A step toward cleaner iron extraction using electricity

"Identifying oxides which can be converted to iron metal at low temperatures is an important step in developing fully electrified processes for steelmaking," says Paul Kempler, the study's corresponding author.

Electrochemical ironmaking isolates the metal by passing electricity through a liquid that holds iron-containing feedstocks. Compared to high-temperature blast furnaces, the electrochemical process could significantly reduce air pollution emissions, such as greenhouse gases, sulfur dioxide and particulate matter, and suggests considerable energy savings. Previously, Kempler and colleagues used this process to convert solutions containing solid iron(III) oxide particles and sodium hydroxide directly into elemental iron at temperatures around 176 to 194 degrees Fahrenheit (80 to 90 degrees Celsius). However, when some natural iron ores with irregularly sized, dense particles and impurities were tested, this low-temperature process wasn't selective enough. So, Kempler and a new team of researchers led by Anastasiia Konovalova and Andrew Goldman wanted to understand which iron ore-like feedstocks could support scalable growth of the process.

First, the researchers prepared high surface area iron oxide particles with internal holes and connective cavities to investigate how the nanoscale morphology of the particles impacted the electrochemical reaction. Then, they converted some of these into micrometer-wide iron oxide particles to mimic the morphology of natural ores. These particles contained only a few trace impurities, such as carbon and barium. The team designed a specialized cathode to pull iron metal from a sodium hydroxide solution containing the iron oxide particles as current passed through it. In experiments, dense iron oxides were reduced, or converted into elemental iron, most selectively at a current density of 50 milliamperes per square centimeter, similar to rapidly charging lithium-ion batteries. Conversely, loose particles with higher porosity, and thus higher surface area, facilitated more efficient electrochemical iron production, as compared to those made to resemble the less porous natural iron ore hematite.

The researchers evaluated the potential cost of their electrochemical ironmaking method. At the current density used in the experiments, they estimated that iron could be produced at less than $600 per metric ton ($0.60 per kilogram), which is comparable to traditional ironmaking. The study showed that much higher current densities, up to 600 milliamperes per square centimeter, similar to those used in industrial electrolysis cells, could be achieved when using particles with nanoscale porosity. Further advances in electrochemical cell design and techniques to make iron oxide feedstocks more porous will be required before the technology sees commercial adoption.

The authors acknowledge funding from the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.