A Guide to Avoiding Quality Pitfalls in Fused Zirconia-Alumina (AZS) Paving Blocks: From Raw Materials to the Furnace

Understanding AZS Bricks: Why Are They the "Ideal Choice" for Furnace Bottom Paving?
Fused Zirconia-Alumina (AZS) bricks—composed of Al₂O₃, ZrO₂, and SiO₂—are high-grade refractory materials produced by melting alumina, zircon sand, and minor additives in an electric arc furnace and casting the mixture into shape. They are categorized into three grades based on ZrO₂ content: AZS-33# (33% ZrO₂), AZS-36#, and AZS-41#; the higher the ZrO₂ content, the greater the resistance to corrosion by molten glass.
There are three key reasons why AZS bricks are the preferred choice for furnace bottom paving:
Extremely high refractoriness: Capable of withstanding temperatures exceeding 1750°C, far surpassing the temperature of molten glass (typically 1450–1600°C);
Superior corrosion resistance: ZrO₂ has extremely low solubility in molten glass, effectively resisting chemical attack from alkali metal vapors and the glass melt itself;
Excellent high-temperature structural strength: Resistant to creep deformation under sustained high-temperature loads.
However, a "quality material" does not automatically guarantee a "quality product." From raw material proportioning to the final factory release, potential defects can arise at any stage of the AZS brick manufacturing process.

Raw Materials: Controlling the Source to Eliminate "Inherent Defects"
1. Zircon sand quality is the critical first step
Zircon sand (ZrSiO₄) is the core raw material for AZS bricks; its purity and particle size directly determine the quality of the finished product.
ZrO₂ content: Should be ≥65.5%, SiO₂ content ≥34%, and total impurities (Fe₂O₃, TiO₂, Na₂O, K₂O, etc.) <0.5%;
Particle size requirements: Fine powder of -200 mesh (below 75μm) is preferred. Excessively coarse particles do not decompose fully during melting, leading to unmelted quartz particles in the finished product that act as stress concentration points;
Radionuclides: Some imported zircon sands contain trace amounts of uranium and thorium, which can cause micro-crack propagation in the brick body under prolonged high-temperature exposure; radiological testing is required.
Improvement suggestions: Establish a grading and inspection system for incoming materials; chemical composition and particle size distribution must be tested for every batch, and non-compliant batches must be rejected. Additionally, create "performance profiles" for zircon sand from different sources to understand variations in melting characteristics.
2. Alumina purity and reactivity
The purity of industrial alumina (Al₂O₃) should be ≥98.5%, with a higher α-Al₂O₃ content being preferable. Insufficiently reactive α-Al₂O₃ leads to incomplete melting, leaving behind significant amounts of needle-like mullite (3Al₂O₃·2SiO₂) crystals; these crystals undergo anisotropic expansion at high temperatures, making them highly prone to causing micro-cracks.
3. The "double-edged sword" effect of additives
Small amounts of additives such as Na₂O, CaO, MgO, and CeO₂ are often introduced to improve the melting performance and crystallization resistance of AZS bricks. Dosage must be precisely controlled within the 0.1%–0.5% range; excessive amounts reduce refractoriness, while insufficient amounts result in uneven melting. Improvement strategy: Promote the use of rare-earth oxides (such as CeO₂) to replace traditional alkali metal additives; this not only facilitates the homogenization of the liquid-liquid phase-separated structure but also suppresses the volume expansion (a catastrophic phase transition involving approximately 4%–5% volume change) associated with the transformation of β-ZrO₂ to the monoclinic phase.

Melting and Casting: A Delicate Balance of Temperature and Time
1. Precise Control of Electric Arc Furnace Melting Temperature
The melting temperature for AZS raw materials typically ranges from 1800°C to 2000°C. If the temperature is too low, the raw materials do not melt completely, leaving "raw" material residues and quartz particles within the brick; if the temperature is too high, excessive decomposition and volatilization of ZrO₂ occur, and SiO₂ evaporation leads to compositional segregation.
Key Parameters:
During the final stage of melting, the temperature should be stabilized at 1900 ± 20°C and maintained for 30–60 minutes;
The furnace atmosphere should be kept slightly reducing (with moderate CO partial pressure) to prevent excessive reduction of ZrO₂;
Stirring should be performed 2–3 times during the melting process to ensure compositional uniformity.
2. Casting Process: Bubbles Are the Primary Enemy
Bubbles within AZS bricks (especially large ones with diameters >1 mm) are the most common and critical defects. There are three sources of bubbles: gases carried by the raw materials themselves, air entrapped during melting, and gases not expelled during casting.
Mitigation Measures:
Preheat raw materials to 300–500°C before charging the furnace to remove adsorbed moisture;
Subject the melt to vacuum degassing prior to casting (vacuum level ≤100 Pa, held for 10–15 minutes);
Control the casting temperature between 1750°C and 1850°C; excessively high temperatures result in low viscosity and slow bubble rise, while excessively low temperatures lead to poor fluidity and air entrapment;
Maintain a slow casting speed—using a "thin stream, slow pour" technique—to avoid turbulence-induced air entrapment;
Coat the inner walls of the casting mold with an anti-stick agent and preheat the mold itself to above 200°C.

3. Annealing Process: A "Slow and Careful" Approach to Relieving Internal Stress
After casting, significant temperature gradients and phase-transformation stresses exist within the AZS brick bodies. Improper annealing leaves behind numerous micro-cracks in the finished products, which can rapidly propagate under thermal shock during service.

Machining Phase: Balancing Precision and Damage
1. Damage Control During Cutting and Grinding
AZS bricks possess extremely high hardness (Mohs hardness of 7.5–8.5); consequently, surface micro-cracks and edge chipping easily occur during machining.
Cutting: Diamond circular saws are recommended; the feed rate must be ≤0.5 m/min, and ample cooling water is required to prevent cracking caused by thermal stress.
Grinding: Use resin-bonded diamond grinding wheels for rough grinding and metal-bonded wheels for finish grinding; the grinding allowance should be controlled within 0.3–0.5 mm.
Post-machining inspection: Each brick must undergo ultrasonic flaw detection and a comprehensive visual inspection; any brick found to have internal cracks or surface chipping/defects is strictly classified as non-conforming.
2. Strict Control of Dimensional Precision
The dimensional precision of furnace bottom paving bricks directly impacts the quality of the masonry. In accordance with industry standards:
Length and width deviation: ≤±0.5 mm;
Thickness deviation: ≤±0.5 mm;
Diagonal difference: ≤1.0 mm;
Flatness: ≤0.3 mm.
Any brick exceeding these tolerances can lead to uneven joints and stress concentrations during masonry installation, thereby accelerating erosion.

Masonry Construction: Even High-Quality Bricks Suffer from Poor Laying
1. Layout Logic for Paving Bricks
AZS paving bricks on the furnace bottom are typically laid in a "herringbone" or "staggered straight-row" pattern to:
Prevent molten glass from seeping through straight joints;
Ensure the load is distributed as evenly as possible across each brick;
Position the joints at an angle relative to the furnace's longitudinal axis (the direction of glass flow) to minimize erosion.
2. Joint Control
Joint width: 1–1.5 mm (excessive width allows glass seepage, while insufficient width leaves no room for thermal expansion);
Joint filling: Use specialized AZS-based refractory mortar; the mortar must be mixed thoroughly and be free of lumps, applied to fill the joint completely without overflowing;
Strictly prohibit the use of mortar made from different materials! Using the wrong material can trigger a chemical reaction with the AZS bricks at high temperatures, forming low-melting-point phases that accelerate erosion.
3. Paving Sequence and Stress Relief
Paving of the furnace bottom should proceed outward from the center toward both sides, with flatness checked using a spirit level after completing each section. Joints between adjacent sections must be staggered by at least one-third of the brick length. Avoid striking the brick surfaces forcefully during paving to prevent internal damage.

Quality Maintenance During Operation
1. Heating-up Schedule: "Slow Means Fast"
When commissioning a new furnace or restarting after a cold repair, the heating process must strictly adhere to the principle of "slow heating and gradual cooling." The entire process of raising the furnace bottom temperature from ambient to operating temperature (approx. 1550°C) should take no less than 30 days. Specific rates are as follows:
Below 600°C: ≤10°C/h (to drive off residual moisture);
600–1000°C: ≤8°C/h (quartz phase transition zone; requires a holding period);
1000–1400°C: ≤5°C/h;
Above 1400°C: ≤3°C/h.
Accelerating the heating process to meet a schedule at any stage may cause new cracks to form in the installed refractory bricks.
2. Monitoring Erosion During Operation
Periodic cold repair inspections: Shut down the furnace for inspection every 12–18 months, focusing on measuring the erosion depth of the furnace bottom bricks (typically, AZS-41# bricks erode at a rate of about 5–8 mm per year; replacement is required if erosion exceeds 15 mm);
Infrared thermography scanning: Use infrared imaging on the furnace exterior during operation to indirectly assess the uniformity of the bottom temperature distribution; localized high-temperature zones often indicate severe brick erosion or glass leakage through brick joints;
Glass melt sampling and analysis: An abnormal rise in Zirconium (Zr) content is often an early indicator of AZS brick erosion.
3. Hot-spot Protection and Localized Repair
If localized erosion at the furnace bottom accelerates, the following measures can be taken:
Adjust burner angles above the hot-spot area to lower the local temperature;
Perform localized repairs using high-ZrO₂ content AZS-41# bricks or rebonded AZS bricks;
In areas with deep erosion, lay a "protective cushion" (using pre-fired AZS fragments) to extend service life.

In recent years, technological advancements in AZS paving blocks have primarily focused on three areas:
Re-fused and rebonded AZS blocks (RBZ): Produced by re-melting and casting used AZS blocks, these offer superior compositional uniformity and a cost reduction of approximately 30%; they are currently being adopted for the bottoms of some large-scale furnaces.
High-ZrO₂ content AZS (AZS-41# or even AZS-45#): Higher ZrO₂ content provides enhanced corrosion resistance but imposes stricter requirements on the melting process and annealing schedule.
Nano-modified AZS blocks: Incorporating nano-ZrO₂ or nano-Al₂O₃ particles into the AZS matrix to improve density and thermal shock resistance; this technology is currently in the pilot-testing stage.

Conclusion
Improving the quality of AZS paving blocks is not a matter of achieving a "breakthrough" at a single stage; rather, it requires "end-to-end control" spanning everything from raw material intake to furnace operation. A high-quality block is the result of a multitude of critical factors—the purity of the zircon sand, the temperature profile of the electric arc furnace, the vacuum level during casting, the extended duration in the annealing kiln, the rotational speed of the diamond saw, the width of the mortar joints during masonry, and the patience exercised during every single degree of the heat-up process. Not one of these elements can be overlooked.
Remember: the furnace bottom serves as the foundation of the entire furnace; if the foundation is unstable, the consequences can be catastrophic. Every penny saved on AZS blocks will sooner or later be paid back twofold during furnace shutdowns and maintenance.