Carbon Dioxide emissions from primary mineral and metal production were estimated to make up 10% of global CO₂emissions in 2018 [1] and CO₂ emissions from concrete production are estimated to form 8% of global CO₂ emissions in 2021 [2]. With ambitious international climate targets set for 2050, it is no small feat to reduce these emissions, especially given that the demand for metals and building resources is only set to increase. Both metal production and mining of aggregate for concrete require the extraction and processing of rock from the Earth’s crust. Extracting the valuable metals and minerals, and processing aggregate into the required sized particles, involves grinding and crushing very large volumes of rock down into smaller and smaller particles, a process which takes up considerable energy.  Our research shows that if this crushing took place in an atmosphere of CO₂ instead of air, the energy that is already being expended in crushing these rocks could also trap CO₂ during rock processing. If adopted globally, the amount of CO₂ that would be captured during rock crushing each year could be as much as 0.5% of global CO₂emissions, which is equivalent to the annual CO₂ uptake from planting a mature forest the size of Germany.

Our recent work published in Nature Sustainability (available here https://www.nature.com/articles/s41893-023-01083-y) investigates the mechanochemical capture of CO₂, that is, the capture of CO₂ by chemical reactions induced by applying a mechanical stimulus, in this case grinding. We show that mechanochemical capture of CO₂ into rock particles is feasible when combined with current mineral and aggregate processing techniques. Rocks composed of multiple minerals (most metamorphic and igneous rocks) capture more carbon in a stable form than rocks made of a single mineral. Our research tests two silicon-containing rock types, basalt, and granite, which have calcium and magnesium contents that are high, and low, respectively. Our experiments show that basalt and granite trap a similar amount of CO₂. This finding is important as it means the mechanochemical trapping mechanism does not require relatively rare rocks with very high calcium and magnesium contents, unlike other geological carbon capture technologies that are using such rocks to chemically trap CO₂. This means that mechanochemical carbon capture into rocks can be applied to a much larger variety of rock types allowing it to be more widely adopted in industrial processes.

How does it work?

Rocks are made of individual crystals bonded together. When rock is crushed, the bonds that hold the crystals together, and well as bonds within the crystals, get fractured. In other words, they are stretched, deformed, and broken. This process generates microscopic holes as well as positive and negative charges on the surface of the crystals. These holes allow CO₂ to pass into the crystal lattice and become trapped within it. The charges also act to attract and stick CO₂ to the surface of the crystals. Equivalent processes can directly trap CO₂ gas onto the surface of a rock, but this requires temperatures in the range of 400-600°C. Our experiments show that trapping during mechanical crushing is efficient at room temperature. This means that if current rock crushing processes are done in an atmosphere of CO₂ gas, instead of air, large amounts of CO₂ can be captured without the need for additional energy (i.e., no more energy is used than in the normal crushing process).  

For the mechanochemical CO₂ trapping process to be viable we needed to ensure the CO₂ would not be released when the crushed rocks are exposed to the environment (i.e., when stored in mining waste heaps, or once aggregate has been mixed with cement to form concrete). To answer this, we performed laboratory experiments which demonstrate that the CO₂ trapped in particles crushed in an atmosphere of CO₂ is not released when mixed with water. For comparison, we tested individual minerals which released up to 93% of their trapped CO₂ when submerged in water. We then slowly heated crushed particles up to 1000°C and measured the CO₂ released as they were heated. We found that for samples that had been crushed in CO₂ instead of air, CO₂ is not released at temperatures lower than 300°C. This means that the CO₂ is firmly trapped within the particles and will not escape under temperatures typical in any climate on Earth.

We still need to understand the conditions that will optimize maximum CO₂ trapping, but from our experiments we predict that mechanochemical carbon trapping can retain at least 0.5 MtCO₂ for every 100Mt of rock that is crushed. Global data are not available for the volumes of igneous and metamorphic rock crushed worldwide. However, based on our calculations for hard rock aggregate production in Norway (where data are published) we could trap as much as 0.5% of global CO₂ annually. This is roughly equivalent to the CO₂ that would be removed from the atmosphere by planting a mature forest the size of Germany.

[1] M. Azadi, S.A. Northey, S.H. Ali, et al. Transparency on greenhouse gas emissions from mining to enable climate change mitigation. Nat. Geosci. 13, 100–104 (2020).

[2] York, I.N. and Europe, I., 2021. Concrete needs to lose its colossal carbon footprint. Nature, 597(7878), pp.593-594.