JP2002302888A - Manufacturing method of bleached pulp - Google Patents

Abstract

(57) [Summary] [PROBLEMS] In a method of bleaching unbleached pulp by alkali oxygen bleaching and then performing bleaching by treating it in a multi-stage bleaching process having a chlorine dioxide bleaching stage only in the first stage and the last stage, bleaching and bleaching efficiently. Provided is a method for producing bleached pulp having improved properties. SOLUTION: The unbleached pulp obtained by digesting a lignocellulosic material is subjected to alkali oxygen bleaching, and then treated in a multistage bleaching step having a chlorine dioxide bleaching stage only in the first stage and the last stage to produce a bleached pulp. The ratio of the chlorine dioxide addition rate of the first stage to the chlorine dioxide addition ratio of the last stage per absolutely dry pulp weight is 3/1 to 20/1, and the washing wastewater after the chlorine dioxide bleaching stage of the last stage is used as the first stage of carbon dioxide. Used as dilution water in the chlorine bleaching stage and / or washing water after the first chlorine dioxide bleaching stage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing bleached pulp of lignocellulosic material. More specifically, the present invention provides bleaching of unbleached pulp obtained by digesting a lignocellulosic material by alkali oxygen bleaching, followed by treatment in a multi-stage bleaching process containing only a chlorine dioxide bleaching stage in the first and final stages. The present invention relates to a method for producing pulp.

[0002]

2. Description of the Related Art In order to use a lignocellulosic material as a raw material for papermaking in many applications, it must be obtained after pulping by a chemical action such as cooking or pulping by a mechanical action using a refiner or the like. The resulting pulp must be bleached with a bleaching chemical to increase whiteness. For example, kraft pulp is usually used as bleached kraft pulp after removal of lignin etc., which are coloring substances contained in pulp, unless used for applications requiring strength such as packaging materials. General.

[0003] When bleached pulp is produced from unbleached pulp, it is necessary to maintain the strength of the pulp fiber itself to a certain degree. Therefore, decomposition of carbohydrates such as cellulose and hemicellulose constituting the pulp fiber is minimized. It is common to employ a multi-stage bleaching method of three to six stages in which bleaching is mildly performed by avoiding extreme one-stage bleaching so that the bleaching chemicals and bleaching conditions are variously combined.

Conventionally, in a multi-stage bleaching method, pulp is first treated with chlorine, lignin contained in the pulp is chlorinated, and 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.

However, in recent years, organochlorine compounds (hereinafter referred to as AOX) contained in the bleaching wastewater from the chlorination stage of pulp
There is a concern about the environmental impact of the pulp bleaching, and there is an increasing trend to use chlorine for pulp bleaching. Also, when hypochlorite is used, chloroform is generated during pulp bleaching, which may have an adverse effect on the environment.Therefore, a bleaching sequence that does not use hypochlorite for pulp bleaching is required. Is coming.

At present, as an alternative to chlorine and hypochlorite,
The use of ozone, chlorine dioxide, hydrogen peroxide and peracids such as peracetic acid and persulfuric acid has been proposed. However, in practice, chlorine dioxide and hydrogen peroxide, which have been proven as chemicals and are relatively easy to handle, are often used as substitutes for chlorine and hypochlorite. In particular, the use of chlorine dioxide as an alternative to chlorine is overwhelmingly large. However, when chlorine dioxide is used as a substitute for chlorine, there is a problem that the bleaching efficiency is reduced and sometimes the bleaching pulp deteriorates in color because the mechanism of delignification is different.

As a method for improving the bleaching efficiency by chlorine dioxide, a method of bleaching chlorine dioxide under oxygen pressure (Japanese Patent Application Laid-Open No. H11-081733) and a method of treating with chlorine dioxide in gaseous form (US Pat. No. 5,474,654). Method), a method of treating at a specific pH (for example, DAHL
PARPERI JA PUU, VOL79, NO.
8, P560-564), a method of dividing and adding (Japanese Patent Laid-Open No.
0-317291) and the like have been proposed. However, in order to actually perform the above-described method, it is necessary to prepare a special device, and in some cases, there is no effect.

As a method of replacing the chlorine stage in the first chlorine stage with chlorine dioxide and not deteriorating the bleaching property of the bleached pulp, a method of performing a xylanase pretreatment (for example, Japanese Patent Application Laid-Open No. 2-26)
4087, JP-A-2-293486), a method of performing an acid pretreatment (for example, Tapauni Vuorinen et al., 1996).
International Pulp Bleaching Conference Proceedin
g 43-51). However,
Even when these methods were used, the discoloration was sometimes deteriorated.

[0009]

In view of this situation, the present inventors have conducted chlorine dioxide bleaching in the first stage and the last stage of the multi-stage bleaching process, and have not reduced the bleaching efficiency, and have further improved the bleaching properties of the bleached pulp. As a result of various studies on methods that do not deteriorate, bleaching was performed so that the bleaching efficiency was improved by bleaching so that the addition rate of chlorine dioxide in the first stage per weight of absolutely dried pulp was at least three times the chlorine dioxide addition ratio in the final stage. It has been found that the bleaching properties of the pulp are also improved. Furthermore, when bleaching is performed so that the chlorine dioxide addition rate in the first stage per bleached pulp weight is at least three times the chlorine dioxide addition ratio in the final stage, the relative pollution degree of the washing wastewater after the chlorine dioxide bleaching in the final stage is relatively high. As the dilution water in the first stage chlorine dioxide bleaching stage,
Further, they have found that they can be substantially reused as washing water after bleaching chlorine dioxide in the first stage, and have completed the present invention.

[0010] It is an object of the present invention to alkali bleach unbleached pulp obtained by cooking lignocellulosic material,
Thereafter, in a method for producing bleached pulp by processing in a multi-stage bleaching step having only a chlorine dioxide bleaching stage in the first stage and the last stage, it is intended to provide a method for producing bleached pulp that efficiently bleaches and does not deteriorate the fading property. is there.

[0011]

The present invention includes the following inventions. (1) A method for producing bleached pulp by subjecting an unbleached pulp obtained by digesting a lignocellulosic substance to alkaline oxygen bleaching, and then treating the unbleached pulp in a multi-stage bleaching step having a chlorine dioxide bleaching stage only in the first stage and the last stage, The ratio of the chlorine dioxide addition rate of the first stage to the chlorine dioxide addition ratio of the last stage per absolutely dry pulp weight is set to 3/1 to 20/1, and the washing wastewater after the chlorine dioxide bleaching stage of the last stage is replaced with the first stage of the carbon dioxide. A method for producing bleached pulp, which is used as dilution water in a chlorine bleaching stage and / or washing water after chlorine dioxide bleaching in a first stage.

(2) The method for producing bleached pulp according to (1), wherein an alkali extraction stage is provided immediately after the first chlorine dioxide bleaching stage.

(3) The multi-stage bleaching step is carried out in three bleaching stages in the order of chlorine dioxide bleaching stage, alkali extraction stage and chlorine dioxide bleaching stage, or (1) or (2).
The method for producing bleached pulp according to the above item.

(4) The method for producing bleached pulp according to the above item (2) or (3), wherein the pulp after the alkali extraction stage has a potassium permanganate value of 3.5 or less.

(5) In the washing after the first and last chlorine dioxide bleaching stages, the pH of the pulp slurry is 2-4.
The method for producing bleached pulp according to any one of items (1) to (4), wherein

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

[0017]

DETAILED DESCRIPTION OF THE INVENTION The lignocellulosic material used in the present invention is preferably hardwood, but may be softwood or non-wood, and is not particularly limited. Cooking methods for obtaining the pulp used in the present invention include kraft cooking, polysulfide cooking,
Known cooking methods such as soda cooking and alkaline sulphite cooking can be used, but in consideration of pulp quality, energy efficiency and the like, kraft cooking or polysulfide cooking is preferably used.

For example, when wood is kraft cooked, the kraft cooking liquor has a sulfuration degree of 5 to 75%, preferably 15 to 75%.
45%, effective alkali addition rate is 5-5
30% by weight, preferably 10 to 25% by weight, the cooking temperature is 130 to 170 ° C, and the cooking method may be either continuous cooking or batch cooking. The cooking method may be modified cooking method, and the method is not particularly limited.

In the cooking, a known cyclic keto compound such as benzoquinone,
Naphthoquinone, anthraquinone, anthrone, phenanthroquinone and alkyl-substituted products such as alkyl and amino of the quinone-based compound, or hydroquinone-based compounds such as anthrahydroquinone which is a reduced form of the quinone-based compound, and further anthraquinone by the Diels-Alder method One or more selected from 9,10-diketohydroanthracene compounds, which are stable compounds obtained as intermediates in the synthesis method, may be added, and the addition rate is a normal addition rate. For example, the content is 0.001 to 1.0% by weight based on the absolute dry weight of the wood chips.

In the present invention, the unbleached chemical pulp obtained by the known digestion method is subjected to washing, roughing and selective steps, and then delignified by the known alkaline oxygen bleaching method. As the alkaline oxygen bleaching method used in the present invention, a known medium concentration method or a high concentration method can be applied as it is, but a medium concentration method currently used generally at a pulp concentration of 8 to 15% by weight is used. preferable.

In the alkaline oxygen bleaching method by the above-mentioned medium concentration method, caustic soda or oxidized kraft white liquor can be used as the alkali, and oxygen from the cryogenic separation method, PSA (Pressure Swing Adsor) can be used as the oxygen gas.
ption), oxygen from VSA (Vacuum Swing Adsorption) and the like can be used. The oxygen gas and the alkali are added to a medium-concentration pulp slurry in a medium-concentration mixer, and after sufficient mixing is performed, the mixture is sent to a reaction tower capable of holding a mixture of pulp, oxygen and alkali under pressure for a certain period of time, and subjected to delignification. Is done.

The rate of addition of oxygen gas is 0.5 to 3% by weight per absolutely dry pulp weight, the rate of alkali addition is 0.5 to 4% by weight, the reaction temperature is 80 to 120 ° C., and the reaction time is 15 to 10%.
At 0 minutes, the pulp concentration is 8 to 15% by weight, and other known conditions can be applied. In the present invention, in the alkaline oxygen bleaching step, it is a preferred embodiment that the alkaline oxygen bleaching is continuously performed a plurality of times, and delignification is advanced as much as possible. The pulp subjected to alkaline oxygen bleaching is then sent to a washing step. After washing, the pulp is sent to an enzyme treatment step, an acid treatment step, or a multi-stage bleaching step.

In the present invention, it is possible to provide an enzyme treatment step after the alkali oxygen bleaching step. The enzymes used in the enzyme treatment step include xylanase, lignin peroxidase, manganese peroxidase,
Those having a bleach accelerating effect such as laccase are preferable, but those which remove pitch components like lipase or modify pulp like cellulase may be used. These enzymes may be used alone or in combination or mixed,
Furthermore, it can be used by dividing into a plurality of times. Of these enzymes, xylan-degrading enzymes called xylanase not only improve the bleaching properties of pulp by reacting with pulp, but also can separately utilize xylooligosaccharides contained in large amounts in treated wastewater. Are preferably used.

In the present invention, an acid treatment step may be provided after the alkali oxygen bleaching step. The acid treatment in the acid treatment step of the present invention has a pH of 2 to 4, preferably 2.5 to 4.
3.5, temperature 70-120 ° C, preferably 85-110
C. and a holding time of 5 to 120 minutes, preferably 20 to 90 minutes. Acid treatment of chemical pulp is UK patent No. 1.
No. 062734, and acid treatment before or before the oxygen bleaching stage of chemical pulp is disclosed in
It is known that acid treatment improves the discoloration of chemical pulp, as is known from EP 10482. The acid used in the acid treatment of the present invention has a pH of 2 to 4 during the acid treatment.
Inorganic acids such as sulfuric acid, nitric acid, hydrochloric acid, sulfurous acid, nitrous acid and the like can be used as long as they can be adjusted, and inorganic acids such as sulfuric acid, nitric acid, sulfurous acid and nitrous acid, among them, sulfuric acid are preferred because they are easy to obtain and handle. The pulp concentration in the acid treatment step of the present invention is in the range of 1 to 30% by weight, preferably 2 to 15% by weight. If the pulp concentration is less than 1% by weight, the treatment requires a large-capacity facility and is not suitable. If the pulp concentration exceeds 30% by weight, a problem occurs in that the pulp and the acid are uniformly mixed, which is not suitable. In order to obtain uniform mixing, a low-concentration mixer, a medium-concentration mixer, a high-concentration mixer, a medium-concentration pump, or the like can be appropriately selected and used according to the type and concentration of pulp at the time of processing.

In the multi-stage bleaching process of the present invention, chlorine dioxide bleaching is always performed in the first stage and the final stage, and the washing wastewater after the final stage chlorine dioxide bleaching is used as the dilution water and / or diluent in the first stage chlorine dioxide bleaching stage. Alternatively, it is reused as washing water after the first-stage chlorine dioxide bleaching. The conditions for chlorine dioxide bleaching in the first stage include a pH of 2 to 6, preferably 3 to 4, and a temperature of 40 to 12;
0 ° C, preferably 60 to 100 ° C, retention time 5 to 120
Minutes, preferably 15-90 minutes, pulp concentration 1-40%,
Preferably it is 8 to 15%, and if necessary, it is possible to add auxiliaries such as acid, alkali and chelating agent. It is also possible to add chlorine dioxide in portions.

On the other hand, the conditions for bleaching chlorine dioxide in the final stage are as follows: pH 3-7, preferably 4-6, temperature 40-12.
0 ° C, preferably 60 to 90 ° C, retention time 5 to 360
Minutes, preferably 60-240 minutes, pulp concentration 1-40
%, Preferably 8 to 15%, and if necessary, an acid,
It is possible to add auxiliaries such as alkalis and chelating agents. It is also possible to add chlorine dioxide in portions.

In the present invention, the ratio of the chlorine dioxide addition rate at the first stage to the chlorine dioxide addition ratio at the last stage per absolutely dry pulp weight is 3/1 to 20/1. When the ratio of the chlorine dioxide addition rate in the first stage to the chlorine dioxide addition ratio in the last stage is less than 3/1, not only the bleaching efficiency is reduced but also the bleaching pulp deteriorates in color, which is not suitable. On the other hand, 20/1
If it is larger, the chlorine dioxide addition rate in the final stage will be too low, and it will be virtually impossible to control.

In the present invention, since the chlorine dioxide addition rate in the final stage per absolutely dry pulp weight becomes relatively low,
The pollution degree of the washing wastewater after the chlorine dioxide bleaching of the last stage also becomes relatively low, and as the dilution water of the chlorine dioxide bleaching stage of the first stage,
Another feature is that it can be reused as washing water after chlorine dioxide bleaching in the first stage.

In the washing stage after the first and last chlorine dioxide bleaching stages in the present invention, the pH of the pulp slurry is preferably controlled to 2 to 4. In the washing stage, by controlling the pH to 2 to 4 to prevent scale due to calcium oxalate in the washing stage, the washing efficiency is improved, and finally the bleaching property of the pulp and the bleached pulp The fading property is improved. In the present invention, the washing wastewater after chlorine dioxide bleaching in the final stage is used as dilution water in the chlorine dioxide bleaching stage in the first stage and / or washing water after chlorine bleaching in the first stage. By controlling the pH of the washing stage to 2 to 4, the pH of the pulp slurry in the washing stage after the initial chlorine dioxide bleaching stage can be easily controlled to 2 to 4.

In the multi-stage bleaching process of the present invention, chlorine dioxide bleaching is always carried out in the first stage and the final stage, except that chlorine dioxide is not used in the other bleaching stages.
There is no particular limitation. However, it is preferred to provide an alkali extraction stage immediately after the first chlorine dioxide bleaching stage. As the alkali extraction stage, a known method can be used.For example, oxygen can be used in combination, hydrogen peroxide can be used in combination, or oxygen and hydrogen peroxide can be used in combination, and it is not particularly limited. Absent.

The bleaching chemicals other than chlorine dioxide (hereinafter referred to as D) used in the multi-stage bleaching process of the present invention include alkali (hereinafter referred to as E), oxygen (hereinafter referred to as O), and peroxide. Examples thereof include hydrogen (hereinafter referred to as P), ozone (hereinafter referred to as Z), organic peracids such as peracetic acid and persulfuric acid (hereinafter referred to as PA), and the like. Not something.

The bleaching sequence in the multi-stage bleaching process in the present invention includes DE / OD, DE / OP
-D, DE / OPD, DE / OP-PD, D
-E / O-ZD, DE / OP-ZD, DE / O
-PA-D, DE / OP-PA-D, DZE-O
-D, DZE-OP / D, D-PA-E / OD,
D-PA-E / OP-D and the like can be mentioned, but of course, it is not limited to these. Further, a chelating agent treatment step using ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA) or the like may be inserted in the multi-stage bleaching step.

In the present invention, the bleaching efficiency is improved and the bleaching property of the bleached pulp is improved by setting the addition ratio of chlorine dioxide in the first stage per weight of the absolutely dried pulp to at least three times the addition ratio of chlorine dioxide in the final stage. We do not know the reason for this without waiting for future research, but we speculate as follows. In the first chlorine dioxide bleaching stage, there are substances that are removed from the pulp after reacting with chlorine dioxide and substances that are extracted only in the subsequent alkali extraction stage. It is considered that this has greatly affected the discoloration of pulp. In other words, by increasing the chlorine dioxide addition rate in the initial chlorine dioxide stage, the amount of substances extracted only in the alkali extraction stage after chlorine dioxide bleaching is increased, and by removing it, bleaching efficiency is improved, It is speculated that the bleaching properties of the bleached pulp have also been improved.

[0034]

EXAMPLES The present invention will be described more specifically with reference to examples and comparative examples below, but of course the present invention is not limited to these examples. In Examples 1 to 3 and Comparative Example 1 shown below, when bleached hardwood kraft pulp after factory oxygen bleaching in the D1-E / O-D2 sequence, D2-stage washing wastewater was collected, and again, When bleaching hardwood kraft pulp in a D1-E / O-D2 sequence after oxygen bleaching made by a factory, it was used as a D1 dilution liquid,
In Examples 4 and 5 and Comparative Example 2, when bleaching hardwood kraft pulp after oxygen bleaching by a factory in the D1-E / O-P-D2 sequence, D2-stage washing wastewater was collected, and again,
Hardwood kraft pulp after oxygen bleaching from factory is D1-E /
It was used as a D1 dilution when bleaching in the OPD2 sequence.

Unless otherwise indicated, the measurement of the kappa number, the measurement of the potassium permanganate value, the measurement of the pulp whiteness, and the evaluation of the bleaching property of the pulp were performed by the following methods. In addition, the addition rates of the chemicals in Examples and Comparative Examples are shown by weight% based on the weight of the absolutely dried pulp.

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

2. Pulp potassium permanganate value (K
Measurement of potassium permanganate value is based on JIS P 8206
It went according to.

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

4. Evaluation of bleaching property of pulp The pulp sheet for whiteness measurement was bleached for 24 hours under the conditions of 80 ° C. and 65% relative humidity, and the PC value was calculated and evaluated from the pulp brightness before and after fading according to the following formula. PC value =
(1 after photobleaching whiteness) ^2/2 / after photobleaching whiteness - (1-fading before whiteness) ^2/2 / fade before whiteness

Example 1 Hardwood kraft pulp after oxygen bleaching made by a factory (mixture of wood: 55% eucalyptus wood, 45% oak wood, whiteness: 48.
9%, kappa number: 10.4) in an absolute dry weight of 70.0 g
Collect, put in a plastic bag, adjust the pulp concentration to 10% using ion-exchanged water, add 1.3% chlorine dioxide per weight of absolutely dry pulp, and immerse in a constant temperature water bath at 70 ° C for 30 minutes Then, D1 stage bleaching was performed. The obtained pulp was diluted to 3% with ion-exchanged water, and then dehydrated and washed with a Buchner funnel.

The pulp after the first stage was adjusted to 10% by adding ion-exchanged water to the pulp, and then 1.0% of caustic soda was added to the absolutely dry pulp weight. And pressurized with a commercially available compressed oxygen gas having a purity of 99.9% so that the gauge pressure becomes 0.15 MPa, and reacted at 70 ° C. for 20 minutes. Thereafter, the pulp slurry was taken out of the autoclave, transferred to a plastic bag, and treated at a temperature of 70 ° C. for 70 minutes in the same manner as in the D1 stage to perform an E / O stage extraction. The obtained pulp was diluted to 3% with ion-exchanged water, and then dehydrated and washed with a Buchner funnel.

Subsequently, the pulp after the E / O stage was placed in a plastic bag in an absolute dry weight of 60.0 g, adjusted to a pulp concentration of 10% using ion-exchanged water, and then chlorine dioxide was extracted per absolute dry pulp weight. 0.2% was added, and the mixture was treated at a temperature of 70 ° C. for 180 minutes in the same manner as in the D1 stage to perform the D2 stage bleaching. The obtained pulp was diluted to 3% with ion-exchanged water, dehydrated and washed with a Buchner funnel, and the first bleaching experiment was completed. The washing filtrate at this time was collected as D2-stage wastewater, and the pulp was discarded.

Again, the same factory oxygen bleached hardwood kraft pulp (wood mix; eucalyptus wood 55%, oak wood 45)
%, Whiteness: 48.9%, kappa number: 10.4)
Was collected in a plastic bag together with 350 ml of the collected D2-stage drainage, and the pulp concentration was adjusted to 10% using ion-exchanged water. % Added and the temperature is 70 ° C
Immersed in a constant temperature water bath for 30 minutes to perform bleaching at D1 stage.
The obtained pulp was diluted to 3% with ion-exchanged water, and then dehydrated and washed with a Buchner funnel.

To the pulp after the first stage of D, ion-exchanged water was added to adjust the pulp concentration to 10%, then 1.0% of caustic soda was added per weight of the absolutely dried pulp, and a 2-liter indirectly heated stainless steel autoclave was used. And pressurized with a commercially available compressed oxygen gas having a purity of 99.9% so that the gauge pressure becomes 0.15 MPa, and reacted at 70 ° C. for 20 minutes. Thereafter, the pulp slurry was taken out of the autoclave, transferred to a plastic bag, and treated at 70 ° C. for 70 minutes to extract an E / O stage. After diluting the obtained pulp to 3% with ion-exchanged water, dehydrate with a Buchner funnel,
Washed. The K value was 1.9.

Subsequently, 60.0 g of the pulp after the E / O stage was placed in a plastic bag in an absolute dry weight, and the pulp concentration was adjusted to 10% using ion-exchanged water. 0.2% was added, and the mixture was treated at 70 ° C. for 180 minutes to perform D2 bleaching. After diluting the obtained pulp to 3% with ion-exchanged water, dehydrate with a Buchner funnel,
After washing, the second bleaching experiment was completed. The whiteness of the bleached pulp was 85.5%. D1 for the second bleaching experiment
Table 1 shows the addition ratios of the D and D stages, the ratio of the D1 and D2 stages, the potassium permanganate value after E / O, the whiteness of the bleached pulp, and the PC value.

Example 2 The chlorine dioxide addition rate in the D1 stage of Example 1 was 1.2.
%, And the same operation as in Example 1 was performed except that the chlorine dioxide addition ratio in the D2 stage was changed to 0.3%. The K value after the E / O stage in the second bleaching experiment was 2.1, and the whiteness of the bleached pulp was 85.4%. D1 stage for the second bleaching experiment,
D2 stage addition ratio, ratio of D1 stage addition ratio and D2 stage addition ratio,
Table 1 shows the potassium permanganate value, the whiteness of the bleached pulp and the PC value after E / O.

Example 3 The chlorine dioxide addition rate in the D1 stage of Example 1 was 1.13.
%, And the same operation as in Example 1 was performed except that the chlorine dioxide addition ratio in the D2 stage was changed to 0.37%. The K value after the E / O stage in the second bleaching experiment was 2.4, and the whiteness of the bleached pulp was 85.3%. D1 for the second bleaching experiment
Table 1 shows the addition ratio of the D and D stages, the ratio of the D1 and D2 stages, the potassium permanganate value after E / O, the whiteness of the bleached pulp, and the PC value.

Example 4 70.0 g of hardwood kraft pulp after factory oxygen bleaching as in Example 1 was collected by absolute dry weight, put in a plastic bag, and the pulp concentration was adjusted to 10% using ion-exchanged water. After that, 1.0% of chlorine dioxide was added per weight of the absolutely dried pulp and immersed in a constant temperature water bath at a temperature of 70 ° C. for 30 minutes to obtain D1.
The steps were bleached. The resulting pulp is deionized with deionized water.
%, And then dehydrated and washed with a Buchner funnel.

The pulp after the first stage was adjusted to a pulp concentration of 10% by adding ion-exchanged water, and then 1.0% of caustic soda was added per weight of the absolutely dried pulp. And pressurized with a commercially available compressed oxygen gas having a purity of 99.9% so that the gauge pressure becomes 0.15 MPa, and reacted at 70 ° C. for 20 minutes. Thereafter, the pulp slurry was taken out of the autoclave, transferred to a plastic bag, and treated at a temperature of 70 ° C. for 70 minutes in the same manner as in the D1 stage to perform an E / O stage extraction. The obtained pulp was diluted to 3% with ion-exchanged water, and then dehydrated and washed with a Buchner funnel.

The pulp after the E / O stage was dried at a dry weight of 60.0%.
g, placed in a plastic bag, adjusted to a pulp concentration of 10% using ion-exchanged water, and then added 0.5% each of hydrogen peroxide and caustic soda to the absolutely dry pulp weight,
The treatment was carried out at a temperature of 70 ° C. for 90 minutes in the same manner as in D1 stage, and P stage bleaching was performed. The obtained pulp is 3% with ion exchange water
, And then dehydrated and washed with a Buchner funnel.

Subsequently, the pulp after the P stage was put into a plastic bag, the pulp concentration was adjusted to 10% using ion-exchanged water, and then 0.2% of chlorine dioxide was added to the absolutely dry pulp weight. In the same manner, the mixture was treated at a temperature of 70 ° C. for 180 minutes to carry out bleaching in D2 stage. The obtained pulp was diluted to 3% with ion-exchanged water, dehydrated and washed with a Buchner funnel, and the first bleaching experiment was completed. The washing filtrate at this time is D
It was collected as two-stage wastewater, and the pulp was discarded.

Again, 70.0 g of the hardwood kraft pulp after oxygen bleaching produced in the same factory as in Example 1 was collected in an absolute dry weight, put into a plastic bag together with 350 ml of the D-stage drainage collected above, and pulp was made using ion-exchanged water. 10% concentration
And then adjust the pulp concentration to 10 using ion-exchanged water.
%, And 1.0% of chlorine dioxide was added per weight of the absolutely dried pulp, and immersed in a constant temperature water bath at a temperature of 70 ° C. for 30 minutes to perform bleaching at D1 stage. The obtained pulp was diluted to 3% with ion-exchanged water, and then dehydrated and washed with a Buchner funnel.

The pulp after the first stage was adjusted to a pulp concentration of 10% by adding ion-exchanged water to the pulp, then 1.0% of caustic soda was added per weight of the absolutely dried pulp, and a 2-liter indirectly heated stainless steel autoclave was used. And pressurized with a commercially available compressed oxygen gas having a purity of 99.9% so that the gauge pressure becomes 0.15 MPa, and reacted at 70 ° C. for 20 minutes. Thereafter, the pulp slurry was taken out of the autoclave, transferred to a plastic bag, and treated at 70 ° C. for 70 minutes to extract an E / O stage. After diluting the obtained pulp to 3% with ion-exchanged water, dehydrate with a Buchner funnel,
Washed. The K value was 2.8.

The pulp after the E / O stage was dried at an absolute dry weight of 60.0%.
g, placed in a plastic bag, adjusted to a pulp concentration of 10% using ion-exchanged water, and then added 0.5% each of hydrogen peroxide and caustic soda to the absolutely dry pulp weight,
The mixture was treated at 70 ° C. for 90 minutes to perform P-stage bleaching. The obtained pulp was diluted to 3% with ion-exchanged water, and then dehydrated and washed with a Buchner funnel.

Subsequently, the pulp after the P stage was put into a plastic bag, and the pulp concentration was adjusted to 10% using ion-exchanged water. Then, 0.2% of chlorine dioxide was added per absolute dry pulp weight, and the mixture was heated at 70 ° C. The mixture was treated for 180 minutes and bleached in two stages of D. The obtained pulp was diluted to 3% with ion-exchanged water, dehydrated and washed with a Buchner funnel, and the second bleaching experiment was completed. The whiteness of the bleached pulp was 85.5%.
Addition rate of D1 stage and D2 stage in the second bleaching experiment, D1
Table 1 shows the ratio of the stage addition ratio to the D2 stage addition ratio, the potassium permanganate value after E / O, the whiteness of the bleached pulp, and the PC value.

Example 5 The chlorine dioxide addition rate in the D1 stage of Example 4 was 0.9.
%, And the same operation as in Example 4 was performed except that the chlorine dioxide addition ratio in the D2 stage was changed to 0.3%. The K value after the E / O stage in the second bleaching experiment was 3.2, and the whiteness of the bleached pulp was 85.4%. D1 stage for the second bleaching experiment,
D2 stage addition ratio, ratio of D1 stage addition ratio and D2 stage addition ratio,
Table 1 shows the potassium permanganate value, the whiteness of the bleached pulp and the PC value after E / O.

Comparative Example 1 The chlorine dioxide addition rate in stage D1 of Example 1 was 0.8.
%, And the same operation as in Example 1 was performed except that the chlorine dioxide addition ratio in the D2 stage was changed to 0.7%. The K value after the E / O stage in the second bleaching experiment was 3.6, and the whiteness of the bleached pulp was 84.1%. D1 stage for the second bleaching experiment,
D2 stage addition ratio, ratio of D1 stage addition ratio and D2 stage addition ratio,
Table 1 shows the potassium permanganate value, the whiteness of the bleached pulp and the PC value after E / O.

Comparative Example 2 The chlorine dioxide addition rate in the D1 stage of Example 4 was 0.6.
%, And the same operation as in Example 4 was performed except that the chlorine dioxide addition ratio in the D2 stage was changed to 0.6%. The K value after the E / O stage in the second bleaching experiment was 4.2, and the whiteness of the bleached pulp was 84.0%. D1 stage for the second bleaching experiment,
D2 stage addition ratio, ratio of D1 stage addition ratio and D2 stage addition ratio,
Table 1 shows the potassium permanganate value, the whiteness of the bleached pulp and the PC value after E / O.

[0059]

[Table 1]

As is clear from comparison of Examples 1 to 3 of Table 1 with Comparative Example 1, and Examples 4 and 5 and Comparative Example 2,
The first stage and the last stage of the multi-stage bleaching process are chlorine dioxide bleaching stages, the total amount of chlorine dioxide added per absolute dry pulp weight is constant, and the chlorine dioxide bleaching addition ratio in the first stage is 3 times the chlorine dioxide addition ratio of the last stage. If it is more than doubled,
It can be seen that even when the washing wastewater after the final stage of chlorine dioxide addition is used as dilution water for the first stage chlorine dioxide bleaching stage, the bleached pulp has high whiteness and improved discoloration. Further, as is clear from comparison of Examples 1 to 3 and Comparative Example 1 in Table 1, when three stages of bleaching are performed in the order of chlorine dioxide bleaching stage-alkali extraction stage-chlorine dioxide bleaching stage,
It is understood that the discoloration is improved by setting the potassium permanganate value after the alkali extraction stage to 3.5 or less.

[0061]

The unbleached pulp obtained by digesting the lignocellulosic material is subjected to alkali oxygen bleaching, and then processed in a multi-stage bleaching step having only a chlorine dioxide bleaching stage in the first stage and the last stage to produce bleached pulp. In the above, the bleached pulp which can be bleached efficiently and has improved bleaching properties can be produced by making the addition ratio of chlorine dioxide in the first stage per weight of the absolutely dried pulp at least three times the addition ratio of chlorine dioxide in the final stage. It has become possible.

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Claims (6)

[Claims] 1. A method for producing bleached pulp by subjecting unbleached pulp obtained by digesting a lignocellulosic substance to alkaline oxygen bleaching and then treating it in a multi-stage bleaching step having chlorine dioxide bleaching stages only in the first and last stages. Wherein the ratio of the chlorine dioxide addition rate in the first stage to the chlorine dioxide addition ratio in the final stage per absolutely dry pulp weight is 3/1 to 20/1, and the washing wastewater after the chlorine dioxide bleaching stage in the final stage is discharged in the first stage. A method for producing bleached pulp, characterized in that the bleached pulp is used as dilution water in the chlorine dioxide bleaching stage and / or washing water after the first stage chlorine dioxide bleaching. 2. The method for producing bleached pulp according to claim 1, wherein an alkali extraction stage is provided immediately after said first chlorine dioxide bleaching stage. 3. The production of bleached pulp according to claim 1, wherein the multi-stage bleaching step is performed in three bleaching stages in the order of chlorine dioxide bleaching stage, alkali extraction stage, and chlorine dioxide bleaching stage. Method. 4. The method for producing bleached pulp according to claim 2, wherein the pulp after the alkali extraction stage has a potassium permanganate value of 3.5 or less. 5. The bleaching method according to claim 1, wherein the pulp slurry has a pH of 2 to 4 in the washing stage after the first and last chlorine dioxide bleaching stages. Pulp manufacturing method. 6. The method for producing bleached pulp according to claim 1, wherein the lignocellulosic substance is a hardwood.