Exploring the Metastability Field for Precipitates Formed from Degassed Serpentinite Leachate

Indirect aqueous mineral carbonation relies on the operation of separate but linked mineral dissolution and precipitation process units. Decarbonation, or the controlled removal of carbon dioxide (CO₂), from aqueous mineral bicarbonate solutions, engenders supersaturation resulting in precipitation. Supersaturation is achieved through the inducement of a pH swing of predictable and measurable effect, directly as a result of decarbonation. Leachates derived from the carbonic acid dissolution of serpentine, an alkaline mineral, results in the formation of Mg bicarbonate solutions which can, in turn, be exploited via decarbonation to produce Mg carbonate. Although magnesite is the most stable Mg carbonate phase predicted to form from degassed serpentinite leachate based on thermodynamic analysis, various metastable Mg carbonate phases having waters of crystallisation and hydroxyl groups in their crystal structures (e.g. nesquehonite, MgCO₃·3H₂O and hydromagnesite Mg₅(CO₃)₄(OH)₂·4H₂O) can form as a result of the slow rate of nucleation and growth of magnesite compared to the hydrated carbonate phases. An understanding of the kinetically controlled precipitates over a range of degassing process operating conditions is needed. Through the use of thermodynamic modelling, we investigate the effect of temperature, solution composition, and the amount of CO₂ degassed from solution on the metastability field for precipitates derived from serpentinite leachate during degassing. Our assessment accounts for the variability of thermodynamic data for mineral solubility, and theoretical yields for various scenarios are calculated. The information thus generated can be used as a basis for development of an experimental campaign, with application to larger scale, to examine mineral phases and the conditions which may lead to their precipitation during degassing.