A method for producing agglomerated lignin

Abstract

The present invention relates to a method for producing agglomerated lignin, the method comprising the steps of: providing moist lignin having a moisture content of at least 25 wt% and an average particle size of less than 100 μm; adding at least one liquid to the moist lignin and mixing the moist lignin and the at least one liquid with a second mixing force; and kneading the moist lignin and water with a third mixing force so as to form moist agglomerated lignin having an average particle size of at least 300 μm. The present disclosure also relates to agglomerated lignin having an average particle size of at least 300 μm, wherein the agglomerated lignin has a roundness of at least 0.6.

Description

[0001]A METHOD FOR PRODUCING AGGLOMERATED LIGNIN Field of the invention The present application relates to a method for producing agglomerated lignin from moist lignin, and agglomerated lignin. Background Lignin is an aromatic polymer, which is a major constituent in e.g. wood and one of the most abundant carbon sources on earth. In recent years, with the development and commercialization of technologies to extract lignin in a highly purified, solid and particularized form from the pulp-making process, it has attracted significant attention as a possible renewable substitute to primarily aromatic chemical precursors currently sourced from the petrochemical industry. Today, the most commercially relevant source of lignin is kraft lignin, obtained from hardwood or softwood through the kraft process. The lignin can be separated from alkaline black liquor using for example membrane- or ultrafiltration. One common separation process is described in WO2006031175 A1. In this process lignin is precipitated from alkaline black liquor by addition of acid and then filtered off. The lignin filter cake is in the next step re-slurried under acidic conditions and washed. The lignin filter cake may then be dried and / or pulverized. One problem with processing lignin in the form of a dry powder is its strong tendency for dust formation during handling. This severely limits the ability to process lignin in an industrially relevant scale due to the risk of dust explosions. One way to overcome the problems associated with dust formation is to form agglomerated lignin from dry lignin powder, as described in WO2020183383 A1. However, since the method involves handling of dry lignin powder, special care must be taken during production of the agglomerated lignin, limiting the versatility of the method. There is thus a need for a more efficient method for providing agglomerated lignin. Lignin may be used as the starting material for obtaining a carbon enriched material. Examples of carbon enriched materials are charcoal, activated carbon and amorphous carbon such as hard carbon. An additional problem with lignin in powder form is that it exhibits undesired thermoplastic behaviour during heating. During thermal conversion of lignin powder into a carbon enriched material, lignin undergoes plastic deformation / melting, aggressive swelling and foaming. Combined with the strong tendency for dust formation during handling, this severely limits processability of lignin in an industrially relevant scale, in terms of equipment dimensioning and process throughput as well as need of intermediate processing. WO2021250604 A1 describes a method of producing carbon from lignin, involving compacting lignin powder and subsequently crushing the compacted lignin to obtain agglomerated lignin. The agglomerated lignin is subsequently heat treated to obtain thermally stabilized agglomerated lignin, which can be converted to a carbon enriched material with retained shape and dimension, avoiding deformation due to melting / swelling. However, the thermal stabilization occurs mainly on the surface of the agglomerated lignin. In addition, the obtained agglomerated lignin has sharp edges which may cause problems with e.g. dust formation during subsequent processing steps. Thus, there is still a need for an improved process of obtaining lignin that can be heat treated while retaining its shape and dimension with no deformation due to melting / swelling. Summary of the invention It is an object of the present invention to provide a method for producing an agglomerated lignin, which method avoids problems with dust formation. It is a further object of the present invention to provide an improved method for producing an agglomerated lignin that can be heat treated while maintaining its shape and dimension. It is a further object of the present invention to provide a method for producing an agglomerated lignin, which method is scalable and thus suitable for large-scale manufacturing. The above mentioned objects, as well as other objects as will be realized by the skilled person in light of the present disclosure, are achieved by the various aspects of the present disclosure. According to a first aspect, the present invention relates to a method for producing agglomerated lignin, the method comprising the steps of: a) providing moist lignin having a moisture content of at least 25 wt% and an average particle size of less than 100 μm; b) adding at least one liquid to the moist lignin and mixing the moist lignin and the at least one liquid with a second mixing force; and c) kneading the moist lignin and the at least one liquid with a third mixing force so as to form moist agglomerated lignin having an average particle size of at least 300 μm. It has surprisingly been found that agglomerated lignin can be formed from moist lignin having an average particle size of less than 100 μm by adding a liquid, such as water, while mixing the moist lignin. Thus, moist precipitated lignin does not need to be dried prior to agglomeration, and problems relating to dust formation when using dried lignin powder can be decreased or avoided. Therefore, production of agglomerated lignin is greatly simplified since no special measures may need to be taken with regards to reduce the risk for e.g. dust explosions associated with handling of lignin powder. Agglomerated lignin is formed by kneading the moist lignin together with the added liquid. Thus, a simple and versatile process that can be used also in large scale manufacturing is enabled. The mechanical properties of the obtained agglomerated lignin are sufficient to enable further downstream processing steps. The obtained agglomerated lignin has a relatively spherical shape with no or fewer sharp edges, which is beneficial during further processing of the lignin since the strength of the obtained spherical agglomerated lignin is improved compared to agglomerated lignin having a shape with sharp edges. Also, any sharp edges present in the agglomerated lignin tend to be torn down during downstream processing of the dried agglomerated lignin, which will increase the dust content in the process and also lead to a risk for dust explosions. The mixing and kneading steps are preferably carried out in an intensive mixer comprising a mixing bowl and a mixing tool. The mixing bowl and the mixing tool are preferably rotating in different directions. When the agglomerated lignin is to be converted to a carbon enriched material, the agglomerated lignin is preferably subjected to a pre-heating step in order to obtain thermally stabilized agglomerated lignin. The agglomerated lignin obtained by the method according to the present invention has a high porosity which facilitates reactions during the pre-heating step due to improved diffusion of reactive species into the lignin. According to a second aspect, the present invention relates to an agglomerated lignin having an average particle size of at least 300 μm, wherein the agglomerated lignin has a roundness of at least 0.6, and optionally a sphericity in the range of from 0.6 to 0.95. The agglomerated lignin according to the second aspect may be dry. The agglomerated lignin according to the second aspect is porous. The agglomerated lignin according to the second aspect is obtainable by the method according to the first aspect. Brief description of figures For a fuller understanding of the present invention and further objects and advantages of it, the detailed description set out below should be read together with the accompanying drawings, in which the same reference notations denote similar items in the various diagrams, and in which: Figure 1 shows the pore size distribution of agglomerated lignin as determined by incremental intrusion of mercury using mercury intrusion porosimetry; Figure 2 shows the pore size distribution of dry compacted lignin as determined by incremental intrusion of mercury using mercury intrusion porosimetry; Figure 3 is a table showing images of agglomerated and dry compacted lignin particles of varying sizes, as well as the roundness and standard deviation of curvature values for these particles. Detailed description Step a) of the method according to the present invention involves providing moist lignin having a moisture content of at least 25 wt% and an average particle size of less than 100 μm, or less than 75 μm, or less than 50 μm. In the present application, the average particle size is defined as the volume average particle size (Dv50). This value refers to the maximum particle size below which 50% of the volume of the sample exists. The particle size is in the context of the present invention taken to be the diameter of the particle. The average particle size may be determined using for example laser diffraction, but is preferably done using sieve analysis under the assumption that mass is proportional to volume (i.e. 50% passing size by mass is equivalent to Dv50). In the context of the present invention, the diameter of a particle 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. It is intended throughout the present disclosure that the term "lignin" refers to any kind of lignin which may be used to produce agglomerated lignin. Examples of said lignin are, but are not limited to, lignin obtained from vegetable raw material such as wood, e.g. softwood lignin, hardwood lignin, and lignin from annular plants. Also, the lignin can be chemically modified. Preferably, the lignin has been purified or isolated before being used in the method according to the present invention. The lignin may be isolated from black liquor and optionally be further purified before being used in the method according to the present invention. The purification is typically such that the purity of the lignin 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 used in the method of the present invention preferably contains less than 10%, preferably less than 5%, more preferably less than 2% impurities, such as cellulose, carbohydrates and inorganic compounds, based on the dry weight of the lignin. The lignin used in method according to the present invention may be obtained through different extraction methods such as an organosolv process or a kraft process. The lignin may also be obtained from processes such as steam explosion or acidic pre-treatment followed by enzymatic hydrolysis. Preferably, the lignin used in the method according to the present invention is kraft lignin, i.e. lignin obtained through the kraft process. The kraft lignin may be obtained from hardwood or softwood. The lignin may be obtained by the process disclosed in WO2006031175 A1 commonly referred to as the LignoBoost process. Typically, this process involves the steps of precipitation of lignin from alkaline black liquor by acidification; separation of the precipitated lignin; and re-slurrying the lignin under acidic conditions at least once. After washing, the precipitated lignin is obtained as a moist filter cake comprising loosely aggregated solid lignin particles. In a preferred embodiment of the present invention, the lignin provided in step a) is obtained by crushing precipitated lignin so that moist lignin with an average particle size less than 100 μm is obtained. The precipitated lignin is preferably the moist filter cake isolated in a LignoBoost process. Crushing precipitated lignin can be carried out using any suitable equipment as known by a person skilled in the art. The crushing may for example be carried out using a rotary granulator, cage mill, beater mill, hammer mill or crusher mill and / or combinations thereof. After crushing, the crushed material is preferably subjected to a sieving step, to remove fine material. In addition, large material may be removed and / or recirculated back to the crushing step. The terms “moist lignin” and “moist agglomerated lignin” as used herein refers to lignin and agglomerated lignin having a moisture content of at least 25 wt%. The moisture in the moist lignin is typically water remaining from the extraction process used to isolate the lignin. For example, the last step in the extraction process may involve washing the isolated lignin, typically with water or an aqueous solution, in order to remove impurities from the lignin. Therefore, the obtained isolated lignin will often be moist, unless dried in a separate step. In the method according to the present invention, the isolated moist lignin can be used without first being dried. The moisture content of the moist lignin is at least 25 wt%, or at least 30 wt% or at least 35 wt%. Preferably, the moisture content of the moist lignin is not higher than 50 wt%, or not higher than 45 wt%. The moisture content refers to the amount of moisture in the moist lignin, based on the total weight of the moist lignin. The moisture content may be determined by the loss on drying method, using for example a halogen dryer to dry the sample. In some embodiments of the present invention, the method according to the first aspect comprises an additional step of pre-mixing the moist lignin with a first mixing force prior to adding the at least one liquid. The term “mixing force” as used herein refers to the force the moist lignin is subjected to during the mixing or kneading process. The mixing force is supplied by the mixing equipment used and depends on a number of factors, such as the mixing speed used, the type of equipment, the type of mixing tool in the mixing equipment, and the mixing time. A high mixing force means a more intensive mixing of the moist lignin. During the pre-mixing step, the moist lignin is subjected to mixing in order to homogenize the material. By carrying out a pre-mixing step, a lignin material with a more uniform particle size distribution is obtained, which in turn facilitates formation of homogenous agglomerated lignin. The first mixing force used during the pre- mixing step is preferably lower than the second mixing force used when mixing the moist lignin with the at least one liquid. The pre-mixing may for example be carried out for at least 5 seconds, or at least 10 seconds, or at least 30 seconds. Step b) in the method according to the first aspect of the present invention involves adding at least one liquid to the moist lignin and mixing the moist lignin and the at least one liquid with a second mixing force. Since at least one liquid is added to the moist lignin in step b), the moisture content of the moist lignin will increase further such that the moisture content of the moist lignin in step b) is higher than of the moist lignin as provided in step a). Any liquid can be used, as long as it will assist in forming agglomerated lignin in step c) and as long as it does not dissolve lignin, or at least does not dissolve lignin to a large extent. The at least one liquid may be selected from at least one of an organic solvent, an aqueous solution and water. Preferably, the at least one liquid is selected from an aqueous solution and water, and even more preferably the at least one liquid is water. The liquid is typically evaporated during any subsequent drying steps, such that it does not remain in the final agglomerated lignin. Therefore, the at least one liquid may preferably have a suitable volatility. However, if the liquid comprises dissolved compounds, such compounds may remain in the agglomerated lignin also after drying. The amount of the at least one liquid added in step b) is preferably at least 3 wt%, or at least 5 wt%, based on the weight of the moist lignin. If too much liquid is added, such as more than 15 wt%, based on the weight of the moist lignin, it may be difficult to form agglomerated lignin in the kneading step. In a preferred embodiment, the amount of the at least one liquid added in step b) is in the range of from 3 to 10 wt%, based on the weight of the moist lignin. For example, 200 g liquid may be added to 3000 g of moist lignin. Since the amount of the at least one liquid added in step b) is typically small compared to the amount of moist lignin, the increase in moisture content will also be small. The at least one liquid is mixed with the moist lignin, thus ensuring that the at least one liquid is quickly incorporated with the moist lignin. The at least one liquid may be added to the moist lignin prior to starting mixing, or at the same time as starting mixing, or after mixing has already been started. The second mixing force used when mixing the at least one liquid and the moist lignin is preferably higher than the first mixing force used during the pre-mixing step, if performed. The second mixing force is preferably higher than the third mixing force used during the kneading step so as to ensure that the moist lignin and the at least one liquid are adequately mixed. It is important that the second mixing force is sufficiently high so that the at least one liquid and the moist lignin are thoroughly mixed prior to the kneading step. The mixing in step b) may for example be carried out for at least 10 seconds, or at least 20 seconds. All the liquid in step b) may be added at the same time, or the liquid may be added in sequential steps, with mixing in between. The total amount of the at least one liquid added to the moist lignin however remains with the ranges discussed above. Preferably, all the liquid is added at the same time. Step c) of the method according to the first aspect of the present invention involves kneading the moist lignin and the at least one liquid with a third mixing force so as to form moist agglomerated lignin having an average particle size (Dv50) of at least 300 μm. In step c), the moist lignin and the at least one liquid are kneaded so as to form agglomerated lignin. The third mixing force used during the kneading is lower than the second mixing force used when adding the at least one liquid to the moist lignin. Kneading is carried out until moist agglomerated lignin is formed, and the at least one liquid has been incorporated in the formed moist agglomerated lignin. In preferred embodiments, all liquid is incorporated in the formed moist agglomerated lignin. Alternatively, not all liquid is incorporated in the formed moist agglomerated lignin. However, if not all liquid is incorporated in the formed moist agglomerated lignin, the remaining amounts of liquid are preferably small. The kneading in step c) may for example be carried out for at least 3 minutes, or at least 5 minutes, or at least 10 minutes. The kneading time needed to form moist agglomerated lignin depends on for example the mixing force used, the equipment used for mixing, the type and amount of liquid added, and the desired average particle size as well as the particle size distribution of the agglomerated lignin. A longer kneading time will typically result in moist agglomerated having a larger average particle size and also a more uniform particle size distribution. In a preferred embodiment, the kneading step comprises a first kneading step and a second kneading step. Preferably, the first kneading step is carried out at a mixing force higher than the mixing force used in the second kneading step. Thus, the third mixing force does in such embodiments have at least two values, both of which are lower than the second mixing force. By such a relationship between the mixing forces used in the first and second kneading steps, formation of moist agglomerated lignin is facilitated. Formation of moist agglomerated lignin typically begins in the first kneading step, but mainly takes place in the second kneading step. Each of the first and the second kneading steps may for example be carried out for at least 1.5 minutes, or at least 2.5 minutes, or at least 5 minutes. In a preferred embodiment of the present invention, all steps involving mixing or kneading are carried out in direct sequence in the same mixer. Mixing is carried out in the optional pre-mixing step, when adding at least one liquid in step b), and in the kneading in step c). Thus, one mixing step is directly followed by the next, without stopping the mixer. Alternatively, the mixing steps may be carried out as discrete steps either in the same mixer or in different mixers. In a preferred embodiment, the method involves pre-mixing moist lignin, mixing at least one liquid and the moist lignin, a first kneading step and a second kneading step. Any suitable mixer may be used to carry out the method according to the first aspect of the present invention. Some suitable mixers may be of the z-blade (sigma), Banbury, planetary, horizontal, paddle or spiral type. The mixer may process the lignin batchwise or continuously. Some mixers may comprise a mixing bowl in which the moist lignin is placed, and a mixing tool that ensures that the moist lignin is mixed within the mixing bowl. The mixing tool may typically rotate, and in certain embodiments the mixing bowl may also rotate. In a preferred embodiment of the present invention, the mixing, i.e. the optional pre-mixing, the mixing in step b) and the kneading in step c), is carried out in a mixer comprising a mixing bowl and a mixing tool, and wherein the mixing bowl is rotating in an opposite direction to the mixing tool. The rotating speed of the mixing bowl is typically lower than the rotating speed of the mixing tool. The mixing in the method according to the first aspect of the invention may be carried out in an intensive mixer. The mixing force is heavily influenced by the rotating speeds and the rotating directions of the mixing tool and the mixing bowl. In addition, the mixing force needed during a certain mixing step depends on the equipment used and the dimensions of the equipment, the mixing time, the amount of material to be mixed etc. It has been found by the present inventors that a certain relation between the mixing forces used in the different mixing steps facilitates formation of agglomerated lignin. Thus, it is preferred that the mixing force in the mixing in step b) is higher than the mixing force(s) used in the kneading in step c). If kneading is carried out in a first kneading step and a second kneading step, it is preferred that the mixing force is lower in the second kneading step. The average particle size of the moist agglomerated lignin is at least 300 μm, or at least 500 μm, or at least 700 μm. A small average particle size may be favourable if a pre-heating step is carried out, since diffusion distances for reactive species are shorter than in a large particle. However, larger particles are typically easier to handle during processing. The obtained moist agglomerated lignin may have an average particle size in the range of from 300 μm to 5.0 mm, or from 300 μm to 3.0 mm, or from 500 μm to 3.0 mm or from 500 μm to 2.0 mm. The moist agglomerated lignin is in the form of relatively round, relatively spherical particles with few or no sharp edges. The roundness of the moist agglomerated lignin is at least 0.6, or at least 0.65, or at least 0.7, or at least 0.75, or at least 0.8. The term “roundness” as used herein refers to a measure on how closely the shape of an object resembles that of a perfect circle. The maximum roundness of the moist agglomerated lignin is not particularly limited, and may be 1 or less, such as 0.95 or less, such as 0.9 or less. The sphericity of the agglomerated lignin may be in the range of from 0.6 to 0.95, or from 0.65 to 0.8 or from 0.7 to 0.95, or from 0.75 to 0.85. The term “sphericity” is a measure of the degree to which a particle resembles the shape of a sphere. The obtained moist agglomerated lignin is relatively spherical, but is not a perfect sphere. The roundness and sphericity are evaluated by means of microscopy and visual estimation as described in for example Liang F. et al, “A comprehensive review on proppant technologies”, Petroleum, 2016, vol.2, pp 26-39. Roundness is determined by automated analysis of optical microscopy images using Otsu’s method for detection of particle form. The moist agglomerated lignin has a larger moisture content than the moist lignin as provided in step a) due to the addition of the at least one liquid in step b). However, since typically only small amounts of the at least one liquid are added, the increase in moisture content is typically also small. The increase in moisture content is typically such that the moisture content of the moist agglomerated lignin is less than 10% higher than the moisture content of the moist lignin provided in step a). There may also be some evaporation of moisture taking place during any of the mixing steps. Therefore, the actual increase in moisture content of the moist agglomerated lignin compared to the moisture content of the moist lignin as provided may be lower than the one calculated from the amount of the at least one liquid added and the moisture content of the moist lignin as provided. The method according to the first aspect may comprise an additional step of adding at least one additive to the moist lignin. Any suitable additives, such as binders or lubricants, may be added to facilitate the subsequent processing and to improve the density and mechanical properties of the agglomerated lignin. In addition, additives having an influence on the properties of the final agglomerated lignin, or any material obtained from the agglomerated lignin, such as functionality-enhancing additives and fillers, may be added. Examples of such functionality-enhancing additives include carbon additives and silicon-containing additives. Carbon additives may be selected from at least one of graphite, graphene, carbon nanotubes, charcoal, biochar, hard carbon, soft carbon, carbon black and electrically conductive carbon. Silicon-containing additives may be selected from at least one of: elemental silicon, a silicon suboxide, a silicon-metal alloy or a silicon-metal carbon alloy. The silicon suboxide may be SiOx with 0 x 2. The silicon-metal alloy may be any suitable silicon-metal alloy, such as e.g. SiFex or SiFexAly. The silicon-metal carbon alloy may be e.g. SiFexCy. The additive may also be a catalyst, such as a graphitization catalyst. In the context of the present invention, an additive is a substance that is added to improve either the processability or the functionality of the obtained materials. Thus, additives are substances that are added, but that are not present in the lignin starting material. Thus, neither moisture, such as water, nor other components already present in the lignin starting material are considered additives in the context of the present invention. The at least one additive may be added in any step during the process, but preferably before drying the moist agglomerated lignin. The at least one additive may be added during step b), at the same time as adding the at least one liquid. The at least one additive may also be mixed with the at least one liquid prior to adding the at least one liquid to the moist lignin. Thus, in the context of the present invention, the at least one liquid is not considered an additive, but it may comprise an additive. The at least one liquid is needed to form the agglomerated lignin, and the optional additive(s) for example provides further processability or functionality to the formed agglomerated lignin, but are not needed for the formation of the agglomerated lignin. For example, in embodiments where the at least one liquid is an aqueous solution, water will act as the liquid needed to form agglomerated lignin, and the compound(s) dissolved in the water will act as additive(s). The at least one additive may also be added to the moist lignin prior to mixing, or during the optional pre-mixing step, or during the kneading step. If more than one additive is added, the different additives may be added during the same method step, or during different method steps. The total amount of the at least one additive added is preferably such that the final agglomerated lignin comprises less than 20 wt%, such as less than 10 wt%, or less than 5 wt%, or less than 2 wt%, additive(s) as based on the total dry weight of the at least one additive and the agglomerated lignin. The agglomerated lignin may thus comprise at least 80 wt%, or at least 90 wt%, or at least 95 wt%, or at least 98 wt% lignin, based on the total dry weight of the at least one additive and the agglomerated lignin. The amount of the at least one additive depends on the type of additive(s) and the reason for adding it. In some embodiments, no additives are added and the moist agglomerated lignin thus consists only of lignin and the at least one liquid. The method according to the first aspect may further comprise an additional step of drying the moist agglomerated lignin obtained in step c) so as to obtain agglomerated lignin. The dried agglomerated lignin preferably has a moisture content of less than 10 wt%, or more preferably less than 5 wt%. The drying may be carried out in any suitable equipment as known in the art. For example, a rotary kiln may be used. Drying may be carried out at a temperature in the range of from 30 to 90°C, or from 30 to 80°C. It is important that the temperature used during drying is not higher than 90°C, or not higher than 80°C, since the obtained agglomerated lignin becomes softer when a higher temperature is used, and hard agglomerated lignin is often important for subsequent processing steps. Temperatures higher than 90°C may however be used during drying if the hardness of the obtained agglomerated lignin is not an issue. The obtained agglomerated lignin is after drying an essentially dry solid material. It has an average particle size of at least 300 μm, or at least 500 μm, or at least 700 μm. The obtained moist agglomerated lignin may have an average particle size in the range of from 300 μm to 5.0 mm, or from 300 μm to 3.0 mm, or from 500 μm to 3.0 mm or from 500 μm to 2.0 mm. The particle size of the obtained moist agglomerated lignin thus remains largely the same after drying. The obtained agglomerated lignin is in the form of relatively round, relatively spherical particles with few or no sharp edges. The roundness of the agglomerated lignin is at least 0.6, or at least 0.65, or at least 0.7, or at least 0.75, or at least 0.8. The maximum roundness of the moist agglomerated lignin is not particularly limited, and may be 1 or less, such as 0.95 or less, such as 0.9 or less. The sphericity of the agglomerated lignin may be in the range of from 0.6 to 0.95, or from 0.65 to 0.8 or from 0.7 to 0.95, or from 0.75 to 0.85. Thus, the values of sphericity and roundness remain largely the same after drying. The relatively round and spherical shape of the agglomerated lignin is advantageous in subsequent processing steps since the mechanical properties are good, and the relative lack of sharp edges reduces problems with dust being generated while processing the dry agglomerated lignin. The obtained agglomerated lignin is porous. Preferably, it has a porosity in the range of from 5 to 50%, such as from 25% to 45%, such as from 30% to 40%. The porosity may be estimated from SEM-images of the material and is measured using a mercury porosimeter in accord with the method described in ISO 15901-1:2016. A porous material is advantageous during the pre-heating step since diffusion of reactive oxidative species into the agglomerated lignin is facilitated, thus in turn enabling more efficient cross-linking of the lignin. As a result, after pre-heating, a highly cross-linked and very stable material is obtained. The pore size distribution of the agglomerated lignin may be essentially monomodal, or may be essentially bimodal, when measured using mercury porosimetry by the method of ISO 15901-1:2016. The modal pore diameter (most frequently occurring pore diameter) in the pore size distribution may be from 0.01 μm to 1 μm, such as from 0.03 μm to 0.3 μm. The bulk density of the agglomerated lignin may be from 0.4 to 1.1 g / cm3, such as from 0.5 to 1.0 g / cm3, such as from 0.6 to 0.9 g / cm3, such as from 0.7 to 0.8 g / cm3. Due to the mixing and kneading, the agglomerated lignin is compacted and thus has a higher bulk density than a lignin material that has been loosely aggregated. The mechanical stability of the obtained agglomerated lignin may be estimated by a friability test. In such a test, the durability of the obtained agglomerated lignin is tested. After the test, the amount of fines is estimated. The fines are present due to degradation and / or pulverization of the agglomerated lignin. Fines are defined in the context of the present application to be particles having a size less than 100 μm. Typically, after the friability test, less than 30 wt%, or less than 20 wt%, or less than 10 wt%, or less than 5 wt% of the tested agglomerated lignin sample is constituted by fines. This means that at least 70 wt%, or at least 80 wt%, or at least 90 wt%, or at least 95 wt% of the agglomerated lignin sample is not degraded and / or pulverized by the friability test. The obtained agglomerated lignin may be used in any suitable application where lignin is typically used. The agglomerated lignin can for example be used in adhesives. The method according to the first aspect may further comprise an additional step of of pre-heating the agglomerated lignin at one or more temperatures in the range of from 140 to 300°C for a period for at least 30 minutes so as to obtain a thermally stabilized agglomerated lignin. The pre-heating is preferably carried out in an oxidative atmosphere. The moist agglomerated lignin is preferably subjected to drying before pre-heating. If not, the moist agglomerated lignin will be dried during the pre-heating step. The pre-heating is carried out such that the agglomerated lignin is heated at one or more temperatures in the range of from 140 to 300°C, preferably from 180 to 250°C. The pre-heating is carried out for at least 30 minutes, i.e. the residence time of the agglomerated lignin inside the equipment used for the pre-heating is at least 30 minutes. In one embodiment, the pre-heating is carried out for at least 1 hour, or at least 1.5 hours. Preferably, the pre-heating is carried out for less than 12 hours, or less than 6 hours. The pre-heating may be carried out at the same temperature throughout the entire pre-heating stage or may be carried out at varying temperature, such as a stepwise increase of the temperature or using a temperature gradient. More preferably, the pre-heating is carried out such that the agglomerated lignin is first heated at a temperature in the range of from 140 to 175°C for a period of at least 15 minutes and subsequently heated at a temperature in the range of from 175 to 250°C for at least 15 minutes. The step of pre-heating the agglomerated lignin can be carried out continuously or in batch mode. The pre-heating can be carried out using methods and equipment as known in the art. For example, the pre-heating may be carried out in a rotary kiln, moving bed furnace or rotary hearth furnace. In some embodiments, the pre-heating may be carried out in the same reactor as used for drying the moist agglomerated lignin. In other embodiments, the moist agglomerated lignin is dried in one reactor and the dried agglomerated lignin is then moved to another reactor for the pre- heating step. Oxidative species that can react so as to cross-link lignin are present in the oxidative atmosphere. The pre-heating may be carried out for example in the presence of oxygen, iodine, ozone, nitrogen dioxide, nitrobenzene, hydrogen peroxide and peracetic acid. Preferably, the pre-heating is carried out in air. Alternatively, any suitable oxidative species may be supplied in a nitrogen atmosphere. The agglomerated lignin may be cross-linked by pre-heating in an oxidative atmosphere. This increases the thermal stability of the agglomerated lignin during the subsequent heat treatment. In particular, the processability of the lignin in terms of avoiding melting / swelling and retaining shape and dimension during the subsequent heat treatment is improved by the pre-heating step. Performing a pre-heating step to obtain thermally stabilized agglomerated lignin is in particular useful when the agglomerated lignin is to be further heat-treated to for example convert it to a carbon enriched material. The thermal stabilization prevents melting / swelling of the agglomerated lignin during the heat treatment. The thermally stabilized agglomerated lignin can also be used as a functional filler, for example in tyres. The thermally stabilized agglomerated lignin typically has similar properties to those of the agglomerated lignin prior to the pre-heating. The sphericity and roundness may increase during the pre-heating step. Depending on the use of the obtained agglomerated lignin or thermally stabilized agglomerated lignin, the material may be pulverized to a smaller particle size. The method according to the first aspect may therefore comprise an additional step of pulverizing the agglomerated lignin or the thermally stabilized agglomerated lignin. Pulverization is preferably carried out after drying. Pulverization may be carried out by any method as known in the art. Some examples include a cutting mill, blade mixer, ball-mill, impact mill, hammer mill and / or jet-mill. Optionally, fine / coarse particle selection by classification and / or sieving may be performed subsequent to the pulverization. After pulverization, the average particle size of the obtained lignin powder may for example be in the range of from 1 to 100 μm, or from 1 to 50 μm, or from 1 to 20 μm. All steps of the method according to the first aspect may be performed at a standard atmosphere, i.e. not in an inert atmosphere. If instead using a method involving lignin in powder form, an inert atmosphere is typically required for safety reasons. According to a second aspect, the present invention relates to an agglomerated lignin having an average particle size of at least 300 μm, wherein the agglomerated lignin has a roundness of at least 0.6 and optionally a sphericity in the range of from 0.6 to 0.95. The agglomerated lignin according to the second aspect may be obtained by the method according to the first aspect. The agglomerated lignin may be further defined as set out above with reference to the first aspect. The agglomerated lignin according to the second aspect may also be thermally stabilized, such that it is a thermally stabilized agglomerated lignin as further outlined above with reference to the first aspect. Examples Example 1 Softwood kraft lignin obtained by a LignoBoost process and having a dry solid content of 65.18% was used.3 kg of the lignin was added to an intensive mixer (Eirich intensive mixer using a z-type mixing tool). The mixer has a mixing tool and a mixing pan, and during the experiments these were rotating in different directions (counterclockwise and clockwise respectively). In a first pre-mixing step, the moist lignin was mixed before 150 g water was added in a second mixing step. Then kneading was carried out in two consecutive steps. All mixing steps were carried out in the intensive mixer and the lignin was not removed between the mixing steps. The mixing speeds and mixing times are listed in table 1. Mixing step Mixing time Mixing tool speed Mixing pan speed (m / s) (m / s) Pre-mixing 20 s 20 0.7 2ndmixing step 30 s 25 0.7 1stkneading step 6 min 20 1.4 2ndkneading step 6 min 15 1.4 Table 1: conditions during mixing in example 1 Moist agglomerated lignin with a spherical shape was obtained. The dry solid content of the obtained lignin was 64.13%.95% of the obtained agglomerates had a particle size larger than 1.0 mm. The moist agglomerated lignin was dried in a rotary kiln at 80°C. The obtained agglomerated lignin has a stability sufficient to withstand mechanical stress during subsequent process steps, such as sieving and thermal stabilization, and was dust free. Example 2 – comparative Softwood kraft lignin obtained from LignoBoost and having a dry solid content of 65.26% was used.3 kg of the lignin was added to an intensive mixer (Eirich intensive mixer using a z-type mixing tool). The mixer has a mixing tool and a mixing pan, and during the experiments these were rotating in different directions (counterclockwise and clockwise respectively). The moist lignin was mixed in two different steps, but no liquid was added. After the first mixing steps, kneading was carried out in two consecutive steps. All mixing steps were carried out in the intensive mixer and the lignin was not removed between the mixing steps. The mixing speeds and mixing times are listed in table 2. Mixing step Mixing time Mixing tool speed Mixing pan speed (m / s) (m / s) Pre-mixing 20 s 20 0 2ndmixing step 20 s 25 0.7 1stkneading step 2 min 20 0.7 2ndkneading step 2 min 15 1.4 Table 2: conditions during mixing in example 2 No agglomerated lignin was obtained when not adding any liquid. The mixing resulted in a smaller particle size of the moist lignin compared to the starting material. The dry solid content after mixing was 67.53%, meaning that drying of the moist lignin had to some extent occurred during the mixing processes. Example 3 – comparative Softwood kraft lignin obtained from LignoBoost and having a dry solid content of 65.26% was used.3 kg of the lignin was added to an intensive mixer (Eirich intensive mixer using a z-type mixing tool). The mixer has a mixing tool and a mixing pan, and during the experiments these were rotating in different directions (counterclockwise and clockwise respectively). In a first step, the moist lignin was mixed before 500 g water was added in a second mixing step. Then kneading was carried out in two consecutive steps. All mixing steps were carried out in the intensive mixer and the lignin was not removed between the mixing steps. The mixing speeds and mixing times are listed in table 3. Mixing step Mixing time Mixing tool speed Mixing pan speed (m / s) (m / s) Pre-mixing 20 s 20 0.7 2ndmixing step 20 s 25 0.7 1stkneading step 2 min 20 0.7 2ndkneading step 2 min 15 1.4 Table 3: conditions during mixing in example 3 No agglomerated lignin was formed, the amount of water was too high and instead of agglomerates a wet mud-like mixture was formed. Example 4 – friability test The dried agglomerated lignin obtained in example 1 was subjected to a friability test, involving subjecting the agglomerated lignin to ball milling (amplitude 30 mm / g, time 15 minutes). After milling, the lignin sample was collected and the particle size distribution was measured by sieving with different sieves. The fraction of fines (material having a size less than 100 μm) was calculated and was found to be less than 5 wt% of the sample. Example 5 – thermal stabilization The agglomerated lignin obtained in example 1 was subjected to a heating step in order to obtain a thermally stabilized agglomerated lignin. The agglomerated lignin was subjected to heating at 250°C in air for 2 hours. It was possible to carry out the heat treatment without the agglomerated lignin breaking apart. The agglomerated lignin further showed no signs of melting / fusing, and no dusting occurred. Example 6 – mercury porosimetry A total of 17 samples of agglomerated lignin produced according to the presently disclosed method in a manner similar to Example 1 (termed hereafter “agglomerated lignin”) were analysed by mercury intrusion porosimetry in accordance with the methods described in ISO 15901-1:2016. The analysis was performed with a MicroActive AutoPore V9600 instrument, with a working range of approximately 1 psia to 60000 psia. For reference, a single sample of lignin produced by the roll compaction method disclosed in WO2020183383 A1 (termed hereafter “dry compacted lignin”) was also analysed. The results are shown in Table 1, as well as Figure 1 (agglomerated lignin) and Figure 2 (dry compacted lignin). Sample Total Intrusion Total pore Volume Bulk Apparent Overall Volume (ml / g) area (m² / g) median pore density density porosity diameter (μm) (g / ml) (g / ml) (%) Dry 0.26 29.47 0.21 1.04 1.42 26.7 compacted agglomerated 0.37 29.03 0.11 0.88 1.31 32.58 (average) agglomerated 0.31–0.62 24.85–33.52 0.07–0.15 0.72–0.97 1.21–1.40 29.6–44.8 (range) Table 1 The main differences can be found in total intrusion volume (how much mercury has entered the pores at a certain pressure), volume median pore diameter (calculated as 4*intrusion volume / pore surface area) and overall porosity. It can be seen that the agglomerated lignins are more porous and have a smaller volume median pore diameter. This is confirmed by the pore diameter distributions obtained from mercury intrusion. It can be seen that for a agglomerated lignin sample (Figure 1), the pore size distribution is essentially monomodal and relatively monodisperse, with the modal pore diameter being approximately 0.1 μm (on the logarithmic scale). The dry compacted sample (Figure 2) shows a bimodal and more disperse size distribution, with a modal pore diameter of approximately 100 μm (on the logarithmic scale). Note that the pore size distribution of the agglomerated lignin is somewhat controllable by i.a. the amount of water added, with more water giving a monomodal, monodisperse distribution with small pore size, and less water giving a bimodal distribution more resembling that of the dry compacted lignin. Example 7 – microscopy and image analysis Samples of agglomerated lignin and dry compacted lignin were classified by particle size (400-1000 μm, 1000-2000 μm and 2000-3150 μm). Images of representative particles from each fraction were obtained by optical microscopy. The images were subjected to analysis to determine i.a. roundness, and standard deviation of curvature. The results are shown in Figure 3. Note that the images of each particle are for illustrative purposed only, and are not to scale. The roundness of each particle was determined using automatic image analysis in accord with the formula: =4 ×( )A perfectly circular of 1. The 2D roundness of a particle will tend to correlate with the 3D sphericity. The roundness of agglomerated lignin ranged from 0.8 to 0.65, whereas dry compacted lignin exhibited a roundness between 0.37 and 0.64. For each size classification, the agglomerated lignin demonstrated greater roundness than the corresponding dry compacted lignin. This data shows that kneaded granules possess a greater degree of roundness compared to their compacted counterparts. The standard deviation of the curvature for each imaged particle was also calculated by automatic image analysis in accord with the following. Firstly, the x,y contour points of the particle were extracted from each image, and for each point along the particle’s contour, the 1stderivatives (direction) x’, y’, and the 2ndderivatives (rate of change of direction) x’’, y’’ were calculated. For each point along the contour, the curvature was then calculated using the following formula: ||=) Finally, the standard deviation of curvature was calculated from all curvature values for each particle. A low standard deviation of curvature indicates a relatively uniformly curved particle, such as a spherical particle, whereas a high standard deviation indicates a non-uniform particle with sharp corners, edges, or complex geometry. The standard deviation of curvature of agglomerated lignin particles ranged from 0.058 to 0.176, whereas dry compacted lignin particles exhibited a standard deviation of between 0.069 and 0.207. For each size classification, the agglomerated lignin demonstrated a smaller standard deviation of curvature as compared to the corresponding dry compacted lignin. This data shows that agglomerated lignin particles possess more uniform curvature as compared to dry compacted lignin particles. In view of the above detailed description of the present invention, other modifications and variations will become apparent to those skilled in the art. However, it should be apparent that such other modifications and variations may be effected without departing from the spirit and scope of the invention.

Claims

Claims 1. A method for producing agglomerated lignin, the method comprising the steps of: a) providing moist lignin having a moisture content of at least 25 wt% and an average particle size of less than 100 μm; b) adding at least one liquid to the moist lignin and mixing the moist lignin and the at least one liquid with a second mixing force; and c) kneading the moist lignin and the at least one liquid with a third mixing force so as to form moist agglomerated lignin having an average particle size of at least 300 μm.

2. The method according to claim 1, wherein the lignin provided in step a) is obtained by crushing precipitated lignin.

3. The method according to any one of claims 1 or 2, wherein the lignin is kraft lignin.

4. The method according to any one of the preceding claims, wherein the method comprises an additional step of pre-mixing the moist lignin provided in step a) with a first mixing force prior to step b).

5. The method according to claim 4, wherein the first mixing force is lower than the second mixing force.

6. The method according to any one of the preceding claims, wherein the at least one liquid is selected from at least one of water and an aqueous solution.

7. The method according to any one of the preceding claims, wherein the method comprises an additional step of adding at least one additive to the moist lignin.

8. The method according to any one of the preceding claims, wherein the third mixing force is lower than the second mixing force.

9. The method according to any one of the preceding claims, wherein the kneading comprises a first kneading step and a second kneading step.

10. The method according to claim 9, wherein the first kneading step is carried out at a mixing force higher than the mixing force used in the second kneading step.

11. The method according to any of the preceding claims, wherein the mixing in step b) and the kneading in step c), as well as the pre-mixing if performed, is carried out in a mixer comprising a mixing bowl and a mixing tool, and wherein the mixing bowl is rotating in an opposite direction to the mixing tool.

12. The method according to any one of the preceding claims, wherein the mixing in step b) and the kneading in step c), as well as the pre-mixing if performed, is carried out in an intensive mixer.

13. The method according to any one of the preceding claims, wherein the method comprises an additional step of drying the moist agglomerated lignin obtained in step c) so as to obtain agglomerated lignin.

14. The method according to claim 13, wherein the drying is carried out at a temperature of 90°C or lower.

15. The method according to any one of the preceding claims, wherein the method comprises an additional step of pre-heating the agglomerated lignin at one or more temperatures in the range of from 140 to 300°C for a period for at least 30 minutes so as to obtain a thermally stabilized agglomerated lignin.

16. The method according to claim 15, wherein the pre-heating is carried out in an oxidative atmosphere.

17. The method according to any one of claims 15 or 16, wherein the pre- heating is carried out by first heating the agglomerated lignin at a temperature in the range of from 140 to 175°C for a period of at least 15minutes and subsequently heating the agglomerated lignin at a temperature in the range of from 175 to 300°C for at least 15 minutes.

18. Agglomerated lignin having an average particle size of at least 300 μm, wherein the agglomerated lignin has a roundness of at least 0.

6.

19. The agglomerated lignin according to claim 18, wherein the agglomerated lignin has a sphericity in the range of from 0.6 to 0.

95.

20. The agglomerated lignin according to claim 18 or claim 19, wherein the agglomerated lignin has a bulk density in the range of from 0.4 to 0.8 g / cm3.

21. The agglomerated lignin according to any one of claims 18 to 20, wherein the agglomerated lignin has a porosity in the range of from 5 to 50%.

22. The agglomerated lignin according to any one of claims 18 to 20, wherein the agglomerated lignin is thermally stabilized agglomerated lignin.

23. The agglomerated lignin according to any one of claims 18 to 22, obtainable by a method according to any one of claims 1-17.

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