Modeling of the Thermal Decomposition of Asbestos Minerals
John P. Sanders, Ph.D. and Denis A. Brosnan, Ph.D. Clemson University, Clemson, SC 29634-0971 USA
While the health effects of asbestos exposure have been known since the 1960's, there remains great interest in the identity of residuals of asbestos species after exposure to high temperatures. Many suspected exposures to asbestos were for persons who demolished industrial furnaces where, for example, asbestos was used as an integral part of the refractory lining. Since the asbestos was typically exposed to high temperatures during the service life of the furnace lining along with the refractories, it is of interest to determine if there was potential exposure to fiber form asbestos during demolition.
It is well known that asbestos minerals exhibit dehydroxylation at moderate temperatures followed by eventual recrystallization at elevated temperatures. Chrysotile asbestos, for example, is reported in most authoritative references to dehydroxylate at about 660oC followed by recrystallation forming forsterite at about 800oC. However, dehydroxylation of chrysotile has been reported at temperatures as low as 400oC. As the kinetics of the dehydroxylation determine the potential for exposures in furnace demolition, research was initiated with chrysotile and amphibole asbestos minerals.
A combination of traditional thermal analysis (TG and DSC) along with evolved gas analysis by FTIR was used to study the thermal decomposition of the minerals. Evolved gas analysis was used to track reaction products during dehydroxylation and from carbonate minerals included in the asbestos. The kinetic models for dehydroxylation were established, and residuals after thermal analysis tests were observed using transmission electron microscopy (TEM) and electron diffraction.
The results of these studies have been used in concert with thermal models of industrial furnaces to demonstrate that, in certain cases, workers could not have been exposed to fiber form minerals. The application of thermal analysis and kinetic modeling to the solution of public health problems has been reinforced in this study.
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