WO1996005364A1

WO1996005364A1 - Bleaching of chemical paper pulp under sulphonating conditions

Info

Publication number: WO1996005364A1
Application number: PCT/SE1995/000486
Authority: WO
Inventors: Petter Tibbling; Ulla EKSTRÖM; Erik Nilsson
Original assignee: Kvaerner Pulping Ab
Priority date: 1994-08-16
Filing date: 1995-05-04
Publication date: 1996-02-22
Also published as: CA2194290A1; AU2755895A; FI970519A7; SE9402726D0; FI970519L; SE9402726L; SE502135C2; FI970519A0; ZA955971B

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

Bleaching of chemical paper pulp under sulphonating conditions

TECHNICAL FIELD

The present invention relates to a method of bleaching chemical paper pulp 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 sulphite-containing stages is carried out under conditions which are in the main sulphonat¬ ing. This is brought about by optimizing the system parameters pressure, temperature, time, pH and sulphur charge in such 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-containing stages can also be included.

STATE OF THE ART AND PROBLEMS

Due to the increasing 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 portrayed as villains in relation to the environment. However, in recent years we have been 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 generating equipment having high capital costs or else entail large costs for purchasing the bleaching chemicals . Other disadvantages are, for example, that ozone and peracetic acid have a degrading effect on the pulp. The disadvantage when bleaching with hydrogen peroxide alone is that the finally bleached pulp will contain a residual lignin content corresponding to 2 - 4 kappa units, which more than doubles the colour reversion induced by heat and UV light. Pulp which is bleached to full brightness, 88 - 90 ISO, using only alkaline peroxide also tends to require higher refining energy and yield inferior strength properties as compared with 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 unbleached pulp first being delignified with oxygen 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 intensive peroxide-bleaching stage. The quantity of hydrogen peroxide required for this method is relatively high, being 15 - 35 kg per ton of pulp, depending on the 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 Boston, MA. EKA Nobel AB has also proposed sequences of the type QPZ or QPZP, using which brightnesses in the range 82 - 87 ISO and 87 - 89 ISO, respectively, can be achieved, depending on the pulp type, see "Non-Chlorine Bleaching", J. Basta, L. Andersson, W. Hermansson; Proceedings March 2 - 5, 1992 - Westin Resort - Hilton Head - South Carolina; Copyright by Miller Freeman Inc.

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

We at Kvaerner Pulping Technologies AB (formerly Kamyr AB) have developed a substantially improved method for peroxide bleaching (see SE 9301960) to full brightness with a surprisingly 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.

SOLUTION AND ADVANTAGES

By means of the present invention, a method is developed in which internally 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 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 provide pulp which has been bleached entirely without using chlorine implies that it is preferred for the invention to be carried out as a part of a total bleaching sequence entirely without any chlorine-containing chemical. However, it is 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 alkaline, oxidizing bleaching stages. The oxidizing stage (s) is/are (a) preferably pressurized peroxide stage (s) (PO) 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 1 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 equal to or greater than 100°C, and more preferably be between 100 and 120°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 SO2 is consumed while HSO3- groups enter as sub¬ stituents into the lignin fragments, whereupon water- soluble compounds of residual lignin arise in the pulp. This results in a substantial kappa reduction directly after washing.

That sulphonation is taking place can readily be established by the fact that the pH 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 (kappa^ - kappa2) / appa!, where kappaη_. 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-containing, reducing bleaching stages are carried out to a degree of sulphonation exceeding 10 %, preferably exceeding 20 %, and still more preferably to 40 - 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-containing, reducing bleaching stages is 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 exceeding 1.2 bar, 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 exceeding 90°C, preferably equal to or greater than 100°C, and more preferably between 100 and 120°C. Such a stage is designated (AR*) and should also contain complex-forming agents for metal elimination, for example EDTA, in accordance with the scheme (S0₂ + Q, E) or (S0₂ + Q) . In this context, E designates alkaline extraction. According 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 NaHSθ3, can, for example, be used as the sulphite source in addition to SO2 • Another possibility is to utilize sulphur-containing process stream from a plant for gasification and combustion of black liquor, for example a so-called Chemrec reactor (see SE-C-448 173) as the sulphur source. Yet another possibility is to generate sulphur dioxide internally 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, alternatively, as acidifier, for 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 sulphonating, 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. While it is true that it is 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 is acidic and it is only the second which is alkaline. Further- more, the sulphite stage is not carried out under sulphonating conditions.

SE 461 991 discloses a method of improving subsequent oxygen-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, the temperature is stated to be between 20°C and 80°C and the pressure is presumed to be atmospheric . That sulphonating conditions are not achieved by this method is evident from the experiments 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 examined:

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 . 2. Q(PO) (AR) (PO) : as in 1, but with an acidic S0₂ stage 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) and a temperature of 105°C. The pulp was washed between the four stages.

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

Experimental conditions and results are shown in Table 1.

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, H2SO4 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. Table 1

1. Additions Sequence 1 Sequence 2 Sequence 3a Sequence 3b

2 . Conditions Q(P0) (PO) Q(PO) (AR) (PO) Q(PO) (AR*) (PO) Q(PO) (AR*) (PO)

3. Results

Q stage (common to 1-3)

1. H S0₄, kg ptp¹ 5

EDTA, kg ptp 2

2. Cone, % 10

Time, min 60

Temp., °C 70

3. Final pH 5.2

PO stage (common to 1- ^■3)

1. H₂0₂, kg ptp 18

DTPA, kg ptp 2

MgS0₄, kg ptp 3

NaOH, kg ptp 19

2. Cone, % 10

Time, hr 2

Temp., °C 120

Pressure, bar (ahis) 6

3. Consumption c ■f

H₂0₂, kg ptp 14

Kappa 4.0

Viscosity, dm³/kg 851

Brightness,

% ISO 82 (AR) or, alternatively, (AR ) stage

1. S0₂, kg ptp 18 18 27

DTPA, kg ptp 2 2 2

MgS0₄, kg ptp 3 3 3

2. Cone, % 10 10 10

Time, hr 1 1 1

Temp. , ^*C 90 105 105

Pressure, barfabs ) 1 6 6

Initial pH 5.2 5.2 4.0

3. Final pH 5.0 3.4 2.7

Consumption of S0 , kg ptp 10 16 24

Kappa 3.7 1.8 1.9

Viscosity, dm³, /kg 829 767 730

Brightness, % : ISO 82.5 82.3 82.4

PO stage

1. H₂0₂, kg ptp 23 14 14 18

DTPA, kg ptp 2 2 2 2

MgS0₄, kg ptp 3 3 3 3

NaOH, kg ptp 22 13 13 18

2. Cone, % 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 11.0 10.5 10.0 10.3

Consumption of

H₂0₂, kg ptp 17 8 8 14

Kappa 3.3 3.4 1.4 1.4

Viscosity, dm³/kg 686 773 723 660

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 3.8 1.7 2.3

Total consumption of

H 0₂, kg ptp 31 22 22 28

1) kg ptp = kilograms per ton of pulp 2) after 3 hours at 105°C, 0 * relative humidi It is evident from the experimental results that, after the preliminary bleaching Q(PO) , the pulp has obtained a kappa number of 4.0, a viscosity of 851 dm³/kg and a brightness of 82 % ISO. 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 of sulphonation in this case is (4.0 - 3.7) /4.0 = 8 %. By contrast, the sulphonating conditions (AR*) in accordance with sequence 3a have reduced the kappa number to less 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 5.0. As can be seen, the degree of sulphonation (4.0 - 1.8) /4.0 = 55 % is on a completely different level.

After the concluding pressurized peroxide stage (PO) 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 H2O2 per ton of pulp but also decreases the loss of brightness 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 sulphonation has been carried out somewhat more vigorously and at a lower pH. This experiment demonstrates that very high brightnesses (90.0 ISO) can be achieved using the method according to the invention.

Good 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) (PO) sequence.

As is evident; 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 produced using ozone or a peroxy acid (peracetic acid, persulphuric acid) . This is illustrated in more detail in diagrams 1 - 4 in which data from bleaching sequences containing ozone (4 kg of O3 ptp) , Q 0 (ZQ) (PO) , and peracetic acid (6 kg as H₂0₂ ptp) , Q(OP) (PaQ) (PO) , are compared with the reference Q(PO) (PO) and a number of sequences in accordance with the invention (AR*) (PO) (AR*) (PO) , Q(PO) (AR*) (PO) . In diagrams 1 - 2 there is also shown an alternative sequence in accordance with the invention, (AR*) O(APO) (PO) , where (APO) 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 SO2 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 shows that about 10 kg of peroxide are required in order to replace 4 kg of ozone. The sequence with a pressurized acetic acid peroxide stage,

(AR*)0(APO) (PO) , is even better in that only about 5 kg of peroxide are required in order to replace 4 kg of ozone .

It can be seen from diagram 2 that the viscosity of pulp which has been bleached in accordance with the invention is approximately the same as that of the ozone-bleached or peracetic acid-bleached pulp but considerably higher than the reference at high brightness (approximately 90 % ISO) .

Diagrams 3 and 4 show that the peracetic acid sequences and sulphonating sequences give approximately the same result as far as colour reversion is concerned and that this result is superior to that for the ozone sequence.

Diagram 5 show the effect on viscosity versus kappa number when using a sulphonating stage in connection 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)D(ED) Q(PO) QO(ZQ) (PO) Q(PO) (AR*) (PO

Kappa no: 3.7 1.5 2.2

Viscosity, dm³/kg: 860 820 740 765

Brightness, %ISO: 90 86 89 89.1

Yield, % on pulp kappa 12: 96 - 95 96

Zero span, kNm/kg: - - 94 98

Evolutions, PFI: 1300 1600 1300 1300

Slowness, °SR: 17.0 17.0 19.5 16.0

Density, kg/dm : 660 680 670 650

Air resistance, sec/lOOml: 3.8 3.8 5.0 4.0

Burst index, MN/kg: 5.9 6.4 6.0 5.5

2 Tear index, Nm /kg: 14.8 12.8 14.2 15.0

Sulphonating sequence - charge SO2 : 10 kg ptp, consumption SO2 : 4 kg ptp, consumption H2O2 : 21 kg ptp. 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 person skilled in the art that there are very many options for varying the construction of the bleaching 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

PATENT CLAIMS

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) .