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Aluminum Oxide
1. General Aspects
Almost 4000 years ago Egyptians and Babylonians used aluminum compounds in various chemicals and medicines. HERODOTUS mentioned alum in the fifth century B.C. and PLINY referred to "alumen," now known as alum, as a mordant to fix dyes to textiles around 80 A.D. In 1754 MARGGRAF showed that a distinct compound existed in both alum and clays. In 1761 the French chemist GUYTON DE MORVEAU proposed the name "alumine" for the base in alum, identified in 1787 by ANTOINE LAVOSIER as the oxide of a then-undiscovered element. By the 1700s the earthy base alumina was recognized as the potential source of a metallic element.
GREVILLE (1798) described a mineral from India that had the composition Al2O3 and named it corundum [1302-74-5], derived from the native name of this stone. HAÜY (1801) called a mineral diaspore [14457-84-2] (from the Greek "diaspora" meaning dispersion) because it decrepitated on heating. Its composition, Al2O3 · H2O, was determined by VAQUELIN in 1802. Gibbsite [14762-49-3], named after the American mineralogist G. GIBBS, was found by DEWEY in 1820; TORREY (1822) showed this mineral to have the composition Al2O3 · 3 H2O. The name hydrargillite [14762-49-3] was given to a similar mineral found later in the Ural Mountains. Using the newly developed technique of X-ray diffraction, BÖHM and NICLASSEN [13] identified a crystalline aluminum oxide hydroxide, Al2O3 · H2O, later named böhmite (boehmite). BÖHM discovered a second type of trihydroxide, Al2O3 · 3 H2O, a year later. FRICKE [14] suggested the name bayerite [20257-20-9] for this compound, believing it to be the product of the Bayer process, which he later identified as gibbsite. Only few occurrences of natural bayerite have been reported [15]. VAN NORDSTRAND et al. [16] reported a third form of trihydroxide, which was later named nordstrandite [13840-05-6] in his honor.
[13] J. Böhm, H. Niclassen, Z. Anorg. Allg. Chem. 132 (1923) 1.
1.3. Aluminum Oxide, Corundum
The hexagonally closest packed a-Al2O3 modification is the only stable oxide in the Al2O3 – H2O system. Corundum is a common mineral in igneous and metamorphic rocks. Red and blue varieties of gem quality are called ruby and sapphire, respectively. The lattice of corundum is composed of hexagonally closest packed oxygen ions forming layers parallel to the (0001) plane. Only two-thirds of the octahedral interstices are occupied by aluminum ions. The structure may be described roughly as consisting of alternating layers of Al and O ions. The corundum structure was determined in the early 1920s [27]; numerous workers later confirmed and refined these data [3]. Properties of corundum are listed in Tables (1) and (2).
[27] W. H. Bragg, J. Chem. Soc. 121 (1922) 2766.
1.4. The Al2O3 – H2O System
Under the equilibrium vapor pressure of water, crystalline Al(OH)3 converts to AlO(OH) at about 375 K. The conversion temperature appears to be the same for all three forms of Al(OH)3 . At temperatures lower than 575 K, boehmite is the prevailing AlO(OH) modification, unless diaspore seed is present. Spontaneous nucleation of diaspore requires temperatures in excess of 575 K and pressures higher than 20 MPa. In the older literature, therefore, diaspore was considered the high-temperature form of AlO(OH). The first reaction diagram of the phase transitions in the Al2O3 – H2O system was published in 1943 [28]. These workers determined the gibbsite ® boehmite conversion temperature to be 428 K. Boehmite transformed to diaspore above 550 K; diaspore converted to corundum, a-Al2O3 , at 725 K. Similar results were reported in 1951 [29].
The system was reinvestigated in 1959 [30] and in 1965 [31]. A phase diagram based on these data is shown in Figure (3). Diaspore is the stable modification of AlO(OH); boehmite is considered metastable, although it is kinetically favored at lower temperatures and pressures. This is because the nucleation energy is lower for boehmite than for the considerably more dense diaspore. Nucleation is additionally facilitated by the possibility that boehmite can grow epitaxially on Al(OH)3 . In the Al2O3 – Fe2O3 – H2O system, the presence of the isostructural goethite, a-FeO(OH), lowers the nucleation energy for diaspore so that this AlO(OH) modification crystallizes at temperatures near 373 K [32]. This observation explains the occurrence of diaspore in clays and bauxite deposits that have never been subjected to high temperatures or pressures.
Nomenclature. Although there is fairly good agreement in the more recent literature on phase fields and structures of the crystalline phases in the Al2O3 – H2O system, the nomenclature is still rather unsystematic.
Bayerite, gibbsite (hydrargillite), and nordstrandite are trihydroxides of aluminum, and not oxide hydrates. The designation "aluminum oxide monohydrate" for boehmite and diaspore is also incorrect. Both are true oxide hydroxides. Molecular water has been determined only in poorly crystallized, nonstoichiometric pseudoboehmite.
The designation of the modifications of aluminum hydroxides and oxides lacks uniformity just as much as does the nomenclature of the compounds. According to the general usage in crystallography, the most densely packed structures are designated as a-modifications [3]. Bayerite, diaspore, and corundum fall within this class. The compounds with cubic packing sequence, gibbsite and boehmite, have been designated by the symbol g. Nordstrandite can be classified as b-Al(OH)3 when regarding this compound not as an intergrowth of bayerite and gibbsite, but as an independent modification.
[28] A. W. Laubengayer, R. S. Weiss, J. Am. Chem. Soc. 65 (1943) 247.
1.5. Thermal Decomposition of Aluminum Hydroxides
When aluminum hydroxides or oxide hydroxides are heated in air at atmospheric pressure, they undergo a series of compositional and structural changes before ultimately being converted to a-Al2O3 . These thermal transformations are topotactic. Despite a loss of 34 or 15 % of mass for the trihydroxides or oxide hydroxides, respectively, the habit of the primary crystals and crystal aggregates changes very little. This leads to considerable internal porosity, which may increase the specific surface area of the material to several hundred m2/g. Structural forms develop that, although not thermodynamically stable, are well reproducible and characteristic for a given temperature range and starting material. These transition aluminas have been the subject of numerous investigations because of their surface activity, sorptive capacity, and usefulness in heterogeneous catalysis. The literature in this field of physical chemistry has been reviewed up to 1987 [3].
The simplest transformation is that of diaspore to corundum. As the structures of these two compounds are very similar, the nucleation of a-Al2O3 requires only minor rearrangement of the oxygen lattice after the hydrogen bonds are broken. A temperature below 860 – 870 K is sufficient for complete conversion. The newly formed corundum grows epitaxially on the decomposing diaspore, with the (0001) plane of Al2O3 parallel to the (010) plane of AlO(OH) [33]. Recent work [34] has shown that transformation to corundum (a-Al2O3) proceeds through an intermediate a'-Al2O3 phase.
The thermal transformation, at ambient pressure, of boehmite and the trihydroxides to a-Al2O3 requires considerably more structural rearrangements and is generally not completed until the temperature reaches at least 1375 – 1400 K. The first step in the reaction sequence is the diffusion of protons to adjacent OH groups and the subsequent formation of water [35], [36]. This process begins at a temperature near 475 K. If this water cannot diffuse rapidly out of larger trihydroxide particles, hydrothermal conditions may develop locally, resulting in the formation of g-AlO(OH). With increasing loss of water, a large internal porosity develops. The lattice voids left by the escaping water are not readily healed because of the slow diffusion in this low temperature range. The voids are oriented parallel and perpendicular to the basal plane of the trihydroxide crystals (Fig. (4)).
The highest surface area and lowest crystalline order of the solid (not counting newly formed boehmite) is obtained at a temperature around 675 K. With increasing temperature the surface area decreases, while the density of the solid shows progressively higher values (Fig. (5)). This trend is the result of progressive reordering and consolidation of the solid.
During the thermally driven consolidation and reordering, the solid goes through structural stages that are influenced by the nature of the starting material as well as by heating rates, furnace atmosphere, and impurities. The general reaction paths are illustrated in Figure (6), which shows the various intermediate transition forms that have been identified during the reordering process.
Transition oxides formed at lower temperatures are mostly two-dimensional, short-range ordered domains within the texture of the decomposed hydroxides. Extensive three-dimensional ordering begins at about 1050 K. Until completely converted to corundum, the solid retains considerable amounts of OH– ions. Most likely protons are retained to maintain electroneutrality in areas deficient of cations. Therefore, the presence of protons may retard the reordering of the cation sublattice. The high surface area (> 75 m2/g) of g-Al2O3 has been shown to provide thermodynamic stability [37]. Addition of fluorine to the furnace atmosphere removes protons. As a result, rapid transition to a-Al2O3 occurs at temperatures as low as 1150 K. Markedly tabular corundum crystals form, possibly because the preceding transition alumina is mostly two-dimensionally ordered [38].
Transition forms other than those shown in Figure (6) can be obtained by hydrothermal treatment [3]. The structures of various transition forms have been investigated [17], [22], [39].
[3] K. Wefers, C. Misra: Oxides and Hydroxides of Aluminum, Alcoa Technical Paper no. 19, revised, Pittsburgh 1987.
Bayer process flow sheet
