Eucalyptus pulp bleaching was modernized in the 90s with the industry's conversion to ECF bleaching and the discovery of the high content of hexenuronic acids (HexA) in eucalyptus pulp. Both acid hydrolysis and ozone bleaching are very selective towards HexA so they have been seen as competing techniques. Today the use of acid hydrolysis for HexA removal is a common practice carried out in the first bleaching stage of sequences such as D_(HT)-EOP-D-P and A/D-EOP-D-P for eucalyptus pulp. Ozone bleaching has also successfully proven to be an economical and sustainable technology and can be widely implemented to treat the high HexA content found in eucalyptus pulp. This paper describes a new way of using ozone: the combination of a hot acid stage A in front of an ozone stage Z. There is clear industrial evidence that maximum efficiency from the chemical cost viewpoint is achieved through an A-Z combination, since the acidic extraction of HexA is intelligently complemented with a selective delignification with ozone.
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