CA2032315C - Process for bleaching of chemical cellulose pulp - Google Patents

Abstract

Up to now, full scale use of the from the environmen-tal point of view excellent bleaching agent ozone, has been hampered e.g. by the comparatively large amounts required and the excessive depolymerization of the pulp. The present invention aims at solving these problems and relates to a three-step method for bleaching chemical pulp with a lignin content corresponding to a kappa number not exceeding 10.
This method is characterized in that the bleaching begins with a chlorine dioxide step at a comparatively high pulp consistency, and that the pulp, after the bulk of chlorine dioxide has been consumed, is intimately contacted with an ozone-containing aqueous solution and brought to react with the ozone at a considerably lower pulp consistency than in the chlorine dioxide bleaching, whereupon the pulp under-goes an alkaline treatment in the presence of oxygen and/or an oxidizing agent giving off oxygen.

Description

- l 2032315 Method for bleaching chemical pulp Background of the Invention The present invention relates to a method for bleach-ing chemical pulp with a lignin content corresponding to a kappa number not exceeding 10, as measured according to SCAN-C 1:77. Chemical pulps form an unambiguous pulp cate-gory and include for instance sulphite pulp, sulphate pulp, and polysulphide pulp. Especially suitable are those chemical pulps which have a high viscosity at a compara-tively low kappa number. This group comprises sulphate pulpdigested at a high sulphidity, modified sulphate pulp digested according to a counter-current process in which white liquor is added also at an advanced stage of the digestion, and alkaline pulps digested in the presence of a catalyst, for example a quinone compound such as anthra-quinone. Other suitable pulps are those designated MSS-AQ
(mini-sulphide-sulphite-anthraquinone), Si-Sa-Si (sulphite-sulphate-sulphite), and PS-Si (polysulphide-sulphite), all of which are mentioned in the Journal "Paperi ja Puu"
(Paper and Timber), 5/1989, pp. 509-513.
Although it is possible to produce, from some ligno-cellulose materials, a pulp which already after the diges-tion (delignification) has a kappa number not exceeding 10, it is not possible, however, in other cases to reduce the lignin content of the pulp thus much during the delig-nification proper, for example if the starting material is wood of high lignin content. In such cases, the ligno-cellulose material in the form of pulp is delignified further, preferably by treating it with chemicals producing spent liquors which, like the spent liquor from the diges-tion, can be recirculated to the chemical recovery system of the pulp mill. Advantageously, this delignification takes place under alkaline conditions in the presence of one or more oxidizing agents, such as oxygen and/or perox-ide. A highly selective and, if so desired, far-reaching delignification of pulp is obtained if the above-mentioned alkaline delignification of the pulp is preceded by a so called activation of the pulp, by which the pulp is brought - into contact with nitrogen dioxide, hydrogen and nitrate ions and, optionally, oxygen.
Description of the Prior Art There exist a large number of bleaching agents and many times this number of bleaching sequences for bleaching pulps of the type described above to obtain an almost or completely lignin-free pulp.
Frequently, a mixture of chlorine (C12) and chlorine dioxide (Cl02) is used in a first bleaching step. Pre-viously, the mixture usually consisted of about 80% chlor-ine and about 20% chlorine dioxide, both calculated as active chlorine. With increasing environmental demands, the amount of chlorine has been reduced and the amount of chlorine dioxide has been increased. From the environmental point of view, the substitution of chlorine dioxide for chlorine as a bleaching agent is to be preferred. This is because the contribution from chlorine dioxide to the amount of organically bound chlorine in the substances contained in the spent bleach liquor, is much smaller than the contribution from elemental chlorine.
Bleaching steps with chlorine dioxide only are widely used, and this applies particularly to the final step of a bleaching sequence.
Frequently, a bleaching step is followed by an alka-line extraction step (E) in which, inter alia, remaininglignin is dissolved from the pulp. In recent years, it has become increasingly common to reinforce the alkali (usually sodium hydroxide) by one or more bleaching agents which are less harmful to the environment, such as oxygen and/or peroxide (Eo, Ep, Eop).
From an environmental point of view, ozone is an excellent bleaching agent. It is, furthermore, very effi-cient and produces an extremely high brightness of the pulp. This bleaching agent has been the subject of many laboratory tests, and tests in pilot plants are also described in the literature. To our knowledge, there has as yet been no commercial break-through for ozone as a bleach-ing agent for pulp.

203231~

-Summary of the Invention Technical Problem The environmental authorities are increasing the legal demands put on producers of bleached chemical pulp, requiring them to reduce their discharge of spent bleach liquors which contain substances where organically bound chlorine is present. In this respect, the situation for the pulp producer would be improved if ozone were used in at least one bleaching step. It has, however, been found as regards ozone as a bleaching agent, that large amounts of ozone are needed for obtaining a given result. Further-more, the cellulose is above all depolymerized to such an extent that the ready-bleached pulp is an unsuitable starting material for many types of paper.
Solution The present invention aims at solving this problem by a method for bleaching chemical pulp with a lignin content corresponding to a kappa number not exceeding 10, the pulp being bleached with chlorine dioxide. The method is charac-terized in that the bleaching with chlorine dioxide occursat a comparatively high pulp consistency, and that the pulp, after the bulk of chlorine dioxide has been con-sumed, is intimately contacted with an ozone-containing aqueous solution and brought to react with the ozone at a considerably lower pulp consistency than in the chlorine dioxide bleaching, whereupon the pulp undergoes an alkaline treatment in the presence of oxygen and/or an oxidizing agent giving off oxygen.
The chlorine dioxide bleaching of the pulp is carried out for 10-600 min. and at a temperature of 50-100C. In a preferred embodiment of the invention, the pulp has such a high pulp consistency that, macroscopically seen, the bleaching takes place in the absence of a free liquid phase. The amount of chlorine dioxide added depends on several factors, inter alia the kappa number of the pulp to be bleached and the desired final brightness, and normally varies within the range of 0.5-4%, calculated as active chlorine on the basis of dry pulp. In the cellulose ~ industry it is customary, that both the amount added and the amount of chlorine dioxide consumed are based on the determination of active chlorine by iodometric analysis, more precisely by adding potassium iodide to an acid environment. By the determination is meant, that after the bulk of chlorine dioxide has been consumed, at least 70% of the active chlorine added has been consumed. The consump-tion may approach 100%, i.e. be almost complete. Suitably, the consumption amounts to 85-98% of the amount of active chlorine added.
It is quite possible, albeit not necessary, to wash the pulp after the chlorine dioxide bleaching step.
The subsequent ozone bleaching of the pulp should take place in a low-viscous pulp suspension with a pulp consistency which preferably is less than half of the pulp consistency in the chlorine dioxide bleaching and not higher than 18%. When the ozone-containing aqueous solution is added to the pulp, the pulp consistency is suitably 1-20%. The amount of ozone added varies within the range of 0.05-2%, as calculated on the basis of dry pulp, and the ozone bleaching is carried out for 0.5-60 min and at a temperature of 5-60C. It is preferred that spent liquor is removed from the pulp after the ozone bleaching step and that it is used, completely or partly, for reducing the pulp consistency after the chlorine dioxide bleaching step.
This is, however, not absolutely necessary, and the pulp may instead be either washed and then taken to the third treatment step, i.e. the alkaline treatment of the pulp, or taken directly to this step. The spent liquor removed from the pulp after the ozone bleaching, or part of this liquor, may advantageously be refreshed with ozone before being used for lowering the pulp consistency after the chlorine dioxide bleaching.
Suitably, the alkaline treatment of the pulp is car-ried out at a pulp consistency of 5-25% for 10-240 min., at a temperature of 50-100C, at an oxygen pressure below 0.6 MPa, and/or with an addition of peroxide of 0.1-1.0%, calculated as H22 on the basis of the dry weight of the pulp. By oxygen is meant oxygen gas which, of course, can be supplied to the pulp in liquid form. By the previously mentioned oxidizing agents giving off oxygen are meant chlorine-free chemical compounds capable of giving off oxygen. This group includes peroxides and superoxides, such as hydrogen peroxide (H2O2), sodium peroxide (Na2O2) and sodium perborate (Na2B2O4(OH)4), and organic peroxy com-pounds, such as peracetic acid (CH3 COOOH).
Then, the pulp is washed as normal. It is perfectly conceivable to end the bleaching of the pulp at this point and, for instance, to transport the pulp to a paper mill attached to the pulp mill or, after optional drying and baling, forward the pulp to an external user.
If one wishes to further increase the brightness and/or purity of the pulp, it is quite possible to bleach the pulp in one or more additional bleaching steps. If, for instance, one further bleaching step is chosen, chlorine dioxide or ozone may be used as a bleaching agent.
Advantages The invention makes it possible to bleach a chemical pulp to a brightness exceeding 90%, while maintaining a sufficiently high viscosity of the pulp, without the use of any other chlorine-containing bleaching agent than chlorine dioxide (the total amount of this bleaching agent consumed being clearly below 2%, calculated as active chlorine on the basis of absolutely dry pulp). From the environmental point of view, this is a clear improvement, since the pulp industry strives to minimize not only the amount of organic material accompanying the spent bleached liquors into the recipient, but also, and above all, the amount of bound chlorine in this organic material.
The ozone step forming part of the bleaching method is of great importance to the excellent result obtained.
According to the invention what is particularly surprising is that already a very insignificant addition of ozone results in a substantial increase in brightness. At the same time, the inevitable attack leading to a depolymeriza-tion of the pulp is reduced. Furthermore, it has been found ~032315 ~ that pulps bleached according to the invention show good brightness stability, i.e. their tendency to yellow is slight.
Description of the Preferred Embodiment In the following, some parts of the invention will be described in more detail and, in connection with this, alternative measures will be stated. Finally, batchwise laboratory tests intended to simulate also continuous bleaching of pulp on a technical scale will be accounted for.
As stated above, it is preferred that the chlorine dioxide bleaching occurs at such high a pulp consistency that, macroscopically seen, the treatment takes place in the gas phase in the absence of a free liquid phase. ThiS
means that the treatment liquid is inside the fibres of the pulp and forms a thin layer on the surface of these fibres. The upper limit of the pulp consistency depends, inter alia, on the water retention value (WRV) of the pulp, which in turn is dependent on the fibre type and hemicellulose content of the pulp. The treatment tempera-ture and the type of apparatus chosen for the bleaching are also of importance. A high pulp consistency is required if the pulp is under a high static pressure, e.g. in a high pulp column. The preferred pulp consistency is within the range of 20-45%, and the pulp is suitably loose and fluffy. Advantageously, chlorine dioxide bleaching is car-ried out in a reaction vessel which, e.g. by means of gas locks, is shielded off from the ambient atmosphere. Suit-ably, the pressure inside the reaction vessel is lower than the atmospheric pressure.
It is possible, albeit not preferred, to carry out the chlorine dioxide bleaching of the pulp at a lower pulp consistency than the one stated above, e.g. in the range of 10-20%. This means, that a free liquid phase is present as well.
As stated above, the chlorine dioxide bleaching should take place at a temperature of 50-100C for 10-600 min. Shorter periods of time are used within the range 7 2~3~315 ~ of 75-100C, while as much as 400-600 min. is to be prefer-red at temperatures within the range of 50-65C. It is particularly suitable to increase the temperature in the course of the bleaching. For instance, the temperature may be 40-60C during an initial period of 5-60 min., whereupon the temperature is elevated to 80C and maintained at this level for a given period of time. It is also possible to do it the other way around, i.e. to lower the temperature from an initial temperature of, for example, 80C, say by evacuation, to for instance 60C, at which level the pulp is stored for a couple of hours. The chlorine dioxide can be supplied to the pulp in gas form or in an aqueous solu-tion.
In the ozone bleaching stage, it is preferred that the pulp consistency is within the range of 0.1-6%. Since ozone is normally supplied to the pulp as an aqueous solution, this additive reduces in itself the pulp consis-tency in relation to the consistency prevalent during the chlorine dioxide bleaching step. For optimum use of the ozone, it is required that the pulp suspension is low-viscous or fluidized. Within the given pulp consistency range, such fluidization can be achieved by means of conventional mixers, e.g. propeller mixers, including various kinds of pumps. If modern mixers in the form of high-intensity mixers, e.g. so-called slit mixers, are used, the pulp consistency can be increased to be in the range of 6-18%.
The ozonic aqueous solution can be supplied to the pulp at the pulp consistency maintained during the chlorine dioxide bleaching step. In this case, however, there is a risk that the ozone bleaching will be uneven. It is there-fore preferred that the pulp consistency is reduced to a certain extent already after the chlorine dioxide bleaching stage by supplying water to the pulp before the ozonic aqueous solution is added.
The manner in which the pulp is fluidized affects the reaction time. A highly intense fluidization permits short reaction times in the order of a fraction of a minute to a 2032~15 ~ couple of minutes. A less intense fluidization of the pulp involves longer reaction times which, however, should be within the time interval previously mentioned.
The third bleaching step, i.e. the alkaline treatment 5 step, may be a conventional oxygen bleaching step carried out at a high pulp consistency, e.g. in the range of 25-37%. However, it is preferred that the pulp consistency is in the range of 5-25%. The temperature should be compara-tively low, e.g. 50-100 C, and the time should be 10-240 min. The oxygen pressure should be lower than 0.6 MPa.
The amount of alkali added, e.g. sodium hydroxide (NaOH), is normally 0. 5-2%, as calculated on the basis of the dry weight of the pulp. It is especially preferred to have a small amount of sodium hydroxide, e.g. 0.5-1%, a low temperature, e.g. 50-75 C, a low oxygen pressure, e.g. 0. 3 MPa, and a long reaction time, e.g. 240 min. At low and average consistency, the oxygen is suitably admixed to the pulp suspension by means of a high-intensity mixer.
The pulp can be further brightened if the above-20 mentioned treatment is reinforced by the addition of a peroxide. The peroxide can be admixed immediately before, during and after the addition of oxygen. Excellent results are obtained if the peroxide is supplied after the oxygen treatment has been in progress for 10-200 min.
As stated above, the oxygen can be excluded and only peroxide supplied to the pulp under the given alkaline conditions.
In the following, the above-mentioned laboratory tests are accounted for in the form of Examples. In the 30 description, claims and examples all percentages and parts are given as percent by weight and parts by weight respec-tively, unless anything else is stated.
Example 1 A sulphate pulp with kappa number 4.9 (measured 35 according to SCAN-C 1:77) and an inherent viscosity of 1025 dm3/kg (measured according to SCAN-CM 15: 88) was used in a series of tests in which the pulp was bleached ac-cording to the invention. For comparative purposes, tests 9 2032~1~
were also made outside the scope of the invention.
The sulphate pulp with kappa number 4.9 originates from a technically produced sulphate pulp with kappa num-ber 30.2 and an inherent viscosity of 1240 dm3/kg. The latter was obtained by a delignification of the wood species Pinus sylvestris. The completely clean-washed sulphate pulp was impregnated with technical black liquor in an amount corresponding to 221 kg per 1000 kg dry pulp to simulate a poorly washed mill pulp. The dry solids content of the black liquor was 19.5%. Then, 2% NO2, as calculated on the basis of the dry weight of the pulp, was supplied to the pulp which subsequently was treated accor-ding to a dilution method (S3) which is described by Samuelson and ~jteg in Proceedings 1989 TAPPI Int. Symp. on Wood and Pulping Chemistry, Raleigh N.C., May 1989, pp 195-204. After being thus activated, the pulp was bleached, in accordance with this publication, with oxygen at average consistency and finally washed.
The pulp had the kappa number and inherent viscosity values stated by way of introduction.
Four batches of pulp were made ready and initially treated with chlorine dioxide corresponding to 1.35% active chlorine, as calculated on the dry weight of the pulp. The bleaching was carried out in a glass reactor which was evacuated before the addition of chlorine dioxide, but after the supply of the hand-fluffed pulp. Then, a glass flask containing a technical solution (mill solution) of chlorine dioxide was connected via a valve. By carefully heating the glass flask for 15 min., all chlorine dioxide was expelled from the glass flask and supplied to the pulp together with a certain amount of expelled water vapour.
During this period, the temperature in the glass reactor was 25C, and the pulp consistency was 30~. Then, the glass reactor was partly immersed in a polyglycol bath with a temperature of 60C, and rotated. During a period of 10 min., the temperature was elevated to 70C and maintained at this level for 120 min. At this point, the glass reactor was removed from the polyglycol bath, and distilled water 20~2315 was supplied to the pulp, thus reducing the pulp consisten-cy to 5%. The glass reactor was drained of the pulp suspen-sion which was passed to a B~chner funnel filtering off liquid. The resulting pulp cake was pressed to a consisten-cy of 30% to obtain a reliable determination of the chlor-ine dioxide consumption and render possible a corresponding determination of the ozone consumption.
In a second step, the pulp with an inherent consis-tency of 30% was treated with ozone. The pulp was intro-duced into a plastic container equipped with a propellermixer. A previously prepared aqueous solution of ozone (gaseous ozone absorbed in water) was supplied to the pulp being agitated in such an amount that the pulp consistency was reduced to 0.5%. The temperature was 22C, and the treatment lasted for 10 min. The pulp suspension was agitated by means of the propeller mixer during the entire reaction time. Then, the pulp suspension was moved to a B~chner funnel for filtering off the spent liquor. At this point, the pulp consistency rose to above 10%. Different amounts of ozone were added, the addition in Test 1 being 0.07%, in Test 2 0.093%, and in Test 3 0.116%, as calcula-ted on the dry weight of the pulp.
In a third step, the pulp was introduced into an autoclave of acid-proof steel. Sodium hydroxide in an amount of 1%, as calculated on the dry weight of the pulp, was added to the pulp in the form of an aqueous solution, resulting in a pulp consistency of 10%. oxygen was sup-plied to the autoclave at room temperature, the resulting oxygen pressure being 0.3 MPa. The autoclave was immersed in a polyglycol bath with a temperature of 70C. After 45 min. at this temperature, the alkaline oxygen deligni-fication of the pulp was interrupted, and the pulp was carefully washed with water.
In the three tests accounted for above, the pulp was treated according to the invention. For comparative purpo-ses, the fourth batch of the pulp was a blank which was treated exactly like the other tests, except that the ozone bleaching step was left out.

~~ As stated above, already these three treatment steps according to the invention yield a pulp which is very useful for various products, e.g. paper of various kinds.
If desired, the pulp can be bleached in at least one more step. Therefore, the above pulp specimen were finally bleached with chlorine dioxide at a pulp consistency of 6%
for 3 h in glass jars immersed in a water bath with a tem-perature of 70C. The glass jars were shaken by hand at short intervals. Then, the pulp was carefully washed with water.
Each batch of pulp was divided into two parts to which was added 0.51% and 0.80% chlorine dioxide, respec-tively, as calculated on absolutely dry pulp.
The results obtained are shown in Table 1.

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-~ The brightness was determined as ISO-brightness according to SCAN-C 11:75. The brightness of the blank after ageing at 80C was determined on the basis of other tests carried out under similar conditions. The total consumption of chlorine dioxide as calculated on the basis of active chlorine, is the total consumption in the first chlorine dioxide bleaching stage and the consumption of chlorine dioxide in the final bleaching stage.
When comparing the pulp in the blank and the pulps in Tests 1-3 after completed oxygen bleaching, one finds that an addition of ozone as low as 0.07% increases the bright-ness by as much as 6 units, while the reduction of the viscosity is no more than 70 units, which is relatively harmless. A further addition of ozone increases the bright-ness by one more unit at a substantially unaltered visco-sity.
In corresponding pulps finally bleached with chlorine dioxide, the difference in brightness is halved, whereas the difference in viscosity of the pulps is substantially reduced. Furthermore, as is apparent from the Table, all the pulps have an excellent brightness stability.
What strikes one first is that only a small addition of the expensive bleaching agent ozone is required in order to increase considerably the brightness of the pulp.
In further tests, the same pulp was treated in a similar manner as stated above including the alkaline oxygen treatment. The amount of ozone added in these tests was the same as in Test 2, i.e. 0.093% as calculated on absolutely dry pulp. However, the pulp consistency during the ozone bleaching step was varied and amounted to 1%, 20%
and 30%, respectively. A pulp consistency of 1% gave the same results as a consistency of 0.5%, i.e. in accordance with Test 2. On the other hand, the results deteriorated considerably at pulp consistencies of 20% and 30%. At a pulp consistency of 20%, the brightness was lower by 1.5 unit and the loss of viscosity 70% greater than in Test 2.
At a pulp consistency of 30%, the brightness was roughly the same as in Test 2, but the loss of viscosity was 100%

greater, resulting in a pulp of insufficient strength. This illustrates the importance of having a low pulp consistency in the ozone bleaching step.
The same pulp was also subjected to a test in which the order of the two first treatment steps of the invention was reversed. In this test, the pulp was first treated with ozone and then with chlorine dioxide, but was otherwise treated in accordance with Tests 1-3, including the final bleaching with chlorine dioxide. Thus, the pulp was first treated with ozone at a pulp consistency of 0.5%. After the treatment liquid had been filtered off and the pulp pressed to a pulp consistency of 30% and fluffed by hand, the pulp was bleached as above with chlorine dioxide con-taining 1.35% active chlorine. To achieve a final bright-ness of 88.5, an addition of 0.3% ozone, as calculated on the dry weight of the pulp, was required, which is to be compared with the 0.07% ozone added in Test 1 according to the invention. In this case, the viscosity of the pulp was reduced to 810 dm3/kg, implying that depolymerization of the pulp had been carried too far. Also the brightnessstability of the pulp was poorer in this test than in the tests according to the invention.
Test 2 according to the invention was repeated, except that the pulp consistency in the initial chlorine dioxide bleaching step was lowered from 30% to 15%. In this test, the chlorine dioxide was supplied in an aqueous solution which was admixed to the pulp, resulting in a consistency of 15%. This yielded a brightness of the pulp of 82.1%, after the oxygen bleaching step. After final bleaching with chlorine dioxide in an amount added of 0.51%
active chlorine as calculated on dry pulp, the brightness was increased to 88.6%. The corresponding values in Test 2 were 82.7% and 89.1%, respectively. Obviously, the diffe-rence in brightness of the pulp is very slight. The visco-sity values of the pulp did not differ significantly fromthose obtained in Test 2. From this it may be concluded that, according to the invention, it is possible to have a pulp consistency of 15% in the chlorine dioxide bleaching step, which is comparatively low.
The amount of active oxygen remaining in the spent liquors after the ozone step was determined iodometrical-ly. This amount corresponded to less than 5% of the added amount of ozone added, determined according to the same method.
Example 2 A sulphate pulp of kappa number 5.0 and a viscosity of 980 dm3/kg, was used in a series of tests according to the invention. ThiS sulphate pulp was of the same type as and produced in a parallel manner to the pulp described in Example 1.
This test series in all parts corresponds to the previously described Test 2, with the addition that a peroxide treatment, with three different amounts added, was introduced after the oxygen bleaching and before the concluding chlorine dioxide bleaching. After the oxygen bleaching, the pulp was diluted with a hydrogen peroxide solution to a pulp consistency of 8%. The temperature during this hydrogen peroxide step was 70C and it lasted for 45 min.. In this series, the amount of chlorine dioxide added in the final bleaching step was 0.52% active chlorine as calculated on dry pulp. The amount of hydrogen peroxide added and the results obtained are apparent from Table 2 below.

Finally bleached pulp H22 Added Test % on the basis Bright- Brightness after visco-of dry pulp ness ageing sity % 80C % 120C dm3/kg 4 0 89.0 85.6 85.5 920 0.20 89.7 86.2 85.1 915 6 0.40 89.9 86.3 86.4 910 7 0.60 90.2 86.5 86.4 910 As will appear, it is possible to further increase the brightness of the pulp by means of hydrogen peroxide, even to above 90%. The decrease in viscosity of the pulp caused by the hydrogen peroxide treatment was slight and lies within the limits of normal analytical errors.

Claims (31)

1. A method for bleaching chemical pulp with a lignin content corresponding to a kappa number not exceeding 10, the pulp being bleached with chlorine dioxide, characterised in that the bleaching with chlorine dioxide occurs at a comparatively high pulp consistency, and that the pulp, after the bulk of chlorine dioxide has been consumed, is intimately contacted with an ozone-containing aqueous solution and brought to react with the ozone at a considerably lower pulp consistency than in the chlorine dioxide bleaching, whereupon the pulp undergoes an alkaline treatment in the presence of at least one member selected from oxygen and an oxidizing agent giving off oxygen.

2. A method according to claim 1, characterised in that the pulp consistency during the chlorine dioxide bleaching is so high that, macroscopically seen, this treatment takes place in the absence of a free liquid phase.

3. A method according to claim 1, characterised in that the bleaching with ozone takes place in a low-viscous pulp suspension with a pulp consistency less than half of the pulp consistency in the chlorine dioxide bleaching, and not higher than 18%.

4. A method according to claim 2, characterised in that the bleaching with ozone takes place in a low-viscous pulp suspension with a pulp consistency less than half of the pulp consistency in the chlorine dioxide bleaching, and not higher than 18%.

5. A method according to claim 1, 2, 3 or 4, characterised in that spent liquor is removed from the pulp after the ozone bleaching and is used, completely or partly, for reducing the pulp consistency after the chlorine dioxide bleaching.

6. A method according to claim 1, 2, 3 or 4, characterised in that the pulp consistency is 1-20%
when the ozone-containing aqueous solution is admixed.

7. A method according to claim 5, characterised in that the pulp consistency is 1-20% when the ozone-containing aqueous solution is admixed.

8. A method according to claim 1, 2, 3, 4 or 7, characterised in that the chlorine dioxide bleaching is carried out for 10-600 min. and at a temperature of 50-100°C.

9. A method according to claim 5, characterised in that the chlorine dioxide bleaching is carried out for 10-600 min. and at a temperature of 50-100°C.

10. A method according to claim 6, characterised in that the chlorine dioxide bleaching is carried out for 10-600 min. and at a temperature of 50-100°C.

11. A method according to claim 1, 2, 3, 4, 7, 9 or 10, characterised in that the ozone bleaching is carried out for 0.5-60 min. and at a temperature of 5-60°C.

12. A method according to claim 5, characterised in that the ozone bleaching is carried out for 0.5-60 min. and at a temperature of 5-60°C.

13. A method according to claim 6, characterised in that the ozone bleaching is carried out for 0.5-60 min. and at a temperature of 5-60°C.

14. A method according to claim 8, characterised in that the ozone bleaching is carried out for 0.5-60 min. and at a temperature of 5-60°C.

15. A method according to claim 1, 2, 3, 7, 9, 10, 12, 13 or 14, characterised in that the alkaline treatment is carried out at a pulp consistency of 5-25% for 10-240 min., at a temperature of 50-100°C, and with at least one condition selected from:
i) at an oxygen pressure below 0.6 MPa, and ii) with an addition of peroxide of 0.1-1.0%, calculated as H2O2 on the basis of the dry weight of the pulp.

16. A method according to claim 5, characterised in that the alkaline treatment is carried out at a pulp consistency of 5-25% for 10-240 min., at a temperature of 50-100°C, and with at least one condition selected from:
i) at an oxygen pressure below 0.6 MPa, and ii) with an addition of peroxide of 0.1-1.0%, calculated as H2O2 on the basis of the dry weight of the pulp.

17. A method according to claim 6, characterised in that the alkaline treatment is carried out at a pulp consistency of 5-25% for 10-240 min., at a temperature of 50-100°C, and with at least one condition selected from:
i) at an oxygen pressure below 0.6 MPa, and ii) with an addition of peroxide of 0.1-1.0%, calculated as H2O2 on the basis of the dry weight of the pulp.

18. A method according to claim 8, characterised in that the alkaline treatment is carried out at a pulp consistency of 5-25% for 10-240 min., at a temperature of 50-100°C, and with at least one condition selected from:
i) at an oxygen pressure below 0.6 MPa, and ii) with an addition of peroxide of 0.1-1.0%, calculated as H2O2 on the basis of the dry weight of the pulp.

19. A method according to claim 11, characterised in that the alkaline treatment is carried out at a pulp consistency of 5-25% for 10-240 min., at a temperature of 50-100°C, and with at least one condition selected from:
i) at an oxygen pressure below 0.6 MPa, and ii) with an addition of peroxide of 0.1-1.0%, calculated as H2O2 on the basis of the dry weight of the pulp.

20. A method according to claim 1, 2, 3, 4, 7, 9, 10, 12, 13, 14, 16, 17, 18 or 19, characterised in that the pulp is finally bleached with chlorine dioxide after the alkaline treatment.

21. A method according to claim 5, characterised in that the pulp is finally bleached with chlorine dioxide after the alkaline treatment.

22. A method according to claim 6, characterised in that the pulp is finally bleached with chlorine dioxide after the alkaline treatment.

23. A method according to claim 8, characterised in that the pulp is finally bleached with chlorine dioxide after the alkaline treatment.

24. A method according to claim 11, characterised in that the pulp is finally bleached with chlorine dioxide after the alkaline treatment.

25. A method according to claim 15, characterised in that the pulp is finally bleached with chlorine dioxide after the alkaline treatment.

26. A method according to claim 1, 2, 3, 4, 7, 9, 10, 12, 13, 14, 16, 17, 18 or 19, characterised in that the pulp is finally bleached with ozone after the alkaline treatment.

27. A method according to claim 5, characterised in that the pulp is finally bleached with ozone after the alkaline treatment.

28. A method according to claim 6, characterised in that the pulp is finally bleached with ozone after the alkaline treatment.

29. A method according to claim 8, characterised in that the pulp is finally bleached with ozone after the alkaline treatment.

30. A method according to claim 11, characterised in that the pulp is finally bleached with ozone after the alkaline treatment.

31. A method according to claim 15, characterised in that the pulp is finally bleached with ozone after the alkaline treatment.