Role of Mg Gangue Minerals in Natural Analogue CO2 Sequestration

Abstract

Mineral carbonation reactions consume CO2 and thus have the potential for the long-term fixation of atmospheric CO2. This paper explores the possibility of integrating industrial-scale carbon storage into mining operations. Ultramafic rocks are typically considered to be one the most promising rocks for carbon capture and storage owing to their high content of Mg-bearing silicate minerals, such as forsterite (Mg2SiO4) and serpentine (Mg3Si2O5(OH)4). Beyond the thermodynamic considerations showing that magnesite (MgCO3) and quartz (SiO2) form from forsterite and serpentine alteration, the degree to which CO2 is carbonated depends on the kinetics of the reaction. For industrial carbon capture and storage to be viable, reasonable carbonation efficiency has to be achieved. To this effect, the reaction rates have to be increased, which can be achieved either by increasing the reactive surface area, increasing the reaction temperature, or using reagents to drive the reactions. However, these approaches are usually energy demanding or not efficient enough. As part of its activities, the mining industry excavates tens or hundreds of million metric tons of rock per mine and in certain areas these mafic rock groups can represent a significant percentage of the waste material left on the surface. This could represent a locally important source of readily available material for carbon capture and storage if the conversion process is sufficiently efficient. To test and quantify the carbonation potential of mine waste, a sample of serpentine skarn waste rock obtained from an iron ore mine in Sweden was reacted for 60 weeks in a laboratory humidity-cell test (HCT) at 20°C. The results show the dissolution of olivine, the precipitation of serpentine, an increase in the neutralization potential of the sample, and the appearance of inorganic carbon during the 60 weeks of testing. At ambient temperatures the sluggish precipitation kinetics of secondary phases will favor the formation of more hydrous Mg silicate phases, such as serpentine (Mg3Si2O5(OH)4), in place of Mg-bearing carbonates. This reaction lowers considerably the efficiency of forsterite carbonation, as only 25% of the Mg released from forsterite dissolution to form carbonate minerals. This study aims to model the carbonation efficiency of Mg silicates through the use of models supported by laboratory testwork and taking the example of a mine site in northern Sweden. This study evaluates the reaction of CO2 with Mg-bearing silicate rocks and the demonstration that carbonation reactions occur with Mg silicate wastes consuming CO2. Consequently, weathering of waste rock may well represent an important sink for carbon in the environment.