EP0728238A1

EP0728238A1 - Method of bleaching chemical paper pulp

Info

Publication number: EP0728238A1
Application number: EP94931025A
Authority: EP
Inventors: François Desprez; Johan Devenyns; Nicholas A. Troughton
Original assignee: Solvay Interox SA
Current assignee: Solvay Chimie SA
Priority date: 1993-11-10
Filing date: 1994-10-28
Publication date: 1996-08-28
Anticipated expiration: 2014-10-28
Also published as: AU7993994A; ATE175738T1; ES2129142T5; NO961889D0; FI961975A0; NO961889L; MA23369A1; CA2176246A1; ES2129142T3; DE69416000D1; FI961975A; EP0728238B1; BR9408027A; JPH10500178A; BE1007757A3; ZA948563B; WO1995013420A1; EP0728238B2

Abstract

Method of bleaching chemical paper pulp having undergone extensive cooking, involving a sequence of processing steps carried out without chlorine reagents. The pulp is subjected to decontamination from its transition metals and then to a step using alkaline hydrogen peroxide.

Description

Process for bleaching a chemical pulp

The invention relates to a process for bleaching a chemical 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.

For about twenty years, it has been proposed to precede the first stage of treatment by means of chlorine or the combination of chlorine and chlorine dioxide by a stage with gaseous oxygen in an alkaline medium. (KIRK-OTHMER Encyclopedia of

Chemical Technology Third Edition Vol. 19, New York 1982, page 415, 3rd paragraph and page 416, 1st and 2nd paragraphs). The delignification rate obtained by this oxygen treatment is however not sufficient if the aim is to produce chemical pulps of high whiteness whose mechanical properties do not are not degraded.

It has been proposed (Gellerstedt G et al., "Chemical Aspects of Hydrogen Peroxide Bleaching. Part II. The Bleaching of Kraft Pulps.", Journal of Wood Chemistry and Technology, Vol. 2, No. 3, 1982, pages 231 to 250 and patent application EP-A1-0456626) to whiten kraft pastes using hydrogen peroxide according to a QP or 0 QP sequence (the acronym Q representing a step with a sequestering agent of metal ions, the acronym P a step at hydrogen peroxide and acronym 0 a step with oxygen). These methods also necessarily include additional steps using chlorinated reagents such as chlorine dioxide.

The invention remedies these drawbacks of known methods by providing a new method for delignification and / or bleaching of chemical paper pulps which makes it possible to achieve high levels of whiteness without degrading the cellulose too strongly and without using chlorinated reagents.

To this end, the invention relates to a process for bleaching a chemical pulp, having undergone extensive cooking, by means of a sequence of treatment steps free of chlorinated reagents, according to which the sequence comprises the steps following steps, carried out in order:

Q P where the acronym Q represents a step of decontaminating the pulp into its transition metals and the acronym P represents a step with alkaline hydrogen peroxide.

By chemical paper pulp is meant the pulp 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 metal salt of sulfurous acid in an acid medium (cooking with sulfite or bisulfite). It is also intended to denote thus the chemical-mechanical pastes and the semi-chemical pastes, for example those where the cooking was carried out using a sulfurous acid salt in a neutral medium (cooking with neutral sulfite also called cooking NSSC) which can also be laundered by the process according to the invention, as well as the pastes obtained by processes using solvents, such as, for example, the 0RGAN0S0LV, ALCELL ^R , 0RGAN0CELL ^R 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 or sulphite cooking.

All types of wood used for the production of chemical pulps are suitable for carrying out the process of the invention and, in particular those used for kraft and sulphite pulps, namely softwoods such as, for example, the various species of pine and fir and deciduous woods like, for example, beech, oak, eucalyptus and hornbeam. According to the invention, it is important that the dough has undergone extensive cooking. The term "extended cooking" is intended to denote any process for cooking chemical pulp mentioned above in which the flow and recycling of the various cooking reagents and liquors, as well as the physical parameters of the process, are regulated so as to modifying the process in order to obtain an improved delignification rate while maintaining the viscosity of the cellulose at an acceptable level. An example of such an extensive cooking process for kraft pasta is described in the work by M. J. KOCUREK "Pulp and Paper Manufac¬ ture", Vol. 5, Alkaline Pulping, 3rd Edition, McGraw-Hill, New-York, 1989, page 122, paragraph 3 (Modifications for low lignin pulping). Pasta which has undergone extensive cooking generally has a kappa index 30 to 50 X lower than that of the same pasta which has undergone normal cooking. Kraft pasta which has undergone extensive cooking most often has a kappa number of 20 or less in the case of softwoods and 14 or less in the case of hardwoods.

According to the invention, the first step is a step of decontamination of the pulp into its transition metals (step Q). According to the invention, step Q consists in treating the pulp with at least one sequestering agent such as a phosphate or polyphosphate inorganic, such as, for example, an alkali metal pyrophosphate or meta-phosphate, a polycarboxylate or an organic aminopolycarboxylate such as, for example, tartaric, citric, gluconic, ethylenediaminetetraacetic, diethylene triaminepentaacetic, cyclohexanediaminetetraacetic and their salts, poly-αhydroxyacrylic acid and its salts or an organic polyphosphonate such as ethylenediaminetetramethylphosphonic acids, diethylenetriaminepenta (methylenephosphonic), cyclohexanediaminetetramethylenephosphonic acids and their salts. Ethylenediaminetetraacetic acid (EDTA) and diethylenetriamine-pentacetic acid (DTPA) have given excellent results.

In addition to the sequestrant, a small amount of acid can also be added in step Q.

Stage Q can also, as a variant, consist of a treatment with an acid free from a sequestrant, followed by the addition of soluble magnesium salt in an amount such that the weight ratio of the amount of Mg to that of Mn present. in the dough is at least 30. Generally, amounts of Mg corresponding to 1 to 4 g MgSθ .7H2θ / 100 g of dry dough are sufficient. The term “acid” is intended to denote the anhydrides or inorganic acids such as sulfur dioxide and sulfuric, sulfurous, hydrochloric, phosphoric and nitric acids or their acid salts, as well as organic acids such as carboxylic or phosphonic acids or their salts acids. Sulfuric acid, sulfur dioxide or alkali or alkaline earth metal bisulfites are well suited. By bisul¬ fite is intended to denote the acid salts of sulfurous acid corresponding to the formula Me (HSθ3) n, in which Me symbolizes a metal atom of valence n, n being an integer having the value 1 or 2.

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 dough is about 5 or more and preferably about 5.5 or more. 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, not exceeding pH 5 and, preferably not 4.5. Generally, in order not to degrade the dough, we should avoid going below pH 1.5 and, preferably, below pH 2.0.

The sequestrant is generally used in step Q in an amount not exceeding 1.5 g of active material per 100 g of dry pulp. Most often, this amount does not exceed 1.0 g of sequestering agent 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 good efficiency of the acid and / or of the sequestrant and, at the same time not too high so as not to degrade the cell. ¬ lose 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 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 strength qualities, it is not advisable to extend the reaction time beyond that necessary for 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, the pretreatment times generally do not exceed 60 minutes and preferably not 40 minutes. A duration of about 30 minutes has given excellent results.

Stage Q is generally carried out at a paste consistency of at least 2% dry matter and, preferably, at least 2.5 X dry matter. In most cases, this consistency does not exceed 15% and preferably not more than 10%. The consistency of approximately 3% of dry matter has given excellent results.

According to the invention, the second 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 quantity of alkali to be used must be sufficient to maintain the pH above 10 and preferably above 11. The quantity of alkali must also be adjusted to ensure sufficient consumption of the peroxide at the end of the reaction. In practice, amounts of alkali of 1 to 4 g of alkali per 100 g of dry pulp are very suitable. In addition to these amounts of alkali, a quantity of hydrogen peroxide of at least 0.3 g H2O2 / 100 g of dry paste and preferably at least 0.5 g / 100 will be used. g dry dough. The amounts of hydrogen peroxide should also generally not exceed 5.0 g H2O2 100 g of dry paste and preferably not 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 must also not exceed 150 ° C and preferably not exceed 135 ^β C. Temperatures of 90 ° C and 120 ° C have given excellent results.

An interesting variant of the process according to the invention consists in carrying out step P of the sequence at a high temperature of at least 100 ° C. According to this variant, the temperature of this step P is preferably at least 110 ° C. It generally does not exceed 140 ° C and preferably not 135 ° C.

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. She will have to also most often do not exceed 600 and preferably 500 minutes.

The consistency of step P is generally chosen to be less than or equal to 50% by weight of dry matter and, preferably, 40% of dry matter. It will often not be less than 5 X and preferably not less than 8 X.

The consistency of step P can, as a variant, be advantageously chosen from high values of 25% by weight of dry matter or more. A consistency of 30 X has given excellent results.

In another variant of the process according to the invention, the sequence can be preceded by 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 step should generally be above 70 ° C and preferably 80 ° C. This temperature should also usually be less 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 deterioration in the structure of the cellulose chains of the pulp. In practice, it will be at least 30 minutes and, preferably, at least 40 minutes. Usually it will not exceed 120 minutes and preferably not 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 dry paste. The consistency of paste in step 0 is generally not less than 8 X by weight of dry matter and, preferably, not less than 10 X. This consistency usually does not exceed 30 X by weight of dry matter and preferably 25 X.

Alternatively, step 0 can also be carried out in the presence of hydrogen peroxide (step Op). The quantity 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, Q 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.

Similarly, step P can also be reinforced by the presence of gaseous oxygen (step Eop). In this case, the oxygen pressure used will most often be at least 20 kPa and at most 1000 kPa.

In another variant of the process according to the invention, an additional enzymatic step consisting in treating the dough with at least one enzyme (step X) can be incorporated at any point in the sequence of treatment steps. This enzymatic treatment can also be carried out before or after the optional oxygen pretreatment step.

The term “enzyme” is intended to denote any enzyme capable of facilitating the delignification, by the stages of treatment subsequent to the stage of treatment with the enzyme, of an unbleached chemical paper pulp originating from the cooking operation. or a chemical pulp that has already been subjected to an oxygen treatment step.

Preferably, an alkalophilic enzyme will be used, that is to say an enzyme whose maximum efficiency lies in the alkaline pH zone, and very particularly at a pH of 7.5 and above.

A category of enzymes well suited to the process according to the invention are hemicellulases. These enzymes are capable of reacting with hemicelluloses on which the lignin present in the dough is fixed. Preferably, the hemicellulases used in the process according to the invention are xylanases, that is to say hemicellulolytic enzymes capable of cutting the xylan bonds which constitute a major part of the interface between lignin and the rest carbohydrates. An example of a xylanase in accordance with the process according to the invention is 1,4-β-D-xylan xylannohydrolase, EC 3.2.1.8.

The xylanases preferred in the methods according to the invention can be of various origins. In particular, they may have been secreted by a wide range of bacteria and fungi.

Xylanases of bacterial origin are particularly interesting. Among the xylanases of bacterial origin, the xylanases secreted by bacteria of the genus Bacillus have given good results.

Xylanases derived from bacteria of the genus Bacillus and of the species pumilus have given excellent results. Of these, xylanases from Bacillus pumilus PRL B12 are particularly interesting. The xylanases of Bacillus pumilus PRL B12 in accordance with the invention can come directly from a strain of Bacillus pumilus PRL B12 or else from a host strain of a different microorganism which has been genetically manipulated beforehand to express the genes coding for degradation xylans from Bacillus pumilus PRL B12.

Preferably, a purified xylanase will be used which does not contain other enzymes. In particular, it is preferred that the xylanase according to the process according to the invention does not contain cellulase so as not to destroy the polymeric cellulose chains of the pulp.

An interesting variant of the process according to the invention consists in carrying out the enzymatic step X in the presence of at least one sequestering agent of metal ions. The metal ion sequestrants can advantageously be chosen from the sequestrants suitable for step Q which are described above. It is also possible to perform step Q in the presence at least one enzyme. In this case, an enzyme can be used which conforms to those described above. It is also possible to combine the incorporation of enzyme in step Q with the addition of an enzymatic step at any point in the sequence. Another variant of the process according to the invention consists in interposing an oxidizing step between step Q and step P. All the oxidizing chemical reagents are suitable for carrying out this oxidizing step. Among the oxidizing reagents known and usually used for delignifying and bleaching paper pulps, it is preferred to use reagents which do not contain chlorine. Peroxyacids and ozone are particularly preferred.

The term “peroxyacids” is intended to denote all the acids comprising in their molecule at least one perhydroxyl group _o_θ-H or also an ammonium salt or any metal of this acid. The peroxyacids according to the invention can indif¬ ferently belong to the family of inorganic or organic peroxyacids.

According to a variant of the invention, the peroxyacid is an inorganic peroxyacid. The inorganic peroxyacids according to the invention can comprise one or more perhydro¬ xyl 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. According to another variant of the invention, the peroxyacid is an organic peroxyacid. The organic peroxyacids in accordance with the invention are selected from performic 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 comprising a single percarboxylic group generally comprise a linear or branched saturated alkyl chain of less than 11 carbon atoms and, preferably 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 designate 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 the oxidizing step can vary over a wide range. It depends on the type of wood used and the effectiveness of the preceding cooking and delignification treatments. In practice, a quantity of peroxyacid is generally used which is not less than 0.2 g of H2O2 equivalent per 100 g of dry paste and, preferably, not less than 0.5 g / 100 g dry paste. Equivalent to H2O2. is intended to denote the amount of hydrogen peroxide which contains an identical 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.

The peroxyacid treatment stage can also be carried out in the presence of one or more additives compatible with peroxyacids such as, for example, surfactants, stabilizers of peroxyacid, depolymerization inhibitors 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 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. Likewise, this temperature generally does not exceed 100 ° C and preferably not 95 ° C. A temperature of 90 ° C has 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 about 60 minutes to about 500 minutes are fine. A duration of 120 minutes has given excellent results.

The pH of the peroxyacid treatment stage can be in the range of both acidic and alkaline pHs. 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 the peroxyacid treatment step is generally chosen to be 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% and preferably not less than 8 X. A consistency of 10 X has given good results. According to another variant of the invention, the ozone treatment step consists in subjecting the dough to a gas stream consisting of a mixture of ozone and oxygen coming from an electric ozone generator supplied with dry oxygen gas. In the laboratory, a generator is advantageously used whose flow rate is from 50 to 100 l / hour and, preferably, from 70 to

90 1 / hour. The amount of ozone used can easily be adjusted by varying the duration of sweeping of the ozone / oxygen mixture stream on the dough. Generally, durations of 20 to 80 minutes are sufficient to use an amount of ozone of 0.4 to 2 g per 100 g of dry paste. On an industrial scale, we will manage to regulate the flow rate of the ozone generators and the duration of the treatment to fix the quantity of ozone used on the dough at values similar to those which we realize in the laboratory. . Thanks to the technique of continuous work, it will be possible in an industrial environment, to substantially reduce the duration of treatment until it drops to durations of the order of 1 minute.

The ozone treatment is preferably carried out in an acid medium. A pH of 0.5 to 5 is very suitable and preferably 1.5 to 4. A pH of 2 to 3 obtained by subjecting the dough to a 30 minute pre-conditioning treatment using an H2SO solution or SO2 at a rate of 0.5 X by weight of SO2 relative to the dry pulp and with a consistency of 3% dry matter has given very good results.

The consistency of the ozone treatment step will be selected from the range of 0.5 to 45% of dry matter and, preferably, from 0.5 to 3% (in the case of low-consistency apparatus) or between 10 to 15 X (case of medium consistency equipment). A consistency of 35% dry matter has given excellent results on a laboratory scale. The temperature of the ozone treatment stage must remain low, otherwise the mechanical properties of the treated pulp will be seriously degraded. This temperature is generally from 2 to 50 ° C and preferably from 10 to 35 ° C. Most often, the ozone treatment is simply carried out at room temperature.

An interesting variant of the process according to the invention consists in preceeding the ozone treatment by a mechanical treatment for opening the dough (called "fluffing" in the English-speaking literature) intended to increase the contact surface of the paste with ozone. This operation is particularly useful when the consistency of the paste during the ozone treatment is at least 15% of dry matter.

The process according to the invention applies to the bleaching of any kind of chemical pulp which has undergone extensive cooking. 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 to 3R (not in accordance with the invention)

A sample of baked softwood pulp normal kraft (initial whiteness 27.9 ° IS0 measured according to ISO 2470-1977 (F), kappa index 26.7 measured according to standard

SCAN Cl-59 and degree of polymerization 1680 expressed in number of glucosic units and measured according to standard SCAN C15-62) was treated according to a sequence of 3 steps 0 Q P under the following conditions:

1st stage: oxygen stage (stage 0): pressure, kPa: 600 NaOH content, g / lOOg dry paste: 4 0 MgSθ content. 7H2θ, g / lOOg dry paste: 0.5 temperature, degrees C: 120 duration, min: 60 consistency, X by weight of dry matter: 12

2nd stage: EDTA stage (stage Q): 5 EDTA content, g / lOOg dry paste: 0.4 temperature, degrees C: 70 duration, min: 45 consistency, X by weight of dry matter: 10

3rd stage: hydrogen peroxide stage (stage P): Q H2O2 content »g / 100g dry paste: example 1R 5.7 example 2R 3 example 3R 3 NaOH content, g / 100g dry paste: example 1R 1, 6 example 2R 2.0 example 3R 1.3 MgSO content .7H2θ, g / 100g dry paste 1.0 temperature, degrees C: example 1R 90 example 2R 120 example 3R 120 duration, min 240 consistency,% by weight of material dryer example 1R 30 example 2R 10 example 3R 30 where DTMPANa represents the heptasodium salt of diethylene triaminepenta (methylene phosphonic) acid.

At the end of each processing step, the dough underwent a washing with demineralized water at room temperature.

After treatment, the kappa number, the whiteness of the dough and its degree of polymerization were determined. The results are shown in the table below.

Example Whiteness Degree Index

Kappa polymerization final number

° IS0 final

1R 85.0 4.91 970

2R 77.1 5.99 1130

3R 84.7 4.76 850

We see that despite the large amount of hydrogen peroxide used, it was not possible to exceed

85 ° IS0 whiteness.

Example 4: (according to the invention)

Another softwood pulp sample having undergone extensive kraft cooking and industrial oxygen bleaching treatment (initial whiteness 58.5 ° IS0 measured according to ISO 2470-1977 (F), kappa index 3.7 measured according to SCAN standard Cl-59 and degree of polymerization 800 expressed in number of glucosic units and measured according to standard SCAN C15-62) was treated according to a sequence of 2 steps QP under the following conditions: step: step at EDTA (step Q): EDTA content, g / 100g dry paste: 0.4 H2SO4 content (for pH 5): 0.34 temperature, degrees C: 70 duration, min: 45 consistency, X by weight of dry matter : 10 2nd stage: hydrogen peroxide stage (stage P): H2O2 content, g / lOOg dry paste: 3.0 NaOH content, g / lOOg dry paste: 1.3 MgSθ4.7H2θ content, g / 100g dry paste: 1.0 Na silicate content 38 ° Bé, g / lOOg dry paste: 3.0 temperature, degrees C: 120 duration, min: 240 consistency, X by weight of dry matter: 30

At the end of each treatment step, the paste was washed with demineralized water at room temperature.

At the end of the sequence, after treatment, the kappa index, the whiteness of the dough and its degree of polymerization were determined.

The results obtained were:

Example Whiteness Index Degree of final No. kappa polymerization

° IS0 final

4 88.0 1.6 710

Examples 5 to 7: (in accordance with the invention)

The same sample of pulp prebleached with oxygen as in Example 4 was bleached according to the Q Paa P sequence, the acronym Paa denoting a step with peracetic acid. The operating conditions carried out were as follows: step: step with EDTA (step Q): EDTA content, g / lOOg dry paste: 0.4 H2SO4 content (for pH 5): 0.34 temperature, degrees C: 70 duration, min: 45 consistency, X by weight of dry matter: 10

2nd stage: peracetic acid stage (Paa stage): CH3CO3H content, g / lOOg dry paste: example 5: 1.0 example 6: 2.0 example 7: 3.0 DTMPANay content, g / lOOg dry paste: 0.1 MgSO4.7H2θ content, g / lOOg dry paste: 0.05 temperature, degrees C: 80 duration, min: 180 consistency, X by weight of dry matter: 10 where DTMPANay represents the heptasodium salt of diethylenetriaminepenta acid (methylenephosphonic). 3rd stage: hydrogen peroxide stage (stage P): H2O2 content, g / 100g dry paste: 2.0 NaOH content, g / 100g dry paste: example 5: 1.0 example 6: 1.2 example 7: 1.6 MgSθ4-7H2θ content, g / lOOg dry paste: 1.0 Na silicate content 38 ° Bé, g / lOOg dry paste: 3.0 temperature, degrees C: 90 duration, min: 240 consistency , X by weight of dry matter: 30

The results obtained were as follows:

Example Whiteness Degree Index

Kappa polymerization final number

° IS0 final

5 86.4 1.7 760

6 90.5 0.9 690

7 91.4 0.7 680

Examples 8 to 10: (in accordance with the invention)

The same sample of pulp prebleached with oxygen as in Examples 5 to 8 was bleached according to the sequence QC ^ P. The operating conditions carried out were as follows: step: step with EDTA (step Q): content in EDTA, g / lOOg dry paste 0.4 H2SO4 content (for pH 5) 0.34 temperature, degrees C 70 duration, min 45 consistency, X by weight of dry matter: 10 2nd step: Caro acid step (step C _A ): H2SO5 content, g / lOOg dry paste: example 8: 1.5 example 9: 3.0 example 10 : 4.5 NaOH content, g / lOOg dry paste: example 8: 2.86 example 9: 5.76 example 10: 8.76 content of DTMPANa, g / lOOg of dry paste: 0.1 content of MgSθ4- 7H2θ, g / lOOg dry paste: 0.05 temperature, degrees C: 80 duration, min: 180 consistency, X by weight of dry matter: 10 where DTMPANa represents the heptasodium salt of diethylene-triaminepenta acid (methylenephosphonic acid). 3rd stage: hydrogen peroxide stage (stage P): H2O2 content, g / lOOg dry paste: 2.0 NaOH content, g / lOOg dry paste: example 8: 1.3 example 9: 1.4 example 10: 1.7 MgSθ4.7H2θ content, g / lOOg dry paste: 1.0 Na silicate content 38 ° Bé, g / lOOg dry paste: 3.0 temperature, degrees C: 90 duration, min: 240 consistency , X by weight of dry matter: 30

At the end of each processing step, the paste was washed with demineralized water at room temperature. At the end of the sequence, after treatment, the kappa index, the whiteness of the dough and its degree of polymerization were determined. The results obtained were as follows: Example Whiteness Degree Index

Kappa polymerization final number

° IS0 final

8 89.4 1.0 700

9 90.6 0.9 710

10 91.6 0.7 690

Claims

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

1 - Process for bleaching a chemical paper pulp, having undergone extensive cooking, by means of a sequence of treatment steps free of chlorinated reagents, characterized in that the sequence comprises the following steps, carried out in the 'order:

2 - Method according to claim 1, characterized in that the pulp subjected to the sequence of treatment steps is selected from kraft pulp of resinous wood with kappa index of 20 or less and kraft pulp of hardwood kappa index of 14 or less.

3 - Process according to claim 1 or 2, characterized in that step P of the sequence is carried out at a temperature of 100 ° C or more.

4 - Method according to any one of claims 1 to 3, characterized in that a step with gaseous oxygen precedes the sequence.

5 - Method according to any one of claims 1 to 4, characterized in that step P is carried out at a paste consistency of at least 25 X by weight of dry matter.

6 - Method according to any one of claims 1 to 5, characterized in that step Q consists of a treatment with an acid free of a sequestrant, followed by the addition of soluble salt of Mg in an amount such that the weight ratio of the amount of Mg to that of Mn present in the dough either 30 or more.

7 - Method according to any one of claims 1 to 6, characterized in that one interposes between step Q and step P an oxidizing step using a reagent selected from peroxyacids and ozone.

8 - Process according to any one of claims 1 to 7, characterized in that step Q is carried out in the presence of an enzyme.

9 - Process according to any one of claims 1 to 8, characterized in that an additional step is incorporated with an enzyme at any point of the sequence.

10 - Application of the method according to any one of claims 1 to 9 to the bleaching of kraft pulp or sulfite.