Phase equilibria and thermodynamic evaluation of the Fe-Ti-V-O system in air

Abstract

In this study, the iron-titanium-vanadium-oxygen (Fe-Ti-V-O) system in equilibrium with air was studied experimentally by high-temperature equilibration, quenching, scanning electron microscope and microprobe analysis coupled with critical assessment and thermodynamic evaluation. The thermodynamic evaluation was performed with FactSage 7.0. The purpose of the study was to develop a set of Gibbs equations for all compounds and solutions of the Fe-Ti-V-O system in equilibrium with air, using the well-known calculation of phase diagram (CALPHAD) technique. The study was categorically divided into three separate investigations. The lower order Fe-V-O and Ti-V-O systems in equilibrium with air were st experimentally investigated and thermodynamically assessed. This was then followed by an experimental investigation and thermodynamic assessment of the Fe-Ti-V-O system in equilibrium with air. The Fe-V-O and Ti-V-O systems in equilibrium with air were studied experimentally in temperatures ranging from 700 C to 1500 C. The measured concentration of Fe in the V-O slag is 35 weight % at 1400 C, and the measured V concentration in the hematite phase reached a maximum of 4.4 weight % at 1350 C. A signi cant amount of precipitation was observed for Fe-V-O samples quenched at 1400 C, causing calculated standard deviations of Fe and V to be more than 1 weight %. The measured concentration of Ti in the V-O slag is less than 6 weight % at 1500 C, and the measured V concentration in the rutile phase reached a maximum of 15 weight % at 1400 C. The solubility of V2O5(s) in the hematite and rutile phases was described with the compound energy formalism. The properties of the liquid phase were described with both the modi ed quasichemical model and the associate species model. A set of self-consistent thermodynamic parameters was estimated within acceptable error limits. The calculated phase diagrams of Fe-V-O and Ti-V-O in equilibrium with air are presented and compared to experimental observations and other literature data. Before experiments in the Fe-Ti-V-O system in equilibrium with air were conducted, the Fe-Ti-O system in equilibrium with air was critically assessed and thermodynamically evaluated. This was due to the slag phase and solid solutions of the Fe-Ti-O system that were previously thermodynamically evaluated only under reducing conditions. However, limited data were available in literature, hence assumptions were required for the evaluation. Nevertheless, an improved phase diagram of the Fe-Ti-O system in equilibrium with air was calculated. Thereafter, isothermal planes were calculated from optimized binary parameters to estimate a range of plausible starting compositions for experiments of the Fe-Ti-V-O system in equilibrium with air. The Gibbs phase rule was carefully applied to avoid redundant experiments. The Fe-Ti-V-O system in equilibrium with air was studied experimentally, ranging from 1000 C to 1400 C. The properties of the liquid phase were successfully described with the quasichemical model by optimizing parameters only related to the Fe-Ti-O system. The model for the rutile solid solution was extended to describe the solubility of Fe2O3(s) and V2O5(s) simultaneously. The model for the hematite solid solution was similarly extended to describe the solubility of TiO2(s) and V2O5(s) simultaneously. The ferropseudobrookite solid solution was modelled with a simple polynomial model to include a small solubility region of V2O5(s). A nal set of self-consistent thermodynamic parameters was estimated within acceptable error limits. Calculated isothermal projections at 1000 C, 1100 C, 1200 C, 1300 C, and 1400 C are presented and compared to experimental observations.