Direct Aqueous Carbonation of High-Calcium Solid Waste in a Slurry Bubble Column: Reactive Mass Transfer and Stage-Dependent Kinetics

CO2 mineralization of high-calcium solid waste was investigated in a slurry bubble column reactor to elucidate the interaction between hydrodynamics, reactive gas–liquid mass transfer, and stage-dependent kinetics. Experiments were conducted using simulated flue gas containing 15 v% CO2 under different gas velocities and solids holdups. Results showed that the process exhibited three stages: rapid alkaline absorption (I), transition (II), and slow late-stage carbonation (III). Stage I was governed primarily by reactive gas–liquid mass transfer, with a volumetric liquid-phase mass transfer coefficient (kL0a) of 0.015–0.040 s–1. Product characterization revealed CaCO3-rich surface layers, supporting a surface coverage model for Stage III, with predicted conversions within ±10% of experimental values. The final conversion reflected competition between reaction acceleration and surface coverage, and the highest conversion was 59.6%, corresponding to 265.7 g CO2·kg–1 of white ash. These findings provide a reactor-scale interpretation and highlight hydrodynamic optimization for improving slurry bubble column carbonation.