FR2474550A1 - Two stage pulp bleaching process - by ozone bleaching of acidified pulp at high consistency, diluting to lower consistency, and making alkaline for peroxy bleaching - Google Patents

Abstract

Pulp bleaching process comprises (a) bleaching the pulp with ozone at an acid pH and a consistency of ca. 20-45%, (b) reducing the consistency of the acid ozone-treated pulp by the addn. of water without raising the pH to 7 or more, and allowing the pulp to stand in this state for at least 10; and (c) adding sufficient alkali to raise the pH over 7 so as to cause peroxy bleaching of the pulp without adding peroxy type chemical bleaching agents. By introducing stage (b), the peroxy functions formed in the system during the acid ozone treatment are not destroyed, so that alkaline peroxy type bleaching can be carried out subsequently without the addn. of further chemical agents.

Description

The present invention relates to whitening

  ozone and peroxygenic from pulp or cell pulp

losique.

Previous ozone bleaching processes

They were made either at a low whore consistency or at a high consistency. The consistency of the dough refers to the water content of the dough. A

high-consistency dough is a fluffy, ty-

  pe solid, with a consistency of 20% to 45%, while

  low consistency generally refers to a mixture

  pumpable age of about 1% to 5% by weight of dough on a

air-dried base.

Low-ozone bleaching processes

consistency consist of large volumes of water in contact with the dough (the dough constituting about 1%

  at 5% by weight of the total) and the ozone is diffused or dispersed

under in the water, so that ozone bleaching

  dough occurs in the aqueous phase.

  Ozone bleaching of the high-concentration type

  sistance is a gaseous ozone bleaching treatment in which the fluffy pulp or pulp, an apparently solid fibrous material, is brought into contact with gaseous ozone in a suitable container, such as the container described in the patent of the States -United

America No. 4,158,597, at an acidic pH during a

  sufficient time to whiten the dough. An ozone bleaching process is described in United States Patent No. 3,451,888 entitled "Bleaching Pulp Having High Consistency With Ozone Having Moisture Content Near 100%", as well as in Canadian Patent No. 966.604 entitled "Kraft Pulp Bleaching and Recovery Process". As described in this Canadian patent, the high consistency paste from the ozone gas bleaching treatment can be washed, if desired, and then treated with an alkali in a

  alkaline extraction treatment or with a solution

  alkaline hydrogen peroxide (see page 3, li-

  gene 22 on page 4, line 11) after ozone bleaching

  born. Washing the dough has been suggested before processing

  alkali but this washing displaces the associated water

dough during bleaching with ozone gas tan-

  say that treatment with an alkali at a pH above 7 destroys the peroxygenic values discovered by

  the plaintiff in water of this type.

The recycling of effluent from various

Various bleaching treatments to reduce the fresh water requirements of bleaching operations are well known, as can be seen in this Canadian patent (page 4, lines 17 to 30). However, the effluent is not obtained from the ozone bleaching treatment because the pulp is at

  very high consistency (20% to 45%) and takes as--

pect of a solid fluffy material. After bleaching

  Water is added to ozone in large quantities.

  aunts with ozone bleached fluffy paste for

  reduce consistency to about 1% to 5%, so

re to obtain a pumpable water / paste mixture, an alkali

  being added immediately for the preparation of the

  alkaline which immediately follows the ozone gas whitening treatment. This addition of alkali

  with the water / pumpable paste mixture destroys the peroxy-

genes contained in it, discovered by the

plaintiff.

Based on the discovery of values

  peroxygenic in acidic water associated with the paste of the

Veteuse emerging from the ozone bleaching reaction

only gaseous at a pH below 7 and on the discovery that

  peroxygenic values are destroyed at pH al-

calin, we have developed a process which uses these peroxygenic values to achieve a certain peroxygen bleaching of the dough without the need to add peroxygenic bleaching chemicals. The improved process sequence includes a step of bleaching with ozone gas, a treatment

of low consistency acid retention in a treatment

  highly alkaline dough from the treatment of

  low consistency acid retention.

  Other details and peculiarities of the in-

  vention will emerge from the description below, given

by way of nonlimiting example and with reference to the appended drawings, in which:

  Figure 1 shows the reten-

acid with low consistency inserted between a

ozone gas bleaching treatment and treatment

alkaline extraction. Also found there

  presented the recycling of the effluent after the treatment

  low consistency acid retention.

  FIG. 2 represents a method of bleaching

discontinuous with recycling of the effluent.

Ozone gas bleaching treatment

  is usually and preferably carried out at a consis-

  high tance (20% to 45% by weight of dough and the remainder in water) and at an acidic pH (less than 7) and bleached with a gaseous mixture containing oxygen and approximately 2% to approximately 4% of ozone . Canadian Patent No. 966,604

  describes an ozone bleaching process of this type.

  The residence time for the fluffy pulp in the reac-

  The ozone content is preferably about 30 seconds at a temperature of about 250C to 301C. Outgoing gas

  of the ozone reactor is essentially devoid of ozo-

ne (for example less than 1% of ozone), Previously, this fluffy pulp bleached with ozone was washed and treated with an alkali, as described in example 1

of Canadian Patent No. 966,604. The whitening sequence

  ment of the present invention deviates from the previous bleaching sequences by interposing a treatment of low consistency acid retention before washing or treating the bleached fluffy pulp.

with ozone with an alkali.

  The bleaching process of the present in-

  vention can be better understood by referring to the fi-

  gure 1 of the attached drawings. In FIG. 1, a fluffy paste 10 with a consistency of 20% to 45% and at an acidic pH is added to an ozone reactor at the same time.

  time that a gaseous mixture 12 containing ozone, or-

  normally around 2% to 4% of ozone, the remainder being oxygen or air. Ozone in

the gas mixture is practically exhausted in the reaction

ozone during the bleaching reaction with the pulp and the gas 16 leaves the ozone reactor

  generally ozone-free. The pulp bleached with ozo-

  ne 20, still at a high consistency (20% to 40% /.), is discharged from the reactor 14 via an unloading device 18 comprising a gas trap, such as a dough or dough plug and water above the discharge device, so as to prevent the release of ozone gas

  same time as the fart. After ozo- whitening

  ne, the consistency of the dough 20 is reduced by the

dition of water usually up to a consistency of at least 12%, preferably until a

  consistency between 1% and 10%. A consistent

  this pumpable of about 1% to 3% turns out to be quite suitable

prayed for the dough. This dough of weaker consis-

  Tance 24 is transferred while still at an acidic pH to a holding or stay zone 26. The pH

ozone bleached paste 20 and 24 is acidic to

se conditions prevailing inside the ozone reactor and, therefore, the pH of the paste in the residence zone 26 is also acidic. It is important that the pH of this ozone-whitened fart is not raised to 7 or more until the retention time in the residence zone 26 is exceeded. The retention time of the dough in the living area 26 east

from about 10 minutes to about 60 minutes, a

12-minute period is particularly preferred

  corn. Therefore, this part of the process in the-

which dough is kept at an acidic pH and is in

tact with the water associated with the fluffy pulp bleached with ozone when it was in the ozone reactor and with a certain amount of complementary water to lower the consistency of the plte, is called

here the low consistency acid retention treatment

this. The low consistency dough 27 exiting the stay zone 26 was subjected to the low consistency acid retention treatment and was bleached to a greater degree than the dough 24 due to bleaching by peroxygenic values (comprising peroxy groups ) available in the water associated with the fluffy paste 20 coming from the ozone reactor. Ozone bleached dough and low consistency peroxygen values (with peroxy groups)

27 is preferably dehydrated at 28 so as to obtain

  hold a dough 29 before being subjected to treatment

  at ordinary alkaline pH, such as an extraction treatment

  alkaline tion, a peroxygen whitening treatment

that or by alkaline peroxide, or else a treatment of

bleaching using an alkaline hypochlorite. The fi-

  gure 1 shows dough 29 entering an ex-

  alkaline traction in which caustic soda is added to raise the pH of the dough above 7 (preferably from 10 to 11). The dehydrated dough 29 is preferably at a consistency of about 10% to 20%, and more preferably at a consistency of about 12%, although the dehydration step can be omitted. The water 25, obtained from the dehydration step of the dough 27, is at an acidic pH but is preferably recycled to bring the water necessary to lower the consistency of the fluffy, bleached dough. '' ozone 20 from

  ozone reactor, because the lowering of the consis-

  This should be done without raising the pH of the ozone bleached fluff to 7 or above

this value.

  The retention time for the dough in the residence zone 26 is preferably made optimal. If the retention times in residence area 26 is reduced to zero, then peroxygen bleaching will not occur and peroxygen values will form

in recycling stream 25 up to a level if-

  significant of about 0.17 g per liter of peroxides

  worn in the form of hydrogen peroxide. In aug-

lying about the retention time of the dough in the holding zone 26, a greater amount of the peroxygenic values in the water associated with the dough 20 will be consumed for bleaching the dough in the holding zone, which reduces the values of peroxide in the recycling stream 25. The control of the peroxide values in the recycling 25 gives the information

  necessary to maximize the whitening of the paper

te in the living area 26. The exact nature of the

  peroxygen or peroxide values is not included

except that they prevent a bleaching effect on the cellulose pulp as a function of time at an acid pH and that they give a positive result when it is tested for peroxygenic values by titration

  following the TAPPI test for hydrogen peroxide.

  The following examples illustrate further

tail of specific embodiments of the process.

  The best mode considered entering the

  dre of the present invention is done continuously

  with recycling of the effluent, as indicated in the figure

gure 1 and described in example 3, in accordance with the

  processing ditions discussed above and exemplified

below. In all of the examples, the brightness levels

  reported spears are Elrepho gloss units, the consistency refers to the percentage by weight of

dough (on an air-dried basis) in the dough mixture

  te / water and all proportions are by weight, except

contrary indications.

Example 1

  A 200 g sample of southern hardwood kraft pulp (base dried in the oven) is treated with water and sulfuric acid to give a paste with a consistency of 35% at a pH of 1.9 . This 35% consistency, acidified paste is brought into contact

with oxygen containing 3% ozone and at a temperature

  rature of 251C for about 1 minute, period during which the dough is continuously agitated to ensure good contact between the dough current and the

  ozone gas flow in the same direction. The amount of ozo-

  not consumed by the dough during the whitening process

ment is of the order of 1% of ozone relative to the weight

oven-dried dough.

  The ozone bleached dough has a shine of

  69.1 after being discharged from the ozone reactor.

The ozone bleached paste is then diluted with

2474550 =

  water so as to bring it from a consistency of 35% to a consistency of 4% 0. After mixing, a sample of the water in the dough mixture is analyzed for a

consistency of 4% to determine the peroxy-

  or peroxide according to the TAPPI titration test for hydrogen peroxide, and the water contains 0.034g per liter of peroxygen or peroxide values reported in the form of hydrogen peroxide. The

  pH of the dough of 4% consistency is 2.7. The pa-

te of 4% of consistency is kept at room temperature without the addition of alkali for 60 minutes and at intervals of 10 minutes, the paste is tested for its gloss and the water is tested for the values

  peroxygenic or peroxide. The results of the

know are given in Table I.

Example 2

  The process of Example 1 is repeated with step

additional pe which consists in submitting each dough sample taken at intervals of 10

minutes to an extraction treatment using an al-

  cali, using a 2% solution of sodium hydroxide

dium, and at a consistency of 12% for the extraction step

  alkaline tion. The brightness of the paste extracted by the alkali and the peroxygenic or peroxide values in the water associated with the paste sample before treatment with the alkali, are given in the

Table II.

Example 3

  FIG. 2 represents the flow diagram of the method of this example. We use a sequence of

  discontinuous treatments in order to approach in the laboratory

  roof, of a continuous bleaching process-using bleaching with ozone gas, a low consistency acid retention treatment, washing of the dough, alkaline extraction, washing of the dough, bleaching using '' an alkaline peroxide and a final washing of the dough. Recycling against the current

effluent from each of the washing steps

  Age of the dough is made as shown in Figure 2. Initially, a first 500 g dough sample, consisting of a southern hard wood pulp of the kraft type, dried in the oven, is diluted with water in a tank 10 to a consistency of 4% to obtain a dough 12. The dough 12 is then washed in

adding 4,500 g of water and compressing in 14 for ex-

  milk 13,000 g of liquid extract 18 and dough 16

  having a consistency of 12.5%. The dough 16 is then

  te acidified and its consistency is up to 35% for the preparation of ozone bleaching. This is carried out by adding 8,500 g of water, sulfuric acid and dough 16 to the tank 20, mixing and then extracting 11,100 g of extract 28 from the press 24 so as to obtain the dough of a consistency of

35%, acidified 26 to a pH of 1.9. The dough 26 is then

  bleached in an ozonator 30 at a temperature of 250C and with an oxygen gas containing 2.9% ozone for a reaction time sufficient to consume essentially all of the ozone in

the gas mixture. The paste and the gas containing the ozo-

  do not flow against the current through the device

ozonization 30. The amount of ozone consumed equi-

  is equal to 1% relative to the dry weight of the dough. We-

  raise a 100 g sample of the blanched dough to

  ozone and the remaining 400 g of ozone-bleached dough

  ne 32 are diluted with 8.900 g of water in a tank 34 (consistency of 4%). Dough 36 is washed with

an additional 4,000 g of water and is com-

  awarded in 38 to obtain 10,800 g of extract 40 and

  dough 42 to a consistency of 12%. We add hydro-

sodium xyde in the pulp 42 in the tank 44 for the extraction with caustic soda. The caustic soda dough 46 is placed in a tank 48 or 6.800 g of additional water is added, this operation being followed by washing the dough 50 with

  4.000 g of additional water and by pressing (ex-

  traction) in the press 52 so as to obtain

  10,800 g of extract 54 from the extraction treatment

  alkaline and washing and so as to produce pulp extracted 56 to a consistency of 12%. Dough

56 is treated with hydrogen peroxide, hy-

  sodium hydroxide and sodium silicate in the re-

actor 58 in order to subject the pulp to a bleaching treatment with alkaline peroxide, ordinary. The pulp bleached with alkaline peroxide 60 is diluted with 6.800 g of water in the tank 62 and the resulting paste 64 is washed with 4.000 g of water and compressed in the press.

  68 so as to obtain 400 g of white dough product

  chi 70 and 10,800 g of extract 66. A second sample of 500 g of southern hardwood kraft pulp is then

  subjected to the same sequence of whitening treatments

  and washing in the same apparatus, as in-

shown in the lower part of the flow diagram shown in Figure 2. The extracts from the bleaching and washing treatments of the first dough sample are collected and used in the treatment of the second dough sample in order to

  reproduce the countercurrent recycling effect of -

the extract. The second 500 g dough sample is combined with 12,500 g of aqueous extract 18 coming from the treatments of the first dough sample so as to obtain dough 104 at a consistency of 4% Y. 4,500 g of water (72 and 76) used to wash the paste 104 are obtained by combining 1,900 g of extract 40 and 2,600 g of extract 28. Washing and pressing of paste 104 gives 16,000 g of extract 92. The remaining 8.500 g

  of extract 28 are used at the same time as a quantity

  sufficient sulfuric acid to acidify the washed paste 106 to a pH of 1.9 before ozonization. 11.100 g of extract 94 are then obtained from the pressing of the acidified paste 108 at a consistency of 35% and at a pH of 1.9 (paste 110). 8,900 g of water 74 used to lower the consistency of the pulp bleached with ozone 112 are obtained from the extract 40 to

from the previous whitening sequence. We ob-

holds an additional 4,000 g of water 80 used to wash the ozone bleached dough 114 at

from extract 54. After extraction with alka

li of the washed dough 116, the dough extracted 118 is diluted with 6.800 g of water obtained from extract 54.

  then wash the diluted paste 120 by the addition of

re 4.000 g of water 84 obtained from extract 66, and it is then compressed. This makes it possible to obtain 10,800 g of aqueous extract 98 and of paste extracted 122 at a consistency of 12%. The paste 122 is then bleached with hydrogen peroxide at an alkaline pH

and in the presence of sodium silicate in the or-

dinary. 6.800 g of water 88 obtained from the extract

66 are used to adjust the consistency of the pa-

bleached with H202 124 before washing and pressing

  wise. 4.000 g of fresh water 90 are added to the dough 126 for washing. Pressing the dough 126 gives a dough product 102 and 10,800 g of extract 100. This process is then repeated six more times (samples 3 to 8), each time using a fresh 500 g dough sample and using

  extracts 92, 94, 96, 98 and 100 from processing

  whitenings of the pre-paste sample

in the same way as we used the extracts

  18, 28, 40, 54 and 66 from the treatments. white

the first sample of dough in the processors

  bleaching elements of the second pulp sample,

as shown in figure 2.

  The sequence of whitening treatments

  and the reuse of the extract in Example 3 re-

  produce approximately a continuous sequence of bleaching treatments using ozone bleaching, alkaline extraction and bleaching

  alkaline hydrogen peroxide with counter recycling

stream of effluents from blanch treatments

  chiment and extraction (washing and pressing

  the dough after adding the appropriate chemicals

prayed are considered part of each

  treatment). Each fraction of pulp bleached with ozo-

ne (paste 32 for the first sample and paste 112 for samples 2 to 8) is kept in the tank 34 at room temperature and at an acidic pH for approximately 12 minutes, while being in contact with an additional quantity of water for dough 32 and

  with a portion 74 of extract 40 for the second sample

tillon and a portion 74 of extract 96 for the samples

  tillons 3 to 8. The portion of extract 74 contains water

associated with the pre-ozone bleached pulp sample

  yield. This leads to peroxygen (or peroxy type) bleaching of the dough in the next reservoir 34

the present invention.

The dilution of each fraction of kraft hardwood pulp bleached with ozone 112 from

  samples 2 to 8 with extract 74 from the state

  eg washing of a previous ozone bleaching treatment at an acidic pH (if necessary, a sufficient quantity of sulfuric acid is added to adjust the pH to 1.9) and the retention of the The paste in the acidified liquid for at least 10 minutes leads to the low consistency acid retention treatment as defined in the context of the present invention. However, the peroxygen bleaching effect (by peroxide or peroxy type) on samples 2 to 8 is greater than the effect obtained

at the start of the low acid retention step

  sistance on dough 32, due to the formation of

  their peroxygenic (or peroxide or peroxy type) in extract 74 due to recycling against the current

effluent extract.

  Although the reuse of effluent 54

from sample 1 and effluent 98 from

  nant of samples 2 to 7 as parts of the wash water used for washing the ozone bleached pulp, causes some addition of soda

caustic with ozone bleached pulp in the tank

  see 34, the amount of caustic soda is not enough

to raise the pH to 7. Some ad-

  addition of caustic soda to the treatment of low consistency acid retention is tolerable provided

that the pH of the extract 74 or the paste of low con-

  sistance 112 to 114 does not reach 7. We add pre-

  ferments sufficient acid to low consistency acid retention treatment to maintain

* the pH between about 1.8 and 3.0.

  Dough samples after being sou-

put to the stages of ozone bleaching, low consistency acid retention, and alkaline extraction

  (dough 56 for sample 1 and dough 122 for the samples

  tillons 2 to 8) are tested for gloss and reverse gloss. The results of these tests are reported in Table III in the columns under the heading 03 / E. Dough samples after

been subjected to ozone treatments, acid retention

Low consistency, alkaline extraction and bleaching by alkaline peroxide (paste 64 for sample 1 and paste 126 for samples 2 to 8) are tested for gloss, reverse gloss and viscosity. The results of the tests are given in Table III under the heading 03 / E / P. In addition, we test the pH of different extracts (18, 28, 40, 54 and 66 for the dough sample 1 and 92, 94, 96, 98

and 100 for dough samples 2 to 8). The -results-

  pH test states are given in Table III

under the header-Extracts.

In Example 3, seven fresh pasta fractions, corresponding to Pasta Samples 2 to 8 in Table III, are treated with extracts from the previous pasta fraction obtained according to the above sequence. Ozone bleaching is carried out at room temperature with 2.9% ozone in the incoming oxygen gas stream. The extraction

  alkaline is carried out at 520C and with a retention time

90 minutes before dilution and washing. The

  bleaching treatment with hydrogen peroxide alka-

  flax is produced at 710C and with a retention time

180 minutes before dilution and washing. The trai-

  low consistency acid retention is achieved

  read at room temperature although it can be used

read high temperatures, for example from 250C to

1000C.

  The low consistency acid retention treatment can be used as part of a bleaching sequence in combination with other ordinary bleaching treatments involving alkaline treatments, such as those previously described,

  and ordinary acid pH treatments such as white

  chlorine dioxide, as long as the

  alkaline substances are not inserted between the treatment

  ozone gas and retention treatment

  weak consistency acid. In addition, we can add

  ter bleaching agents which can be used at an acidic pH to treat acidic retention of low consistency,

  if desired, such as chlorine dioxide. An example

  full of a whitening sequence falling within the scope

  of the present invention in combination with processors

common, involves gaseous ozonization, low acid acid retention treatment, chlorine dioxide bleaching and then

ordinary alkaline treatments.

TABLE I

  Time Brightness Peroxide (peroxy) values <%>

0 60.0 0.17

10 60.9 0.17

61.0 0.12

60.6 0.05

60.4 0.03

59.9 0.02

60 60.0 0.01

1,

TABLE II Brightness Peroxide values (peroxvaéniaues) (m / 1) 69.1 71.2 71.7 71.8

  71.8 71.6 71.2 0.034 0.020 0.014 0.012 0.009 0.007 0.003 Time

10

TABLE III

Extrai dough sample

18 28 40

  (92) 7.7 6.7 6.3 6.8 4.1, 9, 5 6.4 (94) 1.9 1.9 1.8 1.8 1.8 1.8 1.8 1 .8 (96) 2.7 2.6 2.7 2.5 2.7 2.7 3.1 3.4 Lt

54 66

(98) (100)

11.5 10.9

11.8 10.8

11.6 10.5

11.5 11.0

11.9 11.1

12.0 10.9

11.8 11.2

11.9 11.4

03l / E

Shine inv.

68.3 73.2 74.1, 3, 3 78.4 73.0 72.5 67.2 68.4, 2 73.9 72.5, 7, 6 69.7 Gloss, 0 86.8 86 .2 87.1 88.0 87.2 87.2 87.4

0., / E / P

Inv. 82.3 84.9 84.1 83.5 84.6 83.3 83.7 83.6 Viscos. 8.8 7.3 7.0 7.1 7.1 7.3 7.1 8.2 -J r ', -J' c ,. l z

18. 2474550

Claims (10)

  1. - Process for bleaching a dough using   a sequence of pulp treatments involving a   bleaching (14) with gaseous ozone (12) at a paste consistency of about 20% to about 45% - and at an acidic pH followed by an alkaline treatment (32) at a pH above 7 and a dough consistency lower than the dough consistency in the ozone bleaching treatment, this method being characterized in that it consists in using an acid retention treatment (26) with low consistency between the treatment of ozone bleaching and alkaline treatment, the low consistency acid retention treatment of lowering the consistency of the pulp (20) obtained from the ozone bleaching treatment by the addition of water (25) to the dough without neutralizing the dough at a pH of 7 or higher than this value, to maintain the pulp bleached with ozone at this lower consistency (24) and at this acidic pH for a retention period of at least 10 minutes (26), and then treating the dough with a sufficient amount of alkali (30) to raise the pH above 7, so that the peroxy-type bleaching of the pulp is carried out during the retention treatment (26) without the need to add peroxy-type bleaching chemicals in this retention step.   2. - Method according to claim 1, carried out continuously and with a recycling against the current of water obtained by dehydration (28) of the dough (27), this method being characterized in that it consists in removing l water from the paste (27) after the retention time (26), taking the removed water (25) and combining it with the ozone bleached paste (20) entering the retention treatment (26) low consistency acid to lower the consistency of the ozone bleached pulp, so that the use of peroxy (peroxygenic) and acid values in recycled water is maximized when bleaching the pulp in the treatment of low-consistency acid retention (26) tance. 19. 2474550   3. - Method according to claim 1, charac-   terized in that the retention time is of the order of minutes to 60 minutes and in that it is dehydrated to in addition to the dough after the retention time until a consistency   10% or more. 4. - Method according to claim 3, characterized in that one uses the water (25) obtained from the dehydration (28) of the dough (27) as a source of water to lower the consistency of the dough obtained from   ozone bleaching treatment. 5. - Method according to claim 3, charac- terized in that the treatment (32) of the dough with an alkali (30) is part of an alkaline extraction treatment of the dough. 6. - Method according to claim 5, charac-   terized in that the pulp extracted with alkali is subjected to a bleaching process using an alkali peroxide or a alkaline peroxy compound.   7. - Method according to claim 1, charac-   terized in that the consistency of the dough in the acidic retention treatment (26) of low consistency is approximately 4%.   8. - Method according to claim 1, charac-   terized in that the consistency of the dough in processing   bleaching (14) with gaseous ozone (12) is about 35%. 9. - Method according to claim 4, charac-   terized in that the treatment (32) of the dough with an alkali (30) is part of a treatment chosen from the group comprising alkaline extraction, bleaching using an alkaline peroxide, bleaching using a peroxy compound alkaline and bleaching using alkaline hypochlorite.   10. - Method according to claim 9, charac-   terized in that it comprises a bleaching treatment at acid pH of the pulp after the acid retention treatment (26) with low consistency and before the treatment of the pulp with   alkali to raise the pH above 7.