Alumina is a white solid that is a compound of aluminum and oxygen, its chemical formula is Al2O3. In mining, as well as ceramics and materials science, it is also known as bauxite.
Alumina's reserves on earth are only second to silicon oxide. It is one of the raw materials of ceramics. In the oxides, aluminum and oxygen have a strong bond, making alumina have the highest hardness among oxides, it has high chemical stability and excellent resistance to most acids, alkalis, salts and molten solutions.
The content of alumina is the highest in the main component of the aluminum ore, bauxite. Industrially, iron-aluminate is purified as alumina by the Bayer process:
Heating with sodium hydroxide dissolved ore. Iron oxide is insoluble, silica is dissolved as silicate (Si(OH)62−), alumina is dissolved as aluminate, filtered, acid treated, aluminum hydroxide precipitated, and then filtered.
Aluminum hydroxide is heated to give amorphous white alumina powder. Then converted to aluminum metal by the Hall-Heirolt process.
The main components of ruby and sapphire are all aluminum oxide, which shows different colors due to other impurities. Ruby contains chromium oxide and is red, while sapphire contains iron oxide and titanium dioxide and is blue.
Alumina is the reason why metallic aluminum is not easily corroded in the air. The pure aluminum metal reacts easily with oxygen in the air, creating a thin layer of aluminum oxide film that covers the aluminum surface exposed to air. This aluminum oxide film prevents aluminum from being continuously oxidized. Both the thickness and the properties of this oxide film can be enhanced by a process called anodization (anode corrosion).
Aluminum is a good conductor of electricity and heat, and alumina is an insulator between electricity and heat. Alumina crystal form corundum is suitable for abrasives and cutting tools because of its high hardness.
Alumina crystals can also serve as substrates for epitaxy.
Alumina powder is often used as a vehicle for chromatographic analysis.
Emergency braking device for high-speed railway trains (sprays alumina powder to rail to increase rail friction)
Porous alumina with the high specific surface area is called activated alumina and is often used as a catalyst, adsorbent, dehydrating agent and catalyst carrier.
The main components of firebrick