WO2007108760A1

WO2007108760A1 - Final bleaching of cellulose pulp with ozone

Info

Publication number: WO2007108760A1
Application number: PCT/SE2007/050146
Authority: WO
Inventors: Mårten DAHL
Original assignee: Metso Paper, Inc.
Priority date: 2006-03-20
Filing date: 2007-03-13
Publication date: 2007-09-27

Abstract

An improved method for the bleaching of cellulose pulp is provided in which ozone is used in the final bleaching (30) to increase the brightness of a pulp that is prebleached, preferably to kappa number < 5. It is preferred to prebleach with a first addition of ozone in the conventional manner (II) and then also to add ozone to the final bleaching (I), with the second addition being smaller than the first. For example, an ozone addition of approx. 0.5 to approx. 1 kg/tonne of pulp has proven to provide substantially improved effectiveness in the final bleaching. Embodiments in which the ozone stage (36, 38) in the final bleaching (30) is combined with chlorine dioxide bleaching (34) are particularly advantageous as a synergistic effect is achieved by the combination of ozone and chlorine dioxide, with the effectiveness of the bleaching process being improved further. Residual ozone gas is consumed in the chlorine dioxide stage that follows the ozone stage.

Description

Final bleaching of cellulose pulp with ozone

TECHNICAL FIELD

The present invention relates to the bleaching of cellulose pulp, and specifically to the final bleaching of cellulose pulp.

BACKGROUND

In relation to cellulose pulp for use in paper making, requirements generally exist for the pulp to exhibit certain brightness, normally expressed as ISO brightness. This means that pulp, which, after cooking, is brownish or grey- yellow, needs to be bleached. In the past, chlorine gas was often used for this purpose, but chlorine gas has now been replaced by more environmentally friendly bleaching chemicals. Bleaching takes place via a series of bleaching stages, which typically each entails mixing bleaching chemicals using a mixer and also chemical reactions in a bleaching tower. (Together, the bleaching stages form a bleaching sequence.)

Bleaching of paper pulp is normally subdivided into two parts: prebleaching and final bleaching. In the prebleaching, the main task is to lower the kappa number (a measure of the quantity of lignin + hexenuronic acid). Prebleaching may, for example, take place with chlorine dioxide D, ozone Z or combinations of ozone and chlorine dioxide without intermediate washer (DZ) or (ZD), or with a combination of ozone and alkali (ZE). The prebleaching is usually terminated, regardless of whether prebleaching with D or with Z is involved, with some form of alkali extraction (e.g. E, EO, EOP).

In the final bleaching, the main task is instead to increase the brightness of the pulp. Final bleaching is usually performed nowadays in a stage D or in two separate stages DD or DP with washer in between. In certain cases, two D stages are combined (both at pH ~3.5 - 4) without intermediate washer (DD), and extraction/neutralisation may occur between the D stages (DnD). The choice of final bleaching sequence usually depends on what brightness is to be achieved; several bleaching stages generally mean that a higher level of brightness can be achieved.

One problem with known techniques for final bleaching, including separate final bleaching stages with intermediate washer, such as DD, or combined bleaching stages without intermediate washer, such as (DD), is that they are usually associated with major investment costs to achieve the desired brightness in the outgoing cellulose pulp. Several bleaching stages and/or large quantities of bleaching chemicals arc often required; despite this, it can in some cases be difficult to achieve adequate brightness. SUMMARY

A general aim of the invention is to provide an improved method for the bleaching of cellulose pulp. In particular, the invention is intended to bring about more effective final bleaching of cellulose pulp. These aims are achieved in accordance with the accompanying claims.

According to the present invention, it is proposed that ozone be used in the final bleaching of cellulose pulp to increase the brightness of the pulp. It has emerged that a substantial increase in brightness is achieved when a limited quantity of ozone is added to the final bleaching of a cellulose pulp that has been prebleached, preferably to a kappa number < 5. This increase in brightness can be used to reduce the number of final bleaching stages, reduce the chemical cost for the bleaching and/or increase the final brightness. It is preferred to prebleach with a first addition of ozone in the conventional manner and then also add ozone to the final bleaching, with the second addition being smaller than the first. Embodiments in which the ozone stage in the final bleaching is combined with chlorine dioxide bleaching are particularly preferred as a synergistic effect is achieved by the combination of ozone and chlorine dioxide, with the effectiveness of the bleaching process being improved further. It is preferred to arrange a chlorine dioxide stage after the final ozone bleaching as residual ozone gas can then be consumed in the subsequent chlorine dioxide stage and does not need to be specifically purified.

A preferred method in accordance with the invention comprises prebleaching of cellulose pulp with a first addition of ozone substantially to lower the kappa number of the cellulose pulp, and final bleaching of the prebleached cellulose pulp with a second addition of ozone substantially to increase the brightness of the prebleached cellulose pulp. The second ozone addition is smaller than the first ozone addition and the final bleaching step comprises (DzD) or (DznD).

BRIEF DESCRIPTION OF THE DRAWINGS The invention, and further aims and advantages thereof, are best understood by reference to the following description and accompanying drawings, where:

Fig. 1 is a schematic diagram in the form of a block diagram of a process or a system for the treatment of cellulose pulp with final bleaching in accordance with the present invention; Fig. 2 is a schematic block diagram illustrating the bleaching of cellulose pulp in accordance with an exemplifying embodiment of the present invention with addition of ozone at both the prebleaching and final bleaching; and

Fig. 3A-D are schematic block diagrams of various final bleaching sequences in accordance with exemplifying embodiments of the present invention. DETAILED DESCRIPTION

At the end of this section is a list of bleaching stage designations used.

In the drawings, the same reference number is used for similar or corresponding parts. Fig. l is a schematic diagram in the form of a block diagram of a system 100 for the treatment of cellulose pulp with final bleaching 30 in accordance with the invention. Incoming chips are treated in the conventional way in various process steps 10, here illustrated as cooking 12, washing 14, screening 16 and oxygen delignifϊcation 18. Current fibre lines are generally closed systems in which the processes combine with one another. The bleaching plant comprises, as mentioned, prcbleaching 20 and final bleaching 30, in which the prebleaching 20 is adapted to lower the kappa number (a measure of the quantity of lignin and hexenuronic acid) while the final bleaching 30 is mainly adapted to increase the brightness of the pulp.

It should be mentioned, however, that the preceding process steps 10 (the steps before the bleaching) may comprise processes, e.g. oxygen delignifi cation 18, that result in a certain degree of bleaching in the form of a lowered kappa number and/or increased brightness of the pulp.

The basic principle according to the invention is illustrated in Fig. 1 by (1) for the addition of ozone to the final bleaching 30. The final bleaching step is characterised in that incoming cellulose pulp to be treated there for a final increase in brightness exhibits a kappa number of 6 or lower, preferably less than 5. The invention is not limited to a certain cellulose pulp and can thus be used both on sulphate and sulphite pulp. The quantity of ozone added to the final bleaching may be up to 4 kg per tonne of cellulose pulp. Quantities in the range 0.3 - 2 kg/tonne are usually sufficient, and according to a preferred embodiment, an ozone addition of approx. 0.5 to approx. 1 kg/tonne takes place. The ozone addition according to the invention is typically considerably less than the ozone additions that take place according to known techniques for ozone bleaching (with ozone in the prebleaching), which usually total 5 - 8 kg/tonne. As a result of final bleaching with ozone, a more effective increase in brightness is achieved, which can be used to reduce the number of final bleaching stages, reduce chemical consumption and/or increase the final brightness of bleached pulp. This can be brought about with a limited investment cost.

Normally, the final bleaching comprises two or more bleaching stages, one of which consists of an ozone stage z according to the invention. This new ozone stage z can be established in various positions in the final bleaching sequence, in combination with at least one other bleaching stage, which will be exemplified hereinafter. According to a preferred embodiment, the final bleaching comprises an ozone stage z combined with at least one chlorine dioxide stage D, in which a synergistic effect is achieved with respect to the increase in brightness. Such an embodiment is particularly advantageous in relation to reducing the quantity of chemicals (and thus the cost) for the final bleaching and/or supplying pulps of particularly high brightness. The proposed final bleaching sequence may be used to increase the brightness of pulp treated with various bleaching sequences, comprising both prebleaching without ozone and prebleaching with ozone. According to a preferred embodiment of the invention, prebleaching with ozone takes place to lower the kappa number of the pulp, whereupon final bleaching with a smaller addition of ozone z is performed so that outgoing pulp exhibits the desired brightness. It has emerged that if a pulp is bleached with ozone at the prebleaching stage, a substantial increase in brightness can be achieved by adding a limited quantity of ozone during the final bleaching as well. Ozone is thus used as a delignification agent earlier in the bleaching sequence and as a bleaching agent later in the bleaching sequence.

This is illustrated in Fig. 2, which is a schematic block diagram of a bleaching process with addition of ozone both at the prebleaching and at the final bleaching according to a preferred embodiment of the invention. The bleaching process in Fig. 2 comprises prebleaching 20 with ozone Z 22 and alkali E 24 and final bleaching 30, here with washers 32-1, 32-2 before and after, in the form of an ozone stage z 36, 38 enclosed by two D stages 34-1, 34-2 arranged as combined bleaching stage s without intermediate washers. The ozone stage in the prebleaching 20 is adapted for HC ("High Consistency") operation and may for example comprise a press with a shredder screw for dewatering and fluffing the pulp and also an ozone reactor 22 with rotor. The ozone reactor 22 is designed to bring about good mixing and homogeneous delignification. Other embodiments may comprise prebleaching with ozone at MC ("Medium Consistency") by means of one or more mixers. In such cases, the delignification reactions take place in the mixer.

The quantity of ozone may, for example, be within the ranges indicated for ozone in the final bleaching above in connection with Fig. 1. The addition to the final bleaching is smaller than the addition to the prebleaching - according to one embodiment, half the latter or even less.

The washers 32-1 , 32-2 are comprised of equipment fit for the purpose, such as wash presses or drum filters. A relatively small quantity of ozone, e.g. 0.5 - 1.0 kg/tonne, generally suffices for the "smaller" ozone stage in the final bleaching. This means that the ozone may be added in a fiuidising gas mixer 36 in which the brightness-increasing reaction takes place very rapidly. For example, a earner gas containing approx. 10 - 14% by weight of ozone may be used, and addition takes place for example with compressor 38 at approx. 7 bar (I). In general, a pressure in the range 6 - 10 bar is sufficient, and it may be appropriate to choose a pressure based on the system pressure, e.g. so that the addition of ozone takes place at a pressure approx. 1 bar above the system pressure. Any residual ozone gas will react or be broken down in the subsequent chlorine dioxide stage 34-2. This means that no extra equipment will generally be needed to purify the residual gas, which is a major advantage. Another advantage of comparatively low additions of ozone is that the viscosity of the pulp does not risk deteriorating.

It should be mentioned that the ozone stage in the final bleaching can also be performed in other appropriate ways. For example, solutions with two mixers and addition of ozone to one (the first) or both mixers are conceivable. Preferably, ozone source 26, which supplies the prebleaching step 20 with ozone (II in Fig. 2), is also used to supply the final bleaching 30 with (a smaller quantity of) ozone (III). This means that final bleaching with ozone can be introduced in systems that already have ozone bleaching and thus ozone generation by relatively simple and inexpensive measures. In principle, the additional equipment could consist merely of a mixer 36 and a compressor 38. For systems with two or more parallel fibre lines, it is also conceivable for ozone generation in connection with prebleaching in one line to be used for final bleaching in another line, irrespective of whether or not this other line uses ozone for prebleaching. According to an alternative embodiment (IV in Fig. 2), use is made of residual ozone gas from the prebleaching step 20, which is passed from the ozone reactor 22 to the final bleaching 30. This allows further thrift with the system's ozone. The concentration of ozone in gas entering the final bleaching falls to a low level, however, and variants in which III and IV are combined arc conceivable. A specific embodiment of the invention with final bleaching sequence (DzD) was shown in Fig. 2. Fig. 3A-D are schematic block diagrams of other final bleaching sequences according to exemplified embodiments of the invention. (In these cases, too, ozone addition (I) takes place typically with the aid of mixer 36 and the compressor (38 in Fig. 2), but the compressor has been omitted in the interests of clarity.) In Fig. 3A, neutralisation takes place between the ozone stage and the second chlorine dioxide stage in order to improve the effectiveness of the second chlorine dioxide stage. The bleaching sequence there is (DznD). The neutralisation may, for example, take place by means of addition of sodium hydroxide (NaOH) to a mixer 33 arranged before the final chlorine dioxide stage 34-2. Fig. 3B and 3C show final bleaching 30 with ozone combined with a chlorine dioxide stage 34 in the form of the sequences (Dz) and (zD) respectively. The sequence (zD) in Fig. 3 C has the advantage that residual ozone gas can react or be broken down in subsequent chlorine dioxide stage 34. The increased brightness achieved by stage z can in certain cases be used to reduce the number of bleaching stages, whereby sequences such as (Dz) and (zD) may replace longer bleaching sequences.

It is of course also possible for the above bleaching sequences to be combined with additional bleaching stages, with or without intermediate washers. This is illustrated in Fig. 3D by the sequence (zD)P, which has a final peroxide stage 35, and the person skilled in the art realises that other variants are also possible.

Final bleaching with ozone - examples and comparison with conventional techniques

The invention is now exemplified with the aid of analytical data for final bleaching with (DzD).

A hardwood pulp that had been prebleached with the sequence (Z(EO)) in a mill was studied.

Analytical data for this pulp: Kappa number 3.8 Viscosity 658 ml/g

Brightness 65% ISO

This pulp underwent final bleaching in accordance with Table 1.

Table 1

Final bleaching (DnD) with a total of 12 and 24 kg active Cl/tonne of pulp respectively resulted in a brightness of 83.6% ISO and 87.1% ISO respectively (Test 1 and 3). Bleaching with 12 kg active Cl/tonne in combination with 1 kg ozone/tonne

(Test 2) gave an increase in brightness of 2.9% ISO with a viscosity loss of only 20 ml/g, which is a substantial improvement in the brightness.

Bleaching with 24 kg active Cl/tonne in combination with 0.5 kg ozone/tonne (Test 4) also gave an increase in brightness of 2.9% ISO without a viscosity loss. An increase in the ozone addition from 0.5 kg/tonne to 1.0 kg/tonne gave no further increase in brightness. Although the invention has been described with reference to specific illustrated embodiments, it should be emphasised that it also covers equivalents of the distinctive features shown, and changes and variants evident to the person skilled in the art. The scope of the invention is thus limited only by the accompanying claims.

BLEACHING STAGE DESIGNATIONS

D chlorine dioxide stage

E alkali stage P peroxide stage

Z ozone stage z ozone stage, limited addition n neutralisation stage (alkali, limited addition)

Claims

1. A method for the bleaching of cellulose pulp, comprising prebleaching of cellulose pulp with a first addition of ozone substantially to lower the kappa number of the cellulose pulp, and final bleaching of the prebleached cellulose pulp with a second addition of ozone substantially to increase the brightness of the prebleached cellulose pulp, the second ozone addition being smaller than the first ozone addition and the final bleaching step comprising (DzD) or (DznD).

2. The method according to claim 1, wherein cellulose pulp entering the final bleaching step has a kappa number of 6 or lower.

3. The method according to claim 1 or 2, wherein the first and the second addition of ozone take place with ozone from the same ozone source.

4. The method according to any of the previous claims, wherein the final bleaching step comprises addition of up to 4 kg ozone per tonne of cellulose pulp.

5. The method according to claim 4, wherein the final bleaching step comprises addition of 0.3 - 2 kg ozone per tonne of cellulose pulp.

6. The method according to claim 5, wherein the final bleaching step comprises addition of approx. 0.5 to approx. 1.0 kg ozone per tonne of cellulose pulp.