Bauxites vary widely in composition and appearance, but the principal constituents are alumina monohydrate, A12O3H2O, and alumina trihydrate, A12O3-3H2O. The main impurities are oxides of silicon, iron, and titanium. There are usually many trace impurities as well. In order to prepare feed for aluminum electrolytic cells, the crude bauxite must be refined to pure alumina by the Bayer process. The conditions under which leaching of the ore is carried out differ considerably for bauxites that are mainly alumina monohydrate and those that are mainly trihydrate, as shown in Table 1. In either case, however, pure alumina trihydrate is precipitated from the leach solutions, and is then calcined to anhydrous alumina as the final product. Figure 5 shows a flowsheet for the Bayer process. The product is extremely pure, as is shown in Table 2.
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