In the past, wear of superstructure refractories, primarily silica crowns, is probably the most serious concern. Fused cast AZS block has been suggested as an alternative to the traditional silica crown as it has good corrosion against and can slow the wear of superstructure and improve the life of the furnaces.
Superstructure and crown refractories are subjected to corrosive reactions with the vapor species of the batch components and batch carryover. In the melting of soda-lime glasses, the vapor species are primarily soda and sodium sulfates. For borosilicate glasses, Na-tetraborate and B2O3 predominate. Direct attack occurs by condensation of the hot face of the refractory. The chemical changes in this reaction zone result in mechanical stresses and flaking, peeling or spalling of the hot face. Vapor penetration into joints with subsequent condensation and corrosion lowers the mechanical integrity of the superstructure or crown.
Corrosion phenomenon in superstructure have environmental and intrinsic causes. Heavy metals from fossil fuels, volatile component of the glass can reach the refractory surface as vapors, but the most severe attack can be caused by silica, reaching the surface as dust. Silica reacts easily with the corundum crystals of the fused cast AZS and, with the contribution of refractory's soda, produce sodium-alumina-silicates, pealing off the external layer of refractory, exposing deeper layers to corrosion and so forth.
Fused cast AZS block is not exposed to peculiar environmental attack. In some case, due to recurrent thermal cycling and subsequent multiple exudation, the refractory is losing very significant amounts of glassy phase, extruded with a gas-driven mechanism, replacing it with increasing amounts of closed porosity. When this happens, in relatively deep zone under the hot face there is formation of mullite by reaction of silica of the glassy phase and corundum crystals; while minor amounts of alkaline glassy phase are extruded, the increasingly acidic refractory structure undergo progressive mullitization. Volumetric changes and depletion of the liquid phase necessary to absorb the thermal dilatation of crystalline components are concurrent causes of possible cracking and deep spalling of surface, as a consequence of intrinsic corrosion subsequent to exudation.
At temperatures above 1475 ℃, the bond strength between the embedded ZrO2 crystals and the alumina is reduced, attributed to the variation in the thermal expansion characteristics of A12O3 and ZrO2.
While the wear of fused cast AZS block in the superstructure has been a potent and long-term source of glass defects, it is proved a longer service life as superstructure refractories in glass melting furnaces and other similar applications, it still encounters wear in the superstructure.