Clarifying the Five Most Common Misconceptions About AZS Electric Melting Furnaces—These Misconceptions Are Quietly Shortering the Lifespan of Your Furnace

Electric-fused AZS bricks have been used in the glass industry for over half a century. A wealth of hands-on experience has been accumulated regarding this core refractory material. However, some of these practices have been proven by new research and data from kilns with longer service lives to be one-sided, or even harmful. This article addresses five common misconceptions that significantly impact kiln lifespan.

Misconception 1: “The higher the zirconia content, the better the brick.”

Common sense suggests that the erosion resistance of AZS bricks increases with higher ZrO₂ content; consequently, some tender documents indiscriminately specify AZS-41 bricks for all sections.

Erosion resistance is not the only property required for kiln linings. The bricks must work in concert with the backing insulation materials. The thermal expansion coefficient of AZS-41 is slightly higher than that of AZS-36. If the backing clay bricks or lightweight insulating bricks do not provide matching expansion compensation, excessive structural stress will be generated during heating. In severe cases, this can cause the bricks to lift or shift, thereby opening pathways for alkali vapor to penetrate.

A wiser approach is “tiered matching”: use 41# or 36# bricks for areas directly exposed to molten glass, such as tank walls, flow channels, and kiln sills; for breast walls, large arches, and the central section of the regenerator, 33# is often the more cost-effective choice. When ChaoYu proposes brick solutions for kilns, we always recommend specific grades based on the kiln type and glass variety, rather than simply stacking the highest-grade bricks.

Misconception 2: “Glass phase exudation from AZS bricks means the bricks are failing.”

During furnace shutdown inspections, observing pale yellow glass-like streaks on the surface of pool wall bricks leads to the conclusion that the bricks are “not durable.” This confuses normal exudation with abnormal over-exudation.

Electric-fused AZS bricks contain approximately 20% glass phase matrix during manufacturing. Under operating temperatures, minimal softening and seepage of the surface glass phase is normal. Moderate seepage forms a high-viscosity self-glazing protective film on the hot face of the brick, sealing microscopic pores and slowing the inward penetration of the glass melt.

What truly warrants concern is: excessive and persistent seepage accompanied by intense foaming (indicating insufficient oxidation during melting), or the formation of dense voids after seepage (reflecting insufficient density). Identification method: Require suppliers to provide data on “glass phase exudation temperature” and “isothermal exudation volume,” rather than relying solely on visual inspection.

Misconception 3: “Pre-assembly is a waste of money; on-site grinding will suffice.”

The assembly precision of irregular sections—such as flow cavities and feed port corners—directly affects the risk of glass melt leakage. To save costs, some projects require components to be shipped individually according to drawings and assembled on-site through grinding.

On-site cutting cannot guarantee flatness. The joint surfaces of bricks on the side walls of the flow channel typically require a flatness tolerance of ±1 mm. If grinding results in misalignment or excessive brick joints, high-temperature molten glass will seep through the gaps into the backing layer, ultimately causing a sharp rise in the temperature of the kiln bottom steel plate or even material leakage.

Chuangyu’s practice is as follows: all critical assembly components must undergo cold-state fitting at the factory under design temperatures. Alignment lines are drawn, numbers are marked, and photographic records are taken before packaging and shipment. While this may seem cumbersome, it has prevented numerous instances of reduced kiln lifespan caused by poor fit.

Misconception 4: “It’s all the same AZS-36; every supplier is the same.”

While the nominal grade is AZS-36, with ZrO₂ content ranging from 35% to 37%, many buyers compare only chemical composition and price. The problem is that bricks with nearly identical chemical compositions may exhibit completely different erosion rates.

The key differences lie in manufacturing details: insufficient oxidizing atmosphere during electric arc melting can lead to carbon residue and a black core, which become starting points for rapid erosion; conventional casting often results in shrinkage cavities and porous zones, whereas directional solidification technology can significantly increase the proportion of dense material; different annealing cooling rates result in varying levels of residual stress, and high-stress bricks are prone to premature cracking and spalling during furnace heating.

When evaluating suppliers, request quantitative metrics such as oxidation level (Fe²⁺/total iron ratio), cross-sectional metallographic analysis, and ultrasonic penetration depth, rather than relying solely on chemical composition sheets.

Misconception 5: “AZS bricks cannot be used in kilns with pure oxygen combustion.”

In pure oxygen-fired kilns, the concentrations of water vapor and volatile alkalis are far higher than in air-assisted kilns. The SiO₂ component in AZS bricks reacts with high-concentration alkali vapors to form low-melting-point eutectics. Hence, the claim that “AZS is unsuitable for pure oxygen-fired kilns” has emerged.

This claim is only partially correct. AZS-33 is indeed unsuitable for exposure to zones with high concentrations of alkaline vapors and high flow rates; however, AZS-36 and AZS-41 can be used long-term in pure oxygen kilns through measures such as reducing the glass phase ratio, increasing the viscosity of the glass phase, optimizing the brick layout design, and appropriately adding backing insulation. In many pure oxygen kilns that have been in operation for over 10 years, the upper structures made of AZS-41 remain intact.

Conclusion

Refractory technology is a constantly evolving body of knowledge. When selecting materials for new projects or determining replacement lists during intermediate repairs, relying on outdated, static knowledge often results in the loss of months or even years of kiln life. ChaoYu is committed to working alongside partners in the glass industry to dispel these persistent misconceptions through more robust data and meticulous technical practices.