Fixation of CO₂ by dissolution of serpentinite and precipitation of magnesium carbonates

Carbonation of abundant magnesium silicates, such as serpentinites, could provide a respectable reduction of CO₂ emissions in Finland, but most mineral carbonation technologies in development demand high temperatures and high CO₂ pressures. In this work, we have studied the possibility for capturing and storing CO₂ by producing magnesium carbonates from solutions containing dissolved serpentinite. Common acids and bases were tested in search of a solvent for extracting magnesium from serpentinite. All acids tested were able to extract magnesium (3-26 %) from serpentinite in 1 h at room temperature, while no measurable extraction was achieved using any of the bases tested. By raising the temperature to 70 °C all magnesium was extracted in 1-2 h using mineral acids. After dissolving serpentinite in HNO₃ or HCl, the filtered solutions were evaporated, producing a solid salt precipitate, which was dissolved in water. By bubbling CO₂ through the solutions at atmospheric pressure a very pure magnesium carbonate (99% hydromagnesite) precipitated. The optimal alkalinity for producing a carbonate precipitate was found to be pH 9 for both solutions studied. Although the results show that pure carbonates can be produced from serpentinite, the process scheme suggested requires (re)generation of large amounts of sodium hydroxide and acid. Since recycling the spent chemicals cause more CO₂ emissions due to power consumption than is stored in the carbonation process, more research is needed on lowering the requirements for additional chemicals.