Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR REDUCING STARCH CONTENT OF AN AQUEOUS PHASE
REMOVED FROM FIBRE STOCK PREPARATION
The present invention relates to a method for reducing starch content of an
aqueous
phase removed from fibre stock preparation, especially stock preparation of
recycled fibre material and/or broke, according to the preamble of the
enclosed
independent claim.
Paper, board and other cellulosic webs are often surface sized and/or coated
with
compositions that contain starch in order to obtain improved surface and/or
other
properties for the produced webs as well as for the products made from such
webs.
When these products are then recycled and repulped, the obtained fibre stock
from
the repulping process may contain significant amounts of starch originating
from the
compositions applied on the surface of the original webs in the previous
production
process. This starch is often poorly retained on the fibres as it has no
charge or has
a slightly anionic charge. It is easily enrichened to the water circulation of
the pulping
and fibre stock preparation processes, from where it may follow together with
the
effluent to the wastewater treatment.
Starch is also added as a dry strength agent and as a component of an internal
size
to the fibre stocks which are used for production of paper, board and the
other
cellulosic webs. Addition of starch is done in order to improve the properties
of the
formed cellulosic webs, e.g. to increase the strength properties of the formed
cellulosic webs.
In the water circulation of the stock preparation starch may become a suitable
nutrient for various microbial organisms, which increases the risk for
microbial
growth, slime and/or biofilm formation. Microbial organisms may further
negatively
affect both the functioning of the chemistry of papermaking and/or the quality
of the
end product. For example, microbial organisms may produce organic acids, which
lower the pH of the process that may successively lead to dissolution of
calcium
compounds and increased risk for formation of deposits. Presence of
microorganisms may also lead to formation of large stickies, which spoil the
quality
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of the final product and may cause runnability problems. Especially harmful is
the
presence of microbial organisms in the production of paper or board intended
for
packaging, particularly food or beverage packaging, where the presence of
microbial organisms may destroy the product quality of the produced packages
and
make them unsuitable for preserving foodstuffs, even if the packages would be
visually defect-free. Abundant growth of microorganisms in the paper or board
production may further cause severe odour problems.
Furthermore, there is significant loss in yield of the manufacturing process
when
starch present in the fibre stock is destroyed by microbes. Loss of starch
must be
compensated by equivalent addition of fibre material, in order to maintain the
same
solids content for the fibre stock. The loss of starch may also decrease the
strength
of the produced paper or board, which have to be compensated by supplementary
starch addition or by addition of other strength chemicals.
Typically various biocide regimes are used in manufacture of production of
paper,
board and the other cellulosic webs, in order to reduce or eliminate the
problems
associated with high starch content of the fibre stock and microorganisms.
However,
biocide regimes do not improve the starch retention to the fibres.
It is known to use chemical auxiliaries to retain starch liberated at pulping
to the
fibres. For example, EP 2817453 discloses a method where an inorganic
coagulant
is added to a pulp flow in order to interact with the starch having a low
molecular
weight. A polymer flocculant is then added separately to a flow comprising
interacted
coagulant agent for forming starch agglomerates, which are then retained to
the
fibres and/or to the formed web.
However, there is a need for new effective ways to retain starch to the fibres
already
during the stock preparation, especially when recycled fibre material and/or
broke is
used. If the starch is allowed to escape with the aqueous phase which is
removed
from the fibre stock manufacture, e.g. at the thickening of the fibre stock,
the
removed starch ends up to the water treatment process and increases the COD
(chemical oxygen demand) load of the water to be treated. Microbial
degradation of
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starch during stock preparation also increases carbon dioxide emissions from
the
process. Consequently, the efficiency of the starch trapping to the fibres
should be
improved already at the early stages of the manufacture of paper, board,
tissue or
the like.
An object of this invention is to minimise or even eliminate the disadvantages
existing in the prior art.
An object of the invention is to provide a method with which starch is
effectively and
cost-efficiently associated with fibres already during fibre stock
preparation.
These objects are attained with the invention having the characteristics
presented
below in the characterising part of the independent claim.
Some preferred embodiments of the present invention are presented in the
dependent claims.
All the described embodiments and advantages apply to all aspects of the
present
invention, even if not always explicitly stated so.
A typical method according to the present invention for reducing starch
content of
an aqueous phase, which is removed from a fibre stock preparation in a
manufacturing process of paper, board, tissue or the like, the fibre stock
preparation
comprises a thickening step, where a fibre stock comprising cellulosic fibres
originating from recycled fibre material and/or broke as well as starch
dispersed in
an aqueous phase is thickened from a first concentration to a second
concentration
by removing a part of the aqueous phase from the fibre stock,
wherein a cationic polymer obtained by copolymerisation of (meth)acrylamide
and
at least 25 mo1-13/0 of solely cationic monomer(s), having a standard
viscosity SV of
at least 1.7 mPas, is added to the fibre stock at the latest at the thickening
step in
order to associate the starch with the cellulosic fibres of the fibre stock.
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Now it has been surprisingly found that a cationic polymer obtained by
copolyrnerisation of (nneth)acrylarnide and at least 25 nnol-`)/0 of solely
cationic
monomers is able to associate starch with great effectiveness with the
cellulosic
fibres already at the fibre stock preparation stage, provided that the
cationic polymer
has a standard viscosity of at least 1.7 mPas. It is assumed, without wishing
to be
bound by any theory, that the amount of cationic monomers and the size of the
polymer provide optimal polymer structure which is able to both physically and
chemically trap the starch and associate the starch with the fibres. When the
cationic
polymer is added to the fibre stock during the fibre stock preparation, at the
latest at
the thickening step, an unexpected reduction in starch content in the aqueous
phase, which was removed from the fibre stock preparation, was observed.
Effective
removal of starch reduces microbial load in the overall process, especially in
waste
water treatment. Furthermore, it may be possible to reduce use of biocides in
the
process, and for example see a reduction in scale formation in the waste water
treatment of the process.
In the present context the expression "associate with" means and is synonymous
with that the starch present in the aqueous phase of the fibre stock interacts
with
the cationic polymer and the cellulosic fibres. The interaction may be based
on
physical entanglement of the starch and the polymer structure, wherein the
starch
is "trapped" or "caught" by the polymer structure, and/or the interaction may
be
based on chemical interactions, where the starch and/or fibres may be bound to
each other, e.g. by electrostatic forces. The association of the starch and
fibres,
elicited by the cationic polymer, makes it possible for the cellulosic fibres
to carry
the starch forward in the manufacturing process, inhibit its removal from the
process
with the aqueous phase and finally enable its retention to the final web to be
formed.
In the present context it is understood that the cellulosic fibres in the
fibre stock may
originally be produced by any suitable pulping method, i.e. they may originate
from
chemical pulping, mechanical pulping or chemi-mechanical pulping. The
cellulosic
fibres may usually be wood-based fibres, but it is possible that at least some
of them
are non-wood-based fibres, e.g. cellulosic fibres originating from annual
plants. The
fibre stock usually comprises a significant amount of recycled fibres or
fibres
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originating from broke. For example, the fibre stock may comprise from 60
weight-
%, preferably from 75 weight-% or from 90 weight-%, up to 95 weight-% or up to
100 weight-%, of recycled fibres or fibres originating from broke, calculated
from dry
weight of the fibre stock.
5
The cationic polymer used in the present invention is obtained by
copolymerization
of (meth)acrylamide, preferably acrylamide, and cationic monomers. In some
embodiments the cationic copolymer may be obtained by copolymerization of
(meth)acrylamide, cationic monomers and <1 mol-%, preferably <0.5 mol-%, more
preferably <0.1 mol-%, of anionic monomers. According to one preferable
embodiment, the cationic copolymer is free of anionically charged structural
units,
i.e. the copolymerisation is performed in the absence of anionic monomers. The
polymer thus preferably consists of structural units that originate from non-
ionic
monomers, i.e. (meth)acrylamide, and solely from cationic monomers, even in
that
case a minor amount of anionically charged groups may be formed to the polymer
structure during polymer preparation, e.g. during drying.
The cationic polymer may be obtained by copolymerisation of (meth)acrylamide
and
one or more of cationic monomers. The cationic polymer is obtained by
copolymerisation of (meth)acrylamide and at least 25 mol-%, preferably at
least 30
mol-%, more preferably at least 35 mol-% of solely cationic monomer(s). For
example, the cationic polymer may be obtained by copolymerising 5 ¨ 75 mol-%,
preferably 20 ¨75 mol-%, more preferably 30¨ 70 mol-%, even more preferably 40
¨ 70 mol-%, of (meth)acrylamide, preferably acrylamide, and 25 ¨ 95 mol-%,
preferably 25 ¨80 mol-%, more preferably 30¨ 70 mol-%, even more preferably 30
¨ 60 mol-% of cationic monomer(s). According to one embodiment, the
cationic
polymer may be obtained by copolymerising 40¨ 75 mol-%, preferably 45 ¨ 75 mol-
%, of (meth)acrylamide, preferably acrylamide, and 25¨ 60 mol-%, preferably 25
¨
55 mol-%, of cationic monomer(s). It has been observed that when at least 25
mol-
(:)/0 of cationic monomers is present in the polymerisation, the obtained
cationic
polymer is provided with good ability to associate with the starch present in
the
aqueous phase of the fibre stock, e.g. through electrostatic forces, and at
the same
time its ability to interact also with the anionically charged fibres is
improved.
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According to one embodiment of the invention the cationic monomer(s) may be
selected from 2-(dimethylamino)ethyl acrylate (ADAM), [2-(acryloyloxy)ethyl]
trimethylannmonium chloride (ADAM-CI), 2-(dimethylarnino)ethyl acrylate
benzylchloride, 2-(dimethylamino)ethyl acrylate dimethylsulphate, 2-
d imethylaminoethyl methacrylate (MADAM),
[2-(methacryloyloxy)ethyl]
trimethylammonium chloride (MADAM-CI), 2-dimethylaminoethyl methacrylate
dimethylsulphate, [3-(acrylamido)propyl] trimethylammonium chloride (APTAC),
or
[3-(methacrylamido)propyl] trimethylammonium chloride (MAPTAC). Preferably the
cationic monomer(s) may be selected from 2-(dimethylarnino)ethyl acrylate
(ADAM), [2-(acryloyloxy)ethyl] trimethylammoniunn chloride (ADAM-CI), and [3-
(acrylamido)propyl] trimethylammonium chloride (APTAC).
The cationic polymer has a standard viscosity SV of at least 1.7 mPas,
preferably
at least 2.5 mPas, more preferably at least 3.0 mPas. According to one
embodiment
the standard viscosity of the cationic polymer may be in a range of 1.7 ¨ 7.0
mPas,
preferably 2.5¨ 6.0 mPas, more preferably 3.0 ¨ 5.0 mPas. Sometimes the
standard
viscosity of the cationic polymer may be in a range of 3.0 ¨ 7.0 mPas,
preferably 3.5
¨ 6.0 mPas, more preferably 4.0 ¨ 5.5 mPas or 4.5 ¨ 5.5 mPas. Standard
viscosity
is measured at 0.1 weight-% polymer content in an aqueous 1 M NaCI solution,
using Brookfield LV viscometer equipped with UL adapter, at 25 C, using UL
Adapter Spindle and rotational speed 60 rpm. In general, the standard
viscosity of
the polymer gives an indication of the length and/or weight of the polymer
chains of
the polymer. It has been observed that when the standard viscosity SV of the
cationic polymer is at least 1.7 mPas, the polymer is able to effectively
associate
with the starch present in the aqueous phase of the fibre stock. It is assumed
that
the cationic polymer has an improved ability to physically trap the starch and
to
interact at the same time with the starch as well as the anionically charged
cellulosic
fibres of the fibre stock.
A general relationship between the standard viscosity of the cationic polymer
and
its average molecular weight is given in Table A.
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Table A General relationship between the standard viscosity
and the average
molecular weight.
Standard viscosity Average molecular weight
[mPas] [106 g/mol]
2 1.3
2.5 3.4
3 5.5
3.5 7.6
4 9.8
4,5 11.9
5 14.0
5.5 16.1
6 18.3
6.5 20.4
7 22.5
The relationship shown in Table 1 is based on standard viscosity and intrinsic
viscosity measurements and using Mark-Houwink-Sakurada constants K=2.57.10-4
dl/g and a=0.67.
The cationic polymer may be in liquid form or in dry form, preferably in dry
form as
a particulate material. If the cationic polymer is in dry form, it is
dissolved before its
addition to the fibre stock. Irrespective if the polymer is in liquid form or
dry form, it
is usually diluted with water to a suitable dosing concentration before
addition to the
fibre stock. The dosing concentration may be 0 weight-%, for example 0.01 ¨ 10
weight-% or 0.01 ¨ 3 weight-%, sometimes 0.1 ¨ 3 weight-% or 0.5 ¨ 3 weight-%.
The cationic polymer used in the present invention may be obtained by any
suitable
polymerisation method for copolymerisation of (meth)acrylamide and cationic
monomers. The cationic polymer may be obtained suspension polymerisation, such
as solution polymerisation or gel polymerisation; dispersion polymerisation;
or
emulsion polymerisation. Preferably the cationic polymer is obtained by
solution
polymerisation or gel polymerisation.
The cationic polymer is added to the fibre stock at the latest at the
thickening step
of the fibre stock preparation, where a fibre stock comprising cellulosic
fibres
originating from recycled fibre material and/or broke is thickened from a
first
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concentration to a second concentration by removing a part of the aqueous
phase
from the fibre stock in order to associate the starch with the cellulosic
fibre material.
The thickening step is usually the last stage of the stock preparation,
whereafter the
fibre stock is transferred optionally through various storage towers or the
like and
stock blending to the short circulation of the paper or board machine. The
thickening
step usually employs a thickener, such as a disc filter, vacuum disc filter,
gravity
thickener or the like. Thickening step for recycled fibre material may
typically employ
a disc filter as a thickener and for broke the thickener may be a gravity
thickener.
The first concentration for a fibre stock at the stock inlet of the thickener
may be 0.6
¨ 1.4 weight-%, calculated as dry solids. The second concentration at the
stock
outlet of the thickener may be 2 ¨ 13 weight-%, depending on the thickener
used.
For example, the second concentration at the stock outlet of the thickener may
be
8 ¨ 13 weight-%, as dry solids, for a disc filter, or the second concentration
at the
stock outlet of the thickener may be 2 ¨ 6 weight-%, as dry solids, for a
gravity
thickener. According to one preferable embodiment, the cationic polymer may be
added to the fibre stock at the stock inlet or through a separate feed
connection into
the thickener. The cationic polymer can be added, for example, to the fibre
stock at
the stock inlet of the thickener or to the disc filter chamber.
Alternatively, the cationic polymer may be added to the fibre stock at one of
the
steps preceding the thickening step in the fibre stock preparation, e.g. a
screening
step and/or fibre fractioning step of the fibre stock preparation. In the
present
context, fibre stock preparation includes all process steps needed for forming
recycled fibre material in form of an aqueous fibre suspension, which after
optional
dilution with water is suitable for use for manufacture of paper, board,
tissue or the
like. Fibre stock preparation includes also broke handling, where fibre
containing
waste which is generated prior to completion of the manufacturing process is
repulped. For example, the cationic polymer may be added to the fibre stock
directly
after a pulping step, where the recycled fibre material or broke is
transformed into a
fibre stock. In one embodiment, the cationic polymer may be added into a dump
chest. It is advantageous, but not necessary, that the cationic polymer is
added at
a dosage location that allows some time for the association between the
polymer,
starch and the fibres. However, it has been observed that the addition at the
latest
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at the thickener still provides a significant reduction in starch content of
the aqueous
phase removed from the fibre stock preparation.
It is possible that the cationic copolymer is added to the fibre stock in one
or more
dosage locations. For example, the cationic polymer may be added to the dump
chest and at the inlet of the thickener. If the cationic polymer is added to
the fibre
stock in multiple dosage locations, the dosage amount may vary between the
different locations. The cationic polymer may be added in a first dosage
location in
a first amount and in a subsequent dosage location in a subsequent amount, the
first amount and the subsequent amount being different from each other. For
example, the cationic copolymer may be added to the fibre stock in two or more
dosage locations, preferably in different amounts in each dosage location. It
is also
possible to add cationic polymer in multiple dosage locations, the dosage
amount
being constant at each dosage location.
According to one embodiment of the invention the cationic polymer is added at
least
one additional dosage location, situated after the thickening step and before
a wire
section of a web forming machine. This means that it is possible to add at
least one
additional dose of the cationic polymer to the fibre stock after the
thickening step at
an additional dosage location, which is situated after the thickening step and
before
a wire section of a web forming machine. The cationic polymer added after the
thickening step and before the wire section is preferably the same cationic
polymer
which is added to the fibre stock at the latest at the thickening step. For
example,
the additional dose of the same cationic polymer may be added to a storage
tower
or it may be added to a thick stock, preferably having a consistency >3 weight-
%,
preferably 3 ¨ 6 weight-%. According to a preferred embodiment the additional
dose
of the same cationic polymer is added to the cellulosic fibre stock before a
dilution
step, where the fibre stock is diluted to a third concentration of <2 weight-
%.
According to one embodiment, the additional dose of the same cationic polymer
may be even added to the thin stock having concentration <2 weight-%, in which
case the addition of the cationic polymer may provide in addition of starch
retention
even advantageous effects in total retention and/or drainage.
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In one embodiment of the invention, the fibre stock preparation comprises a
fibre
fractioning step, where for example a long fibre fraction is separated from a
short
fibre fraction, and the cationic polymer is added at least to the short fibre
fraction.
Each of the separated fibre fractions may be separately thickened in separate
5 thickening steps, and the cationic polymer may be added separately to the
separate
fibre fractions. In general, the fibre length of long fibre fraction is longer
than the
fibre length of short fibre fraction measured by Kajaani FSA analyzer using
length
weighted distribution. For example, for a recycled fibre material the long
fibre
fraction may have a fibre length in a range of 1.2 ¨ 1.9 mm and the short
fibre fraction
10 may have a fibre length in a range of 0.8¨ 1.1 mm. Cationic polymer may
be added
both to the long fibre fraction and to the short fibre fraction, or only one
of the
fractions. If cationic polymer is added to both fibre fractions, it is
possible that the
cationic polymer is added in different doses in the long fibre fraction and in
the short
fibre fraction, depending for example on the starch content of the fibre
fractions.
Preferably the cationic polymer is added at least to the short fibre fraction.
According
to one preferable embodiment the cationic polymer is added to both the long
fibre
fraction and the short fibre fraction, wherein the cationic polymer is added
to the
short fibre fraction in an amount that is higher than the amount of the
cationic
polymer added to the long fibre fraction. This means that the dose of the
cationic
polymer to the short fibre fraction is higher that the dose of the cationic
polymer to
the long fibre fraction.
According to one preferable embodiment of the invention an amylase enzyme
inhibitor and/or at least one biocide or biocidal agent is added to the fibre
stock
before or after the addition of the cationic polymer, when the cationic
polymer is
added to the fibre stock at the latest at the thickening step. Preferably at
least an
amylase enzyme inhibitor is added to the fibre stock before the thickening
step,
before or after the addition of the cationic polymer.
The fibre stock comprises cellulosic fibres which originate from recycled
fibre
material and/or broke dispersed in an aqueous phase. The method according to
the
present invention is especially suitable for a fibre stocks where cellulosic
fibres
comprise at least 50 weight-%, preferably at least 70 weight-%, more
preferably 100
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weight-%, of recycled fibre material, calculated from total amount of fibres,
as dry.
According to one embodiment the recycled cellulosic fibre material comprises
at
least 40 weight-%, preferably at least 50 weight-%, of fibre material
originating from
old corrugated containers (OCC).
The fibre stock further comprises starch, which is dispersed with the fibres
in the
aqueous phase of the fibre stock. The fibre stocks comprising cellulosic
fibres
originating from recycled fibre material often comprise significant amount of
starch,
which may originate, for example, from surface sizing of the original paper or
board.
According to one embodiment the fibre stock may comprise starch at least 2
weight-
%, preferably at least 2.4 weight-%, more preferably at least 3 weight-%,
calculated
from the dry solid matter, before the addition of the cationic copolymer. The
fibre
stock may comprise starch up to 10 weigh-% or up to 20 weight-%, calculated
from
the dry solid matter, before the addition of the cationic copolymer.
Especially, if an
effective biocide regime, amylase enzyme inhibitor addition or the like is
employed
in the fibre stock preparation process, the starch content of the fibre stock
may
become high, if not associated with the cellulosic fibres of the fibre stock
by using
the present invention.
The starch dispersed in the aqueous phase of the fibre stock may be low
molecular
weight starch, such as oxidized starch or degraded starch. The starch may
have,
for example, a weight average molecular weight in the range of 30 000 - 5 000
000
g/mol, typically 50 000 ¨2 000 000 g/mol. The starch is usually non-ionic or
slightly
anionic, for example with a charge density from -0.25 to 0 meq/g, or from -0.1
to 0
nrieq/g, measured at pH 7.
According to one embodiment the cationic polymer may be added in total amount
of 0.2 ¨ 1.5 kg/ton, preferably 0.3 ¨ 1.2 kg/ton, more preferably 0.4 ¨ 1
kg/ton, even
more preferably 0.5 ¨ 0.8 kg/ton. If cationic polymer is added in several
dosage
locations, the total amount is the sum of additions in each location.
According to one preferable embodiment of the present invention, the method is
free
of addition steps of synthetic organic coagulant or inorganic coagulant, such
as
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aluminium compounds, iron compounds, bentonite and colloidal silica. This
means
that no synthetic organic coagulants or inorganic coagulants are added in the
stock
preparation before or at the latest at the thickening step.
Some embodiments are described more closely in the following schernatical non-
limiting figures, where
Figure 1 shows a conventional stock preparation process
without chemical
additions;
Figure 2 shows a conventional stock preparation process with
biocide and
amylase enzyme inhibitor additions; and
Figure 3 shows a stock preparation process where cationic
polymer is added
according to one embodiment of the present invention.
Figure 1 shows a conventional stock preparation process without chemical
additions. The full arrows in Figure 1 indicate the fibre stock flow through
the stock
preparation process and the dash lines indicate water flows recycled within or
removed from the stock preparation process. The various stock preparation
stages
and apparatuses are indicated with following reference signs: coarse screening
1,
dump tower 2, fine screening 3, thickening 4; stand pipe 5, storage tower 6,
filtrate
tank 7, and pulper water tower 8. It is assumed that the fibre stock flow A
comprises
100 parts of starch when entering the stock preparation process. The numbers
above the arrows indicate the amount of starch (in parts) in the fibre stock
flow at
that location and the percentages in each stock preparation stage/apparatus
indicate the loss percentage for starch in that stage/apparatus. For example,
it is
seen that before fine screening stage 3 the fibre stock flow comprises 135.7
parts
of starch. The starch loss within the fine screening stage 3 is 2%, which
means that
after the fine screening stage 3 the fibre stock flow comprises 133 parts of
starch. It
is seen from Figure 1 that if no chemicals are added at stock preparation, of
100
parts of incoming starch only 5.3 parts remain after the storage tower 6. This
is a
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significant loss of useful starch material, and it may also lead significant
load on
water treatment, here indicated as relative effluent COD value.
Figure 2 shows a conventional stock preparation process with biocide and
amylase
enzyme inhibitor additions. Same stock preparation stages and apparatuses are
indicated with same reference signs as in Figure 1. Sludge thickening step is
indicated with reference number 9. In the stock preparation process of Figure
2 the
amylase enzyme inhibitor addition to coarse screening stage 1 is indicated
with
arrow I. Amylase enzyme inhibitor is added in order to reduce the degradation
of
starch by amylase enzyme. Addition of one or more biocides into the fibre
stock flow
before storage tower 6 is indicated with arrow B. It can be seen that the
addition of
amylase enzyme inhibitor and biocide(s) reduce the loss of starch within the
stock
preparation process. It is calculated that of 100 parts of starch entering the
stock
preparation process approximately 19.5 parts remain in the fibre stock flow
after the
storage tower. This is a clear improvement to the situation of Figure 1, but
still a
significant amount of starch is lost in the stock preparation process.
Figure 3 shows a stock preparation process where cationic polymer is added
according to one embodiment of the present invention. Same stock preparation
stages and apparatuses are indicated with same reference signs as in Figures 1
and 2. At least one biocide and amylase enzyme inhibitor are added in the same
manner as in Figure 2, indicated by arrows I and B. Furthermore, a cationic
polymer
obtained by copolymerisation of (meth)acrylamide and at least 25 mol- /0 of
solely
cationic monomer(s) is added to the fibre stock flow immediately before the
thickening stage 4. It can be seen that the addiction of the polymer
unexpectedly
increases the amount of starch in the fibre stock flow after the storage tower
6 to
56.5 parts. In practice this implies a major improvement in the process and
enables
significant savings due to increased starch retention as well as reduced COD
load
in the water treatment.
EXPERIMENTAL
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Some embodiments of the present invention are described in the following non-
limiting examples.
Example 1
Example 1 demonstrates the effect of high cationic polyacrylamide, when dosed
to
a fibre stock before the thickening stage, for improving the retention of
starch on the
fibres.
The fibre stock was slushed and diluted as follows:
OCC (Old Corrugated Container) material from a European board mill was soaked
for 5 minutes at 2.5 % consistency at 85 C in artificial process water having
conductivity of 4 mS/cm, pH 7. The ratio of salts in the artificial process
was 70 %
calcium acetate, 20 % sodium sulfate and 10% sodium bicarbonate. After 5 min
soaking, while still hot, the disintegration was performed with laboratory
disintegrator, 30000 rotations, wherein a test fibre stock was obtained.
Amylase
enzyme inhibitor (FennoSpec 1200, 100 ppm) and biocide (FennoSan GL10, 100
ppm) were added to the fibre stock after disintegration. Before the
experiments the
fibre stock was cooled to a room temperature (approx. 22 00) and diluted to
consistency of 1.25 %, with the artificial process water described above.
The filtering at a thickener was modelled by using a Dynamic Drainage
Analyzer,
DDA, equipment. DDA parameters used were
- Wire: 0.25 mm openings
- Vacuum: 300 bar
- Follow-up time: 20 s
The test polymers used were cationic polyacrylamides obtained by
polymerisation
of acrylamide and [2-(acryloyloxy)ethyl] trimethylammonium chloride. Their
properties are shown in Table 1, where charge value gives the amount of
cationic
monomer used in the polymerisation and SV is the standard viscosity of the
test
polymer, measured as described elsewhere in this application.
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The test polymer was dosed to 500 ml of fibre stock prepared as described
above
(1.25 % consistency), 60 min before start of the filtering. Thus obtained
sample was
mixed in a beaker using gentle mixing. 60 s before the start of the filtering
the sample
was poured into DDA's vessel and mixing at 1000 rpm was started. The mixing
was
5 stopped when the filtering was started.
After filtering of the sample, 25 ml of the DDA filtrate was added to 10 ml
HCI (conc.
10 weight-%). The mixture stirred for 10 min in 50 ml beaker with a magnetic
stirrer
and filtered by gravitation in a funnel with black ribbon filter paper. 1 ml
of the filtrated
10 mixture was added to 8.5 ml of deionized water, followed by addition of
0.5 ml iodine
reagent (7.5 g/I of KI + 5 g/I of 12). Starch content in the filtrated mixture
was
determined by a spectrophotometer Hach Lange DR 900 by measuring absorbance
value at 610 nm, 30 seconds after the addition of iodine reagent.
15 Non-ionic degraded starch (C*film 07311) was used as a reference to make a
calibration equation for the starch content.
In the present context, the term "starch retention" is used when the starch
reduction
in the DDA filtrate is compared to the starch amount in the water phase of the
fibre
stock before filtration, and the term "starch retention improvement" is used
to
describe the increase of starch retention obtained by test polymer addition in
comparison to the corresponding measurement without any test polymer addition
(0-test). Even in the 0-test a starch retention of a few percentages was
typically
found.
Starch retention and starch retention improvement were determined by using
equations (1) and (2):
Starch retention = (Abspuip ¨ Abstest)/Abspuip x 100%
(1)
where
Abspuip is the absorbance value of the water phase of the fibre stock sample
before
DDA filtration, without any test polymer addition;
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Abstest is the absorbance value of the DDA filtrate of the same fibre stock
sample
after DDA filtration, alternatively with a test polymer addition to the sample
or no test
polymer addition to the sample (0-test).
Starch retention improvement = (Abs-test ¨ Abspolymer)/AbSO-test X 100% (2)
where
AbSo-test is the absorbance value of the DDA filtrate of a sample without test
polymer
addition (0-test) after DDA filtration; and
Abspolymer is the absorbance value of the DDA filtrate of a sample with test
polymer
addition after DDA filtration
The measured absorbance value results and the improvements in starch
retention,
calculated from the absorbance value results and by using the equations (1)
and
(2), are shown in Table 1. Fibre stock without any test polymer addition (0-
test) gave
absorbance value 1.122, corresponding to starch content of 780 mg/I.
Table 1
Test polymer properties, absorbance values and starch retention
results of Example 1.
Starch retention
Charge SV Dosing Absorbance
Polymer improvement
[mol-%] [mPas] [kg/t]* value
[ /0]
Poly-8 38 4.5 0.2 0.972 13
Poly-8 38 4.5 0.4 0.860 23
Poly-8 38 4.5 0.6 0.759 32
Poly-8 38 4.5 0.8 0.688 39
Poly-11 46 3.0 0.2 1.010 10
Poly-11 46 3.0 0.4 0.848 24
Poly-11 46 3.0 0.6 0.733 35
Poly-11 46 3.0 0.8 0.659 41
Poly-2 (ref.) 17 4.5 0.2 1.051 6
Poly-2 (ref.) 17 4.5 0.4 0.935 17
Poly-2 (ref.) 17 4.5 0.6 0.804 28
Poly-2 (ref.) 17 4.5 0.8 0.738 34
* given kg of polymer per ton dry pulp
It can be seen from Table 1 that cationic polymers obtained by polymerising
acrylamide and 38 mol- /0 or more of cationic monomer clearly improve the
starch
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retention. During the experiments it was also observed that some of the tested
polymers gave improved drainage time in filtration. This indicates that
filtering rate
of thickener might also be improved when tested polymers are added before the
thickening stage.
Example 2
Example 2 demonstrates the effect of high cationic polyacrylamide, when dosed
to
a fibre stock before the thickening stage, for improving the retention of
starch on the
fibres.
The fibre stock was prepared in the same way as in Example 1, except that the
slushing was done at 2.0 % consistency and the fibre stock was diluted to 1.2
%
consistency for DDA experiments.
The test polymers were cationic polyacrylamides cationic polyacrylannides
obtained
by polymerisation of acrylamide and [2-(acryloyloxy)ethyl] trimethylammonium
chloride. Their properties are shown in Table 2, where charge value gives the
amount of cationic monomer used in the polymerisation and SV is the standard
viscosity of the test polymer, measured as described elsewhere in this
application.
The same DDA parameters were used as in Example 1. 700 ml of fibre stock,
consistency 1.2 %, was poured into DDA's vessel 120 s before the start of the
filtering and mixing at 500 rpm was started. Test polymer was dosed 40 s
before the
start of the filtering. The used test polymer dosage was 0.6 kg/ton dry fibre
stock.
The starch content and starch retention improvement were determined in the
same
manner as described in Example 1. The measured absorbance value results and
the calculated improvement in starch retention are shown in Table 2. Fibre
stock
without any chemical addition (0-test) gave absorbance value 0.926.
It can be seen from Table 2 that cationic polymers obtained by polymerising of
acrylamide and high amounts of cationic monomers (30 mol- /0 and above)
clearly
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improve the retention of starch from the water phase to the fibre stock. An
improved
drainage rate was also again observed.
Table 2
Test polymer properties, absorbance values and starch retention
results of Example 2.
Starch retention
Charge SV Absorbance
Polymer improvement
[mol-%] [mPas] value
rycd
Poly-6
17 3.8 0.645 30
(reference)
Poly-2
17 4.5 0.664 28
(reference)
Poly-5 30 3.8 0.62 33
Poly-9 31 4.5 0.595 36
Poly-11 46 3.0 0.622 33
Poly-4 48 3.8 0.613 34
Poly-7 48 4.5 0.579 37
Example 3
Example 3 demonstrates the effect of a high cationic polyacrylamide to the
starch
retention on fibres, when the polymer is dosed before the thickening stage of
fibre
stock and used together with a retention system in the sheet forming stage.
The fibre stock was prepared in the same way as in Example 1 and same OCC
material was used as the raw material. Consistency of the fibre stock was 1.25
%,
conductivity 4 mS/cm and pH 7.
The test polymers were cationic polyacrylamides obtained by polymerisation of
acrylamide and [2-(acryloyloxy)ethyl] trimethylammonium chloride. Their
properties
are shown in Table 3, where charge value gives the amount of cationic monomer
used in the polymerisation and SV is the standard viscosity of the test
polymer,
measured as described elsewhere in this application.
Thickening stage was modelled by adding the test polymer to the fibre stock,
dosage
600 g polymer/ton dry fibre stock, 60 min before the start of the sheet
forming. The
test polymer was dosed to 200 ml of fibre stock (1.25 `)/0 consistency). The
fibre stock
was then mixed for 60 min in a beaker using gentle mixing.
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The sheet forming stage was modelled by using DDA equipment, same DDA
parameters as in Example 1 were used. 60 s before the sheet forming the sample
was poured into DDA's vessel and mixing at 500 rpm was started. To model the
short circulation stage of a paper/board mill, the fibre stock was diluted 30
seconds
before the sheet forming to a consistency of 0.5 % with artificial process
water (as
described in Example 1) which also contained ground calcium carbonate GCC in
amount of 1 g/I, and the mixing was increased to 1000 rpm.
In the experiments the retention system was cationic polyacrylamide Poly-1,
dosage
250 g/ton dry fibre stock, and silica microparticles, added 15 s (Poly-1) and
10 s
(nnicroparticles) before the sheet forming.
The starch content and starch retention improvement were determined in the
same
manner as described in Example 1. The measured absorbance value results and
the calculated improvement in starch retention are shown in Table 3. Fibre
stock
without any chemical addition (0-test) gave absorbance value 0.425.
Table 3
Test polymer properties, absorbance values and starch retention
results of Example 3.
Starch retention
Charge SV Absorbance
Test Polymer system
improvement
[mol-%] [mPas] value
[yo]
3-1
(ref.) (Poly-1)* 9 3.3 0.417 2
3-2 Poly-2+(Poly-1)* 17+(9) 4.1 0.318 25
(ref.)
3-3 Poly-8+(Poly-1)* 38+(9) 4.2 0.299 30
3-4 Poly-11+(Poly-1)* 46-F(9) 3.0 0.274 33
* Retention polymer, in brackets, added to 0.5 % consistency stock
It can be seen from Table 3 that the use of a retention polymer alone (test 3-
1) did
not have any significant impact on starch retention, as starch retention
improved
only 2 %. Addition of cationic polymer at thickening stage, however, improves
starch
retention significantly, even over 30 %. It can also be seen that a high
charge for the
cationic polymer is beneficial for starch retention.
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Example 4
Example 4 demonstrates the effect of a high cationic polyacrylamide to the
starch
retention on fibres, when the polymer is dosed before thickening stage of the
fibre
5 stock and also used as a retention polymer in a retention system in the
sheet forming
stage.
The fibre stock was prepared in the same way as in Example 1 and the same OCC
material was used as the raw material. Consistency of the fibre stock was 1.25
%,
10 conductivity 4 mS/cm and pH 7.
The test polymers were cationic polyacrylannides obtained by polymerisation of
acrylamide and [2-(acryloyloxy)ethyl] trimethylammonium chloride. Their
properties
are shown in Table 4, where charge value gives the amount of cationic monomer
15 used in the polymerisation and SV is the standard viscosity of the test
polymer,
measured as described elsewhere in this application.
Thickening stage and sheet forming stage were modelled in the same manner as
in
Example 3.
The retention system in the experiments included the same test polymer that
was
added at the thickening stage, dosage 200 g /ton dry fibre stock, and silica
microparticles, added 15 s (polymer) and 10 s (microparticles) before the
sheet
forming.
The starch content and starch retention improvement were determined in the
same
manner as described in Example 1. The measured absorbance value results and
the calculated improvement in starch retention are shown in Table 4. Fibre
stock
without any chemical addition (0-test) gave absorbance value 0.398.
Furthermore, Table 4 shows total starch retention values, which indicate
retention
of all the materials in the fibre stock, including fibre material, fillers,
starch, etc. The
total retention experiments were performed with Dynamic Drainage Jar (DDJ).
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Similarly, as with DDA, thickening stage was modelled by adding the test
polymer
60 min before the start of the experiment to 200 ml of fibre stock (1.25 %
consistency). The fibre stock was then mixed for 60 min in a beaker using
gentle
mixing. The sheet forming stage was modelled by using DDJ equipment. 60 s
before
end of the experiment, the fibre stock sample was poured into DDJ's vessel and
mixing at 500 rpm was started. To model the short circulation stage at
paper/board
mill, pulp was diluted to consistency of 0.5 % (5 g/l) 30 seconds before end
of the
experiment with artificial process water (as described in Example 1)
containing 1g/1
of ground calcium carbonate GCC, and mixing was increased to 1000 rpm. 100 ml
of filtrate was collected from DDJ. From the filtrate the consistency was
measured
by filtrating the filtrate through weighed black ribbon filtrate paper. Then,
filtrate
paper was dried and weighed for the consistency calculation (equation 3):
Cfiltrate = (Mafter filtration-Mfilter paper)/(volume of the sample)
(3)
By using the obtained consistency value, a total retention was calculated by
using
equation 4:
Total retention = (Cpuip -Cfi !trate). -pulp X 100%
(4)
In equations (3) and (4), m denotes mass and C denotes consistency.
Table 4 Test polymer properties, absorbance values, starch
retention and total
retention results of Example 4.
Starch
Total
Charge SV Absorbance retention
Test Polymer system
retention
[mol-%] [mPas] value improvement
[0A
[ /0]
]
4-1
89
(Poly-1)* 9 3.3 0.368 7
(ref.)
4-2 Poly-1+(Poly-1)* 9 3.3 0.353 11
90
(ref.)
4-3 Poly-2+(Poly-2)* 17 4.1 0.293 26
91
(ref.)
4-4 Poly-8+(Poly-8)* 38 4.2 0.227 43
92
4-5 Poly-9+(Poly-9)* 31 4.1 0.251 37
92
4-6 Poly-11+(Poly-11)* 46 3.0 0.228 42
91
Retention polymer, in brackets, added to 0.5 % consistency stock
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It can be seen from Table 4 that the use of retention system alone (test 4-1)
did not
have a significant impact on starch retention. Addition of the cationic
polymer with
high charge both at the thickening stage and the sheet forming stage, however,
improves the starch retention significantly. It can also be seen that a higher
charge
for the cationic polymer is beneficial for starch retention. Furthermore, it
can be seen
that even if the cationic test polymers provided also a slight increasing
effect on the
total retention, the starch retention was improved significantly more than the
total
retention. This shows that the present invention where the polymer is dosed
before
thickening stage specifically improves starch retention.
Example 5
Example 5 demonstrates the effect of a high cationic polyacrylamide to the
starch
retention on fibres, when the polymer is dosed before thickening stage of the
fibre
stock. The effect of the molecular weight of the cationic polymer on the
starch
retention was studied.
The fibre stock was prepared in the same way as in Example 1 and the same OCC
material was used as the raw material. Consistency of the fibre stock was 1.25
%,
conductivity 4 mS/cm and pH 7.
The test polymers with the names "Poly-X" were cationic polyacrylamides
obtained
by gel polymerisation of acrylamide and [2-(acryloyloxy)ethyl]
trimethylammonium
chloride, and with a high molecular weight (SV >3 mPas). The test polymer
SPoly
was a cationic polyacrylamide obtained by solution polymerisation of
acrylamide and
[2-(acryloyloxy)ethyl] trimethylammonium chloride, and with a lower molecular
weight (SV 1.2 mPas). The test polymer PVArn was a commercial vinylamine
copolymer. The properties of the test polymers are shown in Table 5, where
charge
value gives the amount of cationic monomer used in the polymerisation of
cationic
polyacrylamides and SV is the standard viscosity of the test polymer, measured
as
described elsewhere in this application.
The experiments were conducted in the same manner using DDA as in Example 1.
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The starch content and starch retention improvement were determined in the
same
manner as described in Example 1. The measured absorbance value results and
the calculated improvement in starch retention are shown in Table 5. Fibre
stock
without any chemical addition (0-test) gave absorbance value 0.977.
Table 5
Test polymer properties, absorbance values and starch retention
results of Example 5.
Starch retention
Charge SV Dosing Absorbance
Polymer
improvement
[mol-%] [mPas] [kg/ti value
[ox]
Poly-1
9 3.8 0.6 0.914 9
(reference)
Poly-2
17 4.5 0.6 0.789 19
(reference)
Poly-4 48 3.8 0.6 0.647 34
Poly-5 30 3.8 0.6 0.665 32
Poly-9 31 4.5 0.6 0.643 34
Poly-11 46 3.0 0.6 0.68 30
Poly-12 57 4.2 0.6 0.647 34
PVAm 30 1.2 0.6 0.854 13
SPoly 46 1.2 0.6 0.872 11
It is seen from Table 5 that test polymers having a higher charge were able to
produce improved starch retention in comparison to test polymers with charge
under
mol-%. However, a high charge of the test polymer does not alone guarantee a
high starch retention, but molecular weight of the polymer also has to be high
enough. It is seen from Table 5 that the commercial vinylamine copolymer,
PVAm,
and cationic polyacrylamide SPoly are polymers with high cationic charge, 30
mol-
15 % and 46 mol%, respectively, but their molecular weights are
rather low. The starch
retention improvement obtained with these polymers was significantly lower. It
can
be thus concluded that both the cationic charge and high enough molecular
weight
of the polymer are important for obtaining the desired high starch retention.
20 Even if the invention was described with reference to what at
present seems to be
the most practical and preferred embodiments, it is appreciated that the
invention
shall not be limited to the embodiments described above, but the invention is
intended to cover also different modifications and equivalent technical
solutions
within the scope of the enclosed claims.
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