Note: Descriptions are shown in the official language in which they were submitted.
1339~93
The present invention relates to a procedure for the
bleaching of pulp, in which procedure the oxidating
bleaching chemical is oxygen.
Especially pulp obtained from a sulphate pulping process is
of brown colour, which is mainly due to the lignin remain-
ing in the pulp. Lignin is removed from the pulp by
bieaching, which is a process consisting of several stages.
During this process, the pulp is treated alternately with
oxidizing, lignin-degrading chemicals and chemicals dis-
solving the degradation products. Oxidi~ing agents com-
monly used are oxygen and chlorine-containing chemicals,
whereas alkali solutions are used for eliminating the
degradation products.
In the reactions occurring in bleaching using chemicals
containing chlorine, lignin is converted into organic
chlorine compounds, which remain in the bleaching effluent
removed from the pulp. Bleaching effluents are a problem
in regard of environmental protection because of the toxic
nature of the chlorophenols and other possible organic
chlorine compounds contained in the effluents. Besides,
the chemical oxygen demand in bleaching effluents reaches
detrimental levels. As the measures aimed at reducing the
environmental pollution load resulting from sulphate pulp
production have so far been mainly concentrated on other
parts of the process except bleaching, the relative
significance of bleaching as a polluting factor has been
lncreas lng .
The bleaching effluents causing the worst environmental
pollution load are produced during the washing following
the first chlorination and the first alkali treatment in
the bleaching process. Various methods have been employed
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1339~93
to reduce the pollution load, e.g. by replacing chlorine
gas with chlorine dioxide or using oxygen as the oxidizing
agent in the first bleaching stage, or by biological
purification of the bleaching effluent. However, the
results achieved by these methods have not been completely
satisfactory. Although the amounts of chlorophenols and
other toxic chlorine compounds in the bleaching effluent
have been significantly reduced by employing chlorine
dioxide and oxygen bleaching, it has not been possible to
achieve a sufficient reduction in the chemical oxygen
demand values of the effluents. Therefore, the methods
referred to have required the employment of efficient
biological purification.
The present invention enables the toxic content and the
value of chemical oxygen demand of the bleaching effluent
to be reduced so as to reduce the need for purification of
the effluent. The invention is based on oxygen bleaching
and it is characterized in that oxygen is used in the first
oxidation stage of the bleaching process, that the pulp is
subjected to enzyme treatment, and that after the oxygen
bleaching and enzyme treatment, the pulp is washed.
It has been observed in earlier investigations that by
using enzymes it is possible to separate lignin and/or
hemicellulose from cellulose and thus give the pulp a more
spongy quality. This justifies the assumption that if the
pulp obtained from the digestion process is first subjected
to enzyme treatment, it is possible to reduce the amount of
chemicals needed in the next bleaching stage. According to
the invention, it has now been observed that enzyme treat-
ment involves an essential reduction in the amount of toxic
compounds in the bleaching effluent while at the same time
reducing its chemical oxygen demand, especially when the
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oxidizing chemical used in the first bleaching stage is
oxygen. When the commonest oxidizing chemical, i.e. pure
chlorine gas, is used, enzyme treatment has a substantially
weaker effect on the quality of the bleaching effluent.
According to the invention, the pulp can be subjected to
enzyme treatment and washing before the first oxidation
stage. A preferable alternative solution is to carry out
the enzyme treatment and washing after the first oxidation
stage. The enzyme breaks down hemicellulose and/or lignin
contained in the pulp and renders the pulp more spongy,
thus enhancing the effect of the chemicals in subsequent
oxidation and alkali treatment stages. By washing the pulp
after the enzyme treatment, the degradation products are
removed and can be directed to incineration so that they
will not contribute to the pollution load at all but are
instead utilized in the energy handling system of the
plant.
According to the invention, the subsequent oxidation stages
after oxygen bleaching can be implemented using a bleaching
chemical containing chlorine, e.g. chlorine gas and/or
chlorine dioxide. The production of detrimental chlorine
compounds will thus be limited to these process stages, and
to minimize the amounts of these compounds it is preferable
to use chlorine dioxide to as large an extent as possible.
This makes it possible to reduce the content of organic
chlorine compounds in the bleaching effluent by more than
70% even in the bleaching of softwood pulp. This is a
result unattainable with previously known bleaching
methods.
Except for the enzyme treatment, the bleaching of pulp by
the procedure of the invention can be performed by
1333~33
employing alternate oxidation and alkali phases and washing
the pulp after each of these phases to remove the bleaching
chemicals and degradation products.
The enzyme treatment in the procedure of the invention is
preferably carried out in a temperature range of 10-90~ C,
the most suitable range being 40-80~ C, with pH values in
the range 3.0-10.0, most suitably 4.0-8Ø The enzyme used
can be a hemicellulase, cellulase, pectinase, esterase or a
mixture of these.
The invention also concerns the use of an enzyme in the
bleaching of pulp, with oxygen used as the bleaching chem-
ical used in the first oxidation stage, and especially its
use as a means of reducing the chlorine content of the
bleaching effluent when the bleaching chemical used in the
first oxidation stage is oxygen and in subsequent oxidation
stages a chemical containing chlorine, e.g. chlorine gas or
chlorine dioxide. The enzyme is preferably a hemicellu-
lase, cellulase, pectinase, esterase or a mixture of these,and it is used essentially in the manner explained in the
above description of the bleaching procedure.
In the following, the invention is described in greater
detail by the aid of embodiment examples based on
laboratory experiments.
Example 1
A diluted enzyme mixture KD 50 (hemicellulase) was added to
220 g of dry matter obtained from pine sulphate pulp (with
a dry matter content ~f 30%) so that a mixture with a
consistency of lO~o and a xylanase activity of 5 U/g of pulp
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13~93
dry matter was obtained. The temperature in the enzyme
treatment was 55~ C and the duration of treatment 2 h.
After the enzyme treatment the pulp was subjected to pre-
liminary oxygen bleaching. The acidity of the pulp was
adjusted to pH 12. In the oxygen treatment the temperature
was 100~ C, overpressure 2 bar and duration of treatment 45
min. After oxygen treatment the pulp was washed in a
Buchner funnel with a 20-fold amount of water.
Next, the pulp was subjected to an oxidizing bleaching
treatment using a mixture containing 50 % chlorine dioxide
and 50 % chlorine gas. Of this mixture, a dose equal to
2.0 x kappa number after oxygen bleaching was used. Treat-
ment temperature was 40~ C and duration 45 min. After
oxidation
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1339~93
the pulp was washed in a Buchner funnel with a 20-fold
amount of water.
Next, the mass was subjected to an alkali treatment using a
5 % sodium hydroxide solution in a dose of 0.9 x kappa. Con-
sistency of solution was 10 %, treatment temperature 60 ~C
and duration of treatment 90 min. After the alkali treatment
the pulp was washed in the same way as after oxidation.
After this, the bleaching was continued by repeating the
oxidation and alkali phases and then once more the oxidation
phase and washing the pulp between these phases as described
above. With these arrangements, the amount of chlorine
dioxide in the second oxidation phase was 26 % and in the
third phase 13 % of the amount of chlorine dioxide in the
first oxidation phase. In both cases the dosage of sodium
hydroxide was 1 ~ of the amount of chemical pulp.
The amounts of bleaching chemicals consumed and the analysis
results describing the quality -of the bleached pulp are
presented in Table 1 (experiment 3).
In addition to the above-described expèriment (exp. 3)
illustrating the invention, two reference experiments
(experiments 1 and 2J and an additional experiment (exp. 4)
were carried out, and the results obtained from these are
also presented in Table 1 below. The experiments were
performed as follows:
Experiment 1 (reference): No enzyme treatment and no
preliminary oxygen ble~c~i~g were employed. The dosage of
chlorine chemicals, of which 90 % consisted of chlorine gas
and 10 % of chlorine dioxide, was such that the same target
bleaching degree of 89.0 was reached as in experiments 3 and
4. Otherwise the experiment was analogous to that described
above (exp. 3).
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1339593
Experiment 2 (reference): No enzyme treatment was employed.
In other respects the experiment was analogous to that
described above.
Experiment 4: The pulp was first subjected to preliminary
oxygen bleaching and washing and then enzyme treatment. Each
phase was performed in the manner described above (exp. 3),
only the order of the treatments was reversed. In other
respects the experiment was analogous to that described
above (exp. 3). The present invention comprises a procedure
employing the principle of this experiment.
TABT }~ 1
C/D(90/10)-E-DED O-D/C(50/50)-E-DED Enz-O-D/C(50/50)-E-DED O-enz-D/C(50/50)-E-DED
Exp. 1 Exp. 2 Exp. 3 Exp. 4
CO.~ O.. OF
'ACTIVE chlorine
kg/t of chem. pulp 100 64 54 51
- chlorine gas
kg/t of chem. pulp 63 14.6 13.7 12.6
- C1 02
kg/t of chem. pulp 13.2 18.9 15.4 14.7
Amount of equiva-
lents of chlorine 70 24 21.4 19.9
Saving in equiva-
lents of chlorine % 0 66 70 72
Chlorine (%) saved
due to enzvme 0 11 17
C~3
C~
1339~93'
The results indicate that the amount of chlorine in the
bleaching process can be substantially reduced by subjecting
the pulp as taught by the invention to preliminary oxygen
bleaching and enzyme treatment and replacing part of the
chlorine gas, conventionally used in bleAching~ with chlo-
rine dioxide. It is worth noting that oxygen bleaching and
the use of chlorine dioxide in themselves represent known
technology and that the delignifying effect of enzymes is
known from certain scientific publications, but that the use
of a combination of these as taught by the invention,
leading to particularly favourable results, has not been
known until now.
Furthermore, it is to be noted that, in the bleaching
procedure of the invention, the liquids obtained from the
washing stages phases after oxidation and enzyme treatment
can be burned in a soda recovery boiler, in which case the
amount of organic matter left in the bleaching effluent is
substantially smaller than in conventional chlorine bleach-
ing and also smaller than in oxygen bleaching carried out
without using an enzyme.
Example 2
Preliminary oxygen bleaching of 670 g of brown pine sulphate
pulp (dry matter content 30 %) was performed as follows: The
pH value of the pulp was adjusted to 12 using NaOH, where-
upon the pulp was subjected to a 45-min. oxygen treatment in
an autoclave at a temperature of 100 ~C and an oxygen over-
pressure of 2 bar.
After oxidation the pulp was washed in a Buchner funnel with
a 20-fold amount of water.
Next, the pulp was acidated with sulphuric acid to pH 6. An
amount of diluted enzyme mixture (Trichoderma [r] hemicellu-
lase MK 901, Cultor Oy) was then added to the pulp so that a
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1339593
mixture consistency of 10 % and xylanase activity of 5 U/g
of pulp dry matter was obtained. The pulp was subjected to a
2-hour enzyme treatment at 55 ~C.
After the enzyme treatment the pulp was washed using NaOH in
a dosage of 1.5 % of pulp dry matter. Consistency in the
alkali phase was 10 %, duration of treatment 90 min. and
temperature 60 ~C. After the alkali treatment the pulp was
washed in a Buchner funnel with a 20-fold amount of water.
Next, the pulp was subjected to an oxidating bleaching
treatment using chlorine dioxide in a dosage of 6.6 % of
pulp dry matter. Consistency of the mixture was 10 %, tem-
perature during treatment 70 ~C, and duration of treatment
180 min. After the treatment the pulp was washed in a
Buchner funnel with a 20-fold amount of water.
Next, the pulp was treated with NaOH (amount of NaOH 1 % of
the dry matter of the pulp) for 90 min. Viscosity of the
pulp was 10 %. After treatment, the pulp was washed with a
20-fold amount of water.
As the last bleaching stage, the pulp was subjected to a
chlorine dioxide treatment. The dosage of chlorine dioxide
was 3.3 % of pulp dry matter, mixture viscosity 10 %, treat-
ment temperature 75 ~C and duration 240 min. After treat-
ment, the pulp was again washed with a 20-fold amount of
water.
All of the wash water was collected and analyzed to deter-
mine the quantities of organic chlorine compounds (AOX) and
the chemical oxygen demand (COD) of the water. The results
are presented in Table 2.
In addition to the above-described experiment (exp. 3)
illustrating the invention, two reference experiments (exp.
1 and exp. 2) and two further experiments (experiments 4 and
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1339~93
5) illustrating the invention were carried out. The results
of all these experiments are also presented in Table 2
below.
Experiment 1 (reference): No enzyme treatment and no
preliminary oxygen bleaching of the pulp were employed. The
dosage of chlorine chemicals, of which 90 % consisted of
chlorine gas and 10 % of chlorine dioxide while in the other
bleaching stages the proportion of chlorine dioxide was
100 %, was such that the same target degree 87 of bleAching
was reached as in experiments 2,3,4 and 5.
Experiment 2 (reference): Instead of an enzyme treatment,
the pulp was subjected to normal preliminary oxygen bleach-
ing and chlorine/chlorine dioxide (80t20) treatment. The
preliminary oxygen bleaching was performed as in experiment
3 and the dosage of chlorine chemicals was such that the
target bleaching degree of 87 was achieved.
Experiment 4: The pulp was subjected to preliminary oxygen
bleaching, enzyme treatment and chlorine dioxide ble~ch;ng
as in example 3, but in the alkali washing stage oxygen was
supplied so that a 2-bar oxygen overpressure prevailed in
the reaction vessel.
Experiment 5: The pulp was treated as in experiment 4 except
for the first chlorine chemical treatment phase, in which
the chlorine chemical was 80 % chlorine dioxide and 20 %
chlorine gas. The dosage of active-chlorine was 2.3 x kappa.
In the next chlorine dioxide treatment phase the chemical
was 100 % chlorine dioxide, and the dosage was 2.9 %.
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13~93
As can be seen by the results presented in Table 2, the
amount of organic chlorine compounds in the bleaching
effluent produced in the bleaching of pine sulphate pulp
is reduced to the level of 1.8 kg per ton of chemical pulp
when oxygen bleaching and enzyme treatment are combined.
The same degree of bleaching is achieved by both methods.
The results indicate that the amount of chlorine in the
bleaching process can be substantially reduced if the pulp
is treated as provided by the invention by subjecting it
to preliminary oxygen bleaching, enzyme treatment and
oxygen alkali washing after the enzyme treatment and
replacing all or most of the conventionally used chlorine
gas with chlorine dioxide. Such a procedure allows the
amount of organic chlorine compounds in the bleaching
effluent to be substantially reduced, even by more than
70 %. If the bleaching effluents from all those stages
which precede bleaching stages involving chlorine
chemicals are subjected to burning, then the chemical
oxygen demand of the bleaching effluent will also be
reduced by more than 50 %.
