How to Improve Brightness Stability of ECF bleached Softwood and ...

Paper presented at 2004 APPITA Conference, Canberra
How to Improve Brightness Stability of ECF bleached
Softwood and Hardwood Kraft Pulp
HANS ULRICH SUESS, KURT SCHMIDT, BERND HOPF; Degussa AG, Hanau, Germany
1. ABSTRACT
A comparison of different ECF bleaching sequences regarding the resulting brightness stability
shows an improved stability for brighter pulps and no direct influence of ozone, hot acid
hydrolysis or hot peroxide stages. The brighter the pulp, the more stable brightness becomes.
This is generally valid for Kraft pulps, provided removal of double bonds is effective
and the final Kappa number is below 1.
However, the final bleaching stage and the conditions used in this stage do affect brightness
stability. A pulp bleached in its final stage with chlorine dioxide (D 2 ) more likely loses brightness
in ageing than pulp bleached with a final P stage. Bleaching with chlorine dioxide
leaves quinoid structures in the pulp, with a pronounced tendency for reversion. The oxidation
of quinones is easily accomplished with alkaline hydrogen peroxide, therefore brightness
stability is significantly improved with a final P stage.
Very high temperature in a D stage improves the reversion results. Long retention time is
beneficial as well. Reversion is lowest following a hot D 1 and a final P stage.
2. MATERIALS and METHODS
All trials were made with industrial pulp samples taken after oxygen delignification. D and P
stages were run in plastic bags in water baths. Ozone was added to well fluffed pulp in a
fluidized bed reactor; Eop stages were conducted in a pressurized high-shear mixer. All trials
(except ozone) were made at 10% consistency. Brightness was measured with ISO 2470.
Reversion testing was made with handsheets prepared at pH 6 on a Buchner funnel with a
weight of 280 g/m², Tappi methods UM 200 and T 260 were used.
3. INTRODUCTION
The target of bleaching is a high and stable brightness. Unfortunately, in reality pulp brightness
decreases during storage and paper production. It is therefore important to keep the
intensity of brightness reversion low. Light induced reactions do play only a minor role; these
are limited to the pulp bale's surface. In the presence of light transition metals and oxygen
can cause a chain of oxidation processes generating chromophores [1]. However, these reactions
cannot be of importance inside a pulp bale. Therefore this paper will focus on reversion
in the absence of light.
Inside a pulp bale reversion is the result of chromophores generated by condensation reactions.
This requires available active sites. These can be present as the consequence of poor
washing, and indeed improving washing with cold or hot water has a positive effect. However,
washing alone does not solve a reversion problem.
The conditions required for the oxidation of lignin can cause an oxidation of the cellulose.
However, the content of carbonyl groups in ECF bleached pulp does not correlate with reversion.
This was the result of an analysis with the "CCOA" method [2]. Different ECF sequences
had very little variation of the carbonyl content but different levels of brightness stability.
All results described in this paper are based on ECF bleaching. This method represents
more than 90% of world's bleached pulp production.

2
An analysis of brightness reversion requires reasonable testing conditions. Tappi's "UM 200"
describes ageing under moderate conditions, namely a four hours treatment at 105°C in an
oven. This removes water rapidly and reversion reactions requiring water will not take place.
Therefore this method produces only moderate differences. Another option is the former
Tappi test T 260, a test over boiling water for 2 hours. This moist method produces data
which correlate better with natural reversion [2].
The different test methods are compared in Figure 1. The pulp bleached finally with chlorine
dioxide (D 0 -Eop-D 1 -D 2 ) loses significantly more brightness points compared with the peroxide
bleached pulp (D 0 -Eop-D 1 -P). Because the handsheet's pH has an impact, all other tests
were made with a correction of the pH to 6.
8
7
D: T 260 D: UM 200
P: T 260 P: UM 200
6
losses (points)
5
4
3
2
1
0
5 6 7 8
pH of handsheet
Fig. 1: Impact of aging conditions on reversion of pulp bleached with a final D 2 stage or a
final P stage using different handsheet pH and reversion conditions; UM 200 at 105°C, dry,
4h; T 260, 100°C, 100% humidity, 2h, over boiling water), brightness ahead of testing was
>91 %ISO
__________________________________________________________________________________________________________________________________________________________________
4. COMPARISON of SEQUENCES
4.1 "STANDARD" ECF BLEACHING
Bleaching removes double bonds. This decreases the ageing potential. Brightness reversion
should be higher for a pulp with an incomplete removal of "lignin". However, things are a bit
more complicated. For example, TCF pulps do not have very poor reversion properties despite
of a relative high residual of double bonds. Thus the kind of "double bond" seems to be
of importance as well.
Table 1: "Standard" conditions in ECF bleaching, all at 10% consistency
stage
time
(h)
temperature
(°C)
pH
(begin - end)
D 0 1 50

3
In order to generate a "first base" the following assumption was made: Bleaching with moderate
conditions, intense enough to remove lignin and other double bonds but not too aggressive
to harm the cellulose should produce good brightness stability. Table 1 lists the
"standard" conditions used for this "mild" ECF bleaching. The sequence has neither aggressive
bleaching chemicals nor drastic conditions. Radical side reactions should be avoided as
much as possible.
90
88
86
84
82
80
brightness (%ISO) T260 UM200 bleached
78
1 0,5 P 1,5 0,5 P 2 0,5 P
numbers: D1 + D2 act. Cl in (%)
Fig. 2: Aging of pulp with UM 200 and T 260 after the stages D 1 or D 2 or P in the sequences
D 0 -Eop-D 1 -D 2 or D 0 -Eop-D 1 -P; D 0 with Kappa factor 0.23; see Table 1 for other conditions
_________________________________________________________________________
Bleaching with these conditions generates the reversion results of Figure 2. Following a D 0
stage with high Kappa factor (0.23) the use of increasing amounts of active chlorine in D 1
logically has a positive impact on brightness. These are the observations:
● Brightness is high after just three bleaching stages. However, reversion loss is severe,
especially with the T 260 test.
● Doubling of the D 1 active chlorine charge from 1% to 2% gives a small brightness gain of
just 1.1 point. However, the impact on reversion is positive, losses decrease significantly.
● The application of the D 2 stage has an even more positive impact on brightness and it's
stability. The same amount of active chlorine (e. g. 1.5% to D 1 ) split into two portions (e. g.
1+0.5% to D 1 +D 2 ) gives more than one additional brightness point.
● Cleaning the pulp by oxidation and washing is important for achieving a low reversion.
● Substitution of the D 2 stage with a P stage (replacing 0.5% act. chlorine with 0.25% H 2 O 2 )
results in about the same brightness. However, the peroxide bleached pulp is much less
sensitive to reversion. Losses decrease significantly. Obviously peroxide more effectively
than chlorine dioxide removes compounds important for reversion.
The post colour number [3] is another useful tool to identify differences in reversion. This
method does not compare losses by points of brightness, it uses reflectance and light scattering.
If expressed as post colour number differences become more visible. In Figure 3 the
T 260 test data of Figure 2 are shown as post colour values. The big improvement in stability
with the final P stage is visualised. In combination with the final P stage a very low input of
chlorine dioxide to the D 1 stage is sufficient to achieve a very good final stability.

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2,5 post colour # D1-P
2
D1-D2
D1
1,5
1
0,5
0
1 1,5 2
act Cl in D1 (%)
Fig. 3: Comparison of brightness stability after the T 260 test as post color number
brightness (%ISO)
88
86
84
82
T 260
UM 200
bleached
80
78
D1-D2
D1-P
Fig. 4: Brightness stability of softwood Kraft pulp; bleaching of oxygen delignified pulp with
the sequence D 0 -Eop-D 1 -D 2 or D 0 -Eop-D 1 -P; reversion with T 260 and UM 200
_________________________________________________________________________________________________________________________________________________________________
These effects are valid as well for softwood Kraft pulp. Figure 4 shows an identical reversion
pattern for an oxygen delignified pine pulp after D 0 -Eop-D 1 -D 2 or D 0 -Eop-D 1 -P bleaching.
Losses are higher after the D 2 stage and lower with a final P stage.
➨ Brightness stability improves with higher use of more chemical in D 1 . This becomes very
obvious in the post color numbers.
➨ Following D 1 a final P stage generates the better stability compared with a D 2 stage.
4.2 ECF BLEACHING with AGRESSIVE STAGES
Modern bleach plants frequently use aggressive process conditions. Hardwood pulp mill operate
their D 0 stage above 90°C. The same temperature level is used in peroxide bleaching.
Some mills apply ozone, a chemical with a very high oxidation potential. These conditions
can cause cellulose oxidation which in turn might trigger more brightness reversion.

5
Figure 5 has the results of bleaching with a hot D 0 stage. Two different amounts of chlorine
dioxide were applied in the hot D 0 stage. There is no visible impact on final brightness stability.
Losses in reversion do not differ from the results achieved with the "conventional" ECF
sequence. Again losses are lower for the pulps bleached with a final P stage. Similarly the
more aggressive conditions of a P(O) stage (95°C instead of 80°C in P) do not affect reversion
(Figure 6). The P(O) stage decreases viscosity, however, the lower viscosity has no
impact on aging. On the contrary, the higher input of peroxide to the P(O) stage moderately
improves brightness stability.
90
88
86
84
brightness (%ISO)
82
80
T 260
UM 200
bleached
78
D1-D2 D1-P D1-D2 D1-P
D 0
factor: 0.15 0.23
Fig. 5: Aging of eucalyptus Kraft pulp bleached with hot D 0 -Eop-D 1 -D 2 or -D 1 -P; active chlorine
in D 1 at 2% with low D 0 factor and 1% at high factor; final P stage with 0.25% H 2 O 2
brightness (%ISO)
90
88
86
84
82
80
78
T 260
UM 200
bleached
P P(O) P P(O)
Fig 6: Comparison of reversion following moderate or aggressive conditions in final bleaching
with H 2 O 2 ; sequence hotD 0 -Eop-D 1 -P or D 1 -P(O); P with 0.25% H 2 O 2 at 80°C; P(O) with
0.5% H 2 O 2 at 95°C
__________________________________________________________________________________________________________________________________________________________________
Similarly the use of lower or higher amounts of ozone has no negative impact on reversion.
Figure 7 has the results of reversions tests following bleaching with the sequence Zp/E-D-P.
Viscosity of these pulps is low. However, the only obvious effect is a very good stability of the
brightness. The high input of peroxide in the final P stage has a positive impact. In addition
the old truth is confirmed, the brighter the pulp, the more stable it will be. With a high input of
bleaching chemical, of ozone and chlorine dioxide and peroxide the resulting brightness is
very high and extremely stable.

6
The conclusion is simple. Obviously a high intensity of the bleaching process has a positive
impact on stability and aggressive conditions do not have a negative impact on reversion.
90
88
86
brightness (%ISO)
84
82
80
T260
UM 200
bleached
78
O 3 (%): 0.5 0.65 0.65 0.65 0.65
ClO 2 (%): 1.5 1.0 1.0 1.5 1.5
H 2 O 2 (%): 1.5 0.5 1.0 0.5 1.0
Fig. 7: Reversion following bleaching with the sequence Zp/E-D-P; impact of different
amounts of O 3 , ClO 2 and H 2 O 2
__________________________________________________________________________________________________________________________________________________________________
5. REASONS for REVERSION
Hexenuronic acids have a prominent position among the different theories about the causes
for reversion. Tenkanen [4] or Gellerstedt [5] describe hexenuronic acids as the main source
of reversion. This is hard to understand. As we have seen, reversion of ECF softwood and
hardwood pulp seems to follow a similar pattern. However, softwood pulp always has a lower
content of hexenuronic acids. Bleaching sequences with hot acid hydrolysis or an ozone
treatment will possibly remove hexenuronic acids to a higher extend compared with conventional
sequences. However, there was no difference visible between the reversion of pulp
bleached with our "standard" or with modified ECF conditions.
To analyse the potential impact of hexenuronic acids a pulp bleached with the sequence D 0 -
Eop-D 1 -P ("standard" conditions, Kappa factor 0.26 in D 0 ) was in addition treated for 2 hours
at pH

7
Figures 8 and 9: Simplified reaction schemes for chlorine dioxide and hydrogen peroxide
(perhydroxyl anion) with phenol derivatives to oxidation products and destruction of a quinone
with alkaline peroxide
6 losses (points) 0.5 h 0.5 h 0.5 h 2 h °C
80
5
65
50
4
3
2
0,15 0,3 0,3 + P 0,3 + P
NaOH (%)
Fig. 10: Impact of E or Ep conditions on brightness reversion (T 260) of an D 0 -Eop-D 1 pretreated
pulp. Pulp brightness after D 1 89.6; P = 0.25% H 2 O 2
__________________________________________________________________________________________________________________________________________________________________
A final peroxide stage needs temperature and alkalinity to be effective. The reaction with
chromophores or precursors of chromophores is rather fast. The conditions required for effective
quinone destruction become visible in Figure 10. The presence of peroxide is important,
alkaline conditions only are insufficient. At 80°C the reaction with chromophores or their
precursors needs only 30 minutes to reach the optimum. Lower temperature can be compensated
to a certain extent with longer retention time.
The importance of an alkaline peroxide treatment for quinone removal explains the good
brightness stability of TCF bleached or ECF"light" pulps. These bleaching processes operate
with very intense P stages. The complete degradation of all quinoid structures leaves very
little potential to form new chromophores in the remaining lignin. It secondly explains why

8
neither Tenkanen nor Gellerstedt could detect quinones as reversion source, their ECF
"light" or full TCF pulps had had very intense P stages and thus a complete elimination of
these structures. Instead in the reversion test hexenuronic acids, which are completely unaffected
by a peroxide treatment, showed up as remaining culprit.
The results allow an interpretation of the effects achieved with a final bleaching stage with
peracetic acid. This chemical is in technical use only in Sweden and Finland. It is advertised
as a final brightness booster. Table 2 shows the results of a post-treatment simulating a high
density storage treatment with peracetic acid. High temperature is required to increase
brightness, however, this brightness is not very stable. This becomes visible in the post color
numbers.
Table 2: Post-treatment of a softwood TCF pulp with 0.5% distilled peracetic acid, simulation
of a high density storage treatment at 10% consistency for 3 h
temp. in Paa
(°C)
brightness
(%ISO)
PC #
initially 86,7 0.632
Paa 50 88,1 0.762
Paa 65 88,5 0.695
Paa 80 88,9 0.650
Paa/P* 80 89,5 0.483
* 0.5 h with Paa, 2.5 h at alkaline pH with H 2 O 2
During its reaction with the remaining lignin peracetic acid hydroxylates phenols. If further
oxidized the resulting hydroquinones will easily form chromophores. Thus a Paa stage gives
a limited improvement. However, it can be improved easily with an additional alkaline peroxide
step. This degrades hydroquinones and quinones. The modification from just a Paa
stage to a Paa/P treatment produces an even higher and more stable brightness. Peracetic
acid is a good tool to activate a P stage. It is not the chemical of choice for a better brightness
stability.
6. OPTIONS to IMPROVE BRIGHTNESS STABILITY
These data imply the use of a final treatment with alkaline hydrogen peroxide as the only
available option to improve reversion. However, before this conclusion is drawn, it is necessary
to analyse the potential to improve the chlorine dioxide treatment. Conditions generating
less quinoid structures would certainly be favourable. An option is the application of more
chlorine dioxide. This intensifies oxidation and removal of potential chromophores. It is the
simplest way to improve results (see Figure 2).
The stabilisation of the pH in a D stage has a positive effect. However, in case higher
amounts of chlorine dioxide should be reacted, it is impossible to stop the pH from falling.
There are too many acidic compounds generated by the oxidation process. Another approach
could be chemically. Quinones aren't very stable molecules. Provided the temperature
is high enough they will react further, either by decomposition or as strong oxidisers. A
constant charge of chlorine dioxide is consumed at higher temperature more completely and
produces a better D 1 stage brightness and reversion. Figure 11 shows the impact of the temperature
increase from 60°C to 80°C. It is hard to imagine that the moderate increase in ClO 2
consumption would be responsible for the improvement. Other reactions, like the oxidation of
intermediately formed quinones will have to contribute. The positive impact of these additional
reactions is still visible in brightness and reversion improvements after a final P stage.
The post color number decreases with the higher temperature in D 1 from 1.4 to 0.9. The P
stage decreases the post color # significantly (Figure 12) further to 0.33 and with the high D
stage temperature to only 0.27.

9
0,2
88
0,1
87
0
0,3 residual (%) 60 70 80
60 70 80
temp (°C)
86
89 brightness (%ISO) 60 70 80
1,5 PC #
1,25
1
0,75
0,5
0,25
temp (°C)
0
temp (°C)
Fig. 11: Impact of temperature on chlorine dioxide consumption, brightness and post color #
in a in D 1 stage (T 260); softwood Kraft pulp; Eop Kappa 2.2, constant: 1.5% active chlorine,
10% cons., 2h
90
brightness (%ISO)
D1 P PC #
final post color #
0,6
89
0,4
88
0,2
87
60 70 80
temp. in D1 (°C)
Fig. 12: Impact of temperature in the D 1 stage on brightness and reversion after a final P
stage, reversion test T 260; P stage with 0.25 % H 2 O 2 , 0.3% NaOH, 80°C, 1.5h, 10% cons.
brightness
PC#
post color #
0
0,5
92 brightness (%ISO) 0,2
91
0,4
90
0,3
89
2 4 6
time (h)
Figure 13: Impact of retention time in a D 2 stage. Softwood Kraft pulp, bleached with D 0 -
Eop-D 1 -Ep, D 2 charge constant at 0.5% act. chlorine, 80°C, 10 % cons.

10
The effects become even more pronounced with longer retention time. This was tested in a
five-stage sequence (D 0 -Eop-D 1 -Ep-D 2 ) for the D 2 stage (Figure 13). Because chlorine dioxide
is consumed after about two hours, the impact on brightness is very small. However, the
longer the pulp is kept at 80°C, the less reversion takes place in aging.
The use of very high temperature in chlorine dioxide delignification was first described by
Lachenal [8]. Based on the data of Figure 11 it was only consequent to apply such conditions
in D 1 and D 2 stages. Bleaching a softwood Kraft pulp with the sequence D 0 -Eop-D 1 -E(p)-D 2
at 70°C or 90°C resulted in improved brightness and post colour numbers (Figures 14 and
15). The benefit is very significant for reversion. The advantage of H 2 O 2 addition in the extraction
stage between the D stages stays visible not only in a higher brightness but in addition
in a decrease of the reversion. Logically, as result of new "quinones" formed in the final
D stage the drop is less pronounced compared with a final P stage.
92
70°C
90°C
90°C
90°C
91
90
89
ED2
EpD2
88
87
2+0.5 1.5+0.5 2+0.5 3+0.5
act. Cl, D1+D2 (%)
Fig. 14: Impact of hot D stages (D 1 and D 2 ) in softwood pulp bleaching with D 1 -E-D 2 or D 1 -
Ep-D 2 ; retention time constant at 4 hours in each D stage, all trials at 10% cons.
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0
post colour #
70°C
90°C
90°C
90°C
2+0.5 1.5+0.5 2+0.5 3+0.5
act. Cl, D1+D2 (%)
Fig. 15: Impact of high temperature on post color number (T 260 test)
__________________________________________________________________________________________________________________________________________________________________
The advantage of the addition of H 2 O 2 in the extraction stage between the D stages is visible
not only in a higher brightness but in addition in a decrease of the reversion. Logically, as
result of new "quinones" formed in the final D stage the decrease is less pronounced compared
with a final P stage. Nevertheless, the high D stage temperature obviously degrades
compounds in the remaining "lignin" which trigger reversion. This becomes visible in a comparison
of the reversion results using the aggressive T 260 test and the more moderate UM
200 test. Typically a low temperature (around 70°C) D bleached pulp loses more brightness
points in the T 260 test compared to the UM 200 test. The high temperature D stage evens
EpD2
ED2
93 brightness (%ISO) D1-
D1-

11
out these differences. High temperature obviously degrades potential chromophores (Figure
16). The hot conditions do not affect the pulp's viscosity. There is a very small increase of the
COD, indicating a minor drop in yield by acid hydrolysis, however, the increase is below 2
kg/t, thus at about 0.1% yield.
0
brightness losses (points)
-0,5
-1
-1,5
-2
-2,5
-3
-3,5
70°C
70°C
90°C 90°C
DP DEpD DP DEpD
UM 200
T 260
Fig. 16: Brightness losses in reversion after bleaching softwood Kraft pulp with two or three
final stages (-D 1 -P or -D 1 -Ep-D 2 ) at normal or very high temperature in the D stages; constant
2% act. Cl in D 1 , 0.5% in D 2 , P: 0.5% H 2 O 2 , Ep: 0.25% H 2 O 2
1,2
1
0,8
post colour #
temp in D1/D2 (°C)
90/90 75/90
75/75 P
0,6
0,4
0,2
0
P 2 4 6
time in D 2 (h)
Fig. 17: Impact of time and temperature in D 1 and D 2 stages on post color #; bleaching of
eucalyptus Kraft pulp, sequences D 0 -Eop-D 1 -D 2 or D 0 -Eop-D 1 -P; Eop Kappa 3.6, brightness
81.3 %ISO; D 1 : constant 2% act. Cl, 2h; D 2 : 0.5% act. Cl, time variable, P with 0.25% H 2 O 2
__________________________________________________________________________________________________________________________________________________________________
Additional retention time at very high temperature decreases the post color number further.
Figure 17 shows the impact of time and temperature for a eucalyptus Kraft pulp. Sequences
were D 0 -Eop-D 1 -D 2 or D 0 -Eop-D 1 -P. The potential chromophores are effectively destroyed by
high temperature in the final D stage. Even at the very high temperature of 90°C, an obvious
advantage of an extended retention time can be seen. However, even after six hours at 90°C
in the D 2 stage the post color value remains higher than the value achieved in a final P stage.
This positive effect of high temperature in a D stage on brightness stability is nothing new. It
is described in literature. It is not mentioned by Rapson [9] or Reeve [10] in the "recent" descriptions
of chlorine dioxide bleaching technology. However, it is cited by Rydholm [11]. As
early as 1955 Harrison and Calkin reported the positive impact of very high temperature in a

12
D stage on brightness stability [12]. Apparently they did not publish - as announced - further
work. The obvious difficulty to run a D stage in a mill under the conditions described might
have stopped further work. An application of a hot chlorine dioxide stage is not easy. There
are practical obstacles. Many chlorine dioxide towers are covered with ceramic tiles to prevent
corrosion. The limited stability of the adhesives and the joints typically restricts the temperature
of operation to about 80°C. Very long retention time is another problem. In the huge
mills built nowadays the dimensions of a tower with several hours retention time would be
extreme.
7. SUMMARY and RECOMMENDATIONS
● In ECF bleaching no negative impact of the bleaching process (chemical and temperature)
on brightness stability exists. Quinoid structures seem to be the compounds most important
for reversion.
● The elimination of these remaining quinoid structures is easily achieved with a final P
stage.
● A final P stage generates a very stable brightness.
● Chlorine dioxide bleaching gives lower brightness stability. Very high temperature in the D
stage can improve results, however, the conditions required are impractical.
Very high brightness and good brightness stability will result with these sequences:
● Hardwood pulp: With three stages like Zp/E-D-P or more conventional with four stages
(hot) D 0 -Eop-D 1 -P.
● Softwood pulp: Top levels are achieved with four stages (D 0 -Eop-D 1 -P), very high values
are more easily reached with five stages (D 0 -Eop-D 1 -Ep-D 2 ).
8. REFERENCES
[1] K. Fischer; Vergilbung von Hochausbeutezellstoff, Papier, 44(10A) V11 (1990)
[2] A. Potthast, J. Röhrling, T. Rosenau, P. Kosma, H. Sixta; Determination of carbonyl group
profiles in cellulosics by fluorescence labeling: Novel application of the "CCOA" method, 12 th
ISWPC 2003, proceedings I, 147 - 150 Madison, WI
[3] J.-E. Levlin, L. Söderhjelm, Pulp and Paper Testing, p 128-129, ISBN 952-5216-17-9,
Fapet Oy, Helsinki (1999)
[4] M. Tenkanen, I. Forsskåhl, T. Tamminen, M. Ranua, K. Vuorenvirta, K. Poppius-Levlin;
Heat induced brightness reversion of ECF-light bleached pine kraft pulp; 7 th European Workshop
on Lignocellulosics and Pulp; Proc. 107 - 110 (2002)
[5] G. Gellerstedt, O. Dahlman, Recent hypothesis for brightness reversion of hardwood
pulps; International Colloquium on Eucalyptus Kraft Pulp, Unversidade Federal de Viçosa,
Viçosa, MG, Brasil, Sep. 2003, proceedings
[6] A.-S. Jääskeläinen, A.-M. Saariaho, P. Matousek, A. Parker, M. Towrie, T. Vuorinen;
Characterization of residual lignin structures by UV Raman Spectroscopy and the possibilities
of Raman spectroscopy in the visible region with Kerr-gated fluorescence rejection; 2003
ISWPC, Madison, WI, proceedings 139 - 142
[7] J. Gierer; The Chemistry of Delignification; Holzforschung 36(2) 55 - 64 (1982)
[8] D. Lachenal, C. Chirat; High temperature chlorine dioxide delignification: A breakthrough
in ECF bleaching of hardwood Kraft pulps; 1998 Tappi Pulping Conf. Proceedings, 601 - 604
[9] W. H. Rapson, G. B. Strumila; Chlorine dioxide bleaching; p 113 f; in: The Bleaching of
Pulp, ISBN 0-89852-043-6, Standard Press, Atlanta 1979
[10] D. W. Reeve, Chlorine dioxide in bleaching stages, p 379- 394 in: Pulp bleaching: principles
and practice; ISBN 0-89852-063-0, Tappi Press, Atlanta 1996
[11] S. Rydholm, Pulping Processes, Interscience Publishers ISBN 0 471 74793 9, p. 983
(1965)
[12] W. D. Harrison, C. R. Calkins; A study of variables affecting chlorine dioxide bleaching of
semibleached sulphate pulp, Tappi J. 38 (11) 641 - 648 (1955)