Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
2000-08-11
2002-08-20
Valentine, Donald R. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C204S245000, C204S247000
Reexamination Certificate
active
06436273
ABSTRACT:
FIELD OF THE INVENTION
The present invention concerns a drained cell for the electrowinning of aluminium by the electrolysis of alumina dissolved in a fluoride-based molten electrolyte such as cryolite, having means to improve the distribution of dissolved alumina to enable a uniform electrolysis of alumina.
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 the fact that irregular electromagnetic forces create waves in the molten aluminium pool and the anode-cathode distance (ACD), also called inter-electrode gap (IEG), must be kept at a safe minimum value of approximately 50 mm 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.
Drained cell designs have been proposed to avoid the problems of conventional cells, by replacing the pool with a thin 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 aluminium on its surface in order to reduce wear. In this design, the cathode block stands on the cell bottom.
U.S. Pat. No. 3,400,061 (Lewis/Altos/Hildebrandt) and U.S. Pat. No. 4,602,990 (Boxall/Gamson/Green/Stephen) disclose aluminium electrowinning cells with sloped drained cathodes arranged with the cathodes and facing anode surfaces sloping across the cell. In these cells, the molten aluminium flows down the sloping cathodes into a median longitudinal groove along the centre of the cell, or into lateral longitudinal grooves along the cell sides, for collecting the molten aluminium and delivering it to a sump.
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. U.S. Pat. No. 5,362,366 (de Nora/Sekhar), a double-polar anode-cathode arrangement was disclosed wherein cathode bodies were suspended from the anodes permitting removal and re-immersion of the assembly during operation, such assembly also operating with a drained cathode.
U.S. Pat. No. 5,368,702 (de Nora) proposed a novel multimonopolar cell having upwardly extending cathodes facing and surrounded by or in-between anodes having a relatively large inwardly-facing active anode surface area. In some embodiments, electrolyte circulation was achieved using a tubular anode with suitable openings.
Of course, the active surface of the cathode and of the anode should be at a slope to facilitate the escape of the bubbles of the released gas. Moreover, to have a cathode at a slope and obtain an efficient operation of the cell would be possible only if the surface of the cathode were aluminium-wettable so that the production of aluminium ions would take place on a film of aluminium.
Only recently has it become possible to coat carbon cathodes with a slurry which adheres to the carbon and becomes aluminium-wettable and very hard when the temperature reaches 700-800° C. or even 950-1000° C., as disclosed in U.S. Pat. No. 5,316,718 (Sekhar/de Nora) and U.S. Pat. No. 5,651,874 (de Nora/Sekar). These patents proposed coating components with a slurry-applied coating of refractory boride, which proved excellent for cathode applications. These publications included a number of novel drained cathode configurations, for example including designs where a cathode body with an inclined upper drained cathode surface is placed on or secured to the cell bottom. Further design modifications in the cell construction could lead to obtaining more of the potential advantages of these coatings.
European Patent Application No. 0 393 816 (Stedman) describes another design for a drained cathode cell intended to improve the bubble evacuation. However, the manufacture of the electrodes is difficult since their active surfaces slope along two orthogonal directions of the cell at the same time. Additionally, such a drained cathode configuration cannot ensure optimal distribution of the dissolved alumina.
U.S. Pat. No. 5,683,559 (de Nora) proposed a new cathode design for a drained cathode, where grooves or recesses were incorporated in the surface of blocks forming the cathode surface in order to channel the drained product aluminium. A specific embodiment provides an enhanced anode and drained cathode geometry where aluminium is produced between V-shaped anodes and cathodes and collected in recessed grooves. The V-shaped geometry of the anodes enables on the one hand a good bubble evacuation from underneath the anodes as described in the prior art, and on the other hand it enables the drainage of produced aluminium from cathode surfaces into recessed grooves located at the bottom of the V-shapes.
Whereas conventional cells having an aluminium pool motion require a greater Anode-Cathode Distance to prevent short-circuits between the electrodes, such pools provide sufficient motion in the electrolytic bath to distribute the dissolved alumina over the cathode. Conversely, drained cells have a reduced Anode-Cathode distance but do not have an aluminium pool motion that stirs and distributes alumina-rich electrolyte between the electrodes.
Because drained cells lack stirring means to distribute alumina-rich electrolyte in the Inter-Electrode Gap, areas of the cathodes which are close to the feeding point of alumina into the electrolyte contain greater amounts of alumina than remote areas.
Most of the alumina is electrolysed on the parts of the cathodes close to the dissolution point, whereas remote areas of the cathodes are poorly fed 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 electrolysis of alumina.
While the foregoing references indicate continued efforts to improve the operation of molten cell electrolysis operations, none suggests a design improving the distribution of the dissolved alumina over the whole active surface of a drained cathode configuration.
SUMMARY OF THE INVENTION
It is therefore an 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.
Another object of the invention is to provide a regular flow of the electrolyte containing CO
2
gas towards the gap between the anodes and the subsequent return of electrolyte to the bottom at the lowest point of the anode surface where the alumina-rich electrolyte is formed.
The invention in particular relates to an electrolytic cell for the electrowinning of aluminium from alumina dissolved in a fluoride-based molten electrolyte. Such cell comprises:
a) a cathode cell bottom comprising at least one sloped active cathode surface, and at least one recessed groove or channel below the botto
de Nora Vittorio
Duruz Jean-Jacques
Deshmukh Jayadeep R.
Moltech Invent S.A.
Valentine Donald R.
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