CARBON FIXATION IN MINERAL WASTE FROM THE MOUNT KEITH NICKEL MINE, WESTERN AUSTRALIA, AUSTRALIA

Mineralization of carbon dioxide (CO₂) in mine tailings can occur at a scale that is significant relative to the greenhouse gas production of a mining operation. At the Mount Keith Nickel Mine, Western Australia, the hydrated Mg-carbonate mineral hydromagnesite [Mg₅(CO₃)₄(OH)₂·4H₂O] precipitates within mine tailings as a result of mineral processing. We use stable isotopes of C and O and radiocarbon to fingerprint the source of the carbon in hydromagnesite. Because hydromagnesite is finely intergrown with gangue minerals (including bedrock magnesite, dolomite, and calcite), we employ a sequential extraction method that uses weak acids to selectively analyze CO₂ from hydromagnesite (Barker et al., this session). Our radiocarbon results indicate that more than 80% of carbon in Mount Keith hydromagnesite is sourced from the modern atmosphere.

We have used crystallographic methods to quantify fixation of carbon at Mount Keith in more than 200 representative samples of mine tailings that were selected from a collection of 820 samples. The Rietveld method for quantitative phase analysis and X-ray powder diffraction data provide a measure of the weight-percent abundance of hydromagnesite in a sample. The hydrated Mg-carbonate minerals are highly stoichiometric, allowing the amount of carbon stored within hydromagnesite to be estimated from Rietveld refinement results. Because several of the cells within the tailings storage facilities at Mount Keith contain tailings of known age, we have been able to use Rietveld refinement results (1) to track the mineralogical evolution of the mine tailings and (2) to estimate the rate at which atmospheric CO₂ is fixed within hydromagnesite. Based on our assessment, approximately 58,000 t/year of atmospheric CO₂ are being trapped and stored in the tailings at Mount Keith. This represents an offset of 16% of the mine’s annual greenhouse gas emissions. Our results suggest that passive sequestration resulting from mineral processing can capture and store a significant amount of CO₂. Accelerating the uptake of CO₂ into tailings could further reduce or completely offset the net greenhouse gas emissions at Mount Keith and many other mines.