Method for extracting lignin from lignin source

Abstract

Embodiments of the present disclosure may include a method for extracting lignin from a lignin source, the method including a slurrying step in which the lignin source is mixed with a first acidifying agent, thereby forming a first acidic slurry. Embodiments may also include a first separation step in which lignin is separated from the acidic slurry to form a first lignin separation cake. Embodiments may also include a reslurrying step in which the first lignin separation cake is mixed with a second acidifying agent to form a second acidic slurry. Embodiments may also include a second separation step in which lignin is separated from the second acidic slurry to form a second lignin separation cake. In some embodiments, the temperature of the first acidic slurry of the slurrying stage may be maintained at or below 80 DEG C.

Description

[0001] This disclosure relates to the field of lignin processing, and more specifically, to a method for extracting lignin from lignin sources.

[0002] Lignin is a complex organic polymer and a key structural material for the supporting tissues of most plants. It is particularly important in cell wall formation, especially in wood, as it imparts rigidity and makes the cell walls less prone to decay. Chemically, lignin is a polymer cross-linked from phenolic precursors. Lignin is a product of many industrial processes, including papermaking and biofuel production. Extracted lignin, also known as technical lignin, contains significant amounts of metal ions (such as sodium and iron), which can interfere with subsequent processing and limit its potential applications.

[0003] WO2014 / 116150A1 relates to a method for producing high-purity lignin, the method comprising multiple acidification and corresponding separation stages aimed at reducing the carbohydrate content of the lignin. In this method, a lignin cake may be subjected to additional acidification to bring the pH to at least 2-4 or lower, followed by a third separation to obtain a third lignin cake as an additional treatment step for leaching and washing metals from the lignin cake, said additional treatment being performed before or after obtaining a low-carbohydrate lignin cake. While this method has proven effective to some extent, further technological improvements are needed to further reduce the metal content in the lignin.

[0004] Among its many objectives, this invention aims to provide an improved method for extracting lignin. This invention also aims to extract lignin with reduced metal content, including reduced alkali metal and transition metal content (such as potassium, sodium, iron, and manganese).

[0005] According to a first aspect of this disclosure, a method for extracting lignin is provided, the method comprising the following stages in sequence:

[0006] In the pulping stage, the first lignin source is mixed with the first acidifying agent to form the first acidic pulp.

[0007] The subsequent first separation stage involves separating lignin from the first acidic pulp to form a first lignin separation cake.

[0008] The subsequent repulping stage involves mixing the first lignin separation cake with a second acidifying agent to form a second acidic pulp, and

[0009] The subsequent second separation stage involves separating lignin from the second acidic pulp to form a second lignin separation cake.

[0010] in

[0011] During the pulping stage, the temperature of the first acidic pulp is maintained below 80 °C, and

[0012] During the re-slurrying stage, the second acidic slurry is heated to a temperature range of 80 °C to 140 °C.

[0013] Optionally, during the pulping stage, the purity of the first lignin source is at least 70%.

[0014] In this method, specifically because the temperature of the first acidic pulp is maintained below 80°C, the amount of sodium that can be removed from lignin is significantly increased. When the temperature is above 80°C, although iron can be effectively removed, sodium is found to remain in the lignin. Surprisingly, a sharp increase in sodium concentration was observed when the temperature was around 81°C, compared to the case when the temperature was below 80°C. The repulping stage further reduces the iron content. The sodium content in the second lignin separation cake obtained by this method is below 65 ppm, and can even be below 30 ppm.

[0015] In one embodiment, during the pulping stage, the temperature of the first acidic pulp is maintained at or below 75°C, preferably at or below 72°C, and more preferably at or below 70°C. Thus, the sodium content in the second lignin separation cake obtained by the method is less than 25 ppm.

[0016] Reducing sodium and iron levels offers several benefits. First, sodium and iron ions interfere with chemical reactions in subsequent lignin processing. Second, the presence of these ions alters the physical and chemical properties of lignin, adversely affecting its solubility, reactivity, and other characteristics. Furthermore, metal ions in lignin can cause problems in downstream processing. For example, in the production of biofuels or other bioproducts using lignin, metal ions may poison catalysts, corrode combustion equipment components, and / or reduce processing efficiency. Additionally, lignin can be used to produce carbon, for example, through pyrolysis, and reducing metal impurities can expand the application range of low-metal-content lignin-based carbon production. Applications requiring low impurity / metal content include battery materials, water purification, and the plastics industry.

[0017] When the purity of the primary lignin source is at least 70%, the pulping and repulping stages described herein, as well as the corresponding separation stages, can be considered purification stages to achieve extremely low levels of metals (especially sodium and iron), with sodium content as low as approximately 17 ppm and iron content as low as approximately 18 ppm. Low iron concentrations in lignin are generally a good indicator that other transition metals in the lignin are also present in low concentrations. Examples of lignin sources include kraft lignin, soda lignin, hydrolyzed lignin (such as enzymatically hydrolyzed lignin and steam-exploded lignin), and organic solvent lignin.

[0018] When the lignin purity is less than 70%, it is preferable to add an additional pulping stage and a fourth separation stage (both described below) to the method. When the lignin purity is higher than about 70%, the additional pulping stage may be unnecessary to prepare high-purity lignin using the method. If the lignin purity is higher than 70% but is alkaline, the additional pulping stage and the fourth separation stage may also be added. Alternatively, if the lignin source is alkaline high-purity lignin (e.g., with a purity of at least 85%), the amount of acidifying agent in the pulping stage can be increased.

[0019] In one embodiment, based on the dry weight of the lignin material, the purity of the first lignin source is at least 80%, preferably at least 85%, and most preferably at least 89%. In some embodiments, the purity of the first lignin source can be at least 90%.

[0020] When using lignin of higher purity (such as 85% or 90%), extremely low levels of metals (especially sodium and iron) can be achieved through a combination of purification stages, with sodium content as low as 5 ppm and iron content as low as 5-6 ppm. This is particularly advantageous for applications requiring high-purity lignin (such as battery materials).

[0021] The purity of lignin (such as a primary lignin source or any lignin separation cake) refers to the amount of pure lignin and contaminants or residual chemicals (e.g., salts, sulfides, carbohydrates, organic solvents, etc.) contained based on the dry weight of the lignin material. This purity can be determined by measuring the weight fraction of lignin in the lignin material. This can be done using the test method “L2:2016” published by Innventia AB in 2016, which is incorporated herein by reference in its entirety. The lignin content can be determined according to the method described in paragraph 3.2 of the method, namely, “The total amount of acid-insoluble and acid-soluble matter absorbed at 205 nm after sulfuric acid hydrolysis in the sample is determined by gravimetric and spectrophotometric methods according to the method…”. If the lignin content is expressed in mg lignin / g lignin material (according to the test method), then the purity (%) of the lignin material is equal to the lignin content divided by 10. For example, a lignin content of 700 mg lignin / g lignin material has a purity of 70%. A copy of this test method (published in 2016) is available at: https: / / archive.org / details / biorefinery-test-methods-l-2-2016-kraft-lignins-lignin-and-carbohydrate-content. This test method "L2:2016" relies on test method "L1:2016," also published by Innventia AB in 2016, to determine the dry matter content of lignin. A copy is available here: https: / / archive.org / details / biorefinery-test-methods-L-1-2016.

[0022] In this patent disclosure, various slurries are described as "acidic," such as a first acidic slurry and a second acidic slurry, indicating that these slurries have a pH value below 7. Alkaline slurries have a pH value above 7. This basic chemical or physical principle is understood by the public, especially by those skilled in the art.

[0023] In the method for extracting lignin, the extracted lignin can be extracted into a second lignin separation cake.

[0024] In one embodiment, the temperature of the first acidic slurry during the pulping stage is maintained in the range of 30 °C to 70 °C. This ensures that the amount of sodium that can be leached or eluted is not excessively restricted by kinetics.

[0025] In one embodiment, the temperature of the first acidic slurry is below 65 °C, preferably below 60 °C. When the temperature of the first acidic slurry is maintained below 60 °C, the sodium content in the second lignin separation cake obtained by the method is below 20 ppm.

[0026] In one embodiment, the temperature of the first acidic slurry during the pulping stage is maintained in the range of 30 °C to 65 °C, preferably 40 °C to 60 °C.

[0027] In one embodiment, the second acidic pulp in the re-pulping stage is heated to a temperature range of 80 °C to 120 °C, or a temperature range of 80 °C to 106 °C, such as 75 °C, 80 °C, 85 °C, 90 °C, 95 °C, 100 °C, 105 °C, 106 °C, 110 °C, 115 °C, 120 °C, 125 °C, 130 °C, and 135 °C. Above about 90 °C, the transition metal content of the resulting lignin is significantly reduced. For example, an iron content of less than 15 ppm or lower can be obtained.

[0028] In one embodiment, the second acidic pulp in the re-pulping stage is heated to a temperature range of 80 °C to 98 °C. Observations revealed a slight but measurable increase in the residual sodium content in the purified lignin when the temperature exceeds 98 °C. Simultaneously, the iron content decreases only slightly above this temperature. Therefore, the upper limit of this temperature range, for example, 96 °C, enables optimal removal of both sodium and iron. Furthermore, this also helps reduce the energy requirement compared to higher temperatures.

[0029] In one embodiment, the temperature of the second acidic slurry is between 90 °C and 140 °C. This allows for a further reduction in the transition metal content of the resulting lignin. For example, the iron content in the resulting lignin may be 7 ppm or lower.

[0030] In one embodiment, the holding time of the second acidic slurry at the above-mentioned temperature can be in the range of 10-120 minutes, preferably 15 to 90 minutes, and more preferably 15 to 60 minutes.

[0031] In one implementation, the duration is preferably inversely proportional to the applied temperature.

[0032] In one embodiment, the temperature and duration at which the second acidic slurry is maintained correspond to the P-factor, which is calculated according to the following formula: P-factor = e^(40.48 - 15106 / (273.15 + temperature)). Time / 60 (see Herbert Sixta, Handbook of Pulp, Wiley-VCH, 2006, p. 344, and the table shown on page 7 of WO 2014 / 116150 A1), where the P-factor is greater than 1 and less than 8. For example, at a temperature of 120 °C, 60 minutes corresponds to a P-factor of 8, while 45 minutes corresponds to a P-factor of 6.

[0033] In one implementation, the duration is 10-60 minutes and is inversely proportional to the temperature.

[0034] In one embodiment, the retention time of the first acidic slurry is in the range of 10 to 360 minutes, preferably 20 to 90 minutes, prior to the separation of the lignin from the first acidic slurry in the first separation stage.

[0035] In one embodiment, during the pulping stage, a pH value in the range of 1 to 5, preferably 1 to 4, is established in the first acidic pulp.

[0036] In one embodiment, during the re-slurrying stage, a pH value in the range of 1 to 5, preferably 1 to 4, is established in the second acidic slurry.

[0037] In one embodiment, the amount of the first acidifier added is less than X kg / ton of the first lignin source, where X is equivalent to 100 kg H2SO4 / ton of the first lignin source. This embodiment is useful when limiting the amount of acidifier used. Furthermore, this embodiment can be applied, for example, to lignin sources that have been acid-treated to neutralize / remove any present hydroxide, carbonate, and / or sulfide ions. Such neutralized or acidified lignin is referred to herein as non-alkaline lignin.

[0038] In one embodiment, the amount of the first acidifying agent added may be less than X kg / ton of the first lignin source, where X is equivalent to 50 kg, preferably 20 kg, and more preferably 10 kg H2SO4 / ton of the first lignin source. These upper limits apply to most (if not all) non-alkaline lignin sources, and the process can be used to obtain lignin products of extremely high purity. This embodiment is useful when limiting the amount of acidifying agent used. The specific amount of sulfuric acid or its equivalent can be adjusted according to the desired pH value, which is typically between 2 and 4, but can also be lower, such as 1 or 1.5.

[0039] In one embodiment, the amount of the second acidifying agent added is less than XX kg / ton of the first lignin source, where XX is equivalent to 10 kg H2SO4 / ton of the first lignin source. Preferably, the amount of the second acidifying agent added is less than XX kg / ton of the first lignin source, where XX is equivalent to 20 kg, and more preferably 10 kg H2SO4 / ton of the first lignin source. Since the lignin fed to the repulping stage has already undergone prior acidification treatment in the pulping stage, only a relatively small amount of acid is required at this stage. Alternatively, the amount of these acids can be limited based on the amount of lignin material (i.e., the first lignin separation cake) processed per ton of lignin material in the repulping stage. In this case, the upper limits of the amounts of each second acidifying agent can be similar or the same.

[0040] For the values ​​of X and XX above, if for example, formic acid is used instead, the value of X equivalent to 100 kg H₂SO₄ is converted to approximately 94 kg of formic acid. This can be calculated by taking into account the difference in molecular weight between the two acids and the number of available protons (two for sulfuric acid and one for formic acid). Since formic acid is a weak acid and sulfuric acid is a strong acid, the amount of formic acid required may be more than the calculated value. This is not considered in this explanatory calculation.

[0041] Advantageously, when using any embodiment of the method disclosed herein, the amount of lignin dissolved from the second lignin separation cake is kept below 20%, or even as low as 15%, thereby increasing the yield of lignin.

[0042] In one embodiment, when the temperature of the first acidic slurry is above 65 °C, preferably 60 °C, before the first separation stage, the first acidic slurry is cooled before lignin is subsequently separated from the first acidic slurry in the first separation stage.

[0043] In one embodiment, when the second acidic pulp has a temperature above 60 °C before the second separation stage, the second acidic pulp is cooled before lignin is subsequently separated from the second acidic pulp in the second separation stage.

[0044] This cooling is particularly beneficial when a filter press is used in the corresponding separation stage, as it can extend the service life of the filter.

[0045] In one embodiment, the first lignin source is industrial lignin, preferably selected from the group consisting of alkali lignins such as sulfate-processed lignin and alkali-processed lignin, organic solvent lignin, and hydrolyzed lignin. The alkali lignin may be first neutralized or acidified to form non-alkali lignin, which can be achieved through an additional pulping stage as described below. This neutralization or acidification treatment may also be completed before the first lignin source is fed into the pulping stage, allowing it to proceed directly to the pulping stage. If the alkali lignin has a high purity (as further explained below), the neutralization treatment may alternatively be carried out during the pulping stage, wherein an appropriately increased amount of a first acidifying agent is added to the pulping stage.

[0046] Industrial lignin can refer to solid lignin with a purity of at least 60%, or solid lignin obtained as a product of the extraction process during the processing of biomass materials.

[0047] In one embodiment, the method includes, prior to the first pulping stage, an additional pulping stage in which a second lignin source is mixed with an additional acidifying agent to form a third acidic pulp, followed by a fourth separation stage in which lignin is separated from the third acidic pulp to form a fourth lignin separation cake, wherein the first lignin source mixed with the first acidifying agent in the first pulping stage is the fourth lignin separation cake.

[0048] In one embodiment, the second lignin source is industrial lignin, wherein preferably the industrial lignin is selected from the group consisting of alkali lignins such as sulfate-processed lignin and alkali-processed lignin, organic solvent lignin and hydrolyzed lignin.

[0049] In one embodiment, during the additional pulping stage, the temperature of the third acidic pulp is maintained at or below 75 °C, preferably below 72 °C, more preferably below 70 °C, and most preferably in the range of 55-65 °C.

[0050] In one embodiment, the pH value of the third acidic slurry is in the range of 1 to 4.

[0051] In one embodiment, the amount of the additional acidifying agent added ranges from Y to Z kg / ton of the second lignin source, where Y is equivalent to 100 kg H2SO4 / ton of the second lignin source and Z is equivalent to 400 kg H2SO4 / ton of the second lignin source. If H2SO4 is used, then Y is 100 kg and Z is 400 kg. The amount of sulfuric acid equivalent can be, for example, 150, 200, 250, 300, or 350 kg. If formic acid is used instead, as described above, the Y value of 100 kg H2SO4 is converted to approximately 94 kg of formic acid. Z will be approximately 376 kg of formic acid. If carbonic acid is used instead, it is assumed that one H2CO3 molecule releases one H2O. + If the ions are equal, then approximately 127 kg of carbonic acid would be equivalent to 100 kg of sulfuric acid. This calculation method is the same as the formic acid calculation method described above.

[0052] In an embodiment of black liquor treatment, the method includes, prior to the first pulping stage:

[0053] In the precipitation stage, an acidifying agent is added to the black liquor, which has a first alkaline pH value, to lower the pH value of the black liquor to a second pH level from neutral to alkaline, thereby initiating lignin precipitation.

[0054] In the subsequent third separation stage, the precipitated lignin is separated from the residual liquid phase of the acidified original black liquor, where the pH remains in the neutral to alkaline range, to form a third lignin separation cake.

[0055] In one embodiment, during the precipitation stage, the second pH level is higher than pH 7 and lower than pH 12.

[0056] Lignin typically precipitates at a pH between 11.5 and 12. In this embodiment, the lignin purity of the black liquor typically does not reach at least 70% as described above. The purity of the third lignin separation cake may be at least 70%, but may be lower. If the purity is low, the method preferably includes an additional pulping stage and a fourth separation stage. If an ultra-high purity lignin product is required, the additional pulping stage and the fourth separation stage may also be included.

[0057] In one embodiment, the second lignin source mixed with the additional acidifier during the additional pulping stage is a third lignin separation cake.

[0058] Alternatively, in embodiments without the aforementioned additional acidifier and fourth separation stage, the first lignin source mixed with the first acidifier in the first pulping stage is the third lignin separation cake.

[0059] In one embodiment, the black liquor is raw black liquor. Alternatively, lignin can be partially removed from the black liquor. For example, lignin with a specific molecular weight range may have already been removed during previous precipitation and corresponding separation stages.

[0060] According to a second aspect, a method for extracting lignin from a lignin source is provided, the method comprising:

[0061] An additional pulping stage in which the lignin source is mixed with an additional acidifying agent to form a third acid pulp;

[0062] The subsequent fourth separation stage involves separating lignin from the third acid pulp to form a fourth lignin separation cake;

[0063] In the pulping stage, the fourth lignin separation cake is mixed with the first acidifying agent to form the first acid pulp;

[0064] The subsequent first separation stage involves separating lignin from the first acidic pulp to form a first lignin separation cake;

[0065] The subsequent re-pulping stage involves mixing the first lignin separation cake with a second acidifying agent to form a second acidic pulp; and

[0066] The subsequent second separation stage involves separating lignin from the second acidic pulp to form a second lignin separation cake.

[0067] in

[0068] During the pulping stage, the temperature of the first acidic pulp is maintained below 80 °C.

[0069] In the re-slurrying stage, the second acidic slurry is heated to a temperature range of 80 °C to 140 °C; and

[0070] During the pulping stage, the purity of the lignin source is at least 70%.

[0071] In one embodiment, the lignin source equivalent to the second lignin source described above is industrial lignin, wherein the industrial lignin is preferably selected from the group consisting of alkali lignin (such as sulfate-processed lignin and alkali-processed lignin), organic solvent lignin, and hydrolyzed lignin.

[0072] According to a third aspect, a method for extracting lignin from black liquor is provided, the method comprising the following stages in sequence:

[0073] In the precipitation stage, an acidifying agent is added to the black liquor having a first alkaline pH value to lower the pH value of the black liquor to a second pH level from neutral to alkaline, thereby initiating lignin precipitation.

[0074] In the subsequent third separation stage, the precipitated lignin is separated from the residual liquid phase of the acidified original black liquor, where the pH remains in the neutral to alkaline range, to form a third lignin separation cake. Preferably, the second pH level is higher than pH 7 and lower than pH 12.

[0075] The subsequent additional pulping stage involves mixing the third lignin separation cake with an additional acidifying agent to form a third acid pulp.

[0076] The subsequent fourth separation stage involves separating lignin from the third acid pulp to form a fourth lignin separation cake;

[0077] In the subsequent pulping stage, the fourth lignin separation cake is mixed with a first acidifying agent to form a first acidic pulp.

[0078] The subsequent first separation stage involves separating lignin from the first acidic pulp to form a first lignin separation cake;

[0079] The subsequent re-pulping stage involves mixing the first lignin separation cake with a second acidifying agent to form a second acidic pulp; and

[0080] The subsequent second separation stage involves separating lignin from the second acidic pulp to form a second lignin separation cake.

[0081] in

[0082] During the pulping stage, the temperature of the first acidic pulp is maintained below 95 °C, preferably 80 °C.

[0083] During the additional pulping stage, the temperature of the third acidic pulp is maintained below 95 °C, preferably 80 °C.

[0084] In the re-slurrying stage, the second acidic slurry is heated to 65 °C to 140 °C, preferably 80 °C to 140 °C, more preferably 80 °C to 110 °C, and

[0085] The temperature of the second acidic slurry is higher than the temperature of the third acidic slurry.

[0086] It is understood that, for embodiments that include a sedimentation stage and an additional pulping stage, the method can achieve similar or the same advantages as described above.

[0087] Within these temperature ranges (i.e., the temperatures of the first and third acidic pulps are kept below 95°C), a favorable combination of low sodium content (below 120 ppm) and extremely low transition metal concentrations (iron concentration below 8 ppm) can be achieved in the first lignin separation cake. This means that these contents will be even lower when obtaining the final second lignin separation cake. Depending on the specific application, sometimes it is necessary to remove more transition metals and other metals while allowing for a higher sodium content.

[0088] If the first and third acidic slurries are kept below 80°C, extremely low levels of sodium, as described in the first aspect, any relevant embodiments, and / or examples, can be obtained.

[0089] The upper limit temperature of the second acidic slurry is 110°C, preferably 106°C, which is advantageous because the pressure will be kept at a relatively low level, and the equipment for performing the method can be simpler and less expensive.

[0090] In this specification, the term "separation stage" encompasses any separation method. Preferably, separation is carried out using centrifugation, filter presses, belt filters, rotary filters (such as drum filters or disc filters), settling tanks, or similar equipment, with filter presses being the most preferred. In this specification, the term "raw black liquor" is intended to include waste cooking liquor from the digester, in which most of the lignin derived from the original cellulose material has dissolved. "Raw black liquor" may also contain significant amounts of organic and inorganic matter and may have undergone separation processes to extract turpentine or other specific components while maintaining the total amount of dissolved lignin unchanged.

[0091] In one embodiment, the method is used to extract lignin with reduced metal content, specifically lignin with reduced alkali metal and transition metal content.

[0092] In the above and below aspects, implementation methods and examples, it should also be understood that the names of each stage, acidifier, slurry, acidifier, etc., can be changed without changing their respective technical functions. For example, if necessary, the third separation stage can be named "first separation stage", the third lignin separation cake can be named "first lignin separation cake", the additional pulping stage can be named "first pulping stage", the fourth precipitation stage can be named "second precipitation stage", the third acidic slurry can be named "first acidic slurry", the fourth lignin separation cake can be named "second lignin separation cake", the pulping stage can be named "second pulping stage", the first precipitation stage can be named "third precipitation stage", the first acidic slurry can be named "second acidic slurry", the first lignin separation cake can be named "third lignin separation cake", the re-pulping stage can be named "third pulping stage", the second precipitation stage can be named "fourth precipitation stage", the second acidic slurry can be named "third acidic slurry", and the second lignin separation cake can be named "fourth lignin separation cake". As another example, if necessary, the additional acidifier can be named "first acidifier," the first acidifier "second acidifier," and the second acidifier "third acidifier." In other words, the use of terms such as "first," "second," "third," "fourth," and "additional" is for identification purposes only and does not assign any technical meaning to the relevant features.

[0093] It should be understood that the technical advantages and effects associated with a feature and / or implementation method in one aspect also apply to corresponding, similar, or equivalent features and / or implementation methods in other aspects. It is equally apparent that features of each aspect and / or its implementation method can be applied to other aspects and / or its implementation methods. Attached Figure Description

[0094] The accompanying drawings are provided to illustrate a currently preferred, non-limiting exemplary embodiment of the device disclosed herein. The above and other advantages of the features and objectives of this disclosure will become more apparent, and aspects and embodiments will be better understood, by reading the following detailed description in conjunction with the accompanying drawings, wherein: Figure 1 This is a flowchart illustrating a method for extracting lignin according to some embodiments of the present disclosure; Figure 2 Based on some embodiments of this disclosure, it is described that includes Figure 1 A flowchart of another method for extracting lignin; Figure 3 This is a flowchart illustrating a method for extracting lignin according to some embodiments of the present disclosure; Figure 4It is a bar graph showing the sodium (Na) and iron (Fe) content (unit: ppm) in the produced lignin under different combinations of process temperatures (unit: degrees Celsius); Figure 5 The following is a flowchart illustrating a method for extracting lignin, based on some embodiments of this disclosure; Figure 6 Based on some embodiments of this disclosure, it is described that includes Figure 5 A flowchart of another method for extracting lignin; Figure 7 It is a chart showing the sodium (Na) content (unit: ppm) in lignin produced by methods according to some embodiments of this disclosure; Figure 8 It is a chart showing the iron (Fe) content (unit: ppm) in lignin produced by methods according to some embodiments of this disclosure; Figure 9 This is to show that, according to this disclosure Figure 5 or Figure 6 A chart showing the sodium (Na) content (in ppm) in the lignin produced by this method; and Figure 10 This is to show that, according to this disclosure Figure 5 or Figure 6 A chart showing the iron (Fe) content (in ppm) in lignin produced by this method.

[0095] Detailed description of preferred embodiments

[0096] Figure 1 This is a flowchart illustrating a method 100 for extracting lignin according to some embodiments of the present disclosure. In some embodiments, at 110, the method includes a pulping stage or step. At 120, the method includes a first separation stage or step. At 130, the method includes a re-pulping stage or step. At 140, the method includes a second separation step. In the pulping stage or step 110, a first lignin source is mixed with a first acidifying agent to form a first acidic pulp. In the first separation stage 120, lignin is separated from the acidic pulp to form a first lignin separation cake. In the re-pulping stage 130, the first lignin separation cake may be mixed with a second acidifying agent to form a second acidic pulp. In the second separation stage 140, lignin may be separated from the second acidic pulp to form a second lignin separation cake. The temperature of the first acidic pulp in the pulping stage 110 is maintained at or below 80 °C, preferably 75 °C, more preferably 70 °C. The second acidic slurry in the re-slurrying stage is heated to a temperature range of 80 °C to 140 °C, preferably 80 °C to 110 °C.

[0097] In some embodiments, the temperature of the first acidic slurry may be below 80 °C, preferably below 75 °C. In some embodiments, the temperature of the first acidic slurry during the pulping stage is in the range of 30 °C to 70 °C. In some embodiments, the temperature of the first acidic slurry during the pulping stage is in the range of 30 °C to 65 °C, such as 40 °C to 60 °C.

[0098] In some embodiments, during the pulping stage 110, a pH value ranging from 1 to 5, preferably from 1 to 4, is established in the first acidic pulp. In some embodiments, during the re-pulping stage 130, a pH value ranging from 1 to 5, preferably from 1 to 4, is established in the second acidic pulp. The selected pH value depends on the lignin material used as the first lignin source. For example, a pH value below 1.5 may be beneficial when the lignin material contains undesirable substances such as oxalates (e.g., calcium oxalate).

[0099] Combined with the following text Figure 4 As shown, in some embodiments, the sodium content in the second lignin separation cake is less than 25 ppm. In some embodiments, the iron content in the second lignin separation cake may be less than 10 ppm, specifically less than 7 ppm.

[0100] In some embodiments, the amount of lignin dissolved from the second lignin separation cake may be maintained below 15%. In some embodiments, the second acid slurry may be cooled before lignin is subsequently separated from the second acid slurry in the second separation stage 140. In one embodiment, the first acid slurry is cooled to below 60 °C. When using a filter press, this helps to limit filter degradation.

[0101] In method 100, the first lignin source can be non-alkaline industrial lignin as described above or high-purity lignin that is still alkaline in nature. The industrial lignin can be selected from the group consisting of alkali lignin (such as sulfate-processed lignin and alkali-processed lignin), organic solvent lignin, and hydrolyzed lignin. In some embodiments, the method can be used to extract lignin with reduced metal content, specifically lignin with reduced alkali metal and transition metal content.

[0102] When using non-alkaline lignin as the first lignin source, in some instances, the amount of the first acidifier added can be less than X kg / ton of the first lignin source, where X is equivalent to 100 kg H₂SO₄ / ton of the first lignin source. For example, the amount of the first acidifier added can be or is equivalent to less than 50 kg sulfuric acid / ton of the first lignin source. Depending on the lignin material used as the first lignin source, this amount can be or is equivalent to less than 10 kg sulfuric acid / ton of the first lignin source. These upper limits apply to most (if not all) non-alkaline lignin materials, and the process can be used to obtain lignin products of extremely high purity. The specific amount of sulfuric acid or its equivalent can be adjusted according to the desired pH value, which is typically between 2 and 4.

[0103] In one embodiment, the purity of the first lignin source during the pulping stage is at least 80% based on the dry weight of the lignin material. In some embodiments, the purity of the first lignin source may be at least 89% or 90%. In this way, high-purity lignin is produced using the method 100, as described in more detail in the following examples.

[0104] In some embodiments, the second acidic pulp in the re-slurrying stage 130 may be heated to 70 °C to 150 °C, preferably 80 °C to 140 °C, more preferably 90 °C to 140 °C, such as 85 °C, 95 °C, 106 °C, 110 °C, 115 °C, 120 °C, 125 °C, 130 °C, and 135 °C. In these embodiments, the temperature of the re-slurrying stage is equal to or higher than the temperature of the pulping stage.

[0105] Figure 2 This is a flowchart illustrating a method 200 for extracting lignin according to some embodiments of the present disclosure, the method 200 comprising: Figure 1 Method 100. In some embodiments, at 203, the method may include a precipitation stage or step. At 205, the method may include a third separation stage or step. Stages 203 and 205 are performed prior to the pulping stage 110. In stage 203, an acidifying agent is added to the black liquor having a first alkaline pH value to lower the pH value of the black liquor to a second pH level from neutral to alkaline, initiating lignin precipitation. In stage 205, the precipitated lignin, as a third lignin separation cake, can be separated from the remaining liquid phase of the acidified original black liquor, which remains in the neutral to alkaline pH range. The lignin material mixed with the first acidifying agent in the pulping stage 110 is the third lignin separation cake. The second pH level may be higher than pH 7 and lower than pH 12. Typically, the black liquor in this process will be the original black liquor.

[0106] Because the third lignin separation cake in this embodiment is alkaline, the amount of acidifier added in the pulping stage 110 is typically higher than 100 kg of sulfuric acid, or an equivalent amount of another suitable acid. If a lower pH is used in the pulping stage, the amount of acidifier added will also be higher.

[0107] Figure 3 Other methods 300 according to some embodiments of this disclosure are shown. Unless otherwise stated, method 300 corresponds to method 200. The raw black liquor 301, serving as the lignin material, undergoes a precipitation stage 303, corresponding to precipitation stage 203. An acidifying agent 302 is added to the black liquor; this can be sulfuric acid (H₂SO₄), carbonic acid (from carbon dioxide), aqueous solutions of these acids respectively, or any other suitable acidifying agent. In stage 305, the precipitated lignin is separated from the remaining liquid phase of the acidified raw black liquor as a third lignin separation cake.

[0108] The third lignin separation cake is pulped in pulping stage 310 by adding a first acidifying agent 312 to form a first acidic pulp. The first acidifying agent can be sulfuric acid (H2SO4), formic acid, oxalic acid, aqueous solutions of these acids respectively, or any other suitable acidifying agent. In separation stage 320, lignin is separated from the acidic pulp as the first lignin separation cake. Subsequently, the first lignin separation cake is re-acidified in re-pulping stage 330. A second acidifying agent 332 is added to form a second acidic pulp. The second acidifying agent can be sulfuric acid (H2SO4), formic acid, oxalic acid, aqueous solutions of these acids respectively, or any other suitable acidifying agent.

[0109] Step 335 indicates heating the second acidic slurry to the temperature specified in this disclosure. In some embodiments, the second acidic slurry in the re-slurrying stage 330 may be heated to a temperature range of 335 to 70 °C to 150 °C, preferably 80 °C to 140 °C, more preferably 90 to 140 °C, such as 75 °C, 80 °C, 85 °C, 95 °C, 105 °C, 110 °C, 115 °C, 120 °C, 125 °C, 130 °C, and 135 °C. The second acidic slurry may be held at the above temperatures for a period of 10-120 minutes, which is inversely proportional to the applied temperature.

[0110] After heating the second acidic slurry, it is preferable to cool the second acidic slurry (not shown), followed by separation stage 340. A second lignin separation cake 350 is output from separation stage 340.

[0111] Select the acidification dosage used in steps 302, 312, and 332 to achieve the pH levels described herein for each of the precipitation, pulping, and re-pulping stages. As an example, the sulfuric acid dosage can be compared to that in WO2014 / 116150A1. Figure 2 and / or Figure 5 They are similar to those in the series, and can be changed according to the desired pH value.

[0112] The separation stages 305, 320, and 340 may each include a filtration step and a washing step. Added chemicals, such as acidifiers, can be recovered from the respective filtrates.

[0113] Figure 5 This is a flowchart illustrating a method 500 for extracting or purifying lignin according to some embodiments of the present disclosure. Unless otherwise stated, method 500 is identical to method 100. Method 500 includes an additional pulping stage 502, followed by a fourth separation stage 504, wherein lignin is separated from a third acidic pulp to form a fourth lignin separation cake. At 510, the method includes the pulping stage or step, which is the same as or similar to step 110 of method 100. At 520, the method includes a first separation stage or step, which is the same as or similar to step 120 of method 100. At 530, the method includes the re-pulping stage or step, which is the same as or similar to step 130 of method 100. At 540, the method includes a second separation step, which is the same as or similar to step 140 of method 100.

[0114] In the additional pulping stage 502, the second lignin source is mixed with an additional acidifying agent to form a third acidic pulp. The lignin may remain in the pulping stage 502 for a duration of 30 to 90 minutes. In the fourth separation stage 504, the lignin from the additional pulping stage 502 is separated from the third acidic pulp to form a fourth lignin separation cake. The fourth lignin separation cake serves as the first lignin source in the pulping stage 510. The pH of the third acidic pulp may be between 1 and 4, or preferably between 2 and 4. The purity of the second lignin source is preferably at least 70%. The lignin source used for the additional acidifying agent may be an alkali lignin material (which is alkaline), and the amount of the third acidifying agent is suitable for the lignin source and the desired pH. This amount may, for example, be in the range of 100 to 400 kg sulfuric acid (such as 200 kg sulfuric acid) per ton of the second lignin source. Alternatively, as described above, an equal amount of another acid may be used. In this case, the second lignin source may be lignin precipitated from black liquor, which is discussed below. Figure 6A more detailed description follows. The purity of the fourth lignin separation cake can be at least 80%, and can be equal to or greater than 89%. Alternatively, the lignin material mixed with the additional acidifying agent can be non-alkaline lignin. In this case, depending on the desired pH, less acidifying agent may be required than with alkaline lignin.

[0115] In the pulping stage or step 510, the fourth lignin separation cake is mixed with the first acidifying agent to form a first acidic pulp. The amount of the first acidifying agent added can be less than 100 kg, even less than 20 kg or 10 kg of sulfuric acid, or an equivalent amount of other suitable acidifying agent, since the fourth lignin separation cake has previously been acidified in the additional pulping stage 502. Depending on the process design, more acidifying agent may also be used, for example, in which various acid streams can be recycled. In the first separation stage 520, lignin is separated from the acidic pulp to form the first lignin separation cake. In the re-pulping stage 530, the first lignin separation cake can be mixed with a second acidifying agent to form a second acidic pulp. In the second separation stage 540, lignin can be separated from the second acidic pulp to form a second lignin separation cake. The temperature of the acidic pulp in both the additional pulping stage 502 and the pulping stage 510 is maintained at or below 80 °C. This facilitates the removal of sodium from the lignin, as shown in more detail in the examples below.

[0116] In some embodiments, the temperature of the first acidic slurry may be below 80 °C, preferably below 75 °C. In some embodiments, the temperature of the first acidic slurry during the pulping stage is in the range of 30 °C to 75 °C.

[0117] In some embodiments, during the pulping stage 510, a pH value ranging from 1 to 5, preferably 1 to 4, is established in the first acidic pulp. In some embodiments, during the re-pulping stage 130, a pH value ranging from 1 to 5, preferably 1 to 4, is established in the second acidic pulp. The selected pH level depends on the lignin material to be pulped. For example, a pH value below 1.5 may be beneficial when the lignin material contains undesirable substances such as oxalates (e.g., calcium oxalate).

[0118] Combined with the following text Figure 4 , 7 As shown in Figures 8, 9, and 10, in some embodiments, the sodium content in the second lignin separation cake is less than 25 ppm. In some embodiments, the iron content in the second lignin separation cake may be less than 10 ppm, specifically less than 7 ppm.

[0119] In some embodiments, the amount of lignin dissolved from the second lignin separation cake may be maintained below 15%. In some embodiments, the second acid slurry is cooled before lignin is subsequently separated from the second acid slurry in the second separation stage 140.

[0120] In one embodiment, the first acidic slurry can be cooled to below 60 °C. This helps limit filter degradation when a filter press is used in the respective separation stages. The same cooling treatment can be applied to any or all other slurries (such as the third and second acidic slurries) and the precipitated lignin before lignin separation in their respective separation stages.

[0121] The additional acidifier may be sulfuric acid (H2SO4), formic acid, oxalic acid, or an aqueous solution of each of these acids, or an aqueous mixture of two or more of these acids, or any other suitable acidifier.

[0122] Figure 6 This is a flowchart illustrating a method 600 for extracting lignin according to some embodiments of the present disclosure, the method 600 comprising: Figure 5 Method 500. In this case, the lignin material to be treated is black liquor. In some embodiments, at 603, the method may include the precipitation stage or step, which is the same as step 203 or 303 unless otherwise stated. At 605, the method includes a third separation stage or step, which is the same as stage 205 or 305 unless otherwise stated. Stages 603 and 605 are performed prior to the additional pulping stage 502. In stage 603, an acidifying agent is added to the black liquor having a first alkaline pH value to lower the pH value of the black liquor to a second pH level from neutral to alkaline, initiating lignin precipitation. In stage 605, the precipitated lignin, as a third lignin separation cake, can be separated from the remaining liquid phase of the acidified original black liquor, which remains in the neutral to alkaline pH range. The second lignin source mixed with the additional acidifying agent in the additional pulping stage 502 is the third lignin separation cake. The second pH level may be higher than pH 7 and lower than pH 12. Typically, in this process, the black liquor will be the raw black liquor.

[0123] Example 1

[0124] Use according to Figure 2 and Figure 3 One embodiment of the process employs different temperatures during the pulping and re-pulping stages to produce lignin with reduced metal content. The results of the metal content are shown in Table 1 below. Alternatively, the lignin in this embodiment can be considered as the lignin from the first lignin separation cake during method 500 or 600.

[0125] Alkaline lignin obtained from black liquor was used as the starting lignin material. The alkaline lignin was mixed with the acidifying agent to form a pulp, and each pulp was maintained at the temperatures shown in Table 1 below. The pulps were maintained at their respective temperatures for 60 minutes. The pH value was approximately 2.5 during both the pulping and re-pulping stages.

[0126] Table 1: Concentration of various metals in the second lignin separation cake at different processing temperatures (unit: ppm).

[0127]

[0128] In Table 1, the numbers in the leftmost column represent the temperature combinations (in degrees Celsius) applied to the pulping and repulping stages, respectively. For example, "55+55" means the pulping stage temperature is 55 °C and the repulping stage temperature is 55 °C, "55+95" means the pulping stage temperature is 55 °C and the repulping stage temperature is 95 °C, and so on.

[0129] The reference control in Table 1 refers to the same lignin material, but mixed once with an acidifier at 55 °C to form a slurry, and then separated.

[0130] As can be clearly seen from Table 1, generally, the higher the pulping and re-pulping temperatures, the more effectively transition metals (such as copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn)) are removed from lignin, especially when the re-pulping temperature is above 70 °C (as can be seen in the middle two rows of Table 1, the re-pulping is carried out at 95 °C). If the pulping stage temperature is kept at a lower level (depending on the lignin product requirements, below 100 °C or 70 °C), the content of alkali metals sodium (Na) and potassium (K) can be reduced most effectively. The advantage of processing at higher temperatures is that more other metals can be removed, but compared to lower pulping stage temperatures, slightly more alkali metals may remain.

[0131] Data show that the removal of transition metals is most significant when the temperature during the re-slurrying stage is above approximately 90 °C. Under these conditions, for example, the iron content was reduced to approximately half that of the reference control.

[0132] The data in the iron (Fe) and sodium (Na) columns of Table 1 show... Figure 4In these data, iron and sodium are considered representative of transition metals and alkali metals, respectively. It was observed from these data that sodium was effectively removed, especially when a lower temperature (i.e., 55 °C) was used during the pulping stage. In these cases, the sodium content was 21 ppm and 18 ppm, respectively. When the temperature was increased to 95 °C, the sodium content of the lignin product was 59 ppm. When the temperature was increased to 120 °C, the sodium content of the lignin product was 596 ppm, approximately half the sodium content of the lignin reference control.

[0133] The iron concentration decreases with increasing temperature. Figure 4 As can be seen from the data in Table 1 above, when it is necessary to reduce both iron and sodium to a low level, the temperature of the pulping stage should be relatively low, that is, below 100 °C or even 70 °C (e.g., 55 °C) depending on the product requirements. The temperature of the subsequent pulping stage can be the same or higher, that is, above 70 °C or 80 °C (e.g., 95 °C and 120 °C).

[0134] Example 2

[0135] The lignin is derived from... Figure 5 or Figure 6 The method involves a pulping stage at a temperature of approximately 55 °C, which varies. In this embodiment, the lignin is derived from a first lignin separation cake.

[0136] The results for sodium and iron concentrations are shown in the figures below. Figure 7 and Figure 8 In the middle. For example Figure 7 As shown, when the temperature is above about 75 °C, the sodium concentration increases suddenly. Figure 7 The dashed lines drawn in the image serve as visual guides, with their intersection located at 75 °C. In this case, a sodium concentration of less than approximately 50 ppm can be obtained at temperatures below 80 °C. At 72 °C, the sodium concentration is approximately 24 ppm. This indicates that maintaining the temperature of the additional pulping stage and the pulping stage below a threshold temperature of approximately 80 °C has a beneficial effect on reducing the sodium concentration. If even lower sodium levels are required, then below 75 °C is a suitable threshold temperature.

[0137] like Figure 8 As shown, the iron concentration in the lignin decreases with increasing temperature. Excessive temperature reduction (i.e., below approximately 70 °C) reduces the effectiveness of iron removal from the lignin at each stage. Therefore, it may be desirable to use a temperature range of 70 to 80 °C during the pulping stage to achieve a favorable combination of iron and sodium reduction.

[0138] Example 3

[0139] The lignin is derived from... Figure 5 or Figure 6 The method is similar to that of Example 2, but in this case, the temperature of the pulping stage is approximately 72 °C, while the temperature of the re-pulping stage varies. The lignin tested was obtained from the second lignin separation cake, and the first lignin separation cake was used for comparison.

[0140] The results for sodium and iron concentrations are shown in the figures below. Figure 9 and Figure 10 In the two figures, the left column represents comparative data for a first lignin separation cake sample prepared in a manner similar to that described in Example 2. The temperature of the additional pulping stage is 55 °C, while the temperature of the pulping stage is 72 °C. The re-pulping stages are carried out at 98 °C and 106 °C, respectively. Therefore, in this case, the lignin undergoes a total of three acidic pulping stages. This is done to obtain a high-purity lignin product. For example, the second lignin source entering the additional pulping stage can be alkaline lignin obtained from black liquor with a purity of less than 70%. Although the method including three acidic stages is preferred, similar results can be obtained for lignin with higher purity (e.g., at least 89%), and optionally, the lignin is non-alkaline lignin, fed directly as the first lignin source to the pulping stage.

[0141] like Figure 9 As shown, compared to both stages, the sodium concentration in the second lignin separation cake was below 76 ppm, approximately 5 ppm, at both 98 °C and 106 °C, while the concentration was slightly lower at 98 °C. Therefore, in some applications, maintaining the temperature of the repulping stage below 100 °C may be advantageous. The concentration was reduced by approximately 70% compared to the first lignin separation cake.

[0142] like Figure 10 As shown, the decrease in iron concentration is similar to that of sodium concentration. The repulping stage at 98 °C resulted in an iron concentration of approximately 5.5 ppm in the second lignin separation cake, while the repulping stage at 106 °C resulted in 5 ppm. Therefore, the decrease in iron concentration is relatively moderate for this temperature increase. Compared to the first lignin separation cake, the iron concentration decreased by at least approximately 69%.

[0143] Therefore, the method according to this disclosure can be used to obtain lignin of high purity or ultra-high purity.

[0144] This disclosure further includes the following examples: 1. A method for extracting lignin from a lignin source, the method comprising the following stages performed sequentially: a pulping stage, wherein the lignin source is mixed with a first acidifying agent to form a first acidic pulp; The subsequent first separation stage involves separating lignin from the first acidic pulp to form a first lignin separation cake. The subsequent repulping stage involves mixing the first lignin separation cake with a second acidifying agent to form a second acidic pulp, and The subsequent second separation stage involves separating lignin from the second acidic pulp to form a second lignin separation cake. Its features During the pulping stage, the temperature of the first acidic pulp is maintained at or below 100 °C.

[0145] 2. According to the method described in Example 1, the first acidic pulp is maintained at or below 70°C during the pulping stage.

[0146] 3. The method according to Example 1 or 2, wherein the temperature of the first acidic slurry is below 65 °C, preferably below 60 °C.

[0147] 4. The method according to Example 1 or 2, wherein the temperature of the first acidic slurry is maintained in the range of 30 °C to 70 °C during the pulping stage.

[0148] 5. The method according to any one of the foregoing examples, wherein during the pulping stage, the temperature of the first acidic pulp is maintained in the range of 30 °C to 65 °C, preferably 40 °C to 60 °C.

[0149] 6. The method according to any one of the foregoing examples, wherein in the re-slurrying stage, the second acidic slurry is heated to a temperature of 70 °C to 150 °C, preferably 80 °C to 140 °C, more preferably 90 °C to 140 °C, wherein the temperature of the re-slurrying stage is equal to or higher than the temperature of the slurrying stage.

[0150] 7. The method according to any one of the foregoing examples, wherein a pH value in the range of 1-5, preferably 1-4, is established in the first acidic slurry of the pulping stage.

[0151] 8. The method according to any one of the foregoing examples, wherein a pH value in the range of 1-5, preferably 1-4, is established in the second acidic slurry during the re-slurrying stage.

[0152] 9. The method according to any one of the foregoing examples, wherein the sodium content in the second lignin separation cake is less than 25 ppm.

[0153] 10. The method according to Example 8, which is subordinate to Example 5, wherein the iron content in the second lignin separation cake is less than 15 ppm.

[0154] 11. The method according to any one of the foregoing examples, wherein

[0155] When the first acidic pulp has a temperature above 60 °C before the first separation stage, the first acidic pulp is cooled before lignin is subsequently separated from it in the first separation stage; and / or

[0156] When the second acid pulp has a temperature above 60 °C before the second separation stage, the second acid pulp is cooled before the lignin is subsequently separated from the second acid pulp in the second separation stage.

[0157] 12. The method according to any one of the foregoing examples, wherein the lignin source is industrial lignin, wherein preferably the industrial lignin is selected from the group consisting of alkali lignin such as sulfate-processed lignin and alkali-processed lignin, organic solvent lignin, and hydrolyzed lignin.

[0158] 13. The method according to any one of Examples 1 to 11, wherein black liquor is used as a lignin source, the method comprising, prior to the first pulping stage: In the precipitation stage, an acidifying agent is added to the black liquor, which has a first alkaline pH value, to lower the pH value of the black liquor to a second pH level from neutral to alkaline, thereby initiating lignin precipitation. In the subsequent third separation stage, the precipitated lignin is separated from the residual liquid phase of the acidified original black liquor, where the pH remains in the neutral to alkaline range, to form a third lignin separation cake. The lignin source mixed with the first acidifying agent in the first pulping stage is the third lignin separation cake.

[0159] The preferred second pH level is higher than pH 7 and lower than pH 12.

[0160] 14. The method according to Example 13, wherein the black liquor is raw black liquor or delignified black liquor.

[0161] 15. The method according to any one of the foregoing examples, wherein the method is used to extract lignin with reduced metal content, particularly lignin with reduced alkali metal and transition metal content.

[0162] Although the invention has been described with reference to specific embodiments (which are also shown in the accompanying drawings), those skilled in the art will understand that many variations and modifications can be made within the scope of the specification and as defined by the following claims.

Claims

1. A method for extracting lignin, the method comprising the following stages performed sequentially: a pulping stage, wherein a first lignin source is mixed with a first acidifying agent to form a first acidic pulp; The subsequent first separation stage involves separating lignin from the first acidic pulp to form a first lignin separation cake. The subsequent repulping stage involves mixing the first lignin separation cake with a second acidifying agent to form a second acidic pulp, and The subsequent second separation stage involves separating lignin from the second acidic pulp to form a second lignin separation cake. Its features During the pulping stage, the temperature of the first acidic pulp is maintained below 80 °C. In the re-slurrying stage, the second acidic slurry is heated to a temperature range of 80°C to 140°C; and wherein During the pulping stage, the purity of the first lignin source is at least 70%.

2. The method according to claim 1, wherein during the pulping stage, the temperature of the first acidic pulp is maintained at or below 75°C, preferably at or below 72°C, more preferably at or below 70°C.

3. The method according to claim 1 or 2, wherein, Based on the dry weight of the lignin material, the purity of the first lignin source is at least 80%, preferably at least 85%, and most preferably at least 90%.

4. The method according to claim 1 or 3, wherein during the pulping stage, the temperature of the first acidic pulp is maintained in the range of 30 °C to 75 °C.

5. The method according to any one of the preceding claims, wherein during the pulping stage, the temperature of the first acidic pulp is maintained in the range of 45°C to 75°C, preferably in the range of 55°C to 75°C.

6. The method according to any one of the preceding claims, wherein in the re-slurrying stage, the second acidic slurry is heated to a temperature range of 90 °C to 140 °C, or a temperature range of 80 °C to 120 °C, or a temperature range of 80 °C to 106 °C, or a temperature range of 80 °C to 98 °C.

7. The method according to any one of the preceding claims, wherein the second acidic slurry is maintained for a duration ranging from 10 to 120 minutes, preferably from 15 to 90 minutes, more preferably from 15 to 60 minutes, prior to the separation of the lignin in the second separation stage.

8. The method according to any one of the preceding claims, wherein during the pulping stage, a pH value in the first acidic pulp is established in the range of 1-5, preferably 1-4, more preferably 2-4.

9. The method according to any one of the preceding claims, wherein in the re-slurrying stage, a pH value in the range of 1-5, preferably 1-4, and more preferably 2-4 is established in the second acidic slurry.

10. The method according to any one of the preceding claims, wherein the amount of the first acidifying agent added is less than X kg / ton of the first lignin source, where X is equivalent to 100 kg, preferably 20 kg H2SO4 / ton of the first lignin source.

11. The method according to any one of the preceding claims, wherein the amount of the second acidifying agent added is less than X kg / ton of the first lignin source, where X is equivalent to 100 kg H2SO4 / ton of the first lignin source, wherein preferably the amount of the second acidifying agent added is less than X kg / ton of lignin source, where X is equivalent to 20 kg, preferably 10 kg H2SO4 / ton of the first lignin source.

12. The method according to any one of the preceding claims, wherein the sodium content in the second lignin separation cake is less than 25 ppm.

13. The method according to any one of the preceding claims, wherein the iron content in the second lignin separation cake is less than 15 ppm.

14. The method according to any one of the preceding claims, wherein When the temperature of the first acidic pulp is above 60 °C before the first separation stage, the first acidic pulp is cooled before lignin is subsequently separated from it in the first separation stage; and / or The second acid pulp is cooled before lignin is separated from it in the subsequent second separation stage.

15. The method according to any one of the preceding claims, wherein the method comprises, prior to the first pulping stage: The additional pulping stage involves mixing the second lignin source with an additional acidifying agent to form a third acid pulp. The subsequent fourth separation stage involves separating lignin from the third acidic pulp to form a fourth lignin separation cake. The fourth lignin separation cake is the first lignin source mixed with the first acidifying agent in the first pulping stage.

16. The method according to claim 15, wherein during the additional pulping stage, the temperature of the third acidic pulp is maintained at or below 75 °C, preferably below 70 °C, more preferably below 65 °C, and most preferably in the range of 55-65 °C.

17. The method according to claim 15 or 16, wherein the amount of the additional acidifying agent added is Y to Z kg / ton of lignin source, where Y is equivalent to 100 kg H2SO4 / ton of lignin source and Z is equivalent to 400 kg H2SO4 / ton of lignin source.

18. The method according to any one of claims 1 to 14, wherein the first lignin source is industrial lignin, wherein preferably the industrial lignin is selected from the group consisting of alkali lignin such as sulfate-processed lignin and alkali-processed lignin, organic solvent lignin, and hydrolyzed lignin.

19. The method according to any one of claims 1 to 17, wherein the method comprises, prior to the first pulping stage: In the precipitation stage, an acidifying agent is added to the black liquor, which has a first alkaline pH value, to lower the pH value of the black liquor to a second pH level from neutral to alkaline, thereby initiating lignin precipitation. In the subsequent third separation stage, the precipitated lignin is separated from the residual liquid phase of the acidified original black liquor, where the pH remains in the neutral to alkaline range, to form a third lignin separation cake. The preferred second pH level is higher than pH 7 and lower than pH 12.

20. The method of claim 19, which is dependent on claim 15, wherein the second lignin source mixed with the additional acidifying agent is a third lignin separation cake.

21. The method of claim 19, which is dependent on any one of claims 1 to 14, wherein the first lignin source mixed with the first acidifier in the first pulping stage is a third lignin separation cake.

22. The method according to claim 20, which is dependent on claim 15, The method includes, prior to the first pulping stage: In the precipitation stage, an acidifying agent is added to the black liquor, which has a first alkaline pH value, to lower the pH value of the black liquor to a second pH level from neutral to alkaline, thereby initiating lignin precipitation. In the subsequent third separation stage, the precipitated lignin is separated from the residual liquid phase of the acidified original black liquor, where the pH remains in the neutral to alkaline range, to form a third lignin separation cake. Preferably, the second pH level is higher than pH 7 and lower than pH 12. In the additional pulping stage, the third lignin separation cake serves as the second lignin source and is mixed with an additional acidifying agent to form the third acid pulp. The subsequent fourth separation stage involves separating lignin from the third acidic pulp to form a fourth lignin separation cake. The first lignin source mixed with the first acidifier in the first pulping stage is the fourth lignin separation cake.

23. The method according to any one of claims 19 to 22, wherein the black liquor is raw black liquor or delignified black liquor.

24. The method according to any one of the preceding claims, wherein the method is used to extract lignin having a reduced metal content, particularly lignin having a reduced alkali metal and transition metal content.

25. A method for extracting lignin from a lignin source, the method comprising: An additional pulping stage is performed in which the lignin source is mixed with an additional acidifying agent to form a third acidic pulp. The subsequent fourth separation stage involves separating lignin from the third acid pulp to form a fourth lignin separation cake; In the pulping stage, the fourth lignin separation cake is mixed with the first acidifying agent to form the first acid pulp; The subsequent first separation stage involves separating lignin from the first acidic pulp to form a first lignin separation cake; In the subsequent repulping stage, the first lignin separation cake is mixed with a second acidifying agent to form a second acidic pulp; as well as The subsequent second separation stage involves separating lignin from the second acidic pulp to form a second lignin separation cake. in During the pulping stage, the temperature of the first acidic pulp is maintained below 80 °C. In the re-slurrying stage, the second acidic slurry is heated to a temperature range of 80 °C to 140 °C; and During the pulping stage, the purity of the lignin source is at least 70%. Optionally, the lignin source is industrial lignin, wherein preferably the industrial lignin is selected from the group consisting of alkali lignin such as sulfate-processed lignin and alkali-processed lignin, organic solvent lignin and hydrolyzed lignin.

26. A method for extracting lignin from black liquor, the method comprising, in sequence, the following stages: In the precipitation stage, an acidifying agent is added to the black liquor having a first alkaline pH value to lower the pH value of the black liquor to a second pH level from neutral to alkaline, thereby initiating lignin precipitation. In the subsequent third separation stage, the precipitated lignin is separated from the residual liquid phase of the acidified original black liquor, where the pH remains in the neutral to alkaline range, to form a third lignin separation cake. Preferably, the second pH level is higher than pH 7 and lower than pH 12. The subsequent additional pulping stage involves mixing the third lignin separation cake with an additional acidifying agent to form a third acid pulp. The subsequent fourth separation stage involves separating lignin from the third acid pulp to form a fourth lignin separation cake; In the subsequent pulping stage, the fourth lignin separation cake is mixed with the first acidifying agent to form the first acid pulp; The subsequent first separation stage involves separating lignin from the first acidic pulp to form a first lignin separation cake; In the subsequent repulping stage, the first lignin separation cake is mixed with a second acidifying agent to form a second acidic pulp; as well as The subsequent second separation stage involves separating lignin from the second acidic pulp to form a second lignin separation cake. in During the pulping stage, the temperature of the first acidic pulp is maintained below 95 °C, preferably 80 °C. During the additional pulping stage, the temperature of the third acidic pulp is maintained below 95 °C, preferably 80 °C. In the re-slurrying stage, the second acidic slurry is heated to a temperature range of 65 °C to 140 °C, preferably 80 °C to 110 °C, and The temperature of the second acidic slurry is higher than that of the third acidic slurry.

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