The surface tensions and foaming behavior of melts in the system CaO-FeO-SiO 2
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I. INTRODUCTION
II. EXPERIMENTS
current work was undertaken to obtain accurate measurements of the surface tensions of slags of interest in steelmaking, with a view to providing a better understanding of the phenomenon of slag foaming. The use of a foaming slag in an electric arc furnace to shield the refractories from the arc and to stabilize the arc requires the ability to control the height of the slag foam. Also, in proposed bath smelting processes involving the injection of coal and iron ore into an iron bath, the postcombusted heat could be transferred to and stored in a controlled foaming slag. Kozakevitch[1] has described a foam as being a gas/liquid emulsion in which the volume of the liquid medium is small in comparison with the total volume of the system. The formation and durability of a foam are facilitated by a low surface tension, which can be produced by the formation of adsorbed films of surface-active solutes, and the decay of a foam is caused by the elimination of the liquid in the films by drainage. Kozakevitch considered that a high viscosity is perhaps the most obvious factor in the stabilization of foams, as a high viscosity retards the rate of drainage in the films. The influence of surface tension on foaming has been considered by Gibbs,[2] who showed that any solute that decreases the surface tension of a melt is preferentially adsorbed at the surface of the melt, and the excess surface concentration of the solute is obtained from Gibbs’ adsorption isotherm, which relates the excess surface concentration with the dependence of the surface tension on the thermodynamic activity of the solute in the melt. Kozakevitch[1] has suggested that in silicate melts, in which silica is the surfaceactive component, the adsorbed surface films consist of complex silicate anions on which the Si-O2 bonds are directed toward the bulk phase and that the negative charge is compensated by cations beneath the adsorbed layer. The film is thus an ionic double layer or a bipolar layer, of relatively high viscosity, which contributes to the stability of the foam.
The surface tensions were measured using the dipping cylinder technique, which involves measuring the maximum excess force exerted on a cylinder during its withdrawal from immersion in the melt. Knowledge of this maximum force and the dimensions of the cylinder allow the surface tension, g, of the melt to be calculated as
THE
DAVID SKUPIEN, Engineer, is with Detroit Diesel Corporation, Detroit, MI 48239. D.R. GASKELL, Professor, is with the School of Materials Engineering, Purdue University, West Lafayette, IN 47907. This article is based on a presentation made in the “Geoffrey Belton Memorial Symposium,” held in January 2000, in Sydney, Australia, under the joint sponsorship of ISS and TMS. METALLURGICAL AND MATERIALS TRANSACTIONS B
g5
1 2
F R3 R ?f , 4pR V r
[1]
where F is the maximum excess force required for detachment of the cylinder from the surface of the liquid, R is the average of the inner and outer radii of the cylinder, r is half the w
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