Thermal kaolinite

When supported on kaolinite, Ni observability is independent of the thermal pretreatment used to age the catalyst however, its speciation or interaction with this clay change after steaming. In fact, whereas 2% Ni on kaolin is approximately 60% reducible after calcination, the steamed sample is not, probably because of the formation of a stable Ni-silicates or even a surface spinel phase like Ni-aluminate. 

The AG between the assemblage of muscovite + chlorite at composition y and illite of this is likely to be relatively small and the tendency to recrystallize the muscovite from x to y compositions will be small at sedimentary conditions. However, as more thermal energy is added to the rock system, under conditions of deeper burial, the recrystallization will proceed more rapidly as temperature is increased. Evidence for such an effect can be found in Millot (1964) where sedimentary rocks coming from deeply buried or slightly metamorphosed series show the "chloritization" or kaolinitization" of detrital mica grains in splendid photographs. 

Pelitic rocks investigated in the same areas where corrensites are formed during alpine metamorphism (Kiibler, 1970) revealed the absence of both montmorillonite and kaolinite but the illite or mica fraction was well crystallized as evidenced by measurement of the "sharpness" of the (001) mica reflection (Kiibler, 1968). This observation places the upper thermal stability of the expandable and mixed layered trioctahedral mineral assemblages at least 50°C. above their dioctahedral correlevants. This is valid for rocks of decidedly basic compositions where no dioctahedral clay minerals are present. 

V is characterized by kaolinite-illite-chlorite assemblages beyond the stability of an expanding mixed layered potassic dioctahedral mineral and below the thermal stability of pyrophyllite. The establishment of such conditions will be difficult in that the non-appearance of a mineral is a poor diagnostic and, as we have seen, kaolinite is frequently eliminated from sediments before its upper stability limit in the presence... 

In conclusion thermal degradation studies on Nautilus pompilius indicate that mineralizing matrix and aragonite shell represent a true structural entity. By the sharing of oxygens in protein and mineral lattices we will generate phase boundaries of the type that are present, for instance, in the common clay mineral kaolinite. Here, aluminum octahedra and silica tetrahedra incorporate the same oxygens and hydroxyls, and layers composed of octahedra and tetrahedra arise (Fig. 13). 

Matejka, J. (1922). Thermal analysis as a means of detecting kaolinite in soils. Chem. Listy 16, 8-14. 

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. 

Thus, under mechanical activation, new kaolinite states are formed, which are not observed at thermal treatment. 

Klevtsov D.P., Mastikhin V.M., Krivoruchko O.P. Investigation of the mechanism of solid-phase transformations during thermal treatment of aluminosilicate systems. II. Investigation of the effect of mechanical activation on kaolinite state. Izv. SO AN SSSR, ser. khim. nauk, 1986 3 62-70. 

Klevtzov D.P., Krivorachko O.P., Mastikhin V.M. et al. Influence of mechanochemical activation and thermal treatment of kaolinite and cation distribution of Al(III) and formation Na-A zeolite. React. Kinet. Catal. Letter 1988 36 319-24. 

Cordierite synthesis method based on mechanical activation of mixtures of hydrated oxides of calcium, aluminium and silicon, as well as natural hydrated compounds (talc, kaolinite and gibbsite), has been developed in [2, 3]. Mechanical activation of these mixtures does not lead to the formation of new phases but provides good mixing at the cluster level giving aggregates that form cordierite during the subsequent thermal treatment. 

Presented are the examinations of the multifimctional mineral-earbon and zeolite-carbon sorbents prepared from kaolinite with an admixture of carbonaceous materials industrial waste deposits, municipal sewage sludge and cellulose. The mixture of raw materials was thermally and hydrothermally pretreated in order to facilitate their specific structure. The parameters of capillary structure (micro and mesopores) were determined. For examinations of porous structure the mereury porosimetry method was used. In order to evaluate the solid phase transformation during the each step of sorbent preparation the SEM observation with quantitative X-ray mieroanalysis were made. 

The adsorbents have been prepared fi-om the halloysite (H) - mineral fi-om kaolinite group with an admixture of carbonaceous materials refinery waste deposits (RSI), sediment communal sewage (CSew) and cellulose (Ce), and the fiaction of these mixtures were within 30 - 70 wt.%. The mbcture of raw material was thermally (carbonaceous materials carbonization, 973 K) and hydrothermally (crystallization of the amorphous metahalloysite in alkaline solution to zeolitic structure of NaA type, 373 K) pretreated in order to cilitate their specific structure [1,2]. 

Zeolites, particularly zeolite A, can be manufactured from kaolinitic clays, which as particularly found in Central Europe, Great Britain, Japan, China and USA. To transform kaolin into zeolite, it has to be thermally converted, e.g. by shock heating to > 550°C, to metakaolin. The metakaolin is then su.spended in sodium hydroxide solution and converted at 70 to 100°C into zeolite A. Some of the impurities contained in the natural raw material are retained in the final product. If amorphous silica is added, Si02-rich zeolites are produced. This process enables the transformation of preformed bodies into zeolite materials. 

Figure 4.8. Changes in the Si spectra of kaolinite during its thermal decomposition, showing the progressive formation of the broad metakaolinite resonance envelope - 99 to - 102 ppm) at 650-800°C, the sudden appearance of free Si02 (—110 ppm) at 970°C, and the formation of mullite (— 88 to — 92 ppm) above 1100°C. Adapted from Mackenzie et al. (1985a) by permission...

Figure 5.30. Changes in the Al site occupancy of kaolinite during its thermal decomposition, after Temuujin et al. (1998a) by permission of the copyright owner.

---