Aluminium is the third most common element in the biosphere after oxygen and silicon, and the most common metal in the earth’s crust, but being a base metal, it is not found in its pure form. Aluminium was first purified at the beginning of the nineteenth century, and in 1886 Charles Martin Hall and Paul Héroult developed the fused-salt electrolysis process independently and almost at the same time. After many improvements, theirs is still the standard industrial process. It mainly comprises a redox reaction enforced by connecting a DC power supply. Carbon is used as a reducing agent for the aluminium oxide won from bauxite. The continuously fed carbon anode combines with oxygen to form CO and CO2 gases, and burns down accordingly. Reduced aluminium is deposited on the cathode for subsequent collection. It takes about 500 kg of carbon anode to produce 1 ton of aluminium.
The pure carbon electrodes used in the fused-salt electrolysis process can only be made from special types of coke. Petroleum coke is used industrially because it is available in large quantities. Coal tar pitch is added to the milled petroleum coke as a binding agent. This pitch, a residue from coal tar distillation, is solid at normal temperatures and easily crushed. When heated it melts at 95° to 120°C, depending on molecular weight, to form a low viscosity liquid. The anode paste for electrodes in the correct composition is produced on a respective processing line where the petroleum coke delivered is first crushed, carefully dried, milled, sieved, sifted, and strictly separated for intermediate storage in various bunkers according to grain size.