Slag foaming has become important not only for the modem electric arc furnace but also for basic oxygen steelmaking and new ironmaking processes, such as bath smelting. In the electric arc furnace, foaming practices are widely used to shield the refractories from the arc and to protect metal from the atmosphere. Slag foaming is also used to stabilize the arc in the modem electric arc furnace. The control of the foaming height is required for steady-state operation in the bath-smelting process. Several investigations on slag foaming have been previously carried out. Cooper and Kitchened q measured the foam lives of CaO-SiO2 slags as a function of CaO/SiO2 ratio, concentration of P205, and temperature. The |bam life increased with decreasing temperature and decreasing CaO/SiO2 ratio. Cooper and Kitchener's results indicate that the apparent activation energy for foaming is much higher than that for viscous flow. Swisher and McCabe 121 have also measured the foam life of CaO-SiO 2 slags. Their experimental results indicate that the foam lives increase with decreasing basicity and temperature. Hara and Oginol~l studied the effect of slag composition, surface active additives, and gas composition on the foaming behavior of FeO-SiO2-CaO slags. The foam height increases sharply when the ratio O/Si is decreased below 3.5. Hara and Ogino considered that melt viscosity does not have a role in the foaming behavior of the slags. However, they found that surface tension plays an important role on foam lives. In previous workJ 4.sl the foaming index, ~, and foam life, 7-, were introduced to describe the experimental results. These parameters are defined as