Method for recovering water-absorbing resin

Abstract

Provided is a technique by which it is possible to obtain a water-absorbing resin in which the quantity of impurities mixed therein is further reduced when recovering the water-absorbing resin from a mixture in which the water-absorbing resin and the impurities, such as pulp, are adhered to each other. This method for recovering a water-absorbing resin comprises, in the following order of execution: a crushing step in which a physical force is applied to a mixture formed by a water-absorbing resin and impurities which are adhered and integrated to each other, and in which the mixture is thereby crushed; and a separation step in which the water-absorbing resin and the impurities are separated from the mixture crushed in the crushing step.

Description

Method for recovering water-absorbent resin

[0001] The present invention relates to a method for recovering a water-absorbent resin.

[0002] Articles intended to absorb body fluids, such as disposable diapers, sanitary napkins, and incontinence pads (hereinafter also referred to as "absorbent articles"), are mainly composed of absorbent materials such as pulp and super absorbent polymer (hereinafter also referred to as "SAP"), and exterior materials formed from nonwoven fabrics, resin films, resin-coated paper, etc. In recent years, from the perspective of effective utilization of resources, it has been considered to separate and recover the constituent components of absorbent articles, particularly the pulp and the absorbent resin, and reuse them, rather than incinerating them, from the viewpoint of effective utilization of resources.

[0003] For example, Japanese Patent Application Laid-Open No. 2019-85447 proposes a method for recovering pulp from used absorbent articles by subjecting an inactivated aqueous solution containing pulp and a water-absorbent resin separated from the used absorbent article to solid-liquid separation and then treating it with a specific method. Also, Japanese Patent Application Laid-Open No. 2021-41310 (corresponding to European Patent Application Publication No. 4001354, the same applies hereinafter) proposes a method for producing a recycled superabsorbent polymer by inactivating a used superabsorbent polymer derived from used sanitary products with an acidic solution and then subjecting it to a specific treatment.

[0004] According to the method disclosed in JP 2019-85447 A, the water-absorbent resin is removed from the pulp containing the remaining water-absorbent resin that has been separated from a used absorbent article, and the water-absorbent resin and the pulp are separated. Furthermore, JP 2021-41310 A describes that foreign matter (pulp, etc.) can be separated from the recycled superabsorbent polymer by performing a foreign matter separation step in which foreign matter such as pulp is separated from the recycled superabsorbent polymer after the drying step.

[0005] However, when used absorbent articles are recycled using a wet method, such as the techniques described in the aforementioned Japanese Patent Publications No. 2019-85447 and No. 2021-41310, even after a separation process between the absorbent resin and fibrous materials such as pulp, impurities such as pulp remain attached to the absorbent resin, making it difficult to separate them at high purity. Furthermore, absorbent articles may have to be discarded due to moisture absorption during storage. Even when the absorbent resin and fibrous materials such as pulp are extracted from such absorbent articles, impurities such as pulp remain attached to the absorbent resin, making it difficult to separate them at high purity. Therefore, when attempting to manufacture absorbent articles using the absorbent resin separated and recovered as described above (material recycling), there is a problem in that the physical properties of the absorbent articles are reduced due to impurities such as pulp that were not completely removed. Furthermore, when an attempt is made to chemically decompose the separated and recovered water absorbent resin to newly produce a water absorbent resin (chemical recycling), the remaining impurities cause a decrease in performance of the obtained water absorbent resin and a decrease in workability during production, which become problems.

[0006] Therefore, there is a demand for a technique that can recover a water-absorbent resin containing less impurities from a mixture of the water-absorbent resin and impurities such as pulp adhering thereto.

[0007] Therefore, an object of the present invention is to provide a technology that can obtain a water-absorbent resin with a reduced amount of contaminants mixed in when recovering the water-absorbent resin from a mixture in which the water-absorbent resin and contaminants such as pulp have adhered and integrated with each other.

[0008] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result, have found that the above-mentioned problems can be solved by applying a physical force to a mixture in which a water absorbent resin and impurities are fixed and integrated together, crushing the water absorbent resin and the impurities to break up the integrated state, and then separating the water absorbent resin and the impurities, thereby completing the present invention.

[0009] That is, one aspect for achieving the above-mentioned object is a method for recovering a water-absorbent resin, which comprises, in this order: a crushing step in which a physical force is applied to a mixture in which the water-absorbent resin and impurities are fixed together and integrated, thereby crushing the mixture; and a separation step in which the water-absorbent resin and the impurities are separated from the mixture crushed in the crushing step.

[0010] FIG. 1 is a flow diagram illustrating a method for recovering a water-absorbent resin according to the present invention. In FIG. 1, S1 represents a crushing step, S2 represents an inactivation / dehydration treatment step, S3 represents a cleaning / disinfection step, S4 represents a separation / filtration step, S5 represents an acid treatment step, S6 represents a neutralization step, S7 represents a drying step, S100 represents a preparation step (pretreatment step) for a regenerated water-absorbent resin, S200 represents a crushing step, and S300 represents a separation step. FIG. 2 is a photograph showing the appearance of the dried water-absorbent resin (S1) (mixture) containing pulp obtained in Production Example 1. FIG. 3 is an SEM image (magnification: 30x) of the 850 μm / 500 μm particle portion of the pulverized product (1-1) obtained in Example 1-1. FIG. 4 is an SEM image (magnification: 30x) of the 500 μm / 150 μm particle portion of the pulverized product (1-1) obtained in Example 1-1. FIG. 5 is an SEM image (magnification: 30 times) of the pulverized product (1-1) obtained in Example 1-1, which passed through 150 μm.

[0011] One aspect of the present invention is a method for recovering a water-absorbent resin, which comprises a crushing step in which a physical force is applied to a mixture in which a water-absorbent resin and impurities are fixed together and integrated, thereby crushing the mixture (water-absorbent resin and impurities), and a separation step in which the water-absorbent resin and the impurities are separated from the mixture crushed in the crushing step. In this specification, the method for recovering a water-absorbent resin having such a configuration is also simply referred to as the "recovery method according to the present invention" or the "method according to the present invention." In addition, in this specification, the "water-absorbent resin recovered by the method of the present invention" is also simply referred to as the "water-absorbent resin according to the present invention."

[0012] In absorbent articles such as disposable diapers, water-absorbent resins and pulp are generally used as absorbent materials. To separate these, as in the techniques disclosed in JP 2019-85447 A and JP 2021-41310 A, the water-absorbent resin is treated with an aqueous solution that inactivates the water-absorbent resin (inactivating aqueous solution), and then the water-absorbent resin is separated from fibrous materials such as pulp. The separated water-absorbent resin then undergoes several further treatments and is dried to become a recycled water-absorbent resin. However, even recycled water-absorbent resins obtained by such methods have the problem that a relatively large amount of impurities such as pulp adheres to their interior or surface.

[0013] Therefore, the present inventors have investigated this problem and have concluded that the following causes lead to insufficient separation of the water-absorbent resin from impurities such as pulp. That is, the present inventors have speculated that this is because, when a water-absorbent resin in a hydrated state and impurities such as pulp are mixed together and dried, the water-absorbent resin and impurities such as pulp are strongly adhered to each other and integrated. Specifically, when wet recycling is performed, as in the treatment using the inactivating aqueous solution described above, and then a drying treatment is performed, it is thought that the remaining impurities such as pulp are strongly adhered to and integrated with the water-absorbent resin due to the stickiness (tackiness) of the water-absorbent resin that appears when the water-absorbent resin is hydrated and swollen, or due to the water-soluble components in a free state. Furthermore, even when a water-absorbent resin in a dry state and impurities such as pulp exist independently, it is speculated that the same phenomenon as described above may occur due to moisture absorption from the external environment. That is, the above-mentioned integration (strong adhesion between the water-absorbent resin and impurities such as pulp) can occur not only in the water-absorbent material (water-absorbent resin and pulp) subjected to the above-mentioned wet recycling, but also in the water-absorbent resin contained in an unused absorbent article that has absorbed moisture (due to the external environment) after production.

[0014] In contrast, according to the recovery method of the present invention, a physical force is applied to a mixture in which the water-absorbent resin and impurities such as pulp are fixed together, thereby crushing the water-absorbent resin and impurities such as pulp contained in the mixture. By carrying out this crushing step before the separation step, the water-absorbent resin and the impurities such as pulp can be made to be in an independent state (not fixed together), and therefore the water-absorbent resin and the impurities such as pulp can be efficiently separated in the separation step. As a result, it is presumed that a water-absorbent resin containing a reduced amount of impurities such as pulp can be obtained.

[0015] The above mechanism is merely speculation and does not limit the technical scope of the present invention.

[0016] Preferred embodiments of the present invention will be described below. Note that the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the claims. Furthermore, the embodiments described in this specification can be arbitrarily combined to form other embodiments.

[0017] Throughout this specification, singular expressions should be understood to include the plural concept unless otherwise specified. Therefore, singular articles (e.g., "a," "an," "the," etc. in English) should be understood to include the plural concept unless otherwise specified. Furthermore, terms used in this specification should be understood to be used in the sense commonly used in the art unless otherwise specified. Therefore, unless otherwise defined, all technical and scientific terms used in this specification have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict, the present specification (including definitions) will prevail.

[0018] In this specification, the term "(meth)acrylic" includes both acrylic and methacrylic. Thus, for example, the term "(meth)acrylic acid" includes both acrylic acid and methacrylic acid. In this specification, "acid (salt)" means "acid and / or its salt." Furthermore, unless otherwise specified, concentrations and percentages represent mass concentrations and mass percentages, respectively, and ratios represent mass ratios unless otherwise specified. Furthermore, unless otherwise specified, operations and measurements of physical properties, etc. are performed under conditions of room temperature (20 to 25°C) and a relative humidity of 40 to 55% RH. Furthermore, "A and / or B" means that A and B are included individually and in combination.

[0019] [1] Definition of Terms [1-1] Water-Absorbent Resin The term "water-absorbent resin" refers to a water-swellable, water-insoluble polymer gelling agent, and is not particularly limited, but refers to a commonly used water-absorbent resin having a water absorption capacity of 10 to 1000 times. More specifically, it is preferable that the water-absorbent resin before absorbing the liquid to be absorbed satisfies the physical property of "water-swellability" such that the CRC defined in ERT441.2-02 is 5 g / g or more. The definition of CRC will be described later.

[0020] In this technical field, the term "water-absorbent resin" may refer to each individual particle of the water-absorbent resin, but in this specification, unless otherwise specified, it refers to an aggregate of all particles of the water-absorbent resin.

[0021] The water-absorbent resin may be a polymer derived from a carboxyl group-containing unsaturated monomer. The water-absorbent resin may include a polymer having a partially neutralized carboxyl group. Specific examples of the water-absorbent resin include polyacrylic acid (salt)-based resins, polysulfonic acid (salt)-based resins, maleic anhydride (salt)-based resins, polyacrylamide-based resins, polyvinyl alcohol-based resins, polyethylene oxide-based resins, polyaspartic acid (salt)-based resins, polyglutamic acid (salt)-based resins, polyalginic acid (salt)-based resins, starch-based resins, cellulose-based resins, (meth)acrylate crosslinked polymers, crosslinked products of saponified (meth)acrylate-vinyl acetate copolymers, starch-acrylate graft polymers and crosslinked products thereof.

[0022] In one embodiment of the present invention, the water-absorbing resin is a polymer derived from a carboxyl group-containing unsaturated monomer, and the water-absorbing resin may include a polymer having a partially neutralized carboxyl group. Furthermore, in one embodiment of the present invention, the water-absorbing resin is a polyacrylic acid (salt)-based resin.

[0023] [1-2] Recovered Water-Absorbent Resin The term "recovered water-absorbent resin" refers to a water-absorbent resin that is originally to be discarded. Specifically, it includes a water-absorbent resin contained in a used absorbent article (for example, a water-absorbent resin that has absorbed body fluids such as urine and blood in a used absorbent article) and a water-absorbent resin contained in a discarded absorbent article before use (for example, a water-absorbent resin that has absorbed moisture during storage or has been discarded even if it has not absorbed body fluids such as urine and blood).

[0024] When the recovered water-absorbent resin is a water-absorbent resin contained in a used absorbent article, the recovered water-absorbent resin may be in a state of a hydrous gel in which water such as urine water has been absorbed. Therefore, the water-absorbent resin may include a water-absorbent resin in a state of a hydrous gel.

[0025] The recovered water-absorbent resin is generally recovered by removing components other than the water-absorbent resin, such as an exterior material, from the absorbent article. In this case, the operation of separating the recovered water-absorbent resin from the absorbent article can be performed by a dry separation method or a wet separation method, and any of these methods can be used as a known technique.

[0026] In this specification, the mass of the water-absorbent resin and the recovered water-absorbent resin is a numerical value converted into a solid content unless otherwise specified. For example, when the mass of the recovered water-absorbent resin contained in used disposable diapers is unknown, the mass is calculated using the mass of the water-absorbent resin contained in new (unused) disposable diapers and a general value of the water absorption capacity of the water-absorbent resin in the used disposable diapers, or the mass of the recovered water-absorbent resin is measured by assuming that the content (solid content equivalent value) of the water-absorbent resin relative to the total mass of the new (unused) disposable diapers is 30 mass%.

[0027] [1-3] Absorbent Articles "Absorbent articles" refer to sanitary materials used to absorb body fluids such as urine and blood. Examples of sanitary materials include disposable diapers (for both children and adults), sanitary napkins, adult incontinence products (incontinence pads), nursing pads, etc. Examples of articles similar to sanitary materials include pet sheets and pet diapers, and in the present invention, these animal excrement disposal articles are also included in the sanitary materials.

[0028] An example of the structure of an absorbent article includes a liquid-permeable top sheet (surface material), an absorbent core containing a water-absorbent resin and a fibrous material, and a liquid-impermeable back sheet (waterproof material). The absorbent core is preferably produced by mixing the water-absorbent resin with the fibrous material, or by sandwiching the water-absorbent resin between fibrous materials and molding the mixture into a film, a tube, a sheet, or the like. Examples of the fibrous material include hydrophilic fibers such as pulverized wood pulp, cotton linters, crosslinked cellulose fibers, cotton, wool, and fibers made of rayon, acetate, vinylon, etc. In this specification, these fibrous materials are collectively referred to as "pulp."

[0029] More specifically, an example of the configuration of a disposable diaper, which is one type of absorbent article, includes a surface material such as a nonwoven fabric made of chemical fibers such as polypropylene or polyester; a water-absorbing body containing a water-absorbing material such as a water-absorbing resin or pulp; a waterproof material such as a resin film such as a polyethylene film, or a paper or nonwoven fabric that has been processed with resin, such as polyethylene-laminated paper or polyethylene-laminated nonwoven fabric (resin-treated paper or resin-treated nonwoven fabric); and an adhesive (binder) that bonds each of these components together.

[0030] In this specification, among the components of absorbent articles, components other than the water-absorbing material (water-absorbent resin, pulp), i.e., the surface sheet (surface material), back sheet (waterproof material), and adhesive (binder) formed from nonwoven fabric, resin film, resin-coated paper, resin-coated nonwoven fabric, etc., are also collectively referred to as "exterior materials."

[0031] "Absorbent article" includes both used and unused absorbent articles (especially pre-use discarded absorbent articles).

[0032] [1-4] Used absorbent article A "used absorbent article" is a used sanitary material that has been used by a consumer and has absorbed body fluids such as excrement, blood, etc. In other words, a used absorbent article contains a water-absorbent resin that has swollen with body fluids.

[0033] Here, the state of use is not particularly limited. For example, the state may be one in which solid waste (such as feces) is attached, one in which liquid waste (such as urine or menstrual blood) is attached, or one in which both are attached, but in consideration of the cost and processing efficiency of the recycling process, it is preferable that the state be one in which urine is absorbed as the main component.

[0034] In addition, the used absorbent articles used in the present invention include, for example, those that are collected, recovered, and transported from facilities where users of absorbent articles live or stay, such as ordinary households, hospitals, and welfare facilities.

[0035] [1-5] Impurities As described above, absorbent articles mainly contain a water-absorbent material containing a water-absorbent resin and pulp, and other exterior materials. To recover the water-absorbent resin that has swollen due to use, the following process is generally performed. Specifically, the exterior material of the absorbent article is crushed to roughly crush the exterior material and the water-absorbent material (crushing process). After appropriate processing, the resulting material is separated (filtered) into the water-absorbent resin, pulp, and fine pieces of exterior material that were not completely separated (separation / filtration process). However, even after such processing, the fine crushed pieces of exterior material that are mixed in due to the crushing of the pulp and exterior material cannot be completely separated from the water-absorbent resin, and they remain mixed together. Therefore, in this specification, the crushed pieces of exterior material (fine crushed pieces) as described above, in addition to the pulp used as the water-absorbent material, are collectively referred to as "impurities." Therefore, in one embodiment of the present invention, the contaminants separated from the water-absorbent resin include the exterior material of the absorbent article and pulp. Also, in one embodiment of the present invention, the contaminants separated from the water-absorbent resin include pulp.

[0036] The shape of the impurities is not particularly limited, and examples thereof include fibrous shapes (including straight shapes, wavy shapes, etc.), rod shapes, needle shapes, flat shapes, strip shapes, irregular flake shapes, etc., but generally, most of the impurities mixed in a water absorbent resin are fibrous.

[0037] [1-6] Mixture The "mixture" to be subjected to the method for recovering a water-absorbent resin according to the present invention (more specifically, the disintegration step according to the present invention) is formed by the water-absorbent resin and impurities adhering to each other and integrating with each other. In this specification, the "mixture formed by the water-absorbent resin and impurities adhering to each other and integrating with each other" may be simply referred to as the "integrated mixture according to the present invention" or "mixture". In one embodiment, the mixture to be subjected to the recovery method according to the present invention is in a state in which impurities such as pulp are taken into the water-absorbent resin aggregates and integrated with each other (for example, the state shown in the photograph of FIG. 2). Such a state of the mixture is formed when the water-absorbent resin and pulp etc. in a water-containing state (including a hygroscopic state) are dried, and the pulp etc. is firmly adhering to the water-absorbent resin.

[0038] Here, the specific form (microstructure) of the mixture is not particularly limited, and examples thereof include a form in which impurities such as pulp are attached to the surfaces of particles (irregular particles) of the water-absorbent resin; a form in which particles (irregular particles) of the water-absorbent resin are aggregated to form aggregates and impurities such as pulp are embedded between the particles; a form in which impurities such as pulp penetrate into particles (irregular particles) of the water-absorbent resin; and the like.

[0039] Furthermore, the state in which "the water absorbent resin and the impurities are fixed together" refers to, as an example, a state in which the water absorbent resin and the impurities cannot be separated by a general classification device such as classification by sieve, inertial classification, centrifugal classification, gravity classification or the like.

[0040] As described above, in the mixture to be subjected to the method for recovering a water-absorbent resin according to the present invention (more specifically, the disintegration step according to the present invention), impurities such as pulp are firmly fixed and integrated with the water-absorbent resin, and therefore, even if the mixture is subjected to a separation treatment using a normal separation means such as a sieve, the impurities and the water-absorbent resin cannot be easily separated.

[0041] However, according to the recovery method of the present invention, the impurities such as pulp and the water absorbent resin are separated from each other by a crushing step described in detail later, and then the separation step is carried out, so that the content of the impurities mixed in the water absorbent resin can be reduced.

[0042] In the mixture obtained through the crushing step described in detail later, the water absorbent resin and the impurities may not be completely independent, and may remain as a mixture in which the water absorbent resin and the impurities are integrated in part. Therefore, the "mixture after crushing" obtained through the crushing step may include the water absorbent resin and the impurities which are respectively in an independent state, and the integrated mixture which remains without being completely crushed.

[0043] From the viewpoint of environmental protection and the like, in the present invention, it is preferable that the water-absorbent resin contained in the mixture includes a recovered water-absorbent resin. That is, in one embodiment of the present invention, the water-absorbent resin contained in the mixture includes a recovered water-absorbent resin. Note that, in this embodiment, the mixture may further include a newly produced fresh (unused) water-absorbent resin. Furthermore, in another embodiment of the present invention, the water-absorbent resin contained in the mixture is a recovered water-absorbent resin.

[0044] Furthermore, from the same viewpoint as above, it is preferable that the recovered water-absorbent resin contained in the mixture contains a water-absorbent resin recovered (removed and collected) from used absorbent articles. That is, in one embodiment of the present invention, the mixture contains a recovered water-absorbent resin, and the recovered water-absorbent resin contains a water-absorbent resin recovered (removed and collected) from used absorbent articles.

[0045] Furthermore, in one embodiment of the present invention, it is not possible to completely remove moisture from the mixture subjected to the crushing step. Therefore, the water-absorbent resin contained in the mixture (recovered water-absorbent resin) may be in a state of a hydrous gel in which moisture has been absorbed. Therefore, the water-absorbent resin contained in the mixture may include a water-absorbent resin in a state of a hydrous gel.

[0046] In one embodiment of the present invention, the CRC of the water-absorbent resin (recovered water-absorbent resin) contained in the mixture is 20 g / g or more, 25 g / g or more, 27 g / g or more, or 30 g / g or more. In one embodiment of the present invention, the CRC of the water-absorbent resin (recovered water-absorbent resin) is 70 g / g or less, 60 g / g or less, or 50 g / g or less. The definition of CRC will be described later.

[0047] [1-7] Recycled Water-Absorbent Resin (Recycled SAP) As described above, the recovered water-absorbent resin is generally recovered from the absorbent article by a dry separation method or a wet separation method. In this case, the recovery of the water-absorbent resin that has absorbed water and swollen due to use or the like (the water-absorbent resin contained in a used absorbent article or a water-absorbent resin contained in an absorbent article discarded before use) is carried out by removing components other than the water-absorbent resin (such as an outer covering material that constitutes the absorbent article), and the removal and separation of such components other than the water-absorbent resin can be carried out using a known wet separation method.

[0048] As an example, recovery of a water-absorbent resin by wet separation is carried out through the following steps: that is, an inactivation / dehydration treatment step in which the water-absorbent material containing impurities such as the water-absorbent resin and pulp and the exterior material recovered from the absorbent article are dispersed in a solution (that is, wet), and the water-absorbent resin contained in the absorbent article is subjected to a shrinkage / dehydration treatment (inactivation treatment); a separation / filtration step in which the water-absorbent resin, the pulp, and the exterior material are separated (filtered); and a drying step in which the recovered water-absorbent resin is dried; and the water-absorbent resin is recovered through these steps.

[0049] In this specification, at least the water-absorbent resin recovered by carrying out the wet separation as described above is referred to as a "recycled water-absorbent resin". That is, in one embodiment, the recovered water-absorbent resin may include a recycled water-absorbent resin. In another embodiment, the recovered water-absorbent resin is a recycled water-absorbent resin. Thus, in one embodiment, the water-absorbent resin contained in the mixture is a recycled water-absorbent resin, and the recycled water-absorbent resin includes a recycled water-absorbent resin. In another embodiment, the water-absorbent resin contained in the mixture is a recycled water-absorbent resin, and the recycled water-absorbent resin is a recycled water-absorbent resin (that is, the water-absorbent resin contained in the mixture is a recycled water-absorbent resin).

[0050] In this specification, the steps for obtaining (preparing) the recycled water-absorbent resin as described above are collectively referred to as a "preparation step for recycled water-absorbent resin" or a "pretreatment step."

[0051] In addition, in the recovery of the recycled water-absorbent resin as described above, it is possible to further carry out, as necessary, a crushing step of crushing the exterior material of the absorbent article or the like; a cleaning / disinfecting step of cleaning and / or disinfecting the water-absorbent resin (a water-absorbing material containing the water-absorbent resin); an acid treatment step of treating the water-absorbent resin with an acid; and a neutralization step of neutralizing the remaining acid after the acid treatment step.

[0052] Even if the water absorbent resin has been subjected to the above-described separation and filtration steps, impurities such as pulp are adhered to a part of the recovered water absorbent resin (recycled water absorbent resin). Therefore, in one embodiment, the mixture in the present invention may be a mixture in which the recycled water absorbent resin and the impurities are adhered to each other and integrated.

[0053] [1-8] EDANA and WSP "EDANA" is an abbreviation for European Disposables and Nonwovens Associations, and "WSP" is an abbreviation for World Strategic Partners. EDANA WSP is a European and American standard (almost a global standard) for measuring the physical properties of water-absorbent resins. In the present invention, unless otherwise specified, the physical properties of the water-absorbent resin are measured in accordance with the original EDANA WSP (revised in 2010 / publicly known document).

[0054] [1-8-1] CRC In this specification, the water absorption capacity without load (CRC) is a value measured according to the following method.

[0055] (Measurement of Water Absorption Capacity (CRC) without Load) The "CRC" of a water-absorbent resin is an abbreviation for "Centrifuge Retention Capacity" and indicates the water absorption capacity (unit: g / g) of a water-absorbent resin for 30 minutes under no load with respect to a 0.90 mass % sodium chloride aqueous solution.

[0056] 0.200 g of water-absorbent resin is uniformly placed in a nonwoven fabric bag (85 mm x 60 mm) (manufactured by Nankoku Pulp Industries Co., Ltd., product name: Heatlon Paper, model: GSP-22) and heat-sealed, and then immersed in a large excess (usually about 500 ml) of 0.90% by mass sodium chloride aqueous solution at room temperature. After 30 minutes, the bag is pulled out and drained for 3 minutes using a centrifuge (manufactured by Kokusan Co., Ltd., centrifuge: model H-122) at a centrifugal force (250 G) described in ERT441.2-02, and the mass W1 (g) of the bag is measured. The same operation is also performed without using the water-absorbent resin, and the mass W0 (g) at that time is measured. Then, from these W1 and W0, the CRC (centrifuge retention capacity) is calculated according to the following formula (1):

[0057]

[0058] [1-8-2] Water-soluble content of water-absorbent resin and mixture (16-hour value) In this specification, the water-soluble content (16-hour value) is a value measured according to the following method. In this specification, the "water-soluble content (16-hour value)" is also simply referred to as "water-soluble content" or "water-soluble content" or "soluble polymer amount".

[0059] (Measurement of Water-Extractable Content (16-Hour Value)) 200.0 g of a 0.90% by mass sodium chloride aqueous solution was weighed into a 250 ml glass container with a lid containing a cylindrical rotor having a length of 35 mm and a cross-sectional diameter of 7.5 mm. 1.00 g of a water-absorbent resin or mixture was then added to the aqueous solution, and the container was covered with plastic wrap. The mixture was then stirred (at 500 rpm) for 16 hours at room temperature (25°C) using the cylindrical rotor to extract the water-soluble components in the water-absorbent resin or mixture. After stirring, the extract was filtered using a sheet of filter paper (ADVANTEC Toyo Co., Ltd., product name: (JIS P 3801, No. 2), thickness 0.26 mm, retention particle size 5 μm), and 50.0 g of the resulting filtrate was used as the measurement liquid.

[0060] Next, the measurement solution is titrated with a 0.1N-NaOH aqueous solution until the pH reaches 10, and then titrated with a 0.1N-HCl aqueous solution until the pH reaches 2.7. The titration amounts at this time are calculated as [NaOH] ml and [HCl] ml, respectively.

[0061] Furthermore, the same procedure is carried out using 200.0 g of a 0.90 mass % aqueous sodium chloride solution alone without adding the water-absorbent resin or mixture, and the blank titration amount ([bNaOH] ml and [bHCl] ml) is determined.

[0062] When the water-absorbent resin or mixture is composed of a known amount of acrylic acid and its salt, the amount of water-soluble matter (unit: mass%) in the water-absorbent resin or mixture can be calculated by the following formula (2) based on the average molecular weight of the monomer and the titration amount obtained by the above operation. In the case of a recovered water-absorbent resin, since the average molecular weight of the monomer is unknown, the amount of water-soluble matter as an indicator is calculated by using the neutralization rate calculated by the following formula (3) and the average molecular weight of the monomer calculated by the following formula (4). In the following formula (4), "72" is the molecular weight of acrylic acid, and "94" is the molecular weight of sodium acrylate.

[0063]

[0064]

[0065]

[0066] [1-8-3] AAP0.3 (Water Absorption Capacity Under Pressure) "AAP" of the water absorbent resin of the present invention is an abbreviation for "Absorption Against Pressure" and indicates the water absorption capacity under pressure (unit: g / g) for a 0.90 mass % aqueous sodium chloride solution. Furthermore, "AAP0.3" means the AAP under a pressure of 0.3 psi. The AAP0.3 of the water absorbent resin is measured in accordance with ERT442.2-02. Specifically, a large excess of a 0.9 mass % aqueous sodium chloride solution is used, and 0.9 g of the water absorbent resin is subjected to 2.07 kPa (21 g / cm) for 1 hour. ２ After swelling under a pressure of 0.3 psi (0.3 psi), the AAP (absorbency against pressure) is measured. In this specification, unless otherwise specified, "AAP 0.3", "absorbency against pressure", and "AAP" are treated as synonyms.

[0067] [2] Method for recovering water-absorbent resin according to one embodiment of the first aspect In one embodiment of the present invention, the method for recovering water-absorbent resin includes, in this order, applying a physical force to a mixture in which the water-absorbent resin and impurities are fixed and integrated together, thereby crushing the mixture (crushing step), and separating the water-absorbent resin and the impurities from the crushed mixture (separation step). According to such a recovery method, when recovering a water-absorbent resin from a mixture in which the water-absorbent resin and impurities such as pulp are fixed together, it is possible to obtain a water-absorbent resin (a water-absorbent resin derived from the mixture) in which the amount of impurities mixed in is further reduced.

[0068] Here, the mixture subjected to the disintegration step is a mixture of recovered water-absorbent resin recovered from absorbent articles (used absorbent articles and unused absorbent articles (particularly, absorbent articles discarded before use)) and impurities fixed to and integrated with each other. Specifically, the mixture subjected to the disintegration step may be a mixture of recovered water-absorbent resin recovered from absorbent articles by a known method and impurities, or a mixture of recovered water-absorbent resin and impurities obtained by requesting another company to recover water-absorbent resin from absorbent articles, or a purchased mixture of such recovered water-absorbent resin and impurities. The method for recovering a water-absorbent resin according to the present invention further removes impurities fixed to the recovered water-absorbent resin separated and recovered by a known method, and can obtain a water-absorbent resin with a further reduced amount of impurities mixed in.

[0069] In one embodiment, the mixture subjected to the recovery method according to the present invention may be a mixture of a recycled water absorbent resin and impurities. Therefore, the recovery method according to the present invention may further include a step for obtaining (preparing) a recycled water absorbent resin, i.e., a preparation step (pretreatment step) of a recycled water absorbent resin, before the crushing step.

[0070] Each step will be described below.

[0071] [2-1] Preparation step (pretreatment step) of recycled water-absorbent resin The preparation step (pretreatment step) of recycled water-absorbent resin is an optional step. In this step, recycled water-absorbent resin is obtained by separating the water-absorbent resin (recovered water-absorbent resin) from the constituent members of the absorbent article other than the water-absorbent resin from the absorbent article. In this case, the method for obtaining recycled water-absorbent resin from the absorbent article is not particularly limited, and known methods can be used as is or with appropriate modifications. Note that, since impurities may be fixed to the interior and / or surface of the recycled water-absorbent resin obtained through this step, the product obtained by this step may actually be a mixture of recycled water-absorbent resin and impurities. However, in the following description of this step (pretreatment step), it may also be simply referred to as "recycled water-absorbent resin" or "water-absorbent resin."

[0072] The absorbent article from which the recycled water-absorbent resin is recovered may be a used absorbent article, or a pre-used discarded absorbent article (for example, an absorbent article that has not absorbed bodily fluids such as urine or blood but has absorbed moisture during storage and has been discarded), or a mixture of these. Hereinafter, an example of a method for preparing a recycled water-absorbent resin from an absorbent article (a used absorbent article and / or a pre-used discarded absorbent article) will be described with reference to Fig. 1. Note that the method described below is one example of a method for obtaining a recycled water-absorbent resin, and the method for obtaining a recycled water-absorbent resin to be subjected to the recovery method according to the present invention is not limited to the method described below.

[0073] Fig. 1 is a flow diagram illustrating a method for recovering a water-absorbent resin according to the present invention. As shown in Fig. 1, in one embodiment, the preparation step (pretreatment step) (S100) of the recycled water-absorbent resin includes: a crushing step (S1) of crushing the exterior materials of absorbent articles, etc.; an inactivation / dehydration treatment step (S2) of performing a shrinkage / dehydration treatment (inactivation treatment) on the water-absorbent resin (recovered water-absorbent resin) contained in the absorbent articles crushed in the crushing step (S1); a cleaning / disinfection step (S3) of washing and / or disinfecting the water-absorbent material containing the water-absorbent resin after the inactivation / dehydration treatment step (S2); a separation / filtration step (S4) of separating (filtering) the water-absorbent resin, pulp, and exterior materials; an acid treatment step (S5) of treating the water-absorbent resin recovered in the separation / filtration step (S4) with an acid; a neutralization step (S6) of performing a neutralization reaction; and a drying step (S7) of drying the water-absorbent resin. The order of the above steps may be changed or omitted as necessary, as long as the water-absorbent resin (recovered water-absorbent resin) can be separated from the constituent members of the absorbent article other than the water-absorbent resin. For example, the cleaning / disinfecting step (S3), the acid treatment step (S5), the neutralization step (S6), etc. are optional steps that can be omitted as necessary. In addition to the above, a cleaning step or a concentration step (not shown) may be further performed as necessary.

[0074] (Crushing step (S1)) In this step, the exterior material and other materials constituting the recovered absorbent articles are crushed (shredded). The crushing (shredding) method is not particularly limited, and known methods can be used. At this time, if necessary, the crushed absorbent articles may be dispersed in a solvent to roughly separate the exterior material and the water-absorbing material (rough separation). Here, the separation operation between the exterior material and the water-absorbing material may be performed only once or multiple times. Furthermore, when separating the exterior material from the crushed (shredded) absorbent articles, it may be performed directly without using a solvent, but from the viewpoint of separation efficiency, it is preferable to perform it by a method of dispersing in a solvent (i.e., wet separation). Examples of solvents used for separation include water and organic solvents, but water is preferred. Furthermore, an example of a wet separation method is a method in which the recovered exterior material and water-absorbing material are placed in water and separated into those that sink in the water and those that float on the surface of the water (specific gravity separation).

[0075] (Inactivation and dehydration treatment step (S2)) In this step, the water-absorbent resin (recovered water-absorbent resin) contained in the absorbent article is inactivated and dehydrated (shrunk). Generally, to separate the pulp and water-absorbent resin contained in the absorbent article (water-absorbent material), a method of dispersing the water-absorbent material in a solvent (water or organic solvent) and then separating the material (i.e., a wet separation method) is used. When the absorbent article (water-absorbent material) is dispersed in a solvent in this way, the water-absorbent resin contained in the absorbent article may further swell in the solvent and become a hydrous gel (swollen gel particles). Therefore, when performing the above-mentioned wet separation, from the viewpoint of separation efficiency, it is preferable to separate the water-absorbent resin from components other than the water-absorbent resin, such as an exterior material, by shrinking the water-absorbent resin, thereby utilizing the difference in specific gravity and solubility. The inactivation and dehydration treatment can be performed by a known method. Specifically, the hydrous gel of the water-absorbent resin can be shrunk using a shrinking agent (an acid, an alkali, an inorganic salt, a polyvalent metal salt, etc.) together with the solvent.

[0076] The amount of solvent used (usage amount) is not particularly limited, but as an example, it is preferably 0.1 to 100 times the total mass of the absorbent article, more preferably 0.5 to 50 times, and particularly preferably 1 to 10 times.

[0077] As the shrinking agent, an acid (organic acid or inorganic acid), an alkali, an inorganic salt, a polyvalent metal salt, or the like can be used, and among these, it is preferable to use at least one selected from the group consisting of an organic acid, an inorganic acid, and a polyvalent metal salt.

[0078] As the organic acid or inorganic acid, a water-soluble organic acid or water-soluble inorganic acid having a molecular weight of 500 or less is preferably used. These organic acids and inorganic acids may be used alone or in combination of two or more. The amount of these organic acids and / or inorganic acids used is not particularly limited, but is preferably 0.01 mass % or more and 10 mass % or less with respect to the total mass of these acids (organic acid and inorganic acid) and the water-absorbent resin.

[0079] Examples of the polyvalent metal salt include alkaline earth metal salts and transition metal salts.

[0080] As the alkaline earth metal salt, salts of alkaline earth metals such as beryllium, magnesium, calcium, strontium, and barium can be used, and preferably water-soluble salts are used. Among these, calcium or magnesium salts are preferred. Specific examples of these salts include calcium chloride, calcium nitrate, calcium hydroxide, magnesium chloride, magnesium nitrate, and magnesium hydroxide, and among these, calcium chloride is preferred. The alkaline earth metal salts may be used alone or in combination of two or more.

[0081] As the transition metal salt, salts of transition metals such as iron, cobalt, nickel, copper, etc. can be used, and preferably water-soluble salts are used. Specific examples of these salts include iron salts such as iron lactate, iron chloride, iron sulfate, iron phosphate, and iron nitrate; cobalt salts such as cobalt acetate, cobalt stearate, cobalt chloride, cobalt sulfate, cobalt phosphate, and cobalt nitrate; nickel salts such as nickel acetate, nickel chloride, and nickel sulfate; and copper salts such as copper acetate, copper chloride, and copper sulfate. The transition metal salts may be used alone or in combination of two or more.

[0082] As described above, in one embodiment of the present invention, the water-absorbing resin may include a polymer having a partially neutralized carboxyl group. In this embodiment, the water-absorbing resin may include a polymer having a carboxyl group (—C(═O)OH) and a neutralized carboxyl group (—C(═O)O － A ＋ ; where A ＋ is a monovalent cation (for example, an alkali metal ion such as a sodium ion). In a swollen state, such a water-absorbent resin has a dissociated carboxyl group (C(=O)O － The electrostatic repulsion between the carboxyl groups causes water molecules to enter the gaps, which then absorb and hold the liquid (excrement, etc.). On the other hand, when the polyvalent metal salt is brought into contact with the carboxyl groups, the dissociated carboxyl groups are repelled by polyvalent metal ions (e.g., Ca ２＋ , Mg ２＋ As a result, the swollen water-absorbent resin shrinks, and the liquid that the swollen water-absorbent resin has absorbed is released (i.e., dehydrated).

[0083] The amount of the polyvalent metal salt used is not particularly limited, but is preferably 0.01% by mass or more and 20% by mass or less, and more preferably 0.5% by mass or more and 10% by mass or less, based on the total mass of the polyvalent metal salt and the water-absorbent resin.

[0084] The treatment time in the treatment with the shrinking agent is not particularly limited as long as it is a time sufficient for the water-absorbent resin to shrink and dehydrate, and is, for example, about 5 minutes to 3 hours. The reaction temperature in the treatment with the shrinking agent is not particularly limited as long as it is a temperature sufficient for the water-absorbent resin to shrink and dehydrate, and is, for example, about 5°C to 80°C.

[0085] (Cleaning and disinfection step (S3)) In this step, the water-absorbent resin is washed and / or disinfected. The cleaning and disinfection treatments can be carried out by known methods, for example, by adding water, water containing a disinfectant, or the like and stirring and mixing. In this case, disinfection treatment can also be carried out by adding a disinfectant such as hypochlorous acid, a chlorine dioxide aqueous solution, ozone, hydrogen peroxide water, or electrolyzed water (acidic electrolyzed water) to the solvent. In this case, the amount of disinfectant added is, for example, 0% by mass or more (preferably more than 0% by mass) and 1% by mass or less relative to 100% by mass of the total mass of the absorbent article. The cleaning and disinfection step (S3) may be carried out before the inactivation and dehydration treatment step (S2) or may be omitted.

[0086] (Sorting / filtration step (S4)) In this step, the water-absorbent resin, pulp, and packaging material are separated (sorted). At this time, as described above, the water-absorbent material may contain a large amount of moisture or is in a state of being dispersed in water due to the inactivation / dehydration treatment using a solvent (particularly water) or the washing / disinfection treatment using water. Therefore, in this step, it is preferable that the water-absorbent resin, pulp, and packaging material are each separated by a known method such as filtration, centrifugation, or specific gravity separation.

[0087] The fractionation (separation) method used here can be a known method as is or with appropriate modifications. Examples of equipment used for such fractionation (separation) include a gravity settling tank, a filtration device (e.g., wire mesh), a screw press, a roller press, a rotary drum screen, a belt screen, a vibrating screen, a pressure dehydrator, a vacuum dehydrator, a belt press, a centrifugal concentration dehydrator, and a liquid cyclone.

[0088] (Acid Treatment Step (S5)) In this step, the water absorbent resin recovered in the separation / filtration step (S4) is subjected to an acid treatment. As described above, in the water absorbent resin that has been subjected to the inactivation / dehydration treatment with a shrinking agent (particularly, a polyvalent metal salt), the dissociated carboxyl groups are in an ionically cross-linked state via polyvalent metal ions. Then, such ionically cross-linked carboxyl groups can be converted from the ionically cross-linked state to a free state (C(=O)OH) by the acid treatment. As a result, the water absorption performance (for example, CRC) of the regenerated water absorbent resin can be restored, and the water absorption performance of the regenerated water absorbent resin can be improved.

[0089] The acid treatment is not particularly limited as long as it is in a form that allows contact of an acidic substance with a water absorbent resin, and a known method can be employed as is or with appropriate modification. One example includes a method in which a water absorbent resin (which has been subjected to inactivation and dehydration treatment) is added to a solution containing an acidic substance while stirring the solution, thereby bringing the acidic substance into contact with the water absorbent resin.

[0090] The acidic substance is not particularly limited, and for example, an organic acid and / or an inorganic acid can be used.

[0091] Examples of the organic acid include organic substances having an acid group, such as a carboxyl group or a sulfo group. Examples of the inorganic acid include sulfuric acid, hydrochloric acid, and nitric acid. Specific examples of the organic acid include citric acid, tartaric acid, malic acid, succinic acid, oxalic acid, gluconic acid, pentanoic acid, butanoic acid, propionic acid, glycolic acid, acetic acid, formic acid, and sulfonic acids (e.g., methanesulfonic acid, trifluoromethanesulfonic acid, benzenesulfonic acid, and p-toluenesulfonic acid). These organic acids and inorganic acids may be used alone or in combination of two or more.

[0092] The amount of the acidic substance used is not particularly limited, but is preferably an amount such that the pH of a dispersion containing the acidic substance and a water-absorbing resin falls within the acidic range. The reaction temperature in the treatment with the acidic substance is not particularly limited as long as it is a temperature sufficient for the liberation of carboxyl groups as described above to proceed, and is, for example, about 5°C or higher and 80°C or lower.

[0093] (Neutralization step (S6)) In this step, a neutralization reaction is carried out on the water absorbent resin obtained in the acid treatment step (S5). Specifically, the neutralization step (S6) is a step of adding an alkali metal salt to the water absorbent resin obtained in the acid treatment step (S5). By carrying out such a treatment, a part of the carboxyl groups in a free acid state in the water absorbent resin is neutralized by the alkali metal ions derived from the alkali metal salt. This makes it possible to restore the water absorption performance of the water absorbent resin.

[0094] The alkali metal salt is not particularly limited, and examples thereof include hydroxides, carbonates, and hydrogen carbonates of alkali metals such as lithium, sodium, and potassium. These alkali metal salts may be used alone or in combination of two or more.

[0095] The amount of the alkali metal salt to be added is not particularly limited, but is preferably an amount such that the pH of the water absorbent resin neutralized with the alkali metal salt (regenerated water absorbent resin) is about 4.5 or more and 8.0 or less. In this case, the method for measuring the pH of the water absorbent resin is not particularly limited, and the pH can be measured using, for example, commercially available pH test paper and / or a commercially available pH meter or the like.

[0096] The treatment time for the neutralization with the alkali metal salt is not particularly limited as long as it is a time sufficient for a portion of the carboxyl groups in a free acid state to be neutralized by the alkali metal ions, and is, for example, about 5 minutes to 2 hours. The reaction temperature with the alkali metal salt is also not particularly limited as long as it is a temperature sufficient for a portion of the carboxyl groups in a free acid state to be neutralized by the alkali metal ions, and is, for example, about 5° C. to 90° C.

[0097] After the neutralization treatment, washing treatment with water may be carried out as needed. The type of water used for washing (washing water) may be deionized water, tap water, distilled water, or the like.

[0098] (Drying Step (S7)) In this step, the water-absorbent resin (hydrogel thereof) is dried. This makes it possible to obtain a regenerated water-absorbent resin (actually, a mixture of the regenerated water-absorbent resin and impurities), which is a dried polymer. Note that, although dehydration from the hydrogel of the water-absorbent resin is also performed by the above-mentioned inactivation / dehydration treatment step (S2), sufficient dehydration is not achieved by this step (S2) alone. Therefore, from the viewpoint of improving separation efficiency, in one embodiment of the present invention, the mixture to be subjected to the crushing step (S200) described later is preferably a mixture of the regenerated water-absorbent resin (after the drying step) obtained through the drying step and impurities.

[0099] The method for drying the hydrogel of a water-absorbent resin can be a known method as is or with appropriate modifications, and examples thereof include heat drying, hot air drying, reduced pressure drying, fluidized bed drying, infrared drying, microwave drying, drying by azeotropic dehydration with a hydrophobic organic solvent, and stirring drying in which the material to be dried is dried while being moved. Among these, heat drying, hot air drying, and stirring drying are preferred from the viewpoint of drying efficiency. Examples of devices used for stirring drying include stirring dryers such as paddle dryers and rotary drum dryers.

[0100] From the viewpoint of more efficiently carrying out the crushing step (S200) described below, the drying temperature is preferably 120° C. or higher and 250° C. or lower, more preferably 130° C. or higher and 230° C. or lower, and particularly preferably 140° C. or higher and 200° C. or lower. The drying time is preferably 3 hours or shorter, more preferably 10 minutes or longer and 2 hours or shorter, even more preferably 20 minutes or longer and 1.5 hours or shorter, and particularly preferably 30 minutes or longer and 1 hour or shorter.

[0101] The mixture of the regenerated water-absorbent resin and the impurities obtained as described above is then subjected to a crushing step (S200) and a separation step (S300).

[0102] [2-2] Crushing Step In this step, a physical force is applied to a mixture in which the water-absorbent resin and the impurities are fixed and integrated, and the mixture (i.e., the water-absorbent resin and the impurities) is crushed. In this way, the mixture of the water-absorbent resin and the impurities is crushed to break up the integrated state and make them independent, so that the water-absorbent resin and the impurities can be efficiently separated in the subsequent separation step. Note that the crushing step in the present invention may be any step that performs a treatment to crush a mixture in which the water-absorbent resin and the impurities are fixed and integrated, and at this time, the object to be crushed may be in a state in which the water-absorbent resin and the impurities are each independent. In other words, the object to be subjected to the crushing step may not only be a mixture in which the water-absorbent resin and the impurities are fixed and integrated, but may also further include the water-absorbent resin and the impurities in an independent state.

[0103] In this step, "disintegrating the mixture (a mixture in which the water-absorbent resin and the impurities are bonded together)" refers to disintegrating the bonded state of the mixture in which the water-absorbent resin and the impurities are bonded together, that is, to making the water-absorbent resin and the impurities independently separate from each other. Therefore, even if a physical force is applied to the mixture, if the result is that the water-absorbent resin and the impurities are still bonded together (the state of the mixture is maintained), this operation is not included in the disintegration step according to the present invention. Furthermore, "applying a physical force to the mixture" refers to applying an external force necessary to disintegrate the mixture in which the water-absorbent resin and the impurities are bonded together. In this case, the degree of disintegration is sufficient as long as at least a part of the mixture is separated into the water-absorbent resin and the impurities, and it does not necessarily have to be complete, and may be partial. In this way, when the crushing is partial, as described in the section [1-6] Mixture, the mixture after crushing contains the water absorbent resin and the impurities which are in an independent state, and a mixture which has not been crushed (a mixture in which the water absorbent resin and the impurities are integrated).

[0104] Moreover, from the viewpoint of increasing the separation efficiency of the water absorbent resin and the impurities in the separation step carried out later, it is preferable that the disintegration (preferably pulverization) is carried out without using a solution (in a dry manner) rather than being carried out in a solution such as water.

[0105] In addition, from the viewpoint of ease of crushing, a classification step may be additionally carried out before the crushing step.

[0106] The method of applying a physical force to the integrated mixture is not particularly limited, and examples thereof include stirring, vibration, tearing, cutting (shredding), crushing, and pulverization. Among these, the crushing step is preferably carried out by crushing (preferably pulverizing) the integrated mixture, since this makes it difficult to destroy the pulp, which is the main component of the impurities, and facilitates separation of the water-absorbent resin and the impurities in the subsequent separation step. In this specification, "crushing" is not particularly limited, but refers to processing until the particle diameter of the crushed mixture is preferably less than 4 mm, more preferably less than 3 mm, even more preferably less than 2 mm, particularly preferably 1 mm or less, and most preferably less than 1 mm. Furthermore, when the particle diameter is further reduced (for example, the particle diameter may be less than 500 μm, less than 300 μm, or less than 150 μm), the term "crushing" may be used.

[0107] The apparatus (crushing apparatus) used in the crushing step is not particularly limited, and examples thereof include a pin mill, cyclone mill, roll mill, roller mill, hammer mill, rod mill, ball mill, jet mill, bead mill, vibration mill, knuckle-type crusher, cylindrical mixer, etc. These apparatuses may be used in combination. In particular, the crushing step is preferably carried out using one or more apparatuses selected from the group consisting of a pin mill, cyclone mill, roll mill, roller mill, hammer mill, rod mill, ball mill, jet mill, and bead mill. The use of these apparatuses makes it easy to selectively crush the water-absorbent resin while suppressing destruction of pulp, which is the main component of the impurities. From this perspective, the crushing step is more preferably carried out using one or more apparatuses selected from the group consisting of a pin mill, cyclone mill, roll mill, roller mill, and hammer mill, and a roll mill or hammer mill is particularly preferred.

[0108] The conditions for crushing the mixture are not particularly limited, but as described above, the conditions are preferably such that the water-absorbent resin can be selectively crushed while suppressing the destruction of pulp, which is the main component of the impurities. Specifically, the conditions are preferably such that the particle diameter of the water-absorbent resin contained in the crushed mixture is smaller than the size of the impurities (mainly the size of the pulp) contained in the crushed mixture. That is, the crushing step is preferably performed so that the particle diameter of the water-absorbent resin contained in the crushed mixture is smaller than the size of the impurities (mainly the size of the pulp) contained in the crushed mixture. Note that fibrous materials such as pulp are not necessarily linear, and may be curved, for example. Therefore, when the impurities are fibrous materials such as pulp, the size of the impurities (mainly the size of the pulp) refers to the length (distance) in the long side direction of the smallest rectangle that surrounds the appearance of the impurities (pulp) when contained in the crushed mixture.

[0109] In one embodiment, with regard to the particle size of the water-absorbent resin contained in the mixture after the crushing, 60% by mass or more, more preferably 70% by mass or more, even more preferably 75% by mass or more, particularly preferably 80% by mass or more, and most preferably 90% by mass or more (upper limit: 100% by mass) of the water-absorbent resin contained in the mixture after the crushing may have a particle size (defined by sieve classification) of 3 mm or less, less than 3 mm, 2 mm or less, less than 2 mm, 1 mm or less, less than 1 mm, 0.5 mm (500 μm) or less, less than 0.5 mm (500 μm), 0.3 mm (300 μm) or less, less than 0.3 mm (300 μm), 0.2 mm (200 μm) or less, less than 0.2 mm (200 μm), 0.15 mm (150 μm) or less, or less than 0.15 mm (150 μm). On the other hand, the lower limit of the particle size of the water-absorbing resin contained in the mixture after crushing is not particularly limited, but may be, for example, 0.01 mm (10 μm) or more, or 0.05 mm (50 μm) or more.

[0110] The crushing time is not particularly limited as long as the mixture is crushed. The crushing time can be adjusted as appropriate to obtain the desired effect. At this time, it is preferable to perform crushing while checking the state of the mixture (crushed state) visually or with an SEM, etc. That is, in a preferred embodiment, the recovery method according to the present invention includes observing the crushed mixture after crushing the mixture (after the crushing step) and confirming (or measuring) that the water-absorbent resin and impurities contained in the mixture are in an independent state (not adhered to each other) (observation step). Note that such confirmation (observation) of the crushed state of the mixture may be confirmed by checking whether the crushed mixture passes through a sieve with a predetermined mesh size.

[0111] The temperature during disintegration (temperature of the mixture) is not particularly limited, and is preferably, for example, from room temperature (room temperature: 25°C) to 70°C, more preferably from room temperature (room temperature: 25°C) to 60°C, and particularly preferably from room temperature (room temperature: 25°C) to 50°C.

[0112] From the viewpoint of further improving the separation efficiency, the relative humidity is preferably 65% ​​RH or less, more preferably 60% RH or less, and particularly preferably 55% RH or less. On the other hand, the lower limit is not particularly limited, but an example is 20% RH or more.

[0113] In one embodiment, the mixture subjected to the disintegration step may be a mixture of a recycled water-absorbent resin and impurities. Therefore, in one embodiment of the present invention, the mixture subjected to the disintegration step may be a mixture of a recycled water-absorbent resin obtained through a pretreatment step (a wet separation step for preparing a recycled water-absorbent resin) in which the water-absorbent resin (water-absorbent resin swollen by water absorption) contained in a hydrated absorbent article is separated from the impurities. From the viewpoint of obtaining a water-absorbent resin containing fewer impurities, the mixture of the recycled water-absorbent resin and the impurities is preferably obtained through a wet separation treatment, as exemplified by the explanation in the section [2-1] Preparation step for recycled water-absorbent resin (pretreatment step). In this case, preferred embodiments and conditions of the pretreatment step (preparation step for recycled water-absorbent resin) are as described in the section [2-1] Preparation step for recycled water-absorbent resin (pretreatment step), but the preparation step (pretreatment step) for the recycled water-absorbent resin subjected to the disintegration step is not limited thereto.

[0114] The above-mentioned preparation step (pretreatment step) of the recycled water-absorbent resin can be carried out particularly when obtaining the recycled water-absorbent resin from used absorbent articles. Therefore, in the above-mentioned embodiment, the "hydrated absorbent article" can be a used absorbent article. Therefore, in one embodiment of the present invention, the mixture subjected to the disintegration step can be a mixture of the recycled water-absorbent resin obtained through a pretreatment step (preparation step of the recycled water-absorbent resin by wet separation) of separating the water-absorbent resin contained in the used absorbent article from the impurities, and the impurities.

[0115] Furthermore, the mixture to be subjected to the crushing step may be one prepared without undergoing the above-mentioned wet separation treatment (preparation step (pretreatment step) of recycled water-absorbent resin). For example, in pre-use discarded absorbent articles that have absorbed moisture during storage of the absorbent articles, the water-absorbing material may absorb moisture, and the water-absorbent resin and pulp may be firmly adhered to each other. In order to recover the water-absorbent resin from such pre-use discarded absorbent articles, a mixture may be prepared by crushing (shredding) the exterior material, etc., and then separating the exterior material, etc. and the water-absorbent resin by a known method without undergoing wet separation using a solvent (i.e., dry separation). That is, the water-absorbent resin obtained by crushing pre-use discarded absorbent articles and roughly separating the exterior material, etc. may be directly subjected to the crushing step.

[0116] Therefore, in another embodiment of the present invention, the mixture subjected to the disintegration step may be one obtained by recovering from absorbent articles that have absorbed moisture (particularly, pre-use discarded absorbent articles) (a mixture in which a recovered water-absorbent resin and impurities are integrated). More specifically, in another embodiment of the present invention, the mixture subjected to the disintegration step may be one obtained by recovering from pre-use discarded absorbent articles (in a dry manner) without going through wet separation (a mixture in which a water-absorbent resin and impurities are integrated).

[0117] The mixture (particularly, a mixture of recycled water-absorbent resin and impurities) subjected to the crushing step preferably has a relatively low content of impurities, from the viewpoint of facilitating the production of a water-absorbent resin containing a reduced amount of impurities. Specifically, the content of impurities contained in the mixture (particularly, a mixture of recycled water-absorbent resin and impurities) is preferably less than 20% by mass relative to the total mass of the mixture (total mass being 100% by mass). Furthermore, from the same viewpoint as above, the content of the impurities is more preferably 15% by mass or less, even more preferably 10% by mass or less, and particularly preferably 5% by mass or less, relative to the total mass of the mixture (total mass being 100% by mass). A mixture having an impurity content within the above range can be obtained, for example, by undergoing a wet separation treatment exemplified in the preparation step (pretreatment step) of the recycled water-absorbent resin. That is, in one embodiment, the mixture subjected to the crushing step is a mixture of the recycled water absorbent resin and the impurities, the content of which is within the above range. On the other hand, the lower limit of the content of the impurities is not particularly limited, but, as an example, it is 0.5 mass% or more with respect to the total mass of the mixture (total mass is 100 mass%).

[0118] Furthermore, from the viewpoint of easily obtaining a water-absorbent resin with a reduced amount of impurities, it is preferable that the mixture subjected to the crushing step has a relatively low water-soluble content. The water-soluble content has the effect of adhering impurities to the water-absorbent resin. Therefore, in a mixture with a low water-soluble content (particularly a mixture in which a recycled water-absorbent resin and impurities are integrated), the adhesion between the water-absorbent resin and the impurities is relatively weak, making it easier to separate them. Specifically, the amount of water-soluble content contained in the mixture (particularly a mixture in which a recycled water-absorbent resin and impurities are integrated) is preferably 45% by mass or less relative to the total mass of the mixture (total mass = 100% by mass). Furthermore, from the same viewpoint as above, the amount of water-soluble content is more preferably 35% by mass or less, even more preferably 10% by mass or less, and particularly preferably 5% by mass or less relative to the total mass of the mixture (total mass = 100% by mass). In the case of a mixture of a recycled water-absorbent resin and impurities integrated together obtained by a wet separation treatment exemplified in the preparation step (pretreatment step) of the recycled water-absorbent resin described above, the water-soluble component dissolves in an aqueous solution, separating the water-absorbent resin from the impurities, and therefore the water-soluble component content can achieve the above-mentioned range. That is, in one embodiment, the mixture subjected to the crushing step is a mixture of a recycled water-absorbent resin and impurities integrated together, the water-soluble component content of which is within the above-mentioned range. On the other hand, the lower limit of the water-soluble component content is not particularly limited, but as an example, it is 0.5% by mass or more with respect to the total mass of the mixture (total mass being 100% by mass).

[0119] In the above, the method for measuring the water-soluble content is in accordance with the method described in the section [1-8-2] Water-soluble content of water-absorbent resin and mixture (16-hour value).

[0120] Examples of mixtures in which both the content of impurities and the amount of water-soluble components fall within the preferred ranges include mixtures of recycled water-absorbent resin and impurities obtained by a wet separation process, such as the preparation process (pretreatment process) for the recycled water-absorbent resin described above. In one embodiment, such a mixture of recycled water-absorbent resin and impurities may contain components derived from the shrinking agent because it has undergone an inactivation and dehydration process using a shrinking agent (the inactivation and dehydration process (S2)). Examples of such components include polyvalent metal ions contained in polyvalent metal salts. Therefore, in one embodiment, the content of polyvalent metal ions (when two or more types of polyvalent metal ions are contained, the total amount) contained in the mixture (particularly, the mixture of recycled water-absorbent resin and impurities) subjected to the crushing process may be 0.5% by mass or more relative to the total mass of the mixture (particularly, the mixture of recycled water-absorbent resin and impurities) (the total mass being 100% by mass). The content of the polyvalent metal ions may be 1% by mass or more, or 2% by mass or more, relative to the total mass of the mixture. On the other hand, the upper limit of the content of the polyvalent metal ions contained in the mixture is not particularly limited, but may be, for example, 5% by mass or less.

[0121] Examples of polyvalent metal ions contained in the mixture include alkaline earth metal ions and transition metal ions. For specific examples of alkaline earth metal ions and transition metal ions, see the description in the section (Inactivation and Dehydration Treatment Step (S2)) in the section [2-1] Preparation Step (Pretreatment Step) of Regenerated Water Absorbent Resin. Among these, calcium salts and / or magnesium salts are preferably used as shrinking agents. Therefore, in one embodiment, the content (total amount) of calcium ions and magnesium ions contained in the mixture (particularly, a mixture in which the regenerated water absorbent resin and impurities are integrated) subjected to the crushing step may be 0.3% by mass or more relative to the total mass of the mixture (particularly, a mixture in which the regenerated water absorbent resin and impurities are integrated) (total mass being 100% by mass). Furthermore, the content (total amount) of calcium ions and magnesium ions may be 1% by mass or more, or 1.8% by mass or more, relative to the total mass of the mixture. On the other hand, the upper limit of the content (total amount) of calcium ions and magnesium ions contained in the mixture is not particularly limited, but may be, for example, 5% by mass or less.

[0122] The content of polyvalent metal ions in the mixture is measured using an ICP emission spectrometer as follows: First, 1 g of the mixture is precisely weighed and added to a magnetic crucible, and carbonized in an electric muffle furnace at 550°C for 2 hours. The carbonized mixture is then washed with a 1% by mass (concentration) aqueous nitric acid solution and filtered. The resulting filtrate is then diluted to a volume of 1% by mass in a 50 mL measuring flask. A calibration curve is created using standard solutions of known concentrations (e.g., standard solutions of Ca and Mg), and the amount of polyvalent metals is analyzed using an ICP emission spectrometer.

[0123] Furthermore, from the viewpoint of easily obtaining a water-absorbent resin containing a reduced amount of impurities, the mixture (particularly, a mixture of a recycled water-absorbent resin and impurities) subjected to the crushing step preferably has a low moisture content. In a mixture with a low moisture content, the adhesion between the water-absorbent resin and the impurities is relatively weak, making it easier to separate them. Specifically, the moisture content of the mixture (particularly, a mixture of a recycled water-absorbent resin and impurities) is preferably 20% by mass or less. From the same viewpoint as above, the moisture content of the mixture is more preferably 15% by mass or less, even more preferably 10% by mass or less, particularly preferably 8% by mass or less, and most preferably 5% by mass or less. In order to achieve such a moisture content, the mixture may be dried before being subjected to the crushing step. On the other hand, the lower limit of the moisture content of the mixture is not particularly limited, but, for example, is 3% by mass or more.

[0124] Here, the moisture content of the mixture is determined from the change in mass when 1 g of the mixture is dried by heating at 180° C. for 3 hours. Specifically, it is expressed by the following formula (5).

[0125]

[0126] Furthermore, from the viewpoint of facilitating the production of a water-absorbent resin containing a reduced amount of impurities, the lower the moisture content of the mixture after the crushing (i.e., subjected to the separation step), the more preferable. Also, from the same viewpoint, the smaller the particle size of the mixture after the crushing (i.e., subjected to the separation step), the more preferable. Specifically, the mixture after the crushing (i.e., subjected to the separation step) (particularly, a mixture of a recycled water-absorbent resin and impurities) is preferably in any one of the following forms (I) to (III): (I) In one embodiment, the moisture content of the mixture after the crushing to be subjected to the separation step is 10% by mass or less, and the particle size of the mixture after the crushing to be subjected to the separation step is less than 0.15 mm (150 μm). (II) In one embodiment, the moisture content of the mixture after the crushing to be subjected to the separation step is 8% by mass or less, and the particle size of the mixture after the crushing to be subjected to the separation step is less than 0.3 mm (300 μm). (III) In one embodiment, the water content of the crushed mixture subjected to the separation step is 5% by mass or less, and the particle size of the crushed mixture subjected to the separation step is less than 0.5 mm (500 μm). That is, the crushing step is preferably performed so that the water content and particle size of the crushed mixture are within the above ranges. In each of the above embodiments, the lower limit of the particle size of the crushed mixture subjected to the separation step is not particularly limited, but may be, for example, 0.01 mm (10 μm) or more, or 0.05 mm (50 μm) or more. In this specification, unless otherwise specified, "particle size" refers to the mass average particle size (D50) and is a value calculated by the following procedure. In each of the above embodiments, the lower limit of the water content is not particularly limited, but may be, for example, 3% by mass or more.

[0127] (Method for measuring mass average particle diameter (D50)) 10.0 g of the water-absorbent resin or the crushed mixture is placed on a JIS standard sieve (The IIDA TESTING SIEVE / inner diameter: 80 mm) (JIS Z 8801-1 (2000)) or a sieve equivalent to the JIS standard sieve, and classified for 5 minutes using a vibration classifier (IIDA SIEVE SHAKER / Type; ES-65 type, Ser. No. 0501). The openings of the JIS standard sieve or an equivalent sieve are 850 μm, 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, 212 μm, 150 μm, and 45 μm.

[0128] After the classification, a particle size distribution is determined from the mass of the water-absorbent resin remaining on each sieve (when the measurement target is a mixture after crushing, the mass of the mixture after crushing), and for the obtained particle size distribution, a graph is created by plotting the residual percentage R of each particle size on logarithmic probability paper, and from the graph, the particle size corresponding to R = 50 mass% is read as the mass average particle size (D50). Note that the mass average particle size (D50) is a particle size corresponding to 50 mass% of the entire water-absorbent resin (when the measurement target is a mixture after crushing, the entire mixture after crushing).

[0129] [2-3] Separation step In this step, the water-absorbent resin and impurities are separated from the mixture crushed in the crushing step. The water-absorbent resin and impurities separated from the mixture crushed in the crushing step are in a state in which they are easily separated from each other. Therefore, when separating in this step, the water-absorbent resin and the impurities are easily separated, and a water-absorbent resin containing a reduced amount of impurities can be obtained.

[0130] The water-absorbing resin separated from the crushed mixture may adhere to impurities again when it absorbs water (moisture). Therefore, in order to avoid this, the separation step is preferably carried out in a dry manner.

[0131] The device used for carrying out the separation step is not particularly limited as long as it can separate the water absorbent resin from the impurities, but in one embodiment, the separation step is carried out using a sieve classification device and / or a dry classification device.

[0132] Specific examples include sieve classifiers such as vibrating sieves, low-head screens, electromagnetic screens, and tumbling sieves, as well as dry classification devices such as various air classifiers, including air classifiers that use centrifugal force such as micron separators and cyclone centrifuges, air classifiers that use gravity, and air classifiers that use inertial force. These devices may also be used in combination.

[0133] This step may be performed multiple times (two or more times). Performing the separation step multiple times (two or more times) may enable more efficient removal of impurities. In this case, it is preferable to appropriately change conditions such as the type of sieve and the classification method. In one embodiment, the separation step may be any of the following forms (i) to (iii): (i) comprising at least one separation step using a sieve classifier and at least one separation step using an air classifier, in any order; (ii) comprising at least two separation steps using a sieve classifier; (iii) comprising at least two separation steps using an air classifier. Note that the form (ii) does not comprise a separation step using an air classifier, and the form (iii) does not comprise a separation step using a sieve classifier, and these can be distinguished from the form (i). In the above embodiments (i) to (iii), the upper limit of the number of times (total number of times) the separation step is carried out is not particularly limited, but is, for example, 10 times or less in total.

[0134] As a specific example of the embodiment (i), for example, the first separation may be performed using a sieve classifier, and the second separation may be performed using an air classifier. Alternatively, the separation using the sieve classifier and the separation using the air classifier may be performed in the reverse order. That is, the first separation may be performed using an air classifier, and the second separation may be performed using a sieve classifier.

[0135] Furthermore, the above-mentioned embodiment (i) may be a form in which separation using a sieve classifier is performed two or more times and separation using an air classifier is performed at least once or more times, or a form in which separation using a sieve classifier is performed at least once and separation using an air classifier is performed two or more times. The order of the separation steps using these classifiers / classifiers is not important and may be performed in any order. That is, a total of three or more separation operations (separation steps) may be performed in any order using a sieve classifier and a separation using an air classifier. For example, a method in which separation using a sieve classifier is performed in the first separation, separation using an air classifier is performed in the second separation, and separation using a sieve classifier again in the third separation may be used.

[0136] In the above embodiments (i) to (iii), the type and conditions of the classifying device / classifier may be changed for each separation operation. For example, in the above embodiment (i), the first separation may be performed using a vibrating screen, the second separation may be performed using an air classifier that uses centrifugal force, and the third separation may be performed using an air classifier that uses gravity.

[0137] As the sieve classifier used in the above embodiments (i) and (ii), a vibrating sieve machine is preferably used. As the air classifier used in the above embodiments (i) and (iii), an air classifier using centrifugal force is preferably used, and among these, a cyclone centrifuge is more preferably used. In other words, the separation step using the air classifier is preferably a separation step using a cyclone centrifuge.

[0138] Among the above embodiments (i) to (iii), the separation step according to the present invention is preferably carried out by the above embodiment (i). More preferably, the separation step may include at least one separation step using a sieve classifier and at least one separation step using an air classifier, in this order. In this case, the separation step using the air classifier may include a separation step using a cyclone centrifuge. For example, a method may be used in which the first separation is performed using a vibrating sieve, and the second separation is performed using a cyclone centrifuge. That is, in another embodiment, the separation step may include a separation step using a sieve classifier and a separation step using a cyclone centrifuge, in this order. By combining different separation methods as in the above embodiment, not only can the amount of impurities mixed in be further reduced, but the recovery rate of the water-absorbent resin, as described below, can also be improved. Therefore, according to the present invention, there is also provided a method for improving the recovery rate of a water-absorbent resin, which comprises, in this order: a crushing step of applying a physical force to a mixture in which a water-absorbent resin and impurities are fixed together and integrated, thereby crushing the mixture; and a separation step of separating the water-absorbent resin and the impurities from the mixture crushed in the crushing step, wherein the separation step is in any of the forms (i) to (iii). In this case, the separation step is preferably in the form (i), and more preferably comprises a separation step using a sieve classifier and a separation step using an air classifier, in any order. Furthermore, in the method for improving the recovery rate of a water-absorbent resin, the separation step may comprise a separation step using a sieve classifier and a separation step using a cyclone centrifuge, in any order.

[0139] Here, the recovery rate of the water-absorbent resin (water-absorbent resin recovery rate) indicates the mass ratio (ratio) of the water-absorbent resin recovered after the final separation step to the total mass of the post-disintegration mixture (i.e., the post-disintegration mixture to be subjected to the final separation step), including the water-absorbent resin, impurities, and the integrated mixture that was not completely disintegrated immediately before the final separation step, and is expressed by the following formula (6). Note that the "final separation step" referred to here refers to the separation step when the separation step is performed only once. Furthermore, when the separation step is performed multiple times, the "final separation step" refers to the separation step performed last. For example, when the separation step is performed twice, the "final separation step" refers to the second separation step. In this case, the recovery rate is the mass ratio (ratio) of the water-absorbent resin recovered after the second separation step to the total mass of the post-disintegration mixture immediately before the second separation step.

[0140]

[0141] As described above, one of the reasons why the separation performance of impurities is improved by combining separation using a sieve classifier and separation using an air classifier is the effect of "combining different separation methods." For example, a sieve classifier separates the water-absorbent resin and impurities by utilizing the difference in "size" between them, whereas an air classifier separates them by utilizing the difference in "specific gravity" between them, and therefore it is presumed that a synergistic effect in separation performance can be obtained. However, the smaller the particle size of the water-absorbent resin and the size of the impurities contained in the mixture after crushing become, the smaller the difference in specific gravity between the water-absorbent resin and the impurities becomes, so there is a possibility that the effect obtained by combining a sieve classifier and an air classifier will be reduced.

[0123] In such a case, when it is desired to particularly improve the separation efficiency of impurities (that is, when it is desired to reduce the remaining amount of impurities contained in a water absorbent resin as much as possible, although the recovery rate of the water absorbent resin may be low to some extent), a method may be adopted in which a low particle size part (a fraction having a small particle size) containing a large amount of impurities that are difficult to separate is separated by a sieve classification operation or the like, and then the small particle size part is removed (discarded).

[0142] In addition, the mixture (a mixture in which the remaining water-absorbent resin and impurities that were not completely crushed are integrated) may be crushed between each separation. That is, in one embodiment, the recovery method according to the present invention may have a first crushing step, a first separation step, a second crushing step, and a second separation step in this order. Furthermore, in the embodiment, it is preferable that after performing the first separation step, the state of the mixture after crushing is observed, and then the second crushing step is performed. By adopting such a form, a fraction from which impurities have not been sufficiently separated in the first separation step can be subjected to the second crushing step, and therefore the recovery rate and purity of the water-absorbent resin can be improved.

[0143] The humidity during the separation step is not particularly limited, but from the viewpoint of further improving the separation efficiency, the relative humidity is preferably 65% ​​RH or less, more preferably 60% RH or less, and particularly preferably 55% RH or less. On the other hand, the lower limit is not particularly limited, but an example is 20% RH or more.

[0144] Furthermore, due to the moisture contained in the disintegrated mixture, the disintegrated mixture may adhere to the inner wall surface (inner side wall) of the separation device (generation of deposits), and such adhered material may aggregate during the separation operation and then peel off to form coarse aggregates (generation of agglomerates). For this reason, in one embodiment, the separation device may be used in a heated state or in a state of being kept warm. By heating or keeping the separation device in this manner, the generation of such deposits and aggregates can be effectively suppressed. Here, "heating" refers to heating the inner wall surface of the separation device to 40°C or higher, preferably 50°C or higher. On the other hand, the upper limit of the heating temperature is not particularly limited, but is preferably 100°C or lower in order to suppress denaturation of the water-absorbent resin. Furthermore, "keeping warm" refers to maintaining the inner wall surface of the separation device at 40°C or higher and 100°C or lower, and may be 50°C or higher and 90°C or lower.

[0145] Examples of methods for heating and / or maintaining the temperature of the separation apparatus include providing a heating means or heat-retaining means inside or outside the separation apparatus, or attaching such a means to the separation apparatus. Heating methods using such heating means are not particularly limited as long as they can heat the separation apparatus, and include, for example, providing the separation apparatus with a jacket that can be heated by electricity or steam, or wrapping a heating resistor around the separation apparatus. Furthermore, examples of methods for maintaining the temperature using heat-retaining means include wrapping the separation apparatus with a heat insulating material (thermal insulation). The heat insulating material (thermal insulation material) used is not particularly limited, and examples include fibrous heat insulating materials such as asbestos heat insulating material, rock wool heat insulating material, glass wool heat insulating material, and heat-resistant inorganic fiber heat insulating material; powdery heat insulating materials such as calcium silicate heat insulating material and aqueous perlite heat insulating material; foam heat insulating materials such as polystyrene foam heat insulating material, rigid urethane foam heat insulating material, and multi-cellular glass heat insulating material; and air-layer heat insulating materials such as metal foil heat insulating material and paper honeycomb. Two or more of the heating means / heat-retaining means may be used in combination, or both the heating means and the heat-retaining means may be used.

[0146] Furthermore, as another method for suppressing adhesion (formation of deposits) to the inner wall surface of the separation device and the formation of coarse aggregates due to the moisture contained in the crushed mixture, the following method may be used. That is, in another embodiment, a method of constructing the inner wall surface of the separation device using a hydrophobic material is exemplified. By forming the inner wall surface of the separation device using such a hydrophobic material, the crushed mixture is less likely to adhere, and the formation of the deposits and aggregates can be suppressed. Such a hydrophobic material is preferably a material having a contact angle with water of 60° or more and a heat distortion temperature of 70°C or more. Here, the heat distortion temperature refers to a value measured in accordance with ASTM D648. When the contact angle with water of the contact surface material is 60° or more, the crushed mixture containing moisture is less likely to adhere to the inner wall surface (inner side wall) of the separation device. Furthermore, when the contact surface material has a heat distortion temperature of 70°C or more, the resistance to the heated (high-temperature) separation operation of the crushed mixture is improved, allowing stable separation operation to be continued for a long period of time. The upper limits of the contact angle and the heat distortion temperature are not particularly limited, but may be 100° or less and 150° C. or less, respectively. Examples of such materials (hydrophobic materials) include synthetic resins such as polyethylene, polypropylene, polyester, polyamide, fluororesin, polyvinyl chloride, epoxy resin, and silicone resin; inorganic fillers such as glass, graphite, bronze, and molybdenum disulfide; and composites obtained by mixing organic fillers such as polyimide with the synthetic resins.

[0147] The water-absorbent resin (water-absorbent resin derived from a mixture) obtained through the recovery method according to the present invention as described above has an extremely low content of impurities. Specifically, in the recovered water-absorbent resin (water-absorbent resin derived from a mixture), the content of impurities may be 5% by mass or less relative to the total mass of the water-absorbent resin (including impurities) (total mass being 100% by mass). Furthermore, the content of the impurities may be 3% by mass or less, or 2% by mass or less. Meanwhile, the lower limit thereof may theoretically be 0% by mass.

[0148] [2-4] Recycling of Water Absorbent Resin Derived from Mixture

[0123] There is no problem even if the water absorbent resin recovered through the crushing step and the separation step (in this specification, also referred to as "water absorbent resin derived from the mixture") is reused (recycled) as it is, but particularly, a water absorbent resin having a particle size of less than 150 µm (hereinafter, also simply referred to as "fine powder") is preferably reused through the following fine powder recovery step.

[0149] In general, production of a water absorbent resin can be carried out through, for example, a step of preparing an aqueous monomer solution, a polymerization step, an optional hydrogel crushing step, a drying step, an optional crushing step after drying, an optional classification step, an optional fine powder recovery step, a surface cross-linking step, an optional cooling step, and an optional sizing step. In the production of such a water absorbent resin, it is preferable that the water absorbent resin derived from the mixture obtained through the recovery method according to the present invention is recovered in the fine powder recovery step.

[0150] The monomer aqueous solution preparation step is a step of preparing an aqueous solution (hereinafter referred to as "monomer aqueous solution") containing, as a main component, an unsaturated monomer (preferably a carboxyl group-containing unsaturated monomer, more preferably acrylic acid (salt)) that serves as a raw material for the water-absorbent resin. The polymerization step is a step of polymerizing the unsaturated monomer in the monomer aqueous solution obtained in the monomer aqueous solution preparation step to obtain a hydrogel-like crosslinked polymer (hereinafter also referred to as "hydrogel"). The hydrogel crushing step is a step of gel-crushing the hydrogel obtained in the polymerization step. The drying step is a step of drying the hydrogel and / or hydrogel particles obtained in the polymerization step and / or hydrogel crushing step until the desired resin solid content is reached, thereby obtaining a dried water-absorbent resin product (hereinafter also referred to as "dried product"). The crushing step is a step of appropriately crushing the dried product obtained in the drying step. The classification step is a step of classifying the dried product obtained in the drying step or the pulverized product obtained in the pulverization step after drying to obtain a particulate dried product having a desired particle size. The surface cross-linking step is a step of providing a portion with a higher cross-link density on the surface layer of the particulate dried product (hereinafter also referred to as "water-absorbent resin precursor") obtained through the above-mentioned steps. The cooling step is a step of cooling the surface-cross-linked particulate dried product. The sizing step is a step of adjusting the particle size of the surface-cross-linked water-absorbent resin.

[0151] Hereinafter, a preferred embodiment in which the water-absorbent resin derived from the mixture is recovered in the fine powder recovery step will be described.

[0152] The fine powder collecting step refers to a step of collecting (mixing) a fine powder contained in a water absorbent resin derived from a mixture and / or a water additive thereof in any step (preferably, a step before a drying step) of a general production step of a water absorbent resin. In one embodiment, the fine powder contained in the water absorbent resin derived from the mixture can be collected in a state as it is or after hydration or granulation in any step (preferably, a step before a drying step, more preferably, a step of preparing an aqueous monomer solution, a polymerization step, a hydrous gel crushing step or a drying step, particularly preferably, the polymerization step, the hydrous gel crushing step or the drying step) of a general production step of a water absorbent resin.

[0153] Here, the "fine powder contained in the water absorbent resin derived from the mixture" refers to particles having a particle diameter of less than 150 μm and contained in an amount of 70 mass % or more relative to the total mass of the water absorbent resin derived from the mixture.

[0154] The form of "recovery" of the fine powder contained in the water absorbent resin derived from the mixture includes a form in which the fine powder is recovered as it is, and a form in which the fine powder is recovered as a granulated product. In addition, the granulated product may be recovered in a gelled state by swelling with an appropriate amount of water.

[0155] In one embodiment, depending on the process for recovering the fine powder or granules thereof contained in the water-absorbent resin derived from the mixture, an additive may be mixed with the fine powder or granules thereof. The additive is not particularly limited as long as it is one used in the production of a water-absorbent resin, and examples thereof include water, a crosslinking agent, a binder other than water (e.g., a water-soluble polymer, a thermoplastic resin, etc.), a polymerization initiator, a reducing agent, a chelating agent, a coloring inhibitor, and inorganic fine particles. For example, when water is added, the amount of water added is preferably 1% by mass or more and 1000% by mass or less relative to the total mass (based on 100% by mass) of the water and the solid content of the fine powder (or the solid content of the granules when the fine powder is recovered as a granule). When an additive other than water is added, the amount of the additive added is preferably 0.01% by mass or more and 10% by mass or less relative to the total mass (based on 100% by mass) of the additive other than water and the solid content of the fine powder.

[0156] Methods for recovering fine powder in the preparation step of an aqueous monomer solution are disclosed in WO 92 / 001008, WO 92 / 020723, etc. Methods for recovering fine powder in the polymerization step are disclosed in WO 2007 / 074167, WO 2009 / 109563, WO 2009 / 153196, WO 2010 / 006937, etc. Methods for recovering fine powder in the drying step are disclosed in U.S. Pat. No. 6,228,930, etc.

[0157] The amount of the fine powder or granulated material thereof contained in the recovered water-absorbent resin derived from the mixture is adjusted appropriately depending on the process in which the fine powder or granulated material thereof is recovered.

[0158] In one embodiment, when the fine powder or its granules are recovered in the step of preparing the aqueous monomer solution, the fine powder or its granules is recovered in an amount of preferably 30% by mass or less, more preferably 28% by mass or less, even more preferably 20% by mass or less, particularly preferably 15% by mass or less, and most preferably 10% by mass or less, based on the total mass of the solid content of the monomer and the solid content of the fine powder (or its granules) (total mass being 100% by mass). In this case, the lower limit of the recovered amount is, for example, 1% by mass or more. By recovering the fine powder or its granules within the recovered amount range, a hydrogel-like granule can be obtained. The hydrogel-like granules can then be recovered in a drying step or in a step prior to the drying step (polymerization step, hydrogel crushing step) and dried together with the hydrogel produced in the polymerization step.

[0159] Furthermore, by controlling the recovery amount of the fine powder or a granule thereof, the ratio (solid content ratio) of the primary particles of the hydrogel derived from the monomer to the fine powder of the water-absorbent resin derived from the mixture or a granule thereof in the obtained water-absorbent resin can be appropriately controlled.

[0160]

[0033] In addition, as another embodiment, when a fine powder or a granule thereof contained in a water-absorbent resin derived from a mixture is mixed with a hydrogel derived from a monomer, from the viewpoint of the water absorption performance and productivity of the obtained water-absorbing agent (water-absorbent resin), the amount of the fine powder or a granule thereof relative to the hydrogel is recovered in an amount, in terms of solid content, of preferably 30% by mass or less, more preferably 28% by mass or less, even more preferably 20% by mass or less, particularly preferably 15% by mass or less, and most preferably 10% by mass or less, relative to the total mass of the solid contents of the hydrogel and the fine powder (or a granule thereof) (total mass being 100% by mass). In this case, the lower limit of the recovered amount is, for example, 1% by mass or more.

[0161] [3] Examples of aspects and forms of the present invention The present invention includes the following aspects and forms.

[0162] 1. A method for recovering a water-absorbent resin, comprising, in this order: a crushing step of applying a physical force to a mixture formed by bonding and integrating a water-absorbent resin and impurities to crush the mixture; and a separation step of separating the water-absorbent resin and the impurities from the mixture crushed in the crushing step; 2. The recovery method according to 1. above, wherein the content of the impurities contained in the mixture (the mixture to be subjected to the crushing step) is less than 20 mass% with respect to the total mass of the mixture; 3. The recovery method according to 1. or 2. above, wherein the water content of the mixture (the mixture to be subjected to the crushing step) is 20 mass% or less; 4. The recovery method according to any of 1. to 3. above, wherein the amount of water-soluble content of the mixture (the mixture to be subjected to the crushing step) is 0.5 mass% or more; 5. The recovery method according to any one of 1. to 4. above, wherein the crushing step is carried out using one or more types of equipment selected from the group consisting of a pin mill, a cyclone mill, a roll mill, a roller mill, a hammer mill, a rod mill, a ball mill, a jet mill, and a bead mill; 6. The recovery method according to any one of 1. to 5. above, wherein the separation step is carried out using a sieve classifier and / or a dry classifier; 7. The recovery method according to any one of 1. to 6. above, wherein the separation step is in any of the following forms (i) to (iii): (i) comprising at least one separation step using a sieve classifier and at least one separation step using an air classifier, in any order; (ii) comprising at least two separation steps using a sieve classifier; (iii) comprising at least two separation steps using an air classifier; 8. The recovery method according to 7. above, wherein the separation step using an air classifier is a separation step using a cyclone centrifugal separator; 9. The recovery method according to any one of 1. to 8. above, which satisfies any one of the following (I) to (III): (I) the water content of the crushed mixture to be subjected to the separation step is 10% by mass or less, and the particle size of the crushed mixture to be subjected to the separation step is less than 150 μm; (II) the water content of the crushed mixture to be subjected to the separation step is 8% by mass or less, and the particle size of the crushed mixture to be subjected to the separation step is less than 300 μm;(III) The water content of the crushed mixture to be subjected to the separation step is 5% by mass or less, and the particle size of the crushed mixture to be subjected to the separation step is less than 500 μm; 10. The recovery method according to any one of 1. to 9., wherein the impurities include pulp; 11. The recovery method according to any one of 1. to 10., wherein the mixture (the mixture to be subjected to the crushing step) is obtained through a pretreatment step of separating the water-absorbent resin contained in a water-absorbent article from the impurities; 12. The recovery method according to any one of 1. to 10., wherein the mixture (the mixture to be subjected to the crushing step) is obtained by recovering it from a moisture-absorbent article;

[0163] The effects of the present invention will be explained using the following examples and comparative examples. However, the technical scope of the present invention is not limited to the following examples, and examples obtained by appropriately combining the technical means disclosed in each example are also included in the scope of the present invention. In the following examples, unless otherwise specified, operations were performed at room temperature (25°C). Furthermore, unless otherwise specified, "%" and "parts" mean "% by mass" and "parts by mass," respectively. Furthermore, unless otherwise specified, operations and measurements of physical properties, etc. were performed under conditions of room temperature (20°C or higher and 25°C or lower) and relative humidity of 40% RH or higher and 65% RH or lower.

[0164] [Production Example 1] Preparation of a model of a dried water-absorbent resin containing impurities 1 A commercially available disposable diaper was decomposed to obtain a mixture of pulp and a water-absorbent resin, and then the mixture was dried in a vacuum dryer (temperature inside: 90°C) for 3 hours to obtain a dried product. Thereafter, the dried product was classified using a JIS standard sieve with a mesh size of 2 mm to remove the water-absorbent resin from the mixture, thereby obtaining pulp (preparation of model pulp).

[0165] Subsequently, 5 parts by mass of the pulp obtained by the above procedure was added to 1,500 parts by mass of ion-exchanged water, and the mixture was stirred with a propeller blade for 1 hour to obtain a pulp dispersion. This pulp dispersion was added to 95 parts by mass of a separately prepared water-absorbent resin (A) (polyacrylic acid (salt)-based resin, absorbency (CRC) of 50 g / g, water-soluble content of 25% by mass) using a Lödige mixer (manufactured by Matsubo Co., Ltd.) to obtain a hydrogel (swollen body of water-absorbent resin) (G1) containing the pulp.

[0166] The hydrogel (G1) containing pulp obtained by the above operation was dried in a dryer (temperature inside: 150°C) for 3 hours to obtain a dried water-absorbent resin (S1) containing pulp as an integrated mixture according to the present invention. The dried water-absorbent resin (S1) had a moisture content of 10% by mass and a water-soluble component of 23% by mass. The moisture content and the water-soluble component values ​​were the same for the mixtures after crushing (roughly crushed product (C1), crushed product (1-1), pulverized product (1-1) and pulverized product (1-2)). The content of pulp relative to the total mass of the dried water-absorbent resin (S1) was 5% by mass.

[0167] Comparative Example 1 When the dried water-absorbent resin (S1) containing pulp obtained in Production Example 1 was observed, it was found that the water-absorbent resin and the pulp were fixed and integrated together, as shown in FIG.

[0168] Next, the dried water-absorbent resin containing pulp (S1) was roughly crushed to a degree that could be handled. Specifically, using a jaw crusher (Excel Jaw Crusher manufactured by Yoshida Seisakusho Co., Ltd.), the dried water-absorbent resin containing pulp was roughly crushed to a particle size of 4 to 5 mm to obtain a roughly crushed product (C1).

[0169] When the obtained roughly crushed material (C1) was observed, it was found that the water-absorbent resin and the pulp were integrated together, similar to the dried water-absorbent resin (S1), and the water-absorbent resin and the pulp were not in a crushed state. Furthermore, the roughly crushed material (C1) was subjected to sieve classification and air classification (cyclone classification), but it was not possible to separate the water-absorbent resin and the pulp. Therefore, the rough crushing operation in Comparative Example 1 does not correspond to the crushing step according to the present invention.

[0170] [Example 1-1] (First crushing step) The entire amount of the roughly crushed material (C1) obtained in Comparative Example 1 was further crushed to the extent that it could pass through a JIS standard sieve with a mesh size of 1 mm. Specifically, the crushed material (1-1) was obtained by crushing using a hammer mill (Atomizer manufactured by Dalton Co., Ltd.) equipped with a screen with a mesh size of 2 mm.

[0171] (First Separation Step) The obtained crushed material (1-1) was classified using JIS standard sieves with mesh sizes of 850 μm, 500 μm, and 150 μm. The 850 μm / 500 μm particle fraction ( FIG. 3 ), 500 μm / 150 μm particle fraction ( FIG. 4 ), and 150 μm passing particle fraction ( FIG. 5 ) obtained by this classification were observed with an SEM (magnification: 30x each).

[0172] As shown in Figures 3 and 4, the 850 µm / 500 µm particle fraction and the 500 µm / 150 µm particle fraction had integrated water-absorbent resin and pulp, similar to the roughly crushed material (C1). On the other hand, as shown in Figure 5, for the 150 µm passing particle fraction, almost no integrated water-absorbent resin and pulp were observed, and each was present in a separate state (independent state).

[0173] (Second crushing step) The 850 μm / 500 μm particle fraction and 500 μm / 150 μm particle fraction (i.e., particle fraction that could not pass through a 150 μm mesh sieve) obtained by the above operation were further crushed using the hammer mill used in the first crushing step until they passed through a 150 μm mesh sieve.

[0174] (Second Separation Step) Thereafter, the 150 μm passing particle fraction obtained in the first separation step and the 150 μm passing particle fraction obtained in the second crushing step were mixed to obtain a pulverized product (1-1). The pulp was removed from this pulverized product (1-1) using a cyclone centrifuge.

[0175] The cyclone centrifugal separator has a dome diameter of 600 mm and an air volume of 13 m ３ / min, and a pulp / water-absorbent resin supply rate of 1 kg / min. As a result, the recovery rate of the water-absorbent resin (1-1) derived from the mixture from the bottom of the cyclone was 94 mass%. When the water-absorbent resin (1-1) derived from the mixture from the bottom of the cyclone was observed with an SEM, it was found that there was almost no pulp present, and only a single water-absorbent resin was present.

[0176] Example 1-2 The same procedure as in Example 1-1 was carried out to obtain a pulverized product (1-2) that had passed through a sieve with 150 μm openings.

[0177] In the second separation step of Example 1-1, instead of using a cyclone centrifuge, a JIS standard sieve with a mesh size of 106 μm was used to separate the pulp from the pulverized material (1-2). The recovery rate of the water-absorbent resin (1-2) derived from the mixture that was recovered after passing through the sieve was 82% by mass. Furthermore, when the water-absorbent resin (1-2) derived from the mixture was observed with an SEM, almost no pulp was present, and only a single water-absorbent resin was present.

[0178] [Production Example 2] Preparation of a dried water-absorbent resin model containing impurities (a model of a mixture of recycled water-absorbent resin and impurities) 2 A used absorbent article model was prepared by pouring 200 g of a 0.9% by mass sodium chloride aqueous solution into a commercially available disposable diaper. Approximately 10 g of water-absorbent resin was used per diaper, and the water-absorbent resin had a water absorption capacity (CRC) of 35 g / g. The proportion of the water-absorbent resin in the water-absorbent material composed of pulp and water-absorbent resin was approximately 49% by mass (proportion to the total mass of the water-absorbent material).

[0179] Next, the used absorbent article was further shredded into pieces of 2 to 3 cm, and then a large excess of water (ion-exchanged water) was added to the shredded material and stirred to obtain an aqueous dispersion consisting of pulp, nonwoven fabric, thermoplastic binder, and swollen gel particles of water-absorbent resin.

[0180] Next, 1 g of calcium chloride was added per 100 g of the aqueous dispersion, and the dispersion was treated at room temperature for 1 hour to shrink the swollen gel particles of the water-absorbent resin (partially neutralized sodium polyacrylate crosslinked polymer) to a particle diameter of 2 mm or less. The swollen gel particles were allowed to settle due to the difference in specific gravity, and were separated from the pulp, nonwoven fabric, and thermoplastic binder floating in the water.

[0181] The nonwoven fabric and thermoplastic binder were removed from the separated matter floating in the water by manual sorting, and the remaining pulp was dried for 3 hours in a vacuum dryer at 90° C., and then its mass was measured. By subtracting the mass of the separated pulp from the mass of pulp contained in the original absorbent material of the commercially available diaper, the proportion of pulp contained in the settled swollen gel particles was calculated to be 10% by mass (the proportion relative to the total mass of the water-absorbent resin and pulp, converted into solid content, contained in the gel particles).

[0182] Thereafter, the swollen gel particles shrunk and precipitated by calcium chloride were further filtered through a wire mesh and separated as a shrunk product, thereby obtaining a hydrogel (swollen water-absorbent resin) (G2) containing pulp. The obtained hydrogel (G2) was dried in a dryer (temperature inside the dryer: 150°C) for 40 minutes to obtain a dried water-absorbent resin (S2) containing pulp as an integrated mixture according to the present invention. The dried water-absorbent resin (S2) had a moisture content of 4 mass% and a water-soluble component of 1 mass%. The moisture content and the water-soluble component were the same for the mixture after crushing (coarsely crushed product (C2), crushed product (2-1), pulverized product (2-1), and pulverized product (2-2)). The pulp content relative to the total mass of the dried water-absorbent resin (S2) was 51 mass%.

[0183] [Comparative Example 2] When the dried water absorbent resin (S2) containing pulp obtained in Production Example 2 was observed, it was found that the water absorbent resin and the pulp were fixed together and integrated, similarly to Comparative Example 1, and the water absorbent resin and the pulp were not in a disintegrated state.

[0184] Next, using a jaw crusher in the same manner as in Comparative Example 1, the dried water-absorbent resin (S2) was roughly crushed until the particle size became 4 to 5 mm, thereby obtaining a roughly crushed material (C2). Furthermore, the roughly crushed material (C2) was subjected to sieve classification and air flow classification (cyclone classification), but the roughly crushed material (C2) contained an integrated water-absorbent resin and pulp, and it was not possible to separate the water-absorbent resin and the pulp. Therefore, the rough crushing operation in Comparative Example 2 does not correspond to the crushing step according to the present invention.

[0185] [Example 2-1] (Crushing step) The dried water absorbent resin (S2) containing pulp obtained in Production Example 2 was crushed using a roll mill (manufactured by Inokuchi Giken Co., Ltd.: roll mill) until the particle size became 1000 μm (1 mm) or less, to obtain a crushed product (2-1).

[0186] (Separation Step) Next, the crushed material (2-1) was classified using JIS standard sieves with mesh sizes of 850 μm, 500 μm, 300 μm, and 150 μm to obtain a 850 μm / 500 μm particle fraction, a 500 μm / 300 μm particle fraction, a 300 μm / 150 μm particle fraction, and a 150 μm passing particle fraction.

[0187] When each particle fraction obtained by the classification operation was observed with an SEM, it was found that the water-absorbent resin and pulp were integrated in the 850 μm / 500 μm particle fraction, but in the 500 μm / 300 μm particle fraction, the 300 μm / 150 μm particle fraction, and the 150 μm-passed particle fraction, almost no integrated bodies of the water-absorbent resin and pulp were observed, and each was present in a single state (independent state). In particular, in the 150 μm-passed particle fraction, almost no pulp was present, and only the water-absorbent resin remained.

[0188] The 500 μm / 300 μm particle fraction, 300 μm / 150 μm particle fraction, and 150 μm passing particle fraction obtained by the above operation were mixed to obtain a pulverized product (2-1). Pulp was removed from this pulverized product (2-1) using a cyclone centrifugal separator.

[0189] The cyclone centrifugal separator has a dome diameter of 600 mm and an air volume of 10 m ３ When the cyclone was operated under the conditions of a pulp / water-absorbent resin feed rate of 1 kg / min and a pulp / water-absorbent resin feed rate of 1 kg / min, the recovery rate of the water-absorbent resin (2-1) derived from the mixture from the bottom of the cyclone was 96 mass%. When the water-absorbent resin (2-1) derived from the mixture from the bottom of the cyclone was observed with an SEM, it was found that almost no pulp was present, and only a single water-absorbent resin was present.

[0190] Example 2-2 The same procedure as in Example 2-1 was carried out to obtain a pulverized product (2-2) that had passed through a sieve with 500 μm openings.

[0191] In the (separation step) of Example 2-1, instead of using a cyclone centrifuge, separation of pulp from the pulverized material (2-2) was carried out using a JIS standard sieve with a mesh size of 425 μm. The recovery rate of the mixture-derived water-absorbent resin (2-2) recovered after passing through the sieve was 86 mass%. Furthermore, when the mixture-derived water-absorbent resin (2-2) was observed with an SEM, almost no pulp was present, and only a single water-absorbent resin was present.

[0192] [Production Example 3] The same operation as in Production Example 2 was carried out to obtain a hydrogel (swollen body of water-absorbent resin) (G2) containing pulp. The obtained hydrogel (G2) was dried in a dryer (temperature inside: 140°C) for 60 minutes to obtain a dried water-absorbent resin (S3) containing pulp as an integrated mixture according to the present invention. The dried water-absorbent resin (S3) had a moisture content of 7% by mass and a water-soluble content of 1% by mass. The moisture content and the water-soluble content were the same for the mixtures after crushing (roughly crushed product (C3), crushed product (3-1), pulverized product (3-1) and pulverized product (3-2)).

[0193] [Comparative Example 3] When the dried water-absorbent resin containing pulp (S3) obtained in Production Example 3 was observed, it was found that the water-absorbent resin and the pulp were fixed together and integrated, similarly to Comparative Example 2, and the water-absorbent resin and the pulp were not in a disintegrated state.

[0194] Next, using a jaw crusher in the same manner as in Comparative Example 2, the dried water-absorbent resin (S3) was roughly crushed until the particle size became 4 to 5 mm, thereby obtaining a roughly crushed material (C3). Furthermore, the roughly crushed material (C3) was subjected to sieve classification and air flow classification (cyclone classification), but the roughly crushed material (C3) contained an integrated water-absorbent resin and pulp, and it was not possible to separate the water-absorbent resin and the pulp. Therefore, the rough crushing operation in Comparative Example 3 does not correspond to the crushing step according to the present invention.

[0195] [Example 3-1] (Crushing step) The dried water absorbent resin (S3) containing pulp obtained in Production Example 3 was crushed using a roll mill (manufactured by Inokuchi Giken Co., Ltd.: roll mill) until the particle size became 1000 μm (1 mm) or less, to obtain a crushed product (3-1).

[0196] (Separation Step) Next, the crushed material (3-1) was classified using JIS standard sieves with mesh openings of 850 μm and 300 μm to obtain an 850 μm / 300 μm particle fraction and a 300 μm passing particle fraction.

[0197] When each particle fraction obtained by the above-mentioned classification operation was observed by SEM, integrated bodies of the water absorbent resin and the pulp were occasionally found in the 850 μm / 300 μm particle fraction, but integrated bodies of the water absorbent resin and the pulp were hardly found in the 300 μm passing particle fraction, and each was in a form where they existed in a single state (independent state).

[0198] The 300 μm passing particle fractions obtained by the above procedure were mixed to obtain pulverized material (3-1). Pulp was removed from this pulverized material (3-1) using a cyclone centrifugal separator.

[0199] The cyclone centrifugal separator has a dome diameter of 600 mm and an air volume of 10 m ３ When the cyclone was operated under the conditions of a pulp / water-absorbent resin feed rate of 1 kg / min and a pulp / water-absorbent resin feed rate of 1 kg / min, the recovery rate of the water-absorbent resin (3-1) derived from the mixture from the bottom of the cyclone was 95 mass%. When the water-absorbent resin (3-1) derived from the mixture from the bottom of the cyclone was observed with an SEM, it was found that almost no pulp was present, and only a single water-absorbent resin was present.

[0200] Example 3-2 The same procedure as in Example 3-1 was carried out to obtain a pulverized product (3-2) that had passed through a sieve with 300 μm openings.

[0201] In the (separation step) of Example 3-1, instead of using a cyclone centrifuge, separation of pulp from the pulverized material (3-2) was carried out using a JIS standard sieve with a mesh size of 250 μm. The recovery rate of the mixture-derived water-absorbent resin (3-2) recovered after passing through the sieve was 83% by mass. Furthermore, when the mixture-derived water-absorbent resin (3-2) was observed with an SEM, almost no pulp was present, and only a single water-absorbent resin was present.

[0202] [Production Example 4] A dried water-absorbent resin (S4) containing pulp was obtained by performing the same operation as in Production Example 1, except that the drying time of the hydrogel (G1) containing pulp in a dryer (temperature inside the dryer: 150°C) was shortened to 2 hours. The dried water-absorbent resin (S4) had a moisture content of 15 mass% and a water-soluble content of 22 mass%. The moisture content and the water-soluble content were also the same for the mixture after crushing (crushed product (1-3) and pulverized product (1-3)).

[0203] [Example 1-3] The dried water-absorbent resin containing pulp (S4) obtained in Production Example 4 was subjected to the same operation as in (first crushing step) of Example 1-1 to obtain a crushed product (1-3). Next, the same operations as in (first separation step) and (second crushing step) of Example 1-1 were performed to obtain a crushed product (1-3) that had passed through a sieve with a mesh size of 150 μm. The crushed product (1-3) was subjected to the same operation as in (second separation step) of Example 1-1, and the pulp was separated from the crushed product (1-3) using a cyclone centrifuge. The recovery rate of the recovered water-absorbent resin (1-3) derived from the mixture was 92% by mass. Furthermore, when the water-absorbent resin (1-3) derived from the mixture was observed with an SEM, slight pulp contamination was observed, but almost only the water-absorbent resin was present.

[0204] [Production Example 5] A dried water-absorbent resin (S5) containing pulp was obtained by performing the same operation as in Production Example 2, except that the drying time of the hydrogel (G2) containing pulp in a dryer (temperature inside: 150°C) was shortened to 30 minutes. The dried water-absorbent resin (S5) had a moisture content of 10% by mass and a water-soluble content of 1% by mass. The moisture content and the water-soluble content were also the same for the mixture after crushing (crushed product (2-3) and pulverized product (2-3)).

[0205] [Example 2-3] The dried water-absorbent resin (S5) containing pulp obtained in Production Example 5 was subjected to the same operation as in the (crushing step) of Example 2-1 to obtain a crushed product (2-3). Next, the same operation as the sieve classification in the (separation step) of Example 2-1 was performed to obtain a crushed product (2-3) that passed through a sieve with a mesh size of 500 μm. The crushed product (2-3) was subjected to the same operation as the separation using a cyclone centrifuge in the (separation step) of Example 2-1 to separate the pulp from the crushed product (2-3). The recovery rate of the recovered water-absorbent resin (2-3) derived from the mixture was 91% by mass. Furthermore, when the water-absorbent resin (2-3) derived from the mixture was observed with an SEM, slight pulp contamination was observed, but almost only the water-absorbent resin was present.

[0206] [Production Example 6] A dried water-absorbent resin (S6) containing pulp was obtained by performing the same operation as in Production Example 3, except that the drying time of the hydrogel (G2) containing pulp in a dryer (temperature inside the dryer: 140°C) was shortened to 45 minutes. The dried water-absorbent resin (S6) had a moisture content of 11% by mass and a water-soluble content of 1% by mass. The moisture content and the water-soluble content were also the same for the mixture after crushing (crushed product (3-3) and pulverized product (3-3)).

[0207] [Example 3-3] The dried water-absorbent resin (S6) containing pulp obtained in Production Example 6 was subjected to the same operation as in the (crushing step) of Example 3-1 to obtain a crushed product (3-3). Next, the same operation as the sieve classification in the (separation step) of Example 3-1 was performed to obtain a crushed product (3-3) that passed through a sieve with a mesh size of 300 μm. The crushed product (3-3) was subjected to the same operation as the separation using a cyclone centrifuge in the (separation step) of Example 3-1 to separate the pulp from the crushed product (3-3). The recovery rate of the recovered water-absorbent resin (3-3) derived from the mixture was 94% by mass. Furthermore, when the water-absorbent resin (3-3) derived from the mixture was observed with an SEM, slight pulp contamination was observed, but almost only the water-absorbent resin was present.

[0208] The following Tables 1 to 4 show the conditions and physical property values ​​used in the above Production Examples, Examples, and Comparative Examples.

[0209]

[0210]

[0211]

[0212]

[0213] <Discussion 1> From the results of Comparative Examples 1 to 3 and Examples 1-1, 1-2, 2-1, 2-2, 3-1 and 3-2 shown in Tables 2 to 4 above, it can be seen that the water-absorbent resin derived from the mixture recovered through the crushing step and separation step according to the present invention contains almost no pulp, which is a foreign matter (impurity). Therefore, the recovery method according to the present invention is excellent in pulp separability (it is possible to obtain a water-absorbent resin derived from the mixture with a reduced amount of impurities). Furthermore, from the comparison of Examples 1-1 and 1-2, Examples 2-1 and 2-2, and Examples 3-1 and 3-2, it can be seen that the combination of separation using a sieve classifier and an air classifier (cyclone centrifuge) can maintain good separability of pulp, which is a foreign matter, and further improve the recovery rate of the water-absorbent resin, which is the target of recovery, compared to the case where the separation step is performed using a sieve alone.

[0214] <Discussion 2> From the comparisons of Examples 1-1 and 1-3, Examples 2-1 and 2-3, and Examples 3-1 and 3-3 shown in Tables 2 to 4, it can be seen that when the moisture content of the pulverized product (the mixture after crushing to be subjected to the separation step) is reduced, the recovery rate of the water-absorbent resin remains almost unchanged, but the separability of pulp, which is a foreign substance (impurity), tends to be further improved. From this, it can be seen that the separability of pulp can be further improved by adjusting the moisture content of the mixture after crushing to be subjected to the separation step to a low level. Furthermore, it can be said that the pulp separability and the recovery rate of the water-absorbent resin can be improved by adjusting the moisture content of the mixture after crushing to a low level.

[0215] Reference Example 1: Production Example of Water-Absorbent Resin 244.9 g of acrylic acid, 0.71 g (0.040 mol% relative to acrylic acid) of polyethylene glycol diacrylate (molecular weight 523) as an internal crosslinking agent, 1.83 g of a 1.0 mass% aqueous solution of diethylenetriaminepentaacetic acid trisodium (DTPA 3Na), 103.7 g of a 48.5 mass% aqueous solution of sodium hydroxide, and 389.0 g of deionized water were added to a reaction vessel and mixed to prepare an aqueous monomer solution (a'). Next, the aqueous monomer solution (a') was cooled with stirring. When the liquid temperature reached 40.0 ° C, 100.9 g of a 48.5 mass% aqueous solution of sodium hydroxide adjusted to 40 ° C was added and mixed to prepare an aqueous monomer solution (a). At this time, the temperature of the aqueous monomer solution (a) rose to 77.9 ° C due to the heat of neutralization in the second stage immediately after preparation. Immediately after starting to mix the 48.5% by mass aqueous sodium hydroxide solution, precipitates were observed, but they gradually dissolved to form a transparent, homogeneous solution. Next, 12.1 g of a 4.0% by mass aqueous sodium persulfate solution was added to the stirred monomer aqueous solution (a) and immediately poured into a stainless steel bat-shaped container in an open-to-air system. The time from the start of the second-stage neutralization to pouring the monomer aqueous solution (a) into the bat-shaped container was 55 seconds, and the bat-shaped container was heated using a hot plate until the surface temperature reached 40°C. The polymerization reaction began 70 seconds after the monomer aqueous solution (a) was poured into the bat-shaped container. The polymerization reaction proceeded by the monomer aqueous solution (a) expanding and foaming in all directions while generating steam, and then shrinking to a size slightly larger than the bat-shaped container. Three minutes after the start of the polymerization reaction, the hydrogel-like cross-linked polymer (hereinafter referred to as "hydrogel") (R1) was removed. This series of operations was performed in an open-to-air system. The hydrogel (R1) obtained by the polymerization reaction was cut into strips and pulverized using a meat chopper with a die diameter of 7.5 mm, and then the hydrogel was spread on a 50-mesh wire netting and dried with hot air for 60 minutes at 190° C. Next, the dried product obtained was pulverized using a vibration mill and further passed through a sieve with an opening of 850 μm to obtain an irregularly pulverized water-absorbent resin precursor (R1) having a mass average particle diameter (D50) of 350 μm remaining on the sieve with an opening of 150 μm.

[0216] A surface cross-linking agent solution consisting of 0.030 parts by mass of ethylene glycol diglycidyl ether, 1.35 parts by mass of propylene glycol, and 3.15 parts by mass of deionized water was uniformly mixed with 100 parts by mass of the obtained water-absorbent resin precursor (R1), and the mixture was heat-treated at 100°C for 45 minutes. The mixture was then cooled and passed through a JIS standard sieve with a mesh size of 710 μm to obtain a water-absorbent resin (R1). The CRC of the water-absorbent resin (R1) was 35.9 (g / g), and the AAP0.3 was 29.4 (g / g).

[0217] Reference Example 2 Production example in which fine powder produced from water absorbent resin was recovered in fine powder recovering step A hydrous gel (R2) obtained by the same method as in the hydrous gel (R1) in Reference Example 1 described above was spread on a wire netting, and subjected to hot air drying at 190°C for 60 minutes, and the obtained dried product was pulverized using a roll mill and classified with a 106 µm JIS standard sieve, and the product passing through the 106 µm sieve was recovered, thereby obtaining a water absorbent resin fine powder (R2).

[0218] Next, 100 parts by mass of the water-absorbent resin fine powder (R2) was mixed with 100 parts by mass of hot water at 80 to 90 ° C. under stirring to obtain a fine powder granulated gel (R2) (solid content concentration: approximately 50% by mass) having a particle size of 1 to 3 mm. Separately, a hydrous gel (R2) obtained by the same method as the hydrous gel (R1) in Reference Example 1 was mixed with the fine powder granulated gel (R2) so that the solid content mass ratio (hydrous gel: fine powder granulated gel) was 10:3, and the mixture was dried with hot air at 190 ° C. for 30 minutes. Subsequently, the dried product obtained was pulverized using a roll mill and classified using a JIS standard sieve with a mesh size of 850 to 106 μm to obtain an irregularly pulverized water-absorbent resin precursor (R2) having a mass average particle size (D50) of 380 μm. The CRC of the water-absorbent resin precursor (R2) was 47.2 (g / g).

[0219] A surface cross-linking agent solution consisting of 0.030 parts by mass of ethylene glycol diglycidyl ether, 1.35 parts by mass of propylene glycol, and 3.15 parts by mass of deionized water was uniformly mixed with 100 parts by mass of the obtained water-absorbent resin precursor (R2), and the mixture was heat-treated at 100 ° C. for 45 minutes. The mixture was then cooled and passed through a JIS standard sieve with a mesh size of 710 μm to obtain a water-absorbent resin (R2). The CRC of the water-absorbent resin (R2) was 34.5 (g / g), and the AAP0.3 was 28.1 (g / g).

[0220] Reference Example 3: Production example in which fine powder obtained from a mixture-derived water-absorbent resin (1-1) was recovered in a fine powder recovery step. The mixture-derived water-absorbent resin (1-1) obtained in Example 1-1 was classified using a 106 μm JIS standard sieve, and the material passing through the 106 μm sieve was recovered to obtain a mixture-derived water-absorbent resin fine powder (R3). In Reference Example 2, a fine powder granulated gel (R3) (solid content concentration: approximately 50% by mass) was produced using the mixture-derived water-absorbent resin fine powder (R3) instead of the water-absorbent resin fine powder (R2). Separately, the same operation was carried out as in Reference Example 2 (Reference Example 1) above, except that the hydrous gel (R3) obtained by the same method as Reference Example 2 (Reference Example 1) and the fine powder granulated gel (R3) were mixed so that the solid content mass ratio (hydrous gel:fine powder granulated gel) was 10:3, to obtain a water-absorbent resin (R3). The water-absorbent resin (R3) had a CRC of 35.0 (g / g) and an AAP0.3 of 28.1 (g / g), and it was confirmed that it had water-absorbing properties almost equivalent to those of the water-absorbent resin (R2) of Reference Example 2.

[0221] This application is based on Japanese Patent Application No. 2023-129711, filed on August 9, 2023, the disclosure of which is incorporated by reference in its entirety.

[0222] S1 Crushing process S2 Inactivation / dehydration process S3 Washing / disinfection process S4 Separation / filtration process S5 Acid treatment process S6 Neutralization process S7 Drying process S100 Preparation process for recycled water absorbent resin (pretreatment process) S200 Crushing process S300 Separation process.

Claims

1. A method for recovering a water-absorbent resin, comprising the steps of: a crushing step of applying a physical force to a mixture in which the water-absorbent resin and impurities are adhered to and integrated with each other, thereby crushing the mixture; and a separation step of separating the water-absorbent resin and the impurities from the mixture crushed in the crushing step, in this order.

2. The recovery method according to claim 1, wherein the content of the impurities contained in the mixture is less than 20 mass% based on the total mass of the mixture.

3. The recovery method described in claim 1 or 2, wherein the moisture content of the mixture is 20 mass% or less.

4. A recovery method according to claim 1 or 2, wherein the water-soluble content of the mixture is 0.5 mass% or more.

5. The method according to claim 1 or 2, wherein the disintegration step is carried out using one or more types of apparatus selected from the group consisting of a pin mill, a cyclone mill, a roll mill, a roller mill, a hammer mill, a rod mill, a ball mill, a jet mill, and a bead mill.

6. The recovery method according to claim 1 or 2, wherein the separation step is carried out using a sieve classification device and / or a dry classification device.

7. The recovery method according to claim 1 or 2, wherein the separation step is in any one of the following forms (i) to (iii): (i) having at least one separation step using a sieve classifier and at least one separation step using an air classifier, in any order; (ii) having at least two separation steps using a sieve classifier; (iii) having at least two separation steps using an air classifier.

8. The method according to claim 7, wherein the separation step using the air classifier is a separation step using a cyclone centrifugal separator.

9. The recovery method according to claim 1 or 2, which satisfies any one of the following (I) to (III): (I) the moisture content of the crushed mixture subjected to the separation step is 10% by mass or less, and the particle size of the crushed mixture subjected to the separation step is less than 150 μm; (II) the moisture content of the crushed mixture subjected to the separation step is 8% by mass or less, and the particle size of the crushed mixture subjected to the separation step is less than 300 μm; (III) the moisture content of the crushed mixture subjected to the separation step is 5% by mass or less, and the particle size of the crushed mixture subjected to the separation step is less than 500 μm.

10. The recovery method according to claim 1 or 2, wherein the impurities include pulp.

11. The recovery method according to claim 1 or 2, wherein the mixture is obtained through a pretreatment process for separating the absorbent resin contained in the water-absorbent article from impurities.

12. The method according to claim 1 or 2, wherein the mixture is obtained by recovering the mixture from a moisture-absorbent article.

Images