Thermochimica Acta 510 (2010) 88–92 Contents lists available at ScienceDirect Thermochimica Acta journal homepage: www.elsevier.com/locate/tca Optimization and calculation of the MCl–ZnCl 2 (M = Li, Na, K) phase diagrams Antonio Romero-Serrano ∗ , Aurelio Hernandez-Ramirez, Alejandro Cruz-Ramirez, Manuel Hallen-Lopez, Beatriz Zeifert Metallurgy and Materials Department, Instituto Politecnico Nacional-ESIQIE, Apdo. P. 118-431, 07051 Mexico, D.F., Mexico article info Article history: Received 8 April 2010 Received in revised form 24 June 2010 Accepted 29 June 2010 Available online 4 July 2010 Keywords: Thermodynamic Zinc chloride melts Phase diagram abstract An earlier structural model for binary silicate melts and glasses is extended to zinc chloride–alkali metal chloride systems. The evaluation of the available thermodynamic and phase diagrams data for the MCl–ZnCl 2 (M = Li, Na, K) binary systems have been carried out using the structural model for the liquid phase. This thermodynamic model is based on the assumption that each alkali chloride pro- duces the depolymerization of ZnCl 2 network with a characteristic free-energy change. A least-squares optimization program permits all available thermodynamic and phase diagram data to be optimized simultaneously. In this manner, data for these binary systems have been analysed and represented with a small number of parameters. © 2010 Elsevier B.V. All rights reserved. 1. Introduction There is considerable interest in the application of molten salt reactions using zinc chloride. One of these applications is the electrodeposition of refractory metals such as tungsten and molyb- denum using molten ZnCl 2 –NaCl–KCl system at 523 K as electrolyte [1]. Interest in these systems is also related to the recycling of heavy-metal halide waste generated in industrial processes which must consider phase equilibria for the prevention of environmental pollution and the recovery of resources. During the past decades, several investigations of systems with divalent cations have been performed and a large area of molten salt chemistry was discov- ered. However, the structural and thermodynamic properties and phase diagrams for the zinc chloride–alkali metal chloride systems are not yet accurately known. The thermodynamics of the binary systems MCl–ZnCl 2 (M = Li, Na, K) are of more than usual interest since the component salts are quite dissimilar in character. For example, LiCl is a normal ionic salt while ZnCl 2 is evidently associated since it forms a very viscous liquid which is obviously polymeric [2]. Solid ZnCl 2 has a structure analogous to SiO 2 , in which the Zn 2+ ions are surrounded tetrahe- drally by four chloride ions and each chloride ion is bonded to two zinc ions. Allen et al. [3] used a liquids and amorphous materials diffractometer to show that there exists the tetrahedral coordina- tion of anions around zinc in molten ZnCl 2 . Hefeng et al. [4] used ∗ Corresponding author. Tel.: +52 55 5729 6000x54127; fax: +52 55 5729 6000x55270. E-mail addresses: romeroipn@hotmail.com (A. Romero-Serrano), aurelioh@hotmail.com (A. Hernandez-Ramirez), alcruzr@ipn.mx (A. Cruz-Ramirez), j hallen@yahoo.com (M. Hallen-Lopez), bzeifert@yahoo.com (B. Zeifert). the EXAFS technique and found that the tetrahedrons consisting of four Cl atoms around Zn are dominant and stable in molten ZnCl 2 . The ZnCl 4 2- tetrahedral shares corners to form a three- dimensional network. The liquid might be expected to have a similar polymeric structure due to the high viscosity and low con- ductivity of liquid ZnCl 2 near its melting point. As LiCl or another alkali chloride is added to molten ZnCl 2 , the viscosity drops sharply presumably because bridging chloride linkages are broken and the degree of polymerization decreases in a similar way that of SiO 2 is broken up by the addition of alkali or alkaline earth oxides. Seo et al. [5] used a molecular-dynamic simulation for the calculation of some physicochemical properties in ZnCl 2 -based fluxes, including the enthalpy of solid and liquid ZnCl 2 , self-diffusion coefficient of each ion in FeCl 2 , PbCl 2 and ZnCl 2 , and the Gibbs free energies of mixing and phase diagrams of the PbCl 2 –ZnCl 2 binary system. Lin and Pelton [6] developed a structural model for binary sili- cate systems MO–SiO 2 (M = Ca, Mn, Mg, Fe, etc.). In this model, one single formalism applies over the entire composition range and accounts for two- and three-dimensional silicate network struc- tures. Later, the model was examined in more depth for binary systems [7] and a more general empirical expression for enthalpy was written, and an empirical nonconfigurational excess entropy was added. Then, the purpose of the present article is to use the structural model, formerly developed for silicate melts, to repre- sent the thermodynamic properties of MCl–ZnCl 2 (M = Li, Na, K) melts. 2. Thermodynamic model The model is based on the depolymerization reaction of ZnCl 2 :M + M + Cl - + Zn–Cl–Zn = Zn–Cl–M– Cl–Zn M = Li, Na, K (1) 0040-6031/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2010.06.027