Calcined materials

It is less well known, but certainly no less important, that even with carbon dioxide as a drying agent, the supercritical drying conditions can also affect the properties of a product. Eor example, in the preparation of titania aerogels, temperature, pressure, the use of either Hquid or supercritical CO2, and the drying duration have all been shown to affect the surface area, pore volume, and pore size distributions of both the as-dried and calcined materials (34,35). The specific effect of using either Hquid or supercritical CO2 is shown in Eigure 3 as an iHustration (36). 

Air pollution problems and labor costs have led to the closing of older pyrometaHurgical plants, and to increased electrolytic production. On a worldwide basis, 77% of total 2inc production in 1985 was by the electrolytic process (4). In electrolytic 2inc plants, the calcined material is dissolved in aqueous sulfuric acid, usually spent electrolyte from the electrolytic cells. Residual soHds are generally separated from the leach solution by decantation and the clarified solution is then treated with 2inc dust to remove cadmium and other impurities. 

In 1989, 4,689,000 metric tons of uncalcined gypsum was sold or used 3,229,000 metric tons for use in Pordand cement and the remainder for agriculture and miscellaneous uses. About 17,778,000 metric tons of calcined material was used to produce 1.9 million square meters of board products. Over one million square meters of this material was regular board and about 560,000 m was Type X board. 

Calcined diatomite is produced from natural diatomite, which is then subjected to high temperature calcination in a rotary kiln at about 980°C. The calcined material is then again milled and classified to remove coarse agglomerates as well as extreme fines. 

Fig. 2.25 XRD patterns (a) as-synthesized (b) hydrolyzed (c) calcined materials. Reprinted with permission from [106], Q. Zhang et al.,J. Am. Chem. Soc. 2004, 126, 988. 2004 American Chemical Society.

Structural and textural characterisation of pure SBA-15 and hybrid GFP/SBA-15 Pure SBA-15 and GFP/SBA-15 hybrid were characterised by X-ray powder diffraction, HRTEM and volumetric analysis. Calcined SBA-15 (Fig. 1, curve A) show the typical XRD pattern of an ordered hexagonal network of mesopores with (10), (11) and (20) reflections. The presence of well resolved (11) and (20) peaks indicate that the calcined material used for the preparation of the hybrid materials have a long-range order. The hexagonal XRD pattern was still clearly observed in the hybrid material (GFP/SBA-15), as all the three main reflections were found (Fig. 1, curve B), indicating that the sonication and the GFP physical adsorption does not affect the framework integrity of the material. 

Pyridine sorption studies have shown the presence of both Bronsted and Lewis acid sites in USY zeolites, although to a lesser extent than in the corresponding HY zeolite (51,53). Acidity is maintained even after strong dehydroxylation of USY-B at 820°C. Rehydration of the calcined material did not regenerate significantly Bronsted acid sites, due to irreversible changes in the zeolite framework (51). 

In addition to anionic species, some metal cations can also be removed via adsorption processes with LDH materials. Recently, Lazaridis reported an interesting removal of two anions (P04 , SCN ) and three cations (Cd , Pb ", Ni " ) from aqueous solutions in single batch systems using uncalcined and calcined (773 K) Mg/Al LDH carbonate materials [148]. It was found that the calcined material showed higher sorption capacities than the uncalcined material for all the ions. Since the sorption capacities are relatively high, the author suggested that LDHs could be considered as a potential materials for sorption of both anions and cations in wastewater treatment systems. Seida et al. have also reported the rapid removal of dilute Pb from dilute aqueous solutions by a column packed with a pyroaurite-like Mg/Fe-COs LDH compound over a wide range of space velocity (Sv = 150-800 min" ) [149]. 

In a study of diatomaceous earth workers, those employed in the quarry for more than 5 years and exposed only to natural diatomaceous earth had no significant roentgenologic changes. Of others employed for more than 5 years in the milling process and exposed to calcined material, 17% had simple pneumoconiosis and 23% had the confluent form, probably the result of fibrogenic action of the crystalline silica formed by calcination of the naturally occurring mineral. ... 

Figure 23 shows H and Al CF MAS NMR spectra recorded during the hydration of calcined H-SAPO-34. The H MAS NMR spectrum of the calcined material recorded before the start of the hydration is dominated by a signal of... 

The zinc concentrate is first roasted in a fluid-bed roaster to convert the zinc sulfide to the oxide and a small amount of sulfate. Normally, roasting is carried out with an excess of oxygen below 1000°C so that comparatively htde cadmium is eliminated from the calcined material in this operation (3). Since the advent of the Imperial Smelting Zinc Furnace, the preliminary roasting processes for zinc and zinc—lead concentrates result in cadmium recovery as precipitates from solution or as cadmium—lead fame, respectively, as shown in Figure 1. 

Samples of HY zeolite were exhaustively treated with successive doses of tetramethylsilane in a static reactor at different temperatures in the range 250°-650°C. Rate data for methane evolution were obtained, and the kinetics were discussed. Silicon and some carbon were incorporated, giving gray materials parts of which were calcined in oxygen. Samples of the original H Y, the treated zeolite, and calcined materials were tested for their abilities to accept electrons from perylene and to isomerize cyclopropane and protoadamantane. The treated zeolite had good electron transfer properties but low and high activities for the isomerizations, respectively. However, the opposite was true for the calcined materials. These results are discussed in terms of the acidic properties of the modified zeolites. 

The Ti environment in trimethylsilylated Ti-MCM-41 catalysts with different degrees of silylation was studied by means of diffuse reflectance UV-vis spectroscopy. The UV-vis spectra of trimethylsilylated samples are compared with the calcined material in figure 6. 

The evaluation of the acid properties of calcined materials was based on the assumption that n-butylamine molecules interact with all acid sites, and the total acidity of the sample studied can be determined from the maximum amount adsorbed. Shown in Fig. 2 are thermogravimetric curves for n-butylamine thermodesorption, which were used to evaluate the amount of medium and strong acid sites for both samples. Thermodesorption of n-butylamine from CeMCM-41 exhibits three distinct ranges (i) desorption of physically adsorbed amine bellow 230 °C (ii) desorption of n-butylamine from medium acid sites at 230 - 410 °C (0.25 mmol/g), and (iii) its desorption from strong acid sites at 410 - 590 °C (0.21 mmol/g). However, only one weight loss was observed for pure silica MCM-41 due to thermodesorption of physically adsorbed amine, indicating negligible acidity of this material. 

Figure 1. XRD patterns of MCM-41 a) calcined material, b) pre-silylated and c) functionalized with APTES.

The extracted and the calcined materials show clearly different characteristics. This is exemplified by the pore size distributions, as calculated by the conventional BJH method, in Figure 1. 

It is demonstrated that thermogravimetry data for uncalcined MCM-41 samples can be used to predict the structural quality of the calcined materials. The method is based on the comparison of weight change derivatives for a sample under study with those for a series of well-characterized samples prepared under similar conditions. Thermogravimetry data were found useful for a qualitative estimation of the overall sample quality, phase purity, degree of structural collapse and, in favorable cases, pore size of calcined MCM-41 materials. 

However, low surface areas and pore volumes indicated an extensive structural collapse at some stage of preparation of the calcined materials. 

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). 

The maximum content of titanium in Ti-beta zeolite appears to be higher than in the other materials. A value of x = 0.038 has been reported without formation of extra-framework Ti02. From the characterization of Ti-beta zeolite by XANES and EXAFS, it has been concluded that Tilv in the calcined material is tetrahedrally coordinated, isolated from other TiIV ions, and surrounded by OSi groups. In the presence of H20, Tilv increases its coordination and very likely undergoes hydrolysis of the Ti—O—Si bonds forming TiOH and SiOH groups (Blasco et al., 1993). 

The calcined material has usually undergone a limited amount of sintering and must be milled to give a powder or slip suitable for the shaping stage. The machinery and problems are essentially the same as those discussed above in relation to mixing. The calcine is usually coarser and more abrasive than the raw materials so that precautions against contamination are of more importance. The... 

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