CN1922359A - Method for reducing the extractives content of high-yield pulps and method for producing bleached high-yield pulps - Google Patents

Abstract

The present invention relates to a method for reducing the extractive content of high yield pulp in a peroxide bleaching stage comprising peroxide bleaching and subsequent dewatering or washing, said method comprising during peroxide bleaching, Said dewatering or washing of the bleached pulp is carried out after contacting the pulp with an organic stabilizer and, during or after peroxide bleaching, with a surfactant to remove extractives together with the aqueous phase. The invention also relates to a method of producing bleached high yield pulp.

Description

Method for reducing extractives content of high-yield pulp and method for producing bleached high-yield pulp

field of invention

The present invention relates to a method for reducing the extractives content of high yield mechanical pulp. The present invention also relates to a method of producing bleached high yield pulp with reduced extractives content.

Description of prior art

Extractives are known constituents of wood and pulp materials that can be extracted with organic solvents. The most dominant chemical groups found in the extracts were fatty and resinous acids, terpenes, phytosterols and phenolic compounds. Extracts were found in esterified, glycated and derivatized forms. The most problematic extractives in wood are mainly lipophilic compounds. These are fatty acids, resin acids, terpenes, and sterols, and their function is to protect the tree by preventing attacks by fungi, bacteria, etc. Leafy softwoods contain about 1.5% by weight extract, while temperate hardwoods typically contain 2-2.5% by weight. In general, the extractives in question are often referred to as wood pitch. Problems are usually found in areas of high shear and when the temperature, pH or concentration of wood pitch changes.

Common harmful effects from wood pitch in different areas are described below. In bleaching plants, deposits are formed and the consumption of chemicals can also be increased. In the final product, spots and high levels of extractives are found.

Modern bleaching processes for mechanical pulp often include high consistency peroxide bleaching (PHC). After peroxide bleaching, the pulp is often diluted with white water from the paper machine, after which the pulp slurry is dewatered. This washing step reduces anionic waste carryover to the paper machine and also allows residual peroxide to recycle. The alkaline peroxide oxidizes the extractives and removes the extractives from the pulp quite effectively. However, if the pulp is diluted with white water in the bottom of the PHC bleaching tower or after PCH bleaching, extractives can re-precipitate in the surface of the pulp or process equipment due to the lower pH of the water, high hardness or dielectric (salt) concentration. The precipitated extractives are transferred within the fiber surface to the paper machine causing wet end and paper quality problems. When the PHC wash filtrate is recycled back into earlier stages of the process, extractives that may be found in the wash filtrate may re-precipitate within the process equipment or fibers in an acidic or neutral environment.

US4363699 discloses a method for stabilizing alkaline solutions of peroxide compounds used in bleaching, wherein a basic salt of poly-alpha-hydroxyacrylic acid is added to the solution as a stabilizer. The stable solution can be used for bleaching pulp and textiles. When bleaching textiles, the bleach solution may contain a wetting agent (which is a surfactant).

US4963157 discloses a method of bleaching cellulosic fibrous material, especially cotton, with hydrogen peroxide. The method involves impregnating a fibrous material with a bleach solution comprising a peroxide and a stabilizer. Stabilizers include poly-alpha-hydroxyacrylic acids or their salts or their corresponding polylactones, and organic phosphonic acids or their salts.

DE3423452 discloses solutions avoiding the use of water glass comprising a mixture of polyalpha-hydroxyacrylic acid (PHAA) and a water-soluble homopolymer of acrylic or methacrylic acid or a copolymer of acrylic acid and/or maleic acid. The salts, especially the sodium salt of PHAA and the sodium salt of the polycarboxylate polymer are mixed together. Chelating agents can be added to the mixture, thus obtaining improved stabilizers for bleaching processes. According to DE3423452, the bleaching process is carried out by using an alkaline peroxide bleaching solution containing said stable mixture.

Summary of the invention

The present invention is based on the fact that under alkaline conditions the extract is more soluble and therefore when diluting the pulp with dilution water, for example white water after bleaching, especially when the pH of the dilution water is lower than that of the When the pH of the pulp is lowered, the extractant can redeposit on the fiber surface. According to the present invention, it has been found that some surfactants inhibit the precipitation of extracts and thus keep them in the aqueous phase. The extractives can then be washed out of the pulp at the next dewatering press, which in turn leads to a reduced extractives content in the pulp. Since the extract is dispersed and the particles are stabilized, there is no re-precipitation of the extract when the filtrate is recycled to earlier process stages.

The present invention is particularly advantageous when bleaching pulp without using silicates as stabilizers, since it is noted that silica has a certain dispersing effect on extractives. However, the invention also works well in the presence of silicates.

Detailed description of the invention

According to the present invention, there is provided a method for reducing the extractives content of high yield pulp in a peroxide bleaching stage comprising peroxide bleaching and subsequent dewatering or washing, said method comprising in peroxide bleaching The pulp is contacted with an organic stabilizer and, during or after peroxide bleaching, with a surfactant before said dewatering or washing of the bleached pulp to remove extractives with the aqueous phase.

In the context of the present invention, a peroxide bleaching stage comprises peroxide bleaching of pulp, optionally thinning the pulp, and dewatering or washing the pulp.

According to the present invention there is also provided a process for producing bleached high yield pulp with reduced extractives content, the process comprising bleaching the high yield pulp with peroxide, wherein during peroxide bleaching the pulp is contacted with an organic stabilizer , and contact with surfactants during or after peroxide bleaching and dewater or wash the bleached pulp to remove extractives with the aqueous phase and produce bleaching with reduced extractive content rate pulp.

The extractives removed from the pulp of the present invention are derived from wood pitch. The extract includes esterified, glycated and/or derivatized forms of fatty acids, resin acids, terpenes, phytosterols and phenolic compounds. The most problematic extracts include fatty acids, resin acids, triglycerides, steryl esters, sterols, and lignans.

Organic stabilizers and surfactants can be added independently, or these chemicals can be mixed in advance with the bleach solution. Furthermore, organic stabilizers and surfactants can be added simultaneously or typically as a chemical mixture, ie product.

Organic stabilizers and surfactants can be added to the pulp during or prior to peroxide bleaching.

Surfactants can also be added to the pulp after peroxide bleaching. In this case, the surfactant can be added to the dilution water which is added to the pulp between peroxide bleaching and dewatering or washing. Dilution water can be white water recycled from the paper machine.

The organic peroxide stabilizers used in the process of the invention are preferably polymeric stabilizers such as poly-alpha-hydroxyacrylic acid or its salts or corresponding polylactones, homopolymers of acrylic acid, methacrylic acid or maleic acid or copolymers of acrylic and/or methacrylic acid with unsaturated dicarboxylic acids. The polymeric stabilizer may also be a mixture of poly-alpha-hydroxyacrylic acid and the homopolymer and/or the copolymer. The unsaturated dicarboxylic acid is preferably maleic acid. The salts of poly-alpha-hydroxyacrylic acid are preferably sodium, potassium or ammonium salts. The poly-alpha-hydroxyacrylic acid may have a molecular weight of at least 5000, preferably at least 10000, and more preferably at least 15000 (calculated as the sodium salt form of PHAA). Homopolymers and copolymers may have a molecular weight of at least 4,000, preferably at least 10,000, and more preferably at least 20,000.

Organic stabilizers may also be phosphonic acids or salts thereof. The salts are preferably sodium, potassium or ammonium salts. Suitable phosphonic acids are, for example, (poly)aminophosphonic acids, such as aminotris(methylenephosphonic acid), ethylenediaminetetrakis(methylenephosphonic acid), i.e. EDTMPA or diethylenetriaminepenta(ethylene Methylphosphonic acid), ie DTPMPA.

In addition to the organic stabilizers mentioned, it is also possible to introduce another stabilizer. The other stabilizer may be an alkaline earth metal compound, such as a magnesium and/or calcium compound. The alkaline earth metal compound may be in the form of a salt, for example, sulfate, chloride or any other water soluble salt, or a complex with a polymer or chelating agent. A preferred alkaline earth metal salt is magnesium sulfate.

The organic stabilizer is preferably used in an amount of 0.1 kg to 5 kg per ton of dry pulp, more preferably 0.25 kg to 3 kg per ton of dry pulp, and even more preferably 0.5 kg to 3 kg per ton of dry pulp .

The surfactants used in the process of the invention may be anionic surfactants, such as naphthalenesulfonates or lignosulfonates, or nonionic surfactants, such as O/W emulsifiers, such as fatty alcohol ethoxylates compounds or alkylphenol ethoxylates.

Mixtures of anionic and nonionic surfactants may also be used. Antifoams can be used in combination with surfactants to prevent foaming.

The dosage of the surfactant is preferably 0.005kg-2kg per ton of dry pulp, more preferably 0.05kg-1kg per ton of dry pulp.

In bleached high yield mechanical pulps such as groundwood, stoneground and pressure groundwood (GW, SGW, PGW), refiner mechanical pulp (RMP), thermomechanical pulp (TMP) and chemithermomechanical pulp (CTMP, APMP) using the method of the present invention.

In peroxide bleaching, hydrogen peroxide is preferably used as the bleaching agent. Other chemicals may be used in bleaching, such as bleach stabilizers, chelating agents and/or magnesium sulfate. It is particularly advantageous to add the chemicals prior to bleaching, or at the same time as the bleaching chemicals are being used. If there are several bleaching stages, chemicals can be added in any one bleaching stage or in several stages.

The residence time in bleaching can vary within wide ranges, for example 30-240 minutes, preferably 45-180 minutes, and most preferably 60-120 minutes. Residence time also depends on the temperature used in bleaching.

Bleaching of high yield pulp can be carried out at a temperature of 30-90°C, preferably at a temperature of 50-90°C. Bleaching can be done at a chosen consistency, but is most preferably done at a high consistency, ie, greater than or equal to about 30%. Bleaching can also be carried out in two stages with a dehydration stage in between.

In the alkaline bleaching of the present invention, the pH may be 7-14, preferably 8-11. The ratio of base, typically sodium hydroxide, to peroxide can vary widely, depending on the raw material and the degree of bleaching. Alternative sources of alkalinity, such as sodium carbonate, may also be used.

The invention is illustrated by the following examples. Percentages are % by weight unless otherwise indicated. "kg/t" means kilograms per ton of dry pulp. "Cs" means consistency. "P" stands for peroxide section.

Example

comparative example

The purpose of this comparative example is to clarify the problem to be solved by the present invention. High consistency TMP pulp (spruce) was bleached with peroxide using water glass or poly-alpha-hydroxyacrylate (PHAA) as a stabilizer. Subsequently, the pulp was diluted with ion-exchanged water and then dewatered to a consistency of 30%. Detailed experimental data are shown in the table below.

After dilution, the high turbidity of the filtrate can be attributed to the extract. Analysis of the extracts showed that most of the extracts were removed during bleaching. The results show that the silicate (water glass) has some dispersing power for the extractives, since the extractives content is much higher with the acrylate based stabilizer. This means that especially in the case of silicate-free bleaching, wood pitch dispersants are advantageous.

Example 1

Samples of CTMP-pulp (Aspen) were removed from the attritor and bleached with peroxide to a brightness of 83-84% ISO. The bleaching conditions are as follows: t=120 minutes, 70°C, consistency 30%, H ₂ O ₂ 38kg/t, NaOH 21kg/t, Na ₂ CO ₃ 8kg/t. The pulp had been sequestered in the attritor and had a manganese content of 1.5 ppm and an Fe content of 5 ppm. Additives are fed in the peroxide bleaching section and after bleaching the pulp is diluted to 10% consistency at different pH levels followed by dewatering. The table below shows the turbidity of the filtrate after dehydration. Use 2 kg polymer (poly-α-hydroxy acrylate) as stabilizer and 0.1 kg naphthalene sulfonate + 0.1 kg nonionic surfactant per ton of dry pulp (fatty alcohol ethoxylate 7mol EO, C12-C14, HLB 12.3), carry out the experiment of the present invention. In comparative experiments, water glass and poly-α-hydroxyacrylate were used as stabilizers. In each case, bleaching results and chemical consumption were about the same. Water glass, 25kg/t Acrylate polymer, 2kg/t this invention pH Turbidity, NTU pH Turbidity, NTU pH Turbidity, NTU 6.6 15107.5 14307.8 1465 6.3 13537.3 13177.7 1389 6.5 14647.2 14407.5 1481

From this result it can be seen that silica derived from water glass (sodium silicate) increases turbidity. This can be attributed to the higher extract content in the filtrate. When the silicate is replaced by a polymer-based stabilizer, the turbidity is lower and the extractives content in the pulp is higher. The higher turbidity obtained with the chemical composition according to the invention demonstrates that the extract dissolved in the alkaline peroxide did not reprecipitate after dilution.

Example 2

Samples of CTMP-pulp (Aspen) were removed from the attritor and bleached with peroxide at high consistency. The table below shows the bleaching conditions and results. Surfactants are added in the bleaching section and the pulp is diluted with ion-exchanged water after bleaching. The composition of surfactant here is naphthalenesulfonic acid condensation product (Na salt) and aliphatic alcohol ethoxylate (same as the previous example) in a weight ratio of 2:1 (feeding with 20% aqueous solution dilution). The polymer-based stabilizer was a mixture (1:4w:w) of poly-alpha-hydroxyacrylate and maleate-acrylate copolymer with an active substance content of 25%.

*from pulp

DCM stands for dichloromethane.

The results show that the method of the invention reduces extractives in the pulp without affecting the bleaching results.

Example 3

Samples of CTMP-pulp (Aspen) were removed from the attritor and bleached with peroxide to a brightness of 83-84% ISO. Additives are fed in the peroxide bleaching section and after bleaching the pulp is diluted to 10% consistency at different pH levels followed by dewatering. In one experiment, the surfactant additive was dosed also after dilution. The results are shown in the table below. The surfactant additives used here were naphthalenesulfonic acid condensation products (A) and fatty alcohol ethoxylates (B, 25 mol EO, C16-C18, HLB 16). The polymer-based stabilizer was a mixture (1:4w:w) of poly-alpha-hydroxyacrylate and maleate-acrylate copolymer with an active substance content of 25%.

The results showed that the combination of polymer-based stabilizer and anionic and/or nonionic surfactants gave lower extractables content than water glass.

Claims (20)

1. A method for reducing the extractive content of high-yield pulp in a peroxide bleaching stage comprising peroxide bleaching and subsequent dewatering or washing, said method comprising in peroxide bleaching pulp with The organic stabilizer is contacted and the surfactant is contacted during or after peroxide bleaching before said dewatering or washing of the bleached pulp to remove extractives with the aqueous phase. 2. The method of claim 1, wherein organic stabilizers and surfactants are added to the pulp during peroxide bleaching. 3. The method of claim 1, wherein organic stabilizers and surfactants are added to the pulp prior to peroxide bleaching. 4. The method of claim 1, wherein the surfactant is added to the pulp after peroxide bleaching. 5. The method of any one of claims 1-4, wherein dilution water is added to the pulp between peroxide bleaching and dewatering or washing. 6. The method of claim 5, wherein the surfactant is added to the dilution water. 7. The method of any one of claims 1-6, wherein the organic stabilizer comprises a polymeric stabilizer, for example, poly-alpha-hydroxyacrylic acid or a salt thereof or the corresponding polylactone, acrylic acid, methacrylic acid or maleic acid homopolymers, or copolymers of acrylic acid and/or methacrylic acid and unsaturated dicarboxylic acids, or mixtures of these polymers. 8. The method according to any one of claims 1-7, wherein the organic stabilizer is used in an amount of 0.1 kg-5 kg per ton of dry pulp, preferably 0.25 kg-3 kg per ton of dry pulp. 9. The method of any one of claims 1-8, wherein the surfactant comprises an anionic surfactant, such as naphthalenesulfonate or lignosulfonate, or a nonionic surfactant, such as an O/W emulsifier, such as Fatty alcohol ethoxylates or alkylphenol ethoxylates. 10. The method according to any one of claims 1-9, wherein the surfactant is used in an amount of 0.005 kg-2 kg per ton of dry pulp, preferably 0.05 kg-1 kg per ton of dry pulp. 11. A method of producing bleached high yield pulp with reduced extractives content, the method comprising bleaching the high yield pulp with peroxide, wherein during peroxide bleaching the pulp is contacted with an organic stabilizer, and During oxide bleaching or after peroxide bleaching, contacting with a surfactant and dewatering or washing bleached pulp to remove extractives with the aqueous phase and produce a bleached high yield pulp with reduced extractives content . 12. The method of claim 11, wherein organic stabilizers and surfactants are added to the pulp during peroxide bleaching. 13. The method of claim 11, wherein organic stabilizers and surfactants are added to the pulp prior to peroxide bleaching. 14. The method of claim 11, wherein the surfactant is added to the pulp after peroxide bleaching. 15. The method of any one of claims 11-14, wherein dilution water is added to the pulp between peroxide bleaching and dewatering or washing. 16. The method of claim 15, wherein the surfactant is added to the dilution water. 17. The method of any one of claims 11-16, wherein the organic stabilizer comprises a polymeric stabilizer, for example, poly-alpha-hydroxyacrylic acid or a salt thereof or the corresponding polylactone, acrylic acid, methacrylic acid or maleic acid homopolymers, or copolymers of acrylic acid and/or methacrylic acid and unsaturated dicarboxylic acids, or mixtures of these polymers. 18. The method according to any one of claims 11-17, wherein the organic stabilizer is used in an amount of 0.1 kg-5 kg per ton of dry pulp, preferably 0.25 kg-3 kg per ton of dry pulp. 19. The method of any one of claims 11-18, wherein the surfactant comprises an anionic surfactant, such as naphthalenesulfonate or lignosulfonate, or a nonionic surfactant, such as an O/W emulsifier, such as Fatty alcohol ethoxylates or alkylphenol ethoxylates. 20. The method of any one of claims 11-19, wherein the surfactant is used in an amount of 0.005 kg-2 kg per ton of dry pulp, preferably 0.05 kg-1 kg per ton of dry pulp.