Fundamentals of inclusion removal from liquid steel by bubble flotation

Abstract

Attempting to remove the maximum number of inclusions from liquid steel by bubble flotation is an important means of improving steel cleanliness. There has, however, been no complete theoretical fundamental study on this issue, and the present review represents a tentative step towards this objective. An attempt is made to apply results from the field of mineral processing to the liquid steel–inclusion–bubble system for the case of silica inclusions and argon bubbling. However, it should be noted that this approach has limitations: the size of the bubbles and particles involved and the degree of turbulence, the surface tension of the liquid, and the contact angle between inclusion and bubble are quite different in the two systems. The mechanism of bubble–inclusion interaction, bubble size and rising velocity, liquid film formation and rupture between bubble and inclusion, collision time, film drainage time, sliding time, and collision and adhesion probabilities are discussed. A simple mathematical model of inclusion removal by bubble flotation is described, and the effects of gas flowrate, bubble size, and time on inclusion removal are discussed. Based on the results of the model, it is clear that the optimum bubble diameter for inclusion removal is 1–5 mm. It is proposed that the shroud protecting transfer of liquid metal from ladle to continuous casting tundish is a good place to inject gas to remove inclusions by bubble flotation. The effect of turbulence on the removal of inclusions by bubble flotation is briefly discussed.