Method of producing lignin with reduced amount of odorous substances

Abstract

The present invention relates to a method of producing lignin with reduced amount of odorous substances comprising the steps of: i) providing a solid phase lignin containing starting material to be contacted with a water solution comprising alcohol; ii) dissolving at least guaiacol and etylguaiacol content from the lignin into the water solution, and iii) draining off the water solution with its content of guaiacol and etylguaiacol; iv) obtaining a lignin material with a reduced content guaiacol and etylguaiacol. The alcohol is preferably a C1-C4 alcohol, preferably ethanol. The invention also relates to a lignin product with reduced odour obtained and/or obtainable by the method and to the use of the obtained lignin as a component in polymer blends, an additive or filler in building materials, as binding agent in adhesives, and/or for the production of a carbon fibre, especially in indoor applications.

Claims

1. A method for treatment of lignin to reduce odor from the lignin, wherein: the lignin is maintained in solid phase and treated such that the chemical structure and bonding thereof substantially remain, and the method comprises providing a solid phase lignin containing starting material to be contacted with a water solution comprising alcohol; dissolving at least guaiacol and etylguaiacol content from the lignin into the water solution; draining off the water solution with its content of guaiacol and etylguaiacol; and obtaining a lignin material with a reduced content of guaiacol and etylguaiacol.

2. The method according to claim 1, wherein the lignin containing starting material has dry matter content above 50% (w/w), and a lignin content of this dry matter content exceeding 50% (w/w).

3. The method according to claim 1, wherein the lignin containing starting material is mixed with water and alcohol in any order of mixing, and the resulting mixture is allowed to mature during a residence time of at least 5 minutes before draining off the water solution.

4. The method according to claim 1, wherein the lignin containing starting material is a filter cake subjected to a displacement wash using the water solution comprising alcohol.

5. The method according to claim 1, wherein the content of alcohol in the water solution is in the range of 0.1-50% by weight based on the lignin dry weight.

6. The method according to claim 5, wherein the content of alcohol in the water solution is in the range of 0.5-10% by weight based on the lignin dry weight.

7. The method according to claim 6, wherein the content of alcohol in the water solution is below 5% by weight based on the lignin dry weight.

8. The method according to claim 5, wherein the alcohol content any of ethanol, methanol, propanol or butanol.

9. A method according to claim 1 for treatment of lignin to reduce odor from the lignin, comprising: providing a solid phase lignin containing starting material to be contacted with a water solution comprising alcohol; dissolving at least guaiacol and etylguaiacol content from the lignin into the water solution; draining off the water solution with its content of guaiacol and etylguaiacol; and obtaining a lignin material with a reduced content of guaiacol and etylguaiacol, wherein the lignin containing starting material is obtained from a process comprising the steps of: a. precipitating lignin by acidifying black liquor obtained from an alkaline chemical pulping process; b. dewatering and/or filtrating the obtained lignin to provide a first filter cake; c. re-suspending the lignin; d. adjusting the pH of the obtained suspension in step c) to a pH lower than 6; e. dewatering and/or filtrating the acidic suspension from step d) to provide a second filter cake; and f. washing and dewatering the second filter cake so as to obtain the lignin containing starting material.

10. The method according to claim 1, wherein the water solution is an alcohol containing liquid phase solution, and said alcohol containing liquid phase solution comprises carboxylic acid.

11. The method according to claim 1, wherein the water solution is an alcohol containing liquid phase solution, and wherein the method further comprises utilizing the alcohol containing liquid phase solution to obtain lignin from black liquor in an alkaline chemical pulping process.

12. The method according to claim 1, wherein the water solution is an alcohol containing liquid phase solution, and the alcohol containing liquid phase solution comprises an acid which is not carboxylic acid.

13. A lignin product obtained and/or obtainable by the method according to claim 1.

14. The lignin product according to claim 13, wherein the concentration of guaiacol and etylguaiacol in the obtained lignin product is at least 50% less than in the lignin containing starting material, the concentration being calculated from a peak area of a respective peak in a chromatogram.

15. A polymer blend comprising the lignin material according to claim 13.

16. An additive comprising the lignin product according to claim 13.

17. A filler for a building material or construction material comprising the lignin product according to claim 13.

18. A binding agent for adhesive, the binding agent comprising the lignin product according to claim 13.

19. A carbon fiber comprising the lignin product according to claim 13.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a flow chart illustration of the method according to the invention;

(2) FIG. 2 shows a flow chart illustration of an example method according to the present invention; and

(3) FIG. 3 shows a flow chart illustration of a process for lignin isolation;

(4) FIG. 4 shows how the inventive addition of alcohol, here in form of EtOH, may be added already into the Lignoboost process.

DETAILED DESCRIPTION

(5) In the method of the present invention, which is schematically illustrated in a flow chart in FIG. 1, is lignin treated to obtain a lignin product with reduced odour. By lignin is meant any lignin, which may be pure lignin or lignin with small amounts of impurities, and with dry matter content above 50% and more than 50% of this dry matter content being lignin, also referred to isolated lignin. According to the present method, the amount or concentration of the odorous substances can be reduced in lignin products while it is possible to obtain high yield of lignin.

(6) The less odorous or substantially odourless lignin product can be used in a wide range of applications including indoor applications which is a huge advantage.

(7) Lignin to be treated by the present method is according to one aspect obtained as a by-product from an alkaline chemical pulping process. The alkaline chemical pulping process may be sulphate, also called kraft, process or soda process. Both processes result in alkaline material by cooking the fibre-based material in white liquor at a cooking temperature of from about 130-200 C. to make lignin soluble in the cooking liquor. White liquor is a mixture of sodium hydroxide and sodium sulphide. By black liquor is meant the cooking liquor obtained during cooking from the alkaline chemical pulping process. Black liquor contains residues of white liquor and other pulping chemicals, lignin, hemicelluloses and other extractives from the fibre-based material. Even though the process is exemplified by the above processes may lignin isolated by any process be used in this method, in most cases could the lignin also be treated during the process itself.

(8) The fibre-based material from which the isolated lignin is extracted can be softwood, hardwood or non-wood, such as annual plants. The softwood tree species can be for example, but are not limited to: spruce, pine, fir, larch, cedar, and hemlock. Examples of hardwood species from which pulp useful as a starting material in the present invention can be derived include, but are not limited to: birch, oak, poplar, beech, eucalyptus, acacia, maple, alder, aspen, gum trees and gmelina. Preferably, the fibre-based material mainly comprises softwood. The fibre-based material may comprise a mixture of different softwoods, e.g. pine and spruce. The fibre-based material may also comprise a non-wood raw material, such as bamboo and bagasse. The fibre-based material may also be a mixture of at least two of softwood, hardwood and/or non-wood. Hence, the origin of the lignin is of less importance and the method should be useful for any lignin regardless of its origin.

(9) The amount of odorous or odour containing substances can be reduced in the lignin product by means of extracting. Especially, organic malodorous substances can be extracted by the present method. Extracting is suitably selective, meaning that substantially mainly the odorous or odour containing substances are extracted. The extraction is obtained by means of the addition of alcohol to a water solution, slurry or a solid body comprising lignin. The odorous substances are extracted in or leached by alcohol, e.g. methanol, ethanol, propanol or butanol, in order to remove odorous substances from the lignin. In general, C1-C4 alcohols are considered most suitable for this process. The amount of the alcohol to be added should be kept at a sufficiently low level to avoid that lignin is seriously affected. In general, the amount of alcohol to be added is less than 30%, and the amount may suitably be 0.2 to about 20%, based on the dry weight of lignin. As previously discussed, the amount of alcohol could be decided based on the desired level of purification and not use more than needed to reach this level. The alcohol including the extracted odorous substances is removed from the process and optionally recirculated at least partly back in the process.

(10) Reference is now made to FIG. 2 in which a process involving the method of the present invention is schematically illustrated more in detail

(11) The lignin is preferably slurried in water in stage i), and thereafter is alcohol (EtOH) added in step ii), However the method also works if alcohol is mixed into the water before adding to the lignin. In step iii) may also optionally carboxylic acid (HAc) and/or sodiumsulfite (Na.sub.2SO.sub.3) be added to the water or lignin slurry. The method may comprise a maturing period of desired length that should last for at least 5 minutes, preferably at least 30-60 minutes, or even longer if sufficient storage volume is available. The maturing period can be adjusted by the skilled person to different processes and needs. Depending on the state of the lignin, the maturing may be desired to continue for different times. In case the lignin is dissolved in a solution a shorter time may be needed compared to if the lignin is present as particles, e.g. from a dissolving a wet filter cake. In case the lignin is present as even larger entities, e.g. as a solid, dry filter cake or larger particles, could even longer time be desired to partly disintegrate the larger entities and/or allowing the alcohol solution to be better soaked into the solid matter in order to enable a more efficient leaching operation.

(12) After leaching is the slurry dewatered obtaining a lignin product with fewer odours, and explicitly with a substantial reduction in guaiacol and/or etylguaiacol.

(13) However, in most applications the lignin needs to be transported so that lignin can be used in different applications and solid lignin, either re-precipitated or present as solid matter in the extraction/leaching operation from the process, may thus be subject for dewatering and possibly filtering, washing and drying if desired. As further illustrated in FIG. 2, the method can thus comprise a step iv) in which lignin in the form of a filter cake is dewatered and/or filtrated. Dewatering can be performed by any means to withdraw water. For example, the dewatering is performed by using centrifugation, a filter press apparatus, a band filter, a rotary filter, such as a drum filter, or a sedimentation tank, or similar equipment. Filtration can be performed by using any conventional apparatus suitable for filtration, such as filter press or a band filter. The filtrate from the dewatering step can be re-circulated to a recovery system, and the alcohol can be further recirculated back in the process. Subsequent to dewatering and/or filtration, the obtained lignin in the form of a filter cake is washed in step v). Washing can be performed by using water and/or small amounts of alcohol, e.g. 0.5-10% by weight based on the weight of lignin, such as ethanoic acid. Also during the wash, it is advantageous if the pH is kept acidic, such as from pH 1.5 to pH 5, preferably from pH 1.5 to pH 3.5. In this way the yield of lignin can be further increased. After washing, the obtained filter cake is dried in step vi) and a final lignin product with reduced odour is obtained.

(14) Lignin to be treated according to the present invention can be obtained from a process for separation or isolation of lignin which is illustrated in the flow charts of FIG. 3, and which is also commercially called for LignoBoost process, and FIG. 4 illustrates how the Lignoboost process may be boosted by optionally adding alcohol in differing process positions. In step a) of the process lignin is precipitated by acidifying black liquor obtained from an alkaline chemical pulping process. The chemical process is preferably kraft process. Acidifying can be performed by any means sufficient to acidify black liquor. Preferably the acidifying is performed by adding CO.sub.2 to said black liquor in a carbonating stage. Carbon dioxide is the preferred alternative since this acid may be obtained from the pulp mill in form of exhaust gases from the lime kiln. By using carbon dioxide in step a) so as to acidify the black liquor approximately to a pH between 11.5 and 9, normally around pH 10, a lignin product can be obtained, and the filtrate obtained from subsequent dewatering is still alkaline and may be sent to the black liquor evaporation train in the recovery island without changing the pH level of the black liquor flow to said evaporators.

(15) In stage (c) the dewatered lignin filter cake is (still alkaline) resuspended in an acidic slurry using H.sub.2SO.sub.4.

(16) The lignin product can be used as fuel or for the production of chemicals and has reasonably low ash content and a low tendency to cause corrosion.

(17) Prior to step a), the process optionally comprises a pre-step in which black liquor can be filtrated for example by membrane filtration. By this mechanical separation it is possible to separate for example particulate material, hemicelluloses and/or it is possible to mechanically fractionate lignin and to obtain a specific fraction of lignin. Another way to achieve fractions of lignin is fractionation by different pH levels and select lignins from a certain pH-window for further processing. In this case may of course alcohols also be used which may improve the overall efficiency in removing odorous substances from the lignin.

(18) In the step b) as illustrated in the flow chart, the obtained lignin is dewatered and/or filtrated in a first dewatering and/or filtration step. Also in this connection the dewatering may be performed by any means to withdraw water, for example by using centrifugation, a filter press apparatus, a band filter, a rotary filter, such as a drum filter, or a sedimentation tank, or similar equipment. For example, when using a filter press apparatus the filter cake obtained through dewatering may be blown through by gas or a mixture of gases, preferably compressed air in order to dispose of the remaining liquid, such as black liquor, before resuspending the obtained cake as set out in step c). The filtrate from the dewatering step can be re-circulated to the pulp mill black liquor recovery system. The pH level adjustment before dewatering/filtration, made by addition of acid preferably CO2 (g), can be combined with an adjustment of ion strength, preferably by using alkali metal ions or multivalent alkaline earth metal ions, most preferred calcium ions. Higher ion strength gives at a given pH lower yield losses of lignin as the lignin becomes more stable. Even though it is a dewatering step could small amounts of alcohol be used during this step, e.g. in the end of or after the dewatering step.

(19) As the first Lignoboost product produced was in form of fuel pellets, a reduction of inorganic content was needed as such content could cause corrosion in power boilers. Thus, the step c) comprises re-suspending the lignin to form an acidic suspension by adding H.sub.2SO.sub.4. Generally, by a suspension is meant a heterogeneous mixture containing liquid- and small solid particles, such as about 1 m or larger. The particles in the suspension are able to settle whereby it is possible to obtain a filter cake. Also in this step alcohols could be used to increase removal of odorous substances.

(20) In the step d), the pH level is preferably adjusted to below approximately pH 6, and suitably below approximately pH 4, and preferably below 3.5, e.g. 2-2.5. The pH level is preferably from pH 1.5 to pH 3.5 to ensure that substantially all lignin is re-suspended and to give good filtration properties in the following dewatering filtration step. It is also important in order to isolate a lignin with low content of inorganics. Acidifying can be performed with the same chemicals as in connection with acidifying black liquor and as described above. Alcohols could thus be used herein, possibly together with other acids.

(21) After acidifying, a second dewatering and/or filtration step e) is performed similarly as the above-defined step b). Alcohol could be used also during this step.

(22) In step f), the obtained filter cake is washed and the washing liquid, such as acidified water, can have a pH level of below approximately pH 6, preferably below approximately pH 4. The pH level is most preferred a pH from 1.5 to 3.5. The washing liquid is dewatered and in one 10 embodiment of the invention, the obtained filter cake is treated with the method steps I)-iii) without drying the filter cake before the treatment. This step is suitable to be performed in the presence of alcohols in the washing liquid.

(23) In general, the positive effect of using alcohol treatment in the process above defined by steps a) to f) is probably most efficient if it is used in connection with steps e) and f).

(24) According to another embodiment the method further comprises the step g) of drying the filter cake, whereby the filter cake can be easily e.g. transported.

(25) The filter cake obtained from the final dewatering step above, in connection with the method of treating lignin to reduce odorous substances, either dried in the optional drying step g) or not, may thus be used for the alcohol treatment described in FIGS. 1 and 2. It is to be noted that the acidic reslurrying in stage c and d in the Lignoboost process do not effectively reduce content of guaiacol or etylguaiacol.

(26) With the present method of producing lignin with reduced amount of odorous substances it is possible to obtain high yield, such as over 80% by weight, based on the weight of the isolated start lignin. Even higher yields are possible, such as over 85% and up to about 90-95%. Thus, the method leads only to a minor material loss which is a major advantage. The high yield can be obtained due to the fact that the extraction method is rather moderate meaning that substantially no fractionation of the lignin occurs due to odour reduction process. Mainly only odorous substances, especially organic odorous substances, are extracted. The odorous substances that are extracted with the present method comprise at least guaiacol and etylguaiacol but also one of dimethylsulfide, dimethyldisulphide, dimethyldisulphide, dimethyltetrasulphide, and other phenolic compounds. These substances lead to malodourous gases which have made lignin products difficult to use in indoor applications. The concentration of the odorous substances may be reduced by at least 50%, the concentration being calculated from a peak area of a respective peak in a chromatogram. Preferably, the concentration of the odorous substance is reduced by at least 70%. Thermogravimetry analysis (TGA) of the lignin product produced by the present process shows that the lignin is essentially not affected by the present process. This further supports the conclusion of the present invention that the present process is gentle towards lignin, while the odorous substances can be reduced effectively.

(27) Due to the obtained odour reduction, the lignin product is possible to use in many applications. For example lignin can be used as a component in polymer blends, an additive or filler in building materials, as binding agent in adhesives, and/or for the production of a carbon fibre.

(28) The lignin product can also be used in building materials that are intended for use indoors. Further application areas are for example manufacturing of fibre boards, as a crosslinking agent in vehicle tyres, as antioxidants and as UV-protectors. The application areas are not limited to the above-mentioned areas, other application areas are possible.

(29) The invention is further described in the following example.

EXAMPLE

(30) Similar test has been performed as detailed in previously filed SE1451641-3 (hereby incorporated by reference), having a filing date of Dec. 22, 2014. In SE1451641-3 has been tested to clean lignin obtained from the LignoBoost process, using a process where the lignin is first dissolved in an alkaline solution, adding a 1% (on weight) of EtOH, followed by an additional acidification until lignin precipitates again.

(31) In the tests performed according to the invention, but without dissolving the lignin by an alkaline charge and subsequent acidification for precipitation, the same Lignoboost lignin is simply leached in a water solution with a small charge of EtOH (1% on weight) reaching a reduction of guaiacol in the same order as with the cleaning tests of SE1451641-3. This is performed without extra charge of alkali for dissolution of the lignin, and extra charge of acidifier to precipitate lignin again. Thus a rather modest charge of 10% EtOH and an addition of 0.1% sodiumsulfite in a water solution obtains a yield of over 90% odour free lignin, and the leaching do not affect the physical, chemical or mechanical properties of the odour free lignin produced. The surprising finding that a simple water leaching with a modest charge of alcohol provides the same effect as the more complicated process outlined in SE1451641-3 is not fully understood, but the mechanisms may be related to that the guaiacols are bound in small lignin particles or chemically bound to the lignin chains.