Aluminium is the third most abundant element in the Earth’s crust, but does not occur as a metal naturally. The first step in producing aluminium is mining its ore – bauxite.
Bauxite occurs mainly in tropical and sub-tropical areas, like Africa, the West Indies, South America and Australia – there are some small deposits located in Europe.
Australia is the largest producer of bauxite, with five mines supplying around 23 per cent of global production.
Bauxite mining has five steps: Preparation of the mining area; bauxite mining; crushing; ore transport; and rehabilitation.
Preparation of Mining Area
Pre-Mining Surveys are conducted in all new mining areas, to provide information on fauna and vegetation, to map the extent of dieback disease, and to identify any significant Aboriginal heritage sites. If rare or protected species or significant sites are present, they are avoided or management plans are developed to minimise the impact of mining on them.
The soil profile is made up of a number of layers. The topsoil layer contains a large store of seed and nutrients that is vital to the success of the forest rehabilitation and is about 15cm thick. The overburden layer is 20-80cm of gravely sub-soil material sitting above a solid concrete-like layer called the caprock. These layers are removed separately using scrapers prior to mining.
Scrapers and small excavators are used to remove the remaining overburden and expose the caprock. Depending on the depth of the caprock, it can be broken by blasting, or simply removed with scrapers and excavators.
The bauxite is then mined using excavators or loaders to load the bauxite onto haul trucks and transported to the crusher. Several pits are usually mined simultaneously in order to supply the refinery with a consistent grade of ore.
The crusher is used to break the ore down to a smaller size suitable for transport along the conveyor belt to the refineries. The crusher is made up of a number of components that include a vibrating screen, a jaw crusher and sizers. Fine material drops through the vibrating screen while larger material passes over the screen to the jaw crusher that breaks up the large rocks. The material that passes through the vibrating screen and jaw crusher is collected and passed through the sizer that further reduces the size of the material before it is passed onto the conveyor belt. The final size of the crushed bauxite ore is approximately 7.5cm or less in diameter.
The crushed bauxite is transported via conveyor belts and railway systems, either to the refinery or shipping terminal.
After mining, the edges of the pit are smoothed. Topsoil and overburden are returned to the site and the earth is prepared to prevent soil erosion and for seeding and planting. The logs and rocks that were put aside during clearing are then returned to provide shelter and nesting sites for animals.