Continuous Process for the Aqueous Carbonation of Serpentinite Leachate Derived from Carbonic Acid

Carbonic acid is uniquely placed in that its reaction with alkaline minerals produces carbonate based alkalinity which is an essential substrate in the formation of mineral carbonates.  Moreover, dissolution of divalent metal silicates such as serpentinite using carbonic acid produces a leachate of stoichiometry conforming to that which is required for aqueous precipitation of nesquehonite. 

The serpentinite leachate used in this study was produced in a single stage dissolution process involving the bubbling of gaseous carbon dioxide (CO₂) at a partial pressure of 1 bar into an aqueous suspension of the thermally conditioned serpentinite.  The method used in preparing the thermally conditioned serpentinite involved isothermal heating of the ground mineral (sub 53 µm) at 720°C for a total period of 30 min resulting in a reactive mineral showing some development of forsterite from the predominantly antigorite basis.  During dissolution of the mineral, the CO₂ was applied under open system conditions at moderately elevated temperatures (approximately 50^oC to 80^oC) transitioning to ambient conditions. Incongruent dissolution favouring magnesium extraction was evident, and the magnesium to carbon molar ratio of the leachate was shown as 1 : 2 in conformity to that required for the aqueous carbonation of nesquehonite.  Contact time of the CO₂ with the slurry was varied, with longer contact producing greater magnesium extraction and alkalinity, and there was some evidence of precipitation of carbonate during dissolution for longer duration experiments.  Dissolution over a contact time of 5 min was sufficient to produce an alkalised solution that was degassed in a batch operation yielding nesquehonite as the primary solid phase. Nesquehonite as evident both in degassing experiments using reagents or in solutions derived from carbonic acid dissolution of the thermally activated mineral, was confirmed through XRD, SEM-EDS, TGA-DTG-MS, and FTIR analyses.  XRD, TGA-DTG-MS, and FTIR analyses were also used to assess changes to the activated serpentinite prior to and following dissolution.  ICP-OES analysis, and alkalinity measurements were used to estimate magnesium and carbon elemental balances.   

The use of CO₂ degassing as a method for continuous production of carbonate was demonstrated using prepared magnesium sulfate and sodium bicarbonate solutions of a similar chemical basis to that derived from the carbonic acid dissolution of the thermally conditioned serpentinite.  Kinetic modelling of both dissolution and degassing processes showed that each can be operated on a continuous basis and together as an integrated scheme for the capture and subsequent mineralisation of CO₂.  With a continuous arrangement, mineral saturation in both dissolution and degassing process units is maintained at a fixed and optimal level and the precipitated phase of defined classification is extracted with the solution in the degassing reactor.

This work considered degassing through the sparging or aeration of prepared carbon enriched mineral solutions with an insert gas (N₂) effectively lowering CO₂ partial pressure of the aqueous phase.  Use of aeration allows for the generation of a large difference in the CO₂ activities between the gas and liquid phases and hence high rates of degassing.  However the use of solution depressurisation allows for the recovery of a pure CO₂ stream from the degassing reactor into the process unlike CO₂ degassing from aeration, but requires particular design considerations with regard to vapour-liquid equilibria and degassing nucleation and bubble growth kinetics so as to achieve the necessary rate of induced CO₂ degassing.

An overall scheme for a practical application to large-scale CO₂ storage is envisaged which combines dissolution and degassing process units thereby allowing for their individual optimisation, and optimisation as part of an overall integrated mineral carbonation process.  Both process units in operation enable rapid capture of CO₂ emissions and rapidity in subsequent mineralisation of the captured CO₂ can be achieved at low energy expenditure.  Its application is particularly relevant in view of the large deposits of suitable mineral feedstock available world-wide.