Method for producing recycled super absorbent polymer, method for producing super absorbent polymer using recycled super absorbent polymer, and recycled super absorbent polymer

Abstract

Provided is a method for producing a recycled super absorbent polymer, which can effectively utilize the used super absorbent polymer derived from used sanitary products in products using the super absorbent polymer as a raw material. The method is a method for producing a recycled super absorbent polymer as a raw material for producing the super absorbent polymer from the used super absorbent polymer derived from used sanitary products. The method comprises: an inactivation step (S30) of inactivating the used super absorbent polymer using an acidic solution; an oxidant treatment step (S31) of treating the used super absorbent polymer inactivated by the acidic solution with an oxidant under acidic conditions; and a drying step (S33) of drying the used super absorbent polymer treated with the oxidant to produce a recycled super absorbent polymer.

Description

Technical Field

[0001] 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 a recycled super absorbent polymer. Background Art

[0002] A technique for regenerating used super absorbent polymers from used sanitary products (for example, absorbent articles) is known. For example, Patent Document 1 discloses a method for regenerating used super absorbent polymers. The regeneration method includes the steps of: treating the used super absorbent polymer with a polyvalent metal salt aqueous solution; and treating the super absorbent polymer treated with the polyvalent metal salt aqueous solution with an alkali metal salt aqueous solution.

[0003] On the other hand, it is known that a technology is used to recycle unused super absorbent polymers as part of the raw materials of super absorbent polymers. For example, Patent Document 2 discloses a method for producing resin particles. In this production method, in suspension polymerization, an oily liquid containing resin b is dispersed in an aqueous dispersion of resin particles A composed of resin a, and resin particles B composed of resin b are formed in the aqueous dispersion, and an aqueous dispersion X1 of resin particles C with resin particles A attached to the surface of resin particles B is obtained. At this time, the fine powder removed in the classification process when producing other batches of resin particles C is dispersed in the aqueous dispersion. Patent Document 3 discloses a method for producing a water absorbent. In this production method, a powder containing fine particles of a water absorbent resin is pressurized in the presence of water, dried, and pulverized to produce a water absorbent with an average particle size of 200 μm 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 salts thereof is polymerized in a state where a water-absorbent resin is present 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. In addition, Non-Patent Document 1 discloses a method for producing a highly water-absorbent polymer of a polyacrylate salt system.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2013-198862

[0007] Patent Document 2: Japanese Patent Application Publication No. 2007-246676

[0008] Patent Document 3: Japanese Patent Application Laid-Open No. 10-204184

[0009] Patent Document 4: Japanese Patent Application Laid-Open No. 4-227705

[0010] Patent Document 5: Japanese Patent No. 5996226

[0011] Non-patent literature

[0012] Non-patent document 1: Tadao Shimomura, "Super absorbent polymer", Surface (1991) 495-506 Summary of the invention

[0013] Problem that the invention aims to solve

[0014] In the recycling of used sanitary products (for example, absorbent articles), many used sanitary products are collected, but the types and manufacturing companies of the many used sanitary products collected are various. Therefore, if many super absorbent polymers from used sanitary products are collectively regenerated by the method of Patent Document 1, the super absorbent polymer after regeneration becomes a mixture of various super absorbent polymers. That is, it is difficult to make the performance of the super absorbent polymer after regeneration uniform. Therefore, if the super absorbent polymer after regeneration is directly used in sanitary products using super absorbent polymers as absorbent materials, it may be difficult to make the absorption performance of the sanitary products uniform.

[0015] Therefore, the inventors have studied for the first time this time, for example, in the production methods of super absorbent polymers of Patent Documents 2 to 4 and Non-Patent Document 1, using recycled super absorbent polymers as raw materials (fine powders, fine particles or absorbent resins) of Patent Documents 2 to 4, raw materials or semi-finished products of Non-Patent Document 1. In the production methods of Patent Documents 2 to 4 and Non-Patent Document 1, it is considered to use unused raw materials with very few impurities (for example: super absorbent polymers). On the other hand, recycled super absorbent polymers from used sanitary products sometimes contain unique impurities (for example: odorous substances, colored substances, and miscellaneous bacteria). Therefore, if recycled super absorbent polymers are used in the production methods of Patent Documents 2 to 4 and Non-Patent Document 1 using unused super absorbent polymers with very few impurities, it may not be possible to form high-purity and high-quality super absorbent polymers. In this way, in sanitary products using super absorbent polymers, it may not be possible to utilize the super absorbent polymers produced by the production methods of Patent Documents 2 to 4 and Non-Patent Document 1 using recycled super absorbent polymers.

[0016] An object of the present invention is 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 a used super absorbent polymer derived from used sanitary products in a product using a super absorbent polymer as a raw material.

[0017] Solutions for solving problems

[0018] The present invention is 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 a used sanitary product, wherein the method comprises: an inactivation step of inactivating the used super absorbent polymer using an acidic solution; an oxidant treatment step of treating the used super absorbent polymer inactivated by the acidic solution using an oxidant under acidic conditions; and a drying step of drying the used super absorbent polymer treated by the oxidant to produce the recycled super absorbent polymer.

[0019] The present invention is a method for producing a super absorbent polymer using a recycled super absorbent polymer recycled from a used super absorbent polymer derived from a used sanitary product as a raw material for producing the super absorbent polymer, wherein 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 the semi-finished product for producing the super absorbent polymer in aqueous solution polymerization or cross-linking for producing the super absorbent polymer, and performing the aqueous solution polymerization or cross-linking.

[0020] The present invention provides a recycled super absorbent polymer which is recycled from a used super absorbent polymer derived from a used sanitary product and is used as a raw material for producing a super absorbent polymer, wherein the ash content is 35% by mass or less and the number of viable common bacteria detected by a pouring culture method is below the detection limit.

[0021] Effects of the Invention

[0022] According to the present invention, there can be provided 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 a used super absorbent polymer derived from used sanitary products in a product using a super absorbent polymer as a raw material. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] 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.

[0024] Figure 2 This is a flowchart showing an example of the method for producing a regenerated super absorbent polymer according to the embodiment.

[0025] Figure 3 This is a block diagram showing a configuration example of an apparatus for taking out a used super absorbent polymer according to an embodiment.

[0026] 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

[0027] This embodiment relates to the following aspects.

[0028] [Form 1]

[0029] 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 a used sanitary product, wherein the method comprises: an inactivation step of inactivating the used super absorbent polymer using an acidic solution; an oxidant treatment step of treating the used super absorbent polymer 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 oxidant to produce the recycled super absorbent polymer.

[0030] This method inactivates the used super absorbent polymer from used sanitary products (for example, absorbent articles) with an acidic solution (inactivation step), and then treats it with an oxidant under acidic conditions (oxidant treatment step). That is, this method dehydrates the used super absorbent polymer with an acidic solution to significantly reduce its volume, and then maintains its dehydrated state, that is, the state of reduced volume, under acidic conditions, and deodorizes, decolorizes, and sterilizes the used super absorbent polymer with an oxidant. In this way, by reducing the volume of the used super absorbent polymer, impurities (for example, odorous substances, colored substances, and miscellaneous bacteria) contained in the regenerated super absorbent polymer from the used sanitary products can be easily removed with an oxidant, and a regenerated super absorbent polymer with high purity can be obtained. By using such a high-purity regenerated super absorbent polymer as part of the raw materials in the method of forming a predetermined super absorbent polymer, a high-purity, high-quality super absorbent polymer can be formed. As a method for forming a predetermined super absorbent polymer, there are methods such as those in Patent Documents 2 to 4, and methods for producing a super absorbent polymer using a recycled super absorbent polymer described later, that is, a method for recycling unused super absorbent polymer as a raw material or a part of a semi-finished product of a super absorbent polymer. In this way, used super absorbent polymers from used sanitary products can be effectively used in products using super absorbent polymers.

[0031] [Form 2]

[0032] 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.

[0033] In this method, foreign matter (for example, pulp fibers) is separated from the dried regenerated super absorbent polymer. Therefore, compared with separation in a non-dried state such as separation in a solution, foreign matter can be separated from the regenerated super absorbent polymer more easily, so that a regenerated super absorbent polymer with higher purity can be obtained.

[0034] [Form 3]

[0035] 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.

[0036] In the present method, the used super absorbent polymer is inactivated by a solution containing citric acid (inactivation step). Therefore, not only is the used super absorbent polymer dehydrated and its volume greatly reduced, but also metals (for example, substances derived from the human body contained in excrement) can be easily removed by the chelating effect of citric acid. Moreover, in the present method, the citric acid of the used super absorbent polymer is removed after the inactivation step (citric acid removal step). Therefore, the chelating agent such as citric acid remaining in the regenerated super absorbent polymer can be greatly suppressed. Thus, a regenerated super absorbent polymer with higher purity can be obtained. In particular, since the chelating agent hardly remains in the regenerated super absorbent polymer, in the method of reusing the unused super absorbent polymer as a part of the raw material of the super absorbent polymer, when aqueous solution polymerization, crosslinking, etc. are performed (for example: Patent Documents 2 and 4), the chelating agent can be greatly suppressed from hindering the crosslinking reaction.

[0037] [Form 4]

[0038] 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.

[0039] In the present method, as a removal method for citric acid, the used super absorbent polymer 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. Thus, the used super absorbent polymer can be dehydrated with an acid or an organic solvent, and the citric acid on the surface of the used super absorbent polymer can be washed away. Therefore, the chelating agent such as citric acid can be more significantly suppressed from remaining in the regenerated super absorbent polymer.

[0040] [Form 5]

[0041] The method according to aspect 4, wherein the removal treatment step includes an acid step in which the used superabsorbent polymer is treated with the removal acid or an aqueous solution thereof during or after the oxidizing agent treatment step.

[0042] In the present method, the used super absorbent polymer is treated with a removal acid or an aqueous solution thereof during or after the oxidant treatment step. Thus, citric acid can be removed from the used super absorbent polymer and the used super absorbent polymer can be further dehydrated. Therefore, the subsequent drying step can be performed more easily, and a highly purified regenerated super absorbent polymer can be efficiently obtained.

[0043] [Form 6]

[0044] The method according to aspect 4 or 5, wherein the removing acid is an acid having no carboxyl group or an acid having one carboxyl group in one molecule.

[0045] In this method, the removal acid is an acid having no carboxyl group or an acid having one carboxyl group in one molecule. Therefore, in the removal treatment step, citric acid can be more reliably removed from the used super absorbent polymer and the used super absorbent polymer can be further dehydrated.

[0046] [Form 7]

[0047] 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.

[0048] In the present method, the used super absorbent polymer is treated with an aqueous solution of an organic solvent after the oxidizing agent treatment step and before the drying step. Thus, citric acid can be removed from the used super absorbent polymer and the used super absorbent polymer can be further dehydrated. Therefore, the subsequent drying step can be performed more easily, and thus, a regenerated super absorbent polymer with high purity can be obtained efficiently.

[0049] [Form 8]

[0050] 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.

[0051] In this method, the used super absorbent polymer is treated with an aqueous solution of an organic solvent containing an alkali metal ion supply source. Thus, citric acid can be removed from the used super absorbent polymer, and the used super absorbent polymer can be neutralized with alkali metal ions while being dehydrated. Therefore, for example, when producing a super absorbent polymer, a regenerated super absorbent polymer can be used as a neutralized high-purity raw material.

[0052] [Form 9]

[0053] 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 an acid dissociation constant of an acid group of the super absorbent polymer.

[0054] 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 super absorbent polymer. Therefore, the used super absorbent polymer can be more reliably neutralized with the alkali metal ions.

[0055] [Form 10]

[0056] The method according to any one of aspects 1 to 9, wherein in the oxidizing agent treatment step, the oxidizing agent includes water containing ozone or a hydrogen peroxide solution.

[0057] In this method, the used super absorbent polymer is treated with water containing ozone or hydrogen peroxide solution. As a result, impurities (for example, odorous substances, colored substances, and bacteria) contained in the regenerated super absorbent polymer derived from the used sanitary products can be removed more easily, that is, deodorization, decolorization, and sterilization can be performed more easily. As a result, a regenerated super absorbent polymer with higher purity can be obtained.

[0058] [Form 11]

[0059] A method for producing a super absorbent polymer using a recycled super absorbent polymer recycled from a used super absorbent polymer derived from a used sanitary product as a raw material for producing the super absorbent polymer, wherein the method comprises the steps of: producing the recycled super absorbent polymer by the method according to any one of forms 1 to 10; and mixing the recycled super absorbent polymer into at least one of the raw material and a semi-finished product for producing the super absorbent polymer in aqueous solution polymerization or cross-linking, and performing the aqueous solution polymerization or cross-linking.

[0060] In this method, a recycled super absorbent polymer derived from used sanitary products (for example, absorbent articles) is mixed into at least one of a raw material and a semi-finished product, and a super absorbent polymer is produced by aqueous solution polymerization or cross-linking. In this case, the recycled super absorbent polymer has a high purity because it is produced by the methods of the above-mentioned forms 1 to 10. Therefore, in this method, even if a recycled super absorbent polymer is used, a high-purity, high-quality super absorbent polymer can be formed.

[0061] [Form 12]

[0062] A recycled super absorbent polymer, which is recycled from a used super absorbent polymer derived from a used sanitary product and is used as a raw material for producing a super absorbent polymer, wherein the ash content is 35% by mass or less and the number of viable common bacteria detected by a pouring culture method is below the detection limit.

[0063] In the present regenerated super absorbent polymer derived from used sanitary products (for example, absorbent articles), the ash content is 35% by mass or less, and the number of common viable bacteria detected by the pouring culture method is below the detection limit. Here, it is considered that the amount of ash contained in the interior of a general super absorbent polymer that has not been used after production is about 30% by mass, and the amount of ash contained in the interior of the regenerated super absorbent polymer is also about 30% by mass. Therefore, on the surface of the regenerated super absorbent polymer, there is less unnecessary ash (less than 5% by mass), and in the regenerated super absorbent polymer, the number of common viable bacteria is extremely small (below the detection limit). As a result, the purity of the regenerated super absorbent polymer is very high. Therefore, by mixing such a regenerated super absorbent polymer into at least one of the raw material and the semi-finished product in the suspension polymerization, aqueous solution polymerization or crosslinking used for the production of the super absorbent polymer, a high-purity, high-quality super absorbent polymer can be formed.

[0064] Hereinafter, 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 a used sanitary product, a method for producing a super absorbent polymer using a recycled super absorbent polymer, and a recycled super absorbent polymer according to an embodiment will be described. In this specification, sanitary products refer to articles that contribute to hygiene and are articles containing super absorbent 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 and retained liquid substances, and sanitary products that have not been used but have been discarded. Used super absorbent polymers refer to super absorbent polymers contained in used sanitary products. Recycled super absorbent polymers refer to super absorbent polymers regenerated from used super absorbent polymers derived from used sanitary products. In this embodiment, absorbent articles are described as an example 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 water-absorbent polymer and may further contain pulp fibers.

[0065] First, the structure of the absorbent article is described. The absorbent article has a surface sheet, a back sheet, and an absorber disposed between the surface sheet and the back sheet. As an example of the size of the absorbent article, a length of about 15 cm to 100 cm and a width of 5 cm to 100 cm can be cited. It should be noted that the absorbent article may also include other components of a common absorbent article, such as a diffusion sheet, a leak-proof wall, a side sheet, etc.

[0066] As constituent components 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 components of the back sheet, for example, liquid-impermeable nonwoven fabrics, liquid-impermeable synthetic resin films, and composite sheets thereof can be cited. As constituent components of the diffusion sheet, for example, liquid-permeable nonwoven fabrics can be cited. As constituent components of the leak-proof wall and the side sheet, for example, liquid-impermeable nonwoven fabrics can be cited, and the leak-proof wall can also include elastic components such as rubber. As materials for nonwoven fabrics and synthetic resin films, there are no special restrictions as long as they can be used as absorbent articles, and 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), etc. can be cited. As materials for nonwoven fabrics, natural fibers such as cotton and rayon can also be used. In the present embodiment, an absorbent article in which a constituent member of a back sheet is a film and a constituent member of a top sheet is a nonwoven fabric will be described as an example.

[0067] As constituent members of the absorbent body, absorbent materials, namely pulp fibers and super absorbent polymers can be cited. As pulp fibers, there are no particular restrictions as long as they can be used as absorbent articles, and examples thereof include cellulose fibers. As cellulose fibers, for example, wood pulp, cross-linked pulp, non-wood pulp, regenerated cellulose, semi-synthetic cellulose, etc. can be cited. As the size of pulp fibers, the average value of the major diameter of the fibers can be cited as, for example, several tens of μm, preferably 20 μm to 40 μm, and the average value of the fiber length can be cited as, for example, several mm, preferably 2 mm to 5 mm. As super absorbent polymers (Super Absorbent Polymer: SAP), there are no particular restrictions as long as they can be used as absorbent articles, and examples thereof include polyacrylate-based, polysulfonate-based, and maleic anhydride-based absorbent polymers. As the size of super absorbent polymers (when dry), the average value of the particle size can be cited as, for example, several hundred μm, preferably 200 μm to 500 μm. The absorbent body can also include a core wrap formed by a liquid-permeable sheet.

[0068] One surface and the other surface of the absorbent 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 in a manner that surrounds the absorbent is joined to the portion (peripheral portion) of the back sheet that extends to the outside of the absorbent in a manner that surrounds the absorbent by means of an adhesive. Thus, the absorbent 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 for example, a hot-melt adhesive can be cited. As a hot-melt adhesive, for example, a rubber-based main body such as styrene-ethylene-butadiene-styrene, styrene-butadiene-styrene, styrene-isoprene-styrene, or a pressure-sensitive adhesive or a heat-sensitive adhesive of an olefin-based main body such as polyethylene can be cited.

[0069] 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 a used sanitary product according to an embodiment will be described.

[0070] 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 the method for producing a regenerated super absorbent polymer according to the embodiment.

[0071] The method for producing a regenerated super absorbent polymer includes an inactivation step S30, an oxidant treatment step S31, and a drying step S33, but may further include a foreign matter separation step S34. When citric acid is used in the inactivation step S30, etc., a citric acid removal step S32 may be further included. In this manufacturing method, the citric acid removal step S32 may also include a removal treatment step S40. On the other hand, the system 1 used in the method for producing a regenerated super absorbent polymer includes an inactivation device 30, an oxidant treatment device 31, and a drying device 33, but may further include a foreign matter separation device 34. When citric acid is used in the inactivation device 30, etc., a citric acid removal device 32 may also be provided. In this system 1, it is preferred that the citric acid removal device 32 may also include a removal treatment device 40. The following describes each step in detail.

[0072] The deactivation step S30 is performed by the deactivation device 30. The deactivation step S30 deactivates the used super absorbent polymer using an acidic solution. In this embodiment, the used super absorbent polymer taken out from the used absorbent article is immersed in an acidic aqueous solution as a deactivator to form the deactivated super absorbent polymer 300.

[0073] The acid in the acidic aqueous solution is not particularly limited, and examples thereof include inorganic acids and organic acids. If the super absorbent polymer is inactivated using an acid, it is less likely that ash will remain in the super absorbent polymer and the pulp fiber, compared with the case where the super absorbent polymer is inactivated using lime, calcium chloride, etc. As the inorganic acid, for example, sulfuric acid, hydrochloric acid, and nitric acid can be listed, but sulfuric acid is preferred from the perspective of not containing chlorine and from the perspective of cost. As the organic acid, for example, carboxylic acids having multiple carboxyl groups in one molecule (for example, citric acid, tartaric acid, malic acid, succinic acid, oxalic acid), carboxylic acids having one carboxyl group in one molecule (for example, gluconic acid, valeric acid, butyric acid, propionic acid, glycolic acid, acetic acid, formic acid), sulfonic acids (for example, methanesulfonic acid, trifluoromethanesulfonic acid, benzenesulfonic acid, p-toluenesulfonic acid), etc. can be listed. As an organic acid, it is preferred to have a plurality of carboxyl groups, and more preferably citric acid, from the viewpoint of being easy to form a chelate complex with a divalent or higher metal (for example, calcium) contained in excrement and not allowing ash to remain in the super absorbent polymer and pulp fiber. The citric acid concentration of the acidic aqueous solution is not particularly limited, and for example, 0.5% to 4% by mass can be cited. In the present embodiment, citric acid is used as the acid in the acidic aqueous solution.

[0074] The acidic aqueous solution may be acidic, but preferably has a predetermined pH. As the upper limit of the predetermined pH, it is preferably 4.0, more preferably 3.5, and further preferably 3.0. If the predetermined pH is too high, the inactivation of the superabsorbent polymer cannot be fully carried out, there is a tendency that the discharge of the excrement retained by the superabsorbent polymer becomes insufficient, and there is also a tendency that it is difficult to separate from pulp fibers, etc. On the other hand, as the lower limit of the predetermined pH, it 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 that the reaction of the inactivated superabsorbent polymer with the aqueous solution containing the alkali metal ion supply source takes time. In addition, if the predetermined pH is too low, when recycled pulp fibers are regenerated in addition to the superabsorbent polymer derived 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 super absorbent polymer, the above-mentioned predetermined pH is preferably satisfied at the end of the inactivation step S30. This is from the viewpoint of continuously inactivating the super absorbent polymer.

[0075] As the inactivation device 30 for performing the inactivation step S30, the specific structure is not particularly limited as long as the super absorbent polymer can be immersed in the acidic aqueous solution. The inactivation device 30, for example, has a tank that can be configured to contain the material containing the super absorbent polymer and can store the acidic aqueous solution. In the inactivation step S30, the material containing the super absorbent polymer can be added after the acidic aqueous solution is stored in the tank, or the acidic aqueous solution can be added after the material containing the super absorbent polymer is placed in the tank. The material containing the super absorbent polymer is immersed in the acidic aqueous solution to cause an inactivation reaction.

[0076] In the inactivation step S30, although it also depends on the temperature, in order to uniformly react, the material containing the super absorbent polymer (for example, a used absorbent article) is stirred for about 5 minutes to 60 minutes in a tank containing an acidic aqueous solution, thereby inactivating the super absorbent polymer. In the inactivation step S30, the temperature of the acidic aqueous solution 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 further preferably 70°C to 90°C. As a result, it is easy to sterilize bacteria contained in the acidic aqueous solution and derived from excrement, etc. using the acid in the acidic aqueous solution.

[0077] At the end of the inactivation step S30 , the super absorbent polymer 300 is not completely dehydrated but has absorbed a small amount of water (for example, has absorbed about 10 times the volume of the super absorbent polymer before water absorption), and is in a gel state.

[0078] It should be noted that, when the material to be immersed in the acidic aqueous solution includes other components in addition to the super absorbent polymer, such as pulp fibers, liquid-permeable sheets, liquid-impermeable sheets, etc., a separation step of separating and removing the components other than the super absorbent polymer may be performed before or after the inactivation step S30. The separation step may also be performed simultaneously with the inactivation step S30. As a case where the material to be immersed in the acidic aqueous solution includes other components in addition to the super absorbent polymer, for example, the material is a used absorbent article containing the super absorbent polymer. The separation step will be described later.

[0079] Thereafter, the inactivated super absorbent polymer is separated from the acidic aqueous solution by a sieve (or net) provided in the inactivation device 30 (solid-liquid separation), and then supplied as the super absorbent polymer 300 to the oxidant treatment step S31 (oxidant treatment device 31 ).

[0080] The oxidant treatment step S31 is performed by the oxidant treatment device 31. The oxidant treatment step S31 treats the super absorbent polymer 300 inactivated by the acidic solution with an oxidant under acidic conditions. As the oxidant, there is no particular limitation as long as it can remove impurities (for example, odorous substances, colored substances, and miscellaneous bacteria) attached to the surface of the super absorbent polymer 300, and examples thereof include ozone and hydrogen peroxide. When the oxidant is mixed in water and used as an aqueous solution, the concentration of the oxidant in the aqueous solution can be, for example, 0.5% by mass to 20% by mass. In this embodiment, the super absorbent polymer 300 after inactivation is contacted (immersed) with the aqueous solution of the oxidant, that is, ozone water containing a predetermined concentration of ozone for a predetermined time, to form a super absorbent polymer 301 from which impurities on the surface of the super absorbent polymer 300 are removed. In addition, the acidic condition can be acidic, and examples thereof include an acidic aqueous solution. As the acidic aqueous solution, from the viewpoint of maintaining the inactivated state of the super absorbent polymer 300 and the viewpoint that it is difficult to deactivate ozone when ozone is used, it is preferably an aqueous solution containing the inorganic acid and organic acid listed in the description of the above-mentioned acidic aqueous solution. Then, the pH of the acidic aqueous solution is preferably in a predetermined range, that is, at least 0.5 to 4.0, which is the same as the above-mentioned acidic aqueous solution, and more preferably 1.0 to 3.5. In this embodiment, as the acidic condition, an aqueous solution of citric acid is used. The citric acid concentration of the aqueous solution is not particularly limited, and can be, for example, 0.1% by mass to 5% by mass.

[0081] Bacteria and other organic matter from the excrement of used absorbent articles adhere to the surface of the super absorbent polymer 300 as foreign matter and dirt. If they 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 uniformly 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. That is, in the oxidant treatment step S31, impurities attached to the surface of the super absorbent polymer 300 are removed. In particular, sterilization based on ozone water is called bacteriolysis, and the cell wall (membrane) of the bacteria is destroyed by a chemical reaction between protein and ozone, and the components in the cell leak out, thereby killing the bacteria. Therefore, the sterilization effect is significantly improved, it is not easy to produce drug-resistant bacteria, and the possibility of bacteria re-reproduction and covering the surface after sterilization is extremely low.

[0082] At this time, if the super absorbent polymer is inactivated by an acidic aqueous solution (inactivation step S30), the inactivated super absorbent polymer does not absorb water. Therefore, in the inactivated state, it can be cleaned with ozone water with a low ozone concentration, so it is easy to remove bacteria and the like by cleaning, and bacteria and the like can be removed in a short time. In addition, odor components and pigment components are also decomposed by ozone, so it also has a deodorizing and decolorizing effect. It should be noted that if you want to treat the super absorbent polymer 300 with a gaseous substance, such as ozone gas, the super absorbent polymer 300 is in a block (gel-like) state, so the treatment will not spread throughout the inside of the block, and it is difficult to treat it uniformly as a whole. If you want to treat it with ultraviolet rays or radiation, the super absorbent polymer will decompose at a radiation dose that is strong enough for them to penetrate into the inside of the block. If you want to treat it by heating or high-pressure steam, the dirt of excrement will remain in the super absorbent polymer. If an attempt is made to treat the material with an aqueous sodium hypochlorite solution, chlorine will remain in the super absorbent polymer and the material cannot be used as a sanitary material.

[0083] The ozone concentration in the ozone water is not particularly limited as long as it is a concentration that can remove bacteria, other organic matter, etc. attached to the surface of the super absorbent polymer, and 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 is difficult to remove bacteria, etc., and if the concentration is too high, the super absorbent polymer may begin to decompose. The contact time of the ozone water and the super absorbent polymer is not particularly limited as long as it is a time that can remove bacteria, other organic matter, etc. 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) and the contact time (minutes) in the ozone water (hereinafter also referred to as "CT value") is preferably 0.05ppm·minute to 20ppm·minute, and more preferably 0.08ppm·minute to 10ppm·minute. If the CT value is too small, it is difficult to sterilize, and if the CT value is too large, the super absorbent polymer may be decomposed. In the treatment using ozone water, by removing impurities attached to the surface of the super absorbent polymer 300, it is possible to remove bacteria and the like while also bleaching. Examples of ozone generators that supply ozone to water include the ozone water exposure tester ED-OWX-2 manufactured by ECO Design Co., Ltd. and the ozone generator OS-25V manufactured by Mitsubishi Electric Corporation.

[0084] In the oxidant treatment step S31, the ozone water containing the super absorbent polymer 300 may be stirred in the tank in order to uniformly react. In the oxidant treatment step S31, the temperature of the ozone water is not particularly limited, and can be, for example, room temperature (25°C), preferably 10°C to 40°C. If the temperature of the ozone water is too high, ozone is likely to escape as a gas and is easily inactivated, and if the temperature is too low, the ozone treatment time is likely to be prolonged. Thus, the ozone in the ozone water can easily remove bacteria and organic matter attached to the surface of the super absorbent polymer 300.

[0085] As the oxidant treatment device 31 for performing the oxidant treatment step S31, as long as the inactivated super absorbent polymer 300 can be brought into contact (or immersed) with ozone water without destroying its shape, the specific structure is not particularly limited. As the oxidant treatment device 31, for example, a twin-screw pump (BQ type: manufactured by Fuhu Metal Industry Co., Ltd.) can be cited. The twin-screw pump is a positive displacement self-priming pump. The twin-screw pump comprises a housing and a double-axis screw arranged in a chamber in the housing and extending parallel to each other. Ozone water containing the super absorbent polymer 300 is supplied to the chamber, and after reaching the double-axis screw from the radial direction, it is extruded and discharged in the axial direction by the rotation of the double-axis screw. In the treatment stage of the oxidant treatment step S31, the super absorbent polymer 300 is in a gel state that absorbs water, although not much, as described above, so it is difficult to stir it in a manner that does not destroy the gel using a stirring blade, and it is difficult to react evenly. The twin screw pump has no stirring blades and hardly shears the inactivated super absorbent polymer 300, thereby being able to uniformly mix it with the ozone water without destroying the gel, which is preferred. In the oxidant treatment step S31, the inactivated super absorbent polymer 300 is mixed with the ozone water by the twin screw pump to react.

[0086] 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 the super absorbent polymer 301 to the citric acid removal step S32 (citric acid removal device 32 ).

[0087] The citric acid removal step S32 is performed by the citric acid removal device 32. When the acidic solution used in the inactivation step S30 is a solution containing citric acid, the citric acid removal step S32 removes citric acid from the used super absorbent polymer 301 after the inactivation step S30 and before the drying step S33. In the present embodiment, in the inactivation step S30, a solution containing citric acid is used as the acidic solution, so that not only the used super absorbent polymer is inactivated, but also the metal attached to the surface of the used super absorbent polymer can be easily removed by the chelating effect of citric acid. Moreover, in the citric acid removal step S32, the chelating agent such as citric acid can be greatly suppressed from remaining in the finally obtained regenerated super absorbent polymer.

[0088] Here, the citric acid removal step S32 includes a removal treatment step S40 (removal treatment device 40) of treating the used super absorbent polymer 301 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. In this way, the citric acid can be dissolved in a predetermined acid or organic solvent and removed from the used super absorbent polymer, and the used super absorbent polymer can be further dehydrated.

[0089] As the acid for removing the acid that does not form a chelate structure with the metal ion, for example, there can be listed the acid without a carboxyl group among the inorganic acids and organic acids listed in the description of the acidic aqueous solution above, or the acid having one (not multiple) carboxyl group in one molecule. In other words, the acid for removing can be said to be an acid without multiple carboxyl groups in one molecule among the inorganic acids and organic acids. Therefore, even if it is assumed that the acid for removing remains on the surface of the used super absorbent polymer, it will not function as a chelating agent.

[0090] 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 .

[0091] On the other hand, the organic solvent miscible with water is not particularly limited, but examples thereof include alcohol solvents (examples: methanol, ethanol, propanol and its isomers, butanol and its isomers), ketone solvents (examples: acetone, methyl ethyl ketone), nitrile solvents (examples: acetonitrile), etc. This allows the used super absorbent polymer to be further dehydrated and cleaned. In the present embodiment, methanol is used as an organic solvent miscible with water in the removal treatment step S40 of the citric acid removal step S32.

[0092] The organic solvent may contain other solvents such as water as long as it does not affect the removal of citric acid. The ratio of other solvents to the organic solvent is less than 50% by mass, preferably 10% by mass or less, and more preferably 0% by mass.

[0093] 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 .

[0094] 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 super absorbent polymer is treated with the above-mentioned predetermined removal acid, the above-mentioned predetermined organic solvent, or their aqueous solutions, the used super absorbent polymer after the treatment is further treated with an aqueous solution containing an alkali metal ion supply source capable of supplying alkali metal ions, i.e., reactivated (neutralized). In other words, the removal treatment step S40 may also include a reactivation step after the acid step or the organic solvent step.

[0095] In this case, the super absorbent polymer is deactivated by an acidic aqueous solution (deactivation step S30), and the deactivated super absorbent polymer is neutralized by an aqueous solution containing an alkali metal ion supply source (removal treatment step S40), thereby obtaining a reactivated (neutralized) super absorbent polymer. Therefore, it is not necessary to use polyvalent metal ions for deactivation, so that the occurrence of absorption obstruction due to residual ash can be suppressed.

[0096] Among them, as the alkali metal ion in the alkali metal ion supply source, lithium ion (Li ion), sodium ion (Na ion), potassium ion (K ion), and any combination thereof can be listed. As the alkali metal ion supply source, there is no particular limitation as long as it can supply the aforementioned alkali metal ions, and for example, alkali metal hydroxides, salts of alkali metal hydroxides and acids having an acid dissociation constant greater than that of the acid group of the highly absorbent polymer (hereinafter also referred to as "salts"), etc. can be listed. The acid dissociation constant can adopt the value recorded in the Electrochemical Handbook edited by the Electrochemical Society.

[0097] Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, and potassium hydroxide, and any combination thereof. On the other hand, examples of salts include acidic salts and alkaline salts. Examples of alkali metal hydroxides in salts include lithium hydroxide, sodium hydroxide, and potassium hydroxide, and any combination thereof. Examples of acids in salts include 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.

[0098] When an alkali metal hydroxide is used as the alkali metal ion supply source, the concentration of the hydroxide ion in 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.

[0099] Furthermore, as another embodiment, the citric acid removal step S32 may include a step of treating the used super absorbent polymer with an aqueous solution of an organic solvent containing an alkali metal ion supply source capable of supplying alkali metal ions when the removal treatment step S40 is an organic solvent step. In other words, the organic solvent step of the removal treatment step S40 may include a reactivation step of reactivating the used super absorbent polymer with an alkali metal ion supply source in addition to a dehydration and washing step of dehydrating and washing the used super absorbent polymer with an organic solvent or an aqueous solution thereof. In this way, by simultaneously performing the dehydration and washing step and the reactivation step, citric acid can be removed from the used super absorbent polymer, and the used super absorbent polymer can be neutralized (reactivated) with alkali metal ions while being dehydrated.

[0100] Therefore, in the case of a super absorbent polymer that needs to be reactivated (neutralized) as a regenerated super absorbent polymer, by simultaneously performing the dehydration and washing process and the reactivation process, the super absorbent polymer can be dehydrated without swelling due to water absorption. As a result, the volume of the super absorbent polymer to be reactivated can be reduced, so that the reactivation can be performed efficiently. In addition, the super absorbent polymer can be reactivated in a state where the gel strength is relatively strong, and the gel destruction caused by stirring during the reactivation process can be suppressed. In addition, it is not necessary to enlarge the equipment for reactivation and dehydration and washing, and the amount of organic solvent used can be reduced. Furthermore, by this dehydration, the moisture content near the surface of the super absorbent polymer can be made substantially constant, so that the surface of the super absorbent polymer can be dried relatively uniformly. As a result, the absorption performance of the regenerated super absorbent polymer can be made relatively uniform, and as a result, the relative decrease in absorption performance can be suppressed.

[0101] Furthermore, as another embodiment, the citric acid removal step S32 may include a dehydration and washing step of dehydrating and washing the used super absorbent polymer with an organic solvent or an aqueous solution thereof after the removal treatment step S40 is an acid step. Furthermore, as another embodiment, after the acid step as the removal treatment step S40 is performed and the reactivation step of reactivating (neutralizing) the used super absorbent polymer is performed, a dehydration and washing step of dehydrating and washing the used super absorbent polymer with an organic solvent or an aqueous solution thereof may be included. By including the dehydration and washing step, the subsequent drying step S33 can be performed more easily.

[0102] The temperature of the citric acid removal step S32 (removal treatment step S40) is a predetermined temperature, for example, room temperature (for example: 20°C) to 60°C, preferably 30°C to 50°C. If the temperature is low, the removal time tends to be longer, and if the temperature is high, the water absorption of the super absorbent polymer may be reduced due to dehydration condensation of the acid group of the super absorbent polymer. The time of the citric acid removal step S32 (removal treatment step S40) is a predetermined time, for example, 5 minutes to 300 minutes.

[0103] As a citric acid removal device 32 (removal treatment device 40) for performing the citric acid removal step S32 (removal treatment step S40), the specific structure is not particularly limited. As the citric acid removal device 32 (removal treatment step S40), for example, a mixer (standard mixer NS-PS: manufactured by Tatechs Co., Ltd.) can be cited. The mixer has a mixing tank that can rotate. The super absorbent polymer 301 and the predetermined acid, organic solvent or their aqueous solution for removal are supplied to the mixing tank and mixed by the self-rotation of the mixing tank. In the treatment stage of the citric acid removal step S32 (removal treatment step S40), although the super absorbent polymer 301 is not much, it is in the state of a gel that absorbs water, so it is difficult to stir it with a stirring blade in a manner that does not destroy the gel, and it is difficult to react uniformly. The mixer is preferably capable of uniformly mixing the super absorbent polymer 301 with the predetermined aqueous solution without shearing by the self-rotation of the mixing tank without a stirring blade. In the citric acid removal step S32 (removal treatment step S40 ), the highly water-absorbent polymer 301 is mixed with a predetermined aqueous solution using the mixer.

[0104] 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 ).

[0105] 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 about 2 times, for example, and is in a loose state close to sand.

[0106] The drying step S33 is performed by the drying device 33. The drying step S33 dries the used super absorbent polymer treated with the oxidant to generate a regenerated super absorbent polymer. In the present embodiment, the super absorbent polymer 302 is dried in a heated atmosphere while being stirred.

[0107] In the drying step S33, the drying temperature can be, for example, room temperature (for example: 25°C) to 150°C, preferably 70°C to 120°C. If the drying temperature is low, the drying time tends to be long, and if the drying temperature is high, the acid groups of the super absorbent polymer undergo dehydration condensation, etc., thereby the water absorption of the super absorbent polymer may be reduced. In the drying step S33, the drying time can be, for example, 30 minutes to 300 minutes. From the viewpoint of promoting drying, the drying step S33 can also be carried out under reduced pressure, for example, 0.1 kPa to 100 kPa.

[0108] As the drying device 33 for performing the drying step S33, the specific structure is not particularly limited as long as the super absorbent polymer 302 can be dried. As the drying device 33, for example, a dryer with paddles (paddle dryer NPD-1.6WG: manufactured by Nara Machinery Co., Ltd.) can be cited. The dryer with paddles includes: a housing; and a rotatable double-axis shaft, which is arranged in a room inside the housing and extends in parallel to each other in the axial direction. Each shaft includes a wedge-shaped heat transfer wing (paddle) extending in the radial direction, and the paddles of the two shafts overlap each other when viewed from the axial direction. The super absorbent polymer 302 is supplied into the room, receives hot air, and is stirred and dried by the paddles in a high-temperature atmosphere. In the drying step S33, the super absorbent polymer 302 is dried in a heated atmosphere while being stirred by the paddle dryer in a manner so that the super absorbent polymer 302 does not adhere to each other.

[0109] The drying step S33 is preferably carried out in such a manner that the drying loss of the super absorbent polymer is preferably 15% or less (2.0 g, 105° C., 3 hours). This is from the viewpoint of utilizing the super absorbent polymer. The above-mentioned drying loss is measured according to "7. Drying loss test method" of "Standards for physiological treatment materials" attached as an appendix to the "Standards for physiological treatment materials" issued by the Ministry of Health, Labor and Welfare of Japan on March 25, 2015 as the Notice of the Ministry of Health, Labor and Welfare of Japan on ...

[0110] Thereafter, the dried super absorbent polymer 302 is supplied as super absorbent polymer 303 to the foreign matter separation step S34 (foreign matter separation device 34 ).

[0111] 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. As a method of separating foreign matter, for example, a method of separating foreign matter by a specific gravity difference in gas or by size screening based on sieving can be cited. As foreign matter, for example, pulp fiber can be cited.

[0112] The foreign matter separation device 34 for performing 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, and separates the super absorbent polymer 303 and foreign matter by centrifugal separation in the gas. Thus, a regenerated super absorbent polymer with a further reduced amount of foreign matter is generated.

[0113] As described above, the super absorbent polymer derived from the used absorbent article is regenerated to produce the regenerated super absorbent polymer.

[0114] In the generated regenerated super absorbent polymer (among which, the polymer that has been reactivated (neutralized)), as shown in the examples described later, the ash content is 35% by mass or less, preferably 32% by mass or less, and more preferably 31% by mass or less. In addition, in the generated regenerated super absorbent polymer, the number of common viable bacteria detected by the pouring culture method is below the detection limit. Here, the amount of ash contained in the interior of a general super absorbent polymer (for example: sodium polyacrylate crosslinked body) that has not been used after production is about 30% by mass, so it is believed that the amount of ash contained in the interior of the regenerated super absorbent polymer is also about 30% by mass. In this case, it is believed that the ash of 30% by mass, that is, the ash in the interior of the regenerated super absorbent polymer, is derived from Na (sodium) of the sodium polyacrylate crosslinked body (it should be noted that the ash on the surface of the super absorbent polymer is believed to be inorganic matter derived from excrement, silica originally attached to the surface, etc.). Therefore, in the regenerated super absorbent polymer, the unnecessary ash content is less than about 5% by mass (35% by mass to 30% by mass) on the surface except for the inside. It is preferably less than 2% by mass, more preferably less than 1% by mass, and further preferably less than 0.5% by mass. Furthermore, in the regenerated super absorbent polymer, the number of normal viable bacteria is extremely small (below the detection limit). Therefore, the purity of the regenerated super absorbent polymer is very high. Therefore, the regenerated super absorbent polymer can be used as a regenerated super absorbent polymer as a raw material for the production of super absorbent polymers. In other words, the regenerated super absorbent polymer is a regenerated super absorbent polymer as a raw material for the production of super absorbent polymers, which is regenerated from used super absorbent polymers derived from used sanitary products.

[0115] The method for producing a regenerated super absorbent polymer of the present embodiment inactivates the used super absorbent polymer from used sanitary products (for example, absorbent articles) with an acidic solution (inactivation step S30), treats it with an oxidant under acidic conditions (oxidant treatment step S31), and dries it (drying step S33) to produce a regenerated super absorbent polymer. That is, the present method dehydrates the used super absorbent polymer with an acidic solution to significantly reduce its volume, and then, while maintaining the dehydrated state, i.e., the state of reduced volume, under acidic conditions, deodorizes, decolorizes, and sterilizes the used super absorbent polymer with an oxidant. In this way, by reducing the volume of the used super absorbent polymer, impurities (for example, odorous substances, colored substances, and miscellaneous bacteria) contained in the regenerated super absorbent polymer from the used sanitary products can be easily removed with an oxidant, and a regenerated super absorbent polymer with high purity can be obtained. Such a high-purity recycled super absorbent polymer can be used as a part of the raw material in the method of forming a predetermined super absorbent polymer, thereby forming a high-purity, high-quality super absorbent polymer. As a method of forming a predetermined super absorbent polymer, for example, there can be cited methods such as those in Patent Documents 2 to 4, and a method of producing a super absorbent polymer using a recycled super absorbent polymer described later, that is, a method of reusing unused super absorbent polymer as a part of the raw material of the super absorbent polymer. In this way, the used super absorbent polymer from used sanitary products can be effectively used in products using super absorbent polymers.

[0116] In this embodiment, as a preferred mode, 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 with 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, so that a regenerated super absorbent polymer with higher purity can be obtained.

[0117] In the present embodiment, as a preferred form, the used super absorbent polymer is inactivated by a solution containing citric acid (inactivation step S30). Therefore, not only is the used super absorbent polymer dehydrated and its volume greatly reduced, but also metals (for example, substances derived from the human body contained in excrement) can be easily removed by the chelating effect of citric acid. Furthermore, in the present method, citric acid is removed from the used super absorbent polymer after the inactivation step S30 (citric acid removal step S32). Therefore, it is possible to greatly suppress the chelating agent such as citric acid from remaining in the regenerated super absorbent polymer. Thus, a regenerated super absorbent polymer with higher purity can be obtained. In particular, since the chelating agent hardly remains in the regenerated super absorbent polymer, in the method of reusing the unused super absorbent polymer as part of the raw material of the super absorbent polymer, when performing aqueous solution polymerization, crosslinking, etc., it is possible to greatly suppress the chelating agent from hindering the crosslinking reaction.

[0118] In this embodiment, as a preferred form, as the removal of citric acid (citric acid removal step S32), the used super absorbent polymer 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 (removal treatment step S40). In this way, the used super absorbent polymer can be dehydrated with an acid or an organic solvent, and the citric acid on the surface of the used super absorbent polymer can be washed away. Therefore, the chelating agent such as citric acid can be more significantly suppressed from remaining in the regenerated super absorbent polymer.

[0119] In another embodiment, as the removal treatment step S40, the used super absorbent polymer is treated with a removal acid or an aqueous solution thereof in or after the oxidant treatment step S31 (acid step). In this way, citric acid can be removed from the used super absorbent polymer and the used super absorbent polymer can be further dehydrated. Therefore, the subsequent drying step S33 can be performed more easily, and a highly purified regenerated super absorbent polymer can be efficiently obtained.

[0120] In another embodiment, the removal acid used in the removal treatment step S40 (acid step) is an acid having no carboxyl group or an acid having one carboxyl group in one molecule. Therefore, in the removal treatment step, citric acid can be more reliably removed from the used super absorbent polymer and the used super absorbent polymer can be further dehydrated.

[0121] In this embodiment, as a preferred form, the used super absorbent polymer is treated with an aqueous solution of an organic solvent as a removal treatment step S40 after the oxidant treatment step S31 and before the drying step S33 (organic solvent step). In this way, citric acid can be removed from the used super absorbent polymer and the used super absorbent polymer can be further dehydrated. Therefore, the subsequent drying step S33 can be performed more easily, and thus, a regenerated super absorbent polymer with high purity can be obtained efficiently.

[0122] In another embodiment, in the removal treatment step S40 (organic solvent step), the used super absorbent polymer is treated with an aqueous solution of an organic solvent containing an alkali metal ion supply source. Thus, citric acid can be removed from the used super absorbent polymer, and the used super absorbent polymer can be neutralized with alkali metal ions while being dehydrated. Therefore, for example, when producing super absorbent polymer, a regenerated super absorbent polymer can be used as a neutralized high-purity raw material.

[0123] In another embodiment, 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 super absorbent polymer. Therefore, the used super absorbent polymer can be more reliably neutralized with alkali metal ions.

[0124] In this embodiment, as a preferred form, in the oxidant treatment step S31, the used super absorbent polymer is treated with water containing ozone or hydrogen peroxide solution as an oxidant. This makes it easier to remove impurities (for example, odorous substances, colored substances, and bacteria) contained in the regenerated super absorbent polymer derived from the used sanitary products, that is, deodorization, decolorization, and sterilization can be performed more easily. This makes it possible to obtain a regenerated super absorbent polymer with a higher purity.

[0125] In addition, in the regenerated super absorbent polymer derived from used sanitary products (for example, absorbent articles) of the present embodiment, the ash content is 35% by mass or less, and the number of viable bacteria detected by the pouring culture method is below the detection limit. That is, the purity of the regenerated super absorbent polymer is high. Therefore, by mixing such a regenerated super absorbent polymer into at least one of the raw material and the semi-finished product in the aqueous solution polymerization or cross-linking for the production of the super absorbent polymer, a high-purity, high-quality super absorbent polymer can be formed.

[0126] Next, 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 will be described.

[0127] Among the highly absorbent polymers, the polyacrylate-based highly absorbent polymers are 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).

[0128] (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), and then the cross-linked polyacrylic acid product is neutralized to form a cross-linked sodium polyacrylate product, i.e., a super absorbent polymer. Preferably, the super absorbent polymer thus formed is surface cross-linked 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.

[0129] (2) Acrylic acid (raw material) is neutralized to form sodium acrylate (semi-finished product), and then the sodium acrylate is polymerized and cross-linked in an aqueous solution (polymerization initiator and cross-linking agent), thereby forming a sodium polyacrylate cross-linked body, that is, a super absorbent polymer. Preferably, the super absorbent polymer thus formed is surface cross-linked 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.

[0130] Here, in the method of (1), by mixing the above-mentioned regenerated super absorbent polymer which has not been reactivated (neutralized) into at least one of acrylic acid (raw material) and polyacrylic acid crosslinked product (semi-finished product), the super absorbent polymer can be produced using the regenerated super absorbent polymer. Alternatively, by mixing the above-mentioned regenerated super absorbent polymer which has been reactivated (neutralized) into the super absorbent polymer (semi-finished product) before surface crosslinking, the super absorbent polymer can be produced using the regenerated super absorbent polymer.

[0131] In the method of (2), the super absorbent polymer can be produced by mixing the above-mentioned regenerated super absorbent polymer which has not been reactivated (neutralized) into acrylic acid (raw material). In addition, the super absorbent polymer can be produced by mixing the above-mentioned regenerated super absorbent polymer which has been reactivated (neutralized) into sodium acrylate (semi-finished product) or super absorbent polymer (semi-finished product) before surface cross-linking.

[0132] Alternatively, by mixing the above-mentioned regenerated super absorbent polymer that has not been reactivated (neutralized) and / or the regenerated super absorbent polymer that has been reactivated (neutralized) as the fine powder, granules 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.

[0133] In the method for producing a super absorbent polymer using a recycled super absorbent polymer, a recycled super absorbent polymer derived from a used sanitary product (for example, an absorbent article) is mixed into at least one of a raw material and a semi-finished product, and a super absorbent polymer is produced by aqueous solution polymerization or cross-linking. In this case, the recycled super absorbent polymer is produced by the above-mentioned method for producing a recycled super absorbent polymer, so its purity becomes high. Therefore, in this method, even if the above-mentioned recycled super absorbent polymer is used, a high-quality super absorbent polymer with high purity can be formed.

[0134] 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 the present 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 method shown below is explained.

[0135] Figure 3 1 is a block diagram showing an example of a system 100 for separating materials from used absorbent articles according to the present 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 1 is a flow chart showing an example of a method for separating materials from used absorbent articles according to the present 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 Figure 4 As shown, the separation step S10 includes a hole-making step S11 to a fourth separation step S20. Hereinafter, each step will be described in detail.

[0136] It should be noted that in this embodiment, used absorbent articles are collected and obtained from the outside for reuse (recycling). At this time, a plurality of used absorbent articles are sealed in a collection bag in a manner that excrement, fungi, and odors do not leak to the outside. Each used absorbent article in the collection bag is collected, for example, with the surface sheet on which excrement is excreted being placed inside in a manner that excrement and fungi are not exposed on the surface side and odors do not spread to the surroundings, mainly in a rolled-up state or a folded state.

[0137] The hole-opening step S11 is performed by the bag-breaking device 11. The bag-breaking device 11 includes a solution tank storing an inactivation aqueous solution containing an inactivating agent and a bag-breaking knife rotating in the solution tank. The bag-breaking device 11 uses the bag-breaking knife to open holes in the inactivation aqueous solution in the collection bag put into the solution tank. Thus, a mixed solution 91 of the collection bag into which the inactivation aqueous solution has been immersed from the hole and the inactivation aqueous solution is generated. The inactivation aqueous solution inactivates the superabsorbent polymer of the used absorbent article in the collection bag. The following description takes the case where an acidic aqueous solution is used as the inactivation aqueous solution as an example.

[0138] The crushing step S12 is performed by the crushing device 12. The crushing device 12 includes a double-shaft crusher (for example, a double-shaft rotary crusher, etc.). The crushing device 12 crushes the collection bag of the used absorbent article containing the mixed liquid 91 together with the collection bag. Thus, a mixed liquid 92 having an acidic aqueous solution and crushed materials of the collection bag containing the used absorbent article is generated, and almost all of the super absorbent polymer of the used absorbent article is inactivated. The crushed materials include pulp fibers and super absorbent polymers and other materials (films, nonwoven fabrics, collection bags, etc.).

[0139] The first separation step S13 is performed by the first separation device 13. The first separation device 13 includes a pulping separator having a stirring separation tank that functions as a cleaning tank and a screening tank. The first separation device 13 stirs the mixed liquid 92 to remove excrement and the like from the crushed material, and separates pulp fibers, super absorbent polymers, excrement and an acidic aqueous solution from the mixed liquid 92. Thus, a mixed liquid 93 containing pulp fibers, super absorbent polymers, excrement and an acidic aqueous solution is generated, and the used absorbent article film, nonwoven fabric, raw materials of a collection bag, etc. are recovered.

[0140] It should be noted that the hole-opening step S11 and the crushing step S12 (the bag-breaking device 11 and the crushing device 12) treat the used absorbent article in the inactivation aqueous solution to inactivate the super absorbent polymer, and therefore can be said to be the inactivation step S30 (the inactivation device 30). In addition, the crushing device 12 can also crush the used absorbent article together with the collection bag in a gas (for example: in the air) instead of crushing the used absorbent article in the inactivation aqueous solution. In this case, the bag-breaking device 11 is not required. After crushing, the crushed product of the crushing device 12 and the inactivation aqueous solution are supplied to the first separation device 13 (the first separation step S13) to inactivate the super absorbent polymer. In this case, the first separation device 13 (the first separation step S13) can be said to be the inactivation step S30 (the inactivation device 30).

[0141] 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, and uses a screen to separate the mixed liquid 93 into pulp fibers, super absorbent polymers, excrement, and other materials (foreign matter) in the acidic aqueous solution. Thus, the mixed liquid 94 containing pulp fibers, super absorbent polymers, excrement, and the acidic aqueous solution with a reduced amount of foreign matter is generated, and other materials are removed.

[0142] 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, and uses a screen finer than that of the first dust removal device 14 to separate the mixed liquid 94 into pulp fibers, super absorbent polymers, excrement, and other materials (small foreign matter) in the acidic aqueous solution. Thus, the mixed liquid 95 containing pulp fibers, super absorbent polymers, excrement, and the acidic aqueous solution is generated with a further reduced amount of foreign matter, and other materials are further removed.

[0143] 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, and separates the mixed liquid 95 into pulp fibers, super absorbent polymers, excrement, and other materials (foreign materials with a relatively heavy specific gravity) in the acidic aqueous solution by centrifugal separation. Thus, the mixed liquid 96 containing pulp fibers, super absorbent polymers, excrement, and the acidic aqueous solution is generated, and the other materials with a relatively heavy specific gravity are removed.

[0144] It should be noted that at least one of the first to third dust removing devices 14 to 16 may be omitted depending on the state of the mixed liquid 92 and the like (eg, the amount and size of foreign matter).

[0145] The second separation step S17 is performed by the second separation device 17. The second separation device 17 includes a drum screen separator, and uses a drum screen to separate the mixed liquid 96 into the super absorbent polymer and pulp fibers in the acidic aqueous solution. Thus, a mixed liquid 97 containing the super absorbent polymer, excrement and the acidic aqueous solution is generated, and the pulp fibers are removed as a mixture 98.

[0146] The third separation step S18 is performed by the third separation device 18. The third separation device 18 is equipped with an inclined screen, and uses the screen to separate the mixed liquid 97 into a solid containing a super absorbent polymer and a liquid containing excrement and an acidic aqueous solution. In this way, a super absorbent polymer (SAP) is generated, and the acidic aqueous solution containing excrement and the like is removed.

[0147] 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 oxidant aqueous 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 upper part or the lower part of the treatment tank, and mixes it with the oxidant aqueous solution in the treatment tank. Then, the highly absorbent polymer contained in the pulp fiber is decomposed in the oxidant aqueous solution by using the oxidant supplied from the lower part of the treatment tank by the oxidant supply device, so that it can be dissolved in the oxidant aqueous solution. Thus, a mixed solution 99 having pulp fibers from which the highly absorbent polymer has been removed and an oxidant aqueous solution containing the decomposition product of the highly absorbent polymer is generated. The oxidant is an oxidant that can decompose the highly absorbent polymer, and an example thereof can be ozone. Ozone has high bactericidal power and bleaching power, so it is preferred.

[0148] 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 treatment time using ozone. The lower the ozone concentration, the longer the treatment time using ozone, 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.

[0149] The fourth separation step S20 is performed by the fourth separation device 20. The fourth separation device 20 includes a screen separator and uses a screen to separate the mixed liquid 99 into pulp fibers and an oxidant aqueous solution. In this way, pulp fibers are generated and the oxidant aqueous solution containing decomposition products of the highly absorbent polymer is removed.

[0150] <Ash content>

[0151] 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 physiological treatment product material standard. That is, ash content is measured as follows. Pre-heat 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 the sample, put it into the crucible, and accurately weigh its mass. Remove the lid of the crucible or move it as needed. Initially, weak heating is performed, and the temperature is gradually increased. Heat at 500°C to 550°C for more than 4 hours and ash until no carbide remains. After cooling, accurately weigh its mass. Ash the residue again to a constant weight, cool it, and accurately weigh its mass as the amount of ash content (mass %).

[0152] <Detection of common bacteria>

[0153] Common live bacteria can be detected by the pouring culture method. Examples of common live bacteria include Bacillus cereus, Bacillus subtilis, Staphylococcus aureus, Pseudomonas aeruginosa, glucose non-fermenting Bacillus, Aeromonas, and the like. The above bacteria cannot be detected by the pouring culture method, and thus, the above regenerated super absorbent polymer is unlikely to cause bacteremia, endocarditis, respiratory tract infection, food poisoning, eye infection, and the like, and the user can use the above regenerated super absorbent polymer with peace of mind.

[0154] The pour culture method was carried out in the following manner.

[0155] (1) 500 g of an aqueous dispersion of a regenerated highly water-absorbent polymer having a solid content concentration of 5.0% by mass was prepared in a 1-liter beaker.

[0156] When the regenerated super absorbent polymer is in a dry state, the aqueous dispersion can be prepared 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). Then, when the regenerated super absorbent polymer is in an aqueous solution (for example, in the method for producing the regenerated super absorbent polymer, the regenerated super absorbent polymer is recovered as an aqueous solution) and the solid content concentration of the regenerated super absorbent polymer is 5.0% by mass or more, deionized water or the like is 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.

[0157] Furthermore, when the regenerated super absorbent polymer is present in the form of an aqueous solution and the solid content concentration of the regenerated super absorbent polymer is less than 5.0% by mass, the solid content 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 amount of the serial dilution sample described later can be increased (for example, when the solid content concentration of the regenerated super absorbent polymer is 2.5% by mass, the inoculum amount can be doubled).

[0158] (2) Using an overhead stirrer, stir the aqueous dispersion at 300 rpm for 15 minutes.

[0159] (3) 50 mL of the aqueous dispersion stirred with an overhead stirrer was placed in a sterilization bag with a filter (LMS Corporation, sterilization bag with a filter for homogenizer), and stirred for 5 minutes.

[0160] (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 sterile test tubes to prepare serial dilution samples.

[0161] (5) Determine the number of viable cells by pour culture method.

[0162] Specifically, 1 mL of the serial dilution sample and a standard agar medium (Nippon Pharmaceutical Co., Ltd., 396-00175SCD agar medium for general viability test "DAIGO", 15 g to 20 g) were placed in a culture dish and poured and cultured at 35°C for 48 hours.

[0163] (6) As the normal viable count, the number of colonies grown after cultivation was counted.

[0164] It should be noted that for serial dilutions from 10 to 10 -9 For all serial dilution samples, when the colony count is zero, the target bacteria are judged as "undetectable", that is, the number of normal live bacteria detected by the pouring culture method is below the detection limit. In other words, the number of normal live bacteria is 0 cfu / g.

[0165] (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.

[0166] Example

[0167] Hereinafter, the present invention will be described based on Examples, but the present invention is not limited to the Examples.

[0168] (1) Regarding oxidant treatment

[0169] The effect of the oxidant treatment was evaluated.

[0170] (a) Sample

[0171] The highly absorbent polymer separated from the used diapers in actual use (corresponding to the separation step S10) is used. The highly absorbent polymer is generally a crosslinked sodium polyacrylate. The separated highly absorbent polymer is inactivated by an acidic aqueous solution (citric acid concentration 1%) (corresponding to the inactivation step S30 = corresponding to the opening step S11 + the crushing step S12).

[0172] (b) Evaluation method

[0173] (i) In the sample of the example, 1 g of the super absorbent polymer (deactivated) separated from the used diaper was immersed in ozone water (ozone concentration: 1 ppm) having a mass 10 times the mass of the super absorbent 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 sample of the comparative example, 1 g of the super absorbent polymer (deactivated) separated from the used diaper was not immersed in ozone water.

[0174] (ii) Then, the super absorbent polymers of the samples of the examples and the samples of the comparative examples were treated in an aqueous solution containing methanol and sodium hydroxide (corresponding to the citric acid removal step S32). Thus, citric acid was removed from each super absorbent polymer, and each super absorbent polymer was dehydrated and reactivated (neutralized).

[0175] (iii) Thereafter, the super absorbent polymer of the sample of the example and the super absorbent polymer of the sample of the comparative example were both dried at 105° C. for 60 minutes (corresponding to the drying step S33 ).

[0176] (iv) The ash content and the number of common bacteria of the obtained super absorbent polymers of the samples of the examples and the samples of the comparative examples were measured. The evaluation results are shown in Table 1.

[0177] (c) Evaluation results

[0178] The ash content of the super absorbent polymer of the sample of the example was 30.3% by mass, which was about 10% lower than the ash content of 40.9% by mass of the super absorbent polymer of the sample of the comparative example, and was roughly the same as the amount of ash (about 30% by mass) contained in the interior of a general super absorbent polymer (sodium polyacrylate crosslinked body) that was not used after production. Therefore, it can be seen that there is almost no substance that becomes ash (less than 1% by mass) on the surface of the super absorbent polymer of the sample of the example. That is, considering inorganic substances derived from excrement, silica originally attached to the surface, etc. as the substances that become ash on the surface of the super absorbent polymer, it can be seen that there is almost no inorganic substances and silica derived from excrement on the surface of the super absorbent polymer of the sample of the example. In other words, in the sample of the example, it can be said that the ash content on the surface of the super absorbent polymer is less than 1% by mass. Furthermore, the number of common bacteria in the super absorbent polymer of the sample of the example was below the detection limit (0 (zero) cfu / g), which was an extremely low value compared to the number of common bacteria in the super absorbent polymer of the sample of the comparative example, which was 3400 cfu / g. This shows that the purity of the super absorbent polymer of the sample of the example, that is, the regenerated super absorbent polymer, was very high.

[0179] [Table 1]

[0180] Ash content (mass %) Common bacteria count (cfu / g) Example 30.3 0 Comparative Example 40.9 3400

[0181] The absorbent article of the present invention is not limited to the above-described embodiments, and can be appropriately combined, modified, etc. within a range not departing from the purpose and spirit of the present invention.

[0182] Description of Reference Numerals

[0183] S30 Inactivation process

[0184] S31 Oxidant treatment process

[0185] 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 from a used sanitary product, wherein: The method includes: an inactivation step of inactivating the used superabsorbent polymer taken out of the used sanitary product by using an acidic solution; an oxidant treatment step, 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 oxidant to generate the regenerated super absorbent polymer; The acidic solution is a solution containing citric acid, The method further comprises a citric acid removal step after the inactivation step and before the drying step: removing the citric acid from the used super absorbent polymer.

2. The method according to claim 1, wherein: The method further includes, after the drying step, a foreign matter separation step of separating foreign matter from the regenerated super absorbent polymer.

3. The method according to claim 1, wherein: The citric acid removal step includes a removal treatment step of treating the used super absorbent 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.

4. The method according to claim 3, wherein: The removal treatment step includes an acid step: during or after the oxidant treatment step, the used super absorbent polymer is treated with the removal acid or an aqueous solution thereof.

5. The method according to claim 3 or 4, wherein: The removing acid is an acid having no carboxyl group or an acid having one carboxyl group in one molecule.

6. The method according to claim 3 or 4, 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.

7. The method according to claim 6, 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.

8. The method according to claim 7, 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 highly water-absorbent polymer.

9. 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.

10. A method for producing a super absorbent polymer using a recycled super absorbent polymer recycled from a used super absorbent polymer derived from a used sanitary product 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 any one of claims 1 to 9; 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 the cross-linking is performed.

11. A recycled super absorbent polymer obtained by the method according to any one of claims 1 to 9, which is a recycled super absorbent polymer recycled from a used super absorbent polymer taken out from a used sanitary product and used as a raw material for producing a super absorbent polymer, wherein: The ash content was 35% by mass or less, and the number of viable cells detected by the pouring culture method was below the detection limit.

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