Evaluation of kimberlite residues for carbon dioxide removal

Abstract:

The mining of kimberlite for diamonds produces vast quantities of ultramafic residues that are suitable for carbon dioxide (CO₂) removal (CDR) via enhanced rock weathering and CO₂ mineralization [1–3]. In partnership with De Beers Group of Companies, we are investigating kimberlite residues from Voorspoed and Venetia (South Africa), as well as Jwaneng and Orapa (Botswana), which have the maximum capacity to render these mines carbon neutral. Kimberlite residues are fine-grained (<1 mm) with high surface areas (6.8–13.4 m²/g), contain reactive Mg- and Ca-silicate minerals (e.g., serpentine and diopside), and only trace sulfur and heavy metal content (e.g., Ni and Cr). Kimberlite is also a potential nutrient source for agriculture having P and K contents ranging from 950­–1800 ppm and 0.76–2.10%, respectively. We have developed a reactivity test that consists of CO₂ batch leaches coupled with total inorganic carbon analyses to quantify the release of easily extractable cations from both non-carbonate (desirable) and carbonate (undesirable) sources at ambient conditions. Furthermore, experiments were conducted using CO₂ flux systems to directly measure the removal of atmospheric CO₂ into residues. The most negative CO₂ fluxes ranged from -190 to-4,000 g/m²/yr and were influenced by mineralogy, water content, and whether or not residues had been previously exposed to weathering. Characterization of kimberlite residues and geochemical modelling demonstrate that CDR is limited by mineral dissolution, and more practically the exposure of these residues at the surface. While active mines have limited space (e.g., residue impoundments), spreading of residues at inactive mines or more widely off-site (e.g., agriculture fields) would greatly increase CDR. The estimated cost of using existing kimberlite residues for CDR is approximately $5 (on-site) to $25 (off-site within 10 km) per tonne of CO₂ when considering that mining and comminution have already occurred. Large-scale demonstration pilots are needed to further test acceleration strategies and refine CDR rates and cost estimates.[1] Power et al. (2014), Minerals 4, 399–436. [2] Wilson et al. (2011), Environ. Sci. Technol. 45, 7727–7736. [3] Mervine et al., (2018) Miner. Petrol. 112 (Suppl 2), S755–S765.