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 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.