Use of an anionic lignin-carbohydrate complex as a dispersing agent and method for pulp manufacturing

Abstract

The invention relates to a use of an anionic lignin-carbohydrate complex as a dispersing agent for hydrophobic substances, such as fatty acids and their esters, terpenoids, steryl esters, sterols, in washing of cellulosic pulp in pulp manufacturing. The invention relates also to a method for pulp manufacturing.

Description

[0001] The present invention relates to a use of a lignin-carbohydrate complex as a dispersing agent for lipophilic hydrophobic substances and to a method for pulp manufacturing according to the preambles of the enclosed independent claims.

[0002] Wood comprises, in addition to its main constituents of cellulose, hemicelluloses and lignin, low molecular lipophilic and hydrophobic substances, generally known as extractives or pitch. During mechanical or chemical pulping of wood, the lipophilic and hydrophobic substances are liberated from the wood structure. They may easily form agglomerates, which can deposit on process equipment, block process filters and cause process disturbances, such as process downtime which is needed for additional cleaning of equipment. The lipophilic and hydrophobic substances may even be carried over from the pulping stage to the manufacturing process of the final paper or board, where they may cause spots on the final product and / or negatively affect the strength properties. The term pitch is often used to generally describe both the lipophilic and hydrophobic substances as well as the deposits caused by these substances. The problem with accumulation of lipophilic and hydrophobic substances, i.e. pitch, in the pulping or paper / board manufacturing process is especially large when birch containing betulinol is used as a raw material for pulp, paper or board.

[0003] Deposit problems caused by the hydrophobic substances in manufacture of pulp can be alleviated by using dispersing agents, which keep the lipophilic and hydrophobic substances dispersed in the aqueous phase of the process and prevent or decrease their accumulation into large pitch particles and / or formation of pitch deposits on the process equipment surfaces. Hydrophobic substances dispersed in the liquid phase of the pulp suspension can be removed during various washing stages of pulp. Conventional dispersing agents are fossil-based synthetic polymers. However, there is an increasing trend and desire to reduce or even completely eliminate the use of polymers based on petrochemicals in the industry and to increase the use of sustainable alternatives. Consequently, there is a clear need for non-fossil-based dispersing agents.

[0004] An object of this invention is to minimise or possibly even eliminate the disadvantages existing in the prior art.

[0005] An object of the present invention is also to provide an effective bio-based dispersing agent for dispersing hydrophobic substances present in the manufacturing process of pulp.

[0006] Yet another object of the present invention is to provide a method for pulp manufacturing where the hydrophobic substances can be effectively removed from the process by using a bio-base dispersing agent.

[0007] These objects are attained with the invention having the characteristics presented below in the characterising parts of the independent claims. Some preferable embodiments are disclosed in the dependent claims.

[0008] The features recited in the dependent claims and the embodiments in the description are mutually freely combinable unless otherwise explicitly stated.

[0009] The exemplary embodiments presented in this text and their advantages relate by applicable parts to all aspects of the invention, both the use and the method, even though this is not always separately mentioned.

[0010] A typical use according to the present invention of an anionic lignin-carbohydrate complex, where lignin and carbohydrate are covalently bound with each other, is defined in claim 1, as a dispersing agent for hydrophobic substances, such as fatty acids and their esters, terpenoids, steryl esters, sterols, in washing of cellulosic pulp suspension in pulp manufacturing.

[0011] A typical method according to the present invention for pulp manufacturing, especially for removal of hydrophobic substances in washing of pulp suspension is defined in claim 10. The method comprises, inter alia, obtaining a pulp suspension comprising cellulosic fibres suspended in an aqueous phase, preferably by cooking raw materials comprising cellulosic fibres, chemicals and water, washing the pulp suspension for removal of hydrophobic substances, such as fatty acids and their esters, terpenoids, steryl esters, sterols, from the pulp suspension, and adding to the pulp suspension a dispersion agent comprising an anionic lignin-carbohydrate complex, where lignin and carbohydrate are covalently bound with each other, for dispersing the hydrophobic substances into the aqueous phase of the pulp suspension.

[0012] Now it has been surprisingly found that an anionic lignin-carbohydrate complex, where lignin and carbohydrate are covalently bound with each other, is able to effectively disperse hydrophobic and lipophilic substances into the aqueous liquid phase of the pulp suspension in manufacturing process of pulp. In comparison to conventional dispersing agents, it seems that the same or sometimes even better dispersing effect could be obtained when using the anionic lignin-carbohydrate complex as a dispersing agent. The present invention thus provides a green bio-based alternative for dispersion agents made from fossil-based synthetic polymers and enables an increase of renewable chemicals in manufacturing processes of cellulosic pulp, paper and board.

[0013] In the present context, the term "hydrophobic substances" comprises or consists of pitch, pitch-like substances, wood resins and extractives. The hydrophobic substances comprise or consist of fatty acids and their esters, resin acids, terpenoids, steryl esters, sterols, triglycerides, etc., which are liberated from hardwood and / or softwood cellulosic fibres during chemical and / or mechanical pulping.

[0014] In a pulping process, either mechanical or chemical pulping process, cellulosic fibres are liberated from a raw material, typically wood, and formed into an aqueous cellulosic fibre suspension, i.e. cellulosic pulp suspension. Preferably the pulping process is a chemical pulping process, i.e. the pulp suspension is obtained by chemical pulping, such as such as Kraft or sulphite pulping process, preferably Kraft pulping process. The pulp suspension is thus obtained by cooking raw materials comprising cellulosic fibres, cooking chemicals and water, for example in a digester. The cooking chemicals may comprise sodium hydroxide and sodium sulphide. Preferably the cooking step may have a pH value in a range of 12 - 14 and a temperature 150 - 180 °C. The present invention is, however, fully suitable applicable for manufacture of mechanical pulp, and the lignin-carbohydrate complex is suitable for use as a dispersing agent for hydrophobic substances in manufacturing of mechanical pulp, thermomechanical pulp or chemi-mechanical pulp.

[0015] According to the present invention the dispersing agent comprises an anionic lignin-carbohydrate complex, where lignin and carbohydrate are covalently bound with each other, or the dispersing agent is an anionic lignin-carbohydrate complex, where lignin and carbohydrate are covalently bound with each other.

[0016] The anionic lignin-carbohydrate complex, where lignin and carbohydrate are covalently bound with each other and which is used as a dispersing agent, may be added at any location where the conventional dispersing agents are added during the pulping process. When the pulp suspension is obtained by cooking the raw materials in a digester, the dispersion agent comprising the anionic lignin-carbohydrate complex may be added to the process before the digester. For example, the dispersion agent may be added to at least one of the raw materials entering the digester or to the digester.

[0017] The dispersion agent comprising the anionic lignin-carbohydrate complex is preferably added to the aqueous cellulosic pulp comprising cellulosic fibres suspended in an aqueous phase, i.e. water. According to one embodiment, the anionic lignin-carbohydrate complex may added to the pulp suspension after cooking but before the washing of the pulp suspension. It is also possible that the anionic lignin-carbohydrate complex may added to the pulp suspension during the washing of the pulp suspension. Preferably, the lignin-carbohydrate complex is present during the washing of the cellulosic pulp suspension (brownstock) with water after the cooking in the digester. During the washing step the pH of the pulp suspension may usually be in a range of 10 - 11, and / or the temperature of the pulp suspension may be <100 °C. Addition of the lignin-carbohydrate complex before and / or during the washing step provides milder environment for the interaction between the lignin-carbohydrate complex and the hydrophobic substances than the digester, where the pH is close to 14 and the temperature may be 150 - 180 °C.

[0018] The cellulosic pulp suspension may be bleached or unbleached pulp suspension, preferably unbleached pulp suspension. Cellulosic fibres in the pulp suspension may originate from hardwood, softwood, or their mixtures, preferably from hardwood. Especially, the cellulosic pulp suspension may comprise cellulosic fibres from birch, eucalyptus, acacia, other tropical hardwood species, or any mixtures thereof. According to one preferable embodiment the cellulosic pulp suspension may be unbleached hardwood pulp suspension, preferably unbleached birch pulp suspension.

[0019] The anionic lignin-carbohydrate complex is added to the cellulosic pulp suspension in an amount of 0.05 - 1 kg / ton dry pulp, preferably 0.1 - 0.5 kg / ton dry pulp, more preferably 0.2 - 0.5 kg / ton dry pulp or 0.2 - 0.4 kg / ton dry pulp.

[0020] The anionic lignin-carbohydrate complex suitable for use in the present invention as a dispersing agent is a natural polymeric complex that comprises lignin and one or more carbohydrates, preferably hemicelluloses. The complex comprises lignin and carbohydrate(s) which are covalently bound with each other. The lignin-carbohydrate complex is thus a conjugate of lignin and carbohydrate(s), which are irreversibly bound which each other to a common structure. The anionic lignin-carbohydrate complex may have a branched structure. For example, the lignin or the carbohydrate may form a backbone structure for the complex and the other component, either carbohydrate or lignin, may form pendant groups, which are covalently bound to the backbone structure.

[0021] The lignin-carbohydrate complex may be formed of lignin and one or more of carbohydrates, such as hemicelluloses. The carbohydrate(s) in the lignin-carbohydrate complex are formed from monosaccharides, such as galactose, glucose, mannose, arabinose and / or xylose and / or their fragments or residues; or the carbohydrate(s) may be said monosaccharide(s) and / or their fragments or residues. The exact amounts of the monosaccharides in the lignin-carbohydrate complex and their relative ratios depend on the wood species, e.g. hardwood / softwood, which has been used in the pulping process and from which the lignin-carbohydrate complex originates. The monosaccharides may be present in the lignin-carbohydrate complex as sugar residues, covalently bound to the lignin.

[0022] The lignin-carbohydrate complex comprises various anionic functional groups, which are selected from as sulphonate groups, carboxyl groups and / or phenolic groups. The lignin-carbohydrate complex may comprise, for example, >1300 - 1700 µmol / g, preferably 1400 - 1600 µmol / g of sulphonate groups; 300 - 500 µmol / g, preferably 350 - 450 µmol / g of carboxyl groups; and / or 125 - 250 µmol / g, preferably 150 - 225 µmol / g of phenolic groups.

[0023] The lignin-carbohydrate complex , suitable for use in the present invention, may be obtained from a side stream of a pulping process. In one embodiment, a suitable lignin-carbohydrate complex may be obtained by enzymatic treatment of lignin-carbohydrate material originating from a pulping process. For example, the lignin-carbohydrate complex may be obtained by isolating lignin-carbohydrate material from side streams of wood pulping processes by filtration, such as membrane filtration, and by processing the said isolated lignin-carbohydrate material by enzymatic processing employing preferably laccase enzyme. Alternatively, lignin-carbohydrate complex may be isolated from lignocellulosic material, such as wood or pulp, by using separation and fractionation methods known as such. For example, it is possible to isolate lignin-carbohydrate complexes by fractionating lignin from an industrial process, such as kraft pulping or sulphite pulping. Suitable lignin fractionating methods include, for example, solvent fractionation or precipitation fractionation. In solvent fractionation various organic solvents and their binary mixtures may be employed, such as acetone-hexane, acetone-water, ethanol-water, propyleneglycol monomethyl ether-water. Such a fractionation method is described, inter alia, in Int. J. Biol. Macromolecules 106 (2018) 979-987.

[0024] According to one preferable embodiment of the invention the anionic lignin-carbohydrate complex is an anionic lignosulfonate-carbohydrate complex. It can be obtained, for example, by membrane filtration of a pre-hydrolysis mixture from a sulphite pulping process of wood, and thereafter treated by an enzymatic oxidative treatment, preferably by laccase enzyme. Preferably the filtered pre-hydrolysis mixture is obtained from a sulphite pulping process of wood. The pre-hydrolysis mixture may contain wood-based components and pulping chemicals. Suitable anionic lignosulfonate-carbohydrate complex is disclosed e.g. in BioResources 13(4), 7606 - 7627, 2018, and they are commercially available from Ecohelix AB, Sweden.

[0025] According to one embodiment the anionic lignin-carbohydrate complex may have a branched structure.

[0026] The anionic lignin-carbohydrate complex has an anionic charge density less than - 0.2 meq / g, preferably less than -0.5 meq / g, more preferably less than -0.85 meq / g, measured at pH 7. The anionic charge density of the complex may be from -0.2 meq / g to -2.5 meq / g, preferably from -0.5 meq / g to -2.4 meq / g, more preferably from -0.85 meq / g to -2.3 meq / g measured at pH 7. Sometimes the anionic charge density of the complex may be from -0.5 meq / g to -1.75 meq / g, preferably from -0.85 to -1.5 meq / g, measured at pH 7. The anionic lignin-carbohydrate complex may even have the anionic charge density from -2.0 meq / g to -2.3 meq / g, preferably from -2.1 to - 2.2 meq / g or to -2.15 meq / g, measured at pH 7. All charge density values are given as per dry substance, and measured by using a Mütek Particle Charge Detector.

[0027] According to one preferable embodiment of the lignin-carbohydrate complex may preferably have relatively high molecular weight. It is assumed, without wishing to be bound by a theory, that the high molecular weight, together with the covalent bond between the lignin and carbohydrate, provides at least some of the surprising effects that have been observed. The lignin-carbohydrate complex has a weight average molecular weight MW >15 000 g / mol, sometimes even >20 000 g / mol or >25000 g / mol. The lignin-carbohydrate complex may have the weight average molecular weight MW in a range of 15 000 - 37 000 g / mol. Sometimes the lignin-carbohydrate complex may have the weight average molecular weight MW in a range of 20 000 - 45 000 g / mol, preferably 25 000 - 40 000 g / mol, more preferably 25 000 - 35 000 g / mol or 25 000 - 27 000 g / mol. The lignin-carbohydrate complex has the weight average molecular weight MW in a range of 15 000 - 120 000 g / mol or 20 000 - 90 000 g / mol, preferably 25 000 - 80 000 g / mol, more preferably 30 000 - 70 000 g / mol.

[0028] The lignin-carbohydrate complex comprises lignin and carbohydrates, preferably hemicelluloses, in a ratio from 90:10 to 10:90, preferably from 80:20 to 20:80, more preferably from 75:25 to 25:75 (lignin:carbohydrate). According to one embodiment of the invention the lignin-carbohydrate complex may comprise at least 10 weight-%, preferably at least 15 weight-%, more preferably at least 20 weight-%, of carbohydrates, preferably hemicelluloses, calculated from total dry weight of the complex. The lignin-carbohydrate complex may comprise carbohydrates in a range of 10 - 40 weight-%, preferably 10 - 30 weight-% or 15 - 25 weight-%, calculated from total dry weight of the complex.EXPERIMENTAL

[0029] Some embodiments of the invention are further described in the following nonlimiting experiments.

[0030] Laboratory experiments were performed to compare the performance of three different anionic biobased lignin-carbohydrate complexes (LCC1, LCC2, LCC3 obtained from Ecohelix AB, Sweden) and a commercial dispersing agent for dispersing wood resin in pulping.

[0031] A model compound was used in the experiments to simulate wood resin. The model compound was tall oil fatty acid (Forchem Oyj, Finland) with a high fatty acid content and low content of rosin acids and unsaponifiables. Analysis values for the tall oil fatty acid, as specified in the product datasheet, are given in Table 1. Table 1 Analysis values obtained for tall oil fatty acid used as a model compound and used analysis methods.Analysis Value Method Acid value195SCAN-T 11:72Cloud point, °C-2ASTM D 2500-99Colour Gardner, photometer4ASTM D 6166-97Density at 15°C, kg / m 3< 909SCAN-T 2:65Flash point, closed cup, °C205ASTM D 93-00Free fatty acids, %96ASTM D 5974-00Free rosin acids, %1.9SCAN-T 14:78Iodine value, Wijs154ASTM D 5768-95Pour point, °C-12SCAN-T 5:67Saponification value198SCAN-T 12:72Unsaponifiables, %1.8SCAN-T 13:74Viscosity at 40°C, mPa.s16ASTM D 2196-99

[0032] 100 mg of tall oil fatty acid was added to 49 g of 0.25 M NaOH solution (aq.). A volume of 1 1 of 85°C ultrapure water was then added during fast mixing. Mixing was continued for 5 min. 100 ml sample aliquots were taken from the mixed solution and used for testing of dispersing agent alternatives (LCC samples, commercial dispersing agent) and a reference sample without any dispersing agent. 0.02 mg / l of dispersing agent alternative was added to each 100 ml sample aliquot and allowed to react for 5 min during mixing. 50 ml from each sample aliquot was taken for total organic carbon (TOC) analysis. Quantitative total organic carbon analysis (TOC) was performed by using Shimadzu TOC CPH analyser. The samples were analysed as such without dilution, and the total carbon (TC) calibration area was 0 - 100 mg / l. The pH of the remaining sample aliquots (volume 50 ml) was lowered to pH 3 with H 2 SO 4 , and then mixed for 5 min. Each 50 ml sample aliquot was then filtered through black ribbon filter paper into a bottle containing a pre-defined volume of 0.025 M NaOH. The pH of the filtrates was adjusted back to pH 10 with NaOH and analysed for total organic carbon (TOC). The TOC after filtration was used for estimating the dispersing efficiency of the tested dispersing agents. The more of the TOC was able to penetrate through the filter, the better the dispersing agent.

[0033] The sample aliquots were kept in a water bath at 85°C during the whole test series.

[0034] Total organic carbon results after pH adjustment to pH 3 and filtration are presented in Table 2. Table 2 Results of the experimentsSample Total Organic Carbon after filtration mg / l Reference14Commercial dispersing agent33LCC126LCC223LCC333

[0035] It can be seen from Table 2 that the anionic lignin-carbohydrate complexes were able to effectively disperse the model compound of tall oil fatty acid, even as effectively as a commercial polymer-based dispersing agent.

[0036] The project leading to this application has received funding from the Bio Based Industries Joint Undertaking (JU) under grant agreement No 837866. The JU receives support from the European Union's Horizon 2020 research and innovation programme and the Bio Based Industries Consortium.

Claims

1. Use of an anionic lignin-carbohydrate complex, added in amount of 0.05 - 1 kg / ton dry pulp, as a dispersing agent for hydrophobic substances selected from fatty acids and their esters, terpenoids, steryl esters, and sterols, in washing of cellulosic pulp suspension in pulp manufacturing, in which anionic lignin-carbohydrate complex the lignin and carbohydrate are covalently bound with each other and the carbohydrate(s) in the lignin-carbohydrate complex are formed from monosaccharides, and wherein the anionic lignin-carbohydrate complex - has a lignin:carbohydrate ratio from 90:10 to 10:90 and comprises at least 10 weight-% of carbohydrates, calculated from total dry weight of the complex; - comprises anionic functional groups selected from sulphonate groups, carboxyl groups and / or phenolic groups; and - has a weight average molecular weight MW in a range of 15 000 - 120 000 g / mol and an anionic charge density less than -0.2 meq / g.

2. Use according to claim 1, characterised in that the anionic lignin-carbohydrate complex has the weight average molecular weight MW in a range of 20 000 - 90 000 g / mol, preferably 25 000 - 80 000 g / mol, more preferably 30 000 - 70 000 g / mol.

3. Use according to claim 1 or 2, characterised in that the anionic lignin-carbohydrate complex has the anionic charge density less than -0.5 meq / g, preferably less than -0.85 meq / g, measured at pH 7.

4. Use according to any of preceding claims 1 - 3, characterised in that the anionic lignin-carbohydrate complex comprises at least 15 weight-% of carbohydrates, calculated from total dry weight of the complex.

5. Use according to any of preceding claims 1 - 4, characterised in that the anionic lignin-carbohydrate complex has the lignin:carbohydrate ratio from 80:20 to 20:80, preferably from 75:25 to 25:75.

6. Use according to any of preceding claims 1 - 5, characterised in that lignin-carbohydrate complex comprises galactose, glucose, mannose, arabinose and / or xylose or their fragments or residues.

7. Use according to any of preceding claims 1 - 6, characterised in that the pulp suspension is obtained by chemical pulping, such as Kraft pulping.

8. Use according to any of preceding claims 1 - 7, characterised in that the cellulosic pulp suspension is unbleached hardwood pulp suspension.

9. Use according to any of preceding claims 1 - 8, characterised in that the anionic lignin-carbohydrate complex is added in amount of 0.1 - 0.5 kg / ton dry pulp, preferably 0.2 - 0.5 kg / ton dry pulp or 0.2 - 0.4 kg / ton dry pulp.

10. A method for pulp manufacturing, the method comprising - obtaining a pulp suspension comprising cellulosic fibres suspended in an aqueous phase, preferably by cooking raw materials comprising cellulosic fibres, chemicals and water, - washing the pulp suspension for removal of hydrophobic substances from the pulp suspension, wherein the hydrophobic substances are selected from fatty acids and their esters, terpenoids, steryl esters, and sterols, characterised in that a dispersion agent comprising an anionic lignin-carbohydrate complex, where lignin and carbohydrate are covalently bound with each other, is added to the pulp suspension in amount of 0.05 - 1 kg / ton dry pulp for dispersing the hydrophobic substances into the aqueous phase of the pulp suspension, wherein the carbohydrate(s) in the lignin-carbohydrate complex are formed from monosaccharides, and wherein the anionic lignin-carbohydrate complex - has a lignin:carbohydrate ratio from 90:10 to 10:90 and comprises at least 10 weight-% of carbohydrates, calculated from total dry weight of the complex; - comprises anionic functional groups selected from sulphonate groups, carboxyl groups and / or phenolic groups; and - has a weight average molecular weight MW in a range of 15 000 - 120 000 g / mol and an anionic charge density less than -0.2 meq / g.

11. Method according to claim 10, characterised in that the pulp suspension is obtained by cooking the raw materials in a digester, and the dispersion agent comprising the anionic lignin-carbohydrate complex is added to the digester and / or to at least one of the raw materials entering the digester.

12. Method according to claim 11, characterised in that the cooking step has a pH value in a range of 12 - 14 and a temperature 150 - 180 °C.

13. Method according to claim 10, 11 or 12, characterised in that the anionic lignin-carbohydrate complex is added to the pulp suspension after cooking but before the washing of the pulp suspension.

14. Method according to any of preceding claims 10 - 13, characterised in that the anionic lignin-carbohydrate complex is added to the pulp suspension during the washing of the pulp suspension.

15. Method according to any of preceding claims 10 - 14, characterised in that during the washing step has a pH value in a range of 10 - 11, and / or a temperature <100 °C.