Method for producing regenerated superabsorbent polymer, method for producing superabsorbent polymer using regenerated superabsorbent polymer, and regenerated superabsorbent polymer

Abstract

The present invention relates to a method for producing a regenerated superabsorbent polymer, a method for producing a superabsorbent polymer using a regenerated superabsorbent polymer, and a regenerated superabsorbent polymer. This method is a method for producing a regenerated superabsorbent polymer, which is a raw material for producing a superabsorbent polymer, from a used superabsorbent polymer derived from used hygienic products. The method comprises: an inactivation step (S30) for inactivating a used superabsorbent polymer with an acidic solution; an oxidizing agent treatment step (S31) for treating the used superabsorbent polymer, which has been inactivated with the acidic solution, with an oxidizing agent under acidic conditions; and a drying step (S33) in which the used superabsorbent polymer treated with the oxidizing agent is dried to produce a regenerated superabsorbent polymer.

Description

[0001] This application is a divisional application of an application with an application date of June 11, 2020, application number 202080061821.X, and invention name “Method for manufacturing recycled super absorbent polymer, method for manufacturing super absorbent polymer using recycled super absorbent polymer, and recycled super absorbent polymer”. Technical Field

[0002] The present invention relates to a method for producing a recycled super absorbent polymer from a used super absorbent polymer derived from used sanitary products, a method for producing a super absorbent polymer using the recycled super absorbent polymer, and the recycled super absorbent polymer. Background Art

[0003] Technologies for regenerating used superabsorbent polymers from used sanitary products (e.g., absorbent articles) are known. For example, Patent Document 1 discloses a method for regenerating used superabsorbent polymers. This method includes the steps of: treating the used superabsorbent polymer with an aqueous solution of a polyvalent metal salt; and treating the superabsorbent polymer treated with the aqueous solution of an alkali metal salt.

[0004] Meanwhile, technologies are known for recycling unused superabsorbent polymers as part of the raw materials for superabsorbent polymers. For example, Patent Document 2 discloses a method for producing resin particles. In this method, an oily liquid containing resin b is dispersed in an aqueous dispersion of resin particles A composed of resin a during suspension polymerization. Resin particles B composed of resin b are formed in the aqueous dispersion, resulting in an aqueous dispersion X1 of resin particles C with resin particles A attached to the surfaces of resin particles B. Fine powder removed during the classification process during the production of other batches of resin particles C is dispersed in the aqueous dispersion. Patent Document 3 discloses a method for producing a water-absorbing agent. In this method, a powder containing fine particles of a water-absorbing resin is pressurized in the presence of water, dried, and pulverized to produce a water-absorbing agent with an average particle size of 200 to 1000 μm. Patent Document 4 discloses a method for producing a salt-resistant water-absorbing resin. In this production method, an aqueous solution of at least one monomer component selected from the group consisting of unsaturated carboxylic acids and their salts is polymerized in the presence of a water-absorbent resin at a ratio of 1 to 30 parts by weight of the water-absorbent resin per 100 parts by weight of the monomer component, thereby forming a salt-resistant water-absorbent resin. Furthermore, Non-Patent Document 1 discloses a method for producing a polyacrylate-based superabsorbent polymer.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 2013-198862

[0008] Patent Document 2: Japanese Patent Application Laid-Open No. 2007-246676

[0009] Patent Document 3: Japanese Patent Application Laid-Open No. 10-204184

[0010] Patent Document 4: Japanese Patent Application Laid-Open No. 4-227705

[0011] Patent Document 5: Japanese Patent No. 5996226

[0012] Non-patent literature

[0013] Non-patent document 1: Tadao Shimomura, "Superabsorbent Polymer", Surface (1991) 495-506 Summary of the Invention

[0014] Problems to be solved by the invention

[0015] In the recycling of used sanitary products (e.g., absorbent articles), many used sanitary products are collected. However, the types and manufacturers of these collected used sanitary products vary greatly. Therefore, if the method of Patent Document 1 is used to collectively regenerate a large number of superabsorbent polymers from used sanitary products, the regenerated superabsorbent polymers will become a mixture of various superabsorbent polymers. In other words, it is difficult to achieve uniform properties of the regenerated superabsorbent polymers. Therefore, if the regenerated superabsorbent polymers are directly used in sanitary products that use superabsorbent polymers as absorbent materials, it may be difficult to achieve uniform absorption properties in the sanitary products.

[0016] Therefore, the inventors have conducted the first study to use recycled superabsorbent polymers as the raw materials (fine powders, granules, or absorbent resins) of Patent Documents 2-4 and Non-Patent Document 1, or as the raw materials or semi-finished products of Non-Patent Document 1, in the production methods of superabsorbent polymers, for example. The production methods of Patent Documents 2-4 and Non-Patent Document 1 consider using unused raw materials with very few impurities (for example, superabsorbent polymers). On the other hand, recycled superabsorbent polymers from used sanitary products sometimes contain unique impurities (for example, odorous substances, colored substances, and bacteria). Therefore, if recycled superabsorbent polymers are used in the production methods of Patent Documents 2-4 and Non-Patent Document 1, which use unused superabsorbent polymers with very few impurities, there is a risk that high-purity, high-quality superabsorbent polymers will not be produced. Consequently, in sanitary products using superabsorbent polymers, the superabsorbent polymers produced using the production methods of Patent Documents 2-4 and Non-Patent Document 1 using recycled superabsorbent polymers may not be utilized.

[0017] The present invention aims to provide a method for producing a recycled super absorbent polymer, a method for producing a super absorbent polymer using a recycled super absorbent polymer, and a recycled super absorbent polymer capable of effectively utilizing used super absorbent polymer derived from used sanitary products in products using the super absorbent polymer as a raw material.

[0018] Solutions for solving problems

[0019] The present invention provides a method for producing a recycled superabsorbent polymer, which is used as a raw material for producing a superabsorbent polymer, from used superabsorbent polymer derived from used sanitary products. The method comprises: an inactivation step of inactivating the used superabsorbent polymer using an acidic solution; an oxidant treatment step of treating the used superabsorbent polymer inactivated by the acidic solution with an oxidant under acidic conditions; and a drying step of drying the used superabsorbent polymer treated with the oxidant to produce the recycled superabsorbent polymer.

[0020] The present invention provides a method for producing a super absorbent polymer using a recycled super absorbent polymer, which is recycled from used super absorbent polymers derived from used sanitary products, as a raw material for producing the super absorbent polymer. The method comprises the steps of producing the recycled super absorbent polymer by the method described above; and mixing the recycled super absorbent polymer into at least one of the raw material and a semi-finished product in aqueous solution polymerization or cross-linking for producing the super absorbent polymer, and performing the aqueous solution polymerization or cross-linking.

[0021] The present invention provides a recycled superabsorbent polymer, which is recycled from used superabsorbent polymer derived from used sanitary products and is used as a raw material for producing the superabsorbent polymer. The recycled superabsorbent polymer has an ash content of 35% by mass or less and a normal viable bacterial count detected by a pour culture method of below the detection limit.

[0022] Effects of the Invention

[0023] According to the present invention, a method for producing a recycled super absorbent polymer, a method for producing a super absorbent polymer using a recycled super absorbent polymer, and a recycled super absorbent polymer can be provided, which can effectively utilize used super absorbent polymer derived from used sanitary products in products using super absorbent polymer as a raw material. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a block diagram showing a configuration example of a system used in the method for producing a regenerated super absorbent polymer according to the embodiment.

[0025] Figure 2 This is a flowchart showing an example of a method for producing a regenerated super absorbent polymer according to an embodiment.

[0026] Figure 3 This is a block diagram showing a configuration example of an apparatus for taking out used super absorbent polymer according to an embodiment.

[0027] Figure 4 This is a flowchart showing an example of a method for taking out a used super absorbent polymer according to an embodiment. DETAILED DESCRIPTION

[0028] This embodiment relates to the following aspects.

[0029] [Form 1]

[0030] A method for producing recycled superabsorbent polymer as a raw material for producing superabsorbent polymer from used superabsorbent polymer derived from used sanitary products, the method comprising: an inactivation step of inactivating the used superabsorbent polymer using an acidic solution; an oxidant treatment step of treating the used superabsorbent polymer inactivated by the acidic solution with an oxidant under acidic conditions; and a drying step of drying the used superabsorbent polymer treated with the oxidant to produce the recycled superabsorbent polymer.

[0031] This method inactivates used superabsorbent polymers from used sanitary products (e.g., absorbent articles) with an acidic solution (an inactivation step), followed by treatment with an oxidant under acidic conditions (an oxidant treatment step). Specifically, this method dehydrates the used superabsorbent polymer using an acidic solution, significantly reducing its volume. The dehydrated state, i.e., the reduced volume, is then maintained under acidic conditions while the used superabsorbent polymer is deodorized, decolorized, and sterilized using an oxidant. By reducing the volume of the used superabsorbent polymer in this manner, the oxidant can easily remove impurities (e.g., odorous substances, colored substances, and bacteria) in the recycled superabsorbent polymer from the used sanitary products, resulting in a highly purified recycled superabsorbent polymer. Using this highly purified recycled superabsorbent polymer as part of the raw material in the method for producing a desired superabsorbent polymer enables the production of high-purity, high-quality superabsorbent polymer. Examples of methods for producing a predetermined superabsorbent polymer include methods described in Patent Documents 2 to 4, and methods for producing a superabsorbent polymer using recycled superabsorbent polymer, described below, where unused superabsorbent polymer is reused as a raw material or as part of a semi-finished product for superabsorbent polymer. This allows the effective utilization of used superabsorbent polymer from used sanitary products in products using superabsorbent polymer.

[0032] [Form 2]

[0033] The method according to aspect 1, further comprising, after the drying step, a foreign matter separation step of separating foreign matter from the regenerated super absorbent polymer.

[0034] In this method, foreign matter (e.g., pulp fibers) is separated from the dried regenerated superabsorbent polymer. Therefore, compared to separation in a non-dried state, such as separation in a solution, foreign matter can be separated from the regenerated superabsorbent polymer more easily, resulting in a regenerated superabsorbent polymer of higher purity.

[0035] [Form 3]

[0036] The method according to aspect 1 or 2, wherein the acidic solution is a solution containing citric acid, and further comprising a citric acid removal step of removing the citric acid from the used super absorbent polymer after the inactivation step and before the drying step.

[0037] In this method, used superabsorbent polymer is inactivated using a solution containing citric acid (inactivation step). This not only dehydrates the used superabsorbent polymer, significantly reducing its volume, but also facilitates the removal of metals (e.g., human-derived substances contained in excrement) through the chelating effect of citric acid. Furthermore, in this method, citric acid is removed from the used superabsorbent polymer after the inactivation step (citric acid removal step). This significantly reduces the presence of chelating agents, such as citric acid, in the regenerated superabsorbent polymer. This allows for the production of regenerated superabsorbent polymer with higher purity. In particular, since virtually no chelating agents remain in the regenerated superabsorbent polymer, when reusing unused superabsorbent polymer as part of the raw material for superabsorbent polymer, the chelating agent's ability to inhibit crosslinking reactions, such as in aqueous solution polymerization and crosslinking, can be significantly reduced (e.g., Patent Documents 2 and 4).

[0038] [Form 4]

[0039] The method according to aspect 3, wherein the citric acid removal step includes a removal treatment step of treating the used superabsorbent polymer with a removal acid that does not form a chelate structure with metal ions, an organic solvent miscible with water, or an aqueous solution thereof.

[0040] In this method, citric acid is removed by treating the used superabsorbent polymer with a removal acid that does not form a chelate structure with metal ions, a water-miscible organic solvent, or an aqueous solution thereof. This allows the used superabsorbent polymer to be dehydrated using the acid or organic solvent while simultaneously rinsing the citric acid from the surface of the used superabsorbent polymer. This significantly reduces the presence of chelating agents such as citric acid in the regenerated superabsorbent polymer.

[0041] [Form 5]

[0042] The method according to aspect 4, wherein the removal treatment step includes an acid step in which the used super absorbent polymer is treated with the removal acid or an aqueous solution thereof during or after the oxidizing agent treatment step.

[0043] In this method, the used superabsorbent polymer is treated with a removal acid or its aqueous solution during or after the oxidant treatment step. This removes citric acid from the used superabsorbent polymer and simultaneously further dehydrates the used superabsorbent polymer. This facilitates the subsequent drying step, allowing for efficient production of highly pure regenerated superabsorbent polymer.

[0044] [Form 6]

[0045] The method according to aspect 4 or 5, wherein the removal acid is an acid having no carboxyl group or an acid having one carboxyl group per molecule.

[0046] In this method, the removal acid is an acid without a carboxyl group or an acid having one carboxyl group per molecule. Therefore, in the removal treatment step, citric acid can be more reliably removed from the used superabsorbent polymer and the used superabsorbent polymer can be further dehydrated.

[0047] [Form 7]

[0048] The method according to any one of aspects 4 to 6, wherein the removal treatment step includes an organic solvent step of treating the used super absorbent polymer with an aqueous solution of the organic solvent after the oxidizing agent treatment step and before the drying step.

[0049] In this method, the used superabsorbent polymer is treated with an aqueous solution of an organic solvent after the oxidizing agent treatment step and before the drying step. This removes citric acid from the used superabsorbent polymer and simultaneously further dehydrates the used superabsorbent polymer. This facilitates the subsequent drying step, resulting in highly purified regenerated superabsorbent polymer.

[0050] [Form 8]

[0051] The method according to aspect 7, wherein the organic solvent step includes a step of treating the used super absorbent polymer with an aqueous solution of the organic solvent containing an alkali metal ion supply source capable of supplying alkali metal ions.

[0052] In this method, used superabsorbent polymer is treated with an aqueous solution of an organic solvent containing an alkali metal ion source. This removes citric acid from the used superabsorbent polymer, while simultaneously dehydrating the used superabsorbent polymer and neutralizing it with alkali metal ions. Consequently, for example, recycled superabsorbent polymer can be used as a neutralized, high-purity raw material in the production of superabsorbent polymer.

[0053] [Form 9]

[0054] The method according to aspect 8, wherein the alkali metal ion supply source is an alkali metal hydroxide or a salt of an alkali metal hydroxide and an acid having an acid dissociation constant greater than that of an acid group of the super absorbent polymer.

[0055] In this method, the alkali metal ion supply source is an alkali metal hydroxide, or a salt of an alkali metal hydroxide and an acid having a larger acid dissociation constant than the acid group of the superabsorbent polymer. Therefore, the alkali metal ions can be more reliably used to neutralize the used superabsorbent polymer.

[0056] [Form 10]

[0057] The method according to any one of aspects 1 to 9, wherein in the oxidizing agent treatment step, the oxidizing agent includes ozone-containing water or a hydrogen peroxide solution.

[0058] In this method, used superabsorbent polymer is treated with ozone-containing water or a hydrogen peroxide solution. This makes it easier to remove impurities (e.g., odorous substances, colored substances, and bacteria) from the recycled superabsorbent polymer derived from used sanitary products, thereby facilitating deodorization, decolorization, and sterilization. This results in a regenerated superabsorbent polymer of higher purity.

[0059] [Form 11]

[0060] A method for producing a super absorbent polymer using a recycled super absorbent polymer recycled from used super absorbent polymers derived from used sanitary products as a raw material for producing the super absorbent polymer, the method comprising the steps of: producing the recycled super absorbent polymer by the method of any one of aspects 1 to 10; and mixing the recycled super absorbent polymer into at least one of the raw material and a semi-finished product in aqueous solution polymerization or cross-linking for producing the super absorbent polymer, and performing the aqueous solution polymerization or cross-linking.

[0061] In this method, recycled superabsorbent polymer derived from used sanitary products (e.g., absorbent articles) is mixed with at least one of a raw material and a semi-finished product, and then polymerized or cross-linked in aqueous solution to produce a superabsorbent polymer. In this method, the recycled superabsorbent polymer is produced using the methods of Embodiments 1 to 10, resulting in a high-purity, high-quality superabsorbent polymer. Therefore, even with recycled superabsorbent polymer, this method can produce high-purity, high-quality superabsorbent polymer.

[0062] [Form 12]

[0063] A recycled superabsorbent polymer, which is recycled from used superabsorbent polymer derived from used sanitary products and is used as a raw material for producing the superabsorbent polymer, wherein the recycled superabsorbent polymer has an ash content of 35% by mass or less and a normal viable bacterial count detected by a pour culture method of the polymer is below the detection limit.

[0064] The recycled superabsorbent polymer derived from used sanitary products (e.g., absorbent articles) has an ash content of 35% by mass or less, and the number of viable bacteria detected by the pour culture method is below the detection limit. It is assumed that the ash content of typical unused superabsorbent polymer after production is approximately 30% by mass, and the ash content of the recycled superabsorbent polymer is also approximately 30% by mass. Therefore, the surface of the recycled superabsorbent polymer contains relatively little unwanted ash (5% by mass or less), and the number of viable bacteria in the recycled superabsorbent polymer is extremely low (below the detection limit). Consequently, the purity of the recycled superabsorbent polymer is extremely high. Therefore, by incorporating this recycled superabsorbent polymer into at least one of the raw materials and semi-finished products during suspension polymerization, aqueous solution polymerization, or crosslinking used in the production of the superabsorbent polymer, high-purity, high-quality superabsorbent polymer can be produced.

[0065] Hereinafter, the method for producing a recycled superabsorbent polymer as a raw material for producing a superabsorbent polymer from used superabsorbent polymers derived from used sanitary products, the method for producing a superabsorbent polymer using the recycled superabsorbent polymer, and the recycled superabsorbent polymer according to the embodiment will be described. In this specification, sanitary products refer to articles that contribute to hygiene and are articles containing superabsorbent polymers. Used sanitary products (for example, absorbent articles) refer to sanitary products that have been used by a user and mainly absorb and retain liquid substances (for example, excrement) discharged from the user, including sanitary products that have been used but have not absorbed or retained liquid substances, and sanitary products that have not been used but have been discarded. Used superabsorbent polymer refers to superabsorbent polymers contained in used sanitary products. Recycled superabsorbent polymers refer to superabsorbent polymers that have been recycled from used superabsorbent polymers derived from used sanitary products. In this embodiment, absorbent articles are described as examples of sanitary products. Examples of absorbent articles include disposable diapers, urine-absorbing pads, sanitary napkins, bed sheets, and pet sheets, and these absorbent articles may contain a highly absorbent polymer and may further contain pulp fibers.

[0066] First, an example of the structure of an absorbent article will be described. An absorbent article comprises a topsheet, a backsheet, and an absorber disposed between the topsheet and the backsheet. An example of the size of an absorbent article is a length of approximately 15 cm to 100 cm and a width of 5 cm to 100 cm. It should be noted that an absorbent article may also include other components typically found in absorbent articles, such as a diffuser sheet, leak-proof walls, and side panels.

[0067] As constituent members of the surface sheet, for example, liquid-permeable nonwoven fabrics, synthetic resin films with liquid-permeable holes, and composite sheets thereof can be cited. As constituent members of the back sheet, for example, liquid-impermeable nonwoven fabrics, liquid-impermeable synthetic resin films, and composite sheets thereof can be cited. As constituent members of the diffusion sheet, for example, liquid-permeable nonwoven fabrics can be cited. As constituent members of the leak-proof wall and the side sheet, for example, liquid-impermeable nonwoven fabrics can be cited. The leak-proof wall can also include elastic members such as rubber. As materials for nonwoven fabrics and synthetic resin films, there are no particular restrictions as long as they can be used as absorbent articles. For example, olefin resins such as polyethylene and polypropylene, polyamide resins such as 6-nylon and 6,6-nylon, polyester resins such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) can be cited. As materials for nonwoven fabrics, natural fibers such as cotton and rayon can also be used. In this embodiment, an absorbent article in which a back sheet is formed of a film and a top sheet is formed of a nonwoven fabric will be described as an example.

[0068] Components of the absorbent body include absorbent materials, namely pulp fibers and super absorbent polymers. Pulp fibers are not particularly limited as long as they can be used in absorbent articles. Examples include cellulose fibers. Examples of cellulose fibers include wood pulp, cross-linked pulp, non-wood pulp, regenerated cellulose, and semi-synthetic cellulose. The size of the pulp fibers can be, for example, an average fiber major diameter of several tens of μm, preferably 20 μm to 40 μm, and an average fiber length of several millimeters, preferably 2 mm to 5 mm. Super absorbent polymers (SAPs) are not particularly limited as long as they can be used in absorbent articles. Examples include polyacrylate-based, polysulfonate-based, and maleic anhydride-based absorbent polymers. The size of the super absorbent polymer (when dry) can be, for example, an average particle size of several hundred μm, preferably 200 μm to 500 μm. The absorbent body may also include a core wrap formed from a liquid-permeable sheet.

[0069] One side and the other side of the absorbent body are respectively joined to the surface sheet and the back sheet by means of an adhesive. When viewed from above, the portion (peripheral portion) of the surface sheet that extends to the outside of the absorbent body in a manner that surrounds the absorbent body is joined to the portion (peripheral portion) of the back sheet that extends to the outside of the absorbent body in a manner that surrounds the absorbent body by means of an adhesive. Thus, the absorbent body is wrapped inside the joint of the surface sheet and the back sheet. As an adhesive, there is no particular limitation as long as it can be used as an absorbent article, and examples thereof include hot-melt adhesives. Examples of hot-melt adhesives include rubber-based adhesives such as styrene-ethylene-butadiene-styrene, styrene-butadiene-styrene, and styrene-isoprene-styrene, or pressure-sensitive adhesives or heat-sensitive adhesives with olefin-based adhesives such as polyethylene.

[0070] Next, a method for producing a recycled super absorbent polymer as a raw material for producing a super absorbent polymer from a used super absorbent polymer derived from used sanitary products according to an embodiment will be described.

[0071] Figure 1 This is a block diagram showing a configuration example of a system 1 used in the method for producing a regenerated super absorbent polymer according to the embodiment. Figure 2 This is a flowchart showing an example of a method for producing a regenerated super absorbent polymer according to an embodiment.

[0072] The method for producing regenerated superabsorbent polymer includes an inactivation step S30, an oxidant treatment step S31, and a drying step S33, but may also include a foreign matter separation step S34. When citric acid is used in the inactivation step S30, a citric acid removal step S32 may also be included. In this production method, the citric acid removal step S32 may also include a removal treatment step S40. Meanwhile, the system 1 used in the method for producing regenerated superabsorbent polymer includes an inactivation device 30, an oxidant treatment device 31, and a drying device 33, but may also include a foreign matter separation device 34. When citric acid is used in the inactivation device 30, a citric acid removal device 32 may also be included. In this system 1, the citric acid removal device 32 may preferably also include a removal treatment device 40. Each step is described below in detail.

[0073] The inactivation step S30 is performed by the inactivation device 30. The inactivation step S30 inactivates the used superabsorbent polymer using an acidic solution. In this embodiment, the used superabsorbent polymer removed from the used absorbent article is immersed in an acidic aqueous solution as an inactivator to form the inactivated superabsorbent polymer 300.

[0074] The acid in the acidic aqueous solution is not particularly limited, and examples include inorganic acids and organic acids. Using an acid to inactivate the superabsorbent polymer reduces the likelihood of ash remaining in the superabsorbent polymer and pulp fibers compared to inactivating the superabsorbent polymer using lime, calcium chloride, or the like. Examples of inorganic acids include sulfuric acid, hydrochloric acid, and nitric acid, but sulfuric acid is preferred due to its chlorine-free nature and cost. Examples of organic acids include carboxylic acids having multiple carboxyl groups per molecule (examples include citric acid, tartaric acid, malic acid, succinic acid, and oxalic acid), carboxylic acids having one carboxyl group per molecule (examples include gluconic acid, valeric acid, butyric acid, propionic acid, glycolic acid, acetic acid, and formic acid), and sulfonic acids (examples include methanesulfonic acid, trifluoromethanesulfonic acid, benzenesulfonic acid, and p-toluenesulfonic acid). As an organic acid, from the perspective of easily forming a chelate complex with a divalent or higher metal (for example, calcium) contained in excrement, etc., and thus preventing ash from remaining in the superabsorbent polymer and pulp fibers, an organic acid preferably has multiple carboxyl groups, and citric acid is more preferred. The citric acid concentration of the acidic aqueous solution is not particularly limited, but can be, for example, 0.5% to 4% by mass. In this embodiment, citric acid is used as the acid in the acidic aqueous solution.

[0075] The acidic aqueous solution only needs to be acidic, but preferably has a predetermined pH. The upper limit of the predetermined pH is preferably 4.0, more preferably 3.5, and even more preferably 3.0. If the predetermined pH is too high, the inactivation of the superabsorbent polymer cannot be fully carried out, and there is a tendency for the discharge of excrement retained by the superabsorbent polymer to become insufficient, and there is also a tendency for it to be difficult to separate from pulp fibers and the like. On the other hand, the lower limit of the predetermined pH is preferably 0.5, and more preferably 1.0. If the predetermined pH is too low, the superabsorbent polymer is further inactivated, and in the removal treatment step S40 (described later), there is a tendency for the inactivated superabsorbent polymer to take time to react with the aqueous solution containing the alkali metal ion supply source. In addition, if the predetermined pH is too low, when recycled pulp fibers are regenerated in addition to the superabsorbent polymer from used absorbent articles, the recycled pulp fibers may be damaged. It should be noted that in this specification, pH refers to the value at 25°C. The pH can be measured, for example, using a twin pH meter AS-711 manufactured by Horiba, Ltd. In the method for producing a regenerated superabsorbent polymer, it is preferred that the predetermined pH be met at the end of the inactivation step S30. This is done to ensure continued inactivation of the superabsorbent polymer.

[0076] The inactivation device 30 that performs the inactivation step S30 is not particularly limited in its specific structure, as long as it can immerse the superabsorbent polymer in the acidic aqueous solution. For example, the inactivation device 30 includes a tank that can accommodate the material containing the superabsorbent polymer and store the acidic aqueous solution. In the inactivation step S30, the material containing the superabsorbent polymer can be added after the acidic aqueous solution is stored in the tank, or the material containing the superabsorbent polymer can be placed in the tank and then the acidic aqueous solution is added. The material containing the superabsorbent polymer is immersed in the acidic aqueous solution, causing the inactivation reaction to occur.

[0077] In the inactivation step S30, although temperature is also dependent, the material containing the superabsorbent polymer (e.g., a used absorbent article) is stirred in a tank containing an acidic aqueous solution for approximately 5 to 60 minutes to inactivate the superabsorbent polymer, thereby ensuring a uniform reaction. The temperature of the acidic aqueous solution in the inactivation step S30 is not particularly limited and can be, for example, room temperature (25°C), preferably a temperature higher than room temperature, more preferably 60°C to 95°C, and even more preferably 70°C to 90°C. This facilitates the sterilization of bacteria contained in the acidic aqueous solution, which may originate from excrement, etc.

[0078] At the end of the inactivation step S30 , the super absorbent polymer 300 is not completely dehydrated and has absorbed a small amount of water (for example, approximately 10 times the volume of the super absorbent polymer before water absorption), thus forming a gel.

[0079] It should be noted that if the material to be immersed in the acidic aqueous solution includes other components besides the superabsorbent polymer, such as pulp fibers, a liquid-permeable sheet, or a liquid-impermeable sheet, a separation step may be performed before or after the inactivation step S30 to separate and remove the components other than the superabsorbent polymer. The separation step may also be performed simultaneously with the inactivation step S30. An example of a case where the material to be immersed in the acidic aqueous solution includes other components besides the superabsorbent polymer is a used absorbent article containing the superabsorbent polymer. The separation step will be described later.

[0080] The inactivated super absorbent polymer is then separated from the acidic aqueous solution (solid-liquid separation) by a sieve (or net) provided in the inactivation device 30 , and then supplied as super absorbent polymer 300 to the oxidant treatment step S31 (oxidant treatment device 31 ).

[0081] The oxidizing agent treatment step S31 is performed by the oxidizing agent treatment device 31. In this step, the superabsorbent polymer 300, which has been inactivated with an acidic solution, is treated with an oxidizing agent under acidic conditions. The oxidizing agent is not particularly limited as long as it can remove impurities (e.g., odorous substances, colored substances, and bacteria) adhering to the surface of the superabsorbent polymer 300. Examples include ozone and hydrogen peroxide. When the oxidizing agent is mixed with water and used as an aqueous solution, the concentration of the oxidizing agent in the aqueous solution can be, for example, 0.5% to 20% by mass. In this embodiment, the inactivated superabsorbent polymer 300 is contacted (immersed) with an aqueous solution of the oxidizing agent, i.e., ozone water containing a predetermined concentration of ozone, for a predetermined period of time to form a superabsorbent polymer 301 from which impurities on the surface of the superabsorbent polymer 300 have been removed. The acidic conditions can be any acidic condition, and examples include acidic aqueous solutions. The acidic aqueous solution preferably contains the inorganic and organic acids listed in the description of the acidic aqueous solution, from the perspective of maintaining the inactivated state of the superabsorbent polymer 300 and preventing ozone from deactivating it when used. The pH of the acidic aqueous solution is preferably within a predetermined range, at least 0.5 to 4.0, similar to the aforementioned acidic aqueous solution, and more preferably 1.0 to 3.5. In this embodiment, an aqueous solution of citric acid is used as the acidic condition. The citric acid concentration of the aqueous solution is not particularly limited, but can be, for example, 0.1% to 5% by mass.

[0082] Bacteria and other organic matter from the excreta of used absorbent articles adhere to the surface of the super absorbent polymer 300 as foreign matter and dirt. If these remain on the surface, the purity of the regenerated super absorbent polymer may be reduced. Therefore, in the oxidant treatment step S31, the super absorbent polymer 300 is dispersed roughly evenly in an aqueous solution of an oxidant, in this embodiment, ozone water, at a predetermined concentration (for example, 1 to 10% by mass), and bacteria and other organic matter attached to the surface of the super absorbent polymer 300 are oxidized and decomposed and removed. In other words, in the oxidant treatment step S31, impurities attached to the surface of the super absorbent polymer 300 are removed. In particular, sterilization using ozone water is called bacteriolysis, which destroys the bacterial cell wall (membrane) through a chemical reaction between proteins and ozone, allowing the components inside the cells to leak out, thereby killing the bacteria. Therefore, the sterilization effect is significantly improved, it is less likely to produce drug-resistant bacteria, and the possibility of bacteria re-growth and covering the surface after sterilization is extremely low.

[0083] At this point, if the superabsorbent polymer is inactivated using an acidic aqueous solution (inactivation step S30), the inactivated superabsorbent polymer no longer absorbs water. Therefore, in its inactivated state, it can be cleaned with ozone water having a low ozone concentration, making it easier to remove bacteria and other substances through cleaning, and bacteria and other substances can be removed in a short period of time. Furthermore, odorous and pigment components are also decomposed by ozone, thus achieving a deodorizing and decolorizing effect. It should be noted that if the superabsorbent polymer 300 is treated with a gaseous substance, such as ozone gas, the superabsorbent polymer 300 is in a block (gel-like) form, so the treatment will not reach the entire block, making it difficult to treat uniformly throughout. If ultraviolet rays or radiation are used for treatment, the superabsorbent polymer will decompose at a radiation dose strong enough to penetrate the interior of the block. If treatment is performed using heat or high-pressure steam, fecal matter may remain in the superabsorbent polymer. If an attempt is made to treat it with an aqueous sodium hypochlorite solution, chlorine will remain in the super absorbent polymer, making it unusable as a sanitary material.

[0084] The ozone concentration in the ozone water is not particularly limited as long as it is a concentration that can remove bacteria and other organic matter attached to the surface of the super absorbent polymer. It is preferably 0.3 mass ppm to 2 mass ppm, and more preferably 0.5 mass ppm to 1.5 mass ppm. If the concentration is too low, it will be difficult to remove bacteria, etc., and if the concentration is too high, the super absorbent polymer may begin to decompose. The contact time between the ozone water and the super absorbent polymer is not particularly limited as long as it is a time that can remove bacteria and other organic matter attached to the surface of the super absorbent polymer. The higher the ozone concentration in the ozone water, the shorter the contact time, and the lower the ozone concentration, the longer the contact time. The contact time is preferably 0.3 seconds to 15 minutes, and more preferably 5 seconds to 10 minutes. The product of the ozone concentration (ppm) in the ozone water and the contact time (minutes) (hereinafter also referred to as "CT value") is preferably 0.05 ppm·minute to 20 ppm·minute, and more preferably 0.08 ppm·minute to 10 ppm·minute. If the CT value is too low, sterilization becomes difficult, while if the CT value is too high, the super absorbent polymer may decompose. Ozone water treatment, by removing impurities adhering to the surface of the super absorbent polymer 300, can simultaneously remove bacteria and other substances and bleach the water. Examples of ozone generators that supply ozone to water include the ED-OWX-2 Ozone Water Exposure Tester manufactured by ECO Design Co., Ltd. and the OS-25V Ozone Generator manufactured by Mitsubishi Electric Corporation.

[0085] In the oxidizing agent treatment step S31, the ozone water containing the super absorbent polymer 300 may be stirred in a tank to ensure a uniform reaction. The temperature of the ozone water in the oxidizing agent treatment step S31 is not particularly limited; for example, it can be room temperature (25°C), preferably between 10°C and 40°C. If the temperature of the ozone water is too high, ozone is likely to escape as gas and become inactive. If the temperature is too low, the ozone treatment time tends to be prolonged. This facilitates the removal of bacteria and organic matter adhering to the surface of the super absorbent polymer 300 by the ozone in the ozone water.

[0086] As the oxidant treatment device 31 for performing the oxidant treatment step S31, the specific structure is not particularly limited as long as it can allow the inactivated super absorbent polymer 300 to contact (or immerse) in ozone water without destroying its shape. As an example of the oxidant treatment device 31, a twin-screw pump (BQ type: manufactured by Fuhu Metal Industry Co., Ltd.) can be cited. A twin-screw pump is a positive displacement self-priming pump. The twin-screw pump includes a housing and a double-axis screw arranged in a chamber within the housing and extending parallel to each other. Ozone water containing super absorbent polymer 300 is supplied to the chamber, reaches the double-axis screw from the radial direction, and is then extruded and discharged in the axial direction by the rotation of the double-axis screw. During the treatment stage of the oxidant treatment step S31, as described above, the super absorbent polymer 300 is in a gel state that has absorbed water, although not much. Therefore, it is difficult to stir the gel using a stirring blade without destroying the gel, and it is difficult to carry out a uniform reaction. A twin-screw pump is preferred because it lacks agitating blades and thus does little to shear the deactivated super absorbent polymer 300. This allows for uniform mixing with the ozone water without disrupting the gel. In the oxidant treatment step S31, the deactivated super absorbent polymer 300 is mixed with the ozone water to react using a twin-screw pump.

[0087] Thereafter, the ozone-treated super absorbent polymer 300 is separated from the ozone water (solid-liquid separation) by a sieve (or net) provided in the oxidant treatment device 31 , and then supplied as super absorbent polymer 301 to the citric acid removal step S32 (citric acid removal device 32 ).

[0088] The citric acid removal step S32 is performed by the citric acid removal device 32. If the acidic solution used in the inactivation step S30 contains citric acid, the citric acid removal step S32 removes citric acid from the used superabsorbent polymer 301 after the inactivation step S30 and before the drying step S33. In this embodiment, the use of a solution containing citric acid as the acidic solution in the inactivation step S30 not only inactivates the used superabsorbent polymer but also facilitates the removal of metals adhering to the surface of the used superabsorbent polymer by utilizing the chelating effect of citric acid. Furthermore, the citric acid removal step S32 significantly reduces the presence of chelating agents such as citric acid in the resulting regenerated superabsorbent polymer.

[0089] Here, the citric acid removal step S32 includes a removal treatment step S40 (removal treatment device 40) in which the used super absorbent polymer 301 is treated with a removal acid that does not form a chelate structure with metal ions, an organic solvent miscible with water, or an aqueous solution thereof. This allows the citric acid to be dissolved in the predetermined acid or organic solvent and removed from the used super absorbent polymer, while further dehydrating the used super absorbent polymer.

[0090] Examples of acids for removing acids that do not form a chelate structure with metal ions include the inorganic and organic acids listed above in the description of the acidic aqueous solution, which do not have a carboxyl group, or acids that have one (or more) carboxyl group per molecule. In other words, the acid for removing acids can be inorganic and organic acids that do not have multiple carboxyl groups per molecule. Therefore, even if the acid for removing acids remains on the surface of the used superabsorbent polymer, it will not function as a chelating agent.

[0091] In this case, the removal treatment step S40 may include an acid step of treating the used super absorbent polymer with a removal acid or an aqueous solution thereof after the oxidizing agent treatment step S31 and before the drying step S33 .

[0092] On the other hand, the organic solvent miscible with water is not particularly limited, but examples thereof include alcoholic solvents (examples include methanol, ethanol, propanol and its isomers, butanol and its isomers), ketone solvents (examples include acetone and methyl ethyl ketone), and nitrile solvents (examples include acetonitrile). This allows the used superabsorbent polymer to be further dehydrated and cleaned. In this embodiment, methanol, an organic solvent miscible with water, is used in the removal treatment step S40 of the citric acid removal step S32.

[0093] The organic solvent may contain other solvents such as water as long as they do not affect the removal of citric acid. The proportion of the other solvent relative to the organic solvent is less than 50% by mass, preferably 10% by mass or less, and more preferably 0% by mass.

[0094] In this case, the removal treatment step S40 may include an organic solvent step of treating the used super absorbent polymer with an organic solvent or an aqueous solution thereof after the oxidizing agent treatment step S31 and before the drying step S33 .

[0095] It should be noted that, as another embodiment, the citric acid removal step S32 may also include a reactivation step as the removal treatment step S40: after the used superabsorbent polymer is treated with the predetermined removal acid, the predetermined organic solvent, or an aqueous solution thereof, the treated superabsorbent polymer is further treated with an aqueous solution containing an alkali metal ion source capable of supplying alkali metal ions, i.e., reactivated (neutralized). In other words, the removal treatment step S40 may include a reactivation step after the acid step or the organic solvent step.

[0096] In this case, the superabsorbent polymer is deactivated with an acidic aqueous solution (deactivation step S30), and then neutralized with an aqueous solution containing an alkali metal ion source (removal step S40), thereby obtaining reactivated (neutralized) superabsorbent polymer. This eliminates the need for polyvalent metal ions during deactivation, thus preventing the occurrence of absorption impairment due to residual ash.

[0097] Examples of the alkali metal ion supply source include lithium ions (Li ions), sodium ions (Na ions), potassium ions (K ions), and any combination thereof. The alkali metal ion supply source is not particularly limited as long as it can supply the aforementioned alkali metal ions. Examples include alkali metal hydroxides, and salts of alkali metal hydroxides with acids having a larger acid dissociation constant than the acid group of the superabsorbent polymer (hereinafter referred to as "salts"). The acid dissociation constant can be the value listed in the Electrochemical Handbook compiled by the Society of Electrochemists.

[0098] Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, and potassium hydroxide, and any combination thereof. Examples of salts include acidic salts and alkaline salts. Examples of alkali metal hydroxides in the salt include lithium hydroxide, sodium hydroxide, and potassium hydroxide, and any combination thereof. Examples of acids in the salt include the acids listed in the inactivation step S30 (for example, hydrochloric acid and sulfuric acid), carbonic acid, and the like. Examples of salts include lithium carbonate, sodium carbonate, potassium carbonate, lithium bicarbonate, sodium bicarbonate, potassium bicarbonate, lithium chloride, sodium chloride, and potassium chloride.

[0099] When an alkali metal hydroxide is used as the alkali metal ion supply source, the hydroxide ion concentration of the alkaline aqueous solution is, for example, 0.1 mol / L to 5.0 mol / L, preferably 0.3 mol / L to 3.0 mol / L, and more preferably 0.4 mol / L to 1.0 mol / L.

[0100] Furthermore, as another embodiment, when the removal treatment step S40 is an organic solvent step, the citric acid removal step S32 may also include a step of treating the used superabsorbent polymer with an aqueous solution of an organic solvent containing an alkali metal ion source capable of supplying alkali metal ions. In other words, the organic solvent step of the removal treatment step S40 may include, in addition to a dehydration and cleaning step of dehydrating and cleaning the used superabsorbent polymer with an organic solvent or an aqueous solution thereof, a reactivation step of reactivating the used superabsorbent polymer using the alkali metal ion source. By performing the dehydration and cleaning step and the reactivation step simultaneously in this manner, citric acid can be removed from the used superabsorbent polymer, while simultaneously dehydrating the used superabsorbent polymer and neutralizing (reactivating) it with alkali metal ions.

[0101] Therefore, when reactivating (neutralizing) superabsorbent polymer is required as regenerated superabsorbent polymer, performing the dehydration and cleaning steps simultaneously with the reactivation steps allows the superabsorbent polymer to be dehydrated without swelling due to water absorption. This reduces the volume of the superabsorbent polymer to be reactivated, enabling efficient reactivation. Furthermore, the reactivation process can be performed while the superabsorbent polymer maintains a high gel strength, which can suppress gel breakdown caused by agitation during the reactivation process. Furthermore, the need for larger reactivation and dehydration cleaning equipment can be eliminated, reducing the amount of organic solvent used. Furthermore, this dehydration process maintains a substantially constant moisture content near the surface of the superabsorbent polymer, enabling relatively uniform drying of the surface of the superabsorbent polymer. This ensures relatively uniform absorption performance of the regenerated superabsorbent polymer, ultimately suppressing a relative decrease in absorption performance.

[0102] Furthermore, as another embodiment, when the removal treatment step S40 is an acid step, the citric acid removal step S32 may also include a subsequent dehydration and cleaning step for dehydrating and cleaning the used superabsorbent polymer using an organic solvent or an aqueous solution thereof. Furthermore, as another embodiment, after the acid step as the removal treatment step S40 is performed, and then a reactivation step for reactivating (neutralizing) the used superabsorbent polymer is performed, a dehydration and cleaning step may be included for dehydrating and cleaning the used superabsorbent polymer using an organic solvent or an aqueous solution thereof. Including the dehydration and cleaning step facilitates the subsequent drying step S33.

[0103] The temperature of the citric acid removal step S32 (removal treatment step S40) is a predetermined temperature, for example, room temperature (e.g., 20°C) to 60°C, preferably 30°C to 50°C. Lower temperatures tend to increase the removal time, while higher temperatures may reduce the water absorption of the superabsorbent polymer due to dehydration condensation of the acid groups in the superabsorbent polymer. The duration of the citric acid removal step S32 (removal treatment step S40) is a predetermined time, for example, 5 minutes to 300 minutes.

[0104] The specific structure of the citric acid removal device 32 (removal treatment device 40) that performs the citric acid removal step S32 (removal treatment step S40) is not particularly limited. An example of the citric acid removal device 32 (removal treatment step S40) is a mixer (Standard Mixer NS-PS: manufactured by Tatechs Co., Ltd.). This mixer includes a rotating mixing tank. The superabsorbent polymer 301 and the predetermined removal acid, organic solvent, or aqueous solution thereof are supplied to the mixing tank and mixed by the mixing tank's rotation. During the treatment stage of the citric acid removal step S32 (removal treatment step S40), although the superabsorbent polymer 301 is not large in amount, it is in the form of a gel that has absorbed water. Therefore, it is difficult to stir it with a stirring blade without destroying the gel, making it difficult to achieve a uniform reaction. This mixer is preferably capable of uniformly mixing the superabsorbent polymer 301 with the predetermined aqueous solution with little shearing due to the rotation of the mixing tank, without the need for stirring blades. In the citric acid removal step S32 (removal treatment step S40 ), the super absorbent polymer 301 is mixed with a predetermined aqueous solution using the mixer.

[0105] Thereafter, the super absorbent polymer 301 from which at least citric acid has been removed is separated from the predetermined aqueous solution (solid-liquid separation) by a sieve (or net) provided in the citric acid removal step S32 (removal treatment step S40 ), and then supplied as the super absorbent polymer 302 to the drying step S33 (drying device 33 ).

[0106] At the end of the citric acid removal step S32 (removal step S40 ), the super absorbent polymer 302 is sufficiently dehydrated, and has a water absorption ratio of approximately 2 times, for example, and is in a loose state close to sand.

[0107] The drying step S33 is performed by the drying device 33. The drying step S33 dries the used super absorbent polymer treated with the oxidizing agent to produce a regenerated super absorbent polymer. In this embodiment, the super absorbent polymer 302 is dried in a heated atmosphere while being stirred.

[0108] In the drying step S33, the drying temperature can be, for example, room temperature (e.g., 25°C) to 150°C, preferably 70°C to 120°C. Lower drying temperatures tend to increase the drying time, while higher drying temperatures can cause dehydration condensation of the acid groups in the superabsorbent polymer, which can reduce the water absorption of the superabsorbent polymer. In the drying step S33, the drying time can be, for example, 30 minutes to 300 minutes. To promote drying, the drying step S33 can also be carried out under reduced pressure, for example, 0.1 kPa to 100 kPa.

[0109] The drying device 33 used in the drying step S33 is not particularly limited in its specific structure as long as it can dry the superabsorbent polymer 302. An example of the drying device 33 is a paddle dryer (Paddle Dryer NPD-1.6WG: manufactured by Nara Machinery Co., Ltd.). A paddle dryer comprises a housing and two rotatable shafts arranged within a chamber within the housing and extending axially parallel to each other. Each shaft includes radially extending wedge-shaped heat transfer fins (paddles), with the blades overlapping when viewed axially. The superabsorbent polymer 302 is supplied into the chamber, exposed to hot air, and dried by the paddles in a high-temperature atmosphere. In the drying step S33, the paddle dryer maintains agitation in a heated atmosphere to prevent the superabsorbent polymer 302 from adhering to each other.

[0110] The drying step S33 is preferably carried out so that the weight loss on drying of the superabsorbent polymer is preferably 15% or less (2.0 g, 105°C, 3 hours). This is done to maximize the use of the superabsorbent polymer. The weight loss on drying is measured according to "7. Weight Loss on Drying Test Method," "2. General Test Methods," of the "Standards for Materials of Sanitary Products," attached as an appendix to the Ministry of Health, Labour and Welfare's Notice No. 0325-24, issued on March 25, 2015, regarding the "Standards for Materials of Sanitary Products."

[0111] Thereafter, the dried super absorbent polymer 302 is supplied as the super absorbent polymer 303 to the foreign matter separation step S34 (foreign matter separation device 34 ).

[0112] The foreign matter separation step S34 is performed by the foreign matter separation device 34. The foreign matter separation step S34 separates foreign matter from the dried super absorbent polymer 303. Examples of methods for separating foreign matter include methods that utilize specific gravity differences in gas or screening based on size. Examples of foreign matter include pulp fibers.

[0113] The foreign matter separation device 34 that performs the foreign matter separation step S34 is not particularly limited in its specific structure as long as it can separate foreign matter from the super absorbent polymer 303. The foreign matter separation device 34 is, for example, a cyclone separator that separates the super absorbent polymer 303 from foreign matter by centrifugal separation in the gas. This generates regenerated super absorbent polymer with a further reduced amount of foreign matter.

[0114] As described above, the super absorbent polymer derived from the used absorbent article is regenerated to produce the regenerated super absorbent polymer.

[0115] As shown in the Examples described below, the ash content in the resulting regenerated superabsorbent polymer (including the reactivated (neutralized) polymer) is 35% by mass or less, preferably 32% by mass or less, and more preferably 31% by mass or less. Furthermore, the number of viable bacteria detected in the resulting regenerated superabsorbent polymer by the pour culture method is below the detection limit. Here, the amount of ash contained in a typical superabsorbent polymer (e.g., a crosslinked sodium polyacrylate) before production is approximately 30% by mass, so the amount of ash contained in the regenerated superabsorbent polymer is also approximately 30% by mass. In this case, the 30% by mass ash, i.e., the ash within the regenerated superabsorbent polymer, is believed to be derived from the sodium (Na) in the crosslinked sodium polyacrylate. (Note that the ash on the surface of the superabsorbent polymer is believed to be derived from inorganic matter derived from excrement, silica originally adhering to the surface, and the like.) Therefore, in the regenerated superabsorbent polymer, after removing the internal surface, the amount of unnecessary ash is low, only about 5% by mass (35% to 30% by mass) or less. It is preferably 2% or less, more preferably 1% or less, and even more preferably 0.5% or less. Furthermore, the number of normal viable bacteria in the regenerated superabsorbent polymer is extremely low (below the detection limit). As a result, the purity of the regenerated superabsorbent polymer is extremely high. Therefore, the regenerated superabsorbent polymer can be used as a raw material for the production of superabsorbent polymers. In other words, the regenerated superabsorbent polymer is regenerated from used superabsorbent polymers from used sanitary products and is used as a raw material for the production of superabsorbent polymers.

[0116] The method for producing recycled superabsorbent polymer according to this embodiment inactivates used superabsorbent polymer from used sanitary products (e.g., absorbent articles) with an acidic solution (inactivation step S30), treats it with an oxidant under acidic conditions (oxidant treatment step S31), and then dries it (drying step S33) to produce recycled superabsorbent polymer. Specifically, this method dehydrates the used superabsorbent polymer with an acidic solution, significantly reducing its volume. Subsequently, while maintaining the dehydrated (i.e., reduced) state under acidic conditions, the used superabsorbent polymer is simultaneously deodorized, decolorized, and sterilized with an oxidant. By reducing the volume of the used superabsorbent polymer in this manner, the oxidant can easily remove impurities (e.g., odorous substances, colored substances, and bacteria) present in the recycled superabsorbent polymer from the used sanitary products, resulting in highly pure recycled superabsorbent polymer. Such a highly pure recycled superabsorbent polymer can be used as part of the raw material in a method for producing a predetermined superabsorbent polymer, thereby producing a high-purity, high-quality superabsorbent polymer. Examples of methods for producing a predetermined superabsorbent polymer include the methods described in Patent Documents 2 to 4 and the method for producing a superabsorbent polymer using recycled superabsorbent polymer, described below, which reuses unused superabsorbent polymer as part of the raw material for superabsorbent polymer. This allows the effective utilization of used superabsorbent polymer from used sanitary products in products using superabsorbent polymer.

[0117] In this embodiment, as a preferred embodiment, after the drying step S33, foreign matter (for example, pulp fibers) is separated from the dried regenerated super absorbent polymer (foreign matter separation step S34). Therefore, compared to separation in a non-drying state such as separation in a solution, foreign matter can be separated from the regenerated super absorbent polymer more easily, resulting in a regenerated super absorbent polymer with higher purity.

[0118] In this embodiment, as a preferred aspect, the used superabsorbent polymer is inactivated using a solution containing citric acid (inactivation step S30). This not only dehydrates the used superabsorbent polymer, significantly reducing its volume, but also facilitates the removal of metals (e.g., human-derived substances contained in excrement) through the chelating effect of citric acid. Furthermore, in this method, citric acid is removed from the used superabsorbent polymer after the inactivation step S30 (citric acid removal step S32). This significantly reduces the presence of chelating agents, such as citric acid, in the regenerated superabsorbent polymer. Consequently, a regenerated superabsorbent polymer of higher purity can be obtained. In particular, since virtually no chelating agents remain in the regenerated superabsorbent polymer, when reusing unused superabsorbent polymer as part of the raw material for superabsorbent polymer, the chelating agent can be significantly prevented from interfering with the crosslinking reaction during aqueous solution polymerization, crosslinking, and other processes.

[0119] In this embodiment, as a preferred embodiment, the used superabsorbent polymer is treated (removal treatment step S40) with a removal acid that does not form a chelate structure with metal ions, a water-miscible organic solvent, or an aqueous solution thereof, as a step to remove citric acid (citric acid removal step S32). This allows the used superabsorbent polymer to be dehydrated using the acid or organic solvent, while simultaneously washing away the citric acid on the surface of the used superabsorbent polymer. Consequently, the presence of chelating agents such as citric acid remaining in the regenerated superabsorbent polymer can be significantly reduced.

[0120] In another embodiment, as the removal treatment step S40, the used superabsorbent polymer is treated with a removal acid or its aqueous solution during or after the oxidant treatment step S31 (an acid step). This removes citric acid from the used superabsorbent polymer and further dehydrates the used superabsorbent polymer. This facilitates the subsequent drying step S33, allowing for efficient production of highly pure regenerated superabsorbent polymer.

[0121] In another embodiment, the removal acid used in the removal treatment step S40 (acid step) is an acid without a carboxyl group or an acid having one carboxyl group per molecule. Therefore, in the removal treatment step, citric acid can be more reliably removed from the used superabsorbent polymer and the used superabsorbent polymer can be further dehydrated.

[0122] In this embodiment, as a preferred aspect, the used superabsorbent polymer is treated with an aqueous solution of an organic solvent (organic solvent step) as a removal treatment step S40, after the oxidant treatment step S31 and before the drying step S33. This removes citric acid from the used superabsorbent polymer and further dehydrates the used superabsorbent polymer. This facilitates the subsequent drying step S33, allowing for efficient production of highly pure regenerated superabsorbent polymer.

[0123] In another embodiment, in the removal treatment step S40 (organic solvent step), the used superabsorbent polymer is treated with an aqueous solution of an organic solvent containing an alkali metal ion source. This removes citric acid from the used superabsorbent polymer, while simultaneously neutralizing the used superabsorbent polymer with alkali metal ions. Consequently, for example, recycled superabsorbent polymer can be used as a neutralized, high-purity raw material in the production of superabsorbent polymer.

[0124] In another embodiment, the alkali metal ion source is an alkali metal hydroxide, or a salt of an alkali metal hydroxide and an acid having a larger acid dissociation constant than the acid group of the superabsorbent polymer. Therefore, the alkali metal ions can be more reliably used to neutralize the used superabsorbent polymer.

[0125] In this embodiment, as a preferred aspect, the used superabsorbent polymer is treated with ozone-containing water or hydrogen peroxide solution as an oxidant in the oxidant treatment step S31. This facilitates the removal of impurities (e.g., odorous substances, colored substances, and bacteria) contained in the recycled superabsorbent polymer derived from used sanitary products, thereby facilitating deodorization, decolorization, and sterilization. This results in a regenerated superabsorbent polymer of higher purity.

[0126] Furthermore, the recycled superabsorbent polymer derived from used sanitary products (e.g., absorbent articles) according to this embodiment has an ash content of 35% by mass or less, and the number of viable bacteria detected by the pouring culture method is below the detection limit. This indicates that the purity of the recycled superabsorbent polymer is high. Therefore, by incorporating this recycled superabsorbent polymer into at least one of the raw materials and semi-finished products during aqueous solution polymerization or cross-linking for superabsorbent polymer production, high-purity, high-quality superabsorbent polymer can be produced.

[0127] Next, a method for producing a super absorbent polymer using a recycled super absorbent polymer as a raw material for producing the super absorbent polymer, which is recycled from used super absorbent polymers derived from used sanitary products, will be described.

[0128] Among the highly absorbent polymers, the polyacrylate-based highly absorbent polymer is formed, for example, by aqueous solution polymerization. Examples of the aqueous solution polymerization method include the following methods (1) and (2) (Non-Patent Document 1).

[0129] (1) Acrylic acid (raw material) is polymerized and cross-linked in an aqueous solution (polymerization initiator and cross-linking agent) to form a cross-linked polyacrylic acid product (semi-finished product). The cross-linked polyacrylic acid product is then neutralized to form a cross-linked sodium polyacrylate product, i.e., a super absorbent polymer. Preferably, the super absorbent polymer is then surface-crosslinked using a surface cross-linking agent. In this case, the super absorbent polymer before surface cross-linking can be referred to as a semi-finished product.

[0130] (2) Acrylic acid (raw material) is neutralized to form sodium acrylate (semi-finished product), which is then polymerized and cross-linked in an aqueous solution (with a polymerization initiator and a cross-linking agent) to form a cross-linked sodium polyacrylate, i.e., a super absorbent polymer. Preferably, the super absorbent polymer is then surface-crosslinked using a surface cross-linking agent. In this case, the super absorbent polymer before surface cross-linking can also be referred to as a semi-finished product.

[0131] Here, in the method (1), by mixing the above-mentioned regenerated super absorbent polymer that has not been reactivated (neutralized) into at least one of acrylic acid (raw material) and a cross-linked polyacrylic acid (semi-finished product), a super absorbent polymer can be produced using the regenerated super absorbent polymer. Alternatively, by mixing the above-mentioned regenerated super absorbent polymer that has been reactivated (neutralized) into a super absorbent polymer (semi-finished product) before surface cross-linking, a super absorbent polymer can be produced using the regenerated super absorbent polymer.

[0132] In the method (2), by mixing the above-mentioned regenerated super absorbent polymer that has not been reactivated (neutralized) into acrylic acid (raw material), a super absorbent polymer can be produced using the regenerated super absorbent polymer. Alternatively, by mixing the above-mentioned regenerated super absorbent polymer that has been reactivated (neutralized) into sodium acrylate (semi-finished product) or a super absorbent polymer (semi-finished product) before surface cross-linking, a super absorbent polymer can be produced using the regenerated super absorbent polymer.

[0133] Alternatively, by mixing the above-mentioned unreactivated (neutralized) regenerated super absorbent polymer and / or reactivated (neutralized) regenerated super absorbent polymer as the fine powder, fine particles or water-absorbent resin (raw material) of Patent Documents 2 to 4, a super absorbent polymer can be produced using the regenerated super absorbent polymer.

[0134] In these methods for producing superabsorbent polymer using recycled superabsorbent polymer, recycled superabsorbent polymer derived from used sanitary products (e.g., absorbent articles) is mixed into at least one of a raw material and a semi-finished product, and then polymerized or cross-linked in aqueous solution to produce the superabsorbent polymer. In this case, the recycled superabsorbent polymer is produced using the aforementioned method for producing recycled superabsorbent polymer, resulting in a high-purity, high-quality superabsorbent polymer. Therefore, even with this recycled superabsorbent polymer, this method can produce high-purity, high-quality superabsorbent polymer.

[0135] It should be noted that in the method for producing a recycled superabsorbent polymer from a used sanitary product (for example, an absorbent article) of this embodiment, there is no particular limitation on the method for removing the superabsorbent polymer from the used sanitary product, and any method can be used. Figure 3 and Figure 4 The following method is explained.

[0136] Figure 3 This is a block diagram showing an example of a system 100 for separating materials from used absorbent articles according to this embodiment. The system 100 includes a separation device 10 for separating membranes, nonwoven fabrics, etc., super absorbent polymers (SAP), and pulp fibers from used absorbent articles. Figure 4 This is a flow chart showing an example of a method for separating materials from used absorbent articles according to this embodiment. The method includes a separation step S10 of separating membranes, nonwoven fabrics, etc., super absorbent polymers (SAP), and pulp fibers from used absorbent articles. Figure 3 As shown, the separation device 10 includes a bag breaking device 11 to a fourth separation device 20, and correspondingly, as shown in FIG. Figure 4 As shown, the separation step S10 includes a hole-making step S11 to a fourth separation step S20. Each step will be described in detail below.

[0137] It should be noted that in this embodiment, used absorbent articles are collected and retrieved from an external source for reuse (recycling). In this case, multiple used absorbent articles are sealed in a collection bag to prevent excrement, fungi, and odors from leaking to the outside. Each used absorbent article in the collection bag is collected, for example, with the surface sheet containing excrement facing inward, primarily in a rolled or folded state, to prevent excrement and fungi from being exposed and odors from spreading to the surrounding area.

[0138] The hole-forming step S11 is performed by the bag-breaking device 11. The bag-breaking device 11 includes a solution tank storing an inactivating aqueous solution containing an inactivating agent and a bag-breaking blade that rotates within the solution tank. The bag-breaking device 11 uses the bag-breaking blade to create holes in the inactivating aqueous solution in a collection bag placed within the solution tank. This creates a mixed solution 91 consisting of the collection bag, into which the inactivating aqueous solution has been introduced through the holes, and the inactivating aqueous solution. The inactivating aqueous solution inactivates the superabsorbent polymer of the used absorbent article within the collection bag. The following description uses an acidic aqueous solution as the inactivating aqueous solution as an example.

[0139] The crushing step S12 is performed by the crushing device 12. The crushing device 12 comprises a double-shaft crusher (e.g., a double-shaft rotary crusher). The crushing device 12 crushes the collection bag of the used absorbent article, which contains the mixed liquid 91, along with the collection bag. This produces a mixed liquid 92 containing an acidic aqueous solution and crushed material from the collection bag containing the used absorbent article. This material inactivates substantially all of the superabsorbent polymer in the used absorbent article. The crushed material includes pulp fibers, superabsorbent polymer, and other materials (e.g., films, nonwoven fabrics, and collection bags).

[0140] The first separation step S13 is performed by the first separation device 13. The first separation device 13 includes a pulp separator with an agitation separation tank that functions as both a washing tank and a screening tank. While stirring the mixed liquid 92 to remove excrement and other substances from the crushed material, the first separation device 13 separates the pulp fibers, superabsorbent polymer, excrement, and acidic aqueous solution from the mixed liquid 92. This generates a mixed liquid 93 containing pulp fibers, superabsorbent polymer, excrement, and acidic aqueous solution. Furthermore, the device recovers used absorbent article membranes, nonwoven fabrics, and materials for collection bags.

[0141] It should be noted that the pore-forming step S11 and the crushing step S12 (bag-breaking device 11 and crushing device 12) treat the used absorbent article in an inactivation aqueous solution to inactivate the superabsorbent polymer, and therefore can be considered the inactivation step S30 (inactivation device 30). Alternatively, the crushing device 12 can crush the used absorbent article in a gas (for example, air) along with the collection bag, rather than in an inactivation aqueous solution. In this case, the bag-breaking device 11 is unnecessary. After crushing, the crushed product from the crushing device 12 and the inactivation aqueous solution are supplied to the first separation device 13 (first separation step S13) to inactivate the superabsorbent polymer. In this case, the first separation device 13 (first separation step S13) can be considered the inactivation step S30 (inactivation device 30).

[0142] The first dust removal step S14 is performed by the first dust removal device 14. The first dust removal device 14 includes a screen separator that uses a screen to separate the mixed liquid 93 into pulp fibers, superabsorbent polymer, excrement, and other materials (foreign matter) in the acidic aqueous solution. This produces a mixed liquid 94 containing pulp fibers, superabsorbent polymer, excrement, and the acidic aqueous solution with a reduced amount of foreign matter, while removing other materials.

[0143] The second dust removal step S15 is performed by the second dust removal device 15. The second dust removal device 15 includes a screen separator that uses a finer screen than the first dust removal device 14 to separate the mixed liquid 94 into pulp fibers, superabsorbent polymer, excrement, and other materials (small foreign matter) in the acidic aqueous solution. This further reduces the amount of foreign matter in the mixed liquid 95 containing pulp fibers, superabsorbent polymer, excrement, and the acidic aqueous solution, and further removes other materials.

[0144] The third dust removal step S16 is performed by the third dust removal device 16. The third dust removal device 16 includes a cyclone separator that separates the mixed liquid 95 into pulp fibers, super absorbent polymer, excrement, and other materials (heavier foreign matter) in the acidic aqueous solution through centrifugal separation. This produces a mixed liquid 96 containing pulp fibers, super absorbent polymer, excrement, and the acidic aqueous solution, with a further reduced amount of foreign matter, while removing other heavier foreign matter.

[0145] Note that, depending on the state of the mixed liquid 92 and the like (eg, the amount and size of foreign matter), at least one of the first to third dust removal devices 14 to 16 may be omitted.

[0146] Second separation step S17 is performed by second separation device 17. Second separation device 17 includes a drum screen separator that uses a drum screen to separate mixed liquid 96 into superabsorbent polymer and pulp fibers in an acidic aqueous solution. This generates mixed liquid 97 containing superabsorbent polymer, excrement, and an acidic aqueous solution, from which the pulp fibers are removed as mixture 98.

[0147] The third separation step S18 is performed by the third separation device 18. The third separation device 18 is equipped with an inclined screen that uses the screen to separate the mixed liquid 97 into a solid containing superabsorbent polymer and a liquid containing excrement and an acidic aqueous solution. This generates superabsorbent polymer (SAP) while removing the acidic aqueous solution containing excrement.

[0148] The oxidant treatment step S19 is performed by the oxidant treatment device 19. The oxidant treatment device 19 includes a treatment tank for storing an aqueous oxidant solution and an oxidant supply device for supplying the oxidant into the treatment tank. The oxidant treatment device 19 puts the mixture 98 into the treatment tank from the top or bottom of the treatment tank and mixes it with the aqueous oxidant solution in the treatment tank. Then, the super absorbent polymer contained in the pulp fiber is decomposed in the aqueous oxidant solution by the oxidant supplied from the bottom of the treatment tank by the oxidant supply device, making it soluble in the aqueous oxidant solution. Thus, a mixed solution 99 is generated, which contains pulp fibers from which the super absorbent polymer has been removed and an oxidant aqueous solution containing decomposition products of the super absorbent polymer. The oxidant is an oxidant that can decompose the super absorbent polymer, and an example thereof is ozone. Ozone has high bactericidal and bleaching power and is therefore preferred.

[0149] The ozone concentration in the aqueous oxidant solution is preferably 1 mass ppm to 50 mass ppm. If the concentration is too low, the super absorbent polymer cannot be completely solubilized, and the super absorbent polymer may remain in the pulp fiber. If the concentration is too high, the pulp fiber may be damaged. The higher the ozone concentration in the aqueous oxidant solution, the shorter the ozone treatment time. The lower the ozone concentration, the longer the ozone treatment time, typically 5 minutes to 120 minutes. The product of the ozone concentration (ppm) and the treatment time (minutes) in the aqueous oxidant solution (hereinafter also referred to as "CT value") is preferably 100 ppm·minute to 6000 ppm·minute. If the CT value is too small, the super absorbent polymer cannot be completely solubilized. If the CT value is too large, the pulp fiber may be damaged.

[0150] The fourth separation step S20 is performed by the fourth separation device 20. The fourth separation device 20 includes a screen separator that uses a screen to separate the mixed liquid 99 into pulp fibers and an aqueous oxidant solution. This generates pulp fibers and removes the aqueous oxidant solution containing decomposition products of the highly absorbent polymer.

[0151] <Ash content>

[0152] It should be noted that the method for measuring ash content is as follows. Ash content refers to the amount of inorganic matter or non-combustible residue remaining after organic matter is ashed. Ash content is measured in accordance with "5. Ash content test method" of "2. General test method" of the standard for materials of physiological treatment products. That is, ash content is measured as follows. Preheat a platinum, quartz or magnetic crucible at 500°C to 550°C for 1 hour, and after cooling, accurately weigh its mass. Collect 2g to 4g of sample, put it into the crucible, and accurately weigh its mass. Remove the lid of the crucible or move it as needed, initially perform weak heating, gradually increase the temperature, and ashed at 500°C to 550°C for more than 4 hours until no carbide remains. After cooling, accurately weigh its mass. Ash the residue again to a constant weight, and after cooling, accurately weigh its mass as the amount of ash content (mass %).

[0153] <Detection of common bacteria>

[0154] Common live bacteria can be detected by the pour-over culture method. Examples of common live bacteria include Bacillus cereus, Bacillus subtilis, Staphylococcus aureus, Pseudomonas aeruginosa, glucose non-fermenting Bacillus, and Aeromonas. Since these bacteria cannot be detected by the pour-over culture method, the regenerated superabsorbent polymer is less likely to cause bacteremia, endocarditis, respiratory infections, food poisoning, eye infections, and the like, allowing users to use the regenerated superabsorbent polymer with confidence.

[0155] The pour culture method was carried out as follows.

[0156] (1) 500 g of an aqueous dispersion of a regenerated superabsorbent polymer having a solid content concentration of 5.0% by mass was prepared in a 1-liter beaker.

[0157] When the regenerated super absorbent polymer is in a dry state, the aqueous dispersion can be formed by mixing the regenerated super absorbent polymer (25.0 g in terms of solid content) with deionized water (a total amount of 500.0 g). Furthermore, when the regenerated super absorbent polymer is in an aqueous solution (for example, when the regenerated super absorbent polymer is recovered as an aqueous solution in a method for producing a regenerated super absorbent polymer) and the solid content concentration of the regenerated super absorbent polymer is 5.0% by mass or higher, deionized water or the like can be added to the aqueous solution to prepare an aqueous dispersion having a solid content concentration of the regenerated super absorbent polymer of 5.0% by mass.

[0158] Furthermore, when the regenerated super absorbent polymer is present in an aqueous solution and the solids concentration of the regenerated super absorbent polymer is less than 5.0% by mass, the solids concentration of the regenerated super absorbent polymer can be adjusted to 5.0% by mass by filtration, or the aqueous solution itself can be used as an aqueous dispersion and the inoculum size of the serial dilution sample described below can be increased (for example, when the solids concentration of the regenerated super absorbent polymer is 2.5% by mass, the inoculum size can be doubled).

[0159] (2) Using an overhead stirrer, stir the aqueous dispersion at 300 rpm for 15 minutes.

[0160] (3) 50 mL of the aqueous dispersion stirred with an overhead stirrer was placed in a sterile bag with a filter (manufactured by LMS Co., Ltd., a sterile bag with a filter for homogenizer) and stirred for 5 minutes.

[0161] (4) The filtered aqueous dispersion was dispensed into sterilized test tubes using a sterile bag with a filter, and diluted 10-fold to 10 -9 , dispense into sterilized test tubes to prepare serial dilution samples.

[0162] (5) Determine the number of viable bacteria by pouring culture method.

[0163] Specifically, 1 mL of the serially diluted sample and a standard agar medium (Nippon Pharmaceutical Co., Ltd., 396-00175SCD agar medium "DAIGO" for general viability testing, 15 to 20 g) were placed in a petri dish and poured and cultured at 35°C for 48 hours.

[0164] (6) As the normal viable bacterial count, the number of colonies growing after cultivation was counted.

[0165] It should be noted that for serial dilutions from 10 to 10 -9 For all serial dilution samples, if the colony count is zero, the target bacteria are judged as "undetectable", that is, the number of normal viable bacteria detected by the pour culture method is below the detection limit. In other words, the normal viable bacteria count is 0 cfu / g.

[0166] (7) When colonies of intestinal bacteria or common live bacteria are formed after culture, the type of bacteria can be identified. Identification can be performed by biochemical characterization.

[0167] Example

[0168] Hereinafter, the present invention will be described based on Examples, but the present invention is not limited to the Examples.

[0169] (1) Oxidant treatment

[0170] The effect of the oxidant treatment was evaluated.

[0171] (a) Sample

[0172] Superabsorbent polymer separated from used diapers (corresponding to separation step S10) was used. This superabsorbent polymer is primarily a crosslinked sodium polyacrylate. The separated superabsorbent polymer was inactivated using an acidic aqueous solution (1% citric acid concentration) (corresponding to inactivation step S30, which corresponds to pore-forming step S11 plus crushing step S12).

[0173] (b) Evaluation method

[0174] (i) In the samples of the examples, 1 g of superabsorbent polymer (inactivated) separated from used diapers was immersed in ozone water (ozone concentration: 1 ppm) having a mass 10 times that of the superabsorbent polymer under acidic conditions (citric acid concentration: 1% by mass) for 2 minutes (corresponding to the oxidizing agent treatment step S31). On the other hand, in the samples of the comparative examples, 1 g of superabsorbent polymer (inactivated) separated from used diapers was not immersed in ozone water.

[0175] (ii) Subsequently, the superabsorbent polymers of the example samples and the comparative example samples were treated in an aqueous solution containing methanol and sodium hydroxide (corresponding to the citric acid removal step S32). This removed citric acid from each superabsorbent polymer, while simultaneously dehydrating and reactivating (neutralizing) the superabsorbent polymers.

[0176] (iii) Thereafter, the super absorbent polymers of the samples of Examples and the super absorbent polymers of the samples of Comparative Examples were both dried at 105° C. for 60 minutes (corresponding to the drying step S33 ).

[0177] (iv) The ash content and the number of common bacteria in each of the obtained super absorbent polymer samples of the examples and the super absorbent polymer samples of the comparative examples were measured.

[0178] (c) Evaluation results

[0179] The ash content of the superabsorbent polymer in the Example sample was 30.3% by mass, approximately 10% lower than the ash content of 40.9% in the Comparative Example sample. This is roughly equivalent to the ash content (approximately 30% by mass) found in a typical superabsorbent polymer (crosslinked sodium polyacrylate) before production. Therefore, it can be seen that the surface of the superabsorbent polymer in the Example sample contains almost no ash (less than 1% by mass). Specifically, considering that the surface ash content of the superabsorbent polymer includes inorganic substances derived from feces and silica originally attached to the surface, it can be seen that the surface of the superabsorbent polymer in the Example sample contains almost no inorganic substances or silica derived from feces. In other words, the surface ash content of the superabsorbent polymer in the Example sample can be said to be less than 1% by mass. Furthermore, the number of common bacteria in the superabsorbent polymer of the example sample was below the detection limit (0 (zero) cfu / g), which is an extremely low value compared to the 3400 cfu / g of common bacteria in the superabsorbent polymer of the comparative example sample. This indicates that the purity of the superabsorbent polymer in the example sample, i.e., the regenerated superabsorbent polymer, is very high.

[0180] [Table 1]

[0181] Ash content (mass %) Common bacteria count (cfu / g) Example 30.3 0 Comparative Example 40.9 3400

[0182] The absorbent article of the present invention is not limited to the above-described embodiments, and can be appropriately combined, modified, etc. within a scope not departing from the purpose and spirit of the present invention.

[0183] Description of Reference Numerals

[0184] S30 inactivation process

[0185] S31 Oxidant Treatment Process

[0186] S33 Drying process

Claims

1. A method for producing a recycled super absorbent polymer as a raw material for producing a super absorbent polymer from a used super absorbent polymer taken out of a used sanitary product, wherein: The method includes: an inactivation step of inactivating the used superabsorbent polymer taken out from the used sanitary product using an acidic solution; an oxidant treatment step of treating the used superabsorbent polymer after being inactivated by the acidic solution with an oxidant under acidic conditions; and a drying step of drying the used super absorbent polymer treated with the oxidizing agent to produce the regenerated super absorbent polymer; The method further includes a foreign matter separation step after the drying step: separating foreign matter from the regenerated super absorbent polymer.

2. A method for producing a recycled super absorbent polymer as a raw material for producing a super absorbent polymer from a used super absorbent polymer taken out of a used sanitary product, wherein: The method includes: an inactivation step of inactivating the used superabsorbent polymer taken out from the used sanitary product using an acidic solution; an oxidant treatment step of treating the used superabsorbent polymer after being inactivated by the acidic solution with an oxidant under acidic conditions; and a drying step of drying the used super absorbent polymer treated with the oxidizing agent to produce the regenerated super absorbent polymer; The used superabsorbent polymer removed from the used sanitary article is inactivated when removed from the used sanitary article.

3. The method according to claim 1 or 2, wherein: In the oxidizing agent treatment step, the oxidizing agent includes water containing ozone or a hydrogen peroxide solution.

4. A method for producing a super absorbent polymer using a recycled super absorbent polymer recycled from a used super absorbent polymer derived from used sanitary products as a raw material for producing the super absorbent polymer, wherein: The method comprises the following steps: The regenerated super absorbent polymer is produced by the method according to claim 1 or 2; and In the aqueous solution polymerization or cross-linking for producing the super absorbent polymer, the regenerated super absorbent polymer is mixed into at least one of the raw material and the semi-finished product for production, and the aqueous solution polymerization or cross-linking is performed.

5. A recycled super absorbent polymer produced by the method for producing a recycled super absorbent polymer as a raw material for producing a super absorbent polymer from a used super absorbent polymer taken out from a used sanitary product according to claim 1 or 2, wherein: The ash content was 35% by mass or less, and the number of viable cells detected by the pour culture method was below the detection limit.

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