Achievements in industrial ozone bleaching - Part II

By Emil Germer, Saint-Petersburg State Forest Technical Academy; Alexis Métais, Jean-Christophe Hostachy, ITT Wedeco

BRUSSELS, Aug. 30, 2011 (RISI) - Industrial use of ozone has achieved significant improvements and developments and is now a well-proven technology. But some pulp producers still remember earlier difficulties. In this article, the authors point out practices and technologies that have helped ozone bleaching become an advanced and promising technology. Read Part I here.

In 1998, the Domtar mill in Espanola, ON, switched from the ECF O-A-D-E-DnD sequence to the light-ECF O-A-ZD-E-DnD sequence for bleaching pulp up to 93% ISO brightness. Comparing the two sequences Domtar noticed "ozone has no impact on viscosity or PFI strength properties and in paper machine performance"9.

The Mondi mill in Ruzomberok (Slovakia) produces hardwood and softwood pulps at 89% ISO brightness and switched in 2004 from a D-Eop-D-E-D bleaching to a ZEo-DnD one. It should first be noticed that there was only one washing stage (after ZEo) in the new process. Mondi compared hardwood and softwood pulp strength parameters of the new pulps with the ones of the former pulps set as the reference (100%); only minor differences could be seen in standard pulp properties, Table 118.

Concerning hardwood pulp, energy requirements for refining dropped by 11%, burst was maintained, breaking length and tensile index increased while tear and stiffness decreased. Meanwhile, for softwood pulp, refining energy was again reduced significantly, tensile properties increased marginally while burst, tear and stiffness showed some reduction.

At this stage it is worth remembering that generally, the tensile index is increased by refining while the tear index follows the opposite trend and it is impossible to increase both of them at the same time. Nevertheless the mill reported that the paper machine runnability was not affected and the final paper properties were even better than expected (stiffness)13.

Hardwood pulp properties were particularly excellent and the hardwood proportion in the paper machine furnish was increased over 90%13. In 2005 Mondi Ruzomberok set up a new world record for copy paper production at 1,500 m/min13,18. Of course, these two achievements would not have been possible with a lower pulp quality than before.

Moreover, the Ruzomberok mill has kept improving the Z-stage operation and its bleaching process by installing a pressurized (PO) stage in place of the last D stage in August 2008. Current pulp strength properties are excellent and would make most pulp mills envious, Fig. 6.

Figure 6 - Bleaching line at Mondi Ruzomberok and pulp strength properties

Ozone bleaching process improvements never stopped. The ITC mill in Bhadrachalam (India) started ozone bleaching in 2007 after five years of ECF bleaching operation. Its results are clear19:

• the breaking length increased by 5.5%;
• the burst factor increased by 10%;
• the tear factor increased by 6%;
• the bleaching chemicals cost decreased by about 450 Rs/tonne ($10/tonne).

Operating parameters

Pulp temperature is one important operating parameter that has been tuned. Ozonation was initially carried out at 40°C in the 1990s and such a low temperature was not very convenient as the Z-stage is located right after the 85-95°C oxygen delignification and before an alkaline extraction or a chlorine dioxide stage (after a few MC ozone stages) generally carried out at around 60-80°C. Therefore, it is necessary to cool the pulp before reheating it.

It was thought that a higher temperature would lower the Z-stage efficiency, speed up the ozone decomposition, increase the negative effect of transition metals and ultimately decrease the pulp quality. Slowly, throughout industrial trials, it appeared that such fears were totally unfounded. Several results have demonstrated that for hardwood pulp the Z-stage temperature could be increased up to 60°C and sometimes even higher without any negative effect on pulp strength and brightness9,13,18.

For example, data from Ruzomberok show that an increase of the Z-stage temperature from 43°C to 61°C gives a higher brightness (+1.8% ISO), a lower brightness reversion, a higher stiffness and a better ozone delignification efficiency, with all other properties (PFI rev, Burst Index, Tear Index, Tensile Index and Bulk) unaltered, Table 213,20.

Domtar Espanola first operated the Z-stage at 55°C. When it decided to increase the ozone stage temperature to 60°C, it increased delignification by 5-10% and gained 1 point ISO brightness. Today, it is operating the ozone stage at 70°C9.

All HW pulp mills which set up an ozone stage in the past 5-6 years are running the Z stage at a 55°C minimum. This also resulted in significant steam savings. For example, Ruzomberok reduced its steam requirements by 75% by running the Z-stage at 58-61°C 13.

Metso has shown industrially that steam requirements for light-ECF bleaching are only 25% of steam requirements for conventional ECF bleaching in the case of HW pulp8. While the bleaching sequence DHT-Eop-D requires 249 kg/tonne of low-pressure steam and 40 kg/tonne of medium-pressure steam, 65 kg/tonne of low-pressure steam only are necessary for the Ze-DD bleaching sequence.

As an example, implementation of the ozone stage at Bhadrachalam led to a 50% drop in steam consumption in comparison to the former D-Eop-D bleaching sequence21.

Steam savings are the highest in the case of hardwood pulp bleaching where usually a first A-stage (acidic) or DHT-stage (D-stage at hot temperature) is implemented. Such A or DHT stages have 2 targets:

• removing hexenuronic acids which are responsible for parasite chlorine dioxide consumption and brightness reversion;
• cleaning the pulp from transition metals which are known to decompose H₂O₂ and responsible for weakening of the cellulose during peroxide bleaching.

By removing transition metals the A-stage effectively replaces the use of a chelating agent. Acidic stages were built prior to the ozone stage in the 1990s at the UPM mill in Pietarsaari (Finland), at the Sateri mill in Bahia (Brazil), at Fibria Jacarei and at Espanola. But today there is no doubt that the implementation of an A-stage reduces both pulp strength properties and pulp yield22.

Of course, there is no such problem of hexenuronic acids in the case of softwood pulp bleaching. But the issue of transition metals remains and in the 1990s Swedish mills SCA Östrand and Nordic Paper Säffle as well as the Rosenthal mill in Blankenstein (Germany) chose to start their bleaching sequence with a Q-stage (chelation).

Such A-stages or Q-stages are costly because they require a tower (with 2-3 hours retention time for the later) and the use of expensive chemicals. Yet industrial light-ECF and ozone based TCF experiences have shown that the installation of those A-stages or Q-stages should be avoided.

The hexenuronic acids content is reduced by at least 60% in the ozone stage23. This makes the ozone stage even more efficient than any A-stage and it partly explains why ozone bleached pulps have a lower brightness reversion than other pulps.

Actually, removal of transition metals is efficiently carried out during the ozone stage where the pH is acidic (usually 2.5-3). Awareness of this avoids investing in any acid or chelating stage when ozone is applied, and results in operating cost savings. As a result, only two of the 16 pulp bleaching lines having started ozone since 2000 use an A-stage and none of them use chelating agents.

Bleaching sequences

The proven economical, technical and environmental advantages, the development of oxygen delignification as well as improvements in ozone bleaching and in the whole pulping process over the last 20 years, have allowed the bleaching sequence to be shortened by one, two or even three stages, while simultaneously increasing the brightness ceiling with 2-3 points compared with what was possible.

In the 1990s it was common to fully bleach the pulp with six or even seven stages as in the following sequences:

- A-ZD-Eop-ZD-Ep in Pietarsaari (Finland);
- Q-OP-D-Z-PO-P in Blankenstein;
- A-ZD-Eo-DnD in Espanola.

Bleaching sequences built in the last decade for both hardwood and softwood pulps usually have only three or four stages and vary little from one another:

- Z-D-Ep-D at Nippon PaperYufutsu (Japan);
- ZEop-D-P at Mondi Ruzomberok;
- ZD-E-D at Nippon Paper Maryvale (Australia);
- Ze-DP at ITC Bhadrachalam, Fig. 7;
- Z(EOP)-(PO) at Sniace (Spain, sulphite pulp for dissolving grades);
- Ze-D-P at Celtejo (Portugal).

These bleaching sequences reflect the continuous and impressive developments of ozone application in light-ECF bleaching process and clearly show the worldwide interest of pulp makers in ozone bleaching.

Figure 7 - Line 2 at ITC Bhadrachalam

Conclusions

Ozone bleaching has already been industrially implemented, experienced and improved for 18 years. It is a well proven and safe process, currently used by some reference pulp mills among the most modern in the world. Its advantages in terms of bleaching cost savings, effluent load reduction and ease of use can no longer be questioned. It is applicable to all kind of pulps and has no negative effect on their mechanical properties.

The Z-stage takes place in just one minute and both HC and MC technologies are recognized as fully reliable, safe and stable processes.

High-consistency ozone bleaching is performed at atmospheric pressure. Behind the ozone reactor, it requires only a 5-10 minutes long extraction stage and no intermediate washing is necessary. This solution allows the E filtrate to be recycled back to the brown stock or post-oxygen washing and further to the recovery boiler, and so the total bleaching effluent can drop to 4-6 m³/tonne.

As a result of continuous improvements of the equipment and process automation as well as tuning of the operating conditions over a period of almost 20 years, modern ozone bleaching is now recognised as a state of the art technology for both hardwood and softwood pulps.

There are neither technical nor economical reasons to stick to a 30-year-old conventional ECF process. The use of a light-ECF bleaching sequence can by itself decrease bleaching costs by more than 20% and lead to the same pulp strengths with better optical properties and refining ability. Ozone bleaching is by far the most valuable solution not only for forerunners but for all pulp mills.

LITERATURE
1. MÉTAIS, A., Proceedings of the TAPPSA National Conference 2010, Durban, South Africa.
2. CARRÉ, G. and WENNERSTRÖM, M., Proceedings of the International Pulp Bleaching Conference 2005, Stockholm, Sweden.
3. NORDÉN, S. et al., Eco-Friendly Bleaching with ZeTracTM, paper presented at the IPPTA Annual General Meeting & Seminar 2006, Chennai, India.
4. CAMPO, R. and MARQUES, P., Results Pulp & Paper 3, 32 (2009).
5. BOKSTRÖM, M., African Pulp and Paper Week 2002, Durban, South Africa.
6. HOSTACHY, J.-C., Tappi J., 9(8): 16(2010).
7. CAMPO, R. and SUNDIN, M., Proceedings of the EFPG Days 2004, Grenoble, France.
8. WENNERSTRÖM, M. et al, International Colloquium on Eucalyptus Pulp 2007, Belo Horizonte, Brazil.
9. GARTLEY, B., Ozone Delignification at Domtar Espanola, paper presented at the PAPTAC Bleaching Committee, Oct. 21-23 2002.
10. CHIRAT, C., et al., Proceedings of the ABTCP 2008, Sao Paulo, Brazil.
11. VEHMAA, J. and PIKKA, O., Proceedings of Paperex 2007, New Delhi, India.
12. PIKKA, O., Proceedings of the EFPG Days 2004, Grenoble, France.
13. TOMIS, B. and TUOMI, A., Proceedings of Zellcheming 2006, Wiesbaden, Germany.
14. FERGUSON, K., Pulp & Paper, 66(11): 42 (1992).
15. CHIRAT, C., Proceedings of the EFPG Days 2007, Grenoble, France.
16. LenzingTechnik, Eccentric Mixer - an innovation from LenzingTechnik, commercial presentation.
17. LACHENAL, D., Proceedings of the EFPG Days 2005, Grenoble, France.
18. NORSTEDT, Å. and LINDSTRÖM, L.-Å., ATCP Conference 2006.
19. KUMAR, K.R., Proceedings of Paperex 2009, New Delhi, India.
20. BALAZ, P., Proceedings of Zellcheming 2006, Wiesbaden, Germany.
21. PADMANABHAN, A. et al, paper presented at the National Award for Excellence in Energy Management 2007, 22-23 August 2008, Pune, India.
22. KOBAYASHI, T. et al, Japan Tappi, 59 (2005).
23. NORDÉN, S., ZeTrac Forum 2004, Rochehaut, Belgium.
24. ENGELFELDT A., internal training for ITC.

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