When SiO2 absorbs approximately 37% of CaO, the eutectic equilibrium in the SiO2-CaO system occurs at 1436°C. When SiO2 absorbs about 62% of FeO, the eutectic phase in the SiO2-FeO system is formed at 1178°C. In the case of Al2O3 absorption, when SiO2 combines with up to 55% of Al2O3, the eutectic melting point of the SiO2-3Al2O3·2SiO2 phase reaches 1595°C. These eutectic points play a crucial role in determining the behavior of slag and its interaction with refractory materials.
In systems where the two-phase equilibrium eutectic point exceeds 11,500°C, no liquid phase can form, which minimizes the impact on slag erosion. However, when SiO2 coexists with FeO, the eutectic phase SiO2-2FeO·SiO2 forms at 1178°C, leading to the formation of liquid slag. As the FeO content in the liquid phase increases, SiO2 from the refractory material dissolves into the slag. This dissolution continues until the solubility of SiO2 reaches saturation at a certain temperature. Over time, this process leads to significant erosion of silica bricks. Additionally, when Na2O is present, it forms a low-melting eutectic with SiO2 at 782°C, meaning even small amounts of Na2O can contribute to slag corrosion.
The diffusion theory explains that the dissolution of refractories into slag follows a steady-state diffusion mechanism, forming a diffusion layer at the interface between the slag and the refractory. As SiO2 diffuses into the slag, it gradually reduces the structural integrity of the refractory.
According to data from Jilin Carbon Group over the past decade, slag erosion in the counter-current furnace primarily occurs in the 6th, 7th, and 8th layers, with the erosion decreasing as we move upward. Above the 5th layer, there is minimal to no slag erosion. This is because the upper layers are dominated by the iron reduction reaction, where Fe₂O₃ or Fe₃O₄ reacts with SiO₂ to form an equilibrium phase with a much higher eutectic temperature than the tank wall temperature. Even at 1400°C, this does not lead to slag erosion. The lower layers, however, experience higher temperatures, allowing FeO to form a liquid phase that interacts with SiO₂, causing severe erosion. Despite the long service life of calcined anthracite—often lasting ten years—the presence of high ash and iron content can significantly reduce its lifespan, sometimes by up to 70–80%.
To mitigate slag erosion, reducing the ash content in the slag is essential. Strategies to minimize erosion include lowering the solubility of refractory components in the slag, reducing the diffusion coefficient, and increasing the thickness of the diffusion layer. One practical approach is to lower the flue gas temperature, which decreases solubility and prevents the formation of a liquid phase, thereby reducing erosion. It is recommended to maintain the temperature of the 7th and 8th fire walls between 1200–1280°C and keep the temperature above the 5th floor within acceptable limits.
Due to the iron reduction reactions occurring above the 5th layer, slag erosion is minimal in the upper part of the furnace. The upper furnace wall is typically thicker and more resistant to corrosion compared to the lower sections. Additionally, enhanced magnetic separation of anthracite before it enters the furnace helps remove iron-containing impurities, reducing the rate of dissolution and extending the time it takes for solubility to occur. This slows down the overall erosion process.
The composition of ash varies depending on the coal source, so using anthracite with lower ash and iron content can further reduce slag erosion. By understanding these factors and implementing appropriate measures, the lifespan of the furnace can be significantly extended, ensuring optimal performance during the calcination process.
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