The beneficial stoichiometric effect shown in Figure 10.5 is in a bleach plant obtained by using
several bleaching reactors with a washer between each stage, Figure 10.6. In mill scale the
number of stages in a sequence is usually 3–5 after oxygen delignification if the pulp is
bleached to full brightness i.e. at 88 % ISO or higher. Fewer stages are needed if the brightness
requirement is lower or if the pulp is easy to bleach. Many hardwood pulps like the most commonly
used eucalyptus pulps are easier to bleach than softwood pulps i.e. they require less active
chlorine for bleaching to full brightness compared at a given starting kappa number prior to
bleaching. However, to also obtain brightness stability comparable to softwood pulps hardwood
pulps sometimes need a special treatment which is more discussed below.
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