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Bauxite
Aluminium is one of the world’s most abundant and useful metals. Although it is found in almost every rock on earth, the principal source of aluminium is bauxite, which contains high concentrations of the oxide alumina (Al2O3). Alumina is extracted from bauxite through a refining process and refined alumina is then converted to aluminium metal through a smelting process.
The name bauxite was first applied to a weathered rock containing a mixture of hydrated alumina and iron oxide found near the village of Les Beaux near Arles in France in the mid 1800’s. Bauxite is a natural product of weathering bedrock and is best developed in tropical areas over porous and permeable rocks with an elevated background concentration of elemental aluminium. Bauxite deposits are found in four types as blanket, pocket, interlayered and detrital type deposits. The most extensive and easiest deposit type to mine are blanket deposits like those found under Cape Alumina’s EPM application areas in Cape York.
There are three main mineralogical forms of bauxite and while all three are treated using the Bayer Process for alumina production they each require tailored refineries. The easiest and cheapest form of bauxite to treat is GIBBSITE or trihydrate alumina, which is a true aluminium hydroxide Al(OH)3. The other types, BOHEMITE and DIASPORE both exist in the monohydrate form and are aluminium-oxide hydroxides AlO(OH). Whereas gibbsite can be treated in the Bayer Process at low temperatures <150°C, bohemite and diapsore require much higher temperatures for digestion which leads to higher energy costs.
At Weipa in Cape York the bauxite typically occurs as a bohemitic horizon overlying a mixed gibbsite/bohemite horizon. The mined bauxite typically contains approximately 20% bohemite which is treated at high temperatures of ~250°C. In contrast, Cape Alumina's Pisolite Hills deposit has a much higher ratio of trihydrate gibbsite to bohemite with previous work indicating as much as 95% of the available alumina is present as gibbsite. Bauxite from Pisolite Hills is well suited as blending feed to the new generation low-temperature refineries, recently built in China.
Alumina
Most of the world’s bauxite production is converted to refined alumina via the “Bayer Process” which was discovered and patented by Austrian chemist Karl Bayer in 1893. Refined alumina is then converted to aluminium metal in reduction smelters. In general, it takes 2-3 tonnes of bauxite to produce one tonne of alumina and 2 tonnes of alumina to produce 1 tonne of aluminium metal.
The Bayer Process can be considered in 4 stages. First the crushed bauxite ore is DIGESTED under pressure in hot caustic soda solution within a series of pressurised tanks. The slurry is then CLARIFIED by washing with fresh water in a series of large thickening tanks. These tanks separate the caustic soda and alumina from the waste solid residue which is pumped to a disposal storage as a red mud. The alumina bearing caustic liquor is then further cooled in a series of PRECIPITATION tanks where alumina trihydrate crystals (Al2O3.3H2O) are extracted for the final stage of the process CALCINATION. The calcination process involves washing the hydrated alumina crystals before heating to ~800°C where the water of crystallisation is driven off to leave pure alumina Al2O3.
DIGESTION
Bauxite + caustic soda + steam |
CLARIFICATION
Separation of liquor from solids "Red Mud" |
PRECIPITATION
Lower temperature > precipitation of trihydrate crystals |
CALCINATION
Energy + Al2O3,3H2O > Al2O3 (Pure Alumina) |
Aluminium
Following the Hall-Herault Process developed in 1886, refined alumina is electrolitically reduced in a series of graphite-lined iron pots. Inside the pots, which form the cathode, is a cryolite bath which contains about 5% alumina into which large carbon rods (anodes) are lowered to generate an electric current. The presence of fused cryolite acts to substantially reduce the melting point of alumina. This process reduces the aluminium to liquid metal which is then periodically drawn off to be cast into ingots.
The Bauxite-Aluminium Relationship and Production Process
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