Aluminum and Its Alloys

Aluminum and Its Alloys

Why aluminium alloys?
Abundant element of 8% on earth crust and normally found in oxide forms (Al2O3), i.e., bauxite, kaolinite, nepheline and alunite.
Attractive properties
• High corrosion resistance
• Excellent machining properties
• Light weight
• High thermal/electrical conductivity
• High ductility/easily deformableFrance
Properties and Uses
1. It is a silvery white, light metal, with a density about a third that of steel or brass.
2. It is a very good conductor of electricity; Al is better conductor than copper. Use for heavy conductors and overhead cables.
3. It has a high resistance to corrosion than many other metals, used in manufacture of containers for chemical industry.
4. High heat conductivity: Used in various heat conductivity appliances.
5. Very Ductile, so it can be easily rolled, forged, die cast.
6. Light in weight, widely used in aircraft industry.
7. It becomes hard by cold working.
8. Water and air have practically no effect on it.
9. The milting point of pure Aluminium is about 650°C .
Extraction of allumiiniium
Aluminium can be extracted from bauxite, Kaolinite or nepheline.
Bauxite: 30-50% Alumina (Al2O3)- 3-13% Silica (SiO2) -10-18% Titanium oxide (TiO2)-Balanced water (H2O).
Kaolinite:30-32% Alumina (Al2O3)-Balanced Silica (SiO2) and water (H2O)
Nepheline: 30% Alumina (Al2O3)-40% Silica (SiO2)-20% Na2O + K2O
Bayer process
Alumina (Al2O3) is extracted by digesting crushed bauxite in strong sodium hydroxide (NaOH) solution at Temp upto 240oC.
Schematic diagram of Bayer process
• Bauxite is washed ground and dissolved in caustic soda (NaOH) at high pressure and temperature.
• Sodium aluminate solution containing nonsoluble bauxite residues sink to the bottom ? red mud.
• Seeding agent is added to the clear sodium aluminate solution to give
alumina precipitates.which sink to the bottom of the tank.
• Precipitates are passed through a rotary kiln for calcination at ~1100oC
to give white powder of pure alumina.