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

The thermal decomposition mechanisms and the intermediate morphology of MgCl2 center dot 6H(2)O and MgCl2 center dot 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 center dot 6H(2)O: producing MgCl2 center dot 4H(2)O at 69 C, MgCl2 center dot 2H(2)O at 129 degrees C, MgCl2 center dot nH(2)O (1 <= n <= 2) and MgOHCl at 167 degrees C, the conversion of MgCl2 center dot nH(2)O (1 <= n <= 2) to Mg(OH)Cl center dot 0.3H(2)O by simultaneous dehydration and hydrolysis at 203 degrees C, the dehydration of Mg(OH)Cl center dot 0.3H(2)O 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 center dot H2O, MgCl2 center dot 6H(2)O was first calcined in HCl atmosphere until 203 C when MgCl2 center dot H2O was obtained; HCl gas was then turned off and the calcination process continued, producing Mg3Cl2(OH)(4)center dot 2H(2)O calcined at 203 C, Mg-3(OH)(4)Cl-2 at 220 C and MgO at 360 C. The temperature of producing MgO from calcination of MgCl2 center dot H2O was lower (360 C) than that from MgCl2 center dot 6H(2)O (415 degrees C) because of its more reactive intermediate products: the irregular shape and tiny needle-like Mg3Cl2(OH)(4)center dot 2H(2)O particles and the uneven surface porous Mg-3(OH)(4)Cl-2 particles. The MgO particles obtained at 360 degrees C had a flake structure. (C) 2011 Elsevier B.V. All rights reserved.