Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1
2149649
Process for bleaching of lianocellulose-containing pulp
The present invention relates to a process for
bleaching of lignocellulose-containing pulp, in which the
pulp is first treated with a complexing agent, then delig
nified with a peracid or a salt thereof, and subsequently
bleached with a peroxide-containing compound. Suitably,
delignification is carried out with the strongly oxidising
peracetic acid, giving a considerable increase in bright-
ness and a considerable reduction of the Kappa number
after bleaching with a chlorine-free bleaching agent, such
as hydrogen peroxide. The brightness-increasing effect is
highly selective, i.e. the viscosity of the pulp is main-
tained to a comparatively great extent.
Background of the Invention
Chlorine-free bleaching agents have long been used
for bleaching mechanical pulps. In recent years, it has
become increasingly common to bleach also chemical pulps
with chlorine-free bleaching agents, such as hydrogen
peroxide and ozone, even in the first stages. It has been
considered necessary to pretreat the pulp immediately
after digestion and an optional oxygen-delignifying stage
so as to avoid deteriorated pulp properties and an exces-
sive consumption of the bleaching agent. Pretreatment of
the pulp primarily involves acid treatment and treatment
with a complexing agent or salts of alkaline-earth metals,
optionally in combination. Strongly acid pretreatment
removes desirable as well as undesirable metal ions from
the original positions in the pulp. Treatment with suit-
able complexing agents primarily removes the undesirable
metal ions, while the desirable ones are largely retained.
Treatment with salts of alkaline-earth metals maintains or
reintroduces the desirable metal ions.
EP-A-0 415 149 discloses the use of peroxomono
sulphuric acid before treatment with oxygen and/or
peroxide. The pulp can be pretreated in process stages
where heavy metals and organic impurities are removed. In
the Examples, the pulp is thus treated in the presence of
A
2 2149649
DTPA at a pH of 2 before treatment with peroxomono-
sulphuric acid and oxygen. Such an acid wash removes also
those metal ions, primarily of alkaline-earth metals, that
are necessary for an efficient subsequent bleaching with
peroxide-containing compounds or ozone.
EP-A-0 402 335 describes a process for improving
the effect of an initial alkaline peroxide treatment. The
pulp is pretreated with a complexing agent at pH 3.1-9
and a temperature 26°-100°C. The only exemplified
peroxide-containing compounds are hydrogen peroxide, and
its mixture with oxygen. The purpose of the pretreatment
according to EP-A-0 402 335 is to prepare the pulp for an
initial alkaline peroxide treatment.
WO-A-9 215 752 describes a process where a pulp is
treated with a sequence in two steps. In the first step
peroxomonosulphuric acid, i.e. Caro's acid (=an inorganic
acid containing sulphur), is added. A complexing agent
may be present in the treatment with Caro's acid. In the
second step the pulp is bleached with hydrogen peroxide.
When a peracid is present along with a complexing agent,
i.e. in the same treatment step, the effect of the com-
plexing agent is decreasing dramatically because of the
degradation of complexing agents in presence with
peracids.
With increasingly stringent environmental standards,
there is a growing need for completely chlorine-free
processes for delignifying and bleaching lignocellulose-
containing pulps. To produce fully bleached pulps with
unaltered strength properties in a reasonable number of
stages and with a reasonable consumption of the bleaching
agents, it has become necessary to consider using also
powerful, and hence difficultly-controlled, bleaching
agents having a high delignifying and/or bleaching
capacity.
a
2a
2149649
Description of the Invention
The invention provides a process in which a ligno-
cellulose-containing pulp is delignified and bleached
under conditions whereby a good delignifying and bleaching
effect is obtained even before bleaching with a peroxide-
containing compound.
The inventive process comprises bleaching of ligno-
cellulose-containing pulp with a peroxide-containing
compound, especially hydrogen peroxide, wherein the pulp,
before said bleaching, in a sequence is first treated with
a complexing agent at a pH in the range of from 3.5 up to
about 11 and at a temperature in the range of from 26°C up
to about 100°C and then delignified with an organic
peracid or a salt thereof at a temperature in the range of
from 50°C up to about 140°C, and the pulp, before said
bleaching, is washed at a pH of at least about 4.
The inventive process has made it possible to
delignify the pulp after treatment with a complexing agent
without adversely affecting the conditions, optimised by
the treatment with a complexing agent, for the subsequent
chlorine-free bleaching, taking into consideration the
desirable and undesirable metal ions. Thus, ions of
alkaline-earth metals, especially when in their original
positions in the pulp, are known to have a favourable
effect on the selectivity in bleaching and the consumption
of chlorine-free bleaching agents, such as peroxide-
containing compounds and ozone.
In the invention, peracid or salts thereof include
organic peracids or salts thereof. As organic peracid,
use is made of aliphatic peracids, aromatic peracids or
salts thereof.
a
WO 94/12722 PCT/SE93/01018
3
Suitably, peracetic acid or performic acid is used. Sodium is
suitably used as cation in the salts, since such salts norm-
ally are inexpensive and sodium occurs naturally in the chemi-
cal balance in the pulp mill. Peracetic acid or salts thereof
are preferred, being advantageous in terms of production and
use. In addition, peracetic acid has limited corrosiveness.
Any wastewater containing, inter alia, the degradation prod-
ucts of peracetic acid can be easily used for washing or recy-
cled to the chemical recovery system.
According to the present process, peracetic acid can be
produced by reacting acetic acid and hydrogen peroxide, giving
what is known as equilibrium peracetic acid, by distilling
equilibrium peracetic acid to remove hydrogen peroxide, acetic
acid and sulphuric acid, or by reacting acetic acid anhydride
and hydrogen peroxide directly in the bleaching stage, giving
what is known as in situ peracetic acid. A typical equilibrium
peracetic acid contains about 42% of peracetic acid and about
6% of hydrogen peroxide, i.e. the weight ratio of peracetic
acid to hydrogen peroxide is here about 7:1. When equilibrium
peracetic acid is used in the present process, the weight
ratio between peracetic acid and hydrogen peroxide can be in
the range of from about 10:1 to about 1:60, suitably from 7:1
to 1:15 and preferably from 2.8:1 to 1:2.
The added amount of peracid or salts thereof, should be
in the range of from about 1 kg up to about 100 kg per tonne
of dry pulp, calculated as 100% peracid or salt thereof . Suit
ably, this amount lies in the range of from 2 kg up to 45 kg
per tonne of dry pulp, preferably in the range of from 3 kg up
to 25 kg per tonne of dry pulp, calculated as 100% peracid or
salt thereof.
Suitably, delignification with peracid or salts thereof
is carried out at a pH in the range of from about 2.5 up to
about 12. In preferred embodiments where delignification is
carried out with peracetic acid, the pH suitably lies in the
range of from 3 up to 11, preferably in the range of from 5.5
up to 9. Delignification with the other peracids or salts
thereof mentioned above is carried out within the normal pH
ranges for the respective bleaching agents, these being well-
known to those skilled in the art.
WO 94112722 PCT/SE93/01018
4
In the pulp, manganese ions, inter alia, have a particu-
larly adverse effect on the bleaching with chlorine-free blea-
ching agents, such as alkaline peroxide compounds or ozone.
Thus, compounds forming strong complexes with various manga-
nese ions are primarily used as complexing agents. Such suit-
able complexing agents are nitrogenous organic compounds,
primarily nitrogenous polycarboxylic acids, nitrogenous poly-
phosphonic acids and nitrogenous polyalcohols. Preferred
nitrogenous polycarboxylic acids are diethylenetriaminepenta-
acetic acid (DTPA), ethylenediaminetetraacetic acid (EDTA) or
nitrilotriacetic acid (NTA), DTPA and EDTA being especially
preferred. Diethylenetriaminepentaphosphonic acid is the pre-
ferred nitrogenous 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. Further, such organic acids as are formed
during the pulp treatment with, inter alia, chlorine-free
bleaching agents can also be used as complexing agents.
The pH in the treatment with a complexing agent is of
decisive importance in removing the undesirable trace metal
ions while at the same time retaining the desirable alkaline
earth metal ions. A suitable pH range depends, inter alia, on
the type and the amount of trace metal ions in the incoming
pulp. In the inventive process, however, the treatment with a
complexing agent should be carried out at a pH in the range of
from 3.5 up to about 11, suitably from 3.5 up to 10, and
preferably from 4.5 up to 9. A pH within the range of from 5
up to 7 is especially preferred.
The temperature in the treatment with a complexing agent
is of considerable importance for removal of the undesirable
trace metal ions. Thus, the content of manganese ions decrea
ses with increasing temperature in the treatment with a com
plexing agent, which gives an increase in brightness and a
reduction of the kappa number. For instance, when the tempera
ture is increased from 20°C to 90°C, the viscosity is also
found, surprisingly, to increase noticeably. The treatment
with a complexing agent is carried out at a temperature of
from 26°C up to about 120°C, suitably from 26°C up to
about
100°C, preferably from 40°C up to 95°C, and most
preferably
2149649
from 55°C up to 90°C.
The added amount of complexing agent depends on the
type and amount of trace metal ions of the incoming pulp.
Also, this amount depends on the type of agent used as
5 well as the conditions in the treatment with the complex-
ing agent, such as temperature, residence time and pH.
However, the added amount of complexing agent should be in
the range of from about 0.1 kg up to about 10 kg per tonne
of dry pulp, calculated as 100 complexing agent. Suit-
able, the amount lies in the range of from 0.3 kg up to 5
kg per tonne of dry pulp, and preferably in the range of
from 0.5 up to 1.8 kg per tonne of dry pulp, calculated as
100 complexing agent.
In preferred embodiments where the delignification
with peracid as well as the treatment with a complexing
agent are carried out at a close to neutral pH, the need
of pH adjustment is minimised. As a result, also the
spent liquors from the bleaching and treatment stages can
be used internally for washing. This gives a small total
volume of wastewater, enabling a considerably more closed
system in the pulp mill.
Bleaching with a peroxide-containing compound can
be carried out in an optional sequence or mixture. The
peroxide-containing compound consists of inorganic
peroxide compounds, such as hydrogen peroxide or peroxo-
monosulphuric acid (faro's acid). Preferably, the
peroxide-containing compound is hydrogen peroxide or a
mixture of hydrogen peroxide and oxygen.
When hydrogen peroxide is used as bleaching agent,
the pulp can be treated at a pH of from about 7 up to
about 13, suitably at a pH of from 8 up to 12, and prefer
ably at a pH of from 9.5 up to 11.5. Bleaching with the
other bleaching agents indicated above takes place within
the normal pH ranges for the respective agents, these
being well-known to those skilled in the art.
2149649
The inventive process may also include a bleaching
stage with a peroxide-containing compound or ozone before
the delignification with peracid or salts thereof.
The inventive process is carried out with a washing
stage before the bleaching with a peroxide-containing
compound, such that the washing is carried out at a pH of
at least about 4. Washing efficiently removes the
complexed trace metal ions that have an adverse effect on
the subsequent bleaching with a peroxide-containing
compound, ozone or sodium dithionite, primarily manganese
ions but also ions of e.g. copper and iron. Because the
pH in the washing stage is at least about 4, the alkaline-
earth metal ions that have a favourable effect on the sub-
sequent chlorine-free bleaching, primarily magnesium and
calcium ions, are retained in the pulp. The pH in the
washing stage lies suitably in the range of from 5 up to
about 11, preferably in the range of from 6 up to 10.
Washing before the bleaching with a peroxide
containing compound, is suitably carried out after the
treatment with a complexing agent and before the delig
nification with peracid or salts thereof. In this way,
the complexed trace metal ions are efficiently removed,
while at the same time the remaining peracid or peroxide-
containing compound, if any, can be used in the subsequent
bleaching stage. When washing has to be particularly
effective, it is possible to have a washing stage after
the treatment with a complexing agent as well as after the
delignification with peracid or salts thereof.
The washing liquid may be fresh water, optionally
with an addition of a pH-adjusting chemical, or wastewater
from one or more bleaching stages or extraction stages, in
such a way that a suitable pH in the washing stage is
obtained. Furthermore, the washing liquid may consist of
other types of optionally purified wastewater, provided it
has a low content of undesirable metal ions, such as
manganese, iron and copper.
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6a 2149649
Washing relates to methods for displacing, more or
less completely, the spent liquid in the pulp suspension
to reduce its content of, inter alia, dissolved trace
metal ions in said suspension. The washing methods may
entail an increase of the pulp concentration, e.g. by
sucking-off or pressing, but also a reduction of the pulp
concentration e.g. by dilution with washing liquid. Also,
washing relates to combinations and sequences, in which
the pulp concentration is alternately increased and
reduced, one or more times. In the present process, a
washing method is chosen, which removes not only
to
WO 94/12722 ~ 14 9 6 4 9 PCT/SE93/01018
7
dissolved organic substance, but also the trace metal ions
released in the treatment with a complexing agent, while tak-
ing into account what is suitable in terms of process tech-
nique and economy.
Washing efficiency may be given as the amount of liquid
phase displaced, as compared with the liquid phase present in
the pulp suspension before washing. The total washing efficie-
ncy is calculated as the sum of the efficiency in each washing
stage. Thus, after a treatment stage, dewatering of the pulp
suspension from, say, 10% to 25% pulp concentration gives a
washing efficiency of 66.7%. After a subsequent washing stage
in which the pulp is first diluted to 3 % and then dewatered to
25%, the total washing efficiency is 96.9% with regard to sol-
uble impurities. Washing efficiency should, in the present
process, be at least about 75%, suitably in the range of from
90% up to 100%, and preferably in the range of from 92% up to
100%. It is especially preferred that the washing efficiency
lies in the range of from 96% up to 100%.
By using the inventive process the conditions for the
chlorine-free bleaching, are optimised such that a high
brightness, kappa number reduction and viscosity are achieved
with a minimum consumption of the chlorine-free bleaching
agent. This is possible without the use of any auxiliary che
micals, such as stabilisers and protective agents, in the
chlorine-free bleaching. The remaining bleaching chemicals,
such as hydrogen peroxide and alkali, are advantageously used
directly in the bleaching stage, in the peracid stage or in
some other suitable stage, such that an optimum combination of
process technique and production economy is obtained.
It is also within the scope of the invention, that the
delignification with peracid or salts thereof can be reinforc
ed by an addition of such peroxide-containing compounds as
have been indicated above. Suitably, such reinforcement is
carried out with hydrogen peroxide or a mixture of hydrogen
peroxide and oxygen.
Lignocellulose-containing pulp relates to pulps contain-
ing fibres that have been separated by chemical or mechanical
treatment, or recycled fibres. The fibres may be of hardwood
or softwood. Chemical pulp relates to pulps digested according
PCTlSE93101018
WO 94/12722 214 9 ~ 4 ~
8
to the sulphate, sulphite, soda or organosolv process. Mecha-
nical pulp relates to a pulp produced by refining chips in a
disc refiner (refiner mechanical pulp) or by grinding logs in
a grinder (groundwood pulp). Lignocellulose-containing pulp
also relates to pulps produced by modifications or combina-
tions of the above-mentioned methods or processes. Such pulps
include thermomechanical, chemimechanical and chemi-thermo-
mechanical pulps. Suitably, the lignocellulose-containing pulp
consists of chemically digested pulp, preferably sulphate
l0 pulp. It is especially preferred that the lignocellulose-con-
taining pulp consists of sulphate pulp of softwood.
The inventive process can be applied to pulps with a
yield of up to about 90%, suitably in the range of from 30% up
to 80%, and preferably in the range of from 45% up to 65%.
The inventive process can be carried out in an optional
position in the bleaching sequence, e.g. immediately after the
production of the pulp. When the inventive process is applied
to chemically digested pulp, this is preferably delignified in
an oxygen stage before the treatment with a complexing agent.
The inventive process can be applied to chemically dige-
sted pulps having an initial kappa number in the range of from
about 2 up to about 100, suitably from 5 up to 60, and prefer-
ably from 10 up to 40. The kappa number is then measured
according to the SCAN-C 1:77 Standard Method.
In the inventive process, the treatment with a complex-
ing agent should be carried out for a period of time of from
about 1 min up to about 960 min, suitably from 15 min up to
240 min, and preferably from 35 min up to 120 min. The pulp
concentration in the treatment with a complexing agent may be
from about 1% by weight up to about 60% by weight, suitably
from 2.5% by weight up to 40% by weight, preferably from 3.5%
by weight up to 25 % by weight and most preferably from 5 . 5 % by
weight up to 25% by weight.
In the inventive process, delignification with peracid
should be carried out at a temperature of from 50°C up to
about 140°C, and suitably from 50°C up to about 120°C.
Prefe
rably the pulp is delignified at a temperature of from 50°C up
to about 100°C and more preferably from 50°C up to 90°C.
A
temperature in the range of from 50°C up to 80°C is especially
WO 94/12722 ~ 14 9 6 4 9 PCTISE93/01018
9
preferred. Delignification with peracid should be carried out
for a period of time from about 1 min up to about 960 min,
suitably from 10 min up to 270 min, and preferably from 30 min
up to 150 min. The pulp concentration in the delignification
with peracid may be from about 1% by weight up to about 70% by
weight, suitably from 3% by weight up to 50% by weight, prefe-
rably from 8% by weight up to 35 % by weight and most preferab-
ly from 10% by weight up to 30% by weight.
When the bleaching agent used is hydrogen peroxide, the
pulp should be treated at a temperature of from about 30°C up
to about 140°C, and suitably from about 30°C up to about
120°C. Preferably the pulp is treated at a temperature of from
about 30°C up to about 100°C and more preferably from
60°C up
to 90°C, and for a period of time of from about 5 min up to
about 960 min, suitably from 60 min up to 420 min, and prefer-
ably from 190 min up to 360 min. When the bleaching agent used
is hydrogen peroxide, the pulp concentration may be from about
1% by weight up to about 70% by weight, suitably from 3% by
weight up to 50% by weight, preferably from 8% by weight up to
35 % by weight and most preferably from 10% by weight up to 30 %
by weight . Treatment with the other bleaching agents indicated
above takes place within the normal ranges as to temperature,
time and pulp concentration for the respective agents, these
being well-known to those skilled in the art.
In preferred embodiments using hydrogen peroxide as
bleaching agent, the amount of hydrogen peroxide added in the
bleaching stage should be in the range of from about 1 kg up
to about 60 kg per tonne of dry pulp, calculated as 100% hyd-
rogen peroxide. The upper limit is not critical, but has been
set for reasons of economy. Suitably, the amount of hydrogen
peroxide lies in the range of from 6 kg up to 50 kg per tonne
of dry pulp, and preferably in the range of from 13 kg up to
kg per tonne of dry pulp, calculated as 100% hydrogen
peroxide.
35 In preferred embodiments using hydrogen peroxide as
bleaching agent, the share of peracid added in the delignify-
ing stage to the total amount of peracid and hydrogen peroxide
added in the delignifying and bleaching stages, should be less
than about 60% on a weight-to-weight basis. Here, the amount
PCTISE93101018
WO 94112722 2 1 4 9 ~ 4
of peracid has been recalculated as 100% hydrogen peroxide.
When recalculating peracetic acid as hydrogen peroxide, 1 kg
of peracetic acid is equivalent to 0.45 kg of hydrogen peroxi-
de.
5 In preferred embodiments using ozone as bleaching agent,
the amount of ozone may be in the range of from about 0.5 kg
up to about 30 kg per tonne of dry pulp, suitably in the range
of from 1 kg up to 15 kg per tonne of dry pulp, preferably
from 1.5 kg up to 10 kg per tonne of dry pulp and most prefer-
10 ably from 1.5 kg up to 5 kg per tonne of dry pulp.
After treatment with a complexing agent, delignification
with peracid and subsequent bleaching with a peroxide-contain-
ing compound, ozone or sodium dithionite, the pulp can be used
for direct production of paper. Alternatively, the pulp may be
finally bleached to a desired higher brightness in one or more
stages. Suitably, final bleaching is also carried out by means
of such chlorine-free bleaching agents as are indicated above,
optionally with intermediate extraction stages which can be
reinforced by peroxide and/or oxygen. In this way, the forma-
tion and discharge of AOX is completely eliminated. It is also
possible to use chlorine-containing bleaching agents, such as
chlorine dioxide, in the final bleaching and yet obtain a very
limited formation and discharge of AOX, since the lignin con-
tent of the pulp has been considerably reduced by the present
process.
The invention and its advantages will be illustrated in
more detail by the Examples below which however, are only in-
tended to illustrate the invention without limiting the same.
The percentages and parts stated in the description, claims
and Examples, refer to percent by weight and parts by weight,
respectively, unless otherwise stated. Furthermore, the pH
values given in the description, claims and Examples refer to
the pH at the end of each treatment, unless otherwise stated.
In the Examples below, the kappa number, viscosity and
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, respect
ively. The consumption of hydrogen peroxide and peracetic acid
were established by titration with sodium thiosulphate, and
potassium permanganate and sodium thiosulphate, respectively.
WO 94/12722 PCT/SE93/01018
11
Example 1
Oxygen-delignified sulphate pulp of softwood having a
kappa number of 16.0, a brightness of 37.1% ISO and a viscos-
ity of 1010 dm3/kg was treated with EDTA in accordance with
the invention, delignified with peracetic acid, and bleached
with hydrogen peroxide, in order to illustrate the importance
of the pretreatment for the pulp properties after the present
process. The pulp was treated with 2 kg EDTA per tonne of dry
pulp at a temperature of 90°C, a residence time of 60 min, a
pulp concentration of 10% by weight and varying pH. The amount
of peracetic acid added was 22.4 kg per tonne of dry pulp,
calculated as 100% peracetic acid. In the delignification with
peracetic acid, the pH was 5.5-5.9, the temperature 70°C, the
treatment time 60 min, and the pulp concentration 10% by
weight. Subsequently, the pulp was bleached with hydrogen
peroxide at a temperature of 90°C, a residence time of 240 min
and a pulp concentration of 10% by weight. The addition of
hydrogen peroxide was 25 kg per tonne of dry pulp, calculated
as 100% hydrogen peroxide, and the pH was 10.7-11.6. For com-
parison, the pulp was treated with 2 kg EDTA per tonne of dry
pulp at 25°C and for 30 min at a pH of 6 and 2 (tests 5 and
6). For further comparison, the pulp was treated at a pH of
about 2 in the absence of a complexing agent (test 7) and
without any pretreatment whatsoever (test 8). After each
stage, the pulp was washed with deionised water at a pH of
6Ø At this, the pulp was first dewatered to a pulp concen-
tration of 25% 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. Consequently, the total
washing efficiency was about 97%. The results after bleaching
with hydrogen peroxide appear from the Table below.
TABLE I
Pulp properties after the H20z stage
Test pH Kappa Viscosity Brightness
number
(dm'/kg) (% ISO)
1 1.9 6.1 790 69.1
2 6.4 4.7 890 81.3
3 9.2 5.0 875 77.4
PCTISE93I01018
WO 94/12722
12
TABLE I (cont.)
Pulp properties after the H202 stage
Test pH Kappa Viscosity Brightness
number
(dm3/ka) (% ISO)
4 12.1 6.5 800 68.3
5 6.0 5.0 850 77-0
6 2.0 6.2 785 68.5
7 2.0 6.4 752 67.5
8 --- 6.5 800 65.0
It is evident from the Table that the treatment of soft-
wood pulp with a complexing agent in a separate stage and at
an elevated temperature and at a pH in the range of from 3.5
up to about 11, results in a substantial reduction of the
kappa number and a considerable increase in brightness, as
well as a high viscosity.
Example 2
The oxygen-delignified sulphate pulp of softwood used in
Example 1 was treated with EDTA, delignified with peracetic
acid and bleached with hydrogen peroxide, in order to illus
trate the effect of washing between the separate stages in the
sequence. The conditions in the treatment with a complexing
agent were as in Example 1, except that the pH was 5.7 (test
1). The conditions in the delignification with peracetic acid
and the bleaching with hydrogen peroxide were as in Example 1
throughout. In test 1, the pulp was washed in accordance with
Example 1, both after the treatment with EDTA and after the
delignification with peracetic acid. For comparison, the pulp
was treated with 2 kg EDTA per tonne of dry pulp at a pH of 7,
a temperature of 25°C and a pulp concentration of 8 % by weight
and for 10 min (test 2) . In test 2, the pulp was dewatered
after the treatment with a complexing agent to a pulp concen-
tration of 25% by weight. There was no washing or dewatering
after the delignification with peracetic acid in test 2. Wash-
ing efficiency was about 97% in test 1 and about 74% in test
2. The results after bleaching with hydrogen peroxide (H202)
appear from the Table below.
~149fi49
WO 94/12722 PCTISE93101018
13
TABLE II
Pulp properties after the H20z stage Remaining
Test Kappa Viscosity Brightness H202
number
(dm'/kg) (% ISO) (kg/tonne)
1 4.7 885 81.3 9.2
2 6.0 760 70.7 0
It is evident from the Table that the inventive treat
ment of softwood pulp with a complexing agent at a high tem
perature followed by washing, gives a much greater reduction
of the kappa number and a much higher increase in brightness
with a low consumption of hydrogen peroxide and an essentially
maintained strength of the pulp, than does treatment at room
temperature followed by dewatering.
Example 3
The oxygen-delignified sulphate pulp of softwood used in
Example 1 was treated with EDTA, delignified with peracetic
acid and bleached with hydrogen peroxide, in order to illus-
trate the effect of peracetic acid and separate stages in the
sequence according to the invention. The conditions in the
treatment with a complexing agent were as in Example 1, except
that the pH was 5.7. The conditions in the delignification
with peracetic acid were as in Example 1, except that 11.2 kg
of peracetic acid was added per tonne of dry pulp. The condi-
tions in the bleaching with hydrogen peroxide were as in Exam-
ple 1. In test 1, the pulp Was treated with EDTA, delignified
with peracetic acid and bleached with hydrogen peroxide. In
test 2, the pulp was delignified with peracetic acid in the
presence of EDTA, whereupon the pulp was bleached with hydro-
gen peroxide. In the delignification in the presence of EDTA
in test 2, the pH was 5.1, the temperature 90°C and the treat-
ment time 1 h. In test 3, the pulp was treated with EDTA,
whereupon it was delignified and bleached with peracetic acid
in the presence of hydrogen peroxide. In the delignification
3 5 and the bleaching in test 3 , the temperature was 70 ° C for 1 h,
whereupon it was raised to 90°C and kept there for 4 h, the pH
being 11.1. For comparison, the pulp was treated with EDTA and
bleached with hydrogen peroxide ( test 4 ) . After each stage ,
the pulp was washed in accordance with Example 1. The results
PCTISE93101018
W O 94112722
1~
after the bleaching with hydrogen peroxide (HzOz) appear from
the Table below.
TABLE III
Pulp properties after the H202 stage Remaining
Test Kappa Viscosity Brightness H202
number
(dm3/ka) (% ISO) (ka/tonne)
1 5.0 910 79.7 10.4
2 8,7 800 74.1 0
3 6.1 840 76.2 9.0
4 7.5 890 74.0 6.6
It is evident from the Table that softwood pulp treated
in accordance with the present invention in separate stages
will show a substantial reduction of the kappa number and a
considerable increase in brightness with a low consumption of
hydrogen peroxide and an essentially maintained pulp strength.
Example 4
The oxygen-delignified sulphate pulp of softwood used in
Example 1 was treated with EDTA, delignified with peracetic
acid and bleached with hydrogen peroxide, in order to illus
trate the effect of pH in washing on the brightness of the
pulp after the bleaching stage. The conditions in the treat-
ment with a complexing agent were as in Example 1, except that
the pH was 5.7. The conditions in the delignification with
peracetic acid were as in Example 1, except that the pH was
6.1. The conditions in the bleaching with hydrogen peroxide
were as in Example 1. After each stage, the pulp was washed in
accordance with Example 1, except that the pH was varied in
the washing after the treatment with a complexing agent. The
results after the bleaching with hydrogen peroxide (H202)
appear from the Table below.
TABLE IV
Test pH Brightness after the H,O, stage (% ISO)
1 2.4 65.9
2 3.5 72.0
3 4.1 79.0
4 6.6 82.6
5 g.g 81.4
6 10.7 80.9
~~~9s49
WO 94/12722 PCT/SE93~A1018
It is evident from the Table that softwood pulp treated
in accordance with the present invention shows a substantial
increase in brightness.
Example 5
5 The oxygen-delignified sulphate pulp of softwood used in
Example 1 was treated with EDTA, delignified with peracetic
acid and bleached with hydrogen peroxide, in order to illus-
trate the effect of the weight ratio between peracetic acid
and hydrogen peroxide in the equilibrium peracetic acid and
10 the washing efficiency between delignification and bleaching.
The conditions in the treatment with a complexing agent were
as in Example 1, except that the pH was 5.7. The conditions in
the delignification with peracetic acid were as in Example 1,
except that the pH was 5.2 to 6.3. The conditions in the blea-
15 ching with hydrogen peroxide were as in Example 1, except that
the amount of added hydrogen peroxide was 30 kg per tonne of
dry pulp, calculated as 100% hydrogen peroxide. In tests 1 to
4, the pulp was washed as in Example 1, i.e. with a washing
efficiency of about 97%. The washing efficiency was about 67%
in test 5. There was no washing or dewatering after the delig-
nification with peracetic acid in test 6. The results after
the bleaching with hydrogen peroxide (H202) appear from the
Table below.
TABLE V
Test PAA:H202 Washing efficiency Brightness
after PAA after PAA after H~Oz
(%) (% ISO) (% ISO)
1 11.5:1 97 54.1 78.0
2 5.0:1 97 57.1 81.2
3 2.1:1 97 60.0 82.7
4 0.6:1 97 63.1 84.0
5 2.1:1 67 60.0 79.5
6 2.1:1 0 60.0 78.0
It is evident from the Table that treatment of softwood
pulp in accordance with the invention, results in a consider-
able increase in brightness after delignification with perace-
tic acid as well as after bleaching with hydrogen peroxide.
Example 6
Oxygen-delignified sulphate pulp of softwood having a
WO 94112722 PCT/SE93/01018
214~~ 49 16
kappa number of 16.5, a brightness of 36.0% ISO and a viscos-
ity of 1010 dm3/kg was treated with EDTA in accordance with
the invention, delignified with peracetic acid, and bleached
with hydrogen peroxide, in order to illustrate the importance
of the pretreatment pH for the pulp properties after the pre-
sent process. The conditions in the treatment with a complex-
ing agent were as in Example 1, except that the pH was 4 . 0
(test 1). The peracetic acid was an equilibrium peracetic
acid, with a weight ratio between peracetic acid and hydrogen
peroxide of 4:1. The amount of peracetic acid added was 5 kg
per tonne of dry pulp, calculated as 100% peracetic acid. The
conditions in the delignification with peracetic acid were as
in Example 1, except that the pH was 5.8-6.1. The conditions
in the bleaching with hydrogen peroxide were as in Example 1,
except that the addition of hydrogen peroxide was 35 kg per
tonne of dry pulp, calculated as 100% hydrogen peroxide, and
the pH was 11.2-12Ø For comparison, the pulp was treated
with 2 kg EDTA per tonne of dry pulp under the conditions
stated above, except that the pH was 3.0, i.e. outside the pH
range of the present invention (test 2 ) . After each stage, the
pulp was washed in accordance with Example 1. The results
after bleach-ing with hydrogen peroxide appear from the Table
below.
TABLE VI
Pulp properties after the HZOZ stage
Test pH Kappa Viscosity Brightness
number
(dm3/kcr) ( % ISO)
1 4.0 6.2 860 79.1
2 3.0 7.3 850 64.6
It is evident from the Table that the treatment of soft-
wood pulp with a complexing agent at a pH of at least 3.5,
results in a pulp with superior properties compared to treat-
ment with a complexing agent at a pH which is more acidic.
Example 7
The oxygen-delignified sulphate pulp of softwood used in
Example 6 was treated with EDTA, delignified with peracetic
acid, and bleached with hydrogen peroxide (tests 1-2), in
order to illustrate the effect of residence time and tempera-
WO 94/12722 PCT/SE93101018
~~49649
17
ture in the delignification step. The conditions in the treat-
ment with a complexing agent were as in Example 1, except that
the pH was 5.7. The amount of peracetic acid added was 10 kg
per tonne of dry pulp, calculated as 100% peracetic acid. In
the delignification with peracetic acid, the pH was 6.0-6.5,
the temperature 110°C and the pulp concentration 10% by
weight, whereas the residence time was varied. The conditions
in the bleaching with hydrogen peroxide were as in Example 1,
except that the pH was 11.0-11.1. For comparison, the pulp was
delignified at 40°C, i.e. outside the temperature range of the
present invention (tests 3-4). After each stage, the pulp was
washed in accordance with Example 1. The results after delig-
nification with peracetic acid appear from the Table below.
TABLE VII
Pulp properties after delignification
Test Tempera- Residence Kappa Viscosity Brightness
ture time number
(°C) (min) (dm3/kQ) (o ISO)
1 110 30 13.2 1005 52.0
2 110 75 12.9 1000 52.1
3 40 60 12.5 1005 49.7
4 40 120 12.0 1025 49.6
It is evident from the Table that the treatment of soft
wood pulp with peracetic acid at a temperature in the range of
from 50°C up to about 140°C, results in a pulp with superior
properties compared to treatment at a lower temperature.
Example 8
Oxygen-delignified sulphate pulp of softwood having a
kappa number of 10.3, a brightness of 41.7% ISO and a viscos
ity of 1000 dm3/kg was treated with EDTA, delignified with two
types of peracetic acid, and bleached with hydrogen peroxide,
in order to illustrate the effect of peracetic acid of various
compositions. The conditions in the treatment with a complex-
ing agent were as in Example 1, except that the pH was 5.5 and
the charge of EDTA was 1.5 kg per tonne of dry pulp. The per-
acetic acids used were one equilibrium peracetic acid with a
weight ratio between peracetic acid and hydrogen peroxide of
4:1 (Equil), and one distilled peracetic acid essentially free
of hydrogen peroxide and acetic acid (Dist). With both types
WO 94!12722 214 9 b 4 9 ~T~~93101018
1a
of peracetic acid, the added amount was 10 kg per tonne of dry
pulp, calculated as 100% peracetic acid. The conditions in the
delignification with peracetic acid were as in Example 1,
except that the pH was 6-7. The conditions in the bleaching
with hydrogen peroxide were as in Example 1, except that the
addition of hydrogen peroxide was 35 kg per tonne of dry pulp
calculated as 100% hydrogen peroxide, the pH was 11.5 and the
temperature 110°C in tests 2 and 4. After each stage, the pulp
was washed in accordance with Example 1. The results after
bleaching with hydrogen peroxide appear from the Table below.
TABLE VIII
Pulp prop . after the H202 stage Remaining
Test Perac. Temp. Kappa Viscosity Brightness H20z
acid number
°C (dm3/kcr) (% ISO) (ka/tonne)
1 Dist 90 3.2 810 84.4 14.0
2 Dist 110 2.8 760 86.9 8.4
3 Equil 90 3.4 750 84.0 12.1
4 Equil 110 3.0 700 86.5 7.2
It is evident from the Table that the treatment of soft-
wood pulp according to the invention, results in a pulp with
excellent properties after bleaching in combination with a
moderate consumption of bleaching agents in the sequence.
Examble 9
The oxygen-delignified sulphate pulp of softwood used in
Example 8 was treated with EDTA, delignified with peracetic
acid, and bleached with hydrogen peroxide, in order to illus-
trate the effect of the total amount of bleaching agents and
the distribution of the same between the delignifying (Stage
2) and bleaching stages (Stage 3). The conditions in the
treatment with a complexing agent were as in Example 1, except
that the pH was 5.5 and the charge of EDTA was 1.5 kg per
tonne of dry pulp. The peracetic acid used was a distilled
peracetic acid essentially free of hydrogen peroxide and
acetic acid. The amount of peracetic acid added was varied
between 11 and 80 kg per tonne of dry pulp, calculated as 100%
peracetic acid. The conditions in the delignification with
peracetic acid were as in Example 1, except that the pH was 6-
7. The amount of hydrogen peroxide added was varied between 2
214969
WO 94/12722 PCT/SE93/01018
.... 19
and 30 kg per tonne of dry pulp calculated as 100% hydrogen
peroxide. The conditions in the bleaching with hydrogen per-
oxide were as in Example 1, except that the pH was 11.5. The
total amounts of hydrogen peroxide and calcu-
peracetic acid
lated as 100% hydrogen peroxide were 20 kg per tonne of dry
pulp in tests 1-4 and 40 kg per tonne dry pulp in
of tests 5-
8. The
share
of
peracetic
acid
(calculated
as
100%
hydrogen
perox ide ) in stage 2 , is also given a percentage of the
as
total amount. After each stage, the pulp was washed accor-
in
dance with Example 1. The results after hydro-
bleaching with
gen .
peroxide
appear
from
the
Table
below
TABLE IX
Hydrogen peroxide Prop. after HzOz stage
Remaining
Test Stage 2 Stage 3 Stage 2 Viscosity Brightness H202
(kQ/tonne) (%) (dm3/kcr) (% ISO) (ky/ tonne)
1 5 15 25 930 79.9 7.1
2 10 10 50 940 79.5 5.0
3 15 5 75 935 77.2 3.3
4 18 2 90 950 73.0 1.5
5 10 30 25 870 85.3 12.1
6 20 20 50 880 84.9 9.3
7 30 10 75 890 82.5 5.8
8 36 4 90 895 78.0 3.0
It is evident from the Table that cellent pulp
ex proper-
ties and bleachingaccor-
are
obtainable
after
delignification
ding
to
the
present
invention.
