Bleachability of Pulps Produced From Different Kraft Pulping Methods: A Laboratory Study

Transcription

1 comparison T149 Bleachability of Pulps Produced From Different Kraft Pulping Methods: A Laboratory Study By P. Fatehi, R.O. Malinen, and Y. Ni Abstract: In this work, the difference in the bleachability of Eucalyptus pulps produced in a mill from the SuperBatch (SB) and isothermal cooking (ITC) techniques and in a laboratory from a conventional batch process was evaluated. Two sequences were studied: and Z. The results showed that, under similar conditions, the ITC pulp had the highest bleachability in both sequences. However, the SB pulp had the lowest bleachability in the sequence, and higher bleachability than the lab-made pulps in the Z N owadays, the kraft process is the dominant chemical pulping method in the world. It is generally performed in either batch or continuous systems. These pulping methods have been continuously modified to enhance the pulping efficiency and to reduce operational cost. Two of the modified pulping technologies are SuperBatch (SB) and isothermal cooking (ITC). Each has its own advantages, e.g., the SB technology is flexible for different types of wood species, whereas the ITC process has a lower operational cost [1]. The cooking methodologies of ITC, SB, and laboratory systems are different [1-3]. The impregnation time is longer in the SB digester than in the ITC digester. All white liquor is added prior to the heating-up stage of cooking in the SB digester, while the white liquor is injected at different spots in the ITC digester. The cooking chemicals flow through the fixed bed of chips in the SB digester. On the contrary, the chips and cooking chemicals flow co-currently in the ITC digester until the outlet of black liquor. Then, the chips and washing liquor flow counter-currently to the bottom of the ITC digester. Furthermore, the cooking temperature in the SB digester is normally higher than in the ITC digester [1-3]. Different from the mill practice, in a laboratory set-up, the chips are preheated with black liquor or steam, and all of the cooking chemicals are usually added at the start of a cooking experiment. The pulp reactivity towards chemicals in a sequence, i.e., the bleachability, is defined as the consumption of chemicals to obtain a target brightness [4,5]. It is mainly affected by the residual lignin structures, hexenuronic acid (HexA) content, and lignin-carbohydrate complex (LCC) of pulp [6-8]. Pulping technologies and process conditions are different for the ITC, SB and laboratory systems, which can cause changes in delignification kinetics and carbohydrate degradations, yielding pulps with different compositions and characteristics. They will, in turn, affect the subsequent performance. There are many studies in the literature regarding the bleachability of lab-made pulps treated under different conditions [4,5,9], and commercial pulps produced from different modified technologies [10,11]. To compare the bleachability of lab- and mill-made pulps, the wood chips from the same source have to be cooked by mill and laboratory digesters to the same kappa number. Then, a similar sequence should be applied to obtain a target brightness. Also, it is interesting to investigate the influences of cooking time and temperature on the bleachability of pulps at a constant H-factor [5,9,12]. In this work, two commercial pulp samples produced from the ITC and SB technologies and two lab-made pulp samples from different temperatures, but at the same H-factor, were studied. All of the four samples had similar kappa numbers. They were oxygen-delignified under similar conditions and bleached in the and Z sequences. Finally, the bleachability of the pulps was evaluated based on the formulas available in the literature. EXPERIMENTAL Materials Eucalyptus Camaldulensis chips were received from Advance Agro Company, Thailand. Also, unbleached pulps were collected from the brownstock washers of the ITC and SB digesters of the same company, and washed thoroughly. Pulping The equivalent of 400g o.d. chips was cooked P. FATEHI Dept. of Chemical Engineering & Limerick Pulp and Paper Centre, University of New Brunswick, Fredericton, N.B. R.O. MALINEN Department of Pulp and Paper Technology, Asian Institute of Technology, Pathum thani, Thailand Y. NI, Dept. of Chemical Engineering & Limerick Pulp and Paper Centre, University of New Brunswick, Fredericton, N.B. pulpandpapercanada.com October/November 2009 Pulp & Paper Canada 33

2 T150 comparison in a laboratory set-up consisting of 2.5 litre 6-autoclave air-heated digesters to the target kappa numbers of The sulfidity and effective alkali were 30% and 22%, respectively. The liquid-to-wood ratio was 4:1. The autoclaves were heated from an ambient temperature to 80 C in 12 minutes, and then to 165 C or 155 C in 70 minutes. The cooking step was conducted at 165 C or 155 C for 90 minutes or 225 minutes (H-factor 1018), respectively. The cooked pulps were thoroughly washed with tap water. Pulps were then disintegrated and screened. The screened pulps were centrifuged for 15 minutes and homogenized for 20 minutes. The labmade pulps at 165 C or 155 C were denoted as Lab1 or Lab2 samples, respectively. Oxygen Delignification Oxygen delignification was carried out for all lab- and mill-made pulps in an electrically-heated CSR reactor, where oxygen was pressurized at 8 bars and mixed continuously in the presence of 2% NaOH and 0.4% MgSO 4. The pulp consistency was 10%. Pulp washing, centrifuging, and homogenizing processes followed afterwards. Bleaching Oxygen-delignified pulps were then subjected to the and Z sequences. Chlorine dioxide ( and D 1 ), and extraction (Ep) treatments were all performed in sealed plastic bags in a thermostatically controlled water bath. Ozone gas (Z) was applied at 0.5% on o.d. pulps from a Trailigaz OZC 1001 ozone generator. Ozone was introduced into the pressurized reactor by employing the solid piston compression method [13]. The ozone concentration in the feed and residual ozone was measured based on the iodometric method [13]. The charge of chlorine dioxide in the stage was a kappa factor of 0.2, whereas it was 1.25% based on o.d. pulp in the D 1 stage. Also, the alkali and H 2 O 2 charges were 1.2% and 0.8% on o.d. pulps in the Ep stage. The ph of each stage was adjusted by introducing an appropriate amount of NaOH or H 2 SO 4, and was obtained by conducting the trial and error of initial ph. The consistency for the, D 1, and Ep stages was 10%, whereas it was 8% in the ozone stage. Other conditions are listed in Table I. HexA Analysis The hexenuronic (HexA) content of pulps was measured based on the HUT method [13]. HexA kappa was calculated (11.6 meq/kg HexA responded to one unit of kappa number) in accordance with the method proposed by Li and Gellestedt [14]. Based on the assumption that the total kappa number of unbleached Eucalyptus pulps is from HexA and residual lignin content [15], we calculated the lignin kappa by subtracting the HexA kappa from the total kappa number of pulps. Other required parameters were determined by following the relevant ISO and SCAN standard methods. Bleachability Analysis One method of investigating the bleachability of pulps is to evaluate the brightness development of pulps versus the consumption of chemicals as the sum of oxidation equivalents (OXE) divided by the incoming lignin kappa for each stage in the sequences [5,16]. sequence: OXE/lignin kappa= (1) oxe(d 0 )+oxe(p) + OXE(D 1 ) Lignin kapa(o 2 ) Lignin kappa (Ep) Z sequence: OXE/lignin kappa= (2) oxe(z) + oxe(p) + OXE(D 1 ) Lignin kapa(o 2 ) Lignin kappa ( ) Lignin kappa (Ep) Another method is to investigate the brightness development of pulps against the chlorine dioxide consumption per total kappa number of oxygen delignified pulps. This method is suitable for a sequence having chlorine dioxide stages. Chlorine dioxide charge/total kappa= (3) consumption( ) + consumption (D 1 ) Total Kappa (O 2 ) RESULTS AND DISCUSSION Characteristics of Brownstock Pulp The characteristics of brownstock pulps are listed in Table II. The total kappa numbers of all pulps were fairly close. Since the lignin content of pulp is roughly 0.15 of its kappa number [17], these results imply similar lignin content of pulps. This similarity permitted us to compare their bleachability. It is evident in Table II that the HexA contents differed remarkably. To illustrate the reason for such differences, the preceding cooking procedure of each pulp should be considered. It was reported that the dosage of cooking chemicals and the cooking time affect the HexA formation and its removal during the cooking process [5,18]. In fact, the higher the initial EA concentration, the faster the HexA formation in the initial phase, but the greater the HexA loss happens in the later stage of the cooking process [18]. Since all the fresh cooking chemicals are added at once to the digester in the SB technology, such a process leads to the formation of HexA at the early stage of the cooking, and the degradation of HexA at the later stages, resulting in a low HexA content in the SB pulp (Table II) [19-21]. Buchert et al. also reported a low HexA content during SB pulping [21,22]. For the ITC technology, the EA is added to the digester at different spots in the way that the minimum requirement of fresh EA is always met for pulping [2,3]. As such, the limited presence of the EA during pulping leads to the gradual formation of HexA, causing the ITC pulp with a high HexA content (Table II). The lower HexA content of Lab1 vs. Lab2 pulps is ascribed to their higher temperature during cooking. As reported in the literature, the higher the cooking temperature, the faster the HexA formation at the early stage of cooking, hence the faster degradation of HexA at the later stage of cooking, resulting in a lower HexA content [22]. The influence of cooking chemicals and pulping conditions on the pulp characteristics was reported in the literature [16,23]. For the SB technology, the addition of all chemicals at the beginning of cooking caused perhaps severe carbohydrate degradation, particularly during the initial phase, leading to a very low viscosity in the SB pulp (Table II). The ITC pulp, however, showed the highest pulp viscosity. The higher viscosity of ITC compared to conventional-made Eucalyptus pulp was reported in the literature [1]. Also, the viscosity and HexA content of Lab1 pulp were lower than those of the Lab2 pulp. The lower viscosity of Lab1 pulp is most likely attributed to more serious carbohydrate degradation at a higher cooking temperature (165 C) in comparison with a cooking temperature of 155 C for Lab2 pulp. The influence of cooking temperature on carbohydrate degradation 34 Pulp & Paper Canada October/November 2009 pulpandpapercanada.com

3 peer reviewed T151 Fig. 1: Brightness development in the D 1 stage of the has been reported in the literature [24]. Characteristics of Oxygen Delignified Pulps The characteristics of oxygen delignified pulps are listed in Table III. The kappa number of the ITC pulp was the highest among all, which is due to its highest HexA content. Oxygen delignification is not favorable for HexA removal, as shown in Table III. This is in agreement with the literature results [8,25]. Additionally, the lab-made pulps are much easier to delignify, probably due to less lignin condensation in the pulping process, which results in a higher lignin reactivity during oxygen delignification. Furthermore, less than 10% HexA and viscosity reductions occurred during oxygen delignification (see Table III). These results are consistent with earlier findings about the influencing factors on the bleachability of Eucalyptus pulp, and the HexA impact on the optical properties of pulp under different laboratory cooking and oxygen delignifying conditions [26,27]. Bleachability in Sequence Figure 1 shows the brightness increase of pulps in the D 1 stage of the sequence, calculated in accordance with equations 1 and 3. It is inferred that the ITC pulp had the least chemical requirement (as OXE/lignin kappa and chlorine dioxide consumption/total kappa) to obtain 89% ISO brightness, whereas the SB pulp had the highest consumption. Therefore, the bleachability of the ITC pulp was much higher than that of laband SB-made pulps. Since oxygen delignification was conducted under similar conditions for all pulps, the difference in the bleachability of pulps would be attributed to the difference in their pulping processes. The highest bleachability of the ITC pulp based on both formulas (Fig. 1) implies that its residual chromopheric structures were relatively more reactive than those of other pulps. The higher bleachability of Eucalyptus pulp having higher HexA content was reported in the literature for ECF systems [6,13]. In these systems, the HexA content was claimed to be an influencing factor for the bleachability. The lowest reactivity of the residual lignin structures of the SB pulp can be attributed to the presence of lignin-carbohydrate compounds (LCC) and condensed Fig. 2: HexA and viscosity reductions of pulps versus brightness development in the structures. In the literature, SB pulp was known to contain more condensed lignin structures, resulting in the lower reactivity of residual lignin structure towards alkaline hydrogen peroxide [19,20]. One can also find from Fig. 1 that the two lab-made pulp samples had similar bleachability in the D 1 stage of the sequence, indicating that increasing the maximum cooking temperature from 155 C to 165 C, while maintaining the same H-factor, had a negligible effect on the subsequent processes. Similar results were obtained in the literature that the cooking temperature had a limited influence on the bleachability of Eucalyptus pulp at different H-factors in the E 1 D 1 E 2 D 2 sequence [4]. Figure 2 shows the viscosity and HexA reductions in the One can find that the viscosity and HexA content of pulps were reduced as the brightness increased. The highest viscosity of the ITC pulp was maintained in the subsequent sequence (Fig. 2). In contrast, the SB pulp had the lowest viscosity of the fully bleached pulp after the sequence (Fig. 2). pulpandpapercanada.com October/November 2009 Pulp & Paper Canada 35

4 T152 comparison Table I: Conditions of Oxygen Delignification and Subsequent Bleaching Stages Parameters/stages O D0 Z Ep D1 Temperature, C Time, min End ph Table II: Characteristics of Brownstock Pulps Parameter SB ITC Lab1 Lab2 Total kappa Lignin kappa HexA kappa HexA content, meq/kg Viscosity ml/g Yield N/A* N/A* Fig. 3: Brightness development of pulps versus their chemical consumption in the D 1 stage of the Z *Not Available Table III: Characteristics of Oxygen-Delignified Pulps Parameter SB ITC Lab1 Lab2 Kappa number Lignin kappa HexA kappa HexA content, meq/kg Viscosity, ml/g Yield, % lignin kappa reduction,% HexA kappa reduction,% Viscosity reduction,% Table IV Brightness (%ISO) of Brownstock, Oxygen-Delignified and Fully Bleached Pulps in the and Z Sequences SB ITC Lab1 Lab2 Brownstock Oxygen delignified Z Fig. 4: HexA and viscosity reductions of pulps versus brightness development in the Z sequence Based on the results in Figs. 1 and 2, we concluded that the ITC pulp had the highest reactivity, and also the most carbohydrate degradation and HexA removal in the In contrast, the brightness development, viscosity, and HexA drops were the least for the SB pulp, compared to those of other pulps in this The lower reactivity of the SB pulp was ascribed to the presence of condensed structures [19,20]. Also, the formation of LCC was reported to affect the bleachability of pulp in an ECF sequence [8]. Bleachability in Z Sequence Figure 3 shows the brightness increase in the D 1 stage of the Z sequence versus the chemical consumptions, calculated in accordance with equation 2. It is inferred from the figure that the ITC pulp had the highest bleachability among all pulps to obtain the target brightness (89% ISO). Also, the SB pulp had higher bleachability than the lab-made pulps. Two factors seem to influence the bleachability of pulps in the Z sequence: i) the reactivity of residual lignin structure and ii) the ratio of HexA to lignin. For the ITC pulp, one might expect that it had the most reactive residual lignin structures towards this sequence so that the highest bleachability was obtained. It was reported that ozone reacts preferentially with HexA [28]. As shown in Table III, the SB pulp had the lowest HexA content after oxygen delignification. As a result, more ozone would 36 Pulp & Paper Canada October/November 2009 pulpandpapercanada.com

5 peer reviewed T153 be available for the residual lignin of the SB pulp, increasing its bleachability. Such a phenomenon might be responsible for the higher bleachability of the SB pulp versus the lab-made pulps. Figure 4 shows the HexA drop and viscosity reductions of pulps against the brightness developments in the Z Similar to the sequence, the ITC pulp had the highest HexA and viscosity drop in this sequence, in contrast to those of the SB pulps. The results presented above indicated that the pulp bleachability is different in the two sequences studied, and Z. This is partially due to the fact that chlorine dioxide preferentially reacts with lignin, while ozone is more reactive with HexA [13,29]. Also, it is known that ozone is capable of degrading both phenolic and non-phenolic lignin structures [30,31], whereas chlorine dioxide mainly degrades phenolic lignin structures [17,32]. The viscosity drops for pulps bleached in the sequence were less pronounced than those of pulps bleached in the Z This is because chlorine dioxide is more selective than ozone, and the carbohydrate degradation is more serious in ozone than in chlorine dioxide [11,17,28]. Relation between Brightness of Unbleached and Fully Bleached Pulps The brightness of brownstock, oxygendelignified, and fully-bleached pulps in both sequences is listed in Table IV. The brightness of mill-made brownstock pulps (ITC and SB) was somewhat higher than those of the lab-made brownstock pulps. However, all pulps had approximately similar brightness after the oxygen delignification process. By consuming similar chemicals (700 OXE/lignin kappa) in both sequences, pulps tended to have different final brightness. The final brightness of pulps bleached in the Z sequences correlated well to the unbleached pulp brightness despite their similar brightness after the oxygen delignification stage. On the other hand, there was no correlation between the brightness of brownstock and fully bleached pulps in the In the literature, no correlation was found between the brightness of brownstock and fully bleached pulps from the ECF and TCF sequences [4,9]. CONCLUSIONS The different pulping methods affected the pulp composition and characteristics significantly. ITC-, SB-, and lab-made pulps with similar kappa numbers possessed remarkably different viscosities and HexA contents. The ITC pulp responded to the oxygen delignification the least among all pulps studied. However, it had the highest bleachability in the and Z sequences. On the contrary, the SB pulp had the lowest bleachability in the sequence, while possessing a higher bleachability than the lab-made pulps in the Z The ITC pulp had the highest decreases in viscosity and HexA content in both sequences, while the SB pulp had the least. The viscosity drops in the sequence were less than those in the Z Additionally, the results of lab-made pulps revealed that the temperature of cooking process at a constant H-factor affected the HexA content and viscosity of brownstock pulps. However, it had a negligible effect on bleachability during the subsequent and Z sequences. The final brightness of pulps bleached in the Z sequence was closely correlated to the brightness of unbleached pulps. ACKNOWLEDGEMENTS The authors would like to acknowledge the Finnish government for partially financing this research. The other portion of this work was gratefully supported by the AIT fellowship. Also, Advance Agro Co. is thanked for providing the pulps, chips, and chemicals for this project. LITERATURE 1. GULLICHSEN, J., PAULAPURO, H. Chemical Pulping, Papermaking Science and Technology, B6A, 1st Edition, Finland, Fapet Oy, A493-A570 (1999). 2. NATERCIA, C.P., FERNANDES, J., CASTRO, A.A.M. Steady-State Simulation of a Continuous Moving Bed Reactor in the Pulp and Paper Industry, Chem. Eng. Sci., 55 (18): (1999). 3. TRAN, A.V. Characterization of a Conventional Kamyr Continuous Digester Producing Hardwood Kraft Pulp, Appita J., 58 (1): (2005). 4. NETO P.C., EVTUGUIN, D.V., FUTADO, F.P., SOUSA, A.P.M. Effect of Pulping Conditions on the ECF Bleachability of Eucalyptus Globulus Kraft Pulps, Ind. Eng. Chem. Res., 41: (2002). 5. GUSTAVSSON, C., SJÖSTRÖM, K., AL-DAJANI, W. The Influence of Cooking Conditions on the Bleachability and Chemical Structure of Kraft Pulps, Nordic Pulp Pap. Res. J., 14 (1): (1999). 6. COSTA, M.M., COLODETTE, J.L. The Impact of Kappa Number Composition on Eucalyptus Kraft Pulp Bleachability, Brazilian J. Chem. Eng., 24 (1): (2007). 7. NETO, C.P.; DANIEL A.I.D., EVTUGUIN, D., SILVESTRE, A.J. Influence of Kappa Number of Unbleached Pulp on ECF Bleachability of Eucalyptus Globulus Kraft Pulps, Proc International Pulp Bleaching Conference, PAPTAC, Halifax, Canada, JIANG,Z.H., VAN LIEROP, B., NOLIN A., BERRY, R. A New Insight into the Bleachability of Kraft Pulps, J. Pulp Pap. Sci., 29 (2): (2003). 9. AL-DAJANI, W., GELLERSTEDT, G. The Effect of Polysulphide Pre-treatment and Cooking Parameters on the Chemical Properties and TCF Bleachability of Alkaline Pulps, Pulp Pap. Canada, 102 (8): (2001). 10. FUTADO, F.P., EVTUGUIN, D.V. GOMES, T.M. Effect of the Acid Stage in ECF Bleaching on Eucalyptus Globulus Kraft Pulp Bleachability and Strength, Pulp Pap. Canada, 102 (12): (2001). 11. DA SILVA, M.R., PEOXOTO, M.A.L. Mill Experience Using a Hot Acid Stage for Eucalyptus Kraft Pulp Bleaching, Proc TAPPI International Pulp Bleaching Conference, San Diego, USA, , VUORINEN, T., FAGERSTRÖM, P., BUCHERT J., TENKANEN, M.,TELEMAN, A. Selective Hydrolysis of Hexenuronic Acid Groups and its Application in ECF and TCF Bleaching of Kraft Pulps, J. Pulp Pap. Sci., 25 (5): (1999). 13. KARIM, M.R., MALINEN, R.O. Suitability of the Combined Use of Chlorine Dioxide and Ozone in Pre of Eucalyptus Camaldulensis Kraft Pulp, Appita J., 60 (3): (2007). 14. LI, J., GELLERSTEDT, G. The Contribution to Kappa Number from Hexenuronic Acid Groups in Pulp Xylan, Carbohydrate Res., 302: (1997). 15. COSTA, M.M., COLODETTE, J.L. The Effect of Kraft Pulp Composition on its Bleachability, Proc International Pulp Bleaching Conference, Portland, USA, , SJÖSTRÖM, K. Kraft Cooking with Varying Alkali Concentration-Influence on TCF Bleachability, Nordic Pulp Pap Res., 13 (1): (1998). 17. TOVEN K., GELLERSTEDT, G, KLEPPE, P., Résumé: Nous avons comparé l aptitude au blanchiment de pâtes d eucalyptus produites selon les procédés de cuisson SuperBatch MD (SB) et ITC MD (cuisson isothermique), et aussi en laboratoire à l aide d une méthode classique en discontinue. Nous avons étudié deux séquences de blanchiment : et Z. Les résultats indiquent que, dans des circonstances similaires, la pâte ITC possédait la meilleure aptitude au blanchiment selon les deux séquences. Toutefois, la pâte SB présentait la plus faible aptitude au blanchiment dans la séquence et une plus grande aptitude au blanchiment que les pâtes fabriquées en laboratoire selon la séquence Z. Reference: FATEHI P., MALINEN, R., NI, Y. Bleachability of Pulps Produced from Different Kraft Cooking Methods: A Laboratory Study. Pulp & Paper Canada 110 (8) T149-T154 (October/ November 2009). Paper presented at the 2008 International Pulp Bleaching Conference in Quebec City, Que., June 2-5, Not to be reproduced without permission of PAPTAC. Manuscript received May 05, Revised manuscript approved for publication by the Review Panel April 8, Keywords: ISOTHERMAL COOKING, SUPERBATCH, HEXENURONIC ACID, BLEACH- ABILITY, BLEACHING COMPARISON, PULPING PARAMETERS, LABORATORY COOKING. pulpandpapercanada.com October/November 2009 Pulp & Paper Canada 37

6 T154 comparison MOE, S. Use of Chlorine Dioxide and Ozone in Combination in Pre, J. Pulp Pap. Sci., 28 (9): (2002). 18. CHAI, X.S., LOU, Q., YOON, S.H., ZHU, J.Y. The Fate of Hexenuronic Acid Groups during Kraft Pulping of Hardwoods, J. Pulp Pap. Sci., 27 (12): (2001). 19. WIDSTEN, P., HORTLING, B., POPPIUS-LEVIN, K. Reactions of Conventional and SuperBatch Pulp Residual Lignins with Peroxyformic Acid, J. Wood Chem. Tech., 23 (3): (2003). 20. WIDSTEN, P., HORTLING, B., POPPIUS-LEVIN, K. Treatment of Conventional Kraft and SB Pulp Residual Lignins with Alkaline Hydrogen Peroxide, J. Pulp Pap. Sci., 30 (7): (2004). 21. BUCHERT, J., LAINE, J., TENKANEN, M., VUO- RINEN, T., VIILAI, L. Characterization of Uronic Acids during Kraft and SuperBatch Pulping, Proc International Symposium on Wood Pulping Chemistry, ISWPC, V1, Montreal, Canada, BUCHERT, J., TENKANEN, M., TAMMINEN, T. Characterization of Carboxylic Acids during Kraft and SuperBatch Pulping, Tappi J., 84 (4): (2001). 23. GULLICHSEN, J., PAULAPURO, H. Forest Product Chemistry, Papermaking Science and Technology, B3, 1st edition, Finland, Fapet Oy, (1999). 24. COLODETTE, J.L. Influence of Pulping Conditions on Hardwood Pulp Yield, Quality and Bleachability, Proc International Pulp Bleaching Conference, PAPTAC, Halifax, Canada, SHIN, S.J., SCHROEDER, L.R., LAI, Y.Z. Understanding Factors Contributing to Low Oxygen Delignification of Hardwood Kraft Pulps, J. Wood chem. Tech., 26: 5-20 (2006). 26. MALINEN, R.O., ZHAO, H.P. Evaluation of the Bleachability of Eucalyptus Camaldulensis Kraft Pulp, Proc International Pulp Bleaching Conference, Stockholm, Sweden, MALINEN, R.O., NHAN, T.T. Control of Hexenuronic Acid Content in Bleached Pulp and its Effects in Optical Properties of Pulp, Proc. 60th Appita Annual Conference, Melbourne, Australia, RAGNAR, M. On the Importance of the Structural Composition of Pulp for the Selectivity of Ozone and Chlorine Dioxide Bleaching, Nordic Pulp Pap. Res. J., 16 (1): (2001). 29. FATEHI, P., MALINEN, R. O., NI, Y. Removal of Hexenuronic Acid from Eucalyptus Kraft Pulps during Chlorine Dioxide- and Ozone based ECF Bleaching Sequence, Appita J, 62 (3): (2009). 30. ZHANG, Y., KANG, G., NI, Y., VAN HEININGEN, A.R. P. Degradation of Carbohydrate Model Compounds during Ozone Treatment, J. Pulp Paper Science, 23 (1), J23-27 (1997). 31. KANG, G., ZHANG, Y., NI, Y., and VAN HEININ- GEN, A.R. P. Influence of Lignins on the Degradation of Cellulose during Ozone Bleaching, J. Wood Chem Technol., 15 (4), (1995) 32. NI, Y., SHEN, X., VAN HEININGEN, A.R. P. Studies on the Reaction of Phenolic and Non-Phenolic Lignin Model Compounds with Chlorine Dioxide, J. Wood Chem. Technol., 14 (2), (1994). 38 Pulp & Paper Canada October/November 2009 pulpandpapercanada.com