Calcination step

The cmde diatomite, which may contain up to 60% moisture, is first milled in a method that preserves the intricate stmcture of the diatomite. This material is fed to dryers operating at relatively low temperatures, where virtually all of the moisture is removed (see Drying). Coarse and gritty nondiatomaceous earth material is removed in separators and preliminary particle si2e separation is made in cyclones. For many producers, all of the manufacturing processes, with the exception of the calcination step, take place while the material is being pneumatically conveyed. The resultant material is termed natural product. This is the only type of diatomite made by some producers. 

All the other characterization studies have been performed after the calcination step XRD experiments have shown that the material formed during the synthesis has the MFI structure Si and 27a1 MASNMR spectra indicated that this phase has a Si/Al ratio varying between 550 and 30 as a function of the sample prepared and also that no extra framework A1 is present. 

After the calcination step, experimental data (XRD, 29 i maSNMR) show that a zeolite with the silicalite structure has been formed. 29 i MASNMR indicates for the zeolite material a Si/Al ratio depending on the sample prepared it has been observed that both the natures of the silicon source and of the alumina supports may originate these fluctuations. 

Role of CyDTA during Drying and Calcinations Steps.107... 

It has already been mentioned that the formation of ultrastable Y zeolites has been related to the expulsion of A1 from the framework into the zeolite cages in the presence of steam (8,9), and the filling of framework vacancies by silicon atoms (11,12). This results in a smaller unit cell size and lower ion- exchange capacity (6). It also results in a shift of X-ray diffraction peaks to higher 20 values. Ultrastable Y zeolites prepared with two calcination steps (USY-B) have a more silicious framework than those prepared with a single calcination step (USY-A). Furthermore, since fewer aluminum atoms are left in the USY-B framework, its unit cell size and ion-exchange capacity are also lower and most of the nonframework aluminum is in neutral form (18). 

Finally, the effect of various steps of the deactivation procedures prior to the ReDox cycles on the acidity of the samples was investigated. Catalytic samples were collected after the first calcination step and after the metal impregnation step just... 

In the Infrared spectrum of the sample after calcination at 550°C, a new band was observed at 923 cm (Fig. 4 (b)). AIPO4-5 and crystalline VjO, did not show any band around 920 - 930 cm . The band at 923 cm was detectable for the sample of 0.7 % V O, content and its intensity increased with increasing VjOj content supported on AIPO4-5 (Fig. 5). This indicates strong interaction between the adsorbed vanadium species and the AIPO4-5 surface during the calcination step. 

In this last case, the product was left as a gel, but if the product is to be a glass, a powder, or crystalline material, a high temperature calcining step is required after... 

Fig. 77. Schematic drawing of the liquid-crystal templating mechanism. Hexagonal arrays of cylindrical micelles form (possibly mediated by the presence of silicate ions), with the polar groups of the surfactants (light grey) to the outside. Silicate species (dark grey) then occupy the spaces between the cylinders. The final calcination step burns off the original organic material, leaving hollow cylinders of inorganic material [473]...

The production process for bismuth vanadate pigments consists usually of a precipitation reaction followed by a calcination step. This calcination leads to an appreciable increase in particle size as demonstrated in Figure 30. The calcination step is different from producer to producer and can be completely absent depending on the precipitation process and the desired product properties. 

Preparation of MCM-41 materials [1] usually includes a high temperature calcination step that in fact opens mesopores by removing structure directing molecules (template) as well as results in the structure shrinkage due to high temperature condensation of silanol groups [2], Functionalization of mesoporous materials with organosilanes is often employed to synthesize materials of desired surface properties for advanced adsorption and catalytic applications [3] as well as to improve their stability [4]. 

Cr/silica does not catalyze the polymerization of ethylene until it has been calcined between about 300 and 1000°C. While heat is necessary to cause the esterification of Cr03 to the silica surface, this is probably not the only purpose of the high temperature calcining step. Figure 8 shows how the activity develops as the calcining temperature increases. A respectable activity does not appear until about 500°C, whereas the esterification of Cr(VI) is complete at 300°C (76). Furthermore, other sources of chromium behave similarly in activity even though the particular mechanism and... 

Catalysts generally change from yellow to a deeper orange color as the calcining temperature is raised. On exposure to moisture, however, the orange samples immediately revert back to yellow and lose their activity. A second calcining step then restores the activity and RMIP. 

Ordinarily the chromium binds to the silica by reacting with hydroxyls on a fully hydrated surface, because chromium is impregnated aqueously onto the silica and then calcined. However, a different catalyst results if the chromium attaches instead to a surface already dehydrated by calcining. A large promotional effect, particularly on the termination rate, is obtained (76). To do this the silica is first dehydrated at 900°C, for example, then impregnated with chromium anhydrously so that the surface is not rehydrated. A secondary calcining step at some lower temperature such as 300-600°C then fixes the chromium to the silica. The method is especially effective if the support also contains titania. 

The solidification of waste takes place in two steps, a calcining step and an incorporation of the calcined material into borosilicate glass. Calcining can be done in various ways but primarily involves the removal of volatile products like water or NO3" and conversion of all species in the HLW to solid stable oxides. The oxides are then mixed with SiC>2, B2O3, and the like to make a borosilicate glass, which is then prepared for geologic storage (Fig. 16.13). 

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