Dehydroxylation kaolinite

A number of studies have been devoted to the kinetics of kaolinite dehydroxylation. Duncan and Mackenzie (1969) suggested a kinetic model assuming the rate of nuclea-tion of the new phase as the controlling process, represented by the relationship... 

The sequence of phases appearing in the mixtures composed of natural raw materials was also examined. In the mixture of calcium carbonate with quartz and clay minerals first reactions are the dehydroxylation of the latters. The kaolinite dehydroxylation at the temperature close to 450 °C is transforming in metakaolinite, which has, in disturbed form, the stmcture of initial mineral [18]. It is presenting high reactivity with calcium carbonate and the reaction of these eonrponents will start at this range of temperatures. For these reasons the mixture of marl with kaolinite produce C3S at 1100 °C and with illite even at 1000 °C [19]. 

Multi-component clay-based chemistries involving reactions between clays and lime and pozzolans are of interest in the area of soil stabilization. Thermogramst for kaolinite and montmorillonite treated with lime are presented in Fig. 18. Addition of lime results in the gradual diminution of the primary kaolinite dehydroxylation peak (500-600°C) to a greater extent than can be accounted for by dilution alone. All samples have a small peak at about 130°C and a broad endothermic peak at about 210°C. The decomposition of carbonated lime is associated with endothermic reactions at 700-800°C. 

Dehydroxylation of the clay mineral kaolinite [71,626—629] is predominantly deceleratory and sensitive to PH2o (Table 11). Sharp and co-workers [71,627] conclude that water evolution is diffusion controlled and that an earlier reported obedience to the first-order equation is incorrect. A particularly critical comparison of a—time data is required to distinguish between these possibilities. Anthony and Garn [629] detected a short initial acceleratory stage in the reaction and concluded that at low Ph2o there is random nucelation, which accounts for the reported... 

Fig. 14. Compensation plot for dehydroxylation of kaolinite ( ) and other layer-type silicates (X = montmorillonite, illite and muscovite) data and sources given in Table 11. (Redrawn, with permission, from Advances in Catalysis, ref. 36).

In an early study of the effect of calcination on the surface area of kaolinite, Gregg and Stephens (1953) found a small but progressive decline in the BET area over the temperature range 100-800°C. These results were in contrast to a 12% loss of structural water at 450°C. It was concluded that there was no detectable activation and that the crystallite structure was not broken up as a result of theimal dehydroxylation. 

Abbreviations used cristobalite (crist.) dehydroxylated montmorillonite (dehy. mont.), mata-kaolinite (metakaol.), montmorillonite (mont.), and trace (tr.). 

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. 

Keller, W., Pickett, E. E., and Reesman, A. L. Elevated dehydroxylation temperature of the Keokuk geode kaolinite— a possible reference mineral, Jai Proceedings Internat. Clay Conf. 1, 75-85 (1966). 

Although traces of hydrogen have been identified in the products of dehydroxylation of Mg(OH)2, AlCOH), and kaolinite, MacKenzie [49] detected this product only when the reactant contained impurities. The evolution of may also arise [1] in dehydroxylations where cation oxidation is possible ... 

Figures 3a-d show the td/ins results. Plotted in each case is m/z = 18 which corresponds to water, and the appropriate xa/z for the amine under observation. The amine essentially desorbs intact. The high temperature water peak (k) is due to dehydroxylation of the kaolinite internal standard, as no water is desorbed from the AIPO4-II in this region. The amine is observed to desorb over a large temperature range with a tail up to approximately 400°C.

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. 

MacKenzie (95,96) has developed a method for theTG study of solids in the presence of applied electrical fields. Electric fields of the order of 05 V/m lower the initial decomposition temperature for the dehydroxylation of kaolinite by 60° in some cases. The activation energy for the process is reduced by 3-12 kcal mole-1. Rate constants for the material are increased by electrolysis but this effect falls off at higher temperatures as the normal processes begin to predominate. 

Fig. 2.43 Enthalpy of elinkering process 1, 3, 5, 7, P heating of raw mix, 2 dehydroxylation kaolinite, 4 decarbonisation of CaCOj, 6 mullite crystallization, 8 melt formation, 10 crystallization of clinker phases, 11 clinker cooling, 12 cooling of gaseous CO 13 eooling of water vapour...

When ln[-ln(l - a)] is plotted against In t, if the range of a is limited to 0.15-0.50, the kinetic data give a linear plot with slope m. Dehydroxylation of kaolinite was carried using 91 mg of sample at 427 °C in vacuo, and f(, 5 = 97 min was obtained. A linear plot with a slope (m) of 0.56 was produced, indicating a diffusion-controlled reaction. It is attributable to either D2 or D4 as shown in Table 3.6. 

When kaolinite is heated (Figure 2.5) to above 500 °C it dehydroxylates endothermically, that is, it loses its water of crystallisation forming metakaolinite. This is then stable up to 980 °C, when a defect spinel structure, which is virtually amorphous, forms exothermically. Above 1100 °C there is a slow transformation of the defect spinel with mnllite forming in an amorphous silica matrix. 

Above 500 °C kaolinite starts to lose its water of crystallisation and, by 650 °C, approximately 90% of this dehydroxylation is complete, leaving residual OH groups randomly distributed but isolated so that condensation will not occur readily. The product formed is known as metakaolin. Some crystalline structure is retained [22] but X-ray diffraction patterns are so diffuse and weak that no better, more recent, identification has been made. After these structural changes the aluminium, which was originally in six-fold octahedral sites, occupies four- and five-fold sites almost equally [23]. 

Figure 4.2 Change of the electron density distribution of kaolinite during dehydroxylation to the highly defective metakaolinite structure (Iwai et al, 1971). Reprinted with permission from the International Union of Crystallography.

Soils and clays, in general, when calcined give off adsorbed, interlayer, and hydrated types of water. These effects produce endothermal peaks or loss of weight in DTA and TG, respectively. The endothermal peaks are followed by exothermal peaks that are caused by re-crystalliza-tion. Although many types of clay minerals such as montmorillonite, illite, and some shales show these effects, they are not suitable as pozzolans in concrete. Metakaolin, formed by heating kaolinite, seems to be the most suitable additive material for cement. Heating of kaolinite involves removal of adsorbed water at about 100°C and dehydroxylation at above 600°C, followed by the formation of metakaolinite, an almost amorphous product. The sequence of reactions is as follows ... 

It shows pronounced thermal effects on heating and generally has a more ordered structure than other clay minerals. Figures I and 2 illustrate typical DTA data for kaolinite, halloysite, and montmorillonite. Kaolinite and halloysite lose their hydroxyls between 450 to 600°C. Variations within this range are attributed to differences in entrapped water vapor that is dependent on sample size and shape factors. The loss of hydroxyls from montmo-rillonites in the range of 450 to 650°C is t5q)ical for dioctahedral forms of these minerals. Dehydroxylation is more gradual for trioctahedral forms and can continue to temperatures up to 850°C. 

Conventional Synthesis Without Thermal Dehydroxylation of the Kaolinitic Clay... 

The conventional synthesis process requires the expenditure of thermal energy to dehydroxylate the clay. For this reason, it would be advantageous if the thermal pre-treatment step could be eliminated. Viable geopolymers caimot be prepared from undehydroxylated kaolinite, but several alternative methods have been investigated for pre-treating the clay to render the aluminium... 

The initial geo-material was prepared from a solution containing dehydroxylated kaolinite and KOH pellets (85.7% of purity) dissolved in potassium silicate (Si/K=l.66, density 1.33 g/cc) described in Figure I. The reactive mixture was then placed in a polystyrene sealed mould in an oven at 70°C for 4 hrs so that the polycondensation reaction was complete. 

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