EP2276883A1

EP2276883A1 - Method of bleaching pulp with ozone, chlorine dioxide and peroxide

Info

Publication number: EP2276883A1
Application number: EP09746853A
Authority: EP
Inventors: Solveig NORDÉN
Original assignee: Metso Paper Oy
Current assignee: Valmet Technologies Oy
Priority date: 2008-05-13
Filing date: 2009-04-23
Publication date: 2011-01-26
Also published as: JP2011521116A; CN102027167A; CN102027167B; JP5478610B2; WO2009139693A1; EP2276883A4; SE0801074L; SE532470C2

Abstract

The present disclosure relates to a cost efficient method of bleaching a pulp by means of ozone. The method comprises subjecting an oxygen delignified pulp to an ozone bleaching stage followed by a chlorine dioxide bleaching stage and a peroxide bleaching stage. The peroxide bleaching stage is performed directly after the chlorine dioxide stage without any intermediate wash.

Description

METHOD OF BLEACHING PULP WITH OZONE, CHLORINE DIOXIDE AND PEROXIDE

The present disclosure relates in general to a method of final bleaching of a pulp. More specifically, it relates to a cost efficient method of bleaching an oxygen delignified pulp by means of ozone bleaching to a final brightness of more than 88 % ISO.

Background

In bleaching processes for both softwood and hardwood pulps, the pulps are normally delignified in one or more oxygen stages and thereafter bleached by means of various sequences comprising chlorine dioxide stages, extraction stages, peroxide stages etc.

Hardwood pulps differ from softwood pulps in that they contain high amounts of Hexenuronic Acid (HβxA). The amount of HexA depends of the raw material used and the cooking conditions. Modem methods of cooking, utilizing relatively low cooking temperatures, normally render high contents of HexA. HexA is oxidized by potassium permanganate (KMNO₄) and thereby contributes to the kappa number. In a hardwood pulp with a kappa value of 10, 50-70% of the kappa value could be a result of HexA and only 30-50% of lignin and other compounds. During bleaching, HexA can be reduced by oxidation with bleach

Chemicals such as chlorine dioxide and ozone. An economical way is to degrade HexA by means of acid hydrolysis at high temperature. Therefore, a hot chlorine dioxide step (DHT) is often installed in modem bleach plants in order to perform the acid hydrolysis in combination with chlorine dioxide bleaching. The high temperature in DHT followed by an extraction stage can give a reduction of the kappa number from for example 10.5 to 2.5.

As mentioned above, it is also possible to bleach a pulp by means of ozone. Ozone is a very effective delignification chemical and reacts very fast with the pulp and therefore requires only a short retention time. Ozone bleaching is normally performed at temperatures below 60 °C since the environment is very aggressive. Ozone bleaching is generally combined with a subsequent extraction stage.

End bleaching is normally performed in two separate steps, either in the form of two chlorine dioxide bleaching stages or in the form of a chlorine dioxide bleaching stage followed by a peroxide bleaching stage. In some cases the two chlorine dioxide bleaching stages are performed without any intermediate wash. In the case of the chlorine dioxide bleaching stage being followed by a peroxide bleaching stage, the stages are generally performed with an intermediate wash since the stages are performed at different pH, normally pH 3.5-4 in the chlorine dioxide stage and pH 10-11 in the peroxide stage.

It has been reported that under special circumstances, it is possible to combine a chlorine dioxide bleaching stage with a peroxide bleaching stage without any intermediate wash, see Suess et al., Short sequence bleaching without penalties- options for Eucalyptus pulp. AppKa 2005, p.461-466. This may be performed when a hardwood pulp has been bleached, using a high temperature chlorine dioxide bleaching stage (DHT) followed by an extraction stage integrated with a peroxide bleaching stage, to such a low kappa number that only 5-7.5 kg active CJΛon is required in the chlorine dioxide bleaching stage. In this special case, it is possible to obtain almost the same result when bleaching using a sequence comprising a chlorine dioxide bleaching stage integrated with a peroxide bleaching stage (i.e. without any intermediate wash) as when bleaching using a sequence comprising a chlorine dioxide bleaching stage followed by washing and a peroxide bleaching stage. Suess et ai. also disclose an alternative method comprising direct combination of ozone with hydrogen peroxide and an extraction followed by a combination of chlorine dioxide bleaching and peroxide bleaching. The combination of ozone with hydrogen peroxide and a direct extraction is said to result in a relatively high brightness (69 % ISO) and a low kappa number (<3) after the extraction stage. However, final bleaching is said to require high charge of chlorine dioxide. A total input of 5 kg O₃.20 kg CIO₂.6 kg H₂O₂ and 19 kg NaOH was required to achieve brightness of more than 90 % ISO. As the result of the high demand for chlorine dioxide. Suess et al. concludes that K is impossible to eliminate the washer between the chlorine dioxide bleaching stage and the peroxide bleaching stage.

Therefore, Suess et al. propose to include a chlorine dioxide stage prior to the ozone bleaching stage and thereby achieve a Kappa number below 2 after the extraction. This reduces the demand for chlorine dioxide in the following chlorine dioxide stage which will develop into a polishing stage instead of a delignificatlon step. The lower demand for active chlorine in the chlorine dioxide stage after the extraction stage thus permits to combine said chlorine dioxide stage with the final peroxide stage without any intermediate wash. The proposed bleaching sequence will Include five stages and a total of three washers. A chlorine dioxide bleaching step prior to the ozone bleaching would however lead to presence of chlorides in the filtrate after the extraction step, which would make it more difficult to reuse the filtrate for washing after the oxygen stage and/or burn it

The primary object of the invention is to provide a cost efficient ozone bleaching method of a pulp to a final brightness of more than 88 % ISO.

Summary

The above identified object is achieved by means of the method of bleaching a pulp in accordance with independent claim 1. Preferred embodiments are given in the dependent claims. The method of bleaching a pulp according to the present invention contains one less washing step since the peroxide bleaching stage is integrated with the preceding chlorine dioxide bleaching stage. Hence, the investment costs are substantially reduced. Furthermore, the method according to the invention enables a more efficient handling of the extract from the extraction stage, which follows the ozone bleaching stage, since the extract is free from chlorides.

According to a preferred embodiment, MgSO₄ is added to the pulp just prior to, or during, the chlorine dioxide bleaching stage. This addition results in a higher brightness at the same chlorine dioxide consumption compared to if MgSO₄ is added to the peroxide bleaching stage. Hence, it is also possible to reduce the amount of chlorine dioxide required for a desired final brightness if such an addition of MgSO₄ is made to the pulp.

Even though the method according to the invention is intended for bleaching of hardwood pulp, it is also believed to be suitable for bleaching of other pulps, such as softwood pulp.

Detailed description

In the present disclosure, the term integrated should be interpreted as following directly after the preceding stage without any intermediate wash. The method according to the invention comprises subjecting an oxygen delignified and washed pulp to an ozone bleaching stage followed by an extraction. In contrast to the bleaching method suggested by Suess et al. the method according to the present disclosure does not contain any chlorine dioxide bleaching prior to the ozone bleaching. Therefore, there will be no problems of reusing the filtrate resulting from the extraction stage or destroying the fittrate by means of burning, which would be the case if the fittrate would contain chlorides. Chlorides in the filtrate would make it aggressive and therefore increase the risk of corrosion. The ozone bleaching used in the bleaching method of the present disclosure is preferably performed on a pulp having a pulp consistency of 30-45 %, more preferably 35-40%, also known as a high consistency pulp. This leads to a lower demand of chlorine dioxide in the subsequent chlorine dioxide bleaching step compared to if the ozone bleaching is performed on a wet pulp, such as a pulp having a consistency of 10-12 %.

The ozone bleaching stage is followed by an extraction stage integrated with the ozone bleaching. Preferably, no additions of peroxides are made during these stages as it is not considered necessary in order to achieve the desired result. After the extraction stage, the pulp is washed and thereafter subjected to a chlorine dioxide bleaching stage. The chlorine dioxide bleaching is suitably performed at a pH of 2.5-4.5, preferably 3-4.5, and at a temperature of 70-90 "C, preferably 75-85 °C, with an addition of chlorine dioxide corresponding to 5-25 kg active Ci/ton, preferably 10-25 kg active Cl/ton, more preferably 15-22 kg active CI/ton. The time required for the chlorine dioxide bleaching depends on the factors given above and can easily be determined by the skilled person in the art

The chlorine dioxide bleaching is followed by a peroxide bleaching stage, said peroxide bleaching stage being integrated with said chlorine dioxide bleaching stage, i.e. no washing of the pulp Is made between the chlorine dioxide bleaching stage and the peroxide bleaching stage. Since no wash is required between these two steps, the investment costs are substantially reduced as a result of the requirement of one less washer compared to prior art.

It has been found that, contrary to the opinion expressed in Suess et al., it is possible to integrate the chlorine dioxide bleaching stage and the peroxide bleaching stage while still accomplishing an effective bleaching sequence. Even if comparatively high chlorine dioxide additions are made during the chlorine dioxide bleaching stage, the peroxide bleaching stage will still be effective and may increase the final brightness of the pulp by approximately 2-3 % ISO. The peroxide bleaching stage may suitably be performed at a pH of 9-12, preferably pH 9.5-11 , and at substantially the same temperature as the chlorine dioxide stage. The amount of H₂O₂ may suitably be 1-8 kg/ton pulp, preferably 2-6 kg/ton. The time required for the peroxide bleaching stage can easily be determined by the skilled person. It is commonly known that magnesium in the form of MgSO₄ may be added to the pulp during a peroxide bleaching stage in order to stablize said stage. The addition of MgSO₄ enables an increased brightness and viscosity of the pulp. Moreover, the amount of residual peroxide after the bleaching stage is at a higher and more stable level than In the case of peroxide bleaching without such an addition.

However, it has surprisingly been found that adding MgSO₄ to the pulp already in the chlorine dioxide bleaching stage can further improve the bleaching sequence in the case of the present bleaching method. The same result has not been observed in the case of a bleaching sequence containing a washing stage between the chlorine dioxide bleaching and the peroxide bleaching stages.

Hence, according to a preferred embodiment of the method of bleaching a pulp according to the present disclosure, MgSO₄ is added to the pulp just prior to or during the chlorine dioxide bleaching stage, it has been found that such an addition can result in about 0.5 % higher brightness at the same chlorine dioxide consumption compared to if the MgSO₄ addition is made to the peroxide stage. If comparing at the same brightness, for example 90 % ISO, this means that approximately 2 kg active Cl/ton pulp can be saved if such an addition is made. Suitably 0.5-4 kg/ton MgSO₄ can be added to the pulp for said chlorine dioxide bleaching stage, preferably 1-3 kg/ton is used.

Example 1

A hardwood pulp was oxygen dellgnified and subsequently bleached with HC-ozone to a kappa number of 3.3. Data for the pulp after cooking, oxygen delignlfied stage and ozone bleaching stage, are shown in Table 1. The pulp was thereafter bleached using a sequence S_Inv1 of a chlorine dioxide bleaching stage followed by a peroxide bleaching stage without any intermediate wash in accordance with the invention. The pulp was also bleached in a conventional reference sequence S_Ref1 comprising a chlorine dioxide bleaching stage, a washing stage followed by a peroxide bleaching stage, for comparison. Data for the sequences and the results are shown in Table 2.

The same brightness and viscosity were achieved using both the bleaching sequence according to the invention and the conventional bleaching sequence. However, in the case of the bleaching sequence according to the invention, an additional 2 kg NaOH/ton was required in order to achieve the same final pH.

The results show that it is possible to reduce the investment costs using the bleaching method according to the invention since it does not require any intermediate washing step and consequently one less washer.

Moreover, the measured brightness reversion (Δbrightness) is much lower in the case of the bleaching sequence according to the invention. The measured data could possibly be considered to be somewhat uncertain but indicates that improved Δbrightness could be achieved with the bleaching method according to the invention.

Example 2

A hardwood pulp was oxygen delignifled in a mill and laboratory bleached with HC-ozone to a kappa number of 3.3. The viscosity was 520 ml/g.

The pulp was subsequently subjected to two different end bleaching sequences wherein one of the sequences, S_Inv2, is according to the invention and one is a reference sequence, S_Ref2. S_Inv2 comprised a chlorine dioxide bleaching stage followed by a peroxide bleaching stage integrated with said chlorine dioxide bleaching stage whereas the reference sequence S_Ref2 comprised a chlorine dioxide bleaching stage, washing and subsequently a peroxide bleaching stage. Trials were made with magnesium sulfate additions (MgSO₄) to the different stages of the bleaching sequences. 8

Table 3 shows the data for six trials A-F for the reference sequence S_Ref2 whereas Table 4 comprises the data for six trials G-L for the sequence according to the invention Si_n*. From the results above it is evident that in the case of the reference sequence, S_Ref2, addition of MgSO₄ in the chlorine dioxide stage and in the peroxide bleaching stage, respectively, resulted in approximately the same brightness at the same chlorine dioxide consumption. However, in the case of the sequence according to the Invention, i.e. where no wash is performed between the chlorine dioxide bleaching stage and the peroxide bleaching stage, addition of MgSO₄ to the chlorine dioxide bleaching stage resulted in a substantially higher brightness at a certain chlorine dioxide consumption compared to if the addition of MgSO₄ was made to the peroxide bleaching stage (except in the case of 10 kg active Cl/ton wherein approximately the same brightness was achieved). The results for the sequence according to the invention therefore show that if comparing at the same brightness, making an addition of MgSO₄ to the chlorine dioxide bleaching stage reduces the required amount of chlorine dioxide for said bleaching.

The results also show that only minor differences in viscosity occur depending on when the addition of MgSO₄ is made. Moreover, the peroxide consumption was the highest when the MgSO₄ addition was made to the peroxide stage in the case of the sequence according to the invention.

Claims

1. Method of bleaching a pulp comprising the following steps:

(i) subjecting an oxygen deligntfled and washed pulp to an ozone bleaching stage followed by an extraction stage integrated with said ozone stage, wherein no chlorine dioxide bleaching of the pulp has been performed prior to said ozone bleaching stage, (ii) subjecting the ozone bleached pulp to a chlorine dioxide bleaching stage, and (iii) subjecting the chlorine dioxide bleached pulp to a peroxide bleaching stage integrated with said chlorine dioxide bleaching

2. Method according to claim 1 wherein the ozone bleaching is performed such that the pulp will have a kappa number of 3-4 after said extraction stage.

3. Method according to any of the preceding claims wherein the pulp has a consistency of 30-45 % during the ozone bleaching.

4. Method according to any of the preceding claims wherein the ozone bleaching stage is free from additions of peroxide.

5. Method according to any of the preceding claims wherein MgSO₄ is added to the pulp directly prior to or during the chlorine dioxide bleaching stage.

6. Method according to claim 5 wherein MgSO₄ is added in an amount of 0.5-

4 kg/odt, preferably 1-3 kg/odt.

7. Method according to any of the preceding claims wherein the pulp is a hardwood pulp.

8. Method according to claim 7 wherein the pulp is a Eucalyptus based pulp.