Researchers at the Massachusetts Institute of Technology (MIT) have discovered a way to extract antimony from its ores using electricity. This has for decades been assumed impossible. The discovery could lead to the production of several other useful metals using electrolysis, which would cut the energy costs as well as the hazardous greenhouse gas and sulfur dioxide emissions of conventional ore smelting.

Ores of metals including antimony, copper, nickel, and zinc are often compounds of sulfur. The metals are conventionally extracted by converting the ore into oxides by roasting at high temperatures, during which carbon and sulfur dioxide are emitted. The oxides are then converted to metals by heating them at temperatures over 1000°C.

In comparison, aluminum is typically extracted from its oxide ore alumina using electrolysis, which involves passing electric current through a molten alumina solution. But electrolysis does not work with sulfide compounds. “Metal sulfides have an intolerably high electronic conductivity so they consume electricity without producing metal,” says Donald Sadoway, a materials chemist at MIT.

In the past, researchers have tried to reduce the conductivity of copper sulfide by mixing it with other sulfides that are ionic, so that the ions can trap conductive electrons. But electrolysis of the sulfide mix does not give pure metal.

Sadoway and his colleagues accidentally stumbled upon their new electrolysis method, reported in Nature Communications. The team studies liquid metal batteries, in which the electrodes are liquid metals and electrolytes are molten salts. They were testing a new chemistry for the batteries: a secondary sodium sulfide-doped chloride-based electrolyte (NaCl–KCl–Na₂S) floating on top of a molten antimony sulfide (Sb₂S₃) electrolyte.

The halide electrolyte is an ionic conductor, and when the researchers passed electric current through the antimony sulfide, they found that it decomposed and liquid antimony pooled at the bottom of the cell. “By covering the liquid semiconductor with a second liquid that’s an ionic conductor we blocked the shorting of electrons,” Sadoway says.

In other words, the setup led to electrolysis of the sulfide compound, producing 99.9% pure antimony. Pure sulfur gas, meanwhile, accumulates at the top, where it can be removed for use as a chemical feedstock. So there are no harmful sulfur dioxide emissions.

“The next question is, could we do this for copper,” Sadoway says. The challenge is that copper has a melting temperature of close to 1100°C, almost twice that of antimony’s. So it would be a matter of finding the right inert electrodes and molten salt electrolytes that work at such high temperatures. The researchers are now attempting to pursue experiments with other metal sulfides. “I have no doubt that this is scalable,” he says.

“This is really nice work in electro-metallurgy,” says Dihua Wang, a materials and environmental chemist at Wuhan University in China. “This finding can unlock access to a large class of ore bodies, namely sulfide minerals, in a way that avoids noxious by-products and emissions.” He adds that the same approach could be applied to semiconducting oxides of iron, manganese, and titanium.

And, says Sadoway, while electrolysis is energy-intensive, requiring copious amounts of electricity, the electricity source can be clean hydro, nuclear, or renewables. “The thing about electrolysis is that it’s possible to generate electricity without burning carbon,” he says. “So imagine in the Atacama desert, where there are copper mines, you could have solar panels generating electricity that convert copper sulfide directly into copper, with pure sulfur being produced instead of sulfur dioxide emissions.”

Read the article in Nature Communications.