Method for extracting lignin from sulfate pulping black liquor

Abstract

The invention discloses a method for extracting lignin from sulfate pulping black liquor, which comprises the following steps: heating the black liquor to 50-70 DEG C in the stirring process, then adjusting the pH value of the black liquor to 3.0-4.5 by using organic acid, stopping adding acid, and aging under the conditions of stirring and 50-70 DEG C to obtain aged pulp; centrifuging the aged slurry, discarding the supernatant, collecting the precipitate, then washing the precipitate with acidic water, repeating the steps of centrifuging and washing for several times, and collecting the precipitate to obtain a crude lignin filter cake; and treating the obtained crude lignin filter cake in a phosphoric acid solution, washing the obtained product until the pH value of the filtrate is neutral, and then drying to constant weight to obtain the phosphorylated lignin. According to the invention, low vulcanization, high purity and functionalization (high thermal stability) of the product can be realized at the same time, so that the application barrier of the existing lignin is broken, and the huge potential of the lignin as a renewable resource is released.

Description

Technical Field

[0001] This invention relates to the field of lignin extraction, and more specifically to a method for extracting lignin from black liquor from sulfate pulping. Background Technology

[0002] Lignin is the second most abundant renewable organic polymer on Earth after cellulose, and it is widely found in the cell walls of woody and herbaceous plants. In the chemical pulping industry (such as the sulfate process, or Kraft process), approximately 50 to 70 million tons of lignin are separated annually as a byproduct from cooking waste liquor (commonly known as "black liquor"). Traditionally, this portion of lignin has been mainly used as low-value fuel in factories, with alkali recovered through combustion. Its enormous potential as a chemical raw material has not been fully exploited, resulting in significant waste.

[0003] With the rise of the "biorefining" concept, separating lignin from black liquor and utilizing it for high-value purposes has become a research hotspot in the global academic and industrial communities. High-purity, high-performance lignin can serve as a platform compound for the production of a variety of high-value-added products, such as aromatic chemicals, polymers, carbon materials, and surfactants.

[0004] The mainstream technique for extracting lignin from black liquor produced by the sulfate pulping process is acid precipitation. The basic principle is as follows: in alkaline black liquor, the phenolic hydroxyl groups of lignin are deprotonated to form soluble phenolates; by adding acid to lower the pH of the solution, the phenolic hydroxyl groups are reprotonated, the hydrophobic interactions between lignin molecules are enhanced, and the solubility decreases sharply, thus precipitating out in solid form.

[0005] Among the many acid precipitation technologies, the LignoBoost technology, jointly developed by the Swedish Institute of Innventia and Valmet, is currently the most mature and widely used sulfate lignin extraction process in the world. The process generally includes the following core steps: (1) Pretreatment and primary acidification: First, carbon dioxide (CO2) is used to acidify the black liquor. CO2 dissolves in water to form carbonic acid (H2CO3), which is a weak acid that can lower the pH of the black liquor from about 13-14 to 9-10. At this pH, most of the larger molecular weight lignin will precipitate out, forming a slurry. (2) Solid-liquid separation: The slurry is filtered by equipment such as a filter press to obtain a coarse lignin filter cake and a portion of the filtrate (dilute black liquor). The dilute black liquor is returned to the alkali recovery system. (3) Secondary acidification and purification (key step): The crude lignin filter cake is re-pulped and deeply acidified with dilute sulfuric acid (H2SO4) to further reduce the pH value to about 2-3, so that the low molecular weight lignin that has not been precipitated in the first step can be precipitated, thereby increasing the total yield; more importantly, the lignin is washed in a strong acid environment to dissolve and remove the inorganic salts (such as sodium sulfate and sodium carbonate) and soluble organic impurities (such as sugar degradation products) that are entrained in it, thereby improving the purity of lignin. (4) Secondary solid-liquid separation and washing: The acid-washed lignin slurry is filtered again. The obtained filter cake needs to be washed with a large amount of washing water (usually acidic water) to remove residual sulfuric acid and dissolved salts as much as possible. (5) Drying: The washed lignin filter cake is dried to obtain the final powdered lignin product, namely "Kraft lignin" or "LignoBoost lignin". LignoBoost technology is currently the best existing technology because it enables large-scale, continuous extraction of lignin from the alkali recovery loop of existing pulp mills. The two-step acidification strategy balances yield and purity to some extent. The process is relatively mature, with several commercially viable examples already in operation.

[0006] Besides LignoBoost, some literature and patents mention the use of organic acids for biomass pretreatment or delignification. For example, a mixture of formic acid and acetic acid is used to directly dissolve lignin from wood chips at high temperatures. Some patents also mention using a mixture of organic acids and sulfuric acid as a catalyst, but these processes are complex and have problems such as lignin carbonization. These methods are not directly aimed at the precipitation and purification of lignin in black liquor.

[0007] The three major technological bottlenecks currently facing industrial lignin (especially sulfate-process lignin) in its preparation and application are: (1) Application limitations caused by high sulfur content in the product: In traditional sulfuric acid extraction processes, whether sulfuric acid precipitation is used directly or secondary acidification and washing are performed using sulfuric acid as in the LignoBoost process, sulfate ions will still remain in the final product in the form of adsorption or salt formation, inevitably introducing sulfur into the lignin product. The sulfur content of the final product is usually between 2% and 3%. High sulfur content limits the high-value utilization of lignin. For example, in applications such as catalytic cracking or hydroconversion, sulfur is a typical catalyst poison that can deactivate heavy metal or transition metal catalysts. In the preparation of carbon materials such as carbon fibers and activated carbon, sulfur will affect the degree of graphitization in the carbonization process and the product performance.

[0008] (2) Low product purity (high ash content, high sugar content) leads to unstable performance: Lignin obtained by traditional acid precipitation often contains or adsorbs a large amount of inorganic salts (ash) and residual hemicellulose degradation products (sugars). Even after washing, the ash content is usually still at a high level of 1%-10%, and sugar impurities are difficult to remove completely. These impurities cause large fluctuations in its performance, and the ash and sugar content of different batches of lignin are different. When used as raw materials in fine chemicals or polymer materials, the product performance is unstable. High ash content will increase equipment wear and affect the electrical insulation of the material; high sugar content will reduce the thermal stability of lignin, and it is easy to decompose at high temperatures to produce small molecule acids, aldehydes, etc., which will affect the performance of composite materials.

[0009] (3) The problem of limited application range due to insufficient thermal stability of lignin: The goal of the LignoBoost process is "extraction" rather than "modification". Its product is only unmodified sulfate lignin, and its inherent properties such as thermal stability are not improved. This means that if downstream applications require higher thermal stability, additional independent chemical modification steps must be added, which will undoubtedly increase the total cost and process complexity. Although natural lignin is a huge aromatic polymer, its thermal stability is relatively limited. The thermal decomposition temperature is usually between 200-400°C, and the char residue is not high. This characteristic limits its application as a high-performance flame retardant, thermosetting resin filler, carbon fiber precursor, etc., which require excellent heat resistance. Existing extraction methods do not functionalize lignin to improve its thermal properties.

[0010] (4) Equipment corrosion and environmental issues: The large-scale use of sulfuric acid places high demands on the corrosion resistance of reactors, pipelines and filtration equipment, increasing fixed asset investment. At the same time, the sulfuric acid-containing wastewater generated by pickling needs to be neutralized, increasing the load and cost of wastewater treatment. Summary of the Invention

[0011] Purpose of the invention: The technical problem to be solved by the present invention is to provide a novel lignin extraction and purification technology that addresses the shortcomings of existing technologies. This technology can simultaneously achieve low sulfurization, high purity, and functionalization (high thermal stability) of the product, thereby breaking down existing barriers to the application of lignin and releasing its enormous potential as a renewable resource.

[0012] To address the aforementioned technical problems, this invention discloses a method for extracting lignin from black liquor from sulfate pulping, comprising the following steps: (1) Using black liquor from the sulfate pulping process as raw material, the black liquor is heated to 50-70°C during stirring, and then the pH of the black liquor is adjusted to 3.0-4.5 using organic acid. The acid addition is stopped, and the liquor is aged under stirring and 50-70°C to obtain aged pulp. The purpose of heating is to reduce the viscosity of the black liquor, which is beneficial for mass transfer and mixing, and at the same time promotes the agglomeration of lignin precipitates, making it easier to filter. Excessive temperature (such as above 70°C) may cause some lignin to undergo unnecessary side reactions. The pH value is controlled because the isoelectric point of lignin is approximately in the range of pH = 2-3.5. Lowering the pH to this range can ensure that most of the lignin precipitates out, resulting in a higher yield. Due to the weak acidity of organic acid, it is difficult to quickly and economically lower the pH value to the lowest point. The pH is adjusted to 3.0-4.5 from both economic and yield perspectives. The aging process helps small particles aggregate into large particles that are easy to filter.

[0013] (2) Centrifuge the aged pulp, discard the supernatant, and collect the precipitate. Centrifugation mainly removes water, sodium organic acid, small molecules of lignin degradation, and residual sugars. Then wash the precipitate with acidic water. Repeat the centrifugation and washing steps several times to remove surface-adsorbed and free impurities. Use acidic water instead of pure water for washing to prevent the precipitated lignin from redissolving at a higher pH, thereby ensuring the yield. Collect the precipitate to obtain a crude lignin filter cake. The product at this stage has a low sulfur content (theoretically derived only from trace amounts of sulfur carried by the black liquor itself, not introduced by the process), but still contains a certain amount of ash and sugar impurities.

[0014] (3) The obtained crude lignin filter cake was treated in phosphoric acid solution, the product was washed until the pH of the filtrate was neutral, and then dried to constant weight to obtain phosphorylated lignin.

[0015] The black liquor has a solids content of 15-35 wt%, a pH value of 10.0-13.5, and a lignin concentration of approximately 80-120 g / L.

[0016] The organic acid is any one of acetic acid, citric acid, or lactic acid. The role of the organic acid is to provide hydrogen ions, and its use aims to reduce sulfur content, thereby reducing sulfur pollution at its source. Most organic acids meet the requirements, but some have weak ionization, requiring large amounts of acid to adjust the pH to achieve a certain yield of lignin, making them uneconomical. Preferably, acetic acid, citric acid, and lactic acid offer better yields. Oxalic acid and formic acid are highly corrosive and expensive, therefore their use is not recommended.

[0017] In step (1), the stirring speed is 200 ~ 300 rpm and the aging time is 30 ~ 120 minutes.

[0018] In step (2), the centrifugation conditions are 6000~10000 rpm for 10~20 minutes. Preferably, the centrifugation conditions are 6000 rpm for 15 minutes.

[0019] In step (1), the pH of the organic acidic water is 3.0~4.5.

[0020] In step (3), the concentration of the phosphoric acid solution is 0.1~0.5 mol / L, and the crude lignin obtained in step (2) is used to prepare a lignin slurry of 50-100 g / L with the phosphoric acid solution.

[0021] In step (3), the treatment conditions are: treatment at a temperature of 60-90°C for 1-2 hours.

[0022] Beneficial effects: Compared with the prior art, this application has the following advantages: (1) Achieving “sulfur-free” extraction of lignin: By using non-sulfur organic acids such as acetic acid to replace sulfuric acid as a precipitant, the introduction of sulfur elements is fundamentally eliminated, and low-sulfur lignin is obtained, clearing the way for subsequent catalytic conversion and high-end material preparation.

[0023] (2) Achieving “ultra-high purity” of lignin: By utilizing the unique chemical properties of phosphoric acid, a new purification scheme is designed, which can not only efficiently dissolve and remove inorganic ash and sugar impurities, but also make the purity of the final product stably reach more than 95%, far exceeding the existing technology level.

[0024] (3) Achieving “integration” of extraction and functionalization: During purification, phosphoric acid is introduced into the lignin molecular skeleton in the form of covalent bonds through the esterification reaction of phosphoric acid and lignin, thereby achieving in-situ phosphorylation modification. This “one-step” combination of purification and functionalization aims to significantly improve the thermal stability and char-forming properties of lignin.

[0025] (4) Enhance the application value and scope of the product: Through the above-mentioned technological innovation, the originally low-value industrial by-product - Kraft lignin - is transformed into a low-sulfur, high-purity, and high-thermal-stability "fine chemical" or "high-performance material precursor", enabling it to be competent in high-end application scenarios such as carbon fiber, flame retardant for engineering plastics, epoxy resin curing agent, and catalyst carrier, thereby significantly increasing its economic added value. Attached Figure Description

[0026] Figure 1 The graph shows the optimized aging time (yield per 25 mL black liquor). Figure 2 Graph showing the results of aging temperature optimization (yield per 25 mL black liquor); Figure 3 Thermogravimetric curves for the extraction and purification of lignin from black liquor used in pine sulfate pulping; Figure 4 Thermogravimetric curves for the extraction and purification of lignin from black liquor of eucalyptus sulfate pulping; Figure 5 Thermogravimetric curves for the extraction and purification of lignin from black liquor of poplar sulfate pulping. Detailed Implementation

[0027] The present invention will be further described in detail below through specific embodiments, and the advantages of the present invention in the above and / or other aspects will become clearer. Example

[0028] The raw material was pine sulfate pulping black liquor (initial pH=12.50, solid content 17.33%). The black liquor was placed in a 2L reactor, stirred (200 rpm), and heated to a certain temperature. After the temperature stabilized, a 4mol / L acetic acid solution was slowly added dropwise to adjust the pH to 3.6. During the dropwise addition, an online pH meter was used to monitor the pH in real time. After the target pH was reached, the acid addition was stopped, and the mixture was "aged" or "matured" for a certain period of time at a certain temperature and the same stirring rate.

[0029] To determine the optimal experimental conditions for this scheme, optimization experiments were first conducted. Time optimization: at a temperature of 55°C, the aging times were 1 h, 2 h, 3 h, and 4 h, respectively. Temperature optimization: at aging temperatures of 45°C, 55°C, and 65°C, the reaction time was 2 h. Lignin yield was used as the evaluation index, and the results are shown in [Figure number missing]. Figure 1 , Figure 2 .Depend on Figure 1It can be seen that the lignin yield increases slightly over time (from 0.79g to 0.85g), reaching its maximum at 4h. Under the same reaction time, increasing the temperature initially increases the yield, then decreases, with the highest lignin yield of 0.86g at 55°C. However, the yield does not change significantly with temperature.

[0030] The "matured" slurry (slurry obtained after aging at 55℃ for 2 hours) was centrifuged (6000 rpm, 15 minutes), the supernatant was discarded, and the solid precipitate at the bottom was collected. The precipitate was washed with acidic water at pH 3.0 (adjusted with a small amount of acetic acid), and then centrifuged again. This washing-centrifugation process was repeated 2-3 times to remove surface adsorbed and free impurities, resulting in a low-sulfur crude lignin filter cake.

[0031] Prepare a 0.5 mol / L solution of phosphoric acid using deionized water. Transfer all the low-sulfur crude lignin filter cake obtained in the previous step to a 2L reactor, add the phosphoric acid solution, and prepare a 100 g / L lignin slurry. Proceed to 60°C. o C. Start stirring (200 rpm) and maintain the reaction at this temperature for 60 min. After the reaction is complete, allow it to cool naturally to room temperature. Filter the resulting slurry. Wash the filter cake with plenty of deionized water until the pH of the washing filtrate is neutral (pH ≈ 7.0). Place the washed filter cake in a vacuum oven at 60°C to dry to constant weight, and then pulverize it into a uniform powder. This is the final product—low-sulfur, high-purity, and highly thermally stable phosphorylated lignin.

[0032] To verify the effectiveness of this method, lignin obtained from pine sulfate pulping black liquor as raw material and sulfuric acid as a precipitant was used as a control sample for comparative analysis. The black liquor was placed in a 2L reactor, stirred (200 rpm), and heated to 50 °C. After the temperature stabilized, sulfuric acid solution was slowly added dropwise until the solution pH=2. During the addition, an online pH meter was used for real-time monitoring. Once the target pH was reached, acid addition was stopped, and the mixture was "aged" or "matured" for 120 minutes at the same temperature and stirring rate. The matured pulp was centrifuged (6000 rpm, 15 minutes), and the filter cake was washed with a large amount of deionized water until the pH of the washing filtrate was neutral (pH ≈ 7.0). The washed filter cake was dried in a vacuum oven at 60 °C to constant weight and then pulverized into a uniform powder.

[0033] The basic performance of the above products was analyzed, and the results are as follows: Figure 3 As shown in Table 1.

[0034] Table 1. Basic Performance Analysis of Lignin Extraction and Purification from Black Liquor in Pine Sulfate Pulping Example

[0035] Eucalyptus sulfate pulping black liquor (initial pH=11.30, solid content 15.73%) was placed in a 2L reactor, stirred (200 rpm), and heated to 70°C. After the temperature stabilized, a 20% lactic acid solution was slowly added dropwise to adjust the pH to 3.0. The pH was monitored in real time using an online pH meter during the dropwise addition. Once the target pH was reached, the acid addition was stopped, and the mixture was "aged" or "matured" at 70°C and the same stirring rate for 30 minutes.

[0036] Centrifuge the "matured" slurry (6000 rpm, 15 minutes), discard the supernatant, and collect the solid precipitate at the bottom. Wash the precipitate with acidic water at pH 3.0 (adjusted with a small amount of lactic acid), then centrifuge again. Repeat this washing-centrifugation process 2-3 times to remove surface adsorbed and free impurities.

[0037] Prepare a 0.1 mol / L solution of phosphoric acid using deionized water. Transfer all the low-sulfur crude lignin filter cake obtained in the previous step to a 2L reactor, add the phosphoric acid solution, and prepare a lignin slurry with a concentration of 50 g / L. Proceed to 90°C. o At temperature C, stir at 300 rpm and maintain the reaction at this temperature for 120 min. After the reaction, allow it to cool naturally to room temperature. Filter the resulting slurry. Wash the filter cake with plenty of deionized water until the pH of the washing filtrate is neutral (pH ≈ 7.0). Dry the washed filter cake in a vacuum oven at 60°C until constant weight, then pulverize it into a uniform powder. This is the final product—low-sulfur, high-purity, and highly thermally stable phosphorylated lignin.

[0038] To verify the effectiveness of this method, lignin obtained from pine sulfate pulping black liquor as raw material and sulfuric acid as a precipitant was used as a control sample for comparative analysis. The black liquor was placed in a 2L reactor, stirred (200 rpm), and heated to 70 °C. After the temperature stabilized, a 20% sulfuric acid solution was slowly added dropwise to adjust the pH to 2. The pH was monitored in real-time using an online pH meter. Once the target pH was reached, acid addition was stopped, and the mixture was "aged" or "matured" for 30 minutes at the same temperature and stirring rate. The matured pulp was centrifuged (6000 rpm, 15 minutes), and the filter cake was washed with a large amount of deionized water until the pH of the washing filtrate was neutral (pH ≈ 7.0). The cleaned filter cake was dried in a 60 °C vacuum oven to constant weight and then pulverized into a uniform powder.

[0039] The basic performance of the above products was analyzed, and the results are as follows: Figure 4 As shown in Table 2.

[0040] Table 2. Basic Performance Analysis of Lignin Extraction and Purification from Black Liquor Prepared by Lithium Sulfate Pulping Example

[0041] Poplar sulfate pulping black liquor (initial pH=10.20, solid content 14.58%) was placed in a 2L reactor, stirred (200 rpm), and heated to 60 °C. After the temperature stabilized, a 20% citric acid solution was slowly added dropwise to adjust the pH to 4.5. The pH was monitored in real time using an online pH meter during the dropwise addition. Once the target pH was reached, the acid addition was stopped, and the mixture was "aged" or "matured" at 60 °C and the same stirring rate for 75 minutes.

[0042] Centrifuge the "matured" slurry (6000 rpm, 15 minutes), discard the supernatant, and collect the solid precipitate at the bottom. Wash the precipitate with acidic water at pH 4.5 (adjusted with a small amount of citric acid), then centrifuge again. Repeat this washing-centrifugation process 2-3 times to remove surface adsorbed and free impurities.

[0043] Prepare a 0.3 mol / L solution of phosphoric acid using deionized water. Transfer all the low-sulfur crude lignin filter cake obtained in the previous step to a 2L reactor, add the phosphoric acid solution, and prepare a lignin slurry with a concentration of 67 g / L. Proceed to 75°C. o At temperature C, start stirring (200-300 rpm) and maintain the reaction at this temperature for 90 minutes. After the reaction is complete, allow it to cool naturally to room temperature. Filter the resulting slurry. Wash the filter cake with plenty of deionized water until the pH of the washing filtrate is neutral (pH ≈ 7.0). Place the washed filter cake in a vacuum oven at 60°C to dry to constant weight, and then pulverize it into a uniform powder. This is the final product—low-sulfur, high-purity, and highly thermally stable phosphorylated lignin.

[0044] To verify the effectiveness of this method, lignin obtained from pine sulfate pulping black liquor as raw material and sulfuric acid as a precipitant was used as a control sample for comparative analysis. The black liquor was placed in a 2L reactor, stirred (200 rpm), and heated to 60 °C. After the temperature stabilized, a 20% sulfuric acid solution was slowly added dropwise to adjust the pH to 2. During the addition, an online pH meter was used for real-time monitoring. Once the target pH was reached, acid addition was stopped, and the mixture was "aged" or "matured" for 75 minutes at a specific temperature and stirring rate. The matured pulp was centrifuged (6000 rpm, 15 minutes), and the filter cake was washed with a large amount of deionized water until the pH of the washing filtrate was neutral (pH ≈ 7.0). The washed filter cake was dried in a 60 °C vacuum oven to constant weight and then pulverized into a uniform powder.

[0045] The basic performance of the above products was analyzed, and the results are as follows: Figure 5 As shown in Table 3.

[0046] Table 3. Basic Performance Analysis of Lignin Extraction and Purification from Poplar Sulfate Pulping Black Liquor

[0047] This invention provides a method and approach for extracting lignin from black liquor from sulfate pulping. Many methods and approaches exist for implementing this technical solution; the above description is merely a preferred embodiment of the invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of this invention, and these improvements and modifications should also be considered within the scope of protection of this invention. All components not explicitly stated in this embodiment can be implemented using existing technologies.

Claims

1. A method for extracting lignin from black liquor from sulfate pulping, characterized in that, Includes the following steps: (1) Using black liquor from the sulfate pulping process as raw material, the black liquor is heated to 50-70℃ during stirring, and then the pH of the black liquor is adjusted to 3.0-4.5 using organic acid. The acid addition is stopped, and the liquor is aged under stirring and 50-70℃ conditions to obtain aged pulp. (2) Centrifuge the aged slurry, discard the supernatant, collect the precipitate, and then wash the precipitate with acidic water. Repeat the centrifugation and washing steps several times, and collect the precipitate to obtain a coarse lignin filter cake. (3) The obtained crude lignin filter cake was treated in phosphoric acid solution, the product was washed until the pH of the filtrate was neutral, and then dried to constant weight to obtain phosphorylated lignin.

2. The method according to claim 1, characterized in that, The black liquor has a solids content of 25-35 wt%, a pH value of 11.0-13.5, and a lignin concentration of 80-120 g / L.

3. The method according to claim 1, characterized in that, The organic acid is any one of acetic acid, citric acid, or lactic acid, with a concentration of 2 mol / L to 6 mol / L.

4. The method according to claim 1, characterized in that, In step (1), the stirring speed is 200 ~ 300 rpm and the aging time is 30 ~ 120 minutes.

5. The method according to claim 1, characterized in that, In step (2), the centrifugation conditions are 6000~10000 rpm for 10~20 minutes.

6. The method according to claim 1, characterized in that, In step (2), the pH of the acidic water is 3.0 to 4.5, and the acidic water is obtained by adjusting the pH of the organic acid with water.

7. The method according to claim 6, characterized in that, The organic acid is any one of acetic acid, citric acid, or lactic acid.

8. The method according to claim 1, characterized in that, In step (3), the concentration of the phosphoric acid solution is 0.5~1mol / L, and the crude lignin obtained in step (2) is prepared with the phosphoric acid solution to form a lignin slurry of 50-100 g / L.

9. The method according to claim 1, characterized in that, In step (3), the processing conditions are: at a temperature of 60-90°C, a stirring rate of 200-300 rpm, and a processing time of 1-2 hours.

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