From Lime Scrubbing to Magnesium Bisulfite Circulation: A Critical Review of SO₂ Capture, Sulfite Recovery, and Process Intensification

Wet flue-gas desulfurization remains one of the most mature routes for controlling sulfur dioxide, yet conventional calcium-based systems deliberately combine gas absorption, neutralization, oxidation, crystallization, and solids recirculation in a single slurry loop. This integration is robust but couples SO₂ capture to suspended-solids loading, surface nucleation, scaling, erosion, plugging, mist entrainment, and by-product handling. This critical review evaluates whether magnesium chemistry can be reorganized around a different process principle: capture SO₂ predominantly into a low-solids magnesium-bisulfite-rich liquor and transfer neutralization and controlled magnesium-sulfite precipitation to an external reactor. A structured critical-narrative protocol was applied to 103 scientific and technical sources, combining a 2020–23 August 2026 core corpus with six foundational pre-2020 sources recovered by targeted backward searching. Evidence was classified by industrial, pilot, laboratory, modeling, review, or technical-report level. The synthesis integrates gas–liquid mass transfer, aqueous S(IV) speciation, MgO/Mg(OH)₂ dissolution, electrolyte thermodynamics, hydrate phase behavior, sulfite oxidation, crystallization, solids separation, industrial gas-cleaning integration, circular by-product pathways, and technology readiness. The central finding is not that magnesium is intrinsically superior to calcium, but that spatially separating absorption from precipitation can create independent control of capture and solids formation. The concept is technically plausible but not yet validated for broad industrial deployment. Its feasibility depends on maintaining the absorber below relevant saturation limits, controlling oxygen-driven oxidation, managing impurities and water balance, and demonstrating stable recycle chemistry under dynamic gas loads. A staged validation program is proposed, ranging from electrolyte model validation and crystallization kinetics to closed-loop pilot operation and techno-economic comparison with the best available calcium and regenerative alternatives.