Extraction of Mg(OH)2 from Mg silicate minerals with NaOH assisted with H2O: implications for CO2 capture from exhaust flue gas

Abstract

The utilisation of Mg(OH)₂ to capture exhaust CO₂ has been hindered by the limited availability of brucite, the Mg(OH)₂ mineral in natural deposits. Our previous study demonstrated that Mg(OH)₂ can be obtained from dunite, an ultramafic rock composed of Mg silicate minerals, in highly concentrated NaOH aqueous systems. However, the large quantity of NaOH consumed was considered an obstacle for the implementation of the technology. In the present study, Mg(OH)₂ was extracted from dunite reacted in solid systems with NaOH assisted with H₂O. The consumption of NaOH was reduced by 97% with respect to the NaOH aqueous systems, maintaining a comparable yield of Mg(OH)₂ extraction, i.e. 64.8–66%. The capture of CO₂ from a CO₂–N₂ gas mixture was tested at ambient conditions using a Mg(OH)₂ aqueous slurry. Mg(OH)₂ almost fully dissolved and reacted with dissolved CO₂ by forming Mg(HCO₃)₂ which remained in equilibrium storing the CO₂ in the aqueous solution. The CO₂ balance of the process was assessed from the emissions derived from the power consumption for NaOH production and Mg(OH)₂ extraction together with the CO₂ captured by Mg(OH)₂ derived from dunite. The process resulted as carbon neutral when dunite is reacted at 250 °C for durations of 1 and 3 hours and CO₂ is captured as Mg(HCO₃)₂.