Waste liquid treatment method, boric acid recovery production method, and method for producing polarizing plate

Abstract

This waste liquid treatment method for selectively recovering boric acid from a waste liquid generated in a polarizing plate manufacturing process comprises a first membrane separation step for passing the waste liquid having a pH of 8.5 or less or the waste liquid pH-adjusted to a pH of 8.5 or less through a first separation membrane module provided with a reverse osmosis membrane to separate the waste liquid into a first permeate including boric acid (B1) and a substance (R1) other than boric acid in the waste liquid and a first concentrated liquid including the boric acid (B2) and a substance (R2) other than boric acid in the waste liquid, and a boric acid concentration crystallization step for recovering boric acid by concentration crystallization of the first permeate obtained by the first membrane separation step.

Description

Method for treating waste liquid, method for recovering and manufacturing boric acid, and method for manufacturing polarizing plates

[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 and a method for producing polarizing plates.

[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] Patent Document 3 discloses a wastewater treatment method in which sulfate ions, which are preferentially adsorbed onto layered inorganic hydroxides rather than boron, are removed from the wastewater, and then the wastewater is brought into contact with layered double hydroxides to adsorb and remove boron onto the layered double hydroxides. It is also disclosed that an RO membrane (reverse osmosis membrane) or an NF membrane (nanofiltration membrane) may be used as a method for removing sulfate ions.

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

[0007] This invention provides a wastewater treatment method for selectively recovering boric acid from wastewater using a method different from that of the prior art. It also provides a method for recovering and producing boric acid and a method for producing polarizing plates.

[0008] As a result of diligent research to solve the aforementioned problems, the present inventors have found that boric acid can be recovered in high purity by obtaining an aqueous boric acid solution (more acidic than neutral) from borate in wastewater, concentrating the permeate obtained by passing it through a reverse osmosis membrane, and then separating it by crystallization. That is, the present invention includes the following embodiments.

[0009] 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, and includes: a first membrane separation step (S0-3) in which the wastewater with a pH of 8.5 or less, or the wastewater whose pH has been adjusted to 8.5 or less, is passed through a first separation membrane module (21) equipped with a reverse osmosis membrane to separate it into a first permeate containing the boric acid (B1) and substances other than boric acid in the wastewater (R1), and a first concentrate containing the boric acid (B2) and substances other than boric acid in the wastewater (R2); and a boric acid concentration crystallization step (S1) in which the boric acid is recovered from the first permeate obtained in the first membrane separation step (S0-3) by concentration crystallization.

[0010] With this configuration, high-purity boric acid can be recovered by treating wastewater with a pH of 8.5 or lower with a reverse osmosis membrane and then concentrating and crystallizing the resulting permeate. A wastewater pH of 4 or higher is preferable. A pH below 4 is undesirable because it reduces membrane performance, making it easier for iodine and other impurities to permeate along with the boric acid.

[0011] The amount of substance other than boric acid (R1) in the first permeate may be in the range of 1 / 5 to 1 / 200 of the amount of substance other than boric acid (R2) in the first concentrate.

[0012] The wastewater treatment method is as follows: When the liquid recovery rate of the first permeate in the first membrane separation step (S0-3) is 20% to 70%, the concentration of boric acid (B1) in the first permeate (C Ｂ１) may be 10% to 60% of the concentration (C Ｂ０ ) of boric acid (B0) in the waste liquid input into the first separation membrane module (21), and the concentration (C Ｒ１ ) of substances other than boric acid (R1) in the first permeate may be 20% or less of the concentration (C Ｒ０ ) of substances other than boric acid (R0) in the waste liquid input into the first separation membrane module (21), and the concentration (C Ｂ２ ) of boric acid (B2) in the first concentrate may be 40% to 90% of the concentration (C Ｂ０ ) of boric acid (B0) in the waste liquid input into the first separation membrane module (21), and the concentration (C Ｒ２ ) of substances other than boric acid (R2) in the first concentrate may be 80% or more of the concentration (C Ｒ０ ) of substances other than boric acid (R0) in the waste liquid input into the first separation membrane module (21).

[0013] The method for treating the waste liquid may include a water addition step (S0-4) of adding water to the first concentrate obtained in the first membrane separation step (S0-3) to obtain a water-added concentrate, and passing the water-added concentrate obtained in the water addition step (S0-4) through a second separation membrane module (22) equipped with a reverse osmosis membrane to separate it into a second permeate containing the boric acid (B11) and substances other than boric acid (R11), and a second concentrate containing the boric acid (B21) and substances other than boric acid (R21). The second permeate obtained in the second membrane separation step (S0-5) may be treated in the boric acid concentration crystallization step (S1).

[0014] The amount of substances other than boric acid (R11) in the second permeate may be in the range of 1 / 5 to 1 / 200 of the amount of substances other than boric acid (R21) in the second concentrate.

[0015] The method for treating the waste liquid is such that when the liquid recovery rate of the second permeate in the second membrane separation step (S0-5) is 20% to 70%, the concentration (C Ｂ１１ ) of boric acid (B11) in the second permeate is the concentration (CＢ０１ ) may be 10% to 60%, and the concentration of the substance other than boric acid (R11) in the second permeate (C Ｒ１１ ) is the concentration (C) of substances other than boric acid (R01) in the hydrolyzed concentrate that is fed into the second separation membrane module (22). Ｒ０１ ) may be 20% or less, and the concentration of boric acid (B21) in the second concentrated liquid (C Ｂ２１ ) is the concentration of boric acid (B01) in the hydrolyzed concentrate that is fed into the second separation membrane module (22) (C Ｂ０１ ) may be 40% to 90%, and the concentration of substances other than boric acid (R21) in the second concentrated solution (C Ｒ２１ ) is the concentration (C) of substances other than boric acid (R01) in the hydrolyzed concentrate that is fed into the second separation membrane module (22). Ｒ０１ ) may be 80% or more.

[0016] The waste liquid treatment method may involve adding water to the second concentrated liquid obtained in the second membrane separation step (S0-5) or not, and then putting it back into the first separation membrane module (21) for treatment in the first membrane separation step (S0-3), or adding it to the waste liquid (mixing tank MT1) that is put into the first membrane separation step (S0-3).

[0017] In the wastewater treatment method, the amount of water added in the water addition step (S0-4) may be 50% to 150% of the amount of water in the first permeate, or it may be an amount such that the conductivity of the water-added concentrate obtained by adding water to the first concentrate is 65% to 135% of the conductivity of the raw wastewater. If it is less than 50%, the osmotic pressure will be high, so it will be necessary to increase the liquid pressure in order to maintain the recovery amount. If it exceeds 150%, the membrane permeation time will be long, which will reduce the recovery efficiency and is therefore undesirable.

[0018] The wastewater treatment method may involve adding water to the first concentrated liquid obtained in the first membrane separation step (S0-3) or not, and then introducing it back into the first separation membrane for treatment in the first membrane separation step (S0-3), or adding it to the wastewater (mixing tank MT1) introduced into the first membrane separation step (S0-3).

[0019] The method for treating the waste liquid may include an iodine reduction step (S0-1) of adding an iodine reducing agent to the waste liquid introduced into the first membrane separation step (S0-3) to reduce iodine to the reduction point. The iodine reducing agent may be one or more selected from an aqueous solution of sodium thiosulfate, an aqueous solution of potassium thiosulfate, and an aqueous solution of ascorbic acid.

[0020] The method for treating the waste liquid may be such that the reverse osmosis membrane may be one or more selected from an RO membrane and an NF membrane.

[0021] The boron concentration in the waste liquid is at least 200 ppm or more, preferably 500 ppm or more, more preferably 600 ppm or more. If it is 200 ppm or less, the treatment capacity increases and it is inefficient for industrial use. The upper limit value of the boric acid concentration in the waste liquid is not particularly limited, but examples include 5000 ppm or less, preferably 3000 ppm or less.

[0022] The method for treating the waste liquid may include a pH adjustment step (S0-2) of adjusting the pH of the waste liquid to 8.5 or less. The acid adjuster used for the pH adjustment may be dilute sulfuric acid or concentrated sulfuric acid having a concentration of 20% by mass or more and 70% by mass or less.

[0023] The boric acid concentration crystallization step (S1) may include an evaporation concentration step (S1-1) of evaporating the water in the first permeate (and the second permeate) to obtain a concentrated aqueous boric acid solution, and a cooling crystallization solid-liquid separation step of cooling and crystallizing (S1-2) the concentrated aqueous boric acid solution concentrated to a predetermined concentration and performing solid-liquid separation (S1-3). The means for performing the solid-liquid separation may be, for example, a centrifuge, a filtration device, a filter device, or the like. The method for treating the waste liquid may include a drying step of drying the solid-liquid separated boric acid at a predetermined temperature (for example, from 60°C to 100°C) to obtain boric acid having a purity of 95% by mass or more, a purity of 96% by mass or more, a purity of 97% by mass or more, a purity of 98% by mass or more.

[0024] The method for treating the waste liquid may be such that after the cooling crystallization solid-liquid separation step (S1-2, S1-3), the step of dissolving the boric acid in warm water, cooling and crystallizing, and performing solid-liquid separation (S1-2, S1-3) may be repeated one or more times.

[0025] The method for treating the waste liquid may be to put the separated liquid obtained by separation in the cooling crystallization solid-liquid separation step (S1-2, S1-3) into the first separation membrane (21) again and treat it in the first membrane separation step (S0-3), or it may be added to the waste liquid (mixed tank MT1) input into the first membrane separation step (S0-3).

[0026] The method for recovering and producing boric acid according to the present disclosure is a method for recovering and producing boric acid that selectively recovers boric acid from waste liquid generated in the process of manufacturing a polarizing plate, and may include each step of the method for treating waste liquid that selectively recovers boric acid from the waste liquid generated in the above-mentioned process of manufacturing a polarizing plate.

[0027] The method for manufacturing a polarizing plate according to the present disclosure is a method for manufacturing a polarizing plate including a treatment method for selectively recovering boric acid from waste liquid generated in the process of manufacturing a polarizing plate. The treatment method for selectively recovering boric acid from the waste liquid is selected from the above method for recovering and producing boric acid or the above method for treating waste liquid. The boric acid obtained by the treatment method for selectively recovering boric acid from the waste liquid may be part or all of the raw material of the aqueous boric acid solution used when manufacturing the polarizer constituting the polarizing plate. The method for manufacturing the polarizing plate may be characterized in that the boric acid obtained by the treatment method for selectively recovering boric acid from the waste liquid is the raw material of the aqueous boric acid solution used in one or more steps selected from the boric acid aqueous solution preliminary contact step, dyeing step, cross-linking step, and stretching step of the polarizer constituting the polarizing plate.

[0028] (1) Boric acid can be selectively recovered from waste liquid by a method different from the prior art. (2) The recovered high-purity boric acid can be recycled. (3) The amount of industrial waste can be reduced.

[0029] It is a schematic configuration diagram showing an example of a method for treating waste liquid and a treatment system.

[0030] Hereinafter, embodiments of the present invention will be described.

[0031] (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 such as boric acid and potassium iodide, iodides, and zinc. The waste liquid may also contain alkali metals other than those listed 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.

[0032] In this embodiment, the first waste liquid is mainly waste liquid discharged from the stretcher and has a pH of approximately 4.5 to 5.5. The second waste liquid contains zinc and has a pH of approximately 9 to 10.

[0033] (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).

[0034] (S0) The waste liquid is transferred from the buffer tank to the mixing tank MT1, and the first and second waste liquids are introduced. When the first and second waste liquids are mixed, the mixture becomes alkaline overall. (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).

[0035] (S0-2) The iodine-reduced wastewater is pH-adjusted to 8.5 or lower (pH adjustment step). For example, sulfuric acid or hydrochloric acid can be used as the pH adjusting agent, and the wastewater can be adjusted from, for example, pH 4 to 8.5 or lower. The wastewater, iodine reducing agent, and pH adjusting agent (acid) are mixed. Mixing is performed using a stirrer M1 for a predetermined time and at a predetermined rotational speed.

[0036] Borates in waste liquid are reacted with an acid (e.g., sulfuric acid) to obtain boric acid. When the borate is borax, the following reaction occurs: 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) By lowering the pH of the wastewater to 8.5 or below, boric acid is more easily incorporated into the permeate treated by the reverse osmosis membrane. If the wastewater is alkaline, almost no boric acid will be contained in the permeate, and almost all of the boric acid will be contained in the concentrated solution. In this embodiment, lowering the pH of the wastewater to 8.5 or below can increase the boric acid concentration in the permeate. In this embodiment, the boron concentration in the wastewater discharged during the polarizing plate manufacturing process is at least 200 ppm, preferably 500 ppm or more, and more preferably 600 ppm or more.

[0037] (S0-3) The wastewater, whose pH has been adjusted to 8.5 or lower, is passed through a first separation membrane module 21 equipped with a reverse osmosis membrane to separate it into a first permeate containing boric acid (B1) and substances other than boric acid in the wastewater (R1), and a first concentrate containing boric acid (B2) and substances other than boric acid in the wastewater (R2) (first membrane separation step). The first permeate is sent to the boric acid concentration crystallization step (S1). The first concentrate is sent to the second membrane separation step (S0-5).

[0038] In this embodiment, for example, with a liquid recovery rate of 50%, and assuming each substance is 100% of the amount input to the first separation membrane module 21, the first permeate contains 30% boric acid and 1% or less of other substances. In the first concentrate, it contains approximately 70% boric acid and 99% of other substances.

[0039] When the liquid recovery rate of the first permeate in the first membrane separation step (S0-3) is 20% to 70%, the concentration of boric acid (B1) in the first permeate (C Ｂ１ ) is the concentration of boric acid (B0) in the waste liquid fed into the first separation membrane module 21 (C Ｂ０) may be 10% to 60%. Also, the concentration of substances other than boric acid (R1) in the first permeate (C Ｒ１ ) is the concentration (C) of substances other than boric acid (R0) in the waste liquid fed into the first separation membrane module 21. Ｒ０ It may be 20% or less of the first concentrate. The concentration of boric acid (B2) in the first concentrate (C Ｂ２ ) is the concentration of boric acid (B0) in the waste liquid fed into the first separation membrane module 21 (C Ｂ０ ) may be 40% to 90%. The concentration of the substance other than boric acid (R2) in the first concentrated solution (C Ｒ２ ) is the concentration (C) of substances other than boric acid (R0) in the waste liquid fed into the first separation membrane module 21. Ｒ０ ) can be 80% or more.

[0040] (S0-4) Before sending to the second membrane separation step, water is added to the first concentrate to obtain a hydrolyzed concentrate (hydrolysis step). The amount of water added may be 50% to 150% of the amount of water in the first permeate, or the amount of water added to the first concentrate so that the conductivity of the hydrolyzed concentrate is 65% to 135% of the conductivity of the raw water of the waste liquid. In this embodiment, for example, the same amount of water as the permeate is added.

[0041] (S0-5) The hydrolyzed concentrate is passed through a second separation membrane module 22 equipped with a reverse osmosis membrane to separate it into a second permeate containing boric acid (B11) and a substance other than boric acid (R11), and a second concentrate containing boric acid (B21) and a substance other than boric acid (R21) (second membrane separation step). The second permeate is sent to the boric acid concentration crystallization step (S1). The second concentrate may be sent to the mixing tank MT1, to the first separation membrane module 21, or treated as waste liquid. In this embodiment, for example, with a liquid recovery rate of 50%, and each substance being 100% of the amount input to the second separation membrane module 22, the second permeate contains 21% boric acid (70% x 30%) and other substances contain 1% or less. In the second concentrate, boric acid contains 49% (= 70% x 70%) and other substances contain approximately 99%.

[0042] When the liquid recovery rate of the second permeate in the second membrane separation step (S0-5) is 20% to 70%, the concentration of boric acid (B11) in the second permeate (C Ｂ１１) is the concentration of boric acid (B01) in the hydrolyzed concentrate that is fed into the second separation membrane module 22 (C Ｂ０１ ) may be 10% to 60%. Also, the concentration of substances other than boric acid (R11) in the second permeate (C Ｒ１１ ) is the concentration (C) of substances other than boric acid (R01) in the hydrolyzed concentrate that is fed into the second separation membrane module 22. Ｒ０１ It may be 20% or less of the amount. Also, the concentration of boric acid (B21) in the second concentrate (C Ｂ２１ ) is the concentration of boric acid (B01) in the hydrolyzed concentrate that is fed into the second separation membrane module 22 (C Ｂ０１ ) may be 40% to 90%. Also, the concentration of substances other than boric acid (R21) in the second concentrated solution (C Ｒ２１ ) is the concentration (C) of substances other than boric acid (R01) in the hydrolyzed concentrate that is fed into the second separation membrane module 22. Ｒ０１ ) can be 80% or more.

[0043] (S1) The boric acid concentration crystallization process is shown below. (S1-1) The water in the first permeate and the second permeate is evaporated to obtain a concentrated boric acid aqueous solution (evaporation concentration process).

[0044] (S1-2) The concentrated boric acid aqueous solution obtained in the evaporation concentration step S1-1 is cooled and crystallized at, for example, 15°C to 30°C to obtain crystallized boric acid (cooling and crystallization step). (S1-3) The crystallized boric acid obtained in the cooling and crystallization step S1-2 is separated into a solid component and a liquid component (solid and liquid separation step). The liquid component is, for example, an aqueous sodium sulfate solution, which may be treated as wastewater, put back into the first separation membrane module 21, or added to the waste liquid (mixing tank MT1).

[0045] (Drying process) The solid containing the solid-liquid separated 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.

[0046] The drying process 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.

[0047] (Alternative Embodiments) (1) The configuration may not include the second membrane separation module 22. (2) After the cooling crystallization solid-liquid separation step (S1-2, S1-3), the step of dissolving the boric acid in hot water and cooling crystallizing to separate the solid and liquid (S1-2, S1-3) may be repeated one or more times. (3) If the second wastewater is not treated, the step of adding the pH adjusting agent may be omitted if only the first wastewater is present. (4) The iodine reduction step (S0-1) may be omitted.

[0048] (Wastewater Treatment System) The wastewater treatment system can be suitably used in the above-described wastewater treatment method. The wastewater treatment system is a wastewater treatment system that selectively recovers boric acid from wastewater generated in the polarizing plate manufacturing process. The wastewater treatment system includes a first separation membrane module 21 equipped with a reverse osmosis membrane. Wastewater with a pH of 8.5 or less, or wastewater whose pH has been adjusted to 8.5 or less, is passed through the first separation membrane module 21 to separate it into a first permeate containing boric acid (B1) and substances other than boric acid in the wastewater (R1), and a first concentrate containing boric acid (B2) and substances other than boric acid in the wastewater (R2).

[0049] The wastewater treatment system includes a second separation membrane module 22 equipped with a reverse osmosis membrane. The hydrolyzed concentrate, which is obtained by adding water to the first concentrate, is passed through the second separation membrane module 22 to separate it into a second permeate containing boric acid (B11) and a substance other than boric acid (R11), and a second concentrate containing boric acid (B21) and a substance other than boric acid (R21).

[0050] The separation membranes in the separation membrane module are, for example, RO membranes and NF membranes.

[0051] Any conventional separation membrane can be used. For example, various porous membranes can be used, but a composite semipermeable membrane having a separation function layer on the surface of a porous support is preferred. As for the porous support, one having a polymer porous layer on one side of a nonwoven fabric layer is preferred. The thickness of the separation membrane, especially the composite semipermeable membrane, is preferably about 70 μm to 160 μm, and more preferably 85 μm to 130 μm.

[0052] These composite semipermeable membranes are called RO (reverse osmosis) membranes, NF (nanofiltration) membranes, or FO (forward osmosis) membranes, depending on their filtration performance and treatment method.

[0053] Examples of separation functional layers include polyamide-based, cellulose-based, polyether-based, and silicon-based separation functional layers, but those having a polyamide-based separation functional layer are preferred. Generally, a polyamide-based separation functional layer is a homogeneous film without visible pores and has the desired ion separation ability. This separation functional layer is not particularly limited as long as it is a polyamide-based thin film that does not easily peel off from the polymer porous layer, but for example, a polyamide-based separation functional layer obtained by interfacial polymerization of a polyfunctional amine component and a polyfunctional acid halide component on a porous support film is well known.

[0054] The method for forming the polyamide-based separation functional layer on the surface of the polymer porous layer is not particularly limited, and any known method can be used. For example, methods such as interfacial polymerization, phase separation, and thin-film coating can be used, but in the present invention, interfacial polymerization is particularly preferred. Interfacial polymerization is a method in which, for example, the polymer porous layer is coated with an amine aqueous solution containing a polyfunctional amine component, and then an organic solution containing a polyfunctional acid halide component is brought into contact with this amine aqueous solution coated surface to cause interfacial polymerization and form a skin layer.

[0055] A coating layer made of various polymer components may be provided on the exposed surface of the separation functional layer. The polymer component is not particularly limited as long as it does not dissolve the separation functional layer and the porous support membrane and does not leach out during water treatment operations. Examples include polyvinyl alcohol, polyvinylpyrrolidone, hydroxypropyl cellulose, polyethylene glycol, and saponified polyethylene-vinyl acetate copolymer.

[0056] The nonwoven fabric layer is not particularly limited as long as it provides adequate mechanical strength while maintaining the separation and permeability performance of the composite semipermeable membrane, and commercially available nonwoven fabrics can be used. Examples of such materials include those made of polyolefin, polyester, cellulose, etc., and mixtures of multiple materials can also be used. Polyester is particularly preferable in terms of moldability. Long-fiber nonwoven fabrics and short-fiber nonwoven fabrics can also be used as appropriate, but long-fiber nonwoven fabrics are preferable in terms of preventing fine fuzzing that can cause pinhole defects and maintaining uniformity of the membrane surface.

[0057] The polymer porous layer is not particularly limited as long as it can form the polyamide-based separation functional layer, but is usually a microporous layer having a pore size of about 0.01 μm to 0.4 μm. Examples of materials for forming the microporous layer include polysulfone, polyaryl ethersulfone (exemplified by polyethersulfone), polyimide, and polyvinylidene fluoride. It is particularly preferable to form the polymer porous layer using polysulfone or polyaryl ethersulfone because they are chemically, mechanically, and thermally stable.

[0058] The separation membrane modules 21 and 22 may consist of one or more membrane elements. The membrane elements are typically spiral-type membrane elements. The membrane module may 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 is not limited to the spiral type and may be other types such as hollow fiber type, tubular type, or frame and plate type.

[0059] The wastewater treatment system may include: an evaporation and concentration device that evaporates water from the first and second permeates to concentrate them to a predetermined concentration and obtain a boric acid concentrate; a cooling and crystallization device that cools the concentrated boric acid concentrate to a predetermined temperature (for example, 15°C to 30°C) and crystallizes it to obtain crystallized boric acid; a solid-liquid separation device that separates the solid and liquid components containing the crystallized boric acid; and a drying device that dries the solid component containing the crystallized boric acid separated by the 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 higher, 96% by mass or higher, 97% by mass or higher, or 98% by mass or higher.

[0060] The wastewater treatment system may include a mixing tank (MT1) for storing the wastewater and mixing it with a predetermined agent, and a stirrer.

[0061] Examples of the solid-liquid separation apparatus include a filtration apparatus, a centrifugal separator, and a dewatering apparatus. Examples of the drying apparatus include an electric heater, a hot air device, and a constant temperature bath.

[0062] (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.

[0063] (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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] Examples of surface treatment layers include hard coating, anti-reflective coating, anti-sticking coating, anti-glare coating, and anti-fouling coating.

[0068] (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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 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).

[0073] 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).

[0074] 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).

[0075] (Example) 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), zinc (2 ppm), others (0.8% by mass). Iodine reducing agent added: 20% by mass aqueous solution of sodium thiosulfate Acid for pH adjustment: sulfuric acid (H ２ SO ４: 30% by mass, water: 70% by mass) First membrane separation: First permeate (assuming each substance accounts for 100% of the input amount, the permeate contains 30% boric acid and 1% or less of other substances), first concentrate (assuming each substance accounts for 100% of the input amount, the concentrate contains approximately 70% boric acid and 99% of other substances), using an RO membrane module. Amount of dilution water in the water addition step: Same amount as the first permeate (100%: Examples 1 to 7, Comparative Example 1), 30% of the first permeate (Example 8) Second membrane separation: Second permeate (assuming each substance accounts for 100% of the input amount, the permeate contains 21% boric acid (70% x 30%) and 1% or less of other substances), second concentrate (assuming each substance accounts for 100% of the input amount, the concentrate contains approximately 49% boric acid (= 70% x 70%) and 99% of other substances), using an RO membrane module. Permeate evaporator / concentrator: The permeate was concentrated 62 times under reduced pressure at 70°C to a boric acid concentration of 16% by mass. Cold crystallization of crystallized boric acid: Cold crystallization was performed at 15°C to 30°C for 12 hours. Solid-liquid separation of 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).

[0076] Table 1 shows the treatment conditions for each of the examples and comparative examples, as well as the boron recovery rate, iodine recovery rate, and overall evaluation of the solid components after concentration and crystallization (two RO membrane treatments).

[0077]

[0078] In Examples 1 to 8, boric acid with a purity of 99% by mass was recovered in all cases. Furthermore, the amount of industrial waste to be disposed of was reduced by 50%. In addition, although the boric acid concentration was 1.0% by mass in the above examples, the same procedure can be carried out with a boric acid concentration in the range of 0.1% to 2.5% by mass, for example.

[0079] 21 First separation membrane module 22 Second separation membrane module 100 Polarizing plate manufacturing apparatus

Claims

1. A wastewater treatment method for selectively recovering boric acid from wastewater generated during the polarizing plate manufacturing process, comprising: a first membrane separation step of passing the wastewater having a pH of 8.5 or less, or the wastewater whose pH has been adjusted to 8.5 or less, through a first separation membrane module equipped with a reverse osmosis membrane to separate it into a first permeate containing the boric acid (B1) and substances other than boric acid in the wastewater (R1), and a first concentrate containing the boric acid (B2) and substances other than boric acid in the wastewater (R2); and a boric acid concentration crystallization step of recovering the boric acid from the first permeate obtained in the first membrane separation step by concentration crystallization.

2. The wastewater treatment method is as follows: When the liquid recovery rate of the first permeate in the first membrane separation step is 20% to 70%, the concentration of boric acid (B1) in the first permeate (C Ｂ１ ) is the concentration of boric acid (B0) in the waste liquid fed into the first separation membrane module (C Ｂ０ ) is 10% to 60%, and the concentration of the substance other than boric acid (R1) in the first permeate (C Ｒ１ ) is the concentration (C) of substances other than boric acid (R0) in the waste liquid that is fed into the first separation membrane module. Ｒ０ ) is 20% or less, and the concentration of boric acid (B2) in the first concentrated solution (C Ｂ２ ) is the concentration of boric acid (B0) in the waste liquid fed into the first separation membrane module (C Ｂ０ ) is 40% to 90%, and the concentration of substances other than boric acid (R2) in the first concentrated solution (C Ｒ２ ) is the concentration (C) of substances other than boric acid (R0) in the waste liquid that is fed into the first separation membrane module. Ｒ０ The wastewater treatment method according to claim 1, wherein 80% or more of the amount is [unspecified].

3. The wastewater treatment method according to claim 1, comprising: a hydration step of adding water to the first concentrate obtained in the first membrane separation step to obtain a hydroconcentrated liquid; a second membrane separation step of passing the hydroconcentrated liquid obtained in the hydration step through a second separation membrane module equipped with a reverse osmosis membrane to separate it into a second permeate containing the boric acid (B11) and a substance other than the boric acid (R11), and a second concentrate containing the boric acid (B21) and a substance other than the boric acid (R21); and treating the second permeate obtained in the second membrane separation step in the boric acid concentration crystallization step.

4. When the liquid recovery rate of the second permeate in the second membrane separation step is 20% to 70%, the concentration (C Ｂ１１ ) of boric acid (B11) in the second permeate is 10% to 60% of the concentration (C Ｂ０１ ) of boric acid (B01) in the hydro-concentrate fed into the second separation membrane module; the concentration (C Ｒ１１ ) of substances other than boric acid (R11) in the second permeate is 20% or less of the concentration (C Ｒ０１ ) of substances other than boric acid (R01) in the hydro-concentrate fed into the second separation membrane module; the concentration (C Ｂ２１ ) of boric acid (B21) in the second concentrate is 40% to 90% of the concentration (C Ｂ０１ ) of boric acid (B01) in the hydro-concentrate fed into the second separation membrane module; the concentration (C Ｒ２１ ) of substances other than boric acid (R21) in the second concentrate is 80% or more of the concentration (C Ｒ０１ ) of substances other than boric acid (R01) in the hydro-concentrate fed into the second separation membrane module. The method for treating waste liquid according to claim 3.

5. The wastewater treatment method according to claim 3, wherein the second concentrated liquid obtained in the second membrane separation step is either given water or not given water and put back into the first separation membrane module for treatment in the first membrane separation step, or is added to the wastewater that is put into the first membrane separation step.

6. The wastewater treatment method according to claim 3, wherein in the water addition step, the amount of water added is 50% to 150% of the amount of water in the first permeate, or the amount of water added to the first concentrate so that the conductivity of the diluted concentrate is 65% to 135% of the conductivity of the raw wastewater.

7. The wastewater treatment method according to claim 1, wherein water is added to the first concentrated liquid obtained in the first membrane separation step, or without adding water, and the liquid is put back into the first separation membrane and treated in the first membrane separation step, or the wastewater is added to the wastewater that is put into the first membrane separation step.

8. The wastewater treatment method according to claim 1, wherein the boric acid concentration crystallization step comprises a first evaporation concentration step of evaporating the water in the first permeate to obtain a boric acid concentrated aqueous solution, and a cooling crystallization solid-liquid separation step of cooling and crystallizing the boric acid concentrated aqueous solution concentrated to a predetermined concentration and separating the solid and liquid.

9. The wastewater treatment method according to claim 8, wherein the separated liquid obtained by separating in the cooling crystallization solid-liquid separation step is put back into the first separation membrane and treated in the first membrane separation step, or is added to the wastewater that is put into the first membrane separation step.

10. A method for recovering and producing boric acid, comprising the steps of the waste liquid treatment method described in claim 1 or 2, for selectively recovering boric acid from waste liquid generated during the polarizing plate manufacturing process.

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 claim 1 or 2, 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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