Bayerite

Activation of bayerite [20257-20-9] and nordstrandite [13840-05-6] quaUtatively follows the pattern shown in Figure 4, but the transition sequence is eta-theta-alpha. The stmctures of these transition phases are somewhat different from those obtained from gibbsite, reflecting the differences in crystal stmcture of the hydroxides (16). 

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. 

Fig. 2. Aluminum tiiliydroxides (a) coarse gibbsite from the Bayer process, x 100 (b) Schm h bayerite, x 10, 000.

Bayerite (P-Aluminum Trihydroxide). Bayerite is rarely found in nature. It has been synthesized by several methods A pure product is prepared by the Schm b method (3) in wliich amalgamated aluminum reacts with water at room temperature. Other methods include rapid precipitation from sodium alurninate solution by CO2 gassing, aging of gels produced by neutrali2ation of aluminum salts with NH OH, and rehydration of transition rlio alumina. 

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. 

Nordstrandite. Tlie x-ray diffraction pattern of an aluininum tiiliydroxide wliich differed from the patterns of gibbsite and bayerite was pubhshed (4) prior to the material, named nordstrandite, being found in nature. Tlie nordstrandite structure is also assumed to consist of double layers of hydroxyl ions and aluininum occupies two-tliirds of the octaliedral interstices. Two double layers are stacked with gibbsite sequence followed by two double layers in bayerite sequence. 

Commercial production of bayerite is relatively small and employs CO2 neutralization of caustic aluminate Hquor obtained from either Bayer or sinter processes. The product obtained is about 90% crystalline bayerite having small amounts of gibbsite, pseudoboehmite, and amorphous aluminum hydroxides. 

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

White, friable corrosion products composed of Bayerite AI2O3 3H2O, caustic, and NaA102 cover corroded areas (Fig. 8.3). The white corrosion product and deposit usually test as distinctly alkaline when mixed with distilled water. Corrosion products usually cling tenaciously to the underl3dng metal and do not form voluminous lumps. Instead, corrosion products line and coat generally wasted surfaces below. 

Bayerite, a-Al(OH)3, does not occur in nature but can be made by rapid precipitation from alkaline solutions in the cold ... 

Al,0,-3Hj0 hydrargillite a AljOj-SHjO bayerite b AlA-HjO bohmite c... 

Alumina is not widely used in modem HPLC [48]. Porous gamma alumina is prepared by dehydration and thermal treatment of crystalline bayerite [8,49]. It is available in several types with pore diameters from 6-lS nm, surface areas 70-250 m /g and pore volumes 0.2-0.3 ml/g. After conditioning with acid or base its apparent surface pH can be adjusted between pH 3-9. The alumina surface is more heterogeneous than silica containing both hydroxyl... 

Besides the amorphous alumina films formed in the majority of acidic electrolytes (except those formed in chromic acid and exhibiting traces of y-Al203194), there are possibilities of forming oxides with a more or less pronounced crystallinity. These oxides are formed in alkaline solutions195 and especially in sodium carbonate baths.196 According to the data provided by Hiroshi and Yoshimura,197 these oxides contain a y- A1203 phase easily hydrated and converted into a bayerite-like substance. 

O Reilly et al. (2001) studied the effect of sorption residence time on arsenate desorption by phosphate (phosphate/arsenate molar ratio of 3) from goethite at different pH values. Initially, desorption was very fast (35% arsenate desorbed at pH 6.0 within 24 hrs) and then slowed down. Total desorption increased with time reaching about 65% total desorption after 5 months. These authors found no measurable effect of aging on desorption of arsenate in the presence of phosphate. Furthermore, desorption results at pH 4.0 were similar to the desorption behaviour at pH 6.0. On the contrary, Arai and Sparks (2002) demonstrated that the longer the residence time (3 days-1 year), the greater was the decrease in arsenate desorption by phosphate from a bayerite. 

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