Note: Descriptions are shown in the official language in which they were submitted.
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W O96/35015 PCT/SE9~CIC~
Treatment of pulp with chelating agent and subsequent
bleaching at a ph value above 9
This invention relates to chlorine free bleaching of pulp
from lignocellulosic material. The method comprises treat-
ment of the pulp with chelating agent and subsequent bleach-
ing in at least one step with chlorine free bleaching
agents.
Modern chlorine free bleaching of chemical papermaking
pulp is carried out to a large extent with hydrogen per-
oxide in sequences, which normally also include the bleach-
ing chemicals oxygen and at times even ozone. It is in
future possible that also so-called peroxides, such as per-
acetic acid and peroxymonosulfuric acid will be used.
The use of several bleaching chemicals is due to the the
fact that the lignin, which must be eliminated from the
papermaking pulp in order to achieve a high pulp ISO-
brightness, is strongly bonded. For carrying out the bleach-
ing in a lenient way and with a minimum use of chemicals,
it is favourable to use a combination of several chemic-
als. The individual chemicals are added step-by-step with
a washing of the pulp between the steps. When the bleach-
ing is not required to be driven to full brightness, i.e.
89-90% ISO, the bleaching sequence usually includes only
the chemicals oxygen and hydrogen peroxide.
The greatest charge, counted in kg per ton pulp, normally
is that of hydrogen peroxide, with a magnitude of about
15-45 kg per ton papermaking pulp. Hydrogen peroxide is a
highly effective bleaching chemical when the bleaching
conditions are the right ones. By using only oxygen and
hydrogen peroxide, brightness levels pf substantially
full brightness, i.e. almost 90% ISO, have been achieved
on mill scale. This presupposes, however, very special con-
ditions, such as that the papermaking pulp is subjected
CONFIRMATION
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to far-reaching digestion and oxygen bleaching, i.e.
below kappa number 10, and that the pulp prior to the
peroxide step is made substantially entirely free of
disturbing metals, such as manganese, copper, iron and
other. The presence of such metals implies, that parts
of the hydrogen peroxide are degraded without any bleach-
in8 effect. This tendency increases with increased metal
content, and the effect becomes more obvious wi-th in-
creased charge of hydrogen peroxide. As in many cases the
hydrogen peroxide step is driven very far in order to
bring about a high brightness level, the hydrogen peroxide
steps carried out today are very sensitive even to low
contents of disturbing metals.It has been learned, thus,
by experience that the manganese content in pulp entering
the hydrogen peroxide step must not exceed 1 g/ton pulp,
because this would affect the final brightness level
after the peroxide step. The pulp is desired to have
high brightness, because it can then be sold at a higher
price.
A low metal content in the pulp prior to the hydrogen
peroxide step is obtained by treating the pulp with a
chelating agent, for example EDTA or DTPA, and there-
after to wash the pulp in order to eliminate the released
metal ions before the hydrogen peroxide step. This method
is described in the patent SE 467 006 (EKA Nobel). Accord-
ing to the patent, the treatment with chelating agent
(the so-called Q-step) shall be carr~ed out between
lo_loo~C for a period of up to 6 hours at a pH between
3.1 and 9. EKA Nobel has at a later date, a.o. at the
- I-zernational Pulp BIeaching Conference in Stockholm 1991,
shown more explicitly how the chelatïng age~t treat--
ment is to be carried out. It was shown there that
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the pH value should be about 5-6, and that the longer
the treatment time and the higher the temperature are
during the Q-step, the lower the disturbing metal cont-
ent will be in the pulp before the hydrogen peroxide
1 step, and that a higher pulp brightness can be achiev-
ed at a given hydrogen peroxide charge. The viscosity
of the pulp after the P-step was also shown to be high-
er'when the pH value in the Q-step was about 5-6.
The pulpmaking industry of today manufactures paper-
making pulp mostly by digesting and oxygen bleaching
the pulp to a far-reaching degree, whereafter the blea-
ching proper is carried out by using all towers and
tanks in the existing plants, so that a very long stay-
time in the chelating agent step, and especially in
the hydrogen peroxide step, is obtained. The pulp is
also washed very accurately between the different
bleaching steps. Such washing, however, requires large
amounts of water, and therefore it cannot be used if
and when the bleach plants must be closed at their
effluent, which in most pulp mills will be required
in order to meet various environmental obligations.
It is also desirable to close the bleach plants in
such a way , that, owing to increased contents of
organic materials and process disturbing compounds and
metals, the consumption of bleaching chemicals does
not increase significantly over that of a system
which is not closed, and in such a way, that incrustr-
ations can be prevented at any costs. This latter
problem is strongly connected to increased contents
of a.o. calcium and barium, which readily form precip-
itations (incrusts) of carbonate, sulphate and oxalate
type. The risk Ofincrustration,- increases very much a.o.
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when the system also is exposed to large pH and temp-
erature changes, because then metals etc. can be releas-
ed from the pulp and accumulate to a high content in
the surrounding liquid phase.
The present invention has the object to solve the
aforesaid problems. It implies, that papermaking pulp
can be bleached with chlorine free chemicals in an
effectively closed system without forming disturbing in-
crustrations.
The characterizing features of the invention are app-
arent from the attached claims.
The invention is described in greater detail in the
following by way of some embodiments thereof.
The invention is directed to chlorine free bleaching
pf pulp, where pH is held above 9, suitably between
9 and 11.5, during the entire delignification and
bleaching process. This implies, that the chelating
agent step (Q-step) is carried out at a considerably
higher than normal pH prior to a subsequent peroxide
bleaching (P-step). It was found that this is poss-ible
by using chelating agents, which are active at a con-
siderably higher pH value than that which normally is
used at bleaching according to SE-patent 467 006 (see
above). The chelating agent EDTA normally used is a
poor chelating agent at alkaline conditions and can
be replaced, for example, by DTPA, which is active evwn
at higher pH.
By carrying out the entire digestion and bleaching
process at a relatively uniform pH, and so that the
pH never turns acid, the risk of precipitations of in-
- crustrations can be reduced substantially. Incrustrat-
ions normally become a problem difficult solve when
the bleach plant is closed, because the concentrations y
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of a.o. calcium, barium, oxalate, carbonate and sul-
phate can increase above the solubility maximum, which
results in precipitations to begin to form. Normally,
some type of initiation of the precipitation process
is also required, for example via a pH or temperature
change, in order to cause the precipitation to start,
but in that case the precipitation can proceed very
rapidly. Another advantage is, that the metals and
a.o. calcium and barium, which are bound to the pulp
prior to the bleaching, will remain on the pulp and
are drained off from the closed bleach plant without
being released, which implies that a build-up of cont-
ent is avoided. The transistion metals dangerous for
the process, i.e. especially manganese, further can
be oxidized by adding an oxidation agent, so that
their capacity of degrading peroxide is reduced, alter-
natively entirely eliminated. The oxidation of these
metals is carried out preferably with oxygen gas and/or
hydrogen peroxide at a temperature of above 75 C
prior to the bleaching with chlorine free bleaching
agents, such as hydrogen peroxide.
A suggested suitable sequence comprises: digestion -
Q-step - 02-delignification - Q-step - P-step.
An initiating chelating agent step and an oxygen step,
thus, can be carried out directly after the digestion.
Thereafter the treatment proper with chelating agent
is carried out prior to the hydrogen peroxide bleach-
ing. In this case the pH value shall be held above
9, suitably at 9-11, preferably at 9-11.5, through
the entire sequence. Both Q-steps can advantageously
be carried out in the presence of oxidation agent,
preferably oxygen and/or hydrogen peroxide for the
oxidation of metals present. The metals become thereby
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less disturbing for the bleaching. As an alternative
or a compliment to the final hydrogen peroxide ~ each-
ing, other chlorine free bleachings can be carried out
where oxygen and/or peracids can be used. The bleach-
ing preferably is carried out in at least two oxidiz- ~,
ing steps, where hydrogen peroxide and oxygen can be
used in the same bleaching step.
When DTPA is used as chelating agent, the charge should
be 1-lO kg (100%--product) per ton PU1PJ preferably
2-5 kg per ton pulp.
The invention also implies that an effluent-free pro-
cess can be obtained. The process is preferably closed
in such a way, that washing steps are carried out after
every treatment step (Q-step and, respectively,
bleaching step), and the filtrate from every washing
step is recycled as washing liquid in preceding washing
steps. A high and uniform pH level being maintained
during the entire process, no precipitations of in-
crustrations occur, in spite of the closing of the
bleaching process.
The invention, of course, is not restricted to the em-
bodiments described, but can be varied within the scope
of the invention idea.
