JP2812056B2 - Method for bleaching lignocellulosic material - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

This invention relates to a method for bleaching lignocellulosic materials. More specifically, the present invention, when bleaching the pulp obtained from the lignocellulosic material in a multi-stage bleaching sequence including chlorine bleaching, treated the pulp by mixing the filtrate from the chlorine bleaching stage prior to chlorine bleaching. After the above-mentioned multi-stage bleaching.

[0002]

2. Description of the Related Art In order to use lignocellulosic materials in many applications, lignocellulosic materials are made into pulp by a chemical action such as cooking, or pulp by mechanical action, and then the pulp is bleached. It is necessary to increase the whiteness. For example, kraft pulp is usually bleached with chlorine, hypochlorite (hypo), chlorine dioxide, oxygen, hydrogen peroxide, caustic soda, etc., except when used for applications that require strength, such as packaging materials. It is generally used after bleaching with a bleaching agent and a bleaching aid to remove residual lignin and the like, which are coloring substances of unbleached pulp.

[0003] In the bleaching of chemical pulp which requires strength, in order to keep the strength of the fiber itself of the pulp high, an extreme one-stage bleaching is carried out so as to minimize the influence on the decomposition of carbohydrates (such as cellulose). Generally, a multi-stage bleaching process (sequence) in which bleaching agents and bleaching conditions are mildly changed is employed.

Usually, in a multi-stage bleaching sequence, the lignin contained in the pulp is first chlorinated with chlorine,
After adding solubility to the lignin, the lignin is then dissolved and extracted with alkali to separate the lignin from the pulp. Thereafter, the remaining small amount of lignin is further decomposed and removed using hypochlorite, chlorine dioxide, or the like, to obtain a pulp having high whiteness.

When the chlorination treatment is represented by (C), the alkali treatment is represented by (E), the hypochlorite treatment is represented by (H), the chlorine dioxide treatment is represented by (D), and the hydrogen peroxide treatment is represented by (P). The bleaching sequence combining the bleaching agents includes a bleaching agent and / or a bleaching aid according to the order of the bleaching agents and / or bleaching aids.
-E-H-E-D, C-E-D-E-D, C-E-H-
PD and the like can be mentioned as the most common ones.

Usually, five or six bleaching stages are required to obtain a completely bleached pulp having a Hunter brightness of 80 to 90%. Recently, however, a chlorine stage using chlorine and chlorine dioxide in combination has been required. the method of reduction, i.e., C / D, methods such as _{C D} or _{D C,} or in the alkaline extraction stage following the chlorination stage, oxygen _(E O), or hydrogen peroxide _{(E p)} or hypochlorite (E _H ) or the simultaneous use of them (for example, E _po ) has been developed, and the bleaching stage can be shortened to three or four stages by the combination with the oxygen bleaching technology.

As an example, 1986 Pulping
Conferencing Proceedings, 15
On pages 3, 167, 489 and 491, C / D-
_{E po -D, C / D-} E po -D-P, O 2 -C / D-
Sequence, such as E _O -D and _C D _-E O -D is, 198
7 Pulling Conference Process
The page _{edings15 C-E O -H-P} , C D -E O
Sequences such as -HD, C- _EO -HH have been proposed, and some of the sequences are operating on actual machines.

Recently, the toxicity of organochlorine compounds contained in bleaching wastewater, for example, dibenzo-para-dioxane (commonly called dioxin) has been a problem. Although the effects of dioxins on the human body have not yet been elucidated, various methods have been employed to reduce organochlorine compounds.

A recently developed modified continuous digester (Modif)
ied Continuous Cooking (hereinafter referred to as MCC) method is used when cooking wood chips by the Kraft method, 1) making the alkali concentration relatively uniform during the cooking, and 2) especially in the early stage of the stage where delignification progresses most. Pulp by lowering the concentration of dissolved lignin present in the cooking liquor, especially in the later stages of cooking, 4) lowering the temperature in the early and late stages of cooking, etc. Pulp viscosity, which is an indicator of quality, can produce unbleached pulp having a lignin content of about 30% less than that of pulp obtained from ordinary kraft cooking (1987 Pulp).
ing C on ference Proceeding
s, P249).

As a result, it is said that since the amount of residual lignin in the pulp brought into the bleaching stage is reduced, the pollution load of the bleaching effluent, in particular, the emission of organic chlorine, can be significantly reduced (EUCEPA). Sympos
ium preprintsp16 (1980), Pa
periaa Puu65 (4): 227 (198
3), TappiJ. 66 (9): 97 (198
3)).

In addition, the recently developed oxygen bleaching method can reduce the lignin content of unbleached pulp by almost half, so that the chlorine addition rate in the subsequent chlorine stage can be further reduced.
As a result, the amount of the organic chlorine compound contained in the wastewater can be reduced. For this reason, many oxygen bleaching units are operating in major countries producing pulp and paper in the world, including Japan.
It is expected that the number of operation will increase further in the future.

[0012] The oxygen bleaching process (O ₂ stage) is combined with the conventional chlorine bleaching method, not fully bleached pulp only, and is applied to the production of semi-bleached pulp. For example, Japanese Patent Publication No. 477202 discloses a bleaching sequence of O ₂ -C / DE, and Japanese Patent Publication No. 47-10241 discloses a bleaching sequence of C / DE.
_D-O bleaching sequence of 2 -D-E-D is, JP-B-47
-44441 No. _O 2 _-Dc-O 2 -E bleaching sequence in Publication, Japanese Patent Publication No. Sho 51-17605 _{O 2} -
D / CED bleaching sequences have been proposed respectively.

Furthermore, in order to greatly reduce the residual lignin content of the pulp entering the bleaching stages, a method of combining cooking and oxygen bleaching method by MCC method (O ₂ stage) it is contemplated. For example, 1987 Pulping C on fere
nce Proceedings, page 453, MC
After oxygen bleaching a C cooked pulp, C / _{D-E O} -
It is stated that bleached pulp having a whiteness of 90% ISO could be obtained in three stages of D, and the chlorine addition rate could be greatly reduced.

In a foreign factory operating MCC cooking, softwood pulp is subjected to oxygen bleaching and then ( C / D )-
The bleached pulp having a brightness of 90% ISO is manufactured in the sequence of EDED. According to this method, the amount of the organic chlorine compound is reduced from 2.4 kg / ton pulp to 1.6 kg.
/ Ton pulp is reported to be reduced (Sve
nsk Papersperidn. 87 (10): 30
(1984) p. 453, Oxgen Delignif.
Icitation Symposium Proceed
ings, p. 88 (1985)).

Conventionally, in the oxygen stage, if the kappa number, which is an indicator of the amount of lignin remaining in the pulp, is less than half the kappa number of the pulp to be received in the oxygen stage, the pulp viscosity is greatly reduced and the pulp strength is significantly reduced. So
It was difficult to obtain more than half of the delignification amount in the oxygen stage.

However, it has been reported that pretreatment with nitrogen dioxide before the oxygen bleaching stage improves delignification properties in the oxygen stage, and this method has not yet been operated on an actual scale. In pilot plant tests, the kappa number can be reduced to 36% before oxygen bleaching and the amount of organochlorine compounds can be reduced by 41% compared to normal oxygen bleaching.
It is reported that it was able to decrease (paper and peripheral technology, quarterly 1
No., 1988).

However, this pretreatment method has a problem in that toxic nitrogen dioxide is used. It has also been reported that pretreatment with sulfurous acid or sulfuric acid before oxygen bleaching improves delignification in the oxygen stage as in the case of nitrogen dioxide pretreatment (“The Bleachg of Pu”).
lp "3rd Edition TAPPI PRES
S, 177). However, this method has also not been practically used because it is difficult to recover waste acid in the system as a recovery boiler feed due to the sulfur balance.

As a method of reducing the chlorine addition rate in the chlorine stage, it is possible to use chlorine dioxide in combination with the chlorine stage and increase the use ratio. In recent years, there have been many reports that pulp manufacturing plants in Japan have been operating with an increased proportion of chlorine dioxide, but the existing facilities of the plant have a limited ability to generate chlorine dioxide.
Due to the limited amount of chlorine dioxide required and the high cost of chlorine dioxide production, it is not economical to increase the use of chlorine dioxide to 30% or more of chlorine.

Further, the chlorine addition rate in the chlorine stage is reduced,
In order to compensate for the lack of delignification in the chlorine stage, there is a method in which any of oxygen (E 2 _{O 3} ), hydrogen peroxide (E _P ), hypo (E _H ) and the like are used in combination in the next alkali extraction stage. When using pulp, a special mixer is required because mixing with pulp is important, and hydrogen peroxide is expensive, and in the case of hypo, there is a drawback that the viscosity is greatly reduced.

Therefore, a method that can further reduce the chlorine addition rate by a simple method, in other words, a method that further improves the delignification property in the chlorine stage by a simple pretreatment is desired.

[0021]

The inventors of the present invention have conducted intensive studies on the above-mentioned problems. As a result, when the pulp obtained from the lignocellulosic material is subjected to multi-stage bleaching, the filtrate from the chlorine bleaching stage prior to chlorine bleaching is used. P identified using
The present inventors have found the surprising fact that chlorine bleaching after pretreatment of pulp at H and temperature for a specific period of time significantly improves delignification in the chlorine stage, thereby completing the present invention.

As a result, the chlorine addition rate in the chlorine stage can be reduced, so that the amount of the organic chlorine compound in the wastewater can be greatly reduced. That is, since the present invention improves delignification in the chlorine bleaching stage, even if the chemical addition rate in each bleaching stage or especially the chemical addition ratio in the chlorine stage is reduced to a level below the level of a normal bleaching method, a normal bleaching method can be used. Pulp whiteness comparable to that of the bleaching method can be obtained, and chemical costs can be reduced. In addition, when the chemical addition rate in the chlorine stage is reduced, the amount of the organic chlorine compound in the wastewater can be reduced, the bleaching pulp can be improved in discoloration, and the pulp strength can be improved. Combination with MCC cooking method, combination of ordinary craft method and oxygen bleaching method,
Alternatively, if pulp from lignocellulosic material obtained by a combination of MCC digestion and oxygen bleaching is bleached according to the present invention, not only is the pollution load on the wastewater reduced, but also the chlorine from the chlorine stage is reduced. Since the emission amount of the compound itself is reduced, the quality of the wastewater is greatly improved, and further, the pulp whiteness can be improved and the chemical addition rate can be significantly reduced.

Accordingly, an object of the present invention is to provide a bleaching method capable of producing a bleached pulp having a reduced drainage pollution load, a reduced chemical addition rate, and improved strength and fading.

[0024]

SUMMARY OF THE INVENTION The present invention relates to a method of combining pulp obtained from lignocellulosic material with a multi-stage bleaching sequence comprising chlorine, caustic soda, hypochlorite (hypo), chlorine dioxide, oxygen and hydrogen peroxide. The pulp is subjected to a pH of 2 to 5 and a temperature of 40 to 80 ° C. using the filtrate from the chlorine bleaching stage prior to chlorine bleaching.
After pretreatment with maintaining in the range of 30 to 120 minutes and washing and dewatering, bleaching in the chlorine stage of the multi-stage bleaching sequence. It is a bleaching method.

The lignocellulosic material used in the present invention can be obtained by a modified continuous cooking (MCC) method as well as a chemical pulp such as ordinary kraft pulp (KP), alkali pulp (AP), sulfite pulp (SP) and the like. Any of alkaline pulp, pulp digested by the ordinary Kraft method and oxygen bleached, and chemical pulp digested by the MCC method and oxygen bleached may be used. Further, it goes without saying that the present invention is applicable to both wood (coniferous and hardwood) pulp and non-wood pulp.

The chlorine compounds used in the chlorine stage of the present invention are chlorine and chlorine dioxide, which can be used alone or in any combination. Regarding the method of adding chlorine and chlorine dioxide in the chlorine stage, both may be mixed, and chlorine may be added within a few seconds after the addition of chlorine dioxide. Alternatively, a part of chlorine dioxide may be added, and then the remaining chlorine dioxide and chlorine may be mixed and added.

The absolute dry pulp content of the chlorine compound represented by the active chlorine content is proportional to the lignin content contained in the unbleached pulp, and is appropriately selected and used within the range of 1 to 5%. The reaction temperature in the chlorine stage is in the range of 30C to 60C, the pulp concentration during the reaction is in the range of 1 to 10%, the reaction time is in the range of 5 to 60 minutes, and the final pH is in the range of 2 to 4. After the chlorine stage bleaching step is completed, the pulp is washed and dewatered or compressed by a known method and sent to the next step.

The liquid used in the step of pre-treating the pulp prior to the chlorine stage of the present invention is the filtrate from the chlorine stage, and the type of the chlorine compound contained in the solution is not limited.
The pH is in the range 2-5, preferably in the range 2-4.
If the pH is less than 2, polysaccharides such as hemicellulose are eluted, and a part of the cellulose undergoes hydrolysis, causing fiber damage. If the pH is more than 5, the effect of promoting delignification is lost, which is not suitable.

The higher the temperature in the pretreatment step, the more effective it is in promoting delignification, but it is in the range of 40 to 80 ° C., preferably 50 to 60 ° C. in terms of pulp quality. When the chlorine stage is operated at a temperature of 40 ° C. or lower, it is necessary to heat the filtrate from the chlorine stage in the pulp pretreatment step of the present invention. However, the temperature of unbleached pulp that has undergone the usual digestion and washing steps today is generally in the range of 40 to 60 ° C, and the temperature of pulp treated by the known oxygen bleaching method is 60 to 8
Since the temperature is in the range of 0 ° C., heating is required only at the time of startup to secure the temperature in the pretreatment. If the temperature is lower than 40 ° C., it is difficult to obtain the effect of the pretreatment.
If the temperature exceeds the above range, the pulp quality is deteriorated and the cost is increased thermoeconomically.

The pulp concentration in the pretreatment step is desirably in the range of medium concentration to high concentration. The higher the concentration, the more the pulp comes into contact with each other and the easier it is to remove components that inhibit delignification in the chlorine stage. , concentration desired effect can not be obtained because that mixing of too high filtrate and pulp turn into bad. Also, if the pulp concentration is too low, the desired effect cannot be obtained.
The desirable pulp concentration in the present invention is in the range of 10 to 20%, and the pulp concentration is appropriately selected from these and used in combination with other conditions.

Further, the holding time in the pretreatment step is 30 to 120 minutes. If the holding time is less than 30 minutes, the reaction is not sufficient, and if the holding time exceeds 120 minutes,
It is not suitable because it leads to high costs in terms of equipment. The pulp flow in this pretreatment stage may be upflow or downflow. In the present invention, the liquor mixed and contacted after this pretreatment is not removed, that is, the pretreated pulp is sent to the subsequent chlorine stage without washing. Washing in this process,
Dewatering not only reduces the bleaching ability in the chlorine stage, but also reduces the yield because polysaccharides such as hemicellulose eluted in the liquid are released from the pulp fiber and discharged out of the system. Do not wash or drain.

The alkali used in the alkali extraction stage of the bleaching system of the present invention and the addition ratio thereof can be selected from known ones. When using caustic soda as the alkali,
The alkali addition rate based on the absolute dry weight of the pulp is in the range of 0.5 to 3% in terms of NaOH.

When oxygen is used in the alkali extraction stage,
Either oxygen gas or oxygen-enriched gas can be used, and it is performed under a pressurized or atmospheric pressure by a known method.
Generally, the oxygen addition rate is 0.5 to
It is in the range of 3%. The reaction temperature is in the range of 40 to 70 ° C,
The pulp concentration during the reaction ranges from 5 to 20%, the reaction time ranges from 30 to 120 minutes, and the final pH ranges from 8 to 12. Further, hydrogen peroxide or hypo may be used together in the alkali extraction stage. The pulp from which the alkali extraction has been completed is then washed or dewatered by a known method, and then sent to the next step.

The bleaching system hypochlorite (hypo) of the present invention
The hypochlorite used in the stage can be selected from known hypochlorites such as sodium salt and calcium salt,
The addition rate of hypochlorite was 0.1% per pulp absolute dry weight.
２2%, the reaction temperature is 30３０70 ° C., the pulp concentration during the reaction is 3〜20%, and the reaction time is 15１５.
The initial pH is optimally around 10. After the reaction, washing or compression dewatering is performed by a known method.

As the chlorine dioxide used in the bleaching chlorine dioxide stage of the present invention, chlorine dioxide obtained by a known chlorine dioxide generating method is used. In the range of 0.2-3% by weight,
The reaction temperature ranges from 40 to 90 ° C., the pulp concentration during the reaction ranges from 3 to 20%, and the reaction time ranges from 15 to 300 minutes. An auxiliary alkali may be added so that the final pH falls within the range of 2 to 6. After the chlorine dioxide stage, washing or compression dewatering is performed by a known method as in the above stage.

The bleaching method of the present invention can be carried out as a single bleaching step, or may be substituted as a part of the steps in a conventional bleaching sequence.
In addition, when a pulp with high whiteness is not required, it goes without saying that the treatment step only requires a chlorine stage and an alkali extraction stage.

[0037]

EXAMPLES The present invention will be described in more detail with reference to the following examples, which, of course, are not intended to limit the present invention. In the examples described below, the measurement of the pulp bleaching conditions and the physical properties of the pulp were performed by the following operating means.

(1) Pretreatment with Filtrate from Chlorine Stage The absolutely dry weight of oxygen-bleached hardwood kraft pulp (mixture of domestic and foreign materials, mixing ratio 50/50) is placed in a plastic bag, and 50 g is taken in a plastic bag. Pulp concentration of 15%
After stirring well, the mixture was placed in a thermostat at 50 ° C. and kept for 90 minutes.

(2) Chlorine bleaching stage Each of the pulp which has been pretreated with the filtrate of the chlorine stage and has not been washed and drained, and the pulp which has not been treated with the filtrate from the chlorine stage, 50 g in absolute dry weight, are each placed in a plastic bag. Take, add a predetermined amount of available chlorine (total of chlorine and chlorine dioxide) proportional to the kappa number (kappa number x 0.2%), and add a predetermined amount of water so that the pulp concentration becomes 10%. After stirring well, it was immersed in a thermostat at 50 ° C. and kept for 30 minutes. After the completion of the reaction, the sample was taken out and subjected to centrifugal dehydration washing.

(3) Alkaline extraction stage The C-stage pulp subjected to centrifugal dehydration and washing is put into a plastic bag,
Add a predetermined amount of alkali proportional to the kappa number (kappa number x 0.1%), add a predetermined amount of water so that the pulp concentration becomes 10%, stir well, take out the pulp from the bag, and add 2 liters. The pulp was taken out of the bag and put into a vertical autoclave having a capacity, heated to 60 ° C., and pressurized with oxygen until the pressure became 1.5 kg / cm 2. After stirring under pressure for 10 minutes, the pressure was reduced to atmospheric pressure, and the reaction was further performed for 50 minutes. After the completion of the reaction, the sample was taken out and subjected to centrifugal dehydration washing.

(4) Hypochlorite (hypo) stage The pulp after alkali extraction after centrifugal dehydration and washing was collected in a plastic bag, and sodium hypochlorite was added at 0.3% based on the weight of the absolutely dried pulp. A predetermined amount of alkali is added so that the pH is 9.5 to 10, and the pulp concentration is 10%.
After adding a predetermined amount of water and stirring well, the mixture was kept in a thermostat at 45 ° C. for 2 hours. After the completion of the reaction, the sample was taken out and subjected to centrifugal dehydration washing.

(5) Chlorine dioxide stage The pulp from the hypochlorite stage subjected to centrifugal dehydration and washing is put into a plastic bag, and 0.3% of chlorine dioxide is added to the absolute dry weight of the pulp based on chlorine dioxide. Pulp concentration of 1
A predetermined amount of water was added so as to be 0%, and after stirring well, the mixture was kept in a constant temperature bath at 70 ° C. for 3 hours. After the completion of the reaction, the sample was taken out and subjected to centrifugal dehydration washing.

(6) Measurement of physical properties of pulp Whiteness: Centrifugally dewatered and washed pulp was disaggregated and then Tappi.
According to Test Method T205os-71 (JIS P 8209), a hand-made sheet having a basis weight of 250 g / m ² was prepared.
It was measured according to ISP 8123. Viscosity: Viscosity was measured according to Tappi test method T230 after the centrifuged dewatered washed pulp was loosened by hand. Breaking length and specific tearing strength: Pulverized pulp washed by centrifugal dewatering
Beat to a freeness of 450 ml (Canadian standard freeness) using a FI mill, and weigh 60 g /
to produce a m ² of the sheet, the paper pulp strength test method (J
ISP 8123). Discoloration test: After bleached pulp was heated and bleached for 2 hours in a circulating air oven at 105 ° C., J was measured in the same manner as in the measurement of whiteness.
Measure the whiteness before and after fading according to IS P8123,
Color return (PC value) was calculated according to the Kubelka-Munk equation. Kappa number: Measured according to Scan test method C-1: 77.

Example 1 and Comparative Examples 1 to 3 Examples 1 and Comparative Examples 1 to 3 were hardwood kraft pulp (Kappa number: 10.2; Domestic material: external material = 5
(0:50 mixture) was completely bleached in four stages. In Example 1, the kraft pulp after the oxygen bleaching was pretreated with the filtrate from the chlorine stage at a temperature of 50 ° C., without washing and deliquoring, followed by the CE / O-H-D In this example, bleaching was performed in a sequence.
Comparative Example 2 is an example in which the pulp was pretreated with the filtrate at a temperature of 100 ° C. in the same manner as in Example 1, and bleached in the same bleaching sequence as above without washing and draining. Comparative Example 3 is an example in which the same oxygen bleached hardwood kraft pulp was bleached in the same sequence as in Example 1 without pretreatment with the filtrate from the chlorine stage. Example 1 and Comparative Example 1
Table 1 shows the results obtained in Nos. 1 to 3.

[0045]

[Table 1]

Example 2 and Comparative Examples 4 to 6 In Example 2, oxygen bleached hardwood bleached kraft pulp (Kappa number: 9.3, domestic material: foreign material =
50:50) and pH using the filtrate from the chlorine stage.
In this example, the pulp was pre-treated at 3.0, washed and drained, and then bleached in the CE / O-H-D sequence by the above-described method. Comparative Example 5 performed the pulp pretreatment at pH 6.0,
In this example, bleaching was carried out in the same bleaching sequence as in Example 2 without washing and dewatering after pretreatment as in Example 2. Also,
Comparative Example 6 is an example in which the same hardwood kraft pulp was pretreated with a chlorine stage filtrate under the same conditions as in Example 2, washed and dewatered sufficiently, and then bleached in the same sequence as in Example 2. . Table 2 shows the results obtained in Example 2 and Comparative Examples 4 to 6.

[0047]

[Table 2]

Example 3 and Comparative Example 7 Example 3 and Comparative Example 7 were obtained from domestic and foreign materials (50: 5).
When the mixture of 0) is kraft digested by the MCC method, and the bleached pulp obtained by oxygen bleaching the unbleached pulp (kappa number: 8.9) is bleached in a CE / OHD sequence. The experiment of is shown. Example 3 is an example in which pulp after oxygen bleaching was pretreated with a filtrate from a chlorine stage and bleached without washing and dewatering. Comparative Example 7 was a process in which pulp after oxygen bleaching was filtrated from a chlorine stage. This is an example of bleaching without pretreatment. Table 3 shows the results obtained in Example 3 and Comparative Example 7.

[0049]

[Table 3]

As is evident from Tables 1 to 3, the process of the present invention promotes delignification in the chlorine stage, so using the same chemical addition rate, the Kraft process-oxygen bleached pulp (Examples 1 and 2) and The MCC method-oxygen bleached pulp (Example 3) has a high degree of whiteness of the pulp and finish, significantly improves the fading property, and improves the pulp viscosity and pulp strength (tensile strength and tear strength). Therefore, alternatively, in the method of the present invention, when pulp having the same whiteness and pulp strength as the pulp from the conventional method is obtained, the amount of chemicals can be greatly reduced, and the chemical cost can be reduced. If the reduction in the amount of chemicals used is limited to chlorine added to the chlorine stage, the amount of organic chlorine compounds in the wastewater can be reduced, so that the quality of the wastewater can be greatly improved.

[0051]

As is apparent from the above description, the present invention is superior in pulp whiteness and strength to the conventional method,
Further, in the case of completely bleached pulp, it is possible to obtain a pulp that is hardly faded. Alternatively, when pulp having the same whiteness is used, the amount of chemicals used is greatly reduced. In addition, there is provided an effect of providing a method capable of reducing the discharge of the amount of the organic chlorine compound into the wastewater.

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yosuke Uchida 1-10-6 Shinonome, Koto-ku, Tokyo Oji Paper Co., Ltd. Central Research Laboratory (56) References JP-A-57-121691 (JP, A) 61-12992 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) D21C 9/10

Claims (1)

(57) [Claims] 1. A method for bleaching pulp obtained from a lignocellulosic material by a combination of a multi-stage bleaching sequence comprising chlorine, caustic soda, hypochlorite (hypo), chlorine dioxide, oxygen and hydrogen peroxide. The pulp is adjusted to pH 2-5 using the filtrate from the chlorine bleaching stage prior to
Range and temperature 40-80 ° C and time 30-12
A method for bleaching a lignocellulosic material, comprising pretreating while maintaining the temperature within a range of 0 minutes, and bleaching in a chlorine stage of the multistage bleaching sequence without washing and draining.