The present invention concerns a method for treating fully bleached cellulose pulp according to the introduction to claim 1.

The Prior Art

One problem during the manufacture of, for example, market pulp from a pulp mill for a paper mill that is located at a distance is the tendency of the cellulose pulp to brightness reversion. This problem receives attention also in what are known as "integrated pulp mills" in which the fully bleached pulp is kept in storage for a considerable period until it is led out to the paper machines.

It is particularly the case during the manufacture of fully bleached qualities of pulp, "90-grade pulps" with an ISO brightness greater than 88%, which are suitable for the manufacture of white office paper and for high-quality paper products without any visible trace of yellowing, that extremely high efforts must be made and costs must be borne in order to achieve a fully bleached pulp. It is generally the case that the costs required to increase the ISO brightness by 2-3 percentage points for an ISO 90 grade pulp are greater than the costs required to increase the ISO brightness by 2-3 percentage points for an ISO 80 grade pulp by at least a factor of ten.

Pulp mills strive for this reason to ensure that the fully bleached pulp retains as far as possible the brightness that has been reached after the final bleaching, and to prevent the loss of several ISO units due to brightness reversion, particularly since the costs of achieving the final 2-3 ISO units for a "90-grade" pulp are so high.

Various methods are known for reducing the tendency of the pulp to brightness reversion, where, for example the patent GB1.062.734 revealed in the 1960s various extreme treatment steps with the aim of reducing brightness reversion. A hot alkali/long duration treatment (100/120 ⁰ C, 2 hours, pH 11.5) was used in one variant, while a hot acid/long duration treatment (100 °C, 2 hours, pH 2.25) was used in the second variant.

It is normally so that paper mills desire a neutral or a weakly acidic pulp before the treatment in the paper machines, and for this reason fully bleached pulp that in a final acidic bleaching step, preferably a chlorine dioxide step, does not undergo anything other than a light alkali treatment following a final wash after the final chlorine dioxide step.

Certain suppliers of bleaching systems have considered also a single alkali treatment step in which only alkali is added before a final storage in a pulp tower following the final acidic bleaching step, the storage of the pulp being of a certain duration. A certain limited reduction in brightness reversion has in certain cases been seen in this case.

It has not been considered necessary to treat the pulp with the aim of reducing the brightness reversion in bleaching lines in which a final peroxide step is used in which an increase in brightness of at least 3-5 ISO units is achieved, and in which substantial batches of peroxide are added, greater than 5 kg peroxide per tonne of pulp, possibly with a certain reinforcement using a small addition of oxygen gas, which is typically well under 50% of the batch of peroxide, particularly since chlorine dioxide is used in the preceding bleaching process.

Other bleaching steps have demonstrated positive properties with the aim of reducing brightness reversion. The older chlorine steps and mixed chlorine/chlorine dioxide steps often gave the fully bleached pulp a low tendency to suffer brightness reversion, but these steps are more or less unacceptable due to the known negative effects of the chlorine on the environment. The industry has, with the aim of reducing the use of chlorine, replaced its use with the use of chlorine dioxide, which is a significantly better bleaching agent with a considerably lower negative effect on the environment.

Ozone is another bleaching agent that can, where suitable, contribute to achieving a low level of brightness reversion, but ozone suffers from the disadvantage of being highly toxic. Significant safety measures are required such that it does not constitute a direct risk to health of the operators of the

bleaching line, or in association with the production of the ozone on site in the ozone generators.

Oxygen gas bleaching is an alternative that is very sensitive to the environment from many perspectives, but oxygen gas steps have largely been used exclusively in early delignification, also known as "prebleaching" in which the lignin content is decreased by the oxygen gas from a kappa value of 15-30 units or greater to a kappa value of 8-10. A limit to the degree of delignification possible has, however, been observed and it has proved to be practically impossible in many cases to reach a kappa value lower than approximately 8 by the use of oxygen gas. The oxygen gas has for this reason been excluded as a principal bleaching agent in the latter part of the bleaching, but it has been used as a reinforcing additive in alkali extraction steps, with or without the addition of peroxide (E- EO or EOP steps), and it has been used in pure peroxide steps in which significant amounts of peroxide are added, typically well over 5 kg per tonne of pulp, and often 10-20 kg per tonne of pulp.

Aim and purpose of the invention

The invention is based upon the surprising effect of a relatively simple alkaline oxygen gas step as a treatment step for fully bleached pulp, which has preferably been bleached with chlorine dioxide in the final bleaching step and which maintains an ISO brightness greater than 88% after this final bleaching step, in which it has turned out to be the case that a significant reduction in brightness reversion is obtained for pulp from hard wood, calculated as a reduction in PC number from its original value of 0.4 units down to 0.25 units, a reduction of just over 37%.

This corresponds to a reduction in the ISO of as much as 3.0-3.5 units, from which it will be realised that this has a very great significance for market pulp mills that supply fully bleached "90 grade" pulp (with an ISO brightness greater than 88%).

The present invention intends to offer a treatment step that uses only cheap chemicals that do not harm the environment and that do not add to the environmental load, or that uses bleaching agents that have been already

used and that can be recovered in other steps. The alkali that is added is recovered as a relatively pure alkaline filtrate from the wash after the treatment step and it can be reused without problem in alkali bleaching steps that lie upstream in the bleaching line, and the addition of oxygen gas does not involve any further load on the environment at all, and is currently often already used in the pre-bleaching step or in extraction steps.

The principal aim of the invention is to offer a method that can be used to reduce significantly tendencies to brightness reversion while at the same time allowing the acquirement of a certain increase in brightness through the treatment. A normal simple alkali treatment of the pulp usually gives a certain darkening of the pulp, that is, a reduction in the ISO brightness, but it is possible to reduce this alkali-induced darkening by the addition of oxygen gas; it being even possible to obtain an increase in brightness.

The process can be readily adapted and optimised for any relevant type of wood such as hard wood, soft wood, annual plants and cotton fabrics of the "cotton linter" type.

Description of Drawings

Figure 1 shows how pulp treated by the invention obtains a greater brightness (an increase in ISO value);

Figure 2 shows how pulp treated by the invention obtains a lower tendency to brightness reversion (a decrease in PC number).

Detailed Description of Preferred Embodiments Figure 1 shows the effect on pulp that is obtained following a treatment according to the invention, in which the reduction of brightness reversion is measured in PC units, and the increased brightness is measured in ISO units.

The PC number ("post colour number") is defined, as is explained in "Pulp Bleaching, Principles and Practice", Tappi Press, ISBN 0-89852-063-0, Chapter III 5: Chemistry of Brightness Reversion and its Control, page ^" ! 85;

PC Value = 100 ( (K/S) after aging - (K/S) _be fore aging)!

where:

K is the light absorption of the pulp and S is the light scattering of the pulp and is measured in m ² /kg. The PC number gives fully comparable numerical values for the degree of brightness reversion that has taken place, independently of where in the brightness scale the pulp is located.

Fully bleached pulp of either hard wood or soft wood has been used for laboratory tests, where the soft wood has been cooked, delignified and fully bleached in the following sequence: Kok - OO - D ₀ - (EOP) - Di - EP - D ₂ , and where the hard wood has been cooked, delignified and fully bleached in the following sequence:

KOk - D ₁ - (PE) - D ₂ ; where: Kok = sulphate digestion;

00= two-step oxygen gas delignification;

D= chlorine dioxide step (at the first, second or third D-step; D ₀ /Di or D ₂ );

EOP = alkaline extraction step reinforced with oxygen gas and peroxide;

PE = alkali peroxide step (with a considerably larger batch of peroxide than that normally used in extraction steps).

The treatment step according to the invention is carried out at an elevated temperature of 105 ⁰ C, which is established through a progressive heating for a duration of 30 minutes, followed by maintenance of the pulp at this temperature for 60 minutes, to be subsequently washed.

Thus, the treatment will be active for a period of 90 minutes in total. The treatment takes place in an environment in which oxygen gas is present, which environment is established by the pressurisation of the laboratory vessel (the autoclave) with oxygen gas at a pressure of 0.5 MPa. This laboratory technique of pressurisation by means of externally supplied oxygen gas ensures the presence of oxygen gas throughout the complete test, but for an actual implementation in a reactor at a pulp mill the oxygen gas is added in such an amount that the oxygen gas is present during the principal part of the

treatment, preferably present as long as at least an increase in brightness can be obtained.

Figure 1 presents the ISO values that not only fully bleached hard wood pulp but also fully bleached soft wood pulp have had before the treatment, while Figure 2 presents the PC numbers that these pulps have obtained after the treatment. The value given at 0 kg on the x-axis specifies the properties of the pulp before the treatment, while the effects obtained after the treatment using the addition of 10, 20, 40 or 80 kg of alkali are then specified.

It is clear that the PC number (a measure of the brightness reversion) is reduced in the case of the hard wood pulp from approximately 0.40 to 0.20, which corresponds to a reduction of 50%. This level of the reduction of the brightness reversion corresponds to a loss of brightness of the pulp that lies between 3.0 and 3.5 ISO units. The hard wood pulp has at the same time surprisingly increased its ISO brightness from 87.5% to 89.5%. The principal effect has essentially been obtained at as moderate an addition of alkali of 10-15 kg, while at an addition of 30-40 kg alkali (calculated as kg per "bone dry tonne" or "kg/BDt") the effect has essentially planed out, such that increased addition of alkali does not give any effect that can be shown, something that is confirmed by the addition of 80 kg alkali. The ISO brightness has been increased from 87.5% to 88.5% and the PC number has been reduced from 0.40 to 0.25 by the addition of only 10 kg alkali, where the reduction in brightness reversion (expressed as PC numbers) has been reduced by as much as 37.5%. In the case of the hard wood pulp, it is evident that the PC number (the measure of the brightness reversion tendency) has been reduced from just over 0.25 to 0.15, which corresponds to a reduction of 40%. This reduction is achieved, however, from a lower initial value. This degree of reduction of the brightness reversion corresponds to a loss of brightness of 1.5-2.0 ISO units. The hard wood pulp has at the same time surprisingly increased its ISO brightness from 87.8% to just over 89%. The principal effect has essentially been obtained for hard wood pulp at as moderate an addition of alkali as 10- 15 kg, while at an addition of 30-40 kg alkali (calculated as kg per "bone dry tonne" or "kg/BDt") the effect has essentially planed out, such that increased

addition of alkali does not give any effect that can be shown, something that is confirmed by the addition of 80 kg alkali. The ISO brightness has been increased from 87.5% to 88.4% and the PC number has been reduced from 0.25 to 0.17 by the addition of only 10 kg alkali, where the reduction in brightness reversion (expressed as PC numbers) has thus been reduced by as much as 32%. A certain continuous increase of the ISO brightness does, however, take place, but this is marginal. It does, however, demonstrate that even if the alkali step does occur during a long period, no darkening of the pulp takes place as long as the oxygen gas is present. The tests show that the tendency to brightness reversion can be markedly reduced with as moderate an addition as 10 kg NaOH per tonne of pulp (BDt), where a reduction in the tendency for brightness reversion of over 25% can be obtained, calculated as a reduction in PC number. The tests show that the process can be optimised within reasonable addition of alkali, and that it can by adaptation be minimised in duration, addition of alkali, or degree of heating. The treatments that have been used in the laboratory experiments (30+60 minutes) can be significantly reduced towards times at a level of 5-10 minutes at the correct treatment temperature if, for example, the increase in temperature or the addition of alkali (or both) are carried out and a reduction of brightness reversion of over 25% can still be obtained.

The invention can be modified in a number of ways within the framework of the claims. It is possible, for example, during its practical application to add oxygen gas before it is led to the reactor or storage tower, where the amount of oxygen gas is determined such that it is present during the principal part, greater than 50%, of the treatment time, where the increase in brightness is equivalent to or greater than any post-production darkening effect from the remaining treatment time in the reactor or storage tower that takes place without the presence of oxygen gas. The limit for this amount of added oxygen gas is purely a question of optimisation, depending on the currently used type of wood and how it has been treated in the preceding bleaching steps. Different bleaching sequences can give different effects for the increase in brightness and the reduction in PC number (or both) as a function of time, temperature and the addition of chemicals during the treatment.