January 25, 2010

Aluminum Production

Metallic aluminum is produced by the electrolytic reduction of pure alumina. In a bath of fused cryolite. For pure alumina, possible to reduce alumina with carbon because Al4Cl3 is formed and a back reaction between aluminum vapor and CO2 in the condenser quickly reforms the original aluminum oxide again. The enthalpy change involved in the reaction is equivalent to 20.3 MJ of energy per kilogram of aluminum produced.

Al2O2 + 1 ½ C ==► 2 Al ++ 1 ½ CO2 ∆H = 1100 kJ at 1000 oC

In actual practice, however, some energy is used to bring reactants up to temperature and some is lost in the sensible heat of the products. Some carbon monoxide is formed in the reaction thus increasing the positive ∆H, which amounts, in practice, to 47.5 to 71.4 MJ/kg. consequently this metal can’t be made economically unless low priced requires from 0.5 to 0.6 kg of carbon per kilogram of metal. Although the theoretical amount required by the equation to 0.33 kg, the carbon dioxide evolved contain from 10 to 50% carbon monoxide, so more is required in practice the step involved in the production of the metallic aluminum are:
  •  The cell lining is installed or replaced
  •  Carbon anode are manufactured and used in the cell
  •  The cryolite bath is prepared and the composition controlled
  •  Alumina is dissolved in the molten cryolite bath.
  •  The solution of alumina is fused cryolite is electrolyzed to form metallic aluminum, which serve as the cathode.
  •  The carbon electrode is oxidized by the oxygen liberated
  •  The molten aluminum is tapped from the cells, alloyed off desired, cast into ingots, and cooled.
The electrolytic cells are huge boxy steel containers. Inside each is a cathode compartment lined with either a rammed mix of pitch and anthracite coal or coke baked in place by the passage of electric current, or prebaked cathode blocks cemented together.

This cathode compartment cavity may be from 30 – 50 cm in depth and up to 3 m wide and 9 m long depending on type of the cell and the load for which it is designed. The cavity lining thickness varies from 15 to 25 cm on the side and 26 to 46 cm on the bottom. Thermal insulation consisting of free brick, asbestos blocks, or other similar materials is placed between the cavity lining and the steel cell. Large steel bars, serving as cathode current collectors, are imbedded in the bottom portion of the cavity lining and extend out through to the cathode collectors which is dissolves or penetration of metal out of the steel shell where it leak out around the collectors.

Cell relining is an appreciable part of production expense, including not only the cost of labor, collectors, lining, and insulating materials, but also the loss of electrode materials absorbed by the spent lining (producers now reclaim at least some of these electrolyte materials).

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