Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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March 18, 1991
_
Dr. Hans-Peter Call
Heinsberger Strasse 14a
D-5132 Uebach-Palenberg
A process ~or Enzymatic Bleaching of Celluloses
The present invention concerns a process for enzymatic
bleaching o celluloses.
The biological ~roc~sses for cellulose production known
so far work with micro-organisms, especially fungi. From
DE-PS 31 10 117, for example, there is known a process
for ohtaining cellulose from wood or other plant fibre
materials where the lignocellulose is decomposed with
the aid of pocket rot fungi. However, the processes
working with micro-organisms have considerable disadvan-
tages. For example, it is still not possible to achieve
decomposition and separation of the lignine from its
accompanying polymers (cellulose) without simultaneous
growth of the micro-organisms and without loss of cellu-
lose. The simultaneous growth of the fungus causes long
decomposition periods which can last up to several
weeks.
Owing to the akove-described difficulties of using
micro-organisms, the possibilities of using isolated
enzyme systems have been examined during the last few
years. Especially, the enzymes of the pocket rot fungus
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Phanerochaete chrysosporium were researched and clarifi-
cation obta ned in many details. For example, it is
~nown from "Biotechnology in the Pulp and Paper In-
dustry, ~rd International Conference, Stockholm, 1986"
that the e~uilibrium of the reaction is on the polymeri-
sate side during the decomposition o~ lignine, i.e. non-
cell systems do not decompose lignine, but contribute to
its polymerisation
Since the discovery of the lignolytical enzymes of the
pocket rot fungus Phanerochaete chrysosporium, a number
of enzymatical processes for bleaching cellulose by
means of living ~ungus systems or non-cell systems, too,
have become known in literature. Also, a number of expe-
riments were carried out to blea~h by means of haeme
systems. All these systems re~uire reaction times of
more than 12 hours. This means that all these systems
require considerable time and are costly. The latter
applies es~ecially to the pure heame systems.
Tod~y, bleaching is still carried out purely chemically
in several stages by adding chlorine. However, chlorine
bleaching causes considerable environmental problems. In
the general bleaching processes, the kappa number, i.e.
the lîgnine content, is reduced by removing the chromo-
phore residual lignine conden,ation products that occur
during the koiling process fcr lignine removal, and the
cellulose is thus lightened.
The Present invention now has the object of providing a
process for the enzymatical bleaching of celluloses
which no longer has the disadvantages of the biological
and chemica] bleaching processes described above.
This object is solved by a~ adjusting a redox potential
in the range between 200 and 500 mV while metered addi-
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tion of oxidants and reductives and the addition ofsalts and complexing agents to an aqueous solution con-
taining cellulose takes 21ace simultaneously, b) start-
ing a bleaching reaction by adding lignolytical enzymes,
and c) maintaini~g the reaction for a period of 15
minu~es up to 12 hours during which time it is con-
stantly stirred. This type of process prevents repoly-
merisation of the lignine and makes possible depolyme--
risation in the first place.
.
The redox potential is preferably in the range between
250 and 450 mV. In the process according to the inven-
tion, it can be determined with the aid of a redox
electrode and, by means of a control unit and an actua-
tor, can be kept constant during the entire reaction
time by adding o~idants and reductives, salts and pheno-
lic compounds.
Hydrogen peroxide, oxygen and ozone are preferably used
as oxidants. Suitab]e reductives are ascorbic acid,
dithionite and sodium bisulphite.
The salt added to the aqueous solution containing cellu-
lose is cupric sulphate. Mn(II)sulphate, Mn(III)acetate,
Fe(II)sulphate, Ti(III)chloride, Ce(III)nitrate and
Ce(IV)ammonium nitrate can addi-tionally be used as
salts. Salts containing the elements of zinc, antimon
and lead are also suitable.
Possibly, phenolic compounds can be added to the aqueous
solution containing cellulose. Especially proven com-
pounds of this type are veratryl alcohols.
Further, fatty acids such as oleic acids, haeme com-
pounds such as haemoglobin and bleaching reagents such
as sodium perborate can be added to the aqueous solution
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containing cellulose. Moreover, an after-bleach can be
ca~ried out wi-th conventional bleachirlg agents such as
sodium hypochlorite, 2~ chlorine dioxide, ozone, H202
and sodium dithionite.
Further, it is pointed out that the addition of complex-
ing agents ic very important for the success of the pro-
cess according to the invention. Preferably, ethylene
diamine tetraacetic acid (EDTA) or dieth~lene triamine
pentaacetic acid (DTP~) are used as such complexing
agents. As soon as the required redox potentiaI has been
adjusted, the bleaching process sets in comparatively
quickly. This process can already be completed after a
few minutes, but in that case the kappa figure which,
depending on the cellulose content, can be decreased up
to 90 % after several hours is hardly reduced.
In addition to the above-mentioned substances, further
substances can be added to the aqueous solution con-
taining cellulose. These can be sodium hypochlorite,
sodium perkorate, detergents, surface-active agents and
polysaccharides such as glucanes and xanthane.
LignolyticaI enzymes are preferably used as enzymes in
the process according to the invention. Among others,
these are phenol oxidases, laccases and peroxidases~ The
effectiveness of the process can be further increased by
using pectinases and/or hemicullases. Especially sui~-
able lignolytical enzymes are those obtained from the
pocket rot fungus Phanerochaete chrysosporium. However,
the use o~ such enzymes is already known from U.S.-PS 4
692 413, 4 69~ 895 and 4 687 741. However~ enzymes
obtained from special mutants of Phanerochaete chryso-
sporium are used Lor bleaching according to these print-
ed patent spec~fications. This is not necessary in the
process according to the invention. Rather, all lignoly-
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tical enzymes known today can be used if the above-
described prerequisites are complied with. The main
difference can ke found in the function of the reduc-
tives or oxidants added by metered addition as well as
the mediators which capture the radicals. This is be-
cause these substances prevent repolymerisation and
therefore make possible the decomposition of lignine in
the akove-described manner and quantity within the sti-
pulated short period of time in the first place.
The p~ value is gerlerally between 2 and 5 in the process
according to the inv~ntion. A pH value of 3 is prefer-
red. The temperature is 20 to 60C, preferably 40C. If
these conditions are complied with, a redox potential of
200 to 500 mV is adjusted when the above-mentioned
sukstances are added. It is determined by the ratio of
~he different substances added to the reaction vessel.
It can be maintained during the entire reaction period,
if the addition of oxidants and reductives, salts and
pos$ibly phenolic compounds is monitored and controlled
accordingly.
~ith the aid of the bleaching process described above,
it was possible or the first time to bleach celluloses
within a very short period of time (15 minutes to 2
hours), ak physiological temperatures (40C), without
pressure and with a minimum of chemical sukstances added
in a manner which is inexpensive and, especially impor-
tant, does not affect the environment. Another advantage
of the process according to the invention is the possi-
bility of operating the process continuously.
The process according to the invention is illustrated by
the following e~amples.
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50 a abs dry (atro) cellulose (sulphate cellulose) are
stirred in a stirring vessel at 1 % consistency, approx.
500 rpm and 40C. The pH value is adjusted to pH 3 by
means of 1 n HCL. 0.1 -1.5 % H2O2 based on abs.dry sub-
stance, aoprox 2 ~ 10-5 % to 2 x 10-3 % veratryl alco-
hol (VA), Q.l % EDTA or DTPA and 0.001 - 0.01 % cupric
sulphate based on abs.dry substance are added. After 500
- 5,000 IU lignolytical en~ymes (1 IU = turnover of 1
nmol VA/min. to veratryl aldehyde) have been added, the
bleaching process is started by simultaneous metered
addition of H2O2 and sodium bisulphite solution. The
redox potential of approx. 400 mV is maintained. Once
the process has keen initiated, it is continued for 2
hours. Process control and adjustment is carried out by
means of a redox electrode and a pump control.
