Method for producing recycled pulp fibers, and recycled pulp fibers

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] This disclosure relates to a method for producing recycled pulp fibers from pulp fibers of used sanitary products, and to recycled pulp fibers derived from used sanitary products, including pulp fibers and superabsorbent polymers. [Background technology]

[0002] Technologies for recycling used disposable diapers and other hygiene products are being explored. For example, Patent Document 1 discloses a method for producing recycled pulp that can be reused mainly as a sanitary product. Specifically, Patent Document 1 describes a method for recovering pulp fibers from used sanitary products containing pulp fibers and superabsorbent polymers, and producing recycled pulp that can be reused as a sanitary product, the method comprising the steps of: decomposing the used sanitary product into pulp fibers and other materials by applying physical force to the used sanitary product in an aqueous solution containing polyvalent metal ions or an acidic aqueous solution with a pH of 2.5 or less; separating the pulp fibers from the mixture of pulp fibers and other materials generated in the decomposition step; and treating the separated pulp fibers with an ozone-containing aqueous solution with 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 this 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 an ozone-containing aqueous solution is placed in a treatment tank, and the separated pulp fibers are placed in the ozone-containing aqueous solution. In the above method, it is preferable to stir the ozone-containing aqueous solution appropriately during treatment to create a water flow, and ozone gas may be blown into the aqueous solution in a container, and a water flow may be generated in the ozone-containing aqueous solution by the rising of ozone gas bubbles. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2016-881 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] In used sanitary products, in absorbent materials containing pulp fibers and superabsorbent polymers, (i) the superabsorbent polymer expands as it absorbs bodily fluids or other liquids, enveloping the pulp fibers, and (ii) the expanded superabsorbent polymers envelop the pulp fibers and cause gel blocking, so that multiple superabsorbent polymers and multiple pulp fibers often form a linked structure.

[0006] In the method for producing recycled pulp described in Patent Document 1, when the superabsorbent polymer and pulp fibers form a linked structure, the ozone-containing gas can bleach the pulp fibers that are not surrounded by the superabsorbent polymer, but it is difficult for the gas to come into contact with the pulp fibers surrounded by the superabsorbent polymer, so 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 perceive these less white areas as remaining dirt, which could cause psychological resistance to the user.

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

[0008] The Disclosers provide a method for producing recycled pulp fibers from pulp fibers of used sanitary products, comprising the following steps: a preparation step, preparing a treatment tank having a mixed liquid supply port, a treatment liquid discharge port and an ozone-containing gas supply port located below the mixed liquid supply port; a mixed liquid supply step, supplying a mixed liquid containing superabsorbent polymer and pulp fibers derived from several types of used sanitary products and water to the treatment tank from the mixed liquid supply port; an ozone-containing gas supply step, supplying ozone-containing gas to the treatment liquid in the treatment tank from the ozone-containing gas supply port; and in the treatment tank, the superabsorbent polymer A method has been discovered comprising a recycled pulp fiber formation step, in which the ozone-containing gas is raised while the superabsorbent polymer and pulp fibers are lowered, thereby bringing the ozone-containing gas into contact with the superabsorbent polymer and pulp fibers, dissolving at least a portion of the superabsorbent polymer in the treatment solution, and bleaching the pulp fibers to form recycled pulp fibers; and a treatment solution discharge step, in which the treatment solution containing the recycled pulp fibers is discharged from the treatment solution discharge port, wherein the recycled pulp fibers have a ΔYI of 0 to 10 with respect to a standard white board. [Effects of the Invention]

[0009] The method for producing recycled pulp fibers from used sanitary products disclosed herein results in articles using recycled pulp fibers that tend to have a uniform whiteness, and users are less likely to feel psychological resistance to articles using recycled pulp fibers. [Brief explanation of the drawing]

[0010] [Figure 1] This flowchart shows an embodiment of the method relating to the present disclosure. [Figure 2] Figure 1 is a schematic diagram showing an example of the configuration of the equipment used in the ozone treatment process. [Figure 3] Figure 1 is a schematic diagram showing another example configuration of the ozone treatment process apparatus. [Figure 4] Figure 1 is a schematic diagram showing yet another example of the configuration of the ozone treatment process apparatus. [Mode for Carrying Out the Invention]

[0011] The present disclosure specifically relates to the following aspects. [Aspect 1] A method for producing recycled pulp fibers from pulp fibers of used hygiene articles, comprising the following steps: a preparation step of preparing a treatment tank comprising a mixed liquid supply port, and a treatment liquid discharge port and an ozone-containing gas supply port arranged below the mixed liquid supply port; a mixed liquid supply step of supplying, from the mixed liquid supply port to the treatment tank, a mixed liquid comprising superabsorbent polymer and pulp fibers derived from a plurality of types of used hygiene articles, and water; 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 of bringing the ozone-containing gas into contact with the superabsorbent polymer and the pulp fibers by causing the ozone-containing gas to rise while causing the superabsorbent polymer and the pulp fibers to descend in the treatment tank, dissolving at least part 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; the method comprising, the recycled pulp fibers having a ΔYI of 0 to 10 relative to a standard white board, the method as described above, characterized in that.

[0012] In used hygiene articles, in absorbent bodies and the like containing pulp fibers and superabsorbent polymer, (i) the superabsorbent polymer swells as it absorbs liquids such as body fluids and entrains the pulp fibers, (ii) the swollen superabsorbent polymers cause gel blocking while entraining the pulp fibers, and the like, whereby a plurality of superabsorbent polymers and a plurality of pulp fibers often form a connected structure.

[0013] On the other hand, pulp fibers contained in used hygiene articles may absorb excreta (for example, feces, urine, etc.) and be colored in brownish or yellowish tones. Therefore, in order to reuse the pulp fibers of used hygiene articles as recycled pulp fibers, it is necessary to bleach the coloration caused by excreta. In addition, since users often have psychological resistance to recycled pulp fibers derived from used hygiene articles, from the perspective of reducing users' psychological resistance, it is preferable that the recycled pulp fibers not only have high whiteness, but also have few uneven spots that may be judged as residual stains or the like (i.e., the bleaching degree distribution is narrow).

[0014] In the method described in Patent Document 1, although the ozone-containing gas can bleach free pulp fibers that do not form a connected structure, it is difficult to come into contact with the pulp fibers constituting the connected structure, that is, the pulp fibers surrounded by the superabsorbent polymer, so that the pulp fibers constituting the connected structure may not be sufficiently bleached in some cases. Articles using such recycled pulp fibers may include portions with poor whiteness, and users may judge such portions with poor whiteness as residual stains, which might give users psychological resistance.

[0015] The above production method includes a predetermined recycled pulp fiber forming step. In this recycled pulp fiber forming step, while the superabsorbent polymer and the pulp fibers are descended, the ozone-containing gas is ascended to bring the superabsorbent polymer and the pulp fibers into contact with the ozone-containing gas. Among the free superabsorbent polymer, the free pulp fibers, and the connected structures, the free superabsorbent polymer with a relatively high specific gravity and the connected structures containing the superabsorbent polymer tend to have higher sedimentation properties than the free pulp fibers with a relatively low specific gravity. On the other hand, since the ozone-containing gas rises while consuming ozone to treat the superabsorbent polymer and the pulp fibers, the ozone-containing gas present at a lower position tends to have a higher ozone content (that is, is fresher) than the ozone-containing gas present at an upper position.

[0016] In this specification, the descending speed refers to the downward movement speed of the processing liquid 52 in the processing tank 31, and is generally uniquely determined by the first flow velocity, the second flow velocity, the size of the processing tank, etc. On the other hand, in this specification, settling property refers to the property of the pulp fibers, superabsorbent polymer, and connecting structure contained in the processing liquid 52 in the processing tank 31, which are prone to falling vertically due to gravity, and each of the pulp fibers, superabsorbent polymer, and connecting structure has different settling properties depending on the specific gravity, etc.

[0017] Therefore, in the above manufacturing method, the free superabsorbent polymers and superabsorbent polymers in the linking structure, which have relatively high settling properties, can be oxidized and decomposed with fresher ozone-containing gas, thereby freeing the pulp fibers that constituted the linking structure. At the same time, the free pulp fibers, which have relatively low settling properties and take relatively long to reach the treatment liquid outlet, can be treated by the ozone-containing gas over a longer period of time.

[0018] Furthermore, since pulp fibers generally tend to have a higher specific gravity the higher their lignin content, in the above manufacturing method, pulp fibers with a relatively high lignin content have a relatively higher sedimentation rate than pulp fibers with a relatively low lignin content. As a result, fresher ozone-containing gas can come into contact with the pulp fibers with a relatively high lignin content, decomposing the lignin contained therein and bleaching it.

[0019] Therefore, in the above manufacturing method, even when the superabsorbent polymer and pulp fibers form a linked structure, the ozone in the ozone-containing gas can remove the superabsorbent polymer constituting the linked structure, and the ozone in the ozone-containing gas can act on the pulp fibers constituting the linked structure, bleaching the pulp fibers. As a result, there is less difference between the degree of bleaching of pulp fibers that did not constitute the linked structure and the degree of bleaching of pulp fibers that did constitute the linked structure, the distribution of bleaching degree of recycled pulp fibers contained in the processing liquid becomes narrower (less uneven), and the distribution of bleaching degree becomes narrower (less uneven) in articles in which the recycled pulp fibers are reused.

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

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

[0022] When hygiene products are recycled, the oil from the hot-melt adhesive they contain is easily absorbed by the pulp fibers. In addition, the lignin contained in the pulp fibers is also hydrophobic. In the method described in Patent Document 1, although ozone in the ozone-containing gas can oxidize and decompose hydrophobic components such as oil and lignin in pulp fibers that do not constitute the connecting structure, the ozone in the ozone-containing gas does not easily come into contact with the pulp fibers that constitute the connecting structure, so the hydrophobic components of the pulp fibers may not be sufficiently oxidized and decomposed. Articles using such recycled pulp fibers may have generally poor hydrophilicity or partially poor hydrophilicity, which may cause psychological resistance to the user.

[0023] Because the above manufacturing method includes a predetermined recycled pulp fiber formation step, it is possible to accurately oxidize and decompose free superabsorbent polymers and linked structures, which have relatively high sedimentation properties, with fresher ozone-containing gas to form free pulp fibers. Furthermore, the ozone-containing gas can treat hydrophobic components such as oil and lignin contained in the free pulp fibers, which have relatively low sedimentation properties, over time. In addition, the recycled pulp fibers tend to have a predetermined water contact angle and tend to have uniform hydrophilicity. Consequently, articles using recycled pulp fibers tend to have uniform hydrophilicity, and users are less likely to feel psychological resistance to articles using recycled pulp fibers.

[0024] [Aspect 3] The method according to embodiment 1 or 2, wherein the recycled pulp fibers have a lignin content of 0.1% by mass or less.

[0025] In the above manufacturing method, even when the superabsorbent polymer and pulp fibers form a linked structure, the ozone in the ozone-containing gas can remove the superabsorbent polymer constituting the linked structure, and the ozone in the ozone-containing gas can act on the pulp fibers constituting the linked structure, reducing the lignin content of the pulp fibers. As a result, there is less difference between the lignin content of pulp fibers that did not constitute a linked structure and the lignin content of pulp fibers that did constitute a linked structure, and the distribution of lignin content of recycled pulp fibers contained in the processing liquid becomes narrower (less variability). Furthermore, because the recycled pulp fibers produced by the above manufacturing method have a predetermined lignin content, the recycled pulp fibers tend to be excellent in whiteness and hydrophilicity. Consequently, articles using the above recycled pulp fibers have uniform whiteness and hydrophilicity, and users are less likely to feel psychological resistance to articles made with recycled pulp fibers.

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

[0027] In the above manufacturing method, the recycled pulp fibers have a predetermined rate of reduction in the degree of beating. Therefore, when the recycled pulp fibers are reused, they tend to become fluffier, increasing their surface area. This increases the diffuse reflection of light, making the recycled pulp fibers appear whiter. Consequently, articles using the recycled pulp fibers tend to have a uniform whiteness, and users are less likely to feel psychological resistance to articles made with recycled pulp fibers.

[0028] [Aspect 5] The method according to any one of embodiments 1 to 4, wherein the pulp fibers described above do not contain a coloring agent selected from the group consisting of dyes, pigments, and combinations thereof.

[0029] In the above manufacturing method, since the pulp fibers do not contain the specified coloring agent, the recycled pulp fibers produced by the above manufacturing method tend to have a uniform whiteness, and articles using recycled pulp fibers tend to have a uniform whiteness, and users are less likely to feel psychological resistance to articles using recycled pulp fibers.

[0030] [Aspect 6] The method according to any one of embodiments 1 to 5, wherein at least a portion of the above-mentioned plurality of sanitary products includes pulp fibers containing a coloring agent selected from the group consisting of dyes, pigments and combinations thereof, and further comprises a colored pulp fiber removal step of removing the pulp fibers containing the coloring agent from the plurality of sanitary products before the mixed liquid supply step.

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

[0032] [Aspect 7] At least a portion of the above-mentioned multiple types of sanitary products comprises a liquid-permeable sheet, a liquid-impermeable sheet, and an absorbent core disposed between them, which includes pulp fibers that do not contain the superabsorbent polymer and the coloring agent. The above step of removing colored pulp fibers is carried out in the following steps: A pretreatment step in which the above-mentioned used sanitary products are swelled with water, A disassembly step in which the swollen used sanitary products are subjected to physical impact to break down the used sanitary products into the absorbent core and the rest of the product. A separation step to separate the absorption core mentioned above, The method according to embodiment 6, including the method described in embodiment 6.

[0033] The above manufacturing method includes predetermined pretreatment steps, decomposition steps, and separation steps. Therefore, even if components of a sanitary product other than the absorbent core include pulp fibers containing a coloring agent, these coloring agent-containing pulp fibers are less likely to be mixed into the mixture of the superabsorbent polymer, pulp fibers, and water. As a result, recycled pulp fibers tend to have a uniform whiteness, articles made using recycled pulp fibers tend to have a uniform whiteness, and users are less likely to have psychological resistance to articles made using recycled pulp fibers.

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

[0035] In the above manufacturing method, during the recycled pulp fiber formation step, ozone-containing gas is supplied from the ozone-containing gas supply port as microbubbles or nanobubbles. Therefore, even when the superabsorbent polymer and pulp fibers form a linked structure, the microbubbles or nanobubbles provide buoyancy to the superabsorbent polymer, linked structure, and pulp fibers, reducing their sinking properties. Consequently, the time it takes for the superabsorbent polymer, linked structure, and pulp fibers to reach the treatment liquid discharge port is extended, allowing the ozone to oxidatively decompose the free superabsorbent polymer and the superabsorbent polymer constituting the linked structure, and to sufficiently treat the free pulp fibers and the pulp fibers constituting the linked structure. Therefore, recycled pulp fibers produced by the above manufacturing method, as well as articles using recycled pulp fibers, tend to have uniform whiteness and hydrophilicity, and users are less likely to have psychological resistance to articles using recycled pulp fibers.

[0036] [Aspect 9] The method according to any one of embodiments 1 to 8, wherein in the mixed liquid supply step, the mixed liquid is continuously supplied to the treatment tank from the mixed liquid supply port at a first flow rate, and in the treated liquid discharge step, the treated liquid is continuously discharged from the treated 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 processing tank at a first flow rate, and in the processed liquid discharge step, the processed liquid is continuously discharged from the processed liquid discharge port at a second flow rate. As a result, the processing time for the superabsorbent polymer and pulp fibers to be processed is made uniform, and recycled pulp fibers and articles using recycled pulp fibers tend to have uniform whiteness and hydrophilicity, and users are less likely to have psychological resistance to articles using recycled pulp fibers.

[0038] [Aspect 10] The method according to any one of embodiments 1 to 9, wherein the above-mentioned processing solution is acidic. In the above manufacturing method, the processing solution is acidic (for example, pH 2.5 or lower). Therefore, the superabsorbent polymer to be processed can be inactivated by the acid, or if the superabsorbent polymer to be processed is already inactivated, it can continue to remain inactivated. As a result, even if the superabsorbent polymer and pulp fibers form a linked structure, the ozone in the ozone-containing gas can remove the superabsorbent polymer constituting the linked structure, and the ozone in the ozone-containing gas acts on the pulp fibers that constituted the linked structure, making them easier to bleach.

[0039] [Aspect 11] The method according to any one of embodiments 1 to 10, further comprising an inactivation step of inactivating the superabsorbent polymer with an acid before the above-mentioned mixing supply step.

[0040] The above manufacturing method further includes a predetermined inactivation step. Therefore, even when the superabsorbent polymer and pulp fibers form a linked structure, the ozone in the ozone-containing gas can remove the superabsorbent polymer constituting the linked structure immediately after the mixed liquid containing the superabsorbent polymer derived from used sanitary products, pulp fibers, and water is supplied to the treatment tank. Furthermore, the ozone in the ozone-containing gas acts on the pulp fibers constituting the linked structure, making them easier to bleach.

[0041] [Aspect 12] The method according to embodiment 11, wherein the acid is an acid capable of forming a complex with metal ions contained in excrement.

[0042] In the above manufacturing method, since the acid is capable of forming a complex with metal ions contained in excrement, the recycled pulp fibers produced by this method, as well as articles using recycled pulp fibers, are less likely to contain metal ions and have excellent whiteness and hydrophilicity. As a result, users are less likely to feel psychological resistance to articles using recycled pulp fibers.

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

[0044] In the above manufacturing method, the recycled pulp fibers produced by this method contain an ash content of 0.65% by mass or less, resulting in recycled pulp fibers and articles made using recycled pulp fibers exhibiting excellent whiteness and hydrophilicity. As a result, users are less likely to feel psychological resistance to articles made using recycled pulp fibers.

[0045] [Aspect 14] Recycled pulp fibers derived from used sanitary products containing pulp fibers and superabsorbent polymers, The above recycled pulp fibers have a ΔYI of 0 to 10 relative to a standard whiteboard. The above-mentioned recycled pulp fiber, characterized by the above features.

[0046] The recycled pulp fibers described above, as well as articles made using these recycled pulp fibers, tend to have a uniform whiteness, and users are less likely to feel psychological resistance to articles made using recycled pulp fibers.

[0047] [Aspect 15] The recycled pulp fiber according to embodiment 14, wherein the recycled pulp fiber has a water contact angle of 20° or less.

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

[0049] [Aspect 16] The recycled pulp fiber according to embodiment 14 or 15, wherein the recycled pulp fiber has a lignin content of 0.1% by mass or less.

[0050] Because the above-mentioned recycled pulp fibers have a predetermined lignin content, they tend to be excellent in whiteness and hydrophilicity. Consequently, articles made using these recycled pulp fibers have uniform whiteness and hydrophilicity, and users are less likely to feel psychological resistance to articles made using recycled pulp fibers.

[0051] [Aspect 17] The recycled pulp fibers described above have a beating rate of 300 mL / h or more, as described in any one of embodiments 14 to 16.

[0052] Because the recycled pulp fibers described above have a predetermined rate of reduction in beating degree, when these recycled pulp fibers are reused, they tend to become fluffier, increasing their surface area. This increases the diffuse reflection of light, making the recycled pulp fibers appear whiter. Consequently, articles made using these recycled pulp fibers tend to have a uniform whiteness, and users are less likely to feel psychological resistance to articles made with these recycled pulp fibers.

[0053] [Aspect 18] The recycled pulp fibers described above contain an ash content of 0.65% by mass or less, as described in any one of embodiments 14 to 17.

[0054] Since the recycled pulp fibers described above have a predetermined ash content, the recycled pulp fibers and articles made using them exhibit excellent whiteness and hydrophilicity. As a result, users are less likely to feel psychological resistance to articles made using recycled pulp fibers.

[0055] The following describes 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"). Furthermore, used sanitary products include sanitary products that have been used by a user and have absorbed the user's liquid excrement, as well as products that have been used but have not absorbed excrement, and unused products.

[0056] First, let's describe some examples of the composition of hygiene products. A hygiene product comprises a surface sheet, a backing sheet, and an absorbent material placed between the surface and backing sheets. Examples of hygiene products include disposable diapers, incontinence pads, sanitary napkins, bed sheets, and pet sheets.

[0057] Examples of components for the surface sheet include nonwoven fabrics or films, specifically liquid-permeable nonwoven fabrics, synthetic resin films with liquid-permeable pores, and composite sheets thereof. Examples of components 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] The components of the absorbent include an absorbent core (e.g., pulp fibers and a superabsorbent polymer) and a core wrap. While there are no particular restrictions on the pulp fibers as long as they are usable in sanitary products, examples include cellulose fibers. Examples of cellulose fibers include wood pulp, cross-linked pulp, non-wood pulp, regenerated cellulose, and semi-synthetic cellulose. While there are no particular restrictions on the superabsorbent polymer (SAP) as long as it is usable in sanitary products, examples include polyacrylate-based, polysulfonate-based, and maleate anhydride-based polymers.

[0059] One side and the other side of the absorbent are joined to the surface sheet and the back sheet, respectively, via adhesive. In a plan view, the portion of the surface sheet that extends outward from the absorbent, surrounding it (peripheral portion), is joined to the portion of the back sheet that extends outward from the absorbent, surrounding it (peripheral portion), via adhesive. Therefore, the absorbent is enclosed within the joint of the surface sheet and the back sheet. As for the adhesive, there are no particular restrictions as long as it can be used as a sanitary product and its bonding strength decreases when softened by hot water as described later, but examples include hot-melt adhesives. Examples of hot-melt adhesives include pressure-sensitive or heat-sensitive adhesives mainly composed of rubber such as styrene-ethylene-butadiene-styrene, styrene-butadiene-styrene, and styrene-isoprene-styrene, or olefins such as polyethylene.

[0060] Figure 1 is a flowchart illustrating 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 fibers, and superabsorbent polymer. This material separation method comprises a pretreatment step S11, a decomposition step S12, and a separation step S13. The pretreatment step S11 involves swelling the used sanitary products with water. The decomposition step S12 involves applying physical shock to the swollen used sanitary products to decompose them into film, nonwoven fabric, core wrap, etc., and absorbent core (e.g., pulp fibers and superabsorbent polymer). The separation step S13 separates the film, nonwoven fabric, pulp fibers, and superabsorbent polymer.

[0061] The method for producing recycled pulp fibers described herein is included in the separation step S13 of this material separation method. If a mixture of pulp fibers and superabsorbent polymer has been obtained in advance by some means, the pretreatment step S11, the decomposition step S12, and the steps prior to the method for producing recycled pulp fibers in the separation step S13 are unnecessary. Each step will be described below.

[0062] In the pretreatment step S11, multiple used sanitary products are allowed to absorb water and swell in the same condition as when they were collected from the outside, that is, without being destroyed, cut, or otherwise altered, and without being rolled up or folded if necessary, and without inactivating the superabsorbent polymer of the absorbent material. In this embodiment, the used sanitary products are either allowed to absorb hot water and swell, or the absorbed water is heated after the products have absorbed water and expanded to make hot water. Hot water refers to water at a temperature higher than room temperature (20℃±15℃ (5~35℃): JIS Z 8703).

[0063] Normally, the amount of liquid excrement actually absorbed by used sanitary products is very small compared to the maximum absorption capacity of the sanitary product (for example, about 10-20% by mass of the maximum absorption capacity). In this embodiment, in the pretreatment step S11, the used sanitary product is immersed in hot water to allow it to absorb water up to an amount close to its maximum absorption capacity (for example, 80% or more by mass of the maximum absorption capacity). Alternatively, the used sanitary product is immersed in room temperature water to allow it to absorb water up to an amount close to its maximum absorption capacity, and then the entire used sanitary product is heated to the temperature of the hot water. This allows the used sanitary product to expand considerably in hot water or room temperature water (hereinafter simply referred to as "hot water"). As a result, a very high internal pressure is generated in the used sanitary product. The purpose of using hot water is mainly to weaken the adhesive strength of the adhesive, as will be described later.

[0064] Here, if the used sanitary product is initially rolled up or folded with the back sheet facing outwards (hiding the front sheet inwards), immersing it in hot water causes the absorbent material of the used sanitary product to absorb the hot water and expand. As a result, the internal pressure of the used sanitary product increases, and a force acts on the used sanitary product to open outwards, causing the rolled or folded used sanitary product to open outwards and become mostly flat. In other words, the used sanitary product can be laid flat in hot water. At this time, because the absorbent material of the used sanitary product has absorbed a large amount of hot water and expanded considerably, one of its surfaces, i.e., either the front sheet or the back sheet enclosing the absorbent material, is in a state where it is easily about to burst. In other words, the pretreatment step S11 makes the used sanitary product in a state where one of its surfaces is about to tear or break. If the used sanitary product is laid flat from the beginning, one of its surfaces will remain in that state and will easily burst. This condition cannot occur if the used sanitary product is torn or damaged.

[0065] Furthermore, when used sanitary products are immersed in and / or absorb hot water, the heat of the hot water softens the adhesive (e.g., hot melt adhesive) used to join the components, thereby reducing the bonding strength of the adhesive. For example, the adhesive joining the peripheral portion of the surface sheet to the peripheral portion of the back sheet can be softened by the heat of the hot water, reducing the bonding strength of that adhesive. In addition, the adhesive joining the surface sheet to the absorbent and the adhesive joining the back sheet to the absorbent can be softened by the heat of the hot water, reducing the bonding strength of those adhesives.

[0066] In this way, the pretreatment step S11 can cause the absorbent material of the used sanitary product to expand, resulting in a state where some part of the surface of the used sanitary product is about to burst, and the bonding strength of the adhesive is reduced. When the used sanitary product is in this state, it can be reliably disassembled in the disassembly step described later.

[0067] The temperature of the hot water in the pretreatment step S11 is not particularly limited as long as it can soften the adhesive of the used sanitary product, but for example, it can be 60°C or higher, preferably 70°C or higher and 98°C or lower. By setting the temperature of the hot water to 70°C or higher, the adhesive joining the components can be softened more by the heat of the hot water, and the bonding strength of the adhesive can be further reduced. By setting the temperature of the hot water to 98°C or lower, the hot water is reliably present as a liquid, so the used sanitary product can absorb the hot water more reliably. Due to the expansion of the absorbent and the heat of the hot water, a state in which the surface of the used sanitary product is about to burst and the bonding strength of the adhesive can be reduced more reliably created. For temperature measurement, the temperature of the hot water with the used sanitary product immersed in it is measured, or the temperature 5 mm inside the surface of the used sanitary product that has absorbed water up to close to the maximum absorption amount is measured (by inserting the tip of the temperature sensor).

[0068] Furthermore, sterilization of the constituent materials is extremely important when reusing used sanitary products. It is preferable to use hot water at a temperature of 70°C or higher, as this can effectively sterilize (disinfect) used sanitary products.

[0069] The processing time in the pre-treatment step S11, that is, the time during which the used sanitary products are immersed in warm water, is not particularly limited as long as the absorbent material of the used sanitary products expands, but is, for example, 2 to 60 minutes, preferably 4 to 30 minutes. If the time is too short, the absorbent material will not expand sufficiently, and if it is too long, time will be wasted and processing costs will increase unnecessarily.

[0070] Furthermore, the amount of hot water absorbed by the absorbent in the pretreatment step S11 is not particularly limited as long as the absorbent expands to the extent that it can decompose the used sanitary product in the decomposition step described later. For example, it can be 80% by mass or more of the maximum absorption capacity of the used sanitary product, and preferably 90% by mass or more. This allows the used sanitary product to be fully expanded with water. As a result, an extremely high internal pressure can be generated in the absorbent of the used sanitary product.

[0071] However, the maximum absorption amount is measured using the following procedure. (1) Unused sanitary products are dried in an atmosphere of 100°C or higher, and their mass is measured. (2) If the sanitary product has elastic material that can form pockets that make it difficult for water to reach the absorbent material (for example, elastic material around the legs, waist, etc.), the sanitary product will be made flat by making cuts in the elastic material. (3) Immerse the sanitary items in a bath filled with plenty of tap water, with the surface sheet facing down, and leave for 30 minutes. (4) After standing, place the sanitary product on the mesh with the surface sheet facing down, drain for 20 minutes, and then measure the mass of the sanitary product. The maximum absorption amount is defined as the difference in mass before and after immersion in tap water.

[0072] Next, the decomposition step S12 applies physical impact to the multiple used sanitary products that have been unfolded and swollen by the pretreatment step S11, thereby decomposing the multiple used sanitary products into a film (back sheet), a nonwoven fabric (front sheet), a core wrap, and an absorbent core (e.g., an absorbent material and a superabsorbent polymer).

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

[0074] In this embodiment, the disassembly step S12 includes the steps of loading a plurality of swollen used sanitary products into the bottom of a rotating drum with a horizontal rotation axis, and rotating the rotating drum around its rotation axis to lift the plurality of used sanitary products to the top of the rotating drum and then slamming them against the bottom. This allows for a stable, continuous, and easy application of physical impact to the plurality of used sanitary products. An example of the rotating drum is the rotating drum of the washing tub of a horizontal washing machine, and therefore the disassembly step S12 can be carried out using an existing horizontal washing machine (for example, ECO-22B manufactured by Inamoto Seisakusho Co., Ltd.). The size of the rotating drum is not particularly limited as long as the above impact can be achieved, but the inner diameter and depth can be, for example, 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 impact can be achieved, but can be, for example, 30 to 100 times / minute.

[0075] Furthermore, the temperature of the used sanitary products is kept relatively high by the warm water absorbed into them. However, from the viewpoint of suppressing the temperature drop of the adhesive and maintaining the sterilization effect, the ambient temperature inside the rotating drum is preferably 70°C or higher, and 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, and more preferably 90°C or lower. The amount of water inside the rotating drum is preferably as little as possible, preferably so that the used sanitary products do not fall below the water surface at least at the bottom. If the used sanitary products fall below the water surface, the impact on the used sanitary products is absorbed by the water, making it difficult to impart the desired impact to the used sanitary products. The time for which the rotating drum is rotated is not particularly limited as long as the surface sheet, back sheet, core wrap, etc., and the absorbent core can be disassembled, but for example, it is 2 to 40 minutes, and preferably 4 to 20 minutes.

[0076] When used sanitary products are subjected to physical impact, the joint between the surface sheet (nonwoven fabric) and the back sheet (film) bursts and tears. Simultaneously, the internal pressure of the absorbent material causes the absorbent core (e.g., pulp fibers and superabsorbent polymer) inside the used sanitary product to be ejected (fly out) through the tear. This allows for more reliable decomposition of the used sanitary product into the surface sheet (nonwoven fabric), back sheet (film), core wrap, etc., and the absorbent core (e.g., pulp fibers and superabsorbent polymer).

[0077] Next, separation step S13 separates the multiple films (back sheets) and multiple nonwoven fabrics (front sheets), core wrap, etc. from the absorbent core (e.g., pulp fibers and superabsorbent polymer). However, the nonwoven fabric may only be bonded to the film. The above separation method is not particularly limited, but for example, one method is to use a sieve that passes through the absorbent core but not through the front sheets, back sheets, 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 inactivator 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, the inactivated superabsorbent polymer, and wastewater discharged from the superabsorbent polymer due to inactivation.

[0079] In the inactivation step S31, prior to the first separation step S32, the surface sheet (nonwoven fabric), back sheet (film), and absorbent material (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 adhering to the surface sheet, back sheet, and pulp fibers. As a result, the superabsorbent polymer, which is in a highly viscous state before inactivation, can be reduced to a less viscous state through dehydration caused by inactivation.

[0080] Here, the inactivator is not particularly limited, but examples include acids (e.g., inorganic acids and organic acids), lime, calcium chloride, magnesium sulfate, magnesium chloride, aluminum sulfate, and aluminum chloride. The above acids are preferred because they do not leave ash residue on the pulp fibers. When using an acid as an inactivator, the pH is preferably 2.5 or lower, 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. Also, the sterilization capacity may decrease. If the pH is too low, there is a risk of equipment corrosion, and a large amount of alkaline chemicals will be required for neutralization treatment during wastewater treatment.

[0081] Examples of the inorganic acids mentioned above include sulfuric acid, hydrochloric acid, and nitric acid, but sulfuric acid is preferred from the viewpoint of not containing chlorine and cost. On the other hand, examples of the organic acids mentioned above include citric acid, tartaric acid, glycolic acid, malic acid, succinic acid, acetic acid, and ascorbic acid, but acids that can form complexes with metal ions contained in excrement, such as hydroxycarbonate organic acids like citric acid, tartaric acid, and gluconic acid, are particularly preferred. Calcium ions are an example of metal ions contained in excrement. This is because the chelating effect of acids that can form complexes with metal ions contained in excrement traps and removes metal ions in the excrement. Furthermore, citric acid can be expected to have a high dirt removal effect due to its cleaning effect. Since pH changes with water temperature, the pH referred to in this disclosure is the pH measured at an aqueous solution temperature of 20°C.

[0082] The processing temperature of the inactivation step S31, i.e., the temperature of the aqueous solution containing the inactivator, is not particularly limited as long as the inactivation reaction proceeds. The processing temperature may be at room temperature or higher than room temperature, for example, 15 to 30°C. The processing time of the inactivation step S31, i.e., the time for immersing the surface sheet, back sheet and absorbent in the aqueous solution containing the inactivator, is not particularly limited as long as the superabsorbent polymer is inactivated and dehydrated, for example, 2 to 60 minutes, preferably 5 to 30 minutes. The amount of aqueous solution in the inactivation step S31, i.e., the amount of aqueous solution containing the inactivator, is not particularly limited as long as the inactivation reaction proceeds. The amount of aqueous solution is, for example, preferably 300 to 3000 parts by mass, more preferably 500 to 2500 parts by mass, and even more preferably 1000 to 2000 parts by mass, per 100 parts by mass of used sanitary products.

[0083] In the first separation step S32, the surface 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 due to inactivation. However, 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 hot water.

[0084] In the first separation step S32, the method for separating the surface sheet and back sheet from the pulp fibers, superabsorbent polymer, and wastewater is not particularly limited. For example, the products generated by the inactivation step (surface sheet, back sheet, pulp fibers, superabsorbent polymer, wastewater, etc.) are discharged through a screen with a mesh size of 5 to 100 mm, preferably 10 to 60 mm. This separates the products by allowing the pulp fibers, superabsorbent polymer, and wastewater to remain in the wastewater, while the surface sheet and back sheet remain on the screen. Other large-shaped materials such as nonwoven fabrics and films may remain on the screen. In particular, before inactivation, the superabsorbent polymer is in a highly viscous state, so separating the superabsorbent polymer adhering to the surface sheet, back sheet, and pulp fibers is not easy. However, after inactivation, dehydration reduces the viscosity of the superabsorbent polymer, allowing it to be easily separated from the surface sheet, backing sheet, and pulp fibers. Therefore, the components of sanitary products can be efficiently separated and recovered.

[0085] Furthermore, the manufacturing method disclosed herein applies to multiple types of used sanitary products, and if at least a portion of the multiple types of sanitary products include pulp fibers containing a coloring agent selected from the group consisting of dyes, pigments, and combinations thereof, for example, if Corewrap is composed of pulp fibers containing the above-mentioned coloring agent, it is preferable to remove the pulp fibers containing the coloring agent (e.g., Corewrap) in the separation step S13. This is because the recycled pulp fibers tend to have a uniform whiteness, articles made using recycled pulp fibers tend to have a uniform whiteness, and users are less likely to have psychological resistance to articles made using recycled pulp fibers.

[0086] In this embodiment, separation step S13 may further include a second separation step S33 in which the adhesive at the joint between the film and other members is removed using a solvent that dissolves the adhesive at the joint. In this embodiment, the adhesive at each joint between the film, nonwoven fabric and absorbent 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 surface sheet, surface sheet, absorbent material remaining on the surface of the back sheet, etc.) is removed with a solvent. This allows the film and other components to be separated from each other while maintaining their original shape without tearing or other damage. Therefore, components such as the film of sanitary products can be recovered efficiently. Furthermore, since the film and other components can be separated without leaving any adhesive on the film, the film can be reused as a high-purity resin. This suppresses the adverse effects of the adhesive when the film is reused. The same applies to the nonwoven fabric as to the film.

[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 Tempel is used, and the concentration of Tempel in the aqueous solution is, for example, 0.05% by mass or more and 2% by mass or less. Preferably it is 0.075% to 1% by mass. If the concentration of terpene is too low, it may not be possible to dissolve the adhesive at the joint. If the concentration of terpene is too high, the cost may increase. In addition, Tempel not only dissolves adhesives such as hot melt adhesives, but also has an oil stain cleaning effect. Therefore, for example, if there is printing on the components of sanitary products such as backing sheets, Tempel can also decompose and remove the printing ink.

[0089] Examples of terpene hydrocarbons include myrcene, limonene, pinene, camphor, sapinene, phellandrene, p-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 sedge. Among terpenes, terpene hydrocarbons are preferred, and limonene is particularly preferred. There are three types of limonene: d-limonene, l-limonene, and dipentene (dl-limonene), all of which can be used. Terpenes can be used individually or in combination of two or more types.

[0090] The processing temperature of the second separation step S33, i.e., the temperature of the aqueous solution containing the solvent, is not particularly limited as long as the adhesive dissolves and the used sanitary product is broken down into its components. The processing temperature may be at room temperature or higher than room temperature, for example, 15 to 30°C. The processing time of the second separation step S33, i.e., the time for immersing the surface sheet, back sheet and absorbent in the aqueous solution containing the solvent, is not particularly limited as long as the adhesive dissolves and the used sanitary product is broken down into its components. For example, the processing time may be 2 to 60 minutes, preferably 5 to 30 minutes. The amount of aqueous solution in the second separation step S33, i.e., the amount of aqueous solution containing the solvent, is not particularly limited as long as the adhesive dissolves and the used sanitary product is broken down into its components. The amount of aqueous solution is, for example, preferably 300 to 3000 parts by mass, and more preferably 500 to 2500 parts by mass, per 100 parts by mass of used sanitary product. The second separation step S33 reduces the amount of adhesive remaining on the film, nonwoven fabric, absorbent material, etc. to 1% by mass or less relative to the film, nonwoven fabric, absorbent material, etc.

[0091] In this embodiment, as another preferred embodiment, the second separation step S33 may be performed in conjunction with the inactivation step S31. That is, the superabsorbent polymer attached to the surface sheet, back sheet, and pulp fibers may be inactivated while the adhesive attached to the surface sheet, back sheet, and pulp fibers is dissolved. In this case, the aqueous solution used to immerse the surface sheet, back sheet, pulp fibers, and superabsorbent polymer contains both an inactivator and a solvent. This allows the back sheet (film), surface sheet (nonwoven fabric), and absorbent material (pulp fibers and superabsorbent polymer) to be largely separated in the aqueous solution during the inactivation step S31. Then, in the subsequent first separation step, the back sheet (film) and surface sheet (nonwoven fabric) can be separated from the absorbent material (pulp fibers and superabsorbent polymer), and the second separation step S33 can be omitted. In this case, the back sheet (film) and surface sheet (nonwoven fabric) are substantially separated by the removal of the adhesive.

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

[0093] Sterilization is extremely important when reusing used sanitary products. In the first drying step S34, the separated film (back sheet) and nonwoven fabric (front sheet) are dried in a high-temperature atmosphere or with hot air. The drying temperature is, for example, 105 to 210°C, preferably 110 to 190°C. The drying time depends on the drying temperature, but for example, it is 10 to 120 minutes, preferably 15 to 100 minutes. This not only evaporates and removes any solvent remaining on the surface of the film and nonwoven fabric, but also sterilizes the film and nonwoven fabric with a high-temperature atmosphere or hot air. This makes it possible to remove the solvent while also achieving a sterilizing (disinfecting) effect.

[0094] On the other hand, in this embodiment, the separation step S13 may include a third separation step S35 for separating pulp fibers from the separated mixture. In the third separation step S35, there are no particular limitations on the method for separating the pulp fibers from the separated mixture (including pulp fibers, superabsorbent polymer, and wastewater), 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. As a result, the superabsorbent polymer and wastewater are discharged into the wastewater, and the pulp fibers (mainly with the superabsorbent polymer remaining on the surface) remain on the screen, thereby separating the pulp fibers from the mixture. These pulp fibers contain many impurities, but depending on the application, they can be reused in this state. The separated pulp fibers have superabsorbent polymers attached to them. The separated pulp fibers and the superabsorbent polymers attached to them are mixed with water in a predetermined ratio, and the mixture proceeds to the ozone treatment step S36.

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

[0096] In used sanitary products, in absorbent materials containing pulp fibers and superabsorbent polymers, (i) the superabsorbent polymer expands as it absorbs bodily fluids or other liquids, enveloping the pulp fibers, and (ii) the expanded superabsorbent polymers envelop the pulp fibers and cause gel blocking, so that multiple superabsorbent polymers and multiple pulp fibers often form a linked structure. The above mixture contains not only free pulp fibers and free superabsorbent polymers, but also a linked structure composed of multiple superabsorbent polymers and multiple pulp fibers.

[0097] In the ozone treatment process S36, the superabsorbent polymer contained in the mixed solution (treatment solution) is removed by oxidative decomposition with ozone in the aqueous solution, thereby solubilizing it in the aqueous solution. The state in which the superabsorbent polymer is oxidatively decomposed and soluble in an aqueous solution refers to a state in which the superabsorbent polymer and its linked structures can pass through a 2 mm screen. This removes impurities such as the superabsorbent polymer from the mixed solution (treatment solution), enabling the production of high-purity pulp fibers. Furthermore, ozone treatment can be used to perform secondary sterilization, bleaching, and deodorization of the pulp fibers.

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

[0099] The mixed liquid storage unit 3 includes a mixed liquid tank 12 and a stirrer 13. The mixed liquid tank 12 stores the mixed liquid 51 supplied via piping 61. The stirrer 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 settle at the bottom of the mixed liquid 51.

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

[0101] Specifically, the supply pump 21 continuously supplies the mixed liquid 51 from the mixed liquid tank 12 to the treatment tank 31 at a first flow rate via the piping 62 through the mixed liquid supply port 32. The ozone supply device 41 supplies ozone-containing gas 53 to the treatment tank 31. Examples of the ozone generator 42 of the ozone supply device 41 include the ED-OWX-2 ozone water exposure tester manufactured by Eco Design Co., Ltd. and the OS-25V ozone generator 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 via the piping 65 into the treatment tank 31 and is located at the bottom (preferably the bottom) of the treatment tank 31. The ozone-containing gas supply port 43 continuously supplies the ozone-containing gas 53 as multiple fine bubbles into the treatment liquid 52 (treatment tank 31) from the bottom to the top. The discharge pump 22 continuously discharges the processing liquid 52 in the processing tank 31 from the processing liquid outlet 33 at a second flow rate via the piping 63. The ozone decomposition device 34 receives the ozone-containing gas 53 accumulated at the top of the processing tank 31 via the piping 64, detoxifies the ozone, and releases it to the outside. Before the start of the ozone processing step S36, the processing liquid 52 in the processing tank 31 is only the processing liquid 52, and after the start, it becomes a mixture of the processing liquid 52 and the mixed liquid 51. In this embodiment, however, the liquid in the processing tank 31, including the mixture of the processing liquid 52 and the mixed liquid 51, is referred to as the processing liquid 52.

[0102] Next, the specific method of the ozone treatment process S36 will be described. In the third separation step S35, the pulp fibers and superabsorbent polymer separated are mixed with water to a predetermined concentration to form a mixture 51. The concentration of pulp fibers in the mixture 51 is set to a predetermined concentration when it is introduced into the processing tank 31 and mixed with the processing liquid 52. The mixture 51 is supplied to the mixture tank 12 via piping 61 and stored there. Since the specific gravity of pulp fibers and superabsorbent polymer is greater than 1, the mixture 51 is stirred in the mixture tank 12 by a stirrer 13 to prevent the pulp fibers and superabsorbent polymer from separating from the water.

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

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

[0105] Then, within the processing liquid 52, the pulp fibers and superabsorbent polymer moving downwards, i.e., descending, and the ozone-containing gas 53 moving upwards, i.e., rising, collide with each other as they move in opposition. The ozone-containing gas 53 adheres to the surfaces of the pulp fibers, superabsorbent polymer, and connecting structures. The ozone in the ozone-containing gas 53 oxidizes and decomposes the free superabsorbent polymer, dissolving it in the processing 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 processing tank 31, and the ozone-containing gas 53 escapes into the space above the processing tank 31.

[0106] In free superabsorbent polymers, free pulp fibers, and linked structures, the free superabsorbent polymers, which have a relatively higher specific gravity, and the linked structures containing the superabsorbent polymers tend to settle more readily than the free pulp fibers, which have a relatively lower specific gravity. On the other hand, as the ozone-containing gas rises, it consumes ozone and processes the superabsorbent polymers and pulp fibers; therefore, the ozone-containing gas located at a lower position tends to have a higher ozone content (i.e., be fresher) than the ozone-containing gas located at a higher position.

[0107] Therefore, free superabsorbent polymers and linked structures, which move relatively quickly downwards, can be precisely oxidized and decomposed with fresher ozone-containing gas to form free pulp fibers. On the other hand, since free pulp fibers move relatively slowly downwards, the ozone-containing gas can process the free pulp fibers and the recycled pulp fibers formed over time.

[0108] Specifically, ozone in an ozone-containing gas can bleach pulp fibers (and the recycled pulp fibers produced) by colliding with them while facing each other. Pulp fibers contained in used sanitary products may have absorbed excrement (e.g., feces, urine, etc.) and become discolored in shades of brown or yellow. In order to reuse them as recycled pulp fibers, they need to be bleached. Furthermore, since users may have psychological resistance to recycled pulp fibers derived from used sanitary products, it is preferable for recycled pulp fibers to have a high degree of whiteness from the standpoint of reducing this psychological resistance.

[0109] Subsequently, the processing liquid 52 (containing recycled pulp fibers) at the bottom of the processing tank 31 is continuously discharged at a second flow rate through the processing liquid outlet 33 of the processing tank 31 via the piping 63, controlled by the flow rate of the discharge pump 22. The ozone from the ozone-containing gas 53 accumulated at the top of the processing tank 31 is rendered harmless by the ozone decomposition device 34 and released to the outside.

[0110] In this manner, the mixed liquid 51 is continuously supplied into the treatment tank 31 from the top at a first flow rate, and the treatment liquid 52 is continuously discharged from the bottom of the treatment tank 31 to the outside of the treatment tank 31 at a second flow rate. This forces a continuous and stable flow of fluid (including pulp fibers) from top to bottom into the treatment tank 31.

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

[0112] This method involves continuously supplying a mixture 51 containing at least pulp fibers and an absorbent polymer at a first flow rate into a treatment tank 31 having a treatment liquid 52 capable of dissolving the absorbent polymer, while continuously discharging the treatment liquid 52, which contains recycled pulp fibers from which the absorbent polymer has been removed and low molecular weight organic matter produced by the oxidative decomposition of the absorbent polymer, out of the treatment tank 31 at a second flow rate. This configuration allows for the forced generation of a continuous and stable fluid flow (including pulp fibers) from the mixture supply port 32 that supplies the mixture 51 to the treatment liquid discharge port 33 that discharges the treatment liquid 52 in the treatment tank 31. This fluid flow, i.e., the water flow, allows for the processing (solubilization) of the absorbent polymer and the pulp fibers, even when processing large amounts of pulp fibers and absorbent polymer.

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

[0114] When supplying ozone-containing gas 53 to the treatment solution 52, the ozone concentration in the treatment solution 52 is not particularly limited as long as it is a concentration that can oxidatively decompose the superabsorbent polymer, but for example, it is 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 the treatment solution 52 is too low, the superabsorbent polymer cannot be completely solubilized, and there is a risk that the superabsorbent polymer will remain in the pulp fibers. Conversely, if the ozone concentration in the treatment solution 52 is too high, the oxidizing power will also increase, which may damage the pulp fibers and may also cause safety problems. The ozone treatment temperature is not particularly limited as long as it is a temperature that can oxidatively decompose the superabsorbent polymer, but for example, it may be at room temperature or higher than room temperature.

[0115] The concentration of ozone in the treatment solution 52 (aqueous solution) is measured by the following method. (1) Add 85 mL of treatment solution 52 containing dissolved ozone to a 100 mL graduated cylinder containing approximately 0.15 g of potassium iodide and 5 mL of 10% citric acid solution, and allow to react. (2) Transfer the reaction solution 52 to a 200 mL Erlenmeyer flask, add starch solution to the Erlenmeyer flask, and after it turns purple, titrate with 0.01 mol / L sodium thiosulfate while stirring until it becomes colorless, and record the amount added a (mL). (3) Calculate the concentration of ozone in the aqueous solution using the following formula. The concentration of ozone in an aqueous solution (mass ppm) is given by the following formula: Ozone concentration (mass ppm) in aqueous solution = a(mL) × 0.24 × 0.85(mL) It is calculated by [method].

[0116] The ozone concentration in ozone-containing gas 53 is preferably 40 to 200 g / m³. 3 More preferably, 40-150 g / m² 3 And more preferably 40-100 g / m² 3If the ozone concentration in 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 ozone concentration in ozone-containing gas 53 is too high, it may lead to damage to the pulp fibers, reduced safety, and increased manufacturing costs. The ozone concentration in ozone-containing gas 53 can be measured, for example, using an ultraviolet absorption type ozone meter (e.g., Eco Design Co., Ltd.: Ozone Monitor OZM-5000G).

[0117] The concentrations of pulp fibers and superabsorbent polymer in the treatment solution 52 are not particularly limited, as long as the ozone in the treatment solution 52 can oxidize and decompose the superabsorbent polymer. For example, concentrations of 0.1 to 20% by mass are possible, preferably 0.2 to 10% by mass, and more preferably 0.3 to 5% by mass. If the concentration of pulp fibers is too high, the superabsorbent polymer may not be completely solubilized, and some of the superabsorbent polymer may remain in the pulp fibers. Conversely, if the concentration of pulp fibers is too low, the oxidizing power will also increase, which may damage the pulp fibers and may also pose safety problems. The concentrations of pulp fibers and superabsorbent polymer in the mixed solution 51 are appropriately set based on the concentrations of pulp fibers and superabsorbent polymer in the treatment solution 52 and the amount of treatment solution 52.

[0118] When supplying ozone to a processing solution 52 containing pulp fibers and a superabsorbent polymer, it is preferable that the processing solution 52 is acidic. More preferably, the pH of the processing solution 52 is greater than 0 and 5.0 or less, and even more preferably 1.5 to 2.5. Processing in an acidic state suppresses ozone deactivation, improves the oxidative decomposition effect of the superabsorbent polymer by ozone, and allows the superabsorbent polymer to be oxidatively decomposed in a short time. To maintain the pH of the processing solution, the pH of the mixed solution 51 may be made the same as the pH of the processing solution 52, and the mixed solution 51 may be supplied to the processing tank 31. Alternatively, the pH of the processing solution 52 may be monitored with a pH sensor, and when the pH fluctuates towards neutral, a predetermined acidic solution may be added to the processing solution 52 in an amount corresponding to the fluctuation range.

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

[0120] Flow rate R of ozone-containing gas O (unit: L / min) and the volume V (unit: L) of the treatment liquid 52 in the treatment tank 31 preferably satisfy 0.01≦R O / V≦1.25. It is more preferably 0.03≦R O / V≦1.0, and still more preferably 0.06≦R O / V≦0.75. If R O / V is too small, the superabsorbent polymer may not be completely solubilized and the superabsorbent polymer may remain on the pulp fibers. If R O / V is too large, it may lead to damage to the pulp fibers, reduced safety, and increased manufacturing cost. The flow rate R of the ozone-containing gas O is not particularly limited, and for example, 3 to 6 L / min can be mentioned.

[0121] The time that the pulp fibers are present in the treatment tank 31, that is, the time that the pulp fibers are treated in the treatment liquid 52 (hereinafter also referred to as "in-tank treatment time"), is not particularly limited as long as it is a time that allows for the oxidative decomposition of the superabsorbent polymer. The in-tank treatment time can be short if the ozone concentration of the treatment liquid 52 is high, and long if the ozone concentration of the treatment liquid 52 is low. Examples of in-tank treatment times include 2 minutes to 60 minutes, and preferably 5 minutes to 30 minutes. The product of the ozone concentration (mass ppm) in the treatment liquid 52 and the in-tank treatment time (minutes) (hereinafter also referred to as "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 low, the superabsorbent polymer cannot be completely solubilized, and there is a risk that the superabsorbent polymer will remain in the recovered pulp fibers. If the CT value is too high, it may lead to damage to pulp fibers, reduced safety, and increased manufacturing costs.

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

[0123] In this embodiment, in a preferred configuration, 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. Since the specific gravity of the pulp fibers and superabsorbent polymer in the mixed liquid 51 is greater than the specific gravity of the water in the treatment liquid 52, the pulp fibers, superabsorbent polymer, and connecting structures settle naturally.

[0124] In this embodiment, in a preferred configuration, the processing solution 52 capable of dissolving the superabsorbent polymer is an aqueous solution containing ozone-containing gas that oxidizes and decomposes the superabsorbent polymer in a soluble manner. The ozone treatment step S36 (continuous processing step) further includes a discharge step of continuously discharging multiple bubbles of ozone-containing gas from the bottom to the top of the processing solution 52. In this preferred configuration of the method, in the processing solution 52, the ozone-containing gas rises and the pulp fibers and superabsorbent polymer descend, i.e., there is a countercurrent. 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 settle, the higher the concentration of ozone-containing gas they can come into contact with. Therefore, the superabsorbent polymer that could not be completely dissolved in the processing solution 52 by the ozone-containing gas that came into contact with it at a shallow depth can be brought into contact with a high concentration of ozone-containing gas at a deeper depth in the processing solution 52. This ensures that the superabsorbent polymer is reliably dissolved in the processing solution 52. Therefore, the superabsorbent polymer can be reliably dissolved in the treatment solution and removed from the fibers.

[0125] In this embodiment, in a preferred configuration, the above-described discharge step includes a step of discharging the ozone-containing gas in the form of microbubbles or nanobubbles. However, microbubbles are bubbles with a diameter of about 1 to 1000 μm, preferably about 10 to 500 μm, and nanobubbles are bubbles with a diameter of about 100 to 1000 nm, preferably about 100 to 500 nm. Microbubbles or nanobubbles are such fine bubbles that have a large surface area per unit volume and a slow rate of rising in the liquid. Therefore, in this method, in a preferred configuration, the ozone-containing gas in the form of such fine bubbles is discharged from the bottom to the top of the processing liquid 52 in the processing tank 31.

[0126] Meanwhile, the pulp fibers and superabsorbent polymer move from top to bottom. At this time, because the fine bubbles rise slowly, the probability of the bubbles contacting the pulp fibers is increased. Furthermore, because the area occupied by the fine bubbles on the surface of the pulp fibers is small, more bubbles can come into contact with the surface of the pulp fibers. As a result, the pulp fibers, superabsorbent polymer, and connecting structures can be evenly enveloped by the fine bubbles, and the contact area between them and the ozone-containing gas can be increased. Moreover, as more bubbles come into contact with the surface of the pulp fibers, the buoyancy of the bubbles reduces the settling of the pulp fibers, superabsorbent polymer, and connecting structures, and the contact time between them and the ozone-containing gas can be increased. As a result, the superabsorbent polymer can be more reliably dissolved in the treatment solution 52 and removed from the pulp fibers.

[0127] In this embodiment, a preferred configuration is that the treatment solution 52 is an acidic aqueous solution, for example, an acidic aqueous solution with a pH of 2.5 or lower. In this case, even if the superabsorbent polymer in the mixed solution 51 still retains some water absorption capacity, the water absorption and swelling of the superabsorbent polymer can be suppressed. As a result, the superabsorbent polymer can be dissolved in the treatment solution 52 in a short time, and the superabsorbent polymer can be removed more reliably. In particular, if the treatment solution 52 is an ozone-containing aqueous solution, the ozone in the ozone-containing aqueous solution is less likely to be deactivated, so the superabsorbent polymer can be oxidized and decomposed in a shorter time, dissolved, and removed from the fibers more reliably.

[0128] Furthermore, in another preferred embodiment, the configuration of the treatment tank 31 may be other than that shown in Figure 2. Figure 3 is a schematic diagram showing another example of the configuration of the apparatus 2 for the ozone treatment process shown in Figure 1. The apparatus 2 in Figure 3 differs from the apparatus 2 in Figure 2 in that the piping 63 of the ozone treatment section 4 has a continuous U-tube structure in which two U-tubes are connected inverted and continuously to each other, and the discharge pump 22 is omitted. In this case, when the piping 63 is filled with 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 process tank connected by the piping 63, the treatment liquid 52 is discharged to the next process tank via the piping 63 by the principle of siphon. Therefore, if the liquid level of the processing liquid 52 in the processing tank 31 is initially set to be the same as the liquid level of the liquid in the next process tank before the start of processing, when the mixed liquid 51 is continuously supplied to the processing tank 31 at a first flow rate upon the start of processing, the processing liquid 52 will be discharged to the next process tank via the piping 63 at a second flow rate = first flow rate due to the siphon principle. However, the liquid level of the liquid in the next process tank is maintained at the same level as before the start of processing even during processing. In this case, the discharge pump 22 is unnecessary, and control of the second flow rate of the discharge pump 22 is also unnecessary.

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

[0130] In the fourth separation step S37, there are no particular limitations on the method for separating the pulp fibers from the processing liquid 52 discharged from the processing tank 31. For example, one method is to pass the processing liquid 52 containing recycled pulp fibers through a screen mesh with a mesh size of 0.15 to 2 mm. When the processing liquid 52 containing recycled pulp fibers is passed through a screen mesh with a mesh size of 0.15 to 2 mm, the wastewater containing products from the oxidative decomposition of superabsorbent polymers passes through the screen. On the other hand, 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. The drying temperature is, for example, 105 to 210°C, preferably 110 to 190°C. The drying time depends on the drying temperature, but for example, it is 10 to 120 minutes, preferably 15 to 100 minutes. As a result, any solvent remaining on the surface of the pulp fibers evaporates and is removed, allowing for the recovery of highly pure pulp fibers with an extremely low percentage of superabsorbent polymers. Therefore, the components of sanitary products can be recovered efficiently. In addition, the pulp fibers can be sterilized (disinfected) in a high-temperature atmosphere or with hot air.

[0132] In this disclosure, the recycled pulp fibers have a ΔYI of 0 to 10 relative to a standard whiteboard, preferably 0 to 9.0, more preferably 0 to 7.0, and even more preferably 0 to 5.5. This makes it less likely for users to have psychological resistance to recycled pulp fibers made from pulp fibers of used sanitary products.

[0133] The ΔYI of recycled pulp fibers can be measured as follows: (1) Prepare a Z-300A cross-illumination photometric color difference meter manufactured by Nippon Denshoku Industries, Ltd. in a constant temperature and humidity chamber with a temperature of 20±5℃ and a humidity of 65±5%RH. (2) Spread 4.5g of recycled pulp fibers, dried at 120°C for 60 minutes, evenly across the glass window (40mm in diameter) of the colorimeter's sample stage. (3) Place the black plate attached to the colorimeter (size: 80mm x 80mm, weight: 280g) on ​​top of the laid-out recycled pulp fibers and apply a load to the recycled pulp fibers. (4) Select the colorimeter mode: reflection, transmission window diameter: 30 mm, and for each sample, measure the color difference (absolute value) of the YI value from the standard whiteboard, which is ΔYI (=|[YI value of recycled pulp fiber]-[YI value of standard whiteboard]|). (5) The average value of ΔYI from the 10 samples is used.

[0134] In this disclosure, 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 ensures that articles using recycled pulp fibers have uniform hydrophilicity, thus reducing the likelihood of users having psychological resistance to articles made with recycled pulp fibers. From this viewpoint, the water contact angle of the recycled pulp fibers may also be 0°.

[0135] The water contact angle of recycled pulp fibers can be measured as follows: (1) Prepare an aluminum ring (outer diameter: 43 mm, inner diameter: 40 mm, height: 5 mm) and recycled pulp fibers dried at 120°C for 60 minutes in a constant temperature and humidity chamber at 20 ± 5°C and 65 ± 5% RH, and leave them undisturbed for 24 hours. (2) 1.5 g of recycled pulp fibers are evenly packed into an aluminum ring, and the recycled pulp fibers, along with the aluminum ring, are compressed for 1 minute at a pressure of 3 MPa using a press with a smooth bottom surface to smooth the surface of the recycled pulp fibers. (3) The water contact angle of the compressed recycled pulp fibers is measured in accordance with JIS R 3257:1999, "Test Method for Wettability of Substrate Glass Surfaces," Section 6, the static 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 above water contact angle refers to the value 200 ms after dropping deionized water. (4) Measure the water contact angle in 20 different samples and use the average value.

[0136] In this 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 makes them excellent in whiteness and hydrophilicity, and consequently, articles using the recycled pulp fibers have uniform whiteness and hydrophilicity, making it less likely for users to feel psychological resistance to articles using recycled pulp fibers.

[0137] The lignin content of recycled pulp fibers can be measured according to the method described on pages 85-87 of the "Soil Diagnosis Handbook" published in March 1988 by the Agricultural Guidance Division of the Ehime Prefectural Department of Agriculture, Forestry and Fisheries. A summary is provided below. <Preparing the 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 standing 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, add approximately 400 mL of deionized water, and dissolve the sodium pyrophosphate to prepare an aqueous sodium pyrophosphate solution. (4) Measure 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. Allow the sodium pyrophosphate aqueous solution and sodium hydroxide aqueous solution to cool, add them to a 1 L volumetric flask, and adjust the 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 Erlenmeyer flask A and Erlenmeyer flask B, and shake for 3 minutes. (8) After allowing Erlenmeyer flask A and Erlenmeyer flask B to stand for 15 minutes, filter them using No. 6 filter paper to obtain lignin filtrate and recycled pulp fiber filtrate, respectively.

[0139] <Analysis> (9) Using a volumetric pipette, add 5 mL of lignin filtrate to a 50 mL volumetric flask and dilute to the final volume with deionized water (lignin 10,000 ppm). (10) Using a volumetric pipette, add 1 mL, 2 mL, 5 mL, and 10 mL of the lignin filtrate, which has been reduced to the correct volume, to four 100 mL volumetric flasks. Add deionized water to reduce the volume to the correct level and form calibration solutions (100 ppm, 200 ppm, 500 ppm, 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 as a reference. (12) Create a calibration curve between the concentration of the calibration solution and the 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 commercially available lignin (95%) mentioned above may be replaced with commercially available lignin of a different lignin concentration (for example, lignin from Nacalai Tesque).

[0140] In the manufacturing method of this disclosure, the recycled pulp fibers have a beaten degree reduction 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. As a result, when the recycled pulp fibers are reused, the recycled pulp fibers become more prone to fluffing, increasing their surface area, which in turn increases the diffuse reflection of light, making the recycled pulp fibers appear whiter.

[0141] In the manufacturing method of the present disclosure, the recycled pulp fibers have a beaten degree 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. By doing so, when the recycled pulp fibers are reused, it is possible to suppress the generation of dust and other debris due to excessive fluffing of the recycled pulp fibers. Furthermore, the above-mentioned rate of decrease in the degree of beatenness can be achieved by adjusting the low lignin content of the recycled pulp fibers, the narrow distribution of lignin content, and other factors.

[0142] The above beaten-down rate is measured according to the beaten-down rate test described below. <Test to reduce the degree of percussion> (1) The recycled pulp fibers are beaten for at least one hour, preferably two hours, in accordance with JIS P 8221-1:1998, pulp - beating methods - Part 1: beater method. (2) After the start of beating, a sample shall be taken every 20 minutes, and the degree of beating (Canadian Standard freeness) of each sample shall be measured according to JIS P 8121-2:2012 Pulp - Test method for freeness - Part 2: Canadian Standard freeness method. The test may be stopped when the degree of beating of the sample falls below 100 mL. (3) Plot time (h) on the horizontal axis and beaten degree (mL) on the vertical axis, approximate it with a linear function using the least squares method, and adopt the absolute value of its slope as the beaten degree decrease rate (mL / m). Furthermore, a higher beaten degree reduction rate indicates that the beaten degree decreases faster per unit time, meaning that the recycled pulp fibers are more easily beaten (more prone to fluffing).

[0143] In this disclosure, the recycled pulp fibers preferably have 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, and even more preferably 0.20% by mass or less. As a result, the recycled pulp fibers and articles using recycled pulp fibers have excellent whiteness and hydrophilicity. Consequently, users are less likely to have psychological resistance to articles using recycled pulp fibers. The above ash content can be reduced in the inactivation step S31, which inactivates the superabsorbent polymer, by selecting an acid capable of forming a complex with metal ions contained in the excrement, particularly citric acid, as the inactivator.

[0144] In this specification, ash content refers to the amount of inorganic or non-combustible residue remaining after organic matter has been ashed, and the ash content ratio refers to the ratio (mass ratio) of ash content contained in the material to be accelerated. The above ash content ratio is measured according to "2. General Test Methods" and "5. Ash Content Test Method" of the Physiological Treatment Product Materials Standards. Specifically, the ash content ratio is measured as follows. (1) Preheat a platinum, quartz, or porcelain crucible to 500-550°C for 1 hour, allow it to cool, and then precisely measure its mass. (2) Take 2-4 g of recycled pulp fibers dried at 120°C for 60 minutes, place them in a crucible, weigh them precisely, remove or slightly shift the lid of the crucible if necessary, and heat them slowly at first, then gradually increase the temperature to 500-550°C for more than 4 hours until they are ashed and no carbonized material remains. (3) After cooling, its mass is precisely measured. The residue is again ashed until a constant weight is reached, and after cooling, its mass is precisely measured and expressed as the ash content (mass %).

[0145] In the ozone treatment process S36, it is preferable that the pulp fibers in the mixed liquid 51 supplied from the mixed liquid supply port 32 via the piping 62 do not contain a coloring agent selected from the group consisting of dyes, pigments, and combinations thereof. Since the pulp fibers do not contain a predetermined coloring agent, the recycled pulp fibers tend to have a uniform whiteness, and bleached recycled pulp and articles using recycled pulp fibers tend to have a uniform whiteness, making it less likely for users to feel psychological resistance to articles using recycled pulp fibers.

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

[0147] In this embodiment, in a preferred configuration, the aqueous solution capable of inactivating the water absorption performance of the superabsorbent polymer in the inactivation step S31 is an acidic aqueous solution, for example, an acidic aqueous solution with a pH of 2.5 or lower. Thus, in this method, in a preferred configuration, 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, and thereby the water absorption performance of the superabsorbent polymer can be more reliably suppressed at the inactivation step S31. As a result, the superabsorbent polymer can be more easily dissolved in the treatment solution in a shorter time at 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 with each other. Figure 4 is a schematic diagram showing another configuration example of the apparatus 2 for the ozone treatment process of Figure 1. The apparatus 2 of Figure 4 differs from the apparatus 2 of Figure 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 the 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 the second treated liquid (treated liquid 52-2 from the second treatment tank 31-2), and so on, the mixed liquid 51 is treated in multiple stages. In this case, compared to having one large-capacity processing tank 31, processing is carried out with new processing liquids 52-1 and 52-2 for each of the first and second processing tanks 31-1 and 31-2. For example, superabsorbent polymers that were not completely dissolved in the first processing tank (initial processing tank) 31-1 can be easily dissolved in the second processing tank (next processing tank) 31-2, thus enabling more reliable dissolution of superabsorbent polymers and removal from the fibers.

[0149] In this embodiment, a preferred configuration further involves a material separation step S1, where, in the pretreatment step S11, used sanitary products can be greatly expanded with water without being torn or otherwise damaged, while maintaining their original shape and without inactivating the superabsorbent polymer. This generates very high internal pressure within the used sanitary product, causing one part of its surface to be on the verge of bursting. Then, in the decomposition step S12, a physical impact is applied to the used sanitary product in this state, causing one part of its surface to tear and ejecting the internal absorbent core. This allows the used sanitary product to be decomposed into at least a film (back sheet) and an absorbent core. At this point, the film generally maintains its original shape, so 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 other components while maintaining their original shape without being torn or damaged. Therefore, components such as the film of the sanitary product can be efficiently recovered.

[0150] In this embodiment, a preferred configuration involves using a terpene to remove the adhesive, which allows the hot-melt adhesive used to bond the components of sanitary products to be dissolved at room temperature. This makes it easy and clean to separate the sanitary products, allowing for the separation of pulp fibers and superabsorbent polymers, as well as the nonwoven fabric and film, while maintaining their component forms. In other words, pulp fibers, film, and nonwoven fabric can be easily recovered separately without crushing the sanitary products or going through complex separation processes. When limonene is used as the terpene, a secondary effect of limonene is its refreshing citrus odor, which can mask odors from excrement to some extent, reducing the odor burden on workers and the odor impact on the surrounding area. Since limonene is a monoterpene and has a structure similar to styrene, it can dissolve styrene-based hot-melt adhesives commonly used in sanitary products. Because the cleaning process of sanitary products can be performed at room temperature, energy costs can be reduced, and the generation and diffusion of odors can be suppressed. Terpenes have a high oil stain cleaning effect, and in addition to their ability to dissolve hot melt adhesives, they can also decompose and remove printing inks from films if they have printing on them, and the printed films can be recovered as high-purity plastic material.

[0151] Furthermore, when an organic acid aqueous solution with a pH of 2.5 or lower is used to inactivate superabsorbent polymers, it is less likely to degrade pulp fibers. Also, 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. Additionally, antibacterial and deodorizing effects against alkaline odors can be expected.

[0152] Furthermore, by oxidatively decomposing superabsorbent polymers with ozone, it is possible to prevent contamination of pulp fibers and the rapid increase in wastewater due to water absorption by superabsorbent polymers. By adjusting the ozone concentration, it is possible to simultaneously oxidatively decompose and sterilize superabsorbent polymers. In addition, since no chlorine-based chemicals are used when ozone is employed, it is possible to produce high-quality RPF (Refuse Derived Fuel) from the recovered plastic components that is less likely to damage the combustion furnace. Because no salts are used during the processing, there is no residue in the pulp fibers, and high-quality pulp with low ash content can be recovered.

[0153] Recycled pulp fibers produced by the manufacturing method of the present disclosure are not particularly limited and can be used in a variety of articles. Examples of such articles include tissues, nonwoven fabrics (containing pulp fibers), sanitary products (e.g., disposable diapers, incontinence pads, sanitary napkins, panty liners, pet sheets), corrugated cardboard, and paper (printing paper, packaging paper, books, magazines, etc.). In the case of sanitary products, recycled pulp fibers can be used in absorbents, for example, in absorbents comprising an absorbent core and / or a core wrap, specifically in the core wrap and / or absorbent core. [Examples]

[0154] Recycled pulp fibers were produced from several types of used disposable diapers collected from nursing care facilities, according to the methods shown in Figures 1 and 2. The conditions related to the ozone treatment process S36 were as follows: (i) Mixed liquid 51 • Concentration: 1% by mass (concentration of pulp fibers and superabsorbent polymer) pH: 2.4 (ii) Processing tank 31 ·Capacity: 60L Height: 2.6m • First flow rate: 2 L / min • Second flow rate: 2 L / min • Tank processing time: 30 minutes V / W: 100 ·R O / V:0.033 (iii) Ozone-containing gases • Ozone concentration: 100 g / m³ 3 • Form: Nanobubble

[0155] [Manufacturing Example 1] The deactivation process S31 was carried out with citric acid at pH 2.0, and the ozone treatment process S36 was carried out under the above conditions. The resulting recycled pulp fibers were 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 performed with 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 ozone-containing gas was not supplied in the ozone treatment process S36. [Comparative Manufacturing Example 2] Recycled pulp fiber No. 4 was obtained in the same manner as in Production Example 2, except that ozone-containing gas was not supplied in the ozone treatment process S36. [Comparative Manufacturing Example 3] NBKP's virgin pulp fibers were designated as recycled pulp fibers No. 5.

[0157] [Examples 1 and 2, and Comparative Examples 1-3] 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 method described herein. The results are shown in Table 1. In addition, the rate of decrease in beating degree (mL / h) and the degree of beating degree (mL) after beating for a predetermined time in the beating degree decrease test for recycled pulp fibers No. 1, No. 2, and No. 5 were measured according to the beating degree decrease test described herein. The results are also shown in Table 1.

[0158] [Table 1] [Explanation of Symbols]

[0159] 31 Processing tank 32 Mixed liquid supply port 33 Treatment liquid discharge port 43 Ozone-containing gas supply port 51 Mixed liquid 52 Treatment solution 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: Preparation step: Prepare a treatment tank equipped with a mixed liquid supply port, a treatment liquid discharge port and an ozone-containing gas supply port located below the mixed liquid supply port. A mixed liquid supply step involves supplying a mixed liquid containing superabsorbent polymers and pulp fibers derived from multiple types of used sanitary products, and water, from the mixed liquid supply port to the processing tank. Ozone-containing gas supply step: supplying ozone-containing gas from the ozone-containing gas supply port to the processing liquid in the processing tank, In the processing tank, the superabsorbent polymer and pulp fibers are lowered while the ozone-containing gas is raised, thereby bringing the ozone-containing gas into contact with the superabsorbent polymer and pulp fibers, dissolving at least a portion of the superabsorbent polymer in the processing liquid, and bleaching the pulp fibers to form the recycled pulp fibers, in the recycled pulp fiber formation step. A processing liquid discharge step, wherein the processing liquid containing the recycled pulp fibers is discharged from the processing liquid discharge port. Includes, The recycled pulp fibers have a ΔYI of 0 to 10 relative to a standard whiteboard. The method characterized by the above.

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

3. The method according to claim 1 or 2, wherein the recycled pulp fibers have a lignin content of 0.1% by mass or less.

4. The method according to any one of claims 1 to 3, wherein the recycled pulp fibers have a beaten degree reduction rate of 300 mL / h or more.

5. The method according to any one of claims 1 to 4, wherein the pulp fibers do not contain a coloring agent selected from the group consisting of dyes, pigments and combinations thereof.

6. The method according to any one of claims 1 to 5, wherein at least a portion of the plurality of sanitary products comprises pulp fibers containing a coloring agent selected from the group consisting of dyes, pigments and combinations thereof, and further comprises a colored pulp fiber removal step of removing the pulp fibers containing the coloring agent from the plurality of sanitary products before the mixed liquid supply step.

7. At least a portion of the plurality of sanitary products comprises a liquid-permeable sheet, a liquid-impermeable sheet, and an absorbent core disposed between them, which includes pulp fibers that do not contain the superabsorbent polymer and the colorant, The aforementioned step of removing colored pulp fibers is comprised of the following steps: A pretreatment step of swelling the aforementioned used sanitary products with water, A disassembly step of applying physical impact to the swollen used sanitary products to separate the used sanitary products into the absorbent core and the other parts. A separation step of separating the absorption core, The method according to claim 6, including the method described in claim 6.

8. The method according to 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. The 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 any one of claims 1 to 9, wherein the processing solution is acidic.

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

13. The method according to any one of claims 1 to 12, wherein the recycled pulp fibers contain an ash content of 0.65% by mass or less.

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