Note: Descriptions are shown in the official language in which they were submitted.
2121387
' 1
METHOD FOR BLEACHING LIGNOCELLULOSE-CONTAINING PULP
The present invention relates to a method for bleaching
lignocellulose-containing pulp with a peroxide-containing
compound, in which the pulp, prior to the bleaching and in
optional order, is subjected to acid treatment at a pH of up
to about 5 and treated with a complexing agent in order to
release transition metal ions from their positions in the
pulp. After the acid treatment, a magnesium compound and a
calcium compound are addèd to the pulp at a pH of from about
3.5 to about 8 in order to reintroduce an optimum amount of
magnesium and calcium ions into the pulp. The pulp is there-
after bleached with a peroxide-containing compound at a pH of
from about 7 to about 13.
Backqround of the invention
In the making of lignocellulose-containing pulp of high
brightness, the pulp is bleached in one or more stages. For
quite some time, mechanical pulp has been bleached with
peroxide-containing compounds in alkaline environment, the
intention being to remove chromophoric groups but preserve the
lignin content. For environmental reasons, it has become
increasingly common to treat also chemical pulp as early as in
the first bleaching stages with peroxide-containing compounds
in alkaline environment, thereby to remove chromophoric groups
as well as lignin. Unless the pulp is pretreated, however,
such bleaching is less effective than bleaching with chlorine-
containing bleaching agents. Thus, hydrogen peroxide bleaching
in alkaline environment is disturbed by the presence in the
pulp of ions of certain transition metals, primarily Mn, Cu
and Fe. These metal ions cause the hydrogen peroxide to
disintegrate into undesirable products, thereby reducing the
effectiveness of the peroxide bleaching and increasing the
consumption of peroxide.
The prior art teaches the bleaching of chemical as well
as mechanical pulp with peroxide-containing compounds in
alkaline environment and in the presence of magnesium salts.
Most of the known bleaching processes do not comprise any acid
pretreatment, which preserves the content of desirable as well
as undesirable ions in the pulp.
Furthermore, EP-A-0 402 335 discloses the pretreatment
2I21387
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of chemical pulp with a complexing agent in order to render
more effective subsequent alkaline peroxide bleaching. In this
process, it is important that the treatment with a complexing
agent takes place at an almost neutral pH. Moreover, this
process is not suitable for treating pulp containing large
amounts of transition metal ions.
EP-A-O 511 695 discloses acid treatment of a chemical
pulp, thereafter adding magnesium and subsequently treating
the pulp with alkaline peroxide bleaching.
Therefore, it seems to be obvious to combine the
technical features of the prior art and thus to arrive at a
process which discloses acid treatment of the pulp, thereafter
treating the pulp with a complexing agent and adding magnesium
and subsequently treating the pulp with alkaline peroxide
bleaching. However, such a process does not improve the
brightness of the pulp.
Description of the Invention
The invention relates to a method in which ligno-
cellulose-containing pulp is treated under the conditions
whereby its content of ions having an adverse effect
on subsequent alkaline peroxide bleaching is effectively
reduced, while at the same time its content of favourable
ions is optimised, thereby to obtain a pulp of high
strength and brightness with a low consumption of the
peroxide-containing compound.
Thus, the invention provides a method for bleaching
lignocellulose-containing pulp with a peroxide-containing
compound, in which the pulp, prior to the bleaching and in
optional order, is subjected to acid treatment at a pH of up
to about 5 and treated with a complexing agent, then adding to
the pulp, after the acid treatment, a magnesium compound and
a calcium compound at a pH of from about 3.5 to about 8 in
order to reintroduce magnesium and calcium ions into the pulp,
whereupon the pulp is bleached at a pH of from about 7 to
about 13.
The initial acid treatment combined with the treatment
with a complexing agent in accordance with the invention
effectively releases all types of metal ions in
lignocellulose-containing pulp. By dewatering or washing the
2121387
pulp, the thus-released ions can be effectively removed from
the pulp suspension. It has therefore been found that the
present method effectively reduces the concentration of those
ions of transition metals, in particular manganese, copper and
iron, that have an adverse effect on the subsequent peroxide
bleaching.
In the present method, magnesium ions are reintroduced
into the pulp prior to bleaching, thereby increasing the
brightness and the strength of the pulp while at the same time
reducing the lignin content and the consumption of the
bleaching agent. By also reintroducing calcium ions into the
pulp, there is achieved a surprising improvement of the final
properties of the pulp, especially with respect to brightness
and reduction of the kappa number. In the present method, it
has been found that the content and the positions of magnesium
as well as calcium ions have to be optimised to attain the
synergistic effect enabling effective peroxide bleaching.
Thus, the magnesium and calcium ions have to be added at a pH
of from about 3.5 to about 8 in order that the pulp should
obtain a sufficient content of these ions.
According to the present invention it is possible to
obtain a pulp of high brightness, low lignin content and a on
the whole preserved pulp strength while consuming a reduced
amount of the peroxide-containing compound. Depending on the
type of pulp, inter alia, it is often possible to carry out
the method while using a comparatively small amount of
complexing agent and a simple washing stage before the
peroxide bleaching.
As a result of the acid pretreatment, in combination
with the treatment with a complexing agent and the subsequent
dewatering or washing stage, essentially all ions are released
and removed from the pulp. By the addition of magnesium and
calcium, almost all these ions can be reintroduced into the
pulp, even under the alkaline conditions prevalent in the
peroxide bleaching according to the invention. Thus, the
amount of magnesium charged and the amount of magnesium
present in the pulp in the peroxide bleaching will be essen-
tially the same. Likewise, the amount of calcium charged and
the amount of calcium present in the pulp in the peroxide
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bleaching will be essentially the same.
The acid treatment and the addition of magnesium and
calcium compound are carried out in this order and essentially
within different pH ranges. The treatment with the complexing
agent may be performed whenever suitable from the point of
view of process technique. The treatment with a complexing
agent may be carried out in a separate stage before or after
the acid treatment or after the addition of magnesium and
calcium compounds. Suitably, the treatment with a complexing
agent is combined with the acid treatment or the addition of
magnesium and calcium compounds. Thus, the acid treatment and
the treatment with a complexing agent suitably are performed
in a single stage before the magnesium and calcium compounds
have been added. However, it is preferred that the treatment
with a complexing agent and the addition of magnesium and
calcium compounds take place in a single stage after the acid
treatment. After the pretreatment into two or more stages, the
pulp is suitably dewatered or washed and is then bleached with
a peroxide-containing compound at an alkaline pH.
The weight ratio between added Mg and Ca can be from
about 1:1 to about 1:4 in order to render effective the
alkaline peroxide bleaching. Suitably, the weight ratio of Mg
to Ca is from 1:1.2 to 1:3, preferably from 1:1.5 to 1:2.6.
The amount of magnesium compound charged, as well as the
amount of calcium compound charged, may be up to about 4000
ppm, calculated as alkaline earth metal by weight of dry pulp.
Suitably, the amount of both the magnesium compound and the
calcium compound is in the range 100-3000 ppm, preferably in
the range of 500-2000 ppm.
The amount of magnesium compound charged, as well as the
other conditions, is so chosen that the content of magnesium
in the pulp before the peroxide bleaching amounts to at least
about 50~ of the content of magnesium before carrying out of
the present method (the original content). Suitably, the
amount of magnesium compound charged, as well as the other
conditions, is so-chosen that the content of magnesium in the
pulp before the peroxide bleaching is in the range of 100-300
of the original content, preferably in the range of 130-200~.
The amount of calcium compound charged, as well as the
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other conditions, is so chosen that the content of calcium in
the pulp before the peroxide bleaching amounts to at least
about 25~ of the original content. Suitably, the amount of
calcium compound charged, as well as the other conditions, is
so chosen that the content of calcium in the pulp before the
peroxide bleaching is in the range of 50-150~ of the original
content, preferably in the range of 65-100~.
Suitably, the magnesium-containing compound used is
magnesium sulphate, magnesium chloride, magnesium carbonate or
magnesium nitrate, preferably magnesium sulphate. Suitably,
the calcium-containing compound used is calcium chloride,
calcium nitrate, calcium sulphate or calcium carbonate,
preferably calcium sulphate. Also compounds containing
magnesium and calcium in a suitable ratio may be used, such as
dolomite.
In order to achieve a satisfactory effect, the magnesium
and calcium compounds should be in dissolved form when brought
into contact with the pulp. This can be obtained in various
ways, depending on the type and properties of the pulp, among
other things. By combining a suitable pH of from about 3.5 to
about 8 with suitable temperatures and concentrations of the
magnesium and calcium compounds, these compounds can be
brought into dissolved form.
Within the scope of the invention, all or some of the
magnesium or calcium ions can be added to the pulp suspension
via the water used e.g. for pH adjustment or dilution. Thus,
hard water containing magnesium or calcium ions, or a combina-
tion thereof, may advantageously be used for reintroducing
these ions into the pulp. Examples of such hard water are
fresh water from limestone bedrock, white water from paper-
making machines using lime or chalk as filler, and process
water from sulphite production using magnesium or calcium as
base.
The present method preferably comprise a dewatering or
washing stage after the treatment with the complexing agent
and the addition of magnesium and calcium compounds. As a
result, undesirable metal ions can be effectively removed
before the peroxide bleaching. The pulp may also be dewatered
or washed before and/or after the complexing agent treatment.
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By undesirable metal ions are meant not only transition
metal ions but also any excess of magnesium and calcium ions.
In order that the reintroduced magnesium and calcium ions
should not be released from the pulp in another washing stage,
the pH in the washing liquid must be at least about 4.
Suitably, the pH of the washing liquid is in the range of 5-
13, preferably 6-12.
In the present method, the acid treatment is performed
at a pH of up to about 5, suitably a pH of from 1.5 to 4, and
preferably a pH of from 2 to 3.
In the acid treatment, pH can be adjusted by adding an
acid or an acid liquid to the pulp. Use is then made of
inorganic mineral acids or residual acid from a chlorine
dioxide reactor, either separately or in optional mixture.
Suitably, use is made of an inorganic mineral acid, such as
sulphuric acid, nitric acid or hydrochloric acid, preferably
sulphuric acid.
Manganese ions in the pulp, for example, have an
especially adverse effect on alkaline peroxide bleaching.
Compounds forming strong complexes with different manganese
ions are therefore primarily used as complexing agents.
Suitable complexing agents from this point of view are
nitrogen-containing organic compounds, primarily nitrogen-
containing polycarboxylic acids, nitrogen-containing poly-
phosphonic acids, and nitrogen-containing polyalcohols.
Preferred nitrogen-containing polycarboxylic acids are
diethylenetriamine pentaacetic acid (DTPA), ethylenediamine
tetraacetic acid (EDTA) and nitrilo-triacetic acid (NTA), DTPA
and EDTA being especially preferred. Diethylenetriamine
pentaphosphonic acid is a preferred nitrogen-containing
polyphosphonic acid. Also other compounds can be used as
complexing agents, such as polycarboxylic acids, suitably
oxalic acid, citric acid or tartaric acid, or phosphonic
acids. Such organic acids as are formed in the treatment of
the pulp with chlorine-free bleaching agents, for example, may
also be used as complexing agents. The sodium salts of the
above complexing agents are preferably used, making the bleach
plant a more closed system.
The choice of pH in the treatment with a complexing
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agent is not critical in the present method, since the
magnesium and calcuium ions having a favourable effect on the
subsequent alkaline peroxide bleaching can be optimised both
with respect to amount and position in the pulp. In the
present method, the treatment with a complexing agent can thus
be performed at a pH of from about 1.5 to about 13. As a
result, the treatment with a complexing agent can be combined
with the acid treatment,without there being any appreciable
reduction of the complexing ability.
The amount of complexing agent charged depends on the
type and the amount of transition metal ions present in the
incoming pulp, as well as on the effectiveness of the acid
treatment and the subsequent dewatering or washing stage.
Furthermore, the amount also depends on the type of complexing
agent and on the conditions prevailing in the treatment with
the agent, such as temperature, residence time, and pH.
However, the amount of complexing agent charged can be up to
about 5 kg/tonne of dry pulp, based on a 100~ product.
Suitably, this amount is from 0.05 kg to 2.5 kg/tonne of dry
pulp, preferably from 0.1 kg to 1 kg/tonne of dry pulp, based
on a 100~ product. It is especially preferred that the amount
of complexing agent is in the range of 0.2-0.6 kg/tonne of dry
pulp, based on a 100~ product.
In the present method, the pH in the pulp suspension
before the addition of magnesium and calcium compounds is from
about 3.5 to about 8, suitably in the range of 4-7.5, and
preferably in the range of 4.5-7. It is especially preferred
that the pH is in the range of 5-6.5.
Before the acid treatment in the present method, the
pulp can be subjected to acid treatment in an additional
stage, thereby to obtain a particularly low content of
transition metal ions. Such additional treatment may be
suitable in alkaline peroxide bleaching of pulp in which the
content of transition metal ions is unusually high or in which
these ions are difficult to release. One such instance is
organosolv pulp.
In a preferred mode of carrying out the present method,
the acid treatment is performed in the presence of a deligni-
fying chemical which is effective at a pH of up to about 5.
2 ~ ~ ~ 3 8 7
_ 8
Suitable delignifying chemicals are ozone, acid hydrogen
peroxide, per-acids, such as Caro's acid or peracetic acid,
and salts thereof, chlorine dioxide and chlorine. Preferably,
use is made of ozone, per-acids or salts thereof, or chlorine
dioxide, resulting in far-reaching and selective delignifica-
tion. Ozone and chlorine dioxide are especially preferred.
When chlorine dioxide is used as a delignifying chemical
in the present method, it is possible to bleach the pulp to
full brightness with an initial chlorine dioxide stage
follo~d by a peroxide stage and still reducing the produced
and discharged amount of chlorine containing compounds to an
extremely low level. Thus, the present method optimizes the
conditions for an effective peroxide bleaching, whereby a
reduced adding of chlorine dioxide can be balanced with an
additional, and economically reasonable, adding of peroxide.
A reduced adding of chlorine dioxide means that the content of
chlorine containing compounds in the pulp, which has been
bleached, decreases and the possibilities to close the plants
increases essentially. Thus, with the present method, it is
possible to decrease the total amount of chlorine containing
compounds in the waste water, from all staqes, to less than 5
kg/tonne pulp, based on the element chlorine. The total amount
of chlorine containing compounds in the waste water from all
stages will be decreased suitably to less than 3,5 kg/ton pulp
and preferably to less than 2 kg/tonne pulp, based on elementary
chlorine.
When the present method comprises delignification with
ozone, the amount may be from about 0.5 kg to about 30
kg/tonne of dry pulp, suitably from 1 kg to 20 kg/ tonne of
dry pulp, and preferably from 1.5 kg to 10 kg/ tonne of dry
pulp .
When the present method comprises delignification with
chlorine dioxide, the amount can be indicated as a charging
factor (CF) as follows
CF = total amount of active chlorine in kg/tonne of dry
pulp / ( 1 )
kappa number prior to delignification according to the
present method.
~hus, 1 kg of chlorine dioxide is equivalent to 2.63 kg of
21~1387
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active chlorine. In the present method, the charging factor
may be from about 0.1 to about 10, suitably from 0.5 to 5, and
preferably from 1 to 3.
The present method can include a dewatering or washing
stage after the acid treatment and the treatment with a
complexing agent. The pulp may also be dewatered or washed
before and/or after the complexing agent treatment. Thus,
transition metal ions can be effectively removed prior to the
peroxide bleaching. The washing liquid may consist of fresh
water, optionally with an addition of a pH-adjusting chemical,
or white water from one or more bleaching stages or extraction
stages, giving a suitable pH in the washing stage.
In the present method, the acid treatment, treatment
with complexing agent and the addition of magnesium and
calcium can be carried out at a temperature of from about 10~C
to about 100~C, suitably from 25~C to 90~C, and preferably
from 40~C to 80~C, and for a time of from about 1 min to about
600 min, suitably from 5 min to 120 min, and preferably from
10 min to 60 min.
The pulp concentration may be from about 1~ by weight to
about 60~ by weight and preferably from 3~ by weight to 35~ by
weight.
The bleaching with a peroxide-containing compound is
performed at a pH of from about 7 to about 13, suitably a pH
of from 8 to 12, and preferably a pH of from 9 to 11.
The peroxide-containing compound comprises inorganic
peroxide compounds, such as hydrogen peroxide, sodium peroxide
and peroxo sulphuric acid (Caro's acid), and organic peroxide
compounds, such as peracetic acid and performic acid. The
peroxide-containing compound suitably consists of hydrogen
peroxide or a mixture of hydrogen peroxide and oxygen, and
preferably is hydrogen peroxide.
Conventional bleaching conditions can be used.
After the pretreatment stages and the subsequent
bleaching with a peroxide-containing compound, the pulp may be
used as such for making paper. If so desired, the pulp may
also be finally bleached to a higher brightness in one or more
stages, e.g. by means of hydrogen peroxide, ozone, sodium
dithionite or chlorine dioxide. Final bleaching can also
2121387
include alkaline extraction stages which may be fortified by
peroxide and/or oxygen.
By lignocellulose-containing pulp is meant pulp contain-
ing fibres that have been exposed by known chemical or
mechanical treatment, or recycled fibres. Suitably, the
lignocellulose-containing pulp consists of chemically digested
pulp, preferably chemically digested pulp that has been
delignified with oxygen prior to the present method. It is
especially preferred that the lignocellulose-containing pulp
consists of sulphate pulp of softwood.
The invention and its advantages will be further
illustrated by the following, non-restricting examples. In the
description, the examples and the claims, the figures in per
cent and parts are all by weight, unless otherwise stated.
Furthermore, the pH values given in the description, the
examples and the claims concern the pH at the end of each
treatmentj unless otherwise stated.
In the examples below, the kappa number, the viscosity
and the brightness of the pulp were determined according to
the SCAN Standard Methods C 1:77 R, C 15-16:62 and C 11-75:R,
respectively. The consumption of hydrogen peroxide and
peracetic acid were established by titration with sodium
thiosulphate, and potassium permanganate and sodium
thiosulphate, respectively.
ExamPle 1: Oxygen-delignified sulphate pulp of softwood having
a kappa number of 15.7, a brightness of 37.9~ ISO and a
viscosity of 990 dm3/kg was treated with 15 kg H2SO4/tonne dry
pulp, giving a pH of 2. The acid treatment (A) was performed
at 50~C, a part concentration of 10~ by weight and for 30 min.
After raising the pH to 5, EDTA (Q), magnesium sulphate (Mg)
and calcium sulphate (Ca) were added (test 1). The conditions
were so chosen that magnesium and calcium were dissolved in
the pulp suspension. The treatment took place at 50~C, a pulp
concentration of 10~ by weight and for 60 min. The amount of
EDTA added was 2 kg/tonne dry pulp. The amount of magnesium
and calcium compounds added was, respectively, 1000 ppm and
1500 ppm, based on alkaline-earth metal per dry pulp. Then,
the pulp was bleached with hydrogen peroxide (P) at a tempera-
ture of 90~C, a residence time of 240 min and a pulp concen-
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tration of 10~ by weight. The addition of hydrogen peroxide
was 25 kg/tonne dry pulp, based on 100~ hydrogen peroxide, the
pH was 10.5-11. After each stage, the pulp was washed with
deionised water having a pH of 5.5. Thus, the pulp was first
dewatered to a pulp concentration of 25~ by weight, and then
diluted to a pulp concentration of 3~ by weight. After a few
minutes, the pulp was dewatered to a pulp concentration of 25~
by weight. For comparative purposes, only magnesium sulphate
and calcium sulphate (test 2), as well as only magnesium
sulphate (test 3) were added after the acid treatment, thereby
to illustrate the effect of the complexing agent and of the
complexing agent and the calcium ions. The conditions in tests
2 and 3 were otherwise in keeping with those indicated above.
The results obtained after the bleaching with hydrogen
peroxide appear from the Table below.
TABLE I
Pulp properties after H2O2 Remain-
Test Sequence Kappa Visco- Bright- ing H2O2
number sity ness
(dm3/kq) (~ISO) kq/tonne
1 A-(Q+Mg+Ca)-P 7.1 865 76.4 8.6
2 A-(Mg+Ca)-P 7.3 705 74.9 1.6
3 A-(Mg)-P 8.4 730 71.0 1.6
It is evident from the Table that the pretreatment of
pulp in accordance with the present invention results in a
strong pulp of high brightness and low lignin content, the
consumption of hydrogen peroxide being comparatively low.
Example 2: Oxygen-delignified sulphate pulp of softwood having
a kappa number of 8.2, a brightness of 45~ ISO and a viscosity
of 820 dm3/kg was treated in accordance with the present
method (test 1). The acid treatment, the addition of EDTA,
magnesium sulphate and calcium sulphate in a single stage and
the bleaching with hydrogen peroxide were performed as in
Example 1. After each stage, the pulp was washed as in Example
1. For comparative purposes, only the complexing agent and the
magnesium sulphate (test 2), as well as only the complexing
agent (test 3) were added after the acid treatment. The
conditions in tests 2 and 3 are otherwise in keeping with
those indicated above. The results obtained after the bleach-
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12
ing with hydrogen peroxide appear from the Table below.
TABLE II
Pulp properties after H2O2 Remain-
Test Sequence Kappa Visco- Bright- ing H2O2
number sity ness
(dm3/kq) (~ISO) kq/tonne
1 A-(Q+Mg+Ca)-P 2.2 765 84.9 11.1
2 A-(Q+Mg)-P 2.6 705 81.8 11.0
3 A-(Q)-P 3.4 560 75.0 0
It is evident from the Table that the present treatment
of pulp is an environmentally compatible, selective and
effective method for bleaching pulp.
Exam~le 3: Oxygen-delignified sulphate pulp of softwood having
a kappa number of 14.8, a brightness of 36.4~ ISO and a
viscosity of 1000 dm3/kg was delignified wlth chlorine dioxide
(D), treated with EDTA in the presence of magnesium and
calcium, and bleached with hydrogen peroxide. In delignifica-
tion, chlorine dioxide was added in an amount equivalent to 30
kg active chlorine/tonne dry pulp, i.e. the batching factor
was 2. Delignification was performed at a temperature of 50~C,
a pulp concentration of 10~ by weight and for 50 min, pH at
the end being 3.9. The addition of EDTA, magnesium sulphate
and calcium sulphate in a single stage and the bleaching with
hydrogen peroxide were as in Example 1. After each stage, the
pulp was washed as in Example 1. For comparative purposes, the
pulp was treated without calcium (in test 2), without calcium
or magnesium (in test 3) and without complexing agent, calcium
or magnesium (in test 4). The results obtained after the
bleaching with hydrogen peroxide appear from the Table below.
TABLE III
Pulp properties after H2O2 Remain-
Test Sequence Viscosity Brightness ing H2O2
(dm3/kq) (~ISO) kg/tonne
1 D-(Q+Mg+Ca)-P 850 88.5 11.2
2 D-(Q+Mg)-P 820 85.6 10.7
3 D-Q-P 650 8-0.1 0
4 D-P 700 77.3 0
It is evident from the Table that a preferred mode of
bleaching in which the pulp is delignified with chlorine
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_ 13
dioxide results in a strong pulp of very high brightness, the
consumption of hydrogen peroxide being comparatively low.
Example 4: The sulphate pulp used in Example 3 was delignified
with ozone (Z), treated with EDTA in the presence of magnesium
5 and calcium at different pH values, and bleached with hydrogen
peroxide. Delignification with ozone was performed at a pH of
2.1, the temperature of 25~C, a pulp concentration of about
40~ by weight and for 2-3 min. The amount of ozone charged was
7- 8 kg/tonne dry pulp. The combined treatment with EDTA and
addition of magnesium and calcium sulphate was performed as in
Example 1, except that pH was varied. The bleaching with
hydrogen peroxide was performed as in Example 1. After each
stage, the pulp was washed as in Example 1. The results
obtained after the bleaching with hydrogen peroxide appear
from the Table below.
TABLE IV
Pulp properties after H202 Remain-
Test pH Kappa- Visco- Bright- ing H202
number sity ness
(dm3/kg) (~ISO) kq/tonne
1 3.2 3.1 670 83.5 8.4
2 4.1 3.1 745 84.312.4
3 5.5 3.0 750 85.112.5
4 6.0 3.0 750 85.012.2
7.2 2.7 705 85.0 9.4
6 9.2 2.9 645 84.9 4.8
It is evident from the Table that the treatment of pulp
in the pH range according to the invention results in a pulp
of excellent final properties, involving efficient utilisation
30 of the bleaching capacity of the peroxide.
Example 5: The sulphate pulp used in Example 3 was delignified
with ozone (Z), treated with magnesium and calcium, and
bleached with hydrogen peroxide (P). The complexing agent was
added after Z in the same stage as in Mg and Ca (Q2, test 1),
35 as well as before Z (Ql, test 2), in order to illustrate the
effect of the point of addition of the complexing agent.
Delignification with ozone and the combined treatment with
EDTA and addition of magnesium and calcium sulphate were in
test 1 performed as in Example 1, as were the bleaching with
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_ 14
hydrogen peroxide. In Ql, 15 kg H2SO4 was added per tonne dry
pulp, giving a pH of 2Ø Otherwise, Ql was performed in
keeping with Q2, except that the residence time was 30 min.
After each stage, the pulp was washed as in Example 1. The
5 results obtained after the bleaching with hydrogen peroxide
appear from the Table below.
T~3LE V
Pulp properties after H2O2 Remain-
Test Sequence Kappa Visco- Bright- ing H2O2
number sity ness
(dm3/kq) (~ISO) kq/tonne
1 Z-(Q2+Mg+Ca)-P 3.2 820 82.3 10.8
2 Ql-Z- (Mg+CA)-P 3.1 775 82.9 9.0
It is evident from the Table that a strong and bright
pulp is obtained as a result of the addition of the complexing
agent, either before or after a delignifying stage in a
preferred mode of execution of the present invention.
Example 6: Oxygen-delignified sulphate pulp of softwood having
a kappa number of 7.7, a brightness of 51. 6~ ISO and a
20 viscosity of 825 dm3/kg was treated with 15 kg H2SO4/tonne dry
pulp, giving a pH of 2Ø The acid treatment was performed at
50~C for 30 min and with a pulp concentration of 10% by
weight. After pH had been raised to 5-5.5, 2 kg EDTA/tonne dry
pulp was added together with magnesium sulphate in an amount
25 of 300 ppm, based on magnesium, and calcium sulphate in an
amount of 600 ppm, based on calcium. The treatment took place
at 50~C, for 60 min and with a pulp concentration of 10~ by
weight. Then, the pulp was bleached with hydrogen peroxide at
a temperature of 90~C, for a residence time of 240 min and
with a pulp concentration of 10~ by weight. The addition of
hydrogen peroxide was 25 kg/tonne dry pulp, based on 100~
hydrogen peroxide, and pH was 11. After the acid treatment and
the peroxide bleaching, the pulp was washed with deionised
water having a pH of 5.5. The pulp was first dewatered to a
pulp concentration of 25~ by weight, and was then diluted to
a pulp concentration of 3% by weight. After a few minutes, the
pulp was dewatered to a pulp concentration of 25~ by weight.
For comparative purposes, tests were made involving an
addition of but 900 ppm magnesium (Test 2), an addition of but
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900 ppm calcium (Test 3), and no addition of either magnesium
sulphate or calcium sulphate (Test 4). The results after the
hydrogen peroxide bleaching appear from the Table below.
TABLE VI
5 Test Pulp properties after H2O2
Mg Ca Ratio Viscosity Brightness
pPm Ppm Mq:Ca (dm3/kq) (%ISO)
1 300 600 1:2 750 85.0
2 900 0 --- 740 82.1
3 0 900 --- 670 82.0
4 0 0 --- 665 77.4
It is evident from the Table that the combined addition
of magnesium and calcium to the pulp results in a strong pulp
of high brightness.
Exemple 7: The sulphate pulp used in Example 3 was delignified
with chlorine dioxide (D), treated with EDTA in the presence
of magnesium and calcium, and bleached with hydrogen peroxide
(P), i.e. the sequence D-(Q+Mg+Ca)-P. The pulp was bleached
with different combinations of D and P to illustrate the
effect of the batching factor on the total amount of the
produced chlorine containing compounds at a constant final
brightness of 88~ ISO. The delignification with chlorine
dioxide were as in Example 1, apart from the batching factor
which was from 0.67 up to 2. The combinated addition of EDTA,
magnesium sulphate and calcium sulphate were as in Example 1.
The bleaching with hydrogen peroxide were as in Example 1,
apart from that the temperature was 105~C and that the amount
of hydrogen peroxide was varied. After each stage, the pulp
was washed as in Example 1. The amount of chlorine containing
compounds from the sequence was measured in the total volume
of waste water. The amounts of chlorate and chloride were
measured with ion-chromatography according to the standardized
method. The amount AOX was measured according to SCAN-W 9:89.
The total amount of the elementary chlorine was measured in
each test with titration after reduction with SO2 at room
temperature. As comparison, the same pulp was bleached and
delignified to approximately the same brightness as in the
sequence D-(EOP)-D-(EP)-D (Test 6). The conditions in each
stage were conventional. The amounts of chlorine dioxide and
-- 16 ~g121387
hydrogen peroxide in test 6 are total in the sequence. The
partial pressure of oxygen in the EOP-stage was 0.22. The
results obtained after the bleaching with hydrogen peroxide
appear from the Table below.
T~3ELL VII
Chlorine compounds in waste water
D as active Cl03- Cl- AOX Total count-
Test chlorine P Brightness ed as Cl
(kg/tonne) (kg/tonne) (kg/tonne massa)
1 1 10 40 88,1 0,5 1,4 0,20 1,8
2 15 32 88,3 1,1 2,3 0,30 3,1
3 20 26 88,0 1,6 2,9 0,40 4,0
4 25 20 88,4 2,5 3,2 0,53 4,8
16 88,3 3,1 3,7 0,68 5,7
6 44 4 88,8 4,6 6,4 0,90 8,9
It is evident from the Table that it is possible, with
the present treatment of pulp, to produce a pulp with high
brightness and at the same time reduce the amount of chlorine
containing compounds to a level which permits a highly degree
of closing the cellulose factory.
Exam~le 8: Oxygen-delignified sulphate pulp of softwood having
a kappa number of 9.6, a brightness of 40.5~ ISO and a
viscosity of 890 dm3/kg was delignified with chlorine dioxide
(D) in the presence of EDTA (Q), and bleached with hydrogen
peroxide (P) and oxygen (O) in the presence of magnesium and
kalcium, i.e. in the sequence (D+Q) (P+O+Mg+Ca). The pulp was
bleached with various combinations of D and P to illustrate
the effect on the batching factor on the total amount chlorine
containing compounds produced at a constant final brightness
of about 90% ISO. The delignification with chlorine dioxide
was performed at a pH of 1.8-2.0, and a temperature of 45~C
for 25 min. The batching factor was varied from 1.0 to 2.5.
The added amount of EDTA was 1.5 kg/ tonne dry pulp. After raising
pH to 5-5;5, magnesium sulpate was added in an amount of 300
ppm calculated as magnesium and calcium sulphate was added in
an amount of 600 ppm calculated as calcium. Subsequently
sodium hydroxide and hydrogen peroxide were added and the pulp
concentration was adjusted to 10~ by weight. The used
autoclave was pressurized with oxygen to 0.22 MPa measured at
'~A
71-~;
2 ~ 3 8 7
17
room temperature. After raising the temperature to 110~C, the
pulp was bleached for 240 min. pH became 11-11.5 after
bleaching. After each stage, the pulp was washed as in Example
6. The amount of chlorine containing compounds from the
sequence were measured in the total volume of waste water. The
amounts of chlorate and chloride were measured with ion-
chromatography according to the standardized method. The
amount AOX was measured according to SCAN-W 9: 89. The total
amount of the elementary chlorine was measured in each test
throulgp titration after reduction with SO2 at room tempera-
ture. As comparison, the same pulp was bleached and deligni-
fied to approximately the same brightness as in the sequence
D-(EOP)-D-(EP)-D (Test 5). The conditions in each stage were
conventional. The amounts of chlorine dioxide and hydrogen
peroxide in test 5 are total in the sequence. The partial
pressure of oxygen in the EOP-stage was 0. 22. The results
obtained after the bleaching with hydrogen peroxide appear
from the Table below.
T~ VIII
Chlorine compounds in waste water
D as activeCl03- Cl- AOX Total count-
Test chlorine P Briqhtness ed as Cl
(kg/tonne) (kg/tonne) (kg/tonne massa)
1 9,6 40 89,8 0,5 1,3 0,17 1,8
2 14,4 30 90,1 1,0 2,2 0,28 2,9
3 19,2 25 90,3 1,5 2,8 0,33 3,8
4 24,0 20 90,5 2,4 3,2 0,50 4,7
5 40,0 3,5 89,9 4,4 4,6 0,85 7,7
It is evident from the Table that it is possible, with
30 the present treatment of pulp, to produce a pulp with high
brightness and at the same time reduce the amount of chlorine
containing compounds to a level which permits a highly degree
of closing the cellulose factory.
