Production of magnesium carbonates from serpentinite for long-term storage of CO₂

An approach to capture and storage of CO₂ by precipitation of magnesium carbonate was experimentally studied using aqueous solutions prepared from serpentinite. Serpentinite was first dissolved in 4 M HCl or HNO₃ at 70 °C, after which the excess quantity of solvent was evaporated and the precipitated magnesium salt was mixed with water. CO₂ gas was bubbled through the solution, while the alkalinity of the solutions was controlled using NaOH. A solution pH of 9 was found to be the optimal alkalinity for precipitation of magnesium carbonates from the solution. At this pH, the highest purity of the carbonate product (99 wt.% hydromagnesite), highest amount of CO₂ fixed as carbonate (37 wt.% CO₂ in precipitate), lowest net requirements of NaOH, as well as the highest conversion of magnesium ions to carbonate (94 wt.%) were obtained. Relatively pure iron oxide (88 wt.%) and amorphous silica (82 wt.%) were separated at various stages of the procedure. The high requirements of NaOH (2.4 tonne per tonne CO₂ stored) and make-up acid (2-4 tonne per tonne CO₂ stored) seem to be the largest obstacles to overcome for application of this approach as a CO₂ storage process.