EP0759105B1 - Method for preparing delignified and bleached chemical paper pulps - Google Patents

Description

The invention relates to a preparation process. delignified and bleached chemical pulp.

• les pâtes Kraft ou au sulfate,
• les pâtes au sulfite ou au bisulfite,
• les pâtes semi-chimiques ou au sulfite neutre,
• les pâtes après cuisson avec un solvant comme, celles obtenues par le procédé ORGANOSOLV,

Chemical paper pulps or chemical pulps are those obtained by cooking lignocellulosic materials, in particular wood. Thus among the chemical pastes we distinguish:

• Kraft or sulphate pasta,
• sulfite or bisulfite pastes,
• semi-chemical or neutral sulfite pastes,
• pasta after cooking with a solvent, such as that obtained by the ORGANOSOLV process,

• les pâtes sulfites-Anthraquinone
• les pâtes "Superbatch"

(ULLMANN'S ENCYCLOPEDIA of Industrial Chemistry, 5th Edition, vol.A.18, 1991, pages 568 and 569).

• sulfite-anthraquinone pastes
• "Superbatch" pasta

• les bois de résineux comme les diverses espèces de pins et sapins,
• les bois de feuillus comme par exemple le bouleau, le peuplier, le hêtre et l'eucalyptus.

All types of wood are suitable:

• softwoods such as various species of pine and fir,
• hardwoods such as birch, poplar, beech and eucalyptus.

In a classic way the chemical pastes obtained by cooking are subjected to several stages of minor treatment and / or bleaching.

The first steps are to perfect the delignification resulting from cooking. Steps The following are bleaching steps.

At the end of these delignifying treatments and blanching, the pasta should usually present a whiteness of at least 88-90 ° ISO and a kappa index very weak while retaining good properties mechanical, i.e. without significant degradation of cellulose. This deterioration is detectable by the measure of the viscosity of the dough or its degree polymerization (DP). The PD must remain as high as possible.

Blancheur : norme ISO 2470

Indice kappa : norme SCAN C1-59

Degré de polymérisation (DP) : norme SCAN C 15-12

The definitions of the terms used above and thereafter meet the alternate standards:

Whiteness: ISO 2470 standard

Kappa index: SCAN C1-59 standard

Degree of polymerization (DP): SCAN C 15-12 standard

The first significant steps are usually performed by treatments with Chlorine gas or Chlorine dioxide.

For the preparation of pasta without chlorine it has proposed to replace chlorinated reagents with other oxidizing agents. Article by C.L Forber "Chlorine and chlorine dioxide replacements in kraft pulp bleaching: Emerging technologies or Laboratory curiosities? , TAPPI PRESS, Atlanta, Georgia 1993 ", concludes that there are several compounds with a sufficient bleaching capacity to replace the chlorine for delignification but that replacement chlorine dioxide for bleaching does not appear feasible because no reagent has the efficacy and the cost of chlorine dioxide.

This article shows in particular the powers of delignification and bleaching compared of a series of reagents like chlorine, chlorine dioxide, oxygen, ozone, hydrogen peroxide and indicates also the results generally obtained in terms of kappa index, whiteness and viscosity.

Thus oxygen alone has a capacity for delignification and not appreciably for bleaching. Its chlorine replacement factor (CRF) is 5 and its use results in a reduction of about half of the kappa index (17 from 35), the viscosity obtained being 980 dm ³ / kg (about 37 cps ) and the whiteness of ISO 34 °.

In comparison the use of peroxide of hydrogen is known both in delignification and laundering.

In delignification the kappa index decreases by a little less than half (20 from 35), the viscosity obtained being 900 and more dm ³ / kg (about 30 cps, and the whiteness of 45 ° ISO; we obtain 91 ° ISO for a viscosity of around 600 and more dm ³ / kg.

To lower the kappa index below a value usually between 7 and 14, it is necessary to resort to a second stage of delignification can be effected by ozone (C. Chirat and Lachenal, Proceedings TAPPI, Pulping 1993 Conference, p. 717) or by acid peracetic, or by Caro acid (F. Desprez, S. Devenyns, N Troughton. TAPPI Proceedings, Pulping Conference 1993, p. 443).

Treatment with hydrogen peroxide, as described in patent application EP 0578304 A1, makes it possible to whiten only pulps whose kappa index is less than 5 and whose manganese content is less than or equal to 3 ppm. and whose consistency is at least 25% by weight of dry matter relative to the total weight of the dough. This treatment with H ₂ O ₂ is carried out at a claimed temperature between 50 and 140 ° C.

This manganese content less than or equal to 3 p.p.m. is obtained by an acid pretreatment or a complexing or sequestering agent in an acid medium.

The temperatures exemplified are 80, 90 and 120 ° C. At this temperature of 120 ° C does not correspond any indication of the pressure prevailing in the medium reactive.

This pressure could be the pressure of saturated water vapor at the indicated temperature.

The book: "American Institute of Physics Handbook, Third Edition, Mc Graw-Hill Book Company, page 4-309 indicates saturated water vapor pressures for temperatures between 100 ° C and 374.15 ° C, in particular around 2 bars absolute for 120 ° C and 3.6 bars absolute for 140 ° C.

Furthermore, Examples 18 to 21 of EP0578304A1 using a chemical paste of a consistency of 30%, show that the DP decreases from 1180 at 1030 when the final whiteness increases from 89.8 to 92.6 ° ISO.

Patent application EP0577157A2 describes a process for bleaching a dough having a consistency from 5 to 20%, with hydrogen peroxide in the medium alkaline, at a pressure below 25 bar and preferably less than 14 bars.

However, this process requires pretreatment by ozone under pressure.

The description and the figures do not state the heating means and it can be assumed that the treatment with H ₂ O ₂ is carried out at the same ambient temperature as the pretreatment with ozone.

The article by P. Tibbling and B. Dillner, presented at the 25th EUCEPA conference in Vienna in 1993, shows that bleaching with H ₂ O ₂ in an alkaline medium, at a temperature of 90 to 110 ° C and under a pressure of 5 bars, allows to whiten kappa index pasta up to ISO 85-86 ° whiteness and kappa index pasta up to ISO 90 ° whiteness.

However, the gains in whiteness lead to a notable reduction in the viscosity (dm ³ / kg) of these pastes. Figure 2 shows that the viscosity decreases from 890 to 760 when the ISO whiteness increases from about 78.3 to about 84.7. In addition, an increase in temperature from 100 to 110 ° C does not seem to produce any significant effect.

The document DUOPLAN OY, KVAERNER by B. Dillner, "Chemicals dry solids and solid waste in a low effluent bleach mill, Study review meeting, Helsinki, February 16, 1994 ", shows that PO launderings carried out 105 ° C under a pressure of 5 to 10 bar does not lead to significant effects of pressure. However the viscosity decreases from 970 to 800 when the whiteness increases from 73 to 87 ° ISO.

The treatment of paper pulps with hydrogen peroxide in alkaline medium and in the presence of a silicate has been the subject of numerous publications.

Thus patent application WO-A-93/14262 describes a method of bleaching of a dough according to which the dough, previously delignified, to a Kappa index less than 9.5, is pretreated with a complexing agent in the medium acid, then subjected to a bleaching step with hydrogen peroxide in presence of a silicate and at a temperature below 100 ° C. The pressure to whitening is not indicated.

The bleaching process with hydrogen peroxide, in medium alkaline, in the presence of sodium silicate at a temperature of 50 to 100 ° C and under an oxygen pressure of 1 to 10 bars, disclosed in the patent application EP-A-0087553, allows only a semi-bleached pulp having a content high in lignin.

Patent application WO-A-79/00861 describes a method of bleaching of paper pulp according to which the pulp previously impregnated with chemicals undergoes treatment with hydrogen peroxide at a temperature between 100 and 150 ° C and at a differential pressure of 5 to 400 KPa of vapor containing less than 1% oxygen. This document does not state of the Kappa index of the paste before and after treatment.

The object of the present invention is to provide a process for the preparation of chemical paper pulps delignified and bleached with a high DP, in using oxygen and hydrogen peroxide as oxidizing agents and performing all stages of process treatment in reaction media at pH basic. This process also aims to avoid any stage using a chlorinated derivative as for example chlorine or chlorine dioxide and avoid also any stage of treatment with other agents oxidants including ozone or peracids.

a) un traitement délignifiant par l'oxygène, suivi

b) d'un traitement par un agent complexant ou séquestrant des métaux de transition suivi d'un lavage, puis

c) d'un traitement par le peroxyde d'hydrogène, en présence de silicate de métal alcalin, caractérisé en ce que le traitement par le peroxyde d'hydrogène est effectué à une température t supérieure à 100°C et à une pression p supérieure à 1,5 fois la pression de la vapeur d'eau saturante à la température t.

This object is achieved according to the present invention by a process for the preparation of delignified and bleached chemical paper pulp according to which a lignocellulosic pulp obtained by baking is subjected to the sequence of the following treatments:

a) a delignifying treatment with oxygen, followed

b) treatment with a complexing or sequestering agent of transition metals followed by washing, then

c) a treatment with hydrogen peroxide, in the presence of an alkali metal silicate, characterized in that the treatment with hydrogen peroxide is carried out at a temperature t greater than 100 ° C. and at a pressure p greater at 1.5 times the pressure of the saturated water vapor at temperature t.

This process makes it possible to prepare ECF pasta and TCF without using oxidizing agents other than oxygen and hydrogen peroxide.

Oxygen treatment (a) or delignification oxygen is now widely used in the paper industry and in particular for the development of TCF and ECF pasta as indicated by example, article from "Van Lierop, B, Oxygen delignification, Workshop on Emerging Pulping and Chlorine-free Technology, RALEIGH N.C., March 1-4, 1993) ".

Delignification is generally limited to 50% (as measured by the kappa index) because beyond of this limit the selectivity of oxygen in the middle of alkaline pH drops sharply and cellulose is attacked resulting in a detrimental decrease in the degree of polymerization (DP). The conditions of step (a) of process according to the invention are those known and used in the paper industry.

One or more stages of washing the dough can be added at the end of oxygen treatment in an alkaline pH medium.

Oxygen treatment can also be carried out in several successive stages with oxygen, separated by washing steps.

Complexing or sequestering treatment (b) according to the present invention is carried out by means of a complexing or sequestering agent for metals of transition, such as DTPA, (Sodium diethylenetriaminepentaacetate), EDTA (sodium ethylenediaminetetraacetate), the salts of phosphonic acids.

Several agents can be combined to increase the effectiveness of treatment towards a plus large number of metals.

Advantageously, the amount of complexing agent or sequestering agent is from 0.1% to 1% by weight relative to the dry matter contained in the dough. Preferably this amount is from 0.25 to 0.5%.

The quantities of products and reagents, unless specified otherwise, are always expressed in percent by weight relative to the weight of the dry matter of the dough or dough considered dry.

The consistency of the dough is expressed in hundred by weight of dry matter relative to weight total dough.

Preferably, treatment b) is carried out in a medium having an alkaline pH.

Advantageously, the pH of the dough during the salary b) is greater than 7 and less than or equal to 12.5.

Preferably, the pH in b) is from 8.5 to 9.5. The alkaline pH during the treatment in b) is obtained either by the residual alkalinity of the dough after oxygen treatment, either by the alkalinity of the agent complexing or sequestering, either by addition of a base, for example NaOH.

For most pasta, the alkalinity residual pulp combined with that of DTPA allows to obtain a pH close to 9 without adding sodium hydroxide.

Preferably, the manganese content of the dough before treatment with hydrogen peroxide c) do not not more than 5 p.p.m. in weight relative to the weight of dry matter of this same paste.

The processing temperature b) is generally from 20 to 100 ° C and the preferred temperature from 60 to 90 ° C.

The duration of treatment b) is generally 1 30 minutes and preferably 5 to 15 minutes.

The consistency of the dough during processing b) is generally 2 to 25% and the consistency preferred is 4 to 12%.

At the end of the complexing treatment, the dough is washed with water. Washing is carried out according to known techniques in the paper industry with hot or cold water.

Surprisingly, the fact of operating the treatment c) under pressure minimizes the attack of the cellulosic material of the pulp, and keep a high DP, unlike the results obtained in the prior art.

Advantageously, the kappa index of the dough before treatment with hydrogen peroxide does not exceed 17. In Indeed, the method then allows, in a final step P, to obtain a delignified and very white paste can be used directly for papermaking.

Advantageously, the pressure p is from 5 to 200 absolute bars. This pressure range makes it possible to observe an advantage over maintaining a high DP during carrying out the method according to the invention.

Preferably, for practical reasons of implementation the pressure p is from 25 to 50 bars absolute.

Advantageously, the alkali metal silicate is sodium silicate.

When using sodium silicate, we prefer for convenience, use 0.5 at 10% by weight of a commercial solution at 38 ° Bé par relative to the weight of the dry matter, and better still 4 to 8% by weight of this solution.

Advantageously, the reaction temperature t is 110 ° C to 180 ° C. The preferred range of t is 130 ° C to 160 ° C.

Advantageously, the dough, during the treatment with hydrogen peroxide, has a consistency of 4 to 35% in weight of dry matter relative to the total weight of the wet dough. The process can be carried out effectively at low consistencies of about 4 to about 10% and the very fluid reaction medium, can be easily moved by pumping avoiding any clogging.

Preferably, the consistency is 10 to 20%. This range of consistency optimizes the process yield.

Advantageously, the peroxide treatment of hydrogen lasts from 1 minute to 3 hours. The duration varies in reverse with the increase in temperature.

Preferably the duration is from 15 minutes to 1 hour. These relatively short durations allow increasing the hourly production output of the delignified and bleached dough.

Advantageously, depending on the initial paste used and the process temperature the hydrogen peroxide is committed at a rate of 0.5 to 10% by weight compared to the weight of the dry matter of the dough.

The implementation of step c) of delignification and bleaching according to the invention is carried out continuously or discontinuously (batch) by means of the apparatus generally employed in the pulp and paper industry allowing the dough to be impregnated with the solution aqueous hydrogen peroxide and silicate sodium, at high pressure and temperature for the duration chosen. After this treatment c) the dough is decompressed, cooled and washed with water.

For example, batch bleaching of dough high consistency (20 to 30%) can be made from as follows:

The dough is cold mixed with the peroxide hydrogen and sodium silicate and water so as to obtain the chosen consistency, then introduced into a stainless steel autoclave of so as to fill it as completely as possible. After closing the autoclave, a small amount of water is pumped until a pressure of ten bars then the temperature is brought to the chosen temperature for the selected time. During the temperature rise phase the expansion liquids cause an increase in pressure. This increase can be controlled by the flow a little bit of the liquid phase during heating.

For example, the continuous laundering of low consistency paste (8-10%) can be made from as follows:

DTPA pretreated dough is washed and mixed with hydrogen peroxide silicate and water to have a consistency of 8 to 10% allowing the paste to be pumpable.

The mixture is then introduced under pressure, by a high pressure pump in the reactor by through a heat exchanger which carries the mix at the selected temperature. At the end of tubular reactor whose length is calculated from in order to ensure the duration of the whitening chosen, the bleached dough is decompressed and cooled by dilution with water and then is washed.

Ancillary devices such as heat exchangers heat and vapor recovery cyclones can be added to recover and enhance the heat and the pressure of the dough after bleaching.

The preferred embodiment of the present invention provides a new _basic OQ sequence -P, step P being new in itself.

This preferred sequence has advantages techniques because in the techniques of the prior art there is necessary to perform complexation or sequestration of metals in an acid medium to be able bleach at high levels, hydrogen peroxide.

Patent application EP578304A, already cited above, shows that treatment in an acidic medium at pH controlled is necessary before you can perform the final step with hydrogen peroxide. All examples of EP578304 report processing complexing at acid pH or acid washing at pH 5 before final bleaching.

By the way. D. Lachenal et al., "Optimization of bleaching sequences using peroxide as first stage, 1982, International Pulp Bleaching Conference, TAPPI Proceedings p 145-150, showing (table 4, p. 147) that acid pretreatment also improves the delignification of the dough during bleaching peroxide.

The preferred embodiment of this invention achieves excellent results in bleaching material without requiring an acidic medium to stage b) contrary to the general teaching of prior art.

Being able to perform the processing b) in basic environment is of great industrial interest because this avoids two pH changes during the sequence. Indeed, after the alkaline oxygen treatment, the pH of the dough is basic even after several washes with water, and this avoids a second neutralization of the acidity of the complexing treatment by soda.

Finally, this preferred sequence allows eliminate the corrosion problems in acidic environment of steel fittings and reduces problems concerning the treatment of residual acids and salts for the respect of the environment.

Step c) practiced according to the characteristics of the invention allows thanks to the implementation of a pressure p greater than water vapor pressure saturating to avoid any significant evaporation of liquid reagents.

In conventional methods, for temperatures above 100 ° C, the pressure results from liquid-vapor balance which is established after vaporization of part of the liquid phase during heating. The silicate / high combination temperature / pressure p, provides a unexpected significant delignification while keeping a good DP for the dough. So we get lignin attack with selectivity comparable to that of chlorine gas or dioxide chlorine.

The method according to the present invention allows therefore to obtain delignification and laundering almost complete with chemical pulp after cooking in only 3 steps and using only agents inexpensive oxidants namely oxygen and peroxide hydrogen.

The invention will be better understood using examples, Tables I to X, and Figures 1 to 8 on the following 1/8 to 8/8 plates.

Examples

Examples 1 to 46 which appear in the Tables I to X, were made from three industrial chemical pulps obtained by kraft cooking and oxygen treatment.

The oxygen treatment of the pasta was performed according to standard conditions at 10% consistency, under an oxygen pressure of 3.5 bars, at a temperature of 95 ° C and for a period of 60 to 90 minutes depending on the nature of the wood.

The characteristics of these pastes are reported below in Table A: Type of dough after cooking and treatment with oxygen Dough pH Consistency in% Kappa index DP Whiteness ° ISO Softwood Kraft (KR) 9.4 27 13.1 1010 34.5 Hardwood Kraft (KFI) 8.7 31 9.2 1160 53.7 Kraft hardwood (KFII) 9.6 28 7.4 1170 44.9

General procedure of all examples (unless otherwise stated):

a) Complexing treatment The paste chosen from Table A is suspended at 10% consistency with 0.5% DTPA (commercial solution at 40% by weight of sodium DTPA in water) and heated for 15 minutes (0, 25 hours) at 90 ° C. The final pH is 9 to 9.6 depending on the chosen paste. The chosen paste is then filtered and washed with demineralized water.

b) Bleaching under pressure The paste collected in a) is mixed at room temperature with the charge of hydrogen peroxide, sodium silicate and demineralized water necessary to obtain the consistency chosen for the test, then the reaction medium as well. obtained is placed in a stainless steel autoclave which is completely filled with dough, reducing the dead space as much as possible. A few cm ³ of demineralized water are pumped into the autoclave to reach the chosen pressure. The autoclave is then heated to temperature t for the duration chosen. Heating has the effect of increasing the internal pressure. In order to maintain the pressure at the selected reaction value, the autoclave closing valve is intermittently opened to allow a few cm ³ of liquid to escape. After reaction, the autoclave is cooled and opened, the paste is collected on a filter and washed with demineralized water. The measurements of the whiteness, the kappa index and the DP are then carried out according to the ISO standards of the paper industry, recalled above.

Discussion of the results obtained:

1) Influence of the quantity of sodium silicate on the whiteness obtained and the DP Table I shows the comparative tests 1, 2, 3, 4 and 5 carried out on the KFI paste. Table II shows the tests 6, 7, 8 , 9, 11, 10, 12 and 13 carried out on the KFII paste. Figures 1, 2, 3 based on the tests below clearly show the very significant beneficial effect of sodium silicate and this effect is all the more more important that the bleaching temperature is high. Curve 2 for example shows that for a temperature of 150 ° C. and at 25 bars the addition of 2% of silicate makes it possible to gain 10 points of ISO whiteness. test 1 (comparison 0% silicate) and test 4 (8% silicate) shows that in addition to the very high whiteness gain (73.5 to 89.2 ° ISO) the degree of polymerization of the paste is practically preserved with silicate (DP = 1100) while under the same conditions but without silicate, the DP of the paste drops to 148. The addition of 2% of silicate (test 2) makes it possible to significantly raise the whiteness (85.4 ° ISO) but the DP remains low (DP = 372). With 4% silicate the DP drops slightly less (test 3, DP = 720).

2) Influence of pressure and temperature on whiteness Table III shows the tests 14, 15, 16, 17, 18, 19, 4, 20, 21, carried out on the KFI paste. Tables IV and X show the tests 13, 22, 23, 24, 25, 26, 27, 45 and 46 carried out on the KFII dough. Figures 4, 5 and 6 based on the tests below clearly show the beneficial effect of the pressure. that it is especially from 5 to 50 bars that the effect is most significant. It is therefore preferable to exceed 5 bars of pressure. This effect is recognizable from 110 ° C to 150 ° C. Curves 5 and 6 also show that for the same paste and for otherwise identical conditions, an increase in temperature increases the whiteness. The use of very high temperatures bleaching as 170 ° C allows to obtain very high degrees of ISO whiteness. This result contrasts with the findings of the bleaching of mechanical pulps for which the best whiteness is obtained at temperatures between 60 and 90 ° C.

3) Influence of the quantity of hydrogen peroxide Table V shows the tests 28, 4, 29, 30 carried out on the KFI paste. FIG. 7 based on the tests above shows a notable effect up to 8% in weight of H ₂ O _{2 used} in the reaction medium.

4) Relationship between whiteness and DP The tests in Table I will allow the DP curve to be plotted as a function of whiteness ° ISO. It is noted that the viscosities increase when the whiteness increases, for a temperature of 130 ° C. and a pressure of 100 bars absolute. This result is new and unexpected compared to the teaching of the prior art cited above. Indeed, it was not foreseeable that in the presence of silicate, sodium silicate would make it possible to maintain a good DP under the conditions of high temperature and high pressure. Table VI shows the tests carried out on the KFII paste (tests 31, 32, 33, 34, 35). Table VII shows tests carried out on KFI dough (tests 36, 37, 20, 38). Table VIII shows tests carried out on KFI dough (tests 39, 21, 40, 2, 41, 45, 46). Table IX shows tests carried out on the KR dough (tests 42, 43, 44).

Discussion of other examples

• Tests 34 and 35 in Table VI show that the addition of magnesium (1% magnesium sulfate) a a harmful effect on the painting.
• Tests 31 and 32 of Table VI show that the addition of sodium hydroxide (1% NaOH) has a detrimental effect on whiteness.
• Test 36 in Table VII and test 46 in Table X show that the addition of an additional quantity of DTPA during step b) of the action of H ₂ O ₂ very slightly improves the whiteness.

Comparative test 39 of table VIII, carried out at atmospheric pressure and at low temperature (90 ° C) leads to a much lower whiteness (79.2 ° ISO) than that obtained with the same quantities of reagents test 31 (ISO 88.6 °).

Test 38 of Table VII performed at low temperature but at high pressure gives for the same quantities of reagents a better whiteness (81.9 ° ISO) but much lower than that obtained by conditions of the present invention (test 31 of the table VI, 150 ° C, 100 bar, 88.6 ° ISO).

Test 41 of table VIII carried out under the same experimental conditions as test 4 of table I, but at a consistency of 9.5% instead of 20%, shows that even at low consistency (pumpable paste) the bleaching remains effective, which is not the case for the action of H ₂ O ₂ at atmospheric pressure.

Table IX shows the tests carried out on KR softwood kraft pulp with very low initial whiteness 34.5 ° ISO.
Test 44 shows that the conditions of the invention (temperature of 150 ° C., pressure of 100 bars, and 8% of silicate) make it possible to obtain a high whiteness (89.0 ° ISO), ie a gain of 54, 5 ° ISO and an almost complete delignification (kappa index equal to 1.2). Test 43 shows that with 4% H ₂ O ₂ a gain of 43.8 ° ISO is still obtained.

Tests 23, 26 and 45 show that for the same paste, the efficiency of bleaching increases with temperature. At 170 ° C, a whiteness of ISO 88.3 ° is obtained in only 20 minutes of heating.

Claims (20)

1. Process for the preparation of a delignified and bleached chemical paper pulp according to which a lignocellulose pulp obtained by cooking is subjected to the following series of treatments:

   a) a delignifying treatment with oxygen, followed

   b) by a treatment with a complexing or sequestering agent for transition metals, followed by washing, then

   c) by a treatment with hydrogen peroxide, in the presence of alkali metal silicate, characterized in that the treatment with hydrogen peroxide is carried out at a temperature t greater than 100°C and at a pressure p greater than 1.5 times the saturated vapour pressure of water at the temperature t.
2. Process according to Claim 1, characterized in that the treatment b) is carried out in a medium having an alkaline pH.
3. Process according to Claim 2, characterized in that the alkaline pH of the pulp during the treatment b) is greater than 7 and less than or equal to 12.5.
4. Process according to Claim 3, characterized in that the alkaline pH of the treatment b) is from 8.5 to 9.5.
5. Process according to one of Claims 1 to 4, characterized in that the complexing or sequestering agent is DTPA.
6. Process according to one of Claims 1 to 5, characterized in that the complexing or sequestering agent is used in the treatment b) in an amount from 0.1 to 1 % by weight with respect to the weight of dry matter of the pulp thus treated.
7. Process according to Claim 6, characterized in that the said amount is from 0.25 to 0.5 %.
8. Process according to one of Claims 1 to 7, characterized in that the manganese content of the pulp before treatment with hydrogen peroxide does not exceed 5 ppm by weight with respect to the weight of dry matter.
9. Process according to one of Claims 1 to 8, characterized in that the pressure p is from 5 to 200 bar absolute.
10. Process according to Claim 9, characterized in that the pressure p is from 25 to 50 bar absolute.
11. Process according to one of Claims 1 to 10, characterized in that the alkali metal silicate is sodium silicate.
12. Process according to Claim 11, characterized in that the sodium silicate is added in the form of 0.5 to 10 % by weight of a 38° Be aqueous solution with respect to the weight of the dry matter.
13. Process according to Claim 12, characterized in that the weight of the 38° Be aqueous solution is from 4 to 8 % by weight with respect to the weight of the dry matter.
14. Process according to one of Claims 1 to 13, characterized in that the temperature t is from 110°C to 180°C.
15. Process according to Claim 14, characterized in that the temperature t is from 130°C to 160°C.
16. Process according to one of Claims 1 to 15, characterized in that the pulp, during the treatment with hydrogen peroxide, has a consistency of 4 to 35 % by weight of dry matter.
17. Process according to Claim 16, characterized in that the consistency is from 10 to 20 % by weight of dry matter.
18. Process according to one of Claims 1 to 17, characterized in that the treatment with hydrogen peroxide has a duration of 1 minute to 3 hours.
19. Process according to Claim 18, characterized in that the duration is from 15 minutes to 1 hour.
20. Process according to one of Claims 1 to 19, characterized in that hydrogen peroxide is used in an amount of 0.5 to 10 % by weight with respect to the weight of the dry matter.

Images