JP2000303375A - Production of bleached pulp - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a production process for bleached pulp that can reduce the consumption of the bleaching chemicals in the production of the pulp bleached with ECF, in other words, by bleaching the pulp with a bleaching chemical free from chlorine and/or hypochlorous acid. SOLUTION: In the production of bleached pulp in which a lignocellulose material is cooked and the unbleached pulp is subjected to the oxidative bleaching, then to the multistep bleaching operation in the absence of elementary chlorine to produce the objective bleached pulp, the chlorine dioxide stage is provided on the first step and the immediately following step, that is followed by the extraction treatment under alkaline conditions and the treated pulp is additionally bleached.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing bleached pulp from lignocellulosic materials, especially wood or non-wood. More specifically, the present invention is a method for producing an ECF bleached pulp by oxygen bleaching unbleached pulp obtained by washing after cooking wood or non-wood, and then bleaching without chlorine. Chlorine dioxide is used as a chemical in the first and second stages, and may or may not be washed during the chlorine dioxide treatment. Thereafter, extraction is performed under alkaline conditions, and if necessary, one or more stages. And a method for producing bleached pulp.

In order to use wood pulp or non-wood pulp as a raw material for papermaking in many applications, wood or non-wood is digested with a cooking liquor, and the pulp obtained after pulping by the chemical action of chemicals is bleached with bleaching chemicals. It is necessary to bleach to increase whiteness. For example, kraft pulp obtained by cooking wood chips with kraft cooking liquor is usually used after oxygen delignification with oxygen and alkali, or oxygen, unless used for applications requiring strength such as packaging materials. Chlorine, hypochlorite, chlorine dioxide, oxygen, ozone, hydrogen peroxide,
It is bleached in one or more stages by a bleaching agent selected from a bleaching agent such as caustic soda and a bleaching aid to remove lignin and the like which are coloring agents contained in the pulp and to have a whiteness of 70 to 90%. It is commonly used as a range of semi-bleached or completely bleached kraft pulp.

Conventionally, in a multi-stage bleaching method, pulp is first treated with chlorine, lignin contained in the pulp is chlorinated, lignin is added with solubility, and then chlorinated lignin is dissolved and extracted with alkali. Thus, a method has been adopted in which lignin is separated and removed from the pulp, and a small amount of the remaining lignin is decomposed and removed using hypochlorite, chlorine dioxide and the like to obtain pulp having high whiteness.

[0004] However, in recent years, there has been a concern about the effect of organic chlorine compounds contained in the bleaching wastewater from the pulp chlorination stage on the environment, and there has been much research into bleaching sequences that do not use atomic chlorine for pulp bleaching. . Also, when hypochlorite is used, chloroform is generated during bleaching of the pulp, which may have an adverse effect on the environment. Therefore, a bleaching sequence (ECF:
(Sometimes abbreviated as elementary chlorin free.) Bleaching is being sought.

At present, as an alternative to chlorine and hypochlorite,
Oxygen-based bleaching chemicals, such as ozone, oxygen, peracetic acid and persulfuric acid, have received attention. However, these drugs
Except for oxygen and hydrogen peroxide, it is difficult to handle due to high chemical costs and explosive properties. On the other hand, chlorine dioxide has problems in terms of cost, generator capacity, etc.At present, the use of chlorine dioxide in place of chlorine in Japan has not reached widespread use, but it is promising in the future. Is the way. Compared to chlorine and hypochlorite, chlorine dioxide has less effect on the environment, but is expensive, so that the search for optimal bleaching sequences and conditions to minimize the addition rate has been made.

JP-A-4-245988 discloses a method for reducing the amount of chlorine dioxide used when bleaching chemical pulp in the absence of elemental chlorine by treating the pulp with a hemicellulose-degrading enzyme and then bleaching with chlorine dioxide. , The chlorine dioxide addition rate is about 2
Although a method capable of reducing the amount by 0% is disclosed, problems such as a decrease in pulp viscosity and a decrease in yield due to the use of an enzyme, and a pulp which is not significantly effective due to a substrate specificity in an enzyme treatment reaction appear. Bleaching costs can also increase because the enzymes themselves are not inexpensive reagents.

On the other hand, Japanese Patent Application Laid-Open No. 4-263687 discloses that bleaching is performed by combining ozone with chlorine dioxide as a means for reducing the chlorine dioxide addition rate, whereby the total chlorine dioxide addition rate is reduced by 50%. It is described as possible. However, the pulp viscosity was 27.5 mP
It is not easy to introduce it into existing factories because it is reduced from a · s to 16 mPa · s and equipment for generating ozone is required.

In JP-A-9-87985 and JP-A-9-87986, as a method of reducing the amount of chlorine dioxide used in the production of chemical pulp for papermaking, the pH before bleaching or after bleaching is adjusted to 1 pH. No. 3 to 3, immediately followed by delignification and bleaching with peroxide and oxygen in an alkaline medium, or a method of adding a chelating agent to a chlorine dioxide stage. By this treatment, the whiteness after the treatment was increased by about 5%, and the total chlorine dioxide addition rate could be suppressed to 0.5% by weight with respect to the pulp. However, the comparative example is a bleaching sequence containing chlorine, and ECF was used. Since the comparative example of the bleaching sequence is not bleached to the final stage and compared, the reduction effect in ECF bleaching is not clear.

[0009] Further, such pH adjustment in the chlorine dioxide stage and the combined use of a chelating agent are intended mainly for the effect of removing heavy metals in order to efficiently carry out the reaction between peroxide and oxygen in the subsequent stage. It is not to increase the efficiency of chlorine dioxide bleaching. Even if the treatment is performed by these methods, the peroxide may not be used immediately after that, and in this case, the effect may not be obtained.

It should be noted that a sequence in which the chlorine dioxide stage is continued twice has already been described many times, and is disclosed in JP-A-4-2636.
No. 87, etc., these are provided after the multi-stage bleaching, and do not follow the first stage of the multi-stage bleaching twice as in the present invention.

[0011]

SUMMARY OF THE INVENTION In view of the above background, the present inventors have made various studies on a method for improving bleachability for producing ECF pulp, and as a result, pulp fibers obtained after oxygen bleaching of unbleached pulp. Can be used as a chemical in the first and second stages of ECF bleaching, with or without washing between the first and second stages of chlorine dioxide treatment at different treatment temperatures, and then under alkaline conditions And found that the amount of lignin in the pulp can be reduced and the amount of chlorine dioxide used can be reduced. This is a method with a low environmental load since the emission amount of the organic chlorine compound can be further reduced. An object of the present invention is to provide an economical and environmentally friendly bleaching method that reduces the emission of organic chlorine compounds to wastewater while maintaining high whiteness, stability, and high quality. It is.

[0012]

(1) In the present invention, bleached pulp is produced by subjecting unbleached pulp obtained by digesting a lignocellulosic material to oxygen bleaching and then performing multi-stage bleaching in the absence of elemental chlorine. In the method, there is provided a method for producing bleached pulp, which comprises providing an initial stage of multistage bleaching and a continuous chlorine dioxide stage immediately thereafter, followed by extraction under alkaline conditions, and further bleaching.

(2) In the present invention, the first chlorine dioxide stage of the continuous chlorine dioxide stage is performed at a processing temperature of 70 to 110 ° C. for a processing time of 10 to 120 minutes, and immediately thereafter, a second continuous chlorine dioxide stage is performed. The chlorine dioxide stage is included in the process for producing bleached pulp according to (1), wherein the treatment is carried out at a treatment temperature of 40 to 90 ° C. and a treatment time of 30 to 120 minutes.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION When bleaching wood and then bleaching the pulp obtained by oxygen bleaching with a bleaching chemical containing no chlorine or hypochlorite (ECF bleaching), a chlorine dioxide stage was used as the first stage. Immediately after, compared to the conventional method, which is followed by an extraction process using an alkali treatment or an alkali treatment containing oxygen and peroxide, the chlorine dioxide treatment is performed in two stages, followed by extraction with an alkali that supplements the delignification by the alkali treatment and oxygen. In this method, the chlorine dioxide addition rate in the entire bleaching process can be reduced. Therefore, since the amount of chlorine dioxide used is smaller than that of the conventional method, it is possible to use a smaller chlorine dioxide generator regardless of whether it is newly built or expanded.
It is economical and reduces the amount of organochlorine compounds in the wastewater, making it a useful method for environmental protection.

[0015] The present invention relates to a method of cooking wood by using a chemical such as kraft cooking liquor and subjecting the resulting unbleached pulp to delignification by a known oxygen and alkali oxygen bleaching method, to a two-stage chlorine dioxide process. (The first stage is D1 stage, the next stage is D2 stage)
If it is a stage, it is abbreviated as D1D2) or a process of washing and treating during that period (abbreviated as D1-D2 stage), followed by extraction with oxygen and / or hydrogen peroxide under alkaline and / or alkaline conditions Alternatively, a bleaching pulp having a Hunter whiteness of 70 to 90% is produced by bleaching using a bleaching chemical not containing chlorine and hypochlorite after the extraction.

The pulp used in the present invention is pulp obtained by oxygen-bleaching unbleached pulp from softwood and hardwood, which are lignocellulosic materials, with oxygen and alkali.
As a cooking method for obtaining the pulp used in the present invention, kraft cooking, sulfite cooking, polysulfide cooking, known cooking methods such as soda cooking can be used, but in consideration of pulp quality, energy efficiency and the like. ,
Kraft cooking is preferably used.

For example, when wood chips are kraft cooked, the kraft cooking liquor has a sulfidity of 5 to 75%, preferably 15 to 45%, and an effective alkali addition rate of 5 to 30% by weight, preferably 5 to 30% by weight, based on the weight of the dried wood. 10 to 25% by weight, cooking temperature is 140 to 170 ° C, and the cooking method may be either continuous cooking or batch cooking. When using a continuous cooking furnace, a modified cooking method in which cooking liquor is added at multiple points. However, the method is not particularly limited. At the time of cooking, a known cyclic keto compound may be used in combination with the cooking liquor used as a cooking aid.

As the delignification method by oxygen bleaching used in the present invention, a known medium-concentration method or a high-concentration method using oxygen and alkali can be applied as it is. A medium concentration method performed at 15% by weight is preferred. The alkali used in the delignification method by oxygen bleaching according to the medium concentration method is caustic soda or oxidized kraft white liquor, and oxygen gas and alkali are added to a medium concentration pulp slurry in a medium concentration mixer and mixing is sufficiently performed. After that, it is sent to a delignification reaction tower capable of holding a mixture of pulp, oxygen and alkali under pressure for a certain period of time, and delignified. The above-mentioned medium concentration mixer varies depending on the manufacturer.
It has a rotor that rotates at a high speed of 00 to 1000 rpm,
Applying high shear to the medium strength pulp slurry converts the pulp slurry into a fluid such as water, thereby allowing a good mixing of the pulp, oxygen and alkali. The addition rate of oxygen gas is 0.5 to 3% by weight, the alkali addition rate is 0.5 to 4% by weight, and the reaction temperature is
The temperature is 0 to 120 ° C., the reaction time is 15 to 100 minutes, the pulp concentration is 8 to 15% by weight, and other known conditions can be applied.

The divisional addition or continuous chlorine dioxide treatment in the present invention means that pulp is added with aqueous chlorine dioxide at a pH in an acidic range or impregnated with the aqueous solution, and after maintaining a specific time and temperature, the pulp is washed. No (D1D
2) or performing (D1-D2), re-impregnating the pulp with aqueous chlorine dioxide and maintaining a specific time and temperature.

The pulp concentration in the D1 and D2 stages of the present invention ranges from 5 to 40% by weight, preferably from 8 to 35% by weight, more preferably from 10 to 25% by weight. If the pulp concentration is less than 5% by weight, a large-capacity facility is required for the treatment, which is not suitable. If the pulp concentration exceeds 40% by weight, it is difficult to uniformly mix pulp and chlorine dioxide, and the effect is not sufficiently obtained, which is not suitable.

The pulp concentration in the D1 and D2 stages of the present invention is 5 to 40% by weight, and the processing temperature is 50 to 150%.
C., preferably 70-110 ° C. for the D1 stage, 40-90 ° C. for the D2 stage, and the holding time is preferably 10-120 minutes for the D1 stage and 30-120 minutes for the D2 stage. Will be When the treatment temperature is lower than 40 ° C., the decomposition reaction of lignin by chlorine dioxide becomes insufficient, and the temperature becomes lower than 150 ° C.
If it exceeds, the decomposition of the polysaccharide becomes remarkable, and the strength of the pulp fiber is greatly reduced. Although the treatment time of the chlorine dioxide treatment of the present invention is 10 minutes or more, it is sufficient.
The effect of delignification reaches the upper limit even if it is longer than 120 minutes.

The amount used in the chlorine dioxide treatment of the present invention varies depending on the lignin amount of the pulp before bleaching and the bleaching sequence at the subsequent stage. % By weight, preferably 0.1-1.
0% by weight.

The pulp after the D2 stage is subjected to a lignin extraction / bleaching step using alkali alone and / or oxygen in combination under alkaline conditions (referred to as E stage or E / O stage). At this time, caustic soda, potassium hydroxide, oxidized white liquor and the like can be used as the alkali chemicals under alkaline conditions, but caustic soda is generally used. Also peroxide,
Particularly, hydrogen peroxide may be used in combination. In order to obtain a pulp having a whiteness of 80% or more, the pulp after the alkali extraction step needs to be further subjected to one or more bleaching steps. Examples of the chemical include a bleaching chemical comprising a known bleaching agent such as caustic soda (E), chlorine dioxide (D), oxygen (O), hydrogen peroxide (P), ozone (Z), and organic peracid and a bleaching aid. These are appropriately selected from these and used as a bleaching chemical. As a bleaching sequence, for example, D1D2
-E / OD, D1-D2-E / OD, D1D2-E
-D, D1-D2-ED, D1D2-E / OD-
P, D1-D2-E / ODP, D1D2-E / OP
-D, D1-D2-E / OP-D, D1-D2-E-
P, D1D2-E-P, D1D2-E / O-Z-D, D
A chlorine-free bleaching sequence such as 1-D2-E / O-ZD can be used.

[0024]

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. Examples 1 to 4 shown below are:
The unbleached pulp obtained by digesting wood chips is subjected to oxygen delignification to pulp obtained by dividing addition or continuous chlorine dioxide treatment, and then E / OD or E / O-.
In Comparative Examples 1 and 2, bleaching was carried out in a DP sequence, and in one stage, pulp treated with chlorine dioxide was subjected to E / O-
It is bleached in D or E / ODP sequence. Unless otherwise indicated, the chemical reduction rates in D1 and D2 stages were calculated as follows.

Reduction rate of total chemicals in D1 and D2 stages (vs. D-stage addition ratio) The reduction ratio of chemicals is the sum of chlorine dioxide in D1 and D2 stages used for bleaching with or without two-stage treatment or divided addition. It was calculated from the chemical rate by the following equation (1). Reduction rate of total chemicals in D1 and D2 stages,% = {(total chemical addition ratio in D stage-total chemical addition ratio in D1 and D2 stages in the case of this method) / total chemical addition ratio in D stage} × 100 (1)

Regarding the pulp viscosity, J. A. TappiN
O. 44, AOX (activated carbon adsorbing organic halogen) was performed according to the US Environmental Agency EPA method (TOX-10 manufactured by Mitsubishi Kasei), and PC value was measured before and after accelerated fading in a 105 ° C. oven for 4 hours. Was evaluated according to the following equation (2) using PC value = (1−whiteness after fading) ² / (2 × whiteness after fading)) − (1−whiteness before fading) ² / (2 × whiteness before fading)) (2)

Example 1 Hardwood pulp (Hunter whiteness: 43.8%, kappa number: 1) subjected to kraft digestion and oxygen delignification at a factory
0.8), a two-stage chlorine dioxide treatment was performed under the following conditions. That is, the unbleached pulp is adjusted to a pH of 4 by adding concentrated sulfuric acid to ion-exchanged water to adjust the pulp concentration at the time of treatment to 8%. Then, the mixture was placed in a stainless steel 2-liter indirect heating autoclave, heated at a temperature of 105 ° C. for 60 minutes while maintaining the pressure, and subjected to a D1 stage treatment of the pulp. After cooling the autoclave, D1
The pulp obtained from the treatment was transferred to a plastic bag without washing.

The pulp after the D1 treatment in the plastic bag is 0.30% chlorine dioxide based on the weight of the ion-exchanged water and the absolutely dry pulp so that the pulp concentration at the time of the D2 treatment is 10%.
And immersed in a constant temperature water bath at a temperature of 70 ° C. for 60 minutes.
Two stages of bleaching were performed. The obtained pulp was washed with deionized water and dehydrated. The pulp after D2 stage is put into a plastic bag, and the pulp concentration is adjusted to 10% using ion exchanged water. Then, caustic soda is added to the autoclave in the same manner as in D1 stage. Then, pressurize at 1.5 kg / m ^{2 with} oxygen, treat at 60 ° C for 15 minutes, take out the pulp from the autoclave, transfer it to plastic bag, and treat for 45 minutes in 60 ° C constant temperature water bath. To extract the 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, and the pulp concentration was adjusted to 10% using ion-exchanged water.
%, And chlorine dioxide treatment was performed at a temperature of 70 ° C. for 180 minutes (D3 stage). The obtained pulp was washed and dehydrated using ion-exchanged water to obtain bleached pulp. After the pulp after D3 stage is disintegrated, Tappi test method T205os
-71 (JISP 8209), basis weight 60 g / m
The sheet No. ² was prepared, and the hunter whiteness of the pulp was measured in accordance with JIS P 8123. The result was 85.0%.

Example 2 In the same manner as in Example 1, a D1 stage treatment with an addition rate of 0.45% was performed, followed by washing and dehydration, and further a chlorine dioxide addition rate of 0.2.
D2 two-stage processing of 5% was performed, and the subsequent processing was performed in exactly the same manner as in Example 1, and Hunter whiteness was 85.0.
% Bleached pulp was obtained.

Example 3 After performing D1 stage treatment with an addition rate of 0.20% in the same manner as in Example 1, washing was not performed, and D2 with a chlorine dioxide addition rate of 0.24% was performed.
After performing two-stage treatment and washing the treated pulp with ion-exchanged water,
The pulp after the E / O stage treatment and the D3 stage treatment under completely the same conditions as in Example 1 was put into a plastic bag, and the pulp concentration was adjusted to 10% using ion-exchanged water. 0.35% of hydrogen peroxide and 0.5% of caustic soda were added, and the mixture was treated at 70 ° C. for 120 minutes.
The steps were bleached. The obtained pulp was washed and dehydrated using ion-exchanged water to obtain bleached pulp having a Hunter brightness of 85.0%.

Example 4 As in Example 3, bleaching was performed in the sequence of D1-D2-E / O-D3-P, and washing and dehydration were performed between D1 and D2. The chemical addition rate of each stage is 0.2% for D1 and 0.2% for D2.
Thereafter, the addition was performed at the same addition rate as in Example 3 to obtain bleached pulp having a Hunter brightness of 85.0%.

Comparative Example 1 Unbleached pulp similar to that in Example 1 was placed in a plastic bag.
After adjusting the pulp concentration at the time of treatment to 10% using ion-exchanged water, 0.80% of chlorine dioxide was added to the weight of the absolutely dried pulp and immersed in a constant temperature water bath at a temperature of 70 ° C for 120 minutes to produce a dioxide. The chlorine stage was bleached. The obtained pulp was washed with deionized water and dehydrated. Then E / O-
The D-stage treatment was performed in exactly the same manner as in Example 1 to obtain bleached pulp having a Hunter whiteness of 85.0%.

Comparative Example 2 The same D-stage treatment as in Comparative Example 1 was carried out except that the chlorine dioxide addition ratio was changed to 0.45%, and the subsequent E / O-DP-stage treatment was performed in exactly the same manner as in Example 2. Hunter whiteness 85.
0% bleached pulp was obtained.

Table 1 shows the results of Examples 1 to 4 and Comparative Examples 1 and 2.

[0036]

[Table 1]

As shown in Table 1, although different depending on the sequence, the two-stage chlorine dioxide treatment (D1-D2) can reduce the amount of chemicals by 10% to 14% as compared with the ordinary one-stage treatment. 37% chlorine compounds
About 5 to 5 depending on the D1D2 treatment.
It was possible to reduce the chemicals by 9%, thereby reducing the amount of organochlorine compounds by about 35%. In Examples 3 and 4, the pulp viscosity is improved as compared with Comparative Example 2. Improvement was also observed in the bleaching property of the pulp.

[0038]

According to the present invention, in the ECF bleaching, the amount of chlorine dioxide used can be reduced without decreasing the pulp viscosity by performing two stages of chlorine dioxide treatment in the first stage, and therefore, the amount of organic chlorine compounds is reduced. This provides an effect of providing a method for producing bleached pulp that can reduce the amount of bleached pulp.

Claims (2)

[Claims] An unbleached pulp obtained by digesting a lignocellulosic material is subjected to oxygen bleaching, followed by multi-stage bleaching in the absence of elemental chlorine to produce a bleached pulp. A process for producing bleached pulp, comprising the steps of: providing a continuous chlorine dioxide stage, followed by extraction under alkaline conditions, and further bleaching. 2. The first chlorine dioxide stage of the continuous chlorine dioxide stage has a processing temperature of 70 to 110 ° C. and a processing time of 10 to 1
The process was performed for 20 minutes, and immediately thereafter, the second continuous chlorine dioxide stage was performed at a processing temperature of 40 to 90 ° C. and a processing time of 30 to 12 hours.
2. The process for producing bleached pulp according to claim 1, wherein the process is performed within a range of 0 minutes.