STRATEGIZING CARBON-NEUTRAL MINES: A CASE FOR PILOT PROJECTS

Ultramafic and mafic mine tailings are a valuable feedstock for carbon mineralization that should be used to offset carbon emissions generated by the mining industry. Carbon mineralization occurs via kinetically controlled mineral-microbe-fluid-gas reactions in an environment that is readily accessible for isotopic, mineralogical and geochemical characterization. Observations from mine sites in Canada and Australia suggest passive carbonation of ultramafic mine tailings can be significant: on the order of 10^-4 to 10^-2 tonnes of CO₂ per tonne tailings per year, which can represent a substantial, but not total, offsetting of a mine’s greenhouse gas emissions¹. Carbon mineralization can be accelerated through abiotic and biotic processes that drive silicate and hydroxide mineral dissolution and carbonate precipitation. For instance, silicate dissolution may be accelerated by at least one order of magnitude through bioleaching using Acidithiobacillus spp². Subsequently, phototrophic microbes (algae and cyanobacteria) can promote carbonate precipitation from leachate waters through the alkalinization of their microenvironment². The use of phototrophic microbes has the advantage that biofuel and other value added by-products could be produced concurrently with carbonate minerals. The supply of CO₂ is often rate-limiting for carbon mineralization in mine tailings, yet can be enhanced through tailings management practices and use of CO₂ point sources^1,3. Highly reactive phases, e.g. brucite [Mg(OH)₂], a minor tailings mineral, can be rapidly carbonated by supplying gasses with elevated CO₂ partial pressures into mine tailings³. CO₂ injection into mine tailings could act as a testing facility and offer valuable insights for injection into geological formations. Mining environments that contain vast quantities of pulverized ultramafic rock represent a valuable carbon sink that could be an economically efficient alternative to other technologies currently under development for carbon sequestration. Scenarios for pilot scale projects are proposed with the aim of moving towards carbon-neutral mines.

¹Wilson et al., 2014, Int. J. Greenh. Gas Control 25: 121-140.

²Power et al., 2011, Environ. Sci. Technol. 45: 9061-9068.

³Harrison et al. (2013) Environ. Sci. Technol. 47: 126-134.