Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
  • B01J20/22—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising organic material
  • B01J20/26—Synthetic macromolecular compounds
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B07—SEPARATING SOLIDS FROM SOLIDS; SORTING
  • B07B—SEPARATING SOLIDS FROM SOLIDS BY SIEVING, SCREENING, SIFTING OR BY USING GAS CURRENTS; SEPARATING BY OTHER DRY METHODS APPLICABLE TO BULK MATERIAL, e.g. LOOSE ARTICLES FIT TO BE HANDLED LIKE BULK MATERIAL
  • B07B7/00—Selective separation of solid materials carried by, or dispersed in, gas currents
  • B07B7/08—Selective separation of solid materials carried by, or dispersed in, gas currents using centrifugal force
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B07—SEPARATING SOLIDS FROM SOLIDS; SORTING
  • B07B—SEPARATING SOLIDS FROM SOLIDS BY SIEVING, SCREENING, SIFTING OR BY USING GAS CURRENTS; SEPARATING BY OTHER DRY METHODS APPLICABLE TO BULK MATERIAL, e.g. LOOSE ARTICLES FIT TO BE HANDLED LIKE BULK MATERIAL
  • B07B9/00—Combinations of apparatus for screening or sifting or for separating solids from solids using gas currents; General arrangement of plant, e.g. flow sheets
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B09—DISPOSAL OF SOLID WASTE; RECLAMATION OF CONTAMINATED SOIL
  • B09B—DISPOSAL OF SOLID WASTE NOT OTHERWISE PROVIDED FOR
  • B09B3/00—Destroying solid waste or transforming solid waste into something useful or harmless
  • B09B3/30—Destroying solid waste or transforming solid waste into something useful or harmless involving mechanical treatment
  • B09B3/35—Shredding, crushing or cutting
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J3/00—Processes of treating or compounding macromolecular substances
  • C08J3/12—Powdering or granulating
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02W—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO WASTEWATER TREATMENT OR WASTE MANAGEMENT
  • Y02W30/00—Technologies for solid waste management
  • Y02W30/50—Reuse, recycling or recovery technologies
  • Y02W30/62—Plastics recycling; Rubber recycling

Definitions

• the present invention relates to a method for recovering water-absorbent resin.
• Articles designed to absorb bodily fluids such as disposable diapers, sanitary napkins, and incontinence pads (hereafter also referred to as "absorbent articles"), are primarily composed of absorbent materials such as pulp and super absorbent polymer (hereafter also referred to as "SAP"), and exterior materials made of nonwoven fabric, resin film, resin-coated paper, etc.
• SAP pulp and super absorbent polymer
• exterior materials made of nonwoven fabric, resin film, resin-coated paper, etc.
• JP 2019-85447 A proposes a method for recovering pulp from used absorbent articles by subjecting an inactivated aqueous solution containing pulp and absorbent resin separated from the used absorbent article to solid-liquid separation and then treating it with a specific method.
• JP 2021-41310 A (corresponding to European Patent Application Publication No. 4001354, the same applies below) proposes a method for producing a regenerated 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.
• JP 2019-85447 A the absorbent resin is removed from the pulp in which the absorbent resin remains after being separated from a used absorbent article, and the absorbent resin is separated from the pulp.
• JP 2021-41310 A describes that foreign matter (pulp, etc.) can be separated from the recycled superabsorbent polymer by performing a foreign matter separation process in which foreign matter such as pulp is separated from the recycled superabsorbent polymer after the drying process.
• the present invention aims to provide a technology that can obtain water-absorbent resin with reduced amounts of contaminants when recovering the water-absorbent resin from a mixture in which the water-absorbent resin and contaminants such as pulp have adhered to each other and been integrated.
• the present inventors conducted extensive research to solve the above problems. As a result, they discovered that the above problems could be solved by applying a physical force to a mixture of water-absorbent resin and impurities that are bound 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, which led to the completion of the present invention.
• one aspect for achieving the above-mentioned object is a method for recovering a water-absorbent resin, which includes, 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 to crush the mixture, and a separation step in which the water-absorbent resin and impurities are separated from the mixture crushed in the crushing step.
• Fig. 1 is a flow diagram for explaining the method for recovering water-absorbent resin according to the present invention.
• 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 the regenerated water-absorbent resin
• S200 represents a crushing step
• 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. 1 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 neutral
• FIG. 3 is an SEM image (magnification: 30 times) of the 850 ⁇ m/500 ⁇ m particles of the pulverized product (1-1) obtained in Example 1-1.
• FIG. 4 is an SEM image (magnification: 30 times) of the 500 ⁇ m/150 ⁇ m particles of the pulverized product (1-1) obtained in Example 1-1.
• FIG. 5 is an SEM image (magnification: 30 times) of the 150 ⁇ m passing particles of the pulverized product (1-1) obtained in Example 1-1.
• 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 the water-absorbent resin and impurities are fixed together to break down the mixture (the water-absorbent resin and the impurities), and a separation step in which the water-absorbent resin and the impurities are separated from the mixture crushed in the crushing step.
• the method for recovering a water-absorbent resin having such a configuration is also referred to simply as the “recovery method according to the present invention” or the “method according to the present invention.”
• the "water-absorbent resin recovered by the method according to the present invention” is also simply referred to as the “water-absorbent resin according to the present invention.”
• absorbent resin and pulp are used as absorbent materials.
• the absorbent resin is treated with an aqueous solution that inactivates the absorbent resin (inactivating aqueous solution), and then an operation is performed to separate the absorbent resin from fibrous substances such as pulp.
• the separated absorbent resin is then dried after undergoing several further treatments to become a regenerated absorbent resin.
• the regenerated absorbent resin obtained by such a method has the problem that a relatively large amount of impurities such as pulp is fixed to its interior or surface.
• the present inventors have investigated the problem and have considered that the separation of the water absorbent resin from impurities such as pulp is insufficient due to the following reasons. That is, the present inventors have speculated that when the water absorbent resin in a water-containing state and impurities such as pulp are mixed and dried, the water absorbent resin and impurities such as pulp are strongly fixed and integrated. Specifically, when wet recycling is performed as in the above-mentioned treatment using an inactivating aqueous solution and then a drying treatment is performed, the remaining impurities such as pulp are thought to be strongly fixed and integrated with the water absorbent resin due to the adhesiveness (tackiness) of the water absorbent resin that appears when it is hydrated and swollen, and water-soluble matter in a free state.
• a physical force is applied to a mixture in which the water-absorbent resin and impurities such as pulp are stuck together and integrated, and the water-absorbent resin and impurities such as pulp contained in the mixture are crushed.
• the water-absorbent resin and the impurities such as pulp can be made independent (not stuck together), and therefore the water-absorbent resin and the impurities such as pulp can be efficiently separated in the separation step.
• a water-absorbent resin containing a reduced amount of impurities such as pulp can be obtained.
• the term "(meth)acrylic” includes both acrylic and methacrylic.
• the term “(meth)acrylic acid” includes both acrylic acid and methacrylic acid.
• acid (salt) means “acid and/or its salt.”
• concentrations and percentages represent mass concentrations and mass percentages, respectively, and ratios are mass ratios, unless otherwise specified.
• operations and measurements of physical properties are performed under room temperature (20-25°C)/relative humidity of 40-55% RH conditions.
• a and/or B means that A and B are each included, as well as combinations of these.
• water-absorbent resin refers to a water-swellable, water-insoluble polymer gelling agent, and is not particularly limited, but refers to a conventional 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 a CRC of 5 g/g or more as specified in ERT441.2-02 as "water-swelling property". The definition of CRC will be described later.
• water-absorbent resin may refer to each individual particle of water-absorbent resin, but in this specification, unless otherwise specified, it refers to the entire aggregate of water-absorbent resin particles.
• 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 water-absorbent resins 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, saponified crosslinked products of (meth)acrylate-vinyl acetate copolymers, starch-acrylate graft polymers and crosslinked products thereof.
• the water-absorbent resin is a polymer derived from a carboxyl group-containing unsaturated monomer, and the water-absorbent resin may include a polymer having a partially neutralized carboxyl group. Furthermore, in one embodiment of the present invention, the water-absorbent resin is a polyacrylic acid (salt)-based resin.
• Recovered water-absorbent resin refers to a water-absorbent resin that is originally discarded. Specifically, there is 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).
• 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
• 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
• the recovered absorbent resin When the recovered absorbent resin is a water-absorbent resin contained in a used absorbent article, the recovered 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.
• the recovered water-absorbent resin is generally recovered by removing components other than the water-absorbent resin, such as exterior materials, from the absorbent article.
• the operation of separating the recovered water-absorbent resin from the absorbent article can be performed using a dry separation method or a wet separation method, and any of these methods can be used.
• the mass of the water-absorbent resin and the recovered water-absorbent resin is a numerical value converted into solid content unless otherwise specified. For example, if the mass of the recovered water-absorbent resin contained in a used paper diaper is unknown, the mass is calculated using the mass of the water-absorbent resin contained in a new (unused) paper diaper and the general value of the water absorption ratio of the water-absorbent resin in the used paper diaper, or the mass of the recovered water-absorbent resin is measured by assuming that the content of the water-absorbent resin (converted into solid content) relative to the total mass of the new (unused) paper diaper is 30 mass%.
• Absorbent articles refer to sanitary materials used to absorb body fluids such as urine, blood, etc.
• sanitary materials include paper diapers (for children and adults), sanitary napkins, adult incontinence products (incontinence pads), breast pads, etc.
• articles similar to sanitary materials include pet sheets and pet diapers, and in the present invention, these articles for treating animal urine are also included in the sanitary materials.
• An example of the structure of an absorbent article includes a surface sheet (surface material) with liquid permeability, an absorbent body containing a water-absorbent resin and a fibrous material, and a back sheet (waterproof material) with liquid impermeability.
• the absorbent body is preferably manufactured by mixing the water-absorbent resin with a fibrous material, or by sandwiching the water-absorbent resin between fibrous materials and molding it into a film, a tube, a sheet, or the like.
• the fibrous material include hydrophilic fibers such as crushed 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.”
• an example of the configuration of a disposable diaper which is one type of absorbent product, may include a surface material such as a nonwoven fabric made of chemical fibers such as polypropylene or polyester; a water-absorbing body containing water-absorbing materials such as water-absorbing resin or pulp; a waterproof material such as a resin film such as a polyethylene film, or paper or nonwoven fabric that has been treated 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.
• a surface material such as a nonwoven fabric made of chemical fibers such as polypropylene or polyester
• a water-absorbing body containing water-absorbing materials such as water-absorbing resin or pulp
• a waterproof material such as a resin film such as a polyethylene film, or paper or nonwoven fabric that has been treated with resin, such as polyethylene-laminated paper or polyethylene-laminated nonwoven fabric (resin-treated paper
• the components other than the absorbent material i.e., the surface sheet (surface material), back sheet (waterproof material) and adhesive (binder) formed from nonwoven fabric, resin film, resin-treated paper, resin-treated nonwoven fabric, etc.
• the surface sheet surface material
• back sheet waterproof material
• adhesive binder
• “Absorbent articles” includes both used absorbent articles and unused absorbent articles (especially pre-used discarded absorbent articles).
• Used absorbent article refers to a used sanitary material that has been used by a consumer and has absorbed bodily fluids such as urine, blood, etc.
• a used absorbent article contains a water-absorbent resin that has been swollen by bodily fluids.
• the state of use there is no particular restriction on the state of use.
• it may be attached with solid waste (feces, etc.), liquid waste (urine, menstrual blood, etc.), or both, but in consideration of the cost and efficiency of the regeneration process, it is preferable for it to be in a state where urine is absorbed as the main component.
• the used absorbent articles used in the present invention include, for example, those that are collected, recovered, and transported from facilities where users of the absorbent articles live or stay, such as ordinary households, hospitals, and welfare facilities.
• an absorbent article mainly includes an absorbent material containing a water-absorbent resin and pulp, and other exterior materials.
• the following process is generally performed. Specifically, the exterior material of the absorbent article is crushed, and the exterior material and the absorbent material are roughly crushed (crushing process), and then, after performing an appropriate process, the absorbent resin, pulp, and fine exterior material that could not be separated are separated (filtered) (separation and filtration process).
• the fine crushed material of the exterior material mixed in due to the crushing of the pulp and exterior material cannot be completely separated from the absorbent resin, and they are in a mixed state.
• the crushed material of the exterior material (fine crushed material) as described above is 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.
• the shape of the impurities is not particularly limited, and examples include fibers (including straight and wavy shapes), rods, needles, flat plates, strips, and irregular flakes, but generally, most of the impurities mixed into the water-absorbent resin are fibrous.
• the "mixture” to be subjected to the water-absorbent resin recovery method according to the present invention is formed by the water-absorbent resin and impurities being fixed and integrated together.
• the "mixture formed by the water-absorbent resin and impurities being fixed and integrated together” may be simply referred to as the "integrated mixture according to the present invention” or "mixture”.
• 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 together (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. are firmly fixed to the water-absorbent resin.
• the specific form (microstructure) of the mixture is not particularly limited, but examples include a form in which impurities such as pulp are attached to the surface of water-absorbent resin particles (irregular particles); a form in which water-absorbent resin particles (irregular particles) aggregate to form aggregates with impurities such as pulp penetrating between each particle; and a form in which impurities such as pulp penetrate into water-absorbent resin particles (irregular particles).
• the state in which "the water-absorbent resin and impurities are stuck together” refers to a state in which the water-absorbent resin and impurities cannot be separated using common classification devices such as sieve classification, inertial classification, centrifugal classification, and gravity classification.
• impurities such as pulp are firmly attached and integrated with the water-absorbent resin, so that even if the mixture is subjected to a separation process using a normal separation means such as a sieve, the impurities and the water-absorbent resin cannot be easily separated.
• the impurities such as pulp and the water-absorbent resin are separated from each other through a crushing process described in detail below, and then a separation process is carried out, so the content of impurities mixed in the water-absorbent resin can be reduced.
• the water absorbent resin and the impurities may not be completely independent, and some of the water absorbent resin and the impurities may remain as an integrated mixture. Therefore, the "mixture after crushing" obtained through the crushing process may include the water absorbent resin and the impurities that are each in an independent state, as well as the integrated mixture that has not been completely crushed and remains.
• 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 fresh (unused) water absorbent resin that is newly produced. Also, in another embodiment of the present invention, the water absorbent resin contained in the mixture is a recovered water absorbent resin.
• 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.
• the water-absorbent resin contained in the mixture 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 water-absorbent resin in a state of a hydrous gel.
• the CRC of the absorbent resin (recovered 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 absorbent resin (recovered 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.
• Regenerated water absorbent resin As described above, the recovered water-absorbing resin is generally recovered from the absorbent article by a method based on dry separation or a method based on wet separation. In this case, the recovery of the water-absorbing resin that has absorbed water and swelled due to use or the like (the water-absorbing resin contained in the used absorbent article or the water-absorbing resin contained in the absorbent article discarded before use) is carried out by removing components other than the water-absorbing resin (such as the exterior material constituting the absorbent article), and such removal and separation of components other than the water-absorbing resin can be carried out using a known method based on wet separation.
• recovery of the water-absorbent resin by wet separation is performed through the following steps: an inactivation/dehydration treatment step in which the 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 (i.e., 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 absorbent resin is separated (filtered) into the water-absorbent resin, pulp, and exterior material; and a drying step in which the recovered water-absorbent resin is dried; and the water-absorbent resin is recovered through these steps.
• an inactivation/dehydration treatment step in which the 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 (i.e., wet), and the water-absorbent resin
• the water absorbent resin recovered by carrying out at least the above-mentioned wet separation 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. Also, 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 recovered water absorbent resin includes a recycled water absorbent resin. Also, in another embodiment, the water absorbent resin contained in the mixture is a recycled water absorbent resin, and the recovered 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).
• a crushing step for crushing the exterior materials of the absorbent article, etc.
• a cleaning and disinfecting step for cleaning and/or disinfecting the water-absorbent resin (water-absorbent material containing the water-absorbent resin); an acid treatment step for treating the water-absorbent resin with acid; and a neutralization step for neutralizing the remaining acid after the acid treatment step.
• the mixture in the present invention may be a mixture in which the recycled water-absorbent resin and impurities are adhered and integrated.
• EDANA and WSP are abbreviations 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).
• CRC water absorption capacity 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 in a 0.90 mass % aqueous sodium chloride solution for 30 minutes without pressure.
• water-absorbent resin 0.200 g is uniformly placed in a bag (85 mm x 60 mm) made of nonwoven fabric (manufactured by Nangoku Pulp Kogyo Co., Ltd., product name: Heatlon Paper, model: GSP-22), 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.
• the bag is pulled out and drained for 3 minutes at a centrifugal force (250 G) described in ERT441.2-02 using a centrifuge (manufactured by Kokusan Co., Ltd., centrifuge: model H-122), and the mass W1 (g) of the bag is measured.
• the same operation is performed without using the water-absorbent resin, and the mass W0 (g) at that time is measured.
• the CRC centrifuge retention capacity
• Water-soluble content of water-absorbent resin and mixture (16-hour value) is a value measured according to the following method.
• the "water-soluble content (16-hour value)” is also simply referred to as the “water-soluble content” or “water-soluble content” or “soluble polymer content”.
• the extract is 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 ⁇ m), and 50.0 g of the obtained filtrate is used as the measurement liquid.
• a sheet of filter paper ADVANTEC Toyo Co., Ltd., product name: (JIS P 3801, No. 2), thickness 0.26 mm, retention particle size 5 ⁇ m)
• 50.0 g of the obtained filtrate is used as the measurement liquid.
• 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.
• the amount of water-soluble matter as an index 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).
• "72" is the molecular weight of acrylic acid
• "94" is the molecular weight of sodium acrylate.
• AAP 0.3 water absorption capacity under pressure
• the "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% by mass aqueous sodium chloride solution.
• “AAP0.3” means AAP under a pressure of 0.3 psi.
• the AAP0.3 of the water absorbent resin is measured in accordance with ERT442.2-02.
• 0.9 g of the water absorbent resin is swollen under a pressure of 2.07 kPa (21 g/cm 2 , 0.3 psi) for 1 hour using a large excess of a 0.9% by mass aqueous sodium chloride solution, and then the AAP (water absorption capacity under pressure) is measured.
• AAP0.3 water absorption capacity under pressure
• AAP water absorption capacity under pressure
• 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, to crush the mixture (crushing step), and separating the water-absorbent resin and impurities from the crushed mixture (separation step).
• the mixture subjected to the disintegration step is a mixture of the recovered water-absorbent resin recovered from absorbent articles (used absorbent articles and unused absorbent articles (particularly pre-used discarded absorbent articles)) and impurities that are fixed to each other and integrated together.
• the mixture subjected to the disintegration step may be a mixture of the recovered water-absorbent resin recovered from absorbent articles by a known method and impurities, or a mixture of the recovered water-absorbent resin and impurities obtained by requesting another company to recover the absorbent resin from absorbent articles, or a mixture of such recovered water-absorbent resin and impurities that has been purchased.
• the method for recovering water-absorbent resin according to the present invention can thus further remove impurities fixed to the recovered water-absorbent resin separated and recovered by a known method, thereby obtaining a water-absorbent resin with a further reduced amount of impurities.
• the mixture subjected to the recovery method of the present invention may be a mixture of recycled water-absorbent resin and impurities. Therefore, the recovery method of the present invention may further include a step for obtaining (preparing) recycled water-absorbent resin, i.e., a preparation step (pretreatment step) of recycled water-absorbent resin, prior to the crushing step.
• a preparation step pretreatment step
• Preparation process for recycled water absorbent resin The preparation step (pretreatment step) of the regenerated water absorbent resin is an optional step.
• the regenerated water absorbent resin is obtained by separating the water absorbent resin (recovered water absorbent resin) from the absorbent article and the constituent members of the absorbent article other than the water absorbent resin.
• the method for obtaining the regenerated water absorbent resin from the absorbent article is not particularly limited, and a known method can be used as is or with appropriate modification.
• the product obtained by this step may actually be a mixture of the regenerated water absorbent resin and impurities, but in the following description of this step (pretreatment step), it may also be simply described as "regenerated water absorbent resin” or "water absorbent resin”.
• the absorbent article from which the recycled water-absorbent resin is recovered may be a used absorbent article, a pre-use discarded absorbent article (for example, an absorbent article that has not absorbed bodily fluids such as human waste or blood but has absorbed moisture during storage and has been discarded), or a mixture of these.
• An example of a method for preparing a recycled water-absorbent resin from an absorbent article (used absorbent article and/or pre-use discarded absorbent article) will be described below 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 the recycled water-absorbent resin to be used in the recovery method according to the present invention is not limited to the method described below.
• the preparation process (pretreatment process) (S100) of the recycled water-absorbent resin includes: a crushing process (S1) for crushing the exterior material of the absorbent article; an inactivation/dehydration process (S2) for performing a shrinkage/dehydration process (inactivation process) on the water-absorbent resin (recovered water-absorbent resin) contained in the absorbent article crushed in the crushing process (S1); a cleaning/disinfection process (S3) for cleaning and/or disinfecting the water-absorbent material containing the water-absorbent resin after the inactivation/dehydration process (S2); a separation/filtration process (S4) for separating (filtering) the water-absorbent resin, pulp, and exterior material; an acid treatment process (S5) for performing an acid treatment on the water-absorbent resin recovered in the separation/filtration process (S4)
• the order of the above steps may be changed or omitted as necessary, as long as the absorbent resin (recovered absorbent resin) can be separated from the components constituting the absorbent article other than the absorbent resin.
• the cleaning/disinfecting step (S3), the acid treatment step (S5), the neutralization step (S6), etc. are optional steps and may be omitted as necessary.
• a cleaning step or a concentration step (not shown) may be further performed as necessary.
• crushing step (S1) the exterior material constituting the recovered absorbent article is crushed (shredded).
• the crushing (shredding) method is not particularly limited, and a known method can be used.
• the crushed absorbent article may be dispersed in a solvent to roughly separate the exterior material and the water-absorbing material (rough separation).
• the separation operation between the exterior material and the water-absorbing material may be performed only once or multiple times.
• it when separating the exterior material from the crushed (shredded) absorbent article, it may be performed directly (directly) without using a solvent, but from the viewpoint of separation efficiency, it is preferable to perform the separation by a method of dispersing in a solvent (i.e., wet separation).
• the solvent used for separation examples include water and organic solvents, and preferably water.
• a wet separation method a method (specific gravity separation) in which the recovered exterior material and water-absorbing material are put into water and separated into those that sink in the water and those that float on the water surface can be mentioned.
• the water-absorbing resin (recovered water-absorbing resin) contained in the absorbent article is inactivated and dehydrated (shrinkage).
• the pulp and water-absorbing resin contained in the absorbent article (water-absorbing material) are separated by a method of dispersing the water-absorbing material in a solvent (water or organic solvent) and then separating the material (i.e., a method using wet separation).
• a solvent water or organic solvent
• wet separation i.e., a method using wet separation
• the water-absorbing resin when performing the above-mentioned wet separation, from the viewpoint of separation efficiency, it is preferable to separate the water-absorbing resin from the components other than the water-absorbing resin, such as the exterior material, by shrinking the water-absorbing resin and utilizing the difference in specific gravity and solubility.
• the inactivation and dehydration treatment can be performed by a known method, and specifically, the hydrous gel of the water-absorbing resin can be shrunk by using a shrinking agent (acid, alkali, inorganic salt, polyvalent metal salt, etc.) together with the solvent.
• the amount of the solvent used 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.
• an acid organic acid or inorganic acid
• an alkali an inorganic salt
• a polyvalent metal salt etc.
• 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% by mass or more and 10% by mass or less based on the total mass of these acids (organic acids and inorganic acids) and the water-absorbent resin.
• polyvalent metal salt examples include alkaline earth metal salts and transition metal salts.
• alkaline earth metal salt a salt of an alkaline earth metal such as beryllium, magnesium, calcium, strontium, barium, etc. can be used, and preferably, a water-soluble salt is used.
• a calcium or magnesium salt is preferred.
• Specific examples of these salts include calcium chloride, calcium nitrate, calcium hydroxide, magnesium chloride, magnesium nitrate, magnesium hydroxide, etc., and of these, calcium chloride is preferred.
• the alkaline earth metal salts may be used alone or in combination of two or more.
• a salt of a transition metal such as iron, cobalt, nickel, or copper can be used, and preferably a water-soluble salt is used.
• 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.
• the water-absorbent resin may contain a polymer having a partially neutralized carboxyl group.
• the dissociated carboxyl groups are ionically crosslinked via polyvalent metal ions (e.g., Ca 2+ , Mg 2+ ).
• polyvalent metal ions e.g., Ca 2+ , Mg 2+
• 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.
• the treatment time in the treatment with the shrinking agent is not particularly limited as long as it is sufficient for the 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 sufficient for the absorbent resin to shrink and dehydrate, and is, for example, about 5°C to 80°C.
• washing and disinfecting treatments can be performed by known methods. For example, washing and disinfecting treatments can be performed by adding water, water containing a disinfectant, etc., and stirring and mixing. In this case, disinfecting treatments can also be performed by adding a disinfectant such as hypochlorous acid, an aqueous chlorine dioxide solution, ozone, hydrogen peroxide water, or electrolyzed water (acidic electrolyzed water) to the solvent.
• a disinfectant such as hypochlorous acid, an aqueous chlorine dioxide solution, ozone, hydrogen peroxide water, or electrolyzed water (acidic electrolyzed water)
• 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 with respect to 100% by mass of the total mass of the absorbent article.
• the washing and disinfecting step (S3) may be performed before the inactivation and dehydration treatment step (S2) or may be omitted.
• the absorbent resin, pulp, and exterior material are separated.
• the absorbent material may contain a large amount of water or is dispersed in water due to the inactivation/dehydration treatment using a solvent (especially water) or the washing/disinfection treatment using water. Therefore, in this step, the absorbent resin, pulp, and exterior material are preferably separated from each other by known methods such as filtration, centrifugation, and specific gravity separation.
• the fractionation (separation) method used here can be a known method, either as is or with appropriate modifications.
• equipment used for such fractionation (separation) include gravity settling tanks, filtration equipment (e.g., wire mesh), screw presses, roller presses, rotary drum screens, belt screens, vibrating screens, pressurized dehydrators, vacuum dehydrators, belt presses, centrifugal thickeners, and liquid cyclones.
• the water absorbent resin recovered in the separation/filtration step (S4) is subjected to an acid treatment.
• the dissociated carboxyl groups are in an ionically cross-linked state via polyvalent metal ions.
• the water absorption performance e.g., CRC
• the regenerated water absorbent resin can be restored, and the water absorption performance of the regenerated water absorbent resin can be improved.
• the acid treatment is not particularly limited as long as it is in a form that allows the water-absorbing resin to come into contact with an acidic substance, and a known method can be used as is or with appropriate modifications.
• a method in which the water-absorbing resin (which has been subjected to inactivation and dehydration treatment) is added to a solution containing an acidic substance while stirring, thereby bringing the acidic substance into contact with the water-absorbing resin.
• the acidic substance is not particularly limited, and for example, an organic acid and/or an inorganic acid can be used.
• the organic acids include organic substances having an acid group, such as a carboxyl group or a sulfo group.
• the inorganic acids include, for example, sulfuric acid, hydrochloric acid, and nitric acid.
• Specific examples of organic acids 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, p-toluenesulfonic acid, and the like). These organic acids and inorganic acids may be used alone or in combination of two or more.
• the amount of the acidic substance used is not particularly limited, but is preferably an amount that brings the pH of the dispersion containing the acidic substance and the water-absorbing resin into 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 release of carboxyl groups as described above to proceed, and is, for example, about 5°C to 80°C.
• 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).
• 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).
• 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.
• the amount of the alkali metal salt to be added is not particularly limited, but is preferably an amount that makes the pH of the water-absorbent resin neutralized with the alkali metal salt (regenerated water-absorbent resin) about 4.5 or more and 8.0 or less.
• the method for measuring the pH of the water-absorbent resin is not particularly limited, and can be measured, for example, using commercially available pH test paper and/or a commercially available pH meter.
• the treatment time for neutralization with the alkali metal salt is not particularly limited as long as it is sufficient for some of the carboxyl groups in the 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 sufficient for some of the carboxyl groups in the free acid state to be neutralized by the alkali metal ions, and is, for example, about 5°C to 90°C.
• washing process with water may be performed as necessary.
• the type of water (washing water) used for the washing process may be deionized water, tap water, distilled water, etc.
• the water-absorbent resin (the hydrous gel thereof) is dried. This allows the production of a regenerated water-absorbent resin (actually, a mixture of the regenerated water-absorbent resin and impurities), which is a dried polymer.
• a regenerated water-absorbent resin (actually, a mixture of the regenerated water-absorbent resin and impurities), which is a dried polymer.
• the step (S2) alone does not provide sufficient dehydration.
• 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.
• the method for drying the hydrous gel of the water-absorbent resin can be a known method, either as is or with appropriate modifications.
• Examples 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 moved while drying.
• heat drying, hot air drying, and stirring drying are preferred from the viewpoint of drying efficiency.
• equipment used for stirring drying include stirring dryers such as paddle dryers and rotary drum dryers.
• 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 less, more preferably 10 minutes or higher and 2 hours or lower, even more preferably 20 minutes or higher and 1.5 hours or lower, and particularly preferably 30 minutes or higher and 1 hour or lower.
• the mixture of the recycled water absorbent resin and impurities obtained in the above manner is then subjected to a crushing process (S200) and a separation process (S300).
• [2-2] Crushing step In this step, a physical force is applied to the 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 the integrated state and to make them independent of each other, so that the water absorbent resin and the impurities can be efficiently separated in the separation step, which is the next step.
• the crushing step in the present invention may be a step of performing a treatment to crush the 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 independent of each other in part.
• the object to be subjected to the crushing step may further include not only the mixture in which the water absorbent resin and the impurities are fixed and integrated, but also the water absorbent resin and the impurities in an independent state.
• disintegrating the mixture refers to disintegrating the integrated state of the mixture in which the water-absorbent resin and impurities are fixed together, that is, to making the water-absorbent resin and impurities independent and separated from each other. Therefore, even if a physical force is applied to the mixture, if the result is that the water-absorbent resin and impurities are still integrated (the state of the mixture is maintained), the operation is not included in the disintegration step according to the present invention.
• 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 impurities are fixed together.
• 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.
• the crushing is partial, as described in the above [1-6] Mixture section, the mixture after crushing contains the water-absorbing resin and impurities that are in an independent state, as well as the mixture that has not been crushed (a mixture in which the water-absorbing resin and impurities are integrated).
• the disintegration preferably grinding
• a solution dry type
• an additional classification process may be carried out before the crushing process.
• 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.
• the crushing step is preferably performed by crushing (preferably pulverizing) the integrated mixture, since it is difficult to destroy the pulp, which is the main component of the impurities, and it is easy to separate the water-absorbent resin and the impurities in the subsequent separation step.
• "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.
• the particle diameter when the particle diameter is further reduced (for example, the particle diameter may be less than 500 ⁇ m, may be less than 300 ⁇ m, or may be less than 150 ⁇ m), it may be written as "pulverization”.
• the apparatus (crushing apparatus) used in the crushing step is not particularly limited, and examples thereof include a pin mill, a cyclone mill, a roll mill, a roller mill, a hammer mill, a rod mill, a ball mill, a jet mill, a bead mill, a vibration mill, a knuckle type crusher, a cylindrical mixer, and the like. These apparatuses may be used in combination.
• the crushing step is preferably performed using one or more apparatuses 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.
• the crushing step is more preferably performed using one or more apparatuses selected from the group consisting of a pin mill, a cyclone mill, a roll mill, a roller mill, and a hammer mill, and it is particularly preferable to use a roll mill or a hammer mill.
• the conditions for crushing the mixture are not particularly limited, but as described above, it is preferable that the conditions are such that the absorbent resin can be selectively crushed while suppressing the destruction of the pulp, which is the main component of the impurities. Specifically, it is preferable that the conditions are such that the particle diameter of the absorbent resin contained in the mixture after crushing is smaller than the size of the impurities (mainly the size of the pulp) contained in the mixture after crushing. In other words, it is preferable that the crushing process is performed so that the particle diameter of the absorbent resin contained in the mixture after crushing is smaller than the size of the impurities (mainly the size of the pulp) contained in the mixture after crushing. Note that fibrous substances such as pulp are not necessarily linear, and may be curved, etc.
• the size of the impurities refers to the length (distance) in the long side direction of the smallest rectangle that surrounds the appearance of the impurities (pulp) in the state of being contained in the mixture after crushing.
• the particle diameter of the water absorbent resin contained in the mixture after crushing may have a particle diameter (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 ⁇ m) or less, less than 0.5 mm (500 ⁇ m), 0.3 mm (300 ⁇ m) or less, less than 0.3 mm (300 ⁇ m), 0.2 mm (200 ⁇ m) or less, less than 0.2 mm (200 ⁇ m), 0.15 mm (150 ⁇ m) or less, or less than 0.15 mm (150 ⁇ m).
• 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.
• the 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.
• the temperature during disintegration is not particularly limited, and is preferably 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.
• the relative humidity is preferably 65% RH or less, more preferably 60% RH or less, and particularly preferably 55% RH or less.
• the lower limit is not particularly limited, but an example is 20% RH or more.
• the mixture subjected to the disintegration step may be a mixture of the 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 the recycled water absorbent resin obtained through a pretreatment step (preparation step of the recycled water absorbent resin by wet separation) in which the water absorbent resin (water absorbent resin swollen by water absorption) contained in the water-absorbent article is separated from the impurities, and the impurities.
• a pretreatment step preparation step of the recycled water absorbent resin by wet separation
• Preparation step of the recycled water absorbent resin.
• the preferred embodiments and conditions of the pretreatment process are as described in the above section [2-1] Preparation process for recycled water-absorbent resin (pretreatment process), but the preparation process (pretreatment process) for recycled water-absorbent resin to be subjected to the crushing process is not limited to these.
• the above-mentioned preparation process (pretreatment process) 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 "water-absorbent article" can be a used absorbent article. Therefore, in one embodiment of the present invention, the mixture subjected to the disintegration process can be a mixture of the recycled water absorbent resin obtained through a pretreatment process (preparation process of the recycled water absorbent resin by wet separation) for separating the water absorbent resin contained in the used absorbent article from the impurities, and the impurities.
• the mixture to be subjected to the disintegration step may be prepared without going through the above-mentioned wet separation process (preparation step (pretreatment step) of the recycled water-absorbent resin).
• pretreatment step pretreatment step
• the absorbent material may absorb moisture, and the water-absorbent resin and the pulp may be strongly adhered to each other.
• the exterior material, etc. may be crushed (shredded), and then the exterior material, etc.
• the water-absorbent resin and the water-absorbent resin may be separated by a known method without going through wet separation using a solvent (i.e., dry), to prepare a mixture. That is, the pre-use discarded absorbent article may be crushed, and the water-absorbent resin obtained by roughly separating the exterior material, etc. may be directly subjected to the disintegration step.
• a solvent i.e., dry
• the mixture subjected to the disintegration step may be one obtained by recovering absorbent resin from absorbent articles (particularly pre-use discarded absorbent articles) that have absorbed moisture (a mixture in which recovered 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 absorbent resin from pre-use discarded absorbent articles without going through wet separation (dry method) (a mixture in which absorbent resin and impurities are integrated).
• the mixture (particularly, a mixture in which recycled water absorbent resin and impurities are integrated) subjected to the disintegration step preferably has a relatively low content of impurities from the viewpoint of making it easier to obtain a water absorbent resin with a reduced amount of impurities.
• the content of impurities contained in the mixture (particularly, a mixture in which recycled water absorbent resin and impurities are integrated) is preferably less than 20% by mass with respect to the total mass of the mixture (total mass is 100% by mass).
• 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 with respect to the total mass of the mixture (total mass is 100% by mass).
• a mixture in which the content of impurities is within the above range can be obtained, for example, by passing through a wet separation process exemplified in the preparation process (pretreatment process) of the recycled water absorbent resin. That is, in one embodiment, the mixture subjected to the disintegration step is a mixture in which the regenerated water absorbent resin and impurities, the content of which is within the above range, are integrated.
• the lower limit of the content of impurities 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 is 100% by mass).
• the mixture to be subjected to the crushing step has a relatively small amount of water-soluble content.
• the water-soluble content has the effect of fixing the impurities to the water-absorbent resin. Therefore, in a mixture with a small amount of water-soluble content (particularly, a mixture in which the recycled water-absorbent resin and the impurities are integrated), the adhesion between the water-absorbent resin and the impurities is relatively weak, and it can be said that they are easier to separate.
• the amount of water-soluble content contained in the mixture is 45% by mass or less with respect to the total mass of the mixture (total mass is 100% by mass). Furthermore, from the same viewpoint as above, the amount of water-soluble content is more preferably 35% by mass or less with respect to the total mass of the mixture (total mass is 100% by mass), even more preferably 10% by mass or less, and particularly preferably 5% by mass or less.
• the water absorbent resin and the impurities are separated by dissolving the water soluble content in an aqueous solution, so that the water soluble content can achieve the above range. That is, in one embodiment, the mixture subjected to the disintegration process is a mixture of regenerated water absorbent resin and impurities having a water soluble content within the above range.
• the lower limit of the water soluble 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 is 100% by mass).
• a mixture in which both the content of impurities and the amount of water-soluble matter can be within the preferred range there can be mentioned a mixture of regenerated water absorbent resin and impurities obtained by going through a wet separation process exemplified in the preparation process (pretreatment process) of the regenerated water absorbent resin described above.
• a mixture in which regenerated water absorbent resin and impurities are integrated can contain components derived from the contraction agent because it goes through an inactivation and dehydration process (inactivation and dehydration process step (S2)) using a contraction agent.
• examples of such components include polyvalent metal ions contained in polyvalent metal salts.
• 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 in which regenerated water absorbent resin and impurities are integrated) subjected to the crushing process can be 0.5% by mass or more with respect to the total mass of the mixture (particularly, the mixture in which regenerated water absorbent resin and impurities are integrated) (total mass is taken as 100% by mass).
• the content of the polyvalent metal ions may be 1% by mass or more, or 2% by mass or more, based on the total mass of the mixture.
• the upper limit of the content of the polyvalent metal ions contained in the mixture is not particularly limited, but may be 5% by mass or less, for example.
• the polyvalent metal ions contained in the mixture include alkaline earth metal ions, transition metal ions, etc.
• alkaline earth metal ions and transition metal ions see the description in the (Inactivation and Dehydration Treatment Step (S2)) section in the above section [2-1] Preparation Step (Pretreatment Step) of Regenerated Water Absorbent Resin.
• the content (total amount) of calcium ions and magnesium ions contained in the mixture (particularly, the mixture in which the regenerated water absorbent resin and impurities are integrated) subjected to the disintegration step may be 0.3% by mass or more (total mass is taken as 100% by mass) relative to the total mass of the mixture (particularly, the mixture in which the regenerated water absorbent resin and impurities are integrated).
• 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.
• there is no particular upper limit to the content (total amount) of calcium ions and magnesium ions contained in the mixture but as an example, it can be 5% by mass or less.
• 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 filtrate obtained is then diluted in a 50 mL measuring flask with a 1% by mass aqueous nitric acid solution. 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.
• the mixture (particularly, a mixture in which recycled water-absorbent resin and impurities are integrated) to be subjected to the crushing step preferably has a low moisture content.
• the adhesion between the water-absorbent resin and impurities is relatively weak, and they are easier to separate.
• the moisture content of the mixture (particularly, a mixture in which recycled water-absorbent resin and impurities are integrated) is preferably 20% by mass or less.
• 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.
• the mixture may be dried before being subjected to the crushing step.
• the lower limit of the moisture content of the mixture is not particularly limited, but as an example, it is 3% by mass or more.
• the moisture content of the mixture is calculated from the mass change when 1 g of the mixture is dried by heating it at 180°C for 3 hours. Specifically, it is expressed by the following formula (5).
• the mixture after crushing i.e., subjected to the separation process
• a mixture of recycled water-absorbent resin and impurities is preferably in any of the following forms (I) to (III):
• the moisture content of the mixture after crushing subjected to the separation process is 10% by mass or less, and the particle size of the mixture after crushing subjected to the separation process is less than 0.15 mm (150 ⁇ m).
• the moisture content of the mixture after crushing subjected to the separation process is 8% by mass or less, and the particle size of the mixture after crushing subjected to the separation process is less than 0.3 mm (300 ⁇ m).
• 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 ⁇ m). That is, the crushing step is preferably performed so that the water content of the crushed mixture and the particle size of the crushed mixture are within the above range.
• the lower limit of the particle size of the crushed mixture subjected to the separation step is not particularly limited, but may be 0.01 mm (10 ⁇ m) or more, or 0.05 mm (50 ⁇ m) or more, for example.
• the "particle size” refers to the mass average particle size (D50) unless otherwise specified, and is a value obtained by the following procedure.
• the lower limit of the water content is not particularly limited, but may be 3% by mass or more, for example.
• JIS standard sieve The IIDA TESTING SIEVE/inner diameter: 80 mm
• JIS Z 8801-1 (2000) JIS Z 8801-1 (2000)
• a sieve equivalent to the JIS standard sieve 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 the sieve equivalent thereto are 850 ⁇ m, 710 ⁇ m, 600 ⁇ m, 500 ⁇ m, 425 ⁇ m, 300 ⁇ m, 212 ⁇ m, 150 ⁇ m, and 45 ⁇ m.
• the mass average particle size (D50) is the particle size corresponding to 50 mass% of the entire water-absorbent resin (when the measurement target is a crushed mixture, the entire crushed mixture).
• the absorbent resin separated from the crushed mixture may reattach to impurities if it absorbs water (moisture), so to avoid this, it is preferable to carry out the separation process in a dry manner.
• the device used to carry 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.
• sieve classification devices such as vibrating sieves, low-head screens, electromagnetic screens, and tumbling type sieves
• 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 be used in combination.
• the separation process may be any of the following forms (i) to (iii): (i) having at least one separation process using a sieve classifier and at least one separation process using an air classifier in any order; (ii) having at least two separation processes using a sieve classifier; (iii) having at least two separation processes using an air classifier.
• the form (ii) does not perform a separation process using an air classifier, and the form (iii) does not perform a separation process using a sieve classifier, and thus can be distinguished from the form (i).
• the upper limit of the number of times (total number of times) the separation step is performed is not particularly limited, but is, for example, 10 times or less in total.
• the first separation may be performed using a sieve classifier, and the second separation may be performed using an air classifier.
• the separation using the sieve classifier and the separation using the air classifier may be performed in the reverse order to that described above.
• the first separation may be performed using an air classifier
• the second separation may be performed using a sieve classifier.
• the above-mentioned (i) embodiment may be a form in which separation by a sieve classifier is performed two or more times and separation by an air classifier is performed at least once, or a form in which separation by a sieve classifier is performed at least once and separation by an air classifier is performed two or more times.
• the order of the separation steps by these classifiers/classifiers does not matter, and they may be performed in any order.
• a total of three or more separation operations (separation steps) may be performed in any order using a sieve classifier and separation by an air classifier.
• a method in which separation is performed using a sieve classifier in the first separation, separation is performed using an air classifier in the second separation, and separation by a sieve classifier again in the third separation may be exemplified.
• the type and conditions of the classifier/classifier may be changed for each separation operation.
• the first separation may be performed using a vibrating sieve
• the second separation may be performed using an air classifier that uses centrifugal force
• the third separation may be performed using an air classifier that uses gravity.
• the sieve classifier used in the above embodiments (i) and (ii) it is preferable to use a vibrating sieve machine.
• the air classifier used in the above embodiments (i) and (iii) it is preferable to use an air classifier that uses centrifugal force, and among these, it is more preferable to use a cyclone centrifuge.
• the separation process using the air classifier is preferably a separation process using a cyclone centrifuge.
• the separation step according to the present invention is preferably carried out by the form (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.
• the separation step using the air classifier may include a separation step using a cyclone centrifuge.
• a method in which the first separation is performed using a vibrating sieve machine and the second separation is performed using a cyclone centrifuge can be mentioned. 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.
• a method for improving the recovery rate of 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, and a separation step in which the water-absorbent resin and impurities are separated from the mixture crushed in the crushing step, and the separation step is in any of the forms (i) to (iii).
• the separation step is preferably in the form (i), and it is more preferable that the separation step includes a separation step using a sieve classifier and a separation step using an air classifier in any order.
• the separation step may include a separation step using a sieve classifier and a separation step using a cyclone centrifuge in any order.
• the recovery rate of the water absorbent resin indicates the mass ratio (ratio) of the water absorbent resin recovered after the final separation step to the total mass of the crushed mixture (i.e., the crushed mixture to be subjected to the final separation step) including the water absorbent resin and impurities immediately before the final separation step, and the integrated mixture that was not completely crushed and remains, and is expressed by the following formula (6).
• the "final separation step” here refers to the separation step when the separation step is performed only once.
• 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.
• 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 crushed mixture immediately before the second separation step.
• 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, so it is presumed that a synergistic effect is obtained in terms of separation.
• the mixture (a mixture of the remaining water-absorbent resin that was not completely crushed and impurities integrated together) 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 to observe the state of the mixture after crushing after performing the first separation step, and then perform the second crushing step. 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.
• the humidity during the separation process is not particularly limited, but from the viewpoint of 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.
• the lower limit is not particularly limited, but an example is 20% RH or more.
• the separation device may be used in a heated state or in a warmed state. By heating or warming the separation device in this way, the generation of adhesions and agglomerates as described above can be effectively suppressed.
• heating refers to heating the inner wall surface of the separation device to 40°C or higher, and preferably 50°C or higher.
• the upper limit of the heating temperature is not particularly limited, but in order to suppress the denaturation of the water-absorbing resin, it is preferable that it is 100°C or lower.
• warming 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.
• Methods for heating and/or keeping the separation device in a warm state include, for example, providing a heating means or a heat-retaining means inside or outside the separation device, or attaching these to the separation device.
• the heating method using such a heating means is not particularly limited as long as it can heat the separation device, and examples include a method of providing the separation device with a jacket that can be heated by electricity or steam, and a method of wrapping a heating resistor around the separation device.
• examples of heat-retaining methods using a heat-retaining means include a method of wrapping a heat insulating material (heat-retaining material) around the separation device.
• the heat insulating material (heat-retaining material) used is not particularly limited, and examples of usable materials 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; powder 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-cell glass heat insulating material; air layer heat insulating materials such as metal foil heat insulating material and paper honeycomb.
• two or more of the heating means/insulation means may be used in combination, and both a heating means and an insulation means may be used.
• the following method may be performed. That is, in another embodiment, a method of forming the inner wall surface of the separation device with a hydrophobic material can be mentioned. By forming the inner wall surface of the separation device with such a hydrophobic material, the mixture after crushing is less likely to adhere, and the generation of the adhesions and aggregates can be suppressed.
• a hydrophobic material a material having a contact angle with water of 60° or more and a heat deformation temperature of 70°C or more is preferable.
• the heat deformation temperature is intended to be a value measured in accordance with ASTM D648. If the contact angle with water of the material of the contact surface is 60° or more, the mixture after crushing containing moisture is less likely to adhere to the inner wall surface (inner side wall) of the separation device. In addition, if the material of the contact surface has a heat deformation temperature of 70°C or more, the resistance of the mixture after crushing that is heated (high temperature) in the separation operation is improved, and stable separation operation can be continued for a long 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.
• hydrophobic materials examples 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 and the synthetic resins.
• 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.
• 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).
• the content of the impurities may be 3% by mass or less, or 2% by mass or less. Meanwhile, the lower limit value may theoretically be 0% by mass.
• the water absorbent resin recovered through the crushing step and the separation step may be reused (recycled) as it is, but it is preferable that the water absorbent resin having a particle size of less than 150 ⁇ m (hereinafter, also simply referred to as "fine powder”) is reused through the following fine powder recovery step.
• a water-absorbent resin can be carried out through, for example, a step of preparing an aqueous monomer solution, a polymerization step, an optional step of crushing a hydrous gel, a drying step, an optional step of crushing after drying, an optional step of classification, an optional step of recovering fine powder, a surface cross-linking step, an optional step of cooling, and an optional step of sizing.
• a step of preparing an aqueous monomer solution a polymerization step
• an optional step of crushing a hydrous gel e.g., a drying step, an optional step of crushing after drying, an optional step of classification, an optional step of recovering fine powder, a surface cross-linking step, an optional step of cooling, and an optional step of sizing.
• 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.
• 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 is 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 water-containing gel-like crosslinked polymer (hereinafter also referred to as "water-containing gel").
• the water-containing gel crushing step is a step of gel-crushing the water-containing gel obtained in the polymerization step.
• the drying step is a step of drying the water-containing gel and/or water-containing gel particles obtained in the polymerization step and/or water-containing gel crushing step until the desired resin solid content is reached, thereby obtaining a water-absorbent resin dried product (hereinafter sometimes 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-linking 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 particle size adjustment step is a step of adjusting the particle size of the surface-cross-linked water-absorbent resin.
• the fine powder recovery step refers to a step of recovering (mixing) the fine powder and/or its water additive contained in the mixture-derived water absorbent resin in any step of the general water absorbent resin manufacturing process (preferably a step before the drying step).
• the fine powder contained in the mixture-derived water absorbent resin can be recovered as it is or after hydration or granulation in any step of the general water absorbent resin manufacturing process (preferably a step before the drying step, more preferably a step of preparing an aqueous monomer solution, a polymerization step, a hydrous gel crushing step or a drying step, and particularly preferably a polymerization step, a hydrous gel crushing step or a drying step).
• the "fine powder contained in the water absorbent resin derived from the mixture” refers to particles with a particle diameter of less than 150 ⁇ m, which account for 70 mass% or more of the total mass of the water absorbent resin derived from the mixture.
• the form of "recovery" of the fine powder contained in the water absorbent resin derived from the mixture may be a form in which the fine powder is recovered as it is, or a form in which the fine powder is recovered as a granulated product.
• the granulated product may be recovered in a gelled state by swelling it with an appropriate amount of water.
• additives may be mixed with the fine powder or its granules.
• the additives are not particularly limited as long as they are used in the production of water absorbent resin, and examples thereof include water, crosslinking agents, binders other than water (e.g., water-soluble polymers, thermoplastic resins, etc.), polymerization initiators, reducing agents, chelating agents, coloring inhibitors, inorganic fine particles, etc.
• the amount of water added is preferably 1% by mass or more and 1000% by mass or less based on the total mass (total mass is taken as 100% by mass) of the water and the solid content of the fine powder (when recovered as a granule, the solid content of the granule, the same applies below).
• the amount of the additive added is preferably 0.01% by mass or more and 10% by mass or less based on the total mass (total mass is taken as 100% by mass) of the additives other than water and the solid content of the fine powder.
• Methods for recovering fine powder in the preparation process of the aqueous monomer solution are disclosed in WO 92/001008, WO 92/020723, etc.
• Methods for recovering fine powder in the polymerization process are disclosed in WO 2007/074167, WO 2009/109563, WO 2009/153196, WO 2010/006937, etc.
• Methods for recovering fine powder in the drying process are disclosed in U.S. Patent No. 6,228,930, etc.
• the amount of the fine powder or granulated material contained in the water absorbent resin derived from the recovered mixture is adjusted appropriately depending on the process in which the fine powder or granulated material is recovered.
• the fine powder or its granules when the fine powder or its granules are recovered in the monomer aqueous solution preparation step, the fine powder or its granules are 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).
• the lower limit of the recovery amount is, for example, 1% by mass or more.
• the ratio (solid content ratio) of the primary particles of the monomer-derived hydrogel to the fine powder or its granules of the water-absorbent resin derived from the mixture in the obtained water-absorbent resin can be appropriately controlled.
• the fine powder or its granules contained in the water-absorbing resin derived from the mixture is mixed with a hydrogel derived from a monomer, from the viewpoint of the water-absorbing performance and productivity of the obtained water-absorbing agent (water-absorbing resin), the fine powder or its granules 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, based on the total mass of the solid contents of the hydrogel and the fine powder (or its granules) (total mass being 100% by mass).
• the lower limit of the recovery amount is, for example, 1% by mass or more.
• the method according to 1. or 2. above, wherein the mixture (the mixture to be subjected to the crushing step) has a moisture content of 20% by mass or less; 4.
• the mixture (the mixture to be subjected to the crushing step) has a water-soluble content of 0.5 mass% or more; 5.
• the separation step is in any one of the following forms (i) to (iii): (i) At least one step of separation using a sieve classifier and at least one step of separation using an air classifier, in any order; (ii) Having at least two or more steps of separation using a sieve classification device; (iii) At least two steps of separation using an air classifier; 8.
• the water content of the crushed mixture subjected to the separation step is 10 mass% or less, and the particle size of the crushed mixture subjected to the separation step is less than 150 ⁇ m;
• the water content of the crushed mixture subjected to the separation step is 8 mass% or less, and the particle size of the crushed mixture subjected to the separation step is less than 300 ⁇ m;
• the water content of the mixture after the crushing step to be subjected to the separation step is 5 mass% or less, and the particle size of the mixture after the crushing step to be subjected to the separation step is less than 500 ⁇ m; 10.
• the impurities include pulp; 11. The method according to any one of 1. to 10. above, wherein the mixture (the mixture to be subjected to the disintegration step) is obtained through a pretreatment step of separating the water-absorbent resin contained in the water-absorbent article from impurities; 12. The method according to any one of 1. to 10., wherein the mixture (the mixture to be subjected to the disintegration step) is obtained by recovering the mixture from a moisture-absorbent article.
• 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 content of 23% by mass.
• the moisture content and water-soluble content were the same for the mixtures after crushing (roughly crushed material (C1), crushed material (1-1), crushed material (1-1) and crushed material (1-2)).
• the content of pulp relative to the total mass of the dried water-absorbent resin (S1) was 5% by mass.
• the dried water-absorbent resin material (S1) containing the pulp was roughly crushed to a level that made it easy to handle. Specifically, it was roughly crushed using a jaw crusher (Excel Jaw Crusher, manufactured by Yoshida Seisakusho Co., Ltd.) until the particle size was 4 to 5 mm, to obtain a roughly crushed material (C1).
• a jaw crusher Excel Jaw Crusher, manufactured by Yoshida Seisakusho Co., Ltd.
• 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 an opening 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 an opening of 2 mm.
• the cyclone centrifuge had a dome diameter of 600 mm, and was operated under the conditions of an air volume of 13 m 3 /min and a pulp/water-absorbent resin supply rate of 1 kg/min.
• the recovery rate of the mixture-derived water-absorbent resin (1-1) from the bottom of the cyclone was 94 mass%.
• the mixture-derived water-absorbent resin (1-1) from the bottom of the cyclone was observed with an SEM, almost no pulp was present, and only a single water-absorbent resin was present.
• 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 ⁇ m openings.
• Example 1-1 second separation step
• a JIS standard sieve with a mesh size of 106 ⁇ m was used to separate the pulp from the ground material (1-2).
• the recovery rate of the mixture-derived water-absorbent resin (1-2) that passed through the sieve and was recovered was 82 mass %.
• the mixture-derived water-absorbent resin (1-2) was observed with an SEM, almost no pulp was present, and only a single water-absorbent resin was present.
• the used absorbent article was then further shredded into pieces of 2-3 cm, after which a large excess of water (ion-exchanged water) was added to the shredded material and stirred to obtain an aqueous dispersion of pulp, nonwoven fabric, thermoplastic binder, and swollen gel particles of water-absorbent resin.
• a large excess of water ion-exchanged water
• the nonwoven fabric and thermoplastic binder were removed from the separated material floating in the water by manual sorting, and the remaining pulp was dried in a vacuum dryer at 90°C for three hours and then its mass was measured.
• the ratio of pulp contained in the settled swollen gel particles was calculated to be 10% by mass (the ratio to the total mass of the water-absorbent resin and pulp contained in the gel particles, calculated as the solid content).
• hydrogel (swollen water-absorbent resin) (G2) containing pulp was obtained.
• the obtained hydrogel (G2) was dried in a dryer (temperature inside: 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 water content of 4 mass% and a water-soluble content of 1 mass%.
• the values of the water content and the water-soluble content were the same for the mixture after crushing (roughly crushed product (C2), crushed product (2-1), pulverized product (2-1) and pulverized product (2-2)).
• the content of pulp relative to the total mass of the dried water-absorbent resin (S2) was 51 mass%.
• the dried water-absorbent resin (S2) was roughly crushed until the particle size became 4 to 5 mm, as in Comparative Example 1, to obtain a roughly crushed material (C2). Furthermore, the roughly crushed material (C2) was subjected to sieve classification and air classification (cyclone classification), but the roughly crushed material (C2) contained integrated water-absorbent resin and pulp, and it was not possible to separate the water-absorbent resin and pulp. Therefore, the rough crushing operation in Comparative Example 2 does not correspond to the crushing step according to the present invention.
• Example 2-1 (Crushing process) The dried water absorbent resin containing pulp (S2) obtained in Production Example 2 was crushed using a roll mill (Inokuchi Giken Co., Ltd.: roll mill) until the particle size became 1000 ⁇ m (1 mm) or less, to obtain a crushed product (2-1).
• a roll mill Inokuchi Giken Co., Ltd.: roll mill
• the crushed material (2-1) was classified using JIS standard sieves with mesh sizes of 850 ⁇ m, 500 ⁇ m, 300 ⁇ m, and 150 ⁇ m to obtain a 850 ⁇ m/500 ⁇ m particle fraction, a 500 ⁇ m/300 ⁇ m particle fraction, a 300 ⁇ m/150 ⁇ m particle fraction, and a 150 ⁇ m passing particle fraction.
• the 500 ⁇ m/300 ⁇ m particle fraction, 300 ⁇ m/150 ⁇ m particle fraction, and 150 ⁇ m passing particle fraction obtained by the above operation were mixed to obtain pulverized material (2-1). Pulp was removed from this pulverized material (2-1) using a cyclone centrifuge.
• the cyclone centrifuge had a dome diameter of 600 mm, and was operated under conditions of an air volume of 10 m3 /min and a pulp/water absorbent resin supply rate of 1 kg/min, and the recovery rate of the mixture-derived water absorbent resin (2-1) from the bottom of the cyclone was 96 mass%.
• the mixture-derived water absorbent resin (2-1) from the bottom of the cyclone was observed with an SEM, almost no pulp was present, and only a single water absorbent resin was present.
• 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 ⁇ m openings.
• Example 2-1 In the separation step of Example 2-1, instead of using a cyclone centrifuge, a JIS standard sieve with a mesh size of 425 ⁇ m was used to separate the pulp from the ground material (2-2). The recovery rate of the mixture-derived water-absorbent resin (2-2) that passed through the sieve and was recovered was 86 mass %. In addition, 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.
• the dried water-absorbent resin (S3) was roughly crushed until the particle size became 4 to 5 mm, as in Comparative Example 2, to obtain 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 integrated water-absorbent resin and pulp, and it was not possible to separate the water-absorbent resin and pulp. Therefore, the rough crushing operation in Comparative Example 3 does not correspond to the crushing step according to the present invention.
• Example 3-1 (Crushing process) The dried water absorbent resin containing pulp (S3) obtained in Production Example 3 was crushed using a roll mill (Inokuchi Giken Co., Ltd.: roll mill) until the particle size became 1000 ⁇ m (1 mm) or less, to obtain a crushed product (3-1).
• a roll mill Inokuchi Giken Co., Ltd.: roll mill
• the 300 ⁇ m passing particles 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 centrifuge.
• the cyclone centrifuge had a dome diameter of 600 mm, and was operated under conditions of an air volume of 10 m3 /min and a pulp/water-absorbent resin supply rate of 1 kg/min, and the recovery rate of the mixture-derived water-absorbent resin (3-1) from the bottom of the cyclone was 95 mass%.
• the mixture-derived water-absorbent resin (3-1) from the bottom of the cyclone was observed with an SEM, almost no pulp was present, and only a single water-absorbent resin was present.
• 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 ⁇ m openings.
• Example 3-1 In the separation step of Example 3-1, instead of using a cyclone centrifuge, a JIS standard sieve with a mesh size of 250 ⁇ m was used to separate the pulp from the ground material (3-2). The recovery rate of the mixture-derived water-absorbent resin (3-2) that passed through the sieve and was recovered was 83 mass %. In addition, 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.
• Examples 1-3 The pulp-containing dried water-absorbent resin (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 operation as in (first separation step) and (second crushing step) of Example 1-1 was performed to obtain a crushed product (1-3) that had passed through a sieve with a mesh size of 150 ⁇ m. The crushed product (1-3) was subjected to the same operation as in (second separation step) of Example 1-1 to separate the pulp from the crushed product (1-3) using a cyclone centrifuge. The recovery rate of the recovered mixture-derived water-absorbent resin (1-3) was 92% by mass. In addition, when the mixture-derived water-absorbent resin (1-3) was observed with an SEM, a small amount of pulp was found to be mixed in, but almost only the water-absorbent resin was present.
• Production Example 5 In Production Example 2, the same operation was performed except that the drying time of the hydrogel (G2) containing pulp in the dryer (temperature inside: 150° C.) was shortened to 30 minutes, to obtain a dried water-absorbent resin containing pulp (S5).
• 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 are also the same for the mixture after crushing (crushed product (2-3) and pulverized product (2-3)).
• Example 2-3 The pulp-containing water-absorbent resin dried product (S5) obtained in Production Example 5 was subjected to the same operation as in the (disintegration step) of Example 2-1 to obtain a disintegrated product (2-3). Next, the same operation as the sieve classification in the (separation step) of Example 2-1 was performed to obtain a pulverized product (2-3) that had passed through a sieve with an opening of 500 ⁇ m. The pulverized 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 pulverized product (2-3).
• the recovery rate of the recovered mixture-derived water-absorbent resin (2-3) was 91% by mass.
• the mixture-derived water-absorbent resin (2-3) was observed with an SEM, a small amount of pulp was found to be mixed in, but almost only the water-absorbent resin was present.
• Production Example 6 In Production Example 3, the same operation was performed except that the drying time of the hydrogel (G2) containing pulp in the dryer (temperature inside: 140° C.) was shortened to 45 minutes, to obtain a dried water-absorbent resin containing pulp (S6).
• 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 are also the same for the mixture after crushing (crushed product (3-3) and pulverized product (3-3)).
• Example 3-3 The pulp-containing water-absorbent resin dried product (S6) obtained in Production Example 6 was subjected to the same operation as in the (disintegration step) of Example 3-1 to obtain a disintegrated product (3-3). Next, the same operation as the sieve classification in the (separation step) of Example 3-1 was performed to obtain a pulverized product (3-3) that had passed through a sieve with an opening of 300 ⁇ m. The pulverized 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 pulverized product (3-3).
• the recovery rate of the recovered mixture-derived water-absorbent resin (3-3) was 94% by mass.
• the mixture-derived water-absorbent resin (3-3) was observed with an SEM, a small amount of pulp was found to be mixed in, but almost only the water-absorbent resin was present.
• Tables 1 to 4 below show the conditions and physical properties for the above manufacturing examples, working examples, and comparative examples.
• Reference Example 1 Production Example of Water Absorbent Resin
• 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 1.0 mass% diethylenetriaminepentaacetic acid trisodium (DTPA 3Na) aqueous solution, 103.7 g of 48.5 mass% sodium hydroxide aqueous solution, and 389.0 g of deionized water were added and mixed to prepare an aqueous monomer solution (a'). Next, the aqueous monomer solution (a') was cooled while stirring.
• DTPA 3Na diethylenetriaminepentaacetic acid trisodium
• aqueous monomer solution (a) When the liquid temperature reached 40.0°C, 100.9 g of 48.5 mass% sodium hydroxide aqueous solution 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.
• aqueous sodium persulfate solution was added to the stirred monomer aqueous solution (a), and immediately poured into a stainless steel vat-type container in an open-air system.
• the time from the start of the second stage neutralization to pouring the monomer aqueous solution (a) into the vat-type container was 55 seconds, and the vat-type container was heated using a hot plate until the surface temperature reached 40°C.
• the polymerization reaction started 70 seconds after the monomer aqueous solution (a) was poured into the vat-type container.
• the polymerization reaction proceeded with the monomer aqueous solution (a) expanding and foaming in all directions while generating steam, and then contracted to a size slightly larger than the vat-type container. After 3 minutes had passed since the start of the polymerization reaction, a hydrogel-like crosslinked polymer (hereinafter referred to as "hydrogel”) (R1) was taken out. The series of operations was carried out in an open-air system.
• the hydrogel (R1) obtained by the polymerization reaction was cut into strips and pulverized with a meat chopper having a die diameter of 7.5 mm, and then the hydrogel was spread on a 50-mesh wire net and dried with hot air for 60 minutes at 190° C.
• the dried product obtained was pulverized using a vibration mill and further passed through a sieve with an opening of 850 ⁇ m to obtain an irregularly pulverized water-absorbent resin precursor (R1) having a mass average particle diameter (D50) of 350 ⁇ m remaining on the sieve with an opening of 150 ⁇ m.
• R1 irregularly pulverized water-absorbent resin precursor having a mass average particle diameter (D50) of 350 ⁇ m remaining on the sieve with an opening of 150 ⁇ m.
• water-absorbent resin precursor (R1) 100 parts by mass of the obtained water-absorbent resin precursor (R1) was uniformly mixed with 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, and the mixture was heated at 100°C for 45 minutes. The mixture was then cooled and passed through a JIS standard sieve with a mesh size of 710 ⁇ m to obtain water-absorbent resin (R1). The CRC of water-absorbent resin (R1) was 35.9 (g/g), and the AAP0.3 was 29.4 (g/g).
• 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 ⁇ m to obtain an irregularly pulverized water-absorbent resin precursor (R2) with a mass average particle size (D50) of 380 ⁇ m.
• the CRC of the water-absorbent resin precursor (R2) was 47.2 (g/g).
• water-absorbent resin precursor (R2) 100 parts by mass of the obtained water-absorbent resin precursor (R2) was uniformly mixed with 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, and the mixture was heated at 100°C for 45 minutes. The mixture was then cooled and passed through a JIS standard sieve with a mesh size of 710 ⁇ m to obtain water-absorbent resin (R2). The CRC of water-absorbent resin (R2) was 34.5 (g/g), and the AAP0.3 was 28.1 (g/g).
• ⁇ Reference Example 3> A manufacturing example in which fine powder obtained from the mixture-derived water absorbent resin (1-1) was recovered in the fine powder recovery step.
• the mixture-derived water absorbent resin (1-1) obtained in Example 1-1 was classified with a 106 ⁇ m JIS standard sieve, and the 106 ⁇ m sieve-passing material was recovered to obtain a mixture-derived water absorbent resin fine powder (R3).
• a fine powder granulated gel (R3) solid content concentration of about 50 mass%) was produced using the mixture-derived water absorbent resin fine powder (R3) instead of the water absorbent resin fine powder (R2).
• the same operation was performed as in the above-mentioned Reference Example 2 (Reference Example 1), except that the hydrous gel (R3) obtained by the same method as in the above-mentioned 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, and a water absorbent resin (R3) was obtained.
• 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 the water-absorbent resin (R3) had almost the same water-absorbing properties as the water-absorbent resin (R2) of Reference Example 2.

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