Aluminum hydroxides diaspore

The CAS registry Hsts 5,037 aluminum-containing compounds exclusive of alloys and intermetaUics. Some of these are Hsted in Table 1. Except for nepheline and alunite in the USSR and Poland, bauxite is the raw material for all manufactured aluminum compounds. The term bauxite is used for ores that contain economically recoverable quantities of the aluminum hydroxide mineral gibbsite or the oxide—hydroxide forms boehmite and diaspore. 

Diaspore (P-Aluminum Oxide Hydroxide). Diaspore, found in bauxites of Greece, Cliina, and the USSR, can also be obtained by hydrothemial transfomiation of gibbsite and boehmite. Higher (>200°) temperatures and pressure (>15 AlPa-150bar) are needed for synthesis and the presence of diaspore seed cry stals helps to avoid boehmite fomiation. 

The raw material for the manufacture of aluminum hydroxide is bauxite, a mineral with ca. 45 to 60% AI2O3 in the form of hydrargillite (gibbsite), bdhmite and diaspore with iron oxides or hydroxides, silica (as kaolinite) and titanium dioxide as minor constituents. The largest deposits are to be found in Africa and Australia. 

The term alumina hydrates or hydrated aluminas is used in industry and commerce to designate aluminum hydroxides. These compounds are tme hydroxides and do not contain water of hydration. Several forms are known a general classification is shown in Figure 1. The most well-defined crystalline forms ate the trihydroxides, Al(OH)3 gibbsite [14762-49-3], bayerite [20257-20-9], and nordstrandite [13840-05-6], In addition, two aluminum oxide—hydroxides, AIO(OH), boehmite [1318-23-6] and diaspore [14457-84-2], have been clearly defined. The existence of several other forms of aluminum hydroxides have been claimed. However, there is controversy as to whether they ate truly new phases or structures having distorted lattices containing adsorbed or intedameUar water and impurities. 

The compound is prepared by heating aluminum hydroxide or a-AlaOg to above 1100°C. Diaspore undergoes rapid conversion above 500°C. 

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-... 

Aluminum hydroxide gel contains the equivalent of not less than 3.6g and not more than 4.4g of aluminum oxide (AI2O3), in the form of aluminum hydroxide and hydrated oxide. Aluminum oxide, also known as alumina, occurs in nature in the minerals bauxite, bayerite, boehmite, corundum, diaspore and gibbsite. Hydrated alumina, or aluminum hydroxide has... 

P-Aluminum oxide-hydroxide. See Diaspore Aluminum oxide-hydroxides, 2 421. 

Practically all world production of newly smelted aluminum was made from bauxite. The term bauxite is used for naturally occurring mixtures of aluminum monohydrate (boehmite or diaspore) and trihydrate gibbsite Al(OH)3, including impurities which are typically clay minerals, free silica, iron hydroxides and titania. The luminescence ofboehmite and diaspore and LIBS (Fig. 8.10) maybe used for the detection, sorting and commercial value evaluation of A1 minerals. 

Alkali leach methods axe exemplified by the Bayer process for the preparation of pure a-A C for electrolysis (Section 17.5) from the mineral bauxite. Bauxite consists mainly of a-AlO(OH) (diaspore) and/or 7-A10(0H) (boehmite), the difference between these being essentially that the oxygen atoms form hep and ccp arrays, respectively. The chief contaminants are silica, some clay minerals, and iron(III) oxides/hydroxides, which impart a red-brown color to the mineral. Aluminum (III) is much more soluble than iron(III) or aluminosilicates in alkali, so that it can be leached out with aqueous NaOH (initially 10-15 mol L 1) at 165 °C under approximately 0.6 MPa pressure, leaving a red mud of iron (and other transition metal) oxides/hydroxides and aluminosilicates ... 

There are several important hydrated forms of alumina corresponding to the stoichiometries A1(0)0H and Al(OH)3. Addition of ammonia to a boiling solution of an aluminum salt produces a form of A1(0)0H known as boehmite, which may be prepared in other ways also. A second form of A1(0)0H occurs in Nature as the mineral diaspore. The true hydroxide Al(OH)3 is obtained as a crystalline white precipitate when carbon dioxide is passed into alkaline aluminate solutions. It occurs in Nature as the mineral gibbsite. Materials sometimes referred to as /1-aluminas have other ions such as Na+ and Mg2+ present. They possess the idealized composition Na2011Al203. They can act as ion exchangers, have high electrical conductivity, and are potential solid state electrolytes for batteries. 

Figure 3 Structure of (a) diaspore and (b) boehmite. The aluminum atoms are octahedrally surroimded hy five oxide and one hydroxide anions. The hydrogen bonds are shown as double hues...

The industrially most important minerals for aluminum manufacture are the bauxites (laterite). Bauxite is not a uniform material, but comprises a mixture of different aluminum oxide hydrates and hydroxides [e.g. boehmite, y-AIO(OH), diaspore, a-AlO(OH)] with aluminum silicates, iron and titanium oxides etc. with between 35 and 70% (mostly 50 to 65%) aluminum oxide. 

Since AG" (reaction 2.36) is positive this reaction is not spontaneous when the reactants are in their standard states. Moreover, hydrargillite is expected to dehydrate spontaneously to form diaspore, which is the most stable among aluminum (III) oxides, hydroxides and oxohydroxides. However the practice shows that processes of hydration and dehydration are rather slow at ambient conditions and the unstable forms show sufficient degree of metastability to remain unchanged over the time of typical laboratory experiments. The result obtained for aluminum (III) does not imply that other oxohydroxides are also more stable than oxides or hydroxides. Many oxides do not undergo spontaneous hydration, e.g. 

Gibbsite, boehmite, diaspore, and corundum are minerals found in soil amorphous Al(OH)3 is the form that commonly precipitates first when an aluminum solution is alkalinized soil alnminnm hydroxide refers to an average value introduced by Gustafsson (Gustafsson, Lumsdon, and Simonsson 1998). To evaluate the thermodynamic tendency to precipitate of any solid phase, it is convenient to compute the saturation index, SI, which can be defined as... 

Aluminum forms the hydroxide AI(OH)3, whereas in the case of iron the Fe(OH)3 phase is unstable. It dehydrates spontaneously and crystallizes as a-Fc203 hematite or a-FeOOH goethite. The isostructural a-AlOOH (diaspore) forms only under hydrothermal conditions. 

Figure 12. Hydroxyl absorption of aluminum and iron hydroxides of a chlorite and of scarbroite. (a) Gibbsite (b) bayerite (c) diaspore (d) boehmite (e) scarbroite (f) chlorite (g) goethite (h) lepi-docrocite.

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