Bauxite

Overview

The raw material for virtually all primary aluminium production is bauxite. Bauxite is first refined to pure alumina (aluminium oxide – Al₂O₃) before being electrolytically reduced (smelted) to aluminium metal. The principal cost component for smelting is electricity, hence smelters are generally located in close proximity to a source of cheap power. The lowest cost smelters are those located close to bauxite mines/refineries, thereby minimising freight costs for raw materials, however, quite often alumina is shipped in from refineries located close to bauxite deposits, such as those in Western Australia’s Darling Ranges.

Bauxite Geology and Mineralogy

The principal sources of bauxite are tropical laterite deposits which include those located in Australia, Brazil, Guinea, Venezuela, Jamaica and Indonesia. Laterites are surface residual deposits that have formed as a result of prolonged chemical weathering of underlying silicate rocks such as granite, gneiss, basalt and sediments under hot, wet, humid conditions. Percolating rain water causes chemical breakdown of the primary rock minerals and leaching of the more readily soluble elements including sodium, potassium, calcium, magnesium and silicon. This results in residual concentration of the more insoluble elements such as aluminium and iron which form new minerals that are stable in the weathering environment. Laterites may be iron-rich, which are an important source of iron ore, or aluminium-rich, when they are termed bauxites.

Due to its geological characteristics, lateritic bauxite is mined by strip mining methods from shallow but extensive open pits.

Bauxite Characteristics for Alumina Production

The main value of bauxite lies in its aluminium content, with silica (SiO₂) an undesirable impurity.

In an alumina refinery, bauxite is digested in hot caustic soda (Bayer process) which dissolves the alumina, leaving the iron and titanium oxides and other insoluble impurities to be disposed of as red mud. The alumina is recovered by cooling and precipitation, with the caustic solution recycled. Insoluble silicates discarded with the red mud take caustic soda and alumina with them, thus increasing reagent consumption and reducing process recoveries

Other issues of economic importance in bauxite refining include:

• Disposal of red mud. High iron bauxites produce higher quantities of red mud.
• Handling characteristics and dustiness.
• Crushing characteristics.
• Rate of dissolution of alumina.

Not all bauxite is destined for aluminium production, with about 5% of alumina consumed for water treatment, chemicals production, manufacture of refractories and abrasives and for fluxes in steel making.

The overall bauxite refining and smelting process is shown diagrammatically below.

Global Bauxite Production

Global bauxite production in 2007 was about 202 million tonnes (Mt), from reserves of about 27Bt (Table 1). Although aluminium demand is rapidly increasing, known reserves are in excess of 130 times current annual production. Aluminium recycling has the advantage of lowering the energy cost of production, and for this reason is likely to continue to increase, thus extending the life of reserves. Table 1 shows estimated production for 2008, reserves, and resources including reserves by country, ranked by reserves.

The chart below highlights the untapped potential of Vietnam’s bauxite resources.

Table 1. World Bauxite Production, Reserves & Resources

Source: U.S. Geological Survey, Mineral Commodities Summaries, January 2009.