Aluminum oxide bayerite

The term alumina hydrates or hydrated aluminas is used in industry and commerce to designate aluminum hydroxides. These compounds are tme hydroxides and do not contain water of hydration. Several forms are known a general classification is shown in Figure 1. The most weU-defined crystalline forms ate the trihydroxides, Al(OH) gibbsite [14762-49-3], bayerite [20257-20-9], and nordstrandite [13840-05-6], In addition, two aluminum oxide—hydroxides, AIO(OH), boelimite [1318-23-6] and diaspote [14457-84-2], have been clearly defined. The existence of several other forms of aluminum hydroxides have been claimed. However, there is controversy as to whether they ate truly new phases or stmctures having distorted lattices containing adsorbed or intedameUar water and impurities. 

Bayerite is a commercially available technical product that is produced in small quantities mainly for alumina catalyst manufacture. High purity aluminum [7429-90-5j metal has been converted to bayerite to produce very high purity aluminum oxides. 

The final corrosion product, aluminum oxide trihydrate, is called Bayerite. 

FIGURE 9-2. Reaction of Aluminum in Water to Form Aluminum Oxide Trihydrate, or Bayerite... 

In the aluminum oxide system the precipitation pH is one of the variables which controls the nature of the phase eventually obtained. However, aging conditions of the initially formed amorphous precipitate are at least equally important. In general, it can be stated that precipitation above pH = 8 leads to the formation of bayerite, while precipitation under more acidic conditions favors the subsequent formation of boehmite. Hydrargillite is formed as the product of the Bayer process by seeding a supersaturated alkali containing aluminum solution. The formation of bayerite is strongly facilitated by the presence of alkali cations which stabilize the structure. 

Aluminum oxide occurs naturally as the minerals bauxite, bayerite, boehmite, corundum, diaspore, and gibbsite. 

Aluminum oxide (uh-LOO-min-um OK-side) is white crystalline powder that occurs in nature in a variety of minerals, including boehmite, bayerite, corundum, diaspore, and gibb-site. Corundum is second hardest naturally occurring mineral. Only diamond is harder. Aluminum oxide occurs in a variety of chemical forms in a variety of gemstones, including chryso-beryl, ruby, sapphire, and spinel. The color of these gemstones is a result of impurities, such as chromium (in the case of ruby) and iron and titanium (in the case of sapphire). The colors may also vary depending on the kind and amount of each impurity. 

The a-gel is completely free of electrolytes its surface is quite alkaline. The fresh gel is completely amorphous [19] it ages rapidly to bayerite via the intermediate stage of bohmite, the alkalinity of the surface decreasing considerably in the process [16, 17]. Aluminum oxides prepared from this gel are more active than the usual aluminum oxide catalysts [1]. 

The research [22] revealed high activity of nanoscale powders obtained by electrical explosion of wires (EEW) to reduce polyolefin flammability. Aluminum hydroxide Al(OH)3, bayerite P-AlaOs-SHaO, boehmitey-AlOOH, low-temperature modification of aluminum oxide y-Al203 produced by the method of electrical explosion of wires (EEW) [23, 24] were used as fillers in polypropylene. All additives are resistant to oxidation under heating up to 400 °C, all of them release water in endothermic decomposition, except y-Al203. The results of the smdy indicated that the oxidation rate decreases when polypropylene was filled with gibbsite and bayerite at concentration of 0.5-10 wt%. 

Aluminum hydroxide gel contains the equivalent of not less than 3.6g and not more than 4.4g of aluminum oxide (AI2O3), in the form of aluminum hydroxide and hydrated oxide. Aluminum oxide, also known as alumina, occurs in nature in the minerals bauxite, bayerite, boehmite, corundum, diaspore and gibbsite. Hydrated alumina, or aluminum hydroxide has... 

The first reaction forms the aluminum hydroxide bayerite (Al(OH)3) and hydrogen, the second reaction forms the aluminum hydroxide boehmite (AIO(OH)) and hydrogen, and the third reaction forms aluminum oxide and hydrogen. All these reactions are thermodynamically favorable from room temperature past the melting point of aluminum (660°C). All are also highly exothermic. From room temperature to 280°C, Al(OH)j is the most stable product, while from 280-480°C, AIO(OH) is most stable. Above 480°C, AI2O3 is the most stable product (3) ([15, 16]). The following equation illustrates the combined effect of hydrolysis and hydration on tricalcium silicat. 

The corrosion product is almost always aluminum oxide Irihydroxide (bayerite). Localized corrosion does not usuaUy occur in extremely pure water at ambient temperature or in the absence of oxygen but can occur in more conductive solutions because of the presence of ions such as chloride or sulfate. An exami-... 

Aluminum is a soft, ductile, and relatively inexpensive metal. The surface of aluminum readily oxidizes in the air and water to form a highly resistant oxide film. This oxide film serves to make aluminum resistant to attack when used in environments containing sulfides, sulfur dioxide, carbon dioxide, and other corrosive gases. It is highly resistant to water-initiated corrosion, but is susceptible to galvanic corrosion by trace amounts of copper, tin, lead, nickel, or carbon steel. The reaction of aluminum in water to form Bayerite is shown in FIGURE 9-2. 

In addition to gibbsite there are other routes to manufacture Al(OH>3 and the consecutive transition oxides. One is the precipitation of Al(OH)3 from aluminum salts by adjusting the pH between 7 and 12 by adding bases. Precipitation at elevated temperatures and high pH leads to formation of bayerite, whereas at lower pH pseudoboehmite and subsequently boehmite are formed. By heating, these materials can be converted to the active transition aluminas. 

Sometimes, however, a comparison between the adsorption branch and the desorption branch may lead to a conclusion about the shape of the capillaries. An adsorption branch which has no inflexion point and gives a sharp rise only for relative pressures close to unity, combined with a desorption branch showing a definite inflexion point at medium values of relative pressures, indicates fissure-shaped capillaries (68). Hysteresis curves of this form are, for example, found with a lomerations which consist of disk- or plate-shaped particles, such as montmorillonites, and indeed hysteresis curves published by Barrer and MacLeod (59) show this behavior. Similar curves are found with the dehydration products of many well-crystallized metal oxide hydrates, such as those of the aluminum hydrates (gibbsite, bayerite, boehmite, and diaspora) (60). 

Activated Aluminum Hydroxide. The principal precursor for this class of products is gibbsite derived from the Bayer process although a small amount of bayerite is also used for some specialty catalytic applications. Bayer process gibbsite is available in very large tormages as an intermediate product in aluminum production. It is 99+% pure the main impurity is sodium oxide [1515-59-5], Na.20, at 0.2—0.3% on a Al(OH)2 basis. Low cost, relatively high purity, and availabiUty make gibbsite the raw material of choice for many activated alumina products. 

The chemistry of the precipitation of aluminum hydroxides and oxides is very complex (40). When the solubility is exceeded, gelatinous precipitates, which are found to be amorphous by X-ray diffraction, usually form initially. Al MAS NMR shows the predominance of octahedraUy coordinated Al ions in these amorphous hydroxides, as are present in the crystalline trihydroxides and oxyhydroxides. However, in the amorphous materials some pentacoordinated and tetracoordinated Al ions are also found (15). As discussed above, there are many different crystalline hydroxides or oxyhydroxides, and which of them will be formed depends on the conditions (4f). Primary factors are temperature and pH, as well as aging time however, the nature of the anions present and the possible presence of organic components (42,43) also play a role. At low temperature in an excess of water, the hydroxides are preferentially formed, specifically bayerite at pH values between 5.8 and 9 or gibbsite for pH values smaller than 5.8 or larger than 9. 

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