JP2005076150A - Method for bleaching kraft pulp - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To bleach a pulp component obtained by a kraft cooking method by an inexpensive treating installation at an inexpensive treating cost. <P>SOLUTION: The method for bleaching a kraft pulp comprises subjecting the pulp component obtained by cooking a lignocellulose material by the kraft cooking method to alkali-oxygen delignification, washing the resultant pulp component, treating the washed pulp component in an acidic aqueous solution, concentrating the treated product without washing so that the pulp concentration may be ≥35%, treating the concentrated pulp component with high-density ozone under a condition of ≤50°C, and subjecting the treated pulp component to an alkali extraction step without carrying out the washing after the treatment. Preferably, the pH at the acid treating step is regulated so as to be 2.8-3.2 to unify the pH at the acid treating step with the pH at the high-density ozone treating step to make the regulation of the pH at the high-density ozone treating step unnecessary in the method for bleaching the kraft pulp. The alkali extraction step is preferably carried out at pH 7-8 at the initial stage and at pH 6-7 at the final stage and within a short time of 20-40 min. Broad-leaved tree chips, needle-leaved tree chips or non-wood is preferably used as the lignocellulose material. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for bleaching kraft pulp for bleaching pulp components obtained by cooking lignocellulosic material by kraft cooking, and more specifically, kraft pulp cooked in a batch or continuous digester is subjected to alkaline oxygen desorption. After lignin, washing, and further treating pulp in an acidic aqueous solution under generally known acid treatment conditions, the pulp moves to a high concentration ozone treatment stage without a washing stage, and after this ozone treatment stage The present invention relates to a bleaching method in which the process proceeds to an alkali extraction stage without performing a washing stage, and more preferably, the alkali extraction stage is performed at a low pH and a short processing time.

  The general production process of kraft pulp is a cooking process, oxygen delignification process, washing / selection process and bleaching process. Specifically, it is obtained by digesting lignocellulosic material in a batch kettle or continuous digester in the cooking process. After the pulp component is subjected to alkaline or acidic oxygen delignification, the pulp is washed and carefully selected to increase the whiteness of the pulp in the bleaching step, and a finished bleached pulp is obtained.

  As a conventional chlorine bleaching method, first, a chlorination stage is provided, washed to be soluble by chlorination of lignin, and then most lignin is removed in an alkali extraction stage. Thereafter, the remaining colored impurities are decomposed and removed using hypochlorite (hypo), chlorine dioxide or the like to obtain a pulp having high whiteness (non-patent document 1 described later).

  However, due to the increase in environmental protection, the chlorine-free bleaching method has become the mainstream recently, and in Patent Document 1 described later, medium-concentration ozone after acid treatment for the purpose of removing hexaneuronic acid and heavy metal ions from hardwood kraft pulp Then, depending on the purpose of the pulp quality, it is treated with an extraction stage under alkali or weak acidity, and further treated with chlorine dioxide. A bleaching treatment method that suppresses the decrease is shown. As a specific method, in claim 1 of Patent Document 1, pulp is obtained by digesting a lignocellulosic material with an alkaline chemical and then performing an oxygen delignification step treatment. The kappa number of the product was adjusted to 8-15, and the obtained pulp (referred to as LOKP) was treated in an acidic aqueous solution, The pulp kappa number is reduced by 30% or more in comparison to make the pulp kappa number 3 to 10, and ozone gas treatment and extraction treatment are performed in the subsequent steps, followed by bleaching in a sequence of chlorine dioxide treatment. A method for bleaching lignocellulose is shown.

  And in the said patent document 1, description of Example 1 (paragraph number "0034" -paragraph number "0035") about the specific method in each process, such as an acid process, ozone gas process, an extraction process, and a chlorine dioxide process, is carried out. Has the following description.

Stage A (acid treatment stage)
Additional water and 4N—H ₂ SO ₄ were added to LOKP pulp in an amount of 1.0% in terms of sulfuric acid, and treated for 120 minutes at a pulp concentration of 10% and a treatment temperature of 105 ° C. The obtained pulp was washed with ion-exchanged water and dehydrated and concentrated to a pulp concentration of 20%.

Z stage (medium concentration ozone treatment stage)
The pulp concentration after stage A is adjusted to 10%, and the pulp is put into a stirrer with a sealed pressure-type heating heater, and ozone gas with a concentration of 102 g / m ³ is placed in the tank so that the gauge pressure becomes 3.3 kg / cm ^2. And stirred at 1200 rpm for 10 seconds and then at 600 rpm for 120 seconds to react ozone gas and pulp. Subsequently, the pressure was returned to normal pressure, and the reaction was terminated by replacing unreacted ozone gas in the tank with air. The obtained pulp was washed with ion-exchanged water and dehydrated to a pulp concentration of 30%.

Eb stage (alkali extraction treatment stage)
The pulp concentration after stage Z was adjusted to 14%, 1.0% sodium hydroxide was added, and the pulp concentration was 10% and the treatment temperature was maintained at 60 ° C. for 120 minutes. The obtained pulp was washed with ion-exchanged water and dehydrated and concentrated to a pulp concentration of 20%.

Stage D (chlorine dioxide treatment stage)
Chlorine dioxide was added to the pulp whose Eb stage pulp concentration was adjusted to 14%, and 0.25% in terms of chlorine dioxide was added, and chlorine dioxide treatment was performed at 70% at a pulp concentration of 10%. The filtrate pH after completion of the D-stage reaction was 4.5. Further, after the reaction was completed, the obtained pulp was washed with ion-exchanged water to obtain a finished bleached pulp.

JP 2000-290887 A

Issued by the Pulp and Paper Technology Association "Pulp washing, selection and bleaching", first edition, page 299. As the range of the conditions of the A stage shown in Patent Document 1, the pH is 2.5 to 4.0, and the reaction temperature is 80 ° C. -130 degreeC and the processing time are 30-180 minutes. However, there are several A-stage conditions described in known examples. For example, at the UPM-Kimeni Visa Forest factory in Finland, since July 1996, pH 3.5, temperature 90 ° C., residence time maximum 7 5 hours stage A (Sitala, M., Winberg, K., Alenius, M., Henricson, K., Lonnberg, B., 1998 International Pulp Bleaching Conference, Book 1, 279), then Brazilian Since September 1999, Clavin's Riocell plant has a pH of 3.5, a temperature of 90 ° C, and a residence time of 100-150 minutes (Ratnieks, E., Ventura, JW, Mensch, MR, Zanchin, RA, Pulp Paper Can., 102 (12): 93 (2001)), and further, Ahlstrom (currently changed its name to Andritz) is the condition (pH: 3-4, temperature: 90) of its own AHLTM stage (same as A stage). ~ 110 ° C, residence time: 1-3 hours (Henricson, K., Paperi ja Puu, 79 (8): 546 (1997)) Therefore, the conditions of stage A shown in the above-mentioned Patent Document 1 include known conditions.

  The known kraft pulp bleaching method exemplified in Patent Document 1 generally has a washing stage after the A stage and the Z stage, and the bleaching apparatus is relatively large. The current situation is costing. Moreover, in the conventional kraft pulp bleaching method as described in the above-mentioned known examples, the pH in the Eb stage (alkali extraction stage) after the Z stage is high (usually pH 9.0 to 12.0). A long processing time (usually about 60 to 150 minutes) is a general processing condition. Further, the Z stage described in the above-described known example is a general medium-concentration ozone treatment stage.

  The present invention has been made in order to improve the above-mentioned problems in the conventional kraft pulp bleaching method, and comprises the following means.

  In the bleaching method of kraft pulp of the present invention, the pulp component obtained by digesting the lignocellulosic material by kraft cooking method is subjected to alkaline oxygen delignification, washed and selected, and further processed in an acidic aqueous solution. It may be the same as the conventional kraft pulp bleaching method. In the kraft pulp bleaching method of the present invention, the acid treatment is followed by a high-concentration ozone treatment stage without a washing stage, the pulp is not washed even after the ozone treatment, and the process directly moves to the alkali extraction stage, More preferably, it is recommended that this alkaline extraction stage be performed at a low pH and a short treatment time.

  Hereinafter, the present invention will be described in more detail.

In the kraft pulp bleaching method of the present invention, after the lignocellulosic material is cooked in the cooking liquor, the pulp obtained by performing alkaline oxygen delignification is treated in an acidic aqueous solution (stage A), and then the oxygen in the previous step. The pulp kappa number is 2 to 6 points lower than delignified, pulp is not washed, dehydrated to a pulp concentration of 35% or more, and treated with ozone gas (hereinafter referred to as [
The “high-concentration ozone treatment stage” is referred to as “high-concentration Z-stage” or simply “Z-stage”) as necessary, and thereafter, the weakly alkaline extraction process (E-stage) is performed without washing the pulp. In order to make the whiteness of the pulp after the alkali extraction stage 80% or more, bleaching may be performed using an oxidizing agent such as chlorine dioxide or hydrogen peroxide.

  In addition, as a result of investigating the process of A stage (acid treatment stage) based on the well-known example described above, temperature is 85-90 degreeC, residence time is 150-180 minutes, an acid washing stage, and high concentration ozone. In order to unify the pH of the bleaching stage (high-concentration Z stage), the pH is preferably 2.8 to 3.2 (the pH is not adjusted in the ozone stage). In this case, the pulp after the A stage is The kappa number is 2 to 6 points lower than the one before stage A, and the obtained pulp is not washed, and is dewatered and concentrated so that the pulp concentration is 35% or more in the dehydrator for the high-concentration Z stage at the latter stage. It is recommended.

  It is also recommended to perform ozone bleaching in a high-concentration ozone reactor with a flapper without adjusting the pH of the dehydrated pulp after stage A. In order to prevent a decrease in pulp strength, the temperature of the Z stage is a generally known temperature of 60 ° C. or lower, preferably 50 ° C. or lower. It is preferable to dilute the pulp after ozone bleaching in order to match the concentration (9 to 18%) of the subsequent alkali extraction stage without washing. The alkali extraction stage (stage E) is a condition of short processing time (20 to 40 minutes, preferably about 30 minutes), weak alkaline initial pH (7 to 8) and neutral final pH (6 to 7). Is recommended.

  As the lignocellulosic material used in the method of the present invention, non-wood materials (for example, kenaf, bagasse, bamboo, firewood, etc.) can be used in addition to wood materials (hardwood, conifer, etc.). The cooking method is the conventional kraft cooking method, but the cooking solution is white liquor (main components: caustic soda and sodium sulfide) or orange liquor (polysulfide liquor) alone, orange liquor at the first stage of cooking, white liquor at the later stage of cooking. It can be used.

  In the process of alkaline oxygen delignification and cleaning / selection described in the specification of the present invention, conventionally known equipment and operation methods can be used. Alkaline oxygen delignification may be a conventionally known one-stage system or two-stage system (for example, Dualox, OxyTrac two-stage system, etc.).

  The type of acid used in stage A in the present invention is not particularly limited, but sulfuric acid generally used in paper pulp companies is employed, and the known pH (3.1) and concentration in pulp (described above) When the processing temperature and the processing time were investigated using the conditions of 10 to 14%), as shown in Table 1, the decrease in the kappa number of kraft pulp (hardwood) was small at 80 ° C., and the kappa number at 95 ° C. However, it is difficult to cool the pulp after stage A to 50 ° C. without washing the pulp after stage A, considering the actual operational aspect. Therefore, the temperature of the acid treatment stage is preferably 85 to 90 ° C, and the treatment time is preferably a residence time of 150 to 180 minutes because the decrease in the kappa number is small at 210 minutes. Below, the copper value of the pulp is It has been found to decrease to 6 points.

  In the high concentration Z stage in the method of the present invention, it is generally known that pulp pH 2 to 3 is preferable in order to prevent viscosity reduction of ozone-treated pulp. Are 3-4. When there is no washing stage after stage A as in the present invention, the pH of stage A is preferably adjusted in the vicinity of 3.0 (2.8 to 3.2).

  Regarding the treatment time and temperature, which are other conditions for the high concentration Z stage, the reaction between ozone gas and pulp is fast and the ozone treatment time is generally within a few minutes. Is the main factor. Therefore, unbleached hardwood kraft pulp (kappa number: 11.7) having a concentration of 10% was prepared, adjusted with dilute sulfuric acid (4N) so that the pH was about 3.2, and then heated at a temperature of 50 in a thermostatic bath. The treatment was carried out at 10 ° C. for 10 minutes (pH adjustment and metal ion removal treatment). The obtained pulp was dewatered with a centrifuge, the final pulp concentration was 38.5%, Z-stage treatment was performed at an ozone addition rate of 0.663%, the pulp was washed, and the kappa number and viscosity were measured. did. The results are shown in Table 2. Since the pulp bleached by ozone bleaching at 60 ° C. was inferior in viscosity, it was recognized that high-concentration ozone bleaching was preferably performed at 50 ° C. or less.

  In order to investigate the influence of the washing stage after the high-concentration Z-stage bleaching, the following Z-stage conditions (ozone addition rate: 0.462%, reaction temperature: 50 ° C., pulp pH 2.44, pulp concentration: 38.5) %), The alkali extraction E stage was performed under the following provisional conditions in both cases where there was a pulp washing and no pulp washing. As temporary conditions, extraction temperature: 50 ° C., extraction time: 30 minutes, pulp concentration: 14%, initial pH: about 11.5. As a result, it was found that the Kappa number and the viscosity of the pulp after the E stage were the same regardless of the presence or absence of the washing stage after the previous Z stage, and therefore the washing stage after the high concentration Z stage was unnecessary. The results are shown in Table 3.

  Next, in setting the conditions of the alkali extraction (E) stage, the high concentration Z stage described above (ozone addition rate: 0.462%, reaction temperature: 50 ° C., pulp pH 2.44, pulp concentration: 38.5%) After washing, the pulp was washed and used in the next alkali extraction stage to determine the conditions in the alkali extraction stage. In this alkali extraction stage, oxygen (Eo) and hydrogen peroxide (Ep) may be used alone or in combination (Eop), or may not be used.

  (1) Temperature condition setting of the alkali extraction stage: The following provisional conditions, that is, the initial pH: about 10, pulp concentration: 14%, reaction time: 30 minutes, the alkali extraction stage is performed, the alkali extraction stage kappa number (ZE It has been found that the optimum temperature of the alkali extraction stage is 70 ° C. in terms of the kappa number and pulp viscosity. The results are shown in Table 4.

  (2) pH condition setting of the alkali extraction stage: The following provisional conditions, that is, temperature: 50 ° C., pulp concentration: 14%; reaction time: 30 minutes, the alkali extraction stage is performed, and in terms of ZE kappa number and pulp viscosity It has been found that the optimal initial pH of the alkali extraction stage is 7-8. The results are shown in Table 5.

  (3) Residence time condition setting of the alkali extraction stage: The following conditions are performed: the alkali extraction stage is performed at the temperature: 70 ° C., the initial pH: about 7.7, and the pulp concentration: 14%. From the viewpoint, it was found that the optimum residence time of the alkali extraction stage was 30 minutes. The results are shown in Table 6.

  Summarizing the above results, the reaction temperature is 50 ° C. or less in the high concentration Z stage, pulp washing after the reaction is unnecessary, and the temperature of the next alkali extraction stage is 70 ° C., the residence time is 30 minutes, and the initial pH. Can eliminate the washer after the Z stage under the condition of weak alkalinity 7-8, and the low pH in the alkali extraction stage enables chemicals to be reduced, and the reaction time is short, so the production of pulp can be increased and production It was found that the improvement of the property can be achieved.

  In the embodiment of the present invention, after setting the conditions of the high-concentration Z stage and the subsequent weakly alkaline extraction E stage, the LUKP pulp was used in examining the influence of the washing stage after the acid treatment stage described above. The pH was adjusted to 3.10 with dilute sulfuric acid (4N), and the pulp concentration was adjusted to 10% with water, followed by treatment at a temperature of 90 ° C. for 150 minutes. Half of the obtained pulp was washed and dehydrated (pulp concentration: 37.1%), and after high-concentration ozone bleaching was performed under the conditions of an ozone gas addition rate of 0.535% and a reaction temperature of 50 ° C, the pulp was washed. Without performing the next weakly alkaline extraction E stage, the ZE kappa number was measured. The remaining half of the pulp is not washed, dehydrated (pulp concentration: 38.2%), and ozone bleached under the same Z-stage conditions (ozone gas addition rate: 0.535%, reaction temperature: 50 ° C). The obtained pulp was also subjected to the following weak alkaline extraction E stage without washing, and the ZE kappa number was measured. As shown in the results of Table 7, it was found that the pulp after the acid treatment stage has almost the same ZE kappa value regardless of whether or not it is washed, so that the washing stage after the acid treatment is not necessary.

  The kraft pulp bleaching method of the present invention described above has the following effects.

  (1) In the kraft pulp bleaching method of the present invention, no washing stage is required after the acid treatment stage and the high-concentration ozone (bleaching) treatment stage. Therefore, the bleaching apparatus is simplified and the equipment cost is reduced. it can.

  (2) Also, since the residence time in the alkali extraction stage is short, it is possible to increase pulp production and reduce running costs.

  (3) Furthermore, since the treatment conditions in the alkali extraction stage are performed at a low pH, there is an effect that the cost of chemicals such as caustic soda can be reduced.

  In the above kraft pulp bleaching method, there is no washing stage after stage A (acid treatment stage) and stage Z (high concentration ozone treatment stage), and the next stage E (alkali extraction stage) is weakly alkaline. However, these pulps were found to be able to obtain high whiteness pulp by treatment with chlorine dioxide (abbreviated as D stage) or hydrogen peroxide (abbreviated as P stage) in the latter stage. Some examples are shown below. As comparative examples, an ECF (Elementary Chlorine Free) bleached pulp with chlorine dioxide having a washing stage after stage A and a chlorine bleached pulp not containing stage A are shown.

  The chemical addition rate in each of the following bleaching sequences is% by weight per dry pulp, and a horizontal bar (−) between each stage indicates a washing stage. Other than the high concentration Z stage, the other bleaching stages were carried out in polyethylene bags.

(Example 8)
(AZE) -D ₁ -D ₂ bleaching (1) Pulp used: Sampling of mixed eucalyptus hardwood kraft pulp (abbreviated as LUKP) with an alkaline oxygen delignification and washing of the actual production line with a copper number of 11.7 Stored at 5 ° C. while using.

(2) Stage A (acid treatment stage): LUKP having a pulp concentration of 10.5% was prepared, and then the pH of the pulp slurry was adjusted to about 3.1 with a 4N sulfuric acid solution to adjust the final pulp concentration to 10.0%. . (Sulfuric acid addition rate: 1.10%, pH: 3.10) This pulp was treated in a constant temperature bath at 90 ° C. for 150 minutes, and then washed to a pulp concentration of 38.7% in a centrifuge without washing. Dehydrated. (Copper number: 8.0)
(3) Stage Z (high-concentration ozone treatment stage): The pulp with a concentration of 38.7% after stage A was placed in a 2 liter vertical flask at 60 g BD / batch and attached to a laboratory evaporator. 10% ozone gas was injected into the flask to react ozone and pulp (ozone addition rate: 0.46%). The high-concentration ozone treatment was performed in a constant temperature bath at 50 ° C. Ozone bleached pulp was collected and mixed without washing.

  (4) Stage E (weakly alkaline extraction stage): 0.9% of caustic soda was added to the 38.7% concentrated pulp after the Z stage, and the pulp was held at 70% for 30 minutes at a pulp concentration of 14%. The extracted pulp was washed with tap water (2 liters / 200 g absolute dry pulp) and dehydrated and concentrated to a pulp concentration of 18%.

(5) D _1st stage (chlorine dioxide bleaching stage): 0.2% of chlorine dioxide was added to the pulp after stage E, and chlorine dioxide treatment was performed at a pulp concentration of 14% and 75 ° C. for 117 minutes. The pulp filtrate pH after the reaction was 5.5. The obtained pulp was washed with tap water (2 liters / 200 g absolutely dry pulp) and dehydrated to a pulp concentration of 18%.

(6) D ₂ stage (chlorine dioxide bleaching stage): D ₂ stage pulp concentration, treatment temperature, reaction time, filtrate pH after reaction, and pulp washing method are all the same as D ₁ stage, but chlorine dioxide Only the addition rate of 0.1% was low. As a result, a finished pulp by the (AZE) -D ₁ -D ₂ sequence was obtained.

Example 9
(AZE) bleaching -D-P sequence (1) (AZE) stage: Using been (AZE) -D ₁ -D ₂ sequence (AZE) stage pulp described above.

(2) D stage (chlorine dioxide bleaching stage): Pulp after D ₁ stage of the above (AZE) -D ₁ -D ₂ sequence was used.

  (3) P stage (hydrogen peroxide bleaching stage): 0.15% of hydrogen peroxide was added to the pulp after stage D, and after performing hydrogen peroxide treatment at a pulp concentration of 14% and 70 ° C. for 120 minutes, tap water The pulp was washed and dehydrated with water (2 liters / 200 g absolutely dry pulp) to obtain a finished bleached pulp. The pulp filtrate pH after stage P was 9.5.

(Example 10)
(AZE) bleaching with -P-D sequence (1) (AZE) stage: Using been (AZE) -D ₁ -D ₂ sequence (AZE) stage pulp described above.

  (2) P stage: It implemented on the conditions of P stage of said (AZE) -DP sequence, and the hydrogen peroxide addition rate was 0.55%.

(3) D stage: The conditions of this bleaching stage are the same as those of D ₂ stage of the (Aze) -D ₁ -D ₂ sequence, but the chlorine dioxide addition rate is 0.18%, and the obtained pulp Was washed and dehydrated with tap water to obtain a finished bleached pulp.

(Comparative Example 10)
Bleaching with _{A-D 0 -E-D 1} -D 2 Sequence (1) A stage: take the pulp after A stage of the described (AZE) -D ₁ -D ₂ sequence, were washed with tap water, Dehydrated to a pulp concentration of 18%.

(2) D ₀ stage: 0.366% chlorine dioxide is added to the pulp after stage A, the pH is adjusted to 3.5 with 4% pulp concentration, 4N dilute sulfuric acid, temperature 58 ° C., time 25 minutes After the treatment, the pulp was washed and dehydrated with tap water (2 liter / 200 g absolutely dry pulp).

(3) E stage (alkali extraction stage): 0.72% of caustic soda was added to the pulp after D ₀ stage, and the pulp concentration was 14% and kept at 70 ° C. for 66 minutes. The extracted pulp was washed with tap water and concentrated to a pulp concentration of 18%.

(4) D ₁ stage and D ₂ stage (chlorine dioxide bleaching stage): Pulp after E stage is the same as the conditions of D ₁ stage and D ₂ stage of (AZE) -D ₁ -D ₂ sequence described above was carried out, were each chlorine dioxide addition rate 0.06% (D ₁ stage) and 0.15% (D ₂ stages). The obtained pulp was washed and dehydrated with tap water to obtain a finished bleached pulp.

(Comparative Example 11)
Conventional bleaching by C-E-H-D sequence (1) Stage C: Using LUKP used for the above chlorine-free bleaching, 1.28% of chlorine is added to this pulp, pulp concentration is 4%, temperature is 58 ° C. After chlorination for 25 minutes, the pulp was washed and dehydrated with tap water.

(2) E stage: After alkaline extraction under conditions and caustic soda addition rate 0.96% of the E stage of the _{A-D 0 -E-D 1} -D 2 sequence described above, the pulp in tap water Was washed and concentrated to a concentration of 18%.

  (3) Stage H (sodium hypochlorite or hypo stage): 0.4% of hypo was added to the pulp after stage E, and the pulp concentration was 14%, the temperature was 75 ° C., and the treatment was performed for 36 minutes. The pH of the filtrate after stage H was 10.5, and the obtained pulp was washed with tap water and concentrated to a pulp concentration of 18%.

(4) Stage D (chlorine dioxide bleaching stage): The pulp after stage H is under the condition of stage D _{1 of} the (AZE) -D ₁ -D ₂ sequence described above and the chlorine dioxide addition rate is 0.40%. After performing chlorine dioxide bleaching, the product was washed and dehydrated with tap water. The resulting pulp is a finished bleached pulp with CE-HD sequence.

<Method for measuring pulp quality>
Each finished pulp is beaten with a PFI mill described in “JIS P8221-2”, and after measuring freeness with a Canadian standard freeness (freeness) described in “JIS P8121,” “JIS P8222” and “JIS P8223”. A handsheet at each freeness level was prepared by the method described, and then subjected to a paper quality test. The measurement quality items were kappa number, tear length, specific rupture degree, specific tear degree, folding resistance, whiteness and viscosity. For each quality item, a correlation diagram between the measured value and the freeness was created, and the quality value at a freeness of 500 ml was determined.

  Copper number: The copper number was measured by the method described in “JIS P8211”.

  Breaking length: The breaking length was determined based on the method described in “JIS P8113”.

  Specific rupture degree: indicated as a specific rupture degree using the method described in “JIS P8112”.

  Specific tear degree: The specific tear degree was measured by the method described in “JIS P8116”.

  Folding strength: Folding strength was determined using the method described in “JIS P8115”.

  Whiteness: Hunter whiteness was determined based on the method described in “JIS P8123”.

  Viscosity: Pulp viscosity was measured in accordance with the method described in “Tappi Standards T230 om-89” in the United States.

  As shown in Table 8, the ECF bleaching sequence including the (AZE) stage according to the embodiment of the present invention has a finished whiteness equal to or higher than the ECF bleaching sequence having the washing after the A stage and the conventional chlorine bleaching sequence. In addition, although the pulp viscosity was inferior due to the use of ozone, the results were almost the same in terms of tear length, specific bursting, specific tearing, folding resistance, etc., and the finished pulp had a low kappa number. Therefore, it was found that there is no washing stage after the acid treatment stage and the high-concentration ozone treatment stage, and that the pH in the next extraction stage is low and the residence time is short, which does not affect the bleaching property of kraft pulp. .

Claims (4)

  Pulp components obtained by cooking lignocellulosic material by kraft cooking method are subjected to alkaline oxygen delignification, washed, and further treated in an acidic aqueous solution, and then the pulp concentration is increased to 35% or more without washing. A kraft pulp bleaching method comprising: providing a high-concentration ozone treatment stage that is concentrated so that the high-concentration ozone treatment is performed at a temperature of 50 ° C. or less, and further providing an alkali extraction stage without washing.   In order to unify the pH of the acid treatment stage and the high-concentration ozone treatment stage, the pH in the acid treatment stage is set to 2.8 to 3.2 and the pH of the next high-concentration ozone treatment stage is not adjusted. The bleaching method of the kraft pulp of Claim 1.   The method for bleaching kraft pulp according to claim 1 or 2, wherein the alkali extraction stage is performed at an initial pH of 7 to 8 and a final pH of 6 to 7 in a short time of 20 to 40 minutes. The method for bleaching kraft pulp according to claim 1, 2 or 3, wherein the lignocellulosic material comprises hardwood chips, softwood chips or non-wood.