Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
2000-08-11
2002-06-11
Valentine, Donald R. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S381000, C205S389000, C205S392000, C204S225000, C204S245000, C204S294000
Reexamination Certificate
active
06402927
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for producing aluminium in a cell for the electrowinning of aluminium by the electrolysis of alumina dissolved in a fluoride-based molten electrolyte having a reduced anode-cathode distance such as a drained-cathode cell, having means to improve the distribution of dissolved alumina under the anodes to enable the electrolysis of an alumina-rich bath. The invention also relates to a cell having means so arranged to improve the distribution of the alumina-rich electrolyte under the anodes.
BACKGROUND OF THE INVENTION
The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite containing salts, at temperatures around 950° C. is more than one hundred years old.
This process, conceived almost simultaneously by Hall and Héroult, has not evolved as much as other electrochemical processes, despite the tremendous growth in the total production of aluminium that in fifty years has increased almost one hundred fold. The process and the cell design have not undergone any great change or improvement and carbonaceous materials are still used as electrodes and cell linings.
A major drawback of conventional cells is due to he fact that irregular electromagnetic forces create waves in the molten aluminum pool and the anode-cathode distance (ACD), also called inter-electrode gap (IEG), must be kept at a safe minimum value of approximately 5 cm to avoid short circuiting between the aluminium cathode and the anode or re-oxidation of the metal by contact with the Co
2
gas formed at the anode surface.
Another drawback of the conventional cells is the anode effect which occurs when the electrolyte in the cells contains insufficient dissolved alumina to ensure a continuous electrolysis thereof and therefore allows the electrolysis of the fluoride-based material contained in the electrolyte which produces fluoride-based gas such as CF
4
. The fluoride-based gas accumulates under the anodes and greatly inhibits the current transport between the anodes and the cathodes. The anode effect manifests itself by a sudden increase of the cell voltage. The voltage increase can vary from a 7-8 volts up to 30 V in industrial cells.
However, while the anode effect leads to a high energy consumption for several minutes, it is used in some aluminium production to determine the timing for adding fresh alumina into the electrolyte.
Several methods have been applied in order to overcome the anode effect once it has occurred. In addition to feeding the electrolyte with fresh alumina, it is necessary to stir the electrolyte. It can be done manually by using rakes, wooden poles or compressed air, but it can also be done automatically (Grjotheim et al, Aluminium Electrolysis Fundaments of the Hall-Héroult process (1982), pp. 265-281, Aluminium-Verlag Düsseldorf, 2
nd
Edition).
French Patent No. 2.083.362 (Facsko) describes a method to eliminate the anode effect when it occurs by vibrating the carbon anodes between 1 and 300 Hz preferably 50 Hz at an amplitude comprised between 0.01 and 50 mm preferably 1 mm.
In French Patent No. 782.136 (Ferrand) a permanent or intermittent oscillation of the anodes stirs the electrolyte and inhibits the anode effect.
EP Patent application 0 604 664 (Begunov et al.) discloses a method of feeding allmna to the electrolyte of cell for the electrowinning of aluminium. By periodic vertical anode movements, e.g. every 3 hours, alumina accumlated on the electrolyte crust is poured down through a slot along the perimeter of the anode into the molten electrolyte.
Drained cell design have been proposed to avoid the problems of conventional cells, by replacing the pool with a to layer of aluminium which is drained down the surface of the cathode, enabling the Anode-Cathode Distance to be significantly reduced.
U.S. Pat. No. 4,560,488 (Sane/Wheeler/Kuivila) proposed a drained cathode arrangement in which the surface of a carbon cathode block was covered with a sheath that maintained stagnant album on its surface in order to reduce wear. In this design, the cathode block stands on the cell bottom.
An improvement described in U.S. Pat. No. 5,472,578 (de Nora) consisted in using grid-like bodies which could form a drained cathode surface and simultaneously restrain movement in the aluminium pool.
In drained cells without stirring, means to distribute alumina-rich electrolyte in the Inter-Electrode Gap, the electrolyte in areas of the cathodes which are close to the feeding point of alumina contains greater amounts of alumina than remote areas where electrolysis has taken place.
Most of the alumina is electrolyzed on the parts of the cathodes close to the dissolution point, whereas remote areas of the cathodes are depleted with alumina. This is due to the gradual depletion of the alumina concentration in the electrolyte while the electrolyte is moving between the electrodes where its electrolysis takes place. Consequently, such a gradient of dissolved-alumina concentration over the cathode of a drained cell can cause a non-uniform use of the active surfaces of the cathodes and therefore a non-uniform consumption of the electrodes while increasing the risk of a local anode effect due to a locally insufficient concentration of alumina.
While the foregoing references indicate continued efforts to improve the operation of molten cell electrolysis operations, none suggests a design proving the distribution of the dissolved alumina over the whole active surface of a drained cathode configuration.
SUMMARY OF THE INVENTION
It is therefore a preferred object of the invention to provide a drained cell for the electrowinning of aluminium by the electrolysis of alumina dissolved in a fluoride-based melt such as cryolite, designed to ensure an enhanced distribution of alumina dissolved in electrolyte between the active sloping surfaces of the electrodes.
The invention relates to a method of producing aluminium in an electrolytic cell, in particular by the electrolysis of alumina dissolved in a molten fluoride electrolyte, said cell comprising a cathode having an active cathode surface and facing anodes having active anode surfaces. Each anode is spaced apart in its operative position from the cathode by an anode-cathode distance defining an anode-cathode gap containing the electrolyte.
The method of the invention comprises periodically moving at least one anode from and back into its operative position, feeding alumina into the electrolyte where it is dissolved to enrich the electrolyte with alumina and electrolysing in the anode cathode gap electrolyte containing dissolved alumina.
The method is characterised in that the anode is periodically moved from and back into its operative position such that electrolyte enriched with alumina is intaken into the anode-cathode gap under substantially the entire active anode surface of the anode while it is moved during an intake period.
The method of the invention is preferably applied when the cell has drained cathodes, for instance a drained cell as described in U.S. Pat. No. 5,683,559 (de Nora) which advantageously comprises an aluminium collection storage such as a collection groove or channel to collect the product aluminium.
Likewise the method can even be applied in a cell having an aluminium tool, such as a cell containing grid-like bodies as described in U.S. Pat. No. 5,473,578 (de Nova). The method is in particular designed for any cell configuration which lacks an aluminium pool motion stirring the electrolyte and is therefore usually provided with a reduced anode-cathode distance (ACD), such as an ACD between 1.5 and 4.5 cm, preferably between 2 and 3 cm.
Usually the duration between two consecutive intake periods is longer than the duration of an intake periods. The duration between two consecutive intake periods is preferably comprised between 1 and 20 minutes. In conventional cells the anodes are only moved when an anode effect occurs, the time interval between two consecutive anode effects being usually comprised between 1 and 10 days.
To obtain t
Bello Vittorio
Duruz Jean-Jacques
Deshmukh Jayadep R.
Moltech Invent S.A. Luxembourg
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