In July 2019, Gregory Dipple, a geologist at the University of British Columbia, Vancouver, hopped on a 119-seat charter flight in Yellowknife, Canada, and flew 280 kilometers northeast to the Gahcho Kué diamond mine, just south of the Arctic Circle. Gahcho Kué, which means “place of the big rabbits” in the Dënësu¸łinë language of the region’s native Dené or Chipewyan people, is an expansive open pit mine ringed by sky-blue lakes. There, the mining company De Beers unearths some 4 million carats’ worth of diamonds annually. But Dipple and two students weren’t there for gems. Rather, they were looking to use the mine’s crushed rock waste as a vault to lock up carbon dioxide (CO2) for eternity.
At Gahcho Kué, Dipple’s team bubbled a mix of CO2 and nitrogen gas simulating diesel exhaust through a grayish green slurry of crushed mine waste in water. Over 2 days, the slurry acquired a slight rusty hue—evidence that its iron was oxidizing while its magnesium and calcium were sucking up CO2 and turning it into to carbon-based minerals. The CO2-hungry waste from the diamond mine is an exotic deep-earth rock, shot up to the surface in the volcanic eruptions that bring up diamonds. But a wide array of rock and mudlike wastes from mining, cement and aluminum production, coal burning, and other large-scale industrial processes share a similar affinity for the greenhouse gas. Known as alkaline solid wastes, these materials have a high pH, which causes them to react with CO2, a mild acid. And unlike other schemes for drawing excess CO2 from the atmosphere, these reactive rocks can both capture the gas and store it, locked away permanently in a solid mineral.
“The potential is real,” Dipple says. “It will make an important contribution to lowering CO2.”
If he and others can make the scheme practical, it could address two environmental problems at once. Today, mines and industry generate some 2 billion tons of alkaline solid wastes every year, and more than 90 billion tons are stored behind fragile dams and heaped in waste piles, a threat to people and ecosystems. In 2010, for example, a dam failure in Hungary released a 2-meter-high wall of red mud—an alkaline waste from aluminum production—that killed 10 people and buried villages. And caustic leachates from mountains of steel slag waste have wiped out fish populations in Pennsylvania and the United Kingdom.
Reacting these wastes with CO2 from the air could make them safer by solidifying them—and at the same time help the world avert climate disaster. In the 2015 Paris climate agreement, most of the world’s countries resolved to limit climate warming to below 2°C. For that to happen, the Intergovernmental Panel on Climate Change (IPCC) has determined, cutting greenhouse gas emissions won’t be enough. Countries will also need to employ “negative emissions technologies” (NETs) to pull as much as 10 billion tons (gigatons) of CO2 out of the atmosphere every year toward the end of this century. Possible NETs include planting vast forests, which suck carbon out of the air as they grow; chemically absorbing CO2 from the air or power plant exhaust and pumping it underground; and growing grasses or shrubs, burning them for energy, and capturing and storing the CO2.
But underground storage chambers can leak, and forests can burn. Mineralization is more permanent: Carbon-based minerals, or carbonates, are among the most stable on Earth, adds Siobhan “Sasha” Wilson, a biogeochemist at the University of Alberta, Edmonton. “It’s a really robust place to store CO2,” she says.
And suitable rock waste is plentiful. Start with ultramafic wastes, the calcium- and magnesium-rich rock in which diamonds, along with metals such as nickel, platinum, and palladium are found. A 2019 report on NETs by the U.S. National Academy of Sciences (NAS) described CO2 storage in ultramafic mine wastes as “low-hanging fruit.” Today, some 419 million tons of this and less alkaline “mafic” wastes are produced annually. If fully carbonated, they could lock up 175 million tons of atmospheric CO2 per year. Then there are the alkaline wastes from aluminum, iron, steel, and cement production, which could bring the total up to at least 310 million tons—and by some estimates more than 4 gigatons (GTs)—of CO2 trapped each year. The somewhat less alkaline basalt rock powder generated by coal production could sequester another 2 GTs per year, Phil Renforth of Heriot-Watt University and his colleagues have calculated—meaning alkaline wastes could in principle provide more than half of the negative emissions that IPCC called for.
But there are major hurdles. Governments will need to offer incentives for mineralization on the massive scale needed to make a dent in atmospheric carbon. And engineers will need to figure out how to harness the wastes while preventing the release of heavy metals and radioactivity locked in the material. Still, “Alkaline wastes have tremendous potential,” says Liang-Shih Fan, a chemical engineer at Ohio State University, Columbus. “It’s a potential one should not overlook.
The notion of storing CO2 in minerals isn’t new. Plans to capture the gas from the air or power plant exhaust often call for injecting it into underground rock formations that, like mine waste, react to form carbonates. And certain rocks naturally capture CO2 in a process known as weathering. In Oman, vast ridges of a mineral called peridotite mineralize CO2 from the air, forming white veins resembling marbling in steak. Similar smaller formations dot the globe. Mine wastes behave the same way. In 2014, Wilson and colleagues analyzed mine tailings from the Mount Keith nickel mine in Western Australia and found that the mine’s 11 million tons of tailings produced each year spontaneously react with CO2, locking up about 40,000 tons of the gas. That’s equivalent to about 11% of the CO2 output from the mine’s operations.
Still, weathering is slow, and most alkaline wastes wind up either buried or submerged in ponds, and thus aren’t exposed to air. “It’s a matter of getting those reactions to happen at a faster rate,” says Alison Shaw, a geochemist with Lorax Environmental Services who heads De Beers’s research on mineralizing CO2.
At Gahcho Kué, Dipple and his students tested a way to speed up the process. The mine’s tailings include a wet, siltlike slurry and dry, sandlike grains. Dipple and his students packed a 6-meter-tall column with the greenish slurry and sprayed water on 1 cubic meter of the sand. With both their slurry and dry wastes, they bubbled in a mix of gases—10% CO2 and 90% nitrogen—that matched the exhaust from the local diesel power plant that powers the mine.
The waste soaked up CO2 for as long as 44 hours, they found, converting it into minerals. The newly made magnesium carbonate minerals acted like glue, solidifying the previously free-flowing tailings, much like sand turned to sandstone. Most important, the waste took up CO2 200 times faster than it did through natural weathering, Dipple says.
This summer, he was set to return to Gahcho Kué to scale up the tests and use actual diesel exhaust. But those tests are on hold because of the coronavirus pandemic.
De Beers has funded other projects around the world, Shaw says. For example, Wilson’s team is exploring whether dilute acids speed up weathering. Lab studies suggest the acids could leach magnesium out of mine waste, making it available to react with CO2. Another project, led by Gordon Southam of the University of Queensland, St. Lucia, is adding cyanobacteria to the mix. These photosynthetic bacteria capture CO2 from the atmosphere, and lab studies have shown they speed carbon mineralization. If these efforts work, Shaw says, they could repair mines’ reputation as environmental blights, making them part of a solution to climate change. Anglo American, De Beers’s parent company, has announced it wants to harness alkaline wastes to create the first carbon-neutral mine by 2040.
Diamond mines aren’t the only places where such studies are underway; another is the Woodsreef chrysotile mine in New South Wales in Australia. (Chrysotile is a form of asbestos that is still widely used in building materials in some parts of the world.) Wilson and her colleagues sprayed the mine’s ultramafic rock tailings with dilute sulfuric acid, causing magnesium to leach out. The alkaline tailings then neutralized the acid and locked up CO2 that was bubbled through, as much as 100 times faster than normal weathering.
Jennifer Wilcox, a chemical engineer at Worcester Polytechnic Institute, and her colleagues are pursuing a related strategy at the Stillwater nickel mine in Montana. “The tailings are not particularly reactive,” she says. But CO2 is mildly acidic; bubbling it through the tailings helps release their metals and boosts their affinity for CO2. She and her colleagues are exploring whether adding compounds called oxalates will speed this process further by weakening chemical bonds in the tailings. And they are trying to encourage the growth of CO2-hungry magnesium carbonate crystals by dispersing tiny crystallites of a mineral in the tailings. The crystallites, Wilcox says, are “like a blueprint for making more of what you want.”
Bricks and mortar
Small companies in a nascent industry are using carbon mineralization to capture carbon dioxide (CO2) in construction materials.
|Cambridge Carbon Capture||Fire-retardant materials|
|Mineral Carbonation International||Cement and plasterboard|
|O.C.O. Technology||Construction aggregate|
|Blue Planet||Construction aggregate|
CO2 mineralization could help remediate environmental problems that mining creates, such as the release of heavy metals from pulverized rock. On 1 March in Economic Geology, Wilson and her colleagues reported that techniques that accelerate weathering, such as adding acid, effectively trap heavy metals inside newly formed carbonate minerals, keeping them out of groundwater. Other teams have shown that the carbonates can also trap hazardous residual asbestos fibers in chrysotile mine tailings. “You can lock away just about anything,” Wilson says.
Other industrial wastes, such as red mud from aluminum production and “slags” leftover from making steel and iron, also harbour plenty of chemical reactivity to bind and store CO2. However, according to NAS, fully carbonating these wastes could require building costly plants to speed the reactions.
The rock dust created by pulverizing basalt rock, which is already mined for construction aggregate, could do the job more cheaply, according to Renforth’s team. The researchers suggest adding this dust to agricultural soils around the world, where it would be exposed to the air, could capture up to 2 GTs of CO2 per year. The basalt dust could also fortify soils with nutrients, such as potassium and zinc, depleted by intensive agriculture. And as a bonus, they say, the dust would react with CO2, generating bicarbonate, much of which over time would flow through rivers to the sea; once there, bicarbonate, which is alkaline, could counteract ocean acidification.
In another environmental plus, the NAS panel said, carbonated wastes of all kinds could serve as raw material for concrete and road aggregate. The report noted that replacing 10% of building materials with carbonated minerals could reduce CO2 emissions by 1.6 GTs per year by lowering emissions from cement production. Numerous companies around the globe have already jumped into the field to make and sell the new materials (see table above).
Yet even if large-scale mineralization works, scaling it up will carry daunting costs, both financial and environmental. Quarrying, crushing, and grinding ultramafic rocks would cost only about $10 per ton of CO2 absorbed, Wilson and her team estimate. Moving the rock, stirring it, and other steps to speed mineralization would likely boost the cost to between $55 and $500 per ton of stored CO2. That’s similar to the cost of more traditional direct air capture using liquid amines, which has already gained widespread attention and commercial interest.
But it would take mind-boggling quantities of rock to budge global CO2 levels. According to a report published online on 6 May in Chemical Geology led by Peter Kelemen, a geologist at Columbia University, consuming 1 GT of CO2 would require 10 to 100 GTs of tailings—5 to 50 cubic kilometers of material. That’s enough to bury Washington, D.C., 30 to 300 meters deep—but it could only capture roughly one-fortieth of the CO2 humans spew into the atmosphere every year. “We don’t make anything on the scale that we make CO2,” says Klaus Lackner, a physicist who runs a center at Arizona State University, Tempe, that is evaluating all types of NETs.
All that mining, grinding, and transportation would itself generate CO2, unless it were powered with renewable energy. And if even a tiny bit of the heavy metals from the pulverized rock leached out, mountains of rock waste could risk contaminating groundwater. “Not all rocks are environmentally friendly if you spread them all over,” Lackner says.
“We’re trying to understand the tradeoffs,” Dipple says. “The size of the problem is tens of billions of tons of CO2 per year. The only way to deal with that is to create an industry on the scale of the oil and gas industry. There is more than enough rock to do that. The question is how do you do that in a way that is a net environmental benefit?”
A compromise requiring less land but also less assurance that carbon will remain locked up could come from a hybrid between carbon mineralization and direct air capture. In Chemical Geology as well as a recent patent, Kelemen and his colleagues propose using a mineral called magnesite that, when heated, gives off pure CO2, which could be captured in tanks and pumped underground. That reaction would leave magnesium oxide powder, which when spread thin would rapidly react with CO2 from the atmosphere, re-forming magnesite, completing a cycle that could be repeated over and over. Kelemen and his colleagues calculate that mining and processing 2 GTs of magnesite would enable capture and injection underground of 1 GT of CO2 every year. The cost would be between $24 and $98 per ton of CO2, which is less than traditional direct air capture methods cost. And it would likely require only 4500 to 6100 square kilometers of land, or about four times the size of Gahcho Kué.
Looming just as large as cost is the question of how to entice companies to build a vast carbon-capture industry. Existing government incentives to reduce carbon are little help. The United States offers a tax credit of $50 per ton of CO2 that gets stored underground. California’s low carbon fuel standard also rewards companies that sequester carbon. And carbon taxes in place in 29 countries encourage carbon reductions. But none of those incentives rewards mineralization as a way to lower atmospheric carbon.
There’s reason to hope that could change, says Noah Deich, executive director of Carbon180, a nonprofit firm that is pushing Congress to increase funding and incentives for NETs, including mineralization. If regulators verified mines and other alkaline waste producers as CO2 sequestration sites, Lackner adds, incentives would skyrocket, companies could claim tax benefits, and industry might start to tackle climate change on the grand scale that’s necessary.
To avert the worst damage from climate change, Lackner says, “we need to throw everything we can at it.” Including, perhaps, a lot of rocks.