A flexible pulping method for dissolving pulp

Abstract

This invention discloses a flexible pulping method for dissolving pulp, comprising sulfate pulping and bleaching processes, and one or more stages of combined processing including pre- and post-bleaching refining, alkali treatment, acid treatment, and enzyme treatment. The dissolving pulp preparation process achieves improved yield by controlling the cooking and bleaching intensities, as well as the pre- and post-bleaching process conditions. This invention allows for the coordinated control of pulp properties based on the downstream product quality requirements, and the arrangement of process stages to achieve a flexible dissolving pulping method.

Description

Technical Field

[0001] This invention relates to the field of dissolving pulp, and more specifically to a flexible pulping method for dissolving pulp. Background Technology

[0002] Dissolving pulp is currently the main raw material for obtaining high-purity cellulose, second only to cotton. Cellulose derivatives / functional materials produced from dissolving pulp have been widely used in industries such as textiles, food, daily chemicals, papermaking, and pharmaceuticals, and are green alternatives to many petroleum-based industrial raw materials.

[0003] The key characteristic of dissolving pulp is its high cellulose purity (90-99%, usually expressed as α-cellulose or methyl cellulose content), containing only a small amount of hemicellulose (2-4%) and trace amounts of lignin, extractives, and minerals. The required cellulose purity varies depending on the downstream products. For example, dissolving pulp used in viscose, cellophane, and lyocell production typically contains 90-96% α-cellulose, while products like cellulose acetate and nitrocellulose require higher cellulose purity, exceeding 96%. Another important quality parameter of dissolving pulp is its "reactivity," which reflects its reactivity with downstream product systems and depends on the solvent system of those downstream products. For example, in dissolving pulp for viscose, reactivity typically refers to its ability to react with the main fiber solvent, carbon disulfide, during viscose production; specifically, it refers to the ability of the three hydroxyl groups on each cellulose glucose unit to undergo a xanthation reaction with carbon disulfide to form cellulose xanthate. Reactivity is closely related to parameters that may affect cellulose accessibility / accessibility, such as fiber morphology, pore structure, and degree of cellulose polymerization. Good dissolving pulp reactivity typically translates to higher end-product quality and lower carbon disulfide consumption, resulting in economic and environmental benefits. For example, in dissolving pulp for lyocell fibers, reactivity refers to the pulp's impregnation performance in an NMMO solution system and its subsequent dissolving capacity. Strict requirements are placed on the content of metal ions such as iron and copper in the pulp to prevent the decomposition of the NMMO solvent by these metal ions, which could lead to an explosion in the system. Furthermore, the degree of polymerization (DOP) requirements vary depending on the intended use of the dissolving pulp. For viscose and lyocell grade dissolving pulp, the DOP is controlled within the range of 400–700, while for acetate-grade wood pulp, a DOP of around 1000 is required. Therefore, a single dissolving pulp preparation process cannot meet the diverse requirements of downstream products.

[0004] Currently, the mainstream production methods for dissolving pulp from wood / non-wood fiber raw materials include the sulfite process and the pre-hydrolyzed sulfate process. Due to limitations in equipment and wastewater treatment, domestic dissolving pulp production mainly uses the pre-hydrolyzed sulfate pulping method. This involves adding a pre-hydrolysis stage before the traditional sulfate pulping stage to specifically remove hemicellulose. However, the resulting pulp yield is relatively low, generally only 30-35% (relative to the fiber raw material), which is 12-15% lower than ordinary pulp. Therefore, the raw material cost per ton of pulp is about 33% higher than that of ordinary pulp, significantly reducing the profit margin of dissolving pulp. Using chemical pulping processes to prepare dissolving pulp requires further purification processes, such as cold alkali treatment, enzyme treatment, and acid treatment. However, similarly, a single treatment process cannot meet the quality requirements of different dissolving pulp products.

[0005] Therefore, providing a flexible pulping process that improves yield and allows for the combination and switching of process stages to meet the downstream product demands of dissolving pulp is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0006] In view of this, the present invention provides a flexible pulping method for dissolving pulp, comprising sulfate pulping and bleaching processes, and one or more stages of combined processing including refining, alkali treatment, acid treatment, and enzyme treatment before / after bleaching. This dissolving pulp preparation process achieves improved yield by controlling the cooking and bleaching intensities in a coordinated manner, and by integrating and optimizing the processing conditions of one / more stages before / after bleaching. The present invention can control pulp properties in a coordinated manner before and after processing according to the downstream product quality requirements, and arrange the combination and switching of process stages to achieve a flexible pulping method for dissolving pulp.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] a) Obtain the wood chips to be treated, and cook the wood chips to obtain coarse pulp; b) Perform oxygen delignification treatment on the coarse pulp to obtain oxygen delignified pulp; c) Perform milling and alkali treatment on the oxygen delignified pulp to obtain pretreated pulp; d) Bleach the pretreated pulp to obtain bleached pulp; e) Perform acid strengthening treatment on the bleached pulp to obtain posttreated pulp; f) Perform enzyme treatment on the posttreated pulp to obtain dissolving pulp.

[0009] As a preferred embodiment of the above technical solution, the cooking solution is a mixed solution of sodium hydroxide and sodium sulfide, with a sulfidation degree of 20-30%, an alkali dosage of 15-25% of the dry weight of the material, and a cooking H factor of 600-2400, wherein the cooking H factor for coniferous wood is 1000-2000; the cooking H factor for broadleaf wood is 700-1400; and the cooking H factor for non-wood materials such as bamboo is 600-1400; and the kappa value of the pulp after cooking is 30-40.

[0010] As a preferred embodiment of the above technical solution, the oxygen delignification treatment includes a first-stage oxygen delignification and a second-stage oxygen delignification. The oxygen delignification conditions are controlled according to the kappa value of the pulp after cooking: wherein, in the first-stage oxygen delignification, the amount of alkali used is 2-5% of the oven-dry pulp weight, the oxygen pressure is controlled at 0.6-0.8 MPa, and the reaction temperature is controlled at 80-90℃; in the second-stage oxygen delignification, the amount of alkali used is controlled at 1-4% of the oven-dry pulp weight, the oxygen pressure is controlled at 0.4-0.6 MPa, and the temperature is controlled at 90-100℃.

[0011] As a preferred embodiment of the above technical solution, the pretreatment of the bleaching stage includes refining and alkali treatment: specifically, refining to control the coarse pulp beating degree to 25-50°SR and the coarse pulp consistency to 20-30%; the alkali treatment conditions after refining are: the amount of sodium hydroxide is 1-8% of the weight of the oven-dry pulp, and the reaction is carried out for 60-180 minutes without additional heating.

[0012] As a preferred embodiment of the above technical solution, the bleaching treatment sequentially includes a primary bleaching stage with chlorine dioxide (D0), a primary bleaching stage with alkaline hydrogen peroxide (E1), a secondary bleaching stage with chlorine dioxide (D1), a secondary bleaching stage with alkaline hydrogen peroxide (E2), and a tertiary bleaching stage with chlorine dioxide (D2). The process conditions for stage D0 are: pulp concentration 10–20 wt%, time 120–180 min, temperature 70–95 °C, chlorine dioxide dosage 1.0–2.5% relative to oven-dry pulp mass, and residual chlorine after bleaching 0–0.1 g / L. The process conditions for stage D1 are: pulp concentration 10–20 wt%, time 120–180 min, temperature 70–95 °C, chlorine dioxide dosage 0.5–1.5% relative to oven-dry pulp mass, and residual chlorine after bleaching 0–0.1 g / L. The process conditions for the D2 stage are: pulp concentration 10–20 wt%, time 120–180 min, temperature 70–95℃, chlorine dioxide dosage 0–1.0% relative to oven-dry pulp mass, and residual chlorine after bleaching 0–0.1 g / L; the process conditions for the E1 stage are: pulp concentration 10–20 wt%, time 60–120 min, temperature 70–95℃, sodium hydroxide dosage 1.0–2.0% relative to oven-dry pulp mass, and hydrogen peroxide dosage 1.0–2.0%; the process conditions for the E2 stage are: pulp concentration 10–20 wt%, time 60–120 min, temperature 70–95℃, sodium hydroxide dosage 0–1.5% relative to oven-dry pulp mass, and hydrogen peroxide dosage 0–1.0%.

[0013] As a preferred embodiment of the above technical solution, the post-bleaching treatment sequentially includes an acid strengthening stage and an enzyme treatment stage: wherein the acid strengthening stage extends the reaction time by 30-60 minutes in the final stage of bleaching, and the amount of sulfuric acid added is 0-0.5%; wherein the enzyme treatment stage involves adding one or more hemicellulases such as cellulase and / or xylooligosaccharase, mannosaccharase, etc., after the slurry is washed to pH 4-7 after the acid strengthening stage, without additional heating, with an enzyme dosage of 0-10000 ppm and a reaction time of 30-180 minutes.

[0014] Furthermore, the flexible pulping process for dissolving pulp can be composed of a combination of the above-mentioned processes, wherein the specific schemes for the combined flexible process include:

[0015] (1) The production process of viscose grade dissolving pulp is pulping + oxygen descaling + bleaching + enzyme treatment, specifically the above process steps a+b+d+f.

[0016] (2) The production process of Lyocell-grade dissolving pulp is pulping + oxygen descaling + milling + bleaching + acid treatment + enzyme treatment, specifically the above process steps a+b+c+d+e+f.

[0017] (3) The production process of cellulose acetate dissolving pulp is pulping + oxygen descaling + grinding + bleaching + acid treatment, specifically the above process steps a+b+c+d+e.

[0018] The beneficial effects of this invention include:

[0019] (1) The dissolving pulp prepared by this invention has a higher yield than conventional dissolving pulp. This is mainly achieved by reducing the pre-hydrolysis stage in the traditional dissolving pulp production process, thereby reducing the degradation of carbohydrates in the raw materials by pre-hydrolysis and increasing the pulp yield. At the same time, a light cooking and heavy bleaching pulping and bleaching process is adopted, and the cooking and bleaching stages are controlled in a coordinated manner. The cooking intensity is low, resulting in a coarse pulp with a high kappa value. The oxygen de-oxidation intensity is then increased to further remove lignin while retaining cellulose and hemicellulose, thus ensuring the overall pulping yield. The pulping yield is increased by 1 to 10% compared with conventional dissolving pulp after the combination of the two methods.

[0020] (2) This invention achieves the purification of pulp through pre- and post-bleaching treatments, thereby meeting the demand for diverse downstream products of dissolving pulp: the pre-bleaching treatment purifies cellulose through pulping and alkali treatment, using a lower amount of alkali than in traditional alkali treatment, thus avoiding the conversion of cellulose I to II and improving the reactivity of the pulp; the post-bleaching treatment is to carry out targeted purification based on the needs of downstream products of dissolving pulp, effectively removing impurities such as ash, metal ions, and residual hemicellulose from the pulp; the dissolving pulp obtained by this invention is superior to traditional dissolving pulp in terms of α-cellulose content, reactivity, whiteness, metal ion content, and resin content.

[0021] (3) The pre- and post-processing process of the present invention is integrated with the cooking and bleaching sections. It is based on the original cooking and bleaching sections with minor adjustments. Compared with general pulp refining, it reduces the number of processes and lowers the production cost.

[0022] (4) The pulping process of the present invention can arrange the combination and switching of production processes according to the raw material conditions and product quality grade requirements to realize a flexible pulping process and meet the needs of different dissolving pulps. Detailed Implementation

[0023] The technical solutions in the embodiments of the present invention will be clearly and completely described below. 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 skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] Example 1

[0025] Using moso bamboo as raw material, a) the pulp is cooked at a sulfurization degree of 28%, with an alkali dosage of 18% of the oven-dry weight, and a cooking H factor of 900 to obtain coarse pulp with a kappa value of 44; b) the coarse pulp undergoes a two-stage oxygen delignification treatment. In the first stage, the alkali dosage is 3.0% of the oven-dry pulp weight, the oxygen pressure is controlled at 0.7 MPa, and the reaction temperature is controlled at 85°C; in the second stage, the alkali dosage is controlled at 2.5% of the oven-dry pulp weight, the oxygen pressure is controlled at 0.5 MPa, and the temperature is controlled at 90°C to obtain oxygen-delignified pulp; d) the pretreated pulp is bleached using the D0 stage, where the D0 stage process conditions are a pulp concentration of 10... The process conditions for the D1 stage are: pulp concentration 10 wt%, time 90 min, temperature 75℃, chlorine dioxide dosage 2.0% relative to oven-dry pulp mass, and residual chlorine after bleaching 0.041 g / L; the process conditions for the E1 stage are: pulp concentration 10 wt%, time 90 min, temperature 80℃, chlorine dioxide dosage 1.5% relative to oven-dry pulp mass, and residual chlorine after bleaching 0.032 g / L; the process conditions for the E1 stage are: pulp concentration 10 wt%, time 60 min, temperature 80℃, sodium hydroxide dosage 2.0% relative to oven-dry pulp mass, and hydrogen peroxide dosage 2.0%; bleached pulp is obtained; f) the post-treatment pulp is enzymatically treated. After washing the bleached pulp to pH 4-7, a complex enzyme of cellulase and hemicellulase is added without additional heating. The dosage of cellulase is 100 ppm, the dosage of hemicellulase is 500 ppm, and the reaction time is 90 min, to obtain viscose-grade dissolving pulp.

[0026] Example 2

[0027] Using eucalyptus wood chips as raw material, a) the pulp is cooked at a sulfurization degree of 25%, with an alkali dosage of 15% of the oven-dry weight, and a cooking H factor of 1100 to obtain coarse pulp with a kappa value of 40; b) the coarse pulp undergoes a two-stage oxygen delignification treatment. In the first stage of oxygen delignification, the alkali dosage is 2.5% of the oven-dry pulp weight, the oxygen pressure is controlled at 0.6 MPa, and the reaction temperature is controlled at 90°C; in the second stage of oxygen delignification, the alkali dosage is controlled at 1.5% of the oven-dry pulp weight, and the reaction temperature is controlled at 90°C. The oxygen pressure was 0.4 MPa, and the temperature was controlled at 95℃ to obtain oxygen-delignified pulp; c) The oxygen-delignified pulp was subjected to refining and alkali treatment, specifically including refining to control the coarse pulp freeness at 25°SR and the coarse pulp consistency at 20%; the alkali treatment conditions after refining were: sodium hydroxide dosage was 4% of the oven-dry pulp weight, and the reaction was carried out for 60 minutes without additional heating to obtain pretreated pulp; d) The pretreated pulp was subjected to D0E1D1 bleaching, in which... The process conditions for stage D0 are: pulp concentration 10 wt%, time 120 min, temperature 75℃, chlorine dioxide dosage 1.5% relative to oven-dry pulp mass, and residual chlorine after bleaching 0.035 g / L; the process conditions for stage D1 are: pulp concentration 10 wt%, time 120 min, temperature 85℃, chlorine dioxide dosage 0.5% relative to oven-dry pulp mass, and residual chlorine after bleaching 0.012 g / L; the process conditions for stage E1 are: pulp concentration 10 wt%, time 60 min, temperature 80℃, sodium hydroxide dosage 1.0% relative to oven-dry pulp mass, and hydrogen peroxide dosage 1.0%; bleached pulp is obtained; e) the bleached pulp is acid-strengthened to obtain post-treated pulp, wherein the acid-strengthening stage involves extending the reaction time of stage D1 by 60 min and adding 0.5% sulfuric acid; post-treated pulp is obtained; f) the post-treated pulp is enzymatically treated, and the pulp is washed to pH after the acid-strengthening stage. After 4 to 7 minutes, xylanase was added without additional heating at a dosage of 500 ppm for a reaction time of 120 minutes to obtain Lyocell-grade dissolving slurry.

[0028] Example 3

[0029] Using Southern pine as raw material, a) the pulp was cooked at a sulfur degree of 28%, with an alkali dosage of 20% of the oven-dry weight, and a cooking H factor of 1300, yielding coarse pulp with a kappa value of 43; b) the coarse pulp underwent a two-stage oxygen delignification treatment. In the first stage of oxygen delignification, the alkali dosage was 3.0% of the oven-dry pulp weight, the oxygen pressure was controlled at 0.8 MPa, and the reaction temperature was controlled at 85°C; in the second stage of oxygen delignification, the alkali dosage was controlled at 2.5% of the oven-dry pulp weight, the oxygen pressure was controlled at 0.6 MPa, and the temperature was controlled at 90°C, yielding oxygen-delignified pulp. c) The oxygen-delignified pulp is subjected to refining and alkali treatment, specifically including refining to control the coarse pulp freeness at 30°SR and the coarse pulp concentration at 20%; the alkali treatment conditions after refining are: sodium hydroxide dosage is 6% of the oven-dry pulp weight, and the reaction is carried out for 60 minutes without additional heating to obtain pretreated pulp; d) The pretreated pulp is subjected to D0E1D1E2D2 bleaching, wherein the process conditions for the D0 stage are: pulp concentration 10wt%, time 120min, temperature 75℃, and chlorine dioxide dosage 1.5% relative to the oven-dry pulp weight. The residual chlorine after bleaching is 0.077 g / L; the process conditions for stage D1 are: pulp concentration 10 wt%, time 120 min, temperature 85℃, chlorine dioxide dosage 0.5% relative to oven-dry pulp mass, and the residual chlorine after bleaching is 0.048 g / L; the process conditions for stage D2 are: pulp concentration 10 wt%, time 120 min, temperature 85℃, chlorine dioxide dosage 0.5% relative to oven-dry pulp mass, and the residual chlorine after bleaching is 0.019 g / L; the process conditions for stage E1 are: pulp concentration 10 wt%, time 60 min... min, temperature 80℃, sodium hydroxide dosage 1.0% and hydrogen peroxide dosage 1.0% relative to oven-dry pulp mass; wherein the process conditions of the E2 section are pulp concentration 10wt%, time 60min, temperature 80℃, sodium hydroxide dosage 1.0% and hydrogen peroxide dosage 1.0% relative to oven-dry pulp mass; bleached pulp is obtained; e) acid strengthening treatment is performed on the bleached pulp to obtain post-treated pulp, wherein the acid strengthening section is to extend the reaction time of the bleaching D2 section by 60min and add sulfuric acid dosage of 0.1%; cellulose acetate grade dissolving pulp is obtained.

[0030] The slurry performance results after treatment by Examples 1, 2, and 3 are shown in Table 1 below;

[0031] Table 1

[0032]

[0033]

[0034] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A flexible pulping process for dissolving pulp, characterized in that, The application relates to a dissolving pulp preparation process, which is configured by combining one or more sections of a kraft pulping and bleaching process without a pre-hydrolysis section, and a pulp beating treatment, an alkali treatment, an acid treatment and an enzyme treatment before or after bleaching, and is capable of realizing yield improvement by controlling the cooking and bleaching intensity through a light cooking and heavy bleaching linkage, and realizing yield improvement through process treatment conditions before and after bleaching. a) obtaining wood chips to be treated, and cooking the wood chips to be treated to obtain crude pulp, wherein the light cooking is that cooking liquor of the cooking is a mixed solution of sodium hydroxide and sodium sulfide, the sulfidity is 20-30%, the alkali dosage is 15-25% of the weight of the absolute dry material, and the cooking H factor is 600-2400, wherein the cooking H factor of coniferous wood is 1000-2000, and the cooking H factor of broad-leaved wood is 700-1400, and the Kappa number of the pulp after cooking is 30-40; b) performing oxygen delignification treatment on the crude pulp to obtain pulp after oxygen delignification; c) performing pulp beating and alkali treatment on the pulp after oxygen delignification to obtain pretreated pulp; d) performing bleaching on the pretreated pulp to obtain bleached pulp, wherein the heavy bleaching is that the bleaching treatment comprises, in sequence, chlorine dioxide primary bleaching D0, alkali hydrogen peroxide primary bleaching E1, chlorine dioxide secondary bleaching D1, alkali hydrogen peroxide secondary bleaching E2 and chlorine dioxide tertiary bleaching D2, wherein the process conditions of the D0 section are that the pulp consistency is 10-20 wt%, the time is 120-180 min, the temperature is 70-95 DEG C, the chlorine dioxide dosage is 1.0-2.5% relative to the mass of the absolute dry pulp, and the residual chlorine after bleaching is 0-0.1 g / L; wherein the process conditions of the D1 section are that the pulp consistency is 10-20 wt%, the time is 120-180 min, the temperature is 70-95 DEG C, the chlorine dioxide dosage is 0.5-1.5% relative to the mass of the absolute dry pulp, and the residual chlorine after bleaching is 0-0.1 g / L; wherein the process conditions of the D2 section are that the pulp consistency is 10-20 wt%, the time is 120-180 min, the temperature is 70-95 DEG C, the chlorine dioxide dosage is 0-1.0% relative to the mass of the absolute dry pulp, and the residual chlorine after bleaching is 0-0.1 g / L; wherein the process conditions of the E1 section are that the pulp consistency is 10-20 wt%, the time is 60-120 min, the temperature is 70-95 DEG C, the sodium hydroxide dosage is 1.0-2.0% relative to the mass of the absolute dry pulp, and the hydrogen peroxide dosage is 1.0-2.0%; wherein the process conditions of the E2 section are that the pulp consistency is 10-20 wt%, the time is 60-120 min, the temperature is 70-95 DEG C, the sodium hydroxide dosage is 0-1.5% relative to the mass of the absolute dry pulp, and the hydrogen peroxide dosage is 0-1.0%; e) performing acid strengthening treatment on the bleached pulp to obtain post-treated pulp; f) performing enzyme treatment on the post-treated pulp to obtain dissolving pulp.

2. A flexible pulping process for dissolving pulp according to claim 1, characterized in that, The oxygen delignification treatment includes a first oxygen delignification and a second oxygen delignification, and the oxygen delignification conditions are controlled according to the content of the kappa number of the pulp after cooking, wherein the alkali dosage in the first oxygen delignification is 2-5% of the weight of the absolutely dry pulp, the oxygen pressure is controlled to be 0.6-0.8 MPa, and the reaction temperature is controlled to be 80-90°C; in the second oxygen delignification, the alkali dosage is controlled to be 1-4% of the weight of the absolutely dry pulp, the oxygen pressure is controlled to be 0.4-0.6 MPa, and the temperature is controlled to be 90-100°C.

3. A flexible pulping process for dissolving pulp according to claim 1, characterized in that, The pretreatment of the bleaching section includes grinding and alkali treatment, specifically, grinding to control the beating degree of the coarse pulp to be 25-50°SR, and the pulp consistency of the coarse pulp to be 20-30%; after grinding, the alkali treatment conditions are as follows: the sodium hydroxide dosage is 1-8% of the weight of the absolutely dry pulp, and the reaction is carried out for 60-180 min without additional heating.

4. A flexible pulping process for dissolving pulp according to claim 1, characterized in that, The post-treatment after bleaching includes an acid strengthening section and an enzyme treatment section in sequence, wherein the acid strengthening section is to extend the reaction time of the last section of bleaching by 30-60 min, and the sulfuric acid dosage is 0-0.5%; wherein the enzyme treatment section is to wash the pulp to pH 4-7 after the acid strengthening section, and then to add one or several of cellulase and / or hemicellulase without additional heating, the enzyme dosage is 0-10000 ppm, and the reaction time is 30-180 min.

5. A flexible pulping process for dissolving pulp according to claim 1, characterized in that, The production process of viscose-grade dissolving pulp is step a)+b)+d)+f).

6. A flexible pulping process for dissolving pulp according to claim 1, characterized in that, The production process of lyocell-grade dissolving pulp is step a)+b)+c)+d)+e)+f).

7. A flexible pulping process for dissolving pulp according to claim 1, characterized in that, The production process of acetate-grade dissolving pulp is step a)+b)+c)+d)+e).

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