Seshadri Seetharaman

Estimation of the viscosities of binary metallic melts using Gibbs energies of mixing

In the present work, information on the integral molar Gibbs energies of mixing is employed to calculate the viscosities of binary substitutional metallic melts. A correlation has been established between the second derivative of the integral molar Gibbs energy of mixing with respect to composition and the corresponding function for the Gibbs energy of activation for viscosity. The viscosities predicted from available thermodynamic data in the case of a number of binary metallic systems using this correlation show satisfactory agreement with the values reported from experimental measurements. The value of this correlation in predicting the viscosities of complex metallic melts is also examined.

A Model for Estimation of Viscosities of Complex Metallic and Ionic Melts

In the present work, the viscosities of high-temperature melts, ionic as well as metallic, are estimated by means of a model, which is based on the absolute reaction-rate theory for the description of flow processes. The composition of ionic melts is represented in analogy with Temkins theory. The model has been applied to some multicomponent systems. The results of the calculations illustrate that the model is successful in extrapolating the viscosity data for complex metallic and slag systems both as a function of temperature and composition.

A study of the thermal decomposition of BaCO3

In the present work, the decomposition reaction, BaCO3 (solid) = BaO (solid) + CO2 (gas), was investigated by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) methods. Both shallow powder beds and densely compacted spheres of the carbonate were employed. In the case of the shallow powder beds, TGA and DTA were carried out simultaneously. The DTA curves showed that BaCO3 exhibited two phase transformations, the transformation of orthorhombic to hexagonal occurring at 1079 K and that of hexagonal to cubic at 1237 K. The activation energy and the forward reaction rate constant of the decomposition of BaCO3 were evaluated from the thermogravimetric results of the powder beds. The activation energy of the decomposition was found to be 305(± 14) kJ • mole−1. The experimental results obtained with the compacted spheres were compared with those corresponding to the powder beds. After the initial stages, the formation of liquid due to the eutectic reaction between BaCO3 and BaO appears to play an important role in the reaction kinetics.

Sulfide capacity of high alumina blast furnace slags

Sulfide capacities of high alumina blast furnace slags were experimentally determined using the gas-slag equilibration technique. Two different slag systems were considered for the current study, namely, CaO-SiO2-MgO-Al2O3 quaternary and CaO-SiO2-MgO-Al2O3-TiO2 quinary system. The liquid slag was equilibrated with the Ar-CO-CO2-SO2 gas mixture. Experiments were conducted in the temperature range of 1773 to 1873 K. The effects of temperature, basicity, and the MgO and TiO2 contents of slags on sulfide capacity were studied. As expected, sulfide capacity was found to increase with the increase in temperature and basicity. At the higher experimental temperature, titania decreases the sulfide capacity of slag. However, at the lower temperature, there was no significant effect of titania on the sulfide capacity of slag. Sulfide capacity increases with the increase in MgO content of slag if the MgO content is more than 5 pct.

Experimental studies of viscosities of some CaO-CaF2-SiO2 slags

AbstractIn the present work, the viscosities of some CaO–CaF2–SiO2 slags were measured using the rotating cylinder method. The effects of various volatile fluorine compounds on the change in slag composition are discussed on the basis of thermodynamic calculations. The kinetic factors concerning the escape of the volatile fluorides under the experimental conditions were also examined. It was found that the formation of gaseous species SiF4 was the main source for compositional changes during the viscosity measurements. In the case of CaO–CaF2–SiO2slags, the formation of SiF4 would be somewhat limited owing to the existence of CaO. The composition change during the measurements was only about ±1 wt-% for all components in most cases. Viscosity measurements are reported for slags, based on the post-measurement compositions. It was found that the addition of CaF2 causes a significant decrease in viscosity.

Interdiffusion in the MgO-Al2O3 spinel with or without some dopants

With a view to seek an improved understanding of the DIMOX process, interdiffusion of polycrystalline MgO and Al2O3 in the temperature range 1473 to 1873 K was studied by diffusion couple experiments. The interdiffusivities in the spinel layer were calculated as functions of composition and temperature. The spinel portion of the phase diagram in the system MgO-Al2O3 was determined from carefully measured compositions at the phase boundaries, and the low temperature spinel region of the phase diagram was confirmed from the present results. For Zn2+ as dopant in alumina, the growth rate of spinel thickness seems to increase when compared with that of the diffusion couples without dopant. The samples containing Si4+ as dopant reveal the formation of a glass phase, and the effect of Si4+ on the diffusion process appears to be negligible.

Estimation of viscosity for blast furnace type slags

Abstract A viscosity model based on a new definition of basicity has been proposed for blast furnace type slags. Conceptually, this definition of basicity is close to Bells definition of basicity as used for modelling of sulphide capacity of blast furnace type slags. The model developed in the present work is applicable for wide range of alumina, magnesia and titania containing blast furnace slags, while most of the models available in the literature are mainly applicable for a limited range of slag composition. Viscosity estimation by this model is close to the experimental value for all types of blast furnace slags. This model is based on the chemical composition of slag and is applicable for slags above liquidus temperature.

Thermal diffusivity measurements of liquid silicate melts

The effect of structure on the thermal diffusivities/conductivities for liquid silicates have been summarized based on recent experimental work carried out by the Royal Institute of Technology, Stockholm and the Tokyo Institute of Technology using the laser-flash and the hot-wire methods, respectively. In the former case, the effective thermal diffusivity was measured by a three-layer method. The relationship proposed by Mills that the thermal conductivity of silicates increases with a decrease in the ratio of NBO/T (number of non-bridging oxygens per tetrahedrally coordinated atom) has been well supported by the effective thermal diffusivity data for the liquid CaO-Al2O3-SiO2 slags. However, it has been shown that for the slags having a higher CaO/Al2O3 ratio, the effective thermal diffusivity is roughly constant independent of the ratios of NBO/T. It has been concluded that when the silicate network is largely broken down, the phonon mean free path is not affected by the structure. It has been found by the hot-wire method that the magnitudes of thermal resistivity are in the hierarchy Li2O-SiO2<Na2O-SiO2<K2O-SiO2 despite their similar values of NBO/T. It has been concluded that the ionicity of non-bridging oxygen ions is also a factor controlling the thermal conductivity of silicates as well as the number of broken bridges in the silicate network. The effective thermal diffusivity was measured for the CaO-Al2O3-SiO2-FeO system to elucidate the radiation contribution to the effective thermal diffusivity. It has been found that the effective thermal diffusivity increases with an increase in FeO content. It can be considered that the strong absorption and emission within the liquid slag films caused by the Fe2+ ions enhances the photon heat transfer.

Viscosities of Multicomponent Silicate Melts at High Temperatures

In the present work; the viscosities in the quaternaries CaO–FenO–MgO–SiO2, FenO-MgO–MnO–SiO2, and CaO–MgO–MnO–SiO2 and the quinary CaO–FenO–MgO–MnO–SiO2 were studied. The experimental technique employed was the well-established rotating cylinder method, using a Brookfield digital viscometer mounted over a specially designed graphite furnace. Generally, iron crucibles were used along with iron spindles. Periodic calibrations of the experimental setup were made using the standard reference slag recommended by the European Union. The measurements were carried out up to a maximum temperature of 1773 K in all cases. The reliability of the measurements were checked at different rotation speeds as well as during thermal cycling, and excellent reproducibility of the results was noted. The experimental viscosity values were incorporated into a viscosity model. Equations based on the model for calculating the viscosities of the quarternary systems CaO–FenO–MgO–SiO2, FenO–MgO–MnO–SiO2, and CaO–MgO–MnO–SiO2 and the quinary system CaO–FenO–MgO–MnO–SiO2 are provided.

Application of a nonisothermal thermogravimetric method to the kinetic study of the reduction of metallic oxides: Part II. A theoretical treatment of powder bed reduction and its application to the reduction of tungsten oxide by hydrogen

Theoretical treatment of nonisothermal kinetic studies has been extended in the present work to gas-solid reactions in powder beds. An expression for the activation energies for the reaction has been derived on the basis that the reaction proceeds by the movement of the reaction front, the velocity of the movement being kept constant. The reliability of the method has been tested by applying the same to the reduction of tungsten oxide by hydrogen. The experiments were carried out using thermogravimetric technique under both isothermal and nonisothermal conditions. The reaction front is considered to consist of a thin layer of small particles. The rate of the reduction seems to be controlled by the chemical reaction between the product and the unreacted core existing in each of the small particles. Using the expression derived in the present work, the activation energy of the reaction was calculated from the results of the nonisothermal experiments to be 83.62 kJ · mol−1. This value is in good agreement with the value of 83.17 kJ · mol−1 evaluated from isothermal experiments.