A combined process for the preparation of microcrystalline cellulose and high polymerization degree cellulose

Abstract

The present application is suitable for the technical field of microcrystalline cellulose and high polymerization degree cellulose preparation, and provides a combined preparation process of microcrystalline cellulose and high polymerization degree cellulose, which comprises the following steps: step (1), pretreatment; step (2), cooking; step (3), black liquor extraction; step (4), bleaching and pulp washing; step (5), grading; step (6), mixing, reaction and dilution; step (7), washing; step (8), pressure filtration and concentration; and step (9), drying, crushing and screening. The present application grades the bleached cellulose by using a grading screen, separates the component consisting of short fibers and miscellaneous cells which can produce microcrystalline cellulose and the single high polymerization degree cellulose component, then performs conventional acid hydrolysis and multi-stage countercurrent washing, concentration, drying and crushing on the short fiber and miscellaneous cell mixed component, and then grades the dried microcrystalline cellulose product by using various dry grading equipment, separates the acid hydrolysis microcrystalline cellulose product consisting of short fiber component for sale, and separates the acid hydrolysis microcrystalline cellulose component consisting of miscellaneous cell component for sale as a byproduct of special microcrystalline cellulose.

Description

Technical Field

[0001] This invention relates to the field of microcrystalline cellulose and high-polymerization degree cellulose preparation technology, specifically a combined preparation process of microcrystalline cellulose and high-polymerization degree cellulose. Background Technology

[0002] Microcrystalline cellulose (MCC) is a commonly used food additive and pharmaceutical excipient. It is a non-crystalline cellulose typically produced through chemical treatments such as acid hydrolysis of plant cellulose. Microcrystalline cellulose possesses excellent water absorption, stability, and biocompatibility, and is widely used in pharmaceuticals, food, and cosmetics. In the pharmaceutical industry, microcrystalline cellulose is often used as a filler, disintegrant, and anti-sticking agent in the manufacture of tablets and capsules. In the food industry, it can be used as a thickener, stabilizer, and emulsifier to help improve the texture and mouthfeel of food. Furthermore, microcrystalline cellulose is also used in health supplements and nutritional supplements.

[0003] High-polymer cellulose refers to cellulose-like substances with high molecular weight and degree of polymerization. Cellulose is a polysaccharide composed of glucose units linked by β-1,4 glycosidic bonds and is an important component of plant cell walls. High-polymer cellulose typically exhibits excellent mechanical strength and stability and is widely used in various fields.

[0004] Currently, various plant fiber raw materials are either used to produce single microcrystalline cellulose products or single high-polymerization cellulose products. This traditional preparation method cannot fully utilize the optimal characteristics of both long and short fiber components, resulting in resource waste and increased environmental pollution.

[0005] Based on the above problems, this application proposes a combined preparation process of microcrystalline cellulose and high-polymerization cellulose. Summary of the Invention

[0006] The purpose of this invention is to provide a combined preparation process for microcrystalline cellulose and high-polymerization-degree cellulose, in order to solve the problem that in the prior art, various plant fiber raw materials are either used to produce a single microcrystalline cellulose product or a single high-polymerization-degree cellulose product, which cannot fully utilize the optimal characteristics of the long and short fiber components, thus causing waste of resources and increased environmental pollution load.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a combined preparation process of microcrystalline cellulose and high-polymerization-degree cellulose, comprising the following steps: Step (1), Preprocessing: Select plant fiber raw materials, slice them, and remove impurities; Step (2), steaming: Weigh out the plant fiber raw material granules, add them to the cooking container, add an appropriate amount of cooking agent, and cook. Step (3), black liquor extraction: The cooked pulp is extracted in stages using hot water and then concentrated to a dryness of 5% to 35%. Step (4), bleaching and washing: First, dilute the pulp with waste liquor containing HCl to adjust the pH; then add ClO2 solution, heat to 40-85℃, and bleach for 30-150 minutes; then wash the pulp, and the pulp washing waste liquor is discharged after wastewater treatment to meet the standards. Step (5), grading: Bleached pulp is separated into long fibers and short fibers containing impurities using a grading sieve. The long fibers are used to produce high-polymer cellulose products, while the short fibers containing impurities are concentrated. Step (6), mixing, reacting, and diluting: The concentrated short fiber slurry containing impurities was transferred to a mixer, dilute HCl was added, and the mixture was stirred. The mixture was then transferred to a reaction vessel and reacted at 35℃-115℃ to maintain the concentration of the short fiber solution containing impurities at 4%-20%. Finally, it was transferred to a dilution tank and diluted to 0.5%-4%. Step (7), washing: The pulp is transferred to a multi-stage countercurrent washing equipment for high-speed countercurrent washing to obtain pulp. Part of the washing wastewater is transferred to the No. 1 transition water tank and then pumped to the mixer and dilution tank. The other part is transferred to the No. 2 transition water tank and then pumped to the pulping and bleaching system to adjust the pH value of the ClO2 bleaching pulp. Step (8), pressure filtration and concentration: The washed pulp after bleaching is transferred to a plate and frame filter press for dewatering to 40% to 50% dryness. The wastewater is transferred to the No. 3 transition water tank and then pumped to a multi-stage countercurrent double-net pulp washer. The dewatered semi-dry pulp is transferred to a high-consistency pulp tower. Step (9), drying, pulverizing and sieving: The semi-dry pulp is transferred to a drying system for drying, then pulverized by a pulverizer. After pulverization, it is transferred to a screening machine to separate high-crystallinity MCC containing short fiber components and low-crystallinity special MCC containing impurity cell components.

[0008] Preferably, the cooking temperature in step (2) is 110-180℃, the pressure is 0.2-2.0MPa, and the cooking time is 1-3h; The cooking agent in step (2) is a conventional alkali, acid, or oxygen-alkali mixture.

[0009] Preferably, the grading screen in step (5) is a micro-slit screen with a screen drum and a screen gap of 0.05 to 0.079 mm.

[0010] Preferably, the grading sieve in step (5) is a microporous sieve with a sieve drum hole diameter of 0.5 to 0.8 mm.

[0011] Preferably, the linear velocity of the grading screen rotor in step (5) is ≤11.5m / s; and the screening concentration of the grading screen in step (5) is ≤0.6%.

[0012] Preferably, the grading screen in step (5) is a multi-stage (segment) grading screen.

[0013] Preferably, the rotational speed of the horizontal reactor in step (6) is ≤1 rpm; In step (7), the double-net washer speed is ≥50m / min, the net concentration is ≤1%, and the net dry fiber content is ≤100g / m. 2 .

[0014] Preferably, in step (5), the grading sieve has a grading ratio of 30% to 90% for long fibers and a grading ratio of 70% to 10% for short fibers containing impurities.

[0015] Preferably, the amount of dilute HCl added to the mixer in step (6) is exactly the amount of HCl remaining after acid hydrolysis that meets the amount of HCl required to adjust the pH value of the pulp for pulp bleaching in step (4).

[0016] Preferably, in order to ensure that no excess HCl is left in step (6) and that fresh HCl is not added in the pulp bleaching process in step (4), it is necessary to control the pulp concentration before pulp bleaching in step (4); and to control the ratio of long and short fibers in step (5) so that the waste liquid generated from washing pulp with acid hydrolysis of microcrystalline cellulose just meets the requirements of ClO2 bleaching concentration after all the waste liquid is recycled.

[0017] The present invention has at least the following beneficial effects: This invention provides a combined preparation process for microcrystalline cellulose and high-polymerization cellulose. The process utilizes centrifugal sedimentation or a classifying sieve to classify the acid-hydrolyzed cellulose, separating the microcrystalline cellulose component derived from the acid hydrolysis of impurities and low-polymerization, high-crystallinity components. After concentration and drying, the dried microcrystalline cellulose product is further classified using various dry classification equipment, separating and selling a high-crystallinity, high-quality microcrystalline cellulose product composed of short fibers. The microcrystalline cellulose component composed of impurities is sold as a byproduct of specialty microcrystalline cellulose. This method fully utilizes the optimal characteristics of both long and short fiber components, avoiding resource waste and increased environmental pollution. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the process of the present invention. Detailed Implementation

[0019] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] This invention provides a combined preparation process for microcrystalline cellulose and high-polymerization-degree cellulose. Utilizing plant fiber raw materials, the process first separates the cellulose, removing as many remaining components as possible, particularly lignin. Then, the cellulose and a component containing a small amount of hemicellulose are graded into long and short fibers. The long fiber component is used to produce various cellulose products requiring high polymerization degrees using existing technology; the short fiber component is used to produce microcrystalline cellulose products that require high crystallinity but very low polymerization degrees.

[0021] Based on the above technical solutions, the present invention provides the following embodiments: Example 1 This embodiment provides a combined preparation process of microcrystalline cellulose and high-polymerization-degree cellulose, including the following steps: Step (1), Preprocessing: Select plant fiber raw materials, wash and dry them, then grind them to obtain plant fiber raw material granules; Step (2), steaming: Weigh out the plant fiber raw material granules, add them to the cooking container, add an appropriate amount of cooking agent, and cook at a temperature of 110℃ and a pressure of 0.2MPa for 1 hour. The cooking agent is a conventional alkali, acid, or oxygen-alkali mixture. Step (3), black liquor extraction: The cooked pulp was extracted in stages using hot water and then concentrated to a dryness of 5%. Step (4), bleaching and washing: First, dilute the pulp with waste liquor containing HCl to adjust the pH; then add ClO2 solution, heat to 40℃, and bleach for 30 minutes; then wash the pulp, and the pulp washing waste liquor is discharged after wastewater treatment to meet the standards. Step (5), grading: Bleached pulp is separated into long fibers and short fibers containing impurities using a grading sieve. The long fibers are used to produce high-polymer cellulose products, while the short fibers containing impurities are concentrated. The grading sieve is a micro-slit sieve with a sieve drum slit size of 0.05 mm; the grading sieve is a microporous sieve with a sieve drum aperture diameter of 0.5 mm. The rotor linear velocity of the grading screen is ≤11.5m / s; the screening concentration of the grading screen is ≤0.6%; the equipment type of the grading screen is multi-stage (segment) grading screening; The grading percentage of long fibers is 30%; the grading percentage of short fibers containing impurities is 70%. Step (6), mixing, reacting, and diluting: The concentrated short fiber slurry containing impurities was transferred to a mixer, dilute HCl was added, and the mixture was stirred. The mixture was then transferred to a reaction vessel, and the reaction was carried out at 35°C with a rotation speed of ≤1 rpm, maintaining the concentration of the short fiber solution containing impurities at 4%. Finally, the solution was transferred to a dilution tank and diluted to 0.5%. Step (7), washing: Transfer to a multi-stage countercurrent washing equipment; the screen speed of the double-screen washer is ≥50m / min; the screen concentration is ≤1%; and the dry basis fiber weight is ≤100g / m³. 2 The mixture undergoes high-speed countercurrent washing to obtain slurry. Part of the washing wastewater is transferred to the No. 1 transition water tank and then pumped to the mixer and dilution tank. The other part is transferred to the No. 2 transition water tank and then pumped to the pulping and bleaching system to adjust the pH value of the ClO2 bleaching slurry. Step (8), pressure filtration and concentration: The washed pulp after bleaching is transferred to a plate and frame filter press for dewatering to 40% dryness. The wastewater is transferred to the No. 3 transition water tank and then pumped to a multi-stage countercurrent double-net pulp washer. The dewatered semi-dry pulp is transferred to a high-consistency pulp tower. Step (9), drying, pulverizing and sieving: The semi-dry pulp is transferred to a drying system for drying, then pulverized by a pulverizer. After pulverization, it is transferred to a screening machine to separate high-crystallinity MCC containing short fiber components and low-crystallinity special MCC containing impurity cell components.

[0022] Example 2 This embodiment provides a combined preparation process of microcrystalline cellulose and high-polymerization-degree cellulose, including the following steps: Step (1), Preprocessing: Select plant fiber raw materials, wash and dry them, then grind them to obtain plant fiber raw material granules; Step (2), steaming: Weigh out the plant fiber raw material granules, add them to the cooking container, add an appropriate amount of cooking agent, and cook at a temperature of 160℃ and a pressure of 1.0MPa for 2 hours. The cooking agent is a conventional alkali, acid, or oxygen-alkali mixture. Step (3), black liquor extraction: The cooked pulp was extracted in stages using hot water and then concentrated to 20% dryness. Step (4), bleaching and washing: First, dilute the pulp with waste liquor containing HCl to adjust the pH; then add ClO2 solution, heat to 65℃, and bleach for 100 minutes; then wash the pulp, and the pulp washing waste liquor is discharged after wastewater treatment to meet the standards. Step (5), grading: Bleached pulp is separated into long fibers and short fibers containing impurities using a grading sieve. The long fibers are used to produce high-polymer cellulose products, while the short fibers containing impurities are concentrated. The grading sieve is a micro-slit sieve with a sieve drum slit size of 0.06 mm; the grading sieve is a microporous sieve with a sieve drum aperture diameter of 0.6 mm. The rotor linear velocity of the grading screen is ≤11.5m / s; the screening concentration of the grading screen is ≤0.6%; the equipment type of the grading screen is multi-stage (segment) grading screening; The grading percentage of long fibers was 60%; the grading percentage of short fibers containing impurities was 50%. Step (6), mixing, reacting, and diluting: The concentrated short fiber slurry containing impurities was transferred to a mixer, dilute HCl was added, and the mixture was stirred. The mixture was then transferred to a reaction vessel, and the reaction was carried out at 75°C with a rotation speed of ≤1 rpm, maintaining the concentration of the short fiber solution containing impurities at 10%. Finally, the solution was transferred to a dilution tank and diluted to 1.5%. Step (7), washing: Transfer to a multi-stage countercurrent washing equipment; the screen speed of the double-screen washer is ≥50m / min; the screen concentration is ≤1%; and the dry basis fiber weight is ≤100g / m³. 2 The mixture undergoes high-speed countercurrent washing to obtain slurry. Part of the washing wastewater is transferred to the No. 1 transition water tank and then pumped to the mixer and dilution tank. The other part is transferred to the No. 2 transition water tank and then pumped to the pulping and bleaching system to adjust the pH value of the ClO2 bleaching slurry. Step (8), pressure filtration and concentration: The washed pulp after bleaching is transferred to a plate and frame filter press for dewatering to 45% dryness. The wastewater is transferred to the No. 3 transition water tank and then pumped to a multi-stage countercurrent double-net pulp washer. The dewatered semi-dry pulp is transferred to a high-consistency pulp tower. Step (9), drying, pulverizing and sieving: The semi-dry pulp is transferred to a drying system for drying, then pulverized by a pulverizer. After pulverization, it is transferred to a screening machine to separate high-crystallinity MCC containing short fiber components and low-crystallinity special MCC containing impurity cell components.

[0023] Example 3 This embodiment provides a combined preparation process of microcrystalline cellulose and high-polymerization-degree cellulose, including the following steps: Step (1), Preprocessing: Select plant fiber raw materials, wash and dry them, then grind them to obtain plant fiber raw material granules; Step (2), steaming: Weigh out the plant fiber raw material granules, add them to the cooking container, add an appropriate amount of cooking agent, and cook at a temperature of 180℃ and a pressure of 2.0MPa for 3 hours. The cooking agent is a conventional alkali, acid, or oxygen-alkali mixture. Step (3), black liquor extraction: The cooked pulp was extracted in stages using hot water and then concentrated to a dryness of 35%. Step (4), bleaching and washing: First, dilute the pulp with waste liquor containing HCl to adjust the pH; then add ClO2 solution, heat to 85℃, and bleach for 150 minutes; then wash the pulp, and the pulp washing waste liquor is discharged after wastewater treatment to meet the standards. Step (5), grading: Bleached pulp is separated into long fibers and short fibers containing impurities using a grading sieve. The long fibers are used to produce high-polymer cellulose products, while the short fibers containing impurities are concentrated. The grading sieve is a micro-slit sieve with a sieve drum slit size of 0.079 mm; the grading sieve is a microporous sieve with a sieve drum aperture diameter of 0.8 mm. The rotor linear velocity of the grading screen is ≤11.5m / s; the screening concentration of the grading screen is ≤0.6%; the equipment type of the grading screen is multi-stage (segment) grading screening; The grading percentage of long fibers was 90%; the grading percentage of short fibers containing impurities was 10%. Step (6), mixing, reacting, and diluting: The concentrated short fiber slurry containing impurities was transferred to a mixer, dilute HCl was added, and the mixture was stirred. The mixture was then transferred to a reaction vessel, and the reaction was carried out at 115°C with a rotation speed of ≤1 rpm, maintaining the concentration of the short fiber solution containing impurities at 20%. Finally, the solution was transferred to a dilution tank and diluted to 4%. Step (7), washing: Transfer to a multi-stage countercurrent washing equipment; the screen speed of the double-screen washer is ≥50m / min; the screen concentration is ≤1%; and the dry basis fiber weight is ≤100g / m³. 2 The mixture undergoes high-speed countercurrent washing to obtain slurry. Part of the washing wastewater is transferred to the No. 1 transition water tank and then pumped to the mixer and dilution tank. The other part is transferred to the No. 2 transition water tank and then pumped to the pulping and bleaching system to adjust the pH value of the ClO2 bleaching slurry. Step (8), pressure filtration and concentration: The washed pulp after bleaching is transferred to a plate and frame filter press for dewatering to 50% dryness. The wastewater is transferred to the No. 3 transition water tank and then pumped to a multi-stage countercurrent double-net pulp washer. The dewatered semi-dry pulp is transferred to a high-consistency pulp tower. Step (9), drying, pulverizing and sieving: The semi-dry pulp is transferred to a drying system for drying, then pulverized by a pulverizer. After pulverization, it is transferred to a screening machine to separate high-crystallinity MCC containing short fiber components and low-crystallinity special MCC containing impurity cell components.

[0024] In the above examples 1-3, the amount of dilute HCl added to the mixer in step (6) is exactly the amount of residual HCl after acid hydrolysis, which meets the amount of HCl required to adjust the pH value of the pulp for pulp bleaching in step (4).

[0025] To ensure that no excess HCl is left in step (6) and that fresh HCl is not added in the pulp bleaching process in step (4), it is necessary to control the pulp concentration before pulp bleaching in step (4); and to control the ratio of long and short fibers in step (5) so that the waste liquor generated from washing pulp with acid hydrolysis of microcrystalline cellulose just meets the requirements of ClO2 bleaching concentration after all the waste liquor generated from washing pulp with acid hydrolysis of microcrystalline cellulose is reused.

[0026] The combined preparation process of microcrystalline cellulose and high-polymerization-degree cellulose provided by this invention has the following outstanding advantages: 1. The cellulose in the long fiber component has a higher degree of polymerization, so this type of cellulose can be used to produce high-strength artificial pulp, high-toughness packaging materials, specialty cellulose, kraft paper, cellophane, etc.

[0027] 2. The cellulose in short fiber components has a relatively low degree of polymerization, making it more suitable for producing microcrystalline cellulose products with low polymerization requirements. This means that microcrystalline cellulose products meeting quality requirements can be produced using less inorganic acid and at a lower hydrolysis temperature. Furthermore, experiments have shown that because the cellulose in short fiber components often has a higher degree of crystallinity than that in long fiber components, the resulting products are of better quality.

[0028] 3. Experiments also demonstrated that the content of hemicellulose and lignin in the short fiber component is lower than that in the long fiber component, meaning the α-cellulose content is higher, which improves product quality. However, for certain cellulose products requiring high polymerization degrees, the quality of this component is not very sensitive to the lignin content, especially the hemicellulose content, within a certain range. Furthermore, experiments showed that the crystallinity of the short fiber component containing impurities, as determined by grading and screening, is significantly higher than that of the long fiber component when pulping with high-quality plant fiber raw materials. Even when pulping with straw-based plant fiber raw materials with high impurity content, although it contains a large proportion of impurity components with low crystallinity, its crystallinity is not much lower than that of the long fiber component. Especially in subsequent processes, conventional techniques can be used to further grade this low-crystallinity, acid-hydrolyzed microcrystalline cellulose, mainly composed of impurities, for sale as specialty microcrystalline cellulose, thus solving this drawback. Because these microcrystalline cellulose products, whose main component is heterocellular material, often possess other significant advantages that highly crystalline microcrystalline cellulose products lack, they can be marketed as specialty microcrystalline cellulose products. Of course, the grading of microcrystalline cellulose after acid hydrolysis can also be performed using conventional wet grading methods. This is also within the scope of this technical protection.

[0029] The microcrystalline cellulose production process of this invention generally uses inorganic acids to hydrolyze the amorphous zone of cellulose. The acidic wastewater generated in this process can be directly and entirely reused in the pulping and bleaching stage of the plant fiber raw materials without further water treatment before being discharged into the environment, thus further reducing production costs and pollution load. Moreover, if the long and short fiber components are properly matched, the pulping and bleaching stage (generally using multi-stage ClO2 bleaching) can completely recover 100% of the waste acid and waste heat from the hydrolysis of microcrystalline cellulose for pH adjustment before ClO2 bleaching of the pulp.

[0030] The grading of long and short fibers is equivalent to purifying the long fiber component, which greatly improves the product's various strength properties, including uniformity, and significantly increases the product's added value.

[0031] For the washing of microcrystalline cellulose products after acid hydrolysis, a multi-stage high-speed countercurrent washing system is used, instead of the traditional multiple-unit series washing of plate and frame filter presses, or the traditional horizontal belt washer or multiple-unit series of vacuum washers. This effectively removes the least crystallized impurity cells from the short fiber component, solving this unique weakness in the short fiber component. These impurity cells are also a weakness in the long fiber component.

[0032] To ensure that only impurities are selectively washed away during high-speed washing, two necessary auxiliary measures are taken: first, to use the gentlest stirring method during hydrolysis to prevent the formation of fiber fragments; and second, to prevent the hydrolyzed pulp from being pumped or transported by other means with strong stirring, because the impeller of the pulp pump will cause the fibers with greatly reduced strength after hydrolysis to form fiber fragments.

[0033] This invention utilizes centrifugal sedimentation or a grading sieve to classify acid-hydrolyzed cellulose, separating high-crystallinity MCC products containing short fiber components for external sale; and separating low-crystallinity specialty MCC containing impurity cell components for external sale.

[0034] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or basic characteristics. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

[0035] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A combined preparation process of microcrystalline cellulose and high-polymerization-degree cellulose, characterized in that, Includes the following steps: Step (1), Preprocessing: Select plant fiber raw materials, slice them, and remove impurities; Step (2), steaming: Weigh out the plant fiber raw material granules, add them to the cooking container, add an appropriate amount of cooking agent, and cook. Step (3), black liquor extraction: The cooked pulp is extracted in stages using hot water and then concentrated to a dryness of 5% to 35%. Step (4), bleaching and washing: First, dilute the pulp with waste liquor containing HCl to adjust the pH; then add ClO2 solution, heat to 40-85℃, and bleach for 30-150 minutes; then wash the pulp, and the pulp washing waste liquor is discharged after wastewater treatment to meet the standards. Step (5), grading: Bleached pulp is separated into long fibers and short fibers containing impurities using a grading sieve. The long fibers are used to produce high-polymer cellulose products, while the short fibers containing impurities are concentrated. Step (6), mixing, reacting, and diluting: The concentrated short fiber slurry containing impurities was transferred to a mixer, dilute HCl was added, and the mixture was stirred. The mixture was then transferred to a reaction vessel and reacted at 35℃-115℃ to maintain the concentration of the short fiber solution containing impurities at 4%-20%. Finally, it was transferred to a dilution tank and diluted to 0.5%-4%. Step (7), washing: The pulp is transferred to a multi-stage countercurrent washing equipment for multi-stage countercurrent washing to obtain pulp. Part of the washing wastewater is transferred to the No. 1 transition water tank and then pumped to the mixer and dilution tank. The other part is transferred to the No. 2 transition water tank and then pumped to the pulping and bleaching system to adjust the pH value of the ClO2 bleaching pulp. Step (8), pressure filtration and concentration: The washed pulp after bleaching is transferred to a plate and frame filter press for dewatering to 40% to 50% dryness. The wastewater is transferred to the No. 3 transition water tank and then pumped to a multi-stage countercurrent double-net pulp washer. The dewatered semi-dry pulp is transferred to a high-consistency pulp tower. Step (9), drying, pulverizing and sieving: The semi-dry pulp is transferred to a drying system for drying, then pulverized by a pulverizer. After pulverization, it is transferred to a screening machine to separate high-crystallinity MCC containing short fiber components and low-crystallinity special MCC containing impurity cell components.

2. The combined preparation process of microcrystalline cellulose and high-polymerization-degree cellulose according to claim 1, characterized in that: The grading screen in step (5) is a micro-slit screen with a screen drum and a screen gap of 0.05 to 0.079 mm.

3. The combined preparation process of microcrystalline cellulose and high-polymerization-degree cellulose according to claim 1, characterized in that: The grading sieve in step (5) is a microporous sieve with a sieve drum hole diameter of 0.5 to 0.8 mm.

4. The combined preparation process of microcrystalline cellulose and high-polymerization-degree cellulose according to claim 1, characterized in that: The rotor linear velocity of the grading screen in step (5) is ≤11.5m / s; the screening concentration of the grading screen in step (5) is ≤0.6%.

5. The combined preparation process of microcrystalline cellulose and high-polymerization-degree cellulose according to claim 1, characterized in that: The reaction vessel in step (6) is a horizontal reaction vessel with a rotating body.

6. The combined preparation process of microcrystalline cellulose and high-polymerization-degree cellulose according to claim 5, characterized in that: The rotation speed of the horizontal reactor in step (6) is ≤1 rpm.

7. The combined preparation process of microcrystalline cellulose and high-polymerization-degree cellulose according to claim 1, characterized in that: The multi-stage countercurrent washing equipment in step (7) is a double-net washing machine, and the net speed of the double-net washing machine is ≥50m / min, the net concentration is ≤1%, and the net dry fiber quantity is ≤100g / m 2 .

8. The combined preparation process of microcrystalline cellulose and high-polymerization-degree cellulose according to claim 1, characterized in that: In step (5), the grading sieve has a grading ratio of 30% to 90% for long fibers and a grading ratio of 70% to 10% for short fibers containing impurities.

9. The combined preparation process of microcrystalline cellulose and high-polymerization-degree cellulose according to claim 1, characterized in that: The amount of dilute HCl added to the mixer in step (6) is exactly the amount of residual HCl after acid hydrolysis, which meets the amount of HCl required to adjust the pH value of the pulp for pulp bleaching in step (4).

10. The combined preparation process of microcrystalline cellulose and high-polymerization-degree cellulose according to claim 9, characterized in that: To ensure that no excess HCl is left in step (6) and that fresh HCl is not added in the pulp bleaching process in step (4), it is necessary to control the pulp concentration before pulp bleaching in step (4); and to control the ratio of long and short fibers in step (5) so that the waste liquor generated from washing pulp with acid hydrolysis of microcrystalline cellulose just meets the requirements of ClO2 bleaching concentration after all the waste liquor generated from washing pulp with acid hydrolysis of microcrystalline cellulose is reused.

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