Australia is a major producer of bauxite. Bauxite is the main raw material
used in the commercial production of alumina (Al2O3)
and aluminium metal, although some clays and other materials can be
utilised to produce alumina. Bauxite is a heterogeneous, naturally
occurring material of varying composition that is relatively rich in
aluminium. The principal minerals in bauxite are gibbsite (Al2O3.3H2O),
boehmite (Al2O3.H2O) and diaspore, which
has the same composition as boehmite, but is denser and harder.
Australia Aluminium Deposits and Refining
Lateritic bauxite formation
Pisolithic texture of bauxite
Laterite is formed when heavy tropical rainfall results in intense
weathering of rock and soil.
The chemistry of weathering silica-rich rocks sees the relatively mobile
elements such as calcium, sodium, potassium and magnesium being washed
away, while the immobile metals including aluminium, iron, titanium, and
After many millions of years what is left is called a “laterite”, which
can be either iron-rich, or in extreme cases, aluminium-rich (i.e.
Most lateritic bauxites were formed in a period from the mid-Cretaceous to
the late-Tertiary, that is, 100 million to 2 million years ago. During
this time, laterite formation was not continuous. Instead, they formed
during relatively short periods of intense weathering separated by long
periods of less intense weathering.
Many lateritic bauxites are pisolitic. The word pisolitic comes from the
Greek meaning pea-sized rock particles. When cut or broken open their
inside surfaces reveal concentric bands of different mineral compositions.
Although it has a lower aluminium content than some other minerals,
gibbsite-rich bauxite is cheaper to mine and process.
Lateritic bauxite profile
A “typical” profile is really a misnomer as there are so many variations
Elements of this “typical” profile are seen in most lateritic bauxite
profiles in other parts of the world.
The profile consists of a relatively thin soil layer overlying a horizon
consisting of cemented pisolitic bauxite. The main mineral within this
cemented layer is boehmite or “monohydrate” as the miners call it.
Although one of the most aluminium-rich minerals in lateritic bauxite, the
layer containing this mineral is normally stripped off and discarded
during mining. This is because the layer is typically cemented and would
therefore require crushing. In addition, the caustic soda solution that is
used to extract aluminium from bauxite needs to be much hotter for
boehmite than for gibbsite, so processing costs would be correspondingly
Underlying the cemented bauxite is a layer of loose pisolitic bauxite 1-2m
thick. The main mineral in the pisolitic bauxite is the aluminium-rich
mineral gibbsite or “trihydrate” as it is commonly referred to. Pisolitic
bauxite ore is an ideal ore for various reasons. It is located close to
the surface so very little cover is needed to be removed to expose it. The
bauxite is loose and easily mined with front-end loaders.
Beneficiation of the ore before it is shipped out of port involves simple
screening and washing. Selective mining ensures that only the highest
grades are mined.
Below the pisolitic bauxite horizon is a nodular ironstone layer. At this
level, the bauxite becomes increasingly iron and kaolin-rich. Kaolinite,
which contains a great deal of silica, is also much lower in aluminium
content than bauxite. Kaolinite tends to dissolve relatively easily in the
caustic soda solution during aluminium extraction so uses up this valuable
chemical. For this reason, mining has to be done very carefully as the
ironstone layer is approached because kaolinite in the ore significantly
reduces its value.
Beneath the ironstone layer the profile becomes increasingly less nodular
and more mottled in appearance. The mottled zone has reddish coloured
patches containing hematite and goethite, within a pale coloured
At greater depths below this zone the white mineral kaolinite dominates,
hence the name pallid zone given to the lowest part of the weathering
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