Preparation process of low-dirt-degree kraft paper pulp

Abstract

The invention discloses a preparation process of low-dust-degree craft paper pulp, and belongs to the technical field of craft paper. According to the technical scheme, the method comprises the following steps: 1) pulping and primary screening: fiber raw material waste paper enters a rotary drum pulper from a chain scraper conveyor, and large impurities which are not crushed are screened out; (2) preliminary purification and flotation: enabling the screened slurry to enter two sections of coarse screens through two sections of high-concentration slag removers, enabling the slurry to enter a flotation impurity removing machine, and screening out impurities larger than 14mm through a cylinder screen; 3, fiber grading, wherein the pulp is subjected to grading screening, short fibers, medium fibers and long fibers are screened out, and 4, the short fibers, the medium fibers and the long fibers are treated respectively to obtain the corresponding pulp. Fiber grading fine treatment and efficient deep impurity removal are achieved, and the dust degree of the kraft linerboards can be remarkably reduced.

Description

Technical Field

[0001] This invention belongs to the field of kraft paper technology, specifically relating to a preparation process for low-dust kraft paper pulp. Background Technology

[0002] Kraft paper, as a high-strength packaging paper, has high requirements for its physical strength (such as ring crush strength and bursting strength) and cleanliness. As market demand for kraft paper products with high surface cleanliness gradually increases, the cleanliness requirements for kraft paper raw materials used in high-end tobacco and alcohol packaging boxes, electronic product packaging boxes, and other products are even higher. Dust content, as an important indicator of cleanliness, can reflect the cleanliness of the paper surface to a certain extent. However, the main raw materials for kraft paper production are recycled cardboard waste paper, office waste paper, and other secondary fibers (OCC). These secondary fiber raw materials contain a large amount of impurities such as ink, laminating, coating particles, adhesive particles, and grit, as well as undyed fiber bundles, coating white spots, dust, and wax spots, which seriously affect the cleanliness of the kraft paper surface.

[0003] In existing kraft paper production, the process route for OCC pulp is generally "shredding-coarse screening-(direct) deslagging". Due to the large differences in fiber length and morphology in ungraded pulp, the working environment of heavy-duty deslagging equipment is complex and inefficient. Specifically: a. Long fibers are prone to entanglement and obscuring fine heavy impurities, making them difficult to separate effectively by centrifugation; b. The mass ratio of fibers of different lengths to impurities varies greatly, making it difficult to remove all impurities under fixed deslagging parameters, resulting in incomplete deslagging and a bottleneck in improving paper surface cleanliness; c. Increasing the intensity or number of deslagging cycles in pursuit of cleanliness will lead to increased loss of good pulp fibers and increased energy consumption.

[0004] Therefore, developing a kraft pulp preparation process that can achieve fine fiber grading, efficient and deep impurity removal, and significantly reduce paper dust content has become an urgent problem to be solved in this field. Summary of the Invention

[0005] This invention provides a preparation process for low-dust kraft paper pulp, which achieves fine fiber grading, efficient and deep impurity removal, and can significantly reduce the dust content of kraft paper.

[0006] The technical solution of this invention is as follows: The preparation process of low-dust kraft paper pulp includes the following steps: 1) Pulping and primary screening: Fiber raw material waste paper enters the rotary drum pulper from the chain conveyor. The first section of the rotary drum pulper is the pulping section, which accounts for 2 / 3 of the length of the rotary drum pulper. The pulp concentration is 18-20%. The waste paper separated into fibers enters the screening section, which accounts for 1 / 3 of the length of the rotary drum pulper. The pulp concentration is 3.5-5.0%. The cylindrical screen of the screening section has sieve holes with a diameter of 6-10mm to screen out large impurities that have not been crushed. 2) Preliminary purification and flotation: The screened slurry passes through two high-consistency separators and enters two coarse screens. The slurry concentration in the high-consistency separators is 2.0-4.0%, and the inlet pressure is 0.2-0.35 MPa. The slurry concentration in the coarse screen system is 2.0-3.5%, and the inlet pressure is 0.15-0.25 MPa. The slurry then enters a flotation cleaning machine (to remove light impurities and deink), and is then screened by a cylindrical screen to remove impurities >14mm. 3) Fiber grading: The pulp enters the grading screen, which is divided into a first-stage grading screen and a second-stage grading screen. The first-stage grading screen filters out short fibers with a screen gap size of 0.15-0.20mm. The second-stage grading screen filters out medium and long fibers with a screen gap size of 0.25-0.30mm. Fibers that can pass through the screen gap are medium fibers, while those that cannot pass through will continue to rise and enter the top of the second-stage grading screen. Long fibers are discharged from the pulp outlet pipe. 4) Short fiber treatment: The short fibers pass through a three-stage low-concentration slag remover, the tailings are discharged, and the good pulp enters a multi-disc thickener. After passing through a hot disperser, it enters pulp tower one for use in the core layer and bottom layer. Medium fiber processing: The medium fiber pulp passes through three low-concentration deslagging devices, and the tailings are discharged. The good pulp passes through two light deslagging devices and then enters a multi-disc thickener, and then enters the pulp tower two through a hot disperser for use in the surface and bottom layers. Long fiber processing: Long fibers pass through a five-stage low-consistency deslagging device, the tailings are discharged, the good pulp enters a three-stage fine screen and then enters a multi-disc thickener, then enters the pulp tower three through a hot disperser, is refined by a refiner, and enters the paper machine for use in the core layer.

[0007] Preferably, in step 4), the initial pulp concentration of the first-stage low-concentration separator for short fibers is 0.5-2.0%, the initial pulp pressure is 0.16-0.25 MPa, and the good pulp pressure is 0.03-0.1 MPa; the initial pulp concentration of the second-stage low-concentration separator is 0.4-2.0%, the initial pulp pressure is 0.16-0.25 MPa, and the good pulp pressure is 0.03-0.1 MPa; the initial pulp concentration of the third-stage low-concentration separator is 0.4-2.0%, the initial pulp pressure is 0.16-0.25 MPa, and the good pulp pressure is 0.03-0.1 MPa; the initial pulp concentration of the multi-disc thickener for short fibers is 0.5%-1.5%, and the output pulp concentration is 10%-12%; the temperature of the short fiber thermal disperser is 90-120℃, the inlet concentration of the pressing screw is 8-12%, the outlet concentration is 25-40%, and the torque is 25-40%.

[0008] Preferably, in step 4), the pulp concentration of the first-stage low-consistency separator for medium-fiber fibers is 0.5-2.0%, the pulp pressure is 0.16-0.25 MPa, and the good pulp pressure is 0.03-0.1 MPa; the pulp concentration of the second-stage low-consistency separator is 0.4-2.0%, the pulp pressure is 0.16-0.25 MPa, and the good pulp pressure is 0.03-0.1 MPa; the pulp concentration of the third-stage low-consistency separator is 0.4-2.0%, the pulp pressure is 0.16-0.25 MPa, and the good pulp pressure is 0.03-0.1 MPa; the pulp concentration of the first-stage light-weight separator for medium-fiber fibers is... The pulp concentration is 0.4-2.0%, the feed pressure is 0.16-0.25MPa, and the good pulp pressure is 0.05-0.1MPa; the pulp concentration of the medium fiber two-stage light slag remover is 0.1-1.5%, the feed pressure is 0.15-0.25MPa, and the good pulp pressure is 0.05-0.1MPa; the pulp concentration of the medium fiber multi-disc thickener is 0.5%-1.5%, and the output pulp concentration is 10%-12%; the temperature of the medium fiber hot disperser is 90-120℃, the inlet concentration of the pressing screw is 8-12%, the outlet concentration is 25-40%, and the torque is 25-40%.

[0009] Preferably, when the slurry entering the second slurry tower in step 4) is used for the surface layer, the medium-fiber slurry needs to undergo the following further processing: coarse screening → low-consistency separator → fine screening → multi-disc thickener → surface slurry tower; the screen gap in the coarse screening process is 0.45-0.55mm, the feed pressure is 0.25-0.35bar, the feed concentration in the low-consistency separator is 0.9%, and the feed pressure is 0.2-0.3bar; the screen gap in the fine screening process is 0.15mm, the feed pressure is 0.25-0.35bar, and the feed concentration in the multi-disc thickener is 0.5-1.0%.

[0010] Preferably, in step 4), the initial pulp concentration of the first-stage low-consistency separator for long fibers is 1.0-2.5%, the initial pulp pressure is 0.25-0.4 MPa, and the good pulp pressure is 0.12-0.22 MPa; the initial pulp concentration of the second-stage low-consistency separator is 1.0-2.5%, the initial pulp pressure is 0.15-0.25 MPa, and the good pulp pressure is 0.03-0.15 MPa; the initial pulp concentration of the third-stage low-consistency separator is 0.5-2.0%, and the initial pulp pressure is 0.15-0.25 MPa. MPa, good pulp pressure: 0.03-0.15MPa; long fiber four-stage low-consistency separator feed pulp concentration: 0.5-2.0%, feed pulp pressure: 0.15-0.25MPa, good pulp pressure: 0.02-0.1MPa; long fiber five-stage low-consistency separator feed pulp concentration: 0.5-2.0%, feed pulp pressure: 0.15-0.25MPa, good pulp pressure: 0.02-0.1MPa; long fiber one-stage fine screen feed pulp concentration: 0.7-1.4%, feed pulp pressure: 0.14-0. The feed concentration of the long fiber two-stage fine screen is 0.6-1.3%, and the feed pressure is 0.14-0.21MPa. The feed concentration of the long fiber three-stage fine screen is 0.2-0.8%, and the feed pressure is 0.15-0.25MPa. The feed concentration of the long fiber multi-disc thickener is 0.5%-1.5%, and the output concentration is 10%-12%. The temperature of the long fiber hot disperser is 90-120℃, the inlet concentration of the pressing screw is 8-12%, the outlet concentration is 25-40%, and the torque is 25-40%.

[0011] Short fibers in waste paper pulp are easily lost, so a shorter processing flow is needed than that for long and medium fibers to improve retention rate. Long fibers are prone to entanglement and cover fine heavy impurities, making them more difficult to clean. Medium fibers are easier to clean than long fibers and better retained than short fibers. Medium fibers are also stronger than short fibers and can be used in the surface layer. Therefore, different processes are needed to process fibers of different lengths.

[0012] Compared with the prior art, the present invention has the following advantages: 1. This invention uses a high pulp concentration (18-20%) drum pulping method, which utilizes inter-fiber friction and kneading separation to maximize the preservation of the original strength of the fibers and improve pulp yield.

[0013] 2. This invention proposes a novel process architecture of first grading and then differentiated deep purification, solving the technical problem of low slag removal efficiency in mixed pulp. Fiber grading and optimized allocation are achieved by using first-stage and second-stage grading screens with specific sieve sizes to precisely separate the pulp into short fibers, medium fibers, and long fibers. Based on the characteristics of different fibers and the performance requirements of each paper layer, long and short fibers are used in the core and bottom layers, where high strength is required, while the specially purified medium fibers are used in the top layer, where the highest cleanliness requirements are met. This maximizes the use of materials and optimizes the paper structure.

[0014] 3. This invention employs multi-stage slag removal, setting up three-stage, three-stage, and five-stage low-concentration slag removers for short, medium, and long fibers respectively, as well as a light slag remover for medium fibers, thereby achieving step-by-step, deep removal of both heavy and light impurities.

[0015] For medium-fiber slurry used in the surface layer, an ultra-clean treatment process is added, consisting of a coarse screen, a low-concentration slag remover, and a fine screen. The screen gap size is gradually reduced, which can effectively remove fine impurity particles and reduce the dust content of the surface layer slurry.

[0016] 4. This invention performs thermal dispersion treatment on short, medium and long fibers to effectively disperse hot-melt impurities such as ink and adhesives, making them easier to handle in subsequent processes or on paper machines and reducing dust visibility.

[0017] 5. This invention clarifies and optimizes the key process parameters (concentration, pressure, temperature, torque, etc.) of each stage of the slag remover, thickener, and thermal disperser, ensuring efficient and stable operation of each process and guaranteeing the uniformity and reliability of the final slurry quality.

[0018] 6. This invention achieves a balance of high strength, low dust content, and high yield by combining the synergistic effects of high-concentration pulping, graded screening, and treatment of fibers of different lengths. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the process of the present invention. Detailed Implementation

[0020] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions will be clearly and completely described below in conjunction with the embodiments of this invention.

[0021] Example 1 The preparation process of the low-dust kraft paper pulp includes the following steps: 1) Pulping and primary screening: Waste paper secondary fibers OCC are fed into the rotary drum pulper via a chain conveyor. The pulping section is 2 / 3 the length of the whole machine. Water is added in this section to adjust the pulp concentration to 19%. Under high concentration, the waste paper is disintegrated and separated into fibers by the friction between fibers. The pulp then enters the screening section, which accounts for 1 / 3 of the length. Water is added to dilute the pulp concentration to 4.2%, and the pulp passes through a screen cylinder with an 8mm aperture to remove large pieces of impurities that have not been broken up.

[0022] 2) Preliminary purification and flotation The screened slurry passes sequentially through two high-consistency desanders (feed slurry concentration 3.0%, feed pressure 0.28 MPa) and two coarse screens (feed slurry concentration 2.8%, feed pressure 0.20 MPa); then it enters a flotation cleaning machine to remove light impurities such as ink and lightweight plastic flakes. After flotation, the slurry is further screened through a cylindrical screen (16 mm mesh) to remove larger impurities.

[0023] 3) Fiber grading: The preliminarily purified pulp is pumped into the grading system; First, the pulp is sieved through a grading screen (0.18mm gap) to remove short fiber pulp. Pulp that fails to pass through the first screen opening enters the second-stage grading screen (screen opening 0.28mm), while pulp that passes through the screen opening is medium fiber pulp; Long fibers that cannot pass through the two-stage sieve gaps continue to rise and are discharged from the top pulp outlet pipe, resulting in long fiber pulp. 4) A. Short fiber treatment (for core / bottom layer) The short fiber pulp sequentially passes through three low-consistency separators: the first separator has an inlet pulp concentration of 1.2% and an inlet pressure of 0.20 MPa, with a good pulp pressure of 0.05 MPa; the second separator has an inlet pulp concentration of 1.0% and an inlet pressure of 0.20 MPa, with a good pulp pressure of 0.05 MPa; and the third separator has an inlet pulp concentration of 0.8% and an inlet pressure of 0.20 MPa, with a good pulp pressure of 0.05 MPa. The tailings from the separators are discharged, and the good pulp enters a multi-disc thickener (inlet pulp concentration 1.0%, outlet pulp concentration 11%). After thickening, the pulp is processed by a thermal disperser (temperature 105℃, screw press inlet concentration 10%, outlet concentration 32%, torque 32%), and the processed pulp is sent to pulp tower one for later use.

[0024] B. Medium-fiber treated line (for top / bottom layers) The medium-fiber pulp sequentially passes through three low-consistency separators: the first low-consistency separator has an inlet pulp concentration of 1.6% and an inlet pressure of 0.2 MPa, with a good pulp pressure of 0.06 MPa; the second low-consistency separator has an inlet pulp concentration of 1.5% and an inlet pressure of 0.2 MPa, with a good pulp pressure of 0.06 MPa; the third low-consistency separator has an inlet pulp concentration of 1.2% and an inlet pressure of 0.2 MPa, with a good pulp pressure of 0.06 MPa. The good pulp then enters two light-consistency separators. The feed pulp concentration is 1.0%, the feed pulp pressure is 0.20 MPa, and the good pulp pressure is 0.08 MPa; the feed pulp concentration of the medium fiber secondary light slag remover is 0.12%, the feed pulp pressure is 0.20 MPa, and the good pulp pressure is 0.08 MPa; the good pulp enters the multi-disc thickener (feed pulp concentration 1.2%, output pulp concentration 11%), and then is processed by the hot disperser (temperature 110℃, press screw inlet concentration 11%, outlet concentration 30%, torque 30%) before being sent to the second pulp tower for standby.

[0025] When the fibers in the second pulp tower are used for the surface layer, further processing is required: a portion of the fiber pulp is taken from the second pulp tower and subjected to ultra-purification treatment: first, it passes through a coarse screen (screen gap 0.50mm, feed pressure 0.30bar), and the good pulp enters a low-concentration slag remover (feed concentration 0.9%, feed pressure 0.25bar), the good pulp enters a fine screen (screen gap 0.15mm, feed pressure 0.30bar), and the good pulp enters a multi-disc thickener (feed concentration 0.8%) for processing before being sent to a dedicated surface layer pulping tower.

[0026] C. Long fiber treated yarn (for the core layer) The long fiber pulp passes through five low-consistency separators in sequence: the first low-consistency separator has an inlet pulp concentration of 2.0%, an inlet pulp pressure of 0.32 MPa, and a good pulp pressure of 0.2 MPa; the second low-consistency separator has an inlet pulp concentration of 1.8%, an inlet pulp pressure of 0.20 MPa, and a good pulp pressure of 0.15 MPa; the third low-consistency separator has an inlet pulp concentration of 1.2%, an inlet pulp pressure of 0.20 MPa, and a good pulp pressure of 0.15 MPa; the fourth low-consistency separator has an inlet pulp concentration of 1.0%, an inlet pulp pressure of 0.20 MPa, and a good pulp pressure of 0.05 MPa; and the fifth low-consistency separator has an inlet pulp concentration of 1.0%, an inlet pulp pressure of 0.20 MPa, and a good pulp pressure of 0.05 MPa. The high-quality pulp enters a three-stage fine screening system. The first-stage fine screening for long fibers has a pulp concentration of 1.0% and an inlet pressure of 0.18 MPa; the second-stage fine screening has a pulp concentration of 1.0% and an inlet pressure of 0.16 MPa; and the third-stage fine screening has a pulp concentration of 0.5% and an inlet pressure of 0.20 MPa, further removing fine impurities. The high-quality pulp after fine screening is concentrated in a multi-disc thickener (inlet concentration 1.3%, outlet concentration 12%), and then processed in a thermal disperser (temperature 100℃, inlet concentration of 9% for the press screw, outlet concentration of 28%, torque 35%). Finally, the long-fiber pulp undergoes light refining (freezing degree increased to 28°SR) and is sent to pulp tower three for standby.

[0027] Example 2 The preparation process of the low-dust kraft paper pulp includes the following steps: 1) Pulping and primary screening: Waste paper secondary fibers OCC are fed into the rotary drum pulper via a chain conveyor. The pulping section is 2 / 3 the length of the whole machine. Water is added in this section to adjust the pulp concentration to 20%. Under high concentration, the waste paper is disintegrated and separated into fibers by the friction between the fibers. The pulp then enters the screening section, which accounts for 1 / 3 of the length. Water is added to dilute the pulp concentration to 4.5%, and the pulp passes through a 7mm sieve cylinder to remove large pieces of impurities that have not been broken up.

[0028] 2) Preliminary purification and flotation The screened slurry passes sequentially through two high-consistency desanders (feed slurry concentration 3.5%, feed pressure 0.3 MPa) and two coarse screens (feed slurry concentration 2.5%, feed pressure 0.25 MPa); then it enters a flotation cleaning machine to remove light impurities such as ink and lightweight plastic flakes. After flotation, the slurry is further screened through a cylindrical screen (15 mm mesh) to remove larger impurities.

[0029] 3) Fiber grading: The preliminarily purified pulp is pumped into the grading system; First, the short fiber pulp is sieved through a grading screen (0.2mm gap). Pulp that fails to pass through the first screen opening enters the second-stage grading screen (screen opening 0.25mm), while pulp that passes through the screen opening is medium fiber pulp; Long fibers that cannot pass through the two-stage sieve gaps continue to rise and are discharged from the top pulp outlet pipe, resulting in long fiber pulp. 4) A. Short fiber treatment (for core / bottom layer) The short fiber pulp sequentially passes through three low-consistency separators: the first separator has an inlet pulp concentration of 1.8% and an inlet pressure of 0.25 MPa, with a good pulp pressure of 0.06 MPa; the second separator has an inlet pulp concentration of 1.2% and an inlet pressure of 0.25 MPa, with a good pulp pressure of 0.06 MPa; and the third separator has an inlet pulp concentration of 1.0% and an inlet pressure of 0.25 MPa, with a good pulp pressure of 0.06 MPa. The tailings from the separators are discharged, and the good pulp enters a multi-disc thickener (inlet pulp concentration 1.2%, outlet pulp concentration 12%). After thickening, the pulp is processed by a thermal disperser (temperature 108℃, inlet concentration of the press screw 12%, outlet concentration 30%, torque 30%), and the processed pulp is then sent to pulp tower one for later use.

[0030] B. Medium-fiber treated line (for top / bottom layers) The medium-fiber pulp sequentially passes through three low-consistency separators: the first low-consistency separator has an inlet pulp concentration of 2.0% and an inlet pressure of 0.25 MPa, with a good pulp pressure of 0.08 MPa; the second low-consistency separator has an inlet pulp concentration of 1.5% and an inlet pressure of 0.25 MPa, with a good pulp pressure of 0.08 MPa; the third low-consistency separator has an inlet pulp concentration of 1.2% and an inlet pressure of 0.25 MPa, with a good pulp pressure of 0.08 MPa. The good pulp then enters two light-consistency separators, and the medium-fiber pulp passes through one light-consistency separator. The feed pulp concentration of the medium fiber slurry separator is 2.0%, the feed pulp pressure is 0.25 MPa, and the good pulp pressure is 0.1 MPa; the feed pulp concentration of the medium fiber secondary light slurry separator is 1.5%, the feed pulp pressure is 0.25 MPa, and the good pulp pressure is 0.1 MPa; the good pulp enters the multi-disc thickener (feed pulp concentration 1.5%, output pulp concentration 12%), and then is processed by the hot disperser (temperature 115℃, press screw inlet concentration 12%, outlet concentration 30%, torque 30%) before being sent to the second pulp tower for standby.

[0031] When the fibers in the second pulp tower are used for the surface layer, further processing is required: a portion of the fiber pulp is taken from the second pulp tower and subjected to ultra-purification treatment: first, it passes through a coarse screen (screen gap 0.50mm, feed pressure 0.30bar), and the good pulp enters a low-concentration slag remover (feed concentration 0.9%, feed pressure 0.25bar), the good pulp enters a fine screen (screen gap 0.15mm, feed pressure 0.30bar), and the good pulp enters a multi-disc thickener (feed concentration 0.8%) for processing before being sent to a dedicated surface layer pulping tower.

[0032] C. Long fiber treated yarn (for the core layer) The long-fiber pulp sequentially passes through five low-consistency separators: Stage 1: Inlet pulp concentration 1.8%, inlet pressure 0.25 MPa, good pulp pressure 0.18 MPa; Stage 2: Inlet pulp concentration 1.6%, inlet pressure 0.25 MPa, good pulp pressure 0.15 MPa; Stage 3: Inlet pulp concentration 1.0%, inlet pressure 0.25 MPa, good pulp pressure 0.15 MPa; Stage 4: Inlet pulp concentration 2.0%, inlet pressure 0.25 MPa. The pressure of the good pulp is 0.1 MPa; the feed pulp concentration of the five-stage low-consistency separator for long fibers is 2.0%, the feed pulp pressure is 0.25 MPa, and the good pulp pressure is 0.1 MPa; the good pulp enters the three-stage fine screening system. The feed pulp concentration of the first stage fine screening for long fibers is 1.4%, the feed pulp pressure is 0.21 MPa, the feed pulp concentration of the second stage fine screening for long fibers is 1.3%, the feed pulp pressure is 0.21 MPa, and the feed pulp concentration of the third stage fine screening for long fibers is 0.8%, the feed pulp pressure is 0.25 MPa; further screening removes fine impurities. After fine screening, the good pulp is concentrated by a multi-disc thickener (feed pulp concentration 1.3%, output pulp concentration 12%), and then processed by a thermal disperser (temperature 100℃, press screw inlet concentration 9%, outlet concentration 28%, torque 35%). Finally, the long fiber pulp is lightly refined by a refiner (freezing degree increased to 30°SR) and sent to pulp tower three for standby.

[0033] Comparative Example 1 Fiber grading was not performed in this comparative example. The specific steps are as follows: Steps 1) and 2) are the same as in Example 1; 3) The good pulp, after preliminary purification and flotation in step 2), sequentially passes through three low-consistency separators: First stage: pulp concentration 1.2%, inlet pressure 0.20 MPa, good pulp pressure 0.05 MPa; Second stage: pulp concentration 1.0%, inlet pressure 0.20 MPa, good pulp pressure 0.05 MPa; Third stage: pulp concentration 0.8%, inlet pressure 0.20 MPa, good pulp pressure 0.05 MPa. The tailings from the separators are discharged, and the good pulp enters a multi-disc thickener (inlet pulp concentration 1.0%, outlet pulp concentration 11%). The thickened pulp is then processed by a thermal disperser (temperature 105℃, press screw inlet concentration 10%, outlet concentration 32%, torque 32%). The processed pulp is then fed into the pulp tower for use in each layer of the paper machine.

[0034] Comparative Example 2 Unlike Example 1, in step B of this comparative example, when the fibers in the second slurry tower are used for the surface layer, no further processing of the surface layer slurry is included; the remaining preparation methods and steps are the same as in Example 1.

[0035] Comparative Example 3 Unlike Example 1, in step 1) of this comparative example, the slurry concentration of the pulping section is replaced with "5%" instead of "19%", while the rest of the preparation methods and steps are the same as in Example 1.

[0036] Comparative Example 4 Unlike Example 1, step 3) of this comparative example is as follows: Fiber grading: The preliminarily purified pulp is pumped into the grading system; First, the pulp is sieved through a grading screen (0.18mm gap) to remove short fiber pulp. Pulp that does not pass through a screen gap is used directly as medium- and long-fiber pulp; Step 4) is A. Short fiber treatment (for the core / bottom layer). The short fiber pulp sequentially passes through three low-consistency separators: the first separator has an inlet pulp concentration of 1.2% and an inlet pressure of 0.20 MPa, with a good pulp pressure of 0.05 MPa; the second separator has an inlet pulp concentration of 1.0% and an inlet pressure of 0.20 MPa, with a good pulp pressure of 0.05 MPa; and the third separator has an inlet pulp concentration of 0.8% and an inlet pressure of 0.20 MPa, with a good pulp pressure of 0.05 MPa. The tailings from the separators are discharged, and the good pulp enters a multi-disc thickener (inlet pulp concentration 1.0%, outlet pulp concentration 11%). After thickening, the pulp is processed by a thermal disperser (temperature 105℃, screw press inlet concentration 10%, outlet concentration 32%, torque 32%), and the processed pulp is sent to pulp tower one for later use.

[0037] B. Undifferentiated medium-length blended fibers are processed according to the medium-length fiber processing line. The fiber pulp sequentially passes through three low-consistency desanders: the first low-consistency desander has an inlet pulp concentration of 1.6% and an inlet pressure of 0.2 MPa, with a good pulp pressure of 0.06 MPa; the second low-consistency desander has an inlet pulp concentration of 1.5% and an inlet pressure of 0.2 MPa, with a good pulp pressure of 0.06 MPa; the third low-consistency desander has an inlet pulp concentration of 1.2% and an inlet pressure of 0.2 MPa, with a good pulp pressure of 0.06 MPa. The good pulp then enters two light-consistency desanders, and the fiber pulp undergoes a first-stage light-consistency desisting process. The feed pulp concentration is 1.0%, the feed pulp pressure is 0.20 MPa, and the good pulp pressure is 0.08 MPa; the feed pulp concentration of the fiber stage light slag remover is 0.12%, the feed pulp pressure is 0.20 MPa, and the good pulp pressure is 0.08 MPa; the good pulp enters the multi-disc thickener (feed pulp concentration 1.2%, output pulp concentration 11%), and then is processed by the hot disperser (temperature 110℃, press screw inlet concentration 11%, outlet concentration 30%, torque 30%) before being sent to the pulp tower for standby.

[0038] Further processing when the fiber from the second pulp tower is used for the surface layer: Take a portion of the fiber pulp from the second pulp tower and perform ultra-purification treatment: First, pass it through a coarse screen (screen gap 0.50mm, inlet pressure 0.30bar). The good pulp enters a low-concentration slag remover (inlet concentration 0.9%, inlet pressure 0.25bar). The good pulp enters a fine screen (screen gap 0.15mm, inlet pressure 0.30bar). The good pulp enters a multi-disc thickener (inlet concentration 0.8%) and is then sent to a dedicated surface layer pulping tower.

[0039] The remaining preparation methods and steps are the same as in Example 1.

[0040] The slurries from the examples and comparative examples were prepared to a concentration of 150 g / m³. 2 Kraft paper was used, and its phase performance was tested.

[0041] Dust content testing is performed in accordance with the method of GB / T 1541 Test of dust content in paper and paperboard. The criteria for judging the test results are formulated in combination with industry application requirements, enterprise internal control standards and customer feedback on the cleanliness of high-grade kraft paper.

[0042] Ring crush strength: Refer to GB-T 2679.8-1995 "Determination of ring crush strength of paper and paperboard". Take two paper samples each, 100mm wide and 200mm long, from the operating side, the drive side, and the middle position. Measure the sample thickness and select the inner plate of the sample holder according to the sample thickness. Carefully insert the sample into the sample holder, ensuring it is fully inserted. Place the sample holder in the middle of the lower pressure plate, with the sample ring opening facing the operator. Then start the instrument to compress the sample until it crushes. For electronic instruments with a fixed plate, directly read the pressure value, accurate to 1N.

[0043] Fiber retention rate: Total fiber yield refers to "the percentage of fiber mass ultimately used for paper machine forming relative to the total fiber mass in the initial waste paper raw material". The testing method is based on the general paper industry standards GB / T 2678.1-2011 Determination of Fiber Composition of Paper, Paperboard and Pulp Part 1: Paper and Paperboard and GB / T 7979-2013 Determination of Pulp Ash Content" and is formulated according to process characteristics. The total fiber yield is calculated by accurately weighing the "effective fiber mass" (after deducting raw material ash and non-fiber impurities) in the initial raw material and comparing it with the "total effective fiber mass" of the pulp in each pulp tower before final paper machine forming. The core formula is: .

[0044] Table 1

[0045] Note: Superior grade products are 1.0-4.0mm. 2 Dust particles ≤ 45 / m 2 First-class products are 1.0-4.0mm thick. 2 Dust ≤ 60 particles / m 2 The acceptable thickness is 1.0-4.0mm. 2 61-100 dust particles / m 2 The defective product has a diameter of 1.0-4.0mm. 2 Dust particles ≥ 120 / m 2 .

[0046] Compared with Comparative Example 1, the dust content of Example 1 was significantly lower than that of Comparative Example 1, which directly proves that fiber classification is a prerequisite for efficient slag removal. After classification, the fiber length of the object processed by the slag remover is uniform, avoiding the masking effect of long fibers on impurities. This allows light and heavy impurities to be efficiently separated by centrifugation in fiber slurries of different particle sizes, breaking through the bottleneck of slag removal efficiency pointed out in the background art.

[0047] Compared to Comparative Example 2, the dust content of Example 1 was significantly better than that of Comparative Example 2, demonstrating that the surface ultra-clean treatment (coarse screening → slag removal → fine screening) step is an indispensable key invention. This step specifically removes the fine impurity particles remaining in the medium-fiber pulp that have the greatest impact on visual appearance and printing, thereby reducing the dust content of the kraft paper.

[0048] Compared to Example 1, Comparative Example 3 used conventional low-concentration (5%) pulp. Although the dust content was acceptable, the paper strength was significantly reduced, rendering it unusable. This verifies the necessity of high-concentration (18-20%) pulp. High-concentration pulping relies on inter-fiber frictional disintegration, maximizing the protection of fiber length and strength, thus laying the foundation for the high physical properties of the final product.

[0049] Compared to Example 1, Comparative Example 4 only separated short fibers, while medium and long fibers were mixed. The dust content of the resulting paper was between that of Example 1 and Comparative Example 1. This demonstrates the unique value of two-stage precise separation of medium and long fibers. Due to their strong entanglement, long fibers contain impurities in pulp with different characteristics than medium fibers, and mixing them would interfere with the purification effect. This invention separates them and employs differentiated deep purification methods—"five-stage slag removal, three-stage fine screening" and "three-stage slag removal, two-stage lightweight slag removal"—to achieve optimal impurity removal.

[0050] Although the present invention has been described in detail by way of preferred embodiments, the invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A process for preparing low-dust kraft paper pulp, characterized in that, Includes the following steps: 1) Pulping and primary screening: Fiber raw material waste paper enters the rotary drum pulper from the chain conveyor. The first section of the rotary drum pulper is the pulping section, which accounts for 2 / 3 of the length of the rotary drum pulper. The pulp concentration is 18-20%. The waste paper separated into fibers enters the screening section, which accounts for 1 / 3 of the length of the rotary drum pulper. The pulp concentration is 3.5-5.0%. The cylindrical screen of the screening section has sieve holes with a diameter of 6-10mm to screen out large impurities that have not been crushed. 2) Preliminary purification and flotation: The screened slurry passes through two high-consistency separators and enters two coarse screens. The slurry concentration in the high-consistency separators is 2.0-4.0%, and the inlet pressure is 0.2-0.35 MPa. The slurry concentration in the coarse screen system is 2.0-3.5%, and the inlet pressure is 0.15-0.25 MPa. The slurry then enters the flotation cleaning machine and is further screened by a cylindrical screen to remove impurities >14mm. 3) Fiber grading: The pulp enters the grading screen, which is divided into a first-stage grading screen and a second-stage grading screen. The first-stage grading screen filters out short fibers with a screen gap size of 0.15-0.20mm. The second-stage grading screen filters out medium and long fibers with a screen gap size of 0.25-0.30mm. Fibers that can pass through the screen gap are medium fibers, while those that cannot pass through will continue to rise and enter the top of the second-stage grading screen. Long fibers are discharged from the pulp outlet pipe. 4) Short fiber treatment: The short fibers pass through a three-stage low-concentration slag remover, the tailings are discharged, and the good pulp enters a multi-disc thickener. After passing through a hot disperser, it enters pulp tower one for use in the core layer and bottom layer. Medium fiber processing: The medium fiber pulp passes through three low-concentration deslagging devices, and the tailings are discharged. The good pulp passes through two light deslagging devices and then enters a multi-disc thickener, and then enters the pulp tower two through a hot disperser for use in the surface and bottom layers. Long fiber processing: Long fibers pass through a five-stage low-consistency deslagging device, the tailings are discharged, the good pulp enters a three-stage fine screen and then enters a multi-disc thickener, then enters the pulp tower three through a hot disperser, is refined by a refiner, and enters the paper machine for use in the core layer.

2. The preparation process of low-dust kraft paper pulp as described in claim 1, characterized in that, Step 4) For the short fiber primary low-concentration separator, the feed pulp concentration is 0.5-2.0%, the feed pulp pressure is 0.16-0.25 MPa, and the good pulp pressure is 0.03-0.1 MPa; for the secondary low-concentration separator, the feed pulp concentration is 0.4-2.0%, the feed pulp pressure is 0.16-0.25 MPa, and the good pulp pressure is 0.03-0.1 MPa; for the tertiary low-concentration separator, the feed pulp concentration is 0.4-2.0%, the feed pulp pressure is 0.16-0.25 MPa, and the good pulp pressure is 0.03-0.1 MPa; for the short fiber multi-disc thickener, the feed pulp concentration is 0.5%-1.5%, and the output pulp concentration is 10%-12%; for the short fiber hot disperser, the temperature is 90-120℃, the inlet concentration of the pressing screw is 8-12%, the outlet concentration is 25-40%, and the torque is 25-40%.

3. The preparation process of low-dust kraft paper pulp as described in claim 1, characterized in that, Step 4) The feed pulp concentration for the first-stage low-consistency separator of medium fiber is 0.5-2.0%, the feed pulp pressure is 0.16-0.25 MPa, and the good pulp pressure is 0.03-0.1 MPa; the feed pulp concentration for the second-stage low-consistency separator is 0.4-2.0%, the feed pulp pressure is 0.16-0.25 MPa, and the good pulp pressure is 0.03-0.1 MPa; the feed pulp concentration for the third-stage low-consistency separator is 0.4-2.0%, the feed pulp pressure is 0.16-0.25 MPa, and the good pulp pressure is 0.03-0.1 MPa; the feed pulp concentration for the first-stage light fiber separator of medium fiber is... The feed concentration of the medium fiber two-stage light slurry separator is 0.1-1.5%, the feed pressure is 0.15-0.25MPa, and the good pulp pressure is 0.05-0.1MPa; the feed concentration of the medium fiber multi-disc thickener is 0.5%-1.5%, and the output concentration is 10%-12%; the temperature of the medium fiber thermal disperser is 90-120℃, the inlet concentration of the pressing screw is 8-12%, the outlet concentration is 25-40%, and the torque is 25-40%.

4. The preparation process of low-dust kraft paper pulp as described in claim 3, characterized in that, When the slurry entering the second slurry tower in step 4) is applied to the surface layer, the medium-fiber slurry needs to undergo the following processing: coarse screening → low-consistency separator → fine screening → multi-disc thickener → surface slurry tower; the screen gap in the coarse screening process is 0.45-0.55mm, the feed pressure is 0.25-0.35bar, the feed concentration in the low-consistency separator is 0.9%, and the feed pressure is 0.2-0.3bar; the screen gap in the fine screening process is 0.15mm, the feed pressure is 0.25-0.35bar, and the feed concentration in the multi-disc thickener is 0.5-1.0%.

5. The preparation process of low-dust kraft paper pulp as described in claim 1, characterized in that, Step 4) For the first-stage low-consistency separator for medium-length fibers, the feed pulp concentration is 1.0-2.5%, feed pressure is 0.25-0.4 MPa, and good pulp pressure is 0.12-0.22 MPa; for the second-stage low-consistency separator, the feed pulp concentration is 1.0-2.5%, feed pressure is 0.15-0.25 MPa, and good pulp pressure is 0.03-0.15 MPa; for the third-stage low-consistency separator, the feed pulp concentration is 0.5-2.0%, and the feed pressure is 0.15-0.25 MPa. MPa, good pulp pressure: 0.03-0.15MPa; long fiber four-stage low-consistency separator feed pulp concentration: 0.5-2.0%, feed pulp pressure: 0.15-0.25MPa, good pulp pressure: 0.02-0.1MPa; long fiber five-stage low-consistency separator feed pulp concentration: 0.5-2.0%, feed pulp pressure: 0.15-0.25MPa, good pulp pressure: 0.02-0.1MPa; long fiber one-stage fine screen feed pulp concentration: 0.7-1.4%, feed pulp pressure: 0.14-0. The feed concentration of the long fiber two-stage fine screen is 0.6-1.3%, and the feed pressure is 0.14-0.21MPa. The feed concentration of the long fiber three-stage fine screen is 0.2-0.8%, and the feed pressure is 0.15-0.25MPa. The feed concentration of the long fiber multi-disc thickener is 0.5%-1.5%, and the output concentration is 10%-12%. The temperature of the long fiber hot disperser is 90-120℃, the inlet concentration of the pressing screw is 8-12%, the outlet concentration is 25-40%, and the torque is 25-40%.

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