EA014734B1 - Method for treating of pulp - Google Patents

Abstract

The present invention relates to a method of treating an aqueous suspension of bleached pulp derived from an alkaline pulping process comprising dewatering and drying said suspension, wherein the pH of the suspension to be treated ranges from about 6.5 to about 12.

Description

(57) The present invention relates to a method for treating an aqueous suspension of bleached pulp obtained by alkaline cooking, comprising dewatering and drying said suspension, wherein the pH of the suspension to be treated is in the range of from about 6.5 to about 12.

The present invention relates to a method for reducing whitening or yellowing of bleached pulp.

State of the art

In the field of whitening, the goal is to obtain pulp with high and stable whiteness levels. However, the decrease in the brightness of cellulose to varying degrees occurs during its storage and transportation to the pulp and paper mill, depending, for example, on temperature, humidity, wood species, cooking conditions and bleaching sequence. The conditions for dewatering and drying the pulp after the final bleaching step are also important to maintain the stability of the whiteness of bleached pulp. When commercial pulp is to be transported from its production point to a pulp and paper mill, for economic and quality reasons, it should be substantially dry. It is known that the mode of operation of presses can have a different effect on whiteness, depending on the stability of the pulp and the drying conditions. For example, if cellulose is dried at a high temperature and then laid in bales, thermal whitening continues to occur during storage due to the high temperature and humidity existing in the bale.

Т11С В1ееЫпд о £ Ри1р, ТгбΕάίίί ηοη, Tarр1 ргевв, еббеб бу 8шдЬ, Р.Р. p. 378, 388-390, 1979, discloses that the yellowing of celluloses occurs the faster, the higher the temperature and higher humidity. In this publication, it is further disclosed that the deterioration in color is the higher, the higher the pH of the water from which the cellulosic sheet is obtained. Thus, in this area it is customary to carry out drying at a fairly low pH, regardless of what bleaching sequence was carried out for the pulp to be dried.

In the article UaPaev IcePeel 111e1a1a1e11o \\ 'td o £ TSB-b1easeb Ebg Kgai ri1rv, Ogap51gosh, A, Rougr apb Rareg Vevegeb 1 oigpa1, voo1.16, different factors, acid, 1/2001 dry matter content, temperature, time and pH, affecting the process of reducing whiteness. This article illustrates variable degrees of brightness reduction depending on these factors and, among other things, at pH values from 3 to 6. Hexenuronic acid groups, as mentioned in Ogap51gosh article, are formed, for example, during alkaline cooking, when the conversion of 4-O-methyl groups occurs X-E-glucuronic acid. It was previously assumed that the presence of hexenuronic acid groups is at least partially responsible for the diminution of cellulose whiteness. Therefore, attempts have been made to remove groups of hexenuronic acid, for example, by acid hydrolysis, before bleaching or selecting a bleaching sequence that destroys the groups of hexenuronic acid. All of these attempts, however, lead to a complication of the pulp production system, and in most cases these attempts involve additional costs.

The present invention provides a method for improving the brightness stability of bleached pulps that have been alkaline cooked, in particular those that contain a certain number of hexenuronic acid groups.

Description of the invention

The present invention relates to a method for treating an aqueous suspension of bleached pulp obtained by an alkaline cooking method before paper production, said method comprising dehydrating and drying said suspension, wherein the pH of the suspension to be processed is in the range of from about 6.5 to about 12.

The pH of the cellulosic suspension can be controlled using any standard equipment, such as real-time sensors that continuously measure pH. In one embodiment, the pH is in the range of from about 7.5 to about 12, or from about 8.5 to about 12, for example, from about 9 to about 11, or from about 9.5 to about 10.5. In one embodiment, at least one pH adjusting chemical is added to bleached pulp before or during dehydration and / or drying to adjust the pH, for example, in an amount of from about 0.001 to about 50, from about 0.005 to about 20, or from about 0 01 to about 10 kg / tonne of dry pulp. Amounts of pH regulatory chemicals are given as the amount of active substance. According to one variant of implementation in this method, the pH is constantly maintained in the specified range.

In this application, both the terms dehydration and drying should be understood as denoting techniques in which water is removed from cellulose. Both dehydration and drying, therefore, include dehydration, but usually using different means or devices. While dehydration typically involves compressing or draining water from a cellulosic suspension, drying involves raising the temperature compared to the temperature at which dehydration is performed.

It is understood that the term before paper production means that the steps of the process are carried out before essentially dry pulp is fed to the pulp breaker, where it is pulled in water and then fed to the wet part of the paper machine.

Bleached cellulose can be obtained from a lignocellulose-containing material that can

- 1 014734 may include, for example, wood logs, finely divided raw materials and wood materials such as wood particles, and non-wood such as fibers of annuals or perennials. Wood raw material can be obtained from hard or soft wood, such as birch, beech, aspen, alder, eucalyptus, maple, acacia, mixed tropical hardwood, pine, fir, Tsugi, larch, spruce, and mixtures thereof. Non-woody plant raw material can be obtained, for example, from straws of cereal crops, a double-source of reed, reeds, flax, hemp, kenaf, jute, Chinese nettle, sisal, abacus, coconut fiber, bamboo, beet pulp, or combinations thereof. However, cellulose obtained from recycled cellulose can also be used, either individually or in mixtures with primary celluloses.

Cellulose may be chemical cellulose obtained using an alkaline cooking process. Examples of alkaline cooking methods are sulphate cooking, soda methods or their modifications with optional additives, such as anthraquinone, for example a soda cooking method with the addition of anthraquinone (AO). Chemical cellulose obtained from materials containing lignocellulose can be subjected to one or more stages of oxygen treatment after cooking. This method can be applied to chemically digested celluloses having an initial kappa number, i.e. the number of kappa after cooking, but before oxygen delignification, in the range of from about 5 to about 60, for example, from about 6 to about 50, or from about 8 to about 40, or from about 10 to about 30. The kappa number is measured according to the standard method of Ι8Ο 302.

Bleached cellulose can be obtained by any suitable whitening sequences, including fully chlorine-free (TCP) whitening sequences, including, for example, ozone, peroxide, oxygen, peracids, alkaline extraction, acid hydrolysis, and chelation steps and combinations thereof; elemental chlorine-free bleaching sequences (ECPs), including, for example, ozone, peroxide, oxygen, peracid, alkaline extraction, chlorine dioxide and chelation steps and combinations thereof. The bleaching sequence of both ECP and TCP can be completed with an acidic or alkaline step. In one embodiment, the cellulose can only be bleached using alkaline bleaching steps, such as oxygen and / or peroxide bleaching steps, giving a final pH of from about 7.5 to about 12.

In one embodiment, the final pH in the final bleaching step is in the range of from about 1 to about 8, for example from about 2 to about 7, or from about 3 to about 6.

According to one embodiment, the content of hexenuronic acid in the cellulose before dehydration and drying is in the range of from about 1 to about 150, for example from about 5 to about 100, or from about 10 to about 90, or from about 30 to about 80, such as from about 50 to about 80 mmol / kg dry pulp. The invention has been found to be particularly well suited for celluloses containing hexenuronic acid within the ranges defined herein. Moreover, it is noted that the present invention improves the stability of the pulp whiteness, which makes the step of the prior acid hydrolysis prior to bleaching unnecessary.

Bleached pulp can be diluted before dehydration. In one embodiment, the bleached pulp suspension prior to dehydration comprises dry matter in an amount in the range of from about 0.1 to about 5, for example from about 0.5 to about 4, or from about 1 to about 3% by weight.

Dehydration and drying can be carried out in various ways. In one embodiment, the bleached pulp is cleaned, for example, using a screen, after the final bleaching step, before drying. In this screen, remaining substances such as chips and bark can be removed. In the dehydration step, the cellulose can be subjected to various operations depending on how the cellulose is finally dried. For example, it can be dried in the form of cellulose sheets or flakes. For example, a suspension of bleached pulp can be transferred to a pressure tank, with which a cellulose network or cellulose sheet is formed on a wire in wet presspaste. The formed cellulosic sheet may have a density of 6001000 g / m ² and a dry matter content of from about 5 to about 50 wt.%.

The subsequent drying section may include steam heated dryer cylinders or an air cushion dryer. In an air-cushioned dryer, the cellulose network can make several passes through rows of air-fed booms. Cellulose can be dried so that the solids content is from about 90 to about 100 wt.%. The dried cellulose thus obtained can be cut into sheets and stacked to form bales weighing from 200 to 300 kg.

According to one embodiment, the bleached pulp is compressed, for example, in a squeeze press, to increase the dry matter content of the pulp, which gives a final dry matter content of from about 20 to about 80, for example, from about 30 to about

- 70 01434 or from about 40 to about 60 wt.%. A squeeze press typically presses cellulose to a screen that holds the cellulose fibers but allows water to be forced through itself, optionally with suction air. For example, a squeezing press may take the form of a pair of cylinders designed to compress pulp together. Alternatively, the squeezing press may include a screw conveyor rotating with a screen. Then the cellulose can be formed into sheets or flakes, which are subsequently dried.

The drying step can also be carried out in any other suitable dryer, for example, in an instant steam dryer, drying the cellulose with hot gas, usually air, at a temperature of from about 200 to about 600 ° C. However, according to one embodiment, the drying is not carried out by means of an instant steam dryer. Cellulose, which may be in the form of particles, such as flakes, in an instant steam dryer, encounters hot air created, for example, by burning oil or gas, and passes through a pipe system until it reaches the final cyclone collector. It separates hot humid air, giving a final dryness of up to about 90% by weight. Then the cellulose is fed to a press, in which compressed flakes can form a bale. Then it is treated like a bale of cellulose sheets obtained in presspate, except that wrapping may be necessary, since such bales can more easily disintegrate when handling them. The dehydration and drying operations are further explained in detail, for example, in Räreggshaksd 8s1epse apy Tesypoduodu, SiSsyzep, 1. e! A1, Vook 6A, Sesh1sa1 Ri1rsh§, s.A667-A686, 2000.

In one embodiment, the formed cellulosic sheets are subsequently molded into bales. Bales can have a dry matter content of from about 50 to about 100, or from about 50 to about 98, for example, from about 60 to about 95, or from about 70 to about 93 wt.%.

In one embodiment, at least one desiccant chemical is added to bleached pulp before and / or during dehydration, for example, directly to an aqueous suspension of bleached pulp, for example, in an amount of from about 0.01 to about 10, or from about 0.05 to approximately 5 kg / t dry pulp. It has been found that dehydration and / or drying of bleached pulp in the presence of at least one drying chemical can significantly reduce the drying time, while the stability of whiteness increases due to the reduced exposure time to elevated drying temperatures.

In one embodiment, a combination of drying chemicals is added to the bleached pulp suspension. In one embodiment, the cationic polysaccharide can be used as a drying chemical selected from any polysaccharide known in the art, including, for example, starches, guar gums, celluloses, chitins, chitosans, glycans, galactans, glucans, xanthan gums, pectins, mannans, dextrins, preferably starches and guar gums. Examples of suitable starches include potato, corn, wheat, tapioca, rice, waxy maize, barley starch and so on.

It is satisfactory if the cationic polysaccharide is dispersible in water or preferably it is water soluble. Particularly suitable polysaccharides of the invention include polysaccharides having the general structural formula (I):

(ABOUT

IX 'Ρ - (- Α — Ν ⁺ —K ₂ ) n

I

1¾ g

where P is a polysaccharide residue;

A is a group connecting N to the polysaccharide residue, preferably a chain of atoms including C and H atoms and optionally O and / or атом atoms, usually an alkylene group containing from 2 to 18 and preferably from 2 to 8 carbon atoms, optionally interrupted or substituted with one or more heteroatoms, for example, O or Ν, for example, an alkyleneoxy group or a hydroxypropylene group (—CH2 — CH (OH) —CH2—);

each of K ₁ , K ₂ and K ₃ represents a hydrocarbon group, preferably alkyl containing from 1 to 3 carbon atoms, preferably 1 or 2 carbon atoms; η is an integer from about 2 to about 300000, preferably from 5 to 200000, for example from 6 to 125000, or, alternatively, K ₁ , K ₂ and K ₃ together with N form an aromatic group containing from 5 to 12 carbon atoms ; and X ^- is an anionic counterion, usually a halide such as chloride.

The cationic polysaccharides of the invention may also contain anionic groups, preferably in small amounts. Such anionic groups may be introduced into the polysaccharide by chemical treatment or may be present in the native polysaccharide.

The weight average molecular weight of the cationic polysaccharide can vary within wide limits, inter alia, depending on the type of polymer used, and usually it is at least

- 3 014734 at least about 5,000 and often at least 10,000. More often it is more than 150,000, usually more than 500,000, preferably more than about 700,000 or more than about 1,000,000, or, for example, more than about 2,000,000. The upper limit is not limited; it can be approximately 200000000, usually 150,000,000 and preferably 100,000,000.

The cationic polysaccharide can have a degree of substitution Cato _(Ό8,;). varying in a wide range, inter alia, depending on the type of polymer used; O8 _C can be from 0.005 to 1.0, usually from 0.01 to 0.5, preferably from 0.02 to 0.3, or from 0.025 to 0.2.

According to one embodiment, the charge density of the cationic polysaccharide is in the range from 0.05 to 6.0, preferably from 0.1 to 5.0, or from 0.2 to 4.0 meq / g of dry polymer.

In one embodiment, the anionic polymer is added as a drying chemical, which may be selected from inorganic and organic anionic polymers. Examples of suitable polymers include water-soluble and water-dispersible inorganic and organic anionic polymers.

Examples of suitable polymers include inorganic anionic polymers based on silicic acid and silicate, i.e. polymers based on anionic silica. Suitable anionic silica polymers can be prepared by condensation polymerization of silicic compounds, for example silicic acid and silicates, which can be homopolymerized or copolymerized. In one embodiment, anionic silica polymers include anionic silica particles that have a size in the range of colloidal particle size. Anionic silica particles are typically supplied in the form of aqueous colloidal dispersions, so-called sols. Silica-based sols can be modified to contain other elements, such as aluminum, boron, nitrogen, zirconium, gallium and titanium, which may be present in the aqueous phase and / or in the silica-based particles. Examples of suitable anionic silica particles include polysilicic acids, polysilicon microgels, polysilicates, polysilicon microgels, colloidal silicon dioxide, alumina modified silica, polyaluminosilicates, polyaluminosilicate microgels, polyborosilicates and so on. Examples of suitable anionic silica particles include particles disclosed in US Pat. Nos. 4,388,150; 49,274 98; 4,954,220; 4,961,825; 4980025; 5,277,994; 5,167,891; 5,368,833; 5,447,604; 5,470,435; 5,543,014; 5,571,494; 5,573,674; 5,584,966; 5,603,805; 5,688,482; and 5,707,493, all of which are incorporated herein by reference.

Examples of suitable anionic silica particles include particles having an average size of less than about 100 nm, for example, less than about 20 nm, or in the range of from about 1 to about 10 nm. As is customary in silicate chemistry, particle size refers to the average size of primary particles that can be aggregated or non-aggregated. In one embodiment, the anionic silica polymer comprises anionic silica aggregated particles. The specific surface area of the silica-based particles is preferably at least 50 m ² / g, for example at least 100 m ² / g. In general, the specific surface area can be up to about 1700 m ² / g, preferably up to about 1000 m ² / g. The specific surface area is measured by titration of NaOH as described by C. \ U. 8eag§ in ApA1uDsa1 Schedule 28 (1956): 12, 1981-1983 and in US Pat. No. 5,167,891, after proper removal or correction for the effect of any compounds present in the sample that may interfere with titration, such as aluminum and boron particles. The studied area, thus, reflects the average specific surface area of the particles.

In one embodiment of the invention, anionic silica particles have a specific surface area in the range of 50 to 1000 m ² / g, for example, 100 to 950 m ² / g. Silica-based particles may be present in an ash having an 8-value in the range of 8 to 50%, for example 10 to 40%, containing silica-based particles with a specific surface area in the range of 300 to 1000 m ² / g, preferably from 500 to 950 m ² / g, for example from 750 to 950 m ² / g, where these sols can be modified, as mentioned above. The 8-value is measured and calculated as described by Peg apb EA11op in 1. P11U5. Siet. 60 (1956), 955-957. An 8-value indicates a degree of aggregation or microgel formation, and a lower 8-value indicates a greater degree of aggregation.

In yet another embodiment, the silica particles have a large specific surface area, preferably above about 1000 m ² / g. The specific surface area may be in the range from 1000 to 1700 m ² / g, for example from 1050 to 1600 m ² / g.

Additional examples of suitable polymers include water-soluble and water-dispersible organic anionic polymers obtained by polymerizing an ethylenically unsaturated anionic or potentially anionic monomer or a mixture of monomers including one or more ethylenically unsaturated anionic or potentially anionic monomers, and optionally one or more others

- 4 014734 ethylenically unsaturated monomers. In one embodiment, ethylenically unsaturated monomers are water soluble. Examples of suitable anionic and potentially anionic monomers include ethylenically unsaturated carboxylic acids and their salts, ethylenically unsaturated sulfonic acids and their salts, for example, any of the above monomers. The monomer mixture may contain one or more water-soluble ethylenically unsaturated nonionic monomers. Examples of suitable copolymerizable non-ionic monomers include acrylamide and the aforementioned non-ionic monomers based on acrylamide and based on acrylate and vinylamines. The monomer mixture may also contain one or more water-soluble ethylenically unsaturated cationic and potentially cationic monomers, preferably in small amounts. Examples of suitable copolymerizable cationic monomers include those represented by the aforementioned general structural formula (I) and diallyldialkylammonium halides, for example diallyldimethylammonium chloride. The monomer mixture may also contain one or more polyfunctional crosslinking agents. The presence of a multifunctional crosslinking agent in a mixture of monomers allows the preparation of polymers that are dispersible in water.

Examples of suitable multifunctional crosslinking agents include the aforementioned multifunctional crosslinking agents. These agents may be used in the above amounts. Examples of suitable water dispersible organic anionic polymers include those disclosed in US Pat. No. 5,167,766, which is incorporated herein by reference. Examples of copolymerizable monomers include (meth) acrylamide, and examples of preferred polymers include water-soluble and water-dispersible acrylamide-based anionic polymers.

In one embodiment, an organic anionic polymer is used that is water soluble and has a weight average molecular weight of at least about 500,000. Typically, the weight average molecular weight is at least about 1 million, preferably at least about 2 million, or at least about 5 million. The upper limit is not limited; it can be approximately 50 million, usually 30 million.

In one embodiment, the organic anionic polymer may have a charge density of less than about 14, preferably less than about 10 meq / g, or less than about 4 meq / g. Preferably, the charge density is in the range from 0.5 to 14.0, for example, from 1.0 to 10.0 meq / g.

In one embodiment, the cationic polymer is used as a drying chemical. The cationic polymer may have a charge density in the range of 0.5 to 10.0 or 1.0 to 8.5 meq / g. The cationic polymer may be selected from inorganic and organic cationic polymers and may be water soluble. Examples of suitable cationic polymers include polyaluminium compounds, for example polyaluminium chlorides, polyaluminium sulfates, polyaluminium compounds containing both chloride and sulfate ions, polyaluminium silicatesulfates and mixtures thereof. Further examples of suitable cationic organic polymers include, for example, cationic acrylamide-based polymers; poly (diallyldialkylammonium halides), for example poly (diallyldimethylammonium chloride); polyethyleneimines; polyamidoamines; polyamines and polymers based on vinylamine. Examples of suitable cationic organic polymers include polymers obtained by polymerization of a water-soluble ethylenically unsaturated cationic monomer, or preferably a mixture of monomers, including one or more water-soluble ethylenically unsaturated cationic monomers and, optionally, one or more other water-soluble ethylenically unsaturated monomers. Examples of suitable water-soluble ethylenically unsaturated cationic monomers include diallyldialkylammonium halides, for example diallyldimethylammonium chloride, and cationic monomers represented by the general structural formula (II):

CH ₂ = C-K, K ₂ (II)

I I

0 = С —Α —Β-Ν ⁺ —Рз X '

I g

where Κι represents H or CH ₃ ;

K ₂ and K ₃ each represent H or a hydrocarbon group, preferably alkyl containing from 1 to 3 carbon atoms, for example from 1 to 2 carbon atoms;

A represents O or ΝΗ;

B represents an alkyl or alkylene group containing from 2 to 8 carbon atoms, preferably from 2 to 4 carbon atoms, or a hydroxypropylene group;

A cd is H or a hydrocarbon group, preferably an alkyl group containing from 1 to 4 carbon atoms, preferably 1 to 2 carbon atoms, or an aromatic group-containing substituent, preferably a phenyl or substituted phenyl group, which can be attached to nitrogen via an alkylene group typically containing from 1 to 3 carbon atoms, preferably from 1 to 2 carbon atoms, with a preferred Cd comprising benzyl

- 5 014734 group (-CH2-C6H5); and

X is an anionic counterion, usually a halide, such as chloride.

Examples of suitable monomers represented by the general structural formula (II) include quaternary monomers obtained by treating dialkylaminoalkyl (meth) acrylates, for example dimethylaminoethyl (meth) acrylate, diethylaminoethyl (meth) acrylate and dimethylaminohydroxypropyl (meth) acrylate, and dialkylaminoamylalkyl ( for example dimethylaminoethyl (meth) acrylamide, diethylaminoethyl (meth) acrylamide, dimethylaminopropyl (meth) acrylamide and diethylaminopropyl (meth) acrylamide, methyl chloride or benzyl chloride. Suitable cationic monomers of the general formula (II) include the quaternary salt of dimethylaminoethyl acrylate and methyl chloride, the quaternary salt of dimethylaminoethyl methacrylate and methyl chloride, the quaternary salt of dimethylaminoethyl acrylate and benzyl chloride and the quaternary salt of dimethylaminoethyl methacrylate and benzyl chloride.

The monomer mixture may contain one or more water-soluble ethylenically unsaturated nonionic monomers. Examples of suitable copolymerizable nonionic monomers include acrylamide and acrylamide-based monomers, for example methacrylamide, no. Alkyl (meth) acrylamides, for example methyl (meth) acrylamide, no ethyl (meth) acrylamide, no n-propyl (meth) acrylamide, no isopropyl (meth ) acrylamide, Nn-butyl (meth) acrylamide, Nt-butyl (meth) acrylamide and Nisobutyl (meth) acrylamide; Ν-alkoxyalkyl (meth) acrylamides, for example, No. n-butoxymethyl (meth) acrylamide and No. isobutoxymethyl (meth) acrylamide; ^ No. dialkyl (meth) acrylamides, for example ^ No. dimethyl (meth) acrylamide; dialkylaminoalkyl (meth) acrylamides; acrylate-based monomers such as dialkylaminoalkyl (meth) acrylates; and vinylamines. The monomer mixture may also contain one or more water-soluble ethylenically unsaturated anionic or potentially anionic monomers, preferably in small amounts.

The term “potentially anionic monomer” as used herein is intended to include a monomer bearing a potentially ionizable group that becomes anionic when incorporated into the polymer upon contact of the monomer with a cellulosic suspension. Examples of suitable copolymerizable anionic and potentially anionic monomers include ethylenically unsaturated carboxylic acids and their salts, for example, (meth) acrylic acid and its salts, preferably (meth) sodium acrylate, ethylenically unsaturated sulfonic acids and their salts, for example 2-acrylamido-2methylpropanesulfonate, sulfoethyl ( meth) acrylate, vinyl sulfonic acid and its salts, styrene sulfonate and para-vinyl phenol (hydroxystyrene) and its salts. Examples of copolymerizable monomers include acrylamide and methacrylamide, i.e. (meth) acrylamide, and examples of preferred cationic organic polymers include an acrylamide-based cationic polymer, i.e. a cationic polymer made from a mixture of monomers comprising one or more of acrylamide and acrylamide-based monomers.

The cationic organic polymer may have a weight average molecular weight of at least 10,000, often at least 50,000. More often it is at least 100,000 and usually at least about 500,000, preferably at least about 1 million or more than about 2 million. The upper limit is not limited; it can be approximately 30 million, usually 20 million.

Examples of drainage auxiliary systems include:

(ί) cationic starch and anionic particles based on silicon dioxide;

(i) cationic starch and a water-soluble or water-dispersible acrylamide-based anionic polymer;

(ΐϊί) a cationic polymer based on acrylamide, cationic starch and anionic particles based on silicon dioxide;

(ίν) a cationic polymer based on polyacrylamide and anionic particles based on silicon dioxide;

(ν) a cationic polyaluminium compound, cationic starch and anionic particles based on silicon dioxide;

(νί) an acrylamide-based cationic polymer, cationic starch and a water-soluble or water-dispersible acrylamide-based anionic polymer.

According to one embodiment, the following are added to the aqueous suspension: a) cationic starch or a cationic polymer based on polyacrylamide; and b) anionic particles based on silicon dioxide or an anionic water-soluble or water-dispersible polymer based on polyacrylamide.

In one embodiment, sulfite is not added to the suspension.

The invention also relates to a cellulosic sheet obtained by the method disclosed in this application. The cellulose sheet can be used to produce any type of paper or paperboard.

Based on the above description of the invention, it will be understood that many changes can be made to the invention. Such changes should not be construed as a departure from the essence and scope of the present invention, and it is understood that all such changes, as would be obvious to a person skilled in the art, are included in the scope of the claims. The following examples will now illustrate, without limiting the scope of the invention, how the described invention may be practiced.

All parts and percentages relate to parts and percent by weight unless otherwise indicated.

- 6 014734

Examples

The experiments according to examples 1-3 were carried out on eucalyptus cellulose, previously delignified with oxygen, bleached in the sequence (PP (PP to a final whiteness of 88% by 180. Stage b is a chelation step that removes transition metals that could impair the performance of hydrogen peroxide, used as a bleaching agent in the peroxide (P) stages: Kappa number, 180 whiteness and cellulose hexenuronic acid content before it enters the bleaching sequence constituted 12.2, 51% of 180 and 62.8 mmol / kg of dry cellulose. The bleaching tests were carried out in sealed plastic bags heated in a water bath with the required set temperature.

Processing conditions:

b: 60 min, final pH 5.4, EDTA 2 kg / t dry pulp, cellulose content 10%; P: 240 min, final pH 11.3, H ₂ 0 ₂ 10 kg / t dry cellulose, cellulose content 10% b : 30 min, final pH 6.7, EDTA 1 kg / t dry cellulose, cellulose content 10% P: 960 min, final pH 11.2, H ₂ O ₂ 10 kg / t dry cellulose, cellulose content 10%. Whiteness, kappa number, and hexenuronic acid (HexA) content in bleached pulp were 88.2% at 180, 7.3, and 54 mmol / kg, respectively. After each stage, washing with deionized water was carried out with a washing efficiency of 96 wt.%. The kappa number, viscosity and whiteness were determined by 180 according to 180 302, 180 5351 and 180 2470, respectively. NeXA was determined by acid hydrolysis of cellulose in formate buffer, followed by UV analysis of the resulting 2-furancarboxylic acid in the hydrolyzate. The residual content of hydrogen peroxide was determined by iodometric titration using sodium thiosulfate.

The decrease in cellulose whiteness under the influence of humidity was evaluated under conditions of humidity by measuring the loss of whiteness after the cellulose sheet was kept at 23 ° C and a relative humidity of 50%, sealed in a plastic bag and kept at 70 ° C for 64 hours, and finally removed from the bag and left at room temperature for at least 1 hour before measuring the reduced brightness. Decrease in whiteness was expressed as the difference in units of whiteness between unaged and aged sheets.

The degree of dehydration was determined using Rupaipe Prgashae Lpa1u / er (ERA), available from Akıy, Sweden. For each test, 800 g of a cellulosic suspension with a cellulose content of 1.5% by weight was transferred to a PPA feed tank. Further experiments were performed as described below:

Time s Mixing started at 1500 rpm 0 Download CPAM 10 Silica loading fifteen Stirring stopped, screw removed twenty Valve open 21

CPAM is a cationic polyacrylamide and silica is an aluminum-modified silica sol. When the valve was opened, a vacuum was applied to create a suction force. The time required to extract the water contained in the resulting cellulosic pad was controlled by measuring the vacuum.

Example 1

In the experiments, the sheets were formed from bleached cellulose following the standard 180 method, 80ΑΝ-0Μ 11:95, except that the pH value varied from pH 3 to pH 10.5. The standard method involves forming sheets at pH 5 + 0.3 (80 (-0ΑΝ 11:95). To adjust the pH to the desired levels, H ₂ 80 ₄ or \ 'a011 was used. As can be seen in the table. 1 and in FIG. 1, the decrease in whiteness under the action of humidity was significantly reduced with increasing pH of the formation of the sheet, equal to more than 6.5.

Table 1

leaf formation pH 3.0 4,5 6, 0 7.5 9.0 10.5 White before aging [% Ι3Ο] 88.0 88.6 88.8 89.0 89, 0 88.8 White after wet aging [% 130] 53,2 50,0 55.0 76, 7 84.7 86, 4 Decrease in whiteness [% in units Ι3Ο] 34.8 38.6 33.8 12.3 4.3 2,4

- 7 014734

Example 2

The influence of various approaches to creating pH for sheet formation on the decrease in whiteness under the influence of humidity was also studied. Sheets were prepared as in Example 1, except that the pH was adjusted during sheet formation using buffers instead of Η ₂ 8Ο ₄ or ΝαΟΗ. The buffers used were (borax + ΝαΟΗ) at pH 8.1, 9.1 and 10.0; and (ΝαΗΟΟ ₃ + ΝαΟΗ) at pH 10. FIG. 2 shows that the high pH of sheet formation obtained using buffers also leads to a significantly reduced decrease in brightness caused by moisture.

Example 3

The experiments were carried out using the same bleached pulp that was used in examples 1 and 2. The dehydration time was increased by 20%, while increasing the pH of the cellulose suspension from pH 5 to pH 9. The combination of dehydrating agents, i.e. CPAM (cationic polyacrylamide, having a charge of 1.2 meq / g and a molecular weight of about 7,000,000 Dalton) and silica nanoparticles (colloidal aluminum-modified silica having a specific surface area of about 900 ^m2 / g and 8-value of about 22%) was introduced into the cellulose th suspension at pH 9. The load is 0.5 kg / t pulp for each dewatering chemical. The dehydration time was reduced by 20% relative to the comparison suspension at pH 5. The results are shown in table. 2. Sheets were formed using the resulting cellulose after dehydration. The pH was maintained at pH 5 and pH 9 throughout the entire sheet forming process. The sheets were subjected to a test for decreasing whiteness under the influence of humidity. The results in the table. 2 show that the dehydration time is longer at pH 9 in the absence of drying agents compared to that at pH 9 in the presence of drying agents. If the dehydration time was 10 s, i.e. equal to the dehydration time at pH 5, then after dehydration, more water would remain in the pulp, which in turn would require a long drying time. It is known in the art that a longer drying time and / or drying at elevated temperature in order to obtain the same final dryness as at pH 5 would lead to an increased decrease in brightness. Dehydration of cellulose through the use of dehydrating agents, thus contributes to a decrease in drying time and, as a consequence, increased brightness stability.

table 2

pH 5 pH 9 Dehydrating chemicals not not Yes Decrease in whiteness [% in units of Ι8Ο] 38 6 5 Dynamic drying time [sec] 10 12 8

Example 4

The following experiments were carried out on eucalyptus pulp preliminarily delignified with oxygen, bleached to a final whiteness of 91% Ι8Ο, using the bleaching sequences ΖΌΡ, Ό ΕΟΡ Ό P and Ό ΕΟΡ Ό Ό. The kappa number, Ι8Ο whiteness, and the content of hexenuronic acid in the cellulose before it entered the bleaching sequence were 10.0, 53.6% Ι8Ο, and 63 mmol / kg dry cellulose, respectively. The bleaching conditions are given below in table. 3-7.

Table 3. Sequence ΖΌΡ

Stage Time [minutes] Temperature [° C] Loading [kg / ton] Cellulose Content [%] pH (final) Ζ 4.7 <30 2.9 ϋ 65 80 25 10 3.0 R 111 88 8 10 10.5-11

Table 4. The sequence Ό ΕΟΡ Ό P

Stage Time [minutes] Temperature [° C] Loading [kg / ton] Cellulose Content [%] Pressure [MPa] pH (final) ϋ 45 60 fifteen 10 - 3 ΕΟΡ 60 80 2 10 0.5 11.5 ϋ 120 80 10 10 - 3 Ρ 120 80 3 10 - 11.5

- 8 014734 pH (final)

120

120

Table 5: Sequence Ό ΕΟΡ Ό Ό

Stage

Time [minutes]

Temperature [° C]

Loading [kg / ton]

R EOR

R

R

Cellulose Content [%]

Oxygen [MPa]

0.5

11.5

11.5

96%

After each stage, rinsing with deionized water was carried out with washing efficiency

Table 6. The content of hexenuronic acid after bleaching

Table 7. Decrease in whiteness under the influence of humidity [% in units Ι8Ο]

leaf formation pH ΖϋΡ Ό EOR Ό P Ό EOR Ό Ό 5 10, 5 11,4 10, 9 7 7.2 5, 3 8.5 nine 1.7 3, 1 8.1 eleven 1.8 2, 9 6, 1

From the data of the above table. 7, it can be seen that for the pH of the sheet formation of the present invention, an improvement is achieved in a decrease in whiteness due to moisture.

Example 5

Oxygen delignified softwood pulp with a kappa number of 10.2 and ной8Ο whiteness was bleached in the Ό O PO sequence. The bleaching conditions were conditions according to table. 8.

Table 8

Stage Time [minutes] Temperature [° C] Loading [kg / ton] Oxygen [MPa] Cellulose Content [%] pH (final) R 40 60 20,4 - 12 2.9 60 90 one - 12 5.5 RO 120 one hundred 17 0.5 12 11.5

The whiteness after bleaching was 89.7% %8Ο, and the number of kappa was 1.7. Measurements of the decrease in brightness under the influence of humidity were carried out at pH 5 and 9, and the decrease in brightness was 14 and 3.8% in units of в8Ο, respectively.

Claims (13)

1. A method of treating an aqueous suspension of bleached pulp obtained by alkaline cooking before producing paper in which said suspension is dehydrated and dried, wherein the pH of the suspension to be treated is in the range of about 6.5 to about 12 during said dehydration and drying steps. 2. The method according to claim 1, where the pH of the suspension is in the range from about 7.5 to about 12. 3. The method according to claim 1 or 2, where the pH of the suspension is in the range from about 9.5 to about 10.5. 4. The method according to any one of claims 1 to 3, where at least one pH-regulating chemical is added to the cellulose before or during dehydration and / or drying. 5. The method according to any one of claims 1 to 4, where at least one drying chemical is added to the cellulose before and / or during dehydration. 6. The method according to any one of claims 1 to 5, where at least one desiccant chemical is added in an amount of from about 0.01 to about 10 kg / t dry pulp. 7. The method according to any one of claims 1 to 6, where the suspension is added: a) cationic starch or a cationic polymer based on polyacrylamide and b) anionic particles based on silicon dioxide or an anionic water-soluble or water-dispersible polymer based on polyacrylamide. 8. The method according to any one of claims 1 to 7, where the bleached cellulose has a hexenuronic acid content in the range of from about 1 to about 100 mmol / kg dry cellulose. 9. The method according to any one of claims 1 to 8, where the cellulose is a Kraft pulp. 10. The method according to any one of claims 1 to 9, where the pH-regulating chemical is added in an amount of from about 0.001 to about 50 kg / t dry pulp. - 9 014734 11. The method according to any one of claims 1 to 10, where the pH of the suspension is in the range from about 8.5 to about 12. 12. The method according to any one of claims 1 to 11, wherein the cellulosic sheets are formed during the dehydration step. 13. The method according to any one of claims 1 to 12, where the pH of the suspension to be processed is maintained in the claimed range during the implementation of the method.