Akshoy Kumar Chakraborty

Role of hydrolysis water-alcohol mixture on mullitization of Al2O3-SiO2 monophasic gels

Two kinds of monophasic gels were synthesized from silicon ethyl ester (TEOS) and aluminium nitrate mixture by varying the water-alcohol content of the system. The mullitization reaction paths of the two gels were studied by differential thermal analysis (DTA) and X-ray diffraction techniques. The first kind of monophasic gel exhibited cubic mullite (Si-Al spinel) on the 980 °C exotherm and a further two more broad exotherms at 1150 and 1243 °C. This phenomenon resembles the DTA thermogram of a kaolinitic clay. The later two exotherms have been explained as due to the formation of mullite by two different reaction paths from the two phases appearing in the 980 °C exotherm. Nucleation and crystallization of mullite from amorphous alumino silicate phase takes place at ∼ 1150 °C, and transformation of cubic mullite to orthorhombic mullite takes place at ∼ 1245 °C. The second kind of monophasic gel exhibited only one exotherm and produced exclusively tetragonal mullite at ∼ 980 °C. The results suggest that pure mullite formation is possible when water induced hydrolysis of TEOS is avoided during the gelation process.

Electron Microscopy Study

As shown in Chap. 5, metakaolinite shows amorphous to words X-ray and spinel too is a very weakly crystalline material. In view of it, an extremely powerful technique high resolution electron microscopy is applied for characterization.

New Data on Thermal Effects of Kaolinite in the High temperature Region

DTA study of an Indian Kaolinite has been performed by varying packing density and rate of heating. Both these two parameters influence the intensities of both endothermic and exothermic peaks related to the dehydration, dehydroxylation and then crystallizations of Al-Si spinel, mullite and cristobalite phases. Significantly, the study reveals that mullitization takes place by two separate reactions as indicated by two exotherms in the 1200-1400°C range of DTA trace.

Supplementary alkali extraction studies of 980°C-heated kaolinite by X-ray diffraction analysis

The akali extraction process with the aim of leaching out free SiO2 present in 980 °C-heated kaolinite has been re-examined by corroborating with X-ray diffraction analysis of the leached and heat-treated leached residues of Georgia kaolinite. The validity of the composition of the intermediate Si-Al spinel phase analogous to the composition of 3∶2 mullite has been confirmed.

Radial Electron Density Distribution

The radial electron density distribution (RED) technique was applied to study the structural aspects of the two transition phases in K–M reaction series. This study is perfectly suited for amorphous and poorly crystalline materials like metakaolinite and spinel phases.

Meta Kaolinite Phase

The nature of metakaolin and its structural characteristics are being dealt in this chapter. The first question is: what is occurring on dehydroxylation and what happens to the Si–O–Al linkages present between two layers constituting kaolinite? There are two probabilities. It may happen that kaolinite partially or fully decomposes with disruptions of Si–O–Al bonding during dehydroxylation and liberate-free silica (A) or it may remain as an alumino-silicate (A) compound which decomposes at the vicinity at 980 °C exotherm. Various researchers conducted different chemical dissolution experiments in order to reveal indirectly the chemical linkages existing if any between tetrahedral silica sheet and dehydroxylated alumina layer. Mainly chemical techniques were employed. Since simple X-ray study exhibited amorphous pattern. Thus, it was speculated that metakaolinite be an amorphous mixtures of silica and alumina or amorphous compound. Acid dissolution study showed that alumina layer in metakaolinite either present in highly distorted state or it becomes amorphous. On the other hand, sodium carbonate extraction study showed that silica and alumina packets are not present in free state. Accordingly, two views were prevailed in the olden days.

Scanning Electron Microscopic (SEM) Study

Segnet and Anderson (1971) indicated the nature of changes occurring in the physical structures of a well and a poorly crystallized kaolinite when heated to different temperatures by scanning electron microscopic studies.

Thermal Evolution of Kaolinite

Methods used by various investigators for ascertaining thermal decomposition of kaolinite are chronologically submitted in a comprehensive manner and then a summary is given.

Reasons for First and Second Exothermic Peaks

Metakaolinite is a poorly crystalline material. It is formed from kaolinite by absorption of heat energy and thus it is conceivable that it is metastable with high free energy content.

NAOH Leaching Study

It was thought earlier that kaolinite decomposes during formation of metakaolinite into free silica and alumina. Some also believed that metakaolinite transforms to spinel phase with the liberation of SiO2. Further, the amount of SiO2 exolvation indirectly predicts the nature of spinel phase formed i.e., whether it is γ-Al2O3 or Al–Si spinel. Since the composition of γ-Al2O3 and proposed Al–Si spinel is different. So it is obvious that the amount of amorphous SiO2 would be different. Thus, it is worthwhile to detect and estimate the amount of silica liberated during the formation and subsequent decomposition of metakaolinite. Various researchers have taken recourse to SiO2 estimation by either of the two techniques.