Method for obtaining purified lignin

Abstract

The present invention relates to a method for obtaining purified lignin, wherein the method comprises the steps of: providing black liquor; removing extractives from the black liquor; isolating lignin from the black liquor and dissolving the obtained isolated lignin in a lignin solvent so as to obtain a lignin solution. The method further comprises a step of removing additional extractives from the black liquor obtained prior to isolation of lignin, from the obtained isolated lignin and / or from the lignin solution, so as to obtain purified lignin.

Description

[0001] METHOD FOR OBTAINING PURIFIED LIGNIN

[0002] Field of the invention

[0003] The present invention relates to a method for obtaining purified lignin, wherein additional extractives are removed from lignin or from a solution comprising dissolved lignin so as to obtain purified lignin comprising low amounts of extractives. The obtained purified lignin is suitable for use in high-end applications where lignin of high purity is required.

[0004] Background

[0005] Lignin is an aromatic polymer, which is a major constituent in e.g. wood and one of the most abundant carbon sources on earth. In recent years, with development and commercialization of technologies to extract lignin in a 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 WG2006031175 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. Black liquor contains not only lignin, but also a number of other inorganic or organic substances. The inorganic substances are mainly impurities originating from wood or from the cooking chemicals used during the pulping process. The organic substances include hemicellulose and cellulose residues as well as extractives. The extractives are mainly resin acids and fatty acids. The amount of extractives in the black liquor varies depending on for example the type of wood used as well as on the season when the wood was harvested. The amount of extractives in black liquor is normally rather high, such as in the range of from 35 to 55 kg / ton of black liquor. As the black liquor is alkaline, the extractives are neutralized and converted to their sodium salts. When increasing the solid content of the black liquor, the salts will become insoluble and as the density of the salts is lower than that of the black liquor, they will float to the surface forming a layer typically referred to as resin soap. The resin soap is subsequently removed from the black liquor by processes such as decantation, skimming and / or centrifuging, prior to isolation of lignin from the black liquor. After removal, the extractives may be used for e.g. production of tall oil and turpentine.

[0008] In many potential high-end applications, it is important that the lignin used is as pure as possible. Any impurities present may have an impact on the properties of end products derived from the lignin, and also have a negative impact on the processability of lignin.

[0009] Thus, there is a need for a method for purifying lignin where the obtained purified lignin has a content of impurities that is sufficiently low so that the purified lignin has an improved processability and so that it can be used in high-end applications where a material of high purity is required. In addition, the method should preferably not have a negative impact on the yield of lignin, nor should it involve fractionation of the lignin.

[0010] Summary of the invention

[0011] It is an object of the present invention to provide an improved method for obtaining purified lignin, which method eliminates or alleviates at least some of the disadvantages of the prior art methods.

[0012] It is a further object of the present invention to provide a method for obtaining purified lignin, wherein the obtained lignin has a high purity so that it can be used also in high-end applications.

[0013] It is a further object of the present invention to provide a method for obtaining purified lignin which method does not have a negative impact on the yield of the lignin, or that involves fractionation of the lignin. The above-mentioned object, as well as other objects as will be realized by the person skilled in the art in light of the present invention, are achieved by the various aspects of the present invention.

[0014] According to a first aspect, the present invention is directed to a method for obtaining purified lignin, wherein the method comprises the steps of: a) providing black liquor; b) removing extractives from the black liquor; c) isolating lignin from the black liquor; d) dissolving the obtained isolated lignin in a lignin solvent so as to obtain a lignin solution; wherein the method further comprises a step of removing additional extractives so as to obtain purified lignin; and wherein the additional extractives are removed from the black liquor obtained in step b) prior to isolating lignin from the black liquor; from the isolated lignin obtained in step c; and / or from the lignin solution obtained in step d).

[0015] Extractives are commonly removed from black liquor by processes such as decanting, skimming and / or centrifuging. A large fraction of the extractives present in the black liquor is removed by such processes, but as the processes are rather crude, additional extractives will remain in the black liquor. Further, a fraction of the extractives may be dissolved in the black liquor, and these will not be removed by the above-mentioned processes.

[0016] The inventive method according to the first aspect is based on the surprising realisation that a purified lignin having an improved processability can be obtained by removing additional extractives from black liquor prior to isolation of lignin, from isolated lignin, and / or from a lignin solution comprising dissolved isolated lignin. If not removed, the remaining additional extractives may decrease the quality of the isolated lignin, for example in terms of its processability. If the amount of extractives in the isolated lignin is too high, problems during further processing of the dissolved lignin may occur. For example, there may be problems with phase separation during mixing of the dissolved lignin with other components. In addition, there may be problems with excessive foaming during mixing of lignin solutions comprising too high amounts of extractives. The total amount of extractives present in the lignin is thus further reduced by the inventive method and as a result the processability of a solution comprising the dissolved lignin is improved. Additional extractives can be removed from the black liquor, so that the black liquor is subjected to two steps of removing extractives. After isolation of lignin from the black liquor, purified lignin is obtained. Alternatively, additional extractives can be removed from lignin after the lignin has been isolated from black liquor, so that purified lignin is obtained. Additional extractives can also be removed from a solution comprising dissolved lignin, where the dissolved lignin is lignin that has been isolated from black liquor. After removal of additional extractives, purified (dissolved) lignin is obtained. It is also possible to remove additional extractives at more than one stage. The obtained purified lignin is suitable for use in high-end applications requiring lignin of high purity.

[0017] Detailed description

[0018] The term “black liquor” as used herein refers to the liquid remaining after digestion of wood and separation of cellulose fibres in an alkaline chemical pulping process, such as a kraft process. The black liquor is strongly alkaline, such that the black liquor has a pH value above 13.

[0019] The term "lignin" as used herein refers to any kind of lignin. Examples of said lignin are, but are not limited to, lignin obtained from vegetable raw material such as wood, e.g. softwood lignin, hardwood lignin, and lignin from annular plants. The lignin in the present invention is obtained by isolation from alkaline black liquor. Preferably, lignin used in the method of the present invention is kraft lignin, i.e. lignin obtained through the kraft process. Preferably, the kraft lignin is obtained from hardwood or softwood. The lignin may also be obtained from processes such as steam explosion or acidic pre-treatment followed by enzymatic hydrolysis.

[0020] Lignin may be isolated from black liquor for example by means of filtration or precipitation. Preferably, lignin is isolated from black liquor by means of precipitation.

[0021] The term “precipitation” as used herein refers to a process of isolating lignin from black liquor. When the pH of the black liquor is decreased so that it becomes less alkaline, dissolved lignin will become less soluble and precipitate. Precipitated lignin forms aggregates that can be removed from the black liquor, typically by filtration. Depending on factors such as temperature and ionic strength of the black liquor, molecular weight of lignin, charged groups on the lignin, concentration of lignin in the black liquor and type of lignin, lignin will precipitate at different pH values.

[0022] The term “acidifying” as used herein refers to a process of lowering the pH value of a solution, in this case the alkaline black liquor. The pH of the solution may however still be alkaline, i.e. above 7, even after the acidifying process.

[0023] The term “extractives” as used herein refers to organic species that are extracted from wood during the kraft pulping process and that are subsequently present in the black liquor. The organic species are mainly resin acids and fatty acids. For example, the extractives may comprise about 90 wt% resin acids, and about 10 wt% fatty acids. In addition, the extractives may comprise small amounts of other compounds, such as sterols. The amount and type of organic species present in the extractives may vary depending on the type of wood used and also on the processing conditions. It is intended herein that the term “extractives” covers both the acids and their corresponding salts.

[0024] The term “resin acid” as used herein refers to a mixture of several carboxylic acids present in tree resins. During the pulping process, the resin acids will be extracted from wood during separation of cellulose fibres from the wood.

[0025] According to a first aspect, the present invention is directed to a method for purifying lignin. Step a) of the method according to the first aspect involves providing black liquor. The solid content of the black liquor provided in step a) is typically 30-40 wt% and the black liquor is alkaline, such that the black liquor has a pH value above 13. Black liquor comprises dissolved lignin, that can be isolated from the black liquor by several different means, such as by precipitation or filtration. In addition to lignin, the black liquor also comprises other species from wood such as hemicellulose, extractives and cellulose fractions, as well as inorganics. Roughly one-third of the black liquor is composed of inorganics, whereas the remaining two-thirds are composed of organic material.

[0026] Step b) of the method according to the first aspect involves removing extractives from the black liquor. The extractives present in the black liquor will be neutralized by the strongly alkaline black liquor and be converted to their corresponding salts. The salts have a lower density than the black liquor, and when the solid concentration of the black liquor is increased, a large portion of the salts will become insoluble and will float to the surface of the black liquor, from where they can subsequently be removed.

[0027] In one embodiment, extractives are removed from the black liquor by means of decanting, skimming and / or centrifuging the black liquor. Preferably, extractives are removed by means of decanting and / or skimming the black liquor. Typically, these processes are carried out when the solid content of the black liquor has been increased and the extractives have become insoluble. The processes are carried out using any suitable equipment as known by a person skilled in the art.

[0028] Step c) of the method according to the first aspect involves isolating lignin from the black liquor. Lignin may be isolated from the black liquor by means of precipitation and / or filtration, preferably by means of precipitation.

[0029] In one embodiment, lignin is isolated from the black liquor by acidifying the black liquor, such that the lignin is precipitated. In one embodiment, the black liquor is acidified to a pH in the range of from 9 to 11 . By lowering the pH to a value in the range of from 9 to 11 , a large fraction of lignin in the black liquor is precipitated. Any suitable acid can be used for acidifying the black liquor. In one embodiment, acidifying the black liquor is carried out by adding carbon dioxide to the black liquor.

[0030] After precipitation, the method preferably comprises a step of separating the obtained precipitated lignin from the acidified black liquor. In the separation process, the precipitated lignin is collected and can be further treated or processed. The separation process may involve filtration of the acidified black liquor, such that the precipitated lignin forms a filter cake. A filter press or any other suitable equipment as known by a person skilled in the art may be used for the filtration. Alternatively, separation may involve centrifuging.

[0031] After separation, the method preferably comprises a step of washing the precipitated lignin with an acidic solution so as to remove impurities, such as inorganic impurities, from the precipitated lignin. In addition, any remaining black liquor is also removed from the precipitated lignin. Preferably, the acidic solution has a pH value in the range of from 2 to 4. The acidic solution may comprise any suitable acid, such as sulfuric acid.

[0032] After washing, the method may comprise an additional step of separating the washed precipitated lignin from the acidic solution. The separation process may involve filtration of the acidified black liquor, such that the precipitated lignin forms a filter cake. A filter press or any other suitable equipment as known by a person skilled in the art may be used for the filtration. Alternatively, separation may involve centrifuging. After separation, additional washing steps may be carried out. The precipitated lignin may also be subjected to other treatments such as drying and pulverization.

[0033] Step d) of the method according to the first aspect involves dissolving the obtained isolated lignin so as to obtain a lignin solution. Essentially all the isolated lignin is dissolved in this step. The isolated lignin may be dissolved in any suitable solvent, such as a lignin solvent. The term “lignin solvent” as used herein refers to any solvent that can be used to dissolve lignin. The lignin solvent may for example be selected from at least one of tertiary amine oxides, ionic liquids, aprotic polar solvents, polar protic solvents, dimethylformamide and dimethylacetamide. In some embodiments, only one solvent is used when dissolving the isolated lignin. In other embodiments, at least two solvents are used when dissolving the isolated lignin. In one embodiment, the lignin solvent is an aqueous solution of NaOH.

[0034] Preferably, the isolated lignin is mixed with the solvent(s) in order to dissolve the lignin in the solvent and obtain a lignin solution. Any suitable equipment as known by a person skilled in the art may be used, such as impeller mixers with different geometries, rotor-stator mixers with different mixing elements and various dissolver discs.

[0035] The solvent may be heated during the step of dissolving the obtained isolated lignin in order to facilitate dissolution of lignin. In embodiments where the solvent is water, or where one of the at least two solvents is water, the pH may be adjusted such that the pH of the lignin solution is alkaline. This facilitates dissolution of lignin in the lignin solution. The pH may be adjusted for example by addition of NaOH. The method according to the first aspect further comprises a step of removing additional extractives so as to obtain purified lignin. Additional extractives are removed from the black liquor obtained in step b) prior to isolating lignin from the black liquor; from the isolated lignin obtained in step c; and / or from the lignin solution obtained in step d).

[0036] The term “additional extractives” as used herein refers to extractives that have not been removed from the black liquor during the earlier step of removing extractives. Since the processes, for example decanting, used for removing extractives from black liquor are typically crude, a certain amount of additional extractives will remain in the black liquor. These additional extractives may end up in lignin isolated from the black liquor and have an impact on the processability of the lignin.

[0037] The term “purified lignin” as used herein refers to lignin comprising a reduced amount of additional extractives. Due to the low amount of extractives present in the lignin, the processability of the purified lignin is increased, in particular when the isolated lignin has been dissolved so as to obtain a lignin solution.

[0038] In some embodiments, additional extractives are removed only at one point during the method, such as only from the black liquor, or only from the lignin solution, or only from the isolated lignin. From a process-economy perspective, it may be advantageous to remove additional extractives at only one point during the processing, as additional process steps and equipment is only needed at one stage.

[0039] In some embodiments, additional extractives are removed from the black liquor prior to isolation, or from the lignin solution.

[0040] In some embodiments, additional extractives are removed at more than one point during the method, such as both from the black liquor and from the lignin solution, or from both the black liquor and from the isolated lignin, or from both the isolated lignin and the lignin solution.

[0041] In some embodiments, additional extractives are removed from the black liquor, from the isolated lignin, and from the lignin solution. The total amount of additional extractives that are removed may be increased by removing additional extractives at two or three points during the process. Thus, the total amount of extractives in the obtained purified lignin may be further decreased.

[0042] In a preferred embodiment, additional extractives are removed from the lignin solution. In addition, additional extractives may also be removed from the black liquor and / or from the isolated lignin. It has been found that it is typically most efficient to remove the additional extractives from the lignin solution. Removing additional extractives from the isolated lignin obtained in step c) is often inconvenient since the lignin is in solid form and the extractives may be trapped within the lignin, rendering removal more difficult. The number of useful removal means are also fewer, resulting in a less versatile process. Removing additional extractives from the black liquor may also prove difficult since the black liquor comprises a large number of different compounds. Thus, removal of additional extractives from the black liquor is less efficient due to the other compounds interfering with the removal process, and the obtained lignin may be less pure. Therefore, by removing additional extractives from the lignin solution, a simple and efficient process for obtaining purified lignin is enabled.

[0043] Although for reasons described above it is typically more advantageous to remove additional extractives from the lignin solution, in embodiments where the additional extractives are removed from the lignin solution, the purity of the obtained lignin may be further improved by also removing additional extractives from the black liquor and / or isolated lignin.

[0044] After removal of additional extractives, the purified lignin will contain a lower total amount of extractives compared to lignin prepared from the same black liquor but using a method where additional extractives have not been removed. The term “total amount of extractives” as used herein refers to all extractives in the obtained lignin.

[0045] By removal of additional extractives, the processability of the lignin solution obtained in step d) is improved. As less extractives are present, foaming of the lignin solution during further processing is reduced. This is an advantage for several processing steps such as pumping, spraying, mixing, coating, absorption and adsorption desorption and distillation. In addition, when mixing the lignin solution with other components, high amounts of extractives may have a negative effect on the mixing, so that e.g. phase separation occurs. Such phase separation will have a negative influence on the properties of the final mixture.

[0046] In one embodiment, the yield of lignin in the step of removing additional extractives is at least 90%, such as at least 95%, or at least 99%, or from 90-100%, or from 95- 100%. In embodiments where lignin is present in a dissolved state during the step of removing additional extractives, lignin is not removed along with the extractives to a great extent, but instead largely remain in the black liquor or lignin solution. In embodiments where lignin is instead in solid form, such as when lignin has been isolated from black liquor, during the step of removing additional extractives, the solid lignin is not dissolved and / or dispersed to any great extent during removal of extractives, but instead remains largely in the solid state. It is important to have a high yield in order to have an efficient process and to recover as much lignin as possible.

[0047] In one embodiment, the molar mass distribution of the lignin remains essentially the same after the step of removing additional extractives. The term “remains essentially the same” as used herein refers to that the molar mass distribution of lignin is not significantly altered by carrying out the step of removing additional extractives. Thus, the molar mass distribution of the lignin after the step of removing additional extractives is the same, or essentially the same, as the molar mass distribution of lignin prior to the step of removing additional extractives. This means that no specific fractions of lignin are removed by the method used for removing additional extractives, i.e. that the lignin is not fractionated, and the lignin yield is therefore maximized. In black liquor, the molar mass distribution of the dissolved lignin is broad. If as much lignin as possible is to be recovered from the black liquor, it is of importance that any applied treatments do not remove lignin of a certain molar mass fraction from the black liquor. Also, in some applications it is useful to have a broad molar mass distribution of lignin and for that reason it is important that the molar mass distribution of the lignin remains essentially the same also after the step of removing additional extractives. The same reasoning applies when the additional extractives are removed from a lignin solution.

[0048] In one embodiment, the additional extractives are removed by means of adsorption, foam fractionation, centrifugation and / or solvent extraction. Preferably, the additional extractives are removed by means of adsorption, foam fractionation and / or solvent extraction. Additional extractives may be removed from the black liquor, from the lignin solution and / or from the isolated lignin using only one means, or by a combination of several means. For example, the additional extractives may be removed from the lignin solution first by centrifuging the lignin solution, followed by solvent extraction. By using more than one means to remove additional extractives, a larger amount of additional extractives may be removed, resulting in lignin containing a lower amount of additional extractives.

[0049] In embodiments where additional extractives are removed at more than one point during the method, the additional extractives may be removed at the different points by the same means, or by different means. Additional extractives may also be removed at one point by more than one means. For example, additional extractives may at a first point be removed from black liquor using adsorption, and at a second point be removed from the lignin solution by centrifugation followed by solvent extraction.

[0050] In some embodiments, removal of additional extractives involves adsorption. Removal using adsorption is particularly suitable when the additional extractives are removed from the black liquor and / or the lignin solution. The adsorption is typically carried out such that the additional extractives are adsorbed from the black liquor and / or lignin solution into a matrix for which they have a high affinity. The matrix is chosen such that lignin does not have a high affinity for it and the lignin will thus remain dissolved in the lignin solution or in the black liquor. In one embodiment, additional extractives are removed by adsorption into a nonpolar matrix. Any suitable nonpolar matrix may be used, such as polypropylene. The matrix may be the filter media of a filtration device. In one embodiment, removal of additional extractives involves filtration of the black liquor and / or the lignin solution through a filter media into which additional extractives are adsorbed. Thus, the amount of additional extractives in the black liquor and / or lignin solution is decreased.

[0051] In some embodiments, removal of additional extractives involves foam fractionation. Removal using foam fractionation is particularly suitable when additional extractives are removed from the black liquor and / or the lignin solution. The additional extractives will during foam fractionation be removed from the black liquor and / or lignin solution to a foam. In some embodiments, removal of additional extractives involves centrifugation. Removal using centrifugation is particularly suitable when additional extractives are removed from the black liquor and / or the lignin solution. The additional extractives will during centrifugation be separated as a particular fraction based on their density difference compared to the other black liquor components.

[0052] In some embodiments, removal of additional extractives involves solvent extraction. Removal using solvent extraction is suitable when additional extractives are removed from black liquor, isolated lignin, and / or the lignin solution. The additional extractives will during solvent extraction be extracted from the black liquor, the isolated lignin and / or the lignin solution to a solvent for which the additional extractives have a high affinity. In one embodiment, additional extractives are removed by means of solvent extraction using a nonpolar solvent, such as hexane or heptane.

[0053] The method according to the first aspect may comprise an additional step of removing fibre residues from the black liquor and / or from the lignin solution. The term “fibre residue” as used herein refers to small fragments of cellulose fibres originating from the pulping process. The fibre residues are present in the black liquor and may during isolation of lignin from the black liquor end up in the lignin. Fibre residues may thus be present also in a lignin solution obtained by dissolving the isolated lignin. Remaining fibre residues in the lignin will constitute an impurity which is undesired when the lignin is to be used in high-end applications. If present in a lignin solution, fibre residues may cause problems with clogging of equipment during processing. By removal of fibre residues, the processability of lignin is thus improved further.

[0054] In some embodiments, fibre residues are removed from either one of the black liquor or the lignin solution. In some embodiments, fibre residues are removed from both the black liquor and from the lignin solution.

[0055] The step of removing fibre residues may be carried out by means of filtration, such as microfiltration, and / or centrifugation. Fibre residues are preferably removed by means of filtration, such as microfiltration. Fibre residues may be removed from the black liquor and / or the lignin solution using only filtration, or only centrifugation, or both. In embodiments where fibre residues are removed from both the black liquor and the lignin solution, removal may be by the same means, or different means.

[0056] In embodiments where fibre residues are removed by means of filtration, it is possible to remove additional extractives simultaneously with the fibre residues. In such embodiments, the filter media will act as the matrix into which the additional extractives are adsorbed. By combining removal of fibre residues and additional extractives, a more efficient process is obtained as only one additional process step is required.

[0057] In view of the above detailed description of the present invention, other modifications and variations will become apparent to those skilled in the art. However, it should be apparent that such other modifications and variations may be effected without departing from the spirit and scope of the invention.

Claims

Claims1 . A method for obtaining purified lignin, wherein the method comprises the steps of: a) providing black liquor; b) removing extractives from the black liquor; c) isolating lignin from the black liquor; d) dissolving the obtained isolated lignin in a lignin solvent so as to obtain a lignin solution; wherein the method further comprises a step of removing additional extractives so as to obtain purified lignin; and wherein the additional extractives are removed from the black liquor obtained in step b) prior to isolating lignin from the black liquor; from the isolated lignin obtained in step c); and / or from the lignin solution obtained in step d).

2. The method according to claim 1 , wherein extractives are removed from the black liquor in step b) by means of decanting, skimming and / or centrifuging the black liquor.

3. The method according to any one of the preceding claims, wherein lignin is isolated from the black liquor by acidifying the black liquor.

4. The method according to any one of the preceding claims, wherein the yield of lignin in the step of removing additional extractives is at least 90%.

5. The method according to any one of the preceding claims, wherein the molar mass distribution of the lignin remains essentially the same after the step of removing additional extractives.

6. The method according to any one of the preceding claims, wherein additional extractives are removed by means of adsorption, foam fractionation, centrifugation and / or solvent extraction.

7. The method according to any one of the preceding claims, wherein additional extractives are removed by means of solvent extraction using a nonpolar solvent.

8. The method according to any one of the preceding claims, wherein additional extractives are removed by means of adsorption into a nonpolar matrix.

9. The method according to any one of the preceding claims, wherein the method comprises an additional step of removing fibre residues from the black liquor and / or from the lignin solution.

10. The method according to claim 9, wherein the step of removing fibre residues is carried out by means of filtration and / or centrifugation.