Aluminium encyclopaedia

Extraction

The classical industrially used metals such as iron, copper or zinc are extruded from their oxidic ores (compounds with oxygen) by carbothermic reduction: together with carbon the ore is heated to a high temperature and the carbon reduces the ore to the metal by combining with the oxygen to form carbon dioxide. This is basically not suitable for the extraction of aluminium (so-called "primary aluminium") from bauxite or other aluminium ores because aluminium reacts readily with oxygen. This was something that the aluminium pioneers Davy, Oersted and Wöhler realised at the beginning of the 19th century. They were also barely in a position to achieve the necessary temperatures of more than 2000 °C, which is why they looked for other possible methods of extraction. Since its introduction around 1890, the Bayer"Hall"Héroult process has been the only process used because it is the most economical; the aluminium industry has striven to develop processes with lower energy requirements and consequently lower costs. Of the many different methods investigated, four will be mentioned here.

The standard process

The Bayer"Hall"Héroult process covers two steps: the first is the extraction of aluminium oxide from bauxite in the Bayer process; aluminium is then obtained from this by fused-salt electrolysis (patented by Hall and Héroult in 1886). The energy requirement is some 13 to 16 kilowatt-hours per kilogram of aluminium on average (up to 95 per cent less energy is required to remelt scrap and produce "recycled aluminium" or "secondary aluminium" as it is also known).

Other interesting processes

In 1854 Sainte-Claire Deville developed the first practical process for aluminium production, which was complex, however, and only used up until 1890. If aluminium chloride is heated together with sodium, the latter combines with chlorine to form sodium chloride resulting in the formation of aluminium. The continuing interest in carbothermic reduction is due to the fact that it would make it possible to process bauxite as well as poor aluminium ores directly. Since the first patent was granted to the Briton Monckton in 1862, this approach has proven to be impractical despite many efforts. In 1967 the French company Pechiney closed a test plant in which it had produced over 1000 tonnes of aluminium since 1960 having found that the energy requirement, consumption of raw materials and costs were too high. At the beginning of the 1980s (following the increases in oil prices in the 1970s) carbothermic reduction was investigated anew and evaluated carefully especially in the USA. This showed that the energy requirement (with certain savings possible) was greater than for the Bayer"Hall"Héroult process, and there were certain material problems that still had to be solved. Other detrimental effects are that heating aluminium oxide with carbon at temperatures above 2100 °C produces gaseous carbon monoxide (which contains vaporised aluminium and aluminium oxide and thus represents a loss of raw materials) and the fact that the molten aluminium contains ten per cent of dissolved carbon. The latter solution solidifies to a mixture of aluminium and aluminium carbide; even after these have been separated, which requires considerable effort, the aluminium still has to be cleaned to remove remaining traces of carbon.

In the ASP process (Alcoa Smelting Process), aluminium is precipitated from aluminium chloride (extracted from aluminium oxide or directly from bauxite) by electrolysis. Although the energy requirement is less than in the Bayer"Hall"Héroult process, there are technical problems involved in using the process.