Acetate-grade cellulose, method for preparing same using poplar wood, and application thereof in preparing sustained-release drugs

Abstract

The present invention relates to the technical field of acetate-grade cellulose production, and more particularly to acetate-grade cellulose, a method for producing it from poplar wood, and its use in the preparation of sustained-release drugs. This method uses poplar wood, which is abundant domestically, as raw material. By using an acidic sulfite (magnesium-based) cooking method with the addition of sodium citrate and methanol as cooking aids, the resulting acetate-grade cellulose pulp has a high cellulose content, a high degree of polymerization, and high purity, and has broad application prospects.

Description

Technical Field

[0001] The present invention relates to the technical field of acetate-grade cellulose production, and in particular to acetate-grade cellulose, a preparation method thereof using poplar wood, and application thereof in the preparation of sustained-release drugs. Background Art

[0002] Cellulose acetate is a high-purity cellulose raw material widely used in biology and biopharmaceuticals due to its biocompatibility, plasticity, and biodegradability. In the biopharmaceutical field, cellulose acetate plays a key role in bioprocessing and purification. Its porous structure and functional groups enable efficient adsorption, filtration, and chromatographic separation of biomacromolecules such as proteins, enzymes, and nucleobases. Cellulose acetate membranes are widely used in ultrafiltration, dialysis, and tangential flow filtration for the efficient purification of biopharmaceutical products. Cellulose acetate also serves as a substrate for biosensors, enabling the detection and quantification of biological analytes. By immobilizing enzymes, antibodies, or nucleobases on the cellulose acetate surface, biosensors can detect target molecules with high sensitivity and specificity, showing promising applications in medical diagnostics, environmental monitoring, and food safety. It holds great potential in the future for intelligent drug delivery, 3D-printed customized medical devices, and regenerative medicine. Advances in materials science are enabling novel modification techniques (such as copolymerization and nanocomposites) to further optimize its performance, promoting its development in biology and biopharmaceuticals for the benefit of human health.

[0003] The raw materials for producing acetate-grade cellulose mainly include the following categories: coniferous wood, such as pine, spruce and fir; broadleaf wood, such as eucalyptus and rubber wood, and cotton linters. (1) Coniferous wood raw materials have high cellulose content, long fibers and high strength, which are conducive to uniform distribution of pulp. Disadvantages: The growth cycle of coniferous wood is long, and the supply of resources in my country is very limited, resulting in high costs. In addition, due to its high crystallinity, the esterification reaction performance of coniferous wood acetate-grade cellulose is slightly poor. (2) Broadleaf wood raw materials have a short growth cycle, high yield, sustainable supply, good economic efficiency, and good cellulose esterification reaction performance; its disadvantages: the wood fibers are short and have low strength, which may affect the mechanical properties of cellulose derivatives. Some tree species contain high extractives, such as tannins and gums, which may affect the purity and uniformity of cellulose materials. (3) Cotton linters have the highest cellulose content (>95%) and very few impurities, making them the highest quality cellulose acetate raw material. However, their limited availability and high price also present a problem of slightly poor esterification performance due to their high crystallinity. However, there have been no reports of cellulose acetate being produced from poplar wood.

[0004] Poplars, with their rapid growth, strong adaptability, and short harvesting cycles, are an important afforestation species that can quickly provide timber. Currently, poplars are used only in the production of low-value-added wood-based panels. Therefore, successfully utilizing poplar wood to produce acetate-grade cellulose pulp will address the raw material supply issues facing Chinese papermaking companies, particularly those in northern China.

[0005] Acetate-grade cellulose pulp requires a high cellulose content (>96.5% cellulose alpha), a high degree of polymerization (DP), high brightness, and high reactivity. This requires the chemical pulping and bleaching process to thoroughly remove non-cellulose materials such as hemicellulose, lignin, extractives, and metal ions, while minimizing cellulose degradation. This places high demands on both the raw material and the pulping and bleaching processes. Poplar wood, however, has a low cellulose content, low DP, short fiber length, poor strength, and high hemicellulose content and non-fibrous impurities, making it difficult to meet these pulping requirements. Therefore, producing acetate-grade pulp from poplar wood requires optimized and innovative pulping and bleaching processes to achieve the high DP content, high intrinsic viscosity (7.0-8.5 dL / g), and brightness exceeding 90% required for acetate-grade cellulose pulp. This requires the thorough removal of lignin, hemicellulose, and non-cellulose materials during the pulping and bleaching process, and the purification process must minimize cellulose degradation. This presents significant challenges for the pulping and bleaching production process.

[0006] Therefore, developing a method for preparing acetate-grade cellulose using poplar wood and further applying it to the field of preparing sustained-release drugs has great application prospects. Summary of the Invention

[0007] The present invention aims to overcome the above technical difficulties and provide a process for producing acetyl-grade fiber pulp with a simple process, high efficiency, environmental protection, energy conservation and emission reduction, and high pulp purity and excellent reaction performance. The process is achieved by the following technical solutions:

[0008] A first aspect of the present invention provides a method for preparing acetate-grade cellulose using poplar wood, comprising the following steps:

[0009] S1. Using poplar wood chips as raw materials, washing and desanding the raw materials, and then adding a cooking aid to perform acid sulfite cooking to obtain wood pulp; the cooking aid is sodium citrate and methanol;

[0010] S2, screening, washing, and concentrating the wood pulp from S1 to remove undigested wood knots and fiber pulp clumps to obtain pulp A;

[0011] S3, first stage bleaching: pulp A is subjected to oxidative delignification treatment with a sodium tungstate aqueous solution, and washed until the pulp is no longer viscous and the pH value is neutral, thereby obtaining pulp B;

[0012] S4, second stage bleaching: add dimethyldioxirane to pulp B for bleaching at a temperature of 20-30°C for 60-90 min to obtain pulp C;

[0013] S5. Performing alkali extraction on slurry C to obtain slurry D;

[0014] S6. Treat the pulp D with acid to obtain acetate-grade fiber pulp, and dry it to obtain acetate-grade cellulose.

[0015] As a preferred embodiment, the wood pulp of S1 has a cellulose methyl content of ≥90%, a kappa number of 5-8, and an intrinsic viscosity of 9.0-10.5.

[0016] As a preferred embodiment, in S1, a gradient temperature is used for steaming, wherein the temperature is raised to 95-105°C in the first stage and kept warm for 2-3 hours, and the temperature is raised to the highest temperature of 135-140°C in the second stage and kept warm for 1-1.5 hours.

[0017] Furthermore, the cooking conditions are as follows: the free SO2 of the cooking raw acid is 6.8-7.7%, and the combined SO2 is 1.0-1.5%; the amount of sodium citrate in the cooking aid is 2.5-3.5% of the raw material mass, the amount of methanol is 7.5-8.5% of the raw material mass, and the liquid ratio is 1:6.

[0018] As a preferred embodiment, the specific steps of the first stage bleaching include: adding the dried pulp A to a sodium tungstate aqueous solution with a mass concentration of 3-4% and a pH value of 4.5, reacting at 85-95°C for 60-120 minutes to perform oxidative delignification treatment; after the reaction is completed, washing with clean water until the pulp is no longer viscous and the pH value is measured to be 7±0.5, stopping washing, and obtaining pulp B.

[0019] As a preferred embodiment, the specific steps of the second stage bleaching include: adding dimethyldioxirane to pulp B for bleaching, the pulp concentration is controlled to be 10wt%, the pH value is 7.0-7.5, the temperature is 20-30°C, the amount of dimethyldioxirane relative to pulp B is 2.5-3.5wt%, and the bleaching time is 80-120min.

[0020] As a preferred embodiment, the S6 acetate-grade fiber pulp has a cellulose alpha content of ≥98.0%, an intrinsic viscosity of 7.0-8.5, a cellulose crystallinity of 55-60, and a whiteness of ≥92%.

[0021] As a preferred embodiment, the alkali extraction method includes: using an alkali amount of 6-8% of the mass of slurry C, an extraction temperature of 50-70°C, an extraction slurry concentration of 8-12wt%, and an extraction time of 40-80min.

[0022] As a preferred embodiment, the acid treatment method includes: adding HCl to pulp D, with the amount of HCl being 0.8-1.2wt% and the amount of sodium hexametaphosphate being 1.5-2.5wt% based on the absolute dry pulp, stirring and heating to 40-60°C for 40-80 minutes to obtain acetified fiber pulp.

[0023] The second aspect of the present invention provides an acetate-grade cellulose obtained by the above method.

[0024] A third aspect of the present invention provides a use of acetate-grade cellulose in the preparation of a sustained-release drug, wherein the sustained-release drug is a sustained-release capsule.

[0025] As a preferred embodiment, the method for preparing the sustained-release capsule comprises:

[0026] (1) In a vacuum-sealed polymerization tube, add 1.0 g of pre-vacuum-dried polyethylene glycol methyl ether (molecular weight 2000), 9.0 g of acetyl-grade poplar cellulose, and 700 ppm of stannous isooctanoate as a catalyst. Remove the air from the reactants in the polymerization tube under a vacuum of 0.1 mmHg. Seal the polymerization tube under vacuum and place it in a thermostat at 140°C for 48 hours to obtain a white crystalline uniform polymer. Open the polymerization tube and take out the polymer product.

[0027] (2) The polymer product obtained in step (1) is hot pressed or melt extruded at 100°C and 1500 psi to form a 0.25 mm thick film of cellulose acetate containing glycol ether. The film is annealed at 55°C on a 2.3 mm diameter polytetrafluoroethylene rod to obtain a hollow tube with an inner diameter of 2.3 mm and an outer diameter of 2.5 mm. A 3.2 cm long hollow tube is cut and sealed at one end by heat-melting. 20 mg of the desired drug is loaded from the other end, and the other end is heat-melted again to seal the other end. Long-acting sustained-release drug capsules are thus obtained.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] 1. In the present invention, poplar wood, a highly abundant source in China, is used as the raw material for acidic sulfite (magnesium-based) cooking with the addition of sodium citrate and methanol as cooking aids. In an acidic environment, sodium citrate adjusts the pH, catalyzing aromatic substitution reactions on the benzene rings of lignin and reducing acidic degradation of cellulose. Furthermore, citrate ions chelate with metal ions, reducing their catalytic effect on cellulose degradation. Methanol has a strong affinity for lignin, inhibiting lignin condensation reactions while promoting the dissolution of lignin sulfonates. This significantly increases the delignification reaction rate and extent during acidic sulfite cooking, as well as the dissolution of non-cellulosic substances and fatty compounds. Methanol also stabilizes cellulose, slowing the isomerization of its end groups. In the acidic sulfite (magnesium-based) cooking method with the addition of methanol, the dissolution order of hemicellulose sugars is: arabinose > xylose > glucose; therefore, the acidic sulfite (magnesium-based) cooking method of poplar wood chips with the addition of cooking aids sodium citrate and methanol is of great significance for obtaining acetate-grade cellulose pulp with high cellulose content, high degree of polymerization, and high purity.

[0030] 2. In the present invention, in the first stage of the bleaching process, a sodium tungstate (Na2WO4) solution is used to oxidatively delignify pulp A in step S2. In the second stage of the bleaching process, dimethyldioxirane (DMD) is added to pulp B for bleaching, followed by alkali extraction and acid treatment, thereby obtaining acetate-grade cellulose pulp with high purity, high whiteness, and excellent reactivity. The sodium tungstate oxidative delignification process is carried out under anaerobic conditions. Sodium tungstate is the only oxidant and does not cause auto-oxidation of cellulose free radical chains. Therefore, it has very good delignification selectivity and improves the delignification rate. At the same time, sodium tungstate catalyzes the wet air oxidation (mineralization) of organic matter, making the oxidative degradation products in the pulp easily converted into CO 2 and H₂O, thereby achieving a closed loop and zero discharge of bleaching wastewater. Furthermore, dimethyldioxirane (DMD) is used for bleaching pulp. The electrophilicity of DMD promotes the oxidation of both etherified and non-etherified aromatic rings in the lignin structure. DMD reacts with electron-rich C=C double bonds on aliphatic side chains to form epoxides or aromatic rings to form arene oxides. These epoxides readily hydrolyze into diols in aqueous solution, which further induce ring-opening reactions in dioxirane, promoting the degradation and dissolution of residual lignin. The hydrolyzed diols swell the cellulose and relax the pulp fiber structure, improving the accessibility of chemical reagents to the pulp cellulose, thereby enhancing the effectiveness of subsequent alkaline extraction and acid treatment, and improving the acetification performance of the cellulose pulp. Using this patented acidic sulfite cooking method, the resulting poplar cellulose material achieves a brightness of 78% ISO, thus employing the environmentally friendly Total Chlorine-Free (TCF) bleaching process.

[0031] 3. In the present invention, the poplar cellulose pulp prepared by the process of this scheme has the following characteristics: cellulose alpha content ≥ 98.0%, intrinsic viscosity 7.5-8.5, cellulose crystallinity 55-60, whiteness ≥ 92%, and pulp reaction performance index FACTPV (g / cm 2 ) 800-1000, reaching the performance index of imported hardwood cellulose acetate.

[0032] 4. A unique feature of this invention is its use of a "molecular channel" approach to control drug release rate. A low-molecular-weight polyether substance, capable of controlling drug dissolution rate, is dissolved in a high-molecular-weight cellulose acetate hydrogel. The resulting tube is then heat-pressed into a tube. Drug powder is loaded into one end of the heat-melt tube, and the other end of the tube is then sealed to form a long-acting sustained-release capsule. This capsule is simple to manufacture and suitable for automated industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a physical picture of the acetate-grade cellulose pulp prepared in the present invention.

[0034] Figure 2 This is a physical picture of the acetate-grade cellulose prepared in the present invention.

[0035] Figure 3-4 This is a photo of an industrial product prepared using the acetate-grade cellulose prepared by the present invention. DETAILED DESCRIPTION

[0036] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0037] Example 1

[0038] This embodiment provides a method for preparing acetate-grade cellulose using poplar wood, comprising the following steps:

[0039] S1. Using poplar wood chips (wood chip dimensions of 15-40 mm in length, 7-8 mm in width, and 8-11 mm in thickness, passing through a 5×5 sieve) as raw material, washing and desanding the poplar wood chips, and adding a cooking aid to perform acid sulfite cooking to obtain wood pulp; the cooking aid is sodium citrate and methanol.

[0040] The cooking conditions are as follows: the free SO2 of the cooking raw acid is 7.4±0.2%, and the combined SO2 is 1.3±0.2%; the amount of cooking aid sodium citrate is 3.0% of the raw material mass, the amount of methanol is 8.0% of the raw material mass, and the liquid ratio is 1:6.

[0041] The steaming adopts a multi-stage gradient cooking method: the first stage is set to heat up to 100°C for 1.5 hours and keep warm at 100°C for 2 hours; the second stage is set to heat up to the highest temperature of 138°C for 1.5 hours and keep warm for 1 hour.

[0042] After cooking, the poplar pulp has a cellulose content of 92.5%, a kappa number of 6.5, an intrinsic viscosity of 9.7, and a pulp brightness of 78%; the pulp ash content is 0.29% and the iron ion content is 71ppm.

[0043] S2. Use a pressure screen to screen, wash, and concentrate the pulp obtained in the above pulping process to remove undigested wood knots and fiber pulp clumps, so that the pH value of the pulp solution is about 7, thereby obtaining pulp A.

[0044] S3. First stage bleaching: Pulp A was added to a 3.5% Na2WO4 aqueous solution with a pH of 4.5, with a mass ratio of pulp A to sodium tungstate of 1:4.5. The pulp was reacted at 90°C for 90 minutes to perform oxidative delignification treatment. After the reaction was completed, the pulp was washed four times with 40°C water until the pulp was no longer viscous and the pH was measured at 7±0.5, then washing was stopped to obtain pulp B.

[0045] S4. Second stage bleaching: dimethyldioxirane (DMD) is added to pulp B for bleaching: the pulp concentration is controlled to 10wt%, the amount of DMD relative to pulp B is 3.0%, the pH is 7.0-7.5, the temperature is 25°C, and the bleaching time is 90min to obtain pulp C.

[0046] S5. Perform alkali extraction on slurry C: use 7% of the mass of slurry C as alkali, temperature 60°C, extraction concentration 10%, time 60 min, to obtain slurry D, the alkali is sodium hydroxide.

[0047] S6. Acid treatment of pulp D: add 1.0% HCl (based on absolute dry pulp) and 2.0% sodium hexametaphosphate to pulp D with a pulp concentration of 5%, and heat to 50°C with stirring for 60 minutes to obtain acetate-grade poplar cellulose pulp, which is then dried to obtain acetate-grade cellulose.

[0048] Example 2

[0049] This embodiment provides a method for preparing acetate-grade cellulose using poplar wood, comprising the following steps:

[0050] S1. Using poplar wood chips (wood chip dimensions of 15-40 mm in length, 7-8 mm in width, and 8-11 mm in thickness, passing through a 5×5 sieve) as raw material, washing and desanding the poplar wood chips, and adding a cooking aid to perform acid sulfite cooking to obtain wood pulp; the cooking aid is sodium citrate and methanol.

[0051] The cooking conditions are as follows: the free SO2 of the cooking raw acid is 6.9±0.2%, and the combined SO2 is 1.0±0.2%; the amount of cooking aid sodium citrate is 2.5% of the raw material mass, the amount of methanol is 7.5%, and the liquid ratio is 1:6.

[0052] The steaming adopts a multi-stage gradient cooking method: the first stage is set to heat up to 100°C for 1.5 hours and keep warm at 100°C for 2 hours; the second stage is set to heat up to the highest temperature of 138°C for 1.5 hours and keep warm for 1 hour.

[0053] After cooking, the poplar pulp has a cellulose content of 91.8%, a kappa number of 7.2, an intrinsic viscosity of 10.1, and a pulp brightness of 77%; the pulp ash content is 0.29% and the iron ion content is 71ppm.

[0054] S2. Use a pressure screen to screen, wash, and concentrate the pulp obtained in the above pulping process to remove undigested wood knots and fiber pulp clumps, so that the pH value of the pulp solution is about 7, thereby obtaining pulp A.

[0055] S3. First stage bleaching: add the dried pulp A to a 3.5% Na2WO4 aqueous solution with a pH of 4.5, in a mass ratio of pulp A to sodium tungstate of 1:3.5, and react at 90°C for 90 minutes to perform oxidative delignification treatment; after the reaction is completed, wash the pulp with 40°C water for 4 times until the pulp is no longer viscous and the pH is measured at 7±0.5, then stop washing to obtain pulp B.

[0056] S4. Second stage bleaching: dimethyldioxirane (DMD) is added to pulp B for bleaching: the pulp concentration is controlled to 10wt%, the amount of DMD relative to pulp B is 2.5%, pH 7.0-7.5, temperature 25°C, bleaching time is 90min, and pulp C is obtained.

[0057] S5. Perform alkali extraction on slurry C: use alkali of 6% of the mass of slurry C, temperature of 60°C, extraction concentration of 10%, time of 60 min, to obtain slurry D, and the alkali is sodium hydroxide.

[0058] S6. Acid treatment of pulp D: add 1.0% HCl (based on absolute dry pulp) and 2.0% sodium hexametaphosphate to pulp D with a pulp concentration of 5%, and heat to 50°C with stirring for 60 minutes to obtain acetate-grade poplar cellulose pulp, which is then dried to obtain acetate-grade cellulose.

[0059] Test example

[0060] This test example provides the use of the acetate-grade cellulose prepared in Example 1 in the preparation of sustained-release capsules. The preparation method of the sustained-release capsules comprises:

[0061] (1) In a vacuum-sealed polymerization tube, add 1.0 g of pre-vacuum-dried polyethylene glycol methyl ether (molecular weight 2000), 9.0 g of acetyl-grade poplar cellulose, and 700 ppm of stannous isooctanoate as a catalyst. Remove the air from the reactants in the polymerization tube under a vacuum of 0.1 mmHg. Seal the polymerization tube under vacuum and place it in a thermostat at 140°C for 48 hours to obtain a white crystalline uniform polymer. Open the polymerization tube and take out the polymer product.

[0062] (2) The polymer product obtained in step (1) is subjected to hot pressing or melt extrusion at 100°C and 1500 psi to form a 0.25 mm thick film of cellulose acetate containing glycol ether. The film is annealed at 55°C on a 2.3 mm diameter polytetrafluoroethylene rod to obtain a hollow tube with an inner diameter of 2.3 mm and an outer diameter of 2.5 mm. A 3.2 cm long hollow tube is cut and sealed at one end by heat melting. 20 mg of the desired drug is loaded from the other end, and the other end is again heat melt sealed to produce a long-acting sustained-release drug capsule.

[0063] Comparative Example 1

[0064] The difference between this comparative example and Example 1 is that in Comparative Example 1, the poplar wood chips are cooked using a conventional acidic sulfite (magnesium salt-based) method without adding a cooking aid.

[0065] Cooking conditions were as follows: free SO2 of the crude acid was 7.5±0.2%, combined SO2 was 1.3±0.2%, and the liquid ratio was 1:6. A multi-stage gradient cooking method was used: the first stage was set to heat up to 100°C over 1.5 hours and hold at 100°C for 2 hours; the second stage was set to heat up to a maximum temperature of 138°C over 1.5 hours and hold at that temperature for 1 hour.

[0066] After cooking, the poplar wood pulp is produced, which has a cellulose alpha content of 87.5%, a kappa number of 9.3, an intrinsic viscosity of 10.8, and a pulp brightness of 62.5%; the pulp ash content is 0.33% and the iron ion content is 85ppm.

[0067] The prepared poplar wood pulp was processed according to steps S2-S6 of Example 1 to produce poplar cellulose pulp having a cellulose alpha content of 93.6%, an intrinsic viscosity of 6.74, a brightness of 91.5%, an ash content of 0.03%, and an iron ion content of 8 ppm. The cellulose crystallinity was 56.86%. The Fock reaction performance of the pulp was 410 g / cm 2 (The Fock reaction performance of the pulp does not meet the standards of acetified grade pulp).

[0068] Comparative Example 2

[0069] This comparative example differs from Example 1 only in that sodium citrate, a cooking aid, is not added in step S1 of Example 1. The remaining steps are the same as those of Example 1. Poplar cellulose pulp is prepared, having a cellulose methyl content of 95.8%, an intrinsic viscosity of 7.53, and a whiteness of 91.8%. The ash content is 0.04%, and the iron ion content is 7 ppm. The cellulose crystallinity is 60.79%. The Fock reaction performance of the pulp is 610 g / cm 2 (The Fock reaction performance of the pulp does not meet the standards of acetified grade pulp).

[0070] Comparative Example 3

[0071] This comparative example differs from Example 1 only in that methanol, a cooking aid, is not added in step S1 of Example 1. The remaining steps are the same as those of Example 1. Poplar cellulose pulp is prepared, having a cellulose methyl content of 96.0%, an intrinsic viscosity of 7.93, and a whiteness of 91.8%. The ash content is 0.03%, and the iron ion content is 7 ppm. The cellulose crystallinity is 59.79%. The Fock reaction performance of the pulp is 580 g / cm 2 (The Fock reaction performance of the pulp does not meet the standards of acetified grade pulp).

[0072] Performance Testing

[0073] The physical and chemical properties of the poplar cellulose pulp prepared in the examples and comparative examples were tested using the following method:

[0074] The properties of pulp are tested using the national standard GB / T1548-2016.

[0075] The test results are shown in Table 1.

[0076] Table 1

[0077]

[0078] The above data demonstrate that the acetate-grade cellulose prepared by the present invention has a high cellulose content, high degree of polymerization, and high purity. Furthermore, it exhibits good whiteness, cellulose crystallinity, and reactivity. In Comparative Examples 1-3, the type of cooking aid was adjusted, and the corresponding effects decreased to varying degrees. This demonstrates that acidic sulfite (magnesium-based) cooking of poplar wood chips with the addition of cooking aids sodium citrate and methanol is of great significance for obtaining acetate-grade cellulose pulp with high cellulose content, high degree of polymerization, and high purity.

Claims

1. A method for preparing acetate-grade cellulose using poplar wood, characterized in that: The steps include: S1. Using poplar wood chips as raw materials, washing and desanding the raw materials, and then adding a cooking aid to perform acid sulfite cooking to obtain wood pulp; the cooking aid is sodium citrate and methanol; S2, screening, washing, and concentrating the wood pulp from S1 to remove undigested wood knots and fiber pulp clumps to obtain pulp A; S3. First stage bleaching: The dried pulp A is added to a sodium tungstate aqueous solution having a mass concentration of 3-4% and a pH value of 4.5, and reacted at 85-95°C for 60-120 minutes to perform oxidative delignification treatment. After the reaction, the pulp is washed with clean water until the pulp is no longer viscous and the pH value is measured to be 7±0.

5. Washing is then stopped to obtain pulp B. S4, second stage bleaching: add dimethyldioxirane to pulp B for bleaching at a temperature of 20-30°C for 60-90 min to obtain pulp C; S5. Performing alkali extraction on slurry C to obtain slurry D; S6. Acid-treating the pulp D to obtain acetate-grade fiber pulp, and drying to obtain acetate-grade cellulose; The alkali extraction method includes: using an alkali amount of 6-8% of the mass of the slurry C, an extraction temperature of 50-70° C., an extraction slurry concentration of 8-12wt%, and an extraction time of 40-80 min.

2. The method according to claim 1, characterized in that The wood pulp of S1 has a cellulose alpha content of ≥90%, a kappa number of 5-8, and an intrinsic viscosity of 9.0-10.

5.

3. The method according to claim 2, characterized in that The amount of sodium citrate in the cooking aid is 2.5-3.5% of the mass of the raw material, and the amount of methanol is 7.5-8.5% of the mass of the raw material.

4. The method according to claim 3, characterized in that The specific steps of the second stage bleaching include: adding dimethyldioxirane to pulp B for bleaching, controlling the pulp concentration to 10wt%, the pH value to 7.0-7.5, the temperature to 20-30°C, the amount of dimethyldioxirane relative to pulp B to be 2.5-3.5wt%, and the bleaching time to be 80-90min.

5. The method according to claim 4, characterized in that The S6 acetate-grade fiber pulp has a cellulose A content of ≥98.0%, an intrinsic viscosity of 7.0-8.5, a cellulose crystallinity of 55-60, and a whiteness of ≥92%.

6. The method according to claim 5, characterized in that The acid treatment method comprises: adding HCl to pulp D, wherein the amount of HCl is 0.8-1.2 wt % and the amount of sodium hexametaphosphate is 1.5-2.5 wt % based on the absolute dry pulp, stirring and heating to 40-60° C. for 40-80 minutes to obtain acetified fiber pulp.

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