The thermal transformation of Man Made Vitreous Fibers (MMVF) and safe recycling as secondary raw materials (SRM)

Our plain-English summary of The thermal transformation of Man Made Vitreous Fibers (MMVF) and safe recycling as secondary raw materials (SRM), by A.F. Gualtieri, E. Foresti, I.G. Lesci, et al (Journal of Hazardous Materials, 2009).

The study

• Authors: A.F. Gualtieri, E. Foresti, I.G. Lesci, N. Roveri, M.L. Gualtieri, M. Dondi, M. Zapparoli
• Journal: Journal of Hazardous Materials
• Published: 2009;162(2-3):1494-506
• Cited by: 8 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

This work describes the high temperature reaction sequence of commercial Man Made Vitreous Fibers (MMVF) Cerafiber, Superwool, Rock wool and Glass wool which may be used as substitute for asbestos in some industrial applications. Knowledge of the reaction path and transformation sequence is very important to assess whether carcinogenic crystalline phases are formed during devitrification, which may occur when used as insulators. In addition, knowledge about the nature of the phases formed at high temperature is mandatory to assess if thermally transformed MMVF can be safely recycled as secondary raw material (SRM). In this scenario, this study provides useful information for the optimization of the industrial annealing process aimed to attain a safe, recyclable product. The results of this work show that one of the high-temperature products of Cerafiber and Superwool is cristobalite which is classified as a carcinogenic. It was possible to define the temperature interval at which Cerafiber and Superwool fibers can be safely used as thermal insulators (e.g. insulators in tunnel and/or roller kilns, etc.). As cristobalite is formed in both synthetic fiber products at temperatures higher than 1200 degrees C, their use should be limited to devices operating at lower temperatures. Rock and Glass wool melt upon thermal treatment. As far as the industrial process of inertization is concerned, a maximum firing temperature of 1100 and 600 degrees C is required to melt Rock wool and Glass wool, respectively, with the high-temperature products that can be safely recycled as SRM. Recycling of these products in stoneware tile mixtures were subsequently attempted. The addition of 1-2 wt.% of the melts of Rock and Glass wool gave promising results in terms of viscous sintering reactions and resistance to staining with the only weak characteristic being the color properties of the fired bodies which tend to worsen.

In plain English

This laboratory study heated four commercial man-made vitreous fibre products that the authors note may replace asbestos in some uses: Cerafiber, Superwool, a Rock wool and a Glass wool. Samples were heated for 72 hours per step (Superwool's 1300 °C step was a 1-hour hold), and the crystals formed were identified. Cerafiber formed mullite and around 16 wt.% cristobalite at 1300 °C; the authors describe cristobalite as classified as carcinogenic. Superwool turned mainly into diopside, with around 3 wt.% cristobalite at 1300 °C. Rock wool melted at 1100 °C and Glass wool at around 600 °C. Adding 1–2 wt.% of the melted wools to stoneware tile mixtures gave what the authors called promising results, though tile colour worsened. The authors concluded that heat-treated Cerafiber and Superwool, exceeding the 0.1 wt.% Italian cristobalite limit, cannot be recycled as secondary raw material, but the Rock and Glass wool melts can. Because cristobalite formed above 1200 °C, they concluded these insulators should be limited to lower temperatures, citing producers' continuous limits of 1180 °C (Cerafiber) and 1000 °C (Superwool blanket).

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 materials-science experiment. Commercial fibre samples were heated at set temperatures and analysed by thermal analysis, X-ray powder diffraction, infrared spectroscopy and electron microscopy. The melted wools were then tested as an ingredient in stoneware tile mixtures.
• Who was studied: Four commercial man-made vitreous fibre products: Cerafiber (refractory ceramic fibre), Superwool 607 Max (calcium magnesium silicate fibre), Tervol WF K Rock wool, and Rotaflex Super TFL Glass wool. No human or animal subjects.
• Exposure: Heat treatment of the fibres, 72 hours per step. Cerafiber was heated at 800 to 1300 °C. Superwool was heated at 700 to 1200 °C, followed by a 1-hour hold at 1300 °C. Rock wool was heated at 600 to 1100 °C, and Glass wool at 300 to 600 °C.
• Main results:
  • Cerafiber devitrified (crystallised) into mullite and cristobalite.
  • After firing at 1300 °C, the Cerafiber transformation product contained around 16 wt.% cristobalite, well above the 0.1 wt.% threshold in Italian regulation. The authors identify cristobalite as the carcinogenic substance.
  • Superwool devitrified into diopside. Cristobalite also formed at 1200 °C, and about 3 wt.% cristobalite was present after firing at 1300 °C.
  • Cristobalite formed in both Cerafiber and Superwool after heat treatment at temperatures higher than 1200 °C.
  • Rock wool melted at 1100 °C. Glass wool, because it contains boron, melted at around 600 °C.
  • No crystalline phases were observed in the heated Glass wool.
  • Adding 1 to 2 wt.% of the melted Rock and Glass wool to stoneware tile mixtures gave promising results. Resistance to staining was slightly improved, but the colour properties of the fired tiles tended to be worse.
• The authors' conclusion: The authors concluded that Cerafiber and Superwool form considerable amounts of carcinogenic cristobalite at high temperature and therefore cannot be recycled as secondary raw material, whereas Rock and Glass wool form a melt and can be safely recycled. Because cristobalite formed in Cerafiber and Superwool above 1200 °C, they concluded that use of these fibres as insulators should be limited to lower temperatures. They noted that the producers report continuous maximum service temperatures of 1180 °C for Cerafiber and 1000 °C for Superwool blanket.
• Limitations the authors state:
  • The authors note that respirable crystalline silica is not the only concern. After-service insulation removed from industrial furnaces may contain other potentially toxic constituents, such as heavy metals and petroleum residues.

Topics

Medical terms in this study, explained in our Lexicon: Vitreous, Carcinogenic, Crystalline, Insulator.

Related studies

• In situ ESEM study of the thermal decomposition of chrysotile asbestos in view of safe recycling of the transformation product — A.F. Gualtieri, M.L. Gualtieri, et al. (2008)
• Recycling the product of thermal transformation of cement-asbestos for the preparation of calcium sulfoaluminate clinker — A. Viani, A.F. Gualtieri (2013)
• Microwave thermal inertisation of asbestos containing waste and its recycling in traditional ceramics — C. Leonelli, P. Veronesi, et al. (2006)
• 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)
• Determinants of the pathogenicity of man-made vitreous fibers (MMVF) — G. Oberdörster (2000)
• Cytotoxic and oxidative effects induced by man-made vitreous fibers (MMVFs) in a human mesothelial cell line — D. Cavallo, A. Campopiano, et al. (2004)

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