Metallurgical and Materials Transactions B, February 2015, Volume 46, Issue 1, pp 316-327
Wegener, L. Muhmood, S. Sun, A. V. Deev.
CSIRO Process Science and Engineering, Box 312, Clayton South, VIC, 3169, Australia.
The surface tension of oxide melts is difficult to measure. Even for binary slags with a low degree of polymerisation such as calcia-alumina, there seems no benchmark value for the surface tension available. Published data exhibit considerable scatter even for similar compositions which prevents to build a reliable and consistent database of frequently used physical properties. This study presents an attempt to use three different measurement techniques in the same high-temperature furnace to obtain the surface tension of a binary calcia-alumina slag close to eutectic composition, namely the drop weight method, the pendant drop method, and, for the first time, the oscillating jet method (for circular and elliptic jets) as a function of temperature. The methods used in the present context are explained, and the experimental results evaluated with respect to previously published data. A consistent and coherent trend is observed when the drop weight and the oscillating jet methods are compared.
The surface tension is one of the key parameters in multiphase systems in general and dispersed systems in particular. All surface and interfacial phenomena such as droplet generation, deformation, oscillation, breakage, coalescence, Marangoni effects and so forth are heavily dominated by surface or interfacial tension. Its value is a key ingredient of several important dimensionless numbers, such as the Weber, Bond, Eötvös, Morton or Capillary number, hence, the knowledge of the numerical value is of fundamental importance.
There are well established, precise and accurate surface and interfacial tension measurement techniques available for standard low temperature systems. When carried out with the required care, the numerical value can be measured within acceptable error margins. This becomes a severe issue in high temperature systems (temperatures > 1200°C), as, for example, relevant in iron- and steelmaking processes. The scatter in published data is considerable, due to the manifold sources of uncertainties (e.g. impact of crucible material, surrounding gas atmosphere, presence of surfactants or ion complexes that may act as surfactants, uncertainties in density measurement and determination of exact composition, etc.). Hence, it would be a challenge in itself to choose an applicable surface or interfacial tension value for a given high temperature system due to the lack of reliable benchmark values.
The main idea of the present paper was to use different methods (the drop weight method, the pendant drop method, and – pioneering – the oscillating jet method with circular and elliptic capillary cross section) taking into account their respective pros and cons to measure the surface tension of a synthetic calcia-alumina slag in argon atmosphere. If the data show a consistent trend for all the methods used then the measurements could be considered reliable.