Method for refining lignin

Abstract

The present invention relates to a method for purifying lignin, the method comprising the steps of providing a solid lignin, immersing the lignin in an aqueous acidic solution to remove metals from the lignin and transfer them to the aqueous acidic solution, and separating the purified lignin from the aqueous acidic solution. The present invention also relates to lignin having a total metal content of less than 200 ppm. The purified lignin disclosed in the present invention can be further processed into a carbon-rich material.

Description

[Technical field]

[0001] The present invention relates to a method for purifying lignin, wherein metals are removed from the lignin. The present invention further relates to lignin having a total metal content below 200 ppm. The purified lignin can be further processed to a final product, such as a carbon enriched material. [Background technology]

[0002] Lignin, an aromatic polymer, is the main component in wood, for example, and is the most abundant source of carbon on Earth after cellulose. In recent years, technologies have been developed and commercialized to extract lignin from the pulp manufacturing process in a highly pure, solid, and specific form, and it has attracted considerable attention as a potential renewable alternative to the major aromatic chemical precursors currently supplied by the petrochemical industry.

[0003] Today, the most important commercially available source of lignin is Kraft lignin. It is obtained from hardwoods or softwoods by the Kraft process. Lignin can be separated from alkaline black liquor, for example, by membrane filtration or ultrafiltration. LignoBoost is one common separation process, described for example in WO2006031175 A1. In this process, lignin is precipitated from alkaline black liquor by lowering the pH level (usually by adding carbon dioxide) and then filtered off. The lignin filter cake is reslurried in acidic conditions (usually with sulfuric acid) and washed in a next step. The precipitated and washed lignin can be used as is or can be further dried.

[0004] Black liquor is a cost-effective source of lignin because it is readily available as a by-product from the Kraft process. However, black liquor contains a certain amount of metals (mostly from the wood and from the cooking chemicals used during the pulping process), and therefore the lignin precipitated from black liquor will also contain certain amounts of metals. Lignin generally contains relatively high levels of sodium and potassium, and will also contain small amounts of other metals (e.g. aluminum, calcium, iron, magnesium, and manganese). Lignin may also contain trace amounts of other metals.

[0005] Some metals, such as sodium and potassium, can be largely removed from the precipitated lignin by an acid washing step during the separation process. However, other metals are relatively difficult to remove from the precipitated lignin. For many possible applications, such as biofuels and conversion to carbon-rich materials (such as carbon fibers and carbon powders), it is important that the lignin used is of high purity, especially in terms of metal content, as this may otherwise interfere with the functionality of the material. This is particularly important if it is intended to further convert the lignin into a carbon-rich material (used in energy storage applications). In particular, the presence of transition metals (such as iron and manganese) in energy storage applications involving carbon-rich materials obtained from lignin has a negative effect on the long-term stability of the energy storage device. This is believed to be caused by the precipitation of the transition metals during charging and discharging, and by the decomposition of the electrolyte by reactions catalyzed by the transition metals.

[0006] Lignin can also be obtained by various fractionation methods (such as the organosolv process or hydrolyzed lignin), however the organosolv process is of less commercial interest for producing lignin than the kraft process.

[0007] Various attempts have been made to remove metals from lignin, but what these methods have in common is that they are fairly complicated, involving many additional processing steps, and / or the metal removal efficiency is not sufficiently efficient, the purity level is insufficient, and / or the washing times are long.

[0008] Korean Patent No. 101451299 (KR101451299 B1) discloses a method for obtaining lignin with a low ash content by repeatedly dissolving and precipitating lignin, however, many additional steps are required.

[0009] WO2020013752 A1 discloses a method for dissolving lignin in an acidic aqueous solution. A two-phase system is obtained by phase separation, where one phase is lignin-rich and the other phase is lignin-poor, and this other phase contains metal cations extracted from the lignin. However, many additional steps are required.

[0010] Thus, there is a need for an improved method for purifying lignin, where the metal content of the resulting purified lignin is low enough to allow the purified lignin to be used in applications where high purity materials are required. Moreover, it is desirable that the method be cost-effective and amenable to large-scale production.

[0011] Summary of the Invention It is an object of the present invention to provide an improved process for purifying lignin, which eliminates or mitigates at least some of the disadvantages of the prior art processes.

[0012] It is a further object of the present invention to provide a process for the purification of lignin, wherein the total metal content of the obtained lignin is reduced.

[0013] It is a further object of the present invention to provide a method for purifying lignin, the method being suitable for large-scale production.

[0014] These and other objects are accomplished by the various aspects of the present invention, as will be appreciated by those skilled in the art upon access to the invention.

[0015] According to a first aspect, the present invention relates to a method for purifying lignin, said method comprising the following steps: a) providing a solid lignin; b) providing an acidic aqueous solution having a pH below 6; c) soaking the lignin in an acidic aqueous solution for at least 15 minutes, the temperature of the acidic aqueous solution during soaking being in the range of 40°C to 100°C in order to remove metals from the lignin and transfer them to the acidic aqueous solution, thereby obtaining purified lignin; d) separating the resulting purified lignin from the acidic aqueous solution; and e) Optionally, washing the separated purified lignin wherein the lignin remains solid during all steps of the process.

[0016] The process of the invention according to the first aspect is based on the surprising finding that metals can be removed from solid lignin by immersing the lignin in an acidic solution for at least 15 minutes at a temperature in the range of 40° C. to 100° C. This allows for a rapid lignin purification process with few additional process steps, which can be carried out using existing processing equipment used during the production of lignin from black liquor, making the process suitable for large scale production.

[0017] According to a second aspect, the present invention relates to a lignin having a total metal content of less than or equal to 200 ppm.

[0018] Lignin materials with low metal content are of interest for many different applications, such as biofuels and conversion into carbon-rich materials (such as carbon fibers and powders), particularly for use in energy storage applications.

[0019] Detailed Description Step a) of the method according to the first aspect involves providing a solid lignin. Throughout this specification, the expression "lignin" encompasses all kinds of lignin, for example lignin from broadleaf, coniferous or annular plants. The lignin may also be chemically modified. The lignin is preferably purified or isolated before being used in the process according to the invention. The lignin can be isolated from black liquor and, optionally, further purified before being used in the process according to the invention. The purification is usually carried out so that the purity of the lignin material is at least 90%, preferably at least 95%, more preferably at least 98%, based on the dry weight of the lignin material. Thus, the lignin material used according to the process of the invention preferably contains less than 10%, preferably less than 5%, preferably less than 2% of impurities (e.g. cellulose and inorganic compounds), based on the dry weight of the lignin material.

[0020] The lignin can be obtained by different fractionation methods, such as the organosolv process or the Kraft process. The lignin provided in step a) of the method according to the first aspect is preferably a Kraft lignin, i.e. a lignin obtained by the Kraft process. The Kraft lignin is preferably obtained from hardwoods or softwoods, most preferably from softwoods.

[0021] The lignin is obtained by the process disclosed in WO2006031175 A1, commonly referred to as the Lignoboost process. This process typically involves precipitating the lignin from alkaline black liquor by acidification, separating the precipitated lignin, and reslurrying the lignin at least once under acidic conditions. The obtained lignin can be dried and pulverized to provide solid particles. The lignin obtained by this method typically has a total metal content in the range of 500 ppm to 5000 ppm, the metal content coming mainly from the wood source and from the cooking chemicals added during the pulping process. The pH of the obtained lignin is typically in the range of 3 to 4. The sulfur content of the obtained lignin is typically around 1 to 3% by weight. This lignin is the preferred starting material for the process according to the invention.

[0022] As used herein, the term "total metal content" refers to the total amount of metals present in the lignin. The metals present in the lignin are typically sodium, potassium, magnesium, calcium, aluminum, iron, and manganese. Trace amounts of other metals may also be present. The metal content may be determined by inorganic elemental analysis. The total metal content is determined by combining the amounts of the individual metals. In one embodiment, the lignin provided in step a) of the method of the invention according to the first aspect has a total metal content of at least 500 ppm, preferably at least 600 ppm, more preferably at least 700 ppm. In another embodiment, the lignin provided in step a) of the method of the first aspect has a total metal content in the range of 500 ppm to 5000 ppm, preferably in the range of 600 ppm to 3000 ppm, more preferably in the range of 700 ppm to 1500 ppm.

[0023] In one embodiment, the lignin provided in step a) of the process according to the first embodiment may have an aluminium content of at least 18 ppm or at least 19 ppm, a calcium content of at least 18 ppm or at least 19 ppm, an iron content of at least 16 ppm or at least 17 ppm, a potassium content of at least 30 ppm or at least 50 ppm, a magnesium content of at least 45 ppm or at least 48 ppm, a manganese content of at least 11 ppm or at least 12 ppm, and a sodium content of at least 400 ppm or at least 650 ppm.

[0024] In an alternative embodiment, the lignin provided in step a) of the process according to the first aspect has an aluminium content in the range of 18-25 ppm, preferably in the range of 19-23 ppm, a calcium content in the range of 18-40 ppm, preferably in the range of 19-25 ppm, an iron content in the range of 16-30 ppm, preferably in the range of 17-22 ppm, a potassium content in the range of 30-90 ppm, preferably in the range of 50-70 ppm, a magnesium content in the range of 45-55 ppm, preferably in the range of 48-53 ppm, a manganese content in the range of 11-30 ppm, preferably in the range of 12-16 ppm and a sodium content in the range of 400-2000 ppm, preferably in the range of 650-1000 ppm.

[0025] The lignin provided in step a) of the process according to the first aspect may have a silicon content of at least 90 ppm, or at least 100 ppm, alternatively the silicon content may be in the range 90 to 140 ppm, preferably in the range 100 to 130 ppm.

[0026] The lignin provided in step a) of the method according to the first aspect is in solid form, i.e. not in dissolved state. In one embodiment the lignin is provided in the form of a dry powder. Alternatively, the lignin may be wet or provided in a slurry or suspension. The lignin may also be provided as ground lignin cake obtained from a lignin separation process. The lignin is not dissolved and remains in solid form during any of the steps in the method according to the first aspect.

[0027] Alternatively, the lignin may be dissolved in small amounts during the steps of the method according to the invention such that only a small fraction of the solid lignin is dissolved, and thus the lignin will remain mostly in solid form, which will be removed during the separation step and discarded from the process.

[0028] The lignin provided in step a) of the method according to the first aspect is preferably in particulate, e.g. powder form. The particle size distribution of the lignin particles is preferably such that at least 80% by weight of the particles have a diameter of less than 0.2 mm. In the context of the present invention, the particle diameter is the equivalent spherical diameter of the particle if the particle is not spherical. The equivalent spherical diameter is the diameter of a sphere of equivalent volume.

[0029] Step b) of the method according to the first aspect involves providing an acidic aqueous solution having a pH below 6. As used herein, the term "acidic aqueous solution" refers to any type of aqueous solution having a pH below 7. The acidic aqueous solution in step b) of the method according to the first aspect may be provided by adding at least one acid to an aqueous solution. The acidic aqueous solution provided in step b) of the method according to the first aspect has a pH below 6, preferably below 5, more preferably below 4. In an alternative embodiment, the pH of the acidic aqueous solution may be below 7. In a further alternative embodiment, the pH of the acidic aqueous solution may be in the range of 1 to 6, preferably in the range of 1 to 5, more preferably in the range of 1 to 4.

[0030] Lignin may be added to the aqueous solution after the acid has been added. Alternatively, lignin may be added to the aqueous solution before the acid is added. The aqueous solution is heated either before or after the addition of the acid and lignin. In one embodiment, acid is added to the heated aqueous solution, followed by addition of lignin to the aqueous solution. In another embodiment, lignin is added to the acidic aqueous solution, followed by heating of the aqueous solution. In yet another embodiment, acid is added to the aqueous solution containing lignin. In this embodiment, the aqueous solution is heated either before or after addition of the acid. In a preferred embodiment, lignin is added to the heated aqueous solution, followed by addition of acid to the aqueous solution containing lignin.

[0031] The aqueous acid solution comprises at least one acid such that the pH of the aqueous acid solution is below 7, or below 6, or below 5, or below 4. In one preferred embodiment, the aqueous acid solution comprises at least one organic acid. The organic acid may be a carboxylic acid and may be monoprotic, diprotic, or triprotic. In a preferred embodiment, the pKa value of the organic acid is below 4.75 for at least one acid group. The organic acid may be selected from at least one of acetic acid, citric acid, formic acid, and oxalic acid. Preferably, the aqueous acid solution comprises at least one organic acid selected from formic acid and oxalic acid. It has been found that the use of an organic acid further reduces the metal content in the lignin during the steeping process. Organic acids, such as formic acid and oxalic acid, are cheap, easy to handle, less toxic, and usually less restricted by certain waste disposal regulations than acids containing sulfur, nitrogen, and / or phosphorus.

[0032] In an alternative embodiment, the aqueous acid solution comprises at least one inorganic acid, such as sulfuric acid, hydrochloric acid, or phosphoric acid.

[0033] The aqueous acid solution may also include more than one acid, such as a combination of two or more organic or inorganic acids. The aqueous acid solution may also include a combination of an organic acid and an inorganic acid. In one embodiment, the aqueous acid solution includes both formic acid and oxalic acid.

[0034] In one embodiment of the method according to the first aspect, the total amount of acid in the aqueous acid solution is in the range of 0.1-6 wt.%, preferably in the range of 0.5-5 wt.%, based on the dry weight of the lignin soaked in the aqueous acid solution. As used herein, the term "total amount of acid" refers to the total amount of concentrated acid added to the aqueous acid solution. Increasing the amount of acid allows more metals to be removed from the lignin. However, from a process point of view, it is advantageous to avoid the use of large amounts of acid for cost reasons and to facilitate waste disposal. In addition, increasing the amount of acid in the aqueous acid solution may cause side reactions that may lead to the degradation of the lignin.

[0035] The acidic aqueous solution may further include one or more additives. The additive may be a dispersant, such as glycerol or a fatty acid. The additive may also be an oxidizing agent, such as hydrogen peroxide or EDTA. Other additives include ion exchangers, such as ammonium salts, such as ammonium acetate and ammonium sulfate. The additives in the acidic aqueous solution may enhance the effect of removing metals from lignin.

[0036] Step c) of the method according to the first aspect involves soaking the lignin particles in an acidic aqueous solution for at least 15 minutes, wherein the temperature of the acidic aqueous solution during soaking is in the range of 40°C to 100°C to remove metals from the lignin and transfer them to the acidic aqueous solution to obtain purified lignin.

[0037] As used herein, the term "soaking" refers to the process of contacting solid lignin (e.g., in the form of lignin particles) with an aqueous acidic solution for a period of time. During the soaking step of the method according to the first embodiment, the entire surface area of ​​the lignin is in contact with the aqueous acidic solution, meaning that the lignin is completely immersed in the aqueous acidic solution.

[0038] The lignin is immersed in the acidic aqueous solution at a temperature in the range of 40°C to 100°C, for example in the range of 50°C to 90°C, or in the range of 60°C to 80°C. In one embodiment, the lignin is immersed in the acidic aqueous solution at a temperature in the range of 40°C to 110°C, for example in the range of 80°C to 100°C. The acidic aqueous solution may be heated by any suitable means known to the skilled person. The temperature is kept in the specified range throughout the immersion process. Heating the acidic aqueous solution increases the metal removal effect. However, using too high a temperature may lead to damage or decomposition of the lignin.

[0039] The soaking time in step c) of the method according to the first aspect is at least 15 minutes, preferably at least 30 minutes, or even more preferably at least 1 hour. In alternative embodiments of the method according to the first aspect, the soaking time is in the range of 15 minutes to 6 hours, preferably in the range of 30 minutes to 5 hours, or even more preferably in the range of 1 hour to 4 hours. By increasing the soaking time, the metal removal efficiency can be increased. However, to enable a cost-effective process that can be easily scaled up, it is important that the soaking time is relatively short. However, the soaking time must be long enough to sufficiently remove the metals from the lignin.

[0040] In a preferred embodiment of the method according to the first aspect, the aqueous acidic solution is agitated during the immersion step. Any suitable agitation means known to the skilled person can be used.

[0041] During the soaking step, the metals are removed from the lignin and transferred to the aqueous acidic solution, thus obtaining a purified lignin. As used herein, the term "purified lignin" refers to a lignin material that contains substantially only lignin, for example, at least 99% lignin, based on the dry weight of the lignin material, and less than 1% by weight of other components (e.g., cellulose and inorganic compounds), based on the dry weight of the lignin material. In particular, the purified lignin has a reduced amount of metals. In a preferred embodiment of the method according to the first aspect, the purified lignin obtained in step c) of the method has a total metal content of less than 200 ppm, preferably less than 150 ppm, more preferably less than 100 ppm. As mentioned above, the lignin is not dissolved during the soaking step, which means that the metals are removed from the solid lignin and not from the dissolved lignin. Thus, the purified lignin obtained in step c) of the method according to the first aspect is also in solid, e.g. particulate, form.

[0042] Lignin that is low in metals is obtained by the method of the invention according to the first aspect. The lignin is preferably in particulate form so that lignin particles that are low in metals are obtained.

[0043] Step d) of the method according to the first aspect involves separating the resulting refined lignin from the aqueous acidic solution. The refined lignin is in solid form during separation. As used herein, the term "separation" refers to the process of separating the lignin from the aqueous acidic solution.

[0044] In a preferred embodiment of the method according to the first aspect, the separation in step d) is carried out by filtration. Alternatively, the separation may be carried out by centrifugation or sedimentation or other suitable means known to the skilled person. Since the purified lignin is separated from the aqueous acid solution, the pH of the aqueous acid solution and the lignin during the separation step will be approximately the same as during the soaking step. Thus, the pH of the aqueous acid solution is below 6, preferably below 5, or more preferably below 4 during the step of separating the purified lignin. In one embodiment, the pH of the aqueous acid solution may be in the range of 1-7, preferably in the range of 2-6, more preferably in the range of 2-5, and most preferably in the range of 2-4.

[0045] In one embodiment of the method according to the first aspect, steps c-d and optionally e) are repeated at least once. In this embodiment, the separated lignin is soaked in the aqueous acidic solution (for the second time) before the second separation step. The pH and temperature of the aqueous acidic solution and the time of the soaking step are selected as described above. In an embodiment in which the soaking and separation steps are repeated, the parameters (e.g. pH, temperature, and soaking time) may be the same in all soaking steps or may vary in the different soaking steps. In one embodiment, the same parameters are used during the two soaking steps. In one embodiment, the pH may be relatively lower in the second soaking step than in the first soaking step. In another embodiment, the soaking time may be relatively shorter in the second soaking time than in the first soaking time. In yet another embodiment, the temperature may be relatively higher in the first soaking step than in the second soaking step. The total soaking time is the sum of the soaking times for each step. Separation is preferably performed by filtration after each soaking step. The metal content of the lignin can be further reduced by adding additional soaking steps and optimizing the parameters (e.g. time, temperature and pH of each step).

[0046] Step e) of the method according to the first aspect optionally involves washing the separated purified lignin. In one embodiment of the method according to the first aspect, the separated lignin is subjected to washing with an aqueous washing solution. The aqueous washing solution is preferably water. In one embodiment, the separated purified lignin is washed with water until the pH of the water used for washing is neutral.

[0047] In one embodiment of the method according to the first aspect, the method comprises the additional step of drying the separated and optionally washed refined lignin. The refined lignin can be dried after the separation step or after the optional washing step. Drying of the refined lignin can be carried out by methods and equipment known in the art. The temperature during drying is preferably in the range of 60°C to 160°C, more preferably in the range of 100°C to 120°C. Drying can be carried out at ambient pressure, reduced pressure or under vacuum.

[0048] The refined lignin obtained by the process according to the first aspect has a total metal content of less than 200 ppm, preferably less than 150 ppm, more preferably less than 100 ppm.

[0049] The refined lignin obtainable by the process according to the first aspect may have an aluminium content of less than 17 ppm or less than 13 ppm, a calcium content of less than 16 ppm or less than 10 ppm, a potassium content of less than 10 ppm or less than 5 ppm, a magnesium content of less than 42 ppm or less than 25 ppm and a sodium content of less than 30 ppm or less than 15 ppm.

[0050] The refined lignin obtained by the process according to the first aspect may have an iron content of less than 15 ppm, preferably less than 10 ppm.

[0051] The refined lignin obtained by the process according to the first aspect may have a manganese content of less than 10 ppm, preferably less than 6 ppm.

[0052] In an alternative embodiment, the purified lignin obtained by the method according to the first aspect may have a total metal content in the range of 0 to 200 ppm, such as in the range of 0.1 to 200 ppm, preferably in the range of 0 to 150 ppm, such as in the range of 0.1 to 150 ppm, more preferably in the range of 0 to 100 ppm, such as in the range of 0.1 to 100 ppm.

[0053] In alternative embodiments, the purified lignin obtained by the method according to the first aspect may have an iron content in the range of 0-15 ppm, or in the range of 0-10 ppm, or in the range of 0.1-15 ppm, or in the range of 0.1-10 ppm.

[0054] In alternative embodiments, the refined lignin obtained by the process according to the first aspect may have a manganese content in the range of 0-10 ppm, or in the range of 0-6 ppm, or in the range of 0.1-10 ppm, or in the range of 0.1-6 ppm.

[0055] In an alternative embodiment, the purified lignin obtained by the method according to the first aspect may have an aluminium content in the range of 0-17 ppm or 0-13 ppm, a calcium content in the range of 0-16 ppm or 0-10 ppm, a potassium content in the range of 0-10 ppm or 0-5 ppm, a magnesium content in the range of 0-42 ppm or 0-25 ppm, and a sodium content in the range of 0-30 ppm or 0-15 ppm.

[0056] In an alternative embodiment, the purified lignin obtained by the method according to the first aspect may have an aluminium content in the range of 0.1-17 ppm or 0.1-13 ppm, a calcium content in the range of 0.1-16 ppm or 0.1-10 ppm, a potassium content in the range of 0.1-10 ppm or 0.1-5 ppm, a magnesium content in the range of 0.1-42 ppm or 0.1-25 ppm, and a sodium content in the range of 0.1-30 ppm or 0.1-15 ppm.

[0057] The lignin provided as starting material in step a) of the process according to the first aspect may preferably have a metal content in the range of 500 ppm to 5000 ppm, which means that the reduction in total metal content by the process according to the invention may be at least 60%, preferably at least 70%, more preferably at least 80%.

[0058] Other inorganic impurities, such as silicon, may also be removed from the lignin and transferred to the aqueous acid solution during the steeping step. Thus, in one embodiment of the method according to the first aspect, silicon is also removed from the lignin and transferred to the aqueous acid solution during the steeping step, and the resulting purified lignin has a silicon content of less than 80 ppm, such as in the range of 0-80 ppm or 0.1-80 ppm.

[0059] The refined lignin obtained by the method according to the first aspect may be subjected to further processing, for example various heat treatments.

[0060] The purified lignin obtained by the method according to the first aspect is particularly suitable for further conversion into a carbon-rich material intended for energy storage applications, and for other applications where a lignin material having a total metals content of less than 200 ppm is of interest.

[0061] The lignin according to the second aspect of the invention has a total metal content of 200 ppm or less. The lignin is obtainable by the method according to the first aspect. Due to the low metal content of the lignin, the lignin is suitable for further conversion into a carbon-rich material that can be used in energy storage applications. The lignin according to the second aspect may be further defined with reference to the first aspect as specified above.

[0062] Working Example In all examples, kraft lignin powder obtained from the Lignovoost process was used.

[0063] The contents of minerals (e.g. aluminum, calcium, iron, potassium, magnesium, manganese, sodium and silicon) in the lignin samples (i.e. lignin powders subjected to various treatments) were evaluated by ICP-OES (Inductively Coupled Plasma Optical Emission Spectroscopy). The lignin samples were oxidized by hydrogen peroxide and subsequently wet digested with nitric acid in a microwave oven. The mineral contents were quantified by ICP-OES. The total metal content was calculated by adding up the individual metal contents.

[0064] The inorganic content in the lignin powder starting material before treatment (sample 0) was also evaluated by ICP analysis (see Table 1).

[0065] Table 1: Content of inorganic matter in kraft lignin powder. "Metal" is the total metal content. TIFF2025512919000001.tif40170

[0066] Example 1: Types of Acids Kraft lignin powder from the Lignovoost process (150 g) was added to heated water (1.5 l, 60° C. or 80° C.) with stirring. Then, 7.5 g (5 wt. % based on the dry weight of lignin added to the water) of acid (oxalic, formic, phosphoric or sulfuric) was added and the mixture was stirred for 4 hours while maintaining the temperature. The mixture was then filtered and the lignin was recovered and dried in a vacuum oven at 60° C. overnight. The experimental details are summarized in Table 2 and the results from the ICP analysis are summarized in Table 3. It can be noted that oxalic acid (1d) at 80° C. gave the highest purity lignin, both in terms of the lowest total metal content and the lowest levels of critical transition metals (e.g. Fe and Mn).

[0067] Table 2: Experimental details for Example 1 TIFF2025512919000002.tif59170

[0068] Table 3: Inorganic content in samples from Example 1. "Metal" is the total metal content. TIFF2025512919000003.tif69170

[0069] Example 2: Comparative Example In the comparative example, kraft lignin powder (150 g) from the Lignovoost process was added to heated water (1.5 l) at a temperature of 60° C. (sample 2a) or 80° C. (sample 2b) with stirring. The mixture was stirred for 4 hours while maintaining the temperature. No acid was added. The mixture was then filtered and the lignin was recovered and dried in a vacuum oven at 60° C. overnight. The results from the ICP analysis are summarized in Table 4.

[0070] Table 4: Inorganic content in samples from Example 2. "Metal" is the total metal content. TIFF2025512919000004.tif40170

[0071] Example 3: Amount of acid to be added Kraft lignin powder from the LignoBoost process (150 g) was added to heated water (1.5 l, 60° C. or 80° C.) with stirring. Then, 1.5 g or 7.5 g (1 wt. % or 5 wt. %, respectively, based on the dry weight of lignin added to the water) of acid (oxalic or formic acid) was added and the mixture was stirred for 4 hours while maintaining the temperature. The mixture was then filtered and the lignin was recovered and dried in a vacuum oven at 60° C. overnight. The experimental details are summarized in Table 5 and the results from the ICP analysis are summarized in Table 6. It has been demonstrated that increasing the amount of acid from 1 wt. % to 5 wt. % has little effect when the wash is maintained at 60-80° C. for 4 hours.

[0072] Table 5: Experimental details for Example 3 TIFF2025512919000005.tif50170

[0073] Table 6: Inorganic content in samples from Example 3. "Metal" is the total metal content. TIFF2025512919000006.tif59170

[0074] Example 4: Temperature of an aqueous acid solution Kraft lignin powder (150 g) from the Lignovoost process was added to water (1.5 l) with stirring. The water was heated to 20° C. (sample 4a), 60° C. (sample 4b) or 80° C. (sample 4c). Then, 7.5 g (5 wt. % based on the dry weight of lignin added to the water) of oxalic acid was added and the mixture was stirred for 4 hours while maintaining the temperature. The mixture was then filtered and the lignin was recovered and dried in a vacuum oven at 60° C. overnight. The results from the ICP analysis are summarized in Table 7. It is clear that increasing the temperature from 20° C. to 60° C. or 80° C. reduces the residual metal levels by more than 50%. Increasing the temperature from 60° C. to 80° C. helps further reduce the metal (e.g., Fe, Mg and Mn) content.

[0075] Table 7: Inorganic content in samples from Example 4. "Metal" is the total metal content. TIFF2025512919000007.tif50170

[0076] Example 5: Immersion time Kraft lignin powder from the LignoBoost process (150 g) was added to heated water (1.5 l, 60° C. or 80° C.) with stirring. Then, 7.5 g (5 wt. % based on the dry weight of lignin added to the water) of oxalic or formic acid was added and the mixture was stirred for 15 min to 4 h while maintaining the temperature. The mixture was then filtered, the lignin was recovered and dried in a vacuum oven at 60° C. overnight. For one sample (5d), the lignin was recovered after filtration and subjected to a second steeping. All parameters were the same for both steeping steps, so the total steeping time was 8 h. The experimental details are summarized in Table 8 and the results from the ICP analysis are summarized in Table 9. It can be seen that a 1-h wash is sufficient to reach a low total metal content of 86 ppm. However, it is also clear that a higher temperature in combination with a longer wash time gives the highest purity of lignin. Two 4 hour cleaning trials at 80°C achieved metal levels below 10 ppm except for magnesium with a total metal content of 58 ppm.

[0077] Table 8: Experimental details for Example 5 TIFF2025512919000008.tif89170

[0078] Table 9: Inorganic content in samples from Example 5. "Metal" is the total metal content. TIFF2025512919000009.tif98170

[0079] In light of the above detailed description of the invention, other modifications and variations will be apparent to those skilled in the art. It should be apparent, however, that such other modifications and variations can be made without departing from the spirit and scope of the invention.

Claims

1. 1. A method for purifying lignin, the method comprising the steps of: a) providing solid lignin; b) providing an acidic aqueous solution having a pH below 6; c) soaking the lignin in an acidic aqueous solution for at least 15 minutes, wherein the temperature of the acidic aqueous solution during soaking is in the range of 40°C to 100°C to remove metals from the lignin and transfer them to the acidic aqueous solution, thereby obtaining purified lignin; d) separating the resulting purified lignin from the acidic aqueous solution; and e) optionally washing the separated purified lignin wherein the lignin remains solid during all steps of the method.

2. 2. The method of claim 1, wherein the lignin provided in step a) is kraft lignin.

3. 3. The method of claim 1 or 2, wherein the lignin provided in step a) has a total metal content of at least 500 ppm.

4. 3. The method of claim 1 or 2, wherein the lignin provided in step a) has an aluminum content of at least 18 ppm, a calcium content of at least 18 ppm, an iron content of at least 16 ppm, a potassium content of at least 30 ppm, a magnesium content of at least 45 ppm, a manganese content of at least 11 ppm and a sodium content of at least 400 ppm.

5. 3. The method of claim 1 or 2, wherein the lignin is provided in the form of particles, the particle size distribution of the lignin particles being such that at least 80% by weight of the particles have a diameter of less than 0.2 mm.

6. 3. The method of claim 1, wherein the total amount of acid in the acidic aqueous solution is in the range of 0.1 to 6 wt %, based on the dry weight of the lignin soaked in the acidic aqueous solution.

7. 3. The method of claim 1, wherein the aqueous acidic solution comprises at least one organic acid.

8. 8. The method of claim 7, wherein the at least one organic acid is selected from formic acid and oxalic acid.

9. 3. The method according to claim 1 or 2, wherein the separation in step d) is carried out by filtration.

10. 3. The method of claim 1 or 2, wherein steps cd, and optionally e), are repeated at least once.

11. 3. The method of claim 1 or 2, wherein the separated lignin is subjected to washing with an aqueous washing solution.

12. 3. The method of claim 1 or 2, comprising the additional step of drying the separated and optionally washed lignin.

13. 3. The method of claim 1 or 2, wherein the total metal content of the resulting purified lignin is less than 200 ppm.

14. 3. The method of claim 1 or 2, wherein the iron content of the resulting purified lignin is less than 15 ppm.

15. 3. The method of claim 1 or 2, wherein the manganese content of the resulting purified lignin is less than 10 ppm.

16. 3. The method of claim 1 or 2, wherein the resulting purified lignin has an aluminum content of less than 17 ppm, a calcium content of less than 16 ppm, a potassium content of less than 10 ppm, a magnesium content of less than 42 ppm, and a sodium content of less than 30 ppm.

17. Lignin having a total metal content of less than 200 ppm.

18. 18. The lignin of claim 17, having an iron content of less than 15 ppm.

19. 19. Lignin according to claim 17 or 18, having a manganese content of less than 10 ppm.

20. 19. Lignin according to claim 17 or 18, having an aluminum content less than 17 ppm, a calcium content less than 16 ppm, a potassium content less than 10 ppm, a magnesium content less than 42 ppm and a sodium content less than 30 ppm.

21. 19. The lignin of claim 17 or 18, wherein the lignin is a kraft lignin.