Antoine Allanore, the Thomas B. King Assistant Professor of Metallurgy at MIT, is proposing a direct sulfide electrolysis process to simplify copper extraction and eliminate noxious byproducts. In the traditional process, which still accounts for more than half of copper production, smelters roast a mixture of copper sulfide ore and oxygen. Besides copper, the process produces sulfur oxides—chemical precursors to acid rain. To prevent atmospheric release, the sulfur oxides have to be trapped, filtered, and treated to make sulfuric acid, adding to capital costs. In contrast, the proposed electrochemical process of molten sulfides electrolysis melts the materials at high temperature (above 1,085 ˚C or 1,985 ˚F) and yields pure liquid copper and elemental sulfur gas.
If you look at the energy consumption of a copper smelter today, it’s enormous. They are dependent on electricity already to exist. My approach asks, why don’t we try to do 100 percent electrical, starting from the concentrate and ending with the metal product, if I can use electricity to be more efficient as well as more environmentally friendly?—Antoine Allanore
The idea for direct sulfide electrolysis dates to as early as 1906. In a presentation for the Copper International Conference in Santiago, Chile, in December 2013, Allanore noted the proof of concept for producing copper and sulfur by electrodecomposition of copper sulfide, achieved by T.P. Hoar and R.G. Ward and published in the London-based Transactions of the Institute of Mining and Metallurgy in 1958.
In the electrolysis process, copper sulfide ore is added to an electrolyte, and the chemical transformation of the copper sulfide into its constituent parts takes place on two electrodes in the electrolyte: an anode and a cathode. The crucible, the vessel in which the process occurs, has to be nonreactive both to the electrolyte and to the copper sulfide.
Postdoc associate Sang-Kwon Lee, who is working on the electrolyte design, said that their main strategy is using the multicomponent molten sulfide in order to change the property of the supporting electrolyte and to reduce the melting temperature of the electrolyte.
Allanore addressed the fundamental scientific question of how to promote removal of oxygen from these systems in the paper “Electrochemical engineering of anodic oxygen evolution in molten oxides, published in November 2013 in Electrochimica Acta.
The previous challenge was working with processing of an oxide system. Here, the challenge is to work with a sulfide system, but in the molten state. We know surprisingly little about molten sulfides. We know their properties in the solid state, because they are used for thermoelectrics, magnetic material, and optical materials.
The challenge for us is really to predict the thermodynamic and physical properties of multicomponent sulfides, with the eventual objective of doing the electrolysis, which works very well if you don't have any electronic conductivity.—Antoine Allanore
The fundamental electrolysis process is well understood—at one electrode, an electron reacts with an ionic compound in contact with the electrolyte to release the metal, and at the other electrode, a component in the electrolyte reacts to give back an electron. In an ideal electrochemical process, you don’t have electrons flowing in the electrolyte, as they are stopped at the electrodes, Allanore noted.
However, in liquid sulfides, some free electrons are allowed to pass through the solution without causing the desired chemical transformations on the electrodes; this allows their energy to be wasted as heat but doesn’t contribute to metal separation. The challenge right now is to probe how electrons are either flowing or not flowing in the system to be able to design a system that works for metal extraction, Allanore said.
Experience in the aluminum industry shows that it is actually fairly cheap to make the electrolysis device, Allanore said.
So if you can secure the supply of electrons, if you can make a device that is smaller or cheaper, now we are speaking about a different business scenario. We’re not speaking about 20 or 30 years of investment at several billion [dollars]. It’s something that you will pay once, hopefully, half the price, and therefore, the old business model changes completely. And I think that’s the only way this innovative technology will be implemented, is if it is flexible enough to be integrated anywhere in the country and it has the ability to actually scale from lower capacity to extremely high capacity. And electricity allows that.—Antoine Allanore
The copper research builds on Allanore’s previous work in the steel industry, where he developed new electrochemical processes to make iron without greenhouse gas emissions. One of those efforts, in collaboration with Donald R. Sadoway, John F. Elliott Professor of Materials Chemistry, focused on liquid iron production at 1,600 ˚C, and resulted in a patent, a Nature paper, and a spinoff company, Boston Electrometallurgical Corp. (BEMC), in Woburn, Mass. BEMC is now scaling up the process to 1,000 times laboratory scale to demonstrate its commercial viability.
The molten sulfide electrolysis research received funding from the Office of Naval Research and is continuing with support from an anonymous private investor. The research funds are managed through the Materials Processing Center.
Antoine Allanore (2013) “Electrochemical engineering of anodic oxygen evolution in molten oxides,” Electrochimica Acta, Volume 110Pages 587-592 doi: 10.1016/j.electacta.2013.04.095