Fused cast products based on alumina-zirconia-silica with an...

Details Fused cast products based on alumina-zirconia-silica with an... Fused cast products based on alumina-zirconia-silica with an...

2001-01-30

2003-06-10

Group, Karl (Department: 1755)

Compositions: ceramic

Ceramic compositions

Refractory

C501S104000

Reexamination Certificate

active

06576580

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to AZS (alumina-zirconia-silica) fused cast products having improved characteristics for use in a glass melting furnace. The invention relates more particularly to oxidized AZS products characterized by a homogeneous crystal structure and having elongate dendritic zirconia crystals in the active area thereof. The simultaneous presence of these characteristics provides these products with increased resistance to corrosion by molten glass.
2. Description of the Prior Art
Fused cast products (also referred to as “electro-cast” products) are obtained by melting a mixture of appropriate raw materials in an electric arc furnace or by any other melting technique suited to the products concerned. The molten liquid is then cast in a mold to produce shaped components directly. The product is generally then subjected to a controlled cooling program to cool it to ambient temperature without it breaking. The skilled person refers to this operation as “annealing”.
AZS products have been known in the art for a number of decades and have supplanted products based only on alumina and silica. U.S. Pat. Nos. 2,271,366 and 2,438,552 describe the first improvements to AZS products. FR-A-1 208 577 teaches the production of AZS products under oxidizing conditions. The first generation products tended to release gas bubbles into the molten glass, leading to unacceptable defects in the glass. Changing to oxidizing production conditions improved the strength of the AZS products and the quality of the glass. Oxidized products are generally white-yellow to white-orange in color, whereas reduced products are white to white-gray in color.
AZS refractory products comprise different phases: alpha-alumina crystals (corundum), zirconia crystals and a vitreous phase. The alpha-alumina and the zirconia are partly combined in eutectic crystals.
The prior art provides sometimes contradictory teaching as to the nature and the shape of the crystals. U.S. Pat. No. 2,079,101 indicates that it is preferable to have a highly oriented crystal structure in which the crystals are parallel to each other and perpendicular to the faces of the cast block. FR-A-1 153 488 describes AZS products with an interleaved crystal structure which is advantageous in terms of improved corrosion resistance. The above products are first generation products, i.e. reduced products. However, the inventors of FR-A-1 153 488 disclose their invention only in relation to a very particular block shape and their microstructure analyses relate to only a small area of the block. They indicate that it is the chemical composition of the product that produces the required microstructures. In particular, they specify that the crystal structure of their invention is encountered only in a small area of the Al
2
O
3
-ZrO
2
-SiO
2
system in which the silica content is from 16% to 20%. They also indicate that the presence of too high a proportion of sodium oxide has a harmful effect on corrosion resistance and that the Na
2
O/SiO
2
ratio must be limited to 0.14. U.S. Pat. Nos. 4,791,077 and 5,171,491 indicate that there is a structural difference between the skin and the core of the components. They also teach that a structure with elongate and interleaved crystals is disadvantageous and propose a solution for obtaining components with a fine and uniform microstructure free from dendritic zirconia crystals.
The products commercially available at present are oxidized products, such as our ER-1681, ER-1685 and ER-1711 products, which respectively contain 32%, 36% and 40% by weight of zirconia on average.
The above products contain zirconia which is referred to as “free” or “primary” zirconia (because it is not included in the eutectic crystals). The free zirconia crystals are small and tend to assume a spherical or nodular shape. Eutectic corundum zirconia crystals are also encountered. They have a relatively isotropic shape. Free corundum crystals are often encountered in the products commercially available at present.
AZS refractories are widely used in glass furnaces, in areas in contact with the molten glass. Some new glass compositions are more corrosive with respect to the materials of which the furnace is constructed. Also, glassmakers are seeking much longer working periods (determined by the service life of the refractories). There is therefore still a need for refractories that are more resistant to corrosion by molten glass. The most sensitive area is at the flotation line. The service life of the furnace is often dependent on the wear of the materials at the flotation line. Also, changes in glassmaking furnace design have increased the loads imposed on the hearth of the furnace. Increased insulation of the hearth to limit the consumption of the furnace, the use of bubblers and the increasing number of electrodes passing through the hearth have led to an increase in the temperature of the hearth where it is in contact with the molten glass, which exacerbates the problems of corrosion. There is therefore a need for products having improved corrosion resistance. It is well known in the art that introducing large quantities of zirconia improves corrosion resistance. However, increasing the zirconia content increases the cost and leads to increased segregation in the product, which can reduce industrial feasibility. Also, the increased zirconia content reduces the thermal conductivity, which is disadvantageous from the point of view of the industrial corrosion rate. The rate of corrosion of a material depends on the glass/refractory interface temperature, which is in turn conditioned by the thermal conductivity of the refractory. The more insulative the refractory product and the higher its interface temperature, the greater its rate of corrosion.
There is therefore a requirement for an AZS refractory having improved corrosion resistance with no significant increase in zirconia content.
An object of the invention is to satisfy that requirement.
In-depth studies have shown that it is possible to obtain an oxidized AZS refractory with increased corrosion resistance with the same chemical composition as typically encountered nowadays, the material being characterized by a novel and improved microstructure in the active area.
SUMMARY OF THE INVENTION
The invention provides oxidized alumina-zirconia-silica (AZS) refractories containing 40 wt % to 55 wt % Al
2
O
3
, 32 wt % to 45 wt % ZrO
2
, 10 wt % to less than 16 wt % SiO
2
and 1 wt % to 3 wt % of an alkali metal oxide selected from Na
2
O, K
2
O and mixtures thereof, having a microstructure essentially comprising alpha-alumina crystals, free zirconia crystals, eutectic crystals and an intercrystalline vitreous phase, wherein, at least in the active area, more than 20% by number of the free zirconia crystals have a dendritic shape and are interleaved with each other and with eutectic crystals and at least 40% by number of the dendritic free zirconia crystals have a dimension greater than 300 &mgr;m.
A surface area of 64 mm
2
of the active area of the materials preferably contains at least 200 dendritic free zirconia crystals having a dimension greater than 300 &mgr;m.
The materials claimed preferably contain 45 wt % to 50 wt % Al
2
O
3
, 34 wt % to 38 wt % ZrO
2
, 12 wt % to 15 wt % SiO
2
and 1 wt % to 3 wt % of an alkali metal oxide selected from Na
2
O, K
2
O and mixtures thereof.
For cost reasons, the alkali metal oxide is preferably Na
2
O.
More than 20% of the dendritic free zirconia crystals are preferably longer than 500 &mgr;m.
A surface area of 64 mm
2
of the active area of the materials preferably contains at least 100 dendritic free zirconia crystals having a dimension greater than 500 &mgr;m.
Surprisingly, it has been shown that it is possible to obtain microstructures offering improved corrosion resistance in a reproducible and homogeneous manner in the active area for a given range of chemical composition and using the oxidizing production method. Trials have been conducted and show also that if the microstructure

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