JP2009203583A - Method for cooking lignocellulose material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for cooking a lignocellulose material which improves a kraft cooking effect and pulp yield without any large-scale remodeling of the facility. <P>SOLUTION: The method is such that in cooking a lignocellulose material using a continuous cooker having a permeation zone and a cooking zone in this order, cooking chemical liquids are added to the permeation zone and the cooking zone, respectively; wherein, the cooking chemical liquid to be added to the cooking zone is higher in sulfurization degree than that to be added to the permeation zone by 5% or greater. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

The present invention relates to a method for cooking a lignocellulosic material, and more particularly to a method for cooking a lignocellulosic material capable of improving both the digestibility and pulp yield in kraft cooking.

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 mass production type called a continuous digester and a large one have become mainstream.

As an improved method of the kraft pulp method, an MCC method, an EMCC method, an ITC method, a Lo-Solids method using a continuous digester has been proposed. The MCC method divides the addition of cooking chemicals into the continuous digester into three parts, and a part of it is added counter-currently to the chip from the middle or lower part of the kettle, keeping the alkali concentration in each stage of the cooking process uniform and excessive. It is a method of improving pulp strength by suppressing the disintegration of cellulose in wood due to an alkali. In addition, the Lo-Solids method combines a process of extracting black liquor and a process of replenishing with a liquid with less dissolved organic matter than the extracted black liquor to keep the dissolved organic matter in the cooking liquor small and uniform throughout the cooking process. It is a method of improving pulp strength by making a simple cooking reaction (Patent Documents 1 and 2). These improved methods are methods for improving the pulp quality, but the so-called COMPACT COOKING ^TM method, KOBUDOMARI method (patented) is a method in which the black liquor is returned to the permeation zone, the alkali profile of the cooking is made uniform, and the cooking is carried out at a low temperature for a long time. Documents 3, 4, 5 and Non-Patent Document 1) indicate that the cooking yield can be improved to 102 to 103 as compared with the conventional cooking method. However, when these cooking methods are applied to an existing continuous digester, there is a problem that it involves a major modification of equipment.

In addition, progress has been made in improving cooking liquor and delignification aids aimed at improving pulp yield. The former includes polysulfide cooking, and the latter includes the addition of quinone compounds. Among these, polysulfide cooking is a highly alkaline cooking solution by oxidizing the cooking solution to convert sodium sulfide to polysulfide sulfur or by adding sulfur directly to the cooking solution to protect the end groups of hemicellulose by oxidation. In order to suppress the elution of hemicellulose, the pulp yield is improved (Non-patent Document 2). On the other hand, in quinone cooking in which a quinone compound is added, the quinone compound oxidizes and protects the terminal group of cellulose to suppress the elution of cellulose, and the pulp yield is improved (Patent Documents 6 and 7). Although a cooking method combining these polysulfide cooking and quinone cooking has been studied (Patent Document 8), all of them have a large chemical cost compared with the effect of improving the cooking yield, and the current situation is that the spread is not so much progressed. is there.

The cooking chemicals in kraft cooking consist of caustic soda (NaOH) and sodium sulfide (Na ₂ S), and it has been known for a long time that the strength of the pulp obtained will improve if sulfide is present at the initial stage of kraft cooking (non- Patent Document 3), research on the degree of sulfidization (sodium sulfide concentration / (caustic soda concentration + sodium sulfide concentration)) at the initial stage of cooking has been vigorously conducted. On the other hand, the effect of the degree of sulfidation after the middle stage of cooking on cooking characteristics and pulp yield is not known. As a solution having a degree of sulfidation higher than that of the cooking chemical, there are cooking black liquor and green liquor produced in the recovery process in the pulp mill.
In the COMPACT COOKING ^TM method and the KOBUDOMARI method, a cooking black liquor is added as a sulfide source at the initial stage of cooking, and a method of adding green liquor at the initial stage of cooking has also been studied (Non-Patent Document 4). Even if a solution with a high degree of sulfidation is added to the infiltration zone, which is the initial stage of cooking, much of the sodium sulfide is consumed along with the alkali before reaching the maximum temperature of the infiltration zone and cooking, so that sodium sulfide is used for actual delignification. There is a problem that there are few.
Japanese translation of PCT publication No. 8-511583, Japanese National Patent Publication No. 10-504614 US Pat. No. 6,086,717 US Pat. No. 6,123,807 US Pat. No. 6,159,336 Japanese Patent Laid-Open No. 52-37803 JP-A-53-45404 JP 2002-115190 A Gunobu, “Yield Evaluation Method of Digested Pulp by COMPACT COOKINGTM Method and KOBUDOMARI Method”, 2003 Paper Pulp Technology Association Annual Conference Preliminary Proceedings, p49-59 Akira Yamaguchi, “Polysulfide Cooking of Factory Chips”, Paper-Paper Technology Association, Vol. 33, No. 8, p1-9 Sjoblom, K, et al., "Extended de- ligation in craft cooking through improved selectivity", Paper Puu 65: 4, 177, 1983. Svedman, M, et al., “The use of green liquors and its derivatives in improving craft pulling”, TAPPI J. et al. Vol81 No. 10, 151, 1998

The present invention has been made to solve the above-mentioned problems, and provides a cooking method for improving the digestibility and pulp yield in kraft cooking without major modification of the lignocellulosic material cooking method. For the purpose.

  As a result of various studies paying attention to the degree of sulfidation in kraft cooking, the present inventors, in a continuous cooking apparatus having an infiltration zone and a cooking zone in this order, add delignification by adding a high degree of sulfidizing chemical to the cooking zone. It was found that the pulp yield per kappa number could be greatly increased and the pulp yield per kappa value increased, and the present invention was completed.

The present invention includes the following inventions.
(1) A cooking method of adding a cooking chemical to an infiltration zone and a cooking zone when the lignocellulosic material is cooked in a continuous cooking apparatus having an infiltration zone and a cooking zone in order, and the temperature of the infiltration zone is 90 to 140 A method for cooking a lignocellulosic material, wherein a cooking chemical liquid at 5 ° C. and having a degree of sulfidation of 5% or more higher than the cooking chemical liquid added to the permeation zone is used in the cooking zone.
(2) The method for cooking lignocellulosic material according to (1), wherein green liquor is used as the cooking chemical to be added to the cooking zone.
(3) The method for cooking lignocellulosic substances according to (1), wherein black liquor is used as the cooking chemical to be added to the cooking zone.
(4) The method for cooking lignocellulosic material according to any one of (1) to (3), wherein the cooking zone temperature is 120 ° C to 170 ° C, and the temperature difference between the infiltration zone and the cooking zone is 20 ° C or more.
(5) The method for cooking lignocellulosic material according to any one of (1) to (4), wherein the cooking chemical added to the cooking zone is added before reaching the maximum temperature in the cooking zone.
(6) The lignocellulosic material cooking method according to any one of (1) to (5), wherein the infiltration zone is performed at a low liquid ratio of 2.0 to 2.8.

  According to the present inventor, in a continuous cooking apparatus having an infiltration zone and a cooking zone in order, delignification can be greatly promoted by adding a cooking chemical solution having a high degree of sulfidation to the cooking zone, and the pulp yield per the same kappa number It has become possible to provide a method for cooking lignocellulosic substances that greatly improves the process.

The lignocellulosic material used in the present invention is preferably hardwood and softwood, but may also be called non-wood and is not particularly limited, but softwood is the original kappa value of unbleached pulp. Therefore, the effect of the present invention is large and preferable.

In the present invention, for example, as shown in FIG. 1, a continuous digester having an infiltration zone (A), a cooking zone (B), and a washing zone (C) in this order from the top to the bottom, preferably a cooking zone (B ) Is used as a continuous digester with an upper cooking zone (B1) and a lower cooking zone (B2). Each zone is delimited by a strainer. In the present invention, the permeation vessel having a permeation zone is also suitably used in a two-vessel digester installed in front of a continuous digester. For example, in the case of FIG. 1, the process from the addition of the cooking chemical solution to the lignocellulosic material from the cooking solution introduction pipe 2 to the exit of the strainer 5 from the top 3 of the wood pot is the permeation zone (A), and the strainer 5 to the strainer 6. The process from the upper cooking zone (B1), the process from the strainer 6 to the strainer 7 is the lower cooking zone (B2), and the process from the strainer 7 to the lower kiln 4 is the washing zone (C).

In the present invention, one or more cooking chemicals are always added to each zone excluding the washing zone. For example, in the case of FIG. 1, the cooking chemical (white liquor) is dividedly added to the permeation zone by the cooking chemical introduction pipe 2 and to the cooking zone by the cooking chemical introduction pipe 1. The cooking chemical is added to the permeation zone in an addition ratio of 50% or more. When the ratio of the cooking chemical solution added to the osmosis zone is smaller than this, the alkali in the osmosis zone is insufficient and delignification does not proceed. The total effective alkali addition rate is 5 to 30% by weight, preferably 10 to 25% by weight, based on the weight of the absolutely dry wood. In the present invention, known polysulfides and cyclic keto compounds as cooking aids, for example, benzoquinone, naphthoquinone, anthraquinone, anthrone, phenanthroquinone, and nuclear substitutes such as alkyl and amino of the quinone compound, or reduction of the quinone compound 1 or 2 selected from hydroquinone compounds such as anthrahydroquinone which is a type, and 9,10-diketohydroanthracene compounds which are stable compounds obtained as intermediates in an anthraquinone synthesis method by Diels Alder method More than seeds may be added, and the addition rate is a normal addition rate, for example, 0.001 to 1.5% by weight per the dry weight of the wood chips. Other cooking aids that can be used include chelating agents such as ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA), various surfactants, and the like, and are not particularly limited. These cooking aids can be divided and added in the same manner as the cooking liquor, and the place of addition is not limited.

In the present invention, the sulfidity of the white liquor added to the soaking zone is 15-30%, and the sulfidity of the cooking chemical added to the cooking zone is 5% higher than that of the white liquor added to the infiltration zone. It is characterized by being more expensive. The sodium sulfide component in the white liquor added to the permeation zone exists as hydrosulfide ions (SH ⁻ ), but in the permeation zone, most of it is consumed or adsorbed to the chip component, and the cooking zone where delignification is performed. In this case, there is a shortage of hydrosulfide ions. In order to make up for this shortage, delignification in the cooking zone can be greatly promoted by adding a cooking chemical having a high degree of sulfidation in the cooking zone. When a cooking chemical having the same degree of sulfidization is added to the cooking zone, the effect of supplementing the hydrosulfide ions cannot be obtained, and delignification cannot be promoted.

When white liquor is used as a chemical solution to be added to the cooking zone, two types of white liquor must be produced in advance in order to add white liquor having different degrees of sulfidation in the permeation zone and the cooking zone. Manufacturing facilities need to be expanded. On the other hand, the green liquor produced in the recovery process has a sulfidity of about 50 to 90%, and the use of the green liquor greatly reduces the chemicals and energy required to causticize the green liquor into white liquor. This is preferable because The green liquor at the time when the smelt from the recovery boiler is dissolved contains insoluble impurities called dregs, and these contaminations impair the pulp quality, so it is preferable to use a clarified green liquor. As the clarification method of the green liquor, a known clarifier, a filtration type clarification facility or the like can be used alone or in combination. The cooking black liquor is also preferable because it has a sulfidity of about 50 to 70% and can be added to the cooking zone without requiring any special treatment. When the effects of the present invention are compared between the green liquor and the black liquor, the green liquor usually has a higher effect because the alkali concentration and sulfidity are higher.

In the present invention, the temperature in the infiltration zone is 90 to 140 ° C., and the temperature difference from the maximum temperature in the cooking zone is preferably 20 ° C. or more. The residence time of the lignocellulosic material in the infiltration zone is 20 minutes to 2 hours. is there. When the temperature is lower than 90 ° C. or when the residence time is less than 20 minutes, the infiltration and reaction of the cooking chemical in the infiltration zone is insufficient, and the subsequent cooking zone is heated rapidly, resulting in uneven cooking. Not suitable. On the other hand, if the permeation zone temperature is higher than 140 ° C or if the temperature difference from the maximum temperature in the digestion zone is less than 20 ° C, the digestion reaction starts in the permeation zone before the cooking chemical has sufficiently penetrated into the chip. After all, it is not suitable because it results in uneven cooking. In addition, regarding the infiltration zone having a residence time of 2 hours or more, the equipment cost becomes enormous, which is not realistic.

    In the present invention, the liquid ratio in the permeation zone is 2 to 7, preferably 2 to 2.8. If the liquid ratio is less than 2, the cooking chemical solution does not sufficiently penetrate into the raw material, and the raw material hardly settles in the kettle, resulting in uneven cooking, which is not suitable. On the other hand, if the liquid ratio is larger than 7, the recovery load of black liquor becomes too large, which is not suitable. A liquid ratio of 2.8 or less is preferable because the alkali concentration in the permeation zone increases and the delignification in the subsequent cooking zone tends to proceed.

    In the infiltration zone and the cooking zone 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, countercurrent cooking + countercurrent cooking, countercurrent cooking + cocurrent cooking, cocurrent cooking + cocurrent cooking, cocurrent cooking + countercurrent cooking 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 present invention is 3-10. If the liquid ratio is less than 3, uncooked residue is generated due to insufficient penetration of the cooking liquid. On the other hand, if the liquid ratio exceeds 10, the recovery load of black liquor becomes too large, which is not suitable. The maximum temperature in the cooking zone is, for example, 120 to 150 ° C. when hardwood is used as a raw material, and 140 to 170 ° C. when softwood is used as a raw material. In the present invention, the cooking chemical solution added to the cooking zone has a great effect of promoting delignification in the cooking zone, so it is preferably added before reaching the maximum temperature in the cooking zone. 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, the 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 residual alkali amount 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 cellulose damage increases and the digested pulp yield decreases. Absent.

    The conditions in the cleaning zone of the present invention are not particularly limited, but in consideration of increasing the efficiency of cleaning in the pot, it is a preferred embodiment that countercurrent cleaning is performed.

In the present invention, the unbleached pulp can be delignified by a known alkaline oxygen bleaching method after washing, rough selection and selection steps. As the alkaline oxygen bleaching method used in the present invention, a known medium concentration method or a high concentration method can be applied as it is, but a medium concentration method which is currently used at a pulp concentration of 8 to 15% is widely used. preferable.

In the alkali oxygen bleaching method by the medium concentration method, caustic soda or oxidized kraft white liquor can be used as the alkali. As the oxygen gas, oxygen from a cryogenic separation method, PSA (Pressure Swing Adsorption) Oxygen, oxygen from VSA (Vacuum Swing Adsorption), etc. can be used. The oxygen gas and alkali are added to the pulp slurry in a medium-concentration mixer, and after sufficient mixing, they are sent to a reaction tower capable of holding a mixture of pulp, oxygen and alkali for a certain period of time under pressure and delignified. .

The oxygen gas addition rate is 0.5 to 3% by mass per mass of dry pulp, the alkali addition rate is 0.5 to 4% by mass, the reaction temperature is 80 to 120 ° C., the reaction time is 15 to 100 minutes, and the pulp concentration Is 8 to 15%, and other known conditions can be applied. In the present invention, in the alkali oxygen bleaching step, it is a preferred embodiment that the alkali oxygen bleaching is continuously carried out a plurality of times and delignification proceeds as much as possible.

The pulp that has been subjected to alkaline oxygen bleaching is then sent to a washing step, and after washing, it is sent to a multistage bleaching step, where it can be subjected to a multistage bleaching treatment.

The multi-stage bleaching treatment of the present invention is not particularly limited, but is known such as chlorine dioxide (D), alkali (E), oxygen (O), hydrogen peroxide (P), ozone (Z), peracid, etc. It is preferable to combine a bleaching agent and a bleaching assistant. For example, the first stage of the multistage bleaching process uses a chlorine dioxide bleaching stage (D) or an ozone bleaching stage (Z), the second stage is an alkali extraction stage (E), the hydrogen peroxide stage (P), the third stage or later. A bleaching sequence using chlorine dioxide or hydrogen peroxide is preferably used. The number of stages after the third stage is not particularly limited, but considering energy efficiency, productivity, etc., it is preferable to finish in three or four stages in total. Further, a chelating agent treatment stage with ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA) or the like may be inserted during the multistage bleaching treatment.

EXAMPLES The present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited to these examples. Moreover, unless otherwise indicated, the evaluation of the pulp kappa number and the measurement of the cooking yield and the uncooked residue rate were carried out by the following methods. In addition, the addition rate of the chemical | medical agent in an Example and a comparative example shows the mass% per dry chip mass.

1. Kappa number was measured according to JIS P 8211.

2. Measurement of cooking yield The cooking yield was calculated from the following equation.
Cooking yield (%) = completely dried pulp after cooking (g) / input completely dried chips (g) x 100

3. Measurement of undecomposed residue rate The undecomposed residue rate was calculated from the following equation.
Undigested waste rate (%) = Undigested waste (g) / Dried dry chips (g) x 100

Example 1
Digestion was started using an experimental circulating autoclave as 1.25 liters of cooking chemical solution corresponding to 18% effective alkali, 28% sulfidity, and a liquid ratio of 2.5 for 500 g of Radiata Pine chip absolutely dry. . As conditions for the infiltration zone, the temperature is 125 ° C., the residence time of the chip is 40 minutes, and after completion of the infiltration zone, 0.25 liter of cooking chemical corresponding to 2% effective alkali and 35% sulfidity is added at 140 ° C. Added. Kraft cooking was performed at a temperature of 165 ° C., a liquid ratio of 3 and a residence time of 150 minutes as cooking zone conditions, and a pulp was obtained. Table 1 shows the kappa number, cooking yield, and uncooked residue of the obtained pulp.

Example 2
In Example 1, the same operation as in Example 1 was performed except that 0.25 liter of green liquor corresponding to 2% effective alkali and 70% sulfidity was added as the cooking chemical added after the infiltration zone. Table 1 shows the kappa number, cooking yield, and uncooked residue of the obtained pulp.

Example 3
In Example 1, 19 liters as an effective alkali, 1 liter of cooking chemical corresponding to a sulfidity of 28%, and a liquid ratio of 2 was added to a circulating autoclave for experiments, and the cooking was started. The same operation as in Example 1 was performed except that 0.5 liter of cooking black liquor corresponding to 1% effective alkali and 70% sulfidity was added. Table 1 shows the kappa number, cooking yield, and uncooked residue of the obtained pulp.

Example 4
In Example 1, the same operation as in Example 1 was performed except that the amount of cooking chemical added at the start was 1.5 liters and the liquid ratio was 3. Table 1 shows the kappa number, cooking yield, and uncooked residue of the obtained pulp.

Example 5
In Example 1, the same operation as in Example 1 was performed except that the temperature of the permeation zone was 98 ° C. Table 1 shows the kappa number, cooking yield, and uncooked residue of the obtained pulp.

Example 6
In Example 1, the same operation as in Example 1 was performed except that the cooking chemical was added 5 minutes after the start of cooking at 165 ° C. in the cooking zone. Table 1 shows the kappa number, cooking yield, and uncooked residue of the obtained pulp.

Comparative Example 1
In Example 1, as a cooking chemical solution to be added at the start, 1.5 liters of a cooking chemical solution corresponding to an effective alkali of 20%, a sulfidity of 28% and a liquid ratio of 3 is added, and cooking is performed without adding the cooking chemical solution halfway. The same operation as in Example 1 was performed except that. Table 1 shows the kappa number, cooking yield, and uncooked residue of the obtained pulp.

Comparative Example 2
In Example 1, as a cooking chemical liquid to be added at the start, 1.5 liters of a cooking chemical liquid corresponding to 22% effective alkali, 28% sulfidity, and a liquid ratio of 3 was added, and cooking was performed without adding the cooking chemical liquid halfway. The same operation as in Example 1 was performed except that. Table 1 shows the kappa number, cooking yield, and uncooked residue of the obtained pulp.

Comparative Example 3
In Example 1, the same operation as in Example 1 was carried out except that 0.25 liter of cooking chemical corresponding to 2% effective alkali and 28% sulfidization was added as cooking chemical added after the infiltration zone. Table 1 shows the kappa number, cooking yield, and uncooked residue of the obtained pulp.

Comparative Example 4
In Example 1, the same operation as in Example 1 was performed except that 0.25 liter of cooking chemical corresponding to 2% of effective alkali and 30% sulfidity was added as cooking chemical added after the infiltration zone. Table 1 shows the kappa number, cooking yield, and uncooked residue of the obtained pulp.

Comparative Example 5
In Example 2, the same operation as in Example 2 was performed except that 0.25 liter of green liquor corresponding to 2% effective alkali and 70% sulfidity was added at the start of cooking. Table 1 shows the kappa number, cooking yield, and uncooked residue of the obtained pulp.

Comparative Example 6
In Example 1, the same operation as in Example 1 was performed except that the temperature of the permeation zone was 150 ° C. Table 1 shows the kappa number, cooking yield, and uncooked residue of the obtained pulp.

Comparative Example 7
In Example 1, the same operation as in Example 1 was performed except that the temperature of the permeation zone was 80 ° C. Table 1 shows the kappa number, cooking yield, and uncooked residue of the obtained pulp.

As is clear from comparing Example 1 and Comparative Example 1 in Table 1, the cooking yield is maintained under the same effective alkali addition rate condition by adding a cooking chemical having a high degree of sulfidation after the end of the infiltration zone. It can be seen that the pulp kappa number can be greatly reduced. Next, when Examples 1 and 2 are compared, it can be seen that a higher effect can be obtained by using a green liquor having a higher degree of sulfidation as a cooking chemical to be added after the end of the infiltration zone. From the above comparison, it can be seen that the alkali equivalent to 2% can be reduced as an effective alkali necessary for obtaining a pulp having the same kappa number by adding the green liquor in the middle, and the cooking yield is improved by 1.7%. It can also be seen from Example 3 that the effect of the present invention can be obtained by using black liquor. Further, from Example 4, it can be seen that the effect of the present invention can be obtained even when the liquid ratio of the permeation zone is 3, but a larger effect is obtained in Example 1 where the liquid ratio is as low as 2.5.
On the other hand, from Comparative Examples 3 and 4, when white liquor having the same degree of sulfidation is added after the end of the infiltration zone, the pulp kappa number is increased. From Comparative Example 5, when green liquor is added at the start of cooking, the effect of green liquor is increased. It turns out that it cannot obtain enough. Further, when Examples 1 and 5 are compared with Comparative Examples 6 and 7, there is an optimum range for the temperature of the infiltration zone. If the temperature is too low, cooking is difficult to proceed, and if it is too high, before the cooking chemical penetrates. It can be seen that the cooking reaction started, and there was too much uncooked residue. From Examples 1 and 6, a greater effect can be obtained by adding a cooking agent before reaching the cooking temperature before reaching the cooking temperature.
As described above, the present invention can obtain a low-kappa number pulp in a high yield without remodeling the equipment, and is suitable for a lignocellulosic material cooking method using a continuous cooking apparatus.

Continuous digester

Explanation of symbols

A: Osmosis zone B1: Upper cooking zone B2: Lower cooking zone C: Washing zone 1, 2: White liquor introduction pipe 3: Top part of wooden kettle 4: Lower part of wooden kettle 5-7: Strainers 8-11: Pumps 12-14: heater

Claims (6)

A cooking method in which a cooking chemical is added to an infiltration zone and a cooking zone when the lignocellulosic material is digested in a continuous cooking apparatus having an infiltration zone and a cooking zone in order, and the temperature of the infiltration zone is 90 to 140 ° C. A cooking method for lignocellulosic substances, wherein a cooking chemical liquid having a degree of sulfidation of 5% or more higher than that of the cooking chemical liquid added to the permeation zone is used in the cooking zone. 2. The method of cooking lignocellulosic material according to claim 1, wherein green liquor is used as the cooking chemical added to the cooking zone. 2. The method of cooking lignocellulosic material according to claim 1, wherein black liquor is used as the cooking chemical added to the cooking zone. The method for cooking lignocellulosic material according to any one of claims 1 to 3, wherein the cooking zone temperature is 140 ° C to 170 ° C, and the temperature difference between the permeation zone and the cooking zone is 20 ° C or more. 5. The method for cooking lignocellulosic material according to any one of claims 1 to 4, wherein the cooking chemical added to the cooking zone is added before reaching the maximum temperature in the cooking zone. 6. The method of cooking lignocellulosic material according to any one of claims 1 to 5, wherein the permeation zone is performed at a low liquid ratio of 2.0 to 2.8.

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