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Aluminium-wettable carbon-based bodyRelated Patent Categories: Chemistry: Electrical And Wave Energy, Apparatus, Electrolytic, CellsAluminium-wettable carbon-based body description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060289300, Aluminium-wettable carbon-based body. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF THE INVENTION [0001] The invention relates a carbon-based body having an aluminium-wettable surface. The body can be used as a component of an apparatus which during use contains molten aluminium that comes into contact with the component, for example an aluminium electrowinning cell or an apparatus for treating molten aluminium. BACKGROUND OF THE INVENTION [0002] The production, purification or recycling of aluminium is usually carried out at high temperature in very aggressive environments, in particular in molten aluminium, 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. [0003] Graphite and other carbonaceous materials are commonly used for components, especially conductive components. Unfortunately, carbon components do not resist oxidation and/or corrosion and must be periodically replaced. [0004] Several proposals have been made to reduce wear of carbon components in such technologies to achieve a higher operation efficiency, reduce pollution and the costs of operation. [0005] For the purification of molten metals, in particular 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 (Holz/Duruz), WO01/42168 (de Nora/Duruz), WO01/42531 (Nguyen/Duruz/de Nora) and WO02/096831 (Nguyen/de Nora). [0006] In aluminium production, some components are exposed to molten fluoride-containing electrolyte, molten aluminium and/or anodically produced oxygen. In conventional Hall-Heroult cells these components are still made of consumable carbonaceous materials. [0007] U.S. Pat. Nos. 5,378,327, 5,578,174 and 5,679,224 disclose increasing the resistance of carbon components used in aluminium electrowinning cells by impregnating the carbon with colloidal alumina, ceria, cerium acetate, lithia, silica, yttria, thoria, zirconia, magnesia or monoaluminium phosphate that contain species of sodium, lithium, potassium, aluminium, calcium or amonium; or with a solution, suspension or melt of lithium, sodium or potassium. [0008] The use of titanium diboride and other RHM current-conducting elements to protect or replace carbon components in electrolytic aluminium production cells is described in U.S. Pat. Nos. 2,915,442, 3,028,324, 3,156,639, 3,215,615, 3,274,093, 3,314,876, 3,330,756, 3,400,061, 3,661,736, 4,093,524, 4,308,114, 4,466,996, 5,217,583, 5,310,476, 5,316,718, 5,364,513, 5,374,342, 5,618,403, 5,651,874, 5,728,466, 5,746,895 and 6,436,250. [0009] Recently, the aluminium-wettability of RHM coatings has been significantly improved by the addition of aluminium-reactable metal oxides, such as iron, copper, cobalt, nickel, zinc and manganese oxides, as disclosed in WO01/42168 (de Nora/ Duruz), WO01/42531, (Nguyen/Duruz/de Nora), WO02/096830 (Duruz/Nguyen/de Nora) and WO02/096831 (Nguyen/de Nora). [0010] Such RHM coated bodies have the following attributes: excellent wettability by molten aluminium, resistance to attack by molten aluminium and cryolite, low cost, environmentally safe, ability to absorb thermal and mechanical shocks, durability in the environment of an aluminium production cell, and ease of production and processing. SUMMARY OF THE INVENTION [0011] An object of the invention is to provide a carbon body that is easy to manufacture and that has an aluminium-wettable surface. [0012] A main object of the invention is to provide a carbon body that has the desired mechanical, physical, chemical, and electrochemical characteristics so that it can be used in a molten aluminium environment, in particular in an aluminium electrowinning cell or in an apparatus for treating molten aluminium. [0013] The invention relates to a carbon body having an aluminium-wettable outer part that is made of a carbon-rich mixture containing metal-based particles and carbon. The metal-based particles are made of metal oxide particles and/or partly oxidised metal particles or a precursor thereof. The metal of the metal-based particles are selected from iron, copper, cobalt, nickel, zinc and manganese and combinations thereof. The metal oxide particles and/or partly oxidised metal particles are reactable with molten aluminium when exposed thereto to form a mixture of: aluminium oxide; the metal of said metal-based particles; and aluminium. [0014] Carbon is an aluminium-repellent material. However, it has been found that when carbon material is mixed with an aluminium-reactable metal oxide and/or partly oxidised metal, molten aluminium reacts therewith to form alumina and the metal derived from the metal oxide and/or partly oxidised metal by transfer of oxygen from the metal oxide and/or partly oxidised metal to the aluminium. The metal derived from the metal oxide and/or partly oxidised metal may then be alloyed with unreacted molten aluminium. Hence, the presence of the aluminium-reactable metal oxide and/or partly oxidised metal promotes infiltration of aluminium by reaction therewith into the carbon material. [0015] Moreover, alumina, aluminium and the metal derived from the metal oxide and/or partly oxidised metal form a mixture that remains within the carbon material, in particular at its surface, and maintains it wetted by aluminium after reaction. [0016] It has surprisingly been observed that the carbon body with the aluminium-wettable outer part according to the invention is less exposed to corrosion and erosion by molten aluminium compared to a non-wettable carbon body, despite penetration of aluminium into the body's outer part. [0017] Without being bound to any theory, it is believed that the reduced corrodability and erodability of the carbon body according to the invention has two causes. On the one hand, the mixture of alumina, aluminium and the metal derived from the metal oxide and/or partly oxidised metal, formed at the body's outer part is less reactive than pure aluminium. Hence carbon reacts less with this mixture than it does with pure aluminium to form aluminium carbide. On the other hand, the aluminium layer that continuously wets the body's outer part during use inhibits exposure to the environment, e.g. cryolite, of the formed aluminium carbide and dissolution thereof into the environment. [0018] The carbon body can be a fully manufactured body or a part-manufactured body that is subsequently, for example, subjected to a consolidation or conditioning treatment, e.g. a heat and/or aluminisation treatment. [0019] The carbon-rich mixture is made predominantly of carbon, i.e. carbon is the mixture's main constituent. [0020] The body can comprise an inner part that is substantially free of metal-based particles, the body's outer part having a thickness of 5 to 50 mm, in particular from 10 to 25 mm. Alternatively, the body comprises metal-based particles throughout. [0021] In one embodiment, the metal-based particles are impregnated into the carbon body. In this case, the particles will be found in the body's pores, especially towards the body's surface. Typically, the carbon and the metal-based particles in the outer part are present in a weight ratio metal-based particles/carbon from 1/100 to 1/20, in particular from 1/50 to 1/25. 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