Note: Descriptions are shown in the official language in which they were submitted.
CA 02474933 2004-07-30
WO 03/066962 PCT/EP03/01019
Method for preparing fibers contained in a pulp
suspension
The invention relates to a method for preparing fibers
contained in a pulp suspension and/or for preparing
coating color for coated papers.
During papermaking, fillers such as, in particular,
precipitated calcium carbonate (PCC) or comminuted or
ground calcium carbonate (GCC) are usual substances
which are used for the purpose of reducing the fiber
content and of improving the optical properties of the
paper.
The commercially available PCC or GCC fillers are mass-
produced products which are produced in specific
manufacturing operations, which can be associated with
a paper mill as a satellite plant. However, online
production of PCC has never been or is never considered
in the paper industry, which can be attributed to the
special process properties which are necessary for the
production of PCC. Instead, PCC or GCC is transported
to the paper mills as a bulk material or in the form of
a suspension.
Moreover, PCC and GCC fillers are employed as coating
pigments in sizes of 0.3 um and above. Since the small
particles of GCC fillers do not bring with them the
necessary optical properties, Ti02 is added. During
coating, the necessary optical properties can be
achieved by the use of TiOz, but this is a very
expensive and abrasive pigment, which can be up to 10
times as expensive as the PCC or GCC pigments. Since
the optical properties of the GCC and PCC pigments
which are common at present are limited as a result of
the production methods, hitherto TiOz has been used in
order to improve these properties.
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Loading with an additive, for example a filler, can be
carried out, for example, by means of a chemical
precipitation reaction, that is to say in particular by
means of what is known as a "Fiber LoadingT""" process,
as described inter alia in US-A-5,223,090. In such a
"Fiber LoadingT"'" process, at least one additive, in
particular filler, is deposited on the wetted fiber
surfaces of the fiber material. In the process, the
fillers can be loaded with calcium carbonate, for
example. For this purpose, calcium oxide and/or
calcium hydroxide are added to the moist, disintegrated
fiber material such that at least part thereof
associates with the water present in the fiber
material. The fiber material treated in this way is
subsequently treated with carbon dioxide. During the
addition of the calcium oxide and/or of the medium
containing calcium hydroxide to the pulp suspension, a
chemical reaction with an exothermic property proceeds,
the calcium hydroxide preferably being added in liquid
form (milk of lime). This means that the water possibly
deposited in or on the pulps of the pulp suspension is
not absolutely necessary to cause the chemical reaction
to start and proceed.
It is an object of the invention to provide an improved
method of the type mentioned at the beginning.
According to the invention, this object is achieved by
a method for preparing fillers contained in a pulp
suspension and/or for preparing coating color for
coated papers comprising the following steps:
- providing fibers in the form of a suspension with
a predefined solids concentration,
- loading the fibers with a precipitation product,
- grinding the fibers loaded with the precipitation
product to produce precipitation product particles
with maximum dimensions in a range from about 0.05
to about 5 um,
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- crystalline precipitation product particles being
produced and the production of the crystalline
precipitation product particles being carried out
in an online process directly in the stock
preparation line.
In this case, fox the online process at least one of
the following devices and/or means can be used:
cleaning device, in particular HC cleaner, mixing
device, in particular static mixer, lime slaking
device, press, in particular screw press ox belt press,
balancing reactor, crystallizes, further mixing device,
in particular static mixer, C02 supply device or
additional C02 recovery device, optional C02 heater,
optional chemical bleaching agent addition, press water
tank.
The formation of crystalline precipitation product
particles is associated, inter alia, with the advantage
that, if required, relatively high gloss values for the
end product can be achieved.
It is to be noted that, as a rule, only loaded fibers
are ground. The coating color is not ground as a rule
but can be ground. In general, this depends on the
respective definition but also on the respective
crystallization operation. If CaC03 crystals are
produced in the coating kitchen, then there are no
fibers in the suspension, which means that the pump
crystallizes operates only as a highly efficient
chemical reactor or mixer. Of course, a grinding
component could also be provided in the mixing and
reaction process, specifically by the friction of the
particles in the suspension, assisted by the rotor and
the stator.
According to a preferred practical refinement of the
method according to the invention, the press water is
used as dilution water on the crystallizes side.
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The further mixing device can be used in particular for
the fine adjustment of the pH of the pulp suspension,
preferably in a range between 6 and 8.
The first mixing device is preferably used for mixing
the milk of lime into the pulp suspension.
According to a further expedient refinement of the
method according to the invention, the cleaning device
is used to prevent contamination, occurring during the
process, by heavier materials such as in particular
sand, stones and pieces of metal.
Advantageously, at least some of the C02 needed is
provided by a COZ recovery system. Thus, it can be
recovered for example from the flue gas of boilers or
the flue gas of power plants.
According to a preferred refinement of the method
according to the invention, the precipitation product
is calcium carbonate.
During the addition of the calcium oxide and/or of the
medium containing calcium hydroxide to the pulp
suspension, a chemical reaction with an exothermic
property proceeds, the calcium hydroxide preferably
being added in liquid form (milk of lime). This means
that the water possibly incorporated in or on the pulps
of the pulp suspension is not absolutely necessary to
cause the chemical reaction to start and proceed.
During the addition of the calcium oxide and/or of the
medium containing calcium hydroxide to the pulp
suspension, a chemical reaction with an exothermic
property proceeds, the calcium hydroxide preferably
being added in liquid form (milk of lime). This means
that the water possibly incorporated in or on the pulps
of the pulp suspension is not absolutely necessary to
cause the chemical reaction to start and proceed.
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It is possible, for example, to produce precipitation
product particles of rhombohedral form with a
respective cube size in a range from about 0.05 to
about 2 um. In specific cases, it is also advantageous
to produce precipitation particles of a scalenohedral
form with a respective length in a range from about
0.05 to about 2 um and a respective diameter in a range
from about 0.01 to about 0.05 um.
According to a preferred practical refinement of the
method according to the invention, the solids
concentration of the pulp suspension provided is chosen
to be in a range from about 5 about 60% and preferably
in a range from about 10 to about 35%.
It is advantageous in particular if, in order to load
the fibers with calcium carbonate, calcium oxide and/or
calcium hydroxide is/are added to the pulp suspension
and the precipitation is initiated by treating the pulp
suspension with carbon dioxide.
In the case of, for example, loading the fibers with
filler, it is therefore possible for example for
calcium carbonate (CaC03) to be deposited on the wetted
fiber surfaces by calcium oxide (Ca0) and/or calcium
hydroxide (Ca(OH)2) being added to the wet fiber
material, it being possible fox at least part thereof
to associate with the water of the quantity of pulp.
The fiber material treated in this way can then be
treated with carbon dioxide (C02).
The term "wetted fiber surfaces" can cover all the
wetted surfaces of the individual fibers. In this
case, the case is also covered in which the fibers are
loaded with calcium carbonate or with any other desired
precipitation product both on their outer surface and
in their interior (lumen).
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Accordingly, for example, the fibers can be loaded with
the filler calcium carbonate, for example, the
deposition on the wetted fiber surfaces being carried
out by what is known as a "Fiber LoadingT""" process, as
described as such in US-A-5,223,090. In this "Fiber
LoadingT""" process, for example the carbon dioxide
reacts with the calcium hydroxide to form water and
calcium carbonate.
The calcium hydroxide can be supplied to the pulp
suspension in liquid form or in dry form.
According to an expedient practical refinement of the
method according to the invention, the carbon dioxide
is added to the pulp suspension at a temperature in a
range from about -15 to about 120°C and preferably in a
range from about 20 to about 90°C.
The paper produced can therefore contain fillers of the
order of magnitude of about 0.05 to about 5 um, which
means the optical properties of the end product are
enhanced. The filler can be, in particular, calcium
carbonate, which occurs in nature, for example, as
calcite or talc-spar, aragonite and in the rarer form
vaterite. The filler can be composed mainly of the
form calcite, of which over 300 different crystal forms
are supposed to exist. The shape of the filler
particles used can be, for example, rhombohedral with a
respective cube size range from about 0.05 to about 2
um or, for example, scalenohedral with a respective
length in a range from about 0.05 to about 2 um and a
respective diameter in a range from about 0.01 to about
0.05 um, depending on the grade of paper respectively
to be produced.
The filler is distributed well on, around and within
the fibers, which means that no agglomeration of
crystals in bundles is to be encountered. The
respective filler particle, namely the crystal, is
provided on the fiber spaced apart individually or
CA 02474933 2004-07-30
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separated. The filler particle covers the fiber as a
result of deposition on the fiber, by which means the
optical properties of the end product are improved.
The particle size is therefore important in order to
achieve an optimum opacity. A high opacity is achieved
when the color spectrum of visible light is scattered
well. If the color spectrum is absorbed, then the
result is the color black. If the size of the filler
particles falls below 0.2 to 0.5 um, the result is a
tendency to transparency and higher gloss.
In order to achieve the aforementioned results, the
relevant production process for producing the filler
crystals can be configured as follows, for example, and
can have the following variables:
- moist, that is to say not yet dried, pulp or stock
- calcium hydroxide in liquid or dry form
COa
- gas zone
- rotor
- stator
- production of crystals in a gas atmosphere without
the introduction of mixing energy
- mixing with low shear
- no pressure container
The pulp suspension previously mixed with Ca(OH)2 is
put into a fluffer, a refiner, a disperger or the like
at a consistency or solids concentration in the range
f rom about 5 to about 6 0 % , preferably in a range from
about 10 to about 35%. The Ca(OH)2 can be added in
liquid or dry form. The pulp suspension is treated
with C02. The COz can be added, for example, at
temperatures in a range between about -15 and about
I20°C and preferably at temperatures in a range between
about 20 and about 90°C.
CA 02474933 2004-07-30
The pulp suspension passes into the gas zone, where
each individual fiber is subjected to a gas atmosphere,
followed by the precipitation reaction, with which the
CaC03 results directly. The form of the CaC03 crystals
can be, for example, rhombohedral, scalenohedral or
spherical, the quantity of crystals depending in
particular on the selected temperature range for the
pulp suspension and on the COz content and the Ca (OH) 2
content in the pulp suspension. After the pulp
suspension with the crystals formed has passed through
the gas zone, the PCC formed or the pulp suspension
with the crystals in the lumen, on the fiber and
between the fibers is led through a rotor and a stator,
where the distribution of the crystals in the pulp
suspension is completed by mixing with low shear.
While the pulp/crystal suspension is passing the rotor,
a shear distribution occurs which brings about a size
distribution of the crystals from about 0.05 to about
0.5 um and preferably from about 0.3 to about 2.5 um.
The shape of the filler particles used is, for example,
rhombohedral with a respective cube size in a range
from about 0.05 to about 2 um, or scalenohedral with a
respective length in a range from about 0.05 to about 2
um and a respective diameter in a range from about 0.01
to about 0.5 ~zm, depending on the grade of paper to be
produced.
The further the pulp suspension is to strike the rotor
disk, the lower is the shear, depending on the H20
added for the purpose of dilution. The concentration
of the pulp suspension passing the rotor disk is about
0.1 to about 50°s and preferably about 35 to about 50%.
The pressure acting on the C02 feed line is in
particular in a range from about 0.1 to about 6 bar,
and preferably in a range from about 0.5 to about 3
bar, in order to ensure a constant COZ supply to the
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gas ring for the desired chemical reaction. Just like
the case of water supply via a garden hose, the
pressure has to be increased when there is a high
demand for water, in order to deliver more through the
hose. Since the COz is a compressible gas, the
quantity required can also be increased in order to
ensure a complete reaction. The COz supply and
therefore the precipitation reaction bringing forth the
CaC03 can be controlled and/or regulated via the pH.
For instance, it is possible to consider pH values in a
range from 6.0 to about 10.0, preferably a range from
about pH 7.0 to about 8.5, for the final reaction of
the CaC03 crystals. The energy used for this process
can lie in particular in a range between about 0.3 and
about 8 kWh/t and preferably in a range between about
0.5 and about 4 kWh/t. Dilution water can be added and
mixed with the pulp suspension in order to obtain a
final dilution at which the pulp suspension with filler
produced has a consistency or solids concentration in a
range from about 0.1 to about 16%, for example,
preferably in a range from about 2 to about 6%. The
pulp suspension is then exposed to the atmosphere in a
machine, in a container or the next process machine.
The rotational speed at the external diameter of the
rotor disk can lie in particular in a range from about
20 to 100 m/s and preferably in a range from about 40
to about 60 m/s.
The gap between rotor and stator is, for example, about
0.5 to about 100 mm and preferably about 25 to about 75 mm.
The diameter of the rotor and of the stator can lie in
particular in a range from about 0.5 to about 2 m.
The reaction time is preferably in a range from about
0.001 to 1 min., preferably in a range from about 0.1
to about 10 sec.
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The method described above permits the production of
individual particles, which are spaced apart equally
from one another and are deposited onto the fibers,
covering the f fibers in the required manner in order to
satisfy the requirements of the desired high level of
white or glossy paper. The particle size preferably
lies in a range from about 0.05 to about 5 um, the
preferred size for the rhombohedral form of a cube
lying in a range from about 0.05 to about 2 um or, for
a scalenohedral form, in a range from about 0.05 to
about 2 um with respect to the length and a range from
about 0.01 to about 0.5 um with respect to the
diameter. For high gloss applications, the particle
size should expediently lie below 0.2 to 0.5 um.
In particular, therefore, an online process for the
production of filler particles consisting of
precipitated calcium carbonate directly in the stock
preparation line is specified.
The advantages of the filler particles obtained consist
inter alia in the following:
- It is now possible to distribute the requisite
filler particles uniformly over the fiber surface,
which means that the best optical properties are
achieved online in the stock preparation, it being
possible for the filler level achieved to be below
or above 4 0 °s .
- Since filler particles are also embedded within
the fiber lumen, the tendency to blackening as a
result of calendering is considerably reduced.
- A new way of incorporating pigments is created, in
order to achieve the desired optical properties
and the desired printability in and on the paper
sheet directly during the paper production and not
during the coating process. In the present
exemplary embodiment, the coating process can
therefore be provided only for the fine adjustment
CA 02474933 2004-07-30
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of the properties of the paper surface.
Alternatively or additionally, however, exerting a
corresponding influence in the coating process is
also conceivable.
- Since the filler particles are incorporated in the
fibers, they can no longer be washed out in the
wire section or fourdrinier section of a
papermaking machine, so that it is not necessary
to deal with these particles in the same way as in
connection with the GCC or PCC particles normally
used by means of the coating process, which means
that coating particles can be saved, which can
lead to higher machine speeds, since a lower
amount of coating color has to be applied.
- Since the filler particles are deposited on the
fibers in an online process, that is to say are
crystallized in the pulp preparation system,
economic advantages can be achieved as a result of
savings in retention aids, fibers and sludge, the
reduction of the white water contamination and the
saving of energy and raw material.
- The production of high gloss paper with the filler
particles formed is possible.
- Since the precipitated filler particles are
scouring or abrasive to a lower extent, a longer
lifetime of the coating equipment and of the paper
machine felts and fabrics can be assumed.
- The use of Ti02 can be reduced, since a higher
whiteness and better optical properties can be
achieved.
The method according to the invention can be used in
particular for coating color for coated papers as well.
The PCC production can be part of the coating process,
it being possible again to form the aforementioned
crystal forms.
In this case, it is in particular possible to influence
an online coating machine between the predrying and
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afterdrying, as well as the use of a coating device or
converting machine, independently of the papermaking
machine in the following manner:
- Less TiOz has to be used.
- The paper surface is improved by small crystals.
Less coating slurry is needed.
- The result is better printability, since the
fibers are covered uniformly with crystals.
- Since the fibers are covered uniformly with
crystals, the water absorption and the oil
absorption are also reduced.
- In addition, the wear in the coating equipment and
in the papermaking machine is reduced if online
coating is carried out.
Consequently, the method according to the invention can
in particular also be applied in combined form in a
coating machine and a papermaking machine.
In principle, both offline and online operation are
conceivable.
As distinct from the conventional PCC fillers,
according to the invention special crystal forms are
produced which, inter alia, can be changed in the
desired manner, for example even during the coating
process.
The possible paper grades coming into question include,
amongst others:
Printing and writing papers:
- These can be produced from newsprint.
- Woody or woodfree coated printing and writing
papers.
- Uncoated woody or woodfree printing and writing
papers.
Paper grades determined by groundwood or chemical pulp:
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- Known as woody paper grades with groundwood or
chemical pulp in a range from 25% to 100%.
Chemical pulp is added in order to increase the
strength and the runnability of coating machines
and papermaking machines, etc.
Newsprint grades:
- Can contain up to 100% recycled fibers or up to
100% groundwood or chemical pulp, which can be
either mechanical groundwood, thermomechanical
pulp (TMP), pressure groundwood pulp (PGB), or
CTMP (chemithermomechanical pulp). The use of
chemical pulp can reach as far as 30%. The use of
recycled fibers (RCF) can raise the filler
content.
SC papers:
- These are paper grades which are determined by the
use of chemical pulp and can have a filler content
of up to 30%.
Coated paper grades:
- These paper grades are determined by mechanical
pulp, that is to say mechanical groundwood or
chemical pulp, up to 100%.
Chemical pulp grades:
- These contain up to 10% mechanical pulp. Both
hardwood and softwood chemical pulps are used.
Copy paper:
- This is composed of up to 90 to 100% new chemical
pulp fibers but can contain up to 100% recycled
fibers; a filler content up to about 30% can be
provided.
Printing and writing papers:
- These can be produced from newsprint.
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- Woody or non-woody coated printing and writing
papers.
- Woody and non-woody uncoated printing and writing
papers.
Board grades:
- These contain a top layer made from a mixture of
bleached hardwood (up to 90%) and bleached
softwood (up to 30%) , the top layer or the bottom
layer being coated. It can also be employed in
the underlayer, which can contain mixtures of
deinked pulp, OCC and computer printouts. The
middle layer contains, for example, a mixture of
waste and production broke, while the base layer
can contain unbleached softwood and production
broke as well as OCC.
The method according to the invention can be
implemented, for example, in a "Fiber LoadingT""" system
of the type reproduced in the single figure of the
drawing.
According to said drawing, at least one of the
following devices and/or means can be used for the
online process: cleaning device 10, in particular HC
cleaner (high consistency cleaner), mixing device 12,
in particular static mixer, lime slaking device 14,
press 16, in particular screw press or belt press,
balancing reactor 18, crystallizer 20, further mixing
device 22, in particular static mixer, COZ supply
device 24 or additional COz recovery device, optional
CO2 heater 26, optional chemical bleaching agent
additions, press water tank 28.
The cleaning device 10 or an equivalent device is
preferably equipped with at least one mechanism which
carries out a protective function.
The mixing device 10 and the further mixing device 22
can, in a further refinement, also be constructed in
accordance with the apparatus disclosed in German laid-
CA 02474933 2004-07-30
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open specification DE 41 25 513 A1 for mixing suspended
pulp. An apparatus of this type comprises an
introduction line for a suspended pulp ("thick stock"),
which opens into the wall of a section of pipe, in
particular a curved section of pipe, which carries thin
stock. The speed at which the thick stock flows out of
the introduction line is in this case preferably at
least three times the speed of the thin stock flowing
in the opening area. Furthermore, the introduction
line preferably opens in the central area of the
section of pipe.
In a further refinement, the mixing device 10 and/or
the further mixing device 22 can be equipped with or
without a known buffer chest.
In the figure, it is also possible to see a control
valve 28 provided in a line to the cleaning device 10,
a lime pump 30 provided between the lime slaking device
14 and the first mixing device 12, a press water pump
32 provided between the press water container 28 and
crystallizes 20, a mixing container 34 and also a COZ
pump 36 provided between the C02 supply 24 and the C02
heater 26.
CA 02474933 2004-07-30
WO 03/066962 PCT/EP03/01019
List of designations
Cleaning device
12 First mixing device
14 Lime slaking device
16 Press
18 Balancing reactor
Crystallizer
22 Further mixing device
24 C02 supply device or additional C02 recovery device
26 Optional C02 heater
28 Press water tank
Lime pump
32 Press water pump
34 Mixing container
3 6 COz pump
