Bauxite & alumina history:
|1821||Bauxite discovered near Les Beaux in southern France by Pierre Berthier|
|1847||Armand Dufrénoy names the ore “beauxite”|
|1861||Henri Sainte-Claire Deville renames it as “bauxite”|
|1854||Sainte-Claire Deville discovers a method to separate kilogram amounts of aluminium from its oxide, alumina|
|1886||Charles Martin Hall and Paul Héroult independently develop a process for making aluminium from alumina|
|1887||Karl Josef Bayer patents his process for extracting alumina from bauxite|
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Aluminium is the most abundant metal in the earth’s crust.
The aluminium-containing bauxite ores gibbsite, böhmite and diaspore are the basic raw material for primary aluminium production.
Proven, economically viable reserves of bauxite are sufficient to supply at least another 100 years at current demand. While demand for bauxite is expected to grow as demand for high quality aluminium products increases, new reserves will be discovered or become economically viable.
Gibbsite is an aluminium hydroxide (Al(OH)3), while böhmite and diaspore are both aluminium-oxide-hydroxides (AlO(OH)). The main difference between the latter two is that diaspore has a different crystalline structure to böhmite. Differences in ore composition and presence of iron, silicon and titanium impurities influence their subsequent processing.
90% of the world’s bauxite reserves are concentrated in tropical and sub tropical regions.
Large blanket deposits are found in West Africa, Australia, South America and India as flat layers lying near the surface, extending over an area that can cover many square kilometres. Layer thickness varies from less than a metre to 40 metres in exceptional cases, although 4 – 6 metres is average.
In the Caribbean, as well as in Southern Europe, bauxite is found in smaller pocket deposits, while interlayered deposits occur in the United States, Suriname, Brazil, Guyana, Russia, China, Hungary and the Mediterranean.
Bauxite is generally extracted by open cast mining, being almost always found near the surface, with processes that vary slightly depending on the location. Before mining can commence the land needs to be cleared of timber and vegetation. Alongside this process may be the collection of seeds and/or saplings, for inclusion in a seedbank, which will form the basis of post-mining revegetation of the site. Next the top soil is removed and is usually also stored for replacement during rehabilitation.
The layer under the top soil is known as the “overburden”. On some surface deposits there is no overburden, and on others, the bauxite may be covered by up to 20 metres of rock and clay. On average, overburden thickness is around 2 metres.
The bauxite layer beneath the overburden is broken up using methods such a blasting, drilling and ripping with very large bulldozers. Once the bauxite is loosened into manageable pieces it is generally loaded into trucks, railroad cars or conveyors and transported to crushing and washing plants or to stockpiles, before being shipped to alumina refineries, which are generally located close to bauxite mines.
Unlike the base metal ores, bauxite does not require complex processing because most of the bauxite mined is of an acceptable grade. Ore quality can be improved by relatively simple and inexpensive processes for removing clay, known as “beneficiation”, which include washing, wet screening and mechanical or manual sorting. Beneficiating ore also reduces the amount of material that needs to be transported and processed at the refinery. However, the benefits of beneficiating need to be weighed against the amount of energy and water used in the process and the management of the fine wastes produced.