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.
