A method for improving the bleaching performance of eucalyptus chemical mechanical pulp

Abstract

The present invention discloses a method for improving the bleaching performance of eucalyptus chemimechanical pulp. Prior to the pre-impregnation and dosing steps, the wood chips are first crushed and extruded using a first twin-screw extruder. The chips are then soaked in hot water and concentrated and extruded using a second twin-screw extruder. The resulting pulp is then added with pre-impregnation chemicals to proceed with the normal chemimechanical pulp production process. This application, for the first time, investigates the influence of the water-soluble nitrogen-carbonyl conjugated structure in the eucalyptus raw material on the subsequent bleaching performance of eucalyptus chemimechanical pulp and innovatively proposes a method for improving the bleaching performance of eucalyptus chemimechanical pulp.

Description

Technical Field

[0001] The invention belongs to the field of chemical mechanical pulping, and particularly relates to a method for improving the bleaching performance of eucalyptus chemical mechanical pulp. Background Art

[0002] At present, pulping processes are mainly divided into two categories: chemical pulping and chemical-mechanical pulping (hereinafter referred to as chemical-mechanical pulping). Compared with the traditional chemical pulping process, the chemical-mechanical pulping process can maximize the utilization of plant fiber components and is a high-yield pulping process. Since the first chemical-mechanical pulping production line in China was put into production at Jilin Paper Mill in 1995, the chemical-mechanical pulping process has developed rapidly in China (Fang Guigan, Chemi-mechanical Pulping Process Design Based on the Characteristics of Chinese Fiber Materials and Its Equipment Configuration, China Pulp & Paper Industry, 2021, 42(21): 28-32. FANG GG, Chemi-mechanical Pulping Process Design Based on the Characteristics of Chinese Fiber Materials and Its Equipment Configuration, China pulp & paper industry, 2021, 42(21): 28-32.). The alkaline hydrogen peroxide process is the current mainstream pulping process for bleached chemical-mechanical pulp. Sodium hydroxide provides nucleophilic attack, softening and loosening the originally hard fiber raw material. This helps the oxidizing hydrogen peroxide penetrate deeply, destroying various chromophores. By controlling the dosage of hydrogen peroxide and sodium hydroxide, pulp with varying optical and physical properties can be obtained. It is generally believed that the effective destruction of chromophores by hydrogen peroxide is the primary factor affecting pulp whiteness, while the dosage of sodium hydroxide significantly influences the pulping process yield, as well as the bulk and physical strength of the pulp.

[0003] Most of my country's land is located in the subtropical and temperate regions, and the raw material sources for wood pulp production have gradually formed the characteristics of "eucalyptus in the south and poplar in the north". At present, the area of ​​eucalyptus plantations in China is 5.46 million hectares, mainly distributed in 11 southern provinces (regions), with the largest number in Guangxi and Guangdong, accounting for about 3 / 4 of the total area. The most widely planted species is the Eucalyptus grandis clone, and the annual output of eucalyptus wood exceeds 30 million cubic meters, which exceeds 1 / 3 of the total output of commercial wood in the country. (Liu Tao, Xie Yaojian, Analysis and Prospect of the Rapid Development of Eucalyptus Plantations in China, Eucalyptus Science and Technology, 2020, 37(4): 38-47; Deng Yongjun, Fang Guigan, Han Shanming, et al. Effect of Different Concentrations of Second-Stage Refining on P-RC APMP Pulping Performance [J]. Paper Science and Technology, 2011(3): 1-5) In addition, my country also needs to import a large amount of eucalyptus chips from Southeast Asia and Australia for pulping and papermaking every year. According to Wood Resource Quarterly, Chinese pulping companies imported 13.2 million tons of hardwood chips as pulping raw materials in 2020. Eucalyptus has become the main raw material for wood pulping in my country.

[0004] In the chemical mechanical pulping process, under the condition of a certain amount of hydrogen peroxide, the change trend of pulp brightness with the amount of sodium hydroxide used for different types of fiber raw materials is consistent, that is, the pulp brightness first increases gradually with the amount of sodium hydroxide used, and then gradually decreases after reaching the maximum value. Figure 1 The bleaching performance comparison results of commercial poplar and eucalyptus wood are as follows: hydrogen peroxide dosage: 80kg·t -1 , sodium silicate dosage: 20kg·t -1 DTPA dosage: 5kg·t -1 The reaction temperature is 95°C, the reaction concentration is 20%, and the reaction time is 50 minutes. Therefore, the main indicator for evaluating the bleaching performance of a raw material is the highest whiteness value that can be achieved in the pulp at a certain hydrogen peroxide dosage. This highest whiteness value directly reflects the difficulty of bleaching the raw material.

[0005] When the amount of hydrogen peroxide is 80kg·t -1 Under the conditions, the optimization curve of sodium hydroxide dosage for single-stage bleaching of two raw materials was made in the laboratory to compare the bleaching performance of poplar chips and eucalyptus chips. Figure 1 As can be seen, in the single-stage bleaching process, the bleaching performance of the two raw materials differed significantly. Poplar wood achieved a maximum brightness of 79.42% ISO, while eucalyptus wood only achieved 70.95% ISO. Under current conditions, how to best utilize eucalyptus wood and produce high-brightness pulp (brightness > 78% ISO) at the lowest possible cost to meet the demands of various paper products is an urgent issue facing domestic chemical-mechanical pulp production lines using eucalyptus wood as a raw material.

[0006] The existing mainstream bleached chemimechanical pulp production line is shown in Figure 2.

[0007] The specific process is as follows: qualified wood chips from the stock preparation system are washed with hot water (water temperature does not exceed 70℃) and steamed (105℃, 10-30min). After steaming, the wood chips enter the chemical mechanical pulping process. The production process is roughly as follows:

[0008] (1) After steaming, the material is pre-impregnated with appropriate amounts of chemicals according to the requirements of the pre-impregnation section in Table 1. The pre-impregnation process conditions are: temperature 80-100°C, time 30-60 min, and impregnation system concentration 10-25% (mass ratio of absolute dry wood chips to the total system, the same below).

[0009] (2) After the pre-impregnation, the waste liquid is removed to increase the concentration to 25-40%, and then sent to the conical high-consistency mill for the initial high-consistency pressure refining. The refining concentration is 25-40%, the grinding chamber pressure is 0.1-0.18MPa, and the temperature is 90-105℃.

[0010] (3) After spraying and heat exchange, the resulting pulp is subjected to high-consistency bleaching by adding hydrogen peroxide, sodium hydroxide, sodium silicate, and a chelating agent according to the requirements of the high-consistency bleaching stage in Table 1. The bleaching process is as follows: time 60-150 minutes, temperature 85-95°C, and bleaching system concentration of 15-25% (i.e., the mass ratio of bone dry pulp to the total system).

[0011] (4) After de-latency and dilution, the bleached pulp is sent to a low-consistency disc mill at atmospheric pressure for processing at a grinding temperature of 70-90°C and a consistency of 5-10%. The milled pulp is screened, and the residue is processed through a residue mill and mixed with the bleached pulp before returning to the main process. The good pulp is sent to a multi-disc filter for concentration, and then to a twin-roll extruder for washing and concentration. The finished pulp is then stored in a medium-consistency tower for future use.

[0012] Table 1 Chemical dosage in each stage of conventional chemical mechanical pulping process

[0013]

[0014] Note: The amounts of hydrogen peroxide and sodium hydroxide in the table are based on the amount of dry wood chips after 100% conversion. The chelating agent is generally diethylenetriamine pentaacetic acid (DTPA).

[0015] Currently, large-scale domestic production lines all rely on imported production lines, whose associated processes are designed for perennial European and American softwoods with high initial brightness and a relatively soft texture. However, the raw material for my country's chemical-mechanical pulp is primarily eucalyptus, which has a lower initial brightness, a higher extractive content, and a harder texture. In actual production, a common problem is the low efficiency of bleaching chemicals, making pulp bleaching difficult.

[0016] CN104846676A discloses a method for producing bleached chemical mechanical pulp using eucalyptus chips. The method first washes the eucalyptus chips with hot water at 75-95°C to soften and preheat the chips for easy extrusion, tearing and dehydration. The chips are then extruded, teared and dehydrated using a single screw extruder. The pulp is then subjected to one immersion bleaching step, two high-consistency belt pressure refining steps, and fully mixed with a liquid medicine for bleaching. The invention is based on the idea of ​​processing the material into a multi-gap loose thread-like cotton shape so that the material can strongly absorb chemicals. The pulp is then subjected to processes such as immersion bleaching to obtain a high-whiteness and high-bulk pulp product.

[0017] CN103334328A discloses a method for preparing bleached chemical mechanical pulp using eucalyptus sawdust. The method sequentially subjects the eucalyptus sawdust raw material to the steps of hot water washing, dehydration, pre-steaming, twin-screw extrusion concentration, chemical impregnation, and high-consistency refining. The inventive concept is to remove extractives and impurities from the eucalyptus sawdust as much as possible while minimizing fiber damage, and to impregnate the eucalyptus sawdust raw material with alkaline hydrogen peroxide to fully soften and bleach the eucalyptus sawdust raw material. At the same time, bleaching chemicals are added during the refining process, and the high-consistency mixing effect and heat energy generated during the refining process are utilized to rapidly heat the pulp during the refining process to achieve instantaneous bleaching, thereby improving bleaching efficiency.

[0018] Both of the above-mentioned existing patents are based on the design concept of imported production lines. The idea of ​​improving bleaching efficiency is to enhance the penetration of bleaching solution. It is believed that the chromophores of pulp are mainly conjugated structures in the lignin structural units. As long as the bleaching solution is allowed to contact the chromophores of the wood chips as much as possible, the bleaching efficiency can be improved. Summary of the Invention

[0019] Purpose of the Invention: This application is based on the team's latest research on eucalyptus bleaching chemical-mechanical pulping processes, namely, that small-molecule nitrogen-containing soluble substances in eucalyptus raw materials, particularly structures conjugated with carbonyl groups and carbonyl double bonds (hereinafter referred to as nitrogen-carbonyl conjugated compounds), have a decisive negative impact on subsequent bleaching. It was also discovered that nitrogen-carbonyl conjugated compounds can be removed from the fiber raw materials by fully soaking them at high temperatures before adding chemicals. After this "denitrification" process, the bleaching performance of the eucalyptus raw materials is greatly improved. More importantly, a special property of these substances is that after reacting with conventional pulping chemicals (hydrogen peroxide, sodium hydroxide), they will combine with the pulp to form large, insoluble substances. Once formed, these insoluble substances are not only impossible to remove through conventional water washing and soaking, but also have a persistent negative impact on the bleachability of the pulp. Therefore, these substances must be dissolved from the raw materials as much as possible before adding chemicals, otherwise the optimal bleaching effect of the raw materials will not be achieved. This part of the content is a first in the field of eucalyptus chemical-mechanical pulping.

[0020] Therefore, this application proposes a pretreatment method (high-temperature immersion denitrification) to improve the bleaching performance of eucalyptus chemimechanical pulp. In the preparation of bleached chemimechanical pulp from various eucalyptus wood chips, before using chemicals, the wood chips are first subjected to spiral extrusion to dissociate the macrostructure, then soaked in hot water to dissolve as much nitrogen-carbonyl conjugated compounds as possible, and finally concentrated by extrusion to remove as much nitrogen-carbonyl conjugated compounds as possible. The resulting material then enters the conventional chemimechanical pulping process. This treatment can significantly improve the bleaching performance of eucalyptus wood.

[0021] Technical solution: In order to achieve the above technical objectives, the present application proposes a method for improving the bleaching performance of eucalyptus chemical mechanical pulp. Before the pre-impregnation and dosing steps, the wood chips are first crushed and extruded using a first twin-screw extruder, and then soaked in high-temperature hot water. After soaking, they are concentrated and extruded through a second twin-screw extruder. The resulting pulp is then added with pre-impregnation chemicals to carry out the normal chemical mechanical pulp production process.

[0022] In the first crushing and extrusion, the specific surface area of ​​the eucalyptus wood raw material is reduced from about 1.0 to 1.4×10 -3 m 2 / g increased to 0.10m 2 / g or more, so that the internal structure of the wood chips can be fully exposed.

[0023] The temperature of the high-temperature hot water is 80-100°C.

[0024] Specifically, the conditions for high-temperature hot water soaking are: keeping warm for 30-90 minutes at a system concentration of 1-20%, where the system concentration is the mass ratio of absolute dry fiber raw materials to the total system.

[0025] The material is concentrated and extruded by a second twin-screw extruder to extrude the material to a dryness (mass ratio of absolute dry material divided by the mass of the original material) of not less than 35%.

[0026] In a preferred embodiment, the process is as follows: before the pre-impregnation and dosing step, the wood chips are crushed and extruded using a first twin-screw extruder to increase the specific surface area of ​​the eucalyptus raw material to 0.10 m 2 / g or more, then keep warm at 80-100℃ and system concentration of 1-20% for 30-120min, and after soaking, use a second twin-screw extruder to concentrate and extrude the material until the dryness is greater than 35%. The obtained slurry is then added with pre-impregnation chemicals to carry out normal chemical mechanical pulp production process.

[0027] The chemical mechanical pulp production process after adding the pre-impregnation chemicals includes pre-impregnation, high-concentration refining, high-concentration bleaching, low-concentration refining, screening, washing and concentrating, and pulping.

[0028] Specifically, the conditions for high-concentration refining are: refining concentration 25-40%, grinding chamber pressure 0.1-0.18MPa, temperature 95-105°C; the conditions for high-concentration bleaching are: temperature 85-95°C, bleaching system concentration 15-25%, time 60-150min; the conditions for low-concentration refining are: refining temperature 70-90°C under normal pressure, concentration 5-10%.

[0029] The subsequent step of adding chemicals for the pre-impregnation stage can also refer to the chemical mechanical pulp production process after pre-impregnation in the background art.

[0030] Beneficial effects: This application proposes for the first time the influence of the water-soluble nitrogen-carbonyl conjugated structure in eucalyptus raw materials on the subsequent bleaching performance of eucalyptus chemical-mechanical pulp, and originally proposes a method for improving the bleaching performance of eucalyptus chemical-mechanical pulp, solving the problem that "substances containing carbon-nitrogen double bonds and carbonyl conjugated structures in the initial raw materials affect the bleaching performance of eucalyptus" that has never been found in the industry. In the process of preparing bleached chemical-mechanical pulp by processing various types of eucalyptus wood chips, before using chemicals, a twin-screw extruder is used to destroy the original structure of the eucalyptus chips as much as possible, thereby increasing their specific surface area, and increasing the surface area of ​​the wood chips from about 1.0 to 1.4×10 -3 m 2 / g increased to 0.1m 2 / g or more. The material is then kept warm at 80-100°C and a system concentration of 1-20% for 30-120 minutes. A twin-screw extruder is then used to reduce the dryness of the material to over 35%, removing as much hot water extract as possible. The resulting material then enters the conventional chemical-mechanical pulping process. This treatment can significantly improve the whiteness of the finished eucalyptus pulp. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, and the above and / or other advantages of the present invention will become more apparent.

[0032] Figure 1 To compare the bleaching properties of poplar and eucalyptus;

[0033] Figure 2A This is the process flow chart of mainstream chemical mechanical pulp;

[0034] Figure 2B This is the process flow chart for this application;

[0035] Figure 3 This is the initial state of the Eucalyptus urophylla raw material;

[0036] Figure 4 It is the state of wood chips after being processed by twin-screw extruder;

[0037] Figure 5 The bleaching effect of Examples 1 and 2 is compared with that of the comparative experiment;

[0038] Figure 6 The bleaching effect of Examples 3 and 4 is compared with that of the comparative experiment;

[0039] Figure 7 The states of Eucalyptus grandis wood chips with different degrees of fragmentation;

[0040] Figure 8 The bleaching effects of Examples 2, 5, 6, and 7 are compared with those of Comparative Experiment 2.

[0041] Figure 9 The bleaching effect comparison between Example 8 and Comparative Experiment 5 is shown;

[0042] Figure 10 The bleaching effect comparison between Example 9 and Comparative Experiment 6 is shown. DETAILED DESCRIPTION

[0043] The present invention is described in detail below by way of specific examples. Unless otherwise specified, the reagents used in the following examples are conventional reagents, and the dosage units of the reagents are kg / t odt, i.e., the mass of the chemical relative to the absolute dry raw material, abbreviated as kg / t.

[0044] Figure 2A This is the entire flow chart of the mainstream chemical mechanical pulping process. Figure 2B The whole process flow chart of the present invention is shown in FIG. Unless otherwise specified, in the following embodiments, the pulping process from pre-impregnation onwards is completed according to the following process, which is named “conventional pulping process flow-1”. Figure 2A 、 Figure 2B The same processing steps are followed after the addition of chemicals to the pre-impregnation process. The specific contents are:

[0045] (1) The materials are pre-impregnated with chemicals. The pre-impregnation process conditions are: temperature 90°C, time 40 min, and impregnation system concentration 20% (mass ratio of absolute dry wood chips to the total system, the same below);

[0046] (2) After the pre-impregnation, the impregnation residual liquid (B) is squeezed out, and the material concentration is increased to 32%. The material is then fed into a conical high-consistency mill for initial high-consistency pressure refining. The refining concentration is 32%, the grinding chamber pressure is 0.1 MPa, and the temperature is 95°C. (3) The resulting pulp is subjected to high-consistency bleaching by adding hydrogen peroxide, sodium hydroxide, sodium silicate, and a chelating agent according to the corresponding high-consistency bleaching chemical dosages in Table 2. The bleaching process is as follows: time 60 minutes, temperature 90°C, and bleaching system concentration of 20%.

[0047] (4) After de-latency and dilution, the bleached pulp is sent to a low-consistency disc mill at atmospheric pressure, with a grinding temperature of 80°C and a concentration of 8%. The milled pulp is screened through a vibrating screen with a 0.3mm screen slot. The pulp that passes through the screen slot is good pulp, and the remaining portion is the reject. The reject is processed through an atmospheric reject mill (grinding temperature of 80% and a concentration of 25%) and then mixed with the bleached pulp and returned to the main process. The good pulp is sent to a multi-disc filter and concentrated to a concentration of no less than 30%. The finished pulp is then stored in a medium-consistency stock tower for future use.

[0048] At present, there are two schemes for the dosage and distribution of chemicals in the mainstream chemical-mechanical pulping process, as shown in Table 2, marked as conventional process 1 and conventional process 2 respectively.

[0049] Table 2 Chemical dosage of conventional chemical mechanical pulping process

[0050]

[0051] The present invention is described in detail below through specific embodiments.

[0052] Example 1-4 Effect of water-soluble nitrogen carbonyl conjugated structure in eucalyptus raw material on the subsequent bleaching performance of eucalyptus chemical-mechanical pulp.

[0053] The experimental raw material in this embodiment is Eucalyptus urophylla wood chips.

[0054] Comparative test 1 was conducted with Eucalyptus grandis wood chips (specific surface area 1.12×10 -3 m 2 / g, see Figure 3 ) After screening, washing and steaming, bleached chemical mechanical pulp was prepared according to the "Conventional Pulping Process-1" according to the chemical dosage of Conventional Process 1 in Table 2. The solution squeezed out in the pre-impregnation section was recorded as B1 (i.e., the impregnation residual liquid was removed by squeezing in step (2)).

[0055] In the high temperature soaking denitrification treatment method of Example 1, the Eucalyptus grandis wood chips are processed by a twin-screw extruder to a specific surface area of ​​1.83m 2 / g of filamentous material (see the effect Figure 4 ) was then soaked at 95°C for 60 minutes at a system concentration of 10% (i.e., the mass ratio of the absolute dry fiber raw material to the total system). The material concentration was then increased to 40% using a twin-screw extruder. The extruded solution was designated Solution A. The resulting material was used to prepare bleached chemimechanical pulp according to "Conventional Pulping Process Flow-1," using the chemical dosages specified in Conventional Process 1 in Table 2.

[0056] Comparative test 2 was conducted with Eucalyptus grandis wood chips (specific surface area 1.12×10 -3 m 2After screening, washing, and steaming, bleached chemical-mechanical pulp was prepared according to the "Conventional Pulping Process Flow-1" according to the chemical dosage of Conventional Process 2 in Table 2. The solution extruded in the pre-impregnation stage was recorded as B2.

[0057] In Example 2, the Eucalyptus grandis wood chips were treated with the high-temperature soaking and nitrogen removal process of Example 1. The resulting material was used to prepare bleached chemical-mechanical pulp using the "Conventional Pulping Process Flow-1" according to the chemical dosage of Conventional Process 2 in Table 2.

[0058] Depend on Figure 5 It can be seen that the highest whiteness of Example 1 is 77.62% ISO, which is significantly higher than 75.97% ISO of its comparative experiment 1; the highest whiteness of Example 2 is 77.12% ISO, which is also significantly higher than 74.54% ISO of its comparative experiment 2.

[0059] To explain the reaction mechanism of the above phenomenon, we collected the high-temperature soaking "nitrogen removal" process solution A of the raw materials in Examples 1 and 2 described above, as well as the pre-impregnation solution residue B1 of Comparative Experiment 1 and the pre-impregnation section solution residue B2 of Comparative Experiment 2. These solutions were vacuum-dried to obtain the following: Eucalyptus urophylla high-temperature soaking solution (a), the pre-impregnation section solution (b1) of Comparative Experiment 1, and the pre-impregnation section solution (b2) of Comparative Experiment 2. By comparing the elemental analysis (Table 3) and pyrolysis gas chromatography-mass spectrometry (GC-MS) results of the three solutions, we investigated the differences in their composition and structure.

[0060] Table 3 lists the mass ratios of the four elements (carbon, hydrogen, oxygen, and nitrogen) in the three extracts. As can be seen, the nitrogen content in the hot water extract (a) of Eucalyptus urophylla sawdust is significantly higher than that in the two conventional pre-impregnation extracts (b1 and b2). This indicates that the chemical reaction caused by the addition of chemicals causes some of the nitrogen originally soluble in the raw material to become insoluble.

[0061] Table 3 Elemental analysis of three extracts

[0062] element b1 b2 a C 29.03 28.77 39.59 H 4.136 3.893 4.639 O 42.087 37.233 36.429 N 1.06 1.08 2.78

[0063] Table 4 shows the pyrolysis-GC / MS results for the three extracts. As can be seen from Table 4, a large amount of lignin fragments in the form of phenolic compounds are present in all three extracts. These substances have similar structural characteristics, all resulting from the reaction of lignin in the raw material with the pre-impregnation chemicals. They are present in large quantities in the pre-impregnation pulp and extracts, so their content in the pulp samples has little impact on bleaching. However, the nitrogen forms present in the three extracts differ significantly (in bold black text in Table 4). The nitrogen forms in b1 and b2 completely overlap, but do not overlap with those in a. The nitrogen-containing substances in b1 and b2 are primarily found in compounds 27#, 28#, and 31#, which have longer residence times and larger molecular weights, while the nitrogen-containing substances in a are found in compounds 6#, 7#, 8#, 14#, and 17#, which have shorter residence times and smaller molecular weights. Of particular note, compounds 6#, 7#, and 8# all exhibit conjugated carbon-nitrogen double bonds with carbonyl groups. From the change in the existence form of nitrogen elements from a to b1 and b2, it can be inferred that the 6#, 7#, 8#, 14#, and 17# compounds with smaller molecular weights originally present in the raw materials have transformed into insoluble structures with larger molecular weights after reacting with the pre-impregnation chemicals, among which the 27#, 28#, and 31# compounds with relatively smaller molecular weights have been dissolved.

[0064] Therefore, the present invention effectively improves pulp bleaching performance by dissolving small-molecule nitrogen-carbonyl conjugated compounds through a high-temperature immersion denitrification process. Conventional processes, however, involve adding chemicals to the raw materials. These nitrogen-carbonyl conjugated compounds participate in the immersion reaction, resulting in a product with a higher molecular weight. Only a portion of these compounds can be dissolved, and the remaining portion in the pulp negatively impacts subsequent bleaching performance.

[0065] Table 4 Comparison of main molecular ion peaks of three dissolution products

[0066]

[0067]

[0068]

[0069]

[0070] Note: In the gas chromatogram of the sample, the relative proportion of the molecular ion peak integrated area ≥3% is marked as strong, 3%> molecular ion peak integrated area relative proportion ≥1% is marked as medium, and 1%> molecular ion peak integrated area relative proportion ≥0.8% is marked as weak.

[0071] For a long time, in the field of eucalyptus chemical mechanical pulp bleaching, the prevailing view was that chromophores exist in the lignin structure. Research on the bleaching performance of pulp also focused on the lignin structure, and the idea of ​​improving bleaching efficiency also focused on facilitating the penetration of the bleaching solution. There has been no research on the effects of nitrogen-containing soluble substances in eucalyptus, especially nitrogen-carbonyl conjugated compounds, and their morphological changes after participating in the pre-impregnation chemical reaction on the subsequent bleaching performance of the raw material. Precisely because of the lack of research on this issue, the current pre-mill pre-impregnation process design of chemical mechanical pulping processes is based on softening wood chips, reducing energy consumption, and enhancing penetration, and no hot water soaking denitrification process has ever been included. The related work of the present invention has for the first time proposed the effect of water-soluble nitrogen-carbonyl conjugated structures in eucalyptus raw materials on the subsequent bleaching performance of eucalyptus chemical mechanical pulp, and has originally proposed a method to improve the bleaching performance of eucalyptus chemical mechanical pulp, solving the problem that "substances containing carbon-nitrogen double bonds and carbonyl conjugated structures in the initial raw materials affect the bleaching performance of eucalyptus."

[0072] In order to further prove that if the nitrogen-carbonyl conjugated structure compound is not removed before adding chemicals, but is removed by washing after adding chemicals and completing the pre-impregnation reaction, the bleaching performance of the slurry will be negatively affected to varying degrees, comparative experiment 3 and example 3, comparative experiment 4 and example 4 were specially designed.

[0073] Comparative test 3 was conducted with Eucalyptus grandis wood chips (specific surface area 1.12×10 -3 m 2 After screening, washing, and steaming, the wood chips were processed according to the "Conventional Pulping Process - 1" according to the chemical dosage of Conventional Process 1 in Table 2, wherein after completing the pre-impregnation in step (2), the pulp was completely washed and then sent to a high-consistency disc refiner to continue the subsequent processes until the bleached chemimechanical pulp was prepared.

[0074] The high-temperature soaking treatment method of Example 3 is the same as that of Example 1. The obtained material is pulped according to the chemical dosage of Conventional Process 1 in Table 2 and the pulping process flow of Comparative Experiment 3. That is, after completing the pre-impregnation in step (2), the pulp is completely washed and then sent to a high-consistency disc mill to continue the subsequent process until the bleached chemimechanical pulp is prepared.

[0075] Comparative test 4 was conducted with Eucalyptus grandis wood chips (specific surface area 1.12×10 -3 m 2 / g) were screened, washed, and steamed, and pulping was completed according to the pulping process of comparative experiment 3 and the chemical dosage of conventional process 2 in Table 2.

[0076] The high-temperature soaking treatment method of Example 4 is the same as that of Example 1. The obtained material is pulped according to the chemical dosage of Conventional Process 2 in Table 2 and the pulping process flow of Comparative Experiment 3.

[0077] The results are shown in Figure 6 .

[0078] Depend on Figure 6 As can be seen, the maximum whiteness of Example 3 is 80.21% ISO, significantly higher than the 77.64% ISO of Comparative Experiment 3; the maximum whiteness of Example 4 is 81.05% ISO, also significantly higher than the 79.57% ISO of Comparative Experiment 4. This experiment shows that if the raw material extrusion crushing and high-temperature immersion nitrogen removal process of the present invention is not used, even if the slurry is thoroughly washed after pre-impregnation, the bleaching performance of the slurry will still be negatively affected to a certain extent (as mentioned above, this is because some nitrogen-carbonyl conjugated structures are already bound to the slurry and cannot be removed by washing). That is, in the contents of patents CN104846676A and CN103334328A for the eucalyptus chemical-mechanical pulp production process designed with traditional ideas, even if relevant practitioners can infer that the pulp should be washed after pre-impregnation, because they do not understand the changing law of the effect of the nitrogen-carbonyl conjugated structure compounds in the wood chip raw materials and their products after pre-impregnation reaction on the pulp bleaching performance, which was first discovered by our team, they cannot achieve the improvement effect on the pulp bleaching performance that can be achieved by the present invention. The present invention is something that relevant practitioners cannot reasonably expect and infer from existing knowledge and operations.

[0079] Effect of the degree of wood chip crushing before boiling and soaking on the improvement of subsequent bleaching performance.

[0080] In order to study the effect of the crushing degree of eucalyptus chips before boiling and soaking on the subsequent bleaching performance improvement, a group of Eucalyptus grandis chips with different specific surface areas were prepared by controlling the opening of the spiral blades of the twin-screw extruder. The extrusion effect and the corresponding specific surface area were shown in Figure 2. Figure 7 .

[0081] Example 5: Eucalyptus urophylla wood chips were soaked at 95°C for 60 minutes at a concentration of 10%, and then the concentration was increased to 40% using a twin-screw extruder. Bleached chemimechanical pulp was prepared using "Conventional Pulping Process Flow-1" using the chemical dosages for Conventional Process 2 in Table 2.

[0082] Example 6: Eucalyptus grandis wood chips were processed by a twin-screw extruder to have a specific surface area of ​​5.47×10 -2 m 2 The rod-shaped material (1000 g) was soaked at 95°C for 60 minutes at a concentration of 10%, and then the concentration was increased to 40% using a twin-screw extruder. The resulting material was used to prepare bleached chemical-mechanical pulp using "Conventional Pulping Process Flow-1" according to the chemical dosages specified in Conventional Process 2 in Table 2.

[0083] Example 7: Eucalyptus grandis wood chips were processed by a twin-screw extruder to have a specific surface area of ​​0.245 m 2 / g of rod-shaped material was soaked at 95°C for 60 minutes at a concentration of 10%, and then the material concentration was increased to 40% using a twin-screw extruder. The resulting material was used to prepare bleached chemical-mechanical pulp using "Conventional Pulping Process Flow-1" according to the chemical dosages of Conventional Process 2 in Table 2.

[0084] The results of the above three groups of tests and the comparative experiment 2 and embodiment 2 with the same amount of chemicals are shown in Figure 8 .

[0085] Examples 2, 5, 6 and 7 are comparisons of the bleaching performance of pulp after high temperature soaking and nitrogen removal treatment at different crushing degrees. Figure 7 and Figure 8 The results show that the maximum whiteness of the pulp is positively correlated with the specific surface area of ​​the crushed raw material. The more thorough the crushing of the raw material, the more complete the dissolution of the nitrogen-carbonyl conjugated structure, and the higher the whiteness of the final pulp.

[0086] Example 5 is to soak the logs in high temperature to remove nitrogen. Figure 8 It can be seen that the highest whiteness of the corresponding pulp is very close to that of comparative experiment 2. Therefore, if the macroscopic size of the wood chips is not destroyed, its internal structure is fully exposed, and the nitrogen-carbonyl conjugated structure is fully dissolved, the improvement effect of the bleaching performance of the pulp will be very limited. In the patents CN104846676A and CN103334328A for the production process of eucalyptus chemical mechanical pulp designed with traditional ideas, although there is also a washing process for the wood chips, the purpose of the washing process is to remove the mud and sand on the surface of the wood chips, which is generally hot water spraying, and there is very little soaking residence time. The conditions of Example 5 have increased the hot water temperature, increased the hot water dosage, and increased the residence time, which is equivalent to an enhanced washing and soaking of the wood chips, but the results show that as long as the wood chips are not destroyed, the complete dissolution of the nitrogen-carbonyl conjugated structure cannot be achieved, and the results that can be achieved by the present invention cannot be achieved. That is, the present invention is something that relevant practitioners cannot reasonably expect and infer from existing knowledge and operations.

[0087] Example 8 Impact of High-Temperature Immersion Nitrogen Removal Process on Enterprise Operating Costs

[0088] Comparative experiment 5 was Eucalyptus microphylla wood chips (specific surface area 1.07×10 -3 m 2 / g) According to the chemical dosage of conventional process 1 (total hydrogen peroxide dosage of 80 kg / t) in Table 2, bleached chemimechanical pulp was prepared using "conventional pulping process flow-1".

[0089] In the high temperature soaking treatment of Example 8, the Eucalyptus globulus chips were processed by a twin-screw extruder to a specific surface area of ​​1.65 m 2After the filamentous material (100g) was soaked at 100°C for 90 minutes at a concentration of 1%, the material concentration was then increased to 40% using a twin-screw extruder. The resulting material was used to prepare bleached chemimechanical pulp using "Conventional Pulping Process Flow-1" according to the chemical dosages shown in Table 5 (total hydrogen peroxide dosage: 40kg / t).

[0090] Table 5 Chemical dosage and distribution of Eucalyptus microphylla chemical mechanical pulp Example 8

[0091]

[0092]

[0093] Depend on Figure 9 As can be seen, after the high-temperature immersion denitrification process, the total hydrogen peroxide usage in the pulping process was reduced to 40 kg / t, and the maximum brightness of the resulting pulp was still better than that of the conventional chemical mechanical pulping process using 80 kg / t of hydrogen peroxide. Therefore, this invention has great application value for production enterprises.

[0094] Implementation Case 9

[0095] Comparative test 6 was conducted with Eucalyptus globulus wood chips (specific surface area 1.48×10 -3 m 2 / g) were screened, washed, and steamed, and bleached chemimechanical pulp was prepared according to the “Conventional Pulping Process-1” according to the chemical dosage of Conventional Process 1 in Table 2.

[0096] In the high temperature immersion denitrification treatment method of Example 9, the eucalyptus globulus wood chips are processed by a twin-screw extruder to a specific surface area of ​​0.326 m 2 After the rod-shaped material (g / g) was soaked at 80°C for 30 minutes at a concentration of 20%, the material concentration was increased to 36% using a twin-screw extruder. The resulting material was used to prepare bleached chemical-mechanical pulp using "Conventional Pulping Process 1" according to the chemical dosages of Conventional Process 1 in Table 2.

[0097] Depend on Figure 10 It can be seen that after high-temperature immersion and nitrogen removal treatment, the maximum whiteness of Eucalyptus globulus was significantly improved.

Claims

1. A method for improving the bleaching performance of eucalyptus chemical mechanical pulp, characterized in that: Before the pre-impregnation and dosing step, the first twin-screw extruder is used to crush and extrude the wood chips to increase the specific surface area of ​​the eucalyptus raw material to 0.10 m 2 / g or more, and then keep warm at 80-100℃ and system concentration of 1-20% for 30-120min. The system concentration is the mass ratio of absolute dry fiber raw material to the total system. After soaking, it is concentrated and extruded through a second twin-screw extruder to a material dryness greater than 35%. The resulting slurry is then added with pre-impregnation chemicals to carry out normal chemical mechanical pulp production process.

2. The method according to claim 1, characterized in that The chemical mechanical pulp production process after adding pre-impregnation chemicals includes pre-impregnation, high-consistency refining, high-consistency bleaching, low-consistency refining, screening, washing and concentration, and pulping.

3. The method according to claim 1, characterized in that The conditions for high-concentration refining are: refining concentration 25-40%, grinding chamber pressure 0.1-0.18MPa, temperature 95-105℃; the conditions for high-concentration bleaching are: temperature 85-95℃, bleaching system concentration 15-25%, time 60-150min; the conditions for low-concentration refining are: refining temperature 70-90℃ under normal pressure, concentration 5-10%.

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