WO2013002708A1

WO2013002708A1 - Method for purifying lignin

Info

Publication number: WO2013002708A1
Application number: PCT/SE2012/050679
Authority: WO
Inventors: Niklas Garoff; Carl-Ola DANIELSSON; Kalle Ekman
Original assignee: Stora Enso Oyj
Priority date: 2011-06-30
Filing date: 2012-06-19
Publication date: 2013-01-03
Also published as: AR086801A1; SE1150609A1; UY34149A; TW201302278A

Abstract

A method for producing a purified lignin product by an enzymatic treatment, the method comprising the steps: providing a mixture comprising lignin adjusting and/or maintaining a pH value of said mixture to an optimum value for the enzymatic treatment; adding at least one enzyme to said mixture, wherein said at least one enzyme is directed at hydrolyzing carbohydrates present in said mixture; adjusting and/or maintaining the mixture to/at an optimal working temperature, during an optimal reaction period of time for the enzymatic treatment; and terminating the enzymatic treatment.

Description

METHOD FOR PURIFYING LIGNIN

Technical field

The present disclosure relates to a method for purifying lignin at mild conditions and thereby minimizing the risks for changes in the lignin's chemical structure. More particularly, the present disclosure relates to a method by enzymatic treatment, a lignin product obtainable by such a method and the use of that lignin product.

Background

Wood mainly consists of cellulose, hemicelluloses and lignin. The cellulose is the main constituent of wood. Cellulose is a linear polymer of β- glucopyranoside resuidues conntected with 1 -4 β glucosodic bonds.

(Ljungberg Textbook - Pulp and Paper Chemistry and Technology Book 1 : Wood Chemistry and Wood Biotechnology, M. Ek, G. Gellerstedt, G.

Henriksson; editors, 92, ISSN 1654-1081 , 2007). Hemicellulose is a branched and more heterogeneous macromolecule than cellulose. Its main building blocks are hexoses, pentoses and certain uronic acids. Cellulose and most of the hemicelluloses are structural carbohydrates as they form the bulk of the plant's cell wall. (Ljungberg Textbook - Pulp and Paper Chemistry and Technology Book 1 : Wood Chemistry and Wood Biotechnology, M. Ek, G. Gellerstedt, G. Henriksson; editors, 105, ISSN 1654-1081 , 2007). Lignin is a very heterogeneous macromolecule comprised of three different monolignols that are connected via ether and carbon-carbon bonds to form a three- dimensional network. The biological function of lignin in a plant is to improve hydrophobicity and rigidity. The cellulose macromolecules form crystallites, so called microfibrills, which are the elementary building blocks of a wood fiber. In the wood fiber, these microfibrills are embedded in a matrix of

hemicellulose and lignin. Lignin forms covalent bonds to cellulose and especially hemicellulose through Lignin-Carbohydrate-Complexes, LCC. (Ljungberg Textbook - Pulp and Paper Chemistry and Technology Book 1 : Wood Chemistry and Wood Biotechnology, M. Ek, G. Gellerstedt, G.

Henriksson; editors, 143, ISSN 1654-1081 , 2007). These LCC make it difficult to completely remove lignin from fibers during pulping and bleaching.

Analogously, it can be stated that the LCC impede complete removal hemicellulose and other carbohydrates from lignin.

In chemical pulping wood chips are cooked at elevated temperatures and pressures with a mixture of different chemicals. Lignin and hemicellulose are dissolved and degraded in that process to obtain free cellulose fibers. The dominating chemical pulping process today is the Kraft process (also known as kraft pulping or sulfate process). The Kraft process is carried out with a solution containing sodium hydroxide and sodium sulfide, known as white liquor. After the cooking process the fibers are separated from the so called black liquor that mainly comprises spent cooking chemicals, lignin,

carbohydrates and degradation products formed during the Kraft process. The black liquor is then evaporated to higher dry solids content and burned in the recovery boiler to obtain high pressure steam and sodium sulfide. The lignin in the black liquor is the most important fuel component in black liquor. In the subsequent calcination cycle sodium hydroxide is recovered.

Different technologies exist for separating lignin from black liquor, for example through precipitation. One method of producing precipitated lignin is described in GB66481 1 , which document describes a process for carbonating an alkaline cooking liquor, and dissolving the separated lignin in water and acidifying the resulting solution for precipitation the lignin, the solution is then filtered to obtain the lignin product.

Another method of producing a precipitated lignin is described in WO2006/031 175. In this process the pH of the black liquor is lowered by treating it with CO₂, and the lignin is thereby precipitated. The precipitate is thereafter dewatered by filtering, and the filter cake is dissolved in acid wash water. The lignin containing slurry thereby formed is then once again dewatered and the cake once again dissolved in and washed with acidified water to produce pure lignin cakes. Yet another process for precipitating lignin is disclosed in

US4764596A, where lignin, e.g. from a black liquor, is precipitated from a solution of lignin in a water miscible organic solvent. In the process the lignin- containing solution is diluted with water and an acid to form a solution having a pH of less than 3 and a temperature of less than about 75 °C. The organic solvent may be a lower aliphatic alcohol.

Lignin can be converted to different high-value products, for example absorbents, dispersion agents, adhesives, feedstock for chemicals, e.g.

phenols, aromatics, component in polyurethane foams and carbon fibers. These and other applications require lignin that does not comprise impurities such as hemicellulose and other carbohydrates. Lignin precipitated from black liquor will also comprise some amount of hemicelluloses and other

carbohydrates and there is a desire to produce lignin that does not comprise such impurities.

Lignin can be purified by removing any hemicellulose and other carbohydrates present in a slurry comprising lignin, by treating the lignin slurry with acid or alkaline hydrolysis which will, at least partly, remove the hemicelluloses present in a slurry. However, it is known that the chemical structure of lignin changes due to the combined effect of elevated

temperature and either very acidic or very alkaline pH, respectively. It is well known that such changes are induced by so called condensation reactions. (Sjostrom, Eero, Wood chemistry: fundamentals and applications, p. 126-129, p. 148-149, ISBN 0-12-647481 -8). In condensation reactions, functional end- groups in lignin are converted to carbon-carbon bonds and the molecular weight of lignin increases. This condensation of the lignin may be a

disadvantage in many subsequent processes, as, for example, the glass transition temperature of the lignin will increase, which makes it difficult to spin a precursor fiber for carbon fiber production. In addition, condensation reactions decrease the reactivity and solubility and of lignin which is limiting its use where high reactivity and solubility are required, e.g. for adsorbents, adhesives, feedstock for chemicals.

There is thus a need for a method for producing a purer lignin without affecting the chemistry of the lignin in a negative way. This method should be aimed at selectively removing hemicellulose and other carbohydrates while leaving the lignin unaffected.

Summary

It is an object of the present disclosure, to provide an improved or alternative method for purifying lignin, which eliminates or alleviates at least some of the disadvantages of the prior art.

The object is wholly achieved by a method according to the appended independent claims. Embodiments are set forth in the appended dependent claims and in the following description and examples.

According to a first aspect, there is provided a method for producing a purified lignin product by an enzymatic treatment, the method comprising the steps of providing a mixture comprising lignin; adjusting and/or maintaining a pH value of said mixture to an optimum value for the enzymatic treatment, adding at least one enzyme to said mixture, wherein said at least one enzyme is directed at hydrolyzing carbohydrates present in said mixture, and wherein said enzyme is directed at hydrolyzing hemicellulose, and/or derivatives and degradation products thereof, adjusting and/or maintaining the mixture to/at an optimal working temperature, during an optimal reaction period of time for the enzymatic treatment; and terminating the enzymatic treatment.

By "at least one enzyme" is meant that it may be one single enzyme but also a mixture of different enzymes that is added to the lignin comprising mixture.

By "adjusting" the pH value or the temperature of the mixture is meant that the pH or temperature is either lowered or elevated depending on the optimal working range for the enzyme or mixture of enzymes.

By this method there is provided a way of treating a mixture that comprises lignin, without the need for an acid or alkaline hydrolysis, that efficiently purifies the lignin from impurities and at the same time provides a way for obtaining a lignin in which the lignin chemistry is relatively unaffected, i.e. where virtually no harmful condensation reactions have occurred during the purification process. This is as such a very mild way of treating the lignin comprising mixture or solution. The method thus provides for an enzymatic hydrolysis which is a more moderate treatment compared to the acid and alkaline hydrolysis as the reaction conditions may be chosen to not affect the lignin chemistry to same extent as other treatments for obtaining a pure lignin product. This means that harmful condensation reactions of the lignin may be avoided, such that the glass transition temperature of the lignin may remain unaffected, which makes it easier to spin a precursor fiber for carbon fiber production. In addition, by reducing the harmful condensation reactions the reactivity and solubility and of lignin may be retained which is required in different types of products, such as adsorbents, adhesives, feedstock for chemicals.

As the enzyme is specifically directed at the carbohydrates present in the mixture, the lignin molecules in the mixture should not be substantially affected by the enzymatic treatment. The enzymes used in the process may thus also be tailored for the specific substrate, i.e. the specific carbohydrate (or mixture of carbohydrates) in the lignin comprising mixture that is to be removed.

The method according to the first aspect may further comprise filtrating the mixture; and washing the filtrate with acidic water.

By washing and filtrating the mixture obtained after the enzymatic treatment the impurities may be efficiently washed away, without affecting the lignin chemistry and providing a pure lignin. This washing may also include re-dissolving the filter cake filtering it again.

The step of terminating the enzymatic treatment may comprise adjusting the pH such that the at least one enzyme becomes inactivated.

According to another embodiment the step of terminating the

enzymatic treatment comprises adjusting the temperature such that the at least one enzyme becomes inactivated.

According to yet an embodiment the step of terminating the enzymatic treatment may involve both a pH adjustment and a temperature adjustment.

The at least one enzyme may be an enzyme able to work in a mixture comprising lignin.

The mixture comprising lignin may comprise a precipitated industrial lignin from black liquor. According to one alternative the mixture may further be subjected to a mechanical treatment during the reaction period.

By "mechanical treatment" is meant that the mixture may be stirred or shaken by any suitable means.

According to one embodiment the lignin in said mixture may in a solid form.

According to a different embodiment the lignin in said mixture may in a dissolved form. By having the lignin in a dissolved form the hemicelluloses and other carbohydrates in said mixture may be made more accessible for the enzymes and the enzymatic treatment and thus the purification of the lignin may be more efficient.

The pH may initially be adjusted to above 2, preferably above 3, more preferably above 4, even more preferably above 5 and most preferably above 6. By having a pH during the enzymatic treatment which is above 2 the risk of condensation reactions in the lignin may be reduced.

According to the first aspect the working temperature may be in the range of 20 to 130 °C, preferably in the range of 40 to 80 °C, more preferably in the range of 45 to 75 °C and most preferably in the range of 50 to 70 °C. By having a relatively low reaction or working temperature the risk of

condensation reactions in the lignin may even further be reduced.

The reaction period may be in the range of 6 to 144 hours, preferably in the range of 6 to 72 hours, more preferably in the range of 6 to 48 hours and most preferably in the range of 6 to 24 hours.

The method may be incorporated in an industrial lignin extraction or precipitation process. By the possibility of incorporating the enzymatic treatment method for purifying lignin, a reactive and substantially unaffected lignin may be obtained from virtually any type of industrial process for producing precipitated lignin. This may further be done without any

substantial alterations to existing equipment.

According to a second aspect there is provided a purified lignin product or an intermediate lignin product obtainable by a method according to the first aspect. According to a third aspect there is provided the use of a purified lignin product or intermediate lignin product according to the second aspect.

According to a fourth aspect there is provided a method for removing carbohydrates present in a lignin comprising mixture by an enzymatic treatment , where the enzymatic treatment is directed at hydrolyzing hemicellulose, and/or derivatives and degradation products thereof. By treating the lignin comprising mixture with enzymes carbohydrates present in the mixture may be removed at the same time as a purified lignin, with a relatively unaffected lignin chemistry, may be obtained.

Detailed description

According to one embodiment a slurry comprising lignin from black liqour is treated with at least one enzyme that affects hemicelluloses and other carbohydrates present in the slurry. The treatment is preferebly done in a batch operation, but may, depending on the effectivness of the enzyme be performed in a continous operation as well.

The pH of the slurry is adjusted to or maintained at a level where the enzyme has an optimum working range, through conventional means such as adding a suitable acid or base to the slurry.

When the pH of the slurry has reached an optimal value at least one enzyme is added to the slurry. The amount of enzyme to be added may be based on calculations of the carbohydrate contents of the slurry or original lignin comprising mixture, i.e. the amount may be adjusted from batch to batch depending on the individual amounts of carbohydrates present in the slurry.

To this end the term "carbohydrates" is supposed to encompas all carbohydrates present in the mixture, i.e. mainly celluloses and

hemicelluloses and derivatives and degradation products thereof, even though the enzyme it self may only be specific for one type of carbohydrate, or more specifically only be directed at the hemicelluloses.

The enzymatic reaction is then allowed to proceed for a period of time until a satisfactory breakdown of carbohydrates has been achieved. During the reaction period the slurry or mixture may be heated to or maintained at a temperature where the enzyme has its optimum working range.

That means that the intitial pH value, the reaction period and the reaction temperature will depend on the type of enzyme used in the process and that the amount of enzyme added will be dependent on the carbohydrate contents of the slurry or mixture to be treated.

During the enzymatic reaction the mixture may be stirred or shaken, using any conventional means for performing such a mechanical treatment. This is to ensure an even and well-distributed enzymatic treatment of the entire slurry or mixture batch.

To terminate the enzymatic treatment the pH of the mixture may be adjusted such that the enzyme is no longer active. According to one embodiment also the temperature of the mixture may be adjusted such that the enzyme is no longer active. A combination of pH adjustment and temperature adjustment may also be used to terminate the treatment.

However, for terminating the enzymatic treatment high temperatures in combination with extreme alkaline or acidic pH should be avoided to prevent condensation reactions.

After the enzyme treatment has been terminated the slurry or mixture may be filtrated and the filter cake may then subsequently be washed, redissolved and refiltered. Optionally the second filter cake may be washed and then dewatered again.

According to another embodiment the enzyme treatment may be incorporated into any industrial lignin extraction and/or precipitation process. In case of a Kraft cook or sulfite cook this means that the black liqour may firstly be treated in order to precipitate the lignin, ususally at a low pH. The pH and temperature may then be adjusted and the enzymes added to the lignin precipitate. When the treatment is done, i.e. when the carbohydrate amount has decreased to a acceptable level, acid, such as sulfuric acid may be added to the slurry or the temperature increased to stop the enzymatic activity. The advantage of incorporating the enzymatic treatment into a lignin precipitation process is that no extra equipment is needed. However, it is also possible to perform the enzymatic treatment of the lignin as a subsequent step to a lignin precipitation process.

The carbohydrates (and thus also the lignin) may be in solid or dissolved form during the enzymatic treatment, this depends on the working pH of the enzyme. It may, however, be advantageous to have a high pH so that lignin is in a soluble state and thus also the carbohydrates. When lignin and carbohydrates are in soluble state the enzyme will have better

accessibility and decompose the carbohydrates more effectively. The enzyme will not affect the lignin in any way, thus makes the lignin unaffected and still very useful in following processes.

The enzyme (or enzymes) used in the method may belong to the group of hydrolysases and is also an enzyme that is able to work in an enviroment comprising lignin, which is conventionally considered to be a too harsh substrate for hydrolysases.

Further, the enzyme (or mixture of enzymes) is preferably an enzyme

(or enzyme mixture) that works in a pH range of 2 to 12, and more preferably between 3 and 7. Even further the enzyme (or enzyme mixture) is preferably adapted to work at in a temperature interval of 20 to 130 °C and more preferably between 30 to 80 °C.

The enzymes may belong to the group of hydrolysases. Examples for hydrolysases are endoglucanases, xylanases, glucomannases.

In the case of a mixture of enzymes the process parameters should, of course be adjusted to ranges that are optimal or as close to optimal as possible for the enzymes in the mixture.

The carbohydrate contents and composition of the mixture comprising lignin may of course vary, but generally lignin from industrial black liquor that is taken out at the evaporation unit at about 40% solids content comprises 2- 3% hemicellulose.

Preferred features of each aspect of the invention are as for each of the other aspects mutatis mutandis. The prior art documents mentioned herein are incorporated to the fullest extent permitted by law. The invention is further described in the following examples the only purpose of which is to illustrate the invention and are in no way intended to limit the scope of the invention in any way.

Example 1

In a laboratory experiment, the process parameters are shown in Table 1 , a washed lignin from softwood was treated with an enzyme.

The enzyme was an endo-1 ,4-xylanase.

The lignin was mixed with de-ionized water and the pH was adjusted with sodium hydroxide to pH 4.1 .

The enzyme was then added after the pH adjustement and the reaction mixture was heated to 50 °C and kept there for 72 hrs. During the reaction period the reaction vessels were shaken in a water bath. After the rection period the pH was adjusted to 2.5 with sulfuric acid.

The sample was then filtrated and the filter cake was washed with acidic water at 50 °C. The filter cake was redissolved and the then filtered again at 50 °C and pH 2.5. the second filter cake was finally washed with acidic water at a pH of approximately 2.5.

Table 1 . Process parameters

Lignin contents (% by weight) 10

Initial total carbohydrate contents (% 2.1

by weight of lignin)

Enzyme addition (ml) 0.1

Temperature (°C) 50

Reaction period (h) 72

Adjusted initial pH 4.1

pH after 72 hrs 3.4

Final total carbohydrate contents (% 1 .3

by weight of lignin) The total carbohydrate contents of the lignin sample before the enzymatic treatment was 2,1 % by weight and after the treatment the carbohydrate contents had been reduced to 1 ,3 % by weight, which is a significant reduction. Table 2 shows the total carbohydrate contents and composition of the lignin sample prior to the enzymatic treatment (Ref) and after the enzymatic treatment, as a double sample (A and B).

Table 2. Absolute carbohydrate composition, dry sample

The laboratory example clearly shows that the carbohydrate contents is significantly reduced by the enzymatic treatment.

Nuclear magnetic resonance (NMR) using a phosphorous ( P) derivative was applied to quantify the functional end-groups of the lignin samples. Table 3 shows the amounts of functional end-groups in the lign sample prior to the enzymatic treatment (Ref) and after the enzymatic treatment (Sample A). No significant changes in the composition of the functional end-groups could be observed. Table 3. Functional end-group composition (hydroxyl groups) analysed by 31 P-NMR

The great advantage of this method is that carbohydrates are removed at relatively mild pH and temperature, while leaving the chemical structure of the lignin unaffected.

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

1 . A method for producing a purified lignin product by an enzymatic treatment, the method comprising the steps:

providing a mixture comprising lignin;

adjusting and/or maintaining a pH value of said mixture to an optimum value for the enzymatic treatment;

adding at least one enzyme to said mixture, wherein said at least one enzyme is directed at hydrolyzing carbohydrates present in said mixture, and wherein said enzyme is directed at hydrolyzing hemicellulose, and/or derivatives and degradation products thereof; adjusting and/or maintaining the mixture to/at an optimal working temperature, during an optimal reaction period of time for the enzymatic treatment; and

terminating the enzymatic treatment.

2. The method as claimed in claim 1 , further comprising:

filtrating the mixture; and

washing the filtrate with acidic water.

3. The method as claimed in claim 1 , wherein the step of terminating the enzymatic treatment comprises adjusting the pH such that the at least one enzyme becomes inactivated.

4. The method as claimed in claim 1 , wherein the step of terminating the enzymatic treatment comprises adjusting the temperature such that the at least one enzyme becomes inactivated.

5. The method as claimed in claim 1 , wherein the at least one enzyme is an enzyme able to work in a mixture comprising lignin.

6. The method as claimed in any one of the preceding claims wherein the mixture comprising lignin comprises a precipitated industrial lignin from black liquor.

7. The method as claimed in claim 1 , wherein the mixture is further subjected to a mechanical treatment during the reaction period.

8. The method as claimed in any one of the above claims, wherein the lignin in said mixture is in a solid form.

9. The method as claimed in any one of claims 1 -7, wherein the lignin in said mixture is in a dissolved form.

10. The method as claimed in claim 1 , wherein the pH is initially adjusted to above 2, preferably above 3, more preferably above 4, even more preferably above 5 and most preferably above 6.

1 1 . The method as claimed in claim 1 , wherein the working

temperature is in the range of 20 to 130 °C, preferably in the range of 40 to 80 °C, more preferably in the range of 45 to 75 °C and most preferably in the range of 50 to 70 °C.

12. The method as claimed in claim 1 , wherein the reaction period is in the range of 6 to 144 hours, preferably in the range of 6 to 72 hours, more preferably in the range of 6 to 48 hours and most preferably in the range of 6 to 24 hours.

13. The method as claimed in any one of the preceding claims, wherein the method is incorporated in an industrial lignin extraction or precipitation process.

14. A purified lignin product or an intermediate lignin product obtainable by a method as claimed in any one of claims 1 to 13. claimed in claim 14. 16. A method for removing carbohydrates present in a lignin comprising mixture by an enzymatic treatment, wherein the enzymatic treatment is directed at hydrolyzing hemicellulose, and/or derivates and degradation products thereof.