WO2024090234A1

WO2024090234A1 - Method for recovering superabsorbent polymer from superabsorbent-polymer-containing constituent materials constituting used hygiene products, and method for producing recycled superabsorbent polymer using recovered superabsorbent polymer

Info

Publication number: WO2024090234A1
Application number: PCT/JP2023/037136
Authority: WO
Inventors: 孝義小西; 宜秀石川
Original assignee: ユニ・チャーム株式会社
Priority date: 2022-10-24
Filing date: 2023-10-13
Publication date: 2024-05-02
Also published as: JP2024062047A

Abstract

Provided is a method for improving the recovery rate and purity in a method for recovering a superabsorbent polymer from superabsorbent-polymer-containing constituent materials constituting used hygiene products. This method includes recovering a superabsorbent polymer from superabsorbent-polymer-containing constituent materials constituting used hygiene products, the method comprising a preparation step (S30) for preparing a moist aggregate that contains inactivated-superabsorbent-polymer-containing constituent materials, and a separation step (S20) for applying vibration to the aggregate to separate the superabsorbent polymer from the constituent materials.

Description

A method for recovering a superabsorbent polymer from a component material containing the superabsorbent polymer that constitutes a used sanitary product, and a method for producing a recycled superabsorbent polymer using the recovered superabsorbent polymer

The present invention relates to a method for recovering a superabsorbent polymer from a constituent material containing the superabsorbent polymer that constitutes a used sanitary product, and a method for producing a recycled superabsorbent polymer using the recovered superabsorbent polymer.

Methods for recovering superabsorbent polymers from used sanitary products are known. For example, Patent Document 1 discloses a method for treating used sanitary products, which includes at least a step of disintegrating the sanitary products and dispersing them in water, and a step of separating and recovering the fibers and SAP contained in the sanitary products, and is characterized in that a crosslinking agent and an acidic substance are added in the step of disintegrating the sanitary products and dispersing them in water.

JP 2013-150976 A

In Patent Document 1, first, the sanitary product is disintegrated and dispersed in water by putting the raw material, used disposable diapers, into a pulper and dispersing them in water. Then, in the process of separating and recovering the fibers and SAP contained in the disposable diapers, the diapers are processed using a screen and a cleaner, and the SAP portion is recovered first.

According to Patent Document 1, the screen is a method in which a fluid (liquid, gas) containing the disintegrated constituent materials of sanitary products is supplied, and the fluid is separated into rejects and accepts using a round-hole screen or a slit screen. The cleaner is a method in which a fluid (liquid, gas) containing the disintegrated constituent materials of sanitary products is supplied, and the fluid is separated into materials with different specific gravities using centrifugation.

However, according to the inventor's research, while screens and cleaners can separate and recover superabsorbent polymers from a mixture of disaggregated sanitary product constituent materials (including SAP, i.e., superabsorbent polymers), it is not easy to increase the recovery rate and purity.

The object of the present invention is to provide a method for recovering superabsorbent polymers from constituent materials that contain superabsorbent polymers and that constitute used sanitary products (hereinafter also simply referred to as a "method for recovering superabsorbent polymers") that improves the recovery rate and purity, and a method for producing recycled superabsorbent polymers using the recovered superabsorbent polymers (hereinafter also simply referred to as a "method for producing recycled superabsorbent polymers").

One aspect of the present invention is a method for recovering a superabsorbent polymer from a constituent material that contains the superabsorbent polymer and that constitutes a used sanitary product, the method comprising a preparation step of preparing a wet aggregate that contains the constituent material that contains the inactivated superabsorbent polymer, and a separation step of vibrating the aggregate to separate the superabsorbent polymer from the constituent material.

Another aspect of the present invention is a method for producing a recycled superabsorbent polymer, the method comprising a reactivation step of reactivating the superabsorbent polymer obtained by the above-mentioned method for recovering a superabsorbent polymer from a constituent material containing a superabsorbent polymer that constitutes a used sanitary product, by contacting the superabsorbent polymer with an alkali metal ion source capable of supplying alkali metal ions.

The present invention makes it possible to provide a method for recovering superabsorbent polymers from constituent materials that contain superabsorbent polymers and that constitute used sanitary products, a method for improving the recovery rate and purity, and a method for producing recycled superabsorbent polymers using the recovered superabsorbent polymers.

FIG. 1 is a flow chart showing an example of a method for recovering a superabsorbent polymer according to an embodiment and a method for producing a recycled superabsorbent polymer. FIG. 2 is a flow diagram showing an example of an inactivation step according to an embodiment. FIG. 1 is a block diagram showing an example of the configuration of a system used in a method for recovering a superabsorbent polymer and a method for producing a recycled superabsorbent polymer according to an embodiment. 10 is a flowchart showing a modified example of the first dehydration step according to the embodiment. FIG. 13 is a block diagram showing a modified example of a first dehydration device used in the first dehydration step according to the embodiment.

The present embodiment relates to the following aspects.
[Aspect 1]
A method for recovering a superabsorbent polymer from a constituent material containing the superabsorbent polymer that constitutes a used sanitary product, the method comprising: a preparation step of preparing a wet agglomerate containing the constituent material containing the inactivated superabsorbent polymer; and a separation step of vibrating the agglomerate to separate the superabsorbent polymer from the constituent material.

In this method, the superabsorbent polymer is inactivated (dehydrated) and is in the form of a granule with a small particle size, so that its shape is significantly different from that of the other constituent materials. In addition, the aggregate containing the constituent materials containing the superabsorbent polymer is in a wet state, and therefore moisture is present between the constituent materials. Therefore, the superabsorbent polymer is in a state where it has almost no hydrogen bonds with the other constituent materials. By applying vibration to the aggregate in such a state, the superabsorbent polymer, which has a significantly different (smaller) shape from the other constituent materials and has no hydrogen bonds, can be easily separated from the other constituent materials and separated. In other words, at least the superabsorbent polymer and the other constituent materials can be substantially separated. By creating such a state, it is possible to easily selectively extract the superabsorbent polymer from the constituent materials. This makes it possible to improve the recovery rate and purity of the superabsorbent polymer.

[Aspect 2]
2. The method of claim 1, wherein the separating step comprises a sieving step of forcing the agglomerates through a vibrating sieve having openings through which the superabsorbent polymer can pass.

In this method, the superabsorbent polymer has a shape that is significantly different (smaller) from the other constituent materials, and is in a state in which it has almost no hydrogen bonds. Therefore, when the superabsorbent polymer is sieved, the vibration of the sieve makes it easier to separate the superabsorbent polymer from the other constituent materials. In other words, the superabsorbent polymer and the other constituent materials can be substantially separated. By creating such a state on the sieve, the separated superabsorbent polymer can be more easily sieved out from the other constituent materials. This makes it possible to selectively extract the superabsorbent polymer from the constituent materials, and improve the recovery rate and purity of the superabsorbent polymer. In this case, for example, the other constituent materials such as pulp fibers and sheet members remain on the sieve and are recovered, and the superabsorbent polymer falls below the sieve and is recovered.

[Aspect 3]
The method according to aspect 2, wherein the preparation step includes a solid-liquid separation step of performing solid-liquid separation on an aqueous solution containing the constituent material including the inactivated superabsorbent polymer to obtain the aggregate, which is a solid, and a liquid.

In this method, the preparation step includes a solid-liquid separation step in which an aqueous solution containing constituent materials including an inactivated superabsorbent polymer is subjected to solid-liquid separation to obtain a solid aggregate. Here, the aggregate (solid) is formed from the constituent materials and can retain moisture. Therefore, the amount of liquid (moisture) in the aggregate can be kept to an appropriate amount to create an appropriately moist state. This prevents the superabsorbent polymer from being substantially separated from the other constituent materials, which would otherwise occur if the amount of liquid (moisture) in the aggregate is too much and vibrations are damped, or if the amount is too little and hydrogen bonds between the constituent materials are too strong, making it difficult to separate the superabsorbent polymer.

[Aspect 4]
The method according to aspect 3, wherein the solid-liquid separation step includes a centrifugation step of centrifuging an aqueous solution containing the constituent materials to obtain the aggregate and the liquid.

This method includes a centrifugation step for centrifuging an aqueous solution containing the constituent materials as a solid-liquid separation step. In centrifugation, it is relatively easy to control the force applied to the constituent materials, making it easy to prevent the superabsorbent polymer from being crushed and to make the moisture content (moisture content) of the aggregates within the desired numerical range. This makes it possible to keep the aggregates in an appropriately moist state while minimizing damage to the superabsorbent polymer.

[Aspect 5]
The method according to claim 4, wherein the moisture content of the superabsorbent polymer separated in the centrifugation step is 80% by mass or less.

In this method, the moisture content of the superabsorbent polymer after the centrifugation process is 80% by mass or less, so the superabsorbent polymer is in the form of granules with a small particle size. Also, considering that the moisture content of the inactivated superabsorbent polymer hardly changes before and after the centrifugation process, since the moisture content of the superabsorbent polymer after the centrifugation process is 80% by mass or less, it can be said that the moisture content of the superabsorbent polymer before the centrifugation process is also roughly 80% by mass or less. In other words, because the centrifugation is performed in a state in which the superabsorbent polymer contains almost no moisture, the superabsorbent polymer can be made less likely to be damaged during solid-liquid separation (centrifugation). This makes it easier to recover the superabsorbent polymer in the separation process.

[Aspect 6]
The method according to claim 4, wherein a sieve device having the sieve in the sieving step is disposed below a centrifuge device that performs the centrifugation in the centrifugal separation step.

In this method, a sieving device having a sieve for the sieving separation process is placed under the centrifuge device that performs the centrifugation in the centrifugation process. Therefore, the aggregates after the centrifugation process can be immediately sieved, and the sieving separation process can be performed while suppressing fluctuations in the state of the aggregates after centrifugation (the state of the superabsorbent polymer and the moisture content of the aggregates). This makes it easier to recover the superabsorbent polymer.

[Aspect 7]
4. The method of claim 3, further comprising recovering the superabsorbent polymer from the liquid obtained in the solid-liquid separation step through a screen.

In this method, the superabsorbent polymer is recovered from the liquid obtained in the solid-liquid separation process using a screen. This makes it possible to improve the recovery rate of the superabsorbent polymer.

[Aspect 8]
The method according to any one of aspects 1 to 7, wherein the preparation step includes an inactivation step of inactivating the superabsorbent polymer, and the inactivation step includes a dehydration step of dehydrating the superabsorbent polymer in the constituent material by contacting the constituent material with a polyvalent metal ion source capable of supplying polyvalent metal ions.

In this method, the preparation step includes an inactivation step in which the superabsorbent polymer is inactivated, and the inactivation step includes a dehydration step in which the superabsorbent polymer in the constituent material is dehydrated by contacting the constituent material with a polyvalent metal ion source capable of supplying polyvalent metal ions. By using a polyvalent metal ion source for inactivation (dehydration), the superabsorbent polymer can be more reliably inactivated (dehydrated) to form granules having a small particle size.

[Aspect 9]
The method according to aspect 8, wherein the inactivation step further includes a pre-dehydration step of pre-dehydrating the superabsorbent polymer in the constituent material by immersing the constituent material in an acidic aqueous solution before the dehydration step.

In this method, the inactivation step further includes a pre-dehydration step in which the constituent material is immersed in an acidic aqueous solution prior to the dehydration step to pre-dehydrate the superabsorbent polymer in the constituent material. When the superabsorbent polymer is inactivated (dehydrated) with an acid, the dehydration rate of the superabsorbent polymer is lower (the moisture content is higher) than when the superabsorbent polymer is inactivated (dehydrated) with a polyvalent metal ion. However, by carrying out a pre-dehydration step with an acid prior to the dehydration step with a polyvalent metal ion source, the dehydration rate of the superabsorbent polymer obtained in the inactivation step can be made higher (the moisture content can be made lower).

[Aspect 10]
The method of claim 2, wherein in the sieve separation step, the vibration of the sieve comprises lateral vibration.

In this method, in the sieve separation step, the vibration of the sieve includes not only vertical vibration, i.e., vibration perpendicular to the sieve surface, but also horizontal vibration, i.e., vibration in the in-plane direction parallel to the sieve surface. Therefore, the agglomerates can be shaken not only in a direction perpendicular to the sieve surface, but also in a direction parallel to it. This makes it easier to separate the superabsorbent polymer from the other constituent materials, and makes it easier to make the superabsorbent polymer and the other constituent materials substantially disintegrate. By creating such a state on the sieve, the separated superabsorbent polymer can be more easily sieved away from the other constituent materials.

[Aspect 11]
The method according to claim 2 or 10, wherein the sieve comprises a first sieve portion having meshes through which the superabsorbent polymer can pass, and a second sieve portion located below the first sieve portion and having finer meshes than the first sieve portion and through which the superabsorbent polymer can pass, and the sieving step includes a step of passing the agglomerates through a vibrating sieve to sieve the superabsorbent polymer in two stages, the first sieve portion and the second sieve portion.

In this method, the sieve has a first sieve portion at the top with meshes through which the superabsorbent polymer can pass, and a second sieve portion at the bottom with meshes finer than the first sieve portion and through which the superabsorbent polymer can pass, and in the sieving process, the superabsorbent polymer is sieved in two stages through the first sieve portion and the second sieve portion. This makes it possible to further increase the purity of the superabsorbent polymer that is recovered.

[Aspect 12]
11. The method of claim 2 or 10, wherein the component material comprises pulp fibers, and the sieving step comprises the step of the pulp fibers moving to the outside on the surface of the sieve and the superabsorbent polymer moving to the underside of the sieve and being separated from one another.

In this method, the superabsorbent polymer has a shape significantly different from that of the pulp fibers and is in a state in which there are almost no hydrogen bonds. Therefore, when the superabsorbent polymer is sieved, the vibration of the sieve makes it easier to separate the superabsorbent polymer from the pulp fibers, and the separated superabsorbent polymer can be selectively removed from among the constituent materials. This makes it easier to sieve the superabsorbent polymer from the other constituent materials, and improves the recovery rate and purity of the superabsorbent polymer. In this case, for example, the pulp fibers and sheet members remain on the sieve and are collected, and the superabsorbent polymer falls below the sieve and is collected.

[Aspect 13]
The method according to any one of claims 1 to 12, further comprising a reactivation step of contacting the superabsorbent polymer obtained in the separation step with an alkali metal ion source capable of supplying alkali metal ions to reactivate the superabsorbent polymer.

In this method, the superabsorbent polymer obtained in the separation process is brought into contact with an alkali metal ion source capable of supplying alkali metal ions, thereby reactivating the superabsorbent polymer obtained in the separation process so that it can be used as a water-absorbing material.

[Aspect 14]
A method for producing a recycled superabsorbent polymer, comprising a reactivation step of contacting the superabsorbent polymer obtained by the method for recovering a superabsorbent polymer from a constituent material containing the superabsorbent polymer constituting a used sanitary product according to any one of aspects 1 to 12 with an alkali metal ion source capable of supplying alkali metal ions, thereby reactivating the superabsorbent polymer.

In this method, the superabsorbent polymer obtained by the method for recovering superabsorbent polymer from constituent materials containing superabsorbent polymer that constitute used sanitary products is reactivated by contacting it with an alkali metal ion source capable of supplying alkali metal ions, and it is possible to produce a recycled superabsorbent polymer that can be used as a water-absorbing material using the recovered superabsorbent polymer as a raw material. As a result, the superabsorbent polymer with improved recovery rate and purity is reactivated, and the recovery rate and purity of the recycled superabsorbent polymer can also be improved.

Below, a method for recovering a superabsorbent polymer from a constituent material containing a superabsorbent polymer that constitutes a used sanitary product according to an embodiment, and a method for producing a recycled superabsorbent polymer using the recovered superabsorbent polymer will be described. However, in this specification, a sanitary product refers to an article that contributes to hygiene and contains a superabsorbent polymer. A used sanitary product (e.g., absorbent article) refers to a sanitary product that has been used by a user and that has absorbed and retained liquids (e.g., excrement) released from the user, and includes sanitary products that have been used but do not absorb or retain liquids, and unused but discarded products. A used superabsorbent polymer refers to a superabsorbent polymer that was contained in a used sanitary product. In this embodiment, an absorbent article will be described as an example of a sanitary product. Examples of absorbent articles include paper diapers, urine pads, sanitary napkins, bed sheets, and pet sheets, and contain a superabsorbent polymer and may further contain pulp fibers.

First, an example of the configuration of an absorbent article will be described. The absorbent article includes a top sheet, a back sheet, and an absorbent body that is disposed between the top sheet and the back sheet and contains a superabsorbent polymer. In this embodiment, the absorbent body further contains pulp fibers. Thus, the absorbent article includes, as its constituent materials, a top sheet, a back sheet, and an absorbent body that contains a superabsorbent polymer and pulp fibers. An example of the size of the absorbent article is a length of about 15 to 100 cm and a width of 5 to 100 cm. The absorbent article may further include, as its constituent materials, other members that are included in general absorbent articles, such as a diffusion sheet, a leak-proof wall, a side sheet, an exterior sheet, and a waistband.

The constituent material of the surface sheet may be, for example, a liquid-permeable nonwoven fabric, a synthetic resin film having liquid-permeable holes, or a composite sheet of these. The constituent material of the back sheet or the exterior sheet may be, for example, a liquid-impermeable nonwoven fabric, a liquid-impermeable synthetic resin film, or a composite sheet of these. The constituent material of the diffusion sheet may be, for example, a liquid-permeable nonwoven fabric. The constituent material of the leak-proof wall, side sheet, or waistband may be, for example, a liquid-impermeable nonwoven fabric, and the waistband or the leak-proof wall may include an elastic material such as rubber. There is no particular restriction on the material of the nonwoven fabric or synthetic resin film as long as it can be used as an absorbent article, and examples of the material include olefin-based resins such as polyethylene and polypropylene, polyamide-based resins such as 6-nylon and 6,6-nylon, and polyester-based resins such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT). Natural fibers such as cotton and rayon may be used as the material of the nonwoven fabric. In this embodiment, an absorbent article in which the constituent material of the back sheet is a film and the constituent material of the surface sheet is a nonwoven fabric will be described as an example.

Of the pulp fibers and superabsorbent polymers in the absorbent, there are no particular limitations on the pulp fibers as long as they can be used as absorbent articles, and examples include cellulosic fibers. Examples of cellulosic fibers include wood pulp, crosslinked pulp, non-wood pulp, regenerated cellulose, and semi-synthetic cellulose. The size of the pulp fibers includes an average fiber major axis of several tens of μm, preferably 20 to 40 μm, and an average fiber length of several mm, preferably 2 to 5 mm.

There are no particular limitations on the superabsorbent polymer (SAP) as long as it can be used as an absorbent material in absorbent articles, but examples include polyacrylate-based, polysulfonate-based, and maleic anhydride-based absorbent polymers. The size of the superabsorbent polymer (when dry) can be, for example, an average particle size of several hundred μm, with 200 to 500 μm being preferred. The absorbent may be formed of a liquid-permeable sheet and may include an arrap that encases the absorbent material. However, in used absorbent articles, a part or all of the absorbent, and therefore at least a part or all of the superabsorbent polymer, often absorbs liquid excrement (e.g. urine). In such cases, the superabsorbent polymer that has absorbed the liquid excrement swells to a size of several tens to several hundreds times its original size.

One side and the other side of the absorbent are respectively bonded to the top sheet and the back sheet via an adhesive. In plan view, the portion (peripheral portion) of the top sheet that extends outwardly from the absorbent so as to surround the absorbent is bonded to the portion (peripheral portion) of the back sheet that extends outwardly from the absorbent so as to surround the absorbent via an adhesive. Thus, the absorbent is enclosed inside the bond between the top sheet and the back sheet. There are no particular limitations on the adhesive as long as it can be used as an absorbent article, and examples of the adhesive include hot melt adhesives. Examples of hot melt adhesives include pressure-sensitive adhesives or heat-sensitive adhesives that are mainly rubber-based, such as styrene-ethylene-butadiene-styrene, styrene-butadiene-styrene, and styrene-isoprene-styrene, or mainly olefin-based, such as polyethylene.

Next, a method for recovering a superabsorbent polymer from a constituent material containing the superabsorbent polymer that constitutes a used sanitary product (method for recovering a superabsorbent polymer) and a method for producing a recycled superabsorbent polymer using the recovered superabsorbent polymer (method for producing a recycled superabsorbent polymer) according to an embodiment will be described.

FIG. 1 is a flow diagram showing an example of a method for recovering superabsorbent polymers according to an embodiment, and a method for producing recycled superabsorbent polymers. FIG. 2 is a flow diagram showing an example of an inactivation step in the above method according to an embodiment. FIG. 3 is a block diagram showing an example of the configuration of a system used in the method for recovering superabsorbent polymers according to an embodiment, and a method for producing recycled superabsorbent polymers.

The method for recovering superabsorbent polymer includes a preparation step S30 and a separation step S40, and may further include a reactivation step S50. The system 20 used in the method for recovering superabsorbent polymer includes a preparation device 30 and a separation device 40, and may further include a reactivation device 50. On the other hand, the method for producing recycled superabsorbent polymer includes a reactivation step S50, and may further include a preparation step S30 and a separation step S40. The system 20 used in the method for producing recycled superabsorbent polymer includes a reactivation device 50, and may further include a preparation device 30 and a separation device 40. Note that each device may be a combination of multiple devices. Each step will be described in detail below.

First, the preparation step S30 will be described. The preparation step S30 is performed by the preparation device 30. The preparation step S30 is a step of preparing a wet aggregate containing a constituent material including an inactivated superabsorbent polymer. There are no particular limitations on the preparation device 30, so long as it is a device (including a combination of multiple devices) that can provide a wet aggregate containing a constituent material including an inactivated superabsorbent polymer.

Inactivated superabsorbent polymers are superabsorbent polymers whose absorbency has been reduced. Such superabsorbent polymers are in a state where it is difficult for them to absorb moisture internally, so a superabsorbent polymer that has absorbed moisture (e.g. liquid excrement) will expel the moisture (dehydrate) from its interior by inactivation, and will change from a swollen state to a roughly granular state.

The inactivation method is not particularly limited, but includes a method of immersing the superabsorbent polymer or a constituent material containing the superabsorbent polymer in an aqueous solution containing an inactivating agent (inactivating aqueous solution). Examples of the inactivating agent include acids such as inorganic acids and organic acids, and polyvalent metal ion sources capable of supplying polyvalent metal ions.

The constituent materials include at least some of the above-mentioned top sheet, back sheet, superabsorbent polymer, pulp fiber, etc., and at least superabsorbent polymer is included. Each constituent material (except superabsorbent polymer) forming the aggregate is almost entirely broken into small pieces by decomposing a plurality of used absorbent articles by physical impact or peeling off the adhesive parts with chemicals, etc., and/or crushing or shredding. The size (maximum dimension) of the small pieces is made smaller than a predetermined size. However, the predetermined size (maximum dimension) is, for example, 2 to 30 mm, preferably 3 to 20 mm, and more preferably 4 to 10 mm or less. If the size of the small pieces is too small, it will be difficult to separate them from the superabsorbent polymer, and if it is too large, it will be difficult to transport them between processes. It is not necessary for all of the plurality of used absorbent articles handled in this method to contain superabsorbent polymer, and it is sufficient that some of them contain superabsorbent polymer.

The aggregate is a lump formed by at least a part of the above-mentioned constituent materials. However, the aggregate is in a state where at least the superabsorbent polymer can be separated by vibration described later, and preferably in a state where it can be separated into individual constituent materials. The density of the aggregate is not particularly limited as long as it can be separated by vibration, but the lower limit is 0.10 g/cm ³ , preferably 0.11 g/cm ³ , more preferably 0.12 g/cm ³ , even more preferably 0.13 g/cm ³ , and even more preferably 0.15 g/cm ^3. The upper limit is 1.5 g/cm ³ , preferably 1.2 g/cm ³ , more preferably 1.0 g/cm ³ , and even more preferably 0.80 g/cm ^3. If the density is too high, the constituent materials will adhere to each other and the aggregate will be difficult to decompose, and if the density is too low, the amount of the constituent materials will be reduced and the recovery rate will decrease.

The aggregates are in a wet state, and the moisture content of the aggregates is not particularly limited as long as they can be separated by vibration, but the lower limit is 10% by mass, with 13% being preferred, 15% being more preferred, and 20% being even more preferred. The upper limit is 85% by mass, with 80% being preferred, with 75% by mass or less being more preferred, and 70% by mass or less being even more preferred. However, the method for measuring the moisture content of the aggregates is as follows. If the moisture content is too high, the vibration is attenuated and it becomes difficult to separate the individual materials, and if a sieve is used, the materials tend to stick to the sieve, which can easily cause clogging of the sieve or a decrease in the recovery rate due to sticking, and also tends to decrease production efficiency. If the moisture content is too low, the constituent materials are directly hydrogen bonded to each other, making it difficult to break down the aggregates.

<Moisture content of aggregates>
The moisture content of the aggregates was measured as follows.
(1) Approximately 5 g of the aggregate is weighed out and used as a sample.
(2) The above sample is placed on the sample stage of an infrared moisture meter (FD-720, manufactured by Kett Electric Laboratory).
(3) Set the moisture meter to 120°C and measure the moisture content.
The moisture content (mass %) is calculated by (weight before drying - weight after drying) / weight before drying x 100.

Next, the separation step S40 will be described. The separation step S40 is performed by the separation device 40. The separation step S40 is a step in which vibration is applied to the aggregate to separate the superabsorbent polymer from the constituent materials. There are no particular limitations on the separation device 40, so long as it is a device (including a combination of multiple devices) that can apply vibration to the aggregate and separate the superabsorbent polymer from the constituent materials.

The vibration is applied to the aggregates through a container or sieve on which the aggregates are placed by vibrating the container or sieve. The direction of the vibration can be lateral vibration in a plane parallel to the placement surface (width and/or depth direction of the placement surface), vertical vibration in a direction perpendicular to the placement surface, and/or a combination of these (three-dimensional vibration). There is no restriction on the frequency of the vibration as long as it can be separated, but examples of the frequency include 5 Hz (300 v.p.m) to 500 Hz (30,000 v.p.m), preferably 10 Hz (600 v.p.m) to 250 Hz (15,000 v.p.m), and more preferably 15 Hz (900 v.p.m) to 100 Hz (6,000 v.p.m).

When separating the superabsorbent polymer from the constituent materials, the vibration and the separation of the superabsorbent polymer from the remaining constituent materials may be performed simultaneously or separately. For example, the agglomerates of the constituent materials may be placed in a container and vibrated to turn the agglomerates into aggregates in which the constituent materials are separated from each other and gathered together, and the aggregates may then be separated using a screen or the like, or the agglomerates may be sieved from the beginning and vibration and separation may be performed simultaneously.

In this method, the superabsorbent polymer is inactivated (dehydrated) and is in the form of a granule with a small particle size, so that its shape is significantly different from that of the other constituent materials. In addition, the aggregate containing the constituent materials containing the superabsorbent polymer is in a wet state, and therefore moisture is present between the constituent materials. Therefore, the superabsorbent polymer is in a state where it has almost no hydrogen bonds with the other constituent materials. By applying vibration to the aggregate in such a state, the superabsorbent polymer, which has a significantly different (smaller) shape from the other constituent materials and has no hydrogen bonds, can be easily separated from the other constituent materials. In other words, at least the superabsorbent polymer and the other constituent materials can be substantially separated. By creating such a state, it is possible to easily selectively extract the superabsorbent polymer from the constituent materials. This makes it possible to improve the recovery rate and purity of the superabsorbent polymer.

Next, the reactivation step S50 will be described. The reactivation step S50 is carried out by a reactivation device 50. The reactivation step S50 reactivates the superabsorbent polymer by contacting the superabsorbent polymer obtained in the preparation step S30 and the separation step S40 (a method for recovering a superabsorbent polymer) with an alkali metal ion source capable of supplying alkali metal ions.

The superabsorbent polymer obtained by the preparation step S30 and separation step S40 (method of recovering a superabsorbent polymer) is inactivated, and therefore has low water absorption, making it difficult to use as a polymer that absorbs moisture as is. Therefore, in the reactivation step S50, the inactivated superabsorbent polymer is reactivated by contacting it with an alkali metal ion source. When the superabsorbent polymer is brought into contact with the alkali metal ion source, it may be brought into direct contact with the alkali metal ion source, or it may be immersed in an aqueous solution containing the alkali metal ion source.

In this method, the superabsorbent polymer obtained by the above-mentioned method for recovering a superabsorbent polymer is reactivated by contacting it with an alkali metal ion source capable of supplying alkali metal ions, and the recovered superabsorbent polymer can be used as a raw material to produce a recycled superabsorbent polymer that can be used as a water-absorbing material. This reactivates the superabsorbent polymer with improved recovery rate and purity, and therefore the recovery rate and purity of the recycled superabsorbent polymer can also be improved.

Each process is explained in detail below.

In this embodiment, the preparation process S30 includes an inactivation process S31 and a solid-liquid separation process S32.

First, the inactivation step S31 will be described. The inactivation step S31 is carried out by the inactivation device 31. The inactivation step S31 is a step for inactivating the superabsorbent polymer in the constituent material. When the superabsorbent polymer is derived from used sanitary products, it has absorbed moisture such as liquid excrement inside, but this is discharged (dehydrated) by inactivation. As a result, the superabsorbent polymer changes from a swollen state to a roughly granular state. Therefore, in the inactivation step S31, a constituent material containing an inactivated (dehydrated) superabsorbent polymer (hereinafter simply referred to as an "inactivated material") is produced.

In this embodiment, the inactivation process S31 includes a first dehydration process S311, a second dehydration process S312, and a dilution process S313. The first dehydration process S311, the second dehydration process S312, and the dilution process S313 are respectively performed by the first dehydration device 311, the second dehydration device 312, and the dilution device 313 of the inactivation device 31. However, the first dehydration device 311, the second dehydration device 312, and the dilution device 313 may be independent devices, any two of them may be one device, or they may be one device as a whole.

First, the first dehydration step S311 will be described. In the first dehydration step S311 (pre-dehydration step), the constituent material is immersed in an aqueous solution containing an acid as an inactivating agent, i.e., an acidic aqueous solution, to inactivate and dehydrate the superabsorbent polymer in the constituent material. The superabsorbent polymer in the constituent material is derived from used sanitary products, so the superabsorbent polymer is inactivated by immersing it in the acidic aqueous solution, and liquid waste and the like are expelled to form an inactivated superabsorbent polymer.

The acid that is the inactivating agent is not particularly limited, and examples thereof include inorganic acids and organic acids. When the superabsorbent polymer is inactivated using an acid, it is difficult to leave ash in the high-tax material containing the superabsorbent polymer, compared to when the superabsorbent polymer is inactivated using lime, calcium chloride, or the like. 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, etc. The sulfuric acid concentration of the acidic aqueous solution is not particularly limited, and examples thereof include 0.1 to 0.5 mass% or less. On the other hand, examples of organic acids include carboxylic acids having multiple carboxyl groups in one molecule (examples: citric acid, tartaric acid, malic acid, succinic acid, oxalic acid), carboxylic acids having one carboxyl group in one molecule (examples: gluconic acid, pentanoic acid, butanoic acid, propionic acid, glycolic acid, acetic acid, formic acid), sulfonic acids (examples: methanesulfonic acid, trifluoromethanesulfonic acid, benzenesulfonic acid, p-toluenesulfonic acid), etc. As the organic acid, it is preferable that the organic acid has multiple carboxyl groups, and citric acid is more preferable, from the viewpoint of easily forming a chelate complex with a divalent or higher metal (e.g., calcium) contained in excrement and not easily leaving ash in the superabsorbent polymer and pulp fibers. The citric acid concentration of the acidic aqueous solution is not particularly limited, but may be, for example, 0.5 to 4% by mass. In this embodiment, sulfuric acid is used as the acid in the acidic aqueous solution.

The acidic aqueous solution may be acidic, but preferably has a predetermined pH. The upper limit of the predetermined pH is preferably 4.5, more preferably 4.0, even more preferably 3.5, and even more preferably 3.0. If the predetermined pH is too high, the highly absorbent polymer is not sufficiently inactivated, and the water held by the highly absorbent polymer (e.g., liquid excrement) is likely to be insufficiently discharged, making it difficult to separate the highly absorbent polymer from other constituent materials. On the other hand, the lower limit of the predetermined pH is preferably 0.5, more preferably 1.0. If the predetermined pH is too low, the other constituent materials are likely to be damaged. In this specification, pH refers to a value at 25°C. The pH can be measured using a pH meter (e.g., twin AS-711 manufactured by Horiba, Ltd.). The above-mentioned predetermined pH is measured at the end of the first dehydration step S311.

The first dehydration device 311 that performs the first dehydration step S311 is not particularly limited in its specific configuration as long as it can immerse the superabsorbent polymer in the acidic aqueous solution. The first dehydration device 311 has, for example, a tank in which a constituent material containing a superabsorbent polymer can be placed and in which an acidic aqueous solution can be stored. In the first dehydration device 311, the constituent material containing a superabsorbent polymer may be added after the acidic aqueous solution is stored in the tank, or the constituent material containing a superabsorbent polymer may be placed in the tank and then the acidic aqueous solution may be added.

In the first dehydration step S311, the constituent materials including the superabsorbent polymer are stirred in a tank containing the acidic aqueous solution for, for example, about 5 to 60 minutes to allow the reaction to proceed evenly, depending on the temperature, thereby inactivating the superabsorbent polymer. In the first dehydration step S311, the temperature of the acidic aqueous solution is not particularly limited, and may be, for example, room temperature (25°C), preferably higher than room temperature, more preferably 60 to 95°C, and even more preferably 70 to 90°C. The acid in the acidic aqueous solution makes it easier to disinfect bacteria contained in the acidic aqueous solution that originate from excrement, etc.

In the first dehydration step S311, it is preferable to dehydrate the resulting superabsorbent polymer so that it has a moisture content of preferably 80 to 99% by mass, more preferably 80 to 97% by mass, and even more preferably 80 to 95% by mass. This makes it easier to reduce the moisture content of the superabsorbent polymer obtained in the second dehydration step S312.

However, the moisture content of the superabsorbent polymer can be measured using an infrared moisture meter (FD-720: manufactured by Kett Electric Laboratory Co., Ltd.) in the same way as the moisture content of the aggregate described above.

In this embodiment, the first dehydration step S311 may include an acid separation step in which the superabsorbent polymer in the constituent material is inactivated, dehydrated, and then the constituent material is separated from the acidic aqueous solution (solid-liquid separation using a screen or the like). The superabsorbent polymer may then be inactivated with a strong acid (e.g., pH 1 or so), and if the degree of inactivation is sufficient, the constituent material may be washed with water. That is, the first washing step may further include washing the constituent material with water (hereinafter also referred to as "washing water") several times (e.g., 2 to 10 times) the mass of the constituent material (multiple washings are also possible) and separating the constituent material from the washing water (solid-liquid separation using a screen or the like).

If the moisture content of the superabsorbent polymer is less than 85% by mass at the end of the first dehydration step S311, the superabsorbent polymer can be separated from the other constituent materials in the separation step S40 if the moisture in the other constituent materials is also sufficiently removed in the solid-liquid separation step S32 or the like. In that case, the subsequent second dehydration step S312 (and dilution step S313) can be omitted.

In addition, to inactivate the superabsorbent polymer in the constituent material, the first dehydration step S311 may be omitted and the second dehydration step S312 may be carried out.

Next, the second dehydration step S312 will be described. In the second dehydration step S312 (dehydration step), the constituent material is brought into contact with a polyvalent metal ion source capable of supplying polyvalent metal ions as an inactivating agent, thereby further inactivating and dehydrating the superabsorbent polymer in the constituent material. The superabsorbent polymer after the first dehydration step S311 is further brought into contact with polyvalent metal ions (e.g. calcium ions) from the polyvalent metal ion source, thereby further inactivating the polymer and allowing more liquid waste and the like to be discharged, forming a further inactivated superabsorbent polymer.

In this method, the preparation step S30 includes an inactivation step for inactivating the superabsorbent polymer, and the inactivation step has a second dehydration step S312 for dehydrating the superabsorbent polymer in the constituent material by contacting the constituent material with a polyvalent metal ion source capable of supplying polyvalent metal ions. By using a polyvalent metal ion source for inactivation (dehydration), the superabsorbent polymer can be more reliably inactivated (dehydrated) to form granules having a small particle size.

Examples of the polyvalent metal ions that are inactivating agents include alkaline earth metal ions and transition metal ions. Examples of the alkaline earth metal ions include beryllium ions, magnesium ions, calcium ions, strontium ions, and barium ions. Examples of the transition metal ions include iron ions, cobalt ions, nickel ions, and copper ions.

When the polyvalent metal ion is an alkaline earth metal ion, examples of polyvalent metal ion sources capable of supplying the polyvalent metal ion include alkaline earth metal hydroxides (e.g., calcium hydroxide, magnesium hydroxide), alkaline earth metal hydroxide and acid salts (e.g., calcium chloride, calcium nitrate, magnesium chloride, magnesium nitrate), alkaline earth metal oxides (e.g., calcium oxide, magnesium oxide), etc., with calcium chloride being preferred. Examples of the acid include the acid for the acidic aqueous solution described in the first dehydration step S311.

When the polyvalent metal ion is a transition metal ion, examples of polyvalent metal ion sources capable of supplying the polyvalent metal ion include transition metal hydroxides (e.g., iron hydroxide, cobalt hydroxide, nickel hydroxide, copper hydroxide), transition metal hydroxide and acid salts, transition metal oxides (e.g., iron oxide, cobalt oxide, nickel oxide, copper oxide), etc. Examples of the acid include the acid for the acidic aqueous solution described in the first dehydration step S311.

Specific examples of the hydroxides and acid salts of the transition metals include inorganic acid salts or organic acid salts. Examples of the inorganic acid salts include iron salts such as iron chloride, iron sulfate, iron phosphate, and iron nitrate; cobalt salts such as cobalt chloride, cobalt sulfate, cobalt phosphate, and cobalt nitrate; nickel salts such as nickel chloride and nickel sulfate; and copper salts such as copper chloride and copper sulfate. Examples of the organic acid salts include iron lactate, cobalt acetate, cobalt stearate, nickel acetate, and copper acetate.

In the second dehydration step S312, for example, the superabsorbent polymer dehydrated in the first dehydration step S311 can be directly contacted with a polyvalent metal ion source capable of supplying polyvalent metal ions to form a further inactivated (dehydrated) superabsorbent polymer. Alternatively, in the second dehydration step S312, the superabsorbent polymer dehydrated in the first dehydration step S311 can be stirred in an aqueous solution containing a polyvalent metal ion source capable of supplying polyvalent metal ions for about 5 to 60 minutes, depending on the temperature, to form a further inactivated (dehydrated) superabsorbent polymer.

The aqueous solution containing the polyvalent metal ion source in the second dehydration step S312 may or may not be heated. In addition, the temperature of the aqueous solution containing the polyvalent metal ion source is not particularly limited when it is not heated, and can be, for example, room temperature (25°C) to 40°C, or less than 60°C. When it is heated, it is preferably higher than room temperature, more preferably 60 to 100°C, even more preferably 70 to 95°C, and even more preferably 80 to 90°C. When the solubility of the polyvalent metal ion source in water is low, heating can efficiently supply polyvalent metal ions from the polyvalent metal ion source and further promote the formation of an inactivated (dehydrated) highly water-absorbent polymer.

In the second dehydration step S312, the superabsorbent polymer dehydrated in the first dehydration step S311 is further inactivated (dehydrated) so that the resulting superabsorbent polymer has a moisture content of preferably 50 to 80% by mass, more preferably 50 to 75% by mass, and even more preferably 50 to 70% by mass. This reduces the amount of waste remaining in the resulting superabsorbent polymer, and makes it easier to form a recycled superabsorbent polymer from the superabsorbent polymer. If the moisture content of the superabsorbent polymer is too low, it becomes difficult for the superabsorbent polymer to react with the alkali metal ion source in the reactivation step S50, making it difficult to form a recycled superabsorbent polymer. On the other hand, if the moisture content of the superabsorbent polymer is too high, it becomes difficult for the superabsorbent polymer to be separated from other constituent materials in the separation step S40.

The second dehydration device 312 that performs the second dehydration step S312 is not particularly limited in its specific configuration as long as it can immerse the superabsorbent polymer in the polyvalent metal ions. The second dehydration device 312 has, for example, a tank in which a constituent material containing a superabsorbent polymer can be placed and in which a polyvalent metal ion source or an aqueous solution containing a polyvalent metal ion source can be stored. In the second dehydration device 312, the constituent material containing a superabsorbent polymer may be added after the polyvalent metal ion source or the aqueous solution containing a polyvalent metal ion source is stored in the tank, or the constituent material containing a superabsorbent polymer may be placed in the tank and then the polyvalent metal ion source or the aqueous solution containing a polyvalent metal ion source may be added.

In the second dehydration step S312, when the superabsorbent polymer dehydrated in the first dehydration step S311 is brought into contact with an aqueous solution containing a polyvalent metal ion source, for example when the superabsorbent polymer is put into the aqueous solution, the concentration of the polyvalent metal ion source in the aqueous solution is preferably 1.0 to 30.0% by mass, more preferably 3.0 to 25.0% by mass, and preferably 5.0 to 20.0% by mass. This makes it easier for the superabsorbent polymer obtained in the second dehydration step S312 to have the desired moisture content.

In this embodiment, the second dehydration step S312 may include a polyvalent metal ion separation step in which the superabsorbent polymer in the constituent material is inactivated, the constituent material is dehydrated, and then the constituent material is separated from the aqueous solution containing the polyvalent metal ions (solid-liquid separation using a screen or the like). The second dehydration step may further include a second washing step in which the constituent material is washed (multiple washings are also possible) with several times (e.g., 2 to 10 times) the mass of water (hereinafter also referred to as "washing water") and the constituent material is separated from the washing water (solid-liquid separation using a screen or the like).

Next, the dilution step S313 will be described. In the dilution step S313, the constituent material containing the inactivated superabsorbent polymer obtained in the second dehydration step S312 (or the first dehydration step S311) is diluted with water (hereinafter also referred to as "diluted water") in an amount several times (e.g., 2 to 10 times) the mass of the constituent material. This makes solid-liquid separation easier in the subsequent solid-liquid separation step S32.

The dilution step S313 may be omitted. In that case, if the second dehydration step S312 is omitted, the first washing step of the first dehydration step S311 (however, solid-liquid separation is not performed) may be substituted for the dilution step S313. Alternatively, the second washing step of the second dehydration step S312 (however, solid-liquid separation is not performed) may be substituted for the dilution step S313.

The dilution device 313 that performs the dilution process S313 is not particularly limited in its specific configuration, as long as it can immerse the constituent materials in water several times its mass. The dilution device 313 has, for example, a tank that can store the constituent materials and water. Note that at least two of the first dehydration device 311, the second dehydration device 312, and the dilution device 313 may have the same tank.

The dilution step S313 may also include an oxidizing agent treatment step in which the constituent material containing the superabsorbent polymer is treated with an oxidizing agent. The oxidizing agent is not particularly limited as long as it can remove impurities (e.g., odorous substances, colored substances, and germs) adhering to the surface of the superabsorbent polymer in the constituent material, and examples of the oxidizing agent include ozone and hydrogen peroxide. When the oxidizing agent is mixed with water and used as an aqueous solution, the concentration of the oxidizing agent in the aqueous solution may be, for example, 0.3 to 2 ppm by mass. In this embodiment, the constituent material containing the inactivated superabsorbent polymer is contacted (immersed) for a predetermined time with an aqueous solution of the oxidizing agent, i.e., ozone water containing a predetermined concentration of ozone, to remove impurities on the surface of the superabsorbent polymer.

The ozone concentration in the ozone water is not particularly limited as long as it is a concentration that can remove bacteria and other organic matter adhering to the surface of the superabsorbent polymer, but is preferably 0.3 to 2 mass ppm, more preferably 0.5 to 1.5 mass ppm. If the concentration is too low, it becomes difficult to remove bacteria, and if the concentration is too high, the superabsorbent polymer may begin to decompose. The contact time between the ozone water and the superabsorbent polymer is not particularly limited as long as it is a time that can remove bacteria and other organic matter adhering to the surface of the superabsorbent polymer, but it is shorter if the ozone concentration in the ozone water is high and longer if the ozone concentration is low. The contact time is preferably 0.3 seconds to 15 minutes, more preferably 3 seconds to 10 minutes. The product of the ozone concentration (mass ppm) and the contact time (minutes) in the ozone water (hereinafter also referred to as the "CT value") is preferably 0.01 to 20 ppm/minute, more preferably 0.08 to 10 ppm/minute. If the CT value is too small, sterilization becomes difficult, and if the CT value is too large, the superabsorbent polymer may decompose. However, treatment with ozone water can remove impurities adhering to the surface of the superabsorbent polymer, and can also remove bacteria and bleach the polymer. Examples of ozone generators that supply ozone into water include the ED-OWX-2 Ozone Water Exposure Tester manufactured by Ecodesign Co., Ltd. and the OS-25V Ozone Generator manufactured by Mitsubishi Electric Corporation.

In the oxidizing agent treatment step, the ozone water containing the superabsorbent polymer may be stirred in a tank to ensure that the reaction proceeds evenly. There are no particular limitations on the temperature of the ozone water in the oxidizing agent treatment step, and it may be room temperature (25°C), for example, and is preferably 10 to 40°C. If the ozone water is too hot, the ozone will easily escape as a gas and become inactivated, and if it is too cold, the ozone treatment time will tend to be longer. This makes it easier to remove bacteria and organic matter attached to the surface of the superabsorbent polymer with the ozone in the ozone water.

The oxidant treatment device that performs the oxidant treatment process is not particularly limited in its specific configuration, as long as it can bring the inactivated superabsorbent polymer into contact (or immerse) the ozone water without destroying its shape. An example of an oxidant treatment device is a twin-shaft screw pump (BQ type: manufactured by Futora Metal Industries Co., Ltd.). The twin-shaft screw pump is a positive displacement self-priming pump. The twin-shaft screw pump comprises a casing and twin screws that are arranged in a chamber within the casing and extend parallel to each other. The ozone water containing the superabsorbent polymer is supplied into the chamber, reaches the twin screws from the radial direction, and is then pushed out in the axial direction by the rotation of the twin screws and discharged.

Next, the solid-liquid separation process S32 will be described. The solid-liquid separation process S32 is performed by the solid-liquid separation device 32. In the solid-liquid separation process S32, solid-liquid separation is performed on an aqueous solution containing a constituent material containing an inactivated superabsorbent polymer to obtain a solid aggregate and a liquid. In this embodiment, the constituent material containing an inactivated superabsorbent polymer is a constituent material containing a superabsorbent polymer inactivated in the first dehydration process S311 and/or the second dehydration process S312. Examples of the aqueous solution include the washing water of the first dehydration process S311 (e.g., washing water from the second dehydration process onward, when the second dehydration process S312 is not performed), the washing water of the second dehydration process S312 (e.g., washing water from the second dehydration process onward), and the dilution water of the dilution process S313.

In this method, the preparation step S30 includes a solid-liquid separation step S32 in which an aqueous solution containing constituent materials including an inactivated superabsorbent polymer is subjected to solid-liquid separation to obtain a solid aggregate. Here, the aggregate (solid) is formed from the constituent materials and can retain moisture. Therefore, the amount of liquid (moisture) in the aggregate can be kept to an appropriate amount to create an appropriate moist state. This prevents the superabsorbent polymer from being substantially separated from the other constituent materials due to the amount of moisture in the aggregate being too much, which would dampen vibrations, or the amount being too little, which would cause the hydrogen bonds between the constituent materials to be too strong, making it difficult to separate the superabsorbent polymer.

The solid-liquid separation method in the solid-liquid separation step S32 is not particularly limited as long as it is a method that can obtain, as a solid, an aggregate of constituent materials containing an inactivated highly absorbent polymer, and as a liquid, an aqueous solution. Examples of devices that can realize such a solid-liquid separation method include a centrifugal separator, a vacuum dehydrator, a multiple disk dehydrator, a screw press, and a rotary drum screen. In other words, the solid-liquid separation method in the solid-liquid separation step S32 can be a solid-liquid separation method performed by these devices.

As a solid-liquid separation method in the solid-liquid separation step S32, a method using a centrifugal separator (centrifugal separation method) is preferable. In this embodiment, a method using a Guiner centrifuge is used (example: N-type Guiner continuous centrifuge, manufactured by IHI Corporation). Therefore, the solid-liquid separation step S32 includes a centrifugation step in which an aqueous solution containing the constituent materials is centrifuged as a solid-liquid separation to obtain an aggregate and liquid. In the Guiner centrifuge, the moisture content of the separated aggregate can be adjusted by the rotation speed.

This method includes a centrifugation step for centrifuging an aqueous solution containing the constituent materials as a solid-liquid separation step. In centrifugation, it is relatively easy to control the force applied to the constituent materials, so it is easy to prevent the superabsorbent polymer from being crushed, and it is also easy to adjust the moisture content (moisture content) of the aggregates to a desired range. This makes it possible to keep the aggregates in an appropriately moist state while minimizing damage to the superabsorbent polymer. Furthermore, in centrifugal separation, it is relatively easy to control the force applied to the constituent materials, so it is easy to adjust the density of the aggregates to a desired range.

The moisture content of the aggregate adjusted by centrifugation is 10% by mass or more and less than 85% by mass, preferably 15% by mass or more and 80% by mass or less, more preferably 20% by mass or more and 75% by mass or less. The density of the aggregate adjusted by centrifugation is 0.1 g/ ^cm3 or more and 1.5 g/cm3 ^or less, preferably 0.13 g/ ^cm3 or more and 1.2 g/ ^cm3 or less, and more preferably 0.15 g/ ^cm3 or more and 1.0 g/ ^cm3 or less.

The moisture content of the superabsorbent polymer separated in the solid-liquid separation step S32 (centrifugal separation step) is preferably 80% by mass or less, more preferably 75% by mass or less, and even more preferably 70% by mass or less. The lower the moisture content, the better, so there is no particular lower limit, but it is (in reality) 50% by mass or more, for example.

In this method, the moisture content of the superabsorbent polymer after the centrifugation process is 80% by mass or less, so the superabsorbent polymer is in the form of granules with a small particle size. Also, considering that the moisture content of the inactivated superabsorbent polymer hardly changes before and after the centrifugation process, since the moisture content of the superabsorbent polymer after the centrifugation process is 80% by mass or less, it can be said that the moisture content of the superabsorbent polymer before the centrifugation process is also roughly 80% by mass or less. In other words, because the centrifugation is performed in a state in which the superabsorbent polymer contains almost no moisture, the superabsorbent polymer can be made less likely to be damaged during solid-liquid separation (centrifugation). This makes it easier to recover the superabsorbent polymer in the separation process S40.

Next, the separation process S40 will be described. The separation process S40 includes a sieving process S41. The sieving process S41 is a process in which the agglomerates are passed through a vibrating sieve with meshes through which the superabsorbent polymer can pass. The sieving process S41 is performed by a sieving device 41 (sieving device). There are no particular limitations on the sieving device 41, so long as it is a sieve (including a combination of multiple sieves) that can vibrate the agglomerates and separate the superabsorbent polymer from the constituent materials.

The sieve separation device 41 (sieve device) vibrates the sieve to vibrate the aggregates on the sieve through the sieve. In this embodiment, a Sato-type vibrating sieve machine (cassette type: manufactured by KS Links Co., Ltd.) is used as the sieve separation device 41. The vibration direction can be a lateral vibration in a plane parallel to the sieve surface (width direction and/or depth direction of the sieve surface), a vertical vibration in a direction perpendicular to the sieve surface, and/or a combination thereof (three-dimensional vibration). The sieve separation device 41 is equipped with a vibrating body that generates vibrations, an upper weight with one end attached to the upper part of the vibrating body, and a lower weight with one end attached to the lower part of the vibrating body. The rotation of the upper weight has the role of moving the material supplied to the center of the sieve surface in the rotational direction, and the rotation of the lower weight has the role of generating a vertical vibration of the sieve surface to move it in the outer circumferential direction. The sieve is vibrated by adjusting the relative position (phase difference) between the other end of the upper weight and the other end of the lower weight.

The amplitude of the vibration is, for example, 1 to 20 mm for the lateral vibration in the in-plane direction parallel to the sieve surface (the width direction and/or depth direction of the sieve surface), preferably 1.5 to 15 mm, and more preferably 3 to 10 mm. The amplitude of the vertical vibration in the direction perpendicular to the sieve surface is, for example, 0.5 to 20 mm, preferably 1 to 15 mm, and more preferably 2 to 10 mm. The vibration frequency of the sieve is, for example, 5 Hz (300 v.p.m) to 500 Hz (60,000 v.p.m), preferably 10 Hz (600 v.p.m) to 250 Hz (15,000 v.p.m), and more preferably 15 Hz (900 v.p.m) to 100 Hz (6,000 v.p.m).

In this method, the superabsorbent polymer has a shape that is significantly different (smaller) from the other constituent materials, and is in a state in which it has almost no hydrogen bonds. Therefore, when the superabsorbent polymer is sieved, the vibration of the sieve makes it easier to separate the superabsorbent polymer from the other constituent materials. In other words, the superabsorbent polymer and the other constituent materials can be substantially separated. By creating such a state on the sieve, the separated superabsorbent polymer can be more easily sieved from the other constituent materials. This makes it possible to selectively extract the superabsorbent polymer from the constituent materials, and improve the recovery rate and purity of the superabsorbent polymer. In this case, for example, other constituent materials such as pulp fibers and sheet members (films, nonwoven fabrics, etc.) remain on the sieve and are collected, and the superabsorbent polymer falls below the sieve and is collected. The pulp fibers and sheet members (films, nonwoven fabrics, etc.) may be further separated from each other by another sieve.

In this embodiment, as a preferred aspect, in the sieve separation step S41 of the separation step S40, the vibration of the sieve includes not only vertical vibration, i.e., vibration in a direction perpendicular to the sieve surface, but also horizontal vibration, i.e., vibration in an in-plane direction parallel to the sieve surface. Therefore, the aggregates can be shaken not only in a direction perpendicular to the sieve surface, but also in a direction parallel to it. This makes it easier to separate the superabsorbent polymer from the other constituent materials, and makes it easier to bring the superabsorbent polymer and the other constituent materials into a substantially dispersed state. By creating such a state on the sieve, the separated superabsorbent polymer can be more easily sieved out from the other constituent materials.

There are no particular restrictions on the size of the sieve openings, so long as the superabsorbent polymer can be separated from the other constituent materials, but examples include 1 to 5 mm, with 1.3 to 4 mm being preferable, and 1.6 to 3 mm being more preferable. This allows the superabsorbent polymer to pass through the sieve more easily and the other constituent materials less likely to pass through. If the openings are too small, not only the other constituent materials but also the superabsorbent polymer will have difficulty passing through the sieve, and if they are too large, not only the superabsorbent polymer but also the other constituent materials will easily pass through.

In this embodiment, as a preferred aspect, the sieve comprises a first sieve portion having meshes through which the superabsorbent polymer can pass, and a second sieve portion located below the first sieve portion and having finer meshes than the first sieve portion through which the superabsorbent polymer can pass. In other words, the sieve has two stages. The sieve separation step S41 includes a step of passing the agglomerates through a vibrating sieve to separate the superabsorbent polymer in two stages, the first sieve portion and the second sieve portion. This can further increase the purity of the recovered superabsorbent polymer. The number of stages of the sieve may be even greater, up to three or more stages.

When the sieve has a first sieve portion and a second sieve portion, the size of the sieve openings of the first sieve portion (upper side) can be, for example, 2.5 to 5 mm, preferably 2.8 to 4.5 mm, and more preferably 3 to 4 mm. The size of the sieve openings of the second sieve portion (lower side) can be, for example, 1 to 3 mm, preferably 1.2 to 2.8 mm, and more preferably 1.4 to 2.6 mm. This allows the superabsorbent polymer to be obtained that ultimately passes through the first sieve portion and the second sieve portion (below the second sieve portion) and is extremely low in contamination with other constituent materials.

In this embodiment, as a preferred aspect, when the constituent material contains pulp fibers, the sieve separation step S41 includes a step in which the pulp fibers move to the outside of the surface of the sieve and the superabsorbent polymer moves to the underside of the sieve, and are separated from each other. Such separation can be achieved by the operation of the sieve (the way in which it vibrates, for example, a combination of horizontal and vertical vibrations (phase difference between the upper and lower weights)).

In this method, the superabsorbent polymer has a shape significantly different from that of the pulp fibers, and is in a state in which there are almost no hydrogen bonds. Therefore, when the superabsorbent polymer is sieved, the vibration of the sieve makes it easier to separate the superabsorbent polymer from the pulp fibers, and the separated superabsorbent polymer can be selectively removed from among the constituent materials. This makes it easier to sieve the superabsorbent polymer from the other constituent materials, and improves the recovery rate and purity of the superabsorbent polymer. In this case, for example, the pulp fibers and sheet members remain on the sieve and are collected, and the superabsorbent polymer falls below the sieve and is collected.

In this embodiment, as a preferred aspect, a sieving device 41 (sieving device) (e.g., a Sato-type vibrating sieve machine) having a sieve for the sieving separation process S41 is placed under a centrifuge device (e.g., a Gina continuous centrifuge device) that performs the centrifugation in the centrifugation process. Therefore, the aggregates after the centrifugation process can be immediately sieved, and the sieving separation process S41 can be performed while suppressing fluctuations in the state of the aggregates after centrifugation (the state of the superabsorbent polymer and the moisture content of the aggregates). This makes it easier to recover the superabsorbent polymer.

In this embodiment, as a preferred aspect, a step of recovering the superabsorbent polymer from the liquid (aqueous solution) obtained in the solid-liquid separation step S32 using a screen is further included. The recovered superabsorbent polymer may be supplied to the separation step S40 or returned to the inactivation step S31. This can improve the recovery rate of the superabsorbent polymer.

In the method for recovering superabsorbent polymer according to this embodiment (method for producing recycled superabsorbent polymer), there is no particular limitation on the first dehydration step S311, i.e., the method for extracting constituent materials from used sanitary products and inactivating the superabsorbent polymer in the constituent materials with an acidic aqueous solution, and any method can be used. Below, such a method, i.e., a modified example of the first dehydration step S311 (first dehydration device 311) of the inactivation step S31 (inactivation device 31), will be described with reference to Figures 4 and 5.

FIG. 4 is a flow chart showing a modified example of the first dehydration step S311 according to the embodiment. This modified example includes a step of separating a constituent material A containing a superabsorbent polymer, such as a film or nonwoven fabric, and pulp fibers from a used sanitary product. FIG. 5 is a block diagram showing a modified example of the first dehydration device used in the first dehydration step S311 according to the embodiment. This modified example includes a device for separating a constituent material A containing a superabsorbent polymer, such as a film or nonwoven fabric, and pulp fibers from a used sanitary product. On the other hand, as shown in FIG. 4, the modified example of the first dehydration step S311 includes an opening forming step P11 to a second separation step P17 (preferably a third separation step P18 to a fourth separation step P20), and correspondingly, as shown in FIG. 5, the modified example of the first dehydration device 311 includes a bag breaking device 11 to a second separation device 17 (preferably a third separation device 18 to a fourth separation device 20). Each step will be described in detail below.

However, in this embodiment, used sanitary products are collected and obtained from outside for reuse (recycling). At that time, multiple used sanitary products, particularly used absorbent articles and wet wipes used in connection therewith, are bundled together and sealed in a collection bag to prevent excrement, bacteria, and odors from leaking to the outside. The following mainly describes used absorbent articles.

The opening forming process P11 is carried out by the bag breaking device 11. The bag breaking device 11 is equipped with a solution tank that stores an inactivating aqueous solution containing an inactivating agent, and a bag breaking blade that rotates in the solution tank. The bag breaking device 11 forms an opening in the collection bag placed in the solution tank using the bag breaking blade in the inactivating aqueous solution (or cuts the collection bag). This produces a mixture 91 of the collection bag into which the inactivating aqueous solution has penetrated through the opening, and the inactivating aqueous solution. The inactivating aqueous solution inactivates the superabsorbent polymer of the used absorbent article in the collection bag. Below, an example is described in which an acidic aqueous solution (acid-containing aqueous solution) is used as the inactivating aqueous solution.

The crushing process P12 is performed by the crushing device 12. The crushing device 12 is equipped with a biaxial crusher (e.g., a biaxial rotary crusher, etc.). The crushing device 12 crushes the collection bags containing the used absorbent articles in the mixed liquid 91 together with the collection bags. As a result, a mixed liquid 92 containing crushed pieces of the collection bags containing the used absorbent articles and an acidic aqueous solution is generated. At the same time, almost all of the superabsorbent polymer in the used absorbent articles is inactivated by the acidic aqueous solution. However, the crushed pieces are small pieces of the constituent materials containing the superabsorbent polymer, i.e., the superabsorbent polymer, pulp fibers, films/nonwoven fabrics, etc., and some or all of the collection bags. The size (maximum dimension) of the small pieces can be, for example, 2 to 30 mm, preferably 2 to 20 mm, and more preferably 2 to 10 mm or less. The opening formation process P11 and the crushing process S12 may be performed almost simultaneously. For example, the superabsorbent polymer may be inactivated while the collection bag is crushed in an acidic aqueous solution.

The first separation process P13 is carried out by the first separation device 13. The first separation device 13 is equipped with a pulper separator having an agitation separation tank that functions as a washing tank and a sieve tank. The first separation device 13 agitates the mixed liquid 92, removing waste and the like from the crushed material, while separating the pulp fibers and superabsorbent polymer, which are among the constituent materials, as well as the waste and acidic aqueous solution, from the mixed liquid 92. This produces a mixed liquid 93 containing the pulp fibers and superabsorbent polymer, which are among the constituent materials, as well as the waste and acidic aqueous solution. Separately from this, the film, nonwoven fabric, and other constituent materials, as well as the material of the collection bags, are recovered.

In addition, the hole forming process P11 and the crushing process P12 (bag breaking device 11 and crushing device 12) process the used absorbent article in an acidic aqueous solution to inactivate the superabsorbent polymer. Therefore, if the superabsorbent polymer is sufficiently inactivated and dehydrated at this stage, the hole forming process P11 and the crushing process P12 can be considered as the first dehydration process S311. In that case, the mixed liquid 92 may be subjected to solid-liquid separation, and a constituent material containing the superabsorbent polymer may be extracted from the mixed liquid and supplied to the second dehydration process S312, the dilution process S313, or the solid-liquid separation process S32.

In addition, the crushing process P12 (crushing device 12) may crush the used absorbent articles together with the collection bag in gas (e.g., in air) rather than crushing the used absorbent articles in the inactivating aqueous solution. In this case, the opening forming process P11 (bag breaking device 11) is not necessary. After crushing, the crushed material from the crushing process P12 (crushing device 12) and the inactivating aqueous solution are supplied to the first separation device 13 (first separation process P13), where the superabsorbent polymer is inactivated.

The steps up to the first separation step P13 are carried out in an acidic aqueous solution, during which the superabsorbent polymer is inactivated. If the superabsorbent polymer is sufficiently inactivated and dehydrated at this stage, the first separation device 13 (first separation step P13) or the steps up to that point can be regarded as the first dehydration step S311. In that case, the mixed liquid 93 may be subjected to solid-liquid separation, and a constituent material containing the superabsorbent polymer may be extracted from the mixed liquid and supplied to the second dehydration step S312, the dilution step S313, or the solid-liquid separation step S32.

The first dust removal process P14 is performed by the first dust removal device 14. The first dust removal device 14 is equipped with a screen separator, and the mixed liquid 93 is separated by a screen into pulp fibers, superabsorbent polymer, excrement, and other materials (foreign matter) in the acidic aqueous solution. As a result, a mixed liquid 94 containing pulp fibers, superabsorbent polymer, excrement, and acidic aqueous solution with a reduced amount of foreign matter is generated, and other materials are removed. Next, the second dust removal process P15 is performed by the second dust removal device 15. The second dust removal device 15 is equipped with a screen separator, and the mixed liquid 94 is separated by a screen finer than that of the first dust removal device 14 into pulp fibers, superabsorbent polymer, excrement, and other materials (small foreign matter) in the acidic aqueous solution. As a result, a mixed liquid 95 containing pulp fibers, superabsorbent polymer, excrement, and acidic aqueous solution with a further reduced amount of foreign matter is generated, and other materials are further removed. Next, the third dust removal process P16 is performed by the third dust removal device 16. The third dust removal device 16 is equipped with a cyclone separator, and separates the mixed liquid 95 into pulp fibers, superabsorbent polymer, excrement, and other materials (heavy foreign matter) in the acidic aqueous solution by centrifugation. As a result, a mixed liquid 96 containing pulp fibers, superabsorbent polymer, excrement, and acidic aqueous solution with a reduced amount of foreign matter is generated, and other materials with a high specific gravity are removed. Note that at least one of the first dust removal process P14 to the third dust removal process P16 (first dust removal device 14 to third dust removal device 16) may be omitted depending on the state of the mixed liquid 93, etc. (e.g., the amount and size of foreign matter).

In the first dust removal process P14 to the third dust removal process P16 (first dust removal device 14 to third dust removal device 16), the superabsorbent polymer in each mixed liquid is inactivated (or maintained in an inactivated state) because each mixed liquid contains an acidic aqueous solution. Therefore, if the superabsorbent polymer is sufficiently inactivated and dehydrated in any of these processes, that process (or the processes up to that point) can be considered as the first dehydration process S311. In that case, the mixed liquid in that process may be subjected to solid-liquid separation, and a constituent material containing the superabsorbent polymer may be extracted from the mixed liquid and supplied to the second dehydration process S312, the dilution process S313, or the solid-liquid separation process S32.

The second separation step P17 is performed by the second separation device 17. The second separation device 17 is equipped with a drum screen separator, and the mixed liquid 96 is separated into the superabsorbent polymer in the acidic aqueous solution and the pulp fibers by the drum screen. As a result, a mixed liquid 97 containing the superabsorbent polymer, excrement, and the acidic aqueous solution is generated, and the pulp fibers are removed as a mixture 98. The superabsorbent polymer in the mixed liquid 97 is inactivated by the acidic aqueous solution and is in a dehydrated state. Therefore, when the mixed liquid 97 is subjected to solid-liquid separation and the solid 99 is separated from the mixed liquid 97, the solid 99 is the constituent material A containing the superabsorbent polymer. However, the obtained constituent material A containing the superabsorbent polymer contains pulp fibers (pulp components) and/or films, nonwoven fabrics, etc. (plastic components) that cannot be completely separated. The constituent material A is supplied to the second dehydration step S312. The constituent material A may be supplied to the dilution step S313 or the solid-liquid separation step S32.

The third separation step P18 is carried out by the third separator 18. The third separator 18 is equipped with an inclined screen, and while spraying the cleaning liquid onto the mixture 98, the screen separates the mixture 98 into a solid containing pulp fibers and a liquid containing the cleaning liquid. This produces a mixture 101 containing pulp fibers with reduced dirt and the like, and also removes dirt and the like.

The oxidizing agent treatment process P19 is performed by the oxidizing agent treatment device 19. The oxidizing agent treatment device 19 includes a treatment tank that stores an oxidizing agent aqueous solution, and an oxidizing agent supply device that supplies an oxidizing agent into the treatment tank. The oxidizing agent treatment device 19 introduces the mixture 101 into the treatment tank from the top or bottom of the treatment tank and mixes it with the oxidizing agent aqueous solution in the treatment tank. The oxidizing agent supplied from the bottom of the treatment tank by the oxidizing agent supply device decomposes the superabsorbent polymer contained in the pulp fiber in the oxidizing agent aqueous solution, and solubilizes it in the oxidizing agent aqueous solution. This produces a mixed liquid 102 containing pulp fiber from which the superabsorbent polymer has been removed and an oxidizing agent aqueous solution containing a decomposition product of the superabsorbent polymer. The oxidizing agent is an oxidizing agent that can decompose the superabsorbent polymer, such as ozone, which is preferable because of its high sterilizing and bleaching power.

The ozone concentration in the aqueous oxidizing agent solution is preferably 1 to 50 ppm by mass. If the concentration is too low, the superabsorbent polymer cannot be completely solubilized, and there is a risk that the superabsorbent polymer will remain in the pulp fibers, and if the concentration is too high, there is a risk that the pulp fibers will be damaged. The treatment time with ozone is shorter if the ozone concentration in the aqueous oxidizing agent solution is high, and longer if the ozone concentration is low, typically 5 to 120 minutes. The product of the ozone concentration (ppm) in the aqueous oxidizing agent solution and the treatment time (minutes) (hereinafter also referred to as the "CT value") is preferably 100 to 6000 ppm·min. If the CT value is too small, the remaining superabsorbent polymer in the pulp fibers cannot be completely solubilized, and if the CT value is too large, there is a risk that the pulp fibers will be damaged.

The fourth separation process P20 is carried out by the fourth separation device 20. The fourth separation device 20 is equipped with a screen separator, and separates the mixed liquid 102 into pulp fibers and an aqueous oxidizing agent solution using a screen. This produces recycled pulp fibers and removes the aqueous oxidizing agent solution containing decomposition products of the superabsorbent polymer.

In the modified examples shown in Figures 4 and 5, an aqueous solution containing a polyvalent metal ion source may be used as the inactivating aqueous solution instead of an acidic aqueous solution. In that case, the modified example in Figure 4 is a flow chart showing a modified example of the second dehydration step S312 according to the embodiment, and Figure 5 is a block diagram showing a modified example of the second dehydration device used in the second dehydration step S312 according to the embodiment. In this case, the first dehydration step S311 is omitted. The obtained constituent material A containing the inactivated superabsorbent polymer is supplied to the dilution step S313 or the solid-liquid separation step S32.

In this embodiment, the superabsorbent polymer is separated and recovered by the separation step S40, and at the same time, plastic components (e.g., film, nonwoven fabric, etc.) and/or pulp components can also be recovered. For example, the aggregate obtained by the solid-liquid separation step S32 contains, in addition to the superabsorbent polymer, other substances such as film, nonwoven fabric, etc. (plastic components) and/or pulp fibers (pulp components). Therefore, at the same time as or after the superabsorbent polymer is separated from other substances by sieving in the sieving separation step S41, the film, nonwoven fabric, etc. (plastic components) and/or pulp fibers (pulp components) may be separated from the other separated substances by sieving or other methods. Therefore, this embodiment includes a method for recovering plastic components and also includes a method for recovering pulp components.

The present invention will be explained below based on examples, but the present invention is not limited to these examples.

(1) Sample First, the first dehydration step S311 in FIG. 4 was performed on the used absorbent article to generate the constituent material A. Next, the second dehydration step S312 and the dilution step S313 in FIG. 2 were performed on the constituent material A (a part of which was excluded for measuring the recovery rate (described later)) to generate an inactivated product. Next, the inactivated product was subjected to the solid-liquid separation step S32 in FIG. 1 to generate an aggregate. Next, the aggregate was subjected to the sieve separation step S41 to recover the superabsorbent polymer. However, in the first dehydration step S311, a 0.1% by mass sulfuric acid aqueous solution was used as the inactivating aqueous solution. In the second dehydration step S312, a 7% by mass calcium hydroxide aqueous solution was used as the inactivating aqueous solution. In the dilution step S313, only water was used. In the solid-liquid separation step S32, a Gina centrifuge was used. In the sieve separation step S41, a Sato-type vibrating sieve machine (sieve: two stages) was used. Here, in Examples 1 to 4, the moisture content of the aggregates was set to 10% by mass or more and less than 85% by mass by adjusting the rotation speed of the Guiner centrifuge, and in Comparative Examples 1 and 2, the moisture content of the aggregates was set to less than 10% by mass and 85% by mass or more by adjusting the rotation speed of the Guiner centrifuge.

(2) Results In Examples 1 to 4, the moisture content of the aggregate was 10 mass%, 55 mass%, 65 mass%, and 75 mass%, but good separation was possible in the sieve separation step S41, and the recovery rates of the superabsorbent polymer were 45 mass%, 60 mass%, 40 mass%, and 22 mass%. That is, by setting the moisture content of the aggregate to 10 mass% or more and less than 85 mass%, a high recovery rate of the superabsorbent polymer was obtained. On the other hand, in Comparative Examples 1 and 2, the moisture content of the aggregate was 0 mass% and 85 mass%, but good separation was not possible in the sieve separation step S41, and only 8 mass% and 5 mass% of the superabsorbent polymer were obtained. That is, by setting the moisture content of the aggregate to less than 10 mass% or 85 mass% or more, the recovery rate of the superabsorbent polymer was low.

In this case, the purity of the superabsorbent polymer was high in Examples 1 and 2, and medium in Examples 3 and 4. In other words, superabsorbent polymers of relatively high purity were obtained in Examples 1 to 4. On the other hand, the purity of the superabsorbent polymer was low in Comparative Examples 1 and 2. In other words, only superabsorbent polymers of low purity were obtained in Comparative Examples 1 and 2.

However, the recovery rate of the superabsorbent polymer was measured as follows.
<Recovery rate of superabsorbent polymer>
(1) A predetermined amount (e.g., 100 g) of the constituent material A after the first dehydration step S311 is weighed out as an original material, and two original materials are prepared.
(2) One of the raw materials is dried as is (120°C x 60 minutes) to prepare sample 1.
(3) On the other hand, another raw material is treated with an ozone-containing aqueous solution (ozone concentration in the aqueous solution: 30 ppm x treatment time: 60 minutes (CT value: 1800)) to decompose and remove the contained superabsorbent polymer, and then dried (120°C x 60 minutes) to obtain sample 2.
(4) The mass difference between Sample 1 and Sample 2 (mass of Sample 1 - mass of Sample 2) is the content of the superabsorbent polymer in the raw material.
(5) Calculate the content (mass%) of the superabsorbent polymer in the raw material using the following formula.
Content of superabsorbent polymer (mass%)
= (content of superabsorbent polymer) / (mass of sample 1) × 100
(6) Constituent material A (excluding two pieces of the original raw material) per batch (each of the Examples and Comparative Examples) is dried (120° C.×60 minutes) and its mass is measured.
(7) Multiply the mass of constituent material A by the content (mass %) of the superabsorbent polymer to calculate the mass of the superabsorbent polymer in constituent material A, which is the original mass (dry mass) of the superabsorbent polymer.
(8) The superabsorbent polymer recovered in each batch (each Example and each Comparative Example) is dried (120°C x 60 minutes), and its mass is measured to obtain the recovered amount (dry mass) of the superabsorbent polymer.
(9) The recovery rate (mass%) of the superabsorbent polymer of component material A is calculated using the following formula.
Recovery rate of superabsorbent polymer (mass%)
= Amount of superabsorbent polymer recovered (dry mass) / Original mass of superabsorbent polymer (dry mass) × 100

However, the purity of the superabsorbent polymer is measured as follows.
<Purity of superabsorbent polymer>
(1) A predetermined amount (e.g., 100 g) is weighed out from the superabsorbent polymer ((8) in the method for measuring the recovery rate of superabsorbent polymer) recovered in each batch (each example and each comparative example) and dried (120°C x 60 minutes) to obtain a purity measurement sample 1.
(2) Purity measurement sample 1 is treated with an ozone-containing aqueous solution (ozone concentration in the aqueous solution: 30 ppm x treatment time: 60 minutes (CT value: 1800)) to decompose and remove the contained superabsorbent polymer, and then dried (120°C x 60 minutes) to obtain purity measurement sample 2.
(3) The mass difference between purity measurement sample 1 and purity measurement sample 2 (mass of purity measurement sample 1 - mass of purity measurement sample 2) is the content of superabsorbent polymer in purity measurement sample 1.
(4) Purity Measurement The content (mass%) of the superabsorbent polymer in sample 1 is calculated using the following formula.
Content of superabsorbent polymer (mass%)
= (content of superabsorbent polymer) / (mass of purity measurement sample 1) × 100
This value is taken as the purity of the superabsorbent polymer.

The absorbent article of the present invention is not limited to the above-mentioned embodiments, and can be appropriately combined or modified without departing from the purpose and spirit of the present invention.

S30 Preparation step S40 Separation step

Claims

A method for recovering a superabsorbent polymer from a component material that contains the superabsorbent polymer and that constitutes a used sanitary product, comprising the steps of:
preparing a wet aggregate containing the component material including the inactivated superabsorbent polymer;
a separation step of vibrating the agglomerate to separate the superabsorbent polymer from the constituent materials;
Equipped with
Method.
The separation step includes a sieve separation step of passing the agglomerate through a vibrating sieve having openings through which the superabsorbent polymer can pass.
The method of claim 1.
The preparation step includes a solid-liquid separation step of performing solid-liquid separation on an aqueous solution containing the constituent materials including the inactivated superabsorbent polymer to obtain the aggregate, which is a solid, and a liquid.
The method of claim 2.
The solid-liquid separation step includes a centrifugation step of centrifuging an aqueous solution containing the constituent materials to obtain the aggregate and the liquid,
The method according to claim 3.
The moisture content of the superabsorbent polymer separated in the centrifugation step is 80% by mass or less.
The method according to claim 4.
A sieve device having the sieve of the sieve separation step is disposed below a centrifuge device that performs the centrifugation of the centrifugation step.
The method of claim 4.
The method further includes a step of recovering the superabsorbent polymer from the liquid obtained in the solid-liquid separation step using a screen.
The method according to claim 3.
The preparation step includes an inactivation step of inactivating the superabsorbent polymer,
The inactivation step includes a dehydration step of contacting the constituent material with a polyvalent metal ion source capable of supplying polyvalent metal ions, thereby dehydrating the superabsorbent polymer in the constituent material.
The method according to claim 1 or 2.
The inactivation step further includes a pre-dehydration step of pre-dehydrating the superabsorbent polymer in the constituent material by immersing the constituent material in an acidic aqueous solution before the dehydration step.
The method according to claim 8.
In the sieve separation step, the vibration of the sieve includes lateral vibration.
The method of claim 2.
The sieve is
A first sieve portion having an opening through which the highly absorbent polymer can pass;
a second sieve portion located below the first sieve portion and having a mesh size smaller than that of the first sieve portion and allowing the superabsorbent polymer to pass through;
Equipped with
The sieve separation step includes:
The agglomerate is passed through the vibrating sieve to sieve the superabsorbent polymer in two stages, the first sieve portion and the second sieve portion.
The method according to claim 2 or 10.
The component material includes pulp fibers,
The sieve separation step includes a step in which the pulp fibers move to the outside on the surface of the sieve and the superabsorbent polymer moves to the underside of the sieve and are separated from each other.
The method according to claim 2 or 10.
The method further includes a reactivation step of contacting the superabsorbent polymer obtained in the separation step with an alkali metal ion source capable of supplying alkali metal ions to reactivate the superabsorbent polymer.
The method according to claim 1 or 2.
1. A method for producing a recycled superabsorbent polymer, comprising:
The method includes a reactivation step of contacting the superabsorbent polymer obtained by the method for recovering a superabsorbent polymer from a constituent material containing the superabsorbent polymer constituting a used sanitary product according to claim 1 or 2 with an alkali metal ion source capable of supplying alkali metal ions to reactivate the superabsorbent polymer.
Method.