[eric2011 (OP)]

Hi

What chemicals can lower aluminum oxide melting point?

I know mokybdenum oxide can but where I am is not avsilable.

I want to lower the temperature from 2200 C to at least 800 C

[alancalverd]

Cryolite is the usual additive for the production of electrolytic aluminium at about 900C.

[chiralSPO]

See more here: https://en.wikipedia.org/wiki/Hall%E2%80%93H%C3%A9roult_process

[evan_au]

Cryolite is the usual additive

For a long time, the only commercial quantity of cryolite ore was from one mountain in Greenland.
There are traces of cryolite ore elsewhere, but not in commercial quantities, so today, artificial cryolite is used.

Due to its rarity it is possibly the only mineral on Earth ever to be mined to commercial extinction

See: https://en.wikipedia.org/wiki/Cryolite

[Bored chemist]

Pretty much anything added as an impurity will reduce the melting point of alumina.
 The electrolyte in aluminium production is more realistically described as "cryolite with alumina as an impurity", than the other way round.
https://www.sciencedirect.com/topics/chemistry/cryolite
Nothing is going to produce a mixture that is mainly alumina, but with a melting point as low as 800C.

What problem are you trying to solve?

[eric2011 (OP)]

Pretty much anything added as an impurity will reduce the melting point of alumina.
 The electrolyte in aluminium production is more realistically described as "cryolite with alumina as an impurity", than the other way round.
https://www.sciencedirect.com/topics/chemistry/cryolite
Nothing is going to produce a mixture that is mainly alumina, but with a melting point as low as 800C.

What problem are you trying to solve?

I want to do an experiment to see if I can produce, it doesnt matter the quality, synthetic corrundum. I have a furnace that can go as up as 1400 C degrees. But in order to melt Aluminum oxide it requres temperature of 2200 C degrees. So this is why I want to reduce the temperature to melt aluminum oxide. Initially I wanted to reduce the temperature way lower but it seems it is not possible.

[eric2011 (OP)]

Hi guys. Since I posted this post I have continued researching about this problem. My search led me to some guy mixing aluminum oxide, leed oxide and boron oxide together in a crucible and heat in in a furnace at a temperature of 1300 C and the aluminu. oxide melted/dissolved. I want to try it but I think lead oxide is poisonous. What do you think I can use in place of lead oxide which is not poisonous?

[chiralSPO]

Yes, lead oxide is definitely poisonous (and volatile!) I would highly recommend against using it.

Perhaps you could try a different approach to making synthetic corundums—growing crystals out of solution instead of from a flux or melt.

At room temperature, aluminum oxide has very high solubility in highly alkaline aqueous solution (forming aluminate ions), and can be precipitated out again with a decrease in the pH. Slowly changing the temperature and/or pH of the solution may allow growth of aluminum oxide crystals.

Hydrothermal synthesis of sapphires is a known alternative to synthesis from molten aluminum compounds.

[Bored chemist]

Hydrothermal synthesis of sapphires is a known alternative to synthesis from molten aluminum compounds.

I think you still need a high temperature to avoid production of diaspore.
I guess 500C is a lot better than 2200.

[eric2011 (OP)]

Thank you all guys for your imputs. I think I will do a further research on hydrothermal. I think mixing aluminum oxide with Naoh and heat it may well be a next experiment. Cheers.

[Bored chemist]

What do you plan to do with the sodium aluminate?

[eric2011 (OP)]

Yes, lead oxide is definitely poisonous (and volatile!) I would highly recommend against using it.

Perhaps you could try a different approach to making synthetic corundums—growing crystals out of solution instead of from a flux or melt.

At room temperature, aluminum oxide has very high solubility in highly alkaline aqueous solution (forming aluminate ions), and can be precipitated out again with a decrease in the pH. Slowly changing the temperature and/or pH of the solution may allow growth of aluminum oxide crystals.

Hydrothermal synthesis of sapphires is a known alternative to synthesis from molten aluminum compounds.

Can you explain a bit how can I do this growing aluminum oxide crystals in a home setting if I have 500 C furnace like Bored chemist says. And what aquous solution dissolves aluminum oxide?

If the aluminum oxide is dissolved in aquous solution how do yo crystallize the crystals?