Thermal decomposition mechanisms of MgCl_2·6H_2O and MgCl_2·H_2O

摘要

The thermal decomposition mechanisms and the intermediate morphology of MgCl_2·6H_2O and MgO_2·H_2O 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 MgCl_2·6H_2O: producing MgCl_2·4H_2O at 69 C, MgCl_2·2H_2O at 129 C, MgCl_2·nH_2O (1 ≤ n ≤ 2) and MgOHCl at 167℃, the conversion of MgCl_2·nH_2O (1 ≤n≤2) to Mg(OH)Cl·0.3H_2O by simultaneous dehydration and hydrolysis at 203℃, the dehydration of Mg(OH)Cl·0.3H_2O to MgOHCl at 235 C, and finally the direct conversion of MgOHCl to the cylindrical particles of MgO at 415℃. To restrain the sample hydrolysis and to obtain MgCl_2·H_2O, MgGl_·6H_2O was first calcined in HC1 atmosphere until 203℃ when MgCl_2·H_2O was obtained; HCI gas was then turned off and the calcination process continued, producing Mg_3Cl_2(OH)_4·2H_2O calcined at 203℃, Mg_3(OH)_4Cl_2 at 220 C and MgO at 360℃.The temperature of producing MgO from calcination of MgCl_2·H_2O was lower (360℃) than that from MgCl_2·6H_2O (415℃) because of its more reactive intermediate products: the irregular shape and tiny needle-like Mg_3Cl_2(OH)_4·2H_2O particles and the uneven surface porous Mg_3(OH)_4Cl_2 particles. The MgO particles obtained at 360 C had a flake structure.