Kinetic analysis of thermal dehydration and hydrolysis of MgCl2·6H2O by DTA and TG

Abstract

The thermal decomposition of MgCI 2 9 6H20 (non-dried and partly dried) and the kinetics of the process were studied by DTA, TG, DTG, I R, X-ray diffraction'and chemical analysis of Mg and CI. The reactions which occurred in the course of the thermal analysis were identified as dehydration (in steps), thermal hydrolysis of MgC12"H20 and dehydrochloridization of magnesium hydroxy chlorides, Melting of the phases MgCIz-6HzO, MgC/2" 4HzO and MgC12 was also identified in the thermal curves. Thermal weight loss continued up to 800 ~ in flowing air or nitrogen, but only up to 700 "C in static air. MgO was the end-product of thermal treatment in both cases.The kinetic parameters of the reaction, the activation energy E, pre-exponential factor A and apparent order of reaction b, were computed by several methods. The activation energy and the apparent reaction order of dehydration were found to increase with decreasing hydration number of the Mg. The dehydrochloridization process had the highest activation energy, The thermal decomposition of magnesium chloride hexahydrate has recently been the subject of several studies [1-6}. These studies have elucidated some points which were not cleared up in previous work [7][8][9][10], especially concerning the influence of the experimental conditions on the dehydration and hydrolysis processes. Very Iittle has been done to determine the kinetic parameters of the reactions, however. In the present study the kinetics of the processes are investigated by applying three methods of analysis to DTA and DTG curves: (1) the method of Freeman and Carroll, (2) the initial rate method, and (3) Chen's "half-width" method. In each of these methods, the activation energy E, preexponential factor A and apparent order of reaction b are computed. From a comparison of the results, the reliability of the data can be evaluated. In order to identify the products obtained at various stages of the thermal analysis, infrared 1"John Wiley & Sons, Limited, Chichester AkadOmiai Kiad6, Budapest

A DFT based equilibrium study on the hydrolysis and the dehydration reactions of MgCl2 hydrates

“…Many studies have been carried out to understand the process of hydrolysis during the dehydration step. 18,[23][24][25] Hydrolysis in MgCl 2 hydrates is usually avoided by performing dehydration in the presence of HCl. 5,26 Alternative methods to the direct dehydration of MgCl 2 · 6H 2 O are being proposed for the production of MgCl 2 27, 28 in order to avoid hydrolysis.…”

“…In energy storage systems, this hydrolysis reaction poses a challenge, due to the release of harmful HCl gas, to the salt hydrates as well as to the equipment. However, in most of the experiments mentioned above, the dehydration for industrial applications is performed at relatively high temperatures (above 600 K), whereas in seasonal energy storage systems, the dehydration is carried out at a maximum temperature of 450 K. Kirsh and Shoval, 25 in their experiment, did not see hydrolysis when dehydrating below 473 K, whereas Huang et al 29 observed hydrolysis when calcining MgCl 2 · 6H 2 O isothermally within the range from 440 K to 476 K. Partial hydrolysis was also observed at 473 K when dehydrating MgCl 2 · 4H 2 O in the absence of HCl pressure. Thus, the information about the possibility of the hydrolysis reaction, and its preference over dehydration remains ambiguous.…”

Thermal dehydration and decomposition of nickel chloride hydrate (NiCl2·xH2O)

“…Literature survey reveals that though some work has been carried out on the thermal deaquation of hydrated chlorides ofBa, Mg and Co [10][11][12][13][14][15][16][17], very little work has been reported on the thermal decomposition (including dechlorination) of hydrated chlorides of many important base metals. The present investigation was therefore undertaken to gain further understanding not only on the thermal deaquation of some important metal chlorides, but also on the decomposition of dehydrated or partially dehydrated metal chlorides.…”

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.…”