Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to a procedure for the
bleaching of pulp, in which procedure an oxidating
bleaching chemical containing chlorine is used.
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
bleaching, 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. Oxidizing agents com-
monly used are oxygen and chlorine-containing chemicals
such as pure chlorine gas, chlorine dioxide and sodium and
calcium hypochlorites, whereas alkali solutions are used
for eliminating the degradation products.
In the reactions occurring in bleaching using chlorine-con-
taining chemicals, lignin is converted into organic chlor-
ine compounds, which are dissolved in the bleaching
effluent. 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 liquids. Besides, the chemical oxygen de-
mand 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 concentrated on other parts of the process except
bleaching, the relative significance of bleaching as a pol-
luting factor has been increasing.
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. To reduce the emissions of efflu-
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ents, various methods have been employed, e.g. the so-
called extended digestion, the use of chlorine dioxide as
an oxidating bleaching chemical, oxygen bleaching and
biological purification of the bleaching effluent. How-
ever, the results achieved by these methods are not been
completely satisfactory. Although the amounts of chloro-
phenols 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 3f the effluents. Therefore, the
methods referred to have required the employment of
efficient biological purification.
The present invention enables the toxic content and the
chemical oxygen demand of the bleaching effluent to be
reduced so as to reduce the need for purification of the
effluent. The invention is characterized in that a
chemical with a chlorine dioxide content of at least 50% is
used in the oxidation stage, that the pulp is subjected to
enzyme treatment in conjunction with or before the oxida-
tion, and that after the oxidation and enzyme treatment the
pulp is treated with an alkali.
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 used in the next bleaching phase. According to
the invention, it has now been observed that enzyme treat-
ment substantially reduces the amount of organic chlorine
compounds in the bleaching effluent while at the same time
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reducing its chemical oxygen demand, especially when at
least 50%, preferably at least 70%, of the bleaching
chemical used in the oxidation stage consists of chlorine
dioxide. If 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 is subjected to enzyme
treatment and washing before the first oxidation stage.
The enzyme breaks down hemicellulose and/or lignin con-
tained in the pulp and renders the pulp more spongy, thus
enhancing the effect of the chemicals in subsequent oxida-
tion and alkali treatment stages. By washing the pulp
after the enzyme treatment, the degradation products are
removed and can be burned so that they will not contribute
to the effluent emissions at all.
Except for the enzyme treatment, the bleaching of pulp by
the procedure of the invention can be performed in the
conventional manner by employing alternate oxidation and
alkali treatment phases and washing the pulp after each of
these phases to remove the bleaching chemicals and degra-
tions products.
The enzyme treatment as taught by the invention is prefer-
ably carried out in a temperature range of 10-90~ C, the
most suitable range being 40-75~ C, with pH values in the
range 3.0-10.0, preferably 4.0-9Ø The enzyme used can be
a hemicellulase, cellulase, pectinase, esterase or a
mixture of these.
The invention also concerns the use of an enzyme for
reducing the chlorine content of the bleaching effluent
produced in the bleaching of pulp when an oxidating
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bleaching chemical containing at least 50% of chlorine
dioxide is used. The enzyme is preferably hemicellulase,
cellulase, pectinase, esterase or a mixture of these. The
enzyme 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 examples of embodiments based on
laboratory experiments.
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Example 1
A diluted enzyme mixture (streptomyces hemicellulase) was
added to 220 g of dry matter obtained from birch sulphate
pulp (with a dry matter content of 30%) so that a mixture
with a consistency of 10% and xylanase activity of 5 U/g of
pulp dry matter was obtained. The temperature in the enzyme
treatment was 55 ~C, the duration of treatment 2 h and pH
8Ø
After the enzyme treatment the pulp was subjected to an oxi-
dating bleaching treatment using a mixture contAin;ng 90 %
chlorine dioxide and 10 % chlorine gas, in a dosage equal to
1.4 x post-enzyme-treatment kappa number of the pulp. Treat-
ment temperature was 55 ~C and duration of treatment 45 min.
After the oxidation phase the pulp was washed in a Buchner
funnel with à 20-fold amount of water.
Next, the pulp was subjected to an alkali treatment using a
% sodium hydroxide solution in a dosage of 0.8 x kappa.
Consistency of the mixture was 10 %, treatment temperature
~C and duration of treatment 90 min. After the alkali
treatment the pulp was washed in the same way as after the
oxidation phase.
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. For the combined wash waters, the amount of organic-
ally bound chlorine (AOX) and the chemical oxygen demand
(COD) were determined, and these are presented in Table 1
(experiment 3) below.
In addition to the above-described experiment (exp. 3) il-
lustrating the invention, two reference experiments (experi-
ments 1 and 2) and an additional experiment (exp. 4) were
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carried out, and the AOX and COD values of the combined wash
waters obtained from different stages of the experiments are
also presented in Table 1. The experiments were performed as
follows:
Experiment 2 (reference): No enzyme treatment was employed.
The dosage of bleaching chemicals in different stages of
treatment during the bleaching was 2 x kappa, the commonly
used dosage. In other respects, the experiment was analogous
to that described above (exp. 3).
Experiment 1 (reference): No enzyme treatment was employed.
The bleaching chemical used in the oxidation stages was pure
chlorine gas, in a dosage of 2 x kappa. In other respects,
the experiment was analogous to that described above
(exp. 3.)
-
Experiment 4: The pulp was treated with an enzyme andbleached as described above (exp. 3). In addition, the pulp
was washed after the enzyme treatment,. before the first
oxidating treatment, with a mixture of chlorine dioxide and
chlorine gas. The present invention comprises a procedure
employing the principle of this experiment.
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TABLE 1
Bleaching AOX COD
(kg/t of chemical pulp)
Experiment 1 (reference)
No enzyme treatment 2.6 58.1
Cl2 bleaching
Experiment 2 (reference)
No enzyme treatment
Bleaching with mixture 1.0 55.0
90 % C10z + 10 % Clz
Experiment 3
Enzyme treatment
Bleaching with mixture 0.6 40
90 % C102 + 10 % C12
Experiment 4
Enzyme treatment, washing
Bleaching with mixture -0.6 40
90 % C1 02 + 1 0 % C12
The results indicate that, in comparison to corresponding
bleaching without enzyme treatment, enzyme treatment com-
bined with chlorine dioxide bleaching as provided by the in-
vention substantially reduces the pollution load caused by
the bleaching effluents as measured in terms of AOX and COD.
values. Compared to the commonly used chlorine bleaching,
the improvement achieved is even more distinct. Furthermore,
it is worth noting that in the experiments representing the
invention, the same degree of bleaching was achieved as in
the reference experiments,-which means that enzyme treatment
has no adverse effect on the bleaching result.
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Example 2
A diluted enzyme mixture (streptomyces hemicellulase) was
added to 220 g of dry matter obtained from pine sulphate
pulp (with a dry matter content of 30%) so that a mixture
with a consistency of 10% and a xylanase activity of 5 U/g
of pulp dry matter was obtained. The temperature in the
enzyme treatment was 55 ~C, the duration of treatment 2 h
and pH 8.5.
After the enzyme treatment the pulp was washed in a Buchner
funnel with a 20-fold amount of water.
After the washing, the pulp was subjected to an oxidating
bleaching treatment using a mixture wh-ich contained 80 ~
chlorine dioxide and 20 % chlorine gas. The dosage of the
mixture was 1.4 x kappa number of the pulp after enzyme
treatment. Treatment temperature was 55 ~C and duration of
treatment 45 min. After the oxidating phase the pulp was
washed in a Buchner funnel with a 20-fold amount of water.
Next, the pulp was subjected to an alkali treatment using a
% sodium hydroxide solution in a dosage of 0.9 x kappa.
Consistency of the mixture was 2 %, treatment temperature
45-55 ~C and duration of treatment 90 min. After the alkali
treatment the pulp was washed in the same way as after the
oxidation phase.
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. For the combined wash waters, the amount of organic-
ally bound chlorine (AOX) and the chemical oxygen demand
(COD) were determined, and these are presented in Table 2
(experiment 4) below.
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In addition to the above-described experiment (exp. 4) il-
lustrating the invention, three reference experiments
(experiments 1-3) were carried out, and the AOX and COD
values of the combined wash waters obtained from different
stages of the experiments are also presented in Table 2. The
experiments were performed as follows:
Experiment 3: No enzyme treatment was employed. The dosage
of bleaching chemicals in different phases of treatment
during the bleaching was 2 x kappa, the commonly used
dosage. In other respects, the experiment was analogous to
that described above (exp. 4).
Experiment 2: The pulp was treated with an enzyme as de-
scribed above. The bleaching chemical used in the oxidation
phases was pure chlorine gas, in a dosage of 2 x kappa. In
other respects, the experiment was analogous to that de-
scribed above (exp. 4.)
Experiment 1: No enzyme treatment was employed. The bleach-
ing chemical used in the oxidation phases was pure chlorine
gas, in a dosage of 2 x kappa. In other respects, the
experiment was analogous to that described above (exp. 4.)
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TABLE 2
Bleaching AOX COD
(kg/t of chemical pulp)
Experiment 1 (reference)
No enzyme treatment 4.0 71
Cl2 bleaching
Experiment 2 (reference)
Enzyme treatment, washing 2.9 63
Cl2 bleaching
Experiment 3
No enzyme treatment
Bleaching with mixture 1.7 44
80 % C102 + 20 % C12
Experiment 4
Enzyme treatment, washing
Bleaching with mixture 1.0 36
80 % C102 + 20 % C12
The results indicate that, in comparison to corresponding
bleaching without enzyme treatment, enzyme treatment com-
bined with chlorine dioxide bleaching as provided by the in-
vention substantially reduces the pollution load caused by
the bleaching effluents as measured in terms of AOX and COD
values. Compared to the commonly used chlorine bleAching~
the improvement achieved is even more pronounced. It can
also be seen that when used in conjunction with chlorine
dioxide bleaching, enzyme treatment produces a much more
distinct improvement than when used in conjunction with the
conventional chlorine dioxide bleaching. It should be noted
that in this case, too, the same degree of bleaching of the
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pulp was achieved in the experiment representing the inven-
tion as in the reference experiments, which means that
enzyme treatment had no adverse effect on the bleaching
result.
In the alkali treatment stages phases of the bleaching
process, oxygen may be present in addition to the alkali,
and the duration of the enzyme treatment may vary from a
few minutes to several hours (e.g. 5 min. - 10 h).
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