Bauxite

Bauxite is not a mineral. It is a rock formed from a laterite soil that has been severely leached of silica and other soluble materials in a wet tropical or subtropical climate. It is the primary ore of aluminum. Almost all of the aluminum that has ever been produced has been extracted from bauxite.

Bauxite

Bauxite's Composition

Bauxite does not have a specific composition. It is a mixture of hydrous aluminum oxides, aluminum hydroxides, clay minerals, and insoluble materials such as quartz, hematite, magnetite, siderite, and goethite. The aluminum minerals in bauxite can include: gibbsite Al(OH)3, boehmite AlO(OH), and, diaspore, AlO(OH).

Physical Properties of Bauxite

Bauxite is typically a soft (H:1-3), white to gray to reddish brown material with a pisolitic structure, earthy luster and a low specific gravity (SG: 2.0-2.5). These properties are useful for identifying bauxite; however, they have nothing to do with bauxite's value or usefulness. This is because bauxite is almost always processed into another material with physical properties that are distinctly different from bauxite.


Physical Properties of Bauxite

Color White, gray, sometimes stained yellow, orange, red, pink, or brown by iron or included iron minerals
Streak Usually white, but iron stain can discolor
Luster Dull, earthy
Diaphaneity Opaque
Cleavage None
Mohs Hardness 1 to 3
Specific Gravity 2 to 2.5
Diagnostic Properties Often exhibits pisolitic structure; color
Chemical Composition Variable but always rich in aluminum oxides and aluminum hydroxides
Crystal System n/a
Uses Primary ore of aluminum, also used as an abrasive


Bauxite Used for Aluminum Production

Bauxite is the principal ore of aluminum. The first step in producing aluminum is to crush the bauxite and purify it using the Bayer Process. In the Bayer Process, the bauxite is washed in a hot solution of sodium hydroxide, which leaches aluminum from the bauxite. The aluminum is precipitated out of solution in the form of aluminum hydroxide, Al(OH)3. The aluminum hydroxide is then calcined to form alumina, Al2O3.

Aluminum is smelted from the alumina using the Hall-Heroult Process. In the Hall-Heroult Process, the alumina is dissolved in a molten bath of cryolite (Na3AlF6). Molten aluminum is removed from the solution by electrolysis. This process uses an enormous amount of electricity. Aluminum is usually produced where electricity costs are very low.


Sources: Geology.com

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