Bayerite properties

The sequence of phase transformations shown in Figure 2 is an approximationp largely because process variables such as time, atmosphere and properties of precursor hydroxides are not included. Thus, for example, bayerite and glbbslte may be converted to boehmite and thence to y-alumina during calcination if the particle size Is large and the precipitate Is moist [18]. 

Apps JA, Neil JM, Jun CH (1989) Thermochemical properties of gibbsite, bayerite, boehmite, diaspora, and the aluminate ion between 0 and 350°C. Washington, DC US Nuclear Regulatory Commission (prepared by Lawrence Berkeley Laboratory), p 98... 

Properties Traces of gibbsite and bayerite, median diameter 160 nm, size distribution available [353]. 

Properties Gibbsite with 10-12% of bayerite [562], specific surface area 4 m7g [562,1170], SEM images available [1170]. 

Properties Well-crystallized bayerite, lauric acid adsorption specific surface area 27.9 m /g [1149]. 

Albano, M.P. and Garrido, L.B., Rheological properties of 813X4, pseudoboelimite and bayerite coated 813X4 suspensions with ammonium polyacrylate dispersant. Colloids Surf. A, 203, 117, 2002. 

The solubilities of gibbsite, boehmite and diaspore in alkaline solutions between 20 and 350°C are evaluated and their thermodynamic properties reconciled. The thermodynamic properties of the aluminate ion, Al(OH)4, are derived over the same temperature range and compared with predictions based on the revised Helgeson-Kirkham-Flowers equation of state. Preliminary thermodynamic properties of bayerite and AGf°298 for nordstrandite are also derived from solubility data in alkaline solutions. Log Kg4 values for gibbsite, bayerite, boehmite and diaspore between 0 and 350°C, and thermodynamic data for A1(0H)4 or AIO2, re tabulated for use in distribution-of-species computer codes. 

In this paper, the procedures are summarized by which the solubilities of gibbsite, boehmite, and diapore in alkaline solution are evaluated and used to compute thermodynamic properties of boehmite and the aluminate ion. Published data are then used in conjunction with the derived properties of the aluminate ion given in this paper to calculate preliminary thermodynamic properties for bayerite, and the Gibbs free energy of formation of nordstrandite at 25°C. Aluminum hydroxide and oxyhydroxide, and corundum... 

Bayerite, Al(OH). Bayerite occurs very rarely in nature (26), but is easily synthesized. Little is known regarding its thermodynamic properties. Some heats of decomposition have been measured in relation to gibbsite (73-76). One HF calorimetry study (77) and... 

Amorphous aluminas have moderate thermal stabdity in dry atmospheres, that is, up to temperatures of less than 700 °C. However, the thermal stabihty of amorphous alumina is reported to significantly increase with decreasing particle size (141). Upon hydrothermal treatment, they tend to convert easily into bayerite, Y-AI2O3, or TI-AI2O3 (110,131,142). Amorphous aluminas appear to have only moderate activity as acid—base catalysts (110,130). Surprisingly, they have been reported to have redox properties (143). In summary, amorphous aluminas have so far not shown high activity for any catalytic reaction, and because of their Hmited structural stability, they are also expected to have poor stability as catalysts. However, they can serve as reference materials in the catalysis field. 

Others attempts to prepare new LDH phases or LDH with definite stoichiometiy by this method were unsuccessful. With Zn/Fe/Cl, Cu/Al/Cl, Cu/Fe/Cl systems, mainly hydroxichloride phases are formed, respectively Zn5.xFex(OH)g CI2+X H2O (El Malki 1991), a phase with the structure of the simonkolleite, Zn5(0H)8Cl2.H20 (Nowacki and Silverman 1962), and Cu2(OH)3C1, the paratacamite (Parise and Hyde 1986). With the Mg/Cr/Cl system, a new metastable phase of probable composition MgjjCrj.j (OH)3.3j Ojjy2 (de Roy 1990) is prepared, which displays the structural properties of the Bayerite PA1(0H)3. LDH phases based on divalent metals Ni ([Ni-G-Cl], [Ni-Al-(T1], [Ni-Fe-Cl]) or Co ([Co-Fe-Cl]) were not prepared by this way l cause of the unreactivity of the respective oxides NiO and CoO. 

The stability of aluminum hydroxides shows also a pronounced pH dependence. Hydroxides precipitated from an aqueous solution of AICI3 show the structures of gibbsite at pH 5-7, bayerite at pH 7-11, and nordstrandite at pH 11-13. This means that the controlled dehydration of aluminum hydroxides can lead to alumina products with tailored properties for a large variety of industrial applications. In particular, the size and morphology of the crystalline alumina particles can be adjusted in such a way that, on subsequent sintering, ceramics with predetermined and reproducible properties can be obtained (see also Figure 7.1, steps F to H). 

A passive film formed on the metallic surface block will suppress corrosion. They are very thin films (50 to 100 A). In aluminum, thin films of boehmite (AlOOH) and bayerite (Al(OH)3) are formed which affect the corrosion process. Thick films are liable to breakdown and accelerate corrosion. Copper and nickel also have good passive film forming properties. The stifling of anodic sites by corrosion products, such as iron... 

In prolonged contact with water, it tends to grow, especially at high temperatures, and transforms into bayerite and boehmite (see Section B.5.1). The properties of these two forms of aluminium oxide are given in Table B.1.6. 

The probable composition of the outer layer is a mixture of AljOg and hydrated AI2O3, mostly in the form of amorphous A1(0H)3 or a-Al(OH)j (bayerite). This outer coating of AlOOH-A1(0H)3 is colloidal and porous with poor corrosion resistance and cohesive properties. The inner layer on the other hand... 

Apps, J.A., Neil, J.M. and Jun, C.H. (1989) Thermochemical Properties of Gibbsite, Bayerite, Boehmite, Diaspore and the Aluminate Ion Between 0 and 350 °C. Lawrence Berkeley National Laboratory Report 21482. 

As shown in Fig. 1, the oxide film that develops in normal atnwspheres is composed of two layers (Ref 1). The inner oxide layer next to the metal is a compact amorphous barrier layer of a thickness determined solely by the temperature of the environment. At any given temperature, the limiting barrier thickness is the same in oxygen, dry air, or moist air. Covering the barrier layer is a thicker, more permeable outer layer of hydrated oxide. Most of the interpretation of aluminum corrosion processes has been developed in terms of the chemical properties of these oxide layers. At lower temperatures, the predominant form produced by corrosion is bayerite, aluminum trihydroxide... 

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