Gibbsite dehydration

Whittington, B. and Ilievski, D. (2004) Determination of the gibbsite dehydration pathway at conditions relevant to Bayer refineries. Chem. Eng.]., 98 (1/2), 89-97. 

Besides crystal structure change, heating temperature for gibbsite dehydration also affects the specific surface area of alumina. The BET surface area of alumina samples was tested with a ASAP 2020 V3.01 H analyzer. The BET surface area of alumina is proportional to LOI, as shown in Figure 1.8.5. The specific surface area of alumina works on the dissolution process by affecting the contact area between the alumina particle and molten cryolite. 

Alumina - Alumina forms a variety of oxides and hydroxides whose structures have been characterized by X-ray diffraction (16). From the catalytic viewpoint y-alumina is the most important. This is a metastable phase that is produced from successive dehydration of aluminum trihydroxide (gibbsite) to aluminum oxide hydroxide (boehmite) to y-alumina, or from dehydration of boehmite formed hydrothermally. y-alumina is converted into a-alumina (corundum) at temperatures around 1000 C. 

Dehydration of gibbsite under pressure in moist air produces boehmite (aluminum oxide mono-hydrate). An infrared spectrum of boehmite (Kaiser substrate grade alumina) is shown in Figure 3c. 

When the gibbsite is dehydrated a structural collapse occurs with a large increase in surface area. The boehmite sample has a nominal surface area of 325 m /g. The infrared spectrum of the boehmite shows distinct structure in the OH stretching region, with two peaks located at 3090 and 3320 cm". There are three features at 1648, 1516 and 1392 cm" that are due to adsorbed water and carbonate, which are removed upon heating the boehmite to 350 0 in hydrogen. 

Spherical alumina can also be formed from commercial, low cost aluminum-oxides or even from aluminum-hydroxides. In the latter case energy of the plasma should provide not only the enthalpy of melting but that of dehydration and subsequent phase transformations of alumina as well. Under the aforementioned conditions particles below 45 pm have a good chance to be spherodized. Presumably the wide particle size distribution of starting gibbsite powder accounts for the less spheroidization rate of 70%. 

It appears to have been first noted by Achenbach (1931) that the dehydration of a gibbsite crystal is pseudomorphic the crystal shape and original lattice are retained and... 

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. 

Figure 10.17. Development of the BET-mtrogen surface area during the dehydration, by CRTA, of a sample of fine gibbsite, 1 pm gram size. CRTA conditions controlled pressure indicated in mbar on the curve rate of dehydration, 11.4 mg IT1 g"1 (Rouquerol and Ganteaume, 1977).

Figure 10.18. Development of the BET-nitrogen surface area during the dehydration of a fine gibbsite, versus % of mass loss. Same conditions as for Figure 1.17 (Rouquerol et al., 1979a).

Halse and Pratt (H57) reported SEM observations on pastes hydrated at various temperatures. In those hydrated at 8°C or 23 C, the main feature was fibrous material that was considered to be hydrous alumina, but which could also have been partly dehydrated CAH,q. The hydrating grains of cement were surrounded by shells of hydration products, from w hich they tended to become separated in a manner similar to that observed with Portland cement pastes (Section 7.4.2) though the authors recognized that this could have been partly due to dehydration. Two-day-old pastes hydrated at 40"C showed spheroidal particles of CjAH and thin, flaky plates of gibbsite. In pastes mixed with sea water, hydration took place more slowly, but no other effects on microstructural development were observed. 

The change in the structure of aluminas through the dehydration sequence from either gibbsite or boehmite to, ultimately, a-Al203, is perhaps where Al NMR spectroscopy has been most frequently appUed. The results of a selection these... 

Solvothermal dehydration can avoid this limitation, and dehydration may proceed at a temperature much lower than that required by the hydrothermal reaction. However, thermal dehydration may compete with the solvothermal reaction. When dehydration of hydroxide starts, water formed by dehydration of the starting material may facilitate hydrothermal transformation of the starting material. Therefore complicated reactions may occur simultaneously. As an example, the reaction of gibbsite (a polymorph of aluminum hydroxide, Al(OH)3) in alcohols at 250°C is explained. ... 

Intraparticle hydrothermal reaction originally proposed by de Boer et al. - for the formation of boehmite during thermal dehydration of coarse gibbsite. 

Unhke Mg(OH)2, the rate of dehydration of Al(OH)j (gibbsite) is slightly decreased [5] by an electric field, although it has been suggested that the dehydrations of both solids proceed through a proton-transfer mechanism. The difference is ascribed to... 

Even in the case of a less drastic final step (dehydration as opposed to reduction), Dalmai et al. found that irradiation infiuenced the catalytic properties of the product 182). Aluminum oxide produced from gibbsite, A1(0H)3, by heating at 290-340° was considerably more active in the decomposition of formic acid if the hydroxide had been irradiated in a reactor to 10 nvt or with about 6 X 10 i ev/gm of y-rays. The changes in catalytic activity were accompanied by somewhat complex effects of radiation on the rate of decomposition of the gibbsite 182a). Thus, irradiated samples decomposed more rapidly at low temperatures and less rapidly at high (above 210°) than unirradiated blanks, although the differences were relatively small. 

As expected from the foregoing structural discussion, gibbsite can be dehydrated to boehmite at 100° and to anhydrous at 150°, but... 

Alumina. Alumina has a general formula AhOj, although it usually exists as the trihydrate. Activated aluminas are prepared by dehydrating aluminum hydroxide (Gibbsite) at 370-400 °C in a current of air. The surface areas are 180 mVg (type E) and 90 xrr/g (type T), and the pore sizes range up to 10 gm. 

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