Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 134

Bauxite is an amorphous sedimentary rock that is the chief commercial ore of aluminum. It consists largely of hydrated alumina with variable proportions of iron oxides. In addition, bauxite is the main source of aluminum and gallium. The ore consists mostly of the aluminum minerals gibbsite [Al(OH)₃], boehmite [γ-AlO(OH)], and diaspore [α-AlO(OH)] that are mixed with the two iron oxides: goethite [FeO(OH)] and hematite (Fe₂O₃) as well as the aluminum clay minerals kaolinite [Al₂Si₂O₅(OH)] and small amounts of anatase (TiO₂), and ilmenite (FeTiO₃ or FeO.TiO₂).

The principal aluminum hydroxide minerals found in varying proportions with bauxites are gibbsite and the polymorphs boehmite and diaspore. Bauxite ores are typically classified according to their intended commercial applications such as an abrasive, a cement constituent, an industrial chemical, and a refractory material. The bulk of world bauxite production (approximately 85%) is used as feed for the manufacture of alumina (Al₂O₃) via a wet chemical caustic leach method (the Bayer process). Subsequently, the majority of the resulting alumina produced from this refining process is in turn employed as the feedstock for the production of aluminum metal by the electrolytic reduction of alumina in a molten bath of natural or synthetic cryolite (Na₃AlF₆).

Bauxite is useful in the alternate energy industry insofar as it can be used as a catalyst to convert many different sulfur compounds, in particular mercaptan derivatives (RSH), into hydrogen sulfide (H₂S), which is subsequently removed by a lye treatment (caustic treatment, alkali treatment).

To prepare aluminum oxide, bauxite is heated in pressure vessels with sodium hydroxide solution at 150-200°C through which aluminum is dissolved as aluminate (Bayer process). After separation of ferruginous residue (red mud) by filtering, pure gibbsite is precipitated when the liquor is cooled and seeded with fine-grained aluminum hydroxide. Gibbsite is converted into aluminum oxide by heating. This is molten at approx. 1,000°C (1,830°F) by addition of cryolite as a flux and reduced to metallic aluminum by a highly energy-consumptive electrolytic process (Hall-Héroult process).

During the processing of bauxite to alumina (Al₂O₃) in the Bayer process, gallium accumulates in the sodium hydroxide liquor from which it can be extracted by a variety of methods. Achievable extraction efficiencies critically depend on the original concentration in the bauxite feedstock.

Bauxite ores are industrially important for the supply of aluminum metal, and these ores represent the only raw material used in the production of alumina on a commercial scale. Gallium is a common by-product of the process, and both aluminum and gallium may also be considered as a possible future resource for rare earth elements that find use in renewable energy technologies.

See also: Bauxite Treating Process.