Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
- l 2032315
Method for bleaching chemical pulp
Background of the Invention
The present invention relates to a method for bleach-
ing chemical pulp with a lignin content corresponding to a
kappa number not exceeding 10, as measured according to
SCAN-C 1:77. Chemical pulps form an unambiguous pulp cate-
gory and include for instance sulphite pulp, sulphate pulp,
and polysulphide pulp. Especially suitable are those
chemical pulps which have a high viscosity at a compara-
tively low kappa number. This group comprises sulphate pulpdigested at a high sulphidity, modified sulphate pulp
digested according to a counter-current process in which
white liquor is added also at an advanced stage of the
digestion, and alkaline pulps digested in the presence of
a catalyst, for example a quinone compound such as anthra-
quinone. Other suitable pulps are those designated MSS-AQ
(mini-sulphide-sulphite-anthraquinone), Si-Sa-Si (sulphite-
sulphate-sulphite), and PS-Si (polysulphide-sulphite), all
of which are mentioned in the Journal "Paperi ja Puu"
(Paper and Timber), 5/1989, pp. 509-513.
Although it is possible to produce, from some ligno-
cellulose materials, a pulp which already after the diges-
tion (delignification) has a kappa number not exceeding 10,
it is not possible, however, in other cases to reduce the
lignin content of the pulp thus much during the delig-
nification proper, for example if the starting material is
wood of high lignin content. In such cases, the ligno-
cellulose material in the form of pulp is delignified
further, preferably by treating it with chemicals producing
spent liquors which, like the spent liquor from the diges-
tion, can be recirculated to the chemical recovery system
of the pulp mill. Advantageously, this delignification
takes place under alkaline conditions in the presence of
one or more oxidizing agents, such as oxygen and/or perox-
ide. A highly selective and, if so desired, far-reaching
delignification of pulp is obtained if the above-mentioned
alkaline delignification of the pulp is preceded by a so
called activation of the pulp, by which the pulp is brought
2032315
- into contact with nitrogen dioxide, hydrogen and nitrate
ions and, optionally, oxygen.
Description of the Prior Art
There exist a large number of bleaching agents and
many times this number of bleaching sequences for bleaching
pulps of the type described above to obtain an almost or
completely lignin-free pulp.
Frequently, a mixture of chlorine (C12) and chlorine
dioxide (Cl02) is used in a first bleaching step. Pre-
viously, the mixture usually consisted of about 80% chlor-
ine and about 20% chlorine dioxide, both calculated as
active chlorine. With increasing environmental demands,
the amount of chlorine has been reduced and the amount of
chlorine dioxide has been increased. From the environmental
point of view, the substitution of chlorine dioxide for
chlorine as a bleaching agent is to be preferred. This is
because the contribution from chlorine dioxide to the
amount of organically bound chlorine in the substances
contained in the spent bleach liquor, is much smaller than
the contribution from elemental chlorine.
Bleaching steps with chlorine dioxide only are widely
used, and this applies particularly to the final step of a
bleaching sequence.
Frequently, a bleaching step is followed by an alka-
line extraction step (E) in which, inter alia, remaininglignin is dissolved from the pulp. In recent years, it has
become increasingly common to reinforce the alkali (usually
sodium hydroxide) by one or more bleaching agents which are
less harmful to the environment, such as oxygen and/or
peroxide (Eo, Ep, Eop).
From an environmental point of view, ozone is an
excellent bleaching agent. It is, furthermore, very effi-
cient and produces an extremely high brightness of the
pulp. This bleaching agent has been the subject of many
laboratory tests, and tests in pilot plants are also
described in the literature. To our knowledge, there has as
yet been no commercial break-through for ozone as a bleach-
ing agent for pulp.
203231~
-
Summary of the Invention
Technical Problem
The environmental authorities are increasing the
legal demands put on producers of bleached chemical pulp,
requiring them to reduce their discharge of spent bleach
liquors which contain substances where organically bound
chlorine is present. In this respect, the situation for the
pulp producer would be improved if ozone were used in at
least one bleaching step. It has, however, been found as
regards ozone as a bleaching agent, that large amounts of
ozone are needed for obtaining a given result. Further-
more, the cellulose is above all depolymerized to such an
extent that the ready-bleached pulp is an unsuitable
starting material for many types of paper.
Solution
The present invention aims at solving this problem by
a method for bleaching chemical pulp with a lignin content
corresponding to a kappa number not exceeding 10, the pulp
being bleached with chlorine dioxide. The method is charac-
terized in that the bleaching with chlorine dioxide occursat a comparatively high pulp consistency, and that the
pulp, after the bulk of chlorine dioxide has been con-
sumed, is intimately contacted with an ozone-containing
aqueous solution and brought to react with the ozone at a
considerably lower pulp consistency than in the chlorine
dioxide bleaching, whereupon the pulp undergoes an alkaline
treatment in the presence of oxygen and/or an oxidizing
agent giving off oxygen.
The chlorine dioxide bleaching of the pulp is carried
out for 10-600 min. and at a temperature of 50-100C. In a
preferred embodiment of the invention, the pulp has such a
high pulp consistency that, macroscopically seen, the
bleaching takes place in the absence of a free liquid
phase. The amount of chlorine dioxide added depends on
several factors, inter alia the kappa number of the pulp
to be bleached and the desired final brightness, and
normally varies within the range of 0.5-4%, calculated as
active chlorine on the basis of dry pulp. In the cellulose
2032315
~ industry it is customary, that both the amount added and
the amount of chlorine dioxide consumed are based on the
determination of active chlorine by iodometric analysis,
more precisely by adding potassium iodide to an acid
environment. By the determination is meant, that after the
bulk of chlorine dioxide has been consumed, at least 70% of
the active chlorine added has been consumed. The consump-
tion may approach 100%, i.e. be almost complete. Suitably,
the consumption amounts to 85-98% of the amount of active
chlorine added.
It is quite possible, albeit not necessary, to wash
the pulp after the chlorine dioxide bleaching step.
The subsequent ozone bleaching of the pulp should
take place in a low-viscous pulp suspension with a pulp
consistency which preferably is less than half of the pulp
consistency in the chlorine dioxide bleaching and not
higher than 18%. When the ozone-containing aqueous solution
is added to the pulp, the pulp consistency is suitably 1-
20%. The amount of ozone added varies within the range of
0.05-2%, as calculated on the basis of dry pulp, and the
ozone bleaching is carried out for 0.5-60 min and at a
temperature of 5-60C. It is preferred that spent liquor is
removed from the pulp after the ozone bleaching step and
that it is used, completely or partly, for reducing the
pulp consistency after the chlorine dioxide bleaching step.
This is, however, not absolutely necessary, and the pulp
may instead be either washed and then taken to the third
treatment step, i.e. the alkaline treatment of the pulp, or
taken directly to this step. The spent liquor removed from
the pulp after the ozone bleaching, or part of this liquor,
may advantageously be refreshed with ozone before being
used for lowering the pulp consistency after the chlorine
dioxide bleaching.
Suitably, the alkaline treatment of the pulp is car-
ried out at a pulp consistency of 5-25% for 10-240 min., at
a temperature of 50-100C, at an oxygen pressure below 0.6
MPa, and/or with an addition of peroxide of 0.1-1.0%,
calculated as H22 on the basis of the dry weight of the
2032315
pulp. By oxygen is meant oxygen gas which, of course, can
be supplied to the pulp in liquid form. By the previously
mentioned oxidizing agents giving off oxygen are meant
chlorine-free chemical compounds capable of giving off
oxygen. This group includes peroxides and superoxides, such
as hydrogen peroxide (H2O2), sodium peroxide (Na2O2) and
sodium perborate (Na2B2O4(OH)4), and organic peroxy com-
pounds, such as peracetic acid (CH3 COOOH).
Then, the pulp is washed as normal. It is perfectly
conceivable to end the bleaching of the pulp at this point
and, for instance, to transport the pulp to a paper mill
attached to the pulp mill or, after optional drying and
baling, forward the pulp to an external user.
If one wishes to further increase the brightness
and/or purity of the pulp, it is quite possible to bleach
the pulp in one or more additional bleaching steps. If, for
instance, one further bleaching step is chosen, chlorine
dioxide or ozone may be used as a bleaching agent.
Advantages
The invention makes it possible to bleach a chemical
pulp to a brightness exceeding 90%, while maintaining a
sufficiently high viscosity of the pulp, without the use of
any other chlorine-containing bleaching agent than chlorine
dioxide (the total amount of this bleaching agent consumed
being clearly below 2%, calculated as active chlorine on
the basis of absolutely dry pulp). From the environmental
point of view, this is a clear improvement, since the pulp
industry strives to minimize not only the amount of organic
material accompanying the spent bleached liquors into the
recipient, but also, and above all, the amount of bound
chlorine in this organic material.
The ozone step forming part of the bleaching method
is of great importance to the excellent result obtained.
According to the invention what is particularly surprising
is that already a very insignificant addition of ozone
results in a substantial increase in brightness. At the
same time, the inevitable attack leading to a depolymeriza-
tion of the pulp is reduced. Furthermore, it has been found
~032315
~ that pulps bleached according to the invention show good
brightness stability, i.e. their tendency to yellow is
slight.
Description of the Preferred Embodiment
In the following, some parts of the invention will
be described in more detail and, in connection with this,
alternative measures will be stated. Finally, batchwise
laboratory tests intended to simulate also continuous
bleaching of pulp on a technical scale will be accounted
for.
As stated above, it is preferred that the chlorine
dioxide bleaching occurs at such high a pulp consistency
that, macroscopically seen, the treatment takes place in
the gas phase in the absence of a free liquid phase. ThiS
means that the treatment liquid is inside the fibres of
the pulp and forms a thin layer on the surface of these
fibres. The upper limit of the pulp consistency depends,
inter alia, on the water retention value (WRV) of the
pulp, which in turn is dependent on the fibre type and
hemicellulose content of the pulp. The treatment tempera-
ture and the type of apparatus chosen for the bleaching
are also of importance. A high pulp consistency is required
if the pulp is under a high static pressure, e.g. in a
high pulp column. The preferred pulp consistency is within
the range of 20-45%, and the pulp is suitably loose and
fluffy. Advantageously, chlorine dioxide bleaching is car-
ried out in a reaction vessel which, e.g. by means of gas
locks, is shielded off from the ambient atmosphere. Suit-
ably, the pressure inside the reaction vessel is lower
than the atmospheric pressure.
It is possible, albeit not preferred, to carry out
the chlorine dioxide bleaching of the pulp at a lower pulp
consistency than the one stated above, e.g. in the range
of 10-20%. This means, that a free liquid phase is present
as well.
As stated above, the chlorine dioxide bleaching
should take place at a temperature of 50-100C for 10-
600 min. Shorter periods of time are used within the range
7 2~3~315
~ of 75-100C, while as much as 400-600 min. is to be prefer-
red at temperatures within the range of 50-65C. It is
particularly suitable to increase the temperature in the
course of the bleaching. For instance, the temperature may
be 40-60C during an initial period of 5-60 min., whereupon
the temperature is elevated to 80C and maintained at this
level for a given period of time. It is also possible to
do it the other way around, i.e. to lower the temperature
from an initial temperature of, for example, 80C, say by
evacuation, to for instance 60C, at which level the pulp
is stored for a couple of hours. The chlorine dioxide can
be supplied to the pulp in gas form or in an aqueous solu-
tion.
In the ozone bleaching stage, it is preferred that
the pulp consistency is within the range of 0.1-6%. Since
ozone is normally supplied to the pulp as an aqueous
solution, this additive reduces in itself the pulp consis-
tency in relation to the consistency prevalent during the
chlorine dioxide bleaching step. For optimum use of the
ozone, it is required that the pulp suspension is low-
viscous or fluidized. Within the given pulp consistency
range, such fluidization can be achieved by means of
conventional mixers, e.g. propeller mixers, including
various kinds of pumps. If modern mixers in the form of
high-intensity mixers, e.g. so-called slit mixers, are
used, the pulp consistency can be increased to be in the
range of 6-18%.
The ozonic aqueous solution can be supplied to the
pulp at the pulp consistency maintained during the chlorine
dioxide bleaching step. In this case, however, there is a
risk that the ozone bleaching will be uneven. It is there-
fore preferred that the pulp consistency is reduced to a
certain extent already after the chlorine dioxide bleaching
stage by supplying water to the pulp before the ozonic
aqueous solution is added.
The manner in which the pulp is fluidized affects the
reaction time. A highly intense fluidization permits short
reaction times in the order of a fraction of a minute to a
2032~15
~ couple of minutes. A less intense fluidization of the pulp
involves longer reaction times which, however, should be
within the time interval previously mentioned.
The third bleaching step, i.e. the alkaline treatment
5 step, may be a conventional oxygen bleaching step carried
out at a high pulp consistency, e.g. in the range of 25-
37%. However, it is preferred that the pulp consistency is
in the range of 5-25%. The temperature should be compara-
tively low, e.g. 50-100 C, and the time should be 10-
240 min. The oxygen pressure should be lower than 0.6 MPa.
The amount of alkali added, e.g. sodium hydroxide (NaOH),
is normally 0. 5-2%, as calculated on the basis of the dry
weight of the pulp. It is especially preferred to have a
small amount of sodium hydroxide, e.g. 0.5-1%, a low
temperature, e.g. 50-75 C, a low oxygen pressure, e.g. 0. 3
MPa, and a long reaction time, e.g. 240 min. At low and
average consistency, the oxygen is suitably admixed to the
pulp suspension by means of a high-intensity mixer.
The pulp can be further brightened if the above-
20 mentioned treatment is reinforced by the addition of a
peroxide. The peroxide can be admixed immediately before,
during and after the addition of oxygen. Excellent results
are obtained if the peroxide is supplied after the oxygen
treatment has been in progress for 10-200 min.
As stated above, the oxygen can be excluded and only
peroxide supplied to the pulp under the given alkaline
conditions.
In the following, the above-mentioned laboratory
tests are accounted for in the form of Examples. In the
30 description, claims and examples all percentages and parts
are given as percent by weight and parts by weight respec-
tively, unless anything else is stated.
Example 1
A sulphate pulp with kappa number 4.9 (measured
35 according to SCAN-C 1:77) and an inherent viscosity of
1025 dm3/kg (measured according to SCAN-CM 15: 88) was used
in a series of tests in which the pulp was bleached ac-
cording to the invention. For comparative purposes, tests
9 2032~1~
were also made outside the scope of the invention.
The sulphate pulp with kappa number 4.9 originates
from a technically produced sulphate pulp with kappa num-
ber 30.2 and an inherent viscosity of 1240 dm3/kg. The
latter was obtained by a delignification of the wood
species Pinus sylvestris. The completely clean-washed
sulphate pulp was impregnated with technical black liquor
in an amount corresponding to 221 kg per 1000 kg dry pulp
to simulate a poorly washed mill pulp. The dry solids
content of the black liquor was 19.5%. Then, 2% NO2, as
calculated on the basis of the dry weight of the pulp, was
supplied to the pulp which subsequently was treated accor-
ding to a dilution method (S3) which is described by
Samuelson and ~jteg in Proceedings 1989 TAPPI Int. Symp. on
Wood and Pulping Chemistry, Raleigh N.C., May 1989, pp 195-
204. After being thus activated, the pulp was bleached, in
accordance with this publication, with oxygen at average
consistency and finally washed.
The pulp had the kappa number and inherent viscosity
values stated by way of introduction.
Four batches of pulp were made ready and initially
treated with chlorine dioxide corresponding to 1.35% active
chlorine, as calculated on the dry weight of the pulp. The
bleaching was carried out in a glass reactor which was
evacuated before the addition of chlorine dioxide, but
after the supply of the hand-fluffed pulp. Then, a glass
flask containing a technical solution (mill solution) of
chlorine dioxide was connected via a valve. By carefully
heating the glass flask for 15 min., all chlorine dioxide
was expelled from the glass flask and supplied to the pulp
together with a certain amount of expelled water vapour.
During this period, the temperature in the glass reactor
was 25C, and the pulp consistency was 30~. Then, the glass
reactor was partly immersed in a polyglycol bath with a
temperature of 60C, and rotated. During a period of 10
min., the temperature was elevated to 70C and maintained
at this level for 120 min. At this point, the glass reactor
was removed from the polyglycol bath, and distilled water
20~2315
was supplied to the pulp, thus reducing the pulp consisten-
cy to 5%. The glass reactor was drained of the pulp suspen-
sion which was passed to a B~chner funnel filtering off
liquid. The resulting pulp cake was pressed to a consisten-
cy of 30% to obtain a reliable determination of the chlor-
ine dioxide consumption and render possible a corresponding
determination of the ozone consumption.
In a second step, the pulp with an inherent consis-
tency of 30% was treated with ozone. The pulp was intro-
duced into a plastic container equipped with a propellermixer. A previously prepared aqueous solution of ozone
(gaseous ozone absorbed in water) was supplied to the pulp
being agitated in such an amount that the pulp consistency
was reduced to 0.5%. The temperature was 22C, and the
treatment lasted for 10 min. The pulp suspension was
agitated by means of the propeller mixer during the entire
reaction time. Then, the pulp suspension was moved to a
B~chner funnel for filtering off the spent liquor. At this
point, the pulp consistency rose to above 10%. Different
amounts of ozone were added, the addition in Test 1 being
0.07%, in Test 2 0.093%, and in Test 3 0.116%, as calcula-
ted on the dry weight of the pulp.
In a third step, the pulp was introduced into an
autoclave of acid-proof steel. Sodium hydroxide in an
amount of 1%, as calculated on the dry weight of the pulp,
was added to the pulp in the form of an aqueous solution,
resulting in a pulp consistency of 10%. oxygen was sup-
plied to the autoclave at room temperature, the resulting
oxygen pressure being 0.3 MPa. The autoclave was immersed
in a polyglycol bath with a temperature of 70C. After
45 min. at this temperature, the alkaline oxygen deligni-
fication of the pulp was interrupted, and the pulp was
carefully washed with water.
In the three tests accounted for above, the pulp was
treated according to the invention. For comparative purpo-
ses, the fourth batch of the pulp was a blank which was
treated exactly like the other tests, except that the
ozone bleaching step was left out.
2032315
11
~~ As stated above, already these three treatment steps
according to the invention yield a pulp which is very
useful for various products, e.g. paper of various kinds.
If desired, the pulp can be bleached in at least one
more step. Therefore, the above pulp specimen were finally
bleached with chlorine dioxide at a pulp consistency of 6%
for 3 h in glass jars immersed in a water bath with a tem-
perature of 70C. The glass jars were shaken by hand at
short intervals. Then, the pulp was carefully washed with
water.
Each batch of pulp was divided into two parts to
which was added 0.51% and 0.80% chlorine dioxide, respec-
tively, as calculated on absolutely dry pulp.
The results obtained are shown in Table 1.
oc ~ 2032~15
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2032315
13
-~ The brightness was determined as ISO-brightness
according to SCAN-C 11:75. The brightness of the blank
after ageing at 80C was determined on the basis of other
tests carried out under similar conditions. The total
consumption of chlorine dioxide as calculated on the basis
of active chlorine, is the total consumption in the first
chlorine dioxide bleaching stage and the consumption of
chlorine dioxide in the final bleaching stage.
When comparing the pulp in the blank and the pulps in
Tests 1-3 after completed oxygen bleaching, one finds that
an addition of ozone as low as 0.07% increases the bright-
ness by as much as 6 units, while the reduction of the
viscosity is no more than 70 units, which is relatively
harmless. A further addition of ozone increases the bright-
ness by one more unit at a substantially unaltered visco-
sity.
In corresponding pulps finally bleached with chlorine
dioxide, the difference in brightness is halved, whereas
the difference in viscosity of the pulps is substantially
reduced. Furthermore, as is apparent from the Table, all
the pulps have an excellent brightness stability.
What strikes one first is that only a small addition
of the expensive bleaching agent ozone is required in order
to increase considerably the brightness of the pulp.
In further tests, the same pulp was treated in a
similar manner as stated above including the alkaline
oxygen treatment. The amount of ozone added in these tests
was the same as in Test 2, i.e. 0.093% as calculated on
absolutely dry pulp. However, the pulp consistency during
the ozone bleaching step was varied and amounted to 1%, 20%
and 30%, respectively. A pulp consistency of 1% gave the
same results as a consistency of 0.5%, i.e. in accordance
with Test 2. On the other hand, the results deteriorated
considerably at pulp consistencies of 20% and 30%. At a
pulp consistency of 20%, the brightness was lower by 1.5
unit and the loss of viscosity 70% greater than in Test 2.
At a pulp consistency of 30%, the brightness was roughly
the same as in Test 2, but the loss of viscosity was 100%
14 2032315
greater, resulting in a pulp of insufficient strength. This
illustrates the importance of having a low pulp consistency
in the ozone bleaching step.
The same pulp was also subjected to a test in which
the order of the two first treatment steps of the invention
was reversed. In this test, the pulp was first treated with
ozone and then with chlorine dioxide, but was otherwise
treated in accordance with Tests 1-3, including the final
bleaching with chlorine dioxide. Thus, the pulp was first
treated with ozone at a pulp consistency of 0.5%. After
the treatment liquid had been filtered off and the pulp
pressed to a pulp consistency of 30% and fluffed by hand,
the pulp was bleached as above with chlorine dioxide con-
taining 1.35% active chlorine. To achieve a final bright-
ness of 88.5, an addition of 0.3% ozone, as calculated on
the dry weight of the pulp, was required, which is to be
compared with the 0.07% ozone added in Test 1 according to
the invention. In this case, the viscosity of the pulp was
reduced to 810 dm3/kg, implying that depolymerization of
the pulp had been carried too far. Also the brightnessstability of the pulp was poorer in this test than in the
tests according to the invention.
Test 2 according to the invention was repeated,
except that the pulp consistency in the initial chlorine
dioxide bleaching step was lowered from 30% to 15%. In this
test, the chlorine dioxide was supplied in an aqueous
solution which was admixed to the pulp, resulting in a
consistency of 15%. This yielded a brightness of the pulp
of 82.1%, after the oxygen bleaching step. After final
bleaching with chlorine dioxide in an amount added of 0.51%
active chlorine as calculated on dry pulp, the brightness
was increased to 88.6%. The corresponding values in Test 2
were 82.7% and 89.1%, respectively. Obviously, the diffe-
rence in brightness of the pulp is very slight. The visco-
sity values of the pulp did not differ significantly fromthose obtained in Test 2. From this it may be concluded
that, according to the invention, it is possible to have a
pulp consistency of 15% in the chlorine dioxide bleaching
2032315
step, which is comparatively low.
The amount of active oxygen remaining in the spent
liquors after the ozone step was determined iodometrical-
ly. This amount corresponded to less than 5% of the added
amount of ozone added, determined according to the same
method.
Example 2
A sulphate pulp of kappa number 5.0 and a viscosity
of 980 dm3/kg, was used in a series of tests according to
the invention. ThiS sulphate pulp was of the same type as
and produced in a parallel manner to the pulp described in
Example 1.
This test series in all parts corresponds to the
previously described Test 2, with the addition that a
peroxide treatment, with three different amounts added, was
introduced after the oxygen bleaching and before the
concluding chlorine dioxide bleaching. After the oxygen
bleaching, the pulp was diluted with a hydrogen peroxide
solution to a pulp consistency of 8%. The temperature
during this hydrogen peroxide step was 70C and it lasted
for 45 min.. In this series, the amount of chlorine dioxide
added in the final bleaching step was 0.52% active chlorine
as calculated on dry pulp. The amount of hydrogen peroxide
added and the results obtained are apparent from Table 2
below.
TABLE 2
Finally bleached pulp
H22 Added
Test % on the basis Bright- Brightness after visco-
of dry pulp ness ageing sity
% 80C % 120C dm3/kg
4 0 89.0 85.6 85.5 920
0.20 89.7 86.2 85.1 915
6 0.40 89.9 86.3 86.4 910
7 0.60 90.2 86.5 86.4 910
As will appear, it is possible to further increase
the brightness of the pulp by means of hydrogen peroxide,
even to above 90%. The decrease in viscosity of the pulp
caused by the hydrogen peroxide treatment was slight and
lies within the limits of normal analytical errors.
