Erosion Analysis of Fused Zirconia-Alumina Bricks and Methods to Extend Their Service Life
The crystalline phases of fused zirconia-alumina bricks consist of baddeleyite (ZrO₂), corundum (Al₂O₃), mullite (3Al₂O₃•2SiO₂), and zircon (ZrSiO₄). The main crystalline phase, baddeleyite (ZrO₂), accounts for 32%–33%, corundum (Al₂O₃) for 45%–48%, mullite (3Al₂O₃•2SiO₂) for 0%–2%, and the glass phase for 17%–20%, with a small amount of zircon (ZrSiO₄) introduced from raw materials.
The surface layer of fused zirconia-alumina bricks cools rapidly, resulting in a higher content of the glass phase. However, due to partial crystallization hysteresis, some low-melting-point substances migrate and accumulate at the center, leading to an increase in the glass phase. In addition to the uneven distribution of the glass phase content, there are also differences in its composition and thermal properties.
In the test, the sample was divided into segments of 20 mm from the casting bottom to the center, and the glass phase content was measured as follows: 21.92% for 0–20 mm, 19.22% for 20–40 mm, 18.45% for 40–60 mm, 17.65% for 60–80 mm, and 20.63% for 80–100 mm.
Erosion of Fused Zirconia-Alumina Bricks
The erosion of fused zirconia-alumina bricks can be mainly divided into physical and chemical effects. Physical effects refer to the fact that, during the long-term operation of the glass melting furnace wall, the bricks repeatedly withstand the impact of rapid cooling and heating. As a result, the surface layer inevitably undergoes a process of contraction and expansion. Excessive fatigue causes damage to the structure of the fused zirconia-alumina bricks, leading to an increase in surface cracks and a loosening of the structure. Consequently, the bricks crack and peel off under the scouring action of airflow, materials, and the glass melt, and this process repeats continuously.
The chemical effects on fused zirconia-alumina bricks are more complex and intense, and can be divided into four aspects:
(1) Precipitation of the Glass Phase
During long-term exposure to high-temperature glass melt (>1500°C), the fused zirconia-alumina bricks used for furnace walls undergo two key processes: On one hand, the glass phase within the bricks gradually melts and precipitates (with a minimum precipitation temperature of approximately 1150°C); on the other hand, the alkaline glass melt containing Na₂O infiltrates the bricks through pores and cracks, diffusing and interpenetrating with the precipitated glass phase. This reduces the viscosity and increases the fluidity of the precipitated glass melt, thereby intensifying the erosion and promoting its penetration into the brick interior.
(2) Damage to the Skeleton Structure
As the glass melt's deep penetration and erosion worsen, the skeleton minerals forming the brick structure are gradually wetted and surrounded by the Na₂O-containing glass melt, leading to erosion of the skeleton. First, the dissolved mullite decomposes into α-Al₂O₃ and SiO₂, which further induces the transformation of α-Al₂O₃ into β-Al₂O₃. With increasing temperature, β-Al₂O₃ completely dissolves in the glass melt; the crystal lattices of baddeleyite and corundum are also damaged, resulting in fragmentation, disintegration, and partial melting. β-Al₂O₃ gradually dissolves in the glass at high temperatures and is rarely retained. As the glass continues to diffuse and penetrate, baddeleyite microcrystals become free-some are carried away by the glass melt (potentially forming glass stones), while others remain. Although baddeleyite can dissolve in the glass, its solubility is very low. With temperature fluctuations, ZrO₂ rapidly crystallizes from the glass melt, forming skeleton-like or bead-like baddeleyite crystals.
(3) Crystallization of New Minerals
The partial melting of the brick's skeleton minerals into the glass melt alters the original composition of the glass. Therefore, when the ratio of SiO₂-Al₂O₃-Na₂O in the glass melt approaches the theoretical composition of nepheline, a large number of nepheline crystals precipitate.
Al₂O₃ + 2SiO₂ + Na₂O → 2NaAlSiO₄ (Nepheline)
(4) Erosion and Damage Caused by Nepheline
Since the density of nepheline is lower than that of the brick, the precipitation of nepheline crystals is accompanied by significant volume expansion, leading to a looser brick structure. Although the melting of some crystalline phases in the brick at this stage increases the glass melt's viscosity, providing a certain bonding and protective effect on the loose structure, it cannot fully resist the scouring of furnace gas flow, materials, and glass melt, as well as the action of gravity. Consequently, the brick cracks and spalls into the glass melt, forming glass stones. The exposed surface after spalling continues to be eroded and scoured by the glass melt, leading to further spalling. This ultimately results in the erosion and disintegration of the fused zirconia-alumina bricks.
Extending the Service Life of Fused Zirconia-Alumina Bricks in Glass Electric Melting Furnaces
As is well known, glass tank furnaces use horizontal melting, where the glass surface moves horizontally. Severe erosion occurs at the three-phase interface, except in the throat area, as shown in Figure 1. In contrast, glass electric melting furnaces employ vertical melting, typically using cold-top melting technology. In these furnaces, the glass surface is covered by a layer of raw material, which results in fewer occurrences of three-phase interfaces. Due to the vertical melting mode, erosion on the furnace wall bricks is no longer concentrated at the three-phase interface but becomes more evenly distributed across the entire surface. Therefore, the weak points of fused zirconia-alumina bricks become the focal points for erosion, as illustrated in Figures 2 to 4.
Based on the erosion mechanism of fused zirconia-alumina bricks, the following measures are implemented:
Firstly, strict control is exercised over the Na₂O content in the raw materials of fused zirconia-alumina bricks. National standards require that the Na₂O content be below 1.45% for grade 33#WS and below 1.3% for grade 41#WS. Our company's electric melting furnace standards impose stricter requirements: Na₂O content ≤ 1.35% for 33#WS and ≤ 1.05% for 41#WS.
For the eroded area shown in Figure 2, the ratio of riser to brick must be at least 1.5:1. The pressure from the riser material effectively reduces residual pores in the brick, enhances the erosion resistance of the brick at the sprue, and ensures that no obvious shrinkage cavities remain at the sprue.
For the eroded area shown in Figure 3, brick joints must be strictly inspected during the assembly of fused zirconia-alumina bricks, with a required width of less than 0.3 mm. During the kiln baking process, the expansion differences of each part must be strictly controlled to maintain the tightness of the brick joints. This reduces gas ingress, prevents the formation of three-phase interfaces at the brick joints, and mitigates erosion in the area shown in Figure 3.
For the eroded area shown in Figure 4, the brick width is designed to be no greater than 400 mm-excessive width will result in more residual shrinkage cavities and internal looseness. The ratio of riser to brick must also reach 1.5:1 to improve the internal quality of the brick through pressure and gas discharge efficiency. In the later stage of kiln operation, reduce heat preservation to lower the brick temperature, thereby slowing down the erosion rate.
Conclusion
After analyzing the erosion characteristics of fused zirconia-alumina bricks and the differences in erosion between electric melting furnaces and tank furnaces, the implementation of corresponding measures can prevent and reduce erosion, thereby extending the service life of the furnace. Selecting high-quality fused zirconia-alumina bricks can, to a certain extent, prolong the service life of glass electric melting furnaces and enhance their market competitiveness. Therefore, in addition to improving the quality of raw materials for fused zirconia-alumina bricks, the key measures are to reduce the glass phase content and increase the riser ratio, thus further extending the service life of glass electric melting furnaces.