Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
1999-11-16
2001-07-10
Bell, Bruce F. (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S290010, C204S290030, C204S290120, C204S290130, C204S290150, C264S105000, C264S119000, C264S241000, C264S293000
Reexamination Certificate
active
06258224
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
This invention relates to cathodes used in electrolysis cells, particularly in the cells used for the production of aluminum metal. More particularly, the invention relates to multi-layer cathode structures used in reduction cells of this type.
II. Description of the Prior Art
In metal reduction cells it is usual to line a container with a carbonaceous material, such as anthracite and/or graphite, and to use the carbonaceous layer as a cathode for the cell. A molten electrolyte is held within the container and carbon anodes dip into the molten electrolyte from above. As electrolysis proceeds, molten metal forms a pool above the cathode layer.
The cathode layer, which normally extends along the bottom wall of the cell and possibly up the side walls to a level above the height of the surface of the molten electrolyte, eventually breaks down and the cell has to be taken out of operation for cathode repair or replacement. This is because the surface and joints of the carbonaceous material are attacked and eroded by the molten metal and electrolyte. The erosion/corrosion of the bottom blocks is a particular problem because of movements of the cell contents caused by magneto-hydrodynamic effects (MHD).
Attempts have been made to make cell cathodes more durable by providing the carbonaceous material with a protective lining. The lining must, of course, be electrically-conductive and, to facilitate the operation of self-draining cathode cells, should be wettable by the molten metal.
Lining materials used for this purpose have included refractory composites made of a carbonaceous component and a refractory metal oxide or boride. Because of its desirable erosion resistance and metal wettability, titanium boride (TiB
2
) is a particularly preferred material for use in such composites, despite its extremely high cost. However, the use of this material causes a problem in that it has a different coefficient of thermal expansion compared to that of carbon. During operation at high temperature in the cell, cracks tend to form at the interface of the coating and the underlying cathode carbon, leading to eventual failure of the cathode structure. Thus, the effective service life of the cell is not prolonged as much as would be desired using multi-layer cathode structures of this kind. In fact, although various kinds of cathode structures have been proposed in the past, usually requiring ceramic tiles of TiB
2
adhered to carbon blocks, no such structures are in common use today because the tiles eventually dislodge or crack due to the difference in thermal expansion properties. The same is also true of other composite coating materials, e.g. those containing refractory metals oxides (such as TiO
2
and SiO
2
), silicon, nitrides, etc.
A possible solution to this problem would be to provide cathodes structures made entirely of blocks of the composite materials. However, the high cost of such composites (particularly those based on TiB
2
), has prevented this as a widespread solution.
An attempt to improve the adhesion of the layers is disclosed in U.S. Pat. No.5,527,442 to Sekhar et al., issued on Jun. 18, 1996. This patent relates to the coating of refractory materials (including titanium borides) onto substrates made of different materials, particularly carbonaceous materials, for use in aluminum reduction cells. To avoid adhesion problems, the coating material is applied as a micropyretic slurry to the carbonaceous substrate which, when dried, is ignited to produce condensed matter forming a coating adherent to the surface of the substrate and thus protecting it. However, such a process is expensive, has not been adopted on a significant industrial scale and also this material has a short operational life.
There is, therefore, a need for an improved way of forming multi-layer cathodes that are not subject to unacceptable rates of dislodgement or cracking of the protective layers.
SUMMARY OF THE INVENTION
An object of the present invention is to overcome adhesion and cracking problems in multi-layer cathode structures.
Another object of the present invention is to provide a process of producing multi-layer cathode structures having an acceptable operating life in aluminum production cells.
Yet another object of the invention is to provide multi-layer cathodes in which protective outer layers remain firmly adhered to underlying carbonaceous layers during high temperature use in aluminum production cells.
According to one aspect of the invention, there is provided a process of producing multi-layer cathode structures, which comprises providing a carbonaceous cathode substrate, and forming at least one layer of a metal boride-containing composite refractory material over the substrate, wherein the surface of the carbonaceous substrate to be coated is roughened prior to the formation of the layer overlying the said surface.
According to another aspect of the invention there is provided a process of producing multi-layer cathode structures, which comprises providing a carbonaceous cathode substrate, and forming at least two coating layers of a metal boride-containing composite refractory material successively over the substrate, wherein the content of metal boride in the coating layers increases progressively as the distance of the layer from the substrate increases.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
While the preferred metal boride is TiB
2
, the metal may be selected from the group consisting of titanium, zirconium, vanadium, hafnium, niobium, tantalum, chromium and molybdenum. Thus, where reference is made to TiB
2
, it will be understood that the titanium may be replaced by any of the other above metals.
The cathode is preferably formed in a mould having closed sides and bottom and an open top. A carbonaceous substrate material preferably having a thick, pasty consistency is placed in the bottom of the mould and the top surface of this substrate is then roughened, e.g. by drawing a rake across the surface. The times of the rake form grooves in the surface of the substrate. At least one layer of a TiB
2
-containing composite refractory material is placed over the raked substrate and a weight which is the full internal dimension of the mould is placed on top of the cathode material.
The entire mould unit is then vibrated to compress the material into a green cathode shape, which is then prebaked and machined prior to insertion into an electrolysis cell. In addition to compaction, the vibration step also causes some mixing of the material resulting in a mixed area which is actually thicker than the depths of the grooves formed in the substrate.
A typical rake for the above purpose has times spaced about 25 mm apart and lengths of about 75 to 100 mm. A typical commercial cathode has dimensions of about 43 cm high, 49 cm wide and 131 cm long. When more than one layer of TiB
2
-containing composite is placed on top of the substrate, it is desirable to rake the top surface of each layer before applying a further layer.
It is also preferred that, when more than one coating layer over the substrate is provided, the content of TiB
2
in the layers increase with the distance of the layer from the carbonaceous substrate. That is to say, the outermost coating layer should preferably have the highest TiB
2
content and the innermost coating layer should preferably have the lowest. The other main component of the TiB
2
-containing component is a carbonaceous material, usually in the form of anthracite, pitch or tar. The carbonaceous material of the substrate is also usually in the form of anthracite, graphite, pitch or tar.
Most practically, there should preferably be at least 2 coating layers, and the content of the TiB
2
should increase from about 10-20% by weight in the innermost layer to about 50 to 90% in the outermost layer. For example, a cathode having three TiB
2
-containing layers may have a top layer containing 50-90% TiB
2
and 50-10% carbon, and intermediate layer containing 20-50% TiB
2
and 80-50% carbon and a bottom layer containi
Alcan International Limited
Bell Bruce F.
Cooper & Dunham LLP
LandOfFree
Multi-layer cathode structures does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Multi-layer cathode structures, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Multi-layer cathode structures will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2546864