White paperboard, method for producing same, and paper container

Abstract

The present invention provides: white paperboard that, even when made from a raw material of recovered paper pulp, which is made from a raw material of recovered paper having at least a paper base layer, a metal layer, and a resin layer, has a reduced amount of defibered flakes containing the metal layer; a method for producing white paperboard, and a paper container.　This white paperboard has at least a front layer, a middle layer, and a back layer. The middle layer 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 middle layer has been defibered have a total area of 7.0 mm2 / 0.02 m2 or less, and a maximum area of 2.5 mm2 or less.

Description

White cardboard and its manufacturing method, as well as paper containers. 【0001】 This invention relates to white cardboard, a method for manufacturing the same, and paper containers. 【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, such as aseptic container packaging materials that have metal and resin layers in addition to the paper base, is used for recycling, finely dispersed fragments containing the metal layer derived from the difficult-to-process waste paper are mixed in. Therefore, when paper products obtained from recycled difficult-to-process waste paper, especially white cardboard, are used, the quality is significantly reduced due to the inclusion of these dispersed fragments. 【0005】 Japanese Patent Publication No. 6-228894 did not consider the removal of metal 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, using the waste paper pulp obtained in Japanese Patent Publication Nos. 6-228894, 2010-43368, and 6-220784 as raw material for white cardboard would result in a decrease in the quality of the white cardboard itself. Moreover, if this white cardboard is used for paper containers requiring higher quality (e.g., paper containers for food packaging), it would be rejected by metal detectors during the shipping process of the products (food products packaged in these white cardboard containers), leading to a decrease in product shipping efficiency. 【0006】 The object of the present invention is to provide white cardboard, a method for manufacturing the same, and paper containers, in which the amount of disintegrated fragments having a metal layer is reduced and the quality is improved, 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. 【0007】 The present inventors have developed a white cardboard having at least a surface layer, a middle layer, and a back layer, wherein the middle layer contains difficult-to-treat recycled paper pulp made from difficult-to-treat recycled paper, and the present inventors have developed a 0.05 mm thick handmade sheet having a metal layer in the pulp obtained from the pulp after the middle layer has been disassembled. ２It has been found that the above problems can be solved by setting the total area and the maximum area of the above disintegrated pieces to be not more than a certain value. 【0008】 The present invention provides the following. <1> A white cardboard having at least a surface layer, a middle layer, and a back layer, wherein the middle layer contains a difficult-to-treat waste paper pulp made from difficult-to-treat waste paper, and the difficult-to-treat waste paper has at least a paper base material layer, a metal layer, and a resin layer, and the grammage of 60 g / m ２ obtained from the pulp obtained by disintegrating the middle layer, having a metal layer in the hand-sheet of ２ The total area of the disintegrated pieces of 0.05 mm ２ or more is 7.0 mm ２ or less, and the maximum area is 2.5 mm ２ or less, white cardboard. <2> The number of the disintegrated pieces of 0.05 mm ２ or more having the metal layer in the hand-sheet is 40 pieces / 0.02 m ２ or less, the white cardboard according to <1>. <3> The blending amount of the difficult-to-treat waste paper pulp in the raw material pulp constituting the middle layer is 0.2% by mass or more and 35% by mass or less, the white cardboard according to <1> or <2>. <4> The middle layer is two or more layers, the white cardboard according to any one of <1> to <3>. <5> The paper base material layer of the difficult-to-treat waste paper contains one or more selected from the group consisting of hardwood kraft pulp and softwood kraft pulp, the white cardboard according to any one of <1> to <4>. <6> The white cardboard according to any one of <1> to <5>, having a pigment coating layer on at least one surface of the white cardboard. <7> The 0.05 mm having the metal layer ２The white cardboard according to any one of <1> to <6>, wherein the above-mentioned disintegrated fragments further include a resin layer. <8> The white cardboard according to any one of <1> to <7>, wherein the difficult-to-treat waste paper is aseptic container waste paper. <9> The white cardboard according to any one of <1> to <8>, wherein the difficult-to-treat waste paper has a structure in which a thermoplastic resin layer / pigment coating layer / paper base material layer / adhesive resin layer / aluminum foil layer / adhesive resin layer / thermoplastic resin layer is laminated in this order. <10> A paper container made using the white cardboard according to any one of <1> to <9>. <11> A method for manufacturing white cardboard 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 white cardboard using a paper stock containing the 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. <12> The average size of the difficult-to-process recycled paper used in the disintegration step is 600 cm ２ The method for producing white cardboard as described in <11> is as follows: <13> The method for producing white cardboard as described in <11> or <12>, wherein the processing temperature in the disintegration step is 5°C or more and 80°C or less. <14> The method for producing white cardboard as described in any one of <11> to <13>, wherein the pH of the processing liquid in the disintegration step is 5 or more and 11 or less. <15> The method for producing white cardboard as described in any one of <11> to <14>, wherein, before the disintegration step, 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. <16> 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 ２The method for manufacturing white cardboard as described in <15> is as follows: <17> The method for manufacturing white cardboard as described in <15> or <16>, wherein the method comprises the waste paper washing step 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. <18> The method for manufacturing white cardboard as described in any one of <11> to <17>, wherein the dust removal step comprises the step of removing foreign matter from the pulp slurry after the disintegration step by centrifugal separation and the step of removing foreign matter from the pulp slurry after the disintegration step by screen treatment. <19> The method for manufacturing white cardboard as described in any one of <11> to <18>, wherein the method comprises a dewatering step after the pulp washing step, in which the pulp slurry is dewatered to obtain pulp. 【0009】 [White cardboard] The white cardboard of this embodiment is a white cardboard having at least a surface layer, a middle layer and a back layer, wherein the middle layer contains difficult-to-treat recycled paper pulp made from difficult-to-treat recycled paper, and the difficult-to-treat 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 disintegrating the middle layer. ２ 0.05 mm handmade sheet with a metal layer ２ The total area of ​​the above dissociated fragments is 7.0 mm². ２ / 0.02m ２ The following conditions apply, and the maximum area is 2.5 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 white cardboard, a method for manufacturing white cardboard, and paper containers in which the amount of disintegrated fragments having a metal layer is reduced and the quality is improved. 【0010】 The white cardboard of this embodiment is obtained from pulp with the middle layer disintegrated, 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 dissociated fragments is 7.0 mm². ２ / 0.02m ２ The following conditions apply, and the maximum area is 2.5 mm². ２As a result of the following, even when the product contains difficult-to-treat recycled paper pulp made from difficult-to-treat recycled paper having a metal layer and a resin layer, the amount of disintegrated fragments having a metal layer is reduced, and the quality can be improved. Furthermore, even when the white cardboard of this embodiment contains the aforementioned difficult-to-treat recycled paper pulp, it is difficult for metal detectors to detect it. Therefore, in the shipping process of products that use metal detectors (such as food products packaged in cardboard containers made of this white cardboard), it is possible to suppress the decrease in product shipping efficiency caused by the white cardboard. 【0011】 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. 【0012】 In the white cardboard 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 waste paper, and are generated during the disintegration process when difficult-to-process waste paper is used to produce pulp from difficult-to-process waste paper, when metal layers and other components contained in the difficult-to-process waste 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 white cardboard of this embodiment, the 0.05 mm having a metal layer ２ The above-mentioned disintegrated fragments may contain a metal layer derived from the metal layer of the difficult-to-process waste paper, as well as a resin layer derived from the resin layer of the difficult-to-process waste paper, and may also have a paper base layer derived from the paper base layer of the difficult-to-process waste paper in at least a portion thereof. 【0013】 In the white cardboard of this embodiment, the basis weight obtained from the pulp with the middle layer disintegrated is 60 g / m². ２ 0.05 mm handmade sheet with a metal layer ２ The total area of ​​the above-mentioned disintegrated fragments is 7.0 mm², from the viewpoint of improving the quality of the white cardboard and suppressing the reaction of metal detectors. ２ / 0.02m ２ The following, preferably 5.0 mm２ / 0.02m ２ More preferably, 3.0 mm ２ / 0.02m ２ More preferably 2.5 mm ２ / 0.02m ２ More preferably, 2.0 mm ２ / 0.02m ２ Further, 1.5 mm is more preferable. ２ / 0.02m ２ More preferably, 1.3 mm ２ / 0.02m ２ The following applies: 0.05 mm having the aforementioned metal layer ２ The 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 0.2 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. 【0014】 In the white cardboard of this embodiment, the basis weight obtained from the pulp with the middle layer disintegrated is 60 g / m². ２ A 0.05 mm handmade sheet with a metal layer ２ The maximum area of ​​the above-mentioned detached fragments is 2.5 mm, from the viewpoint of improving the quality of white cardboard and suppressing the reaction of metal detectors. ２ The following, preferably 1.8 mm ２ More preferably 1.2 mm ２ More preferably 1.0 mm ２ More preferably, 0.8 mm ２ Further, 0.65 mm is more preferably used. ２ The following, and from the viewpoint of ease of manufacture, preferably 0.05 mm ２ More preferably 0.1 mm ２ More preferably 0.2 mm ２More preferably 0.3 mm ２ That is all. 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. 【0015】 In the white cardboard of this embodiment, the basis weight obtained from the pulp with the middle layer disintegrated is 60 g / m². ２ A 0.05 mm handmade sheet with a metal layer ２ From the viewpoint of improving the quality of the white cardboard and suppressing the reaction of metal detectors, the number of disintegrated fragments is preferably 40 fragments / 0.02 m. ２ More preferably, 30 pieces / 0.02 m ２ More preferably, 20 pieces / 0.02 m ２ More preferably, 15 pieces / 0.02 m ２ More preferably, 8 pieces / 0.02 m ２ The following applies: And the 0.05 mm having the 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 0 fragments / 0.02 m ２ More preferably, 1 unit / 0.02 m ２ That is all. 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. 【0016】 The white cardboard of this embodiment has at least a surface layer, a middle layer, and a back layer, and is used as a raw material for paper containers, catalog and magazine covers, cards, etc. When used as a raw material for paper containers, the surface layer forms the surface of the paper container, and the back layer forms the inner surface of the paper container. The middle layer is formed between the surface layer and the back layer. 【0017】<Middle Layer> In this embodiment, the white cardboard contains difficult-to-process recycled paper pulp in the middle layer, which is made from difficult-to-process recycled paper. If the white cardboard of this embodiment has multiple middle layers, it is sufficient that at least one of the multiple middle layers contains difficult-to-process recycled paper pulp. In addition, the fact that the middle layer of the white cardboard contains difficult-to-process recycled paper pulp made from difficult-to-process recycled paper can be inferred by performing the following analysis on the middle layer. A 0.05 mm layer that normally has a metal layer ２ The above-mentioned disintegrated fragments are foreign matter in white cardboard and are not intentionally included in the mixture. 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 that recycled paper pulp is present. - Analyze the components of the foreign matter on the white cardboard using micro-FT-IR and identify 0.05 mm with resin layer and metal layer. ２ Identify that the above-mentioned dissociated fragments are present. 【0018】 In this embodiment, the basis weight of the middle layer of the white cardboard is preferably 100 g / m², from the viewpoint of improving the strength of the white cardboard, 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 500g / m ２ More preferably, 150 g / m ２ 450g / m or more ２ More preferably, 250 g / m ２ More than 400g / m ２ More preferably, 300 g / m ２ 350g / m or more ２ The following applies. Note that if the white cardboard in this embodiment has multiple intermediate layers, the basis weight of the intermediate layers of the white cardboard is the sum of the basis weights of each intermediate layer. 【0019】 (Difficult-to-process recycled paper) In the white cardboard of this embodiment, the difficult-to-process recycled paper that serves as the raw material for the 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. 【0020】[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 the 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 raw material pulp (especially softwood kraft pulp), the strength of the white cardboard of this embodiment is further improved. When the paper substrate layer contains hardwood kraft pulp (LKP) and softwood kraft pulp (NKP), from the viewpoint of improving the strength of the white cardboard, 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. 【0021】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. 【0022】 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, and even more preferably 250 μm or more and 450 μm or less, from the viewpoints of improving the strength of the white cardboard, improving the recyclability of the difficult-to-treat waste paper, reducing the disintegrated pieces having a metal layer in the difficult-to-treat waste paper pulp, and improving the quality of the white cardboard. 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. 【0023】From the viewpoints of improving the strength of the white cardboard, improving the recyclability of the difficult-to-recycle waste paper, reducing the disintegrated pieces having a metal layer in the difficult-to-recycle waste paper pulp, and improving the quality of the white cardboard, the content of the paper base material layer in the difficult-to-recycle 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, still more preferably 60% by mass or more and 96% by mass or less, and even more preferably 65% by mass or more and 95% by mass or less. When the difficult-to-recycle waste paper has a plurality of paper base material layers, the content of the paper base material layer in the difficult-to-recycle waste paper is the total amount of the contents of each paper base material layer. 【0024】 [Metal layer] In the difficult-to-recycle waste paper, the metal constituting the metal layer is not particularly limited, but aluminum is preferable from the viewpoint of improving productivity. That is, in the present embodiment, the metal layer is preferably an aluminum layer. As the aluminum layer, one or more selected from the group consisting of an aluminum vapor deposition layer and an aluminum foil layer are preferable, and an aluminum foil layer is more preferable. 【0025】 From the viewpoints of improving the recyclability of the difficult-to-recycle waste paper, reducing the disintegrated pieces having a metal layer in the difficult-to-recycle waste paper pulp, and improving the quality of the white cardboard, the basis weight of the metal layer in the difficult-to-recycle waste paper is preferably 8 g / m ２ or more and 30 g / m ２ or less, more preferably 10 g / m ２ or more and 25 g / m ２ or less, still more preferably 12 g / m ２ or more and 23 g / m ２ or less. When the difficult-to-recycle waste paper has a plurality of metal layers, the basis weight of the metal layer in the difficult-to-recycle waste paper indicates the basis weight of each metal layer. 【0026】 From the viewpoints of improving the recyclability of the difficult-to-recycle waste paper, reducing the disintegrated pieces having a metal layer in the difficult-to-recycle waste paper pulp, and improving the quality of the white cardboard, the thickness of the metal layer in the difficult-to-recycle waste paper is preferably 3 μm or more and 15 μm or less, more preferably 4 μm or more and 12 μm or less, still more preferably 6 μm or more and 10 μm or less. When the difficult-to-recycle waste paper has a plurality of metal layers, the thickness of the metal layer in the difficult-to-recycle waste paper indicates the thickness of each metal layer. 【0027】 The content of the metal layer in 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, 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, thereby improving the quality of white cardboard. 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. 【0028】 [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 containing metal layers in the difficult-to-process waste paper pulp, thereby improving the quality of white cardboard, 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). 【0029】 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 pulp of difficult-to-process waste paper, thereby improving the quality of white cardboard, the basis weight of the resin layer is preferably 5 g / m². ２ 60g / m or more ２ More preferably, 6 g / m ２ 55g / m or more２ 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. 【0030】 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, thereby improving the quality of white cardboard. 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. 【0031】 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 metal layers in difficult-to-process waste paper pulp, thereby improving the quality of white cardboard. 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. 【0032】 [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. 【0033】≪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). 【0034】 When difficult-to-process recycled paper has an adhesive resin layer, the basis weight of the adhesive resin layer in the difficult-to-process recycled paper is preferably 2 g / m², from the viewpoint of improving the recyclability of the difficult-to-process recycled paper and from the viewpoint of reducing the amount of disintegrated fragments containing the metal layer in the difficult-to-process recycled paper pulp, thereby improving the quality of the white cardboard. 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 More than 27g / m 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. 【0035】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 metal fragments in the difficult-to-process waste paper pulp, thereby improving the quality of white cardboard. 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 individual adhesive resin layer. 【0036】 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 reducing the amount of disintegrated fragments having a metal layer in the difficult-to-process waste paper pulp, thereby improving the quality of white cardboard. 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. 【0037】 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 the disintegrated fragments having a metal layer in the difficult-to-process waste paper pulp and improving the quality of white cardboard. 【0038】≪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. 【0039】 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. Note that 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. 【0040】 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. 【0041】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. 【0042】 ≪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. 【0043】 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. 【0044】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. 【0045】 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. 【0046】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. 【0047】 (Difficult-to-process recycled paper pulp) The difficult-to-process recycled paper pulp in this embodiment is obtained from the difficult-to-process recycled paper described above as a raw material, by the manufacturing process of difficult-to-process recycled paper pulp in the method for manufacturing white cardboard in this embodiment, which will be 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 white cardboard of this embodiment has a basis weight of 60 g / m² obtained from pulp with the middle layer disintegrated. ２ A 0.05 mm handmade sheet with a metal layer ２ The total area of ​​the above-mentioned dissociated fragments is 7.0 mm². ２ / 0.02m ２ The following conditions apply, and the maximum area is 2.5 mm². ２The following improvements can be made to the quality of the white cardboard. Furthermore, it can be made into white cardboard that is less likely to trigger metal detectors, making it suitable for use in packaging food and other similar products. 【0048】 In the white cardboard of this embodiment, the middle layer 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 chemical pulps such as unbleached and bleached wood pulp, mechanical pulp, non-wood pulp, 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, paper container recycled paper pulp, and mixed paper 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, paper container recycled paper pulp, and mixed paper recycled paper pulp. 【0049】 In the white cardboard of this embodiment, the amount of difficult-to-treat recycled paper pulp in the pulp constituting the middle layer is preferably 0.1% by mass or more and 50% by mass or less, more preferably 0.2% by mass or more and 35% by mass or less, even more preferably 0.3% by mass or more and 30% by mass or less, even more preferably 0.5% by mass or more and 27% by mass or less, even more preferably 0.5% by mass or more and 15% by mass or less, and even more preferably 0.5% by mass or more and 10% by mass or less. It is preferable that the amount of difficult-to-treat recycled paper pulp in the pulp constituting the middle layer is 0.1% by mass or more because it allows for the inclusion of difficult-to-treat recycled paper pulp, which improves the recyclability of the difficult-to-treat recycled paper pulp. Furthermore, if the amount of difficult-to-treat recycled paper pulp in the pulp constituting the middle layer is 50% by mass or less, the amount of disintegrated fragments having a metal layer in the white cardboard can be further reduced. The amount of difficult-to-process recycled paper pulp used in this blend refers to the amount used in the entire middle layer. For example, if the middle layer consists of multiple layers, the total amount of pulp in all of those layers is considered to be 100% by mass, and the amount of difficult-to-process recycled paper used refers to the proportion of that pulp. 【0050】In the white cardboard of this embodiment, the middle layer may be one layer, but from the viewpoint of improving the strength of the white cardboard, it is preferably two or more layers, and there is no particular upper limit to the number of layers of the middle layer, but from the viewpoint of ease of handling, for example, it is preferably seven layers or less, more preferably five layers or less, and even more preferably four layers or less. 【0051】 (Under-surface layer and under-back layer) Of the middle layers, the layer located immediately below the surface layer and in contact with the surface layer is sometimes called the under-surface layer. Also, of the middle layers, the layer located immediately below the under-back layer and in contact with the under-back layer is sometimes called the under-back layer. In this case, the under-surface layer and under-back layer are layers with a higher degree of whiteness than the other middle layers. To further improve the surface whiteness of the under-surface layer, it is preferable to form it using recycled paper pulp with a higher degree of whiteness among the middle layers mentioned above. In this case, it is preferable to use pulp with a lower degree of whiteness than the pulp of the surface layer but a higher degree of whiteness than the other middle layers. Furthermore, pigments such as white clay, clay, calcined clay, talc, titanium dioxide, silica, and calcium carbonate can also be blended into the under-surface layer. When the whiteness of the underside is increased, it is preferable to form the under-back layer using recycled paper pulp with a higher degree of whiteness among the middle layers mentioned above in order to further improve the whiteness of the underside. This underlayer is usually adjusted to have a lower whiteness than the pulp in the surface layer and a higher whiteness than the other middle layers. However, if the whiteness of the underlayer is to be the same as that of the surface layer, it may have the same composition as the underlayer. Furthermore, pigments such as white clay, clay, fired clay, talc, titanium dioxide, silica, and calcium carbonate can be added to the underlayer. 【0052】 In this embodiment, the basis weight of the front and back sublayers of the white cardboard is preferably 10 g / m², from the viewpoint of improving the strength of the white cardboard, 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 100g / m ２ More preferably, 20 g / m ２ 50g / m or more ２ More preferably, 25 g / m ２ 45g / m or more ２ More preferably, 30 g / m ２ 40g / m or more ２The following applies. Note that if the white cardboard has a front and back sub-layer, the basis weights of the front and back sub-layers are shown as their respective basis weights. 【0053】 <Surface Layer> In the white cardboard of this embodiment, there are no particular restrictions on the raw material pulp that constitutes the surface layer, and examples include the pulp that constitutes the surface layer of conventionally known white cardboard. It is preferable not to use difficult-to-treat recycled paper pulp so that the surface layer does not contain disintegrated fragments having a metal layer and a resin layer. The main component of the pulp that constitutes the surface layer is, for example, one or more selected from the group consisting of deinked pulp and bleached pulp, and examples include the other pulps mentioned above. In this specification, "main component of the pulp that constitutes layer A" means pulp that accounts for 50% by mass or more, preferably 80% by mass or more, and more preferably 90% by mass or more of the total amount of pulp that constitutes layer A (upper limit 100% by mass). Examples of deinked pulp include the recycled paper pulp obtained by deinking as described above, which is recycled paper pulp with high whiteness. Bleaching treatment may be performed to increase the whiteness. As the surface deinking pulp, deinking pulp derived from recycled paper with high whiteness, such as white top white / cards, extra white / medium white / white Manila, imitation / colored top whites, etc., is preferred, and recycled Kent paper pulp is particularly preferred. Examples of bleached pulp include bleached hardwood kraft pulp (LBKP), semi-bleached hardwood kraft pulp (LSBKP), bleached softwood kraft pulp (NBKP), semi-bleached softwood kraft pulp (NSBKP), and bleached chemothermetic pulp (BCTMP), with bleached hardwood kraft pulp (LBKP) and bleached softwood kraft pulp (NBKP) being particularly preferred. In addition, other pulps such as unbleached pulp can be used in combination as long as they do not impair the effects of the present invention. 【0054】 From the viewpoint of improving the strength of the white cardboard, the basis weight of the surface layer of the white cardboard in this embodiment is preferably 10 g / m². ２ More than 100g / m ２ More preferably, 20 g / m ２ 50g / m or more ２ More preferably, 25 g / m ２ 40g / m or more ２ More preferably, 27 g / m ２ 35g / m or more２ The following applies: 【0055】 <Backing Layer> In the white cardboard of this embodiment, there are no particular restrictions on the raw material pulp that constitutes the backing layer, and examples include the pulp that constitutes the backing layer of conventionally known white cardboard. It is preferable not to use difficult-to-process recycled paper pulp so that the backing layer does not contain disintegrated fragments having a metal layer. The main component of the pulp that constitutes the backing layer is, for example, the other pulp mentioned above, and from the viewpoint of further improving recyclability, other recycled paper pulp is preferred, and it is more preferable that it be one or more selected from the group consisting of recycled newspaper pulp and recycled magazine pulp. Although the same degree of whiteness as the surface layer is not required, since it is seen by people, pulp with a higher degree of whiteness than the middle layer is usually used. The backing layer may also contain pulp derived from shredded office paper, etc. Pulp obtained by disintegrating shredded office paper, etc. has a relatively high degree of whiteness even without deinking and bleaching treatment, so it is preferable to blend it into the backing layer. If a higher degree of whiteness is required, the backing layer may be made the same as the surface layer. 【0056】 From the viewpoint of improving the strength of the white cardboard, the basis weight of the backing layer of the white cardboard in this embodiment is preferably 10 g / m². ２ More than 100g / m ２ More preferably, 15 g / m ２ 40g / m or more ２ More preferably, 18 g / m ２ 30g / m or more ２ More preferably, 21 g / m ２ More than 27g / m ２ The following applies: 【0057】 In each layer of the white cardboard of this embodiment, internal additives may be used as needed. For example, yield improvers such as aluminum sulfate, sizing agents such as rosin, paper strength enhancers (dry paper strength enhancers) such as polyacrylamide and starch, filtration yield improvers, wet paper strength enhancers such as polyamide, polyamine, and epichlorohydrin, dyes, fillers, etc. Among these, aluminum sulfate and paper strength enhancers (dry paper strength enhancers) are preferred. 【0058】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 the raw pulp constituting the layer. The content of sizing agent is not particularly limited, but in each layer, it is preferably 0 parts by mass or more and 2.0 parts by mass or less per 100 parts by mass of the raw pulp constituting the layer. The sizing agent is preferably contained in the surface layer, and if so, it is preferably 0.1 parts by mass or more and 2.0 parts by mass or less per 100 parts by mass of the raw pulp constituting the layer. The content of paper strength agent (dry paper strength enhancer) is not particularly limited, but in each layer, it is preferably 0.05 parts by mass or more and 5.0 parts by mass or less per 100 parts by mass of the raw pulp constituting the layer. Known fillers can be used, for example, light calcium carbonate is preferred, and its content is not particularly limited, but in each layer, it is preferably 0 parts by mass or more and 15.0 parts by mass or less per 100 parts by mass of the raw pulp constituting the layer. 【0059】 The basis weight of the white cardboard of this embodiment, which is composed of the surface layer, middle layer, and back layer, is preferably 100 g / m². ２ More than 600g / m ２ More preferably, 200 g / m ２ More than 550g / m ２ More preferably, 250 g / m ２ More than 500g / m ２ More preferably, 300 g / m ２ 450g / m or more ２ The following applies: It is preferable that the basis weight of the white cardboard be within the above range, as this provides superior handling advantages. 【0060】 In the white cardboard of this embodiment, the amount of difficult-to-treat 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 20% by mass or less, and even more preferably 3% by mass or more and 15% 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. Furthermore, when the amount of difficult-to-treat recycled paper pulp is 40% by mass or less, the amount of disintegrated fragments having a metal layer in the white cardboard can be further reduced. 【0061】 <Pigment Coating Layer> Furthermore, a pigment coating layer may be provided on at least one surface of the above-mentioned white cardboard. For example, a pigment coating liquid containing a pigment and a binder may be applied and dried to provide a pigment coating layer, thereby creating white cardboard (coated white cardboard). By providing a pigment coating layer, the color of the intermediate layer can be further concealed, resulting in white cardboard with high surface whiteness, and the printability of the surface can also be improved. The pigment is not particularly limited, but examples include kaolin, calcium carbonate (heavy calcium carbonate, light calcium carbonate), titanium dioxide, aluminum hydroxide, silica, satin white, talc, and other pigments commonly used in the coated paper manufacturing field. Among these, calcium carbonate is preferred because it exhibits excellent printability. The pigment may be used alone or in combination of two or more types. 【0062】 A water-based adhesive is preferred as the binder. The water-based adhesive is composed of water, a polymer, and other additives as needed. Examples of polymers include starches such as oxidized starch, phosphate-esterified starch, hydroxyethyl etherified starch, dextrin, enzyme-modified starch, and water-soluble starch; latexes such as conjugated diene copolymer latex such as styrene-butadiene copolymer latex, methyl methacrylate-butadiene copolymer latex, and styrene-methyl methacrylate-butadiene copolymer latex, and acrylic copolymer latex such as acrylic acid ester and / or methacrylic acid ester copolymer latex; proteins such as casein, gelatin, and soy protein; synthetic resins such as various polyvinyl alcohols, various polyacrylamides, and melamine resins; and various cellulose derivatives such as carboxymethylcellulose. The binder can be one or more selected from the group consisting of these polymers. 【0063】Among these, latex with a glass transition temperature of -50 to 30°C, as measured by Vibron viscoelasticity, is preferred because it increases the flexibility of the coated surface and improves its resistance to breakage. In this case, it is more preferable that the glass transition temperature of the latex used is -50 to 0°C. It is also preferable to blend starch together with the latex. When latex and starch are used together, a balance is achieved between fixing the fine fibers inside the coated layer and the surface strength of the coated layer. The mass ratio of latex to starch is preferably 100:0 to 50:50. 【0064】 The coating liquid for the pigment coating layer may further contain, as needed, one or more of the following: dispersants, pH adjusters such as sodium hydroxide and ammonia water; defoamers; fluorescent dyes; mold release agents; water-resistant agents; fluidity improvers; slime control agents; preservatives; dyes; coloring pigments, etc. A pigment coating layer may also be applied to the surface of the back layer. When applying a pigment coating layer to the surface of the front layer of white cardboard, an example method involves applying the pigment coating liquid to the surface of the front layer, and then drying the coating layer formed by the application to form a pigment coating layer on the surface of the front layer. The pigment coating layer may be formed as a single layer, or it may be formed as a multi-layered pigment coating layer by applying the pigment coating liquid in multiple stages. When applying the pigment coating liquid in multiple stages, for example, in two stages, the coating liquids for the first layer (undercoat) and the second layer (topcoat) may be the same or different. 【0065】 In the coating liquid for the pigment coating layer, the binder content per 100 parts by mass of pigment is preferably 2 to 50 parts by mass, more preferably 5 to 25 parts by mass. When the binder content is 2 parts by mass or more, sufficient strength of the pigment coating layer is ensured, and when the binder content is 50 parts by mass or less, excellent drying properties and suitability for box making are obtained. The coating amount of the coating liquid for the pigment coating layer is preferably 5 to 50 g / m² by dry mass. ２ , more preferably 8 to 40 g / m ２ More preferably 10 to 35 g / m ２The solid content concentration of the coating liquid for the pigment coating layer is preferably 30 to 60% by mass, more preferably 35 to 55% by mass. If the solid content concentration is 30% by mass or more, the opacity of the color of the middle layer is enhanced, and if it is 60% by mass or less, the coatability is good. In addition, the B-type viscosity of the coating liquid for the pigment coating layer is preferably 400 mPa·s or less, more preferably 300 mPa·s or less, in order to ensure workability during coating. The lower limit of the B-type viscosity is not particularly limited, but from the viewpoint of suppressing unevenness in the coating liquid, it is preferably 20 mPa·s or more. When applying the coating liquid for the pigment coating layer multiple times, each of the pigment coating liquids used should satisfy the above content and characteristics, and it is preferable that the total amount of coating from multiple layers satisfies the above range. In addition, when providing multiple pigment coating layers, it is preferable to increase the pigment concentration of the pigment coating layer closer to the surface. For example, when a pigment coating layer consists of two layers, an undercoat layer and a topcoat layer, it is preferable to have a higher pigment concentration in the first layer (undercoat layer) than in the second layer (topcoat layer). 【0066】 [Paper container] The paper container of this embodiment is a paper container made using the white cardboard of this embodiment described above. Because the paper container of this embodiment uses the white cardboard of this embodiment, even if the raw material is difficult-to-process recycled paper pulp made from difficult-to-process recycled paper, it has a metal layer of 0.05 mm ２ The amount of disintegrated fragments is reduced, and the fragments contained are fine. The paper packaging of this embodiment is not particularly limited in its use, and examples include paper boxes used for tissue paper and confectionery boxes. Because the paper packaging of this embodiment uses the white cardboard of this embodiment, it recycles aseptic container packaging material as a raw material for waste paper pulp, while having a metal layer as described above. ２ Because the amount of the aforementioned disintegrated fragments is reduced, and the fragments that are included are extremely fine, it can also be used as paper packaging for food products and the like. 【0067】[Method for manufacturing white cardboard] The method for manufacturing white cardboard according to this embodiment comprises a difficult-to-process recycled paper pulp manufacturing step, in which difficult-to-process recycled paper is used as raw material to manufacture difficult-to-process recycled paper pulp, and a papermaking step, in which white cardboard is made using a paper stock containing the 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. 【0070】 Based on the above, the white cardboard of this embodiment, despite containing difficult-to-process recycled paper pulp in the middle layer, has a basis weight of 60 g / m² obtained from the pulp after disintegrating the middle layer. ２ 0.05 mm handmade sheet with a metal layer ２ The total area of ​​the above dissociated fragments is 7.0 mm². ２ / 0.02m ２ The following conditions apply, and the maximum area is 2.5 mm². ２The following is the result, with a 0.05 mm metal layer. ２ The amount of the above-mentioned disintegrated fragments can be reduced, improving the quality of the white cardboard. Furthermore, the 0.05 mm metal layer in the handmade sheet ２ The maximum area of ​​the above-mentioned dissociated fragments is 2.5 mm². ２ The material consists of minute fragments, which do not pose any problems as a product, and even if a metal detector is used during product quality inspection, it will not be rejected as a defective product. It should be noted that the above mechanism is speculative, and the effects of the present invention are not limited by this mechanism. 【0071】 (Difficult-to-process recycled paper) In the manufacturing process of difficult-to-process recycled paper pulp in this embodiment, the difficult-to-process recycled paper that serves as the raw material for the difficult-to-process recycled paper pulp has at least a paper base layer, a metal layer and a resin layer, and is similar to the difficult-to-process recycled white cardboard described above. 【0072】 In this embodiment, the average size of the difficult-to-treat waste paper subjected to the disintegration process described later is preferably 600 cm². ２ The following, 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, a more preferable length is 40 cm. ２ More than 150cm ２ More preferably, 50 cm ２ More than 100cm ２ The following is particularly preferable: 60 cm ２ More than 90cm 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 ２ As a result, the fineness of the metal and resin layers contained in the difficult-to-treat waste paper is suppressed, and the amount of disintegrated fragments containing the metal layer in the resulting difficult-to-treat waste paper pulp is reduced. The average size of the difficult-to-treat waste paper is measured by the method described in the examples. 【0073】 Difficult-to-process waste paper is usually compressed and packaged into bales of approximately 1 ton in a baler to improve transportation efficiency when shipping to recycling manufacturers. Furthermore, if the difficult-to-process waste paper is used as a liquid container, 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 stacked together (waste paper aggregate). The difficult-to-process waste paper may also be torn. In addition, the difficult-to-process waste paper may be crushed by the crushing process described later, before the disintegration process described later. 【0074】 (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. 【0075】 [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 layer fragments in the raw pulp can be reduced, improving the quality of the resulting white cardboard. 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 More preferably 30 cm ２ More than 200cm ２ More preferably, 40 cm ２ More than 150cm ２ More preferably, 50 cm ２ More than 100cm ２ Further, a more preferable length is 60 cm. ２ More than 90cm ２ The following is true: The average size of difficult-to-process waste paper after shredding is 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, 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. 【0076】 [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 white cardboard. 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. 【0077】 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. Among 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 waste paper washing process are performed simultaneously, it is preferable to use a crushing and washing machine or the like as appropriate. 【0078】 (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. 【0079】 [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 amount of disintegrated fragments having a metal layer in the difficult-to-process recycled paper pulp can be reduced. As a result, in the white cardboard of this embodiment, the amount of disintegrated fragments having a metal layer is reduced (particularly contributing to a reduction in total area and number), and the quality of the white cardboard can be improved. 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 fragments), and the yield of the obtained difficult-to-process recycled paper pulp is improved. The concentration of difficult-to-process waste paper in the disintegration process is preferably 5% by mass or more, 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 having a metal layer in the resulting difficult-to-process waste paper pulp and improving the quality of the white cardboard of this embodiment, and from the viewpoint of improving the yield of the resulting difficult-to-process waste paper pulp, it is preferably 22% by mass or less, more preferably 20% by mass or less, and even more preferably 19% by mass or less. 【0080】[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 recycled paper pulp is improved. Furthermore, 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 ground metal and resin are sufficiently removed, and the amount of disintegrated fragments having a metal layer in the difficult-to-process recycled paper pulp is reduced. As a result, in the white cardboard of this embodiment, the amount of disintegrated fragments having a metal layer is reduced, and the quality of the white cardboard can be improved. 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, and from the viewpoint of reducing the amount of disintegrated fragments having a 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. 【0081】 [Processing Temperature] In the manufacturing process for 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 from the viewpoint of reducing disintegration fragments having a metal layer in the obtained difficult-to-process recycled paper pulp, it 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. 【0082】 [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. Having the pH of the processing solution within this range allows the processing solution to penetrate the difficult-to-process recycled paper more easily. Therefore, the disintegration process is carried out efficiently, improving the yield of the resulting difficult-to-process recycled paper pulp. Furthermore, since the processing time for the disintegration process can be kept within the above range, the miniaturization of the metal and resin layers contained in the difficult-to-process recycled paper is suppressed, reducing the amount of disintegrated fragments containing the metal layer in the resulting difficult-to-process recycled paper pulp. 【0083】In the manufacturing process for difficult-to-process recycled paper pulp, chemicals such as basic compounds and surfactants may be added as needed during the disintegration process. 【0084】 In the disintegration process, adjusting the pH of the processing solution to the above range by adding a basic compound is preferable because it allows for efficient disintegration, improves the yield of the resulting difficult-to-process waste paper pulp, and reduces the amount of disintegrated fragments containing a metal layer in the resulting difficult-to-process waste paper pulp. On the other hand, from the viewpoint of reducing the cost of wastewater treatment, 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 even more 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. 【0085】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, reducing the amount of disintegrated layer 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 reducing the cost of wastewater treatment, 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 even more preferably 0% by mass, relative to the difficult-to-treat waste paper. 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. 【0086】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 and 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. 【0087】 Furthermore, the solid content 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, from the viewpoint of improving transport efficiency. Alternatively, the solid content 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. 【0088】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 improving 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. 【0089】 (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 components derived from the metal layer and resin layer of the difficult-to-process waste paper. 【0090】 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. 【0091】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. From the viewpoint of efficiently removing foreign matter with a specific gravity greater than pulp fibers, heavy foreign matter cleaners and low-concentration ramole cleaners are recommended as cleaners used in the process of removing foreign matter by centrifugal separation. 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, reducing disintegrated fragments containing a metal layer in the resulting recycled paper pulp, and improving the quality of white cardboard. 【0092】The process of removing foreign matter by screening is performed after the process of removing foreign matter by centrifugal separation, for the purpose of removing foreign matter. As the screen used for screening, 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 by screening, reducing the amount of disintegrated fragments containing a metal layer in the resulting difficult-to-process recycled paper pulp, and improving the quality of white cardboard, it is preferable to perform fine screening after coarse screening. As the coarse screen, a round-hole screen or a slit screen is preferred, with a round-hole screen being 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 fewer fragments containing a metal layer are found in the resulting difficult-to-process recycled paper pulp. 【0093】 In the coarse screening process, the solid content concentration of the pulp slurry is preferably 1.0% to 5.0% by mass, more preferably 1.5% to 4.0% by mass, and even more preferably 2.0% to 3.0% by mass, from the viewpoint of efficiently removing foreign matter, reducing the amount of disintegrated fragments with a metal layer in the resulting recycled paper pulp, and improving the quality of white cardboard. In the fine screening process, the solid content concentration of the pulp slurry is preferably 0.2% to 5.0% by mass, more preferably 0.5% to 3.5% by mass, and even more preferably 0.8% to 3.0% by mass, from the viewpoint of efficiently removing foreign matter, reducing the amount of disintegrated fragments with a metal layer in the resulting difficult-to-process recycled paper pulp, and improving the quality of white cardboard. 【0094】 Furthermore, the dust removal process may be performed multiple times. In order to balance the improvement of the yield of the resulting recycled paper pulp with the reduction of disintegrated fragments containing a metal layer in the resulting difficult-to-process recycled paper, it is preferable to perform the process 1 to 3 times, and more preferably 1 time. 【0095】 (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. 【0096】 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. 【0097】 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. 【0098】(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 with a metal layer in the resulting difficult-to-process recycled paper pulp is reduced, the amount of such detached fragments in the resulting white cardboard is reduced, and the quality can be improved. The pulp washing process may, if necessary, alternately repeat washing the pulp in the pulp slurry and dewatering the pulp slurry. 【0099】 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 5.0% by mass or less, more preferably 1.0% by mass or more and 3.0% by mass or less, from the viewpoint of efficiently removing foreign matter from the pulp slurry and reducing the amount of disintegrated fragments having a metal layer in the resulting difficult-to-process recycled paper pulp. 【0100】 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. 【0101】<Papermaking Process> In the manufacturing method of this embodiment, the papermaking process is the process of making the white cardboard of this embodiment described above. That is, this papermaking process is the process of making the white cardboard of this embodiment described above using the difficult-to-process recycled paper pulp obtained by the difficult-to-process recycled paper pulp manufacturing process described above as the paper stock for the middle layer. The white cardboard of this embodiment has the surface layer, middle layer and back layer described above, and is obtained as a multilayer structure by combining three or more paper layers that use pulp as the main raw material. It can be manufactured using a known multilayer papermaking machine, for example, a papermaking machine that combines a long-wire former and a cylinder former. It is also possible to strengthen the strength between each layer by spraying or coating an adhesive such as starch between each layer when they are combined. 【0102】 (Coating Layer Formation Process) In addition, if necessary, a pigment coating layer can be formed on the surface of the white cardboard to make coated white cardboard. When forming a pigment coating layer, a pigment coating liquid is applied to the surface of the surface and dried to form the pigment coating layer. The pigment coating liquid is the same as that described for the pigment coating layer of the white cardboard above, and in the manner described above, the pigment coating liquid is applied to the surface of the surface, and the coating layer formed by the application is dried to form a pigment coating layer on the surface of the surface. 【0103】 As described above, the pigment coating liquid may be applied in multiple layers. When the pigment coating liquid is applied in multiple layers, for example, in two layers, the coating liquids for the first layer (undercoat) and the second layer (topcoat) may be the same or different. The pigment coating liquid may be applied directly to the surface, or it may be applied via a coating film made of size-press liquid applied to the surface. In addition, the surface may be smoothed using a pre-calender before applying the pigment coating liquid. 【0104】The coating device may be on-machine or off-machine, but it is preferable to use an on-machine device that utilizes a known coater part attached to the paper machine. Suitable coating devices for the coater part include, for example, blade coaters, air knife coaters, roll coaters, reverse roll coaters, bar coaters, curtain coaters, slot die coaters, gravure coaters, champlex coaters, brush coaters, slide bead coaters, two-roll or metering blade type size press coaters, bill blade coaters, short dwell coaters, gate roll coaters, and calender nip coaters. Among these, bar coaters, rod metering coaters, and curtain coaters are preferred because they provide a consistent coating amount, thus suppressing uneven coating. The applied coating layer is then dried using a known drying device to form the pigment coating layer. The paper machine may have multiple coater parts. In this case, the pigment coating layer can be applied in multiple stages. 【0105】 A calendering section may be provided after the coater section, if necessary. By providing a calendering section after the coater section, the pigment coating layer is smoothed. Known calendering devices can be used as appropriate for the calendering section after the coater section, such as supercalenders, gloss calenders, soft nip calenders, thermal calenders, and shoe calenders. These may be used in combination. Among these, a soft nip calender equipped with metal rolls and elastic rolls is preferred because it can smooth the pigment coating layer while maintaining the paper thickness. The calendering section after the coater section may be on-machine or off-machine. 【0106】 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%RH unless otherwise specified. 【0107】 <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, washing water volume 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 (KNP-10 type manufactured by Nippon Sharyo Ltd.), and disintegration treatment was carried out under the conditions of a difficult-to-process waste paper concentration of 19% by mass, a processing time of 30 minutes, a 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 (FC100 type manufactured by Aikawa Iron Works Co., Ltd.), and further processed with a coarse sorting screen (MaxFlow-1, Model 1000 type, 1.2 mm round hole manufactured by Aikawa Iron Works Co., Ltd.). The pulp slurry after the coarse screening process 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 the fine screening process 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. 【0108】[Papermaking Process] [Preparation of the top layer (1st layer) pulp] As the raw material pulp, a mixture of 80% by mass of deinked pulp obtained by deinking Kent waste paper and 20% by mass of bleached hardwood kraft pulp was used. To 100 parts by mass of this raw material pulp, 0.2 parts by mass of a sizing agent (product name: Sizing Pine 836, manufactured by Arakawa Chemical Industries, Ltd.), 1.0 part by mass of aluminum sulfate, 0.4 parts by mass of a dry paper strength enhancer (product name: Polystron 1276, manufactured by Arakawa Chemical Industries, Ltd.), and 8.0 parts by mass of light calcium carbonate (product name: Tamapearl TP121-6S, manufactured by Okutama Kogyo Co., Ltd.) was added to prepare the top layer (1st layer) pulp. [Preparation of the middle layer (2nd layer, bottom layer) pulp] As the raw material pulp, deinked pulp obtained by deinking mixed paper waste paper was used. To 100 parts by mass of this pulp, 1.5 parts by mass of aluminum sulfate, 0.1 parts by mass of a dry paper strength enhancer (product name: Polystron 1276, manufactured by Arakawa Chemical Industries, Ltd.), and 3.0 parts by mass of light calcium carbonate (product name: Tamapearl TP121-6S, manufactured by Okutama Kogyo Co., Ltd.) were added to prepare the lower layer (second layer) of the paper stock. [Preparation of the middle layer (third layer) of the paper stock] As the raw material pulp, a pulp was used which was a mixture of magazine waste paper pulp adjusted to 300 mL of CSF and the aforementioned difficult-to-treat waste paper pulp in a mass ratio of 95:5 (magazine waste paper pulp: difficult-to-treat waste paper pulp). To 100 parts by mass of this pulp, 1.5 parts by mass of aluminum sulfate was added to prepare the middle layer (third layer) of the paper stock. [Preparation of the backing (fourth layer) pulp] As raw material pulp, a mixture of 50% by mass of recycled newspaper, 40% by mass of recycled magazines, and 10% by mass of pulp derived from shredded office waste paper was used. To 100 parts by mass of this pulp, 1.0 part by mass of aluminum sulfate and 0.1 parts by mass of a dry paper strength enhancer (product name: Polystron 1276, manufactured by Arakawa Chemical Industries, Ltd.) were added to prepare the backing (fourth layer) pulp. 【0109】 [Manufacturing of White Cardboard] Using the prepared pulp for each layer, white cardboard having a surface layer (1st layer), middle layers (2nd and 3rd layers), and back layer (4th layer) was obtained using a multi-layer paper machine. Here, the basis weight of the surface layer (2nd layer) was 30 g / m². ２ The basis weight of the middle layer (second layer, lower surface layer) is 35 g / m². ２ The basis weight of the middle layer (third layer) is 300 g / m². ２The basis weight of the back layer (4th layer) is 25 g / m². ２ The resulting material is formed into four layers, then machine-calendered to produce a sheet with a width of 3 m, a thickness of 450 μm, and a basis weight of 390 g / m². ２ White cardboard was obtained. The proportion of difficult-to-treat recycled paper pulp in the middle layer (2nd and 3rd layers) of the paper stock at this time was {(0 × 35 / 335) + (0.05 × 300 / 335)} × 100 = 4.5 mass%, and the amount of difficult-to-treat recycled paper pulp in the obtained white cardboard was (0.05 × 300 (set value)) / 390 (measured value) × 100 = 3.9 mass%. 【0110】 [Pigment Coating Layer Formation Process] [Preparation of Coating Solution for Pigment Coating Layer (1st Layer)] 100 parts by mass of heavy calcium carbonate B (product name: FMT90, manufactured by Fimatec Co., Ltd., average particle size 0.78 μm, D75 / D25 = 4.4) was used as the pigment, and a pigment dispersion with a solid content of 70% by mass was prepared using a Coreless disperser. Next, 3 parts by mass of oxidized starch (product name: Oji Ace Y, manufactured by Oji Corn Starch Co., Ltd.) and 14 parts by mass of styrene-butadiene copolymer latex F (product name: BA025, manufactured by Asahi Kasei Chemicals Corporation, glass transition temperature -12°C, average particle size 80 nm) were added to this pigment dispersion on a solid content basis per 100 parts by mass of pigment, and a coating solution for the pigment coating layer (1st layer) with a solid content of 50% by mass and a B-type viscosity of 50 mPa·s was obtained. 【0111】[Preparation of coating solution for the second pigment coating layer] Five parts by mass of heavy calcium carbonate A (product name: FMT-OP, manufactured by Fimatec Co., Ltd., average particle size 0.6 μm, D75 / D25 = 3.0) were used as pigments, 90 parts by mass of kaolin A (product name: Ultra White 90, manufactured by BASF, average particle size 0.32 μm) and 5 parts by mass of titanium dioxide (product name: KA-100, manufactured by Cosmo Chemical Co., Ltd., Korea) were used, and 0.1 parts by mass of sodium polyacrylate was added per 100 parts by mass of pigment as a dispersant. A pigment dispersion with a solid content of 68% by mass was prepared using a Koreth disperser. Next, to this pigment dispersion, 2 parts by mass of oxidized starch (product name: Ace Y, manufactured by Oji Corn Starch Co., Ltd.) and 16 parts by mass of styrene-butadiene copolymer latex B (product name: B1840, manufactured by Asahi Kasei Chemicals Corporation, average particle size 95 nm) were added on a solid content basis per 100 parts by mass of pigment, to obtain a coating solution for the pigment coating layer (second layer) with a solid content concentration of 42% by mass and a B-type viscosity of 40 mPa·s. 【0112】 [Preparation of Pigment Coating Layer] Using the white cardboard obtained above as a base paper for coated white cardboard, the surface of each of its front and back layers is coated with the pigment coating liquid (first layer) using a bar coater at a dry mass of 10 g / m² per side. ２ The surface was coated and dried to form a primer layer. On the primer layer, the aforementioned pigment coating liquid (second layer) was applied using a bar coater at a dry mass of 10 g / m² per side. ２ The paper was coated and dried in such a manner to form a topcoat layer. The base paper on which the pigment coating layers (1st and 2nd layers) were formed was subjected to a soft calendering process with a metal roll surface temperature of 200°C and 2 nips to obtain white cardboard (coated white cardboard) having pigment coating layers on the surface and back. 【0113】 <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. White cardboard was obtained in the same manner as in Example 1. 【0114】<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. White cardboard was obtained in the same manner as in Example 1. 【0115】 <Example 4> White cardboard was obtained in the same manner as in Example 1, except that the disintegration process of the difficult-to-process recycled paper pulp was performed for a processing time of 10 minutes. 【0116】 <Example 5> White cardboard was obtained in the same manner as in Example 1, except that the disintegration process of difficult-to-process recycled paper pulp was performed for a processing time of 60 minutes. 【0117】 <Example 6> White cardboard was obtained in the same manner as in Example 1, except that the disintegration process of the difficult-to-process recycled paper pulp was carried out at a pH of 10. 【0118】 <Example 7> White cardboard was obtained in the same manner as in Example 6, except that the disintegration process of difficult-to-process recycled paper pulp was performed for a processing time of 10 minutes. 【0119】 <Example 8> White cardboard was obtained in the same manner as in Example 1, except that the disintegration process of the difficult-to-process recycled paper pulp was carried out at a processing temperature of 15°C. 【0120】 <Example 9> White cardboard was obtained in the same manner as in Example 1, except that the disintegration process of the difficult-to-process recycled paper pulp was carried out at a processing temperature of 65°C. 【0121】 <Example 10> In the production of difficult-to-process recycled paper pulp, white cardboard 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 203 cm². ２ That was the case. 【0122】 <Example 11> White cardboard was obtained in the same manner as in Example 1, except that in the crushing and waste paper washing processes for the production of difficult-to-process waste paper pulp, only crushing was performed and waste paper washing was omitted. 【0123】<Example 12> In the crushing and waste paper washing processes for the production of difficult-to-process waste paper pulp, white cardboard was obtained in the same manner as in Example 1, except that crushing was omitted and only waste paper washing was performed. In Example 12, the average size of the difficult-to-process waste paper used in the disintegration process was 205 cm². ２ That was the case. 【0124】 <Example 13> In the crushing and waste paper washing processes for the production of difficult-to-process waste paper pulp, white cardboard was obtained in the same manner as in Example 1, except that the number of hooks on the crushing and washing machine was changed to 12. In Example 13, the average size of the difficult-to-process waste paper used for disintegration was 36 cm. ２ That was the case. 【0125】 <Example 14> In the crushing and waste paper washing processes for the production of difficult-to-process waste paper pulp, white cardboard was obtained in the same manner as in Example 1, except that the number of hooks on the crushing and washing machine was changed to 3. In Example 14, the average size of the difficult-to-process waste paper used for disintegration was 113 cm². ２ That was the case. 【0126】 <Example 15> White cardboard was obtained in the same manner as in Example 1, except that the amount of washing water was changed to 27 L / kg in the crushing and washing processes of the difficult-to-process recycled paper pulp. 【0127】<Example 16> In the preparation of the sub-layer (second layer) pulp for the production of white cardboard, the sub-layer (second layer) pulp was prepared in the same manner as in Example 1, except that the raw pulp used was a mixture of pulp derived from mixed recycled paper and the aforementioned difficult-to-treat recycled paper pulp in a mass ratio of 95:5 (pulp derived from mixed recycled paper: difficult-to-treat recycled paper pulp). In addition, in the preparation of the middle layer (third layer) pulp for the production of white cardboard, the middle layer (third layer) pulp was prepared in the same manner as in Example 1, except that the difficult-to-treat recycled paper pulp was not used as the raw pulp. White cardboard was obtained in the same manner as in Example 1, except that the obtained sub-layer (second layer) pulp and middle layer (third layer) pulp were used to form the sub-layer and middle layer. The proportion of difficult-to-treat recycled paper pulp in the middle layer of the paper stock at this time was {(0.05 × 35 / 335) + (0 × 300 / 335)} × 100 = 0.5 mass%, and the amount of difficult-to-treat recycled paper pulp in the resulting white cardboard was (0.05 × 35 (set value)) / 390 (measured value) × 100 = 0.45 mass%. 【0128】 <Example 17> In the preparation of the sub-layer (second layer) pulp for the production of white cardboard, a pulp mixture of pulp derived from miscellaneous recycled paper and the aforementioned difficult-to-treat recycled paper pulp in a mass ratio of 97.5:2.5 (pulp derived from miscellaneous recycled paper: difficult-to-treat recycled paper pulp) was used as the raw material pulp, and in the preparation of the middle layer (third layer) pulp, a pulp mixture of magazine recycled paper pulp adjusted to 300 mL CSF and the aforementioned difficult-to-treat recycled paper pulp in a mass ratio of 97.5:2.5 (magazine recycled paper pulp: difficult-to-treat recycled paper pulp) was used as the raw material pulp, except that the sub-layer (second layer) pulp and middle layer (third layer) pulp were prepared in the same manner as in Example 1. White cardboard was obtained in the same manner as in Example 1, except that the obtained sub-layer (second layer) pulp and middle layer (third layer) pulp were used to form the sub-layer and middle layer. The proportion of difficult-to-treat recycled paper pulp in the middle layer of the paper stock at this time was {(0.025 × 35 / 335) + (0.025 × 300 / 335)} × 100 = 2.5% by mass, and the amount of difficult-to-treat recycled paper pulp in the resulting white cardboard was {(0.025 × 35 (set value)) + (0.025 × 300 (set value))} / 388 (measured value) = 2.2% by mass. 【0129】<Example 18> In the production of white cardboard, white cardboard was obtained in the same manner as in Example 1, except that a pulp was prepared by blending magazine waste paper pulp adjusted to 300 mL of CSF and the aforementioned difficult-to-treat waste paper pulp in a mass ratio of 85:15 (magazine waste paper pulp: difficult-to-treat waste paper pulp) as the raw material pulp. The blending ratio of difficult-to-treat waste paper pulp in the middle layer pulp at this time was {(0 × 35 / 335) + (0.15 × 300 / 335)} × 100 = 13.4 mass%, and the amount of difficult-to-treat waste paper pulp in the obtained white cardboard was (0.15 × 300 (set value)) / 390 (measured value) = 11.5 mass%. 【0130】 <Example 19> In the production of white cardboard, white cardboard was obtained in the same manner as in Example 1, except that a pulp was prepared by blending magazine waste paper pulp adjusted to 300 mL of CSF and the aforementioned difficult-to-treat waste paper pulp in a mass ratio of 70:30 (magazine waste paper pulp: difficult-to-treat waste paper pulp) as the raw material pulp for the middle layer (third layer) of the paper stock. The blending ratio of difficult-to-treat waste paper pulp in the middle layer paper stock at this time was {(0 × 35 / 335) + (0.3 × 300 / 335)} × 100 = 26.9 mass%, and the blending amount of difficult-to-treat waste paper pulp in the obtained white cardboard was (0.3 × 300 (set value)) / 390 (measured value) = 23.1 mass%. 【0131】 <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. White cardboard was obtained in the same manner as in Example 1. 【0132】 <Comparative Example 2> White cardboard was obtained in the same manner as in Example 1, except that the disintegration process of the difficult-to-process recycled paper pulp was performed for a processing time of 3 minutes. 【0133】 <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. ２White cardboard 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. 【0134】 <Comparative Example 4> In the production of white cardboard, white cardboard was obtained in the same manner as in Example 1, except that only pulp containing the aforementioned difficult-to-treat recycled paper pulp was used as the raw material pulp for the middle layer (third layer) of the paper stock. 【0135】 【0136】 <Measurement and Evaluation Method> (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 finest waste paper fragments were 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. 【0137】(Basis weight of white cardboard) The white cardboard 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 white cardboard was measured in accordance with JIS P 8124:2011. As a pretreatment, the white cardboard was soaked in 40°C hot water for 6 hours, then carefully peeled off one layer at a time by hand, and divided into four parts: surface coating layer / surface layer, surface sub-layer, middle layer, and back layer / back coating layer. These were then dried in a dryer set to 105°C for 30 minutes. After that, the white cardboard was conditioned for 24 hours under the humidity-controlled environment specified in JIS P 8111:1998, and the basis weight of each of the four divided white cardboard parts was measured in accordance with JIS P 8124:2011. Next, the surface coating layer / surface layer and back coating layer / back coating layer samples were immersed in a 1 mol / L copper ethylenediamine solution (manufactured by Kanto Chemical Co., Ltd.) for 3 hours to dissolve the surface and back coating layers. The remaining surface and back coating layers were washed with water and dried in a dryer set to 105°C for 30 minutes. After that, the samples were conditioned for 24 hours under the humidity-controlled environment specified in JIS P 8111:1998. Then, in accordance with JIS P 8124:2011, the basis weight of the conditioned surface coating layer and back coating layer was measured, and then subtracted from the previously measured basis weights of the surface coating layer / surface layer and back coating layer / back coating layer to determine the basis weight of each layer of coated white cardboard. 【0138】 (0.05 mm with a metal layer) ２ (Total area, maximum area, and number of the above-mentioned disintegrated pieces) Only the middle layer (including the subsurface layer) was peeled off from the white cardboard obtained in the examples and comparative examples, and the middle layer was disintegrated according to JIS P 8220-1:2012 to obtain a slurry of the pulp from which the middle layer had been disintegrated (concentration: 3% by mass). Using a round sheet preparation device according to JIS P 8222:2015, a round, hand-formed sheet (basis weight 60 g / m²) was made using the slurry of the pulp from which the middle layer had been disintegrated. ２ , area 0.02m ２ 10 sheets (105 μm thick) (basis weight 60 g / m²) ２ ) was produced. By visual inspection, referring to the measurement chart of foreign matter created by the National Printing Bureau, it was determined that each handmade sheet had a 0.05 mm metal layer on both sides. ２The above dissociated fragments were examined, and 0.05 mm of material was found in each handmade sheet. ２ The total area, maximum area, and number of detached pieces having the above-mentioned metal layer were measured. Based on the measurement results for 10 handmade sheets, the 0.05 mm white cardboard 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. 【0139】 (Metal detection) The white cardboard obtained in the examples and comparative examples was cut to the size of a 25cm x 40cm square meter sheet, and 10 sheets were stacked together to form an evaluation sample (total area 1m²). ２ Ten samples were prepared, each measuring approximately 25 cm in length, 40 cm in width, and 4.5 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 φ0.5 mm, SUS φ0.8 mm, conveying speed 20 m / min, conveying belt width 500 mm, opening height 270 mm) and passed through. The white cardboard samples of the examples and comparative examples were evaluated to see if they were detected by the metal detector according to the following criteria. If the result is A to C (metal detector value is between 0 and 100), the white cardboard is judged to have no metallic reaction and will not interfere with foreign object detection by the metal detector when used as packaging material. On the other hand, if the result is D (metal detector value is 100 or higher), it is judged to have a metallic reaction and may interfere with foreign object detection by the metal detector when used as packaging material. [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 【0140】(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 90% or more B: Fiber yield of 80% or more but less than 90% C: Fiber yield of 70% or more but less than 80% D: Fiber yield of less than 70% 【0141】 【0142】 Table 2 shows that Examples 1 to 19 are obtained from pulp with the middle layer disintegrated, with 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 dissociated fragments is 7.0 mm². ２ / 0.02m ２ The following conditions apply, and the maximum area is 2.5 mm². ２ As a result of the following, even when the material contains difficult-to-treat recycled paper pulp made from difficult-to-treat recycled paper having metal and resin layers, the quality of the white cardboard was improved because the disintegrated fragments having metal and resin layers were fine and their content was low. Furthermore, it was confirmed that this white cardboard was less likely to trigger a metal detector. On the other hand, Comparative Examples 1, 3, and 4 had a 0.05 mm metal layer. ２ The total area of ​​the above dissociated fragments is 7.0 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 2.5 mm². ２ Because it exceeded the limit, the metal detector reacted.

Claims

1. White cardboard having at least a surface layer, a middle layer, and a back layer, wherein the middle layer contains difficult-to-treat recycled paper pulp made from difficult-to-treat recycled paper, the difficult-to-treat recycled paper having at least a paper base layer, a metal layer, and a resin layer, and the basis weight obtained from the pulp after 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 dissociated fragments is 7.0 mm². ２ / 0.02m ２ The following conditions apply, and the maximum area is 2.5 mm². ２ The following is white cardboard.

2. The 0.05 mm of the handmade sheet having the metal layer ２ The number of dissociated fragments is 40 fragments / 0.02 m. ２ The white cardboard according to claim 1, which is as follows:

3. The white cardboard according to claim 1, wherein the amount of the difficult-to-process recycled paper pulp in the raw material pulp constituting the middle layer is 0.2% by mass or more and 35% by mass or less.

4. The white cardboard according to claim 1, wherein the middle layer consists of two or more layers.

5. The white cardboard 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.

6. The white cardboard according to claim 1, wherein the white cardboard has a pigment coating layer on at least one surface.

7. 0.05 mm having the aforementioned metal layer ２ The white cardboard according to claim 1, wherein the above-mentioned disintegrated fragments further include a resin layer.

8. The white cardboard according to claim 1, wherein the difficult-to-process waste paper is aseptic container waste paper.

9. The white cardboard 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.

10. A paper container made using white cardboard as described in any one of claims 1 to 9.

11. A method for manufacturing white cardboard according to any one of claims 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 white cardboard using a paper stock containing the 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.

12. The average size of the waste paper to be subjected to the dissociation step is 600 cm ２ or less, the method for producing white cardboard according to claim 11.

13. The method for manufacturing white cardboard according to claim 11, wherein the processing temperature in the disintegration step is 5°C or more and 80°C or less.

14. The method for producing white cardboard according to claim 11, wherein the pH of the processing solution in the disintegration step is 5 or more and 11 or less.

15. A method for manufacturing white cardboard according to claim 11, comprising at least one of the following steps prior to the disintegration step: 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.

16. 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 white cardboard according to claim 15 is as follows:

17. The method for producing white cardboard according to claim 15, wherein the method includes the waste paper washing step prior to 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.

18. The method for manufacturing white cardboard according to claim 11, 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.

19. The method for producing white cardboard according to claim 11, further comprising a dewatering step after the pulp washing step, wherein the pulp slurry is dewatered to obtain pulp.

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