Aluminum oxide (Al2O3) together with silica, is the most important component in the constitution of clays and enamels, conferring resistance and increasing their maturation temperature. It exists in nature in the form of corundum and emery . It has the particularity of being harder than aluminum and the melting point of alumina is 2,000 ° C (2,273.15 K) compared to 660 ° C (933.15 K) for aluminum, so its welding must be done to alternating current .
[ hide ]
- 1 Crystal structure
- 2 Properties
- 3 Production process
- 4 Applications
- 5 Source
Aluminum oxide crystals have a very fine hexagonal crystalline system (nm).
- Density: 3.86 g / cm³.
- Vickers hardness: 1500-1650 kgf mm².
- Modulus of elasticity: 300-400 GPa.
The industry uses the Bayer process to produce alumina from bauxite . Alumina is vital for aluminum production (approximately two tons of alumina are required to produce one ton of aluminum). In the Bayer process, bauxite is washed, pulverized and dissolved in caustic soda ( sodium hydroxide ) at high pressure and temperature; the resulting liquid contains a solution of sodium aluminate and bauxite residues containing iron , silicon, and titanium. These residues are gradually deposited at the bottom of the tank and are then eliminated. They are commonly known as “red mud”. The clarified sodium aluminate solution is pumped into a huge tank called a precipitator. Fine alumina particles are added in order to induce precipitation of pure alumina particles (seeding process), once the liquid cools. The particles are deposited at the bottom of the tank, removed and then subjected to 1,100 ° C (1,373.15 K) in an oven or calciner, in order to eliminate the water they contain, as a result of crystallization. The result is a white powder, pure alumina. The caustic soda is returned to the beginning of the process and used again.
The primary aluminum industry uses alumina primarily as a basic raw material for the production of aluminum. In addition, alumina is used in a complementary way to:
- Thermal insulation for the upper part of the electrolytic cells.
- Protective coating to prevent oxidation of the carbon anodes.
- Absorption of emissions from the tanks.
- It is also used for drying compressed air since it has the property of adsorbing and desorbing water.
- In the health area of dental prostheses, it is used as the basis for the structure of crowns and bridges, providing great hardness and resistance, lightness and translucency.
- In ceramic enamel mills as grinding stones (as the stones that gobble up the birds to crush the grains in the gizzard).
Its regeneration (in the case of adsorption / desorption) is with dry and hot air and has a dew point temperature of -40 ° C (233.15 K).