Basics of gas stirring

Argon is usually bubbled into the molten steel covered with slag either through the top lance or through a porous plug fitted at the bottom. A plume of gas rises upwards when gas is injected through the bottom. The stirring  homogenizes bath composition and temperature. With centrally placed nozzle at the bottom, stirring action is small near the bottom of the ladle. An asymmetrically placed bubble plume gives velocities near the bottom which are greater than for symmetrically placed nozzle.

The liquid flow in the ladle occurs via bulk motion of the metal. Very roughly, characteristic velocity  for gas bubbling is

(1)

Where Q is gas flow rate , g is, H is bath height (m) and V is bath volume (). For gas injection at 50 l/min in a ladle of bath diameter 3.5m and bath height 3.5m, equation 1 calculates 0.14 m/s as the gas bubbling velocity.

At velocities greater than 0.3 m/s at the slag/metal interface slag droplets may be entrained by the metal flowing along the interface and into the melt.

For gas stirring recirculation rate of molten steel is of interest. The mass flux in (tons/s) of entrained steel passing through the top section of the bubble plume can be calculated by the following semi-empirical equation:

(2)

Consider a ladle with bath height 3m and Q=650 l/min (1 atm and 273K). We can calculate. Increasing the flow rate to 800 l/min increases  to 10.3 tons/s.

The calculations show that the recirculation time of a 200 ton melt is 21 s for 600 Nl/min and 20 s for 800 Nl/min gas flow. At higher gas flow rates slag droplets may be entrained in molten steel which may increase the rates of refining reaction because of transitory phase contact (Readers may see the references at the end of this lecture).