Purified lignin

Abstract

The present invention is directed to lignin having iron content below 5.0 ppm and a weight average molecular mass in the range of from 8200 to 15000 g / mol. The purified lignin can be further processed to end products such as carbon enriched materials.

Description

[0001] PURIFIED LIGNIN

[0002] Field of the invention

[0003] The present invention is directed to lignin having iron content below 5.0 ppm and a weight average molecular mass in the range of from 8200 to 15000 g / mol. The purified lignin can be further processed to end products such as carbon enriched materials.

[0004] Background

[0005] Lignin, an aromatic polymer, is a major constituent in e.g. wood, and is the most abundant carbon source on Earth second only to cellulose. In recent years, with 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.

[0006] Today, the most commercially relevant source of lignin is kraft lignin. This lignin is obtained from hardwood or softwood through the kraft process. The lignin can be separated from alkaline black liquor using for example membrane- or ultrafiltration. LignoBoost is one common separation process and is described in for example W02006031175 A1. In this process, lignin is precipitated from alkaline black liquor through reducing the pH level, usually by adding carbon dioxide, and then filtered off. The lignin filter cake is in the next step re-slurried under acidic conditions, commonly using sulfuric acid, and washed. The precipitated washed lignin can be used as it is or further dried.

[0007] Black liquor is readily available as a by-product from the kraft process and is thus a cost-efficient lignin source. However, as black liquor contains a certain amount of metals, mostly originating from wood and from cooking chemicals used during the pulping process, lignin precipitated from the black liquor will also contain a certain amount of metals. Generally, the lignin will comprise relatively high levels of sodium and potassium, as well as lower amounts of other metals such as aluminum, calcium, iron, magnesium and manganese. The lignin may also comprise trace amounts of other metals.

[0008] Some metals, such as sodium and potassium, can to a large extent be removed from precipitated lignin by acidic washing steps during the separation process. Other metals are however harder to remove from the precipitated lignin. In many potential applications, such as in biofuels and for conversion to carbon enriched materials, such as carbon fibers and carbon powders, it is important that the lignin used has a high purity, in particular with regards to the amount of metals, that may otherwise interfere with the functionality of the material. This is of particular importance when lignin is further intended for conversion to carbon enriched materials that in turn is to be used in energy storage applications. Specifically, transition metals such as iron may, if present in energy storage applications involving carbon enriched materials obtained from lignin, have a negative impact on the long-term stability of the energy storage device. It is believed that this is caused by precipitation of transition metals during charging and discharging, and also by decomposition of the electrolyte by reactions in turn catalyzed by transition metals.

[0009] When lignin is intended for conversion to carbon enriched materials that in turn is to be used in energy storage applications it is also important that the melting properties of lignin are suitable for large-scale carbonization processes.

[0010] Lignin may also be obtained through different fractionation methods such as an organosolv process or hydrolysis lignin. The organosolv process is however of less commercial interest for producing lignin compared to the kraft process. Various attempts of removing metals from lignin have been made. Common for these methods is that they are rather complex involving many additional processing steps; and / or that they are not sufficiently efficient in their metal removal efficiency leading to insufficient purity levels and / or long washing times.

[0011] KR101451299 B1 discloses a method where lignin is repeatedly dissolved and precipitated in order to obtain a lignin with low ash content. However, many additional process steps are required.

[0012] W02020013752 A1 discloses a method where lignin is dissolved in an acidic aqueous solvent. Through phase separation a two-phase system is obtained, where one phase is a lignin rich phase, and the other phase is poor in lignin and comprises metal cations extracted from lignin. Many additional process steps are however required.

[0013] LIS20190241595 A1 is directed to a composition comprising Kraft lignin having very low amounts of metal. The lignin is suitable for use in a refinery process to prepare fuel. The purification process used is preferably carried out at room temperature or under moderate heating.

[0014] WO2023 / 277774 A1 is directed to a method for separation of lignin from an aqueous slurry containing lignocellulosic biomass material. In the method, lignin is re-slurried at a temperature of about 65° to 95°C.

[0015] Thus, there is a need for purified lignin that has a metal content, particularly iron content, that is sufficiently low so that the purified lignin can be used also in applications where a material of high purity is required.

[0016] Summary of the invention

[0017] It is an object of the present invention to provide a lignin with improved properties compared to prior art lignins. The present invention is thus directed to a purified lignin comprising less than 5.0 ppm iron and having a weight average molecular mass in the range of from 8200 to 15000 g / mol.

[0018] The purified lignin according to the present invention can be prepared by a method comprising the following steps: a) providing lignin in solid form; b) providing an acidic aqueous solution having a pH below 6; c) immersing the lignin in the acidic aqueous solution for at least 1 minute, wherein the temperature of the acidic aqueous solution during the immersion is in the range of from 111°C to 130°C, to remove metals from lignin to the acidic aqueous solution so as to obtain said purified lignin; d) separating the obtained purified lignin from the acidic aqueous solution; and e) optionally washing the separated purified lignin; wherein the lignin remains in solid form during all steps of the method.

[0019] It has surprisingly been found that a lignin with particularly low iron content and a particularly desirable weight average molecular mass can be obtained.

[0020] A lignin material with a low content of iron is of interest in a number of different applications, particularly for conversion to carbon enriched materials such as carbon fibers and carbon powders, in particular for use in energy storage applications. In such applications, the weight average molecular mass range of the purified lignin according to the present invention is desirable since it is particularly beneficial for the subsequent conversion, such as by carbonization, to carbon enriched materials such as carbon fibers and carbon powders, in particular for use in energy storage applications. The present invention is also directed to a carbon enriched material produced from the purified lignin according to the present invention. Detailed description

[0021] Step a) of the method for preparing the purified lignin according to the present invention involves providing lignin in solid form. It is intended throughout the present description that the expression "lignin" embraces any kind of lignin, e.g. lignin originated from hardwood, softwood or annular plants. Also, lignin can be chemically modified. Preferably, the lignin has been purified or isolated before being used in the process according to the present invention. The lignin may be isolated from black liquor and optionally be further purified before being used in the process according to the present invention. The purification is typically such that the purity of the lignin material is at least 90%, preferably at least 95%, more preferably at least 98%, based on the dry weight of the lignin material, Thus, the lignin material used according to the process of the present invention preferably contains less than 10%, preferably less than 5%, more preferably less than 2% impurities, such as cellulose and inorganic compounds, based on the dry weight of the lignin material.

[0022] The lignin to be purified may be obtained through different fractionation methods such as an organosolv process or a Kraft process or through biorefinery methods. Preferably, the lignin provided in step a) of the method for preparing the purified lignin according to the present invention is Kraft lignin, i.e. lignin obtained through the Kraft process. Preferably, the Kraft lignin is obtained from hardwood or softwood, most preferably from softwood. More specifically, it is preferred that at least 50 wt-%, such as at least 60 wt- % or 70 wt-% or 80 wt-% or 90 wt-% or 95 wt-% or 98 wt-% or 100 wt-% of the wood used to produce the lignin is from softwood.

[0023] The lignin to be purified may be obtained by using 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, preferably at a temperature in the range of from 40°C to 65°C, such as from 50° to 60°C, such as at approximately 55°C at least once. The obtained lignin may be dried and pulverized and thus provided as solid particles. The lignin obtained by this method typically has a total metal content in the range of from 500 ppm to 5000 ppm, originating mainly from the wood source and cooking chemicals added during the pulping process. The pH of the obtained lignin is typically in the range of from 3 to 4. The sulfur content of the obtained lignin is typically in the range of from 1 to 3 wt-%. This lignin is the preferred starting material to obtain the purified lignin of the present invention.

[0024] The term “iron content”, as used herein, refers to the total amount of iron present in the lignin. The iron content may be determined by inorganic elemental analysis. The contents of inorganics can be quantified by ICP- OES.

[0025] The term “total metal content”, as used herein, refers to the total amount of metals present in the lignin. Typically, metals present in lignin are sodium, potassium, magnesium, calcium, aluminum, iron, and manganese. Trace amounts of other metals may also be present. The metal content may be determined by inorganic elemental analysis. The total metal content is determined by summarizing the amounts of all individual metals. In one embodiment, the lignin provided in step a) of the method for preparing the purified lignin according to the present invention has a total metal content of at least 500 ppm, preferably at least 600 ppm and more preferably at least 700 ppm. In another embodiment, the lignin provided in step a) of the method for preparing the purified lignin according to the present invention has a total metal content in the range of from 500 ppm to 5000 ppm, preferably in the range from 600 ppm to 3000 ppm, and more preferably in the range from 700 ppm to 1500 ppm.

[0026] In one embodiment, the lignin provided in step a) of the method for preparing the purified lignin according to the present invention may have an aluminum content of at least 18 ppm, or at least 19 ppm; a calcium content of at least

[0027] 18 ppm, or at least 19 ppm; an iron content of at least 16 ppm, or at least 17 ppm; a potassium content of at least 30 ppm, or at least 50 ppm; a magnesium content of at least 45 ppm, or at least 48 ppm; a manganese content of at least 11 ppm, or at least 12 ppm; and a sodium content of at least 400 ppm, or at least 650 ppm.

[0028] In an alternative embodiment, the lignin provided in step a) of the method for preparing the purified lignin according to the present invention has an aluminum content in the range of from 18 to 25 ppm, preferably from 19 to 23 ppm; a calcium content in the range of from 18 to 40 ppm, preferably from

[0029] 19 to 25 ppm; an iron content in the range of from 16 to 30 ppm, preferably from 17 to 22 ppm; a potassium content in the range of from 30 to 90 ppm, preferably from 50 to 70 ppm; a magnesium content in the range of from 45 to 55 ppm, preferably from 48 to 53 ppm; a manganese content in the range of from 11 to 30 ppm, preferably from 12 to16 ppm; and a sodium content in the range of from 400 ppm to 2000 ppm, preferably from 650 to 1000 ppm.

[0030] The lignin provided in step a) of the method for preparing the purified lignin according to the present invention may also have a silicon content of at least 90 ppm, or at least 100 ppm. Alternatively, the silicon content may be in the range of from 90 to 140 ppm, preferably from 100 to 130 ppm.

[0031] The lignin provided in step a) of the method for preparing the purified lignin according to the present invention is in solid form, i.e. it is not in a dissolved state. In one embodiment, the lignin is provided in the form of a dry powder. Alternatively, the lignin may be moist or provided in a slurry or suspension. The lignin may also be provided as a crushed lignin cake obtained from a lignin separation process. The lignin is not dissolved during any of the steps in the method for preparing the purified lignin according to the present invention but remains in solid form. Alternatively, the lignin may be dissolved to a small extent, such that only small fragments of the solid lignin is dissolved, during the steps of the method according to the present invention. Thus, the lignin will remain largely in solid state. Any dissolved lignin will be removed during the separation step and discarded from the process.

[0032] Preferably, the lignin provided in step a) of the method for preparing the purified lignin according to the present invention is in particulate form, such as in the form of a powder. The particle size distribution of the lignin particles is preferably such that at least 80 wt% of the particles have a diameter less than 0.2 mm. 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.

[0033] Step b) of the method for preparing the purified lignin according to the present invention involves providing an acidic aqueous solution having a pH below 6. The term “acidic aqueous solution” as used herein, refers to any type of aqueous solution having a pH below 7. The acidic aqueous solution in step b) of the method for preparing the purified lignin according to the present invention may be provided by adding at least one acid to an aqueous solution. The pH of the acidic aqueous solution provided in step b) of the method for preparing the purified lignin according to the present invention has a pH below 6, preferably below 5 and more preferably below 4. In an alternative embodiment, the pH of the acidic aqueous solution may be below 7. In yet an alternative embodiment, the pH of the acidic aqueous solution may be in the range of from 1 to 6, preferably from 1 to 5, and more preferably from 1 to 4.

[0034] Lignin may be added to the aqueous solution after the acid has been added. Alternatively, lignin may be added to the aqueous solution prior to adding the acid. The aqueous solution is heated either before or after addition of the acid and lignin. In one embodiment, the acid is added to a heated aqueous solution, to which lignin is subsequently added. In another embodiment, lignin is added to an acidic aqueous solution which is subsequently heated. In yet another embodiment, acid is added to an aqueous solution comprising lignin. In this embodiment, the aqueous solution is heated either before or after addition of acid. In a preferred embodiment, lignin is added to a heated aqueous solution, and acid is subsequently added to the aqueous solution comprising lignin.

[0035] The acidic aqueous solution comprises at least one acid such that the pH of the acidic aqueous solution is below 7, or below 6, or below 5 or below 4. Examples of suitable acids are sulfuric acid, phosphoric acid and hydrochloric acid.

[0036] The acidic aqueous solution may also comprise more than one acid, such as a combination of two or more acids.

[0037] In one embodiment of the method for preparing the purified lignin according to the present invention, the total amount of acid in the acidic aqueous solution is in the range of from 0.1 to 6 wt%, preferably from 0.5 to 5 wt%, based on the dry weight of lignin immersed in the acidic aqueous solution. The term “total amount of acid” as used herein, refers to the total amount of concentrated acid added to the acidic aqueous solution.

[0038] The acidic aqueous solution may further comprise one or more additives.

[0039] Step c) of the method for preparing the purified lignin according to the present invention involves immersing the lignin particles in the acidic aqueous solution for at least 1 minute, wherein the temperature of the acidic aqueous solution during the immersion is in the range of from 111°C to 130°C, to remove metals from lignin to the acidic aqueous solution so as to obtain said purified lignin. The term “immersion” as used herein, refers to a process of contacting lignin in solid form, such as in the form of lignin particles, with an acidic aqueous solution for a certain period of time. During the immersion step of the method for preparing the purified lignin according to the present invention, the entire surface area of the lignin is in contact with the acidic aqueous solution, meaning that the lignin is fully submerged in the acidic aqueous solution.

[0040] The lignin is immersed in the acidic aqueous solution at a temperature in the range of from 111°C to 130°C, such as from 111°C to 129°C, or from 112°C to 124°C or from 112°C to 122°C. The acidic aqueous solution may be heated using any suitable means as known by a person skilled in the art. The heating is carried out in a closed pressurized vessel. The temperature is kept in the defined range during the entire immersion step. By heating the acidic aqueous solution, the metal removal efficiency is increased.

[0041] The immersion time in step c) of the method for preparing the purified lignin according to the present invention is at least 1 minute, preferably at least 10 minutes, or even more preferably at least 30 minutes. In an alternative embodiment of the method for preparing the purified lignin according to the present invention, the immersion time is in the range of from 1 minute to 6 hours, preferably in the range of from 10 minutes to 5 hours, or even more preferably in the range of from 15 minutes to 4 hours. By increasing the immersion time, the metal removal efficiency may be increased. However, in order to enable a cost-efficient method that can easily be scaled up, it is important to have a relatively short immersion time. Still, the immersion time must be long enough for sufficient removal of iron from lignin.

[0042] In a preferred embodiment of the method for preparing the purified lignin according to the present invention, the acidic aqueous solution is stirred during the immersion step. Any suitable stirring means as known by a person skilled in the art may be used. During the immersion step, iron is removed from lignin to the acidic aqueous solution so that purified lignin is obtained. The term “purified lignin” as used herein refers to a lignin material that comprises essentially only lignin, such as at least 99 wt% lignin based on the dry weight of the lignin material and less than 1 wt% of other components such as cellulose, and inorganic compounds, based on the dry weight of the lignin material. Further, the purified lignin obtained in step c) of the method has iron content of less than 5.0 ppm, preferably less than 4.0 ppm and more preferably less than 3.0 ppm, such as an iron content in the range of from 0.01 to 4.9 ppm, such as from 0.1 to 4.0 ppm or 0.1 to 3.0 ppm. As mentioned above, the lignin is not dissolved during the immersion step, which means that metals are removed from solid lignin and not from dissolved lignin. Thus, the purified lignin obtained in step c) of the method for preparing the purified lignin according to the present invention is also in solid form, such as in the form of particles.

[0043] The purified lignin according to the present invention has a weight average molecular mass in the range of from 8200 g / mol to 15000 g / mol, preferably in the range of from 8200 g / mol to 10000 g / mol, such as from 8200 g / mol to 9200 g / mol or from 8300 g / mol to 9000 g / mol or from 8400 g / mol to 8800 g / mol. The weight average molecular mass can be determined as described in Jacobs A. et al., Nordic Pulp & Paper Research Journal (2000), vol 15, p 120-127.

[0044] Step d) of the method for preparing the purified lignin according to the present invention involves separating the obtained purified lignin from the acidic aqueous solution. The purified lignin is in solid form during the separation. The term “separation” as used herein, refers to a process of separating the lignin from the acidic aqueous solution.

[0045] Preferably, the separation in step d) is performed using filtration. Alternatively, the separation may be performed by centrifuging or sedimentation or any other suitable means known by a person skilled in the art. As the purified lignin is separated from the acidic aqueous solution, the pH of the acidic aqueous solution and the lignin during the separation step will be the same as the pH during the immersion step. Thus, the pH of the aqueous acidic solution is below 6, preferably below 5, or more preferably below 4, during the step of separating the purified lignin. In one embodiment, the pH of the aqueous acidic solution may be in the range of from 1 to 6, preferably from 2 to 6, more preferably from 2 to 5, and most preferably from 2 to 4.

[0046] Steps c-d and optionally e) may be repeated at least one time. The separated lignin is in this embodiment immersed in an acidic aqueous solution a second time, before a second separation step. The pH of the acidic aqueous solution and the temperature and time during the immersion step are selected as discussed above. If the immersion and separation steps are repeated, parameters such as pH, temperature and time during the immersion may be the same in all the immersion steps or they may vary between different immersion steps. In one embodiment, the same parameters are used during two immersion steps. In one embodiment, the pH may be lower in a second immersion step than in a first immersion step. In another embodiment, the immersion time may be shorter in a second immersion step than in a first immersion step. In yet another embodiment, the temperature may be higher in a first immersion step than in a second immersion step. The total immersion time is the sum of the immersion times for the individual steps. The separation is preferably performed by filtration after each immersion step. The iron content of the lignin may be further reduced by adding additional immersion steps and by optimizing parameters such as time, temperature and pH of each step.

[0047] Step e) of the method involves optionally washing the separated purified lignin. In one embodiment, the separated lignin is subjected to washing with an aqueous washing solution. The aqueous washing solution is preferably water. In one embodiment, the separated purified lignin is washed with water until the pH of the water used for washing becomes neutral. In one embodiment, the method comprises an additional step of drying the separated, and optionally washed, purified lignin. The purified lignin may be dried after the separation step or after the optional washing step. The drying of the purified lignin may be carried out by methods and equipment known in the art. The temperature during the drying is preferably in the range of from 60°C to 160°C, more preferably in the range of from 100°C to 120°C. The drying may be performed under ambient pressure, reduced pressure or vacuum.

[0048] The purified lignin according to the present invention may have a total metal content in the range of from 0 to 200 ppm, such as from 0.1 to 200 ppm, preferably in the range of from 0 to 150 ppm, such as from 0.1 to 150 ppm and more preferably in the range of from 0 to 100 ppm, such as from 0.1 to 100 ppm.

[0049] Other inorganic impurities may also be removed from lignin to the acidic aqueous solution during the immersion step.

[0050] The purified lignin may be subjected to further treatments, such as various heat treatments.

[0051] The purified lignin according to the present invention is particularly suited for further conversion to carbon enriched materials intended for energy storage applications as well as for other applications where a lignin material having iron content of less than 50 ppm, preferably less than 30 ppm or less than 20 ppm is of interest. The conversion to carbon enriched materials can be carried out according to methods known in the art, such as through WO2021 / 250604.

[0052] The low iron content of the lignin, in combination with its relatively high weight average molecular mass, means that it is particularly suitable for further conversion to a carbon enriched material that can be used in energystorage applications. Examples

[0053] In all examples, softwood Kraft lignin powder obtained from the LignoBoost process was used.

[0054] The content of inorganics, such as aluminium, calcium, iron, potassium, magnesium, manganese, sodium and silicon, in the lignin samples (i.e. lignin powder subjected to various treatments) was evaluated with ICP-OES (Inductively Coupled Plasma-Optical Emission Spectrometry) according to Biorefinery Test Methods L5_2017 from RISE. The lignin samples were oxidized with hydrogen peroxide and subsequently wet digested in a microwave oven with nitric acid. The contents of inorganics were quantified by ICP-OES. The total metal content was calculated by summarizing the contents of individual metals. The elements in the lignin were determined after wet combustion and inductively coupled plasma (ICP) with optical emission spectroscopy detection. The wet digestion was done after oxidising 1g (dry weight) lignin with 2ml of 30% aqueous H2O2 for 10 minutes. Before the H2O2 addition the lignin was wetted with de-ionised water 2-7ml. After the oxidation 5ml of concentrated nitric acid was added and the suspension was heated in a microwave digestion unit to 180°C for 30 minutes, of which 15 min was the ramp time to reach 180°C. The produced lignin solution was transferred to the ICP -OES instrument and the concentration of elements in the lignin was determined by using external standard calibration.

[0055] The content of inorganics in the lignin powder starting material prior to any treatment (sample 0) was also evaluated with ICP analysis, see table 1.

[0056] Table 1 : Content of inorganics in kraft lignin powder, “Metal” refers to total metal content. Example 1 - comparative

[0057] Kraft lignin powder from the LignoBoost process (20g) was added to heated water (80g, 95°C) during stirring. Thereafter 0,02 g (0,1 wt% based on the dry weight of lignin added to the water) of sulfuric acid was added and the mixture was stirred at a maintained temperature for a time period of 1 hours. Subsequently the mixture was filtered, and lignin collected and dried at 40°C overnight in a convection oven. The experimental details are summarized in table 2 and the results from ICP analysis are summarized in table 3.

[0058] The molecular mass was determined.

[0059] Table 2: Experimental details for example 1

[0060] Table 3: Content of iron and weight average molecular mass in samples from example 1.

[0061] Example 2

[0062] Kraft lignin powder from the LignoBoost process (20 g) was added to heated water (80 g, 120°C) during stirring, in a closed pressurized vessel. Thereafter 0,02g (0,1 wt% based on the dry weight of lignin added to the water) of sulfuric acid was added and the mixture was stirred at a maintained temperature for a time period of 1 hours. Subsequently the mixture was filtered, and lignin collected and dried at 40°C in a convection oven overnight. The experimental details are summarized in table 4 and the results from ICP analysis are summarized in table 5.

[0063] Table 4: Experimental details for example 2 Table 5: Content of iron and weight average molecular mass in samples from example 2.

[0064] 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. Lignin having iron content of less than 5.0 ppm and a weight average molecular mass in the range of from 8200 to 15000 g / mol.

2. The lignin according to claim 1, wherein the lignin has an iron content of less than 4.0 ppm.

3. The lignin according to claim 2, wherein the lignin has an iron content of less than 3.0 ppm.

4. The lignin according to claim 1, wherein the lignin has an iron content in the range of from 0.1 to 4.0 ppm.

5. The lignin according to any one of claims 1-4, wherein the lignin has a weight average molecular mass in the range of from 8200 to 10000 g / mol.

6. The lignin according to claim 6, wherein the lignin has a weight average molecular mass in the range of from 8200 to 9200 g / mol.

7. The lignin according to claim 7, wherein the lignin has a weight average molecular mass in the range of from 8300 to 9000 g / mol.

8. The lignin according to any one of claims 1-7, wherein the lignin is Kraft lignin.

9. The lignin according claim 8, wherein the lignin is Kraft lignin from softwood.

10. Carbon enriched material produced from the lignin according to any one of claims 1-9.

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