Calcinations at temperatures

To produce the titanium white mtile pigment, the hydrated Ti02 gel prepared in the presence of mtile seeds is calcined at temperatures of 900—930°C. This temperature is quite important because pigments having a particle size of 200—400 nm are produced at these temperatures. When the calcination is carried out at temperatures above 950°C, the particles of Ti02 become considerably larger and do not have optimum pigmentary properties. 

Two pigment production routes ate in commercial use. In the sulfate process, the ore is dissolved in sulfuric acid, the solution is hydrolyzed to precipitate a microcrystalline titanium dioxide, which in turn is grown by a process of calcination at temperatures of ca 900—1000°C. In the chloride process, titanium tetrachloride, formed by chlorinating the ore, is purified by distillation and is then oxidized at ca 1400—1600°C to form crystals of the required size. In both cases, the taw products are finished by coating with a layer of hydrous oxides, typically a mixture of siUca, alumina, etc. 

Cobalt pigments are usually produced by mixing salts or oxides and calcining at temperatures of 1100—1300°C. The calcined product is then milled to a fine powder. In ceramics, the final color of the pigment maybe quite different after the clay is fired. The materials used for the production of ceramic pigments are... 

Magnesium oxide. The natural minerals, i.e., magnesite (MgCO ), brucite [Mg(OH)9], etc., after being crushed to predetermined size, are calcined at temperatures varying from 1055 to 2000 K, depending upon whether a caustic or a dead-burned produc t (periclase) is being... 

Petroleum coke. In order to eliminate excess volatile matter, petroleum coke is calcined at temperatures of 1475 to 1525 K. This is a sensitive material, and temperature control is difficult to maintain. 

Brown et al. [494] developed a method for the production of hydrated niobium or tantalum pentoxide from fluoride-containing solutions. The essence of the method is that the fluorotantalic or oxyfluoroniobic acid solution is mixed in stages with aqueous ammonia at controlled pH, temperature, and precipitation time. The above conditions enable to produce tantalum or niobium hydroxides with a narrow particle size distribution. The precipitated hydroxides are calcinated at temperatures above 790°C, yielding tantalum oxide powder that is characterized by a pack density of approximately 3 g/cm3. Niobium oxide is obtained by thermal treatment of niobium hydroxide at temperatures above 650°C. The product obtained has a pack density of approximately 1.8 g/cm3. The specific surface area of tantalum oxide and niobium oxide is nominally about 3 or 2 m2/g, respectively. 

Plutonium Oxide Dissolution. All four sites dissolve impure PuO, residues in concentrated HND3 (10 to 14M) containing HF (<0.3M). Whereas material calcined at temperatures of... 

However, in some cases oxidic gold species may be the active sites for CO oxidation. Gates reported that oxidic gold dispersed on La203 by using GG of Au acac complex is active at room temperature [43]. On the other hand, we have recently found that over Au/La coprecipitates calcined at temperatures below 500 K are active even at 193 K [46]. The EXAFS and XANES analyses of the active samples showed that oxidic gold stabilized by La(OH)3 is responsible for low-temperature activity. 

Interpretation of the spectra in Fig. 4.6 is best done by comparison with those of zirconium ethoxide and zirconium oxide reference compounds. Figure 4.7 contains the ZrO+/Zr+ and ZrO/Zr+ ratios from the SIMS spectra of the reference compounds, and of the catalysts as a function of the calcination temperature. The figure clearly shows that catalysts calcined at temperatures up to 200 °C have ZrO+/Zr+ and Zr02+ /Zr+ ratios about equal to those measured from a zirconium ethoxide reference compound. However, samples calcined above 300 °C have intensity ratios close to that of Zr02. 

The effects of calcination temperature on the hydrogenolytic behaviors are shown in Fig. 4. The activity increases as the calcination temperature is increased up to 600°C. Then, the conversion is constant until the temperature reaches 750°C, whereupon it drops rapidly. The values of bja indicate almost the same tendency as the conversion. However, the degree of demeth-ylation is almost constant for all calcination temperatures and a(T/TrMeB) = 1.02, j8 = 2.8-4.7%. The increase of conversion by calcination at temperatures... 

Lithium introduced in the structure of the clay allows to control the density of the pillars and the strength of interaction between the pillar and the clay layer. At low calcination temperature, the interlayer distances and the surface area increased. The thermal stability of the clay, calcined at temperature higher than 400°C, drastically decreases. 

However, the thermal stability of the Li-Zr pillared clays is drastically influenced after calcination at temperatures higher than 400°C. This is mainly due to Li acting as flux. 

It has recently been recognized that crystal structure and particle size can also influence photoelectrochemical activity. For example, titanium dioxide crystals exist in the anatase phase in samples which have been calcined at temperatures below 500 °C, as rutile at calcination temperatures above 600 °C, and as a mixture of the two phases at intermediate temperature ranges. When a range of such samples were examined for photocatalytic oxidation of 2-propanol and reduction of silver sulfate, anatase samples were found to be active for both systems, with increased efficiency observed with crystal growth. The activity for alcohol oxidation, but not silver ion reduction, was observed when the catalyst was partially covered with platinum black. On rutile, comparable activity was observed for Ag, but the activity towards alcohol oxidation was negligibly small . Photoinduced activity could also be correlated with particle size. 

Both zinc sulfide and barium sulfate are insoluble in water. To improve the stability of lithopone, a small amount of a cobalt salt is added to the precipitated mixture. The mixture has to be filtered off, dried, and calcined. The calcination is carried out in rotary calciners at temperatures between 600 and 700°C. During calcination, the particle size of zinc sulfide grows from its original size (about 0.1 Jim) to the pigmentary optimal size of 0.4—0.6 Jim. 

An iron-exchanged mordenite was also studied by Meisel et al. (182), who incorporated Fe3+ into the zeolite structure. Upon calcination at temperatures greater than 500 K the appearance of Fe2+ was noted in the Mdss-bauer spectrum, and for calcination temperatures higher than 770 K the formation of a-Fe203 was observed to take place inside the mordenite. For the iron- mordenite system, it can now be seen that the Mossbauer effect provides information about the chemical state, symmetry, interaction strength with the support, and location on the support of the resonant iron ions. This information enhances the understanding of the catalytic activity of this and other zeolites (178). 

Calcination. After crystallization, the solid contains a substantial amount of organic base which must be removed to give the catalytically active material. The removal can be achieved only by decomposition, and calcination is the most common process used. But calcination at temperatures insufficient to bum all the organic or in-static air generates less active catalysts (Martens et al., 1993). On the other hand, calcination without temperature control can cause the sudden combustion of the organic material, and, at the high temperatures that are reached, the Tilv separates from the crystalline structure to form Ti02. 

The nature of the surface acidity is dependent on the temperature of activation of the NH4-faujasite. With a series of samples of NH4—Y zeolite calcined at temperatures in the range of 200° to 800°C, Ward 148) observed that pyridine-exposed samples calcined below 450°C displayed a strong infrared band at 1545 cm-1, corresponding to pyridine bound at Brpnsted (protonic) sites. As the temperature of calcination was increased, the intensity of the 1545-cm 1 band decreased and a band appeared at 1450 cm-1, resulting from pyridine adsorbed at Lewis (dehydroxylated) sites. The Brtfnsted acidity increased with calcination temperature up to about 325°C. It then remained constant to 500°C, after which it declined to about 1/10 of its maximum value (Fig. 19). The Lewis acidity was virtually nil until a calcination temperature of 450°C was reached, after which it increased slowly and then rapidly at calcination temperatures above 550°C. This behavior was considered to be a result of the combination of two adjacent hydroxyl groups followed by loss of water to form tricoordinate aluminum atoms (structure I) as suggested by Uytterhoeven et al. 146). Support for the proposed dehydroxylation mechanism was provided by Ward s observations of the relationship of Brpnsted site concentration with respect to Lewis site concentration over a range of calcination tem-... 

A maximum in catalytic activity occurred in samples which had been calcined at temperatures above that at which ammonia evolution had ceased. Venuto et al. (160) found that the maximum activity for alkylation... 

Infrared spectral studies of rare earth (RE) ion-exchanged faujasites have been reported by Rabo et al. (214), Christner et al. (217), Ward (211, 212), and Bolton (218). Distinct hydroxyl absorption bands are observed at 3740, 3640, and 3522 cm-1 after calcination at temperatures in the range of 340° to 450°C. As previously discussed, the hydroxyl groups at 3740 cm-1 are attributed to silanol groups either located at lattice termination sites or arising from amorphous silica associated with the structure. The hydroxyl groups that form the 3522 cm-1 band are nonacidic to pyridine or piperidine and are thought to be associated with the rare earth cations. 

Tanabe el al. studied in detail the catalytic action and properties of metal sulfates most of the sulfates showed the maximum acidity and activity by calcination at temperatures below 500°C, with respect to the surface acidity and the acid-catalyzed reaction (118, 119). Other acid-catalyzed reactions were studied with the FeS04 catalyst together with measurement of the surface acidity of the catalyst the substance calcined at 700°C showed the maximum acidity at Ho s 1.5 and proved to be the most active for the polymerization of isobutyl vinyl ether, the isomerization of d-limonene oxide, and the dehydration of 2-propanol (120-122). It is of interest that the catalyst calcined at a slightly higher temperature, 750°C, was completely inactive and zero in acidity in spite of the remarkable activity and acidity when heat treated at 700°C. 

The catalytic activities of the Fe203-I catalyst for the reaction of 2-propanol are shown as a function of calcination temperature of the catalyst in Fig. 2 (56). The maximum activity was observed with calcination at 300 and 450-500°C for the sample treated with sulfuric acid. It is considered that the former activity was due to the catalytic action by the mounted acid, but the latter one was based on acid sites created by strong interaction of the ion with the support. In the case of the sample treated with ammonium sulfate, the maximum activity was observed with calcination at 500°C, and the calcination at temperatures below 300°C did not give activity. The results seem to be reasonable judging from the absence of catalytic effect of ammonium sulfate. 

Starting from an aqueous acidic Al3+ solution (for example an aluminium sulphate solution) precipitation occurs if the pH of the solution is increased above about pH = 3 by addition of a base. The first precipitate is a gel-like substance in which minute crystals of boehmite (A10(0H)) are present. If this is filtered without aging and then calcined at temperatures up to 600°C an X-ray amorphous material is obtained. The material remains amorphous until after firing to temperatures greater than 1100°C. (X-AI2O3 is formed at higher temperatures. 

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