Technical Field of the Invention

The invention relates to a process for recovering and purifying lignin from spent pulping liquors, e.g. black liquor.

Background

Lignocellulosic biomass is a readily available renewable resource and provides us with the two most abundant biopolymers on the planet; lignin and cellulose (the third major component being hemicellulose). Lignin is a class of crosslinked organic polymers containing several aromatic subunits. Wood and bark constitute a type of lignocellulosic biomass from which cellulose and lignin can be recovered. The lignin content in wood is usually between 25 and 35%, but depends on the wood species.

Currently, lignocellulosic materials are often treated via the kraft (or sulphate) process where the lignocellulosic material, for instance wood, is converted into wood pulp by separating cellulose from lignin and hemicellulose. The main achievement has for a long time been to separate the cellulose in the form of pulp for use in the paper making industry, forming other derivative materials such as rayon or cellophane and recently, forming biofuels. Other possible ways to recover cellulose is through other acid treatment processes, alkaline treatments, or organosolv treatments.

The by-product of the kraft (or sulphate) process is an alkaline pulping liquor, containing hemicellulose, lignin etc. In the kraft process, this liquor is known as spent pulping liquor, or simply black liquor due to its color. As lignin has a high energy density, due to its multiple functional groups, interest has been turned to lignin as a source for renewable biofuel. As of today, lignin is recovered in solid form from black liquor. Recovered lignin typically has a high content (> 1 %) of inorganic impurities, including alkali metal (e.g. sodium) originating from the pulping liquor. Recovered lignin is used as a biofuel or as a binder in other energy pellets. A lignin pellet has approximately the same energy contents as a pellet made out of coal, meaning it contains about twice the amount of energy as for a wood pellet. Another use of lignin is in the formation of different composite materials.

Lignin has however found less use in high-end applications, e.g. conversion to carbon fibers. Common for all high end applications of lignin is the requirement of a high purity lignin. As an example, a catalytic process using recovered lignin requires very low amounts of inorganic impurities, including alkali metal (e.g. sodium), in order to avoid catalysts poisoning. In the art, lignin has been considered as a potential feed- stock for oil refineries in producing hydrocarbons, but the high content inorganic impurities has been considered a challenge.

The drawback of the methods available up to know, is that they are expensive, sometimes hard to perform in a larger scale, and often fail to purify lignin to a sufficiently low level. Unfortunately, some methods might further change the physical properties of the material.

As example, the process disclosed in WO 2018/004447 requires a multitude of aqueous washing steps. Further, though metal content of the lignin is lowered, the method does not allow for nearly complete removal of sodium.

Further, US 9 187 512 B2 discloses a process for recovering lignin particles and pellets with moderate ash contents. Lignin is precipitated from black liquid at high pH forming a dense liquid lignin phase, which is further washed with an acid, such as acetic or formic acid. The material is then converted into an energy pellet. Though metal content of the lignin is lowered, the method does not allow for nearly complete removal of sodium.

US 2016/0137680 Al discloses a method to obtain, fractionate, and purify lignin. The method was subsequently also disclosed in Chem. Commun., 2015, 51, 12855. The method comprises one or several separation steps where biomass containing lignin is extracted and fractionated with an aqueous solvent with may comprise an organic acid. The mixture separates and one obtains one liquid phase where high molecular weight lignin is abundant and one phase where the solvent is more abundant, but which phase also comprises low molecular weight lignin. The lignin phase can then be further treated with solvents to obtain a purified lignin. Though metal content of the lignin is lowered, the method does not allow for nearly complete removal of sodium. Further, some lignin is lost, lowering the overall yield in the process.

A study of isolation of lignin as a by-product from ammonia percolation pretreatment of poplar wood has been published in Bioresource Technology vol. 162 of 2014, p. 236-242 by F. P. Bouxin et al. According to F. P. Bouxin et al, the most important factor in the choice of pretreatment technology is how efficiently the lignin is solubilized. In the lignin ammonia percolation pretreatment, the lignin is amino functionalized whereby increasing its solubility; especially at low pH. Further, the ammonia pretreatment increases the cellulose digestibility while avoiding condensation of the lignin. However, the ammonia percolation pretreatment and the resulting amino content hampers downstream catalytic processing of the lignin. Even though systems and processes for recovering and purifying lignin are known in the art, it would be desirable to find an improved technique and method for reducing the amount of contaminants, such as for instance metals, alkali and salts from lignin.

Summary

According to a first aspect of the invention, the above and other objects of the invention are achieved, in full or at least in part, by a providing a process for purifying lignin comprising metal cations, e.g. sodium. The lignin may be lignin obtained from biomass, e.g. wood, in a kraft pulping process. As an example, the feedstock may be alkaline black liquor comprising lignin. The black liquor may be alkaline black liquor from a kraft process or from a soda pulping process. Given the high pH of black liquor due to the use of sodium hydroxide in e.g. the kraft process, at least some of the phenolic hydroxyl groups of the lignin dissolved in black liquor are typically deprotonated with sodium acting as counter ion. At high pH, lignin is water soluble, whereas lignin will precipitate if the pH is lowered as the phenolic hydroxyl groups will become protonated, lowering the water solubility of lignin.

The process for purifying lignin, comprising metal cations, comprises the consecutive steps of:

(a) providing lignin to be purified by acidifying black liquor, and

subsequently separating the thereby formed raw lignin phase;

(b) dissolving the lignin in an acidic, aqueous solvent, at a temperature of at least 50°C, such as at least 60, 70°C, 80°C or 90°C, to provide a liquid one-phase system comprising dissolved lignin;

(c) triggering phase separation by diluting the one-phase system by adding water, and/or by lowering the temperature, to provide a two-phase system, in which two-phase system the first phase is a lignin rich phase, and the second phase is a liquid, aqueous phase poor in lignin and comprising metal cations extracted from the lignin; and

(d) separating the lignin rich phase from the two-phase system to recover purified lignin.

It was found that by employing a step in which lignin is dissolved in a liquid one-phase system to subsequently be separated therefrom, a number of advantages are provided. Firstly, in a process of repeated washing of solid, dispersed lignin, with an acidic aqueous phase (cf. WO 2018/004447) a threshold is reached, as concluded in the experimental part herein below. It seems that entrapped sodium will not be released even if the washing steps are repeated a number of times. However, employing a method in which lignin is dissolved in an acidic, aqueous solvent, efficiently extracts sodium and other metal ions, whereby metal cations efficiently are removed from the lignin, as they remain dissolved once the lignin is separated.

Secondly, if employing many, repeated washing steps, the overall yield of lignin may be lowered. In a process in which a step of dissolving lignin in a liquid one- phase system is included, the overall number of steps may be reduced, whereby improving the yield.

Thirdly, as shown in the experimental part herein below, the washing efficiency is improved by completely dissolving lignin in liquid one-phase system and subsequently triggering phase separation to form a two-phase system, compared to processes in the art (cf. e.g. US 2016/0137680) relying on liquid/liquid two-phase system.

Fourthly, the yield may be improved compared to a system employing liquid/liquid two-phase system.

In order to dissolve lignin, the acidic, aqueous solvent should be heated to at least 50°C, such as at least 60, 70°C, 80°C, or 90°C. Only heating water will however not be sufficient to dissolve lignin. In order to dissolve lignin, the acidic, aqueous solvent should comprise a water soluble organic solvent, increasing the solubility of lignin. The nature of the water soluble organic solvent and the relative amount thereof, as well as the temperature, affect the solubility of lignin. The aqueous solvent may comprise a C1-C3 alkanoic acid (an aliphatic carboxylic acid regarded as derived from an alkane and containing the same number of carbon atoms as the alkane), such as acetic acid, a halogenated C1-C3 alkanoic acid, such as cloroacetic acid, peracetic acid, a C1-C5 alkanol (an aliphatic alcohol regarded as derived from an alkane), such as methanol, etanol, propanol, or butanol, phenol, creosol, a C3-C6 di-alkyl ketone, such as acetone, or a mixture of two or more of any of these solvents. Further, in order to allow for ion-exchange in releasing entrapped cations, e.g. sodium, the aqueous solvent should be acidic. Thus, the aqueous solvent should comprise an acid. The acid may either be an organic solvent, such as acetic acid, or the acid may be a separate acid, such as a mineral acid (e.g. sulfuric acid), or an acidic ion-exchange resin. Though sulfuric acid represents a preferred mineral acid, also other mineral acids, e.g. hydrochloric acid or nitric acid, may be used.

According to an embodiment, the solvent comprises a C1-C3 alkanoic acid. Preferably, the C1-C3 alkanoic acid is acetic acid. As already outlined, the solubility of lignin will vary with the percentage of the water soluble organic solvent as well with the temperature of the solvent. The acidic, aqueous solvent may comprise at least 30 wt%, such as at least 40 wt%, 50 wt%, or 60 wt% acetic acid. In some embodiments, according to which the lignin is to be dissolved at temperatures of less than l00°C, the aqueous solvent may comprise at least 80 wt% acetic acid, such as 80 to 95 wt% acetic acid. In some embodiments, according to which the lignin is to be dissolved at temperatures of more than l20°C, the aqueous solvent may comprise at 30 wt% to 80 wt%, such as 40 wt% to 70 wt%, acetic acid.

According to another embodiment, the solvent comprises a C1-C5 alkanol. Preferably, the C1-C5 alkanol is methanol. Examples of C2-C5 alkanols include, ethanol, n-propanol, and i-propanol. Though the use of methanol as organic solvent requires the addition of a separate acid to the aqueous solvent in order to provide an acidic solvent, it has other advantages. In pulping processes, such as kraft pulping, methanol, is, along with minor amounts of ethanol and acetone, obtained as a by- product. Given the sulfur impurities, the obtained methanol is not used a feedstock in other processes, but typically simply burned in the lime kiln or recovery boiler at the pulping mill to provide heat. The sulfur impurities are however not obstacle for using the methanol in the present process. Actually, the sulfur impurities could provide an advantage as presence of sulfur in the purified lignin may imply that the need to add sulfur as make up in catalytically converting lignin into e.g. a hydrocarbon by means of sulfided catalysts may be dispensed with. Methanol could thus simply be taken out from the pulping process, optionally distilled, used in purifying the lignin, and then transferred back to the evaporation plant at the pulping mill without having to be re- cycled. According to an embodiment, the methanol is thus methanol obtained from a pulping process, such as kraft pulping. Methanol obtained from a pulping process, such as kraft pulping, may comprise turpentine, ethanol and acetone. According to an embodiment the solvent, apart from methanol, comprises acetone as a co-solvent. The solvent may comprise 0.1 to 15 wt.% acetone, such as 0.25 to 10 wt.% acetone or 0.5 to 5 wt.% acetone. Acetone may act as a co-solvent to increase the dissolution of lignin to form a liquid one-phase system. Presence of acetone may further imply that less heating is required to dissolve the lignin. While not necessary, the amount of turpentine may be reduced before using methanol obtained from a pulping process as solvent. The amount of turpentine in the solvent may be less than 10 wt.%, such as less than 5 wt.%, 2.5 wt.%, or 1 wt.%.

For other solvents than methanol, re-cycling may need to be considered in order to provide a cost efficient process.

In embodiments employing aqueous solvent comprising a C1-C5 alkanol, such as methanol, the solvent may further comprise sulfuric acid in order to provide an acidic solvent. The acidic, aqueous solvent may comprise at least 30 wt%, such as at least 40 wt%, 50 wt%, or 60 wt% methanol. Thus, the acidic, aqueous solvent may comprise 30 to 95 wt% methanol, such as 50 to 90 wt% methanol. Further, the aqueous solvent may comprise 0.01 to 5 wt%, 0.1 to 2 wt%, or 0.5 to 1.5 wt% sulfuric acid. In some embodiments, according to which the lignin is to be dissolved in aqueous methanol, the temperature in the step of dissolving the lignin is lower than the boiling point of the aqueous methanol at 1 atm. The temperature in the step of dissolving the lignin may also be higher than the boiling point of the aqueous methanol at 1 atm, provided that the solvent is pressurized.

According to an embodiment, the solvent further comprises a chelating agent. The chelating agent may further improve the extraction of metal ions from the lignin. The chelating agent may be a di- or polyvalent organic acid. Thus, the chelating agent may be selected from the group consisting of oxalic acid, malic acid, citric acid, and tartaric acid, such as from the group consisting of citric acid and tartaric acid. Also aminopolycarboxylic acids, e.g. EDTA (ethylenediaminetetraacetic acid), may be considered as chelating agent.

As already outlined, the temperature in the step of dissolving the lignin is at least 50°C. According to an embodiment, the temperature in the step of dissolving the lignin is at least l00°C, such as 110 to l70°C. Employing a somewhat higher temperature in dissolving lignin implies that less organic solvent needs to be added to the aqueous solvent, which may be advantages, at least for some solvents.

Not only the solvent and the temperature, but also the relative amount of lignin may be of relevance. According to an embodiment, the amount of lignin and the acidic, aqueous solvent are selected in a manner such that the resulting one-phase system comprises 10 to 50 wt%, such as 20 to 40 wt%, lignin after the lignin has been dissolved. It is preferred that the lignin concentration is neither too low nor too high.

When present as liquid one-phase system, remaining impurities, and especially metal cations, may be released from the lignin and dissolved in the aqueous solvent. In order to separate the lignin from the released impurities, phase separation is triggered to provide a two-phase system. Phase separation may be triggered by adding water, and/or by lowering the temperature. By separating a lignin rich phase from the second phase of the two-phase system, purified lignin may be obtained. In the two-phase system, the second phase is a liquid, aqueous phase poor in lignin, which comprises metal cations extracted from the lignin. As already described and as can be seen from the examples provided herein below, it was unexpectedly found that dissolving lignin in a one-phase system and subsequently triggering phase separation and separating the lignin rich phase, provides lignin in high yield, the lignin having very low amounts of metal cations.

Preferably, the phase separation is triggered at least by adding water, i.e. by diluting the water soluble organic solvent and thereby reducing the solubility of lignin. Upon addition of water, a lignin rich phase will separate from the liquid one-phase system to provide a two-phase system. Depending on the temperature, the two-phase system, comprising a liquid, aqueous phase poor in lignin and comprising metal cations extracted from the lignin, may be a liquid/liquid or a liquid/solid system. Upon adding water, the relative amount of lignin is lowered. In an embodiment according to which water is added, the mass ratio of lignin may be reduced by at least 5 percentage units, such as by 10 to 25 percentage units, in diluting the one-phase system.

Further, the phase separation may be triggered at least by lowering the temperature. In an embodiment according which the phase separation is triggered by lowering the temperature, the temperature may be lowered by at least 5°C, such as by at least l0°C, l5°C, 20°C, or 25°C. Thus, the temperature may be lowered by 5°C to 25°C.

While, the phase separation may be triggered by only lowering the

temperature, it is typically combined with addition of water. According to an embodiment, the phase separation is triggered by adding water to dilute the one-phase system and by lowering the temperature of the one-phase system. By adding water, being colder than the heated one-phase system, the dilution of the one-phase system with water will also lower the temperature of the system. Upon diluting the one-phase system with water, the mass ratio of lignin may be reduced by at least 5 percentage units, such as by 10 to 25 percentage units. Further, the temperature may be lowered by at least 5°C, such as by at least l0°C, l5°C, 20°C, or 25°C. The temperature may be lowered by 5°C to 25°C. The steps of dissolving the lignin in a one-phase system, triggering phase separation, and separating a lignin rich phase will provide lignin with significantly less metal cations. However, in order to remove residues of the water soluble organic solvent, sulfates, and/or other impurities, e.g. remaining minor residues of metal cations, it may be preferred to wash the lignin rich phase subsequently to having been separated from the two-phase system. Such an additional washing step may also imply that it not may be necessary to repeat the step of dissolving the lignin in a one-phase system, would the lignin still comprise more metal cations than deemed acceptable.

According to an embodiment, the process thus comprises the further steps of: washing the separated lignin rich phase, subsequent to having been separated from the two-phase system, with a first acidic aqueous phase, and separating a first washed lignin phase. The first acidic aqueous phase preferably comprises at least 90 wt% water, such as at least 95 wt% water. Further, the first acidic aqueous phase may comprise 0.01 to 5 wt%, 0.1 to 2 wt%, or 0.5 to 1.5 wt% sulfuric acid. According to an embodiment, the first acidic aqueous phase further comprises a chelating agent. The chelating agent may further improve the extraction of metal ions from the lignin. The chelating agent may be a di- or polyvalent organic acid. Thus, the chelating agent may be selected from the group consisting of oxalic acid, malic acid, citric acid, and tartaric acid, such as from the group consisting of citric acid and tartaric acid. Also aminopolycarboxylic acids, e.g. EDTA (ethylenediaminetetraacetic acid), may be considered as chelating agent.

The wash with a first acidic aqueous phase may be repeated at least one time, such as 1, 2, or 3 times. Further, the initial wash with a first acidic aqueous phase may be supplemented with a subsequent additional wash with water. A wash with water will serve to remove remaining residues of e.g. sulfate. The water may have a neutral pH, e.g. of about 6 to 8. The pH may be of about 7.

In an embodiment, comprising a step of washing the lignin rich phase separated from the one-phase system, the wash with the first acidic aqueous phase may be performed at temperature of l00°C or less, such as at lower than 90°C, 80°C or 70°C. The temperature the wash with the first acidic aqueous phase may be the performed at temperature of 50 to l00°C. Alternatively, the wash with the first acidic aqueous phase may be performed at temperature of more than l00°C, such as more than 1 l0°C, l20°C, l30°C, or l40°C, and at a pressure higher than 1 atm, such as higher than 1.1 atm, 1.25 atm, or 1.5 atm. While it may be more energy consuming to perform that wash at higher temperature, the lignin phase will be less viscous at higher temperatures, which may be advantageous and facilitate processing of the phases. Further, the lignin may also be washed before being dissolved to provide the one-phase system. Such as pre-wash may serve to remove some metal cations as well as other impurities. Further, a pre-wash may improve the extraction efficiency in the one- phase system. According to an embodiment, the process thus comprises the step of: washing lignin, prior to be being dissolved in the aqueous solvent, with a second acidic aqueous phase; and separating a second washed lignin phase. The second acidic aqueous phase preferably comprises at least 90 wt% water, such as at least 95 wt% water. Further, the second acidic aqueous phase may comprise 0.01 to 5 wt%, 0.1 to 2 wt%, or 0.5 to 1.5 wt% sulfuric acid. According to an embodiment, the second acidic aqueous phase further comprises a chelating agent. The chelating agent may further improve the extraction of metal ions from the lignin. The chelating agent may be a di- or polyvalent organic acid. Thus, the chelating agent may be selected from the group consisting of oxalic acid, malic acid, citric acid, and tartaric acid, such as from the group consisting of citric acid and tartaric acid. Also aminopolycarboxylic acids, e.g. EDTA (ethylenediaminetetraacetic acid), may be considered as chelating agent. The wash with the second acidic aqueous phase may be repeated at least one time, such as 1, 2, or 3 times.

A major source of lignin today is spent pulping liquor. In pulping, cellulose in lignocellulosic biomass is separated from lignin. According to an embodiment, lignin to be purified is provided from a pulping process, such as kraft pulping or soda pulping. According to such an embodiment, lignin may be provided be acidifying a spent alkaline pulping liquor, such as black liquor from kraft pulping or soda pulping, resulting in precipitation of lignin, which may be separated. Black liquor may be acidified by applying pressurized carbon dioxide. The carbon dioxide will dissolve in the black liquor and lower the pH of alkaline black liquor. Sulfuric acid may also be used to lower the pH of black liquor, either on its own or as supplement to carbon dioxide. By lowering the pH of black liquor to precipitate lignin, the formed third lignin phase may be separated. Lignin may be provided from spent pulping liquor, e.g. black liquor, from a soda pulping in a similar manner. According to a preferred embodiment, lignin is provided be acidifying black liquor from kraft pulping, resulting in

precipitation of lignin, which is separated. According to an alternative embodiment, lignin is provided be acidifying black liquor from soda pulping, resulting in

precipitation of lignin, which is separated.

According to some further numbered exemplary embodiments there is provided a process for purifying lignin comprising metal cations. Exemplary embodiment 1 relates to a process for purifying lignin comprising metal cations, the process comprising the consecutive steps of:

(a) providing lignin to be purified by acidifying black liquor, and subsequently separating the thereby formed raw lignin phase (LP3);

(b) dissolving the lignin in an acidic, aqueous solvent, at a temperature of at least 50°C, such as at least 60°C, 70°C, 80°C or 90°C, to provide a liquid one-phase system (OPS) comprising dissolved lignin;

(c) triggering phase separation by diluting the one-phase system by adding water, and/or by lowering the temperature of the one-phase system, to provide a two-phase system, in which two-phase system the first phase is a lignin rich phase (LP1), and the second phase is a liquid, aqueous phase poor in lignin and comprising metal cations extracted from the lignin; and

(d) separating the lignin rich phase (LP1) from the two-phase system to recover purified lignin.

2. The process according to embodiment 1, wherein the aqueous solvent comprises a C1-C3 alkanoic acid, such as acetic acid, a halogenated C1-C3 alkanoic acid, such as cloroacetic acid, peracetic acid, a C1-C5 alkanol, such as methanol, etanol, propanol, or butanol, phenol, creosol, a C3-C6 alkyl ketone, such as acetone, or a mixture of two or more of any of these solvents.

3. The process according to embodiment 2, wherein the solvent comprises a C1-C5 alkanol, preferably the C1-C5 alkanol being methanol.

4. The process according to embodiment 3, preferably the solvent further comprises sulfuric acid (H2SO4) ); preferably the solvent comprising 0.01 to 5 wt%, such as 0.1 to 2 wt%, or 0.5 to 1.5 wt%, sulfuric acid (H2SO4).

5. The process according to any one of the embodiments 2 to 4, wherein the solvent further comprises acetone; preferably the solvent comprising 0.1 to 15 wt.% acetone, such as 0.25 to 10 wt.% acetone or 0.5 to 5 wt.% acetone.

6. The process according to any one of the embodiments 2 to 5, wherein the solvent comprises a C1-C3 alkanoic acid, preferably the C1-C3 alkanoic acid being acetic acid.

7. The process according to any one of the claims 2 to 6, wherein the solvent comprises a chelating agent; preferably the chelating agent being selected from the group consisting of oxalic acid, malic acid, citric acid, and tartaric acid.

8. The process according to any one of the embodiments 1 to 7, wherein the temperature in the step of dissolving the lignin is at least l00°C, such as 110 to l70°C. 9. The process according to any one of the embodiments 1 to 8, wherein the one-phase system comprises 10 to 50 wt%, such as 20 to 40 wt%, lignin after the lignin has been dissolved in the aqueous solvent.

10. The process according to any one of the embodiments 1 to 9, wherein the phase separation is triggered by diluting the one-phase system by adding water;

preferably the mass ratio of lignin is reduced by at least 5 percentage units, such as by 10 to 25 percentage units, in diluting the one-phase system.

11. The process according to any one of the embodiments 1 to 10, wherein the phase separation is triggered by lowering the temperature of the one-phase system by at least 5°C, such as by at least l0°C, l5°C, 20°C, or 25°C.

12. The process according to any one the embodiments 1 to 11, wherein the method further comprises the steps of:

washing the separated lignin rich phase (LP1), subsequent to having been separated from the two-phase system, with a first acidic aqueous phase; and

separating a first washed lignin phase (LP4);

wherein the first acidic aqueous phase preferably comprises at least 90 wt% water, such as at least 95 wt% water, and/or wherein the first acidic aqueous phase preferably comprises 0.01 to 5 wt%, 0.1 to 2 wt%, or 0.5 to 1.5 wt% sulfuric acid.

13. The process according to embodiment 12, wherein first acidic aqueous phase comprises a chelating agent; preferably the chelating agent being selected from the group consisting of oxalic acid, malic acid, citric acid, and tartaric acid.

14. The process according to embodiment 12 or 13, wherein the wash with the first acidic aqueous phase is performed at temperature of less than l00°C, such as lower than 90°C, 80°C, or 70°C.

15. The process according to embodiment 12 or 13, wherein the wash with the first acidic aqueous phase is performed at temperature of more than l00°C, such as more than 1 l0°C, l20°C, l30°C, or l40°C, and at a pressure higher than 1 atm, such as higher than 1.1 atm, 1.25 atm, or 1.5 atm.

16. The process according to any one the embodiments 12 to 15, wherein the first washed separated lignin phase (LP4) is washed with water, subsequent to having been washed with the first acidic aqueous phase.

17. The process according to any one of the embodiments 1 to 12, wherein the process further comprises the step of:

washing the provided raw lignin phase (LP3), prior to be being dissolved in the aqueous solvent, with a second acidic aqueous phase; and separating a second washed lignin phase (LP2) to be dissolved in the aqueous solvent;

wherein the second acidic aqueous phase preferably comprises at least 90 wt% water, such as at least 90 wt% water; and/or wherein the second acidic aqueous water preferably comprises 0.01 to 5 wt%, 0.1 to 2 wt%, or 0.5 to 1.5 wt% sulfuric acid.

18. The process according to embodiment 17, wherein the second acidic aqueous phase comprises a chelating agent; preferably the chelating agent being selected from the group consisting of oxalic acid, malic acid, citric acid, and tartaric acid.

19. The process according to any one of the embodiments 1 to 18, wherein the lignin is provided by acidifying black liquor, such as black liquor from kraft pulping, by applying pressurized carbon dioxide and/or adding sulfuric acid.

20. The process according to embodiment 19, wherein the raw lignin phase (LP3) is provided by applying pressurized carbon dioxide and optionally adding sulfuric acid.

Other objectives, features and advantages of the present invention will appear from the following detailed disclosure, from the attached claims, as well as from the drawings. It is noted that the invention relates to all possible combinations of features.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein

Brief description of the Drawings

By way of example, embodiments of the present invention will now be described with reference to the accompanying drawings, in which:

Fig. 1 is flow scheme schematically depicting purification of lignin from black liquor according to an embodiment; and

Fig. 2 is a process scheme for a process according to a preferred embodiment. Detailed Description of Preferred Embodiments of the invention

The table below lists features of the flow scheme in Fig. 1 and the process scheme Fig. 2

Feature Reference numeral

sedimentation vessel 10

lignin poor phase 11

column 12

acidified black liquor 13

raw lignin phase LP3 first washing vessel 20

sulfuric acid 21

first fresh aqueous phase 22

spent first aqueous phase 23

second washed lignin phase LP2 second washing vessel 30

acidic, aqueous solvent 31

spent second aqueous phase 32

second fresh aqueous phase 33

One-phase system OPS

First lignin rich phase LP1 third washing vessel 40

sulfuric acid 41

spent third aqueous phase 42

third fresh aqueous phase 43

first washed lignin phase LP4 fourth washing vessel 50

fresh water 51

spent third aqueous phase 52

purified lignin 53 According to an embodiment, the lignin to be purified originates from black liquor. In Fig. 1 a flow scheme for purifying lignin from black liquor is provided. In a first step, lignin is precipitated by acidifying the alkaline black liquor to provide a raw lignin phase LP3, which is separated from the black liquor. The alkaline black liquor may, as known in the art, be acidified by applying pressurized carbon dioxide, to precipitate lignin. In addition, sulfuric acid may be used to acidify the alkaline black liquor, either as supplement to carbon dioxide, or on its own. The provided raw lignin phase LP3 is subsequently washed with an acidic aqueous phase to remove salts, e.g. sodium, to provide a second washed lignin phase LP2. The wash may be repeated more than once, such as 1, 2, 3, 4, or 5 times. In the next step, aqueous acetic acid is added to the second lignin phase LP2 and the resulting mixture is heated to dissolve lignin, whereby providing a liquid one-phase system OPS. The solubility of lignin depends on the concentration of acetic acid as well as the temperature. For 80% acetic acid, the mixture is typically heated to at least 90°C, whereas for 65% acetic acid, the mixture is typically heated to at least l20°C. According to an alternative embodiment, the second lignin phase (LP2) is dissolved in aqueous methanol comprising sulfuric acid. By forming a liquid one-phase system OPS, inorganics such as sodium ions, are efficiently released from the lignin. The dwell time once the lignin has been dissolved may be 1 to 60 minutes, such as 5 to 30 minutes. The liquid one-phase system OPS may be stirred to improve the release of inorganics. Subsequently, the liquid one-phase system OPS is diluted by adding water. This lowers the concentration of acetic acid, but typically also the temperature of the system, as the water to be added not is preheated. The addition of water will trigger phase separation and the formation of a first lignin rich phase LP1 and an aqueous phase. The aqueous phase is a liquid, aqueous phase being poor in lignin, but comprising metal cations extracted from the lignin. In the next step, the lignin rich phase LP1 is washed with an aqueous phase to provide a first washed lignin phase LP4. The wash may be repeated more than once, such as 1, 2, 3, 4, or 5 times. At least in the first step, the aqueous phase used in the wash may be an acidic aqueous phase. In the subsequent washing steps, the aqueous phase may comprise only water to remove any remaining sulfates.

In Fig. 2, a more detailed process scheme for carrying out a process according to Fig. 1, is depicted. According to this process scheme of Fig. 2, black liquor is fed to a column 12. Further, also carbon dioxide is fed to the column 12. The black liquor and the carbon dioxide are fed to the column at opposite ends of the column operating in a counter flow manner. The carbon dioxide dissolves in the black liquor to lower its pH. The acidified black liquor 13 is withdrawn from the column 12 and fed to a

sedimentation vessel 10, in which a heavy, raw lignin phase LP3 separates due to the lowered pH. The heavy raw lignin phase LP3 is withdrawn from the sedimentation vessel 10 and fed to a first washing vessel 20. Further, a light, lignin poor phase 11 is withdrawn from the sedimentation vessel 10.

To the first washing vessel 20 sulfuric acid 21 is added. Further, a first aqueous phase added. The first aqueous phase may be a first fresh aqueous phase 22 and/or a spent aqueous phase 42, 52 withdrawn from a downstream washing vessel 40, 50. The mixture of the raw lignin phase LP3 and the first aqueous phase is stirred to extract metal cations from the raw lignin phase LP3. Subsequently, the phases are allowed to separate and a second washed lignin phase LP2 is withdrawn and fed to a stirred second washing vessel 30. Further, a spent first aqueous phase 23 is withdrawn from the first washing vessel 20. The spent first aqueous phase 23, being acidic, is discarded.

To the second washing vessel 30, an acidic, aqueous solvent 31 is added. The acidic, aqueous solvent 31 may be aqueous acetic acid. Alternatively, the acidic, aqueous solvent 31 may be aqueous methanol, comprising sulfuric acid. The resulting mixture, comprising the second washed lignin phase LP2 and the acidic, aqueous solvent 31, is heated and stirred to dissolve lignin to form a one-phase system OPS. Subsequently to having been stirred, the one-phase system OPS is diluted by adding a second fresh aqueous phase 33 and/or a spent third aqueous phase 42 withdrawn from a downstream washing vessel 40, 50. Upon diluting the one-phase system OPS, a heavy, lignin rich phase LP1, separates from the spent second aqueous phase 32. The separated heavy, lignin rich phase LP1 is withdrawn and fed to a stirred third washing vessel 40. Further, the spent second aqueous phase 32 is withdrawn and discarded and/or sent to a recycling unit for recycling the solvent. If the acidic, aqueous solvent 31 comprises methanol, the spent second aqueous phase 32 is typically discarded, whereas the spent second aqueous phase 32 typically is recycled if it comprises acetic acid.

To the third washing vessel 40, sulfuric acid 41 and a third aqueous phase is added. The third aqueous phase may be a third fresh aqueous phase 43 and/or a spent third aqueous phase 52 withdrawn from a downstream washing vessel 50. The mixture of lignin rich phase LP1 and the third aqueous phase is stirred to extract metal cations from the lignin rich phase LP1. Subsequently, the phases are allowed to separate, to provide a separate first washed lignin phase LP4. The separated first washed lignin phase LP4 is withdrawn and fed to a stirred fourth washing vessel 50. Further, the spent third aqueous phase 42 is withdrawn from the third washing vessel 40 and discarded and/or fed to the second washing vessel 30 and/or the first washing vessel 20 (not shown in Fig. 2). Using the spent third aqueous phase 42 to dilute the one-phase system, may reduce the need for fresh water. Similarly, using the spent third aqueous phase 42 for washing the heavy raw lignin phase LP3, may reduce the need for fresh water.

In the fourth washing vessel 50, the first washed lignin phase LP4 is washed with fresh water 51 under stirring to provide a purified lignin 53 phase and a spent third aqueous phase 52. The phases 52 and 53 are separated. Once the phases has been separated, the provided purified lignin 53 is withdrawn from the fourth washing vessel 50. Further, the spent fourth aqueous phase 52 is withdrawn and discarded and/or fed to the third washing vessel 40. The withdrawn, spent fourth aqueous phase 52 may also be fed to the first washing vessel 20 and/or the second washing vessel 30 (not shown in Fig. 2). Using the spent third aqueous phase 52 for washing the lignin rich phase LP1, may reduce the need for fresh washing water.

The pressure vessel 10 may be supplemented with a sedimentation tank, in which the heavy, raw lignin phase LP3 is separated from the light, lignin poor phase 11. Similarly, the washing vessels 20, 30, 40, 50 may consist of two separate parts - a stirring vessel and sedimentation tank, respectively. In the stirring vessel, the lignin mixture is stirred, whereas the washed lignin LP1, LP2, LP4, 53 is separated from spent aqueous phase 23, 32, 42, 52 in the separate sedimentation tank.

Without further elaboration, it is believed that one skilled in the art may, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative and not limitative of the disclosure in any way whatsoever.

Although the present invention has been described above with reference to specific embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the invention is limited only by the accompanying claims and, other embodiments than those specifically described above are equally possible within the scope of these appended claims, e.g. different embodiments than those described above.

In the claims, the term "comprises/comprising" does not exclude the presence of other elements or steps. Additionally, although individual features may be included in different claims, these may possibly advantageously be combined, and the inclusion of features in different claims does not imply that a combination of those features is not feasible and/or advantageous. In addition, singular references do not exclude a plurality. The terms "a", "an",“first”,“second” etc. do not preclude a plurality. Examples

The following examples are mere examples and should by no means be interpreted to limit the scope of the invention, as the invention is limited only by the accompanying claims.

Example 1 (comparative example)

Method

The Aqueous Lignin Purification with Hot Acids process (ALPHA) known in the art (cf. e.g. LTS 2016/0137680) was tested with the method as it is described in the publication A. S. Klett, P. V Chappell, and M. C. Thies,“Recovering ultraclean lignins of controlled molecular weight from Kraft black-liquor lignins ,” Chem. Commun. (Camb)., Vol. 51, No. 64, pp. 12855-8, 2015. The process was tested on two model substances and a further material referred to herein as base-catalysed lignin. The base- catalysed lignin is produced from black liquor, which is concentrated with membrane filtration. The concentrated retentate is base catalysed, using catalysts already present in the retentate. At last, it is separated into two liquid fractions by addition of carbon dioxide. The heavier liquid phase solidifies as it cools and the thus obtained material is used for the further experiments described below. The model substances were two lignin powders, LignoBoost (Valmet) and INDLILIN AT (WestRock, former

MeadWestvaco), respectively.

The lignin materials were mixed with an aqueous solvent comprising 30 wt% acetic acid until it contained approximately l0wt% of lignin material. In some cases where the pH was high, (i.e. > 5 in the starting material) it was adjusted to ~ pH 2.5 by additions of concentrated sulfuric acid. The mixture was stirred and heated to about 90 °C. In successful experiments, particles started to swell upon heating and eventually congregated to form a sticky and heavy phase which separated to the bottom of the vessel. In an unsuccessful experiment, the particles remained unchanged and sedimented to the bottom when the stirring was stopped. The sticky, heavy phase formed in a successful experiment can be removed with a pair of tweezers. The material slowly flows down from the tweezers but immediately solidifies as it cools. What is left in the vessel is a clear liquid.

Results

Successful experiments were conducted with the model substance LignoBoost. The system transforms from being a two-phase solid-liquid system to a two-phase liquid-liquid system. The first experiment with INDULIN AT was unsuccessful. However, the pH of the LignoBoost experiment was 2.9 compared to the pH of the INDULIN AT

experiment where the pH was 7.0. When the pH was adjusted from 7.0 to 2.5, successful experiments were conducted with INDULIN AT as well.

Experiments with base-catalysed lignin were unsuccessful. The pH was initially 8.7 but even though this was adjusted to 2.5, the experiments were not successful. Instead of a liquid lignin-rich heavy phase, the base-catalysed lignin remained as solid particles and the system remained as a solid-liquid system.

Experiments where the acetic acid concentration was increased to 50 wt% were also performed but the result were the same.

A two-step purification was completed for the LignoBoost and base-catalysed lignin (solid/liquid two-phase system), as shown in Table 1. The LignoBoost powder yielded a heavy liquid-phase that was easily separated after both washing steps. Base- catalysed lignin did not yield a heavy phase but separated as solid particles after stirring was turned off. Acetic acid concentration in both washing steps was 30%.

Table 1. A two-step purification was performed on LignoBoost and base-catalysed lignin. Total Dry Solids (TDS), Ash Content on dry basis (AC), Total Lignin Content (TLC),

Base-catalysed lignin (BCL).

Total Dry Solids (TDS) in the base-catalysed lignin heavy phase is lower compared to LignoBoost since no liquid heavy phase was formed and the washing liquid was not pushed out from the grainy bottom cake. The ash content for LignoBoost heavy phase is very low. After the first wash, it dropped slightly. However, after the second wash the weight of the crucible was lower than the starting weight causing the ash content to be negative, and thus leading to the conclusion that the method used is not precise enough to measure such low ash contents reliably. The base-catalysed lignin has a higher ash content compared to LignoBoost. After the first wash ash content is more than reduced by half. After a second wash, the ash content is further reduced by half. As the process has not worked properly, a seven fold decrease in ash content as reported in literature cannot be expected (Chem. Commun., 2015, 51, 12855). The yield that has been calculated is based on TDS and is accumulative, so it compares to the total TDS content in the initial lignin powder added. Yield is high for the LignoBoost experiments showing that small losses can be expected in the ALPHA process. It is much lower for the base-catalysed lignin as the liquid heavy phase did not form.

Example 2 - Improved purification process of lignin material using acetic acid to dissolve lignin

The herein disclosed new and improved process for purifying lignin material including dissolving lignin in an acidic, aqueous solvent was evaluated. As described herein, the method, according to a preferred embodiment, involves washing lignin with for instance sulfuric acid, an alkanoic acid such as acetic acid, and water in different combinations and under different conditions.

The three steps (i.e. prewash, AcOH wash, and postwash, respectively) typically included in the new purification process are described more in detail below. The steps may be combined differently and repeated a number of times. Before each experiment result, the type of step, order of the steps and the number of repetitions used in the particular experiment is stated. The method however remains the same. The only change is if water or 1% sulfuric acid is used in the post-wash, this is also stated before each experiment result.

Method

Prewash

First, lignin material is washed with dilute sulfuric acid (1% v/v) with a ratio of 1 :10 (lignimacid) in a“prewash”. The washing liquid and the lignin material is then separated and the prewash can either be repeated or the lignin material is moved to the second step, the acetic acid wash.

Acetic acid wash

In the acetic acid wash, lignin material is mixed with 80% (w/w) acetic acid so that the mixture contains approximately 30 wt% of lignin material. The mixture is stirred and heated to 90 °C to dissolve lignin and form a one-phase system. Before heating, the mixture, it contains visible dispersed lignin particles. If the stirring is turned off, the visible particles sediment to the bottom of the vessel, i.e. the system is a 2-phase liquid-solid system. When the temperature reaches approximately 70-90 °C, the lignin particles dissolve completely in the acetic acid and a clear dark liquid is formed, i.e. system transforms from a two-phase solid-liquid system to a single-phase liquid system.

When the lignin material has dissolved in the acetic acid it is left to interact with the acid for a short while, approximately 5 min. The acetic acid concentration is lowered by adding water so that it is approximately 50% (w/w). As the added water not is heated, the addition thereof will lower the temperature of the system. Upon diluting the acid concentration and lowering the temperature, phase separation occurs instantly and a two-phase liquid-liquid system is yielded from which a lignin-rich phase may be separated. A heavy and sticky liquid lignin-rich phase separates and sinks to the bottom of the vessel while a lighter lignin-poor liquid phase on top of it. Upon cooling the two- phase system, the fluidity of the liquid lignin-rich phase decreases, at room temperature it is solid.

Post-wash

After the acetic acid wash, a post-wash is performed to wash away remaining acetic acid and metals. This is conducted with the same method as during the prewash but with the addition of milli-Q-water, also with a ratio of 1 : 10.

Results

Prewash

To evaluate the prewash step, two wash series with a total of six consecutive washes were performed on the base-catalysed lignin. The results are presented in

Table 2, which presents the sodium levels in the lignin after each wash step. Sodium measurements are used instead of ash content as the analysis not only is faster, but also more precise. Table 2. Sodium levels at the start and after each washing step.

In Series 1, it can be seen that sodium levels are significantly decreased by the first wash steps but then level out and reach a threshold value at approximately 200 ppm of sodium. In Series 2, the concentrations are overall higher in each step, but a similar decrease in wash efficiency as more washes are performed can be seen. The reason for the higher concentrations in the second series, is believed to be shorter dwell times than in the first series. The dwell time in Series 1 was 5 to 20 min, whereas it was about 2 min in series 2.

Full wash cycle 1

The full wash cycle 1 consisted of three prewashes (1% sulfuric, 1 :10), an acetic acid wash, an intermediary post- wash (water, 1 :10), a second acetic acid wash and finally two post-washes (water, 1 : 10). All post-washes were conducted with milli- Q-water. The sodium levels after each step is presented below in Table 3.

Table 3. Sodium levels after each washing step.

The majority of the metals are removed in the prewash steps. However, as seen previously, the prewash steps are limited to approximately > 200 ppm. After the first acetic acid wash the concentration is half of that, 91 ppm and after a post-wash with water it’s halved, 45 ppm. Thus, it can be concluded that the acetic acid can remove metals which the sulfuric acid pre-washes cannot, at least not without an unreasonable amount number of repetitions. The reason why acetic acid succeeds where sulfuric acid does not, is believed to be due to the single-phase liquid system which is an essential step in the present process. A second acetic acid wash seems to have very limited effect, but with the additional post-washes, the concentration is decreased to below the limit of detection, i.e. < 13 ppm (the instrument used for analysis gave the same response for the blank control sample). Full wash cycle 2

The wash consisted of three prewashes (1% sulfuric, 1 : 10), an acetic acid wash, two post-washes with 1 % sulfuric and one post-wash with milli-Q-water. The sodium level after each step, is presented below in Table 4 Table 4. Sodium level after each washing step.

The results are similar to those above. With a single acetic acid step and two post-washes with sulfuric acid the sodium levels are at 0-12 ppm. Yield

A wash cycle consisting of two prewashes (1% sulphuric acid, 1 : 10), an acetic acid wash and two post- washes (water, 1 : 10) with milli-Q-water. The experiment was conducted to evaluate the yield in the acetic acid step. The yield in the pre- and post washes are deemed very high due to the clear and lightly discoloured washing liquid which is left after separation from the base-catalysed lignin. Thus, the majority of losses are in the acetic acid step. Sodium levels were also analysed and these are presented in

Table 5

Table 5. Sodium levels at the start, after two prewashes and of the final product.

Two prewashes, compared to three or more, result in a higher sodium level on the material which enters the acetic acid wash. The end result after a single acetic acid step and two post-washes are thus 185 ppm, this can be compared to the experiment above where the sodium levels after a single acetic acid wash and a single post-wash is 45 ppm. Thus, the prewash is seemingly a preferred step.

A total of 30 g dry base-catalysed lignin was used for the experiment. After the acetic acid step the material was dried again and a total of 25 g was collected, this results in a yield of 83%. From the light liquid phase another 1.6 g was collected by filtration, adding this material the yield increase to 89%.

Low sodium level from single acetic acid wash

The wash cycle consisted of four prewashes (1% sulfuric, 1 : 10), a single acetic acid wash, two post- washes with 1% sulfuric acid and a single post-wash with milli-Q- water. The sodium levels are presented below in Table 6.

Table 6. Sodium levels at the start, after four prewashes and on the final product.

The experiment shows that it is possible to reach low levels of sodium with a single acetic acid wash, as long as the pre- and post-wash is adequate.

Experiments without pre/post-wash

Though the above described above process consists of a prewash, an acetic acid wash and a post-wash, also a purification process consisting only of repeated acetic acid washes could be envisaged. Thus, a process without any pre/post-wash was evaluated in order to verify the importance of the step of dissolving lignin in a one- phase system.

Base catalysed material was used as starting material and the results from two experiments are presented below. The experiments are identical except for the timing of the water addition. Before water addition the acetic acid concentration is 80% and after it’s 66%. The mixture contains approximately 25 wt% lignin-material at the start of the experiments.

In one of the experiments (inventive example) water was added after the lignin- material had dissolved in the acetic acid, thus the system goes from a two-phase solid- liquid system to a single-phase liquid system as it’s heated. Then after water addition the system separates and forms a two-phase liquid-liquid system. In the second experiment (comparative example), water was added immediately as the lignin-material and the acetic acid was mixed, i.e. a lower concentration of acetic acid was used. The result is now instead that the system moves from a two-phase solid-liquid system to a two-phase liquid-liquid system as it is heated. The heavy phase is formed directly as the temperature reaches 70-90 °C, i.e. no single-phase liquid system is formed. As stated above, the reason for the experiment was to verify the importance of the formation of a single-phase liquid system in the purification.

Table 7. Comparison of experiments were a single-phase liquid system is yielded or not. Total Dry Solids (TDS), Ash Content on dry basis (AC), Total Lignin Content (TLC), Base-catalysed lignin (BCL).

The results show that completely dissolving the lignin has beneficial effects for the purification process. Not only does it result in lower ash content, but also in higher yields, both being desirable. However, the ash content for both experiments were higher than anticipated. This is believed to be due to the relative high lignin to acetic acid ratio used (25wt% lignin-material) and the relatively low TDS of approximately 50% in the heavy phase. The little amount of washing liquid (acetic acid) becomes very

concentrated with dissolved metals and thus if 50% of the heavy phase consists of this concentrated washing liquid the purification is severely haltered. To solve the problem more washing liquid is required. However, in order to improve the overall process economy, pre- and postwashes, respectively, may be added, rather than increasing the amount of acetic acid being used. Temperature and concentration

In most experiments the purification with acetic acid has been performed at about 70-90°C and with an initial acetic acid concentration of 80%. However, it has been found that the concentration of acetic acid required to dissolve lignin is highly dependent on the temperature. Increasing the temperature reduces the required amount of acetic acid. As an example, it has been shown that one-phase system could be formed at l20°C employing 65% acetic acid.

Example 3 - Improved purification process of lignin material using methanol to dissolve lignin

Apart from acetic acid, also methanol is known to dissolve lignin. Further, methanol is water soluble. Thus, methanol was evaluated as solvent in the present process.

Solubility test

An initial experiment was performed in sealed pressure-resistant glass tubes; one tube containing 2 g of lignin powder and 5 ml of 100% methanol and another tube containing 2 g of lignin powder and 5 ml of 80% aqueous methanol. The tubes were heated to 100 °C. In the tube with 100% methanol, spots of semi-dissolved lignin stuck on the walls of the glass tube, while the 80% methanol tube completely dissolved the lignin and a dark clear single-phase liquid was yielded. As the tubes cooled, a heavy liquid lignin-rich phase separated and a two-phase system was yielded. As water was added, more heavy phase separated and the fluidity of the heavy phase decreased.

The 80% methanol tube, which was completely dissolved, was first cooled and then adjusted to 60% methanol by adding water. A heavy lignin-rich phase separated and formed a highly viscous layer in the bottom of the tube. The tube was heated again and the fluidity of the heavy phase increased, at 100 °C it was flowing freely with a syrup-like texture while the surrounding light phase behaves like water.

Complete wash cycle using methanol as solvent

As 80% aqueous methanol dissolved lignin, a similar experimental method as for acetic acid (cf. example 2), was evaluated and a full wash cycle was performed. The wash cycle consisted of three pre-washes, a methanol wash and three post-washes. The pre-wash was done the same way as for the acetic acid experiments (1% H2SO4, 1 : 10 ratio). However, the resident time before centrifugation was longer, i.e. 15 to 30 min. The methanol wash was also performed in the same manner as the experiments with acetic acid.

Lignin material was mixed with 80% aqueous methanol until it comprised approximately 30 wt% lignin. Further, sulfuric acid was added so that the concentration was about 1%, to have an acid present which can act as an ion exchanger. The mixture was heated to about 60 to 70 °C (just under the boiling point of the aqueous methanol) and most of the lignin dissolved. A minor amount of the lignin did however not completely dissolve at 60-70 °C. Thus, at this temperature, the system is formally not a one-phase system, but a two-phase system with a minor separate lignin phase, though most of the lignin was dissolved. Decreasing the concentration somewhat will provide a one-phase system. The mixture was left stirring for a while before the methanol concentration was adjusted to 50% by additions of water to separate dissolved lignin. After some continued heating and stirring, the stirring was turned off and the mixture was left to cool. As it cooled, further lignin separated and the viscosity of the lignin phase increased. At room temperature, the heavy phase could be separated from the lighter liquid phase using tweezers. When left to dry at room temperature the material quickly solidified. It was then crushed and post- washed three times, first with 1% sulfuric acid and then twice with milli-Q-water. Below in Table 8, the resulting sodium levels after the various washed are provided.

Table 8. Sodium levels after each washing step.

After the methanol wash the sodium levels are very low, barely detectible at 12 ppm, and after the first post-wash the levels are below detection limit of the current method.

It can be concluded that the methanol wash behaves very much like the acetic acid wash and that low sodium levels can be achieved.

Yield

An experiment aimed at investigating the yield of the methanol wash step was performed. Lignin material was prewashed three times and then dried overnight. 20 g of dry lignin material was then mixed with 50 g of 80 % aqueous methanol, i.e. about 30 wt% lignin. The mixture was heated to 70°C and the lignin dissolved, as previously described. Thereafter the methanol concentration was lowered to 50% by adding 30 g of water to separate a heavy, lignin-rich liquid phase. The mixture was left to separate as it cooled to room temperature. The heavy phase, which have now solidified due to the increased water concentration and decreased temperature, was separated and weighed. It was then dried overnight and weighed again. Of the original 20.0 g, 18.6 g was remaining. The yield was then calculated to 92.9% and the total dry solids in the heavy phase to 48.2%.

It can be concluded that the methanol wash behaves very much like the acetic acid wash and that low sodium levels can be achieved while retaining the high or even a higher yield.

Example 4 - Improved purification process of lignin material using methanol to dissolve lignin

As an alternative to pure methanol, a methanol side stream (mill methanol) from a pulp mill (mill methanol; MM) was considered for use as solvent. Apart from methanol, mill methanol may comprise turpentine, acetone and ethanol. Especially, the presence of acetone may be positive as it may act as co-solvent. Experiments have shown that pure acetone dissolves the lignin material. Furthermore, it does so at lower temperatures compared to acetic acid and methanol. Experiments have been made where the washing liquid have been spiked with acetone in various concentrations. 5% acetone in the washing liquid dissolves all or most of the dense and sticky liquid phase which surrounded the magnetic stir rod. 20% acetone in the washing liquid dissolves any remaining heavy liquid phase and a single-phase liquid is yielded.

Thus, mill methanol (Methanol 79%, Turpentine 12%, Water 5%, Ethanol 3%, Acetone 1%) was used as solvent in a further experiment. The experiment was performed as the experiments with aqueous methanol. In short, pre-washed lignin material was mixed into mill methanol to approximately 30 wt%. Sulphuric acid was added to 1%. The mixture was heated to 60-70 °C while being stirred. Most of the lignin dissolved at ~65 °C. However, a small amount of dense and sticky liquid phase surrounded the magnetic stir rod. The mill methanol concentration was adjusted to approximately 50% by additions of water. Dissolved lignin separated and could be separated from the washing liquid. No visible differences could be seen compared to experiments with methanol. At room temperature the heavy lignin phase has solidified and could be picked up with a pair of tweezers. The material was then crushed and post- washed in the same way as in the experiments with acetic acid and methanol, respectively. The metal content in the finished material was analyzed.

It was found that the content of metal ions in the purified lignin was the same or lower than in the lignin washed with pure methanol (cf. Table 9 below).

Example 5 - detailed analysis of lignin purity

Further experiments with acetic acid, mill methanol and methanol have been performed in the same manner as outlined above. The procedures were the same for all solvents (except for solvent, i.e. washing liquid, used). In order to provide a more detailed analysis, the purity of the provided lignin was analyzed by ICP-OES

(Inductively coupled plasma - optical emission spectrometry) and the results are provided below in Table 9.

Table 9. Metal concentration in the final lignin product in ppm (pg/g). Comparison between different experiments.

As can be seen in Table 9, the present process provides lignin with very low metal content. The major contaminant sodium (Na) is reduced to about 1 ppm. The relative high content of sulfur is actually a potential advantage, when the lignin is to be treated in a refinery using metal catalysts, as the catalyst typically is operated in sulfided state. Given the sulfur content, there is no or less need to add sulfur as make up in catalytically treating lignin.