Method for separating water-absorbing resin and pulp fiber, device for separating water-absorbing resin and pulp fiber, and method for producing regenerated water-absorbing resin

Abstract

A method for separating a water-absorbing resin and a pulp fiber comprises a separation step in which a mixture that contains a dehydrated water-absorbing resin and a pulp fiber, both derived from a used absorbent article, is brought into contact with a separation liquid that flows from the lower side in the vertical direction toward the upper side in the vertical direction at a flow rate of 0.08 m / min to 0.40 m / min, thereby separating the dehydrated water-absorbing resin and the pulp fiber in the separation liquid. The separation liquid has a density of 0.6 g / cm3 to 1.2 g / cm3.

Description

Method for separating water-absorbent resin from pulp fibers, device for separating water-absorbent resin from pulp fibers, and method for producing recycled water-absorbent resin

[0001] The present invention relates to a method for separating a water-absorbent resin from pulp fibers, an apparatus for separating a water-absorbent resin from pulp fibers, and a method for producing recycled water-absorbent resin.

[0002] Disposable absorbent articles (typically sanitary products) generally contain pulp fibers and a water-absorbent resin between a water-impermeable cover sheet and a water-permeable nonwoven fabric, and the water-absorbent resin absorbs water and swells, thereby absorbing bodily waste. After use, such absorbent articles are typically disposed of by incineration or landfilling. However, recently, research has been conducted into the recovery and reuse of components from used absorbent articles. For example, Patent Document 1 discloses a water-absorbent resin decomposition technology that focuses on scission of crosslinks within the water-absorbent resin, and a technology for recovering water-soluble recycled polymers obtained by decomposing the water-absorbent resin as aggregates by crosslinking with multivalent metal ions. When recovering the water-absorbent resin from used absorbent articles, a typical method involves crushing the used absorbent article to obtain a mixture of the water-absorbent resin and pulp, and then separating the water-absorbent resin and pulp fibers from the mixture. For example, Patent Document 2 discloses a method for separating the water-absorbent resin and pulp fibers contained in used absorbent articles using a drum screen separator (sieve) by utilizing the difference in size between the two.

[0003] JP 2020-049398 A JP 2019-084470 A

[0004] When separating a water-absorbent resin from a mixture of a water-absorbent resin and pulp fibers, the separation method disclosed in Patent Document 2 (a method of separating the water-absorbent resin and pulp fibers using a sieve due to the difference in size) has a problem that it is difficult to separate and recover a high-purity water-absorbent resin.

[0005] One aspect of the present invention relates to a method for separating a water-absorbent resin and pulp fibers, which can recover a highly pure water-absorbent resin from a mixture of the water-absorbent resin and pulp fibers derived from a used absorbent article. Another aspect of the present invention relates to a separation device used in the separation method and a method for producing a recycled water-absorbent resin using the water-absorbent resin obtained by the separation method.

[0006] This specification includes at least the following inventions [1] to [6]: [1] A method for separating a mixture of a dehydrated water-absorbent resin and pulp fibers derived from a used absorbent article, the mixture being brought into contact with a separating liquid flowing vertically from the lower side to the upper side at a flow rate of 0.08 m / min to 0.40 m / min, thereby separating the dehydrated water-absorbent resin and the pulp fibers in the separating liquid, the density of which is 0.6 g / cm ３ ~1.2 g / cm ３[2] The separation method according to [1], wherein the separation liquid is an aqueous solution containing a swelling inhibitor for the dehydrated water absorbent resin. [3] The separation method according to [1] or [2], further comprising, after the separation step, a recovery step of recovering the pulp fibers separated in the separation liquid, wherein the recovery step includes a step of recovering a pulp mixture liquid containing the pulp fibers and the separation liquid downstream of the contact position of the separation liquid with the mixture in the flow path of the separation liquid. [4] The separation method according to [3], further comprising a purification step of removing the pulp fibers from the pulp mixture liquid recovered in the recovery step, and a return step of returning the purified separation liquid purified in the purification step to the separation step. [5] A separation device for separating a water-absorbent resin and pulp fibers, the separation device comprising: a first tank having an outlet and configured so that a separated liquid contained therein can flow vertically from the lower side to the upper side in the vertical direction, a second tank for accommodating the separated liquid flowing out of the outlet of the first tank, a refining section for removing pulp fibers from the separated liquid contained in the second tank, and a return section for returning the purified separated liquid purified by the refining section to the first tank. [6] A method for producing a recycled water-absorbent resin, the method comprising: a preparation step for preparing a water-absorbent resin separated by the separation method according to any one of [1] to [4]; a cleavage step for obtaining a polymer by cleaving a crosslinked structure of a crosslinked polymer contained in the water-absorbent resin; and a crosslinking step for crosslinking the polymer.

[0007] According to one aspect of the present invention, a separation method can be provided that can recover a highly pure water-absorbent resin from a mixture of dehydrated water-absorbent resin and pulp fibers.

[0008] FIG. 1 is a schematic diagram showing an example of a separation apparatus of the present invention, showing a state in which a mixture is contained in a first tank and a separated liquid is contained in a second tank before the start of a spin-drying process. FIG. 2 is a schematic diagram showing a state in which a separation process is being carried out using the separation apparatus of FIG. 1. The outline arrows in the figure indicate the direction in which the separated liquid flows. FIG. 3 is a schematic diagram showing only the first tank of a separation apparatus according to a first modified example. FIG. 4 is a schematic diagram showing only the first tank of a separation apparatus according to a second modified example. FIG. 5 is a schematic diagram showing a separation apparatus according to a third modified example, showing a state in which the separated liquid is contained in the second tank before the start of a spin-drying process. FIG. 6 is a schematic diagram showing a separation apparatus used in a comparative example, showing a state in which the separated liquid is contained in a 10 L bucket before the start of a spin-drying process.

[0009] Hereinafter, embodiments of the present invention will be described. However, the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the present invention.

[0010] The definitions of each term in this specification are as follows. "Top" refers to the top side in the vertical direction (the direction of gravity), and "bottom" refers to the bottom side in the vertical direction. The device shown in each figure is described assuming that the device is placed on a horizontal surface. Unless otherwise specified, the various parameters disclosed in this specification are values ​​measured under an atmosphere of 1 atmosphere, normal temperature, and normal humidity. 1 atmosphere is 101,325 Pa, "normal temperature" means 25°C, and "normal humidity" means 50% RH. The term "process" includes not only independent processes but also processes that cannot be clearly distinguished from other processes, as long as the intended effect of the process is achieved. "(Meth)acrylic" means at least one of acrylic and its corresponding methacrylic. The same applies to other similar expressions such as "(meth)acrylate." "(Poly)" means both cases with and without the prefix "poly." The materials exemplified in this specification may be used alone or in combination of two or more. When a composition contains multiple substances corresponding to each component, the content of each component refers to the total amount of the multiple substances present in the composition unless otherwise specified. A numerical range "A or greater" means a range exceeding A and A. A numerical range "A or less" means a range exceeding A and A. In the numerical ranges described in stages in this specification, the upper or lower limit of a certain numerical range can be arbitrarily combined with the upper or lower limit of a numerical range of another stage. In the numerical ranges described in this specification, the upper or lower limit of the numerical range may be replaced with the value shown in the examples.

[0011] The method for separating a water-absorbent resin and pulp fibers of the present invention includes a separation step of contacting a mixture containing a dehydrated water-absorbent resin and pulp fibers derived from a used absorbent article (hereinafter, sometimes simply referred to as a mixture) with a separating liquid flowing vertically from the bottom to the top at a flow rate of 0.08 m / min to 0.40 m / min, thereby separating the dehydrated water-absorbent resin and pulp fibers in the separating liquid. The separation step can also be said to be a step of contacting the mixture with a separating liquid flowing in the opposite direction to the settling direction of the mixture at a flow rate of 0.08 m / min to 0.40 m / min, thereby separating the dehydrated water-absorbent resin and pulp fibers in the separating liquid. The separation step can also be said to be a step of separating the water-absorbent resin and the pulp fibers dehydrated in the separation liquid by bringing the mixture into contact with a separation liquid that flows vertically from the lower side to the upper side (in the opposite direction to the settling direction of the mixture) at a flow velocity that is greater than the settling velocity of the pulp fibers and less than the settling velocity of the water-absorbent resin. The separation step is typically achieved by a separation device for the water-absorbent resin and the pulp fibers. The separation step will be specifically described below, showing an example of the separation device of the present invention.

[0012] <Separation Device> As shown in FIG. 1 , the separation device 100 of the present invention includes a first tank A having an outlet A4 and configured so that the separated liquid S contained therein can flow vertically from the lower side to the upper side, a second tank B that contains the separated liquid S that flows out from the outlet A4 of the first tank A, a refining section C that removes pulp fibers from the separated liquid S contained in the second tank B, and a return section D that returns the separated liquid S refined by the refining section C (purified separated liquid S2 in FIG. 2 ) to the first tank A.

[0013] (First Tank) In this embodiment, the first tank A has a bottom portion A1, a side portion A2 extending upward from the periphery of the bottom portion A1, an inlet A3 provided at the lower end of the side portion A2, an outlet A4 provided at the upper end of the side portion A2, and a top portion A5. The bottom portion A1 is a portion where a mixture M of dehydrated water-absorbent resin and pulp fibers is placed. A separated liquid S is contained inside the first tank A through the inlet A3 provided at the lower end of the side portion A2 (described later), and contacts the mixture M at the bottom portion A1. In this embodiment, the bottom portion A1 is formed in a flat plate shape parallel to the horizontal plane. The side portion A2, in cooperation with the bottom portion A1 and the top portion A5, serves as a partition wall that partitions the first tank A so that the separated liquid S contained inside the first tank A does not flow out of the first tank A from any location other than the outlet A4 (described later). In this embodiment, the side portion A2 is formed in a cylindrical shape extending vertically and having lower and upper edges with the same shapes as the periphery of the bottom portion A1. As the separated liquid S flows from bottom to top inside the side portion A2, the mixture M placed on the bottom portion A1 also moves from bottom to top, separating the water-absorbent resin M1 and the pulp fibers M2 (see FIG. 2 ). The inlet A3 is a portion to which the separated liquid S is supplied from outside the first tank A. In this embodiment, the lower end of the side portion A2 is provided with a hole-shaped inlet A3, which is connected to the return section D described below. The outlet A4 is a portion through which the separated liquid S flowing from bottom to top inside the first tank A (inside the side portion A2) is discharged to the outside of the first tank A, and is provided above the position where the inlet A3 is provided (in this embodiment, the lower end of the side portion A2). In this embodiment, a hole-shaped outlet A4 is provided at the upper end of the side portion A2, and the separated liquid S flowing out from the outlet A4 is supplied to a second tank B (described later) via a first transfer member E connected to the outlet A4. The top surface A5 is a member that closes the upper end of the side portion A2 to prevent foreign matter from entering the first tank A. In this embodiment, the top surface A5 is a flat plate-shaped member having side edges that are the same shape as the upper edges of the side portion A2.

[0014] As shown in Fig. 2, the separation liquid S supplied to the first tank A through the inlet A3 flows from the bottom to the top inside the side portion A2. In the separation method of the present invention, as will be described later, the flow rate of the separation liquid S is faster than the settling rate of the pulp fibers M2 contained in the mixture M and slower than the settling rate of most of the water absorbent resin M1. Therefore, in the process of transferring the mixture M from the bottom to the top inside the first tank A by the separation liquid S, the water absorbent resin M1 and the pulp fibers M2 are separated. Then, most of the water absorbent resin M1, which has a settling rate faster than the flow rate of the separation liquid S, remains in the first tank A, while most of the pulp fibers M2 are discharged to the outside of the first tank A through the outlet A4 and are stored in the second tank B through the first transfer member E.

[0015] The separated liquid S flows at a specific flow rate between the inlet A3 and the outlet A4 inside the first tank A, and therefore, the longer the distance (ΔA) between the inlet A3 and the outlet A4, the longer the distance that the separated liquid S moves inside the first tank A. Therefore, the longer the distance (ΔA) between the inlet A3 and the outlet A4, the more difficult it becomes for the water absorbent resin M1 separated from the mixture M to be discharged to the outside of the first tank A via the outlet A4, and it is possible to recover (remain in the first tank A) a larger amount of the water absorbent resin M1. From this viewpoint, when the flow rate of the separation liquid S is X m / min, the distance (ΔA) between the inlet A3 and the outlet A4 may be, for example, 5X m to 50X m, 5X m to 40X m, 5X m to 30X m, 5X m to 20X m, 5X m to 10X m, 10X m to 50X m, 10X m to 40X m, 10X m to 30X m, 10X m to 20X m, 15X m to 50X m, 15X m to 40X m, 15X m to 30X m, 15X m to 20X m, 20X m to 50X m, 20X m to 40X m, or 20X m to 30X m. As will be described later, when the flow velocity of the separated liquid S inside the first tank A varies within a range of 0.08 m / min to 0.40 m / min, the fastest flow velocity is used as the basis for calculating the distance (ΔA) between the inlet A3 and the outlet A4. Specifically, the distance (ΔA) between the inlet A3 and the outlet A4 is 0.4 m to 20.0 m, 0.4 m to 15.0 m, 0.4 m to 12.0 m, 0.4 m to 9.0 m, 0.4 m to 6.0 m, 0.4 m to 3.0 m, 0.8 m to 20.0 m, 0.8 m to 15.0 m, 0.8 m to 12.0 m, 0.8 m to 9.0 m, 0.8 m to 6 ... .8m to 3.0m, 1.2m to 20.0m, 1.2m to 15.0m, 1.2m to 12.0m, 1.2m to 9.0m, 1.2m to 6.0m, 1.2m to 3.0m, 1.6m to 20.0m, 1.6m to 15.0m, 1.6m to 12.0m, 1.6m to 9.0m, 1.6m to 6.0m, or 1.6m to 3.0m.

[0016] (Second Tank) The second tank B is a member that stores the separated liquid S (pulp mixture liquid S1 containing pulp fibers M2 separated from the mixture M) discharged from the outlet A4 of the first tank A. In this embodiment, the second tank B has a supply port B1 that receives the pulp mixture liquid S1 discharged from the first tank A, and a discharge port B2 that discharges the separated liquid S (purified separated liquid S2) purified by the purification section C described below from the second tank B to the first tank A. The outlet A4 of the first tank A and the supply port B1 of the second tank B are connected by a first transfer member E (a pipe in this embodiment), and the discharge port B2 of the second tank B and the inlet A3 of the first tank A are connected by a return section D. In this embodiment, the supply port B1 of the second tank B is located below the outlet A4 of the first tank A. Therefore, the separation liquid S that has risen to the outlet A4 of the first tank A flows down through the piping, which is the first transfer member E, by its own weight and is supplied to the second tank B.

[0017] (Purification Section) The purification section C is a component that removes pulp fibers M2 from the separation liquid S (pulp mixture liquid S1) contained in the second tank B to produce a purified separation liquid S2. The purification section C may be, for example, a sieve. The sieve has a mesh size that allows the separation liquid S to pass through but does not allow most of the pulp fibers M2 (e.g., 95% by mass or more, preferably 100% by mass, of the pulp fibers M2 contained in the second tank B to pass through). In this embodiment, a sieve (purification section C) having a surface parallel to the vertical direction is provided in the center of the bottom surface of the second tank B. The sieve divides the interior of the second tank B into a first compartment R1 containing the pulp mixture liquid S1 and a second compartment R2 containing the pulp mixture liquid S1 (purified separated liquid S2) purified by the purification section C, and the supply port B1 of the second tank B described above is provided within the area of ​​the first compartment R1, and the discharge port B2 of the second tank B described above is provided within the area of ​​the second compartment R2.

[0018] (Returning Section) The returning section D is a member that returns the purified separated liquid S2 to the first tank A. In this embodiment, the returning section D includes a second transfer member D1 (in this embodiment, a pipe) having one end connected to the outlet B2 of the second tank B and the other end connected to the inlet A3 of the first tank A, a pump D2 provided in a midway portion of the second transfer member D1, and a valve D3 provided in a midway portion of the second transfer member D1 between the pump D2 and the second tank B. As shown in Fig. 1 , in this embodiment, the separated liquid S is contained in the second tank B in advance before the separation process is started (before the separated liquid S is brought into contact with the mixture M contained in the first tank A), and the valve D3 of the returning section D is configured to prevent the separated liquid S from unexpectedly moving from the second tank B to the first tank A. By opening the valve D3 and driving the pump D2, the purified separated liquid S2 contained in the second compartment R2 of the second tank B is returned to the first tank A through the second transfer member D1, and as shown in FIG. 2 , the separated liquid S flows from bottom to top inside the first tank A. Thereafter, the separated liquid S (pulp mixed liquid S1) is returned to the first compartment R1 of the second tank B through the outlet A4 and the first transfer member E. Thus, in this embodiment, the separated liquid S is circulated between the first tank A and the second tank B by the pump D2. The flow rate of the separated liquid S inside the first tank A can be adjusted, for example, by adjusting the amount of the separated liquid S supplied to the first tank A. The supply amount of the separated liquid S can be adjusted, for example, by changing the driving force (pressure) of the pump D2.

[0019] <Separation Method> (Separation Process) The separation process is a process in which a mixture derived from used absorbent articles is brought into contact with a separation liquid flowing from bottom to top at a flow rate of 0.08 m / min to 0.40 m / min, thereby separating the mixture into a water-absorbent resin and pulp fibers in the separation liquid. The flow rate of the separation liquid in the separation process may be constant or variable, provided that it is within the above-mentioned range. In this embodiment, as shown in FIG. 1 , the separation process is initiated by contacting a mixture M previously placed in the bottom portion A1 of a first tank A of a separation device 100 with a separation liquid S (previously contained in a second tank B) supplied from an inlet A3. The separation liquid S initially supplied to the first tank A may be previously contained in a tank other than the second tank B. In this case, however, a supply means (e.g., a pipe connecting the first tank A to the other tank and a pump provided midway between the pipe and the first tank A) is required to supply the separation liquid S from the other tank to the first tank A.

[0020] The flow velocity (ascending velocity) of the separation liquid S in the first tank A is set to be sufficiently faster than the settling velocity of the pulp fibers M2 contained in the mixture M relative to the separation liquid S and sufficiently slower than the settling velocity of most of the water absorbent resin M1 relative to the separation liquid S. Therefore, as shown in Fig. 2 , the mixture M is separated into the water absorbent resin M1 and the pulp fibers M2 in the separation liquid S in the first tank A, and the pulp fibers M2, which have a settling velocity slower than the flow velocity of the separation liquid S, are transferred to the upper side of the first tank A and are accommodated outside the first tank A (second tank B) via the outlet A4, while the water absorbent resin M1, which has a settling velocity faster than the flow velocity of the separation liquid S, remains in the first tank A. In this way, according to this embodiment, the pulp fibers M2 contained in the mixture M can be separated into the second tank B, and the water absorbent resin M1 can be separated and recovered in the first tank A with high purity (a state in which the amount of pulp fibers M2 remaining in the first tank A is small).

[0021] In the present invention, since the flow rate of the separation liquid is 0.08 m / min to 0.40 m / min, the water absorbent resin can be recovered with high purity, but in order to separate the pulp fibers in a shorter time, the flow rate of the separation liquid can also be set to more than 0.08 m / min. Furthermore, in order to further suppress the separation of the water absorbent resin having a relatively slow settling velocity and a small particle size into the second tank (i.e., to further improve the recovery rate of the water absorbent resin), the flow rate of the separation liquid can also be set to less than 0.40 m / min. From this viewpoint, the flow rate of the separated liquid is set to 0.08 m / min to 0.35 m / min, 0.08 m / min to 0.30 m / min, 0.08 m / min to 0.25 m / min, 0.08 m / min to 0.20 m / min, 0.08 m / min to 0.15 m / min, 0.10 m / min to 0.40 m / min, 0.10 m / min to 0.35 m / min, 0.10 m / min to 0.3 The speed may be 0 m / min, 0.10 m / min to 0.25 m / min, 0.10 m / min to 0.20 m / min, 0.10 m / min to 0.15 m / min, 0.15 m / min to 0.40 m / min, 0.15 m / min to 0.35 m / min, 0.15 m / min to 0.30 m / min, 0.15 m / min to 0.25 m / min, or 0.15 m / min to 0.20 m / min.

[0022] (Mixture) The mixture used in the separation method of the present invention contains a dehydrated water-absorbent resin and pulp fibers. Such a mixture is derived from a used absorbent article. The used absorbent article may be at least one selected from the group consisting of diapers (e.g., disposable diapers), toilet training pants, incontinence pads, sanitary materials (sanitary napkins, tampons, etc.), sweat pads, and pet sheets, and a typical example is a diaper. The mixture can be obtained, for example, by subjecting a used absorbent article to a dehydration step described below (subjecting the used absorbent article to a dehydration treatment and then crushing the absorbent article, or crushing the used absorbent article and then subjecting it to a dehydration treatment).

[0023] (Separated liquid) The separated liquid used in the separation method of the present invention has a density of 0.6 g / cm ３ ~1.2 g / cm ３The density of the separation liquid is a value based on the separation liquid at 25°C. According to the findings of the present inventors, when a separation liquid having the above density is used, the settling velocity of most of the pulp fibers derived from used absorbent articles in the separation liquid is approximately 0.06 m / min, and the settling velocity of most of the dehydrated water-absorbent resin derived from used absorbent articles in the separation liquid is approximately 0.50 m / min. The settling velocity of the pulp fibers and the water-absorbent resin means the velocity at which the pulp fibers and the water-absorbent resin naturally fall (settle) in the separation liquid in an unstirred state. In the separation method of the present invention, the flow velocity of the separation liquid in the separation step is 0.08 m / min to 0.40 m / min (i.e., a velocity at which most of the dehydrated water-absorbent resin settles and almost no pulp fibers settle), and therefore, the water-absorbent resin can be separated and recovered with high purity from the mixture derived from used absorbent articles.

[0024] The density of the separated liquid is 0.6 g / cm ３ ~1.1 g / cm ３ , 0.6 g / cm ３ ~1.0 g / cm ３ , 0.6 g / cm ３ ~0.9 g / cm ３ , 0.6 g / cm ３ ~0.8g / cm ３ , 0.7 g / cm ３ ~1.2 g / cm ３ , 0.7 g / cm ３ ~1.1 g / cm ３ , 0.7 g / cm ３ ~1.0 g / cm ３ , 0.7 g / cm ３ ~0.9 g / cm ３ , 0.7 g / cm ３ ~0.8g / cm ３ , 0.8 g / cm ３ ~1.2 g / cm ３ , 0.8 g / cm ３ ~1.1 g / cm ３ , 0.8 g / cm ３ ~1.0 g / cm ３ , 0.8 g / cm ３ ~0.9 g / cm ３ , 0.9 g / cm ３ ~1.2 g / cm ３, 0.9 g / cm ３ ~1.1 g / cm ３ , 0.9 g / cm ３ ~1.0 g / cm ３ , 1.0 g / cm ３ ~1.2 g / cm ３ , 1.0 g / cm ３ ~1.1 g / cm ３ , or 1.1 g / cm ３ ~1.2 g / cm ３ Examples of such a separation liquid include at least one liquid selected from the group consisting of water and an aqueous solution containing a swelling inhibitor for the water-absorbent resin. From the viewpoint of suppressing a decrease in density of the water-absorbent resin due to absorption of the separation liquid (a decrease in the settling velocity of the water-absorbent resin, which becomes close to the settling velocity of the pulp fibers), the separation liquid may contain a swelling inhibitor.

[0025] Examples of the swelling inhibitor include at least one substance selected from the group consisting of an acid component, a polyvalent metal salt, a monovalent metal salt, and an alcohol. From the viewpoint of more reliably preventing the water absorbent resin from swelling in the separation step by sufficiently inactivating the water absorbent resin (significantly reducing the water absorption capacity of the water absorbent resin), the swelling inhibitor may be at least one substance selected from the group consisting of an acid component and a polyvalent metal salt. When the water absorbent resin obtained through the separation step of the present invention is reused and used as a raw material for recycled water absorbent resin particles, the water absorbent resin obtained through the separation step can be subjected to the cleavage step described below. From the viewpoint of facilitating the cleavage step, the swelling inhibitor may be an acid component.

[0026] The acid component may be an inorganic acid component, an organic acid component, or a combination of both. From the viewpoint of suppressing the destruction of the structure of the water-absorbent resin (reduction in the average molecular weight) and making it easier to obtain a water-absorbent resin that is easily reusable, the inorganic acid component may be, for example, at least one selected from the group consisting of nitric acid, sulfuric acid, hydrochloric acid, phosphoric acid, and boric acid, or at least one of hydrochloric acid and sulfuric acid, or may be sulfuric acid. The organic acid component may be, for example, at least one selected from the group consisting of acetic acid, succinic acid, citric acid, malic acid, lactic acid, maleic acid, and fumaric acid. From the viewpoint of suppressing the destruction of the structure of the water-absorbent resin (reduction in the weight-average molecular weight) and making it easier to obtain a water-absorbent resin that is easily reusable, the acid component may be at least one selected from the group consisting of citric acid, maleic acid, and fumaric acid, or may be citric acid because of its chelating effect and bactericidal effect. One type of acid component may be used alone, or two or more types may be used in combination.

[0027] The polyvalent metal salt is a salt of a divalent or higher metal. The divalent or higher metal may be, for example, at least one selected from the group consisting of calcium, aluminum, magnesium, titanium, chromium, manganese, iron, cobalt, nickel, cadmium, and lead. The polyvalent metal salt may be an inorganic salt or an organic salt. Examples of inorganic salts of polyvalent metals include chlorides of polyvalent metals such as calcium chloride, aluminum chloride, and magnesium chloride; hydroxides of polyvalent metals such as calcium hydroxide, aluminum hydroxide, and magnesium hydroxide; and carbonates of polyvalent metals such as calcium carbonate, aluminum carbonate, and magnesium carbonate. Examples of organic salts of polyvalent metals include carboxylates of polyvalent metals such as calcium formate and calcium acetate. The polyvalent metal salt may be a chloride or calcium chloride from the viewpoint of suppressing destruction of the structure of the water absorbent resin contained in the water absorbent resin (reduction in weight average molecular weight) and facilitating separation of the water absorbent resin for easy reuse. The polyvalent metal salt may be used alone or in combination of two or more types.

[0028] The monovalent metal salt is a salt of a monovalent metal. The monovalent metal may be, for example, at least one of sodium and potassium, and may be sodium. The monovalent metal salt may be an inorganic salt or an organic salt. The inorganic salt of the monovalent metal may be, for example, at least one of sodium chloride and potassium chloride, and may be sodium chloride.

[0029] The alcohol may be, for example, at least one selected from the group consisting of methanol, ethanol, 2-propanol, t-butanol, isobutanol, pentanol, and hexanol.

[0030] The concentration of the swelling inhibitor in the separated liquid is set to 0.6 g / cm ３ ~1.2 g / cm ３ Specific concentrations of the swelling inhibitor are, based on the total mass of the separated liquid, 0.01% by mass to 10.0% by mass, 0.01% by mass to 8.0% by mass, 0.01% by mass to 5.0% by mass, 0.01% by mass to 2.0% by mass, 0.01% by mass to 1.0% by mass, 0.1% by mass to 10.0% by mass, 0.1% by mass to 8.0% by mass, 0.1% by mass to 5.0% by mass, 0. ... .. 1% by mass to 1.0% by mass, 0.3% by mass to 10.0% by mass, 0.3% by mass to 8.0% by mass, 0.3% by mass to 5.0% by mass, 0.3% by mass to 2.0% by mass, 0.3% by mass to 1.0% by mass %, 0.5% to 10.0%, 0.5% to 8.0%, 0.5% to 5.0%, 0.5% to 2.0%, or 0.5% to 1.0%.

[0031] (Recovery Step) The separation method of the present invention may include a recovery step of recovering the pulp fibers separated in the separation liquid after the separation step. The recovery step includes a step of recovering a pulp mixture liquid containing pulp fibers and the separation liquid downstream of the contact position of the separation liquid with the mixture in the flow path of the separation liquid (i.e., the contact position of the separation liquid with the mixture in the flow path of the separation liquid is considered upstream). Specifically, when the recovery step is performed using the separation device 100 shown in FIGS. 1 and 2 , the contact position of the mixture M and the separation liquid S flowing into the first tank A from the inlet A3 is the bottom surface A1 of the first tank A. The pulp mixture liquid S1 flows out from the outlet A4 located above the bottom surface A1 (downstream of the flow path of the separation liquid S), and can be recovered in the second tank B via the first transfer member E.

[0032] (Purification Step) The separation method of the present invention may further include a purification step of removing pulp fibers from the pulp mixture liquid after the recovery step. Specifically, when the purification step is performed using the separation device 100 shown in Figures 1 and 2, the pulp mixture liquid S1 recovered in the first compartment R1 of the second tank B passes through a purification section C (a sieve in this embodiment), whereby the pulp fibers M2 are separated into the first compartment R1 of the second tank B, and the purified separated liquid can be stored in the second compartment R2 of the second tank B.

[0033] (Returning Step) The separation method of the present invention may further include a returning step of returning the purified separated liquid to the separation step after the purification step. The returning step can also be considered a step of supplying the purified separated liquid as a separated liquid for the separation step. Specifically, when the recovery step is performed using the separation apparatus shown in FIGS. 1 and 2 , the purified separated liquid S2 is returned to the first tank A where the separation step is performed by a returning section D provided between the first tank A and the second tank B. Specifically, the returning section D has a second transfer member D1 and a pump D2 and a valve D3 provided midway through the second transfer member D1. By driving the pump D2 with the valve D3 open, the purified separated liquid S2 contained in the second compartment R2 of the second tank B is returned to the first tank A where the separation step is performed.

[0034] As described above, the separation method of the present invention separates the water-absorbent resin and the pulp fibers by focusing on the difference in sedimentation velocity between the two in the separation liquid, and can separate and recover the water-absorbent resin with high purity. Furthermore, by providing the above-mentioned recovery step, purification step, and return step, the separation method of the present invention can continuously carry out the separation steps without preparing a new separation liquid.

[0035] (Dehydration Step) The separation method of the present invention may include a dehydration step prior to the separation step described above. The dehydration step is a step of dehydrating the water-absorbent resin contained in a used absorbent article (hereinafter, sometimes simply referred to as the water-absorbent resin before dehydration), and is achieved by contacting the water-absorbent resin before dehydration with a dehydrating agent. The dehydration step allows a certain amount of liquid (such as urine) to be discharged from the water-absorbent resin before dehydration, thereby obtaining a mixture containing the dehydrated water-absorbent resin and pulp fibers. The dehydration step may be performed while the shape of the used absorbent article is maintained, or may be performed after pulverizing the used absorbent article and separating the mixture containing the water-absorbent resin before dehydration and pulp fibers from other components of the absorbent article. Generally, in a used absorbent article, the water-absorbent resin before dehydration and the pulp fibers are intricately entangled, and therefore, separation of the two is insufficient simply by undergoing the dehydration step. However, by using the separation method of the present invention, the water-absorbent resin can be separated and recovered with high purity from the mixture.

[0036] The dehydrating agent used in the dehydration step may be at least one substance selected from the group consisting of an acid component, a polyvalent metal salt, and an alcohol. Examples of the acid component, the polyvalent metal salt, and the alcohol include the same substances as the swelling inhibitor that may be contained in the separation liquid. The dehydrating agent may be at least one selected from the group consisting of citric acid, maleic acid, and fumaric acid, from the viewpoint of facilitating the preparation of a dehydrated water-absorbent resin having a settling velocity of more than 0.40 m / min by sufficiently dehydrating the water-absorbent resin before dehydration. Alternatively, the dehydrating agent may be citric acid, from the viewpoint of even better solubility in water (easiness in preparing an aqueous solution). The dehydrating agent may be used in a liquid state such as an aqueous solution, or in a solid state such as a powder. The dehydrating agent may be used in a liquid state such as an aqueous solution, from the viewpoint of ensuring uniform contact of the dehydrating agent with the water-absorbent resin before dehydration and easily contacting the water-absorbent resin before dehydration without having to crush the used absorbent article.

[0037] When the dehydrating agent contains an acid component and is liquid (for example, when an aqueous solution containing an acid component is used), the pH of the liquid may be 0.1 or more, 0.3 or more, 0.5 or more, 0.7 or more, 0.8 or more, 0.9 or more, 1.0 or more, or 1.1 or more from the viewpoint of suppressing destruction of the structure of the water absorbent resin before dehydration (the weight average molecular weight becoming too small) and making it easier to obtain a dehydrated water absorbent resin that is easy to reuse, and may be 5.0 or less, 4.5 or less, 4.0 or less, 3.5 or less, 3.0 or less, 2.5 or less, 2.4 or less, or 2.3 or less from the viewpoint of sufficiently dehydrating the water absorbent resin before dehydration. The pH of the liquid may be 0.1 to 5.0, 0.3 to 4.5, 0.5 to 4.0, 0.7 to 3.5, 0.8 to 3.0, 0.9 to 2.5, or 1.0 to 2.4. The amount of the acid component to be contacted with the water absorbent resin before dehydration may be 50 parts by mass or more, 100 parts by mass or more, 150 parts by mass or more, or 200 parts by mass or more relative to 100 parts by mass of the water absorbent resin before dehydration from the viewpoint of sufficiently dehydrating the water absorbent resin before dehydration, and 4000 parts by mass or less, 3000 parts by mass or less, or 2000 parts by mass or less from the viewpoint of suppressing destruction of the structure of the water absorbent resin before dehydration. From these viewpoints, the amount of the acid component to be contacted with the water absorbent resin before dehydration may be 50 parts by mass to 4000 parts by mass, 100 parts by mass to 4000 parts by mass, 150 parts by mass to 3000 parts by mass, or 200 parts by mass to 2000 parts by mass relative to 100 parts by mass of the water absorbent resin before dehydration. When the dehydrating agent contains an acid component and is a liquid, the temperature of the liquid may be 0° C. or higher, 10° C. or higher, or 15° C. or higher from the viewpoint of sufficiently dehydrating the water absorbent resin before dehydration, and may be 100° C. or lower, 80° C. or lower, 60° C. or lower, 40° C. or lower, or 30° C. or lower from the viewpoint of suppressing destruction of the structure of the water absorbent resin before dehydration. From these viewpoints, the temperature of the liquid may be 0° C. to 100° C., 10° C. to 60° C., 10° C. to 40° C., or 15° C. to 30° C.

[0038] When the dehydrating agent contains a polyvalent metal salt and is liquid (for example, when an aqueous solution containing a polyvalent metal salt is used), the pH of the liquid may be 1.0 or more, 3.0 or more, 5.0 or more, 5.5 or more, 6.0 or more, 6.5 or more, 7.0 or more, more than 7.0, 7.5 or more, or 8.0 or more from the viewpoint of suppressing destruction of the structure of the water absorbent resin before dehydration, and from the same viewpoint, it may be 13.0 or less, 12.5 or less, 12.0 or less, 11.5 or less, 11.0 or less, 10.5 or less, or 10.0 or less. The pH of the liquid may be 1.0 to 13.0, 3.0 to 12.5, 5.0 to 12.0, 5.5 to 11.5, 6.0 to 11.0, 6.5 to 10.5, or 7.0 to 10.0. The amount of the polyvalent metal salt to be contacted with the water absorbent resin before dehydration may be 50 parts by mass or more, 100 parts by mass or more, 150 parts by mass or more, or 200 parts by mass or more relative to 100 parts by mass of the water absorbent resin before dehydration, from the viewpoint of sufficiently dehydrating the water absorbent resin before dehydration, and from the viewpoint of suppressing destruction of the structure of the water absorbent resin before dehydration, 4000 parts by mass or less, 3000 parts by mass or less, or 2000 parts by mass or less. From these viewpoints, the amount of the polyvalent metal salt to be contacted with the water absorbent resin before dehydration may be 50 parts by mass to 4000 parts by mass, 100 parts by mass to 4000 parts by mass, 150 parts by mass to 3000 parts by mass, or 200 parts by mass to 2000 parts by mass relative to 100 parts by mass of the water absorbent resin before dehydration. When the dehydrating agent contains a polyvalent metal salt and is a liquid, the temperature of the liquid may be 0° C. or higher, 10° C. or higher, or 15° C. or higher from the viewpoint of sufficiently dehydrating the water absorbent resin before dehydration, and may be 100° C. or lower, 80° C. or lower, 60° C. or lower, 40° C. or lower, or 30° C. or lower from the viewpoint of suppressing destruction of the structure of the water absorbent resin before dehydration. From these viewpoints, the temperature of the liquid may be 0° C. to 100° C., 10° C. to 60° C., 10° C. to 40° C., or 15° C. to 30° C.

[0039] When the dehydrating agent contains an alcohol and is liquid (for example, when a lower alcohol itself is used or an aqueous solution containing an alcohol is used), the temperature of the liquid may be 0 ° C. or higher, 10 ° C. or higher, or 15 ° C. or higher from the viewpoint of sufficiently dehydrating the water absorbent resin before dehydration, and may be 100 ° C. or lower, 80 ° C. or lower, 60 ° C. or lower, 40 ° C. or lower, or 30 ° C. or lower from the viewpoint of suppressing destruction of the structure of the water absorbent resin before dehydration. From these viewpoints, the temperature of the liquid may be 0 ° C. to 100 ° C., 10 ° C. to 60 ° C., 10 ° C. to 40 ° C., or 15 ° C. to 30 ° C. The amount of alcohol to be contacted with the water absorbent resin before dehydration may be 1,000 parts by mass to 1,000,000 parts by mass, or 10,000 parts by mass to 100,000 parts by mass relative to 100 parts by mass of the water absorbent resin before dehydration.

[0040] In this way, a mixture containing a dehydrated water-absorbent resin and pulp fibers can be obtained by passing through a dehydration step before the separation step, but in the separation method of the present invention, the dehydration step and the separation step can also be carried out simultaneously. Specifically, by crushing a used absorbent article and separating the mixture containing the absorbent article before dehydration and pulp fibers from other components that constitute the absorbent article, and then subjecting the mixture to the above-mentioned separation step, both dehydration of the water-absorbent resin and separation from the mixture can be carried out simultaneously. However, in this case, the separating liquid used in the separation step contains a dehydrating agent and has a density of 0.6 g / cm ３ ~1.2 g / cm ３ It must be.

[0041] The water-absorbent resin before dehydration contains, for example, a polymer (crosslinked polymer) of an ethylenically unsaturated monomer. Examples of the ethylenically unsaturated monomer include (meth)acrylic acid and its salts, 2-(meth)acrylamido-2-methylpropanesulfonic acid and its salts, (meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, N-methylol(meth)acrylamide, polyethylene glycol mono(meth)acrylate, N,N-diethylaminoethyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate, and diethylaminopropyl(meth)acrylamide. The ethylenically unsaturated monomer may contain at least one compound selected from the group consisting of acrylic acid and its salts, methacrylic acid and its salts, acrylamide, methacrylamide, and N,N-dimethylacrylamide.

[0042] The crosslinked polymer may contain structural units derived from other than ethylenically unsaturated monomers. The proportion of structural units derived from ethylenically unsaturated monomers in the crosslinked polymer (particularly (meth)acrylic acid and salts thereof) may be 70 mol% to 100 mol% relative to the total amount of monomers used in the crosslinked polymer. The proportion of (meth)acrylic acid and salts thereof among the ethylenically unsaturated monomers may be 70 mol% to 100 mol%. When an ethylenically unsaturated monomer (e.g., (meth)acrylic acid) is used as a constituent material of the crosslinked polymer, the degree of neutralization thereof may be 10 mol% to 100 mol%, 50 mol% to 90 mol%, or 60 mol% to 80 mol% of the acidic groups in the ethylenically unsaturated monomer.

[0043] The water-absorbing resin before dehydration contains, for example, the cross-linked polymer with poly(meth)acrylic acid structure.As such cross-linked polymer, for example, the polymer obtained by polymerizing the monomer composition containing (meth)acrylic acid and the cross-linking agent that can react with the carboxyl group of (meth)acrylic acid to form covalent bond; the polymer obtained by polymerizing the monomer composition containing (meth)acrylic acid to obtain a polymer, and then using the cross-linking agent that can react with the carboxyl group of the polymer to form covalent bond to treat the surface of the polymer; the polymer obtained by polymerizing the monomer composition containing (meth)acrylic acid and the cross-linking agent that can react with the carboxyl group of the (meth)acrylic acid to form covalent bond to obtain a polymer, and then using the cross-linking agent that can react with the carboxyl group of the polymer to form covalent bond to treat the surface of the polymer (surface cross-linking) to obtain a polymer.

[0044] <Washing Step> When the separation method of the present invention includes the dehydration step described above, a washing step may be performed after the dehydration step but before the separation step. The washing step is a step of washing the dehydrated water-absorbent resin obtained by the dehydration step to remove impurities such as urine remaining on the surface. The washing step is performed, for example, by contacting the dehydrated water-absorbent resin with an aqueous solution containing a swelling inhibitor that can be used in the separation step (hereinafter, may be simply referred to as a washing solution). The washing step may be performed once or twice or more times. The swelling inhibitor contained in the washing solution may be the same as or different from the substance that can be used in the separation step. The washing solution may be, for example, at least one selected from the group consisting of an aqueous citric acid solution, an aqueous maleic acid solution, an aqueous fumaric acid solution, an aqueous sulfuric acid solution, an aqueous sodium chloride solution, and an aqueous potassium chloride solution, or may be at least one selected from an aqueous citric acid solution, an aqueous sulfuric acid solution, and an aqueous sodium chloride solution. If the concentration of the washing solution is high, moisture remaining inside the water-absorbent resin is discharged due to the osmotic pressure difference between the inside and outside of the water-absorbent resin, thereby further dehydrating the water-absorbent resin. From this viewpoint, the concentration of the swelling inhibitor in the cleaning solution may be 0.05% by mass to 10.0% by mass, 0.05% by mass to 8.0% by mass, 0.05% by mass to 5.0% by mass, 0.05% by mass to 2.0% by mass, or 0.05% by mass to 1.0% by mass, 0.3% by mass to 10.0% by mass, 0.3% by mass to 8.0% by mass, 0.3% by mass to 5.0% by mass, 0.3% by mass to 2.0% by mass, 0.3% by mass to 1.0% by mass, 0.5% by mass to 10.0% by mass, 0.5% by mass to 8.0% by mass, 0.5% by mass to 5.0% by mass, 0.5% by mass to 2.0% by mass, or 0.5% by mass to 1.0% by mass.

[0045] <Method for producing recycled water-absorbent resin> The method for producing recycled water-absorbent resin of the present invention includes a preparation step of preparing a water-absorbent resin separated by the separation method of the present invention (dehydrated water-absorbent resin), a cleavage step of cleaving a crosslinked structure of a crosslinked polymer contained in the water-absorbent resin to obtain a polymer, and a crosslinking step of crosslinking the polymer.

[0046] (Preparation Step) When separation of a water absorbent resin is performed using the separation device of the present invention, the preparation step is performed, for example, by the following procedure. First, after performing the separation step, a valve provided at a midpoint of the second transfer member is closed, and driving of the pump is stopped to prevent the separated liquid (purified separated liquid) from being supplied to the first tank. Thereafter, the mixed liquid containing the separated liquid and the dehydrated water absorbent resin accommodated in the first tank is filtered, thereby recovering the dehydrated water absorbent resin.

[0047] (Cleavage step) The cleavage step is a step of cleaving the crosslinked structure of the crosslinked polymer contained in the dehydrated water absorbent resin recovered through the preparation step, thereby converting the crosslinked polymer into a linear polymer. The crosslinked structure of the crosslinked polymer can be cleaved by contacting the crosslinked polymer with at least one selected from the group consisting of an acid component (e.g., sulfuric acid), a base component (e.g., sodium hydroxide), an oxidizing agent, and a reducing agent.

[0048] The acid component that can be used in the cleavage step may be, for example, an inorganic acid component. The inorganic acid component may be at least one inorganic acid component selected from the group consisting of acetic acid, nitric acid, sulfuric acid, hydrochloric acid, phosphoric acid, and boric acid, and may be at least one inorganic acid of sulfuric acid and hydrochloric acid. The acid component may be used in the state of an aqueous solution, and when the water-absorbent resin is contacted with the aqueous solution, the temperature of the aqueous solution may be 80°C to 140°C.

[0049] The base component that can be used in the cleavage step may be, for example, an inorganic base component. The inorganic base component may be at least one inorganic base component selected from the group consisting of sodium hydroxide, ammonia, potassium hydroxide, and calcium hydroxide, or may be at least one inorganic base component selected from the group consisting of sodium hydroxide, potassium hydroxide, and calcium hydroxide. The base component may be used in the state of an aqueous solution, and when the water-absorbent resin is contacted with the base component, the temperature of the aqueous solution may be 50°C to 120°C.

[0050] Specifically, for example, when the crosslinked structure of the crosslinked polymer is cleaved by a basic component, the cleavage can be achieved by stirring a solution containing a water-absorbent resin and a basic component at a temperature of 50°C to 150°C and in an environment of pH 10 or more for 1 hour to 50 hours.

[0051] When the water-absorbent resin contains a crosslinked polymer having a poly(meth)acrylic acid structure, the polymer obtained by cleaving the crosslinked polymer has a poly(meth)acrylic acid structure. When the water-absorbent resin contains a crosslinked polymer having a poly(meth)acrylic acid structure, the crosslinked structure of the crosslinked polymer may be cleaved by contacting the water-absorbent resin with a base component, from the viewpoint of facilitating the production of a recycled water-absorbent resin having high water absorption performance and a small soluble content by suppressing cleavage of the poly(meth)acrylic acid structure.

[0052] The weight average molecular weight of the polymer obtained in the cleavage step may be 300,000 or more, 500,000 or more, 700,000 or more, 1,000,000 or more, 1,300,000 or more, 1,500,000 or more, 1,700,000 or more, or 1,900,000 or more. The weight average molecular weight of the polymer may be 10,000,000 or less, 8,000,000 or less, 6,000,000 or less, 4,000,000 or less, 3,000,000 or less, or 2,000,000 or less. The weight average molecular weight of the polymer may be 300,000 to 10,000,000, 500,000 to 6,000,000, or 1,000,000 to 3,000,000. The weight average molecular weight of the polymer is measured by GPC (gel permeation chromatography) under the following conditions: Autosampler: Autosampler AS-11 (manufactured by FLOM Corporation). Degasser: Gastr AG-16 (manufactured by FLOM Corporation). Liquid delivery unit: LC-10AD (Shimadzu Corporation). Column: OHpak SB-807HQ, SB-806HQ, SB-804HQ (Shodex series, manufactured by Resonac Corporation). Detector: Triple Detector TDA 302 (Viscotec). Eluent: NaNO ３(0.2 mol / L) / methylparaben (2 mmol / L) / distilled water. Measurement conditions: injection volume 500 μL, flow rate 0.5 mL / min, column / detector temperature 40°C, dn / dC 0.2270. A sample solution adjusted to contain 0.01 g of polymer was placed in a 300 mL beaker, and the above eluent was added to bring the total volume to 100 mL. The mixture was stirred at 250 rpm for 1 hour. If the pH of the sample solution was not neutral, it was adjusted to pH 7 using hydrochloric acid (Nacalai Tesque, 1 mol / L hydrochloric acid). After stirring, the sample solution was filtered through a 0.8 μm filter syringe and the filtrate was measured by GPC.

[0053] (Adjustment Step) By cleaving the crosslinked structure of the crosslinked polymer, a solution-like polymer (hereinafter sometimes simply referred to as a polymer solution) is obtained, but an adjustment step of adjusting the state of the polymer solution may be carried out before subjecting the polymer solution to the crosslinking step. The adjustment step includes, for example, an impurity removal step. The impurity removal step is a step of removing impurities from the polymer solution, and is carried out, for example, by subjecting the polymer solution to ultrafiltration. Before carrying out the impurity removal step, the pH of the polymer solution may be adjusted to near neutral. The adjustment step includes, for example, a viscosity adjustment step. The viscosity adjustment step is a step of adjusting the viscosity of the polymer solution, and, for example, from the viewpoint of lowering the viscosity of the polymer solution, dilution, heating, addition of salt (a salt containing a monovalent cation), etc. may be carried out.

[0054] (Crosslinking step) The crosslinking step is a step of imparting water absorption performance by crosslinking the polymer contained in the polymer solution (i.e., regenerating the polymer as a water-absorbent resin). In the crosslinking step, for example, a solution containing a crosslinking agent capable of forming a covalent bond with a functional group of the polymer is prepared, and the polymer is crosslinked via the covalent bond to obtain a crosslinked polymer. In the crosslinking step, the polymer solution may gel. In that case, the entire polymer solution loses fluidity, and a gel containing the crosslinked polymer and water is formed.

[0055] Examples of functional groups possessed by the polymer include carboxyl groups. When the polymer has carboxyl groups, the polymer is crosslinked by a reaction between the carboxyl groups and a crosslinking agent and / or a reaction between the carboxyl groups themselves. The covalent bond may be at least one selected from the group consisting of an ester bond, a thioester bond, an amide bond, an ether bond, and a carbon-carbon bond. The polymer may be crosslinked, for example, via at least one group selected from the group consisting of a carboxylic acid ester group, a thioester group, an amide group, an acid anhydride group, an oxyalkylene group, and an oxyarylene group.

[0056] Examples of crosslinking agents capable of forming a covalent bond with a functional group of a polymer include aliphatic polyhydric alcohols such as (poly)ethylene glycol, (poly)propylene glycol, (poly)glycerin, and pentaerythritol; glycidyl compounds such as (poly)ethylene glycol diglycidyl ether, (poly)propylene glycol diglycidyl ether, and (poly)glycerin diglycidyl ether; bisacrylamide compounds such as N,N'-methylenebis(meth)acrylamide; allylated starch; diallyl phthalate; N,N',N"-triallyl isocyanurate; divinylbenzene; ethylenediamine, polyethyleneimine, and glycidyl (meth)acrylate.

[0057] The amount of the crosslinking agent may be 0.0001 parts by mass or more, 0.001 parts by mass or more, or 0.005 parts by mass or more, and may be 10 parts by mass or less, 10 parts by mass or less, or 5 parts by mass or less, relative to 100 parts by mass of the polymer. The amount of the crosslinking agent may be 0.0001 parts by mass to 15 parts by mass, 0.001 parts by mass to 10 parts by mass, or 0.005 parts by mass to 5 parts by mass, relative to 100 parts by mass of the polymer.

[0058] The reaction temperature when crosslinking the polymer is set appropriately depending on the type, amount, etc. of the crosslinking agent used, but may be 50°C or higher, 80°C or higher, or 100°C or higher, and may be 220°C or lower, 200°C or lower, or 180°C or lower. The reaction temperature when crosslinking the polymer may be 50°C to 220°C, 80°C to 200°C, or 100°C to 180°C. The reaction time when crosslinking the polymer is set appropriately depending on the type, amount, reaction temperature, etc. of the crosslinking agent used, but may be 1 minute to 200 minutes, or 5 minutes to 150 minutes.

[0059] After the polymer is regenerated into a crosslinked polymer through the crosslinking step, a surface crosslinking step may be carried out in which the regenerated crosslinked polymer is surface crosslinked. Surface crosslinking can be carried out, for example, by adding a crosslinking agent (surface crosslinking agent) for surface crosslinking to the regenerated crosslinked polymer and allowing it to react. The surface crosslinking agent may be a compound having two or more reactive functional groups reactive with the crosslinked polymer. The surface crosslinking agent may be the same as or different from the crosslinking agent used in the crosslinking step. The reactive functional group of the surface crosslinking agent may be a carbonate group, an alcoholic hydroxyl group, an epoxy group, a halogeno group in a haloepoxy compound, an isocyanate group, an oxetanyl group, an oxazoline group, or a combination thereof. A carbonate group can react with two other molecules and is therefore considered to have two reactive functional groups.

[0060] Examples of surface cross-linking agents having a carbonate group include alkylene carbonates (ethylene carbonate, etc.). Examples of surface cross-linking agents having an alcoholic hydroxyl group include polyol compounds such as ethylene glycol, propylene glycol, 1,4-butanediol, trimethylolpropane, glycerin, polyoxyethylene glycol, polyoxypropylene glycol, and polyglycerin, and hydroxyalkylamide compounds (bis[N,N-di(β-hydroxyethyl)]adipamide, etc.). Examples of surface cross-linking agents having two or more epoxy groups include (poly)ethylene glycol diglycidyl ether, (poly)glycerin diglycidyl ether, (poly)glycerin triglycidyl ether, trimethylolpropane triglycidyl ether, (poly)propylene glycol polyglycidyl ether, and (poly)glycerol polyglycidyl ether. Examples of haloepoxy compounds having an epoxy group and a halogeno group include epichlorohydrin, epibromohydrin, and α-methylepichlorohydrin. Examples of surface crosslinking agents having an isocyanate group include 2,4-tolylene diisocyanate and hexamethylene diisocyanate. Examples of surface crosslinking agents having an oxetanyl group include 3-methyl-3-oxetanemethanol, 3-ethyl-3-oxetanemethanol, 3-butyl-3-oxetanemethanol, 3-methyl-3-oxetaneethanol, 3-ethyl-3-oxetaneethanol, and 3-butyl-3-oxetaneethanol.

[0061] The amount of the surface cross-linking agent may be 0.001 parts by mass or more, 0.005 parts by mass or more, or 0.01 parts by mass or more, and may be 5 parts by mass or less, 3 parts by mass or less, or 1 part by mass or less, relative to 100 parts by mass of the regenerated cross-linked polymer. The amount of the surface cross-linking agent may be 0.001 parts by mass to 5 parts by mass, 0.005 parts by mass to 3 parts by mass, or 0.01 parts by mass to 1 part by mass, relative to 100 parts by mass of the regenerated cross-linked polymer.

[0062] (Drying process and pulverization process) The drying process is a process of removing (drying) water from the regenerated crosslinked polymer to form a water-absorbent resin containing the regenerated crosslinked polymer (hereinafter referred to as regenerated water-absorbent resin). When the reaction liquid subjected to the crosslinking process is an aqueous solution, the reaction liquid itself gels to form a block of crosslinked polymer. The dried product obtained by removing water from this block of crosslinked polymer is the regenerated water-absorbent resin. Furthermore, by subjecting this regenerated water-absorbent resin to a pulverization process in which it is pulverized into particles, a particulate water-absorbent resin (regenerated water-absorbent resin particles) can be obtained.

[0063] The method for drying the regenerated crosslinked polymer may be, for example, a squeezing method such as centrifugation, dehydration using an organic solvent, natural drying, heat drying, air drying, freeze drying, or a combination thereof. The heating temperature for drying may be 80°C to 220°C, 90°C to 200°C, or 100°C to 180°C from the viewpoint of efficiently removing water. The water content of the regenerated water absorbent resin obtained through the drying step may be, for example, 20% by mass or less, 10% by mass or less, or 5% by mass or less. The water content of the regenerated water absorbent resin refers to the proportion of the water content contained in the regenerated water absorbent resin based on the total amount of the regenerated water absorbent resin. The water content of the regenerated water absorbent resin is calculated based on the difference in mass of the crosslinked polymer before and after heating when the regenerated crosslinked polymer is air-dried (heated) at 200°C for 2 hours. Specifically, when the mass of the recycled water absorbent resin before heating is a and the mass of the recycled water absorbent resin after heating is b, the water content (mass%) is calculated from the formula {(a-b) / a} x 100.

[0064] The regenerated crosslinked polymer may be crushed before drying to form a structure (crushed material) of a relatively small size. By crushing the regenerated crosslinked polymer, its specific surface area increases, allowing water to be removed efficiently. The crushing device for crushing the regenerated crosslinked polymer may be, for example, at least one selected from the group consisting of a kneader (e.g., a pressure kneader, a double-arm kneader), a meat chopper, a cutter mill, and a farmer mill.

[0065] Regenerated water-absorbent resin particles are formed by pulverizing the recycled water-absorbent resin. The pulverization method is not particularly limited. For example, the recycled water-absorbent resin can be pulverized using a pulverizer such as a centrifugal pulverizer, a roller mill, a stamp mill, a jet mill, a high-speed rotary pulverizer, or a container-driven mill. The recycled water-absorbent resin particles may be classified. Classification refers to an operation of dividing a particle group (powder) into two or more particle groups having different particle size distributions. A part or all of the recycled water-absorbent resin particles after classification may be pulverized and classified again.

[0066] The classification method is not particularly limited, and may be, for example, screen classification or air classification. Screen classification is a method of classifying particles on a screen into particles that pass through the meshes of the screen and particles that do not pass through by vibrating the screen. Screen classification can be performed using, for example, a vibrating sieve, a rotary sifter, a cylindrical stirring sieve, a blower sifter, or a rotary shaker. Air classification is a method of classifying particles by utilizing an air flow.

[0067] The median particle diameter of the recycled water-absorbent resin particles may be, for example, 200 μm to 500 μm, or 300 μm to 500 μm. The particle size distribution may be adjusted by mixing two or more recycled water-absorbent resin particles having different median particle diameters obtained by classification.

[0068] <Modifications of Separation Apparatus> Modifications of the separation apparatus of the present invention will be described below with reference to the drawings. Note that the separation apparatus of the present invention can adopt any combination of some or all of the configurations of the modifications described below.

[0069] 3 , in the separation device according to the present embodiment, the side surface portion A2 of the first tank A may be extended to form a minor angle α of less than 90° with respect to the vertical direction. Even in this case, the separated liquid flows inside the first tank A from the lower side in the vertical direction to the upper side in the vertical direction, and therefore, the water absorbent resin dehydrated from the mixture can be separated and recovered.

[0070] (Variation 2) As shown in Fig. 4, the separation device according to this embodiment may have a sieve A6 having a surface in a horizontal direction inside the first tank A. The mesh size of this sieve A6 is a size that allows pulp fibers contained in the mixture to pass through but does not allow most of the water-absorbent resin to pass through. By providing such a sieve A6, it is possible to reduce the amount of water-absorbent resin flowing out from the outlet A4 of the first tank A.

[0071] (Variation 3) As shown in FIG. 5 , in the separation device 100 according to this embodiment, the upper end of the first tank A may have an opening facing vertically upward (i.e., the device may not have a top surface). Furthermore, the side surface of the first tank may not have an inlet. In this embodiment, the opening functions as the outlet A4, and the separated liquid S (pulp mixture liquid S1) flowing out from the outlet A4 flows down along the outer surface of the side surface A2 of the first tank A. In this embodiment, the first tank A is provided inside the second tank B. Specifically, the second tank B has a bottom with a surface wider than the bottom surface A1 of the first tank A and a wall extending from the periphery of the wider bottom surface. The first tank A is provided at the center of the bottom of the second tank B. Therefore, in this embodiment, the pulp mixture liquid S1 flowing down along the outer surface of the side surface A2 of the first tank A is contained in the second tank B even without the first transfer member. A refining section C is provided inside the second tank B, between the first tank A and the wall of the second tank B. The refining section C divides the interior of the second tank B into a first compartment R1 containing the pulp mixture liquid and a second compartment R2 containing the purified separated liquid. The second compartment R2 and the first tank A are connected by a return section D. Specifically, one end of a cylindrical member D4 included in the return section D is immersed in the separated liquid S contained in the second compartment R2, and the other end is disposed at the bottom A1 of the first tank A through the outlet A4 (i.e., in this embodiment, the other end of the cylindrical member D4 functions as the inlet of the first tank). The cylindrical member D4 can be flexible, for example, by being formed from a soft material such as rubber. A pump D5 is provided midway through the cylindrical member D4, and by driving the pump D5, the separated liquid S can be supplied from the second tank B to the bottom A1 of the first tank A.

[0072] The present invention will be described in more detail below with reference to examples and comparative examples.

[0073] [Example 1] 7.08 g of a mixture of water-absorbent resin and pulp fiber collected from a new diaper (manufactured by Elleair Products Co., Ltd.: product name "Goon Pants Massarasara Breathable Sizes Larger Than BIG") was placed in a 1 L plastic beaker, and then 1000 g of physiological saline (0.9 mass % sodium chloride aqueous solution) was added to swell the mixture for 30 minutes. Thereafter, the mixture was filtered through a standard sieve with a mesh size of 75 μm, and the swollen mixture of water-absorbent resin and pulp fiber was recovered in a 1 L plastic beaker. Subsequently, 8.7 g of citric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added, and the mixture was stirred at 100 rpm for 30 minutes using a mixer (manufactured by Shinto Scientific Co., Ltd.: product name "BL1200", four inclined paddles with a blade diameter of 75 mm) to dehydrate the swollen water-absorbent resin. Next, the mixture was filtered through a standard sieve with an opening of 75 μm, and the dehydrated mixture of water-absorbent resin and pulp fibers was collected.

[0074] A separation apparatus as shown in FIG. 5 was simulated using various experimental equipment, and a separation process was carried out. Specifically, a plastic tray having a rectangular bottom measuring 300 mm x 400 mm and a wall of 60 mm in height was prepared as the second tank B, and a standard sieve with a mesh size of 75 μm (purification section C) was placed inside the tray to divide the second tank into a first compartment R1 and a second compartment R2. Then, a 2-L plastic graduated cylinder having a circular bottom and a diameter of 90 mm and a height of 450 mm was placed as the first tank A inside the first compartment R1 (the center of the bottom of the second tank B). Next, a separation liquid (0.5% by mass citric acid aqueous solution, density 1.00 g / cm ) was prepared by dissolving citric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) in ion-exchange water. ３) was prepared, and 3 L of the separated liquid S was placed in the second tank B. Furthermore, as a return section D connecting the second compartment R2 of the second tank B and the first tank A, a neoprene tube (manufactured by WATSON MARLOW: product name "Neoprene 9.6 x T3.2 3M", inner diameter 9.6 mm, outer diameter 12.8 mm) serving as a cylindrical member D4 and a tube pump (manufactured by WATSON MARLOW: product name "630S") serving as a pump D5 connected to the middle of the neoprene tube were prepared, and the neoprene tube was placed so that one end of the neoprene tube was immersed in the separated liquid S placed in the second compartment R2 and the other end of the neoprene tube was in contact with the bottom section A1 of the first tank A through the outlet A4 of the first tank A.

[0075] Subsequently, the obtained mixture was placed on the bottom of the first tank. Thereafter, the tube pump was driven to supply a separation liquid to the bottom of the first tank, and the separation liquid was brought into contact with the mixture, thereby starting the separation process. The rotation speed of the tube pump was set so that the flow rate (ascending rate) of the separation liquid in the first tank was 0.2 m / min, and the tube pump was driven for 420 minutes (i.e., the separation process was carried out for 420 minutes), thereby circulating the separation liquid between the first tank and the second tank. By the separation process, the dehydrated water-absorbent resin contained in the mixture was separated into the first tank, and the pulp fibers contained in the mixture were separated into the first compartment of the second tank as a pulp mixture.

[0076] The tube pump was then stopped to terminate the separation process, and the separated liquid contained in the first tank and the separated liquid contained in the second tank were each filtered through a standard sieve with a mesh size of 75 μm to separate the solids remaining in the first tank (first solids) and the solids remaining in the second tank (second solids), which were then collected in 1-L plastic beakers. Subsequently, 500 g of ion-exchanged water was added to each of the 1-L plastic beakers containing the first and second solids. After standing for 10 minutes, the mixture was filtered through a standard sieve with a mesh size of 75 μm, the supernatant was discarded, and 500 g of ion-exchanged water was added again and left to stand for 10 minutes. After performing this 10-minute washing with ion-exchanged water three times in total, the first and second solids were left to stand in 500 g of ion-exchanged water for 30 minutes. After carrying out this series of washing operations, the first solid fraction and the second solid fraction were each tilted obliquely on a standard sieve with a mesh size of 75 μm to drain water for 30 minutes, and immediately thereafter, their weights (W1) were measured. The measurement results are shown in Table 1. After measuring the weights (W1) immediately after draining, the first solid fraction and the second solid fraction were each transferred to a metal tray (having a rectangular bottom of 90 mm x 120 mm and a wall portion with a height of 60 mm) and dried in a vacuum dryer (manufactured by AS ONE Corporation: product name "ETTAS AVO-310N") under an atmosphere of 120 ° C and -0.1 MPa for 5 hours. Immediately thereafter, the weights (W2) of the dried first solid fraction and the second solid fraction were measured. The measurement results are shown in Table 1. After measuring the weights (W2) immediately after drying, the purity and recovery rate of the water absorbent resin remaining in the first tank after the separation step were calculated by the method described below.

[0077] [Example 2] The rotation speed of the tube pump was set so that the flow rate (ascending rate) of the separated liquid in the first tank was 0.3 m / min, and the tube pump was driven for 150 minutes, except that the same operation as in Example 1 was performed to obtain a dried first solid fraction and a dried second solid fraction. Thereafter, the purity and recovery rate of the water-absorbent resin remaining in the first tank after the separation step were calculated by the method described below. The weight (W1) of each solid fraction immediately after draining and the weight (W2) immediately after drying were also measured in the same manner as in Example 1. The measurement results are shown in Table 1.

[0078] Comparative Example 1 In Comparative Example 1, the separation process was performed without flowing the separated liquid from the bottom to the top in the vertical direction. Specifically, the separation apparatus shown in FIG. 6 was first prepared. This separation apparatus 100 included a 10 L bucket F, a standard sieve G with a mesh size of 1.18 mm and a surface parallel to the horizontal direction, installed inside the 10 L bucket F, and an agitator H (manufactured by Shinto Scientific Co., Ltd.: product name "BL1200", four inclined paddles with a blade diameter of 100 mm) installed inside the 10 L bucket F and above the standard sieve. When installing the standard sieve G, its surface was adjusted to be located sufficiently below the water surface of the separated liquid S contained in the 10 L bucket F. When installing the agitator H, it was adjusted so that all of the agitator blades were located below the water surface contained in the 10 L bucket. After preparing the separation device, citric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was dissolved in ion-exchanged water to prepare a separation liquid (1.0 mass% citric acid aqueous solution, density 1.00 g / cm ３ ) was prepared, and the entire separated liquid was placed in a 10 L bucket.

[0079] The same operation as in Example 1 was performed to recover a mixture of dehydrated water-absorbent resin and pulp fibers. The mixture was then placed in a 10-L bucket and placed on the surface of a standard sieve. Immediately afterwards, the agitator was driven and the mixture was stirred at 100 rpm for 1.5 hours to carry out a separation process. The agitator was then stopped, the standard sieve was lifted from the 10-L bucket, and the residue remaining on its surface was collected in a 1-L plastic beaker as the second solid content. The entire separated liquid contained in the 10-L bucket was then filtered through a standard sieve with a mesh size of 75 μm, and the residue remaining on its surface was collected in another 1-L plastic beaker as the first solid content. Subsequently, 500 g of ion-exchanged water was added to each of the 1-L plastic beakers from which the first and second solid contents had been collected. After standing for 10 minutes, the mixture was filtered through a standard sieve with a mesh size of 75 μm, the supernatant was discarded, and 500 g of ion-exchanged water was added again and allowed to stand for 10 minutes. After such 10-minute washing with ion-exchanged water was performed a total of three times, the first solid fraction and the second solid fraction were allowed to stand in 500 g of ion-exchanged water for 30 minutes. After performing this series of washing operations, the first solid fraction and the second solid fraction were each tilted on a standard sieve with 75 μm mesh size to drain for 30 minutes, and immediately thereafter, their weights (W1) were measured. The measurement results are shown in Table 1.

[0080] After measuring the weight (W1) immediately after draining, each of the first solid content and the second solid content was transferred to a metal tub (having a rectangular bottom of 90 mm x 120 mm and a wall portion with a height of 60 mm) and dried for 5 hours in a vacuum dryer (manufactured by AS ONE Corporation: product name "ETTAS AVO-310N") under a pressure (gauge pressure) of -0.1 MPa and an atmosphere of 120 ° C. Immediately thereafter, the weight (W2) of each of the dried first solid content and second solid content was measured. The measurement results are shown in Table 1. After measuring the weight (W2) immediately after drying, the purity and recovery rate of the water absorbent resin remaining on the bottom of the 10 L bucket after the separation step were calculated by the method described below.

[0081]

[0082] <Method of calculating purity of water absorbent resin and recovery rate of water absorbent resin> The purity of the water absorbent resin of each solid content (purity of the water absorbent resin remaining in the first tank or the second tank after the separation step, or remaining in the lower part of the 10 L bucket) and the recovery rate of the water absorbent resin (proportion of the water absorbent resin remaining in the first tank or remaining in the lower part of the 10 L bucket after the separation step, based on the total water absorbent resin contained in the mixture) were calculated by the following method.

[0083] (Measurement of water absorption amount of water-absorbent resin contained in diaper) First, 0.5 g of water-absorbent resin was collected from the new diaper described above, taking care not to mix in pulp fibers. The collected water-absorbent resin was placed in 500 g of physiological saline and allowed to swell for 30 minutes. The swollen water-absorbent resin was then recovered by filtration through a standard sieve with a mesh size of 75 μm. The recovered water-absorbent resin was placed in a 500 mL polybeaker, and 1.09 g of citric acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added. The mixture was stirred for 30 minutes using a mixer (manufactured by Shinto Scientific Co., Ltd.: product name "BL1200", four inclined paddles with a blade diameter of 75 mm) to dehydrate the swollen water-absorbent resin. The mixture was then filtered through a standard sieve with a mesh size of 75 μm, and the dehydrated water-absorbent resin was collected in a 1 L polybeaker. Subsequently, 500 g of ion-exchanged water was added to a 1 L plastic beaker containing the dehydrated water-absorbing resin. After standing for 10 minutes, the mixture was filtered through a standard sieve with a mesh size of 75 μm, the supernatant was discarded, and 500 g of ion-exchanged water was added again and left to stand for 10 minutes. After performing this 10-minute washing with ion-exchanged water three times in total, the water-absorbing resin was left to stand in 500 g of ion-exchanged water for 30 minutes. After performing this series of washing operations, the water-absorbing resin was tilted diagonally on a standard sieve with a mesh size of 75 μm and drained for 30 minutes. The weight of the standard sieve on which the water-absorbing resin was placed was measured, and the weight of the sieve, which had been weighed in advance, was subtracted from the measured value to measure the weight of the water-absorbing resin, and the measured value (g) was designated as a. Thereafter, the dehydrated water-absorbent resin was dried for 5 hours in a vacuum dryer (manufactured by AS ONE Corporation: product name "ETTAS AVO-310N") under a pressure (gauge pressure) of -0.1 MPa and an atmosphere of 120°C, and then the weight of the dried water-absorbent resin was measured, and the measured value (g) was designated as b. Then, the water absorption capacity was calculated by dividing a by b (a / b). The water absorption capacity was 42.2 times.

[0084] (Measurement of Water Absorption of Pulp Fibers Contained in Diapers) Because it was difficult to extract only pulp fibers from the new diapers described above without contaminating them with water-absorbent resin, disintegrated pulp (manufactured by Leonia Co., Ltd.: product name "Layflock"), a pulp fiber commonly used in absorbent articles, was prepared. 1 L of ion-exchanged water was added to a 1 L plastic beaker, followed by 1.0 g of pulp fiber. A magnetic stir bar (8 mm diameter x 30 mm) was added and placed on a magnetic stirrer (manufactured by Iuchi Co., Ltd.: HS-30D). The magnetic stir bar was stirred at 600 rpm for 30 minutes. The absorbed pulp fibers were then transferred to a pre-weighed 75 μm standard sieve, and the sieve was tilted for 30 minutes to drain. The weight of the standard sieve on which the absorbed pulp fibers were placed was then measured, and the weight of the sieve, which had been weighed in advance, was subtracted from the measured weight to calculate the weight of the absorbed pulp fibers. The weight (a) of the pulp fibers was 15.0 g, and the weight (b) of the dried pulp fibers was 1.0 g, resulting in a water absorption ratio of 15.0 times (15.0 / 1.0).

[0085] (Calculation of the contents of water-absorbent resin and pulp fiber contained in each solid fraction immediately after drying) The contents of water-absorbent resin contained in each solid fraction immediately after drying were assumed to be X (g), and the contents of pulp fiber contained in each solid fraction immediately after drying were assumed to be Y (g). Then, the weights W1 (g) of each solid fraction immediately after draining and W2 (g) of each solid fraction immediately after drying, which were actually measured, were substituted into the simultaneous equations consisting of the following formulas (1) and (2), to calculate the water absorption amounts of the water-absorbent resin and pulp fiber contained in each solid fraction immediately after drying. The results are shown in Table 2. Formula (1)... 42.2X + 15.0Y = W1 Formula (2)... X + Y = W2

[0086]

[0087] (Calculation of Purity of Water-Absorbent Resin of Each Solid Content and Recovery Rate of Water-Absorbent Resin) Based on the content (X) of water-absorbent resin contained in the first solid content and the second solid content and the content (Y) of pulp fiber calculated by the above-mentioned method, the purity of the water-absorbent resin in each solid content immediately after drying was calculated from the following formula (3), and the recovery rate of the water-absorbent resin was calculated from the following formula (4). The results are shown in Table 3. Formula (3)... Purity (mass%) of water-absorbent resin = {X / (X + Y)} x 100 Formula (4)... Recovery rate (mass%) of water-absorbent resin = {X of first solid content / (X of first solid content + X of second solid content)} x 100

[0088]

[0089] As shown in Table 3, the purity of the water absorbent resin of the first solid content in Examples 1 and 2 (the purity of the water absorbent resin remaining in the first tank or remaining at the bottom of the 10 L bucket after the separation step) is higher than that of Comparative Example 1 (in which the water absorbent resin and pulp fibers are separated by size). Therefore, when the water absorbent resin is separated from the mixture using the separation method of the present invention, a high-purity water absorbent resin can be separated and recovered. Furthermore, from the comparison between Examples 1 and 2, it can be seen that the recovery rate of the water absorbent resin can be further improved by setting the flow rate of the separation liquid to less than 0.3 m / min.

[0090] 100: Separation device, A: first tank, B: second tank, C: purification section, D: return section, M: mixture, M1: dehydrated water-absorbent resin, M2: pulp fiber, S: separated liquid.

Claims

1. A method for separating a mixture of dehydrated water-absorbent resin and pulp fibers derived from used absorbent articles, by bringing the mixture into contact with a separating liquid flowing vertically from the lower side to the upper side at a flow rate of 0.08 m / min to 0.40 m / min, thereby separating the dehydrated water-absorbent resin and the pulp fibers in the separating liquid, ３ ~1.2 g / cm ３ A method for separating absorbent resin and pulp fibers.

2. The separation method according to claim 1, wherein the separated liquid is an aqueous solution containing a swelling inhibitor for the dehydrated water-absorbent resin.

3. The separation method according to claim 2, further comprising a recovery step of recovering the pulp fibers separated in the separation liquid after the separation step, wherein the recovery step includes a step of recovering a pulp mixture liquid containing the pulp fibers and the separation liquid downstream of the contact position between the separation liquid and the mixture in the flow path of the separation liquid.

4. The separation method described in claim 3, further comprising a purification step of removing the pulp fibers from the pulp mixture liquid recovered in the recovery step, and a return step of returning the separated liquid purified by the purification step to the separation step.

5. A separation device for separating absorbent resin and pulp fibers, comprising: a first tank having an outlet and configured so that a separation liquid contained therein can flow vertically from the lower side to the upper side; a second tank for accommodating the separation liquid flowing out of the outlet of the first tank; a purification section for removing the pulp fibers from the separation liquid accommodated in the second tank; and a return section for returning the separation liquid purified by the purification section to the first tank.

6. A method for producing a recycled water-absorbent resin, comprising: a preparation step of preparing the water-absorbent resin separated by the separation method according to any one of claims 1 to 4; a cleavage step of cleaving the crosslinked structure of a crosslinked polymer contained in the water-absorbent resin to obtain a polymer; and a crosslinking step of crosslinking the polymer.

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