CO2 Sequestration By Mineral Carbonation in Milling Wastes at Thetford Mines (Québec, Canada): Monitoring Experimental Cells Under Atmospheric Conditions

Compared with other technologies, mineral carbonation is considered a permanent option to capture and store atmospheric CO₂.  Mineral carbonation is a process that fixes carbon dioxide through chemical reactions. In most mineral carbonation reactions, CO₂ reacts with dissolved divalent cations such as magnesium, which is a major constituent of ultramafic rocks, to form magnesium carbonates. Often neglected because of its slow reaction rate under ambient conditions, this exothermic reaction occurs naturally and passively in ultramafic milling wastes. In the Thetford Mines area (Québec, Canada), chrysotile mining has produced approximately 0.8 Gt of magnesium-rich milling waste, which mainly consists of poorly sorted grains and fibers of lizardite and chrysotile, with smaller amounts of antigorite, brucite and magnetite. These wastes could serve as significant CO₂ sinks if the amount and rate of CO₂mineral carbonation could be determined.

Mineral carbonation is a combination of several complex coupled processes, including air, water and heat transport in the unsaturated zone, CO₂dissolution in pore water and aqueous geochemical reactions. To improve our knowledge of this natural process under natural conditions, two experimental pilot-scale milling waste cells (200 m³) were built and instrumented to observe the physico-chemical processes involved during mineral carbonation and to propose a conceptual model for mineral carbonation at this scale. To validate this model, numerical simulations with the MIN3P reactive transport code have been carried out.

Inside the cells, five vertical profiles located along a symmetrical cross-section have been equipped with sensors measuring soil temperature and volumetric water content. Gas sampling ports are also located at the same positions as the other instruments in order to measure gas pressure and composition. A geo-membrane with a collector installed at the base of the experimental cell collects the leachate, which was analysed to determine water chemistry. Outside the cells, ambient conditions are recorded by a meteorological station, collecting data on air temperature, precipitation, relative humidity, pressure and snow thickness.

During the summer of 2013, one of the experimental cells was decommissioned and milling waste samples from within the cell were collected. These samples were analyzed in the laboratory for carbon content, to measure the extent of CO₂capture. The results show that the carbon concentration varied between 0.3 and 1.2 %C without any spatial trend, suggesting that the reaction is occurring everywhere in the cell. XRD analyses confirmed the presence of carbonates which included hydromagnesite, sjögrenite and pyroauryte. However, during the SEM analyses, only hydromagnesite was observed, which exhibited a lamellar texture on cemented grain surfaces.

Chemical analysis of leached water shows that the pH is most often above 10, magnesium concentration ranges from 85 to 140 mg/L, and total alkalinity from 260 to 300 mg/L. These results are consistent with active CO₂ mineralization reactions within the cell. The slightly acidic rain water (pH 6 to 6.5) hydrolyses the magnesium minerals (particularly the brucite) which liberates the Mg and yields a very alkaline leachate. CO₂ dissolution can explain the high alkalinity of the leached water. These processes are supported by the highly depleted CO₂ concentration in the interstitial air (ten times lower than the atmospheric CO₂concentrations) within the pile.

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. Diffusion in the pile is facilitated by the fact that the residues are only partially saturated, so the gas can circulate and supply the CO₂ within the cell. These observations have helped to develop a conceptual model for mineral carbonation in the wastes and were used to calibrate a reactive transport model.