Alumina refractories and methods of treatment

#This invention relates to alumina refractories and methods for treating alumina refractories. More particularly, the invention relates to methods of improving the strength of alumina refractories and refractories having improved strength.

#Zircon (ZrSiO4) and zirconia (ZrO2) are recognized as highly refractory materials. Accordingly, shaped bodies of such materials, such as brick, are frequently employed in conjunction with high temperature processes. Zircon is more frequently used because of its stability, ease of fabrication and lower cost.

#Alumina is another known refractory material, and alumina is used in a variety of refractory applications, for example, in furnaces. Furnaces that are used in the manufacture of fused silica optical members, however, typically utilize zircon refractory materials. FIG. 1 shows a furnace 100 for producing fused silica glass. The furnace includes a crown 12 and a plurality of burners 14 projecting from the crown. As noted above, silica particles are generated in a flame when a silicon containing raw material together with a natural gas are passed through the plurality of burners 14 into the furnace chamber 26. These particles are deposited on a hot collection surface of a rotating body where they consolidate to the solid, glass state. The rotating body is in the form of a refractory cup or containment vessel 15 having lateral walls 17 and a collection surface 21 which surround the boule 19 and provide insulation to the glass as it builds up. The refractory insulation ensures that the cup interior and the crown are kept at high temperatures.

#FIG. 1 is a schematic drawing illustrating a fused silica production furnace.

#One embodiment of present invention relates to methods of treating alumina refractory materials. In one embodiment, treatment of alumina refractory materials with a halogen gas substantially improves the strength of the materials, and in some embodiments, by at least 50% when compared with untreated materials. Another embodiment of the invention relates to use of alumina refractory materials in fused silica production furnaces.

#Alumina is of interest in high temperature applications because it can be used in temperatures up to about 1980° C. in air and up to about 1925° C. in reducing conditions. The ability to withstand these temperatures makes alumina a particularly attractive material for used in fused silica furnaces. Alumina also has a high melting point of about 2000° C. It is a single component oxide system that does not dissociate. Alumina refractory materials containing 99% or higher alumina content typically contain less than 2000 parts per million silica and generate less silica-related volatilization when used in furnaces.

#Alumina refractory bricks containing greater than 99% alumina are typically fired at a temperature of about 1650° C. prior to being installed in a furnace. The theoretical density of alumina is 3.98 g/cm3. In comparison to zircon, which has a theoretical density of 4.6 g/cm3, an alumina refractory brick that is the same size and having the same porosity as a zircon refractory brick is lighter than a zircon refractory brick. In addition, alumina is harder than zircon, which should result in a lower potential for imparting solid inclusions into a furnace utilizing alumina refractories when compared to a furnace using zircon refractories. Alumina also has excellent creep resistance compared to zircon.