A pulp manufacturing process based on recycled pulp board

Abstract

This invention relates to the field of pulping technology, specifically to a pulping process based on recycled fiber pulp board. The invention first involves carboxylating, periodate oxidation, and high-pressure homogenization of northern bleached sulfate softwood pulp board to obtain an aldehyde-carboxylated nanocellulose dispersion. Then, low-molecular-weight chitosan and high-molecular-weight trimethyl chitosan are sequentially introduced to form a chitosan-aldehyde-carboxylated nanocellulose dispersion. Simultaneously, 4-O-methyl-D-glucuronic acid-D-xylan is prepared as a locking solution. After initial fragmentation of the recycled fiber pulp board, the above dispersion and locking solution are added sequentially, followed by low-intensity beating, screening, purification, forming, and drying. This invention can improve the retention stability of reinforcing components at the cracks and contact interfaces of recycled fibers, reduce beating load and white water load, and improve the ring crush strength of recycled packaging paper.

Description

Technical Field

[0001] This invention relates to the field of pulping technology, and more specifically to a pulping process based on recycled fiber pulp boards. Background Technology

[0002] Regenerated fiber is an important raw material for the paper industry to save virgin wood pulp and reduce the pressure of waste paper disposal. Regenerated fiber pulp boards, derived from waste corrugated boxes, linerboard scraps, and recycled packaging paper, typically require disintegration, screening, purification, dewatering, pressing, drying, and baling before being used in paper production. Because recycled fibers undergo multiple papermaking, drying, and rewetting processes, the fiber walls are prone to collapse, irreversible hydrogen bonds form on the surface, and fiber flexibility and reswelling capacity decrease, resulting in insufficient effective bonding area between fibers after pulping.

[0003] Current processes often restore fiber bonding by extending the disintegration time, increasing the pulping intensity, or increasing the pulping load, thereby creating more fine fibrous structures on the fiber surface. However, regenerated fibers already suffer from shortened length, significant keratinization, and a high proportion of fine fibers. Excessive mechanical processing will further break down the fibers, increasing the content of fine fibers, colloidal substances, and dissolved impurities in the pulp, resulting in decreased filtration speed, increased white water turbidity, increased system load, and increased energy consumption.

[0004] Existing technologies also employ reinforcing agents such as starch, polyacrylamide, chitosan, hemicellulose, and nanocellulose to improve the strength of recycled paper. However, these components are usually mainly adsorbed on the outer surface of fibers or dispersed in the aqueous phase of pulp, making it difficult for them to effectively penetrate the fiber wrinkles, microcracks, and fine fiber contact interfaces formed after drying and compaction. In particular, although nanocellulose has a high specific surface area, there is still a contradiction between dispersion, retention, and water filtration. If chitosan and other natural polysaccharides such as xylan are combined too early in the aqueous phase, they are also prone to forming large aggregates, reducing the efficiency of interfacial positioning. Summary of the Invention

[0005] In view of this, the purpose of this invention is to propose a pulping process based on recycled fiber pulp board to solve the problem that existing recycled fiber pulp board pulping relies on high-intensity beating to compensate for fiber damage, resulting in high energy consumption and white water load, and limited improvement of ring crush strength of packaging paper.

[0006] To achieve the above objectives, the present invention provides a pulping process based on recycled fiber pulp board, the specific steps of which are as follows:

[0007] (1) The bleached sulfate softwood pulp board from the north was made into a cellulose suspension, and the cellulose suspension was carboxylated by 2,2,6,6-tetramethylpiperidine-1-oxy free radical, sodium bromide and sodium hypochlorite to obtain carboxylated cellulose wet material.

[0008] (2) The carboxylated cellulose wet material is dispersed in water and oxidized with sodium periodate. After the reaction is completed, the reaction is terminated with ethylene glycol. After washing and high-pressure homogenization, an aldehyde-carboxylated nanocellulose dispersion is obtained.

[0009] (3) First add a low molecular weight chitosan acidic solution to the aldehyde-carboxylic nanocellulose dispersion, then add a high molecular weight trimethyl chitosan aqueous solution to obtain a chitosan-aldehyde-carboxylic nanocellulose dispersion.

[0010] (4) 4-O-methyl-D-glucuronic acid-D-xylan was alkalized, dispersed and neutralized to obtain a 4-O-methyl-D-glucuronic acid-D-xylan locked solution;

[0011] (5) After breaking down the recycled fiber pulp board, it is initially crushed to obtain recycled fiber pulp containing fiber bundles and small pulp flakes;

[0012] (6) After the regenerated fiber pulp has been initially broken down and before low-strength beating, add the chitosan-aldehyde-carboxylic acid nanocellulose dispersion to the regenerated fiber pulp and mix, then add the 4-O-methyl-D-glucuronic acid-D-xylan locking solution and continue mixing;

[0013] (7) The obtained pulp is subjected to low-intensity beating, and then screened, purified, shaped and dried to obtain recycled fiber paper sheets;

[0014] Based on 1000kg of oven-dry regenerated fiber, the amount of oven-dry bleached sulfate softwood pulp board is 3000-4200g, the amount of low molecular weight chitosan is 220-340g, the amount of high molecular weight trimethyl chitosan is 70-110g, and the amount of 4-O-methyl-D-glucuronic acid-D-xylan is 800-1200g.

[0015] Preferably, in step (1), the northern bleached sulfate softwood pulp board is torn into pulp sheets of 20-50 mm to form a cellulose suspension with an oven-dry cellulose mass concentration of 8-12 g / L.

[0016] Preferably, the oxidation in step (2) is carried out under light-protected conditions at a system pH of 4-5, at a reaction temperature of 25-32℃, and for a reaction time of 90-150 min.

[0017] Preferably, the high-pressure homogenization parameter in step (2) is 50-70 MPa, and the cycle is 2-4 times.

[0018] Preferably, the solid content of the aldehyde-carboxylic acid nanocellulose dispersion in step (2) is 6-8 g / L.

[0019] Preferably, the low molecular weight chitosan acidic solution in step (3) is a mixture of low molecular weight chitosan, deionized water and glacial acetic acid.

[0020] Preferably, the high molecular weight trimethyl chitosan aqueous solution in step (3) is a mixture of high molecular weight trimethyl chitosan and deionized water.

[0021] Preferably, in step (3), the low molecular weight chitosan acidic solution is added to the aldehyde-carboxylated nanocellulose dispersion within 20-40 min, and stirring is continued for 20-40 min after addition; the high molecular weight trimethyl chitosan aqueous solution is added to the aldehyde-carboxylated nanocellulose dispersion, and stirring is continued for 10-20 min after addition.

[0022] Preferably, the low molecular weight chitosan in step (3) has a molecular weight of 50,000-190,000 Da and a degree of deacetylation of not less than 75%.

[0023] Preferably, the high molecular weight trimethyl chitosan in step (3) has a weight-average molecular weight of 280,000-350,000 Da and a degree of quaternization of 40%-60%.

[0024] Preferably, the alkalization dispersion in step (4) involves adding sodium hydroxide to 4-O-methyl-D-glucuronic acid-D-xylan to convert it into an easily dispersible sodium salt state.

[0025] Preferably, the neutralization treatment in step (4) involves adjusting the pH of the system to 6-7 using a hydrochloric acid aqueous solution with a mass concentration of 300 g / L.

[0026] Preferably, the recycled fiber pulp board in step (5) is a commercially available recycled fiber dry pulp board with recycled fibers from waste corrugated cardboard boxes as the main source. The moisture content of the recycled fiber pulp board is 8%-12%, and the mass ratio of recycled fibers from waste corrugated cardboard boxes in the oven-dry fibers is not less than 80%.

[0027] Preferably, the mass concentration of the regenerated fiber slurry in step (5) is 40-50 g / L.

[0028] Preferably, when adding chitosan-aldehyde-carboxylic acid nanocellulose dispersion in step (6), the mass concentration of the regenerated fiber slurry after initial fragmentation is first adjusted to 36-40 g / L.

[0029] Preferably, the chitosan-aldehyde-carboxylic acid nanocellulose dispersion in step (6) is added to the slurry within 75-110s, and mixing continues for 3-5min after the addition is completed.

[0030] Preferably, the 4-O-methyl-D-glucuronic acid-D-xylan locking solution in step (6) is continuously added to the slurry within 50-75 seconds, and the pH is maintained at 6-7 after addition, and shearing and mixing are continued for 2.5-4 minutes.

[0031] Preferably, in step (7), the pulp temperature is controlled at 35-45°C and the beatness is controlled at 28-35°SR during low-strength pulping.

[0032] Preferably, before performing low-intensity pulping in step (7), the pulp concentration needs to be adjusted to 33-37 g / L.

[0033] Preferably, the screening and purification in step (7) is used to remove incompletely broken slurry lumps, sand particles and conventional adhesive impurities.

[0034] Preferably, the drying temperature in step (7) is 95-105°C, and no pyrolysis, carbonization or roasting is performed.

[0035] Preferably, the amount of the northern bleached sulfate softwood pulp board is based on 3000-4200g of oven-dry cellulose, the amount of 2,2,6,6-tetramethylpiperidine-1-oxy free radical is 5-8g, the amount of sodium bromide is 30-42g, the amount of sodium hypochlorite solution is 19500-27300g, and the amount of sodium periodate is 1200-2100g.

[0036] Preferably, the screening slit width is 0.15 mm, the pulp inlet pressure is 0.20 MPa, the pulp outlet pressure is 0.14 MPa, the screening pressure difference is 0.06 MPa, the screen drum speed is 750 r / min, and the slag discharge rate is controlled at 1.5%, which is used to remove incompletely broken pulp lumps and coarse fiber bundles.

[0037] Preferably, the purification inlet pressure is 0.30 MPa, the good pulp outlet pressure is 0.10 MPa, and the underflow slag discharge rate is controlled at 0.8%; the purification is used to remove sand particles, fine metal shavings, and high-density adhesive impurities.

[0038] Preferably, the pH of the slurry is 6.5 and the temperature is 35°C during the dilution and preparation process. The slurry is then fed into the headbox, and the slurry mass concentration in the headbox is controlled at 5 g / L, the headbox pressure is 8 kPa, and the slurry-to-mesh ratio is 1.02.

[0039] Preferably, the forming process employs a long-net forming method, with the quantitative control being 450 g / m². 2The wet paper web is sequentially dewatered in a vacuum chamber and then in a press. The vacuum chamber has a vacuum level of -35 kPa and a pressing time of 8 seconds. The pressure of the first press line is 120 kN / m, and the pressure of the second press line is 180 kN / m. After pressing, the wet paper web has a dryness of 42%. The pressed wet paper web then enters a multi-cylinder drying section for drying. The surface temperature of the first drying cylinder is controlled at 95℃, the surface temperature of the middle drying cylinder is controlled at 100℃, and the surface temperature of the last drying cylinder is controlled at 105℃. The total drying time is 6 minutes. After drying, the paper web is cooled to below 35℃ by cooling rollers, then trimmed online and cut into recycled fiber paper sheets.

[0040] The beneficial effects of this invention are:

[0041] This invention first carboxylates northern bleached sulfate softwood pulp board, then oxidizes it with sodium periodate to form aldehyde-carboxylated nanocellulose, giving the nanocellulose both aqueous dispersion and near-interface anchoring capabilities. Compared with ordinary nanocellulose used directly as a wet-end reinforcing agent, this structure helps reduce agglomeration and loss in the pulp and provides a reaction basis for the subsequent stable distribution of chitosan.

[0042] This invention introduces low molecular weight chitosan and high molecular weight trimethyl chitosan sequentially onto the surface of aldehyde-carboxylic acid nanocellulose. This treatment can balance structural stability and crack migration ability, making it easier for chitosan-aldehyde-carboxylic acid nanocellulose to enter the fiber wrinkle area, crack area and fine fiber contact interface after the initial fragmentation of the regenerated fiber pulp board, and reducing the aqueous phase flocculation caused by single cationic polymers.

[0043] This invention uses 4-O-methyl-D-glucuronic acid-D-xylan as a delayed addition component, which forms a flexible locking layer after the chitosan-aldehyde-carboxylic nanocellulose completes its interfacial positioning, rather than forming a large particle complex in the aqueous phase of the pulp beforehand. This method is beneficial to improve the retention of the added component at the fiber contact interface and maintain the stability of the interfacial bonding during subsequent low-intensity pulping, screening, shaping and drying processes.

[0044] In summary, compared with existing methods that rely on high-strength pulping or ordinary wet-end strengthening agents to improve the strength of recycled paper, this invention is more suitable for recycled fiber pulp board production scenarios. It can reduce mechanical processing load and white water load without significantly changing existing pulping, refining, screening and forming equipment, thereby improving the ring crush retention capacity and overall strength of recycled packaging papers such as linerboard and corrugated base paper. Detailed Implementation

[0045] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0046] Raw material source:

[0047] Nanocellulose masterbatch: Metsä Pine northern bleached sulfate softwood pulp board is preferred, with an oven dryness of over 90%;

[0048] Regenerated fiber pulp board: Commercially available recycled fiber dry pulp board made primarily from recycled fibers from waste corrugated cardboard boxes, processed through dehydration, pressing, drying, and baling, with a specification of 50kg / bag and a moisture content of 8%-12%, preferably 10%;

[0049] Sodium hypochlorite solution: available chlorine mass fraction is 10%;

[0050] Low molecular weight chitosan: molecular weight 50,000-190,000 Da, degree of deacetylation not less than 75%;

[0051] High molecular weight trimethyl chitosan: weight average molecular weight 280,000-350,000 Da, degree of quaternization 40%-60%;

[0052] 4-O-methyl-D-glucuronic acid-D-xylan: Sigma-Aldrich M5144, derived from beech wood, in powder form.

[0053] The papermaking process includes the following:

[0054] The pressure screen has a slit width of 0.15 mm, an inlet pressure of 0.20 MPa, an outlet pressure of 0.14 MPa, a screening pressure difference of 0.06 MPa, a screen drum speed of 750 r / min, and a slag discharge rate of 1.5%. It is used to remove incompletely broken pulp lumps and coarse fiber bundles.

[0055] The slag removal and purification process has an inlet pressure of 0.30 MPa, a good slurry outlet pressure of 0.10 MPa, and an underflow slag discharge rate controlled at 0.8%. The purification process is used to remove sand particles, fine metal shavings, and high-density adhesive impurities.

[0056] The pH of the diluted slurry was 6.5 and the temperature was 35°C. The slurry was then fed into the headbox, and the slurry mass concentration in the headbox was controlled at 5 g / L, the headbox pressure at 8 kPa, and the slurry-to-mesh ratio at 1.02.

[0057] The mesh is formed using a long mesh forming method, with a quantitative control of 450g / m². 2The wet paper web is sequentially dewatered in a vacuum chamber and then in a press. The vacuum chamber has a vacuum level of -35 kPa and a pressing time of 8 seconds. The pressure of the first press line is 120 kN / m, and the pressure of the second press line is 180 kN / m. After pressing, the wet paper web has a dryness of 42%. The pressed wet paper web then enters a multi-cylinder drying section for drying. The surface temperature of the first drying cylinder is controlled at 95℃, the surface temperature of the middle drying cylinder is controlled at 100℃, and the surface temperature of the last drying cylinder is controlled at 105℃. The total drying time is 6 minutes. After drying, the paper web is cooled to below 35℃ by cooling rollers, then trimmed online and cut into recycled fiber paper sheets.

[0058] Example 1: A pulping process based on recycled fiber pulp board, the specific steps of which are as follows:

[0059] S1. Preparation of carboxylated cellulose wet material:

[0060] Take 3500g of northern bleached sulfate softwood pulp board (based on oven-dry cellulose), tear it into 20-50mm pulp sheets, add deionized water, and stir at 25℃ for 30min to form a cellulose suspension with an oven-dry cellulose concentration of 10g / L. Then, add 6g of 2,2,6,6-tetramethylpiperidine-1-oxy radical and 35g of sodium bromide, and add a 100g / L sodium hydroxide aqueous solution to maintain the pH of the system between 10 and 11. Add 22750g of sodium hypochlorite solution dropwise over 60min at 25℃, and continue the reaction for 120min after the addition is complete. After the reaction is complete, add 2000g of ethanol to terminate the oxidation, then dehydrate and wash three times with 300000g of deionized water to obtain 3500g of carboxylated cellulose wet material (based on oven-dry cellulose).

[0061] S2, Preparation of aldehyde-carboxylic acid nanocellulose dispersion:

[0062] Take all the carboxylated cellulose wet material obtained from S1, add 435,000 g of deionized water for dispersion, and add hydrochloric acid aqueous solution with a mass concentration of 100 g / L to maintain the pH of the system at 4-5; add 1750 g of sodium periodate under light-protected conditions, and react at 25℃ for 120 min; after the reaction is completed, add 700 g of ethylene glycol and continue stirring for 30 min to consume the residual periodate, and then wash with 500,000 g of deionized water 5 times until the washing filtrate shows no obvious oxidizing property when tested with starch potassium iodide test paper; then add deionized water to prepare the obtained material into a dispersion with a solid content of 7 g / L, and use high pressure homogenization to circulate it 3 times at 60 MPa to obtain aldehyde carboxylated nanocellulose dispersion;

[0063] S3, Low molecular weight chitosan anchoring modification:

[0064] Take all of the aldehyde-carboxylated nanocellulose dispersion obtained from S2, stir at 25℃, and add glacial acetic acid to adjust the pH of the system to 5-6; separately take 280g of low molecular weight chitosan, add 19600g of deionized water and 200g of glacial acetic acid, stir at 25℃ for 120min to fully dissolve the low molecular weight chitosan to obtain a low molecular weight chitosan acidic solution; add the obtained low molecular weight chitosan acidic solution to the aldehyde-carboxylated nanocellulose dispersion within 30min, and continue stirring for 30min;

[0065] S4, High molecular weight trimethyl chitosan anchoring modification:

[0066] Take 90g of high molecular weight trimethyl chitosan, add 9000g of deionized water, and stir at 25℃ for 30min to obtain a trimethyl chitosan aqueous solution; add the trimethyl chitosan aqueous solution to the dispersion obtained in S3 within 10min, and continue stirring for 15min to obtain a chitosan-aldehyde-carboxylic acid nanocellulose dispersion;

[0067] Preparation of S5, 4-O-methyl-D-glucuronic acid-D-xylan locking solution:

[0068] Take 1000g of 4-O-methyl-D-glucuronic acid-D-xylan, add 160g of sodium hydroxide and 100000g of deionized water, and stir at 55℃ for 60min to convert 4-O-methyl-D-glucuronic acid-D-xylan into an easily dispersible sodium salt state; after cooling to 30℃, slowly add a 300g / L hydrochloric acid aqueous solution until the pH of the system is adjusted to 6-7, and then add deionized water to make the total mass 110000g, to obtain a 4-O-methyl-D-glucuronic acid-D-xylan locked solution;

[0069] S6. Breaking and initial fragmentation of recycled fiber pulp board:

[0070] Take 1000 kg of recycled fiber pulp board containing oven-dried recycled fiber, break it up and cut it into 30-80 mm pulp board pieces; add process water at 40℃ and pH 6-7 to the hydraulic pulping system, and then add the above pulp board pieces to make the initial pulp mass concentration 45 g / L; disintegrate under shear hydraulic disintegration conditions for 7 min to dissociate the pulp board pieces into fiber bundles and small pulp pieces, but do not perform full pulping;

[0071] S7, Chitosan-aldehyde-carboxylic acid nanocellulose dispersion added:

[0072] After the initial fragmentation is completed in S6, process water at 40℃ is added to adjust the slurry mass concentration to 38g / L. While the slurry is still in a shear cycle state, all the chitosan-aldehyde-carboxylic acid nanocellulose dispersion obtained in S4 is continuously added to the slurry within 90s. After the addition is completed, continue mixing for 4min.

[0073] S8, 4-O-methyl-D-glucuronic acid-D-xylan locking solution added:

[0074] After completing the 4-minute positioning mixing in S7, all the 4-O-methyl-D-glucuronic acid-D-xylan locked solution obtained in S5 was continuously added to the slurry within 60 seconds. After adding, the pH was maintained at 6-7, and shear mixing was continued for 3 minutes.

[0075] S9, Low-strength pulping:

[0076] Add process water at 40°C to the slurry obtained from S8 to adjust the slurry mass concentration to 35g / L; use existing grinding or beating equipment to perform low-intensity beating, control the slurry temperature to 35-45°C, and control the beating degree to 30°SR.

[0077] S10. Screening, purification, shaping, and drying:

[0078] The pulp obtained from S9 is subjected to pressure screening, slag removal and purification, dilution and pulp preparation, wire forming, pressing and drying according to the existing papermaking process. The screening and purification steps are only used to remove incompletely broken pulp lumps, sand particles and adhesive impurities. The drying temperature is controlled at 100℃ and no pyrolysis, carbonization or roasting treatment is performed.

[0079] Example 2:

[0080] S1. Preparation of carboxylated cellulose wet material:

[0081] Take 3800g of northern bleached sulfate softwood pulp board (based on oven-dry cellulose), tear it into 20-50mm pulp sheets, add deionized water, and stir at 25℃ for 30min to form a cellulose suspension with an oven-dry cellulose concentration of 10g / L. Then, add 7g of 2,2,6,6-tetramethylpiperidine-1-oxy radical and 38g of sodium bromide, and add a 100g / L sodium hydroxide aqueous solution to maintain the pH of the system at 10.5. Add 24700g of sodium hypochlorite solution dropwise over 65min at 25℃, and continue the reaction for 120min after the addition is complete. After the reaction is complete, add 2200g of ethanol to terminate the oxidation, then dehydrate and wash three times with 330000g of deionized water to obtain carboxylated cellulose wet material.

[0082] S2, Preparation of aldehyde-carboxylic acid nanocellulose dispersion:

[0083] Take all the carboxylated cellulose wet material obtained from S1, add 480,000 g of deionized water for dispersion, and add hydrochloric acid aqueous solution with a mass concentration of 100 g / L to maintain the pH of the system at 4.5; add 1900 g of sodium periodate under light-protected conditions, and react at 28℃ for 120 min; after the reaction is completed, add 800 g of ethylene glycol and continue stirring for 30 min to consume the residual periodate, and then wash with 550,000 g of deionized water 5 times until the washing filtrate shows no obvious oxidizing property when tested with starch potassium iodide test paper; then add deionized water to prepare the obtained material into a dispersion with a solid content of 7.5 g / L, and use high pressure homogenization to circulate it 3 times at 65 MPa to obtain aldehyde carboxylated nanocellulose dispersion;

[0084] S3, Low molecular weight chitosan anchoring modification:

[0085] Take all of the aldehyde-carboxylated cellulose nanoparticle dispersion obtained in S2, stir at 28℃, and add glacial acetic acid to adjust the pH of the system to 5.5; separately take 300g of low molecular weight chitosan, add 21000g of deionized water and 220g of glacial acetic acid, stir at 25℃ for 120min to fully dissolve the low molecular weight chitosan to obtain a low molecular weight chitosan acidic solution; add the obtained low molecular weight chitosan acidic solution to the aldehyde-carboxylated cellulose nanoparticle dispersion within 30min, and continue stirring for 30min;

[0086] S4, High molecular weight trimethyl chitosan anchoring modification:

[0087] Take 100g of high molecular weight trimethyl chitosan, add 10000g of deionized water, and stir at 25℃ for 30min to obtain a trimethyl chitosan aqueous solution; add the trimethyl chitosan aqueous solution to the dispersion obtained in S3 within 12min, and continue stirring for 15min to obtain a chitosan-aldehyde-carboxylic cellulose nanoparticle dispersion.

[0088] Preparation of S5, 4-O-methyl-D-glucuronic acid-D-xylan locking solution:

[0089] Take 1050g of 4-O-methyl-D-glucuronic acid-D-xylan, add 168g of sodium hydroxide and 105000g of deionized water, stir at 60℃ for 60min to convert 4-O-methyl-D-glucuronic acid-D-xylan into an easily dispersible sodium salt state; after cooling to 30℃, slowly add a 300g / L hydrochloric acid aqueous solution until the pH of the system is adjusted to 6.5, and then add deionized water to make the total mass 115000g, to obtain a 4-O-methyl-D-glucuronic acid-D-xylan locked solution;

[0090] S6. Breaking and initial fragmentation of recycled fiber pulp board:

[0091] Take 1000 kg of recycled fiber pulp board containing oven-dried recycled fiber, break it up and cut or tear it into pulp board sheets of 30-80 mm; add process water at 40℃ and pH 6.5 to the hydraulic pulping system, and then add the above pulp board sheets to make the initial pulp mass concentration 45 g / L; disintegrate for 7 min under shear hydraulic disintegration conditions of 380 r / min rotor speed in the hydraulic pulper to dissociate the pulp board sheets into fiber bundles and small pulp sheets, but do not perform full pulping;

[0092] S7, Chitosan-aldehyde-carboxylic acid nanocellulose dispersion added:

[0093] After the initial fragmentation is completed in S6, process water at 40℃ is added to adjust the slurry mass concentration to 38g / L. While the slurry is still in a shear cycle state, all the chitosan-aldehyde-carboxylic acid nanocellulose dispersion obtained in S4 is continuously added to the slurry within 90s. After the addition is completed, continue mixing for 4min.

[0094] S8, 4-O-methyl-D-glucuronic acid-D-xylan locking solution added:

[0095] After the positioning mixing is completed in S7, all the 4-O-methyl-D-glucuronic acid-D-xylan locked solution obtained in S5 is continuously added to the slurry within 60s. After the addition, the pH is maintained at 6.5, and shear mixing is continued for 3min.

[0096] S9, Low-strength pulping:

[0097] Add process water at 40°C to the slurry obtained from S8 to adjust the slurry mass concentration to 35 g / L; use existing grinding or beating equipment for low-intensity beating, control the slurry temperature at 40°C, and control the beating degree at 32°SR.

[0098] S10. Screening, purification, shaping, and drying:

[0099] The pulp obtained from S9 is subjected to pressure screening, slag removal and purification, dilution and pulp preparation, wire forming, pressing and drying according to the existing papermaking process. The screening and purification steps are only used to remove incompletely broken pulp lumps, sand particles and adhesive impurities. The drying temperature is controlled at 100℃ and no pyrolysis, carbonization or roasting treatment is performed.

[0100] Example 3:

[0101] S1. Preparation of carboxylated cellulose wet material:

[0102] Take 3000g of northern bleached sulfate softwood pulp board (based on oven-dry cellulose), tear it into 20-50mm pulp sheets, add deionized water, and stir at 20℃ for 20min to form a cellulose suspension with an oven-dry cellulose concentration of 8g / L; then add 5g of 2,2,6,6-tetramethylpiperidine-1-oxy radical and 30g of sodium bromide, and add a 100g / L sodium hydroxide aqueous solution to maintain the pH of the system at 10; add 19500g of sodium hypochlorite solution dropwise over 50min at 20℃, and continue the reaction for 90min after the addition is complete; after the reaction is complete, add 1700g of ethanol to terminate the oxidation, then dehydrate and wash three times with 250000g of deionized water to obtain 3000g of carboxylated cellulose wet material (based on oven-dry cellulose);

[0103] S2, Preparation of aldehyde-carboxylic acid nanocellulose dispersion:

[0104] Take all the carboxylated cellulose wet material obtained from S1, add 360,000 g of deionized water for dispersion, and add hydrochloric acid aqueous solution with a mass concentration of 100 g / L to maintain the pH of the system at 4; add 1200 g of sodium periodate under light-protected conditions, and react at 25℃ for 90 min; after the reaction is completed, add 500 g of ethylene glycol and continue stirring for 20 min to consume the residual periodate, and then wash with 400,000 g of deionized water 5 times until the washing filtrate shows no obvious oxidizing property when tested with starch potassium iodide test paper; then add deionized water to prepare the obtained material into a dispersion with a solid content of 6 g / L, and use high pressure homogenization to circulate twice at 50 MPa to obtain aldehyde carboxylated nanocellulose dispersion;

[0105] S3, Low molecular weight chitosan anchoring modification:

[0106] Take all of the aldehyde-carboxylated nanocellulose dispersion obtained from S2, stir at 25℃, and add glacial acetic acid to adjust the pH of the system to 5; separately take 220g of low molecular weight chitosan, add 15400g of deionized water and 160g of glacial acetic acid, stir at 25℃ for 90min to fully dissolve the low molecular weight chitosan to obtain a low molecular weight chitosan acidic solution; add the obtained low molecular weight chitosan acidic solution to the aldehyde-carboxylated nanocellulose dispersion within 20min, and continue stirring for 20min;

[0107] S4, High molecular weight trimethyl chitosan anchoring modification:

[0108] Take 70g of high molecular weight trimethyl chitosan, add 7000g of deionized water, and stir at 25℃ for 20min to obtain a trimethyl chitosan aqueous solution; add the trimethyl chitosan aqueous solution to the dispersion obtained in S3 within 8min, and continue stirring for 10min to obtain a chitosan-aldehyde-carboxylic acid nanocellulose dispersion.

[0109] Preparation of S5, 4-O-methyl-D-glucuronic acid-D-xylan locking solution:

[0110] Take 800g of 4-O-methyl-D-glucuronic acid-D-xylan, add 128g of sodium hydroxide and 80000g of deionized water, stir at 50℃ for 50min to convert 4-O-methyl-D-glucuronic acid-D-xylan into an easily dispersible sodium salt state; after cooling to 25℃, slowly add a 300g / L hydrochloric acid aqueous solution until the pH of the system is adjusted to 6, and then add deionized water to make the total mass 90000g, to obtain a 4-O-methyl-D-glucuronic acid-D-xylan locked solution;

[0111] S6. Breaking and initial fragmentation of recycled fiber pulp board:

[0112] Take 1000 kg of recycled fiber pulp board containing oven-dried recycled fiber, break it open and cut or tear it into 30 mm pulp board pieces; add process water at 35℃ and pH 6 to the hydraulic pulping system, and then add the above pulp board pieces to make the initial pulp mass concentration 40 g / L; disintegrate under shear hydraulic disintegration conditions of 300 r / min in a hydraulic pulper for 6 min to dissociate the pulp board pieces into fiber bundles and small pulp pieces, but do not perform full pulping;

[0113] S7, Chitosan-aldehyde-carboxylic acid nanocellulose dispersion added:

[0114] After the initial fragmentation is completed in S6, process water at a temperature of 35℃ is added to adjust the slurry mass concentration to 36g / L; while the slurry is still in a shear cycle state, all the chitosan-aldehyde-carboxylic acid nanocellulose dispersion obtained in S4 is continuously added to the slurry within 75s, and mixing is continued for 3min after the addition is completed.

[0115] S8, 4-O-methyl-D-glucuronic acid-D-xylan locking solution added:

[0116] After the positioning mixing is completed in S7, all the 4-O-methyl-D-glucuronic acid-D-xylan locked solution obtained in S5 is continuously added to the slurry within 50s. After the addition, the pH is maintained at 6, and shear mixing is continued for 2.5min.

[0117] S9, Low-strength pulping:

[0118] Add process water at 35°C to the slurry obtained from S8 to adjust the slurry mass concentration to 33g / L; use existing grinding or beating equipment for low-intensity beating, control the slurry temperature at 35°C, and control the beating degree at 28°SR.

[0119] S10. Screening, purification, shaping, and drying:

[0120] The pulp obtained from S9 is subjected to pressure screening, slag removal and purification, dilution and pulp preparation, wire forming, pressing and drying according to the existing papermaking process. The screening and purification steps are only used to remove incompletely broken pulp lumps, sand particles and adhesive impurities. The drying temperature is controlled at 95℃ and no pyrolysis, carbonization or roasting treatment is performed.

[0121] Example 4:

[0122] S1. Preparation of carboxylated cellulose wet material:

[0123] 4200g of northern bleached sulfate softwood pulp board (based on oven-dry cellulose) was torn into 20-50mm pulp sheets, and deionized water was added. The mixture was stirred at 30℃ for 40min to form a cellulose suspension with an oven-dry cellulose concentration of 12g / L. Subsequently, 8g of 2,2,6,6-tetramethylpiperidine-1-oxy radical and 42g of sodium bromide were added sequentially, along with a 100g / L sodium hydroxide aqueous solution to maintain the pH of the system at 11. 27300g of sodium hypochlorite solution was added dropwise over 80min at 25℃, and the reaction was continued for 150min after the addition was complete. After the reaction was completed, 2400g of ethanol was added to terminate the oxidation. The mixture was then dehydrated and washed three times with 360000g of deionized water to obtain 4200g of carboxylated cellulose wet material (based on oven-dry cellulose).

[0124] S2, Preparation of aldehyde-carboxylic acid nanocellulose dispersion:

[0125] Take all the carboxylated cellulose wet material obtained from S1, add 540,000 g of deionized water for dispersion, and add hydrochloric acid aqueous solution with a mass concentration of 100 g / L to maintain the pH of the system at 5; add 2100 g of sodium periodate under light-protected conditions, and react at 32℃ for 150 min; after the reaction is completed, add 900 g of ethylene glycol and continue stirring for 40 min to consume the residual periodate, and then wash with 600,000 g of deionized water 5 times until the washing filtrate shows no obvious oxidizing property when tested with starch potassium iodide test paper; then add deionized water to prepare the obtained material into a dispersion with a solid content of 8 g / L, and use high pressure homogenization to circulate 4 times at 70 MPa to obtain aldehyde carboxylated nanocellulose dispersion;

[0126] S3, Low molecular weight chitosan anchoring modification:

[0127] Take all of the aldehyde-carboxylated cellulose nanoparticle dispersion obtained in S2, stir at 30℃, and add glacial acetic acid to adjust the pH of the system to 6; separately take 340g of low molecular weight chitosan, add 23800g of deionized water and 240g of glacial acetic acid, stir at 25℃ for 150min to fully dissolve the low molecular weight chitosan to obtain a low molecular weight chitosan acidic solution; add the obtained low molecular weight chitosan acidic solution to the aldehyde-carboxylated cellulose nanoparticle dispersion within 40min, and continue stirring for 40min;

[0128] S4, High molecular weight trimethyl chitosan anchoring modification:

[0129] Take 110g of high molecular weight trimethyl chitosan, add 11000g of deionized water, and stir at 25℃ for 40min to obtain a trimethyl chitosan aqueous solution; add the trimethyl chitosan aqueous solution to the dispersion obtained in S3 within 15min, and continue stirring for 20min to obtain a chitosan-aldehyde-carboxylic acid nanocellulose dispersion.

[0130] Preparation of S5, 4-O-methyl-D-glucuronic acid-D-xylan locking solution:

[0131] Take 1200g of 4-O-methyl-D-glucuronic acid-D-xylan, add 192g of sodium hydroxide and 120000g of deionized water, and stir at 65℃ for 80min to convert 4-O-methyl-D-glucuronic acid-D-xylan into an easily dispersible sodium salt state; after cooling to 35℃, slowly add a 300g / L hydrochloric acid aqueous solution until the pH of the system is adjusted to 7, and then add deionized water to make the total mass 130000g, to obtain a 4-O-methyl-D-glucuronic acid-D-xylan locked solution;

[0132] S6. Breaking and initial fragmentation of recycled fiber pulp board:

[0133] Take 1000 kg of recycled fiber pulp board containing oven-dried recycled fiber, break it open and cut or tear it into 80 mm pulp board pieces; add process water at 45℃ and pH 7 to the hydraulic pulping system, and then add the above pulp board pieces to make the initial pulp mass concentration 50 g / L; disintegrate for 8 minutes under shear hydraulic disintegration conditions of 450 r / min rotor speed of hydraulic pulper, so that the pulp board pieces are dissociated into fiber bundles and small pulp pieces, but do not perform full pulping;

[0134] S7, Chitosan-aldehyde-carboxylic acid nanocellulose dispersion added:

[0135] After the initial crushing is completed in S6, process water at a temperature of 45℃ is added to adjust the slurry mass concentration to 40g / L; while the slurry is still in a shear cycle state, all the chitosan-aldehyde-carboxylic acid nanocellulose dispersion obtained in S4 is continuously added to the slurry within 110s, and mixing is continued for 5min after the addition is completed.

[0136] S8, 4-O-methyl-D-glucuronic acid-D-xylan locking solution added:

[0137] After the positioning mixing is completed in S7, all the 4-O-methyl-D-glucuronic acid-D-xylan locked solution obtained in S5 is continuously added to the slurry within 75s. After the addition, the pH is maintained at 7, and shear mixing is continued for 4min.

[0138] S9, Low-strength pulping:

[0139] Add process water at 45°C to the slurry obtained from S8 to adjust the slurry mass concentration to 37g / L; use existing grinding or beating equipment for low-intensity beating, control the slurry temperature at 45°C, and control the beating degree at 35°SR.

[0140] S10. Screening, purification, shaping, and drying:

[0141] The pulp obtained from S9 is subjected to pressure screening, slag removal and purification, dilution and pulp preparation, wire forming, pressing and drying according to the existing papermaking process. The screening and purification steps are only used to remove incompletely broken pulp lumps, sand particles and adhesive impurities. The drying temperature is controlled at 105℃ and no pyrolysis, carbonization or roasting treatment is performed.

[0142] The difference between Comparative Example 1 and Example 1 is as follows: 1750g of sodium periodate and 700g of ethylene glycol are not added in S2. The carboxylated cellulose wet material obtained in S1 is washed five times with 500,000g of deionized water, and then deionized water is added to prepare a dispersion with a solid content of 7g / L. The dispersion is then circulated three times under high pressure homogenization at 60MPa. The amount of all raw materials, addition time, temperature, pH and beating degree in S3 to S10 are the same as in Example 1. All other conditions are the same as in Example 1.

[0143] The difference between Comparative Example 2 and Example 1 is as follows: In S3, 280g of low molecular weight chitosan, 19600g of deionized water, and 200g of glacial acetic acid were not added, and an acidic solution of low molecular weight chitosan was not prepared. Instead, all the aldehyde-carboxylic acid nanocellulose dispersion obtained in S2 was stirred at 25°C to 30°C for 30 minutes. In S4, the amount of high molecular weight trimethyl chitosan was adjusted from 90g to 370g, and 37000g of deionized water was added. After stirring at 25°C for 30 minutes, the mixture was added to the dispersion obtained in S3, so that the added oven-dry weight of chitosan was the same as in Example 1. The other conditions were the same as in Example 1.

[0144] The difference between Comparative Example 3 and Example 1 is as follows: In S3, the amount of low molecular weight chitosan was adjusted from 280g to 370g, and 25900g of deionized water and 264g of glacial acetic acid were added. After stirring at 25°C for 120min, the mixture was added to the aldehyde-carboxylic acid nanocellulose dispersion obtained in S2. In S4, 90g of high molecular weight trimethyl chitosan and 9000g of deionized water were not added, and a trimethyl chitosan aqueous solution was not prepared, so that the added oven-dry weight of chitosan was the same as in Example 1. The other conditions were the same as in Example 1.

[0145] The difference between Comparative Example 4 and Example 1 is as follows: all the 4-O-methyl-D-glucuronic acid-D-xylan locking solution obtained in S5 is not added late in S8, but is added to the dispersion obtained in S4 immediately after obtaining the chitosan-aldehyde-carboxylic acid nanocellulose dispersion, and premixed at 25°C for 30 min; in S7, all the premixed dispersion is continuously added to the slurry within 90 s, and mixing continues for 4 min after the addition is completed; in S8, only 110,000 g of process water at 40°C and pH 6-7 is added, and shear mixing continues for 3 min; the other conditions are the same as in Example 1.

[0146] The difference between Comparative Example 5 and Example 1 is as follows: In S7, the chitosan-aldehyde-carboxylic acid nanocellulose dispersion obtained in S4 is not added; only process water at a temperature of 40°C is added and the slurry mass concentration is adjusted to 38 g / L before shearing and mixing for 4 min. In S8, the 4-O-methyl-D-glucuronic acid-D-xylan locking solution obtained in S5 is not added; only the pH is maintained at 6-7 and shearing and mixing is continued for 3 min. After completing low-intensity pulping in S9, all the chitosan-aldehyde-carboxylic acid nanocellulose dispersion obtained in S4 is continuously added to the slurry within 90 s and mixed for 4 min. Then, all the 4-O-methyl-D-glucuronic acid-D-xylan locking solution obtained in S5 is continuously added to the slurry within 60 s and mixed for 3 min before proceeding to S10. The remaining conditions are the same as in Example 1.

[0147] The difference between Comparative Example 6 and Example 1 is as follows: 1000g of 4-O-methyl-D-glucuronic acid-D-xylan is not added in S5; 160g of sodium hydroxide, 100000g of deionized water, and 300g / L hydrochloric acid aqueous solution in S5 are treated according to the same pH procedure as in Example 1, and then deionized water is added to bring the total mass to 110000g; 904g of aldehyde-carboxylic acid nanocellulose dispersion (based on oven-dried cellulose) is taken separately, and 72g of low molecular weight chitosan and 24g of high molecular weight trimethyl chitosan are added. After anchoring modification is completed according to the same pH, temperature, addition time, and stirring time as in S3 and S4, the mixture is added to the chitosan-aldehyde-carboxylic acid nanocellulose dispersion obtained in S4, so that the total amount of oven-dried solids added in S7 and S8 is the same as in Example 1; the other conditions are the same as in Example 1.

[0148] Performance testing

[0149] Pulps from Examples 1 to 4 and Comparative Examples 1 to 6, after low-strength beating in S9, were taken as test samples, with each sample containing at least 500g of oven-dry pulp. The test pulp was diluted to a concentration of 3g / L using standard water for physical testing of pulp conforming to GB / T 22903-2008. After stirring for 10 minutes in a stirrer at 300 rpm, laboratory paper sheets were prepared according to the rapid Kaiser method specified in GB / T 24326-2009, with a target basis weight of 180g / m³. 2 After forming, the paper was wet-pressed at 0.40 MPa for 5 minutes and then vacuum-dried at 95℃ for 10 minutes to obtain paper sheets for physical property testing. All paper sheets were subjected to temperature and humidity treatment under the standard atmospheric conditions specified in GB / T 10739-2023 before testing: 23℃, 50% relative humidity, and 24 hours. At least 20 sheets were prepared for each sample, with 10 sheets used for ring crush strength testing, 5 for tensile strength testing, and 5 for bursting strength and thickness testing.

[0150] White water turbidity: 500 mL of the slurry after shear mixing in S8 of Examples 1 to 4 and Comparative Examples 1 to 6 was taken respectively, stirred at 300 r / min for 1 min, and then filtered through a 200 mesh sieve. The filtrate was collected as the white water sample. The turbidity of the white water was measured using a turbidity meter, and the unit is NTU.

[0151] Freezing degree and beating energy consumption: Freezing degree was determined according to the Schubert-Riegel method specified in GB / T 3332-2004. Slurries obtained from Examples 1 to 4 and Comparative Examples 1 to 6 (S9) were diluted with standard water to a test slurry mass of 2.0 g of oven-dry slurry. The test temperature was controlled at 20℃. Each sample was tested in triplicate, and the arithmetic mean was taken as the freezing degree, expressed in °SR. Beating energy consumption was determined using the same laboratory Walley beater. The beating method followed GB / T 24325-2009. All samples were uniformly beaten to 32.0°SR. After the beater ran unloaded for 3 minutes, the temperature was zeroed and recorded. Then, the test slurry was added, and beating was performed under the same equipment load conditions until the target freezing degree was reached. A 0.5-class three-phase energy meter was used to continuously record the power consumption. After deducting the unloaded power consumption, the energy consumption per unit of oven-dry slurry mass was calculated, expressed in kWh / t.

[0152] Thickness: Thickness was measured in accordance with GB / T 451.3-2002: Five sheets of paper were taken for each sample after standard humidity conditioning, and each sheet was cut into 100mm×100mm samples for testing. Thickness was measured at five different locations on each sheet of paper.

[0153] Ring crush strength: Ring crush strength was determined according to GB / T 2679.8-2016: A standard-conditioned paper sheet was cut longitudinally into 152.4mm × 12.7mm specimens, with 10 specimens tested for each sample. Before testing, the thickness of each specimen was measured, and a ring crush specimen holder matching the thickness was selected. The specimen was inserted into the annular groove and tested on a compression tester at a compression rate of 12.5mm / min. The maximum pressure at which the specimen crushed was recorded, and the ring crush strength was calculated.

[0154] Tensile strength: Tensile strength was determined according to the constant-rate tensile method specified in GB / T 12914-2018. Paper sheets that had undergone standard moisture conditioning were cut into specimens 15 mm wide and 180 mm long, with 10 specimens tested for each sample. The test clamping distance was 100 mm, and the tensile speed was 20 mm / min. Tensile strength, elongation at break, and tensile energy absorption were recorded, and the tensile index was calculated based on the paper's basis weight.

[0155] Bursting strength: Bursting strength was determined according to GB / T 454-2020. For each sample of paper after standard moisture conditioning, 10 locations without creases, holes, or obvious pulp stains were selected for testing. The number of tests was the same on both sides of the sample. The maximum pressure at which the paper broke was recorded.

[0156] Table 1 Performance Test Results

[0157]

[0158] Data Analysis: Table 1 shows that, under similar freeness conditions, the ring crush strength, tensile index, and bursting strength of Examples 1 to 4 are generally higher than those of the comparative examples, while the white water turbidity and unit beating energy consumption are generally lower. Compared with Comparative Example 1, Example 1, without increasing the freeness, shows an increase of approximately 22.6%, 19.1%, and 23.9% in ring crush strength, tensile index, and bursting strength, respectively, and a decrease of approximately 52.6% and 23.9% in white water turbidity and unit beating energy consumption, respectively. This indicates that the aldehyde sites formed by sodium periodate contribute to the stability of the chitosan-nanocellulose structure. Compared with Comparative Example 6, Example 1, with the same total amount of added solids, shows an increase of approximately 20.4%, 13.5%, and 20.6% in ring crush strength, tensile index, and bursting strength, respectively, and a decrease of approximately 43.3% in white water turbidity. This indicates that the locking effect formed by the delayed addition of 4-O-methyl-D-glucuronic acid-D-xylan cannot be achieved by ordinary addition of solids. Compared with Comparative Example 4, Example 1 showed an increase in ring crush strength of approximately 36.4% and a decrease in white water turbidity of approximately 68.8%, indicating that if xylan is pre-complexed with chitosan-aldehyde-carboxylic acid nanocellulose in the aqueous phase, it is prone to agglomeration and reduced fracture interface localization efficiency. This demonstrates a synergistic effect between aldehyde-carboxylic acid dual-site construction, segmented chitosan anchoring, and glucuronic acid xylan delayed locking.

[0159] In terms of application effects, this invention is applicable to the pulping process of recycled fiber pulp board mainly composed of recycled fibers from waste corrugated cardboard boxes. It can improve the ring crush retention capacity and overall strength of packaging paper without significantly changing the existing pulping, refining, screening and forming equipment, and reduce pulping energy consumption and white water load. It is suitable for promotion in recycled packaging paper production lines such as linerboard and corrugated base paper.

[0160] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A pulping process based on recycled fiber pulp board, characterized in that, Includes the following steps: (1) The cellulose suspension was prepared by bleaching sulfate softwood pulp board from the north, and the cellulose suspension was carboxylated by 2,2,6,6-tetramethylpiperidine-1-oxy free radical, sodium bromide and sodium hypochlorite to obtain carboxylated cellulose wet material. (2) The carboxylated cellulose wet material is dispersed in water and oxidized with sodium periodate to obtain an aldehyde-carboxylated nanocellulose dispersion; (3) First add a low molecular weight chitosan acidic solution to the aldehyde-carboxylic nanocellulose dispersion, then add a high molecular weight trimethyl chitosan aqueous solution to obtain a chitosan-aldehyde-carboxylic nanocellulose dispersion. (4) 4-O-methyl-D-glucuronic acid-D-xylan was alkalized, dispersed and neutralized to obtain a 4-O-methyl-D-glucuronic acid-D-xylan locked solution; (5) After breaking down the recycled fiber pulp board, it is initially crushed to obtain recycled fiber pulp containing fiber bundles and small pulp flakes; (6) After the regenerated fiber pulp has been initially broken down and before low-strength beating, add the chitosan-aldehyde-carboxylic acid nanocellulose dispersion to the regenerated fiber pulp and mix, then add the 4-O-methyl-D-glucuronic acid-D-xylan locking solution and continue mixing; (7) The obtained pulp is subjected to low-intensity beating, and then screened, purified, shaped and dried to obtain recycled fiber paper sheets; Based on 1000kg of oven-dry regenerated fiber, the amount of oven-dry bleached sulfate softwood pulp board is 3000-4200g, the amount of low molecular weight chitosan is 220-340g, the amount of high molecular weight trimethyl chitosan is 70-110g, and the amount of 4-O-methyl-D-glucuronic acid-D-xylan is 800-1200g.

2. The pulping process according to claim 1, characterized in that, Step (1) involves tearing the northern bleached sulfate softwood pulp board into 20-50mm pulp sheets to form a cellulose suspension with an oven-dry cellulose mass concentration of 8-12g / L.

3. The pulping process according to claim 1, characterized in that, The oxidation in step (2) is carried out under light-protected conditions at a system pH of 4-5, at a reaction temperature of 25-32℃, and for a reaction time of 90-150 min; the high-pressure homogenization parameters are 50-70 MPa, and the cycle is 2-4 times.

4. The pulping process according to claim 1, characterized in that, The low molecular weight chitosan in step (3) has a molecular weight of 50,000-190,000 Da and a degree of deacetylation of not less than 75%; the high molecular weight trimethyl chitosan in step (3) has a weight-average molecular weight of 280,000-350,000 Da and a degree of quaternization of 40%-60%.

5. The pulping process according to claim 1, characterized in that, The recycled fiber pulp board in step (5) is a commercially available recycled fiber dry pulp board with recycled fibers from waste corrugated cardboard boxes as the main source. The moisture content of the recycled fiber pulp board is 8%-12%, and the mass proportion of recycled fibers from waste corrugated cardboard boxes in the oven-dry fiber is not less than 80%. The mass concentration of the recycled fiber pulp is 40-50 g / L.

6. The pulping process according to claim 1, characterized in that, In step (6), the chitosan-aldehyde-carboxylic acid nanocellulose dispersion is added to the slurry within 75-110s, and mixing continues for 3-5min after the addition is completed; the 4-O-methyl-D-glucuronic acid-D-xylan locking solution is continuously added to the slurry within 50-75s, and the pH is maintained at 6-7 after the addition, and shearing and mixing continues for 2.5-4min.

7. The pulping process according to claim 1, characterized in that, In step (7), the pulp temperature is controlled at 35-45℃ and the beating degree is controlled at 28-35°SR during low-strength pulping.

8. The pulping process according to claim 1, characterized in that, Before performing low-intensity pulping as described in step (7), the pulp concentration needs to be adjusted to 33-37 g / L.

9. The pulping process according to claim 1, characterized in that, The northern bleached sulfate softwood pulp board is based on 3000-4200g of oven-dry cellulose. The amount of 2,2,6,6-tetramethylpiperidine-1-oxy free radical is 5-8g, the amount of sodium bromide is 30-42g, the amount of sodium hypochlorite solution is 19500-27300g, and the amount of sodium periodate is 1200-2100g.

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