A production process for high-concentration hydrogen peroxide bleaching of hardwood semi-chemical pulp

Abstract

This invention relates to the field of pulp and paper technology, and in particular to a high-yield, ultra-high-concentration hydrogen peroxide bleaching process for hardwood semi-chemical pulp. The process includes: pre-treating hardwood chips by separating the fibers to increase their specific surface area; continuous cooking under low liquor ratio conditions; directly introducing the pulp from the cooking tail section into the bleaching system and utilizing the residual heat of cooking for pre-bleaching; conducting a hydrogen peroxide bleaching reaction in a closed ultra-high-consistency bleaching tower, with a hydrogen peroxide mass concentration of 30%–35%, and maintaining a high-temperature, high-consistency reaction environment through a secondary non-contact heating device; adjusting the pH value and reaction time through a control system; and finally, performing multi-stage countercurrent washing and water recycling. This invention achieves energy coupling between the cooking and bleaching processes, improves hydrogen peroxide utilization efficiency, ensures a pulp brightness of over 80 while achieving a pulp yield of over 75%, and significantly reduces water and steam consumption, offering advantages of high efficiency, energy saving, and environmental friendliness.

Description

Technical Field

[0001] This invention relates to the field of pulp and paper technology, and in particular to a production process for high-concentration hydrogen peroxide bleached hardwood semi-chemical pulp. Background Technology

[0002] Hardwood semi-chemical pulp is an important raw material for cultural and household paper products, and its production process has a significant impact on resource consumption, product performance, and the environment. However, existing technologies still have the following prominent problems: It is difficult to balance yield and strength. Hardwood fibers are relatively short and have a high content of impurities, making them prone to excessive degradation during cooking. This results in a pulp yield that is usually below 75%, and the fiber strength also decreases, making it difficult to meet the demands of high-end paper.

[0003] Low bleaching efficiency and high chemical consumption Traditional bleaching processes often use hydrogen peroxide at a concentration of 10% to 25% and react at a low temperature of around 70°C. This results in low oxidation efficiency, and the amount of reagent added needs to be increased to achieve the target whiteness, leading to high bleaching costs.

[0004] Process fragmentation leads to increased energy consumption The cooking and bleaching processes are independent of each other. The residual heat from cooking is not effectively utilized, and the bleaching section needs to be heated separately, resulting in a significant increase in steam consumption and low overall energy efficiency.

[0005] High water and steam consumption Traditional processes involve a large volume of liquid (usually 1:4 or higher), consuming 4 to 5 tons of water and about 1.5 to 2 tons of steam per ton of oven-dried wood chips, resulting in low resource utilization efficiency.

[0006] High-concentration hydrogen peroxide has limited applications. When the hydrogen peroxide concentration exceeds 25%, traditional conveying and reaction equipment is prone to problems such as unstable pumping and uncontrolled decomposition, which limits the application of high-concentration bleaching technology.

[0007] Great pressure on environmental protection Some processes still use chlorine-containing bleaching or inefficient hydrogen peroxide bleaching, resulting in high residual reagent concentrations in wastewater and high treatment costs.

[0008] Therefore, there is an urgent need for a synergistically optimized production process that can achieve high-concentration bleaching, high yield, low energy consumption, and low pollution. Summary of the Invention

[0009] This invention aims to solve the following technical problems: how to improve the yield of hardwood semi-chemical pulp while maintaining strength; how to improve the efficiency of hydrogen peroxide bleaching and reduce chemical consumption; how to achieve energy synergy between cooking and bleaching processes; how to achieve stable delivery and efficient reaction of high-concentration hydrogen peroxide; and how to reduce water consumption, steam consumption, and environmental burden.

[0010] The technical solution of the present invention includes the following: A production process for high-concentration hydrogen peroxide bleaching of hardwood semi-chemical pulp includes the following steps: S1, pre-treat the broadleaf wood chips by splitting them into fiber bundles; S2, the pretreated wood chips are cooked in an inclined continuous steaming device with a low liquid ratio, and the cooking liquid ratio is controlled at 1:3 to 1:3.5; S3, the pulp from the tail end of cooking is directly introduced into the bleaching system to pre-bleach using the residual heat of cooking; S4, adding hydrogen peroxide to a closed ultra-high concentration bleaching tower to carry out a bleaching reaction, wherein the mass concentration of hydrogen peroxide is 30% to 35% and the bleaching temperature is 95 to 100°C. S5, the bleaching system is indirectly heated by a secondary non-contact heating device to maintain high concentration and high temperature reaction conditions; S6, through the control system, adjusts the pH value and reaction time of the bleaching process to make the pulp whiteness reach above 80; S7, the bleached pulp is subjected to countercurrent washing and water recycling treatment; S8 produces high-yield, high-brightness hardwood semi-chemical pulp.

[0011] As a further preferred embodiment of the present invention, the fiber pretreatment involves using a mechanical fiber-splitting device to break down wood chips into fine fibers, thereby increasing the specific surface area by more than 20%.

[0012] As a further preferred embodiment of the present invention, a special cooking catalyst for hardwood is added during the cooking process, which reduces the amount of cooking agent used by 20% to 25% and achieves a crude pulp yield of 78% to 82%.

[0013] As a further preferred embodiment of the present invention, the cooking temperature is 150-170°C, and the cooking liquor ratio is controlled by intermittent water supply.

[0014] As a further preferred embodiment of the present invention, a bleaching activator is added during the bleaching process to improve the oxidation efficiency of hydrogen peroxide and reduce the amount of hydrogen peroxide used by 25% to 30%.

[0015] As a further preferred embodiment of the present invention, the bleaching process is a two-stage hydrogen peroxide bleaching process, with a hydrogen peroxide consumption of 30-35 kg per ton of pulp.

[0016] As a further preferred embodiment of the present invention, the pH value of the bleaching process is controlled within the range of 10.0 to 11.5.

[0017] As a further preferred embodiment of the present invention, the secondary non-contact heating device is an indirect heat exchange structure, which prevents the heating medium from directly contacting the slurry, thereby maintaining a stable slurry concentration.

[0018] As a further preferred embodiment of the present invention, the countercurrent washing process employs a multi-stage washing system, ensuring that the water consumption per ton of pulp does not exceed 8 m³, and that some of the washing water is reused in the cooking process. Beneficial effects

[0019] This invention provides a production process, preparation method, and application of high-concentration hydrogen peroxide bleached hardwood semi-chemical pulp. It has the following beneficial effects: Significantly improves slurry yield while maintaining strength By pre-treating the fibers and cooking them at a low liquid ratio, excessive fiber degradation is reduced, resulting in a pulp yield of 78%–82% and a final pulp yield that is stable at over 75%, while maintaining high fiber integrity and strength properties.

[0020] Significantly improve hydrogen peroxide utilization efficiency Using a 30%–35% ultra-high concentration hydrogen peroxide system, combined with bleaching activators and pH control, the effective oxygen utilization rate is increased by more than 20%, achieving a whiteness of ≥80% while reducing chemical consumption.

[0021] Achieving energy coupling between cooking and bleaching reduces energy consumption. By directly introducing the cooking tail slurry into the bleaching system and utilizing the residual heat for pre-bleaching, the need for secondary heating is reduced, and steam consumption is reduced by approximately 25% to 30%.

[0022] Solving the problems of high-concentration hydrogen peroxide transport and reaction instability By using a closed ultra-high concentration drift tower and a secondary non-contact heating device, the decomposition runaway problem caused by traditional direct heating is avoided, and the high concentration system can be operated stably.

[0023] Significantly reduce water consumption and environmental burden The system employs a low liquid ratio cooking and multi-stage countercurrent washing system, which reduces the water consumption per ton of pulp to below 8m³, while also achieving water recycling and reducing wastewater discharge.

[0024] Improve system continuity and industrial adaptability By combining continuous steaming and continuous bleaching processes, production continuity is improved, making it suitable for large-scale industrial production. Detailed Implementation

[0025] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0026] This invention provides a production process for high-concentration hydrogen peroxide bleaching of hardwood semi-chemical pulp, comprising the following steps: S1, pre-treat the broadleaf wood chips by splitting them into fiber bundles; S2, the pretreated wood chips are cooked in an inclined continuous steaming device with a low liquid ratio, and the cooking liquid ratio is controlled at 1:3 to 1:3.5; S3, the pulp from the tail end of cooking is directly introduced into the bleaching system to pre-bleach using the residual heat of cooking; S4, hydrogen peroxide is added in a closed ultra-high concentration bleaching tower to carry out a bleaching reaction. The mass concentration of hydrogen peroxide is 30% to 35%, and the bleaching temperature is 95 to 100℃. S5, the bleaching system is indirectly heated by a secondary non-contact heating device to maintain high concentration and high temperature reaction conditions; S6, through the control system, adjusts the pH value and reaction time of the bleaching process to make the pulp whiteness reach above 80; S7, the bleached pulp is subjected to countercurrent washing and water recycling treatment; S8 produces high-yield, high-brightness hardwood semi-chemical pulp. Through a continuous integrated design of "fiber separation pretreatment—low-liquid-ratio continuous distillation—residual heat pre-bleaching—ultra-high-concentration hydrogen peroxide bleaching—countercurrent washing," the traditionally decentralized cooking and bleaching processes are coupled energy and synergistically. On one hand, the cooking tail pulp directly enters the bleaching section and utilizes residual heat, maintaining the initial system temperature at a high level and reducing external heat input. On the other hand, the high-concentration system significantly increases the effective contact probability of reactants, thereby improving bleaching efficiency while reducing energy consumption. Therefore, this claim comprehensively ensures a balance between high yield, high brightness, and low energy consumption.

[0027] The pre-treatment of wood chips into fine fibers using a mechanical defiberization device increases the specific surface area by more than 20%. This mechanical defiberization transforms the wood chips from a dense structure to a fibrous bundle structure, further increasing the specific surface area by over 20%. This change significantly improves the penetration properties of the cooking liquor and bleaching agents, making the chemical reaction more uniform and controllable, reducing localized over-reactions, and effectively inhibiting fiber degradation. This is evident in Example 1 and the comparative example, where the tensile index is significantly improved, indicating that this characteristic plays a crucial role in maintaining fiber strength.

[0028] Adding a special hardwood cooking catalyst during the cooking process reduces the amount of cooking agents used by 20%–25%, while achieving a crude pulp yield of 78%–82%. Its mechanism of action involves accelerating the selective dissolution of lignin while reducing non-selective damage to cellulose and hemicellulose. Through catalysis, the cooking reaction achieves the same or even better delignification effect with lower agent dosages, thus stabilizing the crude pulp yield at 78%–82%. Compared to processes without a catalyst, this not only reduces chemical costs but also decreases wastewater load, resulting in significant economic and environmental benefits.

[0029] The cooking temperature is 150–170℃, and the cooking liquor ratio is controlled by intermittent water supply. This differs from traditional high-liquid-ratio processes. Under low-liquid-ratio conditions, the concentration of effective components in the system is increased, making the cooking reaction more efficient, while reducing the heating load on water and thus reducing steam consumption. In addition, the intermittent water supply method avoids local dry-cooking or concentration fluctuations, improving the system's operational stability.

[0030] Adding a bleaching activator during the bleaching process improves the oxidation efficiency of hydrogen peroxide, reducing the amount of hydrogen peroxide used by 25%–30%. Its main function is to promote the decomposition of hydrogen peroxide to generate reactive oxygen free radicals, thereby increasing the oxidation reaction rate. This feature is particularly important in ultra-high concentration systems because it can achieve higher bleaching effects with lower dosages, reducing hydrogen peroxide usage by 25%–30%. Comparative results from examples show that this feature plays a decisive role in improving reagent utilization.

[0031] The bleaching process is a two-stage hydrogen peroxide bleaching process, consuming 30-35 kg of hydrogen peroxide per ton of pulp. By controlling the reaction conditions in stages, the first stage mainly completes the initial oxidation of lignin, while the second stage further improves brightness and stabilizes pulp properties. Compared with single-stage bleaching, this method avoids fiber damage caused by a single high-intensity reaction, thus maintaining a high yield while improving brightness.

[0032] The pH value during the bleaching process is controlled within the range of 10.0 to 11.5. This range represents the balance between the stability and reactivity of hydrogen peroxide. When the pH is too low, the decomposition rate of hydrogen peroxide decreases, resulting in insufficient bleaching efficiency; when the pH is too high, non-selective decomposition is likely to occur, leading to waste of reagents and damage to fibers.

[0033] The secondary non-contact heating device employs an indirect heat exchange structure, ensuring that the heating medium does not directly contact the slurry, thus maintaining a stable slurry concentration. It provides heat through indirect heat exchange, avoiding the localized overheating and concentration dilution problems associated with traditional direct steam injection. This feature is particularly crucial in ultra-high concentration systems, as it not only guarantees a uniform and stable reaction temperature but also prevents the violent decomposition of hydrogen peroxide due to localized high temperatures, thereby improving system safety and reagent utilization efficiency.

[0034] The countercurrent washing process employs a multi-stage washing system, ensuring that the water consumption per ton of pulp does not exceed 8 m³, and that some of the washing water is reused in the cooking process. By achieving counter-current contact between the water and pulp flows, the washing water is utilized in stages, significantly reducing the amount of clean water used. Compared to traditional single-stage washing, this method can control the water consumption per ton of pulp below 8 m³, while simultaneously recovering some heat and chemicals, achieving resource recycling, and reducing the burden of wastewater treatment. Example 1

[0035] Eucalyptus wood chips are selected, with an oven-dry weight of 1000 kg.

[0036] Fiber pretreatment The wood chips are processed using a mechanical defiberization device, which forms a fiber bundle structure, increasing the specific surface area by about 25%.

[0037] Low liquid ratio cooking Cooking was carried out in an inclined continuous steamer: liquor ratio 1:3.2; temperature 160℃; cooking catalyst added (1.5% of wood chips by mass); cooking time 90 min. The pulp yield was 80%.

[0038] Waste heat pre-bleaching The cooking tail pulp (temperature approximately 95°C) is directly fed into the bleaching system for pre-bleaching treatment.

[0039] Ultra-high concentration bleaching: The pulp concentration is adjusted to 32%, and the following are added: H2O2 (30% mass concentration system, dosage 32 kg / t pulp); NaOH is added to adjust the pH to 10.8; 0.5% bleaching activator is added. The reaction is carried out in a closed bleaching tower at a temperature of 98℃ for 120 minutes.

[0040] Non-contact heating is used, and indirect heat exchange is employed to maintain a stable temperature.

[0041] The washing process employs a three-stage countercurrent washing method, with a water consumption of 7.5 m³ / t of pulp.

[0042] result: Whiteness 82% ISO; Overall success rate: 76%; Tensile index: 44 N·m / g.

[0043] Example 2 (Low-concentration bleaching comparison) It is basically the same as Example 1, except that: The bleaching concentration was 15%; no residual heat pre-bleaching step was performed. Results: Whiteness 74% ISO; yield 72%; tensile index 39 N·m / g; steam consumption increased by approximately 28%.

[0044] Comparison conclusion: This demonstrates that the ultra-high concentration system combined with waste heat utilization significantly improves bleaching efficiency and reduces energy consumption. Example 3

[0045] The results were essentially the same as in Example 1, except that direct steam injection heating was used and precise pH control (pH approximately 9.5) was not employed. Results: Whiteness 78% ISO; yield 73%; tensile index 37 N·m / g; hydrogen peroxide decomposition rate significantly increased. Conclusion: This demonstrates that non-contact heating combined with pH control plays a crucial role in stabilizing high-concentration bleaching systems.

[0046] The specific parameter table is as follows:

[0047] .

[0048] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or basic characteristics. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

[0049] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A production process for high-concentration hydrogen peroxide bleaching of hardwood semi-chemical pulp, characterized in that: Includes the following steps: S1, pre-treat the broadleaf wood chips by splitting them into fiber bundles; S2, the pretreated wood chips are cooked in an inclined continuous steaming device with a low liquid ratio, and the cooking liquid ratio is controlled at 1:3 to 1:3.5; S3, the pulp from the tail end of cooking is directly introduced into the bleaching system to pre-bleach using the residual heat of cooking; S4, adding hydrogen peroxide to a closed ultra-high concentration bleaching tower to carry out a bleaching reaction, wherein the mass concentration of hydrogen peroxide is 30% to 35% and the bleaching temperature is 95 to 100°C. S5, the bleaching system is indirectly heated by a secondary non-contact heating device to maintain high concentration and high temperature reaction conditions; S6, through the control system, adjusts the pH value and reaction time of the bleaching process to make the pulp whiteness reach above 80; S7, the bleached pulp is subjected to countercurrent washing and water recycling treatment; S8 produces high-yield, high-brightness hardwood semi-chemical pulp.

2. The production process of high-concentration hydrogen peroxide bleached hardwood semi-chemical pulp according to claim 1, characterized in that, The fiber pretreatment uses a mechanical fiber-splitting device to break down wood chips into fine fibers, increasing the specific surface area by more than 20%.

3. The production process of high-concentration hydrogen peroxide bleached hardwood semi-chemical pulp according to claim 1, characterized in that, The addition of a special cooking catalyst for hardwoods during the cooking process reduces the amount of cooking agent used by 20% to 25% and achieves a crude pulp yield of 78% to 82%.

4. The production process of high-concentration hydrogen peroxide bleached hardwood semi-chemical pulp according to claim 1, characterized in that, The cooking temperature is 150-170℃, and the cooking liquor ratio is controlled by intermittent water supply.

5. The production process of high-concentration hydrogen peroxide bleached hardwood semi-chemical pulp according to claim 1, characterized in that, A bleaching activator is added during the bleaching process to improve the oxidation efficiency of hydrogen peroxide, thereby reducing the amount of hydrogen peroxide used by 25% to 30%.

6. The production process of high-concentration hydrogen peroxide bleached hardwood semi-chemical pulp according to claim 1, characterized in that, The bleaching process is a two-stage hydrogen peroxide bleaching process, with a hydrogen peroxide consumption of 30-35 kg per ton of pulp.

7. The production process of high-concentration hydrogen peroxide bleached hardwood semi-chemical pulp according to claim 1, characterized in that, The pH value of the bleaching process is controlled within the range of 10.0 to 11.

5.

8. The production process of high-concentration hydrogen peroxide bleached hardwood semi-chemical pulp according to claim 1, characterized in that, The secondary non-contact heating device is an indirect heat exchange structure, which prevents the heating medium from directly contacting the slurry, thereby maintaining a stable slurry concentration.

9. The production process of high-concentration hydrogen peroxide bleached hardwood semi-chemical pulp according to claim 1, characterized in that, The countercurrent washing process employs a multi-stage washing system, ensuring that the water consumption per ton of pulp does not exceed 8m³, and that some of the washing water is reused in the cooking process.

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