EP0712453B1 - Method for ozone bleaching paper pulp - Google Patents

Abstract

A method for ozone bleaching unbleached chemical paper pulp in a single ozone stage and in the presence of a tertiary alcohol including four to eight carbon atoms which is fed into the reaction medium at a temperature of - 5 °C to 80 °C prior to the actual ozone treatment stage, whereby the gas-liquid transfer, the exchange potentiality, the lignin oxidation selectivity and the ozone efficiency are improved, and the consitency of the treated pulp undergoing ozone treatment is between 6 and 60 %.

Description

The present invention relates to a process for bleaching paper pulps providing an improvement in the performance of the delignification stage involved in the bleaching sequences.

It is known in the prior art that the bleaching of chemical paper pulps, obtained by cooking lignocellulosic materials, is carried out by means of a sequence of delignifying treatment steps, using oxidizing chemicals. The first step of a conventional bleaching sequence is traditionally carried out by treating the unbleached pulp with chlorine in an acid medium or by a chlorine-chlorine dioxide combination, so as to react with the residual lignin of the pulp, to give chloro -lignins. The latter can be extracted from the dough by solubilization in an alkaline medium during a subsequent treatment step.

The bleaching sequence involves a series of stages corresponding to different chemical treatments which can be coded as follows:

The bleaching sequences currently used up to now comprise five to six stages among which the treatment with molecular chlorine intervenes in the first stage. The most common sequences are: CEHDED and CEDED (for a more complete description of conventional bleaching procedures, we can refer to GA SMOOK, Handbook for Pulp and Paper Technologists, 1982, 153-173, TAPPI, Atlanta, GA).

These bleaching processes provide pulps whose degree of whiteness (Bl), Kappa number (IK), as well as the degree of polymerization (DP) are satisfactory on an industrial level.

To date, existing legislation or legislation in the process of promulgation warns the paper industries to no longer use chlorinated reagents for this type of operation in order to eliminate any risk of pollution due to undesirable by-products in effluents.

Certain by-products of bleaching sequences using chlorine or its derivatives induce toxic compounds (AOX), some of which can be mutagenic, carcinogenic, bioaccumulative. These by-products (AOX) come from the reaction of chlorine or chlorine derivatives with organic matter from the degradation of lignin.

In a first phase, it is known in the prior art that the paper industries have attempted to limit the use of chlorine or its derivatives by replacing, at least partially, the first stage of treatment with chlorine or of the chlorine-chlorine dioxide association, by a pre-delignification stage using oxygen in an alkaline medium.

However, oxygen has been shown to be a less selective delignifying agent than elemental chlorine. In conventional oxygen delignification methods, the reduction in the Kappa index remains limited because it is accompanied by a significant attack on the cellulosic fibers. This is why the pre-treatment of the oxygen pulp is usually followed by a chlorine bleaching stage to obtain a completely bleached pulp, using a lower amount of chlorine. This is how the quantity of AOX produced per tonne of pulp could be reduced significantly. This effort, although substantial, is not sufficient to bring these processes into conformity with the standards or prescriptions decreed by the large industrialized countries.

In recent years, to avoid the use of chlorinated bleaching agents, many research centers have been trying to develop sequences preferably using oxygenated reagents alone or in combination with an acceptable chlorinated oxidant in reduced quantity (carbon dioxide chlorine). The pasta thus produced cannot claim the label "totally chlorine free" (TCF), since chlorine dioxide is still used in these treatments.

It is also known in the prior art to bleach chemical pulp (kraft or sulfite) using sequences using only oxygenated reagents. Oxygenated products conventionally used include oxygen, ozone, hydrogen peroxide, used in combined sequences, that is to say in successive stages without prejudging the relative position of one of the oxidation stages.

Due to its exceptional oxidizing properties, ozone appears to be the ideal delignification reagent for bleaching lignocellulosic fibers, even if until today its relatively high price has limited the development of industrial processes.

Conventionally, described in the prior art, bleaching sequences of the OZEP, OZP, ZOP type are found. But, in this case, either the final dough lacks whiteness, or its degree of polymerization reaches unacceptable levels. , resulting in performance reductions or falls in mechanical characteristics.

This phenomenon is due in particular to the non-selectivity of ozone, which although rapidly attacking the woody structures, correlatively degrades the cellulosic or hemicellulosic structures, which materializes the significant drop in the degree of polymerization. Furthermore, ozone is extremely sensitive to operating conditions such as pH and temperature. Its chemical stability implies perfect control of the ozonation parameters.

According to the literature, the attack of ozone on cellulose is mainly caused by radicals formed during the oxidation of lignin or resulting from the decomposition of ozone in the reaction medium.

Although there is no rule formalizing the relative position of stages involving oxygenated reagents in bleaching sequences, it is generally accepted that these sequences begin with an oxygen stage, the development of which has been the subject of numerous publications. . Generally, it is also accepted that the ozone stage follows the oxygen stage with or without an intermediate stage of washing, alkaline extraction or washing type in the presence of a sequestering agent or a complexing agent.

The purpose of using a sequestrant or a complexing agent is to inhibit the action of the metal cations contained in the paste, said metal cations being able to induce, in the presence of certain reagents, free radicals whose chemical action is well known for parasitic attacks from cellulose and hemicelluloses.

If you want to properly whiten an unbleached chemical pulp using exclusively oxygenated reagents, it is necessary to have specific agents of the protective type or transfer agents capable of protecting the cellulose and hemicelluloses and / or of making the reactions more selective. lignin oxidation.

This type of protective agent or transfer agent must make it possible to avoid and control the drastic drop in the degree of polymerization during the said bleaching stages. Simultaneously with the use of such additives, the operating conditions (choice of physico-chemical parameters, etc.) must be in perfect adequacy in order to optimize the result to be obtained.

In the prior art, certain organic reagents have been tested with more or less success: they are in particular products such as acetone, acetic acid, formic acid, oxalic acid, methyl acetate, acetic anhydride, nitromethane, methanol, ethanol, methyl ethyl ketone, dimethylformamide, dimethyl sulfoxide, propan-2-ol .... (see N. Liebergott, B Van Lierop, A.Skothos, Tappi Journal January 1992, 145-152 and February 1992, 117-124 as well as MV Byrd Jr., JS Gratzl, RP Singh , Tappi Journal March 1992, 207-213).

US-A-4 229 252 reports the use, during an ozone bleaching stage, of alcohols introduced in small quantity into the reaction medium, this in low consistency, ie for 0.01 to 4 , 9% dry matter, suspended in water.

Without taking into account a first analysis of the results obtained in the prior art, it is possible to formulate a certain number of remarks and in particular to specify that many of these products can only be used according to a particular process. treatment.

• la faible consistance (FC), qualifie un stade de blanchiment dans lequel la masse de matière sèche en suspension dans l'eau se situe industriellement entre 3 et 5 % (US-A-4 229 252 par exemple).
• la moyenne consistance (MC), qualifie une suspension dont la teneur en matière sèche se situe industriellement entre 6 et 20 % en masse.
• la haute consistance (HC), caractérise une suspension dont la teneur en matière sèche se situe industriellement entre 20 et 60 % en masse.

It is known that the processes for bleaching paper pulps can be implemented according to three different areas of consistency:

• the low consistency (FC), describes a bleaching stage in which the mass of dry matter suspended in water is industrially between 3 and 5% (US-A-4,229,252 for example).
• the medium consistency (MC), qualifies a suspension whose dry matter content is industrially between 6 and 20% by mass.
• the high consistency (HC), characterizes a suspension whose dry matter content is between 20 and 60% by mass.

The additives used in the prior art have preferably been used for one, or even two, of the consistencies defined above, consistencies where their action has proved to be preponderant. These are generally low and medium consistencies.

The Applicant has discovered that it is possible to use ozone to carry out bleaching sequences in the presence of judiciously chosen additives, and under operating conditions such that the drop in the degree of polymerization is limited, the value obtained being very little diminished, compared to the results previously obtained.

The additives used in the process according to the invention as well as the physicochemical parameters of the ozonation reaction are chosen so as to make it possible to whiten a pulp in a significant manner, essentially using an ozone stage.

Consequently, the invention relates to a process for bleaching unbleached pulp of the chemical type using ozone, characterized in that it uses only an ozone stage exclusively, in the presence of a tertiary alcohol. , comprising from four to eight carbon atoms which is introduced into the reaction medium maintained at a temperature between -5 ° C and 80 ° C, prior to the actual ozonation stage, in order to improve the gas-liquid transfer , the exchange potential, the selectivity of lignin oxidation and the efficiency of ozone, the consistency of the pulp subjected to ozonation being between 6 and 60%.

According to the present invention, said additive is an alcohol comprising from four to eight carbon atoms, preferably four carbon atoms.

According to one embodiment of the method according to the invention, said alcohol is a tertiary alcohol comprising from four to six carbon atoms.

According to the invention, said additive is introduced according to a mass percentage, relative to the dry paste, of between 0.01 and 300%, preferably between 0.1 and 100% and better still between 1 and 10%.

Experience shows that temperature has a predominant influence on the selectivity of the reaction in the presence of additives. According to the invention, the temperature of the reaction medium during ozonation is between - 5 ° C and + 80 ° C, preferably between 0 ° C and 60 ° C and better between 0 ° C and 25 ° C.

The addition, according to the invention, of small quantities of an oxygenated organic product significantly improves the selectivity of the delignification stage of the paper pulp with ozone. In addition, if one operates below ambient temperature, in the presence of such an additive, this delignification becomes very effective.

To date, it has never been shown that it is possible to whiten a chemical pulp by using only an ozone stage, without considerably degrading the pulp thus treated.

In order to demonstrate the surprising effects brought by its invention, the applicant has carried out tests by placing itself in extreme conditions which will not be those of industrial exploitation, namely an almost total delignification of a chemical pulp as well as its bleaching. to acceptable whiteness levels using only an ozone stage. If the procedures of the process according to the invention, as well as the relative products, are conventionally used after a oxygen stage, the results obtained will only be better and will be correlatively accompanied by a reduction in the amount of ozone necessary for this implementation.

An example of implementation of the process according to the invention has been given below, the reagents and / or the physico-chemical parameters of the reactions being given either under direct operating conditions or under oxygen pre-treatment conditions. the pulp.

• un indice Kappa (IK) mesuré selon la norme NF 1S0 302-1981 (F) de : 23,
• un degré de polymérisation (DP) mesuré selon la norme TAPPI T 230 om-82 de : 1610,
• un degré de blancheur (B1) mesuré selon la norme NF Q 50-012 de : 35.

The test was carried out from an unbleached kraft paste comprising no preliminary treatment and characterized by:

• a Kappa index (IK) measured according to standard NF 1S0 302-1981 (F) of: 23,
• a degree of polymerization (DP) measured according to the TAPPI T 230 om-82 standard of: 1610,
• a degree of whiteness (B1) measured according to standard NF Q 50-012 of: 35.

• Une quantité de pâte sèche est mise à tremper dans une solution aqueuse d'acide sulfurique de pH 3 (prétraitement acide).
• Après désintégration de la pâte suivant norme ACPP C-10P, la pâte est séparée de son milieu aqueux puis lavée à l'eau.
• La consistance de ladite pâte est ajustée à 35 % en masse de pâte sèche dans le milieu aqueux.
• La suspension ainsi obtenue est ozonée en maintenant constante la température du milieu réactionnel.
• Après réaction et compte tenu du bilan matière effectué durant cette opération, il est possible de déterminer le taux de traitement réel de la pâte (TT O₃) correspondant à la masse d'ozone ayant effectivement réagi avec la pâte, ramenée à la masse de pâte sèche présente durant la réaction.
• Après réaction la pâte traitée par l'ozone est lavée puis soumise aux tests de détermination de l'indice Kappa, du degré de polymérisation et du degré de blancheur.

During the test, of which the example below gives an account, the following procedure was generally followed with regard to the stages of ozonation.

• A quantity of dry paste is put to soak in an aqueous solution of sulfuric acid of pH 3 (acid pretreatment).
• After disintegration of the paste according to ACPP C-10P standard, the paste is separated from its aqueous medium and then washed with water.
• The consistency of said paste is adjusted to 35% by mass of dry paste in the aqueous medium.
• The suspension thus obtained is ozonated while keeping the temperature of the reaction medium constant.
• After reaction and taking into account the material balance carried out during this operation, it is possible to determine the actual treatment rate of the pulp (TT O ₃ ) corresponding to the mass of ozone having actually reacted with the pulp, reduced to the mass of dry paste present during the reaction.
• After reaction, the ozone-treated paste is washed and then subjected to tests to determine the Kappa number, the degree of polymerization and the degree of whiteness.

The table below gives a summary of the evolution of the characteristics (IK, Bl, DP) of an unbleached kraft pulp after treatment, at room temperature, with ozone (flow rate of 0.3 Nm ³ / h, a concentration of 100 g / Nm ³ charge of 1.5% by mass relative to the dry paste), in the presence of a tertiary alcohol, in particular t-BuOH (2-methyl-propan-2-ol).

Initially this paste has an IK of 23, a DP of 1610, a B1 of 35. It also has a consistency of 35%, and it has undergone an acid pretreatment pH = 2.5. Board Amount of t-BuOH (% by mass) IK BI DP 0 9.6 56.4 1210 5 9 59.3 1220 10 8.5 60.2 1240 20 8 61.3 1250 40 7.4 62.2 1240

A study of this table shows that the consistency (35%) of the mixture has a favorable effect on the overall results; the choice as an additive of t-BuOH (2-methyl-propan-2-ol) verifies its role as transfer agent, that is to say that it ensures better transfer of the ozone gas to the phase liquid, all conditions being equal. The transfer being improved, larger masses of ozone are present in the reaction medium, the exchange potential increases simultaneously and the accessibility of the fibers is thereby facilitated. It lowers the Kappa index (IK) while increasing the degree of whiteness (B1) for an almost constant degree of polymerization (DP).

As it appears in curve 1, the influence of the additive, in this case t-BuOH, is predominant on the selectivity of ozone.

Curve 2 illustrates the evolution of the Kappa index as a function of the percentage of tertiary alcohol (t-BuOH) used.

Claims (10)

1. Process for bleaching unbleached paper pulps of the chemical type with the help of ozone, characterized in that it uses only one exclusively ozone stage, in the presence of a tertiary alcohol comprising from four to eight carbon atoms, which is introduced into the reaction medium maintained at a temperature between -5°C and 80°C, prior to the ozonization stage proper, for the purpose of improving the gas-liquid transfer, the exchange potential, the selectivity of the oxidizing of the lignin and the efficacy of the ozone, the consistency of the pulp subjected to the ozonization being between 6 and 60%.
2. Bleaching process according to claim 1, characterized in that said tertiary alcohol comprises from four to six carbon atoms.
3. Bleaching process according to claim 1, characterized in that said tertiary alcohol comprises four carbon atoms.
4. Bleaching process according to any one of the preceding claims, characterized in that, after reaction, the alcohol is recycled and reused without special treatment in the process.
5. Bleaching process according to any one of the preceding claims, characterized in that the ozone is introduced by successive steps into the reaction medium.
6. Bleaching process according to any one of the preceding claims, characterized in that said alcohol is associated with another oxygenated organic product.
7. Bleaching process according to any one of the preceding claims, characterized in that said alcohol is introduced in a percentage by mass, relative to the dry pulp, of between 0.01 and 300%, preferably between 0.1 and 100%, and more specifically between 1 and 10%.
8. Bleaching process according to any one of the preceding claims, characterized in that the temperature of the reaction medium during the ozonization lies between 0°C and 60°C and preferably between 0°C and 25°C.
9. Bleaching process according to any one of the preceding claims, characterized in that the ozone stage is used after a prior delignification stage using oxygen.
10. Bleaching process according to claim 9, characterized in that said ozone stage is carried out after a pretreatment of the pulp, performed after said predelignification using oxygen.

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