RU2800851C2 - Method for lignin purification - Google Patents

Abstract

FIELD: chemical industry.

SUBSTANCE: invention is directed to a process for obtaining a purified lignin salt from alkaline waste liquor from the chemical pulping of lignocellulosic raw materials. The method includes the steps of reducing the pH value of the spent liquor to form a precipitated lignin suspension, filtering the precipitated lignin to form a filter cake containing lignin and the filtrate, admixing the filter cake containing lignin to a diluting liquid to form an alkaline lignin agglomerate colloid, carrying out an alkaline agglomerate colloid lignin by large-pore membrane filtration, and extracting the concentrate of purified lignin salt.

EFFECT: invention allows avoiding the use of finely porous ultrafiltration membranes and all the difficulties associated with them, separating a relatively pure lignin product as a result of holding lignin agglomerates on a large-pore membrane and allowing low molecular weight organic and inorganic components to pass through and thereby removing them, and avoiding acid washing and the use of sulfuric acid, which would distort the chemical balance of the pulp mill supplying the spent liquor.

14 cl, 1 dwg, 2 tbl, 2 ex

Description

FIELD OF TECHNOLOGY TO WHICH THE INVENTION RELATES

[0001] The present invention relates to the separation and purification of lignin. In particular, the present invention relates to a two-stage process for the production of purified lignin salt from black liquor.

BACKGROUND OF THE INVENTION

[0002] Lignin is a promising raw material for bio-based products. Significant amounts of lignin are available in spent liquors from industrial kraft pulping, ie black liquor. The lignin concentration in black liquor is typically in the range of only 20-40% (w/w) on a dry matter basis, with the remainder of the dry matter containing spent pulping chemicals and dissolved non-lignin wood components. For lignin products, a high degree of purity is important. Thus, cost-effective separation and purification operations are required to achieve industrial scale use of kraft lignin.

[0003] One well-known method for purifying lignin is ultrafiltration as described, among other things, in Luque et al. >12) where the lignin is in dissolved form, a concentrate that is enriched in high molecular weight lignin (e.g., 29% at over 15,000 daltons, Luque et al.), and a permeate that contains more low molecular weight lignin and other low molecular weight black liquor components , inorganic salts and organic acids.

[0004] As is evident, ultrafiltration increases the proportion of high molecular weight lignin in the concentrate, but only slightly, from 15 to 29% lignin at more than 15,000 daltons (Luque et al.). In addition to a rather moderate fractionation result, the ultrafiltration technique using kraft black liquor is difficult due to concentration polarization and membrane fouling. There is a continuous decrease in the flow rate, and ultrafiltration is not a very good fractionation method to obtain a product with a particular molar mass, because the fouling of the membranes leads to a situation where the molecular weight distribution for the concentrated material changes all the time.

[0005] One additional example of conventional ultrafiltration is provided in Rojas et al., "LIGNIN SEPARATION FROM KRAFT BLACK LIQUORS BY TANGENTIAL ULTRAFILTRATION", Science and Technology, La Chimica e l'Industria - Gennaio-Febbraio '06 n. 1 - ANNO 88. (2006). This article offers one example of the problems encountered with this approach. Describes how a typical kraft black liquor was ultrafiltered using typical membranes. The average molecular weight for the considered industrial black liquor ranged from about 1700 to 20000 Da. The Mw value for concentrates and filtrates correlated with the molecular weight cutoff (OMM) for the membrane. However, as found by Rojas et al., the effective molecular size of the filtered particles was smaller than the nominal molecular weight cutoff for the membrane. This difference may be due to typical membrane fouling, which is an inherent problem in this type of application, as well as a reduction in filtration flow due to plugging and/or fouling of the membrane.

[0006] Black liquor contains many inorganic salts and organic aliphatic and phenolic compounds that degrade membrane performance, thus making the molecular weight fractionation process very problematic as described in Aminzadeh et al., Membrane filtration of kraft lignin: Structural charactEristics and antioxidant activity of the low-molecular-weight fraction, Industrial Crops & Products 112 (2018) 200-209.

[0007] Thus, the production of purified lignin from black liquor using ultrafiltration is not very suitable for use. Poor cleaning performance and very limited continuous material flow are very likely, even for occasional washing of UV membranes (Aminzadeh et al.).

[0008] Another method for purifying kraft lignin is the commercial and patented Lignoboost process (WO 2006/038863 and WO 2013/002687). When using the Lignoboost process, the kraft lignin is separated in a two step process. The first stage involves the precipitation of lignin, for example by adding CO ₂ , followed by further filtration. In the second stage, the filter cake is resuspended, usually in a solution of H ₂ SO ₄ -water, followed by an acid wash in a second filtration. The disadvantage of this process is the need for significant costs for processing equipment and for the addition of sulfuric acid, and thus the introduction of excess sulfur into the closed cycle of extraction during kraft pulping, which distorts the Na/S balance in the pulp mill.

DEFINITIONS

[0009] In the present context, the term "large pore membrane filtration" refers to cross-flow filtration for particles formed from agglomerated lignin macromolecules. The cutoff point is at least 40,000 daltons. Advantageously, the cut-off point in the coarse membrane filtration according to the present invention is in the range of 40,000-500,000 Da, preferably 50,000-200,000 Da.

[0010] In the present context, a "diluting liquid" is an aqueous liquid characterized by the absence of significant ionic strength or a significant amount of dissolved organic material. In addition to plain water, examples of dilution fluids commonly found in pulp mills are evaporator condensate, bleach leachate, or process hot water.

[0011] In the present context, a lignin salt is a chemical form of lignin in which a portion of the acid groups in the lignin are dissociated and the negative charge of the dissociated acid groups is offset by cations. The lignin salt may include various cations, but typically there is one major cation that is the same as the cation of the caustic pulping process from which the lignin salt derives its origin. In the case of the kraft lignin salt, the most important cation is sodium (Na ⁺ ).

[0012] In the present context, a precipitated lignin slurry is formed by reducing the initial pulp mill black liquor pH to 9-10.5 using an acidic agent.

[0013] In the present context, as a result of filtration on the filter medium, a filter cake is formed from a suspension of precipitated lignin.

[0014] In the present context, a lignin agglomerate is a combined aggregate of lignin macromolecules in a colloidal state, with the particle size of the aggregate being large enough (>50 kilodaltons) to be filtered (retained on the membrane) by large-pore membrane filtration. It is important to note that these lignin particles of the agglomerates are at least ten times larger than the lignin macromolecules in the original black liquor (typically 5 kilodaltons) that flows through the large pore membrane.

SUMMARY OF THE INVENTION

[0015] The invention is defined by the features of an independent claim. Some specific embodiments are defined in dependent claims.

[0016] The purpose of the invention is precisely to overcome at least some of the shortcomings related to the technical solutions of the prior art, described in the section relating to the prior art.

[0017] In accordance with a first aspect of the present invention, there is provided a process for producing a purified lignin salt from alkaline waste liquor from chemical digestion of lignocellulosic raw materials. The method includes the steps:

- reducing the pH value for the waste liquor with the formation of a suspension of precipitated lignin,

- filtering the precipitated lignin with the formation of a filter cake containing lignin and a filtrate,

- admixing the filter cake containing lignin to the diluting liquid with the formation of a colloid of alkaline lignin agglomerate,

- conducting coarse-pore membrane filtration for the colloid of alkaline lignin agglomerate,

- extracting the concentrate of purified lignin salt.

The purified lignin concentrate constitutes the product. Low molecular weight organic and inorganic components are removed in the flowing fluid.

[0018] It is worth noting a surprising new discovery: admixing a filter cake of precipitated lignin into an aqueous solution under certain controlled conditions, described below, results in the formation of large lignin agglomerates that can be separated and enriched using coarse membrane filtration. Significant advantages are obtained by using the present invention, which makes possible a method that allows:

- to avoid the use of finely porous ordinary ultrafiltration membranes and all the difficulties associated with them,

- to separate a relatively pure lignin product as a result of holding lignin agglomerates on a large-pore membrane and allowing low molecular weight organic and inorganic components to pass through and, thereby, their removal,

- avoid acid washing and the use of sulfuric acid, which would distort the chemical balance of the pulp plant supplying the waste liquor.

[0019] Preferably, the decrease in the pH value of the spent liquor is carried out by adding H ₂ SO ₄ , HCl, SO ₂ or CO ₂ . Preferably, the decrease in pH value is carried out by adding CO ₂ .

[0020] Preferably, the decrease in the pH value of the waste liquor is carried out at a temperature in the range of 40-85°C.

[0021] Advantageously, the pH of the spent liquor is reduced to less than 11. Preferably, the pH of the spent liquor is reduced to 9-10.5.

[0022] Preferably, large-pore membrane filtration is carried out at a temperature in the range of 20-85°C.

[0023] Preferably, large-pore membrane filtration is carried out at a pH value in the range of 7-11.

[0024] Further embodiments will be discussed in more detail with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIGURE 1 is a general schematic diagram of a lignin purification process according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0026] In accordance with the above brief discussion of the invention, a process for preparing a purified alkaline lignin salt is provided. The method is a two-stage technological process in accordance with the General description of the invention in figure 1.

[0027] In the first step of the present invention, a precipitated alkaline lignin slurry is formed from spent alkaline pulping liquor, i.e., black liquor (1), (dry solids content typically in the range of 25-45% (wt.) ) by reducing the pH to 9-10.5 (3) at a temperature in the range of 40-85°C. The formed alkaline lignin slurry is filtered (5), forming a filter cake (7) containing lignin and a permeate liquid (6) containing soluble components of black liquor, which are taken back to the chemical cycle of the pulp mill.

[0028] In the second stage, the aforementioned lignin-containing filter cake (7) is mixed with a dilution liquid (8) using certain controlled mixing and temperature (9) and forming an alkaline lignin agglomerate colloid (10) typically at a dry solids level 10% (mass.). This new unexpected discovery is the existence of alkaline lignin in the form of large colloidal agglomerate particles in a dilute aqueous solution, where these particles are characterized by an average particle mass of 50 kilodaltons, and where they make it possible to produce a purified alkaline lignin salt suitable for use from an industrial and technical point of view.

[0029] Alkaline lignin agglomerate colloid (10) is fed onto a large porous membrane (11) having a cut-off pore size in the range of 50-200 kilodaltons, typically in the temperature range of 20-85°C. Alkaline lignin remains in the concentrate (12), while most of the smaller particles and low molecular weight soluble components pass through the large-pore membrane into the permeate (13). The difference and technical advantage is significant compared to conventional ultrafiltration of black liquor lignin, typically using special technology membranes having a pore size of 5 kilodaltons.

[0030] One significant advantage of the present invention over prior art membrane separation (ultrafiltration) of lignin is the ability to enrich alkali lignin in the form of large colloidal particles, here referred to as alkali lignin agglomerates. This is evident since the enriched concentrate can be made on a large pore membrane having a "molecular weight" cut-off in the range of 50-200 kilodaltons. In this case, this is called large-pore membrane filtration. The term "coarse pore" in practice typically refers to a membrane pore size ten times larger than the pore size used in conventional ultrafiltration.

[0031] Coarse pore membrane filtration according to the present invention differs from ultrafiltration in several improvements and advantages. First of all, dense and technically very demanding membranes are not required to separate macromolecules, which typically have a molecular weight cutoff of 5 kilodaltons. Coarse pore membranes of 50-200 kilodaltons are much easier to operate and maintain. Secondly, the lignin-containing filter cake feedstock has already been purified: many soluble low molecular weight components, especially inorganic salts, have been removed from the filter cake as filtrate. Therefore, the second-stage re-dilution membrane filtration process substantially takes place in a cleaner environment, and thus polarization and fouling of the membrane are largely less pronounced. Thirdly, the end product is an alkaline lignin salt concentrate. There is no need for acid washing or other secondary purification steps that require the use of additional chemicals that distort the recycling and reuse of removed components in the permeate.

[0032] Commercially available conventional types of membranes can be used in the coarse membrane filtration step. The membranes can be constructed from organic materials such as PP, PE, PTFE polymers, polyacrylic material, cellulose acetate, polyacrylonitrile, polysulfone, and the like. Highly durable inorganic membrane materials such as alumina or zirconia can also be used, but such materials are not required.

[0033] Preferably, large-pore membrane filtration is carried out at a pH in the range of 7 to 11 and at a temperature in the range of 20-85°C.

[0034] The concentrate may be used as such or may be subjected to additional processing such as evaporation, freeze drying or spray drying. One possible post-treatment is to further purify the product by diluting the concentrate and repeating the coarse membrane filtration one or more times.

[0035] Various embodiments of the present invention have been described above, but the person skilled in the relevant field of technology is also aware of the existence of additional minor changes that would fall within the scope of the present invention. The scope and scope of the present invention should not be limited by any of the illustrative embodiments described above, but should be defined only in accordance with the claims and their equivalents.

EXAMPLE 1 Production of Purified Kraft Lignin Salt Using Coarse Membrane Filtration

[0036] The pH value for black liquor from kraft softwood pulp was reduced to 10.0 using CO ₂ at 60°C. The precipitated lignin slurry was filtered to form a filter cake containing kraft lignin at a dry matter level of 57%. The filter cake containing lignin was mixed with deionized water to form a lignin agglomerate colloid at a dry solids level of 10%. The lignin agglomerate colloid volume was subjected to pre-filtration and coarse membrane filtration using an Alfa Laval-GR51PP 50 kD cutoff membrane. A large pore membrane filtration test was performed at 50° C. to achieve a volume reduction Vr=0.7 (Vr=Vpermeate/Vinitial). The resulting concentrations of solids, dried in an oven to constant weight, in volumes of concentrate and permeate were, respectively, 19% and 2.7%.

[0037] 81% of the lignin in the feedstock was recovered in the volume of the concentrate, while 63% of the basic inorganic anions (carbonate, sulfate) and 41% of the sodium were converted to permeate. The concentration of lignin in the concentrate was 82% of the total dry matter content compared to 67% in the feed source material. The concentration of the main inorganic anions (carbonate and sulfate) in the permeate was more or less than half of the corresponding concentration in the feed source. The results indicate a significant enrichment of lignin in the concentrate and inorganic matter in the permeate. More detailed results are shown in Table 1 below.

Table 1. Concentration and Yield of Selected Components in Volumes of Feed, Concentrate, and Permeate for the Coarse Membrane Filtration Test of Example 1.

SUPPLY MATERIAL No. 1 CONCENTRATE No. 1 PERMEATE No. 1 Concentration, % of dry matter Concentration, % of dry matter Yield, % of feed source material Concentration, % of dry matter Yield, % of feed source material lignin 67 82 81 eleven 3 Sodium (Na+) 7.6 5.4 47 17 41 Carbonate 4.5 2.0 thirty 15 63 Sulfate 1.2 0.6 37 4.0 63

EXAMPLE 2. Additional purification of lignin salt as a result of repeated large-pore membrane filtration.

[0038] A volume of the concentrate from Example 1 was mixed with deionized water to achieve a dry solids level of 10% and subjected to another coarse membrane filtration test using a 50 kDa Alfa Laval-GR51PP membrane. The test was carried out at 50° C. and continued until the volume reduction Vr=0.7 (Vr=Vpermeate/Vinitial) was achieved. The resulting concentrations of solids, dried in an oven to constant weight, in volumes of concentrate and permeate were, respectively, 21% and 1.3%.

[0039] 95% of the lignin in Feed #2 was recovered in the concentrate, while more than half of the carbonate was transferred through the membrane to the permeate. The concentration of lignin in the concentrate was 90% of the total dry matter content compared to 82% in the feed source material. The results from example 2 indicate that a significant additional purification of lignin was achieved by re-diluting the concentrate and repeating the coarse membrane filtration. More detailed results are shown in Table 2 below.

[0040]

Table 2. Concentration and Yield of Selected Components in Volumes of Feed, Concentrate, and Permeate for the Second Coarse Membrane Filtration Test in Example 2.

FEEDING MATERIAL No. 2 (CONCENTRATE No. 1) CONCENTRATE No. 2 PERMEATE No. 2 Concentration, % of dry matter Concentration, % of dry matter Yield, % of supplied starting material No. 2 Concentration, % of dry matter Yield, % of supplied starting material No. 2 lignin 82 90 95 15 2 Sodium (Na+) 5.4 4.9 78 17 29 Carbonate 2.0 0.8 33 13 53

[0041] As shown in EXAMPLE 1 and EXAMPLE 2, coarse membrane filtration effectively enriches and purifies the lignin agglomerates in the concentrate. High lignin retention is achieved using a 50 kDa membrane. This aspect is exceptional in comparison, for example, with the separation and purification of lignin macromolecules as a result of ultrafiltration of black liquor, which, with the simultaneous use of a finer 15 kDa membrane, results in only a slight enrichment in lignin in the concentrate (20-30% lignin in the supplied source material, Vr=0.9). [Wallberg et al. (2003), Holmqvist et al. (2005)].

[0042] As will be appreciated, embodiments of the disclosed invention are not limited to the specific structures, process steps, or materials disclosed herein, but are extended to their equivalents, as those skilled in the art will recognize. As should also be understood, the terminology used herein is used for the purpose of describing specific embodiments only and is not intended to be limiting.

[0043] Reference throughout this specification to "one embodiment" or "an embodiment" is intended to include a particular feature, structure, or characteristic described in connection with a given embodiment in at least one embodiment. of the present invention. Thus, occurrences of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification do not necessarily refer to the same embodiment in all instances.

[0044] In accordance with the use in this document, many items, structural elements, composite elements and/or materials can be presented for convenience in the General list. However, these lists should be understood as if each item in the list were individually identified as a separate and unique item. Thus, no individual element of such a list should be taken as effectively equivalent to any other element of the same list solely on the basis of their presentation in the general group, unless otherwise indicated. In addition, this document may refer to various embodiments and examples of the present invention in conjunction with alternatives for its various components. As is to be understood, such embodiments, examples, and alternatives are not to be taken as actually equivalent to one another, but are to be considered as separate and stand-alone representations of the present invention.

[0045] In addition, the features, structures, or characteristics described may be combined in any suitable manner for use in one or more embodiments. In the following description of the invention, numerous specific details are given, such as examples of lengths, widths, profiles, and the like, to provide a thorough understanding of embodiments of the invention. However, as will be understood by those skilled in the art, the invention may be practiced without one or more specific details, or using other methods, components, materials, and the like. In other instances, well-known structures, materials, or operations are not shown or described in detail in order to avoid obscuring aspects of the invention.

[0046] Despite the illustrative nature of the above examples of the principles of the present invention in one or more specific applications, it should be obvious to specialists in the relevant field of technology and the possibility of implementing numerous modifications in form, use and details of embodiment without involving the gift of invention and without deviating from the principles and concepts inventions. Accordingly, the invention is not intended to be limiting beyond the use of the claims below.

[0047] The verbs "comprise" and "include" are used herein as non-categorical restrictions that neither exclude nor require the existence of non-listed features as well. The features listed in the dependent claims are freely combinable with one another, unless otherwise explicitly stated. In addition, as should be clear, the use of the terms "one" and "some", that is, the singular form, throughout the course of this document does not exclude a plurality.

LIST OF CITATIONS

Patent Literature

WO2006/038863

W02013/002687

Non-Patent Literature

1. Luque, Rodriquez, Alvarez, Cova, TAPPI Pulping Conference 1994, p.507.

3. Orlando J. Rojas, John Song, Dimitris S. Argyropoulos Forest Biomaterials Laboratory, College of Natural Resources, North Carolina State University, Raleigh, NC 27695-8005, USA, Juan Bullón Department of Chemical Engineering, Universidad de Los Andes, Mérida (Venezuela), "LIGNIN SEPARATION FROM KRAFT BLACK LIQUORS BY TANGENTIAL ULTRAFILTRATION", Science and Technology, La Chimica e l'Industria - Gennaio-Febbraio '06 n. 1 - ANNO 88. (Year 2006).

4. Selda Aminzadeh, Maris Lauberts, Galina Dobelev, Jevgenija Ponomarenkov, Tuve Mattsson, Mikael E. Lindström, Olena Sevastyanova, Membrane filtration of kraft lignin: Structural charactEristics and antioxidant activity of the low-molecular-weight fraction, Industrial Crops & Products 112 (2018) 200-209.

5. Wallberg, O.; Jönsson, A.-S.; Wimmerstedt, R., Ultrafiltration of kraft black liquor with a ceramic membrane, Desalination 2003, 156, 145-153.

6. Holmqvist, A; Wallberg, O.; Jönsson, A.-S., Ultrafiltration of Kraft Black Liquor from Two Swedish Pulp Mills, Chem. Eng. Res. Des. 2005, 83, 994-999.

Claims (19)

1. A method for obtaining a purified lignin salt from alkaline waste liquor from the chemical pulping of lignocellulosic raw materials, comprising the steps: - reducing the pH value of the spent liquor with the formation of a suspension of precipitated lignin, - filtering the precipitated lignin with the formation of a filter cake containing lignin and a filtrate, - admixing the filter cake to the diluting liquid with the formation of a filter cake of a colloid of alkaline lignin agglomerate, - conducting coarse-pore membrane filtration for the colloid of alkaline lignin agglomerate and - extracting the concentrate of purified lignin salt. 2. The method of claim 1 wherein the pH of the waste liquor is reduced to less than 11. 3. The method according to p. 2, where the pH value of the waste liquor is reduced to 9-10.5. 4. The method according to any one of paragraphs. 1-3, where the pH value is reduced by adding H ₂ SO ₄ , SO ₂ , HCl or CO ₂ . 5. The method according to any one of paragraphs. 1-3, where the pH value is reduced by adding CO ₂ . 6. The method of claim 1 wherein the diluting liquid is water. 7. The method according to claim 1, wherein the formed alkaline lignin agglomerate colloid is subjected to coarse pore membrane filtration at nominal cutoff values in the range of 40-400 kDa. 8. The method according to claim 7, where the cutoff value is in the range of 50-200 kDa. 9. The method according to p. 1, where the decrease in pH value is carried out at a temperature in the range of 40-85°C. 10. The method according to p. 1, where coarse membrane filtration is carried out at a temperature in the range of 20-85°C. 11. The method according to p. 1, where large-pore membrane filtration is carried out at a pH value in the range of 7-11. 12. The method of claim 1, further comprising at least one step of re-diluting the purified lignin salt and subjecting it to coarse membrane filtration. 13. The method of claim 1, further comprising treating the concentrate. 14. The method of claim 13 wherein the treatment is evaporation, freeze drying or spray drying.

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