Experimental and calculated solubility isotherms of the quaternary NaBr–MgBr2–Na2SO4–MgSO4–H2O system and its ternary subsystem at 298.15 K

To further complete the phase equilibrium data of bittern system and prepare for the extraction of bromine resources from bittern, the solubility isotherms of the quaternary NaBr–MgBr2–Na2SO4–MgSO4–H2O system and its ternary subsystem MgBr2–MgSO4–H2O at 298.15 K were determined using the isothermal dissolution method. In the phase diagram of the ternary subsystem, there are five invariant points and five single salt crystallization zones corresponding to MgSO4·nH2O (n = 7, 6, 5, 1) and MgBr2·6H2O, where MgSO4·5H2O is a metastable phase. This report supplements and revises the previous data. The phase diagram of the quaternary system belongs to complex type, which comprises nine invariant points and ten crystallization zones including a double salt Na2SO4·MgSO4·4H2O and a metastable phase MgSO4·5H2O. Based on the Pitzer model, the interaction parameter ψ of Mg, Br and SO4 was obtained by fitting. The solubilities of the two systems were calculated, and the results show good agreement with the experimental values.