Alumina is also commonly known as aluminium oxide is a chemical compound primarily comprising of oxygen and aluminium molecules. It is a colourless crystalline substance that is found naturally in a variety of forms namely sapphire and ruby which is usually formed from corundum. The most common naturally occurring crystalline form of alumina or aluminium oxide is corundum. Interestingly, during special circumstances, corundum turns into sapphire and ruby gemstones.

Alumina is also extracted from ores such as bauxite which is found in topsoil in certain tropical and subtropical regions. Extraction and refining of alumina are done using the Bayer process. Here, the bauxite ore is crushed and dissolved in a sodium hydroxide solution. While keeping the temperatures high, the bauxite is then converted into sodium aluminate and the impurities such as slag also known as red mud or Fe2O3  is filtered. As the Bayer liquor is cooled, Al(OH)3 precipitates, leaving the silicates in solution.

Aluminium oxide particles are separated by the heating or calcination of Al(OH)3 at around 1100°C. When refined from bauxite, alumina has an appearance of a white powder just like table salt or granular sugar.

Mineral Description

Bauxite is a naturally occurring, heterogeneous material composed primarily of one or more aluminum hydroxide minerals, plus various mixtures of silica, iron oxide, titania, aluminosilicate, and other impurities in minor or trace amounts.  The principal aluminum hydroxide minerals found in varying proportions with bauxites are gibbsite and the polymorphs boehmite and diaspore.  Bauxites are typically classified according to their intended commercial application:  abrasive, cement, chemical, metallurgical, refractory, etc.  The bulk of world bauxite production (approximately 85%) is used as feed for the manufacture of alumina via a wet chemical caustic leach method commonly known as the Bayer process.   Subsequently, the majority of the resulting alumina produced from this refining process is in turn employed as the feedstock for the production of aluminum metal by the electrolytic reduction of alumina in a molten bath of natural or synthetic cryolite (Na3AlF6), the all-Héroult process.

Applications of Aluminium Oxides

Alumina is the primary material in the production of aluminium. 90% aluminium oxide is used in the manufacture of aluminium metal mostly by the electrolysis process.

They are widely used in Electronic Substrates, Thread and Wire Guides, Seal Rings, Ballistic Armour, Thermometry Sensors, Grinding Media, Furnace Liner Tubes, High Voltage Insulators, Laboratory Instrumentary Tubes.

Various formulation of glass consists of aluminium oxide as an ingredient.

It is used in purification that is the removal of water from gas streams. They are employed in high-performance applications.

It is a favoured filler for plastic. It is sometimes present in cosmetics such as nail polish, lipstick and blush.

They are broadly used as an abrasive. Various sandpaper makes use of aluminium oxide crystals.

They are employed in the manufacturing of titles where they are attached inside the pulverized fuel lines to protect high wearing areas.

Alumina is often used extensively in engineered ceramics which is also called advanced or technical ceramics.