Processing of Bauxite Ore

Processing of Bauxite Ores
To Obtain Aluminum

Bauxite, the primary aluminum ore, uses the Bayer Process to extract alumina from the ore. The processing begins with crushing to 1-2 inch particles and wet screening, to remove some silica fines, which are generally present with bauxite.

From here, the ore is digested in a heated, pressurized vessel at temperatures up to 450 defrees F and pressures of 500 psi. Generally, there are differing types of alumina ores in the bauxite deposit, and these require different temperatures to digest. The low temperature digestor typically operates at 275 to 290 degrees F. Sodium Hydroxide (NaOH) is used to dissolve the alumina in the digestor. The Pregnant liquor is separated from the red mud tailings using a series of thickeners set up in counter current decantation arrangement, to continuously wash the alumina bearing liquor from the tailings, until only barren tailings remain. Various contaminates (iron, organic matter) can play havoc with the process and may require sub routines to remove them.

Filtration, such as vacuum drum or pressure filters, remove the silica and low solids from the clarified alumina bearing liquor. The liquid containing the dissolved alumina is pumped to tanks called crystalization or precipitation tanks. The liquid is cooled with water from the counter current decantation thickeners, and as it cools, alumina hydrate slowly precipitates from the tank, according to the formula:

The precipitated liquor containing the white alumina is then filtered, to remove the solid alumina from the liquid, using vacuum drum filters or rotary pan filters, where it can be washed as it is filtered. The alumina hydrate (AlOH3) is then dried. It can be further calcined to Al2O3, alumina, in a rotary kiln at 800 defrees F.

From the kiln, the alumina is allowed to cool and it is stored for shipment to the aluminum plant. To produce aluminum, the alumina is reduced in a electrical reduction cell which produces pure Al plus carbon dioxide (CO2). Now, aluminum ingots or billets are stored for manufacturers of aluminum products, such as automobiles, pots and pans, and the ubiquitous aluminum siding that covers many houses. The electrical reduction plants are power intensive, using up to 16,000 KW of electricity per ton of aluminum produced.

Information provided by Charles Kubach, Mining and Mineral Processing Engineer
Reference: SME Mineral Processing Handbook  

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