Note: Descriptions are shown in the official language in which they were submitted.
2031~
METHOD OF BLEACHING PULP
Field and Background of the Invention
The present invention relates to a method of bleaching
pulp of cellulosic fibrous material in the production of
chemical pulp in a bleach plant wherein the entering pulp
is fed continuously in a bleaching line thereof and
bleached with a plurality of bleaching agents comprising
chlorine dioxide and ozone.
Chlorine is one of the bleaching agents most frequently
used for bleaching pulp of cellulosic fibrous material,
particularly chemical pulp. However, chlorine, which
forms chlorinated organic compounds, is considered
detrimental to the environmental so that there is
increasing resistance to the use of chlorine. This has
resulted in lower limit values for the emission of
chlorinated organic substances, quantified as kg
adsorbable organic halogen ( AOX ) per ton pulp. Until the
turn of the century a level of 1.0-2.0 kg AOX per ton dry
pulp will probably be accepted by the environment
conservation authorities. Thereafter, and possibly
earlier if technically feasible, one may calculate with
levels of below 1.0 kg AOX per ton pulp.
A number of different methods have been proposed in order
to fulfil the increasingly stringent requirements placed
on the use of chlorine, such as reducing the lignin
content in the pulp prior to chlorine bleaching by
prolonged digestion and by carrying out an initial
delignification with oxygen gas. Other methods include
reducing the quantity of chlorine added or replacing
chlorine by chlorine dioxide.
It is also known that, from the environmental aspect,
ozone is greatly superior to the co~m~rcially used
2031850
bleaching agents chlorine and chlorine dioxide for bleaching
cellulosic pulp. However, in view of the high quality demands
placed on the pulp, particularly brightness, purity and strength,
it is presently impossible to entirely omit chlorine dioxide.
However, the commercial use of ozone has hitherto not been
possible due to the high costs for producing ozone and of the
considerably less selective nature of ozone when conventional
methods of bleaching are used. By "low selectivity" is meant
that, in addition to removing lignin, the ozone also noticeably
breaks down the cellulose. The kappa number of a pulp is a
measure of the lignin content, whereas its viscosity is a measure
of the average chain length of the cellulose and an indication of
its strength. For a bleached pulp of softwood with a brightness
of 90 IS0 and good strength properties, the viscosity ought to be
above 800 SCAN units (dm /kg).
When a pulp bleached with oxygen gas is further bleached
with chlorine dioxide and/or chlorine, four or five separate
stages are necessary for the bleached pulp to achieve a brightness
which fulfils market demand, i.e. approximately 90 IS0. By
"separate stages" it is meant that each of the stages is preceded
and followed by a wash of the pulp. The equipment for washing the
pulp is what requires the highest capital investment in a
bleaching plant.
SummarY of the Invention
The present invention aims to provide an improved method
of bleaching pulp of cellulosic fibrous material which
considerably reduces adsorbable organic halogen and which is more
efficient with respect to the bleaching processes previously used.
A
~.~....
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The invention also aims to provide an improved method of
bleaching pulp of cellulosic fibrous material which reduces the
use of chlorine and chlorine dioxide.
The invention further aims to provide an improved method
of bleaching pulp of cellulosic fibrous material using a fewer
number of bleaching stages in order to obtain a bleached pulp with
brightness and strength common to the market.
The invention also aims to increase the selectivity of
the delignification so that sufficiently high viscosity can be
achieved after the ozone treatment to enable the pulp to be
subjected to additional separate bleaching stages in order to
achieve brightness common to the market.
The method of present invention concerns bleaching pulp
of cellulosic fibrous material in the production of chemical pulp
in a bleach plant wherein the entering pulp is fed continuously in
a bleaching line thereof. The method comprises the step of
bleaching the pulp with chlorine dioxide and ozone in one and the
same stage, said stage being started with chlorine dioxide and
followed by ozone, the chlorine dioxide and ozone being added at
points separated from each other and each at at least one point in
said bleaching line, said one and the same stage being free from
intermediate washing between said points for the addition of
chlorine dioxide and ozone
In a preferred embodiment of the invention ozone is
added at one or more points in said stage which can be defined as
initial stage. A mixer is used at each such point.
It is suitable for the ozone to be added when from 10%
to 99% of the chlorine dioxide has reacted.
20~1850
The ozone is added with the aid of a carrier gas
containing about 2-13 per cent by weight ozone. Preferably, the
pulp is treated with ozone at a pressure in excess of 1 bar over-
pressure. The sequence bleaching of pulp with chlorine dioxide
and ozone is performed at a temperature of 25-70C, preferably
45-65C.
The treatments with chlorine dioxide and ozone are
followed by one or two alkali extractions, at least one of which
may be reinforced with oxygen gas and/or hydrogen peroxide, the
first alkali extraction being performed either in sequence with
the chlorine dioxide and ozone treatments without intermediate
washing or in a separate stage preceded by washing.
The pulp is finally bleached with chlorine dioxide in
one or more stages, usually two stages.
The supplied and treated pulp is preferably of medium
consistency, about 6-15~, and suitably consists of oxygen-
delignified pulp which has been digested continuously in
accordance with a modified process designated MCC or in accordance
with a standard process. The pulp may also be treated with ozone.
In preferred embodiments chlorine dioxide is added in
the one and same stage in an amount of 5-60 kg/air dry metric ton
(ADMT), particularly 10-30 kg/ADMT calculated as active chlorine.
Preferably the ozone is added in an amount of 1-2 kg/ADMT,
particularly 3-10 kg/ADMT.
When alkali extraction is performed in a separate stage
preceded by washing the chlorine dioxide is added for the ultimate
bleaching in an amount of 10-50 kg/ADMT particularly 15-35 kg/ADMT
calculated as active chlorine.
20318S~
4a
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 one embodiment of the method according to the
invention, and
203 1 850
Figure 2 shows another bleaching plant for carrying out a
second embodiment of the method according to the
invention.
Description of Illustrated Embodiments
The bleaching plant shown in Figure 1 comprises a supply
tank 1, a flash tank 2, a first upstream bleaching tower
3 with diffuser washing equipment at the top, a downtube
4 and a second upstream bleaching tower 5 with diffuser
washing equipment at the top. Pulp of medium consistency
(about 6-15~) is fed from the supply tank 1 to the flash
tank 2 through a pipe 6 and then from the flash tank 2 to
the bleaching tower 3 through a pipe 7. The bleaching
tower 3 is connected to the downtube 4 by a pipe 8 and
the downtube being connected to the bottom of the
bleaching tower 5 by a pipe 9 containing a suitable pump
10 arranged close to the downtube 4. The bleached pulp is
removed from the top of the bleaching tower 5 through a
pipe 11. Washing liquid is supplied to the top of the
bleaching towers 3 and 5 through pipes 12 and 13,
respectively. Carrier gas, such as oxygen gas, nitrogen
gas or mixtures thereof, is removed from the top of the
flash tank 2 through a pipe 14 and conducted to a
suitable equipment for possible recovery.
The pulp is fed from the supply tank 1 through a
discharger 15 disposed at the bottom of the tank and from
the flash tank 2 through a similar discharger 16 disposed
at the bottom of the tank. The dischargers 15, 16 are
specially designed for discharging pulp of medium
consistency. Furthermore, pipe 6 contains two mixers 17,
18, disposed one after the other, i.e. apparatus to mix
treating agent homogeneously into the pulp passing
therethrough. The mixer 17 is connected to the mixer 18
located downstream by means of a pipe portion 6a of
predetermined length. A similar mixer 19 is disposed in
the pipe 7 leading from the flash tank 2 to the ~irst
bleaching tower 3. Each mixer 17, 18, 19 comprises
2~31~0
fluidizing means to ensure that the various treating
agents are vigorously and homogeneously mixed into the
pulp. Such an intensive mixer may advantageously comprise
a "Kamyr MC mixer", i.e. a mixer specially designed to
provide highly efficient mixing of treating agent into
pulp of medium consistency.
Close to the outlet from the supply tank 1, if necessary,
an acidification agent such as sulphuric acid is added
via a pipe 20 in order to acidify the pulp to a value
below pH 5, e.g. pH 2-4. Chlorine dioxide is supplied
through a pipe 21 to the first mixer 17 with high mixing
effect, whereas a carrier gas containing ozone is
supplied through a pipe 22 to the following second mixer
18 with high mixing effect. An alkaline agent such as
sodium hydroxide is added through a pipe 23 to the outlet
from the flash tank 2 and oxygen gas may also be added at
the same point through a pipe 24 if desired. Furthermore,
a pipe 25 is connected to the third mixer 19 with high
mixing effect for the supply of hydrogen peroxide, and
another pipe 26 for the supply of steam under high
pressure to the pulp. At the outlet from the downtube 4
the pump 10 is provided with a pipe 27 for the supply of
chlorine dioxide. A reaction vessel 28 may suitably be
disposed in the pipe portion 6b between the second mixer
22 and the flash tank, this vessel being so dimensioned
that the pulp passing therethrough remains therein for
about 0.1-10 minutes, preferably 0.5-5 minutes. If the
pipe portion 6b is sufficiently long to allow the pulp to
remain therein for the required time, the reaction vessel
28 may be omitted.
The pulp acidified to a pH value of 4, for instance, is
homogeneously mixed in the first mixer 17 with chlorine
dioxide and is then, after a relatively short period of
time, mixed with carrier gas containing ozone. This brief
period of time may be from 10 seconds up to a
7 2~31850
few or several minutes, e.g. 10 minutes. It is suitable
for 10% up to 99% of the chlorine dioxide to have reacted
with the pulp before ozone in gaseous form is added. With
current technology for the production of ozone, the
carrier gas may contain about 2-13 per cent by weight
ozone, however, with better methods in the future it
should be possible to increase the portion of ozone, e.g.
up to about 20 per cent by weight. The amount of ozone
supplied can be controlled by the choice of a suitable
gas pressure in that the higher the gas pressure is when
it is mixed into the pulp, the more ozone can be mixed
in. Since none of the carrier gas is consumed, this is
removed through pipe 14 for possible recovery. After the
flash tank 2 sodium hydroxide is added to neutralize the
pulp and increase the pH value up to about 11-12. A small
quantity of oxygen gas may possibly be added to oxidize
the easily oxidized compounds in the pulp so that the
hydrogen peroxide will be utilized more efficient. A
small quantity of the last-mentioned bleaching agent is
added at the third mixer 19. High-pressure steam may also
be added thereto in sufficient amount to increase the
temperature of the pulp to about 60-90C. Until this
point where the high-pressure steam is added, and
particularly before the flash tank 2, the temperature of
the pulp is set at a relatively low level of about
25-70C, preferably 45-65C. An alkali extraction,
reinforced by oxygen gas and peroxide, occurs immediately
at the various additions after the flash tank 2 and up to
the washing equipment in the bleaching tower 3 and is
thus interrupted at the top of the bleaching tower 3 by
the pulp being washed with a washing liquid. The washed
pulp is then bleached with chlorine dioxide in the
bleaching tower 5, this compound being added in a
predetermined amount at the pump 10 located upstream. The
bleaching process described thus consists of a bleaching
sequence with only two bleaching stages, with a washing
stage between them carried
, . . .
8 2031850
out in the top part of the bl~ching tower 3. The
bleaching sequence can thus be illustrated as follows:
( DZEOP )D .
Figure 2 shows a modified bleaching plant which with
respect to that according to Figure 1 has been
supplemented for carrying out a washing stage between the
ozone treatment and the alkali extraction. The parts and
elements substantially equivalent in the two figures are
denoted with the same reference numbers and are therefore
not further explained. As can be seen in Figure 2, the
discharger 16 of the flash tank 2 is provided with a pipe
29 connected to a washing apparatus 30 of diffuser type.
The pipe 29 contains a pump 31 to supply pulp of medium
consistency to the washing apparatus 30, which is
supplied with washing liquid via a pipe 32. The pulp is
carried by a pipe 33 from the washing apparatus 30 to a
downtube 34 connected to the bottom of the bleaching
tower 3 by a pipe 7. The pipe 7 contains a suitable pump
35 disposed close to the downtube 34. Two pipes 23, 24
connected to the pump 35 supply this with an alkaline
agent such as sodium hydroxide, and oxygen gas,
respectively, if desired. The pipe 7 also includes a
mixer 19 with high mixing effect. The mixer has a pipe 25
for the supply of hydrogen peroxide and a pipe 26 for the
supply of steam to the pulp if desired. In the
alternative embodiment of the bleaching process according
to the invention, performed using the bleaching plant
shown in Figure 2, a washing stage is thus introduced
before the alkali addition. The bleaching sequence thus
consists of three bleaching stages, i.e. (DZ)(EOP)D. This
embodiment entails higher investment costs but the
consumption of sodium hydroxide is lower with respect to
the embodiment first described.
The following example illustrates the invention futher
and shows its unexpected results in relation to two
9 20~1850
comparative tests which were performed under equivalent
conditions.
Example
Three tests of bleaching processes were performed by
simulating on a laboratory scale a bleaching plant
substantially in accordance with Figure 1, but
supplemented for treatment with chlorine dioxide in a
further final bleaching stage. In test 2, however, no
chlorine dioxide was added at the first mixer (which was
bypassed). In test 3 the first chlorine dioxide treatment
was performed as a conventional prebleaching stage.
A standard sulphate pulp of softwood was used for the
tests, the pulp having been bleached with oxygen gas and
washed in conventional manner. The sulphate pulp fed into
the supply tank 1 had a kappa number of 18, a viscosity
of 1020 dm3/kg and a consistency of 10%. This consistency
was maintained throughout the bleaching process in all
three tests.
Three tests were carried out according to the folloing
bleaching sequences.
Test 1 Test 2 Test 3
Bleaching sequence (DZEOP) DD ( ZEOP) DD D ( EOP ) DD
Number of bleaching stages 3 3 4
The first bleaching stage with chlorine dioxide (D) in
test 3 was performed, as stated, in conventional manner
at a pH of 3-4, a temperature of 60C and for a period of
60 minutes, followed by a washing stage and a bleaching
stage with alkali extraction reinforced by oxygen gas and
peroxide (EOP). The sulphate pulp had the same
~0~1850
consistency (10%) as in the other two tests as mentioned
above.
In the ozone treatments according to test 1 and test 2,
the temperature of the sulphate pulp fed from the supply
tank was in both cases 28C. In test 1 the pulp was
vigorously mixed with chlorine dioxide in the first stage
(DZEOP) in accordance with the present invention. The
carrier gas cont~;n;ng ozone was supplied at a pressure
of 4.8 bar. The pH value of the sulphate pulp was between
3.5 and 4 and was controlled by the addition of sulphuric
acid at the outlet from supply tank 1. The ozone
treatment took place for a period of 5 minutes and was
interrupted by the addition of alkali when substantially
all ozone had had time to react.
The alkali extraction reinforced by oxygen gas and
peroxide was performed in all three tests at a
temperature of 70C and for a period of 60 minutes,
during which time the pressure was reduced from 3 bar to
atmospheric pressure. The alkali extraction was
interrupted by supplying washing liquid to the upper part
of the bleaching tower 3, and then two bleaching stages
using chlorine dioxide were performed at the previously
set temperature, 70C, for a period of 180 minutes in
each case and the same for the three tests.
Dosage of the various chemicals, final pH value at the
alkali extractions and the partial and final results of
the three tests can be seen in the following table. In
the table and in the rest of the description the initials
ADMT signify "Air Dry Metric Ton", i.e. ton of air dry
pulp, and AOX "Adsorbable Organic Halogens".
.. ,, . , , .. ,, , . . . . , , . . ~ .
2031850
11
TABLE
Test 1 Test 2 Test 3
Bleaching sequence (DZEOP)DD (ZEOP)DD D(EOP)DD
. Stage 1 Stage 1
_ _ _ _
C102, kg/ADMT 20 - 40
Stage 1
0.3, kg/ADMT 10 15
- Stage 2
NaOH, kg/ADMT 35 20 20
2, kg/dm 3 3 3
H202, kg/dm 3
Kappa number 1.9 4.1 2.5
Viscosity, dm3/kg 810 720 980
Stage 2 Stage 2 Stage 3
C102, kg/ADMT 20 45 35
Stage 3 Stage 3 Stage 4
C102, kg/ADMT 10 10 10
Brightness, %ISO89.7 88.9 89.7
Viscosity, dm3/kg810 720 930
Total C102, kg/ADMT 50 55 85
Total AOX, kg/ADMTca 0.1 - 1.5
.. . ..
12 2û31850
It is clear from the table that a 4-stage bleaching
sequence D(EOP)DD according to test 3 requires an
addition of chlorine dioxide, calculated as active
chlorine, in an amount of 85 kg/ADMT pulp in order to
achieve a brightness of about 90 ISO. The amount of AOX
in the outlet from the bleaGhing stage was 1.5 kg/ADMT.
It is also clear that a 3-stage bleaching sequence
(ZEOP)DD according to test 2, with an initial ozone
treatment in the first stage and a total addition of
chlorine dioxide, calculated as active chlorine, of 55
kg/ADMT pulp, gives an even lower brightness (88.9 ISO),
and in particular, the viscosity is at an unacceptably
low level, resulting in poor strength properties. Owing
to these low values it was of no interest to measure AOX.
Finally the table shows that a 3-stage bleaching sequence
(DZEOP)DD according to test 1 (the invention), with a
subsequent ozone treatment in the first stage in
; mm~; ate conjunction with the continuous addition of
chlorine dioxide, surprisingly results in a sufficiently
high brightness level and a viscosity within the
acceptable limit (over 800 dm3/kg). The selectivity of
the delignification has thus been improved. It is also
noted that test 1 results in a surprisingly low quantity
of AOX, about 0.1 kg/ADMT. The reasons for this extremely
favourable result from the environmental aspect have not
been fully understood but a probable explanation might be
that the ozone breaks down a larger proportion of the
amount of AOX initially formed in the chlorine dioxide
treatment.
In the example described above an oxygen-delignified
sulphate pulp of softwood was used, which had been
continuously digested in accordance with a standard
method and which thus required two separate chlorine
dioxide stages. For an oxygen-delignified sulphate pulp
13 ~03185~
of softwood which has been continuously digested in
accordance with a modified process designated MCC
(Modified Continuous Cooking), and which has a kappa
number of 12-14 and a viscosity of 1000-1100 dm3/kg, it
is usually sufficient to perform only a final bleaching
stage with chlorine dioxide in order to achieve
brightness common to the market. Pulp prepared from
hardwood is also usable. The invention can of course also
be applied to sulphate or sulphite pulps which have not
been oxygen-delignified.
