JP2002266271A - Bleached pulp improved in color fading tendency - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a bleached pulp, completely bleached pulp with a lignocellulose matter-derived whiteness degree of ISO 70-89%, markedly improved in color fading tendency as pulp for acidic paper in particular. SOLUTION: This completely bleached pulp is obtained by subjecting unbleached pulp obtained by digesting lignocellulose matter to alkali oxidative bleaching followed by treatment in a multistage bleaching step using neither chlorine nor hypochlorite. The bleached pulp thus obtained has a potassium permanganate value (JIS P-8206) of <=1.5 and ISO whiteness degree of 70-89%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

FIELD OF THE INVENTION The present invention relates to bleached pulp of lignocellulosic material. More specifically, the present invention provides improved bleaching performance by subjecting unbleached pulp obtained by digesting lignocellulosic material to alkaline oxygen bleaching, followed by treatment in a multi-stage bleaching step without using elemental chlorine. About bleached pulp.

[0002]

2. Description of the Related Art In order to use lignocellulosic materials as a raw material for papermaking in many applications, they are 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 generally used as bleached kraft pulp after removal of lignin and the like contained in pulp, except when used for applications requiring strength such as packaging materials.

[0003] When bleached pulp is produced from unbleached pulp, it is necessary to maintain the strength of the pulp fiber itself to a certain extent. Therefore, it is necessary to minimize the decomposition of carbohydrates such as cellulose and hemicellulose that constitute the pulp fiber. In general, a three- to six-stage multi-stage bleaching method is used in which bleaching is performed mildly by using various combinations of bleaching chemicals and bleaching conditions to avoid bleaching in one-stage extreme.

Conventionally, in the multi-stage bleaching method, pulp is first treated with chlorine, lignin contained in the pulp is chlorinated, the lignin is made soluble, 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, there has been a concern about the effects of organic chlorine compounds (hereinafter abbreviated as AOX) contained in the bleaching wastewater from the pulp chlorination stage on the environment, and movements not using chlorine for pulp bleaching have been increasing. 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 that does not use hypochlorite for pulp bleaching is required. Is coming.

At present, as an alternative to chlorine and hypochlorite,
Attention has been paid to ozone, chlorine dioxide, hydrogen peroxide and peracids such as peracetic acid and persulfuric acid. In practice, chlorine dioxide and hydrogen peroxide, which have relatively low chemical costs and are relatively easy to handle, are often used as alternatives. However, since chlorine dioxide and hydrogen peroxide are different from chlorine and hypochlorite, there has been a problem that the bleaching pulp deteriorates after the bleaching chemical is replaced.

As a method for improving the bleaching property of bleached pulp, a method of performing a xylanase pretreatment (for example, Japanese Patent Application Laid-Open No.
JP-A-101185), a method of performing an acid pretreatment (for example, JP-T-10-508346) and the like have been proposed, but even if these methods are used, the discoloration may be deteriorated. None of these methods is practical because the pulp yield after treatment is reduced, the COD of wastewater is increased, and equipment costs are required.

[0007]

SUMMARY OF THE INVENTION In view of such circumstances, the present inventors have developed a method for producing bleached pulp from a lignocellulosic material as a raw material without using elemental chlorine to a desired substantial whiteness. As a result of repeating various studies on a method that does not deteriorate the bleaching property, it was found that the bleaching property of the bleached pulp was significantly improved if the bleached pulp was bleached so that the potassium permanganate value was 1.5 or less. . Furthermore, it has been found that the bleaching pulp has a further improved bleaching property by reducing the amount of hexenuronic acid in the bleached pulp to 10 mmol or less per 1 kg of absolutely dried pulp, thereby completing the present invention.

[0008] It is an object of the present invention to alkali bleach unbleached pulp obtained by cooking lignocellulosic material,
Thereafter, the bleaching treatment is performed in a multi-stage bleaching step without using elemental chlorine to provide a bleached pulp that does not deteriorate in fading.

[0009]

The present invention, which can achieve the above object, includes the following inventions. (1) Unbleached pulp obtained by digesting a lignocellulosic material is subjected to alkali oxygen bleaching, and then bleached without using elemental chlorine in a multi-stage bleaching process. Potassium value (JIS
A bleached pulp for acidic paper having an improved bleaching property, wherein P 8206) is 1.5 or less and ISO whiteness is 70 to 89%.

(2) After bleaching the bleached pulp, a pulp sheet is prepared and treated at 80 ° C. and a relative humidity of 65% for 48 hours, and has a PC value of 10.0 or less ( 1) A bleached pulp for an acidic paper having improved discoloration described in the above.

(3) The bleaching for acidic paper with improved discoloration described in (1) or (2), wherein the amount of hexenuronic acid in the bleached pulp is 10 mmol or less per kg of absolutely dried pulp. pulp.

(4) The bleached pulp for acidic paper according to any one of (1) to (3), wherein the bleached pulp is a hardwood pulp. (5) An acidic paper obtained by making a pulp mainly composed of the bleached pulp for an acidic paper as described in any one of the above (1) to (4). (6) Unbleached pulp obtained by digesting a lignocellulosic material is bleached in an alkaline oxygen bleaching step, and then bleached in a multi-stage bleaching step without using elemental chlorine, whereby the permanganic acid in the bleached pulp is Potassium value (JIS
P 8206) is 1.5 or less and a bleached pulp having an ISO whiteness of 70 to 89% is obtained.

(7) After bleaching the bleached pulp, a pulp sheet is prepared and treated at 80 ° C. and a relative humidity of 65% for 48 hours, and has a PC value of 10.0 or less ( 6) A method for producing bleached pulp for acidic paper having improved fading according to the above.

(8) The acidic paper as described in (6) or (7), wherein the content of hexenuronic acid in the bleached pulp is 10 mmol or less per 1 kg of absolutely dried pulp. For producing bleached pulp.

(9) The method for producing bleached pulp for acidic paper having improved fading according to any one of (6) to (8), wherein the bleached pulp is a hardwood pulp.

(10) The alkaline oxygen bleaching step comprises adding a medium-concentration pulp slurry having a pulp concentration of 8 to 15% by weight to a medium-concentration mixer with a medium-concentration mixer at a concentration of 0.5 to 15 wt.
-3% by weight of oxygen and 0.5-4% by weight of alkali are added, the reaction temperature is 80-120C, the reaction time is 15-100.
The process for producing bleached pulp for acidic paper having improved discoloration according to any one of (6) to (9), wherein the bleaching is performed in a minute.

(11) The bleaching property according to any one of (6) to (10), wherein the alkaline oxygen bleaching step is a step of continuously performing an alkaline oxygen bleaching treatment a plurality of times. Of bleached pulp for acidic paper with improved pulp.

(12) After the alkaline oxygen bleaching step, in the enzyme bleaching step, for example, xylanase,
The bleached pulp for acidic paper with improved discoloration described in any one of (6) to (11), wherein the bleached pulp is improved by enzymatic bleaching using lignin peroxidase, manganese peroxidase, laccase or the like. Production method.

(13) The multi-stage bleaching step using no elemental chlorine includes a chlorine dioxide bleaching stage (D), an alkali bleaching stage (E), an oxygen bleaching stage (O), a hydrogen peroxide bleaching stage (P),
DE / OD, D mainly composed of ozone bleaching stage (Z), etc.
-E / OPD, DE / ODD, DE / O
-DP, DE / OP-D, DE / OZD, Z
-E / OD, ZE / OP-D, ZE / OP-D-
P, ZE / OP-PD, ZDE / OD, Z-
DE / OP-D, Z / DE / OD, Z / DE /
The method for producing bleached pulp for acidic paper according to any one of (6) to (12), which is a bleaching sequence such as OP-D.

(14) The first chlorine dioxide bleaching step in the multi-stage bleaching step without using elemental chlorine is 0.5 to 1.5%, preferably 0.1 to 0.5%, chlorine dioxide per weight of absolutely dry pulp.
The method for producing bleached pulp for acidic paper having improved discoloration described in any one of (6) to (13), wherein the bleaching step is performed by adding 6 to 1.1%.

(15) The first bleaching step in the multi-stage bleaching step without using elemental chlorine is 0.5 to 1.5%, preferably 0.6 to 1%, chlorine dioxide per weight of absolutely dry pulp.
The method for producing bleached pulp for acidic paper having improved discoloration described in any one of (6) to (14), wherein the bleaching step is a chlorine dioxide bleaching step performed by adding 1%. (16) A pulp mainly composed of bleached pulp produced by the method for producing bleached pulp for acidic paper having improved discoloration described in any one of the above (6) to (15). Acid paper.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The lignocellulosic substance used in the present invention is not particularly limited. As the cooking method for obtaining the unbleached pulp used in the present invention, known cooking methods such as kraft cooking, polysulfide cooking, soda cooking, and alkali sulphite cooking can be used, but pulp quality and energy can be used. In consideration of 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 sulfidity 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.

At the time of cooking, the cooking liquor to be used is a known cyclic keto compound as a cooking aid such as benzoquinone, naphthoquinone, anthraquinone, anthrone, phenanthroquinone, and a nucleus-substituted product of the quinone compound such as alkyl and amino. Selected from hydroquinone-based compounds such as anthrahydroquinone, which is a reduced form of quinone-based compounds, and 9,10-diketohydroanthracene compounds, which are stable compounds obtained as intermediates in anthraquinone synthesis by the Diels-Alder method. One or two or more kinds may be added, and the addition rate thereof is 0.001 to 1.0 per absolute dry weight of wood chips.
% By weight.

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

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 Ads) can be used as the oxygen gas.
orption), VSA (Vacuum Swing Adsorptio
The oxygen from n) can be used. The oxygen gas and 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 for a certain period of time under pressure, and degassed. Lignified.

The addition rate of oxygen gas is 0.5 to 3% by weight per absolutely dry pulp weight, the alkali addition rate is 0.5 to 4% by weight, the reaction temperature is 80 to 120 ° C., and the reaction time is 15 to 10%.
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 alkaline oxygen bleaching step. The enzyme used in the enzymatic treatment step may be any enzyme as long as it reacts with pulp to reduce the potassium permanganate value of the pulp measured by JIS P 8206. For example, xylanase, lignin peroxidase, manganese peroxidase, laccase, and the like are known. Of course, these enzymes may be used, and it is needless to say that enzymes that are not yet known may be applicable enzymes. In addition, these enzymes may be used alone, or may be used in combination, mixed, or divided into a plurality of times. Among these enzymes, a xylan-degrading enzyme called xylanase has a bleach accelerating effect at the same time and is preferably used.

In the present invention, it is possible to provide an acid treatment step 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 used in the acid treatment of the present invention,
Any of inorganic acids and organic acids may be used as long as the pH at the time of the acid treatment can be adjusted to 2 to 4. Inorganic acids such as sulfuric acid, nitric acid, hydrochloric acid, sulfurous acid, and nitrous acid, among which sulfuric acid is easily available and easy to handle. Therefore, it is preferably used.

The acid treatment of the present invention can be carried out under pressure using an oxygen-containing gas or a nitrogen-containing gas. The oxygen-containing gas used for pressurization during acid treatment can be used on an industrial scale, such as oxygen from cryogenic separation, oxygen from PSA, oxygen from VSA, etc. An oxygen or oxygen-containing gas having an oxygen purity of 85% by volume or more, an oxygen-containing gas whose oxygen content has been adjusted to exceed 21% by volume using an oxygen production facility using the molecular sieve,
An oxygen-enriched gas produced by mixing oxygen-containing gas having an oxygen purity of 85% by volume or more with air, air having an oxygen content of 20% by volume or more, and the like can be mentioned. Can be used. In the case where the ozone bleaching stage is provided in the multi-stage bleaching process, the exhaust gas containing oxygen can be suitably used.

As the nitrogen-containing gas used in the acid treatment step of the present invention, any gas may be used as long as the nitrogen gas content is 95% or more. From the economic viewpoint, it is used for alkaline oxygen bleaching. Oxygen and PSA from cryogenic separation
Nitrogen-containing gas by-produced when producing oxygen gas such as oxygen from oxygen and oxygen from VSA is preferably used. The pressurizing pressure during the acid treatment with an oxygen-containing gas or a nitrogen-containing gas during the acid treatment is 0.05 to 0.9 MPa (gauge pressure).
And preferably 0.15 to 0.7 MPa.

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, large capacity equipment is required for the treatment, which is not preferable. Pulp concentration 30%
Exceeding the limit is not suitable because a problem arises in uniformly mixing the pulp and the acid. 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. After the acid treatment step, pulp washing is performed, and the washed pulp is sent to a multi-stage bleaching step.

In the multi-stage bleaching process of the present invention, the first stage is a chlorine dioxide bleaching stage (D) or an ozone bleaching stage (Z);
Alternatively, a bleaching stage (Z / D) or the like in which ozone bleaching and chlorine dioxide bleaching are continuously combined is preferably used, an alkali extraction stage (E) is used in the second stage, and a third stage and thereafter are used.
Combinations of chlorine dioxide, hydrogen peroxide and the like are preferably used. The chlorine dioxide used in the chlorine dioxide bleaching stage in the first stage of the multi-stage bleaching process of the present invention can be selected from chlorine dioxide obtained from many chlorine dioxide generation methods known to those skilled in the art. Chlorine dioxide obtained from a generation method that does not produce by-products is used. The pH at the chlorine dioxide stage is from 2 to 6, preferably from 2.5 to 4, and it is also possible to add any acid or alkali to adjust the pH. Other chlorine dioxide bleaching conditions, such as chlorine dioxide treatment time, treatment temperature, and pulp concentration,
All known conditions can be used.

In the alkali extraction stage following the chlorine dioxide bleaching stage in the multi-stage bleaching process of the present invention, there can be selected from many alkali compounds known to those skilled in the art, but caustic soda is most easily used and is preferably used. In the alkali extraction stage of the present invention, oxygen and / or hydrogen peroxide can be used in combination. In addition, the alkali extraction stage of the present invention can be performed under known conditions.

In the third and subsequent bleaching stages following the chlorine dioxide stage and the alkali extraction stage used in the multistage bleaching step of the present invention, any bleaching agent other than chlorine and hypochlorite may be used. However, general bleaching chemicals such as chlorine dioxide, hydrogen peroxide, ozone, and peracid are preferably used. The number of stages after the third stage is not particularly limited. However, in consideration of energy efficiency, productivity, and the like, it is preferable that the process be completed in three or four stages in total.

The bleaching chemicals used in the multi-stage bleaching step of the present invention include chlorine dioxide (D), alkali (E), oxygen (O), hydrogen peroxide (P), excluding chlorine and hypochlorite. Known bleaching agents such as ozone (Z) and organic peracid and bleaching aids can be mentioned. Further, as the bleaching sequence in the multi-stage bleaching process in the present invention, for example, D
-E / OD, DE / OPD, DE / OD-
D, DE / ODP, DE / OP-D, DE /
O-Z-D, ZE / OD, ZE / OP-D, Z-
E / OP-DP, ZE / OP-PD, ZDE
/ OD, ZDE / OP-D, Z / DE / O-
D, Z / DE / OP-D, and the like. Also, ethylenediaminetetraacetic acid (E
DTA), diethylenetriaminepentaacetic acid (DTP
A chelating agent treatment stage according to A) or the like may be inserted.

In the multi-stage bleaching process of the present invention, further studies on the details of the reason why the bleaching property is improved by setting the potassium permanganate value of the unbleached pulp to 1.5 or less have to wait for further studies. It is speculated that the presence of hexeneuronic acid, which is evaluated as a potassium permanganate value, is involved in pulp discoloration. Therefore, a bleaching method in which the content of hexenuronic acid is controlled to a certain level or less is also very effective in improving the bleaching property of bleached pulp.

As a method for removing hexenuronic acid, a method using a strong oxidizing agent such as ozone or peracetic acid, a method of combining hydrogen peroxide and molybdate under acidic conditions, and the like have recently been known. The cost is significantly higher,
Equipment costs are also considerably higher. The present inventors, by adding a large amount of chlorine dioxide mainly to the first chlorine dioxide stage,
It has been found that hexeneuronic acid can be reduced and is employed in the method of the present invention.

As an example of a method for preparing acidic paper using the bleached pulp of the present invention, bleached pulp is beaten with a beater.
SF (Canadian Standard Freeness) 350m
Beating to about 1 to 550 ml, then about 2.5% of sulfate band, rosin sizing agent (for example, size pine E,
Arakawa Chemical) about 0.5%, talc (for example, Illite,
A method of forming acidic paper having a basis weight of about 64 g / m ² by a conventional method by blending in the order of about 20% by Nippon Talc) and about 0.02% by a retention aid (eg, Percoll 182, Kyowa Sangyo). And the like. In making the acidic paper, other bleached pulp may be of course mixed with the bleached pulp of the present invention as long as the excellent properties of the acidic paper are not impaired.

[0040]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but it should be understood that the present invention is not limited to these Examples. Examples 1 to 3 shown below
Are bleached hardwood kraft pulp after factory alkaline alkali bleaching in the DED sequence, and Comparative Examples 1 and 2 were bleached in the D-EP-D sequence. In addition, unless otherwise specified, the measurement of the kappa number, the measurement of the potassium permanganate value, the measurement of the amount of hexenuronic acid in the pulp, the measurement of the pulp whiteness, and the evaluation of the bleaching property of the pulp were performed by the following methods, respectively. Is shown. 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. Measurement of potassium permanganate value of pulp Measurement of potassium permanganate value is based on JIS P820.
Performed according to 6.

3. Determination of the amount of hexeneuronic acid in pulp In a 500 ml SUS container, 5 g of absolutely dried pulp washed with ion-exchanged water was weighed and placed in formic acid-
A total of 300 ml was prepared using a 10 mmol / l sodium formate buffer solution. Thereafter, the inside of the SUS container was replaced with nitrogen gas, and the container was treated at 110 ° C. for 5 hours in an oil thermostat. After cooling the SUS container with running water, the pulp suspension after the treatment is reduced to 500 ml including the washing liquid, and then filtered, and the liquid is subjected to HPLC (high performance liquid chromatography).
Analysis by 2-furoicacid and 5-carb
oxy-2-furaldehyde was quantified. The following formulas and references were used for quantification.

Calculation formula: (concentration of 20 μl of each sample) =
a, b (ng / μl) 1) Amount of 2-furoic acid (mmol / kg)
= A × (500/1000) / (10 × 10−3) / 1
12.08 2) 5-carboxy-2-furaldehyde
Amount (mmol / kg) = b × (500/1000) /
(10 × 10-3) /140.13) Hexeneuronic acid amount (mmol / l) = 2-fur
Oic acid amount + 5-carboxy-2-fur
aldehyde amount

References: Author Vuorinen, T .; Selective hydrolysis of hexenuronic acid grou
ps and its application in ECF and TCF blea
ching of kraft pulps International Pulp Bleaching Conference, April
14-18, 1996, P43-51

4. Preparation method of pulp sheet for measuring whiteness After bleached pulp was disintegrated, a sulfuric acid band was added to the pulp slurry by 3.0% with respect to pulp, and Tappi test method T205os-
71 (JIS P 8209), basis weight 60 g / m
Sheet No. ² was prepared.

5. Measurement of Pulp Whiteness The pulp whiteness was measured according to JIS P 8123.

6 Evaluation of bleaching property of pulp The pulp sheet for whiteness measurement was bleached for 48 hours under the conditions of 80 ° C. and a relative humidity of 65%, and a 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 Pulverization of factory hardwood-Alkali oxygen bleaching After kraft pulp (whiteness: 47.2%, kappa number: 9.9), 80.0 g of absolute dry weight was put in a plastic bag, and ion-exchanged water was used. After adjusting the pulp concentration to 10%, 1.1% of chlorine dioxide was added to the absolutely dry pulp weight, and immersed in a constant temperature water bath at a temperature of 80 ° C. for 60 minutes for the first chlorine dioxide stage (hereinafter, referred to as “the first stage”).
D 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 D stage is put in a plastic bag, and the pulp concentration is adjusted to 10% using ion-exchanged water. Then, caustic soda is added at 1.2% based on the absolutely dry pulp weight, and the temperature is 80 ° C. in the same manner as the D stage. After treating for 90 minutes, an alkali extraction stage (hereinafter abbreviated as E stage) was performed. After diluting the obtained pulp with ion exchange water to adjust the pulp concentration to 3%,
A pulp mat was formed using a Buchner funnel, and dewatered with a cylinder press machine to obtain pulp after E-stage. The pulp concentration after dehydration was 30.0%.

Subsequently, after the E-stage, the pulp was put in 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. And treated at 70 ° C for 180 minutes,
A second D-stage bleaching was performed. The obtained pulp is diluted to 3% with ion-exchanged water and washed using a Buchner funnel.
After dehydration, a bleached pulp having a whiteness of 86.1% was obtained. The potassium permanganate value, the amount of hexenuronic acid, and the PC value of the bleached pulp sheet produced from the bleached pulp after 48 hours were measured, and the results are shown in Table 1.

Example 2 The same operation as in Example 1 was performed except that the chlorine dioxide addition rate in the first stage of the multi-stage bleaching was changed to 1.3%. The pulp brightness after multi-stage bleaching was 88.0%. The potassium permanganate value, the amount of hexenuronic acid, and the PC value of the bleached pulp sheet produced from the bleached pulp after 48 hours were measured, and the results are shown in Table 1.

Example 3 The same operation as in Example 1 was performed except that the chlorine dioxide addition ratio in the first stage of the multi-stage bleaching was changed to 0.6%, and the chlorine dioxide addition ratio in the second stage was changed to 0.1%. Pulp brightness after multi-stage bleaching is 7
1.2%. The potassium permanganate value, the amount of hexenuronic acid, and the PC value of the bleached pulp sheet produced from the bleached pulp after 48 hours were measured, and the results are shown in Table 1.

COMPARATIVE EXAMPLE 1 Pulverization of factory hardwood-alkaline oxygen bleaching After kraft pulp (whiteness: 47.2%, kappa number: 9.9), 80.0 g of absolute dry weight was placed in a plastic bag, and ion-exchanged water was used. After adjusting the pulp concentration to 10%, 0.4% of chlorine dioxide was added to the absolutely dry pulp weight, and immersed in a constant temperature water bath at a temperature of 80 ° C. for 60 minutes to form the first chlorine dioxide stage (hereinafter, referred to as
D 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 D-stage is put in a plastic bag, and the pulp concentration is adjusted to 10% using ion-exchanged water. Then, caustic soda and hydrogen peroxide are each added in an amount of 1.% per weight of the absolutely dried pulp.
Add 2% and 0.5% and add 9% at 80 ° C in the same manner as in stage D.
The mixture was treated for 0 minutes and subjected to alkali extraction and hydrogen peroxide bleaching stage (hereinafter abbreviated as EP stage). After the obtained pulp was diluted with ion-exchanged water to adjust the pulp concentration to 3%, a pulp mat was formed using a Buchner funnel, and dewatered with a cylinder press to obtain pulp after EP stage. The pulp concentration after dehydration was 30.0%.

Subsequently, after the EP stage, the pulp was placed in a plastic bag, and the pulp concentration was adjusted to 10% using ion-exchanged water. Then, 0.2% of chlorine dioxide was added per weight of the absolutely dry pulp. Then, the mixture was treated at a temperature of 70 ° C. for 180 minutes, and the second D-stage bleaching was performed. The obtained pulp was diluted to 3% with ion-exchanged water, washed and dehydrated using a Buchner funnel to obtain bleached pulp having a whiteness of 86.2%. The potassium permanganate value and hexenuronic acid content of the obtained bleached pulp, and the PC value of the bleached pulp sheet after 48 hours were measured.

Comparative Example 2 The same operation as in Comparative Example 1 was carried out except that the chlorine dioxide addition rate in the first stage of the multi-stage bleaching was changed to 0.2%, and the hydrogen peroxide addition rate in the EP stage was changed to 0.3%. Pulp brightness after multi-stage bleaching is 6
It was 8.8%. The potassium permanganate value, the amount of hexenuronic acid, and the PC value of the bleached pulp sheet after 48 hours were measured for the obtained bleached pulp, and the results are shown in Table 1.

[0056]

[Table 1]

As is clear from the comparison between Examples 1 and 2 in Table 1 and Comparative Example 1, when the potassium permanganate value is high even at a high whiteness level, the hexeneuronic acid content is large, and as a result, The PC value is high, and the discoloration of pulp is poor. Also, as can be seen from the comparison between Example 3 and Comparative Example 2, even when the whiteness is low, if the potassium permanganate value is low, the hexeneuronic acid content is low, the PC value is low, and the bleaching property of the pulp is good. .

[0058]

As is clear from the above results, unbleached pulp obtained by digesting lignocellulosic material is subjected to alkaline oxygen bleaching, and then treated in a multi-stage bleaching step using neither chlorine nor hypochlorite. Potassium manganate value (JIS P 820)
6) is 1.5 or less, and the ISO whiteness is 70 to 8
9%, especially as bleached pulp for acidic paper,
It has become possible to provide bleached pulp having significantly improved bleaching properties. Further, after the bleached pulp was disintegrated, a pulp sheet was prepared and treated at a constant temperature and a constant humidity of 80 ° C. and a relative humidity of 65% for 48 hours.
By adjusting the content to 0.0 or less, the bleaching property of the bleached pulp can be further improved.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yosuke Uchida 373 Yoshioka, Yonago-shi, Tottori Oji Paper Co., Ltd. Yonago Mill (72) Inventor Yuki Kisara 1-10-6 Shinonome, Koto-ku, Tokyo Oji Paper Co., Ltd. Shinonome Inside the Research Center (72) Inventor Ayano Kawamura 1-10-6 Shinonome, Koto-ku, Tokyo Oji Paper Co., Ltd. Shinonome Research Center F-term (reference) 4L055 AA03 AC06 AD02 AD07 AD17 BB11 BB13 BB15 BB17 BB18 BB20 BB22 BB25 EA01 EA02 FA04 FA05 FA20

Claims (5)

[Claims] 1. A bleached pulp obtained by subjecting an unbleached pulp obtained by digesting a lignocellulosic substance to alkaline oxygen bleaching and then performing a bleaching treatment without using elemental chlorine in a multistage bleaching step. The pulp has a potassium permanganate value (JISP 8206) of 1.5 or less, and
A bleached pulp for acidic paper having an improved bleaching property, having an ISO whiteness of 70 to 89%. 2. After the bleached pulp is deflocculated, a pulp sheet is prepared, and the pulp sheet is treated under conditions of 80 ° C. and 65% relative humidity.
The bleached pulp for acidic paper with improved discoloration according to claim 1, wherein the PC value after holding for a time is 10.0 or less. 3. The bleached pulp for acidic paper with improved discoloration according to claim 1, wherein the amount of hexenuronic acid in the bleached pulp is 10 mmol or less per kg of the absolutely dried pulp. 4. The bleached pulp for an acidic paper according to claim 1, wherein the bleached pulp is a hardwood pulp. 5. An acidic paper comprising, as a main pulp component, the bleached pulp having improved discoloration according to any one of claims 1 to 4.