Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Patent
1996-02-21
1997-08-19
Valentine, Donald R.
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
205372, 204243R, C25C 300, C25C 306, C25C 308, C25C 700
Patent
active
056584474
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to electrolytic cells for use in the production of metals by electrolysis and to cathodes for use therein. The invention is particularly suitable for use in the production of aluminium.
Aluminium is generally produced by the electrolysis of alumina. Alumina is dissolved in a bath of molten cryolite at a temperature in the range of 950.degree.-1000.degree. C. Carbonaceous electrodes are frequently used for both the cathode and the anode. The anode is placed uppermost in the electrolytic cell and the cathode structure generally forms the bottom floor of the cell.
In operation of the cell, the molten bath of cryolite and dissolved alumina sits between the cathode and the anode. Liquid aluminium metal is electrodeposited at the cathode. The cryolite bath is a very aggressive medium and will readily attack the electrode material at the cell operating temperature. This does not form a major problem with regards to the anodes as the anodes are consumed in the electrolytic reaction and require replacement every few weeks. As the anodes form the upper element of the cell, anode replacement is a relatively simple operation that does not cause great disruption to cell operation.
However, attack of the cathodes by the bath materials can cause severe operational problems. The cathode forms the lower part of the cell and indeed in most aluminium reduction pots, the bottom of the pot consists of a refractory layer having the carbonaceous cathodes being formed as a layer on top. Cathode replacement requires shut-down of the cell and removal of the lining. This procedure is obviously time consuming and represents down-time for the cell. Consequently, aluminium reduction cells are operated under conditions such that cathode life is in the order of 2 to 5 years.
To achieve such cathode life, aluminium reduction cells are generally operated under conditions such that exposure of the cathode to bath materials is substantially avoided. This is obtained in conventional cells by maintaining a pool of molten aluminium above the cathode. Molten aluminium does not attack the cathode to the same extent as the bath materials and hence protects the cathode from the bath. Although providing satisfactory cathode life, maintaining a pool of molten aluminium in the cell requires a number of compromises in cell operation, including the requirement that anode-cathode distance be greater than optimal. Aluminium reduction cells utilise large electric currents which, in turn, can create large electromagnetic fluxes. The electromagnetic fluxes contribute to the formation of wave motion within the pool of molten aluminium, making prediction of the exact depth of the aluminium pool, and therefore the minimum spacing between the anode and the interface between aluminium and cryolite somewhat imprecise. Therefore, in order to prevent the pool of molten aluminium contacting the anode and causing a short circuit in the cell, the anodes are positioned in the cell at a position substantially above the normal or expected position of the aluminium/cryolite interface. This reduces the efficiency of the cell.
A number of proposals have been made to try to reduce the anode--cathode distance. On proposal involves placing a packed bed of material, e.g. TiB.sub.2 rods or rings, into the pool of aluminium to reduce the formation of waves in the aluminium pool. However in such packed bed cells, a safety margin must be incorporated into the anode--cathode distance in order to account for localised disruptions in the aluminium pool. Further, the packing is frequently produced from expensive materials in order to impart resistance to the corrosive effects of the bath materials.
An alternative cell construction which does away with the pool of molten aluminium above the cathode is the drained cathode cell. In such cells, the bulk of the aluminium metal is continuously drained from the cathode as it is formed, leaving only a thin film of molten aluminium on the surface of the cathode. Drained cathode cells permit close anode--cathode spac
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Houston Geoffrey James
Juric Drago Dragutin
Shaw Raymond Walter
Watson Kevin Drew
Comalco Aluminium Limited
Valentine Donald R.
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