Aluminium Gibbsite

Lastly, the previously-mentioned study into the low-surface-area aluminium oxide-hydroxides diaspore and boehmite by CRAMPS (Section 2.2.1) [46] has also been extended to the aluminium hydroxides gibbsite and bayerite, revealing three distinct resonances in a 3 2 1 ratio in these systems and suggesting the presence of Al2(/r-OH) groups with 6-coordinate metal centres. 

The production of flame retardant quahty aluminium hydroxide has recently been reviewed [98]. Various crystal forms of aluminium hydroxide exist, but that used for polymer appHcations is Gibbsite. This occurs widely in nature, usually in the rock bauxite, but the natural form is usually not suitable for direct use and synthetic products are nearly always employed. Most aluminium hydroxide is manufactured through the Bayer process used to make alumina for refractory applications. 

Aluminium oxide is the oldest ceramic material used in medicine. Bauxite and corundum are the main natural sources of aluminium oxide. Bauxite is a mixture of diaspore, gibbsite, iron hydroxides, clay minerals and quartz. It is formed by the tropical weathering of silicate rocks during which quartz and the elements sodium, calcium, magnesium and potassium are largely washed away. This is the reason why the remaining material becomes richer in the resistant elements titanium, iron and aluminium. The latter is extracted from this mixture in the form of aluminium hydroxide. In its turn this compound is converted into aluminium oxide by heating the mixture to 1200-1300 °C, this is called calcination. The hydroxide is thus made anhydrous. 

Phosphorus.—The commonest natural compounds of phosphorus are phosphorite or calcium phosphate, Ca3(P04)2, and gibbsite or aluminium phosphate, A1P04. 

Total concentration of aluminium in equilibrium with gibbsite. In the above calculation a total concentration may have been specified in which the solubility of a mineral phase may have been exceeded therefore, it would be appropriate to calculate the speciation and total aluminium in equilibrium with a specified mineral phase, e.g. gibbsite. 

Figure 5.4 shows [A1]T in equilibrium with gibbsite as a function of pH. The solubility of aluminium decreases as pH increases, up to about pH 7 beyond pH 7 the total concentration of A1 in solution increases as gibbsite becomes more... 

Figure 5.4 Total concentrations of aluminium, [AI]T, in equilibrium with gibbsite in the presence and absence of different concentrations of oxalic acid [H2U.

As the SI is negative gibbsite would not be expected to precipitate in this solution. Perverse though it may seem a more dilute solution of aluminium would actually be supersaturated with respect to gibbsite as the pH would be higher. 

Although various modifications of aluminium trihydroxide, Al(OH)3, have been described in the literature, there are only three common forms gibbsite (originally also called hydrargillite) bayerite and nordstrandite. Gibbsite is die best known and most abundant. It is the main constituent of North and South American bauxite and is obtained as an intermediate product (i.e. Bayer Hydrate ) in the Bayer process for the production of aluminium from bauxite. 

Figure 10.14. Crystalline structure of gibbsite. Small solid circles, aluminium ions large open circles, hydroxyl tons (after Saafeld, 1960).

Brucite [magnesium hydroxide Mg(OH)2] is isostructural with CH. It is formed in Portland cement concrete that has been attacked by magnesium salts, and on hydration of Portland cements high in MgO and possibly of Portland cements in general. It has a = 0.3147 nm, c = 0.4769 nm, Z = 1, >, = 2368kgm", oi = 1.561, e = 1.581 (S63). Three polytypes of aluminium hydroxide [Al(OH)3], gibbsite, bayerite and nordstrandite. contain layers essentially similar to those in brucite, but with an ordered pattern of... 

Bacteria associated with bauxite ore have been shown to reductively mobilize significant amounts of the Fe(III) oxides in the bauxite under anaerobic conditions without mobilizing significant amounts of the Al(III) oxides like gibbsite or boehmite as long as the pH of the reaction system was kept above 4.5. The bauxite residue was thus enriched in aluminium (Ehrlich et al., 1995 Ehrlich Wickert, 1997). 

A substantial fraetion of the ionic character of A1 - O bond and the absenee of the own eleetrons in the valence shell of AF make the complexes kinetieally labile that allows to obtain crystal aluminium hydroxides (gibbsite, bayerite, boemite, norstandite and diaspore) using the reactions in water solutions [33,34]. 

Aluminium hydroxides. Aluminium trihydroxide is known to have three modifications hydrargillite (gibbsite), bayerite and norstandite. Most widely used gibbsite is a natural mineral it can be also synthesized by... 

Cordierite synthesis method based on mechanical activation of mixtures of hydrated oxides of calcium, aluminium and silicon, as well as natural hydrated compounds (talc, kaolinite and gibbsite), has been developed in [2, 3]. Mechanical activation of these mixtures does not lead to the formation of new phases but provides good mixing at the cluster level giving aggregates that form cordierite during the subsequent thermal treatment. 

Natural kaolin is treated by washing, in order to remove the coarser particles and impurities such as quartz, feldspars, etc. For some purposes, use is also made of crude kaolin (e.g. in fireclay manufacture). Clays are more widespread in nature they are usually contaminated with larger amounts of quartz, feldspar, mica, etc. Clays containing larger or smaller amounts of aluminium hydroxides (diaspore or gibbsite) occur and are used as ceramic raw materials in some countries. 

Aluminium hydroxide. A number of forms of Al(0H)3 are recognized which are of two main types, a (bayerite), and y (gibbsite), but there are minor differences within each type arising from different ways of superposing the layers. All forms are built of the same layer, the AX3 layer shown in idealized form in Fig. 14.9. This may be described as a system of octahedral AX coordination... 

Kaolinite is an important constituent of many lateritic profiles, and a common product of hydrolysis it tends to be abundant where there is free-drainage. The reaction progressively separates silica in aqueous solution from aluminium, which remains in the solid phase (as kaolinite or gibbsite). 

I.r. and broad-line n.m.r. studies of the aluminium-containing species gibbsite, bayerite, and nordstrandite have been made.421 The i.r. spectra, in the A1—O stretching region, could be analysed satisfactorily by the factor-group approach. For gibbsite, a model having H atoms between sheets of O atoms is consistent with the i.r. and n.m.r. data, but similar conclusions could not safely be made for the other compounds. 

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... 

The octahedral sheet is composed of cations, usually aluminium, iron or magnesium, arranged equidistant from six oxygen (or OH) anions (Fig. 4.7b). Aluminium is the common cation and the ideal octahedral sheet has the composition of the aluminium hydroxide mineral, gibbsite (Al(OH)s). Where octahedral sites are filled by trivalent aluminium, only two of every three sites are occupied to... 

In an average upper-crustal granodiorite, it is mainly feldspars that weather to form clay minerals (eqns. 4.13 4.14). Since feldspars are framework silicates, the formation of clay minerals (sheet silicates) must involve an intermediate step. This step is not at all well understood although it has been proposed that fulvic acids, from the decay of organic matter in soil, may react with aluminium to form a soluble aluminium-fulvic acid complex, with aluminium in six-fold coordination. This gibbsitic unit may then have Si04 tetrahedra adsorbed on to it to form clay mineral structures. 

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