Producing other metals with thermite reactions

[some bloke]

Hello all,

I am curious as to whether it is possible to create aluminium from a thermite reaction.

Thermite, as standard, uses aluminium and iron oxide, which react together when burnt to create a blob of molten iron. I would like to achieve a similar result, but with a blob of molten aluminium as the end product.

I assume that the reaction is the result of the much more reactive aluminium pulling the oxygen from the iron oxide, and so creating aluminium oxide and iron. If a similar mix of aluminium oxide (alumina) and a more reactive metal than aluminium were used, would it produce the same effect? would it require a higher ignition temperature, or lower? would it be volatile, even?

Note - my final aim, if this is plausible, is to produce less than a teaspoon's worth of the mix, suspended in a flux to a thixotropic gel-like consistency, then use it to try and develop a localised welding technique for cosmetic welding of aluminium, IE filling small holes, in areas which cannot be accessed by conventional TIG welders.

I will understand if this thread is removed, as thermite is not a particularly safe mixture. I did check the posting rules and suspect this one will fall under "to be watched closely".

 

[Bystander]

https://en.wikipedia.org/wiki/Thermite , see Goldschmidt Reaction; little tough to find "oxidants" less noble than Al.

 

[TeethWhitener]

People have done thermite reactions with magnesium, which has a lower (as in more negative) reduction potential than aluminum, so at least thermodynamically, in principle the reaction:
$$Al_2O_3 + 3Mg \longrightarrow 2Al + 3MgO$$
is feasible. I don't know if anyone's tried it, though.

 

[some bloke]

People have done thermite reactions with magnesium, which has a lower (as in more negative) reduction potential than aluminum, so at least thermodynamically, in principle the reaction:
$$Al_2O_3 + 3Mg \longrightarrow 2Al + 3MgO$$
is feasible. I don't know if anyone's tried it, though.

Thanks for the formula, I'm stretching back to my long-since faded chemistry education and am I right in thinking that I will need the molar mass of these elements, then use this to determine the mass of each part to create the correct mixture?

Can anyone speculate as to the result if I were to have too much alumina or too much magnesium? would it simply leave some unreacted, or could it have more dramatic effects?

 

[TeethWhitener]

could it have more dramatic effects

Molten magnesium in air could have very dramatic effects:


The reaction is not very well controlled even with the standard Al/Fe₂O₃ mixture. I can't imagine it would be more controlled with magnesium. Also, magnesium boils at ~1360 K (cf. aluminum ~2740 K), so the reaction would likely produce gaseous magnesium (no bueno). In addition, depending on how much heat/how quickly energy is released, the reaction could run significantly hotter than a standard thermite reaction (~2500 K) and boil the aluminum as well, which in no possible world ends well.

 

[some bloke]

ah, yes, boiling the aluminium would certainly be a bad result. I hope that the small quantities I'm considering using would prevent the reaction from forming too much heat, though. I imagine that a cupful of this stuff would be problematic, but a teaspoonful, I hope, would dissipate its heat quickly enough to not boil.

The alternative I thought of, which is much more difficult to safely construct, would be to use a microwave generator to excite aluminium suspended in a flux, causing it to melt and weld onto the aluminium surface to which it was applied (the flux having stripped the oxide layer), but that has waaaay more safety concerns than thermite (which is saying something!).

 

[TeethWhitener]

but that has waaaay more safety concerns than thermite (which is saying something!).

I’m not so sure that’s true. The problem with the reaction being so much hotter than the boiling point of magnesium is that there’s a chance the magnesium won’t just boil, but superheat and spray molten magnesium and aluminum all over the place.

The microwave method is a neat idea, but I’m not sure how the energy isn’t just going to dissipate into the bulk of the aluminum immediately.

 

[snorkack]

https://en.wikipedia.org/wiki/Thermite , see Goldschmidt Reaction; little tough to find "oxidants" less noble than Al.

Searching for reducers:
Al₂O₃ enthalpy of formation -1675 kJ/mol, making -558 kJ/mol O (linked Wikipedia to Zumdahl), Al boils at 2470 degrees
BeO enthalpy -599 kJ/mol, Be boils at 2470 degrees
MgO enthalpy -602 kJ/mol, Mg boils at 1090 degrees
CaO enthalpy -635 kJ/mol, Ca boils at 1484 degrees
SrO enthalpy -592 kJ/mol, Sr boils at 1377 degrees
BaO enthalpy -582 kJ/mol, Ba boils at 1845 degrees
Li₂O enthalpy -596 kJ/mol, Li boils at 1330 degrees
Na₂O enthalpy -416 kJ/mol (sic!, also Zumdahl), Na boils at 883 degrees
K₂O enthalpy -363 kJ/mol, K boils at 759 degrees

Note:
Alkali metals other than Li have only small affinity for oxygen. Heating metal oxides with metallic Na or K is likely to not work with moderately active metals!
The most active metal turns out to be Ca. And Ca has fairly high boiling point - higher than Mg.
While Ca is fairly reactive in wet air, not so much as Na. So, how useful are thermites based on metallic Ca?

 

[Ygggdrasil]

Here's an interesting youtube video where the youtubers attempt to cast a small silver object from silver oxide in a thermite reaction. Ultimately, they didn't get it to work (though they did produce silver), but perhaps it has some useful information you could use:

 

[TeethWhitener]

So, how useful are thermites based on metallic Ca?

Probably comparable to magnesium, but magnesium is significantly cheaper.

Here's an interesting youtube video where the youtubers attempt to cast a small silver object from silver oxide in a thermite reaction. Ultimately, they didn't get it to work (though they did produce silver), but perhaps it has some useful information you could use:

But silver oxide decomposes to the elements at like 200°C, even without a reducing agent, so Why the thermite? (Other than the temperature needed to melt the silver)

 

[Ygggdrasil]

But silver oxide decomposes to the elements at like 200°C, even without a reducing agent, so Why the thermite? (Other than the temperature needed to melt the silver)

Because the point of YouTube is to make videos that look cool and get clicks, not to do something practical.

 

[tech99]

Aluminium was first made by the reaction of aluminium oxide with potassium amalgam, but later by the reaction with sodium, so it's possible.
Our school safety organisation advises against many alternative thermit reactions these days so it is worth checking.

 

[snorkack]

Aluminium was first made by the reaction of aluminium oxide with potassium amalgam, but later by the reaction with sodium, so it's possible.

No.
Aluminum chloride.