Note: Descriptions are shown in the official language in which they were submitted.
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RPS12761
Process for loading a fibrous slurry with filler
The invention relates to a process for loading a
fibrous slurry according to the precharacterizing
clause of Claim 1.
A number of processes from fibre loading technology for
loading pulp fibres with calcium carbonate are already
known. In US 5,223,090 a process is described in which
fibrous material with elongated fibres having a cell
wall surrounding a cavity is used, the fibres having a
moisture content which is sufficient to form a
dewatered suspension of a pulp. In this case, the
fibres have a moisture content which corresponds to a
proportion of 40 to 50% of the weight of the fibres.
The water is present substantially in the interior of
the fibres and within the fibre walls. Alternatively,
calcium oxide or calcium hydroxide is then added to the
pulp, so that at least some of the calcium oxide or
calcium hydroxide introduced will associate with the
water present in the pulp. The fibrous material is then
brought into connection with carbon dioxide, and
simultaneously subjected to a shear mixing process, in
order to produce a fibre material having a considerable
quantity of calcium carbonate in the hollow interiors
and within the fibre walls of the pulp fibres as a
result of precipitation.
DE 102 04 254 Al discloses a further process for
loading a fibrous slurry. The process comprises the
following steps: introduction of calcium hydroxide in
liquid or dry form or of calcium oxide into the fibrous
slurry, introduction of gaseous carbon dioxide into the
fibrous slurry, precipitation of calcium carbonate by
the carbon dioxide.
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It is an object of the invention to devise a process
for loading a fibrous slurry in which the advantages of
the loading process are increased, possibly a
considerably higher loading effect can also be
achieved.
According to the invention, this object is achieved by
the measures named in Claim 1.
By means of this process, it is possible to produce
fibrous material loaded with calcium carbonate (FLPCC =
fibre loaded precipitated calcium carbonate), in
particular for pulp production or for the use of pulp
in papermaking. The raw fibrous material to be loaded
is produced, for example, from recycled paper, from DIP
(= deinked paper), from secondary fibrous material,
bleached or unbleached chemical pulp, groundwood of any
type, any paper raw chemical pulp, bleached or
unbleached sulphate pulp, finished material broke,
linen, cotton and/or hemp fibres (predominantly used
for cigarette paper) and/or any other raw paper
material which is used in a paper machine. The process
can be employed irrespective of the raw fibrous
material to be loaded.
The area of application of the invention extends to the
production of paper and pulp and the process technology
including the filler produced, and comprises areas of
application in all grades of paper including packaging
paper grades and the broke accumulating during
production, which has a filler content of between 1%
and 60% and/or has a white top layer with a filler
content of between 1% and 60%. The filler content can
preferably lie between 5% and 50%.
If a fibrous slurry is treated with fibre loading
technology during the paper production, this results in
a completely novel product, which has new and improved
properties as compared with the products known on the
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market. Such a process makes it possible to precipitate
filler (calcium carbonate) directly during stock
preparation in a paper mill, the said filler being
distributed uniformly and deposited exclusively on and
in the fibrous material, in particular the paper
fibres.
In the process according to the invention, the
fractionation can beneficially be carried out with
hydrocyclones, which produce a high centrifugal field
which, for example, corresponds to 200 times the effect
of the earth's force of gravity.
Another possibility lies in the application of pressure
screens, in particular if they are designed as fibre
fractionators, that is to say are provided with fine
screen openings. Of course, multi-stage separation
systems and/or combinations of various separation
principles are also conceivable.
The process according to the invention is carried out
particularly advantageously such that the different
effect of the loading of the fibrous material
suspension is either utilised or, by means of
appropriate measures, is partly or completely
compensated again. In the loading process on which this
invention is based, for example a fibrous material
suspension composed of various types of fibres is
formed. On the basis of the different properties, it is
possible that the loading is not the same in all the
participating fibres. In addition, a statistical
influence is conceivable such that, on the basis of
parameter fluctuations during the precipitation
process, be they temporal or local, the precipitation
is not uniform. The different level of loading, that is
to say the quantity of precipitated crystals adhering
to the fibres, permits fractionation following the
loading process, in which at least two fractions with a
different level of loading are formed. Since it is to
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be assumed that more highly loaded fibres are heavier
than less highly loaded ones, the designations heavy
fraction for fibres with a higher level of loading and
light fraction for fibres with.a lower level of loading
are chosen here.
Depending on the requirements on the process, two
possibilities are conceivable in order to treat the
different fractions further.
The first possibility is to carry out the loading
process again by means of recirculation of the light
function in a part of the plant which is located before
the loading, which is advantageous in particular, for
example, when these fibres carried in the light
fraction are able to achieve a significantly higher
level of loading as a result of being loaded once more.
The reason may be that, on account of the fibre
properties, a longer period for loading or a higher
concentration of the chemicals would be more
beneficial. In such cases, it is more economical not to
tune the process to the "heaviest fibres" but to
optimise it economically as a whole.
The second possibility is to use the fractions
performed specifically differently during the paper or
board production. As is known, during the production of
paper for the individual grades and, possibly, also
layers on the paper machine, different quality
requirements are set. It is therefore possible to
improve the entire process of paper production by means
of the fractionation.
As compared with conventional processes for the
production of a fibrous slurry, the loading makes it
possible to achieve a higher level of refining with
energetic benefits; for example, up to 50% of the
refining energy can be saved. This has an effect in
particular on all paper grades which pass through a
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refining process during the production and, above all,
in those which have a high or very high level of
refining, such as FL cigarette papers (FL = fibre
loaded), FL printing and writing paper, FL sack kraft
papers and FL filter papers. In these papers, which do
not need fillers, free filler which is not deposited in
or on the fibres can be removed after the refining
process or before the introduction of the fibrous
slurry into the machine chest or before the supply to
the paper machine. The fibres themselves are, however,
provided with filler on the outside and/or inside and
outside, so that the positive effects of the fibre
loading technology are maintained.
The loading has a positive effect on the production of
all paper grades since, as a result of process-induced
mechanical loadings in the various sections of the
paper machine, such as in the press section, in the
drying section or in the region in which the paper web
is reeled up, the intermediate product produced and the
end product to be produced is highly loaded
mechanically by the use of reeling, winding, rewinding
and converting machines.
A further advantage in using the technology according
to the invention in the paper grades listed above is
that these can also be processed further in a calender.
The fact that, when the fibre loading technology is
used, fibre loading particles are deposited in, around
and on the fibres means that blackening, i.e. calender
blackening, is avoided.
In the following text, a typical example of the loading
of a fibrous slurry will be described:
The starting material used is aqueous fibrous material,
in particular aqueous paper stock, from 0.1 to 20%
consistency, preferably between 2 and 8%. Calcium
hydroxide in aqueous or in dry form or calcium oxide is
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mixed into the aqueous paper stock in a range between
0.01 and 60% of the proportion of solids present. For
the mixing operation, a static mixer, a mixing chest or
a pulper system is used; in this case, a pH in the
range between 7 and 12, preferably between 9 and 12, is
used. The reactivity of the calcium hydroxide is
between 0.01 and 10 minutes, preferably between
1 second and 3 minutes. Dilution water is mixed in in
accordance with predefined reaction parameters.
Carbon dioxide is mixed in in accordance with the
reaction parameters in a wet paper stock dimension. In
the process, calcium carbonate is precipitated in the
carbon dioxide atmosphere.
The process temperature preferably lies between -15 C
and 120 C, in particular between 20 and 90 C.
Rhombohedric, scalenohedric and spherical crystals are
preferably produced, the crystals having dimensions
between 0.05 and 5 pm, in particular between 0.3 and
2.5 pm.
In order to produce a fibrous slurry loaded with
calcium carbonate, static and/or movable, in particular
rotating, mixing elements are used.
The process is preferably carried out in a pressure
range between 0 and 15 bar, in particular between 0 and
6 Bar. Likewise, the process is advantageously carried
out at a pH between 6 and 10, in particular between 6.5
and 9.5. In this case, the reaction time is between
0.03 seconds and 1 minute, in particular between 0.05
and 10 seconds.
It is possible, for example, for precipitation product
particles of rhombohedral form with a respective grain
size in a range from about 0.05 to about 2 pm to be
produced. In specific cases, it is also an advantage to
produce precipitation product particles of a
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scalenohedric 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 m.
According to a preferred refinement of the process
according to the invention, the solids concentration of
the fibrous slurry provided for precipitation is chosen
to lie in a range from about 0.1 to about 60% and
preferably in a range from about 15 to 35%.
According to an expedient practical refinement of the
loading process, the carbon dioxide is added to the
fibrous slurry at a temperature in a range from about -
to about 120 C and preferably in a range from about
15 20 to about 90 C.
The invention and its advantages will be explained by
using drawings, in which:
Fig. 1 shows a schematic drawing of a plant to
illustrate an example of the process according
to the invention;
Fig. 2 shows a variant of the process.
In Fig. 1, a plant having the most important process
steps and apparatuses is illustrated by way of example.
In this plant, the fibrous material P, for example
chemical pulp or deinked wastepaper, together with
water W, is put into a pulper 10. This pulper 10 can be
constructed conventionally, that is to say contain a
pulper screen 11 in its bottom region, and also a
pulper rotor 12, which ensures circulation in the
pulper and keeps the pulper screen 11 free of
blockages. The fibrous material pulped in this way is
fed to a number of cleaning apparatuses in the example
shown here, which are illustrated symbolically as a
stock cleaner 14 and a stock screen 15. There then
follows a screw press 16, in order to raise the
consistency of the fibrous slurry S again. The press
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filtrate 18 from the screw press 16 can be sent back
into the pulper 10 for pulping. The thickened fibrous
material 19 then passes into a crystalliser 20 which,
with the aid of carbon dioxide gas 24 that is supplied,
is used to trigger the desired precipitation of calcium
carbonate in the form of finely distributed filler. The
carbon dioxide gas is provided by a carbon dioxide
supply 21 and can be brought to the desired temperature
in a heat exchanger 22 with the aid of a cooling or
heating medium 23. However, there are also other
possible ways of controlling temperature, for example
the direct addition of steam, if an increase in the
temperature is expedient. The loaded fibrous slurry,
possibly diluted with dilution water 27, is collected
in a storage chest 26.
Then, directly or after intermediate treatment, such as
refining, there follows fractionation of the loaded
fibrous slurry S' . To this end, by way of example and
advantageously, a cleaner plant with highly effective
hydrocyclones 13 is used here. These form a heavy
fraction 2 and a light fraction 1. Fibres which are
more highly loaded with adhering fillers are
specifically heavier than less highly loaded fibres.
They can also be stiffer. The fractionation carried out
in accordance with the invention, in particular when
sedimentation or centrifugal forces are used, makes use
of these differences. It is therefore possible to
assume that the heavy fraction 2 contains fibres more
highly loaded with fillers than the light fraction 1.
Here, the light fraction 1 is recirculated into the
loading process, that is to say led into the feed of
the screw press 16. The heavy fraction 2 is made
available as a fibrous slurry 28, for example to a
paper machine, not shown.
The part of the plant shown in Fig. 1 for loading with
fillers, in particular calcium carbonate, is to be
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viewed only as a simplified example. In many cases,
bleaching can also be integrated. More details relating
to carrying out the loading can be seen from the
publications mentioned at the beginning. The loading
process is advantageously operated as an online process
in the paper mill.
As already mentioned, the fractionation of the loaded
fibrous material can also be used to produce different
stock qualities or to provide different qualities
during paper production. For this procedure, Figure 2
shows a schematic drawing of a plant as an example, in
which, however, the pulping and the pre-cleaning of the
fibrous slurry are no longer illustrated. These can
correspond to Figure 1 or to comparable parts of the
plant known per se. With the aid of the process
according to the invention, it is possible for example
to manage the light fraction 1, that is to say that
with the less loaded fibres, separately from the heavy
fraction 2 as far as the headbox 32 of a paper or board
machine. In this case, the addition of further
components 30, 31 (fibrous materials or additives)
normally has to be carried out. It is also possible
(not shown here) to process, for example to refine, the
fractions differently.
The headbox 32 in this example forms three layers. The
light fraction 1 is advantageously led into the inner
layer and the heavy fraction 2 into the outer layers,
in particular when the higher strength and/or the
better retention of the more highly loaded fibres are
to be used. A plurality of layers can also be formed
with the aid of a plurality of headboxes.
The stock approach system of the paper machine is not
shown here.
