Method for activating and precipitating lignin

Abstract

According to an example aspect of the present invention, there is provided a method of producing reactive lignin from an alkaline lignin containing stream, such as black liquor, e.g. kraft lignin, by using thermal treatment with temperatures between 200 and 250 C. for simultaneous activation (for example by demethylation and/or demethoxylation) and precipitation of the lignin.

Claims

1. A method for activating and recovering lignin from an alkaline lignin-containing feedstock by a thermal treatment, the method comprising: simultaneously activating and precipitating the lignin in the feedstock by a thermal treatment being carried out on the feedstock having a dry matter content of between 10 and 40 wt-% and a pH of 10-12, without added demethylation and demethoxylation enhancing agents, and at a self-generated pressure, by applying temperature between 200 and 220 C. and a retention time between 1 and 7 hours to the feedstock, wherein the pH, temperature, and retention time are selected along with the self-generated pressure to provide simultaneous demethylation and demethoxylation of the lignin.

2. The method of claim 1, wherein the alkaline lignin-containing feedstock is from an alkaline pulping process.

3. The method of claim 1, wherein the dry content of the alkaline lignin-containing feedstock during the activation and precipitation step is between 20 and 40 wt-%.

4. The method of claim 1, comprising operating at a self-generated pressure between 15 and 40 bars.

5. The method of claim 1, comprising operating the thermal treatment step using a retention time of 1 to 4 hours.

6. The method of claim 1, wherein, after the activation and precipitation, the lignin is purified by acidic washing.

7. The method of claim 1, further comprising removing excess filtrate, whereby the lignin is obtained.

8. The method of claim 1, wherein a number of reactive sites of the activated lignin for curing reactions with aldehydes is increased at least 1.5 fold compared to a number of reactive sites of a lignin in untreated feedstock, and wherein the method further comprises substituting the recovered lignin for phenol in a curing reaction with an aldehyde.

9. The method of claim 1, wherein the activated lignin comprises a methoxyl percentage of from 0.5-7 mmol/g.

10. The method of claim 1, wherein the activated lignin comprises at least 1 mmol/g of non-methoxylated catechol-type lignin units and at least 0.5 mmol/g of p-hydroxyphenyl-type lignin units.

11. The method of claim 10, wherein the activated lignin comprises 1.5-1.9 mmol/g of non-methoxylated catechol-type lignin units and 1.0-1.5 mmol/g of p-hydroxyphenyl-type lignin units.

12. The method of claim 1, wherein the activated lignin comprises 0.7-1.0 mmol/g methoxylated guaiacyl units.

13. The method of claim 1, wherein a number of reactive sites of the activated lignin for a reaction with formaldehyde for formation of a phenol formaldehyde resin is increased compared to a number of reactive sites of a lignin in feedstock untreated by the process of claim 1, and wherein the method further comprises reacting the activated lignin with formaldehyde to form the phenol formaldehyde resin.

Description

EMBODIMENTS

(1) The present technology describes a method of producing highly reactive lignin from lignin containing streams, such as black liquor, by thermal treatment and minor pH adjustment.

(2) FIG. 1 illustrates an increase in the proportion of p-hydroxyphenyl and catechol units in comparison to reference SW (softwood) kraft lignin as a result of demethylation and demethoxylation reactions, in accordance with at least some embodiments of the present invention.

(3) FIG. 2 illustrates an increase in the proportion of non-methoxylated p-hydroxyphenyl and catechol units, as well as less methoxylated guaiacyl and methoxycatechol units in comparison to reference HW (hardwood) kraft lignin as a result of demethylation and demethoxylation reactions, in accordance with at least some embodiments of the present invention.

(4) FIG. 3 illustrates the viscosity development in PF (phenol formaldehyde) resin synthesis, in accordance with at least some embodiments of the present invention.

(5) FIG. 4 illustrates resin curing at high phenol substitution levels according to the gel times, in accordance with at least some embodiments of the present invention.

(6) According to one embodiment, lignin with highly reduced methoxyl content can be produced from alkaline lignin containing feedstock, obtained for example from alkaline pulping process, such as kraft black liquor, by using the method of the present invention as herein described, in which lignin is precipitated and activated simultaneously. However, the method can be utilized also for recovery of any other alkaline lignin containing stream.

(7) Lignin precipitation accompanied by activation through demethylation and/or demethoxylation provides better means for utilisation of otherwise less reactive lignin for example in PF resins, and thus being especially beneficial for more methoxylated hardwood lignin separation.

(8) According to one embodiment, the method comprises simultaneous activation and precipitation of lignin from lignin containing liquid stream by applying temperatures between 200 and 250 C. and a retention time between 0.5 and 10 h, more preferably 1-7 hours, and typically more than 1 hour.

(9) Such thermal treatment increases the reactivity, i.e. the amount of reactive sites, of lignin.

(10) According to another embodiment the method operates at a thermal temperature between 200 and 240 C., particularly at a temperature of between 220 and 240 C., during a retention time between 0.5 and 10 h, more preferably 1-7 hours, and typically more than 1 hour. Important features of the invention are that the temperature is not too high, while the retention time should be sufficiently long.

(11) According to a further embodiment, the simultaneous activation and precipitation step is carried out without added demethoxylation and demethylation enhancing agent.

(12) It has been observed by the inventors of the present invention that during lignin activation and precipitation, simultaneous demethylation and demethoxylation occurs, which create new reactive sites that are required especially in PF resin production for reactions with formaldehyde, but being beneficial also in more generally in chemical or enzymatic modification of lignin. Demethylation activates the C2 and C6 position of the aromatic ring, creating catechol and methoxycatechol units, particularly cathecol and methoxycatechol units, depending on the feedstock. Demethoxylation on the other hand activates one or two more ortho position of the lignin phenolic unit, again depending on the feedstock. Few possible examples are shown in scheme 2 below:

(13) ##STR00002##

(14) It is therefore possible to alter the structure of lignin, by first choosing a desired feedstock and then creating new reactive sites by using the method as herein described.

(15) According to one embodiment, the lignin is activated by demethylation and demethoxylation reactions, which generate one or more additional reactive site(s) to C2, C3, C5 and/or C6 positions of the aromatic ring depending on raw material, as described in Scheme 2. The increase in the amount of reactive sites by the method as herein described is at least twofold, such as threefold, or even higher, compared to reactivity of commercial acid precipitated lignins (as shown later in Table 1).

(16) According to a preferred embodiment, the obtained lignin material will have a methoxyl percentage of less than 10, typically 0.5-7, preferably 0.8-5.5.

(17) According to another embodiment, the activation increases the proportion of non-methoxylated units in the lignin, such as p-hydroxyphenyl and catechol units of the lignin with SW lignin. With HW lignin also formation of methoxycatechols is possible, particularly of methoxycatechols.

(18) According to a further preferred embodiment, the obtained lignin material will have an increased proportion of catechol- and p-hydroxyphenyl-type lignin units in its structure of at least 1 mmol/g and 0.5 mmol/g, respectively, preferably more than 1.0 mmol/g and at least 0.8 mmol/g, respectively.

(19) The process is suitable for alkaline lignin containing streams where lignin is either dissolved or colloidal, or alternatively any type of lignin can be dispersed or dissolved in alkaline aqueous solution. Such streams can originate as such from kraft, and soda cooking processes. Alternatively, lignin can be hydrolysis lignin from 2.sup.nd generation bioethanol residues or from breweries if dissolved or dispersed for example for lignin extraction.

(20) Particularly, the selected stream originates from the kraft process, whereby kraft lignin is used.

(21) One important feature of the invention is the dry content of alkaline lignin stream, such as the black liquor, which should be between 10 and 50 wt-%, preferably between 20 and 40 wt-%, most suitably between 20 and 30 wt-%.

(22) According to one embodiment, the thermal treatment method self-generates a pressure between 15 and 40 bars when using the above described process conditions.

(23) According to one embodiment the method is characterized by operating at an alkaline pH, such as between 9 and 13. Yield can be increased and structure of the final lignin can be modified in alkaline raw materials by adding acid catalyst for adjusting the pH to desired level.

(24) According to one embodiment, the present method provides means for increased phenol replacement levels especially in PF resins, which are not achieved with conventional lignins.

(25) According to a further embodiment, after the activation and precipitation, the lignin material can be purified by acidic washing.

(26) According to even further embodiment, the method of the present invention includes the following steps: placing black liquor from softwood kraft pulping process having dry content of 20-40 wt-%, preferably of 20-30 wt-%, into a reactor, adjusting pH of the black liquor between 9-12 by using a pH lowering agent, thermally treating the pH-adjusted black liquor in temperatures between 220 C. and 240 C. under pressures between 15-40 bars for 1 to 7 hours, providing fractions of activated and precipitated lignin and remaining liquid, separating the activated and precipitated lignin fraction from the remaining liquid fraction in a centrifuge, purifying the lignin by using acidic washing, and drying the lignin product.

(27) In addition, a lignin material having low methoxyl content and increased proportion of for example catechol and p-hydroxyphenyl type lignin units in SW lignin structure, and additionally also methoxycatechol structures in HW lignin structure, particularly methoxycatechol structures, belongs to the scope of the present invention. Such lignin material may be used for example in phenol formaldehyde resin applications.

(28) It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

(29) Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases in one embodiment or in an embodiment in various places throughout this specification are not necessarily all referring to the same embodiment. Where reference is made to a numerical value using a term such as, for example, about or substantially, the exact numerical value is also disclosed.

(30) As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and example of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

(31) Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

(32) While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

(33) The verbs to comprise and to include are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of a or an, that is, a singular form, throughout this document does not exclude a plurality.

INDUSTRIAL APPLICABILITY

(34) At least some embodiments of the present invention find industrial application in generating highly reactive lignin, which makes the lignin material more suitable for several industrial applications. Highly reactive lignin can be used for example for phenol formaldehyde (PF) resins for replacement of phenol. Demethoxylation increases the amount of reactive sites in aromatic ring and demethylation also creates new phenolic units that can be utilized in phenolic epoxy resins. This lignin with higher proportion of reactive phenolic functionalities and unoccupied ortho-positions also provides better possibilities for lignin functionalization by means of chemical or enzymatic modification. In addition, the increased amount of phenolic units, especially formation of catechol type units, is expected to improve the antioxidative properties of the produced lignin. This can be utilized in several applications, such as in rubber and plastic products. Similarly the catechol units can improve metal chelation efficiency, and be utilized e.g. in waste water treatment. Other suitable application areas are found among dispersants and surfactants. The structure of the produced lignin can also be optimized in the process by varying process conditions so that the lignin may be used as replacement of fossil-based carbon black, as additive providing reinforcement, UV-stability, antioxidative properties, colouring and thermal stability, and in conventional applications using carbon black, such as rubber, composites, inks and paints. Alternatively, it can be used as a raw material in activated carbon manufacture.

EXAMPLES

Example 1General Method

(35) Raw Material

(36) Black liquor from softwood and hardwood kraft pulping process Dry content of the raw material: 10-50 wt-%, Trials done at 20 wt-% and 30 wt-%
Method Temperature of 200 to 250 C. Residence time 30 min-10 h Self-generated pressure 15-40 bar pH alkaline 9-13 Product purification after the activation and precipitation: the lignin material is purified by acidic washing

(37) In this example, black liquor from softwood kraft pulping process having dry content of 20-30 wt-% was placed into a reactor and pH was adjusted between 9-12 using CO.sub.2 as a pH lowering agent. Then the black liquor was thermally treated in the temperatures between 220 and 240 C. under pressures between 15-40 bars for 1 to 7 hours. Then the activated and precipitated lignin was separated from the remaining liquid in a centrifuge. The separated lignin was purified using acidic washing and dried. Hardwood black liquor was treated similarly at 220 C. and pH 11 for 4 h.

(38) Table 1 shows the amounts of different phenolic hydroxyl group species (mmol/g) and percentage of methoxyl group in softwood lignin samples after the thermal treatment with varying temperature, retention time and pH, determined by .sup.31P NMR. Significantly lower amount of methoxylated guaiacyl units and higher content of non-methoxylated catechol and p-hydroxyphenyl type units was detected together with lower methoxyl content compared to a to typical industrial softwood kraft lignins recovered by traditional acidic precipitation. Total content of phenolic hydroxyl and carboxylic acid groups was also higher, whereas the content of aliphatic hydroxyl groups was lower. Similarly, the demethylation/demethoxylation was evident with hardwood lignin (Table 1). The total amount of phenolic units was significantly increased together with decreased methoxyl content compared to the reference hardwood kraft lignin that was recovered by traditional acid precipitation. The amount of non-methoxylated phenolic units was comparable with the thermally separated softwood lignin.

(39) TABLE-US-00001 TABLE 1 Aliphatic Carboxylic Condensed p-OH- Phenolic Total mmol/g OH acid (+syringyl) Guaiacyl Catechols phenyl OH OH % OCH3.sup.c) Softwood T240-1h-pH 0.5 0.8 2.1 0.7 1.5 1.0 5.3 6.5 4.3 9* T220-1h 0.7 0.8 2.7 0.9 1.9 1.3 6.7 8.3 4.3 pH 11 T220-4h- 0.5 0.9 2.3 1.0 1.6 1.5 6.4 7.8 1.4 pH 11 T220-7h- 0.4 0.9 2.1 0.7 1.5 1.5 5.7 7.0 1.0 pH 11 T220-7h- 0.4 0.961.0 2.2 0.7 1.7 1.5 6.2 7.5 0.8 pH 12 Reference lignins: LignoBoost 1.7 0.4 2.0 2.3 0.00 0.2 4.4 6.5 12.9 Indulin AT.sup.a) 2.0 0.2 1.6 2.3 0.3 4.3 7.6 14.6 Hardwood T220-4h- 0.5 0.8 3.4 0.7 2.2 0.8 7.2 8.5 5.1 pH 11 Reference lignin: HW KL.sup.b) 1.5 0.6 2.4 0.5 0.3 0.2 3.4 5.5 17.4 Kraft 0.9 0.2 2.8 3.8 Lignin.sup.a) *not fully soluble .sup.a)Beis S H (2010) Fast pyrolysis of lignin. BioResources 5(3) 1408-1424 .sup.b)Hardwood kraft lignin precipitated from black liquor at pH 2.5 with hydrochloric acid .sup.c)All values are measured according to Pregl.

(40) Table 2 shows the average molar mass values of softwood lignins determined by SEC in 0.1M NaOH relative to the polystyrene sulphonate standards. Results show that the polymeric nature of lignin is retained, and no lignin degradation was detected. With increasing reaction time and temperature, some condensation takes place, even increasing lignin molecular weight. In optimal reaction conditions, the molar mass is comparable to the typical industrial softwood kraft lignins.

(41) TABLE-US-00002 TABLE 2 Mn MW PD Softwood T240-1h-pH 9 2540 6150 2.4 T220-1h pH11 2420 4190 1.7 T220-4h-pH11 2620 5020 1.9 T220-7h-pH11 2600 5240 2.0 T220-7h-pH12 2580 4870 1.9 Reference lignins: LignoBoost 2290 4450 1.9 Indulin AT.sup.a) 1580 3410 2.2 Hardwood HW KL.sup.b 1260 2310 1.8 T220-4h-pH11 2100 3330 1.6 .sup.a)J. Ropponen, L. Rsnen, S. Rovio, T. Ohra-aho, T. Liittia, H. Mikkonen, D. van de Pas, T. Tamminen, Solvent extraction as a means of preparing homogenous lignin fraction. Holzforschung 65 (2011), 543-549. .sup.bHardwood kraft lignin precipitated from black liquor at pH 2.5 with hydrochloric acid

Example 2Elemental Analysis

(42) The elemental composition of the lignin products was analyzed and compared with the industrial lignins. Feedstock was in most cases softwood kraft lignin. It can be seen from Table 3 that the elemental composition is rather similar to typical industrial softwood kraft lignins.

(43) TABLE-US-00003 TABLE 3 % dry C % H % N % O % S % Ash % Na % Softwood: T220-1h-pH 12 63.9 5.2 n.d. n.d. 2.1 2.0 n.d. T220-1h-pH 10 63.3 5.0 n.d. n.d. 2.3 1.4 n.d. T220-4h-pH 12 65.1 4.9 n.d. n.d. 2.2 2.5 n.d. T220-4h-pH 10 61.3 4.4 n.d. n.d. 2.5 2.5 n.d. T240-1h-pH 9 66.6 4.6 0.2 23.3 1.8 3.5 0.8 T220-1h pH 11 63.5 4.9 0.1 26.0 2.5 3.6 1.1 T220-4h-pH 11 64.1 4.5 0.1 24.2 2.7 3.2 0.9 T220-7h-pH 11 62.1 4.2 0.1 23.6 2.4 2.8 0.6 T220-7h-pH 12 62.9 4.4 0.1 23.4 2.5 3.0 0.7 Reference lignins: LignoBoost.sup.a) 63.6-66.2 5.7-6.2 0.1-0.2 25.9-27.5 1.8-3.2 0.2-1.4 0.17 Indulin AT.sup.b) 64.5 5.4 1.0 24.7 1.9 2.4 0.8 HW KL.sup.b 64.5 6.2 0.3 30.2 2.6 2.1 n.d. Hardwood: T220-4h-pH 11 66.9 5.0 0.2 n.d. 2.1 1.7 n.d. .sup.a)Tomani, P (2010) The LignoBoost process. Chellulose Chem. Technol., 44 (1-3), 53-58 .sup.b)Beis S H (2010) Fast pyrolysis of lignin. BioResources 5(3) 1408-1424 n.d. = not determined

Example 3Viscosity Development in Phenol Formaldehyde Resin Synthesis

(44) PF resin synthesis were performed using 100% phenol (PF Ref), and substituting 50% phenol with softwood LignoBoost lignin or thermally separated and activated lignins (220 C., 1 h, pH 11 and 220 C., 4 h, pH 11). Formaldehyde/phenol ratio of 2 and NaOH/phenol ratio of 0.55 was used. All the other chemicals besides formaldehyde were first mixed and reaction temperature was increased to 55-60 C. Formaldehyde was then added during 10 min, resulting in temperature increase to 70-85 C. The actual condensation phase was carried out at 85 C. and the reaction was followed according to the viscosity.

(45) As shown in FIG. 3 all lignins had faster viscosity increase and shorter reaction time due to already cross-linked polymeric nature compared to phenol. The reactions of thermally separated and activated lignin were faster compared to LignoBoost, supporting faster crosslinking due to the higher amount of reactive sites. As shown in Table 2, the initial molar mass was comparable with LB, and even lower for the sample T220, 1 h, pH 11.

Example 4: Resin Curing at High Phenol Substitution Levels According to the Gel Times

(46) PF resin syntheses were performed by substituting 50% and 70% of phenol with lignin. Commercial softwood kraft lignin and hardwood kraft lignin precipitated at pH 2.5 were compared with the thermally separated and activated softwood and hardwood lignins (220 C., 4 h, pH 11). Formaldehyde/phenol ratio of 2 and NaOH/phenol ratio of 0.55 was used according to Danielson et al (1998). For the lignin part, formaldehyde dosage was calculated according to the reactive functionalities detected by .sup.31P NMR. After complete dissolution of lignin into alkali, the formaldehyde was added at 55-60 C. After that the reaction temperature was increased to 80-85 C. for the actual condensation phase. The reaction was terminated when the target viscosity of 350-450 cP was reached.

(47) The curing rate of resins was evaluated according to gel times. An in-house method was used, where a glass test tube with 5 g of the resin was immersed in water bath at 100 C. and the resin was stirred with a glass rod until the tube was lifted with the rod.

(48) As shown in FIG. 4 all thermally separated and activated lignins had shorter gel times, indicating faster curing rate compared to the acid precipitated reference lignins. Better reactivity of thermally separated and activated lignins was even more emphasised at higher phenol substitution level of 70%. The curing rate of reference lignins was significantly reduced at 70% substitution level unlike with the thermally activated lignins. After thermal activation, the reactivity of softwood and hardwood lignins was comparable. With reference lignins, the reactivtity of hardwood lignin is lower compared to the softwood lignin due to the higher degree of methoxylation.

CITATION LIST

Patent Literature

(49) EP 1794363 WO 2012/091906

Non Patent Literature

(50) Beis, S. H., Mukkamala, S., Hill, N., Joseph, J., Baker, C., Jensen, B., Stemmler, E. A., Wheeler, M. C., Frederick, B. G., van Heiningen, A., Berg, A. G., DeSisto W. J., 2010, Fast pyrolysis of lignin. BioResources 5(3) 1408-1424 Danielson & Simonson, 1998, Kraft lignin in phenol formaldehyde resin. Part 1. Partial replacement of phenol by kraft lignin in phenol formaldehyde adhesives for plywood J. Adhesion Sci. Technol., Vol. 12(9), 923-939. Ropponen, J., Rsnen, L., Rovio, S., Ohra-aho, T., Liiti, T., Mikkonen, H., van de Pas, D., Tamminen, T., 2011, Solvent extraction as a means of preparing homogenous lignin fraction. Holzforschung 65, 543-549 Tomani P., 2010, The LignoBoost process, Cellulose Chem. Technol., 44 (1-3), pp. 53-58.