Laboratory scale study of the degradation of mullite/corundum refractories by reaction with alkali-doped deposit materials

Refractory bricks based on mullite and corundum, commonly used in rotary kilns for iron ore pellet production, and deposit material from an iron ore pellet production kiln, were used in laboratory scale tests to investigate refractory/deposit reactions and the infiltration of deposit components into the refractory bricks. The materials tested were in both monolithic form and in the form of powder. Alkali metal carbonates (containing sodium and potassium) were used as corrosive agents, to increase reaction kinetics. The morphological changes and active chemical reactions at the refractory/deposit interface in the samples were characterized by scanning electron microscopy. X-ray diffraction showed that alkali metals react with the mullite in the bricks, this being more pronounced in the case of sodium than potassium. Phases such as nepheline (Na2O·Al2O3·2SiO2), kalsilite and kaliophilite (both K2O·Al2O3·2SiO2), and leucite (K2O·Al2O3·4SiO2) were formed as a consequence of reactions between alkali metals and the refractory bricks. The formation of these phases causes volume expansions of between 20% and 25% in the brick materials, which accelerate degradation.