Reference Manual

9−4
binds the cellulose wood fibers together. This
process produces stronger pulp than the other
processes, but is darker in color than the other
pulp processes. However, the benefit to this
process is the wide range of fiber sources that can
be used, and the regeneration of cooking liquors.
Woodchips are fed into digesters where they are
impregnated with the cooking liquors of warm
black liquor and white liquor (see chapter 3). The
warm black liquor is spent cooking liquor coming
from the blowtank. White liquor is a mixture of
sodium hydroxide and sodium sulfide, produced in
the Kraft recovery process. Delignification requires
several hours of cooking at 265°F to 355°F. Under
these conditions, the lignin and some
hemicellulose degrade to give fragments solubility
in a strongly basic liquid.
White Liquor:
NaOH ⇔ Na
+
+ OH
−
Na
2
S + H
2
O ⇔ 2Na
+
+ OH
−
+ HS
−
The lignin is removed by the following reaction,
where the HS
−
ion the component that ultimately
removes the lignin.
E1388
The finished cooked wood chips are blown from
the digester, and the action of the cooked wood
chips hitting the walls of the blowtank produce
individual pulp fibers. The solid pulp (about 50%
by weight based on dry wood chips) is then
collected and washed. The washing stages
separate the cooking liquors from the cellulose
fibers, where the pulp is brown after cooking and
is known as brown stock.
The combined liquids, known as black liquor due
to its color, contains lignin fragments,
carbohydrates from the breakdown of
hemicellulose, sodium carbonate, sodium sulfate,
and other inorganic salts.
Recovery Process:
The excess black liquor is concentrated in a
multiple effect evaporator (see chapter 4) into
heavy black liquor, and burned in the recovery
boiler (see chapter 5) to recover the inorganic
chemicals for reuse in the pulping process. More
concentrated black liquor increases the energy
and chemical efficiency of the recovery cycle. The
combustion is carried out such that sodium sulfate
(Na
2
SO
4
) is reduced to sodium sulfide by the
organic carbon in the mixture:
1. Na
2
SO
4
+ 2 C → Na
2
S + 2 CO
2
The molten salts (smelt) from the recovery boiler
are dissolved in process water known as weak
wash (see chapter 6). The solution of Na
2
CO
3
and
sodium sulfide results in green liquor. This liquid is
mixed with calcium hydroxide (Ca(OH)
2
) to
regenerate the white liquor in the pulping process.
2. Na
2
S + Na
2
CO
3
+ Ca(OH)
2
⇔ Na
2
S + 2 NaOH
+ CaCO
3
Calcium carbonate (CaCO
3
) precipitates from the
white liquor and is recovered and heated in a lime
kiln where it is converted to calcium oxide (lime).
3. CaCO
3
⇔ CaO + CO
2
Lime is reacted with water to regenerate the
calcium hydroxide used in reaction 2.
4. CaO + H
2
O → Ca(OH)
2
The combination of reaction 1 through 4 forms a
closed cycle with respect to sodium, sulfur, and
calcium. The recausticizing process where sodium
carbonate is reacted to regenerate sodium
hydroxide is the main reaction in the process
where approximately 98% of the original
chemicals are regenerated.