Processing method of waste liquid and method for manufacturing polarizing film

Abstract

To remove a polyvinyl alcohol-based resin in a waste liquid that contains the polyvinyl alcohol-based resin.SOLUTION: In a processing method of a waste liquid, a waste liquid containing a polyvinyl alcohol-based resin and a process liquid containing boric acid are mixed together to cross-link the polyvinyl alcohol-based resin to precipitate and separate the polyvinyl alcohol-based resin.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for treating waste liquid for reducing the concentration of a polyvinyl alcohol-based resin in the waste liquid containing the polyvinyl alcohol-based resin, and the like.

Background Art

[0002] Polyvinyl alcohol-based resins are used as raw materials for adhesives and polarizing films. For example, Patent Document 1 discloses that an aqueous solution of a polyvinyl alcohol-based resin is applied to one side of a thermoplastic resin film, dried to form a polyvinyl alcohol-based resin layer, and then dyed with a dyeing solution containing iodine and potassium iodide and stretched to produce a polarizing film. In the production process of the polarizing film, a liquid (waste liquid) containing a polyvinyl alcohol-based resin is generated. Waste liquid containing resin components such as polyvinyl alcohol-based resins cannot be discharged directly into the natural environment and requires disposal in accordance with laws and regulations. From the perspective of environmental protection, it is desirable to remove the polyvinyl alcohol-based resin from the waste liquid as much as possible to reduce the volume of the waste liquid. However, waste liquid containing a polyvinyl alcohol-based resin, particularly waste liquid containing a relatively high concentration of the polyvinyl alcohol-based resin, has high viscosity, and it is difficult to remove the polyvinyl alcohol-based resin from the waste liquid.

[0003] For example, Patent Document 2 discloses adding a salt to water containing a water-soluble organic substance to separate the water-soluble organic substance and concentrated brine.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

[0005] The method of Patent Document 2 is a method of precipitating an organic substance dissolved in water by utilizing so-called salting out, and can remove low-molecular-weight organic substances such as isopropyl alcohol from water. However, according to the research of the present inventors, it has been found that the polyvinyl alcohol-based resin in the aqueous solution cannot be sufficiently removed by salting out. In addition, the polyvinyl alcohol-based resin cannot be decomposed using microorganisms. Furthermore, even if the waste liquid containing the polyvinyl alcohol-based resin is heated to evaporate the water and take out the polyvinyl alcohol-based resin, the polyvinyl alcohol-based resin adheres to the treatment apparatus at the initial stage of heating, making it difficult to continue heating. In particular, in the case of a waste liquid with a relatively high concentration of the polyvinyl alcohol-based resin, due to the above reasons, the polyvinyl alcohol-based resin cannot be removed by heat treatment.

Problems to be Solved by the Invention

[0006] An object of the present invention is to provide a treatment method and the like that can separate the polyvinyl alcohol-based resin from a waste liquid containing the polyvinyl alcohol-based resin and reduce the concentration of the polyvinyl alcohol-based resin in the waste liquid.

Means for Solving the Problems

[0007] The treatment method of the waste liquid according to the first aspect is to mix the waste liquid containing the polyvinyl alcohol-based resin and the treatment liquid containing boric acid to crosslink the polyvinyl alcohol-based resin and precipitate and separate the polyvinyl alcohol-based resin.

[0008] The treatment method of the waste liquid according to the second aspect is, in the treatment method of the first aspect, to put the waste liquid containing the polyvinyl alcohol-based resin into the treatment liquid containing boric acid. The treatment method of the waste liquid according to the third aspect is, in the treatment method of the first or second aspect, that the concentration of boric acid in the treatment liquid is 1.0% by weight or more. The method for treating waste liquid according to the fourth embodiment is such that, in any of the treatment methods of the first to third embodiments, the concentration of the polyvinyl alcohol-based resin in the waste liquid is 0.5% by weight or more.

[0009] In another aspect, a method for manufacturing a polarizing film is provided. The method for manufacturing a polarizing film includes a step of manufacturing a polarizing film by immersing a film containing a polyvinyl alcohol-based resin in a liquid, a step of taking out the liquid as waste liquid, and a step of precipitating and separating the polyvinyl alcohol-based resin from the waste liquid by any of the treatment methods of the first to fourth embodiments.

Advantages of the Invention

[0010] According to the method of the present invention, the polyvinyl alcohol-based resin can be easily removed from the waste liquid, and a waste liquid with a very low concentration of the polyvinyl alcohol-based resin can be obtained.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0012] [Waste Liquid] The waste liquid to be treated contains a polyvinyl alcohol-based resin. Hereinafter, the polyvinyl alcohol-based resin may be referred to as "PVA-based resin" in some cases. The waste liquid is, for example, a liquid generated in the process of manufacturing various products, and is not particularly limited as long as it contains a PVA-based resin. In one embodiment, the waste liquid is, for example, a waste liquid generated in the process of manufacturing a PVA-based resin adhesive or a product manufactured using the adhesive; a waste liquid generated in the process of manufacturing an emulsion-based adhesive or an adhesive in which a PVA-based resin is used as an additive or a product manufactured using the same; a waste liquid generated in the process of manufacturing a polarizing film containing a PVA-based resin described in Patent Document 1 (Japanese Patent Application Laid-Open No. 2015-36729) and the like. These waste liquids can be treated individually or in combination of two or more. For example, the method for manufacturing the polarizing film includes a step of manufacturing a polarizing film by immersing a film containing a polyvinyl alcohol-based resin in a liquid, and a step of taking out the liquid as waste liquid. By performing the treatment method described later on the waste liquid obtained in the process of manufacturing this polarizing film, the polyvinyl alcohol-based resin can be precipitated and separated from the waste liquid. In the method for manufacturing the polarizing film, the step of making the film a polarizing film by immersing the film containing a polyvinyl alcohol-based resin in a liquid is called a so-called wet treatment step. Examples of the liquid for immersing the film containing a polyvinyl alcohol-based resin include a swelling liquid, a dyeing liquid, a crosslinking liquid, a stretching liquid, and a cleaning liquid. In the method for manufacturing a polarizing film, at least one liquid selected from the swelling liquid, the dyeing liquid, the crosslinking liquid, the stretching liquid, and the cleaning liquid is taken out as waste liquid.

[0013] Examples of the PVA-based resin include polyvinyl alcohol and its derivatives. Examples of the polyvinyl alcohol include those obtained by saponifying a polyvinyl acetate-based resin. Examples of the derivatives of the polyvinyl alcohol include modified polyvinyl alcohol in which one or more functional groups such as a carboxy group, a hydrocarbon group, an acetoacetyl group, an acrylic group, and a urethane group are introduced into the side chain; modified polyvinyl alcohol in which an alkyl group or the like is introduced at the terminal; cation-modified polyvinyl alcohol in which a cationic monomer is introduced into the molecule. These may be used alone or in combination of two or more. The average degree of polymerization of the PVA-based resin is, for example, about 100 to 10,000, preferably about 1,000 to 10,000. Further, the saponification degree of the PVA-based resin is preferably, for example, about 80 to 100 mol%, more preferably about 95 mol% to 99.95 mol%. The average degree of polymerization and the saponification degree can be determined according to JIS K 6726 (1994).

[0014] The concentration of the PVA-based resin in the waste liquid is not particularly limited, but it is effective to apply the present invention to the treatment of waste liquid with a relatively high concentration. From this perspective, the concentration of the PVA-based resin in the waste liquid is, for example, 0.5% by weight or more, preferably 1% by weight or more, more preferably 3% by weight or more when the total waste liquid is 100% by weight. According to the present invention, it is possible to treat waste liquid with a relatively high PVA-based resin concentration of, for example, 2% by weight or more. There is no particular upper limit to the concentration of the PVA-based resin in the waste liquid, but if the concentration is too high, the viscosity of the waste liquid will become too high and it will lose its fluidity, making it difficult to handle the waste liquid (for example, transferring the waste liquid, etc.). From this perspective, the concentration of the PVA-based resin in the waste liquid is, for example, 15% by weight or less, preferably 10% by weight or less. For waste liquid with a PVA-based resin concentration exceeding 15% by weight, it may be diluted with water to reduce it to an appropriate concentration (for example, 5% by weight, etc.), and then the boric acid-containing treatment liquid may be mixed.

[0015] The waste liquid contains water and a PVA-based resin dissolved in the water. Since the present invention aims to remove the PVA-based resin in the waste liquid, the waste liquid may or may not contain substances other than the PVA-based resin. Examples of substances other than the PVA-based resin include iodine; iodides; resins other than the PVA-based resin; alcohols; organic acids; etc. Examples of the iodides include potassium iodide, sodium iodide, lithium iodide, etc. Iodides such as potassium iodide are often contained in waste liquid generated in the manufacturing process of polarizing films containing PVA-based resins. When a substance other than the PVA-based resin is contained in the waste liquid, the concentration of the substance in the waste liquid is not particularly limited and may be less than, equal to, or greater than the concentration of the PVA-based resin. The concentration of the substance other than the PVA-based resin in the waste liquid is, for example, more than 0 and 5% by weight or less, preferably more than 0 and 3% by weight or less, when the total waste liquid is 100% by weight.

[0016] [Treatment liquid] The treatment liquid contains boric acid. As the treatment liquid, for example, an aqueous solution containing water and boric acid dissolved in water can be used. The concentration of boric acid in the treatment liquid is, for example, 0.5% by weight or more, preferably 1.0% by weight or more, and more preferably 1.5% by weight or more, when the total treatment liquid is 100% by weight. By using the treatment liquid with the boric acid concentration, the removal rate of the PVA-based resin becomes high. The upper limit of the concentration of boric acid in the treatment liquid is the saturation amount of boric acid in water and depends on the solubility of boric acid in water. For example, at 20 °C and atmospheric pressure (hereinafter referred to as the standard state), the solubility of boric acid in water is about 4.7% by weight (this solubility is based on the case where the solubility of boric acid in 100 g of water is 4.9 g at 20 °C). Since the removal rate of the PVA-based resin tends to be higher as the boric acid concentration is higher, it is preferable to use a saturated solution of boric acid as the treatment liquid. Note that since the solubility of boric acid in water is proportional to the temperature, it is also possible to use a treatment liquid with a boric acid concentration higher than the concentration under the standard state by heating when preparing the treatment liquid.

[0017] [Treatment method] The method for treating the waste liquid of the present invention is to mix the waste liquid containing the PVA-based resin and the treatment liquid containing boric acid to crosslink and precipitate the PVA-based resin, and then separate it. After solid-liquid separation, the PVA-based resin is recovered. By precipitating and separating the PVA-based resin, the PVA-based resin dissolved in the waste liquid can be removed from the waste liquid, and a waste liquid with a reduced PVA-based resin concentration can be obtained.

[0018] FIG. 1 shows an example of a treatment facility for removing PVA-based resin from waste liquid. The treatment facility 1 includes a sedimentation tank 2, a waste liquid supply path 31 for supplying waste liquid to the sedimentation tank 2, a treatment liquid supply path 32 for supplying treatment liquid to the sedimentation tank 2, a stirrer 4 provided in the sedimentation tank 2, a discharge path 33 for extracting the PVA-based resin deposited in the sedimentation tank 2, and a water intake path 34 for taking out the treated waste liquid (hereinafter referred to as "treated waste liquid") from which the PVA-based resin in the sedimentation tank 2 has been removed. A solid-liquid separation device 5 such as a screw press dehydrator or a belt filter is provided in the path of the discharge path 33. The sedimentation tank 2 is a tank for mixing waste liquid and treatment liquid. As will be described later, the PVA-based resin supramolecule A accumulates in the lower layer of the sedimentation tank 2. Usually, pipes are used as the respective paths such as the waste liquid supply path 31, the treatment liquid supply path 32, the discharge path 33, and the water intake path 34. The treated waste liquid in the sedimentation tank 2 from the water intake path 34 is transferred from the water intake path 34 to the next treatment facility (the next treatment facility is not shown). The solid-liquid separation device 5 separates the PVA-based resin supramolecule A and moisture (liquid) into solid and liquid. The separated liquid merges into the water intake path 34 through the water supply path 36 and is transferred to the next treatment facility. Note that the liquid obtained by solid-liquid separation may be put back into the sedimentation tank 2 again. In FIG. 1, the flow of each liquid is indicated by a thin arrow, and the flow of the dehydrated PVA-based resin supramolecule A is indicated by a white arrow.

[0019] Next, the case where the treatment method of the present invention is implemented using the treatment facility will be described. Through the waste liquid supply path 31 and the treatment liquid supply path 32, waste liquid and treatment liquid are put into the precipitation tank 2, and the stirrer 4 is rotated. At the beginning of the treatment, the treatment liquid may be put into the precipitation tank 2 first and then the waste liquid, or the waste liquid may be put in first and then the treatment liquid, or the waste liquid and the treatment liquid may be put in simultaneously. Since the PVA-based resin supramolecule A can be efficiently produced and it is difficult for the PVA-based resin to adhere to the inside of the precipitation tank 2, it is preferable to put the treatment liquid into the precipitation tank 2 which is a container first and then put the waste liquid there. Also, since the waste liquid containing the PVA-based resin has a high viscosity, it takes time to store it to a certain extent in the precipitation tank 2 which is a container, while the treatment liquid can be stored in the precipitation tank 2 relatively quickly. Therefore, it is not only convenient to put the treatment liquid into the precipitation tank 2 which is a container first, but also the treatment can be completed in a short time. For example, at the beginning of the treatment, a predetermined amount of the treatment liquid containing boric acid at a predetermined concentration is put into the precipitation tank 2. Then, a predetermined amount of the waste liquid containing the PVA-based resin at a predetermined concentration is supplied to the precipitation tank 2. When the waste liquid is supplied, the boric acid concentration of the treatment liquid previously put into the precipitation tank 2 decreases, so a predetermined amount of the treatment liquid containing boric acid is replenished to the precipitation tank 2. The replenishment of the treatment liquid may be carried out simultaneously and in parallel with the supply of the waste liquid, or may be carried out after a predetermined amount of the waste liquid has been supplied into the precipitation tank 2.

[0020] The boric acid concentration of the treatment liquid and the PVA-based resin concentration of the waste liquid are as described in the columns of [waste liquid] and [treatment liquid] above. The amounts of the treatment liquid and the waste liquid respectively can vary depending on the boric acid concentration and the PVA-based resin concentration. As a reference, for 100 parts by weight of the PVA-based resin (solid content), 3 to 7 parts by weight, preferably 4 to 6 parts by weight of boric acid (solid content) are required. Considering this reference, the boric acid concentration and the PVA-based resin concentration, the amounts of the treatment liquid and the waste liquid are set.

[0021] In the precipitation tank 2, when the treatment liquid and the waste liquid are mixed, the PVA-based resin in the waste liquid reacts with the tetrahydroxyborate ion (B(OH)4 in the treatment liquid -) It is crosslinked by and a plurality of PVA-based resins form supramolecules. The crosslinked PVA-based resin has a significantly reduced solubility in water and precipitates in a gel form. This crosslinked and supramolecularized PVA-based resin (referred to as "PVA-based resin supramolecule" in this specification) accumulates in the lower layer of the precipitation tank 2, so the PVA-based resin can be easily removed from the waste liquid by withdrawing it from the discharge path 33. The PVA-based resin supramolecule A withdrawn from the discharge path 33 is dehydrated by the solid-liquid separation device 5 and then discarded. In addition, according to the research of the present inventors, it has been confirmed that when borax is used, the PVA-based resin and water cannot be separated.

[0022] According to the treatment method of the present invention, it is also possible to remove, for example, 98% or more of the PVA-based resin in the waste liquid by weight ratio. The treated waste liquid after the PVA-based resin is removed is transferred to the next treatment facility through the water intake path 34. The treated waste liquid has a sufficiently reduced PVA-based resin concentration. When the PVA-based resin concentration is below the legal standard, it can be industrially reused or discharged as it is. When the PVA-based resin concentration does not meet the legal standard, a trace amount of PVA-based resin can be removed from the treated waste liquid using a membrane separation method such as reverse osmosis, ultrafiltration, or dialysis, and the obtained water can be industrially reused or discharged. Furthermore, when the waste liquid contains iodides such as potassium iodide, it is also possible to industrially reuse the treated waste liquid as an aqueous solution containing potassium iodide by removing a trace amount of PVA-based resin from the treated waste liquid. The aqueous solution containing potassium iodide can be used, for example, in the production of the above-mentioned polarizing film.

[0023] Next, an example of applying the present invention to actual waste liquid treatment is shown. For example, using a treatment liquid with a boric acid concentration of 4.7% by weight (saturated solution at 20 °C), a waste liquid with a PVA-based resin concentration of 7% by weight is treated at a rate of 3 m per day. 3 In this case, about 20 m of the treatment liquid is added to the precipitation tank. 3Place it. While stirring the treatment liquid with a stirrer, supply the waste liquid to the precipitation tank, and to compensate for the decrease in boric acid, replenish the treatment liquid. When treating 3 m of waste liquid per day, replenish about 1 m of treatment liquid per day. By this treatment, about 900 kg / day of PVA-based resin supramolecules are generated, and about 3.1 m of treated waste liquid is obtained. 3 When treating, replenish about 1 m of treatment liquid per day. 3 of the treatment liquid. By this treatment, about 900 kg / day of PVA-based resin supramolecules are generated, and about 3.1 m 3 of treated waste liquid can be obtained.

Examples

[0024] Hereinafter, examples and comparative examples will be described to explain the present invention in more detail. However, the present invention is not limited to the following examples.

[0025] [Materials Used] (1) 7% PVA waste liquid For the 7% PVA waste liquid, an aqueous solution in which polyvinyl alcohol was dissolved in water was used. The concentration of polyvinyl alcohol in the aqueous solution (waste liquid) was 7% by weight, the viscosity of the aqueous solution was 228 mPa·s, and its pH was 5.0 to 7.0. The saponification degree of the polyvinyl alcohol was 99.0 to 99.5 mol%, and its degree of polymerization was 4000 to 4500.

[0026] (2) 1% PVA waste liquid For the 1% PVA waste liquid, an aqueous solution in which polyvinyl alcohol was dissolved in water was used. The concentration of polyvinyl alcohol in the aqueous solution (waste liquid) was 1% by weight, the viscosity of the aqueous solution was 36 mPa·s, and its pH was 5.0 to 7.0. The saponification degree of the polyvinyl alcohol was 99.0 to 99.5 mol%, and its degree of polymerization was 4000 to 4500.

[0027] (3) 4% modified PVA waste liquid As the 4% modified PVA waste liquid, an aqueous solution in which modified polyvinyl alcohol was dissolved in water was used. The concentration of the modified polyvinyl alcohol in the aqueous solution (waste liquid) was 4% by weight, the viscosity of the aqueous solution was 13.5 mPa·s, and its pH was 4.0 to 6.0. The modified polyvinyl alcohol was polyvinyl alcohol with an acetoacetyl group introduced into the side chain, and its saponification degree was 97.5 to 98.5 mol%, and its degree of polymerization was 1000 to 1500.

[0028] (4) 1% Modified PVA Waste Liquid As the 1% modified PVA waste liquid, an aqueous solution in which modified polyvinyl alcohol was dissolved in water was used. The concentration of the modified polyvinyl alcohol in the aqueous solution (waste liquid) was 1% by weight, the viscosity of the aqueous solution was 3 mPa·s, and its pH was 4.0 to 6.0. The modified polyvinyl alcohol was polyvinyl alcohol with an acetoacetyl group introduced into the side chain, and its saponification degree was 97.5 to 98.5 mol%, and its degree of polymerization was 1000 to 1500.

[0029] However, the viscosity of each waste liquid was measured at 23°C using a B-type viscometer (trade name "TVC-10 viscometer" manufactured by Toki Sangyo Co., Ltd.).

[0030] [Example 1] By dissolving boric acid (reagent) in pure water, an aqueous boric acid solution (treatment liquid) with a boric acid concentration of 0.5% by weight was prepared. Under standard conditions (23°C, 1 atm, 50% RH), 450 milliliters of this aqueous boric acid solution was placed in a beaker. Then, 20 milliliters of 7% PVA waste liquid was injected using a syringe (trade name "Terumo Syringe 50ml" manufactured by Terumo Corporation), and then stirred for about 3 minutes. After that, it was left standing for about 12 hours under standard conditions, and then naturally filtered using No. 5C filter paper to obtain a filtrate. The removal rate of polyvinyl alcohol was calculated from the COD of the filtrate. The results are shown in Table 1. Note that the removal rate is rounded to the first decimal place (the same applies hereinafter).

[0031] Boric acid aqueous solutions (treatment liquids) with boric acid concentrations as shown in Table 1 were each prepared. Except for using these boric acid aqueous solutions with respective concentrations, in the same manner as above, 7% PVA waste liquid was treated, and the removal rate of polyvinyl alcohol was calculated. The results are shown in Table 1.

[0032] [Example 2] Except for using 1% PVA waste liquid instead of 7% PVA waste liquid, in the same manner as Example 1, using the boric acid aqueous solution (treatment liquid) with the boric acid concentration shown in Table 1, 1% PVA waste liquid was treated, and the removal rate of polyvinyl alcohol was calculated. The results are shown in Table 1.

[0033] [Example 3] Except for using 4% modified PVA waste liquid instead of 7% PVA waste liquid, in the same manner as Example 1, using the boric acid aqueous solution (treatment liquid) with the boric acid concentration shown in Table 1, 4% modified PVA waste liquid was treated, and the removal rate of modified polyvinyl alcohol was calculated. The results are shown in Table 1.

[0034] [Example 4] Except for using 1% modified PVA waste liquid instead of 7% PVA waste liquid, in the same manner as Example 1, using the boric acid aqueous solution (treatment liquid) with the boric acid concentration shown in Table 1, 1% modified PVA waste liquid was treated, and the removal rate of modified polyvinyl alcohol was calculated. The results are shown in Table 1.

[0035]

Table 1

[0036] Figure 2 is a graph of the results of Examples 1 to 4. From the results of Examples 1 to 4, it is clear that the higher the boric acid concentration, the higher the removal rate. From the results of Example 1, it can be seen that in the case of 7% PVA waste liquid, by using an aqueous boric acid solution of 1.5 wt% or more, polyvinyl alcohol can be precipitated and separated with a high removal rate. From the results of Example 3, it can be seen that in the case of 4% modified PVA waste liquid, by using an aqueous boric acid solution of 2.0 wt% or more, modified polyvinyl alcohol can be precipitated and separated with a high removal rate. From these results, it is presumed that for waste liquids with a PVA-based resin concentration of 4 wt% or more, and further 5 wt% or more, by using an aqueous boric acid solution of 1.5 wt% or more, preferably 1.7 wt% or more, the PVA-based resin concentration in the waste liquid can be significantly reduced. From the results of Examples 1 to 4, it can be seen that for waste liquids with a higher concentration of PVA-based resin, the PVA-based resin can be precipitated using a relatively low-concentration aqueous boric acid solution.

[0037] Figure 3 is a graph plotting the lower limit values of the boric acid concentration and the PVA-based resin concentration when the removal rate is 70% or more among Examples 1 to 4. The reason for using 70% of the removal rate as a reference is that if the removal rate is 70% or more, it can be sufficiently applied to actual waste liquid treatment. In Figure 3, the horizontal axis represents the PVA-based resin concentration (wt%) of the waste liquid, and the vertical axis represents the boric acid concentration (wt%) of the treatment liquid. (1) in Figure 3 shows the data point where the removal rate in Example 1 was 70% or more (that is, treating 7 wt% PVA waste liquid with a boric acid concentration of 1.5 wt%), (2) shows the data point where the removal rate in Example 2 was 70% or more (that is, treating 1 wt% PVA waste liquid with a boric acid concentration of 3.5 wt%), (3) shows the data point where the removal rate in Example 3 was 70% or more (that is, treating 4 wt% modified PVA waste liquid with a boric acid concentration of 2.0 wt%), and (4) shows the data point where the removal rate in Example 4 was 70% or more (that is, treating 1 wt% modified PVA waste liquid with a boric acid concentration of 4.0 wt%). Since the higher the boric acid concentration, the higher the PVA removal rate as described above, when the boric acid concentration is higher than these data points, the PVA-based resin can be removed with a higher removal rate.

[0038] In one aspect, the approximate line of each data point can be used as an index for determining the boric acid concentration and the PVA-based resin concentration in the waste liquid. Specifically, the straight line (A) indicated by the thick dashed line in FIG. 3 is the approximate line of each of the above data points, and is represented by y = -0.394x + 4.03. However, y represents the boric acid concentration, and x represents the PVA-based resin concentration (the same applies hereinafter). In the region above this straight line (A), it is considered that the removal rate of the PVA-based resin exceeds 70%. That is, when the boric acid concentration in the treatment liquid and the PVA-based resin concentration in the waste liquid satisfy the relationship represented by the formula: y > -0.394x + 4.03, treatment can be performed at a removal rate of 70% or more. From another aspect, the lowest removal rate in the above (1) to (4) is 74%, and it is estimated that the PVA-based resin can be removed at a removal rate of 70% or more at data points higher than this. Specifically, the straight line (B) indicated by the thin dashed line in FIG. 3 is a straight line passing through (2) and (3) that is below the approximate line (straight line (A)), and is represented by y = -0.5x + 4. It is estimated that the removal rate of the PVA-based resin exceeds 70% in the region above this straight line (B). That is, when the boric acid concentration in the treatment liquid and the PVA-based resin concentration in the waste liquid satisfy the relationship represented by the formula: y > -0.5x + 4, it is estimated that treatment can be performed at a removal rate of 70% or more.

[0039] Also, when polyvinyl alcohol and modified polyvinyl alcohol are considered separately, in one aspect, the straight line derived by connecting the data point of (1) and the data point of (2) can be used as an index for determining the boric acid concentration and the polyvinyl alcohol concentration in the waste liquid. In another aspect, the straight line derived by connecting the data point of (3) and the data point of (4) can be used as an index for determining the boric acid concentration and the modified polyvinyl alcohol concentration in the waste liquid. Specifically, the straight line (C) indicated by the dashed line in FIG. 3 is the straight line passing through the data points of (1) and (2) above, and is represented by y = -0.333x + 3.833. In the region above this straight line (C), it is considered that the removal rate of polyvinyl alcohol exceeds 70%. That is, when the boric acid concentration of the treatment liquid and the concentration of polyvinyl alcohol in the waste liquid satisfy the relationship represented by the formula: y > -0.333x + 3.833, the polyvinyl alcohol waste liquid can be treated at a removal rate of 70% or more. The straight line (D) indicated by the long dashed line in FIG. 3 is the straight line passing through the data points of (3) and (4) above, and is represented by y = -0.667x + 4.667. In the region above this straight line (D), it is considered that the removal rate of modified polyvinyl alcohol exceeds 70%. That is, when the boric acid concentration of the treatment liquid and the concentration of modified polyvinyl alcohol in the waste liquid satisfy the relationship represented by the formula: y > -0.667x + 4.667, the modified polyvinyl alcohol waste liquid can be treated at a removal rate of 70% or more. For any of the formulas, the boric acid concentration is preferably, for example, 0.5% by weight or more, more preferably 1% by weight or more, and even more preferably 1.5% by weight or more.

[0040] [Comparative Example 1] By dissolving potassium sulfate (reagent) in pure water, an aqueous potassium sulfate solution with a potassium sulfate concentration of 5% by weight was prepared. Except for using the aqueous potassium sulfate solution instead of the boric acid aqueous solution, a 1% PVA waste liquid was treated in the same manner as in Example 2, and the removal rate of polyvinyl alcohol was calculated. As a result, the removal rate was 0%. An aqueous potassium sulfate solution with a potassium sulfate concentration of 10% by weight was prepared, and a 1% PVA waste liquid was treated in the same manner, and the removal rate of polyvinyl alcohol was calculated. As a result, when potassium sulfate was used, the removal rate was 0% at any concentration.

Explanation of Symbols

[0041] 1 Treatment equipment 2 Settling tank (container) 31 Waste liquid supply line 32 Treatment liquid supply line 33 Discharge line 4 Stirrer 5 Solid-liquid separation device

Claims

1. A method for treating waste liquid, comprising mixing a waste liquid containing a polyvinyl alcohol-based resin and a treatment liquid containing boric acid to crosslink the polyvinyl alcohol-based resin and precipitate and separate the polyvinyl alcohol-based resin.

2. The method for treating waste liquid according to claim 1, wherein the waste liquid containing the polyvinyl alcohol-based resin is put into the treatment liquid containing boric acid.

3. The method for treating waste liquid according to claim 1 or 2, wherein the concentration of boric acid in the treatment liquid is 1.0% by weight or more.

4. The method for treating waste liquid according to claim 1 or 2, wherein the concentration of the polyvinyl alcohol-based resin in the waste liquid is 0.5% by weight or more.

5. A step of manufacturing a polarizing film by immersing a film containing a polyvinyl alcohol-based resin in a liquid, A step of taking out the liquid as a waste liquid and precipitating and separating the polyvinyl alcohol-based resin from the waste liquid by the treatment method according to claim 1. A method for manufacturing a polarizing film.

Images