Molten oxide electrolysis (MOE) is an electrometallurgical technique that enables the direct production of metal in the liquid state from oxide feedstock1,2, and compared with traditional methods of extractive metallurgy offers both a substantial simplification of the process and a significant reduction in energy consumption3. MOE is also considered a promising route for mitigation of CO2 emissions in steelmaking3,4,5, production of metals free of carbon6, and generation of oxygen for extra-terrestrial exploration7,8. Until now, MOE has been demonstrated using anode materials that are consumable (graphite for use with ferro-alloys and titanium6,9) or unaffordable for terrestrial applications (iridium for use with iron10,11). To enable metal production without process carbon, MOE requires an anode material that resists depletion while sustaining oxygen evolution. The challenges for iron production are threefold. First, the process temperature is in excess of 1,538 degrees Celsius (ref. 10). Second, under anodic polarization most metals inevitably corrode in such conditions11,12,13. Third, iron oxide undergoes spontaneous reduction on contact with most refractory metals14 and even carbon. Here we show that anodes comprising chromium-based alloys exhibit limited consumption during iron extraction and oxygen evolution by MOE. The anode stability is due to the formation of an electronically conductive solid solution of chromium(iii) and aluminium oxides in the corundum structure. These findings make practicable larger-scale evaluation of MOE for the production of steel, and potentially provide a key material component enabling mitigation of greenhouse-gas emissions while producing metal of superior metallurgical quality.
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The financial support of the American Iron and Steel Institute is acknowledged; we thank H. Kim and J. Paramore for assistance with the experimental set-up and for discussions.
The authors declare no competing financial interests.
This file contains a description of the Supplementary Video, Supplementary Figures 1-2 showing the influence of the alloy composition and electrolysis duration on the material stability, a section A dedicated to the investigation of the chemical stability of chromium oxide (+III) in the investigated electrolyte, and a section B in which chromium and chromium-iron alloys oxidation kinetics at high temperature are investigated. (PDF 30019 kb)
Observation of the gas evolution obtained with a cell operating with the novel anode material at 1565°C
Video recorded from the top of the tube furnace, it shows from above the entire pool of molten oxide contained in the crucible. The anode is located at the center and is noticeable thanks to its brightness. The electrolyte convection inherited from the gas evolution brings hotter electrolyte toward the cell surface and allows to visualize the gas evolution thanks to the corresponding contrast difference. (MP4 28670 kb)
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Allanore, A., Yin, L. & Sadoway, D. A new anode material for oxygen evolution in molten oxide electrolysis. Nature 497, 353–356 (2013). https://doi.org/10.1038/nature12134
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