A new method for the prediction of the viscosity of coal ash slags, in the Newtonian region, is presented. The technique is modeled on experimental viscosity data less than 1000 Pa s and hence is most reliable in that region. The capability of the model in predicting the viscosity of slags from coal ash was found to be superior to a number of the most commonly used empirical models found in the literature, which are based on simplified oxide melts or British coal ash slags. The method also provides an indication of the relative fluxing strength of the basic oxides usually found in coal ash slags. It was found that the fluxing strength is related to the inverse of the cation radius.

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This paper presents a phase diagram approach to predict the melting temperatures of coal ash/flux mixtures and the viscosity versus temperature characteristics of the molten slags. This is illustrated by calcium oxide fluxing studies of three Australian bituminous coal ashes covering a range of silica to alumina ratios. Reasonable agreement was obtained between the observed melting temperatures of the calcium oxide/coal ash mixtures and predictions based on equilibrium phase diagrams for the ternary system silica, alumina, and calcium oxide or for the quaternary system silica, alumina, calcium oxide, and ferrous oxide. The approach is supported by energy dispersive scanning electron microscopy (SEM), SEM microprobe, and X-ray diffraction (XRD) studies of the slags. Slag viscosity measurements were made with a rotational viscometer over the range 1200-1600°C. The measured viscosities were compared with predicted values using a model based on experimental results for the ternary system silica, alumina, and calcium oxide. The agreement between experimental results and predictions from this approach suggests that sensible estimates can be made of the amount of fluxing agent necessary for satisfactory slag tapping from the ash content and ash composition of the coal.

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