bleaching of pulp
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BLEACHING STAGES AND SEQUENCES
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Unbleached pulps exhibit a wide range of brightness depending on the pulping process and
fibrous raw material used. The sulfite process produces the brightest pulp, up to 65% whereasKraft, soda and semi-chemical may produced pulp of only 15% brightness.
Holocellulose (Cellulose + hemi-cellulose) are inherently white and do not contribute to color. It
is the chromophoric group in lignin which are responsible for color.
Two approaches are used in the chemical bleaching of pulps. One approach utilizes selective
chemicals that destroy some of the chromophoric group but do not attack lignin. The other
approach is to almost totally remove all lignin. The first approach provide around 70% brightness
and retain high pulp yield, while second approach provide 90% + brightness but reduces pulp
yield.
BLEACHING STAGES AND SEQUENCES
SYMBOLS STAGE AMOUNTTIME pH CONSISTENCYTEMPERATURE DESCRIPTION
A Acid WashTo remove metal
element from pulp
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B
Boron
Hydride
NaBH4
C Chlorination5 -7% on
pulp
40
min 2 3 - 4% 20 - 25 0C
Elemental chlorine (Cl2)
is an effective
de-lignifying agent. As it
breaks lignin bonds, it
adds chlorine atoms to
the lignin degradation
products, thus
producing significant
amounts of chlorinated
organic material.
DChlorine
Dioxide
0.6-1.0%
on pulp
180
min
3.5 -
4.010 - 12% 60 - 80
0C
Chlorine dioxide (ClO2)
is a highly selective
chemical that can both
de-lignify and brighten
pulp. It oxidizes lignin,
but does not add
chlorine atoms onto
lignin fragments;
however, small amounts
of elemental chlorine
and other chlorine
compounds formed
during the chlorine
dioxide bleaching
process react with
degraded lignin to form
chlorinated organic
compounds.
CD or C/D
or D/C
Chlorine and chlorine
dioxide added together
EAlkaline
Extraction
3 -4% on
pulp
120
min12 10 - 20% 45 - 95
0C
To remove colored
components from
partially bleached pulps
that have been rendered
soluble in dilute warm
alkali solution.
EOAlkali extraction
reinforced with oxygen
EP
Alkali extraction
reinforced with
hydrogen peroxide
EOP
Alkali extraction
reinforced with oxygen
and hydrogen peroxide
FFormamidine
Sulfinic Acid
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HSodium
Hypo-chlorite
1% on
pulp11-11.5 3 - 15% 30 - 60
0C
Sodium hypochlorite
(NaOCl) is an
inexpensive
de-lignifying agent
formed by mixing
elemental chlorine with
alkali at the mill.
M
Chlorine
Monoxide orHypochlorous
NNitrogen
Compounds
O Oxygen1-5 - 2.0%
on pulp
60
min>7 10 - 15% 85 - 95
0C
Oxygen removes lignin
and modify other
coloring components. In
the oxygen
delignification/bleaching
stage the pulp is treated
with oxygen in apressurized vessel at
elevated temperature in
an alkaline
environment.
P Peroxide
2 -3 % on
pulp
1 -2
hr 10 - 30% 65 - 800
C
Hydrogen peroxide
(H2O2) is mainly used
to brighten pulps in the
final bleaching stages.
Peroxide is often used
at the end of a
conventional bleachingsequence to prevent the
pulp from losing
brightness over time.
Preferred for
mechanical and
recycled fiber.
Paa
Peracetic
Acid,
CH3COOOH
Q Chelatin
To control the
brightness restrictingand reversion effects of
iron salts and other
heavy metals in the
pulp.
W Wash
Pulp is washed almost
every bleaching stage to
remove reactants of
preceding stage.
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X Xylanase
Xylanase-based
enzymatic pretreatment,
in a TCF (Totally
Chlorine Free)
sequence, results in
easier bleaching and
delignification of the
pulp, causing a bleach-
boosting effect.
YSodium
Hydrosulfite
0.5 - 1.2%
on pulp
35-
60
min
5.5 5 - 8% 60 - 750C
Reductive bleaching.
Good for recycled
fibers.
Z Ozone0.1 - 1.0%
on pulp2.5 5 - 15% 90 brightness.
2.
Classical Bleaching Sequences
H
CEH
CEHH
CEHEH
CEHHD
CEHDH
CEDED
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CEHDED
CEDH
ECF
DED
DEDED
(ZD)EopDD
ODEOPDD
TCF
OAPP
OZEPY
OXZEPY
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