Method for pulping by using sugarcane leaves

Abstract

The invention discloses a method for pulping by using sugarcane leaves, and belongs to the technical field of pulping and papermaking. The method comprises the following technical steps: (1) pretreatment of sugarcane leaves: immersing crushed sugarcane leaves into a mixed solution, carrying out microwave irradiation, and filtering to obtain a cooked material; (2) two-stage cooking: putting the cooking material into cooking liquor containing a composite cooking agent, and carrying out first-stage cooking, so as to obtain first slurry; a silicon dispersing agent is added into the first slurry for second-stage cooking, and second slurry is obtained; and (3) pulping: carrying out solid-liquid separation, washing and screening on the second pulp to prepare paper pulp. The method for pulping by using sugarcane leaves has the advantages of high desilicication rate, high lignin removal rate, high cellulose retention rate, small amount of alkali (sodium hydroxide) required in the production process, low viscosity of black liquid, low equipment scaling possibility, low equipment cleaning frequency, high production efficiency and low production cost.

Description

Technical Field

[0001] The invention belongs to the technical field of pulping and papermaking, and particularly relates to a method for pulping using sugarcane leaves. Background Art

[0002] Traditional pulp and papermaking raw materials primarily consist of wood, waste paper, and other fibrous plants. However, with increasing awareness of ecological and environmental protection and the increasing scarcity of resources, the search for new, renewable, and environmentally friendly pulp and paper raw materials has become an urgent need in the industry. Sugarcane leaves, rich in cellulose, are a suitable raw material for pulp and papermaking. The rational use of sugarcane leaves for pulping and papermaking can not only alleviate the tight supply of papermaking raw materials in my country, but also promote the high-value utilization of agricultural waste and contribute to regional air pollution prevention and control.

[0003] Currently, the pulping and papermaking industry generally uses the caustic soda process for pulping. This process involves adding an appropriate amount of sodium hydroxide as a cooking agent. At a specific cooking temperature, this reacts with and degrades the lignin in the fiber plant, dissolving the lignin-sodium lignin in the cooking liquor. As a cleaner production process, caustic soda pulping holds great promise. Research in the document "A Brief Discussion on Pulping and Papermaking with Sugarcane Leaves" (Qiu Yugui) indicates that the ash content in sugarcane leaves is as high as 10.5%, of which silicon (in the form of SiO2) is one of the main components of the ash, accounting for approximately 6.5%. Sodium hydroxide, as a strong alkali, can dissociate lignin from the fiber and dissolve it in the cooking liquor. However, the high silicon content in sugarcane leaves will react with sodium hydroxide to produce sodium silicate (Na2SiO2), which consumes the effective alkali in the cooking process and reduces the efficiency of lignin removal. Sodium silicate forms colloids, which increases the viscosity of the black liquor and leads to increased energy consumption in the evaporation section (requiring 10-15% additional steam). At the same time, silicates (such as calcium silicate and magnesium silicate) are deposited as hard scale at high temperatures, reducing heat transfer efficiency and increasing the frequency of equipment cleaning, thereby significantly increasing the difficulty and cost of pulping.

[0004] Therefore, in view of the high silicon content in sugarcane leaves, the present invention provides a sugarcane leaf pulping method with high silicon removal rate, high lignin removal rate, high production efficiency and low production cost. Summary of the Invention

[0005] In response to the above problems, the present invention provides a method for pulping sugarcane leaves. Through sugarcane leaf pretreatment, two-stage cooking and pulping, the desiliconization rate and lignin removal rate of the sugarcane leaves are high during the pulping process, and the production efficiency is high and the production cost is low.

[0006] The present invention is achieved through the following technical solutions:

[0007] A method for pulping sugarcane leaves, comprising the following steps:

[0008] (1) Pretreatment of sugarcane leaves: After cleaning the sugarcane leaves, air them in a dry and ventilated place. The air-dried sugarcane leaves are crushed and then immersed in the mixed solution and microwave irradiated. After filtering, the pretreated sugarcane leaves are obtained, which are the cooking material;

[0009] (2) Two-stage steaming:

[0010] First stage cooking: the cooking material is placed in a cooking liquid containing a composite cooking agent for first stage cooking, the temperature is raised to 135-145° C., the pressure is 0.45-0.55 MPa, and the temperature is kept for 50-80 minutes to obtain a first slurry (mainly containing cellulose, black liquor, residual lignin, and unreacted cooking agent); the composite cooking agent is a mixture of 3.5-5.0% of sodium hydroxide, 1.0-2.0% of sodium hydrosulfide, and 0.35-1.0% of alkylphenol polyoxyethylene ether, fatty alcohol polyoxyethylene ether, or fatty acid methyl ester ethoxylate based on the total weight of the cooking material;

[0011] Second stage cooking: adding a silicon dispersant to the first slurry for the second stage cooking, heating the slurry to 150-160°C and keeping the temperature for 30-45 minutes to obtain a second slurry (mainly containing cellulose, suspended in black liquor); the silicon dispersant is polyepoxysuccinic acid or polyaspartic acid in an amount of 0.1-0.25% by weight of the total weight of the cooking material;

[0012] (3) Pulping: The second slurry is subjected to solid-liquid separation, water washing and screening to prepare paper pulp.

[0013] Furthermore, in step (2), the composite cooking agent is prepared by mixing 3.5-4.0% of sodium hydroxide, 1.0-1.5% of sodium hydrosulfide and 0.35-0.6% of alkylphenol polyoxyethylene ether based on the total weight of the cooking material.

[0014] Furthermore, in step (2), the liquid-to-material ratio of the cooking liquid to the cooking material is 1:4-6.

[0015] Furthermore, in step (2), the silicon dispersant is 0.1 to 0.15% of the total weight of the cooking material.

[0016] Furthermore, in step (2), the first stage of steaming is as follows: heating to 135-140°C, pressure 0.45-0.50 MPa, and keeping warm for 60-70 minutes;

[0017] The second stage of steaming is as follows: heating to 150-155° C. and keeping the temperature for 35-40 minutes.

[0018] Furthermore, in step (1), the mixed solution contains 0.8-2.5% of citric acid and 1.5-4.2% of acetic acid based on the total weight of the sun-dried sugarcane leaves.

[0019] Furthermore, in step (1), the liquid-to-solid ratio of the mixed liquid to the air-dried sugarcane leaves is 1:4-6.

[0020] Furthermore, in step (1), the microwave irradiation is performed at a microwave frequency of 800 to 1200 MHz for 5 to 10 minutes.

[0021] Furthermore, in step (1), the moisture content of the sugarcane leaves after drying is maintained at 15-20%; and the moisture content of the cooking material is 30-40%.

[0022] Furthermore, in step (1), the sugarcane leaves are crushed to a length of 3 to 5 cm.

[0023] The principle of utilizing sugarcane leaves to make pulp of the present invention is as follows:

[0024] (1) Sugarcane leaf pretreatment: The crushed sugarcane is immersed in a mixture of citric acid and acetic acid and irradiated with microwaves to remove most of the silicon, promote the dissolution of hemicellulose, and retain the integrity of cellulose. In this process, microwaves can accelerate the reaction of silica bodies with H + The ion exchange reaction, citric acid provides H + , forming soluble complexes with silicon, improving desiliconization efficiency. Microwaves can disrupt the hemicellulose-lignin cross-linking structure in sugarcane leaves, while acetic acid hydrolyzes the β-1,4 glycosidic bonds of hemicellulose, increasing hemicellulose dissolution and cellulose retention. Microwaves also expand fiber pores, exposing cellulose microfibrils. Weak acid softens the fibers, reducing mechanical damage and the amount of alkali required for subsequent cooking. By optimizing fiber structure, lowering subsequent processing intensity, and minimizing fiber loss, these processes can effectively increase the final pulp yield.

[0025] (2) Two-stage steaming:

[0026] First stage of cooking: The cooking material is put into the cooking liquid containing the composite cooking agent for the first stage of cooking. During this process, the NaHS in the composite cooking agent releases S 2- and HS - Through reduction, the lignin structure in sugarcane leaves is destroyed, lignin removal is accelerated, and at the same time, the delignification function of NaOH is partially replaced, the amount of strong alkali sodium hydroxide is reduced, the viscosity of black liquor is reduced, and the difficulty of black liquor treatment is reduced; alkylphenol polyoxyethylene ether and the like reduce the surface tension of the cooking liquid, enhance the wettability of the liquid on the sugarcane leaf fiber, promote the rapid penetration of NaOH and NaHS into the fiber, and at the same time synergize with NaOH and NaHS to improve the desiliconization rate, reducing the amount of silicon dispersant in the second stage of cooking.

[0027] Second stage cooking: A silicon dispersant is added to the slurry from the first stage cooking for the second stage cooking. This process further removes residual lignin from the sugarcane leaves. The silicon dispersant inhibits silicate scaling and chelates silicon ions, reducing the thickness of silicon scaling on the equipment walls after cooking and reducing the frequency of equipment cleaning. Compared with existing technologies, the advantages and benefits of this invention are as follows:

[0028] 1. The method of pulping sugarcane leaves of the present invention achieves a high desiliconization rate, high lignin removal efficiency, and high cellulose retention rate during the sugarcane leaf pulping process through sugarcane leaf pretreatment, two-stage steaming, and pulping. In addition, the production process requires a small amount of alkali (sodium hydroxide), has low black liquor viscosity, is less prone to equipment scaling, requires less frequent equipment cleaning, and has high production efficiency and low production costs.

[0029] 2. In the present invention's sugarcane leaf pretreatment, the crushed sugarcane is immersed in a mixture of citric acid and acetic acid and then irradiated with microwaves. This removes most of the silicon, improves desiliconization efficiency, promotes the dissolution of some hemicellulose, reduces fiber loss, increases cellulose retention, and improves the final pulp yield. It also softens the fibers, reduces mechanical damage, and reduces the amount of alkali required for subsequent cooking.

[0030] 3. In the two-stage cooking process of the present invention, a composite cooking agent is first added for the first stage of cooking to further improve the desiliconization rate and accelerate lignin removal. Lignin is degraded into soluble substances that dissolve in the cooking liquor, resulting in a darker liquid color and the formation of black liquor. Because the amount of strong base sodium hydroxide used in this process is small, it cannot react with silicon in the sugarcane leaves to form sodium silicate colloidal substances, thereby reducing the viscosity of the black liquor. A silicon dispersant is then added for the second stage of cooking to further remove residual lignin from the sugarcane leaves, inhibit silicate scaling, chelate silicon ions, and reduce the thickness of silicon scale on the inner walls of the equipment after cooking, thereby reducing the frequency of equipment cleaning and lowering production costs.

[0031] 4. Compared with conventional methods, the present invention increases the desiliconization rate by 89.65%, the lignin removal rate by 19.86%, the cellulose retention rate by 9.91%, the black liquor viscosity by 52.80%, the thickness of silica scale on the equipment walls by 58.52%, and the equipment cleaning cycle by 52.52%. Furthermore, the amount of alkali (sodium hydroxide) used for cooking is reduced by 46.67%, significantly reducing alkali consumption. This shows that compared with conventional methods, the present invention can effectively improve the production efficiency of sugarcane leaf pulping and reduce production costs. DETAILED DESCRIPTION

[0032] The present invention is further described in detail below through examples. These examples are only used to illustrate the present invention and do not limit the scope of protection of the present invention.

[0033] Example 1

[0034] A method for pulping sugarcane leaves, comprising the following steps:

[0035] (1) Pretreatment of sugarcane leaves: After cleaning, sugarcane leaves are air-dried in a dry and ventilated place until the moisture content is 15%. The air-dried sugarcane leaves are crushed to 5 cm and then immersed in a mixture containing 2% citric acid and 4% acetic acid based on the total weight of the air-dried sugarcane leaves, with a liquid-to-material ratio of 1:5. The mixture is irradiated at a microwave frequency of 1000 MHz for 8 minutes. After filtration, the pretreated sugarcane leaves are obtained, which is the cooking material (moisture content 36%).

[0036] (2) Two-stage steaming:

[0037] The first stage of cooking: the cooking material is put into a cooking liquid containing a composite cooking agent for the first stage of cooking, wherein the composite cooking agent is a mixture of 3.8% of the total weight of the cooking material, 1.1% of sodium hydrosulfide and 0.6% of alkylphenol polyoxyethylene ether, and the liquid-to-material ratio of the cooking liquid to the cooking material is 1:4. During cooking, the temperature is raised to 135° C., the pressure is 0.50 MPa, and the temperature is kept for 70 minutes to obtain a first slurry (mainly containing cellulose, black liquor, residual lignin, and unreacted cooking agent);

[0038] Second stage cooking: adding a silicon dispersant to the first slurry for the second stage cooking, wherein the silicon dispersant is polyepoxysuccinic acid at a concentration of 0.1% of the total weight of the cooking material, heating the slurry to 155°C and keeping the temperature for 40 minutes to obtain a second slurry (mainly containing cellulose, suspended in black liquor);

[0039] (3) Pulping: The second slurry is subjected to solid-liquid separation, water washing and screening to prepare paper pulp.

[0040] Example 2

[0041] A method for pulping sugarcane leaves, comprising the following steps:

[0042] (1) Sugarcane leaf pretreatment: After cleaning the sugarcane leaves, air dry them in a dry and ventilated place until the moisture content is 15%. The air-dried sugarcane leaves are crushed to 5 cm and then immersed in a mixture containing 2.5% citric acid and 4% acetic acid based on the total weight of the air-dried sugarcane leaves. The liquid-to-material ratio of the mixture to the sugarcane leaves is 1:6. The mixture is irradiated at a microwave frequency of 1200 MHz for 5 minutes. After filtration, the pretreated sugarcane leaves are obtained, which is the cooking material (moisture content 40%).

[0043] (2) Two-stage steaming:

[0044] The first stage of cooking: the cooking material is put into a cooking liquid containing a composite cooking agent for the first stage of cooking. The composite cooking agent is a mixture of 5.0% sodium hydroxide, 1.3% sodium hydrosulfide and 0.75% fatty alcohol polyoxyethylene ether based on the total weight of the cooking material. The liquid-to-material ratio of the cooking liquid to the cooking material is 1:5. During cooking, the temperature is raised to 135° C., the pressure is 0.45 MPa, and the temperature is maintained for 60 minutes to obtain a first slurry (mainly containing cellulose, black liquor, residual lignin, and unreacted cooking agent);

[0045] Second stage cooking: adding a silicon dispersant to the first slurry for the second stage cooking, wherein the silicon dispersant is polyepoxysuccinic acid at a concentration of 0.2% of the total weight of the cooking material, heating the slurry to 150°C and keeping the temperature for 35 minutes to obtain a second slurry (mainly containing cellulose, suspended in black liquor);

[0046] (3) Pulping: The second slurry is subjected to solid-liquid separation, water washing and screening to prepare paper pulp.

[0047] Example 3

[0048] A method for pulping sugarcane leaves, comprising the following steps:

[0049] (1) Sugarcane leaf pretreatment: After cleaning the sugarcane leaves, air dry them in a dry and ventilated place until the moisture content is 20%. The air-dried sugarcane leaves are crushed to 3 cm and then immersed in a mixture containing 1% citric acid and 3% acetic acid based on the total weight of the air-dried sugarcane leaves, with a liquid-to-material ratio of 1:6. The mixture is irradiated at a microwave frequency of 800 MHz for 10 minutes. After filtration, the pretreated sugarcane leaves are obtained, which is the cooking material (moisture content is 32%).

[0050] (2) Two-stage steaming:

[0051] The first stage of cooking: the cooking material is put into a cooking liquid containing a composite cooking agent for the first stage of cooking. The composite cooking agent is a mixture of 4.5% sodium hydroxide, 1.2% sodium hydrosulfide and 0.5% fatty acid methyl ester ethoxylate based on the total weight of the cooking material. The liquid-to-material ratio of the cooking liquid to the cooking material is 1:6. During cooking, the temperature is raised to 145° C., the pressure is 0.55 MPa, and the temperature is maintained for 50 minutes to obtain a first slurry (mainly containing cellulose, black liquor, residual lignin, and unreacted cooking agent);

[0052] Second stage cooking: adding a silicon dispersant to the first slurry for the second stage cooking, wherein the silicon dispersant is polyaspartic acid at a concentration of 0.25% by weight of the total cooking material, heating the slurry to 160°C and keeping the temperature for 30 minutes to obtain a second slurry (mainly containing cellulose, suspended in black liquor);

[0053] (3) Pulping: The second slurry is subjected to solid-liquid separation, water washing and screening to prepare paper pulp.

[0054] Example 4

[0055] A method for pulping sugarcane leaves, comprising the following steps:

[0056] (1) Pretreatment of sugarcane leaves: After cleaning, sugarcane leaves are air-dried in a dry and ventilated place until the moisture content is 15%. The air-dried sugarcane leaves are crushed to 5 cm and then immersed in a mixture containing 2% citric acid and 3% acetic acid based on the total weight of the air-dried sugarcane leaves, with a liquid-to-material ratio of 1:5. The mixture is irradiated at a microwave frequency of 1000 MHz for 10 minutes. After filtration, the pretreated sugarcane leaves are obtained, which is the cooking material (moisture content is 35%).

[0057] (2) Two-stage steaming:

[0058] The first stage of cooking is to put the cooking material into a cooking liquid containing a composite cooking agent for the first stage of cooking, wherein the composite cooking agent is a mixture of 4.0% of the total weight of the cooking material, 1.5% of sodium hydrosulfide and 0.4% of alkylphenol polyoxyethylene ether, and the liquid-to-material ratio of the cooking liquid to the cooking material is 1:4. During cooking, the temperature is raised to 140° C., the pressure is 0.50 MPa, and the temperature is kept for 70 minutes to obtain a first slurry (mainly containing cellulose, black liquor, residual lignin, and unreacted cooking agent);

[0059] Second stage cooking: adding a silicon dispersant to the first slurry for the second stage cooking, wherein the silicon dispersant is polyepoxysuccinic acid at a concentration of 0.12% by weight of the absolute dry cooking material, heating the slurry to 155°C and keeping the temperature for 40 minutes to obtain a second slurry (mainly containing cellulose, suspended in black liquor);

[0060] (3) Pulping: The second slurry is subjected to solid-liquid separation, water washing and screening to prepare paper pulp.

[0061] Comparative Example 1

[0062] According to the traditional method of acid-treated sugarcane leaf pulping, the sugarcane leaves are washed, air-dried to 15% moisture, crushed to 5 cm, and immersed in a solution containing 5% sulfuric acid at a liquid-to-solid ratio of 1:4. The leaves are then immersed at room temperature for 120 minutes. After filtration, the pretreated sugarcane leaves are obtained, which serves as the cooking material. The cooking material is then added to a cooking liquid containing sodium hydroxide (7.5% by weight of the total cooking material) at a liquid-to-solid ratio of 1:4. The temperature is raised to 160°C and maintained for 120 minutes. The resulting pulp (primarily containing cellulose, black liquor, and residual lignin) is subjected to solid-liquid separation, washed, and sieved before being made into paper pulp.

[0063] The desiliconization rate, lignin removal rate, cellulose retention rate, black liquor viscosity, silica scale thickness on the inner wall of the equipment after cooking and equipment cleaning cycle of the second pulp in the above Examples 1-4 and the pulp in Comparative Example 1 were statistically analyzed, and the results are shown in Table 1 below.

[0064] Table 1 Sugarcane leaf pulping

[0065]

[0066] As shown in Table 1, the present method achieves a desiliconization rate of approximately 59%, a lignin removal rate of approximately 86%, and a cellulose retention rate of approximately 90%. Fiber integrity is high, black liquor viscosity is reduced to approximately 245 mPa·s, and the thickness of silica scale on the equipment walls after cooking is only approximately 1.1 mm. The equipment cleaning cycle is extended to 28 batches, reducing equipment cleaning frequency. Compared with the conventional method in Comparative Example 1, Example 4 of the present invention achieves an 89.65% increase in desiliconization rate, a 19.86% increase in lignin removal rate, a 9.91% increase in cellulose retention rate, a 52.80% decrease in black liquor viscosity, a 58.52% decrease in silica scale thickness on the equipment walls, and a longer equipment cleaning cycle. Furthermore, while the amount of alkali (sodium hydroxide) used for cooking in Comparative Example 1 was 7.5%, Example 4 used only 4.0%, a significant reduction of 46.67%. This demonstrates that, compared with conventional methods, the present invention effectively improves sugarcane leaf pulping production efficiency and reduces production costs.

[0067] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for pulping sugarcane leaves, characterized in that: The following steps are involved: (1) Pretreatment of sugarcane leaves: After cleaning the sugarcane leaves, air them in a dry and ventilated place. The air-dried sugarcane leaves are crushed and then immersed in the mixed solution and microwave irradiated. After filtering, the pretreated sugarcane leaves are obtained, which are the cooking material; (2) Two-stage steaming: First stage cooking: the cooking material is put into a cooking liquid containing a composite cooking agent for first stage cooking, the temperature is raised to 135-145° C., the pressure is 0.45-0.55 MPa, and the temperature is kept for 50-80 minutes to obtain a first slurry; the composite cooking agent is a mixture of 3.5-5.0% of sodium hydroxide, 1.0-2.0% of sodium hydrosulfide, and 0.35-1.0% of alkylphenol polyoxyethylene ether, fatty alcohol polyoxyethylene ether, or fatty acid methyl ester ethoxylate based on the total weight of the cooking material; Second stage cooking: adding a silicon dispersant to the first slurry for a second stage cooking, heating the mixture to 150-160°C and keeping the temperature for 30-45 minutes to obtain a second slurry; the silicon dispersant is polyepoxysuccinic acid or polyaspartic acid in an amount of 0.1-0.25% by weight of the total weight of the cooking material; (3) Pulping: The second slurry is subjected to solid-liquid separation, water washing and screening to prepare paper pulp.

2. The method for pulping sugarcane leaves according to claim 1, wherein: In step (2), the composite cooking agent is prepared by mixing 3.5-4.0% of sodium hydroxide, 1.0-1.5% of sodium hydrosulfide and 0.35-0.6% of alkylphenol polyoxyethylene ether based on the total weight of the cooking material.

3. The method for pulping sugarcane leaves according to claim 1, wherein: In step (2), the liquid-to-material ratio of the cooking liquid to the cooking material is 1:4-6.

4. The method for pulping sugarcane leaves according to claim 1, wherein: In step (2), the silicon dispersant is 0.1 to 0.15% of the total weight of the cooking material.

5. The method for pulping using sugarcane leaves according to claim 1, wherein: In step (2), the first stage of steaming is as follows: heating to 135-140°C, pressure 0.45-0.50 MPa, and keeping warm for 60-70 minutes; The second stage of steaming is as follows: heating to 150-155° C. and keeping the temperature for 35-40 minutes.

6. The method for pulping sugarcane leaves according to claim 1, wherein: In step (1), the mixed solution contains 0.8-2.5% of citric acid and 1.5-4.2% of acetic acid based on the total weight of the sun-dried sugarcane leaves.

7. The method for pulping sugarcane leaves according to claim 1, wherein: In step (1), the liquid-to-solid ratio of the mixed liquid to the air-dried sugarcane leaves is 1:4-6.

8. The method for pulping sugarcane leaves according to claim 1, wherein: In step (1), the microwave irradiation is performed at a microwave frequency of 800 to 1200 MHz for 5 to 10 minutes.

9. The method for pulping sugarcane leaves according to claim 1, wherein: In step (1), the moisture content of the sugarcane leaves after drying is maintained at 15-20%; the moisture content of the cooking material is 30-40%.

10. The method for pulping using sugarcane leaves according to claim 1, wherein: In step (1), the sugarcane leaves are crushed to a length of 3 to 5 cm.

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