Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Patent
1998-06-19
2000-08-15
Gorgos, Kathryn
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
2042431, 2042473, 2042474, 204242, 205233, C25B 900, C25C 700, C25D 1700
Patent
active
061030910
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to the production of bodies of refractory borides for use in cells for the electrowinning of aluminium by electrolysis of alumina dissolved in a fluoride-based molten electrolyte such as cryolite. The invention also relates to the bodies of the refractory borides and cell components made therefrom, as well as cells for the electrowinning of aluminium including these components and their use to produce 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, or with glue.
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 penetration and intercalation of sodium, which 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, 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.
A major drawback of carbon as cathode material is that it is not wetted by aluminium. This necessitates maintaining a deep pool of aluminium (at least 100-250 mm thick) in order to ensure a certain protection of the carbon blocks and an effective contact over the cathode surface. But electromagnetic forces create waves in the molten aluminium and, to avoid short-circuiting with the anode, the anode-to-cathode distance (ACD) must be kept at a safe minimum value, usually 40 to 60 mm. For conventional cells, there is a minimum ACD below which the current efficiency drops drastically, due to short-circuiting between the aluminium pool and the anode. The electrical resistance of the electrolyte in the inter-electrode gap causes a voltage drop from 1.8 to 2.7 volts, which represents from 40 to 60 percent of the total voltage drop, and is the largest single component of the voltage drop in a given cell.
To reduce the ACD and associated voltage drop, extensive research has been carried out with Refractory Hard Metals or Refractory Hard Materials (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 b
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De Nora Vittorio
Duruz Jean-Jacques
Sekhar Jainagesh A.
Deshmukh Jayadeep R.
Gorgos Kathryn
Moltech Invent S.A.
Nicolas Wesley A.
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