Metal Mobility during Passive and Accelerated Carbonation of Ultramafic Mineral Wastes

Ultramafic rocks and mineral wastes rank amongst the most plentiful and most reactive feedstocks for carbon mineralisation. Ultramafic rocks commonly host Ni–Cu–PGE and chromite deposits and they can be enriched in gold, silver and cobalt. Recovery of metal resources during ore processing is not 100% efficient and ore processing circuits are typically designed to only recover one or two key elements or minerals. As such, ultramafic mineral wastes can contain valuable, and potentially toxic, first row transition metals at abundances of several percent by weight. We have undertaken a series of synchrotron, laboratory and field experiments to better understand the mobility – and ultimate fate – of transition metals during passive and accelerated carbonation of ultramafic mineral wastes.

Our laboratory experiments demonstrate that first row transition metals (Cr, Mn, Fe, Co, Ni, Cu) are rapidly incorporated into the structures of hydrated Mg-carbonate and Fe-oxyhydroxide minerals under conditions relevant to accelerated carbonation of ultramafic rocks. Greater than 99 wt.% of aqueous transition metals are sorbed to colloidal carbonate and Fe-oxyhydroxide precipitates within 5 minutes. These are retained during recrystallisation to form mm-scale, euhedral crystals of nesquehonite (MgCO₃â‹…3H₂O) indicating that metal sequestration is likely to be stable during Mg-carbonate phase transitions.

We see the same association of first row transition metals with Mg-carbonate and Fe-oxyhydroxide minerals during passive carbonation in the mineral waste at the Woodsreef Chrysotile Mine, New South Wales, Australia. Our synchrotron X-ray Fluorescence Microscopy data indicate that mobility of transition metals is limited during passive carbonation of ultramafic landscapes. Transition metals are immobilised on lengthscales of tens of micrometres, they are retained by alteration minerals at reaction fronts, and their concentrations are generally below detection in mine pit waters.

Our accelerated carbonation experiments employ weak sulfuric acid leaches to enhance the availability of Mg²⁺ for carbonation reactions. In field-based mesocosm experiments and laboratory-based column experiments, we find that periodic addition of acid can be used to leach Mg from both brucite and serpentine minerals, producing an increasingly deep Mg-depleted horizon. An Fe-oxyhydroxide-rich horizon occurs at the neutralisation front between leached and unreacted ultramafic mineral wastes. Our X-ray fluorescence data confirm that first row transition metals are associated with, and immobilised by, this Fe-oxyhydroxide layer.

These observations suggest there is low risk of transition metal release into process waters (for industrial-style carbon mineralisation) or surface waters (for enhanced weathering of ultramafic landscapes) so long as the neutralisation potential of mineral wastes is not exceeded. The ability of Mg-carbonate minerals to scavenge potentially hazardous transition metals may limit their use by some industries (e.g., pharmaceuticals). Ultimately, understanding the deportment of transition metals during carbonation reactions provides a valuable opportunity to use CO₂ sequestration as a technology to recover increasingly scarce metal resources.