Bubble Characteristics Required for the Complete Removal of Alumina Inclusions from Steel Melts

Gas bubbling can be an effective means to float out alumina inclusions from liquid steel in a ladle. However, large spherical cap bubbles are formed when using porous plugs, as the liquid steel is nonwetting to the porous refractory. These bubbles rise rapidly through the liquid steel, forming a fast‐moving bubble plume, restricting contact times. Sized microbubbles, by contrast, have now been generated in liquid metals by shearing methods, involving linear crossflows to an entering flow of gas, or alternatively by rotational shearing. Combined with these convective shearing forces, local kinetic energy of turbulence can also play an important part in determining final microbubble size distributions. As microbubbles have much smaller rise velocities and present a far greater inclusion capture surface area than those of a single large bubble of the same gross volume, this will allow us to remove sub‐50 μm inclusions from liquid steel. It is expected that this goal will require a redesign of current ladle shrouds. Illustration of burst (or collapsed) microbubbles of argon trapped within a “flash‐frozen” sample of low melting point alloy (freezing point +57 °C), following rapid dipping and withdrawal of a cooled copper bar (≈at −100 °C) from the bath of the molten metal alloy. There, the submerged gas injection system is subjected to rapidly shearing metal flows, which successfully generates microbubbles within the bath.