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Aluminum is not found in nature in its pure state (in other words, it does not exist an "aluminum mine"...) but it is always bond with other elements, the most common being the oxygen. Among the most common aluminum ore there is the alumina, chemical formula Al2O3. The alumina is found in nature most of the time in a kind of soil named bauxite, after the city of Baux in France, where it was first identified. It contains on average 50% in weight of alumina, the rest being other oxides, mainly iron oxides.
With the Hall-Heroult process, through the use of electricity, the chemical bond between aluminum and oxygen in the alumina is broken and the aluminum is then collected for further processing.
The process was discovered at the same time, 1886, and independently by american Charles Martin Hall and french Paul Louis Toussaint Heroult (this the reason why it is called Hall-Heroult process). Nowadays, after more than 100 years, the industrial process is basically the same invented by Hall and Heroult.
Going more in detail, in the Hall-Heroult process the alumina is dissolved in a molten salt, acting as the electrolyte, called cryolite (Na3AlF6) which melts at around 950°C. The molten salt is contained in a vessel (called cell or pot) with the structure made by iron and internally lined with blocks of carbon. The mixture of molten cryolite and dissolved alumina is usually called bath. Into the bath is immersed the anode, another block of carbon, which carries the electrical current into the molten bath driving the electrochemical reactions which make the alumina reduction according to the reaction:
2Al2O3 + 3C -> 4Al + 3CO2
The molten aluminum produced is collected into the bottom of the vessel containing the bath and periodically is tapped.
The electricity flowing from the anode into the bath is then collected by the carbon lining of the vessel.
As can be seen on the previous reaction, the carbon anodes takes itself place on the reaction, being consumed and evolving carbon dioxide in the form of bubbles from the bath. This means that the anode has to be replaced on a regular basis.
The chemical bond between aluminum and oxygen in the alumina is very strong, and this means that a lot of electrical energy is required to produce aluminum. The values depends on the cell technology, ranging from 12.5 to 14.5 kWh/kg in present industry.
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