Carbon Sequestration Potential of Tailings and Mined Rock

In 2015, world leaders reached an agreement at the UN Climate Change Conference in Paris to help slow the effects of climate change. This is now known as the Paris Agreement. The goal of the Paris agreement is to limit global warming to below 2 degrees Celsius, and preferably to 1.5 degrees Celsius (United Nations, 2022, par. 2). Since 2015, 193 countries have signed on to the Paris Agreement including Canada, Australia, New Zealand, and the United States (United Nations Treaty Collection, p. 1, 2022), all of which are home to Okane operations.

One of the most significant factors influencing global warming is carbon emissions, as a result of human activity and industrial processes (Bullock, par 2, 2021).

In order for global warming levels to drop to those defined in the Paris Agreement (<2 degrees Celsius), governments across the world are attempting to find ways to reduce carbon emissions and help curb global warming levels.

Carbon heavy industries, like the mining industry, are actively looking for opportunities to help in the fight to reduce carbon emissions while meeting global mineral and metal demands. An option that is being explored, is the potential sequestration of carbon dioxide (CO₂) through natural weathering processes using tailings or mined rock material.

As Gregg Dipple, a professor and researcher at the University of British Columbia who has been researching carbon mineralization for 20 years says, “It’s technically easily feasible to have mines that are operating net negative in terms of CO₂emissions” (Hiyatel, 2021, par. 5).

Enhanced Weathering

The idea behind using enhanced weathering to remove CO₂ from the air essentially involves speeding up the natural weathering process that occurs between dissolved minerals, water, and atmospheric CO₂. Over time, CO₂ in the air (equation 1) reacts with carbonate and silicate rock, releasing bicarbonate and cations (equations 2 & 3). This process stores atmospheric carbon and increases the alkalinity of contact waters which make their way to the ocean, where the alkalinity also helps to prevent ocean acidification.

The reaction of a carbonate rock and silicate rock with mineral weathering is shown in the chemical formulas below (Bullock, par. 4, 2021):

Dissolution of atmospheric CO₂: CO₂+H₂O⇌H₂CO₃ (1)
Carbonate rock and mineral weathering: H₂CO₃+CaCO₃→Ca₂⁺+2HCO₃^– (2)
Silicate rock and mineral weathering: 2H₂CO₃+CaSiO₃→Ca²⁺+2HCO₃⁻+SiO₂+H₂O (3)

The natural weathering process is slow and occurs over a period of thousands of years (Bullock, par. 5, 2021), which would be far too long to make timely impact on CO₂ levels worldwide. The enhanced weathering process would speed up the above reactions to a rate that would see enough carbon being sequestered to help lower carbon levels in the atmosphere.

Carbon Mineralization

Another potential sink for carbon that can be utilized by the mining industry includes carbon mineralization. Similar to enhanced weathering, minerals in tailings may release cations (e.g., Ca²⁺, Fe²⁺, and Mg²⁺) that react with atmospheric CO₂ under the right conditions to precipitate out minerals that store carbon sourced from the atmosphere in a mineral form where it is very stable. Carbon mineralization may have other positive benefits besides carbon sequestration, including stabilizing tailings storage facilities and dust reduction.

The Application of Enhanced Weathering or Carbon Mineralization to Remove Carbon

The material that has the highest potential to support the weathering process is mined rock with silicate and carbonate properties, finely ground to a dust/gravel like material. Mines that produce waste material with silicate and carbonate properties are generally mines that produce iron, gold, or nickel. Mines that produce significant amounts of tailings with the right geochemistry could also act as a good material candidate for this process, as the rock in tailings is already finely ground.

Theoretically, once enough mined rock or tailings are available, the material could be finely crushed and spread onto large areas of land, beaches, or even sea surfaces (Geoengineering Monitor, 2021, par. 1). This would increase the reactive surface area allowing to accelerate the weathering reactions and remove more carbon dioxide from the atmosphere. While this carbon dioxide removal (CDR) technique is relatively theoretical for the time being, it’s estimated that this has potential to remove one billion tonnes of carbon dioxide from the air every year (Geoengineering Monitor, 2021, par. 1) based on normal weathering process rates.

Passive Carbon Removal Hurdles

This CDR technique remains largely theoretical as there are still many factors to consider before it can be trialed on an industrial scale. For example, there remains potential negative environmental impacts that are associated with spreading large amounts of crushed mined rock or tailings over land or sea areas. Much like on a mine site, mined rock or tailings needs to be properly managed to mitigate the risk of metal leaching into the surrounding environment. This risk remains with application of this CDR technique. If the material being spread is not properly managed, it could increase the risk of water contamination and environmental impact.

Other hurdles to climb include the sheer volume of material needed to make this CDR option viable. It takes around 2 tonnes of suitable rock material to remove 1 tonne of carbon dioxide from the air (Geoengineering Monitor, 2021, par. 11). This would likely mean having to increase the amount of disturbed land through mining operations. The material movement and transportation requirements themselves could also prove costly in carbon emissions.

While these seem like significant hurdles to climb to make carbon removal using mined rock or tailings viable, innovation is at the core of the mining industry. The constant pursuit to improve drives the mining industry. Companies like Okane are always looking for ways to Help Create a Better Tomorrow and will try and figure out ways to help our clients identify feasible opportunities for carbon reduction and sequestration.

How Okane Can Help

While carbon sequestration through enhanced weathering or mineralization are still a theoretical application, it doesn’t mean it’s not a potential opportunity for the mining industry. Okane’s expertise with GeoStudio, gas transport modelling, and other modelling tools such as MIN3P presents an opportunity to quantify CO₂ sequestration potential. We are experienced in modelled diffusive and advective gas transport and consumption of oxygen (O₂) in tailings and mine rock stockpiles to understand acidity generation from oxidation reactions. Similar modelling frameworks can be applied to estimate the potential for CO₂ uptake under varying site conditions.

Reach out to us to learn more about the potential opportunities for carbon management at your site.

www.okaneconsultants.com/contact-us/

References

Bullock, L. & James, R. & Matter, J. & Renforth, P. & Teagle, D. (July 2021). Global Carbon Dioxide Removal Potential from Metal and Diamond Mining. Frontiers in Climate. Retrieved from: fclim-03-694175.pdf

Geoengineering Monitor. (Jan 2021). Enhanced Weathering (Marine and Terrestrial). Geoengineering Technology Briefing. Retrieved from: https://www.geoengineeringmonitor.org/wp-content/uploads/2021/04/enhanced-weathering.pdf

Hiyatel, A. (June 2021). Using Tailings to get to Net Zero. The Canadian Mining Journal. Retrieved from: https://www.canadianminingjournal.com/featured-article/using-tailings-to-get-to-net-zero/

United Nations. (October 2022). The Paris Agreement. United Nations Climate Action. Retrieved from: https://www.un.org/en/climatechange/paris-agreement#:~:text=Today%2C%20193%20Parties%20(192%20countries,strengthen%20their%20commitments%20over%20time

United Nations Treaty Collection. (October 2022). 7. D Paris Agreement. Retrieved from: https://treaties.un.org/Pages/ViewDetails.aspx?src=IND&mtdsg_no=XXVII-7-d&chapter=27&clang=_en