A number of activities, such as the production, purification and recycling of metals, in particular aluminium, are usually carried out at high temperature in very aggressive environments such as molten metal, molten electrolyte and/or corrosive gas. Therefore, the materials used for the manufacture of components exposed to such environments must be thermally and chemically stable.
Graphite and other carbonaceous materials are commonly used for components, especially conductive components. Several proposals have been made to reduce wear of carbon components in such technologies by making them aluminium-wetted so as to achieve a higher operation efficiency, reduce pollution and the costs of operation.
For the purification of molten aluminium, by the injection of a flux removing impurities towards the surface of the molten metal, it has been proposed to coat carbon components which are exposed to the molten metal with a coating of refractory material as disclosed in WO00/63630 (assigned to Moltech Invent S.A.).
In aluminium production, some components are exposed to molten fluoride-containing electrolyte and/or molten aluminium. In conventional Hall-Héroult cells these components are still made of consumable carbonaceous materials.
It has long been recognized that it would be desirable to make (or coat or cover) the cathode of an aluminium electrowinning cell with a refractory boride such as titanium diboride that would render the cathode surface wettable by molten aluminium which in turn would lead to a series of advantages.
For example, U.S. Pat. Nos. 5,310,476, 5,364,513, 5,651,874 and 6,436,250 (all assigned to Moltech Invent S.A.) disclose applying a protective coating of a refractory material such as titanium diboride to a carbon component of an aluminium electrowinning cell, by applying thereto a slurry of particulate refractory material and/or precursors thereof in a colloid in several layers with drying between each layer. WO01/42168, WO01/42531 and WO02/096831 (all assigned to Moltech Invent S.A.) disclose the use of a layer made of particulate oxide of Mn, Fe, Co, Ni, Cu, Zn, Mo or La (−325 mesh) mixed with refractory material and/or on a layer of refractory material. The use of these oxides promotes the wetting of the refractory material by reacting with molten aluminium. These patents also disclose the use of such materials for use in an oxidizing and/or corrosive environment.
U.S. Pat. No. 6,338,785 (assigned to Moltech Invent S.A.) discloses a method of protecting a carbon cathode during cell start-up against oxidation and/or corrosion by applying a start-up layer onto the cathode, in particular an aluminium foil or metallization. It is mentioned that a further layer can be applied to the cathode. The further layer can comprise various materials, inter-alia colloidal alumina, silica, yttria, ceria, thoria, zirconia, magnesia, lithia or monoaluminium phosphate. It is also mentioned that, in order to improve the conductivity of the further start-up layer, such layer may contain a particulate conductor for example particulate aluminium, nickel, iron, titanium, cobalt, chromium, zirconium or copper. Wettability of the cathode by aluminium is achieved by coating the cathode surface with a permanent aluminium-wettable refractory material underneath the start-up layers.
These materials have not as yet found wide commercial acceptance and there is a need to improve the aluminium-wettability of components used in a molten aluminium environment.