Method for producing highly water-absorbent polymer subjected to secondary dehydration for use in producing highly water-absorbent recycled polymer, and method for producing highly water-absorbent recycled polymer

Abstract

The object of the present disclosure is to provide a method for producing a super absorbent polymer for super absorbent recycling polymer that has been dehydrated twice, which can easily form a super absorbent recycling polymer with a small amount of residual excrement. The production method disclosed in the present disclosure has the following technical features. A method for producing a super absorbent polymer for super absorbent recycling polymer that has been dehydrated twice, characterized in that the method includes: a preparation step of preparing a super absorbent polymer that has been dehydrated once, which is obtained by immersing a sanitary product constituent material constituting a sanitary product in an acid-containing aqueous solution containing an acid to form a super absorbent polymer that has been dehydrated once, and separating the super absorbent polymer that has been dehydrated once from the sanitary product constituent material to obtain the super absorbent polymer that has been dehydrated once; a secondary dehydration step of further dehydrating the super absorbent polymer that has been dehydrated once by contacting the super absorbent polymer that has been dehydrated once with a polyvalent metal ion supply source that can supply polyvalent metal ions, thereby forming the super absorbent polymer that has been dehydrated twice.

Description

Technical Field

[0001] The present invention relates to a method for producing a super absorbent recycled polymer subjected to secondary dehydration for use as a super absorbent recycled polymer, a method for producing a super absorbent recycled polymer, and a method for producing a super absorbent recycled polymer from used sanitary products containing pulp fibers and a super absorbent polymer having an acid group. Background Art

[0002] Research is being conducted into the recycling of materials that make up used hygiene products.

[0003] For example, Patent Document 1 discloses a method for regenerating an acid-inactivated superabsorbent polymer into a superabsorbent recycling polymer having a predetermined water absorbency (Claim 1), which comprises: a preparation step of preparing an acid-inactivated superabsorbent polymer having an acid group; a superabsorbent recycling polymer forming step of adding an alkali metal ion supply source capable of supplying alkali metal ions to an aqueous regeneration solution containing the acid-inactivated superabsorbent polymer, thereby forming the superabsorbent recycling polymer in a wet state from the acid-inactivated superabsorbent polymer; and a drying step of drying the wet superabsorbent recycling polymer to form the superabsorbent recycling polymer having the predetermined water absorbency.

[0004] Patent Document 2 discloses a method for treating a water-absorbent resin (Claim 1), characterized by adding and mixing a weak acid alkali metal salt to aggregates of water-absorbent resin powder obtained by agglomerating the water-absorbent resin powder by absorbing water; and dehydrating the aggregates of the water-absorbent resin powder using a divalent metal salt (Claim 2).

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Publication No. 2019-135046

[0008] Patent Document 2: Japanese Patent Application Laid-Open No. 2015-4034 Summary of the Invention

[0009] Problems to be solved by the invention

[0010] In the method described in Patent Document 1, since the super absorbent polymer is deactivated (dehydrated) by acid, ash is less likely to remain in the recycled pulp fibers. However, there is a tendency for the degree of dehydration of the super absorbent polymer to decrease, which may easily lead to the retention of excrement, and the drying energy required to form the super absorbent recycled polymer tends to increase.

[0011] Furthermore, in the method described in Patent Document 2, although the degree of dehydration of the superabsorbent polymer is increased, the separation of the pulp fibers and the superabsorbent polymer tends to be insufficient, and there is a tendency for pulp fibers to remain in the superabsorbent polymer. Furthermore, when recycled pulp fibers are used, there is a tendency for ash to remain in the recycled pulp fibers.

[0012] Therefore, an object of the present disclosure is to provide a method for producing a secondary dehydrated superabsorbent polymer for superabsorbent recycling polymer, which can easily form a superabsorbent recycling polymer with a small amount of residual excrement.

[0013] Solutions for solving problems

[0014] The present inventors have discovered a method for producing a secondarily dehydrated superabsorbent polymer for use as a superabsorbent recycled polymer, the method comprising: a preparation step of preparing a first-dehydrated superabsorbent polymer obtained by immersing a sanitary product component material comprising pulp fibers and a superabsorbent polymer having acidic groups in an acidic aqueous solution containing an acid to form the first-dehydrated superabsorbent polymer, and separating the first-dehydrated superabsorbent polymer from the sanitary product component material to obtain the first-dehydrated superabsorbent polymer; and a second dehydration step of further dehydrating the first-dehydrated superabsorbent polymer by contacting the first-dehydrated superabsorbent polymer with a polyvalent metal ion supply source capable of supplying polyvalent metal ions to obtain the secondarily dehydrated superabsorbent polymer.

[0015] Effects of the Invention

[0016] The method for producing a highly absorbent recycled polymer subjected to secondary dehydration for use in highly absorbent recycled polymer disclosed herein can easily produce a highly absorbent recycled polymer with a low amount of residual excrement. DETAILED DESCRIPTION

[0017] Specifically, the present disclosure relates to the following aspects.

[0018] [Method 1]

[0019] A method for producing a secondarily dehydrated superabsorbent polymer for use as a superabsorbent recycled polymer, the method comprising: a preparation step of preparing a firstly dehydrated superabsorbent polymer obtained by immersing a sanitary product constituent material comprising pulp fibers and a superabsorbent polymer having an acid group in an acid-containing aqueous solution to form the firstly dehydrated superabsorbent polymer, and separating the firstly dehydrated superabsorbent polymer from the sanitary product constituent material, thereby obtaining the firstly dehydrated superabsorbent polymer; and a second dehydration step of further dehydrating the firstly dehydrated superabsorbent polymer by contacting the firstly dehydrated superabsorbent polymer with a polyvalent metal ion supply source capable of supplying polyvalent metal ions, thereby obtaining the secondarily dehydrated superabsorbent polymer.

[0020] In the above-mentioned production method, to produce a superabsorbent recycled polymer, a superabsorbent polymer that has been dehydrated once with an acid is prepared. The superabsorbent polymer that has undergone the primary dehydration is then further dehydrated using a polyvalent metal ion supply source to form a secondarily dehydrated superabsorbent polymer. This secondarily dehydrated superabsorbent polymer has a lower moisture content (higher dehydration rate) than dehydration using only an acid (the same type and amount of acid) or dehydration using a polyvalent metal ion supply source (the same type and amount of polyvalent metal ion supply source). This means that the amount of residual waste in the superabsorbent recycled polymer is reduced, thereby reducing the amount of residual waste in the superabsorbent recycled polymer.

[0021] Furthermore, the highly water-absorbent polymer that has been subjected to secondary dehydration and is formed by the above-mentioned production method can be easily converted into a highly water-absorbent recycled polymer by contacting it with an alkali metal ion supply source or the like.

[0022] In addition, in this specification, a "super absorbent polymer that has been dehydrated once" and a "super absorbent polymer that has been dehydrated twice" may be referred to as a "single-dehydrated super absorbent polymer" and a "double-dehydrated super absorbent polymer," respectively.

[0023] [Method 2]

[0024] The method according to embodiment 1, wherein in the secondary dehydration step, the super absorbent polymer that has undergone the primary dehydration is further dehydrated to a moisture content of 50.0 to 80.0% by mass to form the super absorbent polymer that has undergone the secondary dehydration.

[0025] In this production method, by further dehydrating the primary dehydrated superabsorbent polymer to a predetermined moisture content to form a secondary dehydrated superabsorbent polymer, the amount of residual waste in the secondary dehydrated superabsorbent polymer, and thus in the superabsorbent recycled polymer, can be reduced. Furthermore, by contacting the secondary dehydrated superabsorbent polymer formed by this production method with an alkali metal ion supply source, etc., the superabsorbent recycled polymer can be easily formed.

[0026] [Method 3]

[0027] The method according to embodiment 1 or 2, wherein the polyvalent metal ion supply source is an alkaline earth metal salt or a transition metal hydroxide, a salt of an alkaline earth metal salt or a transition metal hydroxide and an acid, or an alkaline earth metal salt or a transition metal oxide.

[0028] In this production method, since the polyvalent metal ion source is selected from a predetermined substance, the amount of residual waste in the secondary dehydrated superabsorbent polymer and, by extension, the superabsorbent recycled polymer can be reduced. Furthermore, the secondary dehydrated superabsorbent polymer formed by this production method can be easily converted into a superabsorbent recycled polymer by contacting it with an alkali metal ion source or the like.

[0029] [Method 4]

[0030] The method according to any one of aspects 1 to 3, wherein in the secondary dehydration step, the secondary dehydrated super absorbent polymer is formed by contacting the primary dehydrated super absorbent polymer with the polyvalent metal ion source or an aqueous solution of the polyvalent metal ion source.

[0031] By bringing the primary dehydrated superabsorbent polymer into contact with the polyvalent metal ion source itself or an aqueous solution thereof, the above-described production method can reduce the amount of residual waste in the secondary dehydrated superabsorbent polymer, and thus in the superabsorbent recycled polymer. Furthermore, by bringing the secondary dehydrated superabsorbent polymer formed by the above-described production method into contact with an alkali metal ion source, etc., a superabsorbent recycled polymer can be easily formed.

[0032] [Method 5]

[0033] The method according to embodiment 4, wherein the secondary dehydration step further comprises a drying step of drying the super absorbent polymer that has undergone the secondary dehydration.

[0034] In the above production method, since the secondary dehydration step also includes a predetermined drying step, the moisture content of the secondary dehydrated superabsorbent polymer can be further reduced, and the amount of residual waste in the secondary dehydrated superabsorbent polymer and, by extension, the superabsorbent recycled polymer can be further reduced. Furthermore, when the secondary dehydrated superabsorbent polymer formed by the above production method is contacted with an alkali metal ion source to form the superabsorbent recycled polymer, the amount of the alkali metal ion source used can be reduced.

[0035] [Method 6]

[0036] The method according to any one of aspects 1 to 5, wherein in the preparation step, the super absorbent polymer that has been dehydrated once has a moisture content of more than 80.0% by mass and not more than 97.0% by mass.

[0037] In this production method, since the primary dehydrated superabsorbent polymer has a predetermined moisture content during the preparatory step, the moisture content of the secondary dehydrated superabsorbent polymer can be easily reduced (easily reducing the amount of residual waste), thereby reducing the amount of residual waste in the superabsorbent recycled polymer. Furthermore, the secondary dehydrated superabsorbent polymer formed by this production method can be easily converted into a superabsorbent recycled polymer by contacting it with an alkali metal ion supply source, etc.

[0038] [Method 7]

[0039] The method according to any one of aspects 1 to 6, further comprising, before the preparing step, a primary dehydration step of forming the primary dehydrated super absorbent polymer by immersing the sanitary product constituent material in the acid-containing aqueous solution, and separating the primary dehydrated super absorbent polymer from the sanitary product constituent material.

[0040] Because the above production method also includes a predetermined primary dehydration step, it is possible to reduce the amount of residual waste in the secondary dehydrated superabsorbent polymer, and thus in the superabsorbent recycled polymer. Furthermore, the secondary dehydrated superabsorbent polymer formed by the above production method can be easily converted into a superabsorbent recycled polymer by contacting it with an alkali metal ion source, etc.

[0041] [Method 8]

[0042] The method according to any one of aspects 1 to 7, further comprising, after the secondary dehydration step, a super absorbent recycling polymer forming step of mixing the super absorbent polymer that has undergone the secondary dehydration and an alkali metal ion supply source capable of supplying alkali metal ions in an aqueous regeneration solution to form the super absorbent recycling polymer from the super absorbent polymer that has undergone the secondary dehydration.

[0043] The above-mentioned production method further includes a predetermined step of forming a highly absorbent recycled polymer, and thus can easily form a highly absorbent recycled polymer with a small amount of residual excrement.

[0044] [Method 9]

[0045] The method according to embodiment 8, wherein the alkali metal ion supply source is (i) an alkali metal hydroxide having a standard electrode potential difference of 2.0 or less with the polyvalent metal ion; (ii) a salt of an alkali metal hydroxide having a standard electrode potential difference of 2.0 or less with the polyvalent metal ion and an acid having a larger acid dissociation constant than the acid group of the super absorbent polymer; or (iii) an alkali metal oxide having a standard electrode potential difference of 2.0 or less with the polyvalent metal ion.

[0046] In this production method, since the alkali metal source is selected from a predetermined substance, the polyvalent metal ions that crosslink the acid groups of the secondary dehydrated superabsorbent polymer can be easily replaced with alkali metal ions, thereby improving the water absorption of the recycled superabsorbent polymer. As a result, a recycled superabsorbent polymer with excellent water absorption and low residual amounts in excrement can be easily produced.

[0047] [Method 10]

[0048] The method according to embodiment 9, wherein the alkali metal ion is selected from the group consisting of lithium ions, sodium ions, potassium ions, and any combination thereof.

[0049] In the above-mentioned production method, the alkali metal ions are selected from predetermined substances, so that a highly water-absorbent recycling polymer having excellent water absorption and a small amount of residual waste can be easily produced.

[0050] [Method 11]

[0051] A method for producing a superabsorbent recycled polymer, characterized in that the method comprises: preparing a superabsorbent polymer that has been dehydrated twice by immersing a sanitary product constituent material comprising pulp fibers and a superabsorbent polymer having an acid group in an acidic aqueous solution containing an acid to form a superabsorbent polymer that has been dehydrated once; separating the superabsorbent polymer that has been dehydrated once from the sanitary product constituent material; and further dehydrating the superabsorbent polymer that has been dehydrated once by contacting the superabsorbent polymer with a polyvalent metal ion supply source capable of supplying polyvalent metal ions, thereby obtaining the superabsorbent polymer that has been dehydrated twice.

[0052] The super absorbent recycled polymer forming step comprises mixing the super absorbent polymer that has undergone secondary dehydration and an alkali metal ion supply source capable of supplying alkali metal ions in an aqueous regeneration solution to form the super absorbent recycled polymer from the super absorbent polymer that has undergone secondary dehydration.

[0053] The above-mentioned production method can easily form a highly water-absorbent recycling polymer with a small amount of residual excrement.

[0054] [Method 12]

[0055] A method for producing superabsorbent recycled polymer from used sanitary products containing pulp fibers and superabsorbent polymer having acid groups, characterized in that the method comprises:

[0056] a primary dehydration step of immersing a sanitary article constituent material comprising the pulp fibers and the super absorbent polymer in an acidic aqueous solution containing an acid to form a primary dehydrated super absorbent polymer, and separating the primary dehydrated super absorbent polymer from the sanitary article constituent material;

[0057] a secondary dehydration step of further dehydrating the primary dehydrated super absorbent polymer by contacting the primary dehydrated super absorbent polymer with a polyvalent metal ion supply source capable of supplying polyvalent metal ions, thereby forming a secondary dehydrated super absorbent polymer;

[0058] The super absorbent recycled polymer forming step comprises mixing the super absorbent polymer that has undergone secondary dehydration and an alkali metal ion supply source capable of supplying alkali metal ions in an aqueous regeneration solution to form the super absorbent recycled polymer from the super absorbent polymer that has undergone secondary dehydration.

[0059] The above-mentioned production method can easily form a highly water-absorbent recycling polymer with a small amount of residual excrement.

[0060] The following describes in detail (i) the method for producing a super absorbent recycled polymer that has undergone secondary dehydration for use as a super absorbent recycled polymer (hereinafter sometimes referred to as the "method for producing a super absorbent recycled polymer after secondary dehydration"), (ii) the method for producing a super absorbent recycled polymer (hereinafter sometimes referred to as the "method for producing a super absorbent recycled polymer"), and (iii) the method for producing a super absorbent recycled polymer from used sanitary products containing pulp fibers and a super absorbent polymer having an acid group (hereinafter sometimes referred to as the "method for producing a super absorbent recycled polymer from used sanitary products") disclosed herein.

[0061] For ease of explanation, the following will be described in the order of (iii) the method for producing a super absorbent recycled polymer from used sanitary products, (i) the method for producing a secondary dehydrated super absorbent polymer, and (ii) the method for producing a super absorbent recycled polymer.

[0062] In addition, the above three production methods may be collectively referred to as "the production method of the present disclosure."

[0063] <<Method for producing highly absorbent recycled polymer from used sanitary products>>

[0064] The method for preparing superabsorbent recycled polymer from used sanitary products disclosed herein comprises the following steps.

[0065] a primary dehydration step of immersing the sanitary article constituent material comprising the pulp fibers and the super absorbent polymer in an acidic aqueous solution containing an acid to form a primary dehydrated super absorbent polymer, and separating the primary dehydrated super absorbent polymer from the sanitary article constituent material (hereinafter sometimes referred to as the "primary dehydration step");

[0066] a secondary dehydration step of further dehydrating the primary dehydrated super absorbent polymer by bringing the primary dehydrated super absorbent polymer into contact with a polyvalent metal ion supply source capable of supplying polyvalent metal ions, thereby forming the secondary dehydrated super absorbent polymer (hereinafter sometimes referred to as the "secondary dehydration step");

[0067] A super absorbent recycled polymer forming step of mixing the super absorbent polymer that has undergone the second dehydration and an alkali metal ion supply source capable of supplying alkali metal ions in an aqueous regeneration solution to form the super absorbent recycled polymer from the super absorbent polymer that has undergone the second dehydration (hereinafter sometimes referred to as the "super absorbent recycled polymer forming step").

[0068] [1st dehydration step]

[0069] In the primary dehydration step, the sanitary product constituent material containing pulp fibers and a super absorbent polymer having an acid group is immersed in an acidic aqueous solution containing an acid to form a primary dehydrated super absorbent polymer, and the primary dehydrated super absorbent polymer is separated from the sanitary product constituent material.

[0070] During the first dehydration step, the acid groups of the super absorbent polymer change from the salt state (e.g., sodium salt) with the components in the body fluid (e.g., sodium ions) to the free acid state, and thus the water absorption of the super absorbent polymer decreases. If the super absorbent polymer that has absorbed water enters the acidic aqueous solution, the negatively charged hydrophilic groups (e.g., -COO -) is positively charged hydrogen ions (H + ) is neutralized (e.g., -COOH), thereby weakening the ionic repulsion of the hydrophilic group, reducing the water absorption capacity, and dehydrating the super absorbent polymer.

[0071] The acid is not particularly limited, and examples thereof include inorganic acids and organic acids. When the super absorbent polymer is dehydrated (deactivated) using an acid, ash is less likely to remain on the pulp fibers than when the super absorbent polymer is dehydrated (deactivated) using a polyvalent metal ion supply source capable of supplying polyvalent metal ions, such as lime, calcium chloride, magnesium sulfate, magnesium chloride, aluminum sulfate, or aluminum chloride.

[0072] Examples of the inorganic acid include sulfuric acid, hydrochloric acid, and nitric acid. Sulfuric acid is preferred from the perspectives of its lack of chlorine and cost. Examples of the organic acid include those having an acid group, such as a carboxyl group or a sulfonic group. It should be noted that organic acids having a sulfonic group are referred to as sulfonic acids, while organic acids having a carboxyl group and no sulfonic group are referred to as carboxylic acids. From the perspective of protecting equipment, organic acids having a carboxyl group, particularly carboxylic acids, are preferred.

[0073] When the organic acid has a carboxyl group, the organic acid may have one or more carboxyl groups per molecule, and preferably has a plurality of carboxyl groups. This facilitates the formation of chelates between the organic acid and divalent or higher metals, such as calcium, contained in excrement, and, when recycled pulp fibers are formed from used sanitary products, the ash content of the resulting recycled pulp fibers can be reduced.

[0074] Examples of the organic acid include citric acid, tartaric acid, malic acid, succinic acid, oxalic acid (the above carboxylic acids having multiple carboxyl groups), gluconic acid (C6), valeric acid (C5), butyric acid (C4), propionic acid (C3), glycolic acid (C2), acetic acid (C2), glacial acetic acid, formic acid (C1) (the above carboxylic acids having one carboxyl group), methanesulfonic acid, trifluoromethanesulfonic acid, benzenesulfonic acid, and p-toluenesulfonic acid (the above sulfonic acids).

[0075] The acid-containing aqueous solution preferably has a predetermined pH, which is preferably 4.5 or less, more preferably 4.0 or less, even more preferably 3.5 or less, and even more preferably 3.0 or less. If the predetermined pH is too high, the primary dehydration of the superabsorbent polymer may not be sufficiently performed, and separation of the primary dehydrated superabsorbent polymer from the pulp fibers may be difficult.

[0076] The predetermined pH is preferably 0.5 or higher, and more preferably 1.0 or higher. If the predetermined pH is too low, the recycled pulp fibers may be damaged when recycled pulp fibers are formed from used sanitary products.

[0077] In addition, in this specification, pH means the value at 25 degrees C. Moreover, pH can be measured using the twin pH meter AS-711 manufactured by Horiba, Ltd., for example.

[0078] In the production method disclosed herein, the acidic aqueous solution preferably satisfies at least the aforementioned pH at the start of the primary dehydration step, for example, when the sanitary article constituent material is immersed in the acidic aqueous solution. Since the superabsorbent polymer undergoes primary dehydration, insufficient primary dehydration of the superabsorbent polymer can lead to insufficient discharge of fluids such as bodily fluids retained by the superabsorbent polymer, and difficulty in separating the primary dehydrated superabsorbent polymer from pulp fibers and the like.

[0079] In the production method disclosed herein, it is preferred that the pH value described above be satisfied at the end of the primary dehydration step in order to maintain the super absorbent polymer in a state where the polymer has undergone primary dehydration.

[0080] The super absorbent polymer is not particularly limited as long as it is a super absorbent polymer having an acid group in the relevant technical field. Examples thereof include super absorbent polymers containing carboxyl groups, sulfonic groups, etc., and preferably super absorbent polymers containing carboxyl groups.

[0081] Examples of the carboxyl group-containing superabsorbent polymer include polyacrylate-based and polymaleic anhydride-based superabsorbent polymers, and examples of the sulfonic group-containing superabsorbent polymer include polysulfonate-based superabsorbent polymers.

[0082] The pulp fibers are not particularly limited as long as they can be contained in sanitary products.

[0083] It should be noted that the acid used for primary dehydration of the super absorbent polymer preferably has an acid dissociation constant (pK) greater than that of the acid group in the super absorbent polymer in order to effectively dehydrate the super absorbent polymer. a , in water) small acid dissociation constant (pK a , in water).

[0084] When the acid has multiple acid groups, for example, when the acid is a dibasic acid or a tribasic acid, the acid dissociation constant (pK a , the largest acid dissociation constant (pK a , in water) than the acid dissociation constant (pK a In the case where the highly absorbent polymer has multiple acid groups, the acid dissociation constant (pK a , the largest acid dissociation constant (pK a, in water) than the smallest acid dissociation constant (pK a This is from the perspective of the efficiency of primary dehydration of the highly absorbent polymer.

[0085] In this specification, the acid dissociation constant (pK a , in water) can use the values ​​listed in the Electrochemical Handbook compiled by the Electrochemical Society.

[0086] Based on the electrochemical handbook, the acid dissociation constants (pK a , in water, 25°C) as described below.

[0087] [Organic acid]

[0088] Tartaric acid: 2.99 (pK a1 ), 4.44(pK a2 )

[0089] Malic acid: 3.24 (pK a1 ), 4.71(pK a2 )

[0090] Citric acid: 2.87 (pK a1 ), 4.35(pK a2 ), 5.69(pK a3 )

[0091] [Inorganic acid]

[0092] Sulfuric acid: 1.99 (pK a2 )

[0093] The acid dissociation constant (pK) of an acid not listed in the electrochemical handbook a , in water) can be determined by measurement. As the acid dissociation constant (pK a , water) can be measured, for example, by the compound property evaluation and analysis system and T3 manufactured by Sirius.

[0094] In the production method disclosed herein, the specific method is not particularly limited as long as the sanitary product constituent materials can be immersed in the acid-containing aqueous solution. For example, the sanitary product constituent materials can be added to a tank containing the acid-containing aqueous solution, or the acid-containing aqueous solution can be added to a tank where the sanitary product constituent materials are placed.

[0095] The sanitary products are not particularly limited as long as they contain pulp fibers and a highly absorbent polymer, and examples thereof include disposable diapers, disposable pants, sanitary napkins, panty liners, urine-absorbing pads, bed sheets, and pet sheets.

[0096] Examples of the sanitary products include those comprising a liquid-permeable sheet, a liquid-impermeable sheet, and an absorbent body (absorbent core and core cover) therebetween.

[0097] In the manufacturing method disclosed herein, the sanitary product constituent material in the primary dehydration step may be a mixture of pulp fibers and a highly absorbent polymer, such as an absorbent core removed from a used sanitary product. Alternatively, the sanitary product constituent material may be the sanitary product itself.

[0098] When the constituent materials of the sanitary product to be immersed in the acid-containing aqueous solution include, in addition to pulp fibers and superabsorbent polymers (hereinafter sometimes referred to as "specific materials"), additional materials (hereinafter sometimes referred to as "non-specific materials") such as liquid-permeable sheets and liquid-impermeable sheets, the sanitary product itself can be immersed in the acid-containing aqueous solution as the constituent materials of the sanitary product.

[0099] In the primary dehydration step, for example, the sanitary article constituent material is stirred in a primary dehydration tank containing an acidic aqueous solution for about 5 to 60 minutes, depending on the temperature, to thereby primary dehydrate the superabsorbent polymer and form a primary dehydrated superabsorbent polymer.

[0100] The acid-containing aqueous solution in the primary dehydration step may or may not be heated.

[0101] The temperature of the acid-containing aqueous solution in the primary dehydration step may be, for example, room temperature (25° C.) to 40° C., or lower than 60° C., when not heated.

[0102] The temperature of the acid-containing aqueous solution in the primary dehydration step, when heated, is preferably higher than room temperature, more preferably 60 to 100° C., further preferably 70 to 95° C., and even more preferably 80 to 90° C. This facilitates sterilization of bacteria derived from excreta and the like contained in the acid-containing aqueous solution by the acid contained in the acid-containing aqueous solution.

[0103] In the primary dehydration step, it is preferred to perform the primary dehydration so that the primary dehydrated superabsorbent polymer has a moisture content of preferably greater than 80.0% by mass and not more than 97.0% by mass, more preferably 85.0 to 96.0% by mass, and even more preferably 87.0 to 95.0% by mass. This facilitates reducing the moisture content of the secondary dehydrated superabsorbent polymer.

[0104] In this specification, the moisture content is measured using an infrared moisture meter FD-720 manufactured by Kett Electric Laboratory Co. Ltd. Specifically, approximately 5 g of a sample is placed in a sample dish of the FD-720, the temperature is set to 150° C., and the automatic stop mode is selected to measure the moisture content.

[0105] <Second Dehydration Step>

[0106] In the secondary dehydration step, the primary dehydrated super absorbent polymer is brought into contact with a polyvalent metal ion supply source capable of supplying polyvalent metal ions, thereby further dehydrating the primary dehydrated super absorbent polymer to form a secondary dehydrated super absorbent polymer.

[0107] In the secondary dehydration step, the acid groups (eg, -COOH) of the super absorbent polymer having acid groups are cross-linked with polyvalent metal ions (eg, calcium ions), and the primary dehydrated super absorbent polymer is further dehydrated to form a secondary dehydrated super absorbent polymer.

[0108] Examples of the polyvalent metal ions include alkaline earth metal ions and transition metal ions.

[0109] Examples of the alkaline earth metal ions include beryllium ions, magnesium ions, calcium ions, strontium ions, and barium ions. Examples of the transition metal ions include iron ions, cobalt ions, nickel ions, and copper ions.

[0110] When the polyvalent metal ions are alkaline earth metal ions, examples of polyvalent metal ion supply sources capable of supplying the polyvalent metal ions include alkaline earth metal hydroxides (e.g., calcium hydroxide, magnesium hydroxide), salts of alkaline earth metal hydroxides and acids (e.g., calcium chloride, calcium nitrate, magnesium chloride, magnesium nitrate), and alkaline earth metal oxides (e.g., calcium oxide, magnesium oxide). Calcium chloride is preferred. Examples of the acid include the acids used in the acid-containing aqueous solution described in the "primary dehydration step."

[0111] When the polyvalent metal ions are transition metal ions, examples of polyvalent metal ion supply sources capable of supplying the polyvalent metal ions include transition metal hydroxides (e.g., iron hydroxide, cobalt hydroxide, nickel hydroxide, copper hydroxide), salts of transition metal hydroxides and acids, and transition metal oxides (e.g., iron oxide, cobalt oxide, nickel oxide, copper oxide). Examples of the acid include the acids used in the acid-containing aqueous solution described in the "primary dehydration step."

[0112] Specific examples of the salts of the transition metal hydroxides and acids include inorganic acid salts and organic acid salts. Examples of the inorganic acid salts include iron salts such as ferric chloride, ferric sulfate, ferric phosphate, and ferric nitrate; cobalt salts such as cobalt chloride, cobalt sulfate, cobalt phosphate, and cobalt nitrate; nickel salts such as nickel chloride and nickel sulfate; and copper salts such as copper chloride and copper sulfate. Examples of the organic acid salts include iron lactate, cobalt acetate, cobalt stearate, nickel acetate, and copper acetate.

[0113] In the secondary dehydration step, for example, the primary dehydrated super absorbent polymer can be directly contacted with the polyvalent metal ion supply source itself capable of supplying polyvalent metal ions to form the secondary dehydrated super absorbent polymer. Alternatively, in the secondary dehydration step, the primary dehydrated super absorbent polymer can be stirred for approximately 5 to 60 minutes in a secondary dehydration tank containing an aqueous solution containing a polyvalent metal ion supply source capable of supplying polyvalent metal ions, thereby secondary dehydrating the primary dehydrated super absorbent polymer to form the secondary dehydrated super absorbent polymer, depending on the temperature.

[0114] The aqueous solution containing the polyvalent metal ion source in the secondary dehydration step may or may not be heated.

[0115] The temperature of the aqueous solution containing the polyvalent metal ion source in the secondary dehydration step is not particularly limited unless heated, and may be, for example, room temperature (25°C) to 40°C, or lower than 60°C.

[0116] The temperature of the aqueous solution containing the polyvalent metal ion source in the secondary dehydration step, when heated, is preferably higher than room temperature, more preferably 60 to 100°C, even more preferably 70 to 95°C, and even more preferably 80 to 90°C. This allows efficient supply of polyvalent metal ions from the polyvalent metal ion source by heating, even when the polyvalent metal ion source has low solubility in water, thereby promoting the formation of the secondary dehydrated superabsorbent polymer.

[0117] In the secondary dehydration step, the primary dehydrated superabsorbent polymer is dehydrated a second time so that the secondary dehydrated superabsorbent polymer preferably has a moisture content of 50 to 80% by mass, more preferably 50 to 75% by mass, and even more preferably 50 to 67% by mass. This reduces the amount of residual excrement in the secondary dehydrated superabsorbent polymer and facilitates the formation of a superabsorbent recycled polymer from the secondary dehydrated superabsorbent polymer. However, lowering the moisture content of the secondary dehydrated superabsorbent polymer tends to make it difficult for the secondary dehydrated superabsorbent polymer to react with the alkali metal ion supply source in the subsequent superabsorbent recycled polymer formation step, making it difficult to form a superabsorbent recycled polymer.

[0118] In the secondary dehydration step, when the primary dehydrated super absorbent polymer is brought into contact with an aqueous solution containing a polyvalent metal ion source, for example, when the primary dehydrated super absorbent polymer is placed in the aqueous solution containing the polyvalent metal ion source, the concentration of the polyvalent metal ion source in the aqueous solution containing the polyvalent metal ion source is preferably 1.0 to 30.0% by mass, more preferably 3.0 to 25.0% by mass, and even more preferably 5.0 to 20.0% by mass. This makes it easier for the secondary dehydrated super absorbent polymer to have a desired moisture content.

[0119] <Additional drying step>

[0120] In the production method of the present disclosure, after forming the secondary dehydrated super absorbent polymer in the secondary dehydration step, the method may further include a drying step of drying the secondary dehydrated super absorbent polymer to form a dried secondary dehydrated super absorbent polymer.

[0121] In the drying step, the secondary dehydrated super absorbent polymer is preferably dried so that the dried secondary dehydrated super absorbent polymer has a moisture content of preferably 10 to 30% by mass, and more preferably 15 to 25% by mass. This can reduce the amount of the alkali metal ion supply source used to form the super absorbent recycled polymer and maintain the water absorbency of the resulting super absorbent recycled polymer.

[0122] In the drying step, the secondary dehydrated super absorbent polymer is dried at a drying temperature of preferably room temperature (e.g., 25°C) to 120°C, more preferably 30°C to 80°C, and even more preferably 40°C to 60°C. Lowering the drying temperature tends to increase the drying time, while increasing the drying temperature may reduce the water absorbency of the dried super absorbent recycled polymer due to dehydration condensation of the acid groups in the dried super absorbent recycled polymer.

[0123] The drying step can be performed for, for example, 30 to 300 minutes.

[0124] <Highly absorbent recycled polymer formation steps>

[0125] In the super absorbent recycling polymer forming step, the secondarily dehydrated super absorbent polymer and an alkali metal ion supply source capable of supplying alkali metal ions are mixed in the regeneration aqueous solution to form the super absorbent recycling polymer from the secondarily dehydrated super absorbent polymer.

[0126] In addition, in this specification, the super absorbent polymer obtained by recycling the super absorbent polymer which has been dehydrated twice may be referred to as "super absorbent recycled polymer".

[0127] The alkali metal ion supply source is not particularly limited as long as it can supply alkali metal ions, and examples thereof include alkali metal hydroxides, salts of alkali metal hydroxides and acids (hereinafter sometimes simply referred to as "salts"), and alkali metal oxides.

[0128] Examples of the alkali metal ions include lithium ions, sodium ions, potassium ions, and any combinations thereof.

[0129] Examples of the alkali metal hydroxide include lithium hydroxide, sodium hydroxide, potassium hydroxide, and any combination thereof.

[0130] Examples of the salts include acid salts and basic salts.

[0131] Examples of the alkali metal hydroxide in the above salts include lithium hydroxide, sodium hydroxide, potassium hydroxide, and any combination thereof.

[0132] The acid in the salt is not particularly limited, and examples thereof include the acids for the acid-containing aqueous solution described in the "primary dehydration step" (e.g., hydrochloric acid, sulfuric acid), carbonic acid, and the like.

[0133] Examples of the alkali metal oxides include lithium oxide, sodium oxide, potassium oxide, and any combination thereof.

[0134] The alkali metal ion source and the polyvalent metal ion source have a relationship such that the difference in standard electrode potential between the alkali metal ions constituting the alkali metal ion source and the polyvalent metal ions constituting the polyvalent metal ion source is preferably 2.0 or less, more preferably 1.6 or less, even more preferably 1.0 or less, and even more preferably 0.5 or less. This facilitates replacement of the polyvalent metal ions crosslinked with the acid groups of the secondary dehydrated superabsorbent polymer with alkali metal ions, resulting in excellent water absorption of the recycled superabsorbent polymer.

[0135] The acid preferably has an acid dissociation constant (pK) greater than that of the acid group in the highly water-absorbent polymer. a , in water) with a large acid dissociation constant (pK a In this way, the acid group of the highly absorbent polymer easily forms an alkali metal salt.

[0136] When the acid has multiple acid groups, for example, when the acid is a dibasic acid or a tribasic acid, the acid dissociation constant (pK a , the smallest acid dissociation constant (pK a , in water) than the acid dissociation constant (pK a , in water) is large, and when the super absorbent polymer has multiple acid groups, it is preferred that the acid dissociation constant (pKa , in water) is the smallest acid dissociation constant (pK a , in water) is larger than the largest acid dissociation constant (pK a , in water) among the various acid groups of the superabsorbent polymer. This is considered from the viewpoint that the acid groups of the superabsorbent polymer easily form alkali metal salts.

[0137] Carbonic acid as the above-mentioned acid is preferred because it is not likely to remain in the regeneration aqueous solution or can be easily removed by heating or the like.

[0138] In addition, the above-mentioned acids other than carbonic acid also depend on the pH of the superabsorbent recycled polymer. For example, when the superabsorbent recycled polymer is at an acidic pH, antibacterial properties can be imparted to the superabsorbent recycled polymer.

[0139] Examples of the above-mentioned salts include lithium carbonate, sodium carbonate, potassium carbonate, lithium bicarbonate, sodium bicarbonate, potassium bicarbonate, lithium chloride, sodium chloride, potassium chloride, etc.

[0140] In the step of forming the superabsorbent recycled polymer, the regeneration aqueous solution can be acidic (pH < 7.0), neutral (pH = 7.0) or alkaline (7.0 < pH), preferably neutral or alkaline, more preferably alkaline. This is considered from the viewpoint of the effects of the present disclosure.

[0141] In order to make the regeneration aqueous solution acidic, neutral or alkaline, the regeneration aqueous solution can contain an acidic substance, an alkaline substance or a buffer substance (buffer solution), and the above-mentioned alkali metal ion supply source can contain an acidic substance, an alkaline substance or a buffer substance (buffer solution).

[0142] The above-mentioned acidic substance, alkaline substance or buffer substance (buffer solution) is preferred in cases where the alkali metal ion supply source is not likely to change the pH of the regeneration aqueous solution (for example, when the alkali metal ion supply source is lithium chloride, sodium chloride, potassium chloride, etc.), or when it is not desired to change the pH of the regeneration aqueous solution.

[0143] As the above-mentioned acidic substance, there is no particular limitation as long as it can make the pH of deionized water acidic (pH < 7.0), and acidic substances well-known in the technical field can be used. Examples include the acids for the aqueous acid solution described in the "first dehydration step" and carbonic acid.

[0144] As the above-mentioned alkaline substance, there is no particular limitation as long as it can make the pH of deionized water alkaline (7.0 < pH), and alkaline substances well-known in the technical field can be used. Examples include hydroxides or salts of alkali metals or hydroxides or salts of alkaline earth metals. From the viewpoint of not easily deactivating the superabsorbent polymer, hydroxides or salts of alkali metals are preferred. For hydroxides or salts of alkali metals, as described above.

[0145] Examples of the above buffer substance (buffer solution) include, for example, a carbonic acid-bicarbonate buffer solution containing sodium carbonate, sodium bicarbonate, and water.

[0146] If the above regeneration aqueous solution contains water, there is no particular limitation, and it can be a substantially neutral aqueous solution (e.g., tap water, deionized water, etc.), an acidic aqueous solution (aqueous solution with pH < 7.0), such as the acid-containing aqueous solution used in the first dehydration step, an aqueous solution containing the acid for the acid-containing aqueous solution described in the "first dehydration step", such as an inorganic acid or an organic acid aqueous solution, or a basic aqueous solution (aqueous solution with 7.0 < pH), such as an aqueous solution containing the above alkali metal ion supply source.

[0147] By adjusting the pH of the regeneration aqueous solution, the water absorption of the superabsorbent recycled polymer can be adjusted to a predetermined range.

[0148] As the above pH, there is no particular limitation as long as the superabsorbent recycled polymer can achieve a predetermined water absorption (absorption ratio), and it can be any one of an acidic pH region (pH < 7.0), a neutral pH (pH = 7.0), and a basic pH region (7.0 < pH), and the above pH is preferably 5.0 to 14.0.

[0149] When adjusting the above pH within the acidic pH region, the above regeneration aqueous solution is preferably an acidic aqueous solution, and preferably by directly adding or adding in the form of an aqueous solution an alkali metal ion supply source to the regeneration aqueous solution that is an acidic aqueous solution, a superabsorbent recycled polymer is formed.

[0150] When adjusting the above pH within the neutral pH or basic pH region, the above regeneration aqueous solution can be an acidic aqueous solution, a substantially neutral aqueous solution, or a basic aqueous solution, and is preferably a substantially neutral aqueous solution or a basic aqueous solution. This is considered from the viewpoint of effectively using the alkali metal ion supply source for the formation of the superabsorbent recycled polymer.

[0151] In addition, it is preferable to form a superabsorbent recycled polymer by directly adding or adding in the form of an aqueous solution an alkali metal ion supply source to the regeneration aqueous solution that is a substantially neutral aqueous solution or a basic aqueous solution.

[0152] The alkali metal ion supply source can be present in the regeneration aqueous solution as described above, and the alkali metal ion supply source can be directly added or added in the form of an aqueous solution to the regeneration aqueous solution.

[0153] When an alkali metal hydroxide is added to the regeneration aqueous solution as the alkali metal ion supply source, the alkali metal hydroxide is preferably an aqueous solution, and the aqueous solution is added to the regeneration aqueous solution at a hydroxide ion concentration of preferably 0.1 to 5.0 mol / L, more preferably 0.3 to 3.0 mol / L, and even more preferably 0.4 to 1.0 mol / L. This is done from the perspective of ease of adjustment of the pH of the regeneration aqueous solution and, consequently, the water absorbency of the highly absorbent recycled polymer.

[0154] The highly absorbent recycling polymer forming step can be carried out by stirring the regeneration aqueous solution at a predetermined temperature, for example, 2 to 80° C., for a predetermined time, for example, 5 to 60 minutes.

[0155] <Additional drying step>

[0156] The preparation method of the present disclosure may further include a drying step after the super absorbent recycled polymer forming step.

[0157] In the drying step, the super absorbent recycled polymer formed in the super absorbent recycled polymer forming step (hereinafter sometimes referred to as "super absorbent recycled polymer before drying") is dried to form a super absorbent recycled polymer having a predetermined water absorbency (hereinafter sometimes referred to as "super absorbent recycled polymer after drying" for distinction).

[0158] The predetermined water absorbency is not particularly limited. However, considering the practicality of the highly water-absorbent recycled polymer after drying, the water absorbency is preferably 100 times (g / g) or more, more preferably 200 times (g / g) or more, and even more preferably 300 times (g / g) or more relative to deionized water.

[0159] The predetermined water absorption is not particularly limited. However, considering the gel strength of the dried highly water-absorbent recycled polymer when swollen, the water absorption ratio is preferably 500 times (g / g) or less, more preferably 450 times (g / g) or less, and even more preferably 400 times (g / g) or less relative to deionized water.

[0160] In this specification, the water absorption rate is measured as follows.

[0161] (1) A sample (highly water-absorbent recycled polymer) was placed in a mesh and hung for 5 minutes to remove the water adhering to the surface. The mass before drying was measured: m1 (g).

[0162] (2) The sample whose mass before drying was measured was dried at 120°C for 120 minutes, and its mass after drying was measured: m2 (g).

[0163] (3) The water absorption rate (g / g) is calculated by the following formula:

[0164] Water absorption rate (g / g) = m1 / m2.

[0165] In the drying step, the aqueous solution for regeneration containing the super absorbent recycled polymer before drying obtained in the super absorbent recycled polymer forming step is directly dried to obtain the dried super absorbent recycled polymer.

[0166] Alternatively, the super absorbent recycled polymer can be dried by separating the super absorbent recycled polymer before drying from the regeneration aqueous solution containing the super absorbent recycled polymer using a device capable of solid-liquid separation.

[0167] The separated high water-absorbent recycled polymer before drying can be washed with tap water, deionized water, or the like before drying to remove the aqueous solution for regeneration adhering to the surface of the high water-absorbent recycled polymer before drying.

[0168] Furthermore, by contacting the separated, pre-dried super absorbent recycled polymer with a hydrophilic organic solvent (e.g., immersing it in a hydrophilic organic solvent) before drying, the water contained in the separated, pre-dried super absorbent recycled polymer can be dehydrated to a water absorption ratio of preferably 100 times (g / g) or less, more preferably 70 times (g / g) or less, and even more preferably 50 times (g / g) or less. This allows the drying temperature and / or drying time to be reduced.

[0169] As the above-mentioned hydrophilic organic solvent, a hydrophilic organic solvent that is miscible with water is preferred, and examples thereof include alcohol solvents (such as methanol, ethanol, propanol and its isomers, butanol and its isomers), ketone solvents (such as acetone, methyl ethyl ketone), and nitrile solvents (such as acetonitrile).

[0170] In the drying step, the super absorbent recycled polymer is dried at a drying temperature of preferably room temperature (e.g., 25° C.) to 120° C., more preferably 30° C. to 80° C., and even more preferably 40° C. to 60° C. Lowering the drying temperature tends to increase the drying time, while increasing the drying temperature may reduce the water absorbency of the super absorbent recycled polymer after drying due to dehydration condensation of the acid groups of the super absorbent recycled polymer after drying.

[0171] When the separated high water-absorbent recycled polymer before drying is brought into contact with a hydrophilic organic solvent before drying, the drying temperature can be low, for example, room temperature to 60°C.

[0172] The drying step can be performed under reduced pressure, for example, 0.1 kPa or more and less than 100 kPa.

[0173] The drying step can be performed for, for example, 30 to 300 minutes.

[0174] The drying step is preferably carried out so that the drying loss of the highly absorbent recycled polymer is preferably 15% or less (2.0 g, 105° C., 3 hours) from the viewpoint of utilization of the highly absorbent recycled polymer after drying.

[0175] The above-mentioned loss on drying is measured according to "7. Loss on Drying Test Method" under "2. General Test Methods" of "Standards for Materials of Physiological Treatment Products" in the attached sheet of "Standards for Materials of Physiological Treatment Products" issued by the Ministry of Health, Labour and Welfare of Japan as Notice No. 0325-24 on March 25, 2015.

[0176] After the drying step, when the dried highly water-absorbent recycled polymer is adhered or integrated, the dried highly water-absorbent recycled polymer may be pulverized or classified.

[0177] <<Method for producing secondary dehydration superabsorbent polymer>>

[0178] The method for producing a secondary dehydrated superabsorbent polymer disclosed herein includes the following steps.

[0179] a preparation step of preparing a primary dehydrated super absorbent polymer by immersing a sanitary article constituent material comprising pulp fibers and a super absorbent polymer having an acid group in an acidic aqueous solution containing an acid to form the primary dehydrated super absorbent polymer, and separating the primary dehydrated super absorbent polymer from the sanitary article constituent material to obtain the primary dehydrated super absorbent polymer (hereinafter sometimes referred to as the "preparation step");

[0180] A secondary dehydration step of further dehydrating the primary dehydrated super absorbent polymer by bringing the primary dehydrated super absorbent polymer into contact with a polyvalent metal ion supply source capable of supplying polyvalent metal ions, thereby forming the secondary dehydrated super absorbent polymer (hereinafter sometimes referred to as the "secondary dehydration step").

[0181] Preparation Steps

[0182] The primary dehydrated super absorbent polymer in the preparation step can be prepared, for example, by carrying out the “primary dehydration step” in the “method for producing a super absorbent recycled polymer from used sanitary products”.

[0183] <Second Dehydration Step>

[0184] Since the secondary dehydration step is the same as the “secondary dehydration step” in the “method for producing a highly absorbent recycled polymer from used sanitary products”, the description thereof will be omitted.

[0185] <<Method for producing highly absorbent recycled polymer>>

[0186] The method for preparing the highly absorbent recycled polymer disclosed herein includes the following steps.

[0187] a preparation step of preparing a secondarily dehydrated superabsorbent polymer by immersing a sanitary article constituent material comprising pulp fibers and a superabsorbent polymer having an acid group in an acidic aqueous solution containing an acid to form a firstly dehydrated superabsorbent polymer, separating the firstly dehydrated superabsorbent polymer from the sanitary article constituent material, and further dehydrating the firstly dehydrated superabsorbent polymer by contacting the firstly dehydrated superabsorbent polymer with a polyvalent metal ion supply source capable of supplying polyvalent metal ions, thereby obtaining the secondarily dehydrated superabsorbent polymer (hereinafter sometimes referred to as the "preparation step");

[0188] A super absorbent recycled polymer forming step of mixing the super absorbent polymer that has undergone the second dehydration and an alkali metal ion supply source capable of supplying alkali metal ions in an aqueous regeneration solution to form the super absorbent recycled polymer from the super absorbent polymer that has undergone the second dehydration (hereinafter sometimes referred to as the "super absorbent recycled polymer forming step").

[0189] Preparation Steps

[0190] The secondary dehydrated super absorbent polymer in the preparation step can be prepared, for example, by carrying out the “primary dehydration step” and “secondary dehydration step” in the “method for producing a super absorbent recycled polymer from used sanitary products”.

[0191] <Highly absorbent recycled polymer formation steps>

[0192] The step of forming the super absorbent recycled polymer is the same as the step of forming the super absorbent recycled polymer in the method for producing a super absorbent recycled polymer from used sanitary products, and thus its description is omitted.

[0193] The superabsorbent recycled polymer can be used for applications in which superabsorbent polymers are used without particular limitation. Examples of such applications include sanitary products (e.g., disposable diapers, incontinence pads (e.g., light incontinence pads), disposable shorts, sanitary napkins, panty liners, and mattress sheets), pet sheets, cat litter, and soil conditioners.

[0194] Example

[0195] [Example 1]

[0196] 100 g of polyacrylic acid-based superabsorbent polymer (AQUA KEEP, manufactured by Sumitomo Seika Co., Ltd., virgin product, hereinafter referred to as "SAP") was allowed to absorb physiological saline and then filtered through a 200-mesh filter to prepare simulated used SAP No. 1 having a moisture content of 97.5% by mass.

[0197] The simulated used SAP No. 1 was stirred in 4000 mL of a 1.0 mass % aqueous solution of citric acid as an acidic aqueous solution for 5 minutes, allowed to stand for 20 minutes, and then filtered through a 200-mesh filter to obtain a primary dehydrated SAP No. 1. The moisture content of the primary dehydrated SAP No. 1 was 95.0 mass %.

[0198] The primary dehydrated SAP No. 1 was stirred for 5 minutes in 4000 mL of a dispersion solution containing 10.0% by mass of calcium hydroxide as a source of polyvalent metal ions, allowed to stand for 1 hour, filtered through a 200-mesh filter, and then dehydrated at 150 G for 5 minutes using a centrifugal dehydrator to form a secondary dehydrated SAP No. 1. It should be noted that the dispersion solution containing 10.0% by mass of calcium hydroxide (Ca(OH) 2) also contained solid calcium hydroxide. However, it was confirmed that the solid calcium hydroxide was removed by filtering through a 200-mesh filter and then dehydrating using a centrifugal dehydrator. The moisture content of the secondary dehydrated SAP No. 1 was 69.0% by mass.

[0199] The twice-dehydrated SAP No. 1 was dried at 120° C. for 120 minutes to obtain a dried twice-dehydrated SAP No. 1.

[0200] The dryness of the twice-dehydrated SAP after drying was evaluated according to the following criteria. The results are shown in Table 1.

[0201] [Drying properties]

[0202] ◯: The secondary dehydrated SAP after drying was sufficiently dried and was in a powdery form or could be easily formed into a powdery form.

[0203] ×: The secondary dehydrated SAP after drying was insufficiently dried and was in a lumpy state.

[0204] [Examples 2 to 4]

[0205] The acid-containing aqueous solution and the polyvalent metal ion supply source were varied as shown in Table 1. Single-dehydrated SAP Nos. 2 to 4 and double-dehydrated SAP Nos. 2 to 4 were prepared, and their moisture content was measured. Furthermore, dried double-dehydrated SAP Nos. 2 to 4 were prepared, and their drying properties were evaluated. The results are shown in Table 1.

[0206] [Comparative Example 1]

[0207] The simulated used SAP No. 1 was stirred in 4000 mL of a 1.0 mass % aqueous solution of sulfuric acid as an acidic aqueous solution for 5 minutes, allowed to stand for 20 minutes, and then filtered through a 200-mesh filter to obtain a primary dehydrated SAP No. 5. The moisture content of the primary dehydrated SAP No. 5 was 90.0 mass %.

[0208] The dried primary dehydrated SAP No. 5 was dried at 120° C. for 120 minutes to obtain the dried primary dehydrated SAP No. 5, and its drying properties were evaluated. The results are shown in Table 1.

[0209] [Comparative Example 2]

[0210] The simulated used SAP No. 1 was stirred in 4000 mL of a 10.0 mass % dispersion solution of calcium hydroxide as a polyvalent metal ion source for 5 minutes, allowed to stand for 1 hour, passed through a 200-mesh filter, and then dehydrated using a centrifugal dehydrator at 150 G for 5 minutes to form a primary dehydrated SAP No. 6. The moisture content of the primary dehydrated SAP No. 6 was 73.0 mass %.

[0211] The dried primary dehydrated SAP No. 6 was dried at 120° C. for 120 minutes to obtain the dried primary dehydrated SAP No. 6, and its drying properties were evaluated. The results are shown in Table 1.

[0212] [Table 1]

[0213] surface@

[0214]

[0215] The double-dehydrated SAP Nos. 1 to 4 were immersed in a sodium hydroxide aqueous solution to form a highly absorbent recycled polymer. Compared to the single-dehydrated SAP Nos. 5 and 6, the highly absorbent recycled polymer could be formed using (i) a low-concentration sodium hydroxide aqueous solution and (ii) a small amount of sodium hydroxide aqueous solution.

[0216] Furthermore, the dried double-dehydrated SAP No. 1 to No. 4 was immersed in a sodium hydroxide aqueous solution to form a highly absorbent recycled polymer. The results showed that compared to the double-dehydrated SAP No. 1 to No. 4, highly absorbent recycled polymers could be formed using (i) a lower concentration of sodium hydroxide aqueous solution and (ii) a smaller amount of sodium hydroxide aqueous solution.

[0217] [Example 5]

[0218] A mixture of 100 g of a polyacrylic acid-based superabsorbent polymer (AQUA KEEP, manufactured by Sumitomo Seika Co., Ltd., virgin product, hereinafter referred to as "SAP") and 100 g of pulp fibers (coniferous pulp fibers) was allowed to absorb 4000 mL of physiological saline solution. The swollen SAP and pulp fibers became integrated. The mixture, which had absorbed the physiological saline solution, was filtered through a 200-mesh filter to remove any residual physiological saline solution.

[0219] The mixture, from which the saline solution had been removed, was stirred for 5 minutes in 4000 mL of a 1.0% by mass aqueous solution of citric acid, an acidic aqueous solution, and allowed to stand for 20 minutes. The pulp fibers and the primary dehydrated SAP were then separated. The primary dehydrated SAP remaining in the pulp fibers was decomposed using ozone-containing gas according to the oxidant treatment step P19 of Patent Document 1, yielding recycled pulp fiber No. 1. The ash content of recycled pulp fiber No. 1 was 0.12% by mass.

[0220] [Example 6]

[0221] Recycled pulp fiber No. 2 was obtained in the same manner as in Example 5 except that "4000 mL of a 1.0 mass % aqueous solution of citric acid" was replaced with "4000 mL of a 1.0 mass % aqueous solution of sulfuric acid." The ash content of recycled pulp fiber No. 2 was 0.15 mass %.

[0222] [Comparative Example 3]

[0223] Recycled pulp fiber No. 3 was obtained in the same manner as in Example 5 except that "4000 mL of a 1.0 mass % aqueous solution of citric acid" was replaced with "4000 mL of a 10.0 mass % dispersion of calcium hydroxide." The ash content of recycled pulp fiber No. 3 was 12.1 mass %.

Claims

1. A method for producing a highly absorbent polymer having undergone secondary dehydration for use in a highly absorbent recycling polymer, characterized in that: The method comprises: a preparation step of preparing a first dehydrated super absorbent polymer obtained by immersing a sanitary product constituent material containing pulp fibers and a super absorbent polymer having an acid group in an acid-containing aqueous solution containing an acid to form the first dehydrated super absorbent polymer, and separating the first dehydrated super absorbent polymer from the sanitary product constituent material to obtain the first dehydrated super absorbent polymer; The secondary dehydration step further dehydrates the first-dehydrated super absorbent polymer by bringing the first-dehydrated super absorbent polymer into contact with a polyvalent metal ion supply source capable of supplying polyvalent metal ions, thereby forming the second-dehydrated super absorbent polymer.

2. The method according to claim 1, wherein: In the secondary dehydration step, the super absorbent polymer that has undergone the primary dehydration is further dehydrated to a moisture content of 50.0 to 80.0% by mass, thereby forming the super absorbent polymer that has undergone the secondary dehydration.

3. The method according to claim 1 or 2, wherein: The polyvalent metal ion supply source is an alkaline earth metal salt or a transition metal hydroxide, a salt of an alkaline earth metal salt or a transition metal hydroxide and an acid, or an alkaline earth metal salt or a transition metal oxide.

4. The method according to claim 1 or 2, wherein: In the secondary dehydration step, the super absorbent polymer that has undergone the primary dehydration is brought into contact with the polyvalent metal ion supply source or an aqueous solution of the polyvalent metal ion supply source, thereby forming the super absorbent polymer that has undergone the secondary dehydration.

5. The method according to claim 4, wherein: The secondary dehydration step further includes a drying step of drying the highly water-absorbent polymer that has undergone the secondary dehydration.

6. The method according to claim 1 or 2, wherein: In the preparation step, the super absorbent polymer that has been dehydrated once has a moisture content of more than 80.0% by mass and not more than 97.0% by mass.

7. The method according to claim 1 or 2, wherein: The method further includes a primary dehydration step before the preparation step, wherein the sanitary product constituent material is immersed in the acid-containing aqueous solution to form the primary dehydrated super absorbent polymer, and the primary dehydrated super absorbent polymer is separated from the sanitary product constituent material.

8. The method according to claim 1 or 2, wherein: After the secondary dehydration step, the method further includes the step of forming a highly absorbent recycling polymer, wherein the highly absorbent polymer having undergone the secondary dehydration and an alkali metal ion supply source capable of supplying alkali metal ions are mixed in an aqueous regeneration solution to form the highly absorbent recycling polymer from the highly absorbent polymer having undergone the secondary dehydration.

9. The method according to claim 8, wherein: The alkali metal ion supply source is (i) an alkali metal hydroxide whose standard electrode potential difference with the polyvalent metal ion is 2.0 or less; (ii) an alkali metal hydroxide whose standard electrode potential difference with the polyvalent metal ion is 2.0 or less, and a salt of an acid having a larger acid dissociation constant than the acid group of the highly absorbent polymer; or (iii) an alkali metal oxide whose standard electrode potential difference with the polyvalent metal ion is 2.0 or less.

10. The method according to claim 9, wherein: The alkali metal ion is selected from the group consisting of lithium ion, sodium ion and potassium ion, and any combination thereof.

11. A method for producing a highly absorbent recycled polymer, characterized in that: The method comprises: a preparation step of preparing a super absorbent polymer that has been dehydrated twice by immersing a sanitary product constituent material that contains pulp fibers and a super absorbent polymer having an acid group in an acid-containing aqueous solution containing an acid to form a super absorbent polymer that has been dehydrated once, separating the super absorbent polymer that has been dehydrated once from the sanitary product constituent material, and further dehydrating the super absorbent polymer that has been dehydrated once by bringing the super absorbent polymer that has been dehydrated once into contact with a polyvalent metal ion supply source that can supply polyvalent metal ions, thereby obtaining the super absorbent polymer that has been dehydrated twice; The highly absorbent recycling polymer forming step comprises mixing the highly absorbent polymer that has undergone secondary dehydration and an alkali metal ion supply source that can supply alkali metal ions in a regeneration aqueous solution to form the highly absorbent recycling polymer from the highly absorbent polymer that has undergone secondary dehydration.

12. A method for producing a super absorbent recycled polymer from used sanitary products containing pulp fibers and a super absorbent polymer having an acid group, characterized in that: The method comprises: a primary dehydration step of immersing a sanitary product constituent material comprising the pulp fiber and the super absorbent polymer and constituting the sanitary product in an acid-containing aqueous solution containing an acid to form a super absorbent polymer that has undergone primary dehydration, and separating the super absorbent polymer that has undergone primary dehydration from the sanitary product constituent material; a secondary dehydration step of further dehydrating the super absorbent polymer that has undergone primary dehydration by bringing the super absorbent polymer that has undergone primary dehydration into contact with a polyvalent metal ion supply source that can supply polyvalent metal ions, thereby forming a super absorbent polymer that has undergone secondary dehydration; The highly absorbent recycling polymer forming step comprises mixing the highly absorbent polymer that has undergone secondary dehydration and an alkali metal ion supply source that can supply alkali metal ions in a regeneration aqueous solution to form the highly absorbent recycling polymer from the highly absorbent polymer that has undergone secondary dehydration.

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