Note: Descriptions are shown in the official language in which they were submitted.
CA 02443065 2003-09-30
SMB
A Process for the Recycling of Cleaner Rejects
The present invention relates to a process for recycling cleaner rejects from
the
preparation of paper, paperboard and cardboard and to their use for the
prepara-
tion of paper in the paper stock or as a coating slurry for the paper
industry.
In the preparation of paper, the raw material, i.e., wood pulp, wood, fine
straw
pulp or rag pulp, is also admixed with repulped half-stock, fillers and
pigments in
order to achieve a closed surface and thus to improve the properties of the
paper,
especially the whiteness, opacity and printability.
Almost all papers are admixed with fillers which confer a uniform formation,
improved softness, whiteness and touch especially to printing and writing
papers.
These fillers, mostly called "ashes" since they remain as ashes in the
combustion
analysis, are either added to the fiber suspension or applied in the coating
step.
Uncoated papers contain up to 35% by weight of fillers, coated papers contain
from 25 to 50% by weight thereof. The amount of fillers employed is highly
dependent on the intended use of the paper. Highly filled papers have a lower
strength and poorer sizing properties.
The filler content in the paper stock is usually between 5 and 35% by weight
and
consists of primary pigments or recycled coating pigments which may be derived
from coating residuals or from coated rejects. In addition to the whiteness of
the
filler which is important for whitened papers, its grain size plays an
important role
since it has a strong influence on the filler efficiency and the physical
properties of
the paper, in particular porosity. The proportion of filler remaining in the
paper is
between 20 and 80% of the amount added to the fiber suspension.
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As judged by their consumption, the following products have rather great impor-
tance today as fillers and coating pigments: china clay, calcium carbonate,
precipitated calcium carbonate (PCC), artificial aluminum silicates and oxide
hydrates, titanium dioxide, satin white, talcum and calcium silicate.
Subsequently to their coating with coating slurries, papers are often
smoothed,
after drying, with a blade. The so-called cleaner rejects obtained therein,
also
referred to as coated rejects, mainly consist of minerals and fibers and may
comprise from 1 to 4% of the total volume of the paper produced. In most
cases,
they are disposed of in dumps. Thus, for example, it has been proposed to
compress and compact the cleaner rejects by screw extruders in order to reduce
their volume. However, the total amount of the cleaner rejects obtained and
passed to disposal is not reduced.
In the past, another possibility of treating the cleaner rejects was to partly
process
them using centrifugal cleaners in order to separate the fibers, on the one
hand,
and/or the coating pigments and fillers, on the other hand, and reuse them at
least
in part.
It is the object of the present invention to provide a new environmentally
friendly
concept for treating the cleaner rejects, especially from centrifugal
cleaners, and to
enable them to be recycled as completely as possible.
In particular, the object of the invention is to provide a process for the
recycling of
cleaner rejects while energy costs and costs of raw materials, shipping costs
as
well as costs for dumping are saved.
According to the invention, the above object is achieved by a process for
recycling
cleaner rejects, especially from centrifugal cleaners, from the preparation of
paper,
paperboard and cardboard, characterized in that cleaner rejects, which contain
fibers, coating pigments and/or fillers, are milled to the desired grain size
distribu-
tion, optionally with the addition of water, coating pigment, fresh filler
and/or filler
slurry, and employed as a raw material for the paper stock and/or coating
slurry.
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The cleaner rejects, which are usually solid, contain valuable raw materials
which
can be recovered by means of the present invention and, after milling,
recycled to
various processes of papermaking. Especially the cleaner rejects from paper
mills
which prepare coated paper or cardboard grades contain large quantities of
coating
pigment flakes. The thicker the coating layer, the more difficult is the
recovery of
the flakes by the prior art technologies. Double-coated grades or coated
cardboard
grades may be mentioned here as particular examples. Another group of paper
industries are the SC-paper producing plants, which lose large amounts of
fillers
through cleaner rejects.
Now, by means of the present invention, the cleaner rejects are milled and
optionally admixed with water, coating pigment, filler and/or filler slurry.
The
cleaner reject slurry or the milled powder is then recycled to the papermaking
process for the paper stock and/or as a coating slurry.
On a closer inspection of the cleaner rejects, fractions of fillers, coating
pigment
flakes, fiber suspension residuals, such as fiber lumps or splinters, as well
as
foreign matter, for example, sand, can be discovered.
After the milling of the cleaner rejects, the distribution of the particle
sizes of the
cleaner rejects is essentially the same as the original distribution in the
coating
pigments, fillers or filler/pigment slurries. Thus, at the same time, by means
of the
present invention, more than 95% of the total cleaner rejects, or even the
full
amount thereof, can be recovered and recycled to production.
Then, the cleaner rejects processed according to the invention serves, for
example,
as a filler, whereby the consumption of fresh filler or pigment can be reduced
consistently.
According to the invention, the milling of the cleaner rejects is preferably
installed
in a centrifugal cleaner system downstream from the last of usually several
stages
of a centrifugal cleaner. The accepted stock from the additional cleaner
system is
recycled to the previous stage, and the residual amount of the cleaner rejects
forms the final reject.
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In an existing cleaner system, the simplest application area is intended to
com-
prise installing the milling after the last but one cleaner stage and
employing the
last cleaner process as a final stage.
When the process according to the invention was performed, it was found that
the
distribution of particle sizes of the coating pigment flakes had been refined
and
that the original distribution of the pigment particles had been achieved.
This could
be seen from the quantity of finer particles, especially in the differential
distribu-
tion. The fiber knots were extracted and predominantly refined.
In papermaking, it is usual to employ the fillers and coating pigments either
as
powders or in the form of concentrated slurries with a solids content of
preferably
from 30 to 85% by weight for the paper stock or for the preparation of coating
slurries. The fresh fillers and pigments are usually supplied by the
manufacturers
with the desired whiteness and grain size distribution as a slurry or powder.
In the processing of cleaner rejects, it is of course required to separate and
discard
the coarse dirt contents consisting of splinters, sand grains and other
impurities.
For this purpose, multistage centrifugal cleaners are usually employed. The
screenings thus obtained consist of fibers, fillers, pigments, fine sand,
black
particles and agglomerates of fillers and pigments, or pigments, fibers and
fillers.
"Filler" usually means the fine particles employed in the paper stock;
"pigment"
means the fine particles employed in the coat.
Known methods which suggest themselves for the separation of the undesirable
contents of the cleaner rejects are flocculation and sedimentation, filtering,
screening and/or centrifuging. In this case, a mixture of different pigments
is
usually present which often contains china clay, calcium carbonate and talcum.
Agglomerates frequently form during the separation processes due to
flocculation
and charge reversal; they can now be milled according to the invention.
According to the invention, the milling into powders or slurries can be
performed
continuously or discontinuously in usual dry mills or wet mills, especially
agitator
ball mills, for example, having a content of from 700 to 5000 1 or more.
Milling
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media, preferably milling balls, especially having a diameter of from 1 to 4
mm,
are used. I
Screens, preferably sieve bends, for separating impurities (ball crushings,
separat-
ing materials, rust etc.) are usually used for processing. Laser measuring
instru-
ments serve to determine and control the milling fineness during the milling
process and for the computer-based control of the agitator ball mill system.
Therefore, for the preparation of new coating slurries or also, optionally,
for use in
the paper stock, it may be required to enhance the whiteness by per se known
methods. The disruption of agglomerates which adversely affect the flowing
properties of a coating slurry at the blade by forming doctor streaks and
adversely
affect the properties of the coat is particularly preferred. Dispersing aids,
fiilers and
pigments additionally employed in the milling process reduce the overall
consump-
tion of these materials.
In the cleaner rejects, the ratio of fillers and/or pigments to fibers can
vary widely.
It is particularly preferred according to the invention to employ cleaner
rejects
having an optionally enriched concentration of fillers and/or coating pigments
within a range of from 5 to 90% by weight, especially from 30 to 70% by
weight,
based on the solids content. Thus, the fiber content, on the one hand, or the
content of filler and/or coating pigment may vary, for example, from 1 to 99%
by
weight, or from 99 to 1% by weight. "Enrichment" within the meaning of the
present invention especially comprises the addition of fresh filler and/or
fresh
pigment.
Therefore, according to the present invention, china clay, natural or
precipitated
calcium carbonates, artificial or natural aluminum silicates and oxide
hydrates,
titanium dioxide, satin white, dolomite, mica, metal flakes, especially
aluminum
flakes, bentonite, rutile, magnesium hydroxide, gypsum, sheet silicates,
talcum,
calcium silicate and other rocks and earths are preferably used as the fresh
pigment and/or fresh filler before, after and/or during the milling.
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The fresh pigment or fresh filler is preferably employed as a powder, aqueous
fresh-pigment containing and/or fresh-filler containing slurries in the
presence of
the cleaner rejects and optionally usual milling aids and/or dispersing aids
to give a
slurry with a solids content of, for example, from 30 to 85% by weight,
especially
from 40 to 75% by weight.
Preferably, the added amount of fibers, coating pigments and/or fillers
(recycled or
fresh) is from 1 to 100% by weight, based on the cleaner rejects.
Particularly preferred is the addition of water in order to employ the cleaner
rejects
milled according to the invention in a diluted slurry for the stock flow.
Slurries
having a solids content of from 5 to 50% by weight, especially from 10 to 20%
by
weight, are preferably employed. Accordingly, in a preferred embodiment accord-
ing to the invention, the milling is performed in the presence of water in an
amount of from 1 to 1000% by weight, especially from 100 to 1000% by weight,
based on the solids content.
The cleaner rejects are preferably milled to a slurry or powder having a grain
size
distribution of
from 10 to 99% by weight of particles < 1 pm, especially
from 10 to 95% by weight of particles < 1 pm,
respectively based on the equivalent diameter.
From EP 0 625 611 Al, grain size distributions for coating pigments are known
which are also preferably adjusted by means of the present invention. Thus, it
is
particularly preferred according to the present invention for the pigments to
have
the following grain size distribution:
a) from 95 to 100% by weight of particles < 10 pm;
b) from 50 to 100% by weight of particles < 2 pm, especially from 50 to 95% by
weight of particles < 2 pm;
c) from 27 to 95% by weight of particles < 1 pm, especially from 27 to 75% by
weight of particles < 1 pm; and
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d) from 0.1 to 55% by weight of particles < 0.2 pm, especially from 0.1 to 35%
by weight of particles < 0.2 pm;
respectively based on the equivalent diameter of the particles.
In addition, according to the invention, a broad variation of the whiteness
and
grain size distributions is possible, which can be controlled, in particular,
by the
manner and duration of milling. Thus, it is possible to mix a relatively
coarse fresh
filler and water in situ with a large amount of cleaner rejects for
introducing this
slurry into the paper stock after milling. In the same way, it is possible to
use a
smaller amount of cleaner rejects and to perform a finer milling with fresh
pigment
in situ which is then used as a coating pigment, especially in precoating.
Even though per se known wetting agents, stabilizers, milling aids and
dispersing
aids may be employed according to the invention during the mixing and milling
of
the cleaner rejects, as known, for example, from EP 0 625 611 Al, the quantity
thereof required is clearly reduced according to the invention as compared to
the
prior art. The cleaner rejects already contain some amount of the mentioned
agents which are recycled to the papermaking process according to the
invention.
The powders or slurries obtainable according to the present invention may be
employed to particular advantage in the paper industry, especially for the
prepara-
tion of a coat for paper coating or in the paper stock. When the fiber content
is
low, the cleaner rejects milled according to the invention is preferably
employed in
the coating slurry, and when the fiber content is high, it is preferably
employed in
the paper stock. According to the invention, a good retention on the screen is
obtained. Particularly preferred is the use of the cleaner rejects milled
according to
the invention for the preparation of offset paper. In addition, the slurries
according
to the invention are also suitable for the preparation of a coating slurry for
light-
weight coated papers, especially with high coating speeds, and for the
preparation
of rotary offset papers, especially for the preparation of light-weight coated
rotary
offset papers, the coating of cardboard and special papers, such as labels,
wallpa-
pers, silicone base paper, self-copying paper, and for admixture with intaglio
printing paper. Thus, the coating pigment slurries obtainable according to the
invention may be employed, in particular, in sheet-fed offset papers,
especially for
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sheet-fed offset single coating, sheet-fed offset double coating: sheet-fed
offset
precoating and sheet-fed offset top coating; in rotary offset papers,
especially for
LWC rotary offset single coating, rotary offset double coating: rotary offset
precoating and rotary offset top coating; in intaglio printing, especially for
LWC
intaglio single coating, intaglio double coating: intaglio precoating and
intaglio top
coating; in cardboard, especially for cardboard double coating: cardboard
precoat-
ing and cardboard top coating; and for special papers, especially for labels
and
flexible packings.
The process offers the opportunity to employ the pigment slurries prepared
according to the invention without a loss in quality in the base papers,
coatings
and especially final qualities prepared therewith.
In the following, some coating formulations which can be obtained according to
the
present invention are given for illustrative purposes (all figures converted
to weight
parts of solids (atro/active ingredient)). The amounts of fillers and/or
pigment
slurries can be partly or wholly replaced by corresponding amounts of cleaner
rejects milled according to the invention.
1. Sheet-fed offset paper
1.1 Sheet-fed offset single coating
70 parts by weight of commercially available CaCO3 (type 90)
30 parts by weight of commercially available clay (fine, e.g., U.S. No. 1)
11 parts by weight of commercially available latex (acrylate)
0.6 parts by weight of commercially available carboxymethylcellulose (CMC)
0.8 parts by weight of commercially available hardener (urea-formaldehyde,
melamine-formaldehyde, epoxy resin)
0.5 parts by weight of commercially available brightener (opt.)
0.5 parts by weight of commercially available Ca stearate
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solids content: 64%
Brookfield viscosity (100/min): 1,200 mPa=s
pH value: 8.5
1.2 Sheet-fed offset double coating
1.2.1 Sheet-fed offset precoating
100 parts by weight of commercially available CaCO3 (type 60 or 75)
parts by weight of commercially available latex
4 parts by weight of commercially available starch (native, oxidized, corn or
potato
sta rch )
0.8 parts by weight of commercially available hardener (urea-formaldehyde,
melamine-formaldehyde, epoxy resin)
0.5 parts by weight of commercially available brightener (opt.)
solids content: 66%
Brookfield viscosity (100/min): 1,100 mPa=s
pH value: 9.0
1.2.2 Sheet-fed offset top coating
70 parts by weight of commercially available CaCO3 (type 90)
30 parts by weight of commercially available clay (fine, e.g., U.S. No. 1)
10 parts by weight of commercially available latex (acrylate)
0.6 parts by weight of commercially available CMC
0.8 parts by weight of commercially available hardener (urea-formaldehyde,
melamine-formaldehyde, epoxy resin)
0.5 parts by weight of commercially available brightener (opt.)
0.7 parts by weight of commercially available Ca stearate
solids content: 64%
Brookfield viscosity (100/min): 1,200 mPa=s
pH value: 8.5
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2. Rotary offset paper
2.1 LWC rotary offset single coating
50 parts by weight of commercially available CaCO3 (type 90)
50 parts by weight of commercially available clay (fine, Engl. clay)
2 parts by weight of commercially available starch (native, oxidized, corn or
potato
sta rch )
12 parts by weight of commercially available latex (XSB)
0.8 parts by weight of commercially available hardener (urea-formaldehyde,
melamine-formaldehyde, epoxy resin)
0.7 parts by weight of commercially available brightener (opt.)
0.5 parts by weight of commercially available Ca stearate
solids content: 62%
Brookfield viscosity (100/min): 1,400 mPa=s
pH value: 8.5
2.2 Rotary offset double coating
2.2.1 Rotary offset precoating
100 parts by weight of commercially available CaCO3 (type 60 or 75)
4 parts by weight of commercially available starch (native, oxidized, corn or
potato
starch)
12 parts by weight of commercially available latex (XSB)
0.8 parts by weight of commercially available hardener (urea-formaldehyde,
melamine-formaldehyde, epoxy resin)
0.5 parts by weight of commercially available brightener (opt.)
solids content: 66%
Brookfield viscosity (100/min): 1,200 mPa-s
pH value: 9.0
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2.2.2 Rotary offset top coatinq
60 parts by weight of commercially available CaCO3 (type 95)
40 parts by weight of commercially available clay (fine, Engl. clay)
parts by weight of commercially available latex (XSB)
0.6 parts by weight of commercially available CMC
0.8 parts by weight of commercially available hardener (urea-formaldehyde,
melamine-formaldehyde, epoxy resin)
0.5 parts by weight of commercially available brightener (opt.)
0.5 parts by weight of commercially available Ca stearate
solids content: 64%
Brookfield viscosity (100/min): 1,200 mPa=s
pH value: 8.5
3. Intaglio printing paper
3.1 LWC intaglio single coating
70 parts by weight of commercially available clay (normal, Engl. clay)
30 parts by weight of commercially available talcum
5.0 parts by weight of commercially available latex (acrylate sole binder)
0.2 parts by weight of commercially available thickener (synthetic)
1.0 parts by weight of commercially available Ca stearate
solids content: 58%
Brookfield viscosity (100/min): 1,200 mPa-s
pH value: 8.5
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3.2 Intaglio double coating
3.2.1 Intaglio precoating
100 parts by weight of commercially available CaCO3 (type 75)
6.0 parts by weight of commercially available latex (acrylate sole binder)
0.3 parts by weight of commercially available thickener (synthetic)
0.5 parts by weight of commercially available Ca stearate
solids content: 66%
Brookfield viscosity (100/min): 1,200 mPa-s
pH value: 9.0
3.2.2 Intaglio top coating
85 parts by weight of commercially available clay (Engl. clay)
15 parts by weight of commercially available clay (calcined clay)
5.0 parts by weight of commercially available latex (acrylate sole binder)
0.2 parts by weight of commercially available thickener (synthetic)
0.8 parts by weight of commercially available Ca stearate
solids content: 570/o
Brookfield viscosity (100/min): 1,300 mPa=s
pH value: 8.5
4. Cardboard
4.1 Cardboard double coating
4.1.1 Cardboard precoating
100 parts by weight of commercially available CaCO3 (type 75)
3 parts by weight of commercially available starch (native, oxidized, corn or
potato
starch)
14 parts by weight of commercially available latex (XSB)
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0.8 parts by weight of commercially available hardener (urea-formaldehyde,
melamine-formaldehyde, epoxy resin)
0.5 parts by weight of commercially available brightener (opt.)
solids content: 66%
Brookfield viscosity (100/min): 1,000 mPa-s
pH value: 9.0
4.1.2 Cardboard top coating
50 parts by weight of commercially available CaCO3 (type 90)
50 parts by weight of commercially available clay (fine/Engl. clay)
13 parts by weight of commercially available latex (acrylate)
2 parts by weight of commercially available co-binder (acrylate)
0.8 parts by weight of commercially available hardener (urea-formaldehyde,
melamine-formaldehyde, epoxy resin)
0.6 parts by weight of commercially available Ca stearate
solids content: 60%
Brookfield viscosity (100/min): 1,200 mPa-s
pH value: 8.5
5. Special papers
5.1. Labels
70 parts by weight of commercially available clay (normal/Engl. clay)
parts by weight of commercially available TiOZ (rutile)
parts by weight of commercially available CaCO3 (type 90)
16 parts by weight of commercially available latex (XSB)
0.5 parts by weight of commercially available hardener (EH) (urea-
formaldehyde,
melamine-formaldehyde, epoxy resin)
0.6 parts by weight of commercially available Ca stearate
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solids content: 60%
Brookfield viscosity (100/min): 1,200 mPa=s
pH value: 8.5
5.2 Flexible packing
80 parts by weight of commercially available clay (normal, Engl. clay)
20 parts by weight of commercially available CaCO3 (type 90)
14 parts by weight of commercially available latex (acrylate)
0.8 parts by weight of commercially available CMC
0.5 parts by weight of commercially available hardener (urea-formaldehyde,
melamine-formaldehyde, epoxy resin)
0.6 parts by weight of commercially available brightener (opt.)
1.0 parts by weight of commercially available Ca stearate
solids content: 58%
Brookfield viscosity (100/min): 1,200 mPa=s
pH value: 8.5
Example:
Cleaner rejects from the process of a commercially available papermaking
machine
of the prior art were milled in a 15% by weight suspension to a slurry having
a
grain size of < 10 pm and added to the stock flow of the paper stock.
Shortly after the connecting of the milling installation with the cleaner
rejects
milled according to the invention, an increase of the ashes content in the
paper
could be established.
The ashes content increased, and the dosage of filler could be consequently
reduced. From the milling installation, about 16 to 18 I/min of pigment slurry
consisting of milled cleaner rejects was supplied to the pulp chest. To
maintain the
ashes content on the predetermined constant level, the usual dosage of fresh
filler
could be reduced from 10 I/h to 6.0 I/h.
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In the covered period, the milling installation ran without trouble. The ashes
content of the stock flow varied to the usual extent. The retention slightly
de-
creased initially , but increased to the usual values in the course of the
experiment.
The amount saved was about 240 I/h of fresh pigment slurry.
The test run over about 10 hours showed that the processed cleaner rejects can
be
employed again as a filler in the stock by the process according to the
invention
without retention loss.
Statements relating to the composition of the cleaner reject slurries:
solids content: about 24.0%
pigment proportion: about 85.0%, based on the solids content
fiber proportion: about 15.0%, based on the solids content
