Study on the phase equilibria and solid solution in the ternary systems NH4Cl–NH4Br–H2O and NH4Cl–NH4Br–CH3OH at 298.15 K

Abstract

Aiming at utilizing bromine resource in bittern after magnesium extraction, the solubilities of the ternary systems NH4Cl–NH4Br–H2O and NH4Cl–NH4Br–CH3OH at 298.15 K were studied by means of isothermal dissolution equilibrium, and the phase diagrams were depicted. It was found that two phase diagrams are similar, and the solid solution was defined two types, NH4(Brx, Cl1−x) rich of NH4Br and NH4 (Cl1-y, Bry) rich of NH4Cl (x and y are mole fractions of Br− in solid solutions). The phase diagram of water system was delimited two single-phase crystallization zones and contains one invariant point. Under the effect of methanol, a large two-phase cocrystallization area exists in the phase diagram of methanol system. According to the Pitzer model, the solubilities of the ternary NH4Cl–NH4Br–H2O system at 298.15 K were evaluated without the consideration of the ion-interaction parameters θ and ψ, and the results basically coincide with the measured values. Compared with the XRD patterns of the mixture of two salts, the characteristic peaks of the solid solution are weaker and shifted. The thermogravimetry differential scanning calorimetry (TG-DSC) showed that the difference between the solid solutions formed in aqueous solution and methanol is not significant.