Note: Descriptions are shown in the official language in which they were submitted.
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The present invention concerns a procedure for bleaching
cellulose pulp derived from hardwood, a bleaching chemical
containing chlorine being used in said procedure.
Cellulose pulp obtained by sulphate cooking in
particular is brown, mainly owing to lignin remaining in the
pulp. The present understanding is that this residual lignin
is linked to hemicellulose with covalent bonds. Lignin is
removed by bleaching, whereby the pulp is rendered usable for
high-quality paper and cardboard products.
Bleaching is usually accomplished in a multi-stage
process in which the pulp is alternately treated with
oxidizing, lignin-decomposing chemicals and with chemicals
dissolving the substances generated as decomposition
products. Above all, substances containing chlorine have
been used for oxidizing chemicals, such as pure chlorine gas,
chlorine dioxide or sodium or potassium hypochlorite,
although other types of oxidizing bleaching agents containing
no chlorine are also known. For extraction chemicals
removing the decomposition products, alkali solutions have
been used.
In the course of reacting, chemicals containing chlorine
cause chlorination of lignin, as well as of such fatty and
resinous acids which are present in the pulp. Therefore
chlorophenols are produced from lignin in the bleaching
process, which end up in the bleaching waste water escaping
from the pulp. Chlorine is further bound to those residues
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of lignin and acids which are still left in the cellulose
pulp after completed bleaching. In recent time this residual
chlorine has been increasingly regarded as an environmental
detriment at the phase when paper or cardboard products made
from the pulp end up as waste, and restriction imposed on the
allowable chlorine content of pulp are therefore to be
expected on the near future. Such restrictions would
particularly concern pulp used for foodstuff cardboard which
ends up on waste incineration plants and on dumps. This
problem is particularly salient in the case of pulp made from
hardwood, e.g. from birch or eucalyptus wood, which has a
high content of chlorine-binding extractive substances.
The invention is characterized in that the residual
chlorine content of bleached cellulose pulp made of hardwood
is lowered by subjecting the pulp to an enzyme treatment.
It has already been observed in earlier studies that it
is possible with the aid of enzymes to detach lignin from
cellulose. The conclusion herefrom is that when pulp
obtained in a cooking process is first treated with enzyme,
the use of chlorine-containing bleaching chemicals in the
subsequent bleaching step can be eliminated, partly or even
totally. In the present invention now the observation has
been made that when said enzyme treatment is combined with a
bleaching process using chlorine chemicals in substantially
standard manner, the result will be cellulose pulp having a
residual chlorine content which is substantially lowered from
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what it would be in the absence of enzyme treatment.
According to the invention, the enzyme treatment may be
carried out at an arbitrary stage of the bleaching process.
Thus, the pulp may be treated with enzyme in connection with
one of the oxidizing steps or extraction steps (alkali steps)
of the process or between the respective steps, prior to said
bleaching steps, or not until after said steps. The enzyme
to be used is advantageously hemicellulase, cellulase,
esterase or a mixture of these, and as suitable enzyme
products may be mentioned Multifect L 250 and Multifect K .
The temperature of the enzyme treatment may be in the range
of 10 to 90C, preferably 40 to 70C, and pH within the range
of 3.0 to 7.5, preferably 4.0 to 6Ø
The invention provides for the use of enzyme towards
reducing the residual chlorine content of bleached cellulose
pulp made from hardwood. Suitable enzymes include
hemicellulases, cellulases and esterases or mixtures of
these, e.g. the enzyme products Multifect L 250 and
Multifect K . Use of the enzyme is implemented, according to
the invention, in the manner set forth in the preceding
description of the bleaching procedure.
Quite specifically the use of the enzyme as taught by
the invention relates to reduction of the residual chlorine
content in bleached cellulose pulp meant to be processed into
foodstuff packaging cardboard or paper. This is because the
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increasingly stricter provisions regarding residual chlorine
concern particularly said cardboard and paper brands, which
end up as waste for disposal in waste incineration plants,
where the chlorine contained in them, may, when it burns,
produce highly toxic dioxin.
The appropriate enzyme activities (U - unit of activity)
of various enzyme categories that can be used in the present
invention are within the following limits,
Hemicellulases:
e.g. xylanase 0 to 106 U per kg of mass 1)
B-xylosidase 0 to 20,000 U per kg of mass 2)
Esterases: 0 to 100,000 U per kg of mass
Cellulases:
Filter paper
activity 0 to 20,000 U per kg of mass 3)
CMC activity 0 to 500,000 U per kg of mass 4)
B-glucosidase 0 to 20,000 U per kg of mass 5)
~-glucanase 0 to 500,000 U per kg of mass 6)
1) Khan, A.W., Tremblay, D., LeDuy, A., Enzyme Microb.
Technol., 8 (1986) 373-377
2) F. Delyn, M. Claeyssens, J. van Beeumen, C.K. de
Bruyne, Can. J. Biochem 56 43 (1987)
3) Ghose, T.K., Patnak, A.N., Bisaria, V.S. Symposiium
of Enzymatic Hydrolysis of Cellulose, Bailey, M., Enari,
T.M., Linko, M., Eds. (SITRA, Aulanko, Finland, 1975), 111-
136
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4) Mandels, M., Weber, J., Adv. Chem. Ser. 95 (1969)
391-413
5) Berghem, Ler, Pettersson, Eur. J. Biochem 37 (1973)
21-30
6) Sugar determination: Miller, G.L., Anal. Chem. 31
(1959) 426-428
The invention is described in more detail in the
following with the aid of two embodiment examples based on
laboratory experiments.
Example 1
220g pulp dry matter from birch sulphate cooking (dry
weight content of pulp 10%) were suspended into 50 mM of Na
nitrate buffer pH 5 in such manner that the ultimate
concentration of the mixture was 2.5%. Multifect K enzyme
was added so that the xylanase activity of the mixture was 56
U per g of pulp dry matter. The temperature during the
enzyme treatment was 39C and treatment time, 20 h. The pulp
was filtered after enzyme treatment with a Buchner funnel and
the pulp cake thus obtained was washed with ion-exchanged
water so as to obtain a washing ratio of 28.
A similar pulp batch was treated as reference in the
same manner but without enzyme addition.
In Table Ia below are presented results of measurement
from said two pulp stock batches, in this step of treatment.
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TABLE Ia
Enzyme treated No enzyme treatment
(Reference batch)
Kappa number 16.8 21
DKM extract content
(% of dry matter)0.27 0.23
Consistency
(dm3/kg 1065 1280
The enzyme-treated pulp stock batch and the reference
batch were next subjected to bleaching, which took place in
five consecutive oxidizing and extracting steps. In the
first step chlorine and chlorine dioxide were used for
chemicals, in the second step sodium hydroxide, in the third
step chlorine dioxide, in the fourth step sodium hydroxide,
and in the fifth step chlorine dioxide. In the following
Table Ib are presented the total chlorine quantities used in
bleaching, and the results of measurement found with the
bleached pulp batches.
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TABLE Ib
Enzyme treated No enzyme treatment
(Reference batch)
Dosage of active
chlorine,
(mg/g of pulp dry
matter) (2 x kappa) 33.6 42
Yield 93.6% 97.5%
DKM extract content,
% of dry matter 0.26 0.29
Consistency
(dm3/kg) 1035 1210
Residual chlorine
content
(mg/g) 0.845 1.270
The results reveal that the residual chlorine content
after bleaching is clearly lower in the enzyme-treated
cellulose pulp than in the pulp which received no enzyme
treatment. It is further seen that no decisive weakening of
strength values took place in the enzyme-treated pulp.
Example 2
In this experiment three pulp stock batches obtained by
birch sulphate cooking were used, which had the same
consistency as those in the preceding Example 1. Two batches
were treated with enzyme as presented above, while the third
batch constituted a reference batch. The enzymes differed
from those used in Example 1 in that with one of them (in
Batch 1) in the mixture was achieved, in addition to xylanase
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activity 56 U per g of pulp dry matter, cellulase activity
1.5 U per g of pulp dry matter and in the other (Batch 2), in
addition to xylanase activity 56 U per g of pulp dry matter,
cellulase activity 1.1 U per g of pulp dry matter. The
results after enzyme treatment are shown in Table IIa.
TABLE IIa
Enzyme pre- Enzyme pre- No enzyme treatment
treatment treatment (Reference batch)
(Batch 1) (Batch 2)
Kappa number15.8 15.0 18.8
DKM extract
content
(% of dry matter) 0.32 0,32 0.26
Consistency
(dm3/kg) 1025 1130 1305
The pulp batches were hereafter bleached with chlorine
chemicals and sodium hydroxide as in Example 1. The total
chlorine dosage in mg/g was 2 x the kappa number in each
case. The results after bleaching are shown in Table IIb.
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TABLE IIb
Enzyme pre- Enzyme pre- No enzyme treatment
treatment treatment(Reference batch)
(Batch 1) (Batch 2)
Yield 94.6 - 97,4
DKM extract
content
(% of dry matter) 0.36 0,35 0.37
Consistency
(dm3/kg) 980 1065 1210
Total chlorine
content
(mg/g) 1.15 1.15 1.46
The results reveal in this instance, too, that enzyme
treatment substantially reduces the residual chlorine content
of the bleached pulp.
Example 3
Bleached birch sulphate pulp was used in this
experiment. The pulp had been bleached using for bleaching
chemicals in the first step chlorine (90%) and chlorine
dioxide (10%), in the second step sodium hydroxide and in the
third step oxygen.
The bleached pulp was subjected to enzyme treatment
under identical conditions, and using the same enzyme, as in
Example 1. After treatment, the pulp was washed with water
and dried at room temperature. The residual chlorine content
was measured both of the enzyme-treated pulp and of the
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untreated, original bleached sulphate pulp, the result 1.18
being found for the former and 1.56 mg/g for the latter.
Thus, the residual chlorine content of bleached pulp could be
substantially lowered by means of enzyme treatment.
The test demonstrates that the desired result is
attained with the invention also in the case the cellulose
pulp is first bleached and treated with enzyme only
thereafter.
It is obvious to a person skilled in the art that
different embodiments of the invention are not confined to
what has been presented in the foregoing by way of example,
and that they may vary within the scope of the claims
following below. For instance, one may in the first step of
conventional five-step chlorine bleaching use either chlorine
alone or chlorine in mixture with chlorine dioxide. It is
also possible to make the time during which the enzyme is
allowed to act considerably less than the mentioned time of
20 hours. Treatment times of a few hours, or ever shorter
than one hour, are thus conceivable, depending on the enzyme
chosen and on its activity.
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