Dynamic CO2 Sequestration in an Industrial-Scale Chrysotile Mining Waste Pile

Carbon mineralization in ultramafic mine wastes has been studied under controlled laboratory conditions and in small-scale experimental waste cells in the field. Mine waste mineralogy, water saturation, CO₂ availability, and temperature have been identified as key factors controlling the sequestration but can be optimized for more efficient long-term CO₂ storage.

To investigate CO₂ sequestration in an industrial-scale mining waste pile under natural environmental conditions, a 92-m deep borehole was drilled in a 110 Mt chrysotile mine waste pile at Thetford Mines, Québec, Canada. This borehole was instrumented with 20 thermistors for temperature measurements and 9 pneumatic ports for differential pressure measurements and for sampling CO₂ in the interstitial air. The carbon isotope composition of CO₂ in the samples was also measured periodically using a Wavelength Cavity Ring Down Spectroscopy instrument (Picaro G-1101-i). A weather station close to the borehole provided the local environmental conditions.

A nonlinear thermal regime with two thermal gradients was observed in the pile. The thermal gradient for the upper 50 m was 18 °C/km whereas the lower portion had a thermal gradient of 42 °C/km, 1.5 times higher than the expected regional thermal gradient (27 °C/km). These gradients suggest the presence of two thermo-hydro-chemical regimes for the transport and storage of CO₂ within the pile. During cold-weather periods, when the ambient air temperature is less than the temperatures prevailing in the pile (which typically varyfrom 10 to 14 °C), the pressure in the pile is higher than the atmospheric pressure due to thermal expansion of the interstitial air. This thermodynamic effect impedes the inflow of CO₂ from the atmosphere, while the carbonation reactions continue to consume CO₂ and reduce its concentration within the pile. During warm-weather periods, when the ambient air temperature is higher than the pile temperature, a general reversal of differential pressure enhances air inflow and migration of CO₂ into the pile, replenishing CO₂ concentrations. Nevertheless, during this period the carbonation mineralization reactions inside the pile reduce the CO₂ concentration of the inflowing air by 40%. The carbon isotope composition of CO₂ shows a transition from a kinetic, closed-system Rayleigh fractionation during the cold periods to an isotopic system at equilibrium between gas and dissolved CO₂ in interstitial water during the warm periods.

A thermo-hydro-aeraulic numerical model was developed to test the conceptual model and to assess the amount of CO₂ that can be sequestered in such industrial-scale waste piles. The numerical results suggest that about 100 tons of CO₂ per year are stored passively in the Thetford Mines waste pile which is in the same order of magnitude as the estimated storage capacities of other mining waste piles cited in the literature. Model results also suggest that a distributed heat source of at least 0.1 mW/m³ due to the heat released by the exothermic carbon mineralization reactions can explain the thermal regime observed in the pile.