Achievements in industrial ozone bleaching

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

BRUSSELS, Aug. 22, 2011 (RISI) - The first industrial pulp bleaching line including an ozone stage started 19 years ago. Today there are 22 mills worldwide using what is commonly named light-ECF bleaching. Among those 22 mills, 16 produce solely hardwood pulps, four produce both softwood and hardwood pulps while SCA in Östrand (Sweden) and Rosenthal in Blankenstein (Germany) produce exclusively softwood pulp1. Thirteen mills started ozone bleaching in the early years of the 21st century and three mills equipped two of their bleaching lines with ozone: Oji Paper in Tomioka (Japan), Fibria in Jacarei (Brazil) and ITC in Bhadrachalam (India). Fibria selected ozone bleaching in 2002 for Jacarei's new 2,500-tonne/day line C after having operated ozone bleaching as from 1995 on its 900-tonne/day line B. This decision clearly shows that ozone bleaching has met all requirements and expectations on line B for seven years.

In April 2011, four new ozone generation systems have been contracted and will start operating in 2012 for:

- a 700,000-tonne/yr capacity Chinese greenfield mill owned by Oji Paper (the company already uses ozone in its Japanese mills);
- a pulp production capacity increase in Austria at Lenzing;
- two pulp production capacity increases in Brazil (confidential).

Ozone bleaching is efficiently used on hardwood and softwood pulps, on kraft and sulphite pulps dedicated to all kinds of final applications. Pulp producers do not always evaluate the significant ecological advantages of ozone-based bleaching sequences over the traditional ECF bleaching sequence D0-Eop-D1-D2 (or its variants): the quality of wastewater is drastically improved 2,3 as it is possible to lower bleaching effluents to 4-6 m3/air dry tonne4. Of course investors always focus on the return on investment (ROI) and it has now become clear to all ozone users that bleaching costs are reduced by 20-32% when introducing an ozone stage in an ECF bleaching line 3,5,6,7 and even more in the case of TCF bleaching 8. The high bleaching efficiency of ozone allows a drastic reduction in the consumption of expensive bleaching chemicals - chlorine dioxide in ECF bleaching and hydrogen peroxide in TCF bleaching as well as sodium hydroxide in both cases - and the implementation of ozone bleaching also results in reduced of steam requirements during the bleaching process. The ROI in the replacement of a D0-stage by an ozone Z-stage lies between two and four years 1,6.

Without affecting the pulp strength properties in comparison with conventional ECF sequences, bleaching with an ozone stage additionally gives a wide range of opportunities by:

- making very high brightness levels possible (92-93% ISO)2,9;
- decreasing brightness reversion2,8,10;
- reducing drastically the extractive content10,11 by 50-75%8,12;
- reducing energy requirements during refining by at least 10%3,13;
- precisely controlling the viscosity in viscose pulp production.

Nevertheless, arguing that ECF bleaching has already been used in the industry for 30 years, a number of conservative pulp producers still consider the bleaching sequence D0-Eop-D1-D2 and its variants- as the best alternative today. One should however be aware that within the next 20 years, the pulp and paper industry will have to carry out significant modernisations to retain its competitiveness and, in turn such a traditional ECF bleaching will then be over 50 years old. This position could be understood if no better technique was available. But considering the very good results and proven stable operation achieved today by a large number of industrial ozone-based light-ECF and TCF bleach plants, such a conservative attitude can't be justified any longer.

The industrial use of ozone has already undergone a long string of improvements and developments. As with other new technologies, ozone bleaching did not immediately reach its optimal technical efficiency but faced several issues during its early years. All the same, achievements of ozone bleaching have improved year in year out and it is now a well proven technology. Nevertheless, some pulp producers still remember the difficulties faced in the early years and, unfortunately, this still represents a very serious drawback towards the modernization of their pulp mills. By doing a brief survey of ozone application in pulp bleaching development over the past 20 years, the present article aims to draw specialists' and investors' attention to practices and technologies that helped ozone bleaching become the most advanced and promising technology.

Ozone generation

Ozone generation is a pure on-site technology requiring only energy and oxygen (usually also produced on site from a VPSA plant). Ozone (O₃) is produced from oxygen (O₂) in an electrical field at a concentration of 12% by weight, Fig. 1.

Figure 1 - Ozone formation in an electrical field

Modern ozone generators are 50% more efficient than the ones used in the first ozone pulp bleaching applications. Z-Compact-Systems, Fig. 2, were specially designed for the pulp and paper industry and they are able to produce up to 250kg O₃/h (6 tons/day) per unit.

Today, ozone production only requires 7 to 8 kWh/kg of ozone and as a result, 1 kg ozone is now cheaper than 1 kg of chlorine dioxide. Based on a "plug and play" principle, modern ozone generation units are easy to operate and can deliver the full ozone capacity in less than one minute with an availability higher than 99%.

Figure 2 - Z-compact system in Celtejo (Portugal)

High consistency (HC) ozone bleaching

The first commercial HC ozone bleaching started in 1992 at the Union Camp mill in Franklin, VA. According to the C-Free® process implemented there, the pulp was pH adjusted, pressed to high consistency (40%), fluffed and transferred to the ozone paddle reactor operating at atmospheric pressure14. The C-Free® was provided by Sunds Defibrator until the late 1990s in the US, Sweden, South Africa and Germany.

Modern HC ozone bleaching uses the ZeTracTM technology provided by Metso which is a much simplified version of the C-Free, Fig. 35. The experience gained from the first industrial installations showed that ozone requires very short (around 1 minute) contacting time with the pulp and that a 5-10 minute extraction stage after the Z-stage is in most cases sufficient. These observations allowed the size of reactors to be reduced, which lowered investment costs. Then the plug screw feeder, the refiner fluffer and the washing stage prior to the extraction stage could all be eliminated. These drastic simplifications led to significant reduction of the capital expenditure, energy requirements, maintenance costs as well as effluent volume5.

Figure 3 shows the principle of the modern ZeTrac system. The pulp is acidified and then pressed to high consistency (38-42%). Such a high consistency is a prerequisite to facilitate the rapid contact between ozone gas and well fluffed pulp and so preserves the reaction efficiency. Once dewatered, the pulp is fluffed in a shredder screw on the top of the press and fed by gravity into the reactor. Ozone is added to the reactor which is operated at a pressure slightly below atmospheric. After the reactor, the pulp is diluted with alkaline liquor.

Figure 3 - HC ozone bleaching in the 1990S and today

Medium consistency (MC) ozone bleaching

Improvements in MC ozone bleaching consist in fact of alterations to the ozone mixers. This is no wonder since the ozone mixer is the core of the MC Z-stage and the quality of the final pulp depends on its efficiency. It is worth remembering that the very few mills which faced quality issues are those where the first MC ozone bleaching technique was implemented: this was mainly due to a non homogenous mixing and a mixer which mechanically affected the fibers.

Andritz, GL&V and Lenzing Technik are the three suppliers of MC ozone mixers and all MC Z-stages are designed according to the same principles. Industrial practice has shown that Andritz technology requires two mixers in series for a 3-6 kg/tonne ozone dose to obtain the optimal bleaching efficiency while Lenzing Technik considers that one single mixer of its own is sufficient for a 4-5 kg/tonne ozone charge.

Because of the larger amount of filtrate around the fibers at 12% pulp consistency, the reaction must take place in a pressurized (7-8 bar) reactor and consequently the total gas flow (oxygen and ozone) must be compressed accordingly. A typical MC ozone stage from Andritz, Fig. 4, features a MC pump that feeds the pulp to the ozone stage, two ozone mixers in series, a pressurized reaction tube, a flow discharger at the reactor top and a blow tube11.

Figure 4 - Typical MC ozone stage and mixer

It was noticed that the whole reaction process takes place in the mixers while the "reactor" has no effect on the ozone reaction but only guarantees a stable flow to the blow tube11. It is now possible to decrease the gas pressure from 12 bar to 9 bar and thus reduce operating costs.

Andritz improved the original Ahlström mixer efficiency by reducing the gas bubble size without making any fundamental change in the design of the mixer but only increasing turbulence and mitigating the mechanical action12. It makes the mixing a lot more homogenous and maintains pulp strength all along ozonation.

Lenzing started a MC ozone mixing system from Kvaerner in 1992 and has since investigated all possible improvements in MC ozone bleaching. It has been a pioneer in ozone bleaching for almost 20 years. As a result, LenzingTechnik designed its own ozone MC mixers (called Eccentric Mixers) implementing the revolutionary idea of an asymmetric design to increase both fluidization of the pulp and retention time16.

And this proved to work: Lenzing upgraded its two bleaching lines in 2004 with modern Eccentric Mixers and the use of the new mixing technology resulted in a 2.5 ISO brightness points increase for the same ozone dose! Actually, Lenzing reduced its bleaching chemical costs by 50% on line 1 and 38% on line B16.

Pulp quality

Since the first commercial ozone bleaching installation was started, enhancements to the ozone bleaching process have been conducted jointly by laboratories on the chemical side and by both the industry and equipment suppliers on the operational and technical sides. Improvements in ozone bleaching efficiency were boosted by the development of automatic control systems, mainly as a result of achievements in the accuracy and reliability of electronics. It is now possible to adjust very precisely the ozone dosage, pH, retention time and temperature in the Z-stage. These improvements now fully guarantee the pulp quality after ozone bleaching.

When ozone bleaching started to develop in the early 1990s, the use of such a powerful oxidant in combination with yet non-optimized process parameters (the most important issue being the mixing homogeneity) resulted sometimes in uneven pulp delignification and affected the pulp mechanical properties. Practices improved step by step and opportunities soon replaced difficulties, yet many pulp producers still believe that the quality of ozone bleached pulp, especially softwood, is lower than traditional ECF bleached pulp.

Since most of the mills using ozone are producing hardwood pulps, some people even believe that this evidence sustains the alleged lower strength of softwood ozone bleached pulps. The true explanation instead is rather simple: ozone bleaching is mostly implemented in Europe, Brazil, South Africa, Australia, India and Japan, all countries deficient in softwood resources. Thus, such an argument does not stand considering that 25% of the mills using ozone are producing softwood pulps. This proportion is not lower than the ratio of softwood/hardwood bleached pulps produced in those countries.

Numerous studies carried out during the 1990s and in the more recent years have shown that the selectivity of ozone against lignin is very high. It is now established that the reaction kinetic of ozone with typical lignin structures is 10 to 1,000 times higher than with carbohydrates, Fig. 517.

Figure 5 - Main ozone reactions with lignin compounds and carbohydrates

This means that as long as there is some remaining lignin (even only 0.5-1.5%) a well-operated ozone bleaching process does not impact the cellulose more than any other bleaching stage (a chlorine dioxide one for example). In fact, according to the best wood chemistry specialists15,17, the reaction kinetic of ozone with lignin is 1,000 times higher than the kinetic of cellulose oxidation or depolymerisation by ozone. Those scientific results are confirmed by the following industrial experience of ozone bleaching.

To be continued ... Read Part II here.

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