THERMODYNAMIC BEHAVIOUR OF OXYGEN IN METALLIC SOLVENTS

Abstract

Elements such as arsenic, antimony, tin, bismuth, cadmium, copper, zinc and silver occur commonly in asso-ciation in the sulphide lead-deposits and therefore constitute the common imperuties in lead bullion obtained in Bl.ast-furnaces.melting. These elements are also used as important alloy additions to lead, in desired concen-tration ranges, to improve its useful mechanical, physical or chemical characteristics for different industrial applications such as bearing metals, soldering alloys, type metal, fusible alloys, electrodes and electrolytes cell-lining material in batteries and similar other applications in engineering, chemical, electrical and electro-chemical industries. Oxidation behaviour of these elements, when present in dilute concentration range in lead, therefore forms an interesting study useful not only for the development and optimisation of different process steps employed in the extraction and refining of lead but also for development of its alloys to meet the stringent service requirements of its various applications. A critical survey of the available literature on thermodynamic behaviour of oxygen in lead-base alloy solvents revealed only limited studies. It was therefore, felt that there is a need of extensive research input ii iii to generate reliable thermodynamic data for a better understanding of behaviour of oxygen in these metallic solvents, as well as for use in design and development of better production processes. With this in view, the present investigation was undertaken to study the thermodynamic properties of dissolution of oxygen in lead and its binary alloys with alloy additions in dilute concentration range. The systems studied include lead-oxygen, lead-copper-oxygen, lead-tin-oxygen and lead-bismuth-oxygen with copper, tin and bismuth varying between 0-4 weight per-centage in these alloys and the oxygen concentration not exceeding its saturation limit. A suitable experimental set-up using emf tech-nique and employing solid-electrolyte cell with stainless steel (S.S) leads for study in the temperature range 933-1123K was therefore designed and locally fabricated. The cell employed may be represented schematically as, S.S..; Ni-NiO Solid electrolyte 0; Pb(l) or Pb alloy,(PbO); S.S. (Zr02+CaO) — Concentration of oxygen in the different lead melts studied was altered by covering these with synthe-tic slags either PbO-B203 or PbO-Si02 of desired (PbO) activity made specifically for the purpose. However, in case of studies with lead-tin alloys, the oxygen concentration was varied by the addition of Pb-PbO pellet iv to the alloy during the course of experimentation to prevent loss of tin resulting from slag formation. The cell emf was recorded during the course of experiment and metal samples drawn on attainment of equilibrium, were analysed for their oxygen and alloy addition concentrations. These data were used to calculate the desired thermodynamic properties viz, the free energy of dissolution of oxygen in lead bearing metallic solvents and also the binary and ternary interaction parameters. For comparison and analysis of data of present investi-gation, values of activity coefficient of oxygen in different Pb-M-O ternary systems have been calculated employing expressions based on classical thermodynamic concepts, to arrive at the model which is most suitable for theoretical calculation and prediction of thermo-dynamic properties of oxygen in the metallic systems studied. The entire work presented in this dissertation has been devided into sseven chapters. Chapter I, surveys the available literature on thermodynamics of dissolution of oxygen in metallic solvent, different experimental methods of thermodynamic investigation and various theore-tical models proposed by diffeerent workers. Finally the problem of present investigation has been formulated. Chapter II deals with the experimental set up and mate-rials used, experimental procedure adopted and also u the methods of quantitative analysis employed for esti-mation of oxygen and alloy additions. Chapters III to Chapter VI present, for the different metallic systems studied, the experimental results, method of calculation used and the computed values of thermodynamic parameters of present investigation as well as their comparison with those obtained by other workers. While Chapter III presents •data on lead-oxygen system; Chapter IV to Chapter VI deal respectively with the ternary systems Pb-Cu--O, Pb-Bi-O and Pb-Sn-O. In Chapter VII, the major results obtained in the present investigation are summa-rised and important conclusions drawn are lissted. Finally, suggestions for further work in extension of the present investigations are listed.