Method for preparing cotton-stalk biochemomechanical pulp

Abstract

The present application belongs to the technical field of pulping and papermaking, and provides a method for preparing a cotton-stalk biochemomechanical pulp. The present application provides a method for preparing a cotton-stalk biochemomechanical pulp, the method comprising the following steps: cutting cleaned and dried cotton stalks, and then sequentially performing pre-steaming and a screw extrusion fiberization treatment; immersing the defibrated cotton stalks into a buffer solution of cellulase to perform an enzymatic treatment for a set period of time, and then immersing same into a hot alkali solution; sequentially subjecting the cotton stalks, which have been subjected to a hot alkali impregnation treatment, to high-consistency pulp refining and a latency removal treatment, and then subjecting the resulting coarse pulp to a second-stage enzymatic treatment by using a compound enzyme of xylanase and pectinase; and subjecting the pulp, which has been subjected to the second-stage enzymatic treatment, to pulp screening and PFI beating, so as to obtain a cotton-stalk biochemomechanical pulp.

Description

Method for preparing cotton stalk bio-chemi-mechanical pulp

[0001] The present application claims priority to the Chinese patent application No. 202411160323.7, filed on August 22, 2024, and entitled "Method for preparing cotton stalk bio-chemi-mechanical pulp", the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0002] The present application belongs to the technical field of pulping and papermaking, and specifically relates to a method for preparing cotton stalk bio-chemi-mechanical pulp. BACKGROUND

[0003] The statements herein are provided only to further complement the present application and are not necessarily intended to constitute the prior art.

[0004] As the main by-product of cotton, cotton stalks are mainly used for composting, fuel and livestock feeding, while another part is directly burned or discarded in the field, and the value of cotton stalks as a biomass resource has not been fully utilized. Therefore, if cotton stalks are used in the pulp and paper industry, the low-value agricultural waste of cotton stalks can be utilized, which can not only bring certain economic value, but also alleviate the problem of shortage of papermaking raw materials. For cotton stalk raw materials, selecting appropriate pulping and papermaking methods can match the raw materials to produce paper and paperboard suitable for different purposes.

[0005] The pulping method can significantly affect the quality of paper, and is roughly divided into chemical method and mechanical method. The chemical method uses chemicals to treat plant fiber raw materials under certain conditions. The pulp yield of this method is low, and the cellulose content is high. Mechanical pulping is a method that relies on mechanical action to achieve fiber separation. Using this method, most hemicellulose and lignin can be preserved, the yield is high, and the fiber length of the pulp is long. Biotechnology is attracting attention due to its environmental friendliness and can be used to improve the performance of cotton stalk mechanical pulp. There are mainly two kinds of biotechnology used in the pulping process, one is to use fungi for pretreatment, and the other is to directly add biological enzymes during the pulping process. However, the use of fungi treatment in the pulping process has the disadvantages of long processing time and low efficiency.

[0006] In addition to fiber cells, cotton stalks also contain a large number of impurity cells, which will increase the consumption of chemicals during the pulping process and affect the final paper quality. In order to improve the quality of the pulp and remove impurities, a large amount of chemical reagents are often used, which increases the cost and the load of wastewater treatment. SUMMARY

[0007] In view of the deficiencies of the prior art, the purpose of the present application is to provide a method for preparing cotton stalk bio-chemi-mechanical pulp.

[0008] In order to achieve the above-mentioned purpose, the present application is implemented by the following technical solution:

[0009] A method for preparing cotton stalk bio-chemi-mechanical pulp, comprising the following steps:

[0010] After the washed and dried cotton stalks are cut into pieces, the cotton stalks are sequentially subjected to pre-steaming and spiral extrusion defibration treatment;

[0011] The twisted cotton stalks are immersed in a cellulase buffer solution for enzyme treatment, and then subjected to hot alkali immersion treatment in a hot alkali solution;

[0012] The cotton stalks subjected to the hot alkali immersion treatment are sequentially subjected to high consistency refining and latent energy elimination treatment, and the obtained coarse pulp is subjected to two-stage enzyme treatment using a compound enzyme of xylanase and pectinase;

[0013] The pulp subjected to the two-stage enzyme treatment is subjected to screening and PFI beating, and cotton stalk bio-chemi-mechanical pulp is obtained. DETAILED DESCRIPTION

[0014] The application provides a method for preparing cotton stalk bio-chemi-mechanical pulp, comprising the following steps:

[0015] After the washed and dried cotton stalks are cut into pieces, the cotton stalks are sequentially subjected to pre-steaming and spiral extrusion defibration treatment;

[0016] The twisted cotton stalks are immersed in a cellulase buffer solution for enzyme treatment, and then subjected to hot alkali immersion treatment in a hot alkali solution;

[0017] The cotton stalks subjected to the hot alkali immersion treatment are sequentially subjected to high consistency refining and latent energy elimination treatment, and the obtained coarse pulp is subjected to two-stage enzyme treatment using a compound enzyme of xylanase and pectinase;

[0018] The pulp subjected to the two-stage enzyme treatment is subjected to screening and PFI beating, and cotton stalk bio-chemi-mechanical pulp is obtained.

[0019] The application uses cellulase and a compound enzyme to perform two-stage enzyme treatment on cotton stalks, improves the treatment effect of chemicals by using the action of enzymes, promotes the efficiency of refining, and improves the performance of the pulp. The method can improve the utilization rate of cotton stalks, turn waste into treasure, and effectively alleviate the problem of shortage of papermaking raw materials.

[0020] In some embodiments of the application, the length of the cut cotton stalks is 4-6 cm. In an embodiment, the length of the cut cotton stalks is 4 cm, 5 cm or 6 cm.

[0021] In some embodiments of the present application, the solid-liquid ratio of the pre-steaming is 1g:4-8mL, and in an embodiment, the solid-liquid ratio of the pre-steaming is 1g:4mL, 1g:5mL, 1g:6mL, 1g:7mL or 1g:8mL; in some embodiments of the present application, the treatment temperature of the pre-steaming is 90-100℃, and in an embodiment, the treatment temperature of the pre-steaming is 92℃, 95℃ or 98℃; in some embodiments of the present application, the treatment time of the pre-steaming is 30-50min, and in an embodiment, the treatment time of the pre-steaming is 30min, 35min, 40min, 45min or 50min. The present application improves the effect of extruding and separating of cotton stalks by treating the cotton stalks with hot steam.

[0022] In some embodiments of the present application, the compression ratio of the spiral extruding and separating is 1:3-6, and in an embodiment, the compression ratio of the spiral extruding and separating is 1:3, 1:4, 1:5 or 1:6. The present application can fully separate the raw materials by treating the cotton stalks with spiral extruding and separating, which is beneficial to the subsequent treatment.

[0023] In some embodiments of the present application, the buffer solution of the cellulase is a citric acid-sodium citrate solution with a pH value of 4.5-6.5, and in an embodiment, the buffer solution is a citric acid-sodium citrate solution with a pH value of 4.5, 5.5 or 6.5.

[0024] In some embodiments of the present application, the dosage of the cellulase is 5-15U / g relative to the absolute dry material, and in an embodiment, the dosage of the cellulase is 5U / g, 8U / g, 10U / g, 12U / g or 15U / g relative to the absolute dry material.

[0025] In some embodiments of the present application, when the twisted cotton stalks are immersed in the buffer solution of the cellulase for enzyme treatment, the solid-liquid ratio is 1g:4-6mL, the treatment temperature is 45-55℃, and the treatment time is 30-90min; in an embodiment, the solid-liquid ratio is 1g:4mL, 1g:5mL or 1g:6mL, in an embodiment, the treatment temperature is 45℃, 50℃ or 55℃, and in an embodiment, the treatment time is 30min, 40min, 50min, 60min, 70min, 80min or 90min. The present application uses cellulase to treat cotton stalks and hydrolyze part of the amorphous cellulose in the cotton stalks, thereby improving the effect of subsequent hot alkali immersion treatment.

[0026] In some embodiments of the present application, the mass concentration of sodium hydroxide in the hot alkali solution is 2% to 6%, and in an embodiment, the mass concentration of sodium hydroxide in the hot alkali solution is 2%, 3%, 4%, 5% or 6%. The temperature of the hot alkali impregnation treatment is 90 to 100°C, and in an embodiment, the temperature of the hot alkali impregnation treatment is 90°C, 92°C, 95°C, 97°C or 100°C. The time of the hot alkali impregnation treatment is 30 to 50 minutes, and in an embodiment, the time of the hot alkali impregnation treatment is 30 minutes, 35 minutes, 40 minutes, 45 minutes or 50 minutes. The hot alkali impregnation treatment of the cotton stalks is an indispensable step in the chemical mechanical pulp preparation process.

[0027] In some embodiments of the present application, the rotational speed of the high consistency refiner is 3000 to 3500 rpm, and in an embodiment, the rotational speed of the high consistency refiner is 3000 rpm, 3200 rpm or 3500 rpm. In an embodiment of the present application, the refiner is operated in two stages, the gap of the first stage is 0.3 mm, and the gap of the second stage is 0.15 mm.

[0028] In some embodiments of the present application, the ratio of enzyme activity of xylanase to pectinase in the complex enzyme is 1 to 2:1 to 2, and in an embodiment, the ratio of enzyme activity of xylanase to pectinase is 1:1, 1:2, 2:1 or 2:2. The total enzyme dosage of the complex enzyme is 25 to 35 U / g, relative to the absolute dry material, and in an embodiment, the total enzyme dosage of the complex enzyme is 25 U / g, 27 U / g, 33 U / g or 35 U / g.

[0029] In some embodiments of the present application, the buffer solution of the complex enzyme is a citric acid-sodium citrate solution with a pH of 4 to 6, and in an embodiment, the buffer solution of the complex enzyme is a citric acid-sodium citrate solution with a pH of 4, 5 or 6.

[0030] In some embodiments of the present application, the solid-liquid ratio for the two-stage enzyme treatment is 1 g:4 to 6 mL, the treatment temperature is 40 to 60°C, and the treatment time is 30 to 90 minutes, and in an embodiment, the solid-liquid ratio for the two-stage enzyme treatment is 1 g:4 mL, 1 g:5 mL or 1 g:6 mL, the treatment temperature is 40°C, 50°C or 60°C, and the treatment time is 30 minutes, 40 minutes, 50 minutes, 60 minutes, 70 minutes, 80 minutes or 90 minutes. In view of the chemical component structure of the raw material and the high content of pectin in the cotton stalk bark, the pulp is treated with xylanase and pectinase in the present application to significantly improve the pulp quality.

[0031] In some embodiments of the present application, the beating degree of the PFI beating is 40 ± 2°SR, and in an embodiment, the beating degree of the PFI beating is 38°SR, 40°SR or 42°SR.

[0032] The application provides a preparation method of cotton stalk biochemical mechanical pulp, and the cotton stalk is used as raw material and comes from cotton production. The cotton stalk is used for pulp preparation, which can not only alleviate the shortage of papermaking raw materials, but also increase the utilization of cotton stalk and bring good economic value.

[0033] The two-stage enzyme treatment is adopted in the application, the first-stage enzyme treatment adopts cellulase, and the amorphous region of cotton stalk cellulose is hydrolyzed, which can improve the accessibility of the subsequent treatment process, improve the refining efficiency, and improve the pulp quality; the second-stage enzyme treatment adopts compound enzyme, and the compound enzyme is composed of xylanase and pectinase. The xylanase can promote the hydrolysis of hemicellulose, and then promote the removal of part of lignin, and improve the effect of alkali impregnation and refining. The pectinase can degrade pectin in the cotton stalk skin, prevent it from becoming dissolved and colloidal substances, and have adverse effects on the refining quality and the bonding force between pulp fibers. The two enzymes can produce synergistic effect, and more efficiently improve the pulp quality. The two-stage enzyme treatment has different effects, can improve the refining effect, and improve the pulp performance to meet the product requirements.

[0034] The application adopts a treatment method combining biological method and chemical method to invent a cotton stalk biochemical mechanical pulp according to the characteristics of the cotton stalk raw material. The biological method and the chemical method are combined and applied to the preparation process of the cotton stalk mechanical pulp, which not only utilizes the high efficiency, specificity and mildness of the biological enzyme, but also reduces the use of chemical drugs, saves energy consumption, reduces pollution load, and improves the performance of the cotton stalk mechanical pulp.

[0035] It should be pointed out that the following detailed description is exemplary and is intended to provide further explanation of the application. Unless otherwise specified, all technical and scientific terms used in the application have the same meaning as generally understood by those skilled in the art to which the application belongs.

[0036] The application will be further described below in combination with examples.

[0037] The materials involved in the examples can be purchased on the market, and all biological enzymes come from Shandong Longkete Enzyme Preparation Co., Ltd. The enzyme activity of xylanase is 50000 U / mL, the enzyme activity of pectinase is 90000 U / mL, and the enzyme activity of cellulase is 80000 U / mL.

[0038] The cotton stalks used in the following examples are cotton stalks from Dezhou, Shandong, with cellulose content of 42.37%, hemicellulose content of 17.41%, lignin content of 21.87%, and ash content of 2.06%.

[0039] Example 1

[0040] (1) The washed and dried cotton stalks were cut into cotton stalk segments with a length of 4-6 cm.

[0041] (2) The cotton stalks obtained in step (1) were subjected to pre-steaming treatment, the solid-liquid ratio of pre-steaming was 1:5 g / mL, the treatment temperature was 95°C, and the treatment time was 40 min.

[0042] (3) The cotton stalks obtained in step (2) were subjected to spiral extrusion and filament separation treatment before cooling, the compression ratio was 1:4.

[0043] (4) The cotton stalks obtained in step (3) were subjected to enzyme treatment in a buffer solution containing cellulase, the enzyme activity of cellulase was 5 U / g, the buffer solution was a citric acid-sodium citrate solution with a pH value of 4.5, the solid-liquid ratio was 1:5 g / mL, the treatment temperature was 50°C, and the treatment time was 60 min.

[0044] (5) The cotton stalks obtained in step (4) were subjected to hot alkali immersion treatment in hot lye, the amount of sodium hydroxide was 4%, the solid-liquid ratio of immersion was 1:4 g / mL, the treatment temperature was 95°C, and the treatment time was 40 min.

[0045] (6) The cotton stalks obtained in step (5) were subjected to high-concentration grinding, the rotation speed was 3500 rpm, and the gap of two-stage grinding was 0.3 mm and 0.15 mm. After grinding, the latent energy was eliminated.

[0046] (7) The pulp obtained in step (6) was subjected to two-stage enzyme treatment in a buffer solution containing a compound enzyme, the ratio of the compound enzyme (enzyme activity) used was xylanase: pectinase = 2:1, the total enzyme activity was 30 U / g, the buffer solution was a citric acid-sodium citrate solution with a pH value of 6.0, the solid-liquid ratio was 1:5 g / mL, the treatment temperature was 50°C, and the treatment time was 60 min.

[0047] (8) The pulp obtained in step (7) was screened and then subjected to PFI beating to increase the beating degree to 40±2°SR, and the sheet was made.

[0048] Results: The yield of the obtained pulp was 76.89%, the tear index of the obtained sheet was 6.22 mN·m / g, the tensile index was 33.43 N·m / g, and the ring crush index was 11.12 N·m / g. 2

[0049] Example 2

[0050] The difference from Example 1 is step (4), and the other steps are the same as those in Example 1:

[0051] ​(4) The twisted cotton stalks obtained in step (3) are subjected to enzyme treatment in a buffer solution containing cellulase, the enzyme activity of the cellulase is 10 U / g, the buffer solution is a citric acid-sodium citrate solution with a pH value of 6.0, the solid-liquid ratio is 1:6 g / mL, the treatment temperature is 55°C, and the treatment time is 75 min.

[0052] Results: The yield of the obtained pulp is 76.54%, the tear index of the obtained sheet paper is 6.09 mN·m / g, the tensile index is 30.88 N·m / g, and the ring crush index is 10.87 N·m / g. 2

[0053] Example 3

[0054] The difference from Example 1 is step (4), and the other steps are the same as those in Example 1.

[0055] (4) The twisted cotton stalks obtained in step (3) are subjected to enzyme treatment in a buffer solution containing cellulase, the enzyme activity of the cellulase is 15 U / g, the buffer solution is a citric acid-sodium citrate solution with a pH value of 5.0, the solid-liquid ratio is 1:4 g / mL, the treatment temperature is 45°C, and the treatment time is 30 min.

[0056] Results: The yield of the obtained pulp is 77.24%, the tear index of the obtained sheet paper is 6.19 mN·m / g, the tensile index is 32.92 N·m / g, and the ring crush index is 10.96 N·m / g. 2

[0057] Example 4

[0058] The difference from Example 1 is step (7), and the other steps are the same as those in Example 1.

[0059] (7) The pulp obtained in step (6) is subjected to two-stage enzyme treatment in a buffer solution containing a compound enzyme, the proportion (enzyme activity) of the compound enzyme used is xylanase: pectinase = 1:2, the total enzyme activity is 30 U / g, the buffer solution is a citric acid-sodium citrate solution with a pH value of 5.0, the solid-liquid ratio is 1:6 g / mL, the treatment temperature is 55°C, and the treatment time is 90 min.

[0060] Results: The yield of the obtained pulp is 77.04%, the tear index of the obtained sheet paper is 6.14 mN·m / g, the tensile index is 32.73 N·m / g, and the ring crush index is 10.92 N·m / g. 2

[0061] Example 5

[0062] The difference from Example 1 is step (7), and the other steps are the same as those in Example 1.

[0063] ​​​(7) The slurry obtained in step (6) is subjected to two-stage enzyme treatment in a buffer solution containing a complex enzyme, the ratio of the complex enzyme (enzyme activity) used is xylanase: pectinase = 1:1, the total enzyme activity is 30 U / g, the buffer solution is a citric acid-sodium citrate solution with a pH value of 4.5, the solid-liquid ratio is 1:4 g / mL, the treatment temperature is 45°C, and the treatment time is 45 min.

[0064] Results: The yield of the obtained pulp is 77.04%, the tear index of the obtained sheet paper is 6.06 mN·m / g, the tensile index is 30.92 N·m / g, and the ring crush index is 10.61 N·m / g. 2

[0065] Example 6

[0066] The difference from Example 1 is in step (7), and the other steps are the same as those in Example 5.

[0067] (7) The slurry obtained in step (6) is subjected to two-stage enzyme treatment in a buffer solution containing a complex enzyme, the ratio of the complex enzyme (enzyme activity) used is xylanase: pectinase = 1:1, the total enzyme activity is 30 U / g, the buffer solution is a citric acid-sodium citrate solution with a pH value of 4.5, the solid-liquid ratio is 1:4 g / mL, the treatment temperature is 45°C, and the treatment time is 45 min.

[0068] Results: The yield of the obtained pulp is 77.01%, the tear index of the obtained sheet paper is 6.2 mN·m / g, the tensile index is 32.98 N·m / g, and the ring crush index is 11.04 N·m / g. 2

[0069] Comparative Example 1

[0070] The difference from Example 1 is that cellulase is omitted in step (1) and xylanase and pectinase are omitted in step (7), which are as follows:

[0071] (4) The rubbed cotton stalks obtained in step (3) are treated in a buffer solution, the buffer solution is a citric acid-sodium citrate solution with a pH value of 4.5, the solid-liquid ratio is 1:5 g / mL, the treatment temperature is 50°C, and the treatment time is 60 min.

[0072] (7) The slurry obtained in step (6) is subjected to two-stage enzyme treatment in a buffer solution containing a complex enzyme, the ratio of the complex enzyme (enzyme activity) used is xylanase: pectinase = 1:1, the total enzyme activity is 30 U / g, the buffer solution is a citric acid-sodium citrate solution with a pH value of 4.5, the solid-liquid ratio is 1:4 g / mL, the treatment temperature is 45°C, and the treatment time is 45 min.

[0073] Results: The yield of the obtained pulp is 81.17%, the tear index of the obtained sheet paper is 5.47 mN·m / g, the tensile index is 28.62 N·m / g, and the ring crush index is 10.61 N·m / g. 2 ​​Tensile index was 27.11 N-m / g, and ring pressure index was 9.44 N-m / g.

[0074] Comparative Example 2

[0075] The difference from Example 1 was in step (7), and the other steps were the same as in Example 1, and were as follows:

[0076] (7) The slurry obtained in step (6) was treated with a buffer solution, the buffer solution was a citric acid-sodium citrate solution with a pH value of 4.5, the solid-liquid ratio was 1:4 g / mL, the treatment temperature was 45°C, and the treatment time was 45 min.

[0077] Result: The yield of the obtained pulp was 79.65%, the tear index of the obtained sheet paper was 5.78 mN-m / g, the tensile index was 29.65 N-m / g, and the ring pressure index was 9.97 N-m / g. 2

[0078] Comparative Example 3

[0079] The difference from Example 5 was in step (4), and the other steps were the same as in Example 5, and were as follows:

[0080] (4) The rubbed cotton stalks obtained in step (3) were treated in a buffer solution, the buffer solution was a citric acid-sodium citrate solution with a pH value of 4.5, the solid-liquid ratio was 1:5 g / mL, the treatment temperature was 50°C, and the treatment time was 60 min.

[0081] Result: The yield of the obtained pulp was 80.78%, the tear index of the obtained sheet paper was 5.93 mN-m / g, the tensile index was 30.03 N-m / g, and the ring pressure index was 10.17 N-m / g. 2

[0082] Comparative Example 4

[0083] The difference from Example 5 was in step (7), and the other steps were the same as in Example 1.

[0084] (7) The slurry obtained in step (6) was treated with a two-stage enzyme treatment in a buffer solution containing xylanase, the enzyme activity of the xylanase used was 20 U / g, the buffer solution was a citric acid-sodium citrate solution with a pH value of 4.5, the solid-liquid ratio was 1:4 g / mL, the treatment temperature was 45°C, and the treatment time was 45 min.

[0085] Result: The yield of the obtained pulp was 79.79%, the tear index of the obtained sheet paper was 5.98 mN-m / g, the tensile index was 30.65 N-m / g, and the ring pressure index was 10.37 N-m / g. 2

[0086] ​​​Comparative Example 5

[0087] The difference from Example 5 is that Example (7) is different, and the rest is the same as Example 1.

[0088] (7) The slurry obtained in step (6) is subjected to two-stage enzyme treatment in a buffer solution containing pectinase, the enzyme activity of the pectinase used is 20 U / g, the buffer solution is a citric acid-sodium citrate solution with a pH value of 4.5, the solid-liquid ratio is 1:4, the treatment temperature is 45°C, and the treatment time is 45 min.

[0089] Results: The yield of the obtained pulp is 79.95%, the tear index of the obtained sheet paper is 5.88 mN·m / g, the tensile index is 30.28 N·m / g, and the ring crush index is 10.25 N·m / g. 2

[0090] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Those skilled in the art can make various modifications and changes to the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.​

Claims

1. A process for the production of cotton linter biogemcmechanical pulp, characterized by: It comprises the following steps: The washed and dried cotton stalks are cut into pieces, and then pre-steamed and treated by screw extrusion; The cotton stalks treated by screw extrusion are immersed in a buffer solution of cellulase for enzyme treatment, and then immersed in a hot alkali solution for hot alkali treatment; The cotton stalks treated by hot alkali treatment are sequentially subjected to high consistency refining and latent treatment, and then the obtained coarse pulp is subjected to two-stage enzyme treatment by using a compound enzyme of xylanase and pectinase. The pulp treated by two-stage enzyme treatment is screened and PFI beaten to obtain cotton stalk bio-chemi-mechanical pulp.

2. The method of claim 1, wherein: The length of the cut cotton stalks is 4-6 cm.

3. The method of claim 1, wherein: The pre-steaming parameters are as follows: solid-liquid ratio 1g:4-8mL, treatment temperature 90-100℃, and treatment time 30-50min.

4. The method of claim 1, wherein: The compression ratio of the screw extrusion is 1:3-6.

5. The method of claim 1, wherein: The compression ratio of the screw extrusion is 1:

4.

6. The method of claim 1, wherein: The buffer solution is a citric acid-sodium citrate solution with pH 4.5-6.

5.

7. The method of claim 1, wherein: The dosage of cellulase is 5-15U / g, relative to the absolute dry material.

8. The method of claim 1, wherein: When the cotton stalks treated by screw extrusion are immersed in a buffer solution of cellulase for enzyme treatment, the solid-liquid ratio is 1g:4-6mL, the treatment temperature is 45-55℃, and the treatment time is 30-90min.

9. The method of claim 1, wherein: The mass concentration of sodium hydroxide in the hot alkali solution is 2%-6%.

10. The method of claim 1, wherein: The mass concentration of sodium hydroxide in the hot alkali solution is 3-5%.

11. The method of claim 1, wherein: The temperature of the hot alkali treatment is 90-100℃, and the time of the hot alkali treatment is 30-50min.

12. The method of claim 1, wherein: The temperature of the hot alkali treatment is 92-97℃, and the time of the hot alkali treatment is 35-45min.

13. The method of claim 1, wherein: The rotation speed of the high consistency refining is 3000-3500rpm, and the refining is performed in two stages, with a gap of 0.3mm in the first stage and a gap of 0.15mm in the second stage.

14. The method of claim 1, wherein: In the compound enzyme, the enzyme activity ratio of xylanase to pectinase is 1-2:1-2, and the total enzyme dosage of the compound enzyme is 25-35U / g, relative to the absolute dry material.

15. The method of claim 1, wherein: The total enzyme dosage of the compound enzyme is 27-33U / g.

16. The method of claim 1, wherein: The buffer solution of the compound enzyme is a citric acid-sodium citrate solution with pH 4-6.

17. The method of claim 1, wherein: The solid-liquid ratio for two-stage enzyme treatment is 1g:4-6mL, the treatment temperature is 40-60℃, and the treatment time is 30-90min.

18. The method of claim 1, wherein: The beating degree of the PFI beating is 40±2°SR.