Guidance Note on Using the Fused Cast AZS Block in Glass Furnaces

Fused cast AZS block is featured with great corrosion resistance to glass melt. It is the most widely used refractory materials in the glass furnaces. Due to its chemical and physical properties, there are some things that should be paid attention to when used in the glass furnaces. 

 

AZS block made in the reducing method has poor corrosion resistance. When it is in contact with high temperature glass melt, a high viscosity layer will be formed which is the key for the corrosion resistance of fused cast AZS. It requires that the softening temperature of AZS be high and except Al2O3, ZrO2, SiO2 and Na2O, the amount of other components be as small as possible. 

 

However, using graphite lining bricks and graphite electrodes in the glass furnace can produce reducing atmosphere (C, CO), which can reduce the Fe2O3, TiO2 and SiO2 to low-cost oxides and increase the types of components. According to the basic principle of fraction, this will reduce the temperature of the liquid limit. When used at high temperature, the glass phase viscosity of the bricks is great reduced and the protective layer of high viscosity cannot be formed. The low viscosity glass phase can easily spread to the high temperature glass melt. Without the binder, the crystal phases of the brick will spread to the glass melt too, which will cause stripes and stones in the body of the brick. 

 

When making AZS in the oxidizing method, graphite lining bricks are not used. Although the graphite electrodes are still used, with long-arc method, C is transferred into CO2 when combined with O2 in the electric arc. With this method, the carbon content can be reduce to 0.005% and the corrosion resistance can be improved greatly. 

 

AZS block made in the reducing method is easy to cause bubbles in the glass melt. The low-cost oxides and materials with strong reduction such as C in the glass phase can seize the oxygen in the glass melt. This may cause the changes of the gases dissolved in the glass melt. For example, SO3 dissolved in the glass melt is reduced to SO2 and SO2 becomes bubbles as it has low solubility in the glass melt, which can cause glass defects. 

 

AZS is featured with stable and dense structure and high corrosion resistance to glass melt. At 900～1200℃, it will have irregular thermal expansion due to the transformation from monoclinic crystal phase to tetragonal crystal phase. Due to other components of the bricks, the temperature of the crystalline transformation fluctuates in±50℃. Therefore, in the heating and cooling process at 950～1150℃, the temperature changes should be less 15℃/h. 

 

The glass phase can work as a buffer for the irregular expansion and contraction to avoid cracking in the brick. 0.15%～0.20% B2O3 is added to improve the thermal shock resistance of the brick. It also has a cushioning effect for the irregular expansion,but when the temperature fluctuates too much, it will still cause cracking. Therefore, during the preheating of the glass furnaces, temperature should increase slowly at 950～1150℃. 

 

When using the electrode brick to boost the melting, the electrode brick should have good electric insulation. AZS block has great electric insulation and its electric insulation reduces with the increase of the temperature. When producing the soda-lime glass, it can be used as the electrode brick. But when producing high-borosilicate glass, AZS 41# and fused cast high zirconia block are used. 

 

The thermal conductivity of fused cast AZS is twice as big as that of fireclay brick. When using it as the sidewall brick, the air volume of the cooling fan should be twice as big as that when using fireclay brick.

 

When used at high temperature, the corrosion resistance of different refractory materials is as follows: AZS-41#＞AZS-36#＞AZS-33#＞α-β fused cast alumina block. But it does not have as good pollution resistance as fused cast alumina block.