Composite refractory/carbon components of aluminium production c

Details Composite refractory/carbon components of aluminium production c Composite refractory/carbon components of aluminium production c

1995-10-20

1998-05-05

Gorgos, Kathryn

Chemistry: electrical and wave energy

Apparatus

Electrolytic

204290R, 427122, 4271261, 4271262, 4273722, 428689, 428697, 428698, 428699, 428702, C25C 308

Patent

active

057468958

DESCRIPTION:

BRIEF SUMMARY
This is a national stage application of PCT/US93/10993, filed on Nov. 12, 1993.


FIELD OF THE INVENTION

The invention relates to the application of refractory borides to carbon-based components of cells for the production of aluminium by electrolysis of alumina dissolved in a cryolite-based and other molten halide electrolytes, in particular carbon cathodes. The invention also relates to such cells having carbon-based components protected from the corrosive attacks of liquids and/or gaseous components of the electrolyte in the form of elements, ions or compounds, by having refractory borides applied to their surfaces, as well as the use of these cells for the production of aluminium.


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 substrate forming the cell bottom floor. The cathode substrate is usually an anthracite based carbon lining made of prebaked cathode blocks, joined with a ramming mixture of anthracite, coke, and coal tar.
In Hall-Heroult cells, a molten aluminium pool acts as the cathode. The carbon 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 intercalation of sodium, which causes swelling and deformation of the cathode carbon blocks and ramming mix. In additon, 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, 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 ACD.
The use of titanium diboride and other RHM current-conducting elements in electrolytic aluminium production cells is described in U.S. Pat. Nos. 2,915,442, 3,028,324, 3,215,615, 3,314,876, 3,330,756, 3,156,639, 3,274,093 and 3,400,061. Despite extensive efforts and the potential advantages of having surfaces of titanium diboride at the cell cathode bottom, such propositions have not been commercially adopted by the aluminium industry.
Various types of TiB.sub.2 or RHM layers applied to carbon substrates have failed due to poor adherence and to differences in thermal expansion coefficients between the titanium diboride material and the carbon cathode block.
U.S. Pat. No. 3,400,061

REFERENCES:
patent: 4308115 (1981-12-01), Foster, Jr. et al.
patent: 4678760 (1987-07-01), Ray
patent: 4975191 (1990-12-01), Bruckmeyer et al.
patent: 5217583 (1993-06-01), Sekhar et al.

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