What are the functions of each raw material in the castable refractory for the iron outlet trough?

What are the functions of each raw material in the castable refractory for the iron outlet trough?

ASC castable refractory used for iron outlet trenches is generally a low-cement or ultra-low-cement castable refractory, mainly composed of Al2O3 aggregate, SiC, carbon, cement and various additives.

Al₂O₃ Aggregate

Al₂O₃ aggregate is the main component of castables, forming the particle skeleton of the castable. Al₂O₃ aggregate mainly includes fused white corundum, brown corundum, sub-white corundum, sintered alumina, and high-alumina bauxite clinker. The selection should be based on a comprehensive consideration of the material’s application conditions. For high-grade materials, fused dense corundum is used; for medium-grade materials, brown corundum is used; and for low-grade materials, sintered corundum or bauxite clinker is used.

SiC

The main considerations for adding SiC to castables are: (1) it can effectively prevent carbon oxidation and improve the oxidation resistance of refractory materials used in tapping troughs; (2) SiC has a low coefficient of thermal expansion, only half that of Al₂O₃, which can prevent cracking during the heating and cooling process of ASC castables; (3) SiC has high thermal conductivity, which can improve the thermal shock resistance of ASC castables; (4) SiO₂, CO, and CO₂ produced after SiC oxidation can effectively inhibit the oxidation of materials; (5) SiC can effectively improve the erosion resistance of materials. However, when the amount of SiC added is too large, the high-temperature strength of the material decreases, so the amount of SiC added needs to be controlled between 10% and 25%. Research and field use show that a high SiC content can improve the slag erosion resistance of castables. Therefore, the SiC content of castables is often as high as 20% or more.

Silicon powder (silicon)

Carbon in Al2O3-SiC-C castables prevents slag from penetrating into the material’s interior; it confines the slag to the surface of the refractory, improving its erosion resistance; simultaneously, carbon increases the material’s thermal conductivity, enhances its thermal shock resistance, and reduces structural spalling and cracking. Carbon can be added from sources such as graphite, carbon black, and pitch coke. Of course, the effect of carbon in castables depends on the amount added and the type of carbon source. Carbon is typically added in the form of pitch balls or coke, at a concentration of approximately 5%.

Carbon

Al2O3-SiC-C castables for iron troughs typically use high-alumina cement and pure calcium aluminate cement as binders. Cement is added to maintain the material’s low- and medium-temperature strength; however, the addition of cement introduces a small amount of CaO, which is detrimental to the material’s erosion resistance. Furthermore, increased cement content increases the water demand of the castable, leading to increased porosity, decreased bulk density, and reduced erosion resistance. Therefore, Al2O3-SiC-C castables for iron troughs are generally low-cement or ultra-low-cement castables, with the total CaO content controlled below 1.0% to 2.5%.

Cement

Al2O3-SiC-C castables for iron troughs typically use high-alumina cement and pure calcium aluminate cement as binders. Cement is added to maintain the material’s low- and medium-temperature strength; however, the addition of cement introduces a small amount of CaO, which is detrimental to the material’s erosion resistance. Furthermore, increased cement content increases the water demand of the castable, leading to increased porosity, decreased bulk density, and reduced erosion resistance. Therefore, Al2O3-SiC-C castables for iron troughs are generally low-cement or ultra-low-cement castables, with the total CaO content controlled below 1.0% to 2.5%.

Aluminum powder

Aluminum powder reacts with water in the castable to produce H2. The H2, after being expelled, leaves tiny vent holes, facilitating the removal of internal moisture and some free water, while also preventing cracking during baking. The heat released during the reaction also accelerates dehydration, speeds up the solidification and hardening process of the castable, and improves its strength. Furthermore, the Al(OH)3 gel formed after the reaction can form a new binding phase, further enhancing the strength of the castable. However, the amount of aluminum powder added should not be excessive; otherwise, it will release a large amount of hydrogen gas, leaving too many pores, resulting in a loose material structure, reduced strength, and decreased corrosion resistance.

Organic Fibers

Organic fibers prevent the castable from bursting during baking because they are burned off during the drying process, leaving ventilation channels that facilitate the removal of moisture from the castable.

Sodium polyphosphate

When sodium polyphosphate is added to castables, its dispersing and water-reducing effects can increase the bulk density of the castable, reduce the porosity of the material, increase strength, and improve the workability of the castable. The main types of sodium polyphosphate used are sodium tripolyphosphate and sodium hexametaphosphate.

Retarder or Accelerator

Adding retarders or accelerators to castables is to adjust the service life of the castable and improve its workability. Commonly used accelerators for calcium aluminate cement include: NaOH, KOH, Ca(OH)2, Na2CO3, K2CO3, Na2SiO3, etc.; commonly used retarders for calcium aluminate cement include: NaCl, BaCl2, MgCl2, CaCl2, citric acid, tartaric acid, gluconic acid, ethylene glycol, phosphates, and lignoiodates, etc.