Fused Cast AZS Refractory Brick for Glass Melting Furnace
Fused cast AZS (Alumina-Zirconia-Silica) refractory bricks, also known as fused cast zirconia-corundum bricks, are indispensable materials in the construction of glass melting furnaces. Since their first introduction to glass melting tanks in 1928, these refractories have undergone continuous development and remain the most widely used materials in both glass contact areas and superstructures of glass melting furnaces. The term "AZS" is derived from the chemical composition of the ternary system Al₂O₃-ZrO₂-SiO₂.
The refractory lining of a glass melting furnace serves as both thermal and structural protection, holding the liquid glass bath at temperatures exceeding 1500°C. Given the extreme conditions—high temperatures, aggressive chemical corrosion from molten glass, and thermal cycling—the selection of appropriate refractory materials is critical for furnace performance, energy efficiency, and operational longevity.
Manufacturing Process
The production of fused cast AZS bricks involves a sophisticated manufacturing process that fundamentally distinguishes them from sintered refractories. The basic process consists of raw material selection, mold making, arc melting, casting, annealing, finishing, and inspection.
Raw Materials: The primary raw materials are high-purity alumina powder and zircon sand, which typically consists of approximately 65% zirconia (ZrO₂) and 34% silica (SiO₂). These materials are carefully proportioned to achieve the desired final composition.
Melting: The raw material mixture is melted in an electric arc furnace at temperatures reaching approximately 2200°C. This high-temperature melting ensures complete homogenization of the components and the formation of a fully liquid melt.
Casting and Solidification: The molten material is then cast into various molds while still in the liquid state. The term "fused cast" refers precisely to this manufacturing process in which ceramic bonding is achieved through the solidification of the melted mixture. After casting, the blocks undergo controlled cooling (annealing) to relieve residual stresses.
Finishing: Once cooled, the blocks are cut and machined into specific shapes and dimensions according to customer requirements.
It is worth noting that while all AZS manufacturers follow these fundamental process steps, there can be rather significant differences in the actual practice of each step between different companies, which ultimately affects product quality and performance.
Chemical Composition and Classification
Fused cast AZS bricks are classified primarily according to their zirconia (ZrO₂) content, which ranges from 33% to 41%. The three standard grades are:
| Grade | ZrO₂ Content | Key Characteristics |
|---|---|---|
| AZS-33# | ~33% | Standard grade with good corrosion resistance |
| AZS-36# | ~36% | Higher corrosion resistance, low glass pollution |
| AZS-41# | ~41% | Highest corrosion resistance, excellent low pollution |
The zirconia content fundamentally defines the corrosion resistance of the material, as higher ZrO₂ levels provide enhanced protection against molten glass attack. The microstructure of these bricks consists of a eutectic structure of corundum (α-Al₂O₃) and baddeleyite (ZrO₂) phases.
Key Properties and Performance Characteristics
Corrosion Resistance
The most notable property of fused cast AZS refractories is their exceptional resistance to corrosion by molten glass. This resistance stems from their dense microstructure, which exhibits almost no porosity. The low porosity prevents molten glass penetration, significantly slowing the corrosion process.
However, the corrosion mechanism is complex. AZS refractories can be corroded through the meltdown of their own composition, penetration of alkali oxides (such as Na₂O, K₂O, or CaO) from the glass melt, and scouring by the flowing glass. The refractories show relatively weaker resistance against corrosion from oxides like Na₂O, K₂O, and CaO.
Thermal Shock Resistance
Fused cast AZS bricks exhibit good thermal shock resistance, enabling them to withstand the rapid temperature changes that occur during furnace start-up, shutdown, and operation. This property is essential for maintaining structural integrity throughout the furnace campaign.
High-Temperature Mechanical Properties
These refractories maintain good mechanical properties at elevated temperatures, which is crucial for supporting the structural loads within the furnace. The cold crushing strength typically exceeds 300 MPa.
Physical Properties
Typical physical properties include:
Bulk density: ≥3.75 g/cm³ for AZS-33#, ≥3.85 g/cm³ for AZS-36#, ≥4.00 g/cm³ for AZS-41#
Apparent porosity: ≤1.2%
Glass phase exudation temperature: ≥1400°C
Bubble separation ratio (1300°C × 10h): ≤1.2
Oxidation State and Quality
The oxidation state of the product significantly influences its performance. AZS fused cast refractories contain small quantities of impurities (mainly iron or titanium ions, at a few tens of ppm) within their glassy phase. These multivalent ions serve as reliable indicators of the oxidation state of the product. A well-oxidized product displays a light red color due to the presence of ferric ions (Fe³⁺), whereas a reduced product appears greyish due to ferrous ions (Fe²⁺) or metallic iron. The oxidation level strongly influences compactness, blistering tendency, exudation, stoning, and corrosion resistance.
Applications in Glass Melting Furnaces
Fused cast AZS bricks find extensive application throughout glass melting furnaces, serving in both glass contact areas and superstructures.
Glass Contact Areas
In regions that come into direct contact with molten glass, AZS refractories are essential for:
Melting tank walls – providing structural containment for the glass bath
Paving blocks – forming the furnace bottom
Pool walls – resisting corrosion at the glass line
Forehearth channel blocks – maintaining glass quality during delivery
Superstructure Applications
In the upper regions of the furnace, AZS bricks are used for:
Superstructures of melters – with low potential for stoning, blistering, and exudation of the matrix glass
Crowns – the arched roof of the furnace
Side walls and breast walls
Working ends and feeder channels
Doghouse crowns
The specific grade selection depends on the application: AZS-33# is suitable for general applications, AZS-36# offers enhanced performance, and AZS-41# provides the highest corrosion resistance for the most demanding areas.
Quality Control and Performance Optimization
Recent trends in glass manufacturing—including furnace electrification, the production of clearer glass, and extended furnace campaigns—have underscored the importance of rigorous quality control for AZS refractories. Three main quality parameters have been identified that significantly affect refractory lifetime and glass quality: block color (oxidation state), filling density, and microstructure.
Advanced non-destructive evaluation methods, including spectrophotometry for color measurement and radar wave technology for filling assessment, are increasingly employed to ensure consistent quality. These quality controls are critical because AZS refractories are not all equivalent in terms of quality, and even small variations can significantly impact furnace performance and glass quality.
Challenges and Considerations
Despite their excellent properties, fused cast AZS refractories face certain limitations. The glass phase within the material can exude at high temperatures, potentially weakening the refractory by increasing porosity and accelerating corrosion. This exudation can also trigger glass contamination through cords, knots, or secondary zirconia crystals.
Operating conditions significantly influence refractory performance, and computer simulations using finite element methods have proven valuable for understanding thermal and mechanical behavior, particularly regarding crack initiation during furnace heating. Research has shown that maximum principal stress can reach the modulus of rupture of AZS after approximately 42 hours of heating when the hot face temperature reaches 145°C.
