What Are the Functions of Each Raw Material in Taphole Trough Castables?
ASC Castables for Taphole Troughs
ASC castables for taphole troughs are generally low-cement or ultra-low cement castables, primarily composed of Al₂O₃ aggregates, SiC, carbon, cement, and various additives.
1. Al₂O₃ Aggregates
Al₂O₃ aggregates are the main component of castables and form the particle skeleton. These aggregates mainly include fused white corundum, brown fused alumina, sub-white corundum, sintered alumina, and high-alumina bauxite clinker. The selection of aggregates should be determined based on the service conditions. High-grade materials typically use dense fused corundum, medium-grade materials use brown fused alumina, and low-grade materials use sintered corundum or bauxite clinker.
2. SiC
The main reasons for adding SiC to castables are:
It can effectively prevent carbon oxidation and improve the oxidation resistance of taphole trough refractories.
SiC has a low thermal expansion coefficient, which is only half that of Al₂O₃. This helps prevent cracking of ASC castables during heating and cooling.
SiC has high thermal conductivity, which improves the thermal shock resistance of ASC castables.
SiO₂, CO, and CO₂ produced by SiC oxidation can effectively inhibit further oxidation of the material.
SiC can significantly improve the erosion resistance of the material.
However, excessive SiC content can reduce the high-temperature strength of the material, so the addition should be controlled to between 10% and 25%. Research and field applications show that a higher SiC content improves the slag resistance of castables. Therefore, the SiC content in castables is often above 20%.
3. Carbon
In Al₂O₃-SiC-C castables, carbon helps prevent molten slag from penetrating into the interior of the material, confining the slag to the surface of the refractory, and improving the slag resistance of the material. At the same time, carbon increases the thermal conductivity of the material, enhances its thermal shock resistance, and reduces structural spalling and cracking. Carbon sources such as graphite, carbon black, and pitch coke can be used. The effect of carbon in castables is related to both its type and the amount added. Carbon is usually added in the form of pitch pellets or coke, with an addition level of about 5%.
4. Cement
Al₂O₃-SiC-C castables for iron troughs generally use high-alumina cement and pure calcium aluminate cement as binders. Cement is added to maintain the low- and medium-temperature strength of the material. However, the addition of cement introduces a small amount of CaO, which can negatively affect the slag resistance of the material. Additionally, an increase in cement content raises the water demand of the castable, leading to increased porosity, decreased bulk density, and reduced slag resistance. Therefore, Al₂O₃-SiC-C castables for iron troughs are generally low-cement or ultra-low cement castables, with the total CaO content in the castable controlled to below 1.0%–2.5%.
5. Silica Fume
Silica fume can react with carbon in the material at a certain temperature to form SiC. The formed SiC exists in the matrix in two forms: One is very fine SiC whiskers, with a diameter of about 0.1–0.5 μm, which distribute between matrix particles, act as bridges, provide a strong strengthening effect, and improve the high-temperature strength of the castable. The other is worm-like or flocculent SiC, which can improve the microstructure of the castable and form an SiC-bonded Al₂O₃-SiC-C material, thereby enhancing the oxidation resistance and slag resistance of the castable.
6. Aluminum Metal Powder
Aluminum metal powder can react with water in the castable to produce H₂. After the hydrogen escapes, it leaves fine exhaust pores, which facilitate the removal of internal moisture and eliminate some of the free water. At the same time, it can prevent explosive spalling during heating. The heat released during the reaction can also accelerate dehydration, speed up the setting and hardening of the castable, and improve its strength. Additionally, the Al(OH)₃ gel formed after the reaction creates a new bonding phase, which further enhances the strength of the castable. However, the amount of aluminum metal powder added should not be excessive. Otherwise, too much hydrogen will be released, leaving too many pores, which results in a loose structure, reduced strength, and poor erosion resistance.
7. Organic Fiber
Organic fiber prevents explosive spalling of the castable during the drying process. During drying, the organic fiber burns out, leaving exhaust channels that facilitate the removal of moisture from the castable.
8. Sodium Polyphosphate
When added to the castable, sodium polyphosphate acts as a dispersant and water reducer, thereby increasing bulk density, reducing porosity, improving strength, and enhancing the working performance of the castable. The main types used are sodium tripolyphosphate and sodium hexametaphosphate.
9. Retarder or Accelerator
Retarders or accelerators are added to the castable to adjust the working time and improve its constructability.
Common accelerators for calcium aluminate cement include: NaOH, KOH, Ca(OH)₂, Na₂CO₃, K₂CO₃, Na₂SiO₃, etc.
Common retarders for calcium aluminate cement include: NaCl, BaCl₂, MgCl₂, CaCl₂, citric acid, tartaric acid, gluconic acid, ethylene glycol, phosphates, lignin sulfonates, etc.