Carbonation of serpentinite rock from Lithuania and Finland

CO₂ mineral sequestration, or mineral carbonation, is presumably the most important CCS alternative when underground CO₂ sequestration is not an option or considered unattractive. The following advantages must be emphasized: leakage-free CO₂ fixation that does not require post-storage monitoring; the overwhelmingly large capacity offered by mineral resources available worldwide; and as recognized more recently, the possibility to operate with the CO₂ containing gas directly, removing the very expensive CO₂ separation step from the CCS process chain. Also, the solid products can be used in applications ranging from land reclamation to iron- and steelmaking. Here, a staged process for mineral sequestration that resulted from a decade of R&D work in Finland is applied to two serpentinite rocks from Lithuania and Finland, respectively. The process involves production of magnesium hydroxide from the mineral, with ammonium sulphate as the extractant (which is recovered downstream), followed by carbonation in a pressurized fluidized bed at 20-40 bar, 450-550°C. Benefits of this route are that 1) the carbonation reaction heat is taken benefit of, 2) only magnesium hydroxide is carbonated, 3) solid residue, magnesium carbonate and iron oxides are obtained as separate streams, 4) no expensive or non-recoverable chemical additives are used and 5) the pressure in the carbonation process is relatively low. The results show that magnesium hydroxide can be effectively extracted from the two serpentinites at somewhat different extraction process conditions, while the magnesium hydroxide particles produced (∼170μm, ∼340 μm) can be carbonated for 45-50 % within 10 minutes at 20 bar, ∼500°C. Process energy efficiency is similar, or slightly better than (direct, aqueous solution) carbonation processes that were suggested earlier.