Spectroscopic study of the product of thermal transformation of chrysotile-asbestos containing materials (ACM)

In Italy, reclamation of asbestos-containing materials (ACM) such as friable asbestos and cement-asbestos is accomplished by their removal, packaging and dumping in controlled landfills. An alternative way to landfill disposal is the thermal transformation of ACM and recycling of the transformation product as secondary raw material. The aim of this work is to integrate preliminary X-ray diffraction and microscopic investigations on the secondary raw material described earlier with a detailed study on the product of transformation at 1200 degrees C of friable chrysotile-asbestos and cement-asbestos, using (micro) Raman, (micro) FTIR, Fe-57 Mossbauer and XANES at the Fe K-edge. Micro-Raman spectra reveal that the absorption bands generated by chrysotile are no longer present in the high-temperature products, and this is further confirmed by micro-FTIR results. In the core of the former fibres of loose chrysotile asbestos, the newly formed phases are olivine and enstatite, whereas the product of transformation of cement-asbestos is composed of olivine together with several other phases such as hematite and (Ca, Mg, Al)-silicates. The Mossbauer absorption spectra of raw chrysotile reveal that iron is contained in a paramagnetic phase (40 %) as well as in accessory magnetite (60 %). The paramagnetic contribution, attributed to chrysotile, is represented by Fe2+ (10 % of Fe-tot) and Fe3+ (30 % of Fe-tot), both octahedrally coordinated. The spectrum of thermally treated chrysotile clearly shows that the magnetic phases are now oxidized magnetite/maghemite and hematite, and the paramagnetic contribution is quite unaltered, though likely due to the newly formed olivine. The spectrum of untreated cement-asbestos has no evidence of accessory magnetic phases and is made of Fe2+ (15% of Fe-tot) and Fe3+ (85% of Fe-tot), both octahedrally coordinated. In the thermally treated sample all iron is oxidized, but a phase transition occurred, because Fe3+ is tetrahedrally coordinated. Also XANES spectra show that in all samples the dominant iron oxidation state is 3+. XANES data on standard chrysotile are compatible with the possible presence of magnetite. In the high-temperature product of cement-asbestos, the high intensity of the pre-edge peak is comparable with that of the reference compound Fe-silicalite, with ferric iron hosted in the framework. This result indicates that in this product ferric iron is likely hosted in a crystalline phase in four-fold coordination, in agreement with Mossbauer spectroscopy results. Such crystalline phase could be Fe-bearing akermanite-gehlenite.