Fused cast high zirconia block is a new type of fused cast refractory. It is specially designed refractory for glass furnace application. It features excellent corrosion resistance to molten glass due to its dense microstructure.
Fused cast refractory materials are widely used in glass furnaces to resist corrosion by the molten glass. Such a quality is especially due to the low apparent porosity of these materials. First generations of these products is fused cast AZS, which is widely applied to soda-lime furnaces. Then fused cast high zirconia block containing 90% or more of ZrO2 was developed to answer to special needs for the manufacturing of special technical glasses (LCD screens, lead crystal) and very corrosive glasses (boron-silicate glasses).
Fused cast high zirconia block is designed to operate in extreme condition and also require control of the making process. Elaborated by a fused cast process followed by a controlled cooling step, these materials exhibit specific thermal and mechanical properties.
Fused cast high zirconia bricks are made of artificial synthesis high purity raw materials through special casting process. The crystalline texture consists of baddeleyite and with the characteristics of excellent high temperature & corrosion resistance, low blister and stoning potential. These advantages can prevent liquid glass from pollution effectively.
In high zirconia fused cast refractories, the majority of the refractory crystalline structure are composed of baddeleyite crystals (ZrO2). During the cooling process, zirconia transforms successively through cubic, tetragonal and monoclinic form, with tetragonal and monoclinic forms exhibiting anisotropy in thermal expansion along their crystallographic axes. The resulting complex microstructure is made of numerous different crystallographic domains called variants. Finally, the shift from the tetragonal to the monoclinic structure is responsible for a large volume expansion which causes cracks. The manufacturing process must be rigorously controlled, in particular the cooling rate after fusion, in order to limit mechanical stresses induced by important thermal gradients and the martensitic tetragonal–monoclinic transformation of zirconia.
When heated, fused cast high zircnoia block undergoes a volume shrinkage at about 1150℃，due to the crystal transformation from monoclinic crystals to tetragonal crystals in accordance with the crystal transformation specific to baddeleyite crystals. When cooled, it undergoes a volume expansion at about 950℃ due to the crystal transformation from tetragonal crystals to monoclinic crystals. The volume expansion may cause cracking, and in an extreme case, the refractory undergoes a spalling phenomenon due to many cracks.
The transformation of the baddeleyite crystals is absorbed by a matrix glass. SiO2 is essential for forming the matrix glass. However, if the SiO2 content is too high, the corrosion resistance tends to deteriorate. The Al2O3 component plays an important role in adjusting the relation between the temperature and the viscosity of the matrix glass, and reduces the concentration of the ZrO2 component in the matrix glass.
Fused cast high zirconia brick has wide application in a wide range of super high temperature furnaces and kilns, especially special glass and high quality glass furnaces.