AU2018360434B2

AU2018360434B2 - Recovery method for excrement and organic acid, and production method for recycled pulp fibers

Info

Publication number: AU2018360434B2
Application number: AU2018360434A
Authority: AU
Inventors: Toshio HIRAOKA; Takashi Kato; Takayoshi Konishi; Noritomo Kurita
Original assignee: Unicharm Corp
Current assignee: Unicharm Corp
Priority date: 2017-11-01
Filing date: 2018-07-26
Publication date: 2021-10-28
Anticipated expiration: 2038-07-26
Also published as: JP6598836B2; JP2019085343A; WO2019087487A1; CN111344275A; KR102379007B1; PH12020550517A1; CN111344275B; KR20200076692A; AU2018360434A1

Abstract

The purpose of the present disclosure is to provide a recovery method that makes it possible to individually recover excrement and an organic acid from a deactivating aqueous solution that includes the excrement and the organic acid. This recovery method has the following features. A method for recovering excrement and an organic acid from a deactivating aqueous solution that is for use on a highly water-absorbent polymer and includes the excrement and the organic acid, the method being characterized by including: a precipitation step (S1) in which a metal salt or a base is added to the deactivating aqueous solution to precipitate a water-insoluble salt of the organic acid; a mixture collecting step (S2) in which a mixture of the water-insoluble salt of the organic acid and solid excrement that originates from the excrement is collected from the deactivating aqueous solution; an organic acid generation step (S3) in which water and an acid that can generate the free organic acid and a water-insoluble salt are added to the mixture to form an aqueous solution that includes the organic acid, the water-insoluble salt, and the solid excrement; and an organic acid aqueous solution acquisition step (S4) in which the water-insoluble salt and the solid excrement are removed from the aqueous solution to obtain an organic acid aqueous solution that includes the organic acid.

Description

I TITLE RECOVERY METHOD FOR EXCREMENT AND ORGANIC ACID, AND PRODUCTION METHOD FOR RECYCLED PULP FIBERS FIELD

[0001] The present disclosure relates to a method of recovering organic acid and excreta from an inactivating aqueous solution for a superabsorbent polymer containing excreta and an organic acid, and to a method of producing recycled pulp fiber from used absorbent articles while reutilizing the organic acid that inactivates the superabsorbent polymer.

BACKGROUND

[0002] Methods for recovering recycled pulp fiber from used absorbent articles are known. PTL 1, for example, discloses a method in which pulp fiber is recovered from used sanitary products containing pulp fiber and a superabsorbent polymer, and reusable recycled pulp is produced as a sanitary product, wherein the method comprises a step of applying physical force to the used sanitary products in an aqueous solution containing a polyvalent metal ion or an acidic aqueous solution with a pH of 2.5 or lower, thereby disintegrating the used sanitary products to pulp fiber and other member s, a step of separating the pulp fiber from the mixture of pulp fiber and other members generated in the disintegration step, and a step of treating the separated pulp fiber with an ozone-containing aqueous solution at a pH of 2.5 or lower.

[0003] PTL 1 describes including an organic acid in an acidic aqueous solution with a pH of 2.5 or lower, where the organic acid is at least one selected from the group consisting of tartaric acid, glycolic acid, malic acid, citric acid, succinic acid and acetic acid.

[CITATION LIST] [PATENT LITERATURE]

[0004]

[PTL 1] Japanese Unexamined Patent Publication No. 2016-881

[TECHNICAL PROBLEM]

[0005] Since most organic acids function as weak acids and create low burden on the environment, it is preferred to use an organic acid for inactivation of a superabsorbent polymer. With inactivation of a superabsorbent polymer containing excreta of a wearer, however, the excreta held by the superabsorbent polymer is discharged into the inactivating aqueous solution, and therefore sterilizing or other treatment is necessary for disposal of the inactivating aqueous solution containing the excreta. In consideration of the environment, it is preferred to recover both the organic acid and excreta from an inactivating aqueous solution containing excreta and an organic acid.

[0006] Any reference to or discussion of any document, act or item of knowledge in this specification is included solely for the purpose of providing a context for the present invention. It is not suggested or represented that any of these matters or any combination thereof formed at the priority date part of the common general knowledge, or was known to be relevant to an attempt to solve any problem with which this specification is concerned.

SUMMARY

[00071 The present inventors have devised a method of recovering organic acid and excreta from a superabsorbent polymer-inactivating aqueous solution that includes an organic acid and excreta, the method comprises a depositing step in which a metal salt that includes a divalent or higher metal or a base that includes a divalent or higher metal is added to the inactivating aqueous solution, to cause deposition of a water-insoluble salt of the organic acid, a mixture collecting step in which a mixture of the water-insoluble salt of the organic acid and solid excreta from the excreta is collected from the inactivating aqueous solution that has passed through the depositing step, an organic acid-generating step in which an acid capable of generating a free organic acid and a water-insoluble salt, and water, are added to the mixture to form an aqueous solution containing the organic acid, the water-insoluble salt and the solid excreta, and an organic acid aqueous solution-acquiring step in which the water-insoluble salt and the solid excreta are removed from the aqueous solution to obtain an organic acid aqueous solution containing the organic acid.

[0007A] In a further form, the present disclosure provides a method of producing recycled pulp fibers from used absorbent articles while reutilizing organic acid that inactivates a superabsorbent polymer, wherein the method includes: an inactivating step in which a member that includes pulp fibers and a superabsorbent polymer from the used absorbent articles is

-Y

immersed in an inactivating aqueous solution containing an organic acid and having a predetermined pH, for inactivation of the superabsorbent polymer, a recycled pulp fiber-forming step in which recycled pulp fibers are formed from the member that has passed through the inactivating step, a depositing step in which the member is removed from the inactivating aqueous solution that has passed through the inactivating step, and a metal salt that includes a divalent or higher metal or a base that includes a divalent or higher metal is added to the inactivating aqueous solution from which the member has been removed, to deposit the water insoluble salt of the organic acid, a mixture-collecting step in which a mixture of the water insoluble salt of the organic acid and solid excreta from the excreta is collected from the inactivating aqueous solution that has passed through the depositing step, an organic acid generating step in which an acid capable of generating a free organic acid and a water-insoluble salt, and water, are added to the mixture to form an aqueous solution containing the organic acid, the water-insoluble salt and the solid excreta, an organic acid aqueous solution-acquiring step in which the water-insoluble salt and the solid excreta are removed from the aqueous solution to obtain an organic acid aqueous solution containing the organic acid, and a re-inactivating step in which the inactivating step is carried out using the organic acid aqueous solution as the inactivating aqueous solution.

[ADVANTAGEOUS EFFECTS OF INVENTION]

[0008] The method of recovering organic acid and excreta according to the present disclosure allows recovery of an organic acid and excreta from an inactivating aqueous solution containing excreta and an organic acid. The method of producing recycled pulp fibers from used absorbent articles according to this disclosure allows production of recycled pulp fibers while recovering the organic acid and excreta and also reutilizing the organic acid.

BRIEF DESCRIPTION OF DRAWINGS

[0009] FIG. 1 is a diagram showing a flow chart for illustration of the recovering method of the disclosure. FIG. 2 is a diagram showing a flow chart for illustration of additional steps in the recovering method of the disclosure. FIG. 3 is a diagram showing a flow chart for illustration of the method of producing recycled pulp fibers according to the disclosure. FIG. 4 is a block diagram showing an example of a system 1 for carrying out the disclosure. FIG. 5 is a schematic diagram showing a construction example for the bag tearing apparatus 11 and shredding apparatus 12 of FIG. 4. FIG. 6 is a graph showing the results for Examples 1 and 2.

DESCRIPTION OF EMBODIMENTS

[0010]

[Definitions] "Water-insoluble salt" As used herein, "water-insoluble", as it pertains to a "water-insoluble salt", means that it is preferably classified as "insoluble", "highly insoluble" or "essentially insoluble", more preferably classified as "highly insoluble" or "essentially insoluble", and even more preferably classified as "essentially insoluble", according to "General Rule 29" of the "Japanese Pharmacopeia, 15th Edition".

[0011]

_r

Specifically, a "water-insoluble salt" means that the amount of water dissolving 1 g of salt in 30 minutes, when 1 g of a salt solute is placed in a water solvent and vigorously shaken for 30 seconds every 5 minutes at 20±5°C, is as follows. -Insoluble: 100 mL or greater and less than 1000 mL -Highly insoluble: 1000 mL or greater and less than 10,000 mL -Essentially insoluble: 10,000 mL or greater

[0012] -"Water-soluble salt" As used herein, "water-soluble", as it pertains to a "water-soluble salt", means that it is preferably classified as "slightly soluble", "soluble" or "highly soluble", more preferably classified as "soluble" or "highly soluble", and even more preferably classified as "highly soluble", according to "General Rule 29" of the "Japanese Pharmacopeia, 15th Edition".

[0013] Specifically, a "water-soluble salt" means that the amount of water dissolving 1 g of salt in 30 minutes, when 1 g of a salt solute is placed in a water solvent and vigorously shaken for 30 seconds every 5 minutes at 20±5°C, is as follows. -Highly soluble: less than 1 mL -Soluble: 1 mL or greater and less than 10 mL -Slightly soluble: 10 mL or greater and less than 30 mL The following classification is also found in "General Rule 29" of the "Japanese Pharmacopeia, 15th Edition". - Slightly insoluble: 30 mL or greater and less than 100 mL

[0014] -"Inactivation", pertaining to superabsorbent polymer As used herein, the term "inactivation", as it pertains to a superabsorbent polymer (SAP), means that a superabsorbent polymer holding excreta is modified to have an absorption factor of preferably no greater than 50-fold, more preferably no greater than 30-fold and even more preferably no greater than 25-fold, by being caused to discharge the excreta being held or by inhibiting absorption of the inactivating aqueous solution, for example. The absorption factor is measured in the following manner. (1) The inactivated superabsorbent polymer is placed on a mesh and suspended for 5 minutes to remove the moisture adhering to its surface, and the mass before drying: mi (g) is measured.

[0015] (2) The inactivated superabsorbent polymer is dried at 120°C for 10 minutes, and the mass after drying: m2 (g) is measured. (3) The absorption factor (g/g) is calculated by the following formula: Absorption factor (g/g)= 100 x mi/m2 An inactivating aqueous solution is an aqueous solution that inactivates the superabsorbent polymer.

[0016] Specifically, the present disclosure relates to the following aspects.

[Aspect 1] A method of recovering an organic acid and excreta from a superabsorbent polymer inactivating aqueous solution that includes an organic acid and excreta, wherein the method comprises: a depositing step in which a metal salt that includes a divalent or higher metal or a base that includes a divalent or higher metal is added to the inactivating aqueous solution, to cause deposition of a water-insoluble salt of the organic acid, a mixture-collecting step in which a mixture of the water-insoluble salt of the organic acid and solid excreta from the excreta is collected from the inactivating aqueous solution that has passed through the depositing step, an organic acid-generating step in which an acid capable of generating a free organic acid and a water-insoluble salt, and water, are added to the mixture to form an aqueous solution containing the organic acid, and the water-insoluble salt and the solid excreta, and an organic acid aqueous solution-acquiring step in which the water-insoluble salt and the solid excreta are removed from the aqueous solution to obtain an organic acid aqueous solution containing the organic acid.

[00171 Methods for recovering organic acids such as citric acid from organic acid-containing aqueous solutions are known, which employ electrodialysis. With recycling of used absorbent articles, however, the inactivating aqueous solution that includes an organic acid used to inactivate a superabsorbent polymer will also contain excreta from the wearer in addition to the organic acid, and therefore when it is attempted to purify the organic acid from the inactivating aqueous solution by electrodialysis for reuse, the solid excreta from the excreta can potentially clog the ion-exchange membrane that is used in the electrodialysis. When it is attempted to filter the inactivating aqueous solution that includes the organic acid and excreta using a filtration membrane prior to the electrodialysis in order to prevent clogging of the ion-exchange membrane, fine solid excreta tend to clog the filtration membrane, or a very long time is necessary for the filtration.

[0018]

V

In the method described above, the fine solid excreta can be aggregated and collected when the organic acid is converted to a water-insoluble salt of the organic acid and deposited and collected, allowing the organic acid and solid excreta to be separated in the organic acid aqueous solution-forming step. The liquid excreta from the excreta can also be recovered as a liquid in the mixture-collecting step of the method. The method therefore allows recovery of an organic acid and excreta from an inactivating aqueous solution containing excreta and an organic acid.

[0019]

[Aspect 2] The method according to aspect 1, wherein the organic acid is an organic acid with a carboxyl group. Since the organic acid in this method is an organic acid with a carboxyl group, a wider range of options can be available for the acid capable of generating a free organic acid and a water-insoluble salt in the organic acid-generating step, thus allowing the organic acid recoverability to be increased.

[0020]

[Aspect 3] The method according to aspect 1 or 2, wherein in the depositing step, the inactivating aqueous solution is neutralized, and then the metal salt is added to the inactivating aqueous solution, to deposit the water-insoluble salt of the organic acid.

[0021] Since a metal salt is added to the inactivating aqueous solution after neutralizing the inactivating aqueous solution in the depositing step of this method, the organic acid is unlikely to form a chelate complex with the metal of the metal salt, and the recoverability of the organic acid is therefore increased. This method is particularly useful when the organic acid is an organic acid capable of forming chelate complexes with metals.

[0022]

[Aspect 4] The method according to aspect 1 or 2, wherein the organic acid is an organic acid that does not form a chelate complex with a metal, and in the depositing step, the base is added to the inactivating aqueous solution to deposit the water-insoluble salt of the organic acid.

[0023] Since the organic acid in this method is an organic acid that does not form chelate complexes with metals, the base can be directly added to the inactivating aqueous solution to deposit the water-insoluble salt of the organic acid in the depositing step, without neutralization of the inactivating aqueous solution. The method therefore allows recovery of an organic acid and excreta from an inactivating aqueous solution containing excreta and an organic acid, with a fewer number of steps.

[0024]

[Aspect 5] The method according to any one of aspects 1 to 4, wherein the divalent or higher metal is selected from the group consisting of Mg, Ca, Ba, Fe, Ni, Cu, Zn and Al, and any combinations thereof.

[0025] Since the divalent or higher metal in this method is selected from among these specified metals, the water solubility of the water-insoluble salt of the organic acid that is formed is lowered, and the mixture of the water-insoluble salt of the organic acid and the solid excreta can be more easily separated from the water-soluble salt in the subsequent mixture-collecting step.

[0026]

[Aspect 6] The method according to any one of aspects 1 to 5, wherein the acid in the organic acid generating step is an acid having a smaller acid dissociation constant (pKa, in water) than the acid dissociation constant (pKa, in water) of the organic acid.

[00271 Since the acid in this method is an acid having a smaller acid dissociation constant (pKa, in water) than the acid dissociation constant (pKa, in water) of the organic acid, a free organic acid is formed more easily in the organic acid-generating step.

[0028]

[Aspect 7] The method according to any one of aspects 1 to 6, wherein the acid in the organic acid generating step is selected from the group consisting of sulfuric acid, hydrochloric acid, nitric acid, iodic acid and bromic acid.

[0029] Since the acid in the organic acid-generating step in this method is selected from the group consisting of sulfuric acid, hydrochloric acid, nitric acid, iodic acid and bromic acid, a free organic acid is more easily formed in the organic acid-generating step.

[0030]

[Aspect 8] The method according to any one of aspects 1 to 7, which further includes before the depositing step, a concentrating step wherein a pH-adjusting substep in which the organic acid is added to the inactivating aqueous solution to adjust the inactivating aqueous solution to a predetermined pH, and an inactivating substep in which new superabsorbent polymer is inactivated in the inactivating aqueous solution that has passed through the pH-adjusting substep, are alternately repeated to concentrate the organic acid and the excreta in the inactivating aqueous solution.

[0031] This method includes, prior to the depositing step, a concentrating step in which the organic acid and excreta in the inactivating aqueous solution are concentrated while inactivating the superabsorbent polymer in the inactivating aqueous solution. The present inventors have found that a superabsorbent polymer can be inactivated in an inactivating aqueous solution even with a concentration of basic substance-containing excreta, so long as the inactivating aqueous solution is adjusted to a predetermined pH. Therefore, by increasing the concentration of the organic acid and excreta in the inactivating aqueous solution, it is possible to efficiently recover the organic acid and excreta while maintaining inactivation of the superabsorbent polymer.

[0032]

[Aspect 9] The method according to aspect 8, wherein the concentrating step further includes a sterilizing substep in which the inactivating aqueous solution is sterilized.

[0033] An inactivating aqueous solution that includes excreta tends to have proliferation of bacteria in the excreta or bacteria in the environment as time passes, and as the concentration of organic acid and excreta increases. In this method, the concentrating step further includes a sterilizing substep, thus allowing bacteria in the inactivating aqueous solution to be limited to a predetermined amount.

[0034]

[Aspect 10] The method according to aspect 9, wherein in the sterilizing substep, the inactivating aqueous solution is sterilized using ozone, chlorine dioxide, hydrogen peroxide, ultraviolet rays or radiation, or any combination thereof.

[0035] In the sterilizing substep of this method, the inactivating aqueous solution is sterilized using predetermined sterilization means, which allows the bacteria in the inactivating aqueous solution to be limited to a predetermined amount and allows the inactivating aqueous solution to be decolorized and deodorized. Furthermore, since the predetermined sterilization means in this method is sterilization means that essentially does not remain in the inactivating aqueous solution, the sterilization means is unlikely to remain in the recovered organic acid aqueous solution and a step of separating the sterilization means is unnecessary.

[0036]

[Aspect 11] A method of producing recycled pulp fibers from used absorbent articles while reutilizing organic acid that inactivates a superabsorbent polymer, wherein the method includes: an inactivating step in which a member that includes pulp fibers and a superabsorbent polymer from the used absorbent articles is immersed in an inactivating aqueous solution containing an organic acid and having a predetermined pH, for inactivation of the superabsorbent polymer, a recycled pulp fiber-forming step in which recycled pulp fibers are formed from the member that has passed through the inactivating step, a depositing step in which the member is removed from the inactivating aqueous solution that has passed through the inactivating step, and a metal salt that includes a divalent or higher metal or a base that includes a divalent or higher metal is added to the inactivating aqueous solution from which the member has been removed, to deposit the water-insoluble salt of the organic acid, a mixture-collecting step in which a mixture of the water-insoluble salt of the organic acid and solid excreta from the excreta is collected from the inactivating aqueous solution that has passed through the depositing step, an organic acid-generating step in which an acid capable of generating a free organic acid and a water-insoluble salt, and water, are added to the mixture to form an aqueous solution containing the organic acid, and the water-insoluble salt and the solid excreta, an organic acid aqueous solution-acquiring step in which the water-insoluble salt and the solid excreta are removed from the aqueous solution to obtain an organic acid aqueous solution containing the organic acid, and a re-inactivating step in which the inactivating step is carried out using the organic acid aqueous solution as the inactivating aqueous solution.

[00371 This method allows production of recycled pulp fibers from used absorbent articles, while recovering each of the organic acid and excreta and also reutilizing the organic acid.

[0038]

[Aspect 12] The method according to aspect 11, which further includes, after the inactivating step and before the depositing step, a concentrating step wherein a pH-adjusting substep in which the organic acid is added to the inactivating aqueous solution to adjust the inactivating aqueous solution to a predetermined pH, and an inactivating substep in which new superabsorbent

IU

polymer is inactivated in the inactivating aqueous solution that has passed through the pH adjusting substep, are alternately repeated to concentrate the organic acid and the excreta in the inactivating aqueous solution.

[0039] This method further includes a predetermined concentrating step after the inactivating step and before the depositing step. The present inventors have found that a superabsorbent polymer can be inactivated in an inactivating aqueous solution even with a concentration of basic substance-containing excreta, so long as the inactivating aqueous solution is adjusted to a predetermined pH. Therefore, by increasing the concentration of the organic acid and excreta in the inactivating aqueous solution, it is possible to efficiently recover the organic acid and excreta while maintaining inactivation of the superabsorbent polymer.

[0040]

[Aspect 13] The method according to aspect 12, wherein the concentrating step further includes a sterilizing substep in which the inactivating aqueous solution is sterilized.

[0041] An inactivating aqueous solution that includes excreta tends to have proliferation of bacteria in the excreta or bacteria in the environment as time passes, and as the concentration of organic acid and excreta increases. In this method, the concentrating step further includes a sterilizing substep, thus allowing bacteria in the inactivating aqueous solution to be limited to a predetermined amount.

[0042]

[Aspect 14] The method according to aspect 13, wherein in the sterilizing substep, the inactivating aqueous solution is sterilized using ozone, chlorine dioxide, hydrogen peroxide, ultraviolet rays or radiation, or any combination thereof.

[0043] Since the inactivating aqueous solution is sterilized using predetermined sterilization means in the sterilizing substep of this method, it is possible to reduce the amount of bacteria in recycled pulp fibers, and to inhibit coloration and odor of the recycled pulp fibers caused by the inactivating aqueous solution, and thus to reduce the number of steps for washing of the recycled pulp fibers. Furthermore, since the predetermined sterilization means in this method is sterilization means that essentially does not remain in the inactivating aqueous solution, the sterilization means is unlikely to remain in the recycled pulp fibers and a step of separating the sterilization means from the recycled pulp fibers is unnecessary.

[00441 The method of recovering organic acid and excreta from a superabsorbent polymer inactivating aqueous solution containing excreta and an organic acid (hereunder referred to as "method of recovering organic acid and excreta", or simply "recovering method"), and the method of producing recycled pulp fibers from used absorbent articles while reutilizing the organic acid that inactivates the superabsorbent polymer (hereunder also referred to as "method of producing recycled pulp fibers"), according to the present disclosure, will now be described in detail.

[0045] <Method of recovering organic acid and excreta> The method of recovering organic acid and excreta of the disclosure includes the following steps. (Al) A depositing step in which a metal salt that includes a divalent or higher metal or a base that includes a divalent or higher metal is added to an inactivating aqueous solution to deposit a water-insoluble salt of the organic acid (hereunder also referred to as "depositing step"). (A2) A mixture-collecting step in which a mixture of a water-insoluble salt of the organic acid and solid excreta from the excreta is collected from the inactivating aqueous solution that has passed through the depositing step (hereunder also referred to as "mixture-collecting step"). (A3) An organic acid-generating step in which an acid capable of generating a free organic acid and a water-insoluble salt, and water, are added to the mixture to form an aqueous solution containing the organic acid, water-insoluble salt and solid excreta (hereunder also referred to as "organic acid-generating step". (A4) An organic acid aqueous solution-acquiring step in which the water-insoluble salt and solid excreta are removed from the aqueous solution to obtain an organic acid aqueous solution containing the organic acid (hereunder also referred to as "organic acid aqueous solution-acquiring step"). Fig. 1 shows a flow chart for illustration of the recovering method of the disclosure.

[0046] <Depositing step SI> In the depositing step Sl, a metal salt that includes a divalent or higher metal or a base that includes a divalent or higher metal (hereunder also referred to as "water-insoluble salt forming salt") is added to a superabsorbent polymer-inactivating aqueous solution that includes excreta and an organic acid, to cause deposition of a water-insoluble salt of the organic acid. Specifically, by adding a metal salt that includes a divalent or higher metal or a base that includes a divalent or higher metal to the inactivating aqueous solution in the depositing step Si,

I /_

it is possible to obtain an inactivating aqueous solution containing (i) a water-insoluble salt of the organic acid, excreta [(ii) solid excreta and (iii) liquid excreta], and (iv) an aqueous salt.

[00471 The superabsorbent polymer-inactivating aqueous solution that includes excreta and an organic acid can be obtained by inactivating a superabsorbent polymer that has absorbed excreta, in an inactivating aqueous solution containing an organic acid. An organic acid and excreta such as feces and urine are present in a superabsorbent polymer-inactivating aqueous solution that includes excreta and an organic acid. The organic acid is generally dissolved in the inactivating aqueous solution, the (iii) liquid excreta, which are liquids such as urine among the excreta, are generally dissolved in the inactivating aqueous solution, and the (ii) solid excreta, which are solids such as faces, are generally dispersed in the inactivating aqueous solution.

[0048] The organic acid is not particularly restricted so long as it is able to adjust the inactivating aqueous solution to a predetermined pH that allows inactivation of the superabsorbent polymer, and it may be an organic acid that has an acid group, such as a carboxyl or sulfo group. A organic acid with a sulfo group is a sulfonic acid, and an organic acid with a carboxyl group but without a sulfo group is a carboxylic acid. From the viewpoint of carrying out the method of recovering organic acid and excreta of the disclosure, and especially from the viewpoint of widening the options for the acid capable of generating a free organic acid and a water-insoluble salt in the organic acid-generating step, the organic acid is most preferably an organic acid with a carboxyl group, and especially preferably a carboxylic acid.

[0049] When the organic acid has a carboxyl group, the organic acid may have one or more carboxyl groups per molecule, and preferably it has more than one carboxyl group. This will help the organic acid form chelate complexes with divalent or higher metals in the excreta, such as calcium, thus tending to reduce the ash content of the recycled pulp fibers produced from the used absorbent articles.

[0050] Examples of such organic acids include citric acid, tartaric acid, malic acid, succinic acid, oxalic acid (as carboxylic acids having more than one carboxyl group), gluconic acid (C6), pentanoic acid (C 5 ), butanoic acid (C4), propionic acid (C3), glycolic acid (C2), acetic acid (C2) and formic acid (CI) (as carboxylic acids having one carboxyl group), and methanesulfonic acid, trifluoromethanesulfonic acid, benzenesulfonic acid andp-toluenesulfonic acid (as sulfonic acids).

[0051]

I -Y

When a metal salt that includes a divalent or higher metal (hereunder also referred to simply as "metal salt") is added to the inactivating aqueous solution in depositing step Si to deposit (i) a water-insoluble salt of the organic acid, the inactivating aqueous solution may either be neutralized or not neutralized before addition of the metal salt, but preferably a base for neutralization (hereunder also referred to as "neutralizing base") is added to the inactivating aqueous solution to neutralize the inactivating aqueous solution before addition of the metal salt. The organic acid will thus be less likely to form chelate complexes with metals, and the organic acid recoverability will be improved. The method of adding the metal salt after neutralization is particularly useful when the organic acid is an organic acid capable of forming chelate complexes with metals.

[0052] For the purpose of the present specification, a base (neutralizing base or water-insoluble salt-forming base) is preferably a Bronsted-Lowry base, i.e. a substance that can accept protons H.

[0053] When the organic acid is an organic acid capable of forming chelate complexes with metals, the neutralizing base is preferably a base including a monovalent metal, such as lithium hydroxide, sodium hydroxide or potassium hydroxide. This is in order to inhibit formation of chelate complexes by the organic acid.

[0054] The superabsorbent polymer-inactivating aqueous solution that includes excreta and an organic acid may be neutralized to a pH of preferably 5.0 to 10.0, more preferably 6.0 to 9.0 and even more preferably 6.5 to 8.0 by addition of the neutralizing base. This can inhibit formation of chelate complexes by the organic acid, in cases where the organic acid is an organic acid capable of forming chelate complexes with metals.

[0055] The metal salt is not particularly restricted so long as it can form (i) a water-insoluble salt of the organic acid and (iv) a water-soluble salt, by reaction with an organic acid. The metal salt preferably has solubility classified as "highly soluble", "soluble", "slightly soluble" or "slightly insoluble", according to "General Rule 29" of the "Japanese Pharmacopeia, 15th Edition". This is from the viewpoint of shortening the reaction time with the organic acid and making it difficult for unreacted metal salts to remain in the (i) water-insoluble salt of the organic acid.

[0056] The metal salt is preferably a salt of an acid and a base that includes a divalent or higher metal.

I-tr

When the metal salt is added after neutralization of the inactivating aqueous solution, the divalent or higher metal composing the metal salt preferably has an ionization tendency close to that of the metal of the neutralizing base. This will allow the organic acid in the inactivating aqueous solution to be converted to the (i) water-insoluble salt of the organic acid at a high yield.

[00571 The acid capable of forming the metal salt is preferably water-soluble, and it may be either an organic acid or inorganic acid but is preferably an inorganic acid. This is from the viewpoint of recoverability of the organic acid. Inorganic acids include hydrochloric acid, sulfuric acid, nitric acid, iodic acid and bromic acid.

[00581 Examples of divalent or higher metals to form the metal salt include Mg, Ca, Ba, Fe, Ni, Cu, Zn and Al, as well as any combinations thereof. If the metal salt is composed of a divalent or higher metal, the (i) water-insoluble salt of the organic acid (composed of an organic acid and a divalent or higher metal), which is composed of an organic acid and a divalent or higher metal, will be readily deposited in the inactivating aqueous solution, and the (i) water-insoluble salt of the organic acid will be more easily collected in the subsequent mixture-collecting step.

[0059] The metal salt is added in an amount for preferably 0.8 equivalent or greater, more preferably 0.9 equivalent or greater and even more preferably 1.0 equivalent or greater, with respect to the organic acid. The metal salt is also added in an amount for preferably 3.0 equivalents or less, more preferably 2.5 equivalents or less and even more preferably 2.0 equivalents or less, with respect to the organic acid. This is from the viewpoint of efficiently forming the (i) water-insoluble salt of the organic acid. The "equivalents" referred to above are the equivalents between the valency of the metal of the metal salt and the number of acid groups in the organic acid.

[0060] It is preferred to ascertain the amount of organic acid in the inactivating aqueous solution, in order to determine the amount of metal salt to be added. The amount of organic acid in the inactivating aqueous solution can be ascertained by directly measuring the concentration of the organic acid in the inactivating aqueous solution to which the metal salt is to be added, or it can be ascertained by subtracting the amount of inactivating aqueous solution discharged with the superabsorbent polymer and pulp fiber (the amount of discharged organic acid), from the total amount (history) of organic acid added to the inactivating aqueous solution. When the amount of organic acid in the inactivating aqueous solution is unknown, the metal salt may be added in an amount estimated to be in excess with respect to the organic acid, separating the excess metal salt from the organic acid aqueous solution in the subsequent

I -)

mixture-collecting step and organic acid aqueous solution-acquiring step.

[0061] When the organic acid is an organic acid with a carboxyl group, the acid may be hydrochloric acid, sulfuric acid or nitric acid, for example.

[0062] When the organic acid is an organic acid that does not form chelate complexes with metals, the depositing step Si may be carried out by adding a water-insoluble salt-forming base containing a divalent or higher metal to the inactivating aqueous solution, without neutralizing the inactivating aqueous solution, to deposit the (i) water-insoluble salt of the organic acid. This will allow recovery both of an organic acid and excreta from an inactivating aqueous solution containing excreta and an organic acid, with a fewer number of steps.

[0063] Examples of divalent or higher metals to form the water-insoluble salt-forming base include Mg, Ca, Ba, Fe, Ni, Cu, Zn and Al, as well as any combinations thereof. Examples of water-insoluble salt-forming bases include magnesium hydroxide, calcium hydroxide, copper hydroxide and zinc hydroxide.

[0064] The water-insoluble salt-forming base is added in an amount for preferably 0.8 equivalent or greater, more preferably 0.9 equivalent or greater and even more preferably 1.0 equivalent or greater, with respect to the organic acid. The water-insoluble salt-forming base is added in an amount for preferably 3.0 equivalents or less, more preferably 2.5 equivalents or less and even more preferably 2.0 equivalents or less, with respect to the organic acid. This is from the viewpoint of formation of the (i) water-insoluble salt of the organic acid. The "equivalents" referred to above are the equivalents between the valency of the divalent or higher metal composing the water-insoluble salt-forming base, and the number of acid groups in the organic acid.

[0065] Examples of organic acids that do not form chelate complexes with metals include pentanoic acid (C5), butanoic acid (C4), propionic acid (C3), acetic acid (C2) and formic acid

(C). Examples of organic acids that can form chelate complexes with metals include citric acid, oxalic acid, tartaric acid and gluconic acid.

[0066] In the depositing step Sl, the (i) water-insoluble salt of the organic acid is formed and the salt crystallizes and settles. During this time, fine portions of the (ii) solid excreta dispersed in the inactivating aqueous solution adhere onto the surface of the (i) water-insoluble salt of the

IV

organic acid, being taken up and aggregating with the crystals of the (i) water-insoluble salt of the organic acid. In the mixture-collecting step S2, therefore, the fine (ii) solid excreta are collected as solids [that is, the (i) water-insoluble salt of the organic acid and (ii) solid excreta], and are less likely to be included in the liquid [that is, the (iii) liquid excreta and (iv) water soluble salt]. As a result, the suspended solid (SS) concentration in the liquid is lowered, there is less clogging of the filter during solid-liquid separation in which the liquid and solids are separated, and a lower amount of sludge is generated during microbial treatment of the liquid.

[00671 The inactivating aqueous solution that has passed through the depositing step Si includes the (i) water-insoluble salt of the organic acid, the (ii) solid excreta, the (iii) liquid excreta and the (iv) water-soluble salt.

[0068] <Mixture-collecting step S2> In the mixture-collecting step S2, a mixture of the water-insoluble salt of the organic acid and the solid excreta is collected from the inactivating aqueous solution that has passed through the depositing step Sl. Specifically, in the mixture-collecting step S2, the inactivating aqueous solution that includes the (i) water-insoluble salt of the organic acid, the (ii) solid excreta, the (iii) liquid excreta and the (iv) water-soluble salt is separated into the solids [that is, the (i) water-insoluble salt of the organic acid and (ii) solid excreta] and the liquids [that is, the (iii) liquid excreta and (iv) water-soluble salt].

[0069] <Organic acid-generating step S3> In the organic acid-generating step S3, an acid capable of generating a free organic acid and a water-insoluble salt (hereunder also referred to as "free organic acid-generating acid"), and water, are added to the mixture to form an aqueous solution containing the organic acid, water insoluble salt and solid excreta. Specifically, in the organic acid-generating step S3, a free organic acid-generating acid and water are added to the solids [that is, the (i) water-insoluble salt of the organic acid and (ii) solid excreta] to form an aqueous solution containing the (v) organic acid, (vi) water-insoluble salt and (ii) solid excreta.

[00701 The free organic acid-generating acid is not particularly restricted so long as it frees the organic acid from the (i) water-insoluble salt of the organic acid and can form a water-insoluble salt, but it is preferably an acid having a smaller acid dissociation constant (pKa, in water) than the acid dissociation constant (pKa, in water) of the organic acid. This will facilitate freeing of the organic acid in the organic acid-generating step.

[00711 When the organic acid has more than one acid group, such as when the organic acid is a dibasic acid or tribasic acid, the free organic acid-generating acid preferably has a smaller acid dissociation constant (pKa, in water) than the smallest acid dissociation constant (pKa, in water) among the acid dissociation constants (pKa, in water) of the organic acid. This is from the viewpoint of facilitating formation of the free organic acid.

[0072] When the free organic acid-generating acid has more than one acid group, such as when the free organic acid-generating acid is a dibasic acid or tribasic acid, preferably the largest acid dissociation constant (pKa, in water) among the acid dissociation constants (pKa, in water) of the free organic acid-generating acid is smaller than the smallest acid dissociation constant (pKa, in water) among the acid dissociation constants (pKa, in water) of the organic acid. This is from the viewpoint of efficiency of the free organic acid-generating acid.

[0073] An acid having a smaller acid dissociation constant (pKa, in water) than the acid dissociation constant (pKa, in water) of the organic acid may be an organic acid or an inorganic acid, but it is preferably an inorganic acid. Inorganic acids include hydrochloric acid, sulfuric acid, nitric acid, iodic acid and bromic acid.

[0074] The acid dissociation constant (pKa, in water) that is used herein may be the value listed in Denki Kagaku Benran by The Electrochemical Society of Japan. According to Denki Kagaku Benran, the acid dissociation constant (pKa, water, 25°C) of major compounds are as follows.

[Organic acids] -Tartaric acid: 2.99 (pKal), 4.44 (pKa2) -Malic acid: 3.24 (pKal), 4.71 (pKa2) -Citric acid: 2.87 (pKal), 4.35 (pKa2), 5.69 (pKa3)

[Inorganic acid] - Sulfuric acid: 1.99

[00751 The acid dissociation constant (pKa, in water) of acids not listed in Denki Kagaku Benran can be determined by measurement. An example of a device that can be used to measure acid dissociation constants (pKa, in water) is the T3 compound physical analysis system by Sirius Co.

[0076] The free organic acid-generating acid is added in an amount for preferably 0.8 equivalent or greater, more preferably 0.9 equivalent or greater and even more preferably 1.0 equivalent or greater, with respect to the organic acid. This is from the viewpoint of converting the (i) water insoluble salt of the organic acid into the (v) organic acid in its free state.

[00771 The free organic acid-generating acid is also added in an amount for preferably 2.0 equivalents or less, more preferably 1.5 equivalents or less and even more preferably 1.3 equivalents or less, with respect to the organic acid. This is from the viewpoint of making the free organic acid-generating acid less likely to remain in the recovered (vii) organic acid aqueous solution, and of reducing corrosion of the equipment. The equivalents referred to above are the equivalents between the number of acid groups in the free organic acid-generating acid and the number of acid groups of the organic acid.

[0078] The amount of free organic acid-generating acid added can be determined based on the composition of the (i) water-insoluble salt of the organic acid and its dry mass. The free organic acid-generating acid and water added in the organic acid-generating step S3 may be separately added to the mixture, or they may be added together to the mixture, as an aqueous solution of the free organic acid-generating acid.

[00791 <Organic acid aqueous solution-acquiring step S4> In the organic acid aqueous solution-acquiring step S4, water-insoluble salts and solid excreta are removed from the aqueous solution to obtain an organic acid aqueous solution including an organic acid. Specifically, in the organic acid aqueous solution-acquiring step S4, solids, i.e. the (vi) water-insoluble salt and (ii) solid excreta, are removed from an aqueous solution containing (v) an organic acid, (vi) water-insoluble salts and (ii) solid excreta, to obtain liquid, i.e. (vii) an organic acid aqueous solution containing (v) the organic acid.

[0080] The method of recovering organic acid and excreta of the disclosure may also include the following additional step. (A5) A concentrating step in which the organic acid and excreta in the inactivating aqueous solution are concentrated by alternately repeating, before the depositing step, a pH adjusting substep in which an organic acid is added to the inactivating aqueous solution to adjust the inactivating aqueous solution to a predetermined pH (hereunder also referred to as "pH adjusting substep"), and an inactivating substep in which new superabsorbent polymer is inactivated in the inactivating aqueous solution that has passed through the pH-adjusting substep (hereunder also referred to as "inactivating substep") (the above step will also be referred to hereunder as "concentrating step"). The concentrating step may also include a sterilizing substep in which the inactivating aqueous solution is sterilized (hereunder also referred to as "sterilizing substep"). Fig. 2 shows a flow chart for illustration of additional steps in the recovery method of the disclosure.

[00811 <Concentrating step S5> In the concentrating step S5, the organic acid and excreta in the inactivating aqueous solution are concentrated by alternately repeating the pH-adjusting substep S5a and the inactivating substep S5b. By repeating the inactivating substep S5b, excreta held by the superabsorbent polymer are repeatedly released into the inactivating aqueous solution, and the excreta in the inactivating aqueous solution become concentrated. Since the pH of the inactivating aqueous solution increases as the excreta are concentrated, the organic acid is added to adjust the inactivating aqueous solution to the predetermined pH in the pH-adjusting substep S5a, thus resulting in a concentration of the organic acid in the inactivating aqueous solution.

[0082] In the pH-adjusting substep S5a, an organic acid is added to the superabsorbent polymer inactivating aqueous solution that includes an organic acid and excreta, for adjustment of the inactivating aqueous solution to the predetermined pH. Since excreta usually include basic components such as ammonia, inactivation of the superabsorbent polymer in the inactivating aqueous solution and release of the excreta into the inactivating aqueous solution tends to result in a higher pH of the inactivating aqueous solution. The organic acid is therefore added to adjust the inactivating aqueous solution to the predetermined pH.

[0083] The predetermined pH is preferably 4.5 or lower, more preferably 4.0 or lower, even more preferably 3.5 or lower and yet more preferably 3.0 or lower. If the predetermined pH is too high, inactivation of the superabsorbent polymer may be insufficient. The predetermined pH is also preferably 0.5 or higher, and more preferably 1.0 or higher. If the predetermined pH is too low, the recycled pulp fibers can potentially be damaged when, for example, recycled pulps fiber is produced from pulp fibers while inactivating and removing the superabsorbent polymer from a member that includes pulp fibers and a superabsorbent polymer.

[0084] In the inactivating substep S5b, the superabsorbent polymer holding excreta is inactivated. The inactivating substep S5b is preferably carried out at the predetermined pH described above. In the inactivating substep S5b it is possible to inactivate the superabsorbent polymer alone, or to inactivate a superabsorbent polymer in a member that includes a superabsorbent polymer and pulp fiber. The member may be a member comprising a superabsorbent polymer and pulp fiber, such as absorbent cores or absorbent articles (for example, shredded absorbent articles).

[0085] The concentrating step S5 may further include a sterilizing substep S5c in which the inactivating aqueous solution is sterilized. An inactivating aqueous solution that includes excreta tends to have proliferation of bacteria in the excreta or bacteria in the environment as time passes, and as the concentration of organic acid and excreta increases. The sterilizing substep S5c can be carried out using publicly known sterilization means in the technical field, examples of such sterilization means including microbicides, ozone, chlorine dioxide, hydrogen peroxide, ultraviolet rays and radiation, as well as any combinations thereof. Radiation includes electromagnetic radiation (X-ray and y-rays) and particle beams( rays, electron beam, proton beam, deuteron beam, a-rays and neutron rays).

[0086] The sterilizing substep S5c can generally sterilize the inactivating aqueous solution to a bacteria count of preferably 100/mL or less, more preferably 50/mL or less and even more preferably 20/mL or less. The bacteria count is measured according to "General bacterial test methods in water and drainage" of JIS K0350-10-10:2002.

[00871 The sterilization means is preferably sterilization means that essentially does not remain in the inactivating aqueous solution after the sterilizing substep S5c. This is in order to prevent the sterilization means from remaining in the (vii) organic acid aqueous solution that is to be used to recover the organic acid. When the superabsorbent polymer in the member that includes the superabsorbent polymer and pulp fibers is inactivated and recycled pulp fibers is produced in the inactivating substep S5b, the sterilization means can potentially remain in the recycled pulp fibers, sometimes making it necessary to remove the sterilization means from the recycled pulp fibers. Sterilization means that essentially does not remain in the inactivating aqueous solution include microbicides, ozone, chlorine dioxide, hydrogen peroxide, ultraviolet rays and radiation, as well as any combinations thereof.

[0088] The concentrating step S5 may further include a decolorizing step in which the inactivating aqueous solution is decolorized using decolorizing means, and a deodorizing step in which the inactivating aqueous solution is deodorized using deodorizing means. This is because

/- I

coloration and odor from the excreta tend to be produced in the inactivating aqueous solution as the concentration of the excreta increases.

[00891 The decolorizing means is preferably decolorizing means that essentially does not remain in the inactivating aqueous solution after the decolorizing step. The deodorizing means is also preferably deodorizing means that essentially does not remain in the inactivating aqueous solution after the deodorizing step. The sterilization means preferably also serves as the decolorizing means and deodorizing means.

[0090] Sterilization means that also serves as decolorizing means and deodorizing means, and particularly sterilization means (decolorizing means and deodorizing means) that can carry out sterilization, decolorization and deodorization and that essentially does not remain in the inactivating aqueous solution after the sterilizing substep S5c, include ozone, chlorine dioxide, hydrogen peroxide, ultraviolet rays and radiation, as well as any combinations thereof.

[0091] The sterilizing substep S5c can be carried out at any point during the concentrating step S5, such as before the pH-adjusting substep S5a and/or after the pH-adjusting substep S5a. The sterilizing substep S5c can also be carried out once per cycle of the pH-adjusting substep S5a and inactivating substep S5b, or it can be carried out as appropriate for the amount of bacteria, the degree of coloration and the degree of odor of the inactivating aqueous solution, such as once every 5 cycles of the pH-adjusting substep S5a and inactivating substep S5b.

[0092] In the concentrating step S5, the cycle of the pH-adjusting substep S5a and inactivating substep S5b may be repeated until the final concentration of the organic acid in the inactivating aqueous solution reaches preferably 1.5 to 10.0 mass%, more preferably 2.0 to 8.0 mass% and even more preferably 2.3 to 6.0 mass%. This can help avoid inhibited inactivation of the superabsorbent polymer in the inactivating substep S5b and reduced deposition of the organic acid in the inactivating aqueous solution, thereby allowing efficient recovery of the organic acid and excreta.

[0093] <Method of producing recycled pulp fibers> The method of producing recycled pulp fibers of the disclosure includes the following steps. (B1) An inactivating step in which a member that includes pulp fibers and a superabsorbent polymer from used absorbent articles is immersed in an inactivating aqueous solution having a predetermined pH and containing an organic acid, and the superabsorbent polymer is inactivated (hereunder also referred to as "inactivating step"). (B2) A recycled pulp fiber-forming step in which recycled pulp fibers are formed from the member that has passed through the inactivating step (hereunder also referred to as "recycled pulp fiber-forming step").

[0094] (B3) A depositing step in which the member is removed from the inactivating aqueous solution that has passed through the inactivating step, and a metal salt that includes a divalent or higher metal or a base that includes a divalent or higher metal is added to the inactivating aqueous solution from which the member has been removed, to deposit a water-insoluble salt of the organic acid (hereunder also referred to as "depositing step"). (B4) A mixture-collecting step in which a mixture of a water-insoluble salt of the organic acid and solid excreta from the excreta is collected from the inactivating aqueous solution that has passed through the depositing step (hereunder also referred to as "mixture-collecting step"). (B5) An organic acid-generating step in which an acid capable of forming an organic acid and a water-insoluble salt, and water, are added to the mixture to form an aqueous solution containing the organic acid, water-insoluble salt and solid excreta (hereunder also referred to as "organic acid-generating step".

[0095] (B6) An organic acid aqueous solution-acquiring step in which the water-insoluble salt and solid excreta are removed from the aqueous solution to obtain an organic acid aqueous solution containing the organic acid (hereunder also referred to as "organic acid aqueous solution-acquiring step"). (B7) A re-inactivating step in which an inactivating step is carried out using an organic acid aqueous solution as the inactivating aqueous solution (hereunder also referred to as "re inactivating step").

[0096] The method of producing recycled pulp fibers of the disclosure may also further include the following additional step. (B8) A concentrating step in which the organic acid and excreta in the inactivating aqueous solution are concentrated by alternately repeating, after the inactivating step and before the depositing step, a pH-adjusting substep in which an organic acid is added to the inactivating aqueous solution to adjust the inactivating aqueous solution to a predetermined pH (hereunder also referred to as "pH-adjusting substep"), and an inactivating substep in which new superabsorbent polymer is inactivated in the inactivating aqueous solution that has passed through the pH-adjusting substep (hereunder also referred to as "inactivating substep") (the above step will also be referred to hereunder as "concentrating step").

[00971 The concentrating step may also include a sterilizing substep in which the inactivating aqueous solution is sterilized (hereunder also referred to as "sterilizing substep"). Fig. 3 shows a flow chart for illustration of the method of producing recycled pulp fibers according to the disclosure.

[0098] (Inactivating step S101> In the inactivating step S101, a member that includes pulp fibers and a superabsorbent polymer from used absorbent articles is immersed in an inactivating aqueous solution having a predetermined pH and containing an organic acid, and the superabsorbent polymer is inactivated. As mentioned above, the member that includes pulp fibers and a superabsorbent polymer from used absorbent articles may be a member comprising a superabsorbent polymer and pulp fibers (for example, absorbent cores), or absorbent articles (for example, shredded absorbent articles).

[0099] The predetermined pH is preferably 4.5 or lower, more preferably 4.0 or lower, even more preferably 3.5 or lower and yet more preferably 3.0 or lower. If the predetermined pH is too high, inactivation of the superabsorbent polymer may be insufficient. The predetermined pH is also preferably 0.5 or higher, and more preferably 1.0 or higher. If the predetermined pH is too low, the recycled pulp fibers can potentially be damaged when, for example, recycled pulp fibers are produced from pulp fibers while inactivating and removing the superabsorbent polymer from a member that includes pulp fibers and a superabsorbent polymer.

[0100] In the inactivating step S101, for example, the member that includes pulp fibers and a superabsorbent polymer may be stirred for about 5 to 60 minutes in an inactivating tank containing the inactivating aqueous solution, at room temperature, for inactivation of the superabsorbent polymer.

[0101] The superabsorbent polymer is not particularly restricted so long as it is one that is used as a superabsorbent polymer in the technical field, examples of which include those comprising acid groups, such as carboxyl and sulfo groups, and preferably carboxyl groups. Superabsorbent polymers comprising carboxyl groups include polyacrylate-based and polymaleic anhydride salt-based ones, and superabsorbent polymers comprising sulfo groups include polysulfonate-based ones.

[0102]

<Recycled pulp fiber-forming step S102> In the recycled pulp fiber-forming step S102, recycled pulp fibers are formed from the member that has passed through the inactivating step S101. There are no particular restrictions on the specific means for forming the recycled pulp fibers, and it may be formed by any method known in the technical field. For example, by blowing ozone gas into the aqueous solution containing the member that includes pulp fibers and inactivated superabsorbent polymer, which has passed through the inactivating step S101 (for example, the inactivating aqueous solution), as described in PTL 1, it is possible to solubilize the inactivated superabsorbent polymer while sterilizing, bleaching and deodorizing the pulp fibers and forming recycled pulp fibers.

[0103] The concentrating step S103 (pH-adjusting substep S103a, inactivating substep S103b and sterilizing substep S103c), the depositing step S104, the mixture-collecting step S105, the organic acid-generating step S106 and the organic acid aqueous solution-acquiring step S107 are the same as the concentrating step S5 (pH-adjusting substep S5a, inactivating substep S5b and sterilizing substep S5c), the depositing step SI, the mixture-collecting step S2, the organic acid generating step S3 and the organic acid aqueous solution-acquiring step S4 in the method of recovering organic acid and excreta of the present disclosure, and therefore they will not be explained here.

[0104] <Re-inactivating step S108> In the re-inactivating step S108, the organic acid aqueous solution obtained from the organic acid aqueous solution-acquiring step S107 is used as the inactivating aqueous solution for inactivation treatment in the same manner as the inactivating step S101. When the organic acid aqueous solution obtained in the organic acid aqueous solution acquiring step S107 does not have the predetermined pH mentioned above, it is preferably used as the inactivating aqueous solution for the re-inactivating step S108 after adjustment to the predetermined pH.

[0105] A portion of the organic acid aqueous solution obtained in the organic acid aqueous solution-acquiring step S107 is sometimes removed together with the member containing pulp fibers and a superabsorbent polymer, in which case fresh organic acid and water may be supplied to the organic acid aqueous solution obtained in the organic acid aqueous solution-acquiring step S107 for use as the inactivating aqueous solution in the re-inactivating step S108.

[0106] In the re-inactivating step S108, new member that includes pulp fibers and a superabsorbent polymer from used absorbent articles, separate from the one used in the inactivating step S101, is immersed in an inactivating aqueous solution having a predetermined pH and containing an organic acid, and the superabsorbent polymer is inactivated.

[01071 Fig. 4 is a block diagram showing an example of a system 1 for carrying out the disclosure. The system 1 comprises a bag tearing apparatus 11, a shredding apparatus 12, a first separating device 13, a first dust removing device 14, a second dust removing device 15, a third dust removing device 16, a second separating device 17, a third separating device 18, an ozone treatment device 19, a fourth separating device 20, a fifth separating device 21, an ozone treatment device 22, a pH adjusting device 23 and a storage tank 24.

[0108] The bag tearing apparatus 11 opens holes in collection bags containing used absorbent articles, in the inactivating aqueous solution. The shredding apparatus 12 shreds the collection bag which includes the used absorbent articles in the inactivating aqueous solution that has sunk below the liquid surface of the inactivating aqueous solution. Fig. 5 is a schematic diagram showing a construction example for the bag tearing apparatus 11 and shredding apparatus 12 of Fig. 4.

[0109] In the bag tearing apparatus 11, holes are opened in the collection bag A that has filled with the inactivating aqueous solution B and have settled in the inactivating aqueous solution B. The bag tearing apparatus 11 includes a solution tank V and a perforating section 50. The solution tank V holds the inactivating aqueous solution B. The perforating section 50 is provided in the solution tank V and opens holes in the surface of the collection bag A that has contacted with the inactivating aqueous solution B, when the collection bag A has been placed in the solution tank V.

[0110] The perforating section 50 comprises a feeding section 30 and a bag shredder 40. The feeding section 30 feeds (draws) the collection bag A (physically and forcibly) into the inactivating aqueous solution B in the solution tank V. The feeding section 30 may be a stirrer, for example, comprising a stirring blade 33, a support shaft (rotating shaft) 32 that supports the stirring blade 33, and a drive unit 31 that rotates the support shaft 32 around the shaft. Rotation of the stirring blade 33 around the rotating shaft (support shaft 32) by the drive unit 31 produces a swirl flow in the inactivating aqueous solution B. The feeding section 30 draws the collection bag A toward the bottom section direction of the inactivating aqueous solution B (solution tank V) by the swirl flow.

[0111] The bag shredder 40 is situated at the lower end (preferably the bottom) of the solution tank V, and it comprises a bag-shredding blade 41, a support shaft (rotating shaft) 42 that supports the bag-shredding blade 41, and a drive unit 43 that rotates the support shaft 42 around the shaft. By rotating the bag-shredding blade 41 around the rotating shaft (support shaft 42) by the drive unit 43, holes are opened in the collection bag A that has moved to the lowerend of the inactivating aqueous solution B (solution tank V).

[0112] The shredding apparatus 12 shreds the used absorbent articles in the collection bag A that has sunk below the liquid surface of the inactivating aqueous solution B. The shredding apparatus 12 includes a shredder 60 and a pump 63. The shredder 60 is connected with the solution tank V by piping 61, and it shreds the used absorbent articles (liquid mixture 91) in each of the collection bag A in the inactivating aqueous solution B, after the collection bag A has been fed from the solution tank V together with the inactivating aqueous solution B.

[0113] The shredder 60 may be a twin-screw shredder (for example, a twin-screw rotary shredder, twin-screw differential shredder or twin-screw shearing shredder), an example of which is a SUMICUTTER (product of Sumitomo Heavy Industries Environment Co. Ltd.). The pump 63 is connected with the shredder 60 by piping 62, and it draws out the shredded matter obtained by the shredder 60 from the shredder 60, together with the inactivating aqueous solution B (liquid mixture 92), and feeds it to the following step. The shredded matter contains member that includes pulp fibers, superabsorbent polymer, the collection bag A member, films, nonwoven fabrics and elastic solids.

[0114] The first separating device 13 agitates the liquid mixture 92 containing the shredded matter obtained from the shredding apparatus 12 and the inactivating aqueous solution, washing it to remove the contaminants (excreta, etc.) from the shredded matter while separating the inactivating aqueous solution 93 including the pulp fibers and superabsorbent polymer from the liquid mixture 92, and feeds it to the first dust removing device 14.

[0115] The first separating device 13 may be, for example, a washing machine that comprises a washing tank/dewatering tank and a water tank surrounding it. The washing tank/dewatering tank (rotating drum) is used as a washing tank/sifting tank (separation tank). The washing machine may be, for example, an ECO-22B horizontal washing machine (product of Inax Corp.).

[0116] The first dust removing device 14 removes contaminants in the inactivating aqueous z I solution 93 containing pulp fibers and superabsorbent polymer, using a screen having a plurality of openings, thus forming an inactivating aqueous solution 94 containing pulp fibers and superabsorbent polymer with low contaminants. The first dust removing device 14 may be, for example, a screen separator (crude screen separator), and specifically a Pack Pulper (product of Satomi Seisakusho).

[01171 Using a screen having a plurality of openings, the second dust removing device 15 removes the even finer contaminants from the inactivating aqueous solution 94 that contains the pulp fibers and superabsorbent polymer and already has low contaminants, fed out from the first dust removing device 14, to form an inactivating aqueous solution 95 containing the pulp fibers and superabsorbent polymer with even lower contaminants. The second dust removing device 15 may be, for example, a screen separator, and specifically a Lamo Screen (by Aikawa Iron Works Co.).

[0118] The third dust removing device 16 carries out centrifugal separation on the inactivating aqueous solution 95 that contains the pulp fibers and superabsorbent polymer and already has even lower contaminants, fed out from the second dust removing device 15, to form an inactivating aqueous solution 96 containing the pulp fibers and superabsorbent polymer and having yet further lower contaminants. The third dust removing device 16 may be, for example, a cyclone separator, and specifically an ACT Low Concentration Cleaner (by Aikawa Iron Works Co.).

[0119] The second separating device 17 uses a screen with a plurality of openings on the inactivating aqueous solution 96 that contains the pulp fibers and superabsorbent polymer and already has yet further lower contaminants, fed out from the third dust removing device 16, to separate the pulp fibers 97 that includes the remaining inactivating aqueous solution and superabsorbent polymer, from the inactivating aqueous solution 100 that includes the superabsorbent polymer. The second separating device 17 may be, for example, a drum screen separator, and specifically a Drum Screen Dewaterer (by Toyo Screen Kogyo Co., Ltd.).

[0120] The third separating device 18, using a screen with a plurality of openings, separates the pulp fibers 97 that has been fed out from the second separating device 17, into solids 98 including the pulp fibers and superabsorbent polymer and liquids including the remaining superabsorbent polymer and inactivating aqueous solution, while applying pressure to the solids to crush the superabsorbent polymer in the solids. The third separating device 18 may be, for example, a screw press dewaterer, and specifically a screw press dewaterer by Kawaguchi Seiki

Co., Ltd.

[0121] The ozone treatment device 19 treats the solids 98 that have been fed out from the third separating device 18, with an aqueous ozone solution that contains ozone. This oxidatively decomposes the superabsorbent polymer, removing the superabsorbent polymer from the pulp fibers, after which the aqueous ozone solution 99 containing the recycled pulp fibers is discharged.

[0122] The fourth separating device 20 uses a screen with a plurality of openings to separate the recycled pulp fibers from the aqueous ozone solution 99 that has been treated by the ozone treatment device 19. The fourth separating device 20 may be a screen separator, for example.

[0123] The fifth separating device 21, ozone treatment device 22, pH adjusting device 23 and storage tank 24 are devices for regeneration and reutilization of the inactivating aqueous solution that has been used by the system 1. The fifth separating device 21 uses a screen separator to form an inactivating aqueous solution 101 with the superabsorbent polymer removed, from the inactivating aqueous solution 100 that includes the superabsorbent polymer. The ozone treatment device 22 carries out sterilizing treatment with ozone on the inactivating aqueous solution 101 from which the superabsorbent polymer has been removed, to form a sterilized inactivating aqueous solution 102. The pH adjusting device 23 adjusts the sterilized inactivating aqueous solution 102 to a predetermined pH to form a regenerated inactivating aqueous solution 103. The storage tank 24 stores the excess portion of the regenerated inactivating aqueous solution 103.

EXAMPLES

[0124] The present disclosure will now be explained in fuller detail by examples, with the understanding that the disclosure is not meant to be limited to the examples.

[Production Example 1] An inactivating step was carried out a total of 10 times (inactivating step S101 x 1 time, inactivating substep S103b of concentrating step S103 x 9 times), according to the flow chart shown in Fig. 3, to obtain inactivating aqueous solution No. 1. The organic acid was citric acid, the absorbent articles were used disposable diapers, and the superabsorbent polymer was sodium polyacrylate-based. The initial pH of the inactivating aqueous solution (1st time) was adjusted to

2.0, and the pH of the inactivating aqueous solution (2nd to 10th times) was adjusted to about 3.0 in the pH-adjusting substep S103a.

[0125] After the inactivating step S101 and after the inactivating substep S103b of each concentrating step S103, a sterilizing step (sterilizing substep S103c) was carried out with ozone so that the inactivating aqueous solution had a general bacteria count of less than 10/mL, and the inactivating aqueous solution No. 1 was sampled periodically after completing the 1st, 5th and 10th sterilizing steps, to obtain (1st, 5th and 10th) samples of the inactivating aqueous solution No. 1. The inactivated superabsorbent polymer was sampled at each of the 1st to 10th inactivating steps. The inactivating aqueous solution No. 1 that had passed through the total of 10 inactivating steps was analyzed and found to contain citric acid at 2.7 mass%.

[0126]

[Production Example 2] Inactivating aqueous solution No. 2 was prepared in the same manner as Production Example 1, except that the sterilizing step (sterilizing substep S103c) was not carried out. With the inactivating aqueous solution No. 2, (1st, 5th and 10th) samples were periodically taken of the inactivating aqueous solution No. 2 after the 1st, 5th and 10th inactivating steps. The inactivating aqueous solution No. 2 was analyzed and found to contain citric acid at 2.3 mass%.

[01271

[Examples 1 and 2] The general bacteria counts in the (1st, 5th and 10th) periodic samples of inactivating aqueous solution No. 1 and inactivating aqueous solution No. 2 were measured. The results are shown in Fig. 6. The absorption factors (mass ratio) of the inactivated superabsorbent polymers sampled in Production Example 1 were as follows. -First inactivating step S101: ~7.0 -2nd to 10th inactivating substeps S103b: -22.0

[0128] From Fig. 6 it is seen that inactivating aqueous solution No. 1 had a general bacteria count of 0/g (the general bacteria count of the undiluted sample was 0/g), for all of the periodic samples. The periodic samples (1st, 5th and 10th) of inactivating aqueous solution No. 1 were each compared with the periodic samples (1st, 5th and 10th) of inactivating aqueous solution No. 2, and found to have low coloration and odor.

This suggests that when producing recycled pulp fibers, where the number of concentrating steps often increases, it is preferred for the concentrating step to include a sterilizing substep.

[0129] The absorption factors of the inactivated superabsorbent polymer in each of the 2nd to 10th inactivating substeps S103b of Production Example 1 were all approximately 22.0. This indicates that inactivation of the superabsorbent polymer depends on the pH of the inactivating aqueous solution, and is unlikely to be affected by the concentrating step S103 (the discharged excreta).

[0130]

[Example 3] <Depositing step S104> Sodium hydroxide (solid) was added to 2000 g of inactivating aqueous solution No. 1 to adjust the pH to 7. Next, 32 g of calcium chloride was dissolved as a metal salt in inactivating aqueous solution No. 1 while stirring inactivating aqueous solution No. 1, depositing the calcium citrate as a (i) water-insoluble salt of the organic acid, and causing aggregation of the fine (ii) solid excreta.

[0131] (Mixture-collecting step S105> After standing for 24 hours after addition of the calcium chloride, a mesh filter was used for solid-liquid separation of inactivating aqueous solution No. 1, to obtain a (moist) mixture of calcium citrate (tetrahydrate) as the (i) water-insoluble salt of the organic acid with the (ii) solid excreta, and the (moist) mixture was dried at 120°C for 10 minutes to obtain 120 g of a (dry) mixture.

[0132] <Organic acid-generating step S106> A 30 mass% sulfuric acid aqueous solution, as a free organic acid-generating acid, was added to the mixture of the calcium citrate as the (i) water-insoluble salt of the organic acid with the (ii) solid excreta, to 1.0 equivalent with respect to 120 g of the calcium citrate (tetrahydrate). Specifically, considering all of the 120 g of the (dry) mixture to be calcium citrate (tetrahydrate) (= 0.21 mol), the 30 mass% sulfuric acid aqueous solution was added to the (dry) mixture so that the total moles of H+ was 1.26 mol (0.63 mol of sulfuric acid), or 1.0 equivalent with respect to 1.26 mol as the total moles of carboxyl groups in 0.21 mol of calcium citrate (tetrahydrate). Calcium sulfate was deposited as a (vi) water-insoluble salt in the aqueous solution mixture as the 30 mass% sulfuric acid aqueous solution was added.

[0133]

-Y I

<Organic acid aqueous solution-acquiring step S107> The aqueous solution mixture was subjected to solid-liquid separation with a mesh filter to obtain approximately 65 g of a citric acid aqueous solution, as the (vii) organic acid aqueous solution. The pH of the citric acid aqueous solution was 2.1.

REFERENCE SIGNS LIST

[0134] S IDepositing step S2 Mixture-collecting step S3 Organic acid-generating step S4 Organic acid aqueous solution-acquiring step S5 Concentrating step S5a pH-adjusting substep S5b Inactivating substep S5c Sterilizing substep S101 Inactivating step S102 Recycled pulp fiber-forming step S103 Concentrating step S103a pH-adjusting substep S103b Inactivating substep S103c Sterilizing substep S104 Depositing step S105 Mixture-collecting step S106 Organic acid-generating step S107 Organic acid aqueous solution-acquiring step S108 Re-inactivating step 11 Bag tearing apparatus 12 Shredding apparatus 13 First separating device 14 First dust removing device 15 Second dust removing device 16 Third dust removing device 17 Second separating device 18 Third separating device 19 Ozone treatment device

20 Fourth separating device 21 Fifth separating device 22 Ozone treatment device 23 pH adjusting device 24 Storage tank

[0135] In this specification, the terms 'comprises', 'comprising', 'includes', 'including', or similar terms are intended to mean a non-exclusive inclusion, such that a method, system or apparatus that comprises a list of elements does not include those elements solely, but may well include other elements not listed.

Claims

The claims defining the invention are as follows:

[Claim 1] A method of recovering an organic acid and excreta from a superabsorbent polymer inactivating aqueous solution that includes an organic acid and excreta, wherein the method comprises: a depositing step in which a metal salt that includes a divalent or higher metal or a base that includes a divalent or higher metal is added to the inactivating aqueous solution, to cause deposition of a water-insoluble salt of the organic acid, a mixture-collecting step in which a mixture of the water-insoluble salt of the organic acid and solid excreta from the excreta is collected from the inactivating aqueous solution that has passed through the depositing step, an organic acid-generating step in which an acid capable of generating a free organic acid and a water-insoluble salt, and water, are added to the mixture to form an aqueous solution containing the organic acid, the water-insoluble salt and the solid excreta, and an organic acid aqueous solution-acquiring step in which the water-insoluble salt and the solid excreta are removed from the aqueous solution to obtain an organic acid aqueous solution containing the organic acid.
[Claim 2] The method according to claim 1, wherein the organic acid is an organic acid with a carboxyl group.
[Claim 3] The method according to claim 1 or 2, wherein in the depositing step, the inactivating aqueous solution is neutralized, and then the metal salt is added to the inactivating aqueous solution, to deposit the water-insoluble salt of the organic acid.
[Claim 4] The method according to claim 1 or 2, wherein the organic acid is an organic acid that does not form a chelate complex with a metal, and in the depositing step, the base is added to the inactivating aqueous solution to deposit the water-insoluble salt of the organic acid.
[Claim 5] The method according to any one of claims 1 to 4, wherein the divalent or higher metal is selected from the group consisting of Mg, Ca, Ba, Fe, Ni, Cu, Zn and Al, and any combinations thereof.
[Claim 6] The method according to any one of claims 1 to 5, wherein the acid in the organic acid generating step is an acid having a smaller acid dissociation constant (pKa, in water) than the acid dissociation constant (pKa, in water) of the organic acid.
[Claim 7] The method according to any one of claims 1 to 6, wherein the acid in the organic acid generating step is selected from the group consisting of sulfuric acid, hydrochloric acid, nitric acid, iodic acid and bromic acid.
[Claim 8] The method according to any one of claims 1 to 7, which further includes, before the depositing step, a concentrating step wherein a pH-adjusting substep in which the organic acid is added to the inactivating aqueous solution to adjust the inactivating aqueous solution to a predetermined pH, and an inactivating substep in which new superabsorbent polymer is inactivated in the inactivating aqueous solution that has passed through the pH-adjusting substep, are alternately repeated to concentrate the organic acid and the excreta in the inactivating aqueous solution.
[Claim 9] The method according to claim 8, wherein the concentrating step further includes a sterilizing substep in which the inactivating aqueous solution is sterilized.
[Claim 10] The method according to claim 9, wherein in the sterilizing substep, the inactivating aqueous solution is sterilized using ozone, chlorine dioxide, hydrogen peroxide, ultraviolet rays or radiation, or any combination thereof.
[Claim 11] A method of producing recycled pulp fibers from used absorbent articles while reutilizing organic acid that inactivates a superabsorbent polymer, wherein the method includes: an inactivating step in which a member that includes pulp fibers and a superabsorbent polymer from the used absorbent articles is immersed in an inactivating aqueous solution containing an organic acid and having a predetermined pH, for inactivation of the superabsorbent polymer, a recycled pulp fiber-forming step in which recycled pulp fibers are formed from the member that has passed through the inactivating step, a depositing step in which the member is removed from the inactivating aqueous solution that has passed through the inactivating step, and a metal salt that includes a divalent or higher metal or a base that includes a divalent or higher metal is added to the inactivating aqueous solution from which the member has been removed, to deposit the water-insoluble salt of the organic acid, a mixture-collecting step in which a mixture of the water-insoluble salt of the organic acid and solid excreta from the excreta is collected from the inactivating aqueous solution that has passed through the depositing step, an organic acid-generating step in which an acid capable of generating a free organic acid and a water-insoluble salt, and water, are added to the mixture to form an aqueous solution containing the organic acid, the water-insoluble salt and the solid excreta, an organic acid aqueous solution-acquiring step in which the water-insoluble salt and the solid excreta are removed from the aqueous solution to obtain an organic acid aqueous solution containing the organic acid, and a re-inactivating step in which the inactivating step is carried out using the organic acid aqueous solution as the inactivating aqueous solution.
[Claim 12] The method according to claim 11, which further includes, after the inactivating step and before the depositing step, a concentrating step wherein a pH-adjusting substep in which the organic acid is added to the inactivating aqueous solution to adjust the inactivating aqueous solution to a predetermined pH, and an inactivating substep in which new superabsorbent polymer is inactivated in the inactivating aqueous solution that has passed through the pH adjusting substep, are alternately repeated to concentrate the organic acid and the excreta in the inactivating aqueous solution.
[Claim 13] The method according to claim 12, wherein the concentrating step further includes a sterilizing substep in which the inactivating aqueous solution is sterilized.
[Claim 14] The method according to claim 13, wherein in the sterilizing substep, the inactivating aqueous solution is sterilized using ozone, chlorine dioxide, hydrogen peroxide, ultraviolet rays o-r or radiation, or any combination thereof.