In 1886 the American Charles Martin Hall and the Frenchman Paul-Louis-Toussaint Héroult discovered fused-salt electrolysis independently of each other. Since then it has been the most economical process for the extraction of primary aluminium from aluminium oxide and thus the one that has been used exclusively worldwide. It involves the second stage of the Bayer"Hall"Héroult process; the first stage is the Bayer process for the production of aluminium oxide from bauxite. It was fused-salt electrolysis that first made production of aluminium possible on a large scale (see chronology).
The principle of aluminium electrolysis
The term "electrolysis" is used to describe the decomposition of an electrolyte using electric current. An electrolyte is a liquid that conducts electricity, for example a solution of salts or a salt bath. The molecules of the dissolved substance are split into ions, i.e. into electrically charged atoms. If one applies a minimum direct-current voltage that is characteristic of the respective electrolyte (the "decomposition voltage") to two electrodes immersed in the electrolyte, negatively charged ions migrate to the positive electrode (the anode) and the positively charged ions to the negative electrode (the cathode) and a direct electric current flows. The ions exchange their charges with the electrodes and deposit out as electrically neutral substances. Electrolysis is used industrially on a large scale, for example for the extraction of very pure metals (for example electrolytic copper), chemicals (like caustic potash) and gases (such as hydrogen) or for electroplating (see coating).
The process of Hall and Héroult
Hall and Héroult chose cryolite as the electrolyte. This white mineral, an aluminium compound with sodium and fluorine that melts at about 1000 °C is only found naturally in Greenland, but has been produced synthetically for the most part since 1890 (from aluminium oxide, fluorspar and sulphuric acid). In order to act as an electrolyte, cryolite has to be in the molten state, which at the time of Hall and Héroult was described as "fused salt" - hence the expression "fused-salt electrolysis". Molten cryolite can dissolve up to ten per cent aluminium oxide. When a voltage of four to five volts is applied to the carbon electrodes, aluminium precipitates out at the cathode and oxygen at the anode (whereby the oxygen burns with the carbon to form carbon dioxide and carbon monoxide). Due to the electrical resistance in the electrolyte, a part of the electrical energy is converted to heat, which keeps the electrolyte molten.
The process has continually been improved since it was discovered. Today, fluorides of aluminium, calcium and lithium are added to the cryolite, which makes a working temperature of about 950 °C possible and reduces the energy requirement. In the most modern plants, this is between 13 and 15 kilowatt-hours per kilogram of aluminium. Modern pots produce about 2.5 tonnes of unalloyed aluminium a day. Since the mid-1960s, the pots in western industrialised countries have been completely encapsulated (hooded) to collect the fluoride and dust-containing anode gas for cleaning (see emissions).