Monitoring CO2 Sequestration by Mineral Carbonation in Mine Tailings at Thetford Mines, Quebec, Canada

Abstract:

Mineral carbonation is considered a permanent option to capture and store atmospheric CO₂. This reaction occurs naturally under ambient conditions in ultramafic mining and milling waste. In the region of Thetford Mines, Quebec, chrysotile mining has produced approximately 0.8 Gt of magnesium-rich milling waste, which mainly consists of poorly sorted ultramafic rock fragments (< 10 cm) and chrysotile fibers.

To quantify the amount of CO₂ that can be captured in the mine wastes of Thetford Mines, two experimental pilot-scale tailings cells were constructed and instrumented for measuring soil temperature, volumetric water content, gas pressure and gas composition, with ambient conditions recorded by an autonomous meteorological station. The cells were monitored for water geochemistry, carbon content and mineralogy, with the objective to better understand the mineral carbonation processes under natural conditions and to propose a conceptual model for mineral carbonation at the pilot scale. To validate this model, numerical simulations with the MIN3P reactive transport code have been carried out.

The chemical composition of the cell leachate (pH > 10, Mg from 85 to 140 mg.L^-1, and high total alkalinity from 260 to 300 mg.L^-1 CaCO₃) is consistent with active CO₂ mineralization reactions within the cell. SEM analyses show precipitation of dypingite with a lamellar texture and cemented grain surfaces. The milling waste contains up to 1.2% C, which indicates CO₂ sequestration by mineral carbonation. Measured CO₂ concentrations in the interstitial air are also ten times lower than in the atmosphere. Analysis of seasonal variations in fluid flow and heat transfer (essentially by thermal conduction) shows that molecular diffusion is the main process for CO₂ supply within the experimental cells. These observations have helped develop a conceptual model for mineral carbonation in the wastes and were used to calibrate the reactive transport model.