Mechanism and Kinetics of Thermal Decomposition of MgCl2 × 6H2O

Abstract

The reaction mechanism and kinetic behavior of thermal decomposition of MgCl 2 9 6H 2 O were studied by thermal gravimetric analysis. The results showed that the thermal decomposition process of MgCl 2 9 6H 2 O could be divided into six stages. In the first two stages, four crystalline waters were lost. The dehydration and hydrolysis coexisted during the third and fourth stages. The fifth stage corresponded to the evaporation of 0.3 crystalline waters, and one molecular hydrogen chloride was eliminated in the last stage. The kinetic analysis of the thermal decomposition process was performed using the Doyle, Coats-Redfern, and Malek methods. The results suggested that the mechanisms of six stages were two-dimensional phase boundary mechanism, three-dimensional phase boundary mechanism, nucleation and nuclei growth mechanism (Avrami-Erofeev equation n = 3), two-dimensional phase boundary mechanism, three-dimensional diffusion mechanism (cylinder and G-B equation), and nucleation and nuclei growth mechanism (Avrami-Erofeev equation n = 1), respectively. The apparent active energies of six stages were 66.8 kJ 9 mol À1 , 138.0 kJ 9 mol À1 , 77.2 kJ 9 mol À1 , 135.6 kJ 9 mol À1 , 77.4 kJ 9 mol À1 , and 92.2 kJ 9 mol À1 , respectively. The frequency factors were 3.6 9

Reaction kinetics of ZrF4 chloridation at elevated temperatures

“…MgCl 2 ·6H 2 O loses crystal water upon heating through several decomposition steps (Kirsh, Yariv and Shoval, 1987;Huang et al, 2010). The dehydration of MgCl 2 ·6H 2 O is described by Reactions [ii] to [vii] below:…”

“…The formation of gaseous product is reflected on the thermograms as a loss of mass. Kirsh, Yariv and Shoval (1987) and Huang et al (2010) derived kinetic data for the thermal decomposition of MgCl 2 ·6H 2 O. It should be noted that neither was done according to most recent ICTAT specifications (Vyazovkin et al, 2011) and the kinetic triplets reported should be regarded as estimates rather than correct absolutes.…”

“…The kinetic data given by Huang et al (2010) was used for ease of implementation. Their kinetic models for the individual steps were adopted without change, while their pre-exponential factors and activation energies were used as starting values for obtaining a fit to our data.…”

“…The form of the dehydration equations given by Huang et al (2010) can be attributed to the solid-state process mechanism used to fit the decomposition steps. The mechanisms that resulted in the best fit for each step are: two-dimensional phase boundary mechanism, threedimensional phase boundary mechanism, nucleation and nuclei growth mechanism (Avrami-Erofeev equation n=3), two-dimensional phase boundary mechanism, threedimensional diffusion mechanism (cylinder and G-B equation) and nucleation and nuclei growth mechanism (Avrami-Erofeev equation n=1), respectively.…”

“…The mechanisms that resulted in the best fit for each step are: two-dimensional phase boundary mechanism, threedimensional phase boundary mechanism, nucleation and nuclei growth mechanism (Avrami-Erofeev equation n=3), two-dimensional phase boundary mechanism, threedimensional diffusion mechanism (cylinder and G-B equation) and nucleation and nuclei growth mechanism (Avrami-Erofeev equation n=1), respectively. The general form of the kinetic data given by Huang et al (2010) is shown in Equation [9]: [9] where f( ) is termed the kinetic model, for a single step and is given by the above-mentioned mechanisms for solid-state processes. For the chloridation reaction we assumed the simple empirical model, f( )= n .…”