Fused cast refractories.

What are Fused cast refractories?

Fused cast refractory is named by its manufacturing method. The term Fused Cast refers to a manufacturing process in which the ceramic bonding is obtained by the solidification of a mixture which has been melted in an electric furnace and cast in liquid state into molds.
Fused cast refractories have been known and used for many years.

Such refractories have presented many advantages in certain uses over the older type of refractory products. Fused cast refractories are denser and more corrosion-resistant than fired and unfired bonded refractories. Fused cast materials, are the preferred grades in contact with glass melts.

Fused cast refractories usually have a lower apparent porosity (1-3%), a compression strength and a high deformation temperature. They have great corrosion resistance to melts such as glass and molten metal oxides due to compact structure and closed pores. They also offer the lowest blistering potential in many applications.

Fused cast refractories are manufactured by melting mixtures of raw material of the desired composition in an electric furnace at a temperature exceeding 2000°C, casting the melt into moulds where it solidifies and cooling the molten refractory material to form a solidified refractory. High density, small or large shapes are obtained. When appropriate, a finish is made by grinding with diamond tools.
Many different compositions for fused cast refractories have been developed, such as Fused cast alumina and Fused cast high zirconia block.

Fused cast block is the most widely used material both in glass contact and superstructure of glass melting furnaces due to its good corrosion resistance to melten glass and almost no pollution to glass liquid.
Due to good resistance to alkali vapors, fused cast alumina block is the best choice for the downstream part of superstructures of glass furnaces.

Fused cast high zirconia block has been frequently used for a portion of contact with molten glass of a glass melting furnace which is required to have high quality, such as substrate glass for a flat panel display.

Besides glass industry, Fused cast refractory materials are also used in many other industries such as iron & steel, aluminum, petrochemistry or wear resistant applications.

What is the fused cast alumina blocks?

Fused cast alumina block is made of pure alumina powder(>95%) and a small amount of additions (soda sand and quartz sand). There are commercially 3 types of fused cast alumina blocks: Beta fused cast alumina block, Alpha-beta Fused cast Alumina Block and Alpha fused cast alumina block.
The manufacturing process of fused cast alumina block includes mixing the raw material, melting it in the electric furnace, casting, annealing, machining and then getting the final products.
First melt the raw materials in an electric arc furnace. When the material has become molten and its chemical and physical characteristics have been adjusted to the desired condition, pour the molten material into a mold. Then the cast product is processed with machines into the final product.

Alpha-beta Fused cast Alumina Block is formed by the compact structure of alpha alumina and beta alumina crystals in a most ideal proportion which is approximately 50% and 50% respectively,where intertwined crystals of both materials result in a very dense structure. Below 1350 ℃, it has excellent corrosion resistance against molten glass and great performance against contamination to molten glass, thus it is very suitable for paving blocks and fore-hearth channel blocks. When in contact with molten glass, it barely produces any blistering or stones. So, it may be widely used in working tank, feeder channels, superstructures and lipstone, etc.
Beta fused cast alumina block comprises of a majority of beta alumina crystals and a slight portion of alpha alumina crystals in compact structure. Moreover, the intersected texture of large beta-alumina ensures great dimensional stability and great resistance against spalling. Its property of base saturation enables a higher resistance to alkali vapor, thus it has excellent thermal shock resistance and does not form molten droplets. It is the best material for molter crown, port crown,feeder channel,ect.
Its neutrality against soda vapors makes it the best choice for the downstream part of superstructures in demanding applications.

Alpha fused cast alumina block is an ideal product for the lower temperature zones of the glass melting furnaces due to its high density, superior corrosion resistance and low blister potential. It is also an ideal material for Metallurgical Titanium Furnace because the superior thermal stability.

How to manufacture the fused cast alumina blocks ?

Fused cast alumina block is mainly composed of Al2O3 whose contents is more than 94 percent. It is produced by melting and casting high purity alumina in electric furnace at 2000℃.
The producing process of fused cast alumina block includes mixing the raw material, melting it in the electric furnace, casting, annealing, machining and then getting the final products.
In the manufacturing process of fused cast alumina blocks, first melt the raw materials in an electric arc furnace. When the material has become molten and its chemical and physical characteristics have been adjusted to the desired condition, pour the molten material into a mold. The mold is commonly made of graphite or other suitable materials. Maintain the molten material in the interior of an electric arc to accelerate and cause the solidification.
The mold is provided with a riser or header of ample size to enable complete filling of the mold without interference by material freezing in the headers. The cast material is left in its respective mold for heat treatment, or removed from the mold after the outer walls of the casting have solidified and then annealed without other than its own support. The cast parts are generally packed closer together, thereby allowing them to anneal by virtue of their own heat.
After the blocks have cooled, the cast parts are inspected and finished by diamond cutting and/or grinding. The header may be removed shortly after casting or after annealing. The amount of material in the header is often about equal to the amount of material in the desired finished refractory piece. Header material is commonly recycled, but at considerable expense.
Fused cast alumina block has excellent endurance to strong alkali. Under 1350℃, it has strong corrosion resistance and almost does not contaminate to glass melting liquid . Because it does not contain impurities such as Fe2O3 and TiO2, when it comes into contact with glass liquid, air bubbles are rarely found. Therefore, this product has the special characteristic of minimizing contamination to glass liquid.

Composition Of Fused Cast High Zirconia Block

In recent years, fused cast high zirconia block is introduced to decrease the number of glass defects and increase the corrosion resistance of refractories for high quality glasses and special glasses, due to its excellent corrosion resistance and no pollution to glass.
Fused cast high zirconia block is developed to operate in extreme condition and also require control of the making process. It is made of artificial synthesis high purity raw materials through special casting process.
The high zirconia fused cast refractory has a structure in which grain boundaries of relatively coarse baddeleyite crystals are filled with a matrix glass mainly composed of SiO2, Al2O3 and ZrO2 and contains a small amount of Na2O and/or K2O.
ZrO2 adopts a monoclinic crystal structure at room temperature and transitions to tetragonal and cubic at higher temperatures. The larger the content of the ZrO2 component in the refractory, the higher the corrosion resistance against molten glass.
However, when the content of ZrO2 is too much, it becomes difficult to obtain a fused cast refractory having no cracks by casting. When the zirconia crystals undergo reversible transformation between monoclinic crystals and tetragonal crystals, a rapid volume change will happen at a temperature from 1,000℃ to 1,150℃. as the volume expansion rate may cause cracks and a spalling phenomenon.

The Al2O3 component plays an important role in adjusting the relation between the temperature and the viscosity of the matrix glass and provides an effect of reducing the concentration of the ZrO2 component dissolved in the matrix glass. By utilizing this effect of the Al2O3 component, it is possible to prevent precipitation of zircon in the matrix glass and to prevent the change in quality of the matrix glass, whereby it is possible to avoid the chipping off phenomenon of the refractory and cracking due to an accumulation of remaining volume increase.
Na2O and/or K2O provides a function of softening glass. They are important components which influence the viscosity of the matrix glass and also have an effect of controlling the concentration of the ZrO2 component to some extent.

Fused cast high zirconia block has wide application in a wide range of super high temperature furnaces and kilns in military industry,science research, high melting point Metallurgical, laser crystal and electronic,and are used in sidewall, throat cover, throat support, electrode block, dam block parts etc.
Courtesy/AZS.Fused cast refractories.

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