A system and method for producing a three-ply unbleached kraft paper

Abstract

The application discloses a preparation system and a preparation method of three-layer pure cow paper, and the system comprises a surface layer slurry flow feeding branch, a core layer slurry flow feeding branch, a bottom layer slurry flow feeding branch, a first dye preparation and conveying unit, a second dye preparation and conveying unit, a fixing agent supply unit, a two-section slag removal backflow unit and a three-layer composite forming unit, the first dye preparation and conveying unit adds dyes into the surface layer and the bottom layer slurry respectively, the second dye preparation and conveying unit adds dyes into the core layer slurry, each slurry flow feeding branch forms two-section slag removal backflow through a slurry flushing tank, a first-section slag remover and a second-section slag remover, the fixing agent supply unit of the surface layer and the bottom layer is connected between the slurry flushing tank and the first-section slag remover, and three good slurries are conveyed to the three-layer composite forming unit for composite forming, and the application can realize differential dyeing of three-layer slurries, and improve fixing effect and fiber utilization.

Description

Technical Field

[0001] This invention relates to the field of papermaking technology, and in particular to a preparation system and method for three-layer pure kraft paper. Background Technology

[0002] Pure kraft paper is usually formed by multi-layer pulp composite to take into account surface quality, structural strength and material utilization. In three-layer pure kraft paper, the pulp source, pulp flow, fiber composition and contribution to the appearance color of the top layer, core layer and bottom layer are usually different. In order to obtain the target comprehensive color, dyes such as yellow, red and blue can be added to the pulp and the desired hue can be formed by the ratio of each dye.

[0003] In current production, dyes are often prepared into working solutions from existing dye preparation tanks and then sent to the slurry system. If the surface layer, core layer, and bottom layer use the same dye preparation concentration or the same dosage, it is difficult to simultaneously meet the different coloring requirements of the paper surface, bottom layer, and core layer. This can easily lead to problems such as inconsistent hues on the front and back sides, ineffective consumption of dye in the core layer, and fluctuations in dye usage.

[0004] In addition, to improve dye retention on fibers, a fixing agent can be added to the pulp. However, when the fixing agent is added to the tailings tank, it is easy for the fixing agent to react rapidly with the relatively concentrated fine fibers, anionic impurities and unadsorbed dyes in the tailings, thereby increasing the risk of colored fibers, flocs or color spots. Furthermore, the tailings tank needs to enter the second stage of slag removal, and the good pulp from the second stage is returned to the flushing tank. The fixing agent has to go through a long circulation path before re-entering the main pulp flow, resulting in poor stability of the fixing effect. Summary of the Invention

[0005] In order to form a differentiated dye supply for the top, core and bottom layers, and to mix the top and bottom fixing agents in the main pulp stream before entering the slag removal stage, thereby reducing the risk of color fiber, floc or color spot formation and improving the color fixing effect, this application provides a preparation system and method for three-layer pure kraft paper.

[0006] In a first aspect, the present invention provides a preparation system for three-layer pure kraft paper, which adopts the following technical solution: A preparation system for three-layer pure kraft paper includes a surface layer pulp delivery branch, a core layer pulp delivery branch, a bottom layer pulp delivery branch, a first dye preparation and delivery unit, a second dye preparation and delivery unit, a fixing agent supply unit, a two-stage slag removal and recirculation unit, and a three-layer composite forming unit. The first dye preparation and conveying unit includes a first dye preparation tank, a first feeding tank, a surface dye output channel, a bottom dye output channel, a surface high-level tank, and a bottom high-level tank. The first dye preparation tank is connected to the first feeding tank and is used to add surface dye to the surface high-level tank through the surface dye output channel, or to add bottom dye to the bottom high-level tank through the bottom dye output channel. The surface high-level tank and the bottom high-level tank are respectively connected to the surface slurry flow branch and the bottom slurry flow branch. The second dye preparation and conveying unit includes a second dye preparation tank, a second feeding tank, a core layer dye output channel and a core layer high-level box connected in sequence. The second dye preparation and conveying unit is used to add the core layer dye to the core layer slurry conveying branch. The surface layer slurry conveying branch, the core layer slurry conveying branch and the bottom layer slurry conveying branch all flow into the three-layer composite forming unit. Each of the multiple slurry delivery branches is equipped with a two-stage slag removal and return unit. The two-stage slag removal and return unit includes a slurry flushing tank, a first-stage slag remover, and a second-stage slag remover connected in a circulation manner. The inlet of the slurry flushing tank serves as a circulation inlet and is connected to the high-level tank. The good slurry outlet of the first-stage slag remover serves as a circulation outlet and is connected to the three-layer composite forming unit. The inlet and good slurry outlet of the second-stage slag remover are respectively connected to the tail slag outlet of the first-stage slag remover and the inlet of the slurry flushing tank. The surface slurry delivery branch and the bottom slurry delivery branch are both connected to the color-fixing agent supply unit, which is connected between the slurry flushing tank and the first-stage slag remover.

[0007] Preferably, the surface dye output channel and the bottom dye output channel are each equipped with an independently adjustable feeding pump; the surface dye output channel is equipped with a flow meter, and the bottom dye output channel is equipped with a metering device, wherein the metering device is a flow meter or a controller that stores the correspondence between the pump frequency and the output flow rate obtained from actual measurement and calibration.

[0008] Preferably, the fixing agent supply unit is provided with a fixing agent storage tank and a fixing agent output branch. The fixing agent output branch is provided with a metering pump and a flow regulator. The fixing agent output branch is connected to the top / bottom slurry delivery branch through a distributed injection assembly. The distributed injection assembly includes a pre-dilution chamber and an annular distribution chamber connected in sequence. The annular distribution chamber is sleeved around the top / bottom slurry delivery branch. The annular distribution chamber and the top / bottom slurry delivery branch are connected through multiple circumferentially arranged injection joints.

[0009] Preferably, the injection connector is provided with an elastic diaphragm, a throttling boss located in the center of the elastic diaphragm, a valve seat opposite to the throttling boss, and a limiting member that limits the minimum opening of the throttling boss. A throttling gap is formed between the throttling boss and the valve seat. When the pressure difference between the annular distribution chamber and the surface / bottom layer slurry flow branch increases, the elastic diaphragm drives the throttling boss closer to the valve seat and reduces the throttling gap. The limiting member keeps the throttling gap non-zero.

[0010] Preferably, the injection connector is provided with a supply-side isolation valve, the elastic membrane, the valve seat, a backwashing port, and a slurry-side isolation valve in sequence along the injection direction. The backwashing port is connected to a backwashing branch, and the injection connector is also connected to a drain branch. When the injection connector needs to be backwashed, the supply-side isolation valve and the slurry-side isolation valve are closed, and the backwashing branch and the drain branch are opened. The backwashing branch is used to introduce backwashing fluid and clean the inside of the injection connector, and the drain branch is used to discharge the backwashing fluid.

[0011] Preferably, it also includes a control unit, which is connected to each dye output channel, fixing agent output branch, supply-side isolation valve, slurry-side isolation valve, backwash interface of backwash branch and drain branch, and is configured to stop the corresponding fixing agent output when there is no slurry flow in the corresponding top / bottom slurry flow branch.

[0012] Preferably, the supply-side isolation valve and the slurry-side isolation valve are respectively provided with valve position feedback components. The control unit allows the opening of the corresponding backwashing interface and the sewage branch pipe only when both of the closing feedbacks are valid, and prohibits backwashing and outputs an alarm when either of the closing feedbacks is invalid.

[0013] Preferably, the inlet of the slurry flushing tank includes a new slurry inlet and a good slurry return inlet. The control unit is also connected to a first detection component disposed at the new slurry inlet and a second detection component disposed at the good slurry return inlet. The first detection component is used to detect the new slurry flow rate and the new slurry load characterization value. The second detection component is used to detect the good slurry return flow rate and the good slurry load characterization value. The control unit determines the comprehensive pigment load based on the product of the new slurry flow rate and the new slurry load characterization value and the product of the good slurry return flow rate and the good slurry load characterization value. The control unit adjusts the fixing agent input amount based on the comprehensive pigment load and the delivery delay time determined by the effective pipeline volume and the slurry flow rate.

[0014] Preferably, the system further includes a third detection component disposed at the outlet of the slurry flushing tank. The third detection component is used to detect the mixed slurry flow rate and the mixed slurry load characterization value. The control unit determines the actual pigment load based on the mixed slurry flow rate and the mixed slurry load characterization value, and corrects the fixing agent input amount based on the deviation between the actual pigment load and the overall pigment load.

[0015] Secondly, the present invention provides a method for preparing three-layer pure kraft paper, which adopts the following technical solution: A method for preparing three-layer pure kraft paper includes the following steps: S1: Prepare the surface layer slurry, core layer slurry and bottom layer slurry separately, and ensure that the three slurries are transported independently before entering the three-layer composite molding unit; S2: Yellow dye, red dye and blue dye are mixed and diluted in the first dye preparation tank to form the first dye working solution, and then mixed and diluted in the second dye preparation tank to form the second dye working solution; S3: The first dye working solution is quantitatively delivered to the surface layer high-level tank and the bottom layer high-level tank respectively through the surface layer dye output channel and the bottom layer dye output channel, and the second dye working solution is quantitatively delivered to the core layer high-level tank through the core layer dye output channel. S4: After the top layer slurry and the bottom layer slurry pass through their respective dye inlets, the fixing agent working solution and the diluent are mixed to form a pre-diluted solution. The pre-diluted solution enters the annular distribution cavity surrounding the top / bottom layer slurry flow branch and enters the slurry through multiple circumferentially distributed injection joints. Then the slurry passes through a slag remover. S5: The good slurry output from the first stage slag remover enters the three-layer composite forming unit, the tailings output from the first stage slag remover enter the second stage slag remover, and the good slurry output from the second stage slag remover returns to the slurry flushing tank and re-participates in the slag removal cycle. S6: Combine the surface layer pulp, core layer pulp, and bottom layer pulp to form a pure kraft paper with a surface layer, core layer, and bottom layer in sequence.

[0016] The beneficial effects of this invention are as follows: 1. This invention sets up a first dye preparation and conveying unit and a second dye preparation and conveying unit, so that the surface layer, bottom layer and core layer can use different dye formulations and addition amounts according to their respective pulp base color, fiber composition and target hue, avoiding cross-coloring and mutual interference between dyes of different layers, improving the stability of the surface hue and internal overall color tone of the three-layer pure kraft paper, and setting the fixing agent addition position of the surface layer and bottom layer between the pulp flushing tank and the first stage slag remover, so that the fixing agent directly acts on all the mainstream pulp formed by the mixing of new pulp and second stage recirculated good pulp, shortening the circulation path and response time of the fixing agent entering the main pulp flow, reducing the risk of forming colored fibers, flocs or color spots in the tail pulp rich in fine fibers and free dyes; 2. After pre-dilution, the working solution of the fixing agent enters the annular distribution chamber set around the surface / bottom layer slurry flow branch. The slurry is added through multiple circumferentially distributed injection joints, which transforms the single-point, high-concentration fixing agent jet into a multi-point, low-concentration small stream, expands the initial contact range between the fixing agent and the slurry, improves cross-sectional mixing and fixing uniformity, and reduces local excessive flocculation. Each injection joint is also equipped with an elastic membrane, throttling boss, valve seat and limiting component, which can automatically change the throttling gap according to the pressure difference between the annular distribution chamber and the surface / bottom layer slurry flow branch, limit the excessive increase of flow at injection points with large pressure differences, reduce the flow deviation between injection joints, and maintain the continuous injection of fixing agent by maintaining a non-zero opening. 3. The injection joint is equipped with a supply-side isolation valve, a slurry-side isolation valve, a backwash branch, and a drain branch. When sediment blockage occurs at the injection port, valve seat through hole, or throttling gap, a single injection joint can be isolated and backwashed, and the flushing sewage can be discharged independently to prevent flushing water from entering the surface / bottom slurry flow branch without disassembling the joint or stopping the entire distributed injection system. 4. The control unit determines the comprehensive colorant load based on the product of the new pulp flow rate and the new pulp load characterization value, as well as the product of the return pulp flow rate and the return pulp load characterization value. It also compensates for the delivery delay by combining the effective pipeline volume and pulp flow rate, so that the output amount and output time of the fixing agent match the actual pulp load at the addition position, thereby reducing the under-addition or over-addition of fixing agent. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the first dye preparation and delivery unit; Figure 2 This is a schematic diagram of the second dye preparation and delivery unit; Figure 3 This is a schematic diagram of the modules of each slurry flow branch and the three-layer composite forming unit; Figure 4 This is a schematic diagram of the module for the surface / bottom layer slurry delivery branch; Figure 5 This is a schematic diagram showing the connection of the annular distribution chamber, injection joint, and top / bottom layer slurry delivery branch. Figure 6 This is a simplified schematic diagram of the internal structure of the injection connector; Figure 7 This is a flowchart of the preparation method for three-layer pure kraft paper.

[0018] Explanation of reference numerals in the attached drawings: 1. Annular distribution chamber; 2. Injection connector; 21. Elastic membrane; 22. Throttling boss; 23. Valve seat; 24. Supply-side isolation valve; 25. Backwash port; 26. Slurry-side isolation valve; 27. Drainage branch pipe; 28. Downstream pressure channel; 3. Top / bottom slurry delivery branch. Detailed Implementation

[0019] The following will combine Figures 1-7 The present invention will be further illustrated by the embodiments.

[0020] This embodiment discloses a preparation system for three-layer pure kraft paper.

[0021] Reference Figures 1 to 7 The preparation system for three-layer pure kraft paper includes a top layer pulp delivery branch, a core layer pulp delivery branch, a bottom layer pulp delivery branch, and a three-layer composite forming unit. The top, core, and bottom layers are three paper layers arranged sequentially along the paper thickness direction. The top layer constitutes the main appearance of the pure kraft paper, the bottom layer forms the opposite appearance, and the core layer, located between the top and bottom layers, primarily provides thickness and strength support. The three pulp delivery branches for the top, core, and bottom layers operate independently before entering the three-layer composite forming unit. Each layer's dyes undergo independent pulp type ratio, concentration adjustment, dye addition, and wet-end chemical adjustment to prevent cross-contamination of dyes and auxiliaries between different layers before composite forming. After being delivered to the three-layer composite forming unit, the three pulp streams can either merge in layers within a multi-layer headbox or be sprayed separately onto corresponding forming wires or forming areas for wet composite. The resulting wet paper web sequentially forms the top, core, and bottom layers along the thickness direction. Subsequently, after pressing, drying, and necessary surface treatments, the three-layer pure kraft paper is obtained.

[0022] It also includes a first dye preparation and delivery unit, which is used to introduce dye into the surface slurry delivery branch or the bottom slurry delivery branch. Since the surface and bottom layers of the three-layer pure kraft paper are both appearance layers, they usually need to present the same or similar brownish-yellow hue of kraft paper. The proportions of the yellow dye, red dye and blue dye used are similar. Therefore, the surface slurry delivery branch and the bottom slurry delivery branch can share a part of the first dye preparation and delivery unit. Through partial sharing, the basic hue deviation of separately prepared dyes can be reduced, the number of equipment and dye residue can be reduced, while retaining the independent adjustment capability of the amount of dye added to the surface layer and the bottom layer. Specifically, the first dye preparation and conveying unit includes a first dye preparation tank, a first feeding tank, a surface dye output channel or a bottom dye output channel, and a surface high-level box or a bottom high-level box. The first dye preparation and conveying unit is used to add the surface dye formed by mixing yellow dye, red dye and blue dye to the surface slurry conveying branch or add the bottom dye to the bottom slurry conveying branch. Based on this, for the surface slurry conveying branch, its first dye preparation and conveying unit sequentially includes a first dye preparation tank, a first feeding tank, a surface dye output channel and a surface high-level box. For the bottom slurry conveying branch, its first dye preparation and conveying unit sequentially includes a first dye preparation tank, a first feeding tank, a bottom dye output channel and a bottom high-level box, thereby realizing partial sharing of the surface slurry conveying branch and the bottom slurry conveying branch.

[0023] The system also includes a second dye preparation and delivery unit, which comprises a second dye preparation tank, a second feeding tank, a core layer dye output channel, and a core layer high-level box. This unit is used to add the core layer dye, a mixture of yellow, red, and blue dyes, to the core layer slurry flow path. Because the core layer slurry differs from the surface and bottom layer slurries in terms of raw material composition, initial hue, charge characteristics, and dye adsorption capacity, this system provides a separate second dye preparation and delivery unit for the core layer dye. This allows the core layer dye to be prepared and delivered according to independent formulations, concentrations, and flow rates, preventing cross-contamination or residual contamination between the second dye and the first dye supplied to the surface and bottom layers. It also avoids interference with the core layer dye's adjustment of the supply pressure and apparent color control of the surface and bottom layers. Ultimately, this results in a differentiated dye supply for the surface, core, and bottom layers, accommodating the different coloring requirements of the paper's surface, bottom, and core layers, while avoiding problems such as inconsistent hue on both sides, ineffective core layer dye consumption, and fluctuations in dye usage.

[0024] It also includes two slag removal and recirculation units. The surface layer slurry delivery branch, the core layer slurry delivery branch, and the bottom layer slurry delivery branch are all equipped with two slag removal and recirculation units. Specifically, the two slag removal and recirculation units include a slurry flushing tank, a first-stage slag remover, and a second-stage slag remover connected in a circulation manner. The inlet of the slurry flushing tank is connected to the high-level tank as a circulation inlet. The good slurry outlet of the first-stage slag remover is connected to the three-layer composite forming unit as a circulation outlet. The inlet and good slurry outlet of the second-stage slag remover are respectively connected to the tail slag outlet of the first-stage slag remover and the inlet of the slurry flushing tank. The pulp flushing tank receives the new pulp, dilution water, and second-stage good pulp from the second-stage desander, ensuring uniform mixing through agitation or circulation. The flushing tank outlet is connected to the inlet of the first-stage desander via a flushing pump, allowing the second-stage good pulp to return to the flushing tank and be flushed again with the new pulp. The flushing tank also buffers short-term fluctuations in the flow rates of new pulp, second-stage good pulp, and dilution water, providing a relatively stable mixed pulp with consistent concentration and flow rate for the flushing pump and the first-stage desander. The first-stage desander performs the main purification task, removing sand, fiber bundles, adhesives, or other impurities to prevent paper surface spots, holes, scratches, or localized color differences. The second-stage desander further recovers qualified fibers from the pulp residue, returning the good pulp to the flushing tank. This maintains the purity of the pulp while reducing the discharge of high-quality fibers with the pulp residue, thus balancing paper quality and fiber utilization.

[0025] It also includes a fixing agent supply unit. Both the surface pulp flow branch and the bottom pulp flow branch are connected to the fixing agent supply unit. The purpose is that the surface and bottom layers constitute the exposed surfaces of the three-layer pure kraft paper. Their color uniformity, color fastness, water resistance, and abrasion resistance directly affect the appearance and quality of the finished paper. Moreover, the dyes in the surface and bottom layers are easily lost with the white water during the flow and forming process. Therefore, fixing agents are added to each layer to allow the dyes to be more fully adsorbed and retained on the fiber surface, reducing color fading, color difference, and white water staining. After the dye is added to the pulp in the high-level tank, the tail pulp discharged from the first-stage desander usually contains fine fibers, anionic waste, and free dye that has not yet been adsorbed by the fibers. If the fixing agent is added to the tail pulp tank after the first-stage desander, the fixing agent can easily undergo rapid charge neutralization, adsorption bridging, or flocculation with the free dye, fine fibers, and anionic substances under local high concentration conditions. This causes the dye to concentrate and adhere to some fine fibers and form colored flocs. After these colored flocs are returned to the slurry tank with the second-stage good pulp and enter the forming system, they may form colored fibers or spots in the paper. Meanwhile, the addition point is not the direct processing point of the main pulp flow. The fixing agent needs to pass through the second-stage deslagging, good pulp return, and mixing in the flushing tank before it can re-enter the main pulp flow with the mixed pulp. This results in a long transportation and circulation lag, which leads to the fixing agent's untimely response to the dye in the main pulp flow and a decrease in its effective utilization rate, ultimately weakening its fixing effect. Therefore, in this invention, the fixing agent supply unit is preferably connected between the flushing tank and the first-stage deslagging device, so that the fixing agent is added after the new pulp and the second-stage return good pulp have merged and homogenized, and before entering the first-stage deslagging device. This reduces the risk of the formation of colored fibers, flocs, or color spots. At the same time, the fixing agent does not need to go through a long circulation path before re-entering the main pulp flow, thus improving the fixing effect.

[0026] The surface dye output channel and the bottom dye output channel are each equipped with independently adjustable feed pumps, allowing for separate adjustment of the output volume and preventing changes in dye usage in one slurry delivery branch from affecting the other. The surface dye output channel is equipped with a flow meter, and the controller uses the actual surface dye flow rate detected by the flow meter to perform closed-loop regulation of the corresponding feed pump's operating frequency or speed, ensuring that the surface dye addition remains consistent with the set value. The bottom dye output channel is equipped with a metering device, specifically a flow meter, to form a closed-loop control of the bottom dye flow rate. Alternatively, a controller storing the correspondence between measured and calibrated pump frequencies and output flow rates can be used. The controller searches for or calculates the corresponding pump operating frequency based on the target output flow rate and controls the bottom feed pump accordingly, achieving indirect metering of the bottom dye output.

[0027] The fixing agent supply unit is equipped with a fixing agent storage tank for storing the prepared fixing agent solution and fixing agent output branches connected to the surface slurry delivery branch and the bottom slurry delivery branch, respectively. The fixing agent output branch is equipped with a metering pump and a flow regulator. The metering pump is used to continuously deliver the fixing agent in the fixing agent storage tank to the corresponding fixing agent addition point according to the set flow rate. The flow regulator can be a regulating valve, a frequency converter, or a closed-loop regulating component composed of a flow meter and a controller. It is used to adjust the operating frequency, speed, or output stroke of the metering pump according to the slurry flow rate, comprehensive pigment load, and delivery delay of the corresponding surface / bottom slurry delivery branch 3, so that the output of the surface fixing agent and the output of the bottom fixing agent can be controlled independently.

[0028] In a preferred embodiment, the surface slurry is prepared by using 20% ​​by mass of unbleached coniferous natural sulfate slurry and 80% by mass of eucalyptus natural slurry, and the bottom slurry is prepared by using 33% by mass of unbleached coniferous natural sulfate slurry and 67% by mass of eucalyptus natural slurry. The freeness of both the surface slurry and the bottom slurry is controlled to be 24-30°SR.

[0029] The core pulp is a blend of 22% by weight waste paper, 18% by weight eucalyptus natural color pulp, 37% by weight bleached chemothermomechanical pulp (BCTMP) with a brightness of 65%, and 23% by weight bleached chemothermomechanical pulp (BCTMP) with a brightness of 70%. The basis weights of the top, core, and bottom layers are 40 g / m², 220 g / m², and 40 g / m², respectively, for a total basis weight of 300 g / m².

[0030] Based on per ton of paper, the addition amount of cationic starch is 4 kg, aluminum sulfate is 35 kg, rosin is 25 kg, and retention aid is 120 ppm; the working solution concentrations of cationic starch, aluminum sulfate, rosin, and retention aid are 4%, 3.14%, 3.14%, and 0.5%, respectively. Yellow, red, and blue dyes are all liquid direct dyes, diluted to 0.785% before addition. The fixing agent is an aqueous solution based on aliphatic polyamines, diluted to 0.785% to form the fixing agent working solution; the chemicals are added in the relative order of cationic starch, dye, fixing agent, aluminum sulfate, rosin, and retention aid. The fixing agent working solution is added to the surface slurry delivery branch and the bottom slurry delivery branch between the slurry tank and the first-stage deslagging device.

[0031] The first dye working solution is measured separately for the surface and bottom layers of paper, with the amounts of yellow, red, and blue dyes added to the surface, bottom, and bottom layers being 12 kg / t of paper, 3.5 kg / t of paper, and 2 kg / t of paper, respectively. The second dye working solution is measured for the core layer of paper, with the amounts of yellow, red, and blue dyes added to the core layer being 2.4 kg / t of paper, 0.4 kg / t of paper, and 0.4 kg / t of paper, respectively. To ensure color fastness under hot water immersion conditions, the preferred amount of fixing agent is 7-8 kg / t of paper.

[0032] The process of determining the above dye formulation is as follows: Based on the customer's target color sample as the color mixing benchmark, multiple proportions are used for adjustment to obtain the desired effect. Then, based on the overall hue change caused by lamination, the dyes in each layer are fine-tuned to determine the final dye addition amounts for the top / bottom and core layers. See the table below: Table 1. Dye formulation screening scheme and hue measurement results

[0033] In this table, using the customer's target color sample L*=60.96, a*=8.97, b*=24.61 as the color matching benchmark, when using screening scheme No. 4 for the top and bottom layers, the addition ratios of yellow dye, red dye, blue dye, and fixing agent are 1.20%, 0.40%, 0.20%, and 0.40% respectively, resulting in L*=59.24, a*=9.44, and b*=23.48 for the resulting top / bottom layer paper sample. When using screening scheme No. 5 for the core layer, the addition ratios of yellow dye, red dye, and blue dye are 0.24%, 0.04%, and 0.04% respectively, resulting in L*=75.12, a*=3.60, and b*=19.38 for the resulting core layer paper sample. After laminating the above top / bottom and core layer paper samples, the dyes in each layer are fine-tuned based on the overall hue change caused by lamination to determine the final dye addition amounts for the aforementioned top / bottom and core layers.

[0034] Next, a film-making verification was performed. The dyed slurry was mixed with water to 3000 mL in a film-making machine, and after shaping, it was pressed and dried using a hydraulic press. Three parallel samples were prepared for each group. Additionally, 1000 mL of dyeing slurry with a mass concentration of 0.5% was prepared according to the final dyeing scheme, filtered through a forming screen, and the filtrate was collected. The wastewater color was measured according to the colorimetric testing standard and found to be no greater than 30, which is lower than the wastewater color discharge standard of 300.

[0035] Further verification was conducted by hot water immersion: 3g of dyed paper sample was taken, cut into small pieces, and 200mL of 95℃ hot water was added. The color of the water was observed after immersion for 30min and 60min. When the amount of fixing agent added was 4kg / t of paper, the immersion water was light yellow; when the amount of fixing agent added was increased to 7-8kg / t of paper, the immersion water was basically colorless and transparent, indicating that increasing the amount and location of fixing agent addition can improve the anti-fading performance of this formula paper sample under hot water immersion conditions.

[0036] Ultimately, the above verification results show that by setting the dye formulation and addition amount separately for the surface layer, core layer and bottom layer, the hue of each layer can be independently adjusted before lamination, resulting in better performance; and when the fixing agent addition method of this scheme is adopted, the anti-fading performance under hot water immersion conditions can be improved.

[0037] Furthermore, considering that when the fixing agent enters the top / bottom slurry delivery branch 3 through a single interface, the contact range between the fixing agent jet and the slurry flow is limited, and the local concentration peak will vary with the branch flow rate and the dosage flow rate, which may still lead to local flocculation, the fixing agent output branch of this invention is further connected to the top / bottom slurry delivery branch 3 through a distributed injection component. Specifically, the distributed injection component includes a pre-dilution chamber and an annular distribution chamber 1 connected in sequence. The annular distribution chamber 1 is sleeved on the periphery of the top / bottom slurry delivery branch 3, and the annular distribution chamber 1 and the top / bottom slurry delivery branch 3 are connected through multiple circumferentially arranged injection joints 2. Based on this, the working solution of the fixing agent will first be mixed with the diluent in the pre-dilution chamber, and then enter the annular distribution chamber 1 set around the surface / bottom layer slurry flow branch 3. It will then enter the surface / bottom layer slurry flow branch 3 through multiple circumferentially distributed injection joints 2, thereby transforming the single-point high-concentration jet into a low-concentration, multi-directional, small-stream jet, expanding the initial contact area between the fixing agent and the slurry, and effectively reducing local flocculation.

[0038] Multiple injection joints 2 share an annular distribution chamber 1, ensuring that each injection joint 2 receives a basically consistent upstream pressure. However, the slurry pressure, pipeline resistance, and fiber adhesion at each injection port may differ. Therefore, even with the same upstream pressure, the actual flow rate of each injection joint 2 may still vary. The flow rate of each injection joint 2 approximately satisfies: Where q is the injected traffic, and C dΔP is the flow coefficient, A is the effective flow area, and ΔP is the pressure difference between the annular distribution chamber 1 and the surface / bottom layer slurry delivery branch 3. The greater the pressure difference, the greater the flow rate, which can easily cause a certain injection joint 2 to form a high-concentration fixing agent jet. If the flow rate of this injection joint 2 is too large, it will also squeeze the fixing agent injection amount of other injection joints 2. To address this issue, the injection connector 2 is equipped with an elastic diaphragm 21, a throttling boss 22 located at the center of the elastic diaphragm 21, a valve seat 23 opposite to the throttling boss 22, and a limiting element that restricts the minimum opening of the throttling boss 22. A throttling gap is formed between the throttling boss 22 and the valve seat 23. When the pressure difference at a certain injection connector 2 in the annular distribution chamber 1 increases, the elastic diaphragm 21 drives the throttling boss 22 closer to the valve seat 23 and reduces the throttling gap, forming a passive flow regulation mechanism that automatically changes the throttling gap using the pressure difference. This avoids the formation of a high-concentration fixing agent jet at areas with large pressure differences and keeps the fixing agent flow rates of each injection connector 2 as close as possible, achieving a stable flow distribution at each injection port. The limiting element is used to keep the throttling gap non-zero. Specifically, the outer circumference of the elastic diaphragm 21 is sealed and fixed inside the injection connector 2 by a clamping ring, and one side of the elastic diaphragm 21 forms an upstream pressure chamber that communicates with the annular distribution chamber 1. The central region of the elastic membrane 21 is provided with a throttling boss 22 protruding toward the valve seat 23. The valve seat 23 is provided with a valve seat 23 through hole communicating with the downstream liquid outlet channel. A throttling gap is formed between the outer peripheral surface of the throttling boss 22 and the inner peripheral surface or end face of the valve seat 23. The fixing liquid entering the upstream pressure chamber from the annular distribution chamber 1 flows along the guide space between the elastic membrane 21 and the inner wall of the injection joint 2 to the throttling boss 22, and then passes through the throttling gap, the valve seat 23 through hole, and the downstream liquid outlet channel in sequence to enter the surface / bottom layer slurry delivery branch 3. The injection connector 2 is also equipped with a downstream pressure channel 28. One end of the downstream pressure channel 28 is connected to the outlet channel located downstream of the throttling gap, and the other end is connected to the side of the elastic diaphragm 21 facing away from the upstream pressure chamber. This allows the elastic diaphragm 21 to bear the upstream pressure of the annular distribution chamber 1 and the downstream pressure of the outlet channel on both sides, respectively. The downstream pressure channel 28 can be equipped with damping orifices or anti-clogging filters to reduce the frequent vibrations of the diaphragm caused by downstream pressure pulsations. The pressure difference between the two sides acts on the effective pressure-bearing area of ​​the elastic diaphragm 21, causing axial elastic deformation in the central region of the elastic diaphragm 21 and driving the throttling boss 22 to move relative to the valve seat 23. When the pressure difference increases, the throttling boss 22 moves closer to the valve seat 23 and reduces the throttling gap; when the pressure difference decreases, the elastic diaphragm 21 recovers its elasticity and increases the throttling gap, effectively addressing the problem of injection uniformity.

[0039] Furthermore, the injection connector 2 is sequentially equipped with a supply-side isolation valve 24, the aforementioned elastic membrane 21, the aforementioned valve seat 23, a backwash port 25, and a slurry-side isolation valve 26 along the injection direction. The backwash port 25 is connected to a backwash branch, and the injection connector 2 is also connected to a drain branch pipe 27. The backwash branch is equipped with a backwash valve, and the drain branch pipe 27 is equipped with a drain valve. When the injection connector 2 needs to be backwashed, the supply-side isolation valve 24 and the slurry-side isolation valve 26 are closed, and the backwash branch and the drain branch pipe are opened. The backwash branch is used to introduce backwash liquid and clean the inside of the injection connector 2, and the drain branch pipe is used to discharge the backwash liquid. The purpose of backflushing is to remove the deposits from the downstream side to the upstream side of the normal liquid supply direction when the injection joint 2, throttling gap, valve seat 23 through hole or end injection port is blocked by fixing agent precipitates, pulp fibers and other deposits or the flow capacity decreases. This is done by using backflowing flushing water to remove the deposits from the downstream side to the upstream side of the normal liquid supply direction and discharge them through an independent drain branch pipe. This restores the effective flow section and set injection capacity of the injection joint 2, and avoids uneven addition of fixing agent at various points around the perimeter, insufficient local fixing and paper color difference caused by local injection port blockage. During backwashing, by closing the supply-side isolation valve 24 and the slurry-side isolation valve 26, the injection joint 2 to be cleaned is isolated from the annular distribution chamber 1 and the top / bottom slurry delivery branch 3, respectively. The flushing water only flows through the local flow channel of the injection joint 2 and is discharged through the drain branch pipe. This can prevent the flushing water from entering the top / bottom slurry delivery branch 3 and diluting or disturbing the slurry, and also prevent the flushing waste liquid from returning to the fixing agent supply system. At the same time, there is no need to disassemble the injection joint 2 or shut down the entire circumferential injection structure, which helps to shorten maintenance time and maintain continuous and stable system operation.

[0040] It also includes a control unit, which is connected to each dye output channel, fixing agent output branch, supply-side isolation valve 24, slurry-side isolation valve 26, backwash interface of the backwash branch, and drain branch. The control unit receives at least one detection signal from the corresponding slurry delivery branch, including slurry flow rate, fixing agent output flow rate, injection connector 2 pressure, valve opening / closing status, and liquid level status. It coordinates the dye addition, fixing agent addition, and backwashing process of injection connector 2 according to a preset control program. The control unit is also configured to stop the corresponding fixing agent output when there is no slurry flow in the corresponding top / bottom slurry delivery branch 3, preventing the fixing agent from continuously entering the top / bottom slurry delivery branch 3 and accumulating, depositing, or forming a high-concentration fixing area without sufficient slurry carrying and dilution.

[0041] In addition, the supply-side isolation valve 24 and the slurry-side isolation valve 26 are respectively equipped with valve position feedback devices. The control unit allows the corresponding backwashing interface and sewage branch to be opened only when both of the closing feedbacks are valid. Backwashing is prohibited and an alarm is output when either of the closing feedbacks is invalid, so as to prevent backwashing water from entering the surface / bottom slurry delivery branch 3, slurry from flowing back into the backwashing branch, or flushing sewage from entering the color-fixing agent supply system.

[0042] The inlet of the slurry flushing tank includes a fresh slurry inlet and a good slurry return inlet. Therefore, there are actually two streams of slurry in the slurry flushing tank: fresh slurry that continuously enters the slurry flushing tank during normal production and usable slurry that returns to the slurry flushing tank after the tailings have been removed in two stages. The control unit is also connected to a first detection component located at the fresh pulp inlet and a second detection component located at the good pulp return inlet. The first detection component detects the fresh pulp flow rate Qn and the fresh pulp load characterization quantity Bn, and includes a first bypass sampling element, a first electromagnetic flowmeter for acquiring the fresh pulp flow rate, and a first online charge demand detection element for acquiring the fresh pulp load characterization quantity. The second detection component detects the good pulp return flow rate Qr and the good pulp load characterization quantity Br, and includes a second bypass sampling element, a second electromagnetic flowmeter for acquiring the return pulp flow rate, and a second online charge demand detection element for acquiring the return pulp load characterization quantity. The control unit determines the comprehensive pigment load L, L = QnBn + QrBr, based on the product of the fresh pulp flow rate and the fresh pulp load characterization quantity QnBn and the product of the good pulp return flow rate and the good pulp load characterization quantity QrBr, and adjusts the fixing agent input amount according to the comprehensive pigment load. Fixing agent input amount q f Calculated according to the following relationship: Where L is the overall pigment load, and C f K represents the working concentration of the fixing agent, and K is the fixing agent demand coefficient obtained through trial calibration. This allows the control unit to increase the fixing agent input when the return slurry flow rate or return slurry load increases, and vice versa, thus achieving dynamic adjustment of the fixing agent input. It should be noted that the load characterization value is a detection parameter representing the content of effective substances per unit volume or unit mass of slurry that require charge neutralization, adsorption, or fixation by the fixing agent. These effective substances include free dyes not adsorbed by fibers, anionic groups on the surface of fine fibers and fillers, dissolved anionic waste, and other charged substances that consume the fixing agent. A higher value indicates a greater fixing agent demand. Furthermore, considering that the fixing agent addition point is located in the pipeline behind the slurry tank, the slurry detected by the first and second detection components will arrive at the fixing agent addition point after a certain delay. Therefore, the control unit also adjusts the fixing agent input amount based on the delivery delay time determined by the effective pipeline volume and slurry flow rate. For example, if the effective pipeline volume between the detection point and the fixing agent addition point is V, and the slurry flow rate is Q, then when the flow rate is basically stable, the delivery delay time t is: This allows for more precise dynamic adjustment.

[0043] It also includes a third detection component located at the outlet of the slurry flushing tank. The third detection component is used to detect the mixed slurry flow rate and the mixed slurry load characterization. It includes a third bypass sampling device, a third electromagnetic flowmeter for obtaining the mixed slurry flow rate, and a third online charge demand detection device for obtaining the mixed slurry load characterization. The control unit determines the actual pigment load based on the mixed slurry flow rate and the mixed slurry load characterization, and corrects the fixing agent input amount based on the deviation between the actual pigment load and the comprehensive pigment load. The role of the third detection component is not to repeat the detection, but to verify whether the results calculated by the first two detection components are consistent with the actual mixed slurry at the outlet of the slurry flushing tank, and to correct model errors, uneven mixing, and sensor drift.

[0044] This embodiment also discloses a method for preparing three-layer pure kraft paper.

[0045] Reference Figure 7 The preparation method of three-layer pure kraft paper includes the following steps: S1: Prepare the surface layer pulp, core layer pulp, and bottom layer pulp separately, and ensure that the three pulps are transported independently before entering the three-layer composite forming unit. The three pulps are subjected to concentration adjustment, flow control, dyeing, and slag removal treatment respectively, which can prevent the different layers of pulp from mixing or flowing in the wrong direction in advance, and allow the color, basis weight, and fiber composition of each layer of pulp to be adjusted independently, providing a foundation for the subsequent formation of a three-layer paper sheet with stable structure and color.

[0046] S2: Yellow dye, red dye, and blue dye are mixed and diluted in the first dye preparation tank to form the first dye working solution, and mixed and diluted in the second dye preparation tank to form the second dye working solution. The first dye working solution is mainly used to control the overall hue of the paper surface layer and bottom layer, while the second dye working solution is mainly used to cover the base color of the core layer pulp and adjust the overall hue of the paper. By preparing the two dye working solutions separately, the dye composition, concentration, and dilution ratio can be set separately for different dyeing purposes, avoiding color bleeding or residual liquid contamination of the surface layer and bottom layer dyes caused by the dark core layer dye.

[0047] S3: The first dye working solution is quantitatively delivered to the surface layer high-level tank and the bottom layer high-level tank respectively through the surface layer dye output channel and the bottom layer dye output channel. The second dye working solution is quantitatively delivered to the core layer high-level tank through the core layer dye output channel. The dye and the corresponding layer slurry are initially mixed in the high-level tank, and a relatively stable slurry supply condition is formed by utilizing the high-level difference. This can reduce the color difference within the layer caused by fluctuations in the amount of dye added and the slurry flow rate.

[0048] S4: After the top layer slurry and bottom layer slurry pass through their respective dye inlets, the fixing agent working solution and diluent are mixed to form a pre-diluted solution. The fixing agent can be selected from at least one of polydimethyldiallyl ammonium chloride, polyamine fixing agents, polyethyleneimine, dicyandiamide condensation resins, cationic modified polyacrylamide, and their compound formulations. The pre-diluted solution enters the annular distribution cavity 1 surrounding the top / bottom layer slurry flow branch 3 and enters the slurry through multiple circumferentially distributed injection joints 2. Then, the slurry passes through a slag remover. Each injection joint 2 injects the pre-diluted solution into the flowing slurry from different circumferential positions, changing the traditional single-point addition of the fixing agent to a multi-point dispersed addition around the circumference. This shortens the diffusion distance of the fixing agent within the slurry cross-section and improves the interaction between the fixing agent and the dye. The uniformity of material and fiber contact reduces the risk of excessively high local fixative concentrations causing color fiber, color spots, or slurry flocculation. In addition, aluminum sulfate can be added at the outlet of the slurry tank. Its main function is as an inorganic cationic charge regulator and retention coagulant: After entering the slurry, aluminum sulfate hydrolyzes to form positively charged aluminum ions and aluminum hydroxide complexes, which can neutralize the negative charges carried by fine fibers, colloidal substances, dissolved anionic waste, and some free dyes in the slurry, reducing their ineffective consumption of subsequent fixatives, and promoting the adsorption and retention between dyes, fine fibers, and main fibers. At the same time, aluminum sulfate can also adjust the pH value and charge environment of the slurry system, creating more stable reaction conditions for the fixative to perform charge neutralization, adsorption bridging, and dye fixation.

[0049] S5: The good pulp output from the first stage deslagging device enters the three-layer composite forming unit, the tailings output from the first stage deslagging device enter the second stage deslagging device, and the good pulp output from the second stage deslagging device returns to the pulping tank to re-participate in the deslagging cycle. This can not only recover the qualified fibers carried in the tailings of the first stage and reduce fiber loss, but also allow the good pulp of the second stage to undergo the first stage deslagging again, preventing residual impurities in the good pulp of the second stage from bypassing the main purification process.

[0050] S6: The surface layer pulp, core layer pulp, and bottom layer pulp are compounded to form a pure kraft paper with a surface layer, core layer, and bottom layer in sequence. Specifically, the flow rate, concentration, and pressure of the surface layer pulp, core layer pulp, and bottom layer pulp are adjusted respectively so that the three pulps enter the corresponding pulp distribution channels of the three-layer headbox, or are transported to the multilayer forming wire through the corresponding headbox. In the three-layer compound forming unit, the surface layer pulp, core layer pulp, and bottom layer pulp are compounded in sequence according to the preset layer order, so that the core layer is located between the surface layer and the bottom layer. The compounded wet paper web then undergoes dewatering, pressing, drying, and necessary finishing treatments in sequence to obtain a three-layer pure kraft paper with a surface layer, core layer, and bottom layer in sequence. Finally, the obtained three-layer pure kraft paper has a relatively stable surface hue, a low degree of white water coloring, fewer colored fibers and color spots, and good interlayer bonding and appearance quality.

[0051] The above are all preferred embodiments of the present invention and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A preparation system for three-layer pure kraft paper, characterized in that: It includes a surface slurry delivery branch, a core slurry delivery branch, a bottom slurry delivery branch, a first dye preparation and delivery unit, a second dye preparation and delivery unit, a fixing agent supply unit, a two-stage slag removal and recirculation unit, and a three-layer composite forming unit. The first dye preparation and conveying unit includes a first dye preparation tank, a first feeding tank, a surface dye output channel, a bottom dye output channel, a surface high-level tank, and a bottom high-level tank. The first dye preparation tank is connected to the first feeding tank and is used to add surface dye to the surface high-level tank through the surface dye output channel, or to add bottom dye to the bottom high-level tank through the bottom dye output channel. The surface high-level tank and the bottom high-level tank are respectively connected to the surface slurry flow branch and the bottom slurry flow branch. The second dye preparation and conveying unit includes a second dye preparation tank, a second feeding tank, a core layer dye output channel and a core layer high-level box connected in sequence. The second dye preparation and conveying unit is used to add the core layer dye to the core layer slurry conveying branch. The surface layer slurry conveying branch, the core layer slurry conveying branch and the bottom layer slurry conveying branch all flow into the three-layer composite forming unit. Each of the multiple slurry delivery branches is equipped with a two-stage slag removal and return unit. The two-stage slag removal and return unit includes a slurry flushing tank, a first-stage slag remover, and a second-stage slag remover connected in a circulation manner. The inlet of the slurry flushing tank serves as a circulation inlet and is connected to the high-level tank. The good slurry outlet of the first-stage slag remover serves as a circulation outlet and is connected to the three-layer composite forming unit. The inlet and good slurry outlet of the second-stage slag remover are respectively connected to the tail slag outlet of the first-stage slag remover and the inlet of the slurry flushing tank. The surface slurry delivery branch and the bottom slurry delivery branch are both connected to the color-fixing agent supply unit, which is connected between the slurry flushing tank and the first-stage slag remover.

2. The preparation system for a three-layer pure kraft paperboard according to claim 1, characterized in that: The surface dye output channel and the bottom dye output channel are each equipped with an independently adjustable feeding pump; the surface dye output channel is equipped with a flow meter, and the bottom dye output channel is equipped with a metering device, which is also a flow meter or a controller that stores the correspondence between the pump frequency and the output flow rate obtained from actual measurement and calibration.

3. The preparation system for a three-layer pure kraft paperboard according to claim 2, characterized in that: The fixing agent supply unit is equipped with a fixing agent storage tank and a fixing agent output branch. The fixing agent output branch is equipped with a metering pump and a flow regulator. The fixing agent output branch is connected to the top / bottom slurry delivery branch through a distributed injection assembly. The distributed injection assembly includes a pre-dilution chamber and an annular distribution chamber connected in sequence. The annular distribution chamber is sleeved around the top / bottom slurry delivery branch. The annular distribution chamber and the top / bottom slurry delivery branch are connected through multiple circumferentially arranged injection joints.

4. The preparation system for a three-layer pure kraft paperboard according to claim 3, characterized in that: The injection connector is provided with an elastic diaphragm, a throttling boss connected to the center of the elastic diaphragm, a valve seat opposite to the throttling boss, and a limiting member that limits the minimum opening of the throttling boss. A throttling gap is formed between the throttling boss and the valve seat. When the pressure difference between the annular distribution chamber and the surface / bottom layer slurry flow branch increases, the elastic diaphragm drives the throttling boss closer to the valve seat and reduces the throttling gap. The limiting member keeps the throttling gap non-zero.

5. The preparation system for a three-layer pure kraft paperboard according to claim 4, characterized in that: The injection joint is sequentially provided with a supply-side isolation valve, the elastic membrane, the valve seat, a backwashing port, and a slurry-side isolation valve along the injection direction. The backwashing port is connected to a backwashing branch, and the injection joint is also connected to a drain branch. When the injection joint needs to be backwashed, the supply-side isolation valve and the slurry-side isolation valve are closed, and the backwashing branch and the drain branch are opened. The backwashing branch is used to introduce backwashing fluid and clean the inside of the injection joint, and the drain branch is used to discharge the backwashing fluid.

6. The preparation system for a three-layer pure kraft paperboard according to claim 5, characterized in that: It also includes a control unit, which is connected to each dye output channel, fixing agent output branch, supply-side isolation valve, slurry-side isolation valve, backwash interface of backwash branch and drain branch, and is configured to stop the corresponding fixing agent output when there is no slurry flow in the corresponding top / bottom slurry flow branch.

7. The preparation system for a three-layer pure kraft paperboard according to claim 6, characterized in that: The supply-side isolation valve and the slurry-side isolation valve are each equipped with a valve position feedback device. The control unit allows the opening of the corresponding backwashing interface and the sewage branch pipe only when both of the closing feedbacks are valid, and prohibits backwashing and outputs an alarm when either of the closing feedbacks is invalid.

8. The preparation system for a three-layer pure kraft paperboard according to claim 6, characterized in that: The inlet of the slurry tank includes a new slurry inlet and a good slurry return inlet. The control unit is also connected to a first detection component located at the new slurry inlet and a second detection component located at the good slurry return inlet. The first detection component is used to detect the new slurry flow rate and the new slurry load characterization value. The second detection component is used to detect the good slurry return flow rate and the good slurry load characterization value. The control unit determines the comprehensive pigment load based on the product of the new slurry flow rate and the new slurry load characterization value and the product of the good slurry return flow rate and the good slurry load characterization value. The control unit adjusts the fixing agent input amount based on the comprehensive pigment load and the delivery delay time determined by the effective pipeline volume and the slurry flow rate.

9. The preparation system for a three-layer pure kraft paperboard according to claim 8, characterized in that: It also includes a third detection component located at the outlet of the slurry trough. The third detection component is used to detect the mixed slurry flow rate and the mixed slurry load characterization. The control unit determines the actual pigment load based on the mixed slurry flow rate and the mixed slurry load characterization, and corrects the fixing agent input amount based on the deviation between the actual pigment load and the overall pigment load.

10. A method for preparing three-layer pure kraft paper, comprising using the three-layer pure kraft paper preparation system as described in any one of claims 3 to 9, characterized in that: S1: Prepare the surface layer slurry, core layer slurry and bottom layer slurry separately, and ensure that the three slurries are transported independently before entering the three-layer composite molding unit; S2: Yellow dye, red dye and blue dye are mixed and diluted in the first dye preparation tank to form the first dye working solution, and then mixed and diluted in the second dye preparation tank to form the second dye working solution; S3: The first dye working solution is quantitatively delivered to the surface layer high-level tank and the bottom layer high-level tank respectively through the surface layer dye output channel and the bottom layer dye output channel, and the second dye working solution is quantitatively delivered to the core layer high-level tank through the core layer dye output channel. S4: After the top layer slurry and the bottom layer slurry pass through their respective dye inlets, the fixing agent working solution and the diluent are mixed to form a pre-diluted solution. The pre-diluted solution enters the annular distribution cavity surrounding the top / bottom layer slurry flow branch and enters the slurry through multiple circumferentially distributed injection joints. Then the slurry passes through a slag remover. S5: The good slurry output from the first stage slag remover enters the three-layer composite forming unit, the tailings output from the first stage slag remover enter the second stage slag remover, and the good slurry output from the second stage slag remover returns to the slurry flushing tank and re-participates in the slag removal cycle. S6: Combine the surface layer pulp, core layer pulp, and bottom layer pulp to form a pure kraft paper with a surface layer, core layer, and bottom layer in sequence.

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