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.
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 a predetermined configuration (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-Al2O3
fused cast refractories have suitably been used mainly as glass tank furnace refractories.
For example, high alumina fused cast refractories mainly composed of &agr;-Al2O3
crystals and &bgr;-Al2O3 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-Al2O3
fused cast refractories have been focused on densification to minimum porosities
with the aim of increasing corrosion resistance against glass.
describes a fused cast alumina refractory article containing approximately
5% of an alkaline oxide and approximately 1.5% of a commercial sodalime glass, the
remainder of said article being substantially entirely crystalline alumina, said
article having a dense, finely crystalline structure. The crystalline alumina which
exists as a mixture of the alpha and beta crystal forms in approximately equal proportions.
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 erode 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
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
On the other hand, though regular cast high-alumina fused
cast refractories containing shrinkage cavities have low bulk specific gravities,
but there is a problem that cracks are generated along the border of shrinkage cavities
due to 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.
In the meanwhile, increase of the porosities of cast refractories
has been attempted. For example,
proposes a porous high-alumina fused cast refractory having a porosity
of at least 20%. Because the proposed refractory has as low an alkali metal oxide
content as 0.25% or below, the porous high-alumina fused cast refractory is composed
predominately of &agr;-Al2O3 crystals. However, &agr;-Al2O3
crystals readily become &bgr;-Al2O3 crystals through reaction
with an 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 to be used for upper structures of glass tank furnaces.
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.
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.
The present invention provides a porous high-alumina fused
cast refractory comprising from 94 to 98 mass% of Al2O3, from
1 to 6 mass%, in total, of Na2O and/or K2O as chemical components,
which is mainly composed of &agr;-Al2O3 crystals and &bgr;-Al2O3
crystals, has pores dispersed in it and has a porosity of from 5 to 30%, wherein
the ratio of &agr;-Al2O3 crystals to the sum of &agr;-Al2O3
crystals and &bgr;-Al2O3 crystals is from 30 to 70 mass
The present invention also provides a method of producing
a porous high-alumina fused cast refractory comprises from 94 to 98 mass% of Al2O3
from 1 to 6 mass%, in total, of Na2O and/or K2O as chemical
components, which is mainly composed of &agr;-Al2O3 crystals
and &bgr;-Al2O3 crystals, has pores dispersed in it and
has a porosity of from 5 to 30%, wherein the ratio of &agr;-Al2O3
crystals to the sum of &agr;-Al2O3 crystals and &bgr;-Al2O3
crystals is from 30 to 70 mass %, 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.
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 Al2O3,
from 1 to 6%, in total, of Na2O and/or K2O (hereinafter referred
to as alkali metal oxides) as chemical components.
If Al2O3 exceeded 98% or the alkali
metal oxides were less than 1%, the refractory would be mainly composed of &agr;-Al2O3
crystals (corundum crystals, hereinafter referred to as &agr;-crystals) alone,
which readily turn into &bgr;-Al2O3 crystals (R2O·nAl2O3,
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 Al2O3 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 Al2O3 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 SiO2
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 SiO2 content is preferably from 0.3 to 1.5%, particularly
from 0.5 to 1.0%.
The present cast refractory is mainly composed of &agr;-crystals
and &bgr;-crystals. In addition to &agr;-crystals and &bgr;-crystals, the
present cast refractory comprises a matrix glass phase comprising SiO2,
R2O 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 dispersion.
In the present invention, with respect to the mass ratio
of &agr;-crystals to &bgr;-crystals, the ratio of &agr;-crystals/(&agr;-crystals
+ &bgr;-crystals) is from 30 to 70%. It is unfavorable that the mass ratio exceeds
70% because the &agr;-crystals readily turns into &bgr;-crystals by reacting
with an alkali vapor, and the accompanying volume expansion leads to embrittlement.
It is also unfavorable that the mass ratio is less than 30%, because &bgr;-crystals
turn into &agr;-crystals in turn, and the accompanying volume shrinkage leads
to structural embrittlement. The ratio of &agr;-crystals to &bgr;-crystals can
be controlled by adjusting the R2O content.
In the present cast refractory, pores are formed dispersedly,
and the porosity is from 5 to 30%. In the present specification, the porosity is
measured after removal of shrinkage cavities from the refractory. If the porosity
is less than 5%, a porous high-alumina fused cast refractory which is-light in weight
and excellent in thermal shock resistance can not be obtained, and if the porosity
exceeds 30%, the corrosion resistance against an alkali vapor and float components
and the strength are insufficient. The porosity is preferably within the range of
from 7 to 25%. The porosity (%) is calculated as porosity = (1-(d2/d1))×100
from the true specific gravity d1 and the bulk specific gravity d2.
In the present invention, it is preferred that at least
80%, preferably at least 90%, of the pores formed dispersedly in it have diameters
of from 1 µm to 3 mm because pores are formed without remarkable deterioration
in strength while sufficient corrosion resistance is secured. Though pores can have
various shapes, because most pores are oval, the size of a pore means the average
of the longer diameter and the shorter diameter in the present specification.
The present cast refractory is obtainable like ordinary
fused cast refractories by formulating a refractory material of a predetermined
composition, putting the refractory material in an electric furnace at a high temperature
of at least 2000°C until the refractory material melts completely, pouring
the resulting melt into a casting mold having a predetermined shape by casting and
slowly cooling the melt, but is characterized in that a large amount of a gas is
blown into the melt before casting. Blowing of a gas into a melt allows favorable
formation of pores in the refractory and production of a cast refractory having
a much smaller amount of shrinkage cavities.
In the present invention, a gas is blown between fusion
of the refractory material and casting, preferably while the refractory material
is completely molten. A preferable way to blow a gas is to blow a large amount of
a high temperature gas through a ceramic or metal tube inserted in the melt. Appropriate
control of the kind of the gas, the blow time and the amount of the blow gas allows
formation of a given amount of desired pores in the refractory.
In the present invention, though the mechanism of pore
formation is unclear, but it is supposed that the high temperature blow gas dissolved
in the melt in a supersaturated state is released as the solubility decreases upon
cooling. Therefore, oxygen or an oxygen-containing gas containing at least 20 vol%
of oxygen such as air is preferable as the blow gas because improvement of porosity
is great in proportion of the amount of the blow gas by virtue of its low solubility
at low temperature.
It is preferred that the oxygen-containing gas is blown
in an amount of from 0.5 to 2.0 (L/1 kg melt) for a few minutes just before casting.
A longer blow time is preferable in the case of a gas having a low oxygen content.
Now the present invention will be described with reference
to Examples (Examples 1 to 8) and Comparative Examples (Examples 9 to 12).
Bayer's alumina (with a purity of 99% or above) as the
Al2O3 source, silica sand (with a purity of 99% or above)
as the SiO2 source, and other material powders such as Na2CO3,
K2CO3, and CaCO3 were mixed into material blends
having predetermined compositions, and they were fused completely in a single phase
AC 500 KVA arc electric furnace having graphite electrodes at temperatures of from
2000 to 2200°C.
Then, various gases (each having a purity of 99% or above)
were blown in the amounts (L/1 kg melt) shown in Tables 1 and 2 for blow times (min)
shown in Tables 1 and 2. The resulting melts were poured into graphite molds having
internal dimensions of 130 mm × 160 mm × 350 mm, and the resulting casts
were withdrawn from the graphite molds and allowed to cool to around room temperature
in a slow cooling box while insulated with Bayer's alumina powder.
[The evaluation results]
The chemical compositions (%) and the ratio of &agr;-crystals
(%) (i.e. &agr;-crystals/(&agr;-crystals + &bgr;-crystals)) of the various
fused cast refractories and the results of their evaluation are shown in Tables
1 and 2.
Bulk specific density: measured by Archimedes' method.
Compressive strength (MPa): measured in accordance with
Thermal shock resistance (the number of cycles): A 50 mm
× 50 mm × 50 mm specimen was cut out of each fused cast refractory and
subjected to repeated cycles of 15 minutes of incubation in an electric furnace
at an internal temperature of 1500°C and 15 minutes of cooling down in the
atmosphere, and evaluated from the number of cycles which had been repeated when
a crack was first recognized on the surface with the naked eye.
Amount of corrosion (mm): For evaluation of corrosion resistance,
a 50 mm × 50 mm × 50 mm specimen was cut out of each fused cast refractory
and used as the rid of a crucible containing Na2CO3. The crucible
was placed in an electric furnace at 1550°C for 24 hours with the rid on it
and taken out, and the amount of the corrosion of the specimen was measured. For
reference, it is noted that silica bricks commonly used as a ceiling refractory
for conventional glass tank furnaces corroded to a depth of 20 mm.
Appearance of the corroded surface: After measurement of
the amount of corrosion, the samples were inspected for swells and the like with
the naked eye. Those having swells were rated as swollen, and those having no swells
were rated as good.
Pore information: The cast refractories were cut along
the vertical axis of the casting mold and inspected for uniformity of pore distribution
with the naked eye. In Examples 1 to 8, about 95% of the pores were from 1 µm
to 3 mm in size, and the pore distribution was uniform.
CaO and others
Ratio of &agr;-crystals
Bulk specific gravity
Amount of corrosion
Appearance of corroded surface
Thermal shock resistance
CaO and others
Ratio of &agr;-crystals
Bulk specific gravity
Amount of corrosion
Appearance of corroded surface
Thermal shock resistance
The present cast refractory has sufficient corrosion resistance
against alkali vapors and the like and excellent thermal shock resistance and therefore
is suitable as a glass tank furnace refractory. Besides, because of its porous structure
in which fine pores are uniformly dispersed, it has a low bulk specific gravity
and is light in weight and obtainable at low cost compared with dense high Al2O3
fused cast refractories.
Accordingly, it is a perfect refractory for structural
parts of glass tank furnaces such as upper structures, especially for the upper
structure of oxygen-burning glass tank furnaces.
Because no foaming agent is used in its production, dissolution
control is easy. Therefore, the present invention makes it possible to produce porous
high-alumina fused cast refractories having pores dispersed uniformly in it readily
at low cost.