Thermodynamic Modeling of the Na₂O-SiO₂-As₂O₅ System and Its Application to Arsenic Immobilization Using Glass Formation

A critical evaluation and optimization of the Na₂O-SiO₂-As₂O₅ system was conducted using the CALPHAD method to obtain a consistent set of Gibbs energy functions for all solid, liquid and gas phases within the system. The available and reliable thermodynamic and phase diagram data of the unary As₂O₅ system and binary Na₂O-As₂O₅ system were all well reproduced in the present optimization. The phase diagram of the SiO₂-As₂O₅ system was predicted from the SiO₂-P₂O₅ system based on the similarity between As₂O₅ and P₂O₅. The phase diagram and thermodynamic properties of the ternary Na₂O-SiO₂-As₂O₅ system were predicted using the optimization results of the three binary systems. The strong short-range ordering behavior of the liquid Na₂O-SiO₂-As₂O₅ solution was well described with the modified quasichemical model. The phase diagram, arsenic evaporation behavior and energy balance were calculated for the glass-making process related to arsenic immobilization from the present thermodynamic database.