Hydrothermal process development for the treatment of crocidolite asbestos waste

Our plain-English summary of Hydrothermal process development for the treatment of crocidolite asbestos waste, by C.T. Nzogo Metoule, S. Delaby, J.H. Ferrasse, et al (Waste Management & Research, 2019).

The study

• Authors: C.T. Nzogo Metoule, S. Delaby, J.H. Ferrasse, O. Boutin
• Journal: Waste Management & Research
• Published: 2019;37(9):914-924
• Read it: PubMed · Publisher

Abstract

The asbestos-containing waste management is a public health topic for countries which have used this mineral. Treatment of chrysotile (white asbestos), a phyllosilicate from serpentine, crocidolite (blue asbestos, first results on this kind of asbestos), one of the five asbestos varieties of amphibole family and asbestos-containing waste conversion process is proposed by using hydrothermal treatment in supercritical water. All samples were treated in an Inconel Batch Reactor. The treatment durations range is from 1 to 6 hours, temperatures range is from 400°C to 750°C, mass concentration range is from 0.02 to 170 mg. mL-1 and pressures are higher than 23 MPa. Ultrapure water is used for sample preparation. This ultrapure water is used to monitor mineral leaching on the aqueous phase and to avoid particle cross-contamination. Transmission electron microscopy analyses were carried out to check the presence or not of asbestos phase. According to these analyses, the best conditions of conversion were 1 hour and 0.02 mg. mL-1 for chrysotile, 3 hours and 0.02 mg. mL-1 for crocidolite and 1 hour and 20 mg. mL-1 for asbestos-containing waste, at T = 750°C. Supercritical water conditions were maintained during the whole treatment. The X-ray diffraction showed that the main phases present after treatments were riebeckite and magnetite (crocidolite), forsterite and enstatite (chrysotile), and calcite, spurrite and gehlenite (asbestos-containing waste). Finally, a scanning electron microscopy analysis was performed to monitor morphological fibre change. The elongated structure, partially fragmented, was found in all samples.

In plain English

This was a laboratory experiment, not a study of people. The authors heated asbestos in water in a reactor, in most tests above water's critical point (supercritical water), to see whether it would convert to non-asbestos material. They treated chrysotile, crocidolite, and demolition waste containing 0.4 wt% chrysotile. In dilute suspensions (about 0.02 mg per mL), electron microscopy found no chrysotile after 60 minutes at 750°C and 27 MPa. It still found chrysotile after 60 minutes at 400 to 700°C, and after 10 minutes (28 MPa) or 30 minutes (18 MPa) at 750°C. Crocidolite remained after 60 minutes at 750°C but was not found after 180 or 360 minutes. No chrysotile was found in treated waste, even at 200 mg per mL. X-ray diffraction found only forsterite and enstatite in one treated chrysotile sample. Riebeckite was still detected in a crocidolite test at 1 mg per mL, probably, the authors said, because of the concentration difference. Elongated, partly fragmented structures remained. The authors concluded the process was efficient for crystalline conversion, but elongated structures and energy cost need further work.

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: hydrothermal treatment of asbestos samples in water at high temperature and pressure in a batch reactor, with the products analysed by transmission electron microscopy (TEM), X-ray diffraction (XRD) and scanning electron microscopy
• Who was studied: Material samples, not people: Canadian chrysotile B (1.0 g), South African crocidolite (0.1 g) from the Koegas mine, and asbestos-containing waste recovered during asbestos removal from a demolished building, with a chrysotile content of 0.4 wt%. The waste was milled through 80 µm and 2000 µm screens.
• Exposure: Hydrothermal treatment in ultrapure water. Table 1 lists temperatures from 400 to 750°C, pressures from 18 to 28.8 MPa, durations from 10 to 360 minutes, and material-to-water ratios from about 0.02 to 200 mg per mL. The article gives water's critical point as 22.1 MPa and 374°C.
• Main results:
  • Pure chrysotile: TEM no longer found chrysotile after treatment at 750°C for 60 minutes (27 MPa, 0.022 mg per mL). Pure crocidolite: TEM no longer found crocidolite after 180 minutes at 750°C (24 MPa, 0.020 mg per mL).
  • Pure chrysotile was still found by TEM after 60 minutes at 400, 500, 600 and 700°C. It was also still found at 750°C after 10 minutes (28 MPa) and after 30 minutes (18 MPa).
  • Pure crocidolite was still found by TEM after 60 minutes at 750°C (26 MPa, 0.023 mg per mL). It was not found after 360 minutes at 750°C.
  • Asbestos-containing waste: TEM found no chrysotile in any of the seven treated waste samples. These included 1 hour at 750°C at up to 200 mg per mL, and material milled through the 2000 µm screen.
  • XRD of chrysotile after treatment at 740°C, 26 MPa, 20 mg per mL for 6 hours: the characteristic chrysotile peaks were not detected, and only forsterite and enstatite were found.
  • XRD detected riebeckite peaks, indicating crocidolite, both before and after treatment at 750°C, 1 mg per mL for 3 hours. TEM had shown total conversion at 0.02 mg per mL. The authors said the XRD result was probably due to the concentration difference, because XRD needs a minimum amount of material.
  • XRD of the asbestos-containing waste after 6 hours at 750°C, 20 mg per mL and 27 MPa found calcite, spurrite and gehlenite.
  • Scanning electron microscopy: elongated structures in treated chrysotile and crocidolite looked fragmented, but at low magnification a fibrous skeleton remained.
• The authors' conclusion: The authors concluded that the hydrothermal process was efficient for crystalline conversion of chrysotile, crocidolite and asbestos-containing waste, but that elongated structures persist after treatment.
• Limitations the authors state:
  • Elongated structures remained after treatment and need further investigation; the authors suggest testing higher temperatures or ultrasound.
  • Energy consumption and cost were not calculated; accurate energy calculations and comparison with other processes are still needed.
  • The authors note that, depending on operating conditions, the process can be rather expensive.
  • XRD needs a minimum amount of mineral, which limited comparison with TEM results at low concentrations.

Topics

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

Medical terms in this study, explained in our Lexicon: Crocidolite, Chrysotile, Phyllosilicates, Serpentine Group, Amphibole Group, Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD), Riebeckite.

Related studies

• Sustainable bio-hydrothermal sequencing treatment for asbestos-cement wastes — D. Spasiano, V. Luongo, et al. (2019)
• Microwave-assisted acid treatment for the mineral transformation of chrysotile as an alternative for asbestos waste management — S. Essih, L. Pardo, et al. (2024)
• Hydrothermal conversion of chrysotile asbestos using near supercritical conditions — K. Anastasiadou, D. Axiotis, et al. (2010)
• Treatment and recycling of asbestos-cement containing waste — F. Colangelo, R. Cioffi, et al. (2011)
• Assessment of asbestos body formation by high resolution FEG-SEM after exposure of Sprague-Dawley rats to chrysotile, crocidolite, or erionite — N.B. Gandolfi, A.F. Gualtieri, et al. (2016)
• Dark fermentation process as pretreatment for a sustainable denaturation of asbestos containing wastes — D. Spasiano (2018)

Browse more research in Waste Management & Research, or return to medical research.