JP2005336667A - Method of cooking lignocellulosic material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for improving the cooking yield of lignocellulosic material using a continuous digester.
SOLUTION: A method for cooking lignocellulosic material using a two-vessel cooking apparatus having an osmotic vessel and a digester, wherein the digester is provided with at least two strainers, and the extracted black liquor from the strainer is permeated into the osmotic vessel. In the continuous cooking method to be added, the amount of black liquor extracted from the strainer located at the top of the digester is at least 35 when the amount of black liquor added to the permeation vessel is 100, or the top of the permeation zone or Prior to that, white liquor and polysulfide were added to the lignocellulose material, and the addition rate of the polysulfide was 0.3% by mass to 1.5% by mass (absolute dry lignocellulose material). Method of cooking the substance.
[Selection] Figure 2

Description

 The present invention relates to a method for cooking lignocellulosic material. More specifically, it relates to a method for improving the cooking yield of lignocellulosic material using a continuous digester.

  In order to use a lignocellulosic material as a papermaking raw material in many applications, it is necessary to digest it into chemical pulp or mechanically treat it with a refiner or the like to obtain mechanical pulp. These pulps are bleached as necessary, adjusted to a desired whiteness, and then used as a papermaking raw material. At present, the chemical pulping method is mainly used because it is easy to adjust to the desired whiteness and pulp characteristics. Especially, the cooking method called kraft method can regenerate chemicals and there are few restrictions on raw materials used. From the mainstream of chemical pulping. The kraft pulp method has also been developed in terms of equipment, and a large-scale production type called a continuous digester has become mainstream.

As for the kraft pulp method, various improved methods have been proposed so far. For continuous digesters, MCC method, EMCC method, ITC method, Lo-Solids method and the like have been proposed. All of these improvement methods are methods for improving pulp quality.
Of these, the MCC method divides the addition of cooking liquor into a continuous digester into three parts, and adds a portion of the digestion from the middle and bottom of the kettle to the chip to improve pulp strength by making the degree of cooking uniform. I am aiming for. This is because the alkali concentration in each stage of the cooking process is uniform compared to the conventional batch addition method of cooking liquor, and the disintegration of cellulose in the wood due to excessive alkali is suppressed.

  However, this method does not contribute much to the improvement of the pulp yield, and there is a problem that it involves a major modification of equipment when applied to an existing continuous digester. In addition, the Lo-Solids method can maintain a state of less dissolved organic matter throughout the entire cooking process by combining a step of extracting black liquor and a step of replenishing with a solution having less dissolved organic matter than the extracted black liquor. It is disclosed in Patent Document 1 and Patent Document 2 that a uniform cooking reaction can be performed by the above, and a high-strength pulp can be obtained. A high-strength pulp can be obtained, but the cooking yield is uncertain.

  On the other hand, improvements in cooking liquor and delignification aids aimed at improving pulp yield have also been developed. The former includes polysulfide cooking, and the latter includes addition of quinone compounds. Among these, polysulfide cooking is the oxidation of white liquor to convert sodium sulfide to polysulfide sulfur, or by adding sulfur directly to the cooking liquor to protect the end groups of hemicellulose by oxidation, so that the cellulose by alkali during cooking It is said that the pulp yield is improved in order to suppress elution of water. However, in the case of the method of cooking by adding 1.4% polysulfide sulfur (to wood chips) (Non-Patent Document 1), when the cooking method by the conventional cooking method is 100, the cooking by this method The yield is only about 102-104.

Patent Documents 3 to 5 disclose that polysulfide sulfur is generated and used by an electrolytic oxidation method. In this method, the use of an electrode film is indispensable, but the electrode film is easily clogged and has not yet been put into practical use.
Patent Document 6 discloses a method in which a portion of black liquor is gasified to generate hydrogen sulfide, and polysulfide sulfur is formed using an acid gas containing the hydrogen sulfide and carbon dioxide. However, it has not yet been put into practical use because of the safety load of installing a Claus plant and the use of a toxic gas such as hydrogen sulfide, and there is no description of the cooking yield.

As a method for improving the cooking yield, the black liquor is returned to the infiltration zone, the alkali profile of the cooking is made uniform, and the cooking is carried out at a low temperature for a long time, so-called COMPACT COOKING ^TM method, KOBUDO ^MARI method (Patent Documents 7 to 9) However, according to Non-Patent Document 2, when the yield of the conventional cooking method is 100, the cooking yield by the method is only 102 to 103.
Japanese translation of Japanese translation of PCT publication No. 8-51583 Japanese National Patent Publication No. 10-504614 JP-A-10-166374 Japanese Patent Laid-Open No. 11-5016 JP 11-51033 A Special table 2003-526740 gazette US Pat. No. 6,086,717 US Pat. No. 6,123,807 US Pat. No. 6,159,336 Akira Yamaguchi, “Polysulfide Cooking of Factory Chips”, Paper-Paper Technology Association, Vol. 33, No. 8, p1-9 Gunobu, “Yield Evaluation Method for COMPACT COOKINGTM and KOBUDOMARI Methods”, Proceedings of Annual Meeting of Paper and Pulp Technology Association, 2003, p49-59

An object of the present invention is to provide a method for improving the cooking yield of lignocellulosic material using a continuous digester.
More specifically, the lignocellulosic material is digested using a two-vessel cooking apparatus having an osmotic vessel and a digester, and the digester is provided with at least two strainers and permeates the extracted black liquor from the strainer. In the continuous cooking method added to a vessel, it is providing the method of improving the cooking yield of a lignocellulosic substance.

  The present inventors made it difficult to decompose polysulfide in a chemical solution by digesting at a low temperature, which could not be achieved by conventional cooking methods, and returning black liquor containing a large amount of polysulfide to the infiltration zone. As a result, it was found that the effect of the amount of polysulfide actually added was increased and the cooking yield was greatly improved, and the present invention was completed. The present invention includes the following inventions.

(1) A method of digesting lignocellulosic material using a 2-vessel digester having a permeation vessel and a digester having a digestion zone and a washing zone in order from the top to the bottom, wherein the digester has at least two strainers. In the continuous cooking method in which the black liquor extracted from the strainer is added to the permeation vessel, when the amount of black liquor added to the permeation vessel is 100, the strainer from the strainer located at the top of the digester The amount of the extracted black liquor is at least 35, and white liquor and polysulfide are added to the lignocellulosic material at the top of the infiltration zone or before that, and the addition rate of the polysulfide is 0.3 mass% to 1.5 mass% A method for digesting a lignocellulosic material, characterized in that it is an anti-dry lignocellulose material.

(2) The method for digesting a lignocellulosic material according to (1), wherein an anthraquinone derivative is added to the lignocellulosic material at the top of the permeation zone or before the permeation zone.

(3) The method for digesting a lignocellulosic material according to (1) or (2), wherein the polysulfide is prepared by an oxidation catalyst.

(4) Item (1), wherein the amount of black liquor extracted from the strainer located at the top of the digester is 45 or more when the amount of black liquor added to the permeation vessel is 100. The method for digesting a lignocellulosic material according to any one of items (3) to (3).

(5) The method of cooking lignocellulosic material according to any one of (1) to (4), wherein the temperature of the extracted black liquor added to the permeation vessel is 100 to 130 ° C.
(6) The method of cooking lignocellulosic material according to any one of (1) to (5), wherein white liquor is added at the beginning of the cooking zone in the digester.

  According to the method of the present invention, a lignocellulosic material is digested using a two-vessel digester having an osmotic vessel and a digester, wherein the digester is provided with at least two strainers, and the extracted black from the strainer In a continuous cooking method in which a liquid is added to an osmotic vessel, a method for cooking a lignocellulosic material with greatly improved cooking yield is provided.

The lignocellulosic material used in the present invention is preferably hardwood and softwood, but may be called non-wood and is not particularly limited.
Further, as the cooking method of the present invention, known cooking methods such as kraft cooking (polysulfide method), soda cooking, alkali sulfite cooking, etc. can be used, but considering pulp quality, energy efficiency, etc., kraft cooking method Are preferably used.

  In the cooking method of the present invention, for example, as shown in FIG. 1, an infiltration vessel having an infiltration zone (A) from the top to the bottom, and a cooking zone (B) and a washing zone (C) from the top to the bottom. In order, preferably a continuous digester in which the cooking zone (B) is divided into an upper cooking zone (B1), a middle cooking zone (B2) and a lower cooking zone (B3). Each zone is separated by a strainer. For example, in the case of FIG. 1, after the cooking liquor containing polysulfide and anthraquinone is added to the lignocellulosic material from the cooking liquor introduction tube 2, the zone from the permeation zone (A) is the permeation zone. The zone from the top to the strainer-3 is the upper cooking zone (B1), the zone from the strainer 3 to the strainer 4 is the middle cooking zone (B2), the zone from the strainer 4 to the strainer 5 is the lower cooking zone (B3), and the strainer 5 The zone from when the pulp comes out of the wooden kettle is the washing zone (C).

  In the present invention, at least one cooking solution is always added for each of the zones other than the washing zone. For example, in the case of FIG. 1, the cooking liquid is dividedly added to the permeation zone A by the cooking liquid introduction pipe 2 and divided into the cooking zone by the cooking liquid introduction pipe 1. As an addition ratio of the cooking liquor, the white liquor addition ratio to the cooking zone is 55% by mass or more, preferably 60% by mass of the total white liquor. When the white liquor addition rate is less than these, the elution of hemicellulose increases and the cooking yield decreases. As shown in FIG. 1, the cooking solution to be added in a divided manner may be prepared by adding polysulfide to the infiltration zone and further adding anthraquinone, and adding polysulfide or anthraquinone to the cooking zone. Since the penetration of the cooking aid into chips is poor, it is preferable to add only white liquor. As the properties, those having different sulfidities may be used. For example, when kraft cooking wood, the sulfidity of the white liquor is 5 to 75%, preferably 15 to 45%, and the effective alkali addition rate is 5 to 30% by mass per bone dry wood, preferably 10 to 25% by mass.

  Examples of the method for producing polysulfide sulfur used in the present invention include an oxidation catalyst method and an electrolysis method as practical methods. The oxidation catalyst method is a method in which an alkaline solution containing sodium sulfide is oxidized with molecular oxygen such as air in the presence of a catalyst such as activated carbon. In addition, the electrolytic method is a method of electrochemically oxidizing sulfide ions in a solution containing sulfide ions such as an alkaline cooking liquid mainly composed of sodium hydroxide and sodium sulfide or sodium carbonate and sodium sulfide (Patent Literature). 2).

  The former oxidation catalyst method has a problem that sodium thiosulfate, which is a by-product, is produced. Therefore, it can be added only up to 1.5% by mass with respect to the absolutely dry chip, but in Japan it is often Has a track record of adoption at The latter electrolysis method suppresses the reaction of by-products and can obtain polysulfide sulfur at a high concentration, but in Japan, it is not adopted in the factory because of the high electric cost, and the electrolysis method does not use the anode and cathode. The diaphragm is easily clogged and it is difficult to stably produce polysulfide sulfur. Therefore, in the method of the present invention, it was decided to use the polysulfide sulfur produced by the former oxidation catalyst method, but if the electrical cost is reduced due to future changes in the situation or the performance of the diaphragm is improved, it will be produced by the electrolytic method. It is possible to convert to polysulfide sulfur.

  The addition rate of polysulfide sulfur in this invention is 0.3-1.5 mass% with respect to an absolutely dry chip | tip. If the content is lower than 0.3% by mass, the yield is not sufficiently improved. If the content is higher than 1.5% by mass, as shown above, a large amount of by-products are generated, which is not suitable.

  In addition, known cyclic keto compounds as cooking aids, such as benzoquinone, naphthoquinone, anthraquinone, anthrone, phenanthroquinone, and nuclear substitutes by the alkyl group or amino group of the quinone compound, or the reduced form of the quinone compound One or two selected from hydroquinone compounds such as anthrahydroquinone, and 9,10-diketohydroanthracene compound which is a stable compound obtained as an intermediate of an anthraquinone synthesis method by Diels Alder method The above may be added, and the addition rate is a normal addition rate, for example, 0.001 to 1.0% by mass with respect to the absolutely dry mass of the wood chip.

  Other usable cooking aids include chelating agents such as ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA), various surfactants, etc., and green liquor can also be used and is particularly limited. It is not a thing. These cooking aids can be added in the same manner as the cooking liquor, and the place of addition is not limited.

  The infiltration zone of the method of the present invention is characterized in that an operation that satisfies the following conditions is performed. The first is to increase the liquid ratio sufficiently, the second is to add sufficient alkali, and the third is not to raise the temperature too much. If the temperature is too high, the reaction will occur before the chemical solution has sufficiently penetrated the lignocellulosic material, leading to non-uniform digestion and polysulfide degradation. In the method of the present invention, an operation that satisfies the above conditions is performed by adding a black liquor sufficiently containing polysulfide and containing a large amount of residual alkali together with the cooking liquor to the top of the infiltration zone.

  The cooking method of the present invention is a continuous cooking method in which at least two strainers are provided in a digester, and black liquor extracted from the strainers is added to an osmotic vessel. For example, in the case of FIG. 1, the black liquor is extracted from the black liquor extraction tubes 15, 16, 17, and 18 by the pumps 7, 8, 9, and 10. The total amount of black liquor extracted from the black liquor extraction tubes 15 and 16 is added to the permeation vessel, and the total amount of black liquor extracted from the black liquor extraction tubes 17 and 18 is sent to the flash tank 14. Assuming that the total amount of black liquor added to the permeation vessel is 100, the amount of black liquor extracted from the strainer 3 located at the top of the digester is at least 35, preferably 45 or more. This is because when the black liquor extraction amount is less than 35, unreacted polysulfide is ineffectively decomposed through the digester. Black liquor from the black liquor extraction tube 16 may also be added to the permeation zone, but polysulfide has already been decomposed in the process of reaching the strainer 4 and thus is hardly contained in this black liquor.

  In the method of the present invention, the advantage of using the extracted black liquor is that the polysulfide containing polysulfide is extracted and added to the infiltration zone before the polysulfide that has not reacted with the carbohydrates is ineffectively decomposed. The point which can be used effectively is mentioned.

In the method of the present invention, the liquid ratio in the infiltration zone is 4-7. If the liquid ratio is less than 4, the chemical solution may not sufficiently permeate into the raw material. In addition, if a raw material with a small bulk weight is used, the raw material will not easily settle in the kettle, and the raw material will move in the kettle. Is not suitable because of non-uniform digestion resulting in non-uniform cooking. In particular, JAPAN TAPPI paper pulp test method no. This tendency becomes stronger when a raw material having a volume weight measured in accordance with 3 is 450 kg / m ³ or less. Raw materials with a bulk weight of 450 kg / m ³ or less include conifers such as Radiata pine, Kara pine, Todo pine, and Ezo pine, and some eucalyptus such as E.deglupta and E.grandis (for example, paper pa 48, No. 2, P294, Table 5) and Acacia. In addition, the type of digester called “liquid phase kettle” is more difficult to settle than the type of digester called “gas phase kettle”, so that the liquid ratio is preferably set higher. On the other hand, if the liquid ratio is greater than 7, it is not suitable because the black liquor recovery load becomes too large.

  The temperature in the infiltration zone of the method of the present invention is 100 to 130 ° C., and the residence time of the lignocellulosic material is 0.5 to 2 hours. When the temperature is lower than 100 ° C. or when the residence time is shorter than 0.5 hours, the chemical solution permeates and reacts insufficiently in the permeation zone, rapidly heats up in the subsequent cooking zone, and at a high temperature. It is not suitable because it must be cooked and there is a high possibility of uneven cooking. On the other hand, when the temperature is higher than 130 ° C. or when the residence time is longer than 2 hours, polysulfide is ineffectively decomposed, and the cooking reaction starts before the chemical solution sufficiently penetrates into the raw material. It is not suitable because it may not be possible or uniform cooking may not be possible due to side reactions caused by organic substances in the black liquor. In the infiltration zone of the present invention, co-current cooking is always performed from the viewpoint of avoiding channeling.

As the conditions in the cooking zone of the method of the present invention, white liquor is added at least at one place, black liquor is extracted from the middle of the cooking zone, and the black liquor is at least 4 m ³ or more per 1 ton of undried pulp. To the beginning of the permeation zone, extract the remaining black liquor, and extract black liquor at the end of the cooking zone, but not limited, if the amount of black liquor returned is less than 4 m ³ , Since the improvement in yield is not remarkable, it is not suitable. For example, in the case of the method of FIG. 1, the white liquor is added through the white liquor introduction pipe 1 at the beginning of the cooking zone, and most of the black liquor extracted from the strainers 3 and 4 passes through the cooler 13 into the permeation vessel. And a portion of the black liquor is sent to the flash tank 14. Further, in a preferred embodiment, the black liquor (black liquor extraction tube 18) extracted from the strainer 5 is sent to a flash tank.

  In the cooking zone of the method of the present invention, the countercurrent cooking and the cocurrent cooking are not particularly limited, and the countercurrent cooking and the cocurrent cooking are appropriately selected according to the situation. For example, if the cooking zone is divided into upper cooking zone and lower cooking zone, countercurrent cooking + countercurrent cooking, countercurrent cooking + cocurrent cooking, cocurrent cooking + cocurrent cooking, cocurrent cooking + countercurrent cooking These four combinations are possible. These selections are made in consideration of the properties of the lignocellulosic material used as a raw material, operability, economy, pulp quality, and the like.

  The liquid ratio in the cooking zone of the method of the present invention is 4-10. If the liquid ratio is less than 4, uncooked dust is generated due to insufficient penetration of the cooking liquid. On the other hand, if the liquid ratio exceeds 7, the recovery load of black liquor becomes too large, which is not suitable.

  The maximum temperature in the cooking zone of the method of the present invention is, for example, 130 to 150 ° C. when hardwood is used as a raw material, and 140 to 160 ° C. when softwood is used as a raw material. The residence time of the lignocellulosic material in the cooking zone is 2 to 10 hours. If the residence time is shorter than 2 hours, cooking is insufficient and the chips are not pulped. On the other hand, if the residence time is longer than 10 hours, the pulp fibers are damaged by excessive cooking, which is not desirable. The amount of residual alkali in the black liquor extracted at the end of the cooking zone is 5 to 20 g / l, preferably 8 to 12 g / l. When the residual alkali concentration is lower than 5 g / l, the delignification reaction in the cooking zone does not proceed sufficiently. When the residual alkali concentration is higher than 20 g / l, the damage to the cellulose increases and the digested pulp yield decreases. Absent.

  The conditions in the cleaning zone of the method of the present invention are not particularly limited except that the treatment time is 1 hour or less. In the present invention, in order to digest the entire digester at the lowest possible temperature, the cleaning time is inevitably shortened. In the washing zone of the method of the present invention, either counter-current cooking or co-current cooking may be used, and it is not particularly limited. However, considering the efficiency of washing in the kettle, counter-current cooking may be used. This is a preferred embodiment.

  In the method of the present invention, the black liquor extracted from a plurality of locations is sent to the flash tank 14, and after flash vapor is collected, it is sent to the evaporator. After being concentrated by the evaporator, the organic material is recovered as thermal energy by combustion, and the inorganic material is finally regenerated into a cooking liquor.

  The method of the present invention is a method in which lignocellulosic material is digested using a two-vessel digester having an osmotic vessel and a digester, wherein the digester is provided with at least two strainers and permeates the extracted black liquor from the strainer. In the continuous cooking method added to the vessel, the extracted black liquor from the strainer located at the top of the digester used as the black liquor added together with the white liquor and polysulfide to the lignocellulosic material supplied to the osmotic vessel. The amount is at least 35 when the total amount of black liquor is 100, and the addition rate of polysulfide is 0.3 mass% to 1.5 mass% (against dry lignocellulosic material). The reason why the cooking yield was improved by cooking under various conditions is estimated as follows. . In other words, by cooking at low temperatures, which could not be achieved by conventional cooking methods, polysulfide in the chemical solution is difficult to be decomposed, and by returning the extracted black liquor containing a large amount of polysulfide to the infiltration zone, It is presumed that the yield of cooking is greatly improved because the effect of the amount of polysulfide added to the above is brought about, and the disintegration of carbohydrate in lignocellulose is hardly caused by polysulfide.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. Of course, the present invention is not limited to these examples. The cooking was evaluated using a laboratory digester that can reproduce the continuous digester in the factory. “%” And ratio in each example are based on mass.
In Example 1 and Comparative Examples 1 and 2 shown below, the following methods were used to measure the kappa number and cooking yield of unbleached pulp.

1. Measurement of kappa number The kappa number was measured according to JIS P8211.

2. Measurement of cooking yield The cooking yield was calculated from the following equation.

Cooking yield (%) =
[Introduced absolute dry chips (g) / disintegrated absolute pulp (g)] × 100

Example 1
Using the two-column digester shown in FIG. 1, conifer chips consisting of slash pine: radiata pine: domestic conifer = 45: 25: 30 were digested according to the following procedure. First, in the permeation zone A, the cooking liquor corresponding to 5.4 mass% as an effective alkali, polysulfide sulfur 1.0%, anthraquinone 0.03%, and sulfidity 28% is shown in FIG. The black liquor was extracted from the cooking liquor introduction tube 2 and 0.53 and 0.94 liters of black liquor from the black liquor extraction tubes 15 and 16, respectively, and added to the permeation vessel to start the cooking (the liquid ratio was 5). 0.0). In the permeation zone, co-current cooking was performed, the chip residence time in the permeation zone was 68 minutes, and the temperature was 125 ° C.

  Next, in the upper cooking zone, 1.0 liter of cooking liquid (cooking liquid introduction pipe 1) corresponding to 12.6% effective alkali and 28% sulfidization is used through the pump 6 and the heater 12 with respect to 500 g of chip absolutely dry. The liquid was circulated. Thereafter, 2.3 liters of black liquor was extracted from the strainers 4 and 5 through the black liquor extraction tubes 17 and 18 for 500 g of the chip absolutely dry and sent to the flash tank 11. The upper cooking zone B1, the middle cooking zone B2, and the lower cooking zone B3 were co-current cooking. The residence time of the chips in the cooking zone was 352 minutes, and the cooking temperatures were 147 ° C. and 150 ° C. on average.

  Thereafter, in the cleaning zone, the residence time of the chips in the kettle was 0.5 hours. Through the washing filtrate introduction pipes 19 and 20, 2.5 liters of washing filtrate of unbleached pulp was added from the bottom of the kettle as diluted washing water. The unbleached pulp kappa number was 28.1, 22.2 for cooking temperatures of 147 ° C. and 150 ° C., respectively, and the absolute dry yield of unbleached pulp was 260.0 g and 253.0 g. Of 52.0% and 50.6%.

Comparative Example 1
In osmosis zone A, the extraction black liquor was added to the osmosis zone by removing 0.3 and 1.2 liters from the black liquor extraction tubes 15 and 16, respectively, and the temperature of the cooking zone was 148 ° C and 152 ° C. Except that, the same operation as in Example 1 was performed. The unbleached pulp has a kappa number of 25.4 and 21.1 for the cooking temperatures of 148 ° C. and 152 ° C., respectively, and the absolute dry yield of the unbleached pulp is 242.0 g and 236.5 g. Were 48.4% and 47.3%.

Comparative Example 2
In the infiltration zone A, the same operation as in Example 1 was performed except that 0.1% of polysulfide sulfur was added and the temperature of the cooking zone was set to 148 ° C, 152 ° C, and 155 ° C. The kappa number of the obtained unbleached pulp was 27.8, 23.3, 19.4 for cooking temperatures of 148 ° C, 152 ° C and 155 ° C, respectively, and the dry-dry yield of unbleached pulp was 240.5g, 233. The cooking yields were 48.1%, 46.7%, and 46.0%.

  As apparent from Table 1 and FIG. 1, when the lignocellulosic material is digested using a two-vessel digester having an infiltration zone, a digestion zone, and a washing zone in this order, at least two strainers are provided in the digester, In the continuous cooking method in which the black liquor extracted from the strainer is added to the permeation vessel, when the amount of black liquor added to the permeation vessel is 100, the amount of black liquor extracted from the strainer located at the top of the digester is 35. In addition, it is understood that the cooking yield is improved by adding a desired amount of white liquor and polysulfide to the top of the infiltration zone. That is, by carrying out the above cooking method, polysulfide in the chemical solution is difficult to decompose, and the amount of polysulfide actually added by adding black liquor containing a large amount of polysulfide to the infiltration zone from the beginning of the cooking zone. The above effects were brought about, and the cooking yield was greatly improved.

  The method of the present invention can further improve the cooking yield and further improve the relationship between the kappa number and the cooking yield. That is, the present invention can improve the cooking yield at the same kappa number at the same effective alkali addition rate.

It is a figure which shows the relationship between the copper number of unbleached pulp, and a cooking yield. It is a schematic diagram which shows an example of the cooking flow for implementing the cooking of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... White liquor introduction tube 2 ... Polysulfide and anthraquinone containing white liquor introduction tube 3, 4, 5 ... Strainer 6, 7, 8, 9, 10, 11 ... Pump 12 ... Heater 13 ... Cooler 14 ... Flash tank 15,16, 17, 18 ... Black liquor extraction tube 19, 20 ... Washing filtrate introduction tube

Claims (6)

  It is a method of digesting lignocellulosic material using a 2-vessel digester having a permeation vessel and a digester having a digestion zone and a washing zone in order from the top to the bottom, wherein the digester is provided with at least two strainers, In the continuous cooking method in which the black liquor extracted from the strainer is added to the permeation vessel, when the amount of black liquor added to the permeation vessel is 100, the amount of black liquor extracted from the strainer located at the top of the digester Is at least 35 and white liquor and polysulfide are added to the lignocellulosic material at the top of the infiltration zone or earlier, and the addition rate of the polysulfide is 0.3 mass% to 1.5 mass% (vs. Lignocellulosic material), characterized in that it is a lignocellulose material).   2. The method of cooking lignocellulosic material according to claim 1, wherein an anthraquinone derivative is added to the lignocellulosic material at the top of the permeation zone or before.   The method for cooking a lignocellulosic material according to claim 1 or 2, wherein the polysulfide is prepared by an oxidation catalyst.   The amount of black liquor extracted from the strainer located at the top of the digester is 45 or more, where the amount of black liquor added to the permeation vessel is 100 or more. The method for digesting a lignocellulosic material according to any one of the above.   The method for cooking a lignocellulosic material according to any one of claims 1 to 4, wherein the temperature of the extracted black liquor added to the permeation vessel is 100 to 130 ° C. The method for cooking lignocellulosic material according to claim 1, wherein white liquor is added at the beginning of the cooking zone in the digester.

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