Microstructural characterization of alkali metal mediated high temperature reactions in mullite based refractories

Two types of refractory bricks were used in reaction tests with slag from a production kiln for iron ore pellet production. Electron microscopy was used to characterize morphological changes at the slag/brick interface and active chemical reactions. Phases such as kalsilite, nepheline and potassium β-alumina form, in a layered structure, as a consequence of alkali metals migration in the brick. Larger hematite grains (50–100 μm) in the slag remain at the original slag/brick interface, while smaller grains dissolve and move through the partly dissolved brick bulk, and forms micrometer sized needle-shaped crystals deeper in the lining material. Thermodynamic simulations predict the formation of a solid solution between hematite and corundum which is also observed in the reaction zone after extended time periods.