[0001] The present invention relates to a method for treatment of pulp in an alkaline pulping
process of chemical pulp during which process pulp is treated in a chlorine dioxide
stage including an acid treatment at a temperature of over 80°C.
[0002] Pulp mills have recently attempted to abandon the use of elementary chlorine, and
partly also chlorine dioxide, the reasons for this being both aspects of environmental
protection and market factors. Disadvantages caused by elementary chlorine include
both noticeable malodorous gaseous emissions and liquid effluents from chemical pulp
mills into water systems. Liquid chlorine dioxide does not cause odour disadvantages
on such a large scale, it affects primarily water systems. When comparing these chlorine
chemicals with each other by means of the AOX number designating the loading they
incur on water systems, it can be noted that elementary chlorine is many times more
detrimental than chlorine dioxide. The AOX number of chlorine is of the order of 4
- 7 and that of chlorine dioxide is 1 - 1.5, or even below one.
[0003] However, sequences utilizing chlorine dioxide are still popular and environmentally
feasible. There are many reasons for this. The price of chlorine dioxide is very competitive
compared with that of other chemicals. Also the strength and brightness of pulp obtained
by dioxide bleaching are good, in fact at least approximately of the same order as
the ones obtained by using peroxide at the same consumption of chemical (kg/admt).
[0004] Chlorine dioxide bleaching (D) is a well-known and widely used bleaching method.
Conventional chlorine dioxide bleach treatment is usually performed at a temperature
of 50 - 80°C and a with a chlorine dioxide dosage of about 10 - 30 kg per ton of pulp.
Sequences used are for example D
oED
1ED
2 and OD
oED
1ED
2 and various modifications of these. The treatment time in a D
o stage is usually shorter than in other D stages, being for example 30 - 90 minutes.
The pH of the pulp decreases to the range of 1 - 3 towards the end of the D
o stage. In the D
1 and D
2 stages the treatment time is 2-3 hours and the pH a little higher than in the D
o stage. The chlorine dioxide stage is usually performed at a temperature of about
70°C while the treatment time in the D
o stage is 0.5 - 2 hours and in the D
1 and D
2 stages 2 - 3 hours. Higher temperatures have been avoided as the low final pH in
the range of 1 - 3 in the D stage combined with a high temperature and long treatment
time deteriorates the strength properties of the fibers.
[0005] Beside chlorine dioxide bleaching, also bleaching processes utilizing chlorine-free
chemicals, such as peroxide and ozone, have been introduced today. As pulp bleached
with chlorine dioxide has good properties, the development of this process has been
continued in addition to the new processes mentioned. One of the most important objects
is to reduce the amount of chlorine dioxide needed in the bleaching and thus to make
the method more acceptable in view of the environmental impact.
[0006] Finnish patent application no. 944808 and WO publication no. 96/12063 disclose an
acid treatment method for pulp by means of which the consumption of chlorine dioxide
can be reduced. This treatment removes from the pulp hexenuronic acids which can react
with chlorine dioxide and thus increase chemical consumption. The acids may be removed
by adjusting the pH of the pulp to the range of 2 - 5, preferably to 2.5 - 4, and
the temperature to over 80°C, preferably to 90 - 110°C, and by retaining the pulp
in these conditions typically for 30 - 300 minutes. During the treatment the kappa
number of the pulp reduces typically by 2-9 units.
[0007] When the sequence O-A-D-E-D has been used, the consumption of chlorine dioxide in
the bleaching of hard wood pulp has reduced by 30 - 40 % at the brightness of ISO
88 %. The corresponding consumption with soft wood pulp has reduced by 10 - 20 %.
In both cases, the yield remained nearly unchanged compared with bleaching without
an A stage.
[0008] The object of the present invention is to provide an alternative method of combining
a hot acid stage with a bleaching sequence utilizing chlorine dioxide. A further object
of the invention is to provide a method which may be applied in an economical way
particularly in the bleaching plants of existing pulp mills. In other words, the object
of the invention is to fit chlorine dioxide bleaching of pulp into a bleaching plant
in an overall economical and environmentally acceptable way.
[0009] To achieve the objects mentioned, the method of the invention is characterized in
that the chlorine dioxide stage comprises successively at least a first chlorine dioxide
step and an acid treatment step and that the pH of the first chlorine dioxide step
is adjusted so that the final pH of the step is over 4. The conditions during the
first chlorine dioxide treatment are such that the hexenuronic acid groups in the
pulp do not react with chlorine dioxide.
[0010] It has been disclosed above that hexenuronic acid groups are removed by hot acid
treatment prior to the chlorine dioxide treatment. According to the invention we have
now discovered that the ClO
2 treatment can be performed in the bleaching sequences also before the hot acid treatment
if reactions between the hexenuronic acid groups of the pulp and the chlorine dioxide
are prevented. Thus, the D stage comprises successively a D step and an A step. According
to the method of the invention, the chlorine dioxide step of the DA stage is performed
so as to have a final pH of over 4, preferably over 5. The highest pH value is normally
7. Conventionally, the final pH of the first or the second chlorine dioxide bleaching
stage is less than 4, typically 1 - 3.5. Surprisingly, it has been found out that
hexenuronic acids do not react with chlorine dioxide at the pH range of over 4 and
thus no chemical is consumed by reactions with chlorine dioxide. Chlorine dioxide
is reduced to chlorite but it does not decompose further. The chlorine dioxide dose
in this step is preferably 0.1 1.5 % active Cl (1 - 15 kg active Cl/admt), preferably
over 0.5 - 1.0 % active Cl (5 - 10 kg active Cl/admt).
[0011] In the acid treatment (A step), the conditions are typically as follows:
- pH2 - 5, preferably 2.5 - 4
- temperature over 80°C, preferably 90 - 110°C
- time 30 - 300 minutes, preferably at least t minutes,

, in which T (°C) is the temperature of the acid treatment.
[0012] According to the invention, the temperature in the D step of the DA stage is preferably
over 70°C, preferably 80 - 100°C, which is higher than the temperature of a conventional
D stage. Thus, the temperature in the D and the A steps is essentially the same and
there is no remarkable need to cool or to heat the pulp between the steps which is
advantageous for the energy economy. However, the invention is not limited to the
high temperature but the D step may be performed also at the conventional D step temperature
of below 70°C.
[0013] The treatment time in the D step of the invention is short, less than 10 minutes,
preferably 30 seconds to 3 minutes. Conventionally, the chlorine dioxide treatment
time is over 30 minutes, even 120 minutes depending on the temperature and therefore
the treatment requires a reactor of its own. The DA stage according to the invention
may be practiced by performing the acid treatment in the reactor tower but because
of the short retention time the D step preceding the A step may be arranged to take
place for example in the feed line of the A tower. The pulp flowing in the line is
heated to the desired temperature, for example 90°C, and chemicals, such as chlorine
dioxide, and alkali or acid if necessary to adjust the pH, are mixed into it. The
pulp flows under these conditions for the required period, e.g. one minute, and subsequently,
acid is added to the pulp and it is supplied into the acid tower.
[0014] By providing the chlorine dioxide treatment before the acid treatment, the acid demand
in the A step reduces as the reactions of the chlorine dioxide with the lignin produce
hydrochloric acid and organic acids as byproduct.
[0015] It is characteristic of a preferred embodiment of the invention that the chlorine
dioxide stage as described above comprises in addition to the D and A steps described
above also a second D step, i.e. the stage in question comprises a first chlorine
dioxide step, a hot acid treatment step and a second chlorine dioxide step (DAD, could
be marked also D/A/D). The second step is performed in acidic conditions, in other
words the pH may be less that 2 but preferably it is 2 - 4. Thus, no essential adjustment
of the pH is needed after the A step. Adjustment of the temperature is not necessary,
either, but the entire chlorine dioxide stage may be effected essentially at the same
temperature as the A step and no essential adjustments of the temperature are needed
between the steps. Thus facilitates remarkably practicing the step as far as process
technology is concerned.
[0016] The treatment time in the second D step is preferably also short, less than 10 minutes,
preferably 1 - 5 minutes and it may be performed for example in the discharge line
of the A step prior to the washer following the DAD stage. Chlorine dioxide is added
to the pulp flowing in the discharge line and the temperature and the pH are adjusted
if necessary. The required chlorine dioxide dose is typically 0.5 - 2.0 % active Cl
(5 - 20 kg active Cl/admt). The acid treatment removes hexenuronic acids from the
pulp and therefore the amount of chlorine dioxide chemical required is smaller than
with pulp from which hexenuronic acids have not been removed.
[0017] In this (DAD) embodiment of the invention, the chlorine dioxide treatment may be
considered to have been divided into two relatively short partial treatments. Both
steps may be practised at a high temperature; therefore the temperature and the treatment
time must be chosen so as to cause as little viscosity losses in the pulp as possible.
Two partial treatments contribute to a more homogenous brightness of the pulp.
[0018] The DA stage may be practiced also by arranging for the pulp a chelating treatment
with EDTA, DTPA or a corresponding compound after the A step in order to remove harmful
metals; in this case the stage is DAQ. Providing a chelating treatment in connection
with the DA stage is advantageous particularly if the bleaching sequence comprises
later a bleaching stage with a chemical such as peroxide which would be disturbed
by heavy metals. The Q step may be performed at essentially the same temperature as
the preceding D and A steps. The pH may be in the range of 3-6.
[0019] The total amount of chlorine dioxide needed in the DA stage is not greater than the
amount needed in an AD stage although the first D step is performed before the acid
treatment. The final pH of over 4 in the first D step prevents reactions between the
ClO
2 and the hexenuronic acids. A DA stage has produced pulp of at least as good quality
as an AD stage. An advantage provided by a DA stage or a multistep DAD stage is that,
compared with an AD stage, a DA or a DAD stage may be effected with simpler apparatus
as each step does not require a tower of its own. However, the invention may be practised
also so that one or both of the D steps are performed in towers of their own.
[0020] A disadvantage of the AD step may be the release of malodorous gases in the acid
step if the pulp still after washing contains adequate amounts of sulfur compounds
from the digestion. When the D step precedes the A step, malodorous gases are oxidized
by the bleaching chemical during the DA stage.
[0021] The method of the invention is applicable for pulps manufactured by the sulfate process
and other alkaline methods which pulps contain hexenuronic acid compounds. The pulp
to be treated has preferably been oxygen delignified. The DA stage according to the
invention may be at the beginning of the bleaching sequence for example after the
oxygen delignification or later in the sequence for example after a peroxide stage.
Therefore the invention is advantageously applied for example in connection with the
following bleaching sequences or partial bleaching sequences:
digestion-O- DAD-E;
digestion-O- DAD-E-DN-D; or
digestion - O - Q - OP -D/A/Q - PO, in which
- indicates a wash between the stages,
O indicates a delignification stage utilizing at least oxygen,
Q indicates removal of metals by chelating,
A indicates a hot acid treatment,
E indicates an alkaline stage, and
OP or PO indicate a bleaching stage based on oxygen and/or peroxide, possibly at least
part of the steps being pressurized and O indicating oxygen chemical and P peroxide,
and the first letter in each step signifying the main effective bleaching chemical
and the second letter possibly used the bleaching chemical supporting the bleaching
reaction.
[0022] The method of the invention will be described more in detail in the following with
reference to the enclosed drawing figure which illustrates by way of example a preferred
apparatus for carrying out the method of an embodiment of the invention. Pulp is treated
in a DAD stage.
[0023] Pulp is transferred from a preceding treatment stage 10 by means of a high consistency
pump 12 to an acid tower 18. The consistency of the pulp is 6 - 25 %, preferably 8
- 18 %. The preceding stage may be a wash following digestion or more often an oxygen
delignification following digestion or a wash following oxygen delignification.
[0024] Alkali or acid is added to the pulp in order to adjust the initial pH for the chlorine
dioxide treatment or the first D step so that the final pH is over 4, preferably over
5, in order to prevent reactions between chlorine dioxide and hexenuronic acids. The
alkali and the chlorine dioxide may be added directly to the pump 12, it may be injected
into duct 15 between the pump 12 and the tower, or to a particular mixer 14 provided
for this purpose. The typical chlorine dioxide amount added is 0.5 -1.5 % active Cl.
[0025] Steam is added to the pulp in order to raise the temperature typically up to 80 -
100°C. Steam may be added prior to the pump 12 in a steam mixer (not illustrated)
or mixed in the duct 15 following the pump 12. Alternatively, pulp may be heated indirectly
in the duct 15.
[0026] Thus, the first D step is performed in the feed line 15 of the acid tower while the
pulp flows in the line. The treatment time in this step is typically 30 seconds to
3 minutes.
[0027] The D step is followed by an acid treatment in the tower 18. After the chlorine dioxide
treatment the pH of the pulp is decreased to the level 2 - 5 required by the acid
treatment by adding acid (sulfur acid, hydrochloric acid, waste acid from the production
of chlorine dioxide or any suitable organic acid) to the pulp in the mixer 14. Also
the temperature may be adjusted if necessary but according to the invention the steps
of the DAD stage are preferably effected at the same temperature, for example 95°C,
whereby no essential temperature adjustment is required between the steps.
[0028] The feed and flow of pulp to the acid tower should be as even as possible by using
a distributor 16 or a doctor. The distributor mentioned is disclosed for example in
U.S. patent no. 4,964,950, and its use for the above purpose is described in Finnish
patent publication no. 94442.
[0029] If the device 16 has adequate mixing properties and if desired, acid may be added
through it and thus even avoid the use/purchase of the device 14. Pulp flows from
the device 16 to the reactor tank 18 dimensioned for the treatment time required by
the acid treatment, for example 120 minutes at 95°C. Using the distributing feeder
described in the FI publication no. 94442 mentioned ascertains that all portions of
the tower are filled evenly and that the pulp column rises up evenly in the tower
so that harmful channelling can not occur. Correspondingly, the top of the tower has
been provided with a discharger 20 or a discharge doctor to guide pulp to the discharge
line 24 of the tower.
[0030] The second D step is performed in the discharge line 24 in order to bleach the pulp.
Chlorine dioxide may be added to the pulp via the acid tower discharger 20 of a pump
22 in the discharge line. Also a separate mixer (not illustrated) may be arranged
in the line 24 for addition of chemical which is about 0.5 - 2.0 % actie C1
[0031] The addition of chemicals to the pulp is fitted so that a proper treatment time for
carrying out the D step in the discharge line 24 before a washer 26 is reached. The
retention time is less than 10 minutes, preferably 1 - 5 minutes: The treatment temperature
of this D step is preferably the same as the one in the previous D and A steps; thus
no essential adjustment of the temperature after the A step is needed here either.
If desired, pulp may be heated or cooled in a heat exchanger or by direct feeding
of steam. A higher temperature than conventionally is, however, required in order
to obtain adequate bleaching during the short treatment time mentioned.
[0032] The pH of the second D step is lower than that of the first D step. Typically the
pH is about 2 - 4; thus adjustment of the pH is usually not needed after the A step.
If necessary, the chemical adjusting the pH may be added in the same stage as the
chlorine dioxide.
[0033] Pulp has been bleached in a DAD stage the way described above and subsequently the
pulp is treated in the washer 26 and is transported to further treatment. Typically
the subsequent stage is an E stage.
Example:
[0034] Oxygen-bleached pulp was tested in a laboratory, the kappa number of which was 11.9,
viscosity 1061 ml/g and the ISO brightness 49.2 %. The pulp was treated with the following
sequences:
1. DAD-Eo-DN-D
[0035]
- D:
- - ClO2 dose 1.0% as active Cl
- time 1 minute
-final pH 5.3
- temperature 95°C
- A:
- -time 180 minutes
-pH 3.5
- temperature 95°C
- D:
- -ClO2dose 1.25% as active Cl
- time 2 minutes
-final pH 2.5
- temperature 95°C
- Eo
- - 85°C, 60 minutes, 1.25 %NaOH, O2 4 bar, final pH 11.7
- kappa number after treatment 2.3
- viscosity after treatment 890 mg/l
- brightness after treatment 70.2% ISO
- DN
- -75°C, 180 minutes, 1.8% ClO2 as active Cl, 0.3 % NaOH, final pH 3.3
- N (neutralization) pH 8.3
- D
- - 75°C, 180 minutes, 0.8% ClO2 as active Cl, 0.1 % NaOH, final pH 5.1
- Bleached pulp:
- viscosity 868 ml/g, brightness 89,9 % ISO
[0036] In the second test the same pulp was treated
2. AD-Eo-DN-D
[0037]
- A:
- -time 180 minutes
-pH 3.5
- temperature 95°C
- D:
- - ClO2 dose 2.25 % as active Cl
- time 1 minute
-final pH 2.1
- temperature 95°C
- E0
- - 85°C, 60 minutes, 1.25 %NaOH, O2 4 bar, final pH 11.6
- kappa number after treatment 2.7
- viscosity after treatment 890 mg/l
- brightness after treatment 69.4% ISO
- DN
- -75°C, 180 minutes, 1.8 % ClO2 as active Cl, 0.3 % NaOH, final pH 3.4
- N (neutralization) pH 8.3
- D
- - 75°C, 180 minutes, 0.8% ClO2 as active Cl, 0.1 %NaOH, final pH 5.1
- Bleached pulp:
- viscosity 866 ml/g, brightness 89,9 % ISO.
[0038] Based on the test we may state that a DA stage and an AD stage according to the invention
produce pulp of the same quality. Some practical advantages described above may be
obtained by effecting the chlorine dioxide and the acid treatment according to the
invention.
[0039] While the invention has been herein shown and described in what is presently conceived
to be the most practical and preferred embodiment, it will be apparent to those of
ordinary skill in the art that many modifications may be made thereof within the scope
of the invention, which scope is to be accorded the broadest interpretation of the
appended claims so as to encompass all equivalent methods.
1. A method for treatment of pulp in an alkaline pulping process of chemical pulp during
which process pulp is treated in a chlorine dioxide stage including an acid treatment
at a pH of 2-5 and at a temperature of over 80°C, characterized in that the chlorine dioxide stage comprises successively at least a first chlorine
dioxide step and an acid treatment step and that the pH of the first chlorine dioxide
step is adjusted so that the final pH of the step is over 4.
2. A method as recited in claim 1, characterized in that the pH of the chlorine dioxide step is adjusted so that the final pH of the
step is over 5.
3. A method as recited in any of claims 1 - 2, characterized in that the temperature in the chlorine dioxide step is over 70°C.
4. A method as recited in claim 3, characterized in that the temperature in the chlorine dioxide step is 80 - 100°C.
5. A method as recited in any of the preceding claims, characterized in that the treatment time in the chlorine dioxide step is less that 10 minutes,
preferably 30 seconds to 3 minutes.
6. A method as recited in any of the preceding claims, characterized in that the chlorine dioxide stage comprises in addition to the first chlorine dioxide
step also a second chlorine dioxide step after the acid treatment step.
7. A method as recited in claim 6, characterized in that the treatment temperatures in the first chlorine dioxide step, the acid treatment
step, and the second chlorine dioxide step are essentially the same.
8. A method as recited in any of the claims 1 - 4, characterized in that the chlorine dioxide stage comprises a treatment with chelating agent after
the chlorine dioxide step and the acid treatment step.