Bayerite formation

Bayerite formation is favoured at room temperature while gibbsite formation is favoured at 70°C. 

The same kinds of ortho- and oxo-forms were discovered in the mass-spectra of Al(OMe)3. Its most reactive (young) forms were also obtained for Al(OEt)3 obtained under mild conditions — on alcohol interchange from Al(OPii)3 in benzene at 20°C [1640]. In storage in the samples of Al(OMe)3 and Al(OEt)jin the amorphous matrix there occurs crystallization of the polymeric layered structure of Al(OH)3 bayerite type, which on longer storage (after several years) transforms into the Al(OH)3 gjbbsite phase [1366]. The difference in reactivity of old and young Al(OMe)3 is clearly displayed in their complex formation with NaOMe [887]. 

B. S. Hemingway, Gibbs free energies of formation for bayerite, nordstrandite, Al(OH)2+, and Al(OH)2 aluminum mobility, and formation of bauxites and laterites, Adv. Phys. Geochem. 2 285(1982). The mechanistic basis of the GLO Step Rule is discussed in Chap. 6 of W. Stumm, Chemistry of the Solid-Water Interface, Wiley, New York, 1992. See also Chap. 5 in W. Stumm and J. J. Morgan, op. cit.12... 

The formation of sodium aluminate (NaA102) requires that high concentrations (20-40 wt%) of excess sodium hydroxide are used in order to avoid the formation of aluminum hydroxide which precipitates as 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. 

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. 

In the 1950s, de Boer and his co-workers (de Boer et al., 1954, 1956 de Boer, 1957) used a variety of techniques in their studies of the thermal dehydration of gibbsite and bayerite and a more detailed picture was obtained of the conditions under which the two decomposition routes were manifested. For example, it was shown that relatively well-crystallized boehmite could be produced by the treatment of gibbsite or bayerite in saturated steam at temperatures of c. 165°C. These and other findings provided qualitative confirmation that the formation of boehmite involved an intragranular hydrothermal transformation. 

A double hydroxide with the leading bayerite layer (d = 0.88 nm) is formed in Ni(OH)2 - AI(OH)j system obtained by co-precipitation from acidic media enriched with aluminum, while the co-precipitation from alkaline media enriched with nickel results in the formation of the hydroxides with bmcite layer in the packing (0.8 nm). 

The formation conditions, the composition (Ni0 Al203 = 4-14) and the characteristic basal distance d = 0.8 nm of the phase agree with another model of layer packing a continuous brucite layer is a leading layer while bayerite layer possesses discrete structure, i.e., it is not tightly packed with Al cations. 

Figure 1 of [56] indicate the location of possible adsorption sites for metal cations. The octahedral models shown in Figures 3.10 and 3.15 of [57] illustrate the formation of gibbsite, bayerite, and boehmite from solution monomers. Ball-and-stick models of the 100, 010, and 001 surfaces of gibbsite are shown in Figure 9 of [58]. An original model of different planes of a-alumina was used... 

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... 

Comparison of the standard free energies of formation at 298 K of corundum, diaspore, boehmite, gibbsite, and bayerite leads to the following selected values for free energies of formation diaspore —219.5 ... 

At conditions that normally favor the production of bayerite, the addition of chelating agents (e.g., ethylenediamine, ethylene glycol) leads to the formation of nordstrandite (51). A number of different synthesis protocols are reported for nordstrandite preparation (51—53). 

AI2O3 is customarily obtained by calcination of boehmite, bayerite, Y-AI2O3, or TI-AI2O3 at temperatures above 650—800 °C, and its formation is associated with a decrease of the surface area to 70—120 m or less. Apphcations of 0-AI2O3 as a catalyst support are mainly motivated by its stabihty at high temperatures (500—800 °C) namely, it retains a medium-to-high surface area of about 100 in g. ... 

Carbonation of potassium aluminate solutions at elevated temperatures (70-80°C) is reported to result in the formation of large (- SO pm) single crystals of gibbsite (Rosenberg, S.P. private communication, 1993). A study by Wojcik and Pyzalski (5) however, showed bayerite to be the predominant species formed in the carbonation process of potassium aluminate and sodium aluminate solutions between 50 - 80°C. 

For all conditions, bayerite was the predominant species formed at room temperature, while the formation of gibbsite was favoured at 70 C. 

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