How to recycle used hygiene products

Abstract

This invention provides a method for producing regenerated cellulose from pulp fibers, which are components of used sanitary products. [Solution] The present invention provides a method for producing regenerated cellulose, characterized in that it involves mixing crushed used sanitary products containing pulp fibers and a water-absorbent resin with an ionic liquid to dissolve the pulp fibers, separating the water-absorbent resin from the liquid containing the water-absorbent resin and the dissolved pulp fibers to prepare a pulp fiber solution, regenerating cellulose from the pulp fiber solution, and using a salt consisting of an acid with an acid dissociation constant of 4.26 or higher at 25°C in water as the ionic liquid.

Description

[Technical Field]

[0001] This invention relates to a method for recycling used sanitary products. More specifically, it relates to a method for recycling cellulose from pulp fibers of used sanitary products. [Background technology]

[0002] Due to the aging population, the number of people using adult disposable diapers for caregiving is increasing. Globally, the number of people using child disposable diapers is also increasing. In recent years, the amount of disposable diapers used has increased rapidly. These disposable diapers and other hygiene products are soiled with waste after use and are therefore discarded without being reused. Discarded used hygiene products are usually incinerated. However, with the increasing amount of used hygiene products being discarded, attempts are being made to recycle them from an environmental protection perspective. In order to recycle used hygiene products, their constituent components such as pulp fibers are separated and recovered from the used products.

[0003] For example, Patent Document 1 discloses a method for processing used sanitary products, comprising at least the steps of disintegrating the sanitary products and dispersing them in water, and separating and recovering the fibers and SAP contained in the sanitary products, characterized in that a polyvalent metal salt and an acidic substance, which are crosslinking agents, are added in the step of disintegrating the sanitary products and dispersing them in water. Patent Document 1 also discloses that in the step of separating and recovering the fibers and SAP contained in the sanitary products, the process is carried out using a screen or cleaner to first recover the SAP, and then recover the fibers from the dispersion containing the fibers.

[0004] Patent Document 2 discloses a method for recovering pulp fibers from used absorbent articles containing pulp fibers and superabsorbent polymers, the method comprising the steps of: treating the used absorbent articles with an aqueous organic acid solution having a pH of 2.5 or less at a temperature of 80°C or higher to inactivate the superabsorbent polymers; separating the pulp fibers and inactivated superabsorbent polymers from the used absorbent articles after treatment with the aqueous organic acid solution; treating the mixture containing the separated pulp fibers and inactivated superabsorbent polymers with an oxidizing agent (such as ozone) to decompose, reduce the molecular weight of, and solubilize the inactivated superabsorbent polymers; and separating the pulp fibers from the mixture treated with the oxidizing agent.

[0005] Patent Document 3 discloses a decomposition method characterized by contacting a water-absorbing polymer with a decomposition agent containing a periodate as an oxidizing agent, thereby decomposing the water-absorbing polymer.

[0006] Patent Document 4 discloses a method for decomposing a water-absorbing polymer in a water-absorbing polymer or a water-absorbing material containing the same, characterized in that a decomposition agent containing at least one alkaline compound and at least one persulfate compound which is an oxidizing agent is added to the water-absorbing polymer, and the water-absorbing polymer is decomposed in the presence of water.

[0007] Patent Document 5 discloses a method for recycling used disposable diapers, characterized by crushing the used diapers, then placing the crushed used diapers into a polymer decomposition tank where a polymer decomposition agent is mixed with water and stirred, and then decomposing the superabsorbent polymer contained in the diapers into monomers in the polymer decomposition tank before separating and recovering the pulp components contained in the diapers. Calcium chloride is described as an example of the polymer decomposition agent. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2013-150976 [Patent Document 2] Japanese Patent Publication No. 2018-021283 [Patent Document 3] Japanese Patent Publication No. 2001-316519 [Patent Document 4] Japanese Patent Publication No. 2003-321574 [Patent Document 5] Japanese Patent Publication No. 2000-84533 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] With the increasing demand for disposable diapers, the disposal of used disposable diapers is becoming a social problem. The superabsorbent polymer powder used in disposable diapers is water-insoluble and swells when it absorbs water. The absorbed polymer powder becomes sticky and aggregates while incorporating pulp fibers. Therefore, separating the aggregated polymer from the pulp fibers has been difficult.

[0010] The method of oxidatively decomposing superabsorbent polymers had a problem: the oxidizing agent also acted on the hydroxyl groups of pulp fibers, oxidizing them to carboxyl groups. Pulp fibers degraded by this oxidizing agent suffered from reduced quality.

[0011] Therefore, there is a need for efficient methods to recycle pulp fibers and superabsorbent polymers from used hygiene products.

[0012] The present invention has been made in view of the above circumstances, and aims to provide a method for recycling pulp fibers, which are components of used sanitary products. A further objective of the present invention is to provide a method for recycling superabsorbent resins, which are components of used sanitary products. [Means for solving the problem]

[0013] A preferred first aspect of the present invention for solving the above problems is a method for producing regenerated cellulose, comprising: a dissolving step of mixing a mixture of a water-absorbing resin and pulp fibers with an ionic liquid to dissolve the pulp fibers; a water-absorbing resin separation step of separating the water-absorbing resin from a liquid containing the water-absorbing resin and the dissolved pulp fibers to prepare a pulp fiber dissolution liquid; and a step of regenerating cellulose from the pulp fiber dissolution liquid, wherein a salt composed of an acid having an acid dissociation constant of 4.26 or more at 25 °C in water is used as the ionic liquid.

[0014] A preferred second aspect of the present invention for solving the above problems is a method for producing regenerated cellulose, comprising: a dissolving step of mixing a crushed product of a used sanitary product containing pulp fibers and a water-absorbing resin with an ionic liquid to dissolve the pulp fibers; a step of separating the water-absorbing resin from a liquid containing the water-absorbing resin and the dissolved pulp fibers to prepare a pulp fiber dissolution liquid; and a step of regenerating cellulose from the pulp fiber dissolution liquid, wherein a salt composed of an acid having an acid dissociation constant of 4.26 or more at 25 °C in water is used as the ionic liquid.

[0015] The gist of the present invention is that, from a mixture of a water-absorbing resin and pulp fibers, only the pulp fibers can be dissolved using an ionic liquid, so that the water-absorbing resin and the dissolved pulp fibers can be easily separated into solid and liquid. In addition, since the water-absorbing resin used in sanitary products is generally a salt of a crosslinked polymer of polyacrylic acid, by using a salt composed of an acid having an acid dissociation constant of 4.26 or more at 25 °C in water as the ionic liquid, ion exchange between the water-absorbing resin and the ionic liquid can be suppressed, and quality changes of the ionic liquid can be prevented.

Advantages of the Invention

[0016] According to the present invention, regenerated cellulose can be produced from pulp fibers, which are components of used sanitary products. In addition, the water-absorbing resin, which is a component of used sanitary products, can be recycled.

Brief Description of the Drawings

[0017] [Figure 1] Flowchart of the method of the present invention. [Figure 2] Explanatory drawing showing changes in the aggregated state of the water-absorbing resin.

Mode for Carrying Out the Invention

[0018] A preferred first aspect of the present invention is a dissolution step of mixing a mixture of a water-absorbing resin and pulp fibers with an ionic liquid to dissolve the pulp fibers, and separating the water-absorbing resin from a liquid containing the water-absorbing resin and the dissolved pulp fibers to prepare a pulp fiber dissolution solution. A water-absorbing resin separation step, and a step of regenerating cellulose from the pulp fiber dissolution solution, wherein, as the ionic liquid, a salt composed of an acid having an acid dissociation constant of 4.26 or more at 25 ° C. with water is used. It is a method for producing regenerated cellulose characterized by the above. A preferred second aspect of the present invention is a dissolution step of mixing a crushed product of a used sanitary product containing pulp fibers and a water-absorbing resin with an ionic liquid to dissolve the pulp fibers, and a step of separating the water-absorbing resin from a liquid containing the water-absorbing resin and the dissolved pulp fibers to prepare a pulp fiber dissolution solution. And a step of regenerating cellulose from the pulp fiber dissolution solution, wherein, as the ionic liquid, a salt composed of an acid having an acid dissociation constant of 4.26 or more at 25 ° C. with water is used. It is a method for producing regenerated cellulose characterized by the above.

[0019] Figure 1 is a flowchart for explaining the outline of the method for producing regenerated cellulose of the present invention. In a preferred first aspect of the present invention,

[0020] 1) A dissolution step of mixing a mixture of a water-absorbing resin and pulp fibers with an ionic liquid to dissolve the pulp fibers,

[0021] 2) A water-absorbing resin separation step of separating the water-absorbing resin from a liquid containing the water-absorbing resin and the dissolved pulp fibers to prepare a pulp fiber dissolution solution, And a step of regenerating cellulose from the pulp fiber dissolution solution. 3) A step of regenerating cellulose from the pulp fiber dissolution solution. 3) The process includes a step of regenerating cellulose from the pulp fiber dissolution solution (cellulose regeneration step).

[0022] In a preferred second aspect of the present invention, 1) A dissolution step in which crushed used sanitary products containing pulp fibers and superabsorbent resin are mixed with an ionic liquid to dissolve the pulp fibers, 2) A step of separating the water-absorbent resin from a liquid containing the water-absorbent resin and dissolved pulp fibers to prepare a pulp fiber solution (water-absorbent resin separation step), 3) The process includes a step of regenerating cellulose from the pulp fiber dissolution solution (cellulose regeneration step).

[0023] The used sanitary products to which the present invention applies are not particularly limited as long as they contain pulp fibers and superabsorbent resin, and examples include absorbent articles such as disposable paper diapers, incontinence pads, light incontinence pads, sanitary napkins, breast pads, and pet pads. Among these, the present invention can be suitably applied to disposable paper diapers, which use a large amount of pulp fibers.

[0024] 1) Regarding the dissolution process for dissolving pulp fibers In the dissolution process, a mixture of superabsorbent resin and pulp fibers is mixed with an ionic liquid (first embodiment). Examples of the mixture of superabsorbent resin and pulp fibers include absorbents used in absorbent articles such as disposable diapers and sanitary napkins. In used absorbents, the superabsorbent resin absorbs moisture from urine and excrement and coagulates, and the coagulated superabsorbent resin exists in a mixed state with the pulp fibers.

[0025] In a specific industrial embodiment (second embodiment), crushed used sanitary products containing pulp fibers and superabsorbent resin are mixed with an ionic liquid. In used sanitary products, the superabsorbent resin absorbs moisture from urine and excrement and coagulates, and the coagulated superabsorbent resin exists in a mixed state with the pulp fibers.

[0026] Used sanitary products are preferably crushed using a crusher, and more preferably using a single-shaft crusher or a twin-shaft crusher. Specific examples of crushers include, for example, the rotary press crusher manufactured by Oike Iron Works Co., Ltd., or the washing and crushing machine manufactured by Nippon Seam Co., Ltd. The size of the crushed material is preferably about 50 mm to 100 mm square. As a rotary press crusher, for example, the device disclosed in Figure 3 of Japanese Patent No. 7219570 can be used.

[0027] This crushing process shreds the film, nonwoven fabric, pulp fibers, and other materials that make up used sanitary products.

[0028] The pulp fibers used in this invention are those used in the manufacture of absorbent materials for absorbent articles. The pulp fibers are not particularly limited as long as they are fibers composed of cellulose molecules capable of forming a cotton-like mat that is an absorbent material. "Cellulose" is (C6H 10 It is a polymer compound represented as O5)n (where n is a natural number). Examples of the pulp fibers include cellulose fibers such as chemical pulp and dissolved pulp obtained from wood, and artificial cellulose fibers such as rayon and acetate. Examples of raw pulp for the pulp fibers include non-wood pulp such as wood pulp, cotton pulp, or straw pulp. As for wood pulp, softwood kraft pulp is preferred from the viewpoint of forming a cotton-like mat.

[0029] The pulp fibers to be recycled can be short fibers or long fibers, as long as they are made of cellulose molecules, which are polymer compounds. Since short pulp fibers are generally used in hygiene products, the present invention is suitably applied to short pulp fibers.

[0030] The water-absorbing resin to be recycled according to the present invention is not particularly limited, but examples include one made of a crosslinked polymer mainly composed of acrylic acid, in which at least a portion of its carboxyl groups have been neutralized.

[0031] The content of the acrylic acid component constituting the crosslinked polymer is preferably 90% by mass or more, more preferably 95% by mass or more, preferably 99% by mass or less, and even more preferably 97% by mass or less.

[0032] The cations that neutralize at least a portion of the carboxyl groups of the crosslinked polymer are not particularly limited, but examples include alkali metal ions such as lithium, sodium, and potassium, and alkaline earth metal ions such as magnesium and calcium. Among these, it is preferable that at least a portion of the carboxyl groups of the crosslinked polymer are neutralized with sodium ions. The neutralization of the carboxyl groups of the crosslinked polymer may be performed by neutralizing the carboxyl groups of the crosslinked polymer obtained by polymerization, or the crosslinked polymer may be formed using monomers that have been neutralized in advance.

[0033] The degree of neutralization of the carboxyl groups in the crosslinked polymer is preferably 50 mol% or more, more preferably 55 mol% or more, and even more preferably 60 mol% or more. Furthermore, there is no particular upper limit to the degree of neutralization, and all carboxyl groups may be neutralized. The degree of neutralization can be calculated using the following formula. Degree of neutralization (mol%) = 100 × "Number of moles of neutralized carboxyl groups in the crosslinked polymer" / "Total number of moles of carboxyl groups in the crosslinked polymer (including neutralized and unneutralized groups)"

[0034] The superabsorbent polymer contained in used sanitary products is preferably made of a crosslinked polymer with acrylic acid as its main component, and the crosslinked structure (crosslinked chain portion) is preferably formed by a crosslinking reaction between the carboxyl group of acrylic acid and a polyvalent glycidyl compound.

[0035] There are no particular limitations on the shape of the water-absorbing resin, and examples include irregularly shaped crushed, flake-shaped, pearl-shaped, and rice-grain-shaped powders. Of these, the irregularly shaped crushed form is preferred.

[0036] In a mixture of water-absorbent resin and pulp fibers, the mixing ratio of water-absorbent resin to pulp fibers (water-absorbent resin / pulp fibers, mass ratio) is not particularly limited, but is preferably 1 / 99 to 99 / 1, and more preferably 30 / 70 to 50 / 50.

[0037] In crushed used sanitary products containing pulp fibers and superabsorbent resin, the mixing ratio of superabsorbent resin to pulp fibers (superabsorbent resin / pulp fibers, mass ratio) is not particularly limited, but is preferably 1 / 99 to 99 / 1, and more preferably 20 / 80 to 60 / 40.

[0038] The ionic liquid used in the dissolution process of the present invention is not particularly limited as long as it dissolves pulp fibers.

[0039] The aforementioned ionic liquid is a salt that is liquid at room temperature (approximately 25°C). Since it is a salt, the ionic liquid has both a cation and an anion portion.

[0040] Examples of cations in ionic liquids include imidazolium type, quaternary ammonium type, pyridinium type, pyrrolidinium type, and phosphonium type.

[0041] Examples of imidazolium types include 1,3-dimethylimidazolium, 1-ethyl-3-methylimidazolium, 1-methyl-3-propylimidazolium, 1-butyl-3-methylimidazolium, 1-hexyl-3-methylimidazolium, 1-hexyl-3-octylimidazolium, 1-decyl-3-methylimidazolium, 1-dodecyl-3-methylimidazolium, 1-benzyl-3-methylimidazolium, and 1-ethyl-2,3-dimethylimidazolium.

[0042] Examples of pyridinium-type pyridinium include 1-ethylpyridinium, 1-butylpyridinium, 1-butyl-3-methylpyridinium, 1-butyl-4-methylpyridinium, 1-hexyl-4-methylpyridinium, and 1-octyl-4-methylpyridinium.

[0043] Examples of quaternary ammonium compounds include trimethylpropylammonium, tributylmethylammonium, methyltrioctylammonium, tetrabutylammonium, N-ethyl-N-(2-methoxyethyl)-N,N-dimethylammonium, and N-oleyl-N,N-di(2-hydroxyethyl)-N-methylammonium.

[0044] Examples of pyrrolidinium-type pyrrolidinium include 1-ethyl-1-methylpyrrolidinium, 1-methyl-1-propylpyrrolidinium, and 1-butyl-1-methylpyrrolidinium.

[0045] Examples of phosphonium-type phosphorylation include tetrabutylphosphonium and tributyldodecylphosphonium.

[0046] The ionic liquid used in this invention is a salt of an acid with an acid dissociation constant of 4.26 or higher at 25°C in water. That is, the anionic portion of the ionic liquid used in this invention is a proton (H) derived from an acid with an acid dissociation constant of 4.26 or higher at 25°C in water. + It is an anion formed by removing ().

[0047] As the acid component constituting the anion part of the ionic liquid, it is preferable that the acid is of the same degree or weaker than acrylic acid which is a constituent component of the water-absorbing resin powder. That is, as the acid constituting the anion part, it is preferable to use an acid having an acid dissociation constant equal to or higher than that of acrylic acid. By using a metal salt of an acid that is the same as or weaker than acrylic acid, acid substitution between the acid component constituting the anion part of the ionic liquid and the acid component (acrylate) contained in the water-absorbing resin component is prevented. The acid dissociation constant of acrylic acid at 25 °C in water is 4.26. In the present invention, as the ionic liquid, it is preferable to use a salt of an acid having an acid dissociation constant of 4.26 or more at 25 °C in water, more preferably a salt of an acid having an acid dissociation constant of 4.30 or more, and even more preferably a salt of an acid having an acid dissociation constant of 4.50 or more. The upper limit of the acid dissociation constant of the acid constituting the anion component is not particularly limited, but 12 is preferable and 10 is more preferable. For example, for phenol with an acid dissociation constant of 9.95, an effect is recognized, but for ethanol with an acid dissociation constant of 16, no effect is recognized. Acid (H n X n- ) The acid dissociation constant pKa is represented by the following formula. pKa = -log(1 / Ka), Ka = [H + n ·[X n- / [H n X n-

[0048] In the case of an acid that dissociates in multiple stages, it is preferable that at least one-stage acid dissociation constant is in the range of 4.26 to 12, more preferably in the range of 4.3 to 11, and even more preferably in the range of 4.5 to 10. Also, it is preferable that the dissociation constants of all stages are in the range of 4.26 to 12, more preferably in the range of 4.3 to 11, and even more preferably in the range of 4.5 to 10.

[0049] ​​Specific examples of acids with an acid dissociation constant of 4.26 or higher in water at 25°C include, for example, adipic acid (4.26), azelaic acid (4.39), p-anisic acid (4.48), o-aminobenzoic acid (second row: 4.95), p-aminobenzoic acid (4.85), isovaleric acid (4.58), isonicotinic acid (second row: 4.87), isobutyric acid (4.63), octanoic acid (4.89), valeric acid (4.64), quinaldic acid (second row: 4.75), citric acid (second row: 4.35), crotonic acid (4.69), and cinnamic acid (trans) (4.44). Examples include acetic acid (4.56), cyclohexanecarboxylic acid (4.70), (R,R)-tartaric acid (second row: 4.44), p-hydroxybenzoic acid (4.58), pimelic acid (4.31), 2,6-pyridinedicarboxylic acid (third row: 4.68), propionic acid (4.67), hexanoic acid (4.63), heptanoic acid (4.66), o-benzenedicarboxylic acid (second row: 4.93), malonic acid (second row: 5.28), butyric acid (4.63), malic acid (3.24), levulinic acid (4.44), and carbonic acid (6.35).

[0050] Specific examples of ionic liquids used in the present invention include alkylimidazolium-alkylcarboxylic acid, alkylimidazolium-alkylpropionic acid, alkylimidazolium-alkylbutyric acid, alkylimidazolium-alkyl phosphoric acid, and alkylammonium-hexafluorophosphate, with alkylimidazolium-alkylcarboxylic acid and alkylimidazolium-alkyl phosphoric acid being preferred.

[0051] In the dissolution process, it is preferable to mix the mixture of water-absorbent resin and pulp fibers with an ionic liquid, and more preferable to mix the mixture of water-absorbent resin and pulp fibers with an ionic liquid and water. When water is further mixed in the dissolution process, the ratio of ionic liquid to water (ionic liquid / water, mass ratio) is preferably 80 / 20 or more, more preferably 85 / 15 or more, even more preferably 90 / 10 or more, preferably 99 / 1 or less, more preferably 97 / 3 or less, and even more preferably 95 / 5 or less. By setting the ratio (mass ratio) of ionic liquid to water within the above range, the pulp fibers become easier to dissolve.

[0052] Furthermore, in the dissolution process, the amount of pulp fibers to be recycled is preferably 3 parts by mass or more, more preferably 4 parts by mass or more, even more preferably 5 parts by mass or more, preferably 35 parts by mass or less, more preferably 30 parts by mass or less, and even more preferably 25 parts by mass or less, per 100 parts by mass of the ionic liquid. If the amount of pulp fibers to be recycled is within the above range, the solubility of the pulp fibers will be good. In addition, the viscosity of the ionic liquid will decrease, and the workability will be improved.

[0053] The mixing of a mixture of water-absorbent resin and pulp fibers with an ionic liquid, or the mixing of crushed used sanitary products containing pulp fibers and water-absorbent resin with an ionic liquid, can be carried out, for example, using a tank equipped with a stirrer.

[0054] The duration of the dissolution process is preferably 4 hours or more, more preferably 5 hours or more, even more preferably 6 hours or more, preferably 24 hours or less, more preferably 12 hours or less, and even more preferably 10 hours or less.

[0055] The temperature of the dissolution process is preferably 40°C or higher, more preferably 45°C or higher, even more preferably 80°C or higher, preferably 120°C or lower, more preferably 100°C or lower, and even more preferably 90°C or lower. The temperature of the dissolution process is the temperature of the ionic liquid, or the temperature of the mixture obtained by mixing water and the ionic liquid. When mixing with water, care should be taken to prevent evaporation.

[0056] 2) The process of separating the superabsorbent resin from the mixture obtained in the dissolution process and obtaining a pulp fiber solution containing dissolved pulp fibers (superabsorbent resin separation process)

[0057] The mixture obtained in the dissolution step of the present invention contains a water-absorbent resin and dissolved pulp fibers. In the water-absorbent resin separation step, the water-absorbent resin is separated from this mixture to prepare a pulp fiber solution containing dissolved pulp fibers.

[0058] Methods for separating the superabsorbent polymer include, for example, decantation, vacuum filtration, pressure filtration, and atmospheric pressure filtration of the mixture. Specifically, it is preferable to allow the mixture to stand or centrifuge to precipitate the superabsorbent polymer, followed by decantation, vacuum filtration, pressure filtration, and atmospheric pressure filtration. It is even more preferable to separate the polymer by centrifugation followed by vacuum filtration and pressure filtration. If necessary, the separated superabsorbent polymer (cake) may be redispersed in an ionic liquid or a mixture of water and an ionic liquid, washed, and separated again.

[0059] It is also preferable to remove other solid matter from these mixed liquids simultaneously with the water-absorbent resin separation process, or before or after the water-absorbent resin separation process (film removal process). Examples of other solid matter include crushed film and nonwoven fabric that make up used sanitary products, and excrement.

[0060] The film removal process preferably uses a separation mechanism equipped with a perforated plate having multiple holes, and more preferably a separation mechanism equipped with a perforated plate having multiple holes with a diameter of 3 mm to 7 mm. Examples of separation mechanisms equipped with a perforated plate having multiple holes with a diameter of approximately 3 mm to 7 mm include pulpers, rotary drum type separators, and commercial washing and drying machines. For medium-scale equipment, it is preferable to use a pulper or a rotary drum type separator, and for small-scale equipment, it is preferable to use a commercial washing and drying machine.

[0061] In a separation mechanism equipped with a perforated plate having multiple holes with a diameter of approximately 3 mm to 7 mm, dissolved pulp fibers pass through the holes with a diameter of 3 mm to 7 mm, but the cut film and nonwoven fabric do not pass through these holes. For example, perforated metal can be suitably used as the perforated plate having multiple holes with a diameter of approximately 3 mm to 7 mm.

[0062] As a pulper that can be suitably adopted as the separation mechanism, known pulpers can be used. For example, the pulper disclosed in Figure 5 of Japanese Patent No. 7219570 can be used. The pulper preferably comprises a storage tank for storing a mixture obtained by mixing crushed used sanitary products with an ionic liquid, a perforated plate provided at the bottom of the storage tank, and a rotor rotatably provided on the upper part of the perforated plate. The storage tank has a cylindrical shape, and a circular perforated plate is provided in the center of the bottom.

[0063] The perforated plate preferably has multiple holes with a diameter of 3 mm to 7 mm. Below the perforated plate, there is an outlet for the permeable material that passes through the plate. By rotating the rotor, the mixed liquid circulates within the containment tank. The dissolved pulp fibers pass through the holes in the perforated plate and are discharged from the outlet.

[0064] An outlet for discharging cut film and nonwoven fabric is provided at the bottom of the storage tank. Preferably, the outlet for discharging cut film and nonwoven fabric is located on the outer circumference of a circular perforated plate provided in the center of the bottom, and close to the outer wall of the storage tank. A gate valve for opening and closing the outlet is connected to the outlet.

[0065] As an example of a rotary drum type separator that can be used as a separation mechanism, we can refer to the used disposable diaper separation device disclosed in Figures 2 to 6 of Japanese Patent Publication No. 2012-81433.

[0066] Examples of commercial washing and drying machines include those having a rotating drum with multiple holes. By pouring the mixed liquid into the rotating drum and rotating the drum to remove water, the film and nonwoven fabric can be removed. The dissolved pulp fibers are discharged as wastewater from the commercial washing and drying machine by centrifugal force, passing through the holes in the perimeter wall of the rotating drum. On the other hand, the shredded film and nonwoven fabric cannot pass through the holes in the rotating drum and remains inside the rotating drum. It is also preferable to use a commercial washing and drying machine equipped with a drying function to dry the shredded film and nonwoven fabric.

[0067] 3) About the cellulose regeneration process The present invention provides a method for producing regenerated cellulose, comprising the step of regenerating cellulose from the pulp fiber dissolution solution. Specifically, by bringing the pulp fiber dissolution solution into contact with water, the cellulose becomes insoluble and precipitates. The "cellulose" to be regenerated is (C6H 10 This means that the polymer compound is represented by O5)n (where n is a natural number), and its shape (such as fibrous or membranous) is irrelevant. It is preferable to use neutral water as the water.

[0068] The mass ratio of pulp fiber dissolving solution to water (pulp fiber dissolving solution / water) is preferably 80 / 20 or higher, more preferably 82 / 18 or higher, even more preferably 85 / 15 or higher, preferably 100 / 0 or lower, more preferably 97 / 3 or lower, and even more preferably 95 / 5 or lower.

[0069] In a preferred embodiment of the present invention, cellulose is regenerated into fibers. As a method for regenerating cellulose into fibers, for example, fibrous cellulose can be regenerated by continuously releasing a pulp fiber dissolution solution into water using a needle-shaped or tubular member such as a syringe.

[0070] In another preferred embodiment of the present invention, cellulose is regenerated in the form of a film or sheet. A method for regenerating cellulose in the form of a film or sheet is to apply a pulp fiber dissolving solution onto a film or substrate to form a film, and then introduce or immerse the film directly into water.

[0071] It is preferable to wash, dehydrate, and dry the regenerated cellulose as needed. It is also preferable to cut the dried cellulose into the desired size.

[0072] The regenerated cellulose obtained by this invention is of high quality and has few limitations on its reuse applications. Therefore, it can be expected to be applied in various fields. Examples of reuse applications include fibrous articles, film articles, and molded products. Fibrous articles can be divided into short fibers and long fibers. Short fibers can be used as pulp for disposable diapers, airlaid nonwoven fabrics, and thermal bond nonwoven fabrics by blending them with other raw cotton. Examples of use include paper substitutes for business cards and calendars, face masks, cooking paper, wet wipes, hand towels, and wet wipes. Long fibers can be used in spunbond nonwoven fabrics and textiles. Examples of use include gauze, adhesive bandages, oil filters, and drip sheets. Examples of film articles include packaging materials and laminates. Examples of molded products include fixtures and cutlery.

[0073] The present invention also includes a method for regenerating the superabsorbent resin separated in the superabsorbent resin separation process. Superabsorbent resin is used in hygiene products as a powder. When superabsorbent resin powder absorbs water, it swells and exists as aggregates of superabsorbent resin powder. These aggregates of superabsorbent resin powder contain a large amount of water, which makes them difficult to dry. In particular, even if they are dried as is, only the surface dries, and the inside of the aggregates remains dry.

[0074] In the present invention, it is preferable to regenerate the aggregates of the water-absorbent resin powder separated in the water-absorbent resin separation step by processing them as follows.

[0075] First, the aggregates of the superabsorbent polymer powder are mixed with a dehydrating agent to perform a dehydration treatment. This dehydration treatment removes the water-absorbing properties of the superabsorbent polymer powder. Divalent metal salts are preferably used as the dehydrating agent, and calcium chloride is more preferably used. By treating with a divalent metal salt, the carboxyl groups of the acrylic acid that make up the superabsorbent polymer powder are crosslinked. As a result, the volume of the aggregates of the superabsorbent polymer powder is reduced. Due to the change in the volume of the aggregates, water is discharged from the aggregates. This dehydration treatment reduces the water content of the superabsorbent polymer powder from approximately 98% by mass to approximately 70% by mass.

[0076] Next, a weak alkali metal salt is added to the agglomerates of the water-absorbent resin powder after dehydration. The weak alkali metal salt acts on the agglomerates of the water-absorbent resin powder. Treatment with the weak alkali metal salt causes an ion exchange reaction between the divalent metal ions and the alkali metal ions. As a result, the metal crosslinks are broken, making it easy to granulate the agglomerates of the water-absorbent resin powder. The granulated water-absorbent resin has an increased surface area, so it can be easily dried.

[0077] Figure 2 shows the change in the aggregation state of the water-absorbent resin powder. Figure 2(a) shows the aggregates of water-absorbent resin powder 1. Water-absorbent resin powder 1 undergoes metal crosslinking (-COO) due to the divalent metal ions of the dehydrating agent. - ·M 2+ · - It is aggregated by OOC-. Divalent metal ion M 2+ and alkali metal ions M + The metal bridge is broken by ion exchange between (-COO - M + M +- OOC-). As a result, the aggregates of the water-absorbent resin powder change into granular form (Figure 2(b)).

[0078] The amount of alkali metal salt of the weak acid added is preferably 1 part by mass or more, more preferably 3 parts by mass or more, preferably 100 parts by mass or less, and more preferably 30 parts by mass or less, per 100 parts by mass of the water-absorbing resin in a water-containing state treated with a dehydrating agent.

[0079] The mixing of the aggregated water-absorbing superabsorbent resin powder with the weak acid alkali metal salt is not particularly limited, but can be carried out using a rocking mixer such as the BHR type manufactured by Aichi Electric Co., Ltd. The mixing is not particularly limited, but is preferably carried out at a temperature range of 5°C to 130°C for 5 minutes to 6 hours. Dehydration may be carried out simultaneously with the mixing. The dehydration of the mixture is not particularly limited, but is preferably carried out at a temperature range of 80°C to 130°C for 5 minutes to 6 hours. It can be carried out under atmospheric pressure, reduced pressure, or pressurized pressure.

[0080] Upon mixing, the aggregates of the water-absorbing resin powder are transformed into granular or powdered water-absorbing resin. Since the granular or powdered water-absorbing resin powder has a large surface area, it can be easily dried.

[0081] The weak acid component of the alkali metal salt of the aforementioned weak acid is preferably an acid of the same degree as or weaker than acrylic acid, which is a component of the water-absorbent resin powder. That is, it is preferable to use an acid as the weak acid component that has an acid dissociation constant equal to or greater than that of acrylic acid. By using a metal salt of the same or weaker acidity as acrylic acid, the ion exchange reaction with the divalent metal salt used as a dehydrating agent becomes easier to occur. The acid dissociation constant of acrylic acid in water at 25°C is 4.26.

[0082] In other words, it is preferable to mix the aggregate of the water-absorbent resin powder after dehydration with an alkali metal salt of an acid having an acid dissociation constant of 4.26 or higher at 25°C in water. Examples of acids having an acid dissociation constant of 4.26 or higher at 25°C in water include acids that constitute the anionic portion of the ionic liquid used in the dissolution process.

[0083] Examples of alkali metals include lithium, sodium, and potassium. Specific examples of alkali metal salts of weak acids include sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, sodium citrate acetate, and sodium succinate acetate. Furthermore, it is preferable that the alkali metal salt of the weak acid be in solid form. This is because adding and mixing a solid alkali metal salt of the weak acid facilitates the discharge of water from the aggregates of the water-absorbing resin powder due to osmosis.

[0084] The granular water-absorbing resin obtained by the above processing method can be dried to regenerate granular water-absorbing resin powder. The regenerated water-absorbing resin powder also exhibits excellent absorption performance. For example, the absorption ratio of the regenerated water-absorbing resin powder is 15 g / g or higher.

[0085] The recycled water-absorbing resin powder can be used, for example, as a soil conditioner or as a solid fuel. [Examples]

[0086] The present invention will be described in detail below with reference to examples, but the present invention is not limited to the following examples, and any modifications and embodiments that do not depart from the spirit of the present invention are all included within the scope of the present invention.

[0087] (Example 1) The absorbent material was removed from a Saracare Pad Big (lot number: 20240501) manufactured by Ribdo Corporation, and a mixture of 70 parts by mass of pulp fiber and 30 parts by mass of superabsorbent resin was prepared. An aqueous solution of the ionic liquid was prepared by mixing 95 parts by mass of 1-ethyl-3-methylimidazolium acetate (1-ethyl-3-methylimidazolium acetate) and 5 parts by mass of water as an ionic liquid. The aqueous solution of the ionic liquid was heated to 90°C, and 5 parts by mass of the mixture of pulp fiber and superabsorbent resin was added and stirred for 1 hour to obtain a mixture. In the obtained mixture, the pulp fiber was dissolved, but the superabsorbent resin was not dissolved.

[0088] The resulting mixture was filtered to separate the superabsorbent resin, and a pulp fiber solution containing dissolved pulp fibers was obtained.

[0089] Ten mL of the obtained pulp fiber solution was filled into a 20 mL syringe with a needle tip diameter of 2 mm, and released into 500 mL of water to regenerate fibrous cellulose.

[0090] (Example 2) The absorbent material was removed from a Saracare Pad Big (lot number: 20240501) manufactured by Ribdo Corporation, and a mixture of 70 parts by mass of pulp fiber and 30 parts by mass of superabsorbent resin was prepared. An aqueous solution of the ionic liquid was prepared by mixing 95 parts by mass of the ionic liquid 1-ethyl-3-methylimidazolium butanate (1-ethyl-3-methylimidazolium butyrate) with 5 parts by mass of water. The aqueous solution of the ionic liquid was heated to 90°C, and 5 parts by mass of the mixture of pulp fiber and superabsorbent resin was added and stirred for 1 hour to obtain a mixture. In the obtained mixture, the pulp fiber was dissolved, but the superabsorbent resin was not dissolved.

[0091] (Example 3) 95 parts by mass of ionic liquid (SYIL-02, manufactured by Sanyo Chemical Industries, Ltd.) and 5 parts by mass of pulp fibers were mixed. The ionic liquid was heated to 90°C and stirred for 1 hour to dissolve the pulp fibers. Water was added to 100 parts by mass of the resulting pulp fiber solution in amounts of 5, 10, 15, and 20 parts by mass. Pulp fibers precipitated when the pulp fiber solution / water ratio was 80 / 20. The results are shown in Table 1.

[0092] [Table 1]

[0093] (Example 4) 95 parts by mass of ionic liquid (SYIL-21, manufactured by Sanyo Chemical Industries, Ltd.) and 5 parts by mass of pulp fibers were mixed. The ionic liquid was heated to 90°C and stirred for 1 hour to dissolve the pulp fibers. Water was added to 100 parts by mass of the resulting pulp fiber solution in amounts of 5, 10, 15, and 20 parts by mass. Pulp fibers precipitated when the pulp fiber solution / water ratio was 85 / 15. The results are shown in Table 2.

[0094] [Table 2]

[0095] (Example 5) Removal from used disposable diapers After absorbing 300cc of adult male urine into a used diaper (Saracare Pad Big, Lot No.: 20240501) manufactured by Ribdo Corporation, the absorbent material was removed from the used diaper after drying in a 100°C oven with natural airflow. A mixture of 70 parts by mass of pulp fiber and 30 parts by mass of superabsorbent resin was prepared. An aqueous solution of the ionic liquid was prepared by mixing 95 parts by mass of 1-ethyl-3-methylimidazolium acetate (1-ethyl-3-methylimidazolium acetate) with 5 parts by mass of water. The 95 parts by mass of the aqueous solution of the ionic liquid was heated to 90°C, and 5 parts by mass of the pulp fiber and superabsorbent resin mixture was added and stirred for 1 hour to obtain a mixture. In the obtained mixture, the pulp fiber was dissolved, but the superabsorbent resin was not.

[0096] (Comparative Example 1) A mixture was obtained in the same manner as in Example 1, except that 1-ethyl-3-methylimidazolium butanate (1-ethyl-3-methylimidazolium acetate) was replaced with 1-ethyl-3-methylimidazolium chloride (1-ethyl-3-methylimidazolium hydrochloride). In the resulting mixture, the pulp fibers were not completely dissolved, and the superabsorbent polymer was not dissolved. [Industrial applicability]

[0097] The method for producing regenerated cellulose and the method for producing regenerated superabsorbent resin of the present invention can be suitably used for used sanitary products that were used to absorb bodily fluids discharged from the human body, particularly used absorbent articles such as disposable diapers, incontinence pads, light incontinence pads, sanitary napkins, breast pads, and bed pads.

Claims

1. A dissolution step in which a mixture of water-absorbent resin and pulp fibers is mixed with an ionic liquid to dissolve the pulp fibers, A water-absorbing resin separation step is performed to separate the water-absorbing resin from a liquid containing the water-absorbing resin and dissolved pulp fibers in order to prepare a pulp fiber solution. A method for producing regenerated cellulose, comprising the step of regenerating cellulose from the pulp fiber dissolution solution, characterized in that the ionic liquid used is a salt consisting of water and an acid with an acid dissociation constant of 4.26 or higher at 25°C.

2. A method for producing regenerated cellulose according to claim 1, wherein the pulp fiber dissolution solution is brought into contact with water to regenerate cellulose.

3. A method for producing regenerated cellulose according to claim 1, wherein cellulose is regenerated into a fibrous form.

4. A method for producing regenerated cellulose according to claim 1, wherein cellulose is regenerated into a membrane.

5. A method for producing regenerated cellulose according to any one of claims 1 to 4, wherein water is further mixed in the dissolution step.

6. A method for producing a regenerated superabsorbent resin, comprising: a dehydration step of adding a dehydrating agent consisting of a divalent metal salt to the superabsorbent resin obtained in the water-absorbing resin separation step of claim 1 to dehydrate it; and a method for producing a regenerated superabsorbent resin of adding an alkali metal salt of an acid with an acid dissociation constant of 4.26 or higher at 25°C to the dehydrated superabsorbent resin and mixing it to form aggregates of the superabsorbent resin into granules, and drying the resulting granular superabsorbent resin.

7. A dissolution step in which crushed used sanitary products containing pulp fibers and superabsorbent resin are mixed with an ionic liquid to dissolve the pulp fibers, A water-absorbing resin separation step is performed to separate the water-absorbing resin from a liquid containing the water-absorbing resin and dissolved pulp fibers in order to prepare a pulp fiber solution. A method for producing regenerated cellulose, comprising the step of regenerating cellulose from the pulp fiber dissolution solution, characterized in that the ionic liquid used is a salt consisting of water and an acid with an acid dissociation constant of 4.26 or higher at 25°C.

8. A method for producing regenerated cellulose according to claim 7, wherein the pulp fiber solution is brought into contact with water to regenerate cellulose.

9. A method for producing regenerated cellulose according to claim 7, wherein cellulose is regenerated into a fibrous form.

10. A method for producing regenerated cellulose according to claim 7, wherein cellulose is regenerated into a membrane.

11. A method for producing regenerated cellulose according to any one of claims 7 to 10, wherein water is further mixed in the dissolution step.

12. A method for producing a regenerated superabsorbent resin, comprising: a dehydration step of adding a dehydrating agent consisting of a divalent metal salt to the superabsorbent resin obtained in the water-absorbent resin separation step of claim 7 to dehydrate it; and a method for producing a regenerated superabsorbent resin of adding an alkali metal salt of an acid with an acid dissociation constant of 4.26 or higher at 25°C to the dehydrated superabsorbent resin and mixing it to form aggregates of the superabsorbent resin into granules, and drying the resulting granular superabsorbent resin.

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