The mining rights and assets involved with bauxite mining and alumina refining in Australia are 100% owned by Alcoa of Australia Limited (AofA), an affiliate of Alcoa owned by Alcoa World Alumina and Chemicals (AWAC). Prior to Alcoa’s acquisition of Alumina Limited, Alcoa Corporation and Alumina Limited owned 60% and 40%, respectively, of AWAC, an unincorporated global joint venture consisting of a number of affiliated entities that own, operate, or have an interest in bauxite mines and alumina refineries, as well as an aluminum smelter, in seven countries. In August 2024 Alcoa completed the acquisition of Alumina Limited, putting the AWAC joint venture under full control and ownership of Alcoa. As a result, Alcoa owns 100% of AofA and, indirectly, 100% of the mining rights and assets involved with bauxite mining and alumina refining in Australia.
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Summary:
The Darling Range comprises a low incised plateau formed by uplift along the north-south trending Darling Fault, which is a major structural lineament that separates the Pinjarra Orogen to the west, from the Yilgarn Craton to the east. The range extends for over 250 km, from Bindoon in the north to Collie in the south.
Bauxite deposits have been identified throughout the Darling Range and generally occur as erratically distributed alumina-rich lenses within the eroded laterites that mantle the granites to the east of the scarp line. The bauxites are thought to have formed from the lateritization of the peneplained surface of the Western Gneiss Terrane rocks. Lateritization is thought to have commenced during the Cretaceous and continued through to the Eocene, with the subsequent periodic activity of the Darling Fault resulting in the current landform of scarps and deeply incised valleys on the western edge of the Darling Range.
Most of the bauxites display a typical profile comprising the following sequence, from the top down:
• Overburden: A mix of soils, clays, rock fragments and humus that is typically 0.5 m deep, but deeper pockets are common.
• Hardcap: An indurated iron-rich layer that is usually 1 m to 2 m thick. It is generally high in available alumina (AL) and low in reactive silica (SI).
• Friable Zone: A partially leached horizon that usually contains a mix of caprock fragments, clasts, nodules, pisolites, and clays. It is usually a few meters thick but can exceed several meters in places. It is generally high in AL and low in SI.
• Basal Clay: A kaolinitic clay horizon that represents the transition zone between the Friable Zone and the underlying saprolitic material. It is generally high in SI and low in AL.
The Hardcap and Friable Zone are targeted as the ore horizon. Selective mining practices are applied to minimize the inclusion of Overburden, because of its elevated organic carbon levels, and Basal Clay because of its elevated SI concentrations. Within the Hardcap and Friable Zone, the dominant minerals, in order of abundance, are gibbsite, quartz, goethite, kaolinite, and hematite, with lesser amounts of anatase and muscovite.
Mineralization
Weathering, alteration and leaching of the granite bedrock has developed the bauxite mineralization which principally occurs as 65% microcrystalline gibbsite Al(OH)3 with minor to rare boehmite AlO(OH), and accessory minerals of 18% goethite FeO(OH), 7% hematite Fe2O3, 9% quartz SiO2, 1% kaolinite/halloysite Al2Si2O5(OH)4, and 0.5% anatase/rutile TiO2.
Other minerals within the bauxite that may influence the alumina refinery performance include:
• Boehmite: generally occurring below 1%, this can cause premature precipitation of dissolved gibbsite resulting in alumina being lost to the red mud residues.
• Organic Carbon: as oxalate, typically less than 0.2%, (2.0 kg/t, measured as Na2C2O4) this can result in reduced digestion efficiencies and cause crystal growth issues during precipitation.
• Sulphate: generally occurring at 0.25%, this can consume caustic soda during digestion resulting in lower yields.