Behaviour of magnesia–carbon refractories in vacuum–oxygen decarburisation ladle linings

Abstract

Magnesia–chromite bricks are commonly used in the working lining in vacuum–oxygen decarburisation ladles. However, they are expensive and potentially environmentally-unfriendly. The possibilities of employing magnesia–carbon refractories were investigated through industrial testing combined with post-mortem assessments, thermodynamic calculations and mechanical considerations. Pitch bonded magnesia materials cannot be used in the slagline owing to the MgO–C reaction and extrinsic oxidation of carbon by reducible slag components (mainly CrO_x), resulting in carbon removal, slag infiltration and direct MgO corrosion. Combined with turbulent slag motion this leads to hot erosion. Direct carbon burnout and the MgO reaction are the main causes for their failure in the freeboard. However, pitch bonded magnesia refractories are able to compete with magnesia–chromite bricks in the ladle bottom and in the lower metal bath region, where levels of turbulence and slag infiltration are limited. Simultaneously, both the MgO–C reaction and the extrinsic oxidation of carbon are restrained by, respectively, the inhibiting effect of a mechanical barrier and the non-wetting behaviour of steel towards the magnesia refractory component.