Encyclopedia of Renewable Energy. James G. Speight
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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
A benchmark crude oil is a crude oil that is used as the standard crude oil against which the properties of the (liquid) source of any fuel (even a renewable fuel) can be measured or compared. When evaluating the price of any crude oil, it is important to compare the crude oil against an appropriate benchmark crude oil.
The price of crude oil means the spot price of either West Texas Intermediate crude oil as traded on the New York Mercantile Exchange (NYMEX) for delivery in Cushing, Oklahoma, or of Brent crude as traded on the Intercontinental Exchange (ICE, into which the International Petroleum Exchange has been incorporated) for delivery at Sullom Voe. The price of a barrel of oil is highly dependent on both its grade, determined by factors such as its specific gravity or API gravity and sulfur content, as well as location. The vast majority of oil is not traded on an exchange but on an over-the-counter basis, typically with reference to a marker crude oil grade that is typically quoted via pricing. Other important benchmark crude oils include Dubai, Tapis, and the OPEC basket.
For the purposes of pricing, crude oil is generally classified based on the API gravity and sulfur content. For example, light crude oil has low density, low viscosity (there are no exact numbers assigned to this, because the classification is more practical and theoretical), and low sulfur content, making it easier to transport and refine and, therefore, more expensive to purchase. Sweet crude oil has a sulfur content less than 0.5% by weight and is usually (but not always) light crude oil, making it much easier to refine in a way that would meet environmental standards in developed countries - and making it more expensive.
A light crude oil is generally one with an API gravity of less than approximately 40 – for example, Brent crude oil has an API gravity on the order of 38 to 39°. Sweet crude is preferable to sour crude oil because it is also (like light crude) more suited to the production of the most valuable refined products. On the other hand, heavy crude oil has high density, high viscosity, and high sulfur content making it more difficult to transport and refine and cheaper to purchase. Typically, sour crude oil has a sulfur content above 0.5% by weight and is usually heavy crude oil, making it cheaper to purchase but more expensive to refine.
Heavy crude oil will typically have an API gravity of 20 or less - the higher the API gravity, the lower the density. Heavy crude oil is harder to handle (it is too thick to pump easily through pipelines unless diluted with light crude) and is more expensive to refine to produce the most valuable crude oil products such as naphtha (gasoline), kerosene, diesel fuel, and aviation fuel.
Because there are so many different varieties and grades of crude oil, buyers and sellers have found it easier to refer to a limited number of reference, or benchmark, crude oils. Other varieties are then priced at a discount or premium, according to their quality. Thus, crude oil is priced in terms of regional blends, each with different characteristics. Of these, certain blends are followed by traders, as they most reflect the overall value of oil, and therefore affect the way different blends are priced. These are essentially like a Consumer Price Index for different types of oil. Approximately 160 different types of crude are traded around the world; the four primary benchmarks, of which these are priced internationally are (i) Brent blend crude oil, (ii) West Texas intermediate (WTI) crude oil, (iii) Dubai crude oil, and (iv)