Alternative integrated capture − Mineralisation (AICM) for low-energy carbonation of low-reactivity magnesium-rich wastes

Efficient carbon capture and utilization are vital for a low-carbon economy. Unlike traditional CO₂ capture methods that are energy-intensive or amine-based mineralization methods that are limited to calcium-rich wastes, this study presents an aqueous amine-based integrated CO₂ capture and mineralization process (AICM). AICM enables simultaneous CO₂ capture and stable fixation using low-reactivity, magnesium-rich feedstocks, which typically need high temperatures and pressures. A notable advantage of AICM is its ability to regenerate the absorbent by simply adding lime kiln dust (LKD), an abundant waste of lime production. Optimal conditions for CO₂ carbonation with magnesium were achieved with a 4.0 M monoethanolamine (MEA) solution at 60 °C, reaching an 87 % carbonation efficiency. Interestingly, the formation of magnesium carbonate was observed under weakly alkaline conditions, even at pH conditions below 8 where the concentration of free carbonate ions (CO₃²⁻) is negligible. This phenomenon can be attributed to the availability of carbamate, which was potentially reacting with magnesium ions (Mg²⁺) to form magnesium carbonate, thereby facilitating CO₂ desorption in the solution. The carbamate-rich environment was more pronounced at higher MEA concentrations and temperatures investigated here. Following CO₂ desorption, the reactivity of spent MEA solutions has diminished due to consumption of free reactive amine groups by protons released during carbonation. However, LKD can restore these groups by neutralizing protons, allowing regenerated MEA to regain near-full reactivity and CO₂ capture capacity comparable to fresh MEA. Producing valuable magnesium carbonate further enhances the process's economic viability. This study provides valuable insight into new methods to improve traditional thermal regeneration CO₂ capture processes and to reduce the consumption of alkalis compared to conventional pH-swing mineralisation processes, which require significant amounts of strong non-regenerable bases.