Kaolinite, dehydration

The adsorption of transition metal complexes by minerals is often followed by reactions which change the coordination environment around the metal ion. Thus in the adsorption of hexaamminechromium(III) and tris(ethylenediamine) chromium(III) by chlorite, illite and kaolinite, XPS showed that hydrolysis reactions occurred, leading to the formation of aqua complexes (67). In a similar manner, dehydration of hexaaraminecobalt(III) and chloropentaamminecobalt(III) adsorbed on montmorillonite led to the formation of cobalt(II) hydroxide and ammonium ions (68), the reaction being conveniently followed by the IR absorbance of the ammonium ions. Demetallation of complexes can also occur, as in the case of dehydration of tin tetra(4-pyridyl) porphyrin adsorbed on Na hectorite (69). The reaction, which was observed using UV-visible and luminescence spectroscopy, was reversible indicating that the Sn(IV) cation and porphyrin anion remained close to one another after destruction of the complex. 

The zeolite crystals are in the form of fine powders, which would cause a very high pressure drop in a packed bed. They have to be formed into granules of approximately 3 mm in diameter, by using clay binders, such as kaolinite and montmorillonite. The methods consist of pelletization with binders under pressure into short cylinders, wet extrusion with a fluid into continuous cylinders, and granulation by rolling with binders into spheres. They also need to be dehydrated and calcined to remove volatile components before use. 

Interpretation of the mechanisms of the hydrocarbon desorption reactions mentioned above was considered (31,291) with due regard for the possible role of clay dehydration. While this water evolution process is not regarded as a heterogeneous catalytic reaction, it is at least possible that water loss occurs at an interface (293) so that estimations of preexponential factors per unit area can be made. On this assumption, Arrhenius parameters (in the units used throughout the present review) were calculated from the available observations in the literature and it was found (Fig. 9, Table V, S) that compensation trends were present in the kinetic data for the dehydration reactions of illite (+) (294), kaolinite ( ) (293,295 298), montmorillonite (x) (294) and muscovite (O) (299). If these surface reactions are at least partially reversible,... 

Hig. 9. Compensation behavior for the dehydration reactions of kaolinite ( ), illite ( + ), mommorillonite ( x), and muscovite (o), estimated from published kinetic data (293-299). 

Hashimoto, 1. and 3ackson, M.L., 1960. Rapid dissolution of allophane and Kaolinite-Halloysite after dehydration. Clay and Clay Minerals, 7th Conf., pp. 102-113. 

One of the major differences in the reported chemical composition of the kaolinite minerals is in the H20+and H20 values. In part, these variations may be real but many must be due to the presence of halloysite and other impurities, variation in grain size and surface area, and in the methods of dehydration. H20 increases linearly with increase in surface area and with decrease in grain size. 

Dehydrated halloysites have C.E.C. in the range of 6—10 mequiv./lOO g (Van der Marel, 1958 Garrett and Walker, 1959). Garrett and Walker have shown that the exchangable cations are located on the external surfaces of the crystals and not in the interlayer position of halloysite. Until it is possible to obtain accurate chemical analyses of the kaolinite minerals, it will be difficult to determine their exchange capacity and the source of the charge. 

Clays are aluminosilicates with a two-dimensional or layered structure including the common sheet 2 1 alumino- and magnesium- silicates (montmorillonite, hectorite, micas, vermiculites) (figure 7.4) and 1 1 minerals (kaolinites, chlorites). These materials swell in water and polar solvents, up to the point where there remains no mutual interaction between the clay sheets. After dehydration below 393 K, the clay can be restored in its original state, however dehydration at higher temperatures causes irreversible collapse of the structure in the sense that the clay platelets are electrostatically bonded by dehydrated cations and exhibit no adsorption. 

Kaolinite is transformed into X-ray amorphous state when activated in air. According to authors [14,15], amorphization involves the destruction of bonds between tetrahedral and octahedral layers inside the package, till the decomposition into amorphous aluminium and silicon oxides. Other researchers [ 16,17] consider that amorphized kaolinite conserves the initial ordering of the positions of silicon atoms while disordering of the structure is due to the rupture of A1 - OH, Si - O - A1 bonds and the formation of molecular water. Endothermic effect of the dehydration of activated kaolinite is shifted to lower temperatures while intensive exo-effect with a maximum at 980°C still conserves. When mechanically activated kaolinite annealed at 1(X)0°C, only mullite (3Al20j-2Si0j) and X-ray amorphous SiOj are observed. In this case, the phase with spinel structure which is formed under thermal treatment of non-activated kaolinite is not observed thus, mechanical activation leads to the formation of other phases. 

The limit AI2O3 content in fireclay ware is 46 wt.%, which corresponds to the theoretical composition of dehydrated kaolinite having a refractoriness of PCE 35(1770 C). When the Al Og content exceeds this value, the refractoriness increases further and other types of refractories are obtained. 

Calculate the volume of steam produced when a 4.0-kg brick made from pure kaolinite is completely dehydrated at 600°C and a pressure of 1.00 atm. 

Fluids can be released from solids by mineral dehydration thus adding fluids to the compaction-driven fluid flux in a sedimentary column undergoing burial. The relative contribution of such diagenetic processes to the compaction-driven flow, can be estimated. Dehydration of smectite may be important (Burst, 1969), but at greater depths illitization of kaolinite may be significant (Eq. (2)). Bjprlykke et al. (1986) and... 

Compensation behaviour identified for the dehydrations of a number of natural minerals (illite (x), montmorillonite (A), muscovite ( ) and kaolinite (+)) estimated from literature data [53-60]. 

The dehydration of kaolinite has been the subject of several kinetic studies and Brett et al. [1] summarize the salient features of the mechanisms proposed for the sequence of reactions by which kaolinite is converted to mullite (920 to 1370 K). The first step, water loss, is most satisfactorily described by a two-dimensional diffusion equation. Brindley et al. [57] proposed this model from isothermal kinetic measurements (670 to 810 K) and reported a marked increase of in a maintained pressure of water vapour. Anthony and Gam [58] concluded that random nucleation is rate limiting at low pressures of water vapour and that this accounts for reports of first-order kinetic behaviour. Increase in the rate of nucleation, as the (HjO) is increased, is ascribed to a proton transfer mechanism, and acceleration of the growth process may result from contributions due to the onset of the reverse reaction. 

Mineralogical phases formed at different temperatures for each coal sample are summarized in Table III. The major mineral phases detected by XRD in LTA samples are quartz, pyrite, bassanite, kaolinite and plagioclase. The processes responsible for subsequent mineral transformations include oxidation, vaporization, sulfur fixation, dehydration and solid-state Interactions. The temperatures at which specific transformations occur are assigned on the basis of previous experimental work by Mitchell and Gluskoter (4) and published chemical data in the Handbook of Chemistry and Physics ( ). In addition to mineral-mineral interactions it is believed that reactions between minerals and exchangeable cations occur (2) ... 

D. NaC2H302 + Kaolinite 1 1 750 Amorphous + Carnegieite (NaAlSiO4) Dehydration Interstitial infilling in... 

XRD failed to detect calcite in LTA samples possibly due to its extraction by ammonium acetate solution or because the amounts of calcite were below detection limits ( 5%). For the most part, calcium is supplied to the system by gypsum and organically-bound calcium. Calcium, whether in the form of bassanite, calcite, or cations in LTA samples, forms anhydrite in ASTM samples. In HTA samples calcium reacts primarily with dehydrated kaolinite forming aluminosilicates. 

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