Method for producing pulp fiber by using pulp fiber material derived from used absorbent article

Abstract

Provided is a method for producing a pulp fiber that makes it possible to obtain high-quality pulp fiber from a pulp fiber material derived from used absorbent articles. The present invention is a method for producing pulp fiber by using a pulp fiber material derived from used absorbent articles. This method comprises: a preparation step (S1) for preparing a pulp fiber material that includes pulp fiber and impurities, the impurities including multivalent metals, plastics, and superabsorbent polymers crosslinked with multivalent metal ions; a first separation step (S3) for separating, in water, the superabsorbent polymers from the pulp fiber material; and a second separation step (S4) for separating, in an organic acid aqueous solution, the plastics and multivalent metals from the pulp fiber material obtained after the first separation step (S3), thereby obtaining pulp fiber.

Description

Method for producing pulp fiber using pulp fiber material derived from used absorbent articles

[0001] The present invention relates to a method for producing pulp fibers using pulp fiber materials derived from used absorbent articles.

[0002] It is known to regenerate pulp fibers derived from used absorbent articles. For example, Patent Document 1 discloses a method for treating used sanitary products using a rotary drum device having a rotary drum for storing used sanitary products, a water tank containing the rotary drum, and a water supply means for supplying water to the water tank. In this treatment method, used sanitary products, individually packaged chemicals containing chemicals in containers, and water are supplied into the rotary drum, and the rotary drum is rotated to break the containers and diffuse the chemicals into the rotary drum. In this treatment method, used sanitary products are sterilized, decomposed, and separated. As a result, disposable diapers are decomposed into pulp and plastic within the rotary drum. In the examples of Patent Document 1, slaked lime and calcium hypochlorite are used as chemicals.

[0003] JP 2024-15810 A

[0004] In Patent Document 1, disposable diapers are decomposed into pulp and plastics (such as films and nonwoven fabrics), but these decomposition products also contain other constituent materials of the disposable diapers, such as superabsorbent polymers. Therefore, the pulp recovered from the rotating drum may contain superabsorbent polymers and plastics as impurities. Furthermore, Patent Document 1 uses polyvalent metal ions (calcium ions) supplied from a polyvalent metal salt (slaked lime) to inactivate and dehydrate the superabsorbent polymer. Therefore, in the pulp recovered from the rotating drum, the impurity superabsorbent polymer is crosslinked with polyvalent metal ions, and the pulp and the impurity plastics have polyvalent metals (polyvalent metal ions, polyvalent metal compounds, polyvalent metal elements, etc.) attached to them. Thus, the pulp in Patent Document 1 cannot be said to be highly pure, and therefore, its quality cannot be said to be high. Therefore, such pulp may have limited uses.

[0005] Therefore, an object of the present invention is to provide a method for producing pulp fibers that can obtain high-quality pulp fibers from pulp fiber materials derived from used absorbent articles.

[0006] One aspect of the present invention is a method for producing pulp fibers using pulp fiber material derived from used absorbent articles, the method comprising: a preparation step of preparing the pulp fiber material containing pulp fibers and impurities, the impurities including a superabsorbent polymer cross-linked with polyvalent metal ions, plastic, and polyvalent metal; a first separation step of separating the superabsorbent polymer from the pulp fiber material in water; and a second separation step of separating the plastic and the polyvalent metal from the pulp fiber material after the first separation step in an aqueous organic acid solution to obtain pulp fibers.

[0007] According to the present invention, a method for producing pulp fibers can be provided that makes it possible to obtain high-quality pulp fibers from pulp fiber materials derived from used absorbent articles.

[0008] FIG. 1 is a flow diagram showing an example of a method for producing pulp fibers using a pulp fiber material derived from a used absorbent article according to an embodiment.

[0009] The present embodiment relates to the following aspects.

[0010] [Embodiment 1] A method for producing pulp fibers using a pulp fiber material derived from used absorbent articles, the method comprising: a preparation step of preparing the pulp fiber material containing pulp fibers and impurities, the impurities including a superabsorbent polymer cross-linked with polyvalent metal ions, plastics, and polyvalent metals; a first separation step of separating the superabsorbent polymer from the pulp fiber material in water; and a second separation step of separating the plastics and the polyvalent metals from the pulp fiber material after the first separation step in an aqueous organic acid solution to obtain pulp fibers.

[0011] In this method, a superabsorbent polymer is first separated in water from a pulp fiber material containing impurities (first separation step), and then plastics and polyvalent metals are separated from the pulp fiber material from which the superabsorbent polymer has been separated in an organic acid aqueous solution (second separation step). Attempting to separate the superabsorbent polymer from the pulp fiber material in an organic acid aqueous solution first results in polyvalent metal ions being desorbed from the superabsorbent polymer, causing the superabsorbent polymer to absorb water, making it difficult to separate the superabsorbent polymer from the pulp fiber material. Therefore, in this method, the superabsorbent polymer is first separated from the pulp fiber material in water (first separation step), allowing the superabsorbent polymer crosslinked with polyvalent metal ions to be easily separated from the pulp fiber material in water. Furthermore, the polyvalent metals contained in the pulp fibers dissolve in the organic acid aqueous solution. Therefore, in this method, plastics and polyvalent metals are then separated from the pulp fiber material from which the superabsorbent polymer has been separated in an organic acid aqueous solution (second separation step), making it possible to easily remove the polyvalent metals from the pulp fiber material along with the easily separable plastics. In this way, in this method, the superabsorbent polymer, plastics, and polyvalent metals are removed from the pulp fiber material to form pulp fibers, making it possible to improve the quality of pulp fibers derived from used absorbent articles.

[0012] [Aspect 2] The method of aspect 1, wherein the first separation step comprises separating the superabsorbent polymer from the pulp fiber material by sieving and / or gravity separation in the water.

[0013] In this method, the first separation step involves separating the superabsorbent polymer from the pulp fiber material by sieving and / or gravity separation in water. The superabsorbent polymer cross-linked with polyvalent metal ions is smaller in size and has a higher specific gravity than the remaining pulp fiber material (pulp fibers and plastics). Therefore, in water, the superabsorbent polymer settles first, the pulp fibers settle slowly, and the plastics settle very little. Therefore, the superabsorbent polymer can be easily separated from the pulp fiber material by sieving and / or gravity separation.

[0014] [Aspect 3] The method according to Aspect 1 or 2, wherein the second separation step includes separating the plastics and the polyvalent metals from the pulp fiber material after the first separation step by sieving and / or gravity separation in the organic acid aqueous solution.

[0015] In this method, the second separation step includes separating plastics and polyvalent metals from the pulp fiber material from which the superabsorbent polymer has been separated by sieving and / or gravity separation in an organic acid aqueous solution. Here, the polyvalent metals contained in the pulp fibers of the pulp fiber material are dissolved in the organic acid aqueous solution and removed from the pulp fibers. This improves the water absorption of the pulp fibers, making them more likely to settle in the organic acid aqueous solution. Meanwhile, the polyvalent metals contained in the plastics are also dissolved in the organic acid aqueous solution and removed from the plastics. This improves the floating ability of the plastics (reducing their specific gravity), making them more likely to float to the surface of the organic acid aqueous solution. Furthermore, pulp fibers are smaller in size than plastics. Therefore, by sieving and / or gravity separation, the pulp fibers and plastics can be easily separated from each other in the pulp fiber material from which the superabsorbent polymer has been separated.

[0016] [Aspect 4] The method according to aspect 3, wherein the organic acid aqueous solution is an aqueous solution containing an organic acid having a chelating action.

[0017] In this method, the organic acid aqueous solution used in the second separation step contains an organic acid (chelating agent) with chelating activity. Therefore, the chelating activity allows polyvalent metal ions eluted from the superabsorbent polymer and polyvalent metals eluted from pulp fiber materials (pulp fibers and plastics) to remain stable in the organic acid aqueous solution. This prevents the eluted polyvalent metals from being reabsorbed into the superabsorbent polymer, pulp fibers, and plastics.

[0018] [Aspect 5] The method according to aspect 4, wherein the organic acid is citric acid.

[0019] In this method, the organic acid in the organic acid aqueous solution in the second separation step is citric acid. Therefore, polyvalent metal ions eluted from the superabsorbent polymer and polyvalent metals eluted from pulp fiber materials (pulp fibers and plastics) can be more stably present in the organic acid aqueous solution due to the chelating action. This further prevents the eluted polyvalent metals from being reabsorbed into the superabsorbent polymer, pulp fibers, and plastics.

[0020] [Aspect 6] The method of any one of aspects 1 to 5, further comprising a disentangling step of agitating the pulp fiber material in water prior to the first separation step.

[0021] Before the first separation step, the pulp fiber material may be partially or entirely agglomerated due to hydrogen bonding between pulp fibers, etc. In such cases, it is not easy to separate the superabsorbent polymer, plastic, and polyvalent metal from the agglomerated pulp fiber material. Therefore, in this method, in such cases, the pulp fiber material is agitated in water in advance (loosening step) before the first separation step. This loosens the agglomerated pulp fiber material, making it easier to separate the superabsorbent polymer from the pulp fiber material in the subsequent first separation step, and easier to separate the plastic and polyvalent metal from the pulp fiber material in the second separation step.

[0022] [Aspect 7] The method according to any one of aspects 1 to 6, further comprising a dewatering step of dewatering the pulp fibers after the second separation step, and an oxidizing agent treatment step of treating the pulp fibers with an oxidizing agent in the dewatered state.

[0023] In this method, the pulp fibers in a dehydrated state (dewatering step) after the second separation step are treated with an oxidizing agent (oxidizing agent treatment step). In this case, the dehydrated state is different from the dried state and refers to a state containing a certain amount of moisture (semi-dry state). Therefore, the pulp fibers can be contacted with the oxidizing agent at a higher concentration than when the pulp fibers are contacted with the oxidizing agent in an aqueous solution. This allows the pulp fibers to achieve high bleaching, sterilizing, and deodorizing effects, and these effects can be achieved with a short treatment time and a small amount of oxidizing agent. Furthermore, since the dehydrated pulp fibers contain an organic acid aqueous solution, the treatment time required for achieving the bleaching, sterilizing, and deodorizing effects in the oxidizing agent treatment can be shortened. In this way, the pulp fibers obtained from the pulp fiber material are bleached, sterilized, and deodorized in this method, thereby improving the quality of the pulp fibers derived from used absorbent articles.

[0024] [Aspect 8] The method according to aspect 7, wherein the oxidizing agent comprises at least one of hydrogen peroxide, ozone, percarbonate, peracetic acid, chlorine dioxide, and a hypochlorous acid-based agent.

[0025] In this method, the oxidizing agent includes at least one of hydrogen peroxide, ozone, percarbonate, peracetic acid, chlorine dioxide, and hypochlorous acid-based agents, thereby achieving higher bleaching, sterilizing, and deodorizing effects on pulp fibers, and achieving these effects with a shorter treatment time and a smaller amount of oxidizing agent.

[0026] Hereinafter, a method for producing pulp fibers using a pulp fiber material derived from used absorbent articles according to an embodiment will be described.

[0027] First, we will explain used absorbent articles. Used absorbent articles include absorbent articles that have been used, such as those that have been used and contain excrement, and those that have been used but do not contain excrement, as well as unused absorbent articles, such as those that have been manufactured and stored but have been disposed of (factory loss, storage loss, etc.). Absorbent articles are not particularly limited as long as they contain a superabsorbent polymer and pulp fiber as absorbent materials, and examples include disposable diapers, urine absorption pads, sanitary napkins, bed sheets, and pet sheets.

[0028] Next, an example of the configuration of an absorbent article will be described. The absorbent article comprises a topsheet, a backsheet, and an absorbent body disposed between the topsheet and the backsheet. The size of the absorbent article can be, for example, about 15 to 100 cm in length and 5 to 100 cm in width. The absorbent article may further comprise other components that may be included in a typical absorbent article, such as a diffusion sheet, a leak-proof wall, a side sheet, an exterior sheet, or an elastic member.

[0029] The material for the top sheet is not particularly limited as long as it can be used in absorbent articles, and examples thereof include liquid-permeable nonwoven fabrics, synthetic resin films with liquid-permeable holes, and composite sheets thereof. The material for the back sheet is not particularly limited as long as it can be used in absorbent articles, and examples thereof include liquid-impermeable nonwoven fabrics, liquid-impermeable synthetic resin films, and composite sheets thereof. The material for the diffusion sheet is not particularly limited as long as it can be used in absorbent articles, and examples thereof include liquid-permeable nonwoven fabrics. The materials for the leak barrier, side sheets, and exterior sheets are not particularly limited as long as they can be used in absorbent articles, and examples thereof include liquid-impermeable nonwoven fabrics. The material for the elastic member is not particularly limited as long as it can be used in absorbent articles, and examples thereof include rubber threads and flat rubber. The material for the nonwoven fabric and synthetic resin film is not particularly limited as long as it can be used in absorbent articles, and examples thereof include synthetic resins. Examples of synthetic resins include olefin-based resins such as polyethylene and polypropylene, polyamide-based resins such as 6-nylon and 6,6-nylon, and polyester-based resins such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT). Materials for the nonwoven fabric may include natural fibers such as cotton and rayon. Materials for the rubber thread and flat rubber include synthetic rubbers such as styrene-butadiene, butadiene, isoprene, and neoprene. In this embodiment, the plastic includes at least the above-mentioned nonwoven fabric and synthetic resin film, and may also include synthetic rubber.

[0030] Examples of absorbent materials include absorbent materials such as pulp fibers and superabsorbent polymers. Pulp fibers are not particularly limited as long as they can be used in absorbent articles, and examples include cellulosic fibers. Cellulosic fibers include wood pulp, crosslinked pulp, non-wood pulp, regenerated cellulose, and semi-synthetic cellulose. Pulp fiber sizes include an average fiber length of several tens of microns, preferably 20 to 40 microns, and an average fiber length of several millimeters, preferably 2 to 5 mm. Superabsorbent polymers (SAPs) are not particularly limited as long as they are usable in absorbent articles and contain acid groups, and examples include those containing carboxyl groups, sulfo groups, etc., with those containing carboxyl groups being preferred. Examples of superabsorbent polymers include polyacrylate-based (containing carboxyl groups), polysulfonate-based (containing sulfo groups, etc.), and polymaleic anhydride-based (containing carboxyl groups) water-absorbing polymers. The size (when dry) of the superabsorbent polymer may be, for example, several hundred μm in average particle size, preferably 200 to 500 μm. The absorbent body may include a core wrap sheet formed of a liquid-permeable sheet such as tissue, which encloses the absorbent material.

[0031] One side and the other side of the absorbent body are bonded to the top sheet and the back sheet, respectively, via an adhesive. In a plan view, the portion (peripheral portion) of the top sheet that extends outward from the absorbent body so as to surround the absorbent body is bonded via an adhesive to the portion (peripheral portion) of the back sheet that extends outward from the absorbent body so as to surround the absorbent body. Therefore, the absorbent body is enclosed within the bonded body of the top sheet and the back sheet. If the absorbent body has a core wrap sheet, the absorbent material is bonded to the core wrap sheet via an adhesive. The diffusion sheet, leak-proof wall, side sheet, and exterior sheet are also each bonded to other components with an adhesive. The adhesive is not particularly limited as long as it can be used in absorbent articles, and examples thereof include hot-melt adhesives. Examples of hot-melt adhesives include pressure-sensitive adhesives or heat-sensitive adhesives that are primarily rubber-based, such as styrene-ethylene-butadiene-styrene, styrene-butadiene-styrene, and styrene-isoprene-styrene, or olefin-based, such as polyethylene.

[0032] Next, a method for producing pulp fibers using a pulp fiber material derived from used absorbent articles according to an embodiment will be specifically described. Fig. 1 is a flow chart showing an example of the production method. The production method includes a preparation step S1, a first separation step S3, and a second separation step S4, and may further include a loosening step S2, a dehydration step S5, an oxidizing agent treatment step S6, and a drying step S7. Each step will be described below.

[0033] The preparation step S1 is a step of preparing a pulp fiber material derived from used absorbent articles. The pulp fiber material is a material obtained by removing large plastic components for other uses from the constituent materials of used absorbent articles, and includes pulp fibers and impurities. The impurities include superabsorbent polymers crosslinked with polyvalent metal ions, plastics, and polyvalent metals. The polyvalent metal impurities include those in the form of polyvalent metal ions, polyvalent metals themselves, and polyvalent metal compounds. In other words, the pulp fiber material is primarily composed of pulp fibers, with the remainder consisting primarily of the superabsorbent polymers, plastics, and polyvalent metals. The mass ratio of the pulp fibers, superabsorbent polymers, and plastics derived from used absorbent articles contained in the pulp fiber material is not particularly limited, as long as pulp fibers are the primary component. Assuming the total is 100% by mass, an example of the mass ratio is pulp fiber:superabsorbent polymer:plastic = 70-95% by mass:1-25% by mass:1-5% by mass.

[0034] The pulp fiber material may be obtained by mainly extracting pulp fibers from a part or all of a used absorbent article that has been divided into small pieces, or by mainly extracting pulp fibers from a part or all of a used absorbent article that has been disassembled into its constituent parts, or a mixture thereof. The dividing method is not particularly limited, and examples include a method of crushing the absorbent article using equipment such as a biaxial crusher. The disassembling method is not particularly limited, and examples include a method of dissolving the adhesive with an agent such as an organic solvent, or a method using the device described in Patent Document 1. The method of extracting the constituent parts is not particularly limited, and examples include separation using a screen or the like.

[0035] In this embodiment, a defibrating step S2 may be performed. The defibrating step S2 is a step of agitating the pulp fiber material in water prior to the first separation step S3 (described below). The pulp fiber material prepared in the preparation step S1 may be in the form of clumps of a certain size and amount due to reasons such as transportation and storage. By agitating such pulp fiber material in water, the clumps can be defibrated, making it easier to disperse the pulp fiber material in water. Then, the water containing the defibrated and dispersed pulp fiber material may be directly transferred to the first separation step S3. This makes it easier to proceed with the subsequent separation in the first separation step S3.

[0036] The mass ratio of the pulp fiber material to water added to the water is not particularly limited as long as the loosening step S2 can be carried out. Examples of the mass ratio include 0.1 to 10% by mass, and preferably 0.5 to 5% by mass. By setting the ratio to 0.1% by mass or more, the amount of water can be reduced, allowing for efficient processing. By setting the ratio to 10% by mass or less, stirring and dispersion can be facilitated.

[0037] The temperature of the water in the disintegration step S2 is not particularly limited and may be, for example, room temperature (25°C). To facilitate disintegration, the water may be heated to a temperature lower than 100°C. The treatment time is not particularly limited and may be, for example, about 1 to 60 minutes.

[0038] The specific configuration of the device for performing the disentangling step S2 is not particularly limited as long as it can immerse the pulp fiber material in water. The device may, for example, have a tank in which the pulp fiber material can be placed and which can store water, a supply means for supplying water into the tank, a stirring means for stirring the water in the tank and / or a rocking means (e.g., a rotating drum) for rocking, and a control means for controlling each of the means.

[0039] The disintegrating step S2 may include a step of separating the disintegrated pulp fiber material from the water containing the disintegrated pulp fiber material using a screen or the like.

[0040] The first separation step S3 is a step of separating the superabsorbent polymer from the pulp fiber material in water. Because the pulp fiber material becomes more easily dispersible in water, the pulp fibers, superabsorbent polymer, and plastics mixed within the pulp fiber material also become more easily dispersible in water. Here, the superabsorbent polymer is crosslinked with polyvalent metal ions, and therefore has the characteristics of being heavier in specific gravity and smaller in size than pulp fibers and plastics. Therefore, by utilizing these characteristics of the superabsorbent polymer, the superabsorbent polymer can be separated from the pulp fiber material in water.

[0041] The mass ratio of the pulp fiber material to water added to the water is not particularly limited as long as the first separation step S3 can be carried out. Examples of the mass ratio include 0.5 to 30% by mass, and preferably 1 to 20% by mass. By setting the ratio to 0.5% by mass or more, the amount of water can be reduced, allowing for efficient processing. By setting the ratio to 30% by mass or less, separation can be facilitated.

[0042] The temperature of the water in the first separation step S3 is not particularly limited and may be, for example, room temperature (25° C.) The treatment time is not particularly limited and may be, for example, about 1 to 60 minutes.

[0043] In the first separation step S3, the method for separating the superabsorbent polymer from the pulp fiber material in water may be, for example, sieving, gravity separation, or both.

[0044] In sieving, because the size of the superabsorbent polymer cross-linked with polyvalent metal ions is small and the size of the pulp fibers and plastics are large, the pulp fiber material is sieved through a sieve that allows most of the superabsorbent polymer to pass through but most of the pulp fibers and plastics to pass through. In this case, some of the larger pulp fibers may not pass through the sieve. Examples of such sieves include sieves with mesh openings of 500 to 1000 μm, with sieves with mesh openings of 600 to 900 μm being preferred. As a result, most of the plastics and pulp fibers are rejected and most of the superabsorbent polymer and plastics are accepted, increasing the mass ratio of pulp fiber to plastic in the pulp fiber material (reducing the mass ratio of superabsorbent polymer).

[0045] In gravity separation, the specific gravity of the superabsorbent polymer cross-linked with polyvalent metal ions is high, while the specific gravity of the pulp fiber and plastics is low. Therefore, when the pulp fiber material is poured into water, the superabsorbent polymer settles first, followed by the pulp fiber, which then gradually settles, and the plastic remains floating on the water surface. Therefore, after the superabsorbent polymer settles, the settled superabsorbent polymer is separated before the pulp fiber settles. Note that using an organic acid aqueous solution in the second separation step S4 instead of water is not preferable because the polyvalent metal ions in the superabsorbent polymer are released, restoring its water absorption ability and allowing it to absorb water, eliminating the difference in specific gravity with the pulp fiber.

[0046] Sieving can be carried out using a separator using a known screen. Gravity separation can be carried out using a separator using known gravity separation. An example of such an apparatus is the Narutoh Hurricane type vortex water flow gravity separator (manufactured by Nippon Seam Co., Ltd.). When sieving and gravity separation are carried out simultaneously, a separator using known gravity separation using a screen can be used. An example of such an apparatus is the Screening CSM Combi Sorter (manufactured by IHI Voith Paper Technology Co., Ltd.).

[0047] The first separation step S3 may include a step of performing solid-liquid separation of the superabsorbent polymer from the water containing the separated superabsorbent polymer using a screen, etc. The first separation step S3 may also include a step of performing solid-liquid separation of the pulp fibers and plastic from the water containing the separated pulp fibers and plastic using a screen, etc.

[0048] The second separation step S4 is a step of separating plastics and polyvalent metals from the pulp fiber material after the first separation step S3 in an organic acid aqueous solution to obtain pulp fibers. The pulp fiber material after the first separation step S3 has the superabsorbent polymer separated and mainly contains pulp fibers and plastics. Because the pulp fiber material becomes more dispersible in an organic acid aqueous solution, the pulp fibers and plastics mixed within the pulp fiber material also become more dispersible in the organic acid aqueous solution. Here, pulp fibers have the characteristics of being heavier in specific gravity and smaller in size than plastics. Therefore, by utilizing these characteristics of pulp fibers, pulp fibers can be separated from the pulp fiber material in an organic acid aqueous solution.

[0049] The organic acid used in the organic acid aqueous solution is preferably an organic acid having a carboxyl group, particularly a carboxylic acid. When the organic acid has a carboxyl group, the organic acid has one or more carboxyl groups per molecule, preferably multiple carboxyl groups. This makes it easier for the organic acid to form a chelate complex with a polyvalent metal, such as calcium, which is a metal with a valence of two or more contained in pulp fiber materials. Therefore, the organic acid can be said to have a chelating effect. In this case, the polyvalent metal is easily dissolved from the pulp fiber material in the organic acid aqueous solution and, after dissolution, is likely to remain stably in the organic acid aqueous solution. As a result, the ash content of pulp fibers produced from the pulp fiber material can be reduced.

[0050] Examples of organic acids include citric acid, tartaric acid, malic acid, succinic acid, and oxalic acid (all of which are carboxylic acids having multiple carboxyl groups), gluconic acid (C6), pentanoic acid (C5), butanoic acid (C4), propionic acid (C3), glycolic acid (C2), acetic acid (C2), glacial acetic acid, and formic acid (C1) (all of which are carboxylic acids having one carboxyl group). Among these, citric acid is preferred from the viewpoints of chelating action and cleaning effect.

[0051] The concentration of the aqueous organic acid solution is not particularly limited as long as it allows the second separation step S4 to be carried out, and may be, for example, 0.01 to 10 mass %, preferably 0.04 to 5 mass %, and more preferably 0.08 to 2 mass %.

[0052] The organic acid aqueous solution preferably has a pH within a predetermined range. Examples of pH values ​​include 0.2 to 6.0, preferably 0.4 to 5.0, and more preferably 1.0 to 4.0. A pH of 6.0 or less facilitates the removal of polyvalent metals. A pH of 0.2 or more reduces the risk of damaging pulp fibers in pulp fiber resources. The pH is measured at 25°C using, for example, a twin pH meter AS-711 manufactured by Horiba, Ltd.

[0053] The mass ratio of the pulp fiber material to the organic acid aqueous solution to be added to the organic acid aqueous solution is not particularly limited as long as the second separation step S4 can be carried out. Examples of the mass ratio include 0.5 to 30% by mass, and preferably 1 to 20% by mass. By setting the ratio to 0.5% by mass or more, the amount of the organic acid aqueous solution can be reduced, allowing for efficient processing. By setting the ratio to 30% by mass or less, separation can be facilitated.

[0054] The temperature of the aqueous organic acid solution in the second separation step S4 is not particularly limited and may be, for example, room temperature (25° C.). To increase the reaction rate, the aqueous organic acid solution may be heated to a temperature lower than 100° C. The treatment time is not particularly limited and may be, for example, about 1 to 60 minutes.

[0055] In the second separation step S4, the method for separating plastics and polyvalent metals from the pulp fiber material after the first separation step S3 in an organic acid aqueous solution may be sieving, gravity separation, or both.

[0056] In gravity separation, pulp fibers have a high specific gravity and plastics have a low specific gravity. Therefore, when pulp fiber material is placed in an organic acid solution, the pulp fibers gradually settle, while the plastics remain floating on the water surface. In particular, polyvalent metals attached to pulp fibers dissolve in the organic acid solution and can be removed from the pulp fibers. As a result, the water absorption of the pulp fibers improves, making them more likely to settle in the organic acid solution. Furthermore, polyvalent metals attached to plastics dissolve in the organic acid solution, making them more likely to be removed from the plastics. As a result, the specific gravity of the plastics decreases, improving their floating ability and making them more likely to float on the water surface in the organic acid solution. Therefore, the plastics floating on the water surface are recovered as floating matter, and the settled pulp fibers are recovered by solid-liquid separation. Therefore, by utilizing the properties of pulp fibers and plastics, pulp fibers can be separated from pulp fiber material in an organic acid solution.

[0057] In sieving, because pulp fibers are small and plastics are large, the pulp fiber material is passed through a sieve that allows most pulp fibers to pass through but not most plastics. Examples of such sieves include sieves with mesh openings of 900 to 2000 μm, with sieves with mesh openings of 1000 to 1500 μm being preferred. This results in most plastics being rejected and most pulp fibers being accepted, increasing the mass ratio of pulp fibers (reducing the mass ratio of plastics).

[0058] Sieving can be carried out using a separator using a known screen. Gravity separation can be carried out using a separator using known gravity separation. An example of such an apparatus is the Narutoh Hurricane type vortex water flow gravity separator (manufactured by Nippon Seam Co., Ltd.). When sieving and gravity separation are carried out simultaneously, a separator using known gravity separation using a screen can be used. An example of such an apparatus is the Screening Combi Sorter CSM (manufactured by IHI Voith Paper Technology Co., Ltd.).

[0059] The second separation step S4 may include a step of separating the plastic from the separated organic acid aqueous solution containing the plastic into solid and liquid using a screen or the like.

[0060] In this embodiment, a dehydration step S5 may be performed. The dehydration step S5 is a step of dehydrating the pulp fibers after the second separation step S4. In the dehydration step S5, the pulp fibers are dehydrated while undergoing solid-liquid separation from the organic acid aqueous solution containing the pulp fibers separated in the second separation step S4. At this time, the pulp fibers that have reached a certain moisture content (due to the organic acid aqueous solution) through solid-liquid separation are dehydrated by pressing (applying pressure) or the like to adjust the moisture content (due to the organic acid aqueous solution) to fall within a predetermined numerical range. Examples of the predetermined numerical range include 40 to 100% by mass, and 50 to 90% by mass is preferable. A moisture content of 40% by mass or more facilitates the migration of the oxidizing agent to the pulp fibers in the oxidizing agent treatment step S6 described below. A moisture content of 100% by mass or less makes it less likely that the migration of the oxidizing agent to the pulp fibers is inhibited.

[0061] The temperature of the water in the dehydration step S5 is not particularly limited and may be, for example, room temperature (25° C.) The treatment time is not particularly limited and may be, for example, about 1 to 60 minutes.

[0062] The specific configuration of the device for performing the dehydration step S5 is not particularly limited as long as it can dehydrate the pulp fibers in the organic acid aqueous solution while separating them into solid and liquid. Examples of such a device include a screw press dehydrator, specifically, the Reject Processing EX E-Compax (manufactured by Voith IHI Paper Technology Co., Ltd.).

[0063] The oxidizing agent treatment step S6 is a step in which the dehydrated pulp fibers are treated with an oxidizing agent. In the oxidizing agent treatment step S6, the pulp fibers that have been dehydrated to a moisture content within a predetermined range in the dehydration step S5 and are in a semi-dry state containing a certain amount of moisture are brought into contact with an oxidizing agent to be bleached, sterilized, and deodorized. In particular, if the above-mentioned organic acid (aqueous solution) remains in the dehydrated pulp fibers, the bleaching, sterilization, and deodorization are promoted, and bleaching is particularly promoted.

[0064] The oxidizing agent is not particularly limited as long as it has bleaching, sterilizing, and deodorizing effects, and examples thereof include at least one of hydrogen peroxide, ozone, percarbonate, peracetic acid, chlorine dioxide, and hypochlorous acid-based agents. When the pulp fibers are used in sanitary products, the oxidizing agent is preferably hydrogen peroxide or ozone.

[0065] The method for supplying the oxidizing agent is not particularly limited as long as the oxidizing agent can come into contact with the pulp fibers, but examples include a method in which the oxidizing agent is supplied as an aqueous oxidizing agent solution. The pulp fibers may be immersed in the aqueous oxidizing agent solution stored in a tank, or the aqueous oxidizing agent solution may be sprayed onto the pulp fibers. Spraying provides a similarly high bleaching effect as immersion, but is more efficient because a small amount of the aqueous oxidizing agent solution can be distributed throughout the pulp fibers in a short time.

[0066] The concentration of the oxidizing agent aqueous solution is not particularly limited as long as the oxidizing agent treatment step S6 can be carried out. For example, when the oxidizing agent is hydrogen peroxide, the acid (hydrogen peroxide) concentration of the oxidizing agent aqueous solution (hydrogen peroxide solution) can be 1 to 30% by mass, preferably 2 to 20% by mass, and more preferably 3 to 10% by mass. A concentration of 1% by mass or more can easily remove bacteria and other organic matter, and a concentration of 30% by mass or less can suppress damage to the pulp fibers. The treatment time is not particularly limited and is, for example, about 1 to 60 minutes.

[0067] For example, when the oxidizing agent is ozone, the acid (ozone) concentration of the oxidizing agent aqueous solution (ozone water) is 0.3 to 2 mass ppm, more preferably 0.5 to 1.5 mass ppm. A concentration of 0.3 mass ppm or more facilitates the removal of bacteria and other organic matter, while a concentration of 2 mass% or less suppresses damage to the pulp fibers. The contact time between the ozone water and the pulp fibers is not particularly limited as long as it is sufficient to remove bacteria and other organic matter adhering to the surface of the pulp fibers. However, the contact time is preferably shorter when the ozone concentration in the ozone water is high and longer when the ozone concentration is low. The contact time is preferably 0.3 to 15 minutes, more preferably 0.5 to 10 minutes. The product of the ozone concentration (ppm) and the contact time (minutes) in the ozone water (hereinafter also referred to as the "CT value") is preferably 0.1 to 30 ppm·min, more preferably 0.2 to 15 ppm·min. A CT value of 0.1 ppm min or more makes it possible to easily remove bacteria and other organic matter, and a CT value of 30 ppm min can suppress damage to pulp fibers. Examples of ozone generators include the ED-OWX-2 Ozone Water Exposure Tester manufactured by Ecodesign Co., Ltd. and the OS-25V Ozone Generator manufactured by Mitsubishi Electric Corporation.

[0068] The aqueous oxidizing agent solution preferably has a pH within a predetermined range. Examples of pH include 0.2 to 6.0, preferably 0.4 to 5.0, and more preferably 1.0 to 4.0. A pH of 6.0 or less allows the oxidizing agent to easily exert its function. A pH of 0.2 or more makes it difficult for the pulp fibers to be damaged.

[0069] The temperature of the aqueous oxidizing agent solution in the oxidizing agent treatment step S6 is not particularly limited and may be, for example, room temperature (25° C.). In order to enhance the effect of the oxidizing agent, the aqueous oxidizing agent solution may be heated to a temperature range lower than 100° C.

[0070] When immersing the pulp fibers in the aqueous oxidant solution, the specific configuration of the device for performing the oxidant treatment step S6 is not particularly limited as long as it can immerse the pulp fibers in the aqueous oxidant solution. The device may, for example, have a tank in which the pulp fibers can be placed and in which the aqueous oxidant solution can be stored, a supply means for supplying the aqueous oxidant solution into the tank, a stirring means and / or a rocking means for rocking the aqueous oxidant solution in the tank, a heating means for heating the aqueous oxidant solution in the tank, and a control means for controlling each means.

[0071] When spraying the oxidant aqueous solution onto the pulp fibers, the specific configuration of the device for performing the oxidant treatment step S6 is not particularly limited as long as it can spray the oxidant aqueous solution onto the pulp fibers. The device may, for example, include a horizontally placed cylindrical portion, multiple blades disposed inside the cylindrical portion, a liquid supply unit disposed on the upper outer peripheral surface of the cylindrical portion, a pulp supply port disposed on the upper outer peripheral surface of the cylindrical portion on the upstream side, a pulp discharge port disposed on the lower outer peripheral surface of the cylindrical portion on the downstream side, and a heating unit. The multiple blades are disposed approximately perpendicular to the cylindrical axis and aligned in the axial direction of the cylindrical portion, and / or are disposed spirally relative to the cylindrical axis and rotate around the cylindrical axis. The liquid supply unit has multiple liquid supply ports aligned in the axial direction of the cylindrical portion and facing the interior of the cylindrical portion. The heating unit heats the interior of the cylinder. The pulp fibers supplied from the pulp supply port to the upstream side of the cylindrical section are heated, conveyed downstream by the blades, and discharged from the downstream side of the cylindrical section through the pulp discharge port while being sprayed (distributed) with an aqueous oxidizing agent solution from the liquid supply ports. An example of such a device is the Dispersion PLMS Speed ​​Heater (manufactured by Voith IHI Paper Technology Co., Ltd.).

[0072] The oxidizing agent treatment step S6 may include a rinsing step of rinsing the oxidizing agent-treated pulp fibers with water and a dewatering step of dehydrating the rinsed pulp fibers, which can be performed using a known rinsing device or dewatering device.

[0073] The drying step S7 dries the pulp fibers treated in the oxidizing agent treatment step S6. The drying step S7 is carried out by placing the mixture in a dry atmosphere at a temperature higher than room temperature or by blowing dry air at a temperature higher than room temperature onto the mixture. The drying temperature can be, for example, 80 to 120°C, and preferably 100 to 120°C. A drying temperature of 80°C or higher can shorten the drying time. A drying temperature of 120°C or lower can suppress adhesion between components and thermal deterioration of the components. The drying time can be, for example, 30 to 300 minutes. The drying step S7 may be carried out under reduced pressure, for example, at 0.1 to 100 kPa, to accelerate drying. The moisture content of the pulp fibers after the drying step S7 can be, for example, 10 to 70% by mass, and preferably 15 to 50% by mass. A moisture content of 10% by mass or higher makes the mixture less likely to powder and easier to handle. A moisture content of 70% by mass or less makes it difficult for the pulp fibers and water to separate, making handling easier, and also reduces the amount of water transported during transportation, improving transportation efficiency.In addition, a moisture content of 15% by mass or less can suppress the growth of corrosion and mold.

[0074] Through the above steps, pulp fibers can be produced using the pulp fiber material derived from used absorbent articles according to the embodiment.

[0075] The produced pulp fibers can be used as a raw material for tissue, sheet pulp, roll pulp, etc.

[0076] In this method, a pulp fiber material containing impurities is prepared (preparation step S1), and then a superabsorbent polymer is first separated from the pulp fiber material in water (first separation step S3). Then, plastics and polyvalent metals are separated from the pulp fiber material in an organic acid aqueous solution (second separation step S4). If the superabsorbent polymer is first separated from the pulp fiber material in an organic acid aqueous solution, polyvalent metal ions are desorbed from the superabsorbent polymer, causing the superabsorbent polymer to absorb water, making it difficult to separate the superabsorbent polymer from the pulp fiber material. Therefore, in this method, the superabsorbent polymer is first separated from the pulp fiber material in water (first separation step S3), which allows the superabsorbent polymer crosslinked with polyvalent metal ions to be easily separated from the pulp fiber material in water. Furthermore, the polyvalent metals contained in the pulp fibers are dissolved in the organic acid aqueous solution. Therefore, in this method, plastics and polyvalent metals are then separated in an organic acid aqueous solution from the pulp fiber material from which the superabsorbent polymer has been separated (second separation step S4), making it possible to easily remove the polyvalent metals from the pulp fiber material along with the easily separable plastics. In this way, in this method, the interactive relationship between the first separation step and the second separation step allows the superabsorbent polymer, plastics, and polyvalent metals to be removed from the pulp fiber material and pulp fibers to be formed, making it possible to improve the quality of the pulp fibers derived from used absorbent articles.

[0077] In a preferred embodiment of this method, the first separation step S3 includes separating the superabsorbent polymer from the pulp fiber material by sieving in water, gravity separation, or both sieving and gravity separation. The superabsorbent polymer crosslinked with polyvalent metal ions is smaller in size and has a higher specific gravity than the remaining pulp fiber material (pulp fibers and plastics). Therefore, in water, the superabsorbent polymer settles most quickly, the pulp fibers settle slowly, and the plastics settle very little. Therefore, the superabsorbent polymer can be easily separated from the pulp fiber material by sieving and / or gravity separation.

[0078] In a preferred embodiment of this method, the second separation step S4 includes separating plastics and polyvalent metals from the pulp fiber material from which the superabsorbent polymer has been separated by sieving in an organic acid aqueous solution, gravity separation, or both sieving and gravity separation. Here, the polyvalent metals contained in the pulp fibers of the pulp fiber material are dissolved in the organic acid aqueous solution and removed from the pulp fibers. This improves the water absorption of the pulp fibers, making them more likely to settle in the organic acid aqueous solution. Meanwhile, the polyvalent metals contained in the plastics are also dissolved in the organic acid aqueous solution and removed from the plastics. This improves the floating ability of the plastics (reducing their specific gravity), making them more likely to float to the surface of the organic acid aqueous solution. Furthermore, pulp fibers are smaller in size than plastics. Therefore, by sieving and / or gravity separation, the pulp fibers and plastics can be easily separated from each other in the pulp fiber material from which the superabsorbent polymer has been separated.

[0079] In a preferred embodiment of this method, the organic acid aqueous solution in the second separation step S4 is an aqueous solution containing an organic acid (chelating agent) having a chelating effect. Therefore, polyvalent metal ions eluted from the superabsorbent polymer and polyvalent metals eluted from the pulp fiber material (pulp fiber or plastic) can be stably present in the organic acid aqueous solution due to the chelating effect. This prevents the eluted polyvalent metals from being reabsorbed into the superabsorbent polymer, pulp fiber, or plastic.

[0080] In a preferred embodiment of this method, the organic acid in the organic acid aqueous solution in the second separation step S4 is citric acid. Therefore, polyvalent metal ions eluted from the superabsorbent polymer and polyvalent metals eluted from the pulp fiber material (pulp fiber and plastic) can be more stably present in the organic acid aqueous solution due to the chelating action. This further prevents the eluted polyvalent metals from being reabsorbed into the superabsorbent polymer, pulp fiber, and plastic.

[0081] The pulp fiber material before the first separation step S3 may be partially or entirely agglomerated due to hydrogen bonding between pulp fibers, etc. In such cases, it is difficult to separate the superabsorbent polymer, plastic, and polyvalent metal from the agglomerated pulp fiber material. Therefore, in a preferred embodiment of the present method, in such cases, the pulp fiber material is agitated in water in advance (loosening step S2) before the first separation step S3. This loosens the agglomerated pulp fiber material, making it easier to separate the superabsorbent polymer from the pulp fiber material in the subsequent first separation step S3, and easier to separate the plastic and polyvalent metal from the pulp fiber material in the second separation step S4.

[0082] In a preferred embodiment of this method, the pulp fibers in a dehydrated state (dehydration step S5) after the second separation step S4 are treated with an oxidizing agent (oxidizing agent treatment step S6). In this case, the dehydrated state is different from the dried state and refers to a state containing a certain amount of moisture (organic acid aqueous solution) (semi-dry state). Therefore, the pulp fibers can be contacted with the oxidizing agent at a higher concentration than when the pulp fibers are contacted with the oxidizing agent in an aqueous solution. This allows the pulp fibers to achieve high bleaching, sterilizing, and deodorizing effects, and these effects can be achieved with a short treatment time and a small amount of oxidizing agent. Furthermore, since the dehydrated pulp fibers contain the organic acid aqueous solution, the treatment time required for achieving the bleaching, sterilizing, and deodorizing effects in the oxidizing agent treatment can be shortened. In this way, the pulp fibers obtained from the pulp fiber material are bleached, sterilized, and deodorized in this method, thereby improving the quality of the pulp fibers derived from used absorbent articles.

[0083] In a preferred embodiment of the present method, the oxidizing agent includes at least one of hydrogen peroxide, ozone, percarbonate, peracetic acid, chlorine dioxide, and hypochlorous acid-based agents, thereby enabling the present method to achieve higher bleaching, sterilizing, and deodorizing effects on pulp fibers, and to achieve these effects with a shorter treatment time and a smaller amount of oxidizing agent.

[0084] The present invention will be described below based on examples, but the present invention is not limited to these examples.

[0085] (a) Samples (a) Comparative Example 1: A pulp fiber material was prepared. The mass ratio of pulp fibers to impurities (impurities) such as other plastics in the pulp fiber material was 60:40. (b) Example 1: Based on the pulp fiber material of Comparative Example 1, a method for producing pulp fibers using pulp fiber material derived from used absorbent articles according to the above-mentioned embodiment was carried out. However, in Example 1, the oxidizing agent treatment step S6 (using ozone) was not carried out. The mass ratio of pulp fibers to impurities (impurities) in the pulp fiber material was 97:3. (c) Example 2: Based on the pulp fiber material of Comparative Example 1, pulp fibers were produced using pulp fiber material derived from used absorbent articles according to the embodiment. However, in Example 2, the oxidizing agent treatment step S6 (using ozone) was carried out. The mass ratio of pulp fibers to impurities (impurities) in the pulp fiber material was the same as in Example 1.

[0086] (b) Evaluation The obtained pulp fiber material or pulp fiber was evaluated as follows. The evaluation items were appearance / properties (color, foreign matter / off-odor), color elution, pH, fluorescence, ash content, Kjeldahl nitrogen, general bacteria, E. coli, moisture content, arsenic, and heavy metals. The evaluation methods (measurement methods) for each were as follows. However, the standard values ​​for appearance / properties, color elution, pH, fluorescence, Kjeldahl nitrogen, general bacteria, E. coli, arsenic, and heavy metals were based on the quality standards of JIS S 0261:2024 (Recycled pulp for urine absorption products). The ash content was based on the Ministry of Health, Labor, and Welfare's standard for sanitary treatment product materials. The moisture content was a reference value.

[0087] <Method for measuring appearance and properties (color, foreign matter, and unpleasant odor)> Measurement was carried out in accordance with 5.3 Appearance and properties of JIS S 0261:2024 (recycled pulp for urine absorbing products).

[0088] <Method for measuring color elution> This was carried out in accordance with 5.4 Color elution of JIS S 0261:2024 (Recycled pulp for urine absorbing products).

[0089] <Method of measuring pH> The pH was measured in accordance with 5.5 pH of JIS S 0261:2024 (recycled pulp for urine absorbing products).

[0090] <Method of Measuring Fluorescence> The measurement was carried out in accordance with 5.6 Fluorescence Test of JIS S 0261:2024 (Recycled Pulp for Urine Absorbent Products).

[0091] <Method for measuring ash content> Ash content refers to the amount of inorganic or non-combustible residue remaining after organic matter has been incinerated. Ash content is measured in accordance with "5. Ash content test method" under "2. General test methods" in the Standards for Sanitary Treatment Product Materials. Specifically, the procedure is as follows: (1) Ignite a platinum, quartz, or porcelain crucible at 500-550°C for one hour, allow it to cool, and measure its mass, W0. (2) Take 2-4 g of sample (pulp fiber material or pulp fiber), place it in the crucible, and measure its mass, W1. (3) If necessary, remove or shift the lid of the crucible, and heat it gently at first, then gradually increase the temperature and ignite it at 500-550°C for four hours or more until no char remains, and incinerate it. (4) Allow it to cool, and measure its mass. (5) Carbonize the residue again, allow it to cool, and measure its mass. Repeat carbonization, cooling, and weighing until a constant mass is reached. (6) The mass after constant mass is designated as W2. (7) Calculate the ash content (mass%) using the following formula: Ash content (mass%) = (W2 - W0) / (W1 - W0) x 100

[0092] <Method for measuring Kjeldahl nitrogen> Measurement was carried out in accordance with 5.9 Kjeldahl nitrogen (KN) test of JIS S 0261:2024 (recycled pulp for urine absorbing products).

[0093] <Method for measuring general bacteria> Measurement was carried out in accordance with 5.10 General bacteria of JIS S 0261:2024 (recycled pulp for urine absorbing products).

[0094] <Method for measuring coliform bacteria> The measurement was carried out in accordance with 5.11 Escherichia coli of JIS S 0261:2024 (recycled pulp for urine absorbing products).

[0095] <Method for measuring moisture content> Measurements are carried out in an atmosphere of 20°C ± 1°C. (1) Measure the mass A of the container (open container) in which the sample (pulp fiber material or pulp fiber) will be placed. (2) Prepare approximately 5 g of sample and place it in the container whose mass was measured in (1), and measure the mass B of the container containing the sample. (3) Place the container containing the sample in an oven set to a temperature of 105°C ± 3°C for 2 hours. (4) Remove the container containing the sample from the oven and place it in a desiccator (containing a desiccant: colored silica gel) for 30 minutes. (5) Remove the container containing the sample from the desiccator and measure the mass C of the container containing the sample. (6) Calculate the moisture content (mass %) using the following formula: Moisture content (mass %) = (B - C) / (C - A) x 100

[0096] <Method of Arsenic Test> The test was carried out in accordance with 5.7 Arsenic test of JIS S 0261:2024 (Recycled pulp for urine absorbing products).

[0097] <Method of Heavy Metal Test> The test was carried out in accordance with 5.8 Heavy Metal Test of JIS S 0261:2024 (Recycled Pulp for Urine Absorbent Products).

[0098] <Method for measuring the mass ratio of pulp fiber, superabsorbent polymer, and plastic in a pulp fiber material> First, the dry mass of the sample was measured (A), then the pulp fiber in the sample was dissolved and removed and the dry mass was measured (B), and then the superabsorbent polymer in the sample was dissolved and removed and the dry mass was measured (C), and the mass ratio of pulp fiber, superabsorbent polymer, and plastic was calculated from these measured values. However, when it was sufficient to determine the mass ratio of pulp fiber to other impurities (impurities), the dry mass of the sample was measured (A), then the pulp fiber in the sample was dissolved and removed and the dry mass was measured (B), and the mass ratio of pulp fiber to impurities (impurities) was calculated from these measured values.

[0099] First, we will provide an overview of (A) to (C). (A) Dry mass of sample (total mass): The dry mass of the initial sample (plastic, pulp fiber, and superabsorbent polymer) is measured. If the sample is liquid, the sample is filtered using a glass filter or similar, dried, and the mass of the residue is measured to determine the dry mass of the sample. (B) Dry mass after sulfuric acid test (mass after dissolution of pulp fiber): According to the sulfuric acid test method, pulp and other components in the sample are decomposed using 70% sulfuric acid, filtered through a glass filter, and the mass of the residue (plastic and superabsorbent polymer) after drying is measured. This mass is subtracted from the dry mass of the sample in (A) to calculate the dry mass of the pulp fiber. (C) Dry mass after hydrogen peroxide test (mass after dissolution of superabsorbent polymer): According to the hydrogen peroxide test method, the superabsorbent polymer and decomposable organic substances are decomposed using 9% hydrogen peroxide, filtered through a glass filter, and dried, and the mass of the residue (plastic) is measured. The dry mass of the superabsorbent polymer is calculated by subtracting the mass from the dry mass of the sample (B).

[0100] Next, details of (A) will be explained. (A) Sulfuric Acid Test (70% Sulfuric Acid Method (Pulp Dissolution Test Method)) A sample is treated with 70% sulfuric acid to dissolve the pulp, and the pulp mass and (plastic + SAP) mass are roughly determined by gravimetric analysis. The measurement method is as follows: (1) Prepare the sample (solid) to be measured. If the sample is not solid, collect the solids by suction filtration or other methods. (If a glass filter (described below) is used to collect the sample by suction filtration, use the glass filter as the sample for the next step, and perform calculations taking the mass of the glass filter into account.) (2) Prepare a glass filter G100 (90 mm diameter) and measure the mass M1 of the glass filter before use. (3) Place 1 g of the sample (dry mass) in a 500 ml heat-resistant bottle, and then add 100 g of 70% sulfuric acid by mass. (4) Stir the sulfuric acid containing the sample for one hour. If the pulp does not dissolve, slightly increase the temperature of the sulfuric acid and shake the heat-resistant bottle to dissolve it. (5) Attach a glass filter to the suction filtration device (including a suction filtration pump, a drainage bottle, etc.) and moisten it with water. (6) Pour the sulfuric acid containing the sample into the glass filter and filter it by suction. (7) Rinse twice with 50 ml of 70% by weight sulfuric acid, and then rinse with 500 ml of water to wash away the sulfuric acid. (8) Remove the glass filter with tweezers and transfer it to a petri dish. The glass filter contains the residue of the sample that was not dissolved in the sulfuric acid. (9) Dry the glass filter in an oven at 105°C for 1 to 2 hours. (10) Measure the mass M2 of the glass filter (including the residue of the sample). (11) Calculate the mass M01 of the residue of the sample using the following formula: Mass of residue M01 (roughly the mass of plastic + superabsorbent polymer) = M2 - M1 Mass of dissolved material M02 (roughly the mass of pulp) = 1 (g) - M01

[0101] Next, we will explain the details of (B). (B) Hydrogen Peroxide Method (Test Method for Dissolving Superabsorbent Polymers and Organic Matter) By dissolving the superabsorbent polymer and organic matter with hydrogen peroxide, the mass of the superabsorbent polymer and the mass of the plastic are roughly determined gravimetrically. The measurement method is as follows: (1) Measure the mass N1 (M2) of the glass filter (including sample residue) from (10) of the sulfuric acid test above. This glass filter contains a sample from which the pulp fibers have been removed and the plastic and superabsorbent polymer remain. This glass filter is used as is as the sample, and calculations are performed taking into account the mass of the glass filter. (2) Place the entire glass filter (including sample residue) in a 500 ml heat-resistant bottle, and then add 150 g of water. Shake the heat-resistant bottle well to allow the glass filter and water to blend together. (3) Check the pH of the water (hereinafter simply referred to as the "aqueous solution") containing the glass filter (including sample residue) in the heat-resistant bottle using pH test paper or similar to confirm that it is approximately pH 4 to 7. If the solution is more acidic or alkaline than pH 4-7, neutralize it with NaOH (if acidic) or dilute sulfuric acid (if alkaline). (4) After allowing the glass filter and water to soak for at least 10 minutes, add 4 ml of 20% by weight copper sulfate solution to the solution, shake well, and leave for at least 10 minutes. (5) Next, add 50 g of 30-35% by weight hydrogen peroxide solution to the solution, seal the bottle, and stir for 3 hours. Shake occasionally by hand to stir, loosening the stopper as needed to release oxygen. (6) Further, add 10 g of 35% by weight hydrogen peroxide solution to the solution and stir for 3 hours. Loosen the stopper as needed to release oxygen. (7) Repeat step (6) above to allow the solution to react for a total of 2 days (48 hours). (8) After the reaction, add 5 ml of dilute sulfuric acid to the solution. (9) Attach a glass filter G100 (90 mm diameter) to the suction filtration device (including a suction filtration pump, a drainage bottle, etc.) and moisten it with water. Measure the mass N2 of the glass filter before use. (10) Pour the aqueous solution into the glass filter and suction filter it. (11) Rinse with 500 ml of water to wash away the aqueous solution. (12) Remove the glass filter with tweezers and transfer it to a petri dish.(13) Dry the glass filter in an oven at 105°C for 1 to 2 hours. (14) Measure the mass N3 of the glass filter (including the sample residue). (15) Calculate the mass N01 of the sample residue using the following formula: Mass N01 of the residue (generally the mass of plastic) = N3 - N2 Mass N02 of the dissolved material (generally the mass of the superabsorbent polymer) = N1 - N01.

[0102] From the above results, the mass ratio of pulp fiber to impurities (impurities) is pulp fiber:impurities (impurities) = M02:M01. Also, the mass ratio of pulp fiber, superabsorbent polymer, and plastic is pulp fiber:superabsorbent polymer:plastic = M02:N02:N01.

[0103] (c) Results The measurement results are shown in Table 1 below.

[0104]

[0105] As shown in Table 1, it was found that the samples (pulp fibers) of Examples 1 and 2 exhibited good quality, an improvement over the quality of Comparative Example 1. It was also found that the pulp fibers of Examples 1 and 2 can be used in sanitary products.

[0106] The absorbent material and manufacturing method of the present invention are not limited to the above-mentioned embodiments and examples, and can be appropriately combined, modified, etc. within the scope of the purpose and intent of the present invention. In this specification, ordinal numbers such as "first" and "second" are used to distinguish items to which the ordinal numbers are assigned, and do not indicate the order, priority, importance, etc. of each item.

[0107] S1 Preparation process S3 First separation process S4 Second separation process

Claims

1. A method for producing pulp fibers using a pulp fiber material derived from used absorbent articles, comprising: a preparation step of preparing the pulp fiber material containing pulp fibers and impurities, the impurities including a superabsorbent polymer cross-linked with polyvalent metal ions, plastics, and polyvalent metals; a first separation step of separating the superabsorbent polymer from the pulp fiber material in water; and a second separation step of separating the plastics and the polyvalent metals from the pulp fiber material after the first separation step in an aqueous organic acid solution to obtain pulp fibers.

2. The method of claim 1, wherein the first separation step comprises separating the superabsorbent polymer from the pulp fiber material by sieving and / or gravity separation in the water.

3. The method according to claim 1 or 2, wherein the second separation step includes separating the plastics and polyvalent metals from the pulp fiber material after the first separation step by sieving and / or gravity separation in the organic acid aqueous solution.

4. The method according to claim 3, wherein the organic acid aqueous solution is an aqueous solution containing an organic acid having a chelating effect.

5. The method of claim 4, wherein the organic acid is citric acid.

6. The method according to any one of claims 1 to 5, further comprising a disentangling step of agitating the pulp fiber material in water prior to the first separation step.

7. The method according to any one of claims 1 to 6, further comprising: a dewatering step of dewatering the pulp fibers after the second separation step; and an oxidizing agent treatment step of treating the pulp fibers in the dewatered state with an oxidizing agent.

8. The method of claim 7, wherein the oxidizing agent comprises at least one of hydrogen peroxide, ozone, percarbonate, peracetic acid, chlorine dioxide, and hypochlorous acid-based agents.

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