Recycled pulp fiber

Abstract

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

Description

Technical Field

[0001] The present disclosure relates to a method for producing recycled pulp fibers from pulp fibers of used sanitary products, and to recycled pulp fibers derived from used sanitary products containing pulp fibers and superabsorbent polymers.

Background Art

[0002] Techniques for recycling used sanitary products such as disposable diapers have been studied. For example, Patent Document 1 discloses a method for producing recycled pulp that can be mainly reused 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 including decomposing the used sanitary products into pulp fibers and other materials by applying a physical force to the used sanitary products in an aqueous solution containing polyvalent metal ions or an acidic aqueous solution having a pH of 2.5 or less, separating the pulp fibers from the mixture of the pulp fibers and other materials generated in the decomposition step, and treating the separated pulp fibers with an ozone-containing aqueous solution having a pH of 2.5 or less.

[0003] In Patent Document 1, the reason for treating the pulp fibers with an ozone-containing aqueous solution is that a non-negligible amount of superabsorbent polymer remains in the separated pulp fibers, and the superabsorbent polymer is oxidized and decomposed and solubilized to remove it from the pulp fibers. In Patent Document 1, as a method for treating pulp fibers with an ozone-containing aqueous solution, a method is disclosed 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 appropriately stir the ozone-containing aqueous solution during the treatment to create a water flow, and ozone gas may be blown into the aqueous solution placed in a container to generate a water flow in the ozone-containing aqueous solution by the rising of the ozone gas bubbles.

Prior Art Documents

Patent Document

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In used sanitary products, in an absorber containing pulp fibers and a superabsorbent polymer, etc., (i) as the superabsorbent polymer absorbs a liquid such as body fluid, it swells and entangles the pulp fibers, and (ii) the swollen superabsorbent polymers cause gel blocking while entangling the pulp fibers, etc., so that a plurality of superabsorbent polymers and a plurality of pulp fibers often form a connected structure.

[0006] In the method for producing recycled pulp described in Patent Document 1, when a superabsorbent polymer and pulp fibers form a connected structure, although an ozone-containing gas can bleach the pulp fibers not surrounded by the superabsorbent polymer, since it is difficult for the ozone-containing gas to contact the pulp fibers surrounded by the superabsorbent polymer, the pulp fibers surrounded by the superabsorbent polymer may not be sufficiently bleached. In an article using such recycled pulp fibers, there may be a portion with poor whiteness, and the user may judge such a portion with poor whiteness as remaining dirt, which may give the user a psychological resistance.

[0007] Therefore, an object of the present disclosure is to provide a method for producing recycled pulp fibers from the pulp fibers of used sanitary products, in which an article using the recycled pulp fibers is likely to have a uniform whiteness and the user is less likely to have a psychological resistance to the article using the recycled pulp fibers.

Means for Solving the Problems

[0008] The present inventors have developed a method for producing recycled pulp fibers from pulp fibers of used sanitary products, comprising the following steps: a preparation step of preparing a treatment tank having a mixed liquid supply port, a treatment liquid discharge port disposed below the mixed liquid supply port, and an ozone-containing gas supply port; a mixed liquid supply step of supplying a mixed liquid containing a superabsorbent polymer and pulp fibers derived from a plurality of types of used sanitary products and water from the mixed liquid supply port to the treatment tank; an ozone-containing gas supply step of supplying an ozone-containing gas from the ozone-containing gas supply port to the treatment liquid in the treatment tank; a recycled pulp fiber formation step of causing the ozone-containing gas to rise while lowering the superabsorbent polymer and pulp fibers in the treatment tank, thereby bringing the ozone-containing gas into contact with the superabsorbent polymer and pulp fibers, dissolving at least a part of the superabsorbent polymer in the treatment liquid, and bleaching the pulp fibers to form the recycled pulp fibers; and a treatment liquid discharge step of discharging the treatment liquid containing the recycled pulp fibers from the treatment liquid discharge port, wherein the recycled pulp fibers have a ΔYI of 0 to 10 with respect to a standard white board.

Advantages of the Invention

[0009] The method for producing recycled pulp fibers from pulp fibers of used sanitary products according to the present disclosure makes it easy for articles using the recycled pulp fibers to have uniform whiteness, and makes it less likely for users to have a psychological resistance to articles using the recycled pulp fibers.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

Figure 3

Figure 4

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 the pulp fibers of used sanitary products, comprising the following steps: A preparation step of preparing a treatment tank having a mixed liquid supply port, a treatment liquid discharge port disposed below the mixed liquid supply port, and an ozone-containing gas supply port; A mixed liquid supply step of supplying a mixed liquid containing a superabsorbent polymer and pulp fibers derived from a plurality of types of used sanitary products and water from the mixed liquid supply port to the treatment tank; An ozone-containing gas supply step of supplying an ozone-containing gas from the ozone-containing gas supply port to the treatment liquid in the treatment tank; A recycled pulp fiber forming step of bringing the ozone-containing gas into contact with the superabsorbent polymer and the pulp fibers by raising the ozone-containing gas while lowering the superabsorbent polymer and the pulp fibers in the treatment tank, dissolving at least a 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; including the recycled pulp fibers having a ΔYI of 0 to 10 with respect to a standard white board; The method is characterized in that.

[0012] In used sanitary products, in an absorbent body containing pulp fibers and a superabsorbent polymer, etc., (i) as the superabsorbent polymer absorbs a liquid such as body fluid, it swells and entangles the pulp fibers, and (ii) the swollen superabsorbent polymers cause gel blocking while entangling the pulp fibers, etc., so that a plurality of superabsorbent polymers and a plurality of pulp fibers often form a connected structure.

[0013] On the one hand, the pulp fibers contained in used sanitary products absorb excrement (e.g., feces, urine, etc.) and may be colored in a tea color or yellow color. Therefore, in order to reuse the pulp fibers of used sanitary products as recycled pulp fibers, it is necessary to bleach the coloring caused by excrement. In addition, since users often have a psychological resistance to recycled pulp fibers derived from used sanitary products, from the perspective of reducing the users' psychological resistance, it is preferable that the recycled pulp fibers not only have a high whiteness but also have few unevennesses (narrow distribution of bleachability) that can be judged as remaining stains or the like.

[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 contact the pulp fibers constituting the connected structure, that is, the pulp fibers surrounded by the superabsorbent polymer. Therefore, there were cases where the pulp fibers constituting the connected structure could not be sufficiently bleached. In an article using such recycled pulp fibers, there may be a portion with inferior whiteness, and in some cases, the user may judge the portion with inferior whiteness as remaining stains, which may give the user a psychological resistance.

[0015] The above manufacturing method includes a predetermined recycled pulp fiber forming step. In the recycled pulp fiber forming step, while lowering the superabsorbent polymer and the pulp fibers, the ozone-containing gas is raised to bring the superabsorbent polymer and the pulp fibers into contact with the ozone-containing gas. Among free superabsorbent polymers, free pulp fibers, and the connected structure, the free superabsorbent polymer with a relatively high specific gravity and the connected structure containing the superabsorbent polymer tend to have a higher sedimentation property than the free pulp fibers with a relatively low specific gravity. On the other hand, since the ozone-containing gas consumes ozone and rises while treating the superabsorbent polymer and the pulp fibers, the ozone-containing gas existing at a lower position has a higher ozone content (i.e., is fresher) than the ozone-containing gas existing at an upper position.

[0016] In addition, in this specification, the descending speed relates to the downward movement speed of the treatment liquid 52 in the treatment tank 31, and generally, it is uniquely determined by the first flow rate, the second flow rate, the size of the treatment tank, etc. On the other hand, in this specification, sedimentation property means the property representing the ease of falling vertically due to gravity of pulp fibers, superabsorbent polymers, and the connecting structure contained in the treatment liquid 52 in the treatment tank 31. Depending on the specific gravity, etc., each of the pulp fibers, superabsorbent polymers, and the connecting structure has different sedimentation properties.

[0017] Therefore, in the above manufacturing method, free superabsorbent polymers and superabsorbent polymers in the connecting structure with relatively high sedimentation properties can be oxidatively decomposed with fresher ozone-containing gas, and the pulp fibers that constituted the connecting structure can be released. At the same time, free pulp fibers with relatively low sedimentation properties, which take relatively more time to reach the treatment liquid discharge port, can be treated by the ozone-containing gas over time.

[0018] Also, generally, the higher the lignin content rate of pulp fibers, the higher the tendency of the specific gravity. Therefore, in the above manufacturing method, pulp fibers with a relatively high lignin content rate have relatively higher sedimentation properties than pulp fibers with a relatively low lignin content rate. Thus, fresher ozone-containing gas can contact the pulp fibers with a relatively high lignin content rate and decompose and bleach the lignin contained therein.

[0019] Therefore, in the above manufacturing method, even when a superabsorbent polymer and pulp fibers form a connecting structure, ozone in the ozone-containing gas can remove the superabsorbent polymer that constitutes the connecting structure, and ozone in the ozone-containing gas can act on the pulp fibers that constitute the connecting structure to bleach the pulp fibers. As a result, it is difficult for a difference to occur between the brightness of the pulp fibers that did not constitute the connecting structure and the brightness of the pulp fibers that constituted the connecting structure, and the distribution of the brightness of the recycled pulp fibers contained in the treatment liquid becomes narrow (it becomes difficult for unevenness to occur). In the article in which the above recycled pulp fibers are reused, the distribution of the brightness becomes narrow (it becomes difficult for unevenness to occur).

[0020] Furthermore, the recycled pulp fibers produced by the above manufacturing method have an absolute value of a predetermined whiteness in addition to a narrow distribution of the degree of bleaching. Therefore, the recycled pulp fibers produced by the above manufacturing method, as well as the articles using the recycled pulp fibers, are likely to have a uniform whiteness, and it is difficult for the user to have a psychological resistance to the articles using the recycled pulp fibers.

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

[0022] When recycling sanitary products, the oil content of the hot melt adhesive contained in the sanitary products is likely to be absorbed by the pulp fibers. Also, 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 oxidatively decompose hydrophobic components such as the oil content and lignin of pulp fibers that do not constitute the connecting structure, ozone in the ozone-containing gas hardly contacts the pulp fibers that constitute the connecting structure. Therefore, the hydrophobic components of the pulp fibers may not be sufficiently oxidatively decomposed. Articles using such recycled pulp fibers may generally have inferior hydrophilicity or may have partially inferior hydrophilicity, and there is a risk of giving a psychological resistance to the user.

[0023] Since the above manufacturing method includes a predetermined recycled pulp fiber forming step, a free superabsorbent polymer and a connecting structure having relatively high sedimentation can be accurately oxidatively decomposed with fresher ozone-containing gas to form free pulp fibers, and the hydrophobic components such as the oil content and lignin contained in the free pulp fibers having relatively low sedimentation can be processed by the ozone-containing gas over time. In addition, the recycled pulp fibers are likely to have a predetermined water contact angle, and the recycled pulp fibers are likely to have uniform hydrophilicity. Therefore, the articles using the recycled pulp fibers are likely to have uniform hydrophilicity, and it is difficult for the user to have a psychological resistance to the articles using the recycled pulp fibers.

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

[0025] In the above production method, even when the superabsorbent polymer and the pulp fiber form a linked structure, ozone in the ozone-containing gas can remove the superabsorbent polymer constituting the linked structure, and ozone in the ozone-containing gas can act on the pulp fiber constituting the linked structure to reduce the lignin content of the pulp fiber. As a result, a difference is less likely to occur between the lignin content of the pulp fiber that did not constitute the linked structure and the lignin content of the pulp fiber that constituted the linked structure, and the distribution of the lignin content of the recycled pulp fiber contained in the treatment liquid becomes narrow (the variation decreases). In addition, since the recycled pulp fiber produced by the above production method has a predetermined lignin content, the recycled pulp fiber is likely to be excellent in whiteness and hydrophilicity. As a result, an article using the recycled pulp fiber has uniform whiteness and hydrophilicity, and it is less likely for a user to have a psychological resistance to the article using the recycled pulp fiber.

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

[0027] In the above production method, since the recycled pulp fiber has a predetermined beating degree reduction rate, when the recycled pulp fiber is reused, the recycled pulp fiber is likely to have linting and the surface area increases, so the diffuse reflection of light increases and the recycled pulp fiber looks whiter. Therefore, an article using the recycled pulp fiber is likely to have uniform whiteness, and it is less likely for a user to have a psychological resistance to the article using the recycled pulp fiber.

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

[0029] In the above manufacturing method, since the pulp fibers do not contain a predetermined colorant, the recycled pulp fibers produced by the above manufacturing method are likely to have uniform whiteness, and the articles using the recycled pulp fibers are likely to have uniform whiteness, and it is less likely for the user to have a psychological resistance to the articles using the recycled pulp fibers.

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

[0031] In the above manufacturing method, since it further comprises a colored pulp fiber removal step of removing pulp fibers containing the colorant from the plurality of types of sanitary products before the supply step, in the recycled pulp fiber formation step, the pulp fibers are less likely to contain a colorant, the recycled pulp fibers are likely to have uniform whiteness, the articles using the recycled pulp fibers are likely to have uniform whiteness, and it is less likely for the user to have a psychological resistance to the articles using the recycled pulp fibers.

[0032] [Aspect 7] At least a part of the plurality of types of sanitary products includes a liquid-permeable sheet, a liquid-impermeable sheet, and an absorption core disposed therebetween, the absorption core containing the superabsorbent polymer and pulp fibers not containing the colorant. The colored pulp fiber removal step includes the following steps: A pretreatment step of swelling the used plurality of types of sanitary products with water; A decomposition step of applying a physical impact to the swollen used plurality of types of sanitary products to decompose the used plurality of types of sanitary products into the absorption core and the rest. a separation step of separating the absorption core, The method according to aspect 6, comprising:

[0033] In the above manufacturing method, since it includes a predetermined pretreatment step, a decomposition step, and a separation step, even if a component other than the absorption core of the sanitary product contains pulp fibers containing a colorant, the pulp fibers containing the colorant are less likely to be mixed into the mixed liquid containing the superabsorbent polymer, the pulp fibers, and water. As a result, the recycled pulp fibers are likely to have a uniform whiteness, the article using the recycled pulp fibers is likely to have a uniform whiteness, and the user is less likely to have a psychological resistance to the article using the recycled pulp fibers.

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

[0035] In the above manufacturing method, in the step of forming the recycled pulp fibers, since the ozone-containing gas is supplied from the ozone-containing gas supply port as microbubbles or nanobubbles, even when the superabsorbent polymer and the pulp fibers form a connected structure, the microbubbles or nanobubbles give buoyancy to the superabsorbent polymer, the connected structure, and the pulp fibers, and their sedimentation properties are reduced. Therefore, the time for the superabsorbent polymer, the connected structure, and the pulp fibers to reach the treatment liquid discharge port becomes longer, and ozone can oxidatively decompose the free superabsorbent polymer and the superabsorbent polymer constituting the connected structure, and sufficiently treat the free pulp fibers and the pulp fibers constituting the connected structure. Therefore, the recycled pulp fibers produced by the above manufacturing method, as well as the articles using the recycled pulp fibers, are likely to have a uniform whiteness and hydrophilicity, and the user is less likely to have a psychological resistance to the articles using the recycled pulp fibers.

[0036] [Aspect 9] In the above-described 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 above-described treatment liquid discharge step, the treatment liquid is continuously discharged from the treatment liquid discharge port at a second flow rate. The method according to any one of Aspects 1 to 8.

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

[0038] [Aspect 10] The method according to any one of Aspects 1 to 9, wherein the treatment liquid is acidic. In the above manufacturing method, the treatment liquid is acidic (for example, pH 2.5 or less). Therefore, when the superabsorbent polymer to be treated can be inactivated by an acid or when the superabsorbent polymer to be treated is already inactivated, the superabsorbent polymer can continue to be kept in an inactivated state. Thereby, even when the superabsorbent polymer and the pulp fiber form a linked structure, ozone in the ozone-containing gas can remove the superabsorbent polymer constituting the linked structure, and ozone in the ozone-containing gas acts on the pulp fiber constituting the linked structure, making it easier to bleach the pulp fiber.

[0039] [Aspect 11] The method according to any one of Aspects 1 to 10, further including an inactivation step of inactivating the superabsorbent polymer with an acid before the above-described mixed liquid supply step.

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

[0041] [Aspect 12] The method according to aspect 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 an acid capable of forming a complex with metal ions contained in excrement, the recycled pulp fibers produced by the above manufacturing method, as well as articles using the recycled pulp fibers, are less likely to contain metal ions and are excellent in whiteness and hydrophilicity. As a result, it becomes less likely for users to have a psychological resistance to articles using recycled pulp fibers.

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

[0044] In the above manufacturing method, since the recycled pulp fibers produced by the above manufacturing method contain an ash content of 0.65% by mass or less, the recycled pulp fibers, as well as articles using the recycled pulp fibers, are excellent in whiteness and hydrophilicity. As a result, it becomes less likely for users to have a psychological resistance to articles using recycled pulp fibers.

[0045] [Aspect 14] Recycled pulp fibers derived from used sanitary products containing pulp fibers and a superabsorbent polymer, wherein the recycled pulp fibers have a ΔYI of 0 to 10 with respect to a standard white board, The recycled pulp fibers, characterized by the above.

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

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

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

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

[0050] Since the recycled pulp fibers have a predetermined lignin content, they tend to be excellent in whiteness and hydrophilicity. As a result, articles made from the recycled pulp fibers have uniform whiteness and hydrophilicity, and users are less likely to have a psychological resistance to articles made from the recycled pulp fibers.

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

[0052] Since the recycled pulp fibers have a predetermined beating degree reduction rate, when the recycled pulp fibers are reused, the recycled pulp fibers are likely to have linting and an increased surface area, so that the diffuse reflection of light increases and the recycled pulp fibers appear whiter. Therefore, articles made from the recycled pulp fibers tend to have uniform whiteness, and users are less likely to have a psychological resistance to articles made from the recycled pulp fibers.

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

[0054] Since the recycled pulp fiber has a predetermined ash content, the recycled pulp fiber and articles using the recycled pulp fiber are excellent in whiteness and hydrophilicity. As a result, it becomes less likely for the user to have a psychological resistance to articles using the recycled pulp fiber.

[0055] Hereinafter, a method for producing recycled pulp fiber from the pulp fiber of used sanitary products (hereinafter, may be simply referred to as "method for producing recycled pulp fiber") will be described. Note that the used sanitary product is a sanitary product used by a user, including a sanitary product in a state of absorbing the liquid excrement of the user, and includes those that have been used but not absorbed excrement, unused ones, etc.

[0056] First, a configuration example of the sanitary product will be described. The sanitary product includes a surface sheet, a back sheet, and an absorber disposed between the surface sheet and the back sheet. Examples of the sanitary product include disposable diapers, urine pads, sanitary napkins, bed sheets, and pet sheets.

[0057] Examples of the constituent members of the surface sheet include non-woven fabrics or films, specifically, liquid-permeable non-woven fabrics, synthetic resin films having liquid-permeable holes, composite sheets thereof, etc. Examples of the constituent members of the back sheet include non-woven fabrics or films, specifically, liquid-impermeable non-woven fabrics, liquid-impermeable synthetic resin films, composite sheets of these non-woven fabrics and synthetic resin films.

[0058] Examples of the constituent members of the absorber include an absorption core (e.g., pulp fibers and superabsorbent polymer) and a core wrap. There are no particular restrictions on the pulp fibers as long as they can be used in sanitary products. Examples of the pulp fibers include cellulose-based fibers. Examples of the cellulose-based fibers include wood pulp, crosslinked pulp, non-wood pulp, regenerated cellulose, semi-synthetic cellulose, and the like. There are no particular restrictions on the superabsorbent polymer (Super Absorbent Polymer: SAP) as long as it can be used in sanitary products. Examples of the superabsorbent polymer include polyacrylate-based, polysulfonate-based, and maleate anhydride-based polymers.

[0059] One surface and the other surface of the absorber are joined to the top sheet and the back sheet, respectively, via an adhesive. In plan view, the portion of the top sheet that extends outside the absorber so as to surround the absorber (peripheral portion) is joined to the portion of the back sheet that extends outside the absorber so as to surround the absorber (peripheral portion) via an adhesive. Therefore, the absorber is wrapped inside the joined body of the top sheet and the back sheet. There are no particular restrictions on the adhesive as long as it can be used in sanitary products and its bonding strength decreases when softened by warm water as described later. Examples of the adhesive include hot melt adhesives. Examples of the hot melt adhesives include pressure-sensitive adhesives or heat-sensitive adhesives mainly composed of rubber such as styrene-ethylene-butadiene-styrene, styrene-butadiene-styrene, styrene-isoprene-styrene, or mainly composed of olefins such as polyethylene.

[0060] Figure 1 is a flowchart showing a material separation method for separating a used sanitary product into its constituent materials. This material separation method is a method for separating a used sanitary product into a film, a non-woven fabric, pulp fibers, and a superabsorbent polymer. This material separation method includes a pretreatment step S11, a decomposition step S12, and a separation step S13. The pretreatment step S11 swells the used sanitary product with water. The decomposition step S12 applies a physical impact to the swollen used sanitary product to decompose the used sanitary product into a film, a non-woven fabric, a core wrap, etc., and an absorbent core (for example, pulp fibers and a superabsorbent polymer). The separation step S13 separates the film, the non-woven fabric, the pulp fibers, and the superabsorbent polymer.

[0061] The method for manufacturing recycled pulp fibers according to the present disclosure is included in the separation step S13 of this material separation method. In addition, when a mixture of pulp fibers and a superabsorbent polymer is obtained in advance by some method, the steps before the method for manufacturing recycled pulp fibers in the pretreatment step S11, the decomposition step S12, and the separation step S13 are not necessary to be carried out. Hereinafter, each step will be described.

[0062] In the pretreatment step S11, a plurality of used sanitary products are left in the state when they are collected from the outside, that is, without being broken, cut, etc., and if they are in a rolled state or a folded state, they are left in that state, and the superabsorbent polymer of the absorbent body is not inactivated, and water is absorbed to swell. In the present embodiment, the used sanitary product is made to absorb warm water to swell, or the water absorbed and expanded is heated to warm water after absorbing water. Warm water refers to water having a temperature higher than room temperature (20°C ± 15°C (5 to 35°C): JIS Z 8703).

[0063] Generally, the amount of liquid excrement actually absorbed by a used sanitary product 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, by immersing the used sanitary product in warm water, the used sanitary product absorbs water up to an amount close to the maximum absorption capacity (for example, 80% by mass or more of the maximum absorption capacity). Alternatively, the used sanitary product is immersed in water at room temperature, and after the used sanitary product absorbs water up to an amount close to the maximum absorption capacity, the entire used sanitary product is heated to the temperature of warm water. Thereby, the used sanitary product can be in a very expanded state with warm water or water at room temperature (hereinafter, also simply referred to as "warm water"). As a result, a very high internal pressure will be generated in the used sanitary product. Note that the purpose of using warm water is mainly to weaken the adhesive force of the adhesive as described later.

[0064] Here, when the used sanitary product is initially in a state of being rolled up (hiding the surface sheet inside) with the back sheet on the outside or in a folded state, by being immersed in warm water, the absorber of the used sanitary product absorbs the warm water in the warm water and expands. As a result, the internal pressure of the used sanitary product increases, and a force acts on the used sanitary product to open outward, causing the used sanitary product in the rolled-up state or folded state to open outward and become generally flat. That is, the used sanitary product can be in a flatly unfolded state in warm water. At this time, since the absorber of the used sanitary product absorbs a large amount of warm water and expands very much, any part of its surface, that is, either the surface sheet or the back sheet that wraps the absorber, is in a state where it is likely to tear easily. That is, by the pretreatment step S11, the used sanitary product can be in a state where either surface is likely to tear. If the used sanitary product is initially in a flatly unfolded state, any part of its surface will be in a state where it is likely to tear easily. This state cannot occur when the used sanitary product is broken or the like.

[0065] Furthermore, by immersing the used sanitary product in warm water and / or having it absorb warm water, the adhesive (e.g., hot melt adhesive) used for joining between the respective components can be softened by the heat of the warm water, and the bonding strength of the adhesive can be reduced. For example, the adhesive joining the peripheral portion of the surface sheet and the peripheral portion of the back sheet can be softened by the heat of the warm water, and the bonding strength of the adhesive can be reduced. Furthermore, the adhesive joining the surface sheet and the absorber and the adhesive joining the back sheet and the absorber can be softened by the heat of the warm water, and the bonding strengths of these adhesives can be reduced.

[0066] In this way, in the pretreatment step S11, due to the expansion of the absorber of the used sanitary product, it is possible to create a state where any part of the surface of the used sanitary product is about to tear and a state where the bonding strength of the adhesive is reduced. By the used sanitary product being in such a state, in the decomposition step described later, the used sanitary product can be surely decomposed.

[0067] The temperature of the warm water in the pretreatment step S11 is not particularly limited as long as the adhesive of the used sanitary product can be softened. For example, 60°C or higher can be mentioned, preferably 70°C or higher and 98°C or lower. By setting the temperature of the warm water to 70°C or higher, the adhesive joining the components can be softened more by the heat of the warm water, and the bonding strength of the adhesive can be reduced more. By setting the temperature of the warm water to 98°C or lower, since the warm water surely exists as a liquid, the used sanitary product can absorb the warm water more surely. Due to the expansion of the absorber and the heat of the warm water, it is possible to more surely create a state where the surface of the used sanitary product is about to tear and a state where the bonding strength of the adhesive is reduced. Regarding the measurement of the temperature, measure the temperature of the warm water in the state where the used sanitary product is immersed, or measure the temperature within 5 mm from the surface of the used sanitary product that has absorbed water up to an amount close to the maximum absorption amount (insert the tip of the temperature sensor).

[0068] Also, in the reuse of the used sanitary product, sterilization of the constituent materials is extremely important. By setting the temperature of the warm water to 70°C or higher, it is also possible to achieve the effect of sterilizing (disinfecting) the used sanitary product, which is preferable.

[0069] The processing time in the pretreatment step S11, that is, the time for immersing the used sanitary product in warm water, is not particularly limited as long as the absorber of the used sanitary product can expand. For example, it is 2 to 60 minutes, preferably 4 to 30 minutes. If the time is too short, the absorber cannot expand sufficiently. If it is too long, the time is wasted and the processing cost increases unnecessarily.

[0070] In addition, the amount of warm water absorbed by the absorber in the pretreatment step S11 is not particularly limited as long as the absorber can expand to such an extent that the used sanitary product can be decomposed in the decomposition step described later. For example, it is 80% by mass or more of the maximum absorption amount of the used sanitary product, preferably 90% by mass or more. Thereby, the used sanitary product can be made to be in a state of being fully expanded with water. As a result, an extremely high internal pressure can be generated in the absorber of the used sanitary product.

[0071] However, the maximum absorption amount is measured by the following procedure. (1) The unused sanitary product is dried in an atmosphere of 100 °C or higher, and the mass of the sanitary product is measured. (2) When a stretchable material (for example, stretchable members around the legs, around the waist, etc.) that can form a pocket where it is difficult for water to reach the absorber is arranged on the sanitary product, the sanitary product is flattened by making a cut in the stretchable member. (3) The sanitary product is immersed in a water bath filled with sufficient tap water with the surface sheet facing down and left for 30 minutes. (4) After leaving it standing, the sanitary product is placed on a net with the surface sheet facing down, drained for 20 minutes, and then the mass of the sanitary product is measured. Then, the mass difference before and after immersion in tap water is defined as the maximum absorption amount.

[0072] Next, in the decomposition step S12, a physical impact is applied to the plurality of used sanitary products developed and swollen in the pretreatment step S11 to decompose the plurality of used sanitary products into a film (back sheet), a non-woven fabric (surface sheet), and a core wrap, and an absorption core (for example, an absorber and a superabsorbent polymer).

[0073] The used sanitary product is flattened by the pretreatment step S11 and is in a state where any part of the surface is about to tear due to swelling. In this embodiment, in particular, due to the heat of warm water, the bonding strength of the adhesive has been reduced. Therefore, in the decomposition step S12, by applying a physical impact to the used sanitary product in that state, among any parts of the surface, in particular, the joint part between the surface sheet (non-woven fabric) and the back sheet (film) where the bonding strength has been reduced will tear. Thereby, the joint part can be torn (peeled off). The physical impact is not particularly limited, but for example, there are methods such as hitting the used sanitary product against a surface made of a material harder than the used sanitary product, and a method of pressing from both sides while sandwiching the used sanitary product between a pair of rolls arranged facing each other and passing it through.

[0074] In this embodiment, the decomposition step S12 includes a step of putting a plurality of swollen used sanitary products into the bottom of a rotary drum with a horizontal rotation axis, and a step of rotating the rotary drum around the rotation axis to lift the plurality of used sanitary products to the upper part of the rotary drum and then hitting them against the bottom. Thereby, a physical impact can be stably, continuously (continuously) and easily applied to the plurality of used sanitary products. Examples of the rotary drum include the rotary drum of the washing tub of a horizontal washing machine. Therefore, the decomposition 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 rotary drum is not particularly limited as long as the above impact can be realized, but the inner diameter and the depth are, for example, 50 to 150 cm and 30 to 120 cm. The rotation speed of the rotary drum is not particularly limited as long as the above impact can be realized, but for example, 30 revolutions per minute to 100 revolutions per minute can be mentioned.

[0075] In addition, although the temperature of the used sanitary product is kept relatively high by the warm water absorbed in the used sanitary product, from the viewpoints of suppressing the temperature drop of the adhesive and maintaining the sterilization effect, the temperature of the atmosphere in the rotary drum is preferably 70 °C or higher, more preferably 75 °C or higher. From the viewpoint of handling the used sanitary product, the temperature in the rotary drum is preferably 98 °C or lower, more preferably 90 °C or lower. The water in the rotary drum is preferably as little as possible, and is preferably at least as little as the used sanitary product does not sink below the water surface at the bottom. If the used sanitary product sinks below the water surface, the impact on the used sanitary product is absorbed by the water, making it difficult to apply the desired impact to the used sanitary product. The time for rotating the rotary drum is not particularly limited as long as the surface sheet, the back sheet, the core wrap, etc. and the absorbent core can be disassembled. For example, it is 2 to 40 minutes, preferably 4 to 20 minutes.

[0076] Due to physical impact, the joint between the surface sheet (non-woven fabric) and the back sheet (film) of the used sanitary product tears and splits. At the same time, through the tear, due to the internal pressure of the absorbent body, the absorbent core (for example, pulp fibers and superabsorbent polymers) inside the used sanitary product sprays out (pops out). Thereby, the used sanitary product can be more reliably disassembled into the surface sheet (non-woven fabric), the back sheet (film), the core wrap, etc. and the absorbent core (for example, pulp fibers and superabsorbent polymers).

[0077] Next, the separation step S13 separates the plurality of films (back sheets) and the plurality of non-woven fabrics (surface sheets), the core wrap, etc. from the absorbent core (for example, pulp fibers and superabsorbent polymers). However, the non-woven fabric may remain joined to the film. The separation method is not particularly limited, and examples thereof include a method using a sieve that passes through the absorbent core without passing through the surface sheet, the back sheet, the core wrap, etc.

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

[0079] In the inactivation step S31, before the first separation step S32, the surface sheet (nonwoven fabric), the back sheet (film), and the absorber (pulp fibers and superabsorbent polymer) are immersed in an aqueous solution containing an inactivator capable of inactivating the superabsorbent polymer. Thereby, the superabsorbent polymer adhering to the surface sheet, the back sheet, and the pulp fibers can be inactivated. Thereby, the superabsorbent polymer in a highly viscous state before inactivation can be made into a superabsorbent polymer in a low-viscosity state by dehydration due to inactivation.

[0080] Here, the inactivator is not particularly limited, and examples thereof include acids (for example, inorganic acids and organic acids), lime, calcium chloride, magnesium sulfate, magnesium chloride, aluminum sulfate, aluminum chloride, and the like. The above acids are preferable because they do not leave ash on the pulp fibers. When an acid is used as the inactivator, the pH is preferably 2.5 or less, and more preferably 1.3 to 2.4. If the pH is too high, the water absorption ability of the superabsorbent polymer cannot be sufficiently reduced. There is also a possibility that the sterilization ability may decrease. If the pH is too low, there is a risk of corrosion of the equipment, and a large amount of alkaline chemicals are required for the neutralization treatment during wastewater treatment.

[0081] Examples of the inorganic acid include sulfuric acid, hydrochloric acid, and nitric acid. Sulfuric acid is preferred from the viewpoints of not containing chlorine and cost. On the other hand, examples of the organic acid include citric acid, tartaric acid, glycolic acid, malic acid, succinic acid, acetic acid, ascorbic acid, etc. Hydroxycarbonate-based organic acids such as citric acid, tartaric acid, and gluconic acid, which are acids capable of forming a complex with metal ions contained in excreta, are particularly preferred. The metal ions contained in excreta include calcium ions. This is because the metal ions in excreta can be trapped and removed due to the chelating effect of the acid capable of forming a complex with the metal ions contained in excreta. In addition, due to its cleaning effect, citric acid can be expected to have a high dirt component removal effect. Since the pH varies with the water temperature, the pH in the present disclosure refers to the pH measured at an aqueous solution temperature of 20°C.

[0082] The treatment temperature of the inactivation step S31, that is, the temperature of the aqueous solution containing the inactivator, is not particularly limited as long as the inactivation reaction proceeds. The treatment temperature may be room temperature or higher than room temperature, and examples thereof include 15 to 30°C. Also, the treatment time of the inactivation step S31, that is, the time for immersing the surface sheet, the back sheet, and the absorber in the aqueous solution containing the inactivator, is not particularly limited as long as the superabsorbent polymer is inactivated and dehydrated, and examples thereof include 2 to 60 minutes, preferably 5 to 30 minutes. Further, the amount of the aqueous solution in the inactivation step S31, that is, the amount of the aqueous solution containing the inactivator, is not particularly limited as long as the inactivation reaction proceeds. The amount of the aqueous solution is, for example, preferably 300 to 3000 parts by mass, more preferably 500 to 2500 parts by mass, and still more preferably 1000 to 2000 parts by mass with respect to 100 parts by mass of the used sanitary product.

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

[0084] In the first separation step S32, the method for separating the top sheet and the back sheet from the pulp fibers, the superabsorbent polymer, and the wastewater is not particularly limited. For example, the product (top sheet, back sheet, pulp fibers, superabsorbent polymer, wastewater, etc.) generated in the inactivation step is discharged while passing through a screen with a mesh size of 5 to 100 mm, preferably 10 to 60 mm. Thereby, the pulp fibers, the superabsorbent polymer, and the wastewater can be separated into the drainage, and the top sheet and the back sheet remain on the screen, so that these products can be separated. In addition, other large-shaped objects such as non-woven fabrics and films may remain on the screen. In particular, before inactivation, since the superabsorbent polymer is in a highly viscous state, it cannot be said that it is easy to separate the superabsorbent polymer attached to the top sheet, the back sheet, and the pulp fibers. However, after inactivation, due to dehydration, the superabsorbent polymer becomes in a low-viscosity state, so that the superabsorbent polymer attached to the top sheet, the back sheet, and the pulp fibers can be easily separated from the top sheet, the back sheet, and the pulp fibers. Therefore, the constituent members of the sanitary product can be efficiently separated and recovered.

[0085] Incidentally, the manufacturing method of the present disclosure targets multiple types of used sanitary products. When at least a part of the multiple types of sanitary products contains pulp fibers containing a colorant selected from the group consisting of dyes, pigments, and combinations thereof, for example, when the core wrap is composed of pulp fibers containing the above colorant, in the separation step S13, it is preferable to remove the pulp fibers containing the colorant (for example, the core wrap). This is because the recycled pulp fibers are likely to have a uniform whiteness, the articles using the recycled pulp fibers are likely to have a uniform whiteness, and it becomes less likely for users to have a psychological resistance to the articles using the recycled pulp fibers.

[0086] In the present embodiment, the separation step S13 may further include a second separation step S33 of removing the adhesive at the joint portion between the film and the other member by a solvent that dissolves the adhesive at the joint portion. In the present embodiment, the adhesives at the joint portions between the film, the nonwoven fabric, and the absorber are removed by a solvent that dissolves the adhesives.

[0087] In the second separation step S33, the adhesive at the joint portion between the film (back sheet) and the other member (the nonwoven fabric of the front sheet, the front sheet, the absorber remaining on the surface of the back sheet, etc.) is removed by a solvent. Thereby, the film and the other member can be separated from each other while maintaining their original shapes without being broken or the like. Therefore, the component members such as the film of the sanitary product can be efficiently recovered. Further, since the film and the other member can be separated without leaving the adhesive on the film, the film can be reused as a highly pure resin. Thereby, it is possible to suppress the adverse effect 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. For example, terpenes containing at least one of terpene hydrocarbons, terpene aldehydes, and terpene ketones can be mentioned. In this step, an aqueous solution containing terpene is used, and the concentration of terpene in the aqueous solution can be, 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, there is a possibility that the adhesive at the joint portion cannot be dissolved. If the concentration of terpene is too high, the cost may increase. In addition, terpene not only dissolves adhesives such as hot melt adhesives but also has an oil stain cleaning effect. Therefore, for example, when there is printing on a component of a sanitary product such as a back sheet, terpene can also decompose and remove the printing ink.

[0089] Examples of terpene hydrocarbons include myrcene, limonene, pinene, camphor, sabinene, farnesene, paracymene, ocimene, terpinene, caryophyllene, bisabolene, cedrene. Among them, limonene, pinene, terpinene, and caryophyllene are preferred. Examples of terpene aldehydes include citronellal, citral, cyclocitral, safranal, farnal, perillaldehyde, geranial, neral. Examples of terpene ketones include borneol and thujone. Among terpenes, terpene hydrocarbons are preferred, and limonene is particularly preferred. Limonene has three types: d-limonene, l-limonene, and dipentene (dl-limonene), and any of them can be preferably used. Terpenes can be used alone or in combination of two or more.

[0090] The processing temperature of the second separation step S33, that is, the temperature of the aqueous solution containing the solvent, is not particularly limited as long as the dissolution of the adhesive proceeds and the used sanitary product is decomposed into constituent members. The processing temperature may be room temperature or higher than room temperature, and for example, 15 to 30 °C can be mentioned. Also, the processing time of the second separation step S33, that is, the time for immersing the surface sheet, the back sheet, and the absorber in the aqueous solution containing the solvent, is not particularly limited as long as the dissolution of the adhesive proceeds and the used sanitary product is decomposed into constituent members. The processing time is, for example, 2 to 60 minutes, preferably 5 to 30 minutes. The amount of the aqueous solution in the second separation step S33, that is, the amount of the aqueous solution containing the solvent, is not particularly limited as long as the dissolution of the adhesive proceeds and the used sanitary product is decomposed into constituent members. The amount of the aqueous solution is, for example, preferably 300 to 3000 parts by mass, more preferably 500 to 2500 parts by mass, based on 100 parts by mass of the used sanitary product. By the second separation step S33, the amount of the adhesive remaining on the film, nonwoven fabric, absorber, etc. can be reduced to 1% by mass or less with respect to the film, nonwoven fabric, absorber, etc.

[0091] In addition, in this embodiment, as another preferred aspect, in the inactivation step S31, the second separation step S33 may be carried out together. That is, while inactivating the superabsorbent polymer attached to the surface sheet, the back sheet, and the pulp fibers, the adhesive attached to the surface sheet, the back sheet, and the pulp fibers may be dissolved. In this case, as the aqueous solution for immersing the surface sheet, the back sheet, the pulp fibers, and the superabsorbent polymer, an aqueous solution containing both an inactivator and a solvent is used. Thereby, in the inactivation step S31, the back sheet (film), the surface sheet (nonwoven fabric), and the absorber (pulp fibers and superabsorbent polymer) can be brought into a substantially separated state in the aqueous solution. Then, in the subsequent first separation step, the back sheet (film) and the surface sheet (nonwoven fabric) can be separated from the absorber (pulp fibers and superabsorbent polymer), and the second separation step S33 can be omitted. In this case, the back sheet (film) and the surface sheet (nonwoven fabric) are substantially separated by removing the adhesive.

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

[0093] In the reuse of used sanitary products, sterilization is extremely important. In the first drying step S34, a step of drying the separated film (back sheet) and nonwoven fabric (front sheet) in a high-temperature atmosphere or with hot air or the like is performed. The drying temperature is, for example, 105 to 210 ° C, preferably 110 to 190 ° C. The drying time depends on the drying temperature, but is, for example, 10 to 120 minutes, preferably 15 to 100 minutes. Thereby, not only the solvent remaining on the surfaces of the film and the nonwoven fabric is evaporated and removed, but also the film and the nonwoven fabric can be sterilized in a high-temperature atmosphere or with hot air or the like. Thereby, it is also possible to achieve the effect of sterilization (disinfection) while removing the solvent.

[0094] On the other hand, in this embodiment, the separation step S13 may include a third separation step S35 of separating pulp fibers from the separated mixture. In the third separation step S35, the method of separating pulp fibers from the separated mixture (including pulp fibers, superabsorbent polymers, and sewage) is not particularly limited. For example, the separated mixture is discharged while passing through a screen with an opening size of 0.1 to 4 mm, preferably 0.15 to 2 mm. Thereby, the superabsorbent polymer and the sewage are in the drainage, and the pulp fibers (mainly with superabsorbent polymers remaining on the surface) remain on the screen, so that the pulp fibers can be separated from the mixture. Although this pulp fiber contains a lot of impurities, it can be reused in this state depending on the application. The separated pulp fibers have a superabsorbent polymer attached thereto. The separated pulp fibers and the superabsorbent polymer attached to the pulp fibers are mixed with water at a predetermined ratio to form a mixed solution and proceed to the ozone treatment step S36.

[0095] In the present embodiment, the separation step S13 includes an ozone treatment step S36 of treating a mixed solution containing a superabsorbent polymer, pulp fibers, their connected structure, and water with an aqueous solution containing ozone, reducing the molecular weight of the superabsorbent polymer adhering to the pulp fibers, solubilizing it, and removing it.

[0096] In a used sanitary product, in an absorber or the like containing pulp fibers and a superabsorbent polymer, (i) as the superabsorbent polymer absorbs a liquid such as body fluid, it swells and entraps the pulp fibers, and (ii) the swollen superabsorbent polymers cause gel blocking while entrapping the pulp fibers, etc., so that a connected structure is often formed by a plurality of superabsorbent polymers and a plurality of pulp fibers. The above mixed solution contains, in addition to free pulp fibers and free superabsorbent polymers, a connected structure composed of a plurality of superabsorbent polymers and a plurality of pulp fibers.

[0097] In the ozone treatment step S36, the superabsorbent polymer contained in the mixed solution (treatment solution) is removed by oxidative decomposition with ozone in the aqueous solution and solubilization in the aqueous solution. The state where the superabsorbent polymer is oxidatively decomposed and solubilized in the aqueous solution means that the superabsorbent polymer and the connected structure pass through a 2-mm screen. Thereby, impurities such as superabsorbent polymers can be removed from the mixed solution (treatment solution), and high-purity pulp fibers can be produced. Further, ozone treatment can perform secondary sterilization, bleaching, and deodorization of the pulp fibers.

[0098] Figure 2 is a schematic diagram showing an example of the configuration of an apparatus 2 for performing the ozone treatment step S36. The apparatus 2 includes a mixed solution storage unit 3 for storing a mixed solution 51 containing water, pulp fibers separated in the third separation step S35, and a superabsorbent polymer, and an ozone treatment unit 4 for oxidatively decomposing the superabsorbent polymer contained in the mixed solution 51 and removing it from the pulp fibers.

[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 through the pipe 61. The stirrer 13 stirs the mixed liquid 51 in the mixed liquid tank 12 so that the pulp fibers and the superabsorbent polymer in the mixed liquid 51 do not separate from the water and sink below the mixed liquid 51.

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

[0101] Specifically, the supply pump 21 continuously supplies the mixed liquid 51 in the mixed liquid tank 12 into the treatment tank 31 through the pipe 62 from the mixed liquid supply port 32 at a first flow rate. The ozone supply device 41 supplies the ozone-containing gas 53 to the treatment tank 31. Examples of the ozone generator 42 of the ozone supply device 41 include the ozone water exposure tester ED-OWX-2 manufactured by Eco Design Co., Ltd., and the ozone generator OS-25V manufactured by Mitsubishi Electric Corporation. The ozone-containing gas 53 is another type of gas containing ozone, for example, oxygen gas containing ozone. The ozone-containing gas supply port 43 sends the ozone-containing gas 53 supplied to the treatment tank 31 through the pipe 65 into the treatment tank 31 and is arranged at the lower part (preferably the bottom) of the treatment tank 31. The ozone-containing gas supply port 43 continuously supplies the ozone-containing gas 53 as a plurality of fine bubbles from the lower part to the upper part of the treatment liquid 52 into the treatment liquid 52. The delivery pump 22 continuously discharges the treatment liquid 52 in the treatment tank 31 from the treatment liquid discharge port 33 to the outside of the treatment tank 31 at a second flow rate through the pipe 63. The ozone decomposition device 34 receives the ozone-containing gas 53 accumulated in the upper part of the treatment tank 31 via the pipe 64, detoxifies the ozone, and discharges it to the outside. Note that the treatment liquid 52 in the treatment tank 31 is only the treatment liquid 52 before the start of the ozone treatment step S36, and after the start, it becomes a liquid in which the treatment liquid 52 and the mixed liquid 51 are mixed. In the present embodiment, including the liquid in which the treatment liquid 52 and the mixed liquid 51 are mixed, the liquid in the treatment tank 31 is referred to as the treatment liquid 52.

[0102] Next, a specific method of the ozone treatment step S36 will be described. The pulp fibers and the superabsorbent polymer separated in the third separation step S35 are mixed with water so as to have a preset concentration to become the mixed liquid 51. The concentration of the pulp fibers in the mixed liquid 51 is set so as to have a preset concentration in a state where it is put into the treatment tank 31 and mixed with the treatment liquid 52. The mixed liquid 51 is supplied to the mixed liquid tank 12 through the pipe 61 and stored. Since the specific gravity of the pulp fibers and the superabsorbent polymer is greater than 1, the mixed liquid 51 is stirred by the stirrer 13 in the mixed liquid tank 12 so that the pulp fibers and the superabsorbent polymer do not separate from the water.

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

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

[0105] Then, the pulp fibers and the superabsorbent polymer moving downward, that is, descending, in the treatment liquid 52 and the ozone-containing gas 53 moving upward, that is, rising, collide with each other while advancing in opposite directions. Then, the ozone-containing gas 53 adheres to the surfaces of the pulp fibers, the superabsorbent polymer, and the connecting structure. Ozone in the ozone-containing gas 53 oxidizes and decomposes the free superabsorbent polymer and dissolves it in the treatment liquid 52. Thereby, the superabsorbent polymer on the pulp fibers is removed from the pulp fibers. Then, the pulp fibers descend to the bottom of the treatment tank 31, and the ozone-containing gas 53 escapes to the space above the treatment tank 31.

[0106] Among the free superabsorbent polymer, the free pulp fibers, and the connecting structure, the free superabsorbent polymer with a relatively high specific gravity and the connecting structure containing the superabsorbent polymer tend to have a higher sedimentation property than the free pulp fibers with a relatively low specific gravity. On the other hand, since the ozone-containing gas consumes ozone and rises while treating the superabsorbent polymer and the pulp fibers, the ozone-containing gas existing at a lower position has a higher ozone content (that is, is fresher) than the ozone-containing gas existing at an upper position.

[0107] Thus, a free superabsorbent polymer and a connecting structure with a relatively fast downward movement can be accurately oxidized and decomposed with a fresher ozone-containing gas to form free pulp fibers. On the other hand, since the free pulp fibers move downward relatively slowly, the ozone-containing gas can process the free pulp fibers and the recycled pulp fibers to be formed over time.

[0108] Specifically, ozone in the ozone-containing gas can bleach the pulp fibers (as well as the recycled pulp fibers to be produced) by colliding with the pulp fibers while facing them. The pulp fibers contained in the used sanitary products may absorb excreta (e.g., feces, urine, etc.) and may be colored in a tea color or yellow color. In order to reuse them as recycled pulp fibers, it is necessary to bleach them. In addition, since users may have a psychological resistance to the recycled pulp fibers derived from the used sanitary products, from the viewpoint of reducing the psychological resistance of the users, it is preferable that the recycled pulp fibers have a high whiteness.

[0109] Thereafter, the treatment liquid 52 (including recycled pulp fibers) at the bottom of the treatment tank 31 is continuously discharged from the treatment liquid discharge port 33 of the treatment tank 31 to the outside of the treatment tank 31 at a second flow rate by the flow rate control of the delivery pump 22 through the pipe 63. The ozone in the ozone-containing gas 53 accumulated in the upper part of the treatment tank 31 is detoxified by the ozone decomposition device 34 and discharged to the outside.

[0110] In this way, the mixed liquid 51 is continuously supplied into the treatment tank 31 from the upper part of the treatment tank 31 at a first flow rate, and the treatment liquid 52 is continuously discharged from the lower part (bottom) of the treatment tank 31 to the outside of the treatment tank 31 at a second flow rate. Thereby, a continuous and stable fluid (including pulp fibers) flow from the upper part to the lower part can be forcibly generated in the treatment tank 31.

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

[0112] This method continuously supplies a mixed liquid 51 containing at least pulp fibers and a superabsorbent polymer into a treatment tank 31 having a treatment liquid 52 capable of dissolving the superabsorbent polymer at a first flow rate, while continuously discharging, at a second flow rate, a treatment liquid 52 containing recycled pulp fibers from which the superabsorbent polymer has been removed and low-molecular-weight organic substances produced by the oxidative decomposition of the superabsorbent polymer, outside the treatment tank 31. By having such a configuration, it is possible to forcibly generate a continuous and stable fluid (including pulp fibers) flow from the mixed liquid supply port 32 for supplying the mixed liquid 51 in the treatment tank 31 toward the treatment liquid discharge port 33 for discharging the treatment liquid 52. Even if the throughput of the pulp fibers and the superabsorbent polymer is increased by that fluid flow, i.e., the water flow, the superabsorbent polymer can be treated (solubilized) and the pulp fibers can be treated.

[0113] Here, it is preferable that the first flow rate and the second flow rate are the same. By making the first flow rate and the second flow rate the same, the amount of the treatment liquid 52 in the treatment tank 31 can be kept constant, and stable continuous treatment is possible. However, if the amount of the treatment liquid 52 in the treatment tank 31 can be kept substantially constant, that is, if the amount of the treatment liquid 52 in the treatment tank 31 does not increase or decrease significantly, the first flow rate and the second flow rate may vary over time. That is, the first flow rate and the second flow rate do not necessarily have to be exactly the same at all times, and it is sufficient that they are approximately the same on average over time. Here, "approximately the same" means that the difference between the first flow rate and the second flow rate is within 5% by mass. Even in this case, stable continuous treatment is possible.

[0114] When supplying ozone-containing gas 53 to treatment liquid 52, the ozone concentration in treatment liquid 52 is not particularly limited as long as it can oxidatively decompose the superabsorbent polymer. For example, it may be 1 to 50 mass ppm, preferably 2 to 40 mass ppm, and more preferably 3 to 30 mass ppm. If the ozone concentration in treatment liquid 52 is too low, the superabsorbent polymer cannot be completely solubilized, and there is a possibility that the superabsorbent polymer remains in the pulp fibers. Conversely, if the ozone concentration in treatment liquid 52 is too high, since the oxidizing power also increases, there is a possibility of damaging the pulp fibers and causing problems in terms of safety. The ozone treatment temperature is not particularly limited as long as it can oxidatively decompose the superabsorbent polymer. For example, it may remain at room temperature or may be higher than room temperature.

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

[0116] The ozone concentration in ozone-containing gas 53 is preferably 40 to 200 g / m 3 and more preferably 40 to 150 g / m 3 and even more preferably 40 to 100 g / m 3If the ozone concentration in the ozone-containing gas 53 is too low, the highly absorbent polymer cannot be completely solubilized, and there is a risk that the highly absorbent polymer will remain. If the concentration in the ozone-containing gas 53 is too high, it may lead to damage to the pulp fibers, a decrease in safety, and an increase in manufacturing cost. The ozone concentration in the ozone-containing gas 53 can be measured, for example, by an ultraviolet absorption type ozone concentration meter (for example, manufactured by Eco Design Co., Ltd.: Ozone Monitor OZM-5000G).

[0117] The concentrations of the pulp fibers and the highly absorbent polymer in the treatment liquid 52 are not particularly limited as long as they are concentrations at which the highly absorbent polymer can be oxidized and decomposed by ozone in the treatment liquid 52. For example, 0.1 to 20% by mass can be mentioned, preferably 0.2 to 10% by mass, and more preferably 0.3 to 5% by mass. If the concentration of the pulp fibers is too high, the highly absorbent polymer cannot be completely solubilized, and there is a risk that the highly absorbent polymer will remain in the pulp fibers. Conversely, if the concentration of the pulp fibers is too low, the oxidizing power also increases, which may damage the pulp fibers and may also cause problems in safety. The concentrations of the pulp fibers and the highly absorbent polymer in the mixed liquid 51 are appropriately set based on the concentrations of the pulp fibers and the highly absorbent polymer in the treatment liquid 52 and the amount of the treatment liquid 52.

[0118] When supplying ozone to the treatment liquid 52 containing pulp fibers and a highly absorbent polymer, the treatment liquid 52 is preferably acidic. More preferably, the pH of the treatment liquid 52 is greater than 0 and 5.0 or less, and even more preferably 1.5 to 2.5. By treating in an acidic state, the inactivation of ozone is suppressed, the oxidation decomposition effect of the highly absorbent polymer by ozone is improved, and the highly absorbent polymer can be oxidized and decomposed in a short time. In order to maintain the pH of the treatment liquid, the pH of the mixed liquid 51 may be made the same as the pH of the treatment liquid 52, and the mixed liquid 51 may be supplied to the treatment tank 31. Alternatively, the pH of the treatment liquid 52 may be monitored with a pH sensor, and when the pH fluctuates to the neutral side, a predetermined acidic solution may be added to the treatment liquid 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 can oxidatively decompose the superabsorbent polymer. However, 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. 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 there is a risk that the superabsorbent polymer remains. If V / W is too large, the manufacturing cost may increase. The volume V of the treatment tank 31 is not particularly limited, and examples include 50 to 80 L.

[0120] The flow rate R of the 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. 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 there is a risk that the superabsorbent polymer remains in the pulp fibers. If R O / V is too large, it may lead to damage to the pulp fibers, a decrease in safety, and an increase in the manufacturing cost. The flow rate R of the ozone-containing gas O is not particularly limited, and examples include 3 to 6 L / min.

[0121] The time that the pulp fibers exist 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 capable of oxidatively decomposing the superabsorbent polymer. The in-tank treatment time may be short if the ozone concentration of the treatment liquid 52 is high, and a long time is required if the ozone concentration of the treatment liquid 52 is low. Examples of the in-tank treatment time include 2 minutes to 60 minutes, preferably 5 minutes to 30 minutes. The product of the concentration of ozone (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 small, the superabsorbent polymer cannot be completely solubilized, and there is a risk that the superabsorbent polymer remains in the recovered pulp fibers. If the CT value is too large, it may lead to damage to the pulp fibers, a decrease in safety, and an increase in manufacturing cost.

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

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

[0124] In this embodiment, as a preferred aspect, the treatment liquid 52 capable of dissolving the superabsorbent polymer is an aqueous solution containing an ozone-containing gas that oxidatively decomposes the superabsorbent polymer so as to be soluble. The ozone treatment step S36 (continuous treatment step) further includes a feeding step of continuously feeding a plurality of bubbles of the ozone-containing gas from the lower part to the upper part of the treatment liquid 52. In such a preferred aspect of the present method, in the treatment liquid 52, the ozone-containing gas rises while the pulp fibers and the superabsorbent polymer descend, that is, a countercurrent flow is formed. Thereby, the contact probability between the pulp fibers and the superabsorbent polymer and the ozone-containing gas can be increased. Also, the deeper the pulp fibers and the superabsorbent polymer settle, the more they can contact a higher concentration of the ozone-containing gas. Therefore, the superabsorbent polymer that could not be completely dissolved in the treatment liquid 52 only by the ozone-containing gas that contacted at a shallow part in the treatment liquid 52 can be brought into contact with a high-concentration ozone-containing gas at a deep part in the treatment liquid 52. Thereby, the superabsorbent polymer can be surely dissolved in the treatment liquid 52. Thus, the superabsorbent polymer can be surely dissolved in the treatment liquid and removed from the fibers.

[0125] In this embodiment, as a preferred aspect, the above-described feeding step includes a step of feeding the ozone-containing gas in a state of microbubbles or nanobubbles. However, microbubbles are bubbles having a diameter of about 1 to 1000 μm, preferably about 10 to 500 μm, and nanobubbles are bubbles having 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 rising speed in the liquid. Therefore, in this method, as a preferred aspect, such fine ozone-containing gas bubbles are fed from the lower part to the upper part of the treatment liquid 52 in the treatment tank 31.

[0126] On the one hand, the pulp fibers and the superabsorbent polymer move from top to bottom. At this time, since the fine bubbles have a slow rising speed, the probability of the bubbles contacting the pulp fibers can be increased. Further, since the occupied area of the fine bubbles on the surface of the pulp fibers is small, more bubbles can contact the surface of the pulp fibers. Thereby, the pulp fibers, the superabsorbent polymer, and the connecting structure can be evenly wrapped with fine bubbles, and the contact area between them and the ozone-containing gas can be further increased. Further, since more bubbles contact the surface of the pulp fibers, due to the buoyancy of the bubbles, the sedimentation of the pulp fibers, the superabsorbent polymer, and the connecting structure can be reduced, and the contact time between them and the ozone-containing gas can be further increased. As a result, the superabsorbent polymer can be more surely dissolved in the treatment liquid 52 and removed from the pulp fibers.

[0127] In the present embodiment, as a preferred embodiment, the treatment liquid 52 is an acidic aqueous solution, for example, an acidic aqueous solution with a pH of 2.5 or less. In that case, even if the water absorption ability remains partially in the superabsorbent polymer in the mixed liquid 51, the water absorption expansion of the superabsorbent polymer can be suppressed. Thereby, the superabsorbent polymer can be dissolved in the treatment liquid 52 in a short time, and the superabsorbent polymer can be removed more surely. In particular, when the treatment liquid 52 is an ozone-containing aqueous solution, it is difficult to deactivate the ozone in the ozone-containing aqueous solution, so the superabsorbent polymer can be oxidized and decomposed in a shorter time, dissolved, and the superabsorbent polymer can be removed from the fibers more surely.

[0128] As another preferred embodiment, the configuration of the treatment tank 31 may be other configurations than those shown in FIG. 2. FIG. 3 is a schematic diagram showing another configuration example of the apparatus 2 in the ozone treatment step of FIG. 1. The apparatus 2 in FIG. 3 is different from the apparatus 2 in FIG. 2 in that the piping 63 of the ozone treatment unit 4 has a continuous U-shaped pipe structure in which two U-shaped pipes are connected to each other in a reverse and continuous manner, and the delivery pump 22 is omitted. In that case, when the piping 63 is filled with the treatment liquid 52 and the height of the liquid level of the treatment liquid 52 in the treatment tank 31 is higher than the height of the liquid level of the liquid in the tank of the next step connected by the piping 63, the treatment liquid 52 is discharged to the tank of the next step through the piping 63 by the principle of the siphon. Therefore, if the height of the liquid level of the treatment liquid 52 in the treatment tank 31 and the height of the liquid level of the liquid in the tank of the next step are initially made the same before the start of the treatment, and when the mixed liquid 51 is continuously supplied into the treatment tank 31 at the first flow rate at the start of the treatment, the treatment liquid 52 will be discharged to the tank of the next step through the piping 63 at the second flow rate = the first flow rate by the principle of the siphon. However, regarding the height of the liquid level in the tank of the next step, the height before the start of the treatment should be maintained during the treatment. In this case, the delivery pump 22 is unnecessary, and the control of the second flow rate of the delivery pump 22 becomes unnecessary.

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

[0130] In the fourth separation step S37, the method of separating pulp fibers from the treatment liquid 52 discharged from the treatment tank 31 is not particularly limited. For example, a method of passing the treatment liquid 52 containing recycled pulp fibers through a screen mesh with an opening size of 0.15 to 2 mm can be mentioned. When the treatment liquid 52 containing recycled pulp fibers is passed through a screen mesh with an opening size of 0.15 to 2 mm, the drainage containing the products due to the oxidative decomposition of the superabsorbent polymer passes through the screen. On the other hand, the recycled pulp fibers remain on the screen.

[0131] Subsequently, in the second drying step S38, the separated pulp fibers are dried with a high-temperature atmosphere or hot air or the like. The drying temperature is, for example, 105 to 210°C, preferably 110 to 190°C. The drying time depends on the drying temperature, but is, for example, 10 to 120 minutes, preferably 15 to 100 minutes. Thereby, the solvent remaining on the surface of the pulp fibers evaporates and is removed, and high-purity pulp fibers with an extremely low high-absorbency polymer mixing ratio can be recovered. Therefore, the constituent members of the sanitary product can be efficiently recovered. In addition, the pulp fibers can be sterilized (disinfected) with a high-temperature atmosphere or hot air or the like.

[0132] In the present disclosure, the recycled pulp fibers have a ΔYI of 0 to 10 with respect to the standard white board, preferably 0 to 9.0, more preferably 0 to 7.0, and still more preferably 0 to 5.5. By doing so, it is difficult for the user to have a psychological resistance to the recycled pulp fibers produced from the pulp fibers of the used sanitary product.

[0133] The ΔYI of the recycled pulp fibers can be measured as follows. (1) Prepare a spectral reflectance color difference meter Z-300A manufactured by Nippon Denshoku Industries Co., Ltd. in a thermo-hygrostat chamber at a temperature of 20 ± 5°C and a humidity of 65 ± 5%RH. (2) Evenly spread 4.5 g of recycled pulp fibers dried at 120°C for 60 minutes on the glass window (diameter 40 mm) of the sample stage of the color difference meter. (3) Place a black plate (size: 80 mm × 80 mm, mass: 280 g) attached to the color difference meter on the spread recycled pulp fibers to apply a load to the recycled pulp fibers. (4) Select the color difference meter in the mode: reflection, transmission window diameter: 30 mm, and for each sample, measure ΔYI (= |[YI value of recycled pulp fibers] - [YI value of standard white board]|), which is the color difference (absolute value) of the YI value from the standard white board. (5) Adopt the average value of the ΔYI of 10 samples.

[0134] In the present disclosure, the recycled pulp fiber preferably has a water contact angle of 20° or less, more preferably 15° or less, and even more preferably 10° or less. By doing so, since the article using the recycled pulp fiber has uniform hydrophilicity, it becomes less likely for the user to have a psychological resistance to the article using the recycled pulp fiber. From the above viewpoint, the water contact angle of the recycled pulp fiber may be 0°.

[0135] The water contact angle of the recycled pulp fiber can be measured as follows. (1) Prepare an aluminum ring (outer diameter: 43 mm, inner diameter: 40 mm, height: 5 mm) and recycled pulp fiber dried at 120°C for 60 minutes in a thermo-hygrostat chamber at a temperature of 20 ± 5°C and a humidity of 65 ± 5% RH, and leave it standing for 24 hours. (2) Evenly fill 1.5 g of the recycled pulp fiber into the aluminum ring, and compress the recycled pulp fiber together with the aluminum ring for 1 minute at a pressure of 3 Mpa using a press machine with a smooth bottom surface to smooth the surface of the recycled pulp fiber. (3) Measure the water contact angle of the compressed recycled pulp fiber in accordance with the sessile drop method in 6. of "Test Method for Wettability of Substrate Glass Surface" of JIS R 3257:1999. Examples of the contact angle measuring device include the automatic contact angle meter CA-V type manufactured by Kyowa Interface Science Co., Ltd. The above water contact angle means the value 200 ms after dropping deionized water. (4) Measure the water contact angle in 20 different samples and adopt their average value.

[0136] In the present disclosure, the recycled pulp fiber preferably has a lignin content of 0.1 mass% or less, more preferably 0.08 mass% or less, and even more preferably 0.06 mass% or less. By doing so, since the recycled pulp fiber has a predetermined lignin content, it is likely to be excellent in whiteness and hydrophilicity. As a result, the article using the recycled pulp fiber has uniform whiteness and hydrophilicity, and it becomes less likely for the user to have a psychological resistance to the article using the recycled pulp fiber.

[0137] The lignin content of the recycled pulp fibers can be measured according to the method described on pages 85 to 87 of the "Guide to Soil Diagnosis" issued by the Agricultural Guidance Section of the Ministry of Agriculture, Forestry and Fisheries of Ehime Prefecture in March 1988. The following is a summary. <Preparation of Reagents> (1) Prepare recycled pulp fibers dried at 120°C for 60 minutes in a thermostatic and humidistatic chamber at a temperature of 20 ± 5°C and a humidity of 65 ± 5% RH, and let them stand for 24 hours. (2) Prepare commercially available lignin (95%) as a standard reagent. (3) Weigh 22.3 g of sodium pyrophosphate (Na4P2O5·10H2O) into a 500 mL beaker, add approximately 400 mL of deionized water, dissolve the sodium pyrophosphate, and prepare a sodium pyrophosphate aqueous solution. (4) Weigh 10 g of sodium hydroxide into another 500 mL beaker, add approximately 200 mL of deionized water, dissolve the sodium hydroxide, and prepare a sodium hydroxide aqueous solution. Let the sodium pyrophosphate aqueous solution and the sodium hydroxide aqueous solution cool, add them to a 1 L volumetric flask, and make up to the mark with deionized water to prepare a pyrophosphate extract.

[0138] <Pre - operation> (5) Weigh 2 g of lignin into a 100 mL Erlenmeyer flask A. (6) Weigh 2 g of recycled pulp fibers into another 100 mL Erlenmeyer flask B. (7) Add 20 mL of the pyrophosphate extract to each of 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 each through No. 6 filter paper to obtain a lignin filtrate and a recycled pulp fiber filtrate.

[0139] <Analysis> (9) Using a volumetric pipette, add 5 mL of the lignin filtrate to a 50 mL volumetric flask, and make up to the mark with deionized water (lignin 10,000 ppm). (10) Using a female pipette, add the volumetric lignin filtrate to four 100 mL volumetric flasks in amounts of 1 mL, 2 mL, 5 mL, and 10 mL, add deionized water to make up the volume, 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 solutions at a wavelength of 530 nm and convert it to absorbance with reference to the absorbance conversion table on page 235. (12) Create a calibration curve between the calibration solution concentration and the absorbance. (13) Measure the transmittance of the recycled pulp fiber filtrate using deionized water as a blank, convert it to absorbance with reference to the absorbance conversion table on page 235, and calculate the lignin concentration from the calibration curve. (14) Calculate the lignin content (% by mass) from the lignin concentration. Note that the above-mentioned commercially available lignin (95%) may be changed to commercially available lignin such as lignin concentration (for example, lignin from Nacalai Tesque).

[0140] In the manufacturing method of the present disclosure, the recycled pulp fiber preferably has a beating degree reduction rate of 300 mL or more, more preferably 320 mL or more, still more preferably 340 mL or more, and even more preferably 360 mL or more. By doing so, when the recycled pulp fiber is reused, the recycled pulp fiber is prone to fibrillation and the surface area increases, so the diffuse reflection of light increases and the recycled pulp fiber tends to look whiter.

[0141] In the manufacturing method of the present disclosure, the recycled pulp fiber preferably has a beating degree reduction rate of 990 mL or less, more preferably 800 mL or less, still more preferably 700 mL or less, and even more preferably 600 mL or less. By doing so, when the recycled pulp fiber is reused, it is possible to suppress the recycled pulp fiber from being too fibrillated and generating dust and the like. Note that the above-mentioned beating degree reduction rate can be achieved by adjusting the low lignin content of the recycled pulp fiber, the narrowness of the lignin content distribution, etc.

[0142] The beating degree reduction rate is measured according to the following beating degree reduction test. <Beating degree reduction test> (1) Recycled pulp fibers are beaten for 1 hour or more, preferably 2 hours, according to JIS P 8221-1:1998, Pulp - Beating method - Part 1: Beater method. (2) Samples are taken every 20 minutes after the start of beating, and the beating degree (Canadian Standard freeness) of each sample is measured according to JIS P 8121-2:2012, Pulp - Drainability test method - Part 2: Canadian standard drainability method. The test may be stopped when the beating degree of the sample reaches less than 100 mL. (3) Time (h) is plotted on the horizontal axis and beating degree (mL) is plotted on the vertical axis, approximated to a linear function by the least squares method, and the absolute value of the slope is adopted as the beating degree reduction rate (mL / h). Note that the larger the value of the beating degree reduction rate, the faster the reduction of the beating degree per unit time, that is, the recycled pulp fibers are more easily beaten (more likely to have fluff).

[0143] In the present 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, still more preferably 0.30% by mass, and even more preferably 0.20% by mass or less. By doing so, the recycled pulp fibers and articles using the recycled pulp fibers are excellent in whiteness and hydrophilicity. As a result, it becomes less likely for users to have a psychological resistance to articles using recycled pulp fibers. The above ash content can be lowered by selecting an acid, particularly citric acid, which can form a complex with metal ions contained in excrement as an inactivating agent, in the inactivating step S31 for inactivating the superabsorbent polymer.

[0144] In this specification, ash means the amount of inorganic substances or non-combustible residues remaining after the organic matter has been incinerated, and the ash content means the ratio (mass ratio) of the ash contained in the materials to be promoted. The above ash content is measured in accordance with the "5. Ash test method" of the "2. General test method" of the physiological treatment product material standard. Specifically, the ash content is measured as follows. (1) Preheat a crucible made of platinum, quartz, or porcelain at 500 to 550 °C for 1 hour, allow it to cool, and then precisely measure its mass. (2) Collect 2 to 4 g of recycled pulp fibers dried at 120 °C for 60 minutes, put them into the crucible, precisely measure their mass, and if necessary, remove or shift the lid of the crucible. First, heat gently, gradually increase the temperature, and strongly heat at 500 to 550 °C for 4 hours or more until no carbide remains and incinerate. (3) After cooling, precisely measure its mass. Incinerate the residue again until it reaches a constant weight, allow it to cool, and then precisely measure its mass to obtain the ash content (mass %).

[0145] In the ozone treatment step S36, in the mixed liquid 51 supplied from the mixed liquid supply port 32 through the pipe 62, it is preferable that the pulp fibers do not contain a colorant selected from the group consisting of dyes, pigments, and combinations thereof. Since the pulp fibers do not contain a predetermined colorant, the recycled pulp fibers are likely to have a uniform whiteness, and the bleached recycled pulp and the articles using the recycled pulp fibers are likely to have a uniform whiteness, making it less likely for the user to have a psychological resistance to the articles using the recycled pulp fibers.

[0146] In this embodiment, as a preferred aspect, before the ozone treatment step S36 (continuous treatment step), the mixture is treated with an aqueous solution capable of inactivating the water absorption performance of the superabsorbent polymer to inactivate the water absorption performance of the superabsorbent polymer in the mixture, and an inactivation step S31 is provided. Before the ozone treatment step S36 (continuous treatment step), a first separation step S32 is further provided for separating the inactivated superabsorbent polymer and pulp fibers from the aqueous solution. Thus, in this method, as a preferred aspect, in the inactivation step S31, since the water absorption performance of the superabsorbent polymer is suppressed with an aqueous solution capable of inactivating the water absorption performance of the superabsorbent polymer, at the stage of the subsequent ozone treatment step S36 (continuous treatment step), the superabsorbent polymer can be more easily dissolved in the treatment liquid 52 in a short time.

[0147] In this embodiment, as a preferred aspect, in the inactivation step S31, the aqueous solution capable of inactivating the water absorption performance of the superabsorbent polymer is an acidic aqueous solution, for example, an acidic aqueous solution with a pH of 2.5 or less. Thus, in this method, as a preferred aspect, 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, at the stage of the inactivation step S31, the water absorption performance of the superabsorbent polymer can be more reliably suppressed. Thereby, at the stage of the subsequent ozone treatment step S36 (continuous treatment step), the superabsorbent polymer can be more easily dissolved in the treatment liquid in a short time.

[0148] As 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. FIG. 4 is a schematic diagram showing another configuration example of the apparatus 2 in the ozone treatment step of FIG. 1. The apparatus 2 in FIG. 4 is different from the apparatus 2 in FIG. 2 in that two ozone treatment units 4 are joined in series, in other words, the first treatment tank 31-1 and the second treatment tank 31-2 are joined in series. In that case, for example, the first treatment tank 31-1 is supplied with the mixed liquid 51 and discharges the first treated liquid (the treated liquid 52-1 of 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 (the treated liquid 52-2 of the second treatment tank 31-2). In this way, the mixed liquid 51 is treated in multiple stages. In that case, compared with the case of providing one treatment tank 31 with a large capacity, since the treatment is performed with new treated liquids 52-1 and 52-2 for each of the first and second treatment tanks 31-1 and 31-2, for example, the superabsorbent polymer that could not be completely dissolved in the first treatment tank (the first-stage treatment tank) 31-1 can be easily dissolved in the second treatment tank (the second-stage treatment tank) 31-2. Thus, the superabsorbent polymer can be more surely dissolved and removed from the fibers.

[0149] In this embodiment, as a preferred form, in the material separation step S1, in the pretreatment step S11, the used sanitary product can be made to swell extremely with water in its original shape without being broken or the like and without inactivating the superabsorbent polymer. Thereby, a very high internal pressure is generated in the used sanitary product, and any part of its surface can be brought into a state where it is about to burst. Then, in the decomposition step S12, by applying a physical impact to the used sanitary product in such a state, any part of its surface can be torn and the internal absorption core can be ejected to the outside. Thereby, the used sanitary product can be decomposed into at least the film (back sheet) and the absorption core. At this time, since the film generally maintains its original shape, it can be easily separated from the absorption core in the subsequent separation step S13. Thereby, a component such as the film can be separated from other components while maintaining its original shape without being broken or the like. Therefore, components such as the film of the sanitary product can be efficiently recovered.

[0150] In the present embodiment, as a preferred form, by using terpene for removing the adhesive, the hot-melt adhesive that bonds the constituent members of the sanitary product can be melted at room temperature. As a result, the sanitary product can be easily and neatly disassembled, the pulp fibers and the superabsorbent polymer can be separated from the sanitary product, and the nonwoven fabric and the film can be separated while leaving their respective member forms intact. That is, the pulp fibers, the film, and the nonwoven fabric can be easily recovered separately without crushing the sanitary product or going through a complicated separation process. When limonene is used as the terpene, as a secondary effect of limonene, there is a fresh citrus odor, which can cover the odor derived from excrement to a certain extent, reducing the odor burden on the operator and the odor impact on the neighborhood. Since limonene is a monoterpene and has a structure similar to styrene, it can dissolve the styrene-based hot-melt adhesive commonly used in sanitary products. Since the sanitary product can be washed at room temperature, the energy cost can be reduced and the generation and diffusion of odors can be suppressed. Terpene has a high effect on washing oil stains. In addition to the effect of dissolving the hot-melt adhesive, when there is printing on the film, the printing ink can also be decomposed and removed, and the printed film can be recovered as a high-purity plastic material.

[0151] Also, when an aqueous solution of an organic acid with a pH of 2.5 or less is used to inactivate the superabsorbent polymer, it is difficult to deteriorate the pulp fibers. Further, when citric acid is used as the organic acid, due to the chelating effect and detergency of citric acid, an effect of removing dirt components derived from excrement can be expected. Also, a bactericidal effect and a deodorizing effect against alkaline odors can be expected.

[0152] Furthermore, by oxidatively decomposing the superabsorbent polymer with ozone, it is possible to prevent contamination of the pulp fibers and a sharp increase in sewage due to water absorption by the superabsorbent polymer. By adjusting the concentration of ozone, it is possible to simultaneously perform oxidative decomposition and sterilization of the superabsorbent polymer. Also, when ozone is used, since no chlorine-based chemicals are used at all, it is possible to produce high-quality RPF that is difficult to damage the combustion furnace from the recovered plastic members. Since no salts are used during the treatment process, there is no residue on the pulp fibers, and high-quality pulp with low ash content can be recovered.

[0153] The recycled pulp fibers produced by the manufacturing method of the present disclosure are not particularly limited and can be used in various articles. Examples of the above articles include tissues, non-woven fabrics (including pulp fibers), sanitary products (such as disposable diapers, urine pads, sanitary napkins, panty liners, pet sheets), cardboard, and paper (printing paper, packaging paper, books, magazines, etc.). In the above sanitary products, the recycled pulp fibers can be used in absorbents, for example, in an absorbent body including an absorption core and a core wrap, in the core wrap and / or the absorption core.

Example

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

[0155] [Production Example 1] The inactivation step S31 was performed with citric acid at pH 2.0, the ozone treatment step S36 was performed under the above conditions, and the obtained recycled pulp fibers were dried at 120°C for 60 minutes to obtain recycled pulp fiber No. 1. [Production 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 using slaked lime.

[0156] [Comparative Production Example 1] Recycled pulp fiber No. 3 was obtained in the same manner as in Production Example 1, except that the ozone-containing gas was not sent out in the ozone treatment step S36. [Comparative Production Example 2] Recycled pulp fiber No. 4 was obtained in the same manner as in Production Example 2, except that the ozone-containing gas was not sent out in the ozone treatment step S36. [Comparative Production Example 3] Virgin pulp fiber of NBKP was designated as recycled pulp fiber No. 5.

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

[0158]

Table 1

Explanation of Symbols

[0159] 31 Treatment tank 32 Mixed liquid supply port 33 Treatment liquid discharge port 43 Ozone-containing gas supply port 51 Mixed liquid 52 Treatment liquid 53 Ozone-containing gas S36 Ozone treatment step

Claims

1. Recycled pulp fibers derived from used sanitary products containing pulp fibers and superabsorbent polymers, wherein the recycled pulp fibers have a ΔYI of 0 to 5.5, a water contact angle of 10° or less, and a beating degree reduction rate of 360 mL / h or more and 990 mL / h or less with respect to a standard white board, and the recycled pulp fibers have an ash content of 0.65% by mass or less. The recycled pulp fibers are characterized by the above.

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

3. The recycled pulp fibers according to claim 1 or 2, wherein the pulp fibers do not contain a colorant selected from the group consisting of dyes, pigments, and combinations thereof.

4. A tissue containing the recycled pulp fibers according to any one of claims 1 to 3.

5. A non-woven fabric containing the recycled pulp fibers according to any one of claims 1 to 3.

6. A cardboard containing the recycled pulp fibers according to any one of claims 1 to 3.

7. A paper containing the recycled pulp fibers according to any one of claims 1 to 3.

8. A sanitary product containing recycled pulp fibers derived from used sanitary products containing pulp fibers and superabsorbent polymers, wherein the recycled pulp fibers have a ΔYI of 0 to 5.5, a water contact angle of 10° or less, and a beating degree reduction rate of 360 mL / h or more and 990 mL / h or less with respect to a standard white board, and the recycled pulp fibers have an ash content of 0.65% by mass or less. The sanitary product is characterized by the above.

9. The sanitary product according to claim 8, wherein the recycled pulp fibers have a lignin content of 0.1% by mass or less.

10. The sanitary product according to claim 8 or 9, wherein the pulp fibers do not contain a colorant selected from the group consisting of dyes, pigments, and combinations thereof.

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