Aluminium oxide

Aluminium oxide, also known as aluminium(III) oxide, is a chemical compound made from aluminium and oxygen. Its chemical formula is Al₂O₃, and it is the most common type of several aluminium oxides. People often call it alumina, and it also goes by names like aloxide, aloxite, ALOX, or alundum depending on how it is used.

Alumina comes from a material called bauxite. In nature, it appears in a crystal form called α-Al₂O₃, which is known as the mineral corundum. Some types of corundum are very valuable and are cut into gemstones like ruby and sapphire.

Aluminium oxide is very important for many uses. It is used to make aluminium metal, works as an abrasive because it is very hard, and can also stand up to very high heat, making it useful as a refractory material.

Natural occurrence

Corundum is the most common form of aluminium oxide found in nature. Rubies and sapphires are special, shiny types of corundum. Rubies get their deep red color from tiny amounts of chromium, while sapphires can be many different colors because of other tiny bits of materials like iron and titanium mixed in. Although aluminium is very common in the ground, we need to get it from a material called bauxite to make aluminium metal.

History

Aluminium oxide has a long history. People used aluminium salts a long time ago in ancient and medieval times, especially in something called alchemy. Some old books talk about this history.

Properties

Al₂O₃ does not conduct electricity but can move heat well for a ceramic material. It does not dissolve in water. In its most common crystal shape, called corundum or α-aluminium oxide, it is very hard and can be used for scrubbing and in tools that cut materials.

Aluminium oxide helps protect metal aluminium from weather. When aluminium touches air, a very thin layer of aluminium oxide forms right away, keeping the metal safe. Special treatments can make this layer even stronger. Some mixtures of metals use a bit of aluminium to stay safe from rust better.

Aluminium oxide can react with both acids and bases, acting like either to make a new substance.

Al₂O₃ + 6 HF → 2 AlF₃ + 3 H₂O

Al₂O₃ + 2 NaOH + 3 H₂O → 2 NaAl(OH)₄ (sodium aluminate)

Structure

The most common form of aluminium oxide is called corundum. In this form, oxygen atoms are arranged in a nearly hexagonal close-packed structure, with aluminium atoms filling two-thirds of the spaces between them. Each aluminium atom is surrounded by six oxygen atoms in a shape called octahedral.

Aluminium oxide can also exist in other forms, each with its own unique arrangement of atoms and properties. One of these forms, called cubic γ-Al₂O₃, is used in many important technologies.

Production

See also: List of countries by aluminium oxide production

Aluminium oxide comes from minerals found in a rock called bauxite. Bauxite is cleaned using a process called the Bayer process. This process uses special liquids to separate the useful parts from unwanted bits. After cleaning, the material is heated to very high temperatures to make aluminium oxide.

There is also a method called the Sintering Process. This is used when the Bayer Process is not suitable, such as when the bauxite has a lot of silica. In this process, bauxite is mixed with other materials and heated to very high temperatures. The mixture is then cleaned with water to get the needed product, which is later heated again to make aluminium oxide. This method helps make different shapes and types of aluminium oxide.

Applications

Aluminium oxide, also known as alumina, is widely used in many everyday products and industries. Over 90% of the aluminium oxide made each year is used to produce aluminium metal. The rest is used in many special ways because it does not change easily, can handle high temperatures, and does not let electricity pass through.

Aluminium oxide is often added to plastics to make them stronger and whiter. It is also found in sunscreens and cosmetics like blush and nail polish. In glass-making, aluminium oxide helps create strong and clear glass. It can help certain chemical reactions happen, such as turning harmful gases into safer substances in factories. Because it is very hard, aluminium oxide is used to make sandpaper and other scrubbing tools. It is also used in toothpaste to help clean teeth.

In medicine, aluminium oxide is used to make strong and safe materials for artificial joints and other implants. It is also used in some special fibers for very strong materials. Aluminium oxide can be used in body armor to help protect people. It is also used to protect electronic parts from electricity and to make strong coatings for many objects.