Method for treating waste liquid and method for producing polarizing film

Abstract

The purpose of the present invention is to remove a polyvinyl alcohol-based resin from a waste liquid containing the polyvinyl alcohol-based resin. To this end, the waste liquid treatment method according to the present invention comprises mixing a waste liquid containing a polyvinyl alcohol-based resin with a treatment liquid containing boric acid, thereby cross-linking the polyvinyl alcohol-based resin, precipitating the polyvinyl alcohol-based resin, and separating the polyvinyl alcohol-based resin.

Description

Technical Field

[0001] The present invention relates to a method for treating waste liquid containing a polyvinyl alcohol type resin, wherein the concentration of the polyvinyl alcohol type resin in the waste liquid is reduced. Background Art

[0002] Polyvinyl alcohol resins have been used as adhesives, raw materials for polarizing films, etc. For example, Patent Document 1 discloses the following: a polyvinyl alcohol resin aqueous solution is applied to one side of a thermoplastic resin film, dried to form a polyvinyl alcohol resin layer, and then dyed with a dyeing solution containing iodine and potassium iodide and stretched, thereby manufacturing a polarizing film. In the manufacturing process of the above-mentioned polarizing film, a liquid (waste liquid) containing polyvinyl alcohol resin is generated. The waste liquid containing resin components such as polyvinyl alcohol resin cannot be directly discharged into the natural environment and must be disposed of in accordance with laws and regulations. In addition, from the perspective of environmental protection, it is desired to remove the polyvinyl alcohol resin from the above-mentioned waste liquid as much as possible to reduce the volume of the waste liquid.

[0003] However, waste liquid containing polyvinyl alcohol-based resin, especially waste liquid containing polyvinyl alcohol-based resin at a relatively high concentration, has high viscosity, and it is difficult to remove the polyvinyl alcohol-based resin from the waste liquid.

[0004] For example, Patent Document 2 discloses a technique for adding salt to water containing water-soluble organic matter to separate the water-soluble organic matter from concentrated brine.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Publication No. 2015-36729

[0008] Patent Document 2: Japanese Patent Application Publication No. 2020-62624 Summary of the invention

[0009] The method of Patent Document 2 is a method of precipitating organic matter dissolved in water by so-called salting out, and can remove low-molecular organic matter such as isopropyl alcohol from water.

[0010] However, the present inventors have found through studies that the polyvinyl alcohol-based resin in the aqueous solution cannot be sufficiently removed by salting out.

[0011] In addition, it is also impossible to decompose polyvinyl alcohol resin using microorganisms. Furthermore, even if the waste liquid containing polyvinyl alcohol resin is heated to evaporate the water and remove the polyvinyl alcohol resin, it is difficult to continue heating because the polyvinyl alcohol resin adheres to the treatment device in the early stage of heating. In particular, for waste liquid with a relatively high concentration of polyvinyl alcohol resin, the polyvinyl alcohol resin cannot be removed by heating treatment for the above reasons.

[0012] Problems to be solved by the invention

[0013] An object of the present invention is to provide a treatment method and the like capable of separating a polyvinyl alcohol-based resin from a waste liquid containing the polyvinyl alcohol-based resin and reducing the concentration of the polyvinyl alcohol-based resin in the waste liquid.

[0014] Solutions to the problem

[0015] A first aspect of the waste liquid treatment method includes mixing a waste liquid containing a polyvinyl alcohol-based resin with a treatment liquid containing boric acid to crosslink the polyvinyl alcohol-based resin, thereby precipitating and separating the polyvinyl alcohol-based resin.

[0016] A second aspect of the waste liquid treatment method includes: in the treatment method of the first aspect, adding the waste liquid containing the polyvinyl alcohol-based resin to the treatment liquid containing boric acid.

[0017] A third aspect of the waste liquid treatment method includes the following steps: in the treatment method of the first or second aspect, the concentration of boric acid in the treatment liquid is 1.0 wt % or more.

[0018] A fourth aspect of the waste liquid treatment method includes the following steps: in any one of the first to third aspects, wherein the concentration of the polyvinyl alcohol-based resin in the waste liquid is 0.5% by weight or more.

[0019] In another aspect, a method for manufacturing a polarizing film is provided.

[0020] The method for manufacturing a polarizing film comprises: a process of manufacturing the polarizing film by immersing a film containing a polyvinyl alcohol resin in a liquid; and a process of taking out the above-mentioned liquid as waste liquid and separating the polyvinyl alcohol resin from the above-mentioned waste liquid by any treatment method among the above-mentioned first to fourth methods.

[0021] Effects of the Invention

[0022] According to the method of the present invention, the polyvinyl alcohol-based resin can be easily removed from the waste liquid, thereby obtaining the waste liquid in which the concentration of the polyvinyl alcohol-based resin is reduced to a very low level. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1It is a reference schematic diagram of the processing equipment.

[0024] Figure 2 It is a graph showing the relationship between the boric acid concentration and the removal rate in Examples 1 to 4.

[0025] Figure 3 3 is a graph showing the relationship between the boric acid concentration and the PVA-based resin concentration at which the removal rate reaches 70% or more according to Examples 1 to 4 using a formula.

[0026] Explanation of symbols

[0027] 1. Processing equipment

[0028] 2 Sedimentation tank (container)

[0029] 31 Waste liquid supply line

[0030] 32 Treatment liquid supply line

[0031] 33 discharge route

[0032] 4. Agitator

[0033] 5Solid-liquid separation device DETAILED DESCRIPTION

[0034] [Waste liquid]

[0035] The waste liquid to be treated contains a polyvinyl alcohol-based resin, which may be referred to as a "PVA-based resin" hereinafter.

[0036] Waste liquid is, for example, liquid generated in the process of manufacturing various products, and is not particularly limited as long as it is a liquid containing PVA-type resin. In one embodiment, for example, the following waste liquids can be used as treatment objects: waste liquid generated in the manufacturing process of PVA-type resin adhesives or the manufacturing process of products using the above-mentioned adhesives; waste liquid generated in the manufacturing process of using PVA-type resins as additives in emulsion adhesives or adhesives, etc. or the manufacturing process of products using them; waste liquid generated in the manufacturing process of polarizing films containing PVA-type resins described in the above-mentioned patent document 1 (Japanese Patent Publication No. 2015-36729); etc. These waste liquids can be treated alone, or two or more kinds can be mixed and treated.

[0037] For example, the method for manufacturing the polarizing film comprises: a step of manufacturing the polarizing film by immersing a film containing a polyvinyl alcohol resin in a liquid; and a step of taking out the liquid as waste liquid. By subjecting the waste liquid obtained in the process of manufacturing the polarizing film to the treatment method described later, the polyvinyl alcohol resin can be precipitated from the waste liquid and separated. In the method for manufacturing the polarizing film, the step of making the film into a polarizing film by immersing the film containing a polyvinyl alcohol resin in a liquid is called a so-called wet treatment step. Examples of the liquid for immersing the film containing the polyvinyl alcohol resin include swelling liquid, dyeing liquid, cross-linking liquid, stretching liquid, cleaning liquid, and the like. In the method for manufacturing the polarizing film, at least one liquid selected from the swelling liquid, dyeing liquid, cross-linking liquid, stretching liquid, and cleaning liquid is taken out as waste liquid.

[0038] As PVA resins, polyvinyl alcohol and its derivatives can be cited. For example, the polyvinyl alcohol mentioned above can be polyvinyl alcohol obtained by saponifying polyvinyl acetate resin. As derivatives of the polyvinyl alcohol mentioned above, for example, there can be mentioned: modified polyvinyl alcohols obtained by introducing one or more functional groups such as carboxyl groups, hydrocarbon groups, acetoacetyl groups, acrylic groups, and carbamate groups into the side chains; modified polyvinyl alcohols obtained by introducing alkyl groups, etc. into the terminals; cation-modified polyvinyl alcohols obtained by introducing cationic monomers into the molecules; and the like. They can contain one kind alone, or two or more kinds can be contained.

[0039] The average polymerization degree of the PVA-based resin is, for example, about 100 to 10,000, preferably about 1,000 to 10,000. In addition, the saponification degree of the PVA-based resin is, for example, preferably about 80 to 100 mol %, more preferably about 95 mol % to 99.95 mol %. It should be noted that the average polymerization degree and the saponification degree can be obtained according to JIS K 6726 (1994).

[0040] The concentration of the PVA type resin in the waste liquid is not particularly limited, and it is effective to apply the present invention to the treatment of waste liquid with a relatively high concentration. From such a viewpoint, when the waste liquid as a whole is set to 100 weight %, the concentration of the PVA type resin in the waste liquid is, for example, 0.5 weight % or more, preferably 1 weight % or more, and more preferably 3 weight % or more. According to the present invention, waste liquid with a relatively high concentration of PVA type resin, such as 2 weight % or more, can also be treated.

[0041] The upper limit of the concentration of the PVA type resin in the waste liquid is not particularly limited, but if the concentration is too high, the viscosity of the waste liquid becomes too high, the fluidity is lost, and the treatment of the waste liquid (such as the transportation of the waste liquid, etc.) becomes difficult. From this point of view, the concentration of the PVA type resin in the waste liquid is, for example, 15% by weight or less, preferably 10% by weight or less.

[0042] It should be noted that waste liquid with a PVA-based resin concentration exceeding 15 wt % can be diluted with water and then mixed with the boric acid treatment liquid after reducing it to an appropriate concentration (eg, 5 wt %).

[0043] The waste liquid contains water and a PVA-type resin dissolved in the water. The purpose of the present invention is to remove the PVA-type resin from the waste liquid. Therefore, the waste liquid may contain substances other than the PVA-type resin, or may not contain such substances. As substances other than the PVA-type resin, there can be mentioned: iodine; iodides; resins other than the PVA-type resin; alcohols; organic acids; and the like. As the above-mentioned iodides, for example, there can be mentioned: potassium iodide, sodium iodide, lithium iodide, and the like. Iodides such as potassium iodide are often contained in waste liquid generated, for example, in the manufacturing process of a polarizing film containing a PVA-type resin.

[0044] When the waste liquid contains substances other than the PVA resin, the concentration of the substances in the waste liquid is not particularly limited, and may be lower than the concentration of the PVA resin, or may be equal to or higher than the concentration of the PVA resin. When the waste liquid as a whole is set to 100% by weight, the concentration of the substances other than the PVA resin in the waste liquid is, for example, greater than 0 and less than 5% by weight, preferably greater than 0 and less than 3% by weight.

[0045] [Processing liquid]

[0046] The treatment liquid contains boric acid. The treatment liquid may be, for example, an aqueous solution containing water and boric acid dissolved in water. When the entire treatment liquid is set to 100 wt %, the concentration of boric acid in the treatment liquid is, for example, 0.5 wt % or more, preferably 1.0 wt % or more, and more preferably 1.5 wt % or more. By using a treatment liquid having the above boric acid concentration, the removal rate of the PVA-based resin becomes higher.

[0047] The upper limit of the concentration of boric acid in the treatment liquid is the saturated amount of boric acid relative to water, which depends on the solubility of boric acid in water. For example, at 20°C and atmospheric pressure (hereinafter referred to as 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 100g of water at 20°C is 4.9g). Since there is a tendency that the removal rate of PVA-based resins increases with the higher boric acid concentration, a saturated solution of boric acid is preferably used as the treatment liquid.

[0048] It should be noted that the solubility of boric acid in water is proportional to the temperature. Therefore, by heating the treatment liquid when preparing it, it is also possible to use a treatment liquid having a higher boric acid concentration than the concentration under the above-mentioned standard state.

[0049] [Handling method]

[0050] The method for treating waste liquid of the present invention comprises: mixing waste liquid containing PVA resin with a treatment liquid containing boric acid, causing the PVA resin to be cross-linked and precipitated, thereby separating the PVA resin. After solid-liquid separation, the PVA resin is recovered. By separating the PVA resin by precipitating, the PVA resin dissolved in the waste liquid can be removed from the waste liquid, and the waste liquid with a reduced PVA resin concentration can be obtained.

[0051] Figure 1 An example of a treatment facility for removing PVA-based resin from waste liquid is shown.

[0052] The treatment equipment 1 comprises: 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 precipitated 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 is removed in the sedimentation tank 2. A solid-liquid separation device 5 such as a screw press dehydrator and a belt filter is provided on the discharge path 33.

[0053] The above-mentioned sedimentation tank 2 is a tank for mixing waste liquid with treated liquid. As described later, the PVA-type resin supramolecule A will gradually deposit in the lower layer of the sedimentation tank 2. As the above-mentioned waste liquid supply path 31, treated liquid supply path 32, discharge path 33 and water intake path 34, pipes are usually used. The treated waste liquid from the water intake path 34 to the sedimentation tank 2 is sent from the water intake path 34 to the subsequent treatment equipment (the subsequent treatment equipment is not shown). The solid-liquid separation device 5 performs solid-liquid separation on the PVA-type resin supramolecule A and water (liquid). The separated liquid passes through the water supply path 36 and then converges in the above-mentioned water intake path 34 and is sent to the subsequent treatment equipment. It should be noted that the liquid obtained by solid-liquid separation can also be allowed to enter the sedimentation tank 2 again. In Figure 1 In the figure, thin arrows represent the flow of each liquid, and hollow arrows represent the flow of the PVA-based resin supramolecule A after dehydration.

[0054] Next, a description will be given of a case where the processing method of the present invention is carried out using the above-mentioned processing equipment.

[0055] Waste liquid and treatment liquid are added to the sedimentation tank 2 through the waste liquid supply path 31 and the treatment liquid supply path 32, and the agitator 4 is rotated. At the beginning of the treatment, the treatment liquid can be added to the sedimentation tank 2 first and then the waste liquid is added, or the waste liquid can be added first and then the treatment liquid is added, or the waste liquid and the treatment liquid can be added simultaneously. Since the PVA-type resin supramolecular A can be generated efficiently and the PVA-type resin is not easy to adhere to the sedimentation tank 2, it is preferred to first add the treatment liquid to the sedimentation tank 2 as a container, and then add the waste liquid thereto. In addition, since the viscosity of the waste liquid containing the PVA-type resin is high, it takes time to store it in the sedimentation tank 2 as a container to a certain extent, and since the treatment liquid can be stored in the sedimentation tank 2 relatively quickly, therefore, the situation of first adding the treatment liquid to the sedimentation tank 2 as a container is not only more convenient, but also can end the treatment in a short time.

[0056] For example, at the beginning of the treatment, a given amount of a treatment liquid containing boric acid at a given concentration is added to the precipitation tank 2 in advance. Then, a given amount of a waste liquid containing a given concentration of a PVA-based resin is supplied to the precipitation tank 2. When the waste liquid is supplied, the boric acid concentration of the treatment liquid previously added to the precipitation tank 2 decreases, and therefore, a given amount of the treatment liquid containing boric acid at a given concentration is added to the precipitation tank 2. The above-mentioned replenishment of the treatment liquid may be performed simultaneously with the supply of the waste liquid, or may be performed after the supply of the given amount of the waste liquid to the precipitation tank 2 is completed.

[0057] The boric acid concentration of the treatment liquid and the PVA-based resin concentration of the waste liquid are as described in the above-mentioned [Waste liquid] and [Treatment liquid] columns.

[0058] The amount of each of the treatment liquid and the waste liquid can be changed according to the boric acid concentration and the PVA resin concentration. As a benchmark, the amount of the treatment liquid and the waste liquid is set in consideration of the benchmark, the boric acid concentration, and the PVA resin concentration, and the amount of the boric acid (solid content) is 3 to 7 parts by weight, preferably 4 to 6 parts by weight, relative to 100 parts by weight of the PVA resin (solid content).

[0059] In the precipitation tank 2, when the treatment liquid and the waste liquid are mixed, the PVA resin in the waste liquid is separated by the tetrahydroxyborate ions (B(OH) 4 - ) and cross-linked, and multiple PVA-type resins form supramolecules. The solubility of the cross-linked PVA-type resin in water is significantly reduced, and it becomes gel-like and precipitates. The PVA-type resin that has been cross-linked and supramoleculed (referred to as "PVA-type resin supramolecule" in this specification) is deposited in the lower layer of the precipitation tank 2. Therefore, by extracting it from the discharge path 33, the PVA-type resin can be easily removed from the waste liquid. The PVA-type resin supramolecule A extracted from the discharge path 33 is discarded after being dehydrated by the solid-liquid separation device 5.

[0060] It should be noted that the present inventors have confirmed through studies that, when borax is used, the PVA-based resin cannot be separated from water.

[0061] According to the treatment method of the present invention, it is also possible to remove PVA-based resins in the waste liquid in an amount of, for example, 98% or more by weight. The treated waste liquid from which the PVA-based resins have been removed is sent to a subsequent treatment facility through the water intake passage 34 .

[0062] The concentration of PVA-type resins in the treated waste liquid has been sufficiently reduced. When the concentration of PVA-type resins is below the legal standard, it can be reused industrially or discharged directly. When the concentration of PVA-type resins does not meet the legal standards, reverse osmosis, ultrafiltration, dialysis and other membrane separation methods can be used to remove trace amounts of PVA-type resins from the treated waste liquid, and the obtained water can be reused or discharged industrially. In addition, when the waste liquid contains iodides such as potassium iodide, by removing trace amounts of PVA-type resins from the treated waste liquid, the treated waste liquid can also be reused industrially as an aqueous solution containing potassium iodide. The above-mentioned aqueous solution containing potassium iodide can also be used, for example, in the manufacture of the above-mentioned polarizing film.

[0063] Next, an example of the case where the present invention is applied to actual waste liquid treatment will be described.

[0064] For example, using a treatment liquid with a boric acid concentration of 4.7 wt% (saturated solution at 20°C), 3 m3 of waste liquid with a PVA resin concentration of 7 wt% was treated per day. 3 In this case, add about 20 m 3 The above-mentioned treatment liquid is supplied to the precipitation tank while the treatment liquid is stirred by a stirrer, and the above-mentioned treatment liquid is replenished to compensate for the reduction of boric acid. On average, 3m 3 When treating wastewater, about 1m3 is added every day on average. 3 Through this treatment, about 900 kg / day of PVA-based resin supramolecular was generated, and about 3.1 m of treated wastewater was obtained. 3 .

[0065] Example

[0066] Hereinafter, the present invention will be described in further detail by way of examples and comparative examples, but the present invention is not limited to the following examples.

[0067] [Materials used]

[0068] (1) 7% PVA waste liquid

[0069] The 7% PVA waste liquid uses an aqueous solution in which polyvinyl alcohol is dissolved in water. The concentration of polyvinyl alcohol in the aqueous solution (waste liquid) is 7% by weight, the viscosity of the aqueous solution is 228 mPa·s, and the pH thereof is 5.0 to 7.0. The saponification degree of the polyvinyl alcohol is 99.0 to 99.5 mol %, and the degree of polymerization is 4000 to 4500.

[0070] (2) 1% PVA waste liquid

[0071] The 1% PVA waste liquid uses an aqueous solution in which polyvinyl alcohol is dissolved in water. The concentration of polyvinyl alcohol in the aqueous solution (waste liquid) is 1% by weight, the viscosity of the aqueous solution is 36 mPa·s, and the pH thereof is 5.0 to 7.0. The saponification degree of the polyvinyl alcohol is 99.0 to 99.5 mol %, and the degree of polymerization is 4000 to 4500.

[0072] (3) 4% modified PVA waste liquid

[0073] The 4% modified PVA waste liquid uses an aqueous solution in which modified polyvinyl alcohol is dissolved in water. The concentration of the modified polyvinyl alcohol in the aqueous solution (waste liquid) is 4% by weight, the viscosity of the aqueous solution is 13.5 mPa·s, and the pH thereof is 4.0 to 6.0. The modified polyvinyl alcohol is a polyvinyl alcohol having an acetoacetyl group introduced into the side chain, the saponification degree thereof is 97.5 to 98.5 mol %, and the degree of polymerization thereof is 1000 to 1500.

[0074] (4) 1% modified PVA waste liquid

[0075] The 1% modified PVA waste liquid uses an aqueous solution in which modified polyvinyl alcohol is dissolved in water. The concentration of the modified polyvinyl alcohol in the aqueous solution (waste liquid) is 1% by weight, the viscosity of the aqueous solution is 3mPa·s, and the pH thereof is 4.0 to 6.0. The modified polyvinyl alcohol is a polyvinyl alcohol having an acetoacetyl group introduced into the side chain, the saponification degree thereof is 97.5 to 98.5 mol %, and the polymerization degree thereof is 1000 to 1500.

[0076] The viscosity of each waste liquid was measured at 23° C. using a B-type viscometer (trade name “TVC-10 type viscometer” manufactured by Toki Sangyo Co., Ltd.).

[0077] [Example 1]

[0078] A boric acid aqueous solution (treatment solution) with a boric acid concentration of 0.5 wt% was prepared by dissolving boric acid (reagent) in pure water. 450 ml of the boric acid aqueous solution was added to a beaker under standard conditions (23°C, 1 atmosphere, 50% RH). After injecting 20 ml of 7% PVA waste liquid into it using a syringe (trade name "Terumo Syringe 50ml" manufactured by Terumo Co., Ltd.), it was stirred for about 3 minutes. Then, after standing for about 12 hours under standard conditions, 5 types of C filter paper were used for natural filtration to obtain a filtrate. The removal rate of polyvinyl alcohol was calculated based on the COD of the filtrate. The results are shown in Table 1. It should be noted that for the removal rate, one decimal place was rounded off (the same below).

[0079] Boric acid aqueous solutions (treatment solutions) with boric acid concentrations as shown in Table 1 were prepared. The 7% PVA waste liquid was treated in the same manner as above except that the boric acid aqueous solutions with these concentrations were used, and the removal rate of polyvinyl alcohol was calculated. These results are shown in Table 1.

[0080] [Example 2]

[0081] The 1% PVA waste liquid was treated in the same manner as in Example 1 using a boric acid aqueous solution (treatment liquid) having a boric acid concentration shown in Table 1 instead of the 7% PVA waste liquid, and the removal rate of polyvinyl alcohol was calculated. The results are shown in Table 1.

[0082] [Example 3]

[0083] The 4% modified PVA waste liquid was treated with a boric acid aqueous solution (treatment liquid) having a boric acid concentration shown in Table 1 in the same manner as in Example 1, except that the 4% modified PVA waste liquid was used instead of the 7% PVA waste liquid, and the removal rate of polyvinyl alcohol was calculated. These results are shown in Table 1.

[0084] [Example 4]

[0085] The 1% modified PVA waste liquid was treated with a boric acid aqueous solution (treatment liquid) having a boric acid concentration shown in Table 1 in the same manner as in Example 1, except that the 1% modified PVA waste liquid was used instead of the 7% PVA waste liquid, and the removal rate of polyvinyl alcohol was calculated. The results are shown in Table 1.

[0086]

[0087] Figure 2 It is a graph showing the results of Examples 1 to 4. It is clear from the results of Examples 1 to 4 that the removal rate increases as the boric acid concentration increases.

[0088] According to the results of Example 1, in the case of 7% PVA waste liquid, by using a boric acid aqueous solution of 1.5% by weight or more, polyvinyl alcohol can be precipitated and separated at a high removal rate. According to the results of Example 3, in the case of 4% modified PVA waste liquid, by using a boric acid aqueous solution of 2.0% by weight or more, modified polyvinyl alcohol can be precipitated and separated at a high removal rate. According to these results, it can be inferred that for waste liquids with a concentration of PVA resin of 4% by weight or more, and further 5% by weight or more, by using a boric acid aqueous solution of 1.5% by weight or more, preferably 1.7% by weight or more, the concentration of PVA resin in the waste liquid can be significantly reduced.

[0089] The results of Examples 1 to 4 show that, in waste liquids having a higher concentration of PVA-based resin, the PVA-based resin can be precipitated more easily by using an aqueous boric acid solution with a relatively low concentration.

[0090] Figure 3 This is a graph obtained by plotting the lower limit of the boric acid concentration and the PVA resin concentration when the removal rate reaches 70% or more in Examples 1 to 4. It should be noted that the removal rate of 70% is used as a reference because a removal rate of 70% or more can be fully applied to actual waste liquid treatment.

[0091] Figure 3 In the graph, the horizontal axis represents the PVA-based resin concentration (weight %) of the waste liquid, and the vertical axis represents the boric acid concentration (weight %) of the treatment liquid. Figure 3 (1) represents the data point in Example 1 where the removal rate reaches 70% or more (i.e., 7% by weight of PVA wastewater is treated with a boric acid concentration of 1.5% by weight), Figure 3 (2) represents the data point in Example 2 where the removal rate reaches 70% or more (i.e., 1% by weight of PVA wastewater is treated with a boric acid concentration of 3.5% by weight), Figure 3 (3) represents the data point in Example 3 where the removal rate reaches more than 70% (i.e., 4% by weight of the modified PVA waste liquid is treated with a boric acid concentration of 2.0% by weight), Figure 3 (4) represents the data points in Example 4 where the removal rate reaches 70% or more (i.e., 1% by weight of the modified PVA waste liquid is treated with a boric acid concentration of 4.0% by weight). As described above, the higher the boric acid concentration, the higher the PVA removal rate. Therefore, when the boric acid concentration is higher than these data points, the PVA-based resin can be removed at a higher removal rate.

[0092] 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 of the waste liquid.

[0093] Specifically, Figure 3The straight line (A) shown by the thick dotted line is an approximate line of the above data points, which is represented by y=-0.394x+4.03. Among them, y represents the boric acid concentration, and x represents the PVA-type resin concentration (the same below). It can be considered that in the area above the straight line (A), the removal rate of PVA-type resin exceeds 70%. In other words, when the boric acid concentration of the treatment liquid and the PVA-type resin concentration of the waste liquid satisfy the relationship represented by the formula: y>-0.394x+4.03, the treatment can be carried out with a removal rate of more than 70%.

[0094] From another point of view, the lowest removal rate among the above (1) to (4) is 74%, and it can be inferred that at a higher data point, the PVA-based resin can be removed at a removal rate of 70% or more.

[0095] Specifically, Figure 3 The straight line (B) indicated by the thin dotted line is a straight line passing through (2) and (3) and located further below the above approximate line (straight line (A)), and is represented by y=-0.5x+4. It can be inferred that the removal rate of the PVA-type resin exceeds 70% in the area located further above the straight line (B). In other words, it can be inferred that when the boric acid concentration of the treatment liquid and the PVA-type resin concentration of the waste liquid satisfy the relationship represented by the formula: y>-0.5x+4, the treatment can be performed with a removal rate of more than 70%.

[0096] In addition, when considering polyvinyl alcohol and modified polyvinyl alcohol separately, in one viewpoint, a straight line derived by connecting the data points of (1) and (2) can be used as an index for determining the concentration of boric acid and the concentration of polyvinyl alcohol in the waste liquid. In another viewpoint, a straight line derived by connecting the data points of (3) and (4) can be used as an index for determining the concentration of boric acid and the concentration of modified polyvinyl alcohol in the waste liquid.

[0097] Specifically, Figure 3 The straight line (C) indicated by the single-point dashed line is a straight line passing through the data points of (1) and (2) above, and is represented by y=-0.333x+3.833. It can be considered that the removal rate of polyvinyl alcohol exceeds 70% in the area above the straight line (C). In other words, when the concentration of boric acid in 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 with a removal rate of more than 70%.

[0098] Figure 3The straight line (D) indicated by the double-dotted line is a straight line passing through the data points of (3) and (4), and is represented by y=-0.667x+4.667. It can be considered that the removal rate of the modified polyvinyl alcohol exceeds 70% in the area above the straight line (D). In other words, when the boric acid concentration of the treatment liquid and the concentration of the 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 with a removal rate of more than 70%.

[0099] In addition, in any formula, the boric acid concentration is, for example, 0.5 weight % or more, more preferably 1 weight % or more, and further preferably 1.5 weight % or more.

[0100] [Comparative Example 1]

[0101] Potassium sulfate (reagent) was dissolved in pure water to prepare a potassium sulfate aqueous solution having a potassium sulfate concentration of 5 wt%. The 1% PVA waste liquid was treated in the same manner as in Example 2 except that the potassium sulfate aqueous solution was used instead of the boric acid aqueous solution, and the removal rate of polyvinyl alcohol was calculated. The result was that the removal rate was 0%.

[0102] A potassium sulfate aqueous solution with a potassium sulfate concentration of 10% by weight was prepared and treated with 1% PVA wastewater in the same manner, and the removal rate of polyvinyl alcohol was calculated. The result was that when potassium sulfate was used, the removal rate was 0% at any concentration.

Claims

1. A method for treating waste liquid, the method comprising: A waste liquid containing a polyvinyl alcohol-based resin is mixed with a treatment liquid containing boric acid, thereby crosslinking the polyvinyl alcohol-based resin, precipitating the polyvinyl alcohol-based resin, and separating 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 added to 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 wt % 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 method for manufacturing a polarizing film, the method comprising: A process for producing a polarizing film by immersing a film containing a polyvinyl alcohol-based resin in a liquid; The liquid is taken out as waste liquid, and the polyvinyl alcohol-based resin is precipitated and separated from the waste liquid by the treatment method according to claim 1.

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