Kinetics of calcination

The progress of the calcination of magnesite can be divided into five stages. 

Stage 1, Preheating Zone The magnesite is preheated from ambient temperature to between 700 and 900°C by the kiln gases. If firing in 

Stage 2, Calcination Zone When the magnesite reaches a temperature of about 750°C, the pressure of carbon dioxide produced by dissociation of magnesite equals the partial pressure of CO2 in the combustion atmosphere. As the magnesite progresses through the calcination zone, the temperature further rises and the surface layer of the ore begins to decompose. 

Stage 3, Once the temperature of the magnesite has exceeded the decomposition temperature, the partial pressure exceeds one atmosphere and the process of dissociation can proceed beyond the surface of the particles. 

Stage 4, Sintering If all of the magnesite has decomposed to magnesium oxide before it leaves the calcination zone, then the process of sintering begins. For further discussion of sintering see Section 9.2.1. 

The temperature at which the dissociation pressure of MgCOs reaches 1 atmosphere has variously been reported to be between 402 and 550 °C [15.1,15.6]. The heat of dissociation of MgCOs relative to 25 °C has been reported to be 723 kcal/ kgMgO [15.7], 

The decomposition of dolomites and magnesian/dolomitic limestones is more complex. It is reported that some decompose via two discrete stages, others decompose via a single stage, while others decompose in an intermediate manner [15.1,15.7]. 

All dolomites and magnesian/dolomitic limestones decompose at higher temperatures than magnesium carbonate. The onset of calcination can vary from 510 to 750 °C [15.6], and depends on the crystal structure and form of the stone [15.1]. Reaction (15.3) is understood to occur towards the lower end of that range, with reaction (15.4) subsequently occurring at about 900 °C. Reaction (15.5) is reported to occur towards the upper end of the range. This apparent discrepancy may reflect different heat transfer rates, rather than fundamental differences in the physical chemistry of the various limestones. 

The heat of dissociation of dolomite, i.e. heat(i+2) is reported to be 723 kcal/kg of (CaO MgO), relative to 25 °C [15.1]. This is lower than the weighted average of the heats of dissociation of CaCOs and MgC03 — 750 kcal/kg (CaO MgO). This apparent discrepancy may reflect the difference in the heat of formation of dolomite relative to those of calcite and MgCOs. 

The passage of a limestone particle through a lime kiln can be divided into five stages. The following description refers to high-calcium limestone, but parallels can be drawn with magnesian/dolomitic limestones and dolomite. 

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Acceptor Kinetics. The kinetics of calcining and recarbonation of fresh Virginia dolomite at several spot conditions are shown in Figures 11 and 12. The total pressure was 1 atm. in all cases. 

The difficulty in producing a unified theory of the kinetics of calcination is that it would need to account for all of the above processes, any one of which may become rate-determining under particular circumstances. 

Fig. 8. The polymerization kinetics of Cr/silica depends on the temperature of calcining, which controls the surface hydroxyl population.

Here f is a char reactivity factor equal to one unless otherwise stated, while a represents the relative activity of HjO and C02 a typical value would be 3 [7]. Little is known about the kinetics of the reactions involving the CaO and CaS crystallites. Rates of CaS formation in calcined dolomite and limestone have been measured [8,9], but they are likely to be much slower than those possible on the crystallites. Therefore, we use ... 

The bulk density of the catalyst is 1360 kg/m3. The kinetic constants found well describe the operation of a fresh catalyst over less than twenty days. At present, the results are being extended onto a period of a couple of months. In the catalysts studied a drop in activity by 40 % was found when the temperature of calcination was increased from 420 to 440 °C over three days, and by as much as 79 % following the calcination at 460 4C. 

For example, data representing the kinetics of polymerization with the Cr(II) trimethylsilylmethyl compound are shown in Figure 183. The alumina support was first treated with fluoride and calcined at 600 °C, and then it was impregnated with three different loadings of the chromium compound, which actually exists as a tetramer, Cr4(TMSM)8 [660]. These catalysts were then tested for ethylene polymerization, and the observed activity was found to be roughly proportional to the chromium coverage. 

The kinetics of the calcination process can be described by a shrinking shell model, where the process of decomposition proceeds gradually from the outside surface inward to the center. 

Kinetics of the Reaction of Half-Calcined Dolomite with Sulfur Dioxide... 

Kinetics of the reaction of sulfur dioxide with half-calcined dolomite have been studied using gravimetric techniques. The reaction rate depends significantly on the presence of water in the reactant gas mixture. With water, the reaction is first order with respect to the sulfur dioxide concentration. Without water, the reaction rate is slower, and the reaction is 0.76 order with respect to sulfur dioxide concentration. This suggests that the rate-determining step differs depending on whether or not water is present. The reaction has an apparent activation energy of 7.3 kcal/mole with water present in the reactant gas. 

A considerable amount of work, including detailed kinetic studies (1, 2), has been reported on the reaction of calcined limestones with sulfur dioxide (Equation 4)... 

Surface reactivity was measured on selected samples by Pt(acac)2 adsorption. In order to homogenize the surface of the samples, the tested solids were exposed 12 h to a water saturated air at room temperature before calcination 2 h at 500 °C in a diy air stream. The composite was cooled down to room temperature overnight in the same air stream. After the pre-treatment, the solid was impregnated during 8 h at room temperature with a 4 mM solution of platinum bis-acetylacetonate (R(acac)2) in toluene with a solvent to solid ratio of 5 mL g" The kinetics of Pt(acac)2 impregnation, i.e. the evolution of Pt precursor concentration in solution, was followed by UV-vis. Spectra were recorded in transmission mode on a PERKIN-ELMER Lambdall UV-vis spectrometer, using a 1 mm quartz cells. Pt(acac)2 adsorption principle and details on the data treatments can be found in [11]. 

Kinetic data on calcination are given for the second cycle since the first cycle of calcination was unusually slow. The reaction rate was quite... 

As emphasized recently by Corma [26], condensation of benzaldehyde and ethylcyanoacetate, without solvent, is an interesting reaction to check the number and the strength of the basic sites. However, when this reaction is carried out in solution, solvent effect plays an important role on the kinetic of the reaction. For this reason, we have compared the activities of different zeolites calcinated at 5S0°C, using similar experimental conditions (Table 2)... 

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