Note: Descriptions are shown in the official language in which they were submitted.
WO 96/09434 2 2 0 0 3 2 7 pCT~95100506
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METHOD OF BLEACHING KRAFT PULP
Field of invention
The present invention relates to a method of bleaching
sulfate pulp, i.e. so-called kraft pulp, to a brightness
of at least ISO 85 by using elemental chlorine free
chemicals such as, for instance, oxygen and peroxide.
Backctround art
For environmental reasons, among other things, there has
been a tendency to reduce the use of elemental chlorine
in bleaching of cellulose pulp. The first attempts led
to the use of chlorine dioxide which in some sense is a
better alternative in view of protection of the
environment than the use of elemental chlorine. Also
marketing factors direct the development towards giving
up the use of chlorine altogether, both elemental
chlorine and chlorine compounds. Very many customers
appreciate greatly the fact that the product they use has
been produced without using chlorine at all (TCF). The
minimum requirement is that at least no elemental
chlorine in gas form has been used in the manufacture of
the product (ECF).
A few years ago when ozone bleaching at medium
consistencies had been developed to an industrially
applicable mill-scale stage (cf. European patent 0 397
308) most companies developing processes and apparatus
for the cellulose industry concentrated almost
exclusively on developing ozone bleaching to a form which
would be applicable in their own processes.
Ozone, however, has its drawbacks. The most significant
of these is perhaps the high price which is mostly due to
the fairly difficult production method. A further
drawback is its high reactivity which means that
bleaching equipment using ozone, as well as the ones
COt~fiRll~t'tON COPY
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PCT/FZ95/00506
using gaseous chlorine; must be kept absolutely gas
tight. For the same reason, ozone easily damages
cellulose itself if the dose of ozone is not exactly
correct: or if adequate mixing is not provided.
For the reasons discussed above, both peroxide (P) stages and
alkaline oxygen stages activated with peroxide (EOP), and
particularly pressurized stages of this kind, have recently been
included in bleaching sequences (cf. article ~~Medium consistency
Pulp w,~sher generates superior washing efficiency~~, TAPPI
Journal, June 1990; PCT Application No. PCT/FI92/00198, published
January 7, 1993, WO 93/00470; European patent application no.
93301222, filed February 19, 1993, Publication No. 0557112A1,
published August 25, 1993; PCT Application No. PCT/FI93/02222,
published September 15, 1994, WO 94/20673. However, as may be
concluded from the above publications, ozone still has a very
significant role in bleaching sequences.
Disclosure of invention
The present invention pays attention to the environment
protection values which direct the development away from the use
of chlorine chemicals, and to economic aspects which at least to
some extent speak against the use of ozone. A bleaching chemical
cheaper than ozone, and also as to its bleaching properties very
competitive with ozone, is hydrogen peroxide, to name one
preferred alternative in the broad family of peroxides.
Until recently, bleaching sequences utilizing peroxide have
included as an essential part: thereof a so-called Q stage
which removes metals and precedes each peroxide stage and is
separated by an efficient intermediate washing stage from
the peroxide stage. This has been considered necessary also
when the sequence hz~s included several P stages. The Q stage
may involve mere acidification of the pulp at a low pH
(pH approx. 3 - S) or treatment with a complex former such
as EDTA or DTPA at a pH of 4 - E~. In other words, until now it
has been considered necessary to wash away prior to the peroxide
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bleaching stage from the pulp the heavy metals it
contains as the metals were believed to decompose the
peroxide. It has been found out, however, that in
accordance with the method of the present invention it is
not always necessary to pretreat the pulp by a Q stage to
remove heavy metals but the reaction conditions in the
peroxide stage may be made suitable for the peroxide
bleaching process irrespective of the presence of heavy
metals in the pulp. This kind of an improvement in the
peroxide bleach.in:g stage brings a pulp mill remarkable
investment savings as the Q stage, which in view of the
bleaching process itself is unnecessary, is no longer
needed before a peroxide stage whereby also the washer
between the Q stage and the P stage is made unnecessary.
The method of the invention has made it possible to
bleach pulp to a brightness of 85 - 88 ISO, and even
greater, without the use of chlorine and also without the
use of ozone if such is considered a risk.
A characteristic feature of the present invention is that
pulp is at least partially bleached with peroxide alone,
or with peroxide and oxygen, at acidic conditions. This
kind of an acidic peroxide method has been suggested for
example in European Patent No. 433,138 and some
older articles. However, both the patent application
mentioned above and the articles deal with bleaching of
sulfite pulp with peroxide at acidic conditions. In pulp
production, one of the most significant differences
between the sulfate and the sulfite processes is the
acidity of the cook; a sulfate cook is alkaline and a
sulfite cook acidic. When the pH of the cook is acidic,
heavy metals are efficiently separated to the digestion
liquid and most of them are removed during brown stock
washing prior to bleaching. In sulfate digestion, heavy
metals are not separated into the digestion liquid and
thus cannot be removed in the brown stock washing.
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The characteristic features of the method of the present
invention are disclosed by the appended patent claims.
Brief description of drawings
The method of the invention is described in detail below,
by way of example, with reference to the accompanying
drawings of which:
Figs. 1 - 5 illustrate a few bleaching sequences
according to preferred embodiments of the invention; and
Fig. 6 illustrates graphically the brightness and Kappa
numbers obtained by a sequence according to the invention
as compared with two other potential sequences.
Preferred embodiments
Fig. 1 illustrates a bleaching sequence O-Q-(OP)m-PaQ-P,
preceded by a sulfate digestion either in a continuous or
a so-called batch digester to a Kappa number of 30 - 15,
preferably to a Kappa number of approx. 20. It is
possible to reach even lower Kappa numbers by some
advanced digestion methods but in most cases that happens
at the expense of the yield. The O stage at the beginning
of the sequence is a so-called oxygen delignification
stage which aims at bringing the Kappa number down to
below 15, preferably below 12. The oxygen delignification
is followed by a Q stage already described above employed
to remove heavy metals from the cellulose pulp by
chelating (with EDTA, DTPA or corresponding chelating
agents) where metal ions are combined chemically into
3o complexes or by acidifying where metal ions dissolve in
the liquid phase so that heavy metals do not cause
problems in the following peroxide (P) stage. However, we
have found out that a Q stage performed as an acid stage
lowers the Kappa number whereby this phenomenon can be
taken in use when delignifying of pulp is wanted, and
especially in combination with oxygen stages. The Q stage
is preferably carried out at a pH range of 3 - 6 and at a
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temperature of 60 - 100°C and with a treatment time of 20
- 120 min. It is also possible to treat the pulp with
enzymes in the same stage to expose the lignin for
subsequent deligni.fication stages. The Q stage is ended
5 with an efficient washing stage (_) to wash off the heavy
metals separated from the pulp at this stage. Preferably
a so-called DRUM DI~3PLACER~ washer manufactured by A. Ahlstrom
Corporation and disclosed for example in U.S. patents no.
4,919,158 and 5,116,423, is used as the washing device.
The advantage provided by this kind of a fractionating
washer is the washing result which is remarkably more
efficient and precise than the one obtained by a
conventional washer, and the production of filtrate
fractions having different concentrations, the
utilization of which filtrate fractions gives other
advantages to the process.
The subsequent oxygen stage (OP) m in fact comprises one
or several, preferably pressurized, oxygen and/or
peroxide stages with no intermediate washing between them
as is closer describedin Canadian patent no. 2,132,056.
In this oxygen and/or peroxide stage having one
step or several steps, oxygen or peroxide may be added as
supplemental chemical to one step or several steps if
desired. In addition to one-step oxygen and/or peroxide
stage, also for example the following two-step stages may
be used: O/O, O/~?, P/O, P/P, OP/O, PO/O, OP/P, PO/P,
OP/OP, OP/PO, PO/PO, and POOP in which the first
character in each step stands for the main active
bleaching chemical and the possible second character for
the supporting bleaching chemical. In other words, the
substantially delignifying part of the sequence may, for
instance, be O - Q - O, O - Q - OP, O - Q - P, O - Q -
O/O, O - Q - O/P, O - Q - P/O, O - Q - P/P etc. After
the oxygen and/or peroxide stage described above the
Kappa number of the pulp has reduced to the range of 8 -
3.
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The oxygen and/or peroxide stage is followed in the
sequence by a peroxide stage connected to the process = ,
directly after the washer of the oxygen stage without a
metal removal preceding a P stage. The solution allowing
operation without a separate metal removal stage (Q) by
acidifying or chelating prior to a peroxide stage, is
reducing the pH of the peroxide stage acidic to a range
of 2 - 6, preferably to 4 - 6. It is also preferable
that the acidic peroxide stage Pa is pressurized (the
pressure being usually below 10 bar, preferably 3 - 5
bar) and that the temperature is 60 - 130°C, preferably 90
- 110°C. Activators may be added to a peroxide stage of
this kind to disperse hydrogen peroxide so as to produce
a strong oxidant. For example, the peroxide may be
dissolved to water and active oxygen. Pressurizing the
stage ensures that the produced gaseous chemical stays
close to the fiber.
An acidic peroxide stage P~ may be combined with a Q stage
during which for example EDTA or acid is added to~the
pulp to remove heavy metals. Both steps in the PaQ stage,
or quite as well in a QPa stage, are performed at a pH of
below 7, at the same temperature, preferably within the
range mentioned above, in both the steps, and preferably
with at least the P~ step being pressurized, which also
was described above. If necessary, the pH may be
adjusted between the steps if the pH of the Pg step is
very low, i.e, below 3. The pH of the Q step should be 3
- 6, preferably 4 - 6. After a PaQ stage the pulp is
washed (_), preferably with a fractionating washer, to
r
remove the heavy metals treated in the Q step.
The last treatment stage in the sequence described here
is an alkaline peroxide stage employed to increase the
final brightness to a desired level, preferably over 88.
The process conditions in this P stage are: the pH 10 -
12, the temperature 60 - 130°C, preferably 90 - 110°C, the
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pressure usually below 10 bar, preferably 3 - 5 bar, and
the treatment time 30 - 120 min.
Fig. 2 illustrates an embodiment slightly different from
the sequence in 1?ig. 1. The single- or multi-step
oxygen/peroxide stage (OP) m now stands for a multi-step
oxygen stage intensified with peroxide (OP/OP).
Fig. 3 illustrates an alternative sequence in which the
first step of a multi-step oxygen/peroxide stage has been
intensified with pfaroxide (OP/P).
20
Fig. 4 illustrates the one-step oxygen/peroxide stage of
Fig. 1 as a multi.-step peroxide stage (P/P).
Fig. 5 illustrates an embodiment in which the multi-step
peroxide stage of Fig. 4 has been replaced by a multi-
step peroxide stage and in which the first peroxide step
has been intensified with oxygen (PO/P).
The chemical feed rates and the process conditions in the
alternative two-step sequences described,above are the
ones given in t:he Canadian Patent No. 2,132,056
mentioned above, if applicable. However, it must be
noted that the present invention covers also embodiments
in which the bleaching is carried out at its simplest as
a one-step process with one chemical whereby thus the
second chemical d.ase, i.e. either oxygen or peroxide
dose, is 0.
Figure 6 illustrates results received from comparative
laboratory tests performed. The figure illustrates the
changes in Kappa number and brightness obtained from
tests using three sequences. In addition to comparison
of different potential bleaching sequences, the objective
of the test was .also to find out, by stabilizing the
total peroxide volume used in the whole sequence to six
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per cent irrespective of~the number of peroxide stages or
steps, which sequence would give the best bleaching
result. The sequences used in the tests were Q - PO/P -
p~ Q - p0/p - paQ - PO and for comparison a sequence Q -
PO/P - Z - P containing ozone.
The Figure indicates that in all the three tests the PO/P
stage reduced the Kappa number from 12 to 6 and increased
the brightness from 40 to almost 80. It should be noted
that a separate oxygen delignification immediately after
the digestion gives a Kappa level of 6 - 9, thus the
starting point for further bleaching is quite different
from the one with the present invention.
When alkaline peroxide (P) without heavy metals removal
is used as the following stage it is found out that the
Kappa number does not change much and also the brightness
increases only a few per cents. When ozone was used the
Kappa number reduced to approx. three and the brightness
increased to approx. 84. Acidic peroxide (PaQ) seems to
work the same way as ozone although the Kappa number
reduced to about two and the brightness increased to 85.
Two of the sequences were continued by performing the
. bleaching subsequent to the ozone with an alkaline P
stage and the bleaching following the PaQ stage with an
alkaline PO stage. The final Kappa number reached in
both these sequences was below 1 and the brightness in
the sequence utilizing ozone about 90 and in the sequence
utilizing acidic peroxide even about 93.
Yet another interesting and especially promising
bleaching method includes a novel pretreatment with an
oxygen delignification stage followed by a Q stage run as
an acid stage (A) and an oxygen bleaching stage. In this
embodiment, the property of an acid stage to decrease the
Kappa number of pulp is used. The acid stage (A) in
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accordance with this embodiment is different from the Q
° stage discussed earlier as in A stage the main concern is
not to remove heavy metals but to improve bleachability
by lowering the lignin content of pulp. In other words,
the pulp production begins with the following partial
sequence digestion - O - A - O. The following practical
example describes the process in more detail.
Example; laborator5r test
Pulp was digested in accordance with the so-called Kraft
process and thereafter oxygen delignified to a Kappa
number of 14 or less. After the oxygen delignification,
the pulp was treated in an acid stage at a pH of 3 - 5,
preferably 3 - 4, at a temperature of 95°C, the
temperature preferably ranging from 90 to 110°C, and for
a time period of about 1 - 2 hours. The pulp was further
bleached with oxygen whereby a final Kappa number of 5 -
3 was reached. Thus, with a production sequence
digestion - O - A - o pulp was produced which is very
suitable for further bleaching. The purpose of the acid
stage (A) is to improve bleachability, not to remove
metals. In the tests performed the pulp was further
bleached with partial sequences ZQ - P and PaQ - P. Both
sequences produced pulp having a brightness above 88 ISO,
even 90 ISO, or greater.
Thus, pulp having a full brightness was produced with
only two peroxide stages in addition to the pretreatment
of the present invention, i.e. the entire sequence being
digestion - O - A - O - PaQ - P. Another sequence
producing corresponding brightness values, i.e. digestion
- O - A - O - ZQ - P, used only one P stage but an ozone
stage was needed. In applications where only a slight
modification of an existing bleaching plant based on
using chlorine dioxine is needed, the use of a sequence
digestion - O - A - O - DQ - P comes into question which
would drastically reduce the use of chlorine dioxide.
WO 96/09434 2 2 0 ~ 3 2 7 pCT~S/00506
Performed tests have shown that chlorine dioxide dose of
less than 40 kg counted as active chlorine is sufficient,
most often the use of chlorine dioxide dose of 10 to 25
kg/BDMT results in excellent brightness. In other words,
5 the final bleaching stage P may be preceded by P~Q, ZQ, DQ
or any other oxidative treatment combined with a Q stage.
As can be understood from the above description, a new
environmentally friendly bleaching method has been
10 developed which can replace both the bleaching stages
containing elemental chlorine and the bleaching stages
containing chlorine compounds and which can reduce the
use of expensive ozone. However, it should be understood
that only a few preferred embodiments of the bleaching
method of the invention have been described above by way
of example and these embodiments do not in any way intend
to limit the scope of protection of the invention from
what is defined by the appended patent claims. Thus, it
is clear for example that although all the examples
describe only a two-step oxygen and/or peroxide stage,
there may be only one step or more than two steps in the
stage. Further, also the washes not mentioned separately
above may be fractionating washes.
