Influence of Iron Substitution and Solution Composition on Brucite Carbonation

Carbon neutral or negative mining can potentially be achieved by integrating carbon mineralization processes into the mine design, operations, and closure plans. Brucite [Mg(OH)₂] is a highly reactive mineral present in some ultramafic mine tailings with the potential to be rapidly carbonated and can contain significant amounts of ferrous iron [Fe(II)] substituted for Mg; however, the influence of this substitution on carbon mineralization reaction products and efficiency has not been thoroughly constrained. To better assess the efficiency of carbon storage in brucite-bearing tailings, we performed carbonation experiments using synthetic Fe(II)-substituted brucite (0, 6, 23, and 44 mol % Fe) slurries in oxic and anoxic conditions with 10% CO₂. Additionally, the carbonation process was evaluated using different background electrolytes (NaCl, Na₂SO₄, and Na₄SiO₄). Our results indicate that carbonation efficiency decreases with increasing Fe(II) substitution. In oxic conditions, precipitation of ferrihydrite [Fe₁₀^IIIO₁₄(OH)₂] and layered double hydroxides {e.g., pyroaurite [Mg₆Fe₂^III(OH)₁₆CO₃·4H₂O]} limited carbonation efficiency. Carbonation in anoxic environments led to the formation of Fe(II)-substituted nesquehonite (MgCO₃·3H₂O) and dypingite [Mg₅(CO₃)₄(OH)₂·∼5H₂O], as well as chukanovite [Fe₂^IICO₃(OH)₂] in the case of 23 and 44 mol % Fe(II)-brucite carbonation. Carbonation efficiencies were consistent between chloride- and sulfate-rich solutions but declined in the presence of dissolved Si due to the formation of amorphous SiO₂·nH₂O and Fe-Mg silicates. Overall, our results indicate that carbonation efficiency and the long-term fate of stored CO₂ may depend on the amount of substituted Fe(II) in both feedstock minerals and carbonate products.