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 first concentration step of evaporating and concentrating the wastewater from polarizing plate manufacturing to produce a first precipitate; a first solid-liquid separation step of solid-liquid separating the first precipitate from the wastewater 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 solid-liquid separating 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 of adding an iodine reducing agent to the wastewater from polarizing plate manufacturing before the first concentration step and reducing the iodine to its reduction point.

Description

Technical Field

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

Background Art

[0002] The waste liquid generated in the polarizing plate manufacturing process 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 demand for treating the waste liquid obtained in the manufacturing process, recovering potassium iodide, and recycling it to the manufacturing process.

[0003] Patent Documents 1 to 3 disclose a method for treating polarizing plate manufacturing waste liquid that recovers a potassium iodide solution from the polarizing plate manufacturing waste liquid. The treatment method of Patent Document 1 includes a first concentration step of evaporating and concentrating the polarizing plate manufacturing waste liquid to generate a first precipitate mainly containing a boron-containing compound and polyvinyl alcohol, a first solid-liquid separation step of generating a first filtrate obtained by subjecting the first precipitate to solid-liquid separation, a second concentration step of evaporating and concentrating the first filtrate to generate a second precipitate containing potassium iodide, a second solid-liquid separation step of generating a second filtrate obtained by subjecting the second precipitate to solid-liquid separation from the first filtrate, and a recovery step of recovering potassium iodide from the second precipitate. The recovery step includes a step of washing the potassium iodide crystals contained in the second precipitate using a saturated solution of potassium iodide.

[0004] In the treatment method of Patent Document 2, the precipitate produced by concentrating the polarizing plate manufacturing waste liquid and then performing crystallization is subjected to solid-liquid separation to produce a potassium iodide solution with reduced impurities including boron and polyvinyl alcohol, the polyvinyl alcohol remaining in the obtained potassium iodide solution is adsorbed onto a polyvinyl alcohol adsorbent, and the boron remaining in the potassium iodide solution from which polyvinyl alcohol has been adsorbed and removed is adsorbed onto a boron selective adsorption resin. In addition, Patent Document 2 discloses a step of filtering impurities that are precipitated as the saturated solubility decreases due to the endothermic reaction caused by 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 project] [Problems that the invention aims to solve]

[0007] If iodine is present in the evaporated concentrate or filtered liquid, problems can arise such as corrosion of pipes and other equipment. This also deteriorates the quality of the recovered KI water. Furthermore, if iodine remains, it can volatilize and contaminate the evaporated water, potentially leading to problems such as the inability to reuse the water within the factory.

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

[0009] As a result of diligent research to solve the above-mentioned problems, the inventors have found that the influence of iodine can be suppressed by pre-converting iodine into iodide ions using an iodine reducing agent and then performing an evaporation and concentration process. That is, the present invention includes the following embodiments.

[0010] 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, thereby generating a first filtrate containing at least potassium iodide and boric acid. A second concentration step (S2-1) is performed, in which the first filtrate is evaporated and concentrated 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 processing method is: Prior to the first concentration step, the process includes an iodine reduction step (S0-2) in which an iodine reducing agent is added to the waste liquid from the production of polarizing plates to reduce the iodine to its reduction point.

[0011] 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.

[0012] 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.

[0013] 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 become alkaline.

[0014] (effect) According to the above configuration, the volatilization of iodine can be suppressed by using an iodine reducing agent, thereby suppressing equipment corrosion and deterioration of the water quality of the recovered liquid. For example, by using ascorbic acid, the solubility is low, and the quality of recycled potassium iodide can be maintained. Furthermore, by making the waste liquid from polarizing plate manufacturing after the iodine reduction process alkaline, it is possible to prevent the generation of iodine (I2) through re-oxidation.

[0015] The present disclosure is a method for manufacturing a polarizing plate, comprising a treatment method for recovering potassium iodide from the 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 as follows: The above describes a method for treating waste liquid from the production of polarizing plates. The potassium iodide obtained by the above processing method may be 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. The method for manufacturing the polarizing plate is as follows: The potassium iodide obtained by the treatment method for recovering potassium iodide from the waste liquid of polarizing plate manufacturing may be characterized in that it is used as a raw material for an aqueous potassium iodide solution used in one or more processes selected from a boric acid aqueous solution pre-contact step, a dyeing step, a crosslinking step, and a stretching step that contain potassium iodide of the polarizer constituting the polarizing plate. [Brief explanation of the drawing]

[0016] [Figure 1] This is a schematic configuration diagram showing an example of a waste liquid treatment method. [Mode for Carrying Out the Invention]

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

[0018] (Waste Liquid) The polarizing plate production waste liquid in the present embodiment is, for example, a waste liquid generated in the polarizing plate production process. The polarizing plate production waste liquid may contain inorganic substances such as iodine, boron and potassium, organic substances such as polyvinyl alcohol, alcohol components such as glycerin, oil components such as machine oil, and water. The polarizing plate production waste liquid may contain oxides such as boric acid and potassium iodide, and iodides. The polarizing plate production waste liquid may contain alkali metals other than those described above. Examples of the alkali metal include one or more of sodium, lithium and the like. The alkali metal is present in the waste liquid as a cation, but may include those present as fine particles. The polarizing plate production waste liquid may contain monovalent anions. The monovalent anion may be one or more of anions of halogen atoms such as fluorine, chlorine and bromine. The polarizing plate production waste liquid may contain divalent anions and trivalent anions. Hereinafter, the polarizing plate production waste liquid may be referred to as the waste liquid.

[0019] (Treatment Method) Figure 1 shows an example of a waste liquid treatment method. The polarizing plate production 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, a laminating device for a polarizer and a polarizer protective film, and the like. The waste water discharged from this production apparatus is stored in the buffer tank BT.

[0020] (S0) The waste liquid sent from the polarizing plate production apparatus is stored in the raw water tank BT.

[0021] (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.

[0022] (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.

[0023] (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). This prevents the generation of iodine (I2) during re-oxidation.

[0024] (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.

[0025] (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.

[0026] (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 separated first precipitate is a crystalline substance mainly composed of boric acid, containing PVA and some KI crystals. The separated first precipitate also contains approximately 10% to 20% liquid containing KI. Therefore, in this embodiment, KI is further recovered from the first precipitate. Details of this will be described later.

[0027] (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.

[0028] (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.

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

[0030] (S3-2) The redissolved slurry is introduced into a third solid-liquid separator to separate the fourth precipitate from the redissolved slurry into a solid-liquid state (third solid-liquid separation step). As with the first solid-liquid separator, any suitable configuration can be adopted for the third solid-liquid separator. The fourth precipitate, which contains boric acid and PVA, 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 in the polarizing plate manufacturing apparatus (S3-3).

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

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

[0033] (S4-3) The first solution is introduced into the fourth solid-liquid separator to separate the 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 with the first solid-liquid separator, any suitable configuration can be used for the third solid-liquid separator. The third precipitate mainly consists of boric acid, and also contains impurities such as PVA and sulfates, which are then disposed of. The third filtrate mainly contains KI, with trace amounts of boric acid and impurities that did not precipitate.

[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 in the wastewater from the raw water tank BT is measured periodically. The boron concentration in the wastewater can also be calculated from the boron concentration in the third filtrate sent back, the amount returned, the amount of wastewater in the raw water tank, and the processing volume and number of processes in the first concentration step. Since high boron concentrations can cause problems in processing after the first concentration step, the number of repetitions is limited to a certain number. If the boron concentration exceeds 0.8%, preferably 0.7%, the repetition process is omitted until the boron concentration of the wastewater is 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 sent in after the boron concentration of the wastewater in the raw water tank has decreased.

[0035] (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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] Examples of surface treatment layers include hard coating, anti-reflective coating, anti-sticking coating, anti-glare coating, and anti-fouling coating.

[0040] (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.

[0041] 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.

[0042] 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.

[0043] 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.

[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. 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).

[0045] 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).

[0046] 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 release liner (release film) may be attached 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]

[0047] KI was recovered using two types of iodine reducing agents according to the procedure shown in Figure 1. Table 1 shows the results for Examples 1 to 3 and Comparative Example 1. Ascorbic acid in Example 1 yielded the best results. In Example 3, the pH of the wastewater at the time of ascorbic acid addition was 8, resulting in weak iodine-reducing power by ascorbic acid. In the case of thiosulfate 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 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 processing method is: Prior to the first concentration step, the process includes 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. A method for treating waste liquid from polarizing plate manufacturing.

2. The treatment method according to claim 1, wherein the iodine reducing agent is one or more selected from sodium thiosulfate, potassium thiosulfate, and ascorbic acid.

3. The iodine reducing agent is ascorbic acid. The pH of the waste liquid from the production of the polarizing plate prior to the iodine reduction step is less than 7, or Prior to the iodine reduction step, the process includes a pH adjustment step in which the pH of the polarizing plate manufacturing waste liquid is adjusted to less than 7. The processing method according to claim 1.

4. The treatment method according to claim 1, further comprising an alkali adjustment step of adjusting the pH of the waste liquid from the production of polarizing plates after the iodine reduction step to make it alkaline.

5. 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 4. The potassium iodide obtained by the above processing method 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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