The Thermal Kinetics Of Some Metal Hydrated Salts

The Simultaneous thermal analysis techniques TG-DSC-QMS-FTIR was used to study the thermal decomposition of three sulfate hydrates and four nitrate hydrates of metal. The thermal kinetics of CuSO4·5H2O,FeSO4·7H2O,NiSO4·6H2O,Fe(NO3)3·9H2O Zn(NO3)2·6H2O was investigated in depth by using model free Ozawa-Flynn-Wall method and multivariate non-linear regression method. The relationship between the order of the dehydration and the location of the water molecules was also proposed. Results showed that: (1) There were two different location of water molecules in CuSO4·5H2O.Four water molecules were expected to group around the Cu2+, and they were dehydrated in the first two steps, with two molecules in each step. The fifth water molecule is different from the others.And the dehydration of CuSO4·5H2O was a three-step reaction, the active energies of the reaction were 69.86,95.69 and 168KJ/mol, the reaction mechanism of the first two step was Fn while it was CnB of the third reaction. (2) The location of the water molecules in FeSO4·7H2O was similar to that of CuSO4·5H2O except for six water molecules were around the Fe2+. The dehydration of FeSO4·7H2O was also a three-step reaction, the active energies of the reaction were 81,86.5 and 167KJ/mol, the reaction mechanism was F1,F1,Fn. (3) However, all the six water molecules were located around the Ni2+ in the NiSO4·6H2O. The dehydration of NiSO4·6H2O was a three step reaction. The activation energies were 104,83,95 KJ/mol and each of the reaction mechanism was F2. (4) The thermal decomposition of Fe(NO3)3·9H2O was a three-step reaction, the activation energy of was 54.04,183.8, 208.6KJ/mol separately, the pre-exponential factor were 4.99,18.74,20.59, the mechanism of all the three reactions were Fn. (5) The thermal decomposition of Zn(NO3)2·6H2O was also a three-step reaction, the activation energy of was 52.17,159.24,186KJ/mol separately, the Pre-exponential factor were 4.75,10.68,14.7, each of the mechanism of the three reactions was Fn, F1,F1.Analysis of the thermal decomposition of metal hydrated salts could be found:(1) Thermal decomposition of metal sulfate was finished in two stages. The first stage was completely dehydration, and the second stage was the decomposition of the metal sulfate salt which was the product of the first reaction. (2) For coordination water in the same Metal sulfate hydrate, there were different steps of the dehydration reaction, but the mechanism of reactions were the same. (3) The decomposition reaction of the hydrated nitrate salt was more complex. There was N2O gas product in the thermal decomposition of Fe(NO3)3·9H2O, Zn(NO3)2·6H2O, Co(NO3)2·6H2O and Cu(NO3)2·6H2O.The decomposition of the sulfate was sometimes occurred accompanied with the dehydration. The hydrated nitrate salt lost partial of water molecules, and changed into (M(OH)r (NO3)m-r), then it lost all the water molecules and changed into metal oxide.