Reference Manual

14−2
of the remaining lignin can be removed; further
delignification would cause excessive cellulose
degradation. Lignin removal in oxygen
delignification significantly reduces the amount
and cost of the bleaching which follows, and
reduces the load on effluent treatment facilities
because the filtrate from the post-oxygen washers
goes back to the brown stock washers and the
chemical recovery system.
Oxygen delignification is typically done at a
medium to high consistency. Although good
results are obtained from both high and
low-consistency systems, medium-consistency is
favored because of its lower capital costs and
inherently safe operation.
In a medium-consistency system, pulp coming
from the brown stock washer at about 10-14%
consistency is delignified. It is then preheated in a
low pressure steam mixer and pumped through
one or more medium-consistency gas mixers to an
upflow pressurized reaction tower. Steam and
oxygen are added upstream of the consistency
mixer or added directly to the pulp slurry. The
most recent mills have two consecutive stages in
order to improve the chemical efficiency of the
treatment.
Bleaching
Pulps that have or have not been delignified are
bleached in a continuous sequence of process
stages, typically three, four, or five. The chemistry
changes in each stage and the pulp is washed
between stages.
The common bleaching chemicals and
nomenclature are:
Chlorination (C): Reaction with elemental chlorine
in an acidic medium (Cl)
Alkaline Extraction (E): Dissolution of reaction
products with sodium hydroxide (NaOH) - ref
Chlorine Dioxide (D): Reaction with chlorine
dioxide in acidic medium (ClO
2
) - ref
Oxygen (O): Reaction with molecular oxygen at
high pressure in alkaline medium (O
2
) - ref
Hypochlorite (H): Reaction with hypochlorite in
alkaline medium (ClO−) - ref
Peroxide (P): Reaction with peroxide in alkaline
medium - ref
Ozone (Z): Reaction with ozone in acidic medium
(O
3
) - ref
For many years, chlorination was always the first
stage of bleaching. However, since the 1990’s, it
has largely been replaced by chlorine dioxide to
avoid the formation of dioxins. This is due to the
environmental push for all pulp and paper mills to
be ECF, or elemental chlorine free. A few paper
mills are TCF, or totally chlorine free.
Conventional Bleaching
The equipment is the most common aspect of the
stages. This includes: a steam mixer to heat the
pulp suspension with direct steam, a pump to
transport the pulp, a chemical mixer to combine
the pulp with the aqueous bleaching agent, a
retention tower to allow time for the bleaching to
occur, and a washer to separate the spent
bleaching solution from the pulp.
Referring back to the common bleaching
nomenclature, a typical bleach plant has a
DE
OP
DED sequence, or a low-consistency
chlorine dioxide first stage with one or two
chemical mixers, an upflow tower, and a rotary
vacuum drum filter for pulp washing. The chlorine
dioxide comes to the mixer at a solution
concentration of about 10 grams per liter in cold
water. The pulp suspension is around 3.5%
consistency and has been heated to about 140_F.
After mixing, the pulp and chlorine dioxide go to
the retention tower to react for about 45 minutes.
The pulp is then washed afterward.
Each bleaching stage has its own set of process
conditions. The amount of bleaching agent,
consistency of the pulp, pH, temperature, and time
may all vary in each stage. Pulp consistency is
around 3-4% in the chlorine dioxide stage and
about 10% in all subsequent stages. The
temperature is the lowest in the first stage at
140_F, and between 140-176_F, in the other
stages. How much bleaching agent required
depends on which chemical, which stage in the
sequence, and what kind of pulp. With chlorine
dioxide, progressively less is used as you go along
the sequence (figure 14-2).
The objective of bleaching is to remove the
residual lignin from the unbleached pulp. Chlorine
dioxide is the preferred bleaching reagent
worldwide. It is selective in dissolving residual
lignin without degrading the cellulose and
hemicelluloses. In a bleaching sequence, chlorine
dioxide stages are always interspersed with
alkaline extraction stages (see figure 14-2). This