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        Last update : June 19, 2003
 
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            8 - Ozonation of high kappa pulps. New possibilities            
Dinah Nyangiro (EFPG)

Logo EFPGIn order to comply with environmental legislation on AOX levels and mill effluent organic material load (BOD) and with process economy, much interest has been directed towards ECF bleaching process improvement, TCF bleaching and mill effluent recycling.

Oxygen delignification upstream before bleach plant is a common industrial practice. Literature results have shown that stopping the cooking process at high kappa number (40 to 50) instead of the conventional 30 kappa number, then applying an oxygen delignification stage, led to about 2 % bleached pulp yield increase. Pulping to 40 or 50 kappa number followed by two-stage oxygen delignification significantly reduces the boiler load, as wood consumption for a constant bleached pulp production is lower.

Ozone has proved to be an effective delignification agent and is currently applied in about 25 mills, usually in the middle of the bleaching sequence. When ozone is used in ECF sequences to partially substitute chlorine dioxide, it has been shown that 1 kg of ozone could replace about 2 kg of pure chlorine dioxide. Ozone charges are limited to quantities lower than 0.7 % to avoid cellulose degradation.

We proposed to carry out ozonation on high kappa pulp in an effort to improve ozone delignification selectivity and achieve ECF bleaching process improvement by increasing bleached pulp yield. High consistency ozonation at 20°C and at a pH of 2 was carried out on a softwood kraft pulps (Norwegian spruce) of varying kappa numbers. A 2 % ozone charge on dry pulp weight was applied on laboratory prepared kraft pulp of 52 kappa number. An ECF sequence was added to fully bleach the pulp after which pulp and paper properties were determined. The results showed that up to 2.3 % bleached pulp yield increase was obtained, compared to a 27 kappa number pulp bleached by DEDED. The properties of these two pulps were compared. The ozone treated high kappa pulp subjected to an ECF bleaching sequence had superior refining ability compared to classic ECF sequence on a 27 KN pulp. The final viscosity and paper properties were comparable for these two pulps.

Two hypotheses were made to explain the good selectivity of ozone on high kappa pulps:

 

a shorter cooking time should result in a lower amount of free phenolic groups in the lignin. Consequently the reaction of ozone with this pulp should form less harmful hydroxyl radicals, and thus less cellulose degradation should occur. Indeed the reaction of ozone on free phenolic groups has been described to lead to the formation of hydroxyl radicals. 13C NMR analysis of the lignin showed indeed a lower amount of free phenolic groups in the 52 kappa pulp compared to the conventional 27 kappa pulp;

 

a higher quantity of lignin should protect cellulose from direct ozone reaction, as the ozone-cellulose reaction rate is much lower than that of the ozone–lignin reaction.

 

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