Mechanochemical processing of silicate rocks to trap CO2

Milling minerals rich in magnesium and iron within CO2 gas has been proposed to capture carbon as metal-carbonates. We conduct milling experiments in CO2 and show that polymineralic rocks such as granite and basalt, whether high or low in carbonate-forming metals, are more efficient at trapping CO2 than individual minerals. This is because the trapping process is not, as previously thought, based on the carbonation of carbonate-forming metals. Instead, CO2 is chemically adsorbed into the crystal structure, predominantly at the boundaries between different minerals. Leaching experiments on the milled mineral/rock powders show that CO2 trapped in single minerals is mainly soluble, whereas CO2 trapped in polymineralic rocks is not. Under ambient temperature conditions, polymineralic rocks can capture >13.4 mgCO2 g−1 as thermally stable, insoluble CO2. Polymineralic rocks are crushed worldwide to produce construction aggregate. If crushing processes could be conducted within a stream of effluent CO2 gas (as produced from cement manufacture), our findings suggest that for every 100 Mt of hard rock aggregate sold, 0.4–0.5 MtCO2 could be captured as a by-product. Milling of mafic minerals has been proposed as a method to capture carbon dioxide. Hard rocks that are commonly crushed to produce construction aggregate, however, are more efficient at carbon dioxide capture and have the potential to trap substantial CO2 as a by-product of aggregate production.