Note: Descriptions are shown in the official language in which they were submitted.
CA 02315063 2000-06-16
WO 99/32710 PCT/F198/00993
1
BLEACHING OF CHEMICAL PULP WTTH PERACID
The present invention relates to a method for the bleaching of
chemical pulp, wherein the pulp is treated in a plurality of dif-
ferent steps and a solution containing a peracid is used in at
least one of the steps. The invention additionally relates to the
use of a peracid-containing bleaching solution in a process step
according to the invention.
In a prior known pulp delignification process, a raw pulp ob-
tained by digesting cellulose-containing materials with suitable
digestion chemicals is subjected to a treatment wherein the
lignin is removed and a bleaching which involves the use of
oxidative chemicals is carried out. The purpose of conventional
bleaching of chemical pulp is to bring to completion the removal
of lignin from a raw pulp obtained from a digestion procedure.
This bleaching can be carried out by using chlorine or chlorine
dioxide and thereafter alkaline extraction steps in which the
lignin is dissolved out of the pulp. At present, the bleaching of
a chemical pulp is increasingly often carried out by bleaching
methods using no elementary chlorine or chlorine compounds. The
first-mentioned bleaching is called ECF (elementary chlorine
free) bleaching and the latter TCF (totally chlorine free)
bleaching. The chemical pulp is bleached in several successive
steps. Nowadays the bleaching is often started with an oxygen
delignification, whereafter further bleaching can be carried out
by various methods. In TCF bleaching, delignification can be
continued, for example, by using ozone, peracetic acid or
hydrogen peroxide in acid or alkaline conditions. In ECF
bleaching, chlorine dioxide steps and thereafter alkaline
extraction steps are used.
There is a previously known method in which the last bleaching
step for chemical pulp is carried out by using peracetic acid.
CA 02315063 2000-06-16
WO 99/32710 PCT/F198/00993
2
Peracetic acid can be prepared most simply by mixing vinegar and
hydrogen peroxide, whereupon a so-called equilibrium solution is
obtained which contains, in addition to peracetic acid, also
unreacted initial substances. Also a pure peracetic acid solution
can be prepared from this reaction solution by distillation.
There are also known other methods for preparing peracetic acid.
Peracetic acid (PAA) is a highly selective bleaching chemical by
the use of which good strength properties of the pulp can be
maintained. When peracetic acid is used, the optimum pH is
approx. 4-7. At a lower pH peracetic acid is a highly selective
delignifying chemical. At a higher pH its bleaching action is
greater but its selectivity decreases.
By modern bleaching methods, high degrees of brightness can
easily be achieved with both ECF and TCF pulps. However,
achieving very high degrees of brightness by conventional methods
may consume considerable amounts of chemicals. Likewise, after a
normal bleaching the variation of the degree of brightness of the
pulp and the pH of the stock going to the paper machine may cause
runnability problems in the paper machine. Pulp brightness may
also be reduced in storage towers, in which case the pulp must be
bleached clearly beyond the required degree in order for the
reduction of brightness to be compensated for.
A good method for solving these problems is post-bleaching.
Agents suitable for post-bleaching include peracetic acid and
dithionite. The use of hydrogen peroxide is limited by the fact
that in order to function properly it would require alkaline
conditions. With respect to pH, dithionite and peracetic acid are
highly suited for post-bleaching. Dithionite is used commonly in
the bleaching of mechanical pulps, but normally it is not used in
the bleaching of chemical pulp. The sulfur present in it may also
cause problems. Post-bleaching with peracetic acid is a very
effective method of increasing brightness. The consumption of
chemicals for bleached pulps is low, and in addition the reaction
CA 02315063 2006-12-20
WO 99/32710 PCT/F198/00993
3
is rapid even at low consistency. Furthermore, the optimum pH
range for bleaching is precisely suitable for post-bleaching.
However, the peracetic acid post-bleaching of chemical pulps
involves the problem that, within the pH range optimal for the
bleaching result, selectivity is no longer at its best. In the
case of a completely bleached pulp having a very low kappa
number, the use of peracetic acid damages the fibers, causing the
breaking down of carbohydrates and the lowering of pulp strength.
This also causes dissolving of organic matter out of the pulp, a
factor which may disturb the running of the paper machine.
The bleaching method according to the invention, which solves the
problems mentioned above, is characterized in that a peracid is
used in the post-bleaching which is the last step of the
bleaching process and takes place in the presence of one or more
earth-alkali metal compounds. Post-bleaching such as this can
advantageously be carried out as a separate step at the paper
mill to which the pulp has been transferred from a bleach plant.
The earth-alkali metais, such as magnesium and calcium, used
simultaneously with a peracid in the method according to the
invention stabilize carbohydrates so that the reducing of the
pulp strength is prevented. In other words, the strength of the
pulp remains good and the dissolving of organic matter is in-
significant. The brightness and whiteness of the pulp are also
higher than without the use of earth-alkali metals. This de-
creases, for example, the need for using optical brighteners in
the paper machine. When this method is used, the pH of the stock
to be turned into paper, and the brightness and whiteness of the
pulp are more even, in which case the runnability of the paper
machine is considerably improved.
The adding of magnesium to the pulp, for example in oxygen- and
peroxide-based steps, is prior art. Likewise, the use of mag-
nesium in connection with peracetic acid delignification is known
from, for example, the publication Liebergott, N., 81st annual
CA 02315063 2000-06-16
WO 99/32710 PCT/F198/00993
4
meeting, technical section CPPA (1995), B 157 - B 170. According
to the publication, magnesium protects the viscosity of the pulp.
The question is, however, of delignification which is carried out
on a pulp having a high kappa number. This delignification step
is followed by a separate bleaching during which the brightness
is raised to its final level. During the delignification,
transition metals such as Fe and Mn are released from the pulp,
and magnesium prevents their detrimental effects. The addition of
magnesium is used for improving the ratio of magnesium to
transition metal, which improves especially the running of the
subsequent alkaline peroxide step.
In contrast to the foregoing, the invention concerns post-
bleaching which is carried out on an already delignified pulp
having a low kappa number (preferably below 4) and being free of
transition metals, the purpose being to raise the brightness and
whiteness of the pulp to the desired level. In this case the
peracetic acid turns colorless the chromophoric groups in the
pulp, mainly in carbohydrates. In the above-mentioned state-of-
the-art methods the question is of the reactions of peracetic
acid with residual lignin in the pulp.
Another difference between the prior-art peracetic acid delig-
nification method and the present invention is that in the
invention the peracetic acid dose is considerably smaller. In the
method according to the invention, surprisingly even a very small
peracetic acid dose, 0.1-7 kg/tp, preferably 0.5-3 kg/tp, is
sufficient, larger doses being even detrimental.
In methods using peracetic acid known from the literature, it has
been observed that the optimum pH with respect to brightness is
within a neutral or mildly alkaline range; the strength of the
pulp suffers within a slightly acid range. In the method
according to the invention it is possible to operate within a pH
range of 3-8, preferably 4-7, and at the same the decreasing of
the pulp strength is prevented. A preferably acid pH range is
CA 02315063 2000-06-16
WO 99/32710 PCT/F198/00993
essential for the runnability of the paper machine. It has been
observed that by using an earth-alkali metal addition it is
possible to carry out post-bleaching within an acid pH range
without a loss of pulp strength and without organic matter being
5 dissolved out of the pulp. In prior known methods the best
bleaching effect was achieved in mildly alkaline conditions.
Since the stability of peracetic acid decreases as the pH rises,
when small peracetic acid doses are used wasteful consumption of
the chemical decreases brightness. With large peracetic acid
doses the effect of wasteful consumption of the chemical does not
show. Thus by the method according to the invention it is
possible to after-bleach pulp with considerably smaller chemical
doses than by prior known methods, without a loss of pulp
strength.
Another advantage gained through an post-bleaching carried out
using a peracid, e.g. peracetic acid, within an acid pH range is
that the acid released during the reaction serves as an
acidification chemical, whereupon the use of SO2 or H2SO9 as an
acidification chemical is avoided. Thus the total sulfur amount
can be lowered and possible S02 odor problems are avoided.
The peracid used in the method is preferably peracetic acid.
Other usable peracids include performic acid, perpropionic acid
or some longer-chain percarboxylic acid. The peracid may be a so-
called equilibrium solution, i.e. a reaction mixture containing
the said acid, a peracid and hydrogen peroxide, or a pure peracid
solution can be used. The method by which the peracid solution is
prepared does not restrict the use of the method; the peracid may
be prepared, for example, by distilling an equilibrium solution
or from an anhydride and hydrogen peroxide. The peracid may also
be a monopersulfuric acid (Caro's acid) or a mixture of Caro's
acid and a percarboxylic acid, e.g. peracetic acid. Persulfates
may also be used, either as such or as a mixture with any of the
above-mentioned peracids.
CA 02315063 2000-06-16
WO 99/32710 PCT/FI98/00993
6
The earth-alkali metal used in the post-bleaching according to
the invention may be in particular calcium or magnesium. These
may be added to the bleaching solution in the form of sulfate.
acetate, carbonate, oxide or any other compound. It is possible
to use in the method either magnesium or calcium alone or both of
them together at any ratio. Calcium has over magnesium the
advantage that calcium will not precipitate extract-based and
resin-based substances which may be present at the wet end of the
paper machine.
The post-bleaching according to the invention may be preceded by
any known bleaching reaction known from bleaching sequences. The
post-bleaching is especially advantageous for use after chlorine
dioxide or peroxide bleaching. Also, acidification possibly
taking place after the bleaching does not restrict the
application of the method; it can be used on both non-acidified
and acidified stock. The said acidification may be carried out
with, for example, S02, sulfuric acid or any acid suitable for
this purpose. The functioning of the invention is not affected by
a possible wash of the pulp before or after the post-bleaching,
or by the absence of a wash. The consistency of the pulp to be
after-bleached may be 1-30 % and the temperature during the post-
bleaching may be between 30 and 100 C.
The after-bleached pulp may be taken directly to the paper
machine or be dried to produce baled pulp.
The method is suited for use on sulfate and sulfite pulps made
both from softwood and from hardwood, and on various organosolv
pulps.
The said prior art methods in which a peracid and magnesium are
used or in which a peracid has been present in the last steps of
the bleaching sequence are characterized in that they are carried
out in the normal manner in the bleach plant of the pulp mill.
The post-bleaching according to the invention, in which the
CA 02315063 2006-12-20
WO 99/32710 PCT/FI98/00993
7
bleaching reaction is very rapid even at a low consistency, is
not tied to the bleach plant and thus also does not require
investment in equipment for the bleach plant. As was already
pointed out, the invention primarily concerns an post-bleaching
carried out after the bleaching sequence of the bleach plant,
outside the actual bleaching, for example in the pulp flow pipe
during the transfer of pulp, during pulp storage or in the paper
machine. Instead of the bleach plant, the place at which the
post-bleaching according to the invention is carried out is
typically a storage tower for bleached pulp or the paper mill.
The earth-alkall- metal compounds used can be added to the
bleaching solution in any suitable process step. They can be
added to the circulating waters before the water arrives in the
post-bleaching step, or they may, for example, come in the
dilution waters from the paper machine, where they mav be added
in any form. It is also possible to use chelators simultaneouslv
with earth-alkali metals.
In addition to the post-bleaching method described above, the
invention comprises the use of a solution which contains a
peracid and an earth-alkali metal for post-bleaching of delig-
nified pulp at a paper mill.
Example 1
A birch sulfate pulp which had been bleached using a sequence
consisting of an oxygen step, chelation, an oxygen + peroxide
step, a chlorine dioxide step, a peroxide extraction, a chlorine
dioxide step (-O-Q-Op-D-Ep-D) was subjected separately to
post-bleaching at 50 C with a 30 min retention at a consistency
of 5 % with a peracetic acid dose of 3 kg/tp at two different pH
levels (approx. 6.5 and approx. 4.5) . The peracetic acid used had
been distilled. Calcium was added to the pulp in the form of
acetate, and magnesium in the form of sulfate. The figures in the
table indicate the dose of the chemical concerned per one metric
CA 02315063 2000-06-16
WO 99/32710 PCT/F198/00993
8
ton of pulp (kg of 100 % chemical/one metric ton of pulp).
Experiment number 0 stands for pulp which has not been after-
bleached.
Exp. Initial Final CaOAc ~ MgSO4 ~ Brightness ~ Viscosity
No. pH pH kg/tp kg/tp ~ % ISO dm3/kg ~
~ 0 90.3 ~ 930
1 ~ 6.5 ~ 5.4 91.0 766 ~
~ 2 ~ 4.5 ~ 4.3 ( - ~ - ~ 91.5 ~ 845 ~
~ 3 ~ 6.5 ~ 5.3 ~ - 1 ~ 92.5 ~ 925
~ 4 4.5 ~ 4.2 ~ - ~ 1 92.3 ~ 911
~ 5 ~ 6.5 ~ 5.4 1 - ~ 92.8 ~ 899 ~
6 4.5 4.3 1 - ~ 92.6 ~ 901 ~
~ 7 ! 6.5 ~ 5.3 ~ 0.5 ~ 0.5 ~ 92.8 ~ 887 ~
~ 8 4.5 ~ 4.4 ~ 0.5 ~ 0.5 ~ 92.5 ~ 896 ~
As can be seen from the table, with the earth-alkali addition the
viscosity of the pulp can be maintained clearly higher. The
viscosity determined by the copper-ethylene diamine method is
directly proportional to the strength of the pulp, especially
when the same pulp sample is concerned. It is well-founded to
assume that the higher the viscosity of the pulp, the better its
strength. A higher viscosity also means that the yield is better
and that less organic matter has been dissolved out of the pulp.
CA 02315063 2000-06-16
WO 99/32710 PCT/F198/00993
9
ISO brightness was improved by approx. 1 % ISO by the earth-
alkali addition.
Example 2
A birch sulfate pulp which had been bleached by the sequence
-O-Q-Op-D-Ep-D according to Example 1 was subjected to an post-
bleaching at 50 C with a 30 min retention at a consistency of
5$ with a peracetic acid dose of 3 kg/tp at different pH levels.
The magnesium sulfite addition was 1 kg/tp. The peracetic acid
used had been distilled. The brightness of the pulp before the
post-bleaching was 90,3 % ISO and viscosity 930 dm3/kg.
No Mg addition ~ MgSO4 ~
~ Initial ( Viscosity ~ Brightness ~ Viscosity ~ Brightness
pH ~ dm3/kg ~ % ISO dm3/kg ~ % ISO
~ 3 890 91.4 ~ 913 ~ 92.0 ~
~ 3.5 ~ 868 ~ 91.4 ~ 912 ~ 92.1 ~
( 4 850 91.5 907 92.2 ~
~ 4.5 ~ 839 ~ 91.5 ~ 911 ~ 92.3 ~
~ 5 ~ 799 ~ 91.6 1 912 92.4 (
5.5 766 ~ 91.7 ~ 915 92.5 ~
~ 6.5 ~ 750 ~ 91.8 ~ 925 ~ 92.5 (
8 J 802 1 91.2 ~ 916 91.9 ~
' ' ' ' ' ~
CA 02315063 2000-06-16
WO 99/32710 PC.'T/F198/00993
It can be seen from the table that without a magnesium addition
the viscosity of the pulp is lowered considerably; at its lowest
the viscosity is within a pH range of approx. 5.5-6. Owing to the
magnesium addition the viscosity remains high regardless of the
5 pH. Brightness is also clearly higher.
Example 3
A TCF-bleached softwood sulfate pulp which had been bleached
10 using oxygen, ozone and hydrogen peroxide was after-bleached at
70 C with a 240 min retention at a consistency of 10 % by using
two different peracetic acid doses. The conditions and results of
the post-bleaching are shown in Table 3. The brightness of the
pulp before the post-bleaching was 86.8 % ISO and viscosity
642 dm3/kg, and the kappa number was 1.7.
Experiment No. 1 2 3 4
PAA, kg/tp ~ 3 1.5 1.5 3
pH, initial ~ 7.2 7.2 7.2 7.2 ~
pH, final ~ 6.3 6.6 6.8 5.6 ~
MgSO4, kg/tp 1 1 - - ~
Residual PAA, kg/tp 0.7 0.5 0.1 0.1
Kappa 1.6 1.6 1.4 1.3 ~
Viscosity, dm3/kg 633 625 572 564
Brightness, % ISO ~ 89.1 88.9 88.4 88.6
~ '
CA 02315063 2000-06-16
WO 99/32710 PCT/F198/00993
11
As is seen from the table, with the magnesium addition a higher
brightness was obtained and the viscosity remained clearly
better. In post-bleaching with a magnesium addition, no organic
matter was dissolved, which can be seen from the kappa number and
viscosity of the pulp.
Example 4
A TCF-bleached softwood sulfate pulp which had been bleached
using oxygen, ozone and hydrogen peroxide was after-bleached at
70 C with a 240 min retention at a viscosity of 10 % by using
two different peracetic acid doses. The conditions and results of
the post-bleaching are shown in Table 4. The brightness of the
pulp before the post-bleaching was 87.6 % ISO and viscosity
623 dm3/kg, and the kappa number was 1.8.
Experiment No. 1 2 3 4 5 6
PAA, kg/tp 1.5 1.5 1.5 1.5 1.5 1.5
pH, initial 7.2 5.5 5.5 6.1 6.5 7.2
~ . (
pH, final 6.9 4.9 4.9 5 5.5 6.9
( (
MgSO4, kg/tp 1 1 - - - - ~
Residual PAA, kg/tp 0.4 0.4 0.4 0.4 0.4 0.2
Viscosity, dm3/kg 612 609 574 571 561 579
Brightness, % ISO ~ 89.9 89.3 88.6 88.5 88.5 89.0
i ~
As can be seen from the table, a clearly better final viscosity
is obtained with the magnesium addition.
CA 02315063 2000-06-16
WO 99/32710 PCT/F198/00993
12
For a person skilled in the art it is clear that the various
applications of the invention are not limited to those presented
above by way of example only; they may vary within the
accompanying patent claims.
