(321av) Model for Solubility and Solid Phase Composition in High-Temperature Na2Co3-Na2So4 Solutions

Scaling of Na₂SO₄, Na₂CO₃ and their double salts has long been recognized as a problem in black liquor evaporators. We have used published data for Na-CO₃-SO₄-H₂O system to develop a model for predicting the solubility and solid phase composition within the double salt precipitation region. The model is based on a thermodynamic definition of a solubility constant with Pitzer ionic activity coefficients in the solution. It is capable of calculating the solubility and the equilibrium solid composition in Na₂SO₄-Na₂CO₃ aqueous solutions in the temperature range of 100-115°C and 150°C and within the precipitation region of a solid solution consisting of burkeite (2Na₂SO₄?Na₂CO₃) and sodium carbonate salts. The model construction procedure and its possible extension to other Na₂SO₄-Na₂CO₃ precipitates will be addressed.