What is Mineral Carbonation?

Mineral carbonation, also known as carbon mineralization, is a reaction where certain elements from rocks naturally react with carbon dioxide to form carbonate minerals – removing CO2 from the atmosphere.

In nature, rocks are transformed by the natural process of weathering. Geologic structures like the Grand Canyon are formed through millions of years of weathering. Weathering occurs in three different ways: physical, chemical, and biological.

When certain rocks are chemically weathered, through natural exposure to air and rainwater, they dissolve, releasing a portion of their elements into rainwater, some of which then bind with CO2 to form new minerals, as part of the global silicate-weathering process. The elements that are capable of binding with CO2 are members of the divalent metals group and the best suited of these divalent metals are also part of the alkaline earth group, the most abundant of which are Calcium and Magnesium. Silicate minerals, which typically contain calcium and magnesium, are common, making up 95% of the earth’s crust [1], and their natural weathering is thought to remove over 150 million tonnes of CO2 from the atmosphere each year [2]. Over millions of years, this process helps regulate the Earth’s climate by removing and storing atmospheric CO2.

In more detail, natural chemical weathering releases divalent metal ions from silicate minerals, and these ions bind with CO2 to form a new, solid carbonate mineral through the mineral carbonation reaction – safely removing and permanently storing carbon dioxide from the atmosphere. Exterra’s approach – known as engineered carbonation – parallels this natural process but uses technology to minimize the time it takes for a rock to mineralize CO2. This approach is similar to enhanced rock weathering (ERW) but occurs entirely in an engineered system, allowing for greater process control and simplified monitoring, reporting, and verification (MRV). Both engineered carbonation and ERW are specific strategies within the broader category of carbon mineralization.

Unlike natural processes, which occur over geologic timescales, technology can be used to carry out the mineral carbonation reaction in a matter of hours. For the reaction to occur, divalent metals must be freed from the chemical structure of the rock, CO2 must be available, and the reaction environment must not be acidic. Speeding up large-scale mineral carbonation requires two major components: large quantities of well-suited rocks and an activity/process that accelerates the mineral carbonation reaction. 

The proportion of divalent metals contained within a rock feedstock determines the amount of carbon which can be mineralized, while the form of those divalent metals drives the efficiency of the reaction. Even though divalent metals are abundant in many rocks, the chemical structure of those rocks often requires significant effort to extract the metal from the mineral. Potential feedstocks suitable for ex-situ mineral carbonation include:

• Natural minerals – including olivine, serpentine, and wollastonite
• Tailings – from mines including chrysotile, nickel, kimberlite, platinum group metals
• Metallurgical slags – including steel and nickel refining slags

An ideal feedstock material is available in significant quantities and contains a high proportion of labile divalent metals. Accelerating mineral carbonation requires exposing rocks to CO2 and managing the conditions of the reaction environment. CO2 exposure can come directly from the air or can be concentrated beforehand, through existing CO2-rich gases from industrial processes. When CO2 is supplied by air alone, mineral carbonation occurs slower than when the CO2 is concentrated. In engineered systems, other factors like particle size, temperature, pressure, water, and pH can also be optimized to accelerate mineral carbonation.

Exterra Carbon Solutions partners with mines and mineral processors to develop projects that generate high-quality carbon credits through ex-situ engineered carbonation. Our projects implement changes to the handling and treatment of mineral waste that go beyond carbon storage to realize progress towards multiple aspects of sustainable development, including the circular economy, job creation, and ecologic benefits. Additionally, because the conditions of the reaction can be controlled, engineered carbonation projects can mineralize carbon dioxide from dilute sources including directly from flue gas. 

We are currently developing carbon accounting methodologies with leading standard bodies to reliably and transparently determine the carbon benefit of accelerated ex-situ mineral carbonation projects. The methodology will enable projects to generate and sell credible carbon offsets and, ultimately, help reduce global CO2 emissions.

1. Egger, A. E. (2006, April 11). The silicate minerals. Visionlearning; Visionlearning, Inc. https://www.visionlearning.com/en/library/Earth-Science/6/The-Silicate-Minerals/140
2. Hilley, G. E., & Porder, S. (2008). A framework for predicting global silicate weathering and CO2 drawdown rates over geologic time-scales. Proceedings of the National Academy of Sciences of the United States of America, 105(44), 16855–16859. https://doi.org/10.1073/pnas.0801462105
3. Kelemen, P. B., McQueen, N., Wilcox, J., Renforth, P., Dipple, G., & Vankeuren, A. P. (2020). Engineered carbon mineralization in ultramafic rocks for CO2 removal from air: Review and new insights. Chemical Geology, 550(119628), 119628. https://doi.org/10.1016/j.chemgeo.2020.119628
4. Renforth, Phil. (2019). The negative emission potential of alkaline materials. Nature Communications, 10(1), 1401. https://doi.org/10.1038/s41467-019-09475-5
5. Bullock, L. A., James, R. H., Matter, J., Renforth, P., & Teagle, D. A. H. (2021). Global carbon dioxide removal potential of waste materials from metal and Diamond mining. Frontiers in Climate, 3. https://doi.org/10.3389/fclim.2021.694175