TG/DSC study of the thermal behaviour of hazardous mineral fibres

Abstract

For the first time, this paper reports a systematic and comparative study of the thermal behaviour of fibres of social, health, economic and industrial relevance using thermogravimetric and differential scanning calorimetry (TG/DSC). The mineral fibres selected for the study are: three chrysotiles samples, crocidolite, tremolite asbestos, amosite, anthophyllite asbestos and fibrous erionite.Powder X-ray diffraction (PXRD) and scanning electron microscopy combined with energy dispersive spectrometry (SEM/EDS) were used for the characterization of the mineral fibres before and after heating at 1000 or 1100 °C to identify the products of the thermal decomposition at a microscopic and structural scale and characterize their thermal behaviour.TG/DSC data allowed the determination of the structural water content and temperature stability.Furthermore thermal analysis provided a sensitive and reliable technique for the detection of small quantities of different mineral phases occurring as impurities. After thermal treatment, fibrous samples were completely transformed into various iron oxide, cristobalite and other silicate phases which preserved the original overall fibrous morphology (as pseudomorphosis). Only crocidolite at 1100 °C was partially melted and an amorphous surface was observed.

Effect of Grinding on Chrysotile, Amosite and Crocidolite and Implications for Thermal Treatment

“…The X-ray diffraction pattern of the 30 s mechanically ground chrysotile was characterized by sharp peaks (Figure 1), indicating a high level of crystallinity of the materials as already observed in previous studies [46]. After 5 min of grinding, all reflections disappeared except for the (002), (−111) and (004) reflections which were still visible due to texture effects, even if with remarkably reduced intensity ( Figure 1).…”

“…After 5 min of grinding, the absorbed water in the same temperature range increased to 0.50% while it decreased to 0.42% after 10 min of grinding (Table 2). As we can see from the DSC patterns (Figure 7), after 30 s of grinding, the thermal decomposition of crocidolite took place in the range 350-700 • C with two distinct endothermic events: the first stage at 374 • C is related to the partial dehydroxylation during which hydrogen ions and electrons were lost, to form an oxy-crocidolite (crocidolite partially dehydrogenated) [55], while the second endothermic effect at 650 • C represents the main dehydroxylation of crocidolite as already discussed in Bloise et al [46]. The first endothermic peak was no longer observable when the sample was ground for 5 and 10 min.…”

“…The crystallinity, thermal behaviour, and other features of these samples have been already published elsewhere [46][47][48][49]. Dry grinding of the samples (1 g) was conducted using a Bleuler Rotary Mill for 30 s, 5 and 10 min at a speed of 900 revolutions per min (rpm).…”

“…After 30 s of grinding, the DSC curve of the chrysotile ( Figure 4) showed an endothermic event at 660 • C related to chrysotile dehydroxylation [46]. After 5 min of grinding, the structure of chrysotile collapsed at a lower temperature as shown by the endothermic peak at 625 • C, which shifted downward to 622 • C becoming weaker in the sample ground for 10 min (Figure 4).…”

“…After 5 min of grinding, the structure of chrysotile collapsed at a lower temperature as shown by the endothermic peak at 625 • C, which shifted downward to 622 • C becoming weaker in the sample ground for 10 min (Figure 4). The exothermic peaks on the DSC curves (822, 818 and 810 • C) indicate forsterite (Mg 2 SiO 4 ) and enstatite (MgSiO 3 ) crystallization (e.g., [46,52]). Forsterite (JCPDS card 34-0189) and enstatite (JCPDS card 22-0714) were identified by XRPD as the main crystalline phases formed after combined grinding for 30 s, 5 and 10 min and thermal treatments at 1000 • C (Supplementary Figure S1).…”