Dehydration of clay minerals

Table III summarizes the parameters that affect Brrfnsted acid-catalyzed surface reactions. The range of reaction conditions investigated varies widely, from extreme dehydration at high temperatures in studies on the use of clay minerals as industrial catalysts, to fully saturated at ambient temperatures. Table IV lists reactions that have been shown or suggested to be promoted by Br nsted acidity of clay mineral surfaces along with representative examples. Studies have been concerned with the hydrolysis of organophosphate pesticides (70-72), triazines (73), or chemicals which specifically probe neutral, acid-, and base-catalyzed hydrolysis (74). Other reactions have been studied in the context of diagenesis or catagenesis of biological markers (22-24) or of chemical synthesis using clays as the catalysts (34, 36). Mechanistic interpretations of such reactions can be found in the comprehensive review by Solomon and Hawthorne (37).

The reactions described and discussed here have been selected to extend the present analysis beyond surface processes and to include some consideration of certain chemical changes that also involve interactions within lattices of solids. The examples selected include reference both to surface catalytic properties and to dehydration reactions of clay minerals. 

Ash particles are formed through the thermal decomposition or dehydration of inorganic minerals associated with the coal. Calcium carbonate and clay are the most abundant mineral impurities, with lesser amounts of sulfides, chlorides and oxides also present. The shape of the ash particle is dependent on many factors, two of which are the amount of time and temperature to which the coal is exposed in the combustion chamber (Fisher et al., 1978). The spherical shape, most commonly associated with fly ash particles, shows that complete melting of silicates occurs at high temperature. These spheres may be solid, hollow (cenospheres) or encapsulating spheres (plerospheres). 

Thus, under conditions of higher salinity (-28%) in a hydrodynamically stable environment the composition of the clay mineral spectrum may vary considerably. There is a synthesis (or transformation) of clay minerals with a stable lattice when halite is being deposited. This is evidently tied to the physico-chemical conditions leading to the start of crystallization of halite (rock salt). A basin saturated in NaCl is characterized by a low degree of hydratization. The lithification of halite during which any excess water is being eliminated takes place rapidly and favours the dehydration of the interfoliar space of clay minerals which prove to be most apt for transformations. 

The clinker formation process is initiated by the dehydration of gypsum to anhydrite at around 100-120°C, followed by a decomposition of clay minerals at around 300-600°C. The decaibonization of the calcium carbonate that is present starts at about 700°C, and is completed before the temperature reaches 900°C. 

It is possible to prepare saponites with specific surface areas between 100 and 750 nfilg and pore volumes of 0.03-0.32 ml/g, all displaying a house-of-cards structure. The thermal stability of the synthetic saponites is high, which enables us to dehydrate the clay minerals efficiently, prior to the catalytic reaction. 

Between about 100° and 400° C the clay minerals give off their absorptively bound water, including the so-called interlayer water. At higher temperatures, depending on the types of clay mineral concerned, generally between about 400° and 750° C, the chemically combined water (hydroxide groups) is also expelled (dehydration), exemplified by the dehydration of kaolinite-... 

Figure 3. Infrared absorption spectra of Tidinit montmorillonite. (a) Air dried (b) dried at 200°C and for two incidence angles. (From J. M. Serratosa [1962], Dehydration and rehydration studies of clay minerals by infrared absorption spectra. Clays Clay. Min., Proc. 9th Natl. Conf. New York Pergamon Press, p. 413, Figure 1.)...

Changes in X-ray diffraction pattern after controlled heat treatment greatly assist the characterization of clay minerals by that technique, and it seems likely that heat treatment will also assist infrared investigations of clays. Changes in water absorption bands during dehydration of the montmorillonites and vermiculite have been previously discussed. Tettenhorst [1962] reported changes, following loss of interlayer water, in the lattice vibra-... 

Infrared spectra can provide direct evidence on the chemical and physical processes that lead to adsorption on surfaces, and these reactions may, in turn, serve to differentiate the various types of surface present in such heterogenous systems as soil clays. Changes in the concentration or structure of the adsorbed species on aging or other treatments are readily followed in a single small specimen without the necessity for destructive analyses. The surface properties of clay minerals and other colloids of importance in soil have been studied particularly by Fripat and his colleagues, who have made considerable use of infrared spectroscopy. A review (Fripiat [1964]) of their work in this field has appeared. Studies of adsorbed species on silica, silica-alumina, alumina, and zeolites have been principally concerned with the highly dehydrated systems of interest in catalytic applications. This field has been reviewed by Little [1966] and Hair [1967]. 

Dehydration and rehydration studies of clay minerals by infrared absorption spectra. 

J. w. Geus, The interlayer collapse during dehydration of synthetic Na -beidellite a %a solid -state magic-angle spinning NMR study. Clays Clay Minerals. 25 457 (1992). 

Gonzales and Laird145 have shown that smectites abiotically catalyze dehydration of glucose to form furfural under conditions similar to those found in soils. Four smectite clay minerals were used (saturated with Na, Ca, Fe, or Al), and the formation of HMF and furfural was detected by high-pressure liquid chromatography. The polymerization of furfural may thus be a pathway to the formation of new humic materials in soils. 

Nearly all the coals examined are relatively high in ash. Only two of the samples from the Terrace Ridge area contain less than 20% ash, on a moisture-free basis. Most of the remaining samples from this area range from impure coal to coaly shale. Most of the mineral matter is finely divided (dehydrated) clay, silica, and pyrrhotite. This is an important point since the proximate and ultimate analyses for material that deviates so widely from what is normally considered coal cannot be considered particularly reliable for comparison pur-... 

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