Method for refining lignin

Abstract

The present invention relates to a method for purifying lignin, the method comprising the steps of providing an alkaline black liquor, removing inorganic impurities from the black liquor to obtain purified black liquor, acidifying the purified black liquor to precipitate lignin, separating the precipitated lignin from the acidified purified black liquor, mixing the precipitated lignin with an acidic solution to remove inorganic impurities from the precipitated lignin to obtain purified lignin, and separating the resulting purified lignin from the acidic solution. The method according to the present invention reduces the content of inorganic impurities in the purified lignin.

Description

[Technical field]

[0001] The present invention relates to a method for purifying lignin which involves removing inorganic impurities (e.g. metals and other inorganics) from black liquor prior to precipitation of lignin from the black liquor. The resulting purified lignin has low levels of inorganic impurities and is suitable for use in applications requiring pure lignin. [Background technology]

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

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

[0004] Black liquor is a cost-effective source of lignin since it is readily available as a by-product from the Kraft process. Black liquor contains a certain amount of inorganic impurities, mostly from the wood and from the cooking chemicals used during the pulping process. It is believed that inorganic impurities may be present in black liquor as dissolved ions, precipitated in the form of salts, or complexed with lignin. However, the exact form and distribution of all inorganic impurities is not known. Inorganic impurities will also be present in the lignin precipitated from the black liquor. For many possible high-end applications (e.g. bio-binders and bio-fuels) and for converting lignin into carbon-rich materials (carbon fibers and carbon powders), it is important that the lignin used is as pure as possible, especially in relation to the amount of inorganic impurities.

[0005] Some inorganic impurities, such as sodium and potassium, can be largely removed from the precipitated lignin by acid washing during the separation process, however, other inorganic impurities (e.g. aluminum, calcium, iron, manganese, magnesium, zinc, silicon and phosphorus) are more difficult to remove from the precipitated lignin, and thus the total content of inorganic impurities in the lignin obtained after washing may still be too high for many high-end applications.

[0006] Various attempts have been made to remove metals and other impurities from lignin, but what these methods have in common is that they are quite complicated, involving many additional steps, and / or are not efficient enough in their removal efficiency.

[0007] WO2020013752 A1 discloses a method for dissolving lignin in an acidic aqueous solution. A two-phase system is obtained by phase separation, where one phase is lignin-rich and the other phase is lignin-poor, and this other phase contains metal cations extracted from the lignin. However, many additional steps are required.

[0008] Thus, there is a need for a method of lignin purification where the resulting purified lignin has a low enough total inorganic impurity content so that the purified lignin can be used in applications where high purity material is required. In addition, the method should not increase the complexity and desirably does not increase the cost of the lignin production process and desirably does not introduce new or additional chemicals into the lignin production process.

[0009] Summary of the Invention It is an object of the present invention to provide an improved process for obtaining refined lignin, which at least partially eliminates or mitigates the disadvantages of the processes according to the prior art.

[0010] It is a further object of the present invention to provide a method for obtaining refined lignin, the inorganic impurity content of which is sufficiently low so that it can be used in applications requiring low amounts of inorganic impurities, such as starting materials for carbon-rich materials intended for use in energy storage applications.

[0011] It is a further object of the present invention to provide a method for obtaining purified lignin without increasing the complexity of the lignin production process.

[0012] These and other objects are accomplished by the various aspects of the present invention, as will be appreciated by those skilled in the art upon access to the invention.

[0013] According to a first aspect, the present invention relates to a method for obtaining refined lignin, said method comprising the following steps: a) providing alkaline black liquor; b) removing inorganic impurities from the black liquor to obtain purified black liquor; c) acidifying the refined black liquor to a pH value in the range of 9 to 11 to precipitate lignin; d) separating the precipitated lignin from the acidified refined black liquor; e) mixing the precipitated lignin with an acidic solution to remove inorganic impurities from the precipitated lignin to obtain purified lignin; and f) Separating the resulting purified lignin from the acidic solution. Includes.

[0014] The method of the invention according to a first aspect is based on the surprising finding that a purified lignin with a reduced content of inorganic impurities is obtained by removing inorganic impurities from black liquor before precipitating the lignin from said black liquor. Due to the reduced content of inorganic impurities in the black liquor, the amount of such species that can be enriched in the lignin during precipitation is also reduced. After precipitation of the lignin and its subsequent separation, the purified lignin obtained is mixed with an acidic solution in order to remove further inorganic impurities from the precipitated lignin. After separation, a purified lignin is obtained. In particular, the amount of inorganic impurities that are usually difficult to remove from lignin (e.g. aluminum, calcium, iron, magnesium, manganese, zinc, phosphorus and silica) can be reduced by the method according to the invention. The purified lignin thus obtained with a reduced content of inorganic impurities is suitable for use in applications where pure lignin is required.

[0015] Detailed Description Throughout this specification, the term "lignin" is intended to refer to all kinds of lignin. Examples of such lignins are, but are not limited to, lignins obtained from plant materials, such as wood (e.g., softwood lignin, hardwood lignin, and lignins from annular plants). The lignins in the present invention are obtained by precipitation from alkaline black liquor. The lignins used in the process of the present invention are preferably Kraft lignins, i.e. lignins obtained by the Kraft process. Kraft lignins are preferably obtained from hardwoods or softwoods. Lignins are also obtained from processes such as steam explosion or enzymatic hydrolysis followed by acidic pretreatment.

[0016] As used herein, the term "precipitation" refers to the process of separating lignin from black liquor. As the pH of the black liquor decreases and becomes less alkaline, dissolved lignin becomes less soluble and will precipitate. The precipitated lignin forms aggregates that can be separated from the black liquor, usually by filtration. Depending on factors such as the temperature and ionic strength of the black liquor, the molecular weight of the lignin, the charged groups on the lignin, the concentration of lignin in the black liquor, and the type of lignin, lignin will precipitate at various pH values.

[0017] As used herein, the term "acidification" refers to the process of lowering the pH value of a solution (in this case, alkaline black liquor). However, the pH of the solution may still be alkaline, i.e., greater than 7, even after the acidification process.

[0018] According to a first aspect, the present invention relates to a method for lignin purification. Step a) of the method according to the first aspect involves providing an alkaline black liquor. As used herein, the term "black liquor" refers to the liquid remaining after the decomposition of wood and the separation of cellulose fibers in an alkaline chemical pulping process (e.g. the Kraft process). Black liquor is strongly alkaline, the pH of the black liquor being greater than 13. The solids content of the black liquor provided in step a) is typically 30-40% by weight. The black liquor contains dissolved lignin, which can be separated from the black liquor by several different means, e.g. precipitation and filtration. In addition to lignin, the black liquor also contains other extractives from wood, e.g. hemicellulose, fatty acids, cellulose fractions, but also minerals. Approximately one third of the black liquor is composed of minerals, whereas the remaining two thirds is composed of organic matter.

[0019] As used herein, the term "inorganic impurities" refers to inorganic elements, such as metals and other inorganic species, present in black liquor. The amount of inorganic impurities in black liquor can vary depending, for example, on the wood feedstock and pulping process used. Black liquor typically contains high levels of sodium and potassium, and relatively small amounts of metals such as aluminum, calcium, iron, magnesium, manganese, and zinc. Black liquor may also contain trace amounts of other metals. In addition to metals, black liquor typically contains minerals such as phosphorus and silicon.

[0020] Step b) of the method according to the first aspect involves removing inorganic impurities from black liquor to obtain refined black liquor. As used herein, the term "refined black liquor" refers to black liquor from which inorganic impurities have been removed. Thus, the total content of inorganic impurities in refined black liquor is lower than the total content of inorganic impurities in the original black liquor. In one embodiment, the amount of inorganic impurities (e.g., aluminum, calcium, iron, magnesium, manganese, phosphorus, silica, and zinc) in black liquor is reduced by the method according to the first aspect.

[0021] It is believed that during precipitation of lignin from black liquor, inorganic impurities present in the black liquor may be trapped in or complexed with the precipitated lignin, and thus remain in the lignin agglomerates after precipitation. Thus, the inorganic impurities may be enriched in the lignin during precipitation. By removing the inorganic impurities from the black liquor prior to precipitation of the lignin, the amount of inorganic impurities in the precipitated lignin may be reduced.

[0022] In one embodiment, the step of removing inorganic impurities from the black liquor involves microfiltration or ultrafiltration of the black liquor. By filtering the black liquor through a filter, inorganic impurities in the form of salt particles can be removed from the black liquor, thus resulting in purified black liquor. In a preferred embodiment, the step of removing inorganic impurities from the black liquor involves microfiltration of the black liquor.

[0023] In one embodiment, the step of removing inorganic impurities from the black liquor comprises contacting the black liquor with a pre-precipitated lignin fraction to remove inorganic impurities from the black liquor and transfer them to the pre-precipitated lignin. As used herein, the term "pre-precipitated lignin" refers to a particular lignin fraction that is precipitated from black liquor using a higher pH than that used to precipitate the residual lignin fraction. The pre-precipitated lignin will usually constitute a smaller fraction than the residual lignin fraction in the black liquor, depending on the pH during pre-precipitation. At a lower pH, more of the lignin fraction will be precipitated and the pre-precipitated lignin fraction will be larger. Generally, the pre-precipitated lignin will have a higher molecular weight than the lignin in the residual fraction.

[0024] In one embodiment, the pre-precipitation lignin fraction is obtained by acidifying black liquor to a pH value in the range of 11 to 13, wherein the pH value used to acidify the black liquor is higher than the pH value used to acidify the refined black liquor in step c) of the process according to the first aspect. Any suitable acid can be used to acidify the black liquor. In one embodiment, the acidification of the black liquor is carried out by adding carbon dioxide to the black liquor.

[0025] By contacting the black liquor with the pre-precipitated lignin fraction, the amount of inorganic impurities in the black liquor can be reduced. Without being bound to any theory, it is hypothesized that inorganic impurities in the form of solid salts can be separated from the black liquor along with the pre-precipitated lignin. Inorganic impurities in the black liquor can also be trapped within the pre-precipitated lignin during the pre-precipitation process itself. These inorganic impurities can be separated along with the pre-precipitated lignin fraction in order to remove them from the black liquor and obtain purified black liquor.

[0026] As used herein, the term "contacting" refers to the process of placing the pre-precipitated lignin fraction in contact with black liquor. By contacting the black liquor with the pre-precipitated lignin fraction, inorganic impurities are removed from the black liquor and transferred to the pre-precipitated lignin. Contacting can involve processes such as, for example, filtration and centrifugation.

[0027] In one embodiment, contacting the black liquor with the pre-precipitated lignin fraction comprises filtering the black liquor through the pre-precipitated lignin fraction. When filtering black liquor containing the pre-precipitated lignin fraction, the pre-precipitated lignin fraction will form a layer on the filter and the pre-precipitated lignin fraction will act as an additional filter for the black liquor.

[0028] The pre-precipitated lignin fraction may be precipitated from the black liquor prior to or simultaneously with the filtration of the black liquor, whereby a layer of the pre-precipitated lignin fraction is formed on the filter during the filtration of the black liquor after acidification. Alternatively, a pre-formed layer of the pre-precipitated lignin fraction may be applied to the filter prior to the acidification and filtration of the black liquor. The pre-formed layer of the pre-precipitated lignin fraction may be obtained from a previous separation process involving a pre-precipitation step. Following acidification of the black liquor, further pre-precipitated lignin will be deposited on the pre-formed layer on the filter during the filtration of the black liquor.

[0029] Removing inorganic impurities by contacting black liquor with the pre-precipitated lignin fraction provides an efficient method to reduce the content of inorganic impurities in black liquor since only commonly used equipment is used. Furthermore, no additional chemicals are required. This allows for easy integration with existing process lines.

[0030] In one embodiment, the step of removing inorganic impurities from the black liquor involves centrifuging the black liquor. By centrifuging the black liquor, inorganic impurities in the form of salt particles can be removed from the black liquor, thus resulting in purified black liquor.

[0031] It is also within the scope of the present invention to use more than one means for removing inorganic impurities from the black liquor. Thus, the means defined above can be combined in any suitable combination. For example, the black liquor can be first filtered by microfiltration, followed by a step of contacting the black liquor with the pre-precipitated lignin fraction.

[0032] Step c) of the method according to the first aspect involves acidifying the refined black liquor to a pH in the range of 9-11 to precipitate lignin. In this step, the pH is lowered to a lower pH than in the preliminary precipitation step, as further described below. Typically, therefore, a larger amount of lignin will be precipitated in step c) compared to the preliminary precipitation step (if performed). By lowering the pH to a value in the range of 9-11, a larger fraction of the lignin in the refined black liquor will be precipitated. Any suitable acid can be used to acidify the refined black liquor. In one embodiment, the acidification of the refined black liquor is performed by adding carbon dioxide to the black liquor.

[0033] Step d) of the method according to the first aspect involves separating the precipitated lignin from the acidified refined black liquor. As used herein, the term "separation" refers to a process of separating precipitated lignin from a liquid, for example from the acidified refined black liquor. In the present invention, refined lignin is also separated from the acidic solution, as further described below in connection with step f). In the separation process, precipitated lignin is recovered and may be further treated or processed. In a preferred embodiment, the separation involves filtration of the acidified refined black liquor. A filter press or other suitable equipment known to a person skilled in the art can be used for filtration. In this embodiment, the precipitated lignin forms a filter cake. In an alternative embodiment, the separation involves centrifugation.

[0034] Step e) of the method according to the fifth aspect involves mixing the precipitated lignin with an acidic solution and removing inorganic impurities from the precipitated lignin to obtain a purified lignin. Furthermore, any remaining black liquor is also removed from the precipitated lignin. In a preferred embodiment, the acidic solution has a pH value in the range of 2 to 4. The acidic solution may comprise any suitable acid. In one embodiment, the acidic solution comprises sulfuric acid. When the precipitated lignin is mixed with the acidic solution, further inorganic impurities that were not removed by purification of the black liquor in step b) are removed from the precipitated lignin and transferred to the acidic solution. The amount of inorganic impurities in the precipitated lignin is thus reduced. In particular, the amount of water-soluble inorganic impurities (e.g. potassium and sodium) is reduced in step e).

[0035] The combination of removing inorganic impurities from the black liquor and mixing the precipitated lignin with an acidic wash liquor results in a purified lignin having a low total inorganic impurity content that can be used in applications requiring a low inorganic impurity content, such as in biofuels and as a starting material for carbon-rich materials, such as carbon fibers and carbon powders.

[0036] In the process according to the invention, the content of inorganic impurities in the lignin is first reduced by removing the inorganic impurities from the black liquor, so that the lignin precipitated from the refined black liquor has a lower total inorganic impurity content than the lignin precipitated from the unrefined black liquor. In particular, the content of certain inorganic impurities (e.g. aluminum, calcium, iron, magnesium, manganese, zinc, phosphorus and silica) that are difficult to remove once the lignin has been precipitated is reduced in the lignin precipitated from the refined black liquor.

[0037] The content of inorganic impurities in the precipitated lignin is further reduced by mixing the precipitated lignin with an acidic solution, which in particular reduces the content of water-soluble inorganic impurities (e.g., potassium and sodium) in the precipitated lignin. Thus, the content of inorganic impurities in lignin precipitated from refined black liquor and subsequently mixed with an acidic solution is lower than the content of inorganic impurities in lignin precipitated from unrefined black liquor and subsequently mixed with an acidic solution.

[0038] In one embodiment, the refined lignin obtained by the process according to the first aspect contains lower amounts of aluminum, calcium, iron, magnesium, manganese, zinc, phosphorus, and silica compared to lignin precipitated from black liquor where inorganic impurities have not been removed prior to lignin precipitation.

[0039] As used herein, the term "total inorganic impurity content" refers to the total content of inorganic impurities (e.g., metals and other inorganic species) present in the lignin. The content of each inorganic species is determined by elemental analysis (e.g., inductively coupled plasma optical emission spectrometry (ICP-OES). The total inorganic impurity content is determined by summing all of the individual inorganic species.

[0040] Step f) of the method according to the first aspect involves separating the resulting refined lignin from the acidic wash solution. In one embodiment, the separation involves filtration of the acidic solution to isolate the refined lignin from the aqueous acidic solution. In an alternative embodiment, the separation involves centrifugation.

[0041] In one embodiment, the separation of the precipitated lignin in step d) and / or the separation of the purified lignin in step f) is carried out by filtration. A filter press or any other suitable device known to the skilled person can be used for filtration.

[0042] The resulting purified lignin can be subjected to various treatments, such as washing, drying, pulverizing and heating after separation from the acidic solution. In one embodiment, the resulting purified lignin is washed after separation.

[0043] In one embodiment, the resulting purified lignin is dried after separation. Drying can be performed by any suitable means known in the art. In one embodiment, the dried purified lignin is pulverized to obtain a lignin powder.

[0044] In one embodiment, the refined lignin is subjected to heat treatment. By heat treatment at high temperature, carbon-rich material can be obtained. The carbon-rich material obtained from the refined lignin has a sufficiently low content of inorganic impurities so that it is suitable for use in energy storage applications.

[0045] Working Example Example 1 (Comparative Example) Black liquor from softwood kraft digesters (total dry solids content: 30.4%, pH: 13.4, and density: 1.22 g / cm 3 ) was used. Lignin was precipitated from black liquor by lowering the pH of the black liquor to 10.0 at a temperature of about 60° C. with carbon dioxide. The precipitated lignin was separated and recovered by filtration, and the content of each inorganic species in the precipitated lignin was determined gravimetrically. The precipitated lignin was then washed by mixing it with sulfuric acid having a pH value of 2.5 to obtain washed lignin. The content of each inorganic species in the washed lignin was determined using ICP-OES technique after oxidation in hydrogen peroxide followed by wet digestion with nitric acid in a microwave oven. The results from the measurements are summarized in Table 1.

[0046] Example 2 The pre-precipitated lignin fractions were obtained by lowering the pH of the black liquor to 11.2 or 11.8, respectively, by addition of carbon dioxide. The same black liquor as in Example 1 was used. The inorganic impurities were removed from the black liquor by filtration through a lignin cake formed from the pre-precipitated lignin at a feed pressure of 6 bar(g) and a temperature of 50-55°C. The main lignin fractions were subsequently precipitated from the refined black liquor, separated, recovered and washed as described in Example 1. The contents of the individual inorganic species in the precipitated lignin and in the washed lignin were measured by ICP-OES method as described in Example 1. The results from the measurements are summarized in Table 1.

[0047] Table 1: Content of inorganic impurities in lignin precipitated from black liquor (purified lignin) and in precipitated lignin after washing in an acidic aqueous solution (washed lignin). "Pre-precipitated at pH 11.2 / 11.8" refers to lignin precipitated from black liquor purified by filtration through a lignin fraction pre-precipitated at pH 11.2 / 11.8, respectively. TIFF2025511947000001.tif117170

[0048] These examples demonstrate that lignin precipitated from refined black liquor contains lower amounts of inorganic impurities (e.g., aluminum, calcium, magnesium, manganese, phosphorus, and silicon) that are otherwise difficult to remove from lignin by the methods of the present invention than lignin precipitated from unrefined black liquor.

[0049] In light of the above detailed description of the invention, other modifications and variations will be apparent to those skilled in the art. It should be apparent, however, that such other modifications and variations can be made without departing from the spirit and scope of the invention.

Claims

1. A method for purifying lignin, comprising the following steps: a) Steps to prepare alkaline black liquor, b) A step of removing inorganic impurities from the black liquor to obtain purified black liquor, c) A step of acidifying the purified black liquor to a pH value in the range of 9 to 11 to precipitate lignin. d) A step of separating the precipitated lignin from the acidified purified black liquor. e) A step of mixing the precipitated lignin with an acidic solution to remove inorganic impurities from the precipitated lignin and obtain purified lignin, and f) A step to separate the obtained purified lignin from the acidic solution. Methods that include...

2. The method according to claim 1, wherein the step of removing inorganic impurities from the black liquor is accompanied by microfiltration or ultrafiltration of the black liquor.

3. The method according to claim 1, wherein the step of removing inorganic impurities from black liquor includes contacting the black liquor with a pre-precipitated lignin fraction to remove the inorganic impurities from the black liquor and transfer them to the pre-precipitated lignin.

4. The method according to claim 3, wherein the pre-precipitated lignin fraction is obtained by acidifying the black liquor to a pH value in the range of 11 to 13, and the pH value used to acidify the black liquor is higher than the pH value used to acidify the purified black liquor in step c).

5. The method according to claim 3, wherein contacting the black liquor with a pre-precipitated lignin fraction includes filtering the black liquor through the pre-precipitated lignin fraction.

6. The method according to claim 1, wherein the step of removing inorganic impurities from the black liquor involves centrifuging the black liquor.

7. The method according to claim 1, wherein the separation of precipitated lignin in step d) and / or the separation of purified lignin in step f) is performed by filtration.