Surface Tension Measurements of Calcia-Alumina Slags: A Comparison of Dynamic Methods
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NTRODUCTION
SURFACE tension is a fundamental property which has manifestation in all surface and interfacial phenomena. For example, the surface and interfacial tension affect droplet size and drop size distribution via droplet breakage and coalescence behavior, which is important in heavily stirred reactors in process metallurgy to control reaction rates. Moreover, surface active components may have a strong effect on the surface tension and may impose considerable surface tension gradients which give rise to Marangoni effects. The surface tension of slags and steel or the interfacial tension between them plays a dominant role in iron- and steelmaking processes as their absolute values are about 5 to 20 times larger than those of water.[1] Generally, the surface tension of pure oxides and slags (200 to 700 mN m1) is lower than the surface tension of metals, but their viscosities are usually far higher and strongly increase with decreasing temperatures. Compared with dynamic viscosity, the surface tension of slags has not yet been investigated in such detail, resulting in more scatter of experimental data.[2] However, considerable amounts of experimental data have been collected for different systems at various compositions and temperatures; see for example the review by Mills and Keene.[3] From the analytical point of view, predictive models have been developed,[4–7] nevertheless, there often seem to be discrepancies between predicted values and experimental data which underline the need for a more reliable experimental database. M. WEGENER and L. MUHMOOD, Postdoctoral Fellows, S. SUN, Senior Principal Research Scientist, and A.V. DEEV, Project Leader, are with CSIRO Process Science and Engineering, Box 312 Clayton South, VIC 3169 Australia. Contact e-mail: mirco.wegener@ alumni.tu-berlin.de Manuscript submitted April 13, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
In principle, a broad variety of surface tension measurement techniques is available. However, due to the special conditions in high-temperature environments, the technique employed should be appropriate for the experimental conditions.[8] This is why most of the measurements have been carried out using one of the following: sessile drop method, pendant drop method, drop weight method, maximum bubble pressure method, cylinder and ring detachment method. A survey and description of suitable methods for high-temperature systems has been given by Keene[8] and Korenko and Sˇimko.[9] It is an unfortunate matter of fact that there are considerable deviations among the experimental data. These may originate from the presence of uncontrolled surfactants at the surface, from uncertainties in the density value at a given temperature and composition, and from the uncertainty whether the system was in equilibrium or not since chemical reactions and surfactant adsorption have a strong dynamic impact on the surface tension. Similarly, the surface tension depends strongly on the atmosphere used in the experiments. For example, the surface tension of a 40 pct CaO-40
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