Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Process for bleachinq of liqnocellulose-containinq PUlP
This invention relates to a process for
delignification and bleaching of chemically digested
lignocellulose-containing pulp, wherein the pulp is acid
treated at a pH of between about 1 and out 6, whereupon a
water-soluble compound or mixture of compounds containing
an alkaline earth metal or metal is added at a pH of
between about 1 and about 7 before the pulp is treated
with a chlorine-free bleaching agent. The initial acidic
treatment removes the trace metals of the pulp, whereas
the subsequent addition of alkaline earth metal ions in
aqueous solution returns the ions to the positions in the
pulp where they have a particularly beneficial effect on
the preservation of the cellulose chains and, con-
sequently, on the viscosity, as well as on the con-
sumption of bleaching agent in the subsequent bleaching
step. After the treatment according to the invention,
the pulp may be finally bleached to the desired bright-
ness, suitably with a chlorine-free bleaching agent, such
as ozone, to completely avoid formation and discharge of
AOX.
Backqround
In the production of chemical pulp of high bright-
ness, wood chips are first cooked to separate the cellu-
lose fibres. Part of the lignin holding the fibrestogether is thus degraded and modified, such that it can
be removed by subsequent washing. However, in order to
obtain sufficient brightness, more lignin has to be
removed, together with brightness-impairing
(chromophoric) groups. This is frequently effected by
delignification with oxygen, followed by bleaching in
several stages.
For environmental reasons, it has become increas-
ingly common to treat chemical pulp with chlorine-free
bleaching agents already in the first bleaching steps.
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The big advantage is the drastic reduction in the dis-
charges of chlorinated organic substances detrimental to
the environment, owing to the combined effect of a
smaller amount of chlorine-containing bleaching agents
and lower content of lignin, which is the organic sub-
stance primarily reacting with the chlorine.
It is known to use chlorine-free bleaching agents,
such as hydrogen peroxide, peracetic acid or ozone,
already in the prebleaching. However, the delignifi-
cation and consumption of the bleaching agent becomesless effective than with chlorine-containing bleaching
agents, unless the pulp is pretreated. Thus, a hydrogen
peroxide treatment in an alkaline environment is dis-
turbed by the presence in the pulp of ions of certain
metals, such as Mn, Cu and Fe. These metal ions cause
degradation of hydrogen peroxide, thereby reducing the
efficiency of the peroxide treatment and increasing the
consumption of peroxide. According to CA 1,206,704, this
can be counteracted by pretreating the pulp with an acid,
such as sulphuric acid or nitric acid, whereby the con-
centration of all types of metal ions is reduced. How-
ever, by this treatment also metal ions, for example, Mg,
which are advantageous to the peroxide treatment dis-
appear, which ions stabilize the peroxide and increase
the selectivity of the peroxide.
CA 575,636 discloses the addition of magnesium sul-
phate to stabilize alkaline peroxide solutions. However,
the addition is made directly to the bleaching liquor and
in alkaline environment insoluble magnesium hydroxide
precipitates. Furthermore, US 4,222,819 discloses the
addition of magnesium ions to acidic peroxide solutions,
but also in this case the addition is made directly to
the bleaching liquor. None of the related methods makes
possible diffusion of the magnesium ions into the pulp to
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such an extent, that a pulp of high brightness and
strength can be obtained.
The invention
The invention provides a process in which ligno-
cellulose-containing pulp is treated under conditions
whereby the metal ions harmful to the subsequent bleach-
ing are effectively removed and the profile of alkaline
earth metals is restored before the pulp is bleached in a
chlorine-free bleaching step.
Broadly the invention relates to a process for
bleaching of chemically digested lignocellulose-contain-
ing pulp, wherein the pulp is acid treated at a pH in the
range from about 1 up to about 6, whereupon a water
soluble compound or water soluble mixture of compounds
containing an alkaline earth metal or metal is added at a
pH in the range from about 1 up to about 7 and in an
amount of from about 0.01 kg, suitably 0.5 kg, up to
about 10 kg/ton of dry pulp, calculated as alkaline earth
metal, and the pulp subsequently is treated with a
chlorine-free bleaching agent.
In a particular embodiment of this aspect of the
invention there is provided a process for delignification
and bleaching of chemically digested lignocellulose-con-
taining pulp, characterised in that the pulp is bleached
and delignified during an acid treatment at a pH in the
range from about 1 up to about 6, whereupon a water-
soluble chemical containing magnesium is added to a pH in
the range from about 1 up to about 7 and in an amount of
from about 0.01 up to about 10 kg/ton of dry pulp, cal-
culated as magnesium, and that subsequently the pulp isdelignified and bleached with a chlorine-free bleaching
agent comprising hydrogen peroxide, at a pH of from about
8 up to about 12.
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In another embodiment of the invention there is pro-
vided a process for delignification and bleaching of
chemically digested lignocellulose-containing pulp,
characterized in that the pulp is acid treated at a pH in
the range from about 1 up to about 6, whereupon a water-
soluble mixture of a magnesium compound and a calcium
compound is added at a pH in the range from about 1 up to
about 7 and in an amount of from about 0.5 up to about 10
kg/ton of dry pulp, calculated as magnesium and calcium,
and that subsequently the pulp is delignified and
bleached with a chlorine-free bleaching agent comprising
hydrogen peroxide at a pH of from about 8 up to about 12.
In this latter aspect of the invention the water
soluble mixture of a magnesium compound and a calcium
compound is preferably added in an amount of from about 2
to about 10 kg/ton of dry pulp calculated as magnesium
and calcium.
Acid treatment is an effective process to eliminate
metal ions from lignocellulose-containing pulps. At the
same time it is known, that ions of alkaline earth
metals, especially when in their original positions in
the pulp, have a positive influence on the selectivity of
the delignification as well as on the stability and con-
sumption of chlorine-free bleaching agents, such as per-
oxides, ozone and oxygen. The present process presentsan economic solution to the problem of creating a suit-
able trace-metal profile for the subsequent chlorine-free
bleaching, in that non-desirable metal ions are
eliminated while supplied ions of alkaline earth metals
essentially recover the positions in the vicinity of the
cellulose chains previously occupied by ions of alkaline
earth metals. This is achieved by adding the compound
containing an alkaline earth metal at such a pH and such
a temperature that the compound is dissolved in water,
thus enabling the diffusion required to obtain the
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intended effect. Furthermore, an advantage of the pre-
sent process is that the pH adjustment between the treat-
ment with acid and addition of alkaline earth metal ions
becomes very limited or may be left out altogether, which
is advantageous to process technique and economy.
Chlorine-free bleaching agents include inorganic
peroxide compounds, such as hydrogen peroxide and sodium
peroxide, organic peroxide compounds, such as peracetic
acid, as well as ozone, oxygen and sodium dithionite.
Suitably, hydrogen peroxide (P), oxygen(O) and ozone (Z)
are used in an optional sequence or mixture. Preferably,
use is made of hydrogen peroxide or mixtures of hydrogen
peroxide and oxygen (PO). The sequence P-Z or (PO)-Z is
especially preferred.
In the treatment with a chlorine-free bleaching
agent in an alkaline environment, pH is suitably adjusted
by adding to the pulp an alkali or an alkali-containing
liquid, such as sodium carbonate, sodium hydrogen carbon-
ate, sodium hydroxide, oxidized white liquor or magnesium
hydroxide slurry. Suitably, the magnesium hydroxide
slurry is taken from the chemical handling system in the
production of sulphite pulp with magnesium as base, i.e.,
magnefite pulp.
The acid treatment suitably is carried out with an
acid. The acids used are inorganic acids, suitably sul-
phuric acid, nitric acid, hydrochloric acid or residual
acid from a chlorine dioxide reactor, either separately
or in an optional mixture. Preferably, sulfuric acid is
employed.
Compounds containing an alkaline earth metal relate
to water-soluble chemicals containing magnesium or cal-
cium, or mixtures of such chemicals. Use is suitably
made of magnesium-containing compounds, such as magnesium
sulphate or magnesium chloride, or calcium-containing
compounds, such as calcium chloride or calcium oxide.
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Use is preferably made of magnesium sulphate or magnesium
chloride, the use of magnesium sulphate being especially
preferred. The combination of temperature and pH at the
addition of the compound containing an alkaline earth
metal is always so chosen that the compound is in aqueous
solution when contacted with the pulp.
In the process according to the invention, the acid
treatment is carried out at a pH of from about 1 up to
about 6, suitably from 1.5 up to 5, preferably from 2 up
to 4. It is especially preferred that the acid treatment
is carried out at a pH of from 2 up to 3. When magnesium
is the alkaline earth metal in the compound containing an
alkaline earth metal, or a magnesium compound and a cal-
cium compound mixture is employed, the addition is made
at a pH in the range from about 1 up to about 7, suitably
in the range from 2 up to 6, preferably in the range from
2 up to 4. It is especially preferred that the addition
of magnesium compound or mixture of magnesium and calcium
compound is made at a pH of from 2 up to 3.
When the chlorine-free bleaching agent is hydrogen
peroxide, the pulp is suitably treated at a pH of from
about 8 up to about 12, preferably at a pH of from 10 up
to 12. Treatment with the other chlorine-free bleaching
agents mentioned above, is carried out within the normal
pH ranges for each bleaching agent, which are well-known
to the person skilled in the art.
The treatment according to the invention is prefer-
ably carried out with a washing step between the acid
treatment and addition of alkaline earth metal ions, such
that the trace metals that are harmful to the treatment
with a chlorine-free bleaching agent are removed from the
pulp suspension.
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The realization of the acid treatment, a compound
containing an alkaline earth metal and a chlorine-free
bleaching agent, can be carried out at an optional
position in the bleaching sequence, e.g. immediately
after digestion of the pulp or after an oxygen step. The
process according to the invention is preferably applied
to pulp that has been delignified in an oxygen step prior
to the treatment.
According to a particular embodiment of the
invention the pulp is subjected to bleaching and
delignifying treatment, during the acid treatment.
Bleaching and delignifying chemicals active within the pH
range suitable in the acid treatment, are, for example,
chlorine dioxide, ozone, peracetic acid and/or an acid
peroxide-containing compound. Suitably, a combination of
acid treatment and bleaching and/or delignifying
treatment takes place in an ozone step.
Lignocellulose-containing pulps relate to chemical
pulps of softwood and/or hardwood digested according to
the sulphite, sulphate, soda or organosolv process, or
modifications and/or combinations thereof. Use is suit-
ably made of softwood and/or hardwood digested according
to the
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sulphate process, preferably sulphate pulp of hardwood.
The treatment according to the invention can be
applied to lignocellulose-containing pulps having an
initial kappa number within the range from about 5 up to
about 40, suitably 7 up to 32, preferably from 10 up to
20. Here, the kappa number is determined according to the
standard method SCAN-C 1:77.
In the process according to the invention, the acid
treatment is carried out at a temperature of from about 10
up to about 95C, suitably from 20 up to 80~C and prefe-
rably from 40 up to 80C, and for a period of time of from
about 1 up to about 120 min, suitably from 10 up to 120
min and preferably from 20 up to 40 min. The compound
containing an alkaline earth metal is added at a tempera-
ture of from about 10 up to about 95C, preferably from 40
up to 80C, and for a period of time of from about 1 up to
about 180 min, preferably from 20 up to 180 min and prefe-
rably from 30 up to 120 min. When the chlorine-free
bleaching agent is hydrogen peroxide, the pulp is treated
at a temperature of from about 30 up to about 100C,
preferably from 60 up to 90C, and for a period of time of
from about 30 up to about 300 min, suitably from 60 up to
240 min. In the acid treatment and in the addition of
alkaline earth metal ions, the pulp concentration may be
from about 3 up to about 35% by weight, preferably from 3
up to 15% by weight. When the chlorine-free bleaching
agent is hydrogen peroxide, the pulp concentration may be
from about 3 up to about 50% by weight, suitably from 3
up to 35% by weight and preferably from 10 up to 25% by
weight. Treatment with the other chlorine-free bleaching
agents mentioned above, is carried out within the normal
ranges of temperature, time and pulp concentration for
each bleaching agent, which are well-known to the person
skilled in the art.
The amount of compound containing an alkaline earth
metal charged, lies in the range from about 0.01 up to
about 10 kg/ton of dry pulp, calculated as alkaline earth
metal, suitably in the range from 0.5 up to 5 kg/ton of
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dry pulp, calculated as alkaline earth metal, and prefera-
bly in the range from 2 up to 4 kg/ton of dry pulp,
calculated as alkaline earth metal.
In preferred embodiments employing hydrogen peroxide
as the chlorine-free bleaching agent, the amount of
hydrogen peroxide, lies in the range from about 2 up to
about 50 kg/ton of dry pulp, calculated as 100% hydrogen
peroxide. The upper limit is not critical, but has been
set for reasons of economy. The amount of hydrogen peroxi-
de suitably lies in the range from 3 up to 30 kg/ton ofdry pulp and preferably from 4 up to 20 kg/ton of dry
pulp, calculated as 100% hydrogen peroxide.
After the acid treatment, a compound containing an
alkaline earth metal and a chlorine-free bleaching agent,
the pulp can be used for direct production of paper with a
lower demand of brightness. Alternatively, the pulp may be
finally bleached to the desired higher brightness, by
treatment in one or more steps. Suitably, the final
bleaching is also carried out with chlorine-free bleaching
agents of the type mentioned above, optionally with
intermediate alkaline extraction steps, which may be
reinforced with peroxide and/or oxygen. In this way, the
formation and discharge of AOX is completely eliminated.
Suitably, the final bleaching is carried out with ozone in
one or more steps. By the treatment according to the
invention, the lignin content has been reduced to a
sufficiently low level before any chlorine-containing
bleaching agents are used. Therefore, chlorine dioxide
and/or hypochlorite may well be used in one or more final
bleaching steps without causing formation of large amounts
of AOX.
Moreover, use of the process according to the inven-
tion means that the brightness and kappa number of the
resulting pulp is higher and lower, respectively, than
with the processes in which a compound containing an
alkaline earth metal is not added at all or is added at a
higher pH. In a process for bleaching chemical pulps, the
aim is a high brightness as well as a low kappa number,
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the latter meaning a low content of undissolved lignin. At
the same time, the consumption of the chlorine-free
bleaching agent should be as low as possible meaning lower
treatment costs. In the process according to the inven-
tion, these objects are met, as is apparent from theExamples. Furthermore, the strength of the pulp, measured
as viscosity, is sufficient, which means that the pulp
contains cellulose chains which are long enough to give a
strong product.
The invention and its advantages are illustrated in
more detail by the Examples below which, however, are only
intended to illustrate the invention and are not intended
to limit the same. The percentages and parts stated in
the description, claims and examples, refer to percent by
weight and parts by weight, respectively, unless anything
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e se lS s~a e .
Example 1
Sulphate pulp of softwood having a kappa number of
17, a brightness of 35% ISO and a viscosity of 970 dm3/kg
was treated with sulphuric acid at a pH of 2Ø The pulp
was treated at a temperature of 60C for 30 min, the pulp
concentration being 10% by weight. After washing the pulp
with water, magnesium was added in the form of an aqueous
solution containing MgSO4, to give a concentration of
magnesium in the pulp of at least 500 ppm. In the tests,
the pH at the time of the addition was varied between 2.3
and 11.5 by addition of sulphuric acid. Then, the pulp was
bleached with hydrogen peroxide at a temperature of 90C,
the residence time and pulp concentration being 180 min
and 15% by weight, respectively. The final pH was 11.5,
and the addition of hydrogen peroxide was 15 kg/ton of dry
pulp, calculated as 100% hydrogen peroxide. For comparati-
ve purposes, magnesium was added directly to the hydrogen
peroxide step under the conditions stated above, in
accordance with the prior art. To provide a further
comparison, the pulp was also treated with only sulphuric
acid and hydrogen peroxide under the conditions stated
above. The kappa number, viscosity and brightness of the
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pulp were determined according to SCAN Standard Methods,
and the consumption of hydrogen peroxide was determined by
iodometric titration. The test results appear from the
Table below.
TABLE I
pH at the Kappa Viscosity Brightness Residual H2O
addition number step 2 step 2 step 2
of Mg step 2 (dm3/kg) t% ISO) (kg/ton)
2.3 9.1 903 61.0 1.5
4.7 9.2 910 60.0 1.0
9.5 9.8 930 56.1 0.9
11.5 10.0 940 52.2 0.2
---- * 9.8 890 54.1 0.5
2.3 ** 9.9 875 48.2 0.0
15 * Magnesium added directly to the alkaline hydrogen
peroxide step.
** No magnesium added.
As is apparent from the Table, the treatment accord-
ing to the present invention with MgSO4 at a pH in the
range from about 2 up to about 6 is essential to give
maximum increase in brightness and maximum reduction of
the kappa number, as well as minimum decrease in viscosity
and minimum consumption of hydrogen peroxide. Furthermore,
the importance of the magnesium ions for the increase in
brightness appears from the comparison at a pH of 2.3,
where, in the final test, the peroxide treatment was
preceded only by acidic treatment.
Example 2
Oxygen-delignified sulphate pulp of softwood having a
kappa number of 13.7, a brightness of 37.1% ISO and a
viscosity of 1057 dm3/kg, was treated in a first step with
15 kg of sulphuric acid/ton of dry pulp at a pH of 1.9.
The pulp was treated at a temperature of 50C for 60 min,
the pulp concentration being 10% by weight. After washing
the pulp with water, in a second step between 0.1 and 1.5
kg of magnesium/ton of dry pulp was added in the form of
an aqueous solution containing MgSO4. Magnesium was added
at a pH of 4.1, a temperature of 50C for 60 min and with
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a pulp concentration of 3.5% by weight. Then, the pulp was
bleached with hydrogen peroxide at a temperature of 90~C,
the residence time and pulp concentration being 240 min
and 10% by weight, respectively. The final pH was 11.5
and the addition of hydrogen peroxide was 20 kg/ton of dry
pulp calculated as 100% hydrogen peroxide. For comparative
purposes, the pulp was also treated with only sulphuric
acid and hydrogen peroxide under the conditions stated
above. The kappa number, viscosity and brightness were
determined according to SCAN Standard Methods. The results
after bleaching with hydrogen peroxide appear from the
Table below.
TABLE II
Amount of Amount of Kappa no. Viscosity srightness
Mg added Mg in pulp step 3step 3 step 3
in step 2 after step 2
(kg/ton) tppm) (dm3/kg) (% ISO)
0 3,7 9.6 900 51.5
0.1 130 8. 2 870 61. 8
0.3 250 7.8 864 65.1
0.75 370 7.7 855 65.9
1.5 480 7.6 845 67. 3
As is apparent from the Table, acid treatment
followed by addition of dissolved magnesium and bleaching
25 with hydrogen peroxide according to the present invention,
positively influence the pulp as regards kappa number,
viscosity and brightness.
Example 3
The oxygen-delignified sulphate pulp of softwood
used in Example 2, was treated in the sequences D - EOP-
Z - P (test 1) and D - EOP - Z - Mg - P (test 2), where
the conditions in each step were equal in both sequences.
D and EOP relate to a conventional chlorine dioxide step
and a conventional alkaline extraction step reinforced
with hydrogen peroxide and oxygen, respectively. Z relates
to an ozone step with a pH of 2.3. Mg relates to the
addition of 1 kg of magnesium/ton of dry pulp, in the form
of an aqueous solution containing MgSO4. Magnesium was
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added at a pH of 4.1, a temperature of 50C for 30 min,
the pulp concentration being about 3% by weight. P relates
to a hydrogen peroxide step, where the pulp was treated at
a temperature of 80C for 120 min. The final pH was about
11.5 and the addition of hydrogen peroxide was 5 kg/ton of
dry pulp calculated as 100% hydrogen peroxide. The visco-
sity and brightness were determined according to SCAN
Standard Methods. The results after bleaching with hydro-
gen peroxide appear from the Table below.
TABLE III
Test Amount of Amount of Mg Viscosity Brightness
Mg added in pulp after
addition
(kg/ton) (ppm) (dm3/kg) (% ISO)
1 0 16 820 86.3
2 1.0 255 872 88.4
As is apparent from the Table, addition of dissolved
magnesium after an initial acid treatment with ozone and
bleaching with hydrogen peroxide according to the present
invention, positively influence the pulp as regards
viscosity and brightness.
Example 4
The oxygen-delignified sulphate pulp of softwood
used in Example 2, was treated in the sequences Stepl -
- Z - P2 (test 1) and Stepl - Pl - Z - Mg - P2 (test 2),
where the conditions in each step were equal in both
sequences. Stepl relates to treatment with EDTA at a pH of
5. Z relates to an ozone step with a pH of 2.3, the pulp
concentration being 10% by weight. Mg relates to the
addition of 1 kg of magnesium/ton of dry pulp, in the form
of an aqueous solution containing MgSO4. Magnesium was
added at a pH of 4.1, a temperature of 50C for 30 min,
the pulp concentration being about 3% by weight. P2
relates to a hydrogen peroxide step, where the pulp was
treated at a temperature of 80C for 120 min. The final
pH was about 11.5 and the addition of hydrogen peroxide
was 5 kg/ton of dry pulp calculated as 100~ hydrogen per-
oxide. For comparative purposes, the pulp was also treated
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in the sequence Stepl - P1 - Z - (PMg) (test 3). (PMg)
relates to the addition of magnesium in the second alkali-
ne hydrogen peroxide step under the conditions stated
above, in accordance with the prior art. The viscosity and
brightness were determined according to SCAN Standard
Methods, and the consumption of hydrogen peroxide was
determined by iodometric titration. The results after the
second hydrogen peroxide step appear from the Table below.
TABLE IV
10 Test Amount of Amount of Viscosity Bright- H22 con-
Mg added Mg in pulp ness sumed in P2
(kg/ton) (ppm) tdm3/kg) (% ISO) (% of added)
1 0 9 794 80.9 76
2 1.0 300 856 83.9 28
3 1.0 120 809 81.6 60
As is apparent from the Table, addition of magnesium
within the present pH range before bleaching with hydrogen
peroxide, positively influence the pulp as regards visco-
sity and brightness and reduces the consumption of hydro-
gen peroxide.
