SE465224B - Bleaching chemical cellulose pulp with chlorine dioxide and ozone - Google Patents

Abstract

Two of the considerations which have previously impeded the full-scale use of the bleaching agent ozone, which is excellent from an environmental point of view, are the relatively large consumption and the fact that the cellulose pulp is depolymerized too vigorously. The present invention solves these problems and relates to a process for bleaching chemical cellulose pulp having a lignin content corresponding to at most a kappa number of 10 in three stages. The process is characterized in that the bleaching is initiated with a chlorine dioxide stage at a relatively high pulp consistency, and in that, once the bulk of the chlorine dioxide has been consumed, the cellulose pulp is brought into intimate contact with an ozone-containing aqueous solution and is caused to react with the ozone at a substantially lower pulp consistency than when bleaching with the chlorine dioxide, after which the cellulose pulp is subjected to an alkaline treatment in the presence of oxygen and/or oxygen- emitting oxidizing agent.

Description

465 224 2 10 15 20 25 30 35 the content of the cellulose mass so far during the actual digestion. In these cases, the delignification of the lignocellulosic material is continued, and then in the form of cellulose pulp, preferably by treatment with such chemicals, which lead to effluents, which, like the boiling liquor, can be returned to the pulp mill's chemical recycling system. delignification is advantageously carried out under alkaline conditions in the presence of one or more oxidizing agents, such as oxygen and / or selective and, if desired, far-reaching This peroxide. A very delignification of cellulose pulp is obtained if alkaline delignification of the cellulose pulp so-called the cellulose mass inside the cellulose mass. brought into contact with nitrogen - the above is preceded by an activation of carrier, dioxide, hydrogen and nitrate ions and possibly oxygen.

Background Art For bleaching cellulosic pulps of the type described above, a large number of bleaches and a multiple number of bleaching sequences to achieve almost lignin- there is a free or completely lignin-free cellulose pulp.

A mixture of chlorine (C12) and chlorine dioxide (C102) is often used in a first bleaching step. Previously, a mixture consisting of about 80% chlorine and about 20% chlorine dioxide has been used. With stricter environmental requirements, the proportion of chlorine has been reduced and the proportion of chlorine dioxide has been increased. The use of chlorine dioxide instead of chlorine as a bleaching agent is preferable from an environmental point of view, as the contribution from chlorine dioxide to the amount of organically bound chlorine in the bleach-containing constituents is considerably less than what is usually both counted as active chlorine. the case of elemental chlorine.

Bleaching steps where only chlorine dioxide is used are very common, especially when it comes to the last step in a bleaching sequence.

Often a bleaching step is followed by an alkaline extraction (E), where i.a. lignin residues are dissolved out of the cellulose pulp. Recently, it has become increasingly common for alkali (usually sodium hydroxide) to be fortified with an environmentally friendly environmentally friendly bleaching agent, such as and / or peroxide (Eo, EP, EOP).

An environmentally friendly bleach is ozone. It is also an effective bleach, which leads to extremely high brightness of the cellulose pulp. The bleach has been the subject of many laboratory studies and testing in a pilot plant has also been described in the literature. To our knowledge, however, ozone as a bleaching agent for cellulose pulp has not yet had a commercial breakthrough in kind or more oxygen.

Disclosure of the invention Technical problem The environmental authorities are imposing increasingly harsh conditions on manufacturers of bleached chemical cellulose pulp with regard to reducing emissions of bleach liquors containing substances containing organically bound chlorine. A transition to the use of ozone in at least one bleaching step would improve the situation for the cellulosic pulp manufacturer in this respect. However, it has so far been found with regard to the bleaching agent ozone, that on the one hand the consumption is large to obtain a certain result and on the other hand and above all the cellulose is depolymerized so strongly that the finished bleached cellulose pulp is unsuitable as a starting material for many types of paper.

The solution The present invention is a solution to said problem and relates to a process for bleaching chemical cellulose pulp with a lignin content corresponding to at most kappa number 10, wherein the cellulose pulp is bleached with chlorine dioxide, characterized in that the bleaching with chlorine dioxide takes place at a comparatively high pulp consistency that the cellulose pulp, after consuming most of the chlorine dioxide, is brought into intimate ozone-containing aqueous solution and brought to the ozone at a substantially lower mass consistency thereof, and contact with a reactant with 465 224 4 10 15 20 25 30 35 than after bleaching with chlorine dioxide, after which cellulose the pulp is subjected to an alkaline treatment in the presence of oxygen and / or oxygen-releasing oxidizing agents.

The bleaching of the cellulose pulp with chlorine dioxide is carried out for a period of 10-600 minutes and at a temperature of 50-100 ° C. According to a preferred embodiment of the invention, such a high pulp consistency is chosen that the bleaching macroscopically takes place in the absence of a free liquid phase. The amount of chlorine dioxide invested depends on several factors, including to the bleaching kappatal desired final brightness, and normally varies in the range 0.5-4%, calculated as active chlorine of dry-thinking cellulose pulp.

Both addition and consumption of chlorine dioxide are based according to practice in the cellulose industry on determination of active chlorine by iodometric analysis, batch of potassium iodide in the majority of the chlorine dioxide consumed. of added active chlorine has been consumed. 100%, proved to be suitable, added active chlorine.

After the chlorine dioxide bleaching step, it is quite possible to wash the cellulose pulp even if it is not necessary. the cellulose mass and more specifically by addition- With the determination, then that at least 70% Consumption can approach be practically complete. It has that consumption amounts to 85-98% of acidic environment. menas. i.e.

The subsequent ozone bleaching of the cellulose pulp is to take place in a easily mobile pulp suspension and in terms of pulp consistency it is preferred that this is less than half of the pulp consistency in chlorine dioxide bleaching and not more than 18%.

When adding the ozone-containing aqueous solution to the cellulose pulp, it is suitable. that the pulp consistency amounts to 1-20%. The amount of ozone added is varied in the range 0.05-2%, calculated on dry-thinking cellulose pulp, and the ozone bleaching is carried out for a period of 0.5-60 minutes and at a temperature of 5-60 ° C. It is preferred. that the effluent is removed from the cellulose pulp after the ozone bleaching step and that the effluent is used in whole or in part to lower the pulp consistency after the chlorine dioxide bleaching step. However, this measure is not absolutely necessary, but the cellulose pulp can either first be washed and then carried or taken directly to the third treatment step, ie. the alkaline treatment Drainage from the cellulose pulp after the ozone bleaching or part of it can advantageously be refreshed with ozone before it is used to lower the pulp consistency after the chlorine dioxide bleaching.

The alkaline treatment of the pulp suitably takes place at a pulp consistency of 5-25%, for a period of 10-240 minutes, at an oxygen pressure below the cellulose pulp. which is removed at a temperature of 50-100 ° C, rising 0.6 m, and / or at a peroxide addition of 0.1-10%, calculated as H 2 O 2 on the dry weight of the cellulose pulp.

Oxygen here refers to oxygen gas, which can of course be added to the cellulose mass in liquid form, and by previously stated oxygen-releasing oxidizing agents is meant chlorine-free chemical compounds which can give off oxygen. This group includes peroxides and superoxides such as hydrogen peroxide (H 2 O 2), sodium peroxide (Na 2 O 2) and sodium perborate (Na 2 B 2 O 4 (OH) 4) as well as organic peroxy compounds such as peracetic acid (CH 3 'COOOH).

It is full bleaching of the cellulose pulp in this transport the pulp pulp to one or after Then the cellulosic pulp is washed normally. it is possible to end the situation and, for example, to the pulp factory, possibly drying and baling, to collect the cellulose pulp from an external user. paper mill If you wish to further increase the brightness and / or purity of the cellulose pulp, it is entirely possible to bleach the cellulose pulp in one or more further bleaching steps. If, for example, an additional bleaching step is chosen, chlorine dioxide or ozone can be used as bleaching agent.

Advantages By the invention it is possible that without the use of any chlorine-containing bleach other than chlorine dioxide (and in a total consumed amount of this bleach in the form of 465 224 e 10 15 20 25 30 active chlorine well below 2% calculated on absolutely dry mass ) bleach a chemical cellulose pulp to a brightness of over 90%, while maintaining a sufficiently high viscosity of the cellulosic pulp. This describes a clear progress since there is an effort in the industry that follows the environmental point of view, to minimize both the amount of organic material, with the bleach liquors out in the recipient and above all to minimize the amount of bound chlorine in this organic material.

The ozone step included in the bleaching process according to the invention is the result. Particularly surprising is that already. an extremely low addition of 'ozone leads to a sharp increase in brightness. attack, which leads to depolymerization of the cellulose pulp.

Furthermore, it has been found that masses bleached in accordance with the invention brightness stability, tendency to post-yellow is small. of essential importance for the achieved good At the same time, it is limited in itself inevitable, ie. Best Mode for Carrying Out In the following, certain parts of the process according to the invention are described in more detail and in connection therewith alternative alternatives are stated and finally batch experiments performed in laboratory equipment are described, which are intended to simulate continuous bleaching of cellulose pulp on a technical scale.

As can be seen from the foregoing, it is preferred that the chlorine dioxide bleaching take place at such a high pulp consistency that, macroscopically, the treatment takes place in the gas phase in the absence of This means that the treatment liquid is inside the cellulose pulp fibers and as a thin free liquid phase. layers on the surface of these. The upper limit of the pulp consistency is, among other things, dependent on the WRV (water retention value) of the cellulose pulp, which in turn is dependent on the type of fibers in the hemicellulose content. Also important are the treatment temperature and the device- * CYP. stone is required if the cellulose pulp is under high static pressure, for example in a high pulp column. The preferred cellulosic pulp and its selected for the bleaching operation. High pulp consistency The pulp consistency is in the range of 20-45% and it is suitable that the cellulose pulp is loosened and is in fluffy form. The bleaching with chlorine dioxide is advantageously carried out in a reaction vessel, for example by means of gas locks, from the surrounding atmosphere. It is suitable that the pressure inside the reaction vessel is lower than the atmospheric pressure. Although not preferred, it is possible to carry out the chlorine dioxide bleaching of the cellulose pulp at a lower pulp consistency than that indicated above, for example in the range of 10-20%. This means that there is also a free liquid phase. It has previously been stated that the chlorine dioxide bleaching should take place at a temperature of 50-100 ° C for a period of 10-600 minutes. Shorter times are used in the range of 75-100 ° C, while as long as 400-600 minutes are preferred at temperatures in the range of 50-65 ° C. It is particularly suitable to increase the temperature during the bleaching process. For example, an initial period of 5-60 minutes at a temperature of 40-60 ° C can be used and then the temperature is raised to 80 ° C to be kept there for a certain period of time. It is also possible to go the opposite way, ie. to lower the temperature based on a temperature of, for example, 80 ° C, for example by evacuation, for example to 60 ° C and store the cellulose mass at this temperature for a few hours. The chlorine dioxide can be added to the cellulose mass in gaseous form or in aqueous solution.

As for the ozone bleaching step, it is preferred that the pulp consistency be in the range 0.1-6%. Since ozone is normally added to the cellulose pulp in the form of an aqueous solution, this addition in itself means that the pulp consistency is reduced in relation to that prevailing during the chlorine dioxide bleaching step. For maximum utilization of the ozone, the pulp suspension is required to be easily mobile or fluidized. At specified mass consistency ranges, such fluidization can be accomplished by conventional agitator devices, such as propeller agitators, including pumps of various types. If modern mixing equipment in the form of a split mixer, it is used that it falls off intensive mixers, for example so-called is it possible to increase the pulp consistency so, within the range of 6-18%. 465 224 8 10 15 20 25 30 35 It is possible to add the ozone-containing aqueous solution to the cellulose pulp at the pulp consistency maintained during the chlorine dioxide bleaching step. However, in that case there is a certain risk that irregularities in bleaching with the ozone will occur. Therefore, it is preferred that the pulp consistency be reduced to some extent already after the chlorine dioxide bleaching step by adding water to the cellulose pulp before the ozone-containing aqueous solution is added thereto.

The method of fluidizing the cellulose pulp affects the reaction time. A very intensive fluidization enables short reaction times in the order of from a minute to a few minutes. With a gentle fluidization of the cellulose mass, longer reaction times are used, but preferably within the previously stated time interval.

The third pale step, i.e. the alkaline treatment step, may consist of a conventional oxygen bleaching step at high pulp consistency, for example in the range of 25-37%. However, a pulp consistency in the range of 5-25% is preferred. The temperature should be comparatively low, for example 50-100 ° C, and the time 10-240 minutes. The oxygen pressure should be below O.6MPa. The addition of alkali, for example sodium hydroxide (NaOH). normally amounts to 0.5-2%, calculated on the dry weight of the cellulose pulp. Low sodium hydroxide addition, for example 0.5-1%. low temperature, for example 50-75 ° C, low oxygen pressure, for example 0.3MPa, and long reaction time, for example 24O minutes. is especially preferred. At low and medium consistency, the oxygen gas is mixed into the pulp suspension suitably by means of an intensive mixer.

Further increase in brightness of the cellulose pulp can be obtained if the above treatment is enhanced with the addition of some peroxide. The addition can take place immediately before, during and after the oxygen supply. Very good results are obtained if peroxide is added after the oxygen treatment has been going on for a period of 10-200 minutes.

As previously stated, it is possible to exclude the addition of oxygen and only add peroxide to the cellulose pulp under the said alkaline conditions. p »10 15 20 25 30 35 465 224 Below are the previously mentioned laboratory experiments in the form of design examples.

Example 1 A sulphate pulp with a kappa number of 4.9 (measured according to SCAN-C 1:77) and an intrinsic viscosity of 1025 dma / kg (measured according to SCAN-CM 15:88) was used in a series of experiments where the pulp was bleached in accordance with the invention. For comparison, experiments were also made outside the scope of the invention.

The sulphate mass with a kappa number of 4.9 originates from a technically produced sulphate mass with a kappa number of 30.2 and a self-viscosity of 1240 ams / kg. This was in turn obtained from digestion of the wood species Pinus sylvestris. The completely clean-washed sulphate mass was impregnated with technical black liquor in an amount corresponding to 221 kg per 1000 kg dry-dried mass to simulate a poorly washed factory mass. The solids content of 1 black liquor was 19.5%. Then the pulp was added 2% NO2, calculated on the dry weight of the pulp, and treated according to a dilution method (S3), which is described by Samuelson and Öjteg in Proceedings 1989 TAPPI Int. Symp. on Wood and Pulping Chemistry, Raleigh N.C. May 1989, pp. 195-204. After this activation of the pulp, it was oxygen bleached at medium concentration in accordance with what is described in said publication, and finally the pulp was washed.

Said pulp showed the initial values of kappa number and intrinsic viscosity.

Four batches of the pulp were prepared and initially treated with chlorine dioxide corresponding to 1.35% of active chlorine. The bleaching took place in a glass but calculated on the dry weight of the pulp. reactor, which was evacuated before the addition of chlorine dioxide, after supply of the pulp in fluffy (by hand) form. after, a glass flask containing a technical solution (factory solution) of chlorine dioxide was connected via a valve. By gently heating the glass flask for 15 minutes, all the chlorine dioxide was evaporated from the glass flask and the pulp was added together with the temperature in the glass reactor. There was a certain amount of evaporated water vapor. 465 10 15 20 25 30 35 224 1 ° tower was 25 ° C during this period. The pulp consistency was 30%.

The glass reactor was then partially immersed in a polyglycol bath at a temperature of 60 ° C. For a period of 10 minutes, the temperature was raised to 70 ° C. Thereafter, this temperature was maintained for 120 minutes. The glass reactor was removed at this time from the polyglycol bath and distilled water was added to the pulp so that the pulp consistency was reduced to 5%.

The pulp suspension was emptied from the glass reactor into a Büchner funnel where liquid was filtered off. The resulting pulp cake was pressed to a consistency of 30%. The latter was done to obtain a reliable determination of the consumption of and was rotated by chlorine dioxide and also to enable a corresponding determination of the ozone consumption.

In a second step, the pulp with an input consistency of 30% as above was treated with ozone. The mass was lowered into a plastic container equipped with a propeller stirrer. A previously prepared aqueous solution of ozone (gaseous ozone absorbed in water) was added to the pulp with stirring in a drop to 0.5%. A temperature of 22 ° C was maintained and the treatment time was such that the pulp consistency was minutes. The pulp suspension was stirred with the propeller stirrer throughout the board into a Büchner funnel to filter off the liquor.

Ozone was added in various amounts and the addition in Experiment 1 was 0.07%, calculated on during the reaction time. Thereafter, the pulp consistency was then transferred to over 10%. the dry weight of the pulp, and in Experiment 2 it was 0.093% and in Experiment 3 it was 0.16%.

In a third step, the pulp was introduced into an acid-solid steel autoclave. the dry weight of the pulp, sodium hydroxide in an amount of 1%, calculated on, the pulp was added in the form of an aqueous solution so that the pulp consistency became 10%. Oxygen was introduced into the autoclave at room temperature so that an oxygen pressure of 0.3 m was obtained. The autoclave was immersed in a polyglycol bath at a temperature of 70 ° C. After 45 minutes at this temperature, the alkaline oxygen delignification of the pulp was stopped. Then the mass was washed thoroughly with water.

Above, three experiments have been described in which the pulp has been treated in accordance with the invention. For comparison, a zero sample was made with the fourth batch of pulp. The treatment of this pulp sample was completely in accordance with the above with the only difference that the ozone bleaching step was omitted.

As stated earlier, already after these three treatment steps in accordance with the invention, a cellulose pulp is obtained which is very useful for different products, for example paper of different kinds.

However, if desired, the cellulosic pulp can be bleached in at least one more step. Therefore, the mass samples described above were finally bleached with chlorine dioxide at 6% pulp consistency for 3 hours in glass jars immersed in a water bath with a temperature of 70 ° C. The glass jars were shaken by hand at short time intervals. Then the mass was washed thoroughly with water.

Each mass batch was divided into two parts and 0.51 and 0.80% chlorine dioxide, respectively, were added, calculated on absolutely dry mass.

The results obtained are shown in table below l. 12 465 224 næ.- nwm w.mæ m.wæ æ.mw ~ ø.Q ww. ~ Nmm w.mw M.ww h.mw Om.Q mmm æ.Næ W- .DM æ fi. ~ Mmm N.mw m.mæ -.mm cæ.O # w.- ßwm # .nw w.mw F.mæ ~ mQ mmm ß.Nw nmQ-O N Mm. .mæ m.wæ Dw.O ww. ~ Dem N.mw e.nm æ.ww Fm-Q mmm w. ~ w ßO.QF Pæ.- mmm MM® n.Mw h.mw Om.O mw. ~ wwm w.Nw D.nm w.mw Fm.O møop m.mß D> QHQ | Q a 92% ... aaa måna .. aa no fi v. > wuxm uocmp uoom Ho fl x> wuxm: cwussmcox Hawa »uwuwmoxww> .uu fi m nmußø uuzmsw .. umzmz fl .. uumw fifi wh umuwmoxw fi> uu .._ w: w | _: own uwxawvno fi x nu ... mcwm xm h. xmmnwm P 44mm 10 15 20 25 30 35 13 465 224 The brightness has been determined as ISO brightness according to SCAN-Cll: 75.

As for the brightness after aging at 80 ° C of the blank, a calculation has been made on the basis of other experiments performed under similar conditions. The statement of total consumption of chlorine dioxide, as as active chlorine, refers to the sum of the consumption in the first chlorine dioxide bleaching step and the consumption of chlorine dioxide in the final bleaching step.

If a comparison is made between the pulp according to the blank test and the pulps according to experiments 1-3 after completion of oxygen bleaching, it is found that an addition of ozone as low as 0.07% leads to an increase in brightness of as much as 6 units while the viscosity decrease stops at relatively harmless - called. An increased addition of ozone leads to an additional 1 unit of better brightness at substantially unchanged viscosity.

In the case of corresponding masses which have been bleached with chlorine dioxide, the difference in brightness has been reduced by half, but on the other hand the difference has. in the viscosity of the masses shrunk considerably.

Furthermore, it appears from the table. that all masses show very good light stability.

What one is primarily struck by is that only a slight addition of the expensive bleach ozone is required to increase the brightness of the pulp considerably.

Additional experiments were performed on the same mass where it was treated in a similar manner as described above until the alkaline ozone charge in these experiments was the same as in experiment 2, 0.093%, calculated on absolutely dry mass. What was varied was the pulp consistency during the ozone bleaching step, 20% and 30% respectively. 1% mass consistency gave the same result as at 0.5%, ie. in accordance with Experiment 2. However, the results deteriorated significantly at both 20% and 30% pulp consistency.

At 20% pulp consistency, the brightness was 1.5 units lower and the viscosity loss 70% greater than in Experiment 2, while at 30% pulp consistency, the brightness was largely the same as in Experiment 2, but instead the viscosity loss was 100% oxygen treatment. i.e. which amounted to 1%, 465 224 14 10 15 20 25 30 35 larger, leading to a mass' with too poor strength properties. This shows that it is important to have a low pulp consistency in the ozone bleaching step.

An attempt was also made on the same pulp with a reverse step sequence in relation to the first two treatment steps in accordance with the invention. In this experiment the pulp was treated first with ozone and then with chlorine dioxide, while the pulp was otherwise treated according to experiments 1-3 including final bleaching with chlorine dioxide. The pulp was thus first treated with ozone at a pulp consistency of 0.5%. After filtering off the treatment liquid and pressing to 30% pulp consistency and fluffing it by hand, the pulp is bleached with chlorine dioxide, with 1.35 * active chlorine, in the manner previously indicated. To reach a final brightness of 88.5, an addition of 0.3% ozone was required. calculated on the dry weight of the pulp, which is to be compared with the ozone addition of 0.07% in the experiment 1 according to the invention. The viscosity of the pulp dropped in this case to 810 dm3 / kg. which meant an excessively far-reaching depolymerization of the cellulose pulp. The light stability of the pulp was also worse in this experiment than in the experiments according to the invention.

Experiment 2 according to the invention was repeated with the difference that the pulp consistency in the initial chlorine dioxide bleaching step was reduced from 30 to 15%. In this experiment, the chlorine dioxide was added to an aqueous solution which was mixed into the pulp so that 15% consistency was obtained. This led to a brightness of the pulp after the oxygen bleaching step of 82.1% and after final bleaching with chlorine dioxide. in a batch of O.5l% active chlorine calculated on dry mass, the brightness rose to 88.6%. The corresponding values in Experiment 2 were 82.7% and 89.1%, respectively. As can be seen, the difference in brightness of the mass is very modest.

The values of the viscosity of the pulp did not differ significantly from that obtained in Experiment 2. This shows that according to the invention it is possible to use such a comparatively low pulp consistency in the chlorine dioxide bleaching step as 15%.

The amount of active oxygen remaining in the effluents after the ozone step was determined iodometrically. The amount corresponded to less than 5% of the ozone charged determined according to the same method.

Example 2 A series of experiments were carried out in accordance with the invention on a sulphate pulp with a kappa number of 5.0 and a viscosity of 980 dm3 / kg, which sulphate pulp was of the same type and prepared in a parallel manner. with that described in Example 1.

This series of experiments is in all respects consistent with previously described Experiment 2 with the addition that after the oxygen bleaching and before the final chlorine dioxide bleaching, peroxide treatment, with three different batches, has been introduced.

This was done in such a way that after complete bleaching of the pulp, it was diluted with a hydrogen peroxide solution to a pulp consistency of 8%. The temperature in this hydrogen peroxide step was 70 ° C and the time was 45 minutes. The charge of chlorine dioxide in the final bleaching step in this series was 0.52% active chlorine, calculated on dry mass. Investment in hydrogen peroxide and the results achieved are shown in Table 2 below.

Table 2 Final bleached pulp l Experiment _ Brightness Brightness after aging Viscosityl Addition H202 $ on dry pulp 1 00 ° c 1 120 ° c amg / kg É 4 0 09.0 05.0 05.5 920 5 0.20 09.1 00.2 05.1 915 0 0.40 09.9 00.3 00.4 910 1 0.00 90.2 00.0 00.4 910 As can be seen, it is possible to increase the brightness of the pulp further by means of hydrogen peroxide, even that way. that it exceeds 90%. The decrease in viscosity of the pulp as a result of the hydrogen peroxide treatment was small and is within the limits of normal analytical errors.

Claims (10)

465 224 16 10 15 20 25 PATENT REQUIREMENTS 1. A process for bleaching chemical cellulose pulp with a lignin content corresponding to a maximum kappa number of 10, wherein the cellulose pulp is bleached with chlorine dioxide, characterized in that the 1st V, that the bleaching with chlorine dioxide takes place at a comparatively high pulp consistency and that the cellulose pulp, after the bulk of the chlorine dioxide has been consumed, it is brought into intimate contact with an ozone-containing aqueous solution and reacted with the ozone at a substantially lower mass consistency than during the bleaching with chlorine dioxide, after which the cellulose pulp is subjected to an alkaline treatment in the presence of oxygen and / or oxygen-releasing oxidizing agent. . Process according to Claim 1, characterized in that the cellulose mass during the chlorine dioxide bleaching is present at such a high pulp consistency that, macroscopically, the treatment takes place in the absence of a free liquid phase. Process according to Claims 1 to 2, characterized in that the ozone bleaching takes place in an easily mobile pulp suspension and that the pulp consistency thereof amounts to a value which is less than half of the pulp consistency in the chlorine dioxide bleaching and is not more than 18%. . 4. A process according to claims 1-3, characterized in that the liquor is removed from the cellulose pulp after the ozone bleaching and is used in whole or in part to reduce the pulp consistency after the chlorine dioxide bleaching. Process according to Claims 1 to 4, characterized in that the ozone-containing aqueous solution amounts to 1 to 20%. that is, that the pulp consistency when mixing ÅU 10 15 17 465 224 6. A process according to claims 1-5, characterized in that the chlorine dioxide bleaching is carried out for a time of 10-600 minutes and at a temperature of 50-110 ° C. 7. A process according to claims 1-6, characterized in that the ozone bleaching is carried out for a time of 0.5-60 minutes and at a temperature of 5-60 ° C. 8. Process according to claims 1-7, characterized in that the alkaline treatment is carried out at a pulp consistency of 5-25%, for a time of 10-240 minutes. at a temperature of 50-100 ° C. at an oxygen pressure of less than 0.6 MPa and / or at a peroxide addition of 0.1-10%. IÉOZ on the dry weight of the cellulose pulp. n a t d ä r a v, counted as 9. Process according to claims 1-8, characterized in that the cellulose mass after the alkaline treatment is finally bleached by means of chlorine dioxide. 10. A method according to claims 1-8, characterized in that the cellulose mass after the alkaline treatment is finally bleached by means of ozone.