MXPA98007011A - Method for whitening paperwork paste which contains lignin during fabricac - Google Patents

Abstract

The present invention relates to a method for increasing the whiteness of a paste containing lignin, which comprises adding to the aqueous sludge consisting of the pulp containing lignin, during pulp manufacture, a fluorescent whitening agent and optionally a chelating agent.

Description

METHOD FOR BLANKING LUSTAIL PASTE WHICH CONTAINS LIGNIN DURING MANUFACTURE The present invention relates to a method that can be used in pulp or wood pulp mills, for bleaching pulps containing lignin. More particularly, it illustrates the use of fluorescent whitening agents instead of bleaching chemicals to obtain the desired glossy target values, while maintaining convenient fiber characteristics in the pulps containing significant amounts of lignin. Numerous processes are known to convert various types of wood, recycled paper and other fibrous raw materials into pulp, suitable for paper production. In general, these processes can be categorized as mechanical grazing processes, chemical grazing processes and their combinations. The properties of the pulp are determined by the raw materials and the processing parameters. Therefore, the final use of the paper will usually dictate both the raw materials to be used and the convenient processing parameters. In a chemical grazing process to produce kraft paper, much of the lignin and hemicellulose in the wood used is removed or solubilized by a series of chemical treatments. To obtain convenient pulp for white paper such as writing paper, additional bleaching steps are necessary that remove most of the remaining lignin. The resulting papers of low lignin content or substantially lignin-free, possess high strength and a brightness value of 85 or more. However, they are relatively expensive due to the numerous stages of treatment, the costs of effluent treatment and the fact that more than half of the dry weight of the wood is lost during chemical treatments. In mechanical grazing processes, such as the mechanical pulp processes of refiner and thermomechanical pulp (TMP = thermomechanical pulp), the fibers are separated by a combination of thermal and mechanical energy. These processes produce paper at a lower cost since the treatment costs are reduced, and the yield, based on the dry weight of the wood is usually about 95% since there is no removal of chemicals from the wood components. The processes of pulp qui iotermomecánica (CTMP = Chemitheirmomechanical pulp), where some degree of chemical treatment is applied either to chips or pieces of wood before thermomechanical grazing or paste after going, and semi-mechanical pulp processes (SMP = semimechanical pulp), where there is a somewhat higher degree of chemical digestion of wood chips before mechanical grazing, are also employed. The yield, based on the dry weight of the wood is somewhat reduced by these chemical treatments, since there is some removal of the wood components, but it is still substantially higher than that of a purely chemical grazing process. Mechanical process pastes are bleached, if desired, prior to the paper production stage, with chemicals that do not remove lignin, such as alkaline hydrogen peroxide or sodium dithionite, resulting in paper having a brightness value of up to approximately 80. In addition to the lower brightness values, paper from a mechanical grazing process has lower light stability, strength and permanence compared to paper prepared from a chemical grazing process. A main market for paper prepared by mechanical grazing processes is newspaper paper. The general area of bleaching / pulp manufacturing is reviewed very thoroughly in a monograph titled Pulp Bleaching (Bleaching of Pasta), Carlton Dence and Douglas Reeves, Editors, TAPPI Press (1996). For some end uses, pulp which is a mixture of chemical and mechanical pulps is advantageously employed. For example, recycled paper usually contains paper made from both mechanical and chemical pulps, but often predominantly the latter. In this way, the amount of lignin can vary enormously, at one end which is approximately the same as that found in the wood chips from which the pulp is prepared and on the other hand, close to zero in bleached chemical pulps of High brightness suitable for high quality papermaking. The brightness standard is measured as the light reflectance in the blue range (457 nm), compared to magnesium oxide as 100% white. In the U.S., brightness is usually measured with the General Electric brightness meter. In this way, a GE brightness value of 80 corresponds to 80% of the magnesium oxide brightness as measured with the GE meter. For many end users, the color or more precisely the lack of color of the paste is a critical parameter. Therefore, it is highly desirable to be able to increase the whiteness or brilliance of pulps containing lignin, and therefore the whiteness of the resulting products, in an effective cost way. It is also highly convenient to be able to do this without brilliance being the primary determinant of other paste properties. Traditionally, the pastes are bleached to the desired brilliance and the fibers as raw materials are chosen to give approximately appropriate properties, often with physical properties that take a second place with respect to brilliance. For example, typical bleached at high brightness with peroxide and caustic allow gain of brilliance, but at cost of discharge, volume, resistance, opacity and performance. This limits the market potential for pulps that have high lignin contents to relative "lower end" applications, such as paper towels, and avoids entry into high performance end markets, such as printing, writing and printing paper applications. coated. It is well known that paper prepared from chemical pulping processes in combination with bleaching, that is to say paper substantially free of lignin, can and usually is bleached by the addition of fluorescent whitening agents, both to the grazing stage and to the pre-coated leaves. Forged as a surface coating. Undoubtedly, a quantity of fluorescent whitening agents are distributed in the market for this express purpose. However, it has also been common knowledge that fluorescence is inhibited by lignin. This effect has avoided the use of fluorescent whitening agents in paper production from pulps containing significant amounts of lignin, such as those from mechanical pulping processes. Even with bleached kraft-type chemical pulps, fluorescent whitening agents (also referred to as optical brighteners) have traditionally not been used directly in the paper mill, even though the process would be technically straightforward. On the contrary, it has been left to the paper producer (where there is a better understanding of the fluorescent whitening agent chemicals and fibers) to acquire the pastes having an appropriate starting brilliance and to add fluorescent whitening agents and other materials to Obtain the desired levels of brilliance in the resulting paper. The use of optical brighteners as an alternative to bleaching during the manufacture of traditional kraft-type chemical pulps has not typically been practiced. The application co-pending from the US. Serial No. 08 / 766,909 discloses that it is possible to increase the whiteness of paper made from a pulp containing lignin, by adding to a slurry of the lignin-containing pulp, in the paper production stage, a bleaching agent fluorescent. However, the use of optical brighteners as a bleaching alternative in pastas that are high in lignin during manufacture in the pulp mill are unknown. Now in a surprising way, it has been found that it is possible to increase the whiteness of a paste containing lignin by a process consisting of adding to an aqueous sludge comprising a paste containing lignin, during the manufacture of the dough, before the drying stage or stage of papermaking, if the paste is not isolated, an effective amount of a fluorescent whitening agent. While the fluorescent whitening agent can be added to the aqueous sludge comprising the lignin-containing slurry at any processing step, to minimize losses, it is advantageously added in the later stages of pulping, before the final dehydration and drying steps. Preferably, it is added after completing the last bleaching stage. By "a paste containing lignin" is meant any paste that still contains 5% or more of lignin by weight, on a dry basis. By definition, lignin is that portion of the paste that is insoluble in 72% sulfuric acid. Suitable test procedures for lignin content are given in TAPPI T 223 and ASTM D The process of this invention is useful for producing significant bleaching of pastes containing preferably 5% lignin on a dry weight basis up to 100% lignin present in an equivalent amount of wood chips. In this way, the process can be used, for example in relatively low lignin content pastes, such as certain bleached kraft pulps, up to and including higher lignin pulps such as thermomechanical pulps, bleached chemo-thermomechanical pulps and even thermomechanical pulps. bleached detached from ink. Preferably, the pastes contain at least 10% lignin by weight on a dry weight basis; more preferably they contain at least 15%. The range of brightness that can be obtained varies from approximately 5 to 90+, depending on the initial paste brightness and the type of paste used. It is known to employ chelating agents in processes for whitening pastes from mechanical grazing processes. See V. N. Gupta, Pulp Paper Mag. Can., 71 (18), T391-399 (1970). The addition of a chelating agent to a slurry of slurry controls the natural yellowing tendency of the glucuronic, extractive and lignin acids present in the paste by removing or minimizing iron and other heavy metals such as copper, zinc and manganese that catalyze the reactions secondary color formers Iron and other heavy metals are converted in the form of their highly soluble chelates and are substantially removed in the dehydration stages. This decreases the incorporation of the heavy metal ions into the paste. Additionally, the chelating agent sequesters remaining iron and other heavy metal salts that, in their own right, would otherwise relax the excited state of the fluorescent whitening agents and render them ineffective. Depending on the processing parameters used in the pulp mill, this metal control stage can be carried out routinely in grazing processes, where reductive bleaching (for example bisulfite, hydrosulfite, or bleached formamidine sulfite) or oxidative bleaching (for example bleached with peroxide or peroxide) is used. The addition of the chelating agent to a slurry of slurry, if necessary, should be carried out before the addition of fluorescent whitening agent. The background level of iron residual times and other heavy metals and their ions in wood chips, in general is about 10 to 25 ppm, although rather it depends on geographical and species considerations. The amount of iron and other heavy metals and their ions in water used in pulp mills varies widely. Significant additional amounts of iron and other heavy metals and their ions are introduced during mechanical grafting of wood chips as well as in newspaper recycling. In this way, the amount of iron and other heavy metals and their ions in the aqueous paste during manufacture, may be several hundred parts per million, based on the dry weight of the pulp, in some stages of pulp manufacture . It is often not necessary to add a chelating agent before addition of the fluorescent whitening agent due to the common use of peroxy bleaching, which requires prior addition of chelating agents to be effective. However, a chelating agent is advantageously employed if the aqueous sludge comprising the lignin-containing pulp still contains from 25 to 500 ppm by weight, based on the dry weight of the pulp, of iron salts and other heavy metals in the processing step, wherein the fluorescent whitening agent is to be added. At the high end of this range, the brightness gain is moderated by iron relaxation of the fluorescent whitening agent, opacification or dulling of the paste due to the natural color of the heavy metal salts and the catalytic effect of the metals in the species peroxy or reductive species (which in turn react with cellulose and impact paste properties). Initial levels of iron salts and other heavy metal ions of 25 to 100 ppm give the greatest improvement in brilliance when the aqueous slurry slurry is treated with a chelating agent before combination with a fluorescent whitening agent. In general, there is no practical advantage to reduced content of iron and other heavy metals and their ions below the residual bottom level found in wood chips. Heavy metal contents can be determined by standard analytical procedures, such as atomic absorption spectroscopy or inductively coupled plasma analysis. Once the type and quantities of the various heavy metals are known, the amount of chelating agent to be used to reach 100 ppm or less, preferably about 25 ppm or less, can be easily calculated or determined from tables. It is not harmful to use a small excess. In this way, depending on the heavy metal content of the aqueous slurry sludge before the addition of the fluorescent whitening agent, the selected chelating agent and a desired degree of whiteness improvement, from 0 to about 1% by weight, based on the The dry weight of the pulp, of a chelating agent, can be used advantageously. An additional and substantial benefit of the chelate treatment is to open the fiber matrix to make it more accessible to the fluorescent whitening agent. All types of chelating agents are suitable in the present invention, ie those that offer thermodynamic or kinetic control of metal ions. However, preference is given to chelating agents that offer thermodynamic control, that is, chelating agents that form a stable, insulatable complex with a heavy metal ion. In this group is a particularly preferred way to utilize aminocarboxylic acid chelates. Well-known and commercially available members of this class include ethylenediacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), hydroxyethylethylenediaminetriacetic acid (HEDTA) and nitrilotriacetic acid (NTA). Mixtures of thermodynamic and kinetic control chelating agents (eg citrates, keto acids, gluconates, eptagluconates, phosphates, and phosphonates) also work well to reduce the content of free heavy metal ions in the pulp to acceptable levels. An amount of these kinetic control chelating agents is also commercially available. Kinetic control chelating agents are those that do not form a stable, insulatable complex with a heavy metal ion. Fluorescent whitening agents are substances that absorb light in the invisible ultraviolet region of the spectrum and send it back to the visible portion of the spectrum, particularly at blue to blue-violet wavelengths. This provides added brilliance and can displace the natural yellow tint or shade of a substrate such as pulp or paper made from it. Fluorescent bleaching agents useful in the present invention can be selected from a wide range of chemical types such as 4,4'-bis- (triazinyla ino) -stilbene-2, 2'-disulfonic acid sulphonic acids, 4,4'-bis- acids. (triazol-2-yl) stilbene-2, 2'-disulfonic acid, 4,4'-dibenzofuranyl-bifenyls, 4,4'- (difnyl) -stilbenes, 4,4'-styryl-biphenyls, 4-f enyl-4'-benzoxazolyl-stilbenes, stilbennyl-naphthotriazoles, 4-styryl-stilbenes, bis- (benzoxazol-2-yl) derivatives, bis- (benzimidazol-2-yl) derivatives, coumarins, pyrazolines, naphthalimides, triazinyl-pyrenes , 2-styryl-benzoxazoles or -naftoxazoles, benzimidazole-benzofurans and oxanilides, or their mixture. c i d o s p r e f e r i d o s 4,4'-bis- (triazinylamino) -stilbene-2, 2'-disulfonic acids are those having the formula: where R? and R 2, independently, are phenyl, mono- or disulfonated phenyl, phenylamino, phenyl amino mono- or disulfonated, morpholino, -N (CH 2 CH 2 OH) 2, -N (CH 3) (CH 2 CH 2 OH), -NH 2, -N (alkyl with 1 to 4 carbon atoms) 2, -OCH 3, -NH-CH 2 CH 2 S 0 3 H, CH 2 CH 2 OH or ethanolaminopropionic acid amide; and M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium which is mono-, di-, tri- or tetra-substituted by alkyl with 1 to 4 carbon atoms, hydroxyalkyl with alkyl with 1 to 4 carbon atoms or their mixtures. Preferably M is Na, Li or K. Especially preferred compounds of the formula (1) are those in which R is 2,5-disulfophenyl and each R2 is morpholino, -N (C2H5) 2, -N (CH2CH20H) 2 or ethanolaminopropionic acid amide; or each R is 3-sulfophenyl and each R2 is NH (CH2CH20H) or N (CH2CH2OH) 2; or each R ?, is 4-sulfophenyl and each R2 is N (CH2CH2OH) 2, N (CH2CHOHCH3) 2, morpholino, or ethanolamino-propionic acid amide; or each Rx is phenylane and each R2 is morpholino, NH (CH2CH20H), N (CH2CH20H) CH3, N (CH2CH20H) 2 or ethanolaminopropionic acid amide, and in each case the sulphonic group is S03M wherein M is sodium. The compounds of the formulas: they are particularly preferred in particular. Preferred 4,4 '-bis- (triazol-2-yl) stilbene-2, 2'-disulfonic esters are those having the formula: wherein R3 and R4, independently, are H, alkyl having 1 to 4 carbon atoms, phenyl or monosulfonated phenyl; and M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium which is mono-, di-, tri- or tetra-substituted by alkyl with 1 to 4 carbon atoms, or hydroxyalkyl with 1 to 4 atoms of carbon, or its mixture. Preferably M is Na, Li or K. Especially preferred compounds of the formula (2) are those wherein R3 is phenyl, R4 is H and M is sodium.

Preferred 4,4'-dibenzofuranyl biphenyls are those of the formula: wherein Ra and Rb, independently, are H or alkyl with 1 to 4 carbon atoms and M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium which is mono-, di-, tri- or tetra -substituted by alkyl with 1 to 4 carbon atoms, hydroxyalkyl with 1 to 4 carbon atoms, or their mixture. Preferably M is Na, Li or K. Especially preferred are those having the formula: Preferably, the 4,4'-styryl-biphenyls used are those of the formula: wherein R5 and R6, independently, are H, S03M, S02N (with C 1 -C 4 alkyl) 2, 0- (with C 1 -C 4 alkyl) CN, Cl, COO (alkyl with 1 to 4 carbon atoms), CON (C 1-4 alkyl) 2 or 0 (CH 2) 3 N + (CH 3) 2 An, where M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium which is mono-, di-, tri- or tetra-substituted by alkyl with 1 to 4 carbon atoms, hydroxyalkyl with 1 to 4 carbon atoms or its mixture, An ~ is an anion of an organic or inorganic acid or its mixture and n is 1. Preferably M is Na, Li or K and An "is formate anion, acetate, propionate, glycolate, lactate, acrylate, methanphosphonate, phosphite, dimethyl or diethyl phosphite, or their mixture. Especially preferred compounds of the formula ( 4) are those in which each R6 is H and each R5 is a group 2-S03M wherein M is sodium or each R5 is 0 (CH2) 3N + (CH3) 2 An ", where An" is acetate. prefer the compound of the formula: 4-Phenyl-4'-benzoxazolyl-stilbenes having the formula are preferred: wherein R7 and R8 are independently H, Cl, alkyl with 1 to 4 carbon atoms or S02-alkyl with 1 to 4 carbon atoms. Preferably, the stilbenyl-naphthotriazoles used are those of the formula: wherein R9 is H or Cl; R10 is S03M, S02N (alkyl having 1 to 4 carbon atoms) 2, S020-phenyl or CN; R,, is H or S03M; and M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium which is is mono-, di-, tri- or tetra-substituted by aliquil with 1 to 4 carbon atoms, hydroxyalkyl with 1 to 4 atoms of carbon or its mixture. Preferably M is Na, Li or K. Especially preferred compounds of the formula (6) are those wherein R9 and RX1 are H and R10 is 2-S03M wherein M is Na. Preferably, the 4-styryl-stilbenes used are those of the formula: wherein R12 and R13 are independently H, S03M, S02N (alkyl having 1 to 4 carbon atoms) 2, 0- (alkyl with 1 to 4 carbon atoms), CN, Cl, COO (alkyl with 1 to 4 carbon atoms), CON (alkyl with 1 to 4 carbon atoms) 2 or O (CH2) 3N + (CH3) 2 An ", in where An "is an anion of an organic or inorganic acid, in particular a formate, acetate, propionate, glucolate, lactate, acrylate, metanephosphonate, phosphite, dimethyl or diethyl phosphite, or its mixture and M is H, Na, Li, K , Ca, Mg, ammonium, or ammonium which is mono-, di-, tri- or tetra-substituted by alkyl with 1 to 4 carbon atoms, hydroxyalkyl with 1 to 4 carbon atoms or their mixture. Preferably M is Na, Li or K. Especially preferred compounds of the formula (7) are those wherein each of R12 and R13 are 2-cyano or 2-S03M wherein M is sodium or 0 (CH2) 3N + (CH3 ) 2An "where An" is acetate. Bis- (benzoxazol-2-yl) derivatives are those of the formula: wherein R14 is independently H, C (CH3) 3, C (CH3) 2-phenyl, phenyl with 1 to 4 carbon atoms or COO-alkyl with 1 to 4 carbon atoms, and X is -CH = CH- or a group of the formula: H.H Especially preferred compounds of the formula (8) are those wherein each R14 is H and X is -CH = CH-; or a group R14 and each ring is 2-methyl and the other R14 is H and X is -CH = CH-; or a group R14 in each ring is 2-C (CH3) 3 and the other R14 is H and X is; or one group R14 in each ring is 2-methyl and the other R14 is H and X is -CH = CH-; or a group R14 in each ring is 2-C (CH3) 3 and the Preferred bis- (benzimidazol-2-yl) derivatives are those of the formula: wherein R15 and R16, independently, are H, alkyl having 1 to 4 carbon atoms or CH2CH20H; R17 is H or S03M; X, is -CH = CH- or a group of the formula:; and M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium which is mono-, di-, tri- or tetra-substituted by alkyl with 1 to 4 carbon atoms, hydroxyalkyl with 1 to 4 carbon atoms or their mixture. Preferably M is Na, Li or K. Especially preferred compounds of formula (9) are those in which R15 and R16 are each H, R17 is S03M wherein M is sodium and X is -CH = CH- . Preferred coumarins are those of the formula: wherein R18 is H, Cl or CH2C00H, R19 is H, phenyl, COO-alkyl having 1 to 4 carbon atoms or a group of the formula: and R20 is 0-alkyl with 1 to 4 atoms carbon, N (C 1 -C 4 -alkyl) 2, NH-CO-C 1 -C 4 -alkyl or a group of the formula: wherein R x and R 2, independently, are phenyl, mono- or disulfonated phenyl, phenylamino, phenylamino, mono- or disulfonated, morpholino, -N (CH 2 CH 20 H) 2, -N (CH 3) (CH 2 CH 20 H), -NH 2, -N ( alkyl having 1 to 4 carbon atoms) 2, -0CH3, -Cl, -NH-CH2CH2S03H or -NH-CH2CH2OH, R3 and R4, independently, are H, alkyl having 1 to 4 carbon atoms, phenyl or monosulfonated phenyl and R21 is H, alkyl having 1 to 4 carbon atoms, or phenyl. Especially preferred compounds of the formula (10) are those having the formula: Preferably, the pyrazolines used are those having the formula: wherein R22 is H, Cl or N (alkyl having 1 to 4 carbon atoms) 2, R23 is H, Cl, S03M, S02NH2, S02NH- (alkyl having 1 to 4 carbon atoms), COO-alkyl with 1 to 4 carbon atoms, S02-alkyl having 1 to 4 carbon atoms, S02NHCH2CH2CH2N + (CH3) 3 or S02CH2CH2N H (alkyl having 1 to 4 carbon atoms) 2 An, R24 and R25 are the same or different and each one is H , alkyl with 1 to 4 carbon atoms or phenyl, R26 is H or Cl, An "is an anion of an organic or inorganic acid, and M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium that is mono-, di-, tri- or tetra-substituted by alkyl having 1 to 4 carbon atoms, hydroxyaliphenyl with 1 to 4 carbon atoms or its mixture, preferably M is Na, Li or K. Especially preferred compounds of the formula (13) are those in which R22 is Cl, R23 is S02CH2CH2N + H (alkyl having 1 to 4 carbon atoms) 2 An wherein An "is phosphite and R24, R25 and R26 ca <1> 3rd one is H; or those that have the formula: Preferred Naphtali are those of the formula: wherein R27 is alkyl having 1 to 4 carbon atoms or CH2CH2CH2N + (CH3) 3 An "wherein An" is an anion of an organic or inorganic acid, R28 and R29, independently are 0-alkyl having 1 to 4 carbon atoms S03M or NH-CO-alkyl with 1 to 4 carbon atoms; and M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium which is mono-, di-, tri- or tetra-substituted by alkyl with 1 to 4 carbon atoms, hydroxyalkyl with 1 to 4 carbon atoms. carbon, or its mixture. Preferably M is Na, Li or K. Especially preferred compounds of the formula (16) are those having the formula: Preferred 2-styryl-benzoxazole derivatives or -naftoxazole are those that have the formula: wherein R31 is CN, Cl, COO-alkyl having 1 to 4 carbon atoms or phenyl; R32 and R33 are the atoms required to form a fused benzene ring or R33 and R35, independently they are H or alkyl with 1 to 4 carbon atoms; and R34 is H, alkyl with 1 to 4 carbon atoms or phenyl. Preferred benzimidazole-benzofuran derivatives are those having the formula: wherein R36 is alkoxy with 1 to 4 carbon atoms; R37 and R38 are independently alkyl with 1 to 4 carbon atoms; and An "is an anion of an organic or inorganic acid A particularly preferred compound of the formula (21) is that wherein R36 is ethoxy, R37 and R38 each are methyl and An" is methane sulfonate. Preferred oxanilide derivatives include those having the formula: wherein R39 is alkoxy with 1 to 4 carbon atoms; R41 is alkyl with 1 to 4 carbon atoms, or alkoxy with 1 to 4 carbon atoms-S03M wherein M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium which is mono-, di-, tri- or tetra-substituted by alkyl with 1 to 4 carbon atoms, hydroxyalkyl with 1 to 4 carbon atoms or a mixture thereof, preferably Na, Li or K, and R40 and R42 are the same and each is hydrogen , tert-butyl or S03M, where M is as defined for R41. Compounds of the above formulas are known per se and can be prepared by known methods. In the above classes of fluorescent whitening agents, it is advantageous to employ those which have affinity for the cellulose or lignin portion of the pulp fibers. A preferred group of these fluorescent whitening agents are those which are substituted by a sulfonic acid group, especially 2 to 6 sulfonic acid groups. It is especially preferred to employ those having 2 sulfonic acid groups as the primary fluorescent whitening agents. It is known that fluorescent whitening agents can exhibit a green or bluish dye at high dosage levels, for example at a dosage level of about 2% by weight, based on the dry weight of the dough. This is a normal expected effect and is not changed by the presence of a metal chelating agent in the process of the invention. This effect can be counter-attacked by the use of appropriate levels of fluorescent whitening agent blends, particularly blends containing fluorescent whitening agents that have a reddish tint.

A preferred aspect of the present invention is to extend the effectiveness of the primary fluorescent brightener, in particular a fluorescent whitening agent having two sulfonic acid groups, with a more highly active and lower affinity bleach such as a fluorescent whitening agent having 4 or 6 sulfonic acid groups. This allows the polishing mixture to be adjusted to optimize the development of fluorescence and shade, as well as the economy of the process. The use of the mixture of fluorescent whitening agents of formulas (la) and (Ib) is particularly preferred in this regard. Particularly preferred is a mixture comprising 30 to 90 parts by weight of a compound of the formula (la) with 70 to 10 parts by weight of the compound of the formula (Ib), more especially a mixture of 50 to 70 parts by weight of a compound of the formula (la) with 50 to 30 parts by weight of the compound of the formula (Ib). A judicious selection of optical brighteners allows the manufacturer to achieve the desired brightness objectives in the wood pulp mill, while simultaneously balancing other bleaching parameters, to provide the desired typical properties such as volume, rigidity, kinking, opacity, fiber strength, discharge, etc. The use of fluorescent whitening agents also gives benefits in the hue properties, for example in bluing (b * value) which are convenient in the commercialization of these pastes. Another preferred aspect of the present invention is to employ mixtures of the same and different types of fluorescent disulfo bleaching agents. Especially preferred are mixtures comprising a compound of the formula (la) and / or (le) with the compound of the formula (4a) These mixtures can comprise from 5 to 95 parts by weight of a compound of the formula (la) and / or (le), with 95 to 5 parts by weight of the compound of the formula (4a), preferably a mixture of 60 to 90 parts by weight of a compound of the formula (la) and / or (le), with 40 to 10 parts by weight of the compound of the formula 4a (and especially a mixture of 65 to 85 parts by weight of a compound of the formula (la) and / or (le), with 25 to 10 parts by weight of the compound of the formula (4a), the sum in the parts in each case is 100. The fluorescent whitening agents comprising the mixtures can be added to the aqueous paste separately or as a mixture The mixtures of above fluorescent whitening agents give superior results, particularly with respect to bluing (b * value) compared to the same weight of the individual compounds.

The fluorescent whitening agent to be used is preferred to vary from 0.01 to about 2% by weight, based on the dry weight of the pulp, depending on the degree of improvement in whiteness desired. Preferably .1 to 1.5% is oyed; it is more preferably used .2 to .8. 7Before its isolation or forming paper, a paste undergoes a series of chemical treatments and extractions. As indicated above, the fluorescent whitening agent is preferably added in the later stages of the grazing process to minimize physical and chemical losses. More preferably, the fluorescent whitening agent is added after finishing the bleaching steps and before dehydration, swelling and final drying. In this pulp manufacturing step, the pulp usually has a reasonable pH (preferably between 4 and 10) and a very fluid consistency that promotes rapid mixing of the fluorescent whitening agent through the pulp. Typically, in this pulp making step, the solids content is 5 to 15% by weight, based on the dry weight of the pulp. While the fluorescent whitening agent works at high consistencies (> 50% solids by weight, based on the dry weight of the dough) in the present invention, the final dough is more subject to dripping due to incomplete mixing with respect to time which takes the fluorescent whitening agent to fix on the fiber. Therefore, advantageously, the solids content based on the dry weight of the dough is less than 50%, preferably less than 30%. More preferably, it is between 5 and 15% by weight, when the fluorescent whitening agent is added. Another reason for preferring to add the fluorescent whitening agent after the final bleaching step is that the amount of fluorescent whitening agents to be added can be easily varied at this point in the pulping process to adjust the brightness of the appropriate level. While some residual peroxide or reductive whitening agents may still be present at this stage, this does not adversely affect the inventive process. It is also worth noting that variations in trature do not seem to be a negative factor. Fluorescent whitening agents work well within the wide range of tratures, which is typically 21-62 ° C (70-150 ° F), which are encountered during the bleaching and subsequent pulping stages. In the process of the present invention, even if a chelating agent is used in previous steps, for example during bleaching, it is still advantageous to supply additional chelating agent to the lignin-containing pulp before the addition of the fluorescent whitening agent in order to control the amount of iron salts and other heavy metals within the ranges illustrated above. Additionally, the presence of a chelating agent is also advantageous as an aid in dispersing the fluorescent whitening agent in the paste to minimize mottling. This is especially important if the fluorescent whitening agent is added to high consistency pastes, for example after dehydrating and just before going to the foaming and drying stages. Additives that are known to improve the effectiveness of fluorescent whitening agents can also be employed in the present invention, provided they are within the limits of what is acceptable to the end user of the dough. Thus, another preferred aspect of the present invention comprises using a fluorescent whitening agent, in combination with an additive for example an unsubstance used to promote UV absorption and "surface displacement" of the fluorescent whitener in paper or a material that allows Effectively, the optical brightener develops a high degree of fluorescent whitening by cleaning the pulp fibers. Suitable additives include cationic starch, polyvinyl alcohol and enzymes. Suitable enzymes include cellulases and hemicellulases. The addition of the polyvinyl alcohol is particularly preferred. For example, addition of polyvinyl alcohol to the pulp at the 1.25% level, based on the dry weight of the pulp, can increase the effectiveness of an optical brightener by up to 4 more GE units, compared to the same fluorescent whitening agent without the use of the additive. In the following illustrative examples, the parts are parts by weight. Example 1 A slurry of continuous flow slurry containing about 8% by weight, based on the dry weight of the slurry, of a bleached but not dehydrated CTMP, ie a peroxide-bleached chemo-thermomechanical pulp having a iron content of approximately 10 ppm and a lignin content which typically corresponds to 85-90% of the lignin present in an equivalent amount of wood chips, at a temperature between 50 to 65 ° C and a pH between 7 and 8, is added continuously, in proportion of 2.3 parts per 1000 parts of sludge, both based on the "as-is" weight of the slurry slurry, of an aqueous liquid containing 12.5% fluorescent whitening agent Tinopal R HW (Ciba Specialty Chemicals Corp, Consumer Care Division, High Point, NC) of the formula: The time between the addition of Tinopal HW to the slurry of paste before the mixture dehydrates and dries is less than 10 (ten) minutes. As a result of this addition, the brilliance of the resulting paste rises from its initial value of 86 to 90+. EXAMPLE 2 Example 1 is repeated, but employing an aqueous slurry containing approximately 8% of a peroxide-bleached chemithermomechanical pulp. With 5.5 parts per 1000 parts, both based on the weight "as is" of the slurry of slurry, of an aqueous liquid containing 12.5% of the fluorescent whitening agent of the formula: the brightness rises from 86 to 91+, and stabilizes in a relatively short time (approximately (ca. 10 minutes) Example 3 During a batch process operating at 25 ° C, pH 7-8, an aqueous sludge containing about 5% by weight, based on the dry weight of the pulp, of a bleached but not dehydrated CTMP, ie a chemo-thermomechanical pulp bleached with peroxide, having an iron content of about 10 ppm and a lignin content which typically corresponds to 85-90% of the lignin present in an equivalent amount of wood chips, is mixed with 4 parts per 1,000 parts, both based on the "100% dry base" weight of the sludge, of a mixture of aqueous liquid containing a mixture of 80:20"dry base ratio" of 12.5% by weight of the fluorescent whitening agent Tinopal ™ HW and 33% by weight of the fluorescent whitening agent Tinopal ™ SK (Ciba Specialty Chemicals Corp, Consumer Care Division, High Point) , NC) of the f formulas: Í4a) SOaNa S03Na respectively. The Tinopal mixture will be added with the sludge paste for less than 10 (ten) minutes before the mixture dehydrates and dries. As a result of this addition, the brightness of the resulting paste rises from its initial value of 81 to 94+. In addition, blueness as judged by the b * value is increased by 20% over what is obtained by the use of Tinopal H alone.

Claims (1)

1. CLAIMS 1. - A process for increasing the whiteness of the lignin-containing pulp, characterized in that it comprises adding to an aqueous sludge containing the lignin-containing pulp, during pulp manufacture, before the drying step or the production stage of the pulp. paper, an effective amount of a fluorescent whitening agent. 2. - A method according to claim 1, characterized in that the fluorescent whitening agent is added before the final stages of dehydration and drying or the paper production stage. 3. - A method according to claim 1, characterized in that the fluorescent whitening agent is added after finishing the last bleaching stage. . - A method according to claim 1, characterized in that the paste contains 5% or more of lignin by weight in a dry base. 5. - A method according to claim 1, characterized in that it comprises adding to the aqueous sludge consisting of paste containing lignin, an effective amount of a chelating agent to decrease the content of salts of iron and other heavy metals to 100 ppm or less in weight, based on the dry weight of the pulp, before the addition of the fluorescent whitening agent. 6. - A method according to claim 5, wherein the chelating agent is selected from the group consisting of ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, hydroxyethylenediaminetriacetic acid, and nitrilotriacetic acid. 7. - A method according to claim 1, characterized in that the fluorescent whitening agent is selected from the group consisting of 4,4'-bis- (triazinylamino) -stilben-2, 2'-disulfonic acids, acids 4,4 '-bis- (triazol-2-yl) stilbene-2, 2'-disulfonic acid, 4,4'-dibenzofuranyl-biphenyls, 4,4'- (diphenyl) -stilbenes,, 4' -distyryl-biphenyls, 4- phenyl-4'-benzoxazolyl-stilbenes, stilbennyl-naphthotriazoles, 4-styryl-stilbenes, bis- (benzoxazol-2-yl) derivatives, bis- (benzimidazol-2-yl) derivatives, coumarins, pyrazolines, naphthalimides, triazinyl-pyrenes , 2-styryl-benzoxazoles or -naftoxazoles, benzimidazole-benzofurans and oxanilides, or a mixture thereof. 8. - A method according to claim 7, characterized in that the fluorescent whitening agent is of the formula: 4,4'-bis- (triazinylamino) -stilbene-2, 2'-disulfonic is of the formula: wherein Rx and R2, independently, are phenyl, phenyl mono- or disulfonated, phenylamino, phenylamino mono- or disulfonated, morpholino, -N (CH2CH20H) 2, -N (CH3) (CH2CH20H), -NH2, -N (alkyl) with 1 to 4 carbon atoms) 2, -0CH3, -Cl, -NH-CH2CH2S03H, CH2CH20H or ethanolaminopropionic acid amide; and M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium which is mono-, di-, tri- or tetra-substituted by alkyl with 1 to 4 carbon atoms, hydroxyalkyl with alkyl with 1 to 4 carbon atoms or mixtures thereof. 9. - A method according to claim 8, characterized in that the fluorescent whitening agent is of the formula (1), wherein each R? is 2, 5-disulfophenyl and each R2 is morpholino, -N (C2H5) 2, -N (CH2CH20H) 2 or ethanolaminopropionic acid amide; or each R is 3-sulfophenyl and each R2 is NH (CH2CH20H) or N (CH2CH20H) 2; or each Rx is 4-sulfophenyl and each R2 is N (CH2CH20H) 2, N (CH2CHOHCH3) 2, morpholino, or ethanolamino-propionic acid amide; or each R is phenylamino and each is morpholino, NH (CH2CH2OH), N (CH2CH20H) CH3, N (CH2CH20H) 2 or ethanolaminopropionic acid amide, and in each case the sulfo group is S03M wherein M is sodium. 10. - Fluorescent acid whitening agent fluorescent whitening agent is of the formula: 11. - A method according to claim 7, characterized in that the fluorescent whitening agent of 4,4'-bis- (triazol-2-yl) stilbene-2,2'-disulfonic acid is of the formula: S03M SO- wherein R3 and R4, independently, are H, alkyl having 1 to 4 carbon atoms, phenyl or monosulfonated phenyl; and M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium which is mono-, di-, tri- or tetra-substituted by alkyl with 1 to 4 carbon atoms, hydroxyalkyl with 1 to 4 carbon atoms. carbon, or a mixture thereof. 12. - A method according to claim 11, characterized in that the formula (2) wherein R3 is phenyl, R4 is H and M is sodium. 13. - A method according to claim 7, characterized in that the fluorescent whitening agent 4,4 '-dibenzofuranyl-biphenyl is of the formula: wherein Ra and Rb, independently, are H or alkyl with 1 to 4 carbon atoms and M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium which is mono-, di-, tri- or tetra -substituted by alkyl with 1 to 4 carbon atoms, hydroxyalkyl with 1 to 4 carbon atoms, or a mixture thereof. 14. - A method according to claim 7, characterized in that the fluorescent whitening agent is the compound of the formula: 15. - A method according to claim 7, characterized in that the fluorescent whitening agent 4,4 '-styryl-biphenyl is of the formula: wherein R5 and R6, independently, are H, S03M, S02N (alkyl with 1 to 4 carbon atoms) 2, O- (alkyl with 1 to 4 carbon atoms) / CN, Cl, COO (alkyl with 1 to 4) carbon atoms), CON (C 1-4 alkyl) 2 or 0 (CH 2) 3 N + (CH 3) 2 An ", wherein M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium which is mono-, di-, tri- or tetra-substituted by alkyl with 1 to 4 carbon atoms, hydroxyalkyl with 1 to 4 carbon atoms or a mixture thereof, An "is an anion of an organic or inorganic acid or its mixture and n is 1. 16. A process according to claim 15, characterized in that the fluorescent whitening agent is of the formula (4) wherein each R6 is H and each R5 is a 2-S03M group, wherein M is sodium or each R5 is 0 (CH2) 3N + (CH3) 2 An ", where An" is acetate. 17. - A method according to claim 16, characterized in that the fluorescent whitening agent is of the formula: 18. - A method according to claim 7, characterized in that the fluorescent whitening agent is 4-phenyl-4'-benzoxazolyl-stilbene of the formula: wherein R7 and R8 are independently H, Cl, alkyl having 1 to 4 carbon atoms or S02 with 1 to 4 carbon atoms, or a stilbennyl-naphthotriazole of the formula: wherein R9 is H or Cl; R10 is S03M, S02N (alkyl having 1 to 4 carbon atoms) 2, S020-phenyl or CN; R 1 is H or S03M; and M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium which is is mono-, di-, tri- or tetra-substituted by alkyl with 1 to 4 carbon atoms, hydroxyalkyl with 1 to 4 atoms of carbon or a mixture thereof. 19. - A method according to claim 7, characterized in that the fluorescent whitening agent is a 4-styryl-stilbene of the formula: wherein R12 and R13 are independently H, S03M, S02N (C 1 -C 4 alkyl) 2, O- (C 1 -C 4 alkyl), CN, Cl, COO (alkyl with 1 to 4 atoms) carbon), CON (C 1 -C 4 alkyl) 2 or O (CH 2) 3 N + (CH 3) 2 An "wherein An" is an anion of an organic or inorganic acid, or a mixture thereof and M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium which is mono-, di-, tri- or tetra-substituted by alkyl with 1 to 4 carbon atoms, hydroxyalkyl with 1 to 4 carbon atoms or mix of them. 20. A method according to claim 7, characterized in that the fluorescent whitening agent is a bis- (benzoxazol-2-yl) derivative of the formula: wherein R14 is independently H, C (CH3) 3, C (CH3) 2-phenyl, phenyl with 1 to 4 carbon atoms or COO-Cl-alkyl with 1 to 4 carbon atoms, and X is -CH = CH-o a group of the formula: 21. - A method according to claim 7, characterized in that the fluorescent whitening agent is a bis- (benzimidazol-2-yl) derivative of the formula: wherein R15 and R16, independently, are H, alkyl having 1 to 4 carbon atoms or CH2CH20H; R17 is H or S03M; X, is -CH = CH- or a group of the formula:; and M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium which is mono-, di-, tri- or tetra-substituted by alkyl with 1 to 4 carbon atoms, hydroxyalkyl with 1 to 4 carbon atoms or a mixture of the same . 22. - A method according to claim 7, characterized in that the fluorescent whitening agent is a coumarin of the formula: wherein R18 is H, Cl or CH2C00H, R19 is H, phenyl, COO-alkyl having 1 to 4 carbon atoms or a group of the formula: and R20 is 0-alkyl with 1 to 4 atoms carbon, N (C 1 -C 4 -alkyl) 2, NH-CO-C 1 -C 4 -alkyl or a group of the formula: wherein R and R2, independently, are phenyl, mono- or disulfonated phenyl, phenylamino, phenylamino mono- or disulfonated, morpholino, -N (CH2CH20H) 2, -N (CH3) (CH2CH2OH), -NH2, -N (alkyl) with 1 to 4 carbon atoms) 2, -OCH3, -Cl, -NH-CH2CH2S03H or -NH-CH2CH2OH, R3 and R4, independently, are H, alkyl with 1 to 4 carbon atoms, phenyl or monosulfonated phenyl and R21 is H, alkyl having 1 to 4 carbon atoms, or phenyl. 23. - A method according to claim 7, characterized in that the fluorescent whitening agent is a pyrazoline of the formula: wherein R22 is H, Cl or N (alkyl having 1 to 4 carbon atoms) 2, R23 is H 'C1 »S03M, S02 H2, S02NH- (alkyl having 1 to 4 carbon atoms), COO-alkyl with 1 at 4 carbon atoms, S02-alkyl having 1 to 4 carbon atoms, S02NHCH2CH2CH2N + (CH3) 3 or S02CH2CH2N + H (alkyl having 1 to 4 carbon atoms) 2 An ", R24 and 25 are the same or different and each one is H, alkyl with 1 to 4 carbon atoms or phenyl, R26 is H or Cl, An "is an anion of an organic or inorganic acid, and M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium which is mono-, di-, tri- or tetra-substituted by alkyl with 1 to 4 carbon atoms, hydroxyalkyl with 1 to 4 carbon atoms or mixture thereof. 24. - A method according to claim 7, characterized in that the fluorescent whitening agent is a naphthalimide of the formula: wherein R27 is alkyl having 1 to 4 carbon atoms or CH2CH2CH2N + (CH3) 3 An "wherein An" is an anion of an organic or inorganic acid, R28 and R2c, independently are O-alkyl with 1 to 4 carbon atoms; carbon, S03M or NH-CO-alkyl with 1 to 4 carbon atoms; and M is H, Na, Li, K, Ca, Mg, ammonium, or ammonium which is mono-, di-, tri- or tetra-substituted by alkyl with 1 to 4 carbon atoms, hydroxyalkyl with 1 to 4 carbon atoms. carbon, or a mixture thereof. 25. - A method according to claim 7, characterized in that the fluorescent whitening agent is a 2-styryl-benzoxazole- or -naphthoxazole derivative having the formula: wherein R31 is CN, Cl, COO-alkyl having 1 to 4 carbon atoms or phenyl; R32 and R33 are the atoms required to form a fused benzene ring or R33 and R35, independently they are H or alkyl with 1 to 4 carbon atoms; and R34 is H, alkyl with 1 to 4 carbon atoms or phenyl. 26. A method according to claim 7, characterized in that the fluorescent whitening agent is a benzimidazole-benzofuran derivative having the formula: - wherein R36 is alkoxy with 1 to 4 carbon atoms, - R37 and R38, independently are alkyl with 1 to 4 carbon atoms; and An "is an anion of an organic or inorganic acid 27. A process according to claim 7, characterized in that the fluorescent whitening agent is an oxanilide derivative having the formula: wherein R39 is alkoxy with 1 to 4 carbon atoms; R41 is alkyl with 1 to 4 carbon atoms, alkyl with 1 to 4 carbon atoms-S03M, or alkoxy with 1 to 4 carbon atoms-S03M wherein M is H, Na, Li, K, Ca, Mg, ammonium , or ammonium which is mono-, di-, tri- or tetra-substituted by alkyl with 1 to 4 carbon atoms, hydroxyalkyl with 1 to 4 carbon atoms or a mixture thereof, preferably Na, Li or K, and R40 and R42 are the same and each is hydrogen, tert-butyl or S03M, wherein M is as defined for R41. 28. A method according to claim 1, characterized in that the fluorescent whitening agent is replaced by 2 to 6 sulfonic acid groups. 29. A method according to claim 31, characterized in that the fluorescent whitening agent is replaced by 2 sulfonic acid groups. 30. A method according to claim 1, characterized in that the fluorescent whitening agent comprises a mixture of at least 2 different fluorescent whitening compounds. 31. A process according to claim 30, characterized in that the fluorescent whitening agent comprises a mixture of fluorescent whitening agent which is replaced by 2 sulfonic acid groups and a fluorescent whitening agent which is substituted by 4 to 6 sulfonic acid groups . 32. - A method according to claim 31, characterized in that the fluorescent whitening agent comprises a mixture of the compounds of the formulas: 33. - A method according to claim 30, characterized in that the fluorescent whitening agent comprises a mixture of 5 to 95 parts by weight of a compound of the formula: I with 95 to 5 parts by weight a compound of the formula: 34. - A method according to claim 1, characterized in that from 0.01 to about 2% by weight, based on the dry weight of the pulp, of the fluorescent whitening agent is used. 35.- A process according to claim 1, characterized in that the aqueous sludge comprising the pulp containing lignin, has a solid content of less than 50%, based on the dry weight of the pulp. 36.- A process according to claim 35, characterized in that the aqueous sludge comprising the pulp containing lignin, has a pH of 4 to 10 in solids content between 5 to 15%, based on the dry weight of the pasta. 37.- A method according to claim 1, characterized in that it additionally comprises adding to the aqueous sludge comprising a paste containing lignin, and an effective amount of a fluorescent whitening agent, an effective amount of an additive that is known to improve the effectiveness of the fluorescent whitening agent. 38.- A method according to claim 37, characterized in that the additive is cationic starch, polyvinyl alcohol or an enzyme.