Alkaline-based extraction of Mg(OH)2 from magnesium silicate minerals for CO2 capture and storage

Magnesium hydroxide, Mg(OH)2, has great potential for CO2 capture and storage (CCS) as it can capture CO2 in aqueous and solid systems. However, the utilisation of Mg(OH)2 for CCS has been hindered due to its rare occurrence in mineral deposits, and Mg silicate minerals, despite their slow kinetics of carbonation, have been the prime focus of the investigations for CO2 sequestration owing to their abundance in nature.
In the present study, the extraction of Mg(OH)2 from Mg silicate minerals was investigated in NaOH aqueous and solid systems. The effective extraction of Mg(OH)2 from Mg silicate minerals is beneficial because it would offer not only a large CO2 storage capability but also a faster kinetics of carbonation. The direct alkaline-based extraction of Mg(OH)2 is also beneficial, as the acidic extraction commonly used requires the addition of an alkaline reactant to favour the precipitation of Mg(OH)2 and successive carbonation process.
81% yield of Mg(OH)2 extraction was achieved from dunite, an ultramafic rock mainly composed of Mg silicate minerals, reacted with NaOH aqueous systems. Despite the high efficiency of extraction, the large amount of NaOH consumed was identified as a potential obstacle for the implementation of the process.
Dunite was reacted with NaOH solid systems aiming the reduction of NaOH usage. The consumption of NaOH in solid systems was effectively reduced by 94.5 – 97% respect to the aqueous systems, maintaining significant rates of Mg(OH)2 extraction (65 – 73%). The H2O consumption was also reduced or avoided.
Mg(OH)2 carbonation was investigated in aqueous and solid systems. A significant fraction of Mg(OH)2 was converted into hydromagnesite Mg5(CO3)4(OH)2·4H2O when reacted with added H2O under supercritical CO2. The dissolution of Mg in the products from Mg(OH)2 extraction significantly improved respect to Mg silicate minerals previously tested without alkaline pre-treatment. CO2 was captured from a gas mixture of CO2 and N2 at ambient conditions using an aqueous slurry of Mg(OH)2. Mg(OH)2 and CO2 dissolved in H2O and reacted by forming soluble Mg(HCO3)2 which remained in solution under controlled pH.