TCF bleaching of eucalyptus kraft pulp: The role of
hexenuronic and muconic acids on pulp viscosity.
Frederic Pouyet,a,* Dominique Lachenal,a Christine Chirata
An oxygen delignified eucalyptus (Eucalyptus grandis x Eucalyptus urophylla hybrid)
provided by a Brazilian mill was treated with ozone (Z) followed or not by an alkaline
extraction (E) after various pretreatments…
It is concluded that the most probable origin of cellulose degradation during Z is the
direct formation of OH radicals when ozone reacts with the hexenuronic and muconic
acids. These results may be taken into account to design TCF bleaching sequences with a
much lower impact on pulp viscosity.
Keywords: ozone bleaching; viscosity loss; OH radicals; hexenuronic acid; muconic acid.
a
Laboratoire de Génie des Procédés Papetiers (LGP2), Grenoble INP-Pagora, St Martin d’Hères Cedex- France,
* Corresponding author: [email protected].
INTRODUCTION
Unbleached hardwood kraft pulps can be fully bleached with sequences containing
oxygen (O), ozone (Z) and hydrogen peroxide (P) stages (Mokfienski and Demuner 1994;
Rautonen et al. 1995; Bajpai 2005)…
EXPERIMENTAL
Pulps
An oxygen-delignified (hybrid between Eucalyptus grandis x Eucalyptus urophylla) kraft
pulps was provided by a Brazilian pulp mill (Table 1). Two other pulps were prepared from
this oxygen-delignified pulp by applying either a chlorite treatment (holocellulose procedure,
Wise et al. 1946) to remove the lignin, or both a long hot acid stage (Long A stage) to remove
most of the HexA and a chlorite treatment, respectively referred to as lignin-free pulp and
HexA- and lignin-free pulp in this paper.
Table 1. Pulp samples
Pulp
Kraft+O
Lignin-free
HexA- and lignin-free
DP
1630
1630
1420
HexA content (µeq/g)
71.7
48.2
<6
Operating Conditions and Standards
The conditions used for the different stages composing the sequences are listed in
Table 2. For the other stages…
___________________________________________________________________________________________
International Pulp Bleaching Conference – October 29-31, 2014 – Grenoble, France
RESULTS AND DISCUSSION
Viscosity loss during Z stage
In a first set of experiments the eucalyptus pulp was treated by increasing amounts of
ozone (Z stage) after a holocellulose treatment (no lignin left) or after an extensive acidolysis
stage (A) followed by a holocellulose treatment (neither lignin nor HexA left). The cellulose
DP was measured. As seen in Figure 1 there is a severe DP loss in the case of a lignin-free
pulp. This decrease is higher than in the case of the HexA- and lignin-free pulp.
Fig. 1. DP of an HexA- and lignin-free pulp (squares) and of a lignin-free pulp (crosses) at various ozone charges.
Table 4. Effect of splitting ZE into ZEZE on cellulose degradation after treatment
Treatment
Ozone charge on pulp (%)
Brightness (%)
No
51.0
ZE
0.6
64.4
ZEZE
0.3 + 0.3
73.1
DP
1630
1180
1340
Detection of OH radicals during ozonation of model compounds
Measuring the formation of OH radicals during ozonation is not an easy task…
Practical relevance
These findings pave the way for the design of more efficient and attractive TCF
bleaching sequences…
CONCLUSIONS
2.
Hydroxyl radicals are formed when ozone reacts with unsaturated compounds
containing carbon-carbon double bonds.
3.
Consequently the reaction of ozone with lignin, hexenuronic acids and
muconic acid derivatives must be responsible for cellulose degradation during an
ozone bleaching stage.
4.
This finding is confirmed by the positive effect of acidolysis pretreatment prior
to the ozone stage and the split of ozone application, on cellulose DP. It paves the way
for the design of new TCF bleaching sequences with a lower impact on cellulose.
___________________________________________________________________________________________
International Pulp Bleaching Conference – October 29-31, 2014 – Grenoble, France
ACKNOWLEDGMENTS
The authors would like to thank…
REFERENCES
Allison, R. W. (1982) Efficient ozone and peroxide bleaching of alkaline pulps from pinus
radiata. Appita J.36:42-45.
Backa, S., Jansbo, K., Reitberger, T. (1997) Detection of hydroxyl radicals by a
chemiluminescence method – A critical review. Holzforschung 51:557-564.
Eriksson, T., Gierer, J. (1985) Relative rates of ozonation of lignin model compounds.
Proceedings International Symposium on Wood and Pulping Chemistry, Vancouver,
Canada, Poster book. pp. 29-30.
Goldschmid, O. (1971) Ultraviolet spectra. In: Lignins: Occurrence, Formation,
Structure
and Reactions. Eds. Sarkanen, K.V., Ludwig, C.H. John Wiley and Sons, New York. pp. 241266.
___________________________________________________________________________________________
International Pulp Bleaching Conference – October 29-31, 2014 – Grenoble, France