Oxyhydroxide boehmite

The aluminum hydroxide (gibbsite, y-Al(OH)3) and oxyhydroxide (boehmite, y-AlOOH diaspore a-AlOOH) minerals in bauxite ore are selectively extracted from the insoluble components (mostly quartz, clay minerals, and iron and tita-... 

Among different aluminas, y-Al203 (mostly obtained by decomposition of the boehmite oxyhydroxide y-AlOOH) is the most nsed material in any field of... 

Solvothermal reaction of gibbsite in ethanol gives boehmite (AlOOH a polymorph of alnminnm oxyhydroxide), which is also obtained by hydrothermal reaction of the same starting material. The prodnct is comprised of randomly oriented, thin, small crystals of boehmite. This resnlt snggests that a dissolntion-recrystallization mechanism takes place dnring the conversion. 

One can speculate that other soil aluminum oxyhydroxide phases, diaspore, bayerite, and nordstrandite also exhibit low surface free energies, similar to those for gibbsite and boehmite. More calorimetric studies are needed to quantify these relations. 

Several authors [1, 18-20] have surveyed the chemical and crystallographic changes which occur during the dehydroxylation of the three forms of aluminium hydroxide, (Al(OH)3 gibbsite (y), bayerite (a) and norstrandite) and the two forms of aluminium oxyhydroxide, (AlOOH diaspore (a) and boehmite (y)). These phases... 

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... 

The two crystalline Al-oxyhydroxide polymorphs that exist in natural environments are boehmite and diaspore. Gibbsite, boehmite, and diaspore are the three hydrates of Al, which are the main constituents of bauxites and laterites. 

In contrast to the closely related structures of the four trihydroxides, the structures of the two oxyhydroxides are significantly difierent from each other. The structure of boehmite (23,24), depicted in Figure 3.3 (25), is characterized by double layers of oxygen octahedra partially filled with aluminum cations. The stacking arrangement of the three oxygen layers in each... 

At temperatures higher than about 80 °C, the oxyhydroxides become thermodynamically more stable than the trihydroxides (54) and thus tend to form. Poorly crystallized solids designated as pseudoboehmite form at lower temperatures, whereas weU-crystaUized boehmite forms at higher temperatures or after longer aging. The typical conditions for the formation of poorly crystallized, gelatinous pseudoboehmite are pH values of about... 

As stated above, Y-AI2O3 is, by definition, the solid product of the decomposition of boehmite, the oxyhydroxide denoted as y-AlOOH. Since the early investigation by Verwey (145) and subsequent work by Lippens and de Boer (146), the structure ofy-alumina has been conventionally described as a cubic defective (or non-stoichiometric) spinel. The cubic spinel structure (Figure 3.9) is typical of compounds with AB2O4 stoichiometry and belongs to the space group Fd3 m and the number of formula units in the unit cell, Z, is 8. The structure is characterized by a cubic close packed (ccp) sublattice of oxide anions, with A and ions occupying tetrahedral (Ta) and octahedral (Oh) voids, with Wyckoff positions 8a and 16d,... 

TEM shows fumed alumina rich in 8-AI2O3 to have a pronounced globular morphology, whereas 8-AI2O3 produced by calcination of boehmite essentially retains the morphology of the starting oxyhydroxide, such as lamellar, fibrous, or nanorod Hke (57,208). 

Aluminum is another abundant metal that is obtained from the ore bauxite, a mix of the minerals boehmite (aluminum oxyhydroxide) and gibbsite (aluminum hydroxide). The production of aluminum is very expensive because the bauxite must be heated with the compound cryolite to make a molten solution in which the aluminum is concentrated by an electric current. Aluminum is used in products such as cans, foil, windows, vehicles, and household items and mixed with other metals in alloys. 

The results of development work on processes indicate that the two main methods of preventing the duplex microstructure from forming appear to be fast-firing, or increasing the amount of / "-alumina at low temperatures. Based on these results, Duncan et al. [21] and Zyl et al. [22] have described production processes starting from aluminum oxyhydroxides or aluminum hydroxides as precursors for the synthesis of the solid electrolyte "-alumina. Duncan et al. described an alumina precursor which substitutes in part or wholly for a-alumina in an established slurry solution spray-drying process. As a precursor, hydrothermal boehmite, Cera hydrate, has been used. A calcination step is important at the begirming of the process. Boehmite was used both in the as-received condition and after calcination. The effect of the calcination temperature on the fired properties of /S"-alumina can be seen in Table 21.5. 

Aluminum initially forms a few nm thick layer of aluminum oxide, mainly y-Al Oj (boehmite), which in humidified air is covered by aluminum oxyhydroxide, y-AlOOH due to hydrolysis, resulting in a double-layer structure. " Related reactions that occur within the passive film when in contact with humidity or water are as follows ... 

That supposed a stoichiometric hydrate at the initial time of this dehydratioa It may be that, initially, the hydrate starts by losing water by simple variation of its stoichiometiy before breaking up. It is, for example, the case of boehmite which is an aluminmn oxyhydroxide AlOOH, which loses water without phase change and then breaks up into yalumina. We then have the superposition of both transformations which are written, respectively ... 

The structural smdies (X-rays diffraction and infrared spectroscopy) show that there is a phase change around 350°C and that if the high-temperature phase is a hydrated alumina (y-alumina, AI2O3XH2O), the low-temperature phase is an aluminum oxyhydroxide called boehmite, with formula AlOOH. 

Alkalinlzation of an aluminum nitrate solution around pH 6-7 with sodium hydroxide or ammonia causes the immediate formation of a translucent amorphous gel. This unstable gel crystallizes more or less rapidly and forms different crystalline phases depending on the acidity of the medium at pH 6 or 7, it leads to the oxyhydroxide 7-AIOOH (boehmite), whereas at pH < 5 or pH > 8, Al(OH)3 gibbsite or bayerite forms, respectively. The reaction path during transformation of the gel is related to changes in the solubility of aluminum with the pH of the medium (Figure 3.9). 

It is reasonable to assume that boehmite may form by thermolysis of aluminum solutions, starting from [Al40(OH)io(OH2)5] or [Al4(OH)i2(OH2)4]° tetramers. These are good structural models for the oxyhydroxide nuclei and they may be considered in a first approximation as the building blocks of the solid phase. 

Boehmite (y-AlOOH(s)) becomes the stable solid oxyhydroxide phase of aluminium at a temperature of around 100 C. However, solubiUty data at zero ionic strength are available to lower temperatures (on the basis of the reported temperature dependence of the solubility). The relationship between the solubility constants and temperature is illustrated in Figure 13.2. As was the case with gibbsite. 

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