(191i) Thermal Decomposition Mechanisms of MgCl2•6H2 O and MgCl2•H2 O

The thermal decomposition mechanisms and the intermediate morphology of MgCl₂?6H₂O and MgCl₂?H₂O were studied using integrated thermal analysis, calcination by a resistance furnace, x-ray diffraction, scanning electron microscope, energy dispersive x-ray spectrum and chemical analysis. The results were different from the previous' and 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, and the MgCl₂?nH₂O(1≤n≤2) converted to Mg(OH)Cl?0.3H₂O with dehydration and hydrolysis simultaneously at 203°C, Mg(OH)Cl?0.3H₂O dehydrated to MgOHCl at 235°C, MgOHCl directly converted to MgO at 415°C. The morphology of MgOHCl particle was irregular and had porous structure, while Mg(OH)Cl?0.3H₂O particle had relatively flat surface, as shown in Figure 1(a, b). MgCl₂?6H₂O was calcined in HCl atmosphere before 203°C for restraining the hydrolysis of sample to obtain MgCl₂?H₂O, and then turned off HCl gas. MgCl₂?H₂O was calcined in the furnace at 203°C continuously. The calcined products were Mg₃Cl₂(OH)₄?2H₂O at 203°C and Mg₃(OH)₄Cl₂ at 220°C, respectively, which converted to MgO at 360°C. The reaction temperature for obtaining MgO was lower. Mg₃Cl₂(OH)₄?2H₂O particle was irregular and had tiny needle-like structure, while Mg₃(OH)₄Cl₂ particle surface was uneven and had porous, as shown in Figure 1(c, d).