Page 18 - Curriculum Visions Dynamic Book

Aluminium refining
The partly purified ore of bauxite, called alumina, is still a compound of aluminium and oxygen. To refine this to aluminium, the alumina has to be dissolved and the aluminium recovered by electrical means.
The process of using electricity to separate a metal from its rock ore is called electrolysis and takes place inside electrolytic cells.
The alumina has to be liquified so that the aluminium compound will dissociate (break apart) into electrically charged particles called ions. Aluminium ions have a positive charge and can move through a solution to gather at the negatively charged electrode of the cell (the cathode).
Each cell uses a mere four to six volts, about the same as a dry cell used for a flashlight. But the current that flows is sometimes as much as 150,000 amps (equivalent to the maximum consumption of 300 households with all their appliances in use).
An aluminium smelter
Processing alumina happens on
a large scale, but the electrical process cannot be done in a single large vat. Instead hundreds or even thousands of cells are used, made of steel with a carbon lining.
The process turns aluminium oxide into aluminium by removing the oxygen. This is called reduction.
The cells are first filled with a material called cryolite, which is heated to 980°C. Alumina will melt at a lower temperature when mixed with cryolite than if it were melted on its own. This saves electricity.
The alumina and cryolite mixture is then poured into the cell and rows of carbon electrodes are dipped into it. A current flows
from the hanging electrodes to the carbon lining of the cell. At the bottom of the cells, embedded
in the carbon lining, are collector plates (cathodes).
The electrical energy separates the aluminium ions from the oxygen ions and the aluminium collects on the plates at the bottom of the cell.
This process operates continuously, the molten aluminium being siphoned out of the cells and new alumina added from above. The aluminium can then be fed to mills and rolled into sheets, poured into moulds where it cools to make ingots for later use, or mixed with other metals to make alloys.
 This is the “cell room” of an aluminium smelting plant. Each cell is up to ten by four metres. Each cell makes about nine hundred kilograms of aluminium each day.
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