Comparative Analysis of Two CO₂ Sequestration Pathways for Magnesium Slag Based on Kinetics and Life Cycle Assessment

As a metallurgical solid waste rich in active calcium oxide, magnesium slag (MS) is endowed with significant carbon dioxide sequestration potential due to its inherent properties, providing a feasible path for the simultaneous solution of waste residue disposal and carbon dioxide emission reduction. However, current research has neither clarified the kinetic mechanism (core theoretical support for carbon dioxide sequestration industrialization) nor systematically evaluated the life cycle environmental impacts of MS's two carbonation routes (direct or indirect leaching carbonation). To address this, this study explores kinetic laws via the single-factor control variable method, and combines life cycle assessment (LCA) to fill the gap, providing key theoretical support for process optimization and engineering promotion. Kinetic results show indirect carbon dioxide sequestration (ICDS) forms an inert silicon-rich layer (core-shrinkage model, mixed control, 28.4 kJ/mol activation energy), while direct carbon dioxide sequestration (DCDS) involves dual-layer formation and pore blockage (mixed control, 14.0 kJ/mol). The ICDS achieves a higher reaction rate of 89%, compared to 63% for the DCDS. In life cycle assessments, DCDS demonstrates outstanding overall environmental sustainability, particularly excelling in carbon dioxide sequestration and acidification control, while ICDS exhibits significant environmental drawbacks (such as high carbon dioxide emissions and ecological toxicity). However, ICDS possesses advantages such as high feedstock utilization and strong synthesis capabilities for high-value-added products. Through targeted optimization, its environmental indicators can be reduced in the future, making it suitable for specific scenarios like high-end calcium carbonate production and resource utilization of low-grade magnesium slag.