Trace Metal Mobility during Carbon Mineralisation

Carbon-neutral mines could be made possible by accelerating the natural carbon mineralisation that occurs in mine tailings at the Earth’s surface.  Ultramafic mine tailings contain an abundance of Mg-rich minerals such as olivine [(Mg,Fe)₂SiO₄] and serpentine [Mg₃Si₂O₅(OH)₄], which provide an ideal cation source for carbon mineralisation.  These minerals naturally react with meteoric or surface water, releasing Mg²⁺ ions, which in turn react with dissolved carbonate (CO₃^2-) ions to precipitate Mg-carbonate minerals such as nesquehonite (MgCO₃·3H₂O) and hydromagnesite [Mg₅(CO₃)₄(OH)₂·4H₂O].  Carbonation reactions are accelerated with increased mineral surface area, making mine tailings storage facilities an ideal place to observe and promote mineral carbonation.  

Natural carbon mineralisation has been documented at several ultramafic mine tailings sites in Australia, Canada and Norway (Wilson et al. 2009a; Wilson et al. 2009b; Beinlich & Austrheim 2012; Pronost et al. 2012; Oskierski et al. 2013; Wilson et al. 2014).  Whilst natural carbonation rates are slow, they may be accelerated in situ with relatively inexpensive chemical treatments and changes to tailings management practises.  Leaching of industrial and mine wastes with acidic solutions has been widely employed to accelerate mineral dissolution and release of Mg²⁺ for reaction.  However, the resulting low pH may enhance metal mobility. Given that mine tailings typically contain significant concentrations of toxic transition metals, the use of acid to accelerate the dissolution of Mg‑silicate minerals poses an environmental risk.  This concern is also relevant to the in situ injection of CO₂ into basaltic or ultramafic rocks,  which creates an acidic environment, facilitating metal release (Olsson et al. 2014).  As such, we have investigated the mobility of Ni, Cr, Cu, Co and Mn in (1) field samples collected from carbonating mine tailings and (2) laboratory synthesis experiments designed to simulate a worst-case scenario for metal release. 

Field samples were acquired from Woodsreef Chrysotile Mine, NSW, Australia, in order to study baseline metal mobility in material known to be carbonating.  X-ray fluorescence analyses reveal that both serpentinite ore and carbonate crusts from the Woodsreef Mine contain Ni, Cr, Mn and Co, while mine pit water samples contain no detectable trace metals. These metals are concentrated in chromite and inclusions in magnetite grains within serpentinite and harzburgite.  It is expected that these transition metals may substitute for Mg in serpentine minerals and that they may become incorporated into the crystal structures of hydrated Mg-carbonate minerals, such as nesquehonite and hydromagnesite, during carbonation.

Laboratory experiments were conducted to determine whether Mg-carbonate minerals could sequester trace metals from solution during precipitation.  Nesquehonite was synthesised in solutions containing up to 100 mg/L of Ni, Cr, Cu, Co and Mn.  Our Inductively Coupled Plasma – Mass Spectrometry results confirm that generally >99 wt.% of trace metals were sequestered by nesquehonite. The potential for trace metals to be released from nesquehonite may be influenced by the mechanism controlling their initial sorption to the solid phase. Metals incorporated into crystal structures may be less likely to be released post carbonation than metals sorbed onto crystal surfaces.  A combination of X-ray Diffraction and Energy-dispersive X-ray mapping indicates that metals are distributed homogeneously throughout nesquehonite crystals. 

Our results suggest that transition metals are substituting for Mg within the crystal structures of the carbonate weathering products that trap and store CO₂ in mine tailings.  Furthermore, nesquehonite was able to take up high concentrations of metals, commensurate with conditions found at acid and metalliferous drainage sites, within a timescale of minutes to hours. Consequently, if carbonation of metal-rich industrial wastes or mine tailings were to be accelerated, metalliferous drainage is unlikely to pose an environmental risk. These minerals may therefore represent safe and long-term storage option for both CO₂and toxic transition metals.  

Keywords:  carbon mineralisation, metals, nesquehonite, Woodsreef, mine tailings, CO₂sequestration.

References:

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Wilson S. A., Harrison A. L., Dipple G. M., Power I. M., Barker S. L. L., Ulrich Mayer K., Fallon S. J., Raudsepp M. & Southam G. 2014. Offsetting of CO2 emissions by air capture in mine tailings at the Mount Keith Nickel Mine, Western Australia: Rates, controls and prospects for carbon neutral mining. International Journal of Greenhouse Gas Control 25, 121-140.

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