EP0726979B1 - Process for bleaching a chemical pulp - Google Patents

Abstract

Process for bleaching a chemical pulp through a sequence of processing steps comprising, in this order, a step for decontaminating the pulp by removing its transition metals, an alcaline hydrogen peroxide processing step and a processing step using a non peroxygenated acid.

Description

The invention relates to a process for bleaching a dough for chemical paper.

It is known to treat unbleached chemical pulp obtained by cooking cellulosic materials in the presence of chemical reagents through a sequence of processing steps delignifying and whitening implying the implementation of oxidizing chemicals. The first step in a sequence conventional chemical pulp bleaching aims to perfect the delignification of the unbleached dough as it is present after the cooking operation. This first step delignifying is traditionally carried out by treating the dough unbleached by chlorine in an acid medium or by a chlorine association - chlorine dioxide, 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 step further processing.

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

For about twenty years, we have proposed to do precede the first stage of treatment with chlorine or the chlorine - chlorine dioxide association in one step 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 rate of delignification that is obtained by this oxygen treatment however is not enough if one is aiming to produce pasta high whiteness chemicals whose mechanical properties do not   are not degraded.

Patent application EP-A-0 433 138 discloses sequences bleaching including the three successive stages Acid - Alkaline hydrogen peroxide - Acid (page 3, lines 35 to 37). Step P is carried out there at a temperature of 55 to 85 ° C.

Patent application EP-A-0 402 335 discloses sequences bleaching product with two stages: Sequestrant - Peroxide of alkaline hydrogen. Step P is carried out at a temperature from 50 to 130 ° C (page 12, claim 11). The sequence can be followed by traditional bleaching steps including chlorinated reagents (example 7, page 9).

It has been proposed (patent application EP-A1-0456626) to whiten kraft pastes with hydrogen peroxide according to a sequence Q P or O Q P (the acronym Q representing a step with a sequestrant of metal ions, the acronym P a step at hydrogen peroxide and acronym 0 a step with oxygen). It is however difficult with this process to obtain a whiteness final higher than 85 ° ISO, without resorting to additional steps using chlorinated reagents such as chlorine dioxide.

The invention remedies these drawbacks of known methods, by providing a new delignification and / or bleaching of chemical pulp which achieves high whiteness levels without severely degrading the cellulose and without using chlorinated reagents.

To this end, the invention relates to a process for bleaching a chemical paper pulp by means of a sequence of treatment steps comprising at least the following steps, carried out in order:
QPA where the acronym Q represents a step of decontaminating the pulp into its transition metals, the acronym A represents a step with a non-peroxygenated acid and the acronym P represents a step with alkaline hydrogen peroxide carried out at a temperature above 100 ° C.

Preferably, the invention relates to a method for bleaching a chemical pulp by means of a sequence of steps according to which the processing steps are free of chlorinated reagents and the sequence comprises the following steps, carried out in the order:
QPAP

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 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 salt of sulfurous acid 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 or sulfite cooking. All types of wood used for the production of chemical pulp is suitable for implementing the method of the invention and, in particular those used for kraft and sulfite pastes, namely softwoods such as, for example, the various species of pine and fir and hardwoods like, for example, beech, oak, eucalyptus and charm.

According to the invention, the first step is a step of decontamination of the paste into its transition metals (step Q).

According to a first variant of the invention, step Q consists in treating the dough with at least one sequestering agent such than 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, ethylenediaminetetraacetic, diethylenetriaminepentaacetic, cyclohexanediaminetetraacetic and their salts, poly-α-hydroxyacrylic acid and its salts or an organic polyphosphonate such as ethylenediaminetetramethylenephosphonic acids, diethylenetriaminepenta (methylenephosphonic), cyclohexanediaminetetramethylenephosphonic and their salts. Ethylenediaminetetraacetic acids (EDTA) and diethylenetriaminepentaacetic (DTPA) have given excellent results.

The sequestrant is generally used in step Q in not more than 1.5 g of sequestrant per 100 g of dry dough. Most often, this amount is at most 1.0 g of   sequestering agent for 100 g of dry dough. She is often at least minus 0.01 g / 100 g of dry dough.

In addition to the sequestrant, we can also, according to this first variant, add a small amount of acid in step Q. The amount of acid used to do this will generally set to impose a pH of around 5 on the medium.

Step Q can also, according to a second 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. It is also generally not useful for the amount of Mg to exceed that for which the weight ratio Mg / Mn is equal to 10 ⁴ .

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. 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 (HS0 ₃ ) n, in which Me symbolizes a metal atom of valence n, n being an integer having the value 1 or 2.

According to this second variant, the amount of acid to be put will depend on the type of wood and the amount of impurities that it contains. In general, we will implement a amount of acid such that the pH of the dough is at least about 5 and preferably about 5.5. Likewise, we will adjust often the amount of acid so that the pH does not exceed 7 and, preferably not 6.5. When step Q is free of sequestering, the pH will be adjusted so as to make the medium substantially more acidic, that is to say, with a pH at most equal to 5 and, preferably at most 4.5. Generally, we will avoid, in order not to degrade the dough, to drop below pH 2.0 and preferably below pH 2.5.

Stage Q is generally carried out at a pressure close to   atmospheric pressure and at a temperature sufficient to ensure efficient consumption of acid and / or sequestrant and at the same time not too high so as not to degrade the cellulose and not burden the energy cost of the means of heating implemented in said step. In practice, a temperature of at least 40 ° C and preferably at least 50 ° C is fine. Likewise, it is advantageous if the temperature does not not exceed 100 ° C and preferably not 95 ° C. The best results were obtained at 50-55 ° C.

The duration of stage Q must be sufficient to ensure a complete reaction. Although longer durations are without influence on the delignification rate of the dough as well as on its intrinsic resistance qualities, it is not recommended to extend the reaction time beyond that necessary upon completion of the reaction so as to limit costs investment and heating energy costs of the dough. In practice, the duration of the pretreatment can vary within large proportions depending on the type of equipment used, the choice acid, temperature and pressure, for example 15 minutes approximately 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 90 minutes and preferably not 60 minutes. A duration of about 30 minutes has given excellent results.

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

According to the invention, the preferred sequence is a sequence QPAP. In this sequence, the symbols P denote steps with alkaline hydrogen peroxide. The nature of the alkali must be such as this exhibits good extraction efficiency of lignin oxidized together with good solubility. A 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 at a value of at least 10 and, from preferably at least 11. The amount of alkali should also be adjusted to ensure sufficient consumption of peroxide at the end of reaction. In practice, when step P is carried out in the absence of stabilizer, or when sodium silicate is present as a stabilizer, quantities of alkali of at least 1 and at most 3 g of alkali per 100 g of dry paste are suitable good. In the event that a stabilizer giving rise to a reaction acid in aqueous solution is used, the quantities of alkali to maintain a pH of at least 10. In this cases, quantities of alkali up to a maximum of 8 g / 100 g dry paste can be used.

Be used in addition to these amounts of alkali a quantity of hydrogen peroxide of at least 0.3 g H ₂ 0 _2/100 g of dry pulp and preferably at least 0.5 g / 100 g of dry dough. It is also generally the amounts of hydrogen peroxide does not exceed 5.0 g H ₂ 0 _2/100 g of dry pulp and preferably do not exceed 4.0 g / 100 g of dry pulp.

The temperature of steps P must be adjusted so that remain at least 50 ° C and preferably 70 ° C. She should also not exceed 150 ° C and preferably not exceed 135 ° C. A temperature of 90 ° C gave excellent results.

According to the invention at least one of the steps P of the sequence to a temperature above 100 ° C. The temperature of this step P is preferably at least 110 ° C. It generally does not exceed 140 ° C and, from preferably not 135 ° C.

The duration of steps P must be sufficient for the bleaching reaction is as complete as possible. She cannot, however, exceed this reaction time too strongly under penalty of inducing 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. She will have to also most often do not exceed 600 and preferably 500   minutes.

The consistency of steps P is generally chosen to be lower or equal to 50% by weight of dry matter and, preferably, 40% dry matter. She will often not less than 5% and preferably not less than 8%. A consistency of 10% has given good results.

The consistency of the last step P may, alternatively, advantageously be chosen from high values of at least 25% by weight of dry matter and preferably 30% or more. In this variant, the consistency of the other steps P will generally be at most 20%. It will also suit, the most often, that the consistency of the other steps P is at least 8%.

According to the invention, step A is a processing step with a non-peroxygenated acid. By non-peroxygenated acid is meant designate any inorganic or organic acid whose molecule does not does not contain the peroxy group -0-0-. All inorganic acids or usual organic materials used in aqueous solution, alone or in mixture are suitable. Strong inorganic acids such as, for example, sulfuric, phosphoric, nitric or acid hydrochloric acid are suitable. Sulfuric acid, solution aqueous has given excellent results.

The operating conditions of step A must be determined in each particular case according to the type of pulp and wood essence used. In a way general, the choice of acid and the quantity should be fixed implemented to impose a pH on the medium substantially less than 7, for example from at least about 1 to at most about 6.5. Especially advantageous pHs are those of at least about 2.5 and at most about 5.5. The temperature and pressure do not are not critical, atmospheric pressure and temperatures from at least 10 ° C to at most 100 ° C generally suitable. The Pretreatment time can vary widely depending on the type of equipment used, the choice of acid, the temperature and pressure, for example from about 30 minutes to many hours.  

In step A, it is possible to introduce, in addition to the acid, a magnesium salt soluble in aqueous acid medium. Generally the amount of magnesium that can be added does not exceed 0.4 g magnesium per 100 g of dry paste and preferably 0.1 g / 100 g dry paste. When an Mg salt is introduced, its quantity is at least 0.005 g / 100 g of dry paste.

According to another variant of the invention, step A can consist of a simple washing of the dough using a solution acidic aqueous. In the case of a QPAP sequence, this acid wash can be inserted between the two stages P.

Another variant of the process according to the invention consists in repeat steps P and A a second time to achieve the complete sequence Q P A P A P.

In another variant of the process according to the invention, it is can precede the sequence with an oxygen step (step 0). This oxygen step is carried out by bringing the dough with gaseous oxygen at a pressure of at least 20 kPa and at most 1000 kPa in the presence of an alkaline compound in amount such as the weight of alkaline compound relative to the dry dough weight is at least 0.5 and at most 5%.

A particular embodiment of the method consists in recycle all or part of the effluents from the second and / or the third stage P towards a stage P located earlier in the sequence. A similar recycling of the effluents from the second acid step, when present, to the first step sequence acid can also be achieved. Can also combine in a single sequence the recycling of effluents from steps P to a step P above and that of the effluents from an acid step to an acid step above. These recycling has the advantage of saving products chemicals used via recovery of the non-reactive consumed.

The oxygen step temperature should generally be 70 ° C or more and preferably at least 80 ° C. he this temperature should also not exceed 130 ° C and, from preferably does not exceed 120 ° C.  

The duration of the oxygen treatment must be sufficient so that the reaction of oxygen with the lignin contained in the dough is complete. However, it cannot exceed too much strongly this reaction time under penalty of inducing degradations in the structure of the cellulose chains of the pulp. In practice, it will be 30 minutes or more and, preferably, at least 40 minutes. Usually it will not exceed 120 minutes and preferably not more than 80 minutes.

Treatment of the dough 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. The salts of magnesium are preferred, in particular, magnesium sulfate heptahydrate used at a rate of 0.02 to 1% by weight relative to dry dough.

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

Alternatively, step 0 can also be carried out in the presence of hydrogen peroxide (step 0p). The quantity of hydrogen peroxide which can be incorporated in step 0 is generally not less than 0.2 g H ₂ 0 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 0 ₂ per 100 g of dry paste and, most often, not 2 g.

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

In another variant of the method according to the invention, it is can incorporate at any point in the sequence of stages of treatment an additional enzymatic step consisting of treat the dough with at least one enzyme (step X). This treatment enzymatic can also be carried out before or after the stage of   oxygen pretreatment.

The term “enzyme” is intended to denote any enzyme capable of facilitate delignification, through the processing steps subsequent to the step of treatment with the enzyme, from a paste to unbleached chemical paper from the cooking operation or chemical paper pulp that has already been subjected to a step treatment with oxygen.

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

A category of enzymes well suited to the process according to the invention are hemicellulases. These enzymes are able to react with the hemicelluloses on which the lignin present in the dough.

Preferably, the hemicellulases used in the process according to the invention are xylanases, i.e. hemicellulolytic enzymes capable of cutting xylan bonds which constitute a major part of the interface between the lignin and the rest of the carbohydrates. An example of xylanase according to the process according to the invention is 1,4-β-D-xylan xylanohydrolase, EC 3.2.1.8.

Preferred xylanases in the methods according to the invention can be of various origins. They can in particular have been secreted by a wide range of bacteria and mushrooms.

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 contains no other enzymes. In particular, it is preferred that the xylanase according to the process according to the invention does not contain no cellulase so as not to destroy the polymer chains pulp cellulose.

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

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

The following examples are given for the purpose of illustrating the invention, without limiting its scope.

Examples 1R to 8R (not in accordance with the invention)

A sample of softwood pulp having undergone kraft cooking (initial whiteness 27.9 ° ISO measured according to ISO 2470-1977 (F), kappa index 26.7 measured according to SCAN C1-59 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 2 steps 0 P under the following conditions: 1st stage: stage at DTPA (stage Q): DTPA content, g / 100g dry paste 0.2 temperature, degrees C 90 duration, min 60 consistency,% by weight of dry matter 4 2nd stage: hydrogen peroxide stage (stage P): H ₂ 0 _{2 content} , g / 100g dry paste 5 NaOH content, g / 100g dry paste 4 DTMPANa ₇ content, g / 100g of dry paste 0 to 1 MgS0 content ₄ .7H ₂ 0, g / 100g dry paste 0.5 temperature, degrees C 90 and 120 duration, min 120 consistency,% by weight of dry matter 10 where DTMPANa ₇ represents the heptasodium salt of diethylenetriaminepenta acid (methylenephosphonic). The contents of DTMPANa ₇ and the temperatures used in each of Examples 1R to 8R were as follows: Example No. DTMPANa ₇ g / 100g ms Stage temperature P ° C 1R 0 90 2R 0.25 90 3R 0.50 90 4R 1.00 90 5R 0 120 6R 0.25 120 7R 0.50 120 8R 1.00 120

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

After treatment, the kappa number and the whiteness of the dough were determined. The results are shown in the table below. Example No Final whiteness ° ISO Final kappa index 1R 66.5 11.3 2R 68.2 11.2 3R 68.7 11.2 4R 68.2 11.6 5R 71.2 8.1 6R 77.1 7.6 7R 75.5 7.8 8R 72.9 8.1

We see that despite the large amount of peroxide of hydrogen used, it was not possible to exceed 78 ° ISO whiteness.

Example 9: (according to the invention)

The same dough sample as in Examples 1R to 8R was bleached according to the QPA P sequence. The operating conditions carried out were as follows: 1st stage: stage at DTPA (stage Q): H ₂ S0 ₄ (for pH 5) 0.54 DTPA content, g / 100g dry paste 0.2 temperature, degrees C 90 duration, min 60 consistency,% by weight of dry matter 4 2nd stage: hydrogen peroxide stage (stage P): H ₂ 0 _{2 content} , g / 100g dry paste 3 NaOH content, g / 100g dry paste 4 DTMPANa ₇ content, g / 100g of dry paste 0.25 temperature, degrees C 120 duration, min 120 consistency,% by weight of dry matter 10 3rd stage : sulfuric acid stage (stage A): H ₂ S0 ₄ , quantity required to reach pH 3 temperature, degrees C 90 duration, min 120 consistency,% by weight of dry matter 10 4th stage: hydrogen peroxide stage (stage P): H ₂ 0 _{2 content} , g / 100g dry paste 2 NaOH content, g / 100g dry paste 1.7 Na silicate content 38 ° Be, g / 100g of dry paste 3 MgS0 content ₄ .7H ₂ 0, g / 100g dry paste 1 temperature, degrees C 120 duration, min 180 consistency,% by weight of dry matter 30

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

At the end of the sequence, after treatment, it was determined the kappa index and the whiteness of the dough.

The results obtained were: Example No Final whiteness ° ISO Final kappa index 9 84.4 4.4

Example 10R : (not in accordance with the invention)

The same dough sample as in Examples 1R to 9 was bleached according to the sequence 0 QP P. The operating conditions carried out were as follows: 1st stage: oxygen stage (stage 0): pressure, bar 6 NaOH content, g / 100g dry paste 4 MgS0 content ₄ .7H ₂ 0, g / 100g dry paste 0.5 temperature, degrees C 120 duration, min 60 consistency,% by weight of dry matter 12 2nd stage: stage at DTPA (stage Q): DTPA content, g / 100g dry paste 0.12 temperature, degrees C 55 duration, min 30 consistency,% by weight of dry matter 3 3rd stage: hydrogen peroxide stage (stage P): H ₂ 0 _{2 content} , g / 100g dry paste 3 NaOH content, g / 100g dry paste 4 DTMPANa ₇ content, g / 100g of dry paste 0.25 MgS0 content ₄ .7H ₂ 0, g / 100g dry paste 1 temperature, degrees C 120 duration, min 120 consistency,% by weight of dry matter 10 where DTMPANa ₇ represents the heptasodium salt of diethylenetriaminepenta acid (methylenephosphonic). 4th stage: hydrogen peroxide stage (stage P): H ₂ 0 _{2 content} , g / 100g dry paste 1 NaOH content, g / 100g dry paste 1.5 MgS0 content ₄ .7H ₂ 0, g / 100g dry paste 1 temperature, degrees C 120 duration, min 180 consistency,% by weight of dry matter 30

At the end of each processing step, the dough underwent a washing with demineralized water at room temperature. At the end of the sequence, after treatment, it was determined the kappa index and the whiteness of the dough.

The results obtained were as follows: Example No Final whiteness ° ISO Final kappa index 10R 82.6 5.1

Examples 11 to 14 : (in accordance with the invention)

The same dough sample as in Examples 1R to 10R was bleached according to the sequence 0 QPA P. The operating conditions carried out were as follows: 1st stage: oxygen stage (stage O): pressure, bar 6 NaOH content, g / 100g dry paste 4 MgS0 content ₄ .7H ₂ 0, g / 100g dry paste 0.5 temperature, degrees C 120 duration, min 60 consistency,% by weight of dry matter 12 2nd stage: stage at DTPA (stage Q): DTPA content, g / 100g dry paste 0.12 temperature, degrees C 55 duration, min 30 consistency,% by weight of dry matter 3 3rd stage: hydrogen peroxide stage (stage P): H ₂ 0 _{2 content} , g / 100g dry paste 3 NaOH content, g / 100g dry paste 4 DTMPANa ₇ content, g / 100g of dry paste 0.25 MgS0 content ₄ .7H ₂ 0, g / 100g dry paste 1 temperature, degrees C 120 duration, min 120 consistency,% by weight of dry matter 10 where DTMPANa ₇ represents the heptasodium salt of diethylenetriaminepenta acid (methylenephosphonic). 4th step: step with an acid (step A): H ₂ S0 _{4 content} , g / 100g dry paste Ex. 11: 1.08 (pH 3) Ex. 12: 0.52 (pH 5) Ex. 13 and 14: 0 S0 ₂ content, g / 100g dry paste Ex. 11 and 12: 0 Ex. 13: 1.06 (pH 3) Ex. 14: 0.45 (pH 5) MgS0 ₄ .7H ₂ 0 content, g / 100g of dry paste 1 temperature, degrees C 90 duration, min 120 consistency,% by weight of dry matter 10 5th stage : hydrogen peroxide stage (stage P): H ₂ 0 _{2 content} , g / 100g dry paste 1 NaOH content, g / 100g dry paste 1.5 Na silicate content 38 ° Be, g / 100g of dry paste 3 MgS0 content ₄ .7H ₂ 0, g / 100g dry paste 1 temperature, degrees C 120 duration, min 180 consistency,% by weight of dry matter 30

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

At the end of the sequence, after treatment, it was determined the kappa index and the whiteness of the dough.

The results obtained were as follows: Example No Final whiteness ° ISO Final kappa index 11 87.5 3.6 12 86.4 4.6 13 87.8 3.4 14 86.1 4.7

Examples 15 and 16 : (in accordance with the invention)

The same pulp sample as in Examples 1R to 14 was bleached using the sequence 0 XQPAP by using in step X an amount of xylanase derived from Bacillus pumilus PRL B12 corresponding to 10 XU / g of dry pulp . The XU (Xylanase Unit) is defined as the quantity of xylanase which, under the conditions of the test, catalyzes the release of reducing sugars equivalent, in reducing power, to 1 micromole of glucose per minute.

The operating conditions were as follows: 1st stage: oxygen stage (stage O): pressure, bar 6 NaOH content, g / 100g dry paste 4 MgS0 content ₄ .7H ₂ 0, g / 100g dry paste 0.5 temperature, degrees C 120 duration, min 60 consistency,% by weight of dry matter 12 2nd stage: enzyme stage (stage X): H ₂ S0 ₄ for pH 7 xylanase content, XU / g dry paste 10 temperature, degrees C 50 duration, min 120 consistency,% by weight of dry matter 3 3rd stage: stage at EDTA (stage Q): H ₂ S0 ₄ for pH 5 EDTA content, g / 100g dry paste 0.4 temperature, degrees C 50 duration, min 30 consistency,% by weight of dry matter 3 4th stage: hydrogen peroxide stage (stage P): H ₂ 0 _{2 content} , g / 100g dry paste 4 NaOH content, g / 100g dry paste 4 DTMPANa ₇ content, g / 100g of dry paste 0.25 MgS0 content ₄ .7H ₂ 0, g / 100g dry paste 1 temperature, degrees C 120 duration, min 120 consistency,% by weight of dry matter 10 where DTMPANa ₇ represents the heptasodium salt of diethylenetriaminepenta acid (methylenephosphonic). 4th stage: sulfuric acid stage (stage A): H ₂ S0 ₄ , for pH 3 temperature, degrees C 90 duration, min 120 consistency,% by weight of dry matter 10 5th stage : hydrogen peroxide stage (stage P): H ₂ 0 _{2 content} , g / 100g dry paste 1 (Ex. 15) and 2 (Ex. 16) NaOH content, g / 100g dry paste 1.5 Na silicate content 38 ° Be, g / 100g of dry paste 3 MgS0 content ₄ .7H ₂ 0, g / 100g dry paste 1 temperature, degrees C 120 duration, min 180 consistency,% by weight of dry matter 30

At the end of each processing step, the dough underwent a washing with demineralized water at room temperature, except between stages X and Q which were carried out by simple acidification at pH 5 by means of sulfuric acid and by incorporation EDTA to the pulp harvested at the end of stage X, without dilution nor reconcentration.

At the end of the sequence, after treatment, it was determined the kappa index and the whiteness of the dough.

The results obtained were as follows: Example No Final whiteness ° ISO Final kappa index 15 89.6 3.0 16 91.7 2.6

Claims (11)

1. Process for bleaching a chemical paper pulp using a sequence of processing steps comprising at least the following steps, carried out in the order:
      Q P A where the abbreviation Q represents a step of decontamination of the pulp in respect of its transition metals, the abbreviation A represents a step with a non-peroxygenated acid and the abbreviation P represents a step with alkaline hydrogen peroxide performed at a temperature superior to 100 °C.
2. Process for bleaching a chemical paper pulp using a sequence of steps according to Claim 1, characterized in that the processing steps are free of chloro reactants and the sequence comprises the following steps, carried out in the order:
      Q P A P
3. Process according to either of Claims 1 and 2, characterized in that step Q consists of a treatment with an acid free of a sequestering agent, followed by an 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 pulp is at least 30.
4. Process according to either of Claims 1 and 2, characterized in that step Q consists of a treatment with a sequestering agent.
5. Process according to any one of Claims I to 4, characterized in that a step using oxygen gas precedes the sequence.
6. Process according to any one of Claims 2 to 5, characterized in that the final step P is performed at a pulp consistency of at least 25 % solids content by weight.
7. Process according to any one of Claims 2 to 6, characterized in that the final two steps A P are repeated a second time at the end of the sequence.
8. Process according to any one of Claims 2 to 7, characterized in that step A is a simple acidic washing which separates the steps P.
9. Process according to any one of Claims 2 to 8, characterized in that all or some of the effluents of at least one step P or A is recycled into a step using the same reactant and which is situated earlier in the sequence.
10. Process according to any one of Claims 1 to 9, characterized in that an additional step with an enzyme is incorporated at any point in the sequence.
11. Process according to any one of Claims 1 to 10, characterized in that the first step P is reinforced with oxygen.