Molecular Dynamics Investigation of Ca^<b>2+</b> Carbonation in NH^<b>4+</b>–Cl^––H_<b>2</b>O Systems

The steelmaking industry confronts dual challenges of CO₂ emission mitigation and steel slag (SS) utilization. A promising strategy lies in the synergistic waste treatment through Ca²⁺ extraction from SS for subsequent CO₂ sequestration. Ammonium chloride (NH₄Cl) solution has become an effective leaching medium due to its optimal acidity and recyclability. However, the ion association dynamics and crystallization pathways during carbonation in NH₄ ⁺-Cl^--H₂O systems remain poorly understood at the atomic scale. This study presents the first molecular dynamics simulation of Ca²⁺ carbonation in NH₄ ⁺-Cl^--H₂O solutions, revealing the effects of temperature (20-80 °C) and CO₃ ^2- concentration (15-25 vol % in the gas phase) on CaCO₃ crystallization kinetics, binding energies, mean square displacements (MSDs), and diffusion coefficients. Microstructural evolution, bonding configurations, and particle distribution patterns were further characterized through a trajectory analysis. Results demonstrate that elevated temperatures and increased CO₃ ^2- concentrations synergistically enhance Ca²⁺-CO₃ ^2- binding affinity and diffusion coefficients, thereby accelerating CaCO₃ cluster formation. The study on steel slag carbonation experiments found that with the increase of the reaction temperature and CO₃ ^2- concentration, Ca₂SiO₄ in the steel slag undergoes continuous carbonation during the reaction process, leading to an increased degree of carbonation of the steel slag. This conclusion is consistent with the findings from the aforementioned CaCO₃ crystallization kinetics simulation. Notably, the Ca²⁺-CO₃ ^2- binding distance remained stable at 4.05 Å across all tested conditions, suggesting minimal structural dependence on environmental variables.