Chemistry: electrical and wave energy – Apparatus – Electrolytic
Patent
1995-10-06
1997-10-21
Gorgos, Kathryn L.
Chemistry: electrical and wave energy
Apparatus
Electrolytic
204294, 204243R, 204290R, 427113, 4271264, 4273762, 427380, C25D 1700, C25B 1112, C25C 702, B05D 512
Patent
active
056792247
DESCRIPTION:
BRIEF SUMMARY
This is a national stage application of PCT/US93/11380 filed Nov. 23, 1993.
FIELD OF THE INVENTION
This invention relates to carbon or carbon-based cathodic cell components of electrolytic cells for the production of aluminium in particular by the electrolysis of alumina in a sodium-containing molten halide electrolyte such as cryolite.
BACKGROUND ART
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 addition, the penetration of sodium species and other ingredients of cryolite or air leads to the formation of toxic compounds including cyanides.
The problems associated with penetration of sodium into the carbon cathode have been extensively studied and discussed in the literature.
Several papers in Light Metals 1992 published by the The Minerals, Metals and Materials Society discuss these problems. A paper "Sodium, Its Influence on Cathode Life in Theory and Practice" by Mittag et al., page 789, emphasises the advantages of using graphitic carbon over anthracite. Reasons for the superiority of graphitic carbon were also set out in a paper "Change of the Physical Properties and the Structure in Carbon Materials under Electrolysis Test" by Ozaki et al, page 759. another paper "Sodium and Bath Penetration into TiB.sub.2 Carbon Cathodes During Laboratory Aluminium Electrolysis" by Xue et al, page 773, presented results showing that the velocity of sodium penetration increased with increasing TiB.sub.2 content. Another paper "Laboratory Testing of the Expansion Under Pressure due to Sodium Intercalation in Carbon Cathode Materials for Aluminium Smelters" by Peyneau et al, page 801, also discusses these problems and describes methods of measuring the carbon expansion due to intercalation.
There have been several attempts to avoid or reduce the problems associated with the intercalation of sodium in carbon cathodes in aluminum production.
Some proposals have been made to dispense with carbon and instead use a cell bottom made entirely of alumina or a similar refractory material, with a cathode current supply arrangement employing composite current feeders using metals and refractory hard materials. See for example, EP-B-0 145 412, EP-A-0 215 555, EP-B-0 145 411, and EP-A-0 215 590. So far, commercialisation of these promising designs has been hindered due to the high cost of the refractory hard materials and difficulties in producing large pieces of such materials.
Other proposals have been made to re-design the cell bottom making use of alumina or similar refractory materials in such a way as to minimize the amount of carbon used for the cathode--see U.S. Pat. No. 5,071,533. Using these designs will reduce the problems associated with carbon, but the carbon is still subject to attack by sodium during cell start up.
There have been numerous proposals to improve the carbon materials by combining them with TiB.sub.2 or other refractory hard materials, see e.g. U.S. Pat. No. 4,466,996. But, as pointed out in the above-mentioned paper of Xoe et al., with each compo
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Gorgos Kathryn L.
Moltech Invent S.A.
Wong Edna
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