Treatment method for wastewater from polarizing plate manufacturing

Abstract

A method for treating wastewater from the manufacture of polarizing plates, which can improve the recovery rate of potassium iodide. [Solution] A method for treating waste liquid from polarizing plate manufacturing includes a first concentration step of evaporating and concentrating the waste liquid from polarizing plate manufacturing to produce a first precipitate, a first solid-liquid separation step of performing solid-liquid separation of the first precipitate from the waste liquid from polarizing plate manufacturing to produce a first filtrate, a second concentration step of evaporating and concentrating the first filtrate to produce a second precipitate, a second solid-liquid separation step of performing solid-liquid separation of the second precipitate from the first filtrate to produce a second filtrate, and a recovery step of recovering potassium iodide from the separated second precipitate. The treatment method includes an iodine reduction step, prior to the first concentration step, of adding an iodine reducing agent to the waste liquid from polarizing plate manufacturing to reduce iodine to a reduction point.

Description

[Technical Field]

[0001] The present invention relates to a method for treating waste liquid generated in the manufacture of polarizing plates, and more particularly to a method for selectively recovering potassium iodide (KI) from waste liquid generated in the process of manufacturing polarizing plates. [Background technology]

[0002] Wastewater 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 wastewater is treated as industrial waste. However, there is a demand for treating the wastewater obtained during the manufacturing process, recovering potassium iodide, and recycling it into the manufacturing process.

[0003] Patent Documents 1 to 3 disclose a method for treating wastewater from polarizing plate manufacturing, recovering a potassium iodide solution from the wastewater. The method described in Patent Document 1 includes a first concentration step in which the wastewater from polarizing plate manufacturing is evaporated and concentrated to produce a first precipitate containing primarily a boron-containing compound and polyvinyl alcohol; a first solid-liquid separation step in which the first precipitate is separated into solid and liquid to produce a first filtrate; a second concentration step in which the first filtrate is evaporated and concentrated to produce a second precipitate containing potassium iodide; a second solid-liquid separation step in which the second precipitate is separated from the first filtrate to produce a second filtrate; and a recovery step in which potassium iodide is recovered from the second precipitate. The recovery step includes a step of washing potassium iodide crystals contained in the second precipitate with a saturated potassium iodide solution.

[0004] The treatment method of Patent Document 2 involves concentrating wastewater from polarizing plate manufacturing and then crystallizing the resulting precipitate, subjecting the resulting precipitate to solid-liquid separation to produce a potassium iodide solution in which impurities including boron and polyvinyl alcohol have been reduced, adsorbing the polyvinyl alcohol remaining in the resulting potassium iodide solution onto a polyvinyl alcohol adsorbent, and adsorbing the boron remaining in the potassium iodide solution from which the polyvinyl alcohol has been adsorbed onto a boron-selective adsorption resin. Patent Document 2 also discloses a process of filtering impurities that precipitate when the saturated solubility decreases due to an endothermic reaction caused by the dissolution of potassium iodide crystals.

[0005] The treatment method disclosed in Patent Document 3 includes a first concentration step of evaporating and concentrating a polarizing plate manufacturing waste liquid to produce a first precipitate containing mainly a boron-containing compound and polyvinyl alcohol; a first solid-liquid separation step of performing solid-liquid separation of the first precipitate from the polarizing plate manufacturing waste liquid containing the first precipitate to produce a first filtrate; a second concentration step of evaporating and concentrating the first filtrate to produce a second precipitate containing mainly potassium iodide; a second solid-liquid separation step of performing solid-liquid separation of the second precipitate from the first filtrate containing the second precipitate to produce a second filtrate; a filtrate discharge step of discharging at least a portion of the second filtrate to outside the polarizing plate manufacturing waste liquid treatment device; and a recovery step of recovering potassium iodide from the second precipitate by washing the separated second precipitate with an aqueous potassium iodide solution and removing the remaining polyvinyl alcohol. The second filtrate discharged outside the treatment device for waste liquid from the production of polarizing plates is characterized in that it is not returned to the second concentration step and is not reused. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 6650652 [Patent Document 2] Patent No. 7165344 [Patent Document 3] Patent No. 7048950 Summary of the Invention [Problem to be solved by the invention]

[0007] If iodine is contained in the evaporated concentrate or filtrate, it can cause problems such as corrosion of pipes, etc. This can also deteriorate the quality of the recovered KI water. Furthermore, if iodine remains, it can volatilize and become mixed into the evaporated water, causing problems such as making it impossible to reuse the water within the factory.

[0008] The present invention provides a method for treating wastewater from the production of polarizing plates, which can improve the recovery rate of potassium iodide (KI) compared to conventionally known methods. [Means for solving the problem]

[0009] As a result of extensive research aimed at solving the above-mentioned problems, the present inventors have found that the effects of iodine can be suppressed by converting iodine into iodide ions in advance using an iodine reducing agent and then performing an evaporation and concentration step, etc. That is, the present invention includes the following aspects.

[0010] The method for treating waste liquid from the production of polarizing plates according to the present disclosure is a method for treating waste liquid from the production of polarizing plates, which recovers potassium iodide from the waste liquid from the production of polarizing plates, and includes the steps of: a first concentration step (S1-1, S1-2) of evaporating and concentrating the waste liquid from the production of polarizing plates to produce a first precipitate containing at least potassium iodide and boric acid; a first solid-liquid separation step (S1-3) of performing solid-liquid separation on the first precipitate from the polarizing plate manufacturing waste liquid to generate a first filtrate containing at least potassium iodide and boric acid; a second concentration step (S2-1) of evaporating and concentrating the first filtrate to produce a second precipitate containing at least potassium iodide and boric acid; a second solid-liquid separation step (S2-2) of generating a second filtrate by solid-liquid separation of the second precipitate from the first filtrate; and a recovery step (S3) of recovering potassium iodide from the separated second precipitate, The processing method comprises: The method includes an iodine reduction step (S0-2) before the first concentration step, in which an iodine reducing agent is added to the polarizing plate production waste liquid to reduce iodine to a reduction point.

[0011] The iodine reducing agent may be one or more selected from sodium thiosulfate, potassium thiosulfate, ascorbic acid, and oxalic acid, with ascorbic acid being more preferred.

[0012] When the iodine reducing agent is ascorbic acid, the waste liquid from the manufacture of polarizing plates preferably has a pH of less than 7 before the iodine reduction step. When the iodine reducing agent is ascorbic acid, the treatment method may include a pH adjustment step (S0-1) of adjusting the pH of the polarizing plate production waste liquid to a pH of less than 7 before the iodine reduction step.

[0013] The treatment method may include an alkali adjustment step (S0-3) of adjusting the pH of the polarizing plate manufacturing waste liquid after the iodine reduction step so that the waste liquid becomes alkaline.

[0014] (effect) According to the above configuration, the iodine reducing agent can be used to suppress the volatilization of iodine, thereby preventing corrosion of equipment and deterioration of the water quality of the recovered solution. For example, by using ascorbic acid, which has low solubility, the quality of the regenerated potassium iodide can be maintained. In addition, the waste liquid from polarizing plate manufacturing after the iodine reduction process can be made alkaline to prevent iodine (I2) from being generated by reoxidation.

[0015] The method for producing a polarizing plate according to the present disclosure includes a treatment method for recovering potassium iodide from a waste solution produced by producing a polarizing plate, the method comprising: The treatment method for recovering potassium iodide from the waste liquid produced by the production of polarizing plates comprises the steps of: The method for treating wastewater from the production of polarizing plates, The potassium iodide obtained by the treatment method may be a part or all of a raw material for an aqueous potassium iodide solution used in producing a polarizer constituting a polarizing plate. The method for producing the polarizing plate includes the steps of: The method may be characterized in that the potassium iodide obtained by the treatment method for recovering potassium iodide from waste liquid produced in the production of a polarizing plate is used as a raw material for an aqueous potassium iodide solution to be used in one or more steps selected from a pre-contact step with an aqueous boric acid solution containing potassium iodide, a dyeing step, a crosslinking step, and a stretching step of a polarizer constituting a polarizing plate. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a schematic diagram showing an example of a method for treating waste liquid. DETAILED DESCRIPTION OF THE INVENTION

[0017] (Embodiment 1) Hereinafter, a first embodiment of the present invention will be described.

[0018] (waste liquid) The polarizing plate manufacturing waste liquid in this embodiment is, for example, waste liquid generated during the polarizing plate manufacturing process. The polarizing plate manufacturing 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 polarizing plate manufacturing waste liquid may also contain oxides and iodides such as boric acid and potassium iodide. The polarizing plate manufacturing waste liquid may also contain alkali metals other than those mentioned above. Examples of alkali metals include one or more types such as sodium and lithium. The alkali metals exist in the waste liquid as cations, but may also exist as fine particles. The polarizing plate manufacturing waste liquid may also contain monovalent anions. The monovalent anions may be one or more types of anions of halogen atoms such as fluorine, chlorine, and bromine. The polarizing plate manufacturing waste liquid may also contain divalent anions and trivalent anions. Hereinafter, the polarizing plate manufacturing waste liquid may also be referred to as waste liquid.

[0019] (Processing method) An example of a wastewater treatment method is shown in Figure 1. The polarizing plate manufacturing apparatus may include, for example, a dyeing bath, a stretching machine, a crosslinking bath, a cleaning bath, a dryer, a film conveying device, an adhesive coating device, and a device for attaching a polarizer and a polarizer protective film. The wastewater discharged from this manufacturing apparatus is stored in a buffer tank BT.

[0020] (S0) Wastewater sent from the polarizing plate manufacturing equipment is stored in the raw water tank BT.

[0021] (S0-1) The wastewater in the raw water tank BT is measured to see if it has a pH of less than 7. If it does not, the pH is adjusted to less than 7 (pH adjustment step). In the pH adjustment, the waste liquid may be adjusted with an acid (for example, sulfuric acid, hydrochloric acid, nitric acid, etc.) so that the pH is less than 7. In the pH adjustment, the waste liquid may be stirred with a stirring means.

[0022] (S0-2) An iodine reducing agent is added to the wastewater from the raw water tank BT to reduce iodine to the reduction point (iodine reduction step). The iodine reducing agent may be one or more selected from sodium thiosulfate, potassium thiosulfate, and ascorbic acid. When ascorbic acid is used, step S0-1 is essential.

[0023] (S0-3) The pH of the wastewater after the iodine reduction step is adjusted to be alkaline (alkali adjustment step). In the alkali adjustment step, the pH of the waste liquid may be adjusted with an alkali (e.g., potassium hydroxide, sodium hydroxide, etc.) to make it alkaline (e.g., pH 8 or higher). This prevents iodine (I2) from being generated by reoxidation.

[0024] (S1-1) The waste liquid that has undergone the above steps is introduced into a first concentrator and evaporated and concentrated (first concentration step). The first concentrator may have any suitable configuration as long as it is capable of concentrating the waste liquid by evaporation. Specific examples include evaporative concentration devices of the heat pump type, ejector-driven type, steam type, and flash type. When the waste liquid is concentrated in the first concentrator, boron-containing compounds (typically boric acid (H3BO3)) contained in the waste liquid are precipitated.

[0025] (S1-2) The evaporated and concentrated waste liquid is cooled and crystallized in a first cooling crystallizer (first cooling and crystallization step). Examples of the first cooling and crystallization device include jacket-type and vacuum-type cooling crystallizers. In the cooling and crystallization, the waste liquid is cooled to preferably 45°C or less, more preferably 40°C or less, even more preferably 30°C or less, and particularly preferably around room temperature (e.g., 25°C). By performing the cooling and crystallization, boric acid is further crystallized, and the boric acid concentration in the waste liquid can be further reduced. The first precipitate may contain boric acid, PVA, sulfates, and other impurities. Potassium iodide (KI) remains dissolved without being precipitated.

[0026] (S1-3) The waste liquid after evaporation and concentration and cooling crystallization is introduced into a first solid-liquid separation device, where a first precipitate is separated from the waste liquid into solid and liquid, producing a first filtrate containing at least potassium iodide and boric acid (first solid-liquid separation step). The first filtrate has, for example, about 60% to 90% of the boric acid removed and about 50% to 80% of the PVA removed, based on the waste liquid when removed from the raw water tank. The first filtrate may also contain other impurities such as sulfates. Examples of the first solid-liquid separation device include filtration devices (e.g., pressure filtration (filter press) devices, vacuum filtration devices, centrifugal filtration devices, and centrifugal separators (e.g., decanter-type centrifuges). The separated first precipitate is a crystal mainly composed of boric acid, and also contains PVA and a small amount of KI crystals. The separated first precipitate also contains about 10% to 20% of a liquid component containing KI. Therefore, in this embodiment, KI is further recovered from the first precipitate. The details of this will be described later.

[0027] (S2-1) The first filtrate is introduced into a second concentrator and evaporated (second concentration step). The boric acid and PVA concentrations in the first filtrate are sufficiently reduced by the first concentration step, and the first filtrate contains a high concentration of dissolved KI. Therefore, by further concentrating the first filtrate in the second concentrator, the KI becomes supersaturated. As a result, a second precipitate containing primarily KI can be produced. The second precipitate is a crystal composed primarily of KI, containing small amounts of boric acid, PVA, and the like. As with the first concentrator, any suitable configuration can be adopted for the second concentrator.

[0028] (S2-2) The first filtrate (slurry liquid) containing the second precipitate is introduced into a second solid-liquid separator to separate the second precipitate from the first filtrate (second solid-liquid separation step). As the second solid-liquid separator, any appropriate configuration can be adopted, similar to the first solid-liquid separator. The second filtrate may mainly contain KI, boric acid, and PVA. The KI concentration in the second filtrate is about 30% to 55% by weight, the boric acid concentration is about 1% to 5% by weight, and the PVA and sulfates are present in small amounts. (S2-3) The second filtrate may be returned to the raw water tank BT, or a portion thereof may be disposed of.

[0029] (S3-1) The second precipitate is redissolved in a solvent (water or hot water) to produce a redissolved slurry (first redissolution step). Impurities such as boric acid and PVA are crystallized by endothermic cooling during the redissolution.

[0030] (S3-2) The redissolved slurry is introduced into a third solid-liquid separator, and a fourth precipitate is separated from the redissolved slurry (third solid-liquid separation step). As with the first solid-liquid separator, any appropriate configuration may be adopted for the third solid-liquid separator. The fourth precipitate contains boric acid, PVA, etc. and is disposed of as waste. The fourth filtrate contains a high concentration of dissolved KI and low concentrations of impurities such as boric acid and PVA, and can be reused as recycled KI product in the polarizing plate manufacturing equipment (S3-3).

[0031] [Treatment of the first precipitate] (S4-1) The first precipitate is dissolved in a solvent (water or hot water) to produce a first solution (second re-dissolution step). The first precipitate and hot water as a solvent at a temperature of 70°C or higher, 50°C or higher, and preferably above 40°C, are placed in a dissolution tank and dissolved. The first solution may be stirred using a stirrer or the like. The boron is dissolved in the solvent so that the concentration of boron in the first solution is 0.7% or more, preferably 1.5% or more, and is preferably dissolved in the solvent so that the concentration is 3.5% or less, more preferably 3% or less. (S4-1-1) The pH of the first solution is adjusted to a pH of 8 to 12 (first solution pH adjustment step). When the pH of the first solution is not between 8 and 12, it is preferable to adjust the pH with an alkali. The alkali pH adjuster may be, for example, potassium hydroxide or sodium hydroxide. To facilitate precipitation of boric acid by cooling crystallization, the pH of the first solution is adjusted to a pH of 8 to 12, at which the solubility of boric acid and borax is low.

[0032] (S4-2) The first solution is cooled and crystallized in a second cooling and crystallization apparatus to produce a third precipitate (second cooling and crystallization step). The second cooling and crystallization apparatus may have the same configuration as the first cooling and crystallization apparatus. In the cooling and crystallization, the first solution is preferably cooled to 40°C or less, more preferably 30°C or less, and particularly preferably to around room temperature (e.g., 25°C). By performing the cooling and crystallization, boric acid is crystallized, and the boric acid concentration in the first solution is reduced.

[0033] (S4-3) The first solution is introduced into a fourth solid-liquid separator to separate a third precipitate from the first solution and produce a third filtrate containing at least potassium iodide and boric acid (fourth solid-liquid separation step). As the third solid-liquid separator, any appropriate configuration can be adopted, similar to the first solid-liquid separator. The third precipitate is mainly boric acid, and contains impurities such as PVA and sulfates, and is disposed of as waste. The third filtrate contains mainly KI with traces of boric acid and unprecipitated impurities.

[0034] (4-4) The third filtrate is returned to the raw water tank BT and mixed with the waste liquid (repeated process). The boron concentration of the wastewater in the raw water tank BT is measured periodically. The boron concentration in the wastewater can also be calculated from the boron concentration in the returned third filtrate, the amount of this filtrate returned, the amount of wastewater in the raw water tank, and the amount and number of treatments in the first concentration step. Since a high boron concentration can cause problems in the treatment after the first concentration step, the repetition step is limited to a certain number of times. If the boron concentration exceeds 0.8%, preferably 0.7%, the repeating step is omitted until the boron concentration of the wastewater falls to 0.3% or less, preferably 0.2% or less. In this case, the third filtrate may be discarded, or the third filtrate may be stored in a tank and then sent to the raw water tank after the boron concentration of the wastewater in the raw water tank has decreased.

[0035] (polarizing plate) The polarizing plate has, for example, a polarizer and a polarizer protective film provided on one or both of its main surfaces. The polarizing plate may further include an optically functional film provided on the polarizer or the polarizer protective film. The polarizing plate may have a surface treatment layer formed thereon.

[0036] A polarizer is, for example, a resin film containing a dichroic material. 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 resin film or 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 producing a laminate of the resin substrate and the PVA resin layer. This laminate is then stretched and dyed to convert the PVA resin layer into a polarizer.

[0037] Examples of the polarizer protective film include cellulose-based resins such as triacetyl cellulose (TAC), polyester-based, polyvinyl alcohol-based, polycarbonate-based, polyamide-based, polyimide-based, polyethersulfone-based, polysulfone-based, polystyrene-based, cycloolefin-based resins such as polynorbornene, polyolefin-based, (meth)acrylic-based, and acetate-based resins.

[0038] Examples of optically functional films include retardation films and brightness-enhancing films. The polarizing plate may further have a surface protective film on one of its outermost surfaces. A separator film (release film) may be provided on an outermost surface other than the surface protective film. The constituent films of the polarizing plate may be bonded together with an adhesive or pressure-sensitive adhesive.

[0039] Examples of the surface treatment layer include hard coat treatment, anti-reflection treatment, anti-sticking treatment, anti-glare treatment, and anti-fouling treatment.

[0040] (Polarizing Plate Manufacturing Method) An example of a method A for producing a polarizing plate including a polarizer and a polarizer protective film will be described below. The polarizing plate manufacturing method A includes a step of dyeing a polarizer, a step of crosslinking, a step of stretching, a step of adjusting a hue, and a step of drying and shrinking the polarizer, followed by a step of attaching the polarizer and a polarizer protective film. A step of pre-contacting the polarizer with an aqueous boric acid solution may be included before the dyeing step, and one or both of the crosslinking step and the hue adjusting 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.

[0041] For example, the PVA resin film is supplied from a roll on which it is wound, and is transported from upstream to downstream by a plurality of rollers. During this transport, a plurality of processes are performed. The PVA resin film being transported is immersed in a dye bath (dyeing solution), a crosslinking bath (crosslinking solution), a stretching bath (stretching solution), and a hue adjusting bath (hue adjusting solution) in that order, then sent to a heat drying process where it is dried and wound around a polarizer roll.

[0042] The staining solution may be, for example, an aqueous solution containing iodine or an iodine compound, and further containing boric acid. The crosslinking liquid may be, for example, an aqueous solution containing boric acid and an iodine compound. The stretching liquid may be, for example, an aqueous solution containing boric acid and an iodine compound. The color-adjusting liquid may be, for example, an aqueous solution containing an iodine compound. As the boric acid, boric acid recovered by the above method can be suitably used.

[0043] In the stretching step, the degree of stretching may be adjusted by varying the peripheral speed of the upstream and downstream transport rolls, and a uniaxial stretching device or a biaxial stretching device may be used. In the drying and shrinking step, the stretched film is dried and shrunk in the width direction perpendicular to the longitudinal direction by bringing the transport roll into contact with a heated heating roll. In addition to the heating roll, a heating means such as an oven or a heater may be used.

[0044] 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, and an adhesive is applied to one or both of the surfaces to be bonded, thereby bonding the first polarizer protective film to one side of the polarizer and the second polarizer protective film to the other side (first bonding process).

[0045] Alternatively, a pressure-sensitive adhesive may be applied to one or both outer surfaces of the polarizer protective film, and one or more optically functional films may be attached thereto (second laminating step).

[0046] Furthermore, a surface protective film may be attached to the polarizer protective film or the optically functional film on the viewing side via an adhesive (third laminating step). Furthermore, a release liner (release film) may be attached to the polarizer protective film or optically functional film on the device side (liquid crystal device, organic EL device, etc.) via an adhesive (fourth laminating step). [Example]

[0047] KI was recovered using two types of iodine reducing agents according to the procedure shown in Figure 1. Table 1 shows the results of Examples 1 to 3 and Comparative Example 1. The best results were obtained with ascorbic acid in Example 1. In Example 3, the pH of the wastewater was 8 when ascorbic acid was added, so the iodine-reducing power of ascorbic acid was weak. In the case of thiosulfuric acid in Example 2, corrosive components were observed in the recovered KI.

[0048] [Table 1] [Explanation of symbols]

[0049] 100 Polarizing plate manufacturing equipment

Claims

1. A method for treating waste liquid from the manufacture of polarizing plates, which recovers potassium iodide from the waste liquid from the manufacture of polarizing plates, comprising: a first concentration step of evaporating and concentrating the waste liquid from the production of polarizing plates to produce a first precipitate containing at least potassium iodide and boric acid; a first solid-liquid separation step of performing solid-liquid separation on the first precipitate from the polarizing plate production waste liquid to generate a first filtrate containing at least potassium iodide and boric acid; a second concentration step of evaporating and concentrating the first filtrate to produce a second precipitate containing at least potassium iodide and boric acid; a second solid-liquid separation step of generating a second filtrate by solid-liquid separation of the second precipitate from the first filtrate; a recovery step of recovering potassium iodide by re-dissolving the separated second precipitate and recovering the filtrate obtained by solid-liquid separation, The processing method comprises: an iodine reduction step of adding an iodine reducing agent to the polarizing plate production waste liquid and reducing iodine to a reduction point before the first concentration step; Method for treating wastewater from polarizing plate manufacturing.

2. The treatment method according to claim 1 , wherein the iodine reducing agent is at least one selected from the group consisting of sodium thiosulfate, potassium thiosulfate, and ascorbic acid.

3. the iodine reducing agent is ascorbic acid; the waste liquid from the production of polarizing plates has a pH of less than 7 before the iodine reduction step; or a pH adjustment step of adjusting the pH of the polarizing plate production waste liquid to a pH of less than 7 before the iodine reduction step; The processing method according to claim 1 .

4. 2. The treatment method according to claim 1, further comprising an alkali adjustment step of adjusting the pH of the polarizing plate manufacturing waste liquid after the iodine reduction step so that the waste liquid becomes alkaline.

5. A method for producing a polarizing plate, comprising a treatment method for recovering potassium iodide from waste liquid produced in the production of a polarizing plate, The method for recovering potassium iodide from the waste liquid produced by manufacturing a polarizing plate is the method according to any one of claims 1 to 4, The potassium iodide obtained by the treatment method is a part or all of a raw material for a potassium iodide aqueous solution used in producing a polarizer constituting a polarizing plate. A method for manufacturing a polarizing plate.

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