Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Patent
1996-11-12
1999-01-05
Valentine, Donald R.
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
204243R, 204279, 29825, 29861, C25C 316, H01R 4300
Patent
active
058557580
DESCRIPTION:
BRIEF SUMMARY
This is a National Stage Application of PCT/AU95/00339, filed Jun. 9, 1995.
TECHNICAL FIELD
This invention relates to a cathode connection for an aluminium smelter pot or cell.
Aluminium production involves the processing of refined ore (bauxite is refined into alumina by removing mud and sand) into aluminium by an electrolytic process. This process involves the separation of alumina into its component parts of aluminium metal and oxygen gas, by electrolytic reduction in a bath of molten cryolite (sodium aluminium fluoride).
The process is a continuous process with alumina being dissolved in the cryolite bath at a temperature of around 940.degree. C. and the aluminium separated by electrolysis is regularly removed for subsequent alloying and further treatment.
The reduction takes place in "pots" or "cells" (hereinafter referred to as pots) connected electrically in series, with the pots connected in this way being termed a "potline".
Direct current passes from the carbon anodes, through the cryolite bath containing aluminium in solution, to the cathode of one pot and then to the anodes of the next and so on. Steel bars embedded in the cathodes carry the current out of the pots. These bars are themselves connected by bolts or other jointing techniques to flexible straps which are then joined to a bus-bar system.
The pots consist of steel shells, in which the carbon cathode lining is housed. The shells hold the molten cryolite and alumina in solution and the molten aluminium created in the process. The molten aluminium settles in the bottom of the pots and is drawn off by a container called a vacuum crucible.
An electrically insulated superstructure, mounted above the shell, holds the carbon anodes and suspends them in the bath.
As the electrolytic reaction proceeds, aluminium is continuously deposited in a metal pool in the bottom of the pots and oxygen is liberated at the anodes, reacting with the carbon material of the anodes in the form of carbon dioxide.
As the anodes are consumed during the process, they must be continuously lowered to maintain a constant distance between them and the surface of the metal which is, in effect, the cathode.
The typical life of a pot is around 2,000 days, largely dependent on the installation of the pot lining or cathode assembly. For a smelter with some 400 pots, this means that an "old" pot is replaced with a "new" pot around every 5 days on average. For the period of the changeover of a pot, the electric circuit to the pot must be interrupted.
BACKGROUND ART
In a typical pot, around 40 steel bars are embedded in the carbon cathode and are used as current collector bars to carry current from the cell. These collector bars are then individually fastened by bolts to copper tabs fixed to flexible aluminium straps which are welded to the aluminium ring bus-bar system. Alternatively, an aluminium to steel transition joint is employed for the connection of the collector bar to the aluminium strap, this transition joint being welded on installation and cut at the time of pot removal.
For the bolted connections, contact pressure is critical to an acceptable joint.
A low resistance joint, typically 6 to 8 micro ohms, at these collector bar to strap connections (bar/strap connections) is very important to the efficiency of the process. High resistance joints limit the current able to be efficiently passed through the potline, cause higher than normal pot voltage and will cause unstable pot operation.
Collector bars are made from steel in order to withstand the high temperatures encountered during cell operation. Typically, the bar/strap connection effectively operates at a temperature of about 300.degree. C. Steel has relatively poor electrical properties and this makes it difficult to achieve a good connection between the collector bar and the strap.
DISCLOSURE OF THE INVENTION
The present invention provides an improved connection for the cathode of an aluminium smelter pot, including at least one insert received within a hole in the bar of a bar/strap connection for th
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Derwent Abstract Accession No. 84-144757/23, Class M28, SU 1041606 A1, 15 Sep. 1983.
Derwent Soviet Inventions Illustrated, Section 1, Chemical, vol. V, No. 23, Issued 12 Jul. 1974, Metallurgy, p. 11, SU 395500 A, 7 Jan. 1974.
Mitchell John Leslie
Tink John Gregory
JMT Engineering Pty. Ltd.
Valentine Donald R.
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