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Christine Chirat, Dominique Lachenal, Shree Prakash Mishra, Raphaël Passas, Flora Ludovina, Bertine Khelifi
Presented at 14th ISWFPC - Durban, South Africa

KEYWORDS: Ozone bleaching, softwood kraft pulp, eucalyptus kraft pulp, fibre characteristics, physical properties

EFPG has launched a study on the effect of ozone on fibre characteristics and papermaking properties including the refining and behaviour in the wet end part of the paper machine.

In a first part a Nordic softwood kraft pulp with a starting kappa number of 26.4 was used. It was bleached with a control DEDED sequence and an ozone based sequence ZDEDD. The objective being to study the effect of ozone on fibre characteristics, a significant amount of ozone was applied on the unbleached kraft pulp (up to 0,8%). These two bleached pulps were refined in a PFI mill.

During refining the ozone treated pulp presented slightly lower bulk and air permeability values than the DEDED bleached pulp. The strength properties (tear index, breaking length) were similar for the two pulps.

The Morfi equipment was used to analyse some morphological characteristics of the unrefined pulps: the ozone bleached pulp fibres had a slightly higher curl and macro-fibrils content and a slightly lower width than the control bleached pulp fibres. The ESM observation of the fibres concluded that the ozone bleached fibres seemed more flexible and collapsed than the DEDED bleached fibres. Furthermore the ozone treated fibres presented a more pronounced detachment of macrofibrils. These characteristics explain the lower bulk and air permeability.

In a second part a similar study was done on a hardwood pulp (Eucalyptus kraft pulp), which did not lead to the same conclusions as for the softwood kraft pulp.

Ozone is now being used in about 28 bleaching lines for both softwood and hardwood kraft pulps. In some cases there has been some informal reporting on the fact that the ozone bleached pulps would behave differently in the paper mill: the refining ability would be different as well as the runnability on a paper machine. Even though some articles can be found on the effect of ozone on the strength properties of pulps (1-7), the literature does not contain any precise information on the above cited aspects. Furthermore there is also a lack of information on the effect of ozone on fibre characteristics. A study has been launched on the effect of ozone on fibre characteristics and papermaking properties including the refining and behaviour in the wet end part of the paper machine. Some results are presented here: they deal with the effect of ozone on the refining ability, strength properties and fibre morphology of a softwood and a hardwood kraft pulps.

Starting pulps : Two pulps were used: A Nordic spruce kraft pulp, kappa number 26,4 was bleached according to DEDED (control sequence) and ZDEDD, using 0,8% ozone. An Oxygen delignified Eucalyptus Grandis kraft pulp, kappa number 11,9 was bleached by DEDD and ZDED using 0,2, 0,4 and 0,8% ozone.

Bleaching stages :
Z stage : the pulp was acidified to pH 2,5 at a consistency of 5%, and then centrifuged to 40% consistency and fluffled. The ozone treatment was carried out in a spherical glass reactor at room temperature. In the case of the softwood pulp 0,8% ozone was applied. In the case of the hardwood pulp three ozone charges were applied : 0,2, 0.4 and 0.8%
D stages were carried out at 10% pulp consistency in plastic bags placed in a thermoregulated water bath. D0 stages were carried out at 50°C, 1 hour retention time. D1 stages were carried out at 75°C, 2 hours retention time and D2 stages were carried out at 80°C during 3 hours.

Morphological study : the values presented in this study were measured with a Morfi equipment. This equipment analyses 3000 fibres in 3 minutes (10). The fibres pictures were taken using an Environmental Scanning Electron Microscope.

Refining was performed in a PFI mill. The physical properties (brightness, bulk, traction resistance, tearing resistance, air permeability) were measured using the ISO standards.


1. Nordic Spruce kraft pulp
The Nordic spruce kraft pulp was treated directly by an ozone stage, as proposed in a previous study (8) using 0,8% ozone. This level seems high enough to see any potential effect of ozone on pulp properties and realistic at mill scale. The final bleaching was done by a DEDD sequence. The control sequence was DEDED. The bleaching results (Table 1) showed that both sequences led to similar final brightness levels, the ozone being able to replace chlorine dioxide with a replacement ratio of 2. Figure 1 showed that the two pulps had almost a similar refining ability. The bulk of the ozonated pulp was slightly lower than that of the control pulp when the pulp was refined (Figure 2). Gupta and Eckert (1) already noticed that the bulk was lower after an ozone treatment. However they used a much higher charge (more than 2%).

Despite the lower viscosity of the ozonated pulp, the tear Index and breaking length were not affected significantly (Figures 3 and 4). After refining air permeability (Figure 5) of the ozonated pulp appeared lower than for the control pulp, which might be in line with the lower bulk.

Table 1. Bleaching conditions for the control and the ozone treated pulp

Figure 1. Refining ability of the control and ozonated pulps

Figure 2. Evolution of bulk during refining

Figure 3. Evolution of Tear Index during refining

Figure 4. Breaking length versus refining

Figure 5. Air permeability versus refining

The effects of ozone and of refining on the fibre morphological characteristics are illustrated in the next figures. Figure 6 showed that the fibre length (Figure 6) was not affected by the ozone treatment (which is in line with what was observed by Seisto and al (2)). A slightly lower width was observed for the ozonated pulp after refining (Figure 7). A
 higher percent of kinked fibres and curl was found on the ozonated pulp before and at the start of refining (Figures 8 and 9).

The fines content increased slightly more when the ozone treated pulp was refined.(Figure 10). Upon refining the amount of macrofibrils (parts of fibre wall that start to detach from the fibre) became higher for the ozonated pulp compared to the control pulp (Figure 11).

The control (DEDED) and the ozonated (ZDEDD) pulps were observed using Electon Scanning Microscopy (ESM) (Figures 12 and 13). Prior to refining. the ozonated pulp fibres seemed more collapsed than the control pulp fibres. Some macrofibrils started to detach from the fibres, even though no refining has taken place yet. The fact that the ozonated pulps had a tendency to have a lower bulk and air permeability than the control pulp could be explained by these changes in fibre morphology.

Figure 6. Effect of ozonation on fibre length

Figure 7. Effect of ozonation on fibre width

Figure 8. Effect of ozonation on the amount of kinked fibres

Figure 9. Effect of ozonation on curl.

Figure 10. Effect of ozonation on fines

Figure 11. Amount of macrofibrils versus refining time

Figure 12. ESM pictures of the DEDED bleached pulp before refining

Figure 13. ESM pictures of the ZDED bleached pulp, before refining

2. Eucalyptus kraft pulp
Table 2 summarizes the bleaching results for the control DEDD and the ozone based ZDED sequences. Lower ozone charges are generally applied on oxygen bleached hardwood pulps. However the 0.8% level was also included in the study for comparison with the softwood pulp. The replacement ratio of ozone for chlorine dioxide was between 2,0 and 3,3 for the highest and lowest ozone charges respectively, which is consistent with previous results (9). It should be noted that the highest ozone charge (0,8%) led to a substantial drop in viscosity, much bigger than for the softwood pulp.

Table 2. DEDD and ZDED bleaching of the O treated eucalyptus kraft pulp.

The refining ability of the control and the ozone treated pulps are given in Figure 14. For low ozone charges (lower than 0,4%), no difference was observed, whereas for 0,8% ozone the pulp was easier to refine, as previously observed.(8).

Contrary to what was seen with the softwood pulp, the ozone seemed to slightly increase the bulk, prior to and after refining (Figure 15), which was also in line with the slightly higher air permeability observed for the two lower ozone charges (Figure 21).

As far as the strength properties were concerned, the breaking length was not modified significantly, and the tear index showed a 10% decrease only for the highest ozone charge despite the dramatic decrease in viscosity in this latter case (Figures 16 and 17). This confirms the well known fact that the relationship between viscosity and pulp properties depends on the bleaching sequence.

The analysis of fibre morphology did not show any difference between the DEDD and ZDED pulps : the fibre length, width, amount of fines were the same, as were the amount of macrofibrils (Figure 18), kinks and curl (Figures 19 and 20). The same conclusion applied after SEM observation prior and after refining. No morphological differences between the DEDD and the ZDED bleached fibres were seen.

Figure 14. Refining ability of the control and ozonated eucalyptus pulps

Figure 15. Bulk versus refining time for the control and the ozonated pulps

Figure 16. Breaking length versus PFI revolutions

Figure 17. Tear Index versus PFI revolutions

Figure 18. Amount of macrofibrils versus refining

Figure 19. Effect of the ozone treatment on the proportion of kinked fibres

Figure 20. Effect of the ozone treatment on the curl.

Figure 21. Effect of the ozone treatment on air permeability

The results obtained so far showed that ozone (0.8% on pulp) tended to make the softwood fibres more collapsed and flexible. Some macrofibrils detachment from the fibre surface was seen already before refining. It resulted in a slightly reduced bulk and air permeability. Fibres that are difficult to refine should benefit from an ozone treatment.

The same study performed on an oxygen bleached eucalyptus pulp showed that for 0.8% ozone charge, the pulp was easier to refine, but no morphological changes could be observed that could have explained that fact.

The physico-chemical characteristics of these pulps and their behaviour in the wet-end part of a paper machine are under investigation in order to fully characterise these pulps and the impact of ozone.

1.M.K. Gupta and R.C. Eckert, "OZ pre-bleaching : influence on viscosity and sheet strength", 1984 Oxygen Delignification Symposium, San Francisco, Nov 15-16 1984, TAPPI Proceedings, pp133-144, 1984
2.A. Seisto, K. Poppius-Levlin and A. Fuhrmann, "Effect of ozone bleaching on the fibre properties of pine and birch kraft pulp", Cellulosic Pulps, Fibres and Materials, Editors John F. Kennedy, Glyn O., pp 137-147, 2000
3.H. Ryynänen, P.J. Nelson, C.W.J. Chin, "Ozone bleaching of Eucalypt kraft pulps"; Appita Vol. 48 no 6, pp 440-444, November 1995
4.C.A. Lindholm, "Effect of pulp consistency and pH in ozone bleaching. Part 6. Strength properties". Nordic Pulp and Paper Research Journal no 1, vol.5, pp 22-27, March 1990.
5.K. Toven, "Paper properties and swelling properties of ozone-based ECF bleached softwood kraft pulps", TAPPI Journal, vol.2, no 2, pp 3 – 7, February 2003
6.A. Mokfienski and B.J. Demuner, "Pilot –plant experience with ozone in TCF bleaching of eucalypt pulp", Tappi Journal, vol 77, no 11, pp 95-103, Nov 1994
7.A. Fuhrmann, X.L. Li, R. Rautonen, "Effects of ECF and TCF bleaching sequences on the properties of softwood kraft pulp", 1996 International Pulp Bleaching Conference, Washington, Tappi Proceedings, pp 71-79, April 1996
8.C. Chirat, Nyangori, D., Struga, B., Lachenal, D., " The use of Ozone on high kappa softwood and hardwood kraft pulps as a way to improve pulp yield" Inernational Pulp Bleaching Conference, Stockholm, Proceedings pp 74-80, June 2005.
9.Chirat, C., Lachenal, D. Angelier R., Viardin M.T., „" (DZ) and (ZD) bleaching fundamentals and application", Journal of Pulp and Paper Science, 23, 6, pp289-292, 1997.
10.Passas, R., Eymin, G., Voillot, C., Tarrajat, G., Caucal, G., Khelifi, B. MorFi : analyseur morphologique des fibres, Récents Progrès en Génie des Procédé 2001, 15(78), 259-264

The authors wish to thank Jean-Christophe Hostachy and Robert Serfass from WEDECO and Pierre Liechti from OZONIA, for supporting this research.

Ecole Française de Papeterie et des Industries Graphiques, BP 65, 38402 Saint Martin d'Hères Cedex, Tel : +33 476 826 907 - Fax : +33 476 826 933 – e-mail:Christine .Chirat@efpg.inpg.fr

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