By extracting metallic iron without producing carbon dioxide, the new process could even be carbon negative, at least for part of the world's iron production
Making iron, the main ingredient of steel, takes a toll on Earth's delicate atmosphere, producing 8% of all global greenhouse gas emissions. Now, a team of chemists has come up with a way to make the business much more eco-friendly. By using electricity to convert iron ore and salt water into metallic iron and other industrially useful chemicals, researchers report today in Joule that their approach is cost effective, works well with electricity provided by wind and solar farms, and could even be carbon negative, consuming more carbon dioxide (CO2) than it produces.
"It's a very clever approach," says Karthish Manthiram, a chemical engineer at the California Institute of Technology who was not involved with the study. He notes that the process has other advantages, including working at a low temperature, and being amenable to working with intermittent renewable electricity. "It checks all the boxes."
Iron is one of the most abundant elements on Earth, but in its natural state is bound to oxygen in the various minerals that make up iron ore. To extract metallic iron from this ore, workers typically mix it with a high-carbon form of coal called coke and heat the combination to about 1500°C in a blast furnace. At that temperature, the carbon atoms strip the oxygen atoms from the iron, producing CO2 that wafts into the atmosphere and leaves behind the molten metal. Steelmakers then combine this iron with a small amount of carbon and other trace metals to forge steel.
Although this way to make iron and steel is cheap and time tested, it produces significant amounts of CO2. The world mines 2.5 billion tons of iron every year, and reducing it to iron emits as much CO2 as the tailpipes of all passenger vehicles combined. So, scientists are looking for economically viable ways to produce metallic iron that don't generate greenhouse gases.
To that end, Paul Kempler, a chemical engineer at the University of Oregon, and colleagues wondered whether an industrial process for making chlorine from saltwater could be repurposed for ironmaking. In this "chlor-alkali" process, water containing sodium-chloride is placed in an electrochemical cell resembling a battery that contains two electrodes submerged in a liquid electrolyte. The positively charged electrode, the anode, pulls electrons from chloride ions, causing chlorine atoms to pair up into chlorine gas. At the same time, electrons flowing in from the cathode split water molecules into pieces that pair with the sodium ions and one another to make sodium hydroxide and hydrogen gas.
To tweak the setup to purify iron, Kempler's team added iron oxide particles to its cathode. Now, the electrons sent to it would also release the oxygen atoms from iron oxide and again form sodium hydroxide—as well as leave behind solid metallic iron. The process is highly efficient, the researchers claim. In fact, they estimate that selling the chlorine and some of the sodium hydroxide at current market prices should enable the overall process to produce iron at roughly the same price as making it in blast furnaces. And because sodium hydroxide can bind CO2 and convert it into carbon-based minerals, the process could be used to help capture CO2, making it carbon negative.