Method for grading lignin in wheat straw pulping black liquor by using series cationic polyacrylamide

Abstract

The invention discloses a method for grading lignin in wheat straw pulping black liquor by using a series of cationic polyacrylamide, which synthesizes a series of polyacrylamide containing different cations and different cationic degrees through a copolymerization method. By sequentially adding polyacrylamide with different cationic degrees and centrifugally separating lignin with different components, the effect of fractional precipitation of lignin with different molecular weights and hydrophilic and hydrophobic properties in the black liquor is achieved, so that lignin components with different molecular weight intervals and different hydrophilic and hydrophobic properties are obtained, and subsequent utilization of lignin products is facilitated.

Description

Technical Field

[0001] The invention relates to the field of comprehensive resource recycling, and in particular to a method for classifying lignin in wheat straw pulping black liquor by using a series of cationic polyacrylamides. Background Art

[0002] In today's context where environmental protection and resource recycling are increasingly valued, how to efficiently and environmentally friendly treat the black liquor produced during wheat straw pulping has become an urgent problem to be solved in the field of papermaking and comprehensive utilization of agricultural waste. Black liquor, as a by-product of the pulping process, is rich in organic matter such as lignin, hemicellulose and a small amount of cellulose. Direct discharge will not only cause serious pollution to the water environment, but also waste the useful resources therein. Therefore, it is particularly important to develop a technology that can effectively remove pollutants from black liquor and recover valuable chemical components therein. As a low-cost, easy-to-operate and effective physical and chemical treatment method, flocculation has gradually attracted attention in the field of wheat straw pulping black liquor treatment.

[0003] Wheat straw pulping black liquor has the characteristics of 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 difficulty in degradation, and it is difficult to remove it efficiently by traditional treatment methods. However, after it is graded to obtain lignin with narrow molecular weight distribution and hydrophilicity distribution, it can be turned into treasure and can be comprehensively utilized. Therefore, it is key to find a method that can effectively break the structure of lignin and promote its aggregation and precipitation.

[0004] The flocculation method mainly involves adding a specific flocculant to the black liquor to destabilize the dissolved lignin and suspended particles, aggregate them into larger flocs, and then achieve solid-liquid separation by sedimentation or floating. Commonly used flocculants include inorganic flocculants (such as polyaluminium chloride, aluminium sulfate, etc.), organic polymer flocculants (such as polyacrylamide) and biological flocculants. Selecting a suitable flocculant and optimizing the flocculation conditions are the key to improving treatment efficiency. However, it is difficult to effectively grade wheat straw pulping black liquor with a one-time addition of conventional flocculants, and the comprehensive utilization rate of black liquor is low. Summary of the invention

[0005] The technical problem to be solved by the present invention is that, for highly alkaline wheat straw pulping black liquor, although the acid precipitation technology is mature, the acid dosage is large, the equipment requirements are high, and it is difficult to meet environmental emission standards. The addition of too much acid will also produce a large amount of salt, which will increase the difficulty of subsequent treatment processes. The present invention synthesizes a series of cationic polyacrylamides, which sequentially grade and flocculate the wheat straw pulping black liquor, and separate them, so as to achieve the effect of graded precipitation of lignin with different molecular weights and hydrophilicity in the black liquor, which is beneficial to the subsequent utilization of lignin products.

[0006] The present invention provides a method for classifying lignin in wheat straw pulping black liquor using a series of cationic polyacrylamides, comprising the following steps: Step 1: Synthesis of polyacrylamide with different cations and different cationic degrees (1) The synthetic raw materials include: pure water, acrylamide, cationic monomer, initiator, chain transfer agent, oxidant, and reducing agent; wherein the cationic monomer is one of methacryloyloxyethyl trimethyl ammonium chloride (DMC), acryloyloxyethyl trimethyl ammonium chloride (DAC), methacrylamide propyl trimethyl ammonium chloride (MAPTAC), allyl trimethyl ammonium 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 weight, 1000-1100 parts of pure water, 250-280 parts of acrylamide, and 50-100 parts of cationic monomer are mixed evenly and then frozen. After nitrogen is introduced for 15-20 minutes, 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 are added in sequence. After sticky fibers are formed, the introduction of nitrogen is stopped. After the reaction is completed and the mixture is kept warm for 1-2 hours, the rubber block is taken out, granulated, dried, and ground; Step 2: Take 200 mL of wheat straw pulping black liquor and place it in a beaker, use hydrochloric acid to adjust the pH to between 8 and 10, and wait for use; Step 3: Prepare 15-20 g / L mother solution of polyacrylamide with different cations and different cationicity from step 1 (2); Step 4: Take 4-10 mL of the polyacrylamide mother liquor of the third step and add it to the beaker containing the black liquor of the second step, and stir it at 200-220 r / min for 5-8 min, then reduce the speed to 50-70 r / min and stir it slowly for 15-20 min, and let it settle for 1-2 h. Finally, use a centrifuge to separate lignin at a speed of 5000-7000 r / min for 10-15 min. Take the solid component and dry it in a vacuum oven at 40-60°C for 5-8 h to obtain component C1; Step 5: Take 4-10 mL of the polyacrylamide mother solution in the third step and add it to the beaker containing the upper liquid after centrifugation in the fourth step, and stir it at 200-220 r / min for 5-8 min, then reduce the speed to 50-70 r / min and stir it slowly for 15-20 min, and let it settle for 1-2 h. Finally, use a centrifuge to separate lignin at a speed of 5000-7000 r / min for 10-15 min. Take the solid component and dry it in a vacuum oven at 40-60°C for 5-8 h to obtain component C2; Step 6: Take 4-10 mL of the polyacrylamide mother liquor in the third step and add it to the beaker containing the upper liquid after centrifugation in the fifth step, and stir it at 200-220 r / min for 5-8 min, then reduce the speed to 50-70 r / min and stir it slowly for 15-20 min, and let it stand and settle for 1-2 h. Finally, use a centrifuge to separate lignin at a speed of 5000-7000 r / min for 10-15 min. Take the solid component and dry it in a vacuum oven at 40-60°C for 5-8 h to obtain component C3.

[0007] Compared with the prior art, the present invention has the following benefits: (1) The present invention synthesizes a series of cationic polyacrylamides to treat wheat straw pulping black liquor. The product can be operated without excessive acid neutralization and alkalinity, which greatly reduces the requirements for equipment, and also reduces the operating cost and operating risk.

[0008] (2) The flocculation method can be used to grade wheat straw lignin. The high molecular weight and high hydrophobicity lignin can be flocculated and settled first, while the low molecular weight and hydrophilic part can be further flocculated and settled in the subsequent flocculation process, thereby achieving the purpose of grading wheat straw lignin.

[0009] (3) By continuously adding a series of cationic polyacrylamides to separate different components of lignin, lignin components with different molecular weights and different hydrophilicity and hydrophobicity can be obtained, which effectively improves the availability of lignin after classification.

[0010] The invention obtains a series of different cationic polyacrylamides by copolymerizing acrylamide and different cationic monomers, obtains a series of polyacrylamides with different cationic degrees by copolymerizing acrylamide and cationic monomers, or obtains a series of polyacrylamides with different cations and different cationic degrees by copolymerizing acrylamide and different cationic monomers, adds polyacrylamides in sequence, performs sedimentation separation to obtain lignin with narrow molecular weight distribution and different hydrophilicity, and then continues to add cationic polyacrylamide in batches and performs sedimentation separation, thereby effectively classifying wheat straw pulping black liquor and achieving the purpose of comprehensive utilization of the black liquor. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 This is a graph showing the contents of components with different molecular weight ranges and different hydrophilicity and hydrophobicity in Example 1; Figure 2 This is a graph showing the contents of components with different molecular weight ranges and different hydrophilicity and hydrophobicity in Example 2; Figure 3 This is a graph showing the contents of components with different molecular weight ranges and different hydrophilicity and hydrophobicity in Example 3; Figure 4 This is a graph showing the contents of components with different molecular weight ranges and different hydrophilicity and hydrophobicity in Example 4; Figure 5 This is a graph showing the contents of components with different molecular weight ranges and different hydrophilicity and hydrophobicity in Example 5; Figure 6 The figure is a graph showing the contents of components with different molecular weight ranges and different hydrophilicity and hydrophobicity in Comparative Example 1; Figure 7 It is the content diagram of components with different molecular weight ranges and different hydrophilicity in Comparative Example 2; Figure 8 It is the content diagram of components with different molecular weight ranges and different hydrophilicity in comparative example 3; Fig. 9 This is a graph showing the separation rate of lignin in straw pulping black liquor in Examples 1-5 and Comparative Examples 1-3. DETAILED DESCRIPTION

[0012] The following are specific embodiments of the present invention, which specifically describe the operating scheme of the present invention. However, the protection scope of this patent includes but is not limited to these embodiments. All other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this patent.

[0013] Example 1: Synthesis of cationic polyacrylamides with different cations and cationic degrees: (1) Mix 1050 parts by weight of pure water, 280 parts of acrylamide, and 50 parts of DMC evenly, freeze them, and introduce nitrogen for 20 min. Then, 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 introducing nitrogen when sticky threads appear. After the reaction is completed and keep warm for 2 h, take out the rubber block, granulate it, dry it, and grind it.

[0014] (2) Mix 1050 parts of pure water, 280 parts of acrylamide, and 80 parts of DAC evenly, freeze them, and introduce nitrogen for 20 min. Then, 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 introducing nitrogen when sticky threads appear. After the reaction is completed and keep warm for 2 h, take out the rubber block, granulate it, dry it, and grind it.

[0015] (3) Mix 1050 parts by weight of pure water, 280 parts of acrylamide, and 100 parts of MAPTAC evenly, freeze them, and introduce nitrogen for 20 min. Then, 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 introducing nitrogen when sticky threads appear. After the reaction is completed and keep warm for 2 h, take out the rubber blocks, granulate them, dry them, and grind them.

[0016] (4) The cationic polyacrylamides with different cations and different ionicities in the above steps (1) to (3) are prepared into 20 g / L mother liquors, which are named as Example 1-1, Example 1-2 and Example 1-3, respectively. The polymerization reaction formula of the cationic polyacrylamide is: .

[0017] Treatment of black liquor from wheat straw pulping: (5) Take 200 mL of straw pulping black liquor and place it in a beaker. Use hydrochloric acid to adjust the pH to between 8 and 10 for later use.

[0018] (6) 8 mL of the mother liquor of Example 1-1 was added to the beaker containing the black liquor of step (5), and stirred at 200 r / min for 8 min. The speed was then reduced to 50 r / min and stirred slowly for 20 min. The mixture was allowed to settle for 2 h. Finally, lignin was separated by centrifuge at a speed of 7000 r / min for 10 min. The solid component was dried in a vacuum oven at 50 °C for 6 h to obtain component C1.

[0019] (7) Take 6 mL of the mother liquor of Example 1-2 and add it to the beaker containing the upper liquid after centrifugation in step (6), and stir it at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir it slowly for 20 min, and let it settle for 2 h. Finally, use a centrifuge to separate lignin at a speed of 7000 r / min for 10 min. Take the solid component and dry it in a vacuum oven at 50 °C for 6 h to obtain component C2.

[0020] (8) 4 mL of the mother liquor of Example 1-3 was added to the beaker containing the upper layer of liquid after centrifugation in step (7), and stirred at 200 r / min for 8 min. The speed was then reduced to 50 r / min and stirred slowly for 20 min. The mixture was allowed to settle for 2 h. Finally, lignin was separated by centrifuge at a speed of 7000 r / min for 10 min. The solid component was dried in a vacuum oven at 50 °C for 6 h to obtain component C3.

[0021] (9) The molecular weight ranges and the contents of hydrophilic and hydrophobic components of C1, C2 and C3 were determined by ultrafiltration membrane filtration and resin adsorption methods, respectively. The different molecular weight ranges and contents of hydrophilic and hydrophobic components of C1, C2 and C3 are shown in the attached figure. Figure 1 .

[0022] Example 2: Synthesis of cationic polyacrylamides with different cations and cationic degrees: (1) 1050 parts by weight of pure water, 280 parts of acrylamide, and 50 parts of DAC were mixed evenly and frozen. After nitrogen was introduced for 20 min, 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 were added in sequence. When sticky threads appeared, the introduction of nitrogen was stopped. After the reaction was completed and kept warm for 2 h, the rubber blocks were taken out, granulated, dried, and ground.

[0023] (2) Mix 1050 parts by weight of pure water, 280 parts of acrylamide, and 80 parts of MAPTAC evenly, freeze them, and introduce nitrogen for 20 min. Then, 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 introducing nitrogen when sticky threads appear. After the reaction is completed and keep warm for 2 h, take out the rubber blocks, granulate them, dry them, and grind them.

[0024] (3) 1050 parts by weight of pure water, 280 parts of acrylamide, and 100 parts of ATMAC were mixed evenly and then frozen. After nitrogen was introduced for 20 min, 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 were added in sequence. When sticky threads appeared, the introduction of nitrogen was stopped. After the reaction was completed and kept warm for 2 h, the rubber blocks were taken out, granulated, dried, and ground.

[0025] (4) The cationic polyacrylamide with different cations and different ionicity in the above steps (1) to (3) was prepared into 20 g / L mother liquor, which were named as Example 2-1, Example 2-2 and Example 2-3 respectively. The polymerization reaction formula of cationic polyacrylamide is: .

[0026] Treatment of black liquor from wheat straw pulping: (5) Take 200 mL of straw pulping black liquor and place it in a beaker. Use hydrochloric acid to adjust the pH to between 8 and 10 for later use.

[0027] (6) 8 mL of the mother liquor of Example 2-1 was added to the beaker containing the black liquor of step (5), and stirred at 200 r / min for 8 min. The speed was then reduced to 50 r / min and stirred slowly for 20 min. The mixture was allowed to settle for 2 h. Finally, lignin was separated by centrifuge at a speed of 7000 r / min for 10 min. The solid component was dried in a vacuum oven at 50 °C for 6 h to obtain component C1.

[0028] (7) Take 6 mL of the mother liquor of Example 2-2 and add it to the beaker containing the upper liquid after centrifugation in step (6), and stir it at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir it slowly for 20 min, and let it settle for 2 h. Finally, use a centrifuge to separate lignin at a speed of 7000 r / min for 10 min. Take the solid component and dry it in a vacuum oven at 50 °C for 6 h to obtain component C2.

[0029] (8) 4 mL of the mother liquor of Example 2-3 was added to the beaker containing the upper layer of liquid after centrifugation in step (7), and stirred at 200 r / min for 8 min. The speed was then reduced to 50 r / min and stirred slowly for 20 min. The mixture was allowed to settle for 2 h. Finally, lignin was separated by centrifuge at a speed of 7000 r / min for 10 min. The solid component was dried in a vacuum oven at 50 °C for 6 h to obtain component C3.

[0030] (9) The molecular weight ranges and the contents of hydrophilic and hydrophobic components of C1, C2 and C3 were determined by ultrafiltration membrane filtration and resin adsorption methods, respectively. The different molecular weight ranges and contents of hydrophilic and hydrophobic components of C1, C2 and C3 are shown in the attached figure. Figure 2 .

[0031] Example 3: Synthesis of cationic polyacrylamides with different cations and cationic degrees: (1) Mix 1050 parts by weight of pure water, 280 parts of acrylamide, and 50 parts of MAPTAC evenly, freeze them, and introduce nitrogen for 20 min. Then, 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 introducing nitrogen when sticky threads appear. After the reaction is completed and keep warm for 2 h, take out the rubber block, granulate it, dry it, and grind it.

[0032] (2) 1050 parts by weight of pure water, 280 parts of acrylamide, and 80 parts of ATMAC were mixed evenly and frozen. After nitrogen was introduced for 20 min, 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 were added in sequence. When sticky threads appeared, the introduction of nitrogen was stopped. After the reaction was completed and kept warm for 2 h, the rubber blocks were taken out, granulated, dried, and ground.

[0033] (3) Mix 1050 parts by weight of pure water, 280 parts of acrylamide, and 100 parts of [EMIM]Cl evenly, freeze the mixture, and introduce nitrogen for 20 min. Then, 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 introducing nitrogen when sticky threads appear. After the reaction is completed and the mixture is kept warm for 2 h, remove the rubber block, granulate it, dry it, and grind it.

[0034] (4) The cationic polyacrylamide with different cations and different ionicity in the above steps (1) to (3) was prepared into 20 g / L mother liquor, which were named as Example 3-1, Example 3-2 and Example 3-3 respectively. The polymerization reaction formula of cationic polyacrylamide is: .

[0035] Treatment of black liquor from wheat straw pulping: (5) Take 200 mL of straw pulping black liquor and place it in a beaker. Use hydrochloric acid to adjust the pH to between 8 and 10 for later use.

[0036] (6) 8 mL of the mother liquor of Example 3-1 was added to the beaker containing the black liquor of step (5), and stirred at 200 r / min for 8 min. The speed was then reduced to 50 r / min and stirred slowly for 20 min. The mixture was allowed to settle for 2 h. Finally, lignin was separated by centrifuge at a speed of 7000 r / min for 10 min. The solid component was dried in a vacuum oven at 50 °C for 6 h to obtain component C1.

[0037] (7) Take 6 mL of the mother liquor of Example 3-2 and add it to the beaker containing the upper liquid after centrifugation in step (6), and stir it at 200 r / min for 8 min. Then reduce the speed to 50 r / min and stir it slowly for 20 min. Let it stand and settle for 2 h. Finally, use a centrifuge to separate lignin at a speed of 7000 r / min for 10 min. Take the solid component and dry it in a vacuum oven at 50 °C for 6 h to obtain component C2.

[0038] (8) 4 mL of the mother liquor of Example 3-3 was added to the beaker containing the upper layer of liquid after centrifugation in step (7), and stirred at 200 r / min for 8 min. The speed was then reduced to 50 r / min and stirred slowly for 20 min. The mixture was allowed to settle for 2 h. Finally, lignin was separated by centrifuge at a speed of 7000 r / min for 10 min. The solid component was dried in a vacuum oven at 50 °C for 6 h to obtain component C3.

[0039] (9) The molecular weight ranges and the contents of hydrophilic and hydrophobic components of C1, C2 and C3 were determined by ultrafiltration membrane filtration and resin adsorption methods, respectively. The different molecular weight ranges and contents of hydrophilic and hydrophobic components of C1, C2 and C3 are shown in the attached figure. Figure 3 .

[0040] Example 4: Synthesis of cationic polyacrylamides with different cations and cationic degrees: (1) 1050 parts by weight of pure water, 280 parts of acrylamide, and 50 parts of ATMAC were mixed evenly and frozen. After nitrogen was introduced for 20 min, 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 were added in sequence. When sticky threads appeared, the introduction of nitrogen was stopped. After the reaction was completed and kept warm for 2 h, the rubber blocks were taken out, granulated, dried, and ground.

[0041] (2) Mix 1050 parts by weight of pure water, 280 parts by weight of acrylamide, and 80 parts by weight of [EMIM]Cl, freeze the mixture, and introduce nitrogen for 20 min. Then, 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 introducing nitrogen when sticky threads appear. After the reaction is completed and the mixture is kept warm for 2 h, remove the rubber block, granulate it, dry it, and grind it.

[0042] (3) Mix 1050 parts by weight of pure water, 280 parts by weight of acrylamide, and 100 parts by weight of DMC, freeze the mixture, and introduce nitrogen for 20 min. Then, 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 introducing nitrogen when sticky threads appear. After the reaction is completed and the mixture is kept warm for 2 h, remove the rubber block, granulate it, dry it, and grind it.

[0043] (4) The cationic polyacrylamides with different cations and different ionicities in the above steps (1) to (3) are prepared into 20 g / L mother liquors, which are named as Example 4-1, Example 4-2 and Example 4-3 respectively. The polymerization reaction formulas of the above steps (1) to (3) with different cations and different ionicities are as follows: .

[0044] Treatment of black liquor from wheat straw pulping: (5) Take 200 mL of straw pulping black liquor and place it in a beaker. Use hydrochloric acid to adjust the pH to between 8 and 10 for later use.

[0045] (6) 8 mL of the mother liquor of Example 4-1 was added to the beaker containing the black liquor of step (5), and stirred at 200 r / min for 8 min. The speed was then reduced to 50 r / min and stirred slowly for 20 min. The mixture was allowed to settle for 2 h. Finally, lignin was separated by centrifuge at a speed of 7000 r / min for 10 min. The solid component was dried in a vacuum oven at 50 °C for 6 h to obtain component C1.

[0046] (7) Take 6 mL of the mother liquor of Example 4-2 and add it to the beaker containing the upper liquid after centrifugation in step (6), and stir it at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir it slowly for 20 min, and let it settle for 2 h. Finally, use a centrifuge to separate lignin at a speed of 7000 r / min for 10 min. Take the solid component and dry it in a vacuum oven at 50 °C for 6 h to obtain component C2.

[0047] (8) 4 mL of the mother liquor of Example 4-3 was added to the beaker containing the upper layer of liquid after centrifugation in step (7), and stirred at 200 r / min for 8 min. The speed was then reduced to 50 r / min and stirred slowly for 20 min. The mixture was allowed to settle for 2 h. Finally, lignin was separated by centrifuge at a speed of 7000 r / min for 10 min. The solid component was dried in a vacuum oven at 50 °C for 6 h to obtain component C3.

[0048] (9) The molecular weight ranges and the contents of hydrophilic and hydrophobic components of C1, C2 and C3 were determined by ultrafiltration membrane filtration and resin adsorption methods, respectively. The different molecular weight ranges and contents of hydrophilic and hydrophobic components of C1, C2 and C3 are shown in the attached figure. Figure 4 .

[0049] Example 5: Synthesis of cationic polyacrylamides with different cations and cationic degrees: (1) Mix 1050 parts by weight of pure water, 280 parts by weight of acrylamide, and 50 parts by weight of [EMIM]Cl, freeze the mixture, and introduce nitrogen for 20 min. Then, 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 introducing nitrogen when sticky threads appear. After the reaction is completed and the mixture is kept warm for 2 h, remove the rubber block, granulate it, dry it, and grind it.

[0050] (2) Mix 1050 parts of pure water, 280 parts of acrylamide, and 80 parts of DAC evenly, freeze them, and introduce nitrogen for 20 min. Then, 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 introducing nitrogen when sticky threads appear. After the reaction is completed and keep warm for 2 h, take out the rubber block, granulate it, dry it, and grind it.

[0051] (3) 1050 parts by weight of pure water, 280 parts of acrylamide, and 100 parts of ATMAC were mixed evenly and then frozen. After nitrogen was introduced for 20 min, 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 were added in sequence. When sticky threads appeared, the introduction of nitrogen was stopped. After the reaction was completed and kept warm for 2 h, the rubber blocks were taken out, granulated, dried, and ground.

[0052] (4) The cationic polyacrylamide with different cations and different ionicity in the above steps (1) to (3) was prepared into 20 g / L mother liquor, which were named as Example 5-1, Example 5-2 and Example 5-3 respectively. The polymerization reaction formula of cationic polyacrylamide is: .

[0053] Treatment of black liquor from wheat straw pulping: (5) Take 200 mL of straw pulping black liquor and place it in a beaker. Use hydrochloric acid to adjust the pH to between 8 and 10 for later use.

[0054] (6) 8 mL of the mother liquor of Example 5-1 was added to the beaker containing the black liquor of step (5), and stirred at 200 r / min for 8 min. The speed was then reduced to 50 r / min and stirred slowly for 20 min. The mixture was allowed to settle for 2 h. Finally, lignin was separated by centrifuge at a speed of 7000 r / min for 10 min. The solid component was dried in a vacuum oven at 50 °C for 6 h to obtain component C1.

[0055] (7) Take 6 mL of the mother liquor of Example 5-2 and add it to the beaker containing the upper liquid after centrifugation in step (6), and stir it at 200 r / min for 8 min. Then reduce the speed to 50 r / min and stir it slowly for 20 min. Let it stand and settle for 2 h. Finally, use a centrifuge to separate lignin at a speed of 7000 r / min for 10 min. Take the solid component and dry it in a vacuum oven at 50 °C for 6 h to obtain component C2.

[0056] (8) 4 mL of the mother liquor of Example 5-3 was added to the beaker containing the upper layer of liquid after centrifugation in step (7), and stirred at 200 r / min for 8 min. The speed was then reduced to 50 r / min and stirred slowly for 20 min. The mixture was allowed to settle for 2 h. Finally, lignin was separated by centrifuge at a speed of 7000 r / min for 10 min. The solid component was dried in a vacuum oven at 50 °C for 6 h to obtain component C3.

[0057] (9) The molecular weight ranges and the contents of hydrophilic and hydrophobic components of C1, C2 and C3 were determined by ultrafiltration membrane filtration and resin adsorption methods, respectively. The different molecular weight ranges and contents of hydrophilic and hydrophobic components of C1, C2 and C3 are shown in the attached figure. Figure 5 .

[0058] Comparative Example 1: Synthesis of cationic polyacrylamide with different cationic degrees: (1) Mix 1050 parts by weight of pure water, 280 parts of acrylamide, and 50 parts of DMC evenly, freeze them, and introduce nitrogen for 20 min. Then, 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 introducing nitrogen when sticky threads appear. After the reaction is completed and keep warm for 2 h, take out the rubber block, granulate it, dry it, and grind it.

[0059] (2) Mix 1050 parts by weight of pure water, 280 parts by weight of acrylamide, and 80 parts by weight of DMC, freeze the mixture, and introduce nitrogen for 20 min. Then, 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 introducing nitrogen when sticky threads appear. After the reaction is completed and the mixture is kept warm for 2 h, remove the rubber block, granulate it, dry it, and grind it.

[0060] (3) Mix 1050 parts by weight of pure water, 280 parts by weight of acrylamide, and 100 parts by weight of DMC, freeze the mixture, and introduce nitrogen for 20 min. Then, 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 introducing nitrogen when sticky threads appear. After the reaction is completed and the mixture is kept warm for 2 h, remove the rubber block, granulate it, dry it, and grind it.

[0061] (4) The cationic polyacrylamide with different ionicity in the above steps (1) to (3) was prepared into 20 g / L mother liquor, which were named Comparative Example 1-1, Comparative Example 1-2 and Comparative Example 1-3 respectively. The polymerization reaction formula of cationic polyacrylamide is: .

[0062] Treatment of black liquor from wheat straw pulping: (5) Take 200 mL of straw pulping black liquor and place it in a beaker. Use hydrochloric acid to adjust the pH to between 8 and 10 for later use.

[0063] (6) 8 mL of the mother liquor of comparative example 1-1 was added to the beaker containing the black liquor of step (5), and stirred at 200 r / min for 8 min, then the speed was reduced to 50 r / min and stirred slowly for 20 min, and the mixture was allowed to settle for 2 h. Finally, lignin was separated by centrifuge at a speed of 7000 r / min for 10 min. The solid component was dried in a vacuum oven at 50 °C for 6 h to obtain component C1.

[0064] (7) Take 6 mL of the mother liquor of comparative example 1-2 and add it to the beaker containing the upper layer liquid after centrifugation in step (6), and stir it at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir it slowly for 20 min, and let it settle for 2 h. Finally, use a centrifuge to separate lignin at a speed of 7000 r / min for 10 min. Take the solid component and dry it in a vacuum oven at 50 °C for 6 h to obtain component C2'.

[0065] (8) 4 mL of the mother liquor of comparative example 1-3 was added to the beaker containing the upper layer liquid after centrifugation in step (7), and stirred at 200 r / min for 8 min, then the speed was reduced to 50 r / min and stirred slowly for 20 min, and the mixture was allowed to settle for 2 h. Finally, lignin was separated by centrifuge at a speed of 7000 r / min for 10 min. The solid component was dried in a vacuum oven at 50 °C for 6 h to obtain component C3'.

[0066] (9) The molecular weight ranges and the contents of hydrophilic and hydrophobic components of C1, C2' and C3' components were determined by ultrafiltration membrane filtration and resin adsorption methods, respectively. The contents of different molecular weight ranges and different hydrophilic and hydrophobic components of C1, C2' and C3' are shown in the attached figure. Figure 6 .

[0067] Comparative Example 2: The difference between Comparative Example 2 and Example 1 is that ordinary nonionic polyacrylamide is used to classify straw pulping black liquor in steps (5) to (9), and components C1', C2' and C3' are obtained respectively. The molecular weight range and the content of hydrophilic and hydrophobic components of components C1', C2' and C3' are determined by ultrafiltration membrane filtration method and resin adsorption method. The content of components C1', C2' and C3' in different molecular weight ranges and different hydrophilic and hydrophobic components is shown in the attached figure. Figure 7 .

[0068] Comparative Example 3: Synthesis of cationic polyacrylamide: (1) Mix 1050 parts by weight of pure water, 280 parts of acrylamide, and 50 parts of DMC evenly, freeze them, and introduce nitrogen for 20 min. Then, 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 introducing nitrogen when sticky threads appear. After the reaction is completed and keep warm for 2 h, take out the rubber block, granulate it, dry it, and grind it.

[0069] (2) Prepare 20 g / L mother liquor of cationic polyacrylamide, named as comparative example 3-1. The polymerization reaction formula of cationic polyacrylamide is: .

[0070] Treatment of black liquor from wheat straw pulping: (3) Take 200 mL of straw pulping black liquor and place it in a beaker. Use hydrochloric acid to adjust the pH to between 8 and 10 for later use.

[0071] (4) 8 mL of the mother liquor of comparative example 3-1 was added to the beaker containing the black liquor of step (3), and stirred at 200 r / min for 8 min, then the speed was reduced to 50 r / min and stirred slowly for 20 min, and the mixture was allowed to settle for 2 h. Finally, lignin was separated by centrifuge at a speed of 7000 r / min for 10 min. The solid component was dried in a vacuum oven at 50 °C for 6 h to obtain component C1.

[0072] (5) Take 6 mL of the mother liquor of comparative example 3-1 and add it to the beaker containing the upper layer liquid after centrifugation in step (4), and stir it at 200 r / min for 8 min, then reduce the speed to 50 r / min and stir it slowly for 20 min, and let it settle for 2 h. Finally, use a centrifuge to separate lignin at a speed of 7000 r / min for 10 min. Take the solid component and dry it in a vacuum oven at 50 °C for 6 h to obtain component C2'.

[0073] (6) 4 mL of the mother liquor of comparative example 3-1 was added to the beaker containing the upper layer of the liquid after centrifugation in step (5), and stirred at 200 r / min for 8 min, then the speed was reduced to 50 r / min and stirred slowly for 20 min, and the mixture was allowed to settle for 2 h. Finally, lignin was separated by centrifuge at a speed of 7000 r / min for 10 min. The solid component was dried in a vacuum oven at 50 °C for 6 h to obtain component C3'.

[0074] (7) The molecular weight ranges and the contents of hydrophilic and hydrophobic components of C1, C2' and C3' components were determined by ultrafiltration membrane filtration and resin adsorption methods, respectively. The contents of different molecular weight ranges and different hydrophilic and hydrophobic components of C1, C2' and C3' are shown in the attached figure. Figure 8 .

[0075] By comparing the TOC of the original black liquor and the TOC of the final supernatant of Examples 1-5 and Comparative Examples 1-3, it was found that Examples 1-5 can effectively separate lignin from straw pulping black liquor, and the separation rate (TOC 初始黑液 –TOC 最终离心分离后上清液 ) / TOC 初始黑液 ) is about 70%, see attached Fig. 9 . Comparative Example 1 is slightly lower than the embodiment because the use of the same cationic type of polyacrylamide will result in residual flocculants in the supernatant after separation, and the continued addition of the same cationic type of polyacrylamide will inhibit the flocculation effect to a certain extent. The separation rate of Comparative Examples 2-3 is relatively low (about 40%), and it is difficult to effectively separate lignin from straw pulping black liquor. Furthermore, the method of grading straw pulping lignin by flocculation using cationic polyacrylamide as a flocculant will preferentially separate the high molecular weight and strongly hydrophobic part of the lignin, while the lower molecular weight part and the more hydrophilic part must be separated using a polyacrylamide with a higher molecular weight and a higher cationic degree. Therefore, Examples 1-5 all have a good separation effect on straw pulping lignin, while Comparative Example 2 uses ordinary non-ionic polyacrylamide. During the flocculation process, it is difficult to effectively capture the lignin molecules, so the content of the resulting components is greatly reduced, and therefore, it cannot be used in large quantities. Comparative Example 3 effectively separated the high molecular weight and strongly hydrophobic lignin, but the molecular weight and cationicity of the subsequently added polyacrylamide were low, and it was unable to capture the low molecular weight and hydrophilic lignin. Therefore, the subsequent lignin separation effect was not obvious. In summary, the sequential addition of a series of cationic polyacrylamides can effectively grade the wheat straw pulping lignin and improve the comprehensive utilization rate of black liquor.

Claims

1. A method for fractionating lignin in black liquor from wheat straw pulping 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 synthetic raw materials include: pure water, acrylamide, a cationic monomer, an initiator, a chain transfer agent, an oxidant and a reducing agent; wherein the cationic monomer is one of methacryloyloxyethyl trimethyl ammonium chloride (DMC), acryloyloxyethyl trimethyl ammonium chloride (DAC), methacrylamidopropyl trimethyl ammonium chloride (MAPTAC), allyl trimethyl ammonium 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 weight, 1000-1100 parts of pure water, 250-280 parts of acrylamide, and 50-100 parts of cationic monomer are mixed evenly and then frozen. After nitrogen is introduced for 15-20 minutes, 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 are added in sequence. After sticky fibers are formed, the introduction of nitrogen is stopped. After the reaction is completed and the mixture is kept warm for 1-2 hours, the rubber block is taken out, granulated, dried, and ground; Step 2: Take 200 mL of wheat straw pulping black liquor and place it in a beaker, use hydrochloric acid to adjust the pH to between 8 and 10, and wait for use; Step 3: Prepare 15-20 g / L mother solution of polyacrylamide with different cations and different cationicity from step 1 (2); Step 4: Take 4-10 mL of the polyacrylamide mother liquor of the third step and add it to the beaker containing the black liquor of the second step, and stir it at 200-220 r / min for 5-8 min, then reduce the speed to 50-70 r / min and stir it slowly for 15-20 min, and let it settle for 1-2 h. Finally, use a centrifuge to separate lignin at a speed of 5000-7000 r / min for 10-15 min. Take the solid component and dry it in a vacuum oven at 40-60°C for 5-8 h to obtain component C1; Step 5: Take 4-10 mL of the polyacrylamide mother solution in the third step and add it to the beaker containing the upper liquid after centrifugation in the fourth step, and stir it at 200-220 r / min for 5-8 min, then reduce the speed to 50-70 r / min and stir it slowly for 15-20 min, and let it settle for 1-2 h. Finally, use a centrifuge to separate lignin at a speed of 5000-7000 r / min for 10-15 min. Take the solid component and dry it in a vacuum oven at 40-60°C for 5-8 h to obtain component C2; Step 6: Take 4-10 mL of the polyacrylamide mother liquor in the third step and add it to the beaker containing the upper liquid after centrifugation in the fifth step, and stir it at 200-220 r / min for 5-8 min, then reduce the speed to 50-70 r / min and stir it slowly for 15-20 min, and let it stand for 1-2 h. Finally, use a centrifuge to separate lignin at a speed of 5000-7000 r / min for 10-15 min. Take the solid component and dry it in a vacuum oven at 40-60°C for 5-8 h to obtain component C3.

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

3. A method for fractionating lignin in wheat straw pulping black liquor using a series of cationic polyacrylamides according to claim 1, characterized in that: The different molecular weight lignin components C1, C2 and C3 obtained in the six steps are adsorbed by designated resins (DAX-8 and XAD-4) and the total organic carbon (TOC) is calculated to obtain the different hydrophilic and hydrophobic component contents of C1, C2 and C3.

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