Magnesium and its alloys are attractive materials for the transportation industry because of their low density and good mechanical properties. There are two principal magnesium production technologies, electrochemical method and thermal reduction method. The latter is widely used in the world currently while it suffers from high energy consumption and low productivity. High production costs have limited the electrolytic magnesium production.

Researchers from State Key Laboratory of Multi-phase Complex systems, Institution of Process Engineering (IPE) proposed a new method to prepare high-purity anhydrous magnesium chloride using magnesia and ammonium chloride as reactants, alumina as covering agent.

In the preparation, magnesia and ammonium chloride with an appropriate molar ratio were mixed and then putting them into a corundum crucible. Alumina was put on top of the mixture, and then the lid was put on. The crucible was placed in a certain condition for complete reaction, and finally the resultant was obtained.

The process was characterized by X-ray diffraction and thermogravimetric and differential scanning calorimetry (TG-DSC) analysis.

The XRD analysis was carried out in ambient atmosphere at room temperature. Two experiments were carried out for the analysis to determine the products obtained at different temperatures and the effects of alumina on the products. When the temperature was increased to 700 °C, high-purity anhydrous MgCl2 with good crystallinity was formed. As for the effect of alumina as covering agent, comparison was conducted. The result indicated that the unsintered anhydrous MgCl2 could easily react with water vapor in ambient atmosphere to form MgO.

Thermogravimetric analysis (TGA) and differential scanning calorimetry analysis (DSC) were carried out by a simultaneous TGA/DSC apparatus. Two samples were prepared and a 8.15 mg mixture was used for the analysis. DSC curve of NH4Cl and the DSC curve of the mixture (MgO and NH4Cl) were compared. Results showed that the existence of covering agent, which can suppress the emission of ammonium chloride and the existence of covering agent, which can isolate the unsintered anhydrous MgCl2 from ambient atmosphere, wwere necessary.

According to the above XRD and TG-DSC experimental results, the possible reaction mechanism was concluded. Magnesia reacts with ammonium chloride at 200–300 °C with NH4Cl·MgCl2·nH2O (6 > n ≥ 0) as product. Crystallized water and ammonium chloride are gradually released from NH4Cl·MgCl2·nH2O at elevated temperature with anhydrous magnesium chloride as the final product.　

In addition, the main factors that influenced the purity of the anhydrous magnesium chloride were investigated, they included molar ratio of ammonium chloride to magnesia, thickness of alumina, reaction temperature and reaction time. Results showed that the content of magnesia in anhydrous magnesium chloride was achieved 0.014% under the optimum conditions: MgO:NH4Cl molar ratio of 1:5, thickness of alumina of 1.1 cm, reaction temperature of 450 °C, reaction time of 1.5 h, calcination temperature of 700 °C, and calcination time of 0.5 h.

The paper was published in Industrial & Engineering Chemistry Research.