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Phase Equilibria and Phase Diagrams of the Mn2+, Mg2+, NH4+//SO42–H2O System at 298.15, 323.15, and 373.15 K

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journal contribution
posted on 14.05.2020, 21:03 by Min Zhang, Peng Wu, Yaoyao Li, Wenxuan Li, Huan Zhou
Magnesium sulfate exists as a manganese electrolyte, which will be enriched continuously during the mother liquor recycling, thus affecting the electrolytic process and the quality of metal manganese products. In order to separate magnesium salt from manganese electrolyte, the solid–liquid phase equilibrium behaviors of the Mn2+, Mg2+, NH4+//SO42––H2O quaternary system were studied. The complete isothermal phase diagrams with five solid species, three invariable liquid points, and seven univariate curves at the respective temperatures of 298.15, 323.15, and 373.15 K were obtained. The results show that ammonium sulfate prefers to form double-salts with manganese sulfate and magnesium sulfate, such as (MgSO4·(NH4)2SO4·6H2O) and (MnSO4·(NH4)2SO4·6H2O) at 298.15 K and (MgSO4·(NH4)2SO4·6H2O) and (2MnSO4·(NH4)2SO4) at 323.15 and 373.15 K. Magnesium sulfate and manganese sulfate can form solid solutions especially at high temperature, but the phase regions for their double-salts are evident and change significantly with temperature. For instance, MgSO4·(NH4)2SO4·6H2O occupies the greatest part of phase region at low temperature (298.15 K), while 2MnSO4·(NH4)2SO4 covers the largest phase region at high temperature (348.15 and 373.15 K). The process experiments show that the salt-forming behaviors during the evaporation and cooling process are consistent with the quaternary phase diagram obtained in this study. By controlling the temperature, manganese and magnesium salts can be separated from the metal manganese electrolyte via evaporation or crystallization process.