Base paper for corrugated cardboard, method for producing same, and corrugated cardboard formed using base paper for corrugated cardboard

Abstract

The present invention provides: base paper for corrugated cardboard that, even when made from a raw material of hard-to-process recovered paper pulp, which is made from a raw material of hard-to-process recovered paper having at least a paper base layer, a metal layer, and a resin layer, does not readily react to a metal detector and does not interfere with foreign matter detection by a metal detector even when used as a packaging material; a method for producing base paper for corrugated cardboard; and corrugated cardboard formed using the base paper for corrugated cardboard.　This base paper for corrugated cardboard has one or more layers. At least one layer included in the base paper for corrugated cardboard contains hard-to-process recovered paper pulp made from a raw material of hard-to-process recovered paper. The hard-to-process recovered paper has at least a paper base layer, a metal layer, and a resin layer. Defibered flakes of 0.05 mm2 or more containing the metal layer in a handmade sheet of basis weight 60 g / m2 obtained from pulp in which the layer containing the hard-to-process recovered paper pulp has been defibered have a total area of 10.0 mm2 / 0.02 m2 or less, and a maximum area of 4.0 mm2 or less.

Description

Corrugated cardboard base paper and method for manufacturing the same, and corrugated cardboard made using the corrugated cardboard base paper 【0001】 This invention relates to base paper for corrugated cardboard, a method for manufacturing the same, and corrugated cardboard made using the base paper. 【0002】 Traditionally, so-called difficult-to-process waste paper, such as composite paper containing resin or metal layers, has been difficult to recycle. However, in recent years, with growing concern for waste reduction and environmental issues, there has been a desire to recycle more waste paper, and the utilization of such difficult-to-process waste paper is also desired. 【0003】 For example, Japanese Patent Publication No. 6-228894 describes a method for recovering pulp from polyolefins, in which alkali is added to polyolefin laminate waste paper, it is dissociated at a concentration of 10-15% under heating, and after rough sorting and fine sorting processes, it is diluted to a concentration of 0.5-0.85% to obtain the amount of pulp slurry (Lm). ３ ) for the amount of air supplied (GNm ３ A method for processing polyolefin laminate waste paper is disclosed, characterized by performing flotation treatment with a G / L ratio of 5 or more. Furthermore, Japanese Patent Publication No. 2010-43368 discloses a method for recycling metal laminate waste paper, in which at least metal foil is laminated on a paper substrate, and recovering pulp fibers from the metal laminate waste paper, characterized by physically shredding the metal laminate waste paper in advance. Furthermore, Japanese Patent Publication No. Hei 6-220784 discloses a method for recovering waste paper, characterized by recovering paper from waste paper in which at least metal foil and / or plastic is laminated on a paper substrate by the following steps (1) to (5): (1) Disintegrating the waste paper by treating it in an alkaline aqueous solution heated with the addition of a deinking / release agent and a penetrating agent mainly consisting of a silicate compound and a carbonate. (2) Diluting with water. (3) Removing coarse dust. (4) A step in which a degassing agent is added and fine dust is removed using a weight cleaner and a light cleaner. (5) A dealkalization step. 【0004】In recent years, so-called aseptic filling methods have been used for beverage containers and other products. Aseptic container packaging materials have a structure in which metal layers and resin layers are laminated onto a paper base. Aseptic container packaging materials have advantages such as reducing the amount of resin used, making them a viable alternative to conventional plastic containers, and thus demand for them is increasing. On the other hand, when difficult-to-process waste paper, which has metal and resin layers in addition to the paper base, is used for recycling, finely dispersed fragments containing metal layers derived from the difficult-to-process waste paper may be mixed in. As a result, paper products obtained from recycling difficult-to-process waste paper may trigger metal detectors, and there has been a problem in that foreign object detection becomes difficult, especially when used as corrugated cardboard. 【0005】 Japanese Patent Publication No. 6-228894 did not consider the removal of metals and was therefore insufficient for recycling paper substrates containing metal layers. Furthermore, in Japanese Patent Publication Nos. 2010-43368 and 6-220784, the metal and resin layers were pulverized during the recycling process, so dust originating from the metal and resin layers could not be sufficiently removed from the resulting waste paper pulp. For these reasons, if the waste paper pulp obtained in Japanese Patent Publication Nos. 6-228894, 2010-43368, and 6-220784 is used as raw material for corrugated cardboard base paper, corrugated cardboard made from this base paper will be rejected by metal detectors during the shipping process of products filled with packaged goods, leading to a decrease in product shipping efficiency. 【0006】 The object of the present invention is to provide corrugated cardboard base paper, a method for manufacturing the same, and corrugated cardboard made using the corrugated cardboard base paper, in which the amount of disintegrated fragments having a metal layer is reduced, even when using difficult-to-treat recycled paper pulp made from difficult-to-treat recycled paper having at least a paper base layer, a metal layer, and a resin layer as raw material, and the amount of disintegrated fragments having a metal layer is reduced, it is less likely to be detected by metal detectors, it does not interfere with foreign object detection by metal detectors when used as a packaging material, and the quality is improved. 【0007】The inventors of the present invention have found that, in a base paper for corrugated board having one or two or more layers, at least one layer contains a hard-to-treat waste paper pulp made from hard-to-treat waste paper, and by making the total area and the maximum area of the released pieces of 0.05 mm or more in the hand-sheet obtained from the pulp obtained by dissociating the middle layer be below a certain value, the above problems can be solved. ２ above be below a certain value, it has been found that the above problems can be solved. 【0008】 The present invention provides the following. <1> A base paper for corrugated board having one or two or more layers, wherein at least one of the layers of the base paper for corrugated board contains a hard-to-treat waste paper pulp made from hard-to-treat waste paper, the hard-to-treat waste paper has at least a paper base material layer, a metal layer and a resin layer, and the total area of the released pieces having a metal layer in the hand-sheet having a basis weight of 60 g / m ２ obtained from the pulp obtained by dissociating the layer containing the hard-to-treat waste paper pulp is 10.0 mm ２ / 0.02 m ２ or less, and the maximum area is 4.0 mm ２ or less, a base paper for corrugated board. <2> A base paper for corrugated board having at least a surface layer, a middle layer and a back layer, the middle layer contains a hard-to-treat waste paper pulp made from hard-to-treat waste paper, the hard-to-treat waste paper has at least a paper base material layer, a metal layer and a resin layer, and the total area of the released pieces having a metal layer in the hand-sheet having a basis weight of 60 g / m ２ obtained from the pulp obtained by dissociating the middle layer is 10.0 mm ２ / 0.02 m ２ or less, and the maximum area is 4.0 mm ２ or less, the base paper for corrugated board according to <1>. <3> The number of released pieces having the metal layer in the hand-sheet and having a size of 0.05 m ２ or more is 70 pieces / 0.02 m ２ or less. ２ above is 70 pieces / 0.02 m ２The corrugated cardboard base paper according to <1> or <2> below. <4> The corrugated cardboard base paper according to any one of <1> to <3>, wherein the amount of the difficult-to-treat recycled paper pulp in the raw pulp constituting the layer containing the difficult-to-treat recycled paper pulp is 0.2% by mass or more and 50% by mass or less. <5> The corrugated cardboard base paper according to any one of <1> to <4>, wherein the paper base layer of the difficult-to-treat recycled paper contains one or more selected from the group consisting of hardwood kraft pulp and softwood kraft pulp. <6> 0.05 mm having the metal layer ２ The corrugated cardboard base paper according to any one of <1> to <5>, wherein the above-mentioned disintegrated fragments further include a resin layer. <7> The corrugated cardboard base paper according to any one of <1> to <6>, wherein the difficult-to-process waste paper is aseptic container waste paper. <8> The corrugated cardboard base paper according to any one of <1> to <7>, wherein the difficult-to-process waste paper has a structure in which a thermoplastic resin layer / pigment coating layer / paper substrate layer / adhesive resin layer / aluminum foil layer / adhesive resin layer / thermoplastic resin layer is laminated in this order. <9> Corrugated cardboard made using the corrugated cardboard base paper according to any one of <1> to <8>. <10> A method for manufacturing corrugated cardboard base paper as described in any one of <1> to <9>, comprising: a difficult-to-process recycled paper pulp manufacturing step for manufacturing difficult-to-process recycled paper pulp using the difficult-to-process recycled paper as a raw material; and a papermaking step for manufacturing corrugated cardboard base paper using a paper stock containing the difficult-to-process recycled paper pulp as a paper stock for a layer containing the difficult-to-process recycled paper pulp, wherein the difficult-to-process recycled paper pulp manufacturing step comprises: a disintegration step for disintegrating the difficult-to-process recycled paper; a dust removal step for removing foreign matter from the pulp slurry after the disintegration step; and a pulp washing step for washing the pulp in the pulp slurry after the dust removal step, wherein the concentration of the difficult-to-process recycled paper in the disintegration step is 5% by mass or more and 25% by mass or less; and the processing time of the disintegration step is 5 minutes or more and 90 minutes or less. <11> The average size of the difficult-to-process recycled paper used in the disintegration step is 600 cm ２The method for manufacturing corrugated cardboard base paper as described in <10> below. <12> The method for manufacturing corrugated cardboard base paper as described in <10> or <11>, wherein the processing temperature in the disintegration step is 5°C or more and 80°C or less. <13> The method for manufacturing corrugated cardboard base paper as described in any one of <10> to <12>, wherein the pH of the processing liquid in the disintegration step is 5 or more and 11 or less. <14> The method for manufacturing corrugated cardboard base paper as described in any one of <10> to <13>, wherein the method comprises at least one of a crushing step for crushing the difficult-to-process waste paper and a waste paper washing step for washing the difficult-to-process waste paper before the disintegration step. <15> The method comprises the crushing step before the disintegration step, wherein the average size of the difficult-to-process waste paper after crushing is 10 cm ２ More than 400cm ２ The method for manufacturing corrugated cardboard base paper as described in <14> is as follows: <16> The method for manufacturing corrugated cardboard base paper as described in <14> or <15>, wherein the waste paper washing step is performed before the disintegration step, and the amount of washing water per unit mass of difficult-to-process waste paper is 0.5 L / kg or more and 30 L / kg or less. <17> The method for manufacturing corrugated cardboard base paper as described in any one of <10> to <16>, wherein the dust removal step comprises a step of removing foreign matter from the pulp slurry after the disintegration step by centrifugal separation and a step of removing foreign matter from the pulp slurry after the disintegration step by screen treatment. <18> The method for manufacturing corrugated cardboard base paper as described in any one of <10> to <17>, wherein the dewatering step is performed after the pulp washing step, and the pulp slurry is dewatered to obtain pulp. 【0009】 [Base paper for corrugated cardboard] The base paper for corrugated cardboard of this embodiment is a base paper for corrugated cardboard having one or more layers, wherein at least one of the layers of the base paper for corrugated cardboard contains difficult-to-process recycled paper pulp made from difficult-to-process recycled paper (hereinafter also simply referred to as the "layer containing difficult-to-process recycled paper pulp"), wherein the difficult-to-process recycled paper has at least a paper base layer, a metal layer and a resin layer, and the basis weight of 60 g / m² is obtained from the pulp obtained by dissociating the layer containing the difficult-to-process recycled paper pulp. ２ 0.05 mm handmade sheet with a metal layer ２ The total area of ​​the above-mentioned dissociated fragments is 10.0 mm². ２ / 0.02m ２The following conditions apply, and the maximum area is 4.0 mm². ２ The following applies. According to this embodiment, even when using difficult-to-treat recycled paper pulp made from difficult-to-treat recycled paper having at least a paper base layer, a metal layer, and a resin layer as raw material, it is possible to provide corrugated cardboard base paper, a method for manufacturing the same, and corrugated cardboard made using said corrugated cardboard base paper, which is less likely to be detected by metal detectors and does not interfere with foreign object detection by metal detectors when used as packaging material. 【0010】 Furthermore, the corrugated cardboard base paper of this embodiment is a corrugated cardboard base paper having at least a surface layer, a middle layer, and a back layer, and it is preferable that the middle layer is a layer containing difficult-to-process recycled paper pulp made from difficult-to-process recycled paper. That is, the corrugated cardboard base paper of this embodiment is a corrugated cardboard base paper having at least a surface layer, a middle layer, and a back layer, the middle layer contains difficult-to-process recycled paper pulp made from difficult-to-process recycled paper, the difficult-to-process recycled paper has at least a paper base layer, a metal layer, and a resin layer, and the basis weight obtained from the pulp obtained by disintegrating the middle layer is 60 g / m². ２ 0.05 mm handmade sheet with a metal layer ２ The total area of ​​the above-mentioned dissociated fragments is 10.0 mm². ２ / 0.02m ２ The following conditions apply, and the maximum area is 4.0 mm². ２ The following is preferable: It is a base paper for corrugated cardboard. 【0011】 The corrugated cardboard base paper of this embodiment is obtained from pulp obtained by dissociating the layer containing the difficult-to-process recycled paper pulp, and has a basis weight of 60 g / m². ２ In a handmade sheet, a 0.05 mm sheet having a metal layer ２ The total area of ​​the above-mentioned dissociated fragments is 10.0 mm². ２ / 0.02m ２ The following conditions apply, and the maximum area is 4.0 mm². ２ As a result of the following, even if it contains difficult-to-treat recycled paper pulp made from difficult-to-treat recycled paper having metal and resin layers, it is less likely to be detected by metal detectors and does not interfere with foreign object detection by metal detectors when used as packaging material. 【0012】In this specification, a numerical range represented as "X to Y" means a numerical range that includes X as the lower limit and Y as the upper limit. When a numerical range is described in steps, the upper and lower limits of each numerical range can be combined in any way. 【0013】 In the corrugated cardboard base paper of this embodiment, a 0.05 mm layer having a metal layer is provided. ２ The above-mentioned disintegrated fragments are disintegrated fragments derived from difficult-to-process recycled paper pulp, and are generated during the disintegration process when difficult-to-process recycled paper is manufactured from difficult-to-process recycled paper, when metal layers and other components contained in the difficult-to-process recycled paper are finely crushed by the disintegration process. In other words, they are 0.05 mm fragments containing a metal layer. ２ The above dissociated fragments have an area of ​​0.05 mm². ２ The above-mentioned disintegrated fragments contain, at least a portion, a metal layer derived from the metal layer of the difficult-to-process waste paper. Furthermore, the difficult-to-process waste paper has at least a paper base layer, a metal layer, and a resin layer. Therefore, in the corrugated cardboard base paper of this embodiment, a 0.05 mm thick material having a metal layer is used. ２ The above-mentioned disintegrated fragments may contain, in addition to a metal layer derived from the metal layer of the difficult-to-process waste paper, a resin layer derived from the resin layer of the difficult-to-process waste paper, and at least a portion of them may have a paper base layer derived from the paper base layer of the difficult-to-process waste paper. 【0014】 In this embodiment, the base paper for corrugated cardboard is obtained from pulp obtained by dissociating a layer containing difficult-to-process recycled paper pulp, with a basis weight of 60 g / m². ２ 0.05 mm handmade sheet with a metal layer ２ The total area of ​​the above-mentioned dissociated fragments is 10.0 mm², from the viewpoint of suppressing the reaction of metal detectors. ２ / 0.02m ２ The following, preferably 8.0 mm ２ / 0.02m ２ More preferably, 6.0 mm ２ / 0.02m ２ More preferably 4.0 mm ２ / 0.02m ２ More preferably, 3.0 mm ２ / 0.02m ２ The following applies: 0.05 mm having the aforementioned metal layer ２The total area of ​​the above-mentioned dissociated fragments is preferably small, and although there is no particular lower limit, from the viewpoint of ease of manufacture, it is preferably 0.05 mm. ２ / 0.02m ２ More preferably 0.1 mm ２ / 0.02m ２ More preferably 0.3 mm ２ / 0.02m ２ More preferably 0.5 mm ２ / 0.02m ２ That is all. 0.05 mm having the aforementioned metal layer ２ The total area of ​​the above-mentioned dissociated fragments is measured by the method described in the examples. 【0015】 In this embodiment, the base paper for corrugated cardboard is obtained from pulp obtained by dissociating a layer containing difficult-to-process recycled paper pulp, with a basis weight of 60 g / m². ２ 0.05 mm handmade sheet with a metal layer ２ The maximum area of ​​the above-mentioned dissociated fragments is 4.0 mm, from the viewpoint of suppressing the reaction of metal detectors. ２ The following, preferably 2.0 mm ２ More preferably 1.5 mm ２ More preferably 1.0 mm ２ More preferably, 0.7 mm ２ The following applies: 0.05 mm having the aforementioned metal layer ２ The maximum area of ​​the above-mentioned dissociated fragments is preferably small, and is not particularly limited, but from the viewpoint of ease of manufacture, it is preferably 0.05 mm. ２ More preferably 0.1 mm ２ More preferably 0.2 mm ２ More preferably 0.3 mm ２ That is all. The 0.05 mm having the aforementioned metal layer ２ The maximum area of ​​the above-mentioned dissociated fragments is measured by the method described in the examples. 【0016】 In this embodiment, the base paper for corrugated cardboard is obtained from pulp obtained by dissociating a layer containing difficult-to-process recycled paper pulp, with a basis weight of 60 g / m². ２ 0.05 mm handmade sheet with a metal layer ２From the viewpoint of suppressing the reaction of metal detectors, the number of dissociated fragments is preferably 70 fragments / 0.02 m. ２ More preferably, 60 pieces / 0.02 m ２ More preferably, 50 pieces / 0.02 m ２ More preferably, 45 pieces / 0.02 m ２ More preferably, 40 pieces / 0.02 m ２ More preferably, 30 pieces / 0.02 m ２ Further, and more preferably, 20 pieces / 0.02 m ２ The following applies: 0.05 mm having the aforementioned metal layer ２ The number of the above-mentioned dissociated fragments should preferably be small, and there is no particular lower limit, but from the viewpoint of ease of manufacturing, preferably 1 fragment / 0.02 m. ２ Above, 2 pieces / 0.02m ２ More preferably, 3 pieces / 0.02 m ２ That is all. The 0.05 mm having the aforementioned metal layer ２ The number of the above-mentioned dissociated fragments is measured by the method described in the examples. 【0017】 The corrugated cardboard base paper of this embodiment has one or more layers, preferably two or more layers, more preferably three or more layers, and even more preferably three layers. The corrugated cardboard base paper of this embodiment is used, for example, as a liner for corrugated cardboard. When the corrugated cardboard base paper of this embodiment has three or more layers, it is preferable that the corrugated cardboard base paper has at least a surface layer, a middle layer, and a back layer. When used as a liner for corrugated cardboard, the surface layer forms the surface of the corrugated cardboard, and the back layer forms the back surface that is bonded to the core of the corrugated cardboard. The middle layer is formed between the surface layer and the back layer and reinforces the strength of the corrugated cardboard base paper. 【0018】Furthermore, in the corrugated cardboard base paper of this embodiment, at least one of the layers of the corrugated cardboard base paper contains difficult-to-process recycled paper pulp made from difficult-to-process recycled paper. If the corrugated cardboard base paper of this embodiment has one layer, all layers contain difficult-to-process recycled paper pulp. If the corrugated cardboard base paper of this embodiment has two or more layers, at least one layer may contain difficult-to-process recycled paper pulp, or all layers may contain difficult-to-process recycled paper pulp. Furthermore, if the corrugated cardboard base paper of this embodiment has three or more layers, at least one layer may contain difficult-to-process recycled paper pulp, or all layers may contain difficult-to-process recycled paper pulp, and it is preferable that the middle layer contains difficult-to-process recycled paper pulp. 【0019】 <Layer containing difficult-to-process recycled paper pulp made from difficult-to-process recycled paper> In this embodiment, at least one layer of the corrugated cardboard base paper contains difficult-to-process recycled paper pulp made from difficult-to-process recycled paper. It can be inferred that at least one layer of the corrugated cardboard base paper contains difficult-to-process recycled paper pulp made from difficult-to-process recycled paper by performing the following analysis on the layers of the corrugated cardboard base paper. A 0.05 mm layer that normally has a metal layer ２ The above-mentioned disintegrated fragments are considered foreign matter in typical corrugated cardboard base paper and are not intentionally included in the blend. Therefore, it is reasonable to assume that they were integrated with the paper base material and resin layer of the difficult-to-process recycled paper. • Analyze the residual ink content using JIS P 8254:2013 "Pulp and paper - Test method for effective residual ink concentration (ERIC value) by infrared reflectance measurement" and identify the presence of recycled paper pulp. • Analyze the components of the foreign matter on the corrugated cardboard base paper using micro-FT-IR and identify the presence of a 0.05 mm layer containing a resin layer and a metal layer. ２ Identify that the above-mentioned dissociated fragments are present. 【0020】(Difficult-to-process recycled paper) In the corrugated cardboard base paper of this embodiment, the difficult-to-process recycled paper that serves as the raw material for difficult-to-process recycled paper pulp has at least a paper base layer, a metal layer, and a resin layer. Each of these layers may have one or more layers, or two or more layers. 【0021】[Paper Substrate Layer] The paper substrate layer is not particularly limited as long as it is paper that is commonly used with plant-derived wood pulp as the main component. The wood pulp may be one type alone or two or more types. The wood pulp is not particularly limited, but examples include chemical pulps such as hardwood kraft pulp (LKP), softwood kraft pulp (NKP), sulfite pulp (SP), dissolved pulp (DP), soda pulp (AP), bleached kraft pulp (BKP), unbleached kraft pulp (UKP), and oxygen-bleached kraft pulp (OKP); semi-chemical pulps such as semi-chemical pulp (SCP) and chemigroundwood pulp (CGP); and mechanical pulps such as crushed wood pulp (GP), thermomechanical pulp (TMP), and chemothermetic pulp (CTMP). Among these, the paper base layer of difficult-to-process recycled paper preferably contains one or more selected from the group consisting of hardwood kraft pulp (LKP) and softwood kraft pulp (NKP), more preferably one or more selected from the group consisting of unbleached hardwood kraft pulp (LUKP), bleached hardwood kraft pulp (LBKP), unbleached softwood kraft pulp (NUKP), and bleached softwood kraft pulp (NBKP), even more preferably one or more selected from the group consisting of bleached hardwood kraft pulp (LBKP) and bleached softwood kraft pulp (NBKP), and even more preferably bleached hardwood kraft pulp (LBKP) and bleached softwood kraft pulp (NBKP). When the paper base layer contains the above pulp (especially softwood kraft pulp), the strength of the corrugated cardboard base paper of this embodiment is further improved. When the paper base layer contains hardwood kraft pulp (LKP) and softwood kraft pulp (NKP), from the viewpoint of improving the strength of the corrugated cardboard base paper, the mass ratio of hardwood kraft pulp to softwood kraft pulp (LKP:NKP) is preferably 90:10 to 40:60, more preferably 85:15 to 50:50, even more preferably 80:20 to 60:40, and even more preferably 75:25 to 65:35. 【0022】The basis weight of the paper base material layer in the difficult-to-treat waste paper is preferably 65 g / m ２ or more and 500 g / m ２ or less, more preferably 100 g / m ２ or more and 450 g / m ２ or less, even more preferably 150 g / m ２ or more and 400 g / m ２ or less, still more preferably 180 g / m ２ or more and 380 g / m ２ or less, even more preferably 200 g / m ２ or more and 350 g / m ２ or less. When the difficult-to-treat waste paper has a plurality of paper base material layers, the basis weight of the paper base material layer in the difficult-to-treat waste paper indicates the basis weight of each paper base material layer. 【0023】 The thickness of the paper base material layer in the difficult-to-treat waste paper is preferably 80 μm or more and 600 μm or less, more preferably 150 μm or more and 550 μm or less, even more preferably 200 μm or more and 500 μm or less, still more preferably 230 μm or more and 480 μm or less, even more preferably 250 μm or more and 450 μm or less, from the viewpoints of improving the strength of the base paper for cardboard, improving the recyclability of the difficult-to-treat waste paper, and reducing the disintegrated pieces having a metal layer in the difficult-to-treat waste paper pulp. When the difficult-to-treat waste paper has a plurality of paper base material layers, the thickness of the paper base material layer in the difficult-to-treat waste paper indicates the thickness of each paper base material layer. 【0024】 The content of the paper base material layer in the difficult-to-treat waste paper is preferably 50% by mass or more and less than 100% by mass, more preferably 55% by mass or more and 99% by mass or less, even more preferably 60% by mass or more and 96% by mass or less, still more preferably 65% by mass or more and 95% by mass or less, from the viewpoints of improving the strength of the base paper for cardboard, improving the recyclability of the difficult-to-treat waste paper, and reducing the disintegrated pieces having a metal layer in the difficult-to-treat waste paper pulp. When the difficult-to-treat waste paper has a plurality of paper base material layers, the content of the paper base material layer in the difficult-to-treat waste paper is the total amount of the contents of each paper base material layer. 【0025】[Metal Layer] In difficult-to-process recycled paper, the metal constituting the metal layer is not particularly limited, but from the viewpoint of productivity, aluminum is preferred. That is, in this embodiment, the metal layer is preferably an aluminum layer. The aluminum layer is preferably one or more selected from the group consisting of an aluminum vapor-deposited layer and an aluminum foil layer, and an aluminum foil layer is more preferred. 【0026】 From the viewpoint of improving the recyclability of difficult-to-process waste paper and reducing the amount of disintegrated fragments containing the metal layer in difficult-to-process waste paper pulp, the basis weight of the metal layer in difficult-to-process waste paper is preferably 8 g / m². ２ 30g / m or more ２ More preferably, 10 g / m ２ 25g / m or more ２ More preferably, 12 g / m ２ 23g / m or more ２ The following applies. Note that if the difficult-to-process waste paper has multiple metal layers, the basis weight of the metal layers in the difficult-to-process waste paper refers to the basis weight of each individual metal layer. 【0027】 The thickness of the metal layer in difficult-to-process waste paper is preferably 3 μm to 15 μm, more preferably 4 μm to 12 μm, and even more preferably 6 μm to 10 μm, from the viewpoint of improving the recyclability of difficult-to-process waste paper and reducing the amount of disintegrated fragments containing the metal layer in the difficult-to-process waste paper pulp. If the difficult-to-process waste paper has multiple metal layers, the thickness of the metal layer in the difficult-to-process waste paper refers to the thickness of each individual metal layer. 【0028】 From the viewpoint of improving the recyclability of difficult-to-process waste paper and reducing the amount of disintegrated fragments containing metal layers in difficult-to-process waste paper pulp, the content of the metal layer in the difficult-to-process waste paper is preferably 2% to 15% by mass, more preferably 3% to 13% by mass, even more preferably 4% to 11% by mass, or 5% to 9% by mass. If the difficult-to-process waste paper has multiple metal layers, the content of the metal layer in the difficult-to-process waste paper is the sum of the content of each metal layer. 【0029】[Resin Layer] In difficult-to-process waste paper, the resin constituting the resin layer is not particularly limited, but for example, a thermoplastic resin is preferred. Examples of thermoplastic resins include high-density polyethylene (HDPE), medium-density polyethylene (MDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), polyolefins such as polypropylene, polyesters such as polyethylene terephthalate (PET), polyamides such as nylon, ethylene-vinyl alcohol copolymer (EVOH), ethylene-(meth)acrylic acid copolymer, styrene-acrylic copolymer, and ethylene-α-olefin copolymer. Furthermore, if the difficult-to-process waste paper has multiple resin layers, the resins constituting each resin layer may be the same or different. Among these, from the viewpoint of improving the recyclability of difficult-to-process waste paper and reducing disintegrated fragments having a metal layer in the difficult-to-process waste paper pulp, the resin constituting the resin layer is preferably one or more selected from the group consisting of polyolefin, more preferably low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE), and even more preferably low-density polyethylene (LDPE). 【0030】 From the viewpoint of improving the recyclability of difficult-to-process waste paper and reducing the amount of disintegrated fragments containing the metal layer in difficult-to-process waste paper pulp, the basis weight of the resin layer in difficult-to-process waste paper is preferably 5 g / m². ２ 60g / m or more ２ More preferably, 6 g / m ２ 55g / m or more ２ More preferably 8 g / m ２ 50g / m or more ２ More preferably, 10 g / m ２ More than 47g / m ２ The following applies. Note that if the difficult-to-process waste paper has multiple resin layers, the basis weight of the resin layers in the difficult-to-process waste paper refers to the basis weight of each individual resin layer. 【0031】The thickness of the resin layer in difficult-to-process waste paper is preferably 5 μm to 100 μm, more preferably 8 μm to 70 μm, even more preferably 10 μm to 50 μm, and even more preferably 15 μm to 40 μm, from the viewpoint of improving the recyclability of difficult-to-process waste paper and reducing the amount of disintegrated fragments containing the metal layer in the difficult-to-process waste paper pulp. If the difficult-to-process waste paper has multiple resin layers, the thickness of the resin layer in the difficult-to-process waste paper refers to the thickness of each individual resin layer. 【0032】 The resin layer content in difficult-to-process waste paper is preferably 1% to 35% by mass, more preferably 2% to 30% by mass, even more preferably 3% to 25% by mass, and even more preferably 5% to 20% by mass, from the viewpoint of improving the recyclability of difficult-to-process waste paper and reducing disintegrated fragments containing a metal layer in difficult-to-process waste paper pulp. If the difficult-to-process waste paper has multiple resin layers, the resin layer content in the difficult-to-process waste paper is the sum of the contents of each resin layer. 【0033】 [Other Layers] In addition to the paper substrate layer, metal layer, and resin layer described above, difficult-to-process waste paper may also have other layers. Examples of other layers include adhesive resin layers, pigment coating layers, printed layers, adhesive layers, and inorganic oxide vapor deposition layers. 【0034】≪Adhesive Resin Layer≫ In this embodiment, it is preferable that the difficult-to-treat waste paper further has an adhesive resin layer. The adhesive resin layer is formed mainly for bonding the metal layer to the paper substrate layer and / or for bonding the metal layer to the resin layer. The resin constituting the adhesive resin layer is the same as the resin mentioned above for the resin layer. Furthermore, if the difficult-to-treat waste paper has multiple adhesive resin layers, the resins constituting each adhesive resin layer may be the same or different. Among these, the resin constituting the adhesive resin layer is preferably one or more selected from the group consisting of polyolefin, ethylene-acrylic acid copolymer (EAA), ethylene-methacrylic acid copolymer (EMAA), and ethylene-methyl methacrylate copolymer (EMMA), more preferably one or more selected from the group consisting of low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), ethylene-acrylic acid copolymer (EAA), ethylene-methacrylic acid copolymer (EMAA), and ethylene-methyl methacrylate copolymer (EMMA), and even more preferably one or more selected from the group consisting of low-density polyethylene (LDPE) and ethylene-methacrylic acid copolymer (EMAA). 【0035】 When difficult-to-process waste paper has an adhesive resin layer, the basis weight of the adhesive resin layer in the difficult-to-process waste paper is preferably 2 g / m², from the viewpoint of improving the recyclability of the difficult-to-process waste paper and reducing the amount of disintegrated fragments containing the metal layer in the difficult-to-process waste paper pulp. ２ 45g / m or more ２ More preferably, 3 g / m ２ 40g / m or more ２ More preferably 4 g / m ２ 30g / m or more ２ More preferably, 5 g / m ２ 27g / m or more ２ The following applies. If the difficult-to-process waste paper has multiple adhesive resin layers, the basis weight of the adhesive resin layers in the difficult-to-process waste paper refers to the basis weight of each individual adhesive resin layer. 【0036】When difficult-to-process waste paper has an adhesive resin layer, the thickness of the adhesive resin layer is preferably 2 μm to 45 μm, more preferably 3 μm to 40 μm, even more preferably 4 μm to 35 μm, and even more preferably 5 μm to 30 μm, from the viewpoint of improving the recyclability of the difficult-to-process waste paper and reducing the amount of disintegrated fragments containing the metal layer in the difficult-to-process waste paper pulp. When difficult-to-process waste paper has multiple adhesive resin layers, the thickness of the adhesive resin layer in the difficult-to-process waste paper refers to the thickness of each adhesive resin layer. 【0037】 When difficult-to-process waste paper has an adhesive resin layer, the content of the adhesive resin layer in the difficult-to-process waste paper is preferably 1% by mass or more and 12% by mass or less, more preferably 1.5% by mass or more and 10% by mass or less, and even more preferably 2% by mass or more and 9% by mass or less, from the viewpoint of improving the recyclability of the difficult-to-process waste paper and from the viewpoint of reducing disintegrated fragments having a metal layer in the difficult-to-process waste paper pulp. When difficult-to-process waste paper has multiple adhesive resin layers, the content of the adhesive resin layer in the difficult-to-process waste paper is the sum of the content of each adhesive resin layer. 【0038】 Furthermore, when the difficult-to-process waste paper has an adhesive resin layer, the total content of the resin layer and adhesive resin layer in the difficult-to-process waste paper is preferably 5% by mass or more and 40% by mass or less, more preferably 10% by mass or more and 30% by mass or less, and even more preferably 15% by mass or more and 25% by mass or less, from the viewpoint of improving the recyclability of the difficult-to-process waste paper and from the viewpoint of reducing disintegrated fragments having a metal layer in the difficult-to-process waste paper pulp. 【0039】≪Pigment Coating Layer≫ In this embodiment, it is preferable that the difficult-to-treat recycled paper further has a pigment coating layer. Furthermore, when the difficult-to-treat recycled paper has a pigment coating layer, it is more preferable that the pigment coating layer is in contact with the paper substrate layer, and it is even more preferable that the paper substrate layer has a pigment coating layer. The pigment coating layer may be one layer or two or more layers. The pigment coating layer preferably contains a pigment and a binder. The pigment is not particularly limited, but examples include pigments commonly used in the coated paper manufacturing field, such as calcium carbonate (heavy calcium carbonate, light calcium carbonate), titanium dioxide, aluminum hydroxide, silica, satin white, and talc. The binder is not particularly limited, but examples include water-based adhesives containing polymers such as starch, styrene-acrylic copolymer latex, styrene-butadiene copolymer latex, and olefin-unsaturated carboxylic acid copolymer latex. 【0040】 When difficult-to-process waste paper has a pigment coating layer, the basis weight of the pigment coating layer in the difficult-to-process waste paper is preferably 5 g / m², from the viewpoint of improving the recyclability of the difficult-to-process waste paper and reducing the amount of disintegrated fragments containing the metal layer in the difficult-to-process waste paper pulp. ２ 25g / m or more ２ More preferably, 10 g / m ２ 20g / m or more ２ More preferably, 12 g / m ２ 18g / m or more ２ The following applies. Furthermore, if the difficult-to-process waste paper has multiple pigment coating layers, the basis weight of the pigment coating layers in the difficult-to-process waste paper refers to the basis weight of each individual pigment coating layer. 【0041】 When difficult-to-process waste paper has a pigment coating layer, the thickness of the pigment coating layer is preferably 3 μm to 25 μm, more preferably 5 μm to 20 μm, and even more preferably 10 μm to 15 μm, from the viewpoint of improving the recyclability of the difficult-to-process waste paper and reducing the amount of disintegrated fragments containing the metal layer in the difficult-to-process waste paper pulp. When difficult-to-process waste paper has multiple pigment coating layers, the thickness of the pigment coating layer in the difficult-to-process waste paper refers to the thickness of each individual pigment coating layer. 【0042】When difficult-to-process waste paper has a pigment coating layer, the amount of pigment coating layer in the difficult-to-process waste paper is preferably 1% by mass or more and 12% by mass or less, more preferably 2% by mass or more and 10% by mass or less, and even more preferably 3% by mass or more and 8% by mass or less, from the viewpoint of improving the recyclability of the difficult-to-process waste paper and from the viewpoint of reducing disintegrated fragments having a metal layer in the difficult-to-process waste paper pulp. When difficult-to-process waste paper has multiple pigment coating layers, the amount of pigment coating layer in the difficult-to-process waste paper is the sum of the amounts of each pigment coating layer. 【0043】 ≪Printed Layer≫ In this embodiment, the difficult-to-process recycled paper may further have a printed layer. Examples of the printed layer include a printed layer provided on the paper substrate layer or the pigment coating layer. The printed layer may be formed using known inks such as oil-based inks, water-based inks, or biomass inks. The printed content may be patterns, designs, or information (such as ingredients, expiration date, or QR code®). The printed layer may be formed on the entire surface of the layer to which the printed layer is applied, or on only a part of it. 【0044】 When difficult-to-process waste paper has a printed layer, the basis weight of the printed layer in the difficult-to-process waste paper is preferably 0.5 g / m², from the viewpoint of improving the recyclability of the difficult-to-process waste paper and reducing the amount of disintegrated fragments containing the metal layer in the difficult-to-process waste paper pulp. ２ 6g / m or more ２ More preferably, 1 g / m ２ 5g / m or more ２ More preferably, 1.5 g / m ２ 4g / m or more ２ The following applies. Note that if the difficult-to-process waste paper has multiple printing layers, the basis weight of the printing layers in the difficult-to-process waste paper refers to the basis weight of each individual printing layer. 【0045】When difficult-to-process waste paper has a printed layer, the thickness of the printed layer is preferably 0.5 μm to 5 μm, more preferably 1 μm to 4 μm, and even more preferably 1.5 μm to 3 μm, from the viewpoint of improving the recyclability of the difficult-to-process waste paper and reducing the amount of disintegrated fragments containing the metal layer in the difficult-to-process waste paper pulp. When difficult-to-process waste paper has multiple printed layers, the thickness of the printed layer in the difficult-to-process waste paper refers to the thickness of each individual printed layer. 【0046】 When difficult-to-process waste paper has a printed layer, the content of the printed layer in the difficult-to-process waste paper is preferably 0.1% by mass or more and 3% by mass or less, more preferably 0.3% by mass or more and 2% by mass or less, and even more preferably 0.5% by mass or more and 1.5% by mass or less, from the viewpoint of improving the recyclability of the difficult-to-process waste paper and reducing the amount of disintegrated fragments containing a metal layer in the difficult-to-process waste paper pulp. When difficult-to-process waste paper has multiple printed layers, the content of the printed layer in the difficult-to-process waste paper is the sum of the content of each printed layer. 【0047】Examples of difficult-to-process waste paper include waste paper used for liquid containers such as beverage containers, liquid seasoning containers, and detergent containers. Examples of liquid containers such as beverage containers and liquid seasoning containers include aseptic containers and hot packs in which a metal layer and a resin layer are laminated on a paper base layer, with aseptic containers being preferred. An aseptic container has, for example, the following configuration: Thermoplastic resin layer / Pigment coating layer / Paper base layer / Adhesive resin layer / Aluminum foil layer / Adhesive resin layer / Thermoplastic resin layer Thermoplastic resin layer / Printed layer / Pigment coating layer / Paper base layer / Adhesive resin layer / Aluminum foil layer / Adhesive resin layer / Thermoplastic resin layer LDPE layer / Pigment coating layer / Paper base layer / LDPE layer / Aluminum foil layer / EMAA layer / LLDPE layer LDPE layer / Printed layer / Pigment coating layer / Paper base layer / LDPE layer / Aluminum foil layer / EMAA layer / LLDPE layer An aseptic container has, for example, the following configuration: Thermoplastic resin layer / Pigment coating layer / Paper substrate layer / Adhesive resin layer / Aluminum layer / Resin substrate layer / Thermoplastic resin layer Thermoplastic resin layer / Printing layer / Pigment coating layer / Paper substrate layer / Adhesive resin layer / Aluminum layer / Resin substrate layer / Thermoplastic resin layer LDPE layer / Pigment coating layer / Paper substrate layer / LLDPE layer / Aluminum layer / PET layer / LDPE layer LDPE layer / Printing layer / Pigment coating layer / Paper substrate layer / LLDPE layer / Aluminum layer / PET layer / LDPE layer Therefore, as difficult-to-process waste paper, waste paper having the above layer configuration is preferred. 【0048】 (Difficult-to-process recycled paper pulp) The difficult-to-process recycled paper pulp in this embodiment is obtained using the above-mentioned difficult-to-process recycled paper pulp as a raw material and by the manufacturing process of difficult-to-process recycled paper pulp in the method for manufacturing corrugated cardboard base paper of this embodiment described later. Therefore, the difficult-to-process recycled paper pulp in this embodiment has a metal layer and is 0.05 mm thick. ２ The above-mentioned disintegrated fragments are hardly present. Therefore, the corrugated cardboard base paper of this embodiment is obtained from pulp obtained by disintegrating a layer containing difficult-to-process recycled paper pulp, with a basis weight of 60 g / m². ２ 0.05 mm handmade sheet with a metal layer ２ The total area of ​​the above-mentioned dissociated fragments is 10.0 mm². ２ / 0.02m ２ The following conditions apply, and the maximum area is 4.0 mm². ２The following results in a material that is less likely to trigger metal detectors. 【0049】 In the corrugated cardboard base paper of this embodiment, the layer containing difficult-to-process recycled paper pulp may contain other pulps in addition to the difficult-to-process recycled paper pulp described above. (Other pulps) There are no particular limitations on the other pulps, but examples include unbleached and bleached chemical pulps, mechanical pulps, non-wood pulps, and other recycled paper pulps other than the difficult-to-process recycled paper pulp described above. Examples of other recycled paper pulps include recycled paper pulp collected from the trimmings of coated paper for printing generated from bookbinding and printing factories (Kent recycled paper pulp), magazine recycled paper pulp, flyer recycled paper pulp, newspaper recycled paper pulp, office recycled paper pulp, information paper recycled paper pulp, corrugated cardboard recycled paper pulp, and paper container recycled paper pulp. Among these, from the viewpoint of improving recyclability, the other pulps are preferably other recycled paper pulps, and more preferably one or more selected from the group consisting of magazine recycled paper pulp and corrugated cardboard recycled paper pulp. 【0050】 In the corrugated cardboard base paper of this embodiment, the amount of difficult-to-treat recycled paper pulp in the raw pulp constituting the layer containing difficult-to-treat recycled paper pulp is preferably 0.2% by mass or more and 50% by mass or less, more preferably 1% by mass or more and 40% by mass or less, even more preferably 3% by mass or more and 30% by mass or less, even more preferably 5% by mass or more and 20% by mass or less, and even more preferably 10% by mass or more and 18% by mass or less. When the amount of difficult-to-treat recycled paper pulp is 0.2% by mass or more, the difficult-to-treat recycled paper can be used effectively, which is preferable as it improves recyclability. Also, when the amount of difficult-to-treat recycled paper pulp is 50% by mass or less, the corrugated cardboard base paper becomes less likely to be detected by metal detectors. In the corrugated cardboard base paper of this embodiment, when there are two or more layers containing difficult-to-treat recycled paper pulp, the amount of difficult-to-treat recycled paper pulp in the raw pulp constituting the layer containing difficult-to-treat recycled paper pulp refers to the amount of difficult-to-treat recycled paper pulp in each layer containing difficult-to-treat recycled paper pulp. 【0051】 Next, as a preferred embodiment, a preferred embodiment of corrugated cardboard base paper having at least a surface layer, a middle layer, and a back layer will be described in detail. 【0052】<Middle Layer> In the corrugated cardboard base paper of this embodiment, the middle layer may be one layer or two or more layers. 【0053】 When the corrugated cardboard base paper of this embodiment has an intermediate layer, the intermediate layer is preferably a layer containing difficult-to-treat recycled paper pulp, and the amount of difficult-to-treat recycled paper pulp in the raw pulp constituting the intermediate layer is preferably the same as the amount of difficult-to-treat recycled paper pulp in the raw pulp constituting the layer containing the difficult-to-treat recycled paper pulp. 【0054】 In this embodiment, the basis weight of the middle layer of the corrugated cardboard base paper is preferably 30 g / m², from the viewpoint of improving the strength of the corrugated cardboard base paper, improving the recyclability of difficult-to-process waste paper, and reducing the amount of disintegrated fragments containing a metal layer in the difficult-to-process waste paper pulp. ２ More than 200g / m ２ More preferably, 50 g / m ２ More than 170g / m ２ More preferably 90 g / m ２ 140g / m or more ２ More preferably, 105 g / m ２ 130g / m or more ２ The following applies. Note that if the corrugated cardboard base paper of this embodiment has multiple intermediate layers, the basis weight of the intermediate layers of the corrugated cardboard base paper is the sum of the basis weights of each intermediate layer. 【0055】 <Surface Layer> In the corrugated cardboard base paper of this embodiment, if the surface layer is a layer containing difficult-to-process recycled paper pulp, it is preferable that the amount of difficult-to-process recycled paper pulp in the raw pulp constituting the surface layer is the same as the amount of difficult-to-process recycled paper pulp in the raw pulp constituting the layer containing the difficult-to-process recycled paper pulp as described above. On the other hand, in the corrugated cardboard base paper of this embodiment, if the surface layer is not a layer containing difficult-to-process recycled paper pulp, there are no particular restrictions on the raw pulp constituting the surface layer, but it is preferably other pulp, and even more preferably other recycled paper pulp from the viewpoint of recyclability, and even more preferably one or more selected from the group consisting of magazine recycled paper pulp and corrugated cardboard recycled paper pulp, and even more preferably corrugated cardboard recycled paper. 【0056】From the viewpoint of improving the strength of the corrugated cardboard base paper in this embodiment, the basis weight of the surface layer is preferably 10 g / m². ２ More than 100g / m ２ More preferably, 15 g / m ２ 50g / m or more ２ More preferably 20 g / m ２ 40g / m or more ２ More preferably, 25 g / m ２ 35g / m or more ２ The following applies: 【0057】 <Backing Layer> In the corrugated cardboard base paper of this embodiment, if the backing layer is a layer containing difficult-to-process recycled paper pulp, it is preferable that the amount of difficult-to-process recycled paper pulp in the raw pulp constituting the surface layer is the same as the amount of difficult-to-process recycled paper pulp in the raw pulp constituting the layer containing the difficult-to-process recycled paper pulp. On the other hand, in the corrugated cardboard base paper of this embodiment, if the backing layer is not a layer containing difficult-to-process recycled paper pulp, there are no particular restrictions on the raw pulp constituting the backing layer, but examples include the difficult-to-process recycled paper pulp and other pulps mentioned above. Among these, from the viewpoint of suppressing the deterioration of the quality of the corrugated cardboard base paper, other pulps are preferred, and from the viewpoint of recyclability, other recycled paper pulp is even more preferred, even more preferably one or more selected from the group consisting of magazine recycled paper pulp and corrugated cardboard recycled paper pulp, and even more preferably corrugated cardboard recycled paper. 【0058】 From the viewpoint of improving the strength of the corrugated cardboard base paper in this embodiment, the basis weight of the backing layer is preferably 10 g / m². ２ More than 100g / m ２ More preferably, 15 g / m ２ 50g / m or more ２ More preferably, 17 g / m ２ 40g / m or more ２ More preferably, 20 g / m ２ 30g / m or more ２ The following applies: 【0059】In this embodiment, internal additives can be used in each layer of the corrugated cardboard base paper as needed. For example, yield enhancers such as aluminum sulfate, sizing agents such as rosin, paper strength enhancers (dry paper strength enhancers) such as polyacrylamide and starch, water-filtered yield enhancers, wet paper strength enhancers such as polyamide, polyamine, and epichlorohydrin, and dyes can be used. 【0060】 The content of aluminum sulfate is not particularly limited, but in each layer, it is preferably 0.1 parts by mass or more and 5.0 parts by mass or less per 100 parts by mass of raw pulp constituting the layer. The content of sizing agent is not particularly limited, but in each layer, it is preferably 0.1 parts by mass or more and 2.0 parts by mass or less per 100 parts by mass of raw pulp constituting the layer. The content of paper strength agent is not particularly limited, but in each layer, it is preferably 0.1 parts by mass or more and 6.0 parts by mass or less per 100 parts by mass of raw pulp constituting the layer. 【0061】 [Cardboard] The cardboard of this embodiment is made using the base paper for cardboard of this embodiment described above. Because the cardboard of this embodiment uses the base paper for cardboard of this embodiment, it is less likely to be detected by metal detectors, even when difficult-to-process recycled paper pulp is used as raw material. The base paper for cardboard of this embodiment can be used, for example, as a liner placed on at least one outermost surface of the cardboard, or as a corrugated core in the middle layer of the cardboard, thereby providing the cardboard. From the viewpoint of improving recyclability, it is preferable to use the base paper for cardboard as a liner. Cardboard can be single-sided cardboard in which a liner is laminated to only one side of the core, double-sided cardboard in which liners are laminated to both sides of the core, or multi-layer cardboard in which multiple layers of core / liner laminates are provided. The base paper for cardboard of this embodiment is applicable to any type of cardboard. 【0062】 The basis weight of the corrugated cardboard base paper in this embodiment is preferably 100 g / m², from the viewpoint of improving the strength of the corrugated cardboard base paper, improving the recyclability of difficult-to-process waste paper, and reducing the amount of disintegrated fragments containing a metal layer in the difficult-to-process waste paper pulp. ２ More than 400g / m ２ More preferably, 120 g / m² ２ More than 300g / m ２ More preferably, 140 g / m２ More than 250g / m ２ More preferably, 155 g / m ２ More than 200g / m ２ The following applies: 【0063】 In the corrugated cardboard base paper of this embodiment, the amount of difficult-to-process recycled paper pulp is preferably 0.2% by mass or more and 40% by mass or less, more preferably 1% by mass or more and 30% by mass or less, even more preferably 2% by mass or more and 25% by mass or less, even more preferably 3% by mass or more and 20% by mass or less, even more preferably 5% by mass or more and 18% by mass or less, even more preferably 8% by mass or more and 17% by mass or less, and even more preferably 10% by mass or more and 15% by mass or less. When the amount of difficult-to-process recycled paper pulp is 0.2% by mass or more, the difficult-to-process recycled paper can be used effectively, which is preferable as it improves recyclability. Also, when the amount of difficult-to-process recycled paper pulp is 40% by mass or less, the corrugated cardboard base paper becomes less likely to be detected by metal detectors. 【0064】 <Method for Manufacturing Corrugated Cardboard> The method for manufacturing corrugated cardboard in this embodiment can be the conventional method for manufacturing corrugated cardboard. For example, it can be manufactured by a corrugating process in which the core and liner are bonded together with an adhesive. Examples of adhesives include starch paste and synthetic resins. Examples of synthetic resins include polyethylene, polypropylene, polyamide, polyester, ethylene-unsaturated carboxylic acid copolymer, styrene-butadiene copolymer, butadiene-acrylonitrile copolymer, styrene-butadiene-acrylonitrile copolymer, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyacrylic acid ester copolymer, and styrene-acrylic acid ester copolymer. 【0065】 Generally, a method involves forming a corrugated core, coating the core with an emulsion of synthetic resin or an adhesive such as starch paste, bonding it to a liner, and then applying pressure and heat to bond it. 【0066】[Method for Manufacturing Corrugated Cardboard Base Paper] The method for manufacturing corrugated cardboard base paper of the present invention is the method for manufacturing corrugated cardboard base paper described above, comprising: a difficult-to-process recycled paper pulp manufacturing step for manufacturing difficult-to-process recycled paper pulp using difficult-to-process recycled paper as raw material; and a papermaking step for manufacturing corrugated cardboard base paper using a paper stock containing difficult-to-process recycled paper pulp as the paper stock for a layer containing difficult-to-process recycled paper pulp, wherein the difficult-to-process recycled paper pulp manufacturing step comprises: a disintegration step for disintegrating the difficult-to-process recycled paper; a dust removal step for removing foreign matter from the pulp slurry after the disintegration step; and a pulp washing step for washing the pulp in the pulp slurry after the dust removal step, wherein the concentration of the difficult-to-process recycled paper in the disintegration step is 5% by mass or more and 25% by mass or less; and the processing time of the disintegration step is 5 minutes or more and 90 minutes or less. 【0067】 Furthermore, if the corrugated cardboard base paper of this embodiment is a corrugated cardboard base paper having at least a surface layer, a middle layer and a back layer, and the middle layer is a layer containing difficult-to-process recycled paper pulp made from difficult-to-process recycled paper, the method for manufacturing the corrugated cardboard base paper of the present invention comprises a difficult-to-process recycled paper pulp manufacturing step for manufacturing difficult-to-process recycled paper pulp using difficult-to-process recycled paper as raw material, and a papermaking step for making corrugated cardboard base paper using a paper stock containing difficult-to-process recycled paper pulp as a paper stock for the middle layer, wherein the difficult-to-process recycled paper pulp manufacturing step comprises a disintegration step for disintegrating the difficult-to-process recycled paper, a dust removal step for removing foreign matter from the pulp slurry after the disintegration step, and a pulp washing step for washing the pulp in the pulp slurry after the dust removal step, wherein the concentration of the difficult-to-process recycled paper in the disintegration step is 5% by mass or more and 25% by mass or less, and the processing time of the disintegration step is 5 minutes or more and 90 minutes or less. 【0068】 <Manufacturing Process for Difficult-to-Process Recycled Paper Pulp> The manufacturing process for difficult-to-process recycled paper pulp in this embodiment is a recycled paper pulp manufacturing process that uses difficult-to-process recycled paper as a raw material to produce difficult-to-process recycled paper pulp, that is, a manufacturing process that produces the above-mentioned difficult-to-process recycled paper pulp from the above-mentioned difficult-to-process recycled paper. 【0069】Typically, recycled paper pulp production involves a disintegration process. In this process, recycled paper is dispersed in water and agitated to allow it to absorb moisture. Forces such as shearing and impact from dropping are applied to loosen the fibers, leading to disintegration and the production of a pulp slurry. In the case of difficult-to-process recycled paper, the disintegration process also pulverizes the metal and resin layers contained in the paper. Normally, recycled paper pulp production includes a dust removal process after the disintegration process to remove foreign matter from the pulp slurry. However, if the metal and resin layers are particularly finely pulverized, they cannot be removed even in the dust removal process, significantly impairing the quality of the recycled paper pulp. On the other hand, in the screening process, which is one of the dust removal processes, it is possible to efficiently remove the finely pulverized metal and resin layers by reducing the size of the slits in the screen used. However, this increases the amount of pulp captured by the screen, thus reducing the yield of the resulting recycled paper pulp. According to the manufacturing process for difficult-to-process recycled paper pulp in this embodiment, it is possible to reduce the amount of disintegrated fragments containing a metal layer in the difficult-to-process recycled paper pulp by adjusting the conditions of the disintegration process. Specifically, when the concentration of recycled paper in the disintegration process is 5% by mass or more and the processing time of the disintegration process is 90 minutes or less, the force applied to the difficult-to-process recycled paper is adjusted to an appropriate strength, and the excessive disintegration of the recycled paper is suppressed. As a result, the finening of the metal layer and resin layer is suppressed, and the metal layer and resin layer are sufficiently removed in the dust removal process, so it is thought that the amount of disintegrated fragments containing a metal layer in the resulting difficult-to-process recycled paper pulp is reduced. Furthermore, when the concentration of recycled paper in the disintegration process is 25% by mass or less and the processing time of the disintegration process is 5 minutes or more, the disintegration process is sufficient, so it is thought that the yield of the resulting difficult-to-process recycled paper pulp is improved. Based on the above, the corrugated cardboard base paper of this embodiment, despite containing a layer of difficult-to-treat recycled paper pulp, has a basis weight of 60 g / m² obtained from the pulp obtained by disintegrating the layer containing the difficult-to-treat recycled paper pulp. ２ 0.05 mm handmade sheet with a metal layer ２ The total area of ​​the above-mentioned dissociated fragments is 10.0 mm². ２ / 0.02m ２ The following conditions apply, and the maximum area is 4.0 mm². ２The following occurs, making it less likely to be detected by metal detectors. However, the above mechanism is speculative, and the effects of the present invention are not limited by this mechanism. 【0070】 (Difficult-to-process waste paper) In the manufacturing process of difficult-to-process waste paper pulp in this embodiment, the difficult-to-process waste paper that serves as the raw material for the difficult-to-process waste paper pulp has at least a paper base layer, a metal layer and a resin layer, and is similar to the difficult-to-process waste paper used for corrugated cardboard base paper described above. 【0071】 In this embodiment, the average size of the difficult-to-process waste paper used in the disintegration process is preferably 600 cm². ２ More preferably 10 cm ２ More than 400cm ２ More preferably, 20 cm ２ More than 300cm ２ More preferably, 30 cm ２ More than 200cm ２ Further, 40 cm is preferred. ２ More than 150cm ２ More preferably, 50 cm ２ More than 100cm ２ Further, more preferably, 60 cm ２ 90cm or more ２ The following applies: The average size of the difficult-to-process waste paper used in the disintegration process is 600 cm². ２ The following conditions allow the processing solution to penetrate difficult-to-process waste paper more easily, enabling more efficient disintegration processing, and also, 10 cm ２As a result, the fineness of the metal and resin layers contained in the difficult-to-process waste paper is suppressed, and the amount of disintegrated fragments containing the metal layer in the resulting difficult-to-process waste paper pulp is reduced. The average size of the difficult-to-process waste paper is measured by the method described in the examples. Difficult-to-process waste paper is usually compressed and baled into assemblies of about 1 ton using a compression baler to improve transportation efficiency when shipping to recycling manufacturers. In addition, if the difficult-to-process waste paper is used as waste paper for liquid containers, it is preferable that it be washed beforehand. In this embodiment, the difficult-to-process waste paper may be in the form of a single sheet, or it may be in the form of at least two or more sheets of difficult-to-process waste paper stacked on top of each other (waste paper aggregate). The difficult-to-process waste paper may also be torn. Furthermore, the difficult-to-process waste paper may be crushed by the crushing process described later, before the disintegration process described later. 【0072】 (Crushing process and waste paper washing process) In this embodiment, from the viewpoint of efficiently performing the disintegration process described later, it is preferable that the manufacturing method includes at least one of the following before the disintegration process described later: a crushing process for crushing the difficult-to-process waste paper and a waste paper washing process for washing the difficult-to-process waste paper. It is more preferable to include both the crushing process and the waste paper washing process. 【0073】 [Crushing Process] The manufacturing method of this embodiment includes a crushing process, which ensures that the difficult-to-process waste paper used in the disintegration process is of an appropriate size, thereby improving the efficiency of the disintegration process. As a result, the amount of disintegrated metal fragments in the raw pulp can be reduced, making the resulting corrugated cardboard base paper less likely to be detected by metal detectors. Furthermore, the yield of the resulting difficult-to-process waste paper pulp is further improved. The manufacturing method of this embodiment includes a crushing process before the disintegration process described later, and the average size of the difficult-to-process waste paper after crushing is preferably 10 cm. ２ More than 400cm ２ Below, 20cm ２ More than 300cm ２ Further preferably 30 cm ２ More than 200cm ２ More preferably, 40 cm ２ More than 150cm ２ Further, a more preferable length is 50 cm. ２ More than 100cm ２More preferably, 60 cm ２ 90cm or more ２ The following is true: The average size of difficult-to-process waste paper after shredding is 10 cm. ２ As a result, the fineness of the metal and resin layers contained in the difficult-to-process waste paper is suppressed, and the amount of disintegrated fragments containing the metal layer in the resulting difficult-to-process waste paper pulp is reduced, and also, 400 cm ２ The following conditions allow the treatment solution to penetrate the difficult-to-treat waste paper more easily, enabling more efficient disintegration. The average size of the difficult-to-treat waste paper after crushing is measured by the method described in the examples. 【0074】 [Washing Process for Waste Paper] The manufacturing method of this embodiment includes a washing process for waste paper, which allows for the removal of foreign matter adhering to difficult-to-process waste paper, thereby improving the quality of the resulting corrugated cardboard base paper. In the washing process for waste paper, water, acidic washing water, alkaline washing water, washing water containing an oxidizing agent, etc., are typically used as the washing water for washing the waste paper fragments, and water is preferred from the viewpoint of wastewater treatment. The manufacturing method of this embodiment includes a washing process for waste paper before the disintegration process described later, and the amount of washing water per unit mass of difficult-to-process waste paper is preferably 0.5 L / kg or more and 30 L / kg or less, more preferably 5.0 L / kg or more and 25 L / kg or less, and even more preferably 10 L / kg or more and 20 L / kg or less. A washing water amount of 0.5 L / kg or more allows for efficient removal of foreign matter adhering to difficult-to-process waste paper, and a washing water amount of 30 L / kg or less reduces the amount of wastewater used in the waste paper washing process, thereby reducing processing costs and environmental burden. 【0075】 The waste paper washing process may be performed before the crushing process, after the crushing process and before the disintegration process, or simultaneously with the crushing process. Of these, from the viewpoint of improving washing efficiency, it is preferable to perform the waste paper washing process after the crushing process and before the disintegration process, or to perform the crushing process and the waste paper washing process simultaneously, and it is more preferable to perform the crushing process and the waste paper washing process simultaneously. As for the equipment used in the waste paper washing process, any known equipment can be used as appropriate. Furthermore, when the crushing process and the washing process are performed simultaneously, it is preferable to use a crushing and washing machine or the like as appropriate. 【0076】(Disintegration Process) The disintegration process in the manufacturing process of difficult-to-process recycled paper pulp is the process of disintegrating difficult-to-process recycled paper. Disintegration is the process of turning difficult-to-process recycled paper into a slurry. 【0077】 [Recycled Paper Concentration] In the manufacturing process of difficult-to-process recycled paper pulp, the concentration of difficult-to-process recycled paper in the disintegration process (concentration of dry solids of difficult-to-process recycled paper in the processing liquid) is 5% by mass or more and 25% by mass or less. When the concentration of difficult-to-process recycled paper is 5% by mass or more, it prevents excessive disintegration and suppresses the finening of the metal layer and resin layer. Therefore, in the dust removal process described later, the finely granulated metal and resin are sufficiently removed, and the number of disintegrated pieces containing the metal layer in the difficult-to-process recycled paper pulp is reduced (particularly contributing to a reduction in total area and number). As a result, the resulting corrugated cardboard base paper is less likely to be detected by metal detectors. Furthermore, when the concentration of difficult-to-process recycled paper is 25% by mass or less, sufficient disintegration is performed (particularly contributing to the maximum area of ​​the disintegrated pieces), and the yield of the resulting difficult-to-process recycled paper pulp is improved. The concentration of difficult-to-process waste paper in the disintegration process is preferably 7% by mass or more, more preferably 9% by mass or more, and even more preferably 12% by mass or more, from the viewpoint of reducing the amount of disintegrated fragments containing the metal layer in the resulting difficult-to-process waste paper pulp, and preferably 22% by mass or less, and more preferably 20% by mass or less, from the viewpoint of improving the yield of the resulting difficult-to-process waste paper pulp. 【0078】[Processing Time] In the manufacturing process for difficult-to-process recycled paper pulp, the processing time in the disintegration process is 5 minutes or more and 90 minutes or less. If the processing time is 5 minutes or more, sufficient disintegration is performed, and the yield of the obtained difficult-to-process recycled paper pulp is improved. If the processing time is 90 minutes or less, excessive disintegration is prevented, and the fineness of the metal layer and resin layer is suppressed. Therefore, in the dust removal process described later, the finely granulated metal and resin are sufficiently removed, and the amount of disintegrated fragments containing the metal layer in the difficult-to-process recycled paper pulp is reduced. As a result, the resulting corrugated cardboard base paper is less likely to be detected by metal detectors. From the viewpoint of improving the yield of the obtained difficult-to-process recycled paper pulp, the processing time in the disintegration process is preferably 10 minutes or more, more preferably 20 minutes or more, and even more preferably 30 minutes or more. From the viewpoint of reducing the amount of disintegrated fragments containing the metal layer in the obtained difficult-to-process recycled paper pulp, it is preferably 75 minutes or less, more preferably 60 minutes or less, and even more preferably 45 minutes or less. 【0079】 [Processing Temperature] In the process of manufacturing difficult-to-process recycled paper pulp, the processing temperature in the disintegration process is preferably 5°C or higher, more preferably 10°C or higher, even more preferably 20°C or higher, and even more preferably 30°C or higher, from the viewpoint of improving the disintegrability of the difficult-to-process recycled paper and improving the yield of the obtained difficult-to-process recycled paper pulp, and is preferably 80°C or lower, more preferably 70°C or lower, even more preferably 60°C or lower, and even more preferably 50°C or lower, from the viewpoint of reducing disintegration fragments containing the metal layer in the obtained difficult-to-process recycled paper pulp. 【0080】 [pH] In the manufacturing process for difficult-to-process recycled paper pulp, the pH of the processing solution in the disintegration process is preferably between 5 and 11. When the pH of the processing solution is within the above range, the processing solution penetrates the difficult-to-process recycled paper more easily. As a result, the disintegration process is carried out efficiently, and the yield of the resulting difficult-to-process recycled paper pulp is improved. In addition, since the processing time for the disintegration process can be kept within the above range, the fineness of the metal layer and resin layer contained in the difficult-to-process recycled paper is suppressed, and the amount of disintegrated fragments containing the metal layer in the resulting difficult-to-process recycled paper pulp is reduced. 【0081】In the process of manufacturing difficult-to-process recycled paper pulp, chemicals such as basic compounds and surfactants may be added as needed during the disintegration process. 【0082】 In the disintegration process, adding a basic compound is preferable because it adjusts the pH of the processing solution to the above range, allows for efficient disintegration, improves the yield of the resulting difficult-to-process waste paper pulp, and reduces the amount of disintegrated fragments containing the metal layer in the resulting difficult-to-process waste paper pulp. On the other hand, from the viewpoint of wastewater treatment costs, it is preferable not to use a basic compound. Therefore, in the disintegration process, the amount of basic compound added is preferably 1% by mass or less, more preferably 0.5% by mass or less, even more preferably 0.1% by mass or less, and particularly preferably 0% by mass, relative to the difficult-to-process waste paper. As the basic compound, alkali metal hydroxides are preferred, and sodium hydroxide is more preferred. 【0083】 Furthermore, in the disintegration process, adding a surfactant facilitates the penetration of the treatment liquid between the paper substrate of the difficult-to-treat waste paper and the metal and resin layers. This makes it easier to separate the paper substrate from the metal and resin layers, and reduces the amount of disintegrated fragments containing the metal layer in the resulting difficult-to-treat waste paper pulp, which is preferable. Additionally, adding a surfactant is preferable because, for example, if the difficult-to-treat waste paper has a printed layer, the pigments or dyes contained in the printed layer are efficiently removed. On the other hand, from the viewpoint of wastewater treatment costs, it is preferable not to use a surfactant. Therefore, in the disintegration process, the amount of surfactant added is preferably 1% by mass or less, more preferably 0.5% by mass or less, even more preferably 0.1% by mass or less, and particularly preferably 0% by mass, relative to the difficult-to-treat waste paper. 【0084】As the surfactant, a higher alcohol-based surfactant is preferred, a higher alcohol-based nonionic surfactant is more preferred, and a higher alcohol-based amphoteric surfactant is even more preferred. Furthermore, the carbon number of the higher alcohol-based surfactant is preferably 6 to 20, for example. Furthermore, examples of surfactants include higher alcohol-based surfactants used as deinking agents, such as DI-7020, DI-7030, DI-767, DI-7282 manufactured by Kao Corporation, DIA-Z-100, DIA-Z-5000 manufactured by Nisshin Chemical Research Institute Co., Ltd., NeoScore FW-780, NeoScore FW-790, NeoScore FW-795, FT-467, FT-470, FT-487, FT-511, FT-513, FT-514, FT-515, B-B agent manufactured by Toho Chemical Co., Ltd., Daihope 940, Daihope 960 manufactured by Daiichi Kogyo Seiyaku Co., Ltd., and Lipobright DP-810 manufactured by Nikka Co., Ltd., but are not limited to these. 【0085】 In the manufacturing process of difficult-to-process recycled paper pulp, the disintegration process is preferably carried out using a disintegrator called a pulper. From the viewpoint of keeping the recycled paper concentration within the above range, it is preferable to use a medium-concentration pulper or a high-concentration pulper, and more preferably a high-concentration pulper. Here, "concentration" in medium-concentration pulper and high-concentration pulper refers to the range of recycled paper concentrations that can be disintegrated by each pulper. A medium-concentration pulper is generally a pulper that can perform disintegration with a recycled paper concentration in the range of approximately 7 to 9% by mass. A high-concentration pulper is generally a pulper that can perform disintegration with a recycled paper concentration in the range of approximately 10 to 20% by mass. An example of a medium-concentration pulper is one in which an agitator blade called a rotor is attached to the bottom or inner wall of the tank. An example of a high-concentration pulper is one that is similar to a medium-concentration pulper but with a larger rotor shape, a drum pulper in which the disintegration tank is a horizontal drum shape, and a kneading pulper in which agitator blades are installed in multiple stages inside the tank. 【0086】Furthermore, the solid content concentration of each pulp slurry obtained in each step after the disintegration step may be adjusted as appropriate for the next step. For example, when transporting to the next step, water or the like may be added to the pulp slurry obtained in each step to lower the solid content concentration in order to improve transport efficiency. Alternatively, the solid content concentration of the pulp slurry may be lowered by adding water or the like to the pulp slurry to make it suitable for the solid content concentration when carrying out each treatment in each step, or the solid content concentration may be increased by dehydrating the pulp slurry. 【0087】 In this embodiment, a maturation step may be included before the dust removal step described later, in which the pulp slurry after the disintegration step is matured. The maturation step may be carried out by supplying the pulp slurry after the disintegration step to a maturation tower and storing it in the maturation tower. In the maturation step, the pulp slurry after the disintegration step may be left to stand, or it may be gently stirred to maintain the fluidity of the pulp slurry. By including a maturation step, the separation of pulp and foreign matter in the pulp slurry after the disintegration step is further promoted, thereby improving the yield of the resulting recycled paper pulp. From the viewpoint of improving the yield of the resulting recycled paper pulp, the maturation time in the maturation step is preferably 6 hours or more, more preferably 8 hours or more, and even more preferably 10 hours or more. Furthermore, there is no particular upper limit to the maturation time in the maturation step, but from the viewpoint of the productivity of recycled paper pulp, it is preferably 24 hours or less, more preferably 20 hours or less, and even more preferably 16 hours or less. Furthermore, the solid content concentration in the pulp slurry during the maturation process is preferably 8% by mass or more and 25% by mass or less, more preferably 10% by mass or more and 20% by mass or less, and even more preferably 12% by mass or more and 18% by mass or less. Furthermore, the temperature during maturation in the maturation process is preferably 25°C or higher, more preferably 30°C or higher, even more preferably 35°C or higher, and preferably 70°C or lower, more preferably 60°C or lower, and even more preferably 50°C or lower. 【0088】(Dust Removal Process) In this embodiment, the dust removal process is a process of removing foreign matter from the pulp slurry after the disintegration process. In addition, in this embodiment, if there is a maturation process after the disintegration process, the dust removal process is a process of removing foreign matter from the pulp slurry after the maturation process. Here, foreign matter includes disintegrated fragments derived from the metal layer and resin layer of the difficult-to-process waste paper. 【0089】 In this embodiment, the dust removal step preferably includes a step of removing foreign matter from the pulp slurry after the disintegration step by centrifugal separation and a step of removing foreign matter from the pulp slurry after the disintegration step by screen treatment. The same applies in this embodiment if there is a maturation step after the disintegration treatment. 【0090】 The process of removing foreign matter by centrifugal separation primarily involves removing large foreign matter generated after the disintegration process using a cleaner. The cleaner is conical in shape and, by the principle of centrifugal separation, can remove foreign matter with a specific gravity greater than pulp fibers, such as sand and metal particles. As cleaners used in the process of removing foreign matter by centrifugal separation, heavy foreign matter cleaners and low-concentration ramole cleaners are recommended from the viewpoint of efficiently removing foreign matter with a specific gravity greater than pulp fibers. In the process of removing foreign matter by centrifugal separation, the concentration of the pulp slurry is preferably 0.5% by mass or more and 5.0% by mass or less, more preferably 0.8% by mass or more and 4.0% by mass or less, and even more preferably 1.0% by mass or more and 3.0% by mass or less, from the viewpoint of efficiently removing foreign matter and reducing the amount of disintegrated fragments containing a metal layer in the resulting recycled paper pulp. 【0091】The process of removing foreign matter by screen treatment is performed after the process of removing foreign matter by centrifugal separation, for the purpose of further removing foreign matter. As the screen used for screen treatment, for example, a basket-type screen with holes or slits that open to a predetermined opening area can be used, a slit screen can be used, a coarse screen can be used, or a fine screen can be used. From the viewpoint of efficiently removing foreign matter and reducing the amount of disintegrated fragments containing the metal layer in the resulting difficult-to-treat recycled paper pulp, it is preferable to perform a fine screen treatment after the coarse screen treatment. As the coarse screen, a round-hole screen or a slit screen is preferred, and a round-hole screen is more preferred. The diameter of the round holes in the round-hole screen is preferably 0.5 mm or more and 2.5 mm or less, more preferably 0.8 mm or more and 2.0 mm or less. The slit width of the slit screen is preferably 0.1 mm or more and 0.5 mm or less. The slit width of the selection screen is preferably 0.10 mm to 0.25 mm, more preferably 0.10 mm to 0.20 mm, and even more preferably 0.12 mm to 0.18 mm. The wider the slit width of the selection screen, the higher the yield of the resulting recycled paper pulp, and the narrower the slit width, the less fragment containing metal layers is produced in the resulting difficult-to-process recycled paper pulp. 【0092】 In the coarse screening process, the solid content concentration of the pulp slurry is preferably 1.0% by mass or more and 5.0% by mass or less, more preferably 1.5% by mass or more and 4.0% by mass or less, and even more preferably 2.0% by mass or more and 3.0% by mass or less, from the viewpoint of efficiently removing foreign matter and reducing the amount of disintegrated fragments having a metal layer in the resulting recycled paper pulp. In the fine screening process, the solid content concentration of the pulp slurry is preferably 0.2% by mass or more and 5.0% by mass or less, more preferably 0.5% by mass or more and 3.5% by mass or less, and even more preferably 0.8% by mass or more and 3.0% by mass or less, from the viewpoint of efficiently removing foreign matter and reducing the amount of disintegrated fragments having a metal layer in the resulting recycled paper pulp. 【0093】Furthermore, the dust removal process may be performed multiple times. It is preferable to perform the process one to three times, and more preferably once, depending on the balance between improving the yield of the resulting recycled paper pulp and reducing the amount of disintegrated fragments containing a metal layer in the resulting recycled paper. 【0094】 (Deinking Process) In the process of manufacturing difficult-to-process recycled paper pulp, for example, if the difficult-to-process recycled paper has a printed layer, a deinking process may be included after the dust removal process and before the pulp washing process described later, if necessary, to deink the pulp slurry. The deinking process removes the ink contained in the printed layer from the pulp slurry, as well as coarse inorganic foreign matter. The deinking process may be carried out using a flotator or the like. 【0095】 When deinking is performed using a flotator, the solid content concentration of the pulp slurry is preferably 0.5% by mass or more and 2.0% by mass or less, more preferably 0.5% by mass or more and 1.3% by mass or less. In addition, a deinking agent may be added immediately before processing with the flotator during the deinking process. As a deinking agent used immediately before processing with the flotator, one with strong ink coagulation properties is preferred. In the case of fatty acids, examples include DI-254 (oleic acid) and DI-268 manufactured by Kao Corporation, and K-4004-D manufactured by Daiichi Kogyo Seiyaku Co., Ltd. In the case of fatty acid derivatives, examples include DIY-23543 manufactured by Kao Corporation, and Paper Aid W and Daihope 1000 manufactured by Daiichi Kogyo Seiyaku Co., Ltd. In the case of higher alcohols, an example is DI-7020 manufactured by Kao Corporation. In the deinking process, when a deinking agent is added, the amount of deinking agent added is preferably 0.01 parts by mass or more and 0.5 parts by mass or less, and more preferably 0.03 parts by mass or more and 0.3 parts by mass or less, per 100 parts by mass of solid content of the pulp slurry. 【0096】 Furthermore, the deinking process may include a step to remove foreign matter by centrifugal separation after the deinking process, in order to further remove any foreign matter. The step to remove foreign matter by centrifugal separation may be the same as the step to remove foreign matter by centrifugal separation exemplified in the dust removal process described above. 【0097】(Pulp Washing Process) In the process of manufacturing difficult-to-process recycled paper pulp, the pulp washing process is a process of washing the pulp in the pulp slurry after the dust removal process. In this embodiment, if there is a deinking process after the dust removal process, the pulp washing process is a process of washing the pulp in the pulp slurry after the deinking process. The pulp washing process removes more detached fragments originating from the metal layer and resin layer of the pulp in the pulp slurry, as well as foreign matter originating from the printing layer. As a result, the amount of detached fragments containing the metal layer in the resulting difficult-to-process recycled paper pulp is reduced, and the resulting corrugated cardboard base paper is less likely to be detected by metal detectors. The pulp washing process may, if necessary, alternately repeat washing the pulp in the pulp slurry and dewatering the pulp slurry. 【0098】 Examples of equipment used in the pulp washing process include DNT washers, compact washers, fall washers, Variosplit, SP filters, DP Cosmo, gap washers, and disc filters, with disc filters being preferred. In the pulp washing process, the solid content concentration of the pulp slurry is preferably 0.5% by mass or more and 5% by mass or less, from the viewpoint of efficiently removing foreign matter from the pulp slurry and reducing the amount of disintegrated fragments containing metal layers in the resulting difficult-to-process recycled paper pulp. 【0099】 In this embodiment, it is preferable to have a dewatering step after the pulp washing step to dewater the pulp slurry and obtain pulp. By having a dewatering step, the handling properties of the obtained difficult-to-process recycled paper pulp are improved, and when producing paper products using difficult-to-process recycled paper pulp, it becomes easier to blend it with the raw pulp. Examples of equipment used to dewater the pulp slurry in the dewatering step include a double nip thickener, a drum thickener, a disc thickener, a valveless thickener, etc., and a disc thickener is preferred. 【0100】<Papermaking Process> In the manufacturing method of this embodiment, the papermaking process is a process of making corrugated cardboard base paper of this embodiment using the difficult-to-process recycled paper pulp obtained by the difficult-to-process recycled paper pulp manufacturing process described above as a paper stock for the layer containing the difficult-to-process recycled paper pulp. Furthermore, if the corrugated cardboard base paper of this embodiment is a corrugated cardboard base paper having at least a surface layer, a middle layer and a back layer, and the middle layer is a layer containing difficult-to-process recycled paper pulp made from difficult-to-process recycled paper, then in the manufacturing method of this embodiment, the papermaking process is a process of making corrugated cardboard base paper of this embodiment using the difficult-to-process recycled paper pulp obtained by the difficult-to-process recycled paper pulp manufacturing process described above as a paper stock for the middle layer. The corrugated cardboard base paper of this embodiment is a single-layer structure made by making one layer of paper using pulp as the main raw material, or a multi-layer structure made by making two or more layers of paper using pulp as the main raw material, but it can be manufactured using a known multi-layer papermaking machine, for example, a papermaking machine that combines a wire mesh former and a cylinder mesh former. It is also possible to strengthen the strength between each layer by spraying or coating them with an adhesive such as starch during the molding process. 【0101】 The features of the present invention will be further described in detail below with reference to examples and comparative examples. The materials, amounts used, proportions, processing content, and processing procedures shown in the following examples can be modified as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be interpreted as being limited by the specific examples shown below. In addition, the operations in the examples and comparative examples were carried out under conditions of 23±1℃ and 50±2% relative humidity unless otherwise specified. 【0102】 <Example 1> (Manufacturing process for difficult-to-process recycled paper pulp) [Crushing process, recycled paper washing process] Aseptic container recycled paper listed in Table 1 was used as difficult-to-process recycled paper, and it was crushed and washed in a crushing and washing machine (A-Tech Co., Ltd., paper container recycling device PPRS, number of hooks: 6, washing water: water, flow rate 15 L / kg) for 10 cm ２ More than 400cm ２ The following size (average size after crushing: 78 cm) ２) was crushed and washed. [Disintegration process] The crushed and washed difficult-to-process waste paper and water were loaded into a kneading pulper (manufactured by Nippon Sharyo Ltd., KNP-10 type (high-concentration pulper)), and disintegration treatment was carried out under the conditions of difficult-to-process waste paper concentration of 19% by mass, processing time of 30 minutes, processing temperature of 40°C, and pH 6 to obtain a difficult-to-process waste paper pulp slurry. (Dust removal process) After the disintegration process, the pulp slurry was adjusted to a solid content of 2.5% by mass, then processed with a weight foreign matter cleaner (manufactured by Aikawa Iron Works Co., Ltd., FC100 type), and further processed with a coarse sorting screen (manufactured by Aikawa Iron Works Co., Ltd., MaxFlow-1, Model 1000 type, round hole 1.2 mm). The pulp slurry after rough screening was adjusted to a solid content of 1.0% by mass and processed with a fine screening screen (Aikawa Iron Works Co., Ltd., MaxFlow-1, Model 1000, slit 0.15 mm). The pulp slurry after fine screening was processed with a low-concentration rameau cleaner (Aikawa Iron Works Co., Ltd., Bycorn 150). (Pulp washing process, dewatering process) The pulp slurry after the dust removal process was washed using a disc filter (IHI Foyt Paper Technology Co., Ltd., DF520), and then dewatered using a disc thickener (IHI Foyt Paper Technology Co., Ltd., EFK1310) to obtain difficult-to-process recycled paper pulp. 【0103】(Papermaking process) [Preparation of surface pulp] Corrugated cardboard pulp, which was disintegrated at a recycled paper concentration of 4% by mass, pH 6, processing time of 15 minutes, and processing temperature of 50°C, was used as the raw material pulp. To 100 parts by mass of this raw material pulp, 0.35 parts of a sizing agent (product name: Sizing Pine N-817, manufactured by Arakawa Chemical Industries, Ltd.) and 2.5 parts of aluminum sulfate were added to prepare the surface pulp. [Preparation of middle layer pulp] As the raw material pulp, a pulp was used which was a mixture of corrugated cardboard pulp, which was disintegrated at a recycled paper concentration of 4% by mass, pH 6, processing time of 15 minutes, and processing temperature of 50°C, magazine pulp, which was disintegrated at a recycled paper concentration of 4% by mass, pH 6, processing time of 15 minutes, and processing temperature of 50°C, and the aforementioned difficult-to-process recycled paper pulp, in a mass ratio of 60:25:15 (corrugated cardboard pulp: magazine pulp: difficult-to-process recycled paper pulp). To 100 parts by mass of this raw pulp, 1.5 parts of aluminum sulfate were added to prepare the middle layer paper material. [Preparation of backing layer paper material] As the raw pulp, corrugated cardboard pulp that had been disintegrated at a recycled paper concentration of 4% by mass, pH 6, processing time of 15 minutes, and processing temperature of 50°C was used. To 100 parts by mass of this pulp, 1.0 part of aluminum sulfate was added to prepare the backing layer paper material. [Papermaking of base paper for corrugated cardboard] Using the paper materials of each layer prepared above, the basis weight of the surface layer was set to 31 g / m². ２ The basis weight of the middle layer is 115 g / m². ２ The basis weight of the backing layer is 24 g / m². ２ The total basis weight is 170 g / m². ２ The paper was prepared using a short-wire paper machine, and three layers were combined to produce corrugated cardboard base paper. In the papermaking process, a drying process was carried out in a multi-cylinder dryer at a speed of 750 m / min and a temperature of 125°C, resulting in a corrugated cardboard base paper with a width of 4 m and a thickness of 197 μm, wound to a total length of 5000 m. The proportion of difficult-to-treat recycled paper pulp in the middle layer of the paper stock at this time was {15 / (60 + 25 + 15)} × 100 = 15 [mass%], and the amount of difficult-to-treat recycled paper pulp in the obtained corrugated cardboard base paper was 115 [g / m²]. ２ (Set value) × 15 [mass %]) / 170 [g / m³] ２ The measured value was 10.1 [mass%]. 【0104】<Example 2> In the disintegration process for the production of difficult-to-process recycled paper pulp, crushed and washed difficult-to-process recycled paper and water were charged into a high-concentration pulper (HDP-5 manufactured by Aikawa Iron Works Co., Ltd.), and disintegration treatment was carried out under the conditions of a difficult-to-process recycled paper concentration of 19% by mass, a processing time of 30 minutes, a processing temperature of 40°C, and a pH of 6. Otherwise, corrugated cardboard base paper was obtained in the same manner as in Example 1. 【0105】 <Example 3> In the disintegration process for the production of difficult-to-process recycled paper pulp, crushed and washed difficult-to-process recycled paper and water were charged into a medium-concentration pulper (IHI Foit Paper Technology Co., Ltd., IntensaPulper), and disintegration treatment was carried out under the conditions of a difficult-to-process recycled paper concentration of 8% by mass, a processing time of 30 minutes, a processing temperature of 40°C, and a pH of 6. Otherwise, corrugated cardboard base paper was obtained in the same manner as in Example 1. 【0106】 <Example 4> In the disintegration process for manufacturing difficult-to-process recycled paper pulp, base paper for corrugated cardboard was obtained in the same manner as in Example 1, except that the processing time for the disintegration treatment was set to 10 minutes. 【0107】 <Example 5> In the disintegration process for manufacturing difficult-to-process recycled paper pulp, base paper for corrugated cardboard was obtained in the same manner as in Example 1, except that the processing time for the disintegration treatment was set to 60 minutes. 【0108】 <Example 6> In the disintegration process for the production of difficult-to-process recycled paper pulp, base paper for corrugated cardboard was obtained in the same manner as in Example 1, except that the pH of the disintegration treatment was set to 10. 【0109】 <Example 7> In the disintegration process for manufacturing difficult-to-process recycled paper pulp, base paper for corrugated cardboard was obtained in the same manner as in Example 6, except that the processing time for the disintegration treatment was set to 10 minutes. 【0110】 <Example 8> Corrugated cardboard base paper was obtained in the same manner as in Example 1, except that the processing temperature for the disintegration treatment in the disintegration process of the production of difficult-to-process recycled paper pulp was set to 15°C. 【0111】 <Example 9> Base paper for corrugated cardboard was obtained in the same manner as in Example 1, except that the processing temperature for the disintegration treatment in the disintegration process of the production of difficult-to-process recycled paper pulp was set to 65°C. 【0112】<Example 10> In the production of difficult-to-process recycled paper pulp, corrugated cardboard base paper was obtained in the same manner as in Example 1, except that the crushing process and the recycled paper washing process were not performed. In Example 10, the average size of the difficult-to-process recycled paper used for disintegration was 201 cm². ２ That was the case. 【0113】 <Example 11> In the crushing and waste paper washing processes for the production of difficult-to-process waste paper pulp, the waste paper washing process was omitted, and only crushing was performed, except that the same procedure as in Example 1 was used to obtain base paper for corrugated cardboard. 【0114】 <Example 12> In the crushing and washing processes for the production of difficult-to-process recycled paper pulp, the only difference was that only the washing of the recycled paper was performed and no crushing was carried out, otherwise the same procedure as in Example 1 was used to obtain base paper for corrugated cardboard. In Example 12, the average size of the difficult-to-process recycled paper used in the disintegration process was 203 cm². ２ That was the case. 【0115】 <Example 13> In the crushing and waste paper washing processes for the production of difficult-to-process waste paper pulp, the number of hooks on the crushing and washing machine was changed to 12, except that the method was the same as in Example 1 to obtain base paper for corrugated cardboard. In Example 13, the average size of the difficult-to-process waste paper used for disintegration was 31 cm. ２ That was the case. 【0116】 <Example 14> In the crushing and waste paper washing processes for the production of difficult-to-process waste paper pulp, the number of hooks on the crushing and washing machine was changed to 3, except that the method was the same as in Example 1 to obtain base paper for corrugated cardboard. In Example 14, the average size of the difficult-to-process waste paper used for disintegration was 113 cm². ２ That was the case. 【0117】 <Example 15> In the crushing and waste paper washing processes for the production of difficult-to-process waste paper pulp, the flow rate of the washing water was changed to 27 L / kg, but otherwise, base paper for corrugated cardboard was obtained in the same manner as in Example 1. 【0118】<Example 16> In the preparation of the middle layer pulp for the manufacture of corrugated cardboard base paper, corrugated cardboard base paper was obtained in the same manner as in Example 1, except that the raw pulp used was a mixture of corrugated cardboard recycled paper pulp, which was disintegrated at a recycled paper concentration of 4% by mass, pH 6, processing time of 15 minutes, and processing temperature of 50°C; magazine recycled paper pulp, which was disintegrated at a recycled paper concentration of 4% by mass, pH 6, processing time of 15 minutes, and processing temperature of 50°C; and the aforementioned difficult-to-treat recycled paper pulp, blended in a mass ratio of 60:35:5 (corrugated cardboard recycled paper pulp: magazine recycled paper pulp: difficult-to-treat recycled paper pulp). The blending ratio of difficult-to-treat recycled paper pulp in the middle layer pulp at this time was {5 / (60+35+5)}×100 = 5 [mass%], and the amount of difficult-to-treat recycled paper pulp in the obtained corrugated cardboard base paper was 115 [g / m²]. ２ (Set value) × 5 [mass %]) / 170 [g / m³] ２ The measured value was 3.4 [mass%]. 【0119】 <Example 17> In the preparation of the middle layer pulp for the manufacture of corrugated cardboard base paper, corrugated cardboard base paper was obtained in the same manner as in Example 1, except that the raw pulp used was a mixture of corrugated cardboard recycled paper pulp, which was disintegrated at a recycled paper concentration of 4% by mass, pH 6, processing time of 15 minutes, and processing temperature of 50°C, magazine recycled paper pulp, which was disintegrated at a recycled paper concentration of 4% by mass, pH 6, processing time of 15 minutes, and processing temperature of 50°C, and the aforementioned difficult-to-treat recycled paper pulp, in a mass ratio of 60:15:25 (corrugated cardboard recycled paper pulp: magazine recycled paper pulp: difficult-to-treat recycled paper pulp). The blending ratio of difficult-to-treat recycled paper pulp in the middle layer pulp at this time was {25 / (60+15+25)}×100 = 25 [by mass], and the amount of difficult-to-treat recycled paper pulp in the obtained corrugated cardboard base paper was 115 [g / m²]. ２ (Set value) × 25 [mass %]) / 170 [g / m³] ２ The measured value was 16.9 [mass%]. 【0120】<Example 18> In the preparation of the middle layer pulp for the manufacture of corrugated cardboard base paper, corrugated cardboard base paper was obtained in the same manner as in Example 1, except that the raw material pulp used was a mixture of corrugated cardboard waste pulp, which had been disintegrated at a recycled paper concentration of 4% by mass, pH 6, processing time of 15 minutes, and processing temperature of 50°C, and the aforementioned difficult-to-process recycled paper pulp, in a mass ratio of 60:40 (corrugated cardboard waste pulp: difficult-to-process recycled paper pulp). The blending ratio of difficult-to-process recycled paper pulp in the middle layer pulp at this time was {40 / (60+40)} × 100 = 40 [by mass], and the amount of difficult-to-process recycled paper pulp in the obtained corrugated cardboard base paper was (115 [g / m²]). ２ (Set value) × 40 [mass %]) / 170 [g / m³] ２ The measured value was 27.1 [mass%]. 【0121】 <Example 19> Cardboard base paper was obtained in the same manner as in Example 1, except that the papermaking process was changed as follows. (Papermaking process) [Preparation of paper stock] As raw pulp, a pulp was used which consisted of recycled cardboard pulp disintegrated at a recycled paper concentration of 4% by mass, pH 6, processing time of 15 minutes, and processing temperature of 50°C, recycled magazine pulp disintegrated at a recycled paper concentration of 4% by mass, pH 6, processing time of 15 minutes, and processing temperature of 50°C, and the aforementioned difficult-to-process recycled paper pulp, blended in a mass ratio of 60:25:15 (recycled cardboard pulp: recycled magazine pulp: difficult-to-process recycled paper pulp). To 100 parts by mass of this raw pulp, 1.5 parts of aluminum sulfate was added to prepare the paper stock. [Papermaking of cardboard base paper] Using the paper stock prepared above, a total basis weight of 120 g / m² was obtained. ２ The paper was prepared using a short-wire paper machine to obtain corrugated cardboard base paper. In the papermaking process, a drying process was carried out in a multi-cylinder dryer at a speed of 750 m / min and a temperature of 125°C, resulting in a total length of 5000 m of corrugated cardboard base paper with a width of 4 m and a thickness of 170 μm. The proportion of difficult-to-treat recycled paper pulp in the pulp at this time was {15 / (60+25+15)} × 100 = 15 [mass%], and the amount of difficult-to-treat recycled paper pulp in the obtained corrugated cardboard base paper was 120 [g / m²]. ２ (Set value) × 15 [mass %]) / 119 [g / m³] ２ The measured value was 15.1 [mass%]. 【0122】<Example 20> Corrugated cardboard base paper was obtained in the same manner as in Example 1, except that the papermaking process for corrugated cardboard base paper in the papermaking process was changed as follows. [Papermaking of corrugated cardboard base paper] Using the above-prepared paper materials for each layer, the basis weight of the surface layer was 30 g / m². ２ The basis weight of the middle layer is 42 g / m² ２ The basis weight of the backing layer is 48 g / m². ２ The total basis weight is 120 g / m². ２ The paper was prepared using a short-wire paper machine, and three layers were combined to produce corrugated cardboard base paper. In the papermaking process, a drying process was carried out in a multi-cylinder dryer at a speed of 750 m / min and a temperature of 125°C, resulting in a corrugated cardboard base paper with a width of 4 m and a thickness of 169 μm, with a total length of 5000 m being wound up. The proportion of difficult-to-treat recycled paper pulp in the middle layer of the paper stock at this time was {15 / (60+25+15)} × 100 = 15 [mass%], and the amount of difficult-to-treat recycled paper pulp in the obtained corrugated cardboard base paper was (42 [g / m²]). ２ (Set value) × 15 [mass %]) / 120 [g / m ２ The measured value was 5.3 [mass%]. 【0123】 <Comparative Example 1> In the disintegration process for the production of difficult-to-process recycled paper pulp, crushed and washed difficult-to-process recycled paper and water were charged into a low-concentration pulper (AHX Helix Pulper, manufactured by Aikawa Iron Works Co., Ltd.), and disintegration treatment was carried out under the conditions of a difficult-to-process recycled paper concentration of 3% by mass, a processing time of 30 minutes, a processing temperature of 40°C, and a pH of 6. Otherwise, corrugated cardboard base paper was obtained in the same manner as in Example 1. 【0124】 <Comparative Example 2> Corrugated cardboard base paper was obtained in the same manner as in Example 1, except that the processing time for the disintegration treatment in the disintegration process of the difficult-to-process recycled paper pulp was set to 3 minutes. 【0125】 <Comparative Example 3> In the crushing and washing process for manufacturing difficult-to-process recycled paper pulp, the number of hooks in the crushing and washing machine was changed to 24, and the size of the difficult-to-process recycled paper after crushing was 10 cm. ２ Cardboard base paper was obtained in the same manner as in Example 1, except that it was crushed to a size less than 3 cm. In Comparative Example 3, the average size of the difficult-to-treat waste paper used for the disintegration process was 3 cm. ２ That was the case. 【0126】<Comparative Example 4> Corrugated cardboard base paper was obtained in the same manner as in Example 1, except that in the preparation of the middle layer paper material for the manufacture of corrugated cardboard base paper, pulp containing only the aforementioned difficult-to-treat recycled paper pulp was used as the raw material pulp. 【0127】 【0128】 <Measurement and Evaluation> (Average size of difficult-to-process waste paper used in the disintegration process) (Average size of difficult-to-process waste paper) Approximately 100g of waste paper was randomly selected from the difficult-to-process waste paper fragments obtained after the crushing process of the examples and comparative examples. The fine waste paper was removed by sieving (φ: 20 mm), and the remaining difficult-to-process waste paper fragments were scanned using EPSON Scan to acquire images. The scanned images were analyzed using ImageJ to obtain the area of ​​each waste paper fragment, and the average area was calculated. 【0129】 (Basis weight of corrugated cardboard base paper) The corrugated cardboard base paper obtained in the examples and comparative examples was conditioned for 24 hours under the humidity-controlled environment specified in JIS P 8111:1998. The basis weight of the conditioned corrugated cardboard base paper was measured in accordance with JIS P 8124:2011. When the corrugated cardboard base paper had three layers (front layer, middle layer, and back layer), as a pretreatment, the corrugated cardboard base paper was soaked in 40°C hot water for 6 hours, then carefully peeled off one layer at a time by hand, divided into three parts: front layer, middle layer, and back layer, and dried in a dryer set to 105°C for 30 minutes. Subsequently, after 24 hours of humidity control under the humidity-controlled environment specified in JIS P 8111:1998, the basis weight of each layer of the corrugated cardboard base paper after its three-part division was measured in accordance with JIS P 8124:2011. 【0130】 (0.05 mm with a metal layer) ２(Total area, maximum area, and number of the above-mentioned disintegrated pieces) From the corrugated cardboard base paper obtained in the examples and comparative examples, only the layer portion containing difficult-to-treat recycled paper pulp (only the middle layer portion in Examples 1 to 18 and 20, and Comparative Examples 1 to 4; in Example 19, the entire layer as it is a single layer) was taken out, and the layer containing difficult-to-treat recycled paper pulp taken out was disintegrated according to JIS P 8220-1:2012 to obtain a slurry of pulp (concentration: 3 mass%) obtained from the disintegrated layer containing difficult-to-treat recycled paper pulp. In accordance with JIS P 8222:2015, a round sheet preparation device was used to make a round, hand-formed sheet (basis weight 60 g / m²) using the slurry of pulp obtained from the disintegrated layer containing difficult-to-treat recycled paper pulp. ２ , area 0.02m ２ Ten sheets (105 μm thick) were prepared. Visually, referring to the measurement chart of foreign matter created by the National Printing Bureau, the metal layer was found to be 0.05 mm thick on both sides of each handmade sheet. ２ The above-mentioned dissociated fragments were identified, and the 0.05 mm metal layer within each handmade sheet was identified. ２ The total area, maximum area, and number of the above-mentioned disintegrated fragments were measured. Based on the measurement results for 10 handmade sheets, the 0.05 mm thick corrugated cardboard base paper with a metal layer obtained in the examples and comparative examples was measured. ２ The total area, maximum area, and number of the above-mentioned dissociated fragments were calculated. 【0131】 (Metal detection) Ten sheets of corrugated cardboard base paper obtained in the examples and comparative examples were cut to the size of a 25cm x 40cm basis plate and stacked together to form an evaluation sample (total area 1m²). ２Ten samples were prepared, each measuring 25 cm in length, 40 cm in width, and approximately 2 mm in height. Each evaluation sample was placed in the center of the conveyor belt of a metal detector (Nikka Densoku Co., Ltd., model NA1-B500-CS, sensitivity Fe φ1.2 mm, SUS φ1.5 mm, conveying speed 20 m / min, conveying belt width 500 mm, opening height 270 mm) and passed through. The corrugated cardboard base paper of the examples and comparative examples was evaluated to see if it was detected by the metal detector according to the evaluation criteria below. If the result is A to C (metal detector judgment value in the range of 0 or more and less than 100), the corrugated cardboard base paper is judged to have no metallic reaction and will not interfere with foreign object detection by the metal detector even if used as packaging material. On the other hand, if the result is D (metal detector judgment value of 100 or more), it is judged to have a metallic reaction, and if used as packaging material, it may interfere with foreign object detection by the metal detector. [Evaluation Criteria] A: Metal detector detection value is 0 or more and less than 20 B: Metal detector detection value is 20 or more and less than 50 C: Metal detector detection value is 50 or more and less than 100 D: Metal detector detection value is 100 or more 【0132】(Productivity) In the examples and comparative examples, the ratio of the oven-dry mass of the difficult-to-treat recycled paper pulp to the oven-dry mass of the paper base layer contained in the difficult-to-treat recycled paper raw material (fiber yield (%)) was calculated using the following formula and evaluated according to the following evaluation criteria. Fiber yield [%] = Oven-dry mass of difficult-to-treat recycled paper pulp [kg] / Oven-dry mass of the paper base layer contained in the difficult-to-treat recycled paper [kg] × 100 The oven-dry mass of the paper base layer contained in the difficult-to-treat recycled paper and the oven-dry mass of the difficult-to-treat recycled paper pulp were calculated from the moisture content of the difficult-to-treat recycled paper and the moisture content of the difficult-to-treat recycled paper pulp, respectively, using the following formulas. The oven-dry mass [kg] of the paper base layer contained in difficult-to-treat waste paper = Actual mass [kg] of difficult-to-treat waste paper × {(100 - moisture content of difficult-to-treat waste paper [%]) / 100} × (content of paper base layer in difficult-to-treat waste paper [mass %] / 100) The oven-dry mass [kg] of difficult-to-treat waste paper pulp = Actual mass of difficult-to-treat waste paper pulp [kg] × {(100 - moisture content of difficult-to-treat waste paper pulp [%]) / 100} Furthermore, the moisture content was determined by taking samples of the difficult-to-treat waste paper used in the examples and comparative examples and a portion of the obtained difficult-to-treat waste paper pulp, and calculating from their pre-drying mass and post-drying mass after drying in a dryer at 105°C for 30 minutes using the following formula: Moisture content [%] = (pre-drying mass [g] - post-drying mass [g]) / pre-drying mass [g] × 100 Furthermore, the content [mass %] of the paper base layer in difficult-to-treat waste paper was measured by the following method. After manually peeling off the paper base layer of the difficult-to-treat waste paper to expose it, the paper base layer was dissolved by immersion in a 1 mol / L copper ethylenediamine solution (manufactured by Kanto Chemical Co., Ltd.) for 3 hours and removed. The remaining difficult-to-treat waste paper, after the paper base layer was removed, was washed with water, wiped dry with a paper towel, and then dried in a dryer set to 105°C for 5 minutes. Afterward, under humidity control conditions as specified in JIS P 8111:1998 for 24 hours, the basis weight of the remaining material was measured in accordance with JIS P 8124:2011. From the basis weight of the difficult-to-treat waste paper and the basis weight of the remaining material, the paper base layer content [mass %] in the difficult-to-treat waste paper was calculated using the following formula: Paper base layer content [mass %] in difficult-to-treat waste paper = 100 - basis weight [g / m²] of the remaining difficult-to-treat waste paper after the paper base layer was removed. ２ ] / Basis weight of difficult-to-process waste paper [g / m²] ２] x 100 [Evaluation Criteria] A: Fiber yield of 85% or more B: Fiber yield of 75% or more but less than 85% C: Fiber yield of 65% or more but less than 75% D: Fiber yield of less than 65% 【0133】 【0134】 Table 2 shows that Examples 1 to 20 are obtained from pulp with a basis weight of 60 g / m², which is obtained by dissociating a layer containing difficult-to-treat recycled paper pulp. ２ In a handmade sheet, a 0.05 mm sheet having a metal layer ２ The total area of ​​the above-mentioned dissociated fragments is 10.0 mm². ２ / 0.02m ２ The following conditions apply, and the maximum area is 4.0 mm². ２ As a result, it was confirmed that even when the material contains difficult-to-treat recycled paper pulp made from difficult-to-treat recycled paper having a metal layer and a resin layer, metal detectors are less likely to react. On the other hand, Comparative Examples 1, 3, and 4 have a metal layer of 0.05 mm ２ The total area of ​​the above-mentioned dissociated fragments is 10.0 mm². ２ / 0.02m ２ It exceeds the limit, and Comparative Example 2 has a metal layer of 0.05 mm ２ The maximum area of ​​the above-mentioned dissociated fragments is 4.0 mm². ２ Because it exceeded the limit, the metal detector reacted.

Claims

Corrugated cardboard base paper having one or more layers, Of the layers of the corrugated cardboard base paper, at least one layer contains difficult-to-process recycled paper pulp made from difficult-to-process recycled paper. The aforementioned difficult-to-process waste paper has at least a paper base layer, a metal layer, and a resin layer. The basis weight of 60 g / m² is obtained from the pulp obtained by dissociating the layer containing the aforementioned difficult-to-treat recycled paper pulp. ２ 0.05 mm handmade sheet with a metal layer ２ The total area of ​​the above-mentioned dissociated fragments is 10.0 mm². ２ / 0.02m ２ The following conditions apply, and the maximum area is 4.0 mm². ２ The following is the base paper for corrugated cardboard. 　 A base paper for corrugated cardboard having at least a surface layer, a middle layer, and a back layer, The aforementioned middle layer contains difficult-to-process recycled paper pulp made from difficult-to-process recycled paper, The aforementioned difficult-to-process waste paper has at least a paper base layer, a metal layer, and a resin layer. The basis weight of 60 g / m obtained from the pulp obtained by dissociating the middle layer ２ The total area of the dissociation pieces having a metal layer in the hand-skimmed sheet of 0.05 mm ２ or more is 10.0 mm ２ / 0.02 m ２ or less, and the maximum area is 4.0 mm ２ or less. The base paper for corrugated cardboard according to claim 1. The 0.05 mm thick handmade sheet having the metal layer ２ The number of dissociated fragments is 70 fragments / 0.02 m. ２ The corrugated cardboard base paper according to claim 1, which is as follows: 　 The corrugated cardboard base paper according to claim 1, wherein the amount of the difficult-to-process recycled paper pulp in the raw pulp constituting the layer containing the difficult-to-process recycled paper pulp is 0.2% by mass or more and 50% by mass or less. 　 The corrugated cardboard base paper according to claim 1, wherein the paper base layer of the difficult-to-process recycled paper includes one or more selected from the group consisting of hardwood kraft pulp and softwood kraft pulp. 　 0.05 mm having the aforementioned metal layer ２ The corrugated cardboard base paper according to claim 1, wherein the above-mentioned disintegrated fragments further include a resin layer. 　 The corrugated cardboard base paper according to claim 1, wherein the difficult-to-process waste paper is aseptic container waste paper. 　 The corrugated cardboard base paper according to claim 1, wherein the difficult-to-process waste paper has a structure in which a thermoplastic resin layer / pigment coating layer / paper substrate layer / adhesive resin layer / aluminum foil layer / adhesive resin layer / thermoplastic resin layer is laminated in this order. 　 Corrugated cardboard made using corrugated cardboard base paper as described in any one of claims 1 to 8. 　 A method for manufacturing corrugated cardboard base paper according to any one of claims 1 to 8, A process for manufacturing difficult-to-process recycled paper pulp using the aforementioned difficult-to-process recycled paper as raw material, and The papermaking process involves using the aforementioned raw material containing difficult-to-process recycled paper pulp as a raw material for a layer containing difficult-to-process recycled paper pulp, and making base paper for corrugated cardboard. The aforementioned difficult-to-process recycled paper pulp manufacturing process includes a disintegration step for disintegrating the difficult-to-process recycled paper, a dust removal step for removing foreign matter from the pulp slurry after the disintegration step, and a pulp washing step for washing the pulp in the pulp slurry after the dust removal step. The concentration of the difficult-to-process waste paper in the disintegration step is 5% by mass or more and 25% by mass or less, and the processing time of the disintegration step is 5 minutes or more and 90 minutes or less. A method for manufacturing base paper for corrugated cardboard. 　 The average size of the difficult-to-process waste paper used in the disintegration process is 600 cm². ２ The method for manufacturing corrugated cardboard base paper according to claim 10 is as follows: 　 The method for manufacturing corrugated cardboard base paper according to claim 10, wherein the processing temperature in the disintegration step is 5°C or more and 80°C or less. 　 The method for producing corrugated cardboard base paper according to claim 10, wherein the pH of the processing liquid in the disintegration step is 5 or more and 11 or less. 　 A method for manufacturing corrugated cardboard base paper according to claim 10, comprising, before the disintegration step, at least one of a crushing step for crushing the difficult-to-process waste paper and a waste paper washing step for washing the difficult-to-process waste paper. 　 The process includes a crushing step prior to the disintegration step, and the average size of the difficult-to-process waste paper after crushing is 10 cm. ２ More than 400cm ２ The method for manufacturing corrugated cardboard base paper according to claim 14 is as follows: 　 A method for producing base paper for corrugated cardboard according to claim 14, wherein the waste paper washing step is performed before the disintegration step, and the amount of washing water per unit mass of difficult-to-process waste paper is 0.5 L / kg or more and 30 L / kg or less. 　 The method for manufacturing corrugated cardboard base paper according to claim 10, wherein the dust removal step comprises a step of removing foreign matter from the pulp slurry after the disintegration step by centrifugal separation and a step of removing foreign matter from the pulp slurry after the disintegration step by screen treatment. 　 A method for producing corrugated cardboard base paper according to claim 10, further comprising a dewatering step after the pulp washing step, in which the pulp slurry is dewatered to obtain pulp.

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