Massachusetts Institute of Technology (MIT) researchers have identified the proper temperature and chemical mixture to selectively separate pure copper and other metallic trace elements from sulphur-based minerals using molten electrolysis.
This one-step, environmentally-friendly process simplifies metal production and eliminates the toxic byproducts, such as sulphur dioxide.
Postdoc Sulata Sahu and PhD student Brian Chmielowiec decomposed sulphur-rich minerals into pure sulphur and extracted three different metals at very high purity: copper, molybdenum, and rhenium. They also quantified the amount of energy needed to run the extraction process.
An electrolysis cell is a closed circuit, like a battery, but instead of producing electrical energy, it consumes electrical energy to break apart compounds into their elements, for example, splitting water into hydrogen and oxygen.
Such electrolytic processes are the primary method of aluminium production and are used as the final step to remove impurities in copper production. Contrary to aluminium, however, there are no direct electrolytic decomposition processes for copper-containing sulphide minerals to produce liquid copper.
The MIT researchers found a promising method of forming liquid copper metal and sulphur gas in their cell from an electrolyte composed of barium sulphide, lanthanum sulphide, and copper sulphide, which yields greater than 99.9 per cent pure copper. This purity is equivalent to the best current copper production methods.
Their results are published in an Electrochimica Acta paper with senior author Antoine Allanore, assistant professor of metallurgy.
“It is a one-step process, directly just decompose the sulphide to copper and sulphur. Other previous methods are multiple steps,” Sahu explains. “By adopting this process, we are aiming to reduce the cost.”
Copper is in increasing demand for use in electric vehicles, solar energy, consumer electronics and other energy efficiency targets. Most current copper extraction processes burn sulphide minerals in air, which produces sulphur dioxide, a harmful air pollutant that has to be captured and reprocessed, but the new method produces elemental sulphur, which can be safely reused, for example, in fertilisers.
The researchers also used electrolysis to produce rhenium and molybdenum, which are often found in copper sulphides at very small levels.
The new work builds on a 2016 Journal of The Electrochemical Society paper offering proof of electrolytic extraction of copper, authored by Samira Sokhanvaran, Sang-Kwon Lee, Guillaume Lambotte, and Allanore.
They showed that addition of barium sulphide to a copper sulphide melt suppressed copper sulphide’s electrical conductivity enough to extract a small amount of pure copper from the high-temperature electrochemical cell operating at 1105 degrees Celsius.
Sokhanvaran is now a research scientist at Natural Resources Canada-Canmet Mining; Lee is a senior researcher at Korea Atomic Energy Research Institute; and Lambotte is now a senior research engineer at Boston Electrometallurgical Corp.
“This paper was the first one to show that you can use a mixture where presumably electronic conductivity dominates conduction, but there is not actually 100 per cent. There is a tiny fraction that is ionic, which is good enough to make copper,” Allanore explains.
“The new paper shows that we can go further than that and almost make it fully ionic, that is reduce the share of electronic conductivity and therefore increase the efficiency to make metal.”
These sulphide minerals are compounds where the metal and the sulphur elements share electrons.
In their molten state, copper ions are missing one electron, giving them a positive charge, while sulphur ions are carrying two extra electrons, giving them a negative charge.
The desired reaction in an electrolysis cell is to form elemental atoms, by adding electrons to metals such as copper, and taking away electrons from sulphur.
This happens when extra electrons are introduced to the system by the applied voltage. The metal ions are reacting at the cathode, a negatively charged electrode, where they gain electrons in a process called reduction; meanwhile, the negatively charged sulphur ions are reacting at the anode, a positively charged electrode, where they give up electrons in a process called oxidation.
In a cell that used only copper sulphide, for example, because of its high electronic conductivity, the extra electrons would simply flow through the electrolyte without interacting with the individual ions of copper and sulphur at the electrodes and no separation would occur.
The Allanore Group researchers successfully identified other sulphide compounds that, when added to copper sulphide, change the behaviour of the melt so that the ions, rather than electrons, become the primary charge carriers through the system and thus enable the desired chemical reactions.
Technically speaking, the additives raise the bandgap of the copper sulphide so it is no longer electronically conductive, Chmielowiec explains.
The fraction of the electrons engaging in the oxidation and reduction reactions, measured as a percentage of the total current, that is the total electron flow in the cell, is called its faradaic efficiency.
The new work doubles the efficiency for electrolytic extraction of copper reported in the first paper, which was 28 per cent with an electrolyte where only barium sulphide added to the copper sulphide, to 59 per cent in the second paper with both lanthanum sulphide and barium sulphide added to the copper sulphide.