Thermodynamic analysis and phase equilibria investigation in Pb−Zn−Ag system

Abstract

Physico-chemical processes that take place during the refining process in the extractive metallurgy of lead, are connected with ternary Pb−Zn−Ag system, which is necessary to study from the theoretical practical and aspects. Such investigation is important from production point of view, because of the phenomena that occur during desilvering of lead which is one of the important stages during lead refining process. Process of lead desilvering binds to ternary system Pb−Zn−Ag, which was the reason for numerous investigations, both from thermodynamic point of view and in terms of testing and determining the phase diagram, bearing in mind the theoretical, and practical importance of knowledge about the processes which are going in investigated system. The paper presents the results of thermodynamic analysis and investigation of phase equilibria of the Pb−Zn−Ag ternary system using the method of thermodynamic predictions and phase diagrams calculations, respectively, and the experimental results of metalography obtained by optical microscopy. Phase diagram of the vertical section Pb−Zn80Ag20 is presented, obtained by CALPHAD calculation methodology, and using PANDAT thermodynamic software, compared to experimental results obtained by DTA analysis. The results show a pronounced break in solubility, which is characteristic for the whole ternary Pb−Zn−Ag system. Also, it can be noticed that the thermodynamic properties follow the behavior of this system, which is expressed through positive deviation of Raoult’s law, pointing to the lack of lead affinity compared to the other two components in the system. The optical microscopy results of the investigated system show the following: - Sample L1 (weight% Pb = 98): the structure of the observed section shows double eutectic (Pbsol+Zn−Agsol) which lies in the base of the primary crystals of lead (Pbsol) - Samples L2−L5: the structure consists of a dual eutectic (Pbsol+Zn−Agsol) and primary crystals Zn−Agsol Also, it can be noticed certain structural inhomogenity, as a consequence of stratification that occurs in the ternary Pb−Zn−Ag system. However the metalography investigation results confirm the results obtained by defining the phase diagram and mutually agree relatively well in accordance with existing literature data.

Keywords

• Pb−Zn−Ag system
• desilvering of lead
• thermodynamic prediction
• optical microscopy