Comparison of Material Selection and Configuration for Photovoltaic Glass Furnace Sidewall Blocks

01 Introducing the Two Key Players: Fused α-β Corundum Bricks and Fused Zirconia-Corundum Bricks

1.1 Fused α-β Corundum Bricks: The Pure, "Gentle Guardian"

Fused α-β corundum bricks are produced using high-purity alumina (typically >90% Al₂O₃) melted at temperatures between 2000°C and 2200°C. Their crystalline structure consists of an interlocking matrix of 45%–55% α-Al₂O₃ and 45%–60% β-Al₂O₃; the extensive interpenetration of the β-phase transforms the original tubular microstructure into a flaky (lamellar) one, resulting in finer crystals and significantly improved resistance to thermal deformation.

Their core advantage lies in their "purity": the nepheline alteration layer formed on the surface has low viscosity and dissolves easily into the molten glass. This means that even if trace amounts of material detach, they do not cause persistent contamination in the glass melt. They are the undisputed ideal choice for zones where temperatures remain below 1350°C.

However, "gentle" also implies "vulnerable"—the alteration layer washes away too easily, meaning its protective capability is insufficient at high temperatures.

1.2 Fused Zirconia-Corundum Bricks: The Robust "High-Temperature Shield"

Fused zirconia-corundum bricks belong to the Al₂O₃-ZrO₂-SiO₂ system (AZS bricks), exemplified by the "33#" oxidation-method fused zirconia-corundum brick (void-free type). Their primary crystalline phases consist of a dense, coexisting matrix of corundum and baddeleyite (monoclinic ZrO₂). With a ZrO₂ melting point of approximately 2700°C, these bricks possess exceptional corrosion resistance.

The key factor is that the surface alteration layer incorporates ZrO₂; this results in a viscosity significantly higher than that of the nepheline layer found on pure corundum bricks. It acts like a "high-viscosity shield" adhering tightly to the brick surface, resisting both diffusion and spalling. In high-temperature zones, this shield serves as a vital safeguard for the tank wall bricks.

However, there is a downside: if this protective layer does spall, the resulting impurities—containing ZrO₂—exhibit a greater viscosity mismatch with the molten glass, making them much harder to eliminate. Consequently, the risk of contamination cannot be overlooked.

02 The Unique Nature of PV Glass Furnaces: Why Float Glass Practices Don't Apply

If you are transitioning from the float glass industry to PV rolled glass production, do not assume that the configuration of sidewall blocks can simply be "copy-pasted." PV rolled glass furnaces exhibit three significant structural and process differences that fundamentally alter the logic behind selecting sidewall blocks.

2.1 The "Waist" (Throat) is Becoming Shorter and Narrower

While the waist section of traditional float glass furnaces is relatively wide and spacious, the trend for PV rolled glass furnaces is toward shorter and narrower waists. For furnaces of comparable scale, it has become common practice to reduce the waist length from 7,500 mm to 5,000 mm and the width from 4,000 mm to 2,200 mm. A shorter, narrower waist reduces the surface area for heat dissipation as the molten glass passes through and increases flow velocity; consequently, the temperature drop decreases from the 110°C typical of float furnaces to around 90°C.

What does this imply? The temperature of the molten glass near the waist outlet is significantly higher than you might expect.

2.2 Elimination of Stirrers and Dilution Air

Float glass furnaces are typically equipped with horizontal or vertical stirrers, as well as temperature-control mechanisms in the cooling section such as dilution air and space-cooling water jackets. In contrast, many PV rolled glass furnaces have eliminated stirrers and dilution air while enhancing thermal insulation in the working channel. This further limits the temperature drop of the molten glass, causing the high-temperature zone to extend further downstream.

2.3 The "Low-Iron, High-Transmittance" Nature of PV Glass

PV glass features extremely low iron content, resulting in high light transmittance and strong thermal permeability, which drives intense horizontal convection currents. This leads to more aggressive scouring of the sidewalls by the molten glass, placing higher demands on the corrosion resistance of the refractory materials.

The combination of these three factors makes the selection of sidewall blocks near the waist outlet of PV rolled glass furnaces the most challenging aspect of the entire refractory configuration process.

03 1350°C: An Uncrossable Watershed

Discussions regarding configuration inevitably center on a specific temperature figure: 1350°C.

Empirical data reveal a clear pattern: consider two 700 t/d furnaces with identical structures producing the same glass specifications at the same pull rate. A difference in the set refining temperature alone resulted in a 20°C variance in the temperature at the throat outlet. One furnace showed a throat outlet temperature approaching 1370°C, while the other was around 1350°C. In the former case—where fused-cast α-β alumina bricks were used—the rate of corrosion was significantly accelerated.

The reason lies in the material properties: the nepheline alteration layer formed on fused-cast α-β alumina bricks offers insufficient protection at high temperatures, whereas the high-viscosity alteration layer formed on fused-cast AZS (alumina-zirconia-silica) bricks truly acts as a "shield" within this temperature range.

Consequently, an industry "golden rule" has emerged:

Above 1350°C—fused-cast AZS bricks are essential;

Below 1350°C—fused-cast α-β alumina bricks are more economical and safer.

This is not merely a matter of anecdotal experience, but a consensus distilled from extensive empirical data and engineering practice.

04 The 9000mm Rule: A Proven Gold Standard for Configuration

Based on temperature distribution patterns and engineering experience, the industry has established a simple yet efficient configuration guideline known as the "9000mm Rule."

4.1 The Rule Defined

Draw a circle with a radius of 9000mm, centered on the two corner blocks at the throat exit. For the area inside the circle, select 33# oxidation-method, void-free fused zirconia-corundum blocks; for the area outside the circle, select fused α-β corundum blocks.

4.2 Why 9000mm?

Taking a 700 t/d furnace as an example, the average temperature drop of the molten glass from the hot spot to the throat is approximately 7.5°C/m. To ensure a conservative safety margin, the temperature drop near the throat exit is estimated at 6°C/m. Meanwhile, the initial glass-phase precipitation temperature for 33# fused zirconia-corundum blocks is no lower than 1400°C. Using a peak throat exit temperature of 1400°C and a drop rate of 6°C/m, the temperature falls below 1350°C—the safety threshold—at a distance of approximately 9000mm.

4.3 Configuration Diagrams for Typical Furnaces

One-Furnace-Four-Line Configuration: Circles with a 9000mm radius are drawn around the two corner blocks at the throat exit; fused zirconia-corundum blocks are used throughout the area within these circles (including the two main forehearth channels). By the time the molten glass reaches the nearest branch channel, the temperature has dropped below 1350°C, allowing for the use of fused α-β corundum blocks for all branch channel sidewalls.

One-Furnace-Five-Line Configuration: Circles are similarly drawn around the two corner blocks, with zirconia-corundum blocks used inside the circles. Due to the increased number of channels, the coverage area of ​​zirconia-corundum blocks is larger, yet the total cost remains manageable.

4.4 Economic Analysis: Where Are the Savings?

In a one-furnace-four-line setup, zirconia-corundum blocks account for approximately 30%–40% of the total sidewall block volume. Compared to a configuration using only corundum blocks, the total cost is reduced by about 8%–10%, while the service life of critical components is extended by over 30%. The savings go beyond material costs; they also include avoiding the massive losses—often running into the tens of millions—associated with furnace shutdowns during cold repairs.

05 Three Hidden Variables: Look Beyond Just Temperature
When configuring sidewall blocks, temperature is the primary factor, but the following three variables also significantly influence the overall outcome.

5.1 Furnace Tonnage and Cooling Rate

The higher the melting furnace tonnage, the slower the cooling rate of the molten glass. A 700 t/d furnace experiences a temperature drop of approximately 7.5°C per meter, whereas large-tonnage furnaces (1,000–1,200 t/d) exhibit a more gradual temperature decline, meaning the high-temperature zone extends further downstream. In such cases, the "9,000 mm rule" should be adjusted—specifically, the coverage area of ​​the AZS (zirconia-corundum) blocks needs to be extended further downstream.

5.2 Fining Temperature Settings

Measurements indicate that for every 20°C increase in the fining temperature, the temperature at the throat exit rises by approximately 20°C. If the furnace fining temperature is set above 1,455°C, pushing the exit temperature close to 1,370°C, then simply applying the "9,000 mm rule" is insufficient; additional protective measures—such as air-cooling the sidewall blocks to slow the rate of corrosion—must be implemented.

5.3 Balancing Energy Conservation and Thermal Insulation

The more effective the thermal insulation of the forehearth channel, the slower the glass melt cools and the longer the high-temperature zone persists. While this benefits the stability of glass flow and the absorption of micro-bubbles, it also implies that a larger surface area requires AZS block coverage. Striking a balance between energy conservation and the protection of refractory materials is a challenge that every furnace manager must address.

06 Operational Advice: Temperature Control Is More Effective Than Replacing Bricks
If the temperature at your furnace's neck outlet remains consistently high, rather than rushing to replace bricks, consider these three steps first:

First, appropriately lower the refining temperature. Reducing the temperature by 20°C is more effective than replacing any brickwork, and it simultaneously saves energy and reduces consumption.

Second, optimize heat dissipation in the transverse and branch channels. Minimize unnecessary heat loss to prevent the molten glass from cooling excessively before reaching a safe temperature, while also avoiding localized overheating.

Third, implement air-cooling protection for the tank walls in the high-temperature zone. This is a cost-effective, high-impact measure that can significantly delay brick erosion.

Remember: A single cold repair of a furnace costs tens of millions. Getting the temperature regime and material selection right represents the most cost-effective investment.

07. A Broader Perspective: A Systems Approach to Refractories in Furnace Components
Melting tank sidewall blocks are merely one link in the furnace's refractory system. A properly functioning photovoltaic glass furnace relies on the synergistic coordination of materials across its various sections:

Tank Bottom: A multi-layer composite structure; the main layer utilizes large fireclay blocks, topped by protective and wear-resistant layers. Fused-cast AZS blocks are typically used for the wear-resistant layer to ensure direct contact with the molten glass.

Charging Bay: Subjected to the most severe abrasion from batch materials and erosion from glass flow; typically utilizes void-free fused-cast zirconia-corundum blocks (41% ZrO₂) combined with enhanced air cooling.

Throat: Characterized by the highest flow velocity and strongest convection currents; cover blocks should be made of void-free AZS-41 blocks.

Cooling and Forming Sections: Require oxidation-method AZS-33 blocks; these feature a high glass-phase exudation temperature, preventing the formation of bubbles in the molten glass.

Furnace Crown and Breastwalls: High-purity silica bricks are used in high-temperature zones, while fused-cast AZS-33 blocks may be used in secondary high-temperature zones.

A weakness in any single component can become the Achilles' heel of the entire furnace.

09.Choosing the Right Brick: It’s Not About Price, It’s About Suitability
Selecting the material for photovoltaic glass furnace sidewall bricks is never a simple "either-or" decision. It is a dialogue between temperature and material, a balancing act between cost and service life, and a harmonious interplay of process and structure.
Keep these three golden rules in mind:

1,350°C is the dividing line—zirconia-corundum for high-temperature zones, and corundum for low-temperature zones;

9,000 mm is the safety radius—centered on the corner brick, with distinct requirements for areas inside and outside this circle;

Temperature control is more effective than brick replacement—lowering the temperature by 20°C is worth its weight in gold.

Extending the furnace's service life by just two years saves far more than the cost of bricks; it preserves time, production capacity, and a competitive edge in the market. Choosing the right sidewall brick means securing the future of the furnace.