EP0728238B2 - Method of bleaching chemical paper pulp - Google Patents

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

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 presence of chemical reagents by means of a sequence of delignifying and whitening treatment steps involving the use of oxidizing chemicals. The first step in a classic whitening sequence of chemical pulp aims to complete the delignification of the unbleached pulp as it appears after the operation Cooking. This first significant step is traditionally carried out by treating the unbleached dough 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 from the pulp and give rise to chlorolignins which can be extracted from the pulp by solubilization of 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 step significant by 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 of the combination chlorine - chlorine dioxide by a step 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). However, the delignification rate obtained by this oxygen treatment is not not sufficient if the aim is to produce high whiteness chemical pulps with mechanical properties are not degraded.

Patent application EP-A-0 511 695 describes a process for the delignification and the bleaching of a dough for chemical paper according to which the pulp is treated with an acid at pH 1 to 6, it is treated with a compound containing an alkaline earth metal at a pH of 1 to 7, then with a chlorine-free bleach, for example hydrogen peroxide (claims 1 and 2). Patent application EP-A-0 512 590 also describes a process for delignification and bleaching of a chemical paper pulp, According to this process the profile is modified in trace of metals in the dough by treatment with a complexing agent at a pH between 3.1 and 9.0 and bleached then the dough with a hydroene peroxide step followed by an ozone step (claim 2, page 7.) However in these two prior art, no extensive cooking is used, but normal cooking. Cooking extensive gives a higher degree of delignification than in normal cooking.

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 231-250 and patent application EP-A1-0456626) to whiten kraft pulps using hydrogen peroxide according to a Q P or O Q P sequence (the acronym Q represents a step with a sequestering agent of metal ions, the acronym P a step with hydrogen peroxide and the acronym Q a step with oxygen). These methods also necessarily include additional steps using chlorinated reagents such as chlorine dioxide.

The invention overcomes these disadvantages of known methods, by providing a new delignification method. and / or bleaching of chemical pulp which allows high whiteness levels to be achieved without degrade the cellulose too much and without using chlorinated reagents.

To this end, the invention relates to a process for bleaching a chemical pulp of softwood or hardwood, which has undergone extensive cooking and has a kappa number of 20 or less after cooking in the case of softwoods and 14 or less in the case of hardwoods, by means of a sequence of stages of treatment free of chlorinated reagents, comprising the following steps, carried out in order:

Q P where the acronym Q represents a step in decontaminating the pulp into its transition metals and the acronym P represents the final step with alkaline hydrogen peroxide performed at a temperature above 110 ° C and at a dough consistency greater than 25% by weight of dry matter.

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 bleached by the process according to the invention, as well as the pastes obtained by processes using solvents, such as, for example, the ORGANOSOLV, ALCELL ^R , ORGANOCELL ^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 pulp are suitable for the implementation of process of the invention and, in particular those used for kraft and sulfite pulps, namely resinous woods such as, for example, the various species of pine and fir and hardwoods such as, for example, beech, oak, eucalyptus and charm.

According to the invention, it is important that the dough has undergone extensive cuisso. By extensive cooking ("extended cooking"), it is intended to denote any process for cooking chemical pulp mentioned above in which the flows and recycling are regulated. of various cooking reagents and liquors, as well as the physical parameters of the process, so as to modify the process for obtaining an improved delignification rate while maintaining the viscosity of the cellulose at a level acceptable. An example of such an extensive cooking process for kraft pasta is described in the work by M. J. KOCUREK "Pulp and Paper Manufacture", Vol. 5, Alkaline Pulping, 3rd Edition, McGraw-Hill, New-York, 1989, page 122, paragraph 3 (Modifications for low lignin pulping). Pasta that has undergone extensive cooking generally has a kappa index 30 to 50% lower than that of the same dough having undergone normal cooking. Kraft pasta that has undergone extensive cooking has a kappa number of 20 or less in the case of wood softwood and 14 or less in the case of hardwoods.

According to the invention, the first step is a step of decontaminating 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 inorganic polyphosphate, such as, for example, an alkali metal pyrophosphate or metaphosphate, a polycarboxylate or an organic aminopolycarboxylate such as, for example, tartaric, citric, gluconic acids, ethylenediaminetetraacetic, diethylenetriaminepentaacetic, cyclohexanediaminetetraacetic and their salts, acid poly-α-hydroxyacrylic and its salts or an organic polyphosphonate such as ethylenediaminetetramethylenephosphonic acids, diethylenetriaminepenta (methylenephosphonic), cyclohexanediaminetetramethylenephosphonic and their salts. Ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) have given excellent results.

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

Step Q can also, as a variant, consist of a treatment with an acid free of 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 MgSO ₄ .7H ₂ O / 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 bisulfite is intended to denote the acid salts of sulfurous acid corresponding to the formula Me (HSO ₃ ) 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 around 5 or more and, preferably about 5.5 or more. Similarly, we will often adjust the amount of acid so that the pH does not exceed not 7 and preferably not 6.5. When step Q is free of sequestering agents, the pH will be adjusted so as to make the medium substantially more acid, that is to say, not exceeding pH 5 and preferably not 4.5. Generally, we in order not to degrade the dough, 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 dough. Most often, this amount does not exceed 1.0 g of sequestering agent per 100 g of dough dried.

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 sequestrant and, at the same time not too high so as not to degrade the cellulose and not to 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. Of even, 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 resistance qualities, it 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 widely depending on the type of equipment used, the choice of acid, the temperature and pressure, for example from about 15 minutes to several hours. Duration of at least 10 minutes and, preferably at least 15 minutes are generally sufficient. Likewise, the pretreatment times do not exceed generally not 60 minutes and preferably not 40 minutes. A duration of about 30 minutes gave excellent results.

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

According to the invention, the second treatment step represents the final step and 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. Be used in addition to these amounts of alkali a quantity of hydrogen peroxide of at least 0.3 g H ₂ O _2/100 g of dry pulp and preferably at least 0.5 g / 100 g of dry dough. It is also that the amounts of hydrogen peroxide does not generally exceed 5.0 g H ₂ O _2/100 g of dry pulp and preferably 4,0 g / 100 g of dry pulp.

The temperature of step P must be adjusted so as to remain above 110 ° C. It should also not exceed 150 ° C and preferably not exceed 135 ° C. A temperature of 120 ° C has given excellent results.

The temperature of this stage P 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 it will lead to a demotion of the whiteness of the dough. 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.

The consistency of step P is generally chosen to be less than or equal to 50% by weight of dry matter and, preferably 40% dry matter.

A consistency of 30% has given excellent results.

In another variant of the process according to the invention, the sequence can be preceded by an oxygen step (step O). This oxygen step is carried out by bringing the paste into contact with gaseous oxygen under a pressure between 20 and 1000 kPa in the presence of an alkaline compound in an amount such as the weight of compound alkaline relative to the weight of dry dough is between 0.5 and 5%.

The temperature of the oxygen step should generally be above 70 ° C and preferably 80 ° C. he it is also suitable that this temperature is usually 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 dough is complete. However, it cannot exceed this reaction time too strongly, otherwise it will induce degradations 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 soluble magnesium salts, organic sequestering agents such as polycarboxylic or phosphonic acids. Magnesium salts are preferred, in particular, the magnesium sulfate heptahydrate used to 0.02 to 1% by weight based on the dry paste.

The paste consistency during step Q 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 material dry and preferably 25%.

As a variant, step O can also be carried out in the presence of hydrogen peroxide (step Op). The quantity of hydrogen peroxide which can be incorporated in stage O is generally not less than 0.2 g H ₂ O ₂ per 100 g of dry paste and, most often, not less than 0, 5 g. Likewise, we will usually not exceed 2.5 g H ₂ O ₂ 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 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, it is possible to incorporate at any point before the step end of the treatment step sequence an additional enzymatic step consisting in treating the dough with at least one enzyme (step X). This enzymatic treatment can also be carried out before or after the pretreatment step to possible oxygen.

The term “enzyme” is intended to denote any enzyme capable of facilitating delignification, through the processing steps subsequent to the step of treatment with the enzyme, of an unbleached chemical pulp from the operation of baking or chemical paper 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 effectiveness is in the alkaline pH zone, and especially 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 able to react with hemicelluloses on which the lignin present in the paste 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 links which constitute a major part of the interface between lignin and the rest of the carbohydrates. An example of xylanase in accordance with the process according to the invention is 1,4-β-D-xylane xylanohydrolase, EC 3.2.1.8.

The xylanases preferred in the methods according to the invention can be of various origins. They can particular to 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 also 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 chains cellulose pulp polymers.

An interesting variant of the process according to the invention consists in carrying out the enzymatic step X in the presence at least one sequestrant of metal ions. The sequestrants of metal ions can advantageously be chosen from the sequestrants suitable for step Q which are described above.

It is also possible to carry out step Q in the presence of at least one enzyme. In this case, we can use an enzyme conforming 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 the step P. All 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 no 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-O-H or an ammonium salt or any metal of this acid. Peroxyacids according to the invention can either belong to the family of inorganic or organic peroxyacids.

According to a variant of the invention, the peroxyacid is an inorganic peroxyacid. Inorganic peroxyacids in accordance with the invention may contain one or more perhydroxyl groups. Inorganic peroxyacids having only one perhydroxyl group are however preferred. Examples of such inorganic peroxyacids are sulfuric, selenic, telluric, phosphoric, arsenic and silicic peroxyacids. Good results have have been obtained with monoperoxysulfuric acid.

According to another variant of the invention, the peroxyacid is an organic peroxyacid. 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 the peroxyacids comprising from one to three percarboxylic groups.

Aliphatic peroxyacids comprising a single percarboxylic group generally comprise a chain saturated linear or branched alkyl of less than 11 carbon atoms and preferably less than 6 carbon atoms carbon. Examples of such peroxyacids are peroxyacetic, peroxypropanoic, peroxybutanoic acids and peroxypentanoic. Peroxyacetic acid is particularly preferred because of its effectiveness and the relative simplicity of its preparation methods.

The aliphatic peroxyacids comprising two and three percarboxylic groups are selected from the di- and triperoxycarboxylic 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 atoms of carbon located in alpha-omega position relative to each other. Examples of such diperoxyacids are acid 1,6-diperoxyhexanedioic acid, 1,8-diperoxyoctanedioic acid and 1,10-diperoxydecanedioic acid, and acid 1.12-diperoxydodecanedioic. An example of a triperoxyacid is triperoxycitric acid.

The aromatic peroxyacids are selected from those which comprise at least one peroxycarboxylic group by benzene nucleus. Preferably, we will choose aromatic peroxyacids which contain only one only peroxycarboxylic group per benzene nucleus. 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 by any organic functional substitute. By organic functional substituent is meant 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 than 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 peroxyacid solution or alternatively under form of a solution of an ammonium salt, an alkali metal or an alkaline earth metal of this peroxyacid. By solution is intended to denote a solution in water or in an organic solvent. Mixtures of organic solvents also suitable for dissolving peroxyacids in accordance with the invention, as well as 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 on the effectiveness of the preceding cooking and delignification 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 pulp and, preferably, not less than 0.5 g / 100 g dry paste. The expression H ₂ O ₂ equivalent is intended to denote the amount of hydrogen peroxide which contains an identical amount of active oxygen. Usually, a quantity of peroxyacid will not exceed 3 g of H ₂ O ₂ equivalent per 100 g of dry pulp and, preferably, 2 g of H ₂ O ₂ equivalent / 100 g of dry pulp.

The peroxyacid treatment step can also be carried out in the presence of one or more compatible additives with peroxyacids such as, for example, surfactants, peroxyacid stabilizers, inhibitors of depolymerization of cellulosic fibers and anti-corrosion agents. Examples of such additives are surfactants anionics, nonionic surfactants, soluble salts of Mg and sequestrants of metal ions. In as a general rule, when present, the quantity of these additives used does not exceed 3 g per 100 g of dry dough and preferably not more than 2.5 g per 100 g of dry dough.

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 minus 60 ° C. Likewise, this temperature generally does not exceed 100 ° C and preferably not 95 ° 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 type of wood used to prepare the pulp, as well as on the efficiency of the cooking and the preceding stages. Times of approximately 60 minutes to approximately 500 minutes are suitable well. A duration of 120 minutes has given excellent results.

The pH of the peroxyacid treatment stage can be in the range of acid pH as well as alkaline pH. However, moderately acidic pHs are preferred. In practice, we prefer to set the initial pH at a value 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 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.

According to another variant of the invention, the ozone treatment step consists in subjecting the dough to a current gas consisting of a mixture of ozone and oxygen from an oxygen-powered electric ozone generator dry gas. In the laboratory, a generator is advantageously used, the flow rate of which is 50 to 100 l / hour and, preferably from 70 to 90 l / hour. The amount of ozone used can easily be adjusted by varying the duration of the ozone / oxygen mixture stream sweeping over 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 arrange to regulate the flow of ozone generators and the duration of treatment to fix the amount of ozone implementation on the dough at values similar to those that are achieved 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 up to descend 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 suitable and preferably 1.5 to 4. A pH of 2 to 3 obtained by subjecting the dough to a 30-minute conditioning treatment using a solution of H ₂ SO ₄ or SO ₂ at a rate of 0.5% by weight of SO ₂ relative to the dry paste and with a consistency of 3% of dry matter has given very good results.

The consistency of the ozone treatment step will be selected in the range of 0.5 to 45% of materials dry and preferably from 0.5 to 3% (in the case of low consistency apparatus) or between 10 to 15% (in the case of medium consistency equipment). 35% dry matter consistency gave excellent results at the laboratory scale.

The temperature of the ozone treatment stage must remain low, otherwise it will lead to degradation important mechanical properties of the treated pulp. This temperature is generally from 2 to 50 ° C and, from preferably 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 of opening the dough (called "fluffing" in Anglo-Saxon literature) intended to increase the contact surface of the paste with ozone. This operation is particularly useful when the consistency of the dough when treated with ozone is at least 15% dry matter.

The process according to the invention applies to the bleaching of any kind of chemical pulp which has undergone a extensive cooking. It is well suited for delignifying kraft and sulfite pastes. It is particularly well suited to processing of 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 coniferous pulp having undergone normal kraft cooking (initial whiteness 27.9 ° ISO measured according to standard ISO 2470-1977 (F), kappa index 26.7 measured according to standard SCAN C1-59 and degree of polymerization 1680 expressed in number of glucosic units and measured according to the SCAN C15-62 standard) was treated according to a sequence of 3 OQP steps under the following conditions: 1st stage: oxygen stage (stage O): pressure, kPa 600 NaOH content, g / 100g dry paste 4 MgSO ₄ .7H ₂ O content, g / 100g dry paste 0.5 temperature, degrees C 120 duration, min 60 consistency,% by weight of dry matter 12 2nd stage: stage at EDTA (stage Q): EDTA content, g / 100g dry paste 0.4 temperature, degrees C 70 duration, min 45 consistency,% by weight of dry matter 10 3rd stage: hydrogen peroxide stage (stage P): H ₂ O _{2 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 ₄ .7H ₂ O content, 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 dry matter: example 1R 30 example 2R 10 example 3R 30 where DTMPANa ₇ represents the heptasodium salt of diethylenetriaminepenta acid (methylenephosphonic).

At the end of each treatment step, the paste was washed 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 No. Final whiteness ° ISO Final kappa index Degree of polymerization 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 ° ISO 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 ° ISO measured according to ISO 2470-1977 (F), kappa index 3.7 measured according to the SCAN C1-59 standard and degree of polymerization 800 expressed in number of glucosic units and measured according to the SCAN standard C15-62) was treated according to a sequence of 2 steps QP under the following conditions: 1st stage: stage at EDTA (stage Q): EDTA content, g / 100g dry paste 0.4 H ₂ SO _{4 content} (for pH 5) 0.34 temperature, degrees C 70 duration, min 45 consistency,% by weight of dry matter 10 2nd stage: hydrogen peroxide stage (stage P): H ₂ O _{2 content} , g / 100g dry paste 3.0 NaOH content, g / 100g dry paste 1.3 MgSO ₄ .7H ₂ O content, g / 100g dry paste 1.0 Na silicate content 38 ° Be, g / 100g dry paste 3.0 temperature, degrees C 120 duration, min 240 consistency,% 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 determination were determined. polymerization.

The results obtained were: Example No. Final whiteness ° ISO Final kappa index Degree of polymerization 4 88.0 1.6 710

Examples 5 to 7 : (not in accordance with the invention)

The same sample of pulp prebleached with oxygen as in Example 4 was bleached according to the sequence Q Paa P, the acronym Paa designating a step with peracetic acid. The operating conditions carried out were as follows: 1st stage: stage at EDTA (stage Q): EDTA content, g / 100g dry paste 0.4 H ₂ SO _{4 content} (for pH 5) 0.34 temperature, degrees C 70 duration, min 45 consistency,% by weight of dry matter 10 2nd stage: peracetic acid stage (Paa stage): CH ₃ CO ₃ H content, g / 100g dry paste: example 5 1.0 example 6 2.0 example 7 3.0 DTMPANa ₇ content, g / 100g of dry paste 0.1 MgSO ₄ .7H ₂ O content, g / 100g dry paste 0.05 temperature, degrees C 80 duration, min 180 consistency,% by weight of dry matter 10 where DTMPANa ₇ represents the heptasodium salt of diethylenetriaminepenta acid (methylenephosphonic). 3rd stage: hydrogen peroxide stage (stage P): H ₂ O _{2 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 MgSO ₄ .7H ₂ O content, g / 100g dry paste 1.0 Na silicate content 38 ° Be, g / 100g dry paste 3.0 temperature, degrees C 90 duration, min 240 consistency,% 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 determination were determined. polymerization.

The results obtained were as follows: Example No. Final whiteness ° ISO Final kappa index Degree of polymerization 5 86.4 1.7 760 6 90.5 0.9 690 7 91.4 0.7 680

Examples 8 to 10 : (not 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 Q C _A P. The operating conditions carried out were as follows: 1st stage: stage at EDTA (stage Q): EDTA content, g / 100g dry paste 0.4 H ₂ SO _{4 content} (for pH 5) 0.34 temperature, degrees C 70 duration, min 45 consistency,% by weight of dry matter: 10 2nd stage: Caro acid stage (stage C _A ): H ₂ SO _{5 content} , g / 100g dry paste: example 8 1.5 example 9 3.0 example 10 4.5 NaOH content, g / 100g dry paste: example 8 2.86 example 9 5.76 example 10 8.76 DTMPANa ₇ content, g / 100g of dry paste 0.1 MgSO ₄ .7H ₂ O content, g / 100g dry paste 0.05 temperature, degrees C 80 duration, min 180 consistency,% by weight of dry matter 10 where DTMPANa ₇ represents the heptasodium salt of diethylenetriaminepenta acid (methylenephosphonic). 3rd stage: hydrogen peroxide stage (stage P): H ₂ O _{2 content} , g / 100g dry paste 2.0 NaOH content, g / 100g dry paste: example 8 1.3 example 9 1.4 example 10 1.7 MgSO ₄ .7H ₂ O content, g / 100g dry paste 1.0 Na silicate content 38 ° Be, g / 100g dry paste 3.0 temperature, degrees C 90 duration, min 240 consistency,% 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 determination were determined. polymerization.

The results obtained were as follows: Example No. Final whiteness ° ISO Final kappa index Degree of polymerization 8 89.4 1.0 700 9 90.6 0.9 710 10 91.6 0.7 690

Claims (10)

1. Process for bleaching a chemical paper pulp of softwood or hardwood, which has been subjected to extended cooking and which presents, after the cooking, a kappa number of 20 or less in the case of softwood, and of 14 or less in the case of hardwood, by means of a sequence of treatment stages which are free from chlorinated reactants, comprising the following stages, carried out in order:
      Q P
   where the abbreviation Q represents a stage of decontamination of the pulp from its transition metals and the abbreviation P represents the final stage with alkaline hydrogen peroxide performed at a temperature above 110 °C and at a pulp consistency of more than 25 % by weight of solids.
2. Process according to Claim 1, in which the Stage P is carried out in the presence of gaseous oxygen.
3. Process according to Claim 1 or 2, in which the temperature of the Stage P is not higher than 150 °C.
4. Process according to any one of Claims 1 to 3, in which a stage with gaseous oxygen precedes the sequence.
5. Process according to any one of Claims 1 to 4, in which the Stage P is carried out at a pulp consistency of at least 30 % by weight of solids and at most 50 % by weight of solids.
6. Process according to any one of Claims 1 to 5, in which Stage Q consists of a treatment with an acid free from a sequestering agent, followed by an addition of soluble Mg salt in an amount such that the ratio by weight of the amount of Mg to that of Mn present in the pulp is 30 or more.
7. Process according to any one of Claims 1 to 6, in which an oxidizing stage is inserted between Stage Q and Stage P with the aid of an agent chosen from peroxyacids and ozone.
8. Process according to any one of Claims 1 to 7, in which Stage Q is carried out in the presence of an enzyme.
9. Process according to any one of Claims 1 to 8, in which an additionnal stage with an enzyme is incorporated at any point in the sequence.
10. Application of the process according to any one of Claims 1 to 9 to bleaching kraft pulps or sulphite pulps.