Refractory materials for the crown of oxy-fuel glass melting furnaces

Silica Refractories
Silica refractories are primarily used for the main crowns of traditional glass melting furnaces, a subject that has been extensively studied. Research has been conducted to simulate the corrosion of various fused-cast and silica refractories, alongside analyses based on diffusion theory. When silica refractories are used in furnace crowns, exposure to alkali vapors causes surface corrosion; the resulting reaction forms a high-viscosity, SiO₂-rich glassy phase that plugs pores and inhibits the further diffusion of alkali vapors. However, silica undergoes polymorphic transformations with temperature changes, and the associated volume fluctuations can compromise the structural integrity of the bricks. In oxy-fuel combustion, alkali vapor concentrations are four to five times higher, and strong gas flow impingement significantly accelerates the accumulation of the glassy phase on the crown's refractory surface; under the combined influence of gravity and environmental conditions, this accelerates the corrosion of the silica refractories. Nevertheless, some researchers argue that silica refractories offer the advantages of low cost and low density, and that their potential can be fully realized through the optimization of their microstructure and composition.

Corundum-based Refractories
Corundum-based refractories offer advantages such as high melting points, good chemical stability, and excellent high-temperature mechanical properties. Common corundum-based refractories used in glass furnaces include fused-cast AZS, fused-cast α-β alumina, electro-fused corundum, and electro-fused zirconia-corundum. The protective effect of the high-viscosity liquid phase formed on the surface of fused-cast AZS refractories is crucial to ensuring their service life. However, AZS materials contain significant amounts of SiO₂ and are consequently susceptible to corrosion by alkali vapors. Fused-cast α-β alumina refractories are suitable for most sections of the furnace but are expensive.

Among refractories for glass furnaces, electro-fused corundum represents the pinnacle of manufacturing difficulty and technical sophistication; key products include electro-fused α-β alumina and electro-fused β-Al₂O₃. Electro-fused corundum refractories feature a dense structure, effectively resisting corrosion by molten glass or alkali vapors at high temperatures while causing minimal contamination to the glass melt, making them indispensable for oxy-fuel combustion furnaces. Electro-fused α-β alumina is frequently used for the crown of oxy-fuel glass furnaces; similar to electro-fused β-Al₂O₃ bricks, it has a low glass-phase content, minimizing the formation of liquid phases and impurities. In contrast, electro-fused β-Al₂O₃—which exhibits excellent resistance to alkali corrosion—is typically used in the glass cooling zone, as it is highly susceptible to corrosion by dust particles if used in the superstructure of the batch charging area.

Additionally, low-glass-phase electro-fused zirconia-corundum exhibits minimal glass-phase exudation at high temperatures, resulting in superior creep resistance. This allows the furnace operating temperature to be raised from 1600°C to 1650°C, thereby enhancing the stability of the oxy-fuel furnace superstructure; consequently, low-glass-phase electro-fused zirconia-corundum is widely used in this section.

Magnesia-Alumina Spinel Refractories
The unique operating conditions of the main crown in oxy-fuel glass melting furnaces demand that the refractory materials used possess superior resistance to alkali corrosion and a low high-temperature creep rate. When comparing the costs and performance of silica, spinel, and corundum-based refractories, spinel and α-β alumina materials stand out for their exceptional alkali corrosion resistance and favorable thermo-mechanical properties, making them suitable choices for the main crown. However, spinel refractories are often preferred due to their lower cost.

Magnesia-alumina spinel has already been widely applied as a refractory material for the main crowns of glass furnaces—both domestically and internationally—yielding excellent performance results.

Although magnesia-alumina spinel refractories hold great promise for use in the main crowns of oxy-fuel glass furnaces, the material's crystal structure is highly stable; the oxygen ion diffusion coefficient is extremely low at high temperatures, making it difficult to achieve densification during sintering. These characteristics lead to issues such as inferior mechanical properties and high creep rates under prolonged high-temperature loading, thereby driving further research into magnesia-alumina spinel materials.