Note: Descriptions are shown in the official language in which they were submitted.
CA 02665969 2014-03-05
Bleaching of pulp
The present invention relates to a bleaching process which decreases the
brightness reversion or yellowing of an ECF bleached pulp.
Background of the invention
In the art of bleaching, the target is to provide high and stable brightness
levels.
However, brightness reversion or yellowing of pulp occurs to varying extent
during
storage and transportation to the paper mill depending on e.g. temperature,
humidity,
wood species, cooking conditions and bleaching sequence. It is therefore
important to
perform the bleaching so that the brightness reversion becomes as low as
possible.
In the paper "How to Improve Brightness Stability of ECF bleached Softwood and
Hardwood Kraft Pulp", Suess, H.U. et al, 2004, APPITA Conference, Canberra, a
standard elemental chlorine free (ECF) bleaching sequence is disclosed. The
bleached
pulp obtained from this sequence may, however, suffer from considerable
brightness
reversion. Also The Ljungberg Textbook, Pulp Technology, 2004 generally
discloses ECF
bleaching in which brightness reversion may suffer from considerable
brightness
reversion. It would thus be of interest to provide a process further improving
the
brightness stability. The present invention intends to provide such process,
particularly for
decreasing the brightness reversion of pulps derived from an alkaline pulping
process
with a certain content of hexenuronic acid prior to the final bleaching stage.
The invention
The present invention relates to a process of bleaching pulp derived from an
alkaline pulping process having a hexenuronic acid content prior to the final
bleaching
stage from about 1 to about 80 mmol/kg dry pulp, said process comprising three
chlorine
dioxide stages DO, D1, and D2, wherein the chlorine dioxide charges based on
the total
chlorine dioxide charge in
a) DO is from about 40 to about 60 %;
b) D1 is from about 30 to about 55 %;
c) D2 is from about 4 to about 20 %, and wherein the final chlorine dioxide
stage D2
is performed at a pH from about 2 to about 3.5.
The pulp to be bleached may be derived from lignocellulose-containing material
which may comprise e.g. wood logs, finely-divided raw materials, and woody
materials,
such as wood particles and non-wood such as fibres of annual or perennial
plants. The
woody raw material can be derived from hardwood or softwood species, such as
birch,
beech, aspen, alder, eucalyptus, maple, acacia, mixed tropical hardwood, pine,
fir,
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hemlock, larch, spruce, and mixtures thereof. Non-wood plant raw material can
be
provided from e.g. straws of grain crops, reed canary grass, reeds, flax,
hemp, kenaf,
jute, ramie, sisal, abaca, coir, bamboo, bagasse or combinations thereof.
However, also
pulp derived from recycled pulp may be used, either separately or in mixtures
with virgin
pulps. The pulp can be a chemical pulp, produced using an alkaline pulping
process, for
example sulphate, soda processes or modifications thereof with optional
additives such
as anthraquinone, e.g. the soda anthraquinone (AQ) pulping process. The
chemical pulp
obtainable from lignocellulose-containing materials may be subjected to one or
more
oxygen treatment stages subsequent to the pulping process. The process can be
applied
to chemically digested pulps having an initial kappa number, i.e. a kappa
number after
digestion but before oxygen delignification, in the range from about 5 to
about 40, for
example from about 8 to about 30, or from about 8 to about 25 The referred
kappa
number is measured according to the ISO 302 Standard Method.
According to one embodiment, the content of hexenuronic acids in the bleached
pulp prior to the final bleaching stage is from about 5 to about 80 or from
about 5 to about
25, or from about 5 to about 15 mmol/kg dry pulp.
According to one embodiment, the pH of the first chlorine dioxide bleaching
stage ranges from about 2 to about 4, for example from about 2.5 to about 3.5.
According
to one embodiment, the pH of the second chlorine dioxide bleaching stage
ranges from
about 2.5 to about 4.5, for example from about 3 to about 4. According to one
embodiment, the pH of the final chlorine dioxide bleaching stage ranges from
about 2 to
about 4.4 or from about 2 to about 4.3, for example from about 2 to about 4 or
from about
2 to about 3.5, such as from about 2 to about 3.4 or from about 2 to about 3.3
or from
about 2.5 to about 3, or any other sub-range within the range from about 2.5
to about 4.5.
According to one embodiment, the temperature of the first chlorine dioxide
bleaching stage ranges from about 40 to about 90, for example from about 50 to
about 80
or from about 55 to about 70 C. According to one embodiment, the temperature
of the
second chlorine dioxide bleaching stage ranges from about 70 to about 90, for
example
from about 80 to about 90 C. According to one embodiment, the temperature of
the final
chlorine dioxide bleaching stage ranges from about 80 to about 100, for
example from
about 85 to about 100, or from about 87 to about 100 C.
According to one embodiment, the pulp consistency in the chlorine dioxide
bleaching stages ranges from about 1 to about 40, for example from about 3 to
about 30
or from about 5 to about 15 wt%.
According to one embodiment, the charge of chlorine dioxide charge based on
the total chlorine dioxide charge in DO is from about 45 to about 55, for
example from
about 45 to about 50, or from about 45 to about 48 %. According to one
embodiment, the
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charge of chlorine dioxide charge based on the total chlorine dioxide charge
in D1 is from
about 35 to about 50, for example from about 40 to about 50, or from about 45
to about
50 %. According to one embodiment, the charge of chlorine dioxide charge based
on the
total chlorine dioxide charge in D2 is from about 5 to about 15 , for example
from about 5
to about 12, or from about 5 to about 10%.
According to one embodiment, the retention time of the first chlorine dioxide
bleaching stage is from about 20 to about 60, for example from about 30 to
about 50, or
from about 35 to about 45 minutes. According to one embodiment, the retention
time of
the second chlorine dioxide bleaching stage is from about 60 to about 240, for
example
from about 60 to about 120, or from about 60 to about 90 minutes. According to
one
embodiment, the retention time in the final chlorine dioxide bleaching stage
is from about
90 to about 360, for example from about 110 to about 240, or from about 110 to
about
180 minutes. According to one embodiment, the bleaching process may comprise
besides chlorine dioxide stages for example ozone, peracid, oxygen and/or
peroxide-
containing, as well as enzyme-containing bleaching stages either in addition
to the
sequence of three chlorine dioxide stages or substituting any of the initial
chlorine dioxide
stages. According to one embodiment, also washing and/or extraction stages,
acidification and/or chelating agent stages may be present in the bleaching
process
preceeding, between and/or subsequent to any bleaching stages.
According to one embodiment, chlorine dioxide and chelating agent stages can
be carried out without any make-up chemicals added, as both stages can be
performed
at similar pH ranges.
According to one embodiment, a chelating agent stage is carried out
essentially in
absence of chlorine dioxide, as essentially most of the chlorine dioxide is
washed away by
the washing stage.
According to one embodiment, if exceptionally an acid is added to the
treatment
with the chelating agent, an acid readily available in the pulp mill may be
employed.
According to one embodiment, for example subsequent to treatment with a
chelating agent, the pulp can be bleached with a peroxide-containing compound
which can
be an inorganic peroxide compound such as hydrogen peroxide or
peroxomonosulphuric
acid (caro's acid) or an inorganic peroxide compound like aliphatic peracids
and aromatic
peracids and salts thereof. Suitable organic peroxides are peracetic acid and
performic
acid. For example, the peroxide-containing compound is hydrogen peroxide or a
mixture
of hydrogen peroxide and peracetic acid commonly referred to as equilibrium
peracetic
acid. When hydrogen peroxide is used the pH of the pulp suspension is suitably
above 7,
for example in the range from about 7 to about 13, from about 8 to about 12,
or from
about 9.5 to about 11.5. The temperature of the hydrogen peroxide bleaching
may be
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from about 30 to about 130, for example from about 50 to about 100, or from
about 50 to
about 90 C. The length of the hydrogen peroxide treatment may be from about 3
to
about 960, for example from about 10 to about 360, or from about 60 to about
240
minutes. Usually the amount of added hydrogen peroxide is from about 1 to
about 60, for
example from about 6 to about 50, or from about 13 to about 40 kg per tonne
dry pulp. Other
bleaching agents may be added to the hydrogen peroxide bleaching stage such as
oxygen.
According to one embodiment, the bleaching process comprises an ECF
(Elemental Chlorine Free) or a substantially ECF sequence, for example
according to the
following sequence: D0(OP)D1nD2, where DO, D1, and D2 indicate three chlorine
dioxide
(D) stages, OP indicates an oxygen-containing peroxide stage, and n indicates
a neutral
washing stage. According to one embodiment, the bleached pulp is subsequently
treated
as an aqueous suspension prior to papermaking by dewatering and drying said
suspension, wherein the pH of the suspension to be treated ranges from about
6.5 to
about 12. The bleached pulp may be further treated as set out in US published
appl.
US2008/0073043.
According to one embodiment, the invention also relates to a bleached pulp
obtainable by the process defined herein.
The invention being thus described, it will be obvious that the same may be
varied in many ways. Such variations are not to be regarded as a departure
from the gist
and scope of the present invention, and all such modifications as would be
obvious to
one skilled in the art are intended to be included within the scope of the
claims. The
following examples will further illustrate how the described invention may be
performed
without limiting the scope of it.
All parts and percentages refer to part and percent by weight, if not
otherwise
stated.
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Examples
The experiments were carried out employing an industrial oxygen-delignified
eucalyptus
pulp. The characteristics of the oxygen-delignified pulp is shown in table 1.
The pulp
could be characterized as a low bleaching chemical demand pulp with high
content of
hexenuronic acid (HexA). The pulp had an unbleached kappa number of 10.7 and
required 1.5 kg/t H202 and 27 kg/t 0102 in the sequence DO(OP)D1nD2 to reach
the
brightness target 90.5% ISO.
Table 1: Oxygen delignified eucalyptus pulp
Kappa number ISO Brightness Viscosity Hexenuronic acid
(%) dm3/kg mmol/kg
10.7 58.9 1030 71
The aim of the experiments was to improve a standard ECF sequence,
DO(OP)D1nD2, in view of brightness reversion within the limitations of an
existing mill.
The same total dosage of chlorine dioxide was made in all experiments.
The conditions in the D1 and D2 stages and the distribution of chlorine
dioxide were
varied whereas the conditions in the DO and (OP) stages were kept constant.
The
retention time and temperature in the DO stage were 45 minutes and 60 C. The
(OP)
stage was performed at a pressure of 0.5 MPa, a retention time of 60 minutes,
a
temperature of 80 C and a final pH between 11.6 and 11.9. The hydrogen
peroxide
(H202) dosage was 1.5 kg/t on oven dry pulp in all experiments. The pulp and
chemicals
were mixed at 10 wt% pulp consistency and distributed uniformly in
polypropylene bags,
the atmosphere of which was saturated with oxygen. The plastic bags were
sealed and
placed in autoclaves which were pressurized with oxygen. The chlorine dioxide
dosages
were given as active chlorine. The consistency was 10 wt% in all bleaching
stages. All
non-pressurised stages were carried out in sealed plastic polyethylene bags.
After each bleaching stage, the pulp was washed with distilled water. In the
neutral wash
stage (n) between the D1 and D2 stages, the pH was adjusted to between 7 and
10 with
NaOH. The kappa number and ISO brightness were determined according to ISO 302
and ISO 2470 respectively. The hexenuronic acid was determined by acidic
hydrolysis of
the pulp in a formate buffer followed by UV analysis of the 2-furoic acid
formed in the
hydrolysate.
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The accelerated heat-induced brightness reversion was evaluated as humid
brightness
reversion. The "humid brightness reversion" was evaluated by measuring the
brightness
loss after the sheet had been conditioned at 23 C and 50% relative humidity,
sealed in a
polyethylene bag, kept at 70 C for 64h and finally removed from the bag and
allowed to
temperate at room temperature for at least 1 h. The brightness reversion was
reported as
the difference in ISO brightness units before and after reversion treatment.
Example 1
The results after bleaching the pulp according to the invention and the
reference are
set out in table 2. pH and chlorine dioxide distribution in the stages DO, D1,
and D2 were
varied as set out in table 2.
Table 2: Results of different modifications of the standard ECF sequence
DO/D1/D2 Bright Humid Brightness Reversion
Bleaching D1-D2 D1-D2
Distribution % ness ISO units
Approach pH Temp. C
of total charge % ISO
Reference 3.0-6.1 90-90 56/41/3 90.5 14.4
Invention 3.0-3.0 90-90 45/45/10 90.0 10.5
In table 2, the effect of the final pH in the D2 stage and the distribution of
chlorine dioxide
charge in stages DO, D1, and D2 is shown, as well as humid brightness
reversion. The
humid reversion was reduced from 14.4 in the reference to 10.5 units according
to the
invention, thus a considerable improvement.
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