Methods for producing fibers from recycled pulp.

Abstract

Invention details;

Description

Manufacturing method for recycled pulp fiber

[0001] The present disclosure relates to a method for producing recycled pulp fibers from a mixture comprising pulp fibers obtained from hygiene products and a superabsorbent polymer.

[0002] Studies are being conducted on recovering recycled pulp fibers from used sanitary goods. For example, Patent Document 1 describes a method for recovering pulp fibers from used sanitary goods containing pulp fibers and a polymer absorbent, the method comprising the steps of immersing the used sanitary goods in ozone water to decompose the polymer absorbent, discharging the ozone water in which the decomposed polymer absorbent has been dissolved to obtain a residue of the sanitary goods from which the polymer absorbent has been removed, and stirring the residue of the sanitary goods from which the polymer absorbent has been removed in an aqueous solution containing a disinfectant or in water to clean the residue of the sanitary goods and decompose it into its constituent elements.

[0003] JP 2014-217835 A

[0004] The inventors have confirmed that when polymer absorbents are decomposed using ozone water (especially when the polymer absorbents are inactivated using an acidic aqueous solution), in order to oxidatively decompose the superabsorbent polymer, the pulp fibers must be exposed to the oxidizing agent for a long period of time, and as a result, the degree of polymerization, amount of carboxyl groups, alkali solubility rate, etc. of the recycled pulp fibers formed are likely to change.

[0005] Therefore, the present disclosure aims to provide a method for forming recycled pulp fibers that are less likely to leave behind superabsorbent polymers and their residues, and whose properties are less likely to change compared to when the superabsorbent polymer is oxidatively decomposed using only an oxidizing agent.

[0006] The present inventors have discovered a method for producing recycled pulp fibers from a mixture containing pulp fibers obtained from sanitary products and a superabsorbent polymer, the method comprising: a recycled pulp fiber formation step in which an oxidant and ultraviolet light are supplied to an aqueous solution containing the mixture to oxidatively decompose the superabsorbent polymer, and the oxidatively decomposed superabsorbent polymer is dissolved in the aqueous solution to form the recycled pulp fibers; and a recycled pulp fiber recovery step in which the recycled pulp fibers are recovered.

[0007] The method of the present disclosure for producing recycled pulp fibers from a mixture containing pulp fibers obtained from sanitary products and superabsorbent polymers makes it possible to form recycled pulp fibers that are less likely to contain superabsorbent polymers and their residues, and whose properties are less likely to change compared to when the superabsorbent polymer is oxidatively decomposed using only an oxidizing agent.

[0008] Specifically, the present disclosure relates to the following aspects: [Aspect 1] A method for producing recycled pulp fibers from a mixture containing pulp fibers obtained from sanitary products and a superabsorbent polymer, the method comprising: a recycled pulp fiber forming step of supplying an oxidant and ultraviolet light to an aqueous solution containing the mixture to oxidatively decompose the superabsorbent polymer and dissolving the oxidatively decomposed superabsorbent polymer in the aqueous solution to form the recycled pulp fibers; and a recycled pulp fiber recovering step of recovering the recycled pulp fibers.

[0009] The inventors of the present application have discovered that while oxidizing agents are not easily affected by the transparency of the aqueous solution and can oxidatively decompose and remove superabsorbent polymers, in cases where the superabsorbent polymer has been dehydrated, for example, it is necessary to expose the pulp fibers to the oxidizing agent for a long period of time in order to oxidatively decompose the superabsorbent polymer, and as a result, the degree of polymerization, amount of carboxyl groups, alkali solubility rate, etc. of the recycled pulp fibers formed are likely to change.

[0010] The inventors of the present invention also discovered that although ultraviolet light is affected by the transparency of the aqueous solution, it can oxidatively decompose and remove the superabsorbent polymer, while hardly changing the properties of the recycled pulp fibers that are formed.

[0011] In the above-mentioned method, the superabsorbent polymer is oxidatively decomposed using both an oxidizing agent and ultraviolet light, so the superabsorbent polymer is less likely to remain in the aqueous solution, and the resulting recycled pulp fibers are less likely to contain the superabsorbent polymer and its residues. Furthermore, the properties of the recycled pulp fibers change less than when the superabsorbent polymer is oxidatively decomposed using only an oxidizing agent.

[0012] [Aspect 2] The method according to Aspect 1, wherein the oxidizing agent is ozone. In the method, the oxidizing agent is ozone, and therefore the effects of Aspect 1 are enhanced.

[0013] [Aspect 3] The method according to Aspect 1 or 2, wherein the recycled pulp fibers have predetermined properties depending on the intended use, and in the recycled pulp fiber forming step, the ratio of ultraviolet light and an oxidizing agent is adjusted and supplied to the aqueous solution, thereby forming recycled pulp fibers having the predetermined properties depending on the intended use.

[0014] In the recycled pulp fiber forming step, the mixture containing the pulp fibers and the superabsorbent polymer is supplied with an aqueous solution containing an oxidizing agent and ultraviolet light at an adjusted ratio, thereby forming recycled pulp fibers having predetermined properties according to the intended use. Therefore, the formed recycled pulp fibers have the appropriate predetermined properties according to the intended use and are useful for the intended use.

[0015] [Aspect 4] The method according to aspect 3, wherein the application is a cellulose raw material. In the method, the application is a cellulose raw material, and recycled pulp fibers useful as a cellulose raw material can be formed.

[0016] [Aspect 5] The method according to Aspect 4, wherein the cellulose raw material application is selected from the group consisting of cellulose nanofiber, viscose rayon, cellulose derivatives, bioethanol, biobutanol, molding materials, and paper processed products.

[0017] In the above method, the above application is selected from a predetermined group, and recycled pulp fibers useful for the application selected from the predetermined group can be formed.

[0018] [Aspect 6] The method according to any one of aspects 3 to 5, wherein the recycled pulp fibers having the predetermined properties have a carboxyl group amount of 0 to 0.075 mmol / g.

[0019] The above method can produce recycled pulp fibers having a carboxyl group content within a specific range, which makes it possible to produce recycled pulp fibers suitable for applications where the generation of carboxyl groups is undesirable, such as as a raw material for cellulose derivatives such as acetate, rayon, or cellophane.

[0020] [Aspect 7] The method according to any one of aspects 3 to 6, wherein the recycled pulp fibers having the predetermined properties have an increased amount of carboxyl groups of 0 to 0.035 mmol / g based on the pulp fibers.

[0021] The above method can produce recycled pulp fibers having a specific range of carboxyl group content based on the pulp fiber, and therefore can produce recycled pulp fibers suitable for applications where the generation of carboxyl groups is undesirable.

[0022] [Embodiment 8] The method according to any one of embodiments 3 to 7, wherein the recycled pulp fibers having the predetermined properties have a degree of polymerization of 300 or more.

[0023] The above method can produce recycled pulp fibers having a degree of polymerization within a specific range, and therefore can produce recycled pulp fibers suitable for applications where a decrease in the degree of polymerization is undesirable, such as raw materials for rayon, cellophane, and cellulose nanofibers.

[0024] [Aspect 9] The method according to any one of aspects 3 to 8, wherein the recycled pulp fibers having the predetermined properties have a reduction in degree of polymerization of 400 or less based on the pulp fibers.

[0025] The method described above can produce recycled pulp fibers having a degree of polymerization within a specific range based on the pulp fiber, and therefore can produce recycled pulp fibers suitable for applications where a decrease in the degree of polymerization is undesirable.

[0026] [Aspect 10] The method according to any one of Aspects 3 to 9, wherein the recycled pulp fibers having the predetermined properties have an alkali solubility of 0 to 20% by mass.

[0027] The above method can produce recycled pulp fibers having an alkali solubility within a specific range, and therefore can produce recycled pulp fibers suitable for applications where a decrease in alkali solubility is undesirable, such as raw materials for rayon and cellophane.

[0028] [Aspect 11] The method of any one of aspects 1 to 10, wherein the recycled pulp fibers have an increase in alkali solubility of 0 to 16% by weight based on the pulp fibers.

[0029] The method described above can produce recycled pulp fibers having a specific range of alkali solubility based on the pulp fiber, and therefore can produce recycled pulp fibers suitable for applications where a decrease in alkali solubility is undesirable.

[0030] [Aspect 12] The method according to any one of Aspects 1 to 11, wherein the oxidizing agent and the ultraviolet light are supplied to the aqueous solution in an overlapping manner in the recycled pulp fiber forming step.

[0031] In the above method, ultraviolet light and an oxidizing agent are supplied in duplicate, so that the time required for the recycled pulp fiber formation step can be shortened, and the above method can be carried out efficiently.

[0032] [Aspect 13] The method according to any one of Aspects 1 to 12, wherein in the recycled pulp fiber forming step, supply of the oxidizing agent to the aqueous solution is started, and then supply of the ultraviolet light to the aqueous solution is started.

[0033] For example, when the sanitary product is a used sanitary product, an aqueous solution containing a mixture of pulp fibers and a superabsorbent polymer tends to become cloudy due to excrement contained in the used sanitary product. Ultraviolet light tends to be less likely to penetrate a cloudy aqueous solution and therefore tends to be less likely to oxidatively decompose the superabsorbent polymer in the cloudy aqueous solution. In the above method, the supply of ultraviolet light to the aqueous solution is started after the supply of an oxidizing agent to the aqueous solution is started, so that the ultraviolet light can efficiently oxidatively decompose the superabsorbent polymer.

[0034] [Aspect 14] The method according to any one of Aspects 1 to 13, wherein in the recycled pulp fiber forming step, the supply of the oxidizing agent to the aqueous solution is stopped, and then the supply of the ultraviolet light to the aqueous solution is stopped.

[0035] In the above method, the supply of ultraviolet light is stopped after the supply of the oxidizing agent is stopped, so that the oxidizing agent remaining in the aqueous solution can efficiently oxidatively decompose the superabsorbent polymer after the supply of the oxidizing agent is stopped. Furthermore, the recycled pulp fiber recovery step can be carried out after the amount of oxidizing agent remaining in the aqueous solution is reduced, resulting in high safety.

[0036] [Aspect 15] The method according to any one of Aspects 1 to 14, wherein in the recycled pulp fiber forming step, supply of the ultraviolet light to the aqueous solution is started, and then supply of the oxidizing agent to the aqueous solution is started.

[0037] In the above method, an oxidizing agent is supplied to the aqueous solution after the supply of ultraviolet light to the aqueous solution is started, so that the superabsorbent polymer can be oxidatively decomposed without performing a step of dehydrating the superabsorbent polymer before the recycled pulp fiber formation step, for example.

[0038] [Aspect 16] The method according to any one of Aspects 1 to 15, wherein the ultraviolet light includes ultraviolet light having a wavelength of 380 nm or less. In this method, the ultraviolet light includes ultraviolet light having a specific wavelength, and therefore the effect of Aspect 1 is enhanced.

[0039] [Aspect 17] A method for producing recycled pulp fibers having predetermined properties according to an application from a mixture containing pulp fibers obtained from sanitary products and a superabsorbent polymer, the method comprising: a recycled pulp fiber formation step of supplying an oxidizing agent and ultraviolet light to an aqueous solution containing the mixture to oxidatively decompose the superabsorbent polymer, dissolving the oxidatively decomposed superabsorbent polymer in the aqueous solution, and forming recycled pulp fibers having the predetermined properties; and a recycled pulp fiber recovery step of recovering the recycled pulp fibers.

[0040] The inventors of the present application have found that while oxidizing agents are not easily affected by the transparency of the aqueous solution and can oxidatively decompose and remove superabsorbent polymers, in cases where the superabsorbent polymer has been dehydrated, it is necessary to expose the pulp fibers to the oxidizing agent for a long period of time in order to oxidatively decompose the superabsorbent polymer, which results in the recycled pulp fibers being prone to changes in the degree of polymerization, amount of carboxyl groups, alkali solubility, etc. This fact may be desirable depending on the application of the recycled pulp fiber, but may be undesirable depending on the application.

[0041] The inventors of the present invention also discovered that although ultraviolet light is affected by the transparency of the aqueous solution, it can oxidatively decompose and remove the superabsorbent polymer, while hardly changing the properties of the recycled pulp fibers that are formed.

[0042] In the recycled pulp fiber forming step, a mixture containing pulp fibers and a superabsorbent polymer is supplied with an aqueous solution containing an oxidizing agent and ultraviolet light at an adjusted ratio to form recycled pulp fibers having the desired properties for the intended use. Therefore, the formed recycled pulp fibers are less likely to contain superabsorbent polymer and its residues, and have the desired properties appropriate for the intended use, making them useful for the intended use.

[0043] The method for producing recycled pulp fibers from a mixture containing pulp fibers obtained from sanitary products and a superabsorbent polymer in the present disclosure (hereinafter sometimes simply referred to as the "method for producing recycled pulp fibers") will be described in detail below.

[0044] [Method for Producing Recycled Pulp Fiber] The method for producing recycled pulp fiber from a mixture containing pulp fiber obtained from sanitary products and a superabsorbent polymer according to the present disclosure includes the following steps: - A recycled pulp fiber formation step (hereinafter sometimes referred to as the "recycled pulp fiber formation step") in which ultraviolet light and an oxidizing agent are supplied to an aqueous solution containing the mixture to oxidatively decompose the superabsorbent polymer and dissolve the oxidatively decomposed superabsorbent polymer in the aqueous solution to form the recycled pulp fiber; - A recycled pulp fiber recovery step (hereinafter sometimes referred to as the "recycled pulp fiber recovery step") in which the recycled pulp fiber is recovered.

[0045] The method for producing recycled pulp fibers according to the present disclosure may further include the following optional steps: A dehydration step (hereinafter sometimes simply referred to as the "dehydration step") of dehydrating the superabsorbent polymer before the recycled pulp fiber formation step.

[0046] <Recycled Pulp Fiber Formation Step> The sanitary product is not particularly limited as long as it contains pulp fibers and a superabsorbent polymer, and examples thereof include disposable diapers, urine absorption pads, sanitary napkins, bed sheets, pet sheets, etc. The sanitary product includes sanitary products that have been used by a user and have absorbed the user's excrement, sanitary products that have been used by a user but have not absorbed the user's excrement, sanitary products that have been discarded unused, and defective products produced when sanitary products are manufactured.

[0047] The pulp fibers are not particularly limited as long as they are usable in sanitary products, and examples thereof include wood pulp (e.g., softwood pulp, hardwood pulp), non-wood pulp, crosslinked pulp, and the like.

[0048] Examples of the superabsorbent polymer include those used in sanitary products, such as starch-based, cellulose-based, and synthetic polymer-based superabsorbent polymers. Examples of starch-based or cellulose-based superabsorbent polymers include starch-acrylic acid (salt) graft copolymers, saponified starch-acrylonitrile copolymers, and crosslinked products of sodium carboxymethyl cellulose. Examples of synthetic polymer-based superabsorbent polymers include polyacrylate-based, polysulfonate-based, maleic anhydride-based, polyacrylamide-based, polyvinyl alcohol-based, polyethylene oxide-based, polyaspartate-based, polyglutamate-based, polyalginate-based, starch-based, and cellulose-based superabsorbent polymers (SAP, Super Absorbent Polymers).

[0049] The method for obtaining an aqueous solution containing a mixture containing pulp fibers and a superabsorbent polymer from the sanitary product is not particularly limited, and can be obtained, for example, by cutting a used sanitary product, obtaining a mixture containing pulp fibers and a superabsorbent polymer from the absorbent body, and dispersing the mixture in water. Alternatively, for example, a used sanitary product can be cut in water to obtain an aqueous solution containing a mixture containing pulp fibers and a superabsorbent polymer in the absorbent body.

[0050] In the aqueous solution, the superabsorbent polymer may or may not have been dehydrated using a dehydrating agent. If the superabsorbent polymer has not been dehydrated, the dehydration step of the superabsorbent polymer can be omitted, and the resulting recycled pulp fibers are less likely to contain the superabsorbent polymer and its residue, and the dehydrating agent and its residue. If the superabsorbent polymer has been dehydrated, the aqueous solution can be reduced to a predetermined moisture content, and the superabsorbent polymer can be efficiently oxidatively decomposed and removed while maintaining a high concentration of the aqueous solution.

[0051] When the superabsorbent polymer is dehydrated with a dehydrating agent, the superabsorbent polymer is preferably dehydrated to have a water absorption capacity of 50 times or less, more preferably 30 times or less, even more preferably 25 times or less, and even more preferably 20 times or less. When the superabsorbent polymer is dehydrated, the superabsorbent polymer is preferably dehydrated to have a water absorption capacity of 1 time or more, more preferably 2 times or more, even more preferably 3 times or more, and even more preferably 4 times or more. This allows the superabsorbent polymer to be efficiently oxidatively decomposed and removed while maintaining a high concentration in the aqueous solution. The dehydrating agent will be described in the optional dehydration step section.

[0052] When the superabsorbent polymer is dehydrated with a dehydrating agent, the aqueous solution preferably has a solids concentration of 4.0% by mass or less, more preferably 3.5% by mass or less, and even more preferably 3.0% by mass or less. The aqueous solution preferably has a solids concentration of 0.1% by mass or more, more preferably 0.5% by mass or more, and even more preferably 1.0% by mass or more. This allows the superabsorbent polymer to be efficiently oxidatively decomposed and removed while maintaining a high concentration of the aqueous solution. The solids concentration is particularly preferred when the aqueous solution is continuously stirred.

[0053] When the superabsorbent polymer is not dehydrated with a dehydrating agent, the aqueous solution preferably has a solids concentration of 0.1% by mass or less, more preferably 0.075% by mass or less, and even more preferably 0.05% by mass or less. The aqueous solution preferably has a solids concentration of 0.01% by mass or more, more preferably 0.02% by mass or more, and even more preferably 0.03% by mass or more. This allows the superabsorbent polymer to be efficiently oxidatively decomposed and removed. The above solids concentration is particularly preferred when the aqueous solution is continuously stirred.

[0054] In this specification, the solids concentration is calculated by subtracting the moisture content from 100. The moisture content is measured using an infrared moisture meter FD-720 manufactured by Kett Corporation. Specifically, approximately 5 g of sample is placed on the sample tray of the FD-720, the temperature is set to 150°C, the automatic stop mode is selected, and the moisture content of the sample is measured.

[0055] The oxidizing agent used in the recycled pulp fiber formation step is not particularly limited as long as it can oxidatively decompose the superabsorbent polymer, and examples thereof include ozone, hydrogen peroxide, chlorine dioxide, peracetic acid, sodium hypochlorite, etc., with ozone being preferred.

[0056] The ozone is preferably supplied (e.g., blown) into the aqueous solution as an ozone-containing gas, and the ozone-containing gas is preferably supplied (e.g., blown) into the aqueous solution as small bubbles (e.g., microbubbles or nanobubbles).

[0057] The ozone-containing gas is supplied to the aqueous solution so that the CT value, which is the product of the ozone concentration in the ozone-containing gas and the treatment time, exceeds 600 ppm min, preferably 800 ppm min or more, more preferably 900 ppm min or more, even more preferably 1,000 ppm min or more, and even more preferably 1,200 ppm min or more. The ozone-containing gas is also supplied to the aqueous solution so that the CT value, which is the product of the ozone concentration in the ozone-containing gas and the treatment time, is preferably 12,000 ppm min or less, more preferably 8,000 ppm min or less, and even more preferably 6,000 ppm min or less. This facilitates oxidative decomposition of the superabsorbent polymer while maintaining the properties of the recycled pulp fiber within the desired range.

[0058] The ozone concentration in the ozone-containing gas is preferably 30 to 200 ppm (g / m 3 ), more preferably 80 to 200 ppm (g / m 3 ), and more preferably 100 to 200 ppm (g / m 3) This makes it easier to maintain the characteristics of the recycled pulp fiber within the expected range while oxidatively decomposing the superabsorbent polymer. The ozone concentration in the ozone-containing gas can be measured, for example, using an ultraviolet absorption ozone concentration meter (for example, Ozone Monitor OZM-5000G manufactured by Ecodesign Co., Ltd.).

[0059] The ozone concentration in the aqueous solution is preferably 1 to 50 ppm (g / m 3 ), more preferably 2 to 40 ppm (g / m 3 ), and more preferably 3 to 30 ppm (g / m 3 )

[0060] The ozone concentration in an aqueous solution is measured by the following method. (1) 85 mL of the aqueous solution containing dissolved ozone is placed in a 100 mL measuring cylinder containing approximately 0.15 g of potassium iodide and 5 mL of 10% by mass citric acid solution, and the mixture is allowed to react. (2) After the reaction, the aqueous solution is transferred to a 200 mL Erlenmeyer flask, and a starch solution is added to the Erlenmeyer flask to turn it purple. The solution is then titrated with 0.01 mol / L sodium thiosulfate while stirring until it becomes colorless, and the amount added: a (mL) is recorded. (3) The ozone concentration in the aqueous solution is calculated using the following formula. The ozone concentration (ppm by mass) in the aqueous solution is calculated using the following formula: Ozone concentration in aqueous solution (ppm by mass) = a (mL) × 0.24 × 0.85 (mL).

[0061] The ultraviolet light used in the recycled pulp fiber formation step preferably includes ultraviolet light with a wavelength of 380 nm or less, more preferably 290 nm or less, even more preferably 260 nm or less, and even more preferably 200 nm or less. The ultraviolet light preferably includes ultraviolet light with a wavelength of 100 nm or more, more preferably 150 nm or more, and even more preferably 160 nm or more. This allows for the oxidative decomposition of the superabsorbent polymer with highly energy-efficient ultraviolet light, while suppressing changes in the properties of the recycled pulp fiber.

[0062] Examples of sources of ultraviolet light include low-pressure mercury lamps, high-pressure mercury lamps, and deep ultraviolet LEDs. Examples of product names for sources of ultraviolet light include a deep ultraviolet generator manufactured by Nikkiso Giken Co., Ltd., UV-LED irradiation running water sterilization module: PAQ-15ESET (processing capacity: 8 liters per minute). The ultraviolet light can be irradiated onto the aqueous solution by, for example, placing an ultraviolet light irradiating device within the aqueous solution or spaced apart from the aqueous solution.

[0063] The applications of the recycled pulp fibers are not particularly limited, and include pulp fiber applications in which the pulp fibers are used as they are (e.g., the above-mentioned sanitary products) and processing applications (e.g., applications as a cellulose raw material). Examples of the applications of the cellulose raw material include cellulose nanofibers, viscose rayon, cellulose derivatives, bioethanol, biobutanol, molding materials, and processed paper products.

[0064] The oxidizing agent and ultraviolet light can be added to the aqueous solution depending on the intended use of the recycled pulp fibers. The oxidizing agent tends to reduce the degree of polymerization of the recycled pulp fibers, increase the amount of carboxyl groups, and increase the alkali solubility. On the other hand, ultraviolet light tends to reduce the difference in properties between the pulp fibers before and after the recycled pulp fiber formation step, such as the change in the degree of polymerization, the change in the amount of carboxyl groups, and the change in the alkali solubility.

[0065] Therefore, by changing the ratio of oxidizing agent to ultraviolet light during oxidative decomposition of a superabsorbent polymer, recycled pulp fibers suitable for each application can be formed. Examples of applications requiring a relatively higher ratio of oxidizing agent include processing applications (e.g., applications using cellulose as a raw material). Examples of applications requiring a relatively higher ratio of ultraviolet light include pulp fiber applications using recycled pulp fibers as pulp fibers (e.g., the above-mentioned sanitary products).

[0066] In the recycled pulp fiber formation step, the oxidizing agent and the ultraviolet light can be supplied to the aqueous solution in an overlapping manner (hereinafter sometimes referred to as "overlapping supply"), or the oxidizing agent and the ultraviolet light can be supplied to the aqueous solution without overlapping manner (hereinafter sometimes referred to as "non-overlapping supply"). The overlapping supply can shorten the time for the recycled pulp fiber formation step, thereby enabling the method to be carried out efficiently. The non-overlapping supply can, for example, allow the recycled pulp fiber formation step to be carried out while checking the condition of the pulp fibers (recycled pulp fibers) or while checking the properties of the recycled pulp fibers.

[0067] In the overlapping and non-overlapping supplying methods, the supply of the oxidizing agent can be started first, and then the supply of the ultraviolet light can be started. This allows the ultraviolet light to efficiently oxidatively decompose the superabsorbent polymer, for example, when the sanitary product is a used sanitary product. Furthermore, in the overlapping and non-overlapping supplying methods, the supply of the ultraviolet light can be started first, and then the supply of the oxidizing agent can be started. This allows the superabsorbent polymer to be oxidatively decomposed without performing a step of dehydrating the superabsorbent polymer before the recycled pulp fiber formation step, for example.

[0068] In the overlapping and non-overlapping supplying methods, the supply of the oxidizing agent can be stopped, and then the supply of the ultraviolet light can be stopped. This allows the oxidizing agent remaining in the aqueous solution to efficiently oxidize and decompose the superabsorbent polymer. Furthermore, safety is high because the recycled pulp fiber recovery step can be carried out after the amount of oxidizing agent remaining in the aqueous solution has decreased. Furthermore, in the overlapping and non-overlapping supplying methods, the supply of the ultraviolet light can be stopped, and then the supply of the oxidizing agent can be stopped. This allows, for example, after stopping the supply of ultraviolet light, the condition of the pulp fibers (recycled pulp fibers) can be visually checked and the supply of the oxidizing agent can be stopped.

[0069] In the recycled pulp fiber formation step, the recycled pulp fibers can be formed to have a carboxyl group amount appropriate for their intended use. The recycled pulp fibers preferably have a carboxyl group amount within the range of 0 to 0.075 mmol / g, more preferably 0.010 to 0.070 mmol / g, even more preferably 0.010 to 0.065 mmol / g, and even more preferably 0.010 to 0.060 mmol / g. This allows the production of recycled pulp fibers suitable for uses where the generation of carboxyl groups is undesirable, such as as a raw material for cellulose derivatives such as acetate, as a raw material for rayon, or as a raw material for cellophane.

[0070] The recycled pulp fibers preferably have an increased amount of carboxyl groups of 0 to 0.035 mmol / g, more preferably 0.010 to 0.030 mmol / g, even more preferably 0.010 to 0.025 mmol / g, and even more preferably 0.010 to 0.020 mmol / g, based on the pulp fibers before the recycled pulp fiber formation step. This allows the production of recycled pulp fibers suitable for applications where the generation of carboxyl groups is undesirable.

[0071] In this specification, the amount of carboxyl groups, C (mmol / g), of pulp fibers and recycled pulp fibers (hereinafter sometimes simply referred to as "pulp fibers") is measured as follows: (1) Approximately 0.4 g of pulp fibers is added to a container containing 170 mL of deionized water, and the pulp fibers are dispersed in the deionized water. (2) 10 mL of 0.01 M NaCl is added to the container. (3) 0.1 M HCl is added to the container to adjust the pH to 2.8.

[0072] (4) 0.05 M NaOH is added to the container at a rate of 0.1 mL / min to 0.2 mL / min until the pH reaches 11, and the electrical conductivity of the contents of the container is monitored. (5) The amount of 0.05 M NaOH added is plotted on the X-axis and the electrical conductivity on the Y-axis, and the amount of 0.05 M NaOH added: V (mL) at which the electrical conductivity becomes constant is determined. (6) The pulp fibers are recovered by filtration, and their bone dry mass: m1 (g) is measured.

[0073] (7) The amount of carboxyl groups in pulp fibers, C (mmol / g), is calculated using the following formula: C (mmol / g) = (V × 0.05 / 1000) / ml. The pH is measured using a pHashion pH meter, C-62, manufactured by AS ONE Corporation. The electrical conductivity is measured using a portable electrical conductivity meter (CM-31P type) manufactured by DKK-TOA Corporation.

[0074] In the recycled pulp fiber formation step, the recycled pulp fibers can be formed to have a degree of polymerization appropriate for their intended use. The recycled pulp fibers preferably have a degree of polymerization of 300 or more, more preferably 350 or more, even more preferably 400 or more, even more preferably 450 or more, and even more preferably 500 or more. This allows the production of recycled pulp fibers suitable for uses where a decrease in the degree of polymerization is undesirable, such as raw materials for rayon, cellophane, and cellulose nanofibers.

[0075] The recycled pulp fibers have a reduction in degree of polymerization, based on the pulp fibers before the recycled pulp fiber formation step, of preferably 400 or less, more preferably 300 or less, even more preferably 200 or less, and even more preferably 150 or less. This makes it possible to produce recycled pulp fibers suitable for applications where a reduction in degree of polymerization is undesirable.

[0076] In this specification, the degree of polymerization (DP) of pulp fibers and recycled pulp fibers (hereinafter sometimes simply referred to as "pulp fibers") is measured as follows: (1) The viscosity ratio η of the pulp fibers is determined in accordance with "6.4.1 Viscosity ratio" of "Cellulose dilute solution - Intrinsic viscosity number measurement method - Copper ethylenediamine method" specified in JIS 8215:1998. r (=η / η0) is measured.

[0077] (2) The degree of polymerization (DP) of the pulp fiber is calculated using the following formula: Specific viscosity: η sp =η r -1 Intrinsic viscosity: [η] = η sp / (100 × c(1 + 0.28η sp)) Degree of polymerization: DP = 175 × [η]. The above formula is based on the description on page 101 of the Wood Science Experiment Manual (edited by the Japan Wood Research Society, Bun'ei-do Publishing, 2000), where c represents the cellulose concentration (g / mL).

[0078] In the recycled pulp fiber formation step, the recycled pulp fibers can be formed to have an alkali solubility appropriate for their intended use. The recycled pulp fibers preferably have an alkali solubility of 0 to 20% by mass, more preferably 1 to 17% by mass, even more preferably 1 to 14% by mass, and even more preferably 1 to 12% by mass. This allows the production of recycled pulp fibers suitable for uses where a decrease in alkali solubility is undesirable, such as for use as a raw material for rayon or cellophane.

[0079] The recycled pulp fibers have an increase in alkali solubility of preferably 0 to 16% by mass, more preferably 1 to 13% by mass, even more preferably 1 to 10% by mass, and even more preferably 1 to 7% by mass, based on the pulp fibers before the recycled pulp fiber formation step. This allows the production of recycled pulp fibers suitable for applications where a decrease in alkali solubility is undesirable.

[0080] In this specification, the alkali solubility of pulp fiber and recycled pulp fiber (hereinafter sometimes simply referred to as "pulp fiber") is measured as follows: (1) The mass: m2 (g) of pulp fiber (approximately 1.0 g) is measured, immersed in 50 mL of 5 M aqueous sodium hydroxide solution, and allowed to stand for 1 hour. (2) The aqueous sodium hydroxide solution is centrifuged to separate the supernatant and the precipitate, and the precipitate is collected by filtration, neutralized, and then dried at 70°C. (3) The precipitate is dried completely, and its dry mass: m3 (g) is measured. (4) The alkali solubility: S (mass%) is calculated using the following formula: S (mass%) = 100 × (m2 - m3) / m2.

[0081] <Recycled Pulp Fiber Recovery Step> In the recycled pulp fiber recovery step, recycled pulp fibers are recovered. In the recycled pulp fiber recovery step, the recycled pulp fibers can be recovered from the aqueous solution using, for example, a screen having a plurality of openings.

[0082] The manufacturing method according to the present disclosure can further include, as an optional step, a dehydration step of dehydrating the superabsorbent polymer before the recycled pulp fiber forming step. This can reduce the amount of excrement contained in the superabsorbent polymer, as well as the viscosity of the superabsorbent polymer and, ultimately, the viscosity of the aqueous solution containing the mixture of pulp fibers and the superabsorbent polymer, allowing the recycled pulp fiber forming step to be performed in a state where the aqueous solution has a low moisture content (high solids content).

[0083] The dehydration step can be carried out, for example, by adding a dehydrating agent to an aqueous solution containing a mixture of pulp fibers and a superabsorbent polymer, or by immersing the pulp fibers and the superabsorbent polymer (or the sanitary product itself) in an aqueous solution containing the dehydrating agent.

[0084] Examples of the dehydrating agent include acids (e.g., inorganic acids and organic acids), lime, calcium chloride, magnesium sulfate, magnesium chloride, aluminum sulfate, and aluminum chloride. Acids are preferred because they are less likely to leave ash on the recycled pulp fibers. When an acid is used as the dehydrating agent, the aqueous solution preferably has a pH of 2.5 or less, and more preferably 1.3 to 2.4. This allows the water absorption capacity of the superabsorbent polymer to be sufficiently reduced, reducing the risk of equipment corrosion and reducing the need for large amounts of alkaline chemicals for neutralization during wastewater treatment.

[0085] Examples of the inorganic acid include sulfuric acid, hydrochloric acid, and nitric acid, with sulfuric acid being preferred from the standpoint of not containing chlorine and being cost-effective. Examples of the organic acid include citric acid, tartaric acid, glycolic acid, malic acid, succinic acid, acetic acid, and ascorbic acid, with acids capable of forming complexes with metal ions contained in excrement, such as hydroxycarbonate-based organic acids such as citric acid, tartaric acid, and gluconic acid, being particularly preferred. Examples of metal ions contained in excrement include calcium ions. This is because the chelating effect of acids capable of forming complexes with metal ions contained in excrement traps and removes the metal ions in excrement. Furthermore, citric acid is expected to have a high stain removal effect due to its cleaning effect.

[0086] In the dehydration step, the superabsorbent polymer is dehydrated so that the superabsorbent polymer has a water absorption capacity of preferably 50 times or less, more preferably 30 times or less, even more preferably 25 times or less, and even more preferably 20 times or less, and preferably 1 time or more, more preferably 2 times or more, even more preferably 3 times or more, and even more preferably 4 times or more. By doing so, the viscosity of the aqueous solution can be maintained within a predetermined range, and the superabsorbent polymer can be efficiently oxidatively decomposed.

[0087] The water absorption capacity is measured as follows: (1) A superabsorbent polymer is placed in a mesh and hung for 5 minutes, water adhering to the surface is removed, and its mass before drying: m4 (g) is measured. (2) The superabsorbent polymer is dried at 120°C for 10 minutes, and its mass after drying: m5 (g) is measured. (3) The water absorption capacity (g / g) is calculated using the following formula: Water absorption capacity (g / g) = 100 x m4 / m5.

[0088] It should be noted that the optional dewatering step can be performed simultaneously with the recycled pulp fiber formation step, thereby allowing for efficient implementation of the method of the present disclosure.

[0089] The present disclosure will be described below using examples, but the present disclosure is not limited to these examples. [Reference Example 1] A mixture of 1.0 g of pulp fiber (softwood pulp fiber) and 0.5 g of superabsorbent polymer (AQUAKEEP (registered trademark), SA60S, manufactured by Sumitomo Seika Chemicals Co., Ltd.) was placed in a mesh bag (25 cm square, N-No. 250HD, manufactured by NBC Meshtec Co., Ltd.), and the mixture, including the mesh bag, was immersed in 80 mL of physiological saline for 15 minutes to allow the superabsorbent polymer (SAP) to absorb the physiological saline. Next, the mixture was immersed in a sulfuric acid aqueous solution of pH 2.0 containing sulfuric acid as a dehydrating agent for 15 minutes to dehydrate the superabsorbent polymer, forming Mixture No. 1. The pulp fiber and superabsorbent polymer placed in the mesh bag were dried at 120°C for 10 minutes.

[0090] Mixture No. 1 in the mesh bag was heated under a low-pressure mercury lamp (UVL20PH-6, manufactured by Sen Special Light Source Co., Ltd., arc tube power: 0.05 W / cm 2 The mixture was loaded into a photochemical reactor (GT500, container capacity: 500 mL) manufactured by Global Top Chemical Co., Ltd., equipped with a pulp fiber and superabsorbent polymer dispersion having a solids concentration of 0.25% by mass, and deionized water was used to form Aqueous Dispersion No. 1 (600 mL). Aqueous Dispersion No. 1 was continuously stirred with a stirrer. The container capacity of the GT500 refers to the minimum volume of the contents.

[0091] The aqueous dispersion No. 1 was irradiated with ultraviolet light from a low-pressure mercury lamp for 30 minutes to oxidatively decompose the superabsorbent polymer. The contents of the photochemical reactor were then filtered, and the filtrate was washed with deionized water.

[0092] The filtered product washed with deionized water was dried at 120°C for 10 minutes, and the dry mass (m6) (g) of the dried filtered product was measured. The residual rate (R) (mass%) of the filtered product was calculated using the following formula: R (mass%) = 100 × m6 / 1.5. The results are shown in Table 1.

[0093] Reference Example 2 The superabsorbent polymer was oxidatively decomposed in the same manner as in Reference Example 1, except that the superabsorbent polymer was not dehydrated using a dehydrating agent. The residual rate of the filtrate is shown in Table 1.

[0094] Reference Example 3: The residual rate of the filtrate was measured in the same manner as Reference Example 1, except that only 1.0 g of pulp fiber (not including 0.5 g of superabsorbent polymer) was placed in the mesh bag. The results are shown in Table 1. Reference Example 4: The residual rate of the filtrate was measured in the same manner as Reference Example 1, except that only 0.5 g of superabsorbent polymer (not including 1.0 g of pulp fiber) was placed in the mesh bag. The results are shown in Table 1.

[0095] Reference Example 5 Mixture No. 1 was prepared in the same manner as Reference Example 1. Mixture No. 1 in the mesh and deionized water were placed in an ozone gas exposure tank with a volume of 2 L to prepare an aqueous dispersion No. 1 (600 mL) of pulp fiber and superabsorbent polymer having a solids concentration of 0.25% by mass. Ozone at a concentration of 50 g / m was generated from an ozone generator (ozone gas exposure tester: ED-OWX-2, manufactured by Ecodesign Co., Ltd.). 3 An ozone-containing gas (the gas other than ozone was dry air) adjusted to a flow rate of 1 L / min was blown into the aqueous dispersion No. 1 in the ozone gas exposure tank for 30 minutes to oxidatively decompose the superabsorbent polymer. The residual rate of the filtered product is shown in Table 1.

[0096] Reference Example 6 The superabsorbent polymer was oxidatively decomposed in the same manner as in Reference Example 5, except that the superabsorbent polymer was not dehydrated using a dehydrating agent. The residual rate of the filtrate is shown in Table 1.

[0097] Reference Example 7 The residual rate of the filtrate was measured in the same manner as Reference Example 5, except that only 1.0 g of pulp fiber (not including 0.5 g of superabsorbent polymer) was placed in the mesh bag. The results are shown in Table 1. Reference Example 8 The residual rate of the filtrate was measured in the same manner as Reference Example 5, except that only 0.5 g of superabsorbent polymer (not including 1.0 g of pulp fiber) was placed in the mesh bag. The results are shown in Table 1.

[0098]

[0099] It can be seen from Reference Examples 1 to 4 that when UV was used, all of the superabsorbent polymer was oxidatively decomposed and all of the pulp fibers remained. On the other hand, it can be seen from Reference Examples 7 and 8 that when ozone was used, all of the superabsorbent polymer was not oxidatively decomposed and some of the superabsorbent polymer remained, resulting in a high residual rate of the filtrate in Reference Examples 5 and 6.

[0100] Reference Example 9 Pulp fibers (softwood pulp fibers) were dispersed in deionized water to form a 0.25% by mass pulp fiber aqueous dispersion No. 2 (600 mL). The aqueous dispersion No. 2 was heated using a low-pressure mercury lamp (UVL20PH-6, manufactured by Sen Special Light Sources Co., Ltd., arc tube power: 0.05 W / cm). 2 The aqueous dispersion No. 2 was loaded into a photochemical reaction apparatus (GT500, container capacity: 500 mL) manufactured by Global Top Chemical Co., Ltd. equipped with a 1000-kJ / kg ion exchanger (AES). The aqueous dispersion No. 2 was continuously stirred with a stirrer. A low-pressure mercury lamp was used to irradiate the aqueous dispersion No. 2 with ultraviolet light for a predetermined period of time, and the amount of carboxyl groups, degree of polymerization, and alkali solubility were measured at the predetermined time points. The results are shown in Tables 2, 3, and 4.

[0101] Reference Example 10: The low-pressure mercury lamp was replaced with a high-pressure mercury lamp (HL100GL-1, manufactured by Sen Special Light Sources Co., Ltd., arc tube power: 8 W / cm 2 The amount of carboxyl groups, the degree of polymerization and the alkali solubility were measured at predetermined times in the same manner as in Reference Example 9, except that the temperature was changed to 100°C. The results are shown in Tables 2, 3 and 4.

[0102] Reference Example 11 Pulp fibers (softwood pulp fibers) and deionized water were placed in an ozone gas exposure tank with a volume of 2 L to form pulp fiber dispersion No. 3 (600 mL) having a solids concentration of 0.25% by mass. An ozone generator (Ecodesign Co., Ltd., Ozone Gas Exposure Tester: ED-OWX-2) emitted ozone at a concentration of 50 g / m. 3An ozone-containing gas (the gas other than ozone was dry air) adjusted to a flow rate of 1 L / min was blown into the aqueous dispersion No. 3 in the ozone gas exposure chamber for 30 minutes. The ozone concentration in the aqueous dispersion No. 3 was approximately 6 ppm. The amount of carboxyl groups, degree of polymerization, and alkali solubility were monitored over a predetermined period of time. The results are shown in Tables 2, 3, and 4.

[0103]

[0104]

[0105]

[0106] Table 2 shows that when pulp fibers are irradiated with ultraviolet light from a low-pressure mercury lamp and a high-pressure mercury lamp, the amount of carboxyl groups in the pulp fibers is less likely to increase, i.e., the pulp fibers are less likely to be oxidized, compared to when the pulp fibers are exposed to ozone. Also, Table 3 shows that when pulp fibers are irradiated with ultraviolet light from a low-pressure mercury lamp and a high-pressure mercury lamp, the degree of polymerization of the pulp is less likely to decrease, compared to when the pulp fibers are exposed to ozone.

[0107] Furthermore, Table 4 shows that when pulp fibers are irradiated with ultraviolet light from a low-pressure mercury lamp and a high-pressure mercury lamp, the alkali dissolution rate is less likely to increase than when pulp fibers are exposed to ozone.In addition, the pulp fibers of Reference Example 11 had colored bone-dry precipitates.

[0108] Combining Tables 2 to 4 with Table 1, it can be seen that UV is excellent at oxidatively decomposing superabsorbent polymers and is less likely to alter pulp fibers. Also, ozone, while inferior to UV, is excellent at oxidatively decomposing superabsorbent polymers and is prone to altering pulp to some extent.

[0109] From the above, when recycled pulp fibers are used for pulp fiber applications, by increasing the ratio of UV among UV and an oxidizing agent (e.g., ozone), recycled pulp fibers with properties similar to those of the original pulp fibers can be formed, and when recycled pulp fibers are used for processing applications, by increasing the ratio of oxidizing agent among UV and an oxidizing agent (e.g., ozone), recycled pulp fibers with desired properties can be formed.

[0110] Example 1 A mixture of 3.0 g of pulp fiber (softwood pulp fiber) and 1.5 g of superabsorbent polymer (AQUAKEEP®, SA60S, manufactured by Sumitomo Seika Chemicals Co., Ltd.) was placed in a mesh bag (25 cm square, N-No. 250HD, manufactured by NBC Meshtec Co., Ltd.), and the mixture, including the mesh bag, was immersed in 80 mL of saline for 15 minutes to allow the superabsorbent polymer (SAP) to absorb the saline. Next, the mixture was immersed in 700 mL of a sulfuric acid aqueous solution (pH 2.0) containing sulfuric acid as a dehydrating agent for 15 minutes to dehydrate the superabsorbent polymer, forming Mixture No. 2. The pulp fiber and superabsorbent polymer placed in the mesh bag had been dried at 120°C for 10 minutes.

[0111] Mixture No. 2 in the mesh, deionized water, and 10 mL of milk were mixed to form 600 mL of pulp fiber and superabsorbent polymer dispersion No. 4 having a solids concentration of 0.75% by mass. The 10 mL of milk was added to make the dispersion opaque, simulating a situation in which the dispersion was cloudy due to the presence of excrement or the like.

[0112] The transparency of the aqueous dispersion No. 4 was 1.0. The transparency was measured using an acrylic transparency meter AT series AT-1 manufactured by Kasahara Chemical Industries.

[0113] [Ozone Treatment] Pulp fiber and superabsorbent polymer dispersion solution No. 4 (600 mL) was irradiated with a low-pressure mercury lamp (UVL20PH-6, manufactured by Sen Special Light Sources Co., Ltd., arc tube power: 0.05 W / cm 2The aqueous dispersion No. 4 was loaded into a photochemical reaction apparatus (GT500, container capacity: 500 mL) manufactured by Global Top Chemical Co., Ltd. equipped with a 1000-kJ / kg water tank, and the aqueous dispersion No. 4 was continuously stirred with a stirrer.

[0114] An ozone gas exposure tester (ED-OWX-2, manufactured by Ecodesign Co., Ltd.) was used to test the photochemical reaction device. The ozone concentration was 50 g / m. 3 An ozone-containing gas (the gas other than ozone was dry air) adjusted to a flow rate of 1 L / min was blown in for 15 minutes to oxidatively decompose the superabsorbent polymer. Note that during this ozone treatment, ultraviolet light was not irradiated from a low-pressure mercury lamp.

[0115] [UV Treatment] Dispersion No. 4 (600 mL) was irradiated with ultraviolet light from a low-pressure mercury lamp for 15 minutes to oxidatively decompose the superabsorbent polymer. Note that no ozone-containing gas was blown in from the ozone generator during this UV treatment. The contents of the photochemical reactor were then filtered, and the filtrate was washed with deionized water to obtain recycled pulp fibers. Table 5 shows the residual rate of superabsorbent polymer and the transparency (post-treatment transparency) of Dispersion No. 4 after exposure to UV light. The residual rate of superabsorbent polymer was calculated assuming a mass of 3.0 g (the same as the mass of the pulp fiber). In Table 5, the superabsorbent polymer is referred to as "SAP" and the pulp fiber is referred to as "pulp."

[0116] Comparative Example 1: Recycled pulp fibers were obtained in the same manner as in Example 1, except that the ozone treatment time was 30 minutes and UV treatment was not performed. The remaining rate of superabsorbent polymer and the transparency after treatment are shown in Table 5. Comparative Example 2: Recycled pulp fibers were obtained in the same manner as in Example 1, except that 10 mL of milk was not added to Dispersion Aqueous Solution No. 4, the UV treatment time was 30 minutes, and ozone treatment was not performed. The remaining rate of superabsorbent polymer and the transparency after treatment are shown in Table 5.

[0117] [Reference Example 12] The same treatment as in Example 1 was carried out, except that 3.0 g of pulp fiber was not added and a superabsorbent polymer dispersion No. 5 (600 mL) having a solids concentration of 0.25% by mass was prepared. The remaining rate of the superabsorbent polymer and the transparency after treatment are shown in Table 5.

[0118] [Reference Example 13] The same treatment as in Reference Example 12 was carried out, except that the ozone treatment and UV treatment were carried out simultaneously for 30 minutes. The residual rate of the superabsorbent polymer and the transparency after treatment are shown in Table 5. [Reference Example 14] The same treatment as in Reference Example 12 was carried out, except that 10 mL of milk was not added to the aqueous dispersion No. 4, and that the ozone treatment and UV treatment were carried out simultaneously for 30 minutes. The residual rate of the superabsorbent polymer and the transparency after treatment are shown in Table 5.

[0119] [Reference Example 15] The same treatment as in Reference Example 12 was carried out, except that the ozone treatment was carried out for 30 minutes and the UV treatment was not carried out. The residual rate of the superabsorbent polymer and the transparency after the treatment are shown in Table 5. [Reference Example 16] The same treatment as in Reference Example 12 was carried out, except that 10 mL of milk was not added to the aqueous dispersion No. 4, and that the ozone treatment was carried out for 30 minutes and the UV treatment was not carried out. The residual rate of the superabsorbent polymer and the transparency after the treatment are shown in Table 5.

[0120] [Reference Example 17] The same treatment as in Reference Example 12 was carried out, except that the UV treatment was carried out for 30 minutes and the ozone treatment was not carried out. The remaining rate of the superabsorbent polymer and the transparency after treatment are shown in Table 5. [Reference Example 18] The same treatment as in Reference Example 12 was carried out, except that 10 mL of milk was not added to the aqueous dispersion No. 4, and that the UV treatment was carried out for 30 minutes and the ozone treatment was not carried out. The remaining rate of the superabsorbent polymer and the transparency after treatment are shown in Table 5.

[0121]

[0122] A comparison between Example 1 and Comparative Example 1, and a comparison between Reference Examples 12 and 15, shows that when ozone treatment and UV treatment are used in combination, the SAP residual rate is approximately the same as when ozone treatment alone is used. Furthermore, since it is clear from Reference Examples 9 to 11 that the properties of recycled pulp fiber change as the ozone treatment time increases, it can be seen that the properties of recycled pulp fiber can be adjusted by using ozone treatment and UV treatment in combination.

Claims

DEPCT671. Method for the production of recycled pulp fibers from a mixture including pulp fibers and superabsorbent polymers and derived from hygiene products, a method comprising: a procedure for the generation of recycled pulp fibers by oxidizing and ultraviolet radiation of an aqueous solution containing the mixture to oxidize the superabsorbent polymers and dissolve the oxidized superabsorbent polymers in the aqueous solution; and a procedure for the recovery of recycled pulp fibers.

2. Method according to claim 1, where the oxidizing agent is ozone.3.Methods under claim 1 or 2, where recycled pulp fibers have predetermined characteristics based on use, and in the recycled pulp fiber generation process, recycled pulp fibers with predetermined characteristics based on use are generated by adjusting the ratio of ultraviolet radiation and oxidizing agent and the application of ultraviolet radiation and oxidizing agent to the aqueous solution.

4. Method under claim 3, where the use is cellulose raw material.

5. Method under claim 4, where the use of raw materials. Cellulose was selected from a group comprising nanocellulose fibers, viscose rayon, cellulose derivatives, bioethanol, biobutanol, molding materials, and processed paper products.

6. Any one of the methods under claims 3 through 5, where the recycled pulp fibers with predefined characteristics have a carboxyl group content of 0 to 0.075 mmol / g.

7. Any one of the methods under claims 3 through 6, where the recycled pulp fibers with predefined characteristics have an increased carboxyl group content of 0 to 0.0.35 mmol / g based on pulp fibers.

8. Any of the methods under claims 3 to 7, where the recycled pulp fibers with predefined characteristics have a polymerization level of 300 or more.

9. Any of the methods under claims 3 to 8, where the recycled pulp fibers with predefined characteristics have a reduction to a polymerization level of 400 or less based on pulp fibers.

10. Any of the methods under claims 3 to 9, where the recycled pulp fibers with predefined characteristics have an alkali solubility rate of 0 to 20% by mass.

11. Any of the methods under claims 3 to 10, where the recycled pulp fibers have an increase in alkali solubility of 0 to 16% by mass based on pulp fibers.

12. Any of the methods under claims 1 to 11, where in the recycled pulp fiber generation step, the oxidizing agent and ultraviolet radiation are supplied to the aqueous solution in an overlapping manner. 13.

14. Any of the methods under claims 1 through 12, in which, during the recycled pulp fiber formation step, the oxidizing agent supply to the aqueous solution is initiated, and then the ultraviolet radiation supply to the aqueous solution is initiated.

15. Any of the methods under claims 1 through 14, in which, during the recycled pulp fiber formation step, the oxidizing agent supply to the aqueous solution is stopped, and then the ultraviolet radiation supply to the aqueous solution is stopped.

16. Any of the methods under claims 1 through 15, in which the ultraviolet radiation includes ultraviolet radiation with a wavelength of 380 nanometers or less;