Science Interviews

Interview

Thu, 9th May 2013

Environmentally Friendly Steel

Prof Derek Fray, Department of Meterials Science and Metallurgy, University of Cambridge

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Steel is the most widely used metal in the world. In 2011 alone, around a billion tons of it were produced worldwide. But the steel industry accounts for 5% of the worldís carbon dioxide emissions. This week, a team based at the Massachusetts Institute of Technology revealed a more environmentally friendly way of producing it using electricity. To comment on the work, we were joined by Derek Fray whoís a metallurgist from Cambridge University.  

Chris - First of all, what's the process by which we actually make iron that leads to it producing so much CO2 every year?

Derek - Well, itís reduced by carbon. Carbon is an excellent reductant and itís very, very plentiful. As you know, weíre used to burning in our homes to keep the homes warm.

Chris - And weíre doing that by chucking carbon into a furnace and the carbon stealing the oxide off of the iron to leave molten iron.

Derek - That's correct. It ends up as CO2 eventually.

Chris - What's the aspiration then to try and get that CO2 down? How could we do it better?

Derek - Well, this team at MIT has discovered a means of electrically reducing iron oxide at very high temperatures, about 1600 centigrade which is more than white heat. So, they simply dissolve it in a liquid solution and electrolyse it.

Chris - Now, what is electrolysis?

Derek - The iron, when it dissolves in the electrolyte as itís called, is known as iron ions and oxygen ions. And when you pass an electric current through the solution, the iron ions will go to the cathode, discharge and form iron and the oxygen ions will go to the anode and form oxygen gas.

Chris - But we do that with aluminium oxide. We make aluminium metal by electrolysing bauxite which is aluminium oxide. So, why can't we just do that for iron?

Derek - Well, the problem is that well, in the aluminium case, you also produce a lot of carbon dioxide because they use a carbon anode. So, the team in MIT, their goal has been to produce an anode which can evolve oxygen on and you get pure oxygen generated. So, there's no CO2 from actual decomposition of the iron oxide.

Chris - So, the problem that they had to grapple with was, what sort of material to use to do that electrolysis, what to use as the electrodes? So, how did they approach that then?

Derek - The approach they adopted was essentially, use stainless steel. Stainless steel is usually iron, chromium and nickel and what they did, they used an alloy which is iron and chromium, plus a few other minor elements.

Chris - So, you mix those two things together. Why donít they just melt when you dunk those materials into molten iron oxide? Why donít they turn into just liquid iron?

Derek - The melting point is higher than the temperature of the electrolyte.

Chris - So, what happens to these electrodes? Why does this work where others have failed then?

Derek - Well, it forms a very tenacious oxide film which doesnít dissolve in electrolytes and itís conducting, so it can pass the electrons to and fro through it.

Chris - What forms an oxide layer? Just talk me through what's going on in this electrolysis then?

Derek - When you put the iron chromium alloy into the electrolytes, it oxidises a little bit, you get a very, very thin layer which is very stable. So, it doesnít dissolve.

Chris - So, that forms on the surface of the electrodes and this protects the electrodes from the environment they're in. So, what does this mean in terms of how we would actually deploy this?

Derek - It would be very similar to the whole Heroult cell for the reduction of aluminium. We would have a liquid metal pool, you'd have the electrolyte floating on the top, and then your anode would be above that. So, when the oxygen forms on the anode, it just escapes.

Chris - Could you do this in a batch process or would you have to chuck a whole out of the oxide in and then run the electricity through it, and reduce it to metallic iron then pour that off and start again or could you continuously add little bits of iron oxide and keep the process going all the time?

Derek - Well, if you go back to the whole Heroult cell, essentially, thatís a batch process. The aluminium is taken out about every 8 hours and you feed the alumina in about every 8 hours as well.

Chris - If we actually do this, why is this better than doing it the traditional way with carbon because the electricity is presumably going to have a carbon cost attached to it?

Derek - I think the assumption would be, the electricity would come from photovoltaics or wind power or sea power, or tidal power. Obviously, if you're having to use electricity from a coal fired power station, you're no better off.

Chris - Does the discovery of this new material actually mean something in terms of how we could use it in other areas though? Itís not just for iron. Could it be used elsewhere?

Derek - You could use it for other metals.

Chris - What about the fact that it also makes this oxygen? Is that useful?

Derek - Probably not on the earth, but it would be in the Solar System or on the moon, the rockets that took the astronauts to the moon 20 or 30 years ago. They were mainly oxygen. You need about 80 tons of oxygen, about 10 tons of hydrogen for the power. So, if you obviously hope to bring the astronauts back, you have to take enough oxygen up there. And if you could generate the oxygen on the moon, it would be very much easier.

Chris - So, in other words, if we could take moon rock and use this sort of technology, these sorts of electrodes and electrolyse moon rock, it would be very nice because we could get all the metals out as well which could be good for construction.

Derek - Could be used for structural things, yes.

Chris - Plus, we would then get a supply of oxygen to send people off another journeys or even bring them home which is surely nice.

Derek - The advantage of the moon is that gravitational field is 1/6 of that on the Earth. So, you need less power to escape the gravitational field.

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