Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
1999-11-01
2001-07-10
Valentine, Donald R. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S381000, C205S396000, C204S245000, C204S247300, C204S247400
Reexamination Certificate
active
06258246
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to drained-cathode cells for the electrowinning of aluminium by the electrolysis of 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 United States Patents 3,400,061 (Lewis/Altos/Hildebrandt), 4,602,990 (Boxall/ Gamson/Green/Stephen), 5,368,702 (de Nora), 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 by the electrolysis of alumina dissolved in a fluoride-containing molten electrolyte. The drained-cathode cell has a cell bottom which comprises an arrangement for collecting product aluminium surrounded by a peripheral zone of the cell bottom.
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 from the peripheral zone of the cell bottom to form with the cell bottom a trough for containing during operation molten electrolyte and the product aluminium. The or each thermic insulating sidewall is lined with a sidewall lining resistant to molten electrolyte, the or each thermic insulating sidewall inhibiting formation of an electrolyte crust or ledge on the sidewall lining which during operation remains permanently exposed to molten electrolyte.
The peripheral zone of the cell bottom is arranged to keep molten aluminium away from the sidewall lining, whereby the molten aluminium is prevented from reacting with the sidewall lining. The drained-cathode cell design according to the invention thus keeps the molten aluminium away from all cell sidewalls prevention 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 could occur in known designs.
The sidewall lining can be made of tiles containing carbide and/or nitride and/or can comprise a carbide and/or nitride based coating which during cell operation is in contact with the product aluminium.
Alternatively, the sidewall lining may be coated and/or impregnated with one or more phosphates of aluminium, as disclosed in U.S. Pat. No. 5,534,130 (Sekhar). The phosphates of aluminium may be selected from: monoaluminium phosphate, aluminium phosphate, aluminium polyphosphate, and aluminium metaphosphate.
Usually, the drained surface(s) is/are on one or more cathodes which are part of the cell bottom and so arranged that molten aluminium produced thereon drains away from the sidewall lining into the arrangement for collecting molten aluminium. Alternatively, the drained cathode surface(s) can be on one or more cathodes located above the cell bottom, the molten aluminium draining from the cathodes onto the cell bottom and then into the arrangement for collecting molten aluminium.
The cathode and/or the cell bottom can be made of carbonaceous material, such as compacted powdered carbon, a carbon-based paste for example as described in U.S. Pat. No. 5,362,366 (de Nora/Sekhar), prebaked carbon blocks, or graphite blocks, plates or tiles.
It is also possible for the cathode to be made mainly of an electrically-conductive non-carbon material, or of a composite material made of an electrically-conductive material and an electrically non-conductive material.
In such a composit
Berclaz Georges
de Nora Vittorio
Duruz Jean-Jacques
Deshmukh Jayadeep R.
Moltech Invent S.A.
Valentine Donald R.
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