Waste Minimization Approach in the Vacuum Aluminothermic Production of Green Magnesium via CaO–MgO Separation

This work presents a comprehensive study on the sustainable production of metallic magnesium from high purity dolomite ore. The process of chain optimization included calcination-hydration, Ca–Mg separation, and vacuum aluminothermic reduction stages. Optimum calcination was achieved at 1100 °C for 60 min, providing the highest hydration activity (36.78%) through complete dolomite decomposition. A three-step Ca–Mg separation process comprising digestion, carbonization, and crystallization enabled the selective recovery of high-purity MgO and fine particles of CaCO₃ fractions. The effects of raw material particle size, CO₂ flow rate, and solid/liquid ratio were systematically investigated, and an additional washing step reduced MgO contamination in the CaCO₃ phase to 2.37 wt.%. The obtained MgO was further used for aluminothermic reduction under vacuum at 1300 °C for 9 h. Increasing aluminum stoichiometry (75–122% of the theoretical value) enhanced magnesium recovery from 75.05% to 97.70%, while the MgO content in the residue decreased from 26.97 to 2.35%. X-ray diffraction (XRD) analyses revealed high-purity metallic magnesium (magnesium crown) along with Al₂O₃ and MgAl₂O₄ spinel in the residue. In addition, the water-based resulting solutions within the Ca–Mg separation scheme are recyclable within the process chain, establishing a waste minimization, closed-loop approach. The results confirm that this integrated hydrometallurgical–pyrometallurgical route offers an efficient alternative to conventional magnesium metal production, with the potential for reduced emissions and improved waste minimization.