Weathering of mineralogically complex materials and its implications for CO2 mineralization

Abstract:

Several megatons of carbon dioxide (CO₂) can be captured by kimberlite mine wastes [1]. CO₂ reacts with silicates and hydroxide minerals in mine wastes to release calcium (Ca) and magnesium (Mg) that can precipitate as secondary carbonate minerals that capture carbon [2]. However, kimberlites reactivity to CO₂ is strongly impacted by its complex mineralogy, and the determination of CO₂ mineralization rates is challenging. We will present a new method to assess mineralogically complex materials' reactivity and estimate CO₂ mineralization rates for mine-scale projects (km²). Kimberlite residues from De Beers’ Venetia mine (South Africa) were tested, and these kimberlites contain lizardite (28 wt.%), clinochlore (19 wt.%), phlogopite (18 wt.%), diopside (18 wt.%), Na-smectite (9 wt.%), calcite (5 wt.%), and minor amounts of orthoclase, magnetite, tremolite, and quartz.The reactivity test combines CO₂ leaches coupled and total inorganic carbon (TIC) analyses and can help differentiate the non-carbonate (desirable) and carbonate (undesirable) sources for released cations. Ca was predominantly released from carbonates, as suggested by a strong correlation with TIC changes in the solids. Conversely, Mg and Si strong relationship suggests a common silicate source, possibly lizardite and diopside. The cations released from non-carbonate sources suggested a CO₂ mineralization potential of 300-5,000 g CO₂/m²/yr or a 0.5 - 8% CO₂ of the Venetia’s emissions offset. Geochemical modelling of mine water for the last 10 yr predicted mineralization rates of 30 - 700 g CO₂/m²/yr from lizardite or diopside dissolution. These lower rates suggest that kimberlite waste weathering is limited and, by comparison to the reactivity of the mine wastes, greater CO₂ mineralization may be achieved through enhancement strategies. Preliminary results from field tests (1-m³) focused in enhanced weathering of kimberlite waste will be presented. Our approach provides insights into CO₂ mineralization processes at ambient conditions, assists in predicting mineralization rate, and helps identify the need for strategies to promote greater mineral weathering for carbon capture. [1]Mervine et al. (2018), Mineralogy and Petrology 112, 755-765. [2] Power et al. (2014), Minerals 4, 399–436.