Kinetics For Thermal Decomposition Of Manganese Compounds

The thermal decomposition processes of manganese dioxide,manganese oxalate dehydrate and manganese sulfate monohydrate have been studied using simultaneous thermal analysis instrument. The technical measure of XRD,IR were used to confirm the mechanism of the thermal processes. Thermogravimetric data were used to carry out the kinetic analysis. The kinetic triplets of the thermal processes of manganese dioxide and manganese sulfate monohydrate in nitrogen, and the dehydration processes of manganese oxalate dehydrate in air were confirmed.Firstly, the apparent activation energy was calculated by the iso-conversional method; secondly, using the activation energy and P(x) in standard deviation method to confer the reaction mechanism; lastly, the above data were used to get the pre-exponential factor ln(A/s-1).The results are as follows:1.The thermal decomposition processes of manganese dioxide is divided into two stages in the nitrogen and the air atmosphere in the temperature range from 30 to 1100℃. The thermal decomposition processes may be represented as MnO2→Mn2O3→Mn3O4. Because in the reaction processes have emitted the oxygen production, therefore the thermal decomposition temperature of manganese dioxide in air atmosphere is higher than in the nitrogen atmosphere.The kinetic triplets of the thermal decomposition of manganese dioxide in each stage are as follows:First stage MnO2→Mn2O3:the apparent activation energy was 241.604kJ/mol; this process followed the model of random nucleation and growth. The functions are f(α)= n(1-α)·[-ln(1-α)1-1/n, G(α)= [-ln(1-α)1/n where n=1.4; The range of the ln(A/s-1) was from 32.785 to 32.823. Second stage Mn2O3→Mn3O4:the apparent activation energy was 295.384kJ/mol; this process followed the model of random nucleation and growth. The functions are f(α)= n(1-α)·[-ln(1-α)1-1/n, G(α)= [-ln(1-α)]1/n where n=1.4; The range of the ln(A/s-1) was from 32.441 to 32.466.2.The thermal decomposition process of manganese oxalate dihydrate is divided into three stages in the air atmosphere in the temperature range from 30 to 1200℃. The thermal decomposition processes may be represented as MnC2O4·2H2O→MnC2O4→Mn2O3→Mn3O4. The process of MnC2O4 to Mn2O3 was affected by the heating rate. When the heating rate was lower we can see the medium product. This product can create a steady state with MnC2O4. It can be changed into Mn2O3 at about 500℃. The equation can express as MnC2O4+MnO→Mn2O3+ 2CO↑. CO burn in the O2 can emit heat. When the heating rate was higher, MnC2O4 changed into Mn2O3 directly.The kinetic triplets of the dehydration of manganese oxalate dihydrate are as follows:the apparent activation energy was 97.159kJ/mol; this process followed the model of random nucleation and growth. The functions are f(α)=n(1-α)·[-ln(1-α)1-1/n G(α)= [-ln(1-α)]1/n where n=1.7; The range of the ln(A/s-1) was from 28.593 to 28.725.3.The thermal decomposition process of manganese sulfate monohydrate is divided into two stages in the nitrogen and the air atmosphere in the temperature range from 30 to 1100℃. The thermal decomposition processes may be represented as MnSO4·H2O→MnSO4→Mn3O4. In nitrogen atmosphere MnSO4 change into Mn3O4 directly. But in the air atmosphere we can see the medium product. When the heating rate was lower it can appear, disappeared when the heating rate increased. Through analysis we can know the medium product is Mn2O3.The kinetic triplets of the dehydration of manganese sulfate monohydrate are as follows:First stage dehydration:the apparent activation energy was 147.933kJ/mol; this process followed the model of random nucleation and growth. The functions are f(α)= 2(1-α)[-ln(1-α)]0.5和G(α)= [-ln(1-α)]0.5; The range of the ln(A/s-1) was from 32.349 to 32.401. Second stage MnSO4→Mn3O4:the apparent activation energy was 237.341kJ/mol; this process followed the model of random nucleation and growth. The functions are f(α)= n(1-α)·[-ln(1-α)1-1/n, G(α)= [-ln(1-α)]1/n where n=1.7; The range of the ln(A/s-1) was from 23.606 to 23.654.