Alumina

Alumina, also commonly known as aluminium oxide, is a chemical compound primarily comprised 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 are 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 Fe₂O_{3, are} filtered. Once 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.

Overview of Aluminum oxide

Structure of Alumina

As stated above, the most common form of alumina or aluminium oxide is known as corundum, which is in a crystalline form. In this, the oxygen ions tend to form a hexagonal close-packed structure. Meanwhile, the aluminium ions fill two-thirds of the octahedral interstices. Each Al3+ centre is octahedral.

Alternatively, alumina also exists in several different metastable phases, such as the cubic γ and η phases, the hexagonal χ phase, the orthorhombic κ phase, the monoclinic θ phase and the δ phase. Now, these can either be orthorhombic or tetragonal. Each phase will have a unique crystal structure and properties.

Properties of Alumina

• Alumina possesses good or high thermal conductivity. At elevated temperatures, it can withstand alkali attacks and strong acids.
• It consists of excellent dielectric properties.
• Metallic aluminium is highly reactive with atmospheric oxygen.
• It exhibits high stiffness and strength.
• It is available at a purity range from 94%.
• It is insoluble in water.
• Alumina is an amphoteric substance, which means that it can react with both acids and bases.
• Activated alumina is a porous, granular substance.
• Alumina is an excellent ceramic oxide.
• Has high resistance to abrasion and chemical attacks.
• Wear resistance

Reactions of Alumina

1. Reaction with Sodium Hydroxide

The reaction of alumina with sodium hydroxide results in the production of sodium aluminate (salt) and water. This reaction occurs at temperatures between 900-1100°C. Here, aluminium oxide acts as an acid.

Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O

2. Reaction with Hydrochloric Acid

Alumina reacts with heated dilute hydrochloric acid to produce an aluminium chloride solution. In this case, alumina, since it contains oxide ions, reacts with acids in the same way magnesium or sodium oxides do.

Al₂O₃ + 6HCl → 2AlCl₃+ 3H₂O

3. Reaction with Sulphuric Acid

One of the properties of aluminium oxide is that it is an amphoteric oxide. What it means is that it can act both as an acid and base. Interestingly, during its reaction with sulphuric acid, alumina acts as a base. This is basically a form of a neutralization reaction.

Al₂O₃ + H₂SO4 → Al₂(SO₄)₃ + H₂O

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, etc.
• 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 pulverised fuel lines to protect high-wearing areas.
• Alumina is often used extensively in engineered ceramics which is also called advanced or technical ceramics.