Reversible calcite decomposition

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 ... 

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),... 

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 decomposition of dolomite shows many points of similarity with the reactions of calcite and of other single carbonates of Group IIA metals (Sects. 3.1.1 and 3.1.2) the reaction is reversible, occurs at an interface, and both apparent kinetic parameters and reactivity are influenced by the prevailing C02 pressure. 

Maciejewski and Reller confirm [6] that (as yet) there is no experimental evidence for the existence of an amorphous or metastable intermediate in the dissociation of calcite. They show that the CaO formed by CaCOj dissociation under high vacuum reacts relatively rapidly at low temperatures (below 320 K) in the reverse process on CO2 readmission. Carefully controlled conditions are required to isolate and to study any highly active phases that may be formed during solid state decompositions. 

Retorted shale samples were first de-charred at 700 K and then attempts were made to isolate the individual mineral reactions. While this was not too difficult in experiments conducted on the PCM sample, it posed a serious problem with the C-a sample. The problem is best understood by referring to the three sets of reactions which describe the primary mineral reactions occurring the decomposition of ankeritic dolomite (Equation (1)), the reversible decompositon of calcite (Equation... 

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