Method for producing recycled pulp fiber and recycled pulp fiber

Abstract

To provide a method for producing recycled pulp fibers whereby, articles using recycled pulp fibers tend to have uniform whiteness, and the articles using recycled pulp fibers hardly give a feeling of psychological resistance to a user.SOLUTION: The method has a step of supplying a mixed solution 51 containing a superabsorbent polymer and pulp fibers derived from used plural types of hygiene articles to a treatment tank 31 from a mixed solution supply port 32, a step of supplying an ozone-containing gas 53 from an ozone gas supply port 43 to the treatment liquid 52 in the treatment tank 31, a step in which, by raising the ozone-containing gas 53 while lowering the superabsorbent polymer and pulp fiber in the treatment tank 31, the ozone-containing gas 53 is brought into contact with the superabsorbent polymer and pulp fiber, to bleach the pulp fiber to form recycled pulp fibers, and a step of discharging the treatment liquid 52 from the treatment liquid discharge port 33, wherein the recycled pulp fibers have ΔYI of 10 or less with respect to a standard white plate.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a method for producing recycled pulp fibers from post-consumer sanitary article pulp fibers, and recycled pulp fibers derived from post-consumer sanitary articles comprising pulp fibers and superabsorbent polymers. [Background technology]

[0002] BACKGROUND ART Technologies for recycling used sanitary products such as disposable diapers are being investigated. For example, Patent Document 1 discloses a method for producing recycled pulp that can be reused mainly as sanitary products. Specifically, Patent Document 1 describes a method for producing recycled pulp that can be reused as sanitary products by recovering pulp fibers from used sanitary products containing pulp fibers and superabsorbent polymers, the method comprising the steps of: decomposing the used sanitary products into pulp fibers and other materials by applying physical force to the used sanitary products in an aqueous solution containing polyvalent metal ions or an acidic aqueous solution having a pH of 2.5 or less; separating the pulp fibers from the mixture of pulp fibers and other materials produced in the decomposition step; and treating the separated pulp fibers with an ozone-containing aqueous solution having a pH of 2.5 or less.

[0003] In Patent Document 1, the reason for treating pulp fibers with an ozone-containing aqueous solution is that a considerable amount of superabsorbent polymer remains in the separated pulp fibers, and the superabsorbent polymer is removed from the pulp fibers by oxidative decomposition and solubilization. Patent Document 1 discloses a method for treating pulp fibers with an ozone-containing aqueous solution, in which the ozone-containing aqueous solution is placed in a treatment tank and the separated pulp fibers are placed in the ozone-containing aqueous solution. In this method, it is preferable to moderately agitate the ozone-containing aqueous solution during treatment to create a water flow. Alternatively, ozone gas may be blown into the aqueous solution placed in a container, and the rising bubbles of ozone gas may generate a water flow in the ozone-containing aqueous solution. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-881 Summary of the Invention [Problem to be solved by the invention]

[0005] In used sanitary products, in absorbents containing pulp fibers and superabsorbent polymers, (i) the superabsorbent polymers swell as they absorb liquids such as body fluids and entangle the pulp fibers, and (ii) the swelled superabsorbent polymers entangle the pulp fibers with each other, causing gel blocking, and in many cases multiple superabsorbent polymers and multiple pulp fibers form connected structures.

[0006] In the method for producing recycled pulp described in Patent Document 1, when a superabsorbent polymer and pulp fibers form a connected structure, the ozone-containing gas can bleach pulp fibers that are not surrounded by superabsorbent polymer, but because it is difficult for the ozone-containing gas to come into contact with the pulp fibers surrounded by superabsorbent polymer, the pulp fibers surrounded by the superabsorbent polymer may not be sufficiently bleached. Articles using such recycled pulp fibers may contain areas that are less white, and users may judge these less white areas as remaining dirt, which may cause psychological resistance among users.

[0007] Therefore, the present disclosure aims to provide a method for producing recycled pulp fibers from pulp fibers of used sanitary products, which will result in articles using recycled pulp fibers that are more likely to have uniform whiteness and will less likely cause users to feel psychological resistance to articles using recycled pulp fibers. [Means for solving the problem]

[0008] The present inventors have discovered a method for producing recycled pulp fibers from pulp fibers of used sanitary products, which comprises the following steps: a preparation step of preparing a treatment tank having a mixed liquid supply port, a treatment liquid outlet and an ozone-containing gas supply port located below the mixed liquid supply port; a mixed liquid supply step of supplying a mixed liquid containing a superabsorbent polymer and pulp fibers derived from multiple types of used sanitary products, and water, from the mixed liquid supply port to the treatment tank; an ozone-containing gas supply step of supplying an ozone-containing gas from the ozone-containing gas supply port to the treatment liquid in the treatment tank; The present inventors have discovered a method for producing recycled pulp fibers, the method comprising: a recycled pulp fiber forming step of bringing the ozone-containing gas into contact with the superabsorbent polymer and pulp fibers while descending the superabsorbent polymer and pulp fibers, thereby dissolving at least a portion of the superabsorbent polymer in the treatment liquid and bleaching the pulp fibers to form the recycled pulp fibers; and a treatment liquid discharging step of discharging the treatment liquid containing the recycled pulp fibers from the treatment liquid outlet, wherein the recycled pulp fibers have a ΔYI of 0 to 10 relative to a standard white plate. [Effects of the Invention]

[0009] The method of producing recycled pulp fibers from pulp fibers of used sanitary products disclosed herein makes it easier for products using recycled pulp fibers to have uniform whiteness, and users are less likely to feel psychological resistance to products using recycled pulp fibers. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a flow chart illustrating an embodiment of a method of the present disclosure. [Figure 2] FIG. 2 is a schematic diagram showing an example of the configuration of an apparatus for the ozone treatment step of FIG. 1. [Figure 3] FIG. 2 is a schematic diagram showing another example of the configuration of the device for the ozone treatment step in FIG. [Figure 4] 1. FIG. 4 is a schematic diagram showing still another example of the configuration of the apparatus for the ozone treatment step in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0011] Specifically, the present disclosure relates to the following aspects: [Aspect 1] 1. A method for producing recycled pulp fibers from pulp fibers of post-consumer hygiene products, comprising the steps of: a preparation step of preparing a treatment tank including a mixed liquid supply port, and a treatment liquid discharge port and an ozone-containing gas supply port disposed below the mixed liquid supply port; a mixed liquid supply step of supplying a mixed liquid containing a superabsorbent polymer and pulp fibers derived from a plurality of types of used sanitary products and water from the mixed liquid supply port into the treatment tank; an ozone-containing gas supply step of supplying an ozone-containing gas from the ozone-containing gas supply port to the treatment liquid in the treatment tank; a recycled pulp fiber forming step in which the ozone-containing gas is brought into contact with the superabsorbent polymer and pulp fibers by raising the ozone-containing gas while lowering the superabsorbent polymer and pulp fibers in the treatment tank, thereby dissolving at least a portion of the superabsorbent polymer in the treatment liquid and bleaching the pulp fibers to form the recycled pulp fibers; a treatment liquid discharge step of discharging the treatment liquid containing the recycled pulp fibers from the treatment liquid discharge port; Including, The recycled pulp fiber has a ΔYI of 0 to 10 relative to a standard white board. The above method, characterized in that

[0012] In used sanitary products, in absorbents containing pulp fibers and superabsorbent polymers, (i) the superabsorbent polymers swell as they absorb liquids such as body fluids and entangle the pulp fibers, and (ii) the swelled superabsorbent polymers entangle the pulp fibers with each other, causing gel blocking, and in many cases multiple superabsorbent polymers and multiple pulp fibers form connected structures.

[0013] On the other hand, pulp fibers contained in used sanitary products may absorb excrement (e.g., feces, urine, etc.) and become brown or yellow in color. Therefore, in order to reuse pulp fibers from used sanitary products as recycled pulp fibers, it is necessary to bleach the color caused by the excrement. Furthermore, since users often have psychological resistance to recycled pulp fibers derived from used sanitary products, from the perspective of reducing users' psychological resistance, it is preferable that recycled pulp fibers not only have high whiteness but also have little unevenness that could be interpreted as residual dirt (a narrow distribution of bleaching degrees).

[0014] In the method described in Patent Document 1, although the ozone-containing gas can bleach free pulp fibers that do not form connected structures, the ozone-containing gas is unlikely to come into contact with the pulp fibers that form the connected structures, i.e., the pulp fibers surrounded by the superabsorbent polymer, and therefore the pulp fibers that form the connected structures may not be sufficiently bleached. Articles using such recycled pulp fibers may contain areas that are less white, and users may judge these less white areas as remaining soil, which may cause psychological resistance among users.

[0015] The manufacturing method includes a predetermined recycled pulp fiber formation step. In this recycled pulp fiber formation step, an ozone-containing gas is raised while a superabsorbent polymer and pulp fibers are lowered, and the superabsorbent polymer and pulp fibers are brought into contact with the ozone-containing gas. Among free superabsorbent polymers, free pulp fibers, and connected structures, the free superabsorbent polymers, which have a relatively high specific gravity, and the connected structures containing the superabsorbent polymers tend to settle more easily than the free pulp fibers, which have a relatively low specific gravity. On the other hand, because the ozone-containing gas rises while consuming ozone and treating the superabsorbent polymers and pulp fibers, the ozone-containing gas present at a lower position tends to have a higher ozone content (i.e., be fresher) than the ozone-containing gas present at an upper position.

[0016] In this specification, the descending velocity refers to the downward movement velocity of the treatment liquid 52 in the treatment tank 31, and is generally determined uniquely by the first flow velocity, the second flow velocity, the size of the treatment tank, etc. Meanwhile, in this specification, the settling property refers to the property that indicates the ease with which the pulp fibers, superabsorbent polymer, and linked structure contained in the treatment liquid 52 in the treatment tank 31 fall vertically due to gravity, and the pulp fibers, superabsorbent polymer, and linked structure each have different settling properties depending on the specific gravity, etc.

[0017] Therefore, in the above manufacturing method, free superabsorbent polymers and superabsorbent polymers in the connected structure, which have a relatively high sedimentation tendency, can be oxidatively decomposed with fresher ozone-containing gas to liberate the pulp fibers that made up the connected structure, and the ozone-containing gas can slowly treat the free pulp fibers, which have a relatively low sedimentation tendency and take a relatively long time to reach the treatment liquid outlet.

[0018] In addition, since pulp fibers generally tend to have a higher specific gravity as the lignin content increases, in the above-mentioned manufacturing method, pulp fibers with a relatively high lignin content have a relatively higher sedimentation tendency than pulp fibers with a relatively low lignin content, and therefore fresher ozone-containing gas comes into contact with the pulp fibers with a relatively high lignin content, decomposing the lignin contained therein and bleaching them.

[0019] Therefore, in the above-mentioned manufacturing method, even when the superabsorbent polymer and the pulp fibers form a connected structure, the ozone in the ozone-containing gas can remove the superabsorbent polymer that makes up the connected structure, and the ozone in the ozone-containing gas can act on the pulp fibers that make up the connected structure and bleach the pulp fibers. As a result, there is less difference in the bleaching degree between the pulp fibers that did not make up the connected structure and the pulp fibers that did make up the connected structure, and the bleaching degree distribution of the recycled pulp fibers contained in the treatment solution becomes narrow (unevenness is less likely to occur), and the bleaching degree distribution in the article in which the recycled pulp fibers are reused becomes narrow (unevenness is less likely to occur).

[0020] Furthermore, the recycled pulp fibers produced by the above-described production method have a narrow bleaching degree distribution and a predetermined absolute value of whiteness. Therefore, the recycled pulp fibers produced by the above-mentioned production method and the products using the recycled pulp fibers tend to have a uniform whiteness, and users are less likely to feel psychological resistance to the products using the recycled pulp fibers.

[0021] [Aspect 2] 2. The method of claim 1, wherein the recycled pulp fibers have a water contact angle of 20° or less.

[0022] When sanitary products are recycled, the oils in the hot melt adhesives contained in the sanitary products are easily absorbed by the pulp fibers, and the lignin contained in the pulp fibers is also hydrophobic. In the method described in Patent Document 1, although the ozone in the ozone-containing gas can oxidatively decompose hydrophobic components such as oil and lignin in pulp fibers that do not form connected structures, the ozone in the ozone-containing gas is unlikely to come into contact with pulp fibers that form connected structures, and therefore the hydrophobic components of the pulp fibers may not be sufficiently oxidatively decomposed. Articles using such recycled pulp fibers may have poor hydrophilicity overall or may have poor hydrophilicity in parts, which may cause psychological resistance among users.

[0023] Because the manufacturing method includes a predetermined recycled pulp fiber formation step, the free superabsorbent polymer and linked structure, which have relatively high sedimentation tendency, can be accurately oxidatively decomposed with fresher ozone-containing gas to form free pulp fibers, and the ozone-containing gas can treat hydrophobic components such as oil and lignin contained in the free pulp fibers, which have relatively low sedimentation tendency, over time.The recycled pulp fibers are also likely to have a predetermined water contact angle and uniform hydrophilicity.As a result, products using recycled pulp fibers are likely to have uniform hydrophilicity, and users are less likely to feel psychological resistance to products using recycled pulp fibers.

[0024] [Aspect 3] 3. The method of claim 1 or 2, wherein the recycled pulp fibers have a lignin content of less than or equal to 0.1% by weight.

[0025] In the above-described production method, even when the superabsorbent polymer and the pulp fibers form a connected structure, the ozone in the ozone-containing gas can remove the superabsorbent polymer that makes up the connected structure, and the ozone in the ozone-containing gas can act on the pulp fibers that make up the connected structure, reducing the lignin content of the pulp fibers. As a result, there is less difference between the lignin content of the pulp fibers that did not make up the connected structure and the lignin content of the pulp fibers that did make up the connected structure, and the distribution of the lignin content of the recycled pulp fibers contained in the treatment solution becomes narrower (less variation). Furthermore, since the recycled pulp fibers produced by the above manufacturing method have a predetermined lignin content, the recycled pulp fibers tend to have excellent whiteness and hydrophilicity, and as a result, products using the above recycled pulp fibers have uniform whiteness and hydrophilicity, and users are less likely to feel psychological resistance to products using recycled pulp fibers.

[0026] [Aspect 4] Aspect 4. The method according to any one of aspects 1 to 3, wherein the recycled pulp fibers have a freeness reduction rate of 300 mL / h or more.

[0027] In the above-described manufacturing method, the recycled pulp fibers have a predetermined beating degree reduction rate, so that when the recycled pulp fibers are reused, the recycled pulp fibers are likely to fluff and have an increased surface area, which increases the diffuse reflection of light and makes the recycled pulp fibers appear whiter. Therefore, products using the recycled pulp fibers are likely to have a uniform whiteness, and users are less likely to feel psychological resistance to products using the recycled pulp fibers.

[0028] [Aspect 5] Aspect 5. The method of any one of aspects 1-4, wherein the pulp fibers are free of colorants selected from the group consisting of dyes, pigments, and combinations thereof.

[0029] In the above manufacturing method, since the pulp fiber does not contain a specified colorant, the recycled pulp fiber produced by the above manufacturing method tends to have a uniform whiteness, and articles using the recycled pulp fiber tend to have a uniform whiteness, so users are less likely to feel psychological resistance to articles using recycled pulp fiber.

[0030] [Aspect 6] Aspects 6. The method of any one of Aspects 1 to 5, wherein at least a portion of the plurality of sanitary articles comprises pulp fibers containing a colorant selected from the group consisting of dyes, pigments, and combinations thereof, and further comprising, prior to the mixed liquid supply step, a colored pulp fiber removal step of removing the pulp fibers containing the colorant from the plurality of sanitary articles.

[0031] The above manufacturing method further includes a colored pulp fiber removal step in which pulp fibers containing the colorant are removed from multiple types of sanitary products before the supply step.Therefore, in the recycled pulp fiber formation step, the pulp fibers are less likely to contain colorant, the recycled pulp fibers are more likely to have a uniform whiteness, and articles using recycled pulp fibers are more likely to have a uniform whiteness, making it less likely that users will have psychological resistance to articles using recycled pulp fibers.

[0032] [Aspect 7] At least some of the plurality of types of sanitary products comprise a liquid-permeable sheet, a liquid-impermeable sheet, and an absorbent core disposed therebetween, the absorbent core containing the superabsorbent polymer and pulp fibers that do not contain a colorant; The colored pulp fiber removal step comprises the following steps: a pretreatment step of swelling the used sanitary products with water; a decomposition step of applying a physical impact to the swollen used sanitary products to decompose the used sanitary products into the absorbent core and other components; a separating step of separating the absorbent core; 7. The method of embodiment 6, comprising:

[0033] Because the manufacturing method includes a predetermined pretreatment step, a decomposition step, and a separation step, even if components of the sanitary product other than the absorbent core contain pulp fibers containing a colorant, the pulp fibers containing the colorant are less likely to be mixed into a mixture containing a superabsorbent polymer, pulp fibers, and water. As a result, the recycled pulp fibers are more likely to have a uniform whiteness, and articles using recycled pulp fibers are more likely to have a uniform whiteness, making users less likely to feel psychological resistance to articles using recycled pulp fibers.

[0034] [Aspect 8] Aspect 8. The method according to any one of Aspects 1 to 7, wherein in the recycled pulp fiber forming step, the ozone-containing gas is supplied from the ozone-containing gas supply port in the form of microbubbles or nanobubbles.

[0035] In the above-described manufacturing method, in the recycled pulp fiber formation step, ozone-containing gas is supplied as microbubbles or nanobubbles from the ozone-containing gas supply port. Therefore, even when the superabsorbent polymer and the pulp fibers form a connected structure, the microbubbles or nanobubbles provide buoyancy to the superabsorbent polymer, the connected structure, and the pulp fibers, reducing their sedimentation tendency. This lengthens the time it takes for the superabsorbent polymer, the connected structure, and the pulp fibers to reach the treatment liquid outlet, allowing the ozone to oxidatively decompose the free superabsorbent polymer and the superabsorbent polymer that constitutes the connected structure, thereby sufficiently treating the free pulp fibers and the pulp fibers that constitute the connected structure. Therefore, the recycled pulp fibers and articles using the recycled pulp fibers produced by the above-described manufacturing method tend to have uniform whiteness and hydrophilicity, and users are less likely to have a psychological resistance to articles using the recycled pulp fibers.

[0036] [Aspect 9] Aspect 9. The method according to any one of aspects 1 to 8, wherein in the mixed liquid supply step, the mixed liquid is continuously supplied from the mixed liquid supply port to the treatment tank at a first flow rate, and in the treatment liquid discharge step, the treatment liquid is continuously discharged from the treatment liquid discharge port at a second flow rate.

[0037] In the above manufacturing method, in the mixed liquid supply step, the mixed liquid is continuously supplied from the mixed liquid supply port to the treatment tank at a first flow rate, and in the treatment liquid discharge step, the treatment liquid is continuously discharged from the treatment liquid discharge port at a second flow rate.This makes the treatment time for the superabsorbent polymer and pulp fiber to be treated uniform, and the recycled pulp fiber and articles using recycled pulp fiber are more likely to have uniform whiteness and hydrophilicity, making it less likely that users will have psychological resistance to articles using recycled pulp fiber.

[0038] [Aspect 10] A method according to any one of aspects 1 to 9, wherein the treatment liquid is acidic. In the above-mentioned production method, the treatment solution is acidic (for example, pH 2.5 or less). Therefore, the superabsorbent polymer to be treated can be inactivated by the acid, or if the superabsorbent polymer to be treated has already been inactivated, the superabsorbent polymer can be maintained in an inactivated state. As a result, even if the superabsorbent polymer and pulp fibers form a connected structure, the ozone in the ozone-containing gas can remove the superabsorbent polymer that constitutes the connected structure, and the ozone in the ozone-containing gas acts on the pulp fibers that constituted the connected structure, making it easier to bleach the pulp fibers.

[0039] [Aspect 11] Aspect 11. The method of any one of aspects 1 to 10, further comprising, prior to the mixed solution supply step, an inactivation step of inactivating the superabsorbent polymer with an acid.

[0040] Because the above manufacturing method further includes a predetermined inactivation step, even if the superabsorbent polymer and pulp fibers form a connected structure, the ozone in the ozone-containing gas can remove the superabsorbent polymer that makes up the connected structure immediately after the mixed liquid containing the superabsorbent polymer, pulp fibers, and water derived from used sanitary products is supplied to the treatment tank, and the ozone in the ozone-containing gas acts on the pulp fibers that make up the connected structure, making it easier to bleach the pulp fibers.

[0041] [Aspect 12] 12. The method of claim 11, wherein the acid is an acid capable of forming a complex with a metal ion contained in feces.

[0042] In the above-described manufacturing method, the acid is capable of forming a complex with metal ions contained in excrement, so that the recycled pulp fibers and articles using the recycled pulp fibers produced by the above-described manufacturing method are less likely to contain metal ions and have excellent whiteness and hydrophilicity, which reduces users' psychological resistance to the articles using the recycled pulp fibers.

[0043] [Aspect 13] Aspect 13. The method of any one of aspects 1 to 12, wherein the recycled pulp fibers have an ash content of 0.65% by mass or less.

[0044] In the above-described manufacturing method, the recycled pulp fibers produced by the above-described manufacturing method have an ash content of 0.65% by mass or less, and therefore the recycled pulp fibers and articles using the recycled pulp fibers have excellent whiteness and hydrophilicity, which reduces users' psychological resistance to the articles using the recycled pulp fibers.

[0045] [Aspect 14] A recycled pulp fiber derived from a used hygiene product comprising pulp fiber and a superabsorbent polymer, The recycled pulp fiber has a ΔYI of 0 to 10 relative to a standard white board. The recycled pulp fiber is characterized by:

[0046] The recycled pulp fibers and articles using the recycled pulp fibers tend to have uniform whiteness, and users are less likely to have a psychological resistance to articles using recycled pulp fibers.

[0047] [Aspect 15] 15. The recycled pulp fibers of embodiment 14, wherein the recycled pulp fibers have a water contact angle of 20° or less.

[0048] Since the recycled pulp fibers have a predetermined water contact angle, the recycled pulp fibers and articles using the recycled pulp fibers tend to have uniform hydrophilicity, and users are less likely to feel psychological resistance to articles using the recycled pulp fibers.

[0049] [Aspect 16] 16. The recycled pulp fibers of claim 14 or 15, wherein the recycled pulp fibers have a lignin content of 0.1% by mass or less.

[0050] Because the recycled pulp fibers have a predetermined lignin content, they tend to have excellent whiteness and hydrophilicity. As a result, products using the recycled pulp fibers have uniform whiteness and hydrophilicity, and users are less likely to feel psychological resistance to products using recycled pulp fibers.

[0051] [Aspect 17] 17. The recycled pulp fibers according to any one of aspects 14 to 16, wherein the recycled pulp fibers have a freeness reduction rate of 300 mL / h or more.

[0052] Since the recycled pulp fibers have a predetermined beating degree reduction rate, when the recycled pulp fibers are reused, the recycled pulp fibers are likely to fluff and have an increased surface area, which increases the diffuse reflection of light and makes the recycled pulp fibers appear whiter. Therefore, products using the recycled pulp fibers tend to have a uniform whiteness, and users are less likely to feel psychological resistance to products using the recycled pulp fibers.

[0053] [Aspect 18] 18. The recycled pulp fiber according to any one of aspects 14 to 17, wherein the recycled pulp fiber has an ash content of 0.65% by mass or less.

[0054] The recycled pulp fibers have a predetermined ash content, so that the recycled pulp fibers and products using the recycled pulp fibers have excellent whiteness and hydrophilicity, which reduces users' psychological resistance to products using the recycled pulp fibers.

[0055] Hereinafter, a method for producing recycled pulp fibers from pulp fibers of used sanitary products (hereinafter, sometimes simply referred to as "method for producing recycled pulp fibers") will be described. In addition, used sanitary products are sanitary products that have been used by a user and include sanitary products that have absorbed the user's liquid excrement, as well as sanitary products that have been used but have not absorbed excrement, unused products, etc.

[0056] First, we will explain the configuration of a sanitary product. The sanitary product includes a top sheet, a back sheet, and an absorbent body disposed between the top sheet and the back sheet. Examples of sanitary products include disposable diapers, urine absorption pads, sanitary napkins, bed sheets, and pet sheets.

[0057] Examples of constituent materials for the top sheet include nonwoven fabrics or films, specifically liquid-permeable nonwoven fabrics, synthetic resin films having liquid-permeable holes, composite sheets of these, etc. Examples of constituent materials for the back sheet include nonwoven fabrics or films, specifically liquid-impermeable nonwoven fabrics, liquid-impermeable synthetic resin films, and composite sheets of these nonwoven fabrics and synthetic resin films.

[0058] Constituent components of the absorbent body include an absorbent core (e.g., pulp fibers and superabsorbent polymers) and a core wrap. There are no particular limitations on the pulp fibers as long as they can be used in sanitary products, and examples of such fibers include cellulosic fibers. Examples of cellulosic fibers include wood pulp, crosslinked pulp, non-wood pulp, regenerated cellulose, and semi-synthetic cellulose. There are no particular limitations on the superabsorbent polymer (SAP) as long as they can be used in sanitary products, and examples of such superabsorbent polymers include polyacrylates, polysulfonates, and maleic anhydrides.

[0059] One side and the other side of the absorbent body are bonded to the top sheet and the back sheet, respectively, via an adhesive. In plan view, the portion (peripheral portion) of the top sheet that extends outward from the absorbent body so as to surround the absorbent body is bonded via an adhesive to the portion (peripheral portion) of the back sheet that extends outward from the absorbent body so as to surround the absorbent body. Thus, the absorbent body is enclosed within the bonded body of the top sheet and the back sheet. There are no particular limitations on the adhesive as long as it is usable in sanitary products and its bonding strength is reduced by softening or the like in hot water, as described below, but examples of such adhesives include hot melt adhesives. Examples of hot melt adhesives include pressure-sensitive adhesives or heat-sensitive adhesives that are primarily rubber-based, such as styrene-ethylene-butadiene-styrene, styrene-butadiene-styrene, and styrene-isoprene-styrene, or olefin-based, such as polyethylene.

[0060] Fig. 1 is a flowchart showing a material separation method for separating used sanitary products into their constituent materials. This material separation method separates used sanitary products into film, nonwoven fabric, pulp fiber, and superabsorbent polymer. This material separation method includes a pretreatment step S11, a decomposition step S12, and a separation step S13. In the pretreatment step S11, the used sanitary product is swelled with water. In the decomposition step S12, the swollen used sanitary product is subjected to physical impact to decompose the used sanitary product into a film, nonwoven fabric, core wrap, etc., and an absorbent core (e.g., pulp fiber and superabsorbent polymer). In the separation step S13, the film, nonwoven fabric, pulp fiber, and superabsorbent polymer are separated.

[0061] The method for producing recycled pulp fiber according to the present disclosure is included in the separation step S13 of the material separation method. If a mixture of pulp fiber and superabsorbent polymer has been obtained in advance by some method, the pretreatment step S11, decomposition step S12, and separation step S13, which are steps prior to the method for producing recycled pulp fiber, do not need to be performed. Each step will be described below.

[0062] In the pretreatment step S11, multiple used sanitary products are left in the state they were in when collected from the outside, i.e., without being destroyed, cut, or otherwise broken, and if they are rolled or folded, left in that state, and without inactivating the superabsorbent polymer in the absorbent body, and are allowed to absorb water and swell. In this embodiment, the used sanitary products are allowed to absorb warm water and swell, or the water absorbed after absorbing and swelling water is heated to turn it into warm water. Warm water refers to water at a temperature higher than room temperature (20°C ± 15°C (5 to 35°C): JIS Z 8703).

[0063] Typically, the amount of liquid excrement actually absorbed by used sanitary products is much smaller than the maximum absorption capacity of the sanitary products (e.g., approximately 10 to 20% by mass of the maximum absorption capacity). In this embodiment, in the pretreatment step S11, used sanitary products are immersed in warm water to absorb water up to an amount close to the maximum absorption capacity of the used sanitary products (e.g., 80% by mass or more of the maximum absorption capacity). Alternatively, used sanitary products are immersed in room-temperature water to absorb water up to an amount close to the maximum absorption capacity of the used sanitary products, and then the entire used sanitary products are heated to the temperature of the warm water. This allows the used sanitary products to be highly expanded in warm or room-temperature water (hereinafter simply referred to as "warm water"). As a result, very high internal pressure is generated in the used sanitary products. The purpose of using warm water is mainly to weaken the adhesive strength of the adhesive, as described below.

[0064] Here, when a used sanitary product is initially rolled or folded with the back sheet facing outward (with the top sheet hidden inside), it is immersed in warm water, and the absorbent body of the used sanitary product absorbs the warm water and expands in the warm water. As a result, the internal pressure of the used sanitary product increases, and a force acts on the used sanitary product to open outward, causing the rolled or folded used sanitary product to open outward and become roughly flat. In other words, the used sanitary product can be laid out flat in the warm water. At this time, the absorbent body of the used sanitary product has absorbed a large amount of warm water and is highly expanded, so that its surface, i.e., one of the top sheet and back sheet enclosing the absorbent body, is in a state where it is likely to easily burst. In other words, the pre-treatment step S11 can bring one of the surfaces of the used sanitary product to a state where it is likely to tear or rip. Note that if the used sanitary product is initially laid out flat, one of the surfaces will remain in that state and be likely to easily burst. This situation cannot occur if the used sanitary product is broken or otherwise damaged.

[0065] Furthermore, when a used sanitary product is immersed in and / or absorbs warm water, the adhesive (e.g., hot melt adhesive) used to bond the components together can be softened by the heat of the warm water, thereby reducing the adhesive's bonding strength. For example, the adhesive bonding the peripheral edge of the topsheet to the peripheral edge of the backsheet can be softened by the heat of the warm water, thereby reducing the adhesive's bonding strength. Furthermore, the adhesive bonding the topsheet to the absorbent body and the adhesive bonding the backsheet to the absorbent body can be softened by the heat of the warm water, thereby reducing the adhesive's bonding strength.

[0066] In this way, in the pre-treatment step S11, the expansion of the absorbent body of the used sanitary product can cause some part of the surface of the used sanitary product to be in a state where it is about to burst and the adhesive strength is reduced. By putting the used sanitary product in this state, the used sanitary product can be reliably disassembled in the disassembly step described below.

[0067] The temperature of the hot water in the pretreatment step S11 is not particularly limited as long as it can soften the adhesive on the used sanitary products, but examples include 60°C or higher, and preferably 70°C or higher and 98°C or lower. By setting the hot water temperature to 70°C or higher, the heat of the hot water can further soften the adhesive that bonds the components, thereby further reducing the adhesive's bonding strength. By setting the hot water temperature to 98°C or lower, the hot water remains in a liquid state, allowing the used sanitary products to more reliably absorb the hot water. The expansion of the absorbent and the heat of the hot water can more reliably cause the surface of the used sanitary products to burst and the adhesive's bonding strength to be reduced. The temperature is measured either by measuring the temperature of the hot water in which the used sanitary products are immersed, or by measuring the temperature (with the tip of a temperature sensor inserted) 5 mm inside the surface of a used sanitary product that has absorbed water close to its maximum absorption capacity.

[0068] Furthermore, sterilization of the constituent materials is extremely important when reusing used sanitary products. Therefore, setting the temperature of the hot water to 70°C or higher is preferable because it can also sterilize (disinfect) the used sanitary products.

[0069] The treatment time in the pretreatment step S11, i.e., the time for which the used sanitary goods are immersed in warm water, is not particularly limited as long as it allows the absorbent body of the used sanitary goods to expand, but is, for example, 2 to 60 minutes, preferably 4 to 30 minutes. If the time is too short, the absorbent body will not expand sufficiently, and if it is too long, time will be wasted and treatment costs will unnecessarily increase.

[0070] The amount of hot water absorbed by the absorbent body in the pretreatment step S11 is not particularly limited as long as it can expand to the extent that the used sanitary goods can be decomposed in the decomposition step described below, but it can be, for example, 80% by mass or more of the maximum absorption capacity of the used sanitary goods, and preferably 90% by mass or more. This allows the used sanitary goods to be fully expanded with water. As a result, extremely high internal pressure can be generated in the absorbent body of the used sanitary goods.

[0071] However, the maximum absorption amount is measured by the following procedure. (1) Dry unused sanitary products in an atmosphere of 100°C or higher, and measure the mass of the sanitary products. (2) If the sanitary product contains elastic material (e.g., elastic parts around the legs, waist, etc.) that can form pockets that make it difficult for water to reach the absorbent body, the sanitary product can be flattened by making cuts in the elastic parts. (3) Immerse the sanitary product with the surface sheet facing down in a water bath filled with sufficient tap water and leave it for 30 minutes. (4) After leaving it, place the sanitary product on a wire rack with the surface sheet facing down, and after draining for 20 minutes, measure the mass of the sanitary product. The difference in mass before and after immersion in tap water is defined as the maximum absorption amount.

[0072] Next, in the decomposition step S12, a physical impact is applied to the multiple used sanitary products that have been unfolded and swollen in the pre-treatment step S11, and the multiple used sanitary products are decomposed into a film (back sheet), a nonwoven fabric (top sheet), a core wrap, and an absorbent core (e.g., an absorbent body and a superabsorbent polymer).

[0073] The used sanitary products are flattened and unfolded in the pre-treatment step S11, and some part of their surface is about to burst due to expansion. In this embodiment, the adhesive strength is reduced, particularly due to the heat of the hot water. Therefore, in the disassembly step S12, physical impact is applied to the used sanitary products in this state, causing some part of the surface, particularly the bonded part between the top sheet (nonwoven fabric) and the back sheet (film) where the adhesive strength has been reduced, to burst. This allows the bonded part to be torn (peeled off). There are no particular limitations on the physical impact, but examples include hitting the used sanitary products against a surface made of a material harder than the used sanitary products, or pressing the used sanitary products from both sides while sandwiching and passing them between a pair of rolls arranged facing each other.

[0074] In this embodiment, the disassembly step S12 includes the steps of: placing multiple swollen used sanitary products into the bottom of a rotating drum with a horizontal rotation axis; and rotating the rotating drum around the rotation axis to lift the multiple used sanitary products to the top of the rotating drum and slam them against the bottom. This allows for stable, continuous, and easy physical impact on the multiple used sanitary products. An example of a rotating drum is the rotating drum of a horizontal washing machine. Therefore, the disassembly step S12 can be performed using an existing horizontal washing machine (e.g., ECO-22B, manufactured by Inamoto Seisakusho Co., Ltd.). The size of the rotating drum is not particularly limited as long as the above-mentioned impact can be realized, and examples of the inner diameter and depth include 50 to 150 cm and 30 to 120 cm. The rotation speed of the rotating drum is not particularly limited as long as the above-mentioned impact can be realized, and examples include 30 to 100 revolutions per minute.

[0075] Furthermore, the warm water absorbed into the used sanitary products keeps the temperature of the used sanitary products relatively high. However, from the viewpoint of preventing the temperature of the adhesive from dropping and maintaining the sterilization effect, the temperature of the atmosphere inside the rotating drum is preferably 70°C or higher, more preferably 75°C or higher. From the viewpoint of handling the used sanitary products, the temperature inside the rotating drum is preferably 98°C or lower, more preferably 90°C or lower. It is preferable that the amount of water in the rotating drum is as small as possible, and preferably so small that the used sanitary products do not fall below the water level at least at the bottom. If the used sanitary products fall below the water level, impacts on the used sanitary products are absorbed by the water, making it difficult to apply the desired impact to the used sanitary products. The time for which the rotating drum is rotated is not particularly limited as long as it is possible to separate the top sheet, back sheet, core wrap, etc. from the absorbent core, but is, for example, 2 to 40 minutes, preferably 4 to 20 minutes.

[0076] When a used sanitary product is subjected to a physical impact, the joint between the top sheet (nonwoven fabric) and the back sheet (film) bursts and tears. At the same time, the internal pressure of the absorbent body causes the absorbent core (e.g., pulp fiber and superabsorbent polymer) inside the used sanitary product to eject (fly out) through the tear. This allows the used sanitary product to be more reliably decomposed into the top sheet (nonwoven fabric), back sheet (film), core wrap, etc., and the absorbent core (e.g., pulp fiber and superabsorbent polymer).

[0077] Next, in the separation step S13, the absorbent core (e.g., pulp fiber and superabsorbent polymer) is separated from the plurality of films (backsheets), the plurality of nonwoven fabrics (topsheets), the core wrap, etc. However, the nonwoven fabric may simply be bonded to the film. The separation method is not particularly limited, but an example is a method using a sieve that allows the absorbent core to pass but not the topsheet, backsheet, core wrap, etc.

[0078] In this embodiment, the separation step S13 may include an inactivation step S31 in which the superabsorbent polymer is inactivated with an aqueous solution containing an inactivating agent before separating the film, nonwoven fabric, core wrap, etc. from the absorbent core, and a first separation step S32 in which the film and nonwoven fabric are separated from a mixture containing pulp fibers, inactivated superabsorbent polymer, and wastewater discharged from the superabsorbent polymer due to inactivation.

[0079] In the inactivation step S31, before the first separation step S32, the top sheet (nonwoven fabric), back sheet (film), and absorbent body (pulp fibers and superabsorbent polymer) are immersed in an aqueous solution containing an inactivating agent capable of inactivating the superabsorbent polymer. This inactivates the superabsorbent polymer attached to the top sheet, back sheet, and pulp fibers. This allows the highly viscous superabsorbent polymer before inactivation to be dehydrated and converted into a low-viscosity superabsorbent polymer.

[0080] Here, the inactivating agent is not particularly limited, but examples thereof include acids (e.g., inorganic acids and organic acids), lime, calcium chloride, magnesium sulfate, magnesium chloride, aluminum sulfate, and aluminum chloride. The above-mentioned acids are preferred because they do not leave ash residue on the pulp fibers. When an acid is used as the inactivating agent, the pH is preferably 2.5 or less, and more preferably 1.3 to 2.4. If the pH is too high, the water absorption capacity of the superabsorbent polymer cannot be sufficiently reduced. Furthermore, the sterilizing ability may also be reduced. If the pH is too low, there is a risk of corrosion of equipment, and a large amount of alkaline chemicals may be required for neutralization during wastewater treatment.

[0081] Examples of the inorganic acid include sulfuric acid, hydrochloric acid, and nitric acid, with sulfuric acid being preferred from the standpoint of not containing chlorine and cost. On the other hand, examples of the organic acid include citric acid, tartaric acid, glycolic acid, malic acid, succinic acid, acetic acid, and ascorbic acid, with acids capable of forming complexes with metal ions contained in excrement, such as hydroxycarbonate-based organic acids such as citric acid, tartaric acid, and gluconic acid, being particularly preferred. Examples of metal ions contained in excrement include calcium ions. This is because the chelating effect of acids capable of forming complexes with metal ions contained in excrement traps and removes metal ions in excrement. Furthermore, citric acid is expected to have a high stain removal effect due to its cleaning effect. Since the pH changes depending on the water temperature, the pH in this disclosure refers to the pH measured at an aqueous solution temperature of 20°C.

[0082] The treatment temperature of the inactivation step S31, i.e., the temperature of the aqueous solution containing the inactivating agent, is not particularly limited as long as the inactivation reaction proceeds. The treatment temperature may be room temperature or higher, for example, 15 to 30°C. The treatment time of the inactivation step S31, i.e., the time for which the topsheet, backsheet, and absorbent are immersed in the aqueous solution containing the inactivating agent, is not particularly limited as long as the superabsorbent polymer is inactivated and dehydrated, but is, for example, 2 to 60 minutes, preferably 5 to 30 minutes. The amount of the aqueous solution in the inactivation step S31, i.e., the amount of the aqueous solution containing the inactivating agent, is not particularly limited as long as the inactivation reaction proceeds. For example, the amount of the aqueous solution is preferably 300 to 3,000 parts by mass, more preferably 500 to 2,500 parts by mass, and even more preferably 1,000 to 2,000 parts by mass per 100 parts by mass of the used sanitary product.

[0083] In the first separation step S32, the top sheet (nonwoven fabric), back sheet (film), and core wrap are separated from a mixture containing pulp fibers, inactivated superabsorbent polymer, and wastewater discharged from the superabsorbent polymer by inactivation. The wastewater is water released from the superabsorbent polymer by dehydration with an aqueous solution containing an inactivating agent in the inactivation step S31, i.e., wastewater containing liquid derived from excrement and water derived from warm water.

[0084] In the first separation step S32, the method for separating the top sheet and back sheet from the pulp fibers, superabsorbent polymer, and wastewater is not particularly limited. For example, the products (top sheet, back sheet, pulp fibers, superabsorbent polymer, wastewater, etc.) generated in the inactivation step are discharged through a screen with a mesh size of 5 to 100 mm, preferably 10 to 60 mm. This allows the pulp fibers, superabsorbent polymer, and wastewater to be discharged, while the top sheet and back sheet remain on the screen, allowing these products to be separated. Note that other large objects such as nonwoven fabrics and films may remain on the screen. In particular, because the superabsorbent polymer is in a highly viscous state before inactivation, it is not particularly easy to separate the superabsorbent polymer attached to the top sheet, back sheet, and pulp fibers. However, after inactivation, the superabsorbent polymer becomes less viscous due to dehydration, and the superabsorbent polymer attached to the top sheet, back sheet, and pulp fibers can be easily separated from the top sheet, back sheet, and pulp fibers. This allows for efficient separation and recovery of the components of sanitary products.

[0085] The manufacturing method of the present disclosure targets multiple types of used sanitary products, and if at least some of the multiple types of sanitary products contain pulp fibers containing a colorant selected from the group consisting of dyes, pigments, and combinations thereof, for example, if the core wrap is composed of pulp fibers containing the colorant, it is preferable to remove the pulp fibers containing the colorant (e.g., core wrap) in the separation step S13. This is because the recycled pulp fibers tend to have a uniform whiteness, and articles using the recycled pulp fibers tend to have a uniform whiteness, making it less likely that users will have a psychological resistance to articles using the recycled pulp fibers.

[0086] In this embodiment, the separation step S13 may further include a second separation step S33 of removing the adhesive at the joints between the film and other members using a solvent that dissolves the adhesive at the joints. In this embodiment, the adhesive at each joint between the film, nonwoven fabric, and absorbent body is removed using a solvent that dissolves the adhesive at each joint.

[0087] In the second separation step S33, the adhesive at the joint between the film (back sheet) and other components (nonwoven fabric of the top sheet, top sheet, absorbent remaining on the surface of the back sheet, etc.) is removed using a solvent. This allows the film and other components to be separated from each other while maintaining their original shape without breaking or the like. Therefore, components such as films for sanitary products can be efficiently recovered. Furthermore, since the film can be separated from other components without leaving any adhesive on the film, the film can be reused as a high-purity resin. This prevents the adhesive from having a negative effect when the film is reused. The same applies to nonwoven fabrics as to films.

[0088] The solvent used in the second separation step S33 is not particularly limited as long as it can dissolve the adhesive, but examples include terpenes containing at least one of terpene hydrocarbons, terpene aldehydes, and terpene ketones. In this step, an aqueous solution containing Tempe is used, and the concentration of Tempe in the aqueous solution is, for example, 0.05% by mass or more and 2% by mass or less. It is preferably 0.075 to 1% by mass. If the terpene concentration is too low, it may not be possible to dissolve the adhesive at the bonded portion. If the terpene concentration is too high, costs may increase. Tempe not only dissolves adhesives such as hot-melt adhesives, but also has the effect of cleaning oil stains. Therefore, for example, if a component of a sanitary product, such as a backsheet, is printed, Tempe can also decompose and remove the printing ink.

[0089] Examples of terpene hydrocarbons include myrcene, limonene, pinene, camphor, sapinene, phellandrene, para-cymene, ocimene, terpinene, carene, zingiberene, caryophyllene, bisabolene, and cedrene. Among these, limonene, pinene, terpinene, and carene are preferred. Examples of terpene aldehydes include citronellal, citral, cyclocitral, safranal, phellandral, perillaldehyde, geranial, and neral. Examples of terpene ketones include camphor and tsuyoshi. Among terpenes, terpene hydrocarbons are preferred, with limonene being particularly preferred. There are three types of limonene: d-limonene, l-limonene, and dipentene (dl-limonene), and all of these can be used preferably. Terpenes can be used alone or in combination of two or more.

[0090] The treatment temperature in the second separation step S33, i.e., the temperature of the aqueous solution containing a solvent, is not particularly limited, as long as it dissolves the adhesive and decomposes the used sanitary goods into their constituent parts. The treatment temperature may be room temperature or higher, for example, 15 to 30°C. The treatment time in the second separation step S33, i.e., the time the topsheet, backsheet, and absorbent are immersed in the aqueous solution containing a solvent, is not particularly limited, as long as it dissolves the adhesive and decomposes the used sanitary goods into their constituent parts. The treatment time is, for example, 2 to 60 minutes, preferably 5 to 30 minutes. The amount of the aqueous solution in the second separation step S33, i.e., the amount of the aqueous solution containing a solvent, is not particularly limited, as long as it dissolves the adhesive and decomposes the used sanitary goods into their constituent parts. The amount of the aqueous solution is, for example, preferably 300 to 3,000 parts by mass, more preferably 500 to 2,500 parts by mass, per 100 parts by mass of the used sanitary goods. By the second separation step S33, the amount of adhesive remaining on the film, nonwoven fabric, absorbent body, etc. can be reduced to 1% by mass or less relative to the film, nonwoven fabric, absorbent body, etc.

[0091] In another preferred embodiment of the present invention, the second separation step S33 may be performed in conjunction with the inactivation step S31. That is, the adhesive adhering to the topsheet, backsheet, and pulp fibers may be dissolved while inactivating the superabsorbent polymer adhering to the topsheet, backsheet, and pulp fibers. In this case, the aqueous solution in which the topsheet, backsheet, pulp fibers, and superabsorbent polymer are immersed is an aqueous solution containing both an inactivating agent and a solvent. This allows the backsheet (film), topsheet (nonwoven fabric), and absorbent body (pulp fibers and superabsorbent polymer) to be substantially separated in the aqueous solution in the inactivation step S31. Then, in the subsequent first separation step, the backsheet (film), topsheet (nonwoven fabric), and absorbent body (pulp fibers and superabsorbent polymer) can be separated, and the second separation step S33 can be omitted. In this case, the backsheet (film) and topsheet (nonwoven fabric) are substantially separated by removing the adhesive.

[0092] In this embodiment, the separation step S13 may further include a first drying step S34 in which the film is dried in an atmosphere or with hot air at a temperature higher than room temperature to remove the solvent after the step of removing the adhesive from the bonded portions. In this embodiment, the nonwoven fabric is also dried in this step.

[0093] Sterilization is extremely important in the reuse of used sanitary products. In the first drying step S34, the separated film (back sheet) and nonwoven fabric (top sheet) are dried in a high-temperature atmosphere or with hot air or the like. The drying temperature is, for example, 105 to 210°C, and preferably 110 to 190°C. The drying time varies depending on the drying temperature, but is, for example, 10 to 120 minutes, and preferably 15 to 100 minutes. This not only evaporates and removes the solvent remaining on the surfaces of the film and nonwoven fabric, but also sterilizes the film and nonwoven fabric with a high-temperature atmosphere or hot air or the like. This makes it possible to achieve the effect of sterilization (disinfection) while removing the solvent.

[0094] Meanwhile, in this embodiment, the separation step S13 may include a third separation step S35 in which pulp fibers are separated from the separated mixture. In the third separation step S35, the method for separating pulp fibers from the separated mixture (including pulp fibers, superabsorbent polymer, and wastewater) is not particularly limited, but for example, the separated mixture is discharged while passing through a screen with a mesh size of 0.1 to 4 mm, preferably 0.15 to 2 mm. This allows the superabsorbent polymer and wastewater to be discharged, while the pulp fibers (mainly with the superabsorbent polymer remaining on the surface) remain on the screen, thereby separating the pulp fibers from the mixture. Although this pulp fiber contains many impurities, it can be reused in this state depending on the application. The separated pulp fibers have superabsorbent polymer attached to them, and the separated pulp fibers and the superabsorbent polymer attached to the pulp fibers are mixed with water in a predetermined ratio, and the resulting mixture is then sent to the ozone treatment step S36.

[0095] In this embodiment, the separation step S13 includes an ozone treatment step S36 in which a mixture containing a superabsorbent polymer, pulp fibers, their connected structures, and water is treated with an aqueous solution containing ozone to reduce the molecular weight of the superabsorbent polymer attached to the pulp fibers, solubilize it, and remove it.

[0096] In used sanitary products, in absorbents containing pulp fibers and superabsorbent polymers, (i) as the superabsorbent polymer absorbs liquids such as body fluids, it swells and entangles the pulp fibers, and (ii) the swelled superabsorbent polymers entangle the pulp fibers, causing gel blocking, and in many cases, multiple superabsorbent polymers and multiple pulp fibers form connected structures. The above-mentioned mixture contains free pulp fibers and free superabsorbent polymers as well as connected structures composed of multiple superabsorbent polymers and multiple pulp fibers.

[0097] In the ozone treatment step S36, the superabsorbent polymer contained in the mixed liquid (treatment liquid) is oxidatively decomposed by the ozone in the aqueous solution, and is solubilized in the aqueous solution and removed. The state in which the superabsorbent polymer is oxidatively decomposed and solubilized in an aqueous solution refers to the state in which the superabsorbent polymer and the linked structure pass through a 2 mm screen. This allows impurities such as the superabsorbent polymer to be removed from the mixed liquid (treatment liquid), producing high-purity pulp fibers. In addition, ozone treatment can perform secondary sterilization, bleaching, and deodorization of the pulp fibers.

[0098] 2 is a schematic diagram showing an example of the configuration of an apparatus 2 that performs the ozone treatment step S36. The apparatus 2 includes a mixed liquid storage unit 3 that stores a mixed liquid 51 containing water, the pulp fibers separated in the third separation step S35, and a superabsorbent polymer, and an ozone treatment unit 4 that oxidatively decomposes the superabsorbent polymer contained in the mixed liquid 51 and removes it from the pulp fibers.

[0099] The mixed liquid storage unit 3 includes a mixed liquid tank 12 and an agitator 13. The mixed liquid tank 12 stores the mixed liquid 51 supplied via piping 61. The agitator 13 agitates the mixed liquid 51 in the mixed liquid tank 12 so that the pulp fibers and superabsorbent polymer in the mixed liquid 51 do not separate from the water and sink to the bottom of the mixed liquid 51.

[0100] On the other hand, the ozone treatment unit 4 includes a supply pump 21, a treatment tank 31, an ozone supply device 41, a delivery pump 22, and an ozone decomposition device 34. The treatment tank 31 contains an acidic aqueous solution as a treatment liquid 52. The treatment tank 31 is equipped with a mixed liquid supply port 32, a treatment liquid discharge port 33, and an ozone-containing gas supply port 43. The mixed liquid supply port 32 is located at the top of the mixing tank 31 and supplies the mixed liquid 51 to the treatment tank 31. The treatment liquid discharge port 33 is located at the bottom of the mixing tank 31 and discharges the treatment liquid 52. The ozone-containing gas supply port 43 is located at the bottom of the mixing tank 31, specifically, above the treatment liquid discharge port 33, and delivers an ozone-containing gas 53 into the treatment tank 31.

[0101] Specifically, the supply pump 21 continuously supplies the mixed liquid 51 from the mixed liquid tank 12 through the mixed liquid supply port 32 into the treatment tank 31 at a first flow rate via the piping 62. The ozone supply device 41 supplies an ozone-containing gas 53 to the treatment tank 31. Examples of the ozone generator 42 of the ozone supply device 41 include the ozone water exposure tester ED-OWX-2 manufactured by Ecodesign Inc. and the ozone generator OS-25V manufactured by Mitsubishi Electric Corporation. The ozone-containing gas 53 is another type of gas containing ozone, such as oxygen gas containing ozone. The ozone-containing gas supply port 43 delivers the ozone-containing gas 53 supplied to the treatment tank 31 through the piping 65 into the treatment tank 31, and is located at the lower part (preferably the bottom) of the treatment tank 31. The ozone-containing gas supply port 43 continuously supplies the ozone-containing gas 53 into the treatment tank 31 as a plurality of fine bubbles from the lower part to the upper part of the treatment tank 31. The delivery pump 22 continuously discharges the treatment liquid 52 in the treatment tank 31 through the piping 63 and from the treatment liquid outlet 33 to the outside of the treatment tank 31 at a second flow rate. The ozone decomposition device 34 receives the ozone-containing gas 53 accumulated in the upper part of the treatment tank 31 through the piping 64, neutralizes the ozone, and releases it to the outside. Note that the treatment liquid 52 in the treatment tank 31 is only the treatment liquid 52 before the start of the ozone treatment step S36, and becomes a mixture of the treatment liquid 52 and the mixed liquid 51 after the start. However, in this embodiment, the liquid in the treatment tank 31 including the mixture of the treatment liquid 52 and the mixed liquid 51 is referred to as the treatment liquid 52.

[0102] Next, a specific method for the ozone treatment step S36 will be described. The pulp fibers and superabsorbent polymer separated in the third separation step S35 are mixed with water to a preset concentration to form a mixed liquid 51. The concentration of the pulp fibers in mixed liquid 51 is set so that the mixed liquid 51 will have a preset concentration when it is introduced into treatment tank 31 and mixed with treatment liquid 52. Mixed liquid 51 is supplied to mixed liquid tank 12 via piping 61 and stored therein. Because the specific gravities of the pulp fibers and superabsorbent polymer are greater than 1, mixed liquid 51 is stirred by a stirrer 13 in mixed liquid tank 12 to prevent the pulp fibers and superabsorbent polymer from separating from the water.

[0103] The flow rate of the mixed liquid 51 in the mixed liquid tank 12 is controlled by the supply pump 21, and the mixed liquid 51 is continuously supplied at a first flow rate from the mixed liquid supply port 32 to the treatment tank 31 via the pipe 62. The treatment liquid 52 is an acidic aqueous solution, and has a specific gravity of approximately 1. Therefore, the pulp fibers and the superabsorbent polymer settle from the top to the bottom of the treatment liquid 52.

[0104] On the other hand, the ozone-containing gas 53 generated by the ozone generator 42 is supplied to the treatment tank 31 via the pipe 65 and is released in the form of fine bubbles (for example, microbubbles or nanobubbles) from the ozone-containing gas supply port 43 of the treatment tank 31 into the treatment liquid 52. That is, the ozone-containing gas 53 rises from the bottom to the top of the treatment liquid 52.

[0105] The pulp fibers and superabsorbent polymer moving downward, i.e., descending, and the ozone-containing gas 53 moving upward, i.e., ascending, collide with each other while moving in opposite directions within the treatment liquid 52. The ozone-containing gas 53 then adheres to the pulp fibers and superabsorbent polymer, as well as to the surfaces of the connected structure. The ozone in the ozone-containing gas 53 oxidatively decomposes the free superabsorbent polymer and dissolves it in the treatment liquid 52. As a result, the superabsorbent polymer on the pulp fibers is removed from the pulp fibers. The pulp fibers then descend to the bottom of the treatment tank 31, and the ozone-containing gas 53 escapes into the space above the treatment tank 31.

[0106] Among the free superabsorbent polymers, free pulp fibers, and connected structures, the free superabsorbent polymers, which have a relatively high specific gravity, and the connected structures containing superabsorbent polymers, tend to settle more easily than the free pulp fibers, which have a relatively low specific gravity. On the other hand, since the ozone-containing gas rises while consuming ozone and treating the superabsorbent polymers and pulp fibers, the ozone-containing gas at the lower position tends to have a higher ozone content (i.e., be fresher) than the ozone-containing gas at the upper position.

[0107] Therefore, the free superabsorbent polymer and the connecting structure, which move downward relatively quickly, can be oxidatively decomposed by the fresher ozone-containing gas to form free pulp fibers. On the other hand, the free pulp fibers move downward relatively slowly, so the ozone-containing gas can treat the free pulp fibers and the resulting recycled pulp fibers over time.

[0108] Specifically, the ozone in the ozone-containing gas collides with the pulp fibers while facing each other, thereby bleaching the pulp fibers (and the recycled pulp fibers produced). Pulp fibers contained in used sanitary products may have absorbed excrement (e.g., feces, urine, etc.) and become brown or yellow in color, and these fibers must be bleached in order to be reused as recycled pulp fibers. Furthermore, since users may have psychological resistance to recycled pulp fibers derived from used sanitary products, it is preferable that the recycled pulp fibers have a high whiteness, also from the viewpoint of reducing users' psychological resistance.

[0109] Thereafter, the treatment liquid 52 (containing recycled pulp fibers) at the bottom of the treatment tank 31 is continuously discharged at a second flow rate from the treatment liquid outlet 33 of the treatment tank 31 to the outside of the treatment tank 31 via the piping 63 under flow control of the delivery pump 22. The ozone in the ozone-containing gas 53 accumulated in the upper part of the treatment tank 31 is rendered harmless by the ozone decomposition device 34 and released to the outside.

[0110] In this way, the mixed liquid 51 is continuously supplied into the treatment tank 31 from the top of the treatment tank 31 at a first flow rate, and the treatment liquid 52 is continuously discharged from the lower part (bottom) of the treatment tank 31 to the outside of the treatment tank 31 at a second flow rate. This makes it possible to forcibly generate a continuous and stable flow of fluid (including pulp fibers) from the top to the bottom within the treatment tank 31.

[0111] The treatment liquid 52 discharged from the treatment tank 31 contains recycled pulp fibers from which the superabsorbent polymer has been removed, and also contains low-molecular-weight organic matter produced by oxidative decomposition of the superabsorbent polymer. The recycled pulp fibers are recovered in a process downstream of the delivery pump 22, for example, in the fourth separation process S37 described below.

[0112] In this method, a mixed liquid 51 containing at least pulp fibers and a superabsorbent polymer is continuously supplied at a first flow rate into a treatment tank 31 containing a treatment liquid 52 capable of dissolving the superabsorbent polymer, while a treatment liquid 52 containing recycled pulp fibers from which the superabsorbent polymer has been removed and containing low-molecular-weight organic matter produced by oxidative decomposition of the superabsorbent polymer is continuously discharged at a second flow rate out of the treatment tank 31. This configuration makes it possible to forcibly generate a continuous and stable flow of fluid (including pulp fibers) from the mixed liquid supply port 32 that supplies the mixed liquid 51 in the treatment tank 31 toward the treatment liquid discharge port 33 that discharges the treatment liquid 52. This fluid flow, i.e., a water flow, can treat (solubilize) the superabsorbent polymer and treat the pulp fibers, even when the treatment amounts of the pulp fibers and superabsorbent polymer are increased.

[0113] Here, it is preferable that the first flow rate and the second flow rate are the same. By making the first flow rate and the second flow rate the same, the amount of the processing liquid 52 in the processing tank 31 can be kept constant, enabling stable, continuous processing. However, as long as the amount of the processing liquid 52 in the processing tank 31 can be kept substantially constant, i.e., the amount of the processing liquid 52 in the processing tank 31 does not increase or decrease significantly, the first flow rate and the second flow rate may fluctuate over time. In other words, the first flow rate and the second flow rate do not need to be completely the same all the time, but only need to be substantially the same on average over time. Here, "substantially the same" means that the difference between the first flow rate and the second flow rate is within 5% by mass. In this case, stable, continuous processing is also possible.

[0114] When ozone-containing gas 53 is supplied to treatment liquid 52, the ozone concentration in treatment liquid 52 is not particularly limited as long as it is a concentration that can oxidatively decompose the superabsorbent polymer, but may be, for example, 1 to 50 ppm by mass, preferably 2 to 40 ppm by mass, and more preferably 3 to 30 ppm by mass. If the ozone concentration in treatment liquid 52 is too low, the superabsorbent polymer may not be completely solubilized, and the superabsorbent polymer may remain in the pulp fibers. Conversely, if the ozone concentration in treatment liquid 52 is too high, the oxidizing power increases, which may damage the pulp fibers and may also pose safety issues. The ozone treatment temperature is not particularly limited as long as it is a temperature that can oxidatively decompose the superabsorbent polymer, but may be, for example, room temperature or higher.

[0115] The concentration of ozone in the treatment liquid 52 (aqueous solution) is measured by the following method. (1) 85 mL of the treatment liquid 52 in which ozone has been dissolved is placed in a 100 mL measuring cylinder containing about 0.15 g of potassium iodide and 5 mL of 10% citric acid solution, and the mixture is allowed to react. (2) After the reaction, the treated liquid 52 is transferred to a 200 mL Erlenmeyer flask, and the starch solution is added to the Erlenmeyer flask to color it purple. Then, the solution is titrated with 0.01 mol / L sodium thiosulfate while stirring until it becomes colorless, and the amount added, a (mL), is recorded. (3) Calculate the concentration of ozone in the aqueous solution using the following formula: The concentration of ozone in aqueous solution (ppm by mass) was calculated using the following formula: Ozone concentration in aqueous solution (ppm by mass) = a(mL) × 0.24 × 0.85(mL) It is calculated as follows.

[0116] The ozone concentration in the ozone-containing gas 53 is preferably 40 to 200 g / m 3 and more preferably 40 to 150 g / m 3 and more preferably 40 to 100 g / m 3If the ozone concentration in the ozone-containing gas 53 is too low, the superabsorbent polymer may not be completely solubilized, and some of the superabsorbent polymer may remain. If the concentration in the ozone-containing gas 53 is too high, it may damage the pulp fibers, reduce safety, and increase production costs. The ozone concentration in the ozone-containing gas 53 can be measured, for example, by an ultraviolet absorption ozone concentration meter (for example, Ozone Monitor OZM-5000G manufactured by Ecodesign Co., Ltd.).

[0117] The concentrations of the pulp fiber and superabsorbent polymer in the treatment liquid 52 are not particularly limited as long as they are concentrations that allow the superabsorbent polymer to be oxidatively decomposed by the ozone in the treatment liquid 52, but may be, for example, 0.1 to 20% by mass, preferably 0.2 to 10% by mass, and more preferably 0.3 to 5% by mass. If the pulp fiber concentration is too high, the superabsorbent polymer may not be completely solubilized, and the superabsorbent polymer may remain in the pulp fibers. Conversely, if the pulp fiber concentration is too low, the oxidizing power increases, which may damage the pulp fibers and pose safety issues. The concentrations of the pulp fiber and superabsorbent polymer in the mixed liquid 51 are appropriately set based on the concentrations of the pulp fiber and superabsorbent polymer in the treatment liquid 52 and the amount of the treatment liquid 52.

[0118] When ozone is supplied to treatment liquid 52 containing pulp fibers and a superabsorbent polymer, treatment liquid 52 is preferably acidic. More preferably, the pH of treatment liquid 52 is greater than 0 and equal to or less than 5.0, and even more preferably 1.5 to 2.5. Treatment in an acidic state suppresses ozone deactivation, improves the oxidative decomposition effect of ozone on the superabsorbent polymer, and enables the superabsorbent polymer to be oxidatively decomposed in a short period of time. To maintain the pH of the treatment liquid, the pH of mixed liquid 51 may be adjusted to be the same as that of treatment liquid 52, and then mixed liquid 51 may be supplied to treatment tank 31. Alternatively, the pH of treatment liquid 52 may be monitored with a pH sensor, and when the pH fluctuates toward the neutral side, a predetermined acidic solution may be added to treatment liquid 52 in an amount corresponding to the fluctuation range.

[0119] The amount of treatment liquid 52 (including mixed liquid 51) in treatment tank 31 is not particularly limited as long as it is an amount that can oxidatively decompose the superabsorbent polymer. However, it is preferable that the volume V (unit: L) of treatment liquid 52 in treatment tank 31 and the mass W (unit: kg) of pulp fiber satisfy 30≦V / W≦1000. More preferably, it is 50≦V / W≦400, and even more preferably, it is 100≦V / W≦200. If V / W is too small, the superabsorbent polymer may not be completely solubilized, and some superabsorbent polymer may remain. If V / W is too large, it may increase production costs. The volume V of treatment tank 31 is not particularly limited, but may be, for example, 50 to 80 L.

[0120] Flow rate of ozone-containing gas R O (unit: L / min) and the volume V (unit: L) of the treatment solution 52 in the treatment tank 31 are 0.01≦R O / V≦1.25. More preferably, 0.03≦R O / V≦1.0, and more preferably 0.06≦R O / V≦0.75. O If / V is too small, the superabsorbent polymer may not be completely solubilized, and the superabsorbent polymer may remain in the pulp fibers. O If the flow rate R of the ozone-containing gas is too high, it may cause damage to the pulp fibers, reduce safety, and increase production costs. O There are no particular limitations on the flow rate, but examples include 3 to 6 L / min.

[0121] The time the pulp fibers remain in the treatment tank 31, i.e., the time the pulp fibers are treated in the treatment liquid 52 (hereinafter also referred to as the "tank treatment time"), is not particularly limited as long as it is a time that allows the superabsorbent polymer to be oxidatively decomposed. The higher the ozone concentration in the treatment liquid 52, the shorter the tank treatment time may be, but the lower the ozone concentration in the treatment liquid 52, the longer the tank treatment time required. The tank treatment time may be, for example, 2 to 60 minutes, and preferably 5 to 30 minutes. The product of the ozone concentration (ppm by mass) in the treatment liquid 52 and the tank treatment time (minutes) (hereinafter also referred to as the "CT value") is preferably 100 to 6000 ppm·min, more preferably 200 to 4000 ppm·min, and even more preferably 300 to 2000 ppm·min. If the CT value is too small, the superabsorbent polymer may not be completely solubilized, and the superabsorbent polymer may remain in the recovered pulp fibers. If the CT value is too high, it may lead to damage to the pulp fibers, reduced safety, and increased production costs.

[0122] While the pulp fibers are present in the treatment tank 31, the superabsorbent polymer is oxidatively decomposed by the ozone into low molecular weight components, which dissolve in the treatment liquid 52. The low molecular weight components dissolved in the treatment liquid 52 are discharged together with the treatment liquid 52. Furthermore, in this process, the used sanitary products are primarily disinfected by the sterilizing action of ozone.

[0123] In the present embodiment, as a preferred aspect, the ozone treatment step S36 (continuous treatment step) includes a step of continuously supplying the mixed liquid 51 from the top of the treatment tank 31 while continuously discharging the treatment liquid 52 from the bottom of the treatment tank 31. Because the specific gravity of the pulp fibers and the superabsorbent polymer in the mixed liquid 51 is greater than the specific gravity of the water in the treatment liquid 52, the pulp fibers, the superabsorbent polymer, and the connected structure naturally settle.

[0124] In a preferred embodiment of the present invention, the treatment liquid 52 capable of dissolving the superabsorbent polymer is an aqueous solution containing an ozone-containing gas that oxidatively decomposes the superabsorbent polymer to dissolve it. The ozone treatment step S36 (continuous treatment step) further includes a delivery step in which multiple bubbles of ozone-containing gas are continuously delivered from the bottom to the top of the treatment liquid 52. In this preferred embodiment of the present method, the ozone-containing gas rises in the treatment liquid 52, while the pulp fibers and superabsorbent polymer descend, i.e., in a counterflow manner. This increases the probability of contact between the pulp fibers and superabsorbent polymer and the ozone-containing gas. Furthermore, the deeper the pulp fibers and superabsorbent polymer sink, the higher the concentration of the ozone-containing gas they can come into contact with. Therefore, the superabsorbent polymer that was not completely dissolved in the treatment liquid 52 by contacting the ozone-containing gas in a shallow portion of the treatment liquid 52 can be brought into contact with a high concentration of the ozone-containing gas in a deep portion of the treatment liquid 52. This ensures that the superabsorbent polymer is dissolved in the treatment liquid 52. Therefore, the superabsorbent polymer can be reliably dissolved in the treatment liquid and removed from the fibers.

[0125] In the present embodiment, as a preferred aspect, the aforementioned delivery step includes a step of delivering the ozone-containing gas in the form of microbubbles or nanobubbles. Microbubbles are bubbles with a diameter of approximately 1 to 1,000 μm, preferably approximately 10 to 500 μm, and nanobubbles are bubbles with a diameter of approximately 100 to 1,000 nm, preferably approximately 100 to 500 nm. Microbubbles or nanobubbles are such minute bubbles, and have the properties of a large surface area per unit volume and a slow rising speed in the liquid. Therefore, in a preferred aspect of the present method, such minute ozone-containing gas bubbles are delivered from the bottom to the top of the treatment liquid 52 in the treatment tank 31.

[0126] Meanwhile, the pulp fibers and superabsorbent polymer move from top to bottom. At this time, the fine bubbles rise slowly, increasing the probability that the bubbles will come into contact with the pulp fibers. Furthermore, because the fine bubbles occupy a small area on the surface of the pulp fibers, more bubbles can come into contact with the surface of the pulp fibers. This allows the pulp fibers, superabsorbent polymer, and connected structure to be evenly wrapped with the fine bubbles, further increasing the contact area between them and the ozone-containing gas. Furthermore, by allowing more bubbles to come into contact with the surface of the pulp fibers, the buoyancy of the bubbles reduces the sedimentation tendency of the pulp fibers, superabsorbent polymer, and connected structure, further increasing the contact time between them and the ozone-containing gas. These factors allow the superabsorbent polymer to be more reliably dissolved in the treatment liquid 52 and removed from the pulp fibers.

[0127] In the present embodiment, in a preferred embodiment, treatment liquid 52 is an acidic aqueous solution, for example, an acidic aqueous solution with a pH of 2.5 or less. In this case, even if the superabsorbent polymer in mixed liquid 51 partially retains its water absorption capacity, the superabsorbent polymer can be suppressed from absorbing water and expanding. This allows the superabsorbent polymer to be dissolved in treatment liquid 52 in a short time, and the superabsorbent polymer can be removed more reliably. In particular, when treatment liquid 52 is an ozone-containing aqueous solution, the ozone in the ozone-containing aqueous solution is less likely to be deactivated, and the superabsorbent polymer can be oxidatively decomposed and dissolved in a short time, and the superabsorbent polymer can be removed more reliably from the fibers.

[0128] In another preferred embodiment, the configuration of the treatment tank 31 may be other than that shown in FIG. 2. FIG. 3 is a schematic diagram showing another example of the configuration of the ozone treatment device 2 of FIG. 1. The device 2 of FIG. 3 differs from the device 2 of FIG. 2 in that the piping 63 of the ozone treatment unit 4 has a continuous U-shaped pipe structure in which two U-shaped pipes are connected inversely and continuously to each other, and the delivery pump 22 is omitted. In this case, when the piping 63 is filled with the treatment liquid 52 and the liquid level of the treatment liquid 52 in the treatment tank 31 is higher than the liquid level of the liquid in the next step tank connected by the piping 63, the treatment liquid 52 is discharged into the next step tank through the piping 63 due to the siphon principle. Therefore, if the liquid level of processing liquid 52 in processing tank 31 and the liquid level of the liquid in the tank for the next process are initially set to the same level before the start of processing, when mixed liquid 51 is continuously supplied into processing tank 31 at the first flow rate upon the start of processing, processing liquid 52 will be discharged into the tank for the next process through pipe 63 at the second flow rate = the first flow rate due to the siphon principle. However, the liquid level of the liquid in the tank for the next process is maintained at the same level as before the start of processing even during processing. In this case, delivery pump 22 is not required, and control of the second flow rate of delivery pump 22 is not required.

[0129] In this embodiment, the separation step S13 may further include a fourth separation step S37 for separating pulp fibers from the treatment liquid 52 discharged from the treatment tank 31, and a second drying step S38 for drying the separated pulp fibers.

[0130] In the fourth separation step S37, the method for separating the pulp fibers from the treatment liquid 52 discharged from the treatment tank 31 is not particularly limited, but an example is a method in which the treatment liquid 52 containing the recycled pulp fibers is passed through a screen mesh with an opening of 0.15 to 2 mm. When the treatment liquid 52 containing the recycled pulp fibers is passed through a screen mesh with an opening of 0.15 to 2 mm, the wastewater containing products of oxidative decomposition of the superabsorbent polymer passes through the screen. Meanwhile, the recycled pulp fibers remain on the screen.

[0131] In the subsequent second drying step S38, the separated pulp fibers are dried in a high-temperature atmosphere or with hot air, for example. The drying temperature is, for example, 105 to 210°C, and preferably 110 to 190°C. The drying time varies depending on the drying temperature, but is, for example, 10 to 120 minutes, and preferably 15 to 100 minutes. This evaporates and removes the solvent remaining on the surface of the pulp fibers, allowing for the recovery of high-purity pulp fibers with an extremely low superabsorbent polymer content. This allows for efficient recovery of components for sanitary products. The pulp fibers can also be sterilized (disinfected) with a high-temperature atmosphere or hot air, for example.

[0132] In the present disclosure, the recycled pulp fiber has a ΔYI of 0 to 10, preferably 0 to 9.0, more preferably 0 to 7.0, and even more preferably 0 to 5.5, relative to a standard white board. This reduces users' psychological resistance to recycled pulp fiber made from pulp fibers of used sanitary products.

[0133] The ΔYI of recycled pulp fibers can be measured as follows. (1) A Z-300A alternating illumination color difference meter manufactured by Nippon Denshoku Industries Co., Ltd. is prepared in a constant temperature and humidity room with a temperature of 20±5°C and a humidity of 65±5%RH. (2) 4.5 g of recycled pulp fiber dried at 120°C for 60 minutes was spread evenly on the glass window (diameter 40 mm) of the sample stage of the color difference meter. (3) Place the black plate (size: 80 mm x 80 mm, mass: 280 g) attached to the color difference meter on top of the laid recycled pulp fiber, and apply a load to the recycled pulp fiber. (4) Set the color difference meter to reflection mode and a 30 mm transmission window diameter, and measure ΔYI (= |[YI value of recycled pulp fiber] - [YI value of standard white board]|), which is the color difference (absolute value) of the YI value for each sample compared to the standard white board. (5) The average value of ΔYI of the 10 samples is adopted.

[0134] In the present disclosure, the recycled pulp fibers preferably have a water contact angle of 20° or less, more preferably 15° or less, and even more preferably 10° or less. This allows the recycled pulp fibers to have uniform hydrophilicity, making it less likely that users will have a psychological resistance to the recycled pulp fibers. From this perspective, the recycled pulp fibers may have a water contact angle of 0°.

[0135] The water contact angle of recycled pulp fibers can be measured as follows. (1) In a constant temperature and humidity chamber with a temperature of 20±5°C and a humidity of 65±5% RH, an aluminum ring (outer diameter: 43 mm, inner diameter: 40 mm, height: 5 mm) and recycled pulp fiber dried at 120°C for 60 minutes were prepared and left to stand for 24 hours. (2) 1.5 g of recycled pulp fiber is evenly packed into an aluminum ring, and the recycled pulp fiber, along with the aluminum ring, is compressed for 1 minute at a pressure of 3 MPa using a press with a smooth bottom to smooth the surface of the recycled pulp fiber. (3) The water contact angle of the compressed recycled pulp fiber is measured in accordance with JIS R 3257:1999, "Testing Methods for Wettability of Glass Substrate Surfaces," Section 6, Sessile Drop Method. An example of a contact angle measuring device is the CA-V Automatic Contact Angle Meter manufactured by Kyowa Interface Science Co., Ltd. The water contact angle is the value measured 200 ms after deionized water is dropped onto the surface. (4) The water contact angle is measured for 20 different samples, and the average value is used.

[0136] In the present disclosure, the recycled pulp fibers preferably have a lignin content of 0.1% by mass or less, more preferably 0.08% by mass or less, and even more preferably 0.06% by mass or less. By doing so, the recycled pulp fibers have a predetermined lignin content, which tends to result in excellent whiteness and hydrophilicity, and ultimately, articles using the recycled pulp fibers have uniform whiteness and hydrophilicity, making it less likely that users will have psychological resistance to articles using the recycled pulp fibers.

[0137] The lignin content of recycled pulp fiber can be measured according to the method described on pages 85 to 87 of the "Soil Diagnosis Guide" published by the Agricultural Guidance Division of the Agriculture, Forestry and Fisheries Department of Ehime Prefecture in March 1988. An outline of the method is reproduced below. <Preparation of reagents> (1) Prepare recycled pulp fibers dried at 120°C for 60 minutes in a constant temperature and humidity chamber with a temperature of 20±5°C and a humidity of 65±5%RH, and leave them to stand for 24 hours. (2) Prepare commercially available lignin (95%) as a standard reagent. (3) Weigh 22.3 g of sodium pyrophosphate (Na4P2O5·10H2O) into a 500 mL beaker and add approximately 400 mL of deionized water to dissolve the sodium pyrophosphate and prepare an aqueous sodium pyrophosphate solution. (4) Weigh 10 g of sodium hydroxide into another 500 mL beaker, add approximately 200 mL of deionized water, and dissolve the sodium hydroxide to prepare an aqueous sodium hydroxide solution. The aqueous sodium pyrophosphate solution and the aqueous sodium hydroxide solution are allowed to cool, and then added to a 1 L measuring flask and adjusted to a constant volume with deionized water to prepare a pyrophosphate extract.

[0138] <Previous operation> (5) Weigh 2 g of lignin into a 100 mL Erlenmeyer flask A. (6) Weigh 2 g of recycled pulp fiber into another 100 mL Erlenmeyer flask B. (7) Add 20 mL of pyrophosphate extract to each of Erlenmeyer flasks A and B and shake for 3 minutes. (8) After leaving Erlenmeyer flask A and Erlenmeyer flask B to stand for 15 minutes, they are filtered through No. 6 filter paper to obtain a lignin filtrate and a recycled pulp fiber filtrate, respectively.

[0139] <Analysis> (9) Using a volumetric pipette, add 5 mL of the lignin filtrate to a 50 mL volumetric flask and add deionized water to the desired volume (10,000 ppm lignin). (10) Using a measuring pipette, add 1 mL, 2 mL, 5 mL, and 10 mL of the adjusted volume of the lignin filtrate to four 100 mL volumetric flasks, and add deionized water to adjust the volume to the desired volume to form calibration solutions (100 ppm, 200 ppm, 500 ppm, and 1,000 ppm). (11) Using deionized water as a blank, measure the transmittance of the calibration solution at a wavelength of 530 nm and convert it to absorbance using the absorbance conversion table on page 235. (12) Prepare a calibration curve of the calibration solution concentration and absorbance. (13) Measure the transmittance of the recycled pulp fiber filtrate using deionized water as a blank, convert it to absorbance using the absorbance conversion table on page 235, and calculate the lignin concentration from the calibration curve. (14) Calculate the lignin content (mass%) from the lignin concentration. The above-mentioned commercially available lignin (95%) may be changed to a commercially available lignin having a different lignin concentration (for example, lignin from Nacalai Tesque).

[0140] In the manufacturing method of the present disclosure, the recycled pulp fibers have a beating rate of preferably 300 mL or more, more preferably 320 mL or more, even more preferably 340 mL or more, and even more preferably 360 mL or more, so that when the recycled pulp fibers are reused, the recycled pulp fibers tend to fluff and have an increased surface area, which increases the diffuse reflection of light and makes the recycled pulp fibers appear whiter.

[0141] In the manufacturing method of the present disclosure, the recycled pulp fibers have a beating rate reduction rate of preferably 990 mL or less, more preferably 800 mL or less, even more preferably 700 mL or less, and even more preferably 600 mL or less, which can prevent the recycled pulp fibers from becoming too fluffy and generating dust when reused. The above-mentioned rate of reduction in the beating degree can be achieved by adjusting the low lignin content of the recycled pulp fibers, the narrow distribution of the lignin content, and the like.

[0142] The above-mentioned freeness reduction rate is measured according to the following freeness reduction test. <Beating Degree Decrease Test> (1) Recycled pulp fibers are beaten for at least 1 hour, preferably 2 hours, according to JIS P 8221-1:1998 Pulp - Beating Method - Part 1: Beater Method. (2) After beating begins, samples are taken every 20 minutes and the beating degree (Canadian Standard freeness) of each sample is measured in accordance with JIS P 8121-2:2012, Pulp - Freeness Testing Method - Part 2: Canadian Standard Freeness Method. Note that the test may be stopped when the freeness of the sample falls below 100 mL. (3) Plot the time (h) on the horizontal axis and the degree of beating (mL) on the vertical axis, approximate it to a linear function using the least squares method, and use the absolute value of the slope as the rate of decrease in the degree of beating (mL / m). The higher the value of the rate of decrease in the degree of beating, the faster the decrease in the degree of beating per unit time, that is, the easier the recycled pulp fibers are to be beaten (the easier they are to fluff).

[0143] In the present disclosure, the recycled pulp fiber preferably has an ash content of 0.65% by mass or less, more preferably 0.50% by mass or less, even more preferably 0.30% by mass or less, and even more preferably 0.20% by mass or less. This allows the recycled pulp fiber and articles using the recycled pulp fiber to have excellent whiteness and hydrophilicity. As a result, users are less likely to have a psychological resistance to articles using the recycled pulp fiber. The ash content can be reduced by selecting an acid, particularly citric acid, as the inactivating agent in the inactivation step S31 in which the superabsorbent polymer is inactivated, which is capable of forming a complex with metal ions contained in the excrement.

[0144] In this specification, ash content refers to the amount of inorganic or non-combustible residue remaining after organic matter has been incinerated, and ash percentage refers to the proportion (mass ratio) of ash contained in the material to be incinerated. The ash percentage is measured in accordance with "5. Ash Content Test Method" under "2. General Test Methods" in the Sanitary Treatment Product Materials Standards. Specifically, the ash percentage is measured as follows: (1) First, heat a platinum, quartz, or porcelain crucible to 500-550°C for 1 hour, then allow it to cool and accurately measure its mass. (2) Take 2 to 4 g of recycled pulp fiber that has been dried at 120°C for 60 minutes, place it in a crucible, accurately measure the mass, remove or shift the lid of the crucible if necessary, and heat it gently at first, gradually increase the temperature, and heat it to 500 to 550°C for at least 4 hours, until no carbonized material remains, and then ash it. (3) After cooling, accurately measure the mass. The residue is again incinerated until it reaches a constant weight, and after cooling, accurately measure the mass and use this as the ash content (% by mass).

[0145] In the ozone treatment step S36, it is preferable that the pulp fibers in the mixed liquid 51 supplied from the mixed liquid supply port 32 via the pipe 62 do not contain a colorant selected from the group consisting of dyes, pigments, and combinations thereof. Because the pulp fibers do not contain a specific colorant, the recycled pulp fibers tend to have a uniform whiteness, and bleached recycled pulp and products made from recycled pulp fibers tend to have a uniform whiteness, making it less likely that users will have a psychological resistance to products made from recycled pulp fibers.

[0146] In a preferred embodiment, the present method further includes an inactivation step S31, in which the mixture is treated with an aqueous solution capable of inactivating the water absorption performance of the superabsorbent polymer to inactivate the water absorption performance of the superabsorbent polymer in the mixture, prior to the ozone treatment step S36 (continuous treatment step), and a first separation step S32, in which the inactivated superabsorbent polymer and pulp fibers are separated from the aqueous solution, prior to the ozone treatment step S36 (continuous treatment step). Thus, in a preferred embodiment, the present method further includes an inactivation step S31, in which the water absorption performance of the superabsorbent polymer is suppressed with an aqueous solution capable of inactivating the water absorption performance of the superabsorbent polymer, so that the superabsorbent polymer can be more easily dissolved in the treatment liquid 52 in a short time in the subsequent ozone treatment step S36 (continuous treatment step).

[0147] In the present embodiment, in a preferred embodiment, in the inactivation step S31, the aqueous solution capable of inactivating the water absorption performance of the superabsorbent polymer is an acidic aqueous solution, for example, an acidic aqueous solution with a pH of 2.5 or less. Thus, in a preferred embodiment of the present method, since the aqueous solution capable of inactivating the water absorption performance of the superabsorbent polymer is an acidic aqueous solution, the superabsorbent polymer is more easily inactivated, thereby more reliably suppressing the water absorption performance of the superabsorbent polymer in the inactivation step S31. This allows the superabsorbent polymer to be more easily dissolved in the treatment solution in a short time in the subsequent ozone treatment step S36 (continuous treatment step).

[0148] In another preferred embodiment, the treatment tank 31 may include at least a first treatment tank 31-1 and a second treatment tank 31-2 connected in series. FIG. 4 is a schematic diagram showing another example of the configuration of the ozone treatment apparatus 2 of FIG. 1. The apparatus 2 of FIG. 4 differs from the apparatus 2 of FIG. 2 in that two ozone treatment units 4 are connected in series, in other words, the first treatment tank 31-1 and the second treatment tank 31-2 are connected in series. In this case, for example, the first treatment tank 31-1 is supplied with the mixed liquid 51 and discharges a first treated liquid (treated liquid 52-1 from the first treatment tank 31-1), and the second treatment tank 31-2 is supplied with the first treated liquid and discharges a second treated liquid (treated liquid 52-2 from the second treatment tank 31-2), thus treating the mixed liquid 51 in multiple stages. In this case, compared to when one large-capacity treatment tank 31 is provided, treatment is carried out with new treatment liquids 52-1 and 52-2 for each of the first and second treatment tanks 31-1 and 31-2. Therefore, for example, superabsorbent polymer that did not completely dissolve in the first treatment tank (first-stage treatment tank) 31-1 can be easily dissolved in the second treatment tank (next-stage treatment tank) 31-2, and the superabsorbent polymer can be more reliably dissolved and removed from the fibers.

[0149] In a preferred embodiment of the present invention, the material separation step S1 further includes a pretreatment step S11 in which the used sanitary product is highly swollen with water without breaking or otherwise deactivating the superabsorbent polymer. This generates extremely high internal pressure within the used sanitary product, causing some portion of its surface to burst. Then, in the disassembly step S12, a physical impact is applied to the used sanitary product in this state, causing some portion of its surface to tear and ejecting the internal absorbent core to the outside. This allows the used sanitary product to be disassembled into at least the film (backsheet) and the absorbent core. Since the film generally maintains its original shape, it can be easily separated from the absorbent core in the subsequent separation step S13. This allows components such as the film to be separated from the other components while maintaining their original shape without breaking or otherwise deforming them. Therefore, components such as the film of the sanitary product can be efficiently recovered.

[0150] In a preferred embodiment of the present invention, terpene is used to remove the adhesive, enabling the hot melt adhesive used to bond the components of sanitary products to be dissolved at room temperature. This allows the sanitary products to be easily and cleanly disassembled, allowing the pulp fibers and superabsorbent polymer to be separated from the sanitary products, and the nonwoven fabric and film to be separated while retaining their respective component forms. In other words, the pulp fibers, film, and nonwoven fabric can be easily recovered separately without crushing the sanitary products or undergoing complex separation processes. When limonene is used as a terpene, its refreshing citrus odor has the added benefit of masking the odor originating from excrement to some extent, reducing the odor burden on workers and the odor impact on neighbors. Limonene is a monoterpene with a structure similar to styrene, allowing it to dissolve styrene-based hot melt adhesives commonly used in sanitary products. Because sanitary products can be washed at room temperature, energy costs can be reduced and odor generation and diffusion can be suppressed. Terpenes are highly effective at cleaning oil stains, and in addition to dissolving hot melt adhesives, they can also decompose and remove printing ink from films that have been printed, allowing the printed film to be recovered as a high-purity plastic material.

[0151] Furthermore, when an organic acid solution with a pH of 2.5 or less is used to inactivate the superabsorbent polymer, it is less likely to deteriorate the pulp fiber. Furthermore, when citric acid is used as the organic acid, the chelating effect and cleaning power of citric acid can be expected to remove dirt components derived from excrement. It can also be expected to have a disinfecting effect and a deodorizing effect against alkaline odors.

[0152] Furthermore, by oxidatively decomposing superabsorbent polymers with ozone, it is possible to prevent contamination of pulp fibers and a sudden increase in wastewater due to the water absorption of superabsorbent polymers. By adjusting the ozone concentration, it is possible to simultaneously perform oxidative decomposition and sterilization of superabsorbent polymers. Furthermore, since no chlorine-based chemicals are used when using ozone, it is possible to produce high-quality RPF from recovered plastic components that is less likely to damage combustion furnaces. Because no salts are used during the treatment process, no residue remains on the pulp fibers, making it possible to recover high-quality pulp with low ash content.

[0153] The recycled pulp fibers produced by the production method of the present disclosure can be used in a variety of products without any particular limitations. Examples of such products include tissues, nonwoven fabrics (containing pulp fibers), hygiene products (e.g., disposable diapers, urine absorption pads, sanitary napkins, panty liners, pet sheets), cardboard, and paper (printing paper, packaging paper, books, magazines, etc.). In addition, in the hygiene products, the recycled pulp fibers can be used in absorbents, such as core wraps and / or absorbent cores in absorbents that include an absorbent core and a core wrap. [Example]

[0154] Recycled pulp fibers were produced from several types of used disposable diapers collected from nursing homes according to the method shown in Figures 1 and 2. The conditions related to the ozone treatment step S36 were as follows. (i) Mixed liquid 51 Concentration: 1% by mass (concentration of pulp fiber and superabsorbent polymer) pH: 2.4 (ii) Treatment tank 31 ·Capacity: 60L Height: 2.6m Primary flow rate: 2L / min Secondary flow rate: 2L / min Processing time in the tank: 30 minutes V / W:100 ·R O / V:0.033 (iii) Ozone-containing gas Ozone concentration: 100g / m 3 Shape: nanobubbles

[0155] [Manufacturing Example 1] The inactivation step S31 was carried out using citric acid at pH 2.0, and the ozone treatment step S36 was carried out under the above-mentioned conditions. The obtained recycled pulp fiber was dried at 120° C. for 60 minutes to obtain recycled pulp fiber No. 1. [Manufacturing Example 2] Recycled pulp fiber No. 2 was obtained in the same manner as in Production Example 1, except that the inactivation step S31 was carried out using slaked lime.

[0156] [Comparative Manufacturing Example 1] Recycled pulp fiber No. 3 was obtained in the same manner as in Production Example 1, except that no ozone-containing gas was fed in the ozone treatment step S36. [Comparative Manufacturing Example 2] Recycled pulp fiber No. 4 was obtained in the same manner as in Production Example 2, except that no ozone-containing gas was fed in the ozone treatment step S36. [Comparative Manufacturing Example 3] The virgin pulp fiber of NBKP was designated as recycled pulp fiber No. 5.

[0157] [Examples 1 and 2, and Comparative Examples 1 to 3] The ΔYI, water contact angle (°), lignin content (mass%), and ash content (mass%) of recycled pulp fibers No. 1 to No. 5 were measured according to the methods described herein. The results are shown in Table 1. In addition, the beating degree reduction rate (mL / h) of recycled pulp fibers No. 1, No. 2, and No. 5, and the beating degree (mL) after beating for a predetermined time in the beating degree reduction test, were measured according to the beating degree reduction test described herein. The results are also shown in Table 1.

[0158] [Table 1] [Explanation of symbols]

[0159] 31 Treatment tank 32 Mixed liquid supply port 33 Treatment liquid outlet 43 Ozone-containing gas supply port 51 Mixed liquid 52 Processing liquid 53 Ozone-containing gases S36 Ozone treatment process

Claims

1. A method for producing recycled pulp fibers from pulp fibers of used sanitary products, comprising the following steps: a preparation step of preparing a treatment tank including a mixed liquid supply port, and a treatment liquid discharge port and an ozone-containing gas supply port disposed below the mixed liquid supply port; a mixed liquid supply step of supplying a mixed liquid containing a superabsorbent polymer and pulp fibers derived from multiple types of used sanitary products and water from the mixed liquid supply port to the treatment tank; an ozone-containing gas supply step of supplying an ozone-containing gas from the ozone-containing gas supply port to the processing liquid in the processing tank; a recycled pulp fiber forming step in which the ozone-containing gas is brought into contact with the superabsorbent polymer and pulp fibers by causing the ozone-containing gas to rise while causing the superabsorbent polymer and pulp fibers to descend in the treatment tank, thereby dissolving at least a portion of the superabsorbent polymer in the treatment liquid and bleaching the pulp fibers to form the recycled pulp fibers; a treatment liquid discharge step of discharging the treatment liquid containing the recycled pulp fibers from the treatment liquid discharge port; Including, The recycled pulp fibers have a ΔYI of 0 to 10 relative to a standard white board. The method, characterized in that

2. The method described in claim 1, wherein the recycled pulp fibers have a water contact angle of 20° or less.

3. A method described in claim 1 or 2, wherein the recycled pulp fibers have a lignin content of 0.1 mass% or less.

4. A method described in any one of claims 1 to 3, wherein the recycled pulp fibers have a beating degree reduction rate of 300 mL / h or more.

5. A method described in any one of claims 1 to 4, wherein the pulp fibers do not contain a colorant selected from the group consisting of dyes, pigments, and combinations thereof.

6. A method according to any one of claims 1 to 5, wherein at least a portion of the multiple types of sanitary products contain pulp fibers containing a colorant selected from the group consisting of dyes, pigments and combinations thereof, and further comprising a colored pulp fiber removal step of removing pulp fibers containing the colorant from the multiple types of sanitary products before the mixed liquid supply step.

7. At least some of the plurality of types of sanitary products comprise a liquid-permeable sheet, a liquid-impermeable sheet, and an absorbent core disposed therebetween, the absorbent core comprising pulp fibers that do not contain the superabsorbent polymer and the colorant; The colored pulp fiber removal step comprises the following steps: a pretreatment step of swelling the used sanitary products with water; a decomposition step of applying a physical impact to the swollen used sanitary products to decompose the used sanitary products into the absorbent core and other components; a separating step of separating the absorbent core; The method of claim 6, comprising:

8. A method described in any one of claims 1 to 7, wherein in the recycled pulp fiber formation step, the ozone-containing gas is supplied from the ozone-containing gas supply port as microbubbles or nanobubbles.

9. A method according to any one of claims 1 to 8, wherein in the mixed liquid supply step, the mixed liquid is continuously supplied from the mixed liquid supply port to the treatment tank at a first flow rate, and in the treatment liquid discharge step, the treatment liquid is continuously discharged from the treatment liquid discharge port at a second flow rate.

10. The method according to claim 1, wherein the treatment liquid is acidic.

11. A method described in any one of claims 1 to 10, further comprising an inactivation step of inactivating the superabsorbent polymer with an acid before the mixed liquid supply step.

12. The method of claim 11, wherein the acid is an acid capable of forming a complex with a metal ion contained in the excrement.

13. A method described in any one of claims 1 to 12, wherein the recycled pulp fibers have an ash content of 0.65 mass% or less.

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