Method for producing oxidized lignin in kraft pulp mills

Abstract

According to an exemplary embodiment of the present invention, a method for producing oxidized lignin-based high performance surfactants is provided in a techno-economically viable manner within a kraft pulp mill without disrupting the main process.

Description

[Technical Field]

[0001] The present invention relates to a method for producing oxidized lignin in a kraft pulp mill in an industrially feasible manner. [Background technology]

[0002] Dispersants are applied in many processes and products to improve product workability, product properties / performance, and reduce manufacturing costs. Anionic dispersants account for the majority of the dispersant market. Concrete plasticizers are anionic dispersants that disperse cement particles to improve the workability of fresh concrete and ensure excellent strength properties of the final product (mature concrete). Similarly, anionic dispersants are used, for example, in preparing paints and coatings containing a high proportion of different inorganic pigments. Lignosulfonates from sulfite cooking are marketed as bio-based concrete plasticizers and dispersants. Typically, the performance of lignosulfonates is significantly lower than that of synthetic plasticizers / dispersants. Synthetic superplasticizer products include, for example, sulfonated naphthalene formaldehyde condensates or polycarboxylic acid ether copolymers. Polyacrylic acid serves as a typical anionic dispersant for pigments in paints and coatings.

[0003] A simple and cost-effective alkaline O2 oxygen process uses O2 under alkaline conditions to solubilize industrial lignins, which can then be used as surfactants, such as dispersants, for specific end-use applications to enhance processability and product quality. Oxidized kraft lignin has been shown to be a better plasticizer and dispersant than lignosulfonate products or even some synthetic products. However, a feasible industrial process concept for producing kraft lignin-based surfactants that can be integrated into kraft pulp mills without interfering with the main process chemical balance has not yet been fully optimized or explored.

[0004] Bio-based, high-performance surfactants, such as dispersants, are needed for many applications. At the same time, kraft pulp companies are thoroughly examining the possibility of increasing revenues from their operations by pricing lignin, a major process by-product. To provide reasonable profit margins, the viable production of new high-performance kraft lignin-based surfactants should be ensured. Especially when integrated into a pulp mill, the production of new lignin products should not interfere with existing pulp production and chemical balances.

[0005] Typical lignin precipitation poses serious problems for the Na / S balance in modern pulp mills with closed chemical loops and low emissions. After CO2 precipitation, the unwashed kraft lignin contains compounds from the black liquor, which consume sulfuric acid in the next step. This increases the sulfur content in the chemical loop. This excess sulfur needs to be pumped out of the chemical loop. Common methods involve the use of a mixture of mainly Na2SO4 and typically 8-20% Na S The main problem is the removal of electrostatic precipitator dust (ESP) from the recovery boiler containing CO₃. Therefore, there is a significant loss of sodium in the chemical cycle. To compensate, make-up NaOH must be added to the chemical cycle, which is a more significant cost factor than the cost of sulfuric acid.

[0006] The plasticizer should provide the cementitious material with excellent workability and flow properties without side reactions. Traces of organic acids (formed during the oxidation and carryover of unwashed kraft lignin) can, for example, cause delays in cement hydration. Alkaline O2-oxidized lignin has previously been shown to be an efficient plasticizer for concrete (WO 2015 / 049424 A1) and a dispersant for many inorganic pigments (WO 2017 / 077198 A1). However, these disclosures do not disclose the complete set of operations of the present invention, which are designed for a techno-economically improved lignin oxidation process concept and allow its integration into a kraft pulp mill without interfering with the main process.

[0007] Therefore, there is a need for a feasible industrial production of kraft lignin-based surfactants.

[0008] The project leading to this application has received funding from the Bio-based Industries Joint Undertaking under the European Union's Horizon 2020 research and innovation programme under grant agreement no. 745246. Summary of the Invention

[0009] The invention is defined by the features of the independent claims. Some particular embodiments are defined in the dependent claims.

[0010] According to one aspect of the present invention, a method for producing oxidized lignin in a kraft pulp mill is provided.

[0011] According to a further aspect of the present invention, a feasible industrial manufacturing process for kraft lignin-based high performance surface agents in a kraft pulp mill is provided.

[0012] These and other aspects, together with the advantages thereof over known solutions, are accomplished by the present invention, as hereinafter described and claimed.

[0013] The method according to the invention is mainly characterized by what is stated in the characterizing part of claim 1.

[0014] The present invention provides significant advantages. More precisely, it involves the oxidation of unwashed kraft lignin under alkaline conditions using, in part, oxidized white liquor (OWL) or white liquor (WL), followed by post-processing of the oxidized lignin in the kraft pulp mill by filtration to produce a high-performance surfactant. The process concept allows for integration into the kraft pulp mill, thereby enabling the viable production of kraft lignin-based surfactants. The process concept offers various advantages in the lignin separation process, the subsequent lignin oxidation process, and the final oxidized lignin product, which functions primarily as a high-performance concrete plasticizer or dispersant, but also more broadly as other types of surfactants.

[0015] The technology will now be described in more detail with reference to specific embodiments. DETAILED DESCRIPTION OF THE INVENTION

[0016] This technology provides a process concept for oxidizing unwashed kraft lignin under alkaline conditions using OWL (or WL), followed by post-processing of the oxidized lignin in a kraft pulp mill by membrane filtration to produce high-performance surfactants. This concept not only allows integration into a kraft pulp mill without interfering with the main process, but also provides clear cost savings and environmental benefits, making the production of kraft lignin-based surfactants feasible. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a process schematic showing the combined lignin isolation, alkaline O2 oxidation, and filtration of oxidized lignin in a kraft pulp mill. [Figure 2] FIG. 2 displays the sodium and sulfide balances of a reference softwood kraft pulp mill and different lignin recovery and oxidation options. [Figure 3] Figure 3 is a schematic diagram showing two-stage membrane filtration of an oxidized lignin solution using a polymeric membrane NP010 (Microdyn Nadir). The separator unit is a laboratory-scale filtration device equipped with a flat-frame membrane. [Figure 4] FIG. 4 shows the importance of filtration for improving plasticization performance, i.e., concrete plasticization efficiency based on slump measurements of fresh concrete. [Figure 5]Figure 5 shows the Haegerman flow and compressive strength values after acid precipitation and membrane filtration (mortar: standard sand, CEM I5 2.5N, HO (w / c 0.5), plasticizer: 0.30% cement, flow value: after consolidation. LS = lignosulfonate, SNP = PantarhitLKFM. 2d compressive strength was not measured for the reference plasticizers (LS, SNF).

[0018] The present invention works when most of the given features of the process concept described herein are applied to produce lignin-based surfactants in a kraft pulp mill.

[0019] According to one embodiment of the present invention, the method comprises at least - separating unwashed kraft lignin from a kraft pulp mill; - dissolving unwashed kraft lignin in oxidized or unoxidized white liquor to obtain a lignin solution; -In alkaline conditions, a) O2 gas overpressure, and b) oxidizing the lignin solution by introducing NaOH and / or oxidized white liquor as an alternative alkalinity source during oxidation; - treating the oxidized lignin using membrane filtration carried out in one stage or using several filtration stages (membranes) in sequence; recovering the concentrated lignin; -Recovering and / or recycling sodium and other by-products.

[0020] In this specification, unwashed lignin refers to lignin obtained by CO2 precipitation but that has not been subjected to sulfuric acid washing. Acid washing is not necessary in this context, since the high ash content of lignin is not an obstacle to alkaline O2 oxidation. The absence of a washing step can reduce the capital costs of the lignin separation process prior to the alkaline O2 oxidation step by approximately 40%. Furthermore, if washing is not required in the subsequent alkaline O2 oxidation step, the operational costs of lignin separation can be significantly reduced. The absence of washing also results in lower electrostatic precipitator dust (ESP) purging from the recovery boiler, which has beneficial chemical effects. Furthermore, a 15% lower alkali consumption is expected in the alkaline O2 oxidation step.

[0021] In a preferred embodiment of the present invention, the O gas overpressure is maintained below 30 bar, more preferably below 20 bar, and most suitably below or about 18 bar. These O gas pressures have been found to be suitable for lignin oxidation purposes and allow the use of pressure systems typically designed for pulp delignification and therefore already available in kraft pulp mills.

[0022] According to a further embodiment of the invention, the method comprises introducing hydrogen peroxide as an oxidizing agent in combination with the above-mentioned measures a) or a) and b) to oxidize the formed lignin solution under alkaline conditions.

[0023] In one embodiment of the present invention, the oxidized lignin is ultrafiltered (UF) or nanofiltered (NF) at least once through a suitable membrane. The UF / NF membrane filtration process can be repeated as many times as necessary to achieve the desired separation efficiency and concentration of the primary lignin product and improve product quality. Thus, here, the primary product is a concentrated lignin solution, and the by-products include, for example, sodium, sulfur, and traces of organic acids that enter and / or are generated in the oxidation process within the unwashed kraft lignin and are not useful for plasticization purposes. These by-products can be recovered directly or recycled to the kraft pulp mill. Recycling and / or recovery of chemical products is an important element of this process concept.

[0024] In one embodiment of the present invention, the oxidized lignin is acid precipitated and filtered, for example, with a membrane filter press. Thus, here, the primary product is precipitated oxidized lignin, and the by-products include, for example, sodium, sulfur, and traces of organic acids that enter the oxidation process in the unwashed kraft lignin and / or are generated in the oxidation process and are not useful for plasticization purposes. Recycling and / or recovery of chemical products is an important element of this process concept.

[0025] According to one embodiment of the present invention, the filtration step increases the oxidized lignin content in the product lignin to 10 to 40 wt %.

[0026] According to one embodiment of the present invention, the filtration step increases the oxidized lignin content in the product lignin to 10 to 75 wt %.

[0027] The inventors of the present invention have surprisingly found that the methods disclosed herein can be used to - allows the use of impure, i.e. unwashed, kraft lignin, which simplifies the kraft lignin separation process and saves capital and operating costs; - Reduced environmental impact due to reduced Na2SO4 purging with simplified kraft lignin separation without introducing excess sulfur into the process; - Reduces the use of alkali in the kraft lignin alkaline O2 oxidation process, while the lignin stream entering the oxidation is already effectively alkaline with a pH > 10; -Allows the use of oxidized white liquor (OWL) or white liquor (WL) as an alternative alkali source in the lignin oxidation process, which reduces the introduction of excess Na into the process and saves costs. OWL is readily available and is currently used in kraft pulp mills for pulp delignification. Hydroxyl ions (-OH - ), OWL has a buffering capacity (CO3 2- ), which is beneficial for oxidation because it prevents a decrease in pH and lowers the molar mass of the lignin compared to alkaline conditions obtained using only NaOH as the alkali source. Furthermore, the use of OWL results in a higher plasticization capacity for the lignin product. - allowing the use of O2 gas from pressure systems designed for pulp delignification in mills (and in the later stages of the oxidation reaction, H2O2 can be used in addition to O2 to promote anionic charge formation in the oxidized lignin); -Improve product performance through filtration of oxidized lignin and recycle chemical products back into the mill.

[0028] Reference to an embodiment or an embodiment in this specification means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrase "in one embodiment" or "in an embodiment" in various places throughout this specification do not necessarily all refer to the same embodiment. For example, when referring to a numerical value using terms such as about or substantially, the exact numerical value is also disclosed.

[0029] While the foregoing examples illustrate the principles of the present invention in one or more particular applications, it will be apparent to those skilled in the art that numerous changes in the embodiments, uses, and details can be made without the exercise of inventive faculty and without departing from the principles and concepts of the present invention. Accordingly, it is not intended that the present invention be limited except as by the claims set forth below.

[0030] The verbs "comprise" and "include" are used in this document as open limitations that do not require the presence or exclusion of unrecited features. Features recited in dependent claims may be freely combined with one another, unless expressly stated otherwise. Furthermore, it is to be understood that the use of "a" or "an", i.e., the singular, throughout this document does not exclude a plurality. [Industrial Applicability]

[0031] The concepts described herein provide various advantages (as discussed above) in the lignin isolation process, the subsequent lignin oxidation process, and the final oxidized lignin product, which functions as a high-performance concrete plasticizer or dispersant for inorganic (or organic) pigments other than cement, or as a surfactant for processes / materials other than those mentioned above. In summary, the present invention makes the production of kraft lignin-based plasticizers / dispersants within kraft pulp mills techno-economically feasible without process disturbances, offering clear cost and environmental benefits. Furthermore, the efficient production and use of lignin-based dispersants opens new business opportunities for companies throughout the value chain, including lignin producers / pulp mills, machinery suppliers, the chemical industry, and dispersant end-users. [Example]

[0032] Unwashed kraft lignin (alkaline stream, pH <11) obtained from the simplified lignin separation process is subjected to alkaline O2 oxidation. First, lignin is dissolved in oxidized white liquor (OWL) with or without NaOH at a concentration of 0.5 to 40 wt%, more preferably 5 to 25 wt%, to obtain a solution with a pH of 13.0 to 14.0. Prior to reaction with O2, the temperature of the lignin solution is adjusted to below 100°C, most preferably 60 to 80°C. The lignin oxidation process uses O2 gas from the kraft mill pressure system. NaOH or OWL or white liquor (WL) as an alternative alkali source is introduced into the reaction solution during oxidation. The total effective alkali dosage, including OWL, NaOH, and WL, is less than 65 wt%, and most preferably less than 50 wt% for lignin. Oxidation is carried out under an O2 pressure of 18 bar or less. O2 can be supplied to the oxidation reactor during the oxidation reaction period. An example of the composition of an oxidized white liquor (OWL) sample is shown in Table 1.

[0033] [Table 1]

[0034] The beneficial effects of this concept are seen as a reduction in the demand for electrostatic precipitator dust (ESP) purging and make-up NaOH. The use of OWL also maintains the incoming NaOH at acceptable levels. In addition to available alkalinity, oxidized white liquor (OWL) possesses buffering capacity (carbonate ions). The use of OWL proves beneficial for oxidation, preventing a drop in pH and lowering the lignin molar mass compared to the alkaline conditions achieved using only NaOH as the alkalinity source. This also results in a higher plasticization performance of the product obtained when using OWL.

[0035] Table 2 shows the beneficial effect (lower Mw, higher Hagerman flow) of using OWL as part of the alkali (for dissolving lignin before oxidation). Oxidation experiments were performed using kraft lignin MF-KL-1. Mortar fluidity was measured using a Hagerman flow table before consolidation. Mortars were prepared using standard sand, cement (CEM I 42.5N, MHLA SR3 Cementa Ab), and water in a 1:1:0.4 ratio. The dosage of the commercial reference plasticizer (Pantarhit LK FM, WRDA 90D) and MF-KL-1-based oxidized lignin relative to the cement was 0.2 wt%.

[0036] [Table 2]

[0037] Table 3 shows the beneficial effect (low Mw, high Hagerman flow) of using OWL as part of the alkali (for dissolving lignin before oxidation). Experiments were performed using kraft lignin MF-KL-C24. Mortar fluidity was measured using a Hagerman flow table before consolidation. Mortars were prepared using standard sand, cement (CEMI 52.5N, Megasementti Finnementt), and water in a 1:1:0.4 ratio. The dosage of the commercial reference plasticizer and MF-KL-C24-based oxidized lignin relative to the cement was 0.60 wt%. The antifoaming agent tributyl phosphate (TBF) was used in the mortar experiments.

[0038] [Table 3]

[0039] Table 4 shows the Hegerman flow values of mortars plasticized with oxidized lignin (MF-KL-1-JH1) or its concentrated fractions. The mortars were prepared using standard sand, cement (CEMI52.5N, Megasementti Finnsementti), and water in a ratio of 1:1:0.4. The dosage was 0.6 wt.% (based on UV280 lignin content) relative to the cement. The antifoaming agent tributyl lignate (TBF) was used in the mortar experiments.

[0040] [Table 4]

[0041] Post-treatment of the oxidized, unwashed kraft lignin solution was performed by membrane filtration (e.g., in one or two stages). Concentration increased the lignin content in the product solution by 10–40 wt% and simultaneously provided a means for recovering sodium (Na) and sulfur (S) during the Na recovery process after evaporation of the permeate fraction. Membrane concentration separates trace organic acids (which enter the oxidation process within the unwashed kraft lignin and / or are generated during the oxidation process) into the permeate fraction, even though they are not beneficial for dispersion purposes. Concentrated alkali-O2 oxidized lignin solutions demonstrated superior plasticization / dispersion performance compared to synthetic dispersant products. Concentration actually improved performance as a concrete plasticizer by removing trace molecular acids that could be detrimental to hydration kinetics.

[0042] The performance of oxidized lignin was also tested in concrete. The oxidized lignin solution and the membrane-concentrated oxidized lignin solution (membrane-filtered oxidized lignin) allowed a 12% water reduction in concrete when tested according to the superplasticizer standard (SFS-EN934-2). Cement quality CEMI 52.5 (Megasementti) was used to prepare the concrete mixture. The plasticizer dosage relative to cement was 0.36%. Concentration improves the plasticizing efficiency in concrete based on the slump measurement of fresh concrete (see Figure 4).

[0043] Alternatively, the oxidized lignin can be concentrated using membrane filtration after acid precipitation, e.g., using H2SO4. After membrane filtration of this acidic slurry, the lignin content of the plasticizer product can be as high as 75 wt%. Membrane-filtered oxidized lignin, or acid-precipitation followed by membrane filtration, provides enhanced plasticizing performance in mortar / concrete, with acceptable strength development, compared to commercially available plasticized mixtures (see Figure 5).

[0044] Citation List Patent documents WO 2015 / 049424 A1 WO 2017 / 077198 A1

Claims

1. 1. A method for producing oxidized lignin in a kraft pulp mill, comprising at least - separating the unwashed kraft lignin obtained by CO 2 precipitation from the kraft pulp mill; - dissolving the unwashed kraft lignin in oxidized or unoxidized white liquor to obtain an alkaline lignin solution; - Under alkaline conditions, a) O ２ Gas overpressure, and b) NaOH and / or oxidized white liquor as an alternative alkalinity source Oxidizing the lignin solution using - treating the oxidized lignin solution by membrane filtration; - recovering the concentrated lignin solution; - recovering the by-products and chemicals directly or returning the by-products and chemicals to the kraft mill for recycling.

2. 2. The method according to claim 1, characterized in that hydrogen peroxide is introduced as oxidizing agent in step a) or in combination with steps a) and b) to oxidize the lignin solution.

3. The above O ２ 3. A method according to claim 1 or 2, characterised in that the gas pressure is maintained below 30 bar, more preferably below 20 bar, most suitably below or around 18 bar.

4. 4. The method according to any one of claims 1 to 3, characterized in that the oxidized lignin is filtered using a suitable membrane filtration technique or filtered after acid precipitation.

5. 5. The method of any one of claims 1 to 4, wherein the filtration increases the lignin content in the product by at least 10% by weight, more preferably by 10 to 40% by weight, most suitably by 10 to 75% by weight.

6. 6. The method according to any one of claims 1 to 5, characterized in that the unwashed kraft lignin is dissolved in oxidized white liquor (OWL) at a content of 0.5 to 40 wt.%, more preferably 5 to 25 wt.%.

7. The method according to any one of claims 1 to 6, characterized in that a kraft lignin-based surfactant is produced.

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