Chemistry: electrical and wave energy – Apparatus – Electrolytic
Patent
1998-12-08
2000-09-05
Bell, Bruce F.
Chemistry: electrical and wave energy
Apparatus
Electrolytic
2042473, 2042477, 204294, B01D 5940
Patent
active
061137561
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to an electrolytic reduction cell for the production of a metal, such as aluminium. The invention particularly relates to a cathode construction used in such cells.
Aluminium metal is generally produced by the Hall-Heroult process in which electrical current is passed through an electrolytic bath comprising alumina dissolved in molten cryolite to cause the electrodeposition of molten aluminium. Electrolytic reduction cells comprise an outer steel shell that is lined with a layer of insulating material, such as refractory bricks. Carbonaceous blocks are placed on top of the insulating layer and these carbonaceous blocks form the cathode of the cell. The cathode must last for the expected operating life of the cell, which is typically 1000 to 2000 days. A number of consumable anodes are located a short distance above the cathode. In use, the electrolytic bath is located between the cathode and the anodes and the passage of electrical current through the cell causes molten aluminium to form at the cathode. In conventional cells, the molten aluminium collects as a pool on top of the cathode and in operation the pool of molten aluminium acts as the top of the cathode. Aluminium is periodically drained from the cell, typically on a daily basis.
Electrolytic reduction cells are arranged in potlines in which a large number of cells are connected in series. Electrical current enters a cell through the anodes, passes through the electrolytic bath and pool of molten metal and into the cathode. The current in the cathode is collected and passes to an external current carrier and then along to the next cell.
In conventional aluminium reduction cell technology, embedded collector bars are used to collect electrical current from the carbonaceous cathode and conduct it to the external ring bus. The embedding of collector bars, which is performed with the use of cast iron or carbonaceous glue, imposes a number of limitations which adversely affect service life, cost and performance of aluminium reduction cells.
Accommodation of collector bars within the cathode carbon requires a machined groove to be formed in the block and thus increases the cost of cathode blocks and at the same time, the presence of a groove reduces the potential cell life (available erodable lining), in some cases by about 40%. Furthermore, the cathode current density distribution along the length of the cathode blocks is uneven with the outer-most portions of the cathode blocks drawing current at up to three to four times higher density compared to the inner portions of the block.
In embedded collector bar technology, the bar is either cast or glued into a recess on the underside of the cathode block. Under normal operating conditions the electron transfer from the collector bar to the carbon occurs through active spots (a-spots) which are concentrated along the sides of the collector bar and nearest to the block end. The top portion of the collector bar normally does not participate in electron transfer as its own weight and a lack of high-temperature strength causes it to sag. The concentration of a-spots along the sides of the collector bar slots increases the average current path length in the cathode carbon and thus increases cathode voltage loss.
Most of the current transfer from collector bars to carbon occurs near the block end and this leads to uneven current distribution on the surface of the cathode. It is highest nearest to the outer edge of the anode shadow or ledge toe. The uneven cathode current density has a dual effect on cell operation: on the one hand it increases the rate of dissolution of carbon by increasing the chemical activity of sodium (this drives the aluminium carbide forming reaction) in the affected region, and on the other, it increases the rate of transport of dissolved aluminium carbide by inducing circulation of metal and catholyte. This increased circulation can result either from the increased metal pad heave due to interaction in the metal pad of horizontal
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Fridman Michael A.
Juric Drago Dragutin
Lakomsky Victor J.
Paton Boris Eu
Shaw Raymond Walter
Bell Bruce F.
Comalco Aluminium Limited
Plasma Technology Scientific and Engineering Centre of E O Paton
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