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Will surface rust on an iron crucible be converted if used to melt aluminium?

Can one convert bog/lake iron ore into iron using molten aluminium similar to the Thermite reaction?

Or perhaps the real question is what temperature would this occur at?

EDIT: I'm hoping one could use the molten aluminium instead of aluminium dust to achieve the same reaction as occurs in the Thermite process. I know about Thermite steel rail and copper earth connection welding. My goal is to reduce rust to iron by sprinkling it (with attendant sparks) onto a molten aluminium surface. Will the temperature be too low in practice or will the iron oxidise or blow off before consolidating? Trials today were inconclusive due to low temperature and small sample size.

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Short answer: Yes.

Take a look at the Ellingham Diagram.

enter image description here

The line for aluminium oxide is lower than the line for the all the iron oxides. This indicates greater stability of aluminium oxide, or in other words indicates aluminium's greater affinity for oxygen. Thus, based on thermodynamics alone one would expect aluminium to be able to reduce iron oxides.

Based on this figure, one would also expect this reaction to be spontaneous at all temperatures.

Important caveat, the ignition temperature remains critical. Just as a piece of paper will not combust unprovoked, even though the reaction is spontaneous, the thermite reaction needs to reach its ignition temperature.

Additionally, of interest to you would be this paper. Given below is a DSC curve from it

enter image description here

The endothermic peak ca. 660 corresponds to the melting of aluminium, while the exothermic peaks correspond to the thermite reaction.

The activation of the process is reported as 145 kJ/mol, and according to he authors in the study the product is controlled by the diffusion of $\ce{Al}$ in $\ce{FeAl2O4}$

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    $\begingroup$ To the extent that this theory suggests the reaction to be spontaneous, it is woefully inadequate. The reaction is notorious for its high ignition temperature. $\endgroup$
    – feetwet
    Oct 12, 2016 at 17:46
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    $\begingroup$ @feetwet I agree with you completely. In fact, I plan to update my answer with some additional references :) $\endgroup$
    – getafix
    Oct 12, 2016 at 22:29
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    $\begingroup$ If I am reading this correctly it implies that at a temperature somewhere around 850 to 950 degC we can expect the reaction to self start. If there are issues with diffusion it would also suggest to me that this would work best if the components were mixed before they were expected to react. Floating the iron oxide on aluminium might be expected to work at the interface though. I suppose there is hope but it is going to have very hot molten aluminium to get it going. $\endgroup$
    – KalleMP
    Oct 17, 2016 at 15:13
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    $\begingroup$ Yes, I believe so. Although I would expect the ignition temperature to be > 1000 C...the information I could find showed some variance. However, I have no practical experience whatsoever so it may very well be lower. If you are carrying out the reaction, experiment with the temperature and see what results you get. I'd love to hear them myself :) $\endgroup$
    – getafix
    Oct 17, 2016 at 15:17
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Aluminum can indeed reduce iron oxide, but you may not get iron metal. Aluminum and iron constitute one of many pairs of metals that form intermetallic compounds. Reference [1](https://doi.org/10.1007/s11661-003-0215-9) gives a phase diagram:

enter image description here

Thus upon reducing the iron oxide to iron, aluminum can also combine with iron to form several different compounds.

Among the iron-aluminum compounds, $\ce{Fe2Al5}$ plays a significant role in galvanizing steel. Running the steel strip into a pure molten zinc bath forms not a zinc coating, but another series of intermetallic compounds (of iron and zinc this time) which are brittle and easy to powder off. Adding a small amount of aluminum, typically 0.1-0.2% in the molten metallic solution, to the bath forms a very thin layer of $\ce{Fe2Al5}$. This allows the alloying of iron into the zinc to be prevented or moderated, thus obtaining the desired coating properties.

Reference

  1. Han, Qingyou & Viswanathan, Srinath. (2003). "Analysis of the mechanism of die soldering in aluminum die casting". Metallurgical and Materials Transactions A. 34. 139-146. https://doi.org/10.1007/s11661-003-0215-9.
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    $\begingroup$ (+1) You always enlighten me. Very cool about how aluminum facilitates galvanizing. $\endgroup$
    – Ed V
    Sep 4, 2022 at 23:19
  • $\begingroup$ Good find. Thermite is used for welding railway track. The resulting quality of the steel is adequate for the job (though sometimes an alloy insert in the mould adds hardness to the top) so somehow at least at Thermite temperatures there is not a problematic concentration of these intermetalics but at low temperatures like I would hope for they might become significant. $\endgroup$
    – KalleMP
    Sep 13, 2022 at 15:31
  • $\begingroup$ Just had a look at the boiling point of Aluminium is over 2000C and the melting point of rust is about the same as that of Iron. It may be that the temperature where we get the Thermite reaction will be hotter than the temperatures that will favour intermetalics. With enough energy the temperature may boil off aluminium, the burn temperature of Thermite may exceed 2800C so aluminium is likely to be driven off. $\endgroup$
    – KalleMP
    Sep 13, 2022 at 15:59
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    $\begingroup$ @KalleMP that depends on conditions. Moten aluminum can dissolve some iron, and form the intermetallics, without melting the unreacted iron. This would occur at low enough temperature to allow said intermetallics. You must indeed get the termite hot enough to avoid this. $\endgroup$ Sep 13, 2022 at 16:03
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The trick to the thermite reaction of iron oxides with aluminum is the ignition temperature: somewhere over 1600 °C. Since aluminum melts at 660 °C you have to add (and sustain on contact) a lot of heat before it can begin to steal oxygen from rust.

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