Stock material or miscellaneous articles – Composite – Of silicon containing
Patent
1996-08-05
1998-12-22
Lusignan, Michael
Stock material or miscellaneous articles
Composite
Of silicon containing
1062865, 1062871, 4233271, 4233281, 423332, 427299, 4273977, 427421, 427429, 428341, 501125, 501128, 204279, 204290R, B32B 904, B32B 1304
Patent
active
058516778
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD OF INVENTION
The invention relates to oxidation protection of carbon-containing products subjected to a hot, oxidizing environment whose temperature does not exceed 950.degree. C. These are, for example, the parts of anodes not immersed in the molten salt bath and the liners of aluminum electrolysis cells and in general the parts of carbon-containing refractory elements used in electrolysis conducted at less than 950.degree. C. that are not immersed in a molten salt bath. The electrolysis to which the present invention relates is hence, for example, of the type performed to fabricate aluminum, magnesium, calcium, strontium, or alloys such as iron-neodymium or lanthanum-nickel. It may also relate to protection of the hot parts of the carbon or graphite electrodes of electric furnaces used in particular for making steels and ferro-alloys that project from the charge, to the extent that these electrodes are exposed for a long time to an oxidizing atmosphere whose temperature remains below 950.degree. C.
PRIOR ART
In general, the anodes and liners of an aluminum electrolysis cell are not totally protected against oxidation. In cells with "pinholes" the anodes are protected only by a powder "cover layer" composed of alumina powder and ground bath solids, of greater or lesser thickness and permeable to air. The liners are impregnated with retardants effective at temperatures below 600.degree. C.
It should be noted with regard to the anodes that it is particularly at the tapholes and alumina inlet holes, namely the points where the "cover layer" is distinctly thinner or even nonexistent, that protection is most lacking. With regard to the liners, it should be noted that anti-oxidation protection becomes a substantial problem for cells operated at over 200,000 amperes.
The quantity of carbon consumed unnecessarily, namely outside the unavoidable depolarization reaction specific to electrolysis, may be estimated at about sixty kilograms per metric ton of aluminum produced. Half of this over-consumption is due to the carboxy reactivity of the anode in the molten salt bath and the other half to burnup of the non-immersed parts of the anode surfaces. It can be seen that about thirty kilograms of anode carbon could be saved per ton of aluminum produced if these anodes were more effectively protected against oxidation. In addition to the saving in materials, there is an advantageous economy from decreased servicing of the cell due to the longer life of the carbon-containing elements thus protected.
It is well known that coatings may be used to protect carbon-containing surfaces, but these coatings are unsuitable for the application in question, as they are generally intended for use at higher temperatures, expensive to use, and polluting.
Thus, Patent Application WO-A-93/20026 discloses a coating that protects all the cell elements, including cathodes, against corrosion in the salt bath and against oxidation when hot. However, its application requires a dryout phase followed by sintering, which is particularly expensive and difficult to accomplish due to the weight and size of the anodes. Moreover, the numerous elements contained in this coating (borides, carbides, colloidal silica, etc.) have the disadvantage of polluting the aluminum produced by the salt bath and the recycled anode "stubs." Since recycling of the bath and stubs is a determining factor in electrolysis efficiency, a solution that eliminates it loses much of its value.
In addition, Application EP-A-0,269,534 claims a coating that protects the shafts of the anodes and the projecting parts of the anodes used in aluminum production cells for electrolysis of alumina dissolved in molten cryolite melted by the Hall-Heroult process. It can be seen that the coating according to EP-A-0,269,534 indeed plays its protective role by sealing the iron of the anodes but does not effectively protect the carbon of the anodes against oxidation, particularly in the areas closest to the bath where the temperature exceeds 550.degree. C.
Finally, other solu
REFERENCES:
patent: 4888311 (1989-12-01), Davidovits et al.
patent: 5194091 (1993-03-01), Laney
patent: 5352427 (1994-10-01), Davidovits et al.
Orlinski, Joseph, Composites a Matrices Minerales de type Geopolymere, Geopolymer '88, vol. 2,pp. 315-324, 1988.
Laurent Veronique
Paillet Denis
Aluminum Pechiney
Carbone Savoie
Lusignan Michael
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