WO1994020674A1

WO1994020674A1 - Chemical paper pulp delignification method

Info

Publication number: WO1994020674A1
Application number: PCT/EP1994/000546
Authority: WO
Inventors: Johan Devenyns; Nicholas A. Troughton; François Desprez
Original assignee: Solvay Interox S.A.
Priority date: 1993-03-02
Filing date: 1994-02-24
Publication date: 1994-09-15
Also published as: FI954113A0; AU6206094A; NO953440L; JPH08507332A; NO953440D0; BE1006881A3; EP0687322A1; ZA941468B; CA2157291A1; FI954113A; BR9405943A

Abstract

A chemical paper pulp delignification method comprising a series of chlorinated reagent-free steps, i.e. an oxygen treatment step, a pulp decontamination step using a metal ion sequestering agent, an alkaline hydrogen peroxide treatment step and a peroxyacid treatment step.

Description

Process for the delignification of a chemical paper pulp

The invention relates to a method for delignifying a chemical paper pulp.

It is known to treat unbleached chemical paper pulps obtained by cooking cellulosic materials in the presence of chemical reagents by means of a sequence of delignifying and whitening treatment steps involving the use of oxidizing chemicals. The first step of a conventional chemical pulp bleaching sequence aims to complete the delignification of the unbleached pulp as it occurs after the cooking operation. This first delignifying step is traditionally carried out by treating the unbleached pulp with chlorine in an acid medium or by a chlorine-chlorine dioxide association, in mixture or in sequence, so as to react with the residual lignin of the pulp and give rise to chlorolignins which can be extracted from the pulp by dissolving these chlorolignins in an alkaline medium in a subsequent treatment step.

For various reasons, it is useful, in certain situations, to be able to replace this first significant step with a treatment which no longer uses a chlorinated reagent.

It has already been proposed to treat a kraft pulp by a first step with oxygen followed by a treatment step with a sequestering agent of metal ions and then by a third step with alkaline hydrogen peroxide (European patent application EP -0512590 on behalf of EKA NOBEL). We have also proposed an identical sequence in which a second step is carried out using alkaline hydrogen peroxide ("Preprints of the International Pulp Bleaching Conference", "Production of Bleached Chemical Pulp in the Future", Stockholm, June 11-14 1991, vol. 3, pages 23-33, J. BASTA et al., "Reducing Levels of AOX - Part 3 - Lowering of Kappa No. Prior to CIO2 Bleaching").

The pastes obtained after implementation of these known processes are not, however, sufficiently delignified to allow replacement of the delignified pasta with chlorinated reagents. In addition, they have a lower whiteness level than pastes treated with chlorinated reagents.

The invention aims to remedy the drawbacks of the known methods by providing a method which achieves an effective delignification of the unbleached chemical pulp which makes it possible to obtain pulps having high intrinsic qualities without requiring the use of chlorinated reagents.

To this end, the invention relates to a process for the deli¬ gnification of a chemical paper pulp by means of a sequence of treatment steps free of chlorinated reagents according to which reagents chosen from oxygen are used , the sequestrants of metal ions and hydrogen peroxide, characterized in that the sequence comprises the following successive stages: 0 QP À where 0 denotes a stage of treatment with oxygen,

Q denotes a step of decontaminating the paste into its metal ions, P denotes a step of treatment with alkaline hydrogen peroxide, and

A designates a treatment step with a peroxyacid. According to the invention, by chemical paper pulp is meant the pulps having undergone a delignifying treatment in the presence of chemical reagents such as sodium sulfide in alkaline medium (kraft or sulfate cooking), sulfur dioxide or a salt metallic sulfurous acid in an acid medium (cooking with sulfite or bisulfite). Semi-chemical pastes such as those where cooking has been carried out using a sulfurous acid salt in a neutral medium (cooking with neutral sulfite also called NSSC cooking) can also be blanched by the process according to the invention, as well as pasta obtained by methods using solvents, such as 0RGAN0S0LV, ALCELL®, ORGANOCELL® and ASAM pastes described in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, vol. A18, 1991, pages 568 and 569. The invention is particularly intended for pasta which has undergone kraft cooking. All types of wood used for the production of chemical pulp are suitable for carrying out the process of the invention and, in particular those used for kraft pulp, namely softwoods such as, for example, the various species of pine and fir and hardwoods like, for example, beech, oak, eucalyptus and hornbeam.

According to the invention, the first step of the treatment sequence is an oxygen step (step 0). This oxygen step is carried out by bringing the paste into contact with gaseous oxygen at a pressure of between 20 and 1000 kPa in the presence of an alkaline compound in an amount such as the weight of alkaline compound relative to the dry dough weight is between 0.5 and 5 X. The temperature of the oxygen stage must generally be greater than 70 ° C and preferably 80 ° C. This temperature should also be usually lower than 130 ° C and preferably 120 ° C. The duration of the oxygen treatment must be sufficient for the reaction of the oxygen with the lignin contained in the paste to be complete. However, it cannot exceed this reaction time too strongly, otherwise it will cause damage to the structure of the cellulose chains in the pulp. In practice, it will be greater than 30 minutes and preferably 40 minutes. Usually it will also be less than 120 minutes and preferably 80 minutes. The treatment of the pulp with oxygen can also be done in the presence of a cellulose protective agent such as the soluble magnesium salts, organic sequestering agents such as polycarboxylic or phosphonic acids. Magnesium salts are preferred, in particular, magnesium sulfate heptahydrate used in an amount of 0.02 to 1% by weight relative to the paste dried.

The consistency of the paste in step 0 is generally not less than 8% by weight of dry matter and, preferably, not less than 10%. This consistency usually does not exceed 30% by weight of dry matter and preferably 25%.

Alternatively, step 0 can also be carried out in the presence of hydrogen peroxide (step Op). The amount of hydrogen peroxide which can be incorporated in step 0 is generally not less than 0.2 g H2O2 per 100 g of dry paste and, most often, not less than 0.5 g. Likewise, we will usually not exceed 2.5 g H2O2 per 100 g of dry paste and, most often, not 2 g.

According to the invention, the second treatment step is a step of decontaminating the paste into its metal ions

(step Q). According to the invention, step Q consists in treating the pulp with at least one sequestering agent such as an inorganic phosphate or polyphosphate, such as, for example, an alkali metal pyrophosphate or metaphosphate, a polycarboxylate or an organic aminopolycarboxylate such as , for example, tartaric, citric, gluconic, diethylenetriaminepentaacetic acid, cyclohexanediaminetetraacetic acid and their salts, poly-α-hydroxyacrylic acid and its salts or an organic polyphosphonate such as ethylenededia inetetramethylenephosphonic acid, diethylenetriaminepenta (methylenephosphonic acid) and their salts. Diethylene triaminepentaacetic acid (DTPA) has given excellent results.

Step Q may also, as a variant, consist of treatment with an acid free from a sequestrant. The term “acid” is intended to denote the anhydrides or inorganic acids such as sulfur dioxide and sulfuric, sulfurous, hydrochloric and nitric acids or their acid salts, as well as organic acids such as carboxylic or phosphonic acids or their acid salts. Sulfur dioxide or alkali or alkaline earth metal bisulphites are well suited. By bisulfite means the acid salts of sulfurous acid having the formula Me (HS03) n, in which Me symbolizes a metal atom of valence n, n being an integer equal to 1 or 2.

When a sequestrant is present, a small amount of acid can also be added in step Q.

The amount of acid to be used will depend on the type of wood and the amount of metallic impurities it contains. In general, an amount of acid will be used such that the pH of the paste is greater than about 5 and, preferably, about 5.5. Likewise, the amount of acid will often be adjusted so that the pH does not exceed 7 and preferably not 6.5. When step Q is free of sequestering agent, the pH will be adjusted so as to make the medium appreciably more acidic, that is to say, less than pH 5 and, preferably, 4.5. Generally, in order not to degrade the dough, avoid going below pH 2.0 and, preferably, below pH 2.5.

When it is present, the sequestrant is generally used in step Q in an amount less than 1.5 g of sequestrant per 100 g of dry pulp. Most often, this amount is less than 1.0 g of sequestrant per 100 g of dry pulp.

Stage Q is generally carried out at a pressure close to atmospheric pressure and at a temperature sufficient to ensure efficient consumption of the acid and / or of the sequestrant and, at the same time not too high so as not to degrade the cellulose. and not burden the energy cost of the heating means used in said step. In practice, a temperature of at least 40 ° C and preferably at least 50 ° C is suitable. Likewise, it is advantageous that the temperature does not exceed 100 ° C and preferably not 90 ° C. The best results have been obtained at around 60 ° C.

The duration of step Q must be sufficient to ensure a complete reaction. Although longer durations have no influence on the delignification rate of the dough as well as on its intrinsic resistance qualities, it is not advisable to extend the reaction time beyond that necessary at the completion of the reaction so as to limit the investment costs and the energy costs of heating the dough. In practice, the duration of the pretreatment can vary within wide limits depending on the type of equipment used, the choice of acid, the temperature and the pressure, for example from approximately 15 minutes to several hours. Times of at least 10 minutes and preferably at least 15 minutes are generally sufficient. Likewise, it is important that the pretreatment times do not exceed 60 minutes and preferably not 40 minutes. A duration of about 30 minutes has given excellent results.

Step Q is generally carried out at a paste consistency of at least 2% of dry matter and preferably at least 2.5% of dry matter. Most often, this consistency does not exceed 15 X and preferably not 10 X. The consistency of about 3% of dry matter has given excellent results. According to the invention, the third treatment step is a step with alkaline hydrogen peroxide (step P). The nature of the alkali must be such that it has good extraction efficiency for the oxidized lignin at the same time as good solubility. An example of such an alkali is sodium hydroxide in aqueous solution. The amount of alkali to be used must be sufficient to keep the pH above 10 and preferably above 11. The amount of alkali must also be adjusted to ensure complete consumption of the peroxide at the end of the reaction. In practice, amounts of alkali taken between 1 and 3 g of alkali per 100 g of dry paste are very suitable. In addition to these quantities of alca¬ li, an amount of hydrogen peroxide greater than 0.3 g H2O2 IOO g of dry paste and preferably more than 0.5 g / 100 g of dry paste will be used. The amounts of hydrogen peroxide should also generally be less than 5.0 g H2O2 IOO g of dry paste and preferably less than 4.0 g / 100 g of dry paste.

The temperature of step P must be adjusted so as to remain at least equal to 50 ° C. and preferably to 70 ° C. It should also not exceed 100 ° C and preferably not exceed 95 ° C. A temperature of 90 ° C has given excellent results.

The duration of step P must be sufficient for the bleaching reaction to be as complete as possible. However, it cannot exceed this reaction time too strongly, otherwise the demotion of the whiteness of the dough will be reduced. In practice, it will be set at a value of at least 60 minutes and, preferably, at least 90 minutes. It should also most often not exceed 600 and, preferably, 500 minutes. A combination of the temperature and time conditions of about 90 ° C and about 120 minutes has given good results.

The consistency of step P is generally chosen to be less than or equal to 40% by weight of dry matter and, preferably, 30% of dry matter. It will often not be less than 5% and preferably not less than 8%. A consistency of 10% has given good results.

An advantageous variant of the process according to the invention consists in introducing in step P gaseous oxygen, in admixture with hydrogen peroxide (step Po or Op).

According to the invention, the fourth step of the treatment sequence is a step with peroxyacid (step A). The term "peroxyacid" is intended to denote all the acids comprising in their molecule at least one perhydroxyl group -0-0-H or alternatively an ammonium or metal salt of this acid. The peroxyacids according to the invention can either belong to the family of inorganic or organic peroxyacids.

According to a first variant of the invention, the peroxyacid is an inorganic peroxyacid. The inorganic peroxyacids in accordance with the invention may contain one or more perhydroxyl groups. However, inorganic peroxyacids having only one perhydroxyl group are preferred. Examples of such inorganic peroxyacids are sulfuric, selenic, telluric, phosphoric, arsenic and silicic peroxyacids. Good results have been obtained with monoperoxysulfuric acid. When the inorganic peroxyacid is monoperoxy- sulfuric, preferably an aqueous solution of Caro acid is used which generally contains hydrogen peroxide in small quantity and a significant excess of sulfuric acid in mixture with monoperoxysulfuric acid. Aqueous solutions containing 25 to 40% by weight of monoperoxysulfuric acid, 1 to 5% by weight of hydrogen peroxide and 40 to 60% of sulfuric acid are very suitable. However, it is recommended to keep the hydrogen peroxide content low, compared to that of monoperoxysulfuric acid, preferably one for which the weight ratio peroxymonosulfuric acid / hydrogen peroxide is greater than 10, in order to preserve the mechanical properties of the treated dough. According to a second variant of the invention, the peroxyacid is an organic peroxyacid. The organic peroxyacids in accordance with the invention are selected from perforic acid and aliphatic or aromatic peroxyacids.

When the organic peroxyacid is an aliphatic peroxyacid, it is selected from peroxyacids comprising from one to three percarboxylic groups. Aliphatic peroxyacids having a single percarboxylic group generally comprise a linear or branched saturated alkyl chain of less than 11 carbon atoms and, preferably, of less than 6 carbon atoms. Examples of such peroxyacids are peroxyacetic, peroxypropanoic, peroxybutanoic and peroxypentanoic acids. Peroxy¬ acetic acid is particularly preferred because of its efficiency and the relative simplicity of its methods of preparation. The aliphatic peroxyacids comprising two and three percarboxylic groups are selected from di- and triperoxyacidic carboxylic acids comprising a linear or branched alkyl chain of less than 16 carbon atoms. In the case of diperoxyacids, it is preferred that the two percarboxylic groups substitute carbon atoms located in the alpha-omega position relative to one another. Examples of such diperoxyacids are 1,6-diperoxyhexanedioic acid, 1,8-diperoxyoctanedioic acid and 1,10-diperoxydecanedioic acid, and 1,12-diperoxydodecanedioic acid. An example of a triperoxyacid is triperoxycitric acid.

The aromatic peroxyacids are selected from those which comprise at least one peroxycarboxylic group per benzene nucleus. Preferably, the aromatic peroxyacids which have only one peroxycarbo¬ xyl group per benzene nucleus will be chosen. An example of such an acid is peroxybenzoic acid.

Another variant of the process according to the invention consists in choosing an organic peroxyacid substituted with any organic functional substituent. The term “organic functional substituent” is intended to denote a functional group such as the carbonyl group (ketone, aldehyde or carboxylic acid), the alcohol group, the groups containing nitrogen such as the nitrile, nitro, amine and amide groups, the groups containing sulfur such as the sulfo and mercapto groups. Mixtures of different inorganic and / or organic peroxyacids are also well suited.

The peroxyacid can indifferently be used in the form of a solution of peroxyacid or alternatively in the form of a solution of an ammonium salt, of an alkali metal or of an alkaline earth metal of this peroxyacid. By solution is meant a solution in water or in an organic solvent. Mixtures of organic solvents are also suitable for dissolving peroxyacids in accordance with the invention, as are mixtures of water with one or more organic solvents miscible with water. Aqueous solutions are preferred.

The amount of peroxyacid to be used in step A can vary over a wide range. It depends on the type of wood used and on the effectiveness of the preceding cooking and deli¬ gnification treatments. In practice, an amount of peroxyacid is generally used which is not less than 0.2 g of H 2 O 2 equivalent per 100 g of dry paste and, preferably, not less than 0.5 g / 100 g. dry dough. The term “H2O2 equivalent” is intended to denote the quantity of hydrogen peroxide which contains the same amount of active oxygen. Usually, an amount of peroxyacid will not exceed 3 g of H2O2 equivalent per 100 g of dry paste and, preferably, 2 g of H2O2 equivalent / 100 g dry paste. Stage A of peroxyacid treatment can also be carried out in the presence of one or more additives compatible with peroxyacids such as, for example, surfactants, stabilizers of peroxyacid, inhibitors of depolymerization of cellulosic fibers and anti-corrosion agents. Examples of such additives are anionic surfactants, nonionic surfactants, soluble salts of Mg and sequestrants of metal ions. As a general rule, when they are present, the amount of these additives used does not exceed 3 g per 100 g of dry paste and, preferably, does not exceed 2.5 g per 100 g of dry paste.

The peroxyacid treatment step A according to the invention can be carried out over a wide range of temperatures. In general, the peroxyacid treatment will be carried out at a temperature of at least 40 ° C and preferably at least 60 ° C. Similarly, this temperature generally does not exceed 100 ° C and preferably not 95 ^β C. A temperature of 90 ° C led to good results.

Generally, the treatment is carried out with organic peroxyacid at atmospheric pressure. The duration of this treatment depends on the temperature and the essence of the wood used to prepare the dough, as well as the efficiency of the cooking and the steps that preceded it. Times of between about 60 minutes and about 500 minutes are fine. A duration of 120 minutes has given excellent results. The pH of step A of peroxyacid treatment can range from acidic to alkaline pH. However, moderately acidic pHs are preferred. In practice, it is preferable to fix the initial pH at a value of at least 3.5. An initial pH of 5 will generally not be exceeded. An initial pH of 4 has led to good results.

The paste consistency of step A of peroxy treatment acid is generally chosen less than or equal to 40% by weight of dry matter and, preferably, to 30 of dry matter. It will often not be less than 5 X and, preferably, not less than 8 X. A consistency of 10% has given good results.

It may alternatively be advantageous to precede the sequence of treatment steps according to the invention by at least one washing or one decontaminating pretreatment step using an acidic aqueous solution and / or a solution of a sequestrant of metal ions (step Q). The purpose of this washing or this step is to extract from the pulp the impurities present in the form of metal ions which are detrimental to the proper conduct of the bleaching and / or delignification operations. All the inorganic or organic acids used in aqueous solution, alone or as a mixture are suitable. Strong inorganic acids such as, for example, sulfuric acid or hydrochloric acid are well suited.

It is advantageous for the acid decontaminating washing or pretreatment to be carried out in the presence of an agent sequestering metal ions. To this end, organic acids from the class of aminopolycarboxylic or aminopolyphosphonic acids or their alkali metal salts as well as mixtures of these acids or their salts with the strong inorganic acids mentioned above are particularly suitable. Examples of suitable aminopolycarboxylic acids are diethylenetriaminepentaacetic acid, ethylenediaminetetraacetic acid (EDTA), cyclohexanediaminetetraacetic acid (CDTA) and nitrilotriacetic acid (NTA). Diethylenetriaminepentaacetic acid (DTPA) is preferred. Examples of aminopolyphosphonic acids are diethylenetriaminepenta acid (methylenephosphonic acid)

(DTPMPA), ethylenediaminetetra (methylenephosphonic acid) (EDTMPA), cyclohexanediaminetetramethylenephosphonic acid (CDTMPA) and nitrilotri acid (methylenephosphonic). DTPMPA is preferred. The operating conditions of the decontaminating pretreatment are not critical. They must be determined in each particular case depending on the type of pulp and the equipment in which the treatment is carried out. In general, the choice of acid and the quantity used should be fixed to impose on the medium a pH of less than 7, for example between approximately 1 and approximately 6.5. Especially advantageous pHs are those between about 3.0 and about 6.0. Temperature and pressure are not critical, with room temperature and atmospheric pressure generally suitable. The duration of the pretreatment can vary within wide proportions depending on the type of equipment used, the choice of acid, the temperature and the pressure, for example from about 15 minutes to several hours.

According to the invention, it is also possible to have the sequence of treatment steps followed by an additional delignification step of alkaline extraction using a hydroxide or a carbonate of an alkali or alkaline earth metal ( step E). It can also be followed by a step with hydrogen peroxide (step P). It can also be followed by an alkaline extraction step reinforced with hydrogen peroxide (step Ep), with oxygen (step Eo), or by the two reagents oxygen and hydrogen peroxide at the same time ( step Eop).

When the aim is to obtain a pulp of paper deli¬ gified and bleached up to high levels of whiteness, it is also possible, as a variant, to complete the delignification process according to the invention by or several bleaching stages in which known reagents are selected, selected from hydrogen peroxide, chlorine dioxide and sodium hypochlorite.

The process according to the invention applies to the delignification of any kind of chemical pulp. It is well suited for delignifying kraft and sulfite pastes. It is particularly well suited for processing kraft pasta.

The examples which follow are given for the purpose of illustrating the invention, without however limiting its scope. Examples 1R and 2R (not in accordance with the invention)

A sample of baked softwood pulp kraft (initial whiteness 27.9 ° IS0 measured according to standard ISO 2470 (1977), kappa index 26.7 measured according to standard SCAN Cl-59 (1959) and degree of polymerization 1680 expressed in number of glucosic units and measured according to the SCAN C15-62 standard (1962) was treated according to a sequence of 2 stages starting with a stage with gaseous oxygen under pressure (stage 0), followed by a stage with diethylenetriaminepentaacetic acid (DTPA) in acid medium (step Q).

The operating conditions of the first two steps common to the 2 examples 1R and 2R were as follows: Step 1: step with oxygen (step 0): pressure, kPa: 600 NaOH content, g / 100 g dry paste: 4 content MgSθ .7H2Û, g / 100g dry paste: 0.5 temperature, degrees C: 120 duration, min: 60 consistency, X by weight of dry matter: 12 2nd stage: DTPA stage (stage Q): DTPA content, g / lOOg dry paste: 0.5 temperature, degrees C: 60 duration, min: 30 consistency, X by weight of dry matter: 3

A treatment was then carried out using the same quantity of hydrogen peroxide applied in a single step (example 1R) or in two successive steps (example 2R).

Example 1R Example 2R 3rd step: step with hydrogen peroxide (step P): H2O2 content »g / lOOg dry paste: 4 2 NaOH content, g / lOOg dry paste: 3.2 2.2 temperature, degrees C : 90 90 duration, min: 120 120 consistency, X by weight of dry matter: 10 10

4th stage: hydrogen peroxide stage (stage P): H2O2 content, g / lOOg dry paste: - 2 NaOH content, g / lOOg dry paste: - 2.2 temperature, degrees C: - 90 duration, min: - 120 consistency, X by weight of dry matter: - 10

At the end of the treatment, the kappa index of the paste obtained was measured as well as its degree of polymerization and its whiteness.

The results obtained are given in the following table:

Example Whiteness Index DP

Final kappa no. ° IS0

1R 76.5 6.3 1050

2R 77.6 5.7 990

Examples 3 and 4: (in accordance with the invention)

Example 2R was repeated, however replacing the hydrogen peroxide in the fourth step with peroxyacetic acid (Example 3) or with monoperoxysulfuric acid (Caro acid, Example 4). The operating conditions of temperature, duration and consistency were the same as in steps 3 and 4 of Example 2R. The initial pH of the fourth step was 4 in each of the two examples 3 and 4. The amounts of reagent used were as follows:

Example 3 Example 4 CH3-CO3H content, g / 100g dry paste: 2.24 H2SO5 content, g / lOOg dry paste: - 3.35 DTPMPNay content, g / lOOg dry paste: 1.0 MgSθ content. 7H2θ, g / lOOg dry paste: - 0.5 where DTPMPNay symbolizes the heptasodium salt of diethylene triaminepenta acid (methylene phosphonic).

The contents of 2.24 X of CH3-CO3H and 3.35 X of H S0 ₅ represent an amount of active oxygen equivalent to 1% of hydrogen peroxide, that is to say half of what was introduced in the fourth step of Example 2R.

The Caro acid used in Example 4 was a solution aqueous containing 28.3 X by weight of monoperoxysulfuric acid, 1.1 X by weight of hydrogen peroxide and 57.8 X by weight of sulfuric acid.

The results obtained were:

Example Nature of final pH Whiteness DP index

Final peracid number final kappa "ISO final

3 CH3C03H 3.5 75.8 3.4 1090

4 H2S05 2.5 71.0 3.4 1010

Examples 5 and 6: (in accordance with the invention)

The paste obtained in Examples 3 and 4 was subjected to a fifth step of alkaline extraction in the presence of 1.7 X of NaOH and 1 X of hydrogen peroxide.

The results obtained were as follows:

Example Whiteness Index DP

Final No. kappa final

"Final ISO

5 85.1 2.1 1020

6 83.8 2.4 910

The total amount of oxidizing reagents used in the sequences of Examples 1R, 2R, 5 and 6 corresponds to the same amount of active oxygen equivalent to 4 g of hydrogen peroxide per 100 g of dry pulp. It is seen that the blocks 5 and 6 according to the invention make it possible to obtain a dough which is better delimited, of higher whiteness and of degree of polymerization comparable to those of the blocks O Q P and O Q P P of the prior art.

Claims

R E V E N D I C A T I O N S

1 - Process for the delignification of a chemical paper pulp by means of a sequence of treatment steps free of chlorinated reagents according to which reagents chosen from oxygen, sequestrants of metal ions and peroxide are used of hydrogen, characterized in that said sequence comprises the following successive steps:

0 Q P At where 0 denotes an oxygen treatment step,

Q designates a step for decontaminating the paste into its metal ions P designates a treatment step with alkaline hydrogen peroxide À designates a treatment step with a peroxyacid.

2 - Process according to claim 1, characterized in that the peroxyacid is an inorganic peroxyacid.

3 - Process according to claim 1, characterized in that the peroxyacid is an organic peroxyacid.

4 - Process according to claim 2, characterized in that the inorganic peroxyacid is monoperoxysulfuric acid.

5 - Process according to claim 3, characterized in that the organic peroxyacid is peroxyacetic acid.

6 - Method according to any one of claims 1 to 5, characterized in that the first step with oxygen is carried out in the presence of hydrogen peroxide.

7 - Method according to any one of claims 1 to 6, characterized in that one performs before the oxygen treatment step a preliminary treatment step with a sequestrant of metal ions.

8 - Method according to any one of claims 1 to 7, characterized in that one follows the sequence of steps of treatment by an alkaline extraction step using a hydroxide or carbonate of an alkali or alkaline earth metal.

9 - Process according to claim 8, characterized in that the alkaline extraction step is carried out in the presence of oxygen and / or hydrogen peroxide.

10 - Process according to any one of claims 1 to 9, characterized in that the sequence of treatment steps delignifying is followed by one or more bleaching steps in which known reagents are selected selected from peroxide hydrogen, chlorine dioxide and sodium hypochlorite.

11 - Application of the method according to any one of claims 1 to 10 to the delignification and bleaching of kraft pulp and sulphite pulps.