Acid / alkali recovery method, boric acid recovery / manufacturing method, and polarizing plate manufacturing method

Abstract

This acid / alkali recovery method is a method for separating a liquid to be treated, which is obtained by adding an acidic solution and an alkaline solution to a waste liquid containing boric acid and potassium iodide generated in a polarizing plate production process, into an acidic solution, an alkaline solution, and a mainly-boric acid-based treatment liquid by electrodialysis. The acid / alkali recovery method comprises: an NF membrane separation step for passing a liquid to be treated through an NF membrane to separate the liquid to be treated into a permeated liquid containing iodide ions and a concentrated liquid containing iodide ions at a lower concentration than the permeated liquid; and an electrodialysis step for passing the concentrated liquid through an electrodialysis device to separate the concentrated liquid into a mainly-boric acid-based treatment liquid, an acidic treatment liquid, and an alkaline treatment liquid.

Description

Method for recovering acid and alkali, method for recovering and producing boric acid, and method for producing polarizing plate

[0001] The present invention relates to a method for recovering acid and alkali. For example, a treated liquid 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 containing boric acid. The present invention also relates to a method for recovering and producing boric acid and a method for producing a polarizing plate.

[0002] The 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 desire 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 for recovering potassium iodide and boric acid. The treatment method of Patent Document 1 is to solid-liquid separate the precipitate obtained by evaporating and concentrating the waste liquid, recover the filtrate containing potassium iodide, and recover boric acid from the precipitate. In the boric acid recovery step, an acid is added to the treated liquid in which the precipitate is dissolved to adjust the pH, and then cooled to separate the precipitated boric acid crystals. After adding an alkali to the treated 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 adjusts the pH of the wastewater containing iodine and boron to 11 to 14, concentrates it so that the boron concentration becomes 0.5% by mass or more, adds an adsorbent for removing organic substances contained in the liquid obtained by concentration, cools the liquid, and adjusts 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.

[0005] Japanese Patent Application Laid-Open No. 2023-72964, Patent No. 4674168

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

[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 an acid and alkali recovery method which separates a treated liquid, obtained by adding an acidic solution and an alkaline solution to a waste liquid containing boric acid and potassium iodide generated in the polarizing plate manufacturing process, into an acidic solution, an alkaline solution, and a boric acid-based treatment liquid by electrodialysis, and comprises: an NF membrane separation step (S100) in which the treated liquid is passed through an NF membrane to separate it into a permeate containing iodide ions and a concentrated liquid containing iodide ions at a lower concentration than the permeate; and an electrodialysis step (S110) in which the concentrated liquid is passed through an electrodialysis apparatus to separate it into a boric acid-based treatment liquid, an acidic treatment liquid, and an alkaline treatment liquid.

[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 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; and 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 passing it 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 the iodide ions, corrosion of pipes, tanks, etc., is suppressed, and long-term storage becomes possible.

[0014] The liquid to be treated may contain anions derived from the acid (e.g., sulfuric acid) in the acidic solution 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) in which boric acid is selectively adsorbed onto an adsorbent from the wastewater whose pH has been adjusted to be alkaline (S1-1, S1-2); a boric acid desorption step (S3-1, S3-2) in which the boric acid is eluted from the adsorbent into the acidic solution by contacting the adsorbent with an acidic solution to obtain an eluent; an NF membrane separation step (S4-3) in which a base is added to the eluent (S4-1) and the eluent (S4-2) whose pH has been adjusted to be more acidic than neutral is passed through an NF membrane to separate it into a permeate containing boric acid but not containing anions derived from the acid or cations derived from the base, and a concentrated solution containing anions derived from the acid or cations derived from the base; and a second evaporation concentration step (S6-1) in which water is evaporated from the concentrated solution to increase the concentration of boric acid and obtain a concentrated acidic boric acid solution. The wastewater treatment method includes a cooling solid-liquid separation step (S6-2) in which the boric acid concentrated acidic solution is cooled and separated into a boric acid-containing acidic solution and a solid component, wherein the liquid containing the solid component (for example, sodium sulfate, potassium iodide, etc.) may be the liquid to be treated. The wastewater treatment method may also include a boric acid recovery step (S5) in which boric acid is recovered from the permeate.

[0016] The process may include an adsorbent separation step (S2) after the boric acid adsorption step, in which the waste liquid and the adsorbent are separated. The waste liquid (separated liquid) separated in the adsorbent separation step (S2) may be added to 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, and the pH of the waste liquid after pH adjustment may be between 9 and 11. The alkali adjusting agent may also be an aqueous solution. The specified numerical range of pH is due to the requirement for an adsorbent that can selectively adsorb boric acid. If the pH is on the acidic side, boric acid does not exist as ions, so the amount of adsorption will be small, and therefore it is preferable for the pH to be on the alkaline side.

[0018] The acidic solution may be one or more selected from dilute sulfuric acid with a concentration of 1% to 25% or hydrochloric acid with a concentration of 1% to 25%. The acidic solution may be an aqueous solution of sulfuric acid or an aqueous solution of hydrochloric acid. The numerical range of the concentration is determined by the requirements for setting the hydraulic pressure of the NF membrane. The higher the concentration of sulfuric acid or hydrochloric acid, the higher the pressure required. Conversely, lowering the concentration will require a large amount of water for dilution.

[0019] In the NF membrane separation step, the base (pH adjuster) used to adjust the pH to the acidic side from neutral is sodium hydroxide or potassium hydroxide, which is more alkaline than the acidic solution, and the pH of the eluent after pH adjustment may be 3 or more and less than 7. The specified numerical range of pH is for obtaining boric acid in the boric acid concentration and cooling crystallization step, and if it is outside the specified range, borate will be obtained, which is undesirable. Furthermore, if the pH is less than 3, the inhibition rate of other components will decrease due to deterioration of the NF membrane module, and if the pH is 7 or higher, the rate at which boric acid passes through to the permeate side by the NF membrane will decrease extremely, and most of the boric acid will remain on the concentrate side, which is undesirable.

[0020] The boric acid recovery step (S5) may include: a first evaporation concentration step (S5-1) in which water is evaporated from the boric acid aqueous solution in the permeate to obtain a concentrated boric acid aqueous solution; a cooling crystallization solid-liquid separation step (S5-2) in which the concentrated boric acid aqueous solution concentrated to a predetermined concentration is cooled and crystallized to separate the solid and liquid components; and a drying step (S5-3, S5-4) in which the solid-liquid separated boric acid is dried at a predetermined temperature to obtain boric acid with a purity of, for example, 95% or higher, 96% or higher, 97% or higher, or 98% or higher.

[0021] After the cooling crystallization solid-liquid separation step (S5-2), the process of dissolving the boric acid in hot water and performing cooling crystallization to separate the solid and liquid 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 wastewater treatment method 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 wastewater treatment method may 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 present disclosure provides a method for manufacturing a polarizing plate, which includes a recovery manufacturing method for selectively recovering boric acid from wastewater generated during the polarizing plate manufacturing process, and is characterized by reusing the acid or alkali obtained by treating the wastewater generated during the polarizing plate manufacturing process with the acid / alkali recovery method. The acid or alkali recovered by the acid / alkali recovery method may be used as the acid or alkali used in the treatment of recovering boric acid and / or potassium iodide from the wastewater, 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. The acid / alkali recovery method may utilize the obtained acid and alkali as the acidic solution and alkali solution added to the wastewater. The obtained alkali may be used as an alkali adjusting agent used in the step of adjusting the pH to alkaline 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 in the wastewater treatment method and reused as a treatment solution used in the polarizing plate manufacturing process. The obtained boric acid may be part or all of the raw material for the boric acid aqueous solution used when manufacturing the polarizers that constitute the polarizing plate. The boric acid may also be used as a raw material for the boric acid aqueous solution used in one or more processes selected from the boric acid aqueous solution pre-contact process, dyeing process, crosslinking process, and stretching process of the polarizers that constitute the polarizing plate.

[0028] (1) Degradation of ion exchange membranes used in electrodialysis by iodide ions can be suppressed. (2) The boric acid-based treatment solution obtained by electrodialysis and the boric acid-based treatment solution, 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.

[0029] This is a schematic diagram showing an example of an acid / alkali recovery method and treatment system. This is a schematic diagram showing an example of a waste liquid treatment method and treatment system.

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

[0031] (Treatment Liquid) The treatment liquid used in this embodiment is a by-product obtained by the wastewater treatment method described later. This treatment liquid 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 treated is sent to the NF membrane module 220 at a predetermined pressure (for example, 2.5 MPa or more). The configuration of the NF membrane module 220 will be described later. The permeate is an aqueous solution mainly composed of boric acid containing iodide ions (for example, potassium iodide). The concentrate is a boric acid-based treatment solution (1) containing iodide ions at a lower concentration than that of the permeate. Reducing the amount of iodide ions in the concentrate suppresses the effect on the ion exchange membrane of the electrodialysis machine. The boric acid-based treatment solution (1) is sent to the waste liquid treatment method described later and treated again (S100-1). The concentrate is treated in an electrodialysis machine. In the electrodialysis machine, the processing speed decreases due to the buffering effect when boric acid is present in the concentrate. 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 liquid to be treated through the NF membrane module 220, it is possible to suppress 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 concentrations of sulfate and sodium ions.

[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, in the same manner as the boric acid-based treatment solution (1), and is 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 high-concentration base and can be reused as a pH adjuster by being sent to the wastewater treatment method described later (S110-3).

[0035] (Wastewater 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 stretching machine, 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. The wastewater discharged from this manufacturing apparatus is stored in a buffer tank BT. The boric acid-based treatment liquid (1) and boric acid-based treatment liquid (2) described above are stored in the buffer tank BT and provided for the following treatment in the same manner as the wastewater.

[0036] (S1-1) The waste liquid is transferred from the buffer tank BT to the mixing tank MT1. In the mixing tank MT1, the pH of the waste liquid is adjusted to make it alkaline. An aqueous sodium hydroxide solution is used as the alkali adjusting agent. The pH of the waste liquid after pH adjustment is set to 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 so that 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 added to the mixing tank MT1. An example of a boron-selective adsorbent is an ion exchange resin adsorbent. An example of an ion exchange resin is Amberlite® manufactured by Rohm & Haas.

[0039] (S1-4) The alkali-adjusted waste liquid and the boron-selective adsorbent are mixed. 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) Separate the boron-selective adsorbent adsorbed with boric acid from the waste liquid. The solid-liquid separation means used for separation is not particularly limited, and a filtration device may be used. Also, a filter may be incorporated into the drain pipe and the control valve at the bottom of the mixing tank MT1, and the liquid fraction may be transferred to another tank so that the boron-selective adsorbent does not flow through. In this case, the mixing tank MT1 and the mixing tank MT2 may be used in common. The separated waste liquid may be sent to wastewater treatment or may be mixed with the waste water of S1-3 and S1-4 (S2-1).

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

[0042] (S3-2) Mix the acidic solution and the boron-selective adsorbent. Mixing is performed by the stirrer M2 at a predetermined time and a predetermined rotation speed to reduce the adsorption function and elute. Boric acid elutes from the boron-selective adsorbent into the acidic solution, and this acidic solution in which boric acid has eluted is called an eluent.

[0043] (S3-3) Separate the eluent and the boron-selective adsorbent in the mixing tank MT2. The solid-liquid separation means used for separation is not particularly limited, and a filtration device may be used. Also, a filter may be incorporated into the drain pipe and the control valve at the bottom of the mixing tank MT2, and the eluent may be transferred to the mixing tank MT3 so that the boron-selective adsorbent does not flow through.

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

[0045] (S3-3-2) Mix the boron selective adsorbent and the sodium hydroxide aqueous solution. Use the stirrer M4 to perform the mixing at a predetermined rotation speed for a predetermined time. The mixing conditions required for function regeneration are set. (S3-3-3) Separate the boron selective adsorbent and the sodium hydroxide aqueous solution. The solid-liquid separation means used for separation is not particularly limited, and a filtration device may be used. The separated liquid portion may be sent for wastewater treatment or may be used again as the sodium hydroxide aqueous solution in S3-3-1.

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

[0047] (S4-2) Mix the eluate and the sodium hydroxide aqueous solution. Use the stirrer M3 to perform the mixing at a predetermined rotation speed for a predetermined time so that the pH values of the upper, middle, and lower liquids in the tank become uniform or approximately the same.

[0048] (S4-3) The eluate with adjusted pH is sent to the NF membrane at a predetermined pressure (for example, 2.5 MPa or more). The permeate is an aqueous solution containing boric acid and not containing anions derived from sulfuric acid and cations derived from the base. The concentrate is a sulfuric acid aqueous solution containing anions derived from sulfuric acid and cations derived from the base. The sulfuric acid aqueous solution contains boric acid and sodium sulfate. The configuration of the NF membrane will be described later. With respect to the amount input to the NF membrane, the amounts of the permeate and the concentrate can be appropriately controlled. In this embodiment, the permeate is set to, for example, 60% to 80% of the input amount, and the concentrate is set to, for example, 20% to 40% of the input 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. The concentration of boric acid after concentration may also be, for example, 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 evaporator may be used. For example, heating evaporation, reduced-pressure 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 cooled and crystallized. Then, the solid and liquid components are separated. For the cooling and crystallization process, methods such as cooling crystallization, reduced-pressure crystallization, and reaction crystallization can be used, and for the solid and liquid separation process, methods such as centrifugation and filter separation can be used. (S5-2-1) The separated liquid component (aqueous solution containing trace amounts of boric acid) may be sent to be subjected to membrane separation again through an NF membrane. 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) The sulfuric acid solution containing boric acid and sodium sulfate is cooled. Cooling causes the sodium sulfate to solidify. Then, the solid and liquid components are separated. 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 containing boric acid and sodium sulfate (liquid component) may be sent to be subjected to membrane separation again 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 removal step in addition to the steps of Embodiment 1. The adsorption removal step removes at least one of iodine and PVA from the concentrated liquid obtained in the NF membrane separation step by passing it through an adsorbent before the electrodialysis step. This adsorption removal step removes at least one of iodine and PVA from the liquid to be treated by passing it through an adsorbent before passing it through the NF membrane separation step. The adsorbent may consist of one or more of the following, for example: 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 step reduces the iodine contained in the acidic treatment solution separated in the electrodialysis step to iodide ions using a reducing agent. By reducing to iodide ions, corrosion of pipes, tanks, etc. is suppressed, and long-term storage becomes possible.

[0058] (Alternative Embodiments) (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) Instead of an aqueous sodium hydroxide solution, an aqueous potassium hydroxide solution may be used as the alkaline pH adjusting agent.

[0059] (NF Membrane Modules) Examples of NF membrane modules 200 and 220 used in this embodiment are shown below. The membrane separation apparatus equipped with NF membrane modules 200 and 220 is configured to be operated under desired conditions by being equipped with other devices such as pumps, sensors, tanks, control valves, and control devices 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, an NF membrane having such a separation functional layer exhibits 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 type, tubular type, or frame and plate type.

[0066] (Example 1) Boric acid was recovered and by-products were obtained by the method shown in Figure 2. Components 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 solution (H ２ SO ４: 5% by mass, Water: 95% by mass) NF membrane: PRO-XS3 (manufactured by Nitto Denko) Evaporator: Concentrated 71 times under reduced pressure at 70°C to a boric acid concentration of 16% by mass. Cooling crystallization: Performed at 15°C to 22°C for 12 hours until crystallization occurred. Solid-liquid separation: Performed by filter separation. Drying: Dried at 60°C to 100°C. (Performed 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 adsorbent and liquid was allowed for 10 minutes or more. Recovered boric acid: Boric acid with a purity of 99% was recovered. By-products: Sodium sulfate: approximately 9% Boric acid: approximately 4% Potassium iodide (KI): approximately 0.5% Small amount of organic matter (approximately 250 ppm) Water: remainder

[0067] The above by-products are treated by the method shown in Figure 1. Permeate after NF membrane treatment: Boric acid-based treatment solution (1), 4% by mass of boric acid: Sodium sulfate and potassium are greatly reduced: Iodine is maintained or concentrated (due to the significant reduction in sodium sulfate and potassium, the iodine in the permeate is maintained or increased compared to the iodine concentration in the treated solution): Boric acid is roughly maintained (90% maintained): Organic matter (same level) Concentrated solution after NF membrane treatment: Sodium sulfate and potassium are greatly increased (highly concentrated): Iodine is reduced (due to the significant increase in sodium sulfate and potassium, the iodine in the concentrated solution is reduced compared to the iodine concentration 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 (H ２ SO ４ : 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.

[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 in the polarizing plate manufacturing process, into an acidic solution, an alkaline solution, and a boric acid-based treatment liquid by electrodialysis, the method comprising: an NF membrane separation step of passing the treated liquid through an NF membrane to separate it into a permeate containing iodide ions and a concentrated liquid containing iodide ions at a lower concentration than the permeate; and an electrodialysis step of passing the concentrated liquid through an electrodialysis apparatus to separate it into a boric acid-based treatment liquid, an acidic treatment liquid, and an alkaline treatment liquid.

2. The acid / alkali recovery method according to claim 1, further comprising an adsorption removal step, prior to the electrodialysis step, by passing 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, through an adsorbent to remove at least one of iodine and PVA.

3. The acid / alkali recovery method according to claim 1 or 2, further comprising an iodine reduction step of reducing the iodine contained in the acidic treatment solution separated in the electrodialysis step to iodide ions with a reducing agent.

4. The method for recovering acids and alkalis according to any one of claims 1 to 3, wherein the 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; and the alkaline solution is an aqueous solution containing one or two selected from sodium hydroxide and potassium hydroxide.

5. A method for recovering and producing boric acid, comprising selectively recovering boric acid from waste liquid generated during the polarizing plate manufacturing process, characterized in that the acid or alkali obtained by treatment with the acid / alkali recovery method described in claim 1 is reused.

6. A method for manufacturing polarizing plates, comprising a recovery manufacturing method for selectively recovering boric acid from waste liquid generated in the polarizing plate manufacturing process described in claim 5, wherein the acid or alkali recovered from the waste liquid generated in the polarizing plate manufacturing process by 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 waste liquid.

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

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