Making Al2O3

[zoteman94]

I've always wanted to make syntetic gems, so I want to start with transparent sapphire (pure Al2O3). If you can help me to make quartz I would be also happy.

Ideas of making it? Here are mine:

Reacting Al with water and KOH (I don't have NaOH for the moment).

Reacting HCl with K2Al2O4. (I think that's the way is made in the Bayer process) to make KCl and Al(OH)3 and heating to make Al2O3. The problem with this is that I can't measure the amounts and adding excess HCl could make it react with Al(OH)3 to make AlCl3. And I'm planning to make an oxyhydrogen torch to melt it, but while that happens I don't want to have Al2O3 powder, any way to make a single crystal?

Another method would be reacting Al with HCl to make AlCl3 and further heating to give HCl again and Al2O3, I'm waiting for my glass equipment because HCl gas can be a problem...

Another method would be reacting the AlCl3 with KOH to give KCl and Al(OH)3 but I get the problem that if I add too much KOH it would give K2Al2(OH)4.

Another that can be funny is thermite but I can't get the materials easily in my country. (Costa Rica) I know this makes the Al2O3 very impure but iron impurities give it a nice color.

Another can be using a oxidiser but I have the problem of not having powder aluminium and grinding isn't an option because it would be without machines and the mesh is far to be the best.

Some other crazy ideas would be reacting K2Al2O4 with AlCl3, but I think it would be difficult for me to have a decent purity of those.

Another is for the future, for making alkalis. Using an inert atmosphere melting Al and adding NaOH, It would react to give Na2Al2O4 and Na2O and the Na2O further reacts with Al to make Na and Al2O3 and Al with Na2Al2O4 to give the same compounds. That reactions are indeed exothermic.

Any ideas? (I don't have access to a lot of chemicals because I can't buy them from USA (ok I can buy some but shipping is expensive) and here are only a few companies that sell chemicals and I don't have equipment, I'm waiting some friends to give their ones because they were chemists but they don't work anymore.

Thanks.

[ajkoer]

Burn Aluminum and dissolve in pure NH4OH (impure would be to use household ammonia, so heat household ammonia and send the NH3 into distilled H2O to make pure NH4OH). Avoid using an excess of NH4OH as the solution could become clear with an Ammonia Aluminum complex. Otherwise, you should see whitish gelatinous Al(OH)3 and some undissolved residue from the burned Al (most likely silicon impurities). Extract only the pure gelatinous Al(OH)3 (more easily said than done) form the Si residue To this extract add distilled water and CO2. Heat until all the fluid is evaporated. Repeat adding distilled water and CO2. If there is a new residue, this could be either Iron, Magnesium or Manganese Carbonate. In Reynold's wrap Aluminum foil, for example, it is 98% Al and the rest is Si and Fe. Other sources of Al, like Al cans, have Mg and Mn impurities. Hopefully this process of carbonation will separate out these other impurities. Note, Aluminum carbonate is highly unstable decomposing into CO2 and Al(OH)3 upon heating. Heat the final product to yield pure Al2O3 (or dehydrated the Al(OH)3 gel using ethanol).

 

Caution, the process of dissolving of the Al2O3 (and.or the Al) in the NH4OH produces hydrogen gas.

 

Note, the Al2O3 produced from burning Al in air (no not matter how thoroughly you heat it) contains some unreactive Al and Aluminum Nitride. Also, AlN slowly dissolves in H2O to produce NH3. This is why you should dissolve the burned Al in pure NH4OH to separate and purify it. Using Al only also works, but is dissolves more slowly due to possible acrylic & oil coating, and processing techniques to make the protective Al2O3 layer more effective (why?, in my opinion the Al foil manufacturers want to reduce the possible leaching of Al into our food given the developing understanding of the magnitude/or lack thereof of Al toxicity and the potential detrimental, if any, effect on their industry). However, per one Al coating maker, vinegar penetrates the defenses and (I have done this) soaking Al foil in acetic acid for several hours, even though it appears unreactive, does make the Al foil dissolve more rapidly in NH4OH.

 

I probably missed some points in the preparation, so please comment.

Edited June 20, 2011 by ajkoer

[zoteman94]

Thanks for that info!

It would be great... But my problem for now isn't how to purify it... is how to make it... You know, heating Al will not burn it because of passivation.

Also Al2O3 doesn't react with water and CO2 to make Al2(CO3)2 it would be like adding oxygen to salt to make NaClO. If I'm not wrong the way of making it is by double substitution reactions like Al₂(SO₄)₃+3 Na₂CO₃→Al₂(CO₃)₃+3 Na₂SO_4.

Al₂(SO₄)₃ + 3 Na₂CO₃ → Al₂(CO₃)₃ + 3 Na₂SO₄

My frien passivation also happens if I try to add Al to NH4OH or other oxides/hydroxides to oxidise... I think the only way would be thermite or something similar (melting), but I still don't have something to heat at such high temps. (If I had something to reach 1000° I would melt NaCl you know for what, by the way, I'm joking I don't want to electrolyse that, No way of getting rid of Cl2, and impeding Na from reacting with air)