2. Bauxite: Geology, Mineralogy, Resources, Reserves and Beneficiation

In this chapter the Authors summarize the typical features of bauxite which are believed to play roles in mining, beneficiation and mainly in alumina processing, that is, the industrial value of the raw material. Geological curiosities are neglected. General physio-chemical conditions are outlined in order to understand the ore formation being determinant of the quality (chemistry and mineralogy) and quantity of the bauxite ore. Special emphasise is given to its chemical and mineralogical composition make up and their alterations are shown in selected examples, indicating the possible inhomogeneity of the mine product processed in the refineries. In deposit geology laterite, karst and sedimentary/paralic deposits are distinguished requiring significant differences in mining methods and technology applied in processing. In laterite deposits three main types are introduced namely: (1) plateau type deposits developed on morphological terraces and low land interfluves, (2) whale back plateaus and (3) dome shaped hills confined by hilltops and bauxites on hill tops with associated slope bauxites. For estimating the possible supply furnishing of operating refineries or for establishment of green-field plants an overview is given on global bauxite reserves and resources with their grade and mineralogy, amended with their further potential established with the aid of remote sensing technics introduced by the Author in 2006. Principles of physical bauxite beneficiation of the Run of Mine (ROM) bauxite is presented together with the most common unit operations applied. A flowchart example is shown integrating many of the unit operations. To minimize transportation cost and be able to supply more than one refinery, the beneficiation takes place at the mine site before the beneficiated bauxite is exported to the refinery. Principles of thermo-chemical bauxite beneficiation/activation is presented for three distinctive purposes: (1) Reduce organic carbon in the ROM bauxite before feeding the refinery and thus reducing the organic carbon input to the Bayer process liquor before digestion; (2) Activate the content of boehmite in the ROM bauxite to enable low temperature digestion with the side benefit of significantly reducing the organic carbon in the ROM bauxite, and (3) Pyro-genic attack or sintering of the ROM bauxite with limestone and/or sodium carbonate, when the bauxite is composed of mainly mono hydrate bauxites (boehmite or diaspore) and low to high silica content in order to maximize yield of alumina, recovery of caustic and reduced energy consumption. Only the soda-lime sintering process has gained commercial status over time, and especially in China. All the thermo-chemical bauxite beneficiation/activation processes are planned to be built at the refinery site, and in the case of the soda-lime sintering process integrated with the Bayer process in the overall process flowsheet.