CA2194290A1 - Bleaching of chemical paper pulp under sulphonating conditions - Google Patents

Abstract

Method of bleaching chemical paper pulp, preferably entirely without using chlorine, in a bleaching sequence which comprises at least one acidic, sulphite-containing, reducing bleaching stage and at least one alkaline, oxidizing bleaching stage in arbitrary order. At least one of the said acidic, sulphite-containing, reducing bleaching stages is carried out under conditions which are in the main sulphonating to a degree of sulphonation exceeding 10 %, preferably exceeding 20 %, and even more preferably to 40-80 %.

Description

- ~ W096/0s364 ~ P~~ r~FI
Bleaching of chemical paper pulp under sulphonating conditions TECHNICAL FIELD 2 ~ 9 ~ 2 9 0 The present invention relates to a method of bleaching chemical paper pulp in a bleaching se~uence which comprises at least one acidic, s~lphite-containing, reducing bleaching stage and at least one alkaline, oxidizing bleaching stage in arbitrary order At least one of the said sulphite-c~nt~;~;ng stages is carried out under conditions which are in the main sulphonat-ing. Thi=s is brought about by opt;mi7ing the system parameters pressure, temperature, time, pH and sulphur charse in s~ch a manner that a maximum proportion of water-soluble lignin is formed. The bleaching sequence is preferably entirely free of chlorine, but one or more supplementary chlorine-~nt~ining stages can also be ;n~ln~

STATE OF T~E ART AN~D PROBLEMS

Due to the ircreasing interest in the environment, and comprehension of the ecological cycle in nature, a great desire exists both among consumers and among producers to decrease discharges~of pollutants result-ing from human activities.
Producers of pulp and paper have often been portrayedas villains in relation to the environment. However, in ~ recent years we have besn working feverishly to decrease the discharges from our pulp and paper mills, and much progress has also been made. The demand of the market for paper which has been bleached without using chlorine or chlorine dioxide has led to a start being made in the use of alternative bleaching chemicals such as ozone (Z), persulphuric acid (Caro's acid, Ca), peracetic acid (Pa) and hydrogen peroxide (P). However, one of the disadvantages of these bleaching chemicals is that they either require g~n~r~t;ng equipment having high capital costs or else entail large costs for purchasing the bleaching chemicals. Other disadvantages W096/0~364 2 2 1 9 4 2 9 0 r~ l/D~' _. t ~ I

are, for example, that ozone and peracetic acid have a deyrading effect on the :pulp. The disadvantage when bleaching with hydrogen peroxide alone is that the finally bleached puIp will contain a~residual lignin content correspondiny to 2 - 4 kappa units, which more than doubles the colour reversion induced by heat and W light. Pulp which is bleacked to full brightnes6,~88 - 90 ISO, using only alkaline peroxide also tends~to require higher refining energy and yield inferior strength properties as compared wlth sequences including ozone or peracids.

A method of bleaching using hydrogen peroxide has been developed by the Swedish company EKA Nobel AB. The method, which goes under ~ the name LIGNOX (see SE 8902058), involves the urbleached pulp first being delignified with oxygçn and then, after washing, being treated with EDTA or other suitable complex-forming agent ~Q) in order to remove transition metals which are bound in the pulp. Sulphuric acid is used as an acidifier in the~ complex-forming stage. The EDTA stage is followed by an intensiv~peroxide-bleaching stage.
The quantity of hydrogen peroxide re~uired for this method is relatively high, being 15 - 35 kg per ton of pulp, depending on t~e brightness desired and on the bleachability of the pulp. The time required is long, being 4 hours or more, and the quantity of residual peroxide is significant. Only 40 - 60 ~ of the added peroxide is utilized. This method only achieves a limited increase in brightness to 80 - 82 ISO.
A similar technique has also been described by Interox.
In this technique, the pH in the complex-forming stage is adjusted using sulphur dioxide or bisulphite instead of sulphuric acid, see Troughton N. "The efficient use of hydrogen peroxide as a pulp delignification agent":
The Macrox Process; 1992 TAPPI Pulping Conference in Poston, MA.

~ W096l0s364 2 1 9 ~2qO P~

5 EKA Nobel As has also proposed se~uences :of t~e type QPZ or QPZP, using which briyhtne~sses in the range 82 -87 ISO and 87 - 89 ISO, respectlvely, can be achieved, depending on the pulp type, see ~Non-Chlorine Bleaching", J. Basta, ~. Andersson, W. Hermansson;
Proceedings March 2 - 5, 1992 - Westin Resort - Hilton Head - South Carolina; Copyright by Miller Freeman Inc.

EK~ Nobel' 8 patent application SE 9101300 (SE 468 355) additionally describes a bleaching method in which complex-forming agents are used prior to an ozone ar peroxide stage. This application principally relates to 020ne being used directly after a complex-forming stage.

We at Kvaerner Pulping Technologies AB (formerly Kamyr AB) have developed a subst~nt;~lly improved method for peroxide bleaching (see SE 9301960) to full brightness with a surpri~ingly high degree of utilization of added peroxide.-This method, which has aroused a great deal of interest, involves the peroxide bleaching being carried out at elevated pressure and temperature.

Nevertheless, despite the progress which has been achieved using replacement chemicals for chlorine, there still remain the disadvantages of, for example, high process and investment costs and a more or less extensive degradation of the cellulose.

SO~UTION AND ADV~TAGES
By means o~ the present invention, a method is developed in which ;nt~rn~lly generated sulphur compounds, which are available in the pulp mill, are utilized in order to provide, preferably in combination with hydrogen peroxide, pulps which have preferably been bleached entirely without using chlorine-containing chemicals, which method is characterized by the low chemical cost of ozone bleaching, by the 21 942~0 ~ ~

relatively high quality pulp of peracetic acid/chlorine dioxide bleaching, and by the lower capital cost of pure peroxide bleaching. The fact that the bleaching sequence is preferably intended to provlde pulp which has been bleached entirely without using chlorine implies that it is preferred for the i~vention to be carried out as a part of a total bleaching sequence entirely without any chlorine-containing chemical.
However, it i~ conceivable that, in certain cases, it may be desirable for one or more supplementary chlorine-containing stages, for example chlorine gas or chlorine dioxide, to be included in the total bleaching sequence.

In the method according to ~the invention, the total bleaching sequence, or a part of the total bleaching sequence, consists of a combination of acidic, sulphite-containing, reducing bleaching stages and ~lk~l ;n~, oxidizing bleaching stages. The ~ ;7ing stage(s) is/are (a) preferably pressurized peroxide stage(s) (P0) in accordance with SE 9301960, which means that it/they contain(s) peroxide and is~are carried out at a pressure at the top of the bleaching vessel exceeding l bar, preferably exceeding 1.2 bar, and more preferably exceeding 1.5 bar. At the bottom of the bleaching vessel, the pressure should exceed 3 bar, preferably exceed 4 bar, and more preferably exceed 5 bar. The temperature should exceed 90~C, preferably be ecual to or greater than 100~C, and more preferably be between lO0 and l20~C.
The method is characterized by at least one of the acidic, sulphite-containing, reducing bleaching stages being carried out under conditions which are in the main sulphonating. Sulphonation in this context means that S02 is consumed while HSO3- groups enter as sub-stituents into the lignin fragments, whereupon water-soluble compounds of residual lignin arise in the pulp.

~ W09~053~ 2 1 9 4 2 q O rL~ 5 r ~r This results in a s~bsta~tial kappa reduction directly after washing. ' ~
That sulphonation is taking place can readily be est~h~; ~hP~ by the fact that the p~ falls from an initial 4 - 6 to 3 - 4, and by the fact that SO2 is consumed and that the kappa number falls.
Expressed as water-soluble lignin, the degree of sulphonation is defined for a given bleaching stage as (kappa1 - kappa2)/kappal, where kappa1 is the kappa number entering the stage and~ kappa2 is the kappa number leaving the stage.
In the method according to the invention, the sulphonating, acidic, sulphite-crnt~;n;ng, reducing bleaching stages are carried out to a degree of sulphonation exceeding lO ~, preferably exceeding 20 ~, and still more preferably to ~o - 80 %. The reaction time for such a stage is 30 seconds - 120 minutes, preferably 5 - 120 minutes, and still more preferably 30 - 120 minutes. The consumption of sulphur, calcula-ted as sulphur dioxide, during the sulphonating, acidic, sulphite-rrnt~1ning, reducing bleaching stages i8 1 - 40 kg per ton of pulp, preferably 10 - 30 kg per ton of pulp.
The sulphonating conditions are~accomplished in one or more sulphite stages by a pressure in the top of the bleaching vessel exceeding 1 bar~absolute), preferably ~;ng 1.2 bart and more preferably exceeding 1.5 bar, and a pressure in the bottom of the bleaching vessel exceeding 2 bar, preferably between 3 and 15 bar, and more preferably between 5 - 10 bar, and also by a temperature p~rp~;ng 90~C, preferably e~ual to or greater than 100~C, and more preferably between 100 and 120~C. Such a stage is~~deslgna~ed (AR~) and should also contain complex-forming agents for metal elimination, for example EDTA, in accordance with the scheme (SO2 + Q, E) or (SO2 + Q). In this context, E
designates alkaline extraction.

W096~05364 2 1 9 4290 ccording to one aspect of the invention it can have a beneficial effect on the pulp quality not to allow the pH to fall below 3, preferably not below 3.5, in the sulphonating stage. This can be effected by adding alkali, preferably NaOH, as~the pH falls during the sulphonating stage.

Sulphite solutions or bisulphite solutions, such as Na2S03 or NaHS03, can, for= example, be used as the sulphite source in addition to S02. Another ~ossibility is to utilize sulphur-rnnt~ining process stream frQm a plant for gasification and combustion of black liquor, for example a so-called Chemrec reactor (see SE-C-443 173) as the sulphur source.
Yet another pnqc;h;l;ty is to generate sulphur dioxide ;nt~rn~lly by burning sulphur gases, which~have been driven off from black liquor, to, in the main, sulphur dioxide. This sulphur dioxide can then be absorbed in bleaching-plant liquid which is then used in the sulphite stage or~, alternat~ively, as acidifier, for ~s example in connection with a complex-forming stage.
Anthraquinone is also expediently utilized in the sulphite stages in order to improve bleaching selec-tivity still further. Addition of sodium borohydride represents another option for increasing the reducing capability of the sulphite solution.
A sulphite stage which is only acidic~and reducing, without being pressurized, and is consequently not significantly sulphr,n~t;ng, is designated (AR).

The equipment for carrying out the bleaching sequence according to the invention expediently consists of a combination of pressurized reactors with intervening pressure diffusers for washing. In certain cases, partial degasification can be required after one or more pressurized stages, for example after a (PO) stage, to enable satisfactory washing to take place in a subsequent diffuser.

~ W096l0s364 r~ r -While it i8 true that it i8 previously known from WO 92/07139 to carry out a reducing stage of the sulphite or bisulphite type between two peroxide stages, the first peroxide stage in this case i9 acidic and it is only the second which is ~lk~l ;n~. Further-~ l0 more, the sulphite stage is not carried out under snlrhnn~t;ng conditions.

SE 461 991 discloses a method of improving subsequentoxygen-delignification by means ~of alkaline sulphite treatment. This sulphite treatment thus has another purpose and is furthermore not carried out under conditions which are to any appreciable extent sulphonating.

It is known from EP 433 138 to have, in the bleaching sequence, two alkaline stages, for example peroxide, with an intervening acidic stage, for example SO2. The acidic stage is not carried out under sulphonating conditions in this instance either _In the examples, 2s the temperature i9 stated to be between 20~C and 80~C
and the pressure is presumed to be atmospheric.
That s~l3rhnn~t; ng conditions are not achieved by this method is evident from the exp~mPntq which are reported below.
EXPERIMENTS

In a series of experiments, an isothermally cooked pulp was bleached which, after oxygen delignification, had a kappa number of 12.1, a viscosity of 1020 dm3/kg, and a washing carry-over corresponding to 5 kg of COD per ton of pulp. Three bleaching sequences were ~m;n~

1. Q(Po)(PO): elimination of transition metals followed by two pressurized peroxide stages, entirely in accordance with known technique ~SE 9301960). The pulp was washed between the three given stages.

21'~42~0 ~
W096/0~64 p~ r _'rr r-2. Q(PO)(AR)(PO): as in 1, but with an acidic So2 ~tage at 90~C and atmospheric pressure between the two pressurized (PO~ stages, that is similarly to EP 433 138. The pulp was washed between the four given stages.

3a. Q(PO)(AR*)(PO): a preferred bleaching sequence in accordance with the invention, which sequence is carried out in the same way as for experiment 2 but with the conditions for the=sulphite treatment having been chosen so that residual lignin is sulphonated, that is pressure elevated to 6 bar:=(absolute) a~d a temperature of 105~C. The pulp was washed between the four stages. ~ =

3b. Q(PO)(AR*) nFo): the same sequence -as in 3a, but with somewhat different conditions.

Experimental conditions and results are shown in Table l The two initial stages Q~PO) were common to the three experimental sequences. The Q stage was carried out in accordance with the LIGNOX procedure (SE 8902058).
Thus, H2S04 was used in the~laboratory experiment for acidifying in the complex-forming stage. However, in the factory, it is preferred to use sulphur dioxide or returned H+ ions, liberated in association with sulphonating reactions in subsequent stages, for acidifying incoming pulp. Alternatively, the acidifi-cation stage can be carried out without complex-forming agents.

W 096/05364 r~/D~

Table 1 = ~ 3'-= = - ' 1. Ac~ition3 Seo,uence 1 Sequence 2 Sequence.3a Seguence 3b 2. Condition~ Q(PO~(PO~ Q(PO~(AR~(PO~ Q~PO~(AR~(PO~ Q(PO~(AR~)(PO~

3. P~esult~
, .
Q ~tage (common to 1-3~
1- H25O4, kg ptpl 5 EDTA, kg ptp 2 2. Conc., ~ 10 Time, min 60 Temp., ~C 70 3. Final pH 5.2 Po ~tage (common to 1-3~
1. H2O2, kg ptp 18 DTPA, kg ptp 2 MgSO4, kg ptp 3 ~aOH, kg ptp 19 2. Conc., ~ 10 Time, hr 2 Temp., ~C 120 8ressure, bar(abs~ 6 3. Consumption of H2O2, kg ptp lq ~appa 4.0 Viscosity, dm3/kg 851 ~rightness, 21 942qO ~
W 096/05364 ~ 7~ e -(AR) or, ~ltern~tively, ~A~ ) ~t~ge 1. 5~2~ kg ptp 18 18 27 DTPA, kg ptp 2 2 2 Mysog/ kg ptp 3 3 3 2. Conc., ~ 10 10 10 Time, hr Temp., C 90 105 105 Pressure, bar(abs) 1 6 6 Initial pH 5.2 5.2 4.0 3. Final pH 5.0 3.q 2.7 Consumption of 5~2~ kg ptp 10 16 24 Kappa 3.7 1.8 1.9 Viscosity, dm3/kg 829 767 73~
Brightness, ~ ISO ~ 82.5 82.3 82.4 PO atage 1. H2O2, kg ptp 23 14 14 18 DTPA, kg ptp 2 2 Z 2 MgSO4, kg ptp 3 3 3 3 ~
NaOH, kg ptp 22 13 13 18 2. Conc., ~ 10 10 10 10.
Time, hr 2 2 2 2 Temp, 'C 115 115 115 115 Pressure, bar(abs) 6 6 6 6 ~
3. Final pH 11.~ 10.5 8 10.0 10.3 Consumption of H2O2, kg ptp 17 8 8 14 Rappa 3.3 3.4 1.4 l.q Viscosity, dm3/kg 686 773 ~ 723 ~~ ~0 Brightness, ~ ISO 88.0 87.9 88.0 90.0 Colour rever- =
sion, ~ ISO2 84.4 84.1 86.3 87.7 Brightness loss, ~ ISO 3.6 ~ ~.8 1.7 2.3 Total consr~=ption of B2O2, kg ptp 31 22 22 28 1) kg ptp = kilograms per ton of pulp 2~ after 3 hours at 185~C, 0 ~ relative humidity ~ W096/0s364 2 l 9 4 2 9 0 P~ c ,- -It is evident from the experimental results that, after the prPl ; mi n~ry bleaching Q(P0), the pulp has obtained a kappa number of 4.0, a viscosity of 851 dm3/kg and a brightness of 82 ~ IS0.
After sulphite treatment under atmospheric conditions (AR) in accordance with sequence 2, the lignin content of the pulp, expressed as kappa number, has to a large extent been left intact with a value of 3.7. In accordance with the previous definition, the degree o~ sulphonation in this case is (4.0 - 3.7)/4.0 = 8 ~. By contrast, the sulphonating 1~ conditions (AR*) in accordance with sequence 3a have reduced the kappa number to leGs than half, with a value at 1.8.
That sulphonation has taken place is also evident from the fall in the pH to 3.4 as compared with sequence 2, pH 5Ø
As can be seen, the degree of sulphonation:(4.0 - 1.8)/4.0 =
55 ~ is on a completely different level.
After the rn~rln~;ng pressurized peroxide stage (P0) for the three sequences, it is clear that the sulphonating bleaching according to the invention (sequence 3a) not only reduces the peroxide consumption by about 10 kg of H2~2 per ton of pulp but also decreases the 1088 of brightrLess due to aging and also decreases the content of residual lignin to less than half after full bleaching.
The sequence 3b is the same as 3a except that the Gulphonation has been carried out somewhat more vigorously and at a lower pH. This experiment demonstrates that very high brightn~sP~ (90.0 IS0) can be achieved using the method according to the invention.
~ood results (not shown), especially with high brightnesses has also been achieved when substituting the initial Q stage with an (AR*) stage in any of the sequences described in Table 1. For example full brightness can be achieved by a (AR*)(PO)(P0) sequence.
As is e.yident; the degree of sulphonation affects the quality properties of the bleached pulp in a decisive manner and makes the pulp comparable with chlorine-free pulp W096/05364 2 ~ q 4290 ~ c -- -produced using ozone or a pero~y acid ~peracetic acid, persulphuric acid). This is illustrated in more detail in diagrams 1 - 4 in which data from bleaching sequences cn~t~;n;ng ozone (4 kg of 03 ptp), Q 0 (ZQ)(P0), and peracetic acid (6 kg as H202 ptp), Q(OP)(PaQ)(P0), are compared with the reference- Q~PO)(P0) and a numberh of sequences in accordance with the invention (AR*)(Po)(AR*)(Po), Q(PO~(AR*)(P0). In diagrams 1 - 2 there is also shown an alternative sequence in accordance with the invention,(AR~)O(APO)(P0), where (AP0) denotes a pressurized acetic acid peroxide stage. This type of sequence is shown to be almost as efficient and selective as the ozone sequence although it is very cost efficient.

Diagram 1 demonstrates that 16 - 24 kg of S02 consumed per ton of pulp saves approximately 10 kg of hydrogen peroxide as compared with the reference bleached only with alkaline hydrogen peroxide As regards the total consumption of hydrogen peroxide, the sulphonating sequences according to the invention by large give the same result as the peracetic acid sequence. Comparison of the sulphonating sequence and the ozone sequence ~hows that about 10 kg of peroxide:are required in order to replace 4 kg of ozo~e.-The sequence with a pressurized acetic acid peroxide stage, (AR~)O(APO)(P0), is even better in that only about 5 kg of peroxide are required in order to replace 4 Eg of ozone It can be seen from diagram 2 that the viscosity of;pulp which has been bleached in ~cc~ n~P with the invention is approximately the same as that of the ozone-bleached or peracetic acid-h]~a~hpd pulp but considerably higher~than the reference at high brightness (approximately 90 ~ IS0).

Diagrams 3 and 4 show that the peracetic acid sequences and snlph~n~ting sequences give approxima~ely the same result as 21 9~290 ~ W096/05364 .~ 'crr~

far as colour reversion is r~ n~ and that ~his result is superior to that for the ozone sequence.
-Diagram ~ shQw the effect on visc~sity versus kappa numberwhen using a sulphonating stage in c~nn~t ~ ~n with an oxygen delignification stage. The beneficiary effect of preceeding an oxygen stage :with a complex-forming stage is previously known. It has now been found that when replacing the complex-forming stage with a sulphonating stage, even lower kappa numbers can be reached, with acceptable viscosity If an additional sulphonating stage is added after the oxygen stage, the kappa number can be even more lowered.

In Table 2 is shown a comparison of physical properties of ECF (Elementary Chlorine Free) and TCF (Totally Chlorine Free) pulps. The property values have been interpolated to correspond to tensile index 80 kNm/kg.

Table 2 Sequence: D(EOP)DIED) QlP0) QOIZQ)(PO) Q(PO)(AR~)(F
~appa no: 3.7 1.5 2.2 Vicco~ity, d~/kg: 860 B20 740 765 ~ ' , %IS0: 90 86 89 89.1 Yield, % on pulp kappa 12: 96 - 95 g6 2ero sp~n, knm/kg: - - 94 98 Evolutions, PFI: 1300 1600 1300 1300 Slownes~, ~S~: 17.0 17.0 19.5 16.0 Den~ity, kg/dm3: 660 680 670 650 Air ~ P, sec/100~1: 3.8 3.8 5.0 4.0 P,ur~t index, MN/kg: 5.9 6.4 6.0 5.5 T-ar index, Nm /kg: 14.8 12.8 14.2 15.0 ~llphnnAting ~equence - charge S02: 10 kg ptp, ~nn _-;nn 5~2: 4 kg ptp, ~1l~ ;nn H2~2: 21 kg ptp W096/0~64 2 1 9 42~O . ~ f ' As can be seen in Table 2, the sulphonating sequence gives yield and strength properties similar to those of the ECF
sequence and better than those of the ozone sequence.

The invention is not limited by the embodiment in the experiments, but can be varied within the scope of the subsequent patent claims. Thus, it will be readily evident to the pe~son skilled in the art that there are very many options for varying the construction of the bleaching 1~ sequence, especially since the alkaline, oxidizing stage(s) do(es) not need to consist of ~a) peroxide stage(s) but can be constituted by any oxidizing bleaching chemical whatsoever .

Claims (16)

1. Method of bleaching chemical paper pulp, preferably entirely without using chlorine, in a bleaching sequence which comprises at least one acidic, sulphite-containing, reducing bleaching stage and at least one alkaline, oxidizing bleaching stage in arbitrary order.
The method is c h a r a c t e r i z e d i n that at least one of the said acidic, sulphite-containing, reducing bleaching stages is carried out under conditions which are in the main sulphonating to a degree of sulphonation exceeding 10%, preferably exceeding 20 %, and even more preferably to 40 - 80 %.

2. Method according to Claim 1, c h a r a c t e r i z e d i n that the said sulphonating conditions are accomplished by a pressure which, everywhere in the bleaching vessel, exceeds the vapour pressure of the liquid and of gases dissolved in the liquid, and by a temperature exceeding 90°C, preferably equal to or greater than 100°C, and more preferably between 100 and 120°C.

3. Method according to Claim 1, c h a r a c t e r i z e d i n that the said sulphonating conditions are produced by a pressure at the top of the bleaching vessel exceeding 1 bar, preferably exceeding 1.2 bar, and more preferably exceeding 1.5 bar, a pressure at the bottom of the bleaching vessel exceeding 2 bar, preferably between 3 and 15 bar, and more preferably between 5 - 10 bar, and by a temperature exceeding 90°C, preferably equal to or greater than 100°C, and more preferably between 100 and 120°C.

4. Method according to any one of Claims 1 - 3, c h a r a c t e r i z e d i n that the said sulphonating, acidic, sulphite-containing, reducing bleaching stage(s) is/are carried out to a final pH of 1 - 7, preferably 2 - 6.

5. Method according to any one of Claims 1 - 4, c h a r a c t e r i z e d i n that consumption of sulphur, calculated as sulphur dioxide, during one of the said sulphonating, acidic, sulphite-containing, reducing bleaching stages is 1 = 40 kg per ton of pulp, preferably 10 - 30kg per ton of pulp.

6. Method according to any one of Claims 1 - 5, c h a r a c t e r i z e d i n that the reaction time for the said sulphonating, acidic, sulphite-containing, reducing bleaching stage(s) is 30 seconds - 120 minutes, preferably 5 - 120 minutes, and even more preferably 30 - 120minutes.

7. Method according to Claim 1, c h a r a c t e r i z e d i n that the said alkaline, oxidizing bleaching stage(s) contain(s) peroxide, preferably hydrogen peroxide, and in that at least one such stage is carried out at a pressure at the top of the bleaching vessel exceeding 1 bar, preferably exceeding 1.2 bar, and more preferably exceeding 1.5 bar, while the pressure at the bottom of the bleaching vessel exceeds 3 bar, preferably exceeds 4 bar, and more preferably exceeds 5 bar. The temperature in the bleaching vessel should exceed 90°C, preferably be equal to or greater than 100°C, and more preferably be between 100 and 120°C.

8. Method according to Claim 1, c h a r a c t e r i z e d i n that the said acidic, sulphite-containing, reducing bleaching stage(s) contain(s) complex-forming agents.

9. Method according to Claim 1, c h a r a c t e r i z e d i n that the said acidic, sulphite-containing, reducing bleaching stage(s) contain(s) anthraquinone and/or sodium borohydride.

10. Method according to Claim 1, c h a r a c t e r i z e d i n that the pulp is washed between the bleaching stages.

11. Method according to Claim 1, c h a r a c t e r i z e d i n that, before it is treated in the bleaching sequence, the pulp has been oxygen-delignified to a kappa number of 15 or below and a viscosity of 950 dm3/kg or above.

12. Method according to Claim 1, c h a r a c t e r i z e d i n that the bleaching sequence is initiated by the pulp being acidified to pH 4 - 6, preferably in the presence of a complex-forming agent. As acidifying agent is preferably used sulphur dioxide, sulphite, bisulphite, sulphuric acid, effluent from the said acidic, sulphite-containing, reducing bleaching stage(s), or a combination of one or more of these acidifying agents.

13. Method according to Claim 1, c h a r a c t e r i z e d i n that, during the said acidic, sulphite-containing, reducing bleaching stage(s), the pulp has a consistency of between 8 and 16 %, preferably between 9 and 11 %.

14. Method according to Claim 1, c h a r a c t e r i z e d i n that the pulp is not washed between the bleaching stages.

15. Method according to Claim 1, c h a r a c t e r i z e d i n that the bleaching sequence also comprises a pressurized acidic peroxide stage, preferably with the use of acetic acid for acidification.

16. Method according to Claim 1, c h a r a c t e r i z e d i n that the pH is kept above 3, preferably above 3.5, throughout the said acidic, sulphite-containing, reducing bleaching stage(s), preferably by adding NaOH as the pH falls during the stage(s).