Mineral carbonation in water-unsaturated porous media

Abstract:

Ultramafic mine tailings have an untapped capacity to sequester CO₂ directly from air or CO₂-rich gas streams via carbonation of tailings minerals [1]. The CO₂ sequestration capacity of these sites could be exploited simply by increasing the supply of CO₂ into tailings, such as through circulation of air or flue gas from mine site power plants [1,2]. Mine tailings storage facilities typically have heterogeneously distributed pore water [1], affecting both the reactive capacity of the porous medium and the exposure of reactive phases to CO₂ [3]. We examine the physical reaction processes that govern carbonation efficiency in variably saturated porous media using meter-scale column experiments containing the tailings mineral, brucite [Mg(OH)₂], that were supplied with 10% CO₂ gas streams. The experiments were instrumented with water content and gas phase CO₂ sensors to track changes in water saturation and CO₂concentration with time. The precipitation of hydrated Mg-carbonates as rinds encasing brucite particles resulted in passivation of brucite surfaces and an abrupt shut down of the reaction prior to completion. Moreover, the extent of reaction was further limited at low water saturation due to the lack of water available to form hydrated Mg-carbonates, which incorporate water into their crystal structures. Reactive transport modeling using MIN3P-DUSTY [4] revealed that the instantaneous reaction rate was not strongly affected by water saturation, but the reactive capacity was reduced significantly. Surface passivation and water-limited reaction resulted in a highly non-geometric evolution of reactive surface area. The extent of reaction was also limited at high water content because viscous fingering of the gas streams injected at the base of the columns resulted in narrow zones of highly carbonated material, but left a large proportion of brucite unreacted. The implication is that carbonation efficiency in mine tailings could be maximized by targeting an intermediate water content in tailings storage facilities (e.g., ~20-60% saturation).

[1] Wilson et al. (2014) Int. J. Greenhouse Gas Control. 25: 121-140. [2] Harrison et al. (2013) Environ. Sci. Technol. 47: 126-134. [3] Assima et al. (2013) Int. J. Greenhouse Gas Control. 12: 124-135. [4] Molins and Mayer (2007) Water Resour. Res. 43, W05435.