A geochemical assessment of old slimes tailings from two South African kimberlite diamond mines : implications for mineral-carbonation resource estimation

Abstract

The potential of mineral carbonation as a method for capturing anthropogenic CO₂ is still to be developed within the context of South Africa, a country with significant CO₂ mitigation opportunities through the utilisation of suitable current and historic mine tailings. Historical tailings can offer supplementary sequestration capacity, given their geochemical and physiochemical properties are conducive to CO₂ reactivity. Thus, the study focuses on utilising historic tailings from two prominent diamond mines, Cullinan and Finsch, owned and operated by Petra Diamond Limited, to quantify their geochemical and physiochemical suitability for mineral carbonation resource estimation.

Geochemical investigations were conducted, and the major oxide composition analysis revealed SiO₂ as the dominant oxide in Cullinan (42%) and Finsch (38%) tailings. Cullinan tailings had a relatively higher MgO concentration of 23% compared to Finsch (20%), while CaO was significantly higher at 7% for Finsch than Cullinan (3%) tailings. Mineral identification highlighted the modal heterogeneous nature of these tailings, with talc occupying 53 % of Cullinan’s total mineral composition and phlogopite dominating Finsch samples at 28%. The particle size distribution measurements indicated coarser particles in Finsch (D(v,90) = 277 µm) compared to Cullinan (D(v,90) = 85.4 µm), highlighting the variability in particle size between the tailings from different sites. Density measurements were conducted to estimate the tonnage values of the tailings. Therefore, densities of 1.86 g/cm3 for Cullinan and 1.79 g/cm3 for Finsch were used to calculate these values. By employing Digital Elevation Models (DEM) and density measurements, the estimated tonnage of Finsch tailings was ~ 13 times greater than that of Cullinan, which was remotely calculated as 26 702 611 metric tons (Mt). Consequently, the relative CO₂ sequestration capacity for Finsch and Cullinan was estimated to be 434 kgCO₂/t and 428 kgCO₂/t, respectively. Therefore, Finsch possesses a substantial amount of material suitable for carbon dioxide removal (CDR). Simulations of a Direct Mineral Carbonation (DMC) process using talc as the reacting mineral indicated a suboptimal CO₂ capture efficiency of 9%.

The economic evaluation of the DAC process highlighted high operational costs, resulting in a negative present value (NPV), and this was categorised as a high-risk conceptual scoping study following the South African Code for the Reporting of Exploration Results, Mineral Resources and Reserves (SAMREC). The study quantified and characterised historic tailings from Cullinan and Finsch diamond mines, ultimately shedding light on their potential to provide additional sequestration capacity as a mineral carbonation resource. It also underscores the need for a comprehensive techno-economic assessment (TEA) using more reactive mineral feedstock and better-suited carbonation technology. Lastly, it provides a baseline for estimating the economic value proposition of using mine tailings for CDR.