Method for treating waste liquid, method for recovering and producing boric acid, and method for producing polarizing plate

Abstract

A method for treating waste liquid according to the present invention comprises: a boric acid adsorption step for selectively adsorbing boric acid onto an adsorbent from a waste liquid having a pH adjusted to be alkaline; a boric acid desorption step for bringing an acidic solution into contact with the adsorbent to cause the boric acid to elute from the adsorbent into the acidic solution to obtain an eluate; an NF membrane separation step for adding a base to the eluate and passing the eluate that has been pH adjusted further to the acidic side from neutral through a NF membrane to separate the eluate into a permeate containing the boric acid without containing acid-derived anions or base-derived cations, and a concentrate containing acid-derived anions and base-derived cations; and a boric acid recovery step for recovering boric acid from the permeate.

Description

Method for treating waste liquid, method for recovering and producing boric acid, and method for producing polarizing plate

[0001] The present invention relates to a method for treating waste liquid, for example, a method for treating waste liquid that selectively recovers boric acid from waste liquid generated in the process of manufacturing a polarizing plate. It 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 boric acid, 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 that recovers potassium iodide and boric acid from the waste liquid. 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 treated liquid in which the precipitate is dissolved to adjust the pH, followed by cooling 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 method for treating wastewater in Patent Document 2 involves adjusting the pH of the wastewater containing iodine and boron to 11 to 14, concentrating it so that the boron concentration becomes 0.5% by mass or more, adding an adsorbent to remove the 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 the boron component. The obtained precipitate is removed to separate the boron, and chlorine is supplied to the obtained liquid to oxidize the iodine and precipitate it to recover the iodine.

[0005] In the method for treating wastewater in Patent Document 3, the wastewater from which sulfate ions that are preferentially adsorbed by the layered inorganic hydroxide rather than boron have been removed is brought into contact with the layered double hydroxide, and it is disclosed that the layered double hydroxide adsorbs and removes boron. As a method for removing sulfate ions, it is disclosed that an RO membrane (reverse osmosis membrane) or an NF membrane (nanofiltration membrane) may be used.

[0006] Japanese Patent Publication No. 2023-72964, Japanese Patent Publication No. 4674168, Japanese Patent Publication No. 6105500

[0007] However, neither of the processing methods described in Patent Documents 1 and 2 uses an NF membrane (nanofiltration membrane). Patent Document 3 discloses the removal of sulfate ions, which are more readily adsorbed than boron, using an RO membrane (reverse osmosis membrane) or an NF membrane (nanofiltration membrane).

[0008] Therefore, the present invention provides a wastewater treatment method for selectively recovering boric acid from wastewater that has undergone predetermined pretreatment using a method different from that of the prior art. The invention also provides a method for recovering and producing boric acid and a method for producing polarizing plates.

[0009] The present inventors, after diligent research to solve the aforementioned problems, have found that boric acid can be selectively adsorbed and recovered from alkaline-treated wastewater onto a resin, and then, after the eluent obtained by acid treatment of this resin is pH adjusted to the acidic side from neutral, the boric acid-containing water separated by an NF membrane is concentrated, crystallized, and dehydrated by filtration, thereby recovering boric acid in high purity. That is, the present invention includes the following embodiments.

[0010] The wastewater treatment method of this disclosure is a wastewater treatment method for selectively recovering boric acid from wastewater generated in the polarizing plate manufacturing process, comprising: 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) to adjust the pH of the eluent (S4-2) to be acidic rather than neutral, and the eluent 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 of the acidic solution, and a concentrated solution containing anions derived from the acid or cations derived from the base of the acidic solution; and a boric acid recovery step (S5) in which boric acid is recovered from the permeate. Includes.

[0011] 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).

[0012] 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.

[0013] 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.

[0014] 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 the deterioration of the NF membrane module. Furthermore, if the pH is 7 or higher, the rate at which boric acid passes through to the permeate side by the NF membrane decreases extremely, and most of the boric acid remains on the concentrate side, which is undesirable.

[0015] 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.

[0016] 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.

[0017] 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 (S4-3) is performed by passing the separated liquid through the NF membrane.

[0018] The wastewater treatment method may further include an adsorbent regeneration step (S3-3-1, S3-3-2, S3-3-3) in which the adsorbent is regenerated by contacting it with an alkaline solution.

[0019] The adsorbent may be a boron-selective adsorbent.

[0020] The waste liquid treatment method may include: a second evaporation concentration step (S6-1) in which water is evaporated from the concentrated liquid to increase the concentration of boric acid and obtain a concentrated boric acid acidic solution; a cooling solid-liquid separation step (S6-2) in which the concentrated boric acid acidic solution is cooled and separated into a boric acid-containing acidic solution and a solid component; and a separated boric acid re-recovery step (S6-2-1) in which the solid-liquid separated boric acid (boric acid-containing acidic solution) is combined with the pH-adjusted eluent and passed through the NF membrane to perform the NF membrane separation step.

[0021] The boric acid recovery and production method of this disclosure is a method for selectively recovering boric acid from wastewater generated in the polarizing plate manufacturing process, comprising: a boric acid adsorption step of selectively adsorbing boric acid onto an adsorbent from the wastewater whose pH has been adjusted to be alkaline; a boric acid desorption step of contacting an acidic solution with the adsorbent to eluate the boric acid from the adsorbent into the acidic solution to obtain an eluent; an NF membrane separation step of adding a base to the eluent to adjust the pH to be more acidic than neutral and passing the eluent 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 of the acidic solution, and a concentrated solution containing anions derived from the acid or cations derived from the base of the acidic solution; and a boric acid recovery step of recovering boric acid from the permeate. The method may further include each of the above steps of the wastewater treatment method.

[0022] The present disclosure is a method for manufacturing a polarizing plate, comprising a treatment method for selectively recovering boric acid from waste liquid generated during the polarizing plate manufacturing process, wherein the treatment method for selectively recovering boric acid from the waste liquid is selected from the boric acid recovery and manufacturing method described above or the waste liquid treatment method described above, and the boric acid obtained by the treatment method for selectively recovering boric acid from the waste liquid may be characterized in that it becomes part or all of the raw material for the boric acid aqueous solution used when manufacturing the polarizer constituting the polarizing plate. The present disclosure is also characterized in that the boric acid obtained by the treatment method for selectively recovering boric acid from the waste liquid becomes the raw material for the boric acid aqueous solution used in one or more steps selected from the boric acid aqueous solution pre-contact step, dyeing step, crosslinking step, and stretching step of the polarizer constituting the polarizing plate.

[0023] The wastewater treatment system of the present disclosure is a wastewater treatment system for selectively recovering boric acid from wastewater generated in the polarizing plate manufacturing process, comprising: an adsorption means equipped with a boron-selective adsorbent that selectively adsorbs boric acid from the wastewater whose pH has been adjusted to be alkaline; a boric acid desorption means for contacting an acidic solution with the boron-selective adsorbent to eluate the boric acid from the boron-selective adsorbent into the acidic solution to obtain an eluent; an NF membrane separation means for adding a base to the solution to adjust the eluent to a pH more acidic than neutral, and separating the eluent into a permeate containing boric acid but not containing anions derived from the acid or cations derived from the base of the acidic solution, and a concentrated solution containing anions derived from the acid or cations derived from the base of the acidic solution; and a boric acid recovery means for recovering boric acid from the permeate.

[0024] The boric acid recovery means may include: an evaporation and concentration device for evaporating water from the permeate to concentrate the boric acid solution to a predetermined concentration; a cooling and crystallization device for cooling and crystallizing the concentrated boric acid solution; and a solid-liquid separation device (for example, a filtration device, a centrifugal separator, or a dehydration device) for solid-liquid separation of the crystallized boric acid and water.

[0025] The wastewater treatment system may include an evaporator that evaporates water from the concentrated liquid to increase the concentration of boric acid and obtain a concentrated boric acid solution, and a cooling solid-liquid separator that cools the concentrated boric acid solution and separates it into solid boric acid and liquid acid solution.

[0026] (1) Boric acid can be selectively recovered from wastewater that has undergone a predetermined pretreatment using a method different from conventional technology. (2) The recovered high-purity boric acid can be recycled. (3) The amount of industrial waste can be reduced.

[0027] This is a schematic diagram showing an example of a wastewater treatment method and treatment system.

[0028] Embodiments of the present invention will be described below.

[0029] (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.

[0030] (Wastewater Treatment Method and Treatment System) Figure 1 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.

[0031] (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. A sodium hydroxide aqueous solution is used as the alkalinity adjusting agent. The pH of the waste liquid after pH adjustment should be between 9 and 11. In this embodiment, the process is carried out in batches, but it is not limited to this and continuous processing can also be employed.

[0032] (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.

[0033] (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.

[0034] (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.

[0035] (S2) The boron-selective adsorbent on which boric acid has been adsorbed is separated from the wastewater. 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).

[0036] (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.

[0037] (S3-2) The acidic solution and the boron-selective adsorbent are mixed. The mixture is stirred with a stirrer M2 for a predetermined time at a predetermined rotation 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.

[0038] (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.

[0039] (S3-3-1) The separated boron selective adsorbent and the base are introduced into the mixing tank MT4. In this embodiment, the base is an aqueous sodium hydroxide solution that is more alkaline than sulfuric acid (the eluent). The concentration and the amount of the aqueous sodium hydroxide solution are adjusted within the range necessary to regenerate the function of the boron selective adsorbent.

[0040] (S3-3-2) The boron selective adsorbent and the aqueous sodium hydroxide solution are mixed. Mixing is performed by the stirrer M4 at a predetermined rotation speed for a predetermined time. Mixing conditions necessary for function regeneration are set. (S3-3-3) The boron selective adsorbent and the aqueous sodium hydroxide solution are separated. 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 to wastewater treatment or may be used again as the aqueous sodium hydroxide solution in S3-3-1.

[0041] (S4-1) The separated eluent and the base are introduced into the mixing tank MT3. In this embodiment, the base is an aqueous sodium hydroxide solution that is more alkaline than sulfuric acid (the eluent). The pH of the eluent after pH adjustment is 3 or more and less than 7. The ratio of the amount of the aqueous sodium hydroxide solution introduced to the amount of the eluent is adjusted within the above pH range.

[0042] (S4-2) The eluent and the aqueous sodium hydroxide solution are mixed. Mixing is performed by the stirrer M3 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.

[0043] (S4-3) The eluent adjusted in 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 introduced into 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.

[0044] (S5) Recover boric acid from the permeate. In this embodiment, several steps are performed to recover boric acid. (S5-1) Evaporate water from the permeate to concentrate it so that the concentration of boric acid increases. The concentrated permeate is called an aqueous boric acid concentrated solution. For example, the concentration may be 50 times or more, 60 times or more, 70 times or more, 80 times or more. Also, the concentration of boric acid after concentration may, for example, have an upper limit of 25% or less, 20% or less, 16% or less, and a lower limit of 5% or more, 13% or more, 15% or more. The means of evaporation and concentration is not particularly limited, and an evaporation concentrator may be used. For example, for the concentration treatment, heating evaporation, vacuum evaporation, membrane separation using an RO membrane, etc. can be used.

[0045] (S5-2) Cool and crystallize the aqueous boric acid concentrated solution. Then, perform solid-liquid separation into a solid component and a liquid component. For the cooling crystallization treatment, means such as cooling crystallization, vacuum crystallization, reaction crystallization, etc. can be used, and for the solid-liquid separation treatment, means such as centrifugal separation, filter separation, etc. can be used. (S5-2-1) The separated liquid component (aqueous solution containing trace amounts of boric acid) may be sent to be membrane-separated again with an NF membrane. If it is not sent to the NF membrane, it may be treated by wastewater treatment or as industrial waste.

[0046] (S5-3) Dry the solid component (containing some water). The means of drying is not particularly limited, and it may be a drying device or a constant temperature device that can control temperature and humidity.

[0047] (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 manufacturing process of polarizing plates.

[0048] (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.

[0049] (S6-2) The sulfuric acid solution containing boric acid and sodium sulfate is cooled. Cooling solidifies the sodium sulfate. 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 again for membrane separation treatment using an NF membrane. If it is not sent to an NF membrane, it may be treated as industrial waste.

[0050] (S6-3) The separated sodium sulfate (solid component) may be treated as industrial waste.

[0051] (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., 15-22°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.

[0052] (Method for recovering and manufacturing boric acid) The method for recovering and manufacturing boric acid has the same process as the waste liquid treatment method described above. The high-purity boric acid produced can be used in the manufacture of polarizing plates. It can also be used in other manufacturing processes.

[0053] (NF Membrane Module) An example of the NF membrane module 200 used in this embodiment is shown below. The membrane separation apparatus equipped with the NF membrane module 200 is configured to be operated under desired conditions, with other equipment such as pumps, sensors, tanks, control valves, and control devices attached as needed.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] A membrane module 200 using an NF membrane may consist of one or more membrane elements. Typically, the membrane elements are spiral-type membrane elements using an NF membrane. The membrane module 200 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 the NF membrane 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.

[0060] (Polarizing plate) A polarizing plate, for example, has a polarizer and a polarizer protective film provided on one or both of its main surfaces. The polarizing plate may further have an optically functional film provided on the polarizer or the polarizer protective film. The polarizing plate may have a surface treatment layer formed on it.

[0061] A polarizer is, for example, a resin film containing a dichroic substance. Examples of resin films include hydrophilic polymer films such as polyvinyl alcohol (PVA) films, partially formalized PVA films, and partially saponified ethylene-vinyl acetate copolymer films. A polarizer may be made from a single layer of resin film, or it may be made using a laminate of two or more layers. For example, a PVA resin solution is applied to a resin substrate and dried to form a PVA resin layer on the resin substrate, thereby creating a laminate of the resin substrate and the PVA resin layer. This laminate is stretched and dyed to make the PVA resin layer a polarizer.

[0062] Examples of polarizer protective films include cellulose-based resins such as triacetylcellulose (TAC), polyester-based resins, polyvinyl alcohol-based resins, polycarbonate-based resins, polyamide-based resins, polyimide-based resins, polyethersulfone-based resins, polysulfone-based resins, polystyrene-based resins, cycloolefin-based resins such as polynorbornene, polyolefin-based resins, (meth)acrylic-based resins, and acetate-based resins.

[0063] Examples of optically functional films include phase difference films and brightness enhancement films. A polarizing plate may further have a surface protection film on one of its outermost surfaces. A separator film (release film) may be provided on another outermost surface separate from this surface protection film. The constituent films of the polarizing plate may be bonded together with adhesive or a bonding agent.

[0064] Examples of surface treatment layers include hard coating, anti-reflective coating, anti-sticking coating, anti-glare coating, and anti-fouling coating.

[0065] (Method for Manufacturing Polarizing Plates) An example of a method A for manufacturing polarizing plates comprising a polarizer and a polarizer protective film is shown below. Method A for manufacturing polarizing plates includes a dyeing step, a crosslinking step, a stretching step, a hue adjustment step, and a drying shrinkage step for the polarizer, followed by a step of attaching the polarizer and the polarizer protective film. A preliminary contact step with an aqueous boric acid solution may be included before the dyeing step, and one or both of the crosslinking step and the hue adjustment step may be omitted. The dyeing step may be performed two or more times, and the stretching step may also be performed two or more times.

[0066] For example, PVA resin film is supplied from a raw material roll on which it is wound, and transported from upstream to downstream by multiple rollers. Multiple processes are performed during this transport. The transported PVA resin film is immersed in a dyeing bath (dyeing solution), a crosslinking bath (crosslinking solution), a stretching bath (stretching solution), and a color adjustment bath (color adjustment solution) in that order. After that, it is sent to a heat drying process to be dried and then wound onto a polarizer roll.

[0067] The above-mentioned dyeing solution may be an aqueous solution containing, for example, iodine, an iodine compound, and further containing boric acid. The above-mentioned crosslinking solution may be an aqueous solution containing, for example, boric acid and an iodine compound. The above-mentioned stretching solution may be an aqueous solution containing, for example, boric acid and an iodine compound. The above-mentioned color adjusting solution may be an aqueous solution containing, for example, an iodine compound. Boric acid recovered by the above method can be suitably used.

[0068] In the stretching process, the degree of stretching may be adjusted by changing the peripheral speed of the upstream and downstream conveyor rolls, and a uniaxial stretching device or a biaxial stretching device may be used. In the drying shrinkage process, the stretched film is dried and shrunk in the width direction perpendicular to the length direction by bringing the conveyor rolls into contact with heated rolls. In addition to heated rolls, heating means such as an oven or heater may also be used.

[0069] A polarizer film is supplied from a polarizer roll, a first polarizer protective film is supplied from a first protective film roll around which the first polarizer protective film is wound, and a second polarizer protective film is supplied from a second protective film roll around which the second polarizer protective film is wound. Adhesive is applied to one or both of the bonding surfaces to be bonded, and the first polarizer protective film is bonded to one side of the polarizer and the second polarizer protective film to the other side (first bonding step).

[0070] Alternatively, an adhesive may be applied to one or both outer surfaces of the polarizer protective film to bond one or more optical functional films (second bonding step).

[0071] Furthermore, a surface protection film may be bonded to the polarizer protection film or optically functional film on the viewing side via an adhesive (third bonding step). Also, a separator film (release film) may be bonded to the polarizer protection film or optically functional film on the device side (liquid crystal display device, organic EL display device, etc.) via an adhesive (fourth bonding step).

[0072] (Example 1) Boric acid was recovered using the method shown in Figure 1. Components of the 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 (H ２ SO ４: 5% by mass, Water: 95% by mass) NF membrane: PRO-XS3 (manufactured by Nitto Denko Corporation) 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. Waste liquid: Industrial waste treatment was reduced by 50%. Regeneration of adsorbent: Regeneration was possible more than 10 times.

[0073] 100 Polarizing plate manufacturing equipment 200 NF film module

Claims

1. A wastewater treatment method for selectively recovering boric acid from wastewater generated during the polarizing plate manufacturing process, comprising: a boric acid adsorption step of selectively adsorbing boric acid onto an adsorbent from the wastewater whose pH has been adjusted to be alkaline; a boric acid desorption step of contacting an acidic solution with the adsorbent to eluate the boric acid from the adsorbent into the acidic solution to obtain an eluent; an NF membrane separation step of adding a base to the eluent to adjust its pH to be more acidic than neutral and passing the eluent 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 of the acidic solution, and a concentrated solution containing anions derived from the acid or cations derived from the base of the acidic solution; and a boric acid recovery step of recovering boric acid from the permeate.

2. The method for treating wastewater according to claim 1, wherein the alkali adjusting agent used for pH adjustment in the boric acid adsorption step is sodium hydroxide or potassium hydroxide, and the pH of the wastewater after pH adjustment is 9 or more and 11 or less.

3. The wastewater treatment method according to claim 1 or 2, wherein the acidic solution is one or more selected from dilute sulfuric acid with a concentration of 1% to 25% and hydrochloric acid with a concentration of 1% to 25%.

4. The wastewater treatment method according to any one of claims 1 to 3, wherein in the NF membrane separation step, the base 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 is 3 or more and less than 7.

5. The wastewater treatment method according to any one of claims 1 to 4, wherein the boric acid recovery step includes: a first evaporation concentration step of evaporating water from the boric acid aqueous solution in the permeate to obtain a concentrated boric acid aqueous solution; a cooling crystallization solid-liquid separation step of cooling and crystallizing the concentrated boric acid aqueous solution concentrated to a predetermined concentration to separate the solid and liquid; and a drying step of drying the solid-liquid separated boric acid at a predetermined temperature to obtain boric acid with a purity of 95% or more.

6. A wastewater treatment method according to claim 5, comprising a separated liquid recovery step, in which the separated liquid obtained by the cooling crystallization solid-liquid separation step is returned upstream of the first evaporation concentration step and the NF membrane separation step is performed by passing the separated liquid through the NF membrane.

7. A method for treating wastewater according to any one of claims 1 to 6, comprising an adsorbent regeneration step of regenerating the adsorbent by contacting it with an alkaline solution.

8. The method for treating wastewater according to any one of claims 1 to 7, wherein the adsorbent is a boron-selective adsorbent.

9. A method for treating wastewater according to any one of claims 1 to 8, comprising: a second evaporation concentration step of evaporating water from the concentrated liquid to increase the concentration of boric acid and obtain a concentrated boric acid acidic solution; a cooling solid-liquid separation step of cooling the concentrated boric acid acidic solution to separate it into a boric acid-containing acidic solution and a solid component; and a separated boric acid recovery step of combining the solid-liquid separated boric acid with the pH-adjusted eluent and passing it through the NF membrane to perform the NF membrane separation step.

10. A method for recovering boric acid from wastewater generated during the polarizing plate manufacturing process, comprising: a boric acid adsorption step of selectively adsorbing boric acid onto an adsorbent from the wastewater whose pH has been adjusted to be alkaline; a boric acid desorption step of contacting an acidic solution with the adsorbent to eluate the boric acid from the adsorbent into the acidic solution to obtain an eluent; an NF membrane separation step of adding a base to the eluent to adjust its pH to be more acidic than neutral and passing the eluent 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 of the acidic solution, and a concentrated solution containing anions derived from the acid or cations derived from the base of the acidic solution; and a boric acid recovery step of recovering boric acid from the permeate.

11. A method for manufacturing a polarizing plate, comprising a treatment method for selectively recovering boric acid from waste liquid generated during the polarizing plate manufacturing process, wherein the treatment method for selectively recovering boric acid from the waste liquid is selected from the boric acid recovery and manufacturing method described in claim 10 or the waste liquid treatment method described in any one of claims 1 to 9, and the boric acid obtained by the treatment method for selectively recovering boric acid from the waste liquid is used as a raw material for an aqueous boric acid solution used when manufacturing polarizers constituting the polarizing plate.

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