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A method for surface crosslinking water-absorbing resin of the present invention includes a step (1) of obtaining a wet mixture, a step (2) of obtaining a dried particulate composition, and a step (3) of carrying out a surface crosslinking reaction. With this, since a processing time of each step be

A method for surface crosslinking water-absorbing resin of the present invention includes a step (1) of obtaining a wet mixture, a step (2) of obtaining a dried particulate composition, and a step (3) of carrying out a surface crosslinking reaction. With this, since a processing time of each step becomes short, it is possible to mass produce the water-absorbing resin having excellent physical properties. Moreover, a method for manufacturing the water-absorbing resin of the present invention includes a modifying step and a cooling step. The modifying step and/or the cooling step are/is carried out by using stirring means including a rotation axis having a plurality of stirring boards, and the stirring means includes the stirring board having a specific thickness and/or a scraping blade having a specific shape. With this, it is possible to suppress the generation of the fine powder in the modifying step and/or the cooling step.

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What is claimed is: 1. A method for manufacturing water-absorbing resin, comprising: a modifying step that is a step of heating a mixture of a hydrophilic crosslinked polymer containing a carboxyl group and an aqueous solution containing a surface crosslinking agent containing two or more functiona

What is claimed is: 1. A method for manufacturing water-absorbing resin, comprising: a modifying step that is a step of heating a mixture of a hydrophilic crosslinked polymer containing a carboxyl group and an aqueous solution containing a surface crosslinking agent containing two or more functional groups which react with the carboxyl group, so as to crosslink a surface of the hydrophilic crosslinked polymer; and a cooling step of cooling down the hydrophilic crosslinked polymer thus surface crosslinked, at least one of the modifying step and the cooling step being carried out by using a stirring device including a rotation axis and including a plurality of stirring boards, wherein a direction parallel to a longitudinal direction of the rotation axis is a y direction, a direction that is perpendicular to the y direction and is parallel to a circumferential direction of the rotation axis is an x direction, and a direction perpendicular to the x direction and the y direction is a z direction, a ratio of a maximum thickness of the stirring board to a minimum thickness of the stirring board being 5 or less in a cross section taken along a y-z plane of the stirring device, wherein the stirring board is in a disc shape or a disc shape a part of which is lacking. 2. A method for manufacturing water-absorbing resin, comprising: a modifying step that is a step of heating a mixture of a hydrophilic crosslinked polymer containing a carboxyl group and an aqueous solution containing a surface crosslinking agent containing two or more functional groups which react with the carboxyl group, so as to crosslink a surface of the hydrophilic crosslinked polymer; and a cooling step of cooling down the hydrophilic crosslinked polymer thus surface crosslinked, at least one of the modifying step and the cooling step being carried out by using a stirring device including a rotation axis and including a plurality of stirring boards having scraping blades, wherein a direction parallel to a longitudinal direction of the rotation axis is a y direction, a direction that is perpendicular to the y direction and is parallel to a circumferential direction of the rotation axis is an x direction, and a direction perpendicular to the x direction and the y direction is a z direction, (i) a cross-sectional area of the scraping blade in a cross section taken along a y-z plane of the stirring device being 10% or more but less than 50% of an area in the y-z plane between one stirring board from which the scraping blade projects and another stirring board provided so as to face said one stirring board, and (ii) a length of the scraping blade in the y direction in the cross section taken along the y-z plane being 50% or more of a distance in the y-z plane between said one stirring board and said another stirring board. 3. A method for surface crosslinking water-absorbing resin, comprising the sequential steps of: (1) mixing a solution containing a surface crosslinking agent with a water-absorbing resin precursor, so as to obtain a wet mixture; (2) stirring and drying the wet mixture, so as to obtain a dried particulate composition containing the surface crosslinking agent; and (3) carrying out a heat treatment of the dried particulate composition, so as to cause a surface crosslinking reaction of the dried particulate composition, 80 weight % or more of the dried particulate composition obtained in the step (2) being particles which pass through a sieve having a mesh size of 10 mm, each of the steps (2) and (3) being carried out by using at least one respective processing device, and the processing device used for the step (2) and the processing device used for the step (3) being serially connected to each other. 4. The method as set forth in claim 3, wherein the processing device used for the step (2) and the processing device used for the step (3) are different in at least one of type and effective volume from each other. 5. The method as set forth in claim 3, wherein the processing device used for the step (2) and the processing device used for the step (3) are driven under at least one of different heating conditions and different stirring conditions. 6. A method for surface crosslinking water-absorbing resin, comprising the sequential steps of: (1) mixing a solution containing a surface crosslinking agent with a water-absorbing resin precursor, so as to obtain a wet mixture; (2) stirring and drying the wet mixture, so as to obtain a dried particulate composition containing the surface crosslinking agent; and (3) carrying out a heat treatment of the dried particulate composition, so as to cause a surface crosslinking reaction of the dried particulate composition, 80 weight % or more of the dried particulate composition obtained in the step (2) being particles which pass through a sieve having a mesh size of 10 mm, the steps (2) and (3) being carried out by using one processing device, and the steps (2) and (3) being carried out under at least one of different heating conditions and different stirring conditions. 7. A method for manufacturing water-absorbing resin, comprising: a modifying step that (i) is a step of heating a mixture of a hydrophilic crosslinked polymer containing a carboxyl group and an aqueous solution containing a surface crosslinking agent containing two or more functional groups which react with the carboxyl group, so as to crosslink a surface of the hydrophilic crosslinked polymer and (ii) includes a step (2) of stirring and drying the mixture, so as to obtain a dried particulate composition containing the surface crosslinking agent, and a step (3) of carrying out a heat treatment of the dried particulate composition, so as to cause a surface crosslinking reaction of the dried particulate composition; and a cooling step of cooling down the hydrophilic crosslinked polymer thus surface crosslinked, at least one of the step (2), the step (3) and the cooling step being carried out by using a processing device including a stirring device including a rotation axis and including a plurality of stirring boards, wherein a direction parallel to a longitudinal direction of the rotation axis is a y direction, a direction that is perpendicular to the y direction and is parallel to a circumferential direction of the rotation axis is an x direction, and a direction perpendicular to the x direction and the y direction is a z direction, a ratio of a maximum thickness of the stirring board to a minimum thickness of the stirring board being 5 or less in a cross section taken along a y-z plane of the stirring device, the mixture being a wet mixture manufactured in a step (1) of mixing the hydrophilic crosslinked polymer containing the carboxyl group with the aqueous solution containing the surface crosslinking agent containing two or more functional groups which react with the carboxyl group, 80 weight % or more of the dried particulate composition obtained in the step (2) being particles which pass through a sieve having a mesh size of 10 mm, each of the steps (2) and (3) being carried out by using at least one respective processing device, and the processing device used for the step (2) and the processing device used for the step (3) being serially connected to each other, wherein the stirring board is in a disc shape or a disc shape a part of which is lacking. 8. A method for manufacturing water-absorbing resin, comprising: a modifying step that (i) is a step of heating a mixture of a hydrophilic crosslinked polymer containing a carboxyl group and an aqueous solution containing a surface crosslinking agent containing two or more functional groups which react with the carboxyl group, so as to crosslink a surface of the hydrophilic crosslinked polymer and (ii) includes a step (2) of stirring and drying the mixture, so as to obtain a dried particulate composition containing the surface crosslinking agent, and a step (3) of carrying out a heat treatment of the dried particulate composition, so as to cause a surface crosslinking reaction of the dried particulate composition; and a cooling step of cooling down the hydrophilic crosslinked polymer thus surface crosslinked, at least one of the step (2), the step (3) and the cooling step being carried out by using a processing device including a stirring device including a rotation axis and including a plurality of stirring boards having scraping blades, wherein a direction parallel to a longitudinal direction of the rotation axis is a y direction, a direction that is perpendicular to the y direction and is parallel to a circumferential direction of the rotation axis is an x direction, and a direction perpendicular to the x direction and the y direction is a z direction, (a) a cross-sectional area of the scraping blade in a cross section taken along a y-z plane of the stirring device being 10% or more but less than 50% of an area in the y-z plane between one stirring board from which the scraping blade projects and another stirring board provided so as to face said one stirring board, and (b) a length of the scraping blade in the y direction in the cross section taken along the y-z plane being 50% or more of a distance in the y-z plane between said one stirring board and said another stirring board, the mixture being a wet mixture manufactured in a step (1) of mixing the hydrophilic crosslinked polymer containing the carboxyl group with the aqueous solution containing the surface crosslinking agent containing two or more functional groups which react with the carboxyl group, 80 weight % or more of the dried particulate composition obtained in the step (2) being particles which pass through a sieve having a mesh size of 10 mm, each of the steps (2) and (3) being carried out by using at least one respective processing device, and the processing device used for the step (2) and the processing device used for the step (3) being serially connected to each other. 9. A method for manufacturing water-absorbing resin, comprising: a modifying step that is a step of heating a mixture of a hydrophilic crosslinked polymer containing a carboxyl group and an aqueous solution containing a surface crosslinking agent containing two or more functional groups which react with the carboxyl group, so as to crosslink a surface of the hydrophilic crosslinked polymer; and a cooling step of cooling down the hydrophilic crosslinked polymer thus surface crosslinked, at least one of the modifying step and the cooling step being carried out by using a processing device including a stirring device including a rotation axis and including a plurality of stirring boards, wherein a direction parallel to a longitudinal direction of the rotation axis is a y direction, a direction that is perpendicular to the y direction and is parallel to a circumferential direction of the rotation axis is an x direction, and a direction perpendicular to the x direction and the y direction is a z direction, a ratio of a maximum thickness of the stirring board to a minimum thickness of the stirring board being 5 or less in a cross section taken along a y-z plane of the stirring device, the mixture being a wet mixture manufactured in a step (1) of mixing the hydrophilic crosslinked polymer containing the carboxyl group with the aqueous solution containing the surface crosslinking agent containing two or more functional groups which react with the carboxyl group, the modifying step including a step (2) of stirring and drying the wet mixture, so as to obtain a dried particulate composition containing the surface crosslinking agent and a step (3) of carrying out a heat treatment of the dried particulate composition, so as to cause a surface crosslinking reaction of the dried particulate composition, 80 weight % or more of the dried particulate composition obtained in the step (2) being particles which pass through a sieve having a mesh size of 10 mm, the steps (2) and (3) being carried out by using one processing device, and the steps (2) and (3) being carried out under at least one of different heating conditions and different stirring conditions, wherein the stirring board is in a disc shape or a disc shape a part of which is lacking. 10. A method for manufacturing water-absorbing resin, comprising: a modifying step that is a step of heating a mixture of a hydrophilic crosslinked polymer containing a carboxyl group and an aqueous solution containing a surface crosslinking agent containing two or more functional groups which react with the carboxyl group, so as to crosslink a surface of the hydrophilic crosslinked polymer; and a cooling step of cooling down the hydrophilic crosslinked polymer thus surface crosslinked, at least one of the modifying step and the cooling step being carried out by using a processing device including a stirring device including a rotation axis and including a plurality of stirring boards having scraping blades, where a direction parallel to a longitudinal direction of the rotation axis is a y direction, a direction that is perpendicular to the y direction and is parallel to a circumferential direction of the rotation axis is an x direction, and a direction perpendicular to the x direction and the y direction is a z direction, (i) a cross-sectional area of the scraping blade in a cross section taken along a y-z plane of the stirring device being 10% or more but less than 50% of an area in the y-z plane between one stirring board from which the scraping blade projects and another stirring board provided so as to face said one stirring board, and (ii) a length of the scraping blade in the y direction in the cross section taken along the y-z plane being 50% or more of a distance in the y-z plane between said one stirring board and said another stirring board, the mixture being a wet mixture manufactured in a step (1) of mixing the hydrophilic crosslinked polymer containing the carboxyl group with the aqueous solution containing the surface crosslinking agent containing two or more functional groups which react with the carboxyl group, the modifying step including a step (2) of stirring and drying the wet mixture, so as to obtain a dried particulate composition containing the surface crosslinking agent and a step (3) of carrying out a heat treatment of the dried particulate composition, so as to cause a surface crosslinking reaction of the dried particulate composition, 80 weight % or more of the dried particulate composition obtained in the step (2) being particles which pass through a sieve having a mesh size of 10 mm, the steps (2) and (3) being carried out by using one processing device, and the steps (2) and (3) being carried out under at least one of different heating conditions and different stirring conditions. 11. The method as set forth in claim 7, wherein the processing device used for the step (2) and the processing device used for the step (3) are different in at least one of type and effective volume from each other. 12. The method as set forth in claim 7, wherein the processing device used for the step (2) and the processing device used for the step (3) are driven under at least one of different heating conditions and different stirring conditions. 13. The method as set forth in claim 2, wherein a ratio of a maximum thickness of the stirring board to a minimum thickness of the stirring board is 5 or less in the cross section taken along the y-z plane of the stirring device. 14. The method as set forth in claim 1, wherein the stirring board also functions as a heat transferring device. 15. The method as set forth in claim 1, wherein the stirring board also functions as a cooling device. 16. The method as set forth in claim 8, wherein the processing device used for the step (2) and the processing device used for the step (3) are different in at least one of type and effective volume from each other. 17. The method as set forth in claim 8, wherein the processing device used for the step (2) and the processing device used for the step (3) are driven under at least one of different heating conditions and different stirring conditions. 18. The method as set forth in claim 8, wherein a ratio of a maximum thickness of the stirring board to a minimum thickness of the stirring board is 5 or less in the cross section taken along the y-z plane of the stirring device. 19. The method as set forth in claim 10, wherein a ratio of a maximum thickness of the stirring board to a minimum thickness of the stirring board is 5 or less in the cross section taken along the y-z plane of the stirring device. 20. The method as set forth in claim 2, wherein: each of the scraping blades projects from a plurality of the stirring boards, and the adjacent scraping blades among the scraping blades are provided alternately while sandwiching the rotation axis or are provided in parallel with each other, in the cross section taken along the y-z plane of the stirring device. 21. The method as set forth in claim 8, wherein: each of the scraping blades projects from a plurality of the stirring boards, and the adjacent scraping blades among the scraping blades are provided alternately while sandwiching the rotation axis or are provided in parallel with each other, in the cross section taken along the y-z plane of the stirring device. 22. The method as set forth in claim 10, wherein: each of the scraping blades projects from a plurality of the stirring boards, and the adjacent scraping blades among the scraping blades are provided alternately while sandwiching the rotation axis or are provided in parallel with each other, in the cross section taken along the y-z plane of the stirring device. 23. The method as set forth in claim 2, wherein the stirring board is in a disc shape or a disc shape a part of which is lacking. 24. The method as set forth in claim 10, wherein the stirring board is in a disc shape or a disc shape a part of which is lacking. 25. The method as set forth in claim 2, wherein the stirring board also functions as a heat transferring device. 26. The method as set forth in claim 7, wherein the stirring board also functions as a heat transferring device. 27. The method as set forth in claim 10, wherein the stirring board also functions as a heat transferring device.