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bleachingBy EMIL GERMER, ALEXIS MÉTAIS and JEAN-CHRISTOPHE HOSTACHYThe benefits of ozone are now well proven and can inclu...
bleachingsignificant modernisations to retain its competitive-      HIGH CONSISTENCY (HC) OZONEness and, in turn such a tr...
bleaching            Free® was provided by Sunds Defibrator until the late      MEDIUM CONSISTENCY (MC) OZONE            1...
bleachingozone MC mixers (called Eccentric Mixers) imple-           lower than the ratio of softwood/hardwood bleachedment...
bleaching            E-D bleaching to a ZEo-DnD one. It should first be            At this stage it is worth remembering t...
bleaching•	 the	bleaching	chemicals	cost	decreased	by	about	          65 kg/tonne of low-pressure steam only are necessary...
bleaching            have shown that the installation of those A-stages or            In the 1990s it was common to fully ...
bleachingmented, experienced and improved for 18 years. It          4. CAMPO, R. and MARQUES, P Results Pulp & Paper      ...
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Achievements In Industrial Ozone Bleaching Ppi


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Achievements In Industrial Ozone Bleaching Ppi

  1. 1. bleachingBy EMIL GERMER, ALEXIS MÉTAIS and JEAN-CHRISTOPHE HOSTACHYThe benefits of ozone are now well proven and can include costsavings and reduced effluent loadACHIEVEMENTS IN INDUSTRIAL OZONE BLEACHING T he first industrial pulp bleaching line including effluents to 4-6 m3/air dry tonne4. Of course investors an ozone stage started 19 years ago. Today there always focus on the return on investment (ROI) and it are 22 mills worldwide using what is commonly has now become clear to all ozone users that bleach- named light-ECF bleaching. Among those 22 mills, 16 ing costs are reduced by 20-32% when introducing an produce solely hardwood pulps, four produce both soft- ozone stage in an ECF bleaching line 3,5,6,7 and even wood and hardwood pulps while SCA in Östrand (Swe- more in the case of TCF bleaching 8. The high bleach- den) and Rosenthal in Blankenstein (Germany) produce ing efficiency of ozone allows a drastic reduction in exclusively softwood pulp1. Thirteen mills started ozone the consumption of expensive bleaching chemicals bleaching in the early years of the 21st century and three - chlorine dioxide in ECF bleaching and hydrogen mills equipped two of their bleaching lines with ozone: peroxide in TCF bleaching as well as sodium hydrox- Oji Paper in Tomioka (Japan), Fibria in Jacarei (Brazil) ide in both cases - and the implementation of ozone and ITC in Bhadrachalam (India). Fibria selected ozone bleaching also results in reduced of steam require- bleaching in 2002 for Jacarei’s new 2,500-tonne/day line ments during the bleaching process. The ROI in the C after having operated ozone bleaching as from 1995 replacement of a D0-stage by an ozone Z-stage lies on its 900-tonne/day line B. This decision clearly shows between two and four years 1,6. that ozone bleaching has met all requirements and Without affecting the pulp strength properties expectations on line B for seven years. in comparison with conventional ECF sequences, In April 2011, four new ozone generation sys- bleaching with an ozone stage additionally gives a tems have been contracted and will start operating wide range of opportunities by: in 2012 for: - making very high brightness levels possible (92- - a 700,000-tonne/yr capacity Chinese greenfield mill 93% ISO)2,9; owned by Oji Paper (the company already uses - decreasing brightness reversion2,8,10; ozone in its Japanese mills); - reducing drastically the extractive content10,11 by - a pulp production capacity increase in Austria at 50-75%8,12; Lenzing; - reducing energy requirements during refining by at - two pulp production capacity increases in Brazil least 10%3,13; (confidential). - precisely controlling the viscosity in viscose pulp production. Ozone bleaching is efficiently used on hardwood and softwood pulps, on kraft and sulphite pulps dedi- Nevertheless, arguing that ECF bleaching has cated to all kinds of final applications. Pulp produc- already been used in the industry for 30 years, a ers do not always evaluate the significant ecological number of conservative pulp producers still con- advantages of ozone-based bleaching sequences over sider the bleaching sequence D0-Eop-D1-D2 and the traditional ECF bleaching sequence D0-Eop-D1- its variants- as the best alternative today. One should D2 (or its variants): the quality of wastewater is drasti- however be aware that within the next 20 years, cally improved 2,3 as it is possible to lower bleaching the pulp and paper industry will have to carry out34 August 2011 Pulp & Paper International (PPI)
  2. 2. bleachingsignificant modernisations to retain its competitive- HIGH CONSISTENCY (HC) OZONEness and, in turn such a traditional ECF bleaching BLEACHINGwill then be over 50 years old. This position couldbe understood if no better technique was available. The first commercial HC ozone bleaching started inBut considering the very good results and proven 1992 at the Union Camp mill in Franklin, VA. Accord-stable operation achieved today by a large number ing to the C-Free® process implemented there, theof industrial ozone-based light-ECF and TCF bleach pulp was pH adjusted, pressed to high consistencyplants, such a conservative attitude can’t be justified (40%), fluffed and transferred to the ozone paddleany longer. reactor operating at atmospheric pressure14. The C- The industrial use of ozone has already under-gone a long string of improvements and develop-ments. As with other new technologies, ozone bleach-ing did not immediately reach its optimal technicalefficiency but faced several issues during its earlyyears. All the same, achievements of ozone bleachinghave improved year in year out and it is now a wellproven technology. Nevertheless, some pulp produc- Fig. 1 - OZONE FORMATION IN AN ELECTRICAL FIELDers still remember the difficulties faced in the earlyyears and, unfortunately, this still represents a veryserious drawback towards the modernization of theirpulp mills. By doing a brief survey of ozone applica-tion in pulp bleaching development over the past 20years, the present article aims to draw specialists’ andinvestors’ attention to practices and technologies thathelped ozone bleaching become the most advancedand promising technology.O Z O N E G E N E R AT I O NOzone generation is a pure on-site technology requir-ing only energy and oxygen (usually also produced onsite from a VPSA plant). Ozone (O3) is produced fromoxygen (O2) in an electrical field at a concentration of12% by weight, Fig. 1. Modern ozone generators are 50% more efficientthan the ones used in the first ozone pulp bleaching Fig. 2 - Z-COMPACT SYSTEM IN CELTEJO (PORTUGAL)applications. Z-Compact-Systems, Fig. 2, were spe-cially designed for the pulp and paper industry andthey are able to produce up to 250kg O3/h (6 tons/day) per unit. Today, ozone production only requires 7 to 8kWh/kg of ozone and as a result, 1 kg ozone is nowcheaper than 1 kg of chlorine dioxide. Based on a“plug and play” principle, modern ozone genera-tion units are easy to operate and can deliver the fullozone capacity in less than one minute with an avail- Fig. 3- HC OZONE BLEACHING IN THE 1990S AND TODAYability higher than 99%.Pulp & Paper International (PPI) A u g u s t 2 0 1 1 35
  3. 3. bleaching Free® was provided by Sunds Defibrator until the late MEDIUM CONSISTENCY (MC) OZONE 1990s in the US, Sweden, South Africa and Germany. BLEACHING Modern HC ozone bleaching uses the ZeTracTM technology provided by Metso which is a much Improvements in MC ozone bleaching consist in fact simplified version of the C-Free, Fig. 35. The experi- of alterations to the ozone mixers. This is no wonder ence gained from the first industrial installations since the ozone mixer is the core of the MC Z-stage showed that ozone requires very short (around 1 and the quality of the final pulp depends on its ef- minute) contacting time with the pulp and that a 5-10 ficiency. It is worth remembering that the very few minute extraction stage after the Z-stage is in most mills which faced quality issues are those where the cases sufficient. These observations allowed the size first MC ozone bleaching technique was implement- of reactors to be reduced, which lowered investment ed: this was mainly due to a non homogenous mixing costs. Then the plug screw feeder, the refiner fluffer and a mixer which mechanically affected the fibers. and the washing stage prior to the extraction stage Andritz, GL&V and Lenzing Technik are the three could all be eliminated. These drastic simplifications suppliers of MC ozone mixers and all MC Z-stages are led to significant reduction of the capital expenditure, designed according to the same principles. Industrial energy requirements, maintenance costs as well as practice has shown that Andritz technology requires effluent volume5. two mixers in series for a 3-6 kg/tonne ozone dose to Figure 3 shows the principle of the modern obtain the optimal bleaching efficiency while Lenzing ZeTrac system. The pulp is acidified and then pressed Technik considers that one single mixer of its own is to high consistency (38-42%). Such a high consis- sufficient for a 4-5 kg/tonne ozone charge. tency is a prerequisite to facilitate the rapid contact Because of the larger amount of filtrate around between ozone gas and well fluffed pulp and so the fibers at 12% pulp consistency, the reaction must preserves the reaction efficiency. Once dewatered, take place in a pressurized (7-8 bar) reactor and con- the pulp is fluffed in a shredder screw on the top of sequently the total gas flow (oxygen and ozone) must the press and fed by gravity into the reactor. Ozone is be compressed accordingly. A typical MC ozone stage added to the reactor which is operated at a pressure from Andritz, Fig. 4, features a MC pump that feeds slightly below atmospheric. After the reactor, the pulp the pulp to the ozone stage, two ozone mixers in is diluted with alkaline liquor. series, a pressurized reaction tube, a flow discharger at the reactor top and a blow tube11. 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 Fig. 4 - TYPICAL MC OZONE STAGE AND MIXER years. As a result, LenzingTechnik designed its own36 August 2011 Pulp & Paper International (PPI)
  4. 4. bleachingozone MC mixers (called Eccentric Mixers) imple- lower than the ratio of softwood/hardwood bleachedmenting the revolutionary idea of an asymmetric pulps produced in those to increase both fluidization of the pulp and Numerous studies carried out during the 1990sretention time16. and in the more recent years have shown that the se- And this proved to work: Lenzing upgraded its lectivity of ozone against lignin is very high. It is nowtwo bleaching lines in 2004 with modern Eccentric established that the reaction kinetic of ozone withMixers and the use of the new mixing technology typical lignin structures is 10 to 1,000 times higherresulted in a 2.5 ISO brightness points increase for than with carbohydrates, Fig. 517.the same ozone dose! Actually, Lenzing reduced its This means that as long as there is some remain-bleaching chemical costs by 50% on line 1 and 38% ing lignin (even only 0.5-1.5%) a well-operated ozoneon line B16. bleaching process does not impact the cellulose more than any other bleaching stage (a chlorine dioxidePULP QUALITY one for example). In fact, according to the best wood chemistry specialists15,17, the reaction kinetic of ozoneSince the first commercial ozone bleaching instal- with lignin is 1,000 times higher than the kinetic oflation was started, enhancements to the ozone cellulose oxidation or depolymerisation by ozone.bleaching process have been conducted jointly by Those scientific results are confirmed by the follow-laboratories on the chemical side and by both the ing industrial experience of ozone bleaching.industry and equipment suppliers on the operational In 1998, the Domtar mill in Espanola, ON,and technical sides. Improvements in ozone bleach- switched from the ECF O-A-D-E-DnD sequence toing efficiency were boosted by the development the light-ECF O-A-ZD-E-DnD sequence for bleach-of automatic control systems, mainly as a result of ing pulp up to 93% ISO brightness. Comparing theachievements in the accuracy and reliability of elec- two sequences Domtar noticed “ozone has no impacttronics. It is now possible to adjust very precisely the on viscosity or PFI strength properties and in paperozone dosage, pH, retention time and temperature in machine performance”9.the Z-stage. These improvements now fully guarantee The Mondi mill in Ruzomberok (Slovakia)the pulp quality after ozone bleaching. produces hardwood and softwood pulps at 89% ISO When ozone bleaching started to develop in the brightness and switched in 2004 from a D-Eop-D-early 1990s, the use of such a powerful oxidant in com-bination with yet non-optimized process parameters(the most important issue being the mixing homogene-ity) resulted sometimes in uneven pulp delignificationand affected the pulp mechanical properties. Practicesimproved step by step and opportunities soon replaceddifficulties, yet many pulp producers still believe thatthe quality of ozone bleached pulp, especially soft-wood, is lower than traditional ECF bleached pulp. Since most of the mills using ozone are produc-ing hardwood pulps, some people even believe thatthis evidence sustains the alleged lower strength ofsoftwood ozone bleached pulps. The true explana-tion instead is rather simple: ozone bleaching ismostly implemented in Europe, Brazil, South Africa,Australia, India and Japan, all countries deficient insoftwood resources. Thus, such an argument does not Fig. 5 - MAIN OZONE REACTIONS WITH LIGNIN COMPOUNDSstand considering that 25% of the mills using ozone AND CARBOHYDRATESare producing softwood pulps. This proportion is notPulp & Paper International (PPI) A u g u s t 2 0 1 1 37
  5. 5. bleaching E-D bleaching to a ZEo-DnD one. It should first be At this stage it is worth remembering that gener- noticed that there was only one washing stage (after ally, the tensile index is increased by refining while ZEo) in the new process. Mondi compared hardwood the tear index follows the opposite trend and it is and softwood pulp strength parameters of the new impossible to increase both of them at the same time. pulps with the ones of the former pulps set as the Nevertheless the mill reported that the paper machine reference (100%); only minor differences could be runnability was not affected and the final paper prop- seen in standard pulp properties, Table 118. erties were even better than expected (stiffness)13. Hardwood pulp properties were particularly excellent and the hardwood proportion in the paper Table 1 - RELATIVE PULP QUALITY FOR ECF-Z COMPARED WITH ECF AT 27°SR FOR BOTH HW AND SW machine furnish was increased over 90%13. In 2005 Mondi Ruzomberok set up a new world record for Hardwood Softwood copy paper production at 1,500 m/min13,18. Of course, PFI rev 89 90 these two achievements would not have been pos- Burst Index 100 97 sible with a lower pulp quality than before. Tear Index 92 90 Moreover, the Ruzomberok mill has kept improv- Breaking Length 104 102 ing the Z-stage operation and its bleaching process Tensile Index 102 102 by installing a pressurized (PO) stage in place of the Stiffness 93 94 last D stage in August 2008. Current pulp strength properties are excellent and would make most pulp mills envious, Fig. 6. Concerning hardwood pulp, energy requirements Ozone bleaching process improvements never for refining dropped by 11%, burst was maintained, stopped. The ITC mill in Bhadrachalam (India) breaking length and tensile index increased while started ozone bleaching in 2007 after five years of ECF tear and stiffness decreased. Meanwhile, for softwood bleaching operation. Its results are clear19: pulp, refining energy was again reduced significantly, • the breaking length increased by 5.5%; tensile properties increased marginally while burst, • the burst factor increased by 10%; tear and stiffness showed some reduction. • the tear factor increased by 6%; Fig. 6 - BLEACHING LINE AT MONDI RUZOMBEROK AND PULP STRENGTH PROPERTIES38 August 2011 Pulp & Paper International (PPI)
  6. 6. bleaching• the bleaching chemicals cost decreased by about 65 kg/tonne of low-pressure steam only are necessary 450 Rs/tonne ($10/tonne). for the Ze-DD bleaching sequence. As an example, implementation of the ozoneO P E R AT I N G PA R A M E T E R S stage at Bhadrachalam led to a 50% drop in steam consumption in comparison to the former D-Eop-DPulp temperature is one important operating param- bleaching sequence21.eter that has been tuned. Ozonation was initially car- Steam savings are the highest in the case of hard-ried out at 40°C in the 1990s and such a low tempera- wood pulp bleaching where usually a first A-stageture was not very convenient as the Z-stage is located (acidic) or DHT-stage (D-stage at hot temperature) isright after the 85-95°C oxygen delignification and implemented. Such A or DHT stages have 2 targets:before an alkaline extraction or a chlorine dioxide • removing hexenuronic acids which are responsiblestage (after a few MC ozone stages) generally carried for parasite chlorine dioxide consumption andout at around 60-80°C. Therefore, it is necessary to brightness reversion;cool the pulp before reheating it. • cleaning the pulp from transition metals which It was thought that a higher temperature would are known to decompose H2O2 and responsiblelower the Z-stage efficiency, speed up the ozone for weakening of the cellulose during peroxidedecomposition, increase the negative effect of transi- bleaching.tion metals and ultimately decrease the pulp quality.Slowly, throughout industrial trials, it appeared that By removing transition metals the A-stage ef-such fears were totally unfounded. Several results fectively replaces the use of a chelating agent. Acidichave demonstrated that for hardwood pulp the Z- stages were built prior to the ozone stage in thestage temperature could be increased up to 60°C and 1990s at the UPM mill in Pietarsaari (Finland), at thesometimes even higher without any negative effect Sateri mill in Bahia (Brazil), at Fibria Jacarei and aton pulp strength and brightness9,13,18. Espanola. But today there is no doubt that the imple- For example, data from Ruzomberok show that mentation of an A-stage reduces both pulp strengthan increase of the Z-stage temperature from 43°C to properties and pulp yield22.61°C gives a higher brightness (+1.8% ISO), a lower Of course, there is no such problem of hex-brightness reversion, a higher stiffness and a better enuronic acids in the case of softwood pulp bleach-ozone delignification efficiency, with all other proper- ing. But the issue of transition metals remains andties (PFI rev, Burst Index, Tear Index, Tensile Index in the 1990s Swedish mills SCA Östrand and Nordicand Bulk) unaltered, Table 213,20. Paper Säffle as well as the Rosenthal mill in Blan- Domtar Espanola first operated the Z-stage at kenstein (Germany) chose to start their bleaching55°C. When it decided to increase the ozone stage sequence with a Q-stage (chelation).temperature to 60°C, it increased delignification by Such A-stages or Q-stages are costly because they5-10% and gained 1 point ISO brightness. Today, it is require a tower (with 2-3 hours retention time foroperating the ozone stage at 70°C9. the later) and the use of expensive chemicals. Yet in- All HW pulp mills which set up an ozone stage in dustrial light-ECF and ozone based TCF experiencesthe past 5-6 years are running the Z stage at a 55°C mini-mum. This also resulted in significant steam savings. For Table 2 - IMPACT OF TEMPERATURE ON THE KAPPA INDEX, ISO BRIGHTNESSexample, Ruzomberok reduced its steam requirements AND BRIGHTNESS REVERSION AT MONDI RUZOMBEROKby 75% by running the Z-stage at 58-61°C 13. Metso has shown industrially that steam require- Temp in Z 43°C 43°C 58°C 61°Cments for light-ECF bleaching are only 25% of steam Kappa number after Z(EO) 4.1 4.0 4.0 3.8requirements for conventional ECF bleaching in ClO2 charge, kg/adt (DCS data) 24.5 23.5 24.7 24.4the case of HW pulp8. While the bleaching sequence Brightness, % ISO 85.9 85.2 86.6 87.7DHT-Eop-D requires 249 kg/tonne of low-pressure Reverted brightness, % ISO 81.8 82.2 83.9 85.9steam and 40 kg/tonne of medium-pressure steam,Pulp & Paper International (PPI) A u g u s t 2 0 1 1 39
  7. 7. bleaching have shown that the installation of those A-stages or In the 1990s it was common to fully bleach the Q-stages should be avoided. pulp with six or even seven stages as in the following The hexenuronic acids content is reduced by at sequences: least 60% in the ozone stage23. This makes the ozone - A-ZD-Eop-ZD-Ep in Pietarsaari (Finland); stage even more efficient than any A-stage and it - Q-OP-D-Z-PO-P in Blankenstein; partly explains why ozone bleached pulps have a - A-ZD-Eo-DnD in Espanola. lower brightness reversion than other pulps. Actually, removal of transition metals is efficiently Bleaching sequences built in the last decade for carried out during the ozone stage where the pH both hardwood and softwood pulps usually have only is acidic (usually 2.5-3). Awareness of this avoids three or four stages and vary little from one another: investing in any acid or chelating stage when ozone - Z-D-Ep-D at Nippon PaperYufutsu (Japan); is applied, and results in operating cost savings. As a - ZEop-D-P at Mondi Ruzomberok; result, only two of the 16 pulp bleaching lines having - ZD-E-D at Nippon Paper Maryvale (Australia); started ozone since 2000 use an A-stage and none of - Ze-DP at ITC Bhadrachalam, Fig. 7; them use chelating agents. - Z(EOP)-(PO) at Sniace (Spain, sulphite pulp for dissolving grades); BLEACHING SEQUENCES - Ze-D-P at Celtejo (Portugal). The proven economical, technical and environmen- These bleaching sequences reflect the continuous tal advantages, the development of oxygen deligni- and impressive developments of ozone application fication as well as improvements in ozone bleaching in light-ECF bleaching process and clearly show the and in the whole pulping process over the last 20 worldwide interest of pulp makers in ozone bleaching. years, have allowed the bleaching sequence to be shortened by one, two or even three stages, while CONCLUSIONS simultaneously increasing the brightness ceiling with 2-3 points compared with what was possible. Ozone bleaching has already been industrially imple- Fig. 7 - LINE 2 AT ITC BHADRACHALAM40 August 2011 Pulp & Paper International (PPI)
  8. 8. bleachingmented, experienced and improved for 18 years. It 4. CAMPO, R. and MARQUES, P Results Pulp & Paper .,is a well proven and safe process, currently used by 3, 32 (2009).some reference pulp mills among the most modern 5. BOKSTRÖM, M., African Pulp and Paper Weekin the world. Its advantages in terms of bleaching cost 2002, Durban, South Africa.savings, effluent load reduction and ease of use can 6. HOSTACHY, J.-C., Tappi J., 9(8): 16(2010).no longer be questioned. It is applicable to all kind of 7. CAMPO, R. and SUNDIN, M., Proceedings of thepulps and has no negative effect on their mechanical EFPG Days 2004, Grenoble, 8. WENNERSTRÖM, M. et al, International Colloqui- The Z-stage takes place in just one minute and um on Eucalyptus Pulp 2007, Belo Horizonte, Brazil.both HC and MC technologies are recognized as fully 9. GARTLEY, B., Ozone Delignification at Domtarreliable, safe and stable processes. Espanola, paper presented at the PAPTAC Bleaching High-consistency ozone bleaching is performed Committee, Oct. 21-23 atmospheric pressure. Behind the ozone reactor, 10. CHIRAT, C., et al., Proceedings of the ABTCP 2008,it requires only a 5-10 minutes long extraction stage Sao Paulo, Brazil.and no intermediate washing is necessary. This solu- 11. VEHMAA, J. and PIKKA, O., Proceedings of Paperextion allows the E filtrate to be recycled back to the 2007, New Delhi, India.brown stock or post-oxygen washing and further to 12. PIKKA, O., Proceedings of the EFPG Days 2004,the recovery boiler, and so the total bleaching effluent Grenoble, France.can drop to 4-6 m3/tonne. 13. TOMIS, B. and TUOMI, A., Proceedings of As a result of continuous improvements of the Zellcheming 2006, Wiesbaden, and process automation as well as tuning 14. FERGUSON, K., Pulp & Paper, 66(11): 42 (1992).of the operating conditions over a period of almost 20 15. CHIRAT, C., Proceedings of the EFPG Days 2007,years, modern ozone bleaching is now recognised as Grenoble, France.a state of the art technology for both hardwood and 16. LenzingTechnik, Eccentric Mixer - an innovationsoftwood pulps. from LenzingTechnik, commercial presentation. There are neither technical nor economical 17. LACHENAL, D., Proceedings of the EFPG Daysreasons to stick to a 30-year-old conventional ECF 2005, Grenoble, France.process. The use of a light-ECF bleaching sequence 18. NORSTEDT, Å. and LINDSTRÖM, L.-Å., ATCP Con-can by itself decrease bleaching costs by more than ference 2006.20% and lead to the same pulp strengths with better 19. KUMAR, K.R., Proceedings of Paperex 2009, Newoptical properties and refining ability. Ozone bleach- Delhi, is by far the most valuable solution not only for 20. BALAZ, P Proceedings of Zellcheming 2006, Wies- .,forerunners but for all pulp mills. PPI baden, Germany. 21. PADMANABHAN, A. et al, paper presented at theEmil Germer, Saint-Petersburg State Forest Technical National Award for Excellence in Energy ManagementAcademy; Alexis Métais, Jean-Christophe Hostachy, 2007, 22-23 August 2008, Pune, India.ITT Wedeco 22. KOBAYASHI, T. et al, Japan Tappi, 59 (2005). 23. NORDÉN, S., ZeTrac Forum 2004, Rochehaut,LITERATURE Belgium.1. MÉTAIS, A., Proceedings of the TAPPSA National 24. ENGELFELDT A., internal training for ITC.Conference 2010, Durban, South Africa.2. CARRÉ, G. and WENNERSTRÖM, M., Proceedingsof the International Pulp Bleaching Conference 2005,Stockholm, Sweden.3. NORDÉN, S. et al., Eco-Friendly Bleaching with To read more articles on Chemicals, visit our ChemicalsZeTracTM, paper presented at the IPPTA Annual Gen- Technology Channel at technologychannels/chemicalseral Meeting & Seminar 2006, Chennai, India.Pulp & Paper International (PPI) A u g u s t 2 0 1 1 41