JP2000273782A - Production of bleached pulp - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain bleached pulp by subjecting unbleached pulp to an alkali oxygen bleaching, acid treatment and multistage bleaching through setting up washing tiers just before the above respective steps so as to achieve the desired whiteness of the final pulp and reduce the amount of bleaching chemical(s) to be used and effluent load as well. SOLUTION: This bleached pulp is obtained by subjecting unbleached pulp formed by digesting lignocellulosic materials derived from broadleaf trees to treatment in an alkali oxygen bleaching step equipped with a plurality of sequential reactors, acid treatment under pressure with a pressurized gas using sulfuric acid, alkali addition treatment with at least one chemical selected from caustic soda and an oxidized kraft white liquor followed by treatment in a multistage bleaching step; wherein washing tiers are set up just before the above respective steps and the effluents from the respective washing tiers are subjected to counterflow successively so as to be served each as washing water for the preceding upstream washing tier.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing bleached pulp from lignocellulosic material. More specifically, the present invention provides a method for producing bleached pulp from unbleached pulp obtained by digesting a lignocellulosic material, which significantly increases the amount of bleaching chemicals required to bleach the pulp to a desired whiteness. The present invention relates to a method for producing bleached pulp which can significantly reduce the drainage load from the entire production process.

In order to use a lignocellulosic material as a raw material for papermaking in many applications, it is first digested with a cooking liquor and pulped by the chemical action of chemicals, or in the case of wood, wood chips are refined using a refiner or the like. After pulping by mechanical action, the resulting pulp must be bleached with a bleaching chemical to increase whiteness. For example, kraft pulp is usually bleached with alkali oxygen, then used for elemental chlorine, hypochlorite, chlorine dioxide, oxygen, ozone, peroxide, unless used for applications that require strength, such as packaging materials. It is generally used as bleached kraft pulp after being bleached with a bleaching agent such as hydrogen and caustic soda and a bleaching aid to remove lignin and the like, which are coloring substances contained in the pulp.

[0003] When bleached pulp is produced from unbleached pulp, it is necessary to maintain the strength of the pulp fiber itself to a certain extent, so that the decomposition of carbohydrates such as cellulose and hemicellulose constituting the pulp fiber is minimized. It is common to employ a multi-stage bleaching method of three to six stages in which bleaching is avoided by using a bleaching agent and bleaching conditions in various combinations to avoid mild one-stage bleaching.

Conventionally, in a multistage bleaching method, pulp is first treated with elemental chlorine, lignin contained in the pulp is chlorinated, lignin is made soluble, and then chlorinated lignin is dissolved with alkali. Extract and remove lignin from pulp, and further use hypochlorite, chlorine dioxide, etc. to decompose and remove a small amount of residual lignin,
A method for obtaining pulp having high whiteness has been adopted. However, in recent years, there has been a concern about the effect of organic chlorine compounds contained in the bleaching effluent from the pulp chlorination stage on the environment, and there has been an increasing movement to use elemental chlorine for pulp bleaching.
In addition, when hypochlorite is used, chloroform is generated at the time of bleaching pulp, which may have an adverse effect on the environment. Is being sought.

At present, as a substitute for elemental chlorine and hypochlorite, oxygen-based bleaching chemicals such as ozone, oxygen, hydrogen peroxide, peracetic acid, and persulfuric acid have attracted attention. However, the above-mentioned chemicals except oxygen and hydrogen peroxide have not yet been widely used.

In recent years, as a method of reducing the amount of bleaching chemicals used while maintaining the whiteness of pulp after bleaching at a constant level, a pretreatment method for bleaching by acid treatment has attracted attention.
The bleaching pretreatment method by acid treatment is a method in which pulp before bleaching is treated at high temperature and under acidic conditions to selectively remove hexeneuronic acid, which is a xylan side chain in the pulp, and lower the kappa number of the pulp. It is known that this method can reduce the bleaching chemicals required to bleach the pulp to the desired whiteness. (For example, Tapauni Vuor
inen et al., 1996 International Pulp Bleaching Conferen
ce Proceeding 43-51).

As an improved method of the pre-bleaching method by acid treatment, the present inventors have previously proposed a method in which an acid treatment is performed under an oxygen-containing gas pressure in Japanese Patent Application Laid-Open No. 10-251986. This method has a large reduction in the kappa number and a whitening effect of the pulp as compared with the ordinary acid treatment method, so that the effect of reducing the amount of the bleaching chemical used in the bleaching step can be increased.

The present inventors have previously described Japanese Patent Application Laid-Open No. 10-2988.
No. 86 proposes a method of performing an acid treatment under a nitrogen-containing gas under pressure. In this method, the decrease in the kappa number is large compared to the ordinary acid treatment method, and the decrease in the pulp viscosity can be suppressed, so that this method can only increase the effect of reducing the amount of the bleaching chemical used in the bleaching step. And the pulp strength can be kept high.

The present inventors have previously described Japanese Patent Application No. Hei 10-1190.
No. 51 proposes a method of performing acid treatment in the presence of at least one or more chemicals selected from alcohols, aldehydes, and acetone having 1 to 3 carbon atoms. In this method, the decrease in the kappa number is large compared to the ordinary acid treatment method, and the decrease in the pulp viscosity can be suppressed, so that this method can only increase the effect of reducing the amount of the bleaching chemical used in the bleaching step. And the pulp strength can be kept high.

The present inventors have previously described Japanese Patent Application No. 10-2813.
No. 41 proposes a method of performing an alkali oxygen bleaching step using a plurality of continuous reactors prior to the acid treatment in order to effectively perform the bleaching pretreatment by the acid treatment. In this method, the kappa number is lower and the brightness is higher than that of the pulp after the normal alkali oxygen bleaching, so that the pulp kappa value after the acid treatment is lower and the brightness is higher. The amount of bleaching chemicals used in the bleaching process can be significantly reduced.

On the other hand, as a result of repeated studies on the acid treatment, the present inventors have found that the acid treatment step can greatly reduce the bleaching drainage load from the multi-stage bleaching step, but it is surprising that the acid treatment step is surprising. The COD load of the wastewater was found to be significantly higher. Therefore, in order to reduce the wastewater load from the bleached pulp manufacturing process, it is necessary to collect the wastewater from the acid treatment process into the system, but usually, the wastewater from the acid treatment process is acidic, and It is difficult to collect in the system together with the liquor and the alkaline oxygen bleaching wastewater, and there is a problem that a special recovery device must be newly provided to recover the solution alone.

JP-A-6-101186 and JP-A-158573 each disclose acid treatment of pulp after digestion or alkali oxygen bleaching, separating the acid treatment waste water by a dehydrator, and separating the separated acid treatment waste water. Can be used to wash pulp after the acid treatment step and / or the digestion step or after the alkaline oxygen bleaching treatment, and in some cases can be recycled to the digestion step. However, as described above, it is difficult to recover the acid-treated wastewater together with the digestion wastewater and the alkali oxygen bleached wastewater in the system as it is, and a special recovery device must be newly provided to recover the wastewater alone. There was a problem.

[0013]

In view of this situation, the present inventors have conducted various studies on a method for easily recovering the wastewater from the acid treatment step together with the blackened liquor and the alkaline oxygen bleached wastewater. If the pH is increased by adding an alkali just before the end of the treatment process, the wastewater from the acid treatment process can be easily recovered together with the digestion liquor and the alkaline oxygen bleaching wastewater, as well as the reduction of bleaching chemicals in the multi-stage bleaching process. The inventors have found that the effect is even greater, and have completed the present invention.

It is an object of the present invention to provide a method for producing bleached pulp by subjecting unbleached pulp obtained by digesting a lignocellulosic material to an alkali oxygen bleaching step, followed by a multi-stage bleaching step to produce bleached pulp. It is an object of the present invention to provide a method for producing bleached pulp capable of significantly reducing the amount of bleaching chemicals required in the bleaching step required for bleaching and further reducing the drainage load from the entire production step.

[0015]

To achieve the above object, the present invention includes the following inventions. (1) The present invention provides a method for producing bleached pulp by subjecting unbleached pulp obtained by digesting a lignocellulosic substance to an alkali oxygen bleaching step, followed by an acid treatment step, followed by a multi-stage bleaching step. In the method for producing bleached pulp, a washing stage is provided before each of the steps, and in the acid treatment step, an alkali treatment is carried out without washing after the acid treatment.

(2) In the present invention, the wastewater from the washing stage immediately before the multi-stage bleaching step is used as washing water in the washing stage immediately before the acid treatment step, and the wastewater from the washing stage immediately before the acid treatment step is further subjected to alkali oxygen bleaching. The method for producing bleached pulp according to (1), wherein the bleached pulp is used as washing water in a washing stage immediately before the step.

(3) In the present invention, sulfuric acid is used for the acid treatment in the acid treatment step, and caustic soda is used for the alkali addition treatment.
And the method for producing bleached pulp according to the above (1) or (2), wherein at least one chemical selected from oxidized kraft white liquor is used.

(4) The present invention is the method for producing bleached pulp according to any one of the above items (1) to (3), wherein the acid treatment in the acid treatment step is performed under pressure by a pressurized gas.

(5) The present invention is the method for producing bleached pulp according to any one of (1) to (4), wherein the alkaline oxygen bleaching step is performed using a plurality of continuous reactors.

(6) The present invention is the method for producing bleached pulp according to any one of (1) to (5), wherein the lignocellulosic substance is hardwood.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION The lignocellulosic substance used in the present invention is not particularly limited, and may be a non-wood plant such as kenaf, hemp, bagasse or rice, a coniferous wood or a hardwood. Although good, hardwood is preferably used in terms of effect. As a cooking method for obtaining the chemical pulp used in the present invention, known cooking methods such as kraft cooking, polysulfide cooking, soda cooking, and alkali sulphite cooking can be used, but pulp quality, energy efficiency and the like can be used. In consideration, the kraft cooking method is preferably used. For example, when wood is kraft cooked, the degree of sulfuration of the kraft cooking liquor is 5-75.
%, Preferably 15 to 45%, and the effective alkali addition rate is 5 to 30% by weight, preferably 10 to 2% by weight of the dry wood.
5% by weight, the cooking temperature is 140-170 ° C, and the cooking method may be either continuous cooking method or batch cooking method,
When a continuous digester is used, a modified cooking method in which cooking liquor is added at multiple points may be used, and the method is not particularly limited.

In the cooking, a known cyclic keto compound such as benzoquinone,
Naphthoquinone, anthraquinone, anthrone, phenanthroquinone and alkyl-substituted products such as alkyl and amino of the quinone-based compound, or hydroquinone-based compounds such as anthrahydroquinone which is a reduced form of the quinone-based compound, and further anthraquinone by the Diels-Alder method A stable compound obtained as an intermediate in the synthesis method,
One or more selected from 10-diketohydroanthracene compounds and the like may be added, and the addition rate is 0.001 to 1.0% by weight based on the absolute dry weight of the wood chips.
It is. In the present invention, the unbleached chemical pulp obtained by the known digestion method is sent to the alkali oxygen bleaching step through a roughing step and a selective step, and further through a washing step (hereinafter referred to as W1 step).

In the alkaline oxygen bleaching step of the present invention, a known alkaline oxygen bleaching method can be applied, but a medium-concentration method which is currently widely used is preferably used. In the alkali oxygen bleaching step, caustic soda or oxidized kraft white liquor can be used as the alkali,
As oxygen gas, oxygen from cryogenic separation method, PSA (Press
ure Swing Adsorption), VSA (Vacuum Sw
Oxygen from ing Adsorption) can be used. Oxygen gas and alkali are added to the pulp slurry by a mixer, and after sufficient mixing is performed, the mixture is sent under pressure to a reaction tower capable of holding a mixture of pulp, oxygen and alkali for a certain period of time. In the present invention, it is a preferred embodiment that the alkali and oxygen gas are added and mixed to the pulp slurry a plurality of times using a plurality of mixers, and the mixture is treated using a plurality of reaction towers. Further, in the present invention, one of the preferred embodiments is to provide a washing stage between a plurality of reaction towers for treatment. The addition rate of oxygen gas is 0.5 to 3% by weight per absolute dry pulp weight, the alkali addition rate is 0.5 to 4% by weight, the reaction temperature is 80 to 120 ° C, and the reaction time is 15 to 10%.
A known condition can be applied to the other conditions.
The alkali oxygen bleached pulp is sent to an acid treatment step through a washing stage (hereinafter referred to as W2 stage).

The acid treatment step of the present invention is characterized in that an alkali treatment is performed after the acid treatment. The acid treatment in the acid treatment step of the present invention has a pH of 2 to 4, preferably 2.5 to 3.
5, temperature 50-120 ° C, preferably 70-110 ° C,
The reaction is performed under the conditions of a retention time of 5 to 120 minutes, preferably 20 to 90 minutes. The acid used in the acid treatment of the present invention is an inorganic acid as long as the pH at the time of the acid treatment can be adjusted to 2 to 4,
Any of organic acids may be used, but inorganic acids such as sulfuric acid, nitric acid, hydrochloric acid, sulfurous acid, and nitrous acid, among them, sulfuric acid are preferably used because they are easy to obtain and handle, and are easily collected in the system from wastewater. .

In the acid treatment of the present invention, the pressure can be increased using an oxygen-containing gas or a nitrogen-containing gas.
The oxygen-containing gas used for pressurization during the acid treatment includes oxygen from cryogenic separation, oxygen from PSA, and VSA
It can be used on an industrial scale, such as from oxygen, and the oxygen purity currently used for alkaline oxygen bleaching is oxygen or oxygen-containing gas with an oxygen purity of 85% by volume or more, and oxygen production equipment using the molecular sieve is used. The oxygen content to 21
Oxygen-containing gas adjusted to more than 50% by volume, oxygen-enriched gas produced by mixing oxygen-containing gas having an oxygen purity of 85% by volume or more with air, air having an oxygen content of 20% by volume or more, etc. And any of these can be appropriately selected and used. In the case where the ozone bleaching stage is provided in the multi-stage bleaching process, the exhaust gas containing oxygen can be suitably used. As the nitrogen-containing gas used in the acid treatment stage of the present invention, any gas may be used as long as the nitrogen gas content is 95% or more.
From an economic viewpoint, a nitrogen-containing gas by-produced when producing oxygen gas such as oxygen from the cryogenic separation method used for alkaline oxygen bleaching, oxygen from PSA, and oxygen from VSA is preferably used. The pressurizing pressure during the acid treatment with an oxygen-containing gas or a nitrogen-containing gas during the acid treatment is 0.05 to 0.
9 MPa (gauge pressure), preferably 0.15 to
0.7 MPa.

In the acid treatment of the present invention, a peroxide may be added. The peroxide to be added may be any peroxide as long as it is a peroxide, but is relatively easy to handle such as hydrogen peroxide, peracetic acid, formic acid, persulfate, dioxirane, etc., and is preferably used. . These peroxides may be used alone or as a mixture. The addition rate of the above-mentioned chemicals is from 0.01% to pulp absolute dry weight.
1.0%, preferably 0.05% to 0.5%.

The alkali addition treatment after the acid treatment, which is a feature of the present invention, is performed by directly adding an alkali to the pulp slurry after the acid treatment (pH 2 to 4). The pH after alkali addition in the alkali addition treatment of the present invention is 5 to 11, preferably 6 to 8, and the holding time is 0.1 to 30 minutes, preferably 1 to 10 minutes. The processing temperature is not particularly limited. The alkali added in the alkali addition treatment,
Although it is not particularly limited, caustic soda, oxidized kraft white liquor, caustic soda separated from white liquor, sodium carbonate separated from green liquor, and the like are preferable because collection and regeneration in the system from wastewater are easy. Used.

The pulp concentration in the acid treatment step of the present invention is in the range of 5 to 40% by weight, preferably 8 to 35% by weight, more preferably 10 to 25% by weight. In the acid treatment step of the present invention, it is necessary to uniformly mix the pulp slurry and the acid or the pulp slurry and the alkali in order to perform the treatment effectively. In order to obtain uniform mixing, a low-concentration mixer, a medium-concentration mixer, a high-concentration mixer, a medium-concentration pump, or the like can be appropriately selected and used according to the type and concentration of pulp at the time of processing. The pulp that has undergone the acid treatment step is sent to a multi-stage bleaching step through a washing step (hereinafter referred to as W3 step).

The bleaching chemicals used in the multi-stage bleaching process of the present invention include elemental chlorine (C), caustic soda (E), hypochlorite compound (H), chlorine dioxide (D), and oxygen (O). And a bleaching agent comprising a known bleaching agent such as hydrogen peroxide (P), ozone (Z), and organic peracid, and a bleaching aid. The bleaching agent is appropriately selected from these and used as a bleaching agent. As the bleaching sequence in the multi-stage bleaching process in the present invention, for example, CE
A bleaching sequence containing elemental chlorine and a chlorine bleaching chemical such as / OHD, C / DE / OHD can be used, and DED, DE / O-D, E / O
Bleaching sequences that do not contain elemental chlorine, such as -D, EOD, ZD, can also be used. In addition, Z-
A bleaching sequence that does not use any elemental chlorine and chlorine-based bleaching chemicals, such as EP, ZE / OP, and E / OP-PO, can also be used. Needless to say, during the multi-stage bleaching step, an enzyme treatment stage with xylan-degrading enzyme, lignin-degrading enzyme, etc., or a chelating agent treatment stage with ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA) or the like may be inserted. .

In the present invention, in the multi-stage bleaching process,
When an oxidizing agent selected from oxygen and hydrogen peroxide is added to a known alkali extraction stage and bleached in a bleaching sequence using at least one sequence such as E / O, E / P, E / OP or PO The whiteness after bleaching is further improved. The alkali extraction to which oxygen and / or hydrogen peroxide has been added may be performed under pressure in accordance with the alkali oxygen bleaching method. It may be a method.

The cleaning stages (W1 stage, W2 stage) before each process of the present invention
Washer used in the washing stage in the multi-stage bleaching step, a pressure diffuser, a diffusion washer, a pressurized drum washer, a horizontal mesh washer, a press washer, and the like. They are not particularly limited. In each washing stage of the present invention, one washing machine can be used, or a plurality of washing machines can be used.However, the next step, taking care that carryover to the next stage is as small as possible. Used to select.

The washing stages (W1 stage, W2 stage) before each step of the present invention
(W3 stage, W3 stage) is recovered and finally sent to a recovery boiler as cooking black liquor or together with cooking black liquor, and the organic matter contained therein can be used as thermal energy. At the same time, sodium and sulfur can be recycled and reused as cooking white liquor, oxidized white liquor, caustic soda, sulfuric acid and the like. In the present invention, in the multi-stage bleaching step, bleached pulp can be produced in a bleaching sequence not using elemental chlorine as a bleaching chemical, and further in a bleaching sequence not using both elemental chlorine and chlorine-based bleaching chemicals. Therefore, the wastewater from the bleaching can also be collected and the multi-stage bleaching process can be closed, and the entire bleached pulp manufacturing process can also be closed.

In the acid treatment step of the present invention, after the acid treatment,
By performing the alkali addition treatment, not only the in-system recovery of wastewater from the acid treatment step is facilitated, but also the amount of bleaching chemicals required in the multi-stage bleaching step required for bleaching to the desired whiteness is further increased. Although the reason cannot be clearly determined at this time, the present inventors speculate as follows. First, lignin degradation products that were not removed from the pulp by the acid washing after the ordinary acid treatment step were removed by increasing the washing pH by the alkali addition treatment, and the lignin in the pulp after the acid treatment step was removed. Secondly, by reducing the content, secondly, by performing an alkali addition treatment,
This is thought to be due to the fact that the pH was higher than that of the pulp after the ordinary acid treatment step, and the reaction pH in the first bleaching step in the multi-stage bleaching step was optimized.

[0034]

EXAMPLES The present invention will be described more specifically with reference to examples and comparative examples below, but of course the present invention is not limited to these examples. In Examples 1 to 5 and Comparative Examples 1 and 2 shown below, hardwood unbleached kraft pulp was subjected to alkaline oxygen bleaching, followed by acid treatment, followed by D1-E / O-
The result of D2 bleaching is shown. Unless otherwise indicated, the measurement of the kappa number, the measurement of the pulp viscosity, the measurement of the whiteness, and the pulp yield through all the steps were calculated as follows. In addition, the addition rates of the chemicals in Examples and Comparative Examples are shown by weight% based on the weight of the absolutely dried pulp.

1. Measurement of kappa number Kappa number was measured according to JIS P 8211.

[0036] 2. Measurement of Pulp Viscosity Measurement of pulp viscosity is described in J. Am. Performed according to TAPPI 44.

[0037] 3. Measurement of pulp whiteness After pulp was disintegrated, Tappi test method T205os-
71 (JIS P 8209), basis weight 60 g / m
Sheet No. ² was prepared, and the whiteness of the pulp was measured according to JIS P 8123.

[0038] 4. Pulp Yield in All Processes The pulp yield in the alkaline oxygen bleaching process, the acid treatment process and the bleaching process is calculated from the absolute dry weight of the pulp before and after the process according to the formula (1). It was calculated according to (2). Pulp in each process ゜ yield (%) = ｛(pulp absolute dry weight after process) / pulp absolute dry weight before process｝ × 100 (1) Pulp in all processes 工程 yield (%) = alkali oxygen Pulp yield in bleaching step (%) x pulp yield in acid treatment step (%) x pulp yield in multi-stage bleaching step (%) / 10000 ... (2)

Example 1 500 g of hardwood mixed with hardwood consisting of 30% of domestic hardwood and 70% of eucalyptus wood was sampled in an absolute dry state, and the liquid ratio was 4, the effective alkali was 18% per absolute dry chip weight, and the sulphidity of the cooking liquor was 25. %, A cooking temperature of 160 ° C., and a cooking time of 120 minutes using a laboratory autoclave for indirect heating. After the digestion, the digested black liquor and the pulp were dehydrated and separated, and washed with ion exchanged water (W1 stage).
Selected on a flat screen with a cut screen plate, hardwood unbleached kraft pulp (whiteness: 3
7.1%, kappa number; 17.0, pulp viscosity;
8 mPa · s) was obtained in an absolute dry weight of 220 g. 90.0 g of this hardwood unbleached kraft pulp was sampled, 1.5% of caustic soda was added per absolute dry pulp weight, and then diluted with ion-exchanged water to adjust the pulp concentration to 10%. Place the sample in an indirectly heated autoclave and adjust the purity to 9 so that the gauge pressure becomes 0.5 MPa.
Pressurized with 9.9% commercially available compressed oxygen gas,
The mixture was allowed to react for 0 minutes and subjected to alkaline oxygen bleaching. The obtained pulp was washed with ion-exchanged water and then dehydrated (W2
Stage). A pulp having an absolute dry weight of 88.3 g, a whiteness of 45.8%, a kappa number of 9.3, and a pulp viscosity of 22.8 mPa · s was obtained.

80.0 g of this pulp was collected in absolute dry weight, ion-exchanged water and dilute sulfuric acid were added to obtain a pulp slurry having a pH of 3.0 and a pulp concentration of 10%. Autoclave, 95
Acid treatment was performed at 120 ° C. for 120 minutes. The obtained pulp slurry was put in a plastic bag without washing, and 0.50% of caustic soda was added per absolute dry pulp weight, and after stirring well, the mixture was allowed to stand at room temperature for 1 minute to carry out alkali addition treatment. The pulp slurry was taken out of the plastic bag, dehydrated, and the filtrate was separated, washed with ion-exchanged water and dehydrated (W3 stage). Absolute dry weight 79.2g, whiteness 46.3%, kappa number 4.5, pulp viscosity 1
A pulp of 8.1 mPa · s was obtained. The pH of the separated acid-treated filtrate was 7.5.

70.0 g of this pulp was collected in absolute dry weight, placed in a plastic bag, and the pulp concentration was adjusted to 10% using ion-exchanged water. Then, 0.31% of chlorine dioxide was added per absolute dry pulp weight. Then, it was immersed in a constant temperature water bath at a temperature of 70 ° C. for 30 minutes to perform bleaching at D1 stage. The obtained pulp was washed with deionized water and dehydrated. Ion-exchanged water was added to the pulp after the first stage of D to adjust the pulp slurry to a pulp concentration of 10%, and then 0.53% of caustic soda was added to the absolutely dried pulp weight, and a 2-liter stainless steel indirect heating autoclave was used. And the gauge pressure is 0.1
It was pressurized with a commercially available compressed oxygen gas having a purity of 99.9% so as to be 5 MPa, and reacted at 70 ° C. for 20 minutes. Thereafter, the pulp slurry was taken out of the autoclave and transferred to a plastic bag.
The mixture was treated at 70 ° C. for 70 minutes to extract an E / O stage. The obtained pulp was washed and dehydrated using ion exchanged water. Subsequently, the pulp after the E / O stage was put into a plastic bag, the pulp concentration was adjusted to 10% using ion-exchanged water, and then 0.2% of chlorine dioxide was added to the absolutely dry pulp weight. Then, the mixture was treated at a temperature of 70 ° C. for 180 minutes to perform bleaching in D2 stage. The obtained pulp was washed with deionized water, dehydrated, and absolutely dried weight: 69.6 g, whiteness: 85.0%,
A bleached pulp having a pulp viscosity of 16.9 mPa · s was obtained.
Table 1 shows the kappa number, the pulp brightness, the pulp viscosity and the pH of the filtrate after the acid treatment step of the pulp after the alkali oxygen bleaching and the acid treatment step. Table 2 shows the addition ratio, the pulp yield throughout the entire process, and the pulp viscosity and whiteness after bleaching.

Example 2 A hardwood kraft pulp (whiteness: 45.8%, kappa number: 9.3, pulp viscosity: 22.8 mPa · s) after alkaline oxygen bleaching as in Example 1 was used in an absolute dry weight. 80.0
g, and the acid treatment and the alkali addition treatment were performed in the same manner as in Example 1 except that the addition ratio of caustic soda during the alkali addition treatment was changed to 0.8%. After the treatment, it was dehydrated and the filtrate was separated, washed with deionized water and dehydrated (W3 stage).
A pulp having an absolute dry weight of 79.1, a whiteness of 47.0%, a kappa number of 4.2, and a pulp viscosity of 17.8 mPa · s was obtained. The pH of the separated filtrate in the acid treatment step was 10.5.

Example 1 was repeated except that 70.0 g of this pulp was collected in absolute dry weight, placed in a plastic bag, and the chlorine dioxide addition rate per absolute dry pulp weight was changed to 0.27%.
D1 stage bleaching was performed in the same manner as described above. The obtained pulp was washed with deionized water and dehydrated. Put the pulp after D1 stage in a plastic bag, and adjust the pulp concentration to 1 using ion-exchanged water.
After adjusting to 0%, the E / O stage was extracted in the same manner as in Example 1 except that the addition ratio of caustic soda per absolutely dry pulp weight was changed to 0.46%. The obtained pulp was washed with deionized water and dehydrated. Subsequently, the pulp after the E / O stage was placed in a plastic bag and bleached in the D2 stage in the same manner as in Example 1. The obtained pulp is washed using ion-exchanged water,
After dehydration, a bleached pulp having an absolute dry weight of 69.7 g, a whiteness of 85.1%, and a pulp viscosity of 17.0 mPa · s was obtained. Table 1 shows the kappa number, the pulp brightness, the pulp viscosity and the pH of the filtrate after the acid treatment step of the pulp after the alkali oxygen bleaching and the acid treatment step. Table 2 shows the addition ratio, the pulp yield throughout the entire process, and the pulp viscosity and whiteness after bleaching.

Example 3 A hardwood kraft pulp (whiteness: 45.8%, kappa number: 9.3, pulp viscosity; 22.8 mPa · s) after alkaline oxygen bleaching as in Example 1 was absolutely dry weight. 80.0
g, and ion-exchanged water and dilute sulfuric acid were added to adjust the pH to 3.
After a pulp slurry having a pulp concentration of 0% and a pulp slurry of 10% was placed in a 2-liter stainless steel indirect heating autoclave, the purity was adjusted to 9 so that the gauge pressure was 0.5 MPa.
Pressurize with 9.9% commercially available compressed oxygen gas,
The mixture was allowed to react for 0 minutes and subjected to an acid treatment. The obtained pulp slurry was put in a plastic bag without washing, and 0.50% of caustic soda was added per absolute dry pulp weight, and after stirring well, the mixture was allowed to stand at room temperature for 1 minute to carry out alkali addition treatment. The pulp slurry was taken out of the plastic bag, dehydrated, and the filtrate was separated, washed with ion-exchanged water and dehydrated (W3 stage). Absolute dry weight 79.0 g, whiteness 48.8%, kappa number 4.0, pulp viscosity 1
Pulp of 7.3 mPa · s was obtained. Further, the pH of the separated filtrate in the acid treatment step was 7.5.

This pulp was collected in an absolute dry weight of 70.0 g, put in a plastic bag, and the addition rate of chlorine dioxide per absolute dry pulp weight was changed to 0.20%.
D1 stage bleaching was performed in the same manner as described above. The obtained pulp was washed with deionized water and dehydrated. Put the pulp after D1 stage in a plastic bag, and adjust the pulp concentration to 1 using ion-exchanged water.
After adjusting to 0%, the E / O stage was extracted in the same manner as in Example 1 except that the addition ratio of caustic soda per absolutely dry pulp weight was changed to 0.34%. The obtained pulp was washed with deionized water and dehydrated. Subsequently, the pulp after the E / O stage was placed in a plastic bag and bleached in the D2 stage in the same manner as in Example 1. The obtained pulp is washed using ion-exchanged water,
After dehydration, a bleached pulp having an absolute dry weight of 69.8 g, a whiteness of 85.1% and a pulp viscosity of 16.8 mPa · s was obtained. Table 1 shows the kappa number, the pulp brightness, the pulp viscosity and the pH of the filtrate after the acid treatment step of the pulp after the alkali oxygen bleaching and the acid treatment step. Table 2 shows the addition ratio, the pulp yield throughout the entire process, and the pulp viscosity and whiteness after bleaching.

Example 4 Hardwood unbleached kraft pulp as in Example 1 (whiteness; 37.1%, kappa number: 17.0, pulp viscosity;
28.8 mPa · s) was collected in an absolute dry weight of 90.0 g,
Add 1.5% of caustic soda per absolutely dry pulp weight, then dilute with deionized water to adjust the pulp concentration to 10%, put in an indirectly heated autoclave, and purify to a gauge pressure of 0.5 MPa. It was pressurized with 99.9% commercially available compressed oxygen gas and reacted at 100 ° C. for 30 minutes.
Thereafter, the pressure was reduced until the gauge pressure became 0.05 MPa or less, the pulp was taken out of the autoclave, washed and dehydrated using ion-exchanged water. To this pulp, 1.5% caustic soda was added again, and then diluted with ion-exchanged water to adjust the pulp concentration to 10%. Then, the pulp was put into an indirectly heated autoclave, and the purity was adjusted so that the gauge pressure became 0.5 MPa. Is pressurized with 99.9% of commercially available compressed oxygen gas,
The reaction was performed at 00 ° C. for 30 minutes to perform alkaline oxygen bleaching. The obtained pulp was washed with ion-exchanged water and then dehydrated (W2 stage). Absolute dry weight: 88.2 g, whiteness: 48.3
%, Kappa number 8.4, pulp viscosity 22.5mPa.
s pulp was obtained.

80.0 g of the pulp after the above-mentioned alkaline oxygen bleaching was collected in an absolute dry weight, and subjected to an acid treatment and an alkali addition treatment in the same manner as in Example 1. After the treatment, it was dehydrated and the filtrate was separated, washed with deionized water and dehydrated (W3 stage).
A pulp having an absolute dry weight of 79.3 g, a whiteness of 49.0%, a kappa number of 3.6, and a pulp viscosity of 17.5 mPa · s was obtained. Further, the pH of the filtrate obtained in the acid treatment step was 7.4.

Example 1 was repeated except that 70.0 g of this pulp was collected in an absolute dry weight, put in a plastic bag, and the addition rate of chlorine dioxide per absolute dry pulp weight was changed to 0.18%.
D1 stage bleaching was performed in the same manner as described above. The obtained pulp was washed with deionized water and dehydrated. Put the pulp after D1 stage in a plastic bag, and adjust the pulp concentration to 1 using ion-exchanged water.
After adjusting to 0%, the E / O stage was extracted in the same manner as in Example 1 except that the addition ratio of caustic soda per absolutely dry pulp weight was changed to 0.31%. The obtained pulp was washed with deionized water and dehydrated. Subsequently, the pulp after the E / O stage was placed in a plastic bag and bleached in the D2 stage in the same manner as in Example 1. The obtained pulp is washed using ion-exchanged water,
After dehydration, a bleached pulp having an absolute dry weight of 69.8 g, a whiteness of 85.1% and a pulp viscosity of 17.1 mPa · s was obtained. Table 1 shows the kappa number, the pulp brightness, the pulp viscosity and the pH of the filtrate after the acid treatment step of the pulp after the alkali oxygen bleaching and the acid treatment step. Table 2 shows the addition ratio, the pulp yield throughout the entire process, and the pulp viscosity and whiteness after bleaching.

Example 5 A kraft digestion step and an alkali oxygen bleaching were performed in the same manner as in Example 1 except that the washing filtrate of W3 stage was used as the washing water of W2 stage, and the washing filtrate of W2 stage was used as the washing water of W1 stage. The step and the acid treatment step were sequentially performed. A pulp having an absolute dry weight of 79.2, a whiteness of 46.1%, a kappa number of 4.6, and a pulp viscosity of 18.0 mPa · s was obtained. Further, the pH of the separated filtrate in the acid treatment step was 7.5.

Example 1 was repeated except that 70.0 g of this pulp was collected in an absolute dry weight, put in a plastic bag, and the addition rate of chlorine dioxide per absolute dry pulp weight was changed to 0.33%.
D1 stage bleaching was performed in the same manner as described above. The obtained pulp was washed with deionized water and dehydrated. Put the pulp after D1 stage in a plastic bag, and adjust the pulp concentration to 1 using ion-exchanged water.
After adjusting to 0%, the E / O stage was extracted in the same manner as in Example 1 except that the addition ratio of caustic soda per absolutely dry pulp weight was changed to 0.56%. The obtained pulp was washed with deionized water and dehydrated. Subsequently, the pulp after the E / O stage was placed in a plastic bag and bleached in the D2 stage in the same manner as in Example 1. The obtained pulp is washed using ion-exchanged water,
After dehydration, a bleached pulp having an absolute dry weight of 69.6 g, a whiteness of 85.0%, and a pulp viscosity of 16.8 mPa · s was obtained. Table 1 shows the kappa number, the pulp brightness, the pulp viscosity and the pH of the filtrate after the acid treatment step of the pulp after the alkali oxygen bleaching and the acid treatment step. Table 2 shows the addition ratio, the pulp yield throughout the entire process, and the pulp viscosity and whiteness after bleaching.

Comparative Example 1 A hardwood kraft pulp (whiteness: 45.8%, kappa number: 9.3, pulp viscosity: 22.8 mPa · s) after bleaching with alkaline oxygen similar to that in Example 1 was absolutely dried. 80.0
g were collected and subjected to an acid treatment in the same manner as in Example 1, but no alkali addition treatment was performed. After the acid treatment, the filtrate was dehydrated and the filtrate was separated, washed with deionized water and dehydrated (W3 stage).
A pulp having an absolute dry weight of 79.3, a whiteness of 45.9%, a kappa number of 4.9 and a pulp viscosity of 18.3 mPa · s was obtained. Further, the pH of the separated filtrate in the acid treatment step was 3.1.

Example 1 was repeated except that 70.0 g of this pulp was collected in an absolute dry weight, put in a plastic bag, and the addition rate of chlorine dioxide per absolute dry pulp weight was changed to 0.41%.
D1 stage bleaching was performed in the same manner as described above. The obtained pulp was washed with deionized water and dehydrated. Put the pulp after D1 stage in a plastic bag, and adjust the pulp concentration to 1 using ion-exchanged water.
After adjusting to 0%, the E / O stage was extracted in the same manner as in Example 1 except that the addition ratio of caustic soda per absolutely dry pulp weight was changed to 0.70%. The obtained pulp was washed with deionized water and dehydrated. Subsequently, the pulp after the E / O stage was placed in a plastic bag and bleached in the D2 stage in the same manner as in Example 1. The obtained pulp is washed using ion-exchanged water,
After dehydration, a bleached pulp having an absolute dry weight of 69.4 g, a whiteness of 85.0%, and a pulp viscosity of 16.1 mPa · s was obtained. Table 1 shows the kappa number, the pulp brightness, the pulp viscosity and the pH of the filtrate after the acid treatment step of the pulp after the alkali oxygen bleaching and the acid treatment step. Table 2 shows the addition ratio, the pulp yield throughout the entire process, and the pulp viscosity and whiteness after bleaching.

Comparative Example 2 Hardwood kraft pulp (whiteness: 45.8%, kappa number: 9.3, pulp viscosity: 22.8 mPa · s) after alkaline oxygen bleaching as in Example 1 was absolutely dried. 80.0
g, and the acid treatment and the alkali addition treatment were carried out in the same manner as in Example 1 except that washing was not performed after the alkali addition treatment (no W3 stage).

The pulp slurry was placed in a plastic bag, and the addition ratio of chlorine dioxide per weight of the absolutely dried pulp was set at 0.1.
D1 stage bleaching was performed in the same manner as in Example 1 except that the bleaching rate was changed to 65%. The obtained pulp was washed with deionized water and dehydrated. Example 1 Except that the pulp after D1 stage was put in a plastic bag, the pulp concentration was adjusted to 10% using ion-exchanged water, and then the addition ratio of caustic soda per absolute dry pulp weight was changed to 1.11%. The extraction of the E / O stage was performed in the same manner as described above. The obtained pulp was washed with deionized water and dehydrated.
Then, put the pulp after E / O stage in a plastic bag,
D2 stage bleaching was performed in the same manner as in Example 1. The obtained pulp is washed with deionized water, dehydrated, and has an absolute dry weight of 7%.
8.4 g, whiteness 85.0%, pulp viscosity 16.1 mP
As a bleached pulp was obtained. Table 1 shows the kappa number, pulp brightness and pulp viscosity of the pulp after alkaline oxygen bleaching.
Table 2 shows the D1 stage chemical addition ratio and the E / O stage chemical addition ratio in the multi-stage bleaching process, and the pulp yield, pulp viscosity and whiteness after bleaching throughout the entire process.

[0055]

[Table 1]

[0056]

[Table 2]

Examples 1, 2 and 5 in Table 1 and Comparative Example 1,
As is clear from the comparison of No. 2, the acid treatment, alkali addition treatment, and washing can increase the pulp whiteness after the acid treatment and lower the kappa number of the pulp. As a result, as is clear from Table 2, it is possible to reduce the amount of the bleaching chemical used in the multi-stage bleaching process necessary for bleaching to the desired whiteness, and it is also possible to increase the pulp yield in all the processes. Further, the wastewater from the acid treatment step can be easily recovered. Further, the above effect can be enhanced by performing acid treatment under pressure by a pressurized gas (Example 3) or by performing an alkali oxygen bleaching step using a plurality of continuous reactors (Example 4). It is possible.

According to the method for producing bleached pulp, unbleached pulp obtained by digesting a lignocellulose material is treated in an alkali oxygen bleaching step, then treated in an acid treatment step, and then treated in a multi-stage bleaching step. By providing a washing stage before each process and adding alkali just before the end of the acid treatment process, the wastewater from the acid treatment process can be easily collected together with the blackened liquor and the alkaline oxygen bleached wastewater. Further, the amount of the bleaching chemical used in the multi-stage bleaching step required for bleaching to the desired whiteness can be reduced, and a method for producing bleached pulp with less drainage load can be provided.

Claims (6)

[Claims] 1. A method for producing bleached pulp, comprising treating unbleached pulp obtained by digesting a lignocellulosic substance in an alkali oxygen bleaching step, treating it in an acid treatment step, and then treating it in a multi-stage bleaching step. A washing stage is provided before each of the steps, and in the acid treatment step, following the acid treatment,
A method for producing bleached pulp, which comprises performing an alkali addition treatment. 2. The wastewater from the washing stage immediately before the multi-stage bleaching step is used as washing water in the washing stage immediately before the acid treatment step, and the wastewater from the washing stage immediately before the acid treatment step is further used in the washing stage immediately before the alkali oxygen bleaching step. The method for producing bleached pulp according to claim 1, wherein the method is used as washing water. 3. The method according to claim 1, wherein sulfuric acid is used in the acid treatment in the acid treatment step, and at least one chemical selected from caustic soda and oxidized kraft white liquor is used in the alkali addition treatment. Item 1. The method for producing bleached pulp according to Item 1 or 2. 4. The method for producing bleached pulp according to claim 1, wherein the acid treatment in the acid treatment step is performed under pressure by a pressurized gas. 5. The method for producing bleached pulp according to claim 1, wherein the alkaline oxygen bleaching step is performed using a plurality of continuous reactors. 6. The method for producing bleached pulp according to claim 1, wherein said lignocellulosic material is hardwood.