Phase equilibria in the quaternary NaBr–MgBr2–Na2SO4–MgSO4–H2O system at (323.15 and 348.15) K and their application

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To explore the process of extracting bromide salts directly from concentrated bittern, the phase equilibria in the quaternary NaBr–MgBr2–Na2SO4–MgSO4–H2O system at (323.15 and 348.15) K were investigated using the isothermal dissolution equilibrium method. The corresponding equilibrium phase diagrams and water content diagrams were depicted. The phase diagram at 323.15 K consists of eight invariant points, fifteen univariant curves and nine crystallization zones. The phase diagram at 348.15 K has four invariant points, nine univariant curves and six crystallization regions. Double salts were formed in both cases, and either belongs to complex type. The solubility products of the double salts 6Na2SO4·7MgSO4·15H2O and 3Na2SO4·MgSO4 were fitted by adopting the multilinear regression method. The predicted phase diagrams established with the solubilities calculated based on the Pitzer model show good agreement with the measured ones. A new circulation process was proposed to extract NaBr from the concentrated bittern, which provided a theoretical basis for the separation of bromide salts from bittern straightly.