A method for producing a carbon enriched material from agglomerated lignin

Abstract

The present invention relates to a method for producing a carbon enriched material, 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; mixing with a second mixing force; kneading with a third mixing force so as to form moist agglomerated lignin having an average particle size of at least 300 μm; drying; pre-heating at one or more temperatures in the range of from 140 to 300°C for a period for at least 30 minutes; and subjecting to heat treatment at one or more temperatures in the range of from 300 to 1500°C, so as to obtain a carbon enriched material. The disclosure further relates to a negative electrode for a non-aqueous secondary battery comprising such a carbon enriched material, as well as to use of such a carbon enriched material as active material in a negative electrode of a non-aqueous secondary battery.

Description

[0001] A METHOD FOR PRODUCING A CARBON ENRICHED MATERIAL FROM

[0002] AGGLOMERATED LIGNIN

[0003] Field of the invention

[0004] The present invention relates to a method for obtaining a carbon enriched material from agglomerated lignin.

[0005] Background

[0006] 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.

[0007] 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 W02006031175 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.

[0008] One problem with processing lignin in the form of a dry powder is its strong tendency for dust formation during handling. An additional problem with lignin in powder form is that it exhibits undesired thermoplastic behaviour during heating. During heating, lignin undergoes plastic deformation / melting, aggressive swelling and foaming. Combined with the strong tendency for dust formation during handling, which may cause dust explosions, 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. Lignin can be used as a starting material to obtain a carbon enriched material. During such a process, the lignin is converted to carbon by heating, and thus problems related to the thermoplastic behaviour of lignin may arise. Examples of carbon enriched materials are charcoal, activated carbon and amorphous carbon such as hard carbon.

[0009] Activated carbon is a form of porous carbon with high surface area, typically higher than 500 m2 / g. The surface area may also be even higher, such as at least 1000 m2 / g. The unique structure of activated carbon involving a large number of micropores allows it to exhibit excellent adsorptive properties, and activated carbon is frequently used in various filtration and adsorption applications. Due to its high surface area, electrical conductivity and stability, activated carbon is also suitable for use as an electrode material in an electric double layer capacitor (EDLC).

[0010] Amorphous carbons, such as hard carbons (non-graphitizable amorphous carbons) and soft carbons (graphitizable amorphous carbons), which lack long-range graphitic order can for example be used as an alternative to graphite as the material in the negative electrodes of secondary batteries. Common to graphite and amorphous carbons is that the volume changes during charge and discharge are small. This results in a good mechanical stability of the electrode material and helps to maintain good cycling stability. Amorphous carbons can be used as sole active electrode materials or in mixtures with graphite. Hard carbons often have good charge / discharge rate performance which is desired for fast charging and high- power systems.

[0011] One way to overcome the problems with using lignin powder in terms of dust formation and melting / swelling is to form agglomerated lignin from dry lignin powder and subject the agglomerated lignin to a thermal stabilization step 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.

[0012] Thus, there is still a need for an improved process of obtaining a carbon enriched material from lignin, where the lignin retains its shape and dimension during conversion to a carbon enriched material with no melting / swelling deformation, and where the method is more efficient and versatile than prior art methods. Summary of the invention

[0013] It is an object of the present invention to provide a method for producing a carbon enriched material from agglomerated lignin, which method avoids problems with dust formation.

[0014] It is a further object of the present invention to provide an improved method for producing a carbon enriched material from agglomerated lignin where the agglomerated lignin can be heat treated while maintaining its shape and dimension.

[0015] It is a further object of the present invention to provide a method for producing a carbon enriched material from agglomerated lignin, which method is scalable and thus suitable for large-scale manufacturing.

[0016] 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.

[0017] According to a first aspect, the present invention relates to a method for producing a carbon enriched material, 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 pm; 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; 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 pm; d) optionally drying the moist agglomerated lignin so as to obtain agglomerated lignin; e) optionally pre-heating the -optionally moist- agglomerated lignin at one or more temperatures in the range of from 140 to 350°C for a period for at least 30 minutes so as to obtain a thermally stabilized agglomerated lignin; and f) subjecting the moist agglomerated lignin (from step c), the agglomerated lignin (from step d), or the thermally stabilized agglomerated lignin (from step e) to heat treatment at one or more temperatures in the range of from 300 to 1500°C, wherein the heat treatment is carried out for a total time in the range of from 30 minutes to 10 hours, so as to obtain a carbon enriched material.

[0018] It has surprisingly been found that agglomerated lignin can be formed from moist lignin having an average particle size of less than 100 pm 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 agglomerated lignin also has a relatively spherical shape with no or fewer sharp edges, which is beneficial 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 agglomerated lignin, such as in the heating in steps e) and f), which will increase the dust content in the process and also lead to a risk for dust explosions as well as processability issues during heating. The mechanical properties of the obtained agglomerated lignin are sufficient to enable further downstream processing steps to convert the agglomerated lignin to a carbon enriched material.

[0019] Moreover, it has surprisingly been found that the (moist) agglomerated lignin obtained by the method has an improved thermal processability, as indicated i.a. by an increase in the temperature that the lignin may be heated to in a laboratory kiln without inducing excessive melting, swelling and / or fusion. This improved thermal processing ability permits stabilization at higher temperatures, leading to more complete stabilization and potentially improved product quality. Alternatively, the improved thermal processing ability may permit the (moist) agglomerated lignin to be carbonized directly, without requiring any discrete pre-heating step to provide a thermally stabilized lignin. Furthermore, it has surprisingly been found that by providing agglomerated lignin formed from moist lignin, thermal stabilization in step e) is more efficient due to the porosity of the agglomerated lignin, which facilitates reactions during the pre-heating step due to improved diffusion of reactive species into the lignin. Thus, the stability of the obtained thermally stabilized agglomerated lignin is improved. An improved stability is in turn beneficial in the subsequent heat treatment to prevent lignin from melting / swelling.

[0020] According to a second aspect, the present invention relates to a negative electrode for a non-aqueous secondary battery comprising the carbon enriched material obtainable by the method according to the first aspect as active material.

[0021] According to a third aspect, the present invention relates to use of the carbon enriched material obtainable by the method according to the first aspect as active material in a negative electrode of a non-aqueous secondary battery.

[0022] Detailed description

[0023] 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 pm, or less than 75 pm, or less than 50 pm.

[0024] In the present application, the average particle size is defined as the volume average particle size (Dvso). 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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 W02006031175 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.

[0029] 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 pm 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] The term “mixing force” as used herein refers to the force the moist lignin is subjected to during the mixing or kneading process. The first 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.

[0034] 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 mixing force used during the pre-mixing step is preferably lower than the second mixing force used while adding 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.

[0035] 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).

[0036] 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.

[0037] 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.

[0038] 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 present. 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.

[0039] 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.

[0040] 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 pm. 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] The terms “agglomerated lignin” and “moist agglomerated lignin” as used herein refers to macroscopic particles in turn comprising clustered smaller particles of lignin. By providing the lignin in agglomerated form, a more compact and hard material is achieved. Hard agglomerates are advantageous during subsequent processing as they can resist physical impact during processing. In addition, the tendency for dusting is reduced when lignin is provided in agglomerated form.

[0047] The moist agglomerated lignin obtained in step c) has an average particle size of at least 300 pm, or at least 500 pm, or at least 700 pm. A small average particle size may be favourable during the subsequent thermal stabilization in step e), since diffusion distances for reactive species are shorter than in a large particle. However, large particles are typically easier to handle during processing. The moist agglomerated lignin may have an average particle size in the range of from 300 pm to 5.0 mm, or from 300 pm to 3.0 mm, or from 500 pm to 3.0 mm or from 500 pm to 2.0 mm.

[0048] 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 0.6 to 0.8 or from 0.7 to 0.95, or from 0.7 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.

[0049] 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.

[0050] 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.

[0051] 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).

[0052] 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.

[0053] 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.

[0054] In some embodiments, no additives are added and the moist agglomerated lignin thus consists only of lignin and the at least one liquid. Step d) of the method according to the first aspect involves drying the moist agglomerated lignin obtained in step c) so as to obtain agglomerated lignin. This step is optional, and drying of the moist agglomerated lignin may for example be integrated with or incorporated into a thermal processing step such as pre-heating (step e) or heat treatment (step f). 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.

[0055] The obtained agglomerated lignin is after drying an essentially dry solid material. It has an average particle size of at least 300 pm, or at least 500 pm, or at least 700 pm. The obtained moist agglomerated lignin may have an average particle size in the range of from 300 pm to 5.0 mm, or from 300 pm to 3.0 mm, or from 500 pm to 3.0 mm or from 500 pm to 2.0 mm. The particle size of the obtained moist agglomerated lignin thus remains largely the same after drying.

[0056] 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 is in the range of from 0.6 to 0.95, or from 0.6 to 0.8 or from 0.7 to 0.95, or from 0.7 to 0.85. Thus, the values of sphericity and roundness remain largely the same after drying.

[0057] 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.

[0058] 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 pm to 1 pm, such as from 0.03 pm to 0.3 pm.

[0059] 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.

[0060] 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 pm. 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.

[0061] Step e) of the method according to the first aspect involves 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. This step is optional. The pre-heating is preferably carried out in an oxidative atmosphere. Pre-heating may also be referred to as thermal stabilization and is carried out at a temperature lower than the temperature required for conversion of the lignin to a carbon enriched material. The term “thermally stabilized agglomerated lignin” as used herein refers to an agglomerated lignin that has been subjected to a pre-heating (or thermal stabilization) step so as to improve the stability of the lignin and in particular its ability to be subjected to further heat treatment without melting / swelling.

[0062] 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 350°C, such as 140°C 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.

[0063] 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 preheating step.

[0064] 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. Penetration and diffusion of oxidative species are facilitated due to the porosity of the agglomerated lignin used in the present invention. Thus, after the pre-heating step, the degree of cross-linking in the agglomerated lignin is typically high.

[0065] 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.

[0066] Steps a) to e) 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 at least in some steps for safety reasons.

[0067] Step f) of the method according to the first aspect of the present invention involves subjecting the thermally stabilized agglomerated lignin to heat treatment at one or more temperatures in the range of from 300 to 1500°C, wherein the heat treatment is carried out for a total time in the range of from 30 minutes to 10 hours, so as to obtain a carbon enriched material.

[0068] The term “heat treatment” as used herein, refers to a process of heating the thermally stabilized agglomerated lignin at one or more temperatures and for a sufficient time so that the lignin is converted to a carbon enriched material. The process may also be referred to as “carbonization” or “calcination”, depending on the process settings. After heat treatment, the carbon content is higher than 80 wt%, or higher than 90 wt%, or higher than 95 wt%, or higher than 98 wt%, depending on the temperature used and the heating time. Depending on the temperature during the heat treatment, different types of carbon, such as charcoal or hard carbon, can be obtained from the thermally stabilized agglomerated lignin.

[0069] The term “carbon enriched material” as used herein, refers to a carbon material obtained by heat treatment of the thermally stabilized agglomerated lignin. The carbon content of the carbon enriched material is higher than 80 wt%, or higher than 90 wt%, or higher than 95 wt%, or higher than 98 wt%. The carbon enriched material may also comprise for example heteroatoms, such as oxygen, hydrogen, nitrogen or sulphur atoms, inorganic impurities, and functional additives. The carbon enriched material of the present invention may be for example an amorphous (i.e. non-crystalline) carbon, preferably hard carbon, a functional filler, a charcoal material or an activated carbon. Different heat treatments are needed in order to obtain different carbon enriched materials, as further outlined below.

[0070] The heat treatment may be carried out at the same temperature throughout the entire heat treatment or may be carried out at varying temperature, such as a stepwise increase of the temperature or using a temperature gradient. The heat treatment may comprise a temperature ramp from a starting temperature to a target temperature. The heating rate may be 1-100°C / min. For example, the heat treatment may involve several intermediate temperatures, with temperature ramps in between them, before reaching the target temperature needed for carbonization of the thermally stabilized agglomerated lignin. The heat treatment may be carried out as a batch process or a continuous process. Any suitable reactor can be used, such as rotary kiln, moving bed furnace, pusher furnace or rotary hearth furnace. The heat treatment is preferably carried out under inert atmosphere, preferably nitrogen atmosphere.

[0071] The heat treatment may comprise a first heating step carried out at one or more temperatures in the range of from 300 to 800°C, such as from 500 to 700°C. The first heating step is preferably carried out under inert atmosphere, preferably nitrogen atmosphere. The duration of the first heating step is at least 30 minutes and preferably less than 10 hours. The BET specific surface area of the carbon enriched material obtained after this heating step is typically in the range of from 300 to 700 m2 / g, measured as BET using nitrogen gas. In some embodiments, further heating steps are carried out after the first heating step, in other embodiments the carbon enriched material obtained after the first heating step is the final product of the heat treatment. Such a carbon enriched material may be used in applications where a relatively porous carbon enriched material is required. It may also be further processed such as by reaction / coating with various compounds, such as silicon- containing compounds. In some embodiments the first heating step is followed by a final heating step, such that the heat treatment comprises a first heating step and a final heating step. The final heating step is preferably carried out at one or more temperatures in the range of from 800 to 1500°C. The final heating step is preferably carried out under inert atmosphere, preferably nitrogen atmosphere. The duration of the final heating step is at least 30 minutes and preferably less than 10 hours. After a final heating step carried out at 1000°C or higher in an inert atmosphere, the BET specific surface area of the carbon enriched material obtained is typically 50 m2 / g or less. Such a carbon enriched material is typically a hard carbon material.

[0072] The carbon enriched material in the form of hard carbon preferably has a bulk density in the range of from 0.2 to 0.4 g / cm3. This is lower than the bulk density of the agglomerated lignin and the thermally stabilized agglomerated lignin, primarily due to mass loss during the heat treatments.

[0073] The carbon enriched material in the form of hard carbon preferably has a helium true density in the range of from 1 .4 to 2.1 g / cm3, such as from 1 .7 to 2.0 g / cm3. The helium true density may be determined using a pycnometer, as known by a person skilled in the art. It is important to have a helium true density in the range of from 1 .4 to 2.1 g / cm3as the doping and de-doping capacity of the carbon enriched material when used as the active material in the negative electrode of a non-aqueous secondary battery may otherwise be reduced, and the irreversible capacity of the battery may become large. If the density of the carbon enriched material is too low, the energy density of the electrode may also be decreased.

[0074] Alternatively, the final heating step may be an activation step. Preferably, the activation is carried out in an atmosphere comprising at least 10% steam or at least 20 %, or at least 30 % steam. By the activation step, activated carbon is obtained. The steam, i.e. water in gas phase, is added to an inert atmosphere, such that the remainder of the volume is an inert gas, such as nitrogen. In some embodiments, activation is carried out in an atmosphere comprising from 20 to 30 vol% steam. In one embodiment, the activation is carried out in an atmosphere comprising 30 vol% steam and 70 vol% nitrogen. The activation step is preferably carried out at a temperature in the range of from 900 to 1100°C for a time period in the range of from 60 to 180 minutes. The obtained activated carbon typically has a BET specific surface area in the range of 500 to 3000 m2 / g. The obtained activated carbon may be used for example in purification applications and as an electrode material in an electric double layer capacitor.

[0075] It is also within the scope of the present invention to use another type of activation than steam activation. For example, any suitable chemical or physical activation methods as known in the art may be utilized.

[0076] The first and final heating steps may be carried out as discrete steps or as one single step in direct sequence. The first and final heating steps may involve heating at one or more temperatures, as discussed above for the heat treatment. For example, the first heating starts at about 300°C and the temperature is subsequently increased to about 500°C. The final heating step is preferably carried out between 900 and 1300°C, such as at about 1000°C.

[0077] The first and final heating steps may be carried out as batch processes or as continuous processes. Any suitable reactors can be used. The first heating step and the final heating step can be carried out in the same reactor or in separate reactors. If the final heating step is an activation step, the atmosphere is changed in the final heating step. If hard carbon is to be obtained, the first and final heating steps are both preferably carried out in an inert atmosphere.

[0078] The method according to the first aspect may comprise an additional step of pulverizing the carbon enriched material. In many applications, the particle size of the obtained carbon enriched material must be reduced prior to use so as to obtain a carbon powder. For example, when the carbon enriched material of the present invention is to be used as active material in an anode of a secondary battery, the particle size is preferably reduced.

[0079] The pulverization may be performed by any suitable process, using for example 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.

[0080] The pulverization of the carbon enriched material and optional fine / coarse particle selection may be performed so as to obtain a carbon powder comprising powder particles having an average particle size in the range of from 1 to 100 pm, or from 1 to 50 pm, or from 1 to 25 pm.

[0081] In embodiments where the heat treatment comprises a first heating step and a final heating step, the pulverization may be carried out before or after the final heating step. Preferably, pulverization is carried out after the first heating step and before the final heating step.

[0082] It is possible to carry out more than one step of pulverizing or crushing. In addition, the carbon enriched material may be subjected to treatments such as coating or further heat treatments.

[0083] The second aspect of the present invention relates to a negative electrode for a nonaqueous secondary battery comprising the carbon enriched material obtainable by the method according to the first aspect as active material.

[0084] As mentioned above, when subjected to heat treatment so as to obtain hard carbon, the carbon enriched material of the present invention is suitable for use in secondary batteries. The carbon enriched material, preferably in powder form, of the present invention is preferably used as an active material in a negative electrode of a nonaqueous secondary battery, such as a lithium-ion battery or sodium-ion battery. When used for producing such a negative electrode, any suitable method to form such a negative electrode may be utilized. In the formation of the negative electrode, the carbon enriched material may be processed together with further components. Such further components may include, for example, one or more binders to form the carbon enriched material into an electrode, conductive materials, such as carbon black, carbon nanotubes or metal powders, and / or further Li storage materials, such as graphite or lithium. For example, the binders may be selected from, but are not limited to, poly(vinylidene fluoride), poly(tetrafluoroethylene), carboxymethylcellulose, natural butadiene rubber, synthetic butadiene rubber, polyacrylate, poly(acrylic acid), alginate, etc., or from combinations thereof.

[0085] Optionally, a solvent such as e.g. 1-methyl-2-pyrrolidone, 1-ethyl-2-pyrrolidone, water, or acetone is utilized during the processing. The third aspect of the present invention relates to use of the carbon enriched material obtainable by the method according to the first aspect as active material in a negative electrode of a non-aqueous secondary battery.

[0086] Examples

[0087] Example 1

[0088] 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.

[0089] Table 1 : conditions during mixing in example 1

[0090] 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.

[0091] The agglomerated lignin was then subjected to a pre-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. Finally, the thermally stabilized agglomerated lignin was carbonized at 600 °C for 2 hours under a nitrogen atmosphere, then calcined at 1050 °C for a further two hours to provide a hard carbon suitable for use in secondary batteries, such as Li-ion batteries.

[0092] Example 2 - Comparative stabilization studies

[0093] The thermal processability of an agglomerated lignin produced according to the presently disclosed method was compared to that of a dry compacted, crushed lignin sample. The agglomerated lignin was produced in accord with the presently disclosed method in a manner similar to Example 1. The dry compacted lignin was produced in accord with the method disclosed in W02020183383 A1. Each lignin sample was subjected to oxidative stabilization in air in a laboratory kiln to investigate the highest temperature the lignin could tolerate before melting / swelling was induced. The yield of the stabilization reaction was noted, and the glass transition temperature of the resulting stabilized lignin was determined by DSC (ASTM E1269-24 - Standard Test Method for Determining Specific Heat Capacity by Differential Scanning Calorimetry). Finally, each stabilized lignin was subjected to carbonization for 2 hours at 600 °C under a nitrogen atmosphere in order to determine a yield for carbonization.

[0094] It was found that the reference dry compacted carbon could be stabilized at a maximum temperature of 250 °C without encountering excessive melting and swelling. This resulted in a stabilized lignin having a glass transition temperature Tg of 170 °C. Stabilizing the agglomerated lignin under the same conditions resulted in a lower yield of stabilized lignin, but wherein the obtained stabilized lignin had a higher Tg (184 °C). The higher Tg of the stabilized agglomerated lignin indicates that subsequent thermal processability is improved. The higher Tg together with lower stabilization yield also indicates that the conversion of lignin towards carbon is progressing further for the agglomerated lignin as compared to the compacted lignin during stabilization under the same conditions. This is confirmed by the observation that the yield of the subsequent carbonization step was higher for the agglomerated lignin as compared to the dry compacted lignin. Finally, it was found that the agglomerated lignin could be subjected to oxidative stabilization at a maximum temperature of 320 °C without encountering excessive melting and swelling. 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

Claims1. A method for producing a carbon enriched material, 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 pm; 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; 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 pm; d) optionally drying the moist agglomerated lignin so as to obtain agglomerated lignin; e) optionally pre-heating the -optionally moist- agglomerated lignin at one or more temperatures in the range of from 140 to 350°C for a period for at least 30 minutes so as to obtain a thermally stabilized agglomerated lignin; and f) subjecting the moist agglomerated lignin, the agglomerated lignin or the thermally stabilized agglomerated lignin to heat treatment at one or more temperatures in the range of from 300 to 1500°C, wherein the heat treatment is carried out for a total time in the range of from 30 minutes to 10 hours, so as to obtain a carbon enriched material.

2. The method according to claim 1 , wherein step d) and / or step e) are performed.

3. The method according to any one of claims 1-2, wherein the lignin provided in step a) is obtained by crushing precipitated 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 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 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 drying is carried out at a temperature of 90°C or lower.

14. The method according to any one of the preceding claims, wherein the lignin is kraft lignin.

15. The method according to any one of the preceding claims, wherein the preheating in step e) is carried out by first heating the agglomerated lignin to atemperature in the range of from 140 to 175°C for a period of at least 15 minutes and subsequently heating the agglomerated lignin to a temperature in the range of from 175 to 300°C for at least 15 minutes.

16. The method according to any one of the preceding claims, wherein the preheating in step e) is carried out in an oxidative atmosphere.

17. The method according to any one of the preceding claims, wherein the heat treatment comprises a first heating step carried out at a temperature between 400 and 800°C for at least 30 minutes.

18. The method according to claim 17, wherein the first heating step is carried out in an inert atmosphere.

19. The method according to any one of claims 17-18, wherein the heat treatment comprises a final heating step carried out at a temperature between 800 and 1500°C for at least 30 minutes.

20. The method according to claim 19, wherein the final heating step is carried out in an inert atmosphere.

21. The method according to claim 19, wherein the final heating step is an activation step carried out in an atmosphere comprising at least 10% steam.

22. The method according to any one of the preceding claims, wherein the method comprises an additional step of pulverizing the carbon enriched material.

23. A negative electrode for a non-aqueous secondary battery comprising the carbon enriched material obtainable by the method according to any one of claims 1-22 as active material.

24. Use of the carbon enriched material obtainable by the method according to any one of claims 1-22 as active material in a negative electrode of a nonaqueous secondary battery.

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