As critical equipment in glass production, the selection of refractory materials for the crown of a float glass furnace is crucial. Crown refractory materials must not only withstand complex operating conditions such as high temperatures, chemical attack, and mechanical stress, but are also directly impacting the furnace's service life and the quality of the resulting glass. Therefore, an in-depth analysis of crown refractory material selection and its impact on quality is crucial for optimizing float glass production processes and improving production efficiency.
I. Float Glass Furnace Crown Operating Environment
1. High-Temperature Environment: During the production process, the crown of a float glass furnace is subjected to high temperatures of 1550°C to 1630°C for extended periods. Such high operating temperatures require refractory materials with excellent heat resistance; otherwise, they are prone to softening, deformation, or even melting, compromising the furnace's structural stability.
2. Chemical Attack: The molten glass, volatiles, and combustion products within the furnace contain a variety of chemical components, such as alkali metal oxides (Na₂O, K₂O, etc.) and sulfur oxides (SO₂, SO₃, etc.). At high temperatures, these chemicals react with the crown refractory material, damaging its structure and reducing its service life.
3. Mechanical Stress: During the heating and cooling of the furnace, thermal stress is generated due to the different thermal expansion coefficients between the refractory material and the furnace steel structure. Furthermore, during the glass production process, internal airflow and material impact also exert mechanical stress on the crown refractory material. Long-term exposure to this complex stress environment can easily lead to damage such as cracking and spalling.
II. Crown Refractory Material Selection Principles
1. High-Temperature Resistance: The refractoriness should be higher than the actual operating temperature of the furnace, generally requiring a refractoriness of at least 1700°C to prevent softening and deformation at high temperatures. For example, high-alumina refractories, due to their high alumina content, offer excellent high-temperature resistance and are suitable candidates for crown refractory materials.
2. Chemical resistance: The selected refractory should be resistant to chemical attack from molten glass, volatiles, and combustion products. For example, silica refractories are highly resistant to alkali metal oxides and offer advantages in the selection of crown refractory materials for float glass furnaces with high alkali content.
3. Thermal stability: They should exhibit excellent thermal shock resistance and be able to withstand a wide range of furnace temperature increases and decreases without cracking or flaking. For example, magnesia-chromium refractories have high thermal conductivity and a low coefficient of thermal expansion, demonstrating excellent thermal stability.
4. Mechanical strength: They should possess sufficient compressive and flexural strength to withstand the mechanical stresses within the furnace. Normally, a compressive strength of at least 30 MPa and a flexural strength of at least 5 MPa are required.
III. Common Crown Refractories Selection and Quality Impact
1. Silica Brick
Selection Characteristics: Silica bricks are primarily composed of SiO₂ and have a high refractoriness, generally between 1710°C and 1730°C. Their refractoriness under load is also high, approaching their refractoriness. They also offer good resistance to acidic media and are currently a common refractory material used for the crowns in the melting section of float glass furnaces.
Impact on Quality: Their advantage is high-temperature volume stability, which helps maintain the shape of the crown, facilitating high-temperature heating of the molten glass and ensuring consistent quality of the finished glass. However, silica bricks have poor thermal shock resistance and are prone to cracking under frequent temperature fluctuations. Once cracks propagate, they can cause localized damage to the crown, compromising the overall structural safety of the furnace. Silica bricks also have relatively weak resistance to alkaline atmospheres. Under long-term operating temperatures, silica crowns are prone to "rat holes," and silica "crown drops" defects can also occur due to surface erosion by alkaline atmospheres.
2. High-Alumina Bricks
Selection Characteristics: High-alumina bricks are primarily composed of alumina and are graded according to their alumina content. They offer high refractoriness (generally between 1770°C and 1920°C), excellent thermal shock resistance, and mechanical strength. They are suitable for applications requiring high temperatures and corrosion resistance, and are commonly found in small specialty glass furnaces exposed to highly alkaline or acidic atmospheres.
Impact on Quality: The excellent high-temperature and corrosion resistance of high-alumina bricks ensures the crown remains stable during long-term high-temperature operation, providing a stable heating space for the molten glass. Their superior mechanical strength effectively resists mechanical stress within the furnace, reducing temperature fluctuations and unstable glass quality caused by crown damage. However, if the alumina content is inappropriate or the production process is flawed, the high-alumina bricks may react with certain components in the molten glass at high temperatures, affecting its compositional stability and negatively impacting the quality of the glass product, such as reducing its optical properties.
3. Zirconia Corundum Bricks
Selection Characteristics: Zirconia corundum bricks offer high refractoriness (between 1770°C and 2000°C) and corrosion resistance, but their relatively weak thermal vibration stability should be noted. Within the non-phase-transition range (e.g., from room temperature to 900°C or above 1150°C), the linear expansion rate of 33# zirconium corundum bricks increases normally with increasing temperature, and their overall expansion behavior is influenced by the ZrO₂ content. The ZrO₂ content in 33# zirconium corundum bricks is approximately 33%. Their linear expansion rate is lower than that of zirconium corundum bricks with high zirconium content (e.g., 41%), but higher than traditional refractory materials such as clay bricks and high-alumina bricks. Using zirconium corundum bricks in the kiln requires good temperature control. In float glass furnaces, zirconium corundum bricks are often used in areas subject to high temperatures and severe chemical corrosion, such as the kiln's hotspot, the crown. Some all-oxygen furnaces utilize only zirconium corundum bricks to withstand the acidic atmosphere within the furnace. Impact on Quality: Zirconia-corundum bricks have relatively weak thermal shock resistance, requiring precise temperature control within a very narrow range to ensure stable furnace operation, protect the crown structure, and maintain continuous glass production. Their corrosion resistance effectively protects against molten glass and combustion products, extending the furnace's service life and reducing production interruptions caused by crown replacement. However, the silica content of zirconium-corundum bricks can react with alkaline gases in the atmosphere at high temperatures, causing quality issues and, in turn, affecting the quality of the glass product.
IV. The selection of refractory materials for the crown of a float glass furnace requires comprehensive consideration of multiple factors. Different types of refractory materials have their own strengths and weaknesses in terms of high-temperature resistance, chemical resistance, thermal stability, and mechanical strength, and their impact on glass product quality varies. When selecting a refractory material, a comprehensive consideration should be made based on the specific operating conditions of the furnace, the quality requirements of the glass product, and production costs. Selecting the most appropriate refractory material ensures long-term stable furnace operation and high-quality glass production. At the same time, with the continuous development of glass production technology, the performance requirements for canopy refractory materials will continue to increase. In the future, it will be necessary to further develop and apply refractory materials with better performance to meet the development needs of the float glass industry.
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