Thermal decomposition mechanisms of MgCl2·6H2O and MgCl2·H2O

The thermal decomposition mechanisms and the intermediate morphology of MgCl2·6H2O and MgCl2·H2O were studied using integrated thermal analysis, X-ray diffraction, scanning electron microscope and chemical analysis. The results showed that there were six steps in the thermal decomposition of MgCl2·6H2O: producing MgCl2·4H2O at 69 °C, MgCl2·2H2O at 129 °C, MgCl2·nH2O (1 ≤ n ≤ 2) and MgOHCl at 167 °C, the conversion of MgCl2·nH2O (1 ≤ n ≤ 2) to Mg(OH)Cl·0.3H2O by simultaneous dehydration and hydrolysis at 203 °C, the dehydration of Mg(OH)Cl·0.3H2O to MgOHCl at 235 °C, and finally the direct conversion of MgOHCl to the cylindrical particles of MgO at 415 °C. To restrain the sample hydrolysis and to obtain MgCl2·H2O, MgCl2·6H2O was first calcined in HCl atmosphere until 203 °C when MgCl2·H2O was obtained; HCl gas was then turned off and the calcination process continued, producing Mg3Cl2(OH)4·2H2O calcined at 203 °C, Mg3(OH)4Cl2 at 220 °C and MgO at 360 °C. The temperature of producing MgO from calcination of MgCl2·H2O was lower (360 °C) than that from MgCl2·6H2O (415 °C) because of its more reactive intermediate products: the irregular shape and tiny needle-like Mg3Cl2(OH)4·2H2O particles and the uneven surface porous Mg3(OH)4Cl2 particles. The MgO particles obtained at 360 °C had a flake structure.