Method for producing recycled superabsorbent polymer derived from used absorbent articles and recycled superabsorbent polymer derived from used absorbent articles

Abstract

Provided is a method for producing a recycled superabsorbent polymer derived from used absorbent articles, wherein the method has high safety and high washing effectiveness. This method for producing a recycled superabsorbent polymer derived from used absorbent articles includes: a dehydration and washing step (S3); and separation steps (S5, S7). In the dehydration and washing step, a mixed liquid is stirred to dehydrate a superabsorbent polymer, the mixed liquid being formed by mixing: a mixture that contains a superabsorbent polymer of used absorbent articles; and a processing liquid that contains at least a non-flammable hydrophobic organic solvent, a hydrophilic organic solvent, or water. In the separation steps, the superabsorbent polymer is separated from the mixed liquid.

Description

Method for producing recycled superabsorbent polymer derived from used absorbent article and recycled superabsorbent polymer derived from used absorbent article

[0001] The present invention relates to a method for producing a recycled superabsorbent polymer derived from a used absorbent article and a recycled superabsorbent polymer derived from a used absorbent article.

[0002] Methods for producing recycled members derived from used absorbent articles, such as recycled plastic materials, recycled superabsorbent polymers, and recycled pulp fibers, are known. For example, Patent Document 1 discloses a method for recycling a water-absorbing resin derived from a used absorbent article, which comprises discharging the absorbed liquid from the water-absorbing resin containing the absorbed liquid and restoring the water-absorbing capacity of the water-absorbing resin. This method includes: (i) an immersion step of immersing a used absorbent article containing a water-absorbing resin that has absorbed the absorbed liquid in an immersion liquid containing a hydrophilic organic solvent; (ii) a crushing step of crushing the used absorbent article into a crushed product during or before the immersion step; and (iii) a separation step of separating the water-absorbing resin from the mixture of the immersion liquid and the crushed product. In Patent Document 1, in step (iii), one or more members selected from pulp, non-woven fabric, and adhesive may be further separated and recovered, and a sterilization / disinfection step of sterilizing and / or disinfecting the water-absorbing resin may be performed in parallel with or after the immersion step. The separated water-absorbing resin, pulp, and non-woven fabric can become a recycled superabsorbent polymer, recycled pulp fibers, and recycled plastic material, respectively. Examples of the used absorbent article to be the target for recovering the constituent members include used sanitary materials that have absorbed liquids (absorbed liquids) such as urine and blood.

[0003] International Publication No. 2021 / 162082

[0004] In Patent Document 1, by immersing a used absorbent article in an immersion liquid containing a hydrophilic organic solvent, moisture (the absorbed liquid) such as urine is discharged (dehydrated) from the superabsorbent polymer (water-absorbing resin), and the water-absorbing capacity of the superabsorbent polymer is restored.

[0005] However, according to the inventor's research, the following facts were discovered for the first time. Patent Document 1 uses a hydrophilic organic solvent. However, hydrophilic organic solvents are difficult to handle from a safety standpoint due to their high volatility and flammability. In that case, it is conceivable to use a hydrophobic organic solvent instead of a hydrophilic organic solvent. However, hydrophobic organic solvents do not sufficiently remove hydrophilic contaminants from superabsorbent polymers. Therefore, it is difficult to obtain high-quality recycled superabsorbent polymers.

[0006] Therefore, the object of the present invention is to provide a method for producing recycled superabsorbent polymers derived from used absorbent articles that is highly safe and has a high cleaning effect, and to provide a high-quality recycled superabsorbent polymer derived from used absorbent articles.

[0007] One aspect of the present invention is a method for producing recycled superabsorbent polymers derived from used absorbent articles, comprising: a dehydration and washing step of stirring a mixture obtained by mixing a mixture containing a superabsorbent polymer from used absorbent articles with a treatment liquid containing at least one of a non-flammable hydrophobic organic solvent, a hydrophilic organic solvent, and water to dehydrate the superabsorbent polymer; and a separation step of separating the superabsorbent polymer from the mixture.

[0008] Another aspect of the present invention is a recycled superabsorbent polymer derived from used absorbent articles, wherein the polyvalent metal content is 0.06% by mass or less.

[0009] According to the present invention, a method for producing recycled superabsorbent polymers derived from used absorbent articles can be provided that is highly safe and has a high cleaning effect, and can also provide high-quality recycled superabsorbent polymers derived from used absorbent articles.

[0010] This is a flowchart illustrating an example of a method for producing recycled superabsorbent polymers derived from used absorbent articles according to the embodiment.

[0011] This embodiment relates to the following aspects.

[0012] [Aspect 1] A method for producing recycled superabsorbent polymers derived from used absorbent articles, comprising: a dewatering and washing step of stirring a mixture obtained by mixing a mixture containing superabsorbent polymers from used absorbent articles with a treatment liquid containing at least one of a non-flammable hydrophobic organic solvent, a hydrophilic organic solvent, and water to dewater the superabsorbent polymers; and a separation step of separating the superabsorbent polymers from the mixture.

[0013] In this method, first, in the dewatering and washing step, a mixture containing a superabsorbent polymer and a predetermined treatment solution is mixed and stirred to dewater the superabsorbent polymer. However, the mixture may include, for example, used absorbent articles or an assembly of multiple components derived from used absorbent articles.

[0014] In the dewatering and washing process, by adding a non-flammable hydrophobic organic solvent to the treatment solution in addition to a hydrophilic organic solvent, or by using water instead of the hydrophilic organic solvent and adding a non-flammable hydrophobic organic solvent, the volatility and flammability of the treatment solution can be reduced while maintaining the effect of the organic solvent (described later). This enhances the safety of this method. Note that the hydrophilic organic solvent and water may be used simultaneously.

[0015] Furthermore, the non-flammable hydrophobic organic solvent in the treatment solution can break the bonds between the superabsorbent polymer in the mixture and other components, as well as the bonds between the superabsorbent polymers themselves, thus dispersing the superabsorbent polymer in the treatment solution. For example, the adhesive joining the superabsorbent polymer to other components can be dissolved by the non-flammable hydrophobic organic solvent to break the bond. Alternatively, the bonds between superabsorbent polymers can be broken by the non-flammable hydrophobic organic solvent. Then, by stirring the mixture, the superabsorbent polymer can be separated from other components and dispersed freely in the liquid.

[0016] Furthermore, the treatment solution can remove contaminants from the mixture using at least one of a non-flammable hydrophobic organic solvent, a hydrophilic organic solvent, and water. Specifically, lipophilic contaminants on the superabsorbent polymer can be dissolved and removed using the non-flammable hydrophobic organic solvent, while hydrophilic contaminants can be dissolved and removed using at least one of a hydrophilic organic solvent and water. This allows for a high cleaning effect.

[0017] Furthermore, at least one of the non-flammable hydrophobic organic solvents and hydrophilic organic solvents in the treatment solution can be used to dehydrate the water absorbed by the superabsorbent polymer in the mixture. At this time, since dehydration with the organic solvent does not inactivate the superabsorbent polymer, the superabsorbent polymer can maintain its water absorption performance. As a result, there is no need for additional treatment such as reactivation to reuse the superabsorbent polymer as an absorbent material, and there is no risk of contamination with impurities associated with additional treatment. Moreover, since most of the treatment solution during dehydration is at least one of the non-flammable hydrophobic organic solvents and hydrophilic organic solvents, even if a small amount of water remains, it is possible to suppress the superabsorbent polymer from reabsorbing the water.

[0018] Thus, in the dewatering and washing process, dirt is removed under highly safe conditions, resulting in a hygienic superabsorbent polymer. Therefore, by separating it in the separation process, a hygienic recyclable superabsorbent polymer can be obtained.

[0019] Therefore, this method provides a highly safe and effective cleaning method for producing recycled superabsorbent polymers derived from used absorbent articles. Furthermore, since this method generally uses organic solvents for processing the mixture, the amount of water used for processing can be significantly reduced.

[0020] [Aspect 2] The method according to aspect 1, wherein the dewatering and washing step comprises a first stirring step of mixing the non-flammable hydrophobic organic solvent with at least one of the hydrophilic organic solvent and water to form the treatment liquid and stirring the treatment liquid, and a second stirring step of mixing the treatment liquid with the mixture to form the mixed liquid and stirring the mixed liquid.

[0021] In this method, the dewatering and washing process comprises a first stirring step and a second stirring step. First, in the first stirring step, a non-flammable hydrophobic organic solvent is mixed with at least one of a hydrophilic organic solvent and water to form a treatment solution, and this treatment solution is stirred. Due to the relationship between hydrophobicity and hydrophilicity, the non-flammable hydrophobic organic solvent and at least one of the hydrophilic organic solvent and water tend to separate when simply mixed. Therefore, in this method, the separation of the treatment solution can be suppressed by stirring the treatment solution in which the two are mixed. Subsequently, in the second stirring step, the (stirred) treatment solution is mixed with a mixture (containing a superabsorbent polymer) to form a mixed solution, and this mixed solution is stirred. The treatment solution is also prone to separation even when mixed with the mixture. Therefore, in this method, the separation of the treatment solution can be suppressed by stirring the mixed solution in which the treatment solution and the mixture are mixed. Furthermore, the volatility and flammability of the processing solution are further reduced, safety is enhanced, multiple components can be more reliably dispersed in the processing solution, dirt in the mixture can be more reliably removed, and superabsorbent polymers can be more reliably dehydrated.

[0022] [Aspect 3] The method according to aspect 1 or 2, further comprising an oxidizing agent treatment step of treating the mixture in the mixture with an oxidizing agent while stirring the mixture before the separation step.

[0023] This method further includes an oxidizing agent treatment step, which involves treating the mixture (including the superabsorbent polymer) in the mixed liquid with an oxidizing agent while stirring the mixed liquid, prior to the separation step (including simultaneously with the dewatering and washing step). This oxidizing agent treatment step can bleach, sterilize or disinfect, and deodorize the mixture. As a result, the mixture can be made more hygienic. At this time, the oxidizing agent treatment is performed in the presence of at least a non-flammable hydrophobic organic solvent among a non-flammable hydrophobic organic solvent and a hydrophilic organic solvent, so that damage to the superabsorbent polymer by the oxidizing agent can be suppressed. In this way, the oxidizing agent treatment step allows for bleaching, sterilization or disinfection, and deodorization in a highly safe environment while suppressing damage, thus enabling the production of a more hygienic superabsorbent polymer.

[0024] [Aspect 4] The method according to any one of aspects 1 to 3, further comprising an oxidizing agent treatment step of treating the superabsorbent polymer in the mixture with an oxidizing agent while stirring the mixture after the other components of the mixture have been separated from the mixture in the separation step.

[0025] This method further includes an oxidizing agent treatment step in which, after other components of the mixture have been separated from the mixture in the separation step, the superabsorbent polymer in the mixture is treated with an oxidizing agent while the mixture is being stirred. This oxidizing agent treatment step can bleach, sterilize, and deodorize the superabsorbent polymer. As a result, the superabsorbent polymer can be made more hygienic. At this time, the oxidizing agent treatment is performed in the presence of at least a non-flammable hydrophobic organic solvent, which is one of a non-flammable hydrophobic organic solvent and a hydrophilic organic solvent, so that damage to the superabsorbent polymer by the oxidizing agent can be suppressed. In this way, the oxidizing agent treatment step allows for bleaching, sterilization or disinfection and deodorization in a safe environment while suppressing damage, so that a more hygienic superabsorbent polymer can be obtained. At this time, since the pulp fibers are oxidized after other components have been separated from the mixture, there is no influence from other components in the mixture, and the efficiency of oxidation can be increased.

[0026] [Aspect 5] The method according to any one of aspects 1 to 4, further comprising a sanitary treatment step of subjecting the superabsorbent polymer separated from the mixed liquid in the separation step to sanitary treatment.

[0027] This method further includes a sanitary treatment step in which the superabsorbent polymer separated from the mixture in the separation step is subjected to sanitary treatment. Examples of such sanitary treatments include treatment with steam and treatment with ultraviolet light. This sanitary treatment step, that is, treatment to enhance hygiene, can remove bacteria and odor-causing organic matter present in the superabsorbent polymer. As a result, the superabsorbent polymer can be made more hygienic. At this time, since the sanitary treatment is applied to the superabsorbent polymer separated from the mixture, there is no influence from other contaminants in the mixture, and the efficiency of the treatment can be increased. In this way, since bacteria and odor-causing organic matter are removed in a highly safe environment during the sanitary treatment step, a more hygienic superabsorbent polymer can be obtained.

[0028] [Aspect 6] The method according to aspect 5, wherein the sanitary treatment step includes an oxidizing agent treatment step of treating the superabsorbent polymer separated from the mixed liquid with an oxidizing agent.

[0029] In this method, the sanitary treatment step includes an oxidizing agent treatment step in which the superabsorbent polymer separated from the mixture is treated with an oxidizing agent. This oxidizing agent treatment step can bleach, sterilize, and deodorize the superabsorbent polymer. As a result, the superabsorbent polymer can be made more hygienic. At this time, since the superabsorbent polymer separated from the mixture is oxidized, it is not affected by other impurities in the mixture, and the efficiency of oxidation can be increased. Thus, in the oxidizing agent treatment step, bleaching, sterilization, and deodorization are performed under highly safe conditions, so a more hygienic superabsorbent polymer can be obtained.

[0030] [Aspect 7] The method according to any one of aspects 1 to 6, further comprising a pretreatment step of reducing the moisture content of the mixture before the dewatering and washing step.

[0031] If a mixture containing a superabsorbent polymer contains a large amount of water (e.g., urine), the amount of water in subsequent processes may increase, making it difficult to control the composition of the processing solution. Therefore, this method includes a pretreatment step to reduce the water content of the mixture before the dewatering and washing step. This allows for controlling the amount of water released from the mixture in subsequent processes, thereby controlling the composition of the processing solution. Furthermore, if a mixture containing a superabsorbent polymer contains a large amount of water, the volume and mass of the mixture to be processed in subsequent processes may become too large, potentially reducing process efficiency. Therefore, by reducing the water content of the mixture through the pretreatment step, the volume and mass of the mixture in subsequent processes can be reduced, thereby suppressing a decrease in process efficiency.

[0032] [Aspect 8] The method according to any one of aspects 1 to 7, further comprising a bag-breaking step of breaking the packaging bag containing the mixture before the dewatering and washing step.

[0033] Mixtures containing superabsorbent polymers, such as used absorbent articles, are sometimes collected enclosed in packaging bags. Therefore, in order to process such mixtures, it is necessary to remove them from the packaging. In this method, a bag-breaking step is performed to break open the packaging bag containing the mixture before the dewatering and washing step. As a result, the mixture is removed from the packaging bag, making it possible to reliably carry out the subsequent steps on the mixture.

[0034] [Aspect 9] The method according to any one of aspects 1 to 8, further comprising an organic solvent recovery step of separating and recovering the non-flammable hydrophobic organic solvent and / or the hydrophilic organic solvent from the mixed liquid after the separation step.

[0035] This method further includes an organic solvent recovery step in which a non-flammable hydrophobic organic solvent, a hydrophilic organic solvent, or both are separated from the mixed liquid after the separation step and recovered. The recovered organic solvent can be reused, for example, by being supplied to a dehydration and washing step. As a result, this method allows for dehydration, washing, bleaching, sterilization or disinfection, and dehydration of the mixture while suppressing the increase in the cost of organic solvents.

[0036] [Aspect 10] The method according to any one of aspects 1 to 9, wherein the non-flammable hydrophobic organic solvent comprises at least one of a fluorine-based organic solvent and an aromatic organic solvent.

[0037] In this method, since the non-flammable hydrophobic organic solvent contains at least one of a fluorinated organic solvent and an aromatic organic solvent, the superabsorbent polymer can be more reliably dispersed in the treatment solution, becoming loose and more reliably removing lipophilic contaminants.

[0038] [Aspect 11] The method according to any one of aspects 1 to 10, wherein the hydrophilic organic solvent comprises at least one of a ketone-based organic solvent and an alcohol-based organic solvent.

[0039] In this method, since the hydrophilic organic solvent contains at least one of ketone-based organic solvents and alcohol-based organic solvents, the treatment solution can more effectively dehydrate the superabsorbent polymer while further suppressing damage to it, thereby more effectively removing hydrophilic contaminants.

[0040] [Aspect 12] The method according to any one of aspects 1 to 11, wherein the water includes acidic water.

[0041] During the dewatering and washing process, if stirring is stopped, the treatment liquid separates into a non-flammable hydrophobic organic solvent and water. The mixture tends to migrate to the water, so there is a possibility that the superabsorbent polymer will reabsorb water. If this happens, separation in the separation process becomes difficult. Therefore, in this method, acidic water is used in such cases. This allows the superabsorbent polymer to remain in a dewatered state. In addition, the acidic water can disinfect the mixture.

[0042] [Aspect 13] The method according to Aspect 12, comprising an activation step of activating the superabsorbent polymer with a solution containing an activator.

[0043] When water is made into acidic water in the dehydration washing step, the superabsorbent polymer may be inactivated. Therefore, in this method, in that case, it includes an activation step of activating the superabsorbent polymer with a solution containing an activator (for example, a solution containing an alkali metal ion source). The solution containing an activator may be a treatment liquid containing an activator. Thereby, the dehydrated superabsorbent polymer can be activated (capable of absorbing water).

[0044] [Aspect 14] The method according to any one of Aspects 3, 4, and 6, wherein the oxidizing agent contains at least one of ozone, hydrogen peroxide, and chlorine-based substances.

[0045] In this method, as the oxidizing agent, it contains at least one of ozone, hydrogen peroxide, and chlorine-based substances (for example, sodium hypochlorite) that have strong effects of bleaching, sterilizing or disinfecting, and deodorizing. Therefore, the superabsorbent polymer can be more reliably bleached, sterilized or disinfected, and deodorized.

[0046] [Aspect 15] The method according to any one of Aspects 1 to 14, further comprising a drying step of drying the superabsorbent polymer, wherein the drying step is performed at least one of after the dehydration washing step and after the separation step, and is performed by low-temperature drying or reduced-pressure drying.

[0047] In this method, the superabsorbent polymer is dehydrated with an organic solvent in the dehydration washing step, and since the amount of water to be removed is very small, the subsequent drying step of drying the superabsorbent polymer can be performed by low-temperature drying or reduced-pressure drying with mild drying conditions. Thereby, the effect that it is difficult to damage the superabsorbent polymer by drying and it is difficult for the superabsorbent polymer to be colored can be obtained.

[0048] [Aspect 16] A recycled superabsorbent polymer derived from a used absorbent article, wherein the content of polyvalent metal is 0.06% by mass or less.

[0049] The recycled superabsorbent polymer contains superabsorbent polymer derived from used absorbent articles, yet its polyvalent metal content is approximately the same as that of unused superabsorbent polymer, at 0.06% by mass or less. This is because polyvalent metals are not used in the dehydration process, which is essential for the reuse of used superabsorbent polymer. Thus, this recycled superabsorbent polymer, despite being derived from used absorbent articles, has a polyvalent metal content approximately the same as that of unused superabsorbent polymer, and therefore can be said to contain high-quality superabsorbent polymer. As a result, this superabsorbent polymer can be used as a good absorbent material.

[0050] The following describes a method for producing recycled superabsorbent polymers derived from used absorbent articles according to this embodiment, and the recycled superabsorbent polymers derived from used absorbent articles.

[0051] However, used absorbent articles include absorbent articles that have been used, such as those containing excrement and those that do not contain excrement, as well as unused absorbent articles, such as those discarded during the production stage (production loss) and those discarded after storage. Absorbent articles include plastic materials (nonwoven fabric, etc.) and superabsorbent polymers as constituent materials, and may include pulp fibers. Examples include disposable diapers, incontinence pads, sanitary napkins, bed sheets, and pet sheets. Recycled plastic materials refer to plastic materials derived from used absorbent articles, specifically plastic materials recovered from used absorbent articles. Recycled superabsorbent polymers refer to superabsorbent polymers derived from used absorbent articles, specifically superabsorbent polymers recovered from used absorbent articles. Recycled pulp fibers refer to pulp fibers derived from used absorbent articles, specifically pulp fibers recovered from used absorbent articles. Used absorbent articles contain soiling substances. These contaminants include, for example, excrement, sebum, bacteria, odor-causing substances, and coloring substances derived from them, machine oil from manufacturing equipment, and various organic substances.

[0052] The method for producing recycled superabsorbent polymers derived from used absorbent articles according to this embodiment comprises a dewatering and washing step and a separation step. The dewatering and washing step is a step of stirring a mixture obtained by mixing a mixture containing superabsorbent polymers from used absorbent articles with a treatment liquid containing at least one of a non-flammable hydrophobic organic solvent, a hydrophilic organic solvent, and water to dewater the superabsorbent polymers. The separation step is a step of separating the superabsorbent polymers from the mixture. However, the mixture may be, for example, used absorbent articles (themselves) or an assembly of multiple components derived from used absorbent articles.

[0053] The method described above can achieve the following effects. First, in the dewatering and washing step, the superabsorbent polymer is dewatered by stirring the mixture of the mixture and the treatment solution. In this dewatering and washing step, by adding a non-flammable hydrophobic organic solvent to the treatment solution in addition to a hydrophilic organic solvent, or by using water instead of the hydrophilic organic solvent and adding a non-flammable hydrophobic organic solvent, the volatility and flammability of the treatment solution can be reduced while maintaining the effect of the organic solvent (described later). This enhances the safety of the method. Note that the hydrophilic organic solvent and water may be used simultaneously.

[0054] Furthermore, in the dewatering and washing process, the non-flammable hydrophobic organic solvent in the treatment liquid can break the bonds between the superabsorbent polymer in the mixture and other materials (e.g., plastic materials, pulp fibers), as well as the bonds between the superabsorbent polymers themselves, thereby dispersing the superabsorbent polymer in the treatment liquid. For example, the adhesive that joins the superabsorbent polymer to other materials can be dissolved with the non-flammable hydrophobic organic solvent to break the bond. Alternatively, the bonds between the superabsorbent polymers can be broken with the non-flammable hydrophobic organic solvent. Then, by stirring the mixture, the superabsorbent polymer can be separated from the other materials and dispersed freely in the liquid.

[0055] Furthermore, in the dewatering and washing process, contaminants in the mixture can be removed using at least one of a non-flammable hydrophobic organic solvent, a hydrophilic organic solvent, and water in the processing liquid. Specifically, lipophilic contaminants on the superabsorbent polymer can be dissolved and removed using the non-flammable hydrophobic organic solvent, while hydrophilic contaminants can be dissolved and removed using at least one of a hydrophilic organic solvent and water. This allows for a high cleaning effect to be obtained.

[0056] Furthermore, in the dewatering and washing process, at least one non-flammable hydrophobic organic solvent from among the non-flammable hydrophobic organic solvents and hydrophilic organic solvents in the treatment liquid can be used to dewater the water absorbed by the superabsorbent polymer in the mixture. At this time, since dewatering with organic solvents does not inactivate the superabsorbent polymer, the superabsorbent polymer can maintain its water absorption performance. As a result, there is no need for additional treatment such as reactivation in order to reuse the superabsorbent polymer as an absorbent material, and there is no risk of contamination with impurities associated with additional treatment. Note that during dewatering, most of the treatment liquid is at least one non-flammable hydrophobic organic solvent from among the non-flammable hydrophobic organic solvents and hydrophilic organic solvents. Therefore, even if a small amount of water remains, it is possible to suppress the reabsorption of water by the superabsorbent polymer.

[0057] Therefore, in this method, the dewatering and washing process removes dirt under highly safe conditions, yielding a hygienic superabsorbent polymer. Subsequently, a hygienic recycled superabsorbent polymer can be obtained through the separation process.

[0058] Therefore, this method provides a highly safe and highly effective cleaning method for producing recycled superabsorbent polymers derived from used absorbent articles. Furthermore, since this method generally uses organic solvents to treat the mixture in the dewatering and washing step, the amount of water used for treatment can be significantly reduced.

[0059] Next, a method for producing recycled superabsorbent polymers derived from used absorbent articles according to this embodiment will be described in detail.

[0060] First, let's describe an example of the composition of an absorbent article. An absorbent article comprises a surface sheet, a backing sheet, and an absorbent material placed between the surface sheet and the backing sheet. An example of the size of an absorbent article is a length of approximately 15 to 100 cm and a width of 5 to 100 cm. In addition, an absorbent article may further include other components that are generally found in absorbent articles, such as a diffusion sheet to assist in the diffusion of liquids such as urine, a leak-proof wall to prevent lateral leakage of liquids, side sheets or outer sheets to hold the surface sheet, backing sheet, and absorbent material, and elastic members for waist gathers or three-dimensional gathers.

[0061] As for the material of the surface sheet, there are no particular restrictions as long as it can be used for absorbent articles, but examples include liquid-permeable nonwoven fabrics, synthetic resin films with liquid-permeable pores, and composite sheets thereof. As for the material of the back sheet, there are no particular restrictions as long as it can be used for absorbent articles, but examples include liquid-impermeable nonwoven fabrics, liquid-impermeable synthetic resin films, and composite sheets thereof. As for the material of the diffusion sheet, there are no particular restrictions as long as it can be used for absorbent articles, but examples include liquid-permeable nonwoven fabrics. As for the material of the leakproof wall, side sheet, and outer sheet, there are no particular restrictions as long as it can be used for absorbent articles, but examples include liquid-impermeable nonwoven fabrics. As for the material of the elastic member, there are no particular restrictions as long as it can be used for absorbent articles, but examples include elastic threads. As for the material of the nonwoven fabric and synthetic resin film, there are no particular restrictions as long as it can be used for absorbent articles, but examples include synthetic resins. Examples of synthetic resins include olefin resins such as polyethylene and polypropylene, polyamide resins such as 6-nylon and 6,6-nylon, and polyester resins such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT). Natural fibers such as cotton and rayon may be used as the material for the nonwoven fabric. There are no particular restrictions on the material for the elastic thread as long as it can be used in absorbent articles, but examples include styrene-butadiene rubber and urethane rubber. In this embodiment, at least the nonwoven fabric and synthetic resin film described above are referred to as plastic materials, and may include the elastic thread.

[0062] Absorbent materials include absorbent materials, namely pulp fibers and superabsorbent polymers. While there are no particular restrictions on pulp fibers as long as they can be used in absorbent articles, examples include cellulose fibers. Examples of cellulose fibers include wood pulp, cross-linked pulp, non-wood pulp, regenerated cellulose, and semi-synthetic cellulose. The size of the pulp fibers is preferably such that the average length of the fiber is several tens of micrometers, with 20 to 40 micrometers being preferred, and the average length of the fiber is preferably several millimeters, with 2 to 5 mm being preferred. Superabsorbent polymers (SAP) are not particularly limited as long as they can be used in absorbent articles and have acidic groups, but examples include those containing carboxyl groups, sulfo groups, etc., with those containing carboxyl groups being preferred. Examples include polyacrylate-based (containing carboxyl groups), polysulfonate-based (containing sulfo groups, etc.), and polymaleate-based (containing carboxyl groups) superabsorbent polymers. The size of the superabsorbent polymer (when dry) can be such that the average particle size is several hundred μm, and is preferably 200 to 500 μm. The absorbent body may include a core wrap sheet that encloses the absorbent material, formed from a liquid-permeable sheet such as tissue paper.

[0063] One side and the other side of the absorbent are joined to the surface sheet and the back sheet, respectively, via adhesive. In a plan view, the portion of the surface sheet that extends outward from the absorbent, surrounding it (peripheral portion), is joined to the portion of the back sheet that extends outward from the absorbent, surrounding it (peripheral portion), via adhesive. Therefore, the absorbent is enclosed within the joint of the surface sheet and the back sheet. If the absorbent has a core wrap sheet, the absorbent material is joined to the core wrap sheet via adhesive. Each of the diffusion sheets, leak-proof walls, side sheets, outer sheets, and elastic members are also joined to other members with adhesive. There are no particular restrictions on the adhesive as long as it can be used for absorbent articles, but examples include hot-melt adhesives. Examples of hot-melt adhesives include pressure-sensitive or heat-sensitive adhesives mainly composed of rubber such as styrene-ethylene-butadiene-styrene, styrene-butadiene-styrene, and styrene-isoprene-styrene, or olefin such as polyethylene.

[0064] As described above, absorbent articles having the above configuration generally contain some or all of the aforementioned contaminating substances at the stage of used absorbent articles.

[0065] Next, a method for producing recycled superabsorbent polymers derived from used absorbent articles according to this embodiment will be described. Figure 1 is a flowchart showing an example of a method for producing recycled superabsorbent polymers derived from used absorbent articles according to this embodiment.

[0066] A method for producing recycled superabsorbent polymers derived from used absorbent articles comprises a dewatering and washing step S3 and either a first separation step S5 or a second separation step S7. This method may further comprise at least one of the following: a bag breaking step S1, a pretreatment step S2, a first oxidizing agent treatment step S4, a second oxidizing agent treatment step S6, a drying step S8, a recovery step S9, a resupply step S10, and either the other of the first separation step S5 or the second separation step S7. Each step will be described in detail below.

[0067] This method produces recycled superabsorbent polymers derived from used absorbent articles from a mixture containing superabsorbent polymers from used absorbent articles. The mixture is not particularly limited as long as it contains superabsorbent polymers from used absorbent articles. Examples of the mixture include used absorbent articles (themselves) and aggregates of multiple components derived from used absorbent articles. Examples of such aggregates include collections of superabsorbent polymers and other components (e.g., plastic materials, pulp fibers, etc.) extracted from used absorbent articles. Examples of such aggregates include those with a higher proportion of superabsorbent polymers and a lower proportion of other components (plastic materials, pulp fibers) compared to absorbent articles (products).

[0068] (1) Bag-breaking step S1 The bag-breaking step S1 is a step in which the packaging bag containing the mixture is broken before the dewatering and washing step S3. The broken packaging bag is removed and the mixture is taken out. When used absorbent articles are used as the mixture, the used absorbent articles may be packed in a packaging bag to prevent, for example, excrement, bacteria, or odors from leaking out, or to facilitate transport. When an aggregate of multiple components derived from used absorbent articles is used as the mixture, for example, an aggregate of plastic material, superabsorbent polymer, and pulp fibers, the aggregate may be packed in a packaging bag to prevent it from falling apart. In order to process such a mixture, it is necessary to take the mixture out of the packaging bag. Therefore, in this method, the packaging bag is broken to make it possible to take out the mixture. Then the mixture is taken out of the packaging bag so that the subsequent steps (for example, the pre-treatment step S2, the dewatering and washing step S3, etc.) can be carried out reliably. At this time, it is preferable not to crush or damage the mixture as much as possible. This is to avoid damaging the materials and components in the mixture, thereby making them easier to reuse. Furthermore, if the mixture is not enclosed in a packaging bag, or if it is otherwise unnecessary, the bag-breaking step S1 can be omitted.

[0069] An example of a device that performs the bag-breaking process S1 is a bag-breaking device that breaks open a packaging bag containing recyclable waste (e.g., PET bottles, cans, glass bottles, plastic containers and packaging) without damaging the recyclable waste.

[0070] (2) Pretreatment step S2 Pretreatment step S2 is a step to reduce the moisture content of the mixture before the dewatering and washing step S3. If the amount of moisture (e.g., urine) contained in the mixture is large, a large amount of moisture will be released into the processing solution during the dewatering and washing step S3. This may make it difficult to control the composition of the processing solution thereafter. Furthermore, if the amount of moisture contained in the mixture is large, the volume and mass of the mixture to be processed in the steps after the dewatering and washing step S3 may be too large, which may reduce the efficiency of the process. Therefore, in this method, the moisture content of the mixture is reduced (dewatered) in the pretreatment step S2 before the dewatering and washing step S3. For example, the moisture absorbed by the superabsorbent polymer or pulp fibers in the mixture is reduced. This makes it possible to keep the amount of moisture released from the mixture by dewatering in the dewatering and washing step S3 low, making it easier to control the composition of the processing solution. Furthermore, the volume and mass of the mixture in the steps after the dewatering and washing step S3 can be reduced, which can suppress a decrease in process efficiency. However, since the pretreatment step S2 is performed before the dewatering and washing step S3, it is sufficient to dewater the mixture to a certain extent, and complete dewatering is not necessarily required. The moisture content of the mixture after dewatering can be, for example, 30 to 80% by mass. If the moisture content of the mixture is low or if it is not necessary, the pretreatment step S2 can be omitted.

[0071] The apparatus (method) for performing the pretreatment step S2 is not particularly limited as long as it can achieve the moisture content of the mixture described above, but for example, a drying apparatus for drying the mixture can be used. The drying conditions in the drying apparatus are not particularly limited as long as it can achieve the moisture content of the mixture described above, but for example, a temperature of 60 to 110°C can be used. If the temperature is too low, it will take too long, and if it is too high, the components, especially plastic materials, may melt. For example, the time can be 2 to 48 hours.

[0072] (3) Dehydration and washing step S3 The dehydration and washing step S3 is a step in which a mixture of a mixture containing the superabsorbent polymer of the used absorbent article and a treatment liquid containing at least one of a nonflammable hydrophobic organic solvent, a hydrophilic organic solvent, and water is stirred to dehydrate the superabsorbent polymer.

[0073] The mixture may also be an aggregate containing used absorbent articles or superabsorbent polymers that have not undergone the bag-breaking step S1 and / or the pretreatment step S2.

[0074] The treatment solution contains at least one of a non-flammable hydrophobic organic solvent, a hydrophilic organic solvent, and water. Therefore, the treatment solution may contain a non-flammable hydrophobic organic solvent, a hydrophilic organic solvent, and water; it may contain a non-flammable hydrophobic organic solvent and a hydrophilic organic solvent (without water); or it may contain a non-flammable hydrophobic organic solvent and water (without the hydrophilic organic solvent). Other solvents, chemicals, aqueous solutions, etc., may be included as long as they do not affect dehydration and washing. The total proportion of at least one of the non-flammable hydrophobic organic solvent, hydrophilic organic solvent, and water in the treatment solution is, for example, 50% by mass or more. From the viewpoint of the effect of dehydration and washing, a higher proportion is preferable, therefore, 70% by mass or more is preferable, 80% by mass or more is more preferable, and 90% by mass or more is even more preferable.

[0075] In non-flammable hydrophobic organic solvents, "hydrophobic organic solvent" refers to an organic solvent that is miscible with water. The non-flammable hydrophobic organic solvent is not particularly limited as long as it is a liquid and capable of dehydrating and washing the superabsorbent polymer contained in the mixture. The non-flammable hydrophobic organic solvent is thought to work, for example, by separating the water contained in the superabsorbent polymer and transferring it to a hydrophilic organic solvent or water, while simultaneously dissolving and removing contaminants (mainly oily substances) from the superabsorbent polymer. However, the mechanism is not limited to this.

[0076] Examples of non-flammable hydrophobic organic solvents include fluorinated organic solvents, aromatic organic solvents, and combinations of at least two of these. Examples of fluorinated organic solvents include hydrochlorofluoroolefin solvents, such as 1-chloro-2,3,3-trifluoropropene. Alternatively, examples of hydrofluoroolefin solvents include 1,1,1,3,3-pentafluorobutane and 1,3,3,3-tetrafluoropropene. Such non-flammable hydrophobic organic solvents can more reliably disperse superabsorbent polymers in the treatment solution, making them loose, and more reliably remove lipophilic contaminants. Furthermore, fluorinated organic solvents, especially 1-chloro-2,3,3-trifluoropropene, 1,1,1,3,3-pentafluorobutane, and 1,3,3,3-tetrafluoropropene, have low volatility and flammability, making them easy to handle from a safety standpoint. Examples of the above-mentioned aromatic organic solvents include aromatic hydrocarbons, such as benzene-based aromatic hydrocarbons, such as toluene and xylene.

[0077] A hydrophilic solvent is a liquid that is miscible with water. A hydrophilic organic solvent is a liquid, and there are no particular limitations as long as it can dehydrate and wash the superabsorbent polymer contained in the mixture. For example, a hydrophilic organic solvent is thought to dehydrate the superabsorbent polymer by dissolving the water contained within it, and also to remove contaminants (mainly water-based) from the superabsorbent polymer by dissolving them within itself. However, the mechanism is not limited to this.

[0078] Examples of hydrophilic organic solvents include ketone-based organic solvents, alcohol-based organic solvents, and combinations of at least two of these. Examples of ketone-based organic solvents include acetone, methyl ethyl ketone, methyl isobutyl ketone, diethyl ketone, and cyclohexanone. Examples of alcohol-based organic solvents include methanol, ethanol, propanol, butanol, and pentanol. Such hydrophilic organic solvents can more reliably dehydrate superabsorbent polymers in the treatment solution while suppressing damage to the polymers, thereby more reliably removing hydrophilic contaminants. Furthermore, ketone-based organic solvents, particularly acetone, dissolve water well, are not easily oxidized by oxidizing agents, and have the property of protecting functional groups. Therefore, acetone can dehydrate superabsorbent polymers while suppressing their decomposition (dissolution), especially when oxidizing agent treatment is performed simultaneously, and can remove some or all of the contaminants contained in these materials. Furthermore, by combining it with a non-flammable hydrophobic organic solvent and reducing the amount of hydrophilic organic solvent used, the safety of the treatment solution can be improved in terms of volatility and flammability.

[0079] Furthermore, non-flammable hydrophobic organic solvents and hydrophilic organic solvents, particularly non-flammable hydrophobic organic solvents, can dissolve the adhesives that join together the components of a used absorbent article (for example, at least two of the following: plastic material, superabsorbent polymer, and pulp fiber). This makes it easier to separate the components of the used absorbent article from each other. Therefore, if the mixture is the used absorbent article itself, it makes it easier to decompose the used absorbent article into its individual components.

[0080] The lower limit of the proportion of the non-flammable hydrophobic organic solvent in the treatment solution is, for example, 20% by mass, preferably 30% by mass, and more preferably 40% by mass. The upper limit is, for example, 95% by mass, preferably 85% by mass, and more preferably 75% by mass. A proportion of 20% by mass or more makes it easier to remove lipophilic contaminants and adhesives, and when used in combination with a hydrophilic organic solvent, it enhances the safety of the treatment solution in terms of volatility and flammability. A proportion of 95% by mass or less allows it to be used in combination with a hydrophilic organic solvent or water, making it easier to remove hydrophilic contaminants.

[0081] On the other hand, the lower limit of the ratio of at least one of the hydrophilic organic solvent and water in the treatment solution is, for example, 5% by mass, preferably 10% by mass, and more preferably 15% by mass. The upper limit is, for example, 80% by mass, preferably 70% by mass, and more preferably 60% by mass. A ratio of 5% by mass or more makes it easier to remove hydrophilic contaminants. A ratio of 80% by mass or less allows for use in combination with a non-flammable hydrophobic organic solvent, which enhances the safety of the treatment solution in terms of volatility and flammability, and also makes it easier to remove lipophilic contaminants. The mass ratio of the hydrophilic organic solvent to water in the treatment solution depends on the mass ratio of the non-flammable hydrophobic organic solvent to the hydrophilic organic solvent and water, but is generally between 0:100 and 100:0.

[0082] The treatment solution is mainly composed of a hydrophobic agent (non-flammable hydrophobic organic solvent) and a hydrophilic agent (hydrophilic organic solvent, water), so the hydrophobic agent and the hydrophilic agent tend to separate from each other. Therefore, in this method, in the dewatering and washing step S3, the superabsorbent polymer and pulp fibers are dewatered and washed while stirring the mixture of the mixture and the treatment solution, thereby suppressing the separation of the treatment solution during dewatering and washing.

[0083] The preferred state of agitation is one in which the hydrophobic agent and the hydrophilic agent are dispersed almost uniformly, i.e., an emulsified state. If the treatment tank containing the treatment liquid rotates, the agitation speed can be, for example, 20 to 200 rpm, with 40 to 100 rpm being preferred. If agitation is performed within the treatment tank using a stirring blade, the agitation speed can be, for example, 100 to 2000 rpm, with 200 to 1000 rpm being preferred.

[0084] The proportion of the mixture (including the superabsorbent polymer) in the mixed liquid is not particularly limited, as long as the dewatering and washing step S3 can be carried out. For example, the proportion can be 0.1 to 20% by mass, and preferably 1 to 10% by mass. A proportion of 0.1% by mass or more allows for efficient processing. A proportion of 20% by mass or less facilitates dewatering and washing.

[0085] In the dehydration and washing step S3, there are no particular restrictions on the temperature of the dehydration and washing as long as the dehydration and washing can be performed, but it should be at least lower than the boiling point of any of the non-flammable hydrophobic organic solvents, hydrophilic organic solvents, or water. Examples of dehydration and washing temperatures include room temperature (example: 25°C) to 50°C, with 30 to 40°C being preferred. A temperature above room temperature makes it easier to shorten the time required for dehydration and washing. A temperature below 50°C makes it easier to suppress the evaporation (boiling) of organic solvents, suppresses the dehydration condensation of the acid groups of the superabsorbent polymer, and makes it easier to suppress the decrease in its water absorption. There are no particular restrictions on the time of the dehydration and washing as long as the dehydration and washing can be performed, but examples include 1 to 200 minutes.

[0086] The apparatus for performing the dewatering and washing process S3 is not particularly limited in its specific configuration, as long as it can store the processing liquid, immerse the mixture in the processing liquid, and stir the processing liquid containing the mixture. Examples of such apparatus include one having a tank in which the mixture can be placed and the processing liquid can be stored, a supply means for supplying the processing liquid into the tank, and a stirring means for stirring the processing liquid in the tank.

[0087] Here, the dewatering and washing step S3 preferably comprises a first stirring step S31 and a second stirring step S32. However, the first stirring step S31 is a step of mixing a non-flammable hydrophobic organic solvent with at least one of a hydrophilic organic solvent and water to form a treatment liquid, and stirring the treatment liquid. The second stirring step S32 is a step of mixing the treatment liquid stirred in the first stirring step S31 with a mixture to form a mixed liquid, and stirring the mixed liquid.

[0088] Non-flammable hydrophobic organic solvents and at least one of hydrophilic organic solvents and water are easily separated when simply mixed, and also easily separated when mixed with a mixture, due to the relationship between hydrophobicity and hydrophilicity. In other words, the treatment liquid is easily separated into non-flammable hydrophobic organic solvents and at least one of hydrophilic organic solvents and water. Therefore, in this method, in the dewatering and washing step S3, first, the treatment liquid is stirred in the first stirring step S31 to suppress the separation of the treatment liquid. Then, in the second stirring step S32, the treatment liquid containing the mixture is stirred to suppress the separation of the treatment liquid while dewatering and washing the superabsorbent polymer. As a result, the hydrophilic organic solvent can be mixed more reliably with the non-flammable hydrophobic organic solvent (and water), the volatility and flammability of the treatment liquid can be further reduced, and its safety can be further enhanced. At the same time, multiple components can be dispersed more reliably in the treatment liquid, dirt in the mixture can be removed more reliably, and the superabsorbent polymer can be dewatered more reliably.

[0089] Furthermore, in the dewatering and washing step S3, if a large amount of water is used (for example, 50% by mass or more), it is preferable to use acidic water (acidic aqueous solution). This is because, in the dewatering and washing step S3, if stirring is stopped, the treatment liquid tends to separate into a non-flammable hydrophobic organic solvent and water (and a hydrophilic organic solvent), and there is a possibility that the superabsorbent polymer, which easily migrates to water (and a hydrophilic organic solvent), will reabsorb water. In particular, if the proportion of water is high, this water absorption is more likely to occur. If this happens, it may become difficult to separate the superabsorbent polymer in the subsequent separation step. Therefore, when a large amount of water is used, using acidic water (acidic aqueous solution) inactivates the superabsorbent polymer, suppresses its water absorption, and maintains the dewatered state. In addition, the mixture can be disinfected by the acidic water.

[0090] The acid in the acidic water (acidic aqueous solution) is not particularly limited, and examples include acidic substances such as inorganic acids and organic acids. Examples of inorganic acids include sulfuric acid, hydrochloric acid, and nitric acid, but sulfuric acid is preferred from the viewpoint of not containing chlorine and from the viewpoint of cost. Examples of organic acids include carboxylic acids having multiple carboxyl groups (example: citric acid), carboxylic acids having one carboxyl group (example: acetic acid), sulfonic acids (example: methanesulfonic acid), etc. As for organic acids, it is preferable to have multiple carboxyl groups from the viewpoint of readily forming chelate complexes with divalent or higher metals contained in excrement, etc. (example: calcium) and less likely to leave ash residue on the superabsorbent polymer, and citric acid is more preferable. The acid concentration of the acidic aqueous solution is not particularly limited as long as the dehydration function is achieved, and examples include 0.1 to 30% by mass. Furthermore, when an acid is used to inactivate the superabsorbent polymer, the acid should have an acid dissociation constant (pK) of the acid group in the superabsorbent polymer. ａ Acid dissociation constant (pK) smaller than (in water) ａ It is preferable to have (in water).

[0091] The acidic aqueous solution preferably has a pH within a predetermined range. The upper limit of the pH is preferably 6.0, more preferably 5.0. A pH of 6.0 or lower facilitates the dehydration of the superabsorbent polymer. The lower limit of the pH is preferably 0.5, more preferably 1.0. A pH of 0.5 or higher makes it less likely to damage the pulp fibers. However, the pH is measured at 25°C, for example, using a twin pH meter AS-711 manufactured by Horiba, Ltd.

[0092] Furthermore, if the superabsorbent polymer is inactivated by acidic water (acidic aqueous solution), it is preferable to perform an activation treatment step in which the inactivated superabsorbent polymer is activated with a solution containing an activator. The activation treatment step may be included in the dehydration and washing step S3, and therefore may be performed in the middle of the dehydration and washing step S3. In that case, an alkali metal ion source or an aqueous solution containing an alkali metal ion source is added to the treatment solution of the dehydration and washing step S3. Alternatively, the activation treatment step may be performed after the dehydration and washing step S3 by separating the superabsorbent polymer. In that case, the separated superabsorbent polymer is treated with an organic solvent or aqueous solution containing an alkali metal ion source. This makes the dehydrated superabsorbent polymer activated (water-absorbing).

[0093] The activation step specifically involves activating an inactivated superabsorbent polymer with alkali metal ions in an alkaline solution, which is an organic solvent or aqueous solution containing an alkali metal ion source. At this time, the inactivated superabsorbent polymer is neutralized by the substitution of H ions within the polymer with alkali metal ions, resulting in an activated superabsorbent polymer. In other words, the water absorption capacity of the superabsorbent polymer is restored. Treatment in an organic solvent is preferable because it can reduce the water absorption of the superabsorbent polymer when it is activated. Examples of organic solvents include hydrophilic organic solvents.

[0094] Examples of alkali metal ions include lithium ions, sodium ions, potassium ions, and any combination thereof. The alkali metal ion source is not particularly limited as long as it can supply these alkali metal ions; examples include alkali metal hydroxides and salts of alkali metal hydroxides with an acid having a larger acid dissociation constant than the acid groups of the superabsorbent polymer. Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, potassium hydroxide, and any combination thereof. Examples of salts include lithium carbonate, sodium carbonate, potassium carbonate, lithium bicarbonate, sodium bicarbonate, potassium bicarbonate, lithium chloride, sodium chloride, potassium chloride, etc. The aqueous solution containing the alkali metal ion source is preferably neutral or alkaline, and preferably alkaline. The concentration of the alkali metal ion source in the alkaline aqueous solution is not particularly limited as long as the superabsorbent polymer is reactivated; for example, when alkali metal hydroxides are used as the alkali metal ion source, a concentration of 0.5 to 50% by mass is possible.

[0095] The specific configuration of the apparatus for performing the activation process is not particularly limited, as long as it can immerse the inactivated superabsorbent polymer in an alkaline solution. For example, the apparatus may have a tank in which the mixture can be placed and an alkaline aqueous solution can be stored, a supply means for supplying the alkaline solution into the tank, and a stirring means for stirring the alkaline solution in the tank.

[0096] In the activation step, there are no particular restrictions on the temperature of the alkaline aqueous solution; for example, room temperature (25°C) is used. To accelerate the reaction rate, the alkaline aqueous solution may be heated to a temperature below 100°C. There are no particular restrictions on the treatment time; for example, it is about 5 to 60 minutes.

[0097] (4) First oxidizing agent treatment step S4 The first oxidizing agent treatment step S4 (oxidizing agent treatment step) is a step in which the mixture in the mixed liquid is treated with an oxidizing agent while stirring the mixed liquid before the separation step (first separation step S5 or second separation step S7). However, the first oxidizing agent treatment step S4 may be performed at the same time as the dewatering and washing step S3, or after the dewatering and washing step S3. The first oxidizing agent treatment step S4 bleaches, sterilizes or disinfects and deodorizes the mixture in the mixed liquid with an oxidizing agent.

[0098] When the first oxidizing agent treatment step S4 is performed simultaneously with the dehydration and washing step S3, the oxidizing agent is added to the treatment solution before the mixture is dehydrated and washed, or while it is being dehydrated and washed. The mixture is then treated with the oxidizing agent while being dehydrated and washed in the treatment solution. On the other hand, when the first oxidizing agent treatment step S4 is performed after the dehydration and washing step S3, the oxidizing agent is added to the treatment solution after the mixture has been dehydrated and washed in the treatment solution. The mixture is then treated with the oxidizing agent after being dehydrated and washed in the treatment solution.

[0099] The oxidizing agent is not particularly limited as long as it can remove contaminants contained in the mixture while suppressing their effect on each component (e.g., oxidative decomposition of superabsorbent polymers). The oxidizing agent is, for example, a liquid or a gas mixed with a liquid, and is thought to reduce contaminants by oxidative decomposition, lowering their molecular weight and making them more soluble in the solution, but is not limited to this mechanism.

[0100] Examples of oxidizing agents include ozone, hydrogen peroxide, and chlorine-based substances (e.g., sodium hypochlorite), or combinations of two or more of these. Because these oxidizing agents have relatively high oxidizing power, they are effective in reducing contaminants contained in various components of used absorbent articles, such as superabsorbent polymers. This allows for the removal of bacteria and the reduction of odor and color substances, enabling more reliable sterilization, disinfection, deodorization, and decolorization of each component. Note that when using acetone as the hydrophilic organic solvent, hydrogen peroxide should not be used as the oxidizing agent.

[0101] The oxidizing agent may be mixed into the treatment solution, or it may be mixed with another solvent (e.g., an organic solvent, acidic water, or water) before being mixed into the treatment solution.

[0102] When using gaseous ozone as an oxidizing agent and supplying gaseous ozone to the treatment solution to form a treatment solution containing the oxidizing agent, the ozone concentration in the treatment solution is not particularly limited, as long as it is a concentration that can reduce the contaminants contained in each component. The ozone concentration is, for example, 0.2 to 2 ppm by mass, preferably 0.4 to 1.5 ppm by mass. A concentration of 0.2 ppm by mass or higher facilitates the reduction of contaminants, and a concentration of 2 ppm by mass or lower can suppress the oxidative decomposition of superabsorbent polymers. The contact time between the ozone-containing treatment solution and each component is not particularly limited, as long as it is a time that can reduce the contaminants contained in each component. Generally, the contact time is shorter when the ozone concentration is high and longer when the ozone concentration is low. The contact time is, for example, 1 to 200 minutes, preferably 2 to 60 minutes. The product of ozone concentration (mass ppm) and contact time (minutes) (hereinafter also referred to as the "CT value") is preferably 0.2 to 40 mass ppm·min, more preferably 0.5 to 20 mass ppm·min. A CT value of 0.2 mass ppm·min or higher facilitates the reduction of contaminants, and a CT value of 40 mass ppm·min or lower suppresses the oxidative decomposition of each component, especially easily decomposed superabsorbent polymers. Ozone treatment can reduce contaminants contained in each component, for example, by removing bacteria, odor-causing substances, and color-causing substances, and can sterilize, disinfect, deodorize, and decolorize (bleach) each component. Examples of ozone generators that supply gaseous ozone into the treatment machine include the ED-OWX-2 ozone water exposure tester manufactured by Eco Design Co., Ltd., the OS-25V ozone generator manufactured by Mitsubishi Electric Corporation, and the REX MC ozonizer MC-985S manufactured by a subsidiary of Rex Industries Co., Ltd.

[0103] When using a gas (e.g., ozone) as an oxidizing agent, the oxidizing agent may be supplied to the treatment liquid by, for example, the following method. This method involves generating a predetermined amount of oxidizing agent in an oxidizing agent generator, continuously supplying the oxidizing agent to the treatment liquid in the treatment tank at a predetermined airflow rate, while continuously disposing of (discharging) the same amount of waste oxidizing agent (used oxidizing agent) outside the treatment tank. In this case, a flow of oxidizing agent is easily generated, new oxidizing agent is easily continuously supplied to the surface of each component, and reactants are easily continuously discharged from that surface. Therefore, the oxidation reaction on the surface of each component can be carried out more reliably.

[0104] The proportion of the mixture in the above-mentioned treatment solution is not particularly limited, as long as the above-mentioned oxidizing agent treatment can be carried out. For example, the proportion can be 0.1 to 20% by mass, and is preferably 1 to 10% by mass. A proportion of 0.1% by mass or more allows the treatment to proceed efficiently. A proportion of 20% by mass or less facilitates the treatment with the oxidizing agent.

[0105] Dirt, such as excrement, sebum, bacteria, odor-causing substances, and coloring agents derived from them, machine oil from manufacturing equipment, and various organic substances, is expected to be reduced as follows: Dehydration primarily releases dirt absorbed inside each component along with water to the outside. At the same time, dirt attached to each component is also released to the outside along with the released water. Simultaneously, oxidation primarily decomposes dirt attached to each component (including that released from the inside and attached), reducing its molecular weight, solubilizing it, and removing it. This also makes sterilization, disinfection, deodorization, and bleaching of each component possible. However, the reduction of dirt on each component is not limited to this mechanism.

[0106] In the first oxidizing agent treatment step S4, there are no particular restrictions on the temperature of the oxidizing agent treatment as long as the above-described oxidizing agent treatment can be performed, but it should be at least lower than the boiling point of the treatment solution. Examples of oxidizing agent treatment temperatures include room temperature (example: 25°C) to 50°C, with 30 to 40°C being preferred. A temperature above room temperature makes it easier to shorten the time required for the oxidizing agent treatment. A temperature below 50°C makes it easier to suppress the evaporation (boiling) of the organic solvent, thereby suppressing the dehydration condensation of the acid groups of the superabsorbent polymer and thus suppressing the decrease in its water absorption.

[0107] This method includes a first oxidizing agent treatment step S4, in which the mixture (including the superabsorbent polymer) in the mixed liquid is treated with an oxidizing agent while stirring the mixed liquid, before the separation step (first separation step S5 or second separation step S7) (which may also be performed simultaneously with the dewatering and washing step S3). The first oxidizing agent treatment step S4 can bleach, sterilize or disinfect and deodorize the mixture. This makes the mixture more hygienic. At this time, the oxidizing agent treatment is performed in the presence of at least a non-flammable hydrophobic organic solvent among the non-flammable hydrophobic organic solvent and hydrophilic organic solvent, so that damage to the mixture by the oxidizing agent can be suppressed. In this way, in the first oxidizing agent treatment step S4, bleaching, sterilization or disinfection and deodorization are performed in a highly safe environment while suppressing damage, so a more hygienic superabsorbent polymer can be obtained.

[0108] The first oxidizing agent treatment step S4 may be omitted if it is not necessary, such as when the mixture is sufficiently hygienic due to the dewatering and washing step S3, or when oxidizing agent treatment is performed separately in the second oxidizing agent treatment step S6, which will be described later.

[0109] (5) First Separation Step S5 The first separation step S5 (separation step) is a step of separating the superabsorbent polymer and at least one of the other components (e.g., plastic material, pulp fiber) from the mixed liquid. However, the mixed liquid may be the liquid immediately after the dewatering and washing step S3 (the first oxidizing agent treatment step S4 is omitted), or it may be the liquid after both the dewatering and washing step S3 and the first oxidizing agent treatment step S4. The separated superabsorbent polymer and at least one of the other components are taken out from the mixed liquid containing contaminants and other components, and can be said to be recycled components that have been dewatered and washed, and contaminants and other components have been reduced.

[0110] In the first separation step S5, for example, one of the superabsorbent polymer and other components is separated from the mixture of the mixture containing the superabsorbent polymer and other components and the processing liquid. If the superabsorbent polymer is separated as the one component, the separated material can be recycled superabsorbent polymer. If the other component, a plastic material, is separated, the separated material can be recycled plastic material. If the other component, pulp fibers, is separated, the separated material can be recycled pulp fibers.

[0111] Alternatively, in the first separation step S5, for example, two of the superabsorbent polymer and other components are separated from the mixture of the superabsorbent polymer and other components and the processing liquid. If, for example, plastic material and pulp fibers are separated as the two components, the separated materials can be recycled in a mixed state. The two components may be further separated into individual recycled components (recycled plastic material, recycled pulp fibers).

[0112] The apparatus for performing the first separation step S5 is not particularly limited in its specific configuration, as long as it is capable of separating one or two components from the mixed liquid. Examples of separation methods include using one screen or a combination of multiple screens, using specific gravity, using centrifugal force, or a combination thereof.

[0113] (6) Second oxidizing agent treatment step S6 The second oxidizing agent treatment step S6 (oxidizing agent treatment step) is a step in which, after the predetermined components have been separated from the mixture in the first separation step S5, the mixture is stirred and the remaining components in the mixture are treated with an oxidizing agent. The second oxidizing agent treatment step S6 bleaches, sterilizes or disinfects, and deodorizes the remaining components in the mixture. The method and conditions for the oxidizing agent treatment are as described in the first oxidizing agent treatment step S4, but they do not have to be the same as the method and conditions for the first oxidizing agent treatment step S4.

[0114] For example, the second oxidizing agent treatment step S6 is a step in which, after one of the other components, for example, a plastic material, has been separated from the mixture in the first separation step S5, the remaining components in the mixture, for example, a superabsorbent polymer and the remaining other components, are treated with an oxidizing agent while stirring the mixture. In this case, the second oxidizing agent treatment step S6 can bleach, sterilize or disinfect and deodorize the superabsorbent polymer and the remaining other components. This makes the superabsorbent polymer and the remaining other components more hygienic. At this time, since the oxidizing agent treatment is performed in the presence of at least a non-flammable hydrophobic organic solvent, damage to the superabsorbent polymer and the remaining other components by the oxidizing agent can be suppressed. The remaining other components are, for example, pulp fibers.

[0115] Alternatively, for example, the second oxidizing agent treatment step S6 is a step in which, after other components, such as plastic material and pulp fibers, have been separated from the mixture in the first separation step S5, the remaining components in the mixture, such as the superabsorbent polymer, are treated with an oxidizing agent while stirring the mixture. In this case, the second oxidizing agent treatment step S6 can bleach, sterilize or disinfect and deodorize the superabsorbent polymer. This makes the superabsorbent polymer more hygienic. At this time, since the oxidizing agent treatment is performed in the presence of at least a non-flammable hydrophobic organic solvent, damage to the superabsorbent polymer by the oxidizing agent can be suppressed.

[0116] Thus, in the second oxidizing agent treatment step S6, bleaching, sterilization or disinfection, and deodorization are performed in a highly safe environment while suppressing damage, making it possible to obtain a more hygienic superabsorbent polymer.

[0117] Furthermore, the second oxidizing agent treatment step S6 may be omitted if it is not necessary, such as when the mixture is in a sufficiently hygienic state or when oxidizing agent treatment has already been performed in the first oxidizing agent treatment step S4 or elsewhere.

[0118] In another embodiment, the method may further include a sanitary treatment step in which one or two of the plastic material, superabsorbent polymer, and pulp fiber components separated from the mixture in the first separation step S5 are subjected to sanitary treatment (for example, when the first oxidizing agent treatment step S4 is not performed). The sanitary treatment step is a treatment that enhances the sanitary properties of each component, and examples include high-pressure steam treatment, ultraviolet (UV) treatment, and another oxidizing agent treatment.

[0119] High-pressure steam treatment is a process in which high-pressure steam is brought into contact with each component separated from the mixed liquid. This high-pressure steam can decompose at least a portion of the fouling substances (organic matter). The temperature of the high-pressure steam can be, for example, 110 to 180°C, the treatment time can be, for example, 1 to 120 minutes, and the steam pressure can be, for example, 0.2 to 1.2 MPa.

[0120] Ultraviolet (UV) treatment is a process in which each component separated from the mixed solution is immersed in water or an aqueous solution containing a predetermined oxidizing agent, and then irradiated with a predetermined amount of UV light. When water or an aqueous solution containing a predetermined oxidizing agent is irradiated with UV light, active species are generated. For example, when the wavelength of UV light is 253.7 nm, hydroxyl radicals are generated as active species through the reaction of UV light with a predetermined oxidizing agent (ozone + water, hydrogen peroxide). Also, when the wavelength of UV light is 184.9 nm, hydroxyl radicals are generated as active species through the reaction of UV light with water. These hydroxyl radicals can decompose at least a portion of the contaminants (organic matter).

[0121] Another oxidizing agent treatment step involves treating each component separated from the mixture with an oxidizing agent. This treatment may be carried out in acidic water (acidic aqueous solution). Details regarding the acidic water, oxidizing agent, and the method and conditions of the oxidizing agent treatment are as described above.

[0122] This sanitary treatment process can remove bacteria, odor-causing organic matter, etc., present in one or two components, such as a superabsorbent polymer. This makes the components (superabsorbent polymer, etc.) more hygienic. Since the sanitary treatment is applied to the components separated from the mixture, there is no influence from other impurities in the mixture, and the efficiency of the treatment can be increased. In this way, the sanitary treatment process removes bacteria, odor-causing organic matter, etc., in a highly safe environment, so that more hygienic components can be obtained. In addition, if dehydration is necessary for the superabsorbent polymer, an inactivation treatment may be performed, and if activation is necessary, an activation treatment may be performed.

[0123] In the case of a separate oxidizing agent treatment process, one or both of the components can be bleached, sterilized, disinfected, or deodorized. This makes the components more hygienic. In this case, since the components separated from the mixture are treated with the oxidizing agent, there is no influence from other impurities in the mixture, and the efficiency of the oxidizing agent treatment can be increased. Thus, in this oxidizing agent treatment process, bleaching, sterilization, disinfection, and deodorization are carried out under highly safe conditions, so more hygienic components can be obtained.

[0124] (7) Second Separation Step S7 The second separation step S7 (separation step) is a step of separating the mixture (superabsorbent polymer and other components that were not separated in the first separation step S5) from the mixed liquid. However, the mixed liquid may be the liquid immediately after the first separation step S5 (the second oxidizing agent treatment step S6 may be omitted), or it may be the liquid that has gone through both the first separation step S5 and the second oxidizing agent treatment step S6. The separated mixture (components) is removed from the mixed liquid containing contaminants and other components, and can be said to be recycled components that have been dehydrated and washed, and contaminants and other components have been reduced.

[0125] In the second separation step S7, for example, at least one component is separated from the mixture of the treatment liquid and the mixture containing the superabsorbent polymer and other components. If one component is separated, for example, if the superabsorbent polymer is separated, the separated material can be recycled superabsorbent polymer. If plastic material is separated, the separated material can be recycled plastic material. If pulp fibers are separated, the separated material can be recycled pulp fibers.

[0126] Alternatively, if the two components are separated, for example, if a plastic material is separated from a superabsorbent polymer, or if a superabsorbent polymer is separated from pulp fibers, the separated components can be mixed together to form a recycled component. The two separated components may then be further separated to form individual recycled components (recycled plastic material, recycled superabsorbent polymer, recycled pulp fibers).

[0127] The apparatus for performing the second separation step S7 is not particularly limited in its specific configuration, as long as it is capable of separating one or two components from the mixed liquid. Examples of separation methods include using one screen or a combination of multiple screens, using specific gravity, using centrifugal force, or a combination thereof.

[0128] (8) Drying step S8 The drying step S8 is a step of drying the superabsorbent polymer and at least one of the other components. However, the drying step S8 may be performed after the second separation step S7, or after the dehydration and washing step S3, and at least one of the other separation steps, the first separation step S5. Note that the drying step S8 may be omitted if it is unnecessary, such as when the components dry easily by using a large amount of organic solvent in the dehydration and washing step S3.

[0129] Drying step S8 is performed by placing each component in a drying atmosphere at a temperature higher than room temperature, or by blowing dry air at a temperature higher than room temperature onto each component. The drying temperature can be, for example, 40 to 110°C, and preferably 50 to 100°C. A drying temperature of 40°C or higher can shorten the drying time. A drying temperature of 110°C or lower can suppress the components from sticking together and the components from deteriorating due to heat. The drying time can be, for example, 30 to 300 minutes. Drying step S8 may be performed under reduced pressure, for example, 0.1 to 100 kPa, from the viewpoint of promoting drying.

[0130] In this method, a drying step S8, in which at least one of the superabsorbent polymer and other components (e.g., plastic material, pulp fiber) is dried, is performed at least once after the dewatering and washing step S3 and after the separation steps (first separation step S5, second separation step S7). Here, since the superabsorbent polymer and at least one of the other components are dewatered in advance in the dewatering and washing step S3 and are therefore easier to dry, the energy required for drying can be reduced, and the superabsorbent polymer and at least one of the other components can be processed in the next step or reused easily.

[0131] (9) Recovery step S9 The recovery step S9 is a step in which, after the separation step (first separation step S5 or second separation step S7), at least one of the non-flammable hydrophobic organic solvent, hydrophilic organic solvent, and adhesive is separated from the mixed liquid from which the superabsorbent polymer and other components have been removed, and recovered.

[0132] The separated mixture contains a non-flammable hydrophobic organic solvent, a hydrophilic organic solvent, and an adhesive, as well as pollutants such as excrement. Therefore, by heating and / or reducing the pressure of the mixture, at least one of the non-flammable hydrophobic organic solvent and the hydrophilic organic solvent contained in the mixture is boiled and evaporated. Then, by cooling the vapor, at least one of the non-flammable hydrophobic organic solvent and the hydrophilic organic solvent is recovered. That is, at least one of the non-flammable hydrophobic organic solvent and the hydrophilic organic solvent is distilled. The adhesive, on the other hand, is removed as a residue after distillation of the mixture. As a result, the recovered non-flammable hydrophobic organic solvent, hydrophilic organic solvent, and adhesive can be reused because the amount of pollutants and other agents has been reduced. Note that since the non-flammable hydrophobic organic solvent and the hydrophilic organic solvent have different boiling points, they can be recovered separately by fractional distillation.

[0133] The recovery process S9 includes an organic solvent recovery process S91 and an adhesive recovery process S92.

[0134] The organic solvent recovery step S91 is a step in which, after the separation step (first separation step S5 or second separation step S7), the organic solvent (non-flammable hydrophobic organic solvent and / or hydrophilic organic solvent) from the mixed liquid is separated and recovered. When the non-flammable hydrophobic organic solvent and the hydrophilic organic solvent are recovered separately, they are recovered by fractional distillation. The recovered organic solvent can be reused, for example, by supplying it to the dehydration and washing step or by using it for other purposes. As a result, in this method, the increase in the cost of organic solvents is suppressed, and the mixture can be dehydrated, washed, bleached, sterilized, and dehydrated while contributing to the reduction of environmental impact.

[0135] The adhesive recovery step S92 is a step in which the adhesive dissolved in the mixed liquid (processing liquid) is separated from the mixed liquid (processing liquid) and recovered after the separation step (first separation step S5 or second separation step S7). The adhesive is recovered, for example, as the residue obtained by distilling the processing liquid in the organic solvent recovery step S91. The recovered adhesive can be reused, for example, in the manufacture of absorbent articles. This contributes to reducing the environmental burden in this method.

[0136] (10) Resupply process S10 The resupply process S10 is a process of supplying at least one of the non-flammable hydrophobic organic solvent and hydrophilic organic solvent recovered in the recovery process S9 (organic solvent recovery process S91) to the dewatering and washing process S3 for use as a processing liquid. This helps to reduce the environmental burden while suppressing the increase in the cost of organic solvents.

[0137] As described above, a method for producing recycled superabsorbent polymers derived from used absorbent articles according to the embodiment is carried out.

[0138] Next, a recycled superabsorbent polymer derived from used absorbent articles according to the embodiment will be described. The recycled superabsorbent polymer according to this embodiment is produced by the method for producing a recycled superabsorbent polymer derived from used absorbent articles according to the above embodiment.

[0139] The recycled superabsorbent polymer derived from used absorbent articles according to this embodiment has a polyvalent metal content of 0.06% by mass or less. Preferably, the polyvalent metal content is 0.04% by mass or less. However, examples of polyvalent metals include alkaline earth metals and transition metals. Examples of alkaline earth metals include beryllium, magnesium, calcium, strontium, and barium. Examples of transition metals include iron, cobalt, nickel, and copper.

[0140] This recycled superabsorbent polymer contains superabsorbent polymer derived from used absorbent articles, yet its polyvalent metal content is 0.06% by mass or less, which is approximately the same as that of unused superabsorbent polymer. This is because organic solvents are used in the dehydration process, which is essential for the reuse of used superabsorbent polymer, and no polyvalent metals are used. Thus, this recycled superabsorbent polymer, despite being derived from used absorbent articles, has a polyvalent metal content that is approximately the same as that of unused superabsorbent polymer, and therefore can be said to contain high-quality superabsorbent polymer. As a result, this recycled superabsorbent polymer can be used as a good absorbent material.

[0141] Furthermore, in the recycled superabsorbent polymer derived from used absorbent articles according to this embodiment, the ash content is, for example, 40% by mass or less, preferably 35% by mass or less. The average particle size is, for example, 200 to 600 μm, preferably 300 to 500 μm. The amount of saline solution absorbed is, for example, 20 g / g or more, preferably 35 g / g or more. Thus, although the recycled superabsorbent polymer according to this embodiment is derived from used absorbent articles, it has a slightly lower water absorption capacity compared to unused superabsorbent polymers, but its ash content and average particle size are generally similar.

[0142] The present invention will be described below based on examples, but the present invention is not limited to these examples. In the following examples, the dewatering and washing step S3 was examined.

[0143] (1) Sample Example 1: A recycled superabsorbent polymer was prepared by the method for producing a recycled superabsorbent polymer derived from used absorbent articles according to the above embodiment. Specifically, the pretreatment steps S2 to drying step S8 were carried out on used absorbent articles, excluding the first oxidizing agent treatment step S4. However, in the first separation step S5, the plastic material was separated from the mixture, and in the second separation step S7, the superabsorbent polymer and pulp fibers were separated from the mixture. Then, the superabsorbent polymer was separated from the separated superabsorbent polymer and pulp fibers using a sieve to obtain a recycled superabsorbent polymer. Comparative Example 1: A superabsorbent polymer from unused absorbent articles was prepared. Comparative Example 2: A recycled superabsorbent polymer was prepared in the same manner as in Example 1, except that used absorbent articles were crushed in advance, water was used as the treatment liquid (without using non-volatile hydrophobic organic solvents or hydrophilic solvents), and calcium chloride containing calcium ions, which are one of the polyvalent metal ions, was used for the dehydration treatment of the superabsorbent polymer.

[0144] (2) Evaluation (a) Method for measuring polyvalent metal content (mass ratio) Multiple samples were taken from each superabsorbent polymer, and for each sample, the polyvalent metal content (mass ratio) in the superabsorbent polymer was measured using an ICP (Inductively Coupled Plasma) mass spectrometer. (b) Method for measuring ash content (mass ratio) Ash content refers to the mass ratio of inorganic or non-combustible residue remaining after organic matter has been ashed. Ash content is measured according to "5. Ash content test method" of "2. General test methods" of the standards for materials for physiological treatment products. Specifically, it is as follows: (i) A platinum, quartz, or porcelain crucible is heated strongly at 500-550°C for 1 hour, and after cooling, its mass W0 is measured. (ii) 2-4 g of the sample (pulp fiber material or pulp fiber) is taken, placed in the crucible, and its mass W1 is measured. (iii) If necessary, remove or shift the lid of the crucible, heat it gently at first, and gradually increase the temperature to 500-550°C for more than 4 hours, until all the carbonized material is ashed. (iv) After cooling, measure its mass. (v) Carbonize the residue again, and after cooling, measure its mass. Repeat carbonization, cooling, and weighing until a constant weight is reached. (vi) Let the mass after a constant weight is reached be W2. (vii) Calculate the ash content (mass %) using the following formula: Ash content (mass %) = (W2 - W0) / (W1 - W0) × 100

[0145] (3) Results Example 1: The polyvalent metal content was 0.06% by mass or less, and the ash content was 33% by mass. Comparative Example 1: The polyvalent metal content was 0.06% by mass or less, and the ash content was 40.9% by mass. Comparative Example 2: The calcium (Ca) content among the polyvalent metals was 4 to 20% by mass, and the ash content was 51% by mass. In Example 1, the average particle size was 450 μm, and the amount of saline solution absorbed was 40 g / g.

[0146] Thus, the recycled superabsorbent polymer of Example 1, despite being derived from used absorbent articles, had a low polyvalent metal content of 0.06% by mass or less. This value was approximately the same as that of unused superabsorbent polymers. This value was extremely low compared to cases where polyvalent metal ions were used in the dehydration treatment of superabsorbent polymers (4-20% by mass). Furthermore, the recycled superabsorbent polymer of Example 1, despite being derived from used absorbent articles, had a low ash content of 33% by mass. This value was lower than that of unused superabsorbent polymers. It is thought that components that normally adhere to the surface of superabsorbent polymers and remain as ash were removed by solvent treatment. This value was also low compared to cases where polyvalent metal ions were used in the dehydration treatment of superabsorbent polymers (51% by mass). Therefore, it was found that the recycled superabsorbent polymer of Example 1 contains a high-quality superabsorbent polymer. As a result, it was found that this recycled superabsorbent polymer can be used as a good absorbent material.

[0147] The abduction method of the present invention is not limited to the embodiments described above, and can be appropriately combined or modified without departing from the purpose and spirit of the present invention.

[0148] S3 Dehydration and washing process S5 Separation process

Claims

1. A method for producing recycled superabsorbent polymers derived from used absorbent articles, comprising: a dehydration and washing step of stirring a mixture obtained by mixing a mixture containing superabsorbent polymers from used absorbent articles with a treatment liquid containing at least one of a non-flammable hydrophobic organic solvent, a hydrophilic organic solvent, and water to dehydrate the superabsorbent polymers; and a separation step of separating the superabsorbent polymers from the mixture.

2. The method according to claim 1, wherein the dewatering and washing step comprises: a first stirring step of mixing the non-flammable hydrophobic organic solvent with at least one of the hydrophilic organic solvent and water to form the treatment liquid and stirring the treatment liquid; and a second stirring step of mixing the treatment liquid with the mixture to form the mixed liquid and stirring the mixed liquid.

3. The method according to claim 1 or 2, further comprising an oxidizing agent treatment step of treating the mixture in the mixture with an oxidizing agent while stirring the mixture, prior to the separation step.

4. The method according to any one of claims 1 to 3, further comprising an oxidizing agent treatment step of treating the superabsorbent polymer in the mixture with an oxidizing agent while stirring the mixture after the other components of the mixture have been separated from the mixture in the separation step.

5. The method according to any one of claims 1 to 4, further comprising a sanitary treatment step of subjecting the superabsorbent polymer separated from the mixed liquid in the separation step to sanitary treatment.

6. The method according to claim 5, wherein the sanitary treatment step includes an oxidizing agent treatment step of treating the superabsorbent polymer separated from the mixed liquid with an oxidizing agent.

7. The method according to any one of claims 1 to 6, further comprising a pretreatment step of reducing the moisture content of the mixture before the dewatering and washing step.

8. The method according to any one of claims 1 to 7, further comprising a bag-breaking step of breaking a packaging bag containing the mixture before the dewatering and washing step.

9. The method according to any one of claims 1 to 8, further comprising an organic solvent recovery step of separating and recovering the non-flammable hydrophobic organic solvent and / or the hydrophilic organic solvent from the treatment liquid after the separation step.

10. The method according to any one of claims 1 to 9, wherein the non-flammable hydrophobic organic solvent comprises at least one of a fluorinated organic solvent and an aromatic organic solvent.

11. The method according to any one of claims 1 to 10, wherein the hydrophilic organic solvent comprises at least one of a ketone-based organic solvent and an alcohol-based organic solvent.

12. The method according to any one of claims 1 to 11, wherein the water includes acidic water.

13. The method according to claim 12, comprising an activation step of activating the superabsorbent polymer with a solution containing an activator.

14. The method according to any one of claims 3, 4, or 6, wherein the oxidizing agent comprises at least one of ozone, hydrogen peroxide, and a chlorine-based substance.

15. The method according to any one of claims 1 to 14, further comprising a drying step for drying the superabsorbent polymer, wherein the drying step is performed after the dewatering and washing step and after the separation step, and is performed by low-temperature drying or reduced-pressure drying.

16. A recycled superabsorbent polymer derived from used absorbent articles, wherein the polyvalent metal content is 0.06% by mass or less.

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