JPH10317291A - Bleaching of lignocellulosic material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the deterioration in strength of a chemical pulp obtained from a lignocellulosic material and carry out the bleaching to be of a high whiteness degree, by suppressing the pH in the final chlorine dioxide stage of a multistage bleaching sequence and maintaining the optimal conditions. SOLUTION: The oxygen delignifying treatment of a chemical pulp obtained from a lignocellulosic material according to a kraft pulp method is carried out and further the treatment with an enzyme having xylan hydrolyzing activities thereof is performed. The bleaching treatment of the resultant pulp is then conducted in a three-stage bleaching sequence composed of a chlorine stage- alkali extraction/oxygen stage-chlorine dioxide stage. In the final chlorine dioxide stage of the bleaching sequence, the chlorine dioxide is designed for divided addition and an alkali such as caustic soda is added before addition in the latter half thereof so as to maintain the final pH within the range of 4.5-5.6. Thereby, the efficient bleaching is carried out with a small amount of the chlorine dioxide.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pulp bleaching method, and more particularly, to a method for controlling pH in a subsequent chlorine dioxide stage in a three-stage bleaching sequence and reducing fluctuations to optimize pH conditions. The present invention relates to a bleaching method that suppresses a decrease in pulp strength while maintaining high whiteness with a small amount of chlorine dioxide by maintaining the following.

[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.

In recent years, a bleaching method using a non-chlorine bleaching chemical has attracted attention. Ozone, oxygen, hydrogen peroxide, peracid, and the like are known as non-chlorine bleaching chemicals. Among them, the bleaching method using ozone gas requires new equipment, which results in enormous investment costs. Become. Also,
With other chemicals, it is difficult to achieve a high whiteness level, so that it has not been widely used. Methods to reduce the amount of chlorine-based bleaching chemicals used include enhanced washing at each bleaching stage, use of pulp with advanced delignification at the bleaching inlet, and further enzymatic treatment. Significant capital investment was required, such as increasing the amount of wastewater, installing a new washing machine, and changing the digestion plant.

[0006] Regarding the optimum pH of the final D stage, Tappi Pres
s "The Bleaching of Pulp" 3rd Edition (1979
Year) P137 states that the optimum pH of the final chlorine dioxide stage of CEDED is 3.5-4.5. In addition, the same book, P139, describes the D-stage pH in the 3-stage sequence of CED.
Is described as giving the highest whiteness at 3.5.

On the other hand, "Paper Pulp Manufacturing Technology Complete Book", Vol. 5, "Pulp Processing and Bleaching", edited by Japan Society of Paper and Paper Technology (198).
P233-235, the influence of pH is described as one of the conditions for chlorine dioxide bleaching. Here, the highest whiteness is around pH 6, and the pulp viscosity is not much different between pH 3 and 6,
It is described that the maximum value is obtained at around pH 5. Also,
Preferably, chlorine dioxide bleaching is performed at pH 5-6,
Since the fiber is not seriously damaged as long as it is on the acidic side, it is usually industrially started at pH 7, and the pH is lowered by the action of the acid generated as a result of the reaction.
It is described that it often ends in fourth place. On the other hand, the method of keeping the pH weakly acidic, p.
No. 5 discloses that calcium carbonate, magnesium hydroxide, magnesium carbonate, sodium bicarbonate, and the like are used as a buffer.

[0008] However, these methods are the case of a sequence using only the first stage chlorine, and the optimum pH at the last stage in the case of ECF (Elemental Clorine Free) bleaching using no chlorine as in the present invention. There is no description. Moreover, there is no description that the optimum pH is different between when chlorine is used in the first stage and when chlorine is not used. As described later, the actual optimum pH for bleaching is narrower than these pH ranges, and the optimum pH is not always determined by the bleaching sequence.
Are not the same. In addition, it has been found that the above method is inexpensive and easy to apply as a method for adjusting pH, but is not necessarily suitable for controlling pH so that an optimum result can be obtained.

The 1986 Pulping Conference proc
eedings, P153, P167, P489, P491 3 -stage bleaching sequences such as C / D-Eo -D after C _D -Eo -D and oxygen delignification by simplifying the equipment have been proposed to. However, at the end of the D stage of these three stages of bleaching
There is no mention of H and there is no teaching to split chlorine dioxide. Furthermore, even when using the disclosed method, compared with the conventional multi-stage bleached pulp using chlorine-based chemicals, still satisfactory results are obtained in terms of whiteness and strength, the amount of chlorine compounds used, and the drainage load. Not shown.

[0010] On the other hand, regarding the method of dividing and adding chlorine dioxide in the chlorine dioxide stage, see 1987 Pulping Confer.
ence proceedings, P487 to C _D -E- (DED) and C _D -Eo - (DED) are shown. Here, (DED) indicates that there is no cleaning during this time. Also,
JP-A-50-58302 and JP-B-7-6147 disclose a method in which an alkali is added to the lower part of the first stage of a continuous chlorine dioxide stage, and after washing, a chlorine dioxide stage is provided again to perform bleaching. Although disclosed, any of the reactions described in these publications is aimed at performing alkali extraction, for example, adjusting the pH after addition of an alkali to 10 to 12, and the optimal pH for chlorine dioxide bleaching. Not to maintain.

[0011]

SUMMARY OF THE INVENTION The present invention solves the above problem by using a three-stage bleaching sequence and controlling the pH in the final chlorine dioxide stage to reduce the amount of added chemicals. It is an object of the present invention to provide a method for bleaching pulp having high pulp strength while maintaining high whiteness at a high rate.

[0012]

In order to achieve the above-mentioned object, the present invention provides a pulp after oxygen delignification treatment obtained from a lignocellulosic material, which comprises three stages: a chlorine stage, an alkali extraction / oxygen stage and a chlorine dioxide stage. During the bleaching process in the bleaching sequence, chlorine dioxide in the final chlorine dioxide stage is divided and added,
In addition, the present invention provides a method for bleaching a lignocellulosic substance, which comprises adding an alkali before adding chlorine dioxide in the latter half of the division in order to maintain the final pH of the chlorine dioxide stage at 4.5 to 5.5.

The present invention also provides a method for bleaching a lignocellulosic material, which comprises subjecting a chemical pulp after the above-mentioned oxygen delignification treatment to an enzyme treatment and then performing a bleaching treatment in the three-stage sequence.

Further, the present invention provides a method for producing a chemical pulp obtained from a lignocellulosic material, which is subjected to an oxygen delignification treatment followed by a bleaching treatment in a three-stage bleaching sequence of a chlorine dioxide stage-alkali extraction / oxygen stage-chlorine dioxide stage. Of chlorine dioxide in the chlorine dioxide stage at the end of the chlorine dioxide stage,
A method for bleaching lignocellulosic material comprising adding alkali prior to the second half of chlorine dioxide addition to maintain H at 5.5-6.5.

Further, the present invention provides a chemical pulp after the above-mentioned oxygen delignification treatment, which is subjected to an enzyme treatment.
A bleaching method for a lignocellulosic material characterized by performing a bleaching treatment in a step sequence.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The lignocellulosic substance used in the present invention contains polysulfide, or includes a conventional kraft pulping method (KP), a sulphite pulping method (SP), an alkali pulping method (AP) and the like. Pulps derived from the chemical pulping method are preferred, and pulp obtained by the kraft pulping method is particularly preferred. Examples of lignocellulosic materials derived from woody plants used for pulping include pine, spruce, softwoods such as Douglas fir, aspen, beech, acacia, eucalyptus, hardwoods such as mangrove, and lignocellulosic materials derived from herbaceous plants. Include, but are not necessarily limited to, bagasse, kenaf, esparto grass, rice, reeds and the like.

The pulp to be bleached in the three-stage sequence including the chlorine dioxide stage of the present invention has been subjected to an oxygen delignification treatment as a pretreatment so that the kappa number becomes 20 or less, and preferably 15 or less. And more preferably a pulp giving a Kappa number of 12 or less. Also,
A combination of the oxygen delignification treatment and the acid treatment may be used, and there is no particular limitation.

In the present invention, unbleached pulp obtained by a known cooking method undergoes an oxygen delignification treatment step.
Further, it is preferable to perform a treatment with an enzyme having xylan decomposing activity as a pretreatment before performing the multi-stage bleaching. As long as the enzyme has xylan decomposing activity, it may have decomposing activity to cellulose, mannan, pectin and the like.

The enzyme has the following xylan-decomposing activity:
0.1-10U / g of absolutely dry pulp, preferably 0.5-5
It is added to the pulp in the range of U / g of absolutely dried pulp. Here, 1 U means that when an enzyme is allowed to act on xylan,
It refers to the amount of enzyme that produces 1 μmol xylose per minute. The amount of enzyme added was 0.1
When the amount is less than U / g dry pulp, the effect of the xylan from the pulp is insufficient due to insufficient elution, and when the amount exceeds 10 U / g dry pulp, the yield of the pulp in the enzyme treatment step decreases. Is not preferred.

The temperature at which the pulp after oxygen delignification treatment is treated with an enzyme is 30 to 80 ° C., preferably 40 to 60 ° C.
It is in the range of ° C. If the temperature is lower than 30 ° C., the xylan decomposing activity of the enzyme decreases, and the treatment effect becomes insufficient. On the other hand, if the temperature is higher than 80 ° C., it is not suitable for a normal enzyme because the enzyme itself is denatured and becomes inactive. The pH of the pulp solution or the solution contained in the pulp during the treatment is 3
The range is from 10 to 10, preferably from 5 to 9, as long as it has enzyme activity, and is not particularly limited. The treatment time is 10 minutes or more, preferably 30 to 180 minutes, but the time is not particularly limited as long as the enzyme treatment effect is recognized.

The chemical used in the first bleaching stage of the present invention is chlorine alone (stage C) or chlorine dioxide alone (D
₀ stage).

The addition ratio of chlorine compound, expressed as the amount of active chlorine, to absolutely dry pulp is proportional to the amount of lignin in unbleached pulp, and is preferably between 1 and 5% by weight of lignin. The reaction temperature is preferably between 20 and 60 ° C., and the treated pulp concentration during the reaction (pulp absolute dry weight / total weight including moisture, hereinafter abbreviated as pulp concentration) is between 2 and 12%, The reaction time is between 5 minutes and 60 minutes, and the final pH of the bleaching stage is preferably between 2 and 5. After completion of the bleaching stage, a washing or compacting step follows as is known in the art.

The alkali used in the oxygen-enriched alkali extraction stage (Eo stage) in the bleaching system of the present invention can be selected from many alkaline compounds known to those skilled in the art. The alkali addition rate per pulp is NaOH
It is preferably 0.5 to 3% (based on absolute dry pulp) in terms of conversion. Eo
PSA (Pressure SwingAd)
sorption) Oxygen, VSA (Vacuum Swing Adsorption)
) Can be used as long as they can be used on an industrial scale, such as oxygen-enriched air or cryogenic oxygen, and may be used under pressure or under atmospheric pressure. The addition rate is preferably 0.5 to 3% (based on absolutely dry pulp). 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 between 30 and 70 ° C.
The final pH is preferably between 9 and 12 for 120 minutes. After the Eo stage, the washing or compression step is carried out in the same manner as after the first bleaching stage.

The chlorine dioxide used in the final stage of the present invention is the R-8 method, the R-10 method, the soda method, etc., and the alkali type is caustic soda, sodium silicate or the like. Any method is possible as long as it is possible, and the production method is not particularly limited.

As a method of controlling the pH in the chlorine dioxide stage (D ₁ ) in the final stage in the present invention, chlorine dioxide is dividedly added (hereinafter, the first half of the divided chlorine dioxide stage is divided into the d ₁ stage, and the chlorine dioxide stage of the second half is divided into two stages). d ₂ ), and before the addition of chlorine dioxide (d ₂ stage) in the latter half of the division, d ₂
It is characterized in that an alkali is added to adjust the pH after the step (hereinafter, abbreviated as step b for convenience because a basic substance (base) is used). Specifically, in order to apply the present invention to an actual plant, for example, caustic soda is directly fed to a feed pump suction or a medium concentration mixer of a retention tube for the purpose of pH adjustment before the latter half of chlorine dioxide addition in a divided chlorine dioxide stage. injection to, after a certain time reaction is added chlorine dioxide (d ₂ stages). The purpose of adding alkali such as caustic soda is to adjust the pH. Therefore, it is sufficient that the final pH of the chlorine dioxide stage is adjusted to a predetermined value. Not done. For example, the alkali addition rate may be varied depending on the rate at which chlorine dioxide is added. Further, in order to perform the same operation, the retention tubes may be continuously arranged in two or more stages in series.

[0026] For final pH in the chlorine dioxide stage (D ₁ stage) of the first stage of chlorine pH4.5~5.5 For (C stage) is, the first stage chlorine dioxide (D ₀ stage) pH 5.5 A range from to 6.5 is preferred. When the final pH of the chlorine dioxide stage is 4.5 or less, chlorine dioxide is consumed but the whiteness decreases,
On the other hand, when the final pH exceeds 6.5, chlorine dioxide becomes difficult to consume, and the whiteness decreases.

[0027] The treatment time in the chlorine dioxide stage, time 3 split chlorine dioxide treatment in the first half of the addition (d ₁ stage)
It is preferably 30 minutes, particularly preferably 5 to 15 minutes.
If the treatment time is less than 3 minutes, the reaction pH, whiteness, and consumption of chlorine dioxide are unstable, and the final pH and whiteness show unstable values. On the other hand, when the processing time exceeds 30 minutes, the whiteness reaches an equilibrium. Time is preferably 30 to 300 minutes in the second half of the chlorine dioxide treatment (d ₂ stages), particularly preferably 120 to 200 minutes. If it is less than 30 minutes, it is difficult to obtain a pulp with high whiteness. On the other hand, when the processing time is longer than 300 minutes, there is a problem that the whiteness reaches an equilibrium and a color return occurs due to consumption of all the chlorine dioxide.

In the present invention, there is a pH adjustment zone (stage b) with alkali between the first half and the second half of the chlorine dioxide stage, but the treatment time is preferably 0.1 to 10 minutes, particularly preferably 1 to 10 minutes. Minutes to 3 minutes. If the time is less than 0.1 minute, uniform pH adjustment is difficult. On the other hand, beyond 10 minutes, the effect decreases. Incidentally, d ₁ step reaction time period from chlorine dioxide addition to adding an alkali in real plant, as b-stage the time until the addition of further second half of the chlorine dioxide after alkali addition, the further second half of chlorine dioxide The time required from the addition to the end of the washing process is defined as d ₂ stages, and the total of these times is defined as the D ₁ stage chlorine dioxide treatment time.

The processing temperature of the chlorine dioxide stage (d ₁ and d ₂ stages) is preferably 40 to 90 ° C., particularly preferably from 60 ° C. ~
80 ° C. 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.

The pulp concentration is selected from a medium concentration region of 5 to 15%, more preferably 8 to 12%. If it is less than 5%, the reaction between pulp and chlorine dioxide becomes worse, while 15%
If it exceeds, stirring will be poor and uniform reaction will be difficult.

In the present invention, the addition ratio of chlorine dioxide is not particularly limited, and may be appropriately selected depending on the amount of residual lignin and the degree of whiteness of the target final pulp.

As described above, in the conventional multi-stage bleaching sequence including the chlorine dioxide stage, the final D stage is added in a single stage, and the final pH is said to be optimally around 5-6. In contrast, in the present invention, when performing the bleaching process using a three-stage bleaching sequence, the chlorine dioxide in the final chlorine dioxide stage is divided and added, and the chlorine dioxide in the latter half of the divided stage is used in the present invention. (D ₂ ) If the alkali is added before the addition, the fluctuation of pH after the addition of chlorine dioxide can be easily controlled. As a result, in each of the bleaching by the normal method using chlorine and the ECF bleaching using chlorine dioxide, optimum pH in the final chlorine dioxide stage (D ₁ stage)
It has the characteristic that the condition can be maintained.

By controlling the pH in the chlorine dioxide stage, the whiteness can be increased and the decrease in strength can be reduced because the fluctuation can be reduced until the end of the reaction while maintaining the optimum pH of chlorine dioxide. It is estimated that chlorine consumption was significantly reduced. Further, as in the present invention, the reason that the bleachability and strength of each stage have been improved as compared with the conventional single stage bleaching of chlorine dioxide is not sufficiently known, but the multistage bleaching including the conventional chlorine dioxide stage is not known. In the sequence, when chlorine dioxide is added, chloride ions and organic acids increase with a decrease in pH, so that it acts on carbohydrates, and it is considered that a significant decrease in strength was observed, In the case of the present invention, the pH during chlorine dioxide treatment is 5.5 to 6.5 for ECF bleaching, and 4.5 to 5.5 for normal bleaching.
By controlling at high values, rapid oxidation was prevented and the amount of chloride ions was reduced, resulting in less attack on carbohydrates, ultimately increasing whiteness and improving strength It is considered something.

Examples of the bleaching sequence to be bleached this way, the pulp after the oxygen delignification, for example _{C-Eo -D 1, D 0} -Eo -D 1 stage, Ez-C-Eo
_{-D 1, Ez-D 0 -Eo} -D 1 like sequences and the like. Here, (C) is a chlorine stage, and (Eo) is an alkali extraction stage reinforced with oxygen. In addition, (D ₀ ) indicates the first stage of chlorine dioxide, and (D ₁ ) indicates the last stage of chlorine dioxide. Of these, the stage D ₁ is described in detail (d ₁ bd ₂ )
Is being processed. Further, (-) between each stage indicates a washing step. Further, an enzymatic 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.

[0035]

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 addition, the addition ratio in the examples and comparative examples indicates% by weight based on absolute dry pulp.

Example 1 Bleaching by C-Eo-D ₁ sequence (1) Pretreatment: whiteness 50.0%, kappa number 11.
2. A kraft pulp (hereinafter, referred to as LOKP) having a pulp viscosity of 19.7 mPa · s and consisting mainly of beech hardwood after oxygen delignification was diluted with ion-exchanged water to 60 g in absolute dry weight to adjust the pulp concentration to 14.0%. (2) Stage C: Additional water and chlorine are added to the LOKP pulp at 1.1% in terms of chlorine relative to the absolutely dry pulp (hereinafter the addition ratio indicates absolutely dry pulp), pulp concentration 10%, treatment temperature Treated at 55 ° C. for 30 minutes. The obtained pulp was washed with ion-exchanged water. (3) Eo stage: The pulp concentration after C stage was adjusted to 13%,
The pulp was put into a 2-liter sealed pressure-resistant heating stirrer (stirred autoclave) equipped with a heating heater.
The sodium hydroxide was adjusted to 1.3% and the pulp concentration to 10%. Heat while stirring at 300 rpm, 60 ° C,
It was kept under a pressurized oxygen pressure of 1.5 kg / cm ² (gauge pressure) for 20 minutes, then returned to normal pressure, and kept at 60 ° C. for 70 minutes.
The obtained pulp was washed with ion-exchanged water and dehydrated to a pulp concentration of 30%. (4) D ₁ stage: pulp concentration of 14%, the pulp was adjusted to pH 8.1 with chlorine dioxide in the chlorine dioxide converted 0.25
% In addition, the pulp concentration of 12% was maintained for 8 minutes at 70 ° C. (d ₁ stage). PH at the time of d _1-stage end was 5.4. After the reaction is completed, Na is adjusted to pH 6.0.
OH was added and kept for 3 minutes (stage b). Then added 0.25% chlorine dioxide chlorine dioxide converted, subjected to chlorine dioxide treatment 180 min at 70 ° C. After the total pulp concentration of 10% (d ₂ stages). d Filtrate pH after _two- step reaction
Was 4.6. After completion of the reaction, the obtained pulp was washed with ion-exchanged water to obtain a finished bleached pulp.

Example 2 The chlorine dioxide addition rate in the _first stage D was set to 0.2%, and d ₂
The same treatment as in Example 1 was performed except that the pH after the end of the stage reaction was set to 4.9, to obtain a finished bleached pulp. Incidentally, the pH of the reaction in _one step D is d ₁ stage inlet pH is 8.1, d ₁ step reaction completion after pH is 5.5, b stage After completion of the reaction the pH was 6.0.

Example 3 The chlorine dioxide addition rate in the _first stage D was set to 0.2%, and d ₂
The same treatment as in Example 1 was carried out except that the pH after the end of the stage reaction was 5.5, to obtain a finished bleached pulp. Incidentally, the pH of the reaction in _one step D is d ₁ stage inlet pH is 8.2, d ₁ step reaction completion after pH is 5.5, b stage After completion of the reaction the pH was 7.9.

Example 4 The chlorine dioxide addition rate in the _first stage D was set to 0.2%, and d ₂
The same treatment as in Example 1 was performed except that the pH after the end of the stage reaction was set to 4.5, to obtain a finished bleached pulp. The reaction pH at the D ₁ stage was 7.8 at the d ₁ stage entrance pH, 5.2 after the d ₁ stage reaction, and 5.7 after the b stage reaction.

Example 5 As a pretreatment, 2 units of xylanase enzyme were added per absolutely dried pulp to LOKP before the C stage, and the pulp concentration was 1
The same treatment as in Example 1 was performed except that the treatment was performed at 0% and 60 ° C. for 120 minutes, and the chlorine addition rate in the C stage was set to 0.9%.
The finished bleached pulp was obtained. The reaction pH in the D ₁ stage is d
₁ stage inlet pH is 8.0, d ₁ step reaction completion after pH is 5.
2, b-stage reaction was completed after pH is 6.0, d _2-stage reaction was completed after p
H was 4.6.

Example 6 The chlorine dioxide addition rate in the _first stage D was set to 0.2%, and d ₂
The same treatment as in Example 5 was carried out except that the pH after the end of the step reaction was adjusted to 5.0, to obtain a finished bleached pulp. The reaction pH at the D ₁ stage was 8.0 at the d ₁ stage inlet pH, 5.3 after the d ₁ stage reaction, and 6.2 after the b stage reaction.

Example 7 The chlorine in the first stage C of Example 1 was changed to chlorine dioxide, and the first stage D was changed to chlorine dioxide.
₀Step 1.0% addition, D ₁Chlorine dioxide addition rate at each stage
0.2% and d_TwoThe pH after the end of the step reaction was adjusted to 5.5.
Other than the above, the same treatment as in Example 1 was carried out to complete the bleached pulp.
I got Note that D₁The reaction pH at the stage is d₁Step inlet pH
7.8, d₁After the end of the stage reaction, the pH is 5.2, and the stage b reaction
Afterwards the pH was 7.9.

Example 8 The addition ratio of chlorine dioxide in the _first stage D was set to 0.2%, and d ₂
The same treatment as in Example 7 was carried out except that the pH after the end of the step reaction was changed to 6.4, to obtain a finished bleached pulp. The reaction pH at the D ₁ stage was 8.1 at the d ₁ stage inlet pH, 5.7 after the end of the d ₁ stage reaction, and 8.5 after the end of the b stage reaction.

[0044] EXAMPLE 9 D ₀ stage was added 2 units per absolute dry pulp xylanase enzyme as a pre-treatment for the previous LOKP, pulp 10%, 60 ℃, then the process 120 minutes, D ₀ stage of chlorine dioxide The same treatment as in Example 7 was performed except that the addition ratio was 0.7%, to obtain a finished bleached pulp. The reaction pH at the D ₁ stage was 8.1 at the d ₁ stage inlet pH and p after the completion of the d ₁ stage reaction.
H was 5.5, the pH was 8.0 after the completion of the b-stage reaction, and the pH was 5.2 after the completion of the _two- stage reaction.

Example 10 The chlorine dioxide addition rate in _one stage D was set to 0.2%, and d ₂
The same treatment as in Example 9 was carried out except that the pH after the end of the step reaction was changed to 6.4, to obtain a finished bleached pulp. Incidentally, the pH of the reaction in _one step D is d ₁ stage inlet pH is 8.3, d ₁ step reaction completion after pH is 5.8, b stage After completion of the reaction pH was 8.5.

[0046] perform bleaching in Comparative Example 1 C-Eo -D ₁ sequence, and, except that the pH after d ₂ step reaction ended 3.7 was treated in the same manner as in Example 1, a finished bleached pulp Obtained. The reaction pH at the D ₁ stage was 7.8 at the d ₁ stage inlet pH, 5.1 after the d ₁ stage reaction, and 5.3 after the b stage reaction.

[0047] The chlorine dioxide addition rate in Comparative Example 2 D ₁ stage was 0.5% of the D single-stage bleaching, further the pH after completion of the reaction in order to 4.9, D
At the stage entrance, alkali was added in advance to adjust the pH before the reaction to 10.
The same treatment as in Example 2 was performed except that the value was set to 0, to obtain a finished bleached pulp.

[0048] The chlorine dioxide addition rate in Comparative Example 3 D ₁ stage was 0.5% of the D single-stage bleaching, further the pH after completion of the reaction in order to 6.0, D
At the stage entrance, alkali was added in advance to adjust the pH before the reaction to 10.
The same treatment as in Example 2 was carried out except for changing to 8, to obtain a finished bleached pulp.

Comparative Example 4 D The chlorine dioxide addition rate in _one stage was 0.25%, and d
The same treatment as in Example 1 was carried out except that the pH after the completion of the _two- step reaction was changed to 6.0, to obtain a finished bleached pulp. Note that D ₁
The pH of the reaction in stage d ₁ stage inlet pH is 8.1, d ₁ step reaction completion after pH is 5.4, b stage After completion of the reaction pH was 8.4.

[0050] Comparative Example 5 D chlorine dioxide addition rate in the _first stage was 0.5% of the D single-stage bleaching, pre-reaction pH to 7.8, the embodiment except that the pH after completion of the reaction to 3.8 5 In the same manner as described above to obtain a finished bleached pulp.

[0051] Comparative Example 6 D chlorine dioxide addition rate in _one step to 0.4% D single-stage bleaching, pre-reaction pH to 7.8, the embodiment except that the pH after completion of the reaction to 4.0 5 In the same manner as described above to obtain a finished bleached pulp.

[0052] The chlorine dioxide addition rate in Comparative Example 7 D ₁ stage to single-stage bleaching 0.5% D, the pH after completion of the reaction to 5.5, by addition of pre alkaline with D stage inlet The same treatment as in Example 5 was carried out except that the pH before the reaction was changed to 10.6, to obtain a finished bleached pulp.

Comparative Example 8 First stage D in Example 1 was changed from chlorine in the first stage C to chlorine dioxide.
Except that 1.0% of ₀ stage was added, the final stage D was bleached with 0.5% of chlorine dioxide at the _1st stage, and the pH before the reaction was 7.8 and the pH after the reaction was 3.8. Was processed in the same manner as in Example 7 to obtain a finished bleached pulp.

COMPARATIVE EXAMPLE 9 In order to make the chlorine dioxide addition rate in the final D ₁ stage bleach at 0.5% D single stage and to adjust the pH after the reaction to 5.5, alkali was added in advance at the D stage inlet. PH before reaction is 10.6
The same treatment as in Example 7 was carried out except that the finished bleached pulp was obtained.

[0055] Comparative Example 10 Last D chlorine dioxide addition rate in the _first stage was 0.5% of the D single-stage bleaching, pre-reaction pH to 8.2, the pH after completion of the reaction 4.
The same treatment as in Example 9 was performed except that the value was changed to 0, to obtain a finished bleached pulp.

COMPARATIVE EXAMPLE 11 In order to make the chlorine dioxide addition rate in the final D ₁ stage one stage bleaching of 0.5% D and to adjust the pH after the reaction to 5.5, alkali was added in advance at the entrance of the D stage. PH before reaction is 10.5
The same treatment as in Example 9 was carried out except that the finished bleached pulp was obtained.

Comparative Example 12 The chlorine dioxide addition rate in the _first stage D was set to 0.2%, and d ₂
The same treatment as in Example 9 was carried out except that the pH after the completion of the step reaction was changed to 6.8, to obtain a finished bleached pulp. The reaction pH at the D ₁ stage was 8.1 at the d ₁ stage inlet pH, 5.5 after the d ₁ stage reaction was completed, and 9.1 after the b stage reaction was completed.

Comparative Example 13 The chlorine dioxide addition rate in the _first stage D was set to 0.2%, and d ₂
The same treatment as in Example 9 was carried out except that the pH after the end of the step reaction was set to 4.5, to obtain a finished bleached pulp. Incidentally, the pH of the reaction in _one step D is d ₁ stage inlet pH is 7.7, d ₁ step reaction completion after pH is 5.2, b stage After completion of the reaction the pH was 5.8.

[0059]

[Table 1] ──────────────────────────────────── ClO ₂ addition rate Final pH Whiteness Pulp viscosity Specific tear strength Rupture length (%) (ISO)% mPa ・ s km ──────────────────────────────── ──── Example 1 0.5 4.6 86.3 18.2 96 6.5 Example 2 0.4 4.9 85.8 18.4 97 6.6 Example 3 0.4 5.5 85.7 18.3 97 6.6 Example 4 0.4 4.5 85.4 18.4 98 6.6 Example 5 0.5 4.6 86.4 18.7 97 6.7 Example 6 0.4 5.0 86.0 18.9 100 6.7 Example 7 0.4 5.5 85.6 18.4 102 6.6 Example 8 0.4 6.4 85.7 18.6 102 6.6 Example 9 0.4 5.5 85.9 18.8 104 6.7 Example 10 0.4 6.4 85.6 19.0 105 6.8 ────── ────────────────────────────── Comparative Example 1 0.5 3.7 84.7 17.6 92 6.3 Comparative Example 2 0.5 4.9 84.6 17.6 92 6.3 Comparative Example 3 0.5 6.0 84.3 17.3 92 6.3 Comparative Example 4 0.5 6.0 85.2 18.0 94 6.4 Comparative Example 5 0.5 3.8 84.9 18.5 95 6.4 Comparative Example 6 0.4 4.0 84.7 18.6 96 6.5 Comparative Example 7 0.5 5.5 85.1 18.5 94 6.4 Comparative Example 8 0.5 3.8 84.7 17.8 94 6.3 Comparative Example 9 0.5 5.5 85.2 17.9 94 6.3 Comparative Example 10 0.5 4.0 85.1 18.2 96 6.5 Comparative Example 11 0.5 5.5 85.3 18.2 96 6.5 Comparative Example 12 0.4 6.8 85.2 18.6 97 6.6 Comparative Example 13 0.4 4.5 85.1 18.7 97 6.6 ───────────────────────── ───────────

Regarding the method for measuring pulp quality and strength, ISO whiteness was determined using Ellejo 2000 for whiteness. Pulp viscosity is TAPPI STD T230o
The measurement was performed according to m-89. Tensile strength is JIS P8
It was shown as the breaking length using 113. JIS tear strength
It was shown as a specific tear strength using P8116.

[0061] When the first stage of bleaching pulp bleaching sequence according to the present invention As is evident from Table 1 is C stage, at a pH after completion of the reaction of D ₁ stage 4.5 to 5.5, also the first stage as well In the case of D stage (D ₀ ), high whiteness and high viscosity and strength are maintained at a pH of 5.5 to 6.5 after completion of the D ₁ stage reaction (Examples 1 to 6, and Example 7).
-10). On the other hand, when the final stage of the conventional method is a B-stage bleaching process, the whiteness, viscosity and strength are all low even when the pH is changed (Comparative Examples 3, 5, 6, 8, and 10). further,
In the present invention, the whiteness, viscosity and strength are all low in the case of the bleaching treatment in the D-stage even in the place where the pH falls within the optimum pH range (Comparative Examples 2, 7, 9, 11). In addition, D ₁ stage (d ₁ bd
_In the sequence to which ₂ ) was applied, the whiteness, viscosity, and strength were all low in a range other than the optimum pH (Comparative Examples 1, 4, 12, and 13).

[0062]

As is clear from the above description, the bleaching method of the present invention makes it possible to produce bleached pulp with high whiteness and suppressed reduction in pulp strength with less consumption of chlorine dioxide as compared with the conventional method. became.

Claims (4)

[Claims] Claims 1. A chemical pulp obtained from a lignocellulosic material is subjected to an oxygen delignification treatment, followed by a bleaching treatment in a three-stage bleaching sequence of a chlorine stage-alkali extraction / oxygen stage-chlorine dioxide stage. Add chlorine in portions,
And a method for bleaching a lignocellulosic substance, characterized by adding an alkali before adding chlorine dioxide in the latter half of the division in order to maintain the final pH of the chlorine dioxide stage at 4.5 to 5.5. 2. The method for bleaching lignocellulosic material according to claim 1, wherein the chemical pulp after the oxygen delignification treatment is subjected to an enzyme treatment, and then subjected to a bleaching treatment in the three-stage sequence. 3. The chemical pulp obtained from the lignocellulosic material is subjected to oxygen delignification treatment and then subjected to bleaching treatment in a three-stage bleaching sequence of chlorine dioxide stage-alkali extraction / oxygen stage-chlorine dioxide stage. The chlorine dioxide of the stage is dividedly added, and the final pH of the chlorine dioxide stage is 5.5 to 5.5.
A method for bleaching a lignocellulosic material, characterized in that an alkali is added before chlorine dioxide addition in the latter half of the division to maintain 6.5. 4. The method for bleaching a lignocellulosic substance according to claim 3, wherein the chemical pulp after the oxygen delignification treatment is subjected to an enzyme treatment, and then subjected to a bleaching treatment in the three-stage sequence.