Encyclopedia of Renewable Energy. James G. Speight

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      Bauxite

      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.

      Bauxite Treating Process

      Bauxite ore is a mixture of hydrated aluminum oxide derivatives and compounds of other elements such as iron. The aluminum compounds in the bauxite may be present as gibbsite [Al(OH)₃], boehmite [or böhmite, γ-AlO(OH)], or diaspore [α-AlO(OH)]. The different forms of the aluminum component and the impurities dictate the extraction conditions. Aluminum oxides and hydroxides are amphoteric – having both acidic and basic character.

      In the Bayer process, a mixture of the bauxite ore and a sodium hydroxide solution is heated in a pressure vessel at a temperature of 150 to 200°C (300 to 390°F), causing the aluminum to be dissolved as sodium aluminate. After separation of the residue by filtering, gibbsite is precipitated when the liquid is cooled and then seeded with fine-grained aluminum hydroxide crystals from previous extractions.

      The extraction process converts the aluminum oxide in the ore to soluble sodium aluminate (NaAlO₂), and at the same time, silica is dissolved to form sodium silicate (Na₂SiO₃):

      The other components of the bauxite ore do not dissolve.

      Lime may be added to precipitate the silica as calcium silicate (CaSiO₃). The undissolved waste (bauxite tailings) after the aluminum compounds are extracted contains iron oxides, silica, lime (CaO), titania (TiO₂), and some unreacted alumina.

      See also: Bauxite.

      Beavon Process

      The Beavon sulfur removal process for the cleanup of Claus plant tail gas is a two-step process in which the sulfur contaminants are first catalytically hydrolyzed and/or hydrogenated to hydrogen sulfide and the hydrogen sulfide is then converted to elemental sulfur and recovered in a Stretford process unit.

      In the process, which consists of two stages, the sulfur-containing compounds, such as hydrogen sulfide (H₂S), sulfur dioxide (SO₂), carbonyl sulfide (COS), and carbon disulfide (CS₂), are converted to sulfur in over 99.9% efficiency. In the first stage, the various sulfur compounds are either hydrogenated or hydrolyzed to give hydrogen sulfide, while in the second stage, the hydrogen sulfide is oxidized using the Stretford process to give good-quality elemental sulfur. This process can also be utilized in synthetic natural gas plants, natural gas processing, and other similar applications.

      See also: Gas Cleaning, Gas Processing, Gas Treating, Tail Gas Cleaning, Wellman-Lord Process.

      Benchmark Crude Oil

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