Physico-Chemical Property Evaluation of Molten Slags
(a) Viscosity

Viscosity is an important physico-chemical property of slags and plays a major role in the manufacturing operations of glasses. Experimental measurements of viscosities are often very difficult to make and time consuming. Thus it is important to develop mathematical models that will predict viscosities in a self-consistent manner over the entire composition and temperature range. Also, the viscosity of high temperature glass melts is a structure related property. In the present research, theoretical models are developed to predict viscosities of a series of glass melts like Li2O-B2O3, Na2O-B2O3, K2O-B2O3, Al2O3-SiO2, PbO-SiO2, and Na2O-SiO2 based on thermodynamic principles and experiments are carried out to measure the viscosities. Excellent agreement is observed between the experimental and predicted values. A structure based model is also developed to predict the viscosity of Na2O-B2O3-SiO2 ternary system. The calculated results showed an excellent agreement with experimental results for the composition with XSiO2/XNa2O = 2, 1.5 and 1. At XSiO2/XNa2O = 0.5, a large deviation was observed between the experimental and calculated results. This deviation was due to the formation of solid particles in the melt. The type and amount of solid particles formed has been studied by optical microscopy and X-ray diffraction analysis. Based on these results, a structural model has been proposed.


(b) Sulfide Capacity of Molten Slags

For more than three decades, a number of empirical correlations have been proposed between sulfide capacity and the basicity of slags. Because of deviations of data from predictions based on the basicity concepts, ad hoc correlations to these methods are needed. Recently, we (Reddy-Blander Model) showed that sulfide capacities can be calculated a priori, based on a knowledge of the chemical and solution properties of oxides and sulfides. Sulfide capacities have been calculated in a self-consistent manner for slag compositions ranging from the pure basic oxide to acidic oxide melts for the systems CaO-SiO2, FeO-SiO2, MgO-SiO2, MnO-SiO2 and Na2O-SiO2 as a function of temperature. Fundamental equations for the calculation of sulfide capacities, both in basic melts and in the composition range, 0.33 < XSiO2 < 1.00 in MO - SiO2 (where M represents Ca, Fe, Mg, Mn and Na2) systems have been derived. The results showed that our predictions are in excellent agreement with the experimental data. At a given temperature, the sulfide capacity increases with an increase in the concentration of MO in the melt. For a given composition, the sulfide capacity also increases with an increase in temperature. This is largely related to the large increase in the activity of MO in the melt.

It is further concluded that sulfide capacities of slags are directly proportional to (a) the equilibrium constant KM, which is related to the differences in the Gibbs energies of formation of MO and MS; and to (b) values of aMO, which is related to the solution properties; are related to the interaction of the oxides and sulfides with the other solution components. From the analysis presented, it is concluded that the variation of these two independent properties cannot be correlated by a single parameter, such as basicity or optical basicity, but needs to be predicted in a more fundamental manner using known thermodynamic data on the properties of the pure oxides and sulfides and on the solutions. Correlations of the magnitudes and concentration dependence of experimental data with calculations of CS from a basicity function or from the concept of optical basicity appear to be far less accurate than the values deduced from our method. Predictions of CS in aluminates and multi component slag systems were also deduced. Importance of the method we propose for calculating CS in slags a priori, applied to desulfurization of liquid iron and steel in development and production of ferrous alloys in industry are emphasized.