Mechanochemical processing of serpentine with ammonium salts under ambient conditions for CO₂ mineralization

This paper assesses the suitability of mechanochemistry as a convenient low-energy processing option in CO₂ mineralization. Whereas some success has been reported in milling alkaline earth-containing minerals under gaseous CO₂, this work focuses instead on a purely solid-state approach towards two key objectives: (a) Mg extraction from serpentine using ammonium bisulfate; and (b) direct or indirect CO₂ sequestration using ammonium bicarbonate in a natural extension of its role as "CO ₂ carrier" in the chilled ammonia scrubbing process. In Mg extraction work, dry milling of serpentine with ammonium bisulfate gave respectable yields (>60% Mg) as boussingaultite [(NH₄) ₂Mg(SO₄)₂·6H₂O] in 2 to 4 h. In CO₂ sequestration, dry milling anhydrous magnesium sulfate with ammonium bicarbonate yielded only mixed sulfate products. Carbonation of the heptahydrate, epsomite, was found to proceed via ammonium magnesium carbonate hydrate [(NH₄)₂Mg(CO₃)₂· 4H₂O], which dissolves incongruently to yield nesquehonite [MgCO ₃·3H₂O]. The modest conversion (∼30%) is probably due to equipartition of Mg into the double sulfate co-product. A similar route is followed in magnesia and brucite, in which the existence of an amorphous native carbonate precursor to nesquehonite in the same molar ratio (Mg:CO₂ = 1) was inferred from inconsistency in the XRD intensities. This was largely responsible for the high carbonation yields in the unwashed products, ∼70% and ∼85% in MgO and Mg(OH)₂, respectively, as confirmed by TG-FTIR. The same intermediate is probably formed in serpentine, but it is apparently soluble in the aqueous mineral environment. When the unwashed product is subjected to mild thermal consolidation, stable hydromagnesite [Mg₅(CO₃)₄(OH) ₂·4H₂O] is formed in ∼20% yield after milling for 16 h. Possible identities for the amorphous precursor are briefly considered.