EP1322814B1 - Method for bleaching paper pulp - Google Patents

Abstract

The invention concerns a method for bleaching different types of paper pulp in two steps at atmospheric pressure and at a temperature not higher than 100 DEG C. The first step consists in contacting the unbleached paste with a mixture of paracetic acid and performic acid. The second step consists in treating the bleached pulp derived from the first step, with a solution of soda and hydrogen peroxide. The resulting pulps exhibit a high index of whiteness and a degree of polymerisation close to unbleached pulps. The method is environmentally safe and non-polluting.

Description

The invention relates to a process for bleaching paper pulp obtained from lignocellulosic raw materials, in particular vegetable matter from annual plants, agricultural by-products such as cereal straws, sugarcane bagasse or sorghum bagasse. sugar, softwood or hardwood chips, old paper etc ....

The aim of the invention is to provide a process which makes it possible to efficiently and economically bleach any type of paper pulp in a hydro-organic medium with a water content of less than 15% by weight so as to limit or even eliminate pollutant aqueous discharges and obtain pulps. paper with a whiteness index greater than 70 in only two processing steps.

Traditional processes used to bleach pulp use oxidants which are essentially chlorine, oxygen, and their derivatives.

The bleaching of paper pulp proceeds by a delignification operation complementary to that carried out during the thermal and chemical treatment undergone by the plant which leads to the unbleached paper pulp. This operation consists of an oxidative degradation of the colored molecules, essentially formed of fragments of lignins more or less recombined, which are deposited during the baking on the paper fibers as well as the residual lignin present after cooking in the fibers.

Many books and documents describe these operations. We will refer for example for more information to the book Chemical Pulping, J. Gullishen, TAPPI and Paper Editor, pp.1 to 145, 2000 . (www.tappi.org)

Chlorine and its derivatives require considerable amounts of water, exceeding 50 tonnes of water per tonne of pasta. The polluted water must then be discharged into the rivers.

Pollution, even with a chemical and biological treatment of effluents before discharge is considerable with chlorine and sodium hypochlorite, especially because of the toxicity of organochlorine compounds generated.

The use of oxygen, ozone and hydrogen peroxide peroxides and peracids provide a first solution for pollution by chlorine and its derivatives.

These technologies are now known and controlled.

In the same way we will refer for more information to the book Chemical Pulping, J. Gullishen, TAPPI and Paper Editor, p 146-213, 2000 . (www.tappi.org) where these technologies are largely described from very recent references.

The quantities of water used remain high and these processes all require more than two steps of contacting the pulp to be bleached with chemical reagents.

In addition, it is difficult to control the reactivity of ozone and hydrogen peroxide with respect to cellulose and polysaccharides, which causes degradation of the mechanical properties of the paper pulps thus treated.

The number of steps in contacting the pulp to be bleached and the amount of water used and released into the surrounding environment are all factors that penalize these technologies economically and ecologically.

There is another way of making bleached pulp which differs radically from conventional processes and proceeds by oxidative degradation of lignins practiced on the initial vegetable matter. These technologies generally use performic acid and / or peracetic acid generated by direct reaction with hydrogen peroxide, with or without a catalyst.

The work of J. Sundquist and collaborators on this subject published in: Papper 0ch Trä p 88, 2, 1986 ; and summarized in Chemical Pulping, J. Gullishen, TAPPI and Paper Co., p 421-425 (www.tappi.org ), show that this type of pasta has mechanical qualities of lesser quality. In addition, when peracids are used directly on the plant, the consumption of hydrogen peroxide and peracids is very important, which economically condemns this type of process.

The present invention proposes to provide a new process for bleaching paper stocks.

• permet d'obtenir en seulement deux étapes de mise en contact avec des réactifs chimiques des pâtes à papier d'indice de blancheur supérieur à 70 pouvant atteindre pour des papiers de qualité supérieures des valeurs supérieures à 90 ( l'indice de blancheur est défini sur une échelle de 0 à 100 ; la valeur 100 étant celle du Carbonate de Magnésium : MgCO₃ considéré comme la référence absolue en terme de blancheur ).
• permet de conserver les propriétés mécaniques des pâtes écrues sans dégrader notablement les fibres cellulosiques en conservant notamment un degré de polymérisation des dites pâtes proche de celui des pâtes écrues avant blanchiment.
• à un coût économique proche voire inférieur aux procédés industriels actuels dans la mesure où l'essentiel de la lignine a été retirée lors de la fabrication de la pâte à papier.
• est bien adapté notamment à tous les types de pâtes à papiers chimiques et mi-chimiques écrues.

The essential objective of the invention is to provide a new bleaching process which:

• provides in only two stages of contact with chemical reagents pulps with a whiteness index greater than 70, which can reach higher values for papers of higher quality than 90 (the whiteness index is set to a scale from 0 to 100, the value 100 being that of magnesium carbonate: MgCO ₃ considered as the absolute reference in terms of whiteness).
• allows the mechanical properties of unbleached pasta to be preserved without substantially degrading the cellulosic fibers, in particular keeping a degree of polymerization of the said pastes close to that of unbleached pasta before bleaching.
• at an economic cost close to or even less than current industrial processes, since most of the lignin was removed during the manufacture of paper pulp.
• is well suited in particular to all types of unbleached chemical and semi-chemical paper pulps.

Another objective of the invention is to work under conditions of hydration of the lowest possible reaction medium so as to minimize the water consumption and thus the aqueous discharges generated by the bleaching operations.

• la mise en contact de la pâte écrue avec un mélange d'acide peracétique et performique ,d'acide acétique et formique de façon à ce que le mélange obtenu ait une teneur en eau inférieure ou égale à 15% en poids du mélange.
• le traitement de la pâte obtenue après séparation mécanique du milieu réactionnel par une solution d'eau oxygénée en milieu légèrement basique ( 8 < pH < 10 ).

For this purpose the method of the invention is characterized in that it combines two successive processing steps which are:

• contacting the unbleached pulp with a mixture of peracetic and performic acid, acetic and formic acid so that the mixture obtained has a water content of less than or equal to 15% by weight of the mixture.
• the treatment of the paste obtained after mechanical separation of the reaction medium with a solution of hydrogen peroxide in a slightly basic medium (8 <pH <10).

The bleaching phase essentially takes place in the first step corresponding to the contacting of the unbleached pulp with the performic acid and peracetic acid mixture, which are indeed powerful oxidants capable of carrying out the degradation of the residual lignins present, at the same time. origin mostly of the brown color more or less dark unbleached pasta.

The experiments have shown that, surprisingly, the mixture of performic acid and peracetic acid containing little water allowed particularly effective bleaching in a single step without degradation of the cellulose fibers.

This remarkable result in itself is to be attributed to the capacities of these two peracids to act by species reactive generated by the breakdown of the peracidic function including the OH ⁺ ion which degrades, through an electrophilic substitution, aromatic rings and lignin side chains, without significantly attacking the carbonyls and the primary and secondary alcohols functions of sugars.

These reactions, which are very selective for lignins, therefore have no apparent effect on the cellulose chains, and therefore on the quality of the bleached pulp, since in the process of the invention the degree of polymerization of the cellulosic fibers is only slightly or not affected.

The best results have been obtained with a quantity of water limited to 15% by weight of the reaction medium, which differs considerably from the traditional methods, including those using in a peracid bleaching step, a step in which the amount of water present in the reaction medium is much higher.

Under these conditions, the acidity of the medium is buffered by the presence of acetic acid which may explain the very low degradation of the cellulose fibers.

Since the bleaching is carried out with an excess of peracids, the solution separated from the bleached pulp is recycled to the peracid production reactor and treated with 50% by weight hydrogen peroxide to reduce the concentration of peracids to the required value before to be returned to the bleaching reactor.

In this way, the bleaching agent is in large excess with respect to the dyestuffs to be destroyed, which makes it possible to better understand the remarkable efficiency of the process.

The mixture of peracetic acid and performic acid is preferably prepared by contacting acetic acid and formic acid with hydrogen peroxide with a concentration of greater than 50% by weight. A ratio of acetic acid + peracetic acid / formic acid + performic acid of the order of 9/1 by volume leads to the best results observed experimentally.

According to a preferred embodiment, the performic acid and acetic acid solution will circulate against the flow of the pasta stream and will be sent before recirculation in a hydrogen peroxide contact reactor so as to keep permanently an optimal high concentration in contact with the dough in circulation.

• utiliser une concentration en peracides en contact avec la pâte écrue de l'ordre de 20 % en poids par rapport à la pâte sèche.
• mettre en contact les pâtes écrues avec le mélange de peracides circulant à contre courant ou dans un réacteur à lit fixe avec recirculation des acides à travers la pâte.
• réoxyder régulièrement à travers une boucle de dérivation passant dans un réacteur de contact avec de l'eau oxygénée , l'acide acétique et l'acide formique en peracides correspondants.

Another preferred embodiment will lead to:

• use a concentration of peracids in contact with the unbleached pulp of the order of 20% by weight relative to the dry pulp.
• contact the unbleached pasta with the mixture of peracids circulating against the current or in a fixed bed reactor with recirculation of acids through the paste.
• reoxidize regularly through a bypass loop passing into a contact reactor with hydrogen peroxide, acetic acid and formic acid corresponding peracids.

This last operation is made possible by a happy consequence of the process which makes the performac and peracetic acid solutions contain no or little free lignins which allows their reuse in continuous after reoxidation with hydrogen peroxide.

The contact time between the peracid solution and the whitening paste will preferably be between 1 and 3 hours.

The preferred treatment temperature may be between 60 and 90 ° C.

The bleached pulp obtained is then deacidified by vacuum drying

In a preferred embodiment, the second step proceeds by bringing the deacidified paste into contact with a basic aqueous solution at a pH of between 8 and 10, containing 1 to 4% by weight of hydrogen peroxide (calculated by compared to the treated dry pulp). The treatment temperature will preferably be between 60 ° C and 100 ° C

This operation aims to finish the bleaching operation, neutralize the last acid residues and provide, after washing with demineralized water, a stable and neutral paste, or very slightly basic.

The invention extends to neutral or slightly basic pastes obtained by this technology which are characterized by a small decrease in the degree of polymerization, lower than that observed in conventional processes.

The following description with reference to the figure 1 annexed illustrates the method of the invention in an optimal mode of implementation.

The synoptic of the figure 1 illustrates the course of bleaching operations of unbleached pasta. Some operations are known in themselves.

The originality of the process of the invention lies essentially in the production of peracids by recycling the hydro-organic phase containing organic acids and residual peracids whose hydration rate will be controlled.

This makes it possible to have in the final balance sheet a simple consumption of hydrogen peroxide in a process that is very superior in selectivity and effectiveness to a bleaching phase known per se, using directly oxygenated water which can not in a first stage reach the outstanding performance of the process of the invention.

Dried paper pulp, beforehand, so as to have a residual water content of the order of 20% are therefore brought into the presence of the hydro-organic solution containing the peracids at a hydration rate by weight of 1%. 5 to 8% by weight

After a reaction time of the order of 1 to 3 hours, the hydro-organic solution is separated by pressing pasta which already has a whiteness index greater than 65.

The pastes are deacidified in a vacuum chamber (20 to 30KP _a ) brought to a temperature of between 60 ° C and 85 ° C.

The recovered organic acids are added to the liquid phase resulting from the pressing.

It should be noted that residual hydrogen peroxide remains during this operation in the dough.

The solution of organic acids and peracids resulting from the pressing and the vacuum deacidification process is dehydrated by azeotropic distillation using a solvent such as cyclohexane (water / cyclohexane azeotrope: boiling point: 69 ° C., 8 ° C at atmospheric pressure, water content 8.5% by weight).

This distillation is intended to maintain the water concentration of the organic acids to a value of about 4% by weight, before treating with a solution of hydrogen peroxide at 50% by weight so as to increase the rate of peracids in the reactor provided for this purpose before being returned to the acid bleaching reactor.

The deacidified pastes are introduced into the second reactor and placed in the presence of a solution of hydrogen peroxide in a slightly basic medium (pH 8 to 10) (2% by weight of hydrogen peroxide at 100% relative to the dry pulp) The treatment time is between 1 and 3 hours for a temperature ranging from 60 to 100 ° C.

This operation allows the finishing of the whitening by the gain of a few points of whiteness and allows above all to neutralize the pasta before washing with demineralised water.

Neutral pastes separated by pressing are ready for use.

The washing water and that recovered during dehydration have a low organic matter content due to the presence in the bleaching phases of strong oxidants such as peracids and hydrogen peroxide.

It can be fully recycled in the production of pasta which, whatever the method used requires a certain amount of water.

The method of the invention in its optimal implementation therefore does not produce pollutant releases in a high-performance economic environment unlike current bleaching processes.

The examples which follow, used in the laboratory, illustrate the process of the invention.

Example 1 Wheat straw pasta whitening

50 grams of air-dried wheat straw are delignified in an organic acid medium under the conditions described in French Patent No. 97 13658 of October 30, 1997 . (Publication number 2,770,543 ).

When the cooking is complete, the resulting dough (25 grams of dry matter having a kappa number of 30 and a degree of polymerization of 1450) is manually pressed and brought into contact with 500 cm ³ of a solution of acetic and formic acid and peracetic and performic acids having a ratio of acetic acid + peracetic acid / formic acid + performic acid of 9/1 by volume.

The peracids are prepared by bringing the acetic and formic acids into contact with hydrogen peroxide with a concentration of at least 50% by weight (temperature 60 ° C., duration 3 hours).

The concentration of hydrogen peroxide is of the order of 0.35 moles per liter of organic acid mixture.

The water content of the unbleached pasta is less than 10% by weight.

The suspension is maintained at 60 ° C for 3 hours.

When the three hours are up, the dough is filtered, manually pressed and washed with distilled water.

Peracids and residual hydrogen peroxide are metered.

The concentration of peracids in the recovered acids is 0.2 moles per liter while this concentration in the starting acids was 0.3 moles per liter.

That of hydrogen peroxide is 0.08 moles per liter while this concentration was 0.15 moles per liter in the initial mixture of organic acids and peracids.

The recovered acids are enriched with hydrogen peroxide to serve in a new bleaching operation.

The pressed pastes, after washing with distilled water) are brought into contact with a basic solution of hydrogen peroxide at 4% by weight of sodium relative to the dry pulp (PH of the order of 10) and 2% by weight. weight of oxygenated water relative to the dry pulp}.

The liquid / solid ratio is 6/1 by weight.

After two hours of contact time at 90 ° C., the pastes are filtered, pressed and washed with distilled water and dried in air.

The kappa index is less than 1

The degree of polymerization is 1350.

The whiteness index is 88.

Example N ° 2

50 grams of straw are treated as in Example No. 1.

Bleaching of the unbleached straw paste (Kappa index 30, degree of polymerization: 1450) in an acid medium is carried out as in Example No. 1.

After filtration of the pasta and manual pressing to recover the maximum of acids, peracids and residual hydrogen peroxide, the pasta is degassed under vacuum in a rotary evaporator.

The pressure is maintained at an average value of 25 Kpa.

The temperature ranges from 60 ° C to 80 ° C from the beginning to the end of the evaporation phase.

This makes it possible to recover condensates of different concentration of organic acids and peracids.

Hydrogen peroxide is mainly found in pasta after evaporation of acids.

After this evaporation stage, the pastes are brought into contact with a sodium hydroxide solution (pH 10) with a liquid / solid ratio of 7/1.

The treatment temperature is 95 ° C.

The treatment time is 2 hours.

The kappa index obtained is less than 1.

The whiteness index is 88.

The degree of polymerization of the bleached pulp obtained is 1350.

Example N ° 3

Bleaching of raw industrial kraft pulp of softwood.

25 grams of raw industrial softwood kraft pulp (kappa number 35, degree of polymerization 1500), air dried are mixed with 100 cc of a mixture of acetic, formic, peracetic, performic acids prepared as in Example No. 1.

The peracids are prepared by bringing the acetic and formic acids into contact with hydrogen peroxide with a concentration of at least 50% by weight (temperature 60 ° C., duration 3 hours).

The concentration of hydrogen peroxide is of the order of 0.35 moles per liter of organic acid mixture.

The water content of the unbleached pasta is less than 10% by weight.

The suspension is maintained at 70 ° C for 2 hours 30 minutes.

The paste is then filtered, pressed manually and washed with distilled water.

Peracids and residual hydrogen peroxide are metered.

The concentration of peracids in the recovered acids is 0.18 moles per liter while this concentration in the starting acids was 0.3 moles per liter.

That of hydrogen peroxide is 0.07 moles per liter while this concentration was 0.15 moles per liter in the initial mixture of organic acids and peracids.

The recovered acids are enriched with hydrogen peroxide to serve in a new bleaching operation.

The pressed pastes, after washing with distilled water) are brought into contact with a basic solution of hydrogen peroxide at 4% by weight of sodium relative to the dry pulp (PH of the order of 10) and 2% by weight. weight of oxygenated water relative to the dry pulp}.

The liquid / solid ratio is 6/1 by weight.

After two hours of contact time at 85 ° C., the pastes are filtered, pressed and washed with distilled water and dried in air.

The kappa index is less than 1

The degree of polymerization of the bleached pulp obtained is 1350.

The whiteness index is 87.

Example N ° 4

Industrial Ammonium bisulfite pulp bleaching of softwoods.

25 grams of industrial softwood sulphite pulp (Kappa index 30, degree of polymerization 1550) dried in air are treated as in Example No. 3.

The degree of whiteness of the bleached pulp obtained is 90. The kappa number is less than 1.

The degree of polymerization is 1300.

Claims (14)

1. A method for bleaching raw paper pulps with organic peracids, characterized in that it is carried out in two steps at atmospheric pressure and at a temperature of less than or equal to 100°C:

   - the first step is carried out by bringing the raw pulp into contact with a mixture of peracetic acid and performic acid so that the mixture obtained has a water content of less than or equal to 15% by weight;

   - the second step is carried out by treating the bleached pulp obtained, after mechanical separation of the reaction medium obtained at the end of the first step, with a solution of sodium hydroxide and hydrogen peroxide.
2. The method as claimed in claim 1, characterized in that the peracetic acid and the performic acid are obtained by bringing a mixture of acetic acid and formic acid into contact with hydrogen peroxide having a concentration greater than 50% by weight.
3. The method as claimed in claims 1 and 2, characterized in that the acetic acid + peracetic acid/formic acid + performic acid ratio is of the order of 9/1 by volume.
4. The method as claimed in claims 1, 2 and 3, characterized in that the quantity of peracids at the beginning of the first step is greater than or equal to 20% by weight.
5. The method as claimed in claims 1, 2, 3 and 4, characterized in that the raw pulps are brought into contact with the mixture of peracids countercurrentwise with recirculation of the peracids.
6. The method as claimed in claims 1, 2, 3, 4 and 5, characterized in that the solution of peracids after the first bleaching step is enriched with peracids so as to bring the concentration of peracids to the chosen value.
7. The method as claimed in claims 1, 2, 3, 4, 5 and 6, characterized in that the pulps treated during the first step are separated from the peracids by pressing.
8. The method as claimed in claims 1, 2, 3, 4, 5, 6 and 7, characterized in that the pressed pulps are deacidified by drying under vacuum, leaving residual hydrogen peroxide in the pulp.
9. The method as claimed in claims 1, 2, 3, 4, 5, 6, 7 and 8, characterized in that the contact time between the peracids and the raw pulp is between 1 hour and 3 hours.
10. The method as claimed in claims 1, 2, 3, 4, 5, 6, 7, 8 and 9, characterized in that the treatment temperature is between 60°C and 90°C.
11. The method as claimed in claims 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10, characterized in that the second step is carried out by bringing the deacidified pulp into contact with a basic aqueous solution (pH between 8 and 10) containing 1 to 4% by weight of hydrogen peroxide (calculated relative to the dry pulp).
12. The method as claimed in claims 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 and 11, characterized in that the contact time between the pulp and the hydrogen peroxide in a basic medium is between 1 h and 3 h.
13. The method as claimed in claims 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 and 12, characterized in that the pulps obtained during the second step are, after pressing, washed with demineralized water.
14. The method as claimed in claims 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 and 13, characterized in that the water introduced, by the raw pulp, on the one hand, and the hydrogen peroxide, on the other hand, is extracted from the cycle by distillation.

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