Note: Descriptions are shown in the official language in which they were submitted.
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PCT/EP 95/04917 - 1 - 18 November 1996
Description
Method and device for the cleaninq of powders
The invention relates to a method and a device
for the mechanical cleAn; n~ of a powder according to the
precharacterizing clause of Patent Claims 1 and 2.
Such a method is suitable for l~oving impurities
which adhere to the surface of powder particles. Such
methods are employed, for example, in the cleaning of
powder-form aluminium oxide, which is intended for the
production of aluminium.- The powder-form primary
aluminium oxide is, during aluminium production, first
used to clean the flue gas produced during the smelting
electrolysis. In this case the flue gas i8 directed past
the primary alumini I oxide, the particles, in the form
of fluoride, iron, phosphorus, carbon, silicon, vanadium
and nickel, evolved from the smelting electrolysis,
adhering to the surface of the powder-form aluminium
oxide. Before this aluminium oxide is fed to the smelting
electrolysis, the impurities in the form of iron,
phosphorus, carbon, silicon, nickel and vanadium must be
removed, since otherwise the process would be enriched
with them. This would ha~e a de,trimental effect on the
quality of the aluminium and the efficiency of the
process. The fluorine required for carrying out the
smelting electrolysis must be collected and fed back to
the process.
A method for the removal of impurities from the
surface of-powder-formed particles is disclosed by FR-A-
2,499,057, which is identical to Norwegian patent
147,791. In this method, an air stream which is loaded
with the powder to be cleaned is directed against an
impact plate. The effect of the impact is that the
impurities adhering to the surface of the powder are
detached. The impurities and the powder are then
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PCT/EP 95/04917 - 2 - 18 November 1996
separated from each other by screening. This method is
~ery expensive to control and fur~hermQre has an
excessively low efficiency in the case of a throughput of
several tonnes per hour.
French Patent 7,732,072 discloses a method for
the stripping of particulate impurities from the surface
of powders. In this method, the powder to be cleaned is
introduced into two crossed air jets. At the crossover
point of these jets, the powder particles strike one
another. In this case the particles rub on one another,
the impurities adhering to the surface being detached.
Since the stripped impurities are lighter than the powder
particles, they can be removed with the aid of an air
current, while the heavier powder particles fall down-
wards. This method is very difficult when it is used in
a current large-scale process, since it is not designed
for the cleaning of large ~uantities of particles.
DE-A-1,607,465 discloses an impact crusher for
cr~h; ng hard and medium-hardness material. The impact
crusher consists of a housing with impact plates which
are arranged opposite the beater arms of the rotor. The
housing furthermore has a delivery guide for feeding the
material, as well as an output opening. The impact plate
is arranged below the delivery guide and has a roof-
shaped design. The impact plate has from place to placea width which is greater than half the diameter of the
rotor.
US-A-4,361,290 describes a rotating beater mill
with which material can be reduced to three different
sizes and separated. The beater mill is provided with a
rotor, the arms of which have plates for reducing the
material fastened to them.
EP-A-337137 discloses a h~ t~ mill for reducing
the size of ore and similar materials. The device is
bounded by a cylindrical housing. Inside the latter,
impact plates and a rotor are arranged, a plurality of
h~mmers being fastened to the latter at a defined mutual
separa-tion and being used for the ore to have its size
reduced or to be projected against the impact plates.
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PCT/EP 95/04917 - 2a - 18 Nov~her 1996
The object of the invention i8 to provide a
method with which it is possible to strip impurities
adhering to the surface of powders cost-effectively and
with a higher efficiency than hitherto, and to provide a
device, using which this method can be carried out.
The object concerning the method i8 achieved by
the features of Patent Claim 1.
The object concerning the device is achieved by
the features of Patent Claim 2.
When the method according to the invention is
carried out, the powder to be cleaned passes through a
mechanically operating cle~n;ng device. In one embodiment
of the invention, this device comprises a stripping
device, after which a cyclone and/or an air sifter are
connected. The construction and mode of operation of the
stripping device correspond to those of an impact
crusher. In this device, the powder is directed, by a
rotor or a striking mechanism, with a predetermined speed
against impact blades which rotate in the opposite
direction to the striking mechanism. By means of the
rotational speed of the rotor or of the striking
mechanism and a fixed dwell time of the powder in the
stripping device, it is possible to control the impact
speed of the powder and the number of impacts of the
powder on the plates. The impact speed is less than
120 m/s. It is preferably set at 20 to 40 m/s. The
setting of the impact speed is particularly important,
since ~ ~
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it is only by this that it is possible
to achieve the effect that all of the impurities are
removed from the surface of the powder, but without the
powder being broken down. After the cleaned powder and
the impurities detached therefrom have been withdrawn
from the stripping device, the cleaned powder having a
particle size of ~ 10 ~m is separated from powder having
a particle size of c 10 ~m and the impurities, which are
likewise no larger than 10 ,um. This i8 done, for example,
with the aid of a cyclone and/or an air sifter, which are
connected after the stripping device. Air sifters are
very expensive to produce. Costs can be saved by connect-
ing the air sifter after the cyclone. This has the
advantage that a fraction of the powder having a particle
size of ~ 20 to 30 ~m is already stripped in the cyclone.
It iB therefore possible to use a smaller air sifter. In
favourable cases it is also sufficient to connect only a
cyclone after the stripping device.
In a simplified ~hodiment of the cle~n;ng
device, the impact crusher and a cyclone are not
employed. The cle~n; ng device consists only of an air
sifter. In this case, however, the air sifter must be
designed in such a way that the air routing in its
interior causes a disintegration effect. The air sifter
must be built in such a way that the powder to be cleaned
is directed with the above described speed against one or
more surfaces, 80 that the impurities adhering to the
powder are stripped. The advantage of using only an air
~ sifter for the powder cle~n;ng is that separation of the
cleaned powder can be carried out simultaneously there-
with.
All the abovedescribed cle~n; ng devices are
designed in such a way that it is possible to separate
the cleaned powder according to size in such a way that
powder having a particle size of ~ 10 ~m is fed to the
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smelting electrolysis for the production of aluminium.
The powder having a particle size c 10 ~m, and the
impurities, are stored in a tip or further processed as
raw material. Using the method according to the inven-
tion, it is possible to remove at least 25 % of the iron,more than 50 % of the phosphorus and 25 % of the carbon,
which a & eres as impurity to the powder-form aluminium
oxide. Using this method it is also possible to recycle
60 % of the fluorine into the smelting electrolysis. The
quantity of stripped impurities 35, and the quantity of
fluorine which can be recovered using the method accord-
ing to the invention, represents a significant improve-
ment over the results which are obtained using known
methods. Since vanadium and nickel a & ere to iron,
vanadium and nickel are also isolated from the smelting-
electrolysis flue gas in accordance with the stripped
quantity of iron.
Further essential features of the invention are
identified in the subclaims.
The invention is explained in more detail below
with the aid of schematic drawings, in which:
Figure 1 shows a cle~n; ng device according to the
invention,
Figure 2 shows a variant of the cle~n; ng device
represented in Figure 1,
Figure 3 shows a simplified embodiment of the cle~n;ng
device,
Figure 4 shows an air sifter as stripping and separating
device.
Figure 1 shows a cle~n; ng device 1 for the
mechanical detachment of impurities 35 a &ering to the
surface of a powder 25. This cle~n; ng device has a
stripping device 2 and an air sifter 3. As can be seen
from the figure, the air sifter 3 is connected directly
after the stripping device 2. The stripping device 2 is
essentially designed in the manner of an impact crusher.
It has at least one rotor or striking mechanism 2R, with
the aid of which the powder 25 introduced into the
stripping device 2 is directed against stationary impact
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plates or against impact blades 2P rotating in the
opposite direction to the striking mechanism. The
cle~n;ng device 1 represented here is intended for the
cleAn;ng of powder-form primary aluminium oxide 25.
~owever, it is also possible to use it to clean other
powders. The striking mechanism 2R and the impact plates
or blades are made of materials which are particularly
suitable for this treatment of powder-form aluminium
oxide. Preferably, those components of the stripping
device 2 which come into contact with the aluminium oxide
25 are made of a hard metal, a ceramic or a polymer
having the correspon~; ng properties. With the aid of the
cleAn; ng device l, the impurities 35 adhering to the
surface of the powder 25 are detached. These are
particles having a size of ~ 10 ~m. In the primary
aluminium oxide 25, the impurities essentially consist of
fluorine, iron, phosphorus, carbon, silicon, nickel and
vanadium. The powder-form primary aluminium oxide 25 is
first used for cle~n;ng the flue gas evolved by the
smelting electrolysis. It is then itself cleaned and fed
to the smelting electrolysis for the production of
aluminium. CleAn;ng of the flue gas involves tAk;ng up
the abovedescribed impurities. CleAn;ng of the flue gas
involves accumulation of the above described impurities.
It is necessary to clean the aluminium oxide 25 80 that
the smelting electrolysis is not enriched with these
impurities. If this cleAn;ng is not carried out, then the
phosphorus and vanadium lead to a decrease in the current
efficiency during the smelting electrolysis. This means
that the overall efficiency of the process decreases. The
quality of the aluminium is impaired by the iron and the
~ilicon. The powder-form primary aluminium oxide 25 is
~ introduced into the stripping device 2 through a metering
device 40. The throughput of the stripping device 2 shown
here is chosen to be high enough for it to be possible to
clean approximately 20 tonne~ of aln~;nium oxide 25 per
hour. The powder-form aluminium oxide 25 is introduced
automatically. The speed of rotation of the striking
mechanism 2R is in this case set in such a way that the
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aluminium oxide 25 strikes the impact plates or blades 2P
with a speed of 20 to 30 m/s. The impurities 35 adhering
to the surface of the powder-form aluminium oxide 25 are
stripped in the process. Approximately 50 % of the
aluminium oxide 25 to be cleaned has a particle size of
50 ~m. The rem~; n; ng aluminium oxide 25 has a larger
particle size. The speed with which the aluminium oxide
25 is directed against the impact blades or plates 2P is
just large enough for the impurities 35 to be stripped,
but without the powder-form aluminium oxide 25 being
broken down. After the aluminium oxide has been directed
at least a few tens of times per second with a speed of
[lacuna~ 20 and 30 m/s onto the impact plates or blades,
it is removed, together with the stripped impurities 35,
from the stripping device 2 and fed to the air sifter 3.
Both in turn take place automatically. The impurities 35,
which are ~ 10 ~m, and the aluminium oxide 31 having the
same particle size are, as shown in Figure 1, diverted
off to one side with the aid of an air current. The
cleaned powder-form aluminium oxide 30, which has a
particle ~ize of ~ 10 ~m, is diverted downward~ out of
the air sifter 3 under the effect of gravity, and fed to
the smelting electrolysis (not represented here).
In the case of the cle~n; ng device 1 shown in
Figure 2, which is essentially the same in structure as
the cle~n; ng device 1 according to Figure 1, a cyclone 4
is connected between the stripping device 2 and the air
sifter 3. Cleaned aluminium oxide particles having a size
of more than 20 ~m to 30 ~m are isolated by the cyclone
4 and fed to the smelting electrolysis. The rem~;n;ng
powder, having a particle size c 20 to 30 ~m, is fed to
the air sifter for further separation. This cl~n;ng
device 1 has the advantage that, in contra~t to the
cle~n;ng device 1 according to Figure 1, a substantially
smaller air sifter 3 can be used, ~ince the fraction of
the cleaned aluminium oxide whose particles are larger
than 20 to 30 ~m is already fed out directly back to the
smelting electrolysis. Since 50 % of the cleaned
aluminium oxide is larger than 50 ~m, the amount of
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powder to be further treated in the air sifter 3 is
greatly reduced by inter~e~;ate connection of the cyclone
4.
Figure 3 shows a cle~n; ng device 1 which is
essentially the same in structure as the cle~n;ng device
1 according to Figure 1. In this case, only a cyclone 4
is connected after the stripping device 2. It is suitable
to use this cle~n; ng device 1 whenever stripping of
particles ~ 16 ~m is sufficient.1
In a further embodiment according to Figure 4,
only an aïr sifter 3 is provided for stripping the
impurities 35 from the powder 25 and for separating the
powder according to size. The powder 25 to be cleaned is
likewise fed to this sifter via a metering device 40. In
this case use is made of an air sifter 3 which has a
disintegration zone (not represented here). This zone
makes it possible to feed the powder 25 to be cleaned
with the required speed of 20 to 30 m/s against at least
one surface (not represented here), 80 that the
impurities can be stripped. Using this air sifter 3 it is
possible subsequently to carry out separation of the
powder according to particle sizes. Separation down to a
particle size ~ 8 ~m is thereby possible.
With the abovedescribed devices it is possible to
remove at least 25 % of the iron, more than 50 % of the
phosphorus and 25 % of the carbon, which adheres as
impurity 35 to the powder-form aluminium oxide 25. Using
this method it is also possible to recycle 60 % of the
fluorine into the smelting electrolysis. The amount of
stripped impurities 35 and the amount of fluorine which
can be recovered using the method according to the
invention represents a significant improvement over the
- results which are obtained with known methods.
