Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2002-04-27
2004-02-17
Valentine, Donald R (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S247400, C204S247300, C205S381000, C205S384000
Reexamination Certificate
active
06692620
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to drained-cathode cells for the electrowinning of aluminium from alumina dissolved in a molten fluoride-containing electrolyte having sidewalls resistant to molten electrolyte, and methods of operating the cells to produce aluminium.
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.
The electrolytic cell trough is typically made of a steel shell provided with an insulating lining of refractory material covered by prebaked anthracite-graphite or all graphite carbon blocks at the cell floor bottom which acts as cathode. The side walls are also covered with prebaked anthracite-graphite carbon plates.
To increase the efficiency of aluminium production numerous drained-cathode cell designs have been developed, in particular including sloping drained cathode surface, as for instance disclosed in U.S. Pat. No. 3,400,061 (Lewis/Altos/Hildebrandt), U.S. Pat. No. 4,602,990 (Boxall/Gamson/Green/Stephen), U.S. Pat. No. 5,368,702 (de Nora), U.S. Pat. No. 5,683,559 (de Nora), European Patent Application No. 0 393 816 (Stedman), and PCT application WO99/02764 (de Nora/Duruz). These cell designs permit reduction of the inter-electrode gap and consequently reduction of the voltage drop between the anodes and cathodes. However, drained cathode cells have not as yet found significant acceptance in industrial aluminium production.
It has been proposed to decrease energy losses during aluminium production by increasing the thermal insulation of the sidewalls of aluminium production cells. However, suppression of the thermal gradient through the sidewalls prevents bath from freezing on the sidewalls and consequently leads to exposure of the sidewalls to highly aggressive molten electrolyte and molten aluminium.
Several proposals have been made in order to increase the sidewall resistance for ledgeless cell operation. U.S. Pat. No. 2,915,442 (Lewis) discloses inter-alia use of silicon carbide or silicon nitride as sidewall material. U.S. Pat. No. 3,256,173 (Schmitt/Wittner) describes a sidewall lining made of a honeycomb matrix of coke and pitch in which particulate silicon carbide is embedded. U.S. Pat. No. 5,876,584 (Cortellini) discloses sidewall lining material of silicon carbide, silicon nitride or boron carbide having a density of at least 95% and no apparent porosity.
Sidewalls of known ledgeless cells are most exposed to erosion at the interface between the molten electrolyte and the molten aluminium which accumulates on the bottom of the cell. Despite formation of an inert film of aluminium oxide around the molten aluminium metal, cryolite operates as a catalyst which dissolves the protective aluminium oxide film at the aluminium/cryolite interface, allowing the molten aluminium metal to wet the sidewalls along the molten aluminium level. As opposed to aluminium oxide, the oxide-free aluminium metal is reactive at the cell operating temperature and combines with constituents of the sidewalls, which leads to rapid erosion of the sidewalls about the molten aluminium level.
While the foregoing references indicate continued efforts to improve the operation of molten cell electrolysis operations, none suggest the invention and there have been no acceptable proposals for avoiding cell sidewall erosion caused by reaction with molten aluminium metal.
OBJECTS OF THE INVENTION
An object of the invention is to provide a design for an aluminium electrowinning cell in which electrolyte is inhibited from freezing on the sidewalls.
Another object of the invention is to provide a cell configuration for crustless or substantially crustless molten electrolyte resistant sidewalls, in particular carbide and/or nitride-containing sidewalls, which leads to an increased sidewall lifetime.
A further object of the invention is to provide a cell configuration for crustless or substantially crustless molten electrolyte resistant sidewalls, in particular carbide and/or nitride-containing sidewalls, which leads to a reduced erosion, oxidation or corrosion of the sidewalls.
A major object of the invention is to provide a drained cathode cell configuration with sidewalls resistant to molten electrolyte, in particular carbide and/or nitride-containing sidewalls, for crustless or substantially crustless operation.
SUMMARY OF THE INVENTION
One main aspect of the invention concerns a drained-cathode cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte. The drained-cathode cell has a cell bottom which comprises an arrangement for collecting product aluminium and a peripheral upper surface that surrounds the arrangement for collecting product aluminium. At least the part of the cell bottom which is in contact with molten aluminium during operation is made of material resistant to molten aluminium.
Aluminium is produced on at least one drained cathode surface from which the produced aluminium drains into said arrangement for collecting the product aluminium during operation.
The drained-cathode cell further comprises one or more thermic insulating sidewalls extending generally vertically upwards from the peripheral surface of the cell bottom to form with the cell bottom a trough for containing during operation molten electrolyte and the product aluminium. Above the peripheral surface, the or each thermic insulating sidewall is lined with a sidewall lining made of material resistant to molten electrolyte but liable to react with molten aluminium. the or each thermic insulating sidewall inhibits formation of an electrolyte crust or ledge on the sidewall lining that during operation remains permanently exposed to molten electrolyte above and around said peripheral surface.
The peripheral surface of the cell bottom is arranged to keep molten aluminium away from the sidewall lining above and around the entire peripheral surface, whereby the molten aluminium is prevented from reacting with the sidewall lining above and around the entire peripheral surface.
The drained-cathode cell design according to the invention thus keeps the molten aluminium away from all cell sidewalls preventing it from contacting and reacting with the sidewall lining resistant to molten electrolyte, enabling use of a sidewall lining made of a carbide and/or a nitride, such as silicon carbide, silicon nitride or boron nitride, without risk of damage to the sidewall lining by reaction with molten aluminium as can occur with known designs.
Usually the cell comprises four of the above mentioned insulated sidewalls in a generally rectangular arrangement. However, the invention can also be implemented with other sidewall configurations.
The upper surface of the cell bottom for example comprises opposed sloping surfaces leading from opposed sidewalls down into a central channel for the continuous removal of product aluminium, the central channel extending parallel to said opposed sidewalls. This central draining channel (or a side channel or several channels in other embodiments) preferably leads into an aluminium storage sump or space which is internal or external to the cell and from which the aluminium can be tapped from time to time.
Alternatively, the upper surface of the cell bottom comprises a series of oppositely sloping surfaces forming therebetween recesses or channels that extend parallel to opposed sidewalls. The recesses or channels can be of various shapes, for example generally V-shaped.
Usually, the peripheral surface slopes down to a flat or sloping main surface of the cell bottom
Berclaz Georges
De Nora Vittorio
Duruz Jean-Jacques
Moltech Invent S.A.
Valentine Donald R
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