Destigmatizing tailings in carbon capture

Global electrification of an extensive amount of industrial processes necessitates a counterbalance to the wave of new mining required for the electric vehicles and renewable energy infrastructure to meet carbon neutrality imperatives without doing further damage to the environment or simply displacing problems to other areas of the value stream.

Technologies for capturing carbon dioxide are improving, but the captured CO2 still needs a rock-solid alternative to expensive storage that requires further monitoring, as it may not stay put and cause unintended environmental complications over time.

More than 50 billion tons of rock are processed worldwide every year in construction and mining. Various grant prizes, governments, universities, and corporate-funded research groups have been dedicated to expediting the processes of reducing, eliminating and sequestering carbon emissions by making the waste rock from these sectors work harder.

Carbon sequestration is the process of removing carbon from the atmosphere and storing it in another semi-permanent form, as is naturally performed over time by forests. Done artificially by humans, it has historically captured only a fractional percent of emissions with indeterminate long-term efficacy.

New studies, however, show that mining and crushing rocks can significantly enhance CO2 mineralization, which is the formation of stable carbonates through the chemical interaction of CO2 with minerals common in the waste rock from mining, as well as crushed rock at aggregate operations. It is a safer way to store CO2 without concerns over long-term monitoring and liability issues, as in geological storage.

"In the future, we hope that the rock used in concrete to construct high rise buildings and other infrastructure such as roads, bridges and coastal defenses will have undergone this process and trapped CO2, which would otherwise have been released into the atmosphere and contributed to global temperature rise," said Rebecca Lunn, professor of environmental engineering at the University of Strathclyde in Glasgow, Scotland.

Mineral carbonation

Lunn is the principal investigator of new research partly funded by the Engineering and Physical Sciences Research Council's (EPSRC) Doctoral Training Awards Grant in the United Kingdom that suggests around 0.5% of global carbon emissions could be captured by crushing polymineralic rocks commonly used in construction within an effluent stream of CO2 gas in an otherwise normal process.

In an article published this month in Nature Sustainability research proposed that a similar amount of processing could just as easily trap CO2 permanently and with very little difference in energy expended. The resulting powders would then be stored or used for construction.

"This breakthrough research from the University of Strathclyde, which EPSRC has proudly played a part in funding, is truly revelatory. It points to a new process for the construction industry that could significantly reduce global carbon emissions and help us meet our net zero goals," said Lucy Martin, deputy director for cross-council programs at EPSRC.

CO2 capture techniques have improved through increasing demand over the past decade, and mineralization is one of the most efficient methodologies, given that it is thermodynamically favorable with a relatively low overall cost and energy expenditure.

"If the technology was adopted worldwide in aggregate production, it could potentially capture 0.5% of global CO2 emissions – 175 million tonnes (metric tons) of carbon dioxide annually," said Lunn.

This is roughly the equivalent to planting a forest of mature trees the size of Germany.

"Future research can pin this down, as well as optimize the process to trap more carbon," the University of Strathclyde professor added.

Milling experiments show that polymineralic rocks such as granite and basalt, irrespective of carbonate-forming metal content, are more efficient at trapping CO2 than individual minerals.

"There are many industries for which there is currently no low carbon solution, and this research will allow direct gas capture of CO2 from hard to decarbonize industries, where a solution is not going to exist by 2050," said Lunn.

Igneous rock with a high magnesium and iron content, called ultramafic, is what most of Earth's mantle is composed of and happens to be among the largest CO2-storing reservoirs on the planet. The carbon-absorbing potential of these rocks, however, is limited when they are underground. Digging these rocks up and crushing them, however, exposes them to the atmosphere and the CO2 in it.

Research at the University of Strathclyde shows that CO2 is surprisingly absorbed mostly into the crystalline structures between different minerals. Leaching experiments on the resulting milled powders additionally demonstrated that CO2 trapped in single minerals is mainly soluble, whereas CO2 trapped in polymineralic rocks under ambient temperature conditions captures thermally stable, insoluble CO2. The reaction traps the greenhouse gas more effectively, resulting in metal-carbonate sediment – a solid, cement-like mineral that remains in a benign state indefinitely.

"Now we know that CO2 trapping in most hard rock can be done in a lab, we need to optimize the process and push the limits of how much can be trapped through the crushing technique. We then need to understand how this process can be scaled up from the lab to industry, where it can reduce global CO2 emissions," said Mark Stillings, a co-investigator on the study. "If this process was applied, the CO2 footprint associated with building houses and public infrastructure could be greatly reduced, helping to meet global objectives to combat climate change."

Electrochemical mineralization

Newmont, the world's leading gold mining company, is working in partnership with the National Renewable Energy Laboratory (NREL), the U.S. Department of Energy's primary national laboratory for renewable energy and energy research, to explore ways to turn mine waste called tailings into CO2-absorbing assets.

The innovative process being investigated through this partnership is called rapid electrochemical mineralization to form dolomite, or REMineD.

Carbon capture, utilization and sequestration in tailings can produce materials that replace traditional concrete used in construction, which accounts for between 4-8% of total global CO2 emissions coming from its construction and infrastructure.

The REMineD method will be explored in a three-year $4.38 million project co-funded by the DOE Office of Fossil Energy and Carbon Management Technology Commercialization Fund. The project aims to accelerate development of CO2 removal technologies.

Frank Roberto, director of processing at Newmont, says carbon sequestering in tailings is integral to meeting global climate goals for the mining industry.

"Waste rock and tailings are the largest component of residues from our mining operations, and the work for direct air capture of CO2 through tailings carbonation provides a unique opportunity to reduce our and others' emissions throughout the value chain," he noted.

REMineD resources can be deployed on-site, which aids in faster and more efficient development of dolomite aggregate from several varieties of tailings. The process can also create new revenue streams from further recovery of valuable rare earth elements and production of more sustainable building materials instead of waste.

The metals and mining industries generate roughly 8% of the global carbon footprint, while power consumption in the mining industry contributes almost a gigaton of CO2 by itself.

CO2 capture studies, such as those being carried out by the University of Strathclyde and NREL, are critical tools in permanently reducing industrial carbon emissions worldwide, especially where there is too little financial backing or infrastructure to make sweeping changes in this decade.

The research into these two carbon sequestering technologies aims to lower mining's carbon footprint by transforming waste material into a CO2-absorbing asset.