ARPA–E explores paths to emissions-free metal making

The first step in steelmaking is separating iron ore into oxygen and iron metal, which produces CO₂ through both the reduction process and the fossil-fuel burning necessary to create high heat. An ARPA–E solicitation for ideas to clean up that process closed on 14 June. The agency is looking to replace the centuries-old blast furnace with greener technology that can work at the scale of 2 gigatons of steel production annually. It may or may not follow up with a request for research proposals to fund.

Among the many proposed technologies are electrodeposition, reduction using hydrogen in place of carbon, and renewable biomass that would replace the coke—purified coal—in the blast furnace. In a presentation at ARPA–E’s annual summit meeting in May, Chang focused on electrodeposition, also known as electrowinning, in which a current of electrons replaces carbon monoxide as the reductant. To be emissions free, the electricity would come from renewable sources.

Boston Metal, an MIT spinoff backed by the Bill Gates–founded Breakthrough Energy Ventures, is developing a high-temperature iron-electrowinning process. A 12-member consortium known as SIDERWIN, headed by ArcelorMittal, the world’s largest steelmaker, has been working since 2017 to scale up a low-temperature electrowinning process. That effort is funded by the European Union’s Horizon 2020 R&D program (see Physics Today, March 2014, page 26).

The low-temperature process, which proceeds at around 110 °C, occurs in an aqueous alkaline electrolyte and is analogous to water electrolysis. Boston Metal’s process, which is carried out at 1600 °C in a molten oxide electrolyte, is more similar to aluminum smelting, which takes place at about 960 °C. The iron that forms on the cathode in SIDERWIN’s cells is periodically removed as a 1-cm-thick plate; Boston Metal’s process taps batches of molten iron from the bottom of the cells. Neither technology has been commercialized yet in volume steel production.

Chang says electrowinning currently is too costly and can’t achieve the volumes necessary to compete with traditional steelmaking. One possible way to increase output is by using slurry electrodes—particles suspended in the electrolyte—to replace two-dimensional electrodes. That would greatly increase the surface area available for iron deposition.

MIT metallurgist Antoine Allanore worked on the SIDERWIN collaboration and later codeveloped a chromium–iron anode that’s used in Boston Metal’s process. He says scaling up the electrowinning processes could be done by building more and bigger electrolytic cells. The overall footprint might not exceed that of a conventional integrated steel plant and its blast furnaces and coking ovens. Allanore says other alternative iron-making processes have drawbacks. Hydrogen, for example, is difficult to store and transport, and it would be needed in very large quantities, while biomass would be competing with agriculture, forestry, and bioenergy for a finite amount of arable land.

Other potential methods to reduce steelmaking’s energy use include inductive heating and the direct transfer of molten iron into furnaces where alloying into steel takes place. Today, iron from blast furnaces is solidified before it is reheated in basic oxygen furnaces. Making furnaces more efficient would also reduce energy use.

Downstream parts of the steelmaking process, such as hot rolling of slabs into sheet steel and fabrication of sheet into products, are ripe for energy-efficiency improvements, Chang says.

Ultimately, steelmaking might be transformed from its current multiple-vessel process of heating and reheating into one that resembles 3D printing. “It’s totally possible that there’s a black-box technology where you dump in rocks at the top and out comes a steel can, and it breathes out oxygen” instead of CO₂, says Chang.

ARPA–E fellow Elizabeth Troein, a biogeochemist, is pitching so-called hyperaccumulating plants as a domestic source of energy-critical metals that would reduce the need for carbon-intensive hard-rock mining. More than 500 species of such plants accumulate nickel in concentrations greater than 1000 μg/g of dry weight, far above the 50–100 μg/g that’s toxic to most plants. Those same plants also accumulate cobalt, to an extent, and in some cases rare-earth metals.

Most grow in the 2–3% of Earth’s land surface that has serpentine soils, which are nutrient poor and have high concentrations of heavy metals. About 60% of those soils are arable, says Rufus Chaney, a retired Department of Agriculture scientist whose research focused on what he calls phytomining—cultivating crops of hyperaccumulators for their mineral content. Large areas of serpentine soils occur in the Asia-Pacific region and in the Middle East; in the US they are mostly centered in northern California and Oregon.

Cobalt is increasingly in focus because of its scarcity, its price, and the rapidly rising demand for its use in lithium-ion batteries (see Physics Today, May 2021, page 20). Most of the world’s cobalt is mined in the Democratic Republic of the Congo, where social and political instability is high. Troein says that planting hyperaccumulators in a field the size of Rhode Island (2700 km²) could satisfy the current 10 000-ton-per-year US demand for cobalt until the land is depleted in 10 years or so. There are at least 16 000 km² in the US with the type of surface rock (ultramafic) that weathers to produce serpentine soils, she says.

Farming an area of those soils equivalent to the size of Maryland and Delaware (30 000 km²) could produce sufficient cobalt to meet global demand for a decade, Troein says. A second type of metal-rich soil more widely found in the US might also support cobalt accumulators, she adds.

Antony van der Ent, who studies the biopathways of trace elements at the University of Queensland in Australia, offers a somewhat different take. Little R&D has gone into cobalt phytomining to date, he says, and while there is potential, the emphasis should be in central Africa, where large areas with cobalt-enriched soils and mine waste exist. Cobalt hyperaccumulators are native to that area, and they could also be suitable for semiarid and Mediterranean climates.

Van der Ent says that any nickel hyperaccumulator plant will take up some cobalt, but only in appreciable amounts when the nickel content in the soil is very low. The two metals will compete for uptake, and in typical serpentine soils, where the nickel content is 10 times as great as the cobalt content, the plants will accumulate very little cobalt. Chaney says bioengineering could convert one of those species into a cobalt hyperaccumulator that ignores nickel.

Chaney, for whom the 2-meter-tall nickel hyperaccumulating plant Phyllanthus rufuschaneyi is named, says the mining potential of a particular plant can’t be determined until it has been grown as a crop with appropriate fertilizers and soil pH. Several species of hyperaccumulators are not tall plants, making their harvesting difficult and their value low. The hyperaccumulators can be burned and the metals separated from the ash. Alternatively, they can be pressed and the metals extracted from the liquid. Because hyperaccumulators extract the metal of interest from the soil or ore matrix, purifying the metal is much less expensive than conventional refining, he says.

Nickel phytomining is underway in Albania and Indonesia, Chaney says, but it has yet to become commercial in the US. After working with the Department of Agriculture under a cooperative R&D agreement that ended in 2002, the US company Viridian tested two nickel hyperaccumulator species at a site in Oregon. In 2005 the state declared the plants to be noxious weeds after some had been found on nearby lands. The researchers could have avoided the problem if they had harvested the plant before its seeds ripened, Chaney says, adding that the incident effectively brought an end to real-world experiments that used hyperaccumulators in the US.

“The scale of mining is almost incomprehensible, even before the [carbon-free] energy transition,” says Douglas Wicks, an ARPA–E program director. The volume of wastes, or tailings, produced in the world each year is 50 gigatons. Sometimes the piles can lead to disasters, as occurred in 2019, when a tailings dam at an iron mine near Brumadinho, Brazil, burst, killing 270 people (see photo, page 25).

The minerals needed for humanity’s transition to green energy produce some of the largest volumes of wastes on a per-ton basis: Each ton of cobalt, for example, produces 1000 tons of tailings. With demand for the minerals expected to skyrocket, “we’re now on course to leave a trillion tons of waste for the next generation,” Wicks says.

Many mines are “laser focused” on a single metal and leave behind others that they could extract, which would lessen the need to dig up more Earth, says Wicks. When nickel is mined, cobalt, manganese, aluminum, and iron are often left behind in the tailings. Processes could be developed to mine the minerals that are present in lower concentrations, he notes, perhaps through electrochemistry that exploits the different potentials of the metals to fractionate each out from solution. Once all the mineral value has been exhausted, the remaining wastes could be used as building materials. That’s what’s done with steelmaking slag.

The wastes could also serve as sinks for CO₂. Were CO₂ reactions properly integrated into nickel mining, the activity could go from a carbon-emitting process to a carbon-negative one, Wicks says. Olivine rock, in which nickel deposits are found, will absorb up to 63% of its weight in CO₂ under the right conditions, he says. In an ore that is 0.25% nickel, 400 tons of rock will be left behind for every ton of nickel recovered. That rock has the potential to chemically sequester 250 tons of CO₂ per ton of nickel liberated, compared with the 14–20 tons of CO₂ that is currently generated to produce a ton of nickel.

ARPA–E wants industry, academia, inventors, and entrepreneurs to propose new mining processes that require less land, water, and power. “Clean mining is key to a sustainable energy transition,” Wicks says.

Greeshma Gadikota, who directs the Sustainable Energy and Resource Recovery Group at Cornell University, says her research aims to accelerate the time in which chemical reactions such as rock weathering take place from hundreds and thousands of years to as little as hours. Some of those reactions lock up CO₂ in stable carbonates.

“We’ve started to learn more about how we can engineer the reactions to gain control and how we can start to implement them in industrial processes,” Gadikota says. “We are working out the thermodynamic feasibilities and the kinetic limitations. Can we understand the factors that make the reactions go slower or faster, and how expensive will it be to do that? How can we leverage the increasingly low costs of renewable electricity to tune these reactions?”

Updated 21 July 2021: The original version of this article stated incorrectly that ARPA–E has a program titled Steel Made via Emissions-Less Technologies (SMELT). In fact, SMELT is a “request for information” in that subject area, seeking input from the public that could potentially lead to a future program.