Kaolinite heating studies

Study of hydrated kaolinites shows that water molecules adsorbed on a phyllosilicate surface occupy two different structural sites. One type of water, "hole" water, is keyed into the ditrigonal holes of the silicate layer, while the other type of water, "associated" water, is situated between and is hydrogen bonded to the hole water molecules. In contrast, hole water is hydrogen bonded to the silicate layer and is less mobile than associated water. At low temperatures, all water molecules form an ordered structure reminiscent of ice as the temperature increases, the associated water disorders progressively, culminating in a rapid change in heat capacity near 270 K. To the extent that the kao-linite surfaces resemble other silicate surfaces, hydrated kaolinites are useful models for water adsorbed on silicate minerals. 

In our studies, the model substance (montmorillonite) was a calcium bentonite (Istenmezeje, Hungary), the characteristic features of which are given here. X-ray diffraction (intensity of the basal reflection) and thermoanalytical (weight loss upon heating) data show 91% montmorillonite content. The other constituents are 5% calcite, 3% kaolinite, 1% x-ray amorphous silicates, and a trace of quartz. The amorphous phase is silicate particles, which are not crystalline for... 

Several studies of the effects of heating on pure clays have been reported in archaeological literature, but very little systematic work has been done on the effects of admixed mineral impurities upon the clays that constitute ceramic paste. The purpose of this chapter is to study the controlled firing of four measured mixtures of the clays, kaolinite and montmorillonite, with the common carbonates, calcite and dolomite. 

This study is concerned with four different mixtures, including kaolinite and calcite (kc) kaolinite and dolomite (kd) montmorillonite and calcite (me) and montmorillonite and dolomite (md). All the mixtures, by weight, were 95% clay and 5% carbonate mineral. The minerals were first ground to a fine powder and thoroughly mixed by hand before heating in a muffle furnace (temperature controlled to within 20 °C). Each mixture was heated for 1 h, air quenched at room temperature, and analyzed by X-ray diffraction. X-ray films were made in cameras of 11.46-cm diameter with filtered copper radiation and exposure times of 6 h. Several wet mixtures that simulated ceramic paste before firing were heated and studied in like manner, but they showed no differences from the dry mixtures. 

Kaolinite-Calcite (kc) Mixture. Table I shows that there were no appreciable changes for the kaolinite or calcite at 300 or 400 °C. Kaolinite began to disappear between 400 and 500 °C, and it lost its structure by 600 °C. The breakdown of kaolinite upon heating has been studied extensively (numerous papers summarized by Grim, 13) and is caused by the loss of the... 

The transformation of kaolinite to metakaolinite is brought about by heating the clay to about 700 °C causing hydroxyl ions to be removed as water. The rate of dehydroxylation as a function of heating has been studied [3.164]. The reaction can either be a batch process with the clay in crucibles in a directly fired kiln, or a continuous process in a tunnel kiln, rotary kiln, or other furnace. 

However, the synthesis process most extensively studied by solid-state NMR is that of carbothermal reduction of aluminosilicate minerals such as kaolinite, which are mixed with finely divided carbon and heated in nitrogen at > 1400°C (Neal et al. 1994, MacKenzie et al. 1994a). Under carbothermal conditions the clay decomposes to a mixture of mullite and amorphous silica (MacKenzie et al. 1996b), the latter forming SiC which reacts with the mullite to form P-sialon, in some cases via other sialon phases such as X-sialon (see below). The precise reaction sequence and the nature of the intermediates has been shown by the NMR studies to depend on various factors including the nature of the aluminosilicate starting mineral (MacKenzie er a/. 1994a). 

The final test that determines if experimental silico-aluminates can be applied in the formulation of FCC catalysts is the evaluation of their catalytic behavior. Materials deactivated for 4 and 8 hours at 788°C in 100% steam were evaluated in a microreactor at 520°C, with a cat/oil ratio of 5, WHSV of 15.82 h using a modified gas oil feedstock with 5 wt. % of atmospheric residue. The material of reference in this study was the kaolinite with no heat nor chemical treatments coded as Filsttq, which typically is used in the formulation of FCC catalysts. Two experimental silico-aluminates, CARB-11 and Filttq, which are the result of thermal and chemical treatments of the Filsttq kaolinite, were evaluated. 

When heated, kaolinite undergoes a complex series of chemical and physical changes that transform the layered mineral to a combination of crystalline mullite and an amorphous siliceous phase. Though simple conceptually, the study of this reaction sequence... 

Langer and Kerr (65) studied the effect of heating rate on peak temperatures for both the dehydroxylation reaction and the phase transition of kaolinite. These changes to the peak temperature at various heating rates are given in Table 5.1. Both peaks are increased by an increase in heating rate although the dehydroxylation reaction peak is more affected than the other peak. 

Langer and Kerr (65) studied the effect of sample mass on the dehydroxyla-tion peak of kaolinite. As shown in Figure 5.30, an increase in sample mass causes an increase in the peak minimum temperature (7 ), all determined at a heating rate of 25°C/min. The phase transition peak maximum temperature was only slightly affected (5°C) in contrast to the change for the dehydroxyla-tion p,eak temperatures (45°C). 

Red mud is a by-product derived from the aluminum industry and consists mainly of Si, Al, Fe, Ca, and compounds kaolinite and gibbsite. Pera and Momtazil l evaluated the pozzolanic activity of red mud by heating it at different temperatures in the range 600° to 800°C. The cement was mixed with 30-5 0% red mud and cured for different times in water. The products were subjected to DTA and XRD studies. Lime was completely... 

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