Note: Descriptions are shown in the official language in which they were submitted.
_- 1 1 337843
A PROCESS FOR OXYGEN BLEACHING
Field and Background of the Invention
The present invention relates to a process for oxygen bleaching
fibrous cellulose material.
In an article entitled "Oxygen bleaching kinetics at ultra-low
consistency", Tappi Journal, December, 1987, by C.L. Hsu and
Jeffrey S. Hsieh, it is described experiments with oxygen bleach-
ing to study the influence of temperature on the viscosity. The
results show that a high temperature in the initial stage of the
oxygen bleaching has a negative effect on the viscosity.
However, in order to reach the normal range of kappa number in
oxygen bleaching it is not possible to use a reaction temperature
which is too low. For this purpose temperatures of about 100C are
necessary, or special measures must be taken. Temperature control
has thus been proposed to be carried out in oxygen bleaching pulp
of medium consistency, see the article entitled "Improvement of
medium consistency oxygen bleaching through temperature control"
by C.C. Courchene and Y.L. Magnotta, 1984, "Oxygen Delignifica-
tion", pages 11 to 15. The oxygen delignification was performed in
a horizontal tube reactor of laboratory size, to which steam was
supplied at several points along the reactor in order to create
two or more zones in which different temperatures could be main-
tained, the first zone having the lowest temperature. The experi-
ments indicated that the temperature control with a low initial
temperature in the horizontal tube reactor resulted in an improved
yield and improved viscosity with the same retention time. How-
ever, the temperature control described cannot be applied with any
great success on an industrial scale because of the difficulty in
achieving an exact temperature limit between two temperature
zones. This is partly due to the difficulty of efficiently and
quickly mixing steam into the pulp in a uniform manner and also to
1 337843
2 27231-10
the fact that the horizontal tube reactor, which contains a screw
for feeding the pulp, has an upper space which is not filled by
pulp but will instead contain a steam phase which disturbs the
temperature control and extends along the entire length of the
horizontal tube reactor.
Summary of the Invention
The present invention seeks to entirely eliminate the
problems mentioned above and provide a process for oxygen
bleaching which enables industrial utilization of the concept of
having a lower temperature in the initial stage of the oxygen
delignification and which enables efficient adjustment and control
of the temperatures in the various delignification zones so that a
constant low temperature and a constant high temperature,
respectively, are obtained in the delignification zones without a
disruptive steam phase appearing above the delignification zones.
The invention relates to a process for oxygen bleaching
fibrous cellulose pulp comprising the steps of feeding the pulp
through a first vertical reactor containing a first
delignification zone with a predetermined low temperature, and
thereafter through a second vertical reactor containing a second
delignification zone with a predetermined hlgh temperature that is
higher than that in the first delignification zone, the
temperature in the first delignification zone being either
maintained at the temperature that the pulp entering for bleaching
has acquired during a previous treatment before the oxygen
bleaching, or as required being adjusted by the controlled supply
of steam to a mixer disposed in the pipe before the first reactor,
and the temperature in the second delignification zone being
B
2a 1 337843 27231-10
adjusted by the controlled supply of steam to a mixer disposed in
the pipe between said two reactors.
The invention further provides a process for oxygen
bleaching fibrous cellulosic pulp comprising, utilizing a first
vertical reactor having a first delignification zone, and a second
vertical reactor having a second delignification zone, wherein
said first delignification zone is comprised of a first mixer and
said first vertical reactor; and said second delignification zone
is comprised of a second mixer and said vertical second reactor;
and wherein said cellulosic pulp and chemicals are added at said
first mixer and passed directly from said first mixer to said
first vertical reactor and wherein reacted pulp and residual
chemicals from said first vertical reactor are passed directly
from said first vertical reactor to said second mixer and wherein
said reacted pulp and residual chemicals are passed directly from
said second mixer to said second vertical reactor further
comprising the steps of:
(a) feeding the pulp through the first delignification zone
of the first vertical reactor while maintaining a predetermined
low temperature within the range of 70C-90C, while practicing
oxygen delignification;
(b) after step (a), passing the pulp from the first
delignification zone to the second delignification zone without
any intervening filtration being performed during the passage;
(c) after step (b), feeding the pulp through the second
delignification zone of the second vertical reactor while
maintaining the pulp at a predetermined high temperature within
the range of 90C-125C and that is higher than said low
B
2b ~ 3378~3 27231-10
temperature by about 20C-40C, while practising oxygen
delignification;
(d) practising step (a) by introducing pulp into the first
delignification zone that has said predetermined low temperature;
and
(e) maintaining the pulp in the second delignification zone
at said higher temperature by mixing the pulp with high
temperature fluid during step (b).
3 1 3 3 7 8 4 3
Brief Description of the Drawings
The invention will be described further in the following with
reference to the drawings, in which
Figure 1 shows schematically a bleaching plant for carrying out
the process according to the invention.
Figure 2 is a diagram illustrating the relationship between kappa
number and viscosity.
Description of Illustrated Embodiment
The bleaching plant shown in Figure 1 is designed for oxygen
bleaching in two distinct stages and comprises a first reactor 1
and a second reactor 2. The pulp is supplied from a storage tank 3
to the first reactor 1 through a pipe 4 and by means of a pump 5
at the outlet of the storage tank 3, and a mixer 6, i.e. an appa-
ratus for mixing treating agents, into the pipe 4. The mixer 6
contains fluidization means to rapidly and homogenously mixing the
various additives into the pulp. The mixer is preferably a "Kamyr
MC mixer". The pulp to be bleached is thus of medium consistency,
i.e. about 6-15%.
The reactors 1 and 2 are connected to each other by a pipe 7 con-
taining a mixer 8 of the same type as that described above. The
oxygen-delignified pulp is transferred from the second reactor 2
to a blow tank 9 through a pipe 10.
A protective agent for the cellulose, e.g. MgS04, is added via a
pipe 11 at the outlet from the storage tank 3. An alkaline agent
such as NaOH or oxidized white liquor is supplied through a pipe
12 and oxygen gas through a pipe 13 to the first mixer 6 with high
mixing efficiency. Further, a pipe 14 for high-pressure steam is
connected to the mixer 6. Pipes 15, 16 supplying oxygen gas and
high-pressure steam, respectively, are connected to the second
4 1 337843
mixer 8. A pipe 17 may also be provided for the addition of
further alkaline agent to the pulp which has been oxygen-deligni-
fied in a first stage.
The bleaching plant also contains suitable measuring and control
means (not shown) for measuring the temperature in the two reac-
tors and controlling the supply of steam to the mixers to ensure
that the correct different temperatures are maintained in the two
reactors in accordance with the present invention.
The first reactor 1 comprises a first delignification zone with a
low temperature within the interval 70-90C, preferably 75-85C,
while the second reactor 2 comprises a second delignification zone
with a high temperature within the interval 90-125C, preferably
95-110C. The terms "low" and "high" thus relate to the mutual
relationship between the temperatures in the two delignification
zones. The temperature difference between the two delignification
zones shall be about 20-40C, preferably about 30C.
In such cases when the pulp entering for bleaching has a suffi-
cient temperature level, acquired in a previous treatment before
the oxygen bleaching, corresponding to the term "low temperature",
i.e. within the interval 70-90C, no steam need generally be sup-
plied to the first mixer 6 provided the temperature is constant
or substantially constant.
The following example illustrates the invention further.
Example
Oxygen delignification in two stages was carried out in a bleach-
ing plant as shown in Figure 1. The temperature was varied in
three different test series. In the first test the temperature in
the first stage (first delignification zone) was 75C, in the
second test it was 85C and in the third test it was 105C,
whereas the temperature in the second stage (second delignifica-
5 1 337843
tion zone) in all three tests was 105C. The pulp of soft wood to
be oxygen bleached had a kappa number of 28.7, a viscosity of 1141
dm3/kg and a consistency of 10%. The initial pressure (super
atmospheric pressure) was about 0.5 Mpa in both delignification
zones, i.e. stages 1 and 2, and the treatment time in all the
tests was 15 min in stage 1 and 45 min in stage 2. 5 kg MgS04
per ton of dry pulp was added through pipe 11 at pump 5. Each test
was repeated with the single difference that in the first case 20
kg NaOH per ton of dry pulp was used, in the second case 25 kg
NaOH and in the third case 30 kg NaOH per ton of dry pulp, except
in the first test where in the second case 30 kg NaOH was used and
in the third case 35 kg NaOH per ton of dry pulp was used (instead
of 25 and 30 kg, respectively). In general an increased alkali
charge will give a lower kappa number and lower viscosity with
otherwise identical conditions. Further, a fourth test was carried
out in which the oxygen delignification was performed in one stage
at 105C for 60 minutes and with an alkali charge of 30 kg NaOH
per ton of dry pulp. The results can be seen in the diagram in
Figure 2. The values indicated to the right in this diagram thus
refer to the lowest alkali charge of the three tests (20 kg),
whereas the values to the left refer to the largest alkali charge
(35, 30, 30 kg).
The results indicate that a high temperature in the initial stage
of the oxygen delignification has a negative effect on the viscos-
ity and that a low temperature in the initial stage produces an
oxygen-delignified pulp with improved viscosity and with a kappa
number lying within the normal and desired range. Furthermore, the
diagram shows that a low temperature in the first stage of the
oxygen delignification produces 15 to 30 units higher viscosity
measured at the same kappa number when compared with oxygen delig-
nification performed at high temperature in both stages, and 25-40
units higher viscosity measured at the same kappa number when com-
pared with oxygen delignification performed in a single step and
at high temperature. This effect can either be utilized to produce
6 1 337843
a pulp with better strength properties, or to lower the kappa
number of the oxygen-delignified pulp by 1-2 units while retaining
the same viscosity. The latter procedure is interesting from the
environmental aspect since it results in a reduction in the
chlorine consumption in the subsequent bleaching plant and with
that, a corresponding reduction in the discharge of organic
chlorine compounds.
Since the delignification zones are disposed at a distance from
each other, viz. in individual reactors 1, 2, no disruptive steam
phase appears above and between the delignification zones.
Furthermore, effective adjustment and control of the temperatures
in the different delignification zones are achieved by supplying
steam to the mixers 6, 8 before the reactors 1, 2. The mixers
produce homogenous mixing of the steam into the pulp so that a
constant low temperature can be maintained without any problem in
the first reactor, and a constant high temperature can likewise be
maintained without problem in the second reactor.
