Magnesite formation from nesquehonite slurry at 90 °C using some soluble Mg salts: Eitelite as an atypical transient mineral phase

Abstract

The production of engineered magnesite (MgCO3) under mild conditions remains an important fundamental challenge in order to store permanently the industrial captured CO2. Because, the magnesite is the more-known-stable carbonate at the Earth surface conditions and it is ready-to-use as profitable construction materials. In the present study, time-resolved Raman spectroscopy experiments have been conducted to characterize magnesite formation from nesquehonite slurry at 90 °C in an ionic carbonate alkaline medium (CO32–/HCO3– close to 1) using four different soluble Mg salts (acetate, sulphate, nitrate and chloride of magnesium). The investigated experimental conditions have revealed that only nesquehonite (MgCO3·3H2O) via un amorphous phase transformation is produced at room temperature. However, magnesite was systematically formed from nesquehonite slurry when temperature increase to 90 °C (heat-ageing step). Here, hydromagnesite (Mg5(CO3)4(OH)2·4H2O) and eitelite (Na2Mg(CO3)2) with long lifetimes were the main characterized transient phases prior to magnesite formation depending of the counterions. Moreover, eitelite rare mineral has not been reported as transient phase during magnesite formation. In summary, the imposed carbonate concentration, carbonate speciation, counterions, competitive divalent cations and pH have played a critical role on the magnesite formation at 90 °C. The discovered experimental mild conditions are promising; however, 2–3 days were required in single Mg system, except when dissolved calcium is added (Ca/Mg≈0.15) in the chloride counterion system; for such case only one hour is required to produce Ca-magnesite and protodolomite at 90 °C, i.e., the production of anhydrous Mg-Ca carbonates phases in very reduced time. In this latter case, the monohydrocalcite (CaCO3·H2O) and the nesquehonite precursors produced at room temperature were rapidly transformed via concurrent dissolution into Ca-magnesite (CaxMg1-xCO3) and protodolomite (Ca0.5Mg0.5CO3). In conclusion, these novel insights are relevant for fundamental (direct monitoring of multi-nucleation events in complex ionic and/or slurries systems) and applied (permanent storage of CO2) research on the formation of anhydrous Mg carbonates (e.g., magnesite and dolomite) debated in the last two centuries.