Microwave-driven asbestos treatment and its scale-up for use after natural disasters

Our plain-English summary of Microwave-driven asbestos treatment and its scale-up for use after natural disasters, by S. Horikoshi, T. Sumi, S. Ito, et al (Environmental Science & Technology, 2014).

The study

• Authors: S. Horikoshi, T. Sumi, S. Ito, R. Dillert, K. Kashimura, N. Yoshikawa, M. Sato, N. Shinohara
• Journal: Environmental Science & Technology
• Published: 2014;48(12):6882-90
• Cited by: 5 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Asbestos-containing debris generated by the tsunami after the Great East Japan Earthquake of March 11, 2011, was processed by microwave heating. The analysis of the treated samples employing thermo gravimetry, differential thermal analysis, X-ray diffractometry, scanning electron microscopy, and phase-contrast microscopy revealed the rapid detoxification of the waste by conversion of the asbestos fibers to a nonfibrous glassy material. The detoxification by the microwave method occurred at a significantly lower processing temperature than the thermal methods actually established for the treatment of asbestos-containing waste. The lower treatment temperature is considered to be a consequence of the microwave penetration depth into the waste material and the increased intensity of the microwave electric field in the gaps between the asbestos fibers resulting in a rapid heating of the fibers inside the debris. A continuous treatment plant having a capacity of 2000 kg day(-1) of asbestos-containing waste was built in the area affected by the earthquake disaster. This treatment plant consists of a rotary kiln to burn the combustible waste (wood) and a microwave rotary kiln to treat asbestos-containing inorganic materials. The hot flue gas produced by the combustion of wood is introduced into the connected microwave rotary kiln to increase the energy efficiency of the combined process. Successful operation of this combined device with regard to asbestos decomposition is demonstrated.

In plain English

This was a laboratory and engineering study, not a health study. The authors heated asbestos-cement debris from the tsunami that followed the Great East Japan Earthquake of March 11, 2011. The debris contained about 9.5% chrysotile by weight. They compared microwave heating at 800 and 900 °C with electric-furnace heating at 800, 900 and 1000 °C. By X-ray analysis, the chrysotile signal after microwave heating was lower but still present at 800 °C and gone at 900 °C. Furnace samples still showed it at 800 and 900 °C, but not at 1000 °C. After microwave treatment at 900 °C, microscopy found one chrysotile fiber in 3000 particles. The authors built a plant in Natori City, Miyagi Prefecture, able to treat 2000 kg of asbestos-containing waste a day. Its treated waste had 0 chrysotile fibers per 3000 particles at 900 °C and 9 at 750 °C. Japanese law sets 3 or fewer. No asbestos fibers were found in the plant's filter dust. The authors concluded that microwave heating can reach this result at temperatures as low as 800 °C, versus 1600 °C for current thermal treatment.

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 comparing microwave heating with electric-furnace heating of asbestos-containing waste, followed by operation of a full-scale continuous treatment plant (engineering study)
• Who was studied: Asbestos-cement waste separated from debris generated by the tsunami following the Great East Japan Earthquake, supplied by Miyagi Prefecture, Japan. No people were studied.
• Exposure: Chrysotile asbestos in cement waste (about 9.5% by weight). In the laboratory it was treated by microwave heating (2.45 GHz) with results reported at 800 and 900 °C, or by electric-furnace heating at 800, 900 and 1000 °C. Plant runs were at 750 and 900 °C.
• Main results:
  • Laboratory microwave heating to 800 °C: the X-ray signal of chrysotile was significantly lower than in untreated waste but did not totally disappear
  • Laboratory microwave heating to 900 °C: the X-ray signal of chrysotile disappeared
  • Electric-furnace heating: samples at 800 and 900 °C still showed a signal for asbestos fibers despite longer heating. Samples at 1000 °C did not.
  • Laboratory microwave treatment at 900 °C: phase-contrast microscopy found one chrysotile fiber in a random sample of 3000 particles. Under JIS A 1481, fewer than three fibers counts as completely treated.
  • Plant (microwave rotary kiln) operated at 900 °C: 0 chrysotile fibers per 3000 particles
  • Plant operated at 750 °C: 9 chrysotile fibers (range 5 to 15) per 3000 particles. Japanese law stipulates 3 or fewer per 3000 particles.
  • Plant filter dust: maximum dioxin value of 0.78 ng TEQ per gram after several weeks of operation (Japanese limit: lower than 5 ng TEQ per gram)
  • No asbestos fibers were detected in the plant's filter dust
• The authors' conclusion: The authors concluded that microwave heating can reduce chrysotile fibers to the legal limit at temperatures as low as 800 °C, compared with the 1600 °C used in current thermal treatment. They also concluded that the full-scale plant destroyed asbestos fibers to the limit set by Japanese law.

Topics

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

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