Bauxite is ore from which aluminium can be extracted.
Most bauxite, including Australian bauxites are lateritic, formed by intense weathering of various rock formations. The alumina in lateritic bauxite is mainly in the form of gibbsite, an aluminium oxide trihydrate. Other bauxites contain higher levels of boehmite are called monohydrates and involve significantly higher levels of energy to process.
There are currently five bauxite mines in Australia providing feedstock for the six alumina refineries, which in turn supply alumina to the five Australian aluminium smelters and the export market. Australia is the largest producer of bauxite in the world, with 79.4 million tonnes produced in 2012. (Source: AAC Sustainability Report 2012)
The three structural forms of bauxite are gibbsite, boehmite and diaspore. Gibbsite is a true aluminium hydroxide and boehmite and diaspore are both aluminium-oxide-hydroxides.
Extreme lateritic weathering in the Darling Range has resulted in the formation of the trihydrate bauxite Al(OH)3, gibbsite (Refer Figure 1: Typical Darling Range Cross Section)
In an alumina refinery, bauxite is digested using the Bayer Process and involves washing the ore with a pressurised hot solution of sodium hydroxide (caustic soda). Two reasons why refineries that use Darling Range Bauxite operate with lower production costs is:
Figure 2. Process requirements for the extraction of alumina