The Black Lake (Quebec, Canada) mineral carbonation experimental station: CO2 capture in mine waste
Abstract:Passive mineral carbonation of chrysotile mining and milling waste was discovered at the Black Lake mine, southern Québec, 10 years ago. Indurated crusts were found at the surface and within waste piles where mineral and rock fragments are cemented by hydrated magnesium carbonates. A long-term research program has yielded significant insight into the process of CO2 capture from the atmosphere, and how it can be implemented during mining operations.
Laboratory experiments show that the waste mineralogy is crucial, brucite being more reactive than serpentine. Partial water saturation, circa 40%, is also critical to dissolve magnesium from minerals, and transport aqueous CO2 to precipitation sites. Grain armoring by iron oxidation induced by dissolved oxygen prevents further reaction.
Two experimental cells constructed with milling waste and fitted with various monitoring probes (T, H2O content, leachate) and gas sampling ports, have been monitored for more than 3 years, along with environmental conditions. The interstitial gas in the cells remains depleted in CO2 indicating continuous capture of ambient atmospheric CO2 at rates faster than advection to reaction sites.
The energy released by the exothermic mineral carbonation reactions has been observed both in laboratory experiments (up to 4 °C) and in the field. Warm air, depleted to 10 ppmv CO2, vents at the surface of the waste piles, indicating reaction with atmospheric CO2 deep inside the piles. A thermal anomaly, detected by airborne infrared and coincident with a known venting area, was selected for locating a 100 m deep borehole fitted with sensor arrays to monitor active mineral carbonation within the pile. The borehole has intersected areas where mineral carbonation has indurated the milling waste. The borehole will be monitored for the next 3 years to better understand the mineral carbonation process, and its potential to yield recoverable geothermal energy in mining environments.