Calcium Decomposition Kinetics

Methods of EGA using selective sorption, condensation of effluent gases, infrared absorption and thermoparticulate analysis have been reviewed by Lodding [144]. The use of simple gas burette systems should not be forgotten and an Orsat gas analysis apparatus can provide useful measurements in studies of the decomposition of formates [169]. Problems have been encountered in the determination of water released Kiss et al. [170—172] have measured the formation of this compound from infrared analyses of the acetylene evolved following reaction of water with calcium carbide. Kinetic data may be obtained by wet methods ammonia, determined by titration after absorption in an aqueous solution, has been used to measure a—time values for the decomposition of ammonium salts in a fluidized bed [173],... 

The T-S effect becomes apparent under extreme conditions of such kind. The temperature for experiments on the T-S effect is chosen to be much higher than temperatures typical for usual experiments on the decomposition kinetics in the absence of an excess of gaseous product. For crystalline hydrates this excess of temperature may reach 30-50 K, and for calcium carbonate, 100-150 K. This is because the decomposition in the isobaric mode is slower than in the equimolar mode. However, for initial points of the T-S curve corresponding to the absence of gaseous product or to a very low pressure of this gaseous product, this temperature is obviously much higher than the optimal value. That is why self-cooling appears to be well above the common value. 

Isothermal a—time curves were sigmoid [1024] for the anhydrous Ca and Ba salts and also for Sr formate, providing that nucleation during dehydration was prevented by refluxing in 100% formic acid. From the observed obedience to the Avrami—Erofe ev equation [eqn. (6), n = 4], the values of E calculated were 199, 228 and 270 kJ mole"1 for the Ca, Sr and Ba salts, respectively. The value for calcium formate is in good agreement with that obtained [292] for the decomposition of this solid dispersed in a pressed KBr disc. Under the latter conditions, concentrations of both reactant (HCOJ) and product (CO3") were determined by infrared measurements and their variation followed first-order kinetics. 

H2) Freeman, S., and B. Carroll The application of Thermoanalytical techniques to reaction kinetics. The Thermogravimetric Evaluation of the Kinetics of the Decomposition of Calcium Oxalate Monohydrate. J. Phys. Chem. 62, 394/ 397 (1958). 

Reversible reactions. Many solid-gas reactions are reversible, e.g., dehydration of crystal hydrates, so that rate equations for such processes should include terms for the rate of the reverse reaction. If the rates of contributing forward and reverse reactions are comparable, the general set of kinetic models (Table 3.3.) will not be applicable. The decomposition step in a reversible reaction thus needs to be studied [94] under conditions as far removed from equilibrium as possible (e.g. low pressures or high flow rates of carrier gas) and sensitive tests are required for determining whether the kinetics vary with the prevailing conditions. Sinev [95] has calculated that, for the decomposition of calcium carbonate, the rate of the reverse reaction is comparable with that of the forward reaction even when small sample masses (10 mg) and high flow rates (200 cm s ) of inert gas are used. Interpretation of observations becomes more difficult and the reliabihty of conclusions decreases if local inhomogeneities of kinetic behaviour develop within the reactant mass. 

Kinetic data measured for the decomposition of calcium carbonate under isothermal and under programmed-temperature conditions [11] and varied reaction environments influencing the ease of removal of the CO2 product, show that the apparent values of the kinetic parameters k, A and may be influenced by sample heating rate, reactant self-cooling, sample mass, geometry and particle size, which determine the rate because of the reversible nature of the decomposition [12]. These effects can lead to compensation behaviour [13]. 

Other decompositions, which had previously been accepted as simple reactions proceeding in the solid state, have subsequently been shown to be more complicated than was discerned from overall kinetic data. The thermal breakdown of potassium permanganate exhibits almost symmetrical sigmoid curves, now regarded (39) as proceeding with the intermediate formation of K3(Mn04)2 by at least two, possibly consecutive, reactions. Dehydration of calcium oxalate monohydrate proceeds (75) with the loss of H20 molecules from two different types of site by two concurrent reactions that proceed at slightly different rates. 

For a reaction such as the thermal decomposition of calcium carbonate, which obeys linear kinetics, the rate constant may be expressed as... 

Three kinetic methods were evaluated by Sharp and Wentworth (76) using the thermal decomposition of calcium carbonate under various conditions. The physical state of the sample was as a pellet, a powder, or as 1 1 molar ratios with a-aluminum oxide or a-iron(III) oxide. The three methods used were Method 1, Freeman and Carroll Method II, Coats and Redfern and Method HI, Achar et al. (102). The kinetic data calculated by Methods II and III are presented graphically in Figure 2.41 and in Table 2.3. In every case a linear plot was obtained over a wide range of a with n = When these methods were applied with n — j, the range of x was less, especially in the case of Method III, which led to noticeable curvature at... 

A simple example of a kinetic experiment is described here. This was conducted to find the mechanism and kinetic constants for the decomposition of calcium oxalate monohydrate by an isothermal experiment. 

Figure 12 Kinetics of decomposition of calcium oxalate. (A) Reduced time plot and (B) Arrhenius plot...

Three detailed applications of thermogravimetry are described with more quantitative interpretations, i.e., efforts are made to develop information on the kinetics and equilibrium. The calcium oxalate/carbonate decomposition is treated first. The lithium hydrogen phosphate polymerization has been discussed above as a step-reaction in Sect. 3.1 (Figs. 3.16-22). Finally, the method and some examples of lifetime determinations based on TGA are shown at the end of this section. 

Satterfield CN, Eeakes E (1959) Kinetics of the thermal decomposition of calcium carbonate. AIChE 3 5 115-122... 

Freeman, E.S. and Carroll, B. (1958) The application of thermoanalytical techniques to reaction kinetics. The thermogravimetric evaluation of the kinetics of the decomposition of calcium oxalate monohydrate. J. Phys. Chem., 62, 394-397. 

In the UPO3 example, it seemed to be possible to make this assumption. The present example illustrates the more common case that Eq. (6) with the first two factors alone, does not represent the kinetic data for different heating rates and masses. The calcium oxalate decomposition is a reaction with a substantial g(T,p). 

Mineralizers are added to the raw feed to aeeelerate the kinetics of reactions by modifying the solid and liquid state sintering. The temperature of decomposition of calcium carbonate is lowered in the presence of mineralizers. In the synthesis of C3S and C2S, DTA has shown that some phosphates, earbonates, sulfates, and ehlorides deerease the deearbonation temperature and that of the formation of the silieates.[ °l Caleium fluoride acts both as a flux and a mineralizer in promoting the formation the tricalcium silicate phase. DTA thermograms have been applied to study the effect of mineralizers on the melting behaviors and crystallization temperatures of clinker. Early formation of liquid can be established by thermal techniques. 

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