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Others have mentioned that molten oxide electrolysis isn't as flexible with electricity consumption as hydrogen based processes. The even bigger problem, IMO, i

Others have mentioned that molten oxide electrolysis isn't as flexible with electricity consumption as hydrogen based processes. The even bigger problem, IMO, is the incredibly demanding chemical and thermal environment. There are not many combinations of vessel/electrode materials that are stable toward these high temperatures, dissolution by the lava-like electrolyte, and oxidation at the anode. According to the article their molten bath sounds like it is made of silicates. Electrolyzing molten silicates sounds a lot like NASA's "molten regolith electrolysis" [1] concept for in-situ resource utilization on the moon. The NASA concept valued the oxygen product while the produced metal is much more interesting here on Earth. Although conceptually simple, it has been a tremendous challenge finding materials that endure under these operating conditions. If Boston Metal has really cracked the problem, it bodes well for the future of terrestrial and off-planet resource extraction. [1] e.g. https://ntrs.nasa.gov/api/citations/20120003037/downloads/20120003037.pdf https://ntrs.nasa.gov/api/citations/20120003037/downloads/20...

kragen 5y ago
1600° is really about the same temperature as an ordinary blast furnace, and silicates have been commonly used as fluxes and refractories in steelmaking, even if basic oxygen steelmaking is more popular nowadays. If I understand correctly, Bessemer converters commonly used silicates. So I don't think the challenges you're pointing at, even if they have to be solved, are anything new for steelmaking. However, the article suggests that actually the electrolyte might be neutral, containing both calcium and silicon oxides. That can't possibly be right if they're talking about only 1600° because larnite would precipitate out. Electrodes would surely be carbon, just as they are in any old arc furnace. This does result in carbon dioxide emissions, as it does in the making of aluminum. Can they keep this down to an acceptable level? Is there an alternative electrode material, such as zirconia or carborundum? I'd like to know, but the article doesn't say.
- pfdietz 5y ago
  I think carbon electrodes would dissolve in iron. If I recall correctly, it's some sort of oxide-coated metal electrodes.
  - kragen 5y ago
    Well, carbon electrodes don't dissolve fast enough in molten iron to keep them from being standard equipment for decades in EAFs, but you're right that that's not what they're using. Scythe found this paper by Lan Yin, Antoine Allanore, and Sadoway from 02013 that says they're using 90% chromium, 10% iron, forming a refractory but conductive layer of eskolaite/alumina: https://pubmed.ncbi.nlm.nih.gov/23657254/ https://pubmed.ncbi.nlm.nih.gov/23657254/ Boston Metal links this paper and calls out Allanore on their about page: https://www.bostonmetal.com/who-we-are/ https://www.bostonmetal.com/who-we-are/. Sadoway is on Boston Metal's board, but Yin and Allanore evidently aren't involved.
    - pfdietz 5y ago
      Electric arc furnaces for steel don't typically immerse the electrodes in the molten metal (in the slag on top of the molten metal, yes.)
      - kragen 5y ago
        Oh, of course you're correct, thank you. How high is the solubility of graphite in molten iron?
        pfdietz 5y ago
        Iron/carbon phase diagram: http://www.substech.com/dokuwiki/doku.php?id=iron-carbon_phase_diagram http://www.substech.com/dokuwiki/doku.php?id=iron-carbon_pha... The eutectic of iron and carbon (mixture with the minimum melting point) is 4.3% carbon by mass. There's a chapter from Richard Preston's "American Steel" (a 1992 book that I highly recommend about the Nucor minimill in Crawfordsville, Indiana) where they find at one point during the startup of the then-cutting-edge continuous casting line that the steel in arc furnace had too little carbon. They decided to solve that (expensively, for the purpose of the testing) by dipping the electrodes into the steel and letting their ends dissolve.

kragen 5y ago

1600° is really about the same temperature as an ordinary blast furnace, and silicates have been commonly used as fluxes and refractories in steelmaking, even if basic oxygen steelmaking is more popular nowadays. If I understand correctly, Bessemer converters commonly used silicates. So I don't think the challenges you're pointing at, even if they have to be solved, are anything new for steelmaking. However, the article suggests that actually the electrolyte might be neutral, containing both calcium and silicon oxides. That can't possibly be right if they're talking about only 1600° because larnite would precipitate out. Electrodes would surely be carbon, just as they are in any old arc furnace. This does result in carbon dioxide emissions, as it does in the making of aluminum. Can they keep this down to an acceptable level? Is there an alternative electrode material, such as zirconia or carborundum? I'd like to know, but the article doesn't say.

pfdietz 5y ago
I think carbon electrodes would dissolve in iron. If I recall correctly, it's some sort of oxide-coated metal electrodes.
- kragen 5y ago
  Well, carbon electrodes don't dissolve fast enough in molten iron to keep them from being standard equipment for decades in EAFs, but you're right that that's not what they're using. Scythe found this paper by Lan Yin, Antoine Allanore, and Sadoway from 02013 that says they're using 90% chromium, 10% iron, forming a refractory but conductive layer of eskolaite/alumina: https://pubmed.ncbi.nlm.nih.gov/23657254/ https://pubmed.ncbi.nlm.nih.gov/23657254/ Boston Metal links this paper and calls out Allanore on their about page: https://www.bostonmetal.com/who-we-are/ https://www.bostonmetal.com/who-we-are/. Sadoway is on Boston Metal's board, but Yin and Allanore evidently aren't involved.
  - pfdietz 5y ago
    Electric arc furnaces for steel don't typically immerse the electrodes in the molten metal (in the slag on top of the molten metal, yes.)
    - kragen 5y ago
      Oh, of course you're correct, thank you. How high is the solubility of graphite in molten iron?
      - pfdietz 5y ago
        Iron/carbon phase diagram: http://www.substech.com/dokuwiki/doku.php?id=iron-carbon_phase_diagram http://www.substech.com/dokuwiki/doku.php?id=iron-carbon_pha... The eutectic of iron and carbon (mixture with the minimum melting point) is 4.3% carbon by mass. There's a chapter from Richard Preston's "American Steel" (a 1992 book that I highly recommend about the Nucor minimill in Crawfordsville, Indiana) where they find at one point during the startup of the then-cutting-edge continuous casting line that the steel in arc furnace had too little carbon. They decided to solve that (expensively, for the purpose of the testing) by dipping the electrodes into the steel and letting their ends dissolve.