The Hall- Heroult process is an example of Aluminium smelting process and is used industrially. Aluminium cannot be produced by an aqueous electrolytic process because hydrogen is Electrochemically much nobler than aluminium. Thus, liquid aluminium is produced by the electrolytic reduction of alumina dissolved in an electrolyte (bath) mainly containing Cryolite . The overall chemical reaction can be written as:
Aluminium is formed at about 900°C, but once formed has a melting point of only 660°C. In some smelters this spare heat is used to melt recycled metal, which is then blended with the new metal. Recycled metal requires only 5 percent of the energy required to make new metal. Blending recycled metal with new metal alloys considerable energy savings, as well as the efficient use of the extra heat available. When it comes to quality, there is no difference between primary metal and recycled metal.
The smelting process required to produce aluminium from the alumina is continuous, the outline is usually kept in production for 24 hours a day year around. A smelter cannot be easily stopped and restarted. if production is interrupted by a power supply failure of more than four hours, the metal in the pots will solidify, often requiring an expensive rebuilding process.
There are two basic technology- Prebaked Technology and Soderberg Technology. In prebaked technology the anodes used are termed as prebaked anodes which are made from a mixture of petroleum coke, aggregate and coal tar pitch binder moulded into blocks and baked in separate anode baking furnace at about 1120 °C. An aluminium rod with iron studs is then cast or rammed into grooves in the top of the anode block in order to support the anode and conduct the electric current to the anode when it has been positioned in the cell.
Prebaked anodes have to be removed at regular intervals, when they have reacted down to one third or one fourth of their original size. These remaining anodes are termed as butts and are usually cleaned outside the cell in a separate cleaning station to be able to recirculate the adhering bath materials removed from the cell. The cleaned butts are then crushed and used as a raw material in the manufacturing of new anodes. A typical prebaked cell is shown below:
The major raw material required for aluminium production is alumina, carbon, power, aluminium fluoride and cryolite.
|Raw Material||Alumina||Carbon||Cryolite||Aluminium Fluoride||Energy|
|Theoretical Consumption (Kg per Ton of Aluminium)||1930||415||2||20||13460 KWH/t-AI|
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