Mechanisms of bottom paving corrosion
The corrosion of bottom paving is a critical parameter for controlling and optimizing the lifespan of glass furnaces. As the use of electrical boosting in furnaces increases, the wear of refractory materials is accelerated due to harsher operational environments. A comprehensive understanding of the mechanisms driving the degradation of bottom paving is essential for identifying effective strategies and solutions to mitigate associated risks.
Direct Corrosion
The wear of the bottom lining is primarily driven by corrosion of the refractory material by the glass. The corrosion mechanism of fused-cast AZS refractories by sodalime glass is well-documented and can be delineated through several sequential steps:
Migration/Penetration of Aggressive Species
Alkaline species, particularly sodium (Na), migrate into the refractory material via the glassy phase.
Partial Dissolution of Weaker Phases
The glassy phase of AZS and ZrO2-Al2O3 eutectic crystals undergo partial dissolution.
Formation of an Intermediate Layer
An intermediate layer forms at the interface between the molten glass and the refractory, characterized by a glass matrix heavily loaded with refractory species (Al2O3 and ZrO2).
Dissolution and Migration of Refractory Species
The final dissolution of ZrO2 crystals occurs, leading to the assimilation and dilution of these dissolved species into the glass melt.
The primary advantage of AZS over other fused-cast materials lies in the formation of this intermediate layer, which consists of glass with a high refractory content. The presence of ZrO2 and Al2O3 significantly increases the viscosity of this altered glass layer compared to sodalime glass. This elevated viscosity retards both the migration of aggressive species from the glass melt toward the refractories and the dissolution and migration of ZrO2 crystals into the glass melt. This passivation layer is crucial for the durability of fused-cast AZS products in sodalime glass applications. However, the stability of this passivation layer can be affected by operating conditions. High glass velocities or changes in flow patterns -due to job changes or modification in pull rate- can destabilize the protective layer of AZS refractories, leading to higher corrosion rates.
The migration of alkaline species within the ceramic structure and the dissolution of refractory components in the glass melt are influenced by temperature and concentration gradients, which are further intensified by interface renewal in scenarios of high glass velocity. Numerical simulations can be employed to assess the lifespan of refractory paving under varying glass melt properties, with typical corrosion rates ranging from 5 to 15 mm/year, contingent upon operating conditions. These results can inform adjustments to the fused-cast paving material and thickness to align with industrial and economic objectives.
Upward Drilling
Upward drilling corrosion occurs when molten glass containing gas bubbles becomes trapped beneath a horizontal refractory surface. The presence of a triple point, where liquid, solid, and gas coexist, disrupts the formation of the protective layer, leading to localized acceleration of corrosion. Similar to the conditions at the melt line on sidewall blocks, the presence of gas bubbles induces a Marangoni effect, resulting in continuous contact between "fresh" molten glass—rich in aggressive alkalis—and the refractory material (Figure 1).
Quantifying the corrosion rate in this context is complex, as it depends on several parameters, including bubbles size and concentration. However, empirical evidence from industrial furnaces suggests that the corrosion rate can exceed 50 mm/year, making upward drilling of bottom paving a significant contributor to premature maintenance or shutdown of furnaces.
Utilizing an internally developed laboratory test, SEFPRO’s R&D teams successfully replicated the conditions leading to upward drilling on U-shaped refractory samples. Quantitative analysis of the resulting corrosion through 3D scanning indicates that upward drilling is a temperature-driven phenomenon, with its effects being limited below 1250°C (Figure 2).
Downward Drilling
The increasing incorporation of cullet in glass batches has led to recurrent issues of metallic contamination in the glass melt, particularly among container glass manufacturers. Alterations in the redox state of the glass melt due to metallic contamination can result in the formation of metallic nodules which sink and accumulate at the bottom of the glass bath. When in contact with the fused-cast tiles, a triple point is established, involving solid refractory, molten glass, and molten metal which locally accelerates the corrosion of refractories.
Downward drilling is challenging to predict, as it is linked to cullet quantity and quality fed to the furnace during operations. The selection of high-end refractory materials can help increase the corrosion resistance of bottom paving.
Reinforced bottom paving assemblies
To address the challenges of bottom paving corrosion in traditional furnace, bottom paving assemblies are usually constructed according to the following pattern (Figure 3):
Fused-cast paving tile
Lining of the furnace, this layer must exhibit high corrosion resistant at elevated temperature (>1350°C) and excellent glass contact properties to prevent glass defects. Specific tight-joint machining of the tiles is also crucial to prevent glass infiltration.
Refractory concrete
Composed of ERSOL SL self-leveling concrete, this layer must demonstrate temperature stability. In case of infiltration, ERSOL’s fused cast AZS grains provide good corrosion resistance against glass in the 1000-1200°C temperature range.
Safety layer
Serving as the last line of defense before insulation, this layer should exhibit corrosion resistance against glass at lower temperatures (<900°C) in the event of complete infiltration. The selection of a bonded AZS product—made from AZS fused-cast grains, such as ERMOLD —offers advantages in terms of corrosion resistance and blistering properties compared to zircon mullite sintered products.
Insulation
This layer provides no glass contact resistance and is used to reduce thermal losses and enhance energy efficiency of the furnace.
The modular layer system described above can be adapted to suit the operating conditions of each furnace. Increasing the thickness of the paving tiles is the first measure to improve the expected lifespan of the bottom paving. With more refractory material, the resistance of the lining against direct corrosion and downward drilling will improve.
Additionally, Figure 4 presents the benefits of using thicker paving for high-boosting operations.
In this example, considering a glass temperature of 1330°C (high electrical boosting furnace), a 200 mm-thick paving can maintain the temperature below the tiles around 1250°C. Minimizing the temperature beneath the paving is critical to mitigating the risks of failure in case of glass infiltration. If the temperature exceeds 1300°C, the ERSOL layer may sinter and shrink (see Figure 5), leading to joint openings in the concrete layer. In such cases, molten glass can infiltrate through the open joints and spread beneath the surface of the paving. Furthermore, elevated temperatures below paving will accelerate upward drilling kinetics, resulting in rapid corrosion of the bottom paving, which could be entirely compromised within months. By maintaining temperatures around or below 1250°C, thicker paving tiles allow to mitigate the risks of upward drilling by slowing the corrosion mechanism and preventing spread of molten glass I the concrete layer.
Double-layer paving concept
Increasing the thickness of the bottom paving is an effective strategy to increase the lifespan of the paving and mitigate the risk of upward drilling in case of glass infiltration. However, transitioning from 75 mm or 120 mm-thick tiles to 200mm blocks incurs significant economic drawbacks with increased manufacturing costs per ton. Additionally, increasing the thermal gradient along the thickness of the paving during the application complicates joint closure due to thermal expansion variations between glass contact and bottom of the block.
To address these challenges, an alternative consists in adopting a double-layer paving design for high thickness applications. Utilizing an assembly of 75 mm and 120 mm tiles helps reduce the temperature gradient within the paving, facilitating joint closure. Moreover, a double-paving design with crossed joints creates a more tortuous path for glass to reach the concrete sub-layer.
Finally, using two distinct layers of fused-cast tile allows for the functionalization of the top layer, thereby limiting the costs associated with a material upgrade compared to a single-layer design of identical thickness. In this context, the use of yttria-stabilized AZS, such as ER 2010 RIC, offers several key benefits to further enhance the lifetime and performance of bottom pavings.
Thanks to its yttria content, the zirconia phase change in ER 2010 RIC occurs at lower temperatures (approximately 100°C lower) compared to standard AZS products (Figure 6). Moreover, the volume change induced by the zirconia transition is 40% lower than that of standard AZS products. These phenomena facilitate complete joint closure at operating temperatures, as the thermal expansion matches the maximum expansion encountered prior to the zirconia transition. Additionally, ER 2010 RIC provides advantages in terms of corrosion resistance and lower temperatures beneath the paving. The 39% ZrO2 content enables corrosion resistance performance comparable to standard AZS 40 products while also resulting in lower thermal conductivity compared to AZS 33 alternatives, leading to reduced temperatures beneath paving for identical thicknesses.
The enhanced corrosion resistance can also be leveraged to optimize the thickness of the first layer in the paving assembly while still benefiting from the optimized joint closure properties of ER 2010 RIC.
Conclusion
By identifying the mechanisms responsible for bottom paving corrosion, glassmakers can effectively address challenges and mitigate risks during the design phase of the furnace. An increased thickness of fused-cast material is typically necessary to achieve a long lifespan in high-temperature environments. For high-boosting and hybrid furnaces, designs may require a minimum of 150 to 200 mm-thick paving. Such solutions provide satisfactory corrosion resistance along with controlled temperatures beneath the paving, which mitigates the risk of upward drilling.
For high-thickness applications, the choice of double-layer paving can offer several advantages. Crossed joints between the two layers create a tortuous path that slows glass infiltration to the concrete sub-layers. The top layer can also be functionalized with premium AZS products, offering higher corrosion resistance and optimized joint closure while maintaining an affordable base ER 1681 layer. Double-layer designs can be assembled by dry-stacking two layer of fused-cast tiles. This option requires machining of the tiles’ large face and offers excellent contact properties between the layers. Alternatively, specific refractory cement can be used to ensure proper contact between the layers without additional machining.
For all studies and projects, numerical simulation of the temperature and corrosion behavior of bottom paving is essential to make informed decisions and align refractory design and selection with industrial and economic targets.
