Note: Descriptions are shown in the official language in which they were submitted.
~ 1 0 9 1 ~
.
Process for bleachinq of liqnocellulose-containinq pulp
The present invention relates to a process for bleaching
of lignocellulose-containing pulp, in which the pulp is treat-
ed at a pH between about 1 and about 7 in the presence of a
magnesium compound, whereupon the pulp is washed and subse-
quently bleached with a peroxide-containing compound. The ini~
tial treatment removes from the pulp ~hose trace metal ions
that have a negative effect on the subsequent bleaching with i~
a peroxide-containing compound. Owing to the presence o mag-
nesium ions in dissolved form during the initial treatment,
the magnesium ions are retained in those positions in the pulp
where they have a particularly positive effect on the effec-
tiveness of the bleaching stage.
Backqround of the Invention 1
In the making of lignocellulose-containing pulp of high
brightness, the pulp is bleached in one or more stages. For
quite some time, mechanical pulps have been bleached with
chlorine-free bleaching agents, the intention in this case
being to remove chromophoric groups while preserving the con-
tent of lignin. For environmental reasons, it has become in-
creasingly common to treat also chemical pulps with chlorine-
free bleaching agents, such as hydrogen peroxide, peracetic
acid and ozone, already in the first bleaching stages. Unless
the pulp is pretreated, however, bleaching with chlorine-free l~
bleaching agents is less effective. Thus, hydrogen peroxide
bleaching in alkaline environment is disturbed by the presence ~ `~
in the pulp of ions of certain trace metals, primarily Mn, Cu
and Fe. These metal ions cause degradation of hydrogen per- ;
oxide into undesirable products, thereby reducing the effecti-
veness of the peroxide bleaching and increasing the consump-
tion of peroxide.
The prior art teaches bleaching of chemical as well as
mechanical pulps in the presence of magnesium salts. Most of
the known bleaching processes do not comprise acid pretreat-
35 ment, which preserves the content of undesirab].e trace metals ~ `
in the pulp. E'urthermore, bleaching with chlorine-free bleach-
ing agents is normally performed in alkaline environment. As
a result, it becomes impossible to retain or reintroduce the
especially desirable magnesium ions, since these are precipi- ;;~
~ ~,
2 ~ 1 0 9 1 7 9 ; ~;;
tated at an alkaline pH and thus cannot diffuse into the pulp
to such an extent that a pulp of high brightness and strength -
can be obtained.
,:: ,:
Description of the Invention
The inventio~ provides a process for treating ligno~
cellulose-containing pulp unde.r the conditions disclosed in
the claims, whereby the content oE trace metal iOllS ill the
pulp i9 selectively altered to render more effective the
subsequent bleaching with a peroxide-containing compound.
I'hus, the invention relates to a process for bleaching
of lignocellulose-containing pulp, whereby the pulp is treated
at a pH within the range of from about 1 up to about 7 in the
presence of a magnesium compound, whereupon the pulp is washed
and subsequently the pulp is bleached with a peroxide-contain-
ing compound.
An initial acid treatment effectively removes metal ions
from lignocellulose-containing pulps. However, magnesium ions,
especially when in their original positions of the pulp, are
known to have a positive influence on the selectivity of the
bleaching and the consumption of peroxide-contalning com-
pounds. By the present process, undesirable metal ions are
removed from the pulp suspension, while the magnesium ions are
largely retained in their original positions. The latter is
achieved by the presence of a magnesium compound in the treat-
ment liquid, at such a pH and temperature that the compound isin dissolved form when contacted with the pulp. The present
process further has the advantage of reducing the number of
treatment stages compared with prior-art processes.
In the initial treatment, the magnesium compound should
be in dissolved form when contacted with the pulp to produce
a good effect. This can be achieved in various ways, depend-
ing, inter alia, on the type and properties of the pulp. The
magnesium compound can be brought to dissolved form by a
suitable choice of pH within the range of from about 1 up to
about 7, in combination with a suitable temperature and
concentration of the magnesium compound.
Besides a magnesium compound, the initial treatment may
be carried out in the presence of at least one more compound
containing an alkaline earth metal. Thus, a compound contain-
2 1 0 9 ~ 7 9
r . 3
- , ~: . ... :
ing calcium or barium may also be present. Preferably, the
pulp is treated in the presence of a magnesium compound and
calcium compound in optional sequence or mixture.
The magnesium-containing compound used is suitably
magnesium sulphate, magnesium chloride, magnesium carbonate or
magnesium nitrate, preferably magnesium sulphate. The calcium-
containing compound used is suitably calcium chloride, calcium
nitrate, calcium sulphate or calcium carbonate, preferably
calcium chloride.
In the process according to the invention, the initial
treatment is performed at a pH of from about 1 up to about 7,
suitably from 1.5 up to 6, and preferably from 2 up to 5. The
initial treatment may also be performed at a pH of from 3.1 up
to 4.
In the initial treatment in the presence of a magnesium
compound, the pH can be adjusted by adding an acid or an acid
liquid to the pulp. The acid used may be an inorganic mineral
acid 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.
The amount of magnesium compound added in the initial
treatment may be within the range of from about 100 ppm up to
about 4000 ppm, calculated as part by weight of magnesium on
part by weight of dry pulp. Suitably, the amount of the magne-
sium compound added lies within the range of from 200 ppm up
to 3000 ppm, preferably within the range of from 500 ppm up to
2000 ppm.
The amount of an optional compound containing an alkali-
ne earth metal added in the initial treatment may be wi.thinthe range of from about 100 ppm up to about 4000 ppm, calcu-
lated as part by weight of alkaline earth metal on part by
weight of dry pulp. Suitably, the amount of the compound con-
taining an alkaline earth metal added lies within the range of
from 200 ppm up to 3000 ppm, preferably within the range of
from 500 ppm up to 2000 ppm.
The amount of magnesium compound added as well as the
other conditions in the initial treatment are so chosen thak
the magnesium content of the pulp prior to the bleaching with
..: ~ : ..
r ~ C ~ ~ ~
2 1 0 9 1 7 9
a peroxide-containing compound amounts to at least about 50%
of the initial content. Suitably, the amount of magnesium
compound added as well as the other conditions are so chosen
that the magnesium content of the pulp prior to the bleaching
with a peroxide-containing compound lies within the range of
from 100% up to 300% of the initial content, preferably within
the range of from 130i~ up to 200i~
The peroxide-containing eompound ineludes inorganic
peroxide eompounds sueh as hydroyen peroxide, sodium peroxide
or peroxosulphurie aeid (Caro's aeid), or organie peroxide
eompounds, sueh as peraeetie aeid. Preferably, the peroxide~
eontaining eompound is hydrogen peroxide or a mixture oE
hydrogen peroxide and oxygen.
Bleaehing with a peroxide-containing compound is suitab-
ly performed at a pH equal to or higher than the pH in theinitial treatment in the presenee of a magnesium eompound. In
this way, the trace metal ions having a positive effect on the
bleaching are retained also in the bleaching stage itself.
When the peroxide-containing compound is hydrogen peroxide,
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, preferably at a pH of
from 9.5 up to 11. Bleaehing with the other peroxide-eontain-
ing compounds indicated above takes place within the normal pH
ranges for each bleaching agentj which are well-known to the
expert.
Suitably, bleaching with a peroxide-containing compound
takes plaee in the presenee of yet another compound containing
an alkaline earth metal. The compound containing an alkaline
earth metal present in the bleaching, may be the same compound
that was used in the initial treatment or another compound.
The alkaline earth metal suitably is calcium, magnesium or
barium, or a mixture thereof. This further improves the selec-
tivity of the bleaching and reduces the consumption of the
peroxide-containing compound. It is especially preferred that
the bleaching takes plaee in the presence of a calcium com-
pound. The caleium eompound ean be added to the pulp suspen-
sion outside the bleaching tower, e.g. by introduction into
the pipeline leading to the bleaehing tower. Also, the cal-
eium eompound can be added to the pulp suspension inside the
': :::...' ~ '"
2 1 ~ 9 1 7 9
bleaching tower, before the bleaching has commenced.
The amount of the compound containing an alkaline earth
metal added in the bleaching, may lie withln the range of from
about 100 ppm up to about 4000 ppm, calculated as part by
weight of alkaline earth metal on part by weight of dry pulp.
Suitably, the a~ount of the compound containing an alkaline
earth metal added lies within the range of from 300 ppm up to
3000 ppm, preferably within the range of from 600 ppm up to
2000 ppm.
When a calcium compound is present in the bleaching with
a peroxide-containing compound, this compound suitably is cal-
cium chloride, calcium nitrate, calcium sulphate or calcium
carbonate. Calcium chloride is preferably used. Furthermore,
the calcium compound is suitably added at such a combination
of pH, temperature and concentration of the calcium compound
that said compound is in active form when contacted with the
pulp. Thus, suitably the pH lies within the range of from
about 7 up to about 11. However, a positive effect is obtained
also when the calcium compound is precipitated in the pulp
suspension, e.g. as calcium carbonate.
The amount of calcium added as well as the other condi-
tions in connection with the addition of calcium are so chosen
that the calcium content of the pulp prior to the bleaching
with a peroxide-containing compound amounts to at least about
25~ of the calcium content prior to the initial treatment. At
this, the pulp may be treated with calcium in the initial
treatmént as well as in the bleaching with a peroxide-contain-
ing compound. The amount of calcium compound added as well as
the other conditions are suitably so chosen that the calcium
content of the pulp prior to the bleaching with a peroxide~
containing compound lies within the range of from 40% up to
150% of the original content, preferably within the range of
from 50% up to 120% and most preferably within the range of
from 50% up to 70%.
It is also within the scope of the invention, that all
or a part of the amount of magnesium, and optionally any of
the other alkaline earth metals, may be added to the pulp sus-
pension by way of the water used for e.g. pH adjustment, wash-
ing or dilution. Thus, hard water primarily containing magne-
::
~a~l7s . ~
~`` 6
sium or calcium ions, or a combination thereof, is advanta-
geously used in the initial treatment, for dilution in the
washing stage, or for achieving a suitable pulp concentration
in the bleaching stage. In this way, it is possible to reduce
the added amount of the corresponding alkaline earth metal,
provlded that the combination oE pH, temperature and conce.n-
tration of the alkaline earth metal can be so chosen that said
allcaline earth metal is in dissolved form when contacted with
the pulp. Such hard waters include fresh water from limestone
bedrock, white water from papermaking machines using Ch.ina
clay or chalk as filler, or process water from production of
sulphite pulp based on magnesium or calcium.
The process according to the invention is performed with
a washing stage after the initial treatment and before the
bleaching with a peroxide-containing compound. Washing effec-
tively removes those trace metal ions that have a negative
effect on the subsequent bleaching with a peroxide-contalning
compound, e.g. ions of manganese, copper and iron. In order to
retain the trace metal ions that have a positive effect on the
subsequent bleaching with a peroxide-containing compound, pri-
marily magnesium and calcium ions, the pH in the washing
liquid is preferably equal to or higher than the pH in the
initial treatment. However, the pH in the washing liquid
should lie within the range of from about 3 up to about 10,
suitably within the range of from 5 up to 10. It is especially
preferred that the pH in the washing liquid is at least about
2 pH units higher than the pH in the initial treatment.
The washing liquid may be fresh water, optionally con-
taining some added pH-adjusting chemical, or wastewater from
one or more bleaching stages or extraction stages, giving a
suitable pH in the washing stage. Furthermore, the washing
liquid may consist of other types of wastewater, optionally
purified, provided that it has a low content of undesirable
ions of metal, such as manganese, iron and copper.
Washing between the initial treatment and bleaching with
a peroxide-containing compound relates to methods -for remov-
ing, more or less completely, the liquid phase from the pulp
suspension in order to reduce the content of dissolved trace
metal ions in said suspension. Washing may involve raising the
7 2~0917~ : :
pulp concentration, e.g. by sucking-off or pressing the sus-
pension through a filter, or lowering the pulp concentration,
e.g. by dilution with a washing liquid. Washing also relates
to combinations and sequences in which the pulp concentration
5 i9 alternatingly raised and lowered, one or more times. In the
present process, a washing method should be chosen which
effectively removes the trace metal ions released in the ini-
tial treatment, aspects of process technique and economy being
considered.
The efectiveness of the washing may be indicated as the
amount o liquid phase removed, compared with the liquid phase
present in the pulp suspension prior to washing. Washing
effectiveness can be at least about 80%, and suitably lies
within the range of from 90% up to 100%, preferably within the
range of from 95% up to 100%.
The initial treatment in the presence of a magnesium
compound can also be performed in the presence of a bleaching
agent. Bleaching agents that are active within the pH range
suitable for the initial treatment include chlorine dioxide,
ozone and acid peroxide-containing compounds. Acid peroxide-
containing compounds include such organic compounds as per-
acetic acid and such inorganic compounds as hydrogen peroxide
and peroxosulphuric acid (Caro's acid). Suitably, the initial
treatment takes place in the presence of ozone or peroxosul-
phuric acid, preferably ozone, since these bleaching agentsaffect but to a minor extent the content of ions of alkaline
earth metals in the pulp.
The process according to the invention may also involve
an extraction stage before the bleaching with a peroxide-con~
taining compound. This is especially convenient when the ini-
tial treatment is performed in the presence of a bleaching
agent. Suitably, the extraction is performed at a pH that is
equal to or higher than the pH in the initial treatment. Sui-
tably, the pH in the extraction stage lies within the range of
from about 7 up to about 11, preferably within the range of
from 8 up to 10.
Regardless of the number of stages or the type of treat-
ment following the initial treatment, the pH in a subsequent
stage suitably is equal to or higher than the pH in the prece-
~' 8 2 1 0 9 ~ 7 9
.
ding stage, in order to preserve the content of ions of alka-
line earth metals in the pulp. In a sequence in which the pulp
is first treated at a pH of about 2 in the presence of a mag-
nesium compound and then washed and bleached with hydrogen
peroxide, the pH in the washing stage should be at least about
g, and the pH in the bleachi.ng stage should be at least about
8.
The term lignocellulose-containing pulp relates to pulps
containing fibres that have been separated by chemical or
mechanical treatment, or recycled fibres. The fibres may be of
hardwood or softwood. The term chemical pulp relates to pulps
digested according to the sulphate, sulphite, soda or organo-
solv process. The term mechanical pulp relates to pulps pro-
duced by refining of chips in a disc refiner (refiner mechani-
cal pulp) or by grinding of logs in a grinder (groundwoodpulp). The term lignocellulose-containing pulp also relates to
pulps produced by modifications or combinations of the above
methods or processes. Such pulps include thermomechanical,
chemimechanical and chemi-thermomechanical pulp9. Suitably,
the lignocellulose-containing pulp consists of chemically
digested pulp, preferably sulphate pulp of softwood.
The process according to the invention, can be applied
to pulps with a yield of up to about 90%, suitably a yield
within the range of from 40% up to 80%, preferably within the
range of from 45% up to 65%.
The process according the invention, can be performed at
an optional position in the bleaching sequence, e.g. immedia-
tely after the making of the pulp. When the process according
to the invention is applied to a chemically digested pulp,
this is preferably delignified in an oxygen stage before
applying the process according to the invention.
The process according to the invention can be applied to
chemically digested pulps having an initial kappa number with-
in the range of from about 3 up to about 100, suitably from 4
up to 60, preferably from 5 up to 40. The kappa number is
established according to the Standard Method SCAN-C 1:77.
In the process according to the invention, the initial
treatment can be performed at a temperature of from about 10C
up to about 100C, suitably from 25C up to 90C, preferably
--~ 9 ~1~9179
'! ' `
from 40~C up to 80C. The initial treatment can be performed
for a period of time of from about 1 s up to about 600 min,
sultably from 1 min up to 120 min, preferably from 10 min up
to 60 min. The pulp concentration in the initial treatment can
be from about 1~ by weight up to about 60% by weight, suitably
from 2% by weight up to 40~ by weight, preferably from 3% by
weight up to 35% by weight.
When hydrogen peroxide .is used as peroxide-containing
compound, the pulp i9 treated at a temperature of from about
30C up to about 100C, preEerably from 60C up to 90C, and
for a period oE time of from about 30 min up to about 960 min,
suitably from 60 min up to 360 min. Further, when hydrogen
peroxide is used as peroxide-containing compound, the pulp
concentration can be from about 1% by weight up to about 70%
by weight, suitably from 3% by weight up to 60% by weight,
preferably from 10~ by weight up to 50% by weight. Treatment
with the other peroxide-containing compounds indicated above
takes place within the normal ranges as to temperature, period
of time and pulp concentration for each bleaching agent, which
are well-known to the expert.
In preferred embodiments involving the use of hydrogen
peroxide as peroxide-containing compound, the amount of hydro~
gen peroxide lies within the range of from about 1 kg up to
about 60 kg per tonne of dry pulp, calculated as 100% hydrogen
peroxide. The upper limit is not critical, but has been set
for economic reasons. Suitably, the amount of hydrogen peroxi-
de lies within the range of from 2 kg up to 50 kg per tonne of
dry pulp, preferably from 5 kg up to 40 kg per tonne of dry
pulp, calculated as 100% hydrogen peroxide.
After the initial treatment in the presence of magnesium
and subsequent bleaching with a peroxide-containing compound,
the pulp can be used as such for making paper of lower bright-
ness. Alternatively, the pulp can be finally bleached to the
desired higher brightness by treatment in one or more stages.
Suitably, the ~inal bleaching involves chlorine-free bleaching
agents such as the peroxide-containing compounds indicated
above, ozone, oxygen or sodium dithionite, optionally with
intermediate alkaline extraction stages which may be reinforc-
ed with peroxide and/or oxygen. In this way, the formation and
2~1q~ '
1 0 v
discharge of AOX is completely eliminated.
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, reEer 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 following Examples, the kappa number, viscosity
and brightness of the pulp were established according to SCAN
Standard Methods, and the consumption of hydrogen peroxide was
determined by iodometric titration.
Example 1
Oxygen-delignified sulphate pulp of softwood having a
kappa number of 18, a brightness of 34.2% ISO and a viscosity
of 1000 dm3/kg was treated at a pH of 2.1 ~/-0.1 in the pre-
sence of various alkaline earth metals. After oxygen deligni-
fication, the sulphate pulp had a pH of about 11, which was
adjusted by an addition of 15 kg sulphuric acid/tonne dry
pulp. The amount of alkaline earth metal added was 1000 ppm,
calculated as part by weight of each alkaline earth metal on
part by weight of dry pulp. In the initial treatment, the
temperature was 50C, the treatment time was 30 min and the
pulp concentration was 10% by weight. In the initial treat-
ment, the general conditions as well as the compound contain-
ing an alkaline earth metal were so chosen that the alkaline
earth metal present was dissolved in the pulp suspension.
After the initial treatment, the pulp was washed at a pH of 6-
7, in order to remove those metal ions that have a negativeeffect on the subsequent chlorine-free bleaching. Subsequent-
ly, the pulp was bleached with hydrogen peroxide at a tempera-
ture of 90~C, a residence time of 240 min, and a pulp concen-
tration of 10% by weight. The addition of hydrogen peroxide
was 25 kg/ tonne dry pulp, calculated as 100~ hydrogen
peroxide, and the pH was about 10.5. For comparative purposes,
the pulp was treated without addition of an alkaline earth
metal, and subsequently bleached with hydrogen peroxide under
the conditions stated above (Test 4). For further comparison,
2~09~7~ ~
:.. . 11
the pulp was treated with magnesium as in Test 1, but without
an intermediate washing stage (Test 5). The results of the ;
tests appear from Table I below.
TABLE I
5Alkaline Pulp properties after peroxide bleaching
Test earth Kappa number Viscosity Brightness
metal
_ ~dm3/kq)(~ ISO)
1 Mg 9.2 830 63.4 ` ;
10 2 Ca 10.5 800 53.0
3 Ba 11.4 880 51.0
4 --- * 12.8 840 45,5
Mg ** 12.2 880 41.0 ~ ~:
* No alkaline earth metal added. ;;
** No washing stage after the magnesium treatment.
As is evident from the Table, pulp treatment according
to the invention at a pH of about 2.1 in the presence of a ~ `p
magnesium compound, results in a much higher increase in ' ~
brightness and a more substantial reduction of the kappa num- ;
20 ber than does a treatment in the absence of such a compound. `~
Example 2 ~ ;~
The oxygen-delignified sulphate pulp of softwood employ-
ed in Example 1 was treated in the presence of a magnesium
compound at a pH within the range of from 1.5 up to 11 in
order to illustrate the effect of pH in the initial treatment.
The amount of magnesium added was 1000 ppm, calculated as part
by weight of magnesium on part by weight of dry pulp. The con-
ditions in the initial treatment were identical with those in
Example 1, except the pH variation. Likewise, the conditions
in the subsequent hydrogen peroxide bleaching were identical
with those in Example 1. After the initial treatment, the pulp
was washed at a pH that was about 2 pH units higher than that
in the initial treatment. The results of the tests appear from
Table II below.
- - 12 ~0~79 ~.
TABLE II
pH in the Pulp properties after peroxide bleaching
Test initial Kappa number Viscosity Brightness
treatment `~
(dm3/kq)(% ISO)
1 1.5 9.1 795 64.0
2 2.0 9.2 835 63.2
3 3.3 9.6 820 61.0
4 ~.4 9.7 815 60.2
6.1 9.9 825 59.3 ~ "
6 6.8 10.2 8~0 57.5
7 10.8 12.1 880 43.6
As is evident from the Table, pulp treatment according
to the invention at a pH within the range of from about 1 up
15 to about 7, results in a much higher increase in brightness `~
and a more substantial reduction of the kappa number than does ~ ~ `
treatment at a pH above this range.
Exam~le 3
The oxygen-delignified sulphate pulp of softwood employ-
ed in Example 1 was treated in the presence of a magnesiumcompound at a pH of 2.1 +/- 0.1 in order to illustrate the
effect of pH in the subsequent washing stage. In the initial l~
treatment, the pH was adjusted by addition of sulphuric acid. !~
The amount of magnesium added was 1000 ppm, calculated as part !~
by weight of magnesium on part by weight of dry pulp. The con~
ditions in the initial treatment as well as in the hydrogen
peroxide bleaching were identical with those in Example 1.
After the initial treatment, the pulp was washed at a pH with~
in the range of from 3 up to about 11 where appropriate by de~
ionised water to which sodium hydroxide had been added. The
results of the tests appear from Table III below.
,
~ 13 2109~ 79
~ . . .
TABLE III
pH in the Pulp properties after peroxide bleaching
Test washing Kappa number Viscosity Brightness
stage `~
_(dm3/kq) (~ ISO) _
1 3~0 11.9 890 52.9
2 4.3 10.2 830 58.9
3 4.8 10.2 840 59.4
4 6.9 9.0 835 63.4 ~
8.2 9.0 830 64.0 ~ `
6 10.8 9.0 800 59.5 ~` `;``
As is evident from the Table, pulp treatment according ` ;`~
to the invention with an intermediate washing stage at a pH
within the range of from about 3 up to about 10 results in a
15 considerable increase in brightness and a considerable reduc~
tion of the kappa number.
Example 4 ~!;
The oxygen-delignified sulphate pulp of softwood employ-
ed in Example 1 was treated in the presence of a magnesium
20 compound at a pH of 2.1 ~/- 0.1 in order to illustrate the
effect of pH in the bleaching with hydrogen peroxide within
the range of from 7 up to 12. The amount of magnesium added
was 1000 ppm, calculated as part by weight of magnesium on
part by weight of dry pulp. The conditions in the initial i
25 treatment were identical with those in Example 1, as were the ~ K conditions in the hydrogen peroxide bleaching, except the pH
variation. After the initial treatment, the pulp was washed at
a pH of 6-7. The results of the tests appear from Table IV
below.
TABLE IV
pH in the Pulp properties after peroxide bleaching
Test bleaching Kappa number Viscosity Brightness
stage
(dm3/kq)(~ ISO)
1 7.1 12.0 900 50.2
2 8.2 9.9 830 58.0
3 10.3 9.2 825 63.1
4 11.9 9.1 780 62.5
As is evident from the Table, pulp treatment according
:~-``'-' .
~- 14 ~109179
to the present invention using hydrogen peroxide as the per-
oxide-containing compound, results in a very high increase in
brightness and a considerable reduction of the kappa number.
Exam~le 5
The oxygen-delignified sulphate pulp of softwood employ- ~ ` ~
ed in Example 1 was treated in the presence of a magnesium ~`
compound at a pH of 2.1 ~ 0.1 in order to illustrate the
effect o a calcium compound on the hydrogen peroxide bleach~
ing (Test 2). The amount of magnesium added was 1000 ppm, cal~
10 culated as part by weight of magnesium on part by weight of ~;
dry pulp. After washing, calcium was added in an amount of
1000 ppm, calculated as part by weight of calcium on part by
weight of dry pulp. The conditions in the initial treatment ~ `
were identical with those in Example 1, as were the conditions ` ~
15 in the hydrogen peroxide bleaching, except the presence of a ~ `
calcium compound. After the initial treatment, the pulp was
washed at a pH of 6-7. For comparative purposes, the pulp was ; `~
also bleached in the absence of the calcium compound (Test 1). ~` ~ ```
The results of the tests appear from Table V below. `~
TABLE V `
Pulp properties after peroxide bleaching
Test Kappa number Viscosity Brightness Remaining
H22
(dm3/kq) (~ ISO) (kq/tonne) !`~
25 1 9.2 830 63.1 0
2 8.9 850 68.2 3.1
As is evident from the Table, pulp treatment according `
to the invention using a calcium compound in the hydrogen
peroxide bleaching, results in a much higher brightness at a `
lower consumption of hydrogen peroxide than does bleaching in
the absence of calcium.
Example 6
The oxygen-delignified sulphate pulp of softwood employ-
ed in Example 1 was treated in the presence of a magnesium ~ -
compound at a pH of 2.1 +/- 0.1, washed and bleached with
hydrogen peroxide, in order to illustrate the effect of a
selective metal profile in the pulp prior to bleaching. The
amount of magnesium added was 1000 ppm, calculated as part by
weight of magnesium on part by weight of dry pulp. After wash- ;
:: ::
~" 15 2109179
ing, calcium was added in an amount of looO ppm, calculated as
part by weight of calcium on part by weight of dry pulp (Test
4). The conditions in the initial treatment were identical
with those in Example 1, as were the conditions in the hydro-
gen peroxide bleaching, except the presence of a calcium com-
pound in Test 4. After the initial treatment, the pulp was
washed at a pH of about 3 (Test 1) and 6-7 (Tests 2-4),
respectively. For comparative purposes, the pulp was treated
without any magnesium in the initial treatment and without any
calcium in the bleaching stage (Tests 1 and 2). The content of
certain metal ions in the sulphate pulp before (test 0) and
after the initial treatment, as well as the brightness after
the hydrogen peroxide bleaching, appear from Table VI below.
TABLE VI
Test Metal content before bleaching, ppm Brightness after
Mg Ca Mn Fe Cu bleaching
~ISO~
0 3001800 68 8.31.5 ----
l 30 220 6.5 5.8~ 0.5 45.5
2 lO0 640 20 7.00.9 45.7
3 550 690 18 6.30.7 63.2
4 5501700 18 6.30.7 68.2
As is evident from the Table, the process according to
the invention involving treatment in the presence of a magne-
sium compound, followed by hydrogen peroxide bleaching, resul-
ts in a much higher brightness at a lower consumption of
hydrogen peroxide than does bleaching in the absence of
calcium.
