EP0514609A1 - Process for hydrogen peroxide bleaching of high-yield pulps - Google Patents

Abstract

High-yield pulp bleaching in which said pulp is subjected to a pretreatment using a complexing agent for metal ions followed by washing, then to a treatment using hydrogen peroxide in an alkaline medium, characterized in that between the start and the end of said treatment, an additional addition of hydrogen peroxide and alkaline agent is carried out, without interrupting the treatment, at a time when 60% to 85% of the initial quantity of hydrogen peroxide were consumed, the additional amount of hydrogen peroxide being equal to or less than said initial amount.

Description

The present invention relates to a process for bleaching high-yield pasta using hydrogen peroxide in an alkaline medium.

By high-yield pulp is meant here the pulp obtained with a yield by weight in the dry state relative to the starting lignocellulosic material, such as wood generally in the form of chips, counted in the dry state, which is greater than approximately 85% and often at least 90%.

Such pastes are produced by defibering the starting material, wood chips most often, by means of a grinding wheel or a disc shredder, associated or not with a chemical and / or heat treatment.

They include those that the industry describes as mechanical. thermomechanics, chemothermothermics.

• le premier consiste à faire agir sur la pâte un agent réducteur, classiquement l'hydrosulfite, dans des conditions douces et en milieu neutre ou légèrement acide. Il en résuite un blanchiment partiel qui pour certaines applications est néanmoins suffisant.
• le deuxième procédé consiste à faire agir sur la pâte le peroxyde d'hydrogène en milieu alcalin. Le blanchiment obtenu est plus important que dans le procédé précédent ce qui explique qu'aujourd'hui, pour satisfaire les exigences de qualités des papiers, le procédé au peroxyde soit de plus en plus utilisé.

Two types of process are used today to carry out their bleaching:

• the first consists in causing a reducing agent, conventionally hydrosulfite, to act on the paste, under mild conditions and in a neutral or slightly acid medium. This results in partial bleaching, which for certain applications is nevertheless sufficient.
• the second method consists in causing the hydrogen peroxide to act on the paste in an alkaline medium. The whitening obtained is more important than in the previous process, which explains why today, to meet the quality requirements of the papers, the peroxide process is used more and more.

However, hydrogen peroxide is an expensive product whose stability decreases as the pH increases. If it is admitted that the pH zone in which bleaching by hydrogen peroxide is normally carried out extends from approximately 9 to approximately 11 as recalls "The bleaching of pulp", TAPPI Press, SINGH ed., Atlanta USA, 1970, p.227, research in this context of an ever more efficient use of hydrogen peroxide is of permanent interest for the industry.

Thus it has been proposed to involve hydrogen peroxide in at least two distinct bleaching phases.

For example, in patent application WO-84.02366, a first phase is proposed in which the alkalinity conditions are abnormally strong with a view to improving the mechanical quality of the fibers and a second phase under normal alkalinity conditions to correct the bleaching failure resulting from the first phase.

In TAPPI Journal, March 1987, pages 119 et seq. D. LACHENAL also describes a two-phase process but in which an amount of sodium hydroxide is much greater than that normally accepted for a first phase is implemented in the second phase.

Finally french patent ^No. 2537177 recommends maintaining conditions uniform as possible throughout the laundering by performing a succession of distinct phases.

In each of the proposed processes, a phase is distinguished from the following in that the products which have served in the first, in particular hydrogen peroxide and alkaline agents such as sodium hydroxide, are eliminated from the pulp, at least for the most part, for example 90%, normally by washing and / or pressing said paste. The energy and investment costs which result therefrom oppose the advantage of improving the efficiency of use of hydrogen peroxide.

The process of the present invention comprises two phases of action of hydrogen peroxide on the dough but does not require the above intermediate operation.

It is a high-yield pulp bleaching process in which the bleaching pulp is subjected to a pretreatment using a metal ion complexing agent followed by washing, then to a treatment using hydrogen peroxide in an alkaline medium, characterized in that between the start and the end of said treatment there is an additional addition to the paste of hydrogen peroxide and alkaline agent without interruption of the treatment, at a time when 60% to 85% of the initial amount of hydrogen peroxide was consumed, the additional amount of hydrogen peroxide added being equal to or less than said initial amount.

In the definition given above of the process of the invention, as in all that follows, the term “paste” means the assembly consisting of lignocellulosic material counted in the dry state and of the liquid present with it.

In all that follows, unless specified or obvious, the quantities of material are expressed in percent by weight relative to the weight of dough taken in the dry state.

The pretreatment using a metal ion complexing agent is generally carried out at a temperature normally lower than 100 ° C but higher than 20 ° C, for example between 50 ° C and 95 ° C, at a consistency, percentage of dough in the dry state, between 5% and 30% and most often between 10% and 15%, for a period which depends on the other parameters but which is most often between 5 minutes and 2 hours, at a pH between 4 and 8. The complexing agent is used at a rate generally from 0.1% to 1%. most often at 0.2% to 0.5% and it is most often chosen from sodium tripolyphosphate, sodium tetrapyrophosphate, sodium salts of citric acids, nitrilotriacetic, ethylenediaminetetraacetic, diethylenetriaminepentaacetic (DTPA).

By washing following the pretreatment using a complexing agent is understood the operation which consists in eliminating more or less completely, and here as efficiently as possible, the liquid present in the dough, for example by pressing that -this on filter or in a dilution-pressing sequence, repeated or not, in which the dilution is very generally ensured by water. By efficiency is meant, expressed in percent, the degree of elimination of the liquid present in the dough before washing. In the process of the invention, the efficiency targeted for washing is at least 75%.

The amount of hydrogen peroxide H₂O₂ engaged at the start of treatment of the pretreated dough and washed as above, can be between 0.5% and 6%, preferably between 3% and 5%.

The additional amount of H₂O₂ added during treatment with H₂O₂ in an alkaline medium according to the process of the invention is often between 0.25% and 3% and is in any case and as already mentioned, equal to or less than the amount of H₂O₂ engaged at the start of treatment.

The alkaline agent mainly chosen to accompany H₂O₂ is sodium hydroxide NaOH.

The amount of NaOH added to the pulp with H₂O₂ at the start of treatment, as well as the additional amount of NaOH added during treatment with additional H₂O₂, essentially depend on the amount of H₂O₂ and also on the nature of the wood and other parameters of treatment. The first of these two quantities of NaOH can be between 0.5% and 6%, most often between 1.5% and 4%; the second can also be between 0.5% and 6% and preferably between 1% and 3%.

As in the known processes, the treatment of the paste with H₂O₂ in an alkaline medium can take place in the presence of a stabilizer of H₂O₂, such as for example sodium silicate, as well as an agent complexing ions metallic, such as DTPA in the form of the sodium salt. For example, from 1% to 4% of an aqueous sodium silicate solution of density 1.33 and from 0% to 0.5% of an aqueous solution at 40% by weight of sodium salt of DTPA can be added with H₂O₂ and NaOH at the start of treatment, from 0% to 4% of said silicate solution and from 0% to 0.5% of said DTPA salt solution can be added during bleaching with the additional amounts of H₂O₂ and NaOH added to dough.

The treatment using H₂O₂ in an alkaline medium is normally carried out at a temperature between 60 ° C and 90 ° C, between 65 ° C and 75 ° C most often, at the highest possible consistency, for example between 15% and 45%. The duration of action of H₂O₂ added with NaOH at the start of treatment, like that of H₂O₂ added with NaOH during treatment according to the process of the invention, mainly depend on the temperature and the amount of H₂O₂; each of them is most often between 1 hour and 6 hours.

It has been found that it is often advantageous to add additional quantities of H supplémentairesO₂ and NaOH during the treatment when about 70% to 80% of the quantity of H₂O₂ initially used is consumed.

The following examples, given as an indication but not limiting, either illustrate the invention, or are given for comparison.

In each of these examples, the pulp subjected to the bleaching process is a mechanical grinding pulp formed, by weight, of 75% of softwood and 25% of hardwood, and whose degree of whiteness is equal to 63 , 2 ° ISO. In each of these examples again, the dough is subjected before treatment with H₂O₂ in an alkaline medium, to a pretreatment with a consistency of 10% using 0.5% of an aqueous solution at 40% by weight of salt sodium DTPA, at 90 ° C for 15 minutes followed by a washing efficiency of 90%.

The treatment using H₂O₂ in an alkaline medium was each time carried out in the absence of complexing agent for metal ions, at a temperature equal to 70 ° C., at a consistency successively equal to 20% before the additional additions H₂O₂ and NaOH were made, 15% after these additions were made.

The amounts of H₂O₂, NaOH, sodium silicate solution of density 1.33 designated by silicate, expressed as usual here in% by weight relative to the dough in the dry state are given in the single table below in which also appear the degrees of whiteness of the dough which has been subjected either to the process of the invention, or to a known process comprising, unlike the process according to the invention, a washing designated by L, of equal effectiveness at 90%, before additional addition of at least H₂O₂ and NaOH.

The duration of the bleaching treatment was each 7 hours.

Claims (8)

Process for bleaching high-yield paper pulps in which the pulp to be bleached is pretreated with a metal ion complexing agent followed by washing, followed by treatment with peroxide hydrogen in an alkaline medium, characterized in that between the start and the end of the said treatment an additional addition to the paste of hydrogen peroxide and alkaline agent is carried out without interruption of the treatment, at a time when 60% 85% of the initial amount of hydrogen peroxide was consumed, the additional amount of hydrogen peroxide added being equal to or less than said initial amount. A method according to claim 1, characterized in that the additional addition of hydrogen peroxide and alkaline agent takes place when 70% to 80% of the initial amount of hydrogen peroxide has been consumed. Method according to one of claims 1 and 2, characterized in that the initial amount of hydrogen peroxide is between 0.5% and 6% by weight relative to the dough in the dry state. Process according to claim 3, characterized in that the initial quantity of hydrogen peroxide is between 3% and 5%. Method according to one of claims 1 to 4, characterized in that the additional quantity of hydrogen peroxide added is between 0.25% and 3% by weight relative to the dough in the dry state. Method according to one of claims 1 to 5, characterized in that the alkaline agent which accompanies the hydrogen peroxide is sodium hydroxide used in an amount of between 0.5% and 6% by weight compared to the dough in the dry state at the start of treatment as an amount initially committed as during treatment as an additional amount added. Process according to claim 6, characterized in that the quantity of sodium hydroxide used at the start of treatment is between 1.5% and 4%. Method according to one of claims 6 and 7, characterized in that the additional amount of sodium hydroxide added is between 1% and 3% by weight relative to the dough in the dry state.