Ozone reacts very rapidly with the pulp and the reaction takes place more or less instantly. The
importance of a highly efficient mixing of the pulp and the ozone gas is therefore very high. In
addition, ozone is sensitive towards high temperatures that cause spontaneous decomposition
into oxygen. For this reason ozone stages are run at low temperature, which typically means
around 30–60 °C. The stage is carried out at a pH of 2–3 and the retention time is a few seconds
up to 1 minute, see the box below. An advantage with ozone is that extractives are removed
very efficiently. This decreases the odour and taste in the pulp. Another advantage is its ability
to efficiently remove hexenuronic acids in the pulp which improves the brightness stability of
the final bleached pulp and reduce the ClO2–demand in the bleach plant. Removal of hexenuronic
acids is especially interesting for hardwood kraft pulps.