Factors Controlling the Dissolution of Thermally Activated Serpentinite Under High Pressure and Temperature Carbonic Acid

The Albany NETL mineral carbonation process¹ remains a prospective technology for CO₂ storage by thermally activated serpentinite (3MgO.2SiO₂). To date however, this process has rarely exceeded over 60 % overall conversion, i.e. based on the conversion of MgO to MgCO₃. This reflects a knowledge gap in the controlling factors that govern both the dissolution and precipitation (in the form of MgCO₃) of Mg and Si from the thermally activated material. Our work focuses on the dissolution side of this knowledge gap, with the aim of enhancing the understanding of the fundamentals that control this process.

Thermal activation of serpentinite involves the thermally induced (600-800 °C) dehydroxylation of the serpentine phase (Mg₃Si₂O₅(OH)₄) resulting in the formation of the amorphous product meta-serpentine (3MgO.2SiO₂). This phase, in general, displays high leachability of both Mg and Si but can undergo rapid recrystallisation towards the relatively inert, crystalline product forsterite (Mg₂SiO₄). The extent of this recrystallisation is dependent on the heat treatment temperature (HTT) with a general correlation of decreasing activity in the activated serpentinite with increasing HTT over the range of 630-770 °C. This translates to a lower extent of carbonation, i.e. the amount of MgCO₃ formed per unit Mg₃Si₂O₅(OH)₄, resulting in an overall decrease in efficiency.

We show that this loss of activity is a result of a coupled dissolution process. At NETL temperatures and pressures, dissolution of Mg and Si proceeds through two defined dissolution rate profiles, most notable over the HTT range of 630 and 750 °C². For these materials, we first observe an initially fast rate which progressively declines on increasing extraction. This continues up to a defined extent of extraction, beyond which, a transition to a new, revived rate profile begins. Most striking, is that this transition point is directly correlated to the ratio of amorphous and forsteritic Mg in the activated material. This is equally true for Si. Based on this observation, it is concluded that dissolution first proceeds through the removal of the amorphous fraction, which then allows access for the subsequent dissolution of recrystallised forsterite. We have confirmed this picture through extensive quantitative microscopy and porosimetry, where analysis of the leached product reveals a remnant conglomerate material consisting exclusively of nano-sized forsterite granules. Continuing from this, we have developed a simple method for deconvoluting the amorphous and forsterite components. This method allows for the isolation of the individual rates based on reaction stoichiometry and the initial amorphous and forsteritic Mg ratio.

¹NaHCO3 buffered, carbonic acid system operating at 150 °C and 150 barg.
²Above this range, the activate material is essentially forsteritic in nature.