Effect of temperature on slag foaming
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Effect of Temperature on Slag Foaming
E = 1.83 tr0.2d0.-------~
BAHRI OZTURK and R.J. FRUEHAN 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
[3]
where p is the density of the slag, tr is the surface tension, v is the viscosity, and d is the bubble size. Using Eq. [3], it is possible to predict the foaming index for a slag at a given bubble size. Similar equations have also been derived by Jiang and Fruehan ~7~ for bath-smelting slags, such as CaO-SiO2-A1203-FeO, in their experimental setup. The data in the literature indicate that the effect of temperature on the foaming behavior of slags used in ironmaking and steelmaking operations has not been investigated in detail. In particular, most of the work done by Ito and Fmehant4,s] was done at temperatures less than 1723 K, whereas 1823 to 1923 K is more typical of steelmaking processes. Therefore, the effect of temperature on slag foaming was studied. Also, there is some controversy as to the effect of slag depth on foaming. Equations [1] and [2] indicate that the foam height is independent of slag depth as long as there is a sufficient amount of slag to form foam. Some interpretations of foaming suggest it depends on slag volume. Therefore, experiments were conducted as a function of slag weight. A schematic of the experimental apparatus is shown in Figure l. A molybdenum disilicide resistance furnace was used in the experiments. The hot zone of the furnace was 10 cm. T
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