Method for producing carbon-rich material from lignin

Abstract

The invention relates to a method for producing a carbon-rich material, said method comprising the following steps: a) providing lignin; b) mixing lignin with the recycled carbon-rich material fraction to obtain a lignin-carbon mixture; c) forming an agglomerated lignin-carbon composite comprising lignin, a recycled carbon-rich material fraction, and optionally at least one additive; d) subjecting the agglomerated lignin-carbon composite material to a heat treatment at one or more temperatures in the range of 300 to 1500 DEG C, where the total time of the heat treatment is in the range of 30 minutes to 10 hours, to obtain a carbon-rich material; e) grinding the obtained carbon-rich material to reduce the average particle size of the carbon-rich material and to obtain at least a first fraction of the carbon-rich material and a second fraction of the carbon-rich material, and f) recycling at least a portion of the first fraction obtained in step e) to step b).

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for producing a carbon-rich material from lignin. The method comprises the step of recycling at least a portion of the obtained carbon-rich material and mixing it with lignin prior to the earlier process stage of converting lignin into the carbon-rich material. The invention further relates to a negative electrode of a non-aqueous secondary battery comprising the obtained carbon-rich material as active material, and the use of the obtained carbon-rich material as active material in a negative electrode of a non-aqueous secondary battery. BACKGROUND

[0002] Secondary batteries, such as lithium ion batteries, are batteries that can be charged and discharged multiple times, i.e. they are rechargeable batteries. In lithium ion batteries, lithium ions flow from the negative electrode through the electrolyte to the positive electrode during discharging and return upon charging. Nowadays, lithium compounds, in particular lithium metal oxides, such as lithium nickel manganese cobalt oxide (NMC) or, alternatively, lithium iron phosphate (LFP), are commonly used as material for the positive electrode, and carbon-rich materials are used as material for the negative electrode.

[0003] Graphite (natural or synthetic graphite) is nowadays used as material for the negative electrode in most lithium ion batteries due to its high energy density and stable charge / discharge performance over time. Alternatives to graphite are amorphous carbon materials, such as hard carbon (non-graphitizable amorphous carbon) and soft carbon (graphitizable amorphous carbon), which lack long-range graphite order. Graphite and amorphous carbon have in common that the volume change during charging and discharging is small. This leads to good mechanical stability of the electrode material and contributes to maintaining good cycle stability. Amorphous carbon can be used as the only active electrode material or in combination with graphite. Hard carbon typically has good charge / discharge rate performance, which is desirable for fast charging and high power systems.

[0004] Amorphous carbon can be derived from lignin. Lignin is an aromatic polymer which is the main constituent in, for example, wood and one of the most abundant sources of carbon on earth. In recent years, with the development and commercialization of technologies to extract lignin in highly purified, solid and particularized form from pulping processes, it has attracted great interest as a possible renewable alternative to the current main aromatic chemical precursors from the petrochemical industry. Amorphous carbon derived from lignin is typically non-graphitizable, i.e. hard carbon. Thus, hard carbon derived from lignin enables a more sustainable anode material selection compared to graphite, which is currently commonly used in secondary batteries.

[0005] Today, the most relevant source of lignin commercially is Kraft lignin, obtained by the Kraft process from either hardwood or softwood. Lignin can be separated from the alkaline black liquor using, for example, membrane filtration or ultrafiltration. A common separation method is described in WO2006031175 Al. In this method, lignin is precipitated from the alkaline black liquor by the addition of acid, which is then filtered off. The lignin filter cake is reslurried under acidic conditions in a next step, and washed before drying and comminution.

[0006] One problem with using lignin as a precursor for carbon-rich materials is that it is not suitable to use lignin in the form of a fine powder directly, as it exhibits an undesired thermoplastic behavior. During thermal conversion of the lignin powder into a carbon-rich material, the lignin undergoes plastic deformation / melting, erosive expansion and foaming. This severely limits the processability of lignin on an industrially relevant scale in terms of equipment size and process throughput as well as the need for intermediate processing.

[0007] Therefore, there is still room for improvement in the process of producing carbon-rich materials from lignin. The process should avoid that lignin undergoes plastic deformation and melting, erosive expansion and foaming during heating and upon conversion of the lignin into a carbon-rich material. Furthermore, the process should be usable in large-scale manufacturing. Material waste during the process should be minimized to enable a sustainable manufacturing. SUMMARY

[0008] It is an object of the present invention to provide an improved process of producing carbon-rich materials, which allows the use of a renewable carbon source, and which process eliminates or mitigates at least some of the drawbacks of prior art processes.

[0009] It is another object of the present invention to provide a process of obtaining an improved carbon-rich material from lignin, which carbon-rich material is suitable for use as an active material in the negative electrode of a secondary battery, such as a lithium-ion battery.

[0010] It is another object of the present invention to provide a process of producing carbon-rich materials from lignin, which process allows for heat treatment of the lignin while maintaining the shape and size of the lignin.

[0011] It is another object of the present invention to provide a process of improving the thermal processability of lignin.

[0012] It is another object of the present invention to provide a process of producing carbon-rich materials from lignin, which process is scalable and thus suitable for large-scale manufacturing.

[0013] It is another object of the present invention to provide a process of producing carbon-rich materials from lignin, wherein material waste during the process is reduced, enabling a more sustainable process.

[0014] The above objects, and other objects which will be apparent to those skilled in the art in view of the present disclosure, are achieved by the various aspects of the present disclosure.

[0015] According to a first aspect, the present invention relates to a method of producing a carbon-rich material, the method comprising the steps of:

[0016] a) providing lignin;

[0017] b) mixing the lignin with a recycled fraction of carbon-rich material to obtain a lignin-carbon mixture;

[0018] c) forming an agglomerated lignin-carbon composite material comprising the lignin, the recycled fraction of carbon-rich material and optionally at least one additive;

[0019] d) subjecting the agglomerated lignin-carbon composite material to a heat treatment at one or more temperatures in the range of 300-1500 °C, wherein the heat treatment is performed for a total time in the range of 30 minutes to 10 hours, to obtain a carbon-rich material;

[0020] e) grinding the obtained carbon-rich material to reduce the average particle size of the carbon-rich material and to obtain at least a first fraction of carbon-rich material and a second fraction of carbon-rich material; and

[0021] f) recycling at least a part of the first fraction obtained in step e) to step b).

[0022] Surprisingly, it was found that by mixing lignin with a carbon-rich material and agglomerating the mixture, the thermal processability of the lignin is improved. The carbon-rich material is dispersed within the lignin matrix, thereby forming an agglomerated lignin-carbon composite material. The carbon-rich material improves the heat resistance of the agglomerated lignin-carbon composite material by reducing the melting / swelling behavior of the lignin upon heating, which in turn improves the processability of the lignin on an industrial scale.

[0023] In the method of the present invention, the carbon-rich material mixed with the lignin in step b) is recycled from step e). After the grinding step, at least two fractions of carbon-rich material are obtained. The at least two fractions of carbon-rich material can have different average particle sizes. By recycling at least a part of the first fraction, material waste of the method is minimized and the yield is improved, as carbon-rich material that would otherwise be discarded (e.g. carbon fines) is reintroduced into the method. This increases the profitability of the method and also reduces the environmental footprint. A more sustainable method can thus be achieved. The second fraction of carbon-rich material can be further processed into an end product.

[0024] Typically, after milling, the fraction of the carbon-rich material having an average particle size in the range of 2 to 4 pm, i.e. the fine fraction, is discarded, while the fraction of the carbon-rich material having a larger particle size is further processed into the final product. The discarded fraction can be used in various applications, e.g. as an energy source or as a replacement for carbon derived from the petrochemical industry. By alternatively recycling the fine fraction, material waste of the process can be reduced, enabling a more sustainable process.

[0025] According to a second aspect, the present invention relates to a negative electrode for a non-aqueous secondary battery, comprising a carbon-rich material obtainable by the method according to the first aspect.

[0026] According to a third aspect, the present invention relates to the use of a carbon-rich material obtainable by the method according to the first aspect as an active material in a negative electrode of a non-aqueous secondary battery. DETAILED DESCRIPTION

[0027] Step a) of the method according to the first aspect of the present invention comprises providing a lignin. Throughout the present disclosure, it is intended that the term “lignin” refers to any kind of lignin that can be used as a carbon source for the preparation of the carbon-rich material. Examples of the lignin are, but are not limited to, lignin obtained from plant raw materials, such as wood, e.g. softwood lignin, hardwood lignin and lignin from annular plants. Furthermore, the lignin can be chemically modified.

[0028] The lignin used in the present invention can be obtained by different extraction methods, such as an organic solvent method or a kraft method. The lignin can also be obtained from methods such as steam explosion or acid pretreatment followed by enzymatic hydrolysis. Preferably, the lignin used in the method of the present invention is kraft lignin, i.e. lignin obtained by the kraft method. The kraft lignin can be obtained from hardwood or softwood. The lignin can be obtained by the method disclosed in WO2006031175A1, commonly referred to as the LignoBoost method. Typically, the method comprises the steps of precipitating lignin from an alkaline black liquor by acidification; separating the precipitated lignin; and reslurrying the lignin at least once under acidic conditions. The obtained lignin can be dried and comminuted, and thus provided as solid particles in the form of a powder.

[0029] Preferably, the lignin has been purified or isolated before being used in the method according to the present invention. The lignin can be separated from the black liquor and, optionally, further purified before being used in the method according to the present invention. The purification typically results in a purity of the lignin of at least 90%, preferably at least 95%, more preferably at least 98%, based on the dry weight of the lignin. Thus, the lignin used in the method according to the present invention preferably contains less than 10%, preferably less than 5%, more preferably less than 2% of impurities, such as cellulose, carbohydrates and inorganic compounds, based on the dry weight of the lignin.

[0030] In one embodiment, the lignin provided in step a) is in powder form. The particle size distribution of the lignin in powder form can be such that at least 80 wt-% of the particles have a diameter of less than 0.2 mm. The lignin in powder form preferably has a moisture content of less than 45 wt%, or less than 25 wt%, or less than 10 wt%, or less than 8 wt%. In the context of the present application, if the particles are not spherical, the diameter of the particles is the equivalent spherical diameter of the particles. The equivalent spherical diameter is the diameter of a sphere of equivalent volume.

[0031] In one embodiment, the lignin in step a) is provided in the form of a slurry. The term "slurry" as used herein refers to solid lignin particles suspended in a liquid, preferably an aqueous solution. For example, the lignin slurry provided in step a) can be a process stream in a lignin extraction process, such as the LignoBoost process. In embodiments where the lignin is provided in the form of a slurry, the lignin must be dried prior to the agglomeration in step c). Once dried, the lignin can also be comminuted to obtain a lignin powder.

[0032] In one embodiment, the lignin provided in step a) is dissolved in a solution. The lignin can be dissolved in any suitable solvent in which lignin is soluble. For example, the lignin can be dissolved in black liquor. In embodiments where the lignin is dissolved in a solution, the lignin must be separated from the solution, for example by precipitation, and dried prior to the agglomeration in step c). Once dried, the lignin can also be comminuted to obtain a lignin powder.

[0033] Thus, the lignin provided in step a) can be in solid form, or it can be dissolved. In step c), the lignin must be in powder form.

[0034] Step b) of the method according to the first aspect of the present application comprises mixing the lignin with a recycled carbon-rich material fraction to obtain a lignin-carbon mixture. The terms "recycled" and "recycled" as used herein refer to the process of collecting a fraction of the produced material and introducing it at an earlier stage of the method. Thus, the recycled carbon-rich material fraction has been produced by a heat treatment at a later stage of the method, collected and recycled, so that it is introduced back into the method at the earlier stage, where it is mixed with the lignin.

[0035] The term "lignin-carbon mixture" as used herein refers to a mixture of lignin and carbon-rich material, wherein the carbon-rich material has been recycled. The recycled carbon-rich material fraction is in the form of a solid powder and is mixed with the lignin by any suitable mixing means. The mixing means can be chosen depending on the form in which the lignin is provided. In the agglomerated lignin-carbon composite material, it is important that the distribution of the carbon-rich material within the lignin matrix is uniform. This is in turn ensured by sufficient mixing time and mixing speed during the step of mixing the lignin with the recycled carbon-rich material fraction.

[0036] The lignin-carbon mixture can be a powder mixture, or a mixture comprising carbon-rich material dispersed in a solution, in which the lignin is dispersed or dissolved. Thus, the recycled carbon-rich material fraction can be introduced back into the process at any suitable stage. For example, the recycled carbon-rich material fraction can be introduced after the lignin has been separated and dried, i.e. outside the lignin separation equipment, e.g. outside the LignoBoost equipment. In another example, the recycled carbon-rich material fraction can be introduced inside the lignin separation equipment, e.g. inside the LignoBoost equipment. When introduced inside the lignin separation equipment, the recycled carbon-rich material can be introduced into the process stream before or after the lignin has been separated from the black liquor.

[0037] In embodiments where the lignin is provided in powder form, the recycled carbon-rich material fraction and the lignin powder can be mixed by a dry mixing method. By providing the lignin in powder form, the mixing with the recycled carbon-rich material fraction is simplified, as both materials are in a dry state and the mixing takes place outside the lignin separation equipment.

[0038] In embodiments where the lignin is provided in slurry form, the recycled carbon-rich material fraction can be mixed with the lignin by adding the carbon-rich material to the lignin slurry. Thus, the slurry will comprise lignin particles and carbon-rich material particles. The lignin-carbon mixture can be separated from the solution by, e.g. filtration or any other suitable separation means. Thereafter, the lignin-carbon mixture is dried before agglomeration in step c). In embodiments where the lignin slurry is a process stream in the LignoBoost equipment, the recycled carbon-rich material fraction can be introduced at any suitable stage in the process after the lignin has been precipitated from the black liquor.

[0039] In embodiments where the lignin is provided in a form dissolved in a solution, the recycled carbon-rich material fraction can be mixed with the lignin by adding the carbon-rich material to the solution comprising the dissolved lignin. Thus, the solution will comprise dissolved lignin and dispersed carbon-rich material. Prior to the agglomeration in step c), the lignin must be separated from the solution, e.g. by precipitation or any other suitable means. Once the lignin is in solid form, the lignin-carbon mixture can be separated from the solution by, e.g. filtration or any other suitable separation method. Thereafter, the lignin-carbon mixture is dried prior to the agglomeration in step c). In embodiments where the solution is black liquor, the recycled carbon-rich material fraction must be introduced to the black liquor prior to the lignin being precipitated from the black liquor. For example, the carbon-rich material can be introduced to the black liquor prior to the black liquor being introduced to the LignoBoost apparatus, or while the black liquor is already introduced to the LignoBoost apparatus.

[0040] Mixing the recycled carbon-rich material with the lignin can be advantageous when the lignin is in the form of a slurry or dissolved in a solution, as the interaction between the carbon-rich material and the lignin can be improved.

[0041] Step c) of the method according to the first aspect of the present invention comprises forming an agglomerated lignin-carbon composite material comprising lignin, the recycled carbon-rich material fraction, and optionally at least one additive.

[0042] The term "lignin-carbon composite material" as used herein in phrases such as "agglomerated lignin-carbon composite material" and "thermally stable agglomerated lignin-carbon composite material" refers to a composite material comprising lignin and carbon-rich material. The term "lignin-carbon composite material" also refers to a material comprising essentially only lignin and carbon-rich material, such that at least 95 wt%, or at least 98 wt% of the lignin-carbon composite material, based on the dry weight of the lignin-carbon composite material, consists of lignin and carbon-rich material. The lignin-carbon composite material can also optionally comprise small amounts, e.g. less than 5 wt%, or less than 2 wt%, of at least one additive. In the lignin-carbon composite material, the carbon-rich material is uniformly dispersed within a lignin matrix.

[0043] The term "agglomerated lignin-carbon composite material" as used herein refers to a macroscopic particle, which in turn comprises a cluster of smaller lignin particles, wherein particles of carbon-rich material are dispersed within a lignin matrix. Optionally, small amounts of at least one additive can be present in the agglomerated lignin-carbon composite material.

[0044] The agglomerated lignin-carbon composite material comprises, based on the total weight of the agglomerated lignin-carbon composite material, 1 to 50 wt%, or 1 to 40 wt%, or 1 to 30 wt%, or 5 to 30 wt% of the recycled carbon-rich material; 50 to 99 wt%, or 60 to 99 wt%, or 70 to 95 wt%, or 70 to 99 wt% of the lignin; and optionally less than 5 wt%, or less than 2 wt% of the at least one additive.

[0045] The agglomerated lignin-carbon composite material can have an average particle size in the range of 0.05 to 5.0 mm, or 0.05 to 1.0 mm, or 0.2 to 1.0 mm, or 0.2 to 5.0 mm, or 0.5 to 2.0 mm, or 0.5 to 5.0 mm. A preferred average particle size is in the range of 0.5-2.0 mm. In the present application, average particle size is defined as the volume average particle size (D v50 ). This value refers to the largest particle size below which 50% by volume of the sample exists. In the context of the present invention, particle size is considered to be the diameter of the particle. Particle size distribution can be determined using, for example, laser diffraction.

[0046] The agglomerated lignin-carbon composite material has a bulk density in the range of 0.4-0.8 g / cm 3 . The bulk density of the agglomerated lignin-carbon composite material is increased compared to the lignin-carbon mixture prior to forming the agglomerated material.

[0047] By forming the agglomerated lignin-carbon composite material, a denser and harder material is achieved. Hard agglomerates are advantageous during subsequent processing as they can withstand physical impact during processing. The interaction between particles, for example between lignin and carbon-rich material, is enhanced in the agglomerated composite material as the particles are in close proximity to each other. Another advantage is that the tendency for dusting is reduced when lignin is provided in agglomerated form.

[0048] It has been found that the use of carbon-rich material together with lignin, as in the agglomerated lignin-carbon composite material, can improve the thermal processability of the lignin. The agglomerated lignin-carbon composite material has improved thermal resistance by reducing the melting / swelling behavior upon heating compared to lignin agglomerated without the addition of carbon-rich material. This facilitates and improves the processability of the lignin on an industrial scale.

[0049] The agglomerated lignin-carbon composite material can comprise at least one additive, or no additive. In the context of the present invention, an additive is a substance that is added to improve the processability or functionality of the obtained material. Thus, an additive is a substance that is added but not present in the lignin raw material. Thus, in the context of the present invention, moisture, such as water, and other components already present in the lignin raw material are not considered to be additives. In the context of the present invention, recycled carbon-rich material is not considered to be an additive.

[0050] The total amount of additive is preferably less than 5 wt%, such as 0 to 5 wt%, or 0.1 to 5 wt%, or less than 2 wt%, such as 0 to 2 wt%, or 0.1 to 2 wt%, based on the total dry weight of the agglomerated lignin-carbon composite material. Thus, the agglomerated lignin-carbon composite material comprises at least 95 wt%, such as at least 98 %, of lignin and carbon-rich material, based on the total dry weight of the agglomerated lignin-carbon composite material.

[0051] In embodiments where the agglomerated lignin-carbon composite material comprises at least one additive, the additive can be added to the lignin-carbon mixture at any time. The additive can also be added to the lignin or the recycled carbon-rich material fraction before the lignin is mixed with the recycled carbon-rich material fraction. The additive can also be added at any stage during the formation of the agglomerated lignin-carbon composite material. Any suitable additive can be added, such as a binder or a lubricant, to facilitate the subsequent compaction process and to improve the density and mechanical properties of the obtained lignin-carbon composite material. Furthermore, additives that have an influence on the properties of the final material can be added, such as functional enhancing additives.

[0052] The agglomerated lignin-carbon composite material can be formed by any suitable method. For example, the lignin-carbon composite material can be formed by compacting a dry lignin-carbon mixture and crushing the compacted material, or by e.g. a pellet press.

[0053] In a preferred embodiment, the agglomerated lignin-carbon composite material is formed by a method comprising compacting and crushing. The steps of forming the agglomerated lignin-carbon composite material can comprise the following steps:

[0054] - optionally drying the lignin-carbon mixture;

[0055] - compacting the lignin-carbon mixture to obtain a lignin-carbon composite material; and

[0056] - crushing the lignin-carbon composite material to obtain the agglomerated lignin-carbon composite material.

[0057] The first step in forming the agglomerated lignin-carbon composite material comprises optionally drying the lignin-carbon mixture. In embodiments where the recycled carbon-rich material fraction is mixed with lignin in solution or slurry, the lignin-carbon mixture has to be dried before the compacted lignin-carbon mixture is formed. Also in embodiments where the recycled carbon-rich material fraction is mixed with lignin in powder form, the lignin-carbon mixture can be dried before compaction. Preferably, the lignin-carbon mixture has a moisture content of less than 45 wt%, or less than 25 wt%, or less than 10 wt%, or less than 8 wt% before the lignin-carbon mixture is compacted. Preferably, the moisture content is at least 1 wt%, for example at least 5 wt%. Drying is performed using methods and equipment known in the art. The temperature during drying is preferably in the range of 80 to 160 °C, more preferably in the range of 100 to 120 °C. The lignin-carbon mixture can also be comminuted before compaction.

[0058] The second step in forming the agglomerated lignin-carbon composite material comprises compacting the lignin-carbon mixture to obtain the lignin-carbon composite material. Since the lignin-carbon mixture has been dried before compaction, the lignin-carbon mixture is preferably in powder form at the start of the compaction. Compaction of the lignin-carbon powder mixture is preferably performed by roll compaction. Roll compaction of the lignin-carbon powder mixture can be achieved by a roll compactor to compact the lignin-carbon powder mixture into a composite material.

[0059] In the compaction step, a compacted lignin-carbon composite material is generated. Here, the lignin-carbon powder mixture is typically fed through a hopper and conveyed through a horizontal or vertical feed screw into a compaction zone where the material is compacted into a sheet by compaction rolls having a defined gap. By controlling the feed screw speed and the pressure development in the compaction zone, a sheet with a uniform density can be obtained. The pressure development in the compaction zone can preferably be monitored and controlled by the rotational speed of the compaction rolls. As the powder is dragged between the rolls, it enters a so-called nip area where the density of the material increases and the powder is transformed into a sheet or a band. The rolls used have cavities. The depth of each cavity used in the roll compaction is 0.1 mm to 10 mm, preferably 1 mm to 8 mm, more preferably 1 mm to 5 mm or 1 mm to 3 mm. The specific pressure applied during compaction can vary depending on the equipment used for compaction, but can be in the range of 1 kN / cm to 100 kN / cm. Equipment suitable for performing the compaction is known in the art.

[0060] In a preferred embodiment, the temperature of the lignin-carbon powder mixture is kept below 150 °C, for example below 100 °C, during the compaction process.

[0061] In one embodiment of the roller compaction, the roller configuration is such that the first roller has a ring-shaped edge in this configuration, so that the lignin-carbon powder mixture in the nip region is sealed along the roller surface in axial direction.

[0062] In one embodiment, the roller configuration is such that the nip region is sealed with static plates along the roller surface in axial direction. By ensuring that the nip region is sealed, the loss of lignin-carbon powder at the axial ends of the rollers is minimized compared to rollers that are completely cylindrical.

[0063] During compaction, when the lignin particles and carbon-rich material particles are pressed together by mechanical pressure, a lignin-carbon composite material is formed. When the particles of the respective powders are pressed into close proximity to each other, the dispersion of the carbon-rich material and the lignin is improved. It has been found that the carbon-rich material can facilitate the compaction process by reducing the internal friction between the lignin particles.

[0064] Due to the rearrangement of the primary particles and the plastic deformation caused by the mechanical forces, the compaction can also serve to enhance the interaction between the lignin particles and the carbon-rich material in the composite material. The compaction will further serve to ensure that the homogeneous distribution achieved in the mixing step is maintained until the lignin-carbon composite material can be further stabilized, i.e. by the thermal stabilization step.

[0065] The lignin-carbon powder mixture can be subjected to compaction without the addition of additives. Alternatively, it can be subjected to a lignin-carbon powder mixture that further comprises a small amount of at least one additive, for example less than 5 wt%, or less than 2 wt%, based on the total dry weight of the lignin-carbon powder mixture.

[0066] The second step of forming the agglomerated lignin-carbon composite material comprises crushing the lignin-carbon composite material to obtain the agglomerated lignin-carbon composite material. In the crushing step, the compacted lignin-carbon composite material from the compaction step is subjected to crushing or milling, for example by a rotary granulator, a cage mill, a beater mill, a hammer mill or a crusher mill and / or combinations thereof. In this step, the compacted lignin-carbon composite material is crushed into agglomerates as the compacted lignin-carbon composite material is crushed, resulting in the agglomerated lignin-carbon composite material.

[0067] After crushing, the crushed material is preferably subjected to a sieving step to remove fine material, which can be recycled back to the compaction step. Furthermore, large material, for example agglomerates with a diameter larger than 2.0 mm or 5.0 mm, can be removed and / or recycled back to the crushing step.

[0068] In the sieving step, the agglomerated lignin-carbon composite material from the crushing step is sieved by physical fractionation, such as sieving, also known as sifting, to obtain a product of agglomerated lignin-carbon composite material having a defined particle size set by the aperture size of the sieve or screen in this step. The sieve or screen is chosen such that most of the particles having a diameter below 50 pm, or below 200 pm, or below 500 pm, pass through the screen and are rejected and preferably returned to the compacting step, while most of the particles having a diameter above 50 pm, or above 200 pm, or above 500 pm, are retained and subjected to subsequent steps of the method according to the present application. Sieving can be performed in more than one step, i.e. sieving can be performed such that the crushed material from the crushing step is sequentially passed through more than one screen, sieve or classifier.

[0069] During the compacting process, the bulk density of the material increases as the material is compacted tightly together. In one embodiment, the bulk density of the agglomerated lignin-carbon composite material is in the range of 0.5-0.8 g / cm3. 3 Due to the compaction of the lignin-carbon powder mixture during the preparation of the agglomerated lignin-carbon composite material, the bulk density of the lignin-carbon powder mixture will increase as pressure is applied to the powder. This means that the agglomerated lignin-carbon composite material will have a higher bulk density than the lignin-carbon powder mixture. A denser material can be beneficial during the subsequent processing into carbon-rich material, as it has been found that the agglomerated lignin-carbon composite material maintains its shape and size without melting or swelling. The agglomerated lignin-carbon composite material will also have a relatively high hardness after compaction. Hard agglomerates are advantageous during the subsequent processing, as they can resist physical impact during processing. As mentioned above, the carbon-rich material will also improve the thermal processability of the lignin.

[0070] In a preferred embodiment, the method according to the first aspect comprises the additional step of pre-heating the agglomerated lignin-carbon composite material to a temperature in the range of 140-300 °C for at least 30 minutes to obtain a thermally stabilized agglomerated lignin-carbon composite material.

[0071] As used herein, the term "thermally stabilized" refers to a material obtained by a method of pre-heating the agglomerated lignin-carbon composite material at a temperature lower than the temperature required for carbonization. By performing such pre-heating, which can also be referred to as thermal stabilization, the obtained thermally stabilized agglomerated lignin-carbon composite material can be subjected to heat treatment at higher temperatures while maintaining shape and size, avoiding melting / swelling and deformation during subsequent carbonization. The thermally stabilized agglomerated lignin-carbon composite material is cross-linked.

[0072] In one embodiment, the preheating is performed in an oxidizing atmosphere. Due to the oxidation and the thermal effect, cross-linking of the lignin will occur during the preheating. The cross-linking is promoted by the combined oxidation and thermal effect. Due to the cross-linking, the lignin within the agglomerate will become hard and will not melt / swell during the subsequent carbonization. Prior to the preheating, the lignin agglomerate behaves as a thermoplastic material, while after the thermal stabilization, the lignin agglomerate behaves instead as a thermoset material.

[0073] In this oxidizing atmosphere, oxidizing species are present that can react to cross-link the lignin. The preheating can for example be performed in the presence of oxygen, iodine, ozone, nitrogen dioxide, nitrobenzene, hydrogen peroxide and peracetic acid. Preferably, the heating is performed in air. Alternatively, any suitable oxidizing species can be supplied in a nitrogen atmosphere.

[0074] Preferably, the preheating is performed such that the agglomerated lignin-carbon composite is fully thermally stabilized, i.e. fully cross-linked. As used herein, the term "fully thermally stabilized" means that the agglomerated lignin-carbon composite has been thermally stabilized to the extent that the same degree of cross-linking is achieved throughout the material. This means that the material properties, such as structure and hardness, will be the same throughout the fully thermally stabilized agglomerated lignin-carbon composite. For example, the core of the fully thermally stabilized agglomerated lignin-carbon composite will have the same hardness and the same degree of cross-linking as the shell. Thus, the fully thermally stabilized agglomerated lignin-carbon composite is structurally homogeneous. The fully thermally stabilized agglomerated lignin-carbon composite can be obtained by providing agglomerated lignin-carbon composite of relatively small size, e.g. having an average particle size in the range of 50 pm to 1.0 mm, and / or by preheating the agglomerated lignin-carbon composite for a sufficient long time.

[0075] The preheating is performed such that the agglomerated lignin-carbon composite is heated to a temperature in the range of 140-300 °C, preferably 180-260 °C. The preheating is performed for at least 30 minutes, i.e. the residence time of the agglomerated lignin within the equipment used for the preheating is at least 30 minutes. In one embodiment, the preheating is performed for at least 1 hour, or at least 1.5 hours. Preferably, the preheating is performed for less than 12 hours. The preheating can be performed at the same temperature throughout the preheating phase, or can be performed at varying temperatures, e.g. increasing the temperature stepwise or using a temperature gradient. More preferably, the preheating is performed such that the agglomerated lignin-carbon composite is first heated to a temperature in the range of 140-175 °C for at least 15 minutes, and subsequently heated to a temperature in the range of 175-300 °C for at least 15 minutes.

[0076] The thermal processability of lignin is improved by providing lignin in the form of agglomerated lignin, by mixing lignin with carbon-rich material to obtain a lignin-carbon composite, and by a combination of pre-heating to thermally stabilize the lignin. Thus, the thermally stabilized agglomerated lignin-carbon composite can be subjected to further heat treatment to carbonize the material without melting / swelling behavior. This improves the processability on an industrial scale. Depending on the amount and particle size distribution of the carbon-rich material present in the agglomerated lignin-carbon composite, sufficient processability can also be obtained without pre-heating.

[0077] Step d) of the method according to the first aspect of the present invention involves subjecting the agglomerated lignin-carbon composite to heat treatment at one or more temperatures in the range of 300-1500 °C, wherein the total time of heat treatment is in the range of 30 minutes to 10 hours, to obtain a carbon-rich material. In embodiments where the agglomerated lignin-carbon composite has been subjected to pre-heating such that a thermally stabilized agglomerated lignin-carbon composite has been obtained, the thermally stabilized agglomerated lignin-carbon composite is subjected to heat treatment.

[0078] The term "heat treatment" as used herein refers to the process of heating the agglomerated lignin-carbon composite at one or more temperatures and for a sufficient time such that the lignin in the composite is converted to a carbon-rich material. The process can also be referred to as carbonization or calcination. After heat treatment, the carbon content of the material is higher than 80 wt%, or higher than 90 wt%, or higher than 95 wt%, or higher than 98 wt%. Depending on the temperature during heat treatment, different types of carbon can be obtained from the lignin, such as charcoal or hard carbon.

[0079] The term "carbon-rich material" as used herein refers to a carbon material obtained by heat treatment of lignin. The carbon content of the carbon-rich material is higher than 80 wt%, or higher than 90 wt%, or higher than 95 wt%, or higher than 98 wt%. The carbon-rich material can also comprise, for example, heteroatoms (e.g. O and N), inorganic impurities, and functional additives. The carbon-rich material of the present invention is amorphous (i.e. non-crystalline) carbon, preferably hard carbon. The carbon-rich material obtained in step d) of the method according to the first aspect comprises recycled carbon-rich material and carbon-rich material obtained by heat treatment of lignin, wherein the recycled material comprises carbon-rich material obtained in a previous heat treatment of lignin. Once the lignin has been completely converted to carbon-rich material (e.g. at a temperature of at least about 1000 °C), it is not possible to distinguish between recycled carbon-rich material and newly obtained carbon-rich material.

[0080] The heat treatment can be performed at the same temperature throughout the heat treatment, or can be performed at different temperatures, for example with a stepwise increase in temperature or using a temperature gradient. The heat treatment can comprise a temperature ramp from a starting temperature to a target temperature. The heating rate can be 1-100°C / min. For example, the heat treatment can comprise several intermediate temperatures with a temperature ramp between them, before reaching the target temperature required for carbonization of the agglomerated lignin-carbon composite. The heat treatment can be performed as a batch process or as a continuous process. Any suitable reactor can be used, for example a rotary kiln, a moving bed furnace, a pusher furnace or a rotary hearth furnace. The heat treatment is preferably performed under an inert atmosphere, preferably a nitrogen atmosphere.

[0081] Preferably, the heat treatment comprises a preliminary heating step, preferably followed by a final heating step. The preliminary heating step is preferably performed at one or more temperatures in the range of 300 to 800°C, for example 500 to 700°C. The preliminary heating step is preferably performed under an inert atmosphere, preferably under a nitrogen atmosphere. The duration of the preliminary heating step is at least 30 minutes, and preferably less than 10 hours. The surface area of the carbon-rich material obtained after the preliminary heating step is typically in the range of 300 m 2 / g to 700 m 2 / g, measured using nitrogen BET.

[0082] The final heating step is preferably performed at one or more temperatures in the range of 800 to 3000°C. The final heating step is preferably performed under an inert atmosphere, preferably under a nitrogen atmosphere. The duration of the final heating step is at least 30 minutes, and preferably less than 10 hours. The surface area of the carbon-rich material obtained after the final heating step performed at 1000°C or higher is typically 50 m 2 / g or less.

[0083] The preliminary and final heating steps can be performed as discrete steps or as one single step in direct sequence. The preliminary and final heating steps can comprise heating at one or more temperatures, as discussed above for the heat treatment. For example, the preliminary heating starts at about 300°C, followed by an increase in temperature to about 500°C. The final heating step is preferably performed between 900 and 1300°C, for example at about 1000°C.

[0084] The conversion of lignin to carbon-rich material starts at about 250°C. The amount of lignin converted to carbon depends mainly on the temperature and time used during the heat treatment. The properties of the obtained carbon-rich material also depend on the temperature and time used during the heat treatment. Thus, the carbon-rich material obtained after the preliminary heating step can differ in some aspects from the carbon-rich material obtained after the final heating step. For example, the carbon content can be higher after the final heating step, and the structure of the carbon in the carbon-rich material can be different.

[0085] The preliminary and final heating steps can be performed as batch processes or as continuous processes. Any suitable reactor can be used. The preliminary heating step and the final heating step can be performed in the same reactor or in separate reactors.

[0086] The carbon-rich material preferably has a bulk density in the range of 0.2 g / cm 3 to 0.4 g / cm 3 . This is lower than the bulk density of the agglomerated lignin-carbon composite material, mainly due to the mass loss during the heat treatment.

[0087] The helium true density of the obtained carbon-rich material is preferably in the range of 1.4 to 2.1 g / cm 3 , for example 1.7 to 2.0 g / cm 3 . The helium true density can be determined using a pycnometer, as known to the person skilled in the art. It is important that the helium true density is in the range of 1.4 to 2.1 g / cm 3 , because otherwise the doping and dedoping capacity of the carbon-rich material can be reduced and the irreversible capacity of the battery can be large when used as active material in the negative electrode of a non-aqueous secondary battery. If the density of the carbon-rich material is too low, the energy density of the electrode can also be reduced.

[0088] By providing the lignin in agglomerated form, and by mixing the lignin with a recycled carbon-rich material fraction, it is ensured that the lignin does not melt / swell during the heat treatment to convert it into carbon-rich material. By performing a pre-heat step prior to the heat treatment, the dimensional stability of the lignin can be further improved, thereby further reducing the risk of melting / swelling. Depending on the amount of recycled carbon-rich material mixed with the lignin and the particle size distribution of the recycled fraction, a pre-heat can not be needed to reduce the melting / swelling of the lignin during the heat treatment.

[0089] Step e) of the method according to the first aspect of the application comprises milling the obtained carbon-rich material to reduce the average particle size of the carbon-rich material and to obtain at least a first fraction of carbon-rich material and a second fraction of carbon-rich material. The milling can be performed using any suitable equipment, for example using a cutting mill, a blade mixer, a ball mill, an impact mill, a hammer mill and / or a jet mill. The milling can also be referred to as for example crushing, crushing or milling. The first fraction of carbon-rich material can have a first average particle size and the second fraction of carbon-rich material can have a second average particle size.

[0090] The first fraction of the carbon-rich material can have an average particle size in the range of 1 to 100 pm, or 1 to 80 pm, or 1 to 50 pm, or 1 to 20 pm. The second fraction of the carbon-rich material can have an average particle size in the range of 1 to 100 pm, or 1 to 80 pm, or 1 to 50 pm, or 1 to 20 pm.

[0091] The average particle size of the first fraction and the second fraction of the carbon-rich material can be the same or different. For example, the first average particle size can be smaller than the second average particle size.

[0092] The first fraction and the second fraction can be separated by any suitable means, for example classification and / or sieving. Such a process can be performed after and / or during the milling. Suitable equipment that can be used to separate the first fraction from the second fraction include air classifier equipment, for example gravity, centrifugal, cyclonic and / or gyratory air classifier systems, and sieving equipment, for example tumbling and / or vibrating sieving instruments. The separation can be performed in several steps.

[0093] The obtained carbon-rich material is milled to reduce the average particle size, which facilitates the use of the carbon-rich material as an active material in the negative electrode of a secondary battery. During the milling, particles having a large span size are produced. Typically, particles having a small particle size, often referred to as carbon fines, are removed and discarded, while particles having a size in the desired range are further processed. However, in some applications, it is also of interest to utilize particles having a small particle size, for example carbon fines. For example, the electrode density can be increased by using active material of small particle size. Furthermore, when using active material of small particle size, the charge / discharge rate performance of the electrode can be improved due to faster metal diffusion. Thus, the particle size of the first fraction and the second fraction is selected depending on the end use of the carbon-rich material produced by the method according to the present application.

[0094] In a preferred embodiment, the first fraction of the carbon-rich material has an average particle size in the range of 1 to 4 pm. This corresponds to carbon fines. By recycling the carbon fines, material waste of the process is reduced, as the carbon fines would otherwise be discarded. A more sustainable process is thus achieved, while also improving the thermal processability of the agglomerated lignin-carbon composite material. In such an embodiment, the second part of the carbon-rich material can have an average particle size in the range of 4 to 100 pm, or 4 to 80 pm, or 4 to 50 pm, or 4 to 20 pm. Such a fraction can for example be obtained by performing grinding and fractioning simultaneously, such that most of the particles having a size of less than 100 pm, or less than 80 pm, or less than 50 pm, or less than 20 pm will pass the first fractioning unit. Particles having a larger size are retained in the grinding unit until the size has been further reduced. The particles that have passed the first fractioning unit are further fractioned in a second fractioning unit, wherein a first fraction is separated from a second fraction. Here, the first fraction of the carbon-rich material having a small average particle size, for example below 4 pm, passes the fractioning unit and is recycled. The second fraction of the carbon-rich material having a larger average particle size, like above 4 pm, can continue to a subsequent process step, or can alternatively be collected as a final product.

[0095] In another embodiment, the first fraction of the carbon-rich material can have an average particle size in the range of 1 to 100 pm, or 1 to 80 pm, or 1 to 50 pm, or 1 to 20 pm. The second fraction of the carbon-rich material can also have an average particle size in the range of 1 to 100 pm, or 1 to 80 pm, or 1 to 50 pm, or 1 to 20 pm. Thus, in such an embodiment, the first fraction and the second fraction can comprise carbon-rich material having the same average particle size. Thus, carbon fines can be included in both the first and the second fraction. In this embodiment, recycling is thus not performed to reduce material loss of the process, but to improve the processability of the obtained agglomerated lignin-carbon composite material.

[0096] In another embodiment, the first fraction of carbon-rich material can have an average particle size in the range of 4 to 100 pm, or 4 to 80 pm, or 4 to 50 pm, or 4 to 20 pm, such that carbon fines are predominantly present in the second fraction. In such an embodiment, the second fraction of carbon-rich material can have an average particle size in the range of 1 pm to 20 pm. In such an embodiment, the recycled fraction can comprise carbon-rich material having an average particle size that is larger than the average particle size of the final product. If the desired average particle size of the final product is small, for example in the range of 1 to 5 pm, a large amount of grinding is required, resulting in high energy consumption, to bring all carbon-rich material to the desired particle size. If the carbon-rich material cannot be ground sufficiently, for example due to limitations of the equipment, the carbon-rich material that is too large in particle size will be discarded. By recycling the carbon-rich material of larger particle size instead, as in the process of the present invention, less grinding is required and energy consumption is reduced, while also reducing material loss. Furthermore, as mentioned above, the recycled carbon-rich material improves the thermal processability of the obtained agglomerated lignin-carbon composite.

[0097] In embodiments wherein the heat treatment in step d) comprises a preliminary heating step followed by a final heating step, the grinding step is preferably performed after the preliminary heating step and before the final heating step. Thus, the first fraction can be recycled after the preliminary heating step, while the second fraction is subjected to the final heating step.

[0098] Step f) of the process according to the present invention comprises recycling at least a part of the first fraction obtained in step e) to step b). For example, at least 80% of the first fraction is recycled. Preferably, at least 90% is recycled, and more preferably at least 98% is recycled. This means that preferably the entire first fraction is recycled back, such that it is mixed with lignin. In embodiments wherein the first fraction would otherwise be discarded, recycling the entire first fraction or a large part of the first fraction enables a more sustainable process, as material loss is reduced. The yield is also improved, as all or almost all of the lignin that enters the process will be converted to carbon and constitutes the final product.

[0099] The process according to the present invention can be a batch process or a continuous process. In a batch process, the recycling comprises collecting the first fraction and storing it until it is subsequently mixed with lignin in the next batch. In a continuous process, the first fraction collected after the grinding step is continuously recycled back to the mixing step, where it is introduced into the lignin process stream.

[0100] In a preferred embodiment, the heat treatment comprises a preliminary heating step and a final heating step, and the grinding is performed after the preliminary heating step and before the final heating step. In such an embodiment, the first fraction is preferably recycled after the preliminary heating step, while the second fraction is subjected to the final heating step.

[0101] During the heat treatment, lignin is converted into carbon-rich material. The properties of the carbon-rich material depend on, for example, the temperature and the time of the heat treatment. Heating at high temperatures, for example above 1000 °C, leads to hard materials. Thus, due to the temperature-dependent change of the carbon structure during the heat treatment, the carbon-rich material obtained after the final heating step is harder than the carbon-rich material obtained after the preliminary heating step. It has been found that when grinding after the preliminary heating step and recycling the obtained first fraction before the final heating step, the mixing of the recycled fraction with lignin is facilitated due to the softer carbon-rich material in the recycled fraction. Moreover, the formation of agglomerated lignin-carbon composites is facilitated due to the improved interaction between lignin and carbon-rich material.

[0102] Thus, in a preferred embodiment, the method according to the present application comprises the following steps:

[0103] a) providing lignin;

[0104] b) mixing the lignin with a recycled fraction of carbon-rich material to obtain a lignin-carbon mixture;

[0105] c) forming an agglomerated lignin-carbon composite comprising lignin, the recycled fraction of carbon-rich material and optionally at least one additive;

[0106] d) optionally pre-heating the agglomerated lignin-carbon composite to a temperature in the range of 140 to 300 °C for a period of at least 30 minutes to obtain a thermally stabilized agglomerated lignin-carbon composite;

[0107] e) subjecting the agglomerated lignin-carbon composite or the thermally stabilized agglomerated lignin-carbon composite to a preliminary heating step at one or more temperatures in the range of 400 °C to 800 °C for at least 30 minutes to obtain a carbon-rich material;

[0108] f) grinding the obtained carbon-rich material to reduce the average particle size of the carbon-rich material and to obtain at least a first fraction of carbon-rich material and a second fraction of carbon-rich material; and

[0109] g) recycling at least a part of the first fraction obtained in step f) to step b);

[0110] h) subjecting the second fraction obtained in step f) to a final heating step at one or more temperatures in the range of 800 to 1500 °C for at least 30 minutes.

[0111] Example

[0112] Example 1

[0113] Lignin powder (softwood kraft lignin) from the LignoBoost process was mixed together with 5 wt% of carbon-rich material powder using a conical screw mixer (200 RPM, 15 min). The carbon-rich material powder was recycled from a previous batch, where the agglomerated lignin was heated at 500 °C for 1 h in a nitrogen atmosphere, followed by comminution to D v50 5 pm. No other additives were added. The mixture was then compacted using a Lab Compactor by roller compaction at 50 kN to obtain a composite material, which was then crushed using a flaker and sieved into agglomerates with a particle size distribution of 0.5 to 1.5 mm.

[0114] The agglomerated lignin-carbon composite material was further heat stabilized by heating to 235 °C for 2 h inside a rotary kiln in air. During this process, the agglomerated lignin did not exhibit any melting behavior and kept its original shape. The individual agglomerates were found not to fuse together and remained free-flowing. The material gradually turned black during processing until it was completely black and odorless.

[0115] The heat stabilized agglomerated lignin-carbon composite material was then heat treated at 500 °C for 1 h under an inert atmosphere to carbonize the material. This resulted in a granular carbon-carbon composite material compared to the heat stabilized agglomerated lignin-carbon composite material before carbonization, without any melting or fusing of the granules and with the shape / size kept.

[0116] Other modifications and changes will occur to those skilled in the art upon reading the foregoing description. However, it will be apparent that, in light of the foregoing description, other modifications and changes will occur to those skilled in the art without departing from the spirit and scope of the present application.

Claims

1. Process for producing a carbon-rich material, the process comprising the steps of: a) providing a lignin; b) mixing the lignin with a recycled carbon-rich material fraction to obtain a lignin-carbon mixture; c) forming an agglomerated lignin-carbon composite material comprising the lignin, the recycled carbon-rich material fraction and optionally at least one additive; d) subjecting the agglomerated lignin-carbon composite material to a heat treatment at one or more temperatures in the range of 300-1500 °C, wherein the heat treatment is performed for a total time in the range of 30 minutes to 10 hours, to obtain a carbon-rich material; e) grinding the obtained carbon-rich material to reduce the average particle size of the carbon-rich material and to obtain at least a first fraction of the carbon-rich material and a second fraction of the carbon-rich material; and f) recycling at least a part of the first fraction obtained in step e) to step b).

2. Process according to claim 1, wherein the lignin is a kraft lignin.

3. Process according to any one of claims 1 or 2, wherein the lignin provided in step a) is in the form of a powder.

4. Process according to any one of claims 1 or 2, wherein the lignin provided in step a) is in the form of a slurry.

5. Process according to any one of claims 1 or 2, wherein the lignin provided in step a) is dissolved in a solution.

6. Process according to any one of the preceding claims, wherein the first fraction of the carbon-rich material has an average particle size in the range of 1 to 100 pm.

7. Process according to any one of the preceding claims, wherein the first fraction of the carbon-rich material has an average particle size in the range of 1 to 4 pm. The agglomerated lignin-carbon composite material comprises 1 to 50 wt% of the recycled carbon-rich material; 50 to 99 wt% of the lignin; and optionally less than 5 wt% of the at least one additive, based on the total weight of the agglomerated lignin-carbon composite material.

8. The method of any of the preceding claims, wherein, 9. Process according to any one of the preceding claims, wherein the step of forming the agglomerated lignin-carbon composite material comprises the steps of: - optionally drying the lignin-carbon mixture; - compacting the lignin-carbon mixture to obtain a lignin-carbon composite material; and - crushing the lignin-carbon composite material to obtain the agglomerated lignin-carbon composite material.

10. Process according to any one of the preceding claims, wherein the process comprises the additional step of pre-heating the agglomerated lignin-carbon composite material to a temperature in the range of 140 to 300 °C for a time period of at least 30 minutes to obtain a thermally stabilized agglomerated lignin-carbon composite material.

11. Process according to claim 10, wherein the pre-heating is performed in an oxidizing atmosphere. ​ ​ 12. The method according to any one of claims 10 or 11, wherein the pre-heating is performed by first heating the agglomerated lignin-carbon composite to a temperature in the range of 140 to 175 °C for a time period of at least 15 minutes and subsequently heating the agglomerated lignin-carbon composite to a temperature in the range of 175 to 300 °C for at least 15 minutes.

13. The method according to any one of the preceding claims, wherein the heat treatment in step d) is performed in an inert atmosphere.

14. The method according to any one of the preceding claims, wherein the heat treatment in step d) comprises a preliminary heating step followed by a final heating step.

15. The method according to claim 14, wherein the preliminary heating step is performed at a temperature between 400 and 800 °C for at least 30 minutes.

16. The method according to any one of claims 15 or 16, wherein the final heating step is performed at a temperature between 800 °C and 1500 °C for at least 30 minutes.

17. The method according to any one of claims 14-16, wherein a grinding step is performed after the preliminary heating step and before the final heating step.

18. The method according to claim 17, wherein at least a portion of the first fraction of carbon-rich material is recycled before the final heating step.

19. An anode for a non-aqueous secondary battery comprising a carbon-rich material obtainable by the method according to any one of claims 1-18 as active material.

20. Use of a carbon-rich material obtainable by the method according to any one of claims 1-18 as active material in an anode for a non-aqueous secondary battery.