Physical and Chemical Controls on Mineral Carbonation in Mine Tailings from the Pore to Field Scale

Mineralization of carbon in solid, stable carbonate minerals through reaction of CO₂ with Mg-rich mining wastes is a promising CO₂ sequestration strategy that could render certain mines greenhouse gas neutral [1]. Here, the physical and chemical controls on rates of and capacity for CO₂ sequestration in systems representative of mine tailings are examined from the pore to field scale using a combination of experimental and reactive transport modeling techniques. Experimental and field data are used to develop a comprehensive reactive transport model that captures the processes governing carbon mineralization in the shallow subsurface. Microfluidic pore scale and decimeter to meter scale column carbonation experiments using brucite [Mg(OH)₂] revealed that the primary controls on carbonation include the CO₂ supply rate, the distribution of the reactive phase, the mineral grain size/surface area, and the availability and distribution of water. The rate-limiting step during carbonation varied from CO₂ supply to mineral dissolution depending on the experimental variables [2]. 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 meter scale columns resulted in narrow zones of highly carbonated material, but left a large proportion of brucite unreacted. The incorporation of water consumption, water-limited reactivity, and surface passivation functions into the reactive transport code, MIN3P [3], and the capability to simulate the impact of climate on carbonation rates allows more robust predictions of the CO₂ sequestration rate and capacity at the field scale. This research imparts a better understanding of fundamental mechanisms and chemical processes relevant to CO₂ sequestration in mine tailings, with implications for mineral carbonation in other settings that have greater CO₂ sequestration capacity, such as shallow subsurface formations with similar mineralogy.

[1] Wilson et al. (2009) Econ. Geol. 104: 95-104. [2] Harrison et al. (in press) Geochim. Cosmochim. Acta. [3] Mayer et al. (2002) Water Resour. Res. 38: 1174.