In tapping channel castables, various raw materials serve specific functions to ensure the performance and durability of the final product. Here is a breakdown of the roles of each component in Al₂O₃-SiC-C (ASC) castables typically used for tapping channels:
1.Al₂O₃ Aggregates: These form the primary component of the castable, providing the structural framework. The aggregates used may include fused white corundum, brown corundum, semi-white corundum, sintered alumina, and high-alumina bauxite clinker. Selection depends on the required quality and application conditions. High-grade materials typically use fused dense corundum, medium-grade materials use brown corundum, and lower-grade materials use sintered corundum or bauxite clinker.
2.SiC (Silicon Carbide): SiC is added for several reasons:
It effectively prevents the oxidation of carbon, enhancing the castable's oxidation resistance.
SiC has a low thermal expansion coefficient, about half that of Al₂O₃, which helps prevent cracking during heating and cooling.
SiC's high thermal conductivity improves the castable's thermal shock resistance.
SiC oxidation produces SiO₂, CO, and CO₂, which further inhibit material oxidation.
SiC enhances the material's resistance to erosion. However, excessive SiC can reduce high-temperature strength, so its content is generally controlled between 10% and 25%. Research and practical use suggest that higher SiC content improves slag erosion resistance, often reaching over 20% in castables.
3.Carbon: In ASC castables, carbon plays a critical role in:
Preventing slag penetration into the material, thereby enhancing erosion resistance.
Increasing thermal conductivity and improving thermal shock resistance.
Reducing structural spalling and cracking. Carbon is typically added as graphite, carbon black, or pitch coke, with an addition amount around 5%.
4.Cement: High-alumina cement or pure calcium aluminate cement is used as a binder to maintain low to medium-temperature strength. However, the addition of cement also introduces a small amount of CaO, which can negatively impact erosion resistance. Increased cement content raises the water demand, leading to higher porosity, lower bulk density, and reduced erosion resistance. Therefore, ASC castables for tapping channels usually have low or ultra-low cement content, with total CaO content controlled below 1.0% to 2.5%.
5.Silica Fume (Silicon Powder): Silica fume reacts with carbon at certain temperatures to form SiC. The resulting SiC exists in two forms:
Fine SiC whiskers (~0.1 to 0.5 μm in diameter) that bridge between the matrix particles, providing significant reinforcement and improving high-temperature strength.
Worm-like or flocculent SiC, which enhances the microstructure, forming a SiC-bonded Al₂O₃-SiC-C material that improves oxidation resistance and slag resistance.
6.Metallic Aluminum Powder: Aluminum powder reacts with water in the castable to generate hydrogen, which leaves fine vent channels after being expelled, facilitating the removal of moisture and preventing explosive spalling during drying. The reaction also generates heat, accelerating dehydration, setting, and hardening, thus improving the castable's strength. However, excessive aluminum powder can generate too much hydrogen, leading to excessive porosity, weakened structure, and reduced strength and erosion resistance.
7.Organic Fibers: Organic fibers prevent explosive spalling during drying by burning out and leaving exhaust channels, which help to expel moisture from the castable.
8.Sodium Polyphosphate: This additive, primarily consisting of sodium tripolyphosphate and sodium hexametaphosphate, acts as a dispersant and water reducer, improving the castable's bulk density, reducing porosity, and enhancing strength and workability.
9.Set Retarders or Accelerators: These additives adjust the working time of the castable to improve construction performance. Common accelerators for calcium aluminate cement include NaOH, KOH, Ca(OH)₂, Na₂CO₃, K₂CO₃, and Na₂SiO₃. Common retarders include NaCl, BaCl₂, MgCl₂, CaCl₂, citric acid, tartaric acid, gluconic acid, ethylene glycol, phosphates, and lignin iodates.
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