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

Abstract

This invention provides a wastewater treatment method that selectively recovers boric acid from wastewater by performing a predetermined treatment on borates using a method different from conventional technologies. [Solution] The wastewater treatment method is a wastewater treatment method for selectively recovering boric acid from wastewater generated in the polarizing plate manufacturing process, and includes 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.

Description

Technical Field

[0001] The present invention relates to a method for treating waste liquid, for example, a method for treating waste liquid to selectively recover boric acid from waste liquid generated in the process of manufacturing a polarizing plate. It also relates to a method for recovering and manufacturing boric acid.

Background Art

[0002] Waste liquid generated in the manufacturing process of 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 to recover potassium iodide and boric acid. The treatment method of Patent Document 1 involves solid-liquid separation of the precipitate obtained by evaporating and concentrating the waste liquid, recovering the filtrate containing potassium iodide, and recovering boric acid from the precipitate. The boric acid recovery step involves adding an acid to the treated liquid in which the precipitate is dissolved to adjust the pH, cooling to precipitate boric acid crystals, separating the boric acid crystals, adding an alkali to the treated liquid from which the boric acid crystals have been separated to neutralize it, and then performing electrodialysis.

[0004] The waste water treatment method of Patent Document 2 adjusts the pH of the waste water containing iodine and boron to 11 to 14, concentrates it so that the boron concentration becomes 0.5 mass% or more, adds an adsorbent to remove the 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, removes the obtained precipitate to separate boron, and supplies chlorine to the obtained liquid to oxidize iodine and precipitate and recover iodine.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

[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. [Means for solving the problem]

[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 described herein is a wastewater treatment method for selectively recovering boric acid from wastewater generated in the polarizing plate manufacturing process, A wastewater concentration step (S0) is performed to concentrate the aforementioned wastewater to obtain a borate concentrate, A borate separation step (S1) is performed to separate the borate (for example, sodium tetraborate) from the borate concentrate, The process includes a concentration and crystallization step (S2) in which an aqueous boric acid solution is produced from the separated borate, and the aqueous boric acid solution is concentrated and crystallized 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 is: The process may also 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 aforementioned wastewater concentration step (S0) is performed as follows: The aforementioned waste liquid is subjected to an iodine reduction step (S0-1) in which an iodine reducing agent is added and the iodine is reduced to its reduction point. The waste liquid that has undergone the iodine reduction step is subjected to an alkalinity adjustment step (S0-2) in which the pH is adjusted to be alkaline (for example, pH 8.5 or higher), The process may also include a borate concentration step (S0-3) in which the waste liquid obtained in the alkali adjustment step (for example, until the amount of borax in the waste liquid 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 aqueous sodium thiosulfate, aqueous potassium thiosulfate, and aqueous ascorbic acid.

[0014] The alkaline 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 a pH of 13 or lower.

[0015] In the borate concentration step (S0-3), the water is evaporated and concentrated under reduced pressure at 65°C to 80°C, 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) is performed as follows: The borate concentrate may be subjected to a pH adjustment treatment followed by a cooling treatment to precipitate the borate. The borate separation step (S1) is performed as follows: For example, a pH adjustment step (S1-1) is performed in which an acid (for example, sulfuric acid, hydrochloric acid) is added to the borate concentrate obtained in the borate concentration step (for example, a strong alkali with a pH above 12) to adjust the pH to a weak alkali (for example, pH 8 or higher and pH 12 or lower), Cool the pH-adjusted borate concentrate (e.g., at 15°C to 30°C) to precipitate borate from the borate concentrate in a cooling and borate precipitation step (S1-2). A borate solid-liquid separation step (S1-3) of solid-liquid separating the borate concentrate containing the precipitated borate into a solid component containing borate and a liquid component may be included.

[0017] React the borate concentrate with an acid (e.g., sulfuric acid) to obtain boric acid. In the following reaction formula (1), borax is shown as an example of the borate. Na2B4O7·10H2O + H2SO4 → 4H3BO3 + Na2SO4 + 5H2O ··· (1)

[0018] The pH adjustment step (S1-1) is The borate concentrate (e.g., a strong alkali with a pH exceeding 12) may be pH-adjusted to a pH of 8 or more and 12 or less, preferably 9 or more and 11 or less. If the pH is less than 8, there is concern that iodine may be oxidized and device corrosion due to volatilization may occur. If the pH exceeds 12, the amount of precipitated borate tends to be small.

[0019] The concentration and crystallization step (S2) is A borate aqueous solution generation step (S2-1) of dissolving the solid component containing the borate separated in the borate separation step (S1) in water at a predetermined temperature (e.g., from 20°C to 70°C) to generate an aqueous solution containing borate. Add an acid (e.g., dilute sulfuric acid, 20% dilute hydrochloric acid) to the aqueous solution containing borate, adjust the pH to 6 or less (pH 3 or more, preferably 4 or more and 5 or less), react the borate (e.g., sodium tetraborate) to boric acid, and precipitate impurities to obtain a first boric acid-containing acidic solution in a borate-boric acid reaction step (S2-2). Evaporate and concentrate the first boric acid-containing acidic solution to obtain a first boric acid concentrate in a first boric acid concentration step (S2-4). Cool and crystallize the first boric acid concentrate (e.g., at 15°C to 30°C) to obtain first crystallized boric acid in a first crystallized boric acid cooling crystallization step (S2-5). A first crystallized boric acid solid-liquid separation step (S2-6) of solid-liquid separating the solid content containing the first crystallized boric acid obtained in the first boric acid cooling crystallization step into a solid component and a liquid component (for example, an acidic solution mainly composed of sodium sulfate) may be included. The concentration crystallization step (S2) Between the borate·boric acid reaction step (S2-2) and the first boric acid concentration step (S2-4), An impurity removal step (S2-3) of removing the impurities (for example, PVA residue) from the first boric acid-containing acidic solution obtained by reacting in the borate·boric acid reaction step may be included.

[0020] The concentration crystallization step (S2) A first crystallized boric acid drying step of drying the boric acid (solid content containing the first crystallized boric acid) separated in the first crystallized boric acid solid-liquid separation step 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 may be included.

[0021] The recrystallization step (S3) A second boric acid aqueous solution generation step (S3-1) of dissolving the solid content containing the first crystallized boric acid separated in the first crystallized boric acid solid-liquid separation step in water at a predetermined temperature (for example, from 20°C to 70°C) to generate a second boric acid aqueous solution, A second crystallized boric acid cooling crystallization step (S3-2) of cooling and crystallizing (for example, from 7°C to 20°C) the second boric acid aqueous solution generated in the second boric acid aqueous solution generation step to obtain second crystallized boric acid, A second crystallized boric acid solid-liquid separation step (S3-3) of solid-liquid separating the solid content containing the second crystallized boric acid obtained in the second crystallized boric acid cooling crystallization step into a solid component and a liquid component (for example, mainly an aqueous solution) may be included.

[0022] The recrystallization step (S3) The process may also include a second crystallized boric acid drying step (S3-4) in which the boric acid (solid component containing the 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 higher, 96% by mass or higher, 97% by mass or higher, or 98% by mass or higher.

[0023] The boric acid recovery and manufacturing method disclosed herein is: A method for recovering and producing boric acid, which selectively recovers boric acid from waste liquid generated during the polarizing plate manufacturing process, The method may include each step of a wastewater treatment method for selectively recovering boric acid from the wastewater generated during the polarizing plate manufacturing process described above.

[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, The treatment method for selectively recovering boric acid from the aforementioned waste liquid is selected from the boric acid recovery and production method or the waste liquid treatment method described above. 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 materials for the boric acid aqueous solution used when manufacturing polarizers that constitute a polarizing plate. The method for manufacturing the polarizing plate is as follows: The boric acid obtained by the treatment method for selectively recovering boric acid from the waste liquid may be characterized in that it is used as a raw material for an aqueous boric acid solution used in one or more processes selected from a pre-contact step of an aqueous boric acid solution for polarizers constituting a polarizer, a dyeing step, a crosslinking step, and a stretching step. [Effects of the Invention]

[0025] (1) Boric acid can be selectively recovered from wastewater 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. [Brief explanation of the drawing]

[0026] [Figure 1] This is a schematic diagram showing an example of a wastewater treatment method and treatment system. [Modes for carrying out the invention]

[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] (Waste liquid 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 stretcher, a crosslinking bath, a washing bath, a dryer, a film transport device, an adhesive coating device, and a device for attaching polarizers and polarizer protective films. Wastewater discharged from this manufacturing apparatus is stored in a buffer tank (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 process). (S0-1) In the mixing tank MT1, an iodine reducing agent is added to the waste liquid to reduce the iodine to its reduction point (iodine reduction process). (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 carried out using a stirrer M1 for a predetermined time and at a predetermined rotational speed. (S0-3) The waste liquid, which has been adjusted to be alkaline, is evaporated and concentrated until, for example, the borate content 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). The water is evaporated and concentrated under reduced pressure at 65°C to 80°C to adjust the borate concentrate to a pH range of 9 to 14 and a specific gravity range of 1.35 to 1.55. In this embodiment, the process is carried out in batches, 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 and adjust the pH to a weak alkali, for example, pH 8 to pH 12. 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) The borate concentrate after pH adjustment is cooled, for example, to 15°C to 30°C, and borate is precipitated from the borate concentrate (cooling and borate precipitation step). (S1-3) The borate concentrate containing the precipitated borate is separated into a solid component and a liquid component (borate solid-liquid separation step). The liquid component may be treated as wastewater.

[0032] (S2) A boric acid aqueous solution is produced from the borate separated in the borate solid-liquid separation step (S1-3), and the boric acid aqueous solution is concentrated and crystallized to obtain boric acid (concentration and crystallization step). (S2-1) The borate-containing solid 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 the mixing container MT2 to produce a borate-containing aqueous solution (borate aqueous solution production step). The mixture is mixed using a stirrer M2 for a predetermined time at a predetermined rotational speed. (S2-2) An acid is added to the borate-containing aqueous solution to adjust the pH to an acidic level, for example, pH 6 or lower, to react the borate with boric acid and precipitate impurities, thereby obtaining 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 sulfuric acid solution or an aqueous hydrochloric acid solution. Mixing is carried out with a stirrer M2 for a predetermined time 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 impurities are removed, the acidic solution containing primary boric acid 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 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 and crystallization step S2 is dissolved in water and recrystallized to recover the boric acid (recrystallization step). (S3-1) The solid 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). The mixture is mixed using a stirrer M3 for a predetermined time at a predetermined rotational speed. (S3-2) Secondary boric acid aqueous solution production step: The secondary boric acid aqueous solution produced in S3-1 is cooled and crystallized at, for example, 7°C to 20°C to obtain secondary crystallized boric acid (secondary crystallized boric acid cooling and crystallization step). (S3-3) Second crystallization boric acid cooling crystallization step S3-2 is separated into a solid component and a liquid component (second crystallization 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 higher, 96% by mass or higher, 97% by mass or higher, or 98% by mass or higher (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 boric acid with a purity of, for example, 95% by mass or higher.

[0036] (Waste liquid treatment system) The wastewater treatment system can be suitably used in the wastewater treatment method described above. The wastewater treatment system is a wastewater treatment system that selectively recovers boric acid from wastewater generated during the polarizing plate manufacturing process. The wastewater treatment system may include: a first evaporation and concentration device that evaporates water from the wastewater to concentrate it to a predetermined concentration (borate content of 5% to 70% by mass) to obtain a borate concentrate; a pH adjustment means that adds acid to the concentrated borate concentrate to adjust it to a predetermined pH (for example, 8 to 12); a first cooling and crystallization device that cools the pH-adjusted borate concentrate to a predetermined temperature (for example, 15°C to 30°C) and cools and crystallizes the borate from the borate concentrate; and a first solid-liquid separation device that separates the cooled and crystallized (precipitated) borate (solid) from the water.

[0037] The wastewater treatment system includes a reaction vessel for dissolving the borate from the solid separated by the first solid-liquid separator in water to form a borate aqueous solution, adding acid to adjust the pH, reacting the borate with boric acid, and precipitating impurities; an impurity removal device for removing the impurities from the boric acid-containing acidic solution discharged from the reaction vessel; and a second evaporation device for evaporating water from the first boric acid-containing acidic solution from which impurities have been removed by the impurity removal device to concentrate it to a predetermined concentration (for example, a boric acid content of 5% to 70% by mass) and obtain a first boric acid concentrate. The apparatus may also include a concentrator, a second cooling crystallization apparatus for cooling and crystallizing the concentrated first boric acid concentrate at a predetermined temperature (e.g., 15°C to 30°C) to obtain first crystallized boric acid, a second solid-liquid separation apparatus for solid-liquid separation of the solid and liquid components containing the crystallized first crystallized boric acid, and a first crystallized boric acid drying apparatus for drying the solid component containing the first crystallized boric acid separated by the second solid-liquid separation apparatus at a predetermined temperature (e.g., 60°C to 100°C) to obtain boric acid with a purity of, for example, 95% by mass or higher, 96% by mass or higher, 97% by mass or higher, or 98% by mass or higher.

[0038] The wastewater treatment system may include: a production container for dissolving the solid portion containing the first crystallized boric acid separated in the second solid-liquid separation step in water at a predetermined temperature (e.g., 20°C to 70°C) to produce a second boric acid aqueous solution; a third cooling crystallization apparatus for cooling and crystallizing (e.g., 7°C to 20°C) the second boric acid aqueous solution discharged from the production container to obtain second crystallized boric acid; a third solid-liquid separation apparatus for solid-liquid separation of the solid portion containing the crystallized second crystallized boric acid and the liquid portion; and a second crystallized boric acid drying apparatus for drying the solid portion containing the second crystallized boric acid separated in the third solid-liquid separation apparatus at a predetermined temperature (e.g., 60°C to 100°C) to obtain boric acid with a purity of, for example, 95% by mass or higher, 96% by mass or higher, 97% by mass or higher, or 98% by mass or higher.

[0039] Examples of solid-liquid separation devices include filtration devices, centrifugal separators, and dewatering devices. Examples of drying devices include electric heaters, hot air devices, and constant temperature baths. Examples of impurity removal devices include mesh filters, adsorption devices, centrifugal separators, and filtration devices.

[0040] (Method for recovering and manufacturing boric acid) The method for recovering and manufacturing boric acid involves the same steps as the wastewater 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.

[0041] (Polarizing plate) A polarizing plate, for example, comprises 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 also have a surface treatment layer formed on it.

[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 of manufacturing polarizing plates) An example of a method A for manufacturing a polarizing plate comprising a polarizer and a polarizer protective film is shown below. Method A for manufacturing a polarizing plate 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 to a polarizer protective film. A preliminary contact step with an aqueous boric acid solution may be included before the dyeing step, and either the crosslinking step or the hue adjustment step, or both, 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 the film is wound, and then 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 staining solution may, for example, be an aqueous solution containing iodine, an iodine compound, and further containing boric acid. The crosslinking solution may be, for example, an aqueous solution containing boric acid and an iodine compound. The above-mentioned stretching solution may be, for example, an aqueous solution containing boric acid and an iodine compound. The hue adjusting solution described above may be, for example, an aqueous solution containing an iodine compound. Boric acid recovered by the above method can be suitably used.

[0049] The stretching process 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 and shrinking process, the stretched film is dried and shrunk in the width direction perpendicular to the length direction by bringing the conveying roll into contact with a heated roll. In addition to the heated roll, 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 process).

[0051] Alternatively, an adhesive may be applied to one or both outer surfaces of the polarizer protective film, and one or more optical functional films may be bonded to it (second bonding step).

[0052] Alternatively, 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). Alternatively, a separator film (release film) may be bonded to the polarizer protective film or optical functional film on the device side (LCD device, OLED device, etc.) via an adhesive (fourth bonding step). [Examples]

[0053] Boric acid was recovered using the procedure shown in Figure 1. An example is shown where the borate is borax (sodium tetraborate decahydrate). Composition of waste liquid: Water (95% by mass), boric acid (1.0% by mass), iodine (0.8% by mass), : PVA (1.2% by mass), potassium iodide (1.2% by mass), Other (0.8% by mass). Iodine reducing agent added: 20% by mass aqueous solution of sodium thiosulfate Alkali addition: Sodium hydroxide aqueous solution (NaOH: 10% by mass, water: 90% by mass) Evaporator for borate concentrate: The solution was concentrated 20 times under reduced pressure at 70°C to achieve a borate (borax) content of 15% by mass. pH adjustment (acid injection): Sulfuric acid aqueous solution 70% (H2SO4: 70% by mass, water: 30% by mass) Examples 1 to 7 were pH 7 to pH 13, while 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 injection): Sulfuric acid aqueous solution 20% (H2SO4: 20% by mass, water: 80% by mass) Examples 1 to 7 were adjusted to pH 4 to pH 7. Impurity removal: A filter was used. Evaporator for boric acid-containing solution: The solution was concentrated three times under reduced pressure at 70°C to a boric acid concentration of 16% by mass. First crystallization of boric acid by cooling: Cooling crystallization was performed at 15°C to 30°C for 12 hours. Solid-liquid separation of primary crystallized boric acid: A centrifuge was used. Secondary crystallization of boric acid by cooling: Cooling crystallization was performed at 7°C to 20°C for 12 hours. Solid-liquid separation of secondary crystallized boric acid: A centrifuge was used. Drying: Dry 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. It was adjusted to achieve 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 solid form after drying / Amount of boric acid in waste liquid per 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] [Table 1]

[0057] In Examples 2, 3, and 5, potassium salts were obtained along with sodium salts because the pH was adjusted to 7, 8, and 13 during the borate separation process. 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 to 5, the pH in the borate-boric acid reaction step was kept constant at pH 4, so it was found 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 in Examples 6 and 7 at pH 6 and 7. In Examples 6 and 7, the pH was adjusted to 9 in the borate separation step and to 6 and 7 in the borate-boric acid reaction step, respectively. It was found that a larger precipitate was produced at pH 6.

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

[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, A wastewater concentration step is performed to concentrate the aforementioned wastewater and obtain a borate concentrate, A borate separation step for separating borate from the borate concentrate, The process includes a concentration and crystallization step of generating an aqueous boric acid solution from the separated borate, concentrating and crystallizing the aqueous boric acid solution to obtain boric acid, Methods for disposing of waste liquid.

2. The waste liquid treatment method is: The process includes a recrystallization step in which the boric acid obtained in the concentration crystallization step is dissolved in water and recrystallized to recover the boric acid, The method for treating waste liquid according to claim 1.

3. The aforementioned wastewater concentration step is, An iodine reduction step is performed by adding an iodine reducing agent to the waste liquid and reducing the iodine to its reduction point. The waste liquid that has undergone the iodine reduction step is subjected to an alkalinity adjustment step to adjust its pH to be alkaline, The process includes a borate concentration step in which the waste liquid obtained in the alkali adjustment step is evaporated and concentrated to obtain a borate concentrate, The method for treating waste liquid according to claim 1.

4. The borate separation step is, The borate concentrate is subjected to a pH adjustment treatment followed by a cooling treatment to precipitate the borate. The wastewater treatment method according to claim 1.

5. The pH adjustment process described above is The borate concentrate is adjusted to a pH of 8 or higher and a pH of 12 or lower. The method for treating waste liquid according to claim 4.

6. The aforementioned concentration and crystallization step is A borate aqueous solution production step involves dissolving the solid containing the borate solution separated in the borate separation step in water to produce a borate-containing aqueous solution, A borate-boric acid reaction step is performed in which an acid is added to the aforementioned borate-containing aqueous solution to adjust the pH to 6 or less, the borate reacts with boric acid, and impurities are precipitated to obtain a primary boric acid-containing acidic solution. A first boric acid concentration step is performed by evaporating and concentrating the aforementioned first boric acid-containing acidic solution to obtain a first boric acid concentrate. A first cooling crystallization step is performed to obtain first crystallized boric acid by cooling and crystallizing the first boric acid concentrate, The process includes a solid-liquid separation step of first crystallized boric acid, which separates the first crystallized boric acid obtained by the first cooling crystallization into a solid and a liquid. The method for treating waste liquid according to claim 1.

7. The aforementioned concentration and crystallization step is The process further includes 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 higher. The method for treating waste liquid according to claim 6.

8. The aforementioned recrystallization step is The process involves dissolving the solid containing the first crystalline boric acid separated in the first crystalline boric acid solid-liquid separation step in water to produce a second crystalline boric acid aqueous solution, and The process includes a cooling and crystallization step to obtain secondary crystallized boric acid by cooling and crystallizing the secondary boric acid aqueous solution produced in the secondary boric acid aqueous solution production step, The process includes a solid-liquid separation step of secondary crystallized boric acid, which separates the secondary crystallized boric acid obtained by the secondary crystallization of secondary crystallized boric acid into a solid and a liquid. The method for treating waste liquid according to claim 2.

9. The process further includes a drying step for the second crystallized boric acid, in which the solid containing the separated second crystallized boric acid is dried to obtain boric acid with a purity of 98% by mass or higher. The method for treating waste liquid according to claim 8.

10. A method for recovering and producing boric acid, which selectively recovers boric acid from waste liquid generated during the polarizing plate manufacturing process, The method for treating wastewater according to claim 1 or 2 includes each step of the above-mentioned steps, Method for recovering and manufacturing boric acid.

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