ES2612270T3 - Paste treatment - Google Patents

Abstract

Method of treating an aqueous suspension of bleached pulp obtained by an alkaline cooking process, said method comprising stripping and drying said suspension before papermaking, in which the pH of the suspension to be treated is in the range 6, 5 to 12, in which the removal of said suspension by pressure increases the dry content of the resulting paste to a final dry content of the paste from 20 to 80% by weight and the drying of said suspension is carried out at a temperature from 200 to 600 ° C.

Description

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DESCRIPTION

Paste treatment

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

Background of the invention

In the bleaching technique, the objective is to provide paste with high and stable whiteness levels. However, during storage and transport to the paper mill, the whiteness of the pulp is reverted to a variable degree that depends, for example, on the temperature, humidity, wood species, cooking conditions and bleaching sequence. . Also, for the stability of the whiteness of the bleached pulp, the conditions of the draining and drying of the pulp after the final stage of bleaching are important. When commercial pulp is to be transported from its manufacturing site to a paper mill, it must be substantially dry for economic and quality reasons. It is known that the operation of drying equipment can affect whiteness to a variable degree that depends on the stability of the paste and the drying conditions. For example, if the paste is dried at high temperature and then packed, thermal reversion of the whiteness continues during storage due to the high temperature and the moisture in the bullet.

"The Bleaching of Pulp", third edition, Tappi Press, edited by R. P. Singh, pages 388-390, 1979, describes that the yellowing of the paste is faster at higher temperatures and at higher humidity. It is also described that the color conversion is greater at a higher pH of the water from which the pulp sheet is manufactured. Therefore, it is conventional in the art to carry out drying at a fairly low pH irrespective of the bleaching sequence that has been performed on the paste to be dried.

In the article “Variables affecting the thermal yellowing of TFC-bleached birch kraft pulps, A. Granstrom, Nordic Pulp and Paper Research Journal, volume 16, number 1/2001, various variables that influence the process of whiteness reversion are discussed , such as hexenuronic acid content, dry content, temperature, time and pH. This article shows varying degrees of whiteness reversion that depend on these variables, inter alia at pH values in the range of 3 to 6. As mentioned in the Granstrom article, groups of hexenuronic acid are formed, for example, during alkaline cooking when xylan 4-O-methyl-D-glucuronic acid groups are converted. Previously it had been suspected that the presence of hexenuronic acid groups is responsible, at least partially, for the reversal of the whiteness of the pastes.

Therefore, attempts have been made to remove hexenuronic acid groups, for example, by acid hydrolysis before bleaching or by selecting a bleaching sequence that degrades hexenuronic acid groups. However, all these attempts originate a more complicated pasta production system and, in most cases, involve additional investments.

US 2003/213568 describes a process for making a paste that has a low chemical oxygen demand (COD). The process involves impregnating and washing pasta in alkaline water for a sufficient time for the COD to be, after impregnation and washing, equal to or less than 2.0 kg / ton of dry pasta. This process includes impregnating and repeatedly washing a raw paste in alkaline water whose pH is never dropped below 7.0. In this process, the paste is impregnated and washed for a total of at least 220 minutes at an elevated temperature, to produce a paste having a low COD.

US Patent 6,077,394 describes a method for making alkaline thin paper. The method comprises adding during the papermaking process (i) a glyoxylated acrylamido-diallyldimethylammonium chloride resin and (ii) an aminopolyamide-epichlorohydrin resin to a suspension of alkaline fine paper pulp containing filler material. The resins are added in an amount and at a speed sufficient to increase the retention of the load on the paper and the drainage, of the suspension, of process water.

US Patent 5,695,609 describes a process for the production of paper by adding auxiliary agents to an aqueous fiber suspension to improve retention and / or wear. The auxiliary agents are a long chain cationic polyacrylamide and an aluminum salt and by tearing the fiber suspension during the sheet forming stage the paper is produced.

US Patent 4,964,954 describes a method for the production of paper by forming and tearing on a mesh a suspension of paper fibers. The formation and drainage takes place in the presence of a synthetic cationic polymeric retention agent, preferably a cationic polyacrylamide, an anionic inorganic colloid and a poly (aluminum compound). The process is carried out at a pH of the pulp suspension greater than 5 and results in better drainage and better retention of fine fibers and optional loads.

US Patent 5,035,772 describes a method for treating a suspension of bleached pulp containing lignin, to solve the yellowing problems that frequently occur with such pastes.

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high performance

The present invention provides a method for improving the whiteness stability of bleached pastes that have been subjected to alkaline cooking, particularly those that contain a certain degree of hexenuronic acid groups.

The invention

The present invention relates to a method of treating an aqueous suspension of bleached pulp obtained from an alkaline cooking process according to claim 1.

The pH of the paste suspension can be monitored using any conventional technique, such as line sensors that continuously measure the pH. According to one embodiment, the pH is in the range of about 7.5 to about 12 or about 8.5 to about 12, for example, about 9 to about 11 or about 9.5 to about 10.5 . According to one embodiment, a pH-adjusted chemical product is added to the bleached pulp before or during dripping and / or drying to adjust the pH, for example, in an amount of about 0.001 to about 50, about 0.005 to about 20 or about 0.01 to about 10 kg / ton of dry pasta. The quantities of pH adjustment chemicals are given as the amount of active substance. According to one embodiment, the pH is maintained within the range claimed throughout the method.

In the present application, it should be understood that the terms "drain" and "drying" mean processes in which water is removed from the pulp. Therefore, "wear" and "drying" imply "wear" but usually using different means or devices. While the dewatering involves pressing or draining the water paste suspension, drying implies raising the temperature with respect to the temperature at which the dewatering is performed.

It is intended that the term "before papermaking" means that the steps of the method are performed before the substantially dry pulp is delivered to a disintegrator where it disintegrates in water and is then supplied to the wetted part of a paper machine.

Bleached pulp can be obtained from lignocellulose-containing material, which can comprise, for example, wood logs, finely divided raw materials and woody materials, such as wood particles and other than wood, as annual or perennial plant fibers . The woody raw material can be obtained from common and leafy species, such as birch, beech, poplar, alder, eucalyptus, maple, acacia, mixed hardwood, pine, fir, “tsuga” (Tsuga heterophylla), larch, white pine and mixtures of these species. Non-woody raw material can be obtained, for example, from straw from cereal crops, birdseed, gray hair, flax, hemp, kenaf, jute, ramie, sisal, abaca, coir, bamboo, bagasse or combinations of these materials. However, pulp obtained from recycled pulp, alone or mixed with virgin pastes, can also be used.

The pasta can be a chemical paste produced using an alkaline cooking process. Examples of alkaline cooking processes are sulphate cooking, soda or modifications of these processes with optional additives, such as anthraquinone, for example, baking processes with soda with anthraquinone (AQ). The chemical paste obtainable from materials containing lignocellulose may be subjected to one or more stages of oxygen treatment after the cooking process. The process can be applied, for example, to chemically far-off pastes that have an initial Kappa index, that is, a Kappa index after cooking but before oxygen delignification, in the range of 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 index is measured according to the standard ISO 302 method.

The paste may have been obtained by any suitable bleaching sequences, including completely chlorine-free bleaching sequences (TCF) comprising, for example, ozone, peroxide, oxygen, peracides, alkaline extraction, acid hydrolysis and chelation steps, and combinations of these stages; Elemental chlorine-free bleaching sequences (ECF) that include, for example, ozone, peroxide, oxygen, peracide, alkaline extraction, chlorine dioxide and chelation stages, and combinations of these stages. The bleaching sequence, both of ECF and TCF, can be terminated with an acid stage or an alkaline stage. According to one embodiment, the paste can be bleached only using alkaline bleaching stages, for example, oxygen and / or peroxide bleaching stages that give rise to a final pH of about 7.5 to about 12.

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

According to one embodiment, the content of hexenuronic acid in the pulp, prior to draining and drying, is in the range of about 1 to about 150, for example, about 5 to

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about 100 or about 10 to about 90 or about 20 to about 80, like about 50 to about 80 mmol / kg of dry pasta. It has been found that the invention works particularly with pastes that have a hexenuronic acid content within the ranges defined herein. Furthermore, it has been indicated that the present invention improves the stability of the whiteness of a paste by making a prior stage of acid hydrolysis unnecessary before bleaching.

Bleached pulp can be diluted before weeding. In accordance with one embodiment, the bleached pulp suspension has a dry content, before weaning, in the range of about 0.1 to about 5, for example, about 0.5 to about 4 or about 1 to about 3 % in weigh.

Weeding and drying can be done in various ways. According to one embodiment, the bleached pulp is purified, for example, by means of a sieve after the final bleaching stage and before drying. In said sieve, remnants can be removed, such as fibrous bundles and bark. In the stage of draining, a paste can be subjected to different operations that depend on how the paste is finally dried. For example, the paste can be dried in the form of pasta sheets or flakes. For example, the bleached pulp suspension can be transferred to the feed box of a paper machine in whose wetted portion a continuous sheet or strip of pulp is formed on a mesh. The formed sheet of paste has a weight of 600-1,000 g / m2 and a dry content of about 5 to about 50% by weight. A subsequent drying section may involve drying cylinders heated by steam or an air floating dryer. In an air flotation dryer, the continuous pulp band can pass several times over rows of air bars. The paste can be dried to a dryness of about 90 to about 100% by weight. The dried pasta thus obtained can be cut into sheets and stacked forming bullets of 200 to 300 kg.

According to one embodiment, the bleached pulp is pressed, for example, in a degassing press, to increase the dryness of the resulting pulp to a final dryness from about 20 to about 80, for example, from about 30 to about 70 or about 40 to about 60% by weight. Generally, the degassing press presses the pulp against a felt that retains the pulp fibers but forces the water through the felt, optionally with the suction air. For example, the degassing press may be formed by a pair of cylinders arranged to press the pulp between them. Alternatively, the degassing press may comprise a conveyor screw that rotates within a sieve. After drying, the paste can be formed into leaves or flakes.

The drying step can also be carried out in any other suitable dryer, for example, a dryer known as a "flash" dryer, which dries the paste by a hot gas, generally air, at a temperature of about 200 to about 600 ° C. However, according to another embodiment, drying is not performed by means of a "flash" dryer. The paste, which can appear in the form of particles, finds in the "flash" dryer hot air produced, for example, burning an oil or gas, and passes through the duct until it finds the final cyclone. This separates moist hot air giving a final dryness of up to about 90% by weight. The paste is subsequently transferred to a press in which the pressed flakes can form a bullet. This is then treated like a bullet of pasta sheets from a pasta dryer except that it may need to be wrapped since such bullets can more easily disintegrate during handling. The draining and drying operations are well explained, for example, in "Papermaking Science and Technology", J. Gullichsen et al., Volume 6A, Chemical Pulping, pages A667-A86, 2000.

According to one embodiment, the formed pasta sheets are then formed into bullets. The bullets may have a dryness of about 50 to about 100 or about 50 to about 90, for example, about 60 to about 95 or about 70 to about 93% by weight.

According to one embodiment, a bleached chemical is added to the bleached pulp before and / or during draining, for example, directly to the aqueous bleached pulp suspension, for example, in an amount of about 0.01 to about 10 or about 0.05 to about 5 kg / ton of dry pasta. It has been found that the draining and / or drying of the bleached pulp in the presence of at least one chemical drainage product can significantly shorten the drying time also improving the stability of the whiteness due to the shorter exposure to high drying temperatures. .

According to one embodiment, a combination of chemical drainage products is added to the bleached pulp suspension. According to one embodiment, as a chemical drainage product, a cationic polysaccharide selected from polysaccharides known in the art may be used, including, for example, starches, guar gums, celluloses, chitins, chitosans, glycans, galactans, glucans, gums of xanthan, pectins, mannans, dextrins, preferably starches and guar gums. Examples of suitable starches include potato, corn, wheat, tapioca, rice, corn flour, barley, etc. starches. Conveniently, the polysaccharide

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cationic is water dispersible or, preferably, water soluble. Particularly suitable polysaccharides according to the invention include those comprising the general structural formula (I):

Ri 0)

I X '

p _ (_ A-N + -R2) n

I

r3

wherein P is a residue of a polysaccharide, A is a group that links N to the rest of the polysaccharide, conveniently a chain comprising atoms of C and atoms of H and optionally atoms of O and / or N, usually an alkylene group with 2 to 18 and conveniently with 2 to 8 carbon atoms, optionally interrupted or substituted by one or two heteroatoms, for example, O or N, for example, an alkyleneoxy group or a hydroxypropylene group (—CH2— CH (OH) —CH2— ); each of R1, R2 and R3 are H or a hydrocarbon group, conveniently alkyl, having 1 to 3 carbon atoms, conveniently 1 or 2 carbon atoms; n is an integer from about 2 to about 300,000, conveniently from 5 to 200,000, for example, from 6 to 125,000 or, alternatively R1, R2 and R3 together with N form an aromatic group having 5 to 12 carbon atoms; and X- is an anion, usually a halide, such as chloride.

The cationic polysaccharides according to the invention may also contain anionic fluids, preferably in a small amount. Said anionic groups can be introduced into the polysaccharide by chemical treatment or they can be present in the natural polysaccharide.

The weight average molecular weight of the cationic polysaccharide can vary within wide limits depending, inter alia, on the type of polymer used and is usually at least about 5,000 and often at least 10,000. More frequently, it is greater than 150,000, usually greater than 500,000, conveniently greater than approximately 700,000 or greater than approximately 1,000,000 or, for example, greater than approximately 2,000,000. The upper limit is not critical; it can be approximately 200,000,000, usually

150,000 000 and conveniently 100,000,000.

The cationic polysaccharide can have a degree of cationic substitution (DS;) that varies within a wide range depending, inter alia, on the type of polymer used; DSc may be 0.005 to 1.0, usually 0.01 to 0.5, conveniently 0.02 to 0.3 or 0.025 to 0.2.

According to one embodiment, the charge density of the cationic polysaccharide is within the range of 0.05 to

6.0, conveniently from 0.1 to 5.0 or from 0.2 to 4.0 meq / g dry polymer.

According to one embodiment, an anionic polymer that can be selected from inorganic and organic anionic polymers is added as a drainage chemical. 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 silica and silicates, that is, silica-based anionic polymers. Suitable anionic polymers based on silica can be prepared by condensation polymerization of silica compounds, for example, silica acids and silicates, and can be homopolymers or copolymers. According to one embodiment, the sflce-based anionic polymers comprise anionic silicon-based particles that have a particle size within the colloidal range. Silicon-based anionic particles are supplied in the form of colloidal aqueous dispersions, called soles. Silica-based soles can be modified and contain other elements, for example, aluminum, boron, nitrogen, zirconium, gallium and titanium, which may be present in the aqueous phase and / or in silica-based particles. Examples of suitable anionic silica-based particles include poly (silphic acids), poly (silphic acids) microgels, polysilicates, polysilicate microgels, colloidal silica, aluminum-modified colloidal silica, polyaluminosilicates, polyaluminosilicate microgels, polyiboratosilicates, etc. Examples of suitable silicon-based anionic particles include those described in U.S. Patent Nos. 4,388,150, 4,927,498, 4,954,220, 4,961,825, 4,998 .025, 5,127,994, 5,176,891, 5,368 .833, 5,447,604, 5,470,435, 5543,014, 5,571,494, 5,573,674, 5,584,966, 5,603,805, 5,688,482 and 5,707,493, which are incorporated herein by reference.

Examples of suitable anionic silica-based particles include those having an average particle size below about 100 nm, for example, below about 20 nm or in the range of about 1 to about 10 nm. As is conventional in the chemistry of silica, particle size refers to the average size of the primary particles, which can be aggregated or not aggregated. According to one embodiment, the silica-based anionic polymer comprises anionic particles based on silica. The specific surface area of silica-based particles is conveniently at least 50 m2 / g, per

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for example, up to 50 m2 / g, for example, at least 100 m2 / g. Generally, the specific surface area can be up to about 1,700 m2 / g, conveniently up to about 1,000 m2 / g. The specific surface area is measured by titration with NaOH, as described in GW Sears in Analytical Chemistry, 28 (1956), 12, 1981-1983, and in U.S. Patent No. 5,176,891, after properly removing or adjusting any compounds present in the sample that may alter the titration, such as aluminum and boron compounds. The surface thus obtained represents the average specific surface area of the particles.

In an embodiment of the invention, the silica-based anionic particles have a specific surface area in the range of 500 to 1,000 m2 / g, for example, 100 to 950 m2 / g. The silica-based particles may be present in a sun having an S 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 at 1,000 m2 / g, conveniently from 500 to 950 m2 / g, for example, from 750 to 950 m2 / g, The S value is measured and calculated as described by Iler & Dalton in Phys. Chem. 60 (1956), 955- 957 The value S indicates the degree of aggregation or formation of the microgel, indicating a value S under a greater degree of aggregation.

Also in another embodiment of the invention, silica-based particles have a high, convenient surface area greater than about 1,000 m2 / g. The specific surface area may be in the range of 1,000 to 1,700 m2 / g, for example, 1,050 to 1,600 m2 / g.

Other examples of suitable polymers include water soluble and water dispersible anionic organic polymers obtained by polymerizing an anionic monomer or an ethylenically unsaturated potentially anionic monomer or a mixture comprising one or more ethylenically unsaturated anionic or potentially anionic monomers and optionally one or more other ethylenically unsaturated monomers. According to one embodiment, ethylenically unsaturated monomers are water soluble. Examples of suitable anionic or potentially anionic monomers include ethylenically unsaturated carboxylic acids and their salts, ethylenically unsaturated sulfonic acids and their salts, for example, any one of the aforementioned. The monomer mixture may contain one or more ethylenically water-soluble unsaturated non-ionic monomers. Examples of suitable copolymerizable non-ionic monomers include acrylamide and the aforementioned non-ionic monomers based on acrylamide and acrylates and vinylamines. The monomer mixture may also contain one or more ethylenically water-soluble cationic and potentially cationic monomers, preferably in small amounts. Examples of suitable copolymerizable cationic monomers include the monomers represented by the general structural formula (I) and diallyldialkylammonium halides, for example, diallyldimethylammonium chloride. The monomer mixture may also contain one or more cross-linking polyfunctional agents. The presence of a cross-linking polyfunctional agent in the monomer mixture makes it possible to prepare polymers that are dispersible in water.

Examples of suitable cross-linking polyfunctional agents include the aforementioned cross-linking polyfunctional agents. These agents can be used in the aforementioned amounts. Examples of suitable water dispersible anionic organic polymers include those described in US Patent No. 5,167,766, which is included herein by reference. Examples of polymerizable monomers include (meth) acrylamide and examples of preferred polymers include water soluble and water dispersible anionic polymers based on acrylamides.

According to one embodiment, an anionic organic polymer is used that is water soluble and has a weight average molecular weight of at least about 500,000. Usually, the weight average molecular weight is at least about 1 million, conveniently at least about 2 million or at least about 5 million. The upper limit is not critical; It can be approximately 50 million, usually 30 million.

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

According to one embodiment, a cationic polymer is used as a drainage 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 can be selected from inorganic and organic cationic polymers and can be water soluble. Examples of suitable cationic polymers include poly (aluminum compounds), for example, poly (aluminum chlorides), poly (aluminum sulfates), poly (aluminum compounds) containing chloride and sulfate ions, poly (aluminum silicate sulfates) and mixtures of these polymers. Other examples of suitable cationic organic polymers include, for example, acrylamide-based cationic polymers; poly (diallyldialkylammonium halides), for example, poly (diallyl dimethyl ammonium chloride); polyethyleneimines; polyamidoamines; polyamines; and polymers based on vinylamines. Examples of suitable cationic organic polymers include polymers prepared by polymerization of ethylenically unsaturated cationic monomers or, preferably, a mixture of monomers comprising one or more ethylenically unsaturated cationic monomers, water soluble, and optionally one or more of others.

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ethylenically unsaturated, water soluble monomers. Examples of ethylenically unsaturated, water-soluble cationic monomers include diallyl dimethyl ammonium halides, for example, diallyldimethylammonium chloride, and cationic monomers represented by structural formula (II):

CH2 = C-R-, R2 (II)

I I

O = C-A-B-N + -R3 X '

I

R4

in which Ri is H or CH3; each of R2 and R3 is H or a hydrocarbon group, conveniently alkyl, having 1 to 3 carbon atoms, conveniently 2 to 4 carbon atoms, or a hydroxypropylene group; R4 is H or a hydrocarbon group, conveniently alkyl, having 1 to 4 carbon atoms, preferably 1 to 2 carbon atoms, or a substituent containing an aromatic group, conveniently a substituted phenyl or phenyl group, which may be attached to the nitrogen by an alkylene group usually having 1 to 3 carbon atoms, conveniently 1 to 2 carbon atoms, the appropriate R4 including a group (—CH2 — C6H5); and X— is an anion, usually a halide, such as chloride.

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

The monomer mixture may contain one or more ethylenically unsaturated non-ionic monomers. Examples of suitable copolymerizable non-ionic monomers include acrylamide and acrylamide-based monomers, for example, methacrylamide, N-alkyl (meth) acrylamides, for example, N-methyl (meth) acrylamide, N-ethyl (meth) acrylamide, Nn- propyl (meth) acrylamide, N-isopropyl (meth) acrylamide, Nn-butyl (meth) acrylamide, Nt-butyl (meth) acrylamide and N-isobutyl (meth) acrylamide; N-alkoxyalkyl (meth) acrylamides, for example, N-n-butoxymethyl (meth) acrylamide and N-isobutoxymethyl (meth) acrylamide; N, N-dialkyl (meth) acrylamides, for example, N, N-dimethyl (meth) acrylamide and dialkylaminoalkyl (meth) acrylamide; acrylate-based monomers, such as (meth) dialkylaminoalkyl acrylates; and vinylamines. The monomer mixture can also contain one or more ethylenically unsaturated anionic or potentially anionic monomers, soluble in water, conveniently in small amounts. Here, the term "potentially anionic monomer" means a monomer that carries a potentially ionizable group that becomes anionic when included in a polymer when applied to the cellulosic suspension. Examples of suitable anionic and potentially anionic monomers include ethylenically unsaturated carboxylic acids and their salts, for example, (meth) acrylic acid and its salts, conveniently (meth) sodium acrylate, ethylenically unsaturated sulfonic acids and salts thereof, for example, 2-propane sulfonate. -acrylamido-2-methyl, vinyl sulfonic acid and its salts, styrenesulfonate and paravinylphenol (hydroxystyrene) and its salts. Examples of copolymerizable monomers include acrylamide and methacrylamide, that is, (meth) acrylamide, and examples of preferred cationic organic polymers include cationic acrylamide-based polymers, that is, cationic polymers prepared from a mixture of monomers comprising one or more of acrylamide and acrylamide based monomers.

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

Examples of drainage adjuvant systems include:

(i) cationic starch and silica-based particles,

(ii) cationic starch and water soluble or water dispersible anionic polymer based on acrylamide,

(iii) cationic polymer based on acrylamide, cationic starch and anionic silica-based particles,

(iv) polyacrylamide-based cationic polymer and silica-based anionic particles,

(v) cationic poly (aluminum compound), cationic starch and silicon-based anionic particles,

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(vi) cationic polymer based on acrylamide, cationic starch and water-soluble anionic polymer or

water dispersible, acrylamide based.

According to one embodiment, a cationic starch or a polyacrylamide-based cationic polymer and (b) silica-based anionic particles or a water-soluble or water-dispersible anionic polymer are added to the aqueous suspension. of polyacrylamide.

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

The invention also relates to a sheet of pulp obtainable from the method described herein. The pulp sheet can be used for the production of any type of paper or cardboard.

Having thus described the invention, it should be evident that it can be varied in many ways. Such variations should not be considered as deviation from the essence and scope of the present invention and should be apparent to those skilled in the art that are included in the scope of the claims. The following examples further illustrate how the described invention can be performed without limiting its scope.

All parts and percentages refer to parts and percentages by weight, unless otherwise specified.

Examples

Experiments according to examples 1-3 were performed with oxygen-delignated eucalyptus paste and bleached with the QPQP sequence to an ISO final whiteness of 88%. Stage Q is a chelation stage that eliminates transition metals that could deteriorate the behavior of hydrogen peroxide used as a bleaching agent in the peroxide (P) stages, and the Kappa index, ISO whiteness and hexenuronic acid content of The pulp entering the bleaching sequence were, respectively, 12.2, 51% and 62.8 mmol / kg of dry pulp. The bleaching tests were carried out in hermetically sealed polyethylene bags and heated in a Marfa bath with the desired preset temperature.

The treatment conditions were:

Q: 60 minutes, final pH 5.4, EDTA 2 kg / ton of dry pasta, paste consistency 10%

P: 240 minutes, final pH 11.5, H2O210 kg / ton of dry pasta, paste consistency 10%

Q: 30 minutes, final pH 6.7, EDTA 3 kg / ton of dry pasta, paste consistency 10%

P: 960 minutes, final pH 11 ,, 2, H2O210 kg / ton of dry pasta, paste consistency 10%

The ISO whiteness, Kappa index and hexenuronic acid (HexA) content of the bleached pulp were, respectively, 88.2%, 7.3 and 54 mmol / kg. After each stage, the paste was washed with deionized water, resulting in a washing efficiency of 96% by weight. The Kappa index, viscosity and ISO whiteness were determined according to ISO 302, ISO 5351 and ISO 2470, respectively. The HexA was determined by acid hydrolysis of the paste in a formate buffer, followed by UV analysis of the 2-furoic acid formed in the hydrolyzate. Residual levels of hydrogen peroxide were established by iodometric titration with sodium thiosulfate.

The reversion of the whiteness of the paste into wet was evaluated under wet conditions by measuring the loss of whiteness after conditioning the paste sheet at 23 ° C and 50% relative humidity, tightly wrapping it in a polythene bag and keeping it at 70 ° C for 64 hours, and finally take it out of the bag and condition it at room temperature for at least 1 hour before measuring the whiteness conversion. Whiteness reversal was expressed in units of whiteness between aged and non-aged leaves.

The drainage was determined using a dynamic drainage analyzer (DDA), available from Akribi, Sweden. In each test 800 g of a paste suspension with a consistency of 1.5% by weight was transferred to the feed jar of the DDA. The experiments were then performed as described below:

 Time (s)

 Start of agitation at 1,500 rpm

 0

 CPAM load

 10

 Silica loading

 fifteen

 End of agitation

 twenty

 Open valve

 twenty-one

In this table, CPAM is a cationic polyacrylamide and silica is an aluminum modified silica sol. When

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opened the valve was applied vacuum to obtain suction force. The time required to remove the water in the resulting paste was monitored by measuring the vacuum.

Example 1

In the experimented leaves were formed from the bleached pulp following the standard ISO method, SCAN-CM 11:95, except that the pH was varied from 3 to 10.5. The standard method involves forming the sheet at pH 5 ± 0.3 (SCAN-CM 11:95). To adjust the pH to the desired levels, H2SO4 or NaOH was used. As can be seen in Table 1 and in Figure 1, the wetness whiteness conversion decreased substantially at a pH of the sheet formation above 6.5.

Table 1

 pH of leaf formation

 3.0 4.5 6.0 7.5 9.0 10.5

 Whiteness before aging (% ISO)

 88.0 88.6 88.8 89.0 89.0 88.8

 Whiteness after wet aging (% ISO)

 53.2 50.0 55.0 76.7 84.7 86.4

 Whiteness Reversion (% of ISO units)

 34.8 38.6 33.8 12.3 4.3 2.4

Example 2

It was also investigated as different proposals to obtain the pH of the formation of the leaves affected the reversion of the whiteness in wet. The sheets were prepared as in Example 1, except that the pH of the sheet formation was established using buffers instead of H2SO4 and NaOH. The buffers used were (Borax + NaOH) at pH 8.1, 9.1 and 10.0, and (NaHCO3 + NaOH) at pH 10. Figure 2 shows that the high leaf formation pH obtained using buffers originates also a substantially less humid whiteness reversal.

Example 3

The experiment was carried out using the same bleached paste used in examples 1 and 2. The drainage time was increased by 20% when the pH of the pasta suspension was increased from 5 to 9. The paste suspension was applied to pH 9 a combination of two dropping agents, namely a CPAM (cationic polyacrylamide having a charge density of 1.2 meq / g and a molecular weight of approximately 7,000,000 Da) and silica nanoparticles (aluminum-modified colloidal silica ) that has a specific surface area of approximately 900 m2 / g and an S value of approximately 22%. The loads were 0.5 kg of each chemical waste / tonne paste. The drainage time decreased by 20% compared to that of the reference suspension at pH 5. The results are shown in Table 2. Sheets were formed using the resulting paste after degassing. The pH was maintained at 5 and 9 throughout the leaf formation process. The leaves were subjected to the wet reversion test. The results in Table 2 show that the drain time is longer at pH 9 in the absence of waste agents compared to the waste time at pH 9 in the presence of waste agents. If the drain time has been interrupted after 10 seconds, that is, the drain time at pH 5, more water may be kept in the paste after the drain, which, in turn, may require more drying time. It is known in the art that a longer drying time and / or a drying at a higher temperature level to obtain the same final dryness as at pH 5 could cause greater whiteness reversion. The draining of the pulp using draining agents thus contributes to shorten the drying time and, as a consequence, to a greater stability of the whiteness.

Table 2

 pH 5 pH 9

 Chemical waste products

 No No Sf

 Whiteness Reversion (% of ISO units)

 38 6 5

 Dynamic drain time (s)

 10 12 8

Example 4

The following experiments were carried out on eucalyptus paste elucidated with oxygen and bleached to a final ISO whiteness of 91% using the bleaching sequence ZDP, D EOP DP and D EOP D D. The Kappa index, ISO whiteness and hexenuronic acid content of the paste were, respectively, 10.0, 53.6% and 63 mmol / kg of

dry pasta. The bleaching conditions are specified in the following tables 3-8. Table 3

Sequence Z D P

 Stage

 Time (min) Temperature (° C) Load (kg / t) Paste consistency (%) pH (final)

 Z

     4.7 <30 2.9

 D

 65 80 25 10 3.0

 P

 111 88 8 10 10.5-11

5 Table 4

Sequence D EOP D P

 Stage

 Time (min) Temperature (° C) Load (kg / t) Paste consistency (%) Pressure (MPa) pH (final)

 D

 45 60 15 10 - 3

 EOP

 60 80 2 10 0.5 11.5

 D

 120 80 10 10 - 3

 P

 120 80 3 10 - 11.5

Table 5

Sequence D EOP D D

 Stage

 Time (min) Temperature (° C) Load (kg / t) Paste consistency (%) Oxygen (MPa) pH (final)

 D

 45 60 16 10 - 3

 EOP

 60 80 2 10 0.5 11.5

 D

 120 80 12 10 - 3

 D

 120 80 4 10 - 11.5

10

After each stage, a wash was performed, causing a washing efficiency of 96%. Table 6

Content of hexenuronic acid after bleaching

 Sequence

 Hexenuronic acid (mmol / kg)

 Z D P

 7.2

 D EOP D P

 12.5

 D EOP D D

 3.8

15 Table 7

Reversion of whiteness in wet (% of ISO units)

 pH in leaf formation

 ZDP D EOP D P D EOP D D

 5

 10.5 11.4 10.9

 7

 7.2 5.3 8.5

 9

 1.7 3.1 8.1

 eleven

 1.8 2.0 6.1

From table 7 above it can be seen that a better reversion of wet whiteness is obtained for the pH of the leaf formation according to the present invention.

20 Example 5

A delignified cornflake paste was bleached with oxygen, with a Kappa index of 10.2 and an ISO whiteness of 45%, with a D Q PO sequence. The bleaching conditions were according to table 8

Table 8

 Stage

 Time (min) Temperature (° C) Load (kg / t) Oxygen (MPa) Paste consistency (%) pH (final)

 D

 40 60 20.4 - 12 2.9

 Q

 60 90 1 - 12 5.5

 PO

 129 100 17 0.5 12 11.5

The ISO whiteness after bleaching was 89.7% and the Kappa index was 1.7. Measurements of the wetness whiteness conversion at pH 5 and 9 were made and the whiteness reversals were, respectively, 14 and 3.8% 5 of ISO units.

Claims (14)

5 10 fifteen twenty 25 30 1. Method of treating an aqueous suspension of bleached pulp obtained by an alkaline cooking process, said method comprising removing and drying said suspension before papermaking, in which the pH of the suspension to be treated is in the range 6.5 to 12, in which the draining of said suspension by pressure increases the dry content of the resulting paste to a final dry content of the paste from 20 to 80% by weight and the drying of said suspension is carried out at a temperature of 200 to 600 ° C. 2. Method according to claim 1, wherein the pH of the suspension is in the range of 7.5 to 12. 3. Method according to claim 1 or 2, wherein the pH of the suspension is in the range of 9.5 to 10.5. 4. Method according to any one of claims 1 to 3, wherein at least one chemical pH adjustment product is added to the pulp before or during dripping and / or drying. 5. Method according to any one of claims 1 to 4, wherein a chemical drainage product is added to the pulp before and / or during wear. 6. Method according to any one of claims 1 to 5, wherein at least one chemical drainage product is added in an amount of 0.01 to 10 kg / ton of dry pulp. 7. Method according to any one of claims 1 to 6, wherein a cationic starch or a cationic polyacrylamide-based polymer is added to the suspension; and (b) particles based on anionic silica or a water soluble or water dispersible anionic polymer based on polyacrylamide. 8. Method according to any one of claims 1 to 7, wherein the bleached pulp has a hexenuronic acid content of 1 to 100 mmol / kg of dry pulp. 9. Method according to any one of claims 1 to 8, wherein the pulp is a kraft pulp. 10. Method according to any one of claims 1 to 9, wherein a chemical pH adjustment product is added in an amount of 0.001 to 50 kg / ton of dry pulp. 11. Method according to any one of claims 1 to 9, wherein the pH of the suspension is in the range of 8.5 to 12. 12. Method according to any one of claims 1 to 11, wherein drying is not performed by means of a flash dryer. 13. Method according to any one of claims 1 to 12, wherein sulfite is not added during the method. 14. Method according to any one of claims 1 to 13, wherein pulp sheets are formed during the stage of draining.