Kinetics of the thermal dehydration of magnesium oxalate dihydrate in a flowing atmosphere of dry nitrogen

Abstract

The kinetics of isothermal dehydration of MgC204-2H20 in a dry nitrogen flow have been studied in detail by use of many accurate thermogravimetric data acquired on a microcomputer. This dehydration proceeded as a two-dimensional phase boundary reaction, R2. In general, the nucleation in the phase boundary reaction occurs so rapidly that the reaction rate is only determined by following the chemical process occurring at the reactant-product interface. However, the present dehydration was affected by the nucleation process, and its dehydration fraction curve showed a sigmoidal character especially at lower temperatures than ca. 160 °C. We succeeded in separately evaluating the rate constants kN for the nucleation and k2 for the R2 process by detailed analysis of the dehydration fraction curve. The value of kN became comparable to that of k2 at lower temperatures, whereas at higher temperature the former became much larger than the latter and the overall dehydration had a characteristic of natural R2 reaction. The values of activation energies and preexponential factors for both processes were determined from the temperature dependency of kN and k2. Those were 430 kJ mol'1 and 3.73 X 1049 s'1 for the nucleation process, and 111 kJ mol'1 and 3.40 X 109 s'1 for two-dimensional phase boundary process, respectively.

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