The role of hydroxyl radicals in selectivity of ozone delignification

Damien Lorcaa, Anne Dombaslea, Eric Jordelb*

a Laboratoire de Génie des Procédés Papetiers (LGP2), Grenoble INP-Pagora, St Martin d’Hères - France,

b Chemical Engineering Laboratory, University of Grenoble-Alpes, Grenoble - France

* Corresponding author:

ABSTRACT

Ozone is a powerful delignifying agent because it reacts very readily with unsaturated organic compounds. Applied to the phenolic moieties, this reaction causes lignin degradation and dissolution…

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Keywords: ozone bleaching; TCF bleaching; OH radicals; hexenuronic acid; muconic acid

INTRODUCTION

Ozone can be used to bleach pulp, both in ECF and TCF sequences, in what is called a Z stage. It was implemented on an industrial scale for the first time in 1992 (Nutt et al. 1993)…Recent work showed that this effect is not primarily caused by ozone itself, but more probably by radicals (Ragnar et al. 1999b; Kang et al. 1995).

Regarding lignin, it is generally admitted that a charge transfer occurs between ozone and the phenolic rings (Ragnar et al. 1999c), leading to the ozone anion radical...

Several methods can be used to detect the formation of OH radicals. One of them relies on the use of TNM (Tetranitromethane) and was proposed by Ragnar et al. (1999a). Another method uses phtalhydrazide and was used by several authors (Reitberger and Gierer 1988, Marc 1994)…

EXPERIMENTAL

Pulps

An oxygen-delignified (hybrid between Eucalyptus grandis x Eucalyptus urophylla) kraft pulps was provided by a Brazilian pulp mill (DPv: 1630; Brightness: 51%, HexA content: 71.7µeq/g; Kappa number: 9.3). The associated commercial bleached ECF pulp was also provided by the mill (DPv: 1130; Brightness: 91.5%).

Operating Conditions and Standards

The conditions used for different stages composing the sequences suggested in this study are listed below (Table 1).

The following standards were used for pulp characterization: Kappa number (ISO 302-1981), viscosity (ISO 5351-1981), translated here into DPv, brightness (ISO 3688 – 1977)…

Table 1. Conditions used during the bleaching stages

Stage / pH / T (°C) / Consistency (%) / Time (h) / Reagents
A / 3 / 90 / 10 / 2 / H2SO4

ESR Spectroscopy

Ozonations of model compounds were followed by ESR spectroscopy. Acetovanillone (Aldrich) was used as a model for residual lignin after oxygen delignification… Citraconic acid (Alfa Aesar) and 2, 5-dimethyl 2, 4-hexadienedioic acid (Alfa Aesar) were also used.

X‐band ESR spectra were recorded at room temperature with a Bruker EMX Plus spectrometer equipped with the ER-4102ST Bruker cavity…

RESULTS AND DISCUSSION

Detection of radical species during ozonation of lignin models

In a first set of experiments, ESR spectroscopy was used…

Formation of radical species with non-aromatic structures

In a second set of experiments, non-aromatic compounds were used…

Origin of the radical species detected in ESR trials

Several hypotheses could explain the generation of hydroxyl radical…

TCF bleaching sequence

Such a sequence is illustrated in Figure 1. …

Fig. 1. Possible flowsheet of AQ(ZE)(ZE)P sequence with high effluent recirculation(courtesy Andritz)

CONCLUSIONS

1.  Hydroxyl radicals are formed when ozone reacts with unsaturated compounds containing carbon-carbon double bonds. The mechanism should involve a charge transfer.

2.  The reaction of ozone with lignin, muconic acid derivatives and hexenuronic acid must be responsible for cellulose degradation during an ozone bleaching stage.

3.  Selective TCF sequences based on the use of ozone were designed. They should prove to be very attractive in terms of cost and environmental impact.

ACKNOWLEDGMENTS

The authors would like to thank …..

REFERENCES

Bose, S. K., Leavitt, A., Stromberg, B., Kanungo, D., and Francis, R. C. (2011). Inclusion of a pressurized acidolysis stage in chemical pulp bleaching. BioResources, 6(1), 823–840.

Cole, B.J.W., and Kwon, H. (1995) The Periodate Oxidation Method: Does It Really Measure Phenolic Hydroxyl Content. In: Proc. 8th Int. Symp. Wood and Pulping Chemistry, Helsinki, Finland, June 6-9. vol. I, pp. 541-548.

Ragnar, M., Eriksson, T., and Reitberger, T. (1999a). Detection of radicals generated by strong oxidants in acidic media. Holzforschung, 53(3), 285–291.

Ragnar, M., Eriksson, T., and Reitberger, T. (1999b). Radical formation in ozone reactions with lignin and carbohydrate model compounds. Holzforschung, 53(3), 292–298.

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European Workshop on Lignocellulosics and Pulp - June 28-July 1, 2016 - Autrans, France