Papermaking chitosan-based retention and drainage step-by-step regulation method

Abstract

The present application relates to the technical field of papermaking industry, and discloses a kind of papermaking chitosan-based retention and drainage step-by-step regulation method, and the pulp characteristics of different pulp layers in papermaking are differentiated pretreated, such as surface layer, core layer and bottom layer, using high molecular bridging and particle filling step-by-step regulation process, layered precision addition modified chitosan and inorganic particles, supporting online charge closed-loop control, the present application solves the defects that traditional retention system is poor in environmental protection, weak in shear resistance and cannot adapt to the layered needs of high-speed paper machine;With completely biodegradable bio-based materials as the standard, high retention rate and paper performance are considered, the stability of paper machine operation is improved, production consumption is reduced, and the requirements of clean production of papermaking are met.

Description

Technical Field

[0001] This invention relates to the field of papermaking technology, specifically to a step-by-step control method for papermaking chitosan-based retention and filtration aids. Background Technology

[0002] As the paper industry develops towards high-speed, closed, and green directions, the high-turbulence and high-shear forming environment places extremely high demands on the shear resistance and fine particle retention capacity of the wet end retention and filtration system. At the same time, the increased use of secondary fibers and the greater degree of closed-loop white water circulation lead to the continuous accumulation of anionic impurities in the wet end, resulting in deterioration of pulp filtration performance and worsening of paper forming conditions.

[0003] Among the existing mainstream retention and filtration aid systems, cationic polyacrylamide (CPAM) unit systems and ordinary anionic-cationic dipolymer systems have poor shear resistance and are prone to excessive flocculation, leading to decreased paper uniformity and strength, making them unsuitable for high-speed paper machine production. While the traditional CPAM-bentonite microparticle system improves shear resistance, its additive CPAM is a fossil-based synthetic polymer that is non-biodegradable, posing a significant environmental burden and failing to meet the requirements of clean production and sustainable development in the paper industry. Chitosan, as a natural cationic bio-based polymer, possesses advantages such as complete biodegradability, high cationic charge density, and strong fiber bonding ability, making it an ideal material to replace traditional fossil-based retention aids.

[0004] However, existing chitosan-based retention aids have significant drawbacks: (1) Most of them adopt a single-system one-step addition process, which cannot form shear-resistant dense flocs, resulting in unstable retention effect and difficulty in adapting to high-speed paper machines; (2) The different characteristics of the pulp on the surface, core and bottom of the linerboard are not controlled in layers, and the retention rate of the whole pulp layer and the paper performance cannot be taken into account. (3) The closed-loop control logic has not been formed, and it cannot adapt to the real-time fluctuations of pulp conditions, resulting in poor paper machine operation stability.

[0005] Therefore, there is an urgent need to develop an environmentally friendly, biodegradable, high-shear paper machine-compatible, layered, precisely controllable, chitosan-based retention and filtration aid process that balances high retention rate and paper performance. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a step-by-step control method for papermaking chitosan-based retention and filtration aids. This method achieves high retention rate and low white water contaminants while adapting to high-speed paper machine high-shear conditions, compensating for the loss of paper strength caused by high retention, and meeting the dual requirements of clean production and product quality.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A stepwise control method for papermaking chitosan-based retention and filtration aids includes the following steps: S1. The pulps for the papermaking surface layer, core layer, and bottom layer are subjected to beating, concentration adjustment, pH adjustment, and temperature adjustment to obtain three groups of pulps to be treated. The freeness of the surface layer pulp is 35-45°SR, the freeness of the core layer and bottom layer pulps is 28-38°SR, the oven-dry mass concentration of the three groups of pulps is 0.8%-1.2%, the pH value is 5.5-8.0, and the temperature is 35-55℃. S2. Prepare modified chitosan working solution and inorganic microparticle dispersion separately. The modified chitosan working solution has a degree of deacetylation of 80%-95% and a weight-average molecular weight ≥1×10⁻⁶. 6 The acetic acid-water solution of highly linear cationic chitosan has a solid content of 0.5%-2.0% and a pH of 4.0-6.0; the inorganic particulate dispersion is an aqueous dispersion of sodium-based bentonite or nano-silica with a solid content of 2%-5%. S3. Modified chitosan working solution is continuously added to the surface layer, core layer and bottom layer of the slurry to be treated according to the corresponding ratio. The mixture is stirred and mixed under the set shear conditions to complete the charge neutralization and polymer bridging of the fibers and fine components in the slurry, and the pretreated slurry is obtained. S4. After pressure screening of the pretreated slurry, inorganic microparticle dispersion is continuously added to the surface layer, core layer and bottom layer of the pretreated slurry according to the corresponding ratio. The mixture is stirred and mixed under the set shear conditions. Through the microparticle filling effect, a dense and shear-resistant fiber flocculent is formed to obtain the slurry to be formed. S5. Simultaneously transport the surface layer, core layer and bottom layer of the pulp to be formed to the forming section of the paper machine to complete the papermaking, pressing, drying, sizing and calendering processes to obtain the finished paper.

[0008] Optionally, in step S1, the pH value of the surface layer slurry is 6.0-7.0, the pH value of the core layer and bottom layer slurry is 5.5-6.5, and the temperature of the three slurries is 40-50℃.

[0009] Optionally, in step S3, the amount of modified chitosan working solution added is based on the oven-dry weight: 0.05%-0.15% of the oven-dry slurry for the surface layer, 0.1%-0.2% of the oven-dry slurry for the core layer, and 0.15%-0.3% of the oven-dry slurry for the bottom layer.

[0010] Optionally, in step S3, the modified chitosan working solution is added at the main pipeline of the slurry layer forming tank outlet, the shear rate of stirring and mixing is 500-1500 r / min, and the mixing time is 10-30 s.

[0011] Optionally, in step S4, the amount of inorganic microparticle dispersion added is based on the oven-dry mass: 0.2%-0.5% of the oven-dry slurry for the surface layer, 0.3%-0.8% of the oven-dry slurry for the core layer, and 0.5%-1.0% of the oven-dry slurry for the bottom layer.

[0012] Optionally, in step S4, the inorganic microparticle dispersion is added at the slurry pipeline at the outlet of the pressure screen of the corresponding slurry layer, the shear rate of stirring and mixing is 1000-2000 r / min, and the mixing time is 5-20 s.

[0013] Optionally, when the required cationic content of the pretreated slurry is >400 μmol / L, the amount of modified chitosan working solution added should be increased accordingly; when the required cationic content of the slurry is <100 μmol / L, the amount of modified chitosan working solution added should be decreased accordingly.

[0014] Optionally, in step S2, the purified highly linear cationic chitosan is dissolved in an aqueous solution of acetic acid with a mass fraction of 1%-2%, stirred until completely dissolved, filtered to remove insoluble matter, adjusted to the target solid content with deionized water, and the pH value is adjusted to 4.0-6.0 with acetic acid or sodium hydroxide to obtain the modified chitosan working solution.

[0015] Optionally, in step S2, the sodium-based bentonite has a sieve mesh size ≥ 2000 mesh, and the average particle size of the nano-silica is 10-100 nm. When preparing the inorganic microparticle dispersion, the inorganic microparticles are first added to deionized water under high-speed stirring conditions, stirred and dispersed for 20-40 min, and then allowed to stand for more than 30 min for later use.

[0016] Optionally, in step S3, a low molecular weight cationic polyacrylamide solution is also added simultaneously, with the addition amount being 0.01%-0.05% of the oven-dry slurry mass, added and mixed at the same point and through the same pipeline as the modified chitosan working solution.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) In this invention, a step-by-step control design of polymer bridging and microparticle filling is adopted. At the same time, a layered precise adaptation scheme is set for the characteristics of different pulp layers in papermaking. The resulting flocs have excellent shear resistance and can be perfectly adapted to the high turbulent production conditions of high-speed paper machines, which greatly improves the stability of continuous operation of paper machines. Moreover, there is no need to carry out large-scale transformation of existing production lines, which has good industrial adaptability and promotion. (2) In this invention, the dual characteristics of chitosan molecules, which have the functions of retention aid and filtration aid and fiber reinforcement, are fully utilized. While significantly improving the retention rate of fibers, fine components and fillers, reducing pulp loss and white water pollutant load, it can effectively compensate for the loss of paper strength and uniformity caused by high retention conditions. It takes into account both cost reduction and efficiency improvement on the production side and quality improvement on the product side, and achieves synergistic improvement of retention effect and paper performance. Attached Figure Description

[0018] Figure 1 This is a schematic flowchart of the step-by-step control method for papermaking chitosan-based retention and filtration aids in an embodiment of the present invention. Detailed Implementation

[0019] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. These drawings are simplified schematic diagrams, which are only used to illustrate the basic structure of the present invention and therefore only show the components relevant to the present invention.

[0020] like Figure 1 As shown, a step-by-step control method for papermaking chitosan-based retention and filtration aids includes the following steps: Step S1 (Pulp Pretreatment): The pulps of the papermaking surface layer, core layer, and bottom layer are respectively subjected to beating, concentration adjustment, pH adjustment, and temperature adjustment to obtain three groups of pulps to be treated; the beating degree of the surface layer pulp is controlled at 35-45°SR, the beating degree of the core / bottom layer pulp is controlled at 28-38°SR, the oven-dry mass concentration of the three groups of pulps is controlled at 0.8%-1.2%, the pH value is controlled at 5.5-8.0, and the temperature is controlled at 35-55℃.

[0021] The surface layer pulp is mainly composed of bleached softwood pulp and hardwood pulp, while the core / bottom layer pulp is mainly composed of recycled waste cardboard pulp. The pH value of the surface layer pulp is controlled at 6.0-7.0, and the pH value of the core / bottom layer pulp is controlled at 5.5-6.5. The temperature of all three pulps is controlled at 40-50℃ to ensure the solubility and charge neutralization efficiency of chitosan.

[0022] Step S2 (Preparation of the auxiliary agent system): Prepare the modified chitosan working solution and the inorganic particulate dispersion separately; the modified chitosan working solution has a degree of deacetylation of 80%-95% and a weight-average molecular weight ≥1×10⁻⁶. 6 The acetic acid-water solution of highly linear cationic chitosan has a solid content of 0.5%-2.0% and a pH of 4.0-6.0; the inorganic particulate dispersion is an aqueous dispersion of sodium bentonite or nano silica with a solid content of 2%-5%.

[0023] The preparation process of the modified chitosan working solution is as follows: the purified highly linear cationic chitosan is dissolved in an aqueous solution of acetic acid with a mass fraction of 1%-2%, stirred at 25-35℃ until completely dissolved, vacuum filtered to remove insoluble matter, the solid content is adjusted to the target value with deionized water, and the pH value is adjusted to 4.0-6.0 with acetic acid or sodium hydroxide. It is prepared and used immediately.

[0024] When preparing inorganic microparticle dispersions, inorganic microparticles are first added to deionized water under high-speed stirring at 3000-5000 r / min. After stirring and dispersing for 20-40 min, the mixture is allowed to stand and mature for at least 30 min before use to ensure the dispersibility of the microparticles.

[0025] Step S3 (Bridging Adsorption Treatment): Using a layered branch synchronous conveying process, modified chitosan working solution is continuously added to the surface layer, core layer, and bottom layer of the slurry to be treated according to the corresponding ratio. The mixture is stirred and mixed under set shear conditions to complete the charge neutralization and polymer bridging of the fibers and fine molecules in the slurry, thus obtaining the pretreated slurry.

[0026] The amount of modified chitosan working solution added, based on oven-dry mass, is 0.05%-0.15% of the oven-dry slurry for the surface layer, 0.1%-0.2% for the core layer, and 0.15%-0.3% for the bottom layer. The addition point is the main pipeline at the outlet of the slurry pool corresponding to the slurry layer. The shear rate of stirring and mixing is 500-1500 r / min, and the mixing time is 10-30 s, to ensure that chitosan molecules are fully adsorbed on the surface of fibers and fine molecules to form preliminary bridged flocs.

[0027] Furthermore, a low molecular weight cationic polyacrylamide (CPAM) solution can be added simultaneously in this step, at a rate of 0.01%-0.05% of the oven-dry slurry mass. It should be added and mixed at the same location and in the same pipeline as the modified chitosan working solution to further improve charge neutralization efficiency and make it suitable for recycled slurries with high anionic impurity content.

[0028] Step S4 (microparticle filling and flocculation treatment): After pressure screening of the pretreated slurry, inorganic microparticle dispersion is continuously added to the surface layer, core layer and bottom layer of the slurry according to the corresponding ratio. The mixture is stirred and mixed under the set shear conditions. Through the microparticle filling effect, dense and shear-resistant fiber flocs are formed to obtain the slurry to be formed.

[0029] The above-mentioned amount of inorganic microparticle dispersion added is based on the oven-dry weight: 0.2%-0.5% of the oven-dry pulp for the surface layer, 0.3%-0.8% for the core layer, and 0.5%-1.0% for the bottom layer. The addition point is the pulp pipeline corresponding to the outlet of the pressure screen of the pulp layer. The shear rate of stirring and mixing is 1000-2000 r / min, and the mixing time is 5-20 s. The high shear effect of the pressure screen is used to disperse the large flocs that are over-flocculated. Then, through the electrostatic interaction between inorganic microparticles and chitosan molecules, microparticles are formed between the fibers to fill the gaps, generating small and dense flocs with strong shear resistance, thus taking into account both retention rate and paper uniformity.

[0030] Step S5 (Paper Forming): The surface layer, core layer, and bottom layer of pulp to be formed are simultaneously transported to the forming section of the paper machine to complete the paper forming, pressing, drying, sizing, and calendering processes to obtain the finished paper.

[0031] Furthermore, between steps S3 and S4, the present invention also includes an online detection and closed-loop control step: the cation demand (PCD), temperature, and pH value of the pretreated slurry are detected in real time by an online sensor. When the cation demand of the slurry is >400 μmol / L, the central control system correspondingly increases the amount of modified chitosan working solution added; when the cation demand of the slurry is <100 μmol / L, the amount of modified chitosan working solution added is correspondingly reduced to ensure the stability of the wet-end charge balance.

[0032] Example 1: This example describes a retention and filtration aid process for surface slurry. The specific steps are as follows: Pulp pretreatment: Take the surface bleached wood pulp, beat it to a freeness of 40°SR, thicken it to an oven-dry mass concentration of 1.0%, adjust the pH value to 6.5, and heat it to 45℃ to obtain the pulp to be treated; Preparation of the auxiliary agent system: Prepare a modified chitosan working solution with a solid content of 1.0% and a pH of 5.0; prepare a sodium-based bentonite dispersion with a solid content of 3%, stir at high speed for 30 min and then mature for 30 min for later use; Bridged adsorption treatment: Add modified chitosan working solution to the slurry to be treated. The amount added is 0.1% of the oven-dry slurry. The addition point is the outlet of the slurry forming tank. Mix at a shear rate of 1000 r / min for 20 s to obtain the pretreated slurry. Online closed-loop control: The PCD value of the pretreated slurry was detected to be 280 μmol / L, which is within the target range, and there is no need to adjust the amount of chitosan added; Microparticle-filled flocculation treatment: After the pretreated slurry is treated by pressure sieving, sodium-based bentonite dispersion is added at a rate of 0.3% of the oven-dry slurry. The addition point is the outlet of the pressure sieving. The mixture is mixed for 15 seconds at a shear rate of 1500 r / min to obtain the slurry to be formed. Paper forming: The pulp to be formed is formed into a paper sheet with a basis weight of 120g / m² in a standard paper sheet forming machine. After pressing and drying, the finished paper sheet is obtained.

[0033] Example 2: This example describes a retention and filtration aid process for core layer slurry. The specific steps are as follows: Slurry pretreatment: Take the core layer OCC recycled slurry, beat it to a freeness of 32°SR, thicken it to an oven-dry mass concentration of 1.0%, adjust the pH value to 6.0, and heat it to 45℃ to obtain the slurry to be treated; Preparation of the auxiliary agent system: Prepare a modified chitosan working solution with a solid content of 1.0% and a pH of 5.0; prepare a sodium-based bentonite dispersion with a solid content of 3%, stir at high speed for 30 min and then mature for 30 min for later use; Bridged adsorption treatment: Add modified chitosan working solution to the slurry to be treated, the amount of which is 0.15% of the oven-dry slurry, and simultaneously add low molecular weight CPAM, the amount of which is 0.03% of the oven-dry slurry. The addition point is the outlet of the slurry forming tank. Mix at a shear rate of 1200 r / min for 20 s to obtain the pretreated slurry. Online closed-loop control: The PCD value of the pretreated slurry was detected to be 180 μmol / L, which is within the target range, and no adjustment of the addition amount is required; Microparticle-filled flocculation treatment: After the pretreated slurry is treated by pressure sieving, sodium-based bentonite dispersion is added at a rate of 0.5% of the oven-dry slurry. The addition point is the outlet of the pressure sieving. The mixture is mixed for 10 seconds at a shear rate of 1800 r / min to obtain the slurry to be formed. Forming: Same as in Example 1, form into paper sheets with a basis weight of 120g / m².

[0034] Example 3: This example describes a retention and filtration aid process for the bottom slurry. The specific steps are as follows: Slurry pretreatment: Take the bottom OCC recycled slurry, beat it to a freeness of 30°SR, thicken it to an oven-dry mass concentration of 1.0%, adjust the pH value to 5.8, and heat it to 45℃ to obtain the slurry to be treated; Preparation of the auxiliary agent system: Prepare a modified chitosan working solution with a solid content of 1.0% and a pH of 5.0; prepare a nano-silica dispersion with a solid content of 3%, stir at high speed for 30 min and then mature for 30 min for later use; Bridged adsorption treatment: Add modified chitosan working solution to the slurry to be treated at a rate of 0.2% of the oven-dry slurry, and simultaneously add low molecular weight CPAM at a rate of 0.05% of the oven-dry slurry. The addition point is the outlet of the slurry forming tank. Mix at a shear rate of 1200 r / min for 20 s to obtain the pretreated slurry. Online closed-loop control: The PCD value of the pretreated slurry was detected to be 150 μmol / L, which is within the target range, and no adjustment of the addition amount is required; Microparticle-filled flocculation treatment: After the pretreated slurry is treated by pressure sieving, nano silica dispersion is added at a rate of 0.8% of the oven-dry slurry. The addition point is the outlet of the pressure sieving. The mixture is mixed for 10 seconds at a shear rate of 1800 r / min to obtain the slurry to be formed. Forming: Same as in Example 1, form into paper sheets with a basis weight of 120g / m².

[0035] Example 4 illustrates the simultaneous application of this technology across the entire pulp layer in an industrial production line. It was implemented on a linerboard production line with a speed of 1200 m / min. The specific steps are as follows: Slurry pretreatment: Surface layer slurry with a freeness of 40°SR, a concentration of 1.0%, a pH of 6.5, and a temperature of 45℃; Core layer slurry with a freeness of 32°SR, a concentration of 1.0%, a pH of 6.0, and a temperature of 45℃; Bottom layer slurry with a freeness of 30°SR, a concentration of 1.0%, a pH of 5.8, and a temperature of 45℃. Preparation of the auxiliary agent system: Modified chitosan working solution with a solid content of 1.0% and pH 5.0; Sodium-based bentonite dispersion with a solid content of 3%, continuously dispersed at high speed and then matured for later use; Layered and step-by-step addition: Top layer: 0.1% chitosan, 0.3% bentonite; Core layer: 0.15% chitosan, 0.5% bentonite, and 0.03% CPAM added simultaneously; Bottom layer: 0.2% chitosan, 0.8% bentonite, and 0.05% CPAM added simultaneously; Chitosan is added at the outlet of the slurry tank for each layer, and bentonite is added at the outlet of the pressure screen for each layer. Online closed-loop control: Real-time detection of PCD value of each layer of pretreated slurry, dynamic adjustment of chitosan addition amount to ensure that PCD of surface layer < 400 μmol / L, and PCD of core layer and bottom layer < 200 μmol / L; Continuous papermaking: Each layer of pulp enters the forming section simultaneously to complete the pressing, drying, sizing, and calendering processes, continuously producing 250g / m² linerboard, running continuously for 72 hours.

[0036] Example 5: This example demonstrates parameter boundary verification. The specific steps are as follows: Slurry pretreatment: Surface layer slurry with a freeness of 35°SR, a concentration of 0.8%, a pH of 5.5, and a temperature of 35℃; Core layer slurry with a freeness of 28°SR, a concentration of 1.2%, a pH of 8.0, and a temperature of 55℃; Preparation of the additive system: The modified chitosan working solution has a solid content of 0.5% and a pH value of 4.0; the nano-silica dispersion has a solid content of 2%. Bridged adsorption treatment: Chitosan addition amount of surface layer is 0.05%, shear rate is 500 r / min, and mixing time is 30 s; Chitosan addition amount of core layer is 0.3%, shear rate is 1500 r / min, and mixing time is 10 s; Microparticle-filled flocculation treatment: Surface layer nano-silica addition amount 0.2%, shear rate 1000 r / min, mixing for 20 s; Core layer nano-silica addition amount 1.0%, shear rate 2000 r / min, mixing for 5 s; Copying and forming: Same as in Example 1, copy the corresponding paper pages.

[0037] Comparative Example 1: This comparative example is a traditional CPAM-bentonite microparticle system, used as a comparison with Example 2. The specific steps are as follows: Slurry pretreatment: Same as in Example 2, take the core layer OCC recycled slurry, with a freeness of 32°SR, a concentration of 1.0%, a pH of 6.0, and a temperature of 45°C; Preparation of additives: Prepare a high molecular weight CPAM working solution (weight average molecular weight 8000 kDa, cationicity 15%) with a solid content of 0.1%; prepare a bentonite dispersion as in Example 2; Stepwise addition: The amount of CPAM added is 0.03% of the oven-dry slurry, and the addition point is the outlet of the slurry forming tank; the amount of bentonite added is 0.5% of the oven-dry slurry, and the addition point is the outlet of the pressure screen; the shearing conditions are the same as in Example 2; Forming: Same as in Example 2, form paper sheets with a basis weight of 120g / m².

[0038] Comparative Example 2: This comparative example is a one-step chitosan single-system addition process, compared with Example 2. The specific steps are as follows: Slurry pretreatment: Same as in Example 2; Preparation of additives: Same as the modified chitosan working solution in Example 2; One-step addition: Chitosan working solution is added to the slurry in one step. The amount added is 0.15% of the oven-dry slurry. The addition point is the outlet of the slurry forming tank. The shear mixing conditions are the same as the bridging adsorption treatment in Example 2. Forming: Same as in Example 2, form paper sheets with a basis weight of 120g / m².

[0039] Comparative Example 3: This comparative example uses a one-step chitosan-bentonite addition process, compared with Example 2. The specific steps are as follows: Slurry pretreatment: Same as in Example 2; Preparation of additives: Same as the chitosan working solution and bentonite dispersion in Example 2; One-step addition: After mixing the chitosan working solution and bentonite dispersion in advance, add them to the slurry all at once. The total amount added is the same as in Example 2. The addition point is the outlet of the slurry tank. The shear mixing conditions are the same as the bridging adsorption treatment in Example 2. Forming: Same as in Example 2, form paper sheets with a basis weight of 120g / m².

[0040] In the above embodiments and comparative examples, the raw materials and equipment used were all commercially available conventional products: Chitosan: High linear cationic chitosan is used, with a degree of deacetylation of 88% and a weight-average molecular weight of 1200 kDa; Sodium bentonite: 2000 mesh; Nano-silica: average particle size 30nm; Low molecular weight CPAM: weight average molecular weight 500 kDa, cationicity 20%; Pulp: The surface layer is made of bleached softwood pulp and hardwood pulp (ratio 3:7), and the core and bottom layers are made of OCC recycled waste cardboard pulp; Testing equipment: dynamic water filter, particle charge meter, paper physical property tester; Testing standards: First-pass retention rate is tested according to GB / T2678.2-2021, physical properties of finished paper are tested according to GB / T13024-2016 "Corrugated Board", and PCD value of white water is tested according to industry standards.

[0041] The laboratory performance test results of Examples 1-3 and Comparative Examples 1-3 are shown in the table below:

[0042] The results of continuous operation testing of the industrial production line in Example 4 are as follows: the average retention rate of the surface layer is 52.8%, the core layer is 68.7%, and the bottom layer is 78.5%, all meeting the project's technical requirements; the average PCD of the white water under the wire is 315 μmol / L for the surface layer, 172 μmol / L for the core layer, and 128 μmol / L for the bottom layer, which is stably controlled within the target range; the average operating efficiency of the paper machine is 96.8%, an increase of 1.2% compared to before the modification; the wet end paper breakage rate decreased by 12.3% compared to before the modification, and the overall usage of retention aid decreased by 11.5%; all physical indicators of the finished paper are better than the enterprise standard, and there are no quality abnormalities in continuous 72-hour production, demonstrating excellent process stability.

[0043] Example 5 Parameter boundary verification results: First pass retention rate of surface layer 51.2%, PCD of white water 380 μmol / L; First pass retention rate of core layer 67.8%, PCD of white water 185 μmol / L; The physical properties of the finished paper all meet the enterprise standard requirements, proving that the process parameter range of the present invention has good applicability and stability.

[0044] In summary, the papermaking chitosan-based retention and filtration aid stepwise control method proposed in this invention achieves high retention rate and low white water contaminants, while adapting to high-speed paper machine high-shear conditions, compensating for the loss of paper strength caused by high retention, and meeting the dual requirements of clean production and product quality.

[0045] The embodiments of the present invention have been described above with reference to the accompanying drawings. However, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many improvements and modifications under the guidance of the present invention without departing from the spirit and scope of the claims. These improvements and modifications should also be considered within the scope of protection of the present invention.

[0046] In the description of this invention, it should be understood that the terms "center", "longitudinal", "lateral", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0047] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0048] Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

Claims

1. A stepwise control method for papermaking chitosan-based retention and filtration aids, characterized in that, Includes the following steps: S1. The pulps for the papermaking surface layer, core layer, and bottom layer are subjected to beating, concentration adjustment, pH adjustment, and temperature adjustment to obtain three groups of pulps to be treated. The freeness of the surface layer pulp is 35-45°SR, the freeness of the core layer and bottom layer pulps is 28-38°SR, the oven-dry mass concentration of the three groups of pulps is 0.8%-1.2%, the pH value is 5.5-8.0, and the temperature is 35-55℃. S2. Prepare a modified chitosan working solution and an inorganic microparticle dispersion, respectively. The modified chitosan working solution has a degree of deacetylation of 80%-95% and a weight-average molecular weight ≥1×10⁻⁶. 6 The acetic acid-water solution of highly linear cationic chitosan has a solid content of 0.5%-2.0% and a pH of 4.0-6.0; the inorganic particulate dispersion is an aqueous dispersion of sodium-based bentonite or nano-silica with a solid content of 2%-5%. S3. Add the modified chitosan working solution to the surface layer, core layer and bottom layer of the slurry to be treated in the corresponding proportions, respectively, and stir and mix under the set shear conditions to complete the charge neutralization and polymer bridging of the fibers and fine components in the slurry, and obtain the pretreated slurry. S4. After pressure screening of the pretreated slurry, the inorganic microparticle dispersion is continuously added to the surface layer, core layer and bottom layer of the pretreated slurry according to the corresponding ratio. The mixture is stirred and mixed under the set shear conditions. The dense and shear-resistant fiber flocs are formed through the microparticle filling effect to obtain the slurry to be formed. S5. Simultaneously convey the slurry to be formed for the surface layer, core layer and bottom layer to the forming section of the paper machine to complete the papermaking, pressing, drying, sizing and calendering processes to obtain the finished paper.

2. The step-by-step control method for papermaking chitosan-based retention and filtration aids according to claim 1, characterized in that, In step S1, the pH value of the surface layer slurry is 6.0-7.0, the pH value of the core layer and bottom layer slurry is 5.5-6.5, and the temperature of the three slurries is 40-50℃.

3. The stepwise control method for papermaking chitosan-based retention and filtration aids according to claim 1, characterized in that, In step S3, the amount of modified chitosan working solution added is based on the oven-dry weight: 0.05%-0.15% of the oven-dry slurry for the surface layer, 0.1%-0.2% of the oven-dry slurry for the core layer, and 0.15%-0.3% of the oven-dry slurry for the bottom layer.

4. The step-by-step control method for papermaking chitosan-based retention and filtration aids according to claim 1, characterized in that, In step S3, the modified chitosan working solution is added at the main pipeline of the slurry layer outlet, the shear rate of stirring and mixing is 500-1500 r / min, and the mixing time is 10-30 s.

5. The stepwise control method for papermaking chitosan-based retention and filtration aids according to claim 1, characterized in that, In step S4, the amount of inorganic microparticle dispersion added is based on the oven-dry mass: 0.2%-0.5% of the oven-dry slurry for the surface layer, 0.3%-0.8% of the oven-dry slurry for the core layer, and 0.5%-1.0% of the oven-dry slurry for the bottom layer.

6. The stepwise control method for papermaking chitosan-based retention and filtration aids according to claim 1, characterized in that, In step S4, the inorganic microparticle dispersion is added at the slurry pipeline at the outlet of the pressure screen of the corresponding slurry layer, the shear rate of stirring and mixing is 1000-2000 r / min, and the mixing time is 5-20 s.

7. The step-by-step control method for papermaking chitosan-based retention and filtration aids according to claim 1, characterized in that, When the required cationic content of the pretreated slurry is >400 μmol / L, the amount of modified chitosan working solution added should be increased accordingly; when the required cationic content of the slurry is <100 μmol / L, the amount of modified chitosan working solution added should be decreased accordingly.

8. The stepwise control method for papermaking chitosan-based retention and filtration aids according to claim 1, characterized in that, In step S2, the purified highly linear cationic chitosan is dissolved in an aqueous solution of acetic acid with a mass fraction of 1%-2%. After stirring until completely dissolved, the insoluble matter is removed by filtration. The solid content is adjusted to the target value with deionized water, and the pH value is adjusted to 4.0-6.0 with acetic acid or sodium hydroxide to obtain the modified chitosan working solution.

9. The stepwise control method for papermaking chitosan-based retention and filtration aids according to claim 1, characterized in that, In step S2, the sodium-based bentonite has a sieve mesh size ≥ 2000 mesh, and the average particle size of the nano-silica is 10-100 nm. When preparing the inorganic microparticle dispersion, the inorganic microparticles are first added to deionized water under high-speed stirring conditions, stirred and dispersed for 20-40 min, and then allowed to stand for more than 30 min for later use.

10. The stepwise control method for papermaking chitosan-based retention and filtration aids according to claim 1, characterized in that, In step S3, a low molecular weight cationic polyacrylamide solution is also added simultaneously, with an addition amount of 0.01%-0.05% of the oven-dry slurry mass. It is added and mixed at the same point and through the same pipeline as the modified chitosan working solution.

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