A general model based on the Urbain formalism has been developed, which enables the viscosities of liquid slags to be predicted for all compositions in the Al 2 O 3 -CaO-'FeO'-SiO 2 system in equilibrium with metallic iron. Available experimental viscosity data have been analyzed and critically reviewed. The Urbain formalism has been modified to include compositional dependent model parameters. Experimental data in unaries, binaries, ternaries, and the quaternary system have been described by the model over the whole compositional and temperature ranges using one set of model parameters. This viscosity model can now be applied to various industrial slag systems.

A structurally based viscosity model for fully liquid silicate slags has been proposed and applied to the Al 2 O 3 -CaO-'FeO'-SiO 2 system at metallic iron saturation. The model links the slag viscosity to the internal structure of melts through the concentrations of various anion/cation structural units (SUs). The concentrations of structural units are equivalent to the second nearest neighbor bond concentrations calculated by the quasi-chemical thermodynamic model. This viscosity model describes experimental data over the entire temperature and composition range within the Al 2 O 3 -CaO-'FeO'-SiO 2 system at metallic iron saturation and can be extended to other industrial slag systems.

A new mathematical formalism has been developed to describe the viscosities of molten oxides as a function of composition and temperature.1,2) The formalism, hereafter referred to as the quasi-chemical viscosity (QCV) model, enables the internal structural information of complex slags to be related to this physicochemical property. The internal structural information changes systematically with changing bulk composition and temperature and is derived from consideration of the thermodynamic equilibrium in the system. The QCV model provides a link between fundamental thermodynamic and physicochemical properties of the slags, and enables the slag viscosities for all compositions and temperatures in the chemical system to be predicted using the same set of model parameters. Application of this approach and the evaluation of QCV model parameters for the system Al 2 O 3 -CaO-'FeO'-SiO 2 have previously been reported by the authors.2)The present paper is the first in a series describing the effect of MgO on slag viscosities and extending the QCV model to the quaternary system Al 2 O 3 -CaO-MgO-SiO 2 . The present paper provides descriptions of the quasi-chemical viscosity model and optimisation procedure, outlines the principles applied in the evaluation of experimental data and the resultant viscosity models for the MgO, Al 2 O 3 -MgO, CaO-MgO and MgO-SiO 2 systems.Many of the materials that are used as feedstocks for pyrometallurgical processes include MgO-containing minerals. In some cases MgO is deliberately added to these hightemperature processes to adjust and to optimise the chemical and/or physicochemical characteristics of the systems. The influence of MgO on melt viscosity is therefore significant not only from a fundamental scientific point of view, but also because of its relevance to a wide range of industrial applications.A number of viscosity models found in the literature on the Al 2 O 3 -CaO-'FeO'-MgO-SiO 2 system have recently been reviewed by the authors.3) It would appear that at present no viscosity model accurately describes this property over the whole range of chemical compositions. The Urbain formalism is one of the most widely used slag viscosity models and is based on the application of polynomial functions of slag composition to describe the pre-exponential and exponential terms in the Weymann-Frenkel equation. 4)Urbain reported 4) different model parameters for each ternary system Al 2 O 3 -MgO-SiO 2 and Al 2 O 3 -CaO-SiO 2 , and suggested a method of extrapolating model predictions to multi-component systems. However, this method does not provide close agreement between the model predictions and experimental data in the Al 2 O 3 -CaO-MgO-SiO 2 systems over the whole compositional range. Several viscosity models have been developed on the basis of the Urbain formalism. 5,6) The Kalmanovich-Frank model 5) adopted the Urbain model for viscosity of the particular coal ash slags. Riboud et al. 6) corrected the Urbain formalism to describe the viscosities of some industrial mould fluxes (in th...

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