What happens when you burn Al2O3?

[Mebing]

I have a bag of Al2O3 from the aluminium smelter. I have heard of activating Al2O3 by burning it. What exactly is happening to Al2O3 chemically when you burn it? And how is it better than when it's not activated? What does "activation" means when you burn Al2O3?

 

[ardilesd]

Just guessing...
Perhaps they refer to some kind of thermal treatment, e.g. process Al2O3 for some hours an a given temperature (700 °C). This could have some effect on activating or promoting some acidic sites in the solid which are responsibles for asdortive properties of alumina (mainly used in heteregeneous catalysis)

 

[Chuck56]

Hydrated alumina Al2O3*3(H2O) can be flash activated, or calcined, by heating. This activation results in Al2O3*1/2H2O. The common desiccant in plant compressed air dryers is Activated Alumina. With various modifications, it is used in a wide variety of adsorption and purification processes. The activation does result in a number of active, charged surface sites, and a greatly increased surface area (a gram of activated alumina has 350 m2 of surface area!). Activated alumina has high affinity for water.

If you have a bag of Smelter Grade Alumina (SGA) or plain hydrated alumina, it is really not practical to activate it yourself, though you certainly could try. If you need a desiccant, it is easily available.

Hope this helps.

Chuck

 

[Mebing]

Does anyone know any books on activation of alumina? What I'm interested in is how by burning it results in active, charged surface and how it increase in area. Any journals or books that i can read about this?

thanks a lot. It has been helpful...

 

[kenvlach]

Mebing,
I agree with Chuck, you are probably wasting your time. One does not ‘activate’ alumina by burning it – alumina does not burn. Possibly, heating in very high pressure steam will activate the surface, but it will do little for the bulk structure. Heating activated alumina at excessive temperature will de-activate it; the material will tend to a more stable, lower energy structure by decreasing surface area via sintering. I am not sure what ‘alumina from an aluminum smelter’ is (maybe dross?); but have used granular alumina (like sand) in some foundry work, and it is totally unsuitable for activation.

Activated alumina is produced by first precipitating aluminum hydroxide from aqueous solution, and then briefly heating the hydroxide to drive off the water, creating a very porous network structure. If you actually want to use your material, you need to characterize it better before proceeding. My guess is that the only method would be to dissolve it in acid, neutralize to precipitate Al(OH)₃, rinse with DI water (or perhaps some anti-sintering agent), then briefly heat the aluminum hydroxide to drive off the water. The high surface area is a result of the loose network structure of the aluminum hydroxide precipitate and its subsequent decomposition (think popcorn).

“Activated Alumina-
A very high surface area alumina that is used in a variety of adsorbent and catalyst applications. This product normally is supplied in the chi crystalline phase and has a surface area of up to 300 m2/g. The product is produced by performing a special flash calcination on alumina trihydrate. After activation, the product typically is formed into granules or pellets.”

http://www.alcanchemicals.com/products/glossary/gloss1.htm

 

[25362]

If you pay a visit to the internet by asking for thermal activation of alumina, you'll most certainly find information on the principles by which alumina becomes activated.