PROCEDURE FOR THE BLEACHING OF PULP INVOLVING THE USE OF LACCASE, A LIGNIN-MODIFYING OXIDATION-REDUCTION ENZYME

PROCESSUS POUR BLANCHIR LA PATE A L'AIDE DE LACCASE, UNE ENZYME D'OXYDO-REDUCTION MODIFIANT LA LIGNINE

English Abstract

The invention concerns a procedure for the bleaching of
pulp, in which procedure an oxidating bleaching chemical
containing chlorine is used. The essential feature of the
invention is that the pulp is treated, either in conjuction
with or before oxidation, with a lignin-modifying oxidation-
reduction enzyme, such as the laccase produced by the white-
rot fungus. The oxidating bleaching chemical used in the
procedure may be chlorine dioxide or a mixture of chlorine
dioxide and chlorine gas. The use of said enzyme reduces the
amount of chlorophenols and other forms of organically bound
chlorine in the bleaching effluent, simultaneously lowering
its chemical oxygen demand.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1 A process for bleaching pulp, comprising using an
oxidizing bleaching chemical containing chlorine,
wherein during or before oxidation, the pulp is enzyme
treated with the phenoloxidase, laccase, produced by
Trametes hirsuta white-rot fungus.
2. The process of claim 1, wherein the redox potential of
the enzymatic reaction is below 400 mV.
3. The process of claim 2, wherein the redox potential is
below 250 mV.
4. The process of claim 1, 2 or 3, wherein the pulp is
treated with laccase and washed before a first oxidation
stage.
5. The process of claim 1, 2 or 3, wherein the bleaching
chemical is chlorine dioxide or a mixture thereof with
chlorine gas, and between oxidation stages the pulp is
treated with an alkali.
6. The process of claim 1, 2 or 3, wherein the enzyme
treatment is at a temperature of 10 to 90°C and a pH of
3.0 to 10Ø

7. The process of claim 6, wherein the temperature is 40 to
80°C and the pH is 4.0 to 8Ø
8. The process of claim 1, 2, 3 or 7, wherein the pulp is
softwood pulp.
9. The process of claim 8, wherein the pulp is pine
sulphate pulp.
10. Use of the phenoloxidase, laccase, produced by Trametes
hirsuta white-rot fungus in a pulp bleaching process
using an oxidizing bleaching chemical containing
chlorine.

Description

1- 133518~
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
remaining in the pulp. Lignin is removed from the pulp by
bleaching, which is generally a process consisting of several
stages. During this process, the pulp is treated alternately
with oxidizing, lignin-degrading chemicals and chemicals
dissolving the degradation products. Oxidizing agents
commonly used are chlorine-containing chemicals and oxygen,
whereas alkali solutions are used for eliminating the
degradation products.
In the reactions occurring in bleaching using chlorine-
containing chemicals, lignin is converted into organicchlorine compounds, which remain in the spent bleach liquor.
Bleaching effluents are a problem in regard of environmental
protection because of the possible toxic nature of the
organic chlorine compounds contained in the effluents. More-
over, the chemical oxygen demand in bleaching effluentsreaches detrimental levels.
The bleaching effluents causing the worst environmental
pollution load are produced in the washing stages following
the first chlorination and the first alkali treatment in the
bleaching process. To reduce the pollution load, various
methods have been employed, including the use of chlorine
dioxide instead of chlorine gas, the use of oxygen as an
oxidating chemical in the first bleaching stage, and
biological purification of the bleaching effluents. However,
these methods have not yielded completely satisfactory
results. Although the amounts of chlorophenols and other
toxic chlorine compounds in the bleaching effluents have been
significantly reduced by using chlorine dioxide and employ-

C~ 1 335 1 84
ing oxygen bleaching, it has not been possible to achieve a sufficient reduction in thechemical oxygen demand values of the effluents. Therefore, the methods referred to
have required the employment of efficient biological purification.
The present invention provides a pulp bleaching procedure that enables the toxiccontent and chemical oxygen demand of the bleaching effluent to be reduced so asto reduce the need for purification of the liquor. The invention is characterized in
that, in conjunction with or before oxidation, the pulp is subjected to an enzyme
treatment using an oxidation-reduction enzyme modifying lignin.
It has been observed in earlier investigations that by using enzymes degrading
hemicellulose it is possible to separate lignin and/or hemicellulose from cellulose and
thus make the pulp more easily bleachable in the subsequent bleaching stages. This
effect has not been observed before in conjunction with the use of lignin-degrading
enzymes.
According to the invention, it has now been observed that a treatment with a suitable
redox enzyme reduces the amount of chlorine chemicals required for bleaching,
thereby also reducing the amounts of organic chlorine compounds left in the bleaching
effluent.
According to the invention, the redox potential of the enzymatic reaction should be
below 400 mV, preferably below 250 mV. Suitable redox enzymes are e.g.
phenoloxidases, such as the laccase produced by the white-rot fungus.
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 stages and washing the pulp after each of these stages to remove
the bleaching chemicals and degradation products.

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The enzyme treatment as taught by the invention is preferably
carried out in a temperature range of 10-90 C, the most
suitable range being 40-80C, with pH values in the range
3.0-10.0, preferably 4.0-8Ø The enzyme used is a lignin-
modifying oxidation-reduction enzyme, e.g. a laccase as
mentioned above.
The invention also concerns the use of a lignin-modifying
enzyme, e.g. the laccase produced by the white-rot fungus, in
the bleaching of pulp using an oxidating bleaching chemical
containing chlorine.
In the following, the invention is described in greater
detail by the aid of examples of embodiments based on
laboratory experiments.
Example 1
A diluted enzyme mixture of laccase produced by the Trametes
hirsuta white-rot fungus 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
laccase activity of 60 U/kg of pulp dry matter was obtained.
The temperature in the enzyme treatment was 55 C and the
duration of treatment 2h. After the enzyme treatment the
pulp was filtered in a Buchner funnel and the pulp cake
obtained was washed with ion-exchange-treated water so that a
wash ratio of 20 was obtained.
For reference, a corresponding portion of pulp was treated in
the same way except that no enzyme was added.
Next, both the pulp quantity which had undergone enzyme
treatment and the reference portion were bleached by a
process consisting of five successive stages of oxidation and
leaching. In the first stage, the chemicals used were
X

1335184
chlorine (90%) and chlorine dioxide (10%), in the second
stage sodium hydroxide, in the third stage chlorine dioxide,
in the fourth stage sodium hydroxide and in the fifth stage
chlorine dioxide.
The consumption of bleaching chemicals and the analysis
results representing the quality of the bleached pulp are
presented in Table 1.
The experiment shows that by using a lignin-modifying enzyme,
i.e. laccase, a substantial reduction in the amount of
chlorine has been achieved.
g

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TABLE 1
Enzyme treatment Final pH Viscosity Kappa Yield%
pine sulphate pulp,
kappa 27.4
Reference portion 8.70 1020 26.4 100.84
Enzyme-treated pulp 8.60 1045 26.5 100.63
Experiment continues with bleaching sequences D/C-E-D-E-D
Phase D/C 10/90ReferenceEnzyme treated
D/C 2 kappa pulp
D/C dosage % 5.28 5.30
consumption % 5.28 5.30
pH 1.65 1.55
consistency 3.5 3.5
temperature C 55 55
duration min. 45 45
Phase*E
E 0.9 kappa
E dosage % 2.38 2.39
pH 11.65 11.3
kappa 9.6 8.2
yield % 96.13 96.81
Phase D
dosage % 3.0 3.0
consumption % 3.0 3.0
pH 1.9 1.8
consistency % 10 10
temperature C 70 70
duration min. 180 180
Phase E2
dosage % 1.0 1.0
pH 11.2 11.3
consistency % 10 10
temperature C 60 60
duration min. 90 90
Phase D2
dosage % 1.0 1.5
consumption % 1.5 1.42
pH 2.2 2.2
consistency % 10 10
temperature C 75 75
duration min. 240 240
acidation pH 4.2 3
yield % 97.96 98.18
X

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(continued) TABLE 1
Phase D/C 10/90ReferenceEnzyme treated
total yield %94.96 95.65
degree of bleaching 83.8 87.1
viscosity
TC1 mg/kg 345 265
DKM extract %