Methods for recovering acids and alkalis, methods for recovering and manufacturing boric acid, and methods for manufacturing polarizing plates

Abstract

This invention provides an acid / alkali recovery method that can suppress the degradation of ion exchange membranes used in electrodialysis due to iodide ions. [Solution] The acid / alkali recovery method involves separating a treated liquid, which is a waste liquid containing boric acid and potassium iodide generated during the polarizing plate manufacturing process, into an acidic solution, an alkaline solution, and a boric acid-based treatment solution by electrodialysis. The acid / alkali recovery method includes an NF membrane separation step in which the treated liquid is passed through an NF membrane to separate it into a permeate containing iodide ions and a concentrated solution containing iodide ions at a lower concentration than the permeate, and an electrodialysis step in which the concentrated solution is passed through an electrodialysis apparatus to separate it into a boric acid-based treatment solution, an acidic treatment solution, and an alkaline treatment solution.

Description

Technical Field

[0001] The present invention relates to a method for recovering acids and alkalis. For example, the present invention relates to a method for recovering acids and alkalis in which a liquid to be treated obtained by adding an acidic solution and an alkaline solution to a waste liquid containing boric acid and potassium iodide generated in the process of manufacturing a polarizing plate is separated by electrodialysis into an acidic solution, an alkaline solution, and a treatment liquid mainly composed of boric acid. The present invention also relates to a method for recovering and manufacturing boric acid and a method for manufacturing a polarizing plate.

Background Art

[0002] Waste liquid generated in the process of manufacturing a polarizing plate contains inorganic substances such as iodine, boron, and potassium, and organic substances such as polyvinyl alcohol. Such waste liquid is treated as industrial waste. On the other hand, there is a demand to treat the waste liquid obtained in the manufacturing process, recover boron, and recycle it to the manufacturing process.

[0003] Patent Document 1 discloses a method for treating waste liquid from the production of a polarizing plate to recover potassium iodide and boric acid. The treatment method of Patent Document 1 involves solid-liquid separation of the precipitate obtained by evaporating and concentrating the waste liquid, recovering the filtrate containing potassium iodide, and recovering boric acid from the precipitate. In the boric acid recovery step, an acid is added to the liquid to be treated in which the precipitate is dissolved to adjust the pH, and then the solution is cooled to separate precipitated boric acid crystals. After adding an alkali to the liquid from which the boric acid crystals have been separated to neutralize it, electrodialysis is performed.

[0004] The wastewater treatment method of Patent Document 2 involves adjusting the pH of wastewater containing iodine and boron to 11 to 14, concentrating it so that the boron concentration becomes 0.5 mass% or more, adding an adsorbent to remove organic substances contained in the liquid obtained by concentration, cooling the liquid, and adjusting the pH of the liquid to 1 to 7 to precipitate boron components. The obtained precipitate is removed to separate boron, and chlorine is supplied to the obtained liquid to oxidize iodine and precipitate it to recover iodine.

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Japanese Patent Publication No. 2023-72964 [Patent Document 2] Patent No. 4674168 [Overview of the project] [Problems that the invention aims to solve]

[0006] A problem with ion exchange membranes used in electrodialysis is that they deteriorate due to iodide ions. Furthermore, the presence of borate ions reduces the processing speed of electrodialysis due to buffering, and also lowers current efficiency. However, neither Patent Documents 1 nor 2 address this issue, nor do they implement any countermeasures.

[0007] Therefore, the present invention provides an acid / alkali recovery method that can suppress the degradation of ion exchange membranes used in electrodialysis by iodide ions. Furthermore, the present invention provides an acid / alkali recovery method that can suppress the reduction in processing speed and current efficiency of electrodialysis caused by borate ions. It also provides a method for recovering and producing boric acid and a method for producing polarizing plates. [Means for solving the problem]

[0008] The inventors of the present invention have conducted extensive research to solve the above problems and have found that the above problems can be solved by passing the liquid to be treated through an NF membrane to reduce potassium iodide before performing electrodialysis. That is, the present invention includes the following embodiments.

[0009] The acid and alkali recovery method of this disclosure is a method for recovering acid and alkali by electrodialysis, in which an acidic solution and an alkaline solution are added to a waste liquid containing boric acid and potassium iodide generated in the polarizing plate manufacturing process, and the treated liquid is separated into an acidic solution, an alkaline solution and a boric acid-based treatment liquid. The process includes an NF membrane separation step (S100) in which the liquid to be treated is passed through an NF membrane to separate it into a permeate containing iodide ions and a concentrated solution containing iodide ions at a lower concentration than the permeate, The aforementioned concentrated liquid is passed through an electrodialysis apparatus to separate it into a boric acid-based treatment solution, an acidic treatment solution, and an alkaline treatment solution in an electrodialysis process (S110). Includes.

[0010] In the NF membrane separation step (S100), the concentration of boron ions in the concentrated solution may be the same as or less than the concentration of boron ions in the permeate after passing the liquid to be treated through the NF membrane.

[0011] The aforementioned waste liquid contains boric acid, potassium iodide, and polyvinyl alcohol. The acidic solution is an aqueous solution containing one or two selected from sulfuric acid and hydrochloric acid. The alkaline solution may be an aqueous solution containing one or two selected from sodium hydroxide and potassium hydroxide.

[0012] Prior to the electrodialysis step, the method may include an adsorption removal step in which the concentrated liquid obtained in the NF membrane separation step, or the liquid to be treated before being passed through the NF membrane separation step, is passed through an adsorbent to remove at least one of iodine and PVA.

[0013] The process may also include an iodine reduction step in which iodine contained in the acidic treatment solution separated in the electrodialysis step is reduced to iodide ions using a reducing agent. By reducing it to iodide ions, corrosion of pipes, tanks, etc., is suppressed, and long-term storage becomes possible.

[0014] The liquid to be treated is The acidic solution may contain anions derived from the acid (e.g., sulfuric acid) and cations derived from the base (e.g., sodium sulfate). The alkaline solution may be, for example, an aqueous solution of sodium hydroxide or an aqueous solution of potassium hydroxide.

[0015] The liquid to be treated may be a liquid obtained by the following wastewater treatment method (boric acid recovery method). A boric acid adsorption step (S1-3, S1-4) of selectively adsorbing boric acid from the waste liquid whose pH has been adjusted to alkaline (S1-1, S1-2) onto an adsorbent, A boric acid desorption step (S3-1, S3-2) of contacting an acidic solution with the adsorbent to elute the boric acid from the adsorbent into the acidic solution to obtain an eluate, Adding a base to the eluate (S4-1), passing the eluate whose pH has been adjusted to the acidic side relative to neutrality (S4-2) through an NF membrane, and separating it into a permeate containing the boric acid and no anions derived from acids or cations derived from bases, and a concentrate containing anions derived from acids and cations derived from bases in an NF membrane separation step (S4-3), A second evaporation concentration step (S6-1) of evaporating water from the concentrate to increase the concentration of boric acid to obtain a boric acid concentrated acidic solution, A cooling solid-liquid separation step (S6-2) of cooling the boric acid concentrated acidic solution and performing solid-liquid separation into a boric acid-containing acidic solution and a solid content, including, The liquid containing the solid content (for example, sodium sulfate, potassium iodide, etc.) may be the liquid to be treated. The method for treating the waste liquid is, It may include a boric acid recovery step (S5) of recovering boric acid from the permeate.

[0016] After the boric acid adsorption step, it may include an adsorbent separation step (S2) of separating the waste liquid and the adsorbent. The waste liquid (separation liquid) separated in the adsorbent separation step (S2) may be included in the waste liquid of the boric acid adsorption step (S2-1).

[0017] In the boric acid adsorption step, the alkali adjusting agent used for pH adjustment is sodium hydroxide or potassium hydroxide, The pH of the waste liquid after pH adjustment may be 9 or more and 11 or less. The alkali adjusting agent may be an aqueous solution. The definition of the numerical range of the pH is based on the requirements of an adsorbent that can selectively adsorb boric acid. When the pH is on the acidic side, boric acid does not exist as an ion, so the adsorption amount is small, and therefore the alkaline side is preferred.

[0018] The acidic solution may be one or more selected from dilute sulfuric acid with a concentration of 1% or more and 25% or less, and hydrochloric acid with a concentration of 1% or more and 25% or less. The acidic solution may be an aqueous sulfuric acid solution or an aqueous hydrochloric acid solution. The definition of the numerical range of the concentration is based on the requirements of the hydraulic pressure setting of the NF membrane. The higher the concentration of sulfuric acid or hydrochloric acid is set, the higher the pressure needs to be. Conversely, if their concentrations are lowered, a large amount of water for dilution is required.

[0019] In the NF membrane separation step, the base (pH adjuster) used to adjust the pH to the acidic side rather than neutral is sodium hydroxide or potassium hydroxide that is more alkaline than the acidic solution. The pH of the eluate after pH adjustment may be 3 or more and less than 7. The definition of the numerical range of the pH is for obtaining boric acid in the concentration and cooling crystallization of boric acid in the boric acid recovery step. If it is outside the defined range, borate will form, which is not preferable. Also, when the pH is less than 3, the rejection rate of other components decreases due to the deterioration of the NF membrane module. When the pH is 7 or more, the rate of allowing boric acid to pass through the NF membrane to the permeate side decreases extremely, and most of the boric acid remains on the concentrate side, which is not preferable.

[0020] The boric acid recovery step (S5) A first evaporation concentration step (S5-1) of evaporating water from the aqueous boric acid solution in the permeate to obtain a concentrated aqueous boric acid solution, A cooling crystallization solid-liquid separation step (S5-2) of cooling and crystallizing the concentrated aqueous boric acid solution concentrated to a predetermined concentration and performing solid-liquid separation, A drying step (S5-3, S5-4) of drying the solid-liquid separated boric acid at a predetermined temperature to obtain boric acid with a purity of, for example, 95% or more, 96% or more, 97% or more, or 98% or more may be included.

[0021] After the cooling crystallization solid-liquid separation step (S5-2), the step of dissolving the boric acid in warm water, cooling and crystallizing, and performing solid-liquid separation may be repeated one or more times.

[0022] The process may also include a separation liquid recovery step (S5-2-1), in which the separated liquid obtained in the cooling crystallization solid-liquid separation step (S5-2) is returned upstream of the first evaporation concentration step (S5-1), and the NF membrane separation step (S4) is performed by passing the separated liquid through the NF membrane.

[0023] The waste liquid treatment method is: The process may further include an adsorbent regeneration step (S3-3-1, S3-2-2, S3-3-3) in which the adsorbent is regenerated by contacting it with an alkaline solution.

[0024] The adsorbent may be a boron-selective adsorbent.

[0025] The waste liquid treatment method is: The method may also include a separated boric acid recovery step (S6-2-1), in which the boric acid (boric acid-containing acidic solution) separated in the cooling solid-liquid separation step (S6-2) is combined with the pH-adjusted eluent and passed through the NF membrane to perform the NF membrane separation step.

[0026] The boric acid recovery and manufacturing method of this disclosure is a method for selectively recovering boric acid from waste liquid generated in the polarizing plate manufacturing process, characterized in that the acid or alkali obtained by treatment with the acid / alkali recovery method is reused. The boric acid recovery and manufacturing method may include each step of the waste liquid treatment method described above.

[0027] The method for manufacturing a polarizing plate according to this disclosure includes a recovery manufacturing method for selectively recovering boric acid from waste liquid generated during the polarizing plate manufacturing process. This method is characterized by reusing the acid or alkali obtained by treating wastewater generated during the polarizing plate manufacturing process using the aforementioned acid / alkali recovery method. The acid or alkali recovered by the acid / alkali recovery method may be used as an acid or alkali for the treatment of recovering boric acid and / or potassium iodide from the waste liquid, and the obtained boric acid or potassium iodide may be used in the polarizing plate manufacturing process. The acid may be sulfuric acid and / or hydrochloric acid, and the alkali may be sodium hydroxide and / or potassium hydroxide. In the above-mentioned acid and alkali recovery method, the obtained acid and alkali may be used as the acidic solution and alkali solution to be added to the waste liquid. The obtained alkali may be used as an alkali adjusting agent in the step of adjusting the pH to an alkaline state in the wastewater treatment method. The obtained acid may be used as the acidic solution in the wastewater treatment method. Furthermore, the potassium iodide and / or boric acid may be contained in the permeate that has passed through the NF separation membrane or in the boric acid-based treatment solution after electrodialysis, and the potassium iodide and / or boric acid may be recovered by the wastewater treatment method and reused as a treatment solution used in the polarizing plate manufacturing process. The resulting boric acid may be part or all of the raw materials for the boric acid aqueous solution used in the production of polarizers that make up a polarizing plate. The boric acid may also be used as a raw material for an aqueous boric acid solution used in one or more processes selected from a preliminary contact step, a dyeing step, a crosslinking step, and a stretching step of the polarizer constituting the polarizer. [Effects of the Invention]

[0028] (1) It can suppress the degradation of ion exchange membranes used in electrodialysis due to iodide ions. (2) The boric acid-based treatment solution obtained by electrodialysis, which is the permeate from the NF membrane, can be supplied to the wastewater treatment method described above for retreatment. The recovery rate of boric acid can be improved. (3) The acidic treatment solution (e.g., sulfuric acid) and alkaline treatment solution (e.g., sodium hydroxide) obtained by electrodialysis can be recycled. (4) The amount of industrial waste can be reduced. [Brief explanation of the drawing]

[0029] [Figure 1]This is a schematic diagram showing an example of an acid / alkali recovery method and treatment system. [Figure 2] This is a schematic diagram showing an example of a wastewater treatment method and treatment system. [Modes for carrying out the invention]

[0030] Embodiment 1 of the present invention will be described below.

[0031] (The liquid being treated) The liquid to be treated in this implementation is a by-product obtained by the wastewater treatment method described later. This liquid to be treated is, for example, an aqueous solution mainly composed of boric acid and contains organic substances such as sodium sulfate, potassium iodide, PVA, zinc, potassium sulfate, and sodium thiosulfate (iodine reducing agent).

[0032] (Waste liquid) The waste liquid in this embodiment is, for example, waste liquid generated during the polarizing plate manufacturing process. The waste liquid may contain inorganic substances such as iodine, boron, and potassium, organic substances such as polyvinyl alcohol, alcohols such as glycerin, oils such as machine oil, and water. The waste liquid may also contain oxides and iodides such as boric acid and potassium iodide. The waste liquid may also contain alkali metals other than those mentioned above. Examples of alkali metals include sodium and lithium, one or more of which are present in the waste liquid. The alkali metals exist as cations in the waste liquid, but may also exist as fine particles. The waste liquid may also contain monovalent anions. Examples of monovalent anions may include one or more anions of halogen atoms such as fluorine, chlorine, and bromine. The waste liquid may also contain divalent and trivalent anions.

[0033] (Acid / alkali recovery method) Figure 1 shows an example of an acid / alkali recovery method. (S100) The liquid to be processed is sent to the NF membrane module 220 at a predetermined pressure (for example, 2.5 MPa or higher). The configuration of the NF membrane module 220 will be described later. The permeate is a boric acid-based aqueous solution containing iodide ions (e.g., potassium iodide). The concentrate is a boric acid-based treatment solution (1) containing a lower concentration of iodide ions than the permeate. Reducing the iodide ions in the concentrate suppresses the impact on the ion exchange membrane of the electrodialysis machine. The boric acid-based treatment solution (1) is sent to the wastewater treatment method described later and treated again (S100-1). The concentrated solution is processed in an electrodialysis machine. In the electrodialysis machine, the processing speed decreases due to the buffering effect when boric acid is present in the concentrated solution. Therefore, by removing boron in the NF membrane module 220, the processing in the subsequent electrodialysis machine can be optimized. In other words, by passing the solution to be processed through the NF membrane module 220, various problems such as deterioration of the ion exchange membrane due to iodide ions, a decrease in processing speed and current efficiency due to boric acid, and a decrease in current efficiency due to low sulfate and sodium ion concentrations are suppressed.

[0034] (S110) The concentrated solution is passed through an electrodialysis machine to separate it into a boric acid-based treatment solution (2) containing iodide ions, an acidic treatment solution, and an alkaline treatment solution. The boric acid-based treatment solution (2) is sent to the wastewater treatment method described later, similar to the boric acid-based treatment solution (1), and treated again (S110-1). The acidic treatment solution is an aqueous solution of high-concentration acid and can be reused as a pH adjuster by being sent to the wastewater treatment method described later (S110-2). The alkaline treatment solution is an aqueous solution of a high concentration of base and can be reused as a pH adjuster by being sent to the wastewater treatment method described later (S110-3).

[0035] (Waste liquid treatment method and treatment system) Figure 2 shows an example of a wastewater treatment method and treatment system. (S0) The polarizing plate manufacturing apparatus may include, for example, a dyeing bath, a stretcher, a crosslinking bath, a washing bath, a dryer, a film transport device, an adhesive coating device, and a device for attaching polarizers and polarizer protective films. Wastewater discharged from this manufacturing apparatus is stored in a buffer tank BT. The boric acid-based treatment solutions (1) and (2) described above are stored in buffer tank BT and provided for the following treatments, similar to wastewater.

[0036] (S1-1) Transfer the waste liquid from buffer tank BT to mixing tank MT1. In mixing tank MT1, adjust the pH of the waste liquid to make it alkaline. Use an aqueous sodium hydroxide solution as the alkali adjusting agent. The pH of the waste liquid after pH adjustment should be between 9 and 11. Here, the alkaline treatment solution obtained in S110-3 above can be used as the alkali adjusting agent. In this embodiment, the treatment is carried out in a batch manner, but it is not limited to this and continuous treatment can also be employed.

[0037] (S1-2) Mix the alkali adjusting agent (sodium hydroxide aqueous solution) with the waste liquid. Using the agitator M1, mix at a predetermined rotation speed for a predetermined time until the pH values ​​of the liquids in the upper, middle, and lower parts of the tank become uniform or approximately the same.

[0038] (S1-3) A boron-selective adsorbent is introduced into the mixing tank MT1. Examples of boron-selective adsorbents include ion exchange resin adsorbents. Examples of ion exchange resins include Amberlite® manufactured by Rohm & Haas.

[0039] (S1-4) The alkali-adjusted waste liquid is mixed with a boron-selective adsorbent. The mixture is stirred using a stirrer M1 for a predetermined time and at a predetermined rotational speed. Boric acid is selectively adsorbed onto the boron-selective adsorbent.

[0040] (S2) The boron-selective adsorbent on which boric acid has been adsorbed is separated from the waste liquid. The solid-liquid separation means used for separation is not particularly limited and may be a filtration device. Alternatively, a filter may be incorporated into the drain pipe and gate valve at the bottom of the mixing tank MT1 to transfer the liquid to another tank so that the boron-selective adsorbent is not washed away. In this case, mixing tanks MT1 and MT2 may be used interchangeably. The separated wastewater may be sent to wastewater treatment or mixed with the wastewater from S1-3 and S1-4 (S2-1).

[0041] (S3-1) The acidic solution and the boron-selective adsorbent on which boric acid has been adsorbed are added to the mixing tank MT2. The acidic solution is dilute sulfuric acid (sulfuric acid aqueous solution) with a concentration of 1% to 25%. The ratio of the amount of dilute sulfuric acid added to the amount of boron-selective adsorbent added may be set depending on the mixing conditions and the amount of boric acid contained in the eluent. Here, the acidic treatment solution obtained in S110-2 above can be used as the dilute sulfuric acid.

[0042] (S3-2) The acidic solution and the boron-selective adsorbent are mixed. The mixture is stirred using a stirrer M2 at a predetermined time and rotational speed to reduce the adsorption function and induce elution. Boric acid is eluted from the boron-selective adsorbent into the acidic solution, and this acidic solution from which the boric acid has been eluted is called the eluent.

[0043] (S3-3) The eluent and boron-selective adsorbent in the mixing tank MT2 are separated. The solid-liquid separation means used for separation is not particularly limited and may be a filtration device. Alternatively, a filter may be incorporated into the drain pipe and gate valve at the bottom of the mixing tank MT2 to transfer the eluent to the mixing tank MT3, so as not to wash away the boron-selective adsorbent.

[0044] (S3-3-1) The separated boron-selective adsorbent and base are added to the mixing tank MT4. In this embodiment, the base is an aqueous sodium hydroxide solution which is more alkaline than sulfuric acid (eluent). The concentration and amount of the aqueous sodium hydroxide solution added are adjusted to the extent necessary to regenerate the function of the boron-selective adsorbent. Here, the alkaline treatment solution obtained in S110-3 above can be used as the base.

[0045] (S3-3-2) Mix the boron-selective adsorbent with an aqueous sodium hydroxide solution. Mixing is performed using stirrer M4 for a predetermined time and at a predetermined rotational speed. The mixing conditions necessary for functional regeneration are set. (S3-3-3) Separate the boron-selective adsorbent from the sodium hydroxide aqueous solution. The solid-liquid separation means used for separation is not particularly limited and may include a filtration device. The separated liquid may be sent to wastewater treatment, or it may be used again as the sodium hydroxide aqueous solution S3-3-1.

[0046] (S4-1) The separated eluent and base are added to the mixing tank MT3. In this embodiment, the base is an aqueous sodium hydroxide solution which is more alkaline than sulfuric acid (eluent). The pH of the eluent after pH adjustment is 3 or more and less than 7. The ratio of the amount of aqueous sodium hydroxide solution added to the amount of eluent added is adjusted within the above pH range. Here, the alkaline treatment solution obtained in S110-3 above can be used as the base.

[0047] (S4-2) Mix the eluent and the sodium hydroxide aqueous solution. Using the stirrer M3, mix at a predetermined rotation speed for a predetermined time until the pH values ​​of the liquids in the upper, middle, and lower parts of the tank become uniform or approximately the same.

[0048] (S4-3) The pH-adjusted eluent is delivered to the NF membrane at a predetermined pressure (e.g., 2.5 MPa or higher). The permeate is an aqueous solution containing boric acid, without anions derived from sulfuric acid or cations derived from bases. The concentrate is an aqueous sulfuric acid solution containing anions derived from sulfuric acid and cations derived from bases. The aqueous sulfuric acid solution contains boric acid and sodium sulfate. The composition of the NF membrane will be described later. The amounts of permeate and concentrate can be controlled as appropriate with respect to the amount introduced into the NF membrane. In this embodiment, the permeate is set to, for example, 60% to 80% of the introduced amount, and the concentrate is set to, for example, 20% to 40% of the introduced amount.

[0049] (S5) Recover boric acid from the permeate. In this embodiment, several steps are performed to recover boric acid. (S5-1) Evaporate the water from the permeate to concentrate it so that the concentration of boric acid is high. The concentrated permeate is called a concentrated boric acid aqueous solution. For example, the concentration may be 50 times or more, 60 times or more, 70 times or more, or 80 times or more. Also, the concentration of boric acid after concentration may have an upper limit of 25% or less, 20% or less, or 16% or less, and a lower limit of 5% or more, 13% or more, or 15% or more. The means of evaporation concentration are not particularly limited, and an evaporative concentrator may be used. For example, heating evaporation, vacuum evaporation, or membrane separation using an RO membrane can be used for the concentration process.

[0050] (S5-2) The concentrated boric acid aqueous solution is subjected to cooling crystallization. Subsequently, the solid and liquid components are separated. For the cooling crystallization process, methods such as cooling crystallization, reduced-pressure crystallization, and reaction crystallization can be used, and for the solid-liquid separation process, methods such as centrifugation and filter separation can be used. (S5-2-1) The separated liquid (aqueous solution containing trace amounts of boric acid) may be sent to an NF membrane for further membrane separation treatment. If it is not sent to an NF membrane, it may be treated as wastewater or industrial waste.

[0051] (S5-3) Dry the solid components (which contain some moisture). The means of drying are not particularly limited and may include a drying apparatus capable of controlling temperature and humidity, or a constant temperature apparatus.

[0052] (S5-4) Boron with a moisture content of 5% or less and a purity of 95% or more can be recovered by drying. (S5-4-1) The recovered high-purity boron can be provided for recycling in the polarizing plate manufacturing process.

[0053] (S6-1) The water is evaporated from the concentrated solution (sulfuric acid solution) to concentrate it so that the concentrations of sodium sulfate and boric acid are high. The concentrated solution is called a concentrated boric acid acidic solution (sulfuric acid solution containing boric acid and sodium sulfate). For example, the concentration may be 50 times or more, 60 times or more, 70 times or more, or 80 times or more. The means of evaporation concentration are not particularly limited, and an evaporative concentrator may be used. For example, heating evaporation, vacuum evaporation, and membrane separation using an RO membrane can be used for the concentration process. Boron is contained in the permeate, and similarly in the concentrated solution (sulfuric acid solution). Also, the concentrated solution contains sodium sulfate, while the permeate does not contain sodium sulfate.

[0054] (S6-2) Cool the sulfuric acid solution containing boric acid and sodium sulfate. The sodium sulfate solidifies upon cooling. Then, separate the solid and liquid components. For the cooling process, methods such as cooling crystallization, reduced-pressure crystallization, and reaction crystallization can be used, and for the solid-liquid separation process, methods such as centrifugation and filter separation can be used. (S6-2-1) The separated sulfuric acid solution (liquid portion) containing boric acid and sodium sulfate may be sent again to be subjected to membrane separation treatment using an NF membrane.

[0055] (S6-3) The separated sodium sulfate (solid component) may be sent as a by-product to the acid / alkali recovery method described above.

[0056] (Embodiment 2) Embodiment 2 includes an adsorption and removal step in addition to the steps of Embodiment 1. The adsorption removal step, performed before the electrodialysis step, removes at least one of iodine and PVA from the concentrated solution obtained in the NF membrane separation step by passing it through an adsorbent. Furthermore, this adsorption removal step removes at least one of iodine and PVA from the solution to be treated before passing it through the NF membrane separation step by passing it through an adsorbent. The adsorbent may consist of one or more of the following: ion exchange resin, activated carbon, activated alumina, silica gel, etc.

[0057] (Embodiment 3) Embodiment 3 includes an iodine reduction step in addition to the steps of Embodiment 1 or 2. The iodine reduction process reduces the iodine contained in the acidic treatment solution separated in the electrodialysis process to iodide ions using a reducing agent. By reducing the iodine to iodide ions, corrosion of pipes, tanks, etc., is suppressed, and long-term storage becomes possible.

[0058] (Another embodiment) (1) After the cooling crystallization solid-liquid separation step (S5-2), the process of dissolving boric acid in warm water (e.g., 65-75°C) and performing cooling crystallization (e.g., 15°C-22°C) to separate the solid and liquid may be repeated one or more times. (2) Instead of dilute sulfuric acid, an aqueous hydrochloric acid solution may be used as the acidic solution. (3) Potassium hydroxide solution may be used instead of sodium hydroxide solution as an alkaline pH adjuster.

[0059] (NF film module) Examples of NF film modules 200 and 220 used in this embodiment are shown below. The membrane separation system, equipped with NF membrane modules 200 and 220, is configured to operate under desired conditions, with additional equipment such as pumps, sensors, tanks, control valves, and control devices attached as needed.

[0060] An NF membrane may be a composite semipermeable membrane comprising, for example, a porous support membrane and a separation functional layer, wherein the separation functional layer is supported by the porous support membrane. The material and structure of the porous support membrane are not particularly limited. As the porous support membrane, for example, an ultrafiltration membrane is used in which a microporous layer having an average pore size of 0.01 to 0.4 μm is formed on a nonwoven fabric. Examples of materials for forming the microporous layer include polysulfone, polyaryl ethersulfone such as polyethersulfone, polyimide, polyvinylidene fluoride, and polytetrafluoroethylene.

[0061] Examples of negatively charged NF films include those having a separation functional layer with anionic groups. Examples of anionic groups include sulfonic acid groups and carboxylic acid groups, but sulfonic acid groups, which are strong acidic groups, are preferred.

[0062] Furthermore, examples of resins constituting the separation functional layer include polysulfone resins, polyamides, cellulose acetate, and polyvinyl alcohol, but polysulfone resins are particularly preferred from the viewpoint of chemical, mechanical, and thermal stability. Examples of polysulfone resins include polysulfone, polyethersulfone, and polyphenylsulfone.

[0063] In other words, a preferred separation functional layer for an NF membrane is one that contains a polysulfone resin having sulfonic acid groups. In particular, NF membranes having such a separation functional layer exhibit higher durability against alkaline cleaning solutions and chlorine-based cleaning solutions.

[0064] As for NF films in which the separation functional layer is made of sulfonated polyethersulfone having a negative fixed charge, those described in Japanese Patent Publication No. 61-4505 and Japanese Patent Publication No. 61-4506 are particularly preferred.

[0065] The membrane modules 200 and 220 using NF membranes may consist of one or more membrane elements. The membrane elements are typically spiral-type membrane elements using NF membranes. The membrane modules 200 and 220 may also consist of a pressure vessel and one or more spiral-type membrane elements arranged inside the pressure vessel. However, the structure of the membrane elements containing NF membranes is not limited to the spiral type and may be of other types such as hollow fiber, tubular, or frame-and-plate. [Examples]

[0066] (Example 1) Boric acid was recovered using the method shown in Figure 2, and by-products were obtained. Composition of waste liquid: Water (95% by mass), boric acid (1.0% by mass), iodine (0.8% by mass), : PVA (1.2% by mass), potassium iodide (1.2% by mass), Other (0.8% by mass) pH adjuster: Sodium hydroxide aqueous solution (NaOH: 10% by mass, water: 80% by mass) Acidic solution: Sulfuric acid water (H2SO4: 5% by mass, water: 95% by mass) NF film: PRO-XS3 (manufactured by Nitto Denko) Evaporator: The solution was concentrated 71 times under reduced pressure at 70°C to a boric acid concentration of 16% by mass. Cooling crystallization: This was performed at 15°C to 22°C for 12 hours until crystallization occurred. Solid-liquid separation: This was performed using filter separation. Drying: Drying was performed at 60°C to 100°C. (This was done at a temperature at which boric acid does not decompose, e.g., 66°C) Mixing of each step: Mixing of solutions was allowed for 10 minutes, and mixing of the adsorbent and the liquid was allowed for 10 minutes or more. Recovered boric acid: We were able to recover boric acid with a purity of 99%. By-product: Sodium sulfate: approximately 9% Boric acid: Approximately 4% Potassium iodide (KI): Approximately 0.5% : Small amount of organic matter (about 250ppm) :Water: Rest

[0067] The above by-products are processed using the method shown in Figure 1. Permeate after NF film treatment: Boric acid-based treatment solution (1), 4% by mass of boric acid Sodium sulfate and potassium levels decrease significantly. Iodine levels are maintained or concentrated (due to a significant decrease in sodium sulfate and potassium, the iodine concentration in the permeate is maintained or increased compared to the iodine concentration in the treated solution). Boric acid levels remain largely unchanged (90% maintained). :Organic matter (same level) Concentrated solution after NF membrane treatment: Sodium sulfate and potassium levels increase significantly (high concentration). Iodine levels decreased (due to a significant increase in sodium sulfate and potassium, the iodine concentration in the concentrate was lower than that in the treated solution). Boric acid is maintained or slightly concentrated. :Organic matter (same level) Boric acid-based treatment solution after electrodialysis (2): Aqueous solution of 4% by mass of boric acid, also containing KI. Acidic treatment solution after electrodialysis: Sulfuric acid (H2SO4: 5% by mass or more) Alkaline treatment solution after electrodialysis: Sodium hydroxide aqueous solution (NaOH: 4% by mass or more)

[0068] As described above, boric acid-based treatment solutions, acidic treatment solutions, and alkaline treatment solutions can be suitably obtained while suppressing the deterioration of the ion exchange membrane of the electrodialysis apparatus. [Explanation of Symbols]

[0069] 100 Polarizing plate manufacturing equipment 200 NF film module 220 NF film module

Claims

1. An acid and alkali recovery method comprising separating a treated liquid, which is a waste liquid containing boric acid and potassium iodide generated during the polarizing plate manufacturing process, into an acidic solution, an alkaline solution, and a boric acid-based treatment liquid by electrodialysis, wherein the treated liquid is a waste liquid containing boric acid and potassium iodide, to which an acidic solution and an alkaline solution are added, The process includes an NF membrane separation step in which the liquid to be treated is passed through an NF membrane to separate it into a permeate containing iodide ions and a concentrated solution containing iodide ions at a lower concentration than the permeate, The aforementioned concentrated liquid is passed through an electrodialysis apparatus to separate it into a boric acid-based treatment solution, an acidic treatment solution, and an alkaline treatment solution in an electrodialysis process. including, Acid and alkali recovery methods.

2. Prior to the electrodialysis step, the concentrated liquid obtained in the NF membrane separation step, or the liquid to be treated before passing it through the NF membrane separation step, is subjected to an adsorption removal step in which at least one of iodine and PVA is removed by passing it through an adsorbent. The method for recovering acids and alkalis according to claim 1.

3. The process includes an iodine reduction step in which iodine contained in the acidic treatment solution separated in the electrodialysis step is reduced to iodide ions using a reducing agent. The method for recovering acids and alkalis according to claim 1.

4. The aforementioned waste liquid contains boric acid, potassium iodide, and polyvinyl alcohol. The acidic solution is an aqueous solution containing one or two selected from sulfuric acid and hydrochloric acid. The aforementioned alkaline solution is an aqueous solution containing one or two substances selected from sodium hydroxide and potassium hydroxide. The method for recovering acids and alkalis according to claim 1.

5. A method for recovering and producing boric acid, which selectively recovers boric acid from waste liquid generated during the polarizing plate manufacturing process, A method for recovering and producing boric acid, characterized by reusing the acid or alkali obtained by processing with the acid / alkali recovery method described in claim 1.

6. The present invention includes a recovery and manufacturing method for selectively recovering boric acid from waste liquid generated during the polarizing plate manufacturing process described in claim 5, The acid or alkali recovered from the wastewater generated in the polarizing plate manufacturing process using the acid / alkali recovery method described in claim 1 is used as an acid / alkali used in the treatment of recovering boric acid and / or potassium iodide from the wastewater. A method for manufacturing polarizing plates.

7. The boric acid and / or potassium iodide obtained from the treatment of recovering boric acid and / or potassium iodide from the aforementioned waste liquid are used in the polarizing plate manufacturing process. The method for manufacturing a polarizing plate according to claim 6.

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