Alumina's universal ore
From craggy rock to universal ore
Bauxite owes its name to Les Baux ('craggy rocks'), the picturesque region in Southern France first recognised as a potential source of aluminous minerals; fortunately, it was never extensively mined.
Rich in alkali-soluble aluminous minerals, bauxite is the ore that feeds the Bayer process to produce alumina (Al2O3), from which aluminium metal is won by the Hall-Herault process of electrolytic smelting.
The values in bauxite are the 'hydrated aluminas', aluminium hydroxide (Al(OH)3 (gibbsite)) and the oxy-hydroxides boehmite and diaspore (AlO(OH)), formed by the breakdown of alumino-silicate minerals by millions of years of rainwater leaching.
There are two distinct types of bauxite, according to differences in the parent alumino-silicates and weathering conditions:
karst bauxites formed with limestone and dolomite and their associated underground drainage labyrinths, and
lateritic bauxites formed from surface weathered igneous and sedimentary alumino-silicates.
The best bauxite is high in gibbsite, the most soluble of the 'hydrated aluminas'. Gibbsitic bauxites are mostly found in tropical regions where high rainfall and acidic drainage waters have created optimal leaching conditions over the millennia. They are processed in the 'Low Temperature' (LT) Bayer process, in which the bauxite is digested at temperatures in the range 100-150 degrees C.
Karst bauxites are mostly found at higher latitudes (the Jamaican bauxite province is a notable exception). They tend to be higher in boehmite and diaspore, and require higher digestion temperatures (230-250 degrees C for boehmite and >270 degrees C for diaspore).
Most bauxite are orange to red in colour, because iron is the predominant second element, present mostly as red hematite Fe2O3 and yellow-orange goethite AlO(OH). Iron is usually followed (in typical order of abundance) by silicon, titanium, calcium (high in karst bauxites only), phosphorus and vanadium. Silicon manifests in clays (primarily kaolinite Al2Si2O5(OH)4) and quartz (SiO2). Titanium is usually present as anatase and rutile (both TiO2), and sometimes combined with iron in ilmenite. Calcium in karst bauxites is mostly as calcite, CaCO3. Phosphorus, vanadium and host of minor elements may present as traces of individual minerals, or more often as substituents in major minerals.
Gallium is worthy of particular note. As a close chemical relative of aluminium, Ga is found as a minor constituent of Al minerals, and it follows Al into the liquor in Bayer digestion. Extraction from Bayer liquors in China is a major source of global Ga.
Major mineral species in bauxites: