A study of hydromagnesite and nesquehonite precipitation in indirect aqueous carbonation of thermally-activated serpentine in a batch mode

•The grain size of magnesium carbonates depends on both temperature and supersaturation.•Impurities present in parent solution didn’t interfere with the precipitation phase.•Transition phenomenon from nesquehonite to hydromagnesite is affected by temperature only.•The grain size distribution of the obtained minerals are larger than those issued from synthetic solutions in literature. To control and decrease the amount of greenhouse gases in the environment, the process of carbonation is attracting particular attention. Via carbonation, it is possible to control carbon dioxide (CO2) emissions by using mining residues. This study focuses on the precipitation step of aqueous indirect carbonation of serpentine mining wastes using the actual carbonated solution. To our knowledge, studies of magnesium carbonate precipitation have been based on synthetic solutions only, unlike the process in the present study. Three solutions prepared from the carbonation step with different concentrations were precipitated at temperatures ranging from 25 to 90 °C. The solution contained dissolved magnesium and inorganic carbon in addition to impurities, such as calcium and silica that are initially present in serpentine rock. Nesquehonite precipitated at reaction temperatures of 25, 40 and 50 °C. Hydromagnesite was precipitated at 60, 70, 80, and 90 °C via transition from nesquehonite. The study showed that increasing temperatures accelerate precipitation kinetics and hastens the transition period from nesquehonite to hydromagnesite. The investigation of the effects of the initial supersaturation showed that higher supersaturation promotes a better reaction yield and consequently a better sequestration potential, but lower supersaturation is better for crystal growth. However, supersaturation had no effect on the transition phenomenon from nesquehonite to hydromagnesite. Also, the grain sizes of the resulting minerals had different ranges from those obtained in synthetic solutions. The average size of the nesquehonite in our experiments was around 120 μm, while nesquehonite’s size was around 25 µm when prepared from a synthetic solution under relatively comparable operating conditions. The average size of the hydromagnesite was around 50 μm, while its size was around 30 µm when prepared from a synthetic solution, under relatively comparable the same operating conditions.