavy metal content of the solution in the strong acid solution tank 7 at the end of the electrodialysis was 100 ppm or less. The amount of Na in the solution in each tank was not measured.

[0135] [Table 4]

[0136] In Example 2, the ratio (Q1 / Q2) of the amount of permeated nitric acid to the amount of formic acid in the strong acid solution finally obtained 270 minutes after the start of electrodialysis reached 100 or more.

[0137] [Example 3] In Example 3, the contents corresponding to the embodiment 3 were carried out. An electrodialysis device 10 similar to that used in Example 1 was used, except that a cation exchange membrane (Astrom CMB) was placed between the anion exchange membrane and the anion exchange layer of the bipolar membrane. Ten anion exchange membranes and ten cation exchange membranes, and eleven bipolar membranes were used.

[0138] The electrodialysis conditions were as follows: Voltage: Max 30V ·Current: Max 4.4A - 20 L of water to be treated (composition etc. shown in Table 5 below) was initially added to the water to be treated tank 6. Initially, 5 L of treated acid solution and 10 L of pure water are added to the strong acid solution tank 7. The treated acid solution is a strong acid solution obtained by initially adding 20 L of water to be treated to the water to be treated tank 6, 15 L of pure water to the strong acid solution tank 7, and 15 L of pure water to the alkaline solution tank 9, and carrying out electrodialysis under the same conditions as those employed in Example 3 (electrodialysis device 10, voltage, current, temperature of each tank and the solution flowing in the electrodialysis device 10 during electrodialysis, and treatment time). The alkaline solution tank 9 initially contains 5 L of treated alkaline solution and 10 L of pure water. The treated alkaline solution is the alkaline solution obtained by electrodialysis carried out to obtain the treated acid solution. Temperature of the liquid flowing through each tank and the electrodialysis device 10 during electrodialysis: 20 to 37°C Treatment time: 20 hours (1 hour per 1L of treated water)

[0139] The reason for using the treated acid solution and the treated alkaline solution is to facilitate the energization and to complete the first stage quickly. The composition of the liquid in each tank obtained by the above electrodialysis is shown in Table 5 below. The pH of the liquid in the treated water tank one hour after the start of electrodialysis was 8.70, and the pH was 4.0 four hours later. The heavy metal content of the liquid in the strong acid liquid tank 7 at the end of electrodialysis was 100 ppm or less. The amount of Na in the various liquids shown in Table 5 has not been measured.

[0140] [Table 5]

[0141] [Example 4] In Example 4, the same procedure as in Example 3 was carried out. Astom AHO (electrical resistance 20 Ω cm) was used as the anion exchange membrane. 2An electrodialysis apparatus 10 similar to that used in Example 1 was used, except that a cation exchange membrane (Astom CMB) was placed between the anion exchange membrane and the anion exchange layer of the bipolar membrane. Ten anion exchange membranes and ten cation exchange membranes, and eleven bipolar membranes were used.

[0142] The electrodialysis conditions were as follows: Voltage: Max 30V ·Current: Max 4.4A · 500 mL of treated liquid and 500 mL of pure water are initially added to the treated water tank 6. The treated liquid is the liquid in the treated water tank 6 obtained by initially adding 1 L of water to be treated to the treated water tank 6, 1 L of pure water to the strong acid liquid tank 7, and 1 L of pure water to the alkaline liquid tank 9, and carrying out electrodialysis under the same electrodialysis conditions (electrodialysis device 10, voltage, current, temperature of each tank and the liquid flowing in the electrodialysis device 10 during electrodialysis, and treatment time) as those employed in Example 4. In the initial state, 500 mL of treated acid solution and 500 mL of pure water are added to the strong acid solution tank 7. The treated acid solution is a strong acid solution obtained by electrodialysis carried out to obtain the treated solution. In the initial state, 500 mL of treated alkaline liquid and 500 mL of pure water are added to the alkaline liquid tank 9. The treated alkaline liquid is the alkaline liquid obtained by electrodialysis carried out to obtain the treated liquid. Temperature of the liquid flowing through each tank and the electrodialysis device 10 during electrodialysis: 20 to 37°C Processing time: 100 minutes

[0143] The composition of the liquid in each tank before and after the electrodialysis is shown in Table 6 below. The heavy metal content of the liquid in the tank for treated water before electrodialysis and the liquid in the strong acid tank 7 at the end of electrodialysis was both 100 ppm or less. The amount of Na in these various liquids has not been measured. [Table 6]

[0144] In Example 3, AHA with a relatively low electrical resistance was used as the anion exchange membrane. At the end of the latter stage of Example 3, the ratio δ (=α / β) of the concentration α of nitric acid to the concentration β of formic acid in the strong acid tank 7 was about 40. On the other hand, in Example 4, in which AHO with a relatively high electrical resistance was used as the anion exchange membrane, the ratio δ remained almost unchanged from the start of the electrodialysis process, and the ratio δ was maintained at 100 or more, and nitric acid was sufficiently selectively transferred to the strong acid tank. In addition, ammonia was maintained at a low concentration in the strong acid tank, and was sufficiently selectively transferred to the alkaline tank. The treated acid solution used in the electrodialysis of this example is a mixture of strong acid solution and water obtained by performing electrodialysis similar to that of this example under the conditions of adding 1 L of treated water (ratio δ in the range of 1 to 10) to the treated water tank 6 in the initial state and adding 1 L of pure water to the strong acid tank 7, as described above, and is a solution with a ratio δ exceeding 100. [Explanation of symbols]

[0145] 1 Left end bipolar membrane 1a Left anion exchange layer 1b Leftmost cation exchange layer 2. Left anion exchange membrane 2´ Left cation exchange membrane 3 Central bipolar membrane 3a Central anion exchange layer 3b Central cation exchange layer 4 Right side anion exchange membrane 4´ Right side cation exchange membrane 5 Right end bipolar membrane 5a Right anion exchange layer 5b Right end cation exchange layer 6. Treated water tank (weak acid tank) 7 Strong Acid Tank 8. Acid tank 9 Alkaline Liquid Tank 10 Electrodialysis device 11 Anode 12 Cathode

Claims

1. The acid dissociation constant pKa of the acid A is larger than that of the acid B, and the heavy metal content of the water to be treated is 1000 ppm or less. Ａ and the acid dissociation constant pKa of the acid B is Ｂ The method for treating water to be treated comprises electrodialyzing the water to be treated at a smaller pH (X) using a bipolar membrane having an anion exchange layer and a cation exchange layer, and an electrodialysis device having an anion exchange membrane, to obtain a strong acid solution containing acid A that has permeated the anion exchange membrane.

2. The acid A and the acid B having a larger acid dissociation constant than the acid A, the heavy metal content is 1000 ppm or less, and the pH is 100% or less than the acid dissociation constant pKa of the acid B. Ｂ a first electrodialysis step in which the above-mentioned water to be treated is electrodialyzed by an electrodialysis device equipped with a bipolar membrane having an anion exchange layer and a cation exchange layer to obtain an acid solution having a lower pH than the water to be treated and containing the acid A and the acid B; The acid dissociation constant pKa of the acid A with respect to the acid solution Ａ and the acid dissociation constant pKa of the acid B is Ｂ a second electrodialysis step in which the solution is electrodialyzed at a lower pH (X) using an electrodialysis device equipped with a bipolar membrane having an anion exchange layer and a cation exchange layer, and an anion exchange membrane, to obtain a strong acid solution containing acid A that has permeated the anion exchange membrane; The method for treating water to be treated comprises the steps of:

3. The acid A and the acid B having a larger acid dissociation constant than the acid A, the heavy metal content is 1000 ppm or less, and the pH is 100% or less than the acid dissociation constant pKa of the acid B. Ｂ The method includes an electrodialysis step of electrodialyzing the above-mentioned treated water using an electrodialysis device including a bipolar membrane having an anion exchange layer and a cation exchange layer, a cation exchange membrane, and an anion exchange membrane, In the early stage of the electrodialysis process, a high pH liquid having a higher pH than the water to be treated before the start of the electrodialysis is obtained, while the pH of the water to be treated is adjusted to the acid dissociation constant pKa of the acid A. Ａ and the acid dissociation constant pKa of the acid B is Ｂ to a smaller pH (X), In the latter stage of the electrodialysis process, the acid A in the water to be treated having the pH (X) is permeated through the anion exchange membrane to obtain a strong acid solution. A method for treating the water to be treated.

4. The electrodialysis device includes a plurality of the bipolar membranes, 4. The method for treating water to be treated according to claim 1, wherein the anion exchange membrane is disposed between two of the bipolar membranes.

5. the electrodialysis apparatus has a membrane structure in which the bipolar membrane, the anion exchange membrane, and the cation exchange membrane are repeatedly arranged in this order; the anion exchange membrane faces the cation exchange layer of the bipolar membrane and faces the cation exchange membrane; The method for treating water to be treated according to claim 3 , wherein the cation exchange membrane faces the anion exchange layer of the bipolar membrane.

6. The method for treating water to be treated according to any one of claims 1 to 3, wherein the acid A is at least one selected from the group consisting of nitric acid, hydrochloric acid, and sulfuric acid, and the acid B is at least one selected from the group consisting of formic acid and acetic acid.

7. The pKa Ｂ to the pKa Ａ The method for treating water to be treated according to any one of claims 1 to 3, wherein the value obtained by subtracting

8. The pH(X) to the pKa Ａ The method for treating water to be treated according to any one of claims 1 to 3, wherein the value obtained by subtracting

9. The pKa Ｂ The method for treating water to be treated according to any one of claims 1 to 3, wherein a value obtained by subtracting the pH(X) from is 0.4 or more.

10. The method for treating water according to any one of claims 1 to 3, wherein Q1 is 30 times or more as large as Q2, where Q1 is the amount of acid A permeating through the anion exchange membrane and Q2 is the amount of acid B permeating through the anion exchange membrane.

11. 4. The method for treating water to be treated according to claim 1, wherein the concentration α of the acid A in the strong acid solution is 52 g / L or more.

12. 4. The method for treating water to be treated according to claim 1, wherein the content of heavy metals in the water to be treated that is subjected to the electrodialysis is 100 ppm or less.

13. The electrical resistance of the anion exchange membrane is 10 Ωcm ２ The method for treating water to be treated according to claim 3 .

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