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BAUXITE

 

Aluminium accounts for about 7% of Earth’s crust making it the third most abundant element after oxygen and silicon.

Due to its high chemical reactivity, aluminium is generally found as aluminium silicates or aluminium hydrates.

Bauxites contain hydrated forms of aluminium oxide and occur in three main forms, namely gibbsite, boehmite or boehmite and diaspore.

 

The table below compares these three forms of bauxite.

Comparison of Bauxite forms:

 

Description

Unit

Gibbsite

Boehmite

Diaspore

Composition

 

Al2O3.3H2O

Al2O3.H2O

Al2O3.H2O

Maximum Alumina Content

%

65.4

85.0

85.0

Crystal System

 

Monoclinic

Orthorhombic

Orthorhombic

Density

Gram / Cubic Centimetre

2.42

3.01

3.44

Temperature for rapid Dehydration

0C

150

350

450

Product Of Dehydration

 

α-Al2O3

γ –Al2O3

α-Al2O3

Solubility in 100g/litre Na2O at 125ºC

Al2O3 gram/litre

128

34

Virtually Insoluble

Hardness

Moh

2.5-3.5

3.5-4

6.5-7

 

Bauxite is a naturally occurring, heterogeneous material composed primarily of one or more aluminium hydroxide minerals, plus various mixtures of silica, iron oxide, Titania, aluminium silicates, and other impurities in minor or trace amounts. Bauxite is a sedimentary rock produced by in situ chemical weathering typically under tropical to subtropical climate conditions.

The principal aluminium hydroxide minerals found in varying proportions with bauxites are gibbsite and the polymorphs boehmite and diaspore. Bauxites are typically classified according to their intended commercial application: abrasive, cement, chemical, metallurgical, refractory, etc.
The bulk of world bauxite production (approximately 85%) is processed into aluminium oxide (Al2O3, also known as alumina) via a wet chemical, caustic leach method (the Bayer process). The resulting Al2O3 is then reduced to aluminium metal (Al) using an electrolytic process, the Hall Heroult process.
Bauxite is the raw material most widely used in the production of aluminium on a commercial scale. Other raw materials, such as anorthosite, alunite, coal wastes, and oil shales, offer additional potential Al2O3 sources. Although it would require new facilities and technology, Al2O3 from these non-bauxite materials could satisfy the demand for primary metal, refractories, aluminium chemicals, and abrasives. Synthetic mullite, produced from kyanite and sillimanite, substitutes for bauxite- based refractories. Although more costly, silicon carbide and alumina-zirconia substitute for bauxite based abrasives.
Bauxite is a light mineral; its specific gravity is 2.6 to 3.5. It is usually an amorphous or clay like substance which is, however, non plastic. The usual color of bauxite is pink, but if it is of low iron content it may tend to become whitish in color, and with increase in iron it is reddish-brown-in-color.
Very commonly bauxite deposits are associate with the rock type known as laterite which forms a kind of ferruginous crust over any type of rock whether igneous, sedimentary, or metamorphic as a result of weathering under tropical or subtropical monsoonal climate conditions.
It is well known that when alumina-rich igneous rocks weather the feldspars of such rocks are usually koalonized, but under tropical monsoonal conditions, the weathering goes a step further and results in residium rich in hydroxides of aluminum together with oxides of iron, manganese and titanium. Where there is a sufficient concentration of the aluminum hydroxides, economic deposits of bauxite originate. This process of “bauxitization“ as it is called thrives well on a topographically elevated well-drained place and that is why, most bauxite deposits occur, on large plateau of low relief. In the world, bauxite deposits have been found in rocks of Tertiary age, Mesozoic age, and even in Paleozoic age.