TWI859279B - Method for producing bleached pulp - Google Patents

Abstract

The present invention provides a novel method for producing bleached pulp, which has excellent whiteness, fading properties and other properties. The above-mentioned problem is solved by the following method for producing bleached pulp. That is, it is a method for producing bleached pulp, which comprises: a digestion step of digesting wood fiber material, an alkali bleaching step of bleaching the unbleached pulp obtained by the digestion step, and a chlorine dioxide treatment step of treating the pulp obtained by the alkali bleaching step with chlorine dioxide and persulfuric acid, wherein the pH value of the treatment liquid in the chlorine dioxide treatment step is in the range of 2 to 8.

Description

Method for producing bleached pulp

The present invention relates to a method for producing bleached pulp using wood cellulose materials such as wood as raw materials.

In the past, methods for producing bleached pulp for papermaking have been known (e.g., patent documents 1 to 5). In the conventional methods for producing bleached pulp, unbleached pulp is subjected to various treatments to produce bleached pulp. [Prior art document] [Patent document]

[Patent Document 1] WO2009/081714 [Patent Document 2] WO2007/132836 [Patent Document 3] Japanese Patent Publication No. 2010-270410 [Patent Document 4] Japanese Patent Publication No. 2010-265564 [Patent Document 5] Japanese Patent Publication No. 2008-088606

[The problem that the invention wants to solve]

In the conventional bleached pulp production method, there are various treatment steps for unbleached pulp, and there are many combinations of these steps. In addition, it is necessary to develop a new method for producing bleached pulp that is superior in terms of various properties such as whiteness and fading resistance. [Means for solving the problem]

The present invention provides the following method for producing bleached pulp. (1) A method for producing bleached pulp, comprising: a digestion step of digesting a woody cellulose material, an alkali-oxygen bleaching step of bleaching the unbleached pulp obtained by the digestion step, and a chlorine dioxide treatment step of treating the pulp obtained by the alkali-oxygen bleaching step with chlorine dioxide and persulfuric acid, wherein the pH value of the treatment liquid in the chlorine dioxide treatment step is in the range of 2 to 8. (2) A method for producing bleached pulp as described in (1), wherein in the chlorine dioxide treatment step, the amount of persulfuric acid added is 0.01-2.00 mass % relative to the absolute dry pulp mass, and the amount of chlorine dioxide added is 0.01-2.00 mass % relative to the absolute dry pulp mass. (3) A method for producing bleached pulp as described in (1) or (2), wherein the time of the chlorine dioxide treatment step is 20-200 minutes, and the temperature in the chlorine dioxide treatment step is 40-70°C. (4) A method for producing bleached pulp as described in any one of (1) to (3), wherein the bleached pulp is papermaking pulp. (5) The method for producing bleached pulp as described in any one of (1) to (4), further comprising a pH measuring step of measuring the pH value of the treatment liquid in the chlorine dioxide treatment step. (6) The method for producing bleached pulp as described in any one of (1) to (5), wherein the pH value of the treatment liquid before the treatment in the chlorine dioxide treatment step, i.e., the pre-treatment pH value, and the pH value of the treatment liquid after the treatment, i.e., the post-treatment pH value, are both within the range of 2 to 8. (7) The method for producing bleached pulp as described in any one of (1) to (6), wherein the difference between the pH value of the treatment liquid before the treatment in the chlorine dioxide treatment step, i.e., the pre-treatment pH value, and the pH value of the treatment liquid after the treatment in the chlorine dioxide treatment step, i.e., the post-treatment pH value, is 3.0 or less. (8) The method for producing bleached pulp according to (7), wherein the difference between the pH value before the treatment and the pH value after the treatment is less than 2.0. (9) A method for producing bleached pulp, comprising: a digestion step of digesting a woody cellulose material, an alkali-oxygen bleaching step of bleaching the unbleached pulp obtained by the digestion step, and a chlorine dioxide treatment step of treating the pulp obtained by the alkali-oxygen bleaching step with chlorine dioxide and permonosulfuric acid, wherein the pH value of the treatment solution in the chlorine dioxide treatment step is in the range of 2 to 8, and the pH value of the treatment solution before the treatment in the chlorine dioxide treatment step, i.e., the pH value before the treatment, is 3.0 or less. The difference between the pH value before the treatment and the pH value of the treatment solution after the treatment in the chlorine dioxide treatment step, i.e., the pH value after the treatment, is 1.0 or less. When the pH value before the treatment is higher than 3.0 and less than or equal to 5.5, the difference between the pH value before the treatment and the pH value after the treatment is 2.0 or less. When the pH value before the treatment is higher than 5.5 and less than or equal to 8.0, the difference between the pH value before the treatment and the pH value after the treatment is 2.5 or less. (10) The method for producing bleached pulp as described in (9), wherein when the pH value before treatment is 3.0 or less, the difference between the pH value before treatment and the pH value after treatment is 0.5 or less; when the pH value before treatment is higher than 3.0 and lower than or equal to 5.5, the difference between the pH value before treatment and the pH value after treatment is 1.6 or less; when the pH value before treatment is higher than 5.5 and lower than or equal to 8.0, the difference between the pH value before treatment and the pH value after treatment is 2.0 or less. (11) The method for producing bleached pulp as described in any one of (1) to (10), wherein the ISO brightness of the bleached pulp after the chlorine dioxide treatment step is 60% or more. (12) The method for producing bleached pulp as described in any one of (1) to (11), wherein the K value of the bleached pulp after the chlorine dioxide treatment step is 1.50 to 4.50. (13) The method for producing bleached pulp as described in any one of (1) to (12), further comprising a K value measuring step of measuring the K value of the pulp treated in the chlorine dioxide treatment step before the treatment, i.e., the K value before treatment, and the K value of the bleached pulp after the treatment in the chlorine dioxide treatment step, i.e., the K value after treatment. (14) The method for producing bleached pulp as described in (13), wherein the K value residual ratio (K ₂ /K ₁ × 100) calculated from the K value before the treatment (K ₁ ) and the K value after the treatment (K ₂ ) is less than 80%. (15) The method for producing bleached pulp as described in any one of (1) to (14), wherein the hexenuronic acid content in the bleached pulp after the chlorine dioxide treatment step is less than 30.00 μmol/g. (16) A method for producing bleached pulp according to any one of (1) to (15), wherein the residual ratio of hexenuronic acid content (H2/H1×100) calculated from the hexenuronic acid content _(H1 (μmol/g)) of the pulp before treatment and the hexenuronic acid content ( _H2 (μmol/g)) of the bleached pulp after treatment in _{the chlorine} dioxide treatment step is 90% or less. (17) A method for producing bleached pulp according to any one of (1) to (16), wherein a value of a brightness increase rate (W 2 /W 1 × 100) calculated from an ISO brightness value (W ₁ %) of the pulp before treatment in the chlorine dioxide treatment step and an ISO brightness value ₍ W ₂ %) of the bleached pulp after treatment in the _chlorine dioxide treatment step is 115% or more. (18) The method for producing bleached pulp as described in any one of (1) to (17), wherein the viscosity retention ratio (A 2 /A 1 × 100) calculated from the viscosity value of the pulp before treatment (A ₁ (cP)) and the viscosity value of the bleached pulp after treatment (A ₂ (cP)) is 60% or more. (19) The method for producing bleached pulp as described in any _one of ( ₁ ) to (18), wherein the weight ratio of chlorine dioxide to peroxymonosulfuric acid used in the chlorine dioxide treatment step is 1 or more. (20) A method for producing bleached pulp as described in any one of (1) to (19), wherein after the chlorine dioxide treatment step, there is further a hydrogen peroxide treatment step, wherein the pulp treated with chlorine dioxide is treated with hydrogen peroxide. (21) A method for producing bleached pulp as described in (5), wherein in the pH measurement step, the pH value of the treatment solution before treatment, i.e., the pre-treatment pH value, and the pH value of the treatment solution after treatment, i.e., the post-treatment pH value, are measured. [Effect of the Invention]

According to the method for producing bleached pulp of the present invention, by adjusting the conditions of the pulp treatment steps, pulp having generally excellent properties such as high whiteness and excellent fading property can be obtained.

The present invention is described in detail below. The present invention is not limited to the following embodiments, and can be arbitrarily modified and implemented within the scope of the effects of the invention.

[Method for producing bleached pulp] The method for producing bleached pulp of the present invention comprises: a digestion step of digesting wood fiber material, an alkali-oxygen bleaching step of bleaching unbleached pulp, and a chlorine dioxide treatment step of treating the pulp obtained by the alkali-oxygen bleaching step. In the chlorine dioxide treatment step, the pH value of the treatment liquid is adjusted to a predetermined range.

[Wooden Cellulose Material] The woody cellulosic material used in the present invention is preferably a broad-leaved tree material containing a large amount of methyl glucuronic acid that generates hexenuronic acid, or a coniferous tree material. Furthermore, the woody cellulosic material may be non-wood such as bamboo and hemp, and may also be a mixture of the above-mentioned broad-leaved tree material, coniferous tree material, and non-wood, without particular limitation.

[Digestion step] In the present invention, unbleached pulp can be obtained by a digestion step of digesting wood cellulose material. As for the digestion method used in the digestion step to obtain unbleached pulp, known digestion methods such as kraft digestion, polysulfide digestion, soda digestion, acidic sulfite digestion, alkaline sulfite digestion, etc. can be used. Considering pulp quality, energy efficiency, etc., it is preferable to use the kraft digestion method or polysulfide digestion.

For example, when 100% of the wood cellulose of broad-leaved wood is subjected to sulfate digestion, the sulfidation degree of the sulfate digestion liquid is 5-75% by mass, preferably 15-45% by mass, and the effective alkali addition rate is 5-30% by mass per unit mass of absolutely dry wood, preferably 10-25% by mass. In addition, the digestion temperature is, for example, 130-170°C, and the digestion method can be either a continuous digestion method or a batch digestion method. During the digestion, when a continuous digestion kettle is used, a modified digestion method in which the digestion liquid is added at multiple points can also be used, and the method is not particularly limited.

In the digestion step of the woody cellulose material, a delignification treatment is performed. Finally, when the pulp for papermaking is obtained as bleached pulp, hemicellulose from the woody cellulose material can also be removed together with lignin in advance in the digestion step. When it is desired to perform a hemicellulose removal treatment, for example, a hydrolysis treatment step can be set up before the step of removing lignin by an alkaline digestion method such as a sulfate digestion method, and the hemicellulose can be removed. In addition, if lignin is removed by an acidic sulfite digestion method, hemicellulose and lignin can be removed at the same time, so a digestion step using an acidic sulfite digestion method can also be used. The object of the method for producing bleached pulp of the present invention is not particularly limited, and it is preferably pulp for papermaking. In addition, the pulp for papermaking herein also includes items 47.01 to 47.06 in the Harmonized System Code (H.S. Code) for the names and classification of international trade commodities, that is, any one of mechanical wood pulp, chemical wood pulp, wood pulp produced by combining mechanical and chemical pulping steps, waste paper pulp, and pulp using other cellulose fibers as raw materials.

In the steaming step, 5-30 mass% of active base is preferably added to the absolute dry pulp unit mass. In the steaming step, 10-25 mass% of active base is preferably used, 12-20 mass% is more preferably used, and 15-18 mass% of active base is more preferably used. There is no particular limitation on the active base in the steaming step, for example, sodium hydroxide, potassium hydroxide, etc. can be used. In the steaming step, sodium hydroxide is preferably used as the active base.

Furthermore, in the digestion step of the lignocellulosic material, known cyclic keto compounds as digestion aids may be used in the digestion liquid used, such as benzoquinone, naphthoquinone, anthraquinone, anthraquinone, phenanthrenequinone, and core-substituted products of the aforementioned quinone compounds such as alkyl and amine groups, or reduced forms of the aforementioned quinone compounds, i.e., anthrahydroquinone, etc. In addition, one or more of the stable compounds selected from 9,10-diketohydroanthracene compounds obtained as intermediates in the anthraquinone synthesis method of the Diels-Alder process may be added. The addition rate of these digestion aids is a normal addition rate, for example, 0.001 to 1.0 mass % per absolute dry pulp unit mass of the lignocellulosic material such as wood chips.

[Oxygen-alkali bleaching step] The unbleached pulp obtained by the above-mentioned digestion is subjected to washing, roughing, and fine selection steps as needed. Thereafter, it is further subjected to delignification treatment and bleaching by the oxygen-alkali bleaching step. For the oxygen-alkali bleaching step, the well-known medium-concentration method or high-concentration method can be used, and the pulp concentration in the treatment liquid is preferably 8-40% by mass, and the pulp concentration is more preferably 10-35% by mass.

In the alkali-oxygen bleaching step, sodium hydroxide (caustic sodium), potassium hydroxide, oxidized sulfate liquor, etc. can be used as the alkali added to the pulp. In addition, oxygen and alkali can be added together in the alkali-oxygen bleaching step. As for oxygen, oxygen from a low-temperature refining process, oxygen from PSA (Pressure Swing Adsorption), oxygen from VSA (Vacuum Swing Adsorption), etc. can be used.

In the oxygen-alkali bleaching step, alkali and oxygen used therewith are added to the pulp slurry, for example, in a mixer. After sufficient mixing, the mixture of pulp, oxygen and alkali is sent to a reaction tower that can be maintained for a fixed time under pressure to perform a delignification reaction on the pulp. The reaction conditions of the oxygen-alkali bleaching step are described as follows. That is, the oxygen addition rate is 0.5-3 mass% based on the absolute dry (BD; bonedry) pulp unit mass, the alkali addition rate is 0.5-4 mass%, the reaction temperature is 80-120°C, the reaction time is 15-100 minutes, the pulp concentration is 8-40 mass%, preferably 10-35 mass%, and other conditions can use known ones.

In the alkali-oxygen bleaching step, it is preferred to continuously perform the above-mentioned alkali-oxygen bleaching treatment for multiple times to delignify as much as possible and reduce the content of heavy metals in advance. The pulp after the alkali-oxygen treatment, that is, the crude pulp obtained by the alkali-oxygen treatment step, is preferably sent to the washing step.

[Chlorine dioxide treatment step] In the chlorine dioxide treatment step, the pulp washed in the washing step is preferably subjected to chlorine dioxide treatment as follows. In the chlorine dioxide treatment, peroxymonosulfuric acid and chlorine dioxide are added to the pulp obtained by the oxyalkali bleaching step. In the chlorine dioxide treatment step, the amount of peroxymonosulfuric acid added per unit mass of the absolute dry pulp of the crude pulp to be treated is preferably 0.01-2.00 mass%, more preferably 0.02-1.00 mass%, more preferably 0.05-0.80 mass%, and particularly preferably 0.1-0.6 mass%.

Peroxymonosulfuric acid is also called peroxymonosulfuric acid. It can be produced by hydrolyzing peroxydisulfuric acid or by mixing hydrogen peroxide and sulfuric acid in any ratio. The production method is not particularly limited. In addition, a double salt of peroxymonosulfuric acid (2KHSO ₅ ･KHSO ₄ ･K ₂ SO ₄ ) such as OXONE can also be used. However, considering the economical efficiency, it is better to mix cheap high-concentration hydrogen peroxide and cheap high-concentration sulfuric acid to produce and use peroxymonosulfuric acid at low cost.

As for the method of mixing high-concentration hydrogen peroxide and high-concentration sulfuric acid to produce peroxymonosulfuric acid, it is ideal to add concentrated sulfuric acid with a concentration of 80-98 mass%, preferably 93-98 mass%, to hydrogen peroxide with a concentration of 20-70 mass%, preferably 35-70 mass%, and mix them. The molar ratio of the above-mentioned sulfuric acid and hydrogen peroxide is preferably 1:1-5:1, and more preferably 2:1-4:1. If the hydrogen peroxide and sulfuric acid are used at low concentrations, the production efficiency of peroxymonosulfuric acid will decrease, so it is not suitable. Moreover, if these concentrations are too high, the risk of fire will increase, so it is not suitable. In addition, when the molar ratio of sulfuric acid and hydrogen peroxide is outside the range of 1:1-5:1, the production efficiency of peroxymonosulfuric acid will decrease, so it is also not suitable.

In the chlorine dioxide treatment step, it is preferred to use permonosulfuric acid and chlorine dioxide together for the washed pulp. The amount of chlorine dioxide used in the chlorine dioxide treatment step is preferably 0.01-2.00 mass%, more preferably 0.05-1.50 mass%, more preferably 0.1-1.00 mass%, and particularly preferably 0.3-0.80 mass%, based on the absolute dry pulp unit mass of the crude pulp to be treated. In this way, the chlorine dioxide treatment step used in the present invention is cost-effective because the amount of expensive chlorine dioxide used can be suppressed. In addition, by using chlorine dioxide and permonosulfuric acid together, pulp can also be bleached efficiently. Furthermore, in the present invention, not only molecular chlorine is not required, but the amount of chlorine-based compounds, i.e., chlorine dioxide, is also used at the minimum, which not only ensures safety but is also recognized to have the effect of suppressing environmental load. In addition, the mass ratio of chlorine dioxide to peroxymonosulfuric acid used in the chlorine dioxide treatment step (mass of chlorine dioxide/mass of peroxymonosulfuric acid) is greater than 1, that is, it is preferable to use more chlorine dioxide than peroxymonosulfuric acid, and the above ratio is preferably greater than 2.

In the chlorine dioxide treatment step, by adding inorganic peroxyacid and/or its salt (permonosulfuric acid and/or its salt) and chlorine dioxide to the reaction solution, although there is a possibility that the two bleaching agents react and are wasted, the addition of permonosulfuric acid can reduce the amount of expensive chlorine dioxide used. Therefore, it is generally advantageous in terms of cost. Furthermore, if the existing equipment used for the treatment step using only chlorine dioxide is introduced into the treatment step using only permonosulfuric acid, the initial equipment investment such as the additional bleaching tower will become huge. However, the method of adding permonosulfuric acid and chlorine dioxide to the reaction solution and using them together can be easily introduced by only slightly modifying the existing equipment used for chlorine dioxide treatment, such as adding a permonosulfuric acid injection pipeline, and can therefore be introduced into many factories.

The duration of the chlorine dioxide treatment step is preferably 10 to 200 minutes, more preferably 15 to 180 minutes, even more preferably 20 to 100 minutes, and particularly preferably 25 to 60 minutes. In the chlorine dioxide treatment step, the treatment temperature is preferably 20 to 100°C, more preferably 30 to 90°C, and particularly preferably 40 to 70°C. There is no particular limitation on the concentration of the pulp to be treated in the chlorine dioxide treatment step, but it is usually 5 to 30% by mass, and is preferably 8 to 15% by mass from the perspective of operability.

[pH measurement step and pH adjustment] During the chlorine dioxide treatment step, the pH value of the treatment solution should be maintained within a good range. Specifically, the pH value of the treatment solution in the chlorine dioxide treatment step is adjusted to 2-8, preferably 3-8, more preferably 4-8, more preferably 4.5-7.5, and particularly preferably 5-7. In addition, since chlorine dioxide can efficiently generate ions with bleaching activity if the pH value is within a predetermined range, in the chlorine dioxide treatment step, not only the pH value during the treatment step, but also the pH value of the treatment solution before the treatment step, i.e., the pre-treatment pH value, especially the pH value at the beginning or just before the treatment step, and the pH value after the treatment, i.e., the post-treatment pH value, especially the pH value of the treatment solution just after the treatment, should all be adjusted to the above range of 2 to 8. However, at least one of the pre-treatment pH value and the post-treatment pH value may be within the above range of 2 to 8. In addition, during the treatment of the chlorine dioxide treatment step, the pH value of the treatment solution should always be within the above range of 2 to 8. The chlorine dioxide treatment step preferably further includes a pH measurement step for measuring the pH value of the treatment object, i.e., the treatment liquid.

In the chlorine dioxide treatment step, the pH value of the treatment liquid is preferably adjusted to 3.5 to 7.5, for example, 3.5 or more than 3.5 and less than 7, more preferably 4 to 7, for example, 4 or more than 4 and less than 7, from the viewpoint of achieving high whiteness of the treated pulp. In addition, in the chlorine dioxide treatment step, the pH value of the treatment liquid is preferably adjusted to 2 to 6.5, more preferably 2.5 to 6, and more preferably 3 to 5.5, from the viewpoint of suppressing the K value and hexenuronic acid content of the treated pulp. In this way, by always adjusting the pH value of the treatment solution in the treatment step to an ideal range while paying attention to the change of the pH value over time, the pulp can be efficiently bleached without exposing it to excessively severe conditions during the chlorine dioxide treatment step.

In the chlorine dioxide treatment step, it is not necessary to measure the pH value multiple times. For example, according to the rule of thumb, as long as the pH value range can be suppressed within the desired range, it is not necessary to measure the pH value. However, in the pH measurement step, it is advisable to measure the pH value of the treatment solution multiple times, such as the pH value before treatment and the pH value after treatment, especially the pH value after treatment, to ensure that the ideal pH range is ensured. In addition, the pH value can be measured at least before the chlorine dioxide treatment (before the treatment is about to start, or just after the treatment starts), during the treatment (for example, when half of the time required for the chlorine dioxide treatment step has passed), and after the treatment (just after the treatment is about to end, or just before the treatment is about to end). Furthermore, in order to adjust the pH of the treatment solution in the chlorine dioxide treatment step, known alkalis and acids may be added to the treatment solution.

Regarding the chlorine dioxide treatment step, the difference between the pH value before treatment and the pH value after treatment is preferably less than 3.0, and the pH value should be adjusted to such a value as needed. In addition, the pH value of the treatment solution can be adjusted according to the above-mentioned known methods, but based on the rule of thumb, it is advisable to use a treatment solution that can maintain the pH value within the ideal range in advance. The difference between the pH value before treatment and the pH value after treatment is more preferably less than 2.5, more preferably less than 2.0, and particularly preferably less than 1.5. When the pH value before treatment is less than 3.0, the difference between the pH value before treatment and the pH value after treatment is preferably less than 1.0, and the difference between the pH value before treatment and the pH value after treatment is more preferably less than 0.5. When the pH value before treatment is higher than 3.0 and lower than 5.5, the difference between the pH value before treatment and the pH value after treatment is preferably 2.0 or lower, and the difference between the pH value before treatment and the pH value after treatment is more preferably 1.6 or lower. In addition, when the pH value before treatment is higher than 5.5 and lower than 8.0, the difference between the pH value before treatment and the pH value after treatment is preferably 2.5 or lower, and the difference between the pH value before treatment and the pH value after treatment is more preferably 2.0 or lower, and particularly preferably 1.6 or lower.

[Pulp properties] The ISO whiteness of the bleached pulp after the chlorine dioxide treatment step, i.e., the pulp obtained by the chlorine dioxide treatment, is preferably 60 (%) or more, more preferably 65 (%) or more, more preferably 70 (%) or more, and particularly preferably 72 (%) or more. In addition, the ISO whiteness of the bleached pulp is preferably high, so its upper limit is not important, but the upper limit of the ISO whiteness of the bleached pulp after the chlorine dioxide treatment step is, for example, 90 (%) or less.

In the chlorine dioxide treatment step, the whiteness increase ratio W 2 /W 1 × 100 calculated from the ISO whiteness value W ₁ (%) of the pulp before treatment and the ISO whiteness value W ₂ (%) of the bleached pulp after treatment in _{the chlorine} dioxide treatment step is preferably 115% or more. The whiteness increase ratio is more preferably 120% or more, more preferably 125% or more, more preferably 130% or more, and particularly preferably 135% or more. The higher the whiteness increase ratio, the better, so its upper limit is not important, but the upper limit of the whiteness increase ratio is, for example, 160% or less. Furthermore, the chlorine dioxide treatment step preferably includes a whiteness measuring step for measuring the whiteness value of the pulp, and preferably includes a whiteness measuring step for measuring the ISO whiteness value of the pulp before treatment (especially just before the treatment begins) and the ISO whiteness value of the pulp after treatment (especially just after the treatment ends) in the chlorine dioxide treatment step.

The K value of the bleached pulp after the chlorine dioxide treatment step, i.e. the pulp obtained by the chlorine dioxide treatment, is preferably 1.50-4.50, for example, below 4.50, more preferably 1.80-4.10, for example, below 4.10 or below 4.00, further preferably 2.10-3.80, particularly preferably 2.50-3.50.

Regarding the chlorine dioxide treatment step, it is preferable to further include a K value measuring step for measuring the K value of the pulp to be treated before treatment (especially immediately before the treatment begins), i.e., the K value before treatment, and the K value after treatment (especially immediately after the treatment ends) in the chlorine dioxide treatment step, i.e., the K value after treatment. In addition, the K value residual rate, i.e., K2/ _K1 × ₁₀₀ , calculated from the K value before treatment, i.e., _{K1 ,} and the K value after treatment, i.e., K2, is preferably 80% or less, preferably 70% or less. The K value residual rate is more preferably 65% or less, and even more preferably 60% or less, 50% or less, for example, 45% or less. The K value residual rate is preferably lower, so its lower limit is not important, but the K value residual rate is, for example, 20% or more. Furthermore, the viscosity value (cP) of the pulp obtained by the chlorine dioxide treatment step is preferably 10 to 30, more preferably 15 to 25.

The hexenuronic acid content in the bleached pulp after the chlorine dioxide treatment step, i.e., the pulp obtained by the chlorine dioxide treatment, is preferably 30.00 μmol/g or less. The hexenuronic acid content is more preferably 25.00 μmol/g or less, and even more preferably 20.00 μmol/g or less. In addition, the hexenuronic acid content is preferably low, so its lower limit is not important, but the hexenuronic acid content in the bleached pulp after the chlorine dioxide treatment step is, for example, 10.00 μmol/g or more.

The residual hexenuronic acid content calculated from the hexenuronic acid content value of the treated pulp in the chlorine dioxide treatment step before treatment, i.e. _H1 (μmol/g), and the hexenuronic acid content value of the bleached pulp after treatment in the chlorine dioxide treatment step, i.e. the pulp obtained by the chlorine dioxide treatment, i.e. _H2 (μmol/g), is preferably 90 _% or less. The residual hexenuronic acid content, i.e. _H2 / _{H1 ×} 100, is more preferably 80% or less, more preferably 75% or less, and particularly preferably 70% or less, for example 60% or less or 50% or less. The hexenuronic acid content is preferably low, and thus the lower limit thereof is not critical, but the hexenuronic acid content is, for example, 20% or more.

The retention rate of the viscosity value (cP) of the pulp in the chlorine dioxide treatment step, that is, the ratio of the viscosity value A ₂ (cP) of the pulp obtained by the chlorine dioxide treatment step to the viscosity value A ₁ (cP) of the pulp immediately before the chlorine dioxide treatment step (A ₂ /A ₁ ×100 (%)) (cP) is preferably 60% or more. The above retention rate is more preferably 80% or more, more preferably 85% or more, and particularly preferably 90% or more, for example, 95% or more. In addition, the retention rate of the viscosity of the pulp is preferably high, so its upper limit is not important, but the retention rate of the viscosity of the pulp after the chlorine dioxide treatment step is, for example, 100% or less. In addition, the chlorine dioxide treatment step preferably further includes a viscosity measurement step for measuring the viscosity of the pulp. In the chlorine dioxide treatment step, the viscosity of the pulp during the treatment step, the viscosity of the pulp before the treatment step, i.e., the viscosity before treatment, especially the viscosity at the beginning or just before the beginning of the treatment step, and the viscosity after the treatment, i.e., the viscosity after treatment, especially the viscosity of the pulp just after the treatment, are preferably measured. In addition, the viscosity value (cP) of the pulp obtained by the chlorine dioxide treatment step is preferably 10 or more and 30 or less, and more preferably 15 or more and 25 or less.

[Hydrogen peroxide treatment step] In the hydrogen peroxide treatment step, hydrogen peroxide is added to the pulp that has been washed after the previous treatment. In addition, the hydrogen peroxide treatment step is preferably performed after the chlorine dioxide treatment step. At this time, the pulp that has been treated with chlorine dioxide at least once becomes the object of the hydrogen peroxide treatment step.

The addition rate of hydrogen peroxide in the hydrogen peroxide treatment step is preferably 0.1-2.0 mass %, more preferably 0.2-1.0 mass %, based on the absolute dry pulp unit mass of the crude pulp to be treated. In addition, the treatment temperature in the hydrogen peroxide treatment step is preferably 20-100°C, more preferably 40-90°C. In the hydrogen peroxide treatment step, the pH value of the reaction system is preferably adjusted to 8-14, and the pH value is more preferably adjusted to 10.5-12.0. Known alkalis and acids can be used for pH adjustment. In addition, there is no particular limitation on the pulp concentration in the hydrogen peroxide treatment step. Considering the operability, it is preferably 8-15 mass %.

In the hydrogen peroxide treatment step, at least one of magnesium salt and sodium silicate (soda sieve) is preferably used. _{As for magnesium salt, for example, magnesium sulfate is preferably added to the pulp. As for sodium silicate, for example, any one of compounds such as Na2SiO3, Na4SiO4, Na2Si2O5 , Na2Si4O9 is preferably added to the} pulp. If magnesium salt or sodium silicate is added _to the reaction system containing pulp _, it is possible to prevent the viscosity of the pulp from being reduced due to the reaction of heavy metals and transition metals such as copper, iron, and manganese with oxygen or hydrogen peroxide in an alkaline system to generate active oxygen free radicals. The addition rate of magnesium salt in the hydrogen peroxide treatment step is preferably 0.01-1.0 mass % of Mg relative to the absolute dry pulp mass of the crude pulp to be treated, more preferably 0.02-0.5 mass % of Mg relative to the absolute dry pulp mass, and even more preferably 0.05-0.2 mass % of Mg relative to the absolute dry pulp mass. Furthermore, the addition rate of the sodium silicate compound in the hydrogen peroxide treatment step is preferably 0.1-2.0 mass % relative to the absolute dry pulp mass of the crude pulp to be treated, more preferably 0.2-1.5 mass % relative to the absolute dry pulp mass, and even more preferably 0.5-1.0 mass % relative to the absolute dry pulp mass.

In the hydrogen peroxide treatment step, the reaction system is preferably kept alkaline as described above, and the mass of hydrogen peroxide is preferably 0.1-5.0 mass %, more preferably 0.2-3.0 mass %, and more preferably 0.3-2.0 mass % of the alkaline component is used. As the alkaline component, sodium hydroxide (caustic soda), potassium hydroxide, etc. can be used.

[Types of various steps and number of stages] In the method for producing bleached pulp, the chlorine dioxide treatment step preferably has only one stage. In this case, the steps of the method for producing bleached pulp can be simplified. However, in the method for producing bleached pulp, for pulp bleached by the alkali-oxygen method, a second chlorine dioxide treatment step of treating with persulfuric acid may be further included. In addition, in addition to the above-mentioned hydrogen peroxide treatment step, a second hydrogen peroxide treatment step of further treating with hydrogen peroxide may be included. Furthermore, in the method for producing bleached pulp comprising a second chlorine dioxide treatment step and a second hydrogen peroxide treatment step, it is also preferred that at least one of the hydrogen peroxide treatment steps is performed after the chlorine dioxide treatment.

Furthermore, the method for producing bleached pulp may also include a bleaching treatment step other than the hydrogen peroxide treatment step and the chlorine dioxide treatment step, such as an ozone treatment step of treating unbleached pulp with ozone, a chlorine treatment step, a hypochlorite treatment step, etc. However, in order to simplify the steps of the bleached pulp production method and ensure safety, it is preferable not to include the ozone treatment step, the chlorine treatment step, and the hypochlorite treatment step. [Example]

1. Production of peroxymonosulfuric acid 60.2 g of 98% sulfuric acid (special grade for reagents, manufactured by KISHIDA Chemical) was added to 15.2 g of 45% by mass hydrogen peroxide solution (manufactured by Mitsubishi Gas Chemical) to produce peroxymonosulfuric acid. The composition of the produced peroxymonosulfuric acid aqueous solution was 23.4% by mass peroxymonosulfuric acid, 1.8% by mass hydrogen peroxide, 59.8% by mass sulfuric acid, and 15.0% by mass water.

2. Determination of pulp whiteness After the bleached pulp is dissolved, two sheets with a basis weight of 400 g/ ^m2 are prepared according to ISO3688-1977, and the pulp whiteness (%) is determined according to JIS P 8148.

3. Determination of potassium permanganate value (K value) of pulp The determination of potassium permanganate value is carried out in accordance with TAPPI UM 253.

4. Determination of Hexenuronic Acid (HexA) in Pulp The completely washed pulp was weighed accurately to 0.8 g on an absolute dry basis. The pulp was placed in a pressure-resistant container, 80 mL of pure water was added, and then formic acid was added to adjust the pH to 3. The pressure-resistant container was placed in an oven and treated at 120°C for 4 hours to hydrolyze HexA with acid. After treatment, the solution was filtered and the acid hydrolyzates of HexA, namely 2-furancarboxylic acid and 5-carboxy-2-furancarboxaldehyde, were quantified by HPLC. The original amount of HexA was obtained by summing the molar amounts of these products.

5. Determination of pulp viscosity The determination of pulp viscosity is carried out in accordance with J.TAPPI No.44 method.

6. Physical properties of unbleached pulp used The physical properties of the unbleached pulp used are shown below. Unbleached pulp: pulp bleached by oxygen-alkali method Pulp 1: ISO brightness 48.0%, K value 8.0, HexA content 44.9μmol/pulp g, viscosity 24.19cP Pulp 2: ISO brightness 54.1%, K value 6.6, HexA content 37.8μmol/pulp g, viscosity 19.07cP

7. Cleaning conditions Pure water was added to each pulp (pulp 1 or pulp 2) after the above bleaching treatment to make the pulp concentration 2.4%, and then dehydrated until the pulp concentration reached 20% (cleaning rate 90%).

(Example 1) An absolute dry mass of 16 g of unbleached pulp 1 was sampled in a polyethylene bag. After adding the hollow fiber membrane filtered water required for bleaching the pulp at a concentration of 12%, an aqueous sodium hydroxide solution was added in an amount such that the pH at the start of the reaction became about 4.5 and mixed thoroughly. Thereafter, 0.75% by mass of chlorine dioxide and 0.4% by mass of permonosulfuric acid were added relative to the mass of the pulp, and further mixed. The pH at this time was 4.8. The bleaching treatment (chlorine dioxide treatment step) was performed by immersion in a constant temperature water tank at 60°C for about 36 minutes. The bleached pulp washed under the above washing conditions was diluted to 2L with pure water, adjusted to pH 5.5 with sulfite water, and made into two pulp sheets (acid papermaking) on a Buchner funnel. After air drying for 12 hours, the ISO brightness, K value, and HexA content of the papermaking samples were measured.

(Example 2) An absolute dry mass of 16 g of unbleached pulp 1 was sampled from a polyethylene bag. After adding the hollow fiber membrane filtered water required for bleaching the pulp at a concentration of 12%, an aqueous sodium hydroxide solution in an amount such that the pH at the start of the reaction becomes about 6 was added and mixed thoroughly. Thereafter, 0.75% by mass of chlorine dioxide and 0.4% by mass of peroxymonosulfuric acid were added relative to the mass of the pulp, and further mixed. The pH at this time was 6.3. Thereafter, the same operation as in Example 1 was performed.

(Example 3) An absolute dry mass of 16 g of unbleached pulp 2 was sampled in a polyethylene bag. After adding the hollow fiber membrane filtered water required for bleaching the pulp at a concentration of 10%, sulfuric acid was added in an amount such that the pH at the start of the reaction became about 2.5 and mixed thoroughly. Thereafter, 0.75% by mass of chlorine dioxide and 0.4% by mass of persulfuric acid were added relative to the mass of the pulp, and further mixed. The pH at this time was 2.5. The bleaching treatment (chlorine dioxide treatment step) was carried out by immersion in a constant temperature water tank at 60°C for about 30 minutes. Thereafter, the same operation as in Example 1 was carried out.

(Example 4) 16 g of absolute dry mass of unbleached pulp 2 was sampled in a polyethylene bag. After adding the hollow fiber membrane filtered water required for bleaching the pulp at a concentration of 10%, an aqueous sodium hydroxide solution was added in an amount to make the pH at the start of the reaction about 5.0 and mixed thoroughly. Thereafter, 0.75% by mass of chlorine dioxide and 0.4% by mass of permonosulfuric acid were added relative to the mass of the pulp, and further mixed. The pH at this time was 5.2. Thereafter, the same operation as in Example 3 was performed.

(Example 5) The unbleached pulp 2 was sampled in a polyethylene bag with an absolute dry mass of 16 g. After adding the hollow fiber membrane filtered water required for bleaching the pulp at a concentration of 10%, an aqueous sodium hydroxide solution was added in an amount that the pH at the start of the reaction was about 6.5 and mixed thoroughly. Thereafter, 0.75% by mass of chlorine dioxide and 0.4% by mass of permonosulfuric acid were added relative to the mass of the pulp, and further mixed. The pH at this time was 6.6. Thereafter, the same operation as in Example 3 was performed.

(Comparative Example 1) 16 g of absolute dry mass of unbleached pulp 1 was sampled in a polyethylene bag. After adding the hollow fiber membrane filtered water required for bleaching the pulp at a concentration of 12%, sulfuric acid was added in an amount to make the pH at the start of the reaction about 1.5 and mixed thoroughly. Thereafter, 0.75% by mass of chlorine dioxide and 0.4% by mass of persulfuric acid were added relative to the mass of the pulp, and further mixed. The pH at this time was 1.4. Thereafter, the same operation as in Example 1 was performed.

(Comparative Example 2) An absolute dry mass of 16 g of unbleached pulp 1 was sampled in a polyethylene bag. After adding the hollow fiber membrane filtered water required for bleaching the pulp at a concentration of 12%, an aqueous sodium hydroxide solution was added in an amount such that the pH at the start of the reaction became about 8.0 and mixed thoroughly. Thereafter, 0.75% by mass of chlorine dioxide and 0.4% by mass of permonosulfuric acid were added relative to the mass of the pulp, and further mixed. The pH at this time was 8.1. Thereafter, the same operation as in Example 1 was performed.

(Comparative Example 3) The unbleached pulp 2 was sampled in a polyethylene bag with an absolute dry mass of 16 g. After adding the hollow fiber membrane filtered water required for bleaching the pulp at a concentration of 10%, sulfuric acid was added in an amount to make the pH at the start of the reaction about 1.5 and mixed thoroughly. Thereafter, 0.75% by mass of chlorine dioxide and 0.4% by mass of persulfuric acid were added relative to the mass of the pulp, and further mixed. The pH at this time was 1.3. Thereafter, the same operation as in Example 3 was performed.

(Comparative Example 4) 16 g of absolute dry mass of unbleached pulp 2 was sampled in a polyethylene bag. After adding the hollow fiber membrane filtered water required for bleaching the pulp at a concentration of 10%, an aqueous sodium hydroxide solution was added in an amount such that the pH at the start of the reaction became about 8.0 and mixed thoroughly. Thereafter, 0.75% by mass of chlorine dioxide and 0.4% by mass of peroxymonosulfuric acid were added relative to the mass of the pulp and further mixed. The pH at this time was 8.2. Thereafter, the same operation as in Example 3 was performed.

The results of Examples 1 and 2 and Comparative Examples 1 and 2 are shown in Table 1, and the results of Examples 3 to 5 and Comparative Examples 3 and 4 are shown in Table 2. In addition, the results of Examples 1 to 5 and Comparative Examples 1 to 4 are shown in Figures 1 to 3 for whiteness, K value, and hexenuronic acid content. In each of the graphs of Figures 1 to 3, the points on the left and right ends represent the results of the comparative examples, and the points other than these represent the results of the examples. [Table 1] [Table 2]

In each example, the whiteness was maintained at a good level, and at the same time, the K value and the hexenuronic acid content were suppressed, confirming that bleached pulp with generally excellent properties could be obtained.

without

[Figure 1] is a graph showing the ISO brightness (%) of the bleached pulp of each Example and Comparative Example. [Figure 2] is a graph showing the K value of the bleached pulp of each Example and Comparative Example. [Figure 3] is a graph showing the amount of hexenuronic acid (HexA) in the bleached pulp of each Example and Comparative Example.

Claims (19)

A method for producing bleached pulp comprises: a digestion step of digesting wood cellulose material, an alkali-oxygen bleaching step of bleaching unbleached pulp obtained by the digestion step by alkali-oxygen bleaching, and a chlorine dioxide treatment step of treating the pulp obtained by the alkali-oxygen bleaching step with chlorine dioxide and persulfuric acid, wherein the pH value of the treatment liquid in the chlorine dioxide treatment step is in the range of 2 to 8, and the pH value of the treatment liquid before the treatment in the chlorine dioxide treatment step is That is, when the pH value before treatment is below 3.0, the difference between the pH value before treatment and the pH value of the treatment solution after treatment in the chlorine dioxide treatment step, i.e., the pH value after treatment, is below 1.0; when the pH value before treatment is higher than 3.0 and less than or equal to 5.5, the difference between the pH value before treatment and the pH value after treatment is below 2.0; when the pH value before treatment is higher than 5.5 and less than or equal to 8.0, the difference between the pH value before treatment and the pH value after treatment is below 2.5. The method for producing bleached pulp as claimed in claim 1, wherein in the chlorine dioxide treatment step, the amount of persulfuric acid added is 0.01-2.00 mass % relative to the absolute dry pulp mass, and the amount of chlorine dioxide added is 0.01-2.00 mass % relative to the absolute dry pulp mass. The method for producing bleached pulp as claimed in claim 1 or 2, wherein the duration of the chlorine dioxide treatment step is 20 to 200 minutes, and the temperature in the chlorine dioxide treatment step is 40 to 70°C. A method for producing bleached pulp as claimed in claim 1 or 2, wherein the bleached pulp is pulp for papermaking. The method for producing bleached pulp as claimed in claim 1 or 2 further comprises a pH measurement step for measuring the pH value of the treatment object, i.e., the treatment liquid, in the chlorine dioxide treatment step. The method for producing bleached pulp as claimed in claim 1 or 2, wherein the pH value of the treatment liquid before treatment in the chlorine dioxide treatment step, i.e., the pH value before treatment, and the pH value of the treatment liquid after treatment, i.e., the pH value after treatment, are both within the range of 2 to 8. The method for producing bleached pulp as claimed in claim 1 or 2, wherein the difference between the pH value of the treatment liquid before the treatment in the chlorine dioxide treatment step, i.e., the pH value before treatment, and the pH value of the treatment liquid after the treatment in the chlorine dioxide treatment step, i.e., the pH value after treatment, is less than 2.0. For the method for producing bleached pulp as claimed in claim 1, when the pH value before treatment is below 3.0, the difference between the pH value before treatment and the pH value after treatment is below 0.5; when the pH value before treatment is higher than 3.0 and less than or equal to 5.5, the difference between the pH value before treatment and the pH value after treatment is below 1.6; when the pH value before treatment is higher than 5.5 and less than or equal to 8.0, the difference between the pH value before treatment and the pH value after treatment is below 2.0. The method for producing bleached pulp as claimed in claim 1 or 2, wherein the ISO whiteness of the bleached pulp after the chlorine dioxide treatment step is above 60%. The method for producing bleached pulp as claimed in claim 1 or 2, wherein the K value of the bleached pulp after the chlorine dioxide treatment step is 1.50~4.50. The method for producing bleached pulp of claim 1 or 2 further comprises a K value determination step, which determines the K value of the pulp before treatment in the chlorine dioxide treatment step, i.e., the K value before treatment, and the K value of the bleached pulp after treatment in the chlorine dioxide treatment step, i.e., the K value after treatment. In the method for producing bleached pulp of claim 11, the K value residual rate (K ₂ /K ₁ × 100 ₎ calculated from the K value before the treatment (K ₁ ) and the K value after the treatment (K 2 ) is less than 80%. The method for producing bleached pulp as claimed in claim 1 or 2, wherein the hexenuronic acid content in the bleached pulp after the chlorine dioxide treatment step is less than 30.00 μmol/g. The method for producing bleached pulp of claim 1 or 2, wherein the residual rate of hexenuronic acid content, i.e., H2/H1×100, calculated from the hexenuronic acid content value, i.e., _H1 (μmol/g), of the pulp to be treated in the chlorine dioxide treatment step before treatment and the hexenuronic acid content value, i.e., _H2 ( _μmol /g) in the bleached pulp after treatment in the chlorine dioxide treatment step is ₉₀ % or less. The method for producing bleached pulp of claim 1 or 2, wherein the value of the whiteness increase rate (W 2 /W 1 × 100) calculated from the ISO whiteness value (W ₁ (%) of the pulp before the treatment in the chlorine dioxide treatment step) and the ISO whiteness value (W ₂ (%) of the bleached pulp after the treatment in the _chlorine dioxide treatment _step ) is 115% or more. The method for producing bleached pulp of claim 1 or 2, wherein the viscosity retention rate (A ₂ /A 1 × 100) calculated from the viscosity value of the pulp before treatment (A ₁ ( _cP )) and the viscosity value of the bleached pulp after treatment (A ₂ (cP)) is 60% or more. A method for producing bleached pulp as claimed in claim 1 or 2, wherein the weight ratio of chlorine dioxide to permonosulfuric acid used in the chlorine dioxide treatment step is greater than 1. The method for producing bleached pulp as claimed in claim 1 or 2, wherein after the chlorine dioxide treatment step, there is a hydrogen peroxide treatment step, wherein the pulp treated with chlorine dioxide is treated with hydrogen peroxide. The method for producing bleached pulp as claimed in claim 5, wherein in the pH measurement step, the pH value of the treatment liquid before treatment, i.e., the pH value before treatment, and the pH value of the treatment liquid after treatment, i.e., the pH value after treatment, are measured.

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