Discharge aiding agent for bleaching process of cellulosic fibres

Abstract

A method for improving drainage in an acidic stage of a bleaching process of cellulosic fibres followed by at least one scrubber stage, in which method a polymer comprising cationic functional groups is added to the acidic stage of the bleaching process, and the polymer comprising cationic functional groups has a mass average molecular weight of 4000000-17000000 g / mol at pH 7, a cationic net charge, and a total degree of ionization of 5-35 mol-%.

Description

Technical Field

[0001] This invention relates to the use of polymers containing cationic functional groups as drainage aids in the bleaching process of cellulose fibers, and to methods for improving drainage during the acidic phase of the bleaching process of cellulose fibers, according to the independent claims presented below. Background Technology

[0002] Cellulose fibers obtained from chemical pulping are bleached during a bleaching process, in which residual lignin is removed from the fibers to improve their brightness. Bleaching is a staged process in which various bleaching chemicals are typically used in different successive stages to break down lignin into smaller structures. The breakdown of lignin into smaller structures is water-soluble, especially at pH > 7, and is removed from the fibers and the bleaching process in an alkaline extraction stage located after or between the bleaching stages. Therefore, the bleaching process for cellulose fibers typically involves multiple bleaching stages with washing stages in between. Multi-stage bleaching produces optimal quality and cost-effective results.

[0003] However, bleaching plants may encounter drainage and dewatering issues in the washers connected to each bleaching stage. In bleaching processes where the acidic bleaching stage is followed by the alkaline washing stage, the washing efficiency of the alkaline stage may be lower if there are drainage problems in the washers of the previous acidic stage. Poor drainage reduces the consistency of the pulp in the next stage, leading to lower bleaching efficiency and thus higher chemical consumption. Furthermore, chemicals from the acidic stage and impurities from poor washing can enter the subsequent alkaline stage, also resulting in higher chemical consumption. Increased chemical consumption leads to higher costs for the bleaching plant and also has an environmental impact. In addition, the deposition of inorganic impurities throughout the bleaching process can cause operational performance problems. Summary of the Invention

[0004] The purpose of this invention is to reduce or even eliminate the problems mentioned above in the prior art.

[0005] The purpose of this invention is to reduce drainage and dehydration problems in the washing machine connected to the acid bleaching stage during the bleaching process, thereby improving the operational performance of the bleaching plant.

[0006] The objective of this invention is to improve the quality of the filtrate obtained from the washer in the bleaching stage. Improved removal of water and impurities reduces chemical consumption in subsequent stages of the bleaching process, thus providing a more cost-effective bleaching process. Furthermore, the environmental impact of the bleaching process is reduced due to lower chemical consumption.

[0007] To achieve the objectives presented above, the present invention is characterized by the contents presented in the appended independent claims.

[0008] Some preferred embodiments of the present invention will be described in the other claims.

[0009] The embodiments and advantages mentioned herein relate to all aspects of the invention where applicable, even if not always specifically mentioned.

[0010] A typical method according to the invention for improving drainage during the acidic stage of a bleaching process of cellulose fibers includes...

[0011] - Cellulose fibers are obtained from the pulping process and formed into an aqueous cellulose fiber suspension. - The cellulose fiber suspension is bleached during a bleaching process that includes at least one acidic stage, wherein the pH of the cellulose fiber suspension is < 6, followed by at least one washing stage, and - Add a drainage aid to the cellulose fiber suspension entering the acidic stage of the bleaching process, wherein the drainage aid comprises a polymer containing cationic functional groups and has a mass-average molecular weight of 4,000,000–17,000,000 g / mol, a net cationic charge, and a total ionization of 5–35 mol-% at pH 7.

[0012] A typical use of the cationic functional group-containing polymer according to the invention is as a bleaching aid in the bleaching process of cellulose fibers, wherein the polymer is added to a cellulose fiber suspension during the acidic phase of the bleaching process, wherein the cellulose fiber suspension has a pH < 6, and wherein the cationic functional group-containing polymer has a mass-average molecular weight of 4,000,000–17,000,000 g / mol, a net cationic charge, and a total ionization degree of 5–35 mol-% at pH 7.

[0013] It has now been found that using polymers containing cationic functional groups and having a mass-average molecular weight of 4,000,000–17,000,000 g / mol, a net cationic charge, and a total ionization degree of 5–35 mol-% as drainage aids improves drainage in the acidic stage of the scrubber. The cationic functional group-containing polymers used as drainage aids in this invention are added to the acidic stage of the bleaching process at least before the acidic stage scrubber, where improved drainage is observed as an increase in the consistency of the cellulose fiber suspension.

[0014] It has also been observed that the polymers containing cationic functional groups are suitable for improving drainage during the acidic stage in all bleached cellulose pulp grades. According to one embodiment of the invention, the cellulose fibers to be bleached can be derived from hardwood, softwood, or mixtures thereof. Cellulose fibers are obtained from the pulping process and formed into an aqueous cellulose fiber suspension. Preferably, the cellulose fibers are derived from a chemical pulping process, such as sulfate or sulfite pulping, preferably sulfate pulping (Kraft pulping). When the cellulose fibers are derived from sulfate pulping, the unbleached fibers are separated from the cooking chemicals (so-called black liquor) by washing the fibers (brownstock) with water. The resulting unbleached fiber suspension is then transferred to the bleaching process.

[0015] The bleaching process involves bleaching a cellulose fiber suspension, which includes at least one acidic stage, wherein the pH of the cellulose fiber suspension is < 6, preferably < 5. The acidic stage can be, for example, an acid hydrolysis stage, typically denoted as stage A; and / or a chlorine dioxide stage, typically denoted as stage D. The bleaching process may include one, two, or more acidic stages. The bleaching process may further include one, two, or more alkaline extraction stages, located after the acidic stages or between two acidic stages. In the alkaline extraction stages, the pH of the cellulose fiber suspension is raised to at least pH 9, preferably at least pH 10. Typically, the acidic stage includes a scrubber arranged after the acidic stage reactor in the bleaching process.

[0016] The cationic functional group-containing polymer, used as a discharge aid in this invention, is added to a cellulose fiber suspension during the acidic phase of the bleaching process, wherein the pH of the cellulose fiber suspension is < 6, preferably < 5. Adding to the acidic phase indicates that the discharge aid is connected to the cellulose fiber suspension in the acidic phase. Typically, the cationic functional group-containing polymer is added to the cellulose fiber suspension during the acidic phase before the acidic phase washer. According to the invention, the discharge aid can be added to any acidic phase of the bleaching process. In one embodiment of the invention, the cationic functional group-containing polymer is added to an aqueous cellulose fiber suspension at least before the acidic phase reactor. In another embodiment of the invention, the cationic functional group-containing polymer can be added to an aqueous cellulose fiber suspension after the acidic phase reactor but before the acidic phase washer. In one embodiment of the invention, the cationic functional group-containing polymer, used as a discharge aid, is added at least during the first acidic phase of the bleaching process, typically at least before the first acidic phase washer. In one embodiment of the invention, the drainage aid is added to the first acidic stage, and can be further added to other acidic stages of the bleaching process. In another embodiment, the drainage aid is added to all acidic stages of the bleaching process. The drainage aid can be added to the cellulose fiber suspension as a single dose, or it can be added to the cellulose fiber suspension as multiple doses at multiple feeding locations, or it can be added continuously at one or more feeding locations. Preferably, the drainage aid is added continuously to the cellulose fiber suspension.

[0017] The discharge aid according to the present invention comprises at least one polymer containing a cationic functional group. The cationic functional group-containing polymer may be a cationic polymer and / or an amphoteric polymer. According to one embodiment, the discharge aid may be a mixture of two or more polymers. The discharge aid according to the present invention may contain at least 25 wt%, preferably at least 50 wt%, more preferably at least 75 wt%, and even more preferably at least 90 wt% of a cationic functional group-containing polymer, calculated by the dry weight of the total polymer in the discharge aid. According to a preferred embodiment, the discharge aid consists of one or more cationic polymers and / or one or more amphoteric polymers having a net cationic charge at pH 7. The net charge of the polymer is positive, even if the polymer contains anionic groups. An amphoteric polymer is a polymer containing both cationic and anionic functional groups. The net cationic amphoteric polymer has a positive charge, even if the polymer contains anionic groups. The net charge of the polymer is calculated as the sum of the charges of the present cationic and anionic groups. In one embodiment of the present invention, the total degree of ionization of the cationic functional group-containing polymer is in the range of 5–35 mol-%, and the net charge is cationic.

[0018] According to one embodiment of the invention, the polymer containing cationic functional groups comprises a cationic polymer, preferably a cationic polyacrylamide obtained by copolymerization of (meth)acrylamide and at least one cationic monomer. According to another embodiment of the invention, the polymer containing cationic functional groups comprises a net cationic amphoteric polymer, preferably a net cationic amphoteric polyacrylamide obtained by copolymerization of (meth)acrylamide with cationic and anionic monomers.

[0019] According to one embodiment of the invention, the cationic group of the cationic or amphoteric polymer is derived from a monomer selected from the group consisting of: 2-(dimethylamino)ethyl acrylate (ADAM), [2-(acryloyloxy)ethyl]trimethylammonium chloride (ADAM-Cl), 2-(dimethylamino)ethyl acrylate benzyl chloride, 2-(dimethylamino)ethyl acrylate dimethyl sulfate, 2-dimethylamino)ethyl methacrylate (MADAM), [2-(methacryloyloxy)ethyl]trimethylammonium chloride (MADAM-Cl), 2-dimethylamino)ethyl methacrylate dimethyl sulfate, [3-(acryloylamino)propyl]trimethylammonium chloride (APTAC), and [3-(methacryloylamino)propyl]trimethylammonium chloride (MAPTAC), and any combination thereof. The extrusion aid may comprise polyacrylamide obtained by copolymerizing acrylamide or methacrylamide with a cationic monomer. The monomer may be copolymerized with acrylamide and / or methacrylamide monomers to obtain copolymers having cationic functional groups.

[0020] According to a preferred embodiment of the invention, the polymer containing a cationic functional group comprises net cationic amphoteric polyacrylamide. Net cationic amphoteric polyacrylamide can be obtained by copolymerizing acrylamide or methacrylamide with both anionic and cationic monomers. Preferably, the amphoteric polyacrylamide is obtained by copolymerizing acrylamide with both anionic and cationic monomers. The cationic group can be derived from one or more cationic monomers listed above in this application. The anionic group in the amphoteric polymer can be derived from monomers selected from the group consisting of unsaturated monocarboxylic acids or dicarboxylic acids, such as acrylic acid, maleic acid, fumaric acid, itaconic acid, aconitic acid, citric acid, crotonic acid, isocrotonic acid, angelic acid, and tigrinic acid, and any combination thereof. Preferably, the anionic group is derived from acrylic acid or itaconic acid.

[0021] According to one embodiment of the invention, the polymer containing cationic functional groups has a weight-average molecular weight (MW) of 4,000,000–17,000,000 g / mol and a total ionization degree of 5–35 mol-%. In one embodiment, the polymer containing cationic functional groups has a weight-average molecular weight (MW) of 4,000,000–10,000,000 g / mol or 4,000,000–8,000,000 g / mol and a total ionization degree of 5–10 mol-%. According to another embodiment of the invention, the polymer containing cationic functional groups has a weight-average molecular weight (MW) of 11,000,000–17,000,000 g / mol and a total ionization degree of 20–35 mol-% or 25–35 mol-%. It has been observed that polymers with the aforementioned molecular weights and total ionization degrees have a net cationic charge, improving drainage in the acidic phase of the bleaching process. In this application, the "weight-average molecular weight" value is used to describe the size of the polymer chain length. The weight-average molecular weight was calculated from the intrinsic viscosity measured in 1N NaCl at 25°C using an Ubbelohde capillary viscometer in a known manner. The selected capillary was appropriate, and in the measurements used in this application, an Ubbelohde capillary viscometer with a constant K = 0.005228 was employed. The average molecular weight was then calculated from the intrinsic viscosity using the Mark-Houwink equation [η] = K·Ma in a known manner, where [η] is the intrinsic viscosity, M is the molecular weight (g / mol), and K and a are parameters given for poly(acrylamide) in *Polymer Handbook, Fourth Edition, Volume 2, Editors: J. Brandrup, EH Immergut and EA Grulke, John Wiley & Sons, Inc., USA, 1999, p. VII / 11*. Therefore, the value of parameter K is 0.0191 ml / g and the value of parameter “a” is 0.71. The average molecular weight given for the parameters used is 490,000–3,200,000 g / mol, but the same parameters are also used to describe molecular weights outside this range. The pH of the polymer solution used for determining intrinsic viscosity was adjusted to 2.7 using formic acid.

[0022] In this invention, the cationic functional group-containing polymer used as a drainage aid may comprise cationic polyacrylamide, which is obtained by copolymerizing (meth)acrylamide with 5–35 mol-% of a cationic monomer (calculated by the total amount of monomer). According to one embodiment, the cationic polyacrylamide is obtained by copolymerizing (meth)acrylamide with 5–10 mol-% of a cationic monomer, meaning that the cationic polyacrylamide has a total degree of ionization in the range of 5–10 mol-%. In another embodiment, the cationic polyacrylamide is obtained by copolymerizing (meth)acrylamide with 20–35 mol-% or 25–35 mol-% of a cationic monomer, wherein the cationic polyacrylamide has a total degree of ionization in the range of 20–35 mol-% or 25–35 mol-%. Accordingly, amphoteric polyacrylamide is obtained by further copolymerization of (meth)acrylamide with 5–35 mol-% or 5–10 ml-% or 20–35 mol-% or 25–35 mol-% cationic monomers (calculated based on the total amount of monomers) in the presence of anionic monomers.

[0023] According to one embodiment of the invention, a bleaching aid is added during the bleaching process in an amount of < 500 g polymer / ton dry pulp, preferably < 300 g polymer / ton dry pulp, more preferably < 200 g polymer / ton dry pulp or < 100 g polymer / ton dry pulp. Detailed Implementation

[0024] Some embodiments of the present invention are described in the following non-limiting examples.

[0025] Background of the experiment

[0026] Tests demonstrating drainage characteristics were conducted at the Northern Kraft pulp mill. The mill produces bleached softwood (SW) pulp and bleached hardwood (HW) pulp. The bleaching process comprises bleaching stages D0-EOP-D1-P (acidic stages D0 and D1, alkaline stages EOP and P). For both SW and HW grades, tests were conducted at stage D0. The polymer, used as a drainage aid, was fed before the chemical mixer, with bleaching chemicals and pH control agents added after the polymer feed point.

[0027] In factory tests, improved drainage can be observed as a decrease in the rotational speed of the washer at stage D0. This effect can also be observed as an increase in solids or a change in pressure.

[0028] Example 1: Cork Test

[0029] In the first experiment, the polymer used as a filter aid was amphoteric polyacrylamide (MW 4–8 Mg / mol, net cationic, degree of ionization 5 mol-%). The cellulose fibers were derived from cork. The experiment lasted for 4 hours (production 1150 tons / day).

[0030] During the first test, the rotational speed of the D0 washer was Figure 1 Presented in the shaded area. The polymer feed rate (g / ton dry pulp) is also shown. Figure 1 As shown in the figure, when polymers are fed into a fiber suspension, the rotational speed (rev / min) decreases due to the increased drainage of the cellulose fiber suspension.

[0031] Example 2: Cork Test

[0032] In the second experiment, the polymer used as a filter aid was amphoteric polyacrylamide (MW 4–8 Mg / mol, net cationic, ionization degree 5 mol-%). The cellulose fibers were derived from cork. The experiment lasted for 28 hours (production capacity 900 tons / day).

[0033] During the second test, the rotational speed of the D0 washer was Figure 2 Presented in the shaded area. The polymer feed rate (g / ton dry pulp) is also shown. Figure 2 As shown in the diagram, the rotational speed increases slowly only when a flow aid is added to the fiber suspension, and increases rapidly after the polymer addition is stopped.

[0034] Example 3: Hardwood Test

[0035] In the third experiment, the polymer used as a filter aid was amphoteric polyacrylamide (MW 4–8 Mg / mol, net cationic, degree of ionization 5 mol-%). The cellulose fibers were derived from hardwood. The experiment lasted for 6 hours (production capacity 1400 tons / day).

[0036] During the third test, the rotational speed of the D0 washer was Figure 3 Presented in the shaded area. The polymer feed rate (g / ton dry pulp) is also shown. Figure 3 As shown in the figure, when polymers are fed into a fiber suspension, the rotational speed (rev / min) decreases due to the increased drainage of the cellulose fiber suspension.

[0037] Example 4: Drainage Time

[0038] In this embodiment, oxygen-delignified birch pulp (8% consistency) was bleached in the laboratory using chlorine dioxide (D0 stage). The ClO2 dosage was 24.8 kg active Cl / t. During bleaching, the temperature was maintained at 80°C and the retention time was 30 min.

[0039] Prior to acid bleaching, the polymer to be tested was added to the pulp liquor as a flow aid. The polymers with a net cationic charge used in this example are shown in Table 1. The reference is the D0 stage without added polymer. The polymer addition amount was 500 g / ton of dry pulp.

[0040] After the D0 stage, the pulp was not washed. A sample taken from the bleached pulp was diluted to a consistency of 1%. Drainage time was studied using a dynamic drainage analyzer (DDA) (AB Akribi Kemikonsuter, Sweden). The time between the application of vacuum and the point of vacuum breaking was measured. The change in vacuum indicates the time of wet fiber web formation until air passes through the thickened web, indicating drainage time. The mixing speed was 2000 rpm, and the line was 0.25 mm. Chemicals were added 15 s before drainage, the vacuum was 200 mbar, and the suction time was 25 s.

[0041]

[0042] The results of the drainage test are in Figure 4 The results show that if the polymer has a higher molecular weight, a higher charge is also required to achieve improved drainage time. Lower molecular weight and lower charge also improve drainage time.

Claims

1. A method for improving drainage during the acidic stage of a bleaching process of cellulose fibers, the method comprising: - Cellulose fibers are obtained from the pulping process and formed into an aqueous cellulose fiber suspension. - The cellulose fiber suspension is bleached during a bleaching process, the bleaching process including at least one acidic stage, wherein the pH of the cellulose fiber suspension is < 6, followed by at least one washing stage. Its features The method further includes - Add a drainage aid to the cellulose fiber suspension at least in the acidic phase of the bleaching process, wherein the drainage aid comprises a polymer containing cationic functional groups and has a mass-average molecular weight of 4,000,000–17,000,000 g / mol, a net cationic charge, and a total ionization of 5–35 mol-% at pH 7.

2. The method according to claim 1, characterized in that, The polymer containing cationic functional groups includes cationic polyacrylamide obtained by copolymerization of (meth)acrylamide and at least one cationic monomer.

3. The method according to claim 1 or 2, characterized in that, The polymers containing cationic functional groups include amphoteric polyacrylamides obtained by copolymerization of (meth)acrylamide with cationic and anionic monomers.

4. The method according to any one of the preceding claims, characterized in that, The cationic functional group of the polymer is derived from monomers selected from the group consisting of: 2-(dimethylamino)ethyl acrylate (ADAM), [2-(acryloyloxy)ethyl]trimethylammonium chloride (ADAM-Cl), 2-(dimethylamino)ethyl acrylate benzyl chloride, 2-(dimethylamino)ethyl acrylate dimethyl sulfate, 2-dimethylaminoethyl methacrylate (MADAM), [2-(methacryloyloxy)ethyl]trimethylammonium chloride (MADAM-Cl), 2-dimethylaminoethyl methacrylate dimethyl sulfate, [3-(acryloylamino)propyl]trimethylammonium chloride (APTAC), and [3-(methacryloylamino)propyl]trimethylammonium chloride (MAPTAC), and any combination thereof.

5. The method according to any one of the preceding claims, characterized in that, The anionic functional groups of amphoteric polyacrylamide are derived from monomers selected from the group consisting of unsaturated monocarboxylic acids or dicarboxylic acids, such as acrylic acid, maleic acid, fumaric acid, itaconic acid, aconitic acid, zeaxanthin, citraconic acid, crotonic acid, isocrotonic acid, angelic acid, and tigrinic acid, and any combination thereof.

6. The method according to any one of the preceding claims, characterized in that, The polymer containing cationic functional groups is added to the cellulose fiber suspension in the acidic stage before the acidic stage scrubber.

7. The method according to any one of the preceding claims, characterized in that, The polymer containing cationic functional groups is added to the cellulose fiber suspension at least during the first acidic phase of the bleaching process.

8. The method according to any one of the preceding claims, characterized in that, The cellulose fibers are derived from hardwood, softwood, or mixtures thereof.

9. The method according to any one of the preceding claims, characterized in that, The cellulose fibers are derived from chemical pulping processes, such as sulfate pulping.

10. The method according to any one of the preceding claims, characterized in that, The discharge aid is added in an amount of < 500 g / ton dry pulp, preferably < 300 g polymer / ton dry pulp, more preferably < 200 g polymer / ton dry pulp or < 100 g polymer / ton dry pulp.

11. Use of a polymer containing cationic functional groups as a bleaching aid in a bleaching process of cellulose fibers, wherein the polymer is added to a cellulose fiber suspension during an acidic phase of the bleaching process, wherein the cellulose fiber suspension has a pH < 6, and wherein the polymer containing cationic functional groups has a mass-average molecular weight of 4,000,000–17,000,000 g / mol, a net cationic charge, and a total ionization degree of 5–35 mol-% at pH 7.

12. The use according to claim 11, characterized in that, The polymer containing cationic functional groups includes cationic polyacrylamide obtained by copolymerization of (meth)acrylamide and at least one cationic monomer.

13. The use according to claim 11 or 12, characterized in that, The polymer containing cationic functional groups includes amphoteric polyacrylamide obtained by copolymerization of (meth)acrylamide with cationic and anionic monomers.

14. The use according to any one of claims 11-13, characterized in that, The cationic functional group of the polymer is derived from monomers selected from the group consisting of: 2-(dimethylamino)ethyl acrylate (ADAM), [2-(acryloyloxy)ethyl]trimethylammonium chloride (ADAM-Cl), 2-(dimethylamino)ethyl acrylate benzyl chloride, 2-(dimethylamino)ethyl acrylate dimethyl sulfate, 2-dimethylaminoethyl methacrylate (MADAM), [2-(methacryloyloxy)ethyl]trimethylammonium chloride (MADAM-Cl), 2-dimethylaminoethyl methacrylate dimethyl sulfate, [3-(acryloylamino)propyl]trimethylammonium chloride (APTAC), and [3-(methacryloylamino)propyl]trimethylammonium chloride (MAPTAC), and any combination thereof.

15. The use according to any one of claims 11-14, characterized in that, The anionic functional group of the amphoteric polyacrylamide is derived from monomers selected from the group consisting of unsaturated monocarboxylic acids or dicarboxylic acids, such as acrylic acid, maleic acid, fumaric acid, itaconic acid, aconitic acid, succinic acid, citraconic acid, crotonic acid, isocrotonic acid, angelic acid and tigrinic acid, and any combination thereof.

16. The use according to any one of claims 11-15, characterized in that, The polymer containing cationic functional groups is added to the cellulose fiber suspension in the acidic stage before the acidic stage scrubber.

17. The use according to any one of claims 11-16, characterized in that, The polymer containing cationic functional groups is added to the cellulose fiber suspension at least during the first acidic stage of the bleaching process.

18. The use according to any one of claims 11-17, characterized in that, The cellulose fibers are derived from hardwood, softwood, or mixtures thereof.

19. The use according to any one of claims 11-18, characterized in that, The cellulose fibers are derived from chemical pulping processes, such as sulfate pulping.

20. The use according to any one of the preceding claims, characterized in that, The discharge aid is added in an amount of < 500 g / ton dry pulp, preferably < 300 g polymer / ton dry pulp, more preferably < 200 g polymer / ton dry pulp or < 100 g polymer / ton dry pulp.

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