Mullite decomposition

Figure 7.1 Typical thermal expansion trace kyanite (AljCVSiOj) + quartz (Si02) at 5°C/min. The a-0 quartz inversion is apparent at 573°C. Kyanite converts to mullite (3Alj03-2Si02) and residual glass starting at 1100°C, reaching a maximum rate at 1400°C [1]. The sharp contraction starting at 1100°C is interpreted to correspond to sintering. At 1320°C, the rapid formation of the less dense decomposition products of kyanite cause a temporary expansion [2].

Kaolinite is transformed into X-ray amorphous state when activated in air. According to authors [14,15], amorphization involves the destruction of bonds between tetrahedral and octahedral layers inside the package, till the decomposition into amorphous aluminium and silicon oxides. Other researchers [ 16,17] consider that amorphized kaolinite conserves the initial ordering of the positions of silicon atoms while disordering of the structure is due to the rupture of A1 - OH, Si - O - A1 bonds and the formation of molecular water. Endothermic effect of the dehydration of activated kaolinite is shifted to lower temperatures while intensive exo-effect with a maximum at 980°C still conserves. When mechanically activated kaolinite annealed at 1(X)0°C, only mullite (3Al20j-2Si0j) and X-ray amorphous SiOj are observed. In this case, the phase with spinel structure which is formed under thermal treatment of non-activated kaolinite is not observed thus, mechanical activation leads to the formation of other phases. 

Thermal decomposition of two starting materials in inert organic solvent may provide a convenient route for the synthesis of mixed oxides or precursor of mixed oxides. For example, when a mixture of aluminum isopropoxide and tetraethoxysilane (tetraethylorthosilicate) in a 3 1 ratio is decomposed in toluene, an amorphous product is obtained." Note that thermal decomposition of the former compound alone yields %-alumina, while the latter compound alone does not decompose at the reaction temperature." Mullite is crystallized by calcination of the product at 900°C." It is known that the crystallization behavior of mullite from the precursor gel depends on the homogeneity of mixing of aluminum and silicon atoms in the precursor when the precursor gel has atomic scale homogeneity, mullite is crystallized at 900°C, and the gel with homogeneity in a nano... 

Figure 4.8. Changes in the Si spectra of kaolinite during its thermal decomposition, showing the progressive formation of the broad metakaolinite resonance envelope - 99 to - 102 ppm) at 650-800°C, the sudden appearance of free Si02 (—110 ppm) at 970°C, and the formation of mullite (— 88 to — 92 ppm) above 1100°C. Adapted from Mackenzie et al. (1985a) by permission...

The reactions of several other minerals which thermally decompose to form mullite have been studied by Si and Al NMR. These include the mica mineral muscovite, which also contained sufficient iron to permit a complementary Fe Mossbauer study (MacKenzie et al. 1987), the hydroxyfluoride mineral topaz (Day et al. 1995) and the semi-amorphous aluminosilicate minerals allophane (MacKenzie et al. 1991) and imogolite (MacKenzie et al. 1989). The same combination of NMR nuclei has been used to study the thermal decomposition of other aluminosilicates including an illite-rich clay (Roch et al. 1998), montmorillonite (Brown et al. 1987), and a related mineral, Fuller s Earth (Drachman et al. 1997). NMR has also been used to study the effect of water vapour on the thermal decomposition of montmorillonite clay compacts (Temuujin et al. 2000a). 

Similar to the procedure utilized by Rigby and Hutton, Lambertson [30] ground a 60% alumina-diaspore, fireclay brick and mixed this powder with sodium sulfate. The mixture was heated in a platinum crucible to 1300°C. X-ray and microscopic examination showed the decomposition of mullite to form glass and corundum. As the reaction proceeded, nephelite replaced the glass. 

The three sillimanite minerals are structurally similar and have structures that are related to that of mullite. It is not surprising that they all form mullite upon decomposition. Kyanite crystallizes in the triclinic system, while sillimanite, andalusite, as well as mullite have orthorhombic crystal structures. In these structures, all the Si4+ cations are in fourfold coordination with 02 anions, but the Al3+ cations exist in four-, five-, and sixfold coordination with 02 anions, and therein lie the structural differences. The fivefold coordination of some Al3+ cations within A105 polyhedra is rather unusual, perhaps the result of formation at high pressures. The other structural differences among the three minerals are quite small. They are associated with the double chain structures of these three minerals and the linkages of the chains to one another by different alumina and silica polyhedra. Those concepts are readily extended to mullite. 

The structure of mullite is similar to that of the sillimanites, consistent with the fact that they decompose to form mullite at high temperatures and 1 atm pressure. It has been suggested that the double A106 octahedral chain structure is preserved during the decomposition. The mullite structure is, however, somewhat complicated by its extensive stability over a wide range of stoichiometries. The composition of mullite can be expressed as... 

Differential Thermal Analysis. Details revealed in the differential thermograms of the 4 MA-Y zeolites (Figure 2) are consistent with the TGA results. They all exhibited an endotherm (122°-190°C) caused by loss of adsorbed water. Over the region where slow decomposition occurred, there were weak endotherms or inflections in the curve, and the rapid decomposition was confirmed by a sharp endotherm (516°-577°C). Dehydroxylation was clearly revealed by the endotherm at 785°C in MMA-Y, but was not as well resolved in the others. The exotherm (977°-1026°C) was owing to mullite transformation (I). However, all the samples were found to be amorphous (by x-ray) after 1 hour at 900°C. 

Monolithic supports were obtained om CTI Company (Salindres, France). The chosen material is mullite, 3Al203-2Si02, and the monolith displays 7x7 square channels of 1 mm internal side length. The overall length of the decomposition chamber is 15 mm. The advantages of mullite are good thermal stability and mechanical properties. The monoliths are put for 1 h in a solution of nitric acid 1 mol.L then heated at 300°C for 1 h. 

However, the previously mentioned hydrolytic reaction is very sensitive to experimental conditions, such as the presence of acid or base catalysts, reaction temperature, and molar ratio of alkoxides to H2O. Three critical experimental parameters (pH, temperature, and reaction time) that affected the rate of hydrolysis were systematically studied by Paulick et al. (34). The volume ratio of 100 ml mixed alkoxide [Al(OC3H7)3 and tetraethylorthosilicate (TEOS)] solution to 500 ml of an H2O/CH3OH (3 1 ratio) solution was used for all the experiments. It was found that in an acid environment (pH 2) the hydrolytic decomposition was completed within 7 h at room temperature. Stoichiometric mullite was the only product obtained, as evidenced by both elemental analysis and x-ray diffraction. Under alkaline conditions (pH 10), the degree of hydrolytic decomposition was both time and temperature dependent. The TEOS was only partially hydrolyzed even after heating at 70°C for 20 h. The oxide powder obtained was composed of 65% AI2O3 and 35% mullite. The degree of hydrolysis is only moderately affected by temperature, and it is not affected by the reaction time. 

However, each polymorph exhibits a different decomposition behavior. Actually, the decomposition of kyanite is unpredictable it first starts to decompose slowly at 1310 C, and the reaction disrupts at about 1350 to 1380 C with an important volume expansion of 17 vol.%. For that reason, kyanite must always be calcined prior to being incorporated into a refractory in order to avoid blistering and spalling. By contrast, andalusite decomposes gradually from 1380 tol400"C with a low volume increase of 5 to 6 vol.%, while sillimanite does not change into mullite until the temperature reaches 1545"C with a volume expansion of 5 to 6 vol.%. 

The thermal techniques are routinely used for quality control purposes. A typical TG/DTA analysis is shown in Fig. 13 for a Thessalian brick clay.t None of these clays contains kaolin and the characteristic mullite peak is absent. The endothermic peak between lOOand 160°Cisdue to the removal of adsorbed water, and its size is dependent on the surface area and crystallinity of the clay. The peak (400-700°C) is attributed to the dehydration of the combined water (dehydroxylation of the silicate lattice) and decomposition of the clay. The third peak (800°C) indicates decomposition of the carbonates and other salts present in the clays. 

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