Bauxite is the most important aluminum ore. This form of rock consists mostly of the minerals gibbsite Al(OH)3, boehmite γ-AlO(OH), and diaspore α-AlO(OH), in a mixture that usually includes the two iron oxides goethite and hematite, and may include the clay mineral kaolinite, and small amounts of the titanium minerals anatase TiO2, Ilmenite, FeTi 03, and FeO Ti02. Bauxite was named after the village Les Baux in southern France, where it was first recognised as containing aluminium and named by the French geologist Pierre Berthier in 1821.
Lateritic bauxites (silicate bauxites) are distinguished from karst bauxite ores (carbonate bauxites).
The carbonate bauxites occur predominantly in Europe and Jamaica above carbonate rocks (limestone and dolomite), where they were formed by lateritic weathering and residual accumulation of intercalated clay layers – dispersed clays which were concentrated as the enclosing limestones gradually dissolved during chemical weathering.
The lateritic bauxites are found mostly in the countries of the tropics. They were formed by lateritization of various silicate rocks such as granite, gneiss, basalt, syenite, and shale. In comparison with the iron-rich laterites, the formation of bauxites depends even more on intense weathering conditions in a location with very good drainage. This enables the dissolution of the kaolinite and the precipitation of the gibbsite. Zones with highest aluminium content are frequently located below a ferruginous surface layer.
The aluminium hydroxide in the lateritic bauxite deposits is almost exclusively gibbsite.
Bauxite is the main raw material used in the commercial production of alumina (Al2O3) and aluminium metal, although some clays and other materials can be utilised to produce alumina. Bauxite is a heterogeneous, naturally occurring material of varying composition that is relatively rich in aluminium.
Australia is the world's largest producer of bauxite, representing 30% of global production in 2012. The large bauxite resources at Weipa with more than 3000 million tonnes (Mt) in Queensland (Qld) and Gove (>200 Mt) in the Northern Territory (NT) have average grades between 49 and 53% Al2O3 and are amongst the world's highest grade deposits. Other large deposits (each >500 Mt) are located in Western Australia (WA) in the Darling Range, the Mitchell Plateau and at Cape Bougainville, of which the latter two have not been developed. The bauxite mines in the Darling Range have the world's lowest grade bauxite mined on a commercial scale (around 27-30% Al2O3). Despite the low grade, the mines accounted for 23% of global alumina production as they also have low reactive silica, making the bauxite relatively easy to refine. Bauxite resources also occur in New South Wales (NSW) and Tasmania (Tas) but these are small (<25 Mt).
More than 85% of the bauxite mined globally is converted to alumina for the production of aluminium metal. The industry in Australia is geared to serve world demand for alumina and aluminium with more than 80% of production exported. Australia's aluminium industry is a highly integrated sector of mining, refining, smelting and semi-fabrication centres and is of major economic importance nationally and globally.
Bauxite is mined through the following series of processes: - Exploration Drilling - Mine Planning - Clearing - Top Soil and Overburden Removal - Secondary Overburden Removal (SOBR) - Backfilling and Drilling - Breaking Caprock - Mining, Crushing and Conveying.
If you look around you will be able to see some of these processes and stages not too far away.
The first step in the mining process is locating deposits of bauxite which is carried out through explorative drilling. Once a bauxite deposit is located, the area is logged, removing any worthwhile products including sawlogs, fence posts and firewood. Some of the remaining wood is used to produce charcoal. Other logs, stumps and rocks are put aside to be used an animal habitats in rehabilitated mined areas.
After the area has been logged, the area is cleared of all other trees and vegetation. This process is referred to as clearing.
The topsoil (0-15cm) is stripped from the area to be mined and it is usually directly returned to a nearby pit undergoing rehabilitation. The underlying layer is called overburden (~15-100cm); this overburden is stripped and stockpiled for later use when the pit is rehabilitated.
The caprock forms a hard sheet so it needs to be broken to enable the ore to be transported. To break the ore it is either blasted or ripped with a very large bulldozer.
The 3-5m layer of caprock and friable bauxite is removed using large excavators or front-end loaders.
The ore is taken to the crusher in haul trucks where it is crushed into smaller pieces that can be transported along the conveyor belt to the refinery.
When mining is completed, the pit is rehabilitated. Topsoil and overburden are returned to the site and the earth is prepared to prevent soil erosion and for seeding and planting. Logs and rocks put aside during clearing are now returned to provide shelter and nesting sites for animals.
To log this Earth Cache we require you to wander around and look at the area and what is available to you visually, consider the information given and perhaps if you need to do some research of your own, then message us with the following answers to the best of your ability;
1. Find some Bauxite and examine it, what characteristics does it have?
2. Can you see a ship being loaded today, what is the name of the ship and it’s port of origin ?
3. Is the Bauxite here in Weipa Carbonate bauxite or Lateritic bauxite, explain why you chose this answer?
4. A photo of your team, GPS near GZ with your log and answers. (optional)
You are welcome to log your answers straight away to keep your TB's and Stats in order but please message us with your answers within 24 hours. Cachers who do not fulfil the Earth Cache requirement will have their logs deleted.
Source: Wikipedia, australianminesatlas.gov.au, alcoa.com, geology.com
