Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Patent
1997-03-14
1999-02-02
Valentine, Donald R.
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
205381, 205384, 204243R, 204284, 204290R, 204294, C25C 308
Patent
active
058659811
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to cells for the production of aluminium by the electrolysis of a molten electrolyte, in particular the electrolysis of alumina dissolved in a molten fluoride-based electrolyte such as cryolite, comprising anodes immersed in the molten electrolyte above a cell bottom whereon molten product aluminium is collected in a pool or layer which contains bodies of aluminium-resistant material.
BACKGROUND OF THE INVENTION
Aluminium is produced conventionally by the Hall-Heroult process, by the electrolysis of alumina dissolved in cryolite-based molten electrolytes at temperatures up to around 950.degree. C. A Hall-Heroult reduction cell typically has a steel shell provided with an insulating lining of refractory material, which in turn has a lining of carbon which contacts the molten constituents. Conductor bars connected to the negative pole of a direct current source are embedded in the carbon cathode forming the cell bottom floor. The cathode is usually an anthracite or graphite based carbon lining made of prebaked cathode blocks, joined with a ramming mixture or glue.
In Hall-Heroult cells, a molten aluminium pool acts as the cathode surface. The carbon bottom lining or cathode material has a useful life of three to eight years, or even less under adverse conditions. The deterioration of the cathode bottom is due to erosion and penetration of electrolyte and liquid aluminium as well as penetration of sodium into the carbon, which by chemical reaction and intercalation causes swelling, deformation and disintegration of the cathode carbon blocks and ramming mix. In addition, the penetration of sodium species and other ingredients of cryolite or air leads to the formation of toxic compounds including cyanides.
Difficulties in operation also arise from the accumulation of undissolved alumina sludge on the surface of the carbon cathode beneath the aluminium pool which forms insulating regions on the cell bottom. Penetration of cryolite and aluminium through the carbon body and the deformation of the cathode carbon blocks also cause displacement of such cathode blocks. Due to displacement of the cathode blocks and the formation of cracks, aluminium reaches the steel cathode conductor bars causing corrosion thereof leading to deterioration of the electrical contact, non uniformity in current distribution and an excessive iron content in the aluminium metal produced.
Extensive research has been carried out with Refractory Hard Metals (RHM) such as TiB.sub.2 as cathode materials. TiB.sub.2 and other RHM's are practically insoluble in aluminium, have a low electrical resistance, and are wetted by aluminium. This should allow aluminium to be electrolytically deposited directly on an RHM cathode surface, and should avoid the necessity for a deep aluminium pool.
Because titanium diboride and similar Refractory Hard Metals are wettable by aluminium, resistant to the corrosive environment of an aluminium production cell, and are good electrical conductors, numerous cell designs utilizing Refractory Hard Metal have been proposed, which would present many advantages, notably including the saving of energy by reducing the anode-cathode distance (ACD).
U.S. Pat. No. 3,856,650 proposed lining a carbon cell bottom with a ceramic coating upon which parallel rows of tiles are placed, in the molten aluminium, and spaced apart from one another by expansion gaps in a grating-like arrangement. The purpose of this "grating" was to protect the ceramic coating against mechanical effects due, for example, to movements of the aluminium pool.
U.S. Pat. No. 4,243,502 described designs for aluminium-wettable cathodes some of which had a generally horizontal active surface supported by one or more supporting plates, usually connected to a current supply by an extension protruding from the top of the electrolyte, between the anodes. Such designs were not practicable.
U.S. Pat. No. 4,410,412 described wettable cathodes made of aluminide materials. These cathodes were supposed to be exchangeable, by ho
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Moltech Invent S.A.
Valentine Donald R.
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