Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a process for bleaching
paper stock at elevated temperature and at a stock density
of more than 20% during intensive mechanical or thermomech-
anical treatment of the stock.
More particularly, the invention relates to the raw
stock from wastepaper typically used for papermaking, for
which there are already several known bleaching processes
and physicomechanical cleaning processes:
(1) Helmling 0., Suss U., Berndt W.:
Wochenblatt fur Papierfabrikation 113 (1985), 657-661.
(2) Melzer J., Tibbling P., Jokio P.:
Wochenblatt fur Papierfabrikation 113 (1985), 684-688.
For example, there is the separation of printing ink
in which the printing inks are removed from the stock
suspension by flotation or washing.
Hydrogen peroxide, sodium dithionite or formamidine
sulfinic acid is often used as the bleach.
A separate mixing unit, generally with additional
heating, and a separate bleaching vessel (for example a
bleaching tower) are used for carrying out these processes.
In addition, certain stock densities have to be established
for each bleaching chemical to obtain optimal effective-
ness. This applies in particular to bleaching with reduc-
ing bleaching chemicals, such as sodium dithionite, where
bleaching has to be carried out at low stock densities in
order largely to keep out atmospheric oxygen. For stock
densities of more than 15~, an involved procedure, such as
operation ln vacuo or in an inert gas atmosphere (DE-OS 36
10 940 Al, US-PS 2,963,395), has been described as neces-
sary.
EP-B-O 120 132 describes a process for bleaching raw
paper stock with formamidine sulfinic acid. However, this
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process can only be carried out at a maximum stock density
of 20%.
~ Now, in the regeneration of wastepaper, there are
units operating at stock densities of > 20% by weight which
are known as dispersers, refiners, kneaders, single-shaft
fiberizers, Frotapulpers~R~, etc. and in which the stock is
homogenized, refined or mechanically activated under the
effect of strong frictional forces.
Impurities such as printing inks, stickies or wood
specks are size-reduced to below the visible limit by this
treatment.
In these cases, size reduction of the inhomogeneities
and the relatively intensive fibrillation of the fibers by
the grinding effect leads to a loss of whiteness. In
addition, the stock can contain unwanted tints and, gener-
ally, can have an inadequate whiteness level.
The question arises of whether these machines, which
are intended for the intensive mechanical treatment of the
stock, can also be used for bleaching purposes.
Now, DE-OS 36 10 940 describes a process in which
stock thickened to more than 20~ by weight is treated with
hydrogen peroxide or dithionite in such a unit, in this
case a high-speed short-time intensive mixer, thereby
saving additional equipment for bleaching.
However, it has been found in practice that this
process does not lead to the desired improvement in white-
ness, even where bleach stabilizers or complexing agents
are used.
Accordingly, there is still a need to be able to use
existing wastepaper regenerating plant for bleaching paper
stock.
~he present invention relates to a process for bleach-
ing raw paper stock at elevated temperature and at a stock
density of more than 20% by weight during intensive mechan-
ical treatment of the stock, characterized in that bleach-
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5 89 108 BL
ing is carried out using formamidine sulfinic acid, gener-
ally in a concentration of 0.05 to 2.5% by weight and more
especially in a concentration of 0.2 to 1.2% by weight,
based on bone-dry stock.
sleaching is carried out at an initial pH value of
from 9 to 10 which falls during the reaction to 6 to 8.
Bleach stabilizers or complexing agents are not
necessary.
The process according to the invention is carried out
in existing stock processing machines in which intense
mechnical or thermomechanical treatment is applied without
any change in the treatment. Examples of machines of this
type are, for example, dispersers, refiners, kneaders,
Frotapulpers~R~ or similar machines in which the stock to be
processed can be dispersed and homogenized after concentra-
tion to a density of more than 20% by weight and up to 38
by weight and preferably to a density of 25 to 30% by
weight.
The intense frictional forces in these machines
produce an increase in temperature of approximately 10 to
20C. However, the stock may also be heated beforehand,
for example in heating screws, so that homogenization of
the stock takes place at temperatures in the range from 80
to 99C (in the absence of pressure) or above 100C (under
pressure).
High temperatures of up to 120C can briefly occur
during the mechanical aftertreatment step.
The bleaching process takes place smoothly at tempera-
tures in the range from 50 to 120~C.
The bleach may be added in the form of a powder,
slurry or suspension in water either separately or together
with the alkaline compound, such as sodium hydroxide for
example, depending on the conditions prevailing before the
dispersion step, and can be optimally mixed during the
mechanical treatment of the fibers, so that it is then able
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6 89 108 BL
to act directly on the new fiber surfaces formed. The
bleach may also be added after the dispersion step provid-
ing the bleach is adequately incorporated by pumps, stir-
rers or special mixing units. -
At temperatures of more than 90~C during the disper-
sion step, the reaction time is approximately l to 5
minutes, so that the bleaching reaction is ended in the
c~llecting vessels (such as pipes, vats or stacking towers)
following the units acting as intensive mixers.
lo Under the intense mixing effect of the units, the heat
of friction in the event of purely mechanical stock proces-
sing and the temperature level during the thermomechanical
stock treatment, more than 90% of the bleaching process is
complete after leaving the mixing unit by virtue of the
immanent high reactivity of the FSA. -
Where formamidine sulfinic acid is used as the bleach,
for example in contrast to hydrogen peroxide, the stock
density may be lowered immediately after passing through
one of the units described above without any loss of white-
ness in cases where this is necessitated by the general
course of the process.
The optimal bleaching parameters for each individual
case may be selected on the basis of the foregoing observa-
tions.
This process is preferably used to bleach a wastepaper
stock which has been thickened to around 25 to 30~ by
weight, based on the total quantity, after preliminary
mechanical cleaning, which may include a deinking step,
such as flotation for example.
If the wastepaper used as raw stock is repeatedly
floated and if dispersers or similar machines operating at
the high stock densities are present between these process
steps, bleaching by the new process may alternatively be
carried out with advantage in those machines.
Despite the high stock densities, the process accord-
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ing to the invention surprisingly leads to a distinct
increase in whiteness although the high sensitivity to
oxidation by the atmospheric oxygen present had been
expected to result in a smaller increase than that obtained
using the less reactive dithionite (DE-OS 36 10 940).
Examples
Example 1 ~ -
Comparison of the bleaching effect of sodium dithionite and
formamidine sulfinic acid
A tint-free wastepaper stock of 50% newapapers and 50%
magazines produced in a conventional flotation deinking
process is repeatedly evacuated at a stock density of 20%
and at a pH value of 6.6, subsequently reaerated with
nitrogen gas to remove all atmospheric oxygen and then
subjected to short-time microwave heating under nitrogen to
70OC, after which a freshly prepared sodium dithionite
solution containing complexing agent (Na5DTPA) is added
under nitrogen at 95C in a laboratory kneader so that the
quantity of sodium dithionite, based on stock, was 0.1%
(commercial product) and the quantity of complexing agent
(NasDTPA, commercial product, 40%) 0.2%, based on stock.
A freshly prepared, alkali-activated FSA solution with
no complexing agent added was similarly used instead of the
sodium dithionite solution in a quantity of 0.5% FSA + 0.2%
NaOH (100%), based on stock.
Samples were taken after kneading times of l, 5, 10
and 20 minutes and laboratory sheets formed. The whiteness
values are shown in Table 1.
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Table 1: Evolution of whiteness during the high-tempera-
ture high-consistency bleaching of deinked stock
with sodium dithionite and FSA
5 Kneader time Whiteness ISO R457 (%)
(mins.) with 1% Na dithionite with 0.5% FSA
0 (unbleached) 61.0 61.0
1 62.3 63.4
2 63.1 64.2
63.8 65.4
62.9 66.3
62.4 66.8
The results show that the bleaching effect of sodium
dithionite is distinctly inferior to that of FSA.
Example 2a:
Comparison of the bleaching activity of hydrogen peroxide
20 and formamidine sulfinic acid in a hot disperser bleaching
process
In a deinking plant (capacity 180 tonnes/day), hydro-
gen peroxide bleaching solutions are added to the deinked,
25 undyed and highly thickened wood-containing wastepaper
stock after the heating screw of a hot disperser plant.
After the disperser, the stock is immediately diluted and,
after passing through an intermediate vat, is further
processed. The increase in whiteness obtained by addition
30 of the bleach is the difference in the whiteness of the
stock before entering the heating screw and immediately on
leaving the dispersing disc before entering the inter~
mediate vat.
Accordingly, the bleaching time just corresponds to
35 the time taken to pass through the dispersing unit.
Instead of the H202 bleaching solution, an alkali-activated
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FSA bleaching solution was then added at the same point
(tests 4 and 5). The increases in whiteness are shown in
Table 2a (stock density in the hot disperser: 25% by
weight).
Table 2a: Increase in whiteness during hot disperser
bleaching with hydrogen peroxide or FSA
Bleaching solution Whiteness ISO R457 (%)
Before After Brightness
disperser
1) 0-7% HzO2 52.5 53.6 + 1.1
0.1% NaOH
0.2% stabilizer
2) 0.8% H2O2 51.6 53.3 + 1.7
0.3% NaOH
no stabilizer
3) 0.7% H2O2 54.1 55.0 + 0.9
0.3% NaOH
0.2% stabilizer
4) 0.5% FSA 52.6 57.7 + 5.1
0.2% NaOH
5) 0.5% FSA 50.7 55.5 + 4.8
0.3% NaOH
This Example shows that the effectiveness of bleaching
with hydrogen peroxide is only minimal if the stock is
diluted to low stock densities directly after passing
through the hot disperser plant.
Example 2b:
Hot disperser bleaching of~wood-free deinked stock with
hydrogen peroxide and FSA
The procedure was as in Example 2a, except that
largely wood-free, undyed deinked stock was used. Changes
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in whiteness of + 0.3, + 3.0 and - 0.3 points were obtained
in the three tests using hydrogen peroxide bleaching
solutions containing 0.8% - 1.0% hydrogen peroxide (100).
By contrast, hot disperser bleaching with FSA in four tests
leads to increases in whiteness of 3 to 4 points, the final
whiteness values of the bleached stocks lying at a level of
73 to 7s%.
Exammple 2c:
Hot disperser bleaching of wood-free, pulp-dyed deinked
stock with hydrogen peroxide and FSA
A wood-free deinked stock containing a yellow pulp dye
is bleached in the same deinking plant as described in
Example 2a. Accordingly, the starting whiteness is only
between 62 and 63% IS0. 0.5% to 1.0% hydrogen peroxide
produced hardly any increase in whiteness. An addition of
0.3% to 0.5% alkali-activated FSA solution produced an
increase in whiteness from 62 - 63% ISO to 83 - 86% ISO (~
brightness = + 20 to + 23 points), the yellow dye having
been bleached out by the time the stock leaves the disper-
ser.
Example 3:
FSA bleaching in a unit for the thermomechanical treatment
of deinked stock
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In a deinking plant (capacity: 60 tones/day) compris-
ing a unit for the thermomechanical aftertreatment of
deinked stock made by a different manufacturer than in ~ -
Example 2, 0.4% FSA produces an increase in whiteness from
64% to more than 72% IS0 (R457) in the case of the deinked
stock of low wood content and a corresponding increase from
84% to more than 90% in the case of wood-free wastepaper
stock. Bleaching is terminated immediately the stock
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leaves the plant because the stock is subsequently trans-
ported in the heated state at a density of 30% into a
stacking tower by an open conveyor belt. Under these
conditions, any further bleaching effect of the reducing
bleach can be largely ruled as a result of decomposition by
atmospheric oxygen. The effective bleaching time at a
treatment temperature of approximately 98C is thus less
than 0.5 minute.
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