Method for disposing of waste liquid from polarizing plate manufacturing

Abstract

This invention provides a method for treating waste liquid from polarizing plate manufacturing that can improve the recovery rate of potassium iodide. [Solution] A method for treating wastewater from polarizing plate manufacturing comprises a recovery step of evaporating and concentrating the wastewater from polarizing plate manufacturing to produce a first precipitate, separating the first precipitate into solid and liquid to produce a first filtrate, evaporating and concentrating the first filtrate to produce a second precipitate, separating the second precipitate from the first filtrate into solid and liquid to produce a second filtrate, and recovering potassium iodide from the second precipitate. The recovery step includes a redissolution solution generation step of dissolving the second precipitate in a solvent to produce a redissolution solution so that the amount of the second precipitate is 30% to 60% by mass, a cooling and crystallization step of cooling and crystallizing the redissolution solution with a pH of 7 or less at 30°C or less to produce a fourth precipitate, and a fourth solid and liquid separation step of separating the fourth precipitate from the redissolution solution to produce a fourth filtrate containing at least potassium iodide.

Description

Technical Field

[0001] The present invention relates to a method for treating waste liquid generated in the production of polarizing plates, for example, a method for selectively recovering potassium iodide (KI) from waste liquid generated in the process of producing polarizing plates.

Background Art

[0002] The waste liquid generated in the manufacturing process of polarizing plates 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 potassium iodide, and recycle it to the manufacturing process.

[0003] Patent Documents 1 to 3 disclose a method for treating waste liquid from the production of polarizing plates to recover a potassium iodide solution from the waste liquid. The treatment method of Patent Document 1 includes a first concentration step of evaporating and concentrating the waste liquid from the production of polarizing plates to produce a first precipitate mainly containing a boron-containing compound and polyvinyl alcohol, a first solid-liquid separation step of performing solid-liquid separation on 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 potassium iodide, a second solid-liquid separation step of performing solid-liquid separation on the second precipitate from the first filtrate to produce a second filtrate, and a recovery step of recovering potassium iodide from the second precipitate. The recovery step includes a step of washing the crystals of potassium iodide contained in the second precipitate using a saturated solution of potassium iodide.

[0004] The treatment method of Patent Document 2 involves concentrating the waste liquid from the production of polarizing plates and then performing crystallization to separate the resulting precipitate by solid-liquid separation to produce a potassium iodide solution with reduced impurities containing boron and polyvinyl alcohol, adsorbing the polyvinyl alcohol remaining in the obtained 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 and removed onto a boron-selective adsorption resin. Further, Patent Document 2 discloses a step of filtering impurities precipitated due to a decrease in the saturated solubility by an endothermic reaction caused by the dissolution of potassium iodide crystals.

[0005] The processing method of Patent Document 3 includes: a first concentration step of evaporating and concentrating the wastewater from polarizing plate manufacturing to produce a first precipitate mainly containing a boron-containing compound and polyvinyl alcohol; a first solid-liquid separation step of solid-liquid separating the first precipitate from the wastewater from polarizing plate manufacturing 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 mainly containing potassium iodide; a second solid-liquid separation step of solid-liquid separating 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 outside the processing device for wastewater from polarizing plate manufacturing; and a recovery step of washing the separated second precipitate with an aqueous potassium iodide solution to remove any remaining polyvinyl alcohol and recover the potassium iodide from the second precipitate. A key feature of this system is that the second filtrate, which is discharged outside the treatment device for the waste liquid from polarizing plate manufacturing, is not returned to the second concentration process 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 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] The present invention provides a method for treating wastewater from polarizing plate manufacturing that can improve the purity of potassium iodide (KI) compared to conventionally known methods. [Means for solving the problem]

[0008] As a result of diligent research to solve the above-mentioned problems, the present inventors have found that potassium iodide can be recovered by dissolving the second precipitate in a solvent in a recovery process so that the amount of the second precipitate is 30% to 60% by mass, adjusting the pH of the redissolved solution to 6 or less, cooling and crystallizing the redissolved solution at 30°C or below, separating the fourth precipitate that has been cooled and crystallized from the redissolved solution, and generating a fourth filtrate containing potassium iodide. That is, the present invention includes the following embodiments.

[0009] The method for treating wastewater from polarizing plates according to this disclosure is a method for treating wastewater from polarizing plates that recovers potassium iodide from the wastewater from polarizing plates, A first concentration step (S1-1, S1-2) involves evaporating and concentrating the waste liquid from polarizing plate manufacturing to produce a first precipitate containing at least potassium iodide and boric acid, A first solid-liquid separation step (S1-3) is performed to separate the first precipitate from the waste liquid used in the production of the polarizing plate and to produce a first filtrate containing at least potassium iodide and boric acid. A second concentration step (S2-1) is performed by 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) is performed to produce a second filtrate obtained by solid-liquid separation of the second precipitate from the first filtrate, The process includes a recovery step (S3) for recovering potassium iodide from the separated second precipitate, The aforementioned recovery process (S3) is: A redissolving solution generation step (S3-1) is performed to generate a redissolving solution (100% by mass) by dissolving the second precipitate in a solvent (70% to 40% by mass) so that the amount of the second precipitate becomes 30% to 60% by mass, The redissolving solution pH adjustment step (S3-2) involves adjusting the pH of the redissolving solution so that it is pH 7 (preferably pH 6) or lower. A cooling crystallization step (S3-3) is performed in which the redissolved solution with a pH of 7 or less is cooled and crystallized at 30°C or below to produce a fourth precipitate, A fourth solid-liquid separation step (S3-4) is performed to separate the fourth precipitate from the redissolving solution and produce a fourth filtrate containing at least potassium iodide, Includes.

[0010] In the fourth solid-liquid separation step (S3-4), impurities crystallized in the redissolved solution may be removed by filtration.

[0011] The fourth filtrate may have a pH of 4.5 to 7.0. The process may include a pH adjustment step (S3-5) in which the pH of the fourth filtrate is adjusted to be between pH 4.5 and pH 7.0.

[0012] The fourth filtrate, with a pH of 4.5 to 7.0, may be returned to the polarizing plate manufacturing apparatus and reused (S3-6).

[0013] The processing method described above is: Prior to the first concentration step, the process may include an iodine reduction step in which an iodine reducing agent is added to the waste liquid from the production of the polarizing plate, and the iodine is reduced to its reduction point. The iodine reducing agent may be one or more selected from sodium thiosulfate, potassium thiosulfate, ascorbic acid, and oxalic acid. Ascorbic acid is more preferred.

[0014] When the iodine reducing agent is ascorbic acid, it is preferable that the waste liquid from the production of polarizing plates prior to the iodine reduction step has a pH of less than 7. When the iodine reducing agent is ascorbic acid, the treatment method may include a pH adjustment step (S0-1) before the iodine reduction step, in which the pH of the polarizing plate manufacturing waste liquid is adjusted to less than 7.

[0015] The above treatment method may include an alkali adjustment step (S0-3) in which the pH of the polarizing plate manufacturing waste liquid after the iodine reduction step is adjusted to be alkaline (for example, pH 8 or higher and pH 11 or lower).

[0016] (effect) According to the above configuration, a re-dissolved solution in which the second precipitate is dissolved to 30% to 60% by mass is cooled after pH adjustment to stably crystallize impurities (for example, boric acid, PVA, etc.), and solid-liquid separation (for example, filtration) is performed, thereby increasing the purity of the recovered potassium iodide.

[0017] 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 recovering potassium iodide from a waste liquid for manufacturing a polarizing plate, The treatment method for recovering potassium iodide from the waste liquid for manufacturing a polarizing plate is the above-described treatment method for the waste liquid for manufacturing a polarizing plate, It may be characterized in that the potassium iodide obtained by the treatment method becomes part or all of the raw material of the aqueous potassium iodide solution used when manufacturing the polarizer constituting the polarizing plate. The method for manufacturing the polarizing plate is, It may be characterized in that the potassium iodide obtained by the treatment method for recovering potassium iodide from the waste liquid for manufacturing a polarizing plate becomes the raw material of the aqueous potassium iodide solution used in one or more steps selected from the boric acid aqueous solution preliminary contact step, dyeing step, crosslinking step, and stretching step of the polarizer constituting the polarizing plate.

[0018] Another method for manufacturing a polarizing plate is a method for manufacturing a polarizing plate including a treatment method for recovering potassium iodide from a waste liquid for manufacturing a polarizing plate, The treatment method for recovering potassium iodide from the waste liquid for manufacturing a polarizing plate is the above-described treatment method for the waste liquid for manufacturing a polarizing plate, It may be characterized in that the fourth filtrate having a pH of 4.5 to pH 7.0 obtained by the treatment method, or the fourth filtrate whose pH is adjusted to 4.5 to pH 7.0, becomes part or all of the raw material of the aqueous potassium iodide solution used when manufacturing the polarizer constituting the polarizing plate.

Brief Description of the Drawings

[0019] [Figure 1] It is a schematic configuration diagram showing an example of a treatment method and a treatment system for waste liquid. [Modes for carrying out the invention]

[0020] (Embodiment 1) Embodiment 1 of the present invention will be described below.

[0021] (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 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 polarizing plate manufacturing 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 polarizing plate manufacturing waste liquid may also contain divalent and trivalent anions. Hereafter, the polarizing plate manufacturing waste liquid may be referred to as waste liquid.

[0022] (Processing method and processing 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 BT.

[0023] (S0) The waste liquid sent from the polarizing plate manufacturing equipment is stored in the raw water tank BT.

[0024] (S0-1) Measure whether the wastewater from the raw water tank BT has a pH of less than 7. If it does not have a pH of less than 7, adjust the pH so that it becomes less than 7 (pH adjustment step). In pH adjustment, the waste liquid may be adjusted to a pH of less than 7 using an acid (e.g., sulfuric acid, hydrochloric acid, nitric acid, etc.). Stirring may also be used during pH adjustment.

[0025] (S0-2) Add an iodine reducing agent to the wastewater from the raw water tank BT and reduce the iodine to its reduction point (iodine reduction step). The iodine reducing agent may be one or more selected from sodium thiosulfate, potassium thiosulfate, and ascorbic acid. When using ascorbic acid, step S0-1 is mandatory.

[0026] (S0-3) The wastewater after the iodine reduction process is pH adjusted to become alkaline (alkalinity adjustment process). In the alkali adjustment process, 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 and pH 11 or lower). This prevents the generation of iodine (I2) through re-oxidation.

[0027] (S1-1) The waste liquid obtained through the above process is introduced into the first concentration device and concentrated by evaporation (first concentration process). As the first concentration device, any suitable configuration can be adopted, as long as the waste liquid can be concentrated by evaporation. Specific examples include evaporative concentration devices such as heat pump type, ejector-driven type, steam type, and flash type. When the wastewater is concentrated in the first concentration device, boron-containing compounds (typically boric acid (H3BO3)) contained in the wastewater precipitate.

[0028] (S1-2) The evaporated and concentrated waste liquid is subjected to cooling and crystallization in a first cooling and crystallization apparatus (first cooling and crystallization step). Examples of the first cooling and crystallization apparatus include jacket type and vacuum type cooling and crystallization apparatuses. For cooling and crystallization, it is preferable to cool the waste liquid to a temperature of preferably 45°C or lower, more preferably 40°C or lower, even more preferably 30°C or lower, and particularly preferably to around room temperature (for example, 25°C). By performing cooling and crystallization, boric acid is further crystallized, so the concentration of boric acid 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 and does not precipitate.

[0029] (S1-3) The wastewater after evaporation concentration and cooling crystallization is introduced into a first solid-liquid separation device to separate the first precipitate from the wastewater into solid and liquid to produce a first filtrate containing at least potassium iodide and boric acid (first solid-liquid separation step). In the first filtrate, approximately 60% to 90% of boric acid and approximately 50% to 80% of PVA are removed, compared to the wastewater taken 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 a filtration device (e.g., a pressure filtration (filter press) device, a vacuum filtration device, a centrifugal filtration device, a centrifuge (e.g., a decanter-type centrifuge)). The first precipitate that is separated is a crystalline substance mainly composed of boric acid and is disposed of as waste.

[0030] (S2-1) The first filtrate is introduced into the second concentration device and concentrated by evaporation (second concentration step). The concentration of boric acid and PVA in the first filtrate is sufficiently reduced by the first concentration step, and since KI is dissolved in the first filtrate at a high concentration, further concentration in the second concentration device causes KI to become supersaturated. As a result, a second precipitate mainly containing KI can be produced. The second precipitate is a crystal mainly composed of KI, with some boric acid and PVA. As with the first concentration device, any appropriate configuration can be used for the second concentration device.

[0031] (S2-2) The first filtrate (slurry) 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). The second solid-liquid separator can be any suitable configuration, 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 approximately 30% to 55% by weight, the boric acid concentration is approximately 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, and a portion of it may be discarded.

[0032] [S3: Recovery Process] (S3-1) The second precipitate is redissolved in a solvent (water or warm water) to produce a redissolved solution (redissolved solution production step). The redissolved solution is 30% to 60% by mass of the second precipitate and 70% to 40% by mass of the solvent, with 100% by mass of the redissolved solution. The solvent and the second precipitate may be placed in a dissolution tank and stirred with a stirring device.

[0033] (S3-2) Adjust the pH of the redissolved solution to be pH 7 or lower, preferably pH 6 or lower, more preferably pH 6 or lower, and pH 4 or higher (redissolved solution pH adjustment step). If the pH of the redissolved solution is 7 or lower, pH adjustment is not necessary. In the redissolved solution pH adjustment step, the pH of the redissolved solution may be adjusted with an acid (e.g., sulfuric acid, hydrochloric acid, nitric acid, etc.) so that the pH of the redissolved solution is 7, preferably 6 or lower. In pH adjustment, the solution may be stirred in the tank using a stirring means. By lowering the pH to 7 or lower, more precipitates can be generated. At pH 4 or lower, iodine may volatilize, potentially degrading the process piping.

[0034] (S3-3) The redissolved solution with a pH of 7 or lower is cooled and crystallized at 30°C or lower in a third cooling crystallizer to produce a fourth precipitate (cooling crystallization step). The third cooling crystallizer may have the same configuration as the first cooling crystallizer. For cooling crystallization, it is preferable to cool the redissolved solution to 30°C or lower, preferably to room temperature (for example, around 25°C). By performing cooling crystallization, impurities such as boric acid and PVA are crystallized, and the concentration of impurities such as boric acid in the redissolved solution is reduced.

[0035] (S3-4) The redissolved liquid is introduced into a third solid-liquid separator to separate the fourth precipitate from the redissolved liquid (fourth solid-liquid separation step). The third solid-liquid separator can be any suitable configuration, similar to the first solid-liquid separator. The fourth precipitate, which contains boric acid and PVA, is disposed of as waste.

[0036] (S3-5) If the fourth filtrate is not at pH 4.5 to pH 7.0, the pH of the fourth filtrate is adjusted to pH 4.5 to pH 7.0 (pH adjustment step). In the pH adjustment step, the pH of the fourth filtrate may be adjusted with an acid (e.g., sulfuric acid, hydrochloric acid, nitric acid, etc.) to pH 4.5 to pH 7.0. Stirring may be performed using a stirring means during pH adjustment.

[0037] (S3-6) The fourth filtrate contains a high concentration of dissolved KI and is reused as recycled KI in the polarizing plate manufacturing apparatus. If the fourth filtrate has a pH of 4.5 to 7.0, it can be reused as recycled KI in the polarizing plate manufacturing apparatus after adjusting the concentration as needed. Alternatively, KI may be precipitated as a solid by evaporation crystallization.

[0038] (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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] Examples of surface treatment layers include hard coating, anti-reflective coating, anti-sticking coating, anti-glare coating, and anti-fouling coating.

[0043] (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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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).

[0048] 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).

[0049] 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]

[0050] KI was recovered according to the procedure shown in Figure 1. Table 1 shows the results for Examples 1 to 6 and Comparative Examples 1 to 4. Ascorbic acid was used as the iodine reducing agent.

[0051] The overall evaluation criteria were as follows: "◎" if all three items were met, "〇" if two were met, "△" if one was met, "△" if the polarizer quality was poor, and "×" if none of the three items were met. However, even if the values ​​for the other items were met, a "×" was given if the purity of the recycled KI was less than 96% or the recycled KI recovery rate was less than 90%. Recycled KI purity of 97% or higher Recycled KI recovery rate of over 90% The polarizing plate quality is good (no problems).

[0052] The recovery rates of KI in Examples 1 to 3, 5, and 6 were high at 98%, while that of Example 4 was 95%. The purity of KI in Examples 1 to 4 was high at 99%, while that of Examples 5 and 6 was 97%. Examples 1, 4, 5, and 6 had good polarizing plate quality, while that of Examples 2 and 3 worsened due to humidification. The overall evaluation was "◎" for Example 1, "〇" for Examples 4, 5, and 6, and "△" for Examples 2 and 3. In Comparative Example 1, the concentration of the second precipitate in the redissolved solution was high at 70%, and the recovery rate of KI was low at 88%. In Comparative Example 2, the cooling temperature of the redissolved solution was high at 40°C, and the purity of KI was low at 95%. In Comparative Example 3, endothermic cooling was performed during dissolution, so the purity of KI was unstable at 95-99%. All of Comparative Examples 1 to 3 received an overall "X" rating.

[0053] [Table 1] [Explanation of Symbols]

[0054] 100 Polarizing plate manufacturing equipment

Claims

1. A method for treating waste liquid from polarizing plate manufacturing, which recovers potassium iodide from the waste liquid from polarizing plate manufacturing, A first concentration step involves 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 involves separating the first precipitate from the waste liquid used in the production of the polarizing plate to produce a first filtrate containing at least potassium iodide and boric acid, A second concentration step involves 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 is performed to produce a second filtrate obtained by solid-liquid separation of the second precipitate from the first filtrate, The process includes a recovery step for recovering potassium iodide from the separated second precipitate, The aforementioned recovery process is, A redissolving solution generation step is performed by dissolving the second precipitate in a solvent so that it is 30% to 60% by mass, and A redissolving solution pH adjustment step is performed to adjust the pH of the redissolving solution so that the pH of the redissolving solution is 7 or less. A cooling crystallization step is performed in which the redissolved solution with a pH of 7 or less is cooled and crystallized at 30°C or below to produce a fourth precipitate, A fourth solid-liquid separation step involves separating the fourth precipitate from the redissolving solution to produce a fourth filtrate containing at least potassium iodide, including, A method for treating waste liquid from polarizing plate manufacturing.

2. The processing method according to claim 1, wherein in the fourth solid-liquid separation step, impurities crystallized in the redissolved solution are removed by filtration.

3. The fourth filtrate has a pH of 4.5 to 7.0, or The processing method according to claim 1, further comprising a pH adjustment step of adjusting the pH of the fourth filtrate so that the pH is between 4.5 and 7.

0.

4. A method for manufacturing polarizing plates, which includes a treatment method for recovering potassium iodide from waste liquid used in the manufacture of polarizing plates, The treatment method for recovering potassium iodide from the waste liquid used in the production of polarizing plates is the treatment method described in any one of claims 1 to 3. The fourth filtrate obtained by the above processing method, having a pH of 4.5 to 7.0, or the fourth filtrate whose pH has been adjusted to 4.5 to 7.0, is characterized in that it becomes part or all of the raw materials for the potassium iodide aqueous solution used when manufacturing polarizers that constitute a polarizing plate. A method for manufacturing polarizing plates.

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