A method for producing a superabsorbent recycled polymer regenerated from used superabsorbent polymers containing acid groups, the superabsorbent recycled polymer, and cat litter.

Abstract

To provide a method for producing superabsorbent recycled polymers, which can be easily manufactured from used superabsorbent polymers containing acid groups. [Solution] A method for producing a superabsorbent recycled polymer regenerated from a used superabsorbent polymer having an acid group, comprising: a preparation step of preparing the superabsorbent polymer; and a superabsorbent recycled polymer forming step of mixing the superabsorbent polymer with powder of an alkali metal salt of a peracid to form the superabsorbent recycled polymer.

Description

[Technical Field]

[0001] This disclosure relates to a method for producing a superabsorbent recycled polymer regenerated from used superabsorbent polymers, a superabsorbent recycled polymer, and cat litter comprising the superabsorbent recycled polymer. [Background technology]

[0002] Recycling used superabsorbent polymers is being considered. For example, Patent Document 1 discloses a method for regenerating a superabsorbent polymer inactivated by an acid into a superabsorbent recycled polymer having a predetermined water absorption capacity, the method comprising: a preparation step of preparing a superabsorbent polymer having acid groups and being inactivated by an acid; a superabsorbent recycled polymer formation step of adding an alkali metal ion source capable of supplying alkali metal ions to a regeneration aqueous solution containing the acid-inactivated superabsorbent polymer to form the superabsorbent recycled polymer in a wet state from the acid-inactivated superabsorbent polymer; and a drying step of drying the superabsorbent recycled polymer in a wet state to form the superabsorbent recycled polymer having the predetermined water absorption capacity.

[0003] Furthermore, although it does not involve recycling used superabsorbent polymers, Patent Document 2 lists sodium percarbonate as a disinfectant, deodorizer, etc., in a method for processing used disposable diapers. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2019-135046 [Patent Document 2] Japanese Patent Publication No. 2003-19169 [Overview of the project] [Problems that the invention aims to solve]

[0005] Patent documents 1 and 2 do not disclose a method for producing a superabsorbent recycled polymer regenerated from used superabsorbent polymers related to this disclosure. Accordingly, the present disclosure aims to provide a method for producing a superabsorbent recycled polymer, which is recycled from used, acid-group-containing superabsorbent polymers, and which allows for the easy production of superabsorbent recycled polymers. [Means for solving the problem]

[0006] The Disclosers have found a method for producing a superabsorbent recycled polymer regenerated from a used superabsorbent polymer having an acid group, comprising: a preparation step of preparing the superabsorbent polymer; and a superabsorbent recycled polymer forming step of mixing the superabsorbent polymer with a powder of an alkali metal salt of a peracid to form the superabsorbent recycled polymer. [Effects of the Invention]

[0007] The method for producing a superabsorbent recycled polymer regenerated from a used superabsorbent polymer having acid groups, as disclosed herein, allows for the easy production of the superabsorbent recycled polymer. [Modes for carrying out the invention]

[0008] Specifically, this disclosure relates to the following aspects: [Aspect 1] A method for producing a superabsorbent recycled polymer regenerated from a used superabsorbent polymer having an acid group, Preparation steps for preparing the above superabsorbent polymer, A superabsorbent polymer is mixed with powder of an alkali metal salt of a peracid to form the superabsorbent recycled polymer. A method characterized by including the following.

[0009] In the above manufacturing method, in the step of forming the superabsorbent recycled polymer, the superabsorbent polymer is mixed with a powder of an alkali metal salt of peracid. Therefore, in the step of forming the superabsorbent recycled polymer, the superabsorbent polymer can be dehydrated, sterilized and bleached while restoring the water absorption of the superabsorbent polymer to form the superabsorbent recycled polymer. As a result, the above manufacturing method can easily manufacture the superabsorbent recycled polymer.

[0010] [Aspect 2] The method according to Aspect 1, wherein the peracid is percarbonic acid.

[0011] In the above manufacturing method, the peracid is percarbonic acid. Since the alkali metal salt of percarbonic acid volatilizes as carbon dioxide after reacting with the superabsorbent polymer, the step of removing percarbonic acid from the superabsorbent recycled polymer is unnecessary. Therefore, the above manufacturing method can easily manufacture the superabsorbent recycled polymer.

[0012] [Aspect 3] The method according to Aspect 1 or 2, wherein in the step of forming the superabsorbent recycled polymer, the superabsorbent polymer and the powder are mixed so that the concentration of the powder is 1 to 30% by mass based on the dry mass of the superabsorbent polymer.

[0013] In the above manufacturing method, in the step of forming the superabsorbent recycled polymer, the superabsorbent polymer and the powder are mixed at a predetermined ratio. Therefore, the water retention ratio of the superabsorbent recycled polymer is likely to be improved, and the whiteness of the superabsorbent recycled polymer is likely to be improved.

[0014] [Aspect 4] The method according to any one of Aspects 1 to 3, wherein in the above preparation step, the superabsorbent polymer has a moisture content of 35 ± 10% by mass.

[0015] In the above manufacturing method, in the above preparation step, the superabsorbent polymer has a predetermined moisture content. Therefore, while suppressing the adhesion of the superabsorbent polymers to each other due to the adhesiveness of the superabsorbent polymer, in the next superabsorbent recycled polymer formation step, the reaction between the superabsorbent polymer and the powder of the alkali metal salt of peracid can proceed accurately. From the above, the above manufacturing method can easily manufacture a superabsorbent recycled polymer.

[0016] [Aspect 5] The method according to any one of Aspects 1 to 4, further including a moisture content adjustment step of adjusting the moisture content of the superabsorbent polymer before the above preparation step.

[0017] The above manufacturing method further includes a predetermined moisture content adjustment step before the above preparation step. Thereby, while suppressing the adhesion of the superabsorbent polymers to each other due to the adhesiveness of the superabsorbent polymer, in the next formation step, the reaction between the superabsorbent polymer and the powder of the alkali metal salt of peracid can proceed accurately. From the above, the above manufacturing method can easily manufacture a superabsorbent recycled polymer.

[0018] [Aspect 6] The method according to any one of Aspects 1 to 5, further including a dehydration step of contacting the superabsorbent polymer with a polyvalent metal ion source capable of supplying polyvalent metal ions before the above preparation step, dehydrating the superabsorbent polymer, and forming a dehydrated superabsorbent polymer.

[0019] In the above manufacturing method, before the preparation step, it further includes a predetermined dehydration step. Therefore, the liquid absorbed by the superabsorbent polymer, for example, body fluid, can be discharged. Further, when the above polyvalent metal ion source can exhibit strong alkalinity, such as quicklime, slaked lime, etc., the superabsorbent polymer can be sterilized. As a result, the above manufacturing method can easily manufacture a superabsorbent recycled polymer.

[0020] [Aspect 7] The method according to embodiment 6, further comprising a washing step of washing the dehydrated superabsorbent polymer in an acidic aqueous solution after the dehydration step and before the preparation step, to form a dehydrated superabsorbent polymer after washing.

[0021] The above manufacturing method further includes a predetermined washing step after the dehydration step and before the preparation step. This cleans the superabsorbent polymer, reduces the amount of polyvalent metals crosslinking the acid groups of the superabsorbent polymer, and adjusts the water absorption of the superabsorbent recycled polymer formed in the superabsorbent recycled polymer formation step.

[0022] [Aspect 8] The above-mentioned acidic aqueous solution has a pH of 1.5 to 5.0, according to the method of embodiment 7.

[0023] The above acidic aqueous solution has a predetermined pH. This allows for the purification of the superabsorbent polymer, a more precise reduction in the amount of polyvalent metals crosslinking the acid groups of the superabsorbent polymer, and a more precise adjustment of the water absorption of the superabsorbent recycled polymer formed in the superabsorbent recycled polymer formation step.

[0024] [Aspect 9] The above-mentioned acidic aqueous solution is the method according to embodiment 7 or 8, having a temperature of 15 to 60°C.

[0025] The above acidic aqueous solution has a predetermined temperature. This allows for the purification of the superabsorbent polymer, a more precise reduction in the amount of polyvalent metals crosslinking the acid groups of the superabsorbent polymer, and a more precise adjustment of the water absorption of the superabsorbent recycled polymer formed in the superabsorbent recycled polymer formation step.

[0026] [Aspect 10] The superabsorbent recycled polymer described above contains 10.0 to 20.0% by mass of polyvalent metals and 3.0 to 14.0% by mass of alkali metals in a dry state, according to any one of embodiments 6 to 9.

[0027] In the above manufacturing method, the superabsorbent recycled polymer contains predetermined amounts of polyvalent metals and alkali metals. The polyvalent metals and alkali metals can maintain the hydrophilicity of the superabsorbent recycled polymer and suppress the water absorption rate of the superabsorbent recycled polymer. As a result, the superabsorbent recycled polymer exhibits excellent shape retention, maintaining its shape after absorbing water.

[0028] [Aspect 11] The superabsorbent recycled polymer described above has a W value of 80 or more in a dry state, according to the method according to any one of embodiments 1 to 10. In the above manufacturing method, since the superabsorbent recycled polymer has a predetermined W value, users are less likely to feel aversion towards the superabsorbent recycled polymer.

[0029] [Aspect 12] A superabsorbent recycled polymer regenerated from used superabsorbent polymers having acid groups, The above superabsorbent recycled polymer, in its dry state, contains 10.0 to 20.0% by mass of polyvalent metals and 3.0 to 14.0% by mass of alkali metals. A superabsorbent recycled polymer characterized by the following features.

[0030] The superabsorbent recycled polymer described above contains predetermined amounts of polyvalent metals and alkali metals. The polyvalent metals and alkali metals can maintain the hydrophilicity of the superabsorbent recycled polymer and suppress the water absorption rate of the superabsorbent recycled polymer. As a result, the superabsorbent recycled polymer exhibits excellent shape retention, maintaining its shape after absorbing water.

[0031] [Aspect 13] The superabsorbent recycled polymer described above has a water retention ratio of 2.0 to 20.0 (g / g) in a dry state, as described in Embodiment 12.

[0032] The above-mentioned superabsorbent recycled polymer has a predetermined water retention rate, and therefore exhibits superior shape retention when it absorbs water.

[0033] [Aspect 14] The superabsorbent recycled polymer described above has a W value of 80 or more in a dry state, as described in embodiment 12 or 13. Because the above-mentioned superabsorbent recycled polymer has a predetermined W value, users are less likely to feel aversion towards the superabsorbent recycled polymer.

[0034] [Aspect 15] A superabsorbent recycled polymer for use in cat litter, as described in any one of embodiments 12 to 14.

[0035] When the above-mentioned superabsorbent recycled polymer is used in cat litter (preferably the core layer of cat litter), it exhibits superior shape retention when absorbing cat urine.

[0036] [Aspect 16] Cat litter comprising a core layer and a shell layer, The superabsorbent recycled polymer described in any one of embodiments 12 to 15 is provided in the core layer. Cat litter characterized by the following features.

[0037] The above cat litter exhibits superior shape retention when it absorbs cat urine.

[0038] The following describes in detail the method for producing a superabsorbent recycled polymer regenerated from used superabsorbent polymers having acid groups (hereinafter sometimes simply referred to as "method for producing superabsorbent recycled polymer"), the superabsorbent recycled polymer, and cat litter containing the above-mentioned superabsorbent recycled polymer (hereinafter sometimes simply referred to as "cat litter").

[0039] The method for producing the superabsorbent recycled polymer described herein includes the following steps. - Preparation step for preparing the above-mentioned superabsorbent polymer (hereinafter referred to as the "preparation step") - A superabsorbent polymer is mixed with powder of an alkali metal salt of a peracid to form the superabsorbent recycled polymer (hereinafter referred to as the "superabsorbent recycled polymer forming step").

[0040] The method for producing a superabsorbent recycled polymer according to this disclosure may further include the following steps as optional steps. - A dehydration step (hereinafter referred to as the "dehydration step") prior to the above preparation step, in which the superabsorbent polymer is brought into contact with a polyvalent metal ion source capable of supplying polyvalent metal ions, the superabsorbent polymer is dehydrated, and a dehydrated superabsorbent polymer is formed. - A washing step (hereinafter referred to as the "washing step") is performed after the above dehydration step and before the above preparation step, in which the dehydrated superabsorbent polymer is washed with an acidic aqueous solution to form a dehydrated superabsorbent polymer after washing. - A moisture content adjustment step (hereinafter referred to as the "moisture content adjustment step") to adjust the moisture content of the superabsorbent polymer, prior to the above preparation step.

[0041] [Preparation Steps] In the preparation step, a used, acid-containing superabsorbent polymer is prepared. The superabsorbent polymer having the acid group described above is not particularly limited as long as it is used in the art as a superabsorbent polymer having an acid group, and examples include those containing a carboxyl group, a sulfo group, etc., with those containing a carboxyl group being preferred. Examples of superabsorbent polymers containing carboxyl groups include polyacrylate-based and polymaleate-based polymers, while examples of superabsorbent polymers containing sulfo groups include polysulfonate-based polymers.

[0042] The above-mentioned used superabsorbent polymer having acidic groups can be prepared, for example, by extracting the superabsorbent polymer having acidic groups containing bodily fluids from used sanitary products. It should be noted that in used sanitary products, not all of the superabsorbent polymer contained in the absorbent material absorbs bodily fluids; the above-mentioned used superabsorbent polymer having acidic groups only needs to have absorbed a portion of the bodily fluids. The above-mentioned hygiene products are not particularly limited as long as they contain a superabsorbent polymer with acid groups, and examples include disposable diapers, disposable underwear, sanitary napkins, panty liners, incontinence pads, bed sheets, pet sheets, etc.

[0043] The above-mentioned used superabsorbent polymer having acidic groups preferably has a moisture content of 35±10% by mass, more preferably 35±7% by mass, and even more preferably 35±5% by mass. This suppresses adhesion between the superabsorbent polymers due to their tackiness, while allowing the reaction between the superabsorbent polymer and the alkali metal salt powder of the peracid to proceed accurately in the subsequent superabsorbent recycled polymer formation step. In this disclosure, the above moisture content is measured using Kett's FD-720 infrared moisture meter. Specifically, approximately 5 g of the sample is placed in the sample dish of the FD-720, the set temperature is set to 150°C, and the automatic stop mode is selected to measure the moisture content.

[0044] In the above preparation step, the used superabsorbent polymer having acid groups may have undergone at least one of the following: a dehydration step, a washing step, and a moisture content adjustment step.

[0045] [Dehydration step] In the optional dehydration step, prior to the preparation step, the used superabsorbent polymer having acidic groups is brought into contact with a polyvalent metal ion source capable of supplying polyvalent metal ions to dehydrate the superabsorbent polymer and form a dehydrated superabsorbent polymer. This allows the liquid absorbed by the superabsorbent polymer, such as bodily fluids, to be expelled. Furthermore, if the polyvalent metal ion source can be strongly alkaline, such as quicklime or slaked lime, the superabsorbent polymer can be sterilized.

[0046] In the dehydration step described above, the acidic groups (e.g., -COOH) of the superabsorbent polymer having acidic groups are crosslinked by polyvalent metal ions (e.g., calcium ions), the superabsorbent polymer having acidic groups is dehydrated, and a dehydrated superabsorbent polymer is formed.

[0047] Examples of the polyvalent metal ions mentioned above include alkaline earth metal ions, transition metal ions, and post-transition metal ions. Examples of the above-mentioned alkaline earth metal ions include beryllium ions, magnesium ions, calcium ions, strontium ions, and barium ions. Examples of the above-mentioned transition metal ions include iron ions, cobalt ions, nickel ions, and copper ions. Examples of the above-mentioned post-transition metal ions include aluminum ions.

[0048] When the above-mentioned polyvalent metal ions are alkaline earth metal ions, examples of polyvalent metal ion sources capable of supplying the above-mentioned polyvalent metal ions include hydroxides of alkaline earth metals (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 oxides of alkaline earth metals (e.g., calcium oxide, magnesium oxide), with calcium chloride being preferred.

[0049] The above-mentioned acids are not particularly limited, but examples include inorganic acids and organic acids. Examples of inorganic acids include sulfuric acid, hydrochloric acid, and nitric acid, with sulfuric acid being preferred from the viewpoint of not containing chlorine and cost. Examples of organic acids include those having an acid group, such as a carboxyl group or a sulfo group. Organic acids having a sulfo group are called sulfonic acids, and organic acids having a carboxyl group but not a sulfo group are called carboxylic acids. From the viewpoint of protecting equipment, organic acids having a carboxyl group, and especially carboxylic acids, are preferred.

[0050] Examples of the above organic acids include citric acid, tartaric acid, malic acid, succinic acid, oxalic acid (all carboxylic acids having multiple carboxyl groups), gluconic acid (C6), pentanoic acid (C5), butanoic acid (C4), propionic acid (C3), glycolic acid (C2), acetic acid (C2), glacial acetic acid, formic acid (C1) (all carboxylic acids having one carboxyl group), methanesulfonic acid, trifluoromethanesulfonic acid, benzenesulfonic acid, p-toluenesulfonic acid (all sulfonic acids), etc.

[0051] When the above-mentioned polyvalent metal ions are transition metal ions, examples of polyvalent metal ion sources capable of supplying these ions include transition metal hydroxides (e.g., iron hydroxide, cobalt hydroxide, nickel hydroxide, copper hydroxide), transition metal hydroxides and acid salts, and transition metal oxides (e.g., iron oxide, cobalt oxide, nickel oxide, copper oxide). Examples of the above-mentioned acids include those listed under "Alkaline Earth Metal Hydroxides and Acid Salts."

[0052] Specific examples of the above-mentioned transition metal hydroxides and acid salts include inorganic salts and organic salts. Examples of the inorganic salts include iron salts such as iron chloride, iron sulfate, iron phosphate, and iron 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 salts include iron lactate, cobalt acetate, cobalt stearate, nickel acetate, and copper acetate.

[0053] When the above-mentioned polyvalent metal ions are post-transition metal ions, examples of polyvalent metal ion sources capable of supplying these ions include post-transition metal hydroxides (e.g., aluminum hydroxide), post-transition metal hydroxides and acid salts, and post-transition metal oxides. Examples of the above-mentioned acids include those listed under "Alkaline Earth Metal Hydroxides and Acid Salts."

[0054] Specific examples of the above-mentioned post-transition metal hydroxides and acid salts include inorganic acid salts and organic acid salts. Examples of the inorganic acid salts include aluminum salts such as aluminum chloride, aluminum sulfate, aluminum phosphate, and aluminum nitrate. Examples of the organic acid salts include aluminum lactate, aluminum acetate, and aluminum stearate.

[0055] In the dehydration step described above, for example, a used superabsorbent polymer having acidic groups can be brought into direct contact with a polyvalent metal ion source capable of supplying polyvalent metal ions to form a dehydrated superabsorbent polymer. Alternatively, in the dehydration step, a used superabsorbent polymer having acidic groups can be dehydrated and a dehydrated superabsorbent polymer can be formed by stirring the used superabsorbent polymer having acidic groups in a dehydration tank containing an aqueous solution of a polyvalent metal ion source capable of supplying polyvalent metal ions for approximately 5 to 60 minutes, depending on the temperature.

[0056] The aqueous solution containing the polyvalent metal ion source in the above dehydration step may or may not be heated. The temperature of the aqueous solution containing the polyvalent metal ion source in the dehydration step described above is not particularly limited if it is not heated, and can be, for example, room temperature (25°C) to 40°C, or less than 60°C.

[0057] In the dehydration step described above, the temperature of the aqueous solution containing the polyvalent metal ion source is preferably higher than room temperature, more preferably 60-100°C, even more preferably 70-95°C, and even more preferably 80-90°C, if heating is performed. If the solubility of the polyvalent metal ion source in water is low, heating can efficiently supply polyvalent metal ions from the source and promote the formation of the dehydrated superabsorbent polymer.

[0058] In the dehydration step described above, the used superabsorbent polymer having acidic groups is dehydrated so that the dehydrated superabsorbent polymer has a moisture content of preferably 55% by mass or less, more preferably 50% by mass or less, and even more preferably 48% by mass or less. This reduces the amount of waste remaining in the dehydrated superabsorbent polymer and makes it easier to form a superabsorbent recycled polymer from the dehydrated superabsorbent polymer. However, if the moisture content of the dehydrated superabsorbent polymer is reduced too much, it may become difficult to form a superabsorbent recycled polymer in the subsequent superabsorbent recycled polymer formation step.

[0059] In the dehydration step described above, when a used, acidic, dehydrated superabsorbent polymer is brought into contact with an aqueous solution containing a polyvalent metal ion source, for example, when a used, acidic, superabsorbent polymer is added to an aqueous solution containing a 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 preferably 5.0 to 20.0% by mass. This makes it easier for the dehydrated superabsorbent polymer to have the desired moisture content.

[0060] [Washing Step] In the optional washing step described above, the dehydrated superabsorbent polymer is washed with an acidic aqueous solution after the dehydration step and before the preparation step to form a post-wash dehydrated superabsorbent polymer. By washing the dehydrated superabsorbent polymer with an acidic aqueous solution, at least a portion of the acid groups [e.g., (-COO)2Ca] of the dehydrated superabsorbent polymer that were crosslinked by polyvalent metal ions (e.g., calcium ions) can be returned to free acid groups (e.g., -COOH). This cleans the superabsorbent polymer and reduces the amount of polyvalent metal crosslinking the acid groups of the superabsorbent polymer, thereby adjusting the water absorption of the superabsorbent recycled polymer formed in the superabsorbent recycled polymer formation step.

[0061] The above-mentioned acidic aqueous solution refers to an aqueous solution containing an acid. Examples of acids include the hydroxides and acid salts of the above-mentioned alkaline earth metals.

[0062] If the above acid is an organic acid having a carboxyl group, the organic acid may have one or more carboxyl groups per molecule, and it is preferable that it has multiple carboxyl groups. By doing so, the organic acid is more likely to form a chelate complex with polyvalent metal ions, such as calcium, and it becomes easier to remove polyvalent metal ions that were crosslinking the acid groups of the dehydrated superabsorbent polymer, or polyvalent metal ions that were attached to the dehydrated superabsorbent polymer.

[0063] The acid used to wash the dehydrated superabsorbent polymer returns the acid groups that were crosslinked by polyvalent metal ions back to free acid groups, thus affecting the acid dissociation constant (pK) of the acid groups in the superabsorbent polymer. a Acid dissociation constant (pK) smaller than (in water) a It is preferable to have (in water).

[0064] If the above acid has multiple acid groups, for example, if the above acid is a dibasic acid or a tribasic acid, the acid dissociation constant (pK) of the above acid is a The largest acid dissociation constant (pK) among those in water. a, in water) is preferably smaller than the acid dissociation constant (pK a , in water) of the acid group of the superabsorbent polymer. When the superabsorbent polymer has a plurality of types of acid groups, the acid dissociation constant (pK a , in water) among them, the largest acid dissociation constant (pK a , in water) is preferably smaller than the smallest acid dissociation constant (pK a , in water) of the plurality of types of acid groups of the superabsorbent polymer. This is from the viewpoint of returning the acid group of the superabsorbent polymer to the free acid group.

[0065] In the present disclosure, the acid dissociation constant (pK a , in water) can adopt the value described in the Electrochemical Handbook edited by The Electrochemical Society. According to the Electrochemical Handbook, the acid dissociation constants (pK a , in water, 25 °C) of the main compounds are as follows. [Organic acid] · Tartaric acid: 2.99 (pK a1 ), 4.44 (pK a2 ) · Malic acid: 3.24 (pK a1 ), 4.71 (pK a2 ) · Citric acid: 2.87 (pK a1 ), 4.35 (pK a2 ), 5.69 (pK a3 ) [Inorganic acid] · Sulfuric acid: 1.99 (pK a2 )

[0066] The acid dissociation constant (pK a , in water) of an acid not described in the Electrochemical Handbook can be determined by measurement. As an instrument capable of measuring the acid dissociation constant (pK a , in water) of an acid, for example, the Compound Physical Property Evaluation Analysis System, T3 manufactured by Sirius can be mentioned.

[0067] In the above cleaning, the ratio of the dehydrated superabsorbent polymer to the acidic aqueous solution is not particularly limited, but preferably, the acidic aqueous solution is used in a mass ratio of preferably 5 times or more, more preferably 7 times or more, and even more preferably 10 times or more, relative to the dehydrated superabsorbent polymer. This cleans the superabsorbent polymer and reduces the amount of polyvalent metals crosslinking the acid groups of the superabsorbent polymer, making it easier to adjust the water absorption of the superabsorbent recycled polymer formed in the superabsorbent recycled polymer formation step.

[0068] The above acidic aqueous solution preferably has a predetermined pH. The predetermined pH is preferably 5.0 or less, more preferably 4.0 or less, even more preferably 3.0 or less, and even more preferably 2.0 or less. Alternatively, the predetermined pH is preferably 1.0 or higher, more preferably 1.2 or higher, even more preferably 1.5 or higher, and even more preferably 1.8 or higher. This allows for the purification of the superabsorbent polymer, a more precise reduction in the amount of polyvalent metals crosslinking the acid groups of the superabsorbent polymer, and a more precise adjustment of the water absorption of the superabsorbent recycled polymer formed in the superabsorbent recycled polymer formation step. In this specification, pH refers to the value at 25°C. Furthermore, pH can be measured using, for example, a twin pH meter AS-711 manufactured by Horiba, Ltd.

[0069] The above acidic aqueous solution preferably has a predetermined temperature. The predetermined temperature is preferably 15°C or higher, more preferably 20°C or higher, even more preferably 25°C or higher, and even more preferably 30°C or higher. Alternatively, the predetermined temperature is preferably 90°C or lower, more preferably 80°C or lower, even more preferably 70°C or lower, and even more preferably 60°C or lower. This allows for the purification of the superabsorbent polymer, a more precise reduction in the amount of polyvalent metals crosslinking the acid groups of the superabsorbent polymer, and a more precise adjustment of the water absorption of the superabsorbent recycled polymer formed in the superabsorbent recycled polymer formation step.

[0070] In the above washing step, the method for washing the dehydrated superabsorbent polymer with the acidic aqueous solution is not particularly limited, and can be carried out, for example, by immersing the dehydrated superabsorbent polymer in the acidic aqueous solution. Furthermore, when immersing the dehydrated superabsorbent polymer in the acidic aqueous solution, the acidic aqueous solution may be stirred. The immersion time for the dehydrated superabsorbent polymer in the acidic aqueous solution can be, for example, 0.5 to 2.0 hours.

[0071] In the washing step described above, the dehydrated superabsorbent polymer after washing preferably has a moisture content of 55% by mass or less, more preferably 50% by mass or less, and even more preferably 48% by mass or less. This reduces the amount of residual waste, unreacted polyvalent metal ion sources in the dehydration step, and polyvalent metal ions with cross-linked acid groups in the dehydration step, and also restores the water absorption properties of the dehydrated superabsorbent polymer after washing, making it easier to form a superabsorbent recycled polymer from the dehydrated superabsorbent polymer after washing. However, if the moisture content of the dehydrated superabsorbent polymer after washing is reduced too much, it may become difficult to form a superabsorbent recycled polymer in the subsequent superabsorbent recycled polymer formation step.

[0072] [Moisture content adjustment step] In the optional moisture content adjustment step, (i) the moisture content of the superabsorbent polymer can be adjusted before the preparation step, (ii) the moisture content of the dehydrated superabsorbent polymer can be adjusted after the dehydration step and before the preparation step, or (iii) the moisture content of the washed and dehydrated superabsorbent polymer can be adjusted after the washing step and before the preparation step. This suppresses adhesion between the superabsorbent polymer, the dehydrated superabsorbent polymer, or the washed and dehydrated superabsorbent polymer (hereinafter, "the superabsorbent polymer, the dehydrated superabsorbent polymer, or the washed and dehydrated superabsorbent polymer" may be referred to as "superabsorbent polymer, etc.") due to their tackiness, while allowing the reaction between the superabsorbent polymer and the alkali metal salt powder of the peracid to proceed accurately in the next forming step.

[0073] The method for adjusting the moisture content is not particularly limited and includes, for example, natural drying, heat drying, reduced-pressure drying, immersion drying in a hydrophilic organic solvent, microwave drying, and freeze-drying of the superabsorbent polymer.

[0074] For example, the above-mentioned heat drying method involves drying the superabsorbent polymer in a dryer at 60 to 200°C for 1 to 24 hours. For example, the above-mentioned natural drying method involves drying the superabsorbent polymer at room temperature for 12 to 24 hours. Natural drying can also be carried out under forced air. For example, the above-mentioned vacuum drying method involves drying the superabsorbent polymer under reduced pressure at 60 to 80°C for 4 to 12 hours. For example, the above-mentioned immersion drying method involves immersing the superabsorbent polymer in a solvent for 5 to 60 minutes. From the viewpoint of adjusting the moisture content, the solvent is preferably of a purity of 95% by mass or higher and a water content of less than 5% by mass.

[0075] In the moisture content adjustment step described above, the superabsorbent polymer preferably has a moisture content of 35±10% by mass, more preferably 35±7% by mass, and even more preferably 35±5% by mass. This suppresses adhesion between the superabsorbent polymers due to their tackiness, while allowing the reaction between the superabsorbent polymer and the alkali metal salt powder of the peracid to proceed accurately in the subsequent superabsorbent recycled polymer formation step.

[0076] [Superabsorbent recycled polymer formation step] In the above step of forming a superabsorbent recycled polymer, the superabsorbent polymer is mixed with powder of an alkali metal salt of a peracid to form the superabsorbent recycled polymer. The peracid mentioned above is not particularly limited and includes, for example, percarbonate, perboric acid, peracetic acid, and persulfuric acid. Percarbonate is preferred as the peracid. The tackiness of the superabsorbent polymers suppresses adhesion between the superabsorbent polymers themselves, while allowing the reaction between the superabsorbent polymer and the alkali metal salt powder of the peracid to proceed accurately in the next step of forming a superabsorbent recycled polymer. Examples of alkali metals that make up the alkali metal salts mentioned above include sodium, potassium, and lithium.

[0077] The above powder preferably has an average particle size of 1 μm or more, and more preferably 10 μm or more. Furthermore, the above powder preferably has an average particle size of 1,000 μm or less, and more preferably 800 μm or less. This allows for the efficient formation of a superabsorbent recycled polymer. The average particle size mentioned above is measured using the Mastersizer 3000+Lab laser diffraction particle size distribution analyzer manufactured by Malvern Panalytical.

[0078] The mixing of the superabsorbent polymer and the alkali metal salt powder of the peracid is not particularly limited and can be carried out, for example, by putting the superabsorbent polymer and the alkali metal salt powder of the peracid into a container and stirring as appropriate. The mixing of the superabsorbent polymer and the alkali metal salt powder of the peracid can preferably be carried out for 30 minutes or more, and more preferably for 60 minutes or more. Alternatively, the mixing of the superabsorbent polymer and the alkali metal salt powder of the peracid can preferably be carried out for 120 minutes or less, and more preferably for 100 minutes or less.

[0079] The mixing of the superabsorbent polymer and the alkali metal salt powder of the peracid can be carried out at room temperature (e.g., 25°C) without heating, or with heating. When the mixing of the superabsorbent polymer and the alkali metal salt powder of the peracid is carried out without heating, it is preferable to heat the superabsorbent recycled polymer after the mixing in order to remove any remaining alkali metal salt of the peracid that has not reacted with the superabsorbent polymer.

[0080] When the mixing of the superabsorbent polymer and the alkali metal salt powder of the peracid is carried out by heating, the temperature is preferably 60°C or higher, more preferably 70°C or higher, and even more preferably 75°C or higher. Alternatively, the temperature can be preferably 120°C or lower, more preferably 115°C or lower, and even more preferably 110°C or lower. By heating the mixing at the above temperatures, hydrogen peroxide can be generated from the remaining alkali metal salt powder of the peracid that does not react with the superabsorbent polymer, and excess alkali metal salt of the peracid can be removed. Furthermore, by heating the mixing at the above temperatures, it becomes easier to improve the water retention ratio of the superabsorbent recycled polymer and to improve the whiteness of the superabsorbent recycled polymer.

[0081] The mixing ratio of the superabsorbent polymer to the alkali metal salt powder of the superacid varies depending on the performance of the superabsorbent recycled polymer formed, but is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more, relative to the superabsorbent polymer in its dry state (dry mass of the superabsorbent polymer). Furthermore, the mixing ratio of the superabsorbent polymer to the alkali metal salt powder of the superacid also varies depending on the performance of the superabsorbent recycled polymer formed, but is preferably 30% by mass or less, more preferably 20% by mass or less, and even more preferably 15% by mass or less, relative to the superabsorbent polymer in its dry state (dry mass of the superabsorbent polymer). This makes it easier to improve the water retention ratio of the superabsorbent recycled polymer and to improve the whiteness of the superabsorbent recycled polymer. However, if the ratio of alkali metal salt powder of the superacid is too low, the whiteness of the superabsorbent recycled polymer may be insufficient. Furthermore, if the proportion of alkali metal salt powder of the peracid is too high, the hydrogen peroxide generated from the alkali metal salt of the peracid may decompose the superabsorbent recycled polymer, increasing its viscosity and potentially causing the polymers to bond together.

[0082] In this disclosure, superabsorbent polymers in a dry state can be obtained by drying them at 120°C for 120 minutes. Furthermore, in this disclosure, the mass of superabsorbent polymers in a dry state may simply be referred to as the dry mass.

[0083] The above step of forming a superabsorbent recycled polymer may include a drying substep in which the superabsorbent polymer is mixed with a powder of an alkali metal salt of a peracid, and then the superabsorbent polymer is dried. By performing the drying substep, the moisture content of the formed superabsorbent recycled polymer can be adjusted. The above drying substep may involve drying the superabsorbent polymer, etc., in a dryer at 60-200°C for 1-24 hours, for example.

[0084] The moisture content of the superabsorbent recycled polymer is preferably 10% by mass or less, more preferably 5% by mass or less. This prevents the superabsorbent recycled polymer from becoming sticky, ensures its absorption capacity, and suppresses the growth of mold.

[0085] The above superabsorbent recycled polymer contains polyvalent metals in a dry state, preferably in a ratio of 10.0% by mass or more, more preferably 12.0% by mass or more, and even more preferably 14.0% by mass or more. Furthermore, the above superabsorbent recycled polymer contains polyvalent metals in a ratio of preferably 20.0% by mass or less, more preferably 18.0% by mass or less, and even more preferably 16.0% by mass or less.

[0086] The above-mentioned superabsorbent recycled polymer contains alkali metals in a dry state, preferably in a ratio of 3.0% by mass or more, more preferably 6.0% by mass or more, and even more preferably 10.0% by mass or more. Furthermore, the above-mentioned superabsorbent recycled polymer contains alkali metals in a ratio of preferably 14.0% by mass or less, more preferably 13.0% by mass or less, and even more preferably 12.0% by mass or less. By containing polyvalent metals and alkali metals within the above ranges, the superabsorbent recycled polymer exhibits excellent shape retention, maintaining its shape when it absorbs water.

[0087] The ratio of polyvalent metals in the superabsorbent recycled polymer can be adjusted, for example, by the amount of polyvalent metal ion source in the dehydration step, the contact time with the polyvalent metal ion source, etc., during the washing step. The ratio of alkali metals in the superabsorbent recycled polymer can be adjusted, for example, by the amount of alkali metal salt powder of the peracid in the superabsorbent recycled polymer formation step.

[0088] In this disclosure, the ratio of polyvalent metals and alkali metals in the superabsorbent recycled polymer is measured as follows: [Pre-processing] (1) Accurately weigh out approximately 1 g (mass: m(g)) of superabsorbent recycled polymer into a conical beaker. (2) Add 5 mL of nitric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., for the measurement of harmful metals) and 10 mL of hydrochloric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., for the measurement of harmful metals) to a conical beaker using a graduated pipette, and heat the contents of the conical beaker to decompose them.

[0089] (3) After disassembling the contents of the conical beaker, 0.5 mL of nitric acid (for measuring heavy metals) is added to the conical beaker using a pipette, and then approximately 5 mL of ultrapure water is added. The ultrapure water is produced from tap water using a pure water production system (Merck KGaA, Elix Essential UV5), and then from the above pure water using an ultrapure water production system (Merck KGaA, Simplicity UV). Hereinafter, it will simply be referred to as "ultrapure water".

[0090] (4) After allowing the contents of the conical beaker to cool, transfer the contents of the conical beaker to a 50 mL volumetric flask. Add 0.25 μL of mixed internal standard solution (Yttrium standard solution, Y1000, concentration: 1,000 mg / L, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) to the volumetric flask using a pipette, and then make up the volumetric flask with ultrapure water to prepare the test sample. If the contents of the volumetric flask are cloudy, filter the contents of the volumetric flask through filter paper (ADVANTEC quantitative filter paper No. 5C) after making up the volume, and use the filtrate as the test sample. (5) A blank sample is prepared by performing steps (1) to (5) using the same amount of ultrapure water as the superabsorbent recycled polymer instead of the superabsorbent recycled polymer.

[0091] [Preparation of standard solutions for calibration curves] (1) Dispense 0.5 mL of nitric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., for the measurement of harmful metals) using a pipette and place it into four 50 mL volumetric flasks. Next, dispense 0.25 μL of mixed internal standard solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., yttrium standard solution, Y1000, yttrium concentration: 1,000 mg / L) using a pipette and place it into the four 50 mL volumetric flasks containing the nitric acid. (2) If the metal to be measured is sodium as an alkali metal, take 0.05 mL, 0.1 mL, 0.2 mL, and 0.5 mL of sodium standard solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., Na1000, sodium concentration: 1,000 mg / L) using a pipette and add them to each of the four 50 mL volumetric flasks to which nitric acid and mixed internal standard solution were added in (1).

[0092] [measurement] (1) Following the operating procedure of the ICP mass spectrometer (Thermo Fisher Scientific Inc., iCAPQc), a calibration curve is prepared from the ionic intensity ratio (ionic intensity of sodium mass number / ionic intensity of yttrium mass number) and concentration of the calibration standard solution. The measured mass number of sodium is 23, and the measured mass number of yttrium is 89. (2) Determine the ionic strength ratio of the test sample (ionic strength of sodium by mass number / ionic strength of yttrium by mass number), and calculate the sodium concentration from the calibration curve.

[0093] (3) If the output of the test sample exceeds the upper limit of the concentration range of the calibration curve, the test sample shall be diluted at the specified dilution ratio: Z (times), and the measurement shall be repeated from the pretreatment stage, and the sodium concentration shall be calculated using the following formula. concentration ={((Ionic intensity ratio of test sample - a) / b)×Z}-{(Ionic intensity ratio of blank sample - a) / b} a: Slope of the calibration curve b: Intercept of the calibration curve (4) Determine the sodium concentration of the blank sample and confirm that it is below the lower limit of the concentration range of the calibration curve. (5) Divide the sodium concentration by m(g) and round the result according to JIS Z 8401:2019 "Rounding of Numerical Values". Round to two significant figures, up to the number of decimal places of the lower limit of quantification.

[0094] The above describes the method for measuring the concentration when the metal to be measured is sodium as an alkali metal. However, if the metal to be measured is not sodium as an alkali metal, the sodium standard solution can be modified as follows and the measurement can be performed in the same manner. If the metal to be measured is potassium as an alkali metal, replace the sodium standard solution with a potassium standard solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., Ka1000, potassium concentration: 1,000 mg / L). If the metal to be measured is lithium as an alkali metal, replace the sodium standard solution with a lithium standard solution (Li1000, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., lithium concentration: 1,000 mg / L).

[0095] If the metal to be measured is calcium as a polyvalent metal, the sodium standard solution should be changed to a calcium standard solution (Fujifilm Wako Pure Chemical Industries, Ltd., Ca1000, calcium concentration: 1,000 mg / L). The number of calcium mass measurements should be 43 and 44. If the metal to be measured is magnesium as a polyvalent metal, the sodium standard solution should be changed to a magnesium standard solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., Mg1000, magnesium concentration: 1,000 mg / L). The number of mass measurements for magnesium should be 24 and 25.

[0096] If the metal to be measured is aluminum as a polyvalent metal, replace the sodium standard solution with aluminum standard solution (Al1000, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., aluminum concentration: 1,000 mg / L), and take 0.001 mL, 0.004 mL, 0.010 mL, and 0.040 mL using a pipette, and add them to each of the four 50 mL volumetric flasks to which nitric acid and the mixed internal standard solution were added in (1). The number of mass measurements for aluminum is 27. If the metal to be measured is iron as a polyvalent metal, the sodium standard solution is changed to an iron standard solution (Fujifilm Wako Pure Chemical Industries, Ltd., Fe1000, iron concentration: 1,000 mg / L), and 0.001 mL, 0.004 mL, 0.010 mL, and 0.040 mL are taken using a pipette and added to each of the four 50 mL volumetric flasks to which nitric acid and the mixed internal standard solution were added in (1). The number of mass measurements for iron is 54 and 56.

[0097] The above-mentioned superabsorbent polymer has a water retention ratio of preferably 2.0 g / g or more, more preferably 4.0 g / g or more, and even more preferably 6.0 g / g or more in a dry state. Furthermore, the above-mentioned superabsorbent polymer has a water retention ratio of preferably 20.0 g / g or less, more preferably 15.0 g / g or less, and even more preferably 12.0 g / g or less. As a result, the above-mentioned superabsorbent recycled polymer exhibits superior shape retention when it absorbs water. Furthermore, the water retention rate in the above-mentioned superabsorbent recycled polymer can be achieved by adjusting the ratio of polyvalent metals and alkali metals.

[0098] In this disclosure, the water retention ratio is measured as follows: (1) Prepare a bag (200mm x 200mm) made of nylon net (250 mesh nylon net manufactured by NBC Mesh Tech Co., Ltd.) and measure its mass: N0 (g). (2) Place approximately 5 g of the sample in a dry state (dried at 120°C for 120 minutes) into a nylon net bag, and measure the mass including the nylon net bag: A0 (g). (3) Pour 1 liter of physiological saline into a beaker, immerse the nylon net bag containing the sample in it, and let it stand for 3 minutes. (4) Pull up the nylon net bag, place it in a drain net, and hang it for 3 minutes to drain the water.

[0099] (5) The sample, which was placed in a drain net and suspended for 3 minutes, was dehydrated using a centrifuge (separator manufactured by Kokusan Centrifugal Co., Ltd., model H130, rotation speed 850 rpm = 150 G) for 90 seconds at 150 G, and its mass B (g) was measured. (6) Prepare another set of nylon net bags cut to the same size, and perform steps (3) to (5) in the same manner without putting the sample inside. Measure the mass of the nylon net bags alone after draining the water: N(g). (7) Water retention ratio: WR (g / g) is given by the following formula: WR(g / g)=(BN-(A0-N0)) / (A0-N0) It is calculated by [method]. (8) Measurements will be taken a total of 10 times, and the average of the 10 measurements will be used as the water retention ratio.

[0100] The above superabsorbent recycled polymer has a W value of preferably 80 or higher, more preferably 82 or higher, even more preferably 84 or higher, even more preferably 85 or higher, and even more preferably 86 or higher in a dry state. Furthermore, the above superabsorbent recycled polymer has a W value of preferably 100 or lower, more preferably 98 or lower, even more preferably 96 or lower, even more preferably 94 or lower, and even more preferably 93 or lower. As a result, the superabsorbent recycled polymer has the same whiteness as virgin superabsorbent polymer, making it less likely for users to feel aversion towards the superabsorbent recycled polymer.

[0101] In this disclosure, the above W value is measured as follows: (1) Prepare a Z-300A cross-illumination photometric color difference meter manufactured by Nippon Denshoku Industries, Ltd. in a constant temperature and humidity chamber with a temperature of 20±5℃ and a humidity of 65±5%RH. (2) A sample of the superabsorbent recycled polymer, dried at 120°C for 120 minutes, is left standing in the above-mentioned constant temperature and humidity chamber for 24 hours. (3) Spread 2.0 g of the sample evenly in a glass petri dish with a diameter of 30 mm.

[0102] (4) Set the mode of the above colorimeter to reflection mode. (5) Place the glass petri dish containing the sample into the colorimeter's measurement case and close the lid of the colorimeter. (6) Measure the W value of the sample. (7) Measure the W value a total of 10 times using different samples and use the average value.

[0103] The uses of the above-mentioned superabsorbent recycled polymer are not particularly limited, but it is useful in applications where a certain degree of water retention is preferable, such as cat litter, toilets (e.g., emergency toilets), water bags, and soil conditioners.

[0104] The cat litter described above includes a core layer and a shell layer, the core layer containing the superabsorbent recycled polymer. Preferably, the cat litter is of a type that absorbs excrement, especially urine. [Examples]

[0105] The following examples illustrate this disclosure, but this disclosure is not limited to these examples. [Manufacturing Example 1] Superabsorbent polymer that absorbed excrement was recovered from used disposable diapers. Approximately 6 kg (dry weight: approximately 1.2 kg) of the recovered superabsorbent polymer was immersed for 60 minutes in a container containing 32 L of 4.0% by mass slaked lime (Ca(OH)2) aqueous solution. Then, 18 times the amount of deionized water was added to the container, and the contents of the container were sieved to perform a dehydration step, thereby obtaining dehydrated superabsorbent polymer No. 1.

[0106] Dehydrated superabsorbent polymer No. 1 was immersed in 5 L of deionized water at room temperature, and the deionized water was stirred for 30 minutes to perform a washing step, thereby obtaining dehydrated superabsorbent polymer No. 1 after washing. After washing, dehydrated superabsorbent polymer No. 1 was dried at 110°C for 90 minutes to perform a moisture content adjustment step, thereby obtaining superabsorbent polymer No. 1 with a moisture content of approximately 35% by mass.

[0107] Superabsorbent polymer No. 1 was placed in a container, and 5 g of sodium percarbonate (manufactured by Hayashi Pure Chemical Industries, Ltd., average particle size: 100 μm, sodium percarbonate) was added to the container for every 100 g of dry mass of superabsorbent polymer No. 1. The contents of the container were stirred at 80°C for 90 minutes, and then the contents were dried at 120°C for 120 minutes (performing a drying substep (drying SS)) to carry out the superabsorbent recycled polymer formation step and obtain superabsorbent recycled polymer No. 1.

[0108] [Manufacturing Examples 2-4] Superabsorbent recycled polymers No. 2 to No. 4 were obtained in the same manner as in Production Example 1, except that the temperature in the washing step and the superabsorbent recycled polymer formation step were as shown in Table 1.

[0109] [Manufacturing Examples 5 and 6] Superabsorbent recycled polymers No. 5 and No. 6 were obtained in the same manner as in Production Example 1, except that in the washing step, "5 L of deionized water at room temperature" was changed to "5 L of 0.1% by mass citric acid aqueous solution at 60°C," and the amount of sodium percarbonate in the superabsorbent recycled polymer formation step was as shown in Table 1.

[0110] [Manufacturing Example 7] Superabsorbent recycled polymer No. 7 was obtained in the same manner as in Production Example 1, except that in the washing step, "5 L of deionized water at room temperature" was replaced with "5 L of 0.02% by mass sulfuric acid aqueous solution at 60°C". Furthermore, in order to increase the surface area of ​​the superabsorbent recycled polymer No. 7, it was crushed using the Super Free Mill (SJM) manufactured by Nara Machinery Works Co., Ltd.

[0111] [Comparative Manufacturing Example 1] Superabsorbent recycled polymer No. 8 was obtained in the same manner as in Production Example 1, except that the moisture content adjustment step and the superabsorbent recycled polymer formation step were omitted, and drying was performed at 120°C for 120 minutes, which corresponds to the drying substep. In the table, NA means not applicable. The same applies to the other examples.

[0112] [Comparative Manufacturing Examples 2-4] Superabsorbent recycled polymers No. 9 to No. 11 were obtained in the same manner as in Production Example 1, except that the temperature in the washing step was as shown in Table 1, the moisture content adjustment step and the superabsorbent recycled polymer formation step were not performed, and drying was carried out at 120°C for 120 minutes, which corresponds to the drying substep.

[0113] [Comparative Manufacturing Example 5] Superabsorbent polymers that have absorbed excrement were recovered from used disposable diapers, sieved, and dried at 120°C for 120 minutes (corresponding to a drying substep). This was designated as Superabsorbent Recycled Polymer No. 12. [Reference production example 1] A commercially available superabsorbent polymer (Sumitomo Seika Co., Ltd., AquaKeep®, SA60S) was designated as Superabsorbent Recycled Polymer No. 13.

[0114] [Examples 1-7, Comparative Examples 1-5, and Reference Example 1] The amounts of calcium (Ca) and sodium (Na) in superabsorbent recycled polymers No. 1 to No. 13 were measured. The results are shown in Table 1. In addition, the water retention ratio and W value of superabsorbent recycled polymers No. 1 to No. 13 were measured. The results are shown in Table 1.

[0115] [Table 1]

Claims

1. A method for producing a superabsorbent recycled polymer regenerated from a used superabsorbent polymer having an acid group, Preparation step of preparing the superabsorbent polymer, A superabsorbent polymer forming step involves mixing the superabsorbent polymer with powder of an alkali metal salt of a peracid to form the superabsorbent recycled polymer. A method characterized by including the following.

2. The method according to claim 1, wherein the peracid is percarbonate.

3. The method according to claim 1, wherein in the step of forming the superabsorbent recycled polymer, the superabsorbent polymer and the powder are mixed such that the concentration of the powder is 1 to 30% by mass with respect to the dry mass of the superabsorbent polymer.

4. The method according to claim 1, wherein in the preparation step, the superabsorbent polymer has a moisture content of 35 ± 10% by mass.

5. The method according to claim 4, further comprising a moisture content adjustment step of adjusting the moisture content of the superabsorbent polymer before the preparation step.

6. The method according to claim 1, further comprising a dehydration step, prior to the preparation step, of contacting the superabsorbent polymer with a source of polyvalent metal ions capable of supplying polyvalent metal ions to dehydrate the superabsorbent polymer and form a dehydrated superabsorbent polymer.

7. The method according to claim 6, further comprising a washing step of washing the dehydrated superabsorbent polymer with an acidic aqueous solution after the dehydration step and before the preparation step to form a dehydrated superabsorbent polymer after washing.

8. The method according to claim 7, wherein the acidic aqueous solution has a pH of 1.0 to 5.

0.

9. The method according to claim 7, wherein the acidic aqueous solution has a temperature of 15 to 60°C.

10. The method according to any one of claims 6 to 9, wherein the superabsorbent recycled polymer, in a dry state, contains 10.0 to 20.0% by mass of polyvalent metals and 3.0 to 14.0% by mass of alkali metals.

11. The method according to claim 1, wherein the superabsorbent recycled polymer has a W value of 80 or more in a dry state.

12. A superabsorbent recycled polymer regenerated from used superabsorbent polymers having acid groups, The superabsorbent recycled polymer, in its dry state, contains 10.0 to 20.0% by mass of polyvalent metals and 3.0 to 14.0% by mass of alkali metals. A superabsorbent recycled polymer characterized by the following features.

13. The superabsorbent recycled polymer according to claim 12, wherein the superabsorbent recycled polymer has a water retention ratio of 2.0 to 20.0 (g / g) in a dry state.

14. The superabsorbent recycled polymer according to claim 12, wherein the superabsorbent recycled polymer has a W value of 80 or more in a dry state.

15. The superabsorbent recycled polymer according to claim 12, for use in cat litter.

16. Cat litter comprising a core layer and a shell layer, The core layer comprises a superabsorbent recycled polymer according to any one of claims 12 to 15. Cat litter characterized by the following features.

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