As described in section 10.4 the bleaching result that can be obtained in a single bleaching stage
is limited and it is usually not sufficient for a pulp mill that produce bleached kraft pulp. Therefore,
a sequence of stages are in most cases used and the number of stages is usually 3–5. The
maximum brightness that can be obtained is called the brightness ceiling and that brightness is
dependent on the wood raw material, the pulping method, the bleaching sequence and the
bleaching conditions. The bleaching chemical charge in a given stage is usually aimed for
bleaching well below the brightness ceiling of economical reasons. If a higher pulp ceiling is
needed more stages, new bleaching chemicals or more intense conditions are needed in the sequence.
A chlorine dioxide charge can for example become more efficient if the charge is split
up from one to two stages with an alkaline stage between. This split of acidic and alkaline stages
results in significantly better bleaching efficiency measured for example as a higher brightness
for a given consumption of bleaching chemicals, Figure 10.5. The kappa factor on the xaxis
in the figure is defined as the active chlorine consumption given in % on the pulp divided
chemical applied per unit of pulp brightness lignin content chemical applied per unit of pulp
pulp viscosity by the kappa number of the pulp before bleaching. Thus, if the kappa number of the pulp is 12
and the kappa factor 0.2 the active chlorine charge is 2.4 % or 24 kg per ton of pulp