Mineral carbonation of ultramafic rocks provides an environmentally safe and permanent solution for CO(2) sequestration. In order to assess the carbonation potential of ultramafic waste material produced by industrial processing, we designed a laboratory-scale method, using a modified eudiometer, to measure continuous CO(2) consumption in samples at atmospheric pressure and near ambient temperature. The eudiometer allows monitoring the CO(2) partial pressure during mineral carbonation reactions. The maximum amount of carbonation and the reaction rate of different samples were measured in a range of experimental conditions: humidity from dry to submerged, temperatures of 21 and 33 °C, and the proportion of CO(2) in the air from 4.4 to 33.6 mol %. The most reactive samples contained ca. 8 wt % CO(2) after carbonation. The modal proportion of brucite in the mining residue is the main parameter determining maximum storage capacity of CO(2). The reaction rate depends primarily on the proportion of CO(2) in the gas mixture and secondarily on parameters controlling the diffusion of CO(2) in the sample, such as relative saturation of water in pore space. Nesquehonite was the dominant carbonate for reactions at 21 °C, whereas dypingite was most common at 33 °C.

We have discovered diffuse warm air vents at the surface of a chrysotile milling waste heap at the Black Lake mine, Thetford Mines, Québec, Canada. The venting areas are inconspicuous, except in winter when the vents form snow-free areas of unfrozen ground, each with a surface area of 1-15 m 2 . The temperature and chemical composition of the warm air vents have been monitored from March 2009 to July 2010. The temperature of the warm air and ground surface at the venting sites ranged from 6.6 to 20.0 °C, whereas that of the ambient air ranged from -13.2 to 20.0 °C. The difference between atmospheric and vent air temperatures is greater in cold-weather months. The warm air has low CO 2 content, but has otherwise normal atmospheric gas composition. Warm air volumetric fl ow varies from 2.1 to 19.9 L/m 2 /s in winter, when it contains between <10 and 18 ppm CO 2 . In summer, the venting areas are more diffuse, with volumetric fl ow rates ranging from 0.5 to 1.5 L/m 2 /s, and are less depleted in CO 2 (260-370 ppm). Frozen ground is likely focusing airfl ow in winter compared to summer. We present a conceptual model in which air enters the steep fl anks of the chrysotile milling waste heap, into which CO 2 reacts with Mg-rich minerals, stripping CO 2 from air by exothermic mineral carbonation reactions. Considering the surface area of summer and winter venting areas, fl ow rates, and concentration of CO 2 in warm air vents, we estimate that the Black Lake mine heap passively captures at least 0.6 kt CO 2 per year.