Waste paper pulping and deslagging treatment system and treatment method thereof

Abstract

The invention belongs to the technical field of waste paper pulping, and particularly relates to a waste paper pulping and deslagging treatment system and a treatment method thereof. Comprising the following steps: step 1, concentrating the finely screened tailings through a first rotary screen concentrator; 2, the concentrated pulp is conveyed to a double-disc pulping machine to be subjected to defibering and fibrillation treatment; step 3, feeding the slurry treated in the step 2 into an up-flow pressure screen for screening light impurities; 4, the screened slurry enters a sticky substance removing screen and then is conveyed to a light slag remover; and 5, concentrating the pulp obtained in the step 4 through a second rotary screen concentrator, and conveying the concentrated pulp to a pulp forming pool for making corrugated base paper. The pressure screen, the sticky substance treatment screen and the light slag separator are combined, so that the fine screening tailings which are originally discharged out of a production system can be recycled into the production system, and the quality index of the recycled paper completely reaches the product quality standard, so that the raw materials are effectively saved.

Description

Technical Field

[0001] The invention belongs to the technical field of waste paper pulping, and in particular relates to a waste paper pulping slag processing system and a processing method thereof. Background Art

[0002] In the conventional corrugated paper waste pulping process, the tailings discharged from the fine screening process are usually discharged directly into the ditch and enter the sewage treatment system of the environmental protection workshop. This treatment method has the following problems: First, the tailings are rich in fiber components, and direct discharge leads to the loss of fiber resources, increasing raw material waste and production costs; second, the fiber and other organic compounds contained in the tailings greatly increase the organic matter content in the sewage, increase the sewage treatment load, reduce the treatment efficiency and significantly increase the environmental protection cost.

[0003] In addition, the fine screening tailings are mixed with a large number of light impurities (including furfural, plastic skin, rags, asbestos, etc.) and adhesives (including ink, hot melt adhesive, latex, wax, etc.). If they are directly mixed into good pulp for corrugated paper production without deep treatment, the light impurities will destroy the bonding force between fibers, resulting in insufficient tension, reduced ring compression strength, increased impurities on the paper surface, and other quality problems. The product indicators are difficult to meet user needs. At the same time, the accumulation of adhesives in pulping equipment and paper machine systems can easily cause frequent paper breaks and equipment blockages, which seriously restricts production continuity and production efficiency.

[0004] In the existing technology, there is no efficient and systematic solution for the recycling of fine screening tailings, especially in the coordinated removal of light impurities and stickies, fiber quality recovery and process adaptability. Therefore, it is urgent to develop a tailings treatment technology that can both efficiently recover fibers and deeply remove impurities. Summary of the invention

[0005] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a waste paper pulping slag processing system, which combines a pressure screen, a stickies processing screen and a lightweight slag remover to allow the fine screening tailings that were originally excluded from the production system to be reused in the production system, and the quality indicators of the recycled paper fully meet the product quality standards, thereby effectively realizing the saving of raw materials.

[0006] Another object of the present invention is to provide a method for treating waste paper pulping slag.

[0007] The technical solution adopted by the present invention is as follows:

[0008] The waste paper pulping slag treatment method uses a treatment system comprising a first cylinder thickener, a double-disc refiner, an upflow pressure screen, a stickies removal screen, a light slag remover and a second cylinder thickener connected in sequence, wherein the discharge port of the second cylinder thickener is connected to a pulping tank;

[0009] The described method for treating waste paper pulp residue includes the following steps:

[0010] Step 1: Concentrate the fine screen tailings through a first round screen concentrator;

[0011] Step 2: Transport the concentrated pulp to a double-disc refiner to defiber and fibrillate the coarser fibers and fiber bundles in the fine screen tailings. At the same time, knead the relatively large light impurities to make them finer and smaller for screening;

[0012] Step 3: Feed the pulp treated in Step 2 into an upflow pressure screen for light impurity screening; The upflow pressure screen feeds pulp from the lower part and discharges pulp from the upper part, with high screening efficiency, gentle screening, and little damage to fibers. At the same time, the sieve drum of the upflow pressure screen is selected as a corrugated slot screen with a screen slot specification of 0.15 mm, and the removal rate of light impurities can reach more than 95%, meeting the production requirements of corrugating base paper;

[0013] Step 4: After the fine screen tailings treated by the upflow pressure screen, most of the light impurities are removed, but the adhesives cannot be removed. Therefore, the screened pulp enters an adhesive removal screen. The sieve drum of the adhesive removal screen is selected as a corrugated screen plate with a screen slot specification of 0.25 mm, and more than 95% of the adhesives can be removed.

[0014] Then it is transported to a light impurity remover. The light impurities and adhesives that cannot be removed by the upflow pressure screen and the adhesive removal screen are further removed by the light impurity remover. The pulp residue enters the interior of the light impurity remover from the top of the light impurity remover, forms a high-speed rotation by means of centrifugal force. The light impurities and adhesives have a relatively small specific gravity and are discharged from the top, while the pulp is discharged from the bottom;

[0015] Step 5: After the pulp obtained in Step 4 is concentrated by a second round screen concentrator, it is transported to a pulp chest for papermaking corrugating base paper.

[0016] The parameters of the fine screen tailings before treatment are: concentration 0.8 - 1.2%, beating degree 23 - 26°SR, wet weight 2 - 3 g, light impurity content 15 - 20%, adhesive content 5 - 10%.

[0017] The inlet pulp concentration of the first round screen concentrator is 0.8 - 1.2%, the outlet pulp concentration is 4 - 6%, the mesh specification is 60 mesh, and the filtration area is 20 m 2 。

[0018] The double-disc refiner is of DD550 type, the grinding disc adopts a fan-shaped direct-through defibering grinding disc, the inlet pulp concentration is 4 - 6%, the outlet pulp concentration is 4 - 6%, the grinding disc gap is 3 - 5 mm, and the rotation speed is 900 rpm.

[0019] The feed consistency of the upflow pressure screen is 2.5 - 3.5%, the discharge consistency is 2 - 3%, the screening area is 0.8 m 2 , the screen slot specification is 0.15 mm, the rotation speed is 800 rpm, and the pressure is 0.15 MPa.

[0020] The feed consistency of the adhesive removal screen is 1.5 - 2.5%, the discharge consistency is 1 - 2%, the screening area is 0.5 m 2 , and the screen slot specification is 0.25 mm.

[0021] The processing flow rate of the light impurity remover is 150 L / min, the feed consistency is 0.8 - 1.2%, and the discharge consistency is 0.8 - 1.0%.

[0022] The feed consistency of the second cylinder screen concentrator is 0.8 - 1.2%, the discharge consistency is 4 - 6%, the mesh specification is 60 mesh, and the filtration area is 20 m 2 .

[0023] The parameters of the pulp transported to the finished pulp tank are: consistency 4 - 6%, beating degree 28 - 30°SR, wet weight 2.8 - 3.5 g, light impurity content 0.5 - 0.8%, and adhesive content 0.3 - 0.5%.

[0024] The treatment system used in the treatment method for waste paper pulping slag removal includes a first cylinder screen concentrator, a double-disc refiner, an upflow pressure screen, an adhesive removal screen, a light impurity remover, and a second cylinder screen concentrator connected in sequence. The discharge port of the second cylinder screen concentrator is connected to the finished pulp tank.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] By adopting the synergistic combination of an upflow pressure screen, an adhesive removal screen, and a light impurity remover, the present invention realizes the efficient recovery and deep purification of fine screening tailings. Among them, the upflow pressure screen with a screening area of 0.8 m 2 can remove more than 95% of light impurities through a 0.15 mm screen slot; the adhesive removal screen also has an adhesive removal rate of more than 95%; combined with a light impurity remover with a small flow rate of 150 L / min to further deeply remove residual impurities, finally significantly improving the fiber quality of the treated pulp, fully meeting the production requirements of corrugated base paper. This technology converts the direct discharge of tailings into system reuse, not only reducing fiber loss and saving raw material costs, but also significantly reducing the organic matter load in sewage and significantly improving the environmental protection benefits. The indexes such as basis weight, ring crush strength, and breaking length of the recycled pulp paper meet the standard requirements. Brief Description of the Drawings

[0027] Figure 1 It is a structural schematic diagram of the treatment system used in the present invention;

[0028] In the figure: 1. First cylinder thickener; 2. Double-disc refiner; 3. Upflow pressure screen; 4. Adhesive removal screen; 5. Light impurity remover; 6. Second cylinder thickener; 7. Finished pulp tank. Specific embodiments

[0029] The present invention will be further described below in conjunction with embodiments, but it does not limit the implementation of the present invention.

[0030] The raw materials used in the examples and comparative examples are all conventional commercially available raw materials unless otherwise specified, and the process methods used in the examples and comparative examples are all conventional methods in the art unless otherwise specified.

[0031] The fine screen tailings used in the examples and comparative examples are from a conventional corrugated base paper waste pulp production line, specifically the tailings discharged during the fine screening process.

[0032] Example 1

[0033] For the treatment method of waste paper pulp slag, the treatment system used is as Figure 1 shown, including a first cylinder thickener 1, a double-disc refiner 2, an upflow pressure screen 3, an adhesive removal screen 4, a light impurity remover 5, and a second cylinder thickener 6 connected in sequence. The discharge port of the second cylinder thickener 6 is connected to the finished pulp tank 7;

[0034] The treatment method of waste paper pulp slag includes the following steps:

[0035] Step 1: Concentrate the fine screen tailings with a concentration of 0.8% through the first cylinder thickener 1. The mesh size of the first cylinder thickener 1 is 60 meshes, and the filtration area is 20 m 2 , and the pulp concentration of the discharged pulp is 4%;

[0036] Step 2: Transport the concentrated pulp with a concentration of 4% to a DD550 type double-disc refiner 2 for defibering and fibrillation treatment. The disc gap is 3 mm, the rotational speed is 900 rpm, and the pulp concentration of the discharged pulp is 4%;

[0037] Step 3: The pulp treated in Step 2 passes through a pulp tank and is mixed with white water to obtain a pulp with a concentration of 2.5%. Then it is sent to the upflow pressure screen 3 for screening of light impurities. The screening area is 0.8 m 2 , the screen slot size is 0.15 mm, the rotational speed is 800 rpm, the pressure is 0.15 MPa, and the pulp concentration of the discharged pulp is 2%;

[0038] Step 4: The screened pulp passes through a pulp tank and is mixed with white water to obtain a pulp with a concentration of 1.5%. Then it is sent to the adhesive removal screen 4. The screening area of the adhesive removal screen 4 is 0.5 m 2, the sieve slot size is 0.25 mm and the pulp output concentration is 1%;

[0039] Then the pulp passes through the pulp chest and is mixed with white water to obtain pulp with a concentration of 0.8%. It is transported to the light impurity remover 5. The processing flow rate of the light impurity remover 5 is 150 L / min and the pulp output concentration is 0.8%;

[0040] Step Five: Send the pulp with a concentration of 0.8% obtained in Step Four to the second cylinder screen concentrator 6 for concentration. The mesh size of the second cylinder screen concentrator 6 is 60 meshes and the filtration area is 20 m 2 , the pulp output concentration is 4%. After concentration, it is transported to the finished pulp chest 7 for papermaking corrugating medium.

[0041] The parameters of the refined screen tailings before treatment are: concentration 0.8%, beating degree 23°SR, wet weight 2 g, light impurity content 15%, and adhesive content 5%.

[0042] The parameters of the pulp transported to the finished pulp chest 7 are: concentration 4%, beating degree 28°SR, wet weight 2.8 g, light impurity content 0.5%, and adhesive content 0.3%.

[0043] Example 2

[0044] For the treatment method of waste paper pulp residue, the treatment system used is the same as that in Example 1;

[0045] The treatment method of waste paper pulp residue includes the following steps:

[0046] Step One: Concentrate the refined screen tailings with a concentration of 1.2% through the first cylinder screen concentrator 1. The mesh size of the first cylinder screen concentrator 1 is 60 meshes and the filtration area is 20 m 2 , the pulp output concentration is 6%;

[0047] Step Two: Transport the concentrated pulp with a concentration of 6% to the DD550 double-disc refiner 2 for defibration and fibrillation treatment. The disc gap is 5 mm, the rotation speed is 900 rpm, and the pulp output concentration is 6%;

[0048] Step Three: The pulp treated in Step Two passes through the pulp chest and is mixed with white water to obtain pulp with a concentration of 3.5%. It is sent to the upflow pressure screen 3 for light impurity screening. The screening area is 0.8 m 2 , the sieve slot size is 0.15 mm, the rotation speed is 800 rpm, the pressure is 0.15 MPa, and the pulp output concentration is 3%;

[0049] Step Four: The screened pulp passes through the pulp chest and is mixed with white water to obtain pulp with a concentration of 2.5%. It is sent to the adhesive removal screen 4. The screening area of the adhesive removal screen 4 is 0.5 m 2, the sieve slot size is 0.25 mm, and the pulp discharge concentration is 2%;

[0050] Then the pulp passes through the pulp chest, mixes with white water, and obtains pulp with a concentration of 1.2%. It is transported to the light impurity remover 5. The processing flow rate of the light impurity remover 5 is 150 L / min, and the pulp discharge concentration is 1.0%;

[0051] Step Five: Send the pulp with a concentration of 1.0% obtained in Step Four to the second cylinder screen concentrator 6 for concentration. The mesh size of the second cylinder screen concentrator 6 is 60 mesh, and the filtration area is 20 m 2 , the pulp discharge concentration is 6%, and after concentration, it is transported to the finished pulp chest 7 for making corrugating base paper.

[0052] The parameters of the refined screen tailings before treatment are: concentration 1.2%, beating degree 26°SR, wet weight 3 g, light impurity content 20%, and adhesive content 10%.

[0053] The parameters of the pulp transported to the finished pulp chest 7 are: concentration 6%, beating degree 30°SR, wet weight 3.5 g, light impurity content 0.8%, and adhesive content 0.5%.

[0054] Example 3

[0055] The treatment method of waste paper pulp residue uses the same treatment system as in Example 1;

[0056] The treatment method of waste paper pulp residue includes the following steps:

[0057] Step One: Concentrate the refined screen tailings with a concentration of 1.0% through the first cylinder screen concentrator 1. The mesh size of the first cylinder screen concentrator 1 is 60 mesh, and the filtration area is 20 m 2 , and the pulp discharge concentration is 5%;

[0058] Step Two: Transport the concentrated pulp with a concentration of 5% to the DD550 double-disc refiner 2 for defibration and fibrillation treatment. The disc gap is 4 mm, the rotation speed is 900 rpm, and the pulp discharge concentration is 5%;

[0059] Step Three: The pulp treated in Step Two passes through the pulp chest, mixes with white water, and obtains pulp with a concentration of 3%. Send it to the upflow pressure screen 3 for light impurity screening. The screening area is 0.8 m 2 , the sieve slot size is 0.15 mm, the rotation speed is 800 rpm, the pressure is 0.15 MPa, and the pulp discharge concentration is 2.5%;

[0060] Step Four: The screened pulp passes through the pulp chest, mixes with white water, and obtains pulp with a concentration of 2.0%. Send it to the adhesive removal screen 4. The screening area of the adhesive removal screen 4 is 0.5 m 2, the sieve slot specification is 0.25 mm, and the pulp output concentration is 1.5%;

[0061] Then the pulp passes through the pulp chest, mixes with white water to obtain pulp with a concentration of 1.0%, and is transported to the light impurity remover 5. The processing flow rate of the light impurity remover 5 is 150 L / min, and the pulp output concentration is 0.9%;

[0062] Step Five: Send the pulp with a concentration of 0.9% obtained in Step Four to the second cylinder screen concentrator 6 for concentration. The mesh specification of the second cylinder screen concentrator 6 is 60 mesh, and the filtration area is 20 m 2 , the pulp output concentration is 5%, and after concentration, it is transported to the finished pulp chest 7 for making corrugating base paper.

[0063] The parameters of the refined screen tailings before treatment are: concentration 1.0%, beating degree 24.5°SR, wet weight 2.5 g, light impurity content 17%, and adhesive content 8%.

[0064] The parameters of the pulp transported to the finished pulp chest 7 are: concentration 5%, beating degree 29°SR, wet weight 3.2 g, light impurity content 0.6%, and adhesive content 0.4%.

[0065] Comparative Example 1

[0066] The difference from Example 3 is that the upflow pressure screen with a sieve slot specification of 0.15 mm in Step Three is replaced with an upflow pressure screen with a sieve slot specification of 0.25 mm, and the others are the same as Example 3.

[0067] Comparative Example 2

[0068] The difference from Example 3 is that the used device does not include the adhesive remover screen 4, and the pulp enters the light impurity remover 5 from the upflow pressure screen 3, and the others are the same as Example 3.

[0069] Comparative Example 3

[0070] The difference from Example 3 is that the used device does not include the light impurity remover 5, and the pulp enters the second cylinder screen concentrator 6 from the adhesive remover screen 4, and the others are the same as Example 3.

[0071] Comparative Example 4

[0072] The difference from Example 3 is that the upflow pressure screen with a sieve slot specification of 0.15 mm in Step Three is replaced with an upflow pressure screen with a sieve slot specification of 0.25 mm, the used device does not include the adhesive remover screen 4 and the light impurity remover 5, and the pulp enters the second cylinder screen concentrator 6 from the upflow pressure screen 3, and the others are the same as Example 3.

[0073] Use the tailings treated in Examples 1 - 3 and Comparative Examples 1 - 4 respectively to prepare corrugating base paper. The preparation method includes the following steps:

[0074] S1. Waste paper treatment: The waste cartons are conveyed by a chain conveyor to a D-type hydraulic pulper for pulping to obtain primary pulp.

[0075] S2. Purification and screening: The primary pulp is successively passed through a high-concentration sand remover to remove heavy impurities, a coarse screen to remove large-size impurities, a low-concentration sand remover to further remove light impurities, and a fine screen for fine screening to obtain purified pulp.

[0076] S3. Concentration and pulp storage: The purified pulp is concentrated by a multi-disk concentrator and then conveyed to a pulp storage tank for temporary storage.

[0077] S4. Refining treatment: The pulp in the pulp storage tank is subjected to fiber fibrillation treatment by a double-disc refiner. After refining, it is conveyed to a finished pulp tank and mixed evenly with the treated tailings in the examples or comparative examples. The weight ratio of the pulp to the tailings is 8:2.

[0078] S5. Pre-sheet preparation: The pulp in the finished pulp tank is sent to a pre-sheet tank and mixed with the white water recycled from the off-machine white water tank, and then conveyed by a filling pump to a headbox screen for homogenization treatment.

[0079] S6. Sheet forming: The homogenized pulp is evenly distributed to the wire section through a headbox. The lip opening of the headbox is controlled at 13 mm, the pulp-to-wire speed ratio is 0.95, and the pulp concentration on the wire is 1.1%. After dewatering by the wire section, a wet paper web is formed, and the dryness at the exit of the couch roll reaches 22%.

[0080] S7. Press dewatering: The wet paper web enters the press section for further dewatering. The line pressure of the large roll in the press section is 290 kN / m, and the dryness at the exit of the press section is increased to 50%.

[0081] S8. Front-end drying: The dewatered paper web is dried by the front dryer to a dryness of 94%.

[0082] S9. Surface sizing: The dried paper web is fed into a size press. The line pressure of the size press roll is controlled at 110 kN / m, the dryness at the entrance of the size press is 94%, and the dryness at the exit of the size press is reduced to 67%.

[0083] S10. Rear-end drying and finished paper: The sized paper web is dried by the rear dryer to a dryness of 94% of the finished paper.

[0084] S11. Reeling and post-treatment: The dried paper web is reeled by the reeling section, and then rewound, slit, and packed into the warehouse.

[0085] A 4400-type fourdrinier multi-cylinder corrugating paper machine is adopted, and the process parameters of each section are as follows:

[0086] Line pressure of the large roll in the press section: 290 kN / m;

[0087] Line pressure of the size press roll: 110 kN / m;

[0088] Dryness of the formed paper: 94%.

[0089] Perform performance tests on the corrugating base paper made from the tailings treated in Examples 1-3 and Comparative Examples 1-4 respectively. The test methods are as follows:

[0090] Grammage (g / m 2 ): Test according to GB / T 451.2-2002;

[0091] Bulk density (g / cm 3 ): Test the thickness according to GB / T 451.3-2002, and bulk density = grammage / thickness;

[0092] Water absorption (g / m2): Test according to GB / T 1540-2002;

[0093] Ring crush strength (N·m / g): Test according to GB / T 2679.8-2016;

[0094] Breaking length (km): Test according to GB / T 12914-2008;

[0095] Moisture content (wt.%): Test according to GB / T 462-2023;

[0096] Ash content (wt.%): Test according to GB / T 463-1989;

[0097] The test results are shown in Table 1.

[0098] Table 1 Performance test results

[0099]

[0100] It can be seen from the data in Table 1 that the corrugating base paper made in Examples 1-3 meets the formed paper index standards in terms of ring crush strength and breaking length.

[0101] In Comparative Examples 1-3, the ring crush strength of the formed paper is slightly higher than 9 N·m / g, and the breaking length is slightly higher than 4.3 km. Although it meets the formed paper standard, changing Comparative Examples 1-3 can significantly affect the main formed paper strength indicators of the corrugating base paper; to further illustrate the effect of the present invention, in Comparative Example 4, three changes are made simultaneously, and it can be seen that the ring crush strength and breaking length are significantly reduced and cannot meet the formed paper standard, resulting in non-conforming products.

Claims

1. A method for treating waste paper pulping slag, characterized in that: The processing system used therein comprises a first cylinder thickener (1), a double-disc refiner (2), an upflow pressure screen (3), a stickies removal screen (4), a light slag remover (5) and a second cylinder thickener (6) which are connected in sequence, and the discharge port of the second cylinder thickener (6) is connected to a pulping tank (7); The method for treating waste paper pulping slag comprises the following steps: Step 1: Concentrating the fine screening tailings through a first round screen concentrator (1); Step 2: conveying the concentrated pulp to a double-disc refiner (2) for debonding and silk-forming treatment; Step 3, sending the slurry treated in step 2 to an upflow pressure screen (3) with a screen gap specification of 0.15 mm to screen light impurities; Step 4: The screened slurry enters the sticky matter removal screen (4), and then is transported to the light slag remover (5); Step 5: The slurry obtained in step 4 is concentrated by a second rotary screen concentrator (6), and then transported to a pulping tank (7) for making corrugated paper.

2. The method for treating waste paper pulping slag according to claim 1, characterized in that: The parameters of the fine screening tailings before treatment are: concentration 0.8-1.2%, beating degree 23-26°SR, wet weight 2-3g, light impurity content 15-20%, and sticky matter content 5-10%.

3. The method for treating waste paper pulping slag according to claim 1, characterized in that: The first rotary screen thickener (1) has an inlet concentration of 0.8-1.2%, an outlet concentration of 4-6%, a mesh size of 60 meshes, and a filtration area of ​​20 m 2 .

4. The method for treating waste paper pulping slag according to claim 1, characterized in that: The inlet pulp concentration of the double-disc refiner (2) is 4-6%, the outlet pulp concentration is 4-6%, the clearance between the grinding discs is 3-5 mm, and the rotation speed is 900 rpm.

5. The method for treating waste paper pulping slag according to claim 1, characterized in that: The upflow pressure screen (3) has a pulp inlet concentration of 2.5-3.5%, a pulp outlet concentration of 2-3%, and a screening area of ​​0.8 m 2 , rotation speed is 800rpm and pressure is 0.15MPa.

6. The method for treating waste paper pulping slag according to claim 1, characterized in that: The sticky matter removal screen (4) has a pulp inlet concentration of 1.5-2.5%, a pulp outlet concentration of 1-2%, and a screening area of ​​0.5 m 2 , the sieve gap specification is 0.25mm.

7. The method for treating waste paper pulping slag according to claim 1, characterized in that: The light slag remover (5) has a processing flow rate of 150 L / min, an inlet slurry concentration of 0.8-1.2%, and an outlet slurry concentration of 0.8-1.0%.

8. The method for treating waste paper pulping slag according to claim 1, characterized in that: The second rotary screen thickener (6) has an inlet concentration of 0.8-1.2%, an outlet concentration of 4-6%, a mesh size of 60 meshes, and a filtration area of ​​20 m 2 .

9. The method for treating waste paper pulping slag according to claim 1, characterized in that: The parameters of the slurry transported to the pulping tank (7) are: concentration 4-6%, beating degree 28-30°SR, wet weight 2.8-3.5 g, light impurity content 0.5-0.8%, and sticky matter content 0.3-0.5%.

10. A treatment system used in the method for treating waste paper pulping slag according to any one of claims 1 to 9, characterized in that: The invention comprises a first cylinder thickener (1), a double-disc refiner (2), an upflow pressure screen (3), a stickies removal screen (4), a light slag remover (5) and a second cylinder thickener (6) which are connected in sequence, wherein the discharge port of the second cylinder thickener (6) is connected to a pulping tank (7).

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