JP2000290887A - Bleaching of lignocellulose - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for bleaching a lignocellulose material which suppresses a decrease in pulp strength while maintaining high whiteness with a small amount of added ozone gas. SOLUTION: After digestion of lignocellulosic material with an alkaline chemical, oxygen delignification is performed to obtain a kappa number of 8 to 15, and the obtained pulp is treated in an acidic aqueous solution, and compared with the oxygen delignification step in the previous step. Reduce the pulp kappa number by 30% or more and reduce the kappa number by 3
After the ozone gas treatment in the subsequent process,
A method for bleaching lignocellulose, comprising performing bleaching in a sequence of performing an extraction treatment and further performing a chlorine dioxide treatment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for bleaching a lignocellulosic material, and more particularly, to an acid treatment for delignification of pulp, followed by an ozone treatment, and then to an alkali or a pulp depending on the purpose of pulp quality. The present invention relates to a bleaching method for suppressing a decrease in pulp strength while maintaining high whiteness with a small amount of ozone gas by treating in an extraction stage under weak acidity and further treating with chlorine dioxide.

[0002]

2. Description of the Related Art In order to use lignocellulosic materials in many applications, bleaching of pulp is carried out after pulping by a chemical action such as cooking or pulping by a mechanical action using a refiner or the like. It is necessary to increase the whiteness. For example, kraft pulp is used for applications that require strength, such as packaging materials,
Normally bleaching is performed with bleaching agents such as chlorine, hypochlorite, chlorine dioxide, oxygen, hydrogen peroxide, and caustic soda, and bleaching aids, and bleaching is performed after lignin, which is a coloring agent contained in pulp, is removed. It is commonly used as kraft pulp.

[0003] Also, in bleached pulp, it is necessary to maintain the strength of the pulp fiber itself to a certain extent. For this reason, extreme simple pulp fibers are required to minimize the decomposition of carbohydrates (such as cellulose) constituting the pulp fiber. It is common to employ a multi-stage bleaching step in which mild bleaching is performed while avoiding bleaching in stages and changing bleaching agents and bleaching conditions.

[0004] Usually, in the multi-stage bleaching process, lignin contained in pulp is first chlorinated with chlorine (stage C),
After adding solubility to the lignin, the lignin is then dissolved and extracted with alkali (E stage) to separate the lignin from the pulp, and hypochlorite (H stage), chlorine dioxide (D stage), etc. Use, further decompose and remove a small amount of remaining lignin,
A method of obtaining pulp having high whiteness has been adopted.

[0005] In recent years, attention has been paid to a method of reducing the amount of chlorine-based bleaching chemicals used due to an increase in environmental problems. Intensification of washing at each bleaching stage, use of pulp with a higher degree of delignification at the bleaching inlet, and furthermore, Although methods such as enzyme treatment are applied, significant capital investment was required, such as increasing the amount of wastewater, installing a new washing machine, and changing the digestion plant. On the other hand, a bleaching method using a non-chlorine bleaching chemical has attracted attention. As non-chlorine bleaching chemicals, ozone, oxygen, hydrogen peroxide, peracid, etc. are known. Among them, chemicals other than ozone are generally difficult to achieve a high whiteness level, and thus are generally used. It has not yet spread. Further, the bleaching method using ozone gas requires a high addition rate despite the fact that the chemical is expensive, and thus there are not many cases where it has been introduced despite being excellent as a bleaching chemical.

Up to now, acid bleaching has been performed in the former stage or simultaneously with the ozone stage in ozone bleaching of pulp. For example, “Paper and peripheral technology” (4) P2-9, (19)
87), in Takagi's paper entitled "About Ozone Bleaching of Pulp", regarding the pH, "Ozone is relatively stable under acidic conditions, but becomes more easily decomposed with increasing pH at pH 4 and higher. Performing ozone bleaching at a pH of 2 to 3 is also effective in suppressing the decrease in pulp viscosity. "It is recommended that the ozone treatment be performed under acidic conditions. In connection with this, a technique for removing a metal ion that decomposes ozone gas by adding a washing stage after acidification as a pretreatment of ozone treatment has also been proposed.

Further, as a bleaching sequence containing ozone, Japanese Patent Application Laid-Open No. 6-346391 discloses a technique including performing an ozone treatment at an acidic pH of 1 to 3. Japanese Patent Application Laid-Open No. 148785/1993 discloses that a chemically pulped lignocellulose-containing pulp can be treated with p.
A method is disclosed in which the pulp is treated with a complexing agent in the range of H3.1 to 9.0 to change the trace metal content of the pulp, and then the pulp is ozonated. Further, Japanese Patent Application Laid-Open No. 57-154489 discloses an acidic p-pulverized material having a pulp concentration of about 20% to about 45%.
A method for performing ozone treatment of pulp with H is disclosed.

[0008] However, the acid treatment performed during the ozone treatment disclosed in the above-mentioned literature and the gazette is set to the acidic side for the purpose of suppressing the decomposition of ozone or the decrease in viscosity of the pulp in the ozone stage. Is only a thing. In other words, it is clearly different from the acid treatment technique for positively removing lignin, hexenuronic acid and the like.

On the other hand, as an acid treatment for positively removing lignin, hexenuronic acid, etc., which are substances that hinder bleaching, for example, Japanese Unexamined Patent Publication (Kokai) No. 6-280177 discloses that unbleached pulp after digestion is treated with an inorganic acid having a pH of 1 or less. 0.0 to 1.6, temperature 8
0 ° C ~ boiling temperature of acid treatment solution, pulp concentration 5 ~ 20%,
There is disclosed a method of bleaching pulp by performing an acid treatment under a condition of 1 to 3 hours and then performing an alkali extraction treatment of lignin. However, in this method, as an acid treatment for delignification, for example, a treatment pH of 1.0
The conditions are severe as in the case of ~ 1.6, and there is a concern that the pulp strength may be reduced due to acid hydrolysis. Further, the treatment of the present invention in which lignin, hexenuronic acid and the like are selectively removed by combining the acid treatment and the ozone treatment is clearly different from the treatment of the present invention.

[0010]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a bleaching method for suppressing a decrease in pulp strength while maintaining high whiteness with a small amount of added ozone in order to solve the above-mentioned problems. It is assumed that.

[0011]

According to the present invention, an ozone treatment is carried out after an acid treatment for the purpose of delignification, followed by an extraction stage under an alkali or weak acid according to the purpose of pulp quality. It is characterized by being treated with chlorine dioxide, and includes the following inventions.

(1) After digesting a lignocellulosic substance with an alkaline chemical, an oxygen delignification step is performed to adjust the kappa number of the pulp to 8 to 15, and the obtained pulp is treated in an acidic aqueous solution. In comparison with the oxygen delignification step, the pulp is reduced by 30% or more in terms of the kappa number so that the kappa number of the pulp is 3 to 10, the ozone gas treatment and the extraction treatment are performed in the subsequent steps, and the bleaching is performed in a sequence of chlorine dioxide treatment. A method for bleaching lignocellulose, characterized in that:

(2) The treatment in an acidic aqueous solution has a pH of 2.5
(4) The method for bleaching lignocellulose according to item (1), wherein the reaction temperature is from 80 to 130 ° C. (3) The extraction treatment after the ozone gas reaction is performed under alkaline pH 9 to 12, or (1) or
(2) The method for bleaching lignocellulose according to item (2). (4) The method for bleaching lignocellulose according to item (1) or (2), wherein the extraction treatment after the ozone gas reaction is performed under an acidic condition of pH 4 to 6.5.

[0014]

DETAILED DESCRIPTION OF THE INVENTION As the lignocellulosic substance used in the present invention, pulp containing polysulfide or ordinary kraft pulping method (KP), sulphite pulping method (SP), alkali pulping method (AP) Pulp derived from a chemical pulping method such as pulp is particularly preferred, and pulp obtained by a kraft pulping method is particularly preferred. As the lignocellulosic material derived from woody plants used for pulping, hardwood materials such as aspen, beech, acacia, eucalyptus, and mangrove are preferable, softwood materials such as pine, spruce, Douglas fir, and further, bagasse, kenaf, and esparto. It can also be applied to lignocellose materials derived from herbaceous plants such as grasses, rice, reeds, etc. if the conditions are satisfied.

In the present invention, unbleached pulp obtained by a known digestion method is subjected to an oxygen delignification treatment step. At this time, the oxygen delignification treatment is performed so that the kappa number becomes 8 to 15 or less. . This oxygen delignification treatment step is preferably a treatment step for providing pulp having a kappa number of 10 to 12. If the kappa number of the pulp in this processing step exceeds 15, it is not preferable because chemicals are unnecessarily consumed in the subsequent bleaching step. Conversely, if the kappa number is less than 8, the strength and yield of the pulp in the stage before bleaching are reduced. This is not preferable because it significantly decreases.

[0016] The acid treatment in the present invention means that the pulp is treated with pH.
It refers to a treatment step of immersing in an acidic solution in the range of 2.5 to 4 and positively removing substances such as lignin that consumes potassium permanganate when measuring the Kappa number. The acid treatment conditions will be described in detail. The temperature when the pulp after the oxygen delignification treatment is treated with an acid is 80 to 130.
° C, preferably in the range of 90 to 120 ° C. Temperature 8
If the temperature is lower than 0 ° C., the reaction with the acid-soluble substance in the pulp is small and the treatment effect is insufficient. On the other hand, if the temperature is higher than 130 ° C., the cellulose in the pulp deteriorates significantly, which is not suitable.

[0017] The treatment pH is selected from 2.5-4.
When the pH is less than 2.5, the kappa number after the acid treatment becomes small, but the acid hydrolysis of the pulp proceeds, and the viscosity and strength of the pulp are greatly deteriorated, which is not preferable. On the other hand, if the pH exceeds 4, the decrease in the Kappa number becomes small even if the temperature or pressure is increased, and the effect on the subsequent bleaching properties is also undesirably reduced. Processing time is 10 minutes or more, preferably 30 to
Although it is 180 minutes, the time is not particularly limited as long as the acid treatment effect is recognized. In addition, the effect of the present invention is maintained under the acid treatment conditions selected from the above even in the method of pressurizing the acid treatment step with oxygen or nitrogen.

The acidic substance used in the acid treatment step of the present invention may be any of an inorganic acid and an organic acid as long as the pH can be adjusted from 2.5 to 4, but may be sulfuric acid, nitric acid, hydrochloric acid, sulfurous acid, or the like. Inorganic acids such as nitrous acid, especially sulfuric acid, are preferred because they are easy to obtain and handle.

The pulp used in the ozone stage (Z stage), which is the first bleaching stage of the present invention, is subjected to an acid treatment on the pulp having a kappa number in the range of 8 to 15 in the oxygen delignification step, Further cleaning was performed. Further, it is an essential condition that the kappa number after the acid treatment is 3 to 10 and that the kappa number is 30% or more lower than the kappa number before the acid treatment. Even if the kappa number after the acid treatment is reduced by 30% or more, if the kappa number is less than 3, the viscosity and strength of the pulp are greatly reduced. On the other hand, when the reduction rate of the kappa number is less than 30% and the kappa number exceeds 10, even after the acid treatment, there are a large amount of substances that promote consumption of the bleaching chemicals such as lignin and hexenuronic acid, so that the bleaching chemicals in the subsequent process are mischievous. Undesirably consumed. Further, even if the kappa number after acid treatment is 3 to 10, when the above-mentioned kappa number reduction rate is less than 30%, the difference in the amount of the bleaching chemical used in the finished pulp with or without acid treatment is small. Since the contribution to the whiteness is small, there is little significance in performing the acid treatment. The acid-treated pulp exerts the effects of the present invention regardless of whether or not the pulp is washed. However, in order to further enhance the effect, it is preferable that the pulp be continuously washed or compressed.

The gas and the apparatus used for the ozone treatment of the present invention can be selected from ozone generation methods and reactors known to those skilled in the art, for example, corona discharge in the presence of oxygen. The ozone addition rate is not particularly limited because it varies depending on the target quality of the finished pulp. For example, assuming that a high-quality varieties are used, as a guide, when the ISO whiteness is 70% to 87%, the weight per pulp is Is selected from about 0.1 to 1.5%. Since the pH of the washed pulp after the acid treatment is in an acidic atmosphere, the reaction pH is 2.5 to 4.
Adjusted to 5. The reaction is preferably performed at a reaction temperature of 60 ° C. or less, at which ozone decomposition is small, and the reaction is preferably performed with stirring for a time necessary for consuming the added ozone, usually several minutes.

The extraction stage under the alkaline conditions of the present invention [hereinafter referred to as the Eb stage for convenience because the extraction stage using a basic substance (base) is performed. ] Can be selected from many alkaline compounds known to those skilled in the art, for example, NaOH. The alkali addition ratio per pulp is 0.5 to
3% (vs. dry pulp) is preferred. An alkali extraction stage (Eo stage) reinforced with bleaching oxygen is also a preferred embodiment of the present invention, but the oxygen used in the Eo stage is PSA (Pressure Swing Adsorption) oxygen, VSA
(Vacuum Swing Adsorption) Any oxygen-enriched air such as oxygen or cryogenic oxygen can be used as long as it can be used on an industrial scale, and it can be used under pressure or under atmospheric pressure. good. Oxygen addition rate is 0.5-3%
(Vs. absolutely dry pulp) is preferred. The reaction temperature of the Eb stage or the Eo stage is preferably between 40 and 70 ° C, the pulp concentration during the reaction is preferably between 5 and 15%, the reaction time is between 30 and 120 minutes, and the final pH is Preferably between 9 and 12. After the Eb stage, the washing or compression step is performed subsequently, as in the first bleaching stage.

In the present invention, the pulp after the ozone treatment is washed and further extracted with an acidity without making it alkaline (an extraction (Extraction) stage is carried out in an acidic state. Abbreviated. ] Is also one of the preferred embodiments. The washed pulp after the acid treatment usually contains a component that recolors in an alkaline state, and the whiteness of the pulp is significantly reduced even temporarily by placing the pulp in an alkaline state. It is also known that if the pulp after the ozone treatment is made alkaline in a subsequent step, the pulp viscosity is significantly reduced. Therefore, as a means for maintaining strength, the pulp is kept weakly acidic to prevent a decrease in the degree of polymerization of cellulose and hemicellulose due to alkali, and is prevented from lowering the pulp viscosity by extracting before coloring of the coloring substance, thereby achieving high whiteness. As a means for maintaining acidity, it is preferable to add an alkaline compound to the pulp that has been washed after ozone treatment.
From 6.5 to 6.5 can be selected. The reaction temperature is preferably between 40 and 70 ° C., the pulp concentration during the reaction is preferably between 5 and 15%, and the reaction time is 30.
~ 120 minutes, with a final pH between 4 and 6.5 being preferred. After the Ea stage, the washing or compressing step is performed in the same manner as after the first bleaching stage.

In the present invention, the rate of addition of the chlorine dioxide stage (hereinafter abbreviated as D stage), which is performed subsequent to the alkali extraction stage or the weakly acidic extraction stage, is not particularly limited, and the amount of residual lignin and the purpose thereof are not limited. It may be appropriately selected depending on the degree of whiteness of the final pulp to be produced.

The chlorine dioxide used in the final stage of the present invention includes R-8 method, R-10 method, sodium bicarbonate method and the like.
As the kind of alkali for adjusting H, any of caustic soda and sodium silicate may be used as long as it can be used on an industrial scale, and the production method is not particularly limited.

The final pH in the chlorine dioxide stage (D stage) is pH
It is a preferred embodiment that the treatment is performed in the range of 4.0 to 5.5. If the final pH of the chlorine dioxide stage is less than 4.0, chlorine dioxide is consumed but the increase in whiteness is small. On the other hand, when the final pH exceeds 5.5, chlorine dioxide is not easily consumed, and the increase in whiteness is small. The stage D may be subjected to a pressure treatment according to the purpose of bleaching.

The treatment time in the chlorine dioxide stage is 30 to
The time is preferably 300 minutes, particularly preferably 120 to 180 minutes. If the time is less than 30 minutes, it is difficult to obtain pulp with high whiteness.
On the other hand, if the processing time is longer than 300 minutes, there is a problem that the whiteness reaches equilibrium and there is a problem of color return due to consumption of all chlorine dioxide, which is not preferable. The processing temperature of each chlorine dioxide stage is preferably from 40 to 90C, particularly preferably from 60 to 80C. If the temperature is lower than 40 ° C., the reaction rate of chlorine dioxide is low. If the temperature is higher than 90 ° C., chlorine dioxide is decomposed, so that the effective chlorine dioxide amount to the pulp decreases, and the whiteness and strength decrease. Further, the pulp concentration is 5 to 15%, more preferably 8 to 12%.
% Of the medium density region. If it is less than 5%, the reaction between pulp and chlorine dioxide will be poor, while if it exceeds 15%, stirring will be poor and uniform reaction will be difficult.

As described above, the acid treatment before the ozone stage in the conventional multistage bleaching sequence including ozone is carried out by the p ozone stage.
While it is limited to those that are pH-adjusting to perform H in the acidic region, or those that are relatively mild with little delignification for removing metal ions before the ozone stage, In the present invention, a bleached pulp having both high whiteness and high strength is produced by performing acid treatment and ozone treatment for the purpose of active delignification, and performing an alkali or acid extraction stage and chlorine dioxide treatment. Has the features that can be.

In the present invention, the whiteness of the finished pulp can be increased and the strength of the finished pulp can be reduced by performing the alkali extraction stage after the acid treatment and further the ozone treatment. This is presumed to be because the ozone treatment causes the colored component to become decomposed or easily decomposed and thus easily extracted under alkali.

The reason why the whiteness and the strength are increased by performing the acidic extraction after the ozone treatment is that the coloring components in the pulp subjected to the acid treatment under the strong acid and the ozone treatment cannot be extracted under the same strong acid. Since it has the property that it can be extracted under weak acidity, and the component that develops color by making it alkaline after the acid treatment step does not develop color under weak acidity, the decrease in the degree of polymerization of pulp fibers caused by placing it under alkaline after ozone treatment is reduced. It is presumed that this does not occur because of weak acidity.

In any of the treatments, chlorine dioxide treatment is performed after the alkali extraction stage or the weakly acidic extraction. However, since the D stage is performed under an acidic condition, the color does not reappear due to the alkali, and the color is stably high. It is believed that whiteness is obtained. Also, after the chlorine dioxide treatment, the chromogenic substance is removed or transformed into a substance that is less likely to develop color, so even if the chlorine dioxide stage is the final stage, a bleaching stage is added afterwards to achieve the desired whiteness Is also good.

The bleaching sequence thus bleached includes, for example, AZ-Eb-D, AZ-Eo-D, and AZ-E for pulp after oxygen delignification.
a-D, AZ-Eb-DP, AZ-Eo-D-
P, AZ-Ea-DP, AZ-Eb-DD, A
-Z-Eo-DD, AZ-Ea-DD, AZ-
Sequences such as Eb-PD and AZ-Ea-PD are exemplified. Here, (A) is an acid treatment stage, (Z) is an ozone treatment stage, (Ea) is an acidic extraction stage, (Eb) is an alkali extraction stage, and (Eo) is an alkali extraction stage reinforced with oxygen. In addition, (D) shows a chlorine dioxide stage,
(P) shows the hydrogen peroxide stage. Further, (-) between each stage indicates a washing step. Further, an enzyme treatment (Ez) stage may be provided in a part of the bleaching step for the purpose of adding chemicals and improving pulp quality in the above sequence, and a chlorine dioxide stage of D stage is dividedly added (dnD, DND, dED etc.).

[0032]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but of course, the present invention is not limited to these Examples. In addition, the addition rates in Examples and Comparative Examples are shown by weight% based on absolute dry pulp.

Example 1 Bleaching by AZ-Eb-D sequence [Hereinafter, (-) in the sequence indicates a washing stage. (1) Pretreatment: whiteness 44.6%, kappa number 12.
1. A beech-based hardwood having a pulp viscosity of 19.9 mPa · s after oxygen delignification Kraft pulp (hereinafter referred to as LOKP) is diluted to 130 g by absolute dry weight with ion-exchanged water and adjusted to a pulp concentration of 14.0%. did.

(2) Stage A (acid treatment stage): The LOKP pulp is supplemented with additional water and 4N-H ₂ SO ₄ by 1.0% in terms of sulfuric acid, based on the absolute dry pulp (hereinafter, the addition ratio is absolute). Shows dry pulp.) Added, pulp concentration 10%, treatment temperature 105
Treated at 120C for 120 minutes. The obtained pulp was washed with ion-exchanged water and dehydrated and concentrated to a pulp concentration of 20%. At this time, the pulp copper value after dehydration was 6.2. (3) Z stage (ozone treatment stage): The pulp concentration after A stage is 1
The pulp was adjusted to 0%, and the pulp was placed in a 3 liter in-house stirrer (stirring autoclave) with a closed pressure-resistant heating heater, and 102 g / m ³ ozone gas was supplied to a gauge pressure of 3.3 kg / cm ² . Sealed in a tank so that
Stirring was performed at 00 rpm for 10 seconds, and then at 600 rpm for 120 seconds to react the ozone gas with the pulp. Then, the pressure was returned to normal pressure, and the unreacted ozone gas in the tank was replaced with air to terminate the reaction. The obtained pulp was washed with ion-exchanged water and dehydrated to a pulp concentration of 30%. At this time, the ozone addition rate was 0.4%.

(4) Eb stage (alkali extraction stage): Z
The pulp concentration after the step was adjusted to 14%, 1.0% of sodium hydroxide was added, the pulp concentration was 10%, and the treatment temperature was 60 ° C.
For 120 minutes. The obtained pulp was washed with ion-exchanged water and dehydrated and concentrated to a pulp concentration of 20%. (5) Stage D (chlorine dioxide treatment stage): 0.25% of chlorine dioxide equivalent to chlorine dioxide is added to the pulp having the pulp concentration adjusted to 14% after the Eb stage, and the pulp concentration is 10% and the pulp concentration is 1 at 70 ° C.
A chlorine dioxide treatment was performed for 80 minutes. The filtrate pH after the completion of the D-stage reaction was 4.5. After completion of the reaction, the obtained pulp was washed with ion-exchanged water to obtain a finished bleached pulp.

Example 2 Bleaching by AZ-Ea-D Sequence The final pH of the E stage after the Z stage (ozone treatment stage) was adjusted to 0.14% with sodium hydroxide added so as not to be alkaline.
Then, the same treatment as in Example 1 was performed except that the stage was an Ea stage (an extraction stage under acidic conditions) to obtain a finished pulp. The final pH in the Ea stage was 4.7.

Example 3 Bleaching by AZ-Eb-D Sequence The processing temperature of the A stage (acid treatment stage) was set to 90 ° C., and the inlet kappa number of the subsequent Z stage (ozone treatment stage) was set to 8, and further ozone was added. The same treatment as in Example 1 was performed except that the rate was changed to 0.5%, to obtain a finished pulp.

Example 4 Bleaching by AZ-Eb-D sequence Whiteness 47.8%, Kappa number 10.0, Pulp viscosity 1
The same processing as in Example 1 except that the beaker-based LOKP of 9.3 mPa · s was used, the inlet kappa number of the Z stage (ozone treatment stage) was 4.0, and the ozone addition rate was 0.2%. To obtain a finished pulp.

Example 5 Bleaching by AZ-Ea-D Sequence The final pH of the E stage after the Z stage (ozone treatment stage) was adjusted so that the sodium hydroxide addition rate was 0.14% so as not to be alkaline.
Then, the same treatment as in Example 4 was carried out except that the stage was an Ea stage (an extraction stage under acidic conditions) to obtain a finished pulp. The final pH in the Ea stage was 4.7.

Example 6 Bleaching by AZ-Eb-D Sequence The processing temperature of the A stage (acid treatment stage) was set to 90 ° C., and the inlet kappa number of the subsequent Z stage (ozone treatment stage) was set to 6.5. The same treatment as in Example 4 was performed except that the ozone addition rate was changed to 0.4%, to obtain a finished pulp.

COMPARATIVE EXAMPLE 1 Bleaching by aZ-Eb-D sequence (1) Pretreatment: Whiteness 44.6%, Kappa number 12.
1. Beech-based LOK with pulp viscosity of 19.9 mPa · s
P was diluted with 130 g in absolute dry weight and ion-exchanged water to adjust the pulp concentration to 14%. (2) aZ stage (acid treatment and ozone treatment stage): This LOK
To the P pulp, additional water and 4N-H ₂ SO ₄ were added to the absolutely dry pulp in terms of sulfuric acid in an amount of 1.0% (hereinafter the addition ratio indicates absolutely dry pulp), and ion-exchanged water was additionally added. The pulp concentration was adjusted to 10%, and the pulp was placed in a 3-liter closed-type stirrer with a pressure-resistant heating heater (stirring autoclave) without washing, and 148 g / m ³ ozone gas at a gauge pressure of 4 g. 0.0kg / cm ² , sealed at 1200 rpm for 10 seconds,
Stirring was performed at 0 rpm for 120 seconds to react the ozone gas with the pulp. Then, the pressure was returned to normal pressure, and the unreacted ozone gas in the tank was replaced with air to terminate the reaction. The obtained pulp was washed with ion-exchanged water and dehydrated to a pulp concentration of 30%. At this time, the ozone addition rate was 0.77%.

(3) Eb stage (alkali extraction stage): Z
The pulp concentration after the step was adjusted to 14%, 1.0% of sodium hydroxide was added, the pulp concentration was 10%, and the treatment temperature was 60 ° C.
For 120 minutes. The obtained pulp was washed with ion-exchanged water and dehydrated and concentrated to a pulp concentration of 20%. (4) Stage D (chlorine dioxide treatment stage): 0.25% of chlorine dioxide equivalent to chlorine dioxide is added to the pulp having the pulp concentration adjusted to 14% after the Eb stage, and the pulp concentration is 10% and the pulp concentration is 1 at 70 ° C.
A chlorine dioxide treatment was performed for 80 minutes. The filtrate pH after the completion of the D-stage reaction was 4.5. After completion of the reaction, the obtained pulp was washed with ion-exchanged water to obtain a finished bleached pulp.

Comparative Example 2 Bleaching by the a-Z-Eb-D sequence Stage a (acid treatment stage): Additional water and 4N were added to LOKP pulp.
The -H ₂ SO ₄ with sulfuric acid in terms of relative bone dry pulp 1.0%
(Hereinafter, the addition ratio indicates absolutely dry pulp.) The mixture was treated at a pulp concentration of 10% at a treatment temperature of 50 ° C. for 60 minutes. The obtained pulp is washed with ion-exchanged water and has a pulp concentration of 2
Dehydration concentration was performed to 0%. At this time, the pulp copper value after dehydration was 10.9. Further, the same treatment as in Example 1 was carried out except that the stage A in Example 1 was replaced by the stage a and the ozone addition rate in the Z stage was changed to 0.73%, to obtain a finished pulp.

Comparative Example 3 Bleaching by the a-Z-Ea-D sequence The bleaching was carried out except that the A treatment described in Example 5 was replaced with the a treatment described in Comparative Example 2 and the ozone addition rate in the Z stage was 0.6%. The same treatment as in Example 5 was performed to obtain a finished pulp. The kappa number at the entrance of the Z stage was 9.0.

Comparative Example 4 Bleaching by aZ-Eb-D sequence. The A treatment and the Z treatment described in Example 4 were replaced with the aZ described in Comparative Example 1.
The same treatment as in Example 4 was performed, except that the ozone addition rate was changed to 0.6%, to obtain a finished pulp.

The pulp quality and strength measurement methods shown in Table 1 are as follows. Whiteness: Five sheets of 200 g per square meter were prepared, and the ISO whiteness was determined using a spectral white clock SC-10WN manufactured by Suga Test Instruments Co., Ltd. Pulp viscosity: TAPPI STDT230om-89
The measurement was performed according to the following. Sheet strength: Twenty sheets of 60 g of absolutely dry rice tsubo were prepared, and the tensile strength was indicated as a tear length using JISP8113. The tear strength was shown as a specific tear strength using JIS P8116.

[0047]

[Table 1]

As is clear from Table 1, the bleached pulp according to the present invention suppresses the decrease in pulp strength while maintaining high whiteness with a small amount of added ozone (Examples 1 to 5).
6). When the Eb stage after the Z stage is replaced with the Ea stage, the decrease in whiteness is small and the intensity is improved. On the contrary,
In the case of the conventional bleaching containing ozone, when the whiteness is increased, the viscosity and the strength are reduced (Comparative Example 1). Further, even in the case of conventional mild acid treatment for the purpose of removing metal ions, when the whiteness is increased, the viscosity and strength are reduced (Comparative Example 2). Furthermore, when trying to maintain the strength, the whiteness decreases (Comparative Example 4). Even when the Eb stage in Comparative Example 4 was replaced with the Ea stage, the intensity was hardly improved and the whiteness was low (Comparative Example 3).

[0049]

As is apparent from the above description, according to the bleaching method of the present invention, a decrease in pulp strength can be suppressed while maintaining high whiteness with a small amount of added ozone as compared with the conventional method. Becomes possible.

Claims (4)

[Claims] Claims 1. A lignocellulosic substance is digested with an alkaline chemical and then subjected to an oxygen delignification step treatment to adjust the kappa number of the pulp to 8 to 15, and the obtained pulp is treated in an aqueous acidic solution to obtain To reduce the pulp kappa number to 3 to 10 by reducing the pulp kappa number by 30% or more in comparison with the oxygen delignification step, perform ozone gas treatment and extraction treatment in the subsequent steps, and bleach in a sequence of chlorine dioxide treatment. A method for bleaching lignocellulose, characterized in that: 2. The treatment in an acidic aqueous solution is carried out at pH 2.5 to
The method for bleaching lignocellulose according to claim 1, wherein the method is carried out under a condition of 4.0 and a reaction temperature of 80C to 130C. 3. The extraction treatment after the ozone gas reaction is performed at pH 9
The method for bleaching lignocellulose according to claim 1 or 2, wherein the method is performed under alkaline conditions of from 1 to 12. 4. The extraction treatment after the ozone gas reaction is carried out at pH 4
The method for bleaching lignocellulose according to claim 1 or 2, wherein the method is performed under an acidity of from 6.5 to 6.5.