JP7809595B2 - Method for separating water-absorbent resin - Google Patents

Description

The present invention relates to recycling technology for absorbent articles.

From the perspective of efficient resource utilization and reducing greenhouse gas emissions, there is a desire to recycle used absorbent articles such as disposable diapers and sanitary napkins. Patent Document 1 discloses a method for producing recycled superabsorbent polymers regenerated from used superabsorbent polymers, in which the used superabsorbent polymers are dehydrated using an acid or base, superabsorbent polymer particles are attached to the dehydrated polymer, and the polymer is then dried.

Japanese Patent Application Laid-Open No. 2021-166968

Absorbent articles contain not only water-absorbent polymers but also thermoplastic resins, cellulose, etc., and have an integrated structure. However, Patent Document 1 does not disclose a means for extracting superabsorbent polymers from used absorbent articles. Furthermore, the method described in Patent Document 1 may require large amounts of processing agents and large equipment, and more efficient recycling technology for absorbent articles is desired.

The present invention relates to a technology for efficiently separating water-absorbent resin from waste materials containing absorbent articles.

A method for separating a water-absorbent resin according to one embodiment of the present invention is a method for separating the water-absorbent resin from a processing target including an absorbent article that has absorbed water and contains a water-absorbent resin and a thermoplastic resin.
The method for separating the water-absorbent resin comprises:
a drying step of stirring and drying the object to be treated;
a heating step of heating the object to be treated at a temperature equal to or higher than the softening point of the thermoplastic resin and lower than the thermal decomposition temperature of the water-absorbing resin;
a granulation step of stirring the material to be treated after the heating step and cooling it to a temperature lower than the softening point of the thermoplastic resin to obtain granules;
and a separation step of separating the water-absorbent resin based on the particle size of the granular material.

The present invention makes it possible to efficiently separate water-absorbent resin from waste containing absorbent articles.

1 is a schematic diagram of a granulation device that can be used in one embodiment of the present invention. FIG. 2 is a schematic diagram of the granulation device. 4 is a flowchart showing a method for separating a water-absorbent resin according to the embodiment. 10 is a flowchart showing a method for producing water-absorbent resin-containing granules according to a modified example of the embodiment. 10 is a flowchart showing a method for producing a granular material with a low water-absorbent resin content (granular material) according to a modified example of the embodiment.

[Outline of an embodiment of the present invention]
A method for separating a water-absorbent resin according to one embodiment of the present invention is configured to granulate a material to be treated, including absorbent articles that have absorbed water, to form granules of different particle sizes, and separate the water-absorbent resin from other materials, such as thermoplastic resin, based on particle size. This allows the water-absorbent resin to be efficiently separated from absorbent articles that are formed by integrating multiple components, thereby enabling the water-absorbent resin to be recycled. Furthermore, the present embodiment also allows the recovery of granules with a low content of water-absorbent resin, which can be effectively used as recycled material. Therefore, the separation method according to the present embodiment contributes to promoting the recycling of waste, including used absorbent articles, and ultimately contributes to the realization of a resource-circulating society.

In this embodiment, the absorbent article refers to an article that contains a water-absorbent resin and a thermoplastic resin and is capable of absorbing liquid. Absorbent articles according to this embodiment include, for example, one or more articles selected from disposable diapers, sanitary napkins, urine-absorbing pads, panty liners, pet toilet sheets, portable toilets, drip sheets, ice packs, and deodorizers. Note that a "drip sheet" refers to a water-absorbent sheet that absorbs excess water (such as body fluids) from food ingredients such as meat and fish. In this embodiment, an absorbent article that has absorbed water refers to an absorbent article that has absorbed water primarily by a water-absorbent resin. The water referred to here may be body fluids such as urine or menstrual blood. In other words, an absorbent article that has absorbed water may be a used absorbent article.

Absorbent articles used to absorb bodily fluids, such as disposable diapers and sanitary napkins, generally comprise an absorbent body containing a water-absorbent resin and sheet materials covering the front and back of the absorbent body, with these components joined together with adhesives or the like. The sheet materials may contain, for example, thermoplastic resin and/or cellulose. The absorbent body includes at least an absorbent core containing a water-absorbent resin and may further include a core wrap sheet covering the absorbent core. The core wrap sheet is made of a hydrophilic material, such as cellulose. The absorbent core may also contain a hydrophilic material, such as cellulose pulp, in addition to the water-absorbent resin.

In this embodiment, the water-absorbent resin is a resin having water-absorbing properties, and includes, for example, one or more water-absorbent resins selected from polyacrylic acid, polyacrylic acid salts, partially crosslinked polymeric compounds having carboxyl groups or salts thereof, partially crosslinked polysaccharides, etc. Partially crosslinked polymeric compounds having carboxyl groups or salts thereof include crosslinked polyacrylates, poly(vinyl alcohol/acrylate) copolymers (crosslinked), starch-acrylate graft copolymers (crosslinked), and polyvinyl alcohol-polymaleic anhydride graft copolymers (crosslinked). Partially crosslinked polysaccharides include crosslinked carboxymethyl cellulose salts, etc.
Furthermore, the "salt" constituting the water absorbent resin includes, for example, one or more salts selected from alkali metal salts (sodium salts, potassium salts, lithium salts, etc.), alkaline earth metal salts (calcium salts, magnesium salts, barium salts, etc.), ammonium salts (quaternary ammonium salts, quaternary alkylammonium salts, etc.), etc.
The water-absorbing resin of the present invention preferably contains one or more water-absorbing resins selected from polyacrylic acid, sodium polyacrylate, or a crosslinked sodium polyacrylate.

In this embodiment, the thermoplastic resin is not limited to a specific type, and examples include polyolefin, polyester, polystyrene, etc. Examples of polyolefin include polyethylene and polypropylene. Examples of polyester include polyethylene terephthalate, etc. These thermoplastic resins are used, for example, as sheet materials or exterior materials in absorbent articles. Thermoplastic resins may also be contained in objects to be treated other than absorbent articles.

The object to be treated in this embodiment includes at least absorbent articles that have absorbed water, and may also include other waste, secondary materials, etc. The other waste is not particularly limited and may include, for example, used packaging containers (food containers, bottles, etc.), marine debris, and other waste generated by households, businesses, etc.

In this embodiment, the material to be treated preferably contains cellulose. When the material to be treated contains cellulose, it becomes easier to carbonize a portion of the material to be treated in heating step S2, as described below. The cellulose may be contained in an absorbent article, or may be contained in a material to be treated other than an absorbent article.

In this embodiment, "granular" refers to the shape of each fragment of the crushed material. Furthermore, "granular body" refers to an aggregate of multiple fragments that are separated from each other. Furthermore, "granulation" refers to converting the material into granular bodies. In this embodiment, the maximum diameter of the granular bodies is preferably 5 cm or less, more preferably 1 cm or less, and even more preferably 0.5 cm or less, from the standpoint of ease of handling.

In this embodiment, the particle size of a granular material refers to the degree of size of the granular material. In other words, the particle size of a granular material serves as a standard for separating granular materials by size. The size of a granular material here may be defined as a range of particle sizes equivalent to the diameter of each granular material if it were assumed to be a sphere, but it does not have to be strictly defined by particle size as long as the granular material can be classified by size. For example, the particle size of a granular material may be determined by the opening of a sieve or the classification point set in a classifier.

[Configuration of Granulation Device]
First, a granulation apparatus 100 will be described as an example of a processing apparatus that can be used in the separation method according to this embodiment. FIG. 1 shows the granulation apparatus 100 from the front, and FIG. 2 shows the granulation apparatus 100 from the side. The granulation apparatus 100 includes a processing tank 101. The processing tank 101 includes a wall 101a and a storage section 101b. The storage section 101b can store materials to be granulated by the granulation apparatus 100, and is configured as an internal space of the processing tank 101 surrounded by the wall 101a. FIGS. 1 and 2 each show a vertical cross section of the wall 101a, illustrating the interior of the storage section 101b. In the processing tank 101, a portion of the wall 101a is configured to be openable and closable, allowing external access to the storage section 101b for loading and unloading materials.

The granulation device 100 also includes an agitator shaft 102. The agitator shaft 102 has a shaft portion 102a and multiple blade portions 102b. The shaft portion 102a is configured as a round bar-shaped member that can rotate around a rotation axis C that extends horizontally to the side. Both ends of the shaft portion 102a are supported by the wall portion 101a on the sides of the storage portion 101b, and the portion between the both ends supported by the wall portion 101a is located within the storage portion 101b. The multiple blade portions 102b are spaced apart along the longitudinal direction, with no substantial bias over the entire portion of the shaft portion 102a located within the storage portion 101b. Furthermore, each blade portion 102b protrudes in various radial directions from the outer circumferential surface of the shaft portion 102a.

The granulation device 100 also has a heater 103. The heater 103 is attached to the outside of the wall 101a of the treatment tank 101. The heater 103 is configured so that it can raise the temperature inside the storage section 101b through the wall 101a when current is applied. In other words, in the granulation device 100, the heater 103 can heat the workpiece stored in the storage section 101b. Note that the configuration of the heater 103, such as the heating method and installation position, can be determined in various ways depending on the configuration of the treatment tank 101 and the properties of the workpiece.

In addition, the granulation device 100 further includes a blower mechanism 104. The blower mechanism 104 includes an air supply section 104a and an exhaust section 104b. The air supply section 104a and the exhaust section 104b are each provided on the wall section 101a of the treatment tank 101 and face each other laterally across the storage section 101b. The blower mechanism 104 is configured to supply air into the storage section 101b through the air supply section 104a and exhaust air from the treatment tank 101 through the exhaust section 104b, thereby blowing air into the storage section 101b. Therefore, in the granulation device 100, the blower mechanism 104 can promote a decrease in temperature when cooling the workpiece. Note that the gas used to blow air by the blower mechanism 104 in the granulation device 100 is not limited to air and may be, for example, an inert gas such as nitrogen.

With the above-described configuration, the granulation device 100 rotates the shaft portion 102a around the rotation axis C, allowing the interaction of gravity and the force applied by the multiple blade portions 102b within the storage portion 101b to effectively agitate the material to be processed. Therefore, the granulation device 100 can combine temperature control using the heater 103 and/or the air blowing mechanism 104 with agitation, and can perform the drying step S1, heating step S2, and granulation step S3 described below.

The configuration of the granulation device 100 is not limited to the above and can be modified in various ways. For example, in the granulation device 100, the rotation axis C of the shaft portion 102a does not necessarily have to extend horizontally; it may be tilted relative to the horizontal plane. However, to effectively utilize the stirring effect of gravity in the granulation device 100, it is necessary that at least the rotation axis C of the shaft portion 102a be tilted relative to the vertical direction, and it is preferable that the angle of the rotation axis C of the shaft portion 102a relative to the horizontal plane be small. Specifically, in the granulation device 100, it is preferable that the angle of the rotation axis C of the shaft portion 102a relative to the horizontal plane be 30° or less.

[Method for separating water-absorbent resin]
3, the method for separating a water-absorbent resin according to this embodiment includes a drying step S1, a heating step S2, a granulation step S3, and a separation step S4. In this embodiment, an example will be described in which the granulation device 100 is used in the drying step S1, the heating step S2, and the granulation step S3. In this example, first, the object to be treated, including absorbent articles, is placed in the storage section 101b of the treatment tank 101, and the drying step S1 is performed.

(Drying step S1)
In the drying step S1, the material is agitated and dried. This evaporates the moisture contained in the material, reducing the material's weight, and the agitation crushes the material. In this embodiment, the heater 103 of the granulation device 100 adjusts the temperature inside the storage section 101b, while the agitation shaft 102 rotates to agitate the material stored in the storage section 101b of the treatment tank 101.

The drying temperature in this step is not particularly limited, but can be 40°C or higher and lower than the softening point of the thermoplastic resin described below. The drying temperature may be maintained approximately constant, but from the perspective of smoothly transitioning to heating step S2, it is preferable to set it so that it increases continuously or in stages. When increasing the temperature, the rate of temperature increase is preferably 0.001°C/min or higher and 20°C/min or lower.

If drying were performed without stirring, when the moisture held in the water-absorbent resin seeps out, the water-absorbent resin would easily bond with other highly hydrophilic materials such as pulp or core wrap sheets via the moisture. If drying were performed in this state, the water-absorbent resin and other materials would adhere to each other and form clumps, making it difficult to separate the water-absorbent resin. In contrast, in this embodiment, drying while stirring can promote separation of the water-absorbent resin from other materials while reducing the amount of moisture in the water-absorbent resin.

Furthermore, as mentioned above, in absorbent articles, the water-absorbent resin is often integrated with pulp or sheet material. For this reason, in this step, it is preferable to crush the absorbent article to expose the water-absorbent resin. This makes it possible to more efficiently reduce the moisture content of the water-absorbent resin and further promote separation of the water-absorbent resin from other materials.

(Heating step S2)
Next, in heating step S2, the workpiece is heated to a temperature equal to or higher than the softening point of the thermoplastic resin and lower than the thermal decomposition temperature of the water-absorbent resin. This softens the thermoplastic resin and makes it more deformable, while the water-absorbent resin maintains its molecular structure without thermal decomposition. This allows the physical properties of the thermoplastic resin to be significantly altered relative to the water-absorbent resin. In this embodiment, the temperature of the workpiece is adjusted by, for example, a heater 103. Also in this step, it is preferable to stir the workpiece by, for example, rotating the stirring shaft 102. This allows the workpiece to be heated uniformly and makes it easier to crush.

In this embodiment, "softening point" refers to the temperature at which the thermoplastic resin contained in the workpiece begins to rapidly soften. When the workpiece contains multiple thermoplastic resins, the softening point of any of the thermoplastic resins can be selected as the "softening point of the thermoplastic resin." However, the softening point of the thermoplastic resin with the highest softening point can be selected. If the thermoplastic resin contained in the workpiece can be identified, the softening point of the thermoplastic resin can be determined by measuring its Vicat softening temperature in advance. The Vicat softening temperature can be measured in accordance with JIS K 7206:2016 (ISO 306:2013). Furthermore, if the thermoplastic resin contained in the workpiece cannot be identified, the softening point of the thermoplastic resin contained in the workpiece can be determined by removing the resin from the workpiece and measuring its Vicat softening temperature. For example, the softening point of polypropylene is 160-200°C, and the softening point of polyethylene terephthalate is 200-250°C.

In this embodiment, the "thermal decomposition temperature" of a water-absorbent resin is the temperature at which heat causes molecular chains to break down and become smaller molecules. The thermal decomposition temperature is the temperature at which the weight of a water-absorbent resin sample decreases by 5% when heated in an air atmosphere at a predetermined heating rate (5-10°C/min) using a thermogravimetric and differential thermal analyzer (TG-DTA). For example, in this embodiment, the thermal decomposition temperature of the water-absorbent resin may be 450-500°C. Water-absorbent resins that have a crosslinked structure and no softening point may experience significant changes in their physical properties at the thermal decomposition temperature.

The heating temperature in this step may be maintained approximately constant, or may vary within a range above the softening point of the thermoplastic resin and below the thermal decomposition temperature of the water-absorbent resin. Specifically, the maximum temperature in this step is preferably 150°C or higher, more preferably 200°C or higher, and preferably 450°C or lower, more preferably 350°C or lower. Furthermore, the time for which the maximum temperature is maintained in this step is preferably 60 minutes or longer, more preferably 120 minutes or longer, and preferably 480 minutes or shorter, more preferably 300 minutes or shorter.

In this step, a portion of the material to be treated may be carbonized by performing the step under temperature conditions that promote carbonization of the material. In particular, if the material to be treated contains easily carbonized components such as cellulose, carbonized material is more likely to be produced in this step. By carbonizing a portion of the material to be treated, highly brittle carbonized material is produced, which makes it easier to form fine granules in the subsequent granulation step S3.

(Granulation step S3)
The heating step S2 is followed by the granulation step S3. In the granulation step S3, the workpiece is agitated and cooled below the softening point of the thermoplastic resin to obtain granules. This step promotes homogenization, densification, and weight reduction of the workpiece, while also producing granules that are separated from one another. In the granulation step S3 of this embodiment, for example, heating of the workpiece by the heater 103 is stopped, while the workpiece is agitated by the agitation shaft 102. Furthermore, in the granulation step S3 of this embodiment, it is preferable to blow air using the air blowing mechanism 104. This allows the temperature of the heated workpiece in the storage section 101b to be quickly reduced. However, the use of the air blowing mechanism 104 in this step is not essential.

As the thermoplastic resin softened by heat cools, hardening due to the drop in temperature and an increase in viscosity due to thixotropy occur simultaneously. In the granulation step S3 of this embodiment, stirring is performed as the resin cools, allowing the thermoplastic resin to harden while maintaining a low viscosity due to its thixotropy. This allows the thermoplastic resin to be finely crushed to form small-particle-size granules without forming large, clumped granules. Furthermore, if highly brittle carbides are produced in the heating step S2, the stirring in this step can also finely crush the carbides, forming small-particle-size granules.

On the other hand, non-pyrolyzed water-absorbent resin is less likely to be crushed than thermoplastic resin or carbonized material. By stirring while cooling in this step, the water-absorbent resin is prevented from bonding with other granules and forming clumps. Therefore, this step allows the water-absorbent resin to form granules that are larger in size than the other granules.

In this way, in this step, granules of different particle sizes are formed. Larger granules contain a large amount of water-absorbent resin, while smaller granules are mainly composed of thermoplastic resin, carbide, etc. These granules are dry and have a form that makes them easy to separate from each other. Therefore, by separating these granules by particle size, it becomes possible to easily separate the granules that contain a large amount of water-absorbent resin.

(Separation step S4)
Subsequently, in a separation step S4, the water-absorbent resin is separated based on the particle size of the granules. The separation of the water-absorbent resin in this embodiment is not limited to a mode in which granules composed only of the water-absorbent resin are separated, but also includes a mode in which a part of the granules is separated so that the content of the water-absorbent resin in the first granules after separation is higher than that in the homogeneously mixed granules before separation.

In this embodiment, for example, the granular material is removed from the storage section 101b of the granulation device 100, and the granular material is separated by particle size using tools or devices such as a sieve or classifier. The separation method may be either dry or wet, but a dry method is preferred because dry granular material is being handled. In this step, it is preferable to use a vibrating sieve machine, as it allows for automatic separation of the water-absorbent resin with a relatively simple device configuration. In this step, it is sufficient to separate the granular material into at least two particle sizes, but it may also be separated into three or more particle sizes.

In this embodiment, for example, it is possible to separate first granules having a first particle size from second granules having a second particle size smaller than the first particle size. The first granules are configured as water-absorbent-polymer-containing granules containing a water-absorbent polymer. The second granules are configured as low-absorbent-polymer granules containing a lower water-absorbent polymer content than the first granules and primarily containing components other than the water-absorbent polymer. The water-absorbent polymer content in the first granules is preferably 31% by mass or more, more preferably 50% by mass or more. The second granules contain, for example, one or more components selected from thermoplastic resins, carbides, etc. The water-absorbent polymer content in the second granules is preferably 30% by mass or less, more preferably 25% by mass or less.

In this embodiment, for example, it is preferable that the particle size distribution of the granular material after granulation step S3 has a first peak corresponding to the first particle size and a second peak corresponding to the second particle size. The peak in the particle size distribution refers to the peak that forms an upward convex mountain when a particle size distribution graph is created with particle size on the horizontal axis and frequency (%) on the vertical axis. From among the multiple particle size distributions, two particle sizes are selected in descending order of peak height, with the larger particle size designated as the first particle size and the other designated as the second particle size. Furthermore, "the first particle size corresponds to the first peak" means that the first particle size is set to include a particle size corresponding to the first peak. "The second particle size corresponds to the second peak" means that the second particle size is set to include a particle size corresponding to the second peak. Having a particle size distribution with two or more peaks in this manner allows for more reliable separation of the first granular material, which contains a large amount of water-absorbent resin, from the second granular material.

The lower limit of the particle size of the first particle size may also be set based on a particle size distribution graph. Specifically, the lower limit of the particle size of the first particle size is preferably set between the particle size corresponding to the first peak and the particle size corresponding to the second peak, and more preferably set to a particle size corresponding to the downward convex portion of the particle size distribution graph located between the first and second peaks. The particle size distribution of the granular material can be confirmed using a known particle size distribution measurement method.

Specifically, the lower limit of the particle size of the first particle size is preferably 106 μm or more, more preferably 150 μm or more, and preferably 600 μm or less, more preferably 300 μm or less. For example, if a sieve is used in this step, the sieve openings are preferably 106 μm or more, more preferably 150 μm or more, and preferably 600 μm or less, more preferably 300 μm or less. If a sieve is used, the granules obtained in granulation step S3 can be passed through a sieve with the above openings, and the granules remaining on the sieve can be referred to as the first granules, and the granules that passed through the sieve can be referred to as the second granules.

[Method for producing granular material]
In this embodiment, the first granules and/or the second granules can be produced using the above-described method for separating a water-absorbent resin.
4, the method for producing the first granules (water-absorbent resin-containing granules) includes the drying step S1, heating step S2, and granulation step S3 described above, as well as a recovery step S41 of the first granules. In the recovery step S41, the first granules and the second granules are separated based on the particle size of the granules, and the first granules are recovered as water-absorbent resin-containing granules. In other words, the first granules can be obtained by recovering the first granules separated in the separation step S4 described above.

As shown in FIG. 5, the method for producing the second granules (granules) includes the drying step S1, heating step S2, and granulation step S3 described above, as well as a recovery step S42 of the second granules. In the recovery step S42, the first granules and the second granules are separated based on the particle size of the granules, and the second granules are recovered. In other words, by recovering the second granules separated in the separation step S4 described above, the second granules can be obtained as granules with a low water-absorbent resin content.

[Effects of this embodiment]
According to this embodiment, by subjecting the object to be treated, including the absorbent article, to the drying step S1, heating step S2, and granulation step S3 described above, the object to be treated can be treated as lightweight, finely divided granules. Furthermore, by setting the temperature in the heating step S2 to a temperature equal to or higher than the softening point of the thermoplastic resin and lower than the thermal decomposition temperature of the water-absorbent resin, the water-absorbent resin becomes less likely to be crushed into small pieces in the granulation step S3, and large-particle-sized first granules can be formed. As a result, the water-absorbent resin can be efficiently separated based on the difference in particle size in the separation step S4.

By recovering the first granules containing a large amount of water-absorbent resin from the material to be treated, the first granules can be used as a material containing water-absorbent resin. This allows the water-absorbent resin to be reused, making effective use of resources. It also reduces greenhouse gas emissions associated with the production of new water-absorbent resin.

On the other hand, the recovered second granules can be reused, for example, as fuel or fertilizer. Furthermore, if the second granules contain carbonized material, they can also be effectively used as, for example, water purification treatment agents, building materials such as insulation, adsorbents, detoxifiers, deodorizers, raw materials for disposable hand warmers, activated carbon, etc. In addition to having high homogeneity, the second granules can suppress swelling and moisture absorption caused by water-absorbent resins, making them an easy-to-handle recycled material.

In addition, in this embodiment, granules from which the water-absorbent resin can be separated are formed using a dry process. As a result, in this embodiment, the volume required to store the material to be treated can be reduced compared to wet processes, and the size of the equipment can be kept from increasing. The dry process also prevents water pollution due to wastewater, etc. Furthermore, in this embodiment, granulation increases the density, so the volume required for storage after treatment is significantly reduced, and the mass is reduced due to the evaporation of water during the drying step S1 and heating step S2. As a result, the granules of the material to be treated obtained in this embodiment are easy to store and transport, making them a recycled material with excellent handleability.

[Additional Description of This Embodiment]
When the granulation device 100 accommodates the workpiece in the drying step S1, it is preferable to accommodate a relatively large amount of workpiece in the accommodation section 101b. Specifically, the amount of workpiece accommodated in the accommodation section 101b at the start of the drying step S1 is preferably set so that the volume of the granular material in the accommodation section 101b at the end of the granulation step S3 exceeds the shaft portion 102a. The volume of the granular material in the accommodation section 101b is obtained, for example, as shown in Figures 1 and 2, as the vertical height L of the surface of the granular material smoothed along a horizontal plane in the accommodation section 101b. In this way, by determining the amount of workpiece accommodated in the accommodation section 101b at the start of the drying step S1 based on the state at the end of the granulation step S3, when the volume of the workpiece is at its lowest due to the progress of crushing, the shaft portion 102a is maintained submerged in the workpiece during agitation throughout the entire process of the drying step S1, the heating step S2, and the granulation step S3. This allows the workpiece to be efficiently stirred in the granulation step S3. From the perspective of efficiently stirring the workpiece, it is preferable that the volume of the granular material of the workpiece exceeds the entire shaft portion 102a in the storage section 101b at the end of the granulation step S3, i.e., the shaft portion 102a is completely buried in the granular material of the workpiece. However, in cases such as when the shaft portion 102a is tilted relative to the horizontal plane as described above, the effect of efficiently stirring the workpiece can be achieved as long as the volume of the granular material of the workpiece exceeds at least a portion of the shaft portion 102a in the storage section 101b at the end of the granulation step S3.

The object to be processed in this embodiment is not limited to waste containing used absorbent articles, but may also include bulk-increasing secondary materials. This ensures a sufficient amount of object to be processed in the storage section 101b. The secondary materials can be selected arbitrarily. For example, objects made of highly heat-resistant, hard materials such as ceramics that do not change during the drying, heating, and granulation processes (e.g., ceramic balls such as alumina balls, zirconia balls, silicon nitride balls, and silicon carbide balls) can be used as secondary materials. By leaving such secondary materials in the storage section 101b after the granulation step S3, they can be repeatedly used as secondary materials for bulk-increasing. In addition, any object other than absorbent articles (e.g., food waste, agricultural waste, etc.) can be used as secondary materials. Furthermore, when carbonizing the material to be treated in heating step S2, carbonization of the material can be accelerated by using, as a secondary material, an object whose main component is easily carbonized cellulose (e.g., wood pellets, paper waste, selected branches, etc.) or an already carbonized material (e.g., carbonized fuel pellets, activated carbon, etc.).

Furthermore, the granulated material obtained in granulation step S3 of a previous granulation process can be used as a secondary material in a subsequent granulation process. In particular, when multiple granulation processes are performed consecutively, by leaving at least a portion of the granulated material in storage section 101b after granulation step S3 of the previous granulation process and adding new material in drying step S1, the granulated material obtained in the previous granulation process can be used as a secondary material. This allows for the amount of material to be used as a secondary material, even if there is variation in the amount of material to be newly added in drying step S1. By recovering an amount of granulated material from storage section 101b that is calculated backward from the amount of waste to be newly added, the amount of material to be used in storage section 101b can be kept constant. This enables stable, repeated granulation processes to be performed.

Furthermore, when carbonizing a portion of the workpiece in heating step S2, it is preferable to fix as much of the carbon that constitutes the carbon-resistant thermoplastic resin as part of the char. This reduces the amount of carbon dioxide emitted from the carbon that constitutes the thermoplastic resin. From this perspective, it is preferable to add at least one of phosphoric acid and a phosphate salt as a catalyst to the workpiece in storage section 101b. Phosphoric acid and a phosphate salt have the effect of promoting the desorption of hydrogen, water, and other elements from the thermoplastic resin. Therefore, using phosphoric acid or a phosphate salt as a catalyst promotes the desorption of hydrogen, water, and other elements from the thermoplastic resin contained in the waste during heating step S2, making it easier for the thermoplastic resin to be fixed integrally as elemental carbon on the surface of the char. The phosphate salt added to the workpiece as a catalyst can be, for example, at least one of ammonium phosphate, potassium phosphate, and sodium phosphate. Examples of ammonium phosphate include ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and triammonium phosphate. Examples of potassium phosphate include potassium dihydrogen phosphate, dipotassium hydrogen phosphate, and tripotassium phosphate. Examples of sodium phosphate include sodium dihydrogen phosphate, disodium hydrogen phosphate, and trisodium phosphate. Phosphoric acid and phosphate salts condense when heated to form polyphosphoric acid and polyphosphate salts. However, polyphosphoric acid and polyphosphate salts soften in the heating step S2 and, like thermoplastic resins, tend to bond fragments together during the cooling process. However, by stirring in the granulation step S3 of this embodiment, the treated material remains in a granular state composed of separate fragments.

Furthermore, when the workpiece contains two or more thermoplastic resins with different softening points, as described above, the heating temperature in heating step S2 can be set based on the softening point of any of the thermoplastic resins. Therefore, the workpiece may contain a thermoplastic resin (e.g., polyethylene terephthalate) with a softening point higher than the maximum temperature of heating step S2. Such thermoplastic resins that do not reach their softening point in heating step S2 may form fibrous or string-like objects rather than granular bodies after granulation step S3. These fibrous or string-like thermoplastic resins can be removed based on their shape differences from the granular bodies, and can be easily separated from the granular bodies. In other words, in this embodiment, even when the workpiece contains a thermoplastic resin with a high softening point, separation of the water-absorbent resin based on particle size is possible in separation step S4.

The above describes an embodiment of the present invention, but the present invention is not limited to the above embodiment, and various modifications can of course be made without departing from the spirit of the present invention.

[Examples and Comparative Examples]
Examples and comparative examples of the method for separating a water-absorbent resin according to the present embodiment will be described. In each example and comparative example, a sample of the material to be treated was prepared, simulating an actual used absorbent article. Specifically, the sample of the material to be treated in each of Examples 1 to 4 and Comparative Examples 1 to 5 was prepared by allowing 15 kg of unused disposable diapers (Merrys Pants, size L, manufactured by Kao Corporation) to absorb 7.5 kg of tap water. In addition, 0.6 kg of ammonium dihydrogen phosphate was added to the sample to reproduce the nutritional components contained in urine and promote carbonization.

The samples of the workpieces in Examples 1 to 4 were subjected to all of the steps including the drying step S1, the heating step S2, the granulating step S3, and the separating step S4.
On the other hand, as shown in Table 1, at least one of the drying step S1, heating step S2, granulation step S3, and separation step S4 described above was not performed on the treated material samples of Comparative Examples 1 to 5. Specifically, none of the steps were performed in Comparative Example 1. In Comparative Example 2, the drying step S1 and heating step S2 were performed, but stirring was not performed in the granulation step S3, and the separation step S4, as described below, was also not performed. In Comparative Example 3, the drying step S1, heating step S2, and granulation step S3 were performed, but the separation step S4 was not performed. In Comparative Examples 4 and 5, none of the steps were performed, and instead a wet separation process, as described below, was performed.

In Examples 1 to 4 and Comparative Examples 2 and 3, the drying step S1 and heating step S2 were performed consecutively. In these steps, the temperature was raised from 20°C to 200°C at a rate of 0.5°C/min and maintained at 200°C for 180 minutes. The material to be treated was continuously stirred during these steps. In the granulation step S3 in Examples 1 to 4 and Comparative Examples 2 and 3, the temperature was lowered from 200°C to 30°C at a rate of 0.2°C/min while blowing air. Of these, in Examples 1 to 4 and Comparative Example 3, the material to be treated was stirred during cooling. On the other hand, in Comparative Example 2, the material to be treated was not stirred during cooling.

For Comparative Examples 4 and 5, one diaper was removed from the treated material sample and cut into 5 cm pieces. The cut diapers were placed in a 5% aqueous CaCl solution and stirred. After leaving to stand, the floating matter and sediment were separated. The floating matter and sediment were each placed in a 250-mesh nylon mesh bag and dehydrated in a stretch dehydrator to obtain samples. The floating matter sample was designated Comparative Example 4, and the sediment sample was designated Comparative Example 5.

[Separation process]
The granular samples after the granulation step S3 in Examples 1 to 4 were subjected to a separation process using a sieve (separation step S4). In Examples 1 and 3, the separation process was performed using a sieve with 150 μm openings. In Examples 2 and 4, the separation process was performed using a sieve with 300 μm openings. In Examples 1 and 2, the granular material (residue) remaining on the sieve was collected. In Examples 3 and 4, the granular material that passed through the sieve was collected.

On the other hand, the sample of Comparative Example 1 maintained the shape of the diaper itself, and the water-absorbent resin could not be separated. Furthermore, the sample of Comparative Example 2 was not stirred during cooling in the granulation step S3, so the material was in clumps and could not be separated by sieving. In Comparative Example 3, as described above, no separation by sieving was performed, and all of the granules were recovered.

[Measurement of calorific value]
The calorific value of the recovered samples of each Example and Comparative Example was measured. The results are shown in Table 1. The calorific value is affected by the moisture content of the sample. In addition, since the calorific value tends to decrease when the water-absorbent resin content is high and increase when the water-absorbent resin content is low, the calorific value is also affected by the content of the water-absorbent resin in the sample. The calorific value was measured as the total calorific value based on JIS Z 7302-2.

As shown in Table 1, the calorific values of the samples of Examples 1 and 2, which are made of granular material with large particle sizes, were smaller than those of the sample of Comparative Example 3, which contained all granular material. From these results, it is believed that the samples of Examples 1 and 2 contain more water-absorbent resin than the sample of Comparative Example 3. Furthermore, the granulated sample of Comparative Example 3 and the dried sample of Comparative Example 2 had a lower moisture content than Comparative Example 1, which is the diaper itself containing absorbed water-absorbent resin, and therefore had a higher calorific value.

Furthermore, as shown in Table 1, the calorific values of the samples of Examples 3 and 4, which are small-particle-sized granules, were higher than those of Comparative Example 3 and Examples 1 and 2. This is thought to be because the samples of Examples 3 and 4 contained a lower amount of water-absorbent resin than the samples of Comparative Example 3 and Examples 1 and 2. Furthermore, the results in Table 1 show that the samples of Examples 3 and 4 had a higher calorific value than the samples of Examples 1 and 2 and Comparative Examples 1 to 4, and were therefore more suitable as fuel.

Furthermore, the heat generation values of the wet-separated samples of Comparative Examples 4 and 5 were lower than those of Comparative Examples 1 to 3 and Examples 1 to 4. This is thought to be because the samples of Comparative Examples 4 and 5 had a higher moisture content than those of Comparative Examples 1 to 3 and Examples 1 to 4. The heat generation value of the sample of Comparative Example 4 was higher than that of the sample of Comparative Example 5. This is thought to be because the sample of Comparative Example 4 had a lower water-absorbent resin content than the sample of Comparative Example 5.

[Calculation of water-absorbent resin content]
Furthermore, the content of the water-absorbent resin in each of the collected samples of Examples and Comparative Examples was calculated. The results are shown in Table 1.
The content of the water absorbent resin was calculated as follows. First, all sodium in the sample was assumed to be derived from the water absorbent resin, and the content of the water absorbent resin was calculated from the sodium ratio. The sodium ratio was measured using IPC optical emission spectrometry based on "Refuse-derived fuel - Part 5: Test method for metal content (JIS Z 7302-5:2002)".

As shown in Table 1, the samples recovered in Examples 1 and 2 had a water-absorbent resin content of 50% by mass or more, which was higher than the samples in Comparative Examples 2 and 3, which did not undergo separation step S4. On the other hand, the samples recovered in Examples 3 and 4 had a water-absorbent resin content of less than 30%, which was lower than the other samples. These results demonstrate that by performing separation step S4 using sieves with 150 μm and 300 μm mesh sizes after performing drying step S1, heating step S2, and granulation step S3, it is possible to separate granules with a high water-absorbent resin content from granules with a low content.

100... granulation device 101... treatment tank 101b... storage section 102... stirring shaft 102a... shaft section 102b... blade section 103... heater 104... air blowing mechanism C... rotation shaft

Claims (10)

A method for separating a water-absorbent resin from a processing object including an absorbent article that has absorbed water and includes a water-absorbent resin and a thermoplastic resin, comprising:
a drying step of stirring and drying the object to be treated;
a heating step of heating the object to be treated at a temperature equal to or higher than the softening point of the thermoplastic resin and lower than the thermal decomposition temperature of the water-absorbing resin;
a granulation step of stirring the material to be treated after the heating step and cooling it to a temperature lower than the softening point of the thermoplastic resin to obtain granules;
a separation step of separating the water-absorbent resin based on the particle size of the granules. 2. The method for separating a water-absorbent resin according to claim 1, wherein the separation step separates a first granular body containing the water-absorbent resin and having a first particle size from a second granular body having a lower content of the water-absorbent resin than the first granular body and a second particle size smaller than the first particle size. The method for separating a water-absorbent resin according to claim 2, wherein a particle size distribution of the granular material has a first peak corresponding to the first particle size and a second peak corresponding to the second particle size. The method for separating a water-absorbent resin according to claim 1 or 2 , wherein in the drying step, the absorbent article is crushed to expose the water-absorbent resin. The method for separating a water-absorbent resin according to claim 1 or 2 , wherein the heating step carbonizes a part of the object to be treated. The method for separating a water-absorbent resin according to claim 1 or 2 , wherein the material to be treated contains cellulose. The method for separating a water-absorbent resin according to claim 1 or 2 , wherein a vibrating screen is used in the separating step. The method for separating a water-absorbent resin according to claim 1 or 2 , wherein in the separating step, the water-absorbent resin is separated using a sieve having an opening of 106 μm or more. A method for producing a water-absorbent resin-containing granule containing a water-absorbent resin from a processing object including an absorbent article having a water-absorbent resin and a thermoplastic resin, the method comprising:
a drying step of stirring and drying the object to be treated;
a heating step of heating the object to be treated at a temperature equal to or higher than the softening point of the thermoplastic resin and lower than the thermal decomposition temperature of the water-absorbing resin;
a granulation step of stirring the material to be treated after the heating step and cooling it to a temperature lower than the softening point of the thermoplastic resin to obtain granules;
A method for producing a water-absorbent resin-containing granular material, comprising: a recovery step of separating a first granular material from a second granular material having a smaller particle size than the first granular material based on the particle size of the granular material, and recovering the first granular material as a water-absorbent resin-containing granular material. A method for producing granular materials from a processing object including an absorbent article that has absorbed water and contains a water-absorbent resin and a thermoplastic resin, comprising:
a drying step of stirring and drying the object to be treated;
A heating step of maintaining the temperature of the object to be treated at a temperature equal to or higher than the softening point of the thermoplastic resin and lower than the thermal decomposition temperature of the water-absorbing resin;
a granulation step of stirring the material to be treated after the heating step and cooling it to a temperature lower than the softening point of the thermoplastic resin to obtain granules;
A method for manufacturing granular materials, comprising: a recovery step of separating first granular materials from second granular materials having a smaller particle size than the first granular materials based on the particle size of the granular materials, and recovering the second granular materials.