Method for extracting lignin from a lignin source

Abstract

Embodiments of the present disclosure may include a method for extracting lignin from a lignin source, including a slurrying in which the lignin source is mixed with a first acidifier thereby forming a first acidic slurry Embodiments may also include a first separation step in 5 which lignin is separated from the acidic slurry forming a first lignin cake. Embodiments may also include a re-slurrying step in which the first lignin cake is mixed with a second acidifier thereby forming a second acidic slurry. Embodiments may also include a second separation step in which the lignin is separated from the second acidic slurry forming a second lignin cake. In some embodiments, a temperature of the first acidic slurry in the slurrying stage may 10 be kept equal to or lower than 80 °C.

Description

[0001] METHOD FOR EXTRACTING LIGNIN FROM A LIGNIN SOURCE

[0002] The present disclosure relates to the field of lignin processing, and more specifically, to a method for extracting lignin from a lignin source.

[0003] Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood, because they lend rigidity and do not rot easily. Chemically, lignins are polymers made by cross-linking phenolic precursors. Lignin is a product of numerous industrial processes, including papermaking and biofuel production. Extracted lignin, or so called technical lignin, contains significant amounts of metal ions, such as sodium and iron, which can interfere with its further processing and limit its potential applications.

[0004] WO 2014 / 116150 Al concerns a method for producing high purity lignin which comprises several acidification and corresponding separation stages, with the goal of lowering the carbohydrate content of the lignin. In the method, the lignin cake may be subjected to an additional acidification to a pH of at least 2-4 or lower followed by a third separation of a third lignin cake as an additional treatment step for leaching and washing metals from said lignin cake, wherein the additional treatment is made either before or after obtaining the low carbohydrate lignin cake. While this method has proven effective to some extent, there is still a need for improved techniques that can reduce the metal content in lignin even further.

[0005] It is an object, among objects, to provide an improved method for extracting lignin. It is also an object to extract lignin having a reduced metal content, including a reduced alkali and transition metals, such as potassium, sodium, iron, and manganese, content.

[0006] In the present disclosure, in accordance with a first aspect, there is provided a method for extracting lignin, comprising the following stages in sequence: a slurrying stage wherein a first lignin source is mixed with a first acidifier thereby forming a first acidic slurry, followed by a first separation stage in which the lignin is separated from the first acidic slurry forming a first lignin cake, followed by a re-slurrying stage wherein the first lignin cake is mixed with a second acidifier thereby forming a second acidic slurry, and followed by a second separation stage in which the lignin is separated from the second acidic slurry forming a second lignin cake, wherein a temperature of the first acidic slurry in the slurrying stage is kept lower than 80°C, and the second acidic slurry in the re-slurrying stage is heated to a temperature in the range of 80 °C to 140 °C. wherein optionally a purity of the first lignin source in the slurrying stage is at least 70%.

[0007] In this method, in particular due to the first acidic slurry being kept at a temperature below 80 °C, the amount of sodium that can be taken out of the lignin is greatly increased. When the temperature is above this value of 80 °C, although iron can be effectively removed, it is found that sodium remains in the lignin. It was found that, surprisingly, when the temperature was about 81 °C, a sudden increase of the sodium concentrations was observed compared to when temperatures below 80 °C were applied. The re-slurrying stage allows for the iron content to be lowered further. The sodium content in the second lignin cake obtained from the method is below 65 ppm and may be even lower than 30 ppm.

[0008] In an embodiment, the temperature of the first acidic slurry in the slurrying stage is kept equal to or lower than 75 °C, preferably equal to or lower than 72 °C, more preferred equal to or lower than 70°C. In this way, the sodium content in the second lignin cake obtained from the method is below 25 ppm.

[0009] The reduced sodium and iron levels are beneficial for several reasons. One is that sodium and iron ions can interfere with the chemical reactions involved in further processing of the lignin. Another is that the presence of these ions can alter the physical and chemical properties of lignin, affecting its solubility, reactivity, and other characteristics in an unwanted way. In addition, the metal ions in lignin can cause issues in downstream processes. For example, in the production of biofuels or other bioproducts from lignin, metal ions may poison catalysts, corrode parts of combustion plants, and / or reduce process efficiency. In addition, carbon can be produced from lignin, for instance by pyrolysis, and reduction of metal impurities increases the range of applications of carbon produced from the lignin with reduced metal content. Applications wherein a low impurity / me tai content is wanted may include battery materials, water purification, and the plastics industry.

[0010] When the purity of the first lignin source is at least 70%, then the each of the slurrying and reslurrying stages and respective separation stages as described herein can be seen as purification stages to reach a very low level of metals, in particular of sodium and iron, as low as about 17 ppm for sodium and about 18 ppm for iron. A low iron concentration in lignin is generally a good indicator that other transition metals in the lignin also have a low concentration. Examples of lignin sources may include kraft lignin, soda lignin, hydrolysis lignin (such as enzymatic hydrolysis lignin and steam explosion lignin), and organosolv lignin.

[0011] When the lignin purity is below 70%, then it is preferred to include the additional slurrying stage and the fourth separation stage, both of which are described below, in the method. When the lignin purity is larger than approximately 70%, the additional slurrying stage may not be required to produce the high purity lignin with the method. In case lignin has a purity larger than 70%, but is alkaline, then it is an option to include the additional slurrying stage and the fourth separation stage as well. Alternatively, the amount of acidifier to the slurrying stage can be increased in case of a lignin source being an alkaline, high-purity lignin, such as having a purity of at least 85%.

[0012] In an embodiment, a purity of the first lignin source is at least 80%, preferably at least 85%, most preferably at least 89%, based on the dry weight of the lignin material. In some embodiments, the purity of the first lignin source can be at least 90%.

[0013] When lignin of higher purity, such as 85% or 90%, is used, the combination of the purification stages allows to reach a very low level of metals, in particular of sodium and iron, as low as 5 ppm for sodium and 5-6 ppm for iron. This is especially beneficial for applications for the lignin where such a high purity is desired, such as for battery materials.

[0014] The purity of lignin, such as of the first lignin source, or any of the lignin cakes, refers to how much, based on the dry weight of the lignin material, is pure lignin and how much is contaminants or residual chemicals (e.g. salts, sulphur compounds, carbohydrates, organic solvents, etc.). The purity may be determined by obtaining a weight fraction of the lignin in the lignin material. This may be done using the test method “L 2:2016” as published by Innventia AB in 2016, which is incorporated herein by reference in its entirety. The lignin content can be determined as described under paragraph 3.2 thereof, i.e. by “the sum of the amount of acidinsoluble matter and acid-soluble matter, absorbing at 205 nm, after sulphuric acid hydrolysis, as determined by gravimetry and spectrophotometry according to the method, in a sample...”. If the lignin content is in mg lignin per gram lignin material as per the test method, then the purity in % of the lignin material is equal to the lignin content divided by 10. For example, a lignin content of 700 mg lignin per gram lignin material is equal to a purity of 70%. A copy of this testing method as published in 2016 may be found here: https: / / archive.org / details / biorefmery-test-methods-l-2-2016-kraft-lignins-lignin-and- carbohydrate-content. The testing method “L 2:2016” relies on testing method “L 1:2016” for determining the dry matter content of lignins, also published by Innventia AB in 2016. A copy is available here: https : / / archive . org / details / biorefinerv-test-methods-L- 1-2016.

[0015] In the present patent disclosure, the various slurries being “acidic”, such as the first acidic slurry and the second acidic slurry, indicates that such slurries have a pH below 7. A basic, or alkaline, slurry would have a pH above 7. This basic chemical or physical principle would be understood by the general public, in particular a person skilled in the art.

[0016] In the method for extracting lignin, the extracted lignin may be extracted as the second lignin cake.

[0017] In an embodiment, the temperature of the first acidic slurry in the slurrying stage is kept in a range of 30 °C to 70 °C. In this way, the amount of sodium that can be leached or washed out is not limited too much by kinetics.

[0018] In an embodiment, the temperature of the first acidic slurry is lower than 65 °C, preferably lower than 60 °C. When the temperature of the first acidic slurry is kept below 60 °C, the sodium content in the second lignin cake obtained from the method is below 20 ppm.

[0019] In an embodiment, the temperature of the first acidic slurry in the slurrying stage is kept in a range of 30 °C to 65 °C, preferably 40 °C to 60 °C.

[0020] In an embodiment, the second acidic slurry in the re-slurrying stage is heated to a temperature in the range of 80 °C to 120 °C, or in the 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 produced lignin is lowered substantially. For example, an iron content below 15 ppm or lower is obtained.

[0021] In an embodiment, the second acidic slurry in the re-slurrying stage is heated to a temperature in the range of in the range of 80 °C to 98 °C. It is observed that, at a temperature above 98 °C, there is a small but measurable increase in the sodium level remaining in the purified lignin. At the same time, there is only a minor decrease in the iron level above this temperature. Therefore, the upper end of the range, for example 96 °C, provides an optimum of both the removal sodium and iron. In addition, beneficially, the thermal energy requirements are reduced compared to higher temperatures. In an embodiment, the temperature of the second acidic slurry is in the range of 90 °C to 140 °C. In this way, the transition metal content of the produced lignin may be lowered even further. For example, iron may be present in the produced lignin at 7 ppm or lower.

[0022] In an embodiment, the time at which the second acidic slurry is kept at the above temperature may lie in the range of 10-120 minutes, preferably 15 to 90 minutes, more preferably 15 to 60 minutes.

[0023] In an embodiment, the time period is preferably inversely proportional to the applied temperature.

[0024] In an embodiment, the temperature and time period at which the second acidic slurry is kept correspond to a P-factor, as calculated by the formula P-factor = eA(40.48- 15106 / (273.15 +temp))*time / 60 (cf. Herbert Sixta, Handbook of Pulp, Wiley-VCH, 2006, p. 344, and the table presented on page 7 of WO 2014 / 116150 Al), wherein the P-factor is above 1 and below 8. For example, at a temperature of 120 °C, a time of 60 minutes corresponds to a P-factor of 8, while a time of 45 minutes corresponds to a P-factor of 6.

[0025] In an embodiment, the time period is in the range of 10-60 minutes and in inverse proportion to temperature.

[0026] In an embodiment, the time period at which the first acidic slurry is kept before the lignin is separated from the first acidic slurry in the first separation stage lies in the range of 10 to 360 minutes, preferably 20 minutes to 90 minutes.

[0027] In an embodiment, in the slurrying stage in the first acidic slurry a pH value in the range of 1 to 5, preferably 1 to 4, is established.

[0028] In an embodiment, the re-slurrying stage in the second acidic slurry a pH value in the range of 1 to 5, preferably 1 to 4, is established.

[0029] In an embodiment, the first acidifier is added in an amount less than X kg per ton of the first lignin source, X being equivalent to an amount of 100 kg H2SO4 per ton of the first lignin source. This embodiment is useful in case the amount of acidifier is to be limited. Also, this embodiment may, for example, be suited for use with a lignin source that is already treated with acid to neutralize / remove any hydroxide, carbonate and / or sulfide ions that were present. This neutralized or acidified lignin is referred to herein as non-alkaline lignin. In an embodiment, the first acidifier may be added in an amount less than X kg per ton of the first lignin source, X being equivalent to an amount of 50 kg, preferably 20 kg, more preferably 10 kg, H2SO4 per ton of the first lignin source. These upper limits fit most if not all of the non- alkaline lignin sources and the process can be used to obtain a lignin product with a very high purity. This embodiment is useful in case the amount of acidifier is to be limited. The specific amounts of sulfuric acid or equivalent may be adapted to the desired pH, which generally lies within the range of 2 to 4, but can be lower, such as 1 or 1.5.

[0030] In an embodiment, the second acidifier is added in an amount less than XX kg per ton of the first lignin source, XX being equivalent to an amount of 10 kg H2SO4 per ton of the first lignin source. Preferably, the second acidifier is added in an amount less than XX kg per ton of the first lignin source, XX being equivalent to an amount of 20 kg, more preferably 10 kg, H2SO4 per ton of the first lignin source. Since the lignin fed to the re-slurrying stage has undergone a previous acidic processing in the slurrying stage, relatively small amounts of acid are required in this stage. Alternatively, these amounts of acid can be defined per ton of lignin material processed in the re-slurrying stage, i.e. first lignin cake. In that case the upper limits to the respective amounts of second acidifier may be similar or the same.

[0031] For the above values of X and XX, if instead, for example, formic acid is used, then the value of X being equivalent to 100 kg H2SO4 translates to about 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 protons available (two for sulfuric acid, and one for formic acid). Since formic acid is a weak acid, and sulfuric acid a strong acid, more formic acid than calculated may be required. This is disregarded in this explanatory calculation.

[0032] Beneficially, when using any one of the embodiments of the method disclosed herein, the amount of lignin dissolved from the second lignin cake is kept below 20%, even as low as 15%, thus increasing the lignin yield.

[0033] In an embodiment, when the first acidic slurry has a temperature above 65 °C, preferably 60 °C, before the first separation stage, the first acidic slurry is subjected to cooling before subsequent separation of the lignin from the first acidic slurry in the first separation stage.

[0034] In an embodiment, when the second acidic slurry has a temperature above 60 °C before the second separation stage, the second acidic slurry is subjected to cooling before subsequent separation of the lignin from the second acidic slurry in the second separation stage. This cooling is especially beneficial when press filters are used in the respective separation stage, in order to save the lifetime of the filters.

[0035] In an embodiment, the first lignin source is technical lignin, wherein preferably the technical lignin is selected from the group consisting of alkali lignin, such as kraft lignin and soda lignin, organosolv lignin, and hydrolysis lignin. The alkali lignin may first be neutralized or acidified to form non-alkaline lignin, as can be done with the additional slurrying stage described below. This neutralization or acidification can also be already done before to the first lignin source, such that it can be fed directly to the slurrying stage. If the purity of the alkali lignin is high, as further explained below, then the neutralization can be alternatively performed in the slurrying stage, with an appropriately increased amount of first acidifier added to the slurrying stage.

[0036] Technical lignin may refer to solid lignin having a purity of at least 60%, or solid lignin that is the product of an extraction process, obtained during processing of the biomass material.

[0037] In an embodiment, the method comprises, before the first slurrying stage: an additional slurrying stage wherein a second lignin source is mixed with an additional acidifier thereby forming a third acidic slurry, followed by a fourth separation stage in which lignin is separated from the third acidic slurry forming a fourth lignin cake, wherein the first lignin source that is mixed with a first acidifier in the first slurrying stage is the fourth lignin cake.

[0038] In an embodiment, the second lignin source is technical lignin, wherein preferably the technical lignin is selected from the group consisting of alkali lignin, such as kraft lignin and soda lignin, organosolv lignin, and hydrolysis lignin.

[0039] In an embodiment, a temperature of the third acidic slurry in the additional slurrying stage is kept equal to or lower than 75 °C, preferably below 72 °C, more preferably below 70 °C, most preferred in a range of 55-65 °C.

[0040] In an embodiment, a pH of the third acidic slurry lies in a range of 1 to 4.

[0041] In an embodiment, the additional acidifier is added in an amount in the range of Y to Z kg per ton of the second lignin source, Y being equivalent to an amount of load of 100 kg H2SO4 per ton of the second lignin source, Z being equivalent to an amount of 400 kg H2SO4 per 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 instead, for example, formic acid is used, then the value of Y of 100 kg H2SO4 translates to about 94 kg formic acid as described above. Z would be about 376 kg formic acid. If instead carbonic acid is used, then approximately 127 kg of carbonic acid would be equivalent to 100 kg of sulfuric acid, assuming the carbonic acid (H2CO3) releases one H+ion per molecule. This calculation is done in the same way as described above for formic acid.

[0042] In an embodiment where black liquor is processed, the method comprising, before the first slurrying stage: a precipitation stage wherein an acidifier charge is added to the black liquor, having a first alkaline pH value, in order to decrease the pH value of the black liquor to a second pH level ranging from neutral to alkaline, initiating precipitation of lignin, followed by a third separation stage wherein the precipitated lignin is separated as a third lignin cake from the remaining liquid phase of the acidified original black liquor still kept in the pH range from neutral to alkaline,

[0043] In an embodiment, in the precipitation stage the second pH level is above pH 7 and below 12. Lignin generally precipitates from a pH between 11.5 and 12. In this embodiment, the black liquor generally does not yet have a lignin purity of at least 70% as described above. The third lignin cake may have a purity of at least 70%, but it may also be lower. If it is lower, it is preferred that the method includes the additional slurrying stage and the fourth separation stage. The additional slurrying stage and the fourth separation stage may also be included in case an ultra high purity lignin product is desired.

[0044] In an embodiment, the second lignin source that is mixed with the additional acidifier in the additional slurrying stage is the third lignin cake.

[0045] Alternatively, in an embodiment without the additional acidifier and the fourth separation stage, the first lignin source that is mixed with the first acidifier in the first slurrying stage is the third lignin cake.

[0046] In an embodiment, the black liquor is original black liquor. Alternatively, the black liquor may be partially depleted of lignin. For instance, lignin with a certain molecular weight range may have been removed in a preceding precipitation stage followed by a respective separation stage.

[0047] According to a second aspect, there is provided a method for extracting lignin from a lignin source, comprising: an additional slurrying stage wherein the lignin source is mixed with an additional acidifier thereby forming a third acidic slurry; followed by a fourth separation stage in which lignin is separated from the third acidic slurry forming a fourth lignin cake; a slurrying stage wherein the fourth lignin cake is mixed with a first acidifier thereby forming a first acidic slurry; followed by a first separation stage in which the lignin is separated from the first acidic slurry forming a first lignin cake; followed by a re-slurrying stage wherein the first lignin cake is mixed with a second acidifier thereby forming a second acidic slurry; and followed by a second separation stage in which the lignin is separated from the second acidic slurry forming a second lignin cake, wherein a temperature of the first acidic slurry in the slurrying stage is kept lower than 80 °C, the second acidic slurry in the re-slurrying stage is heated to a temperature in the range of 80 °C to 140 °C; and a purity of the lignin source in the slurrying stage is at least 70%,

[0048] In an embodiment, the lignin source, which is equivalent to the second lignin source described above, is technical lignin, wherein preferably the technical lignin is selected from the group consisting of alkali lignin, such as kraft lignin and soda lignin, organosolv lignin, and hydrolysis lignin.

[0049] According to a third aspect, there is provided a method for extracting lignin from black liquor, the method comprising the following stages in sequence: a precipitation stage wherein an acidifier charge is added to the black liquor, having a first alkaline pH value, in order to decrease the pH value of the black liquor to a second pH level ranging from neutral to alkaline, initiating precipitation of lignin; followed by a third separation stage wherein the precipitated lignin is separated as a third lignin cake from the remaining liquid phase of the acidified original black liquor still kept in the pH range from neutral to alkaline, wherein preferably the second pH level is above pH 7 and below 12; followed by an additional slurrying stage wherein the third lignin cake is mixed with an additional acidifier thereby forming a third acidic slurry; followed by a fourth separation stage in which lignin is separated from the third acidic slurry forming a fourth lignin cake; followed by a slurrying stage wherein the fourth lignin cake is mixed with a first acidifier thereby forming a first acidic slurry; followed by a first separation stage in which the lignin is separated from the first acidic slurry forming a first lignin cake; followed by a re-slurrying stage wherein the first lignin cake is mixed with a second acidifier thereby forming a second acidic slurry; and followed by a second separation stage in which the lignin is separated from the second acidic slurry forming a second lignin cake, wherein a temperature of the first acidic slurry in the slurrying stage is kept lower than 95 °C, preferably 80 °C, a temperature of the third acidic slurry in the additional slurrying stage is kept lower than 95 °C, preferably 80 °C, the second acidic slurry in the re-slurrying stage is heated to a temperature in the range of 65 °C to 140 °C, preferably 80 °C to 140 °C, more preferably 80 °C to 110 °C, and the temperature of the second acidic slurry is higher than the temperature of the third acidic slurry.

[0050] It will be understood that similar or the same advantages as described above for embodiments encompassing both the precipitation stage and the additional slurrying stage are achieved for this method.

[0051] With these temperature ranges, i.e. the first and third acidic slurries kept below 95 °C, an advantageous combination of a reasonably low sodium level (below 120 ppm) and very low transition metal concentration (iron concentration below 8 ppm) can already be achieved in the first lignin cake. This means these levels will be even lower once the final second lignin cake is achieved. Depending on the application, it can be desirable to remove more of the transition and other metals, while the sodium levels are allowed to remain higher.

[0052] If the first and third acidic slurries are kept below 80 °C, then a very low sodium level can be obtained as described in relation to the first aspect, any of the relevant embodiments and / or examples.

[0053] An upper level of 110 °C, preferably 106 °C, for the second acidic slurry is beneficial since then the pressure remains reasonably low and the equipment to perform the method can be simpler and less costly. It is intended throughout the present description that the expression "separation stage" embraces any means of separation. Preferably the separation is performed by using centrifugation, a fdter press apparatus, a band fdter, a rotary filter, such as a drum filter or a disk filter, or a sedimentation tank, or similar equipment, most preferred a filter press apparatus is used. It is intended throughout the present description that the expression "original black liquor" embraces spent cooking liquor from a digester, having most of the lignin from the original cellulose material dissolved in the "original black liquor". The "original black liquor" may also have a large content of organic and inorganic material and may also have passed through separation processes for extracting turpentine or other specific constituents, while keeping the bulk volume of dissolved lignin unaltered.

[0054] In an embodiment, the method is for extracting lignin with a reduced metal content, in particular lignin with a reduced alkali metal and transition metal content.

[0055] In the above and below aspects, embodiments and examples, it will furthermore be understood that the names for the various stages, acidifiers, slurries, acidifiers, etc., may be altered without changing their respective technical functions. For example, the third separation stage could be named “the first separation stage”, the third lignin cake could be named “the first lignin cake”, the additional slurrying stage could be named “the first slurrying stage” , the fourth precipitation stage could be named “the second precipitation stage”, the third acidic slurry could be named “the first acidic slurry”, the fourth lignin cake could be named “the second lignin cake”, the slurrying stage could be named “the second slurrying stage” , the first precipitation stage could be named “the third precipitation stage”, the first acidic slurry could be named “the second acidic slurry”, the first lignin cake could be named “the third lignin cake”, the re-slurrying stage could be named “the third slurrying stage” , the second precipitation stage could be named “the fourth precipitation stage”, the second acidic slurry could be named “the third acidic slurry”, and the second lignin cake could be named “the fourth lignin cake”, if so desired. Other examples are the additional acidifier, which could be named “the first acidifier”, the first acidifier could be named “the second acidifier”, and the second acidifier could be named “the third acidifier”, if so desired. In other words, the use of “first”, “second”, “third”, “fourth”, and “additional”, are used for identification purposes and do not give any technical meaning to the associated features.

[0056] It will be understood that technical advantages and effects associated with features and / or embodiments of one aspect, apply to the corresponding, similar or equivalent features and / or embodiments the other aspects. It will also be apparent that the features of the various aspects and / or embodiments thereof may be applied to the other aspects and / or embodiments thereof. Brief Description of the Drawings

[0057] The accompanying drawings are used to illustrate presently preferred non-limiting exemplary embodiments of devices of the present disclosure. The above and other advantages of the features and objects of the disclosure will become more apparent, and the aspects and embodiments will be better understood from the following detailed description when read in conjunction with the accompanying drawings, in which:

[0058] FIG. 1 is a flowchart illustrating a method for extracting lignin, according to some embodiments of the present disclosure;

[0059] FIG. 2 is a flowchart illustrating a method including the method for extracting lignin of FIG. 1, according to some embodiments of the present disclosure;

[0060] FIG. 3 is a flowchart illustrating a method for extracting lignin, according to some embodiments of the present disclosure;

[0061] FIG. 4 is a bar plot showing sodium (Na) and iron (Fe) content in ppm of produced lignin for various combinations of process temperatures in degrees Celsius;

[0062] FIG. 5 is a flowchart further illustrating a method for extracting lignin, according to some embodiments of the present disclosure;

[0063] FIG. 6 is a flowchart illustrating a method for extracting lignin including the method of FIG. 5, according to some embodiments of the present disclosure;

[0064] FIG. 7 is a plot showing sodium (Na) content in ppm of produced lignin produced by a method according to some embodiments of the present disclosure;

[0065] FIG. 8 is a plot showing iron (Fe) content in ppm of produced lignin produced by a method according to some embodiments of the present disclosure;

[0066] FIG. 9 is a plot showing sodium (Na) content in ppm of produced lignin produced by a method according to FIGS. 5 or 6 of the present disclosure; and

[0067] FIG. 10 is a plot showing iron (Fe) content in ppm of produced lignin produced by a method according to FIGS. 5 or 6 of the present disclosure.

[0068] Detailed Description of Preferred Embodiments

[0069] FIG. 1 is a flowchart that describes a method 100 for extracting lignin, according to some embodiments of the present disclosure. In some embodiments, at 110, the method includes a slurrying stage or step. At 120, the method includes a first separation stage or step. At 130, the method includes a re-slurrying stage or step. At 140, the method includes a second separation step. In the slurrying stage or step 110, a first lignin source is mixed with a first acidifier, thereby forming a first acidic slurry. In first separation stage 120, the lignin is separated from the acidic slurry forming a first lignin cake. The first lignin cake may be mixed with a second acidifier in the re-slurrying stage 130, thereby forming a second acidic slurry. The lignin may be separated from the second acidic slurry in the second separation stage 140, thereby forming a second lignin cake. A temperature of the first acidic slurry in the slurrying stage 110 is kept equal to or lower than 80 °C, preferably 75 °C, more preferred 70 °C. The second acidic slurry in the reslurrying stage is heated to a temperature in the range of 80 °C to 140 °C, preferably 80 °C to 110 °C.

[0070] In some embodiments, the temperature of the first acidic slurry may be lower than 80 °C, preferably lower than 75 °C. In some embodiments, the temperature of the first acidic slurry in the slurrying stage is in a range of 30 °C to 70 °C. In some embodiments, the temperature of the first acidic slurry in the slurrying stage is in a range of 30 °C to 65 °C, such as 40 °C to 60 °C.

[0071] In some embodiments, in the slurrying stage 110, in the first acidic slurry a pH value in the range of 1 to 5, preferably 1 to 4, is established. In some embodiments, in the re-slurrying stage 130, in the second acidic slurry a pH value in the range of 1 to 5, preferably 1 to 4, is established. The chosen pH levels depend on the lignin material used as the first lignin source. For example, a pH below 1.5 may be beneficial when the lignin material contains substances like oxalates, e.g. calcium oxalate, which may be undesired.

[0072] As will be shown in relation to FIG. 4 below, in some embodiments, a sodium content in the second lignin cake is below 25 ppm. In some embodiments, an iron content in the second lignin cake may be below 10 ppm, in particular below 7 ppm.

[0073] In some embodiments, the amount of lignin dissolved from the second lignin cake may be kept below 15%. In some embodiments, the second acidic slurry may be subjected to cooling before subsequent separation of the lignin from the second acidic slurry in the second separation stage 140. In an embodiment, the first acidic slurry is cooled to below 60 °C. This is beneficial when filter presses are used, in order to limit the degradation of the filters.

[0074] In the method 100, the first lignin source may be either non-alkali technical lignin or high purity lignin that is still alkaline in nature, as described above. The technical lignin may be selected from the group consisting of alkali lignin, such as kraft lignin and soda lignin, organosolv lignin, and hydrolysis lignin. In some embodiments, the method may be for extracting lignin with a reduced metal content, in particular lignin with a reduced alkali metal and transition metal content.

[0075] In case non-alkali lignin being used as the first lignin source, according to some examples the first acidifier may be added in an amount less than X kg per ton of the first lignin source, X being equivalent to an amount of 100 kg H2SO4 per ton of the first lignin source. For example, the first acidifier may be added in an amount of, or equivalent to, less than 50 kg sulfuric acid per ton of the first lignin source. Depending on the lignin material used as the first lignin source, this amount may be, or may be equivalent to, less than 10 kg of sulfuric per ton of the first lignin source. These upper limits fit most if not all of the non-alkaline lignin materials and the process can be used to obtain a lignin product with a very high purity. The specific amounts of sulfuric acid or equivalent may be adapted to the desired pH, which generally lies within the range of 2 to 4.

[0076] In an embodiment, a purity of the first lignin source in the slurrying stage is at least 80%, based on the dry weight of the lignin material. In some embodiments, the purity of the first lignin source can be at least 89%, or 90%. In this way, a high purity lignin is produced by the method 100, as explained in more detail in the below examples.

[0077] In some embodiments, the second acidic slurry in the re-slurrying stage 130 may be heated to a temperature in the range of 70 °C to 150 °C, preferably in the range of 80 °C to 140 °C, more preferred in the range of 90 to 140 °C, such as 85 °C, 95 °C, 106 °C, 110 °C, 115 °C, 120 °C, 125 °C, 130 °C, and 135 °C. The temperature of the re-slurrying stage is, in these embodiments, equal to or higher than the temperature of the slurrying stage.

[0078] FIG. 2 is a flowchart depicting a method 200 for extracting lignin, the method 200 comprising the method 100 from FIG. 1, according to some embodiments of the present disclosure. 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. Stage 203 and 205 are performed before the slurrying stage 110. In stage 203, an acidifier charge may be added to black liquor, having a first alkaline pH value, in order to decrease the pH value of the black liquor to a second pH level ranging from neutral to alkaline, initiating precipitation of lignin. The precipitated lignin may be separated in stage 205 as a third lignin cake from the remaining liquid phase of the acidified original black liquor still kept in the pH range from neutral to alkaline. The lignin material that may be mixed with a first acidifier in the slurrying stage 110 is the third lignin cake. The second pH level may be above pH 7 and below 12. Generally, in this process the black liquor will be original black liquor. Since in this embodiment, the third lignin cake is alkaline, the amount acidifier added in the slurrying stage 110 is typically higher than 100 kg sulfuric acid, or an amount of another suitable acid that is equivalent thereto. The amount of acidifier will also be higher in case a lower pH is applied in the slurrying stage.

[0079] FIG. 3 shows a further method 300, according to some embodiments of the present disclosure. The method 300 corresponds to method 200 unless stated otherwise. Original black liquor 301, a lignin material, is subjected to precipitation stage 303, which corresponds to precipitation stage 203. To the black liquor is added 302 an acidifier charge, which may be sulfuric acid (H2SO4), carbonic acid (from carbon dioxide), respective aqueous solutions of these acids, or any other suitable acidifier. In stage305, the precipitated lignin is separated as a third lignin cake from the remaining liquid phase of the acidified original black liquor.

[0080] The third lignin cake is slurried in slurrying stage 310, by the addition 312 of a first acidifier, to form a first acidic slurry. The first acidifier may be sulfuric acid (H2SO4), formic acid, oxalic acid, respective aqueous solutions of these acids, or any other suitable acidifier. Lignin is separated from the acidic slurry as a first lignin cake in separation stage 320. The first lignin cake is thereafter re-acidified in re-slurrying stage 330. A second acidifier is added 332, thereby forming a second acidic slurry. The second acidifier may be sulfuric acid (H2SO4), formic acid, oxalic acid, respective aqueous solutions of these acids, or any other suitable acidifier.

[0081] Step 335 indicates the heating of the second acidic slurry to a temperature as specified in the present disclosure. In some embodiments, the second acidic slurry in the re-slurrying stage 330 may be heated 335 to a temperature in the range of 70 °C to 150 °C, preferably in the range of 80 °C to 140 °C, more preferred in the range of 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 time at which the second acidic slurry is kept at the above temperature may lie in the range of 10-120 minutes, which time is inversely proportional to the applied temperature.

[0082] After the heating of the second acidic slurry, the second acidic slurry is preferably cooled (not shown) and thereafter subjected to separation stage 340. From separation stage 340 the second lignin cake is output 350.

[0083] The amount of acidifier used in steps 302, 312 and 332 is chosen in order to achieve the pH levels described herein for the respective precipitation, slurrying, and re-slurrying stages. As examples, the values for the amount of sulfuric acid may be similar to those in FIG. 2 and / or FIG. 5 of WO 2014 / 116150 Al, and may be altered according to the desired pH.

[0084] Each of the separation stages 305, 320, and 340 may comprise respectively a filtration step and a washing step. The added chemicals such the acidifier may be recovered from the respective filtrates.

[0085] FIG. 5 is a flowchart that describes a method 500 for extracting or purifying lignin, according to some embodiments of the present disclosure. The method 500 is the same as method 100, unless stated otherwise. The method 500 comprises the additional slurrying stage 502 followed by the fourth separation stage 504 in which lignin is separated from the third acidic slurry forming a fourth lignin cake. At 510, the method includes the slurrying stage or step, which is the same as, or similar to, the step 110 of method 100. At 520, the method includes the first separation stage or step, which is the same as, or similar to, the step 120 of method 100. At 530, the method includes the re-slurrying stage or step, which is the same as, or similar to, the step 130 of method 100. At 540, the method includes the second separation step, which is the same as, or similar to, the step 140 of method 100.

[0086] In the additional slurrying stage 502, a second lignin source is mixed with an additional acidifier, thereby forming a third acidic slurry. The lignin may reside in the slurrying stage 502 for a time period between 30 minutes to 90 minutes. In the fourth separation stage 504, the lignin from the additional slurrying stage 502 is separated from the third acidic slurry forming a fourth lignin cake. The fourth lignin cake is used as the first lignin source for the slurrying stage 510. The pH of the third acidic slurry may lie in the range of 1 to 4, or preferably 2 - 4. The purity of the second lignin source is preferably at least 70%. The lignin source for the additional acidifier may be an alkali lignin material, which is alkaline, and the amount of the third acidifier is suitable for the lignin source and the desired pH. The amount may, for example, lie in a range of 100 to 400 kg sulfuric acid, such as 200 kg sulfuric acid, per ton of the second lignin source. Alternatively, an equivalent amount of another acid may be used, as described above. In this case, the second lignin source may be precipitated lignin from black liquor, which is described in more detail in relation to FIG. 6 below. A purity of the fourth lignin cake can be at least 80%, and may be equal to or higher than 89%. Alternatively, the lignin material mixed with the additional acidifier may be non-alkaline lignin. Less acidifier may be required in this case than for alkaline lignin, depending on the desired pH.

[0087] In the slurrying stage or step 510, the fourth lignin cake is mixed with a first acidifier, thereby forming a first acidic slurry. The amount of first acidifier can be below 100 kg, or even below 20 kg, or 10 kg, sulfuric acid, or an equivalent amount of other suitable acidifier, since the fourth lignin cake has previously been acidified in the additional slurrying stage 502. More acidifier can also be used, depending on the design of the process, in which, for example, various acid streams can be recycled. In the first separation stage 520, the lignin is separated from the acidic slurry forming a first lignin cake. The first lignin cake may be mixed with a second acidifier in the re-slurrying stage 530, thereby forming a second acidic slurry. The lignin may be separated from the second acidic slurry in the second separation stage 540, thereby forming a second lignin cake. A temperature of the acidic slurry in both the additional slurrying stage 502 and the slurrying stage 510 is kept equal to or lower than 80 °C. This is beneficial to the amount of sodium that can be removed from the lignin, as is shown in more detail in the below examples.

[0088] In some embodiments, the temperature of the first acidic slurry may be lower than 80 °C, preferably lower than 75 °C. In some embodiments, the temperature of the first acidic slurry in the slurrying stage is in a range of 30 °C to 75 °C.

[0089] In some embodiments, in the slurrying stage 510, in the first acidic slurry a pH value in the range of 1 to 5, preferably 1 to 4, is established. In some embodiments, in the re-slurrying stage 130, in the second acidic slurry a pH value in the range of 1 to 5, preferably 1 to 4, is established. The chosen pH levels depend on the lignin material to be slurried. For example, a pH below 1.5 may be beneficial when the lignin material contains substances like oxalates, e.g. calcium oxalate, which may be undesired.

[0090] As will be shown in relation to FIGS. 4, 7, 8, 9 and 10 below, in some embodiments, a sodium content in the second lignin cake is below 25 ppm. In some embodiments, an iron content in the second lignin cake may be below 10 ppm, in particular below 7 ppm.

[0091] In some embodiments, the amount of lignin dissolved from the second lignin cake may be kept below 15%. In some embodiments, the second acidic slurry may be subjected to cooling before subsequent separation of the lignin from the second acidic slurry in the second separation stage 140.

[0092] In an embodiment, the first acidic slurry may be cooled to below 60 °C. This is beneficial when filter presses are used in the respective separation stage, in order to limit the degradation of the filters. The same cooling can be done to any one or all of the other slurries, such as the third acidic slurry and the second acidic slurry, and the precipitated lignin, before the lignin is separated in the respective corresponding separation stage. The additional acidifier may be sulfuric acid (H2SO4), formic acid, oxalic acid, or respective aqueous solutions of these acids, or aqueous mixtures of two or more of these acids, or any other suitable acidifier.

[0093] FIG. 6 is a flowchart depicting a method 600 for extracting lignin, the method 600 comprising the method 500 from FIG. 5, according to some embodiments of the present disclosure. The lignin material to be processed in this case 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 stated otherwise. At 605, the method includes the third separation stage or step, which is the same as stage 205 or 305 unless stated otherwise. Stage 603 and 605 are performed before the additional slurrying stage 502. In stage 603, an acidifier charge may be added to the black liquor, having a first alkaline pH value, in order to decrease the pH value of the black liquor to a second pH level ranging from neutral to alkaline, initiating precipitation of lignin. The precipitated lignin may be separated in stage 605 as a third lignin cake from the remaining liquid phase of the acidified original black liquor still kept in the pH range from neutral to alkaline. The second lignin source that is mixed with the additional acidifier in the additional slurrying stage 502 is the third lignin cake. The second pH level may be above pH 7 and below 12. Generally, in this process the black liquor will be original black liquor.

[0094] Example 1

[0095] Lignin with reduced metal content was produced using the process according to one of the embodiments of FIGS. 2 and 3 with varying temperatures for the slurrying and re-slurrying stages respectively. The results of the metal content are shown in Table 1 below. Alternatively, the lignin of this example can be seen as lignin from the first lignin cake, when performing the method 500 or 600.

[0096] Alkaline lignin obtained from black liquor was taken as a starting lignin material. The alkaline lignin was mixed with the acidifier to a slurry and kept at the respective temperature as given in the Table 1 below. The slurry was kept at the respective temperatures for 60 minutes. The pH was about 2.5 for both the slurrying stage and the re-slurrying stage.

[0097] Table 1: Concentrations of various metals in ppm of the second lignin cake at various processing temperatures.

[0098] In Table 1, the numbers in the utmost left column indicate the combination of temperatures in degrees Celsius applied respectively in the slurrying stage and in the re-slurrying stage. For example, “55 + 55” means a temperature of 55 °C in the slurrying stage and a temperature of 55 °C in the re-slurrying stage, “55 + 95” means a temperature of 55 °C in the slurrying stage and a temperature of 95 °C in the re-slurrying stage, etc.

[0099] The reference in Table 1 is for the same lignin material, but mixed with an acidifier to form a slurry once at 55 °C, and thereafter separated.

[0100] From Table 1, it is clear that transition metals such as Cu, Fe, Mn, and Zn are removed from the lignin more effectively the higher the temperatures of the slurrying and re-slurrying in general, but especially when the temperature of the re-slurrying is above 70 °C, as can be seen from the middle two rows of Table 1, where the re-slurrying was performed at 95 °C. The alkali metals Na and K are reduced most effectively if the temperature of the slurrying stage is kept low, below 100 °C or 70 °C depending on the lignin product requirements. The advantage of treating at a higher temperature is that more of the other metals can be removed, while a bit more of the alkali metals may remain compared to a lower temperature for the slurrying stage.

[0101] Data shows that the transition metals are most effectively removed at temperatures above about 90 °C of the re-slurrying stage. In these conditions, for instance, the iron content is reduced to about half of that of the reference.

[0102] The data from the columns for Fe and Na of Table 1 are shown in FIG. 4, wherein Fe and Na are taken as a proxy for respectively the transition metals and alkali metals. What is observed from these data is that the sodium is removed well when in particular a lower temperature for the slurrying stage is used, being 55 °C. In those cases, the sodium content is respectively 21 and 18 ppm. When the temperature is increased to 95 °C, the sodium content of the lignin product is 59 ppm. When the temperature is increased to 120 °C, the sodium content of the lignin product is 596 ppm, about half of the sodium content of the lignin reference.

[0103] The iron concentration lowers with increasing temperature. From the data of FIG. 4 and Table 1 above, it follows that when both iron and sodium are to be reduced to relatively low levels, the temperature of the slurrying stage should be relatively low, i.e. below 100 °C, or even 70 °C, depending on the product requirements, such as 55 °C, and thereafter the temperature of the reslurrying stage may be the same or higher, i.e. above 70 °C or 80 °C, such as 95 °C and 120 °C.

[0104] Example 2

[0105] The lignin is taken from a method according to FIGS. 5 or 6T The temperature of the additional slurrying stage was about 55 °C, while the temperature of the slurrying stage was varied. The lignin of this example is taken from the first lignin cake.

[0106] Results for the sodium and iron concentrations are respectively shown in FIG. 7 and FIG. 8. As can be seen from FIG. 7, the sodium concentration increases suddenly when the temperature is above approximately 75 °C. The dashed lines drawn in FIG. 7 are guides to the eye, intercepting at 75 °C. At temperatures below 80 °C, already a sodium concentration of less than about 50 ppm could be obtained in this case. At 72 °C, the sodium concentration was about 24 ppm. This shows the beneficial effect to the sodium concentration of keeping the temperature of the additional slurrying stage and the slurrying stage below a threshold temperature, lying below approximately 80 °C. If a lower sodium level is desired, then a temperature below 75 °C is a suitable threshold temperature.

[0107] As can be seen in FIG. 8, the iron concentration of the lignin is reduced with increasing temperature. Lowering the temperature too much, i.e. below approximately 70 °C, reduces the effectiveness of the stages to remove iron from the lignin. Therefore, it may be desirable to use a temperature range between 70 and 80 °C in the slurrying stage to have a beneficial combination of the iron and sodium reduction.

[0108] Example 3

[0109] The lignin is taken from a method according to FIGS. 5 or 6, similar to example 2, but in this case the temperature of the slurrying stage was about 72 °C, while the temperature of the reslurrying stage was varied. The tested lignin is from the second lignin cake and the first lignin cake for comparison.

[0110] The results for sodium and iron concentrations are shown in FIGS. 9 and 10, respectively. In both figures, the left column represents comparative data obtained for a sample of the first lignin cake produced similarly as described for Example 2. The temperature of the additional slurrying stage was 55 degrees, while the slurrying stage was at 72 °C. The reslurrying stage was performed at 98 °C and at 106 °C. So, in this case, the lignin is processed in a total of total of three acidic slurrying stages. This was done in order to achieve a high purity lignin product. For example, the second lignin source to the additional slurrying stage could be alkaline lignin obtained from black liquor, with a purity below 70%. Although the method including the three acidic stages are preferred, similar results may be obtained for lignin of higher purity (e.g. at least 89%), and optionally the lignin being non-alkaline lignin, fed to the slurrying stage as the first lignin source directly.

[0111] As can be seen in FIG. 9, compared to the two stages, the sodium level of the second lignin cake has a concentration below 76 ppm, of approximately 5 ppm, for both 98 °C and 106 °C, the concentration at 98 °C being slightly lower. It may therefore be advantageous in some applications to keep the temperature of the re-slurrying stage below 100 °C. Compared to the first lignin cake, the concentration is reduced by approximately 70%.

[0112] The iron concentration is similarly reduced as the sodium concentration, as can be seen in FIG. 10. The re-slurrying stage at 98 °C resulted in the lignin of the second lignin cake having iron at a concentration of about 5,5 ppm, and 5 ppm for the re-slurrying stage at 106 °C. The reduction of the iron concentration is thus relatively moderate for this increase in temperature. Compared to the first lignin cake, the iron concentration is reduced by at least approximately 69%.

[0113] Thus, the method according to the present disclosure can be used to obtain highly, or ultra-, pure lignin.

[0114] The disclosure further comprises the following examples:

[0115] 1. Method for extracting lignin from a lignin source, comprising the following stages in sequence: a slurrying stage wherein the lignin source is mixed with a first acidifier thereby forming a first acidic slurry, followed by a first separation stage in which the lignin is separated from the first acidic slurry forming a first lignin cake, followed by a re-slurrying stage wherein the first lignin cake is mixed with a second acidifier thereby forming a second acidic slurry, and followed by a second separation stage in which the lignin is separated from the second acidic slurry forming a second lignin cake, characterized in that a temperature of the first acidic slurry in the slurrying stage is kept equal to or lower than 100 °C. 2. Method according to example 1, the first acidic slurry in the slurrying stage is kept equal to or lower than 70 °C.

[0116] 3. Method according to example 1 or 2, wherein the temperature of the first acidic slurry is lower than 65 °C, preferably lower than 60 °C.

[0117] 4. Method according to example 1 or 2, wherein the temperature of the first acidic slurry in the slurrying stage is kept in a range of 30 °C to 70 °C.

[0118] 5. Method according to any one of the preceding examples, wherein the temperature of the first acidic slurry in the slurrying stage is kept in a range of 30 °C to 65 °C, preferably 40 °C to 60 °C.

[0119] 6. Method according any one of the preceding examples, wherein the second acidic slurry in the re-slurrying stage is heated to a temperature in the range of 70 °C to 150 °C, preferably in the range of 80 °C to 140 °C, more preferred in the range of 90 to 140 °C, wherein the temperature of the re-slurrying stage is equal to or higher than the temperature of the slurrying stage.

[0120] 7. Method according to any one of the preceding examples, wherein in the slurrying stage in the first acidic slurry a pH value in the range of 1-5, preferably 1-4, is established.

[0121] 8. Method according to any one of the preceding examples, wherein in the re-slurrying stage in the second acidic slurry a pH value in the range of 1-5, preferably 1-4, is established.

[0122] 9. Method according to any one of the preceding examples, wherein a sodium content in the second lignin cake is below 25 ppm.

[0123] 10. Method according to example 8, in dependence of example 5, wherein an iron content in the second lignin cake is below 15 ppm.

[0124] 11. Method according to any one of the preceding examples, wherein when the first acidic slurry has a temperature above 60 °C before the first separation stage, the first acidic slurry is subjected to cooling before subsequent separation of the lignin from the first acidic slurry in the first separation stage; and / or when the second acidic slurry has a temperature above 60 °C before the second separation stage, the second acidic slurry is subjected to cooling before subsequent separation of the lignin from the second acidic slurry in the second separation stage.

[0125] 12. Method according to any one of the preceding examples, wherein the lignin source is technical lignin, wherein preferably the technical lignin is selected from the group consisting of alkali lignin, such as kraft lignin and soda lignin, organosolv lignin, and hydrolysis lignin.

[0126] 13. Method according to any one of examples 1 to 11 , wherein black liquor is used as a lignin source, the method comprising, before the first slurrying stage: a precipitation stage wherein an acidifier charge is added to black liquor, having a first alkaline pH value, in order to decrease the pH value of the black liquor to a second pH level ranging from neutral to alkaline, initiating precipitation of lignin, followed by a third separation stage wherein the precipitated lignin is separated as a third lignin cake from the remaining liquid phase of the acidified original black liquor still kept in the pH range from neutral to alkaline, wherein the lignin source that is mixed with a first acidifier in the first slurrying stage is the third lignin cake, wherein preferably the second pH level is above pH 7 and below 12.

[0127] 14. Method according to example 13, wherein the black liquor is original black liquor or lignin depleted black liquor.

[0128] 15. Method according to any one of the preceding examples, wherein the method is for extracting lignin with a reduced metal content, in particular lignin with a reduced alkali metal and transition metal content.

[0129] Although the present invention has been described with reference to specific embodiments, also shown in the appended drawings, it will be apparent to those skilled in the art that many variations and modifications can be done within the scope of the invention as described in the specification and defined with reference to the claims below.

Claims

CLAIMS1. Method for extracting lignin, comprising the following stages in sequence: a slurrying stage wherein a first lignin source is mixed with a first acidifier thereby forming a first acidic slurry, followed by a first separation stage in which the lignin is separated from the first acidic slurry forming a first lignin cake, followed by a re-slurrying stage wherein the first lignin cake is mixed with a second acidifier thereby forming a second acidic slurry, and followed by a second separation stage in which the lignin is separated from the second acidic slurry forming a second lignin cake, characterized in that a temperature of the first acidic slurry in the slurrying stage is kept lower than 80 °C, the second acidic slurry in the re-slurrying stage is heated to a temperature in the range of 80 °C to 140 °C; and in that a purity of the first lignin source in the slurrying stage is at least 70%.

2. Method according to claim 1, the first acidic slurry in the slurrying stage is kept equal to or lower than 75 °C, preferably equal to or lower than 72 °C, more preferred equal to or lower than 70 °C.

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

4. Method according to claim 1 or 3, wherein the temperature of the first acidic slurry in the slurrying stage is kept in a range of 30 °C to 75 °C.

5. Method according to any one of the preceding claims, wherein the temperature of the first acidic slurry in the slurrying stage is kept in a range of 45 °C to 75 °C, preferably 55 °C to 75 °C.

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

7. Method according to any one of the preceding claims, wherein a time period at which the second acidic slurry is kept before the lignin is separated in the second separation stage lies in the range of 10 to 120 minutes, preferably 15 to 90 minutes, more preferably 15 to 60 minutes.

8. Method according to any one of the preceding claims, wherein in the slurrying stage in the first acidic slurry a pH value in the range of 1-5, preferably 1-4, more preferably 2 - 4, is established.

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

10. Method according to any one of the preceding claims, wherein the first acidifier is added in an amount less than X kg per ton of the first lignin source, X being equivalent to an amount of 100 kg, preferably 20 kg, H2SO4 per ton of the first lignin source.

11. Method according to any one of the preceding claims, wherein the second acidifier is added in an amount less than X kg per ton of the first lignin source, X being equivalent to an amount of 100 kg H2SO4 per ton of the first lignin source, wherein preferably the second acidifier is added in an amount less than X kg per ton of lignin source, X being equivalent to an amount of 20 kg, preferably 10 kg, H2SO4 per ton of the first lignin source.

12. Method according to any one of the preceding claims, wherein a sodium content in the second lignin cake is below 25 ppm.

13. Method according to any one of the preceding claims, wherein an iron content in the second lignin cake is below 15 ppm.

14. Method according to any one of the preceding claims, wherein when the first acidic slurry has a temperature above 60 °C before the first separation stage, the first acidic slurry is subjected to cooling before subsequent separation of the lignin from the first acidic slurry in the first separation stage; and / or the second acidic slurry is subjected to cooling before subsequent separation of the lignin from the second acidic slurry in the second separation stage.

15. Method according to any one of the preceding claims, comprising, before the first slurrying stage:an additional slurrying stage wherein a second lignin source is mixed with an additional acidifier thereby forming a third acidic slurry, followed by a fourth separation stage in which lignin is separated from the third acidic slurry forming a fourth lignin cake, wherein the fourth lignin cake is the first lignin source that is mixed with a first acidifier in the first slurrying stage.

16. Method according to claim 15, wherein a temperature of the third acidic slurry in the additional slurrying stage is kept equal to or lower than 75 °C, preferably below 70 °C, more preferably below 65 °C, most preferred in a range of 55-65 °C.

17. Method according to claim 15 or 16, wherein the additional acidifier is added in an amount in the range of Y to Z kg per ton of lignin source, Y being equivalent to an amount of 100 kg H2SO4 per ton of lignin source, Z being equivalent to an amount of 400 kg H2SO4 per ton of lignin source.

18. Method according to any one of claims 1 to 14, wherein the first lignin source is technical lignin, wherein preferably the technical lignin is selected from the group consisting of alkali lignin, such as kraft lignin and soda lignin, organosolv lignin, and hydrolysis lignin.

19. Method according to any one of claims 1 to 17, the method comprising, before the first slurrying stage: a precipitation stage wherein an acidifier charge is added to black liquor, having a first alkaline pH value, in order to decrease the pH value of the black liquor to a second pH level ranging from neutral to alkaline, initiating precipitation of lignin, followed by a third separation stage wherein the precipitated lignin is separated as a third lignin cake from the remaining liquid phase of the acidified original black liquor still kept in the pH range from neutral to alkaline, wherein preferably the second pH level is above pH 7 and below 12.

20. Method according to claim 19, in dependence of claim 15, wherein the second lignin source that is mixed with the additional acidifier is the third lignin cake.

21. Method according to claim 19, in dependence of any one of claims 1 to 14, wherein the first lignin source that is mixed with the first acidifier in the first slurrying stage is the third lignin cake.

22. Method according to claim 20, in dependence of claim 15, the method comprising, before the first slurrying stage: a precipitation stage wherein an acidifier charge is added to black liquor, having a first alkaline pH value, in order to decrease the pH value of the black liquor to a second pH level ranging from neutral to alkaline, initiating precipitation of lignin, followed by a third separation stage wherein the precipitated lignin is separated as a third lignin cake from the remaining liquid phase of the acidified original black liquor still kept in the pH range from neutral to alkaline, wherein preferably the second pH level is above pH 7 and below 12, an additional slurrying stage wherein the third lignin cake is the second lignin source and is mixed with an additional acidifier thereby forming a third acidic slurry, followed by a fourth separation stage in which lignin is separated from the third acidic slurry forming a fourth lignin cake, wherein the first lignin source that is mixed with a first acidifier in the first slurrying stage is the fourth lignin cake.

23. Method according to any one of claims 19 to 22, wherein the black liquor is original black liquor or lignin depleted black liquor.

24. Method according to any one of the preceding claims, wherein the method is for extracting lignin with a reduced metal content, in particular lignin with a reduced alkali metal and transition metal content.

25. Method for extracting lignin from a lignin source, comprising: an additional slurrying stage wherein the lignin source is mixed with an additional acidifier thereby forming a third acidic slurry; followed by a fourth separation stage in which lignin is separated from the third acidic slurry forming a fourth lignin cake; a slurrying stage wherein the fourth lignin cake is mixed with a first acidifier thereby forming a first acidic slurry; followed by a first separation stage in which the lignin is separated from the first acidic slurry forming a first lignin cake; followed by a re-slurrying stage wherein the first lignin cake is mixed with a second acidifier thereby forming a second acidic slurry; and followed by a second separation stage in which the lignin is separated from the second acidic slurry forming a second lignin cake, whereina temperature of the first acidic slurry in the slurrying stage is kept lower than 80 °C, the second acidic slurry in the re-slurrying stage is heated to a temperature in the range of 80 °C to 140 °C; and a purity of the lignin source in the slurrying stage is at least 70%, wherein optionally the lignin source is technical lignin, wherein preferably the technical lignin is selected from the group consisting of alkali lignin, such as kraft lignin and soda lignin, organosolv lignin, and hydrolysis lignin.

26. Method for extracting lignin from black liquor, the method comprising the following stages in sequence: a precipitation stage wherein an acidifier charge is added to black liquor, having a first alkaline pH value, in order to decrease the pH value of the black liquor to a second pH level ranging from neutral to alkaline, initiating precipitation of lignin; followed by a third separation stage wherein the precipitated lignin is separated as a third lignin cake from the remaining liquid phase of the acidified original black liquor still kept in the pH range from neutral to alkaline, wherein preferably the second pH level is above pH 7 and below 12; followed by an additional slurrying stage wherein the third lignin cake is mixed with an additional acidifier thereby forming a third acidic slurry; followed by a fourth separation stage in which lignin is separated from the third acidic slurry forming a fourth lignin cake; followed by a slurrying stage wherein the fourth lignin cake is mixed with a first acidifier thereby forming a first acidic slurry; followed by a first separation stage in which the lignin is separated from the first acidic slurry forming a first lignin cake; followed by a re-slurrying stage wherein the first lignin cake is mixed with a second acidifier thereby forming a second acidic slurry; and followed by a second separation stage in which the lignin is separated from the second acidic slurry forming a second lignin cake, wherein a temperature of the first acidic slurry in the slurrying stage is kept lower than 95 °C, preferably 80 °C, a temperature of the third acidic slurry in the additional slurrying stage is kept lower than 95 °C, preferably 80 °C, the second acidic slurry in the re-slurrying stage is heated to a temperature in the range of 65 °C to 140 °C, preferably 80 °C to 110 °C, andthe temperature of the second acidic slurry is higher than the temperature of the third acidic slurry.

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