Bleached pulp manufacturing method

Abstract

The present invention provides a means for producing beached pulp that has an excellent degree of whiteness.　A method for producing beached pulp, said method comprising: an alkaline oxygen bleaching step wherein unbleached pulp is subjected to alkaline oxygen bleaching; a permonosulfuric acid bleaching step wherein a treatment with permonosulfuric acid bleaching is performed; and a first chlorine dioxide bleaching step wherein a treatment with chlorine dioxide is performed. With respect to this method for producing beached pulp, the ratio of the addition rate (% by mass) of the chlorine dioxide relative to the absolute dry mass of the unbleached pulp to the addition rate (% by mass) of the permonosulfuric acid relative to the absolute dry mass of the unbleached pulp, namely (addition rate of chlorine dioxide) / (addition rate of permonosulfuric acid) is from 0.25 to 4.0.

Description

[Technical field]

[0001] The present invention relates to a method for producing bleached pulp. [Background technology]

[0002] Due to the recent increase in environmental awareness, pulp bleaching is generally carried out using elemental chlorine free (ECF) bleaching, which does not use chlorine itself or hypochlorite.

[0003] For example, Patent Document 1 describes an invention relating to a method for producing bleached pulp, which comprises subjecting unbleached pulp obtained by cooking lignocellulosic materials to alkaline oxygen bleaching, then treating the pulp with an inorganic peroxyacid and / or its salt without adding a chelating agent, washing the pulp, and then subjecting the pulp to a multi-stage bleaching treatment starting with ozone treatment.

[0004] Patent Document 1 describes that the use of chlorine dioxide (ClO2) has become mainstream in ECF bleaching, and that light ECF bleaching, which reduces the amount of chlorine dioxide used as much as possible, is attracting attention.

[0005] According to the invention described in Patent Document 1, by carrying out ozone bleaching after performing an acid washing with an inorganic peroxyacid as a pretreatment, it is possible to improve the reaction selectivity of the ozone bleaching and increase the reaction efficiency, thereby making it possible to reduce the amount of chlorine dioxide used in the latter stage.

[0006] Furthermore, Patent Document 2 describes an invention relating to a method for producing bleached pulp, which comprises subjecting unbleached pulp obtained by cooking lignocellulosic materials to alkaline oxygen bleaching, then treating it with an inorganic peroxyacid and / or its salt, and then further subjecting it to a multi-stage bleaching treatment starting with a chlorine dioxide treatment.

[0007] According to the invention described in Patent Document 2, when pulp bleached with alkaline oxygen is bleached with chlorine dioxide, pretreatment with an inorganic peroxyacid is described to promote delignification and removal of hexenuronic acid in the chlorine dioxide stage. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2007-308815 A [Patent Document 2] JP 2007-308824 A Summary of the Invention [Problem to be solved by the invention]

[0009] Conventionally, in ECF bleaching, pulp has been bleached by combining various bleaching methods with chlorine dioxide as described above, but a method for suitably producing bleached pulp with higher brightness is required.

[0010] Therefore, the present invention provides a means for producing bleached pulp with excellent brightness. [Means for solving the problem]

[0011] The present inventors have conducted extensive research to solve the above problems. As a result, they have found that the above problems can be solved by combining a bleaching step using monopersulfuric acid and a bleaching step using chlorine dioxide, and setting the ratio of the addition rate of monopersulfuric acid and the addition rate of chlorine dioxide in each step within a predetermined range, and have completed the present invention. That is, the present invention is, for example, as follows.

[0012] [1] An alkaline oxygen bleaching process for alkaline oxygen bleaching unbleached pulp; a monopersulfuric acid bleaching step of treating with monopersulfuric acid; a first chlorine dioxide bleaching step of treating with chlorine dioxide; Including, A method for producing bleached pulp, wherein the ratio of the addition rate (mass%) of monopersulfuric acid to the bone dry mass of the unbleached pulp to the addition rate (mass%) of chlorine dioxide to the bone dry mass of the unbleached pulp (chlorine dioxide addition rate / monopersulfuric acid addition rate) is 0.25 to 4.0. [2] An alkaline oxygen bleaching process for alkaline oxygen bleaching unbleached pulp; a monopersulfuric acid bleaching step of treating with monopersulfuric acid; a first chlorine dioxide bleaching step of treating with chlorine dioxide; Including, the ratio of the addition rate of monopersulfuric acid (mass%) to the bone dry mass of the unbleached pulp to the addition rate of chlorine dioxide (mass%) to the bone dry mass of the unbleached pulp (chlorine dioxide addition rate / monopersulfuric acid addition rate) is 0.25 to 2.0; A method for producing bleached pulp, wherein the treatment temperature in the peroxymonosulfate bleaching step is 70 to 98°C. [3] The method according to [2] above, further comprising an alkaline hydrogen peroxide bleaching step of treating with alkaline hydrogen peroxide. [4] The method according to [2] or [3] above, further comprising an ozone bleaching step of treating with ozone. [5] An alkaline oxygen bleaching process for alkaline oxygen bleaching the unbleached pulp; a monopersulfuric acid bleaching step of treating with monopersulfuric acid; a first chlorine dioxide bleaching step of treating with chlorine dioxide; an alkaline hydrogen peroxide bleaching step of treating with alkaline hydrogen peroxide; a second chlorine dioxide bleaching step of treating with chlorine dioxide; in this order, a ratio (% by mass) of monopersulfuric acid to the bone dry mass of the unbleached pulp and a ratio (% by mass) of chlorine dioxide to the bone dry mass of the unbleached pulp in the first chlorine dioxide bleaching step (chlorine dioxide addition rate / monopersulfuric acid addition rate) is 0.25 to 2.0; A method for producing bleached pulp, wherein the treatment temperature in the peroxymonosulfate bleaching step is 70 to 98°C. [6] An alkaline oxygen bleaching process for alkaline oxygen bleaching the unbleached pulp; a monopersulfuric acid bleaching step of treating with monopersulfuric acid; a first chlorine dioxide bleaching step of treating with chlorine dioxide; Including, The addition rate (mass%) of monopersulfuric acid to the bone dry mass of the unbleached pulp is 0.30 to 1.75 mass%, A method for producing bleached pulp, wherein the ratio of the addition rate (mass%) of monopersulfuric acid to the bone dry mass of the unbleached pulp to the addition rate (mass%) of chlorine dioxide to the bone dry mass of the unbleached pulp (chlorine dioxide addition rate / monopersulfuric acid addition rate) is 0.5 to 2.0. [7] The method according to [6] above, further comprising an alkaline hydrogen peroxide bleaching step of treating with alkaline hydrogen peroxide. [8] The method according to [6] or [7] above, further comprising an ozone bleaching step of treating with ozone. [9] The method according to any one of [6] to [8] above, wherein the treatment temperature in the monopersulfuric acid bleaching step is 40 to 98°C.

[10] The manufacturing method according to any one of the above [1] to [9], wherein the TOC increase rate (TOC1 / TOC2×100) of the total organic carbon (TOC2) of the wastewater obtained in the peroxymonosulfuric acid bleaching step and the first chlorine dioxide bleaching step in a reference method in which a sulfuric acid bleaching step is performed in place of the peroxymonosulfuric acid bleaching step to treat the total organic carbon (TOC1) of the wastewater obtained in the first chlorine dioxide bleaching step with sulfuric acid is more than 100%.

[11] The method according to

[10] above, further comprising reusing at least a portion of the wastewater obtained in the peroxymonosulfuric acid bleaching step and the wastewater obtained in the first chlorine dioxide bleaching step in the peroxymonosulfuric acid bleaching step.

[12] The method according to any one of the above [1] to

[11] , wherein the treatment time of the monopersulfuric acid bleaching step is 2 to 200 minutes.

[13] The method according to any one of the above [1] to

[12] , wherein the treatment pH in the monopersulfuric acid bleaching step is 2 to 5.

[0013] Furthermore, the present invention may include, for example, the following aspects.

[0014] [1'] an alkaline oxygen bleaching step of alkaline oxygen bleaching unbleached pulp; a monopersulfuric acid bleaching step of treating with monopersulfuric acid; a first chlorine dioxide bleaching step of treating with chlorine dioxide; Including, A method for producing bleached pulp, wherein the ratio of the addition rate (mass%) of monopersulfuric acid to the bone dry mass of the unbleached pulp to the addition rate (mass%) of chlorine dioxide to the bone dry mass of the unbleached pulp (chlorine dioxide addition rate / monopersulfuric acid addition rate) is 0.25 to 4.0. [2'] The manufacturing method according to the above [1'], in which the TOC increase rate (TOC1 / TOC2 x 100) of the total organic carbon (TOC2) of the wastewater obtained in the peroxymonosulfuric acid bleaching step and the first chlorine dioxide bleaching step in a reference method in which a sulfuric acid bleaching step is performed in place of the peroxymonosulfuric acid bleaching step to treat the wastewater with sulfuric acid is more than 100%. [3'] The method according to [2'] above, further comprising reusing at least a part of the wastewater obtained in the peroxymonosulfuric acid bleaching step and the wastewater obtained in the first chlorine dioxide bleaching step in the peroxymonosulfuric acid bleaching step. [4'] The method according to any one of the above [1'] to [3'], wherein the treatment time in the monopersulfuric acid bleaching step is 2 to 200 minutes. [5'] The method according to any one of the above [1'] to [4'], wherein the treatment temperature in the monopersulfuric acid bleaching step is 40 to 98°C. [6'] The method according to any one of the above [1'] to [5'], wherein the treatment pH in the monopersulfuric acid bleaching step is 2 to 5. [7'] The method according to any one of the above [1'] to [6'], further comprising an alkaline hydrogen peroxide bleaching step of treating with alkaline hydrogen peroxide. [8'] The method according to any one of the above [1'] to [7'], further comprising an ozone bleaching step of treating with ozone. Effect of the Invention

[0015] The present invention provides a means for producing bleached pulp with excellent brightness. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Hereinafter, an embodiment of the present invention will be described in detail.

[0017] <Bleached pulp manufacturing method> The method for producing bleached pulp of the present invention includes an alkaline oxygen bleaching step of alkaline oxygen bleaching unbleached pulp, a monopersulfuric acid bleaching step of treating unbleached pulp with monopersulfuric acid, and a first chlorine dioxide bleaching step of treating unbleached pulp with chlorine dioxide, characterized in that the ratio of the monopersulfuric acid addition rate (mass%) to the bone dry mass of the unbleached pulp and the chlorine dioxide addition rate (mass%) to the bone dry mass of the unbleached pulp (chlorine dioxide addition rate / monopersulfuric acid addition rate) is 0.25 to 4.0.

[0018] The method for producing bleached pulp may further include an unbleached pulp preparation step, an alkaline hydrogen peroxide bleaching step in which the pulp is treated with alkaline hydrogen peroxide, and an ozone bleaching step in which the pulp is treated with ozone. In this case, each step may be repeated two or more times. For example, when the chlorine dioxide bleaching step is performed twice, the method may include a first chlorine dioxide bleaching step and a second chlorine dioxide bleaching step.

[0019] In one embodiment, the method for producing bleached pulp includes an unbleached pulp preparation step, an alkaline oxygen bleaching step (O step), a monopersulfuric acid bleaching step (MPS step), an ozone bleaching step (Z step), a chlorine dioxide bleaching step (D step), and an alkaline hydrogen peroxide bleaching step (Ep step) in this order. Each step will be described below.

[0020] [Unbleached pulp preparation process] The unbleached pulp preparation process is a process for preparing unbleached pulp primarily from lignocellulosic materials.

[0021] (Lignocellulosic materials) Lignocellulosic materials include wood, non-wood, waste paper, and the like.

[0022] The wood is not particularly limited, but examples thereof include broad-leaved trees such as eucalyptus, acacia, poplar, beech, maple, chestnut, paulownia, birch, and elm; and coniferous trees such as cedar, pine, fir, cypress, and hemlock.

[0023] The non-wood material is not particularly limited, but examples thereof include straw, reed, mulberry, kenaf, orange, and mandarin orange.

[0024] Of these, the lignocellulosic material is preferably wood or waste paper, more preferably wood, and even more preferably hardwood.

[0025] The above-mentioned lignocellulosic materials may be used alone or in combination of two or more kinds.

[0026] (Preparation of Unbleached Pulp) The unbleached pulp is prepared by separating (digesting) at least a portion of lignin from a lignocellulosic material that contains cellulose, which forms the backbone of the lignocellulosic material, hemicellulose, which is a cross-linking component, and lignin, which is an adhesive component. That is, in one embodiment, the unbleached pulp preparation process includes a process of digesting the lignocellulosic material. The unbleached pulp obtained after digestion is appropriately washed.

[0027] The cooking method is not particularly limited, but examples thereof include soda cooking, kraft cooking, and sulfite cooking.

[0028] The soda cooking is a cooking method that uses sodium hydroxide and sodium carbonate.

[0029] The kraft cooking is a cooking method using sodium hydroxide and sodium sulfide and / or sodium polysulfide. Unbleached pulp obtained by kraft cooking is generally called kraft pulp (KP).

[0030] The sulfite cooking is a cooking method using sulfurous acid or its salts. Examples of the sulfite cooking include alkaline sulfite cooking using Na2SO3; neutral sulfite cooking using Na2SO3 and NaHSO3; slightly acidic sulfite cooking using NaHSO3 or Mg(HSO3)2; acidic sulfite cooking using NaHSO3 and SO2, Mg(HSO3)2 and SO2, or Ca(HSO3)2 and SO2; sodium polysulfide (Na2S x The unbleached pulp obtained by sulfite cooking is generally called sulfite pulp (SP).

[0031] In addition, a cooking aid may be used in the cooking. The cooking aid is not particularly limited, and examples thereof include benzoquinone, naphthoquinone, anthraquinone, anthrone, and phenanthroquinone. Specific examples of the cooking aid include anthraquinone (AQ) and 1,4-dihydro-9,10-dihydroxyanthracene (DDA). These cooking aids may be used alone or in combination of two or more.

[0032] Among these, the cooking method is preferably kraft cooking or sulfite cooking, and more preferably kraft cooking or polysulfite cooking, from the viewpoints of pulp quality, energy efficiency, etc. The above cooking methods may be carried out alone or in combination of two or more.

[0033] In preparing the unbleached pulp, hydrolysis may be performed in order to further remove at least a portion of the hemicellulose of the lignocellulosic material. The hydrolysis is usually performed before cooking. That is, in one embodiment, the unbleached pulp preparation process includes a step of hydrolyzing the lignocellulosic material to obtain a hydrolysate and a step of cooking the hydrolysate. The hydrolysate and / or the unbleached pulp obtained after cooking are appropriately washed.

[0034] The hydrolysis method is not particularly limited, but may be a method of adding water to the lignocellulosic material and heating it. This causes the acetyl group to be released from the hemicellulose, producing acetic acid, which makes the liquid acidic, and the acid hydrolysis proceeds. Note that an acid may be added to promote the hydrolysis.

[0035] (Unbleached pulp) The whiteness of the unbleached pulp is preferably 30 to 60%, and more preferably 40 to 55%. When the whiteness of the unbleached pulp is 30% or more, it is preferable because the cost of the bleaching treatment can be reduced. When the whiteness of the unbleached pulp is 60% or less, it is preferable because the cost of cooking can be reduced. In this specification, the whiteness of the unbleached pulp is measured by the following method. That is, according to ISO3688:1977, the whiteness of the unbleached pulp is measured by the following method. 2 Using the obtained sheets, the brightness (%) of the pulp is measured according to JISP8148:2001.

[0036] The potassium permanganate value (K value) of the unbleached pulp is preferably 5 to 10, and more preferably 6 to 9. When the K value of the unbleached pulp is 5 or more, the cooking cost can be reduced, which is preferable. When the K value of the unbleached pulp is 10 or less, the amount of chlorine dioxide used and the cost of the bleaching treatment can be reduced, which is preferable. In this specification, the K value of the unbleached pulp is measured by the following method. That is, the K value of the unbleached pulp is measured in accordance with TAPPI UM 253:2010.

[0037] Furthermore, the viscosity of the unbleached pulp is preferably 15 cP or more, more preferably 17 cP or more, and even more preferably 20 cP or more. When the viscosity of the unbleached pulp is 15 cP or more, it is preferable because the freeness can be maintained. There is no particular upper limit for the viscosity of the unbleached pulp, but it is preferably 25 cP or less. When the viscosity of the unbleached pulp is 25 cP or less, it is preferable because the viscosity of the paper can be maintained. In this specification, the viscosity of the unbleached pulp is measured by the following method. That is, the viscosity of the unbleached pulp is measured in accordance with the J.TAPPI No. 44 method.

[0038] The hexenuronic acid content (HexA content) of the unbleached pulp is preferably 25 to 50 μmol / g, more preferably 30 to 40 μmol / g. If the HexA content of the unbleached pulp is 25 μmol / g or more, it is preferable because the cooking cost can be reduced. If the HexA content of the unbleached pulp is 50 μmol / g or less, it is preferable because the bleaching cost can be reduced. In this specification, the HexA content of the unbleached pulp is measured by the following method. That is, after the unbleached pulp is completely dried, 0.8 g of bleached pulp in terms of completely dried mass is precisely weighed and placed in a pressure-resistant container. Next, 80 mL of pure water is added, and then formic acid is added to adjust the pH value to 3. The pressure-resistant container is placed in an oven and treated at 120° C. for 4 hours to acid-hydrolyze HexA. After treatment, the solution is filtered and the acid hydrolysates of HexA, 2-furancarboxylic acid and 5-carboxy-2-furanaldehyde, present in the filtered solution are quantified by HPLC, and the original HexA content is calculated from the sum of their molar amounts.

[0039] [Alkaline oxygen bleaching process (O process)] The alkaline oxygen bleaching process is a process in which lignin and the like contained in pulp is removed by oxygen bleaching under alkaline conditions. The alkaline oxygen bleaching process is usually carried out by adding oxygen to a pulp slurry containing alkaline pulp and carrying out a bleaching treatment. The pulp obtained after the bleaching treatment is appropriately washed.

[0040] (Pulp Slurry) The pulp slurry includes pulp and water.

[0041] (pulp) The pulp is not particularly limited and may be unbleached pulp or pulp that has been subjected to another bleaching process, but is preferably unbleached pulp.

[0042] The pulp concentration is preferably from 8 to 40% by mass, and more preferably from 10 to 35% by mass, based on the total mass of the pulp slurry.

[0043] The pulp slurry has an alkaline pH, specifically, preferably 8 to 14, and more preferably 10 to 14.

[0044] The pH of the pulp slurry can be adjusted using bases such as sodium hydroxide (caustic soda) and potassium hydroxide; acids such as sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, boric acid and carbonic acid; and oxidized kraft white liquor.

[0045] (Bleaching treatment) The method of bleaching is not particularly limited, but includes a medium consistency method (pulp consistency: 8 to 25% by mass, preferably 10 to 15% by mass) and a high consistency method (pulp consistency: 25 to 40% by mass). Of these, the medium consistency method is preferred.

[0046] The oxygen gas addition rate is preferably 0.5 to 3 mass % based on the bone dry mass of the unbleached pulp.

[0047] In this case, the oxygen used is not particularly limited, but examples thereof include oxygen obtained by cryogenic separation, oxygen obtained by pressure swing adsorption (PSA), and oxygen obtained by vacuum swing adsorption (VSA).

[0048] The mixing method is not particularly limited, but it is preferable to use a high shear mixer.Specific examples include the Kamyr method, the Sunds-Defibrator method, the Rauma-Repola method, and the Impcpo method.

[0049] The treatment temperature in the alkaline oxygen bleaching step is preferably 80 to 120°C.

[0050] The treatment time in the alkaline oxygen bleaching step is preferably 15 to 100 minutes.

[0051] [Monopersulfuric acid bleaching process (MPS process)] The monopersulfuric acid bleaching process is a process in which hexaneuronic acid (HexA) and other substances contained in pulp are removed using monopersulfuric acid (MPS). Hexenuronic acid (HexA) is a substance produced by demethanolization of methylglucuronic acid in the xylan side chain of hemicellulose during cooking. The content of hexenuronic acid (HexA) in bleached pulp correlates with the fading of the bleached pulp.

[0052] The monopersulfuric acid bleaching step (MPS step) is usually carried out by adding monopersulfuric acid (MPS) to a pulp slurry and carrying out a bleaching treatment. The pulp obtained after the bleaching treatment is appropriately washed.

[0053] (Pulp Slurry) The pulp slurry contains pulp and water, and may further contain other additives.

[0054] (pulp) The pulp is not particularly limited and may be unbleached pulp or pulp that has been subjected to another bleaching process, but is preferably unbleached pulp or pulp that has been subjected to an alkaline oxygen bleaching process.

[0055] The pulp concentration is preferably 5 to 30% by mass, and more preferably 8 to 15% by mass, based on the total mass of the pulp slurry, from the viewpoint of operability.

[0056] Additives Additives include lignin, bleaching-derived substances, and the like.

[0057] The bleaching-derived substances include, but are not limited to, lignin derivatives, 2-methoxyphenol, 3-methoxyphenol, 4-methoxyphenol, 5-amino-methoxyphenol, muconic acid, 2,5-dimethyl-2,4-hexadienedioic acid, and oxides thereof.

[0058] The lignin derivative is not particularly limited, but examples thereof include calcium lignin sulfonate, sodium lignin sulfonate, lignin sulfonic acid, sodium lignin sulfonate acetate, and organosolv lignin.

[0059] The addition of lignin and bleaching-derived substances can improve the removal ability of hexuronic acid (HexA) by monopersulfuric acid (MPS).

[0060] Although the lignin and bleaching-derived substances may be separately prepared or prepared and used as additives, wastewater obtained in the bleaching step of the bleaching pulp manufacturing method may be used. Since lignin and bleaching-derived substances are removed from the pulp in each bleaching step of the bleaching pulp manufacturing method, the wastewater obtained may contain lignin and bleaching-derived substances.

[0061] The wastewater containing lignin and bleaching-derived substances is not particularly limited, but may be from an unbleached pulp preparation step, an alkaline oxygen bleaching step (O step), a monopersulfuric acid bleaching step (MPS step), an ozone bleaching step (Z step), a chlorine dioxide bleaching step (D step), and an alkaline hydrogen peroxide bleaching step (Ep step). The "wastewater" is usually derived from the washing liquid of pulp after bleaching treatment. Of these, it is preferable to reuse the wastewater from the monopersulfuric acid bleaching step (MPS step) and the chlorine dioxide bleaching step (D step), and it is more preferable to reuse the wastewater from the monopersulfuric acid bleaching step (MPS step) and the first chlorine dioxide bleaching step (D step). That is, in a preferred embodiment, the monopersulfuric acid bleaching step (MPS step) includes reusing at least a part of the wastewater obtained in the monopersulfuric acid bleaching step and the wastewater obtained in the first chlorine dioxide bleaching step in the monopersulfuric acid bleaching step.

[0062] The total organic carbon (TOC) of the wastewater is preferably 50 mg / L or more, more preferably 500 mg / L or less, and even more preferably 95 to 300 mg / L. The total organic carbon (TOC) includes lignin and bleaching-derived substances. The concentration of the total organic carbon (TOC) in the wastewater is measured by the same method as that described in the Examples.

[0063] In addition, the present invention can produce bleached pulp with excellent brightness. In other words, the wastewater obtained from the bleaching process of the present invention contains a large amount of lignin and bleaching-derived substances, and therefore has a high concentration of total organic carbon. By reusing the wastewater in the peroxymonosulfate bleaching process, HexA can be removed at low cost and with high efficiency.

[0064] The pH of the pulp slurry is preferably 1 to 6, and more preferably 2 to 5. That is, in one embodiment, the treatment pH of the monopersulfuric acid bleaching step is preferably 1 to 6, and more preferably 2 to 5. It is preferable that the treatment pH is within the above range because HexA can be suitably removed.

[0065] An acid can be added to adjust the pH of the pulp slurry. Examples of the acid include sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, boric acid, and carbonic acid. Of these, the acid used to adjust the pH is preferably sulfuric acid. The acid used to adjust the pH may be used alone or in combination of two or more kinds.

[0066] (Bleaching treatment) The bleaching treatment is preferably carried out by adding monopersulfuric acid (MPS) to the pulp slurry and mixing it.

[0067] Monopersulfuric acid (MPS), also known as peroxymonosulfuric acid, is represented by H2SO5.

[0068] The amount of monopersulfuric acid (MPS) added is preferably 0.01 to 5 mass%, more preferably 0.05 to 3 mass%, even more preferably 0.10 to 2 mass%, particularly preferably 0.15 to 1.75 mass%, and most preferably 0.30 to 1.75 mass%, based on the bone dry mass of the unbleached pulp.

[0069] In one embodiment, the amount of monopersulfuric acid (MPS) added is preferably 0.15 to 1.75 mass% based on the bone dry mass of the unbleached pulp, more preferably 0.22 to 1.0 mass%, even more preferably 0.22 to 0.75 mass%, and particularly preferably 0.3 to 0.5 mass%. When the amount of monopersulfuric acid (MPS) added is within the above range, for example, lignin and / or hexenuronic acid (HexA) can be suitably removed from the unbleached pulp. As a result, the obtained bleached pulp can have effects such as a higher brightness, a lower K number residual rate, and a lower HexA content residual rate. In addition, a decrease in viscosity can be prevented or suppressed. When the ratio of the monopersulfuric acid addition rate (mass%) to the bone dry mass of unbleached pulp and the chlorine dioxide addition rate (mass%) to the bone dry mass of unbleached pulp (chlorine dioxide addition rate / monopersulfuric acid addition rate) is within a predetermined range, if the amount of monopersulfuric acid (MPS) added falls within the above-mentioned predetermined range, the amount of chlorine dioxide added in the chlorine dioxide bleaching step (step D) described below will be a predetermined value. In other words, if the addition rate falls within the predetermined range, the bleaching effect in both the monopersulfuric acid bleaching step (MPS step) and the chlorine dioxide bleaching step (step D step) can be effectively obtained.

[0070] Monopersulfuric acid (MPS) can be produced by a known method. A specific example of a method for producing monopersulfuric acid (MPS) is, for example, a method of dropping sulfuric acid into hydrogen peroxide solution and mixing them. In this case, the concentration of the hydrogen peroxide solution is preferably 20 to 70 mass%, more preferably 35 to 70 mass%. The concentration of the sulfuric acid is preferably 80 to 98 mass%, more preferably 93 to 98 mass%. The mixing molar ratio of sulfuric acid to hydrogen peroxide is preferably 1:1 to 5:1, more preferably 2:1 to 4:1.

[0071] The processing temperature in the monopersulfuric acid bleaching step is preferably 40 to 110°C, more preferably 40 to 105°C, further preferably 40 to 98°C, and particularly preferably 70 to 98°C.

[0072] In one embodiment, the treatment temperature in the monopersulfuric acid bleaching step is preferably 40 to 98° C., more preferably 55 to 98° C., even more preferably 65 to 98° C., and particularly preferably 80 to 98° C. When the treatment temperature in the monopersulfuric acid bleaching step is within the above range, it can have effects such as lowering the K number residual rate and lowering the HexA content residual rate of the obtained bleached pulp. In addition, it can be more difficult for the obtained bleached pulp to deteriorate, for example, to decrease in brightness and viscosity.

[0073] The treatment time for the monopersulfuric acid bleaching step is preferably 2 to 200 minutes, more preferably 5 to 180 minutes, even more preferably 10 to 170 minutes, particularly preferably 70 to 160 minutes, and most preferably 100 to 150 minutes.

[0074] In one embodiment, the treatment time for the monopersulfuric acid bleaching step is preferably 30 to 180 minutes, more preferably 40 to 160 minutes, even more preferably 60 to 150 minutes, particularly preferably 80 to 140 minutes, and most preferably 100 to 130 minutes.

[0075] In another embodiment, the treatment time in the monopersulfuric acid bleaching step is preferably from 30 to 120 minutes, more preferably from 60 to 120 minutes, and even more preferably from 90 to 120 minutes, from the viewpoint of productivity.

[0076] [Ozone bleaching process (Z process)] The ozone bleaching step is a step of removing lignin and the like contained in pulp by using ozone. The ozone bleaching step is usually carried out by adding ozone to a pulp slurry containing pulp and carrying out a bleaching treatment. In one embodiment, the method for producing bleached pulp preferably further includes an ozone bleaching step of treating with ozone. The pulp obtained after the bleaching treatment is appropriately washed.

[0077] (Pulp Slurry) The pulp slurry includes pulp and water.

[0078] (pulp) The pulp is usually pulp that has been subjected to other bleaching steps, and preferably pulp that has been subjected to at least an alkaline oxygen bleaching step and a monopersulfuric acid bleaching step.

[0079] The pulp concentration is preferably from 8 to 40% by mass, and more preferably from 10 to 35% by mass, based on the total mass of the pulp slurry.

[0080] The pH of the pulp slurry is preferably 1-6, more preferably 1-5, and even more preferably 1-3.

[0081] The pH of the pulp slurry can be adjusted using bases such as sodium hydroxide (caustic soda) and potassium hydroxide; and acids such as sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, boric acid, and carbonic acid.

[0082] (Bleaching treatment) The method of bleaching is not particularly limited, but includes a medium consistency method (pulp consistency: 8 to 25% by mass, preferably 10 to 15% by mass) and a high consistency method (pulp consistency: 25 to 40% by mass). Of these, the medium consistency method is preferred.

[0083] Since ozone has a short life span, it is preferable to prepare ozone when it is used. The method for preparing ozone is not particularly limited, but it is preferable to prepare ozone by subjecting oxygen gas to corona discharge.

[0084] In this case, the oxygen used may be liquid oxygen, oxygen obtained by cryogenic separation, oxygen obtained by pressure swing adsorption (PSA), oxygen obtained by vacuum swing adsorption (VSA), or the like.

[0085] The addition rate of ozone is preferably 0.05 to 20 mass%, more preferably 0.1 to 15 mass%, further preferably 0.1 to 10 mass%, and particularly preferably 0.1 to 1 mass%, based on the bone dry mass of unbleached pulp.

[0086] The treatment temperature in the ozone bleaching step is preferably 0 to 100°C, more preferably 10 to 80°C, further preferably 30 to 80°C, and particularly preferably 50 to 80°C.

[0087] The processing time in the ozone bleaching step is preferably from 5 seconds to 60 minutes, and more preferably from 10 seconds to 10 minutes.

[0088] [Chlorine dioxide bleaching process (D process)] The chlorine dioxide bleaching step is a step in which lignin and the like contained in pulp is removed by chlorine dioxide. By using chlorine dioxide instead of chlorine itself or hypochlorous acid, ECF bleaching can be achieved. In addition, by appropriately combining other bleaching steps, the amount of chlorine dioxide used in the chlorine dioxide bleaching step can be reduced, resulting in light ECF bleaching. Here, in this specification, the "first chlorine dioxide bleaching step" refers to the chlorine dioxide bleaching step that is carried out first in the bleaching step (bleaching sequence).

[0089] The chlorine dioxide bleaching step is usually carried out by adding chlorine dioxide to a pulp slurry containing pulp and carrying out a bleaching treatment. The pulp obtained after the bleaching treatment is appropriately washed.

[0090] (Pulp Slurry) The pulp slurry includes pulp and water.

[0091] (pulp) The pulp is usually pulp that has been subjected to other bleaching steps, and preferably pulp that has been subjected to at least an alkaline oxygen bleaching step and a monopersulfuric acid bleaching step.

[0092] The pulp concentration is preferably 5 to 30% by mass, and from the viewpoint of operability, more preferably 8 to 15% by mass, based on the total mass of the pulp slurry.

[0093] The pH of the pulp slurry is preferably 1.5 to 6, more preferably 2 to 6 since the decomposition of cellulose can be inhibited, and further preferably 3 to 5 since the decomposition of hexenuronic acid can be enhanced.

[0094] The pH of the pulp slurry can be adjusted using bases such as sodium hydroxide (caustic soda) and potassium hydroxide; and acids such as sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, boric acid, and carbonic acid.

[0095] (Bleaching treatment) The bleaching treatment is preferably carried out by adding chlorine dioxide to the pulp slurry and mixing.

[0096] The amount of chlorine dioxide added is preferably 0.01 to 2.0 mass%, more preferably 0.05 to 1.5 mass%, even more preferably 0.1 to 1.0 mass%, and particularly preferably 0.1 to 0.8 mass%, based on the bone dry mass of the unbleached pulp.

[0097] In one embodiment, the amount of chlorine dioxide added is preferably 0.1 to 2.0 mass%, more preferably 0.1 to 1.0 mass%, even more preferably 0.2 to 0.8 mass%, and particularly preferably 0.3 to 0.7 mass%, relative to the bone dry mass of the unbleached pulp.

[0098] In addition, in relation to the preceding monopersulfuric acid bleaching step, by setting the ratio of the monopersulfuric acid addition rate and the chlorine dioxide addition rate within a predetermined range, bleached pulp with better whiteness can be produced. Specifically, the ratio (addition rate ratio) of the monopersulfuric acid addition rate (mass%) to the bone dry mass of the unbleached pulp represented by the following formula to the chlorine dioxide addition rate (mass%) to the bone dry mass of the unbleached pulp is 0.25 to 4.0, preferably 0.25 to 3.5, more preferably 0.25 to 4.0, and even more preferably 0.5 to 4.0. When the bleached pulp production method includes two or more chlorine dioxide bleaching steps (step D), the chlorine dioxide addition rate refers to the chlorine dioxide addition rate in the first chlorine dioxide bleaching step (step D).

[0099]

number

[0100] If the addition rate ratio is in the range of 0.25 or less, lignin is repolymerized in the monopersulfuric acid bleaching step, and the addition rate of chlorine dioxide is increased in the subsequent chlorine dioxide bleaching step. If the addition rate ratio is in the range of 4.0 or more, sufficient bleaching cannot be achieved in the monopersulfuric acid bleaching step due to a shortage of monopersulfuric acid. As a result, bleached pulp with excellent brightness can be produced at the above addition rate ratio. In addition, the HexA content can be reduced, and bleached pulp that is less likely to fade can be produced.

[0101] In one embodiment, the addition rate ratio is preferably 0.3 to 3.0, more preferably 0.5 to 2.3, even more preferably 0.5 to 1.8, particularly preferably 0.9 to 1.8, and most preferably 1.0 to 1.5. When the addition rate ratio is within the above range, the bleaching balance between the monopersulfuric acid bleaching step and the chlorine dioxide bleaching step becomes favorable, and, for example, lignin and / or hexenuronic acid (HexA) can be favorably removed from the unbleached pulp. As a result, the obtained bleached pulp can have the effects of increasing the brightness, decreasing the K number residual rate, decreasing the HexA content residual rate, and the like. In addition, a decrease in viscosity can be prevented or suppressed.

[0102] In another embodiment, the addition ratio is preferably 0.25 to 2.0, more preferably 0.25 to 1.5, even more preferably 0.25 to 1.0, particularly preferably 0.25 to 0.5, and most preferably 0.25 to 0.3. The addition ratio within the above range is preferable because the bleaching balance between the monopersulfuric acid bleaching step and the chlorine dioxide bleaching step is favorable, and, for example, the bleached pulp is less likely to deteriorate. Specifically, it is preferable because the whiteness of the bleached pulp is less likely to deteriorate, and the viscosity is less likely to decrease due to deterioration.

[0103] Chlorine dioxide can be prepared, for example, by reacting sodium sulfite with sulfur dioxide, by reacting sodium chlorite with sulfuric acid, by reacting calcium chlorate with hydrochloric acid, by reacting sodium chlorate, sodium chloride with sulfuric acid, by reacting sodium chlorate, methanol with sulfuric acid, or by reacting sodium chlorate, hydrogen peroxide with sulfuric acid.

[0104] The treatment temperature in the chlorine dioxide bleaching step is preferably 20 to 100°C, and more preferably 40 to 90°C.

[0105] In one embodiment, the treatment temperature in the chlorine dioxide bleaching step is preferably 30 to 90°C, more preferably 40 to 90°C, particularly preferably 50 to 80°C, and most preferably 65 to 75°C.

[0106] The treatment time for the chlorine dioxide bleaching step is preferably 1 minute to 5 hours, more preferably 10 to 180 minutes, even more preferably 20 to 150 minutes, particularly preferably 20 to 120 minutes, and most preferably 50 to 130 minutes.

[0107] In one embodiment, the treatment time of the chlorine dioxide bleaching step is preferably 10 to 150 minutes, more preferably 10 to 130 minutes, even more preferably 15 to 100 minutes, particularly preferably 20 to 80 minutes, and most preferably 30 to 60 minutes. If the treatment time of the chlorine dioxide bleaching step is 10 minutes or more, the resulting bleached pulp may have a higher brightness and may be less susceptible to deterioration. On the other hand, if the treatment time of the chlorine dioxide bleaching step is 150 minutes or less, it is preferable from the viewpoints of efficiently producing bleached pulp with excellent brightness and excellent productivity.

[0108] [Alkaline hydrogen peroxide bleaching process (Ep process)] The alkaline hydrogen peroxide bleaching step is a step in which lignin and the like contained in pulp is removed by hydrogen peroxide bleaching under alkaline conditions. Alkaline hydrogen peroxide bleaching is usually carried out by adding hydrogen peroxide to a pulp slurry containing alkaline pulp and carrying out a bleaching treatment. In one embodiment, the method for producing bleached pulp preferably further includes an alkaline hydrogen peroxide bleaching step of treating with alkaline hydrogen peroxide. The pulp obtained after the bleaching treatment is appropriately washed.

[0109] (Pulp Slurry) The pulp slurry includes pulp and water.

[0110] (pulp) The pulp is not particularly limited and may be unbleached pulp or pulp that has been subjected to another bleaching process, but is preferably unbleached pulp.

[0111] The pulp concentration is preferably from 8 to 40% by mass, and more preferably from 10 to 35% by mass, based on the total mass of the pulp slurry.

[0112] The pH of the pulp slurry is alkaline. Specifically, the pH of the pulp slurry is preferably 8 to 14, and more preferably 10 to 14.

[0113] The pH of the pulp slurry can be adjusted using bases such as sodium hydroxide (caustic soda) and potassium hydroxide; and acids such as sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, boric acid, and carbonic acid.

[0114] (Bleaching treatment) The method of bleaching is not particularly limited, but includes a medium consistency method (pulp consistency: 8 to 25% by mass, preferably 10 to 15% by mass, more preferably 10 to 15% by mass) and a high consistency method (pulp consistency: 25 to 40% by mass). Of these, the medium consistency method is preferred.

[0115] Hydrogen peroxide is preferably added in the form of aqueous hydrogen peroxide. In this case, the aqueous hydrogen peroxide contains hydrogen peroxide and water. The aqueous hydrogen peroxide may further contain an additive.

[0116] The concentration of hydrogen peroxide in the hydrogen peroxide solution is preferably 1 to 80 mass %, more preferably 10 to 70 mass %, and even more preferably 30 to 65 mass %, based on the total mass of the hydrogen peroxide solution.

[0117] The additives are not particularly limited, and examples thereof include stabilizers such as sodium silicate, magnesium silicate, nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), aminotri(methylenesulfonic acid) (ATMP), diethylenetriaminepenta(methylenephosphonic acid) (DTPMP), sodium poly-α-hydroxyacrylate (PHAS), etc. These stabilizers may be used alone or in combination of two or more kinds.

[0118] The concentration of the additive in the hydrogen peroxide solution is preferably 0.01 to 5 mass %, more preferably 0.05 to 3 mass %, and even more preferably 0.1 to 1 mass %, based on the total mass of the hydrogen peroxide solution.

[0119] The amount of hydrogen peroxide added is preferably 0.01 to 10% by mass, more preferably 0.03 to 5% by mass, and even more preferably 0.05 to 2% by mass, based on the bone dry mass of the unbleached pulp.

[0120] In the alkaline hydrogen peroxide bleaching step, oxygen may be added together with hydrogen peroxide. By adding oxygen, bleaching with oxygen can be performed at the same time. When oxygen is used in combination with the alkaline hydrogen peroxide bleaching step (Ep step), it is sometimes referred to as Eop step.

[0121] The oxygen gas addition rate is preferably 0.1 to 3 mass % based on the bone dry mass of the unbleached pulp.

[0122] In this case, the oxygen used is not particularly limited, but examples thereof include oxygen obtained by cryogenic separation, oxygen obtained by pressure swing adsorption (PSA), and oxygen obtained by vacuum swing adsorption (VSA).

[0123] The treatment temperature in the alkaline hydrogen peroxide bleaching step is preferably 40 to 120°C, more preferably 50 to 100°C, and even more preferably 60 to 90°C.

[0124] The treatment time in the alkaline hydrogen peroxide bleaching step is preferably from 15 to 150 minutes, more preferably from 15 to 120 minutes, and even more preferably from 15 to 100 minutes.

[0125] [Variations] In the above embodiment, the unbleached pulp preparation step, the alkaline oxygen bleaching step (O step), the monopersulfuric acid bleaching step (MPS step), the ozone bleaching step (Z step), the chlorine dioxide bleaching step (D step), and the alkaline hydrogen peroxide bleaching step (Ep step) are described in this order, but the present invention can be modified by changing the presence or absence of each bleaching step, the order, and the repetition of each step. This allows the physical properties of the resulting bleached pulp, the amount of chlorine dioxide used, and the like to be appropriately controlled.

[0126] In one embodiment, the method for producing bleached pulp includes, in this order, an O step, an MPS step, a first D step, an Ep step, and a second D step. By omitting the Z step, the production cost can be reduced.

[0127] In another embodiment, the method for producing bleached pulp includes, in this order, an O step, an MPS step, a Z step, a first D step, an Eop step, and a second D step. In this case, the Eop step is a step in which oxygen is used in combination with the Ep step, as described above. It is also preferable to carry out the first D step without washing after the Z step.

[0128] In another embodiment, the method for producing bleached pulp includes, in this order, an O step, an MPS step, a Z step, a first D step, an Ep step, and a second D step. In this case, it is preferable to carry out the first D step without washing after the Z step.

[0129] In another embodiment, the method for producing bleached pulp includes, in this order, an O step, an MPS step, a Z step, an Ep step, and a first D step. In this case, it is preferable to carry out the Ep step without washing after the Z step.

[0130] [Bleached pulp] The bleached pulp produced by the above-mentioned production method has excellent whiteness. Specifically, the whiteness of the bleached pulp is preferably 50% or more, more preferably 50 to 80%, and even more preferably 55 to 70%. When the whiteness of the bleached pulp is 50% or more, the bleached pulp has excellent whiteness, which is preferable. In this specification, the whiteness of the bleached pulp is a value measured by the method in the Examples.

[0131] The increase in the whiteness of the bleached pulp (whiteness of bleached pulp / whiteness of unbleached pulp × 100) is preferably 115% or more, more preferably 117% or more, even more preferably 119% or more, particularly preferably 120% or more, and most preferably 121% or more.

[0132] The potassium permanganate value (K value) of the bleached pulp is preferably 6 or less, more preferably 5 or less, even more preferably 1 to 4, and particularly preferably 2 to 3. When the K value of the bleached pulp is 6 or less, the bleached pulp has excellent whiteness, which is preferable. The K value of the bleached pulp is an index representing the amount of residual lignin in the pulp. In addition, in this specification, the K value of the bleached pulp is a value measured by the method in the Examples.

[0133] The residual rate of the K value of the bleached pulp (K value of bleached pulp / K value of unbleached pulp × 100) is preferably 65% ​​or less, more preferably 56% or less, even more preferably 40% or less, particularly preferably 35% or less, and most preferably 33% or less.

[0134] Furthermore, the viscosity of the bleached pulp is preferably 10 cP or more, more preferably 12 cP or more, and even more preferably 14 to 20 cP. If the viscosity of the bleached pulp is 10 cP or more, it is preferable because the strength of the paper can be maintained. In this specification, the viscosity of the bleached pulp is a value measured by the method in the Examples.

[0135] The viscosity retention rate of bleached pulp (viscosity of bleached pulp / viscosity of unbleached pulp x 100) is preferably 60% or more, more preferably 70% or more, even more preferably 80% or more, and particularly preferably 85% or more.

[0136] The hexenuronic acid content (HexA content) of the bleached pulp is preferably 30 μmol / g or less, more preferably 20 μmol / g or less, and even more preferably 1 to 10 μmol / g. If the HexA content of the bleached pulp is 30 μmol / g or less, the bleached pulp is less likely to fade, which is preferable. In this specification, the HexA content of the bleached pulp is a value measured by the method in the Examples.

[0137] The residual rate of HexA content of bleached pulp (HexA content of bleached pulp / HexA content of unbleached pulp × 100) is preferably 75% or less, more preferably 65% ​​or less, even more preferably 60% or less, particularly preferably 30% or less, and most preferably 15% or less.

[0138] In the present invention, the ratio of the monopersulfuric acid addition rate in the monopersulfuric acid bleaching step (MPS step) and the chlorine dioxide addition rate in the first chlorine dioxide bleaching step (D step) is set within a predetermined range, so that lignin and the like can be removed, and bleached pulp with excellent whiteness can be produced. This effect is significantly higher than that of the conventional technology in which the monopersulfuric acid bleaching step (MPS step) of the present invention is replaced with a sulfuric acid bleaching step in which treatment with sulfuric acid is performed. This can be confirmed by comparing the ratio of total organic carbon (TOC) in each method. Specifically, first, in the method of the present invention, TOC1 is calculated as the sum of the TOC of the wastewater (washing wastewater of pulp after bleaching) obtained in the monopersulfuric acid bleaching step and the TOC of the wastewater (washing wastewater of pulp after bleaching) obtained in the first chlorine dioxide bleaching step. In addition, in the conventional method (specifically, a method in which a sulfuric acid bleaching step is performed instead of the MPS step of the method of the present invention, hereinafter also referred to as the "reference method"), TOC2 is calculated as the sum of the TOC of the wastewater (washing wastewater of pulp after bleaching) obtained in the sulfuric acid bleaching step and the TOC of the wastewater (washing wastewater of pulp after bleaching) obtained in the first chlorine dioxide bleaching step. When the TOC increase rate (TOC1 / TOC2×100) is calculated, it is preferably more than 100%, more preferably more than 110%, and even more preferably more than 115%. Here, the TOC in the wastewater includes lignin and bleaching-derived substances (lignin derivatives, etc.). Therefore, if the TOC increase rate is more than 100%, it means that lignin, etc. can be removed more suitably than the conventional technique in which a sulfuric acid bleaching step is performed. That is, in a preferred embodiment, the TOC increase rate (TOC1 / TOC2×100) of the total organic carbon (TOC2) of the wastewater obtained in the sulfuric acid bleaching step and the first chlorine dioxide bleaching step in a reference method in which a sulfuric acid bleaching step is performed in place of the monopersulfuric acid bleaching step, is preferably greater than 100%, more preferably greater than 110%, and even more preferably greater than 115%. EXAMPLES

[0139] The present invention will be described in detail below with reference to examples, but the present invention is not limited thereto. In addition, unless otherwise specified, "%" is based on mass.

[0140] [Example 1] The pulp (hereinafter referred to as "raw pulp") after the alkaline oxygen bleaching process (process O), in which unbleached pulp was bleached with alkaline oxygen, was used to produce bleached pulp through a monopersulfate bleaching process (process MPS) and a first chlorine dioxide bleaching process (process D).

[0141] (Monopersulfuric acid bleaching process (MPS process)) 34 g of bone dry mass of raw pulp (brightness: 49.2%, K value: 7.0, viscosity: 18 cP, HexA content: 32.6 μmol / g) was sampled in a polyethylene bag. Hollow fiber membrane filtered water required for bleaching at a pulp concentration of 10% was added, and then an amount of sulfuric acid aqueous solution was added so that the pH at the start of the reaction was about 3, and mixed well. After that, 0.13 mass% of monopersulfuric acid was added based on the bone dry mass of the unbleached pulp, and further mixed.

[0142] The fabric was bleached by immersing it in a constant temperature water bath at 98° C. for 120 minutes.

[0143] The pulp after the MPS process was washed. Specifically, pure water was added until the pulp concentration became 2.4%, and then the pulp was dewatered until the pulp concentration became 20% (washing rate 90%).

[0144] (First chlorine dioxide bleaching process (D process)) The bleached pulp obtained above was sampled in an amount of 16 g by bone dry weight in a polyethylene bag. The hollow fiber membrane filtered water necessary for bleaching the pulp at a concentration of 10% was added, and then an aqueous sulfuric acid solution was added in an amount such that the pH at the start of the reaction was about 3, and mixed thoroughly. Thereafter, 0.50% by mass of chlorine dioxide was added to the bone dry mass of the unbleached pulp, and further mixed. The ratio of the addition rate of monopersulfuric acid to the bone dry mass of the unbleached pulp (0.13% by mass) to the addition rate of chlorine dioxide to the bone dry mass of the unbleached pulp (0.50% by mass) (chlorine dioxide addition rate / monopersulfuric acid addition rate) was 3.85.

[0145] The pulp was bleached by immersing it in a thermostatic water bath at 70°C for 120 minutes, and then washed in the same manner as in the MPS process.

[0146] The ISO brightness, potassium permanganate value (K value), viscosity, and hexenuronic acid content (HexA content) of the bleached pulp were measured by the following methods. The measurement sample for each measurement was a pulp sheet (acidic paper) prepared by the following method. Here, the pulp sheet (acidic paper) was prepared by diluting the bleached pulp in 2 L of pure water and adjusting the pH to 5.5 with sulfurous acid water. Two sheets were then prepared on a Buchner funnel and air-dried for 12 hours.

[0147] [ISO whiteness] The brightness (%) was measured according to JISP8148:2018. As a result, the ISO brightness was 57.4% (increase in brightness relative to the raw pulp: 117%).

[0148] [Potassium permanganate value (K value)] The K value was measured according to TAPPI UM 253:2010. The result was that the K value was 2.6 (residual rate of K value relative to raw pulp: 37%).

[0149] [viscosity] The viscosity was measured according to J.TAPPI No. 44. The result was that the viscosity was 15 cP (viscosity retention rate relative to the raw pulp: 83%).

[0150] [Hexenuronic acid content (HexA content)] After drying the pulp sheet (acidic paper), 0.8 g of the pulp sheet (acidic paper) was weighed out and placed in a pressure vessel. 80 mL of pure water was then added, and formic acid was added to adjust the pH to 3. The pressure vessel was placed in an oven and treated at 120°C for 4 hours to acid-hydrolyze HexA. After treatment, the solution was filtered, and the acid hydrolyzates of HexA, 2-furancarboxylic acid and 5-carboxy-2-furanaldehyde, present in the filtered solution, were quantified by HPLC, and the original HexA content was calculated from the sum of their molar amounts. As a result, the HexA content of the bleached pulp was 6.4 μmol / g (residual HexA content relative to the raw pulp: 20%).

[0151] [Total organic carbon (TOC) in wastewater] 50mL of washing wastewater (MPS process wastewater) obtained by dehydrating the pulp washing after the MPS process was collected. The collected wastewater from the MPS process was measured for TOC in the wastewater from the MPS process using a total organic carbon meter TOV-VCN (Shimadzu) in NPOC mode, which measures the sample as non-volatile organic carbon (Non-Purgeable Organic Carbon: NPOC) by volatilizing inorganic carbon through the addition of acid and aeration. In accordance with JIS K0551:1994, potassium hydrogen phthalate and sodium hydrogen carbonate were used as standard samples.

[0152] In addition, 50 mL of the washing wastewater (wastewater from process D) obtained by dewatering during the pulp washing in process D was collected, and the TOC in the wastewater from process D was measured in the same manner as for the wastewater from the MPS process.

[0153] The sum of the TOC in the wastewater from the MPS process and the TOC in the wastewater from the D process (hereinafter, sometimes simply referred to as "total organic carbon (TOC) in the wastewater") was 289 mg / L.

[0154] Here, the TOC increase rate in the wastewater in Example 1 was calculated based on Reference Example 1 (Reference Method), in which a sulfuric acid bleaching step is performed using sulfuric acid instead of the MPS step in Example 1. Reference Example 1 was carried out as follows.

[0155] Reference example 1 A bleached pulp was produced in the same manner as in Example 1, except that in the monopersulfuric acid bleaching step (MPS step), monopersulfuric acid was not added.

[0156] The ISO brightness, potassium permanganate value (K value), viscosity, and hexenuronic acid content (HexA content) of the bleached pulp, as well as the total organic carbon (TOC) in the wastewater were measured in the same manner as above. The results were that the ISO brightness was 56.4% (increase rate relative to the raw pulp: 115%), the K value was 2.6 (residual rate relative to the raw pulp: 37%), the viscosity was 14cP (retention rate relative to the raw pulp: 78%), and the HexA content was 4.8μmol / g (residual rate relative to the raw pulp: 15%). In addition, the TOC in the wastewater from the MPS process (sulfuric acid bleaching process) and the TOC in the wastewater from the D process were measured in the same manner as above, and the sum of these (total organic carbon (TOC) in the wastewater) was calculated to be 257mg / L.

[0157] The TOC increase rate in the wastewater in Example 1 was calculated to be 112% by the following formula.

[0158]

number

[0159] [Example 2] Except for changing the amount of monopersulfuric acid added in the monopersulfuric acid bleaching step (MPS step) to 0.20 mass% based on the bone dry mass of the unbleached pulp, bleached pulp was produced in the same manner as in Example 1. The chlorine dioxide addition ratio / monopersulfuric acid addition ratio was 2.50.

[0160] The ISO brightness, potassium permanganate value (K value), viscosity, and hexenuronic acid content (HexA content) of the bleached pulp, as well as the total organic carbon (TOC) in the wastewater were measured in the same manner as in Example 1. As a result, the ISO brightness was 58.3% (increase rate relative to the raw pulp: 119%), the K value was 2.4 (residual rate relative to the raw pulp: 35%), the viscosity was 15 cP (retention rate relative to the raw pulp: 83%), the HexA content was 5.6 μmol / g (residual rate relative to the raw pulp: 17%), and the TOC in the wastewater was 295 mg / L (TOC increase rate relative to Reference Example 1: 115%).

[0161] [Example 3] Except for changing the amount of monopersulfuric acid added in the monopersulfuric acid bleaching step (MPS step) to 0.25% by mass based on the bone dry mass of the unbleached pulp, bleached pulp was produced in the same manner as in Example 1. The ratio of chlorine dioxide to monopersulfuric acid was 2.00.

[0162] The ISO brightness, potassium permanganate value (K value), viscosity, and hexenuronic acid content (HexA content) of the bleached pulp, as well as the total organic carbon (TOC) in the wastewater were measured in the same manner as in Example 1. As a result, the ISO brightness was 59.0% (increase rate relative to the raw pulp: 120%), the K value was 2.38 (residual rate relative to the raw pulp: 34%), the viscosity was 14.6 cP (retention rate relative to the raw pulp: 81%), the HexA content was 4.8 μmol / g (residual rate relative to the raw pulp: 15%), and the TOC in the wastewater was 295 mg / L (TOC increase rate relative to Reference Example 1: 115%).

[0163] [Example 4] Except for changing the pH at the start of the reaction in the monopersulfuric acid bleaching step (MPS step) to about 3.5 and changing the amount of monopersulfuric acid added to 0.40 mass% based on the bone dry mass of the unbleached pulp, bleached pulp was produced in the same manner as in Example 1. The chlorine dioxide addition ratio / monopersulfuric acid addition ratio was 1.25.

[0164] The ISO brightness, potassium permanganate value (K value), viscosity, and hexenuronic acid content (HexA content) of the bleached pulp, as well as the total organic carbon (TOC) in the wastewater were measured in the same manner as in Example 1. As a result, the ISO brightness was 60.6% (increase rate relative to the raw pulp: 123%), the K value was 2.2 (residual rate relative to the raw pulp: 32%), the viscosity was 14 cP (retention rate relative to the raw pulp: 78%), the HexA content was 3.9 μmol / g (residual rate relative to the raw pulp: 12%), and the TOC in the wastewater was 297 mg / L (TOC increase rate relative to Reference Example 1: 116%).

[0165] [Example 5] Except for changing the amount of monopersulfuric acid added in the monopersulfuric acid bleaching step (MPS step) to 0.60 mass% based on the bone dry mass of the unbleached pulp, bleached pulp was produced in the same manner as in Example 1. The chlorine dioxide addition ratio / monopersulfuric acid addition ratio was 0.83.

[0166] The ISO brightness, potassium permanganate value (K value), viscosity, and hexenuronic acid content (HexA content) of the bleached pulp, as well as the total organic carbon (TOC) in the wastewater were measured in the same manner as in Example 1. As a result, the ISO brightness was 60.0% (increase rate relative to the raw pulp: 122%), the K value was 2.2 (residual rate relative to the raw pulp: 32%), the viscosity was 13.2 cP (retention rate relative to the raw pulp: 73%), the HexA content was 3.2 μmol / g (residual rate relative to the raw pulp: 10%), and the TOC in the wastewater was 295 mg / L (TOC increase rate relative to Reference Example 1: 115%).

[0167] [Example 6] Except for changing the pH at the start of the reaction in the monopersulfuric acid bleaching step (MPS step) to about 3.2 and changing the amount of monopersulfuric acid added to 1.50 mass% based on the bone dry mass of the unbleached pulp, bleached pulp was produced in the same manner as in Example 1. The chlorine dioxide addition ratio / monopersulfuric acid addition ratio was 0.33.

[0168] The ISO brightness, potassium permanganate value (K value), viscosity, and hexenuronic acid content (HexA content) of the bleached pulp, as well as the total organic carbon (TOC) in the wastewater were measured in the same manner as in Example 1. As a result, the ISO brightness was 59.2% (increase rate relative to the raw pulp: 120%), the K value was 2.3 (residual rate relative to the raw pulp: 33%), the viscosity was 12 cP (retention rate relative to the raw pulp: 67%), the HexA content was 2.0 μmol / g (residual rate relative to the raw pulp: 6%), and the TOC in the wastewater was 287 mg / L (TOC increase rate relative to Reference Example 1: 112%).

[0169] [Comparative Example 1] Bleached pulp was produced in the same manner as in Example 1, except that the pH at the start of the reaction in the monopersulfuric acid bleaching step (MPS step) was changed to about 3.2, the amount of monopersulfuric acid added was changed to 2.00 mass% based on the bone dry mass of the unbleached pulp, and the amount of chlorine dioxide added in the first chlorine dioxide bleaching step (D step) was changed to 0.25 mass% based on the bone dry mass of the unbleached pulp. The chlorine dioxide addition rate / monopersulfuric acid addition rate was 0.13.

[0170] The ISO brightness, potassium permanganate value (K value), viscosity, and hexenuronic acid content (HexA content) of the bleached pulp, as well as the total organic carbon (TOC) in the wastewater were measured in the same manner as in Example 1. As a result, the ISO brightness was 56.2% (increase rate relative to the raw pulp: 114%), the K value was 2.6 (residual rate relative to the raw pulp: 37%), the viscosity was 11 cP (retention rate relative to the raw pulp: 61%), the HexA content was 2.5 μmol / g (residual rate relative to the raw pulp: 8%), and the TOC in the wastewater was 271 mg / L (TOC increase rate relative to Reference Example 1: 105%).

[0171] [Comparative Example 2] A bleached pulp was produced in the same manner as in Example 1, except that the amount of monopersulfuric acid added in the monopersulfuric acid bleaching step (MPS step) was changed to 0.10% by mass based on the bone dry mass of the unbleached pulp. The ratio of chlorine dioxide to monopersulfuric acid was 5.00.

[0172] The ISO brightness, potassium permanganate value (K value), viscosity, and hexenuronic acid content (HexA content) of the bleached pulp, as well as the total organic carbon (TOC) in the wastewater were measured in the same manner as in Example 1. As a result, the ISO brightness was 55.8% (increase rate relative to the raw pulp: 113%), the K value was 2.7 (residual rate relative to the raw pulp: 39%), the viscosity was 15 cP (retention rate relative to the raw pulp: 83%), the HexA content was 6.7 μmol / g (residual rate relative to the raw pulp: 21%), and the TOC in the wastewater was 276 mg / L (TOC increase rate relative to Reference Example 1: 107%).

[0173] [Example 7] A raw material pulp (brightness: 54.7%, K number: 7.0, viscosity: 19 cP, HexA content: 36.1 μmol / g) was used, and bleached pulp was produced in the same manner as in Example 1, except that the bleaching temperature in the monopersulfuric acid bleaching step (MPS step) was changed to 50° C. and the bleaching time to 90 minutes, and the bleaching temperature in the first chlorine dioxide bleaching step (D step) was changed to 60° C. and the bleaching time to 35 minutes. The chlorine dioxide addition ratio / monopersulfuric acid addition ratio was 3.85.

[0174] The ISO brightness, potassium permanganate value (K value), viscosity, and hexenuronic acid content (HexA content) of the bleached pulp were measured in the same manner as in Example 1. As a result, the ISO brightness was 63.2% (increase rate relative to the raw pulp: 116%), the K value was 4.0 (residual rate relative to the raw pulp: 57%), the viscosity was 17 cP (retention rate relative to the raw pulp: 89%), and the HexA content was 25.0 μmol / g (residual rate relative to the raw pulp: 69%).

[0175] The total organic carbon (TOC) in the wastewater was measured in the same manner as in Example 1 and found to be 106 mg / L. The TOC increase rate in Example 7 was calculated based on that in Reference Example 2 (Reference Method), in which a sulfuric acid bleaching step was performed using sulfuric acid instead of the MPS step in Example 7. Reference Example 2 was carried out as follows.

[0176] Reference example 2 A bleached pulp was produced in the same manner as in Example 7, except that in the monopersulfuric acid bleaching step (MPS step), monopersulfuric acid was not added.

[0177] The ISO brightness, potassium permanganate value (K value), viscosity, and hexenuronic acid content (HexA content) of the bleached pulp, as well as the total organic carbon (TOC) in the wastewater were measured in the same manner as above. The results were that the ISO brightness was 62.4% (increase rate relative to the raw pulp: 114%), the K value was 4.3 (residual rate relative to the raw pulp: 61%), the viscosity was 17cP (retention rate relative to the raw pulp: 89%), and the HexA content was 28.9μmol / g (residual rate relative to the raw pulp: 80%). In addition, the TOC in the wastewater from the MPS process (sulfuric acid bleaching process) and the TOC in the wastewater from the D process were measured in the same manner as above, and the sum of these (total organic carbon (TOC) in the wastewater) was calculated to be 91mg / L.

[0178] The TOC increase rate in the wastewater in Example 7 was calculated to be 116% by the following formula.

[0179]

number

[0180] [Example 8] Bleached pulp was produced in the same manner as in Example 7, except that the amount of monopersulfuric acid added in the monopersulfuric acid bleaching step (MPS step) was changed to 0.20 mass% based on the bone dry mass of the unbleached pulp. The chlorine dioxide addition ratio / monopersulfuric acid addition ratio was 2.50.

[0181] The ISO brightness, potassium permanganate value (K value), viscosity, and hexenuronic acid content (HexA content) of the bleached pulp, as well as the total organic carbon (TOC) in the wastewater were measured in the same manner as in Example 7. As a result, the ISO brightness was 64.2% (increase rate relative to the raw pulp: 117%), the K value was 3.8 (residual rate relative to the raw pulp: 54%), the viscosity was 17 cP (retention rate relative to the raw pulp: 89%), the HexA content was 23.5 μmol / g (residual rate relative to the raw pulp: 65%), and the TOC in the wastewater was 108 mg / L (TOC increase rate relative to Reference Example 2: 119%).

[0182] [Example 9] Bleached pulp was produced in the same manner as in Example 7, except that the amount of monopersulfuric acid added in the monopersulfuric acid bleaching step (MPS step) was changed to 0.25% by mass based on the bone dry mass of the unbleached pulp. The chlorine dioxide addition rate / monopersulfuric acid addition rate was 2.00.

[0183] The ISO brightness, potassium permanganate value (K value), viscosity, and hexenuronic acid content (HexA content) of the bleached pulp, as well as the total organic carbon (TOC) in the wastewater were measured in the same manner as in Example 7. As a result, the ISO brightness was 65.1% (increase rate relative to the raw pulp: 119%), the K value was 3.7 (residual rate relative to the raw pulp: 53%), the viscosity was 16.5 cP (retention rate relative to the raw pulp: 87%), the HexA content was 22.7 μmol / g (residual rate relative to the raw pulp: 63%), and the TOC in the wastewater was 109 mg / L (TOC increase rate relative to Reference Example 2: 120%).

[0184] [Example 10] Bleached pulp was produced in the same manner as in Example 7, except that the pH at the start of the reaction in the monopersulfuric acid bleaching step (MPS step) was changed to about 3.5, and the amount of monopersulfuric acid added was changed to 0.40 mass% based on the bone dry mass of the unbleached pulp. The chlorine dioxide addition ratio / monopersulfuric acid addition ratio was 1.25.

[0185] The ISO brightness, potassium permanganate value (K value), viscosity, and hexenuronic acid content (HexA content) of the bleached pulp, as well as the total organic carbon (TOC) in the wastewater were measured in the same manner as in Example 7. As a result, the ISO brightness was 66.9% (increase rate relative to the raw pulp: 122%), the K value was 3.6 (residual rate relative to the raw pulp: 51%), the viscosity was 16 cP (retention rate relative to the raw pulp: 84%), the HexA content was 21.0 μmol / g (residual rate relative to the raw pulp: 58%), and the TOC in the wastewater was 110 mg / L (TOC increase rate relative to Reference Example 2: 121%).

[0186] [Example 11] Bleached pulp was produced in the same manner as in Example 7, except that the amount of monopersulfuric acid added in the monopersulfuric acid bleaching step (MPS step) was changed to 0.60% by mass based on the bone dry mass of the unbleached pulp. The chlorine dioxide addition ratio / monopersulfuric acid addition ratio was 0.83.

[0187] The ISO brightness, potassium permanganate value (K value), viscosity, and hexenuronic acid content (HexA content) of the bleached pulp, as well as the total organic carbon (TOC) in the wastewater were measured in the same manner as in Example 7. As a result, the ISO brightness was 66.2% (increase rate relative to the raw pulp: 121%), the K value was 3.64 (residual rate relative to the raw pulp: 52%), the viscosity was 15.8 cP (retention rate relative to the raw pulp: 83%), the HexA content was 18.8 μmol / g (residual rate relative to the raw pulp: 52%), and the TOC in the wastewater was 108 mg / L (TOC increase rate relative to Reference Example 2: 119%).

[0188] [Example 12] Bleached pulp was produced in the same manner as in Example 7, except that the pH at the start of the reaction in the monopersulfuric acid bleaching step (MPS step) was changed to about 3.2, and the amount of monopersulfuric acid added was changed to 1.50 mass% based on the bone dry mass of the unbleached pulp. The chlorine dioxide addition ratio / monopersulfuric acid addition ratio was 0.33.

[0189] The ISO brightness, potassium permanganate value (K value), viscosity, and hexenuronic acid content (HexA content) of the bleached pulp, as well as the total organic carbon (TOC) in the wastewater were measured in the same manner as in Example 7. As a result, the ISO brightness was 64.0% (increase rate relative to the raw pulp: 117%), the K value was 3.9 (residual rate relative to the raw pulp: 56%), the viscosity was 16 cP (retention rate relative to the raw pulp: 84%), the HexA content was 8.4 μmol / g (residual rate relative to the raw pulp: 23%), and the TOC in the wastewater was 107 mg / L (TOC increase rate relative to Reference Example 2: 118%).

[0190] [Comparative Example 3] Bleached pulp was produced in the same manner as in Example 7, except that the amount of monopersulfuric acid added in the monopersulfuric acid bleaching step (MPS step) was changed to 2.00 mass% based on the bone dry mass of the unbleached pulp, and the amount of chlorine dioxide added in the first chlorine dioxide bleaching step (D step) was changed to 0.25 mass% based on the bone dry mass of the unbleached pulp. The chlorine dioxide addition rate / monopersulfuric acid addition rate was 0.13.

[0191] The ISO brightness, potassium permanganate value (K value), viscosity, and hexenuronic acid content (HexA content) of the bleached pulp, as well as the total organic carbon (TOC) in the wastewater were measured in the same manner as in Example 7. As a result, the ISO brightness was 62.1% (increase rate relative to the raw pulp: 114%), the K value was 4.0 (residual rate relative to the raw pulp: 57%), the viscosity was 15 cP (retention rate relative to the raw pulp: 79%), the HexA content was 10.5 μmol / g (residual rate relative to the raw pulp: 29%), and the TOC in the wastewater was 98 mg / L (TOC increase rate relative to Reference Example 2: 108%).

[0192] [Comparative Example 4] Bleached pulp was produced in the same manner as in Example 7, except that the amount of monopersulfuric acid added in the monopersulfuric acid bleaching step (MPS step) was changed to 0.10 mass% based on the bone dry mass of the unbleached pulp. The chlorine dioxide addition ratio / monopersulfuric acid addition ratio was 5.00.

[0193] The ISO brightness, potassium permanganate value (K value), viscosity, and hexenuronic acid content (HexA content) of the bleached pulp, as well as the total organic carbon (TOC) in the wastewater were measured in the same manner as in Example 7. As a result, the ISO brightness was 62.6% (increase rate relative to the raw pulp: 114%), the K value was 4.0 (residual rate relative to the raw pulp: 57%), the viscosity was 17 cP (retention rate relative to the raw pulp: 89%), the HexA content was 26.2 μmol / g (residual rate relative to the raw pulp: 73%), and the TOC in the wastewater was 101 mg / L (TOC increase rate relative to Reference Example 2: 111%).

[0194] [Example 13] Bleached pulp was produced in the same manner as in Example 4, except that the bleaching time in the monopersulfuric acid bleaching step (MPS step) was changed to 2 minutes. The chlorine dioxide addition ratio / monopersulfuric acid addition ratio was 1.25.

[0195] The ISO brightness, potassium permanganate value (K value), viscosity, and hexenuronic acid content (HexA content) of the bleached pulp, as well as the total organic carbon (TOC) in the wastewater were measured in the same manner as in Example 1. As a result, the ISO brightness was 58.0% (increase rate relative to the raw pulp: 118%), the K value was 4.3 (residual rate relative to the raw pulp: 62%), the viscosity was 17 cP (retention rate relative to the raw pulp: 94%), the HexA content was 23.1 μmol / g (residual rate relative to the raw pulp: 71%), and the TOC in the wastewater was 277 mg / L (TOC increase rate relative to Reference Example 1: 108%).

[0196] [Example 14] Bleached pulp was produced in the same manner as in Example 4, except that the bleaching time in the monopersulfuric acid bleaching step (MPS step) was changed to 7 minutes. The chlorine dioxide addition ratio / monopersulfuric acid addition ratio was 1.25.

[0197] The ISO brightness, potassium permanganate value (K value), viscosity, and hexenuronic acid content (HexA content) of the bleached pulp, as well as the total organic carbon (TOC) in the wastewater were measured in the same manner as in Example 1. As a result, the ISO brightness was 58.3% (increase rate relative to the raw pulp: 118%), the K value was 4.3 (residual rate relative to the raw pulp: 61%), the viscosity was 16 cP (retention rate relative to the raw pulp: 89%), the HexA content was 19.1 μmol / g (residual rate relative to the raw pulp: 59%), and the TOC in the wastewater was 285 mg / L (TOC increase rate relative to Reference Example 1: 111%).

[0198] The results obtained are shown in Table 1 below.

[0199] [Table 1]

[0200] The results in Table 1 show that the bleached pulps produced in Examples 1 to 14 have excellent brightness.

[0201] In Examples 1, 2, 4, and 6, the absorbance at a wavelength of 280 nm of the mixed wastewater of 50 mL of the wastewater from the MPS process and 50 mL of the wastewater from the D process was measured using a spectrophotometer (UV-2450; Shimadzu Corporation). The results are shown in Table 2 below.

[0202] [Table 2]

[0203] The results in Table 2 show that the wastewater contains aromatic compounds, which are presumed to be lignin or its derivatives.

[0204] [ISO brightness and viscosity after deterioration test] The ISO brightness and viscosity of the bleached pulps of the above Examples 1, 2, 4, 6, 7, 10, and 12, Comparative Examples 2 and 4, Reference Examples 1 and 2, and the following Examples 15 to 23 were measured. In addition, a deterioration test was conducted, and the ISO brightness and viscosity of the bleached pulp after the deterioration test were measured. In each measurement, a pulp sheet (acidic paper) prepared in the same manner as in Example 1 was used as the measurement sample.

[0205] Example 15 Except for changing the amount of monopersulfuric acid added in the monopersulfuric acid bleaching step (MPS step) to 2.00 mass% based on the bone dry mass of the unbleached pulp, bleached pulp was produced in the same manner as in Example 1. The chlorine dioxide addition rate / monopersulfuric acid addition rate was 0.25.

[0206] Example 16 Bleached pulp was produced in the same manner as in Example 4, except that the bleaching temperature in the first chlorine dioxide bleaching step (step D) was changed to 60°C and the bleaching time to 35 minutes. The chlorine dioxide addition ratio / monopersulfuric acid addition ratio was 1.25.

[0207] (Example 17) Bleached pulp was produced in the same manner as in Example 6, except that the bleaching temperature in the first chlorine dioxide bleaching step (step D) was changed to 60°C and the bleaching time to 35 minutes. The chlorine dioxide addition ratio / monopersulfuric acid addition ratio was 0.33.

[0208] (Example 18) A bleached pulp was produced in the same manner as in Example 6, except that the bleaching time in the monopersulfuric acid bleaching step (MPS step) was changed to 90 minutes. The chlorine dioxide addition ratio / monopersulfuric acid addition ratio was 0.33.

[0209] (Example 19) Bleached pulp was produced in the same manner as in Example 1, except that the amount of monopersulfuric acid added in the monopersulfuric acid bleaching step (MPS step) was changed to 0.40 mass% based on the bone dry mass of the unbleached pulp, the bleaching temperature was changed to 70°C, the bleaching time was changed to 90 minutes, and the bleaching temperature in the first chlorine dioxide bleaching step (D step) was changed to 60°C, and the bleaching time was changed to 35 minutes. The chlorine dioxide addition ratio / monopersulfuric acid addition ratio was 1.25.

[0210] (Example 20) Bleached pulp was produced in the same manner as in Example 19, except that the amount of monopersulfuric acid added in the monopersulfuric acid bleaching step (MPS step) was changed to 1.50 mass% based on the bone dry mass of the unbleached pulp. The chlorine dioxide addition rate / monopersulfuric acid addition rate was 0.33.

[0211] Example 21 Bleached pulp was produced in the same manner as in Example 19, except that the amount of monopersulfuric acid added in the monopersulfuric acid bleaching step (MPS step) was changed to 1.50 mass% based on the bone dry mass of the unbleached pulp, the bleaching treatment time was changed to 120 minutes, and the bleaching treatment temperature in the first chlorine dioxide bleaching step (D step) was changed to 70°C and the bleaching treatment time to 120 minutes. The chlorine dioxide addition rate / monopersulfuric acid addition rate was 0.33.

[0212] Example 22 Bleached pulp was produced in the same manner as in Example 6, except that the bleaching temperature in the monopersulfuric acid bleaching step (MPS step) was changed to 60° C. The chlorine dioxide addition ratio / monopersulfuric acid addition ratio was 0.33.

[0213] (Example 23) Bleached pulp was produced in the same manner as in Example 7, except that the amount of monopersulfuric acid added in the monopersulfuric acid bleaching step (MPS step) was changed to 2.00 mass% based on the bone dry mass of the unbleached pulp. The chlorine dioxide addition rate / monopersulfuric acid addition rate was 0.25.

[0214] (Deterioration test) The deterioration test was carried out according to the accelerated deterioration treatment method for paper and paperboard (JIS8154-3:2008), by keeping the acidic paper at 80°C and 65% relative humidity for 24 hours.

[0215] (ISO whiteness) The ISO whiteness index was measured before and after the deterioration test in the same manner as in Example 1, and the whiteness change rate was calculated by the following formula. The obtained results are shown in Table 3 below.

[0216]

number

[0217] (viscosity) In addition, the viscosity before and after the deterioration test was measured in the same manner as in Example 1, and the viscosity difference was calculated by the following formula. The obtained results are shown in Table 3 below.

[0218]

number

[0219] [Table 3]

[0220] The results in Table 3 show that the bleached pulps produced in Examples 1, 2, 4, 6, 7, 10, 12, and 15 to 23 were less likely to lose brightness even after the deterioration test.

Claims

1. an alkaline oxygen bleaching step of alkaline oxygen bleaching the unbleached pulp; a monopersulfuric acid bleaching step of treating with monopersulfuric acid; a first chlorine dioxide bleaching step of treating with chlorine dioxide; Including, a ratio of an addition rate (mass%) of monopersulfuric acid to the bone dry mass of the unbleached pulp to an addition rate (mass%) of chlorine dioxide to the bone dry mass of the unbleached pulp (chlorine dioxide addition rate / monopersulfuric acid addition rate) is 0.25 to 2.0; The method for producing bleached pulp, wherein the treatment temperature in the monopersulfuric acid bleaching step is 70 to 98°C.

2. The method according to claim 1, further comprising an alkaline hydrogen peroxide bleaching step of treating with alkaline hydrogen peroxide.

3. The method according to claim 1 or 2, further comprising an ozone bleaching step of treating with ozone.

4. an alkaline oxygen bleaching step of alkaline oxygen bleaching the unbleached pulp; a monopersulfuric acid bleaching step of treating with monopersulfuric acid; a first chlorine dioxide bleaching step of treating with chlorine dioxide; an alkaline hydrogen peroxide bleaching step of treating with alkaline hydrogen peroxide; a second chlorine dioxide bleaching step of treating with chlorine dioxide; in this order, a ratio (% by mass) of monopersulfuric acid to the bone dry mass of the unbleached pulp and a ratio (% by mass) of chlorine dioxide to the bone dry mass of the unbleached pulp in the first chlorine dioxide bleaching step (chlorine dioxide addition rate / monopersulfuric acid addition rate) is 0.25 to 2.0; The method for producing bleached pulp, wherein the treatment temperature in the monopersulfuric acid bleaching step is 70 to 98°C.

5. an alkaline oxygen bleaching step of alkaline oxygen bleaching the unbleached pulp; a monopersulfuric acid bleaching step of treating with monopersulfuric acid; a first chlorine dioxide bleaching step of treating with chlorine dioxide; Including, The addition rate (mass%) of monopersulfuric acid to the bone dry mass of the unbleached pulp is 0.30 to 1.75 mass%, The method for producing bleached pulp comprises a ratio (% by mass) of monopersulfuric acid to the bone dry mass of the unbleached pulp and a ratio (% by mass) of chlorine dioxide to the bone dry mass of the unbleached pulp (chlorine dioxide addition rate / monopersulfuric acid addition rate) of 0.5 to 2.

0.

6. The alkaline oxygen bleaching step; the monopersulfuric acid bleaching step; the first chlorine dioxide bleaching step; an alkaline hydrogen peroxide bleaching step of treating with alkaline hydrogen peroxide; a second chlorine dioxide bleaching step of treating with chlorine dioxide; The method of claim 5 , comprising, in this order:

7. The method according to claim 5 or 6, further comprising an alkaline hydrogen peroxide bleaching step of treating with alkaline hydrogen peroxide.

8. The method according to any one of claims 5 to 7, further comprising an ozone bleaching step of treating with ozone.

9. The method according to any one of claims 5 to 8, wherein the treatment temperature in the monopersulfuric acid bleaching step is 40 to 98°C.

10. The total organic carbon (TOC) of the wastewater obtained in the monopersulfuric acid bleaching step and the first chlorine dioxide bleaching step is １ The total organic carbon (TOC) content of wastewater obtained in the sulfuric acid bleaching step and the first chlorine dioxide bleaching step in the reference process is calculated by replacing the monopersulfuric acid bleaching step with a sulfuric acid bleaching step. ２ TOC increase rate (TOC １ / TOC ２ The method according to any one of claims 1 to 9, wherein the ratio of the total number of particles per unit area to the total number of particles per unit area is 100%.

11. The method according to claim 10, further comprising reusing at least a portion of the wastewater from the monopersulfuric acid bleaching step and the wastewater from the first chlorine dioxide bleaching step in the monopersulfuric acid bleaching step.

12. The method according to any one of claims 1 to 11, wherein the treatment time of the monopersulfuric acid bleaching step is 2 to 200 minutes.

13. The method according to any one of claims 1 to 12, wherein the treatment pH in the monopersulfuric acid bleaching step is 2 to 5.

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