According to one of the world’s leading carbon removal buyers, the sector’s future lies in piles of industrial waste.
When Frontier, the Stripe-led coalition of carbon removal supporters, announced its latest $915 million funding commitment, it took the opportunity to lay out the five technologies it views as most promising. I was familiar with four of them — ocean alkalinity enhancement, biomass carbon removal and storage, enhanced rock weathering, and direct air capture. Heatmap has covered them all. But the name on the very top of the list stumped me: surficial mineralization.
It sounds technical, and like all methods of carbon removal, it is — sort of. The idea is to take advantage of the tailings ponds and slag heaps left behind by the mining and steelmaking industries. These piles of calcium- or magnesium-rich debris naturally capture and store carbon from the air — not enough to change the trajectory of our warming planet without any human intervention, but managed well, they could one day capture carbon at a significant scale.
How significant, exactly? While there’s been very little action in the space to date, Frontier says surficial mineralization has the potential to remove over 10 gigatons of carbon from the atmosphere per year — as much or more than any other pathway — at an eventual cost of $80 to $120 per ton. That would put it among the cheapest approaches on Frontier’s list, in part because those heaps of industrial waste alone could absorb anywhere from a gigaton to 4 gigatons of carbon before there’s a need to mine rocks solely for carbon removal purposes.
“The beauty of surficial mineralization is twofold,” Hannah Bebbington Valori, who heads the Frontier coalition, told me. “One, we are working with an abundant source of highly reactive rock, and so there is a significant opportunity for carbon dioxide drawdown. And two, it is carbonating in place, and so sufficient mineralization technologies can be considered closed system approaches, and have generally more straightforward measurement reporting and verification infrastructure.”
At a chemical level, the process resembles other carbon removal pathways Frontier champions, such as enhanced rock weathering and ocean alkalinity enhancement. All three rely on alkaline minerals reacting with moisture and ambient carbon dioxide to form stable carbonate compounds that permanently lock away the gas. The difference is exactly where this reaction takes place: While surficial mineralization contains it to waste piles at industrial sites, the other approaches disperse the reaction across open, difficult-to-monitor systems such as farmland soils and the ocean.
That makes measurement, reporting, and verification — known as MRV — far more challenging and expensive for ocean- and soil-based systems, as scientists must track carbon uptake across ecologically complex environments where countless biological and chemical processes are unfolding simultaneously. These intersecting processes makes it difficult to demonstrate that human intervention was responsible for any given ton of carbon removed, as opposed to natural variability. MRV for these pathways thus relies heavily on modeling, which can never provide the same level of certainty as direct measurement.
Surficial mineralization, however, can be measured much more directly. On-site sensors continuously monitor CO2 concentrations above mine tailings or steel slag, providing a real-time signal of how quickly and to what degree the materials are drawing down carbon. Scientists can then validate these measurements in the lab by comparing physical samples of the material taken before and after the reaction, quantifying exactly how much solid carbonate formed as a result of various engineered interventions. The primary tool for this is X-ray diffraction — a well-established geological technique that identifies a sample’s mineral composition like a chemical fingerprint, making it possible to directly measure how much carbon the material locked away.
Don’t mistake the relative simplicity of the MRV framework for evidence that surficial mineralization is a proven carbon removal pathway — the reality is far from it. While mineralization may look simpler than, say, direct air capture, which typically uses giant fans and specialized sorbents to pull CO2 from the air, there are very few companies working in this space today. All are extremely early stage, and the time and capital required to secure feedstock partnerships, gain site access, and acquire necessary industrial equipment remain significant barriers to getting these projects off the ground.
Why is this heavy equipment needed in the first place? Because these waste piles won’t do much carbon capture work if they’re simply left untouched. That’s because the minerals at the pile’s surface will begin to slowly carbonate, eventually becoming fully saturated and acting as a seal that blocks carbon from reaching the reactive minerals below. As yet there’s no consensus on how to most quickly and cost-effectively break through this natural process to maximize carbon uptake — companies are testing a range of approaches, from crushing and spreading material to maximize air exposure (similar to enhanced rock weathering) to actively churning piles of waste to constantly reveal fresh reactive surfaces.
“Understanding exactly what is the best system to use to maximize your carbon removal efficiency and minimize your cost — this is what we need real-world deployment to do, and to understand,” Bebbington Valori told me.
One of the seed-stage startups Frontier has supported with a small pre-purchase agreement, Arca, spun out of the University of British Columbia to commercialize its approach to carbon removal from mine tailings. The company’s focus is ultramafic waste — magnesium- and iron-rich rock that locks away carbon dioxide as stable magnesium carbonate. “My pathway for interest on that was knowing that there was already about 2 billion tons of ultramafic mine waste sitting on the surface of the Earth in Canada alone,” Greg Dipple, Arca’s co-founder and head of science, told me.
Arca proposes to increase the surface area available for carbon capture in two ways. The first is by using customized robots to continuously till and churn tailings piles, constantly exposing fresh feedstock to the air to maximize carbon uptake before the next layer of tailings is deposited on top. That strategy, Dipple told me, “can give us a five- to 10-fold increase in the rate of CO2 capture” at active mine sites.
It successfully demonstrated this approach in an 18-month pilot project with Australian mining giant BHP at an active mine in the country's Northern Goldfields region where Arca says it increased the tailings’ mineralization rate by an order of magnitude. But the startup plans to push the efficacy of its tech further through what it calls “mineral activation.” This technique uses industrial-scale microwaves to heat the minerals rapidly enough to drive off the water that’s chemically bound within their crystal structure. This essentially blows apart the minerals from the inside out, exposing fresh magnesium-rich surfaces primed to react with carbon dioxide. The expected result is faster mineralization and more carbon captured per ton of mine tailings — but the startup has yet to test it in the field.
“Essentially we’re making microwave popcorn out of silicate minerals,” Dipple explained. “The microwaves cause the release of that water in the same way that when you make popcorn, you’re essentially boiling the water out of the center of the kernel, and that’s what blows the kernel up and creates this high surface area.” The idea is to eventually integrate this step into the mine’s tailings processing stream, with minerals moving through the giant microwave before they’re deposited at the storage facility.
Dipple told me that mineral activation will be a core part of Arca’s future projects, including those intended to fulfill the company’s 10-year carbon removal offtake agreement with Microsoft. Signed last October, the deal calls for Arca to deliver nearly 300,000 metric tons of carbon removal to the software giant.
While no other startup in the space has landed an offtake agreement of that scale, several have secured early backing from Frontier through pre-purchase agreements. One of them, Karbonetiq, is working to capture carbon from steel slag, the calcium-rich byproduct of steel production that accumulates in large piles at processing sites. Like the magnesium-rich minerals in mine tailings, calcium compounds in steel slag naturally react with moisture and carbon dioxide to form a stable calcium carbonate — a.k.a. limestone — permanently locking up the CO2.
Unlike mine tailings however, slag doesn’t begin as a fine powder. Instead, the molten byproducts poured off from high-temperature steel furnaces cool into chunks the size of large rocks, leaving only their outer surfaces exposed to the air and able to react with CO2. Karbonetiq’s strategy is essentially to crush and disperse those rocks to increase their reactive surface area. As the company’s commercial vice president, Luke Rondel, explained, “We crush [the slag] down so you get smaller particle sizes. We then spread that out in a field of material, and we till that material with a tractor and plow, which is just turning over new surfaces.”
Each pathway has its advantages — while Arca’s magnesium-rich mine tailings are the most abundant feedstock, Rondel told me that the calcium-based reactions in slag happen significantly faster. For its part, Frontier hopes to test and evaluate a range of approaches at its new Surficial Mineralization Hub in Quebec, which it announced at the end of April. Located at a former asbestos mine, the hub will give participating startups access to “10,000 tons of serpentinite tailings and space for pilot scale testing,” Bebbington Valori told me, as well as local labs with specialized equipment.
This should eliminate some of the hurdles facing the nascent sector, chief among them being access to the right kinds of reactive rocks. Small startups “really need to either partner with large academic labs or with large mining companies to get access to that feedstock,” Bebbington Valori told me — a difficult and expensive proposition for a company that’s just getting off the ground.
While Frontier has yet to announce the cohort of participating startups, both Arca and Karbonetiq told me they hope to test their technology there, with the latter planning what would be one of its first mine tailings pilots through the program. Ultimately the goal is to generate the proof points needed to give both the startups and Frontier a clearer roadmap for which approaches can realistically scale — and what kind of support they’ll need to get there.
It certainly won’t be a straightforward process — bringing new technology into old-school industries never is — and the economics will only start to pencil if their operations reach meaningful scale. In theory, mining companies could benefit from hosting surficial mineralization projects, whether through site access fees, outsourcing elements of waste management, or even critical minerals recovery. Miners could even develop and scale the technology themselves, if they so desire. But the sector has historically been reluctant to adopt new tech. “The classic quote is, in mining you always want to be No. 2, you don’t want to be the first one,” Dipple told me. “You don’t want to put up a $2 billion plant that doesn’t work.”
So like nearly everything in the carbon removal space, early execution is falling to the startups that aren’t afraid of a little risk. “They’re watching for sure,” Dipple said of the mining industry at large. “But they want to be No. 2. We’re going to have to be No. 1.”
