A Review on the Kaolinite-Mullite Transformation

Mullitisation behaviour of calcined clay-alumina mixtures

Abstract: The mullitisation behavior of three south Indian calcined clays with three different alumina sources has been studied. The calcined clays were ground and then mixed with different alumina sources such as reactive alumina, gibbsite and boehmite. Rectangular bars were prepared and sintered at 1600 °C/3 h. The X-ray diffraction and microstructural analysis were carried out to understand the mullitisation behavior. Thermogravimetric and differential thermal analyses of the powders were also carried out. The physical and mechanical properties were measured and discussed. The calcined clay (meta kaolin) derived samples show better strength and density than the uncalcined clay derived sample. The microstructure also shows a higher aspect ratio of mullite crystals. Among the three clay combinations, the calcined Neyveli clay and fine reactive alumina mixture was found to exhibit a better mullitisation behavior compared to other combinations.

Phase‐Pure Mullite from Kaolinite

Abstract: Sintering of kaolinite in the presence of certain carbonate mineralizers, viz., CaCO 3 , Na 2 CO 3 , and K 2 CO 3 , has been conducted at 950°-1350°C. The influence of these mineralizers at the above temperatures is evaluated using XRD and SEM techniques. A comparative study of phase formation of the above compositions shows that the sodium- and calcium-fluxed samples give rise to multiphase systems, while K 2 CO 3 -incorporated samples give phase-pure mullite. The observation that kaolinite in the presence of K 2 CO 3 can act as a precursor material for phase-pure mullite is of great industrial significance.

Abrasive Tools and Bonding Systems

Abstract: The manufacture of high performance grinding wheels is heavily dependent upon the selecting the correct abrasive grain in terms of cutting ability, fracturing ability and the shape of the grain that controls metal removal rate, dressing interval, coolant delivery to the wheel-workpiece interface and chip evacuation. This chapter explains the recent developments in abrasive grains and bonding systems and should allow the reader to better select the correct abrasive grain and bonding system to economically machine materials.

Effect of Bi2O3 on phase formation and microstructure evolution of mullite ceramics from mechanochemically activated oxide mixtures

Abstract: Mullite ceramics with hollow whisker structure have been synthesized firstly through ordinary sintering process. The effects of Bi 2 O 3 and processing, on mullitization behavior and morphology development of mullite ceramics, derived from the mechanochemically activated mixture of Al 2 O 3 and SiO 2 , were investigated in this paper. When the content of Bi 2 O 3 was less than 10 mol%, the mullite grains show a short rod-like morphology, without the formation of whisker. As the content of Bi 2 O 3 was increased to more than 10 mol%, the formation temperature of mullite was decreased from 1400 °C to 1100 °C. After sintering at 1400 °C, well-developed mullite whiskers with hollow structure were formed. The formation process and growth mechanism of hollow structural whiskers in mullite ceramic doped with high content of Bi 2 O 3 were discussed in detail.

THE SYSTEM: Al2O3.SiO2

Abstract: This paper deals with a study of the equilibrium relations of mixtures of pure alumina and silica at high temperatures. The results are expressed concisely in the form of an equilibrium, diagram and their bearing on ceramic problems is discussed. The principal feature of the diagram is the absence of the compound Al2O3.SiO2, the only compound being 3Al2O3.2SiO2. Crystals of this latter compound occur in all alumina-silica refractories. The optical properties of these crystals have been determined and are compared with those of sillimanite, Al2O3.SiO2, which has hitherto been regarded as the crystalline compound occurring in refractories and clay bodies in general. The behavior of natural sillimanite on heating is discussed.

Interpretation of the Kaolinite-Mullite Reaction Sequence from Infrared Absorption Spectra

Abstract: The phases in the kaolinite-mullite reaction sequence were reexamined by ir absorption spectrophotometry. Particular attention was paid to the controversial intermediate Al-containing phases. Amorphous materials were leached from fired kaolinite samples with NaOH to help identify crystalline phases. Metakaolinite partially decomposes, releasing amorphous γ-Al2O3 and SiO2, before the “950°C” exothermic reaction in which metakaolinite is completely decomposed. The resulting spinel-type phase, which is associated with amorphous SiO2 and some poorly crystalline “primary” mullite, is γ-Al203 (crystalline) rather than an Al-Si spinel. There is some evidence, however, that a fraction of the γ-Al2O3, may be an Al-Si spinel. At ≥1100°C secondary mullite therefore forms primarily from the γ-Al2O3/amorphous SiO2 reaction and the recrystallization of primary mullite, whereas excess amorphous SiO2 eventually crystallizes as cristobalite.

The KaoIinite‐MuIIite Reaction Series: Ill, The High‐Temperature Phases

Abstract: X-ray diffraction data for the high-temperature phases show that a spinel-type structure develops with marked orientation at about 925°C. This phase is considered to be an aluminum silicon spinel with vacant cation sites. Mullite is thought to be formed by the decomposition of the spinel. Silica is eliminated progressively as metakaolin transforms to the spinel phase and thence to mullite. The X-ray data show variation in the mullite parameters between 1200° and 1400°C.; at 1400°C., the composition probably approximates 3A12O3·2SiO2. The nature of the intermediate spinel-type phase is discussed in relation to the crystal chemistry of spinels.