Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
2000-03-30
2002-03-19
Valentine, Donald R. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S381000, C205S384000, C205S386000, C205S387000, C205S390000, C204S244000, C204S245000, C204S247300, C204S247400, C204S247500, C204S291000, C204S292000, C204S293000, C204S294000, C029S592100
Reexamination Certificate
active
06358393
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the production of aluminium by the electrolysis of an aluminium compound dissolved in a molten electrolyte, for example alumina dissolved in a molten fluoride-based electrolyte. It concerns in particular, but not exclusively, cells of the type having a drained cathode having sloping drained cathode surfaces. The invention also relates to cathodes of such cells, their manufacture, 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 and to which the negative pole of a direct current source is connected by means of steel conductor bars embedded in the carbon blocks. The side walls are also covered with prebaked anthracite-graphite carbon plates or silicon carbide plates.
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 which decreases at lower temperatures and to have a higher conductivity of the electrolyte.
The carbonaceous materials used in Hall-Héroult cells as cell lining deteriorate under the existing adverse operating conditions and limit the cell life.
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.
The carbon lining of the cathode bottom has a useful life of a few years after which the operation of the entire cell must be stopped and the cell relined at great cost. Despite an aluminium pool having a thickness of 10 to 20 cm maintained over the cathode, the deterioration of the cathode carbon blocks cannot be avoided because of penetration of sodium into the carbon which by chemical reaction and intercalation causes swelling, deformation and disintegration of the cathode carbon blocks, and because of penetration of cryolite and liquid aluminium.
The carbonaceous blocks of the cell side wall do not resist oxidation and attack by cryolite and a layer of solidified cryolite has to be maintained on the cell side walls to protect them. In addition, when cells are rebuilt, there are problems of disposal of the carbon cathodes which contain toxic compounds including cyanides.
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 growth of this important metal.
In the second largest electrochemical industry following aluminium, namely the caustic and chlorine industry, the invention of the dimensionally stable anodes (DSA®) based on noble metal activated titanium metal, which were developed around 1970, permitted a revolutionary progress in the chlorine cell technology resulting in a substantial increase in cell energy efficiency, in cell life and in chlorine-caustic purity. The substitution of graphite anodes with DSA® increased drastically the life of the anodes and reduced substantially the cost of operating the cells. Rapid growth of the chlorine caustic industry was retarded only by ecological concerns.
In the case of aluminium production, pollution is not due to the aluminium produced, but to the materials and the manufacturing processes used and to the cell design and operation.
However, progress has been reported in the operation of modern aluminium plants which utilize cells where the gases emanating from the cells are in large part collected and adequately scrubbed and where the emission of highly polluting gases during the manufacture of the carbon anodes and cathodes is carefully controlled.
While progress has been reported in the use of carbon cathodes to which have been applied coatings or layers of new aluminium wettable materials which are also a barrier to sodium penetration during electrolysis, very little progress has been achieved in design of cathodes for aluminium production cells with a view to improving the overall cell efficiency, simplifying assembly of the cathodes in the cell, simplifying the removal and disposal of used cathodes, 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. No. 3,400,061 (Lewis et al) and U.S. Pat. No. 4,602,990 (Boxall et al) 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 center of the cell, or into lateral longitudinal grooves along the cell sides, for collecting the molten aluminium and delivering it to a sump.
U.S. Pat. No. 4,544,457 (Sane et al) 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. 5,203,971 (de Nora et al) discloses an aluminium electrowinning cell having a partly refractory and partly carbon based cell lining. The carbon-based part of the cell bottom may be recessed in respect to the refractory part, which assists in reducing movement of the aluminium pool.
U.S. Pat. No. 3,856,650 (Kugler) proposed lining a carbon cell bottom with a ceramic coating upon which parallel rows of tiles are placed, in the molten aluminium, in a grating-like arrangement in an attempt to reduce wear due to movements of the aluminium pool.
To restrict movement in a “deep” cathodic pool of molten aluminium, U.S. Pat. No. 4,824,531 (Duruz et al) proposed filling the cell bottom with a packed bed of loose pieces of refractory material. Such a design has many potential advantages but, because of the risk of forming a sludge by detachment of particles from the packed bed, the design has not found acceptance. U.S. Pat. No. 4,443,313 (Dewing et al) sought to avoid this disadvantage of the previously mentioned loose packed bed by providing a monolayer of closely packed small ceramic shap
Berclaz Georges
de Nora Vittorio
Deshmukh Jayadeep R.
Moltech Invent S.A.
Valentine Donald R.
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