The invention relates to a high-concentration mixer for producing a homogeneous
suspension of cellulose in an aqueous solution of a tertiary amine oxide, comprising a
stationary container and an agitator arranged in a container axis, which agitator is connected
to a motor via a flange provided in the region of the container bottom, which motor is
designed for rotationally driving the agitator for mixing the suspension provided in the
container, wherein the agitator has flow breakers which bring about a vertical downward
flow of the suspension in the centre of the container and a vertical upward flow of the
suspension in the region of the container wall.
Document EP 0 853 642 B1 discloses a method of producing a cellulose suspension, wherein
a container comprising an agitator is used for mixing and dissolving pulp in an aqueous
solution. As a solvent, N-methylmorpholine-N-oxide (NMMO) is primarily used, into which
pulp is placed and which is admixed to the cellulose suspension. According to the teaching
of said patent, the container is rotationally driven in one direction for achieving a particularly
thorough mixture, and the agitator is rotationally driven in the opposite direction.
Furthermore, it is disclosed to provide the agitator outside of the container axis. It has proved
to be a disadvantage of this known method that only a relatively small cellulose content is
obtained in the cellulose suspension.
Document WO 2013/131113 A1 likewise discloses a method of producing a cellulose
suspension, wherein a cellulose suspension with a cellulose content of from 4.0 to 9.0% by
weight is obtained in a high-concentration mixer. This cellulose suspension is applied to a
double-wire belt press and is squeezed there to a cellulose content of from 9.0 to 15.0% by
weight. It has proved to be a disadvantage of this known method that the double-wire belt
press is technically complex and consequently is prone to failure and expensive during
operation.
Furthermore, a high-concentration mixer is known from the field of papermaking, wherein
paper to be recycled is introduced into an aqueous solution for the preparation of a cellulose
suspension. Such a mixer has been placed on the market, for example, by the company
Vaahto. This mixer comprises an agitator arranged in the container axis and comprising coils
and flow breakers symmetrically distributed on the container at the circumference thereof,
which bring about a vertical downward flow of the cellulose suspension in the centre of the
container and a vertical upward flow of the cellulose suspension in the region of the
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container wall. It has proved to be a disadvantage of the known mixer that, with a higher
cellulose content, scaling may appear in the cellulose suspension between the rotor and the
container bottom. The cellulose suspension was no longer circulated and exchanged in this
gap, whereby it was compressed strongly and damaged. This led to disruptions in the process
and fire hazard due to the emerging frictional heat. The desired high cellulose content in the
cellulose suspension was impossible to achieve in this way.
The invention is based on the object of providing a high-concentration mixer for the
preparation of a homogeneous suspension of cellulose in an aqueous solution of a tertiary
amine oxide, by means of which a cellulose concentration of more than 9.0% by weight can
be obtained in the high-concentration mixer.
According to the invention, this object is achieved in that the flange comprises at least one
blade provided on its circumference, which is designed for wiping the cellulose suspension
from the container and for supporting the vertical upward flow of the cellulose suspension in
the region of the container wall, and that a spacer is provided between the container bottom
and the flange, which spacer ensures a gap of a substantially consistent width between the
container bottom and the flange.
The invention is based on the finding that an increased pressure is generated in the cellulose
suspension by the rotor in the region of the container bottom, the increased pressure
contributing to the flow of the cellulose suspension from the lower central region of the
container to the lower peripheral region of the container and from there along the container
wall vertically upwards in the container. However, in mixers according to the prior art and
with cellulose concentrations of more than, e.g., 9% by weight, the increased pressure in the
lower region of the container causes the relatively viscous cellulose suspension to be pressed
into the gap between the flange and the container bottom and the suspension to be damaged
therein, whereby the vertical flow along the container comes to a standstill. This effect is
counteracted by providing, according to the invention, blades on the circumference of the
flange, whereby the high-concentration mixer is rendered suitable also for the production of
a cellulose suspension in an aqueous NMMO solution having a cellulose concentration of
more than 9% by weight and up to 15% by weight and more.
As the increased pressure in the cellulose suspension in the lower central region of the
container is always highest at the place where the helical coils of the rotor end on the flange,
it has turned out to be advantageous to attach the blades to the flange exactly at those
positions. In this way, the entry of a relatively viscous cellulose suspension into the gap
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between the flange and the container bottom is prevented particularly effectively, and the
existing pressure energy is converted into a vertical upward flow on the container wall.
By providing a spacer between the flange and the container bottom, the distance of the
flange from the container bottom is widened from a gap to a width which still causes
insignificantly small frictional forces on the rotationally driven agitator even if the cellulose
suspension is viscous.
According to another exemplary embodiment, the spacer is omitted, but, instead, a flush
connection is installed in the container bottom underneath the flange. During operation, an
aqueous solution, for example, NMMO, or a suspension is continuously pumped out of the
representational container into the gap between the flange and the container bottom, thus
making sure that no viscous cellulose suspension will deposit in the gap in a frictionincreasing
manner. Of course, the technical measures of the spacer and the flush connection
in the container bottom can also be combined in order to be able to intermix particularly high
cellulose concentrations.
It has proved to be advantageous to provide prongs or ripper teeth at the free peripheral edge
of the helical coil for tearing up the pulp parts. In this way, a particularly high cellulose
concentration can be achieved within a short period of time.
By providing a scraper at the end of the agitator which is opposite to the flange, the
advantage is obtained that pulp parts which might deposit on the agitator above the liquid
level of the cellulose suspension during the stirring process are reintroduced into the
cellulose suspension.
An auxiliary agitator with a separate motor on the container wall serves for supporting the
vertical downward flow of the cellulose suspension in the centre of the container and the
vertical upward flow of the cellulose suspension in the region of the container wall and
consequently accelerates the mixing process for enriching a high cellulose concentration.
Further advantageous embodiments of the high-concentration mixer according to the
invention are illustrated hereinbelow in further detail on the basis of the figures.
Fig. 1 shows a sectional side view of a high-concentration mixer.
Fig. 2 shows a top view of the high-concentration mixer according to Figure 1.
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Fig. 1 shows a high-concentration mixer or, respectively, in abbreviated form a mixer 1 in a
sectional side view A-A, and Fig. 2 shows a top view of the mixer 1. The mixer 1 is suitable
for mixing different suspensions, but is particularly well suited for the preparation of a
cellulose suspension, that is, a homogeneous suspension 2 of cellulose in an aqueous
solution of a tertiary amine oxide, specifically N-methylmorpholine-N-oxide (NMMO). The
liquid level of the suspension 2 in a container 3 of the mixer 1 is shown in Fig. 1. The
container 3 has a cylindrical design, with the container bottom 4 exhibiting chamfers 5. Flow
breakers 6 on the container wall serve for a better blending of the suspension 2.
The mixer 1 comprises an agitator 8 arranged in a container axis 7, which agitator is
connected to a motor not illustrated in the figures via a flange 9 provided in the region of the
container bottom 4, which motor is designed for rotationally driving the agitator 8 for mixing
the suspension 2 provided in the container 3. The motor drives the agitator 8 about its axis of
rotation in a rotational direction 10. The agitator 8 has three helical coils 11 attached to the
circumference of the axis of the agitator 8 in a rotationally symmetric way, which bring
about a vertical downward flow 12 of the suspension 2 in the centre of the container 3 and a
vertical upward flow 13 of the suspension 2 in the region of the container wall. The axis of
the agitator 8 widens conically in the lower region 14, whereby the pressure in the
downwardly flowing suspension 2 rises and the flow of the suspension 2 in the container 3 as
described is enhanced further. The ratio of the diameter of the agitator 8 to the diameter of
the container 3 is typically 0.4 to 0.8.
The flange 9 of the mixer 1 now has three blades 15 provided on its circumference, which
are designed for wiping the suspension 2 from the container 3 and, in doing so, particularly
for wiping the suspension 2 from the chamfer 5. Portions of the suspension 2 which are
rather heavy or, respectively, viscous and, in this connection, in particular portions of the
cellulose to be dissolved in the solvent NMMO are thereby reintroduced into the vertical
upward flow 13 of the suspension 2 in the region of the container wall. In this way, those
rather viscous or even solid parts are advantageously prevented from getting into the gap
between the flange 9 and the container bottom 4 and from remaining therein, where they
would increase the friction between the rotating flange 9 and the stationary container bottom
4.
In the mixer 1, furthermore, a spacer 16 is provided between the flange and the container
bottom 4, whereby the gap, which otherwise is only very narrow, is widened to a width 17
which still causes insignificantly small frictional forces on the rotationally driven agitator 8
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even if the cellulose suspension is viscous. The width 17 must be at least 20 mm and
increases with the diameter of the agitator 8.
According to another exemplary embodiment of the invention, the spacer 16 might be
omitted, but, instead, a flush connection could be installed in the container bottom 4
underneath the flange 9. In this exemplary embodiment, an aqueous solution, for example,
NMMO, or a suspension would continuously be pumped during operation out of the
representational container into the gap between the flange 9 and the container bottom 4 in
order to make sure that no viscous cellulose suspension will deposit in the gap in a frictionincreasing
manner. Of course, the technical measures of the spacer 16 and the flush
connection in the container bottom 4 could also be combined.
The pressure in the suspension 2 in the lower central region of the container 3 which has
been increased by the axis of the agitator 8, which axis is widened conically in the lower
region 14, is always highest at the place where the helical coils 11 of the rotor end on the
flange 9. For this reason, it has proved to be advantageous to attach the blades 5 to the flange
9 exactly at those positions, as can be seen in Fig. 2. In this way, the entry of a relatively
viscous suspension 2 into the gap between the flange 9 and the container bottom 4 is
prevented particularly effectively, and the existing pressure energy is converted into a
vertical upward flow on the container wall.
Furthermore, the mixer 1 now has prongs 18 at the free peripheral edge of the helical coil 11
for tearing up pulp parts contained in the suspension. The prongs could also be formed by
recesses of the peripheral edge or by other shapes. In this way, a particularly high cellulose
concentration can be achieved within a short period of time.
According to an application example, an 8% suspension 2 was initially produced from a
spruce pulp and a 76% NMMO solution by means of an agitator without ripper teeth at the
agitator. The suspension quality was poor, since pulp sheets collapsed and formed large
lumps which failed to dissolve and, in addition, blocked the outlet during emptying. By
equipping the agitator 8 with ripper teeth 18, the formation of lumps could be prevented
effectively and the drawing of the pulp sheets into the suspension 2 could be improved.
Furthermore, the agitator 8 of the mixer 1 now has a scraper 19 at its end opposite to the
flange 9, which scraper is designed for wiping off pulp parts of the suspension 2. When they
are introduced, those pulp parts typically have a size of 60 x 80 cm or 75 x 100 cm and may
stick to the agitator 8. However, it is also possible that pulp parts which already have
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dissolved or, respectively, been shredded partially deposit at the end of the agitator 8 which
is located just above the liquid level. Only by providing the scraper 19, it is ensured that all
pulp parts introduced into the container 3 will be dissolved in the suspension 2.
Furthermore, the mixer 1 comprises an auxiliary agitator 20 with a separate motor on the
container wall for supporting the vertical downward flow 12 of the suspension 2 in the centre
of the container 3 and the vertical upward flow 13 of the suspension 2 in the region of the
container wall. By the auxiliary agitator 20, the flow is enhanced even further and the
blending of the suspension 2 is accelerated. However, it is stated explicitly that the mixer
will achieve the object according to the invention of producing a cellulose suspension in an
aqueous NMMO solution having a cellulose concentration of more than 9% by weight and
up to 15% by weight and more also without an auxiliary motor.
In an application example for the mixer 1, cellulose was introduced into the aqueous NMMO
solution of 72% to 80% as a mixing partner in pieces of a size of 60 x 80 cm without precrushing
in a dry state or with a moisture content of up to 50%. The agitator 8 was driven at
100 to 500 revolutions per minute, with the number of revolutions to be selected depending
on the size of the container 3. The cellulose can be introduced into the mixer 1 also as rather
large pulp sheets of 75 x 100 cm or even larger in order to obtain a homogeneous suspension
2 of cellulose in an aqueous solution of a tertiary amine oxide.
It may be mentioned that at least one of the blades can have a special form. The tip of this
blade is not bent downwards in the rotational direction, whereas a baffle is mounted to the
bottom side, which baffle presses the suspension accumulating underneath the blade against
an outlet opening of the container, as the agitator rotates. This causes a very extensive
evacuation of highly viscous suspension.
The distance from the end of a blade 15, as viewed in the rotational direction 10, to the
beginning of the next blade 15 must be chosen large enough so that an exchange of the
suspension 2 underneath the flange 9 is promoted. At least 30% of the circumference should
be freed in this way.
Furthermore, it may be mentioned that the container wall can be heated. According to a
further application example, a 12.7% suspension 2 was prepared from a short-fibre pulp and
a 78% NMMO solution at a temperature of 75°C. At this temperature, the suspension is
clearly more flowable, and the driving power on the agitator 8 is significantly smaller than
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with a temperature of the suspension 2 of 65°C. Already at 65°C, the suspension 2 achieved
a very high stiffness.
The high-concentration mixer may be operated both discontinuously and continuously.
It may be mentioned that the agitator might also have alternative design variants with one or
several helical coils. According to one design variant, the one helical coil does not run or
several helical coils do not run as far as to the flange, but end(s) further up. Those helical
coils generate only a vertical downward flow in the centre of the container, whereas one or
several further helical coil(s) generate(s) a horizontal flow from the centre of the container to
the container wall directly at the flange, whereby the vertical upward flow on the container
wall is supported.

We Claim: -
1. A high-concentration mixer (1) for producing a homogeneous suspension (2) of
cellulose in an aqueous solution of a tertiary amine oxide, comprising a stationary container
(3) and an agitator (8) arranged in a container axis (7), which agitator is connected to a motor
via a flange (9) provided in the region of the container bottom (4), which motor is designed
for rotationally driving the agitator (8) for mixing the suspension (2) provided in the
container (3), wherein the agitator (8) has at least one helical coil (11) which brings about a
vertical downward flow (12) of the suspension (2) in the centre of the container (3) and a
vertical upward flow (13) of the suspension (2) in the region of the container wall,
characterized in that the flange (9) comprises at least one blade (15) provided on its
circumference, which is designed for wiping the suspension (2) from the container (3) and
for supporting the vertical upward flow (13) of the suspension (2) in the region of the
container wall, and that a spacer (16) is provided between the container bottom (4) and the
flange (9), which spacer ensures a gap of a substantially consistent width (17) between the
container bottom (4) and the flange (9).
2. A high-concentration mixer according to claim 1, characterized in that, in the
container bottom, a flush connection is provided via which an aqueous NMMO solution or a
suspension can be withdrawn from the container and/or introduced into the container by
means of a pump.
3. A high-concentration mixer (1) according to any of the preceding claims, wherein the
at least one helical coil (11) is mounted on the circumference of the axis of the agitator (8)
and ends on the flange (9), characterized in that, in each case, one of the blades (15) is
provided at the position or, respectively, at the positions where the one or several helical
coil(s) (11) end(s) on the flange (9).
4. A high-concentration mixer (1) according to claim 3, characterized in that the free
peripheral edge of the helical coil (11) has prongs or ripper teeth (18) for tearing up pulp
parts contained in the suspension (2).
5. A high-concentration mixer (1) according to any of the preceding claims,
characterized in that the agitator (8) has a scraper (19) at its end opposite to the flange (9),
which scraper is designed for wiping off pulp parts of the suspension (2).
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6. A high-concentration mixer (1) according to any of the preceding claims,
characterized in that an auxiliary agitator (20) with a separate motor is provided on the
container wall for supporting the vertical downward flow (12) of the suspension (2) in the
centre of the container (3) and the vertical upward flow (13) of the suspension (2) in the
region of the container wall.
7. A method of using a high-concentration mixer (1) according to any of claims 1 to 6,
characterized in that cellulose is introduced into the aqueous NMMO solution of about 72%
to 80% as a mixing partner in pulp pieces of a size of about 60 x 80 cm to about 75 x 100 cm
without pre-crushing in a dry state or with a moisture content of up to 50% for the
preparation of a homogeneous suspension.