Calcite decompositions

From the more recent reports cited below, further references to the extensive literature concerned with calcite decomposition may be traced. Other modifications of CaC03 (aragonite and vaterite) undergo solid phase transitions to calcite at temperatures of 728 K and 623—673 K respectively [733], below those of onset of decomposition (>900 K). There is strong evidence [742] that the reaction... 

In reviewing reported values of E for calcite decompositions, Beruto and Searcy [121] find that most are close to the dissociation enthalpy. They suggest, as a possible explanation, that if product gas removal is not rapid and complete, readsorption of C02 on CaO may establish dissociation equilibria within the pores and channels of the layer of residual phase. The rate of gas diffusion across this barrier is modified accordingly and is not characteristic of the dissociation step at the interface. 

Where large samples of reactant are used and/or where C02 withdrawal is not rapid or complete, the rates of calcite decomposition can be controlled by the rate of heat transfer [748] or C02 removal [749], Draper [748] has shown that the shapes of a—time curves can be altered by varying the reactant geometry and supply of heat to the reactant mass. Under the conditions used, heat flow, rather than product escape, was identified as rate-limiting. Using large ( 100 g) samples, Hills [749] concluded that the reaction rate was controlled by both the diffusion of heat to the interface and C02 from it. The proposed models were consistent with independently measured values of the transport parameters [750—752] whether these results are transfenable to small samples is questionable. 

Basu and Searcy [736] have applied the torsion—effusion and torsion— Langmuir techniques, referred to above for calcite decomposition [121], to the comparable reaction of BaC03, which had not been studied previously. The reaction rate at the (001) faces of single crystals was constant up to a product layer thickness of 1 mm. The magnitude of E (225.9 kJ mole-1) was appreciably less than the enthalpy of the reaction (252.1 kJ mole-1). This observation, unique for carbonates, led to the conclusion that the slowest step in BaC03 vacuum decomposition at 1160—1210 K is diffusion of one of the reaction components in a condensed phase or a surface reaction of C02 prior to desorption. 

Therefore, later in the test program, we added carbon dioxide to the feed gas (at about the 5-mol% level) in an attempt to reduce total mineral carbonate decomposition suppress calcite decomposition) since this had been successfully done in laboratory thermobalance tests. Mineral carbonate decomposition was reduced about 25% by adding carbon dioxide to the feed gas. 

The mechanism and kinetics of calcite decomposition have been much studied. The reaction proceeds by the movement inwards from the surface of an interface, behind which the material is converted into lime, thus producing a highly porous pseudomorph. The interface moves at a constant rate, implying that the rate at any instant is proportional to the area of the interface. In principle, the rate is controlled by the slowest of the following five steps ... 

For the calcite decomposition reaction, the equilibrium constant. Kg, has the same definition as the distribution coefficient, K, given in equation (5.7) giving the equilibrium partial pressure of CO2. The dependence of the equilibrium constant on temperature is given by the Clausius—Claperon equation [2] ... 

The kinetics of reversible decompositions are often highly sensitive to reaction conditions [43]. For example, the values of and E, for the decomposition of CaCOj show unusually wide variations, owing to the sensitivity of reaction rate to the availability of COj [44,45]. The spread of apparent E values is considerable [46] and some values are close to the dissociation enthalpy [1]. However, Beruto and Searcy [47] concluded that, under high vacuum conditions, the constant rate of interface advance in large crystals was probably controlled by the dissociation step in the absence of a perceptible contribution from the reverse process. The decomposition activation energy (205 kJ mol ) was appreciably larger than the dissociation enthalpy (178 kJ mol ). This is probably the most precise kinetic measurement for the calcite decomposition [48]. 

Two Colorado oil shale samples one from the Parachute Creek Member and the other from the C-a tract, were retorted, de-charred and then subjected to temperatures between 800 K and 1100 K in order to study the mineral reactions which take place. Comparisions between these two samples include the reversible nature of ankeritic dolomite and free calcite as well as the temperatures at which significant silication takes place. Results for the C-a tract samples indicated silication appears to take place in stages and that ankeritic dolomite decomposition can be prevented by relatively low CO2 concentrations. Ankeritic dolomite and calcite decomposition rates were similar for the two samples and there was strong evidence that calcite recarbonation takes place via non-activated chemisorption of C(>2 ... 

Ankerite/Calcite Decomposition Since X-ray diffraction data indicated that the ankeritic dolomite in the C-a sample was closer to ankerite (x >. 5 in Eq. (1)) than to dolomite (x < 3), we shall refer to it as ankerite in the C-a sample and as dolomite in the PCM sample. 

Figure 3 is a comparison of first order decomposition plots for both ankerite and calcite in the C-a sample with that predicted by Soni and Thomson (12) for dolomite in the PCM sample. In both samples calcite decomposes at the same rate as the complete decomposition of ankerite (Eq (1)) at 850 K. However, it should be kept in mind that the data for calcite decomposition shown in Figure 3 is for calcite which was recarbonated from the CaO produced by Equations (1) and (2). Note also that the decomposition rates are about 30% lower in the C-a sample. 

That is, since dolomite decomposition was unaffected by the presence of C02 and calcite decomposition can be prevented by a sufficient CO2 over-pressure, Equation (4) was expected to prevail if ankerite decomposition was carried out at low temperatures (< 900 K) in a CO2 environment. Figure 4 shows the results for ankerite decomposition carried out at two different temperatures with and without CC>2 Note that the presence of CO2 completely prevents decomposition at 853 K and severely inhibits decomposition at 900 K. 

Reversible Calcite Decomposition. The reversible nature of calcite decomposition was studied in both the PCM and C-a samples. In the former this was achieved by carrying out the decomposition at C02 pressures less than the equilibrium values. In the latter, the calcite was decomposed to completion in a C02 free atmosphere and then recarbonated at various C02 pressures and temperatures. The data obtained with the PCM sample were fit to the expression shown in Equation (5),... 

The decomposition of pure phase carbonate minerals has been extensively studied and reviewed (17). The influence of these minerals on oil shale pyrolysis kinetics has not been extensively studied, but the studies of Jukkola et al. (18) and Campbell (15) are notable. The results of both these studies indicate that the major calcite decomposition step is through reaction with silicate minerals in shale to produce Ca- and Ca-, Mg-silicates. The observed enhancement in pyrolysis yield after carbonate removal may be indicative of the catalytic role of silicate minerals in paraffinic and aromatic compound decompositions. In effect, an apparent preference for calcite-silicate interactions in raw shale limits silicate-catalyzed organic reactions which would presumably result in enhanced oil yields. It should be noted, however, that the silicate/carbonate ratio is increasing with net pyrolysis yield for the raw shales, Table I. This may reflect excess silicates becoming free to catalyze organic decomposition. 

Equation (6), dolomite decomposition," is irreversible and takes place at T >875K. Equation (7), "calcite decomposition," is reversible and can be prevented if there is a sufficient CO2 overpressure. Equation (8), "silication," is irreversible and takes place at higher temperatures (>1050K) provided that calcite decomposition is prevented. Equation (9) occurs at lower temperatures and is significant because the iron oxides that result can... 

The chemical reactions below 1300°C are calcination, decomposition of clay minerals as well as the reaction of calcium carbonate (calcite) CaCOg or calcium oxide (lime) CaO with quartz and clay mineral decomposition products. Calcination of calcite, decomposition of clay minerals are endothermic reactions, while reaction of calcife or lime with quartz and clay mineral decomposition products are exothermic. Calcination of pure calcium carbonate is done according to the reaction ... 

Hyperbolic Dependence of the Decomposition rate on the External CO2 Pressnre The results presented in studies [16, 18, 19] confirm the second consequence, which for calcite decomposition in the presence of CO2 predicts a hyperbolic dependence between the rate and Data taken from the paper by Hyatt et al. [18] are presented in Fig. 5.3. If we exclude from consideration the rates of CaCOa decomposition at 900 °C in the absence of C02(Pco2 = Oatm), which are clearly underestimated owing to the self-cooling of the sample caused by the low thermal conductivity of gas and by the... 

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