Ankerite decomposition

Based on past experience with the PCM sample, it was expected to be able to separate the ankerite decomposition into two steps. 

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. 

However, when the temperature was raised to 933 K, ankerite decomposition was unaffected by the presence of C02 This afforded an opportunity to separate Reactions (4) and (2) but only in the narrow temperature range between 935 and 975 K (since 975 K was close to the minimum silication temperature). These results indicated that the rates of Equations (2) and (4) were almost identical and thus, if the reaction does proceed in two steps, the rate of (4) is the controlling step at these temperatures. 

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 ... 

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... 

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. 

In comparing these shales, the most significant finding was that ankeritic dolomite decomposition could be prevented in the C-a sample below 930 K, with relatively low concentrations of C02 The enthalpy of reaction for Equation (4) is... 

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