Waste liquid treatment method and boric acid recovery production method

Abstract

This waste liquid treatment method for selectively recovering boric acid from a waste liquid generated in a polarizing plate manufacturing process includes: a waste liquid concentration step of concentrating the waste liquid to obtain a borate concentrated liquid; a borate separation step of separating borate from the borate concentrated liquid; and a concentration and crystallization step of generating a boric acid aqueous solution from the separated borate, and concentrating and crystallizing the boric acid aqueous solution to obtain boric acid.

Description

Method for treating waste liquid and method for recovering and manufacturing boric acid

[0001] The present invention relates to a method for treating waste liquid, and more particularly to a method for selectively recovering boric acid from waste liquid generated during the polarizing plate manufacturing process. It also relates to a method for recovering and producing boric acid.

[0002] The waste liquid generated during the manufacturing process of polarizing plates contains inorganic substances such as iodine, boron, and potassium, as well as organic substances such as polyvinyl alcohol. Such waste liquid is treated as industrial waste. However, there is a demand to process the waste liquid obtained from the manufacturing process, recover boric acid, and recycle it into the manufacturing process.

[0003] Patent Document 1 discloses a method for treating wastewater from polarizing plate manufacturing, which recovers potassium iodide and boric acid from the wastewater. The treatment method in Patent Document 1 involves evaporating and concentrating the wastewater, separating the precipitate into solid and liquid components, 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 containing the dissolved precipitate to adjust the pH, then it is cooled to separate the precipitated boric acid crystals, an alkali is added to the treated liquid from which the boric acid crystals have been separated to neutralize it, and then electrodialysis is performed.

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

[0005] Japanese Patent Publication No. 2023-72964, Japanese Patent No. 4674168

[0006] The present invention provides a wastewater treatment method that selectively recovers boric acid from wastewater by performing a predetermined treatment on borates in a manner different from the prior art. It also provides a method for recovering and producing boric acid.

[0007] As a result of diligent research to solve the above-mentioned problems, the present inventors have found that borate can be recovered in high purity by concentrating and separating borate from wastewater to obtain an aqueous boric acid solution, and then concentrating the aqueous boric acid solution. That is, the present invention includes the following embodiments.

[0008] The wastewater treatment method of the present disclosure is a wastewater treatment method for selectively recovering boric acid from wastewater generated in the polarizing plate manufacturing process, comprising: a wastewater concentration step (S0) of concentrating the wastewater to obtain a borate concentrate; a borate separation step (S1) of separating borate (for example, sodium tetraborate) from the borate concentrate; and a concentration and crystallization step (S2) of generating a boric acid aqueous solution from the separated borate, concentrating and crystallizing the boric acid aqueous solution to obtain boric acid.

[0009] This configuration involves concentrating and separating borate from waste liquid, generating a boric acid aqueous solution from the separated borate, and then concentrating and separating boric acid from the boric acid aqueous solution. This allows for the concentration and separation of high-purity boric acid.

[0010] The waste liquid treatment method may include a recrystallization step (S3) in which the boric acid obtained in the concentration crystallization step (S2) is dissolved in water and recrystallized to recover the boric acid.

[0011] With this configuration, higher purity boric acid can be recovered by dissolving the boric acid obtained in the concentration and crystallization process in water and recrystallizing it.

[0012] The wastewater concentration step (S0) may include: an iodine reduction step (S0-1) in which an iodine reducing agent is added to the wastewater and the iodine is reduced to its reduction point; an alkali adjustment step (S0-2) in which the pH of the wastewater that has gone through the iodine reduction step is adjusted to be alkaline (for example, pH 8.5 or higher); and a borate concentration step (S0-3) in which the wastewater obtained in the alkali adjustment step (for example, until the amount of borax in the wastewater is 4.7% by mass or more and less than 37% by mass) is evaporated and concentrated to obtain a borate concentrate.

[0013] The iodine reducing agent used in the iodine reduction step (S0-1) may be one or more selected from an aqueous sodium thiosulfate solution, an aqueous potassium thiosulfate solution, and an aqueous ascorbic acid solution.

[0014] The alkali agent used in the alkali adjustment step (S0-2) may be an aqueous sodium hydroxide solution and / or an aqueous potassium hydroxide solution, and the waste liquid may be adjusted to a pH of 8.5 or higher and 13 or lower.

[0015] In the borate concentration step (S0-3), water is evaporated and concentrated at 65°C to 80°C under reduced pressure, and the borate concentrate may have a pH of 9 to 14 and a specific gravity of 1.35 to 1.55.

[0016] The borate separation step (S1) may be a process of performing a cooling treatment on the borate concentrate after pH adjustment treatment to precipitate borate. The borate separation step (S1) may include, for example, a pH adjustment step (S1-1) of adding an acid (for example, sulfuric acid, hydrochloric acid) to the borate concentrate obtained in the borate concentration step (for example, a strong alkali with a pH exceeding 12) to adjust the pH to a weakly alkaline state (for example, a pH of 8 or higher and 12 or lower), a cooling / borate precipitation step (S1-2) of cooling the pH-adjusted borate concentrate (for example, at 15°C to 30°C) to precipitate borate from the borate concentrate, and a borate solid-liquid separation step (S1-3) of performing solid-liquid separation of the borate concentrate containing the precipitated borate into a solid component containing borate and a liquid component.

[0017] React the borate concentrate with an acid (for example, sulfuric acid) to obtain boric acid. In the following reaction formula (I), borax is shown as an example of the borate. Na ２ B ４ O ７ ·10H ２ O + H ２ SO ４ →4H ３ BO ３ + Na ２ SO ４ +5H ２ O...(1)

[0018] The pH adjustment step (S1-1) may involve adjusting the pH of the borate concentrate (for example, a strong alkali with a pH exceeding 12) to a pH of 8 or higher and 12 or lower, preferably a pH of 9 or higher and 11 or lower. If the pH is below 8, there is a concern that iodine will be oxidized and corrosion of the equipment will occur due to volatilization. If the pH exceeds 12, the amount of borate precipitate tends to be small.

[0019] The concentration and crystallization step (S2) comprises: a borate aqueous solution production step (S2-1) in which the solid containing the borate separated in the borate separation step (S1) is dissolved in water at a predetermined temperature (for example, 20°C to 70°C) to produce a borate-containing aqueous solution; a borate-boric acid reaction step (S2-2) in which an acid (for example, dilute sulfuric acid or 20% dilute hydrochloric acid) is added to the borate-containing aqueous solution, the pH is adjusted to 6 or less (pH 3 or higher, preferably pH 4 or higher to 5 or lower), the borate (for example, sodium tetraborate) is reacted with boric acid, and impurities are precipitated to obtain a primary boric acid-containing acidic solution; a primary boric acid concentration step (S2-4) in which the primary boric acid-containing acidic solution is evaporated and concentrated to obtain a primary boric acid concentrate; and a primary crystallized boric acid cooling and crystallization step (S2-5) in which the primary boric acid concentrate is cooled and crystallized (for example, at 15°C to 30°C) to obtain primary crystallized boric acid. The first crystallized boric acid solid-liquid separation step (S2-6) may include a solid-liquid separation step (S2-6) which separates the first crystallized boric acid obtained in the first crystallized boric acid cooling crystallization step into a solid and a liquid (for example, mainly an acidic solution such as sodium sulfate). The concentration crystallization step (S2) may include an impurity removal step (S2-3) between the borate-boric acid reaction step (S2-2) and the first boric acid concentration step (S2-4), which removes the impurities (for example, PVA residue) from the first boric acid-containing acidic solution obtained by the reaction in the borate-boric acid reaction step.

[0020] The concentration crystallization step (S2) may include a first crystallized boric acid drying step in which the boric acid (solid component containing the first crystallized boric acid) separated in the first crystallized boric acid solid-liquid separation step is dried at a predetermined temperature (for example, 60°C to 100°C) to obtain boric acid with a purity of, for example, 95% by mass or more, 96% by mass or more, 97% by mass or more, or 98% by mass or more.

[0021] The recrystallization step (S3) may include: a second boric acid aqueous solution production step (S3-1) in which the solid portion containing the first crystalline boric acid separated in the first crystalline boric acid solid-liquid separation step is dissolved in water at a predetermined temperature (for example, 20°C to 70°C) to produce a second boric acid aqueous solution; a second crystalline boric acid cooling and crystallization step (S3-2) in which the second boric acid aqueous solution produced in the second crystalline boric acid aqueous solution production step is cooled and crystallized (for example, 7°C to 20°C) to obtain second crystalline boric acid; and a second crystalline boric acid solid-liquid separation step (S3-3) in which the solid portion containing the second crystalline boric acid obtained in the second crystalline boric acid cooling and crystallization step is separated into a solid portion and a liquid portion (for example, mainly an aqueous solution).

[0022] The recrystallization step (S3) may include a second crystallized boric acid drying step (S3-4) in which the boric acid (solid component containing second crystallized boric acid) separated in the second crystallized boric acid solid-liquid separation step is dried at a predetermined temperature (for example, 60°C to 100°C) to obtain boric acid with a purity of, for example, 95% by mass or more, 96% by mass or more, 97% by mass or more, or 98% by mass or more.

[0023] 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, and may include each step of a waste liquid treatment method for selectively recovering boric acid from the waste liquid generated in the polarizing plate manufacturing process.

[0024] 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.

[0025] (1) Boric acid can be selectively recovered from waste liquid 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.

[0026] This is a schematic diagram showing an example of a wastewater treatment method and treatment system.

[0027] (Embodiment 1) Embodiment 1 of the present invention will be described below.

[0028] (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.

[0029] (Wastewater Treatment Method and Treatment System) Figure 1 shows an example of a wastewater treatment method and treatment system. 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 (not shown).

[0030] (S0) The waste liquid is transferred from the buffer tank to the mixing tank MT1, and the waste liquid is pretreated to obtain a borate concentrate (waste liquid concentration step). (S0-1) In the mixing tank MT1, an iodine reducing agent is added to the waste liquid and the iodine is reduced to its reduction point (iodine reduction step). (S0-2) The iodine-reduced waste liquid is pH adjusted to be alkaline (for example, pH 8.5 or higher) (alkaline adjustment step). For example, an aqueous sodium hydroxide solution or an aqueous potassium hydroxide solution can be used as the alkaline agent, and the waste liquid can be adjusted to a pH of 8.5 or higher and pH 13 or lower. The waste liquid, iodine reducing agent and alkaline agent are mixed. Mixing is performed using a stirrer M1 for a predetermined time and at a predetermined rotation speed. (S0-3) The alkaline-adjusted waste liquid is evaporated and concentrated, for example, until the amount of borate in the waste liquid is 4.7% by mass or more and less than 37% by mass, to obtain a borate concentrate (borate concentration step). By evaporating the water under reduced pressure at 65°C to 80°C, the borate concentrate can be adjusted to a pH of 9 to 14 and a specific gravity of 1.35 to 1.55. In this embodiment, processing is performed in batch mode, but it is not limited to this and continuous processing can also be employed.

[0031] (S1) Separate the borate from the borate concentrate. (S1-1) Add an acid to the borate concentrate to adjust the pH to a weak alkali, for example, pH 8 or higher and pH 12 or lower. The acid may be used as an acidic solution. The acidic solution may be selected from, for example, dilute sulfuric acid with a concentration of 1% to 25% by mass, or hydrochloric acid with a concentration of 1% to 25% by mass. The acidic solution may be an aqueous solution of sulfuric acid or an aqueous solution of hydrochloric acid. (S1-2) Cool the borate concentrate after pH adjustment, for example, at 15°C to 30°C, to precipitate the borate from the borate concentrate (cooling and borate precipitation step). (S1-3) Separate the borate concentrate containing the precipitated borate into a solid component containing borate and a liquid component (borate solid-liquid separation step). The liquid component may be treated as wastewater.

[0032] (S2) An aqueous boric acid solution is produced from the borate separated in the borate solid-liquid separation step (S1-3), and the aqueous boric acid solution is concentrated and crystallized to obtain boric acid (concentration and crystallization step). (S2-1) The solid component containing the borate separated in the borate solid-liquid separation step (S1-3) is dissolved in water at a predetermined temperature, for example, 20°C to 70°C, in a mixing container MT2 to produce an aqueous borate solution (aqueous borate solution production step). Mixing is carried out with a stirrer M2 for a predetermined time and at a predetermined rotation speed. (S2-2) Acid is added to the aqueous borate solution to adjust the pH to acidic, for example, pH 6 or lower, to react the borate with boric acid and precipitate impurities to obtain a primary boric acid-containing acidic solution (borate-boric acid reaction step). The acid may be used as an acidic solution. The acidic solution may be selected from, for example, dilute sulfuric acid with a concentration of 1% to 25% by mass, or hydrochloric acid with a concentration of 1% to 25% by mass. The acidic solution may be an aqueous solution of sulfuric acid or an aqueous solution of hydrochloric acid. Mixing is carried out with a stirrer M2 for a predetermined time and at a predetermined rotational speed. Examples of impurities include PVA residue and precipitated iodine. (S2-3) Impurities are removed from the primary boric acid-containing acidic solution obtained by the reaction in the borate-boric acid reaction step (S2-2) (impurity removal step). (S2-4) After the impurities have been removed, the primary boric acid-containing acidic solution is evaporated and concentrated to obtain a primary boric acid concentrate (primary boric acid concentration step). (S2-5) The primary boric acid concentrate obtained in the primary boric acid concentration step S2-4 is cooled and crystallized at, for example, 15°C to 30°C to obtain primary crystallized boric acid (primary crystallized boric acid cooling and crystallization step). (S2-6) The first boric acid obtained in the first crystallized boric acid cooling crystallization step S2-5 is separated into a solid component and a liquid component (first crystallized boric acid solid-liquid separation step). The liquid component is, for example, an aqueous sodium sulfate solution and may be treated as wastewater.

[0033] (S3) The boric acid obtained in the concentration crystallization step S2 is dissolved in water and recrystallized to recover the boric acid (recrystallization step). (S3-1) The solid portion containing the first crystallized boric acid separated in the first crystallized boric acid solid-liquid separation step S2-6 is dissolved in water at a predetermined temperature, for example, 20°C to 70°C, in the mixing container MT3 to produce a second boric acid aqueous solution (second boric acid aqueous solution production step). Mixing is carried out with a stirrer M3 for a predetermined time and at a predetermined rotation speed. (S3-2) The second boric acid aqueous solution produced in the second boric acid aqueous solution production step S3-1 is cooled and crystallized, for example, at 7°C to 20°C to obtain second crystallized boric acid (second crystallized boric acid cooling crystallization step). (S3-3) The solid portion containing the second crystallized boric acid obtained in the second crystallized boric acid cooling crystallization step S3-2 is separated into a solid and a liquid portion (second crystallized boric acid solid-liquid separation step). The liquid component is, for example, an aqueous solution and may be treated as wastewater. (S3-4) Second crystallized boric acid solid-liquid separation step The boric acid separated in S3-3 (solid component containing second crystallized boric acid) is dried at a predetermined temperature, for example, 60°C to 100°C to obtain boric acid with a purity of, for example, 95% by mass or more, 96% by mass or more, 97% by mass or more, or 98% by mass or more (second crystallized boric acid drying step).

[0034] The drying process in the second crystallization boric acid drying step allows for the recovery of boron with a moisture content of 5% by mass or less and a purity of 95% by mass or more. The recovered high-purity boron can be provided for recycling in the polarizing plate manufacturing process.

[0035] (Another embodiment) (1) The boric acid separated in the first crystallized boric acid solid-liquid separation step (solid component containing the first crystallized boric acid) may be dried at a predetermined temperature (for example, 60°C to 100°C) to obtain, for example, boric acid with a purity of 95% by mass or more.

[0036] (Waste liquid treatment system) The waste liquid treatment system can be suitably used in the above waste liquid treatment method. The waste liquid treatment system is a waste liquid treatment system that selectively recovers boric acid from the waste liquid generated in the polarizing plate manufacturing process. The waste liquid treatment system includes a first evaporation concentration device that evaporates moisture from the waste liquid and concentrates it to a predetermined concentration (borate content concentration of 5% to 70% by mass) to obtain a borate concentrate, a pH adjustment means that adds an acid to the concentrated borate concentrate to adjust it to a predetermined pH (for example, 8 or more and 12 or less), a first cooling crystallization device that cools the pH-adjusted borate concentrate at a predetermined temperature (for example, at 15°C to 30°C) and cools and crystallizes borate from the borate concentrate, and a first solid-liquid separation device that separates the cooled and crystallized borate (solid content) and moisture into solid and liquid phases, and may be provided.

[0037] The waste liquid treatment system dissolves the solid borate separated by the first solid-liquid separation device in water to form an aqueous borate solution, adds an acid to adjust the pH, reacts the borate to form boric acid, and precipitates impurities, a reaction vessel for this purpose, an impurity removal device that removes the impurities from the boric acid-containing acidic solution derived from the reaction vessel, a second evaporation concentration device that evaporates moisture from the first boric acid-containing acidic solution from which the impurities have been removed by the impurity removal device and concentrates it to a predetermined concentration (for example, boric acid content concentration of 5% to 70% by mass) to obtain a first boric acid concentrate, a second cooling crystallization device that cools and crystallizes the concentrated first boric acid concentrate at a predetermined temperature (for example, 15°C to 30°C) to obtain first crystallized boric acid, a second solid-liquid separation device that separates the solid and liquid phases including the crystallized first crystallized boric acid into solid and liquid phases, a first crystallized boric acid drying device that dries the solid content including the first crystallized boric acid separated by the second solid-liquid separation device at a predetermined temperature (for example, from 60°C to 100°C) to obtain boric acid with a purity of, for example, 95% by mass or more, 96% by mass or more, 97% by mass or more, 98% by mass or more, and may be provided.

[0038] The waste liquid treatment system may include a production vessel for dissolving the solid content containing the first crystallized boric acid separated in the second solid-liquid separation step in water at a predetermined temperature (for example, 20°C to 70°C) to produce a second aqueous boric acid solution, a third cooling crystallization device for cooling and crystallizing (for example, 7°C to 20°C) the second aqueous boric acid solution derived from the production vessel to obtain the second crystallized boric acid, a third solid-liquid separation device for separating the solid content and the liquid content containing the crystallized second crystallized boric acid, and a second crystallized boric acid drying device for drying the solid content containing the second crystallized boric acid separated by the third solid-liquid separation device at a predetermined temperature (for example, 60°C to 100°C) to obtain boric acid with a purity of, for example, 95% by mass or more, 96% by mass or more, 97% by mass or more, 98% by mass or more.

[0039] Examples of the solid-liquid separation device include a filtration device, a centrifugal separation device, a dehydration device, and the like. Examples of the drying device include an electric heater, a hot air device, a constant temperature bath, and the like. Examples of the impurity removal device include a mesh filter, an adsorption device, a centrifuge, a filtration device, and the like.

[0040] (Method for recovering and producing boric acid) The method for recovering and producing boric acid has the same steps as the above waste liquid treatment method. The produced high-purity boric acid can be used in the production of polarizing plates. It can also be used in other productions.

[0041] (Polarizing plate) A polarizing plate has, for example, a polarizer and a polarizer protection film provided on one or both main surfaces thereof. The polarizing plate may further have an optical functional film provided on the polarizer or the polarizer protection film. The polarizing plate may have a surface treatment layer formed thereon.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] Examples of surface treatment layers include hard coating, anti-reflective coating, anti-sticking coating, anti-glare coating, and anti-fouling coating.

[0046] (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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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).

[0051] 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).

[0052] 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).

[0053] Boric acid was recovered using the procedure shown in Figure 1. An example is shown where the borate is borax (sodium tetraborate decahydrate). 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). Addition of iodine reducing agent: 20% by mass aqueous solution of sodium thiosulfate. Addition of alkali: aqueous solution of sodium hydroxide (NaOH: 10% by mass, water: 90% by mass). Evaporator for borate concentrate: concentrated 20 times under reduced pressure at 70°C to a borate (borax) content of 15% by mass. pH adjustment (acid addition): 70% (H) sulfuric acid aqueous solution. ２ SO ４ :70% by mass, water:30% by mass. Examples 1 to 7 were pH 7 to pH 13, and Comparative Example 1 was pH 10. Cooling and precipitation of borax: Cooling and crystallization was performed at 15°C to 30°C for 12 hours. Solid-liquid separation of borax: A centrifuge was used. pH adjustment (acid addition): 20% (H) sulfuric acid aqueous solution ２ SO ４: 20% by mass, Water: 80% by mass. pH was adjusted from 4 to 7 in Examples 1 to 7. Impurity removal: A filter was used. Evaporation and concentration of boric acid-containing solution: Concentrated three times under reduced pressure at 70°C to a boric acid concentration of 16% by mass. Cooling crystallization of first crystallized boric acid: Cooling crystallization was performed at 15°C to 30°C for 12 hours. Solid-liquid separation of first crystallized boric acid: A centrifuge was used. Cooling crystallization of second crystallized boric acid: Cooling crystallization was performed at 7°C to 20°C for 12 hours. Solid-liquid separation of second crystallized boric acid: A centrifuge was used. Drying: Drying was performed at 60°C to 100°C (at a temperature at which boric acid does not decompose, e.g., 66°C).

[0054] Table 1 shows the results for Examples 1 to 7 and Comparative Example 1. The samples were adjusted to a purity of 99% by mass. Examples 1 and 6 achieved a recovery rate of 60% or more after drying. The recovery rate can be calculated using the following formula: Recovery rate = Amount of boric acid in the solid portion after drying / Amount of boric acid in the waste liquid of one batch × 100 (%). The amount of boric acid in the waste liquid was calculated from the concentration measurement of the waste liquid.

[0055] In Example 1, the amount of liquid wastewater discharged was reduced by 50% in the treatment of industrial waste.

[0056]

[0057] In Examples 2, 3, and 5, potassium salts were obtained along with sodium salts. This was because the pH in the borate separation step was adjusted to 7, 8, and 13, respectively. In contrast, in Examples 1, 4, 6, and 7, only sodium salts were obtained by adjusting the pH to 10, 12, and 9. In Examples 1 through 5, the pH in the borate-boric acid reaction step was kept constant at pH 4, indicating that the difference in recovery rates was due to the difference in pH adjustment in the borate separation step. Furthermore, adjusting the pH in the borate-boric acid reaction step to pH 4 resulted in a greater amount of boric acid precipitate than adjusting the pH to 6 and 7 in Examples 6 and 7. In Examples 6 and 7, the pH in the borate separation step was adjusted to pH 9, while the pH in the borate-boric acid reaction step was adjusted to pH 6 and 7, respectively. It was found that pH 6 resulted in a greater amount of precipitate.

[0058] In Comparative Example 1, since the borate concentration step was not performed, no borate precipitated during the cooling and precipitation after adjusting the pH to 10 in the borate separation step.

[0059] 100 Polarizing plate manufacturing equipment

Claims

1. A wastewater treatment method for selectively recovering boric acid from wastewater generated during the polarizing plate manufacturing process, comprising: a wastewater concentration step of concentrating the wastewater to obtain a borate concentrate; a borate separation step of separating borate from the borate concentrate; and a concentration and crystallization step of generating a boric acid aqueous solution from the separated borate, concentrating and crystallizing the boric acid aqueous solution to obtain boric acid.

2. The wastewater treatment method according to claim 1, comprising a recrystallization step of dissolving the boric acid obtained in the concentration crystallization step in water and recrystallizing it to recover the boric acid.

3. The wastewater treatment method according to claim 1 or 2, wherein the wastewater concentration step includes: an iodine reduction step of adding an iodine reducing agent to the wastewater and reducing iodine to its reduction point; an alkali adjustment step of adjusting the pH of the wastewater that has undergone the iodine reduction step to be alkaline; and a borate concentration step of evaporating and concentrating the wastewater obtained in the alkali adjustment step to obtain a borate concentrate.

4. The wastewater treatment method according to any one of claims 1 to 3, wherein the borate separation step includes a process of applying a pH adjustment treatment to the borate concentrate followed by a cooling treatment to precipitate the borate.

5. The method for treating wastewater according to claim 4, wherein the pH adjustment treatment adjusts the pH of the borate concentrate to pH 8 or higher and pH 12 or lower.

6. The method for treating wastewater according to any one of claims 1 to 5, wherein the concentration and crystallization step comprises: a borate aqueous solution production step of dissolving the solid containing the boric acid solution separated in the borate separation step in water to produce a borate-containing aqueous solution; a borate-boric acid reaction step of adding acid to the borate-containing aqueous solution to adjust the pH to 6 or less, reacting the borate with boric acid, and precipitating impurities to obtain a primary boric acid-containing acidic solution; a primary boric acid concentration step of evaporating and concentrating the primary boric acid-containing acidic solution to obtain a primary boric acid concentrate; a primary cooling and crystallization step of cooling and crystallizing the primary boric acid concentrate to obtain primary crystallized boric acid; and a primary crystallized boric acid solid-liquid separation step of separating the solid and liquid components containing the primary crystallized boric acid obtained in the primary cooling and crystallization.

7. The wastewater treatment method according to claim 6, wherein the concentration crystallization step further comprises a first crystallized boric acid drying step, in which the solid portion containing the first crystallized boric acid separated from the first crystallized boric acid solid-liquid is dried to obtain boric acid with a purity of 95% by mass or more.

8. The wastewater treatment method according to claim 2, wherein the recrystallization step comprises: a second boric acid aqueous solution production step of dissolving the solid portion containing the first crystalline boric acid separated in the first crystalline boric acid solid-liquid separation step in water to produce a second boric acid aqueous solution; a second crystalline boric acid cooling crystallization step of cooling and crystallizing the second boric acid aqueous solution produced in the second boric acid aqueous solution production step to obtain second crystalline boric acid; and a second crystalline boric acid solid-liquid separation step of separating the solid portion containing the second crystalline boric acid obtained in the second crystalline boric acid cooling crystallization into a solid portion and a liquid portion.

9. The wastewater treatment method according to claim 8, further comprising a second crystallized boric acid drying step of drying the solid containing the second crystallized boric acid separated from the second crystallized boric acid solid-liquid to obtain boric acid with a purity of 98% by mass or more.

10. A method for recovering and producing boric acid, comprising the steps of the waste liquid treatment method described in any one of claims 1 to 9, for selectively recovering boric acid from waste liquid generated during the polarizing plate manufacturing process.

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