Role of Silicon Carbide in Al₂O₃-SiC-C Refractories

Abstract

Al₂O₃-SiC-C refractories are used in ladle for hot metal pretreatments. In the Al₂O₃-SiC-C refractories used in such treatments, the surface of SiC grains is corroded in a comb-like shape, and deposits of carbon are found in the affected section. The Na₂O-Al₂O₃-SiO₂ compounds also deposited on the surface of Al₂O₃ grains and inside the micropores. The following remarks have been obtained based on the microscopic observation of the refractories used and the thermodynamic study in the system SiO₂-SiC-C.
(1) Carbon is oxidized at the hot surface to generate CO(g), forming a reducing atmosphere. A part of the generated CO(g) reacts with SiC(s) to deposit carbon in the refractories by the following reaction.
SiC(s)+CO(g)→SiO(g)+2C(s)
(2) SiC grains in the refractories are corroded by the reaction with CO(g) within the refractories to form SiO(g) and carbon. Consequently, the affected section is filled with carbon.
(3) The generated SiO(g) is oxidized by CO(g) to SiO₂(s), which reacts with Na₂O and Al₂O₃ to form Na₂O-Al₂O₃-SiO₂ compounds on the surface of Al₂O₃(s) grains and inside the micropores. These compounds, which create a denser structure, prevent the gases from passing through the refractories and suppress the oxidation of carbon.