JP2024095100A - Method for producing recycled water absorbent resin and method for determining production conditions in said method - Google Patents

Abstract

The present invention aims to fully restore the water absorbing performance of a water absorbent resin contained in a used absorbent article, and to prevent an excessive increase in the cost required for producing the recycled water absorbent resin.
[Solution] A method for producing a regenerated water absorbent resin, comprising: a dehydration step (1) of treating a used water absorbent resin with a polyvalent metal salt; an acid treatment step (2) of treating a dispersion of the water absorbent resin treated with the polyvalent metal salt with an acidic substance so that the pH of the dispersion becomes -1.0 to 3.0; a water washing step (3) of washing the water absorbent resin treated with the acidic substance with water; a neutralization step (4) of neutralizing the water-washed water absorbent resin with an alkali metal salt; and a drying step (5) of drying the water absorbent resin neutralized with the alkali metal salt.
[Selected Figure] Figure 1

Description

The present invention relates to a method for producing recycled water-absorbent resin and a method for determining the production conditions in the production method.

Normally, absorbent articles contain not only absorbent resins, but also a large amount of pulp components to aid in the absorption of human waste and other waste. Currently, most absorbent articles after use are incinerated. However, from the standpoint of resource conservation and environmental protection, it is desirable to separate and recover the pulp components and absorbent resins contained in used absorbent articles, produce recycled pulp components and recycled absorbent resins, and reuse these.

As an existing method for separating and recovering pulp components and water-absorbent polymers from used absorbent articles, for example, a method described in Patent Document 1 is known, which includes a step of disintegrating and dispersing used absorbent articles (sanitary products) in water, and a step of separating and recovering the pulp components (fibers) and water-absorbent polymers (water-absorbent resins) contained in the absorbent articles.

On the other hand, the absorbent resin contained in the used absorbent article described in Patent Document 1 has a reduced water absorption performance. Therefore, in order to reuse the above-mentioned absorbent resin as a recycled absorbent resin, it is necessary to restore the absorption performance of the absorbent resin and regenerate it.

As a conventional method for regenerating the above water-absorbent resin and producing a regenerated water-absorbent resin, for example, the following method can be mentioned.
A method in which a dehydration treatment step, an acid treatment step, and an alkali treatment step are carried out in this order in which a transition metal salt alone or a mixture of a transition metal salt and an alkali metal salt or an alkaline earth metal salt is added to a dispersion of a water-absorbent resin obtained from a used absorbent article (Patent Document 2).
A method of carrying out an operation selected from an operation of changing the pH of the water absorbent resin obtained from a used absorbent article, an operation of changing the temperature of the water absorbent resin, and an operation of contacting the water absorbent resin with a hydrophilic organic solvent, and placing the water absorbent resin in an environment in which the absorbed liquid is discharged (Patent Document 3).
A method including a step of treating a water-absorbent resin obtained from a used absorbent article with an aqueous solution of a polyvalent metal salt, and a step of treating the water-absorbent resin treated with the aqueous solution of a polyvalent metal salt with an aqueous solution of an alkali metal salt (Patent Document 4).
A method including the steps of: inactivating a water-absorbent resin obtained from a used absorbent article with an acid; adding an alkali metal ion source to the water-absorbent resin inactivated with the acid to form a highly absorbent recycled polymer in a wet state; and drying the highly absorbent recycled polymer in a wet state (Patent Document 5).

Of the methods listed above, the method described in Patent Document 2 in particular has the potential to be used with systems that are already being operated using the existing methods described in Patent Document 1 and other documents, and is expected to be put into practical use soon.

JP 2013-150976 A JP 2003-225645 A JP 2003-326161 A JP 2013-198862 A JP 2019-135046 A

In general, the absorbent resin obtained from the recovered used absorbent articles, which are the raw material for the recycled absorbent resin, varies in the content of absorbent liquid such as human waste and/or the content of absorbent resin for each absorbent article. In addition, in the method described in Patent Document 2, the used absorbent articles brought in may be directly subjected to the dehydration process, and in that case, the content of absorbent resin in the used absorbent articles may be high or low for each lot to be treated, and is not stable. Therefore, in the above-mentioned conventional method, it is difficult to determine the manufacturing conditions such as the necessary amount of the agent used in the acid treatment process. Therefore, in the above-mentioned conventional method, if the amount of the agent added is less than the required amount, the recovery of the absorbent performance of the absorbent resin may be insufficient, and the absorbent performance of the obtained recycled absorbent resin may be reduced. In addition, in the above-mentioned conventional method, if the amount of the agent added is excessive compared to the required amount, the cost may increase more than necessary and the absorbent performance of the recycled absorbent resin may be reduced.

One embodiment of the present invention has been made in consideration of the above problems, and its purpose is to provide a method for producing recycled water-absorbent resin that can determine the amount of additive (such as the above-mentioned acid) to be used in the treatment in the method for producing recycled water-absorbent resin even when the content of water-absorbent resin contained in used absorbent articles is unknown, and as a result, can fully recover water-absorbent performance, and can efficiently produce recycled water-absorbent resin with excellent water-absorbent performance, and a method for determining the production conditions in the production method.

As a result of intensive research, the inventors discovered that the above-mentioned problems with the amount of acid used in the acid treatment step in the method described in Patent Document 2 can be solved by adjusting the amount of acid added so that the pH of the dispersion falls within a specific range, and thus arrived at the present invention.

That is, one aspect of the present invention is a method for producing a regenerated water-absorbent resin from a water-absorbent resin contained in a used absorbent article, the water-absorbent resin containing a polymer having a partially neutralized carboxyl group, the method including a dehydration step (1) of treating the water-absorbent resin with a polyvalent metal salt, an acid treatment step (2) of treating the water-absorbent resin treated with the polyvalent metal salt with an acidic substance, a water washing step (3) of washing the water-absorbent resin treated with the acidic substance with water, a neutralization step (4) of neutralizing the water-washed water-absorbent resin with an alkali metal salt, and a drying step (5) of drying the water-absorbent resin neutralized with the alkali metal salt to obtain a regenerated water-absorbent resin, and in the acid treatment step (2), the acidic substance is added to a dispersion of the water-absorbent resin treated with the polyvalent metal salt so that the pH of the dispersion is -1.0 to 3.0.

Another aspect of the present invention is a method for determining the manufacturing conditions in a method for producing a regenerated water-absorbent resin from a water-absorbent resin contained in a used absorbent article, the water-absorbent resin containing a polymer having a partially neutralized carboxyl group, the method for producing a regenerated water-absorbent resin including a dehydration step (1) for treating the water-absorbent resin with a polyvalent metal salt, an acid treatment step (2) for treating the water-absorbent resin treated with the polyvalent metal salt with an acidic substance, a water washing step (3) for washing the water-absorbent resin treated with the acidic substance with water, a neutralization step (4) for neutralizing the water-washed water-absorbent resin with an alkali metal salt, and a drying step (5) for drying the water-absorbent resin neutralized with the alkali metal salt to obtain a regenerated water-absorbent resin, the method for determining the manufacturing conditions in the method for producing a regenerated water-absorbent resin including determining the end point of the acid treatment step (2) based on the pH of the dispersion of the water-absorbent resin treated with the polyvalent metal salt to which the acidic substance has been added during the acid treatment step (2).

According to one embodiment of the present invention, even if the content of the water-absorbent resin contained in a used absorbent article is unknown, the amount of additive (such as the above-mentioned acid) to be used in the regeneration process can be determined, and as a result, the water-absorbent performance is sufficiently restored, making it possible to produce a regenerated water-absorbent resin with excellent water-absorbent performance, and preventing an excessive increase in the cost required for producing the regenerated water-absorbent resin.

FIG. 1 is a schematic diagram showing an embodiment of each step constituting an example of a method for producing a recycled water-absorbent resin according to an embodiment of the present invention.

One embodiment of the present invention will be described below, but the present invention is not limited to this. The present invention is not limited to each of the configurations described below, and various modifications are possible within the scope of the claims. The technical scope of the present invention also includes embodiments obtained by appropriately combining the technical means disclosed in different embodiments. In this specification, unless otherwise specified, "A to B" representing a numerical range means "greater than or equal to A and less than or equal to B."

1. Definitions of Terms
[1-1. Water-absorbent resin]
In this specification, the term "water-absorbent resin" refers to a polymer gelling agent having a water swelling capacity (CRC) of 5 g/g or more as defined by NWSP 241.0.R2(15) and a water-soluble component (Ext) of 70% by weight or less as defined by NWSP 270.0.R2(15).

"NWSP" stands for "Non-Woven Standard Procedures-Edition 2015." NWSP was jointly published by EDANA (European Disposables And Nonwovens Association) and INDA (Association of the Nonwoven Fabrics Industry) to standardize evaluation methods for nonwoven fabrics and their products in the United States and Europe, and indicates standard measurement methods for water-absorbent resins. Unless otherwise specified, the physical properties of water-absorbent resins are measured in accordance with NWSP in this specification.

In this specification, the term "water-absorbent resin" is not limited to the case where the total amount (100% by weight) is the water-absorbent resin alone, but may be a water-absorbent resin composition containing additives. In addition, in this specification, the term "water-absorbent resin" refers not only to the water-absorbent resin contained in a used absorbent article, but also to the water-absorbent resin subjected to each step constituting the method for producing a recycled water-absorbent resin according to one embodiment of the present invention, that is, to an intermediate in the method for producing a recycled water-absorbent resin, and may include the recycled water-absorbent resin obtained by the method. In addition, the intermediate water-absorbent resin may be in a state of a hydrous gel in which water is taken in. Therefore, the water-absorbent resin may include a water-absorbent resin in a state of a hydrous gel. The water-absorbent resin includes a polymer having a partially neutralized carboxyl group.

[1-2. Used absorbent articles]
In this specification, the term "absorbent article" refers to an article that contains a water-absorbent resin as a constituent material and has liquid-absorbing properties. The "absorbent article" generally contains pulp, nonwoven fabric, plastic, etc. as constituent materials other than the water-absorbent resin. The absorbent article is not particularly limited, and examples thereof include sanitary materials such as paper diapers, incontinence pads, and sanitary napkins.

In this specification, the term "used absorbent article" refers to an absorbent article that has absorbed liquid. The liquid is not particularly limited, and examples include body fluids such as urine, blood, and sweat, and physiological saline.

In this specification, used absorbent articles that have been crushed by the crushing process described below are also included in the term "used absorbent articles."

[1-3. Others]
In this specification, the range "X to Y" means "greater than or equal to X and less than or equal to Y."

In this specification, "acid (salt)" means "acid and/or its salt." "(Meth)acrylic" means "acrylic and/or methacrylic." "Poly...-based water-absorbent resin" means a water-absorbent resin whose main component is the monomer described in "..." as a repeating unit. Specifically, it means a water-absorbent resin in which the molar ratio of the monomer described in "..." to the total monomers (excluding crosslinking agents) is preferably 50 to 100 mol%, more preferably 70 to 100 mol%, even more preferably 90 to 100 mol%, and particularly preferably substantially 100 mol%.

In this specification, the unit of volume "liter" may be written as "l" or "L".

[2. Manufacturing method of recycled water absorbent resin]
A method for producing a recycled water absorbent resin according to one embodiment of the present invention is a method for producing a recycled water absorbent resin from a water absorbent resin contained in a used absorbent article, the water absorbent resin containing a polymer having a partially neutralized carboxyl group, the method including a dehydration step (1) of treating the water absorbent resin with a polyvalent metal salt, an acid treatment step (2) of treating the water absorbent resin treated with the polyvalent metal salt with an acidic substance, a water washing step (3) of washing the water absorbent resin treated with the acidic substance with water, a neutralization step (4) of neutralizing the water absorbent resin washed with water with an alkali metal salt, and a drying step (5) of drying the water absorbent resin neutralized with the alkali metal salt to obtain a recycled water absorbent resin, and in the acid treatment step (2), the acidic substance is added to a dispersion of the water absorbent resin treated with the polyvalent metal salt so that the pH of the dispersion is -1.0 to 3.0.

Each step constituting the method for producing the recycled water-absorbent resin will be described in detail below. In the following description, reference will be made as appropriate to FIG. 1, which is a schematic diagram showing an aspect of each step constituting an example of a method for producing a recycled water-absorbent resin according to one embodiment of the present invention.

The reactions in each step may be carried out as batch reactions, or may be carried out continuously if the time required for the reactions can be secured.

[2-1. Dehydration process]
The dehydration step (1) in one embodiment of the present invention is a step of treating the water-absorbent resin contained in a used absorbent article with a polyvalent metal salt. means that a polyvalent metal salt is added to drain (dehydrate) at least a portion of the liquid absorbed in the water-absorbent resin contained in the used absorbent article.

The water-absorbing resin to be subjected to the dehydration step (1) may be a water-absorbing resin previously separated from a used absorbent article, or may be a water-absorbing resin in a form contained in a used absorbent article. The term "subjecting a water-absorbing resin in a form contained in a used absorbent article" to the dehydration step (1) means subjecting a used absorbent article, or a used absorbent article crushed by a crushing step described below, to the dehydration step (1) as it is. In that case, the water-absorbing resin is then separated from the used absorbent article treated with the polyvalent metal salt to obtain a water-absorbing resin treated with the polyvalent metal salt. The water-absorbing resin previously separated from a used absorbent article may be, for example, a water-absorbing resin obtained by a crushing step and/or a separation step described below.

The method for separating the water-absorbent resin from the used absorbent article described above is not particularly limited, and any known method can be used. Examples of the known method include the following method including a crushing step and/or a separation step.

The crushing step is a step of crushing the used absorbent article. The crushing step is performed by physical forces such as stirring, cutting, shredding, and crushing. The crushing step may be performed while the used absorbent article is immersed in water (aqueous solution). In addition, in order to facilitate the separation of the water-absorbent resin in the separation step described below, the crushing step may be performed by crushing the used absorbent article while stirring the absorbent article.

The separation process is a process of separating the crushed used absorbent article into each component material including the water-absorbent resin, and recovering at least the water-absorbent resin. In the separation process, each component material other than the water-absorbent resin may be recovered. The water-absorbent resin recovered in the separation process is in the form of a hydrous gel.

The method for separating the aforementioned constituent materials is not particularly limited, and examples thereof include a method for separating and recovering the constituent materials using a screen, and a method for separating and recovering the constituent materials from the crushed absorbent article dispersed in water by utilizing the difference in specific gravity of each of the constituent materials. In addition, for example, the crushing step may involve crushing the absorbent article in water to prepare the crushed absorbent article dispersed in water, and the separation step may be carried out at the same time.

The used absorbent article may be washed and/or sterilized before the dehydration step (1) or before the crushing step when the crushing step is performed before the dehydration step (1). The above-mentioned washing and/or sterilization may be performed by placing the used absorbent article in a high-temperature and high-pressure environment to kill harmful bacteria contained in the article, or may be performed by using a chemical agent to kill the harmful bacteria. When a chemical agent is used, the chemical agent is not particularly limited as long as the desired cleanliness can be obtained. Examples of the chemical agent include an aqueous solution in which a disinfectant such as sodium hypochlorite and chlorine dioxide is dissolved, ozone water, and electrolyzed water (acidic electrolyzed water). Here, when the washing and/or sterilization is performed using the chemical agent, a solution (dispersion) containing the water-absorbing resin and the chemical agent can be obtained. In particular, when the washing and/or sterilization is performed using ozone water, it is preferable to perform the washing and/or sterilization under acidic conditions. Furthermore, it is preferable that the above-mentioned cleaning and/or sterilization be carried out in a tank having an agitating blade and/or that the solution be circulated to efficiently mix the absorbent article with the drug.

After the above-mentioned washing and/or sterilization, the washed and/or sterilized absorbent article is separated from the liquid by a known method such as a filtration operation. The separated absorbent article can be subjected to the dehydration step (1) or the crushing step if the crushing step is carried out before the dehydration step (1).

The water-absorbing resin includes a polymer having a partially neutralized carboxyl group. The polymer is not particularly limited, and examples thereof include crosslinked poly(meth)acrylate-based polymers, isobutylene/maleate-based copolymers, starch/polyacrylate-based copolymers, polyvinyl alcohol (PVA)/polyacrylate-based copolymers, starch/polyacrylonitrile copolymers (nitrile groups are hydrolyzed to carboxylate salts), crosslinked carboxymethyl cellulose (cellulose to which carboxymethyl groups are bonded), and the like. The water-absorbing resin may be a three-dimensional crosslinked body having a crosslinked structure. The water-absorbing resin is preferably a three-dimensional crosslinked body of a poly(meth)acrylic acid-based polymer.

The water-absorbing resin includes a carboxyl group (-C(=O)OH) and a neutralized carboxyl group (a group consisting of -C(=O) ^O- and A ⁺ , A ⁺ : cation). Here, the A ⁺ : cation is a monovalent cation, for example, an alkali metal ion such as a sodium ion (Na ⁺ ). Thus, the water-absorbing resin may be a polymer including a carboxyl group partially neutralized with an alkali metal salt such as sodium hydroxide.

The water-absorbent resin contained in the used absorbent article (hereinafter referred to as "used water-absorbent resin") is in a swollen state by absorbing liquid (such as human waste). Here, the "used water-absorbent resin" is the water-absorbent resin to be subjected to the dehydration step (1). The swollen water-absorbent resin absorbs and holds liquid (such as human waste) due to the osmotic pressure generated by dissociated A ⁺ and the electrostatic repulsion of dissociated carboxyl groups (C(=O) ^O- ). The osmotic pressure acts in a direction to eliminate the concentration difference between the inside and outside of the water-absorbent resin because the ion concentration [A ⁺ , C(=O)O-] inside the water-absorbent resin is higher than the ion concentration of the liquid present outside the water-absorbent resin. In other words, the osmotic pressure acts so that the external liquid enters the inside of the water-absorbent resin, that is, the water-absorbent resin absorbs and holds the external liquid. On the other hand, the electrostatic repulsion acts in a direction in which the dissociated carboxyl groups repel each other inside the water-absorbing resin that has absorbed water and become swollen, widening the distance between the dissociated carboxyl groups. Therefore, gaps are generated between the dissociated carboxyl groups, and the external liquid enters the gaps. At the same time, electrostatic attraction is generated between the dissociated carboxyl groups and the molecules (e.g., water molecules) constituting the external liquid, and the molecules constituting the external liquid are trapped in the gaps. Therefore, the electrostatic repulsion acts so that the water-absorbing resin absorbs and holds the external liquid. As a result, the liquid is absorbed and held by the "used water-absorbing resin". Here, when the swollen water-absorbing resin comes into contact with a polyvalent metal ion, as shown in the structural formula before and after the "dehydration process" in FIG. 1, a plurality of dissociated carboxyl groups are ionically bridged via a polyvalent metal ion (e.g., Ca ²⁺ ). By the ionic cross-linking, the swollen water absorbent resin shrinks, and as a result, the liquid absorbed by the swollen water absorbent resin is released. Therefore, when the "used water absorbent resin" is treated with the polyvalent metal salt, it comes into contact with the polyvalent metal ions, and as a result, the absorbed liquid is released and the resin is dehydrated. Hereinafter, the operation of releasing the liquid absorbed by the swollen water absorbent resin is referred to as the "dehydration operation".

In the dehydration operation, the method of treating with the polyvalent metal salt, i.e., adding the polyvalent metal salt, can be, for example, a method of contacting the used absorbent article or used water-absorbent resin with an aqueous solution containing the polyvalent metal salt. Specifically, a method of immersing the used absorbent article or used water-absorbent resin in an aqueous solution containing the polyvalent metal salt can be exemplified. In addition, it is preferable to contact the used absorbent article or used water-absorbent resin with an aqueous solution containing a polyvalent metal compound under stirring.

As the polyvalent metal salt, alkaline earth metal salts, transition metal salts, etc. can be used.

As the alkaline earth metal salt, water-soluble salts of beryllium, magnesium, calcium, strontium, barium, etc. can be used. One of these may be selected and used, or two or more may be used in combination. Examples of preferred alkaline earth metal salts include calcium chloride, calcium nitrate, magnesium chloride, magnesium nitrate, etc., and calcium chloride is more preferred.

Examples of the transition metal salt include water-soluble salts of iron, cobalt, nickel, copper, etc. As the transition metal salt, any salt, such as an inorganic acid salt, an organic acid salt, or a complex, can be used as long as it can be incorporated into the water-absorbent polymer. Among them, in terms of cost and availability, inorganic acid salts or organic acid salts are preferred as the transition metal salt. Examples of the inorganic acid salt include iron salts such as iron chloride, iron sulfate, iron phosphate, and iron nitrate; cobalt salts such as cobalt chloride, cobalt sulfate, cobalt phosphate, and cobalt nitrate; nickel salts such as nickel chloride and nickel sulfate; and copper salts such as copper chloride and copper sulfate. Examples of the organic acid salts include iron lactate, cobalt acetate, cobalt stearate, nickel acetate, and copper acetate.

The amount of the polyvalent metal salt used may be determined from the contraction state of the absorbent resin after the dehydration operation, i.e., after treatment with the polyvalent metal salt. Here, when a polyvalent metal salt is added to a used absorbent article, the absorbent resin after treatment with the polyvalent metal salt is the absorbent resin contained in the used absorbent article after the polyvalent metal salt is added. Also, when a polyvalent metal salt is added to a used absorbent resin, the absorbent resin after treatment with the polyvalent metal salt is the absorbent resin after the polyvalent metal salt is added. The amount of the polyvalent metal salt used may be determined, for example, by performing the dehydration operation using a certain amount of polyvalent metal salt on a used absorbent article or a used absorbent resin, and then taking out a part of the absorbent resin after treatment with the polyvalent metal salt and visually checking the contraction state. If the water absorbent resin does not shrink sufficiently after being treated with the polyvalent metal salt, additional polyvalent metal salt is added to the water absorbent resin after being treated with the polyvalent metal salt, and the dehydration operation is carried out again to sufficiently dehydrate the resin. However, if the amount of polyvalent metal salt is excessive, the excess polyvalent metal ions remain in the treatment liquid without being incorporated into the used water absorbent resin, which leads to a waste of the polyvalent metal salt and increases the treatment cost.

The treatment time in the treatment with the polyvalent metal salt is not particularly limited as long as it is sufficient for the polyvalent metal ions to be incorporated into the used water absorbent resin, and is preferably 5 minutes to 3 hours, more preferably 5 minutes to 2 hours, and even more preferably 5 minutes to 1 hour. If the treatment time in the treatment with the polyvalent metal salt is too short, the dehydration of the used water absorbent resin is insufficient, and a large amount of liquid components remains inside. If the treatment time in the treatment with the polyvalent metal salt is too long, the amount of polyvalent metal ions incorporated into the used water absorbent resin becomes saturated, so a treatment time exceeding that value is economically undesirable. The treatment time in the treatment with the polyvalent metal salt means the time from the time when the addition of the polyvalent metal salt to the used absorbent article or the used water absorbent resin is started to the time when the operation of removing the water absorbent resin treated with the polyvalent metal salt described later is performed.

The reaction temperature in the treatment with the polyvalent metal salt is not particularly limited as long as it is a temperature at which polyvalent metal ions are incorporated into the used water-absorbent resin. The reaction temperature in the treatment with the polyvalent metal salt is preferably 5 to 80°C, more preferably 10 to 70°C, and even more preferably 15 to 60°C. The reaction temperature refers to the temperature of a mixture, such as an aqueous solution, that contains the polyvalent metal salt and the used absorbent article or used water-absorbent resin.

The water-absorbent resin after being treated with the polyvalent metal salt is in a state of being present in a dispersion containing a liquid such as water (hereinafter referred to as the "dispersion in the dehydration step (1)"). The water-absorbent resin after being treated with the polyvalent metal salt obtained in the dehydration step (1) may remain in the dispersion in the dehydration step (1). The water-absorbent resin after being treated with the polyvalent metal salt obtained in the dehydration step (1) may be in a state of being taken out from the dispersion in the dehydration step (1). Here, when the dehydration operation is performed on a used absorbent article, the operation for taking out the water-absorbent resin after being treated with the polyvalent metal salt can be, for example, the above-mentioned separation operation or the crushing operation and separation operation. When the dehydration operation is performed on a used water-absorbent resin, the operation for taking out the water-absorbent resin after being treated with the polyvalent metal salt can be, for example, a known method such as a filtration operation.

[2-2. Acid treatment process]
The acid treatment step (2) in one embodiment of the present invention is a step of treating the water absorbent resin treated with the polyvalent metal salt with an acidic substance. Here, the term "treating with an acidic substance" generally means adding the acidic substance to a dispersion (containing a dispersion solvent such as water) of the water absorbent resin treated with the polyvalent metal salt.

In the acid treatment step (2), after preparing a dispersion of the water-absorbent resin treated with the polyvalent metal salt (hereinafter also referred to as the "dispersion in the acid treatment step (2)"), an acidic substance may be added to the dispersion in the acid treatment step (2), so that the water-absorbent resin in the dispersion in the acid treatment step (2) is brought into contact with the acidic substance and treated with the acidic substance. The preparation of the dispersion in the acid treatment step (2) may be performed by adding the water-absorbent resin treated with the polyvalent metal salt to a dispersion solvent. In addition, the preparation of the dispersion in the acid treatment step (2) and the addition of the acidic substance may be performed simultaneously, so that the water-absorbent resin in the dispersion in the acid treatment step (2) is brought into contact with the acidic substance and treated with the acidic substance. The simultaneous preparation of the dispersion in the acid treatment step (2) and the addition of the acidic substance means that the water absorbent resin treated with the polyvalent metal salt is added to a solution containing the acidic substance to prepare the dispersion in the acid treatment step (2), and the water absorbent resin in the dispersion in the acid treatment step (2) is brought into contact with the acidic substance. The dispersion solvent and/or the solvent in the solution containing the acidic substance are not particularly limited as long as they are solvents that can prepare the dispersion in the acid treatment step (2), and for example, water can be used.

In addition, when the water-absorbing resin treated with the polyvalent metal salt remains in the dispersion in the dehydration step (1), the water-absorbing resin in the dispersion in the dehydration step (1) may be brought into contact with the acidic substance by adding the acidic substance to the dispersion in the dehydration step (1), and treated with the acidic substance.

In the acid treatment step (2), it is more preferable to add the water-absorbing resin treated with the polyvalent metal salt while stirring the solution containing the acidic substance, thereby bringing the acidic substance into contact with the water-absorbing resin treated with the polyvalent metal salt.

In the acid treatment step (2), as shown in the structural formulas before and after the "acid treatment step" in Fig. 1, the water absorbent resin treated with the polyvalent metal salt is treated with an acidic substance to dissociate bonds between the dissociated carboxyl groups (C(=O) ^O- ) in the water absorbent resin treated with the polyvalent metal salt and the polyvalent metal ions (e.g., Ca2 ⁺ ). As a result, the polyvalent metal ions are released from the water absorbent resin treated with the polyvalent metal salt, and a water absorbent resin having carboxyl groups (C(=O)OH) in a free acid state is obtained.

The acidic substance is not particularly limited, and for example, inorganic acids and/or organic acids can be used.

Examples of the inorganic acid include sulfuric acid, hydrochloric acid, and nitric acid. Examples of the organic acid include organic substances having an acid group, such as a carboxyl group or a sulfo group. As the acidic substance, one acidic substance selected from the inorganic acids and organic acids described above may be used, or a mixture of two or more acidic substances selected may be used. As the acidic substance, an inorganic acid and an organic acid may be used in combination. Specific examples of the organic acid include citric acid, tartaric acid, malic acid, succinic acid, oxalic acid, gluconic acid, pentanoic acid, butanoic acid, propionic acid, glycolic acid, acetic acid, formic acid, and sulfonic acid. Examples of the sulfonic acid include methanesulfonic acid, trifluoromethanesulfonic acid, benzenesulfonic acid, and p-toluenesulfonic acid.

The acidic substance is added to the water-absorbent resin treated with the polyvalent metal salt so that the pH of the dispersion liquid (hereinafter also referred to as the "reaction liquid") after the addition of the acidic substance is -1.0 to 3.0, preferably 0 to 2.8, and more preferably 0.5 to 2.5. Here, the pH of the reaction liquid can be controlled by adjusting the amount of the acidic substance added. Therefore, in the acid treatment step (2), it is preferable to add the acidic substance to the dispersion liquid in the acid treatment step (2) in an amount such that the pH of the reaction liquid is within the above-mentioned preferred range.

A low pH of the reaction solution means that the concentration of protons (H ⁺ ) present in the reaction solution is high. Here, the dissociation reaction is a reversible reaction represented by the reaction formula (COOX) _n + nH ⁺ ⇔ nCOOH + X ^{n +} (wherein X means a polyvalent metal and n represents the valence of the polyvalent metal). When the pH of the reaction solution is low, the equilibrium in the reaction formula is biased to the right, i.e., the dissociation side, and the amount of carboxyl groups (-COOH) in the free acid state present in the reaction solution increases. In other words, the equilibrium in the dissociation reaction formula is the equilibrium of the bond between the carboxyl group and the polyvalent metal ion.

The carboxyl groups in the free acid state are neutralized at a specific neutralization rate in the neutralization process described below. The neutralized carboxyl groups have the function of absorbing liquids such as human waste. On the other hand, the carboxyl groups in an ionically cross-linked state bonded with polyvalent metal ions lose their ability to absorb liquids even after the neutralization process. Therefore, if the above-mentioned bonds are not sufficiently dissociated, the recovery of the water absorption performance of the regenerated water absorbent resin obtained in the drying process described below becomes insufficient, and the water absorption performance of the regenerated water absorbent resin decreases.

When the pH of the reaction solution is equal to or lower than the upper limit of the aforementioned range, the equilibrium of the bond between the carboxyl group and the polyvalent metal ion is largely biased toward dissociation, meaning that the dissociation of the bond has progressed sufficiently. Therefore, by having the pH of the reaction solution equal to or lower than the upper limit of the aforementioned range, the water absorption performance (e.g., CRC) of the regenerated water absorbent resin is sufficiently restored, and the water absorption performance of the regenerated water absorbent resin can be improved. In addition, having the pH of the reaction solution equal to or higher than the lower limit of the aforementioned range is economically preferable, since it is possible to prevent an excessive increase in costs due to the use of more acidic substances than necessary.

The method for measuring the pH of the reaction solution is not particularly limited, and can be measured, for example, using a commercially available pH test paper and/or a commercially available pH meter. Here, for example, the acidic substance is added to the water absorbent resin treated with the polyvalent metal salt by the method described in the section [3. Method for determining the manufacturing conditions in the method for manufacturing the regenerated water absorbent resin] described later, so that the pH of the reaction solution falls within a predetermined range.

The reaction temperature in the treatment with the acidic substance is not particularly limited as long as it is a temperature at which the bond between the carboxyl group of the water absorbent resin treated with the polyvalent metal salt and the polyvalent metal ion can be dissociated. The reaction temperature in the treatment with the acidic substance is preferably 5 to 80°C, more preferably 10 to 70°C, and even more preferably 15 to 60°C. The reaction temperature in the treatment with the acidic substance means the temperature of the dispersion liquid in the acid treatment step (2) immediately before and after the addition of the acidic substance (or the temperature from immediately before the addition of the acidic substance to the end of the acid treatment step).

The water absorbent resin treated with the acidic substance obtained in the acid treatment step (2) may remain in the dispersion liquid in the acid treatment step (2) after the treatment with the acidic substance is completed. The water absorbent resin treated with the acidic substance obtained in the acid treatment step (2) may be in a state of being taken out from the dispersion liquid in the acid treatment step (2) after the treatment with the acidic substance is completed. Here, the operation of taking out the water absorbent resin treated with the acidic substance is not particularly limited, and a known method such as a filtration operation can be used. A specific example of the filtration operation is, for example, an operation of filtering the dispersion liquid in the acid treatment step (2) after the treatment with the acidic substance is completed using a 100-mesh wire net and leaving it for 1 minute.

[2-3. Water washing process]
The water-washing step (3) in one embodiment of the present invention is a step of washing the water-absorbent resin treated with the acidic substance with water to obtain a water-washed water-absorbent resin.

In the water washing step (3), it is preferable to wash the water absorbent resin treated with the acidic substance with water so that the ratio (E1/E0) of the extract conductivity E1 of the water absorbent resin washed with water obtained in the water washing step (3) to the extract conductivity E0 of the water absorbent resin treated with the acidic substance obtained in the acid treatment step (2) is less than 0.40. In that case, before carrying out the water washing step (3), it is necessary to measure the extract conductivity E0 of the water absorbent resin treated with the acidic substance in advance by the method described later. The extract conductivity E0 is a parameter that represents the amount of the polyvalent metal ions remaining in the water absorbent resin treated with the acidic substance.

The water washing step (2) is a step of washing the water absorbent resin treated with the acidic substance with water to wash away polyvalent metal ions present on the surface of the water absorbent resin treated with the acidic substance, inside the water absorbent resin, and/or in the gaps between particles of the water absorbent resin.

If a large amount of the polyvalent metal ions remain in the regenerated water absorbent resin, the water absorption performance (e.g., water absorption capacity without load (CRC) and/or water absorption speed, etc.) of the regenerated water absorbent resin may decrease. The water absorption speed is expressed, for example, by the vortex value. Therefore, by carrying out the water washing step, the polyvalent metal ions remaining in the regenerated water absorbent resin can be removed or the content thereof can be reduced. As a result, the water absorption performance (e.g., water absorption capacity without load (CRC) and/or water absorption speed, etc.) of the regenerated water absorbent resin can be improved.

Here, the amount of the polyvalent metal ions remaining in the regenerated water absorbent resin is determined by the amount of the polyvalent metal ions remaining in the water absorbent resin washed with water. The amount of the polyvalent metal ions remaining in the water absorbent resin is correlated with the electrical conductivity E1 of the extract of the water absorbent resin washed with water, which is measured by the method described below.

The electrical conductivity E0 of the extract of the water absorbent resin treated with the acidic substance or the electrical conductivity E1 of the extract of the water absorbent resin washed with water is measured by a method including the steps of (i) to (iii) below, when the water absorbent resin is obtained in a state where the water absorbent resin is present in a dispersion in the acid treatment step (2) or the water washing step (3);
(i) a step of filtering the dispersion of the water absorbent resin obtained in the acid treatment step (2) or the water washing step (3) using a 100 mesh wire screen and leaving the resultant for 1 minute to obtain a filtered water absorbent resin.

(ii) Add the filtered water-absorbent resin obtained in step (i) and deionized water in an amount four times the weight of the filtered water-absorbent resin to a beaker, and stir with a stirrer at 25°C for 20 minutes to obtain an extract.

(iii) A step of measuring the conductivity of the extract obtained in step (ii) using a conductivity meter.

Further, the extract electrical conductivity E0 of the water absorbent resin treated with the acidic substance or the extract electrical conductivity E1 of the water absorbent resin washed with water is measured by a method including the steps of (iv) and (v) below, in the case where the water absorbent resin is obtained in a state of being taken out from the dispersion in the acid treatment step (2) or the water washing step (3);
(iv) adding the water absorbent resin obtained in the acid treatment step (2) or the water washing step (3) and deionized water having a weight four times that of the water absorbent resin to a beaker, and stirring the mixture with a stirrer at a temperature of 25° C. for 20 minutes to obtain an extract.

(v) A step of measuring the conductivity of the extract obtained in step (iv) using a conductivity meter.

When measuring the extract conductivity E0 of the water-absorbent resin treated with the acidic substance, the operations for obtaining the extract in the steps (ii) and (iv) also correspond to the washing operation described later. Therefore, the water-absorbent resin obtained by carrying out the operation of extracting the water-washed water-absorbent resin described later from the extract after measuring the extract conductivity E0 of the water-absorbent resin treated with the acidic substance can also be called the water-washed water-absorbent resin.

In the water washing step (3), the water absorbent resin treated with the acidic substance is washed with water so that the ratio of the extract conductivity E1 to the extract conductivity E0 (E1/E0) is preferably less than 0.40, more preferably 0.35 or less, and even more preferably 0.30 or less.

The ratio (E1/E0) being within the above range means that a large amount of the polyvalent metal ions have been removed in the water washing step (3). Therefore, the amount of the polyvalent metal ions remaining in the water-washed water-absorbent resin is suitably reduced, and as a result, the water-absorption performance (e.g., water-absorption capacity without load (CRC) and/or water-absorption speed, etc.) of the regenerated water-absorbent resin can be more suitably improved.

The upper limit of the extract conductivity E0 is preferably 50 mS/cm or less, more preferably 40 mS/cm or less, and even more preferably 30 mS/cm or less. Furthermore, the lower limit of the extract conductivity E0 is usually 10 mS/cm or more.

When the ratio (E1/E0) is within the above-mentioned range and the extract conductivity E0 is within the above-mentioned range, it means that the extract conductivity E1 is a smaller value. Therefore, in this case, the amount of the polyvalent metal ions remaining in the water-washed water-absorbent resin is more suitably reduced. As a result, the water-absorption performance (e.g., water absorption capacity without load (CRC) and/or water absorption speed, etc.) of the regenerated water-absorbent resin is further suitably improved.

In the water washing step (3), the operation of washing the water absorbent resin treated with the acidic substance with water (hereinafter referred to as the "washing operation") is not particularly limited, and a known method can be used. Examples of the washing operation include an operation of immersing the water absorbent resin treated with the acidic substance in water and stirring it, and an operation of washing the water absorbent resin treated with the acidic substance using a water jet or shower. The washing operation may be repeated. By repeatedly performing the washing operation, the washing effect is improved, and a larger amount of the polyvalent metal ions can be removed from the water absorbent resin treated with the acidic substance.

The type of water (washing water) used in the washing operation can be deionized water, tap water, distilled water, etc. When the electrical conductivity E1 of the extract is measured after that, the amount of remaining polyvalent metal ions is determined by electrical conductivity measurement, so it is more preferable to use water that does not contain ions as the washing water. Here, water that does not contain ions means water that contains so little ions that it hardly affects the ratio (E1/E0). Here, the content of ions in water and the electrical conductivity of the water are correlated. Therefore, when the electrical conductivity of water is a small value below a specific value, it means that the water contains so little ions that it hardly affects the ratio (E1/E0). Specifically, the electrical conductivity of the washing water is preferably 5 mS/cm or less, more preferably 3 mS/cm or less.

The temperature during the cleaning operation can be from room temperature to a high temperature of 100°C. Here, the temperature during the cleaning operation means the temperature of the cleaning water. In addition, high-temperature steam can be used as the cleaning water in the cleaning operation. The temperature during the cleaning operation is preferably 5 to 80°C, more preferably 10 to 60°C, and even more preferably 15 to 40°C. When high-temperature cleaning water or high-temperature steam is used in the cleaning operation, not only a cleaning effect but also a sterilizing effect can be expected.

The water absorbent resin after the washing operation is in a state of being present in a dispersion containing the washing water (hereinafter referred to as "dispersion in the water washing step (3)"). In the water washing step (3), the water absorbent resin washed with water can be extracted by performing an operation of removing water (washing water) from the dispersion in the water washing step (3). At this time, the polyvalent metal salt is removed together with the removed water. The operation of removing water (washing water) from the dispersion in the water washing step (3) is not particularly limited. From the viewpoint of reducing the amount of the polyvalent metal ions remaining in the water absorbent resin washed with water, it is preferable to remove as much water as possible from the dispersion in the water washing step (3), in other words, to reduce the water content in the water absorbent resin washed with water as much as possible. As the operation of removing the water, for example, a conventional solid-liquid separation means, for example, a filtration operation, a centrifugation operation, etc. can be applied. As a specific example of the operation of removing the water, for example, an operation of filtering the dispersion in the water washing step (3) using a 100-mesh wire net and leaving it for 1 minute can be mentioned.

[2-4. Neutralization process]
The neutralization step (4) in one embodiment of the present invention is a step of neutralizing the water-washed water-absorbent resin with an alkali metal salt.

The neutralization step (4) is a step of adding an alkali metal salt to the absorbent resin washed with water. In the neutralization step (4), as shown in the structural formula before and after the "neutralization step" in FIG. 1, some of the carboxyl groups in the free acid state in the absorbent resin washed with water are neutralized by the alkali metal ions derived from the alkali metal salt. More specifically, the protons in some of the carboxyl groups in the free acid state are replaced by the alkali metal ions. As described above, the carboxyl groups neutralized by the alkali metal ions have the function of absorbing liquids (such as human waste). Therefore, in the neutralization step (4), some of the carboxyl groups in the free acid state are neutralized, thereby restoring the water absorption performance of the absorbent resin washed with water.

The alkali metal salt is not particularly limited, and examples thereof include hydroxides, carbonates, and hydrogen carbonates of alkali metals. The alkali metal may be at least one selected from the group consisting of lithium, sodium, and potassium. The alkali metal salt may be one type, or a mixture of multiple types of alkali metal salts. The method of adding the alkali metal salt may be a method of adding an alkali metal salt as an aqueous solution of the alkali metal salt prepared by mixing the alkali metal salt with water, or a method of adding a powdered or granular alkali metal salt as a single substance. From the viewpoint of saving energy required for drying in the drying step (5), it is preferable to add the alkali metal salt alone to the water-washed water-absorbent resin.

The amount of alkali metal salt added may be adjusted so that the pH of the absorbent resin neutralized with the alkali metal salt falls within the range of 4.5 to 8.0. Here, the pH of the absorbent resin neutralized with the alkali metal salt represents the ratio (neutralization rate) of the number of neutralized carboxyl groups to the total number of carboxyl groups in the absorbent resin neutralized with the alkali metal salt. It is known that when the neutralization rate is sufficiently high, specifically, about 70%, the regenerated absorbent resin obtained from the absorbent resin neutralized with the alkali metal salt has sufficient water absorption performance. By having the pH of the absorbent resin neutralized with the alkali metal salt be 4.5 or more, the neutralization rate in the absorbent resin neutralized with the alkali metal salt becomes about 70%, and as a result, a regenerated absorbent resin with sufficient water absorption performance can be obtained.

On the other hand, it is desirable to prevent the obtained regenerated water absorbent resin from becoming strongly alkaline from the viewpoint of preventing adverse effects on the human body. By making the pH of the water absorbent resin neutralized with the alkali metal salt 8.0 or less, it is possible to prevent the obtained regenerated water absorbent resin from becoming strongly alkaline.

The method for measuring the pH of the water-absorbent resin neutralized with the alkali metal salt is not particularly limited, and can be measured, for example, using a commercially available pH test paper and/or a commercially available pH meter.

The treatment time for neutralization with the alkali metal salt is not particularly limited as long as it is sufficient for some of the carboxyl groups in the free acid state in the water-washed water-absorbent resin to be neutralized by the alkali metal ions. The treatment time for neutralization with the alkali metal salt is preferably 5 minutes to 2 hours, more preferably 10 minutes to 1 hour, and even more preferably 15 minutes to 45 minutes. If the treatment time for neutralization with the alkali metal salt is too short, the neutralization may be insufficient, and the water absorption performance of the regenerated water-absorbent resin may be reduced. If the treatment time for neutralization with the alkali metal salt is too long, the alkali metal salt is used more than necessary, which is economically undesirable. The treatment time for neutralization with the alkali metal salt means the time from the time when the addition of the alkali metal salt to the water-washed water-absorbent resin is started to the time when the water-absorbent resin neutralized with the alkali metal salt is provided in the drying step described below and the drying step is started.

The reaction temperature in the neutralization with the alkali metal salt is not particularly limited as long as it is a temperature sufficient for some of the carboxyl groups in the free acid state in the water-absorbent resin washed with water to be neutralized by the alkali metal ions. The reaction temperature in the neutralization with the alkali metal salt is preferably 5 to 90°C, more preferably 10 to 80°C, and even more preferably 15 to 70°C. The alkali metal salt can be added in the form of an aqueous solution or powder, and as a result, the neutralized product is obtained in the form of a dispersion, a gel-like hydrate, or a water-absorbent resin. The reaction temperature in the neutralization with the alkali metal salt refers to the temperature of the dispersion, the gel-like hydrate, or the water-absorbent resin.

Between the water washing step (3) and the neutralization step (4), a concentration step may be included to increase the solid content concentration of the water absorbent resin aqueous dispersion (e.g., the dispersion in the water washing step (3) and/or the dispersion containing the water absorbent resin washed with water to be subjected to the neutralization step (4), etc.). The concentration method is not particularly limited. As the concentration method, for example, a method using a solid-liquid separation mechanism that allows water to pass through, such as a wedge wire screen, and capable of increasing the concentration of the water absorbent resin aqueous dispersion is preferable. By carrying out this concentration step, it becomes possible to improve the processing efficiency and/or drying efficiency in the neutralization step (4) and/or the drying step (5).

[2-5. Drying process]
The drying step (5) in one embodiment of the present invention is a step of drying the water-absorbent resin neutralized with the alkali metal salt to obtain a regenerated water-absorbent resin.

The drying step (5) is a step of drying the water absorbent resin neutralized with the alkali metal salt to a desired water content to obtain a regenerated water absorbent resin, which is a dried polymer. The water content is determined from the weight change when a drying operation is performed by heating 1 g of the water absorbent resin at 180° C. for 3 hours. Specifically, it is represented by the following formula (1).
Moisture content [wt%] = [1 - {(weight of water absorbent resin after drying operation [g]) / (weight of water absorbent resin before drying operation [g])} × 100 (1)
The water content of the regenerated water absorbent resin is preferably 5 to 30% by weight, more preferably 7 to 25% by weight, and further preferably 10 to 25% by weight.

The method for drying the water-absorbent resin neutralized with the alkali metal salt is not particularly limited, and examples thereof include heat drying, hot air drying, reduced pressure drying, fluidized bed drying, infrared drying, microwave drying, and drum dryer drying.

The drying temperature in the drying step (5) is preferably 120 to 250°C, more preferably 150 to 200°C, from the viewpoint of the color tone and/or drying efficiency of the resulting regenerated water absorbent resin.

Here, for example, when hot air drying is used to perform the drying step (5), the drying temperature means the temperature of the hot air blown onto the water-absorbent resin neutralized with the alkali metal salt. In addition, when a method other than hot air drying is used among the drying methods listed above, the drying temperature means the temperature of the atmosphere inside the heating device.

The drying time in the drying step (5) is preferably 20 minutes to 180 minutes, more preferably 25 minutes to 120 minutes, and even more preferably 30 minutes to 60 minutes. By drying the neutralized water absorbent resin at a drying temperature within the above-mentioned preferred range for the above-mentioned preferred drying time, a regenerated water absorbent resin having a moisture content within the above-mentioned preferred range can be suitably obtained.

[3. Method for determining manufacturing conditions in the method for manufacturing recycled water absorbent resin]
A method for determining the manufacturing conditions in a method for producing a recycled water absorbent resin according to one embodiment of the present invention is a method for determining the manufacturing conditions in a method for producing a recycled water absorbent resin from a water absorbent resin contained in a used absorbent article, the water absorbent resin includes a polymer having a partially neutralized carboxyl group, and the method for producing a recycled water absorbent resin includes a dehydration step (1) of treating the water absorbent resin with a polyvalent metal salt, an acid treatment step (2) of treating the water absorbent resin treated with the polyvalent metal salt with an acidic substance, a water washing step (3) of washing the water absorbent resin treated with the acidic substance with water, a neutralization step (4) of neutralizing the water absorbent resin washed with water with an alkali metal salt, and a drying step (5) of drying the water absorbent resin neutralized with the alkali metal salt to obtain a regenerated water absorbent resin, the method includes determining an end point of the acid treatment step (2) based on a pH of a dispersion of the water absorbent resin treated with the polyvalent metal salt to which the acidic substance has been added during the acid treatment step (2).

In the method for determining the manufacturing conditions in the method for producing the recycled water absorbent resin, the items related to each step in the manufacturing method for the recycled water absorbent resin according to one embodiment of the present invention shown above can be used for each step of the "dehydration step (1)", "acid treatment step (2)", "water washing step (3)", "neutralization step (4)" and "drying step (5)".

The end point of the acid treatment step (2) can be determined by the pH of the dispersion of the water-absorbent resin treated with the polyvalent metal salt to which the acidic substance has been added during the acid treatment step (2) by the method shown below. The dispersion of the water-absorbent resin treated with the polyvalent metal salt to which the acidic substance has been added during the acid treatment step (2) may be the dispersion to which the acidic substance is gradually added from the start of the acid treatment step (2) in the method described below. The dispersion of the water-absorbent resin treated with the polyvalent metal salt to which the acidic substance has been added during the acid treatment step (2) may be a dispersion to which an acidic substance has been added as appropriate after the addition of the acidic substance has been completed (for example, if the dispersion does not fall within the target pH range, an additional acidic substance has been added).

That is, the acidic substance is gradually added to the dispersion of the water-absorbent resin treated with the polyvalent metal salt prepared in the acid treatment step (2) while measuring the pH. Thereafter, when the pH reaches a target value, the addition of the acidic substance is temporarily stopped, and during the period during which the acid treatment is performed, an appropriate amount of acidic substance is added so that the pH falls within the target range. The method for measuring the pH is not particularly limited, and for example, commercially available pH test paper and/or a commercially available pH meter, etc., can be used. The end point of the acid treatment step (2) refers to the point at which the addition of the acidic substance is completely completed.

Therefore, determining the end point of the acid treatment step (2) based on the pH of the dispersion when the acidic substance is added means determining the amount of the acidic substance to be added so that the pH of the dispersion reaches a target value. Here, the amount of the acidic substance to be added corresponds to the manufacturing conditions in the method for manufacturing the regenerated water absorbent resin.

Therefore, by applying the method for determining the manufacturing conditions, in the acid treatment step (2) in the method for producing the regenerated water-absorbent resin, the acidic substance can be added to the dispersion of the water-absorbent resin treated with the polyvalent metal salt so that the pH of the dispersion reaches a target value.

The target pH value is -1.0 to 3.0, preferably 0 to 2.8, and more preferably 0.5 to 2.5 (0.6 to 2.3, or 0.8 to 2.0).

Therefore, as described in the above section [2-2. Acid treatment step] of [2. Method for producing recycled water-absorbent resin], by applying the method for determining the production conditions in the method for producing the recycled water-absorbent resin, the water-absorbing performance of the resulting recycled water-absorbent resin can be improved, and excessive increases in costs due to the use of more acidic substances than necessary can be prevented.

It is preferable that the method for determining the manufacturing conditions in the method for producing a regenerated water absorbent resin according to one embodiment of the present invention further includes determining the end point of the water washing step (3) based on the ratio (E1/E0) of the electrical conductivity E1 of the extract of the water absorbent resin washed with water obtained in the water washing step (3) to the electrical conductivity E0 of the extract of the water absorbent resin treated with the acidic substance obtained in the acid treatment step (2).

The end point of the water washing step (3) can be determined by the ratio (E1/E0) in the following manner. In the determination, the extract conductivity E1 of the water-washed water-absorbent resin includes not only the extract conductivity of the water-washed water-absorbent resin obtained after the water washing step (3) is performed, but also the extract conductivity of the water-washed water-absorbent resin during the water washing step (3). The end point of the water washing step (3) refers to the time when the washing with water is completed before the operation of removing the water-washed water-absorbent resin is performed.

Before carrying out the water washing step (3), the extract conductivity E0 of the water absorbent resin treated with the acidic substance is measured in advance. Then, under specific conditions, the extract conductivity E1 of the washed water absorbent resin obtained as a result of carrying out the washing operation is measured. Then, it is confirmed whether the ratio of E1/E0 has reached the target value. If the ratio of E1/E0 has reached the target value, that point is set as the end point of the water washing step (3). On the other hand, if the ratio of E1/E0 has not reached the target value, the washing operation is repeated under specific conditions until the ratio of E1/E0 reaches the target value, or the amount of washing water is increased.

Therefore, determining the end point of the water washing step (3) based on the ratio (E1/E0) means determining the conditions in the washing operation (e.g., the number of times the washing operation is performed, the type of water used, the amount of water used, the temperature during the washing operation, etc.) so that the ratio (E1/E0) becomes a target value. Here, the conditions in the washing operation correspond to the manufacturing conditions in the method for manufacturing the regenerated water absorbent resin.

Therefore, by applying the determination of the end point of the water washing step (3) based on the ratio (E1/E0), the washing operation can be performed so that the ratio (E1/E0) of the electrical conductivity of the extract of the water absorbent resin washed with water obtained in the water washing step (3) to the electrical conductivity of the extract of the water absorbent resin treated with the acidic substance obtained in the acid treatment step (2) E0 is a target value.

The target value of the ratio (E1/E0) is preferably less than 0.40, more preferably 0.35 or less, and even more preferably 0.30 or less (0.25 or less, 0.20 or less, or 0.15 or less). The lower limit of the target value of the ratio (E1/E0) is preferably 0.05, 0.08, or 0.10, from the viewpoint of reducing the amount of cleaning water used and being cost-effective.

Therefore, as described in the above section [2-3. Water washing step] of [2. Manufacturing method of recycled water absorbent resin], by determining the end point of the water washing step (3) based on the ratio (E1/E0), the water absorption performance (e.g., water absorption capacity without load (CRC) and/or water absorption speed, etc.) of the resulting recycled water absorbent resin can be more suitably improved.

The extract conductivity E0 and the extract conductivity E1 are measured by the method described in the section [2-3. Water washing step] of [2. Manufacturing method of regenerated water absorbent resin] above.

One embodiment of the present invention may include the inventions shown in [1] to [6] below.

[1] A method for producing a recycled water-absorbent resin from a water-absorbent resin contained in a used absorbent article, comprising:
The water-absorbing resin contains a polymer having a partially neutralized carboxyl group,
A dehydration step (1) of treating the water absorbent resin with a polyvalent metal salt;
an acid treatment step (2) of treating the water absorbent resin treated with the polyvalent metal salt with an acidic substance;
A water washing step (3) of washing the water absorbent resin treated with the acidic substance with water;
a neutralization step (4) of neutralizing the water-washed water-absorbent resin with an alkali metal salt;
A drying step (5) of drying the water absorbent resin neutralized with the alkali metal salt to obtain a regenerated water absorbent resin,
In the acid treatment step (2), the acidic substance is added to the dispersion of the water absorbent resin treated with the polyvalent metal salt so that the pH of the dispersion becomes −1.0 to 3.0.

[2] In the water washing step (3),
The method for producing a regenerated water absorbent resin according to [1], wherein the water absorbent resin treated with an acidic substance is washed with water so that a ratio (E1/E0) of an extract electrical conductivity E1 of the water absorbent resin washed with water obtained in the water washing step (3) to an extract electrical conductivity E0 of the water absorbent resin treated with the acidic substance obtained in the acid treatment step (2) is less than 0.40.

(Here, in the case where the water absorbent resin is obtained in a state in which it exists in a dispersion in the acid treatment step (2) or the water washing step (3), the extract conductivity of the water absorbent resin is measured by a method including the steps shown in the following (i) to (iii);
(i) a step of filtering the dispersion of the water absorbent resin obtained in the acid treatment step (2) or the water washing step (3) using a 100 mesh wire screen and leaving the resultant for 1 minute to obtain a filtered water absorbent resin.

(ii) Add the filtered water-absorbent resin obtained in step (i) and deionized water in an amount four times the weight of the filtered water-absorbent resin to a beaker, and stir with a stirrer at 25°C for 20 minutes to obtain an extract.

(iii) measuring the conductivity of the extract obtained in step (ii) using a conductivity meter.
In addition, in the case where the water absorbent resin is obtained in a state in which it is taken out from the dispersion in the acid treatment step (2) or the water washing step (3), the extract conductivity of the water absorbent resin is measured by a method including the steps shown in the following (iv) and (v);
(iv) adding the water absorbent resin obtained in the acid treatment step (2) or the water washing step (3) and deionized water having a weight four times that of the water absorbent resin to a beaker, and stirring the mixture with a stirrer at a temperature of 25° C. for 20 minutes to obtain an extract.

(v) measuring the conductivity of the boiled extract obtained in step (iv) using a conductivity meter.
[3] In the dehydration step (1),
The method for producing a regenerated water absorbent resin according to [1] or [2], wherein the polyvalent metal salt is an alkaline earth metal salt.

[4] In the acid treatment step (2),
The method for producing a recycled water absorbent resin according to any one of [1] to [3], wherein the acidic substance is at least one selected from the group consisting of organic acids and inorganic acids.

[5] A method for determining manufacturing conditions in a method for manufacturing a recycled water-absorbent resin from a water-absorbent resin contained in a used absorbent article, comprising:
The water-absorbing resin contains a polymer having a partially neutralized carboxyl group,
A dehydration step (1) of treating the water absorbent resin with a polyvalent metal salt;
an acid treatment step (2) of treating the water absorbent resin treated with the polyvalent metal salt with an acidic substance;
A water washing step (3) of washing the water absorbent resin treated with the acidic substance with water;
a neutralization step (4) of neutralizing the water-washed water-absorbent resin with an alkali metal salt;
A drying step (5) of drying the water absorbent resin neutralized with the alkali metal salt to obtain a regenerated water absorbent resin,
A method for determining a production condition in a method for producing a regenerated water absorbent resin, comprising: determining an end point of the acid treatment step (2) based on a pH of a dispersion of the water absorbent resin treated with the polyvalent metal salt to which the acidic substance has been added during the acid treatment step (2).

[6] The method for determining the manufacturing conditions described in [5] further includes determining the end point of the water washing step (3) based on the ratio (E1/E0) of the electrical conductivity E1 of the extract of the water absorbent resin washed with water obtained in the water washing step (3) to the electrical conductivity E0 of the extract of the water absorbent resin treated with the acidic substance obtained in the acid treatment step (2).

(Here, in the case where the water absorbent resin is obtained in a state in which it exists in a dispersion in the acid treatment step (2) or the water washing step (3), the extract conductivity of the water absorbent resin is measured by a method including the steps shown in the following (i) to (iii);
(i) a step of filtering the dispersion of the water absorbent resin obtained in the acid treatment step (2) or the water washing step (3) using a 100 mesh wire screen and leaving the resultant for 1 minute to obtain a filtered water absorbent resin.

(ii) Add the filtered water-absorbent resin obtained in step (i) and deionized water in an amount four times the weight of the filtered water-absorbent resin to a beaker, and stir with a stirrer at 25°C for 20 minutes to obtain an extract.

(iii) measuring the conductivity of the extract obtained in step (ii) using a conductivity meter.
In addition, in the case where the water absorbent resin is obtained in a state in which it is taken out from the dispersion in the acid treatment step (2) or the water washing step (3), the extract conductivity of the water absorbent resin is measured by a method including the steps shown in the following (iv) and (v);
(iv) adding the water absorbent resin obtained in the acid treatment step (2) or the water washing step (3) and deionized water having a weight four times that of the water absorbent resin to a beaker, and stirring the mixture with a stirrer at a temperature of 25° C. for 20 minutes to obtain an extract.

(v) measuring the conductivity of the boiled extract obtained in step (iv) using a conductivity meter.
The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

<Evaluation method>
The water absorption performance of the regenerated water-absorbent resins obtained in Examples 1 to 10, Comparative Examples 1 to 3, and Reference Example 1 described later was evaluated by the method shown below.

[Water absorption capacity without load (CRC)]
The "CRC" of the water-absorbent resin of the present invention is an abbreviation for Centrifuge Retention Capacity, and indicates the absorption capacity of the water-absorbent resin for 0.90 mass % saline solution under no pressure for 30 minutes. Specifically, the "CRC" of the water-absorbent resin was measured using the measurement method shown below.

0.200 g of water-absorbent resin was uniformly placed in a bag (85 mm x 60 mm) made of nonwoven fabric (manufactured by Nangoku Pulp Kogyo Co., Ltd., product name: Heatlon Paper, model: GSP-22) and heat-sealed, and then immersed in a large excess (usually about 500 ml) of 0.90% by mass sodium chloride aqueous solution at room temperature. After 30 minutes, the bag was pulled out and drained for 3 minutes at a centrifugal force (250 G) described in Edana ABSORBENCY II 441.1-99 using a centrifuge (manufactured by Kokusan Co., Ltd., centrifuge: model H-122), and the mass W1 (g) of the bag was measured. In addition, the same operation was performed without using a water-absorbent resin, and the mass W0 (g) of the bag at that time was measured. Then, from these W1 and W0, the centrifuge retention capacity (CRC) (g / g) was calculated according to the formula (2) shown below.
CRC (g/g) = [{W1 (g) - W0 (g)} / {mass of water-absorbent resin (g)}] - 1 (2)
[Water absorption speed]
0.02 parts by weight of edible blue No. 1, a food additive, was added to 1000 parts by weight of 0.90% by weight sodium chloride aqueous solution (physiological saline), and the liquid temperature was adjusted to 22°C. 50 ml of the physiological saline was weighed into a 100 ml beaker, and 2.0 g of a sample was added while stirring at 600 rpm using a cylindrical stirrer with a length of 40 mm and a thickness (diameter of the cross section perpendicular to the length direction of the cylinder) of 8 mm, and the vortex (absorption rate: seconds) of the sample (water-absorbent resin) was measured. The end point of the measurement of the vortex (absorption rate) was measured in accordance with the standard described in JIS K 7224-1996 "Explanation of the method for testing the absorption rate of superabsorbent resins", and the time (vortex) until the sample (water-absorbent resin particles, water-absorbent resin particle composition, etc.) absorbed the physiological saline and covered the stirrer tip with the test liquid was measured as the absorption rate (seconds).

<Production Example 1: Production of water-absorbent resin treated with polyvalent metal salt>
150 g of physiological saline was added to an "Attends" adult paper diaper manufactured by Daio Paper Corporation to swell the water-absorbent resin, and the mixture was left to stand overnight to create a simulated used paper diaper.

5000 g of 0.5 wt % calcium chloride aqueous solution was added to a 10 L container. Next, the used paper diapers were cut into 5 cm square pieces with scissors. The cut used paper diapers were then added to the container to prepare dispersion liquid (1), which was then stirred for 30 minutes at a stirring speed of 500 rpm and a temperature of 25°C to disperse the cut paper diapers in the liquid (dehydration process of the absorbent resin in the used paper diapers).

After dispersing the cut paper diapers in the liquid, it was confirmed that the absorbent resin treated with the polyvalent metal salt present in the dispersion liquid (1) was dehydrated. Specifically, it was visually confirmed that the absorbent resin treated with the polyvalent metal salt had shrunk compared to the absorbent resin contained in the cut used paper diapers.

Next, the water absorbent resin treated with the polyvalent metal salt was separated and collected from the bottom of the container by utilizing the difference in specific gravity between the water absorbent resin treated with the polyvalent metal salt and other constituent materials of the disposable diaper. The collected water absorbent resin treated with the polyvalent metal salt was designated as water absorbent resin (1) treated with the polyvalent metal salt. The water content of the obtained water absorbent resin (1) treated with the polyvalent metal salt was 85.5% by mass.

<Production of recycled water absorbent resin>
[Example 1]
50.0 g of the water absorbent resin (1) treated with the polyvalent metal salt obtained in Production Example 1 and 300.0 g of deionized water were added to a reaction vessel equipped with a pH meter (LAQUA D-71 manufactured by HORIBA, Inc.) to obtain a dispersion (2). The content of the water absorbent resin in the dispersion (2) was 2.1% by weight: [{50×(1−0.855)}g/350.0g×100].

6M aqueous hydrochloric acid solution was gradually added to dispersion (2) while stirring under conditions of stirring speed: 500 rpm, temperature of dispersion (2): 25°C, and when the pH meter reading reached 1.43, the addition of 6M aqueous hydrochloric acid solution was stopped. The amount of 6M aqueous hydrochloric acid solution added was 13.3 g.

Stirring was then continued for 15 minutes under the same conditions as above. During stirring, the pH meter reading was within the range of 1.45±0.05. The contents (dispersion liquid (2) after addition of 6M hydrochloric acid aqueous solution) were then filtered using a 100-mesh stainless steel wire mesh to obtain the residue on the wire mesh (water-absorbent resin (1) treated with an acidic substance) (filtration operation). The acid treatment step and filtration operation were carried out in the above manner.

Then, the water-absorbent resin (1) treated with the acidic substance was returned to the reaction vessel, and 237 g of deionized water was added to obtain a dispersion (3). The dispersion (3) was stirred for 5 minutes at a stirring speed of 500 rpm and a temperature of 25°C. Thereafter, the contents (dispersion (3) after stirring) were filtered using a 100-mesh stainless steel wire mesh, and the residue on the wire mesh (water-washed water-absorbent resin (1)) was obtained (filtration operation). The water washing process and filtration operation were carried out in the above manner.

Then, the absorbent resin (1) washed with water was placed in another container, and while stirring the absorbent resin (1) washed with water with a spatula, 2.95 g of sodium carbonate powder was gradually added thereto at a temperature of 25°C, and stirring was continued for another 2 minutes. After the addition of the sodium carbonate powder was completed, the mixture was left to stand for 20 minutes. As a result, the absorbent resin (1) neutralized with an alkali metal salt was obtained. The time from the start of the addition of the sodium carbonate powder to the end of the leaving (processing time for neutralization) was about 23 minutes.

Then, the obtained water-absorbent resin (1) neutralized with the alkali metal salt was spread on the bottom surface of a stainless steel tray, and the stainless steel tray was placed in a hot air circulating oven with the hot air temperature set to 180°C. Thereafter, the water-absorbent resin (1) neutralized with the alkali metal salt was dried for 60 minutes in the hot air circulating oven. As a result, a regenerated water-absorbent resin (1) was obtained. The CRC of the regenerated water-absorbent resin (1) was 48.1 (g/g).

[Example 2]
The addition of 6M hydrochloric acid to the dispersion (2) was continued until the pH meter showed a value of 2.14, and the pH range during stirring was 2.15±0.05. The same method as in Example 1 was carried out to obtain a regenerated water absorbent resin (2). The amount of the 6M hydrochloric acid aqueous solution added in Example 2 was 10.6 g. The CRC of the regenerated water absorbent resin (2) was 47.6 (g/g).

[Example 3]
The addition of 6M hydrochloric acid to the dispersion (2) was continued until the pH meter showed a value of 2.38, and the pH range during stirring was 2.40±0.05. The same method as in Example 1 was carried out to obtain a regenerated water absorbent resin (3). The amount of the 6M hydrochloric acid in Example 3 was 10.0 g. The CRC of the regenerated water absorbent resin (3) was 47.5 (g/g).

[Example 4]
The addition of 6M hydrochloric acid to the dispersion (2) was continued until the pH meter showed a value of 0.53, and the pH range during stirring was 0.55±0.05. The same method as in Example 1 was carried out to obtain a regenerated water absorbent resin (4). The amount of the 6M hydrochloric acid aqueous solution added in Example 4 was 39.8 g. The CRC of the regenerated water absorbent resin (4) was 44.3 (g/g).

[Example 5]
The addition of 6M hydrochloric acid to the dispersion (2) was continued until the pH meter showed a value of 2.86, and the pH range during stirring was 2.85±0.05. The same method as in Example 1 was carried out to obtain a regenerated water absorbent resin (5). The amount of the 6M hydrochloric acid aqueous solution added in Example 5 was 9.3 g. The CRC of the regenerated water absorbent resin (5) was 39.7 (g/g).

[Comparative Example 1]
The same method as in Example 1 was carried out to obtain a comparative regenerated water absorbent resin (1), except that the addition of 6M hydrochloric acid to the dispersion (2) was carried out until the pH meter showed 3.25 and the pH range during stirring was 3.25±0.05. The amount of the 6M hydrochloric acid aqueous solution added in Comparative Example 1 was 8.0 g. The CRC of the comparative regenerated water absorbent resin (1) was 18.4 (g/g).

[Comparative Example 2]
The same method as in Example 1 was carried out to obtain a comparative regenerated water absorbent resin (2), except that the addition of 6M hydrochloric acid aqueous solution to the dispersion (2) was carried out until the pH meter showed a value of 3.52 and the pH range during stirring was 3.50±0.05. The amount of the 6M hydrochloric acid aqueous solution added in Comparative Example 2 was 6.6 g. The CRC of the comparative regenerated water absorbent resin (2) was 6.9 (g/g).

[Reference Example 1]
The dehydration step and the acid treatment step were carried out in the same manner as in Example 1, except that the addition of 6M hydrochloric acid to the dispersion (2) was carried out until the pH meter showed a value of 1.44 and the pH range during stirring was 1.45±0.05, and the mixture was filtered using a 100-mesh stainless steel wire screen (filtration operation), to obtain a water absorbent resin treated with an acidic substance for reference. The amount of the 6M hydrochloric acid aqueous solution added in Reference Example 1 was 13.1 g.

20.0 g of the obtained water-absorbent resin treated with the reference acidic substance was placed in a beaker, 80.0 g of deionized water was added, and the mixture was stirred for 15 minutes at room temperature (25°C) to obtain an extract (1) for measuring electrical conductivity. The electrical conductivity of the extract (1) for measuring electrical conductivity was measured using an ES-51 electrical conductivity system manufactured by HORIBA, and the value was 10.2 mS/cm. This electrical conductivity was designated as the electrical conductivity E0 of the extract of the water-absorbent resin treated with the reference acidic substance.

The obtained water absorbent resin was treated with the reference acidic substance and filtered, and 40.0 g of the water absorbent resin treated with the reference acidic substance was placed in another container without performing a water washing process. While stirring the water absorbent resin treated with the reference acidic substance with a spatula, 1.70 g of sodium carbonate powder was gradually added thereto at a temperature of 25°C, and stirring was continued for another 2 minutes. After the addition of the sodium carbonate powder was completed, the mixture was left for 20 minutes. As a result, a water absorbent resin neutralized with a reference alkali metal salt was obtained. That is, the water absorbent resin neutralized with the reference alkali metal salt is a water absorbent resin treated with an acidic substance that was not subjected to a water washing process. The time from the start of the addition of the sodium carbonate powder to the end of the leaving (the processing time in neutralization) was about 23 minutes.

Then, the obtained reference water-absorbent resin neutralized with the alkali metal salt was dried in the same manner as in the drying process of Example 1. As a result, a reference regenerated water-absorbent resin was obtained. The CRC of the reference regenerated water-absorbent resin was 46.1 (g/g), and the vortex (water absorption speed) was 163 (seconds).

[Example 6]
The dehydration step and the acid treatment step were carried out in the same manner as in Example 1, except that the addition of 6M hydrochloric acid aqueous solution to the dispersion liquid (2) was carried out until the value of the pH meter indicated 1.38 and the pH range during stirring was 1.40±0.05, thereby obtaining a water absorbent resin (2) treated with an acidic substance. The amount of the 6M hydrochloric acid aqueous solution added in Example 6 was 13.3 g.

When the electrical conductivity of the extract E0 of the water-absorbent resin (2) treated with an acidic substance was measured using the same method as in Reference Example 1, the value was 10.2 mS/cm. This electrical conductivity was defined as the electrical conductivity E0 of the extract of the water-absorbent resin (2) treated with an acidic substance.

Next, the water washing step and filtration operation were carried out in the same manner as in Example 1, except that the amount of deionized water used for the water absorbent resin (2) treated with the acidic substance was changed to 21.5 g, and water-washed water absorbent resin (2) was obtained.

20.0 g of the water-washed water-absorbent resin (2) was placed in a beaker, 80.0 g of deionized water was added, and the mixture was stirred for 15 minutes to obtain an extract (6) for measuring electrical conductivity. The electrical conductivity of the extract (6) for measuring electrical conductivity was measured using the same method as in Reference Example 1, and the value was 6.7 mS/cm. This electrical conductivity was designated as the electrical conductivity E1 of the extract of the water-treated water-absorbent resin (2).

Therefore, the ratio (E1/E0) of the extract conductivity E1 of the water absorbent resin (2) treated with water to the extract conductivity E0 of the water absorbent resin (2) treated with an acidic substance (extract conductivity of the water absorbent resin treated with a reference acidic substance) is 6.7/10.2 ≒ 0.66.

40.0 g of the water-treated water-absorbent resin (2) was placed in another container, and 1.70 g of sodium carbonate powder was gradually added thereto at a temperature of 25°C while stirring the water-treated water-absorbent resin (2) with a spatula. After the addition of the sodium carbonate powder was completed, the mixture was left to stand for 20 minutes. As a result, water-absorbent resin (2) neutralized with an alkali metal salt was obtained. The time from the start of the addition of the sodium carbonate powder to the end of the standing (neutralization processing time) was about 23 minutes.

Then, the water-absorbent resin (2) neutralized with the obtained alkali metal salt was dried in the same manner as in the drying process of Example 1. As a result, a regenerated water-absorbent resin (6) was obtained. The CRC of the regenerated water-absorbent resin (6) was 46.2 (g/g), and the vortex (water absorption speed) was 124 (seconds).

[Example 7]
The dehydration step and the acid treatment step were carried out in the same manner as in Example 1, except that the addition of 6M hydrochloric acid aqueous solution to the dispersion liquid (2) was carried out until the pH meter showed a value of 1.40 and the pH range during stirring was 1.40±0.05, thereby obtaining a water absorbent resin (3) treated with an acidic substance. The amount of the 6M hydrochloric acid aqueous solution added in Example 7 was 13.3 g.

When the electrical conductivity of the extract E0 of the water-absorbent resin (3) treated with an acidic substance was measured using the same method as in Reference Example 1, the value was 10.2 mS/cm. This electrical conductivity was defined as the electrical conductivity E0 of the extract of the water-absorbent resin (3) treated with an acidic substance.

A water washing step was carried out on the obtained water absorbent resin (3) treated with an acidic substance in the same manner as in Example 1, except that the amount of deionized water used was changed to 70.0 g, to obtain a water absorbent resin (3) washed with water.

20.0 g of the water-washed water-absorbent resin (3) was used to obtain an extract (3) for measuring electrical conductivity by the same method as in Example 6. The electrical conductivity of the extract (3) for measuring electrical conductivity was measured by the same method as in Reference Example 1, and the value was 3.5 mS/cm. This electrical conductivity is defined as the electrical conductivity E1 of the extract of the water-treated water-absorbent resin (3).

Therefore, the ratio (E1/E0) of the extract conductivity E1 of the water-absorbent resin (3) treated with water to the extract conductivity E0 of the water-absorbent resin (3) treated with an acidic substance (extract conductivity of the water-absorbent resin treated with a reference acidic substance) is 3.5/10.2 ≒ 0.34.

Furthermore, 40.0 g of the water-washed water-absorbent resin (3) was used to obtain a regenerated water-absorbent resin (7) by the same method as in Example 6. The CRC of the regenerated water-absorbent resin (7) was 47.2 (g/g) and the vortex (water absorption rate) was 71 (seconds).

[Example 8]
The dehydration step and the acid treatment step were carried out in the same manner as in Example 1, except that the addition of 6M hydrochloric acid aqueous solution to the dispersion liquid (2) was continued until the value of the pH meter indicated 1.41 and the pH range during stirring was 1.40±0.05, thereby obtaining a water absorbent resin (4) treated with an acidic substance. The amount of the 6M hydrochloric acid aqueous solution added in Example 8 was 13.1 g.

When the electrical conductivity of the extract E0 of the water-absorbent resin (4) treated with an acidic substance was measured using the same method as in Reference Example 1, the value was 10.2 mS/cm. This electrical conductivity was defined as the electrical conductivity E0 of the extract of the water-absorbent resin (4) treated with an acidic substance.

A water washing step was carried out on the obtained water absorbent resin (4) treated with an acidic substance in the same manner as in Example 1, except that the amount of deionized water used was changed to 120.0 g, to obtain a water absorbent resin (4) washed with water.

20.0 g of the water-washed water-absorbent resin (4) was used to obtain an extract (4) for measuring electrical conductivity by the same method as in Example 6. The electrical conductivity of the extract (4) for measuring electrical conductivity was measured by the same method as in Reference Example 1, and the value was 2.9 mS/cm. This electrical conductivity is defined as the electrical conductivity E1 of the extract of the water-treated water-absorbent resin (4).

Therefore, the ratio (E1/E0) of the extract conductivity E1 of the water absorbent resin (4) treated with water to the extract conductivity E0 of the water absorbent resin (4) treated with an acidic substance (extract conductivity of the water absorbent resin treated with a reference acidic substance) is 2.9/10.2 ≒ 0.28.

Furthermore, 40.0 g of the water-washed water-absorbent resin (4) was used to obtain a regenerated water-absorbent resin (8) by the same method as in Example 5. The CRC of the regenerated water-absorbent resin (8) was 47.1 (g/g) and the vortex (water absorption rate) was 59 (seconds).

[Example 9]
The dehydration step and the acid treatment step were carried out in the same manner as in Example 1, except that the addition of 6M hydrochloric acid aqueous solution to the dispersion liquid (2) was continued until the pH meter showed a value of 1.44 and the pH range during stirring was 1.45±0.05, thereby obtaining a water absorbent resin (5) treated with an acidic substance. The amount of the 6M hydrochloric acid aqueous solution added in Example 9 was 13.0 g.

When the electrical conductivity of the extract E0 of the water-absorbent resin (5) treated with an acidic substance was measured using the same method as in Reference Example 1, the value was 10.2 mS/cm. This electrical conductivity was defined as the electrical conductivity E0 of the extract of the water-absorbent resin (5) treated with an acidic substance.

A water washing step was carried out on the obtained water absorbent resin (5) treated with an acidic substance in the same manner as in Example 1, except that the amount of deionized water used was changed to 260.0 g, to obtain a water absorbent resin (5) washed with water.

20.0 g of the water-washed water-absorbent resin (5) was used to obtain an extract (5) for measuring electrical conductivity by the same method as in Example 5. The electrical conductivity of the extract (5) for measuring electrical conductivity was measured by the same method as in Reference Example 1, and the value was 2.3 mS/cm. This electrical conductivity is defined as the electrical conductivity E1 of the extract of the water-treated water-absorbent resin (5).

Therefore, the ratio (E1/E0) of the extract conductivity E1 of the water absorbent resin (5) treated with water to the extract conductivity E0 of the water absorbent resin (5) treated with an acidic substance (extract conductivity of the water absorbent resin treated with a reference acidic substance) is 2.3/10.2 ≒ 0.23.

Furthermore, 40.0 g of the water-washed water-absorbent resin (5) was used to obtain a regenerated water-absorbent resin (9) by the same method as in Example 5. The CRC of the regenerated water-absorbent resin (9) was 47.5 (g/g) and the vortex (water absorption rate) was 52 (seconds).

[Example 10]
The dehydration step and the acid treatment step were carried out in the same manner as in Example 1, except that the addition of 6M aqueous hydrochloric acid to the dispersion (2) was continued until the pH meter showed a value of 1.42 and the pH range during stirring was 1.40±0.05, thereby obtaining a water absorbent resin (6) treated with an acidic substance. The amount of the 6M aqueous hydrochloric acid added in Example 10 was 12.8 g.

When the electrical conductivity of the extract E0 of the water-absorbent resin (6) treated with an acidic substance was measured using the same method as in Reference Example 1, the value was 10.2 mS/cm. This electrical conductivity was defined as the electrical conductivity E0 of the extract of the water-absorbent resin (6) treated with an acidic substance.

A water washing step was carried out on the obtained water absorbent resin (6) treated with an acidic substance in the same manner as in Example 1, except that the amount of deionized water used was changed to 550.0 g, to obtain a water absorbent resin (6) washed with water.

20.0 g of the water-washed water-absorbent resin (6) was used to obtain an extract (6) for measuring electrical conductivity by the same method as in Example 5. The electrical conductivity of the extract (6) for measuring electrical conductivity was measured by the same method as in Reference Example 1, and the value was 1.9 mS/cm. This electrical conductivity is defined as the electrical conductivity E1 of the extract of the water-treated water-absorbent resin (6).

Therefore, the ratio (E1/E0) of the extract conductivity E1 of the water absorbent resin (6) treated with water to the extract conductivity E0 of the water absorbent resin (6) treated with an acidic substance (extract conductivity of the water absorbent resin treated with a reference acidic substance) is 1.9/10.2 ≒ 0.19.

Furthermore, 40.0 g of the water-washed water-absorbent resin (6) was used to obtain a regenerated water-absorbent resin (10) by the same method as in Example 6. The CRC of the regenerated water-absorbent resin (10) was 47.6 (g/g) and the vortex (water absorption speed) was 50 (seconds).

[Example 11]
The dehydration step and the acid treatment step were carried out in the same manner as in Example 1, except that the addition of 6M hydrochloric acid aqueous solution to the dispersion liquid (2) was continued until the pH meter showed a value of 1.39 and the pH range during stirring was 1.40±0.05, thereby obtaining a water absorbent resin (7) treated with an acidic substance. The amount of the 6M hydrochloric acid aqueous solution added in Example 11 was 12.8 g.

When the electrical conductivity of the extract E0 of the water-absorbent resin (7) treated with an acidic substance was measured using the same method as in Reference Example 1, the value was 10.2 mS/cm. This electrical conductivity was defined as the electrical conductivity E0 of the extract of the water-absorbent resin (7) treated with an acidic substance.

A water washing step was carried out on the obtained water absorbent resin (7) treated with an acidic substance in the same manner as in Example 1, except that the amount of deionized water used was changed to 1,150 g, to obtain a water absorbent resin (7) washed with water.

20.0 g of the water-washed water-absorbent resin (7) was used to obtain an extract (7) for measuring electrical conductivity by the same method as in Example 6. The electrical conductivity of the extract (7) for measuring electrical conductivity was measured by the same method as in Reference Example 1, and the value was 1.5 mS/cm. This electrical conductivity is defined as the electrical conductivity E1 of the extract of the water-treated water-absorbent resin (7).

Therefore, the ratio (E1/E0) of the extract conductivity E1 of the water-absorbent resin (7) treated with water to the extract conductivity E0 of the water-absorbent resin (7) treated with an acidic substance (extract conductivity of the water-absorbent resin treated with a reference acidic substance) is 1.5/10.2 ≒ 0.15.

Furthermore, 40.0 g of the water-washed water-absorbent resin (7) was used to obtain a regenerated water-absorbent resin (11) by the same method as in Example 5. The CRC of the regenerated water-absorbent resin (11) was 47.3 (g/g) and the vortex (water absorption rate) was 50 (seconds).

[result]
The production conditions for Examples 1 to 5 and Comparative Examples 1 and 2 are shown in Table 1 below. The results of evaluating the water absorption performance, CRC, of the obtained recycled water absorbent resins (1) to (5) and the comparative recycled water absorbent resins (1) and (2) are also shown in Table 1 below.
As shown in Table 1, in Examples 1 to 5, the pH of the dispersion at the end of the addition of the hydrochloric acid aqueous solution and during the acid treatment step is in the range of -1.0 to 3.0. In other words, the production methods in Examples 1 to 5 have a feature (hereinafter referred to as "Feature A") in which an acidic substance is added to the dispersion of the water absorbent resin treated with a polyvalent metal salt in the acid treatment step so that the pH of the dispersion is -1.0 to 3.0. Thus, the production methods in Examples 1 to 5 correspond to the production method of the regenerated water absorbent resin according to one embodiment of the present invention. On the other hand, the production methods in Comparative Examples 1 and 2 do not have Feature A and do not correspond to the production method of the regenerated water absorbent resin according to one embodiment of the present invention. In addition, the regenerated water absorbent resins (1) to (5) produced in Examples 1 to 5 have a higher CRC than the comparative regenerated water absorbent resins (1) and (2) produced in Comparative Examples 1 and 2.

From the above, it can be understood that the method for producing recycled water-absorbent resin according to one embodiment of the present invention, by having feature A, can solve the problem of the present invention, which is to sufficiently recover the water absorption performance of the water-absorbent resin contained in used absorbent articles. It can also be understood that the method for producing recycled water-absorbent resin according to one embodiment of the present invention can solve the problem of the present invention, which is to prevent the cost required for producing the recycled water-absorbent resin from increasing excessively.

The production conditions for Reference Example 1 and Examples 6 to 11 are shown in Table 2 below. The results of evaluation of the water absorption performance, CRC and vortex (water absorption rate), of the obtained reference regenerated water absorbent resins and regenerated water absorbent resins (6) to (11) are also shown in Table 2 below. Note that a water washing step was not performed in the Reference Examples. Therefore, in the Reference Example column in Table 2, the amount of water used, and the values of "E1" and "E1/E0" for the extract conductivity of the water absorbent resin treated with water obtained in the water washing step are recorded as "-".
As shown in Table 2, in Reference Example 1 and Examples 6 to 11, the same amount of acidic substance was added in the acid treatment step. In addition, while the water washing step was not performed in Reference Example 1, the water washing step was performed in Examples 6 to 11. Furthermore, compared with the reference regenerated water absorbent resin obtained in Reference Example 1, the regenerated water absorbent resins (6) to (11) obtained in Examples 6 to 11 have increased CRC values and significantly smaller vortex values. Here, a significantly smaller vortex value means that the water absorption rate is improved. The production method in Examples 6 to 11 corresponds to a production method for a regenerated water absorbent resin according to one embodiment of the present invention.

<Production of recycled water absorbent resin by successively performing an acid treatment process, a water washing process, and a neutralization process>
[Example 12]
A water absorbent resin (1) treated with a polyvalent metal salt, i.e., a water absorbent resin gel after a dehydration step, was obtained by carrying out the same operation as in Production Example 1. 1000 g of the obtained water absorbent resin (1) treated with a polyvalent metal salt was mixed with 1300 g of deionized water to prepare a dispersion (4) having a solid content concentration of 6.3% by weight. The dispersion (4) is an aqueous dispersion of the water absorbent resin gel after the dehydration step.

To a reaction processor equipped with a pH meter and a stirrer, and equipped with a raw material supply port and an outlet on the opposite side of the raw material supply port to enable continuous processing, the dispersion liquid (4) was supplied at a supply rate of 43.0 g/min, and 1.1 wt% hydrochloric acid was simultaneously supplied at a supply rate of 95.1 g/min, to carry out an acid reaction. As a result, in the reaction processor, a dispersion liquid (5), which is a dispersion liquid of a water-absorbent resin gel after the acid reaction process, was obtained. 10 minutes after starting to supply the raw material dispersion liquid (4) and 1.1 wt% hydrochloric acid from the supply port of the reaction processor into the reaction processor, it was confirmed that the pH meter indicated a value of 1.21. Thereafter, the dispersion liquid (5) was continuously extracted from the outlet of the reaction processor for 20 minutes. The extracted dispersion liquid (5) was sampled, and the extract conductivity E0 was measured. The value of the extract conductivity E0 obtained as a result of the measurement was 11.1 mS/cm. Subsequently, the extracted dispersion (5) was continuously supplied to a water washing tank at a supply rate of 43.0 g/min, and deionized water was continuously supplied as washing water at a supply rate of 380 g/min. As a result, the water absorbent resin gel after the acid reaction step contained in the dispersion (5) was continuously washed to obtain a water-washed product. The obtained water-washed product was subjected to filtration and collection using a 100-mesh stainless steel wire mesh for 20 minutes to obtain a water absorbent resin gel after the washing step (12). Note that the value of the pH meter was within the range of 1.20±0.10 for 20 minutes while the dispersion (5) was extracted from the reaction processor. In addition, the extract liquid conductivity E1 of the water absorbent resin gel (12) after the washing step was 3.3, and E1/E0 was 0.30. The solid content concentration of the obtained water absorbent resin gel (12) after the washing step was measured and found to be 10.1% by weight. Here, solids concentration (wt%) = 100 - water content (wt%).

The water absorbent resin gel (12) after the washing process was continuously fed to a meat chopper at a feed rate of 170 g/min, and at the same time, sodium carbonate powder was continuously fed to the meat chopper at a feed rate of 8.8 g/min. Next, in the meat chopper, the water absorbent resin gel (12) after the washing process and the sodium carbonate powder were kneaded, and a neutralization reaction was carried out on the water absorbent resin gel (12) after the washing process, to obtain the water absorbent resin gel (12) after the neutralization process.

The water-absorbent resin gel (12) obtained after the neutralization process was spread on a stainless steel tray and dried for 60 minutes in a hot air circulating oven set at 180°C to obtain a water-absorbent resin (12) after the drying process. The water-absorbent resin (12) after the drying process was used as a regenerated water-absorbent resin (12).

The CRC of the recycled water-absorbent resin (12) was 48.5 (g/g) and the Vortex (absorption speed) was 62 (seconds).

As described above, it was found that when the reactions in each step of the process for producing the regenerated water-absorbent resin of the present invention are carried out continuously, a regenerated water-absorbent resin with excellent water-absorbing properties can be obtained, just as in the case of a palindrome reaction.

From the above, it can be understood that the method for producing a recycled water-absorbent resin according to one embodiment of the present invention can improve the water absorption performance, particularly the water absorption speed, of the resulting recycled water-absorbent resin by including a water washing step.

In addition, in Examples 7 to 12, the water washing step was performed so that the E1/E0 value was less than 0.40, while in Example 6, the E1/E0 value in the water washing step exceeded 0.40. Compared with the regenerated water absorbent resin (6) obtained in Example 6, the regenerated water absorbent resins (7) to (12) obtained in Examples 7 to 12 had slightly higher CRC values and smaller vortex (water absorption rate) values.

From the above, it can be seen that in the method for producing a recycled water-absorbent resin according to one embodiment of the present invention, the water-absorbent resin that has been treated with an acidic substance so that the E1/E0 value is less than 0.40 is washed with water in a water washing step, and the water-absorption performance, particularly the water-absorption speed, of the resulting recycled water-absorbent resin can be significantly improved.

According to one embodiment of the present invention, it is possible to produce a recycled water-absorbent resin with excellent water-absorbent performance, in which the water-absorbent performance has been sufficiently restored, from the water-absorbent resin contained in a used absorbent article, and it is possible to prevent an excessive increase in the cost required for producing the recycled water-absorbent resin. Therefore, one embodiment of the present invention can be used to regenerate water-absorbent resin with sufficiently restored water-absorbent performance from a used absorbent article.

Claims (6)

A method for producing a recycled water-absorbent resin from a water-absorbent resin contained in a used absorbent article, comprising the steps of:
The water-absorbing resin contains a polymer having a partially neutralized carboxyl group,
A dehydration step (1) of treating the water absorbent resin with a polyvalent metal salt;
an acid treatment step (2) of treating the water absorbent resin treated with the polyvalent metal salt with an acidic substance;
A water washing step (3) of washing the water absorbent resin treated with the acidic substance with water;
a neutralization step (4) of neutralizing the water-washed water-absorbent resin with an alkali metal salt;
A drying step (5) of drying the water absorbent resin neutralized with the alkali metal salt to obtain a regenerated water absorbent resin,
In the acid treatment step (2), the acidic substance is added to a dispersion of the water absorbent resin treated with the polyvalent metal salt so that the pH of the dispersion is −1.0 to 3.0. In the water washing step (3),
2. The method for producing a regenerated water absorbent resin according to claim 1, further comprising a step of washing the water absorbent resin treated with the acidic substance with water such that a ratio (E1/E0) of an extract conductivity E1 of the water absorbent resin washed with water obtained in the water washing step (3) to an extract conductivity E0 of the water absorbent resin treated with the acidic substance obtained in the acid treatment step (2) is less than 0.40.
(Here, in the case where the water absorbent resin is obtained in a state in which it exists in a dispersion in the acid treatment step (2) or the water washing step (3), the extract conductivity of the water absorbent resin is measured by a method including the steps shown in the following (i) to (iii);
(i) a step of filtering the dispersion of the water absorbent resin obtained in the acid treatment step (2) or the water washing step (3) using a 100 mesh wire screen and leaving the resultant for 1 minute to obtain a filtered water absorbent resin.
(ii) adding the filtered water absorbent resin obtained in the step (i) and deionized water having a weight four times the weight of the filtered water absorbent resin to a beaker, and stirring the mixture with a stirrer at a temperature of 25° C. for 20 minutes to obtain an extract.
(iii) measuring the conductivity of the extract obtained in step (ii) using a conductivity meter.
In addition, in the case where the water absorbent resin is obtained in a state in which it is taken out from the dispersion in the acid treatment step (2) or the water washing step (3), the extract conductivity of the water absorbent resin is measured by a method including the steps shown in the following (iv) and (v);
(iv) adding the water absorbent resin obtained in the acid treatment step (2) or the water washing step (3) and deionized water having a weight four times that of the water absorbent resin to a beaker, and stirring the mixture with a stirrer at a temperature of 25° C. for 20 minutes to obtain an extract.
(v) measuring the conductivity of the boiled extract obtained in step (iv) using a conductivity meter. In the dehydration step (1),
The method for producing a regenerated water absorbent resin according to claim 1 or 2, wherein the polyvalent metal salt is an alkaline earth metal salt. In the acid treatment step (2),
The method for producing a regenerated water absorbent resin according to any one of claims 1 to 3, wherein the acidic substance is one or more acids selected from the group consisting of organic acids and inorganic acids. A method for determining manufacturing conditions in a method for manufacturing a recycled water-absorbent resin from a water-absorbent resin contained in a used absorbent article, comprising:
The water-absorbing resin contains a polymer having a partially neutralized carboxyl group,
A dehydration step (1) of treating the water absorbent resin with a polyvalent metal salt;
an acid treatment step (2) of treating the water absorbent resin treated with the polyvalent metal salt with an acidic substance;
A water washing step (3) of washing the water absorbent resin treated with the acidic substance with water;
a neutralization step (4) of neutralizing the water-washed water-absorbent resin with an alkali metal salt;
A drying step (5) of drying the water absorbent resin neutralized with the alkali metal salt to obtain a regenerated water absorbent resin,
A method for determining a production condition in a method for producing a regenerated water absorbent resin, comprising: determining an end point of the acid treatment step (2) based on a pH of a dispersion of the water absorbent resin treated with the polyvalent metal salt to which the acidic substance has been added during the acid treatment step (2). The method for determining a production condition according to claim 5, further comprising determining an end point of the water washing step (3) based on a ratio (E1/E0) of an extract conductivity E1 of the water absorbent resin obtained in the water washing step (3) to an extract conductivity E0 of the water absorbent resin treated with the acidic substance obtained in the acid treatment step (2).
(Here, in the case where the water absorbent resin is obtained in a state in which it exists in a dispersion in the acid treatment step (2) or the water washing step (3), the extract conductivity of the water absorbent resin is measured by a method including the steps shown in the following (i) to (iii);
(i) a step of filtering the dispersion of the water absorbent resin obtained in the acid treatment step (2) or the water washing step (3) using a 100 mesh wire screen and leaving the resultant for 1 minute to obtain a filtered water absorbent resin.
(ii) adding the filtered water absorbent resin obtained in the step (i) and deionized water in an amount four times the weight of the filtered water absorbent resin to a beaker, and stirring the mixture with a stirrer at a temperature of 25° C. for 20 minutes to obtain an extract.
(iii) measuring the conductivity of the extract obtained in step (ii) using a conductivity meter.
In addition, in the case where the water absorbent resin is obtained in a state in which it is taken out from the dispersion in the acid treatment step (2) or the water washing step (3), the extract conductivity of the water absorbent resin is measured by a method including the steps shown in the following (iv) and (v);
(iv) adding the water absorbent resin obtained in the acid treatment step (2) or the water washing step (3) and deionized water having a weight four times that of the water absorbent resin to a beaker, and stirring the mixture with a stirrer at a temperature of 25° C. for 20 minutes to obtain an extract.
(v) measuring the conductivity of the boiled extract obtained in step (iv) using a conductivity meter.

Images