For sodium hydroxide, chelating agents, peracetic acid and sometimes hydrogen peroxide
the chemical can also be added to the pulp prior to the pump or in the pump. The requirement on
the mixing efficiency is high as poor mixing leads to uneven distribution of bleaching chemicals
in the pulp and this results in local over-bleaching in some areas and local under-bleaching in
other areas. The final result of poor mixing is that the consumption of bleaching chemicals increase
and the pulp strength is reduced compared at a given bleaching result. The requirements
are especially high if gas (for example oxygen) is added to the medium consistency pulp slurry
or if the bleaching reaction is very fast and the risk for uneven bleaching is obvious. The latter
case is particularly valid for bleaching with ozone that reacts extremely fast with the pulp.
When the chemical, or chemicals, are added to the pulp the reaction starts and a certain reaction
time is needed. Depending on the chemical used and how far the pulp has been bleached
prior to the stage the time requirements vary considerably. The total time requirement is for example
less than 1 minute for ozone bleaching and 1–3 hours for final bleaching with chlorine dioxide.
The reactor shown in