Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DEVICE FOR COLLECTING AND REMOVING GASES IN AN
ALUMINUM REDUCTION CELL
Field of the Invention
The invention relates to non-ferrous metallurgy, in particular, to the
production of aluminum in reduction cells with prebaked anodes, and can be
used
to reduce the volumes of removed gases from the reduction cell while
maintaining
high removal efficiency both between routine operations and durinu, routine
operations when reduction cells are unsealed, i.e. hooding covers are open.
Prior Art
A device is known, wherein a gas-collecting cap contacts the crust at the
crust opening. The purpose of this invention is to collect off-gases from the
reduction cell in order to strip them of alumina fluorine compounds (patent
US 4,770,752, 1988).
The drawbacks of the known device include limitations of said invention
related to maintenance of the alumina feed system, located inside the cap, and
possible damage during anode replacement since the cap is situated close to
the
anodes and crust.
A device is known, wherein a standard amount of process gases is removed
between routine operations, and an increased amount of process gases is
removed
when opening the anode shell covers by activating an additional exhausting
fan. A
separator wall is mounted inside the hooding to direct the streams upward into
gas
channels to reduce emissions into the potroom (patent RU 2251593 C2,
1PC C25C3/20, Publ. 11.15.2000).
The drawback of the analog device is that, upon unsealing the reduction cell,
a stream of air drawn under the hooding displaces the electrolysis gases along
the
reduction cell and creates stagnant regions with elevated concentrations of
electrolysis gases under the bottom flange of the collector beam. In these
regions,
the gases escape into the working zone of the potroom through the gaps between
the anode rods and the flange, as well as through the gaps between hooding
covers.
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A device for collecting and removing gases from an aluminum reduction cell
is known as disclosed in patent RU 2553137, C25C 3/22 Publ. 06.10.2015,
comprising a collector beam with top and bottom stiffening rings and double
vertical walls, between which, in the top part of the collector beam along the
vertical walls, the main gas duct channels of the variable cross-section are
formed
with confusors located along the longitudinal axis of the collector beam above
the
anodes, with one end secured at the fairing inlet and the other end with
openings at
the bottom stiffening ring, with the height of the main gas duct channels
increasing
towards the end of the collector beam connected to the gas stripping system.
Two
additional gas duct channels of the variable cross-section are located between
the
top and bottom stiffening rings symmetrically with respect to the longitudinal
axis
of the collector beam, which are connected to the bottom stiffening ring with
confusors equipped with shutters and located along the longitudinal axis above
the
anodes between the confusors of the main gas duct channel, wherein each main
gas
duct channel has a confusor mounted at its front side in the metal extraction
zone.
The drawback of the provided device is that a local increase of rarefaction in
the zone of anode replacement is provided in the device without equalizing the
rarefaction under the entire hooding. To this end, additional gas ducts are
included,
connected to confusors that are equipped with automatic shutters. Through the
openings, with doors removed for anode replacement, a large volume of air is
drawn under the hooding, with swirling streams forming and propagating under
the
entire hooding. As a result, confusors in other zones of the hooding cannot
manage
gas removal, and the gas escapes into the potroom through leaks in the
hooding.
The provided device does not ensure efficient gas removal during anode
replacement.
The closest invention to the claimed invention in terms of technical essence
is a device for collecting hot off-gas emerging in the course of reduction
disclosed
in patent WO 2010/033037 Al, IPC C25C3/22, Publ. 03.25.2010, which has been
selected as the prior art. In the device, the gas removal cap is located
immediately
above the crust opening and ensures a lower total volume of the gas removed
from
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the reduction cell at a higher concentration of CO2 and elevated temperature,
reducing the number of gas stripping units and increasing the heat exchange
potential. In addition, the gas-collecting cap has at least two inlets, i. e.
it has
double walls. The inlet speed between the double walls significantly exceeds
the
speed at the cap center, providing additional draft that creates an artificial
air wall,
ensuring more efficient collection of off-gases and reducing interference from
transversal streams.
Drawbacks of the prior art device:
The prior art cap design ensures efficient removal of electrolysis gases by
i0 the
double walls, which create an artificial air wall and reduce interference from
transversal streams. In this case, cap displacement away from the breakers
will
render gas removal inefficient, and during operation in aggressive
environments,
positioning the cap immediately above the crust opening will result in damage
to
the control mechanisms for the streams of the reduction cell reflector and
feeder.
The desirable range of cap positioning above the crust, being 10-1,000 mm,
is determined by the speed of alumina carry-over and the possibility of
replacing
the anodes. To meet this requirement, the cap mounting height should be
changed
for different operations, which disturbs the tightness of the device for
removing
gases.
Efficient gas removal from the reduction cell requires equal volumes of gas
removal from all the caps positioned under the hooding of the reduction cell.
This
requires equal rarefaction levels at each cap inlet; however, the prior art
device has
no system to control uniform gas intake under the hooding of the reduction
cell.
In the device disclosed in WO 2010/033037, the cap for process gas
collection may be mounted next to the feeder, rather than being an integral
part of
it. In this case, the bulk of the gases will be drawn at the cap periphery;
however,
this requires uniform gas intake along the cap base length.
When replacing the anodes, the hooding doors are opened and air drawn
through the resulting opening forms swirling streams that displace the gas
into one
end of the reduction cell such that the caps in that zone cannot manage
removal of
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the increased gas volume. Furthermore, the air wall prevents drawing in the
transversal streams of gas that evolves through large breaks formed in the
electrolyte crust where the anodes are being removed.
Thus, the prior art device does not ensure efficient removal of electrolysis
gases, both between routine operations of reduction cell operation and during
routine operations. Furthermore, the equipment positioned under the hooding
experiences high stresses from an aggressive environment.
Disclosure of the Invention
The invention is intended to reduce heat losses with off-gases from the
reduction cell and to relieve the stress on gas stripping units, while
maintaining gas
removal efficiency.
The stress on the gas stripping facilities is determined by the volume of
gases removed from the reduction cells. The total volume of gases removed from
a
reduction cell with baked anodes is 3,000-20,000 nm3/h, of which only 1-2% are
electrolysis gases, with the remainder being air. The efficiency of gas
removal
from a reduction cell is determined by the efficiency coefficient of the
system of
gas removal from the reduction cell. This value is usually 98%.
The technical result of the claimed invention is a several-fold reduction in
the total volume of gases removed from a reduction cell due to decreased air
volume, while ensuring the following conditions.
1. The efficiency coefficient of the gas removal system is maintained at
98% or higher, both between routine operations of reduction cell operation and
during routine operations.
2. The stresses exerted by an aggressive environment on the equipment
positioned under the hooding do not increase.
To ensure the -first condition, the provided technical solution maintains
uniformity of gas intake along the reduction cell hooding length and along the
length of each gas-collecting cap base between routine operations of reduction
cell
operation.
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To ensure the second condition, the gas-collecting caps are positioned in
such a manner that the mechanisms for feeding alumina are outside the gas
intake
zone, while the cap design ensures uniformity of gas intake along the length
of
each gas-collecting cap base for said positioning¨outside the gas intake zone.
In one of the embodiments, the posed problem is solved by a device for
collecting and removing gases in an aluminum reduction cell, which comprises a
system of gas ducts comprising horizontal main and additional gas ducts
configured to cut in/out the main and additional gas ducts; and gas-collecting
caps
wherein each gas-collecting cap is connected by a first channel to a
horizontal
to main gas duct to form a main gas removal loop, and is connected by a second
channel to an additional vertical gas duct to form an additional gas removal
loop.
The height of each subsequent first channel of the main loop is increased
along the
gas stream by 16-24% of the height of the preceding first channel, and the
height
of each subsequent second channel of the additional loop is increased along
the gas
15 stream by 24-26% of the height of the preceding second channel.
Separator plates
are mounted at the bottom part on the internal surface of the longitudinal
sides of at
least one gas-collecting cap along the direction of the gas flow, having a
length of
not more than 50% of the height of the gas-collecting cap, with at least two
separator plates mounted symmetrically at each side of the central axis of the
gas-
20 collecting cap, and the length of each subsequent plate in the direction
of the
central axis of the cap decreasing with respect to the preceding one by 25-
35%,
According to one embodiment of the suggested invention, the caps are made
in the form of confusors.
According to one embodiment of the suggested invention, the main and
25 additional loops are combined at the top part of the caps.
According to one embodiment of the suggested invention, the distance
between the separator plates is at least 15% of the length of the gas-
collecting cap
base.
In addition, a system for collecting and removing gases in an aluminum
30 reduction cell is provided, which comprises a reduction cell comprising
at least
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anodes and electrolyte crust breakers, a hooding of the reduction cell made of
removable covers, a system of gas ducts, including horizontal main and
additional
gas ducts configured to cut in/out the main and additional gas ducts, and gas-
collecting caps positioned under the hooding of the reduction cell along its
longitudinal axis between the electrolyte crust breakers to form a gas intake
zone at
the center of the reduction cell. Guiding elements are mounted at the internal
side
of the removable hooding covers horizontally with respect to the electrolyte
crust,
which are configured to guide gas streams into the gas intake zone, wherein
each
of the gas-collecting caps is connected by a first channel to a horizontal
main gas
I() duct to form a main gas removal loop, and by a second channel to an
additional
vertical gas duct to -am an additional gas removal loop. The height of each
subsequent first channel of the main loop is increased along the gas stream by
16-
24% of the height of the preceding first channel, and the height of each
subsequent.
second channel of the additional loop is increased along the gas stream by 24-
26%
of the height of the preceding second channel. Separator plates are mounted at
the
bottom part on the internal surface of the longitudinal sides of at least one
gas-
collecting cap along the direction of the gas flow, having a length of not
more than
50% of the height of the gas-collecting cap. At least two separator plates are
mounted symmetrically on each side of the central axis of the gas-collecting
cap,
2() with
the length of each subsequent plate in the direction of the central axis of
the
cap decreasing with respect to the preceding one by 25-35%.
According to one embodiment of the suggested invention, the gas-collecting
caps in the system are positioned above the electrolyte crust at a distance
equal to
0.5-1.5 of the height of a new anode of the reduction cell.
According to one embodiment of the suggested invention, the height of the
hooding with respect to the electrolyte level is equal to 1,5-2 of the height
of a
new anode.
Also, a reduction cell is provided, which includes the device for collecting
and removing gases in an aluminum reduction cell described above.
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The listed embodiments of the present invention are not the only possible
ones. Various modifications and improvements are envisioned without departing
from the scope of the invention as defined by the independent claims.
Brief Description of the Draivings
The inventive essence is illustrated by the following drawings.
Fig. I shows a general view of the reduction cell that includes the device for
collecting and removing gases.
Fig. 2 shows the arrangement of elements of the structure for collecting and
removing gases inside the reduction cell.
Fig. 3 shows the elements of the main and additional gas removal loops.
Fig. 4 shows an embodiment of the guiding element of the device for collecting
and removing gases made in the form of a projection (cross-sectional view of
the
reduction cell).
Fig. 5 shows an embodiment of the guiding element of the device for collecting
and removing gases made in the form of plates (cross-sectional view of the
reduction cell).
Fig. 6 shows the arrangement of the separator plates in the gas-collecting cap
(cross-sectional view of the cap).
Fig. 7 shows a longitudinal view of the main and additional gas removal loops
with
gas-collecting caps.
Embodiments of the Invention
The device for collecting and removing gases is mounted in a reduction cell.
A reduction cell is a device for producing aluminum by reduction of melts,
typically comprising anodes, point alumina feeders with breakers, a collector
beam
with gas ducts and gas-collecting caps, and a hooding. All structural elements
of
the claimed device are secured on the collector beam (1). The hooding of the
reduction cell (2) is made of separate covers with guiding elements 3 rigidly
mounted on their internal side horizontally with respect to the electrolyte
crust of
the melt. The guiding elements may be structurally made in the form of plates
or
projections (Fig. 4 and Fig. 5) made of materials used to manufacture the
hooding
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covers, such as aluminum. The number of guiding elements is determined by the
speed of streams between the guides, and must ensure air stream speed higher
than
2 m/s and lower than 7 m/s to exclude gas escape from the reduction cell and
alumina carry-over from the crust surface. The position and length of the
guiding
elements are determined by the hooding configuration and provision of uniform
flow around the anodes.
The gas-collecting caps (4) are positioned under the hooding of the reduction
cell along its longitudinal axis between the electrolyte crust breakers above
the
anodes (5) and between the breakers (6) to form a gas intake zone at the
center of
the reduction cell.
Each gas-collecting cap (4) is connected by a first channel (8) to a
horizontal
main gas duct (9) to form a main gas removal loop, and is connected by a
second
channel (8') to an additional vertical gas duct (10) to form an additional gas
removal loop (Fig. 3). The height of each subsequent first channel of the main
loop
is increased along the gas stream by 16-24% of the height of the preceding
first
channel, and the height of each subsequent second channel of the additional
loop is
increased along the gas stream by 24-26% of the height of the preceding second
channel. The main (9) and additional (10) horizontal gas ducts are connected
to the
potroom system of gas removal (not shown).
Separator plates are mounted at the bottom part on the internal surface of the
longitudinal sides of at least one gas-collecting cap along the direction of
the gas
flow, having a length of not more than 50% of the height of the gas-collecting
cap
(Fig. 7). The separator plates are rigidly secured at each side of the central
axis of
the gas-collecting cap, with the length of each subsequent plate in the
direction of
the central axis of the cap decreasing with respect to the preceding one by 25-
35%.
The plates are positioned symmetrically with respect to the central axis of
the cap,
ensuring equal conditions for gas intake along the cap base length. The number
of
plates must be at least two at each side of the central axis to exclude
stagnant zones
in the cap. The separator plates mounted in such a way divide the cap volume
into
sectors (channels) with equal speeds at the cap base, not forming stagnant
zones
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through which gas is not removed, ensuring equal efficiency of gas intake at
the
cap center and periphery. The width of each parallel channel should preferably
be
at least 15% of the cap base length. The obtained effect allows the caps to be
positioned between the breakers, whilst the mechanisms for -feeding alumina
are
outside the gas intake zone and the stress from an aggressive environment
thereupon is minimal.
The inventors unexpectedly -found that for different plate position and
dimensions, the effect of the uniform gas intake at the cap base is not
obtained.
Thus, an increase in plate length or directing the plates towards the cap
center resulted in collision of streams from individual channels and therefore
the
formation of positive pressure zones. In this case, the gas escaped back into
the
hooding. Positioning the plates at the cap top resulted in increased speeds
upon
exiting the cap. As a consequence, the gas-dynamic resistance of the cap
increased,
and thus the stress on the gas-stripping resistance increased as well. The non-
uniformity of gas intake at the cap base also increased, as in this case,
zones with
different degrees of rarefaction are fOrmed at the cap inlet. Furthermore, air
walls
similar to air walls in the prior art device may form, preventing gas intake
at the
cap periphery.
The gas-collecting caps (4) are mounted above the electrolyte crust, for
example, at a distance equal to 0.5-1.5 of the height of a new anode, and the
height
of the hooding is equal to 1.5-2.0 of the height of a new anode. An increase
in
hooding height will increase the volume of air in the removed gases, the gas
removal will require an increase in rarefaction to levels present in the
current
designs of reduction cells, with increased stress on the gas-stripping
facilities as
well as increased heat losses. In case of a decrease in volume, anode
replacement
will become impossible.
In one of the embodiments designed in the course of developing the
invention, the gas-collecting cap was made in the form of a confusor--a device
wherein the channel gradually tapers, which increases the speed of the gases
being
removed and reduces losses of energy spent on gas removal. The cap height was
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800 mm, and the base length was 1,800 mm at a base width of 170 mm. The
separator plates were secured at a height of 30 mm from the cap base and were
directed along the side walls, forming parallel channels for the gas flow. The
length of the first plate was 400 mm, which is 50% of the cap height. The
lengths
of the second and third plates were 260 mm and 170 mm, respectively. The
rectangular shape of the cap base was defined by the need to perform anode
replacement and position mechanisms for feeding alumina under the hooding,
with
the caps covering the entire space in the reduction cell center between the
anodes,
excluding the space fbr operating the breakers and feeders. The width of each
of
i0 the parallel channels was 180 mm. The caps were mounted at a height of
50 mm
above the anodes,
The device operates as follows. Between routine operations of reduction cell
operation, the gas coming under the hooding of the reduction cell (2) through
crust
openings punctured by the breakers (6) mixes with air coming through the
hooding
1 5 leaks, and is drawn into the caps (4) of the main horizontal gas duct
(9) for gas
removal and further proceeds to the potroom gas removal system (not shown).
The
separator plates (7) in the caps (4) uniformly distribute the gas stream over
the cap
base. The uniformity of gas streams distribution among the caps (4) is ensured
by
distributing the resistance along the length of the main horizontal gas duct
(9) by
20 varying the heights of channels (8) and (8') to the main gas duct. The
caps (4) are
preferably positioned above the crust at a height of 0.5-1.5 of the anode (5)
height,
between the breakers (6), ensuring a degree of rarefaction sufficient for
complete
gas removal.
When performing operations with the reduction cell involving partial
25 unsealing of the reduction cell, air is drawn under the hooding (2) and
mixes with.
anode gases to form vertical and horizontal swirling streams. The guiding
elements (3) secured to the internal side of the hooding covers break down the
swirling streams and direct them to the caps (4). When performing operations
(such as anode replacement), the gas is simultaneously removed through the
30 main (9) and additional (10) gas ducts. Thus, the degree of rarefaction
under the
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hooding can be increased by a factor of 3-4, which is sufficient for gas
removal
upon partial unsealing of the hooding. Known mechanisms, such as dampers, cut
the main and additional gas ducts in and out.
Implementing the claimed device decreases the volume of gases removed
from one reduction cell by a factor of 2-4 due to the combined effect of
reducing
the volume of gases to be removed, redistributing gas streams into the gas
intake
zone, and positioning the caps close to the location of electrolysis gas
evolution
under the hooding. The obtained efficiency coefficient of gas removal is 98%-
99%, depending on operations performed with the reduction cell.
Mounting the caps under the hoodin2, close to the location of electrolysis
gas evolution, increases gas removal efficiency at a lower volume of removed
gas,
and said height of cap mounting above the electrolyte crust ensures the
possibility
of anode replacement without damaging the caps. The plates inside the caps
ensure
uniform gas intake along the cap base length.
The caps are simultaneously included in the main and additional gas removal
system, ensuring efficient gas removal, both between routine operations and
during
routine operations.
Said range of variation of the height of channels connecting to the caps
ensures efficient operation of all the caps, i. e. efficient gas removal along
the
length of the reduction cell.
The main and additional gas ducts connect to the caps at the top part of the
caps (at the con fusor vertex). The speed of the streams at the exit from
confusors
has already been equalized, and cutting in an additional loop will not change
the
character of gas flow inside the confusor, i. e. will have no effect on the
uniformity
of gas intake at the cap base. The gas redistributes over the loops
proportionally to
the rarefaction in the loops and the gas duct cross-sectional area.
Cap positioning between the breakers at a height equal to 0.5-1.5 of the
anode height allows the anodes to be replaced without damaging the structural
elements of the device for removing gases and decreases the effect of
temperatures
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and abrasive particles on the normal operation of breakers and feeders of the
reduction cell.
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