Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
2001-10-16
2004-01-27
Nguyen, Nam (Department: 1753)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S372000, C205S380000, C205S391000, C204S245000, C204S247000, C204S247300, C204S280000
Reexamination Certificate
active
06682643
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte having oxygen evolving metallic anodes facing a cell bottom with an aluminium-wettable drained cathode surface and an aluminium reservoir, and a method to produce aluminium in such an aluminium electrowinning cell.
BACKGROUND ART
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 anodes are still made of carbonaceous material and must be replaced every few weeks. The operating temperature is still approximately 950° C. in order to have a sufficiently high rate of dissolution of alumina and a higher conductivity of the electrolyte.
The anodes have a very short life because during electrolysis the oxygen which should evolve on the anode surface combines with the carbon to form CO
2
and small amounts of CO. The actual consumption of the anode is approximately 450 kg/ton of aluminium produced which is more than ⅓ higher than the theoretical amount.
Another major drawback, however, is due to the fact that irregular electromagnetic forces create waves in the molten aluminium pool and the anode-cathode distance (ACD), also called interelectrode 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 reoxidation of the metal by contact with the CO
2
gas formed at the anode surface, leading to a lower current efficiency.
The high electrical resistivity of the electrolyte, which is about 0.4 ohm. cm., causes a voltage drop which alone represents more than 40% of the total voltage drop with a resulting high energy consumption which is close to 13 kWh/kgAl in the most modern cells. The cost of energy consumption has become an even bigger item in the total manufacturing cost of aluminium since the oil crisis, and has decreased the rate of production growth of this important metal.
While progress has been reported in the use of carbon cathodes to which have been applied coatings or layers of aluminium wettable materials which are also a barrier to sodium penetration during electrolysis, very little progress has been achieved in design of cathodes with a view to improving the overall cell efficiency, as well as restraining movement of the molten aluminium in order to reduce the interelectrode gap and the rate of wear of its surface.
U.S. Pat. No. 3,202,600 (Ransley) proposed the use of refractory borides and carbides as cathode materials, including a drained cathode cell design wherein a wedge-shaped consumable carbon anode was suspended facing a cathode made of plates of refractory boride or carbide in V-configuration.
U.S. Pat. Nos. 3,400,061 (Lewis/Altos/Hildebrandt) and 4,602,990 (Boxall/Gamson/Green/Stephen) disclose aluminium electrowinning cells with sloped drained cathodes arranged 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 located at one end of the cell.
By inclining the active surface of the cathode and of the anode the escape of the bubbles of the released gas is facilitated. 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 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 better 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/Sekhar). These patents proposed coating cell cathodes with a slurry-applied 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 with slopes as suggested is difficult. Additionally, such a drained cathode configuration cannot ensure optimal distribution of the dissolved alumina.
WO98/53120 (Berclaz/de Nora) discloses a cell provided with a cathode mass supported on a cathode shell or plate, the cathode mass being V-shaped and having along the bottom of the V-shape a central channel extending along the cell for draining molten aluminium.
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, 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.
OBJECTS OF THE INVENTION
It is an object of the invention to provide an aluminium electrowinning cell with oxygen-evolving anodes and having an aluminium-wettable drained cathode bottom and an aluminium collection reservoir from which molten aluminium is tapped.
A major object of the invention is to provide an aluminium electrowinning cell having an aluminium-wettable drained cathode which is made of conventional cell blocks which can be easily retrofitted in existing cells.
A further object of the invention is to provide an aluminium electrowinning cell having an aluminium collection reservoir from which molten aluminium can be tapped, without the risk to freeze and at a location where the reservoir can be easily retrofitted in existing cells.
Another object of the invention is to provide an aluminium-wettable cell bottom for such aluminium electrowinning cells.
Yet another object of the invention is to provide a method to produce aluminium in an aluminium electrowinning cell provided with such a cell bottom.
SUMMARY OF THE INVENTION
The invention provides a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte. The cell comprises a plurality of metal-based anodes provided with an oxygen evolving electrochemically active structure having a series of substantially vertical through-openings for the escape of anodically produced gaseous oxygen. The electrochemically active anode structure face and are spaced apart from an aluminium-wettable drained cathode surface on which aluminium is produced. The drained cathode surface is formed along the cell by upper surfaces of a series of juxtaposed carbon cathode blocks, the cathode blocks extending across the cell, for instance single blocks or pairs of blocks end-to-end extending across the entire width of drained cathode surface. The cathode blocks comprise means for connection to an external electric current suppl
Deshmukh Jayadeep R.
Moltech Invent S.A.
Mutschler Brian L
Nguyen Nam
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