Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
1996-06-17
2002-06-11
Valentine, Donald R. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S384000, C205S388000
Reexamination Certificate
active
06402926
ABSTRACT:
FIELD OF THE INVENTION
Generally, the invention relates to the development of a coating technology to apply different compositions of refractory materials such as those containing hard metals, particularly titanium borides, metallic alloys, intermetallic compounds, cermets, oxides, metals and ceramics to the surface of substrates made of different materials such as carbonaceous materials, refractory materials, ceramics, cermets, oxides, metallic alloys (particularly those of iron, nickel, aluminum, and copper) and intermetallic compounds.
Such substrates may in particular be components of electrolytic cells operating at high temperatures, particularly aluminium production cells. The present invention thus more specifically relates to a novel method of application of adherent protective coatings of refractory material to the surface of substrates of components of electrolytic cells for molten salt electrolysis for the electrowinning of metals and operating at high temperatures, particularly for the production of aluminium and as well to novel designs of such cells and their operation.
The protective coating is a refractory material or a combination of refractory materials containing aluminum-wettable hard metals, particularly titanium borides or other materials consisting of metallic alloys, intermetallic compounds, cermets, oxides and ceramics on the surface of the substrates e.g. of electrolytic cell components, in particular an adherent protective coating of aluminium-wettable refractory material on the surface of a carbonaceous or refractory substrate lining the cell bottom floor of an aluminium production cell.
The invention also relates to composite materials comprising a carbonaceous or refractory substrate coated with an aluminium-wettable refractory material and to the use of the coated composite materials in such cells.
BACKGROUND OF THE INVENTION
Among the metals obtained in electrolytic cells operating at high temperature in a molten salt electrolyte containing an oxide or compound of the metal to be electrowon, aluminium is the most important and the invention will describe in particular the protection of components of aluminium cells, more particularly the protection of the cell cathode bottom by applying an aluminium wettable, adherent coating.
Aluminium is produced conventionally by the Hall-Héroult process, by the electrolysis of alumina dissolved in molten salt containing cryolite at temperatures around 950° C. A Hall-Héroult 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-Héroult 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 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 (RHM) such as TiB2 as cathode materials. TiB
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.
The non-acceptance of tiles and other methods of applying layers of TiB
2
and other RHM materials on the surface of aluminium production cells is due to their lack of stability in the operating conditions, in addition to their cost. The failure of these materials is associated with penetration of the electrolyte when not perfectly wetted by aluminium, and attack by aluminium because of impurities in the RHM structure. In RHM pieces such as tiles, oxygen impurities tend to segregate along grain boundaries leading to rapid attack by aluminium metal and/or by cryolite. To combat disintegration, it has been proposed to use highly pure TiB
2
powder to make materials containing less than 50 ppm oxygen. Such fabrication further increases the cost of the already-expensive materials. No cell utilizing TiB
2
tiles as cathode is known to have operated for long periods without loss of adhesion of the tiles, or their disintegration. Other reasons for failure of RHM tiles have been the lack of mechanical strength and resistance to thermal shock.
Various types of TiB
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 describes a cell without an aluminium pool but with a drained cathode of Refractory Hard Metal which consists of a mixture of Refractory Hard Metal, at least 5 percent carbon, and 10 to 20% by weight of pitch binder, baked at 900° C. or more and rammed into place in the cell bottom. Such composite cathodes have found no commercial use probably due to susceptibility to attack by the electrolytic bath.
U.S. Pat. No. 4,093,524 discloses bonding tiles of titanium diboride and other Refractory Hard Metals to a conductive substrat
de Nora Vittorio
Sekhar Jaihagesh A.
Deshmukh Jayadeep R.
Moltech Invent S.A.
Valentine Donald R.
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