Mechanochemical processing of silicate rocks to trap CO₂

Abstract

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.