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 step of: providing a solid phase lignin containing starting material to be contacted with a water solution comprising carboxylic acid; dissolving the guaiacol and etylguaiacol content from the lignin into the water solution, and draining off the water solution with its content of guaiacol or etylguaiacol; obtaining a lignin material with a reduced content of guaiacol or etylguaiacol. The invention also relates to a lignin product with reduced odour obtained and/or obtainable by the method 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. Method for treatment of lignin in order to reduce the amount of guaiacol and etylguaiacol from the lignin characterized in that said method comprises the step of: providing a solid phase lignin containing starting material to be contacted with a water solution comprising carboxylic acid; dissolving the guaiacol and etylguaiacol content from the lignin into the water solution, and draining off the water solution with its content of guaiacol or etylguaiacol; obtaining a lignin material with a reduced content of guaiacol or etylguaiacol.

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

3. A method according to claim 1 wherein the lignin containing starting material is mixed with water and carboxylic acid 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. A 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 carboxylic acid.

5. A method according to claim 1 wherein the content of carboxylic acid in the water solution is in the range of 0.1-50% by weight.

6. A method according to claim 5 wherein the content of carboxylic acid in the water solution is in the range of 0.5-10% by weight.

7. A method according to claim 6 wherein the content of carboxylic acid in the water solution is below 5% by weight.

8. A method according to claim 5 wherein the carboxylic acid content is an acetic acid.

9. A method according to claim 5 wherein the carboxylic acid is a methanoic acid or ethanoic acid.

10. A method according to claim 5 wherein the carboxylic acid content is combined with an addition of an alcohol.

11. A method according to claim 1 characterised in that the carboxylic acid containing liquid phase solution also comprises another acid.

12. A method according to claim 1 wherein the lignin containing starting material is obtained from a process comprising the steps of: i. precipitating lignin by acidifying black liquor obtained from the alkaline chemical pulping process; ii. dewatering and/or filtrating the obtained lignin to provide a first filter cake; iii. re-suspending the lignin; iv. adjusting the pH of the obtained suspension in step c) to a pH lower than 6; v. dewatering and/or filtrating the acidic suspension from step d) to provide a second filter cake; and vi. washing and dewatering the second filter cake; whereby the lignin containing starting material is obtained.

13. Lignin product obtained and/or obtainable by the method according to claim 12.

14. 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. Use of the lignin product according to claim 13 as a component in polymer blends, an additive or filler in building and construction materials, as binding agent in adhesives, and/or for the production of a carbon fibre.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0060] FIG. 1 shows a flow chart illustration of the method according to the invention;

[0061] FIG. 2 shows a flow chart illustration of an example method according to the present invention; and

[0062] FIG. 3 shows a flow chart illustration of a process for lignin isolation using the Lignoboost process;

[0063] FIG. 4 shows how the inventive addition of carboxylic acid, here in form of HAc, may be added already into the Lignoboost process.

DETAILED DESCRIPTION

[0064] 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. 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.

[0065] 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 130-200C 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.

[0066] 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.

[0067] 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 carboxylic acid to a water solution, slurry or a solid body comprising lignin. The odorous substances are extracted in or leached by carboxylic acid, preferably methanoic acid or ethanoic acid, in order to remove odorous substances from the lignin. The amount of the carboxylic acid to be added should be kept at a sufficiently low level to avoid that lignin is seriously affected. In general, the amount of carboxylic acid 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 carboxylic acid could be decided based on the desired level of purification and not use more than needed to reach this level. The carboxylic acid including the extracted odorous substances is removed from the process and optionally recirculated at least partly back in the process.

[0068] Reference is now made to FIG. 2 in which the method according to the present invention is schematically illustrated more in detail

[0069] The lignin is preferably slurried in water in stage i), and thereafter is carboxylic acid (HAc) added in step ii), However the method also works if HAc is mixed into the water before adding to the lignin. In step iii) may also optionally alcohol (EtOH) 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 carboxylic acid to be better soaked into the solid matter and to enable a more efficient leaching operation.

[0070] After leaching is the slurry dewatered obtaining a lignin product with fewer odours, and explicitly with a substantial reduction in guaiacol and/or etylguaiacol.

[0071] The lignin cake obtained after dewatering or filtering will have a reduced concentration of odorous substances after this treatment. However, in most applications the lignin may need washing and drying if desired. 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 carboxylic acid 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 iii). Washing can be performed by using water and/or small amounts of carboxylic acid, 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 iv) and a final lignin product with reduced odour is obtained.

[0072] 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 chart 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 HAc in differing process positions. In step a) of the process lignin is precipitated from alkaline black liquor 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.

[0073] In stage (c) the dewatered lignin filter cake (still alkaline) is resuspended in an acidic slurry using H.sub.2SO.sub.4.

[0074] 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.

[0075] 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 carboxylic acids also be used, together with other acids, which may improve the overall efficiency in removing odorous substances from the lignin.

[0076] 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 carboxylic acid be used during this step (see FIG. 4), e.g. in the end of or after the dewatering step.

[0077] 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 carboxylic acids could be used to increase removal of odorous substances.

[0078] 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. The acidic conditions establish an efficient leaching of metal ions from the lignin. Acidifying can be performed with the same chemicals as in connection with acidifying black liquor and as described above. Carboxylic acids could thus be used herein, possibly together with other acids.

[0079] After acidifying, a second dewatering and/or filtration step e) is performed similarly as the above-defined step b). Carboxylic acid could be used also during this step.

[0080] 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 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 carboxylic acids in the washing liquid.

[0081] In general, the positive effect of using carboxylic acid 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).

[0082] 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.

[0083] 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 carboxylic acid 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.

[0084] 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 minor material losses which are 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, and the cleaning process may be validated by measuring the content of guaiacol and etylguaiacol before and after the cleaning process using spectrum analysis. The odorous substances that are extracted with the present method comprise at least guaiacol and etylguaiacol but also one of dimethyl sulphide, 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.

[0085] 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. 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.

[0086] The invention is further described in the following example.

Example

[0087] 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.

[0088] In the tests performed according to the invention the same Lignoboost lignin is simply leached in a water solution with a small charge of HAc (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 1% HAc in a water solution obtains a yield of 95% odour free lignin, and the leaching do not affect the physical, chemical or mechanical properties of the odour free lignin produced.