Method and treatment device for wastewater from polarizing plate manufacturing

Abstract

To provide a processing method of a polarizer production waste liquid capable of efficiently recovering boric acid while suppressing generation of wastes on recovery of potassium iodide from a polarizer production waste liquid.SOLUTION: A processing method of a polarizer production waste liquid for recovery of potassium iodide and boric acid from a polarizer production waste liquid includes a KI recovery step of recovering a first filtrate containing potassium iodide by evaporative concentration of the polarizer production waste liquid, followed by solid liquid separation of a first precipitate having been formed; and a boric acid recovery step of recovering boric acid from the first precipitate. The boric acid recovery step includes a cooling crystallization step of precipitation of boric acid crystal by acid addition to a target liquid where the first precipitate is dissolved for pH control, followed by cooling; a crystal separation step of separating boric acid crystal having been precipitated; and an electrodialysis step of recovering an acid and an alkali by alkali addition to a target liquid from which boric acid crystal has been separated, followed by neutralization and electrodialysis.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

[0002] The wastewater generated during the polarizing plate manufacturing process contains inorganic components such as iodine, boron, and potassium, as well as organic components such as polyvinyl alcohol (PVA), and methods for treating such wastewater from polarizing plate manufacturing have been studied for some time.

[0003] For example, Patent Document 1 discloses a method for recovering potassium iodide by evaporating and concentrating waste liquid from the production of polarizing plates to produce a first precipitate containing boric acid and polyvinyl alcohol, performing solid-liquid separation of the first precipitate to produce a first filtrate, evaporating and concentrating the first filtrate to produce a second precipitate containing potassium iodide, and performing solid-liquid separation of the second precipitate. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2018-89602 Summary of the Invention [Problem to be solved by the invention]

[0005] The method for treating waste liquid from the manufacture of polarizing plates disclosed in Patent Document 1 can easily and efficiently recover potassium iodide from waste liquid from the manufacture of polarizing plates, and also recovers boric acid-based crystals that are generated in the process of recovering potassium iodide without discarding them, thereby reducing the environmental burden.

[0006] However, in the past, it has been difficult to recover high-purity boric acid from boric acid-based crystals without generating waste products. For example, in a method of recovering boric acid by dissolving boric acid-based crystals in heated water and then performing cooling crystallization, high-purity boric acid can be obtained by adding an acid such as sulfuric acid before cooling crystallization, but there is a problem in that salt wastewater containing potassium sulfate and the like as a main component is generated as waste.

[0007] Therefore, an object of the present invention is to provide a method and an apparatus for treating waste liquid produced in the manufacture of polarizing plates, which can efficiently recover boric acid while suppressing the generation of waste when recovering potassium iodide from the waste liquid produced in the manufacture of polarizing plates. [Means for solving the problem]

[0008] The above-mentioned object of the present invention is achieved by a method for treating waste liquid from the manufacture of polarizing plates, which recovers potassium iodide and boric acid from the waste liquid from the manufacture of polarizing plates. The method comprises: a KI recovery step of performing solid-liquid separation on a first precipitate containing boric acid and polyvinyl alcohol, produced by evaporating and concentrating the waste liquid from the manufacture of polarizing plates, to recover a first filtrate containing potassium iodide; and a boric acid recovery step of recovering boric acid from the first precipitate. The boric acid recovery step comprises a cooling and crystallization step of adding an acid to a liquid to be treated, in which the first precipitate has been dissolved, to adjust the pH, and then cooling the liquid to precipitate boric acid crystals; a crystal separation step of separating the precipitated boric acid crystals; and an electrodialysis step of adding an alkali to the liquid to be treated, from which the boric acid crystals have been separated, to neutralize it, and then performing electrodialysis to recover the acid and alkali.

[0009] In this method for treating waste liquid from the production of polarizing plates, it is preferable to adjust the pH of the liquid to be treated in the cooling and crystallization step using the acid recovered in the electrodialysis step.

[0010] It is also preferable that the alkali recovered in the electrodialysis step is used to neutralize the liquid to be treated in the electrodialysis step.

[0011] The electrodialysis step preferably includes a divalent cation removal step for removing divalent cations contained in the neutralized liquid to be treated.

[0012] The electrodialysis step preferably includes a PVA removal step for removing polyvinyl alcohol contained in the neutralized liquid to be treated.

[0013] It is preferable that the liquid to be treated that has been desalted in the electrodialysis step be merged with the waste liquid from the manufacture of polarizing plates before being evaporated and concentrated in the first precipitate recovery step.

[0014] The above-mentioned object of the present invention is also achieved by a treatment device for polarizing plate manufacturing waste liquid that recovers potassium iodide and boric acid from polarizing plate manufacturing waste liquid, the treatment device comprising: a KI recovery device that performs solid-liquid separation on a first precipitate containing boric acid and polyvinyl alcohol produced by evaporating and concentrating the polarizing plate manufacturing waste liquid, and recovers a first filtrate containing potassium iodide; and a boric acid recovery device that recovers boric acid from the first precipitate, the boric acid recovery device comprising: a cooling crystallization device that adds an acid to a treatment liquid in which the first precipitate has been dissolved to adjust the pH, and then cools the liquid to precipitate boric acid crystals; a crystal separation device that separates the precipitated boric acid crystals; and an electrodialysis device that adds an alkali to the treatment liquid from which the boric acid crystals have been separated to neutralize it, and then performs electrodialysis to recover the acid and alkali. [Effects of the Invention]

[0015] According to the present invention, it is possible to provide a method and an apparatus for treating waste liquid produced during the manufacture of polarizing plates, which can efficiently recover boric acid while suppressing the generation of waste when recovering potassium iodide from the waste liquid produced during the manufacture of polarizing plates. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a block diagram showing a schematic configuration of a treatment device for waste liquid from the production of polarizing plates according to one embodiment of the present invention. [Figure 2]2 is a block diagram showing the configuration of a main part of the treatment device for waste liquid from the production of polarizing plates shown in FIG. 1. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0017] An embodiment of the present invention will now be described with reference to the accompanying drawings. Fig. 1 is a block diagram of a treatment device for waste liquid from the manufacture of polarizing plates according to one embodiment of the present invention. As shown in Fig. 1, the treatment device 1 for waste liquid from the manufacture of polarizing plates includes a KI recovery device 2 and a boric acid recovery device 3. A method for treating waste liquid from the manufacture of polarizing plates using this treatment device 1 for waste liquid from polarizing plate manufacture is as follows.

[0018] First, a KI recovery step is performed in which the polarizing plate manufacturing waste liquid stored in a raw liquid pit is introduced into the KI recovery device 2, where it is evaporated and concentrated to produce a first precipitate, which is then subjected to solid-liquid separation to recover a first filtrate containing potassium iodide. The KI recovery device 2 includes a first precipitate recovery device 4 that recovers the first precipitate separated from the first filtrate, and a second precipitate recovery device 5 that recovers a second precipitate produced by further evaporating and concentrating the first filtrate, which is subjected to solid-liquid separation to recover the second precipitate.

[0019] Polarizing plate manufacturing waste liquid is waste liquid generated during the manufacturing process of polarizing plates used in liquid crystal displays and the like. In the polarizing plate manufacturing process, a film made of polyvinyl alcohol (PVA) is typically immersed in a potassium iodide (KI) solution, then stretched in a boric acid (H3BO3) aqueous solution, washed with water, and dried to produce a polarizing plate. Therefore, polarizing plate manufacturing waste liquid contains PVA, as well as KI and H3BO3, mainly in ionic form. The pH of polarizing plate manufacturing waste liquid is in the range of 3.5 to 8.0, and is usually acidic due to the inclusion of boric acid solution, but it can also be near-neutral. To prevent corrosion of manufacturing equipment, a neutralizing agent such as potassium hydroxide may be added to the polarizing plate manufacturing waste liquid.

[0020] The first precipitate recovery device 4, into which the waste liquid from polarizing plate production is introduced, is equipped with an evaporative concentration device capable of concentrating the waste liquid from polarizing plate production by evaporation. The evaporative concentration device can be configured using one or more of known devices, such as a heat pump type, an ejector-driven type, a steam type, or a flash type.

[0021] When the waste liquid from the manufacture of polarizing plates is concentrated by the evaporation concentrator, much of the H3BO3 and PVA contained in the waste liquid becomes sludge, and a first precipitate containing these substances is generated in the waste liquid from the manufacture of polarizing plates. The first precipitate may also contain impurities other than H3BO3 and PVA.

[0022] The first precipitate recovery device 4 further includes a cooling crystallizer that cools and crystallizes the polarizing plate manufacturing waste liquid containing the first precipitate. Examples of cooling crystallizers include known types such as jacket-type and vacuum-type. If the HBO concentration in the polarizing plate manufacturing waste liquid can be sufficiently reduced by the evaporation and concentration device, the system may not include a cooling crystallizer.

[0023] The first precipitate recovery device 4 further includes a solid-liquid separation device that separates the first precipitate from the polarizing plate manufacturing waste liquid, which has been evaporated, concentrated, and then appropriately cooled and crystallized, from the first filtrate. Examples of solid-liquid separation devices include various types of filtration devices such as pressure filtration (filter press), vacuum filtration, and centrifugal filtration, as well as decanter-type centrifugal separators. When the polarizing plate manufacturing waste liquid passes through the solid-liquid separation device, the first precipitate is separated from the first filtrate and recovered. By separating the first precipitate, approximately 60 to 90% of H3BO3 has been removed, and approximately 40 to 60% of PVA has been removed from the first filtrate. The separated first precipitate is a crystal composed mainly of H3BO3 and contains PVA.

[0024] The first filtrate produced by the first precipitate recovery apparatus 4 is introduced into the second precipitate recovery apparatus 5. The second precipitate recovery apparatus 5 includes an evaporative concentration apparatus that evaporates and concentrates the first filtrate to produce a second precipitate containing KI crystals, and a solid-liquid separation apparatus that separates the produced second precipitate from the second filtrate. The evaporative concentration apparatus and solid-liquid separation apparatus included in the second precipitate recovery apparatus 5 can be configured similarly to the evaporative concentration apparatus and solid-liquid separation apparatus included in the first precipitate recovery apparatus 4. Because the concentrations of H3BO3 and PVA contained in the first filtrate have been sufficiently reduced, crystals mainly composed of KI can be recovered as the second precipitate.

[0025] Since the first filtrate produced by the first precipitate recovery device 4 contains a high concentration of dissolved KI, if the first filtrate can be reused as is in the polarizing plate manufacturing process, the configuration does not need to include the second precipitate recovery device 5. In other words, the KI recovery step can be performed using only the first precipitate recovery device 4.

[0026] In the method for treating waste liquid from the production of polarizing plates according to this embodiment, after the first precipitate recovery step is performed, a boric acid recovery step is performed in which boric acid is recovered from the first precipitate by a boric acid recovery device 3.

[0027] Fig. 2 is a block diagram showing a schematic configuration of the boric acid recovery apparatus 3. As shown in Fig. 2, the boric acid recovery apparatus 3 includes a crystal dissolving tank 11, a filter 12, a cooling crystallization apparatus 13, a crystal separation apparatus 14, a boric acid recovery container 15, a tank for treated liquid 16, a concentrator 17, a PVA removal apparatus 18, a divalent cation removal apparatus 19, and a bipolar membrane electrodialysis apparatus 20. The boric acid recovery process performed by the boric acid recovery apparatus 3 includes a cooling crystallization step S1, a crystal separation step S2, and an electrodialysis step S3.

[0028] In the cooling and crystallization step S1, an acid is added to the liquid to be treated in which the first precipitate has been dissolved to adjust the pH, and then the liquid is cooled to precipitate boric acid crystals. First, the first precipitate is introduced into a crystal dissolving tank 11 and dissolved in water to produce a liquid to be treated. Steam is preferably introduced into the crystal dissolving tank 11 together with pure water to dissolve the first precipitate in warm water, and the boric acid concentration in the liquid to be treated is preferably near the saturated concentration.

[0029] The liquid to be treated produced in the crystal dissolving tank 11 is passed through a filter 12 to remove insoluble matter, and then a pH adjuster is added to adjust the pH to the acidic side (for example, about pH 4), before being introduced into a cooling crystallizer 13. Sulfuric acid is preferably used as the pH adjuster, but other acids may be used as long as they do not cause any problems in the process.

[0030] The cooling crystallizer 13 can be a known device similar to the cooling crystallizer provided in the KI recovery unit 2, and highly pure boric acid crystals can be precipitated by cooling the pH-adjusted liquid through heat exchange with cooling water. The cooling temperature of the liquid to be treated in the cooling crystallizer 13 is preferably 20°C or lower, more preferably 10°C or lower. There is no particular lower limit to the cooling temperature, but it can be set to, for example, -10°C or higher, and in practice it is 0°C or higher.

[0031] In the crystal separation step S2, the boric acid crystals precipitated in the cooling crystallization step S1 are separated from the liquid to be treated by a crystal separation device 14. The crystal separation device 14 preferably separates the crystals while maintaining the temperature of the liquid to be treated at the above-mentioned cooling temperature. In this embodiment, a centrifuge is used as the crystal separation device 14, but other devices capable of separating solid components from a liquid, such as filtration or sedimentation, may also be used. The separated boric acid crystals are collected in a boric acid collection container 15, such as a flexible container. The collected boric acid is appropriately dried using a dryer or the like.

[0032] The liquid to be treated from which boric acid crystals have been separated in the crystal separation device 14 is stored in the liquid to be treated tank 16. Since the liquid to be treated supplied to the liquid to be treated tank 16 contains a small amount of unprecipitated boric acid, the recovery rate of boric acid can be increased by concentrating the liquid from the liquid to be treated tank 16 in the concentrator 17 and then cooling and crystallizing it again in the cooling crystallizer 13. The concentrator 17 can be configured, for example, similar to the evaporative concentration device provided in the first precipitate recovery device 4. Repeating the concentration and cooling crystallization of the liquid to be treated from which crystals have been separated by batch processing gradually reduces the recovery rate and purity of boric acid. Therefore, the number of repetitions is preferably, for example, about 3 to 4 times. Purified water may be appropriately replenished to the liquid to be treated as the liquid to be treated is concentrated. Furthermore, the boric acid recovered in the boric acid recovery container 15 may be supplied to the liquid to be treated tank 16 depending on its purity, where it is dissolved in the liquid to be treated and recrystallized.

[0033] In the electrodialysis step S3, acid and alkali are recovered from the liquid to be treated after the boric acid crystals have been separated in the crystal separation step S2. The liquid to be treated in the tank 16 for treatment is neutralized by adding a neutralizing agent, and then passes through a PVA removal device 18 and a divalent cation removal device 19 before being supplied to a bipolar membrane electrodialysis device 20. Potassium hydroxide (KOH) is preferably used as the neutralizing agent, but if the recovered alkali is to be used outside the system, other alkaline aqueous solutions such as sodium hydroxide (NaOH) may also be used.

[0034] The PVA removal device 18 is a device that selectively removes PVA contained in the liquid to be treated, and can suitably use, for example, granular or honeycomb-shaped activated carbon that adsorbs and removes PVA by contacting the liquid to be treated. The material of the activated carbon is not particularly limited, and for example, natural products such as wood or burned synthetic resins can be used.

[0035] The divalent cation removal device 19 is a device that selectively removes divalent cations, such as magnesium ions and calcium ions, contained in the liquid to be treated. For example, a known device such as a resin tower filled with a chelating resin can be used. By passing the liquid to be treated through the divalent cation removal device 19, it is possible to suppress a decrease in membrane performance due to the precipitation of magnesium ions, calcium ions, etc. in the bipolar membrane electrodialysis device 20, thereby maintaining good acid and alkali recovery efficiency. Furthermore, by providing a PVA removal device 18 upstream of the divalent cation removal device 19, as in this embodiment, it is possible to maintain good performance of the chelating resin, etc., in the divalent cation removal device 19.

[0036] The bipolar membrane electrodialysis device 20 has a known three-chamber configuration, for example, in which an anion exchange membrane and a cation exchange membrane are disposed between two bipolar membranes, with an acid chamber, a salt chamber, and an alkaline chamber formed between them. By supplying the liquid to be treated to the salt chamber and pure water to the acid chamber and the alkaline chamber, acid (sulfuric acid in this embodiment) is discharged from the acid chamber and alkali (KOH in this embodiment) is discharged from the alkaline chamber, and the acid and alkali are recovered in the acid tank and the alkaline tank, respectively.

[0037] The liquid to be treated that has passed through the salt chamber of the bipolar membrane electrodialysis device 20 and been desalinated is returned to the raw liquid pit shown in Fig. 1 and merges with the waste liquid from the manufacture of polarizing plates, whereby the small amount of KI contained therein is recovered by the KI recovery device 2. A portion of the liquid to be treated that has been discharged from the tank 16 for the liquid to be treated and neutralized may be returned to the raw liquid pit. Impurities that accumulate when the liquid to be treated is returned to the raw liquid pit from the tank 16 for the liquid to be treated can be discarded by diverting a portion of the liquid to be treated that is returned to the raw liquid pit.

[0038] As described above, the method for treating waste liquid from polarizing plate manufacturing according to the present embodiment is configured such that, in the boric acid recovery step, alkali is added to the liquid to be treated from which boric acid crystals have been separated to neutralize it, and then the acid and alkali are recovered by electrodialysis. This makes it possible to suppress the generation of new waste materials associated with the recovery of boric acid, thereby reducing the environmental burden.

[0039] The acid and alkali recovered by electrodialysis can be used outside the system, but are preferably used inside the polarizing plate manufacturing waste liquid treatment device 1. That is, the acid recovered by electrodialysis can be used as a pH adjuster for the liquid to be treated in the cooling and crystallization step S1, and the alkali recovered by electrodialysis can be used as a neutralizer for the liquid to be treated in the electrodialysis step S3. In this way, by effectively utilizing the products from the polarizing plate manufacturing waste liquid without discharging them to the outside, it is possible to achieve zero liquid discharge (ZLD) in the waste liquid treatment and reuse of waste materials. [Explanation of symbols]

[0040] 1. Polarizing plate manufacturing wastewater treatment equipment 2 KI recovery device 3 Boric acid recovery device 13 Cooling crystallizer 14 Crystal separation device 18 PVA removal equipment 19 Divalent cation removal device 20 Bipolar membrane electrodialysis device

Claims

1. A method for treating waste liquid produced in the manufacture of polarizing plates, which recovers potassium iodide and boric acid from the waste liquid produced in the manufacture of polarizing plates, comprising: a potassium iodide recovery step of subjecting a first precipitate containing boric acid and polyvinyl alcohol, which is produced by evaporating and concentrating the waste liquid from the production of polarizing plates, to solid-liquid separation to recover a first filtrate containing potassium iodide; and a boric acid recovery step of recovering boric acid from the first precipitate separated by the solid-liquid separation, The boric acid recovery step includes: a cooling and crystallization step of adding an acid to the liquid to be treated in which the first precipitate separated by the solid-liquid separation has been dissolved, adjusting the pH of the liquid, and then cooling the liquid to precipitate boric acid crystals; a crystal separation step of separating the precipitated boric acid crystals; and an electrodialysis step of adding an alkali to the liquid to be treated from which the boric acid crystals have been separated to neutralize the liquid, and then performing electrodialysis to recover the acid and alkali.

2. 2. The method for treating waste liquid produced in the manufacture of polarizing plates according to claim 1, wherein the acid recovered in the electrodialysis step is used to adjust the pH of the liquid to be treated in the cooling and crystallization step.

3. 3. The method for treating waste liquid from the production of polarizing plates according to claim 1, wherein the alkali recovered in the electrodialysis step is used to neutralize the liquid to be treated in the electrodialysis step.

4. 4. The method for treating waste liquid from the production of polarizing plates according to claim 1, wherein the electrodialysis step includes a divalent cation removal step for removing divalent cations contained in the neutralized liquid to be treated.

5. 5. The method for treating waste liquid produced in the manufacture of polarizing plates according to claim 1, wherein the electrodialysis step includes a polyvinyl alcohol removal step of removing polyvinyl alcohol contained in the neutralized liquid to be treated.

6. 6. The method for treating waste liquid from the manufacture of polarizing plates according to claim 1, wherein the liquid to be treated that has been desalted in the electrodialysis step is merged with the waste liquid from the manufacture of polarizing plates before being evaporated and concentrated in the first precipitate recovery step.

7. A treatment device for polarizing plate manufacturing waste liquid, which recovers potassium iodide and boric acid from polarizing plate manufacturing waste liquid, a potassium iodide recovery device that performs solid-liquid separation on a first precipitate containing boric acid and polyvinyl alcohol, which is produced by evaporating and concentrating a waste liquid from the production of polarizing plates, and recovers a first filtrate containing potassium iodide; a boric acid recovery device that recovers boric acid from the first precipitate separated by the solid-liquid separation, The boric acid recovery device includes: a cooling crystallization device that adds an acid to a liquid to be treated in which the first precipitate separated by the solid-liquid separation has been dissolved, adjusts the pH of the liquid, and then cools the liquid to precipitate boric acid crystals; a crystal separation device for separating the precipitated boric acid crystals; and an electrodialysis device that adds an alkali to the liquid to be treated from which the boric acid crystals have been separated to neutralize the liquid, and then performs electrodialysis to recover the acid and alkali.

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