Low-temperature gas–solid carbonation of magnesia and magnesium hydroxide promoted by non-immersive contact with water

From gas–solid carbonation studies and product characterization by XRD, carbon elemental analysis, and TG-FTIR profiling of evolved CO₂, the presence of water vapour at high relative humidity (>25% RH) was shown to cause a drastic acceleration in the rate of CO₂ absorption into MgO and Mg(OH)₂ producing magnesite and hydrocarbonated precursors. From thermogravimetric experiments in the vicinity of the dew point, carbonation was shown to proceeded in steps triggered by spontaneous condensation (and re-evaporation) of water. Almost complete conversion to magnesite (MgCO₃) and/or hydromagnesite [HM = 4MgCO₃·Mg(OH)₂·4H₂O] was observed in a few hours under rather mild conditions of pressure (P_CO2 ≤ 10 bar) and temperature (T ≤ 150 °C). Unwashed (sulphate-contaminated) Mg(OH)₂ extracted from serpentinite mineral via the Åbo Akademi sulphation/precipitation route yielded MgCO₃ selectively. Washed extracts and commercial samples formed mainly HM. Carbonation was even measurable below 50 °C using sub-atmospheric pressures of CO₂ typical of flue gas. The rate was quite insensitive to P_CO2. Complete conversion to nesquehonite [MgCO₃·3H₂O] and/or dypingite [4MgCO₃·Mg(OH)₂·5H₂O] was achieved by overnight treatment. High-pressure scale-up tests (1–35 g) on commercial Mg(OH)₂ (Aldrich, >95%) from 70–150 °C under equilibrated batch conditions at 100% RH yielded HM in the 1^st hour. At longer times, HM seemed to transform spontaneously into MgCO₃. Judiciously pre-dampened samples were more reactive, re-affirming the importance of ambient water. Water as a promoter in the more speculative direct gas–solid mineral carbonation is briefly evaluated.