THERMODYNAMICS OF DILUTE SOLUTION OF SULPHUR IN MULTICOMPONENT MOLTEN ALLOY SYSTEMS

Abstract

Studies on the properties of pure sulphides, inter-actions between different sulphides or the different gas-sulphide or liquid-sulphide systems and also the behaviour of sulphur in molten as well as solid metals and alloys have been, for a long time, of pirticular concern to the geologists, chemists, metallurgists and material scientists for a better understanding of the formation and characteristics of mineral deposits, extraction and refining of pure metalsfrom sulphur-bearing raw-materials and development of new metallic materials for specific applications. A critical study of the published work indicated that although extensive work has been done on the properties of pure sulphides and their mixtures,yet only few studies have been conducted on the behaviour of sulphur in the dilute solution range. Here again most of the work reported has been carried out on the behaviour of sulphur in ferrous-systems only. Thus little is known on the beha-viour of sulphur in non-ferrous metals and their alloys, although many important metals of this group are recovered from their sulphide deposits occurring in nature. The present work has, therefore, been taken up to systematically study the behaviour of sulphur in non-ferrous metallic systems. Three pure metals namely lead, silver and copper and their binary and ternary alloys of different com-positions have been used for the experimental studies at different temperatures for dissolution of sulphur from the gaseous phase of known sulphur potentials in equilibrium with the raoltem metallic solvents. Further, for a quantitative interpretation of the experimental data so obtained, several statistical thermodynamic solution models based on free-volume theory have been developed. The entire work reported in this dissertation has been spread over five chapters. Chapter-1 deals with the review of the available literature on different studies made on sulphide systems and methods employed for this purpose, different theoretical thermodynamic models developed to interpret properties of actual solutions and finally the formlation of the problems Chapter-II describes the experimental set-up, materials and methods used for carrying out the equilibria runs and for chemical analysis for sulphur in the equilibrated metal-alloy samples. Chapter-III presents the results of experimentation in tabular fortes and also the different thermodynamic proper-ties of sulphur computed from these, namely, the equilibrium constants, interaction parameters, free energy, enthalpy and entropy for the dissolution reaction equilibria at differerit temperatures for pure metallic solvents and their binary and ternary alloys of different compositions. The results obtained in the present work have also been compared with the available data obtained by others workers4 In Chapter-IV, for interpretation of the experimental data, presented in Chapter-III, eight different thermodynamic models for multicomponent substitutional solutions have been developed based on free volume theory and different simpli-fying assumptions. Expressions for the free energy of dis-solution of sulphuy in binary, ternary and multicomponent solvents have been derived assuming linear and non-linear forms of energy and free volume and also preferential amd random distribution of solute and solvent atoms. Calculated values of the thermodynamic parameters, based on the dif-ferent expressions,derived, have been plotted and compared with the experimental values to ascertain and establish the applicability of the theoretical solution models developed. Chapter-V summarizes the different results obtained and conclusions drawn from the present work. Finally, suggestions for further work in this direction have been incorporated.