Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
2002-01-31
2004-11-02
Marcantoni, Paul (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C501S084000
Reexamination Certificate
active
06812177
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a refractory used for glass tank furnaces, in particular, to a porous high-alumina fused cast refractory suitable for upper structures of glass tank furnaces, and a method of its production.
DESCRIPTION OF THE BACKGROUND
Fused cast refractories are provided by melting formulated refractory raw materials in an electric arc furnace completely, pouring the resulting melt into casting molds having predetermined configurations (casting), and solidifying the melt by cooling to ordinary temperature, in many cases, with heat insulation. It is widely known that fused cast refractories are denser and more corrosion-resistant than fired and unfired bonded refractories.
Among these fused cast refractories, high-Al
2
O
3
fused cast refractories have suitably been used mainly as glass tank furnace refractories. For example, high alumina fused cast refractories mainly composed of &agr;-Al
2
O
3
crystals and &bgr;-Al
2
O
3
crystals are frequently used at portions of glass tank furnaces which contact with molten glass and have such dense structures that they have porosities of 4% and less, provided that pores called shrinkage cavities inevitably formed during the cooling step after casting are ignored.
Therefore, improvements of high-Al
2
O
3
fused cast refractories have been focused on densification to minimum porosities with the aim of increasing corrosion resistance against glass.
In recent years, the application of the technique of oxygen burning to glass tank furnaces has generated a new demand on glass tank furnace refractories. Namely, though conventional glass tank furnaces usually use silica bricks having bulk specific gravities of about 2 for ceilings and other upper structures (such as crowns), there is a problem that high concentrations of alkali vapor in glass tank furnaces utilizing the technique of oxygen burning erodes silica bricks considerably. As a countermeasure, use of high-alumina fused cast refractories excellent in corrosion resistance against alkali vapor for these upper structures is considered. Conventional high-alumina fused cast refractories are grouped into two classes: those called void-free which are dense residues of refractories obtained by cutting off shrinkage cavities, and so-called regular casts, which partly contain shrinkage cavities.
It is unadvisable to use void-free high-alumina fused cast refractories for upper structures of glass tank furnaces because such low-porosity refractories having higher bulk specific gravities than silica bricks are heavy in weight and require upper structure supports having high mechanical strength. Another disadvantage of them is their poor thermal shock resistance due to their dense structures.
On the other hand, although regular cast high-alumina fused cast refractories containing shrinkage cavities have low bulk specific gravities, a problem that occurs is that cracks form along the border of the shrinkage cavities because of the great difference in physical properties across the border during the operation of the furnace.
Namely, conventional high-alumina fused cast refractories are advantageous in view of corrosion resistance against glass by virtue of their low porosity and denseness but their high bulk specific gravities is disadvantageous to their use for parts which do not require so much corrosion resistance such as upper structures in view of structural strength and cost.
Meanwhile, increases in the porosities of cast refractories have been attempted. For example, JP-A-59-88360 proposes a porous high-alumina fused case refractory having a porosity of at least 20%. Because the proposed refractory has an alkali metal oxide content as low as 0.25% or below, the porous high-alumina fused cast refractory is composed predominantely of &agr;-Al
2
O
3
crystals. However, &agr;-Al
2
O
3
crystals readily become &bgr;-Al
2
O
3
crystals through reaction with alkali vapor while expanding in volume to form a brittle structure. Therefore, the proposed porous high-alumina fused cast refractory does not have enough corrosion resistance for use in the upper structure of glass tank furnaces.
JP-A-3-208869 proposes the use of a foaming agent such as a metal, carbon and a carbide to form pores. The use of a foaming agent has a problem in the production process because the vigorous foaming reaction between a foaming agent and a melt which involves generation of carbon dioxide or the like makes it difficult to control the melting.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a porous high-alumina fused cast refractory which has sufficient corrosion resistance against an alkali vapor or the like, is light in weight and has excellent thermal shock resistance and a method of producing it.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention provides a porous high-alumina fused cast refractory comprising from 94 to 98 mass % of Al
2
O
3
, from 1 to 6 mass %, in total, of Na
2
O and/or K
2
O as chemical components, which is mainly composed of &agr;-Al
2
O
3
crystals and &bgr;-Al
2
O
3
crystals, has pores dispersed in it and has a porosity of from 5 to 30%.
The present invention also provides a method of producing a porous high-alumina fused cast refractory comprises from 94 to 98 mass % of Al
2
O
3
from 1 to 6 mass %, in total, of Na
2
O and/or K
2
O as chemical components, which is mainly composed of &agr;-Al
2
O
3
crystals and &bgr;-Al
2
O
3
crystals, has pores dispersed in it and has a porosity of from 5 to 30%, which comprises blowing a gas, especially a gas containing oxygen, into a molten refractory material, casting and slowly cooling the refractory material to form pores in it dispersedly.
BEST MODE FOR CARRYING OUT THE INVENTION
The porous high-alumina fused cast refractory of the present invention (hereinafter referred to as the present cast refractory) comprises from 94 to 98 mass % (hereinafter abbreviated simply as %) of Al
2
O
3
, from 1 to 6%, in total, of Na
2
O and/or K
2
O (hereinafter referred to as alkali metal oxides) as chemical components.
If Al
2
O
3
exceeded 98% or the alkali metal oxides were less than 1%, the refractory would be mainly composed of &agr;-Al
2
O
3
crystals (corundum crystals, hereinafter referred to as &agr;-crystals) alone, which readily turn into &bgr;-Al
2
O
3
crystals (R
2
O·nAl
2
O
3
, wherein R is Na or K, and n is a real number around 11, herein after referred to as &bgr;-crystals) upon contact with an alkali vapor while expanding in volume when used for upper structures of a glass tank furnace, and the corrosion resistance would become inadequate due to the resulting structural embrittlement.
On the other hand, if Al
2
O
3
were 94% or less or the alkali metal oxides exceeded 6%, the present cast refractory would be mainly composed of &bgr;-crystals alone and have such a low compressive strength as 30 MPa or below, and use of the present cast refractory for upper structures of a glass furnace would make a problem in view of mechanical strength. It is preferred that Al
2
O
3
is from 94.5 to 96.5%, and the alkali metal oxides are from 2.5 to 4.5%.
The present cast refractory preferably comprises SiO
2
as another component to form a matrix glass phase. The matrix glass phase helps formation of a crack-free refractory by relaxing strain stress which occurs during the annealing. The SiO
2
content is preferably from 0.3 to 1.5%, particularly from 0.5 to 1.0%.
The present cast refractory is mainly composed of &bgr;-crystals and &bgr;-crystals. In addition to &agr;-crystals and &bgr;-crystals, the present cast refractory comprises a matrix glass phase comprising SiO
2
, R
2
O and CaO as main components (hereinafter the present matrix glass phase) and pores and has such a structure that the matrix glass phase fills gaps between the crystals, and pores are dispersed between the &agr;-crystals, &bgr;-crystals and the present matrix glass phase. It is preferred that pores are dispersed uniformly because the durability of the refractory increases with the uniformity of pore dispers
Asahi Glass Company Limited
Marcantoni Paul
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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