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Kinetic analysis has been performed on TG and DTG diagrams of the forward reaction MnCO 3 ~ MnO + CO2, recorded at different pressures of COs ranging form 2.6.10 -~ Pa to 26.6 kPa. The results obtained show that this reaction follows a firstorder kinetic law, independently of the COs pressure used in carrying out the experiments. On the other hand, the activation energy increases on increase of the COs pressure, from 117 kJ/mol up to an asymptotic value of 292 kJ/mol at about 26.6 Pa.This finding cannot be explained by considering the influence of the reverse reaction of formation of MnCO 8, for under the described experimental conditions the ratio PcoJPequa. is very close to zero. A mechanism that takes into account the adsorption of COs on the phase boundary has been proposed in order to interpret the results.A large number of authors [1 -10] have studied the thermal decomposition of MnCOa because of the industrial interest of rhodochrosite as a starting material for the preparation of different manganese oxides. However, papers relating to the decomposition kinetics of this compound are rather scarce [11 -13] and there is a lack of agreement among the kinetic parameters obtained by different authors. By way of example, values ranging from 121 kJ/mol [13] to 213 kJ/mol [11] have been reported for the activation energy of the thermal decomposition of MnCO~ under nitrogen atmosphere. This behaviour could perhaps be explained by considering the influence of the CO2 generated during the salt decomposition. In fact, in a previous paper [14] we have demonstrated that the removal of the gases generated during the thermal decomposition of solids is not attained even by using a very large flow of an inert gas.The scope of the present paper is to study the influence of the CO 2 pressure on the thermal decomposition of MnCQ, in order to achieve a better understanding of the reaction mechanism.Experimental D'Hemio a.r. MnCQ was used. Rhodochrosite was the only phase observed by powder X-ray diffraction analysis.A Mettler thermoanalyzer that allows a vacuum better than 2.66.10 -4 Pa and the selection of heating rates ranging from 0.5~ to 25~ was employed. A Cahn electrobalance, model RG, equipped with a derivation adapter Mark II, was used as well. This instrument permits the simultaneous recording of both & Thermal Anal 24, 1982

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