Aluminium encyclopaedia

Bauxite / bauxite mining

Deposits

In 1822 a French geologist, Pierre Berthier (1782-1861), discovered a reddish coloured mineral containing about 50 per cent aluminium oxide (alumina) near the village of Les Baux in southern France. His fellow countryman Dufresnoy gave the mineral the name "bauxite" in 1847. Bauxite mining, which began around 1860 near Marseille, provided raw material for the aluminium compound aluminium sulphate as well as for aluminium extraction according to the Sainte-Claire Deville process. With the introduction of fused-salt electrolysis around 1887, the growth in aluminium production also led to a rapid increase in the demand for bauxite. Subsequently deposits were developed in Great Britain and the USA, and then later also in Guyana, Greece, Italy, Surinam, Hungary, Yugoslavia and the Caribbean (for example Jamaica). In the 1950s and 1960s, Australia, Guinea, India, Indonesia, Malaysia and Sierra Leone developed into the main mining areas, followed in the 1970s by Brazil. The main mining areas today are Australia, Guinea, Brazil and Jamaica.

Known bauxite reserves, which are spread across all continents and more than 60 countries, are estimated to be over 22 billion tonnes.

Formation

Bauxite is a weathering product of calc-silicate rock with an aluminium oxide (Al2O3) content often above 50 per cent and is mainly opencast mined. In hot and humid climates, rainwater and naturally occurring acids dissolve metals such as manganese and potassium out of the rocks and wash them into the sea, while aluminium, iron and titanium remain behind, become enriched and combine with oxygen to form oxides. The main constituents of bauxite are thus aluminium hydroxides (aluminium oxide containing water) and iron oxide, as well as small amounts of titanium oxide and other oxides. As a result of the differences in the conditions under which they are formed and their composition, the consistency of bauxites ranges from earthy-soft to as hard as crystal, and the colour can be light grey, yellow, pink or a dark reddish brown.

Use

At the moment, mining of bauxites is only economical if they are easily accessible and contain more than 30 per cent aluminium oxide (in the case of on-site processing) or more than 50 per cent (where there is long-range transport before processing). Over 90 per cent of the world"s annual production is used for aluminium extraction, the rest is used for chemicals, refractory linings, abrasives and aggregate in cement manufacturing. Clays and many other minerals, which are found almost everywhere, are also aluminium ores. They can be mined economically but it is not possible to process them to aluminium oxide economically.

Rehabilitation

Opencast mining requires areas of land that have to be rehabilitated once the bauxite mining has been completed. Rehabilitation is part and parcel of bauxite mining. Appropriate measures to facilitate the desired type of rehabilitation and subsequent use are taken long before actually mining the ore. This involves steps such as careful removal of vegetation, collection of seeds, systematic removal of topsoil and overburden, and the temporary storage of these top layers.

Only then is the bauxite deposit mined. Subsequently there are several steps that are an important part of bauxite mining, such as the adaptation of the site to the landscape by spreading the temporarily stored overburden und top soil over the whole area and planting the seeds (covering with vegetation).

This is essential for efficient rehabilitation. Today about 85 percent of the bauxite mining area is reforested and a further 10 per cent is developed for agricultural purposes. The remaining approximately five per cent is used, for example, for recreational, residential or industrial areas for social or economic development.

Bauxite mining is accompanied by continuous environmental monitoring, which includes erosion control and water and waste management. Moreover, the mine operators have their own nurseries for plants and trees, which enable different types of plants and saplings to be grown.

If, for example, one wants to put the mining area to agricultural use, research projects are carried out to test and optimise the yield of fruit trees or grasses under the specific geographic conditions.