A thermodynamic model for high temperature corrosion applications: The (Na₂SO₄ + K₂SO₄ + ZnSO₄ + PbSO₄) system

The (Na₂SO₄ + K₂SO₄ + ZnSO₄ + PbSO₄) salt system is important for deposit buildup, fouling and associated corrosion of metallic structural and heat exchanger materials of municipal solid waste incinerators, low-grade fuel firing boilers, as well as metallurgical heat recovery boilers. In this study, a complete critical evaluation of all available phase diagram and thermodynamic data has been performed for all condensed phases of the (Na₂SO₄ + K₂SO₄ + ZnSO₄ + PbSO₄) system, and optimized model parameters have been found. The Modified Quasichemical Model for short-range ordering was used for the molten salt phase while most of the relevant solid solutions were modeled using the Compound Energy Formalism. The (Na₂SO₄ + K₂SO₄) sub-system has been critically evaluated in a previous article. A (60 mol% PbSO₄ + 40 mol% ZnSO₄) binary mixture was investigated by DTA-TGA since no thermodynamic data were available in the literature for this particular sub-system. The model parameters obtained for the binary and ternary sub-systems of the (Na₂SO₄ + K₂SO₄ + ZnSO₄ + PbSO₄) system can be used to predict thermodynamic properties and phase equilibria for the multicomponent system. In particular, ternary and quaternary eutectics (corresponding to the formation of a liquid phase at low temperatures) may be predicted, thus permitting to avoid “catastrophic” corrosion. This work is part of a large thermodynamic model for the Na⁺, K⁺, Zn²⁺, Pb²⁺ // Cl⁻, SO₄²⁻ system.