In situ ESEM study of the thermal decomposition of chrysotile asbestos in view of safe recycling of the transformation product

Our plain-English summary of In situ ESEM study of the thermal decomposition of chrysotile asbestos in view of safe recycling of the transformation product, by A.F. Gualtieri, M.L. Gualtieri, M. Tonelli (Journal of Hazardous Materials, 2008).

The study

• Authors: A.F. Gualtieri, M.L. Gualtieri, M. Tonelli
• Journal: Journal of Hazardous Materials
• Published: 2008;156(1-3):260-6
• Cited by: 13 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

The thermal transformation of asbestos into non-hazardous crystalline phases and their recycling is a promising solution for the "asbestos problem". The most common asbestos-containing industrial material produced worldwide is cement-asbestos. Knowledge of the kinetics of thermal transformation of asbestos fibers in cement-asbestos is of paramount importance for the optimization of the firing process at industrial scale. Here, environmental scanning electron microscopy (ESEM) was used for the first time to follow in situ the thermal transformation of chrysotile fibers present in cement-asbestos. It was found that the reaction kinetics of thermal transformation of chrysotile was highly slowed down in the presence of water vapor in the experimental chamber with respect to He. This was explained by chemisorbed water on the surface of the fibers which affected the dehydroxylation reaction and consequently the recrystallization into Mg-silicates. In the attempt to investigate alternative and faster firing routes for the decomposition of asbestos, a low melting glass was mixed with cement-asbestos and studied in situ to assess to which extent the decomposition of asbestos is favored. It was found that the addition of a low melting glass to cement-asbestos greatly improved the decomposition reaction and decreased the transformation temperatures.

In plain English

This laboratory experiment followed how chrysotile asbestos fibers in commercial cement-asbestos slates change when heated, to help optimize industrial heat treatment of asbestos waste. Small powdered samples were heated inside an environmental scanning electron microscope and photographed. They were then checked with conventional electron microscopy and X-ray diffraction. Each experiment was run twice with reproducible results. In helium gas, new crystals appeared on the fiber surfaces at about 1000 °C, and crystallization continued up to 1150 °C. In water vapor, the transformation was markedly slower. It was not completed during heating to 1300 °C and was seen only after prolonged holding at 1000 °C. The authors attributed this, as probable, to inhibition of chrysotile's dehydroxylation (its loss of chemically bound water). When the cement-asbestos was mixed with a low melting glass, the mixture melted at 600 to 750 °C and the chrysotile fibers dissolved into the glassy phase. The authors concluded that such a glass may greatly decrease the melting temperature of chrysotile. They noted that the microscope shows newly formed crystals only after they reach a certain size.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory experiment: in situ environmental scanning electron microscopy (ESEM) of heated samples, with conventional SEM and X-ray powder diffraction before and after heating
• Who was studied: Samples of commercial cement-asbestos slates (a material sample, not people). Three heating experiments: in helium, in water vapor, and mixed with a low melting glass. Each was repeated twice.
• Exposure: Chrysotile (serpentine asbestos clinochrysotile, 13.6 wt% of the slate according to the authors' earlier analysis) in cement-asbestos, heated at about 20 °C per minute. Conditions were helium (1.9 to 3.5 Torr, up to 1150 °C), water vapor (2.5 to 3.4 Torr, up to 1300 °C), or a 5:1 cement-asbestos and low melting glass mixture in water vapor (up to 850 °C).
• Main results:
  • In helium, newly formed crystals appeared on the fiber surface at about 1000 °C, and crystallization continued up to 1150 °C.
  • In water vapor, the reaction kinetics were markedly slowed compared with helium.
  • In water vapor, recrystallization was observed only after prolonged heat treatment at 1000 °C (images after 1.5 h and 3 h).
  • After the water vapor experiment, newly formed crystals indicated complete transformation of the asbestos fibers.
  • After heating, X-ray diffraction showed the new phases akermanite, merwinite, silicocarnotite and ferrite, which the authors took as confirmation that thermal transformation had occurred.
  • The authors attributed the higher reaction temperature in water vapor, as probable, to inhibition of chrysotile dehydroxylation.
  • With the low melting glass added, the system melted at 600 to 750 °C and the chrysotile fibers dissolved inside the glassy phase.
  • After the glass experiment, the sample was completely amorphous on X-ray diffraction, which the authors described as complete dissolution of the chrysotile fibers.
  • The controlled ESEM experiments were each repeated twice and gave reproducible results.
• The authors' conclusion: The authors concluded that a low melting glass may greatly decrease the melting temperature of chrysotile, and that applying it to cement-asbestos slates may increase the efficiency of industrial thermal destruction processes.
• Limitations the authors state:
  • Because of resolution limits, in situ imaging shows newly formed crystals only after they reach a certain size. The authors call this a limitation compared with X-ray diffraction for dynamic studies.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Chrysotile.

Medical terms in this study, explained in our Lexicon: Chrysotile, Crystalline, Scanning Electron Microscopy (SEM), Dehydroxylation, Silicate.

Related studies

• The thermal transformation of Man Made Vitreous Fibers (MMVF) and safe recycling as secondary raw materials (SRM) — A.F. Gualtieri, E. Foresti, et al. (2009)
• Recycling the product of thermal transformation of cement-asbestos for the preparation of calcium sulfoaluminate clinker — A. Viani, A.F. Gualtieri (2013)
• The transformation sequence of cement-asbestos slates up to 1200 degrees C and safe recycling of the reaction product in stoneware tile mixtures — A.F. Gualtieri, C. Cavenati, et al. (2008)
• Microwave thermal inertisation of asbestos containing waste and its recycling in traditional ceramics — C. Leonelli, P. Veronesi, et al. (2006)
• In vitro biodurability of the product of thermal transformation of cement-asbestos — A.F. Gualtieri, A. Viani, et al. (2012)
• Recycling of the product of thermal inertization of cement-asbestos for various industrial applications — A.F. Gualtieri, C. Giacobbe, et al. (2011)

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