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Author: Subject: Dissolving Al2O3 ?
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[*] posted on 2-11-2021 at 01:01
Dissolving Al2O3 ?

Probably a question which is asked before, but still I ask it.

I did a thermite for making silicon by reducing SiO2 with Al powder.
On this site I found tips.

First I tried "A mixture of Silicon Dioxide, Aluminum powder, and Sulfur in the ratio of 9 : 10 : 12 by weight respectively, works well and is (relatively) easy to ignite." .
That worked but, as they said, a horrible H2S stench appeared when I dropped the result in dilute HCl.
Leaving it bubble overnight outdoors I isolated the crud.
The second experiment I tried was SiO2, Al, KClO3 using the ratio of 100 : 96 : 81 without the CaF2 (which I don't have).
That was even faster and dumping the result in dilute HCl sol'n was almost odorless.
But here the same: How can I dissolve the Al2O3 to separate it from Si 'metal' ?
No strong acid or base attacks it.
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[*] posted on 2-11-2021 at 04:45

Quote: Originally posted by metalresearcher  

No strong acid or base attacks it.

Both strong acids and strong bases react

Reflux condenser?? I barely know her!
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[*] posted on 2-11-2021 at 04:59

Wikipedia tells me that Si metal "does not react with most aqueous acids" but "readily dissolves in hot aqueous alkali to form silicates" so definitely don't use base. I would suggest refluxing in dilute HCl or maybe acetic acid? because it would be milder. It might just take a while while refluxing. Some of the crystal structures of Al2O3 such as corundum (sapphire) are very resistant to hydrolysis.
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[*] posted on 2-11-2021 at 22:32

Quote: Originally posted by njl  

Both strong acids and strong bases react

Al2O3 is actually quite stubborn.

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[*] posted on 3-11-2021 at 16:10

After you initial reaction you need to remelt the silicon.
The aluminum oxide will not melt anywhere close to the silicon melting temperature.
The temperature required is around 1500C which is not easy for the amateur.
Industrially, this is how silicon is purified.
They use a zone melting technique, by repeatedly melting and refreezing, you can get very pure silicon.
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[*] posted on 3-11-2021 at 19:53

Al2O3 dissolves in all sorts of things at equilibrium — HCl, NaOH, etc. It just takes its sweet time reaching equilibrium.

You need to grind it up as finely as you can and then it should dissolve in hydrochloric acid in a reasonable amount of time. Good luck though — it's infamously hard.

EDIT: Beware that the freed Si may be pyrophoric

EDIT2: Some references mention Al forming complexes with alpha-hydroxy acids, including lactic acid. One mentions the use of lactic acid to clean oxides off Al metal!
[Edited on 4-11-2021 by clearly_not_atara]

[Edited on 4-11-2021 by clearly_not_atara]

[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 9-11-2021 at 08:07

sodium aluminate
when working with NaOH its good to know 316 stainless steel is practically invulnerable, even to molten NaOH- which you may wanna give a go

NaOH and Al forms sodium aluminate
iirc you can ppt that back out again with an acid, shifting pH, the resulting hydroxide can then be evaporated to dryness where it stops being a gel and you can dissolve soluble compounds previously stuck in the gel

~25 drops = 1mL @dH2O viscocity - STP
Truth is ever growing - but without context theres barely any such.
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