SE1150609A1 - Procedure for the purification of lignin - Google Patents

Abstract

14 ABSTRACT A method tor producing a puritied iignin product by an enzyrnatiotreatment, the method comprieing the steps: providing a mixture oomprisingiignin adjusting andlor rnaintaining a pi-i vaiue ot said mixture to an optimumvalue for the enzymatio treatment; adding at least one enzyme to saidrnixture, wherein the at Eeast one enzyme is direoted at hydroiyzingcarbohydrates present in said mixture; adjusting andior maintaining themixture to/at an optimai working temperature, during an optirnai reactionperiod of tirhe for the enzymatio treatment; and terrninating the enzynfiatictreatment

Description

METHOD FOR PURiFYENG LiGNiN Technics! fieid The present disciosure reiates to a method for purifying iignin at miidconditions and thereby minimizing the risks for changes in the iigninsohemicei structure. fi/iore particularly, the present disciosure reietes to emethod by enzyrnatio treatment, a iignin product obtainabie by such a methodand the use of that iignin product. ëaetateausttdWood mainiy consists of oeiiuiose, hemiceiiuioses and iignin. The ceiiuiose ie the main constituent of wood. Ceiiuiose is a iinear poiyrner of ß-giucopyranoside resuidues conntected with 1-4 ß giucosodic bonde.(Ljungberg Textbook -- Puip end Paper Chemistry and Technoiogy Book 1:Wood Chemistry and Wood Biotechnoiogy, tvi. Ek, G. Geiierstedt, G.Henriksson; editors, 92, EBSN 1654-1081, 2007). i-riemiceiiuiose is a branchedand more heterogeneous rnecromoiecuie than ceiiuiose. its main buiidingbiooks ere hexoses, pentoses and certain uronic acids. Ceiiuiose and most ofthe hemiceiiuioses are structure! carbohydrates as they form the buik of thepients ceii wait. (Ljungberg Textbook - Pulp and Paper Chemistry andTechnology Book t: Wood Chemistry and Wood Bioteohnoiogy, M. Ek, G.Geiierstedt, G. Henriksson; editors, 105, iSSN 1654-1081, 2007). Lignin is avery heterogeneous macromoiecuie cornprised of three different monoiignoisthat are connected vie ether and carbon-carbon bonde to form a three-dirnensionai network. The bioiogicai function of iignin in e piant is to improvehydrophobicity and rigidity. The ceiiuiose macromoiecuies form crystaiiitee, socaiied microfibriiis, which are the eiementary buiiding biocks of a wood fiber.in the wood fiber, these microfibriiis are embedded in e matrix ofherniceiluioee and iignin. Lignin forms covaient bonde to ceiiuiose andespecieiiy hemiceiiuiose through Lignin-Carbohydrete-Compiexes, LCC.(Ljungberg Textbook -- Puip and Paper Chemistry and Technoiogy Book 1:Wood Chemistry and Wood Biotechnoiogy, ivi. Ek, G. Geiierstedt, G.Henriksson; editors, 143, iBBN 1654-1081, 2007). These LCC make it difficuitto cornpieteiy remove iignin from fibers during puiping and bieaching. 2G 3G 2 Analogously, it can be stated that the LCIC irnpede complete removalhemicelluiose and other carbohydrates from lignin. in chemical pulping wood chips are cooked at elevated temperaturesand pressures with a mixture of different chemlcals. Lignin and hemicelluloseare dissolved and degraded in that process to obtain free cellulosa fibers. Thedominating chemical pulping process today is the Kraft process (also knownas kraft pulping or sulfate process) The Kraft process is carried out with asolution containing sodium hydroxide and sodium sulflde, known as whiteliquor. After the cooklng process the fibers are separatecl from the so calledblack liquor that mainly cornprises spent booking chemicals, llgnin,carbohydrates and degradation products formed during the Kraft process.The black liquor is then evaporated to higher dry solids content and burned inthe recovery boiler to obtain high pressure steam and sodium sulfite. Thelignin in the black liquor is the most important fuel component in black liquor.ln the subseouent calcination cycle sodium hydroxide is recovered.

Different technologies exist for separating iignin from black liquor, forexample through preclpitation. One method of producing precipitated lignln isdescribed in 68664811, which document describes a process for carbonatingan alkaline cooking iiquor, and dissolying the separated lignin in water andacidifying the resulting solution for precipitatlon the lignin, the solution is thenfiltered to obtain the lignin product Another method of producing a precipitated lignin ls described inWOZGOS/Oêl 175. ln this process the pl-i of the black liquor is lowered bytreating it with C02, and the llgnln is thereby precipitated. The precipitate isthereafter dewatered by filtering, and the filter cake is dissclved in acid washwater. The iignin containing slurry thereby formed is then once againdewatered and the cake once again dlssolved in and washed with acidifiedwater to produce pure lignin cakes.

Yet another process for precipitating lignin is disclosed lnUS4764596A, where lignin, eg. from a black liouor, is precipltated from asolution of lignin in a water miscible organic solvent. ln the process the lignln-containing solution is diluted with water and an acid to form a solution havinga pl-t of less than 3 and a temperature of less than about 75°C. The organicsolverrt may be a lower aliphatic alcohol.

Lignin can be converted to different high-value products, for exampleabsorbents, dispersion agents, adhesives, feedstock for chemlcals, e.g.phenols, aromatics, component in polyurethane foams and carbon fibers. 'lO 3G 3 These and other applications require lignin that does not cornprise impuritiessuch as hemiceiiuiose and other carbohydrates, Lignin precipitated from blackliquor vvili also comprise some amount of hemioeiluloses and othercarbohydrates and there is a desire to produce iignin that does not comprisesuch impurities.

Lignin can be purified by removing any hemiceilulose and otheroarbohydrates present in a slurry cornprising lignin, by treating the iigninslurry with acid or alltaiine hydroiysis which will, at least partiy, remove thehernicelluloses present in a siurry. However, it is known that the chemicalstructure of iignin changes due to the combined effect of eievatedtemperature and either very acidic or very alltaiine pH, respectively. it is wellknown that such changes are induced by so called condensation reactions.(Sjöström, Eero, Wood chemistry: fundamentais and appiications, p. 126-129,p. 148-149, lSBN 0-12-647481-8). in condensation reactions, functional end-groups in iignln are converted to carbon-carbon bonde and the moiecularweight of lignin increases. This condensatlon of the iignin may be adisadvantage in many subsequent processes, as, for exarnpie, the glasstransition temperature of the lignin will increase, which makes it difficult tospin a precursor fiber for carbon fiber production. in addition, condensationreactions decrease the reaotivity and solubiiity and of iignin which is iimitingits use where high reactivity and soiubiiity are required, eg. for adsorbents,ad hesives, feedstock for chemicais.

There is thus a need for a method for producing a purer iignin withoutaffecting the chemistry of the lignin in a negative way. This method should beaimed at selectiveiy removing hemiceliulose and other carbohydrates whileleaving the iignln unaffected. lt is an object of the present disciosure, to provide an improved oralternative method for purifying lignln, which eiiminates or alieviates at leastsome of the disadvantages of the prior art.

The object is whoily achieved by a method according to the appendedindependent ciairns. tšmbodiments are set forth in the appended dependentciaims and in the following description and examples.

According to a first aspect, there is provided a method for producing apurified iignin product by an enzymatic treatment, the method comprising the steps of providing a mixture comprising iignin; acljusting andlor maintaining a 4 pi-t vaiue of said mixture to an optimum vaiue for the enzymatic treatment,adding at ieast one enzyme to said mixture, wherein the at ieast one enzymeis directed at hydroiyzing carhohydrates present in said mixture, adiustingandfor maintaining the mixture to/at an optimai working temperature, duringan optimai reaction period of time for the enzymatic treatment; andterminating the enzymatic treatment.

By “at ieast one enzyme” is meant that it may he one singie enzymehut aiso a mixture of different enzymes that is added to the iignin comprisingmixture.

By “adjusting” the pit vaiue or the temperature ot the mixture is meantthat the pH or temperature is either iowered or eievated depending on theoptimai working range tor the enzyme or mixture of enzymes.

By this method there is provided a way of treating a mixture thatoomprises iignin, without the need for an acid or aikaiine hydroiysis, thatefficientiy purifies the iignin from impurities and at the same time provides away for ohtaining a iignin in which the iignin Chemistry is reiativeiy unaffected,i.e. where virtuaiiy no harmtui oondensation reactions have occurred duringthe purification process. This is as such a very miid way of treating the iignin comprising mixture or soiution.

The method thus provides for an enzymatic hydroiysis which is a moremoderate treatment compared to the acid and aikaiine hydroiysis as thereaction conditions may he chosen to not affect the iignin chemistry to sameextent as other treatments for ohtaining a pure iignin product. This means thatharmtui condensation reactions of the iignin may he avoided, such that thegiass transition temperature of the iignin may remain unatfected, whichmakes it easier to spin a preoursor tiher for carbon fiber production. inaddition, hy reducing the harmtui condensation reactions the reactivity andsoiuhiiity and of iignin may he retained which is required in different types ofproducts, such as adsorhents, adhesives, feedstock for chemicais.

As the enzyme is speciticaiiy directed at the carhohydrates present inthe mixture, the iignin moiecuies in the mixture should not he suhstantiaiiyatfected hy the enzymatic treatment. The enzymes used in the process may thus aiso he taiiored for the specific suhstrate, i.e. the specific carhohydrate 2G 3G 5 (or mixture of carbohydrates) in the iignin comprising mixture that is to beremoved.

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

By washing and fiitrating the mixture obtained after the enzymatictreatment the impurities may be efficientiy washed away, without affecting theiignin Chemistry and providing a pure Eignin. This washing may aiso inciuderedissoiving the fiiter cake fiitering it again.

The step of terminating the enzymatic treatment may compriseadjusting the pH such that the at ieast one enzyme becomes inactivated.

According to another embodiment the step of terminating theenzymatic treatment comprises adiusting the temperature such that the atieast one enzyrne becomes inactivated.

According to yet an embodiment the step of terminating the enzymatictreatment may invoiye both a pH adjustment and a temperature adjustment.

According to the first aspect the at ieast one enzyme directed athydroiyzing carbohydrates may be directed at hydroiyzing hemiceiluiose,andior derivatives and degradation products thereof.

The at ieast one enzyme may be an enzyme abie to work in a mixturecomprising iignin.

The mixture comprising iignin may comprise a precipitated industriaiiignin from biack iiquor.

According to one aiternative the mixture may further be subiected to amechanicai treatment during the reaction period.

By “mechanicai treatment” is meant that the mixture may be stirred orshaken by any suitable means.

According to one embodiment the iignin in said mixture may in a soiidform.

According to a different embodiment the iignin in said mixture may in adissoived form. By having the iigniri in a dissoived form the hemicetiuiosesand other carbohydrates in said mixture may be made more accessible forthe enzymes and the enzymatic treatment and thus the purification of the iignin may be more efficient. it) 2G 3G 6 The pt-i may initiaiiy be adjusted to above 2, preferabiy above 3, morepreferabiy above 4, even more preferabiy above 5 and most preferabiy above6. By having a pi-i during the enzymatio treatment which is above 2 the risk ofcondensation reactions in the iignin may be reduced.

According to the first aspect the working temperature may be in therange of 20 to 130%, preferabiy in the range of 4G to 80°C, more preferabiyin the range of 45 to 75°C and most preferabiy in the range of 50 to ?'Û°C. Byhaving a reiativeiy iow reaction or working temperature the risk ofcondensation reactions in the iignin may even further be reduced.

The reaction period may be in the range of 12 to 144 hours, preferabiyin the range of 12 to 72 hours, more preferabiy in the range of 6 to 48 hoursand most preferabiy in the range of d to 24 hours.

The method may be incorporated in an industriai iignin extraction orprecipitation process. By the possibiiity of incorporating the enzymatiotreatment method for purifying iignin, a reactive and substantiaiiy unaffectediignin may be obtained from virtuaiiy any type of industriai process forproducing precipitated iignin., This may further be done without anysubstantiai aiterations to existing equipment.

According to a second aspect there is provided a purified iignin productor an intermediate iignin product obtainabie by a method according to the firstaspect According to a third aspect there is provided the use of a purified iigninproduct or intermediate iignin product according to the second aspect.

According to a fourth aspect there is provided a method for removingcarbohydrates present in a iignin comprising mixture by an enzymatictreatment. By treating the iignin comprising mixture with enzymescarbohydrates present in the mixture may be removed at the same time as apurified iignin, With a reiativeiy unaffected iignin chemistnr, may be obtained Detaiied descriptionAccording to one embodiment a siurry comprising iignin from biack iigour is treated with at ieast one enzyme that effects hemiceiiuloses andother carbohydrates present in the siurry. The treatment is preferebiy done ina batch operation, but may, depending on the effectivness of the enzyme beperformed in a continous operation as vveii. 'lÜ 3G 7 The pi-l of the slurry is adjusted to or maintalned at a level where theenzyme has an optimum working range, through conventional means such asadding a suitable acid or base to the siurry.

When the pl-i of the siurry has reached an optimal value at least oneenzyme is added to the slurry. The amount of enzyme to be added may bebased on caicuiations ot the carbohydrate contents of the slurry or originallignin comprlsing mixture, i.e. the amount may be adjusted from batch tobatch depending oh the individual amounts of carbohydrates present in theslurry.

To this end the term "carbohydrates" is supposed to encompas allcarbohydrates present in the mixture, i.e. mainly celiuloses andhemiceiluloses and derivatives and degradation products thereof, eventhough the enzyme it sett may only be specific for one type ot carbohydrate,or more specificaliy only be directecl at the hemlcelluloses.

The enzymatic reaction is then allowed to proceed tor a period of timeuntil a satisfactory breakdown ot carbohydrates has been achieved, Duringthe reaction period the slurry or mixture may be heated to or maintained at atemperature where the enzyme has its optimum working range.

That means that the intitial pl-l value, the reaction period and thereaction temperature will depend on the type of enzyme used in the processand that the amount ot enzyme added will be dependent on the carbohydratecontents of the siurry 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 enzymatlc treatment of theentire siurry or mixture batch.

To terminate the enzymatic treatment the pl-l of the mixture may beadjusted such that the enzyme is no longer active. According to oneembodiment also the temperature of the mixture may be adjusted such thatthe enzyme is no longer active. A combination of pl-l adjustment andtemperature adjustment may also be used to terminate the treatment.l-lowever, for terminating the enzymatic treatment high temperatures incombination with extreme aikaiine or acidic pi-l should be avoiclecl to prevent condensation reactions.After the enzyme treatment has been terminated the siurry or mixturemay be tiitrated and the filter cake may then subseguently be vvashed, 8 redissoived and refiitered. Optionaiiy the second fiiter cake may be washedand then dewatered again.

According to another embodiment the enzyme treatment may beincorporated into any industrial iignin extraotion andlor precipitation process.in case of a Kraft cook or suifite cooi: this means that the biack Iigour mayfirstiy be treated in order to precipitate the iignin, ususaiiy at a iow pH. The pi-iand temperature may then be adjusted and the enzymes added to the iigninprecipitate. When the treatment is done, i.e. when the carbohydrate amounthas decreased to a acceptabie ievei, acid, such as suifuric acid may beadded to the siurry or the temperature increased to stop the enzymaticactivity. The advantage of incorporating the enzymatio treatment into a iigninprecipitation process is that no extra equipment is needed.

However, it is aiso possibie to perform the enzymatic treatment of theiignin as a subseouent step to a iignin precipitation process.

The carbohydrates (and thus aiso the iignin) may be in soiid ordissoived form during the enzymatic treatment, this depends on the workingpi-i of the enzyme. it may, however, be advantageous to have a high pt-i sothat tignin is in a soiubie state and thus aiso the carbohydrates. When Iigninand carbohydrates are in soiubie state the enzyme wiii have betteraccessibiiity and decompose the carbohydrates more effectiveiy. The enzymewiii not affect the iignin in any way, thus makes the iignin unaffected and stiiivery usefui in toilowing processes.

The enzyme (or enzymes) used in the method may betong to the groupof hydroiysases and is aiso an enzyme that is abie to work in an envirornentcomprising iignin, which is conventionaiiy considered to be a too harshsubstrate for hydroiysases.

Further, the enzyme (or mixture of enzymes) is preferabiy an enzyme(or enzyme mixture) that works in a pH range of 2 to 12, and more preferabiybetween 3 and 7. Even further the enzyme (or enzyme mixture) is preferabiyadapted to work at in a temperature intervai of 20 to 130%? and morepreferabiy between 30 to 8G °C.

The ehzymes may betong to the group of hydroiysases. Exampies forhydroiysases are endogiucanases, xyianases, giucomannases. in the case of a rnixture of enzymes the process parameters shouid, ofcourse be adjusted to ranges that are optimai or as ciose to optimai aspossibie for the enzymes in the mixture. 1G 2G 9 The carbohydrate contents and composition of the mixture cornprisingiignin may of course vary, but generaily iignin from industriai biack liquor thatis taken out at the evaporation unit at about 46% soiids content comprises 2-3% hemiceiiuiose.

Exampie tin a iaboratory experiment, the process parameters are shown in Tabie t, a washed iignin trorn softwood was treated with an enzyme.

The enzyme was an endo-t A-xyianase.

The iignin was mixed with de-ionized water and the ptt was adjustedwith sodium hydroxide to pH 4.1.

The enzyme was then added efter the pt-i adjusternent and the reactionmixture was heated to 5G°C and kept there for 72 hrs. During the reactionperiod the reaction vesseis were shaken in a water bath. After the rectionperiod the pi-i was adjusted to 2.5 with sulfuric acid.

The sampie was then fiitrated and the fiiter cake was washed withacidic water at 5G°C. The fiiter cake was redissoived and the then fiiteredagain at 50°C and pi-i 2.5. the second fiiter cake was finaity washed withacidic water at a pH of approximateiy 2.5.

Tabie t. Process oaranteters Lignin contents r(% by weight) tt) initiai totai carhohydrate contents (% 2.1by weight of iignin) Enzyme addition (mi) ÛfiTemperature CC) 50Reaction period (h) 72Adjusted initiai pi-i 4.1pt-i after 72 hrs _ 3.4 Finai totai carbohydrate contents (% “ 1.3by weight of iignin) i The totai carbohydrate contents of the iignin sampie before theenzymatic treatment was 2,1 % by weight and after the treatment thecarbohydrate contents had been reduced to 1,3 % by weight, which is asignificant reduction. Tabie 2 shows the totai carbohydrate contents and 'iÛ oornposition of the iignin sample prior to the enzymatic treatment and after theenzymatic treatment, as a doubie sarnpie (A and B).

Table 2. Absoiute carbohydrate compoattšon, dry sample Contents and Prior to treatment After treatment % After treatment % composition % by weight of by weight of lignin hy weight of _ g lignin -»~ Sample A ligninm Sample BTotai 2.16 1.26 1.23Cåitbøhyëfaífïe .

Ptrabinose 0.46 0.28 6.28 Qaiactose 0.90 6.56 0.55Giucose 9.16 0.11 0.11Xyiose 0.54 0.2? Û.27Mannose 6.05 CLÛE D35 The iaboratory example oieariy shows that the carhohydrate contentsis significantiy reduced by the enzyrnatic treatment. The great advantage ofthis method is that carbohydrates are removed at reiativeiy miid pi-i andtemperature, and thus minimizing the risk for changes in the iignins ohemicai structure. in view of the above detailed description of the present invehtion, other modifications and Variations will become apparent to those skiiied in the art.However, it should loe apparent that such other modifications and variationsmay he effected without departing from the spirit and scope of the invention

Claims (16)

1. G 2G 11CLAEMS t. A method tor producing a puritied iignin product hy an enzymatiotreatment, the method oomprising the steps: providing a mixture oomprising iignin; adiusting and/or maintaining a pH vaiue ot said mixture to an optimumvaiue tor the enzymatio treatment; adding at ieast one enzyme to said mixture, Wherein the at ieast oneenzyme is direoted at hydroiyzing oarpohydrates present in said mixture; adjusting and/or maintaihing the rnixture tolat an optimai workingtemperature, during an optimai reaction period ot time tor the enzymatiotreatment; and terminating the enzymatio treatment.

2. The method as oiaimed in oiaim t, further comprising:tiitrating the mixture; andwashing the tiitrate With acidic water.

3. The method as oiaimed in oiaim 1, wherein the step ot terminatingthe enzyrnatio treatment oornprises adjusting the pi-i such that the at ieast one enzyme becomes inaotivated.

4. The method as oiaimed in oiaim 1, wherein the step ot terminatingthe enzymatic treatment oomprises adiusting the temperature such that the atieast one enzyme becomes inaotivated.

5. The method as oiaimed in any one ot the above ciaims, wherein theat ieast one enzyme directed at hydroiyzing carhohydrates is direeted athydroiyzing hemioeiiuiose, and/or derivates and degradation products thereot. 2G 12

6. The method as oiaimed in oiaim 5, wherein the at ieast one enzyme is an enzyme abie to work in a mixture oomprising iignin.

7. The method as otairned in any one of the oreoeding ciaims whereinthe mixture oomprising iignin cornprises a oreoipitated industriai iignin fromblack iiquor.

8.The method as oiaimed in oiairn t, wherein the mixture is further subjeoted to a meohanioai treatment during the reaction period.

9. The method as oiaimed in any one of the above oiaims, vvherein the iignin in said mixture is in a soiid form.

10. The method as oiaimed in anv one of ciaims 1-8, vvherein the iignin in said mixture is in a dissoived form. tf.

11. The method as oiaimed in oiaim t, wherein the oi-i is initiativadjusted to above 2, preferabiy above 3, more preferabiy above 4, even more oreferabiy above 5 and most oreferabiy above 6.

12. The method as oieirned in oiaim t, wherein the Workingtemperature is in the range of 20 to t3G°C, oreferabiy in the range ot” 40 to80%, more preferabiy in the range of 45 to 75°C and most preferabiy in therange of 50 to 7G°C.

13. The method as oiaimed in ciaim t, wherein the reaction period is inthe range of 12 to 144 hours, preferabiy in the range of 12 to '72 hours, morepreferabiv in the range of 6 to 48 hours and most preferabiy in the range of 6to 24 hours.

14. The method as oiaimed in any one of the preoeding oiaims,vvherein the method is incorporated in an industriai iignin extraction or preoioitation process. 1315, A puråfšed Eigrršn prøduct ar an intermediära äägnirr prsfiucisbtašnabâe pby a methød as cåašmeá in any sne crf câašms 1 te 14. 16, Use af a purâfšed Ešgrašrs praduct ar intermedâate Ešgnin prsdrrct asS: ciašmfed En ciašm

15. ”šï .A rrrerhcæd før rerrrovrrrg carbøhyfirates present in a išgrršncfirrrpršsšng mšxture by an enzyrrratâs treatment; "iQ