The goal was a new battery, but researchers at the Massachusetts Institute of Technology’s Department of Materials Science and Engineering instead discovered a new way to produce antimony – and the method could be used to smelt other metals, too.
Researchers hoping to charge an experimental battery instead produced antimony. They soon discovered they could add an ionic conductor on top of this molten semiconductor layer to separate the metal out of the sulfide compound to produce a pool of 99.9 percent pure antimony at the bottom of the cell and pure sulfur gas at the top for chemical feedstock.
The group, comprised of Donald Sadoway, the John F. Elliott Professor of Materials Chemistry; postdoctoral student Huayi Yin; and visiting scholar Brice Chung, intended to develop a different electrochemistry for a battery.
Sadoway’s team has been trying to develop this electrochemistry as an extension of the variety of chemical formulations for the all-liquid, high-temperature storage batteries in development for several years. Different parts of these batteries are made up of molten metals or salts with varying densities, so they form separate layers, Sadoway noted in an MIT statement.
“We wanted to investigate the utility of putting a second electrolyte between the positive and negative electrodes [of the liquid battery],” Sadoway said in the statement. “We found that when we went to charge this putative battery, we were in fact producing liquid antimony instead of charging the battery.”
Antimony sulfide is a molten semiconductor and would not ordinarily allow for the kind of electrolytic process used to produce aluminum and some other metals through the application of an electric current, said the MIT statement.
“Antimony sulfide is a very good conductor of electrons,” Sadoway noted. “But if you want to do electrolysis, you only want an ionic conductor [a material good at conducting molecules that have a net electric charge].”
Under typical smelting processes, the sulfur produced on top would bond with oxygen to form the pollutant sulfur dioxide, the main source of acid rain. Instead, this contained process provides highly purified metal without the need for scrubbing out the polluting gas, MIT’s report said.
If this process can be applied to industrial metals like copper, it could lower prices and reduce air pollution and greenhouse gas emissions, Sadoway predicts.
“The thing that made this such an exciting finding is that we could imagine doing the same for copper and nickel, metals that are used in large quantities,” Sadoway said in the MIT statement.
Antimony, with its lower melting point of 631 degrees Celsius compared to copper’s 1,085 degrees C, made sense as a starting point. Higher melting temperatures could add some complications to the process even though the underlying physical principles are the same, he noted.