Arrangement for gas collection in aluminium reduction cells having self baking

The present invention relates to an arrangement for gas collection in electrolytic aluminium reduction furnaces equipped with Soderberg anode. The arrangement comprises a plurality of liftable cover plates 8-11 which cover the complete area between the sidewalls of the furnace and the anode casing 2. The cover plates 8-11 are gas tight sealed against the circumference of the anode casing 2 and against the furnace sidewalls 7. The cover plates 8-11 are on their upper side equipped with a plurality of ribs 12. A channel 13 comprising of a lower plate 14, and inclined plate 15 and an upper plate 16 is inserted in recesses in the ribs 12.

The present invention relates to an arrangement for gas collection in 
furnaces for electrolytic production of aluminium, which furnaces are 
equipped with Soderberg selfbaking carbon anodes where electric current is 
supplied to the anode by means of vertical contact bolts which also are 
holding the weight of the anode. Such furnaces are equipped with a 
permanent iron casing through which the anode is gradually slipped through 
at a rate corresponding to the rate which the anode is consumed during the 
electrolysis process. 
BACKGROUND OF THE INVENTION 
In furnaces of the above type, the furnace gases are usually collecting 
under a gas shirt which surrounds the Soderberg anode. The gas shirt is 
connected to the lower part of the casing and the lower part of the gas 
shirt extends down to about 5-10 cm above the level of the electrolytic 
bath. When a solid crust of electrolyte has been formed on the top of the 
bath, sealing between the lower end of the gas shirt and the crust is 
obtaining by addition of aluminium oxide on the top of the crust. From the 
gas shirt, the gas is usually forwarded to a burner where the content of 
CO in the gas is combusted to CO.sub.2 by the supply of air. 
In furnaces equipped with Soderberg anodes, the furnace gas will contain 
volatiles of pitch which also will be combusted in the burner. From the 
burner, the combustion gas is forwarded to a gas cleaning unit where the 
gas is subjected to wet cleaning with water or with a solution which 
contains alkali or alkali earth compounds, or the gas is subjected to dry 
cleaning such as absorption on aluminium oxide. The purpose of the 
cleaning process is to remove dust and gaseous fluorine components from 
the gas in order to be able to let the gas into the atmosphere without 
harming the environment. The collected fluorine compounds can be cleaned 
and returned to the electrolytic furnace. 
The above described arrangement for gas collection has a number of 
disadvantages and drawbacks. One of the disadvantages is that when the 
crust on the outside of the gas shirt from time to time has to be broken 
down in order to supply aluminium oxide to the bath, the gas shirt will be 
open to the atmosphere and furnace gases and dust will escape to the 
furnace building. If point feeding of aluminium oxide through the gas 
shirt is used, one will easily have a build-up of oxide under the gas 
shirt which will prevent the flow of gases in the gas channel under the 
gas shirt, and there will be a risk of increased gas pressure in the 
channel which may press the furnace gas out under the gass shirt and into 
the furnace building. Such a blockage of the gas channel can also happen 
due to splashing of molten electrolyte. In addition to increased gas 
pressure and leackage of gases to the furnace building, there may be an 
increased suction in other parts of the gas channel with the result that 
aluminium oxide fed by point feeders may be sucked out into the gas 
collection system. It is a time consuming job to clean a blocked gas 
channel, and in addition, this job is unpleasant due to the fluorine 
containing gas and heat stress. 
Splashing of the electrolyte bath happens particularly often when the 
so-called anode effect occurs. The anode effect can be observed by an 
increase in the furnace voltage from about 5 V to 15-60 V. The reason for 
the occurance of the anode effect is a build-up of an insulating gas layer 
on the anode. This gas layer has to be removed in order to stop the anode 
effect. The strong increase in the voltage can, when an anode effect 
occurs, cause a partly melting of the side crust in the furnace resulting 
in an increased electrolytic bath level. The increased bath level together 
with splashing can result in that the electrolytic bath comes into contact 
with the gas shirt which strongly cases increased wear of the gas shirt 
resulting in iron contamination of the produced aluminium. In order to 
maintain a good environment in the furnace building, the gas shirt, 
therefore, has to be replaced every second or third year. 
It has also been experienced that it is difficult to keep the flanges 
between each of the gas shirt sections sealed. Air can, therefore, be 
sucked in between the sections of the gas shirt resulting in combustion of 
the furnace gases under the gas shirt. This combustion causes an increased 
wear of the anode and thereby an increased consumption of anode paste. 
It has further been found that the sealing obtained by supplying aluminium 
oxide in the area of the lower end of the gas shirt will not be entirely 
gas tight. In addition, it is necessary to break the crust in order to tap 
the produced metal and in order to inspect the anode. By conventional 
Soderberg operation the complete crust will have to be broken into at 
regular intervals, for example every second to fourth day in order to 
charge oxide. A new crust, therefore, has to be built up in order to 
obtain a sealing of the gas shirt. By point feeding of oxide, this regular 
breakage of the crust will not be necessary. It will, however, be 
difficult to close openings made in the crust for allowing tapping of 
metal etc, as the oxide will flow through such openings without forming a 
new crust which will block the openings. This causes a pollution of the 
environment and loss of fluorides which otherwise would have been 
collected in the gas collection system and returned to the furnace. 
SUMMARY OF THE INVENTION 
By the present invention there is provided an arrangement for gas 
collection in electrolytic aluminium furnaces equipped with Soderberg 
anodes where the gas shirt is eliminated and where the outlet of furnace 
gases is substantially reduced compared to the known state of art for this 
kind of aluminium furnaces. 
Accordingly, the present invention relates to an arrangement for gas 
collection in electrolytic aluminium reduction furnaces equipped with 
Soderberg anodes, which arrangement comprises a plurality of liftable 
cover plates which cover the complete area between the circumference of 
the anode and the top of the furnace side walls, which cover plates are 
sealed against the anode casing and against the sidewalls of the furnace. 
Preferably, there are arranged four cover plates, one for each of the four 
sidewalls of the furnace. Further embodiment of the present invention will 
be evident from the claims.

In the figures, there is shown an electrolytic furnace for aluminium 
production equipped with a Soderberg anode 1 having an anode casing 2. The 
anode casing 2 shown on FIG. 1 has vertical recesses 3, 4 in which there 
are arranged point feeders 5, 6 for aluminium oxide. Between the anode 
casing 2 and the sidewalls 7 of the furnace there is arranged four cover 
plates 8-11 for closing the surface of the furnace. Even though FIG. 1 
shows four cover plates, it should be appreciated that according to the 
present invention the number of cover plates can be more than four, for 
example six, eight or more. The cover plates are equipped with closable 
openings 28 which can be opened in order to tap metal from the furnace, 
and for inspection etc. 
Due to the risk for gas leakages between adjacent cover plates, it is 
preferred to use four cover plates, one for each of the long sides, and 
one for each of the short sides of the electrolytic furnace. 
The gas sealing between the cover plates 8-11 and the anode casing 2 and 
between the cover plates 8-11 and the furnace sidewalls 7 preferably 
consist of a layer of particulate material such as for example aluminium 
oxide. 
The cover plates 8-11 will now be further described in connection with 
FIGS. 3 and 4. 
In FIGS. 3 and 4, there are shown one cover plate 8. The cover plate 8 is 
preferably made from steel. To the top of the cover plate 8, there is 
welded a plurality of ribs 12 made from steel. An iron channel 13 is 
inserted into grooves in the ribs 12. The channel 13 is made from a lower 
plate 14, and inclined plate 15 and an upper plate 16. The lower plate 14 
of the channel 13 is kept in place by means of the grooves in the ribs 12 
and is dimensioned in order that the cover plate with the ribs 12 are 
allowed to expand without being stuck to the channel 13. The cover plate 8 
is thus free to move relatively to the channel 13. Heat expansion of the 
cover plate 8 can, thus, take place freely and without causing substantial 
deformation of the cover plate 8. The channel 13 is lesser exposed to heat 
than the cover plate 8 and is more solidly designed, whereby the channel 
13 forms a rigid profile which will prevent the cover plate 8 from being 
deformed due to the heat exposure. The closeable openings 28 in the cover 
plates 8-11 are shown in more details in FIG. 5. As shown in FIG. 5 the 
opening 28 comprises a pipe 33 which extends through the cover plate 8 and 
the lower and upper plates 14, 16 of the channel 13. The pipe 33 is 
normally closed by a lid 34 arranged in the annulus between two rings 35, 
36 which are welded to the upper plate 16 of the channel 13. In order to 
obtain a good gas seal the annulus between the rings 35, 36 is filled with 
an electric insulating particulate material, preferably aluminium oxide. 
The pipe 33 also has the function of a fixed point between the channel 13 
and the cover plate 8. To the upper plate 16 of the channel 13, there are 
bolts 17, 18 connected to a holding and lifting arm 19. The holding and 
lifting arm 19 is electrically insualted from the cover plate 8 by means 
of an insulating layer 32. The holding and lifting arm 19 is rotatably 
connected to the anode casing 2 at 20. The holding and lifting arm 19 is 
further equipped with a member 21 for limiting the lowering of the arm 19. 
In FIG. 3, the cover plates are shown in closed position, In this closed 
position the upper and inner end 22 of the cover plate 8 will be at a 
distance of 3-10 mm from a bracket 23 which is arranged about the 
circumference of the anode casing 2. The cover plate 8 is sealed against 
the anode casing 2 by filling aluminium oxide into the bracket 23 as shown 
at 24. In the closed position, the lower end 25 of the cover plate 8 will 
be in located about 3-10 mm above the top of the furnace sidewall 7. The 
cover plate 8 is sealed against the furnace sidewall 7 by means of a layer 
26 of aluminium oxide. When all the cover plates 8-11 are in the position 
shown on FIG. 3, the surface of the furnace is effectively sealed against 
the atmosphere. The reaction gases which form during the electrolysis are 
sucked out through a gas outlet 27, and the gases are then combusted by 
supplying air, whereafter the gas is cleaned in a conventional way before 
it is released to the atmosphere. 
During normal operation, the cover plates 8-11 are in the closed position. 
Tapping of the metal, inspection etc. are done through the closeable 
openings 28 in the cover plates 8-11. From time to time, it will, however, 
be necessary to open the cover plates in order to remove carbon particles 
etc. from the electrolytic bath. The cover plates 8-11 can then be lifted 
to an upper open position as shown in FIG. 4. The lifting is preferably 
done by means of hydraulic or pneumatic cylinders 29, 30 as shown in in 
FIG. 1. In order to lock the cover plates 8-11 in open position there is 
arranged a movable bracket 31 for engagement with an opening in the 
inclined plate 15 of the channel 13. The arrangement according to the 
present invention is compact and simple to operate. The lifetime for the 
cover plates are long as they are not directly exposed to the melt. Due to 
a large increased volume below the cover plates compared to the volume 
below conventional gas shirt, the risk of clogging is eliminated.