Calcining time

Table 2.4. Microstructural Characteristics of Alumina Membranes as a Function of Calcination Time and Temperature (Leenaars et al. 1984, Burggraaf et al. 1989,...

II Anhydrite 11 insoluble anhydrite inactive anhydrite dead-burned gypsum chemical anhydrite mineral anhydrite Produced hy calcining at 250-1.000 C. Relatively inert. Reactivity depends upon calcining-time-lemperaiure relationship and particle size. 

Fig. 3. Changes of calcination time with temperature for the BPH samples containing about 2.5 wt.% Ca2+.

The term mechanical activation should only be used for processes where a solid is pretreated prior to a subsequent reaction. The mechanical activation of solids is often used on an industrial scale to effect the calcination times of ceramics, or the solubility of ores. For a review of hydrometallurgical applications, see Ref. [38],... 

Hydroxyl-Zr bentonite catalyst for esterification was prepared. Effects of the preparation conditions (calcination temperature, calcination time and ageing time) on interlayer distance, specific surface area and surface total acidity of catalyst have been studied. XRD, BET method and potentiometric titration were respectively used to detennine interlayer distance, specific surface area and surface total acidity of catalyst. Scanning Electron Microscopy was performed to observe the images of samples. 

With the variation in calcination time, the interlayer distances of hydroxyl-Zr bentonite remain nearly constant (see Table 3). 

Table 3. Effects of the different calcination times (l) on interlayer distances (d)...

From Table 5 and Table 6 it can be seen that all the samples of different calcination time and aging time possess nearly the same acidity at a particular calcination temperature, revealing the negligible effect of the calcination time and aging time on the surface total acidity. However, Table 7 shows that the total number of acid sites and the number of the BrOnsted acid sites on the catalysts decrease with an increase in calcination temperature. It is known that the surface acid sites of hydroxyl-Zr bentonite mainly stem from surface hydroxyl and exposed metal cation [6], and the Bronsted acid sites result from protons on the surface of the octahedral layers. When the calcination temperature increases, the migration of protons to the octahedral layers of montmorillonite will become easier, leading to the decrease of the number of BrGnsted acid sites. 

Table 5. Effects of the different calcination time (t) on the total acidity (n).

Calcinations time has no effect on specific surface area of the samples, as can be seen from Table 10. 

Zhu et al. prepared nanosized La2Cu04 perovskite oxide using an amorphous heteronuclear complex La2Cu(DTPA)i 6 6H20 as a precursor at a low temperature of 650 °C (Zhu et al., 2000). The grain size was in nano-scale even though the precursor was calcined at 900 °C. The crystal size barely changes with calcination temperature above 650 °C and the calcination time. 

Powder X-ray diffraction studies of USbOs, USb30io and Sb204 as a function of calcination time and temperature allowed investigation of the mechanism of formation of the phases, USbOs and USbsOio- Accordingly, USbOs, which was formed at about 675 °C, was considered to be the precursor of the USbsOio phase [43]. At this temperature Sb20s was also initially present and since it was not stable it reacted with USbOs to produce USb30io. The USbsOio phase increased in concentration as the temperature was increased. At 980 °C the USbsOio phase was decomposed to re-form USbOs. 

Zeosil SDAs reaction mixture SiOizSDArHiO Reaction temp. m Reaction time [d] Calcination temp. [°C] Calcination time [d] Unit cell composition Ii Si02... 

Modern rotary kiln plants have capacities of up to ca. 500 t/d, continuous grate furnaces up to 1000 t/d. The residence times are less than 1 hour. In kettles throughputs up to 150 t/d with residence times of several hours are aimed for. Box shaft kilns are no longer viable due to the lengthy (several day) calcination times. 

The TOP of a commercial scale rotary calciner is typically 30-60 min and depends primarily on feed rate and tube length. The TOP in a rotary calciner is much shorter than that in a belt calciner. A continuous multipass operation is sometimes employed for materials requiring a long calcination time. 

J.2.4 Evaporation and Calcination of Acidified RiiCI, Solution RiiCI was dissolved in 1 M HCl. The solution was gently heated to dryness and then heated for 1 h at 450°C. The precipitate was crushed, milled, and heated again (total calcination time 3 h). It was then water-washed. 

When silica is heated in air, it does not become dehydroxylated immediately. An annealing process is involved that can take up to 24 h to reach equilibrium, depending on the temperature. The consequences of this behavior are explained in more detail in Section 20. However, in a series of experiments, represented in Figure 127, the calcination time in CO at 871 °C was varied from 0.5 to about 7 h, to determine whether this form of dehydroxylation is also time dependent. The curve in Figure 127 shows... 

Figure 5.4 Effect of calcination time and temperature on surface area of MgO using seawater magnesium hydroxide precursor.

PuOj from direct calcination of Pu(N03 )4. The precipitation steps of the above processes can be avoided by the direct calcination of the plutonium nitrate solution to PuOa. Calcination has been carried out at 350 C in a liquid-phase screw calciner. Half a mole of ammonium sulfate per mole of plutonium is added to the feed solution to increase the production of reactive PuOi. The calcination time and temperature must be low enough to minimize sintering, which would otherwise reduce the chemical reactivity of the oxide particles for subsequent conversion to a halide. 

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