Mantel raises $18.3M to make molten-salt carbon capture repeatable

Why it matters

Mantel is testing whether better materials and reusable process heat can beat established amine systems. The $18.3M round funds the industrial proof needed before much larger project financing can follow.

Mantel Capture has raised $18.3 million to turn its high-temperature carbon-capture chemistry into standardized industrial plants, according to an SEC filing submitted Monday.

The Cambridge, Massachusetts-based startup, led by co-founder and CEO Cameron Halliday, sold $18,336,764 of equity to seven investors after the offering began on July 10th. Mantel set a maximum of $24,999,995, leaving room for another $6.7 million under the filing. Mantel declined to disclose its revenue range, and the filing does not include a valuation.

In a separate announcement, Mantel described the financing as an $18 million strategic investment from Constellation Technology Ventures and Azimut Investments, alongside existing backers. The company said the deal brings its total funding to $50 million. Axios characterized the financing as an extension to Mantel's Series A.

Halliday founded Mantel in 2022 with COO Danielle Rapson and CTO Sean Robertson (@rubes2929), spinning the technology out of MIT's Hatton Research Group. Halliday had spent years watching prospective high-temperature capture materials deteriorate through repeated use. The experiment that changed the founders' direction came in 2019, when a lithium-sodium borate salt captured more than 95% of CO2 and showed little degradation after 1,000 cycles, according to MIT's account of Mantel's origins.

The material remained stable because it became liquid under operating conditions, avoiding the cracking that undermined solid high-temperature sorbents. Robertson brought a complementary background in molten salts: his MIT doctoral work studied the thermophysical properties of fluoride salts after earlier training in chemistry and nuclear engineering.

That discovery gave Mantel a credible piece of chemistry. The new financing is meant to address the harder commercial problem: building the same system repeatedly inside facilities that were never designed to host carbon-capture equipment.

The product is an industrial heat system

Most post-combustion capture systems work after fuel has been burned. Flue gas is treated with a solvent or sorbent that binds to CO2, after which energy is applied to release a concentrated stream for transportation, reuse or underground storage.

The energy required to regenerate the capture material has weighed on the economics of the entire category. Conventional aqueous amine systems typically cool exhaust before absorption and consume steam to release the captured CO2. Those processes are established and widely understood, but they impose additional equipment, energy demand and operating costs on the host facility.

Mantel's molten-borate process is designed to operate inside the high-temperature environment of boilers, kilns, furnaces and gas turbines. Hot salt absorbs CO2 at around 500 degrees Celsius. Mantel then heats the CO2-rich salt to roughly 800 degrees Celsius in a desorber, releasing a concentrated stream and circulating the regenerated material back through the system.

Heat generated and transferred through the loop is recovered as steam. That steam can return to the host plant, feed another industrial process or generate electricity. Mantel is effectively asking customers to evaluate the system as carbon equipment and thermal infrastructure in the same capital decision.

Mantel claims the design produces a CO2 stream above 99.9% purity, captures roughly 95% of emissions across its intended applications and reduces energy losses by 97% compared with conventional capture. It projects a capture cost below $50 per tonne once deployed at scale. Those figures appear in Mantel's May 2026 technical paper and remain company projections rather than results from a full commercial plant.

The distinction matters. Carbon-capture projects accumulate costs outside the core separation chemistry, including flue-gas preparation, compression, pipelines, storage, permitting, construction and financing. Mantel does not provide transportation or sequestration. Its economic case rests on cutting the capture plant's energy penalty and returning useful steam to customers.

Mantel is entering a crowded race around the incumbent process

Amine-based capture remains the reference point because large engineering groups know how to design it, obtain performance guarantees and integrate it into industrial sites. Linde, Honeywell and SLB Capturi offer capture systems backed by decades of process-engineering experience.

SLB Capturi, formed from SLB and Aker Carbon Capture, combines amine-based modular plants with newer solvent and sorbent work. Its advantage is delivery experience: the business can sell engineering, integration and operating history alongside the underlying chemistry.

Startups and growth-stage companies are attacking different parts of the same cost structure. Carbon Clean uses proprietary solvents and rotating packed beds to shrink the towers found in conventional plants. Carbon Clean says its prefabricated CycloneCC units can handle 75 to 855 tonnes of CO2 per day per train. Its strategy is to keep liquid-solvent capture while reducing footprint, steel requirements and on-site construction.

Svante replaces liquid absorption towers with structured filters coated in solid sorbents. Its rotary machines use rapid temperature-swing adsorption, with low-grade heat regenerating the filters. Svante and Samsung E&A are developing standardized, skid-mounted plants, another attempt to replace bespoke construction with manufactured modules.

Mantel's position is narrower and potentially valuable. Its strongest fit is a facility where high temperatures and steam are already central to production. A pulp mill, thermal oil operation or power station can use the recovered heat directly. A site with colder exhaust, irregular operations or limited demand for steam may see less of the economic advantage that Mantel advertises.

That focus gives Halliday's team a way to avoid competing application by application with every solvent and sorbent supplier. Mantel is building around a specific industrial condition: customers that cannot easily electrify their heat and would benefit from receiving steam back from the capture process.

The competitive pressure has shifted beyond capture rates in controlled tests. Carbon Clean is standardizing complete modules. Svante has built filter-manufacturing capacity and is working with a major engineering contractor. SLB Capturi sells into customers that already buy large industrial projects from SLB. Mantel must prove that molten salts can circulate reliably through large equipment, tolerate contaminants, protect surrounding materials and deliver the promised heat balance over long operating periods.

The round follows the contractors

Mantel's recent partnerships show how Halliday intends to close the execution gap.

In May, Mantel named Wood its preferred partner for integrating the process into fired equipment. Wood is conducting front-end engineering work for a project at a steam-assisted gravity drainage facility in Western Canada. Mantel says that system is designed to capture approximately 60,000 tonnes of CO2 and produce 150,000 tonnes of high-pressure steam annually.

In July, Mantel signed a supply agreement with Aalborg CSP to create road-transportable salt-tank systems. The goal is to move instrumentation, storage and heating equipment into shop-built packages rather than constructing each component at the customer's site.

That work is central to the financing thesis. The molten borate is Mantel's technical differentiator, while modular engineering determines whether the process can become a repeatable product. Halliday put the problem plainly in announcing the Aalborg agreement: the whole system has to be transportable and repeatable from one deployment to the next.

Mantel's first industrial demonstration is located at Kruger's Wayagamack pulp and paper mill in Trois-Rivieres, Quebec. Kruger announced the C$23.75 million project in November 2024, with C$17.65 million coming from Canadian and Quebec government programs and C$6.1 million from Kruger and Mantel.

Mantel currently rates the demonstration for more than 2,000 tonnes of CO2 per year and one megawatt of steam returned to the mill. Its website also lists the Western Canadian project at approximately 60,000 tonnes per year, a thirtyfold jump in stated capture capacity.

Mantel has also been selected as the capture technology provider for the proposed 1.6-gigawatt TerraSpark Energy Campus in West Virginia. The U.S. Department of Energy selected that project for up to $18.5 million to support engineering, permitting and early studies. TerraSpark remains an early-stage development project rather than an operating reference plant.

These deployments cover different commercial questions. Kruger tests whether the salt loop and steam recovery work continuously inside an industrial mill. The Western Canadian project tests a much larger system in an application where steam is essential to production. TerraSpark would place Mantel inside the design of a new power plant, avoiding some of the constraints of retrofitting an existing facility.

Strategic investors can open doors, but customers will demand operating data

Constellation Technology Ventures gives Mantel another relationship inside the North American power sector. Constellation is a large electricity producer and already evaluates technologies against the reliability requirements of operating generation assets. Its investment does not constitute a deployment commitment, but it puts a technically demanding potential customer and partner on Mantel's shareholder list.

Azimut adds a different capability. Commercial carbon-capture plants require far more capital than venture-backed technology development alone can provide. Projects typically combine customer spending, government support, tax incentives, debt and infrastructure capital. Mantel said Azimut will contribute access to capital markets and project-finance expertise as it pursues deployments outside North America.

The investor mix now spans both sides of the industry Mantel wants to sell into. Shell Ventures and Eni Next co-led Mantel's $30 million Series A in September 2024, with participation from Engine Ventures, New Climate Ventures, Hartree, bp Ventures, Arosa Ventures, Vale Ventures, Newlab and MCJ Collective. The strategic shareholders bring potential industrial sites, engineering knowledge and introductions. Their involvement also reflects the continuing search among oil, gas, mining and power companies for capture systems that consume less energy than conventional plants.

The current round still looks like bridge capital for a much larger financing requirement. Halliday told Axios that Mantel plans to seek between $50 million and $200 million in 2027. The SEC filing's $25 million ceiling also leaves the present offering open to additional sales, although Mantel has announced only the $18 million investment.

That next raise will depend heavily on what Mantel can show from its first plants. Investors can underwrite engineering studies and demonstration equipment on projections. Financing a commercial fleet requires operating hours, maintenance records, construction schedules and evidence that recovered steam has measurable value to the customer.

Halliday's original insight came from finding a salt that kept working after the graph for other materials pointed steadily downward. Mantel has spent four years scaling that result from laboratory equipment toward industrial projects. The new capital buys the founders time to prove that the same durability can survive the pipes, tanks, contractors and operating demands of a real plant. In carbon capture, that is where the competition is decided.