Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2139246
This invention relates to a process for bleaching of
lignocellulose-containing pulp.
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This application is a division of Canadian Patent
Application 2,067,296, filed April 27, 1992.
The present invention relates to a process for delig~
nification and bleaching of chemically digested lignocellulo-
se-containing pulp, wherein the pulp is acid treated at a pH
of between about 1 and about 6, whereupon a water-soluble
mixture of a magnesium compound and a calcium compound is
added at a pH of between about 1 and about 7 before the pulp
is treated with a chlorine-free bleaching agent compri~ing
hydrogen peroxide at a pH of from about 8 up to about 12. 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, consequently, on the
viscosity, as well as on the consumption of bleaching agent in
the subsequent bleaching step. After the treatment according
to the invention, the pulp may be finally bleached to the
desired brightness, suitably with a chlorine-free bleaching
agent, such a ozone, to completely avoid formation and
discharge of AOX.
Back~round
In the production of chemical pulp of high brightness,
wood chips are first cooked to separate the cellulose fibres.
Part of the lignin holding the fibres together is thus
degraded and modified, such that it can be removed by subse-
quent washing. However, in order to obtain sufficient bright-
ness, 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.
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For environmental reasons, it has become increasingly
common to treat chemical pulp with chlorine-free bleaching
agents already in the first bleaching steps. The big advantage
is the drastic reduction in the discharges 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 substance primarily reacting with the chlorine.
It is known to use chlorine-free bleaching agents, such
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as hydrogen peroxide, peracetic acid or ozone, already in the
prebleaching. However, the delignification and consumption of
the bleaching agent become less effective than with chlorine-
containing bleaching agents, unless the pulp is pretreated.
Thus, a hydrogen peroxide treatment in an alkaline environment
is disturbed by the presence in the pulp of ions of certain
metals, such as Mn, Cu and Fe. These metal ions cause degrada-
tion 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 concentration of all types of
metal ions is reduced. However, by this treatment also metal
ions, for example Mg, which are advantageous to the peroxide
treatment disappear, which ions stabilize the peroxide and
increase the selectivity of the peroxide.
CA 575,636 discloses the addition of magnesium sulphate
to stabilize alkaline peroxide solutions. However, the
addition is made directly to the bleaching liquor and in
alkaline environment insoluble magnesium hydroxide precipi-
tates. 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 such an extent, that a pulp of
high brightnes~ and strength can be obtained.
The invention
The invention provides a process in which lignocellulose-
containing pulp is treated under the conditions disclosed in
the claims, whereby~thelmetal ions~harmful to the subsequent
bleaching are effectively removed and the profile of alkaline
earth metals is restored before the pulp is bleached in a
chlorine-free bleaching step.
The invention relates to a process for bleaching of
chemically digested lignocellulose-containing pulp, wherein
the pulp is acid treated at a pH in the range from about 1 up
to about 6, whereupon a mixture containing 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 0.01
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kg, suitably 0.5 kg, up to about 10 kg/ton of dry pulp,
calculated as magnesium and calcium, and that the pulp
subsequently is treated with a chlorine-free bleaching agent
comprising hydrogen peroxide at a pH of from about 8 up to
about 12.
Acid treatment is an effective process to eliminate metal
ions from lignocellulose-containing pulps. At the same time it
i6 known, that ions of magnesium and calcium, especially when
in their original positi.ons in the pulp, have a positive
influence on the selectivity of the delignification as well as
on the stability and consumption of chlorine-free bleaching
agents, such as peroxides, ozone and oxygen. The present
process presents an economic solution to the problem of
creating a suitable trace-metal profile for the subsequent
chlorine-free bleaching, in that non-desirable metal ions are
eliminated while supplied ions of magnesium and calcium
essentially recover the positions in the vicinity of the
cellulose chains previously occupied by ions of alkaline earth
metals. This is achieved by adding a mixture of a magnesium
compound and a calcium compound at such a pH and such a
temperature that the compound is dissolved in water, thus
enabling the diffusion required to obtain the intended effect. ~i
Furthermore, an advantage of the present process is that the
pH adjustment between the treatment 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 ;j ~ ;
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 (0) and ozone (Z) are used in an
optiondl sequence or mixture. Preferably, use is made of
hydrogen peroxide or mixtures of hydrogen peroxide and oxygen ;
(P0). The sequence P-Z or (P0)-Z are 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 carbonate, sodium hydroxide,
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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 sulphuric acid,
nitric acid, hydrochloric acid or residual acid from a
chlorine dioxide reactor, either separately or in an optional
mixture. Preferably, sulphuric acid is employed.
Use is suitably made of magne~ium-containing compounds,
such as magnesium sulphate or magnesium chloride, and calcium-
containing compounds, such as calcium chloride or calcium
oxide. The combination of temperature and pH at the addition
of the mixture of a magnesium compound and a calcium compound
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, 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 is made at a pH of from 2 up to 3. When
the chlorine-free bleaching agent is hydrogen peroxide, the
pulp is 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 preferably
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.
The realization of the acid treatment, a mixture of a
magnesium compound and a calcium compound and a chlorine-free
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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 deligni-
fied in an oxygen step prior to the treatment.
It is also within the scope of the invention, that the
pulp in the acid treatment also can be subjected to bleaching
and/or delignifying treatment. Bleaching and/or delignifying
chemicals active within the pH range suitable in the acid
treatment, are e.g. chlorine dioxide, ozone, peracetic acid
and/or an acid peroxide-containing compound. Suitably, a
combination of acid treatment and bleaching and/or delig-
nifying 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 modifi-
cations and/or combinations thereof. Use is suitably made of
softwood and/or hardwood digested according to the 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 i~ determined according to the standard method SCAN-C
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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 80C and preferably from
40 up to 80C, and for a period of time of from about 1 up to
about 120 min, su!itablylfrom 10 up to 120 min and preferably
from 20 up to 40 min. The mixture of a magnesium compound and
a calcium compound is added at a temperature 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 preferably 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
2~39~!46
up to 240 min. In the acid treatment and in the addition of a
mixture of magnesium and calcium 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 mixture containing a magnesium compound and
a calcium compound charged, lies in the range from about 0.01
up to about 10 kg/ton of dry pulp, suitably in the range from
0.5 up to 5 kg/ton of dry pulp, and preferably in the range
from 2 up to 4 kg/ton of dry pulp, calculated as magnesium and
calcium.
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 peroxide suitably lies in the
range from 3 up to 30 kg/ton of dry pulp and preferably from
4 up to 20 kg/ton of dry pulp, calculated as 100~ hydrogen
peroxide.
After the acid treatment, a mixture of a magnesium
compound and a calcium compound, 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 finaliy 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
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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 proce3s according to the invention
means that the brightness and kappa number of the resulting
pulp is higher and lower, respectively, than with the pro-
cesses in which a mixture of a magnesium compound and a
calcium compound 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, 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 invention, these objects are met, as
is apparent from the Examples. 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 else is stated.
Exam~le 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 concen-
tration 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
sul~huric 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.
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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 comparative purposes, magnesium wa~ 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 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 Ma ste~ 2 (dm3/ka) (~ ISO) (kq/ton) ~
2.3 9.1 903 61.0 1.5 ~; i
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
* Magnesium added directly to the alkaline hydrogen
peroxide step.
** No magnesium added.
A~ 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; ;
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the kappa number, as well as minimum decrease in vi~co~
sit~ and minimum consumption of hydrogen peroxide.
Furthermore, the importance of the magnesium ions for the
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increase in brightness appears from the comparison at a
pH of 2.3, where, in the final test, the peroxide treat-
ment was preceded only by acidic treatment.
Exam~le 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 ~ ~-
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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
a pulp concentration of 3.5~ by weight, Then, the pulp
.. . . . .
was bleached with hydrogen peroxide at a temperature of
90C, 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 Brightness
Mg added Mg in pulp step 3 step 3step 3
in step 2 after step 2
(kq/ton) (~E~_ (dm3/kq) (~ ISO)
~5 0 3,7 9.6 900 51.S
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 o~ dissolved magnesium and bleaching
with hydrogen peroxide according to the present inven~
tion, positively influence the pulp as regards kappa
number, viscosity and brightness.
Exam~le 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.
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D and EOP relate to a conventional chlorine dioxide step
and a conventional alkaline e~traction step reinforced
with hydrogen peroxide and oxygen, respectively. Z
relates to an ozone step with a pH of 2.3. Mg relates to j
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. P q
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 pu]p calculated as 100% hydrogen per~
oxide. The viscoRity and brightness were determined
according to SCAN Standard Methods. The results after
bleaching with hydrogen peroxide appear from the Table
below.
TABLE III
Test Amount of Amount of Mg Viscosity Brightness
Mg added in pulp after
addition
_ (ka/ton) (ppm) (dm3/kq) 1 ~ ISO)
1 0 16 820 86.3
2 1.0 255 872 88.4 ,,
As is apparent from the Table, addition of dis- ,
solved 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.
Exam~le 4
The oxygen-delignified sulphate pulp of softwood
used in Example 2, was treated in the ~sequences 9tepl -
P1 - Z - P2 (test 1) and Stepl - P1 - 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
2~39Z46
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 in the sequence Stepl - Pl - Z - (PMg) (test 3).
(PMg) relates to the addition of magnesium in the second
alkaline 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.
TAsLE IV
Test Amount of Amount of Viscosity Bright- H2O2 con-
Mg added Mg in pulp ness sumed in P2
(kg/ton) (p~m) (dm3/ka) (~ ISO) (~ of added)
10 9 794 80.9 76
21.0 300 856 83.9 28
31.0 120 809 81.6 60
As is apparent from the Table, addition of mag~
nesium within the present pH range before bleaching with
hydrogen peroxide, positively influence the pulp as
regards viscosity and brightness and reduces the consump-
tion of hydrogen peroxide.
Example 5
Sulphate pulp of softwood having a kappa number of
10, a brightness of 42.5 ~ ISO and a viscosity of 986
dm3/kg was treated with 15 kg/ton suIphuric acid at a
temperature of 50C for 30 min and the pulp concentration
being 10% by weight. pH was 5-5.5. After washing the pulp
with water, magnesium, calcium or a mixture of them, was
added. 1 kg/ton magnesium respectively 1.5 kg/ton calcium
was added, or the total amount of the chemicals when a
mixture is added. pH at the time of the addition was
between 5 and 5.5! at a temperature of 60C for 60 min.
Then, the pulp was bleached with hydrogen peroxide at a
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12
temperature of 90C, the residence time and pulp concen- `
tration being 240 min and 15% by weight, respectively. pH ~-
was 10.5-11 and the addition of hydrogen peroxide wa~ 35 -~
kg/ton of dry pulp, calculated as 100% hydrogen peroxide. -~
The kappa number and brightness of the pulp were deter- `
mined according to SCAN Standard Methods, and the con- ;~ ~-
sumption of hydrogen peroxide was determined by
iodometric titration. The test results appear from the
Table below.
TABLE V
Sequence Brightness Kappa No. Residual H2O2 -
~ISO kq/t
A-Ca-P 70,5 4,58 0,15
A-Mg-P 77,5 4,44 7,3
A-(Mg+Ca)-P 81,3 4,02 8,7 ~
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