THERMODYNAMICS OF DILUTE SOLUTIONS OF ZINC IN MOLTEN BINARY SUBSTITUTIONAL SOLVENTS AT 173°K

Abstract

Metallic solutions and their oxide melts especially those involvingsilicate systems are of primary interest to extrac-tive metallurgists for optimization of existing unit processes and development of newer techniques of extraction and refining of metals and also to material scientists for development of new materials especially alloys, cermets9 ceramic materials etc. for their widely varying applications. For past several decades, therefo re9 attempts have been made to study these systems, especially the binary metallic solutions and a host of selected data has been compiled. However, only a limited number of studies have been made in the dilute solution range in binary, ternary and other higher component metallic solvents in their entire range of composition, primarily because the different experi-mental techniques are not only tedious due to the stringent requirements of elevated temperatures and high-purity materials due to the limited accuracies in the techniques of measurement used, but also time consuming and costly. Consequently several simultaneous attempts have been made in the recent past to propose theoretical models with different degrees of success for calculation and theoretical prediction of properties of such solutions. Zinc is a highly versatile engineering material due to a unique combination of its physical, mechanical and chemical properties and finds extensive industrial applications in the metallic form as wrought zinc sheets and strips or its alloys, as zinc dust and also as chemicals in widely diversant fields ranging from batteries, machine parts in automotive, electrical, electronic and refrigeration industries, buildings and architecture or household appliances and for their protection against corrosion to paints, pigments, rubber and ceramic industries and even in agriculture and medicine. A critical survey of available literature revealed only limited studies on properties of this important metal in binary and higher component alloy solvents. Because of its association with cadmium, bismuth, tin, lead, copper,silver and occasionally germanium etc. in its major ore depositsp especially in the 'sulphide-values' and also in many of its important alloys used industrially, it was decided to study its properties in low temperature molten systems involving binary solvents of bismuth, cadmium and tin at 7730K in the present work. Emf technique using fused salt electrolyte (LiC1+KC1 mixture containing ZnC12) has been selected for the studies, due to its high versatility over other experimental techniques$ using first dilute solutions of zinc in pure metals and then in binary bismuth-cadmium, bismuth-tin and cadmium-tin solvents of specific compositions. A 'suitable experimental set-up involving pyrex-glass H-tube emf cells using fused salt electrolyte and a gas-purifiction train for use of inert/reducing atmospheres during actual emf measurements/alloy preparation was designed and fabricated. Emfs of two cells of the type Pure Zn/LiC1 t KCl + ZnC12 electrolyte / _, 0.1 NZn zinc'base alloys and - 0.lNZn zinc base alloy/LiCl+KCl+ZnCl2 electrolyte/N 'zinc alloy under study, in the desired solvents (pure metals or binaries of specific compositions) were measured to arrive at the required emf value with reference to pure zinc electrode to improve accuracy of measurement with the available equipment. From the values of emf so obtained, ' relevant empirical thermodynamic properties of zinc have been calculated, tabulated and plotted. For theoretical calculation and prediction of properties of dilute solutions of zinc in binary solvents of any desired composition, a.theo retical model, based on 'free-volume theory? has been developed. The theoretical computed values, using expressions developed in the present work show good correspondence with the empirical values obtained, reflecting to good predictive value of the model developed. Partial molar properties of the components of the binary solvents needed for such calculations have been computed at 773°K from the data compiled by Hu ltgrcnet. al. The entire work has been divided into five chapters as follows. Chapter-I surveys the available literature on determinat-ion of thermodynamic properties of zinc including methods employed for this purpose, reviews very briefly yet critically, the different theoretical models proposed for calculation of properties of solute in metallic solutions and finally ends-up with the formulation of problem. For statistical thermodynamic analysis of the substi-tutional ternary systems, involving a solute in dilute concentrations in binary solvents, a theoretical model has been developed in Chapter-II, based on the 'free-volume theory' and assuming non-linear forms of energies and free volumes and random distribution of feebly inter-acting atoms, constituting such systems. Suitable expressions have been arrived at for the excess-partial molar free energy of solute at infinite dilution and functions 1T and 0 defined in the text, using first theoretical parameters involving changes in energies and free-volumes of the constituent atoms on mixing and then in terms of empirically determinable parameters such as partial molar free energies, interaction parameters etc. in pure and binary metallic solvents. Chapter-III deals with the experimental set-up, materials used and techniques adopted for experimentation for deter-mination of thermodynamic properties of zinc in dilute solution range 'using emf technique. In Chapter-IV, the different experimental results are presented. Various thermodynamic parameters are computed from these data and values so obtained are compared with the available data obtained by other workers, wherever possible. Chapter-V summarizes the different results obtained and lists the major conclusions drawn from the present investigation. Finally suggestions for further work in extension of the present one are listed.