A method for fractionating lignin in wheat straw pulping black liquor using a series of cationic polyacrylamides

Abstract

The application discloses a method for fractionating lignin in wheat straw pulping black liquor by using a series of cationic polyacrylamides, and a series of polyacrylamides containing different cations and different cationic degrees are synthesized by a copolymerization method. By adding the polyacrylamides with different cationic degrees in sequence and centrifuging lignin components, the effect of fractionating and precipitating lignin with different molecular weights and hydrophilic or hydrophobic properties in the black liquor is achieved, so that lignin components with different molecular weight intervals and different hydrophilic or hydrophobic properties are obtained, which is beneficial to subsequent utilization of the lignin products.

Description

Technical Field

[0001] This invention relates to the field of comprehensive resource recycling, specifically to a method for classifying lignin in wheat straw pulping black liquor using a series of cationic polyacrylamides. Background Technology

[0002] In the context of increasing emphasis on environmental protection and resource recycling, the efficient and environmentally friendly treatment of black liquor generated during wheat straw pulping has become a pressing issue in the papermaking and agricultural waste comprehensive utilization fields. Black liquor, a byproduct of the pulping process, is rich in lignin, hemicellulose, and small amounts of cellulose, among other organic matter. Direct discharge not only severely pollutes water bodies but also wastes valuable resources. Therefore, developing a technology that can effectively remove pollutants from black liquor while recovering valuable chemical components is crucial. Flocculation, as a low-cost, simple, and effective physicochemical treatment method, is gaining increasing attention for its application in the treatment of black liquor from wheat straw pulping.

[0003] Wheat straw pulping black liquor is characterized by high concentration, high alkalinity, and high organic load, among which lignin is one of the main factors affecting its treatment difficulty. Lignin is a complex organic polymer with extremely high stability and recalcitrant degradation, making it difficult to remove efficiently using traditional methods. However, by fractionating lignin to obtain lignin with narrow molecular weight distribution and hydrophilic / hydrophobic distribution, it can be transformed from waste into a valuable resource for comprehensive utilization. Therefore, finding a method that can effectively break down the lignin structure and promote its aggregation and precipitation is crucial.

[0004] Flocculation primarily involves adding specific flocculants to black liquor to destabilize dissolved lignin and suspended particles, causing them to aggregate into larger flocs. Solid-liquid separation is then achieved through sedimentation or flotation. Commonly used flocculants include inorganic flocculants (such as polyaluminum chloride and aluminum sulfate), organic polymeric flocculants (such as polyacrylamide), and bio-flocculators. Selecting a suitable flocculant and optimizing flocculation conditions are crucial for improving treatment efficiency. However, conventional flocculants, when added in a single application, are insufficient for effectively classifying wheat straw pulping black liquor, resulting in a low overall utilization rate of the black liquor. Summary of the Invention

[0005] The technical problem this invention aims to solve is that, while acid precipitation technology is mature for highly alkaline wheat straw pulping black liquor, it requires large amounts of acid, sophisticated equipment, and is difficult to meet environmental emission standards. Excessive acid addition also generates large amounts of salt, increasing the difficulty of subsequent treatment processes. This invention synthesizes a series of cationic polyacrylamides, which are used to sequentially grade and flocculate wheat straw pulping black liquor, achieving the effect of graded precipitation of lignin with different molecular weights and hydrophilic / hydrophobic properties in the black liquor, thus facilitating the subsequent utilization of lignin products.

[0006] This invention provides a method for lignin fractionation in wheat straw pulping black liquor using a series of cationic polyacrylamides, comprising the following steps:

[0007] Step 1: Synthesis of polyacrylamide with different cations and different cationic degrees

[0008] (1) The raw materials for synthesis include: pure water, acrylamide, cationic monomer, initiator, chain transfer agent, oxidant, and reducing agent; wherein the cationic monomer is one of methacryloyloxyethyltrimethylammonium chloride (DMC), acryloyloxyethyltrimethylammonium chloride (DAC), methacrylamidepropyltrimethylammonium chloride (MAPTAC), allyltrimethylammonium chloride (ATMAC), and N-trimethyl-(4-vinylbenzyl)ammonium chloride ([EMIM]Cl), the initiator is azobisisobutyronitrile, the chain transfer agent is sodium hypophosphite, the oxidant is tert-butyl hydroperoxide, and the reducing agent is sodium sulfite.

[0009] (2) Synthesis steps: By mass, mix 1000-1100 parts of pure water, 250-280 parts of acrylamide, and 50-100 parts of cationic monomer evenly and freeze. After purging with nitrogen for 15-20 min, add 0.3-0.5 parts of initiator, 0.05-0.1 parts of chain transfer agent, 0.002-0.006 parts of oxidant, and 0.002-0.006 parts of reducing agent in sequence. After the mixture becomes sticky, stop purging with nitrogen. After the reaction is completed, keep it warm for 1-2 h, take out the gel block, granulate, dry, and grind it.

[0010] Step 2: Take 200 mL of wheat straw pulping black liquor and place it in a beaker. Adjust the pH to between 8 and 10 using hydrochloric acid. Set aside for use.

[0011] Step 3: Prepare a mother liquor of 15-20 g / L using polyacrylamide with different cations and different cationic degrees from step 1 (2);

[0012] Step 4: Add 4-10 mL of the polyacrylamide mother liquor from Step 3 to a beaker containing the black liquor from Step 2, and stir rapidly at 200-220 r / min for 5-8 min. Then reduce the speed to 50-70 r / min and stir slowly for 15-20 min, and let it stand to settle for 1-2 h. Finally, use a centrifuge to separate the lignin at 5000-7000 r / min for 10-15 min. Take the solid component and dry it in a vacuum oven at 40-60℃ for 5-8 h to obtain component C1.

[0013] Step 5: Add 4-10 mL of the polyacrylamide mother liquor from Step 3 to a beaker containing the supernatant liquid from Step 4 after centrifugation, and stir rapidly at 200-220 r / min for 5-8 min. Then reduce the speed to 50-70 r / min and stir slowly for 15-20 min, and let it stand to settle for 1-2 h. Finally, use a centrifuge to separate the lignin at 5000-7000 r / min for 10-15 min. Take the solid component and dry it in a vacuum oven at 40-60℃ for 5-8 h to obtain component C2.

[0014] Step 6: Add 4-10 mL of the polyacrylamide mother liquor from Step 3 to a beaker containing the supernatant liquid from Step 5 after centrifugation, and stir rapidly at 200-220 r / min for 5-8 min. Then reduce the speed to 50-70 r / min and stir slowly for 15-20 min, and let it stand to settle for 1-2 h. Finally, use a centrifuge to separate the lignin at 5000-7000 r / min for 10-15 min. Take the solid component and dry it in a vacuum oven at 40-60℃ for 5-8 h to obtain component C3.

[0015] Compared with the prior art, the present invention has the following advantages:

[0016] (1) The present invention synthesizes a series of cationic polyacrylamides to treat wheat straw pulping black liquor. This product can be operated without excessive acid neutralization and alkalinity, which greatly reduces the requirements for equipment, as well as the operating cost and the degree of operation danger.

[0017] (2) The flocculation method for classifying wheat straw lignin can utilize the high molecular weight and high hydrophobic lignin to be preferentially flocculated and settled, while the low molecular weight and hydrophilic lignin are further flocculated and settled in the subsequent flocculation process, thereby achieving the purpose of classifying wheat straw lignin.

[0018] (3) By continuously adding a series of cationic polyacrylamides to separate lignin components of different molecular weights and different hydrophilicity and hydrophobicity, the availability of lignin after fractionation is effectively improved.

[0019] This invention obtains a series of different cationic polyacrylamides by copolymerizing acrylamide with different cationic monomers, or by copolymerizing acrylamide with cationic monomers to obtain a series of polyacrylamides with different cationic degrees, or by copolymerizing acrylamide with different cationic monomers to obtain a series of polyacrylamides with different cationic and cationic degrees. By sequentially adding polyacrylamides and then performing sedimentation separation to obtain lignin with narrow molecular weight distribution and different hydrophilicity and hydrophobicity, and by continuing to add cationic polyacrylamides and perform sedimentation separation in stages, the black liquor from wheat straw pulping can be effectively graded, achieving the purpose of comprehensive utilization of black liquor. Attached Figure Description

[0020] Figure 1 This is a graph showing the content of different molecular weight ranges and different hydrophilic and hydrophobic components in Example 1;

[0021] Figure 2 This is a graph showing the content of different molecular weight ranges and different hydrophilic and hydrophobic components in Example 2;

[0022] Figure 3 This is a graph showing the content of different molecular weight ranges and different hydrophilic and hydrophobic components in Example 3;

[0023] Figure 4 This is a graph showing the content of different molecular weight ranges and different hydrophilic and hydrophobic components in Example 4;

[0024] Figure 5 This is a graph showing the content of different molecular weight ranges and different hydrophilic and hydrophobic components in Example 5;

[0025] Figure 6 This is a comparative example showing the content of components with different molecular weight ranges and different hydrophilicity / hydrophobicity.

[0026] Figure 7 This is a graph showing the content of different molecular weight ranges and different hydrophilic and hydrophobic components in Comparative Example 2.

[0027] Figure 8 The graph shows the content of different molecular weight ranges and different hydrophilic and hydrophobic components in Comparative Example 3.

[0028] Figure 9 The graphs show the separation rates of lignin in straw pulping black liquor in Examples 1-5 and Comparative Examples 1-3. Detailed Implementation

[0029] The following are specific embodiments of the present invention, which describe the operation scheme of the present invention in detail. However, the scope of protection of this patent includes but is not limited to these embodiments. All other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this patent.

[0030] Example 1: Synthesis of cationic polyacrylamides with different cationicities and cationic degrees:

[0031] (1) Mix 1050 parts of pure water, 280 parts of acrylamide, and 50 parts of DMC evenly and freeze. After passing nitrogen gas for 20 min, add 0.4 parts of azobisisobutyronitrile, 0.1 parts of sodium hypophosphite, 0.004 parts of tert-butyl hydroperoxide, and 0.004 parts of sodium sulfite in sequence. Stop passing nitrogen gas after the sticky fibers appear. After the reaction is completed and kept warm for 2 h, take out the gel block, granulate, dry, and grind it.

[0032] (2) Mix 1050 parts of pure water, 280 parts of acrylamide, and 80 parts of DAC evenly and freeze. After passing nitrogen gas for 20 min, add 0.4 parts of azobisisobutyronitrile, 0.07 parts of sodium hypophosphite, 0.004 parts of tert-butyl hydroperoxide, and 0.004 parts of sodium sulfite in sequence. Stop passing nitrogen gas after the sticky threads appear. After the reaction is completed and kept at the temperature for 2 h, take out the gel block, granulate, dry, and grind it.

[0033] (3) Mix 1050 parts of pure water, 280 parts of acrylamide, and 100 parts of MAPTAC evenly and freeze. After passing nitrogen gas for 20 min, add 0.4 parts of azobisisobutyronitrile, 0.05 parts of sodium hypophosphite, 0.004 parts of tert-butyl hydroperoxide, and 0.004 parts of sodium sulfite in sequence. Stop passing nitrogen gas after the sticky fibers appear. After the reaction is completed and kept warm for 2 h, take out the gel block, granulate, dry, and grind it.

[0034] (4) Prepare a 20 g / L mother liquor using cationic polyacrylamides with different cations and different ionic degrees from steps (1)-(3) above, and name them Example 1-1, Example 1-2, and Example 1-3 respectively. The polymerization reaction formula of the cationic polyacrylamide is: .

[0035] Treatment of black liquor from wheat straw pulping:

[0036] (5) Take 200 mL of straw pulping black liquor and put it in a beaker. Adjust the pH to between 8 and 10 with hydrochloric acid and wait for use.

[0037] (6) Take 8 mL of the mother liquor from Example 1-1 and add it to the beaker containing the black liquor from step (5). Stir quickly at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir slowly for 20 min. Let it stand and settle for 2 h. Finally, use a centrifuge to separate the lignin. The centrifuge speed is 7000 r / min and the time is 10 min. Take the solid component and dry it in a vacuum oven at 50 ℃ for 6 h to obtain component C1.

[0038] (7) Take 6 mL of the mother liquor from Examples 1-2 and add it to a beaker containing the supernatant liquid after centrifugation in step (6). Stir quickly at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir slowly for 20 min. Let it stand and settle for 2 h. Finally, use a centrifuge to separate the lignin at 7000 r / min for 10 min. Take the solid component and dry it in a vacuum oven at 50 ℃ for 6 h to obtain component C2.

[0039] (8) Take 4 mL of the mother liquor from Examples 1-3 and add it to a beaker containing the supernatant liquid after centrifugation in step (7). Stir quickly at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir slowly for 20 min. Let it stand and settle for 2 h. Finally, use a centrifuge to separate the lignin at 7000 r / min for 10 min. Take the solid component and dry it in a vacuum oven at 50 ℃ for 6 h to obtain component C3.

[0040] (9) The molecular weight ranges and hydrophilic / hydrophobic components of components C1, C2, and C3 were determined using ultrafiltration and resin adsorption methods, respectively. See the attached graph for the different molecular weight ranges and contents of different hydrophilic / hydrophobic components of C1, C2, and C3. Figure 1 .

[0041] Example 2: Synthesis of cationic polyacrylamides with different cations and cationic degrees:

[0042] (1) Mix 1050 parts of pure water, 280 parts of acrylamide, and 50 parts of DAC evenly and freeze. After passing nitrogen gas for 20 min, add 0.4 parts of azobisisobutyronitrile, 0.1 parts of sodium hypophosphite, 0.004 parts of tert-butyl hydroperoxide, and 0.004 parts of sodium sulfite in sequence. Stop passing nitrogen gas after the sticky threads appear. After the reaction is completed and kept warm for 2 h, take out the gel block, granulate, dry, and grind it.

[0043] (2) Mix 1050 parts of pure water, 280 parts of acrylamide, and 80 parts of MAPTAC evenly and freeze. After passing nitrogen gas for 20 min, add 0.4 parts of azobisisobutyronitrile, 0.07 parts of sodium hypophosphite, 0.004 parts of tert-butyl hydroperoxide, and 0.004 parts of sodium sulfite in sequence. Stop passing nitrogen gas after the sticky fibers appear. After the reaction is completed and kept at the temperature for 2 h, take out the gel block, granulate, dry, and grind it.

[0044] (3) Mix 1050 parts of pure water, 280 parts of acrylamide, and 100 parts of ATMAC evenly and freeze. After passing nitrogen gas for 20 min, add 0.4 parts of azobisisobutyronitrile, 0.05 parts of sodium hypophosphite, 0.004 parts of tert-butyl hydroperoxide, and 0.004 parts of sodium sulfite in sequence. Stop passing nitrogen gas after the sticky threads appear. After the reaction is completed and kept warm for 2 h, take out the gel block, granulate, dry, and grind it.

[0045] (4) Prepare 20 g / L mother liquors of cationic polyacrylamide with different cations and different ionic degrees from steps (1)-(3) above, and name them Example 2-1, Example 2-2, and Example 2-3 respectively. The polymerization reaction formula of cationic polyacrylamide is: .

[0046] Treatment of black liquor from wheat straw pulping:

[0047] (5) Take 200 mL of straw pulping black liquor and put it in a beaker. Adjust the pH to between 8 and 10 with hydrochloric acid and wait for use.

[0048] (6) Take 8 mL of the mother liquor from Example 2-1 and add it to the beaker containing the black liquor from step (5). Stir quickly at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir slowly for 20 min. Let it stand and settle for 2 h. Finally, use a centrifuge to separate the lignin. The centrifuge speed is 7000 r / min and the time is 10 min. Take the solid component and dry it in a vacuum oven at 50 ℃ for 6 h to obtain component C1.

[0049] (7) Take 6 mL of the mother liquor from Example 2-2 and add it to a beaker containing the supernatant liquid after centrifugation in step (6). Stir quickly at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir slowly for 20 min. Let it stand and settle for 2 h. Finally, use a centrifuge to separate the lignin at 7000 r / min for 10 min. Take the solid component and dry it in a vacuum oven at 50 ℃ for 6 h to obtain component C2.

[0050] (8) Take 4 mL of the mother liquor from Example 2-3 and add it to a beaker containing the supernatant liquid after centrifugation in step (7). Stir quickly at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir slowly for 20 min. Let it stand and settle for 2 h. Finally, use a centrifuge to separate the lignin at 7000 r / min for 10 min. Take the solid component and dry it in a vacuum oven at 50 ℃ for 6 h to obtain component C3.

[0051] (9) The molecular weight ranges and hydrophilic / hydrophobic components of components C1, C2, and C3 were determined using ultrafiltration and resin adsorption methods, respectively. See the attached graph for the different molecular weight ranges and contents of different hydrophilic / hydrophobic components of C1, C2, and C3. Figure 2 .

[0052] Example 3: Synthesis of cationic polyacrylamides with different cations and cationic degrees:

[0053] (1) Mix 1050 parts of pure water, 280 parts of acrylamide, and 50 parts of MAPTAC evenly and freeze. After passing nitrogen gas for 20 min, add 0.4 parts of azobisisobutyronitrile, 0.1 parts of sodium hypophosphite, 0.004 parts of tert-butyl hydroperoxide, and 0.004 parts of sodium sulfite in sequence. Stop passing nitrogen gas after the sticky threads appear. After the reaction is completed and kept at the temperature for 2 h, take out the gel block, granulate, dry, and grind it.

[0054] (2) Mix 1050 parts of pure water, 280 parts of acrylamide, and 80 parts of ATMAC evenly and freeze. After passing nitrogen gas for 20 min, add 0.4 parts of azobisisobutyronitrile, 0.07 parts of sodium hypophosphite, 0.004 parts of tert-butyl hydroperoxide, and 0.004 parts of sodium sulfite in sequence. Stop passing nitrogen gas after the sticky fibers appear. After the reaction is completed and kept warm for 2 h, take out the gel block, granulate, dry, and grind it.

[0055] (3) Mix 1050 parts of pure water, 280 parts of acrylamide, and 100 parts of [EMIM]Cl evenly and freeze. After passing nitrogen gas for 20 min, add 0.4 parts of azobisisobutyronitrile, 0.05 parts of sodium hypophosphite, 0.004 parts of tert-butyl hydroperoxide, and 0.004 parts of sodium sulfite in sequence. Stop passing nitrogen gas after the sticky fibers appear. After the reaction is completed and kept at the temperature for 2 h, take out the gel block, granulate, dry, and grind it.

[0056] (4) Prepare 20 g / L mother liquors of cationic polyacrylamide with different cations and different ionic degrees from steps (1)-(3) above, and name them Example 3-1, Example 3-2, and Example 3-3 respectively. The polymerization reaction formula of cationic polyacrylamide is: .

[0057] Treatment of black liquor from wheat straw pulping:

[0058] (5) Take 200 mL of straw pulping black liquor and put it in a beaker. Adjust the pH to between 8 and 10 with hydrochloric acid and wait for use.

[0059] (6) Take 8 mL of the mother liquor from Example 3-1 and add it to the beaker containing the black liquor from step (5). Stir quickly at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir slowly for 20 min. Let it stand and settle for 2 h. Finally, use a centrifuge to separate the lignin. The centrifuge speed is 7000 r / min and the time is 10 min. Take the solid component and dry it in a vacuum oven at 50 ℃ for 6 h to obtain component C1.

[0060] (7) Take 6 mL of the mother liquor from Example 3-2 and add it to a beaker containing the supernatant liquid after centrifugation in step (6). Stir quickly at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir slowly for 20 min. Let it stand and settle for 2 h. Finally, use a centrifuge to separate the lignin at 7000 r / min for 10 min. Take the solid component and dry it in a vacuum oven at 50 ℃ for 6 h to obtain component C2.

[0061] (8) Take 4 mL of the mother liquor from Example 3-3 and add it to a beaker containing the supernatant liquid after centrifugation in step (7). Stir quickly at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir slowly for 20 min. Let it stand and settle for 2 h. Finally, use a centrifuge to separate the lignin at 7000 r / min for 10 min. Take the solid component and dry it in a vacuum oven at 50 ℃ for 6 h to obtain component C3.

[0062] (9) The molecular weight ranges and hydrophilic / hydrophobic components of components C1, C2, and C3 were determined using ultrafiltration and resin adsorption methods, respectively. See the attached graph for the different molecular weight ranges and contents of different hydrophilic / hydrophobic components of C1, C2, and C3. Figure 3 .

[0063] Example 4: Synthesis of cationic polyacrylamides with different cationicities and cationic degrees:

[0064] (1) Mix 1050 parts of pure water, 280 parts of acrylamide, and 50 parts of ATMAC evenly and freeze. After passing nitrogen gas for 20 min, add 0.4 parts of azobisisobutyronitrile, 0.1 parts of sodium hypophosphite, 0.004 parts of tert-butyl hydroperoxide, and 0.004 parts of sodium sulfite in sequence. Stop passing nitrogen gas after the sticky fibers appear. After the reaction is completed and kept warm for 2 h, take out the gel block, granulate, dry, and grind it.

[0065] (2) Mix 1050 parts of pure water, 280 parts of acrylamide, and 80 parts of [EMIM]Cl evenly and freeze. After passing nitrogen gas for 20 min, add 0.4 parts of azobisisobutyronitrile, 0.07 parts of sodium hypophosphite, 0.004 parts of tert-butyl hydroperoxide, and 0.004 parts of sodium sulfite in sequence. After the mixture becomes sticky, stop passing nitrogen gas. After the reaction is completed, keep it warm for 2 h, take out the gel block, granulate, dry, and grind it.

[0066] (3) Mix 1050 parts of pure water, 280 parts of acrylamide, and 100 parts of DMC evenly and freeze. After passing nitrogen gas for 20 min, add 0.4 parts of azobisisobutyronitrile, 0.05 parts of sodium hypophosphite, 0.004 parts of tert-butyl hydroperoxide, and 0.004 parts of sodium sulfite in sequence. Stop passing nitrogen gas after the sticky fibers appear. After the reaction is completed and kept at the temperature for 2 h, take out the gel block, granulate, dry, and grind it.

[0067] (4) Prepare 20 g / L mother liquors of cationic polyacrylamide with different cations and different ionic degrees from steps (1)-(3) above, and name them Example 4-1, Example 4-2, and Example 4-3 respectively. The polymerization reaction formulas of cationic polyacrylamide with different cations and different ionic degrees from steps (1)-(3) above are as follows: .

[0068] Treatment of black liquor from wheat straw pulping:

[0069] (5) Take 200 mL of straw pulping black liquor and put it in a beaker. Adjust the pH to between 8 and 10 with hydrochloric acid and wait for use.

[0070] (6) Take 8 mL of the mother liquor from Example 4-1 and add it to the beaker containing the black liquor from step (5). Stir quickly at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir slowly for 20 min. Let it stand and settle for 2 h. Finally, use a centrifuge to separate the lignin. The centrifuge speed is 7000 r / min and the time is 10 min. Take the solid component and dry it in a vacuum oven at 50 ℃ for 6 h to obtain component C1.

[0071] (7) Take 6 mL of the mother liquor from Example 4-2 and add it to a beaker containing the supernatant liquid after centrifugation in step (6). Stir quickly at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir slowly for 20 min. Let it stand and settle for 2 h. Finally, use a centrifuge to separate the lignin at 7000 r / min for 10 min. Take the solid component and dry it in a vacuum oven at 50 ℃ for 6 h to obtain component C2.

[0072] (8) Take 4 mL of the mother liquor from Example 4-3 and add it to a beaker containing the supernatant liquid after centrifugation in step (7). Stir quickly at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir slowly for 20 min. Let it stand and settle for 2 h. Finally, use a centrifuge to separate the lignin at 7000 r / min for 10 min. Take the solid component and dry it in a vacuum oven at 50 ℃ for 6 h to obtain component C3.

[0073] (9) The molecular weight ranges and hydrophilic / hydrophobic components of components C1, C2, and C3 were determined using ultrafiltration and resin adsorption methods, respectively. See the attached graph for the different molecular weight ranges and contents of different hydrophilic / hydrophobic components of C1, C2, and C3. Figure 4 .

[0074] Example 5: Synthesis of cationic polyacrylamides with different cationicities and cationic degrees:

[0075] (1) Mix 1050 parts of pure water, 280 parts of acrylamide, and 50 parts of [EMIM]Cl evenly and freeze. After passing nitrogen gas for 20 min, add 0.4 parts of azobisisobutyronitrile, 0.1 parts of sodium hypophosphite, 0.004 parts of tert-butyl hydroperoxide, and 0.004 parts of sodium sulfite in sequence. Stop passing nitrogen gas after the sticky fibers appear. After the reaction is completed and kept at the temperature for 2 h, take out the gel block, granulate, dry, and grind it.

[0076] (2) Mix 1050 parts of pure water, 280 parts of acrylamide, and 80 parts of DAC evenly and freeze. After passing nitrogen gas for 20 min, add 0.4 parts of azobisisobutyronitrile, 0.07 parts of sodium hypophosphite, 0.004 parts of tert-butyl hydroperoxide, and 0.004 parts of sodium sulfite in sequence. Stop passing nitrogen gas after the sticky threads appear. After the reaction is completed and kept at the temperature for 2 h, take out the gel block, granulate, dry, and grind it.

[0077] (3) Mix 1050 parts of pure water, 280 parts of acrylamide, and 100 parts of ATMAC evenly and freeze. After passing nitrogen gas for 20 min, add 0.4 parts of azobisisobutyronitrile, 0.05 parts of sodium hypophosphite, 0.004 parts of tert-butyl hydroperoxide, and 0.004 parts of sodium sulfite in sequence. Stop passing nitrogen gas after the sticky threads appear. After the reaction is completed and kept warm for 2 h, take out the gel block, granulate, dry, and grind it.

[0078] (4) Prepare 20 g / L mother liquors of cationic polyacrylamide with different cations and different ionic degrees from steps (1)-(3) above, and name them Example 5-1, Example 5-2, and Example 5-3 respectively. The polymerization reaction formula of cationic polyacrylamide is: .

[0079] Treatment of black liquor from wheat straw pulping:

[0080] (5) Take 200 mL of straw pulping black liquor and put it in a beaker. Adjust the pH to between 8 and 10 with hydrochloric acid and wait for use.

[0081] (6) Take 8 mL of the mother liquor from Example 5-1 and add it to the beaker containing the black liquor from step (5). Stir quickly at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir slowly for 20 min. Let it stand and settle for 2 h. Finally, use a centrifuge to separate the lignin. The centrifuge speed is 7000 r / min and the time is 10 min. Take the solid component and dry it in a vacuum oven at 50 ℃ for 6 h to obtain component C1.

[0082] (7) Take 6 mL of the mother liquor from Example 5-2 and add it to a beaker containing the supernatant liquid after centrifugation in step (6). Stir quickly at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir slowly for 20 min. Let it stand and settle for 2 h. Finally, use a centrifuge to separate the lignin at 7000 r / min for 10 min. Take the solid component and dry it in a vacuum oven at 50 ℃ for 6 h to obtain component C2.

[0083] (8) Take 4 mL of the mother liquor from Example 5-3 and add it to a beaker containing the supernatant liquid after centrifugation in step (7). Stir quickly at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir slowly for 20 min. Let it stand and settle for 2 h. Finally, use a centrifuge to separate the lignin at 7000 r / min for 10 min. Take the solid component and dry it in a vacuum oven at 50 ℃ for 6 h to obtain component C3.

[0084] (9) The molecular weight ranges and hydrophilic / hydrophobic components of components C1, C2, and C3 were determined using ultrafiltration and resin adsorption methods, respectively. See the attached graph for the different molecular weight ranges and contents of different hydrophilic / hydrophobic components of C1, C2, and C3. Figure 5 .

[0085] Comparative Example 1:

[0086] Synthesis of cationic polyacrylamides with different cationic degrees:

[0087] (1) Mix 1050 parts of pure water, 280 parts of acrylamide, and 50 parts of DMC evenly and freeze. After passing nitrogen gas for 20 min, add 0.4 parts of azobisisobutyronitrile, 0.1 parts of sodium hypophosphite, 0.004 parts of tert-butyl hydroperoxide, and 0.004 parts of sodium sulfite in sequence. Stop passing nitrogen gas after the sticky fibers appear. After the reaction is completed and kept warm for 2 h, take out the gel block, granulate, dry, and grind it.

[0088] (2) Mix 1050 parts of pure water, 280 parts of acrylamide, and 80 parts of DMC evenly and freeze. After passing nitrogen gas for 20 min, add 0.4 parts of azobisisobutyronitrile, 0.07 parts of sodium hypophosphite, 0.004 parts of tert-butyl hydroperoxide, and 0.004 parts of sodium sulfite in sequence. Stop passing nitrogen gas after the sticky threads appear. After the reaction is completed and kept at the temperature for 2 h, take out the gel block, granulate, dry, and grind it.

[0089] (3) Mix 1050 parts of pure water, 280 parts of acrylamide, and 100 parts of DMC evenly and freeze. After passing nitrogen gas for 20 min, add 0.4 parts of azobisisobutyronitrile, 0.05 parts of sodium hypophosphite, 0.004 parts of tert-butyl hydroperoxide, and 0.004 parts of sodium sulfite in sequence. Stop passing nitrogen gas after the sticky fibers appear. After the reaction is completed and kept at the temperature for 2 h, take out the gel block, granulate, dry, and grind it.

[0090] (4) Prepare 20 g / L mother liquors of cationic polyacrylamide with different ionic degrees from steps (1)-(3) above, and name them Comparative Example 1-1, Comparative Example 1-2, and Comparative Example 1-3, respectively. The polymerization reaction formula of cationic polyacrylamide is: .

[0091] Treatment of black liquor from wheat straw pulping:

[0092] (5) Take 200 mL of straw pulping black liquor and put it in a beaker. Adjust the pH to between 8 and 10 with hydrochloric acid and wait for use.

[0093] (6) Take 8 mL of the mother liquor of Comparative Example 1-1 and add it to the beaker containing the black liquor of step (5). Stir quickly at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir slowly for 20 min. Let it stand and settle for 2 h. Finally, use a centrifuge to separate the lignin. The centrifuge speed is 7000 r / min and the time is 10 min. Take the solid component and dry it in a vacuum oven at 50 ℃ for 6 h to obtain component C1.

[0094] (7) Take 6 mL of the mother liquor of Comparative Example 1-2 and add it to the beaker containing the liquid above the centrifuged liquid in step (6). Stir quickly at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir slowly for 20 min. Let it stand and settle for 2 h. Finally, use a centrifuge to separate the lignin. The centrifuge speed is 7000 r / min and the time is 10 min. Take the solid component and dry it in a vacuum oven at 50 ℃ for 6 h to obtain component C2'.

[0095] (8) Take 4 mL of the mother liquor of Comparative Example 1-3 and add it to a beaker containing the liquid above the centrifuged liquid in step (7). Stir quickly at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir slowly for 20 min. Let it stand and settle for 2 h. Finally, use a centrifuge to separate the lignin. The centrifuge speed is 7000 r / min and the time is 10 min. Take the solid component and dry it in a vacuum oven at 50 ℃ for 6 h to obtain component C3'.

[0096] (9) The molecular weight ranges and hydrophilic / hydrophobic components of C1, C2', and C3' were determined using ultrafiltration and resin adsorption methods, respectively. See the attached graph for the content of different molecular weight ranges and different hydrophilic / hydrophobic components of C1, C2', and C3'. Figure 6 .

[0097] Comparative Example 2:

[0098] The difference between Comparative Example 2 and Example 1 is that ordinary nonionic polyacrylamide was used to classify the straw pulping black liquor in steps (5)-(9) to obtain components C1', C2', and C3'. Ultrafiltration membrane filtration and resin adsorption were used to determine the molecular weight range and the content of hydrophilic and hydrophobic components of components C1', C2', and C3'. The graphs of different molecular weight ranges and the contents of different hydrophilic and hydrophobic components of C1', C2', and C3' are shown in Appendix 1. Figure 7 .

[0099] Comparative Example 3:

[0100] Synthesis of cationic polyacrylamide:

[0101] (1) Mix 1050 parts of pure water, 280 parts of acrylamide, and 50 parts of DMC evenly and freeze. After passing nitrogen gas for 20 min, add 0.4 parts of azobisisobutyronitrile, 0.1 parts of sodium hypophosphite, 0.004 parts of tert-butyl hydroperoxide, and 0.004 parts of sodium sulfite in sequence. Stop passing nitrogen gas after the sticky fibers appear. After the reaction is completed and kept warm for 2 h, take out the gel block, granulate, dry, and grind it.

[0102] (2) Prepare a 20 g / L mother liquor of cationic polyacrylamide, named Comparative Example 3-1. The polymerization reaction formula of cationic polyacrylamide is: .

[0103] Treatment of black liquor from wheat straw pulping:

[0104] (3) Take 200 mL of straw pulping black liquor and put it in a beaker. Adjust the pH to between 8 and 10 with hydrochloric acid and wait for use.

[0105] (4) Take 8 mL of the mother liquor of Comparative Example 3-1 and add it to the beaker containing the black liquor of step (3). Stir quickly at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir slowly for 20 min. Let it stand and settle for 2 h. Finally, use a centrifuge to separate the lignin. The centrifuge speed is 7000 r / min and the time is 10 min. Take the solid component and dry it in a vacuum oven at 50 ℃ for 6 h to obtain component C1.

[0106] (5) Take 6 mL of the mother liquor of Comparative Example 3-1 and add it to a beaker containing the supernatant liquid after centrifugation in step (4). Stir quickly at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir slowly for 20 min. Let it stand and settle for 2 h. Finally, use a centrifuge to separate the lignin. The centrifuge speed is 7000 r / min and the time is 10 min. Take the solid component and dry it in a vacuum oven at 50 ℃ for 6 h to obtain component C2'.

[0107] (6) Take 4 mL of the mother liquor of Comparative Example 3-1 and add it to a beaker containing the supernatant liquid after centrifugation in step (5). Stir quickly at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir slowly for 20 min. Let it stand and settle for 2 h. Finally, use a centrifuge to separate the lignin. The centrifuge speed is 7000 r / min and the time is 10 min. Take the solid component and dry it in a vacuum oven at 50 ℃ for 6 h to obtain component C3'.

[0108] (7) The molecular weight ranges and hydrophilic / hydrophobic components of C1, C2', and C3' were determined using ultrafiltration and resin adsorption methods, respectively. See the attached graph for the content of different molecular weight ranges and different hydrophilic / hydrophobic components of C1, C2', and C3'. Figure 8 .

[0109] By comparing the TOC of the raw black liquor and the TOC of the final supernatant in Examples 1-5 and Comparative Examples 1-3, the results showed that Examples 1-5 can effectively separate lignin from straw pulping black liquor, with a separation rate (TOC... 初始黑液 –TOC 最终离心分离后上清液 ) / TOC 初始黑液 Approximately 70% (see attached image) Figure 9Comparative Example 1 showed a slightly lower separation rate than the Example 1 because using the same cationic polyacrylamide would result in flocculant residue in the supernatant after separation, and continued addition of the same cationic polyacrylamide would inhibit the flocculation effect to some extent. Comparative Examples 2-3 had a low separation rate (approximately 40%), making it difficult to effectively separate lignin from straw pulping black liquor. Furthermore, the flocculation method for classifying straw pulping lignin using cationic polyacrylamide as a flocculant preferentially separates the high molecular weight, strongly hydrophobic lignin, while the lower molecular weight and more hydrophilic fractions require the use of polyacrylamide with higher molecular weight and higher cationicity. Therefore, Examples 1-5 all showed good separation effects on straw pulping lignin, while Comparative Example 2 used ordinary nonionic polyacrylamide, which had difficulty effectively capturing lignin molecules during flocculation, resulting in a significantly reduced component content. Therefore, it could not be used in large quantities. Comparative Example 3 effectively separated high molecular weight and strongly hydrophobic lignin. However, the subsequently added polyacrylamide had a lower molecular weight and cationicity, and could not capture low molecular weight and hydrophilic lignin. Therefore, the effect on subsequent lignin separation was not significant. In summary, the sequential addition of polyacrylamide with varying cationicity can effectively classify lignin from wheat straw pulping and improve the comprehensive utilization rate of black liquor.

Claims

1. A method for lignin fractionation in wheat straw pulping black liquor using a series of cationic polyacrylamides, characterized in that... Includes the following steps: Step 1: Synthesis of polyacrylamide with different cations and different cationic degrees: (1) The raw materials for synthesis include: pure water, acrylamide, cationic monomer, initiator, chain transfer agent, oxidant and reducing agent; wherein the cationic monomer is one of methacryloyloxyethyltrimethylammonium chloride (DMC), acryloyloxyethyltrimethylammonium chloride (DAC), methacrylamidopropyltrimethylammonium chloride (MAPTAC), allyltrimethylammonium chloride (ATMAC), and N-trimethyl-(4-vinylbenzyl)ammonium chloride ([EMIM]Cl), the initiator is azobisisobutyronitrile, the chain transfer agent is sodium hypophosphite, the oxidant is tert-butyl hydroperoxide, and the reducing agent is sodium sulfite; (2) Synthesis steps: By mass, mix 1000-1100 parts of pure water, 250-280 parts of acrylamide, and 50-100 parts of cationic monomer evenly and freeze. After purging with nitrogen for 15-20 min, add 0.3-0.5 parts of initiator, 0.05-0.1 parts of chain transfer agent, 0.002-0.006 parts of oxidant, and 0.002-0.006 parts of reducing agent in sequence. After the mixture becomes sticky, stop purging with nitrogen. After the reaction is completed, keep it warm for 1-2 h, take out the gel block, granulate, dry, and grind it. Step 2: Take 200 mL of wheat straw pulping black liquor and place it in a beaker. Adjust the pH to between 8 and 10 using hydrochloric acid. Set aside for use. Step 3: Prepare a mother liquor of 15-20 g / L using polyacrylamide with different cations and different cationic degrees from step 1 (2); Step 4: Add 4-10 mL of the polyacrylamide mother liquor from Step 3 to a beaker containing the black liquor from Step 2, and stir rapidly at 200-220 r / min for 5-8 min. Then reduce the speed to 50-70 r / min and stir slowly for 15-20 min, and let it stand to settle for 1-2 h. Finally, use a centrifuge to separate the lignin at 5000-7000 r / min for 10-15 min. Take the solid component and dry it in a vacuum oven at 40-60℃ for 5-8 h to obtain component C1. Step 5: Add 4-10 mL of the polyacrylamide mother liquor from Step 3 to a beaker containing the supernatant liquid from Step 4 after centrifugation, and stir rapidly at 200-220 r / min for 5-8 min. Then reduce the speed to 50-70 r / min and stir slowly for 15-20 min, and let it stand to settle for 1-2 h. Finally, use a centrifuge to separate the lignin at 5000-7000 r / min for 10-15 min. Take the solid component and dry it in a vacuum oven at 40-60℃ for 5-8 h to obtain component C2. Step 6: Add 4-10 mL of the polyacrylamide mother liquor from Step 3 to a beaker containing the supernatant liquid from Step 5 after centrifugation, and stir rapidly at 200-220 r / min for 5-8 min. Then reduce the speed to 50-70 r / min and stir slowly for 15-20 min, and let it stand to settle for 1-2 h. Finally, use a centrifuge to separate the lignin at 5000-7000 r / min for 10-15 min. Take the solid component and dry it in a vacuum oven at 40-60℃ for 5-8 h to obtain component C3. In steps four, five, and six, the molecular weight and cationicity of the polyacrylamide mother liquor used in step three increase sequentially.

2. A method for lignin fractionation in wheat straw pulping black liquor using a series of cationic polyacrylamides according to claim 1, characterized in that: The lignin components C1, C2, and C3 with different molecular weights obtained in the six steps were sequentially passed through ultrafiltration membranes of 5000 Da and 1000 Da to obtain the contents of C1, C2, and C3 in < 1000 Da, 1000 Da-5000 Da, and > 5000 Da.

3. The method for lignin fractionation in wheat straw pulping black liquor using a series of cationic polyacrylamides according to claim 1, characterized in that: The six steps yielded lignin components C1, C2, and C3 with different molecular weights. The adsorption methods of the specified resins (DAX-8 and XAD-4) were used to calculate the total organic carbon (TOC) and obtain the contents of different hydrophilic and hydrophobic components of C1, C2, and C3.

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