LUENTO 2 (11.9.)
Chlorine dioxide as a bleaching agent
„
Chlorine dioxide bleaching
„
ClO2 is a radical bleaching agent
ClO2 is manufactured by several methods:
„
„
„
„
ClO2 is an unstable gas and therefore it must be
stored in solution.
„
Less AOX load than in chlorine bleaching,
although there is chlorine present also in chlorine
dioxide bleaching.
„
Process conditions:
„
pH
„
pre-bleaching stage pH 3-4
„
final bleaching stage pH 9-10
„
„
„
Temperature
„
(40-85 ºC, often 70 ºC)
„
Time
„
Dosage
Consistency
„
„
(long reaction time, 3-5 hours)
7-10 g/l solution water
Chlorine dioxide as a bleaching agent
„
Non-phenolic lignin
Phenolic lignin
Mathiesson process
R3/SPV generator
Etc.
„
ClO2 is an oxidant and it is reduced during
bleaching reactions.
Chemistry of chlorine dioxide bleaching is
complex since due to the reduction there are
several intermediates present during the process:
Chlorate
chlorine dioxide
Chlorite
Hypochlorous acid
Chlorine
Chloride
ClO3ClO2
ClO2HClO
Cl2
Cl-
(+5)
(+4)
(+3)
(+1)
(0)
(-1)
1
LUENTO 2 (11.9.)
Reactions with wood compounds
I
Lignin
„
II
Reactions with lignin
„
Reacts with phenolic and non-phenolic lignin
structures
Carbohydrates
„
III
IV
Not particularly reactive with carbohydrates
As chlorine dioxide oxidizes lignin it is
reduced to chlorite. Depending on pH
chlorite (ClO2-) is partly converted to
chlorous acid (HClO2) (pK1~2).
Hexenuronic acid groups
Extractives
„
K1
Reacts and removes wood extractives
especially during the pre-bleaching (low pH)
Conversion of chlorite ion to
chlorine and hypochlorous acid
[ClO ]⋅ [H O ]
=
−
2
+
3
[HClO 2 ]
ClO2 and lignin
„
„
„
ClO2 reacts readily with non-phenolic
lignin structures.
Chlorine dioxide reacts even more easily
with phenolic lignin and ionized lignin
structures.
Neither chlorite nor chlorous acid are
reactive toward lignin.
] [
]
Lign phenolic
d [ClO2 ]
= −k1 Lignnon − phenolic − (k 2 + k 3 K 2 / H 3O + ) ⋅
dt
1 + K 2 / H 3O +
[
]
[
[
]
2
LUENTO 2 (11.9.)
ClO2 and lignin
R
Reactions with lignin
R
„
O
R
OR'
.
OH
O
OR'
ClO2
OR'
H
R
[
R
O
OCl
O
R
O
-ClO-
OR'
R
O
OR'
.
O
R
ClO2
„
R
OR'
OH
OR'
HO-
-ClO2-
ClO2
OR
O
HO-, ClO3 -
OR
[HOCl ]⋅ [H 3O + ]⋅ [Cl − ]
[Cl 2 ]
HOCl + H2O ' ClO- + H3O+
K4 =
(1)
ClO2 and lignin
Cl2 + 2H2O ' HOCl + H3O+ + Cl-
K3 =
ClO2
OR'
ClO2 and lignin
[ ]
Formation hypochlorous acid, chlorine dioxide,
chlorate, chloride, etc. Hypochlorous acid is in
balance with hypochlorite (pK4 ~ 7.5) and
chlorine.
R
O
O
]
d ClO2−
2
= − k 4 [HClO2 ] − k 5 [HClO2 ] ⋅ Cl −
dt
The reaction is followed by several other
reactions
O
O
-ClO2-
R
O
OClO
OR'
R
ClO2
In acidic conditions chlorite is decomposed via
formation of chlorous acid. Reaction is partly
catalyzed by chloride ions.
[ClO ]⋅ [H O ]
−
+
3
[HOCl ]
(2)
„
(3)
„
(4)
(5)
Both electrophiles (HOCl, Cl2) and
nucleophiles (ClO-) are formed during the
chlorine dioxide bleaching stages.
The final pH of D0 stage is typically 2-3
and ClO2 exists mainly as chlorine. During
other D stages the final pH is so high (4-5)
that the proportion of chlorine is low.
3
LUENTO 2 (11.9.)
ClO2 and lignin
Chlorination and hydroxylation
„
ClO2 and lignin
Chlorination and hydroxylation
d [LignCl ]
= k 6 [Lign] ⋅ [Cl 2 ]
dt
Reactions of chlorine with lignin lead to
chlorination, whereas hypochlorous acid
increases the amount of (phenolic)
hydroxyl groups in lignin
d [LignOH ]
= k7 [Lign]⋅ [HOCl ]
dt
(6)+(7)
Or also
d [LignCl ]
k [Cl ]
= 6 2
d [LignOH ] k 7 [HOCl ]
[
][ ]
d [LignCl ] k 6 H 3O + ⋅ Cl −
=
d [LignOH ]
k7 K 3
(6)
(7)
(8)
(9)
hypochlorous acid=alikloorihapoke
ClO2 and lignin
Chlorination and hydroxylation
„
„
The degree of lignin chlorination is linearly
dependent on the hydrogen and chloride
ion concentrations
Increasing chemical dosage increases the
amount of chloride present in the chlorine
dioxide stage.
Degree of chlorination increases.
AOX load increases
ClO2 and HexA
„
„
„
„
ClO2 is a very selective lignin oxidant.
Reactions with carbohydrates are limited.
Chlorine dioxide is able to react with
hexenuronic acid groups of xylan.
Degradation products of HexA are
chlorinated and hydroxylated dicarboxylic
acids.
4
LUENTO 2 (11.9.)
Reactions of hexenuronic acid groups
O
OH
High pH
OH
OH
O
Low pH
O
OH
„
O
„
HO
OH
HO
Cl
OH
OH
O
OH
O Cl
OH
„
OH
HO
OH
O
O
OH
O
HO
Reactions of hexenuronic acid
groups
Cl
OH
AOX
Chlorine or chlorous acid reacts first with
electrophilic carbon-carbon double bonds.
Chlorine dioxide does not oxidize hexenuronic
acid groups.
In a normal chlorine dioxide stage both lignin and
HexA contents decrease in a same way. If the
final pH is above 6, HexA doesn’t react because
under these conditions chlorite is not able to
degrade into hypochlorous acid and chlorine.
ClO2 and extractives
„
„
„
Chlorine dioxide reacts with unsaturated
lipophilic extractive (i.e. resin) components.
ClO2 favours oxidation reactions. The reaction
products of resin comprise a mixture of oxidized
products containing hydroxyl, carboxyl or
carbonyl groups.
The reaction products are more hydrophilic than
the original compounds and therefore they are
easier to wash out of the pulp.
5